Thèse Hedieh Teymoorzadeh Doctorat en génie chimique

Composites and Foams based on Polylactic Acid (PLA)

Thèse

Hedieh Teymoorzadeh

Doctorat en génie chimique Philosophiae doctor (Ph.D.)

Québec, Canada

© Hedieh Teymoorzadeh, 2016

iii

Résumé Cette étude est destinée à la production et à la caractérisation des composites d'acide

polylactique (PLA) et des fibres naturelles (lin, poudre de bois). Le moussage du PLA et

ses composites ont également été étudiés afin d'évaluer les effets des conditions de

moulage par injection et du renfort sur les propriétés finales de ces matériaux. Dans la

première partie, les composites constitués de PLA et des fibres de lin ont été produits par

extrusion suivit par un moulage en injection. L'effet de la variation du taux de charge (15,

25 et 40% en poids) sur les caractéristiques morphologique, mécanique, thermique et

rhéologique des composites a été évalué. Dans la deuxième étape, la poudre de bois (WF)

a été choisie pour renforcer le PLA. La préparation des composites de PLA et WF a été

effectuée comme dans la première partie et une série complète de caractérisations

morphologique, mécanique, thermique et l'analyse mécanique dynamique ont été

effectués afin d'obtenir une évaluation complète de l'effet du taux de charge (15, 25 et

40% en poids) sur les propriétés du PLA. Finalement, la troisième partie de cette étude

porte sur les composites de PLA et de renfort naturel afin de produire des composites

moussés. Ces mousses ont été réalisées à l'aide d'un agent moussant exothermique

(azodicarbonamide) via le moulage par injection, suite à un mélange du PLA et de fibres

naturelles. Dans ce cas, la charge d'injection (quantité de matière injectée dans le moule:

31, 33, 36, 38 et 43% de la capacité de la presse à injection) et la concentration en poudre

de bois (15, 25 et 40% en poids) ont été variées. La caractérisation des propriétés

mécanique et thermique a été effectuée et les résultats ont démontré que les renforts

naturels étudiés (lin et poudre de bois) permettaient d'améliorer les propriétés mécaniques

des composites, notamment le module de flexion et la résistance au choc du polymère

(PLA). En outre, la formation de la mousse était également efficace pour le PLA vierge et

ses composites car les masses volumiques ont été significativement réduites.

v

Abstract This study reports on the production and characterization of natural fiber reinforced

polylactic acid (PLA) composites. Foaming PLA and its composites was also undertaken

to investigate the effect of injection molding conditions (shot size) and natural fiber (flax

and wood flour) content on the final properties of the final products.

In the first part, PLA was mixed with flax fiber via extrusion and further processed by

injection molding to manufacture the final parts. The effect of flax fiber content (15, 25,

and 40% wt.) on the morphological, mechanical, thermal, and rheological properties of

the composites was evaluated.

In the second step, wood flour (WF) was selected to reinforce PLA. Compounding of

PLA and WF was carried out in a twin-screw extruder followed by injection molding to

obtain the test specimens. A complete series of morphological, mechanical, thermal, and

dynamic mechanical analysis was performed to get a complete evaluation of WF addition

(15, 25, and 40% wt.) on the properties.

Finally, the last step studied PLA composites with natural fibers for the purpose of

foaming. Foaming was carried out using an exothermic foaming agent

(azodicarbonamide) via injection molding. Injection foaming proceeded after mixing

PLA and natural fibers by extrusion. In this case, the shot size (amount of material

injected into the mold: 31, 33, 36, 38, and 43% of the machine capacity) and

reinforcement content (15, 25, and 40% wt.) were varied. The characterization included

mechanical and thermal properties. The results showed that both flax and wood flour led

to increased mechanical properties including flexural modulus and impact strength.

Moreover, foaming was also effective for neat PLA and PLA composites, i.e. the overall

density of the parts was significantly reduced.

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Table of Contents Résumé..........................................................................................................................................................iii

Abstract...........................................................................................................................................................v

Table of Contents......................................................................................................................................vii

List of tables.................................................................................................................................................xi

List of figures............................................................................................................................................xiii

Abbreviations..............................................................................................................................................xv

Acknowledgement...................................................................................................................................xix

Foreword.....................................................................................................................................................xxi

Chapter 1. Introduction on biopolymers..............................................................................................1

1.1 Plastics......................................................................................................................................................1

1.2 Renewable-resource-based plastics................................................................................................3

1.2.1 Polysaccharides.................................................................................................................................4

1.2.1.1 Polysaccharides from vegetal sources..........................................................................4

1.2.1.2 Polysaccharides from marine sources...........................................................................5

1.2.2 Proteins.................................................................................................................................................6

1.2.2.1 Proteins from animal sources..........................................................................................6

1.2.2.2 Proteins from vegetal sources..........................................................................................7

1.2.3 Fatty-acid based polymers.............................................................................................................7

1.2.4 Biotechnologically-derived polymers........................................................................................9

1.2.5 Bacterial polymers.........................................................................................................................10

1.2.5.1 Polyhydroxyalkanoates (PHA)....................................................................................11

1.2.5.2 Polylactic acid (PLA)......................................................................................................12

Chapter 2. Polylactic acid (PLA).........................................................................................................14

2.1 Synthesis of lactic acid....................................................................................................................14

2.2 Synthesis of lactide monomers.....................................................................................................15

2.3 Polymerization of lactide monomers..........................................................................................16

2.3.1 Polycondensation...........................................................................................................................17

2.3.2 Ring-opening...................................................................................................................................20

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Chapter 3. PLA modification................................................................................................................22

3.1 Totally biodegradable PLA composites.....................................................................................22

3.2 PLA composite foams......................................................................................................................26

3.3 Thesis Objectives...............................................................................................................................32

Chapter 4. Biocomposites of Flax Fiber and Polylactic Acid: Processing and Properties34

Résumé.........................................................................................................................................................34

Abstract........................................................................................................................................................35

4.1 Introduction.........................................................................................................................................36

4.2 Materials and methods.....................................................................................................................39

4.2.1 Composite preparation.................................................................................................................39

4.2.2 Mechanical characterization.......................................................................................................39

4.2.3 Thermal properties.........................................................................................................................40

4.2.4 Scanning electron microscopy (SEM)....................................................................................40

4.2.5 Density...............................................................................................................................................41

4.2.6 Rheology...........................................................................................................................................41

4.3 Results and discussion.....................................................................................................................41


4.3.2 Density...............................................................................................................................................43

4.3.3 Mechanical properties..................................................................................................................43

4.3.4 Thermal Properties........................................................................................................................44

4.3.5 Rheology...........................................................................................................................................45

4.4 Conclusion...........................................................................................................................................51

Acknowledgement....................................................................................................................................52

Chapter 5. Biocomposites of Wood Flour and Polylactic Acid: Processing and Properties

.........................................................................................................................................................................53

Résumé.........................................................................................................................................................53

Abstract........................................................................................................................................................54

5.1 Introduction.........................................................................................................................................55


5.2.1 Sample preparation........................................................................................................................57

5.2.2 Mechanical characterization.......................................................................................................58

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5.2.4 Dynamic mechanical analysis (DMA)....................................................................................60


5.2.6 Density...............................................................................................................................................62

5.3 Results and discussion.....................................................................................................................62


5.3.2 Density...............................................................................................................................................63

5.3.3 Mechanical properties..................................................................................................................63

5.3.4 Dynamic mechanical properties................................................................................................66


5.4 Conclusion...........................................................................................................................................68

Acknowledgment......................................................................................................................................68

Chapter 6. Morphological, Mechanical, and Thermal Properties of Injection Molded PLA

Foams/Composites based on Wood Flour........................................................................................69

Résumé.........................................................................................................................................................69

Abstract........................................................................................................................................................70

6.1 Introduction.........................................................................................................................................71


6.2.1 Sample Preparation........................................................................................................................73

6.2.2 Scanning Electron Microscopy (SEM)...................................................................................76

6.2.3 Foam Cell Characterization........................................................................................................76

6.2.4 Mechanical Characterization......................................................................................................76


6.2.6 Density...............................................................................................................................................77

6.3 Results and Discussion....................................................................................................................77

6.3.1 Density...............................................................................................................................................77

6.3.2 Foam cell morphology.................................................................................................................79

6.3.3 Cell Diameter..................................................................................................................................82

6.3.4 Cell-Population Density...............................................................................................................84

6.3.5 Skin thickness..................................................................................................................................85

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6.3.6 Mechanical Properties..................................................................................................................86

6.3.6.1 Shot size...............................................................................................................................86

6.3.6.2 Wood Flour Reinforcement...........................................................................................88


6.3.7.1 Shot size...............................................................................................................................91

6.3.7.2 Wood Flour.........................................................................................................................92

6.4 Conclusion...........................................................................................................................................93

Acknowledgements..................................................................................................................................94

Chapter 7. Conclusions and recommendations...............................................................................95

7.1 Conclusion...........................................................................................................................................95

7.2 Recommendations.............................................................................................................................97

References...................................................................................................................................................99

Appendix (A): Injection molding condition..................................................................................109

A.1 PLA/wood flour and flax fiber composites...........................................................................109

A.2 Neat foamed PLA..........................................................................................................................110

A.3 Foamed PLA/wood flour composites.....................................................................................111

Appendix (B): Comparison of mechanical properties between commercialized

composites and PLA based composites reinforced with wood flour and flax fiber........112

B.1 Introduction......................................................................................................................................112

B.2 Discussion........................................................................................................................................112

B.3 Reference..........................................................................................................................................116

Appendix (C): DSC thermographs of the samples.....................................................................118

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List of tables

Table 1.1 Characteristics of some commercial fatty acids [21]. ......................................... 8

Table 3.1 Comparison of the mechanical properties between various natural fibers and E-

glass fiber [47]. ......................................................................................................... 24

Table 4.1 Extrusion and injection molding temperature profile used. .............................. 40

Table 4.2 Density of the materials. ................................................................................... 43

Table 4.3 Effect of flax content on the thermal properties of PLA. ................................. 47

Table 5.1 Extrusion and injection molding temperature profiles. .................................... 57

Table 5.2 Injection molding pressure and speed. .............................................................. 58

Table 5.3 Density of neat PLA, wood flour, and PLA/wood flour composites. ............... 61

Table 5.4 Dynamic mechanical analysis of neat PLA and PLA/wood flour composites. 64

Table 5.5 Thermal properties of neat PLA and PLA/wood flour composites. ................. 67

Table 6.1 Extrusion and injection molding temperature profile. ...................................... 74

Table 6.2 Shot sizes of unfoamed and foamed PLA samples with coding. ...................... 75

Table 6.3 Injection molding pressure. ............................................................................... 75

Table 6.4 Density (±0.01 g/cm3) of the samples produced. .............................................. 78

Table 6.5 Specific mechanical properties of foamed and unfoamed PLA with different

shot sizes (see Table 6.2 for the definition). ............................................................. 88

Table 6.6 Specific mechanical properties of PLA and PLA/wood flour composites (see

Table 6.2 for the definition). ..................................................................................... 91

Table 6.7 Thermal properties of the sample produced (see Table 6.2 for the definition). 93

Table B.1 Comparison of mechanical properties between PE/wood flour and PLA/wood

flour composite. ...................................................................................................... 115

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List of figures Figure 1.1 Classification of bioplastics with typical examples [6]. .................................... 2

Figure 1.2 Diagram of sustainability [5]. ............................................................................ 3

Figure 1.3 Production of biobased polymers with typical examples for each class [8]. ..... 4

Figure 1.4 Typical chemical structure of a triglyceride molecule [21]. .............................. 8

Figure 1.5 Chemical structure of some common polyhydroxyalkanoates [23]. ............... 12

Figure 2.1 Chemical structure of L, D, and DL lactic acid [35]. ...................................... 14

Figure 2.2 Chemical synthesis of lactide via lactonitrile [35]. ......................................... 15

Figure 2.3 Chemical structures of various lactides [35]. .................................................. 15

Figure 2.4 Synthesis approach to lactide from lactic acid via oligolactide [35]. .............. 15

Figure 2.5 Examples of microstructures of lactide and PLA [35]. ................................... 17

Figure 2.6 Equilibrium reactions between PLLA, L-lactic acid, and L-lactide [35]. ....... 18

Figure 2.7 Diagram of the azeotropic dehydration polycondensation of LA [36]. ........... 19

Figure 3.1 Schematic representation of a lignocellulosic fiber [48]. ................................ 23

Figure 4.1 SEM images of: a) PLA/15% flax (x25), b) PLA/15% flax (x250), c)

PLA/25% flax (x25), d) PLA/25% flax (x250), e) PLA/40% flax (x25), f) PLA/40%

flax (x250). ................................................................................................................ 42

Figure 4.2 Effect of flax fiber content on the flexural modulus of PLA. ......................... 46

Figure 4.3 DSC thermographs for PLA and PLA/flax fiber composites. ......................... 46

Figure 4.4 Dynamic viscosity as a function of frequency and flax fiber content (170 oC).

................................................................................................................................... 47


................................................................................................................................... 48

Figure 4.6 Storage modulus as a function of frequency and flax fiber content (170 oC). 49

Figure 4.7 Storage modulus as a function of frequency and flax fiber content (190 oC). 50

Figure 5.1 SEM images of a) PLA/15% WF (x25), b) PLA/15% WF (x250), c) PLA/25%

WF (x25), d) PLA/25% WF (x250), e) PLA/40% WF (x25), and f) PLA/40% WF

(x250). ....................................................................................................................... 59

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Figure 5.2 Flexural modulus of neat PLA and PLA composites. ..................................... 61

Figure 5.3 Notched Izod impact strength of neat PLA and PLA composites. .................. 62

Figure 5.4 Shore D hardness of neat PLA and PLA composites. ..................................... 63

Figure 5.5 Dynamic storage modulus of neat PLA and PLA/WF composites. ................ 65

Figure 6.1 SEM images at different magnification of the foamed samples: a,b) SS1, c,d)

SS2, e,f) SS3, g,h) SS4, i,j) SS5, k,l) foamed PLA/15% WF, m,n) foamed PLA/25%

WF, and o,p) foamed PLA/40% WF (see Table 6.2 for the definition). .................. 82

Figure 6.2 Cell diameter of the samples produced (see Table 6.2 for the definition). ..... 83

Figure 6.3 Cell-population density of the samples produced (see Table 6.2 for the

definition). ................................................................................................................. 85

Figure 6.4 Skin thickness of foamed PLA and foamed PLA/WF composites (see Table

6.2 for the definition). ............................................................................................... 86

Figure B.1 DSC thermogram of HDPE/wood flour composites at different wood flour

content: a) heating and b) cooling [5]. .................................................................... 114

Figure B.2 Shear viscosity as a function of wood flour content [5]. .............................. 116

Figure C.1 Foamed neat PLA samples. .......................................................................... 118

Figure C.2 Unfoamed PLA/wood flour composites. ...................................................... 118

Figure C.3 Foamed PLA/wood flour composites. .......................................................... 119

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Abbreviations BC

CA

CBA

DMA

DOE

DSC

FAD

FAT

FIM

FRQNT

HDT

3-HHx

3-HV

MCL

MDF

MSW

Nf

NBSK

NSERC

OLLA

PALF

PBA

PBAT

PBS

PCL

PDLA

PDLLA

Bamboo charcoal

Cellulose acetate

Chemical blowing agent

Dynamic mechanical analysis

Department of energy

Differential scanning calorimetry

Fatty acid dimer

Fatty acid trimer

Foam injection molding

Fonds de Recherche Nature et Technologie du Québec

Heat deflection temperature

3-hydroxyhexanoate

3-hydroxyvalerate

Medium chain length

Medium density fiberboard

Municipal solid waste

Cell-population density

Northern bleached softwood Kraft

National Science and Engineering Research Council of Canada

Oligo(lactic acid)

Pattawia pineapple leaf fiber

Physical blowing agent

Poly(butylene adipate-co-terephthalate)

Polybutylene succinate

Polycaprolactone

Poly(D-lactic acid)

Poly(DL-lactic acid)

xvi

PE

PEG

PET

PHA

PHB

PHBV

PLA

PP

PTT

PVOH

ROP

Sb-PLA

SCL

Sc-PLA

SEM

SSP

Tc

Tg

TGA

Tm

TSA

WF

ηo*

χc

Polyethylene

Polyethylene glycol

Polyethylene terephthalate

Polyhydroxyalkanoate

Polyhydroxybutyrate

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

Polylactic acid

Polypropylene

Polytrimethylene terephthalate

Poly(vinyl alcohol)

Ring opening polymerization

Stereo-block polylactic acid

Short chain length

Stereo-complex polylactic acid

Scanning electron microscopy

Solid state polymerization

Crystallization temperature

Glass transition temperature

Thermogravimetric analysis

Melting temperature

p-toluenesulfonic acid

Wood flour

Zero shear viscosity

Degree of crystallinity

xvii

To my Parents Homa and Hooshang

xix

Acknowledgement First and most of all, I would like to express my sincere gratitude to Professor Denis

Rodrigue whom this work would never have been done without his support. During the

past three years, he has wisely guided me through different steps of my work and I am so

thankful for being part of his research group.

I would also like to thank the Department of Chemical Engineering at Université Laval,

for providing me with exceptional academic opportunities.

I am also grateful to Mr. Yann Giroux for all his technical help and kindness throughout

my laboratory works.

I acknowledge the financial support from Fonds de Recherche Nature et Technologie du

Québec (FRQNT) and Natural Sciences and Engineering Research Council of Canada

(NSERC).

I would also like to thank all my good friends at Université Laval who were genuinely

supportive during my studies, especially Atieh, Amir, Fatima, and Désiré.

Above all, my genuine appreciation goes to my parents for their love and support

throughout my life. A very special thanks goes to my siblings, especially Hilda for all her

kindness and motivational talks.

xxi

Foreword This thesis is composed of seven chapters. In the first three chapters, a detail-oriented

literature review on biodegradable and natural resource-based polymers, their composites

and foam structures is presented. Chapters 4, 5, and 6 showcase our published

experimental works in international academic journals. Finally, chapter 7 consists of

conclusions of the work, as well as suggestions for future works.

Chapter 4

In this chapter, composites of PLA/flax fiber (15, 25, and 40% wt.) are produced through

extrusion and injection molding. A complete series of characterizations are undertaken to

evaluate the mechanical, thermal, and rheological properties of the aforementioned

composites. Scanning electron microscopy is also used to show that good interaction

between both PLA and flax fiber phases was obtain even without the use of any coupling

agents/fiber treatments. This study is published as:

H. Teymoorzadeh, and D. Rodrigue, Biocomposites of flax fiber and polylactic acid:

processing and properties. J. Renew. Mater., 2(4), 270-277 (2014).

Chapter 5

Throughout this chapter, PLA reinforced with wood flour is manufactured via extrusion

and injection molding at three wood flour concentrations (15, 25, and 40% wt.).

Mechanical, thermal, and dynamic mechanical analyses of PLA/wood flour composites

are presented to investigate the effect of wood flour addition on the aforementioned

properties. Scanning electron microscopy images are also presented to describe that good

interaction between PLA and wood flour was obtained. The results of this work are

published as:

xxii

H. Teymoorzadeh and D. Rodrigue, Biocomposites of wood flour and polylactic acid:

processing and properties. J. Biobased Mater. Bioenergy, 9(2), 252-257 (2015).

Chapter 6

This chapter investigates the effect of injection molding shot size and wood flour

reinforcement on the foaming behavior of PLA. Samples are produced through extrusion

and injection molding and a complete series of characterizations including morphology,

mechanical, and thermal properties was performed to study the effect of injection

molding shot size and wood flour addition on the properties of the foamed PLA and

foamed PLA composites. All the data and information on this work can be found in:

H. Teymoorzadeh, and D. Rodrigue, Morphological, Mechanical, and Thermal Properties

of Injection Molded PLA Foams/Composites based on Wood Flour, J. Cell. Plast.,

Submitted (2015).

Chapter 7

The final chapter is devoted to general conclusions on the work performed, followed by

the presentation of some suggestions for future works.

1

Chapter 1. Introduction on biopolymers

1.1 Plastics Plastics have been considered as suitable and promising materials designed for the

manufacture of various applications from automotive and construction to packaging and

medical appliances. Since their first appearance in 1907, plastics gradually replaced

series of materials such as wood, metals, and ceramics as a result of their durability,

lightweight, application versatility, and easy processing [1]. In addition, the use of

plastics within different industrial sectors resulted in economical efficiency of the

production due to their low cost. In transportation, for example, the use of plastics in the

production of different parts reduces weight and therefore decreases fuel consumption [1,

2]. Both synthetic and semi-synthetic plastics are generally derived from fossil carbon

sources such as crude oil and natural gas [3]. Although, these petroleum-based plastics

are classified as biodegradable, their degradation rate is very low to the point that they

can virtually be considered as non-biodegradable [4].

Biodegradability of plastics is defined as the capability of these materials to undergo

microbial chain scission or enzymatic decomposition in a defined period of time when

the former leads to mineralization, photodegradation, oxidation, and hydrolysis while the

latter results in the production of carbon dioxide (CO2), methane, inorganic compounds,

or biomass [5]. The resistance of petroleum-based plastics to biological degradation

brings about challenges in sustainable waste management operations. It is estimated that

approximately 8% of the 31 million tons of annually generated plastic waste is recycled

in the U.S. [3]. As a result, a considerable amount of plastic waste is currently landfilled

or released into the nature. Based on a report by the World Bank, about 8-12% of all

municipal solid waste (MSW) belong to plastic waste, which is also predicted to reach 9-

13% by 2025 [1]. Moreover, waste management systems have been faced with another

serious problem: a large amount of plastic waste is commingled with organic wastes such

as food, paper, soil, etc., which makes it difficult to recycle both plastics and organic

fractions without the use of expensive cleaning, separating, and sanitizing procedures. In

2

addition to the above mentioned drawbacks of petroleum-based plastics, the hazardous

effects of these materials on the environment due to the emission of carbon dioxide (CO2)

along with their production based on non-renewable resources having highly fluctuating

prices due to political and environmental conflicts, have initiated a global search for

outstanding candidates offering better biodegradability, renewability, and low costs [3].

Prior to exclusively focus on biobased plastics from renewable and natural resources, one

must distinguish the difference between the aforementioned plastics and bioplastics.

Generally, bioplastics are composed of [6]:

1. Petroleum-based biodegradable polymers, which are synthesized from petroleum

resources but are biodegradable,

2. Renewable-resource-based polymers, which are either obtained naturally from

plants and animals or completely synthesized from renewable resources,

3. Polymers from mixed sources, which are composed of renewable-resource-based

and petroleum-based monomers.

Figure 1.1 presents a classification of these materials with typical examples for each

class.

Figure 1.1 Classification of bioplastics with typical examples [6].

3

1.2 Renewable-resource-based plastics The design and development of the next generation of materials and processes rely on

sustainability, eco-efficiency, industrial ecology, and green chemistry [5]. Biodegradable

plastics emerging from annually renewable agricultural and biomass feedstocks guarantee

the sustainability and eco-efficiency of the products while maintaining a competition with

conventional yet hazardous petroleum-based plastics in many sectors. The sustainability

of renewable-based plastics is defined as their recycling capability, biodegradability,

commercial viability, and environmental acceptability (Figure 1.2).

The Technology Road Map for Plant/Crop-based Renewable Resources 2020, sponsored

by the U.S. Department of Energy (DOE), has the objective to produce 10% of the basic

chemical building blocks from plant-derived renewable resources by 2020, which will be

increased to 50% by 2050 [7]. Furthermore, the U.S. agricultural, forestry, life science,

and chemical communities have proposed a strategic plan to use crops, agricultural

residues, and biomass feedstocks in the manufacturing of sustainable industrial products

[5].

Figure 1.2 Diagram of sustainability [5].

Plastics based on renewable resources are currently competing with commodity resins

such as polyethylene (PE) and polypropylene (PP). The best examples of renewable-

resource-based plastics are polysaccharides (from marine and vegetal sources), proteins

(from animal and vegetal resources), and bacterial polymers (polylactic acid and

microbial polyesters). Today, three different procedures can be used to produce

renewable-resource-based plastics [8]:

4

1. Using natural renewable-resource-based polymers with partial modification

(cellulose),

2. Producing monomers by fermentation or conventional chemistry following

polymerization (polylactic acid),

3. Producing renewable-based polymers directly from bacteria

(polyhydroxyalkanoates).

Figure 1.3 presents this classification with typical examples for each class.

Figure 1.3 Production of biobased polymers with typical examples for each class [8].

In the next sections, a concise review of the most important biodegradable polymers

originating from natural resources including polysaccharides, and proteins will be given

[9].

1.2.1 Polysaccharides

1.2.1.1 Polysaccharides from vegetal sources

Cellulose and starch are the most commonly used materials in this category. Cellulose is

a linear polymer with long macromolecular chains of cellobiose as the repeating units

[10]. Cellulose is a crystalline, infusible, and insoluble substance in organic solvents.

Biodegradation of cellulose proceeds by both enzymatic oxidation and bacteria

degradation. Due to its insolubility and infusibility, cellulose should be modified on one

or more of the hydroxyl groups in its repeating units before it can be processable as a

cellulose-based plastic. Ethers, esters, and acetals are the main cellulose derivatives [9].

5

Cellulose esters are modified polysaccharides, which have different degrees of

substitution. As the degree of substitution increases, their mechanical properties and

biodegradability decrease. One of the most important cellulose derivatives is cellulose

acetate (CA). Cellulose acetate can be derived from agricultural by-products. Through the

conversion of lignocellulose to ethanol via a four-step procedure including enzymatic

saccharification and fermentation of hemicellulose sugars, cellulose is obtained as a

residue, which can be further processed to produce cellulose acetates. The degree of

substitution in cellulose acetates also determines the mechanical properties and the

biodegradation rate of these polymers. Commercially available cellulose acetate has a

degree of substitution between 1.7 and 3 [11]. Tensile strength of cellulose acetate films

is similar to polystyrene. Cellulose acetate is mainly used in fiber and film applications.

Starch is an abundantly available polysaccharide and one of the cheapest biodegradable

polymers. Starch is mainly extracted from potatoes, corn, wheat, and rice [9]. It is

composed of a linear and crystalline polymer known as amylose (poly-𝛼𝛼 -1,4-D-

glucopyranoside) and a branched and amorphous polymer known as amylopectine (poly-

𝛼𝛼-1,4-D-glucopyranoside and 𝛼𝛼-1,6-D-glucopyranoside). Various amounts and molar

masses of amylose and amylopectine are found in different starch sources, which

correspondingly alter the mechanical properties and biodegradation of their polymers. It

was shown that by increasing the amylose content, the elongation at break and strength

also increase [12, 13]. Biodegradation of starch is obtained by hydrolysis at the acetal

link under the effect of enzymes. The 𝛼𝛼 -1,4 link is attacked by amylases while

glucosidases attack the 𝛼𝛼 -1,6 link. However, starch-based polymers have some

disadvantages such as water sensibility, brittleness, and poor mechanical properties [9].

There are several approaches to overcome the aforementioned drawbacks including

modification of starch by acetylation, grafting with monomers (styrene and

methylmethacrylate), and blending with synthetic biodegradable and non-biodegradable

polymers [14, 15].

1.2.1.2 Polysaccharides from marine sources Although cellulose and starch are the main polysaccharides used in the fabrication of bio-

based polymers, two other molecules have been studied on a more limited scale. Chitin is

6

the second abundantly available natural biopolymer, which is mainly found in the shells

of crabs, shrimp, crawfish, and insects [9]. Recent studies suggest that by advances in

fermentation technology, the culture of fungi can provide an alternative source of chitin

[16]. Chitin is a linear copolymer of N-acetyl-glucosamine and N-glucosamine with 𝛽𝛽-1,4

linkage, which has poor solubility in solvents. According to the processing method, these

units are randomly or block distributed throughout the polymer chain. An enzyme, known

as chitinase, can degrade the chitin biopolymer [9].

By partial alkaline N-deacetylation of chitin, chitosan is produced. The glucosamine units

in chitosan are predominant to acetyl glucosamide. The ratio of glucosamide to acetyl

glucosamide defines the degree of deacetylation. Depending on the preparation method

this degree could range from 30 to 100%, which influences crystallinity, surface energy,

and degradation rate of chitosan [17]. Due to the presence of a rigid and compact

crystalline structure combined with strong intra- and inter-molecular hydrogen bonding,

chitosan is insoluble in water and alkaline media. The degradation of chitosan relies on

the effect of enzymes such as chitosanase or lyzozymes. Generally, due to the insolubility

of chitin and chitosan in most solvents, they have limited applications in cosmetics and

wound treatment.

1.2.2 Proteins Proteins are thermoplastic heteropolymers consisting of both polar and non-polar 𝛼𝛼-

aminoacids [9]. The ability of aminoacids to form various intermolecular linkages results

in a wide range of applications and functional properties. Due to the insolubility and

infusibility of the aforementioned polymers, they are generally used in their natural form

(silk, wool, and collagen). The proteins can be classified in two categories depending on

their source: animal or vegetal.

1.2.2.1 Proteins from animal sources Collagen is the primary constituent of animal connective tissues. Collagen is composed

of various polypeptides which mainly contains glycine, proline, hydroxyproline, and

lysine [18]. Enzymatically degradable collagen has outstanding biological properties and

extensive studies on its biomedical applications are available. Another example of protein

7

from animal sources is gelatin which is a high molecular weight polypeptide produced

via denaturation and/or physico-chemical degradation of collagen. Gelatine is soluble in

water with good film forming abilities. Gelatine is composed of 19 aminoacids for which

composition and molecular weight distribution control the mechanical and barrier

properties of this polymer [9]. Proteases can degrade gelatin by hydrolyzing the amide

function. Elastin, albumin, and fibrin are also protein-based polymers from animal

sources, which have been investigated for biomedical applications.

1.2.2.2 Proteins from vegetal sources Proteins from vegetal sources are abundantly available with annual production rates of

kilotons. One of the most important examples of this category is wheat gluten which is a

by-product of starch processing. Wheat gluten consists of two main groups of proteins:

gliadin and glutenin [9]. Gliadins are protein molecules with disulphide bonds having low

molecular weight and a low level of aminoacids with charged sides. Glutenins are three-

dimensional structures with higher molecular weight. Wheat gluten materials are fully

biodegradable with excellent film forming properties. Soy proteins are another type of

vegetal protein-based polymers, which can be formed as films. However, their

mechanical and barrier properties are lower than most protein due to their hydrophilic

nature [19].

1.2.3 Fatty-acid based polymers One of the ideal alternatives to fossil fuel is natural oil from plants and animals. In the

United States, approximately 30 billion pounds of soy oil and 25 billion pounds of corn

oil are produced [20]. Table 1.1 presents a list of commercial plant oils with their

characteristics. Plant oils consist of dominant triglyceride molecules. The structure of a

triglyceride molecule is shown in Figure 1.4 [20, 21]. Triglycerides are ester products

composed of three fatty acids linked at a glycerol link. Fatty acids are monocarboxylic

acids with long unbranched aliphatic tails (hydrocarbon chain) which are either saturated

or unsaturated. The number of carbon atoms in the hydrocarbon chain varies from 10 to

30, but normally for commonly used fatty acids, the value is between 12 and 18 carbons

[20]. About 94 to 96% of the total weight of a triglyceride molecule is dominated by fatty

8

acids [21]. Therefore, fatty acid distribution has a major effect on the physical and

chemical properties of plant oils and further on the properties of the synthesized polymer.

Biopolymers from triglycerides (fatty acids) offer biodegradability as well as

biocompatibility (suitable for biomedical).

Most fatty acids are monofunctional and act as chain terminator during polymerization

(stearic acid). Therefore, to use them as monomers in polymer synthesis, functional

groups must be introduced to their structure. Other fatty acids, namely erucic acid and

oleic acid contain double bonds, which could be converted to fatty acid dimer (FAD) or

trimer (FAT), with two or three carboxylic groups, respectively. Two different types of

fatty acids exist in nature, one without the double bond in its structure (saturated), and the

other with one or more than one double bond (unsaturated) [21].

Figure 1.4 Typical chemical structure of a triglyceride molecule [21].

Table 1.1 Characteristics of some commercial fatty acids [21].

Since fatty acids play a dominant role in the determination of plant oils properties, the

choice of a triglyceride oils is really crucial in controlling the physical and mechanical

properties of the final polymers [21]. For triglyceride oils to be used in biopolymer

synthesis, a series of chemical techniques must be applied. To achieve triglyceride-based

monomers suitable for polymer synthesis, several methods are used to activate

9

triglycerides [20]:

1. The first method is associated to the presence of active sites in the structure of

triglycerides (double bonds, allylic carbons, and ester groups). These active sites are

used to introduce polymerizable functional groups. For instance, it is possible to add

epoxy or hydroxyl functionalities to the double bonds of the triglycerides. This

functionalized triglyceride could react in a ring-opening or polycondensation

reaction, leading to the formation of a polymer.

2. In the second method, the triglyceride molecule is converted to a monoglyceride

through glycerolysis. The double bonds of the resulted monoglyceride may

contribute to a polymerization reaction. In addition, a polycondensation reaction

between the alcohol functional groups and the monoglyceride and its co-monomer

(epoxy, diacid, or anhydride) results in the synthesis of polymers.

1.2.4 Biotechnologically-derived polymers Nowadays, emerging advances in biotechnologies have opened a new approach in the

production of conventional petroleum-based polymers from biological sources.

Poly(butylene succinate) (PBS), polyethylene (PE), polypropylene (PP), polyethylene

terephthalate (PET), and biobased polyamides are some examples.

Biobased poly(butylene succinate) (PBS) is synthesized through direct polymerization of

succinic acid and 1,4-butandiol. Currently, several attempts to biologically obtain

succinic acid from natural feedstocks such as corn starch, cane molasses, corn steep

liquor, whey, glycerol, cereals, and straw hydrolysis by the use of bacteria including

Actinobacillus succinogenes, Mannheimia succiniciproducens, and Anaerobiospirillum

succiniciproducens have been proposed [22]. Both polyethylene (PE) and polypropylene

(PP) from petroleum-based resources have been widely used in different applications.

However, with the help of biotechnology, both ethylene and propylene monomers can be

obtained from biological feedstocks such as starchy crops, sugar crops, and

lignocellulosic materials throughout similar polymerization routes which have been used

in the production of their conventional counterparts. Polyethylene terephthalate (PET) is

one of the most important polyesters with applications in the packaging industry.

10

Through biotechnological routes, biobased ethylene glycerol is produced which can

undergo a polyesterification process to synthesize a biobased polyethylene terephthalate

(PET). Biobased ethylene glycol can be obtained by oxidization of bio-ethylene to

ethylene oxide and its hydrolysis. Polyamides (Nylon), which can be obtained from

castor oil, are also widely used in automotive, packaging, and electrical appliances [6].

1.2.5 Bacterial polymers Various sources of carbon are efficiently converted by bacteria to a wide range of

polymers with different chemical and physical properties [10]. Four main classes of

polymers can be produced by bacteria: polysaccharides, polyesters, polyamides, and

inorganic polyanhydrides. Polyesters with a wide range of applications are considered as

the most important examples of bacterial polymers. Good mechanical and thermal

properties of bio-polyesters along with biodegradability (presence of ester linkage) and

biocompatibility have attracted the interest of many researchers [23]. These biopolymers

have a wide range of applications from drug delivery to food packaging [20, 24]. Bio-

polyesters, which are also known as aliphatic polyesters, have ester functional groups in

their backbone. Based on the chemistry of the structural units connecting these functional

groups, various types of bio-polyesters are produced, but each one has its own

applicability from biomedical to fiber and high temperature resistant materials [25].

Polylactic acid (PLA) and polyhydroxyalkanoates (PHA) are examples of bio-polyesters

which have been used in recent years. Bio-polyesters from renewable resources are

divided in three groups [26]:

1. Bio-polyesters from biomass (particularly from agro-resources),

2. Bio-polyesters from microbial production,

3. Synthetic bio-polyesters based on monomers derived from agro-resources.

Among all these polyesters, polyhydroxyalkanoates (PHA) have attracted the most

attention due to their outstanding properties such as high biodegradability in different

environment and processing versatility. PHA can be formulated and processed for use in

different industrial sectors such as packaging, molded products, paper coatings, non-

woven fabrics, adhesives, films, and performance additives. Compared to other

11

renewable-based polymers, good thermomechanical, and barrier properties of PHA offer

great potential in the packaging industry [10].

1.2.5.1 Polyhydroxyalkanoates (PHA) Polyhydroxyalkanoates are known to be bacterial polyesters of various hydroxyalkanoate

monomers accumulating as energy/carbon storage materials by granular inclusions in the

cytoplasm of different bacterial cells, usually under unbalanced growth conditions [27].

These polyesters are potential substitutes of some petroleum-based polymers such as

polypropylene (PP) since they have very similar physico-chemical properties [28]. There

are approximately 150 different types of polyhydroxyalkanoates as components of the

bacterial storage polyesters. Depending on the number of carbon atoms in the monomer

units, bacterial PHA are divided in two groups: short-chain-length (SCL) PHA consisting

of 3-5 carbon atoms; and medium-chain-length (MCL) PHA consisting of 6-14 carbon

atoms [29]. Various Gram-positive and Gram-negative bacteria can synthetize PHA,

while over 300 different microorganisms are known to synthesize and accumulate PHA

intracellularly including Azotobacter sp., Pseudomonas sp., Bacillus sp., and

Methylobacterium sp. [27]. The composition of PHA depends on the producing

microorganism, media composition, and culture conditions. Thus, by choosing the

appropriate microorganism, carbon source, co-substrate, and culture conditions, a variety

of polymers are obtained with different physico-chemical properties [28, 30]. Figure 1.5

presents some of the most important copolymers of polyhydroxyalkanoates.

Figure 1.5a shows the structure of poly(3-hydroxybutyrate) (PHB), a linear polyester of

D (-)-3-hydroxybutyric acid, which is the most widespread and characterized member of

the polyhydroxyalkanote family. It is accumulated in intracellular granules by various

Gram-positive and Gram-negative organisms under nutrient limited conditions [23,29].

PHB is a thermoplastic polymer, which can be degraded in compost and different

environments such as marine water. Therefore, PHB attracted much attention due to its

unique properties. PHB is produced by microorganisms (e.g., Ralstonia eutrophus or

Bacillus megaterium) due to physiological stress-related conditions as an energy storage

molecule to be metabolized when other energy sources are not available. The molecular

12

weight of PHB relies on the organisms, growth conditions, and extraction method. It also

varies from approximately 50000 to over a million Daltons [23].

In the microbial biosynthesis of PHB, two molecules of acetyl-CoA condensate to form

acetoacetyl-CoA, which later reduce to hydroxybutyryl-CoA. Thereafter,

hydroxybutyryl-CoA is used as the monomer in the polymerization of PHB [10]. One

disadvantage of PHB is its high crystalline structure as well as its brittleness and high

melting temperature of 180oC. It is believed that the introduction of various HA

monomers including 3-hydroxyvalerate (3-HV) or 3-hydroxyhexanoate (3HHx) into the

PHB structure can effectively improve its properties. For instance, poly(3-

hydroxybutyrate-co-3-hydroxyvalerate), known as PHBV, has lower crystallinity and

melting temperature compared to PHB [29]. Figures 1.5b and 1.55c present the structures

of PHB copolymers [23].

Figure 1.5 Chemical structure of some common polyhydroxyalkanoates [23].

PHA is totally biodegradable and can be degraded to water and carbon dioxide by micro-

organisms found in nature [30]. Polyhydroxyalkanoates also have some disadvantages

such as brittleness and unsatisfactory mechanical properties, which influence their

properties [31].

1.2.5.2 Polylactic acid (PLA) Polylactic acid (PLA) belongs to the family of aliphatic polyesters made from 𝛼𝛼-hydroxy

acids. PLA is a thermoplastic, high-strength, and high modulus polymer originating from

annually renewable resources with wide range of applicability from industrial packaging

13

to biocompatible and bioabsorbable medical devices [32]. Due to easy degradability,

versatility, low cost, and acceptable performance, polylactic acid has been considered as

the most promising replacement for petroleum-based plastics [33]. The degradation of

PLA is simply carried out by hydrolysis of the ester bond without the use of any enzymes

to catalyze the reaction. The degradation rate relies on the size and shape of PLA, isomer

ratio (D,L), and hydrolysis temperature [32]. Carothers first discovered PLA in 1932 at

DuPont, as a low-molecular weight polymer, which was obtained by heating lactic acid

under vacuum [33]. Subsequently, another technique known as ring opening of lactide

was introduced to synthesize PLA with higher molecular weight. As opposed to other

biobased polymers, the production of PLA has several advantages including low cost,

low energy consumption, and the possibility to recycle PLA to lactic acid by hydrolysis

or alcoholysis [34]. Polylactic acid is commonly synthesized via two different routes such

as direct polycondensation of lactic acid and ring opening polymerization (ROP) of

lactide. In the former, the production of a high-molecular-weight polymer needs severe

conditions such as high temperature (180-200oC), low pressure (5 mmHg), and long

reaction times (30-40 h); while the latter can produce high-molecular-weight PLA based

on mild conditions (lower temperature of 130oC and shorter reaction times of 3-16 h) [32,

35, 36]. Therefore, the ROP of PLA is preferred by the industry.

The following two chapters consist of discussions on polylactic acid synthesis routes and

PLA modifications to improve the mechanical and thermal properties.

14

Chapter 2. Polylactic acid (PLA)

2.1 Synthesis of lactic acid Lactic acid (2-hydroxypropanoic acid) is a simple 2-hydroxycarboxylic acid with a chiral

carbon atom having two optically active stereoisomers (L and D enantiomers). Figure 2.1

shows the structures of the different lactic acids. The L- and D-lactic acids are generally

synthesized through fermentation by suitable micro-organisms. DL-lactic acid consists of

the equimolar mixture of D- and L-lactic acid which is chemically synthesized and has

different characteristics than those of the optically active ones [35].

Figure 2.1 Chemical structure of L, D, and DL lactic acid [35].

Fermentation of lactic acid is a bacterial reaction done by series of bacteria such as

Lactobacillus, Streptococcus, Pediococcus, Aerococcus, Leuconostoc, and Coryne

species. These bacteria are basically classified in terms of their cell morphology. Most of

these bacteria produce L-lactic acid, while some produce D- or DL-lactic acids. The

fermentation of lactic acid yields 85-90% of L- and 70-80% D-lactic acid according to

carbon use, respectively [37]. The fermentation liquor contains lactic acid with different

impurities including unreacted raw materials, cells, and culture media-derived

saccharides, amino acids, carboxylic acids, proteins, and inorganic salts. Consequently,

the fermentation liquor must be purified by two approaches such as precipitation of metal

lactates followed by a neutralizing reaction with sulfuric acid or by esterification with

alcohol, distillation and hydrolysis of the ester, or by electro-dialysis [35, 36]. As

mentioned earlier, DL-lactic acid can be chemically synthesized as shown in Figure 2.2.

The DL-lactic acid is produced by hydrolysis of lactonitrile, which is generally formed by

15

the addition reaction of acetaldehyde and hydrogen cyanide. The obtained lactic acid is

thus purified by distillation of its ester [35].

Figure 2.2 Chemical synthesis of lactide via lactonitrile [35].

2.2 Synthesis of lactide monomers Each lactide is composed of different stereoisometric lactic acid units (Figure 2.3). L- and

D-lactides consist of two L- and D-lactic acids, respectively; while meso-lactide consists

of both L- and D-lactic acids. Racemic lactide (rac-lactide) is an equimolar mixture of D-

and L-lactides [35].

Figure 2.3 Chemical structures of various lactides [35].

Generally, lactides are synthesized by depolymerization of the corresponding oligo(lactic

acid) (OLLA) derived by polycondensation of lactic acid followed by a purification step

(Figure 2.4) [38].

Figure 2.4 Synthesis approach to lactide from lactic acid via oligolactide [35].

16

2.3 Polymerization of lactide monomers As mentioned earlier, the polymerization of polylactic acid is carried out by two various

routes: direct polycondensation of lactic acid or ring opening (ROP) of lactide (Figure

2.5). However, the latter is mainly considered for industrial production of polylactic acid

due to the production of a high-molecular-weight polymer. The polymerization of

optically pure L- and D-lactides results in the production of isotactic homopolymers of

PLLA and PDLA, respectively. PLLA and PDLA are both crystalline, with a melting

temperature of around 180oC [39]. But their crystallinity and melting temperature

decrease with decreasing optical purity (OP) of the lactate units. On the contrary,

optically inactive poly(DL-lactide) (PDLLA), prepared from rac- and meso- lactides, is

an amorphous polymer with an atactic sequence of D and L units. It is also possible to

obtain crystalline polymers based on stereo-regular control of both D and L units.

Furthermore, by mixing isostatic PLLA and PDLA in a 1:1 ratio, stereo-complex crystals

(sc-PLA) are formed with a melting temperature of about 50oC higher than PLLA and

PDLA [35]. Stereo-block copolymers (sb-PLA), consisting of isotactic PLLA and PDLA

sequences, are also synthesized by stereo-regular polymerization techniques involving

copolymerization. The aforementioned structural varieties of PLA lead to the formation

of a broad range of physico-chemical properties of PLA materials when processed [35,

40].

17

Figure 2.5 Examples of microstructures of lactide and PLA [35].

2.3.1 Polycondensation

Direct polycondensation of lactic acid is carried out in bulk by distillation of the

condensation water with or without a catalyst while gradually increasing vacuum and

temperature. The polycondensation of LA consists of two reaction equilibria namely as

dehydration equilibrium of esterification and ring-chain equilibrium starting the

18

depolymerization of PLA to lactide (Figure 2.6) [35]. In order to obtain high molecular

weight PLA, both dehydration and ring-chain equilibrium must be controlled. As shown

in Figure 2.6, in the polycondensation reaction of PLA, water is produced which could

negatively influence the balance between the two aforementioned equilibria. Therefore,

water removal is a crucial step in producing high molecular weight PLA.

Figure 2.6 Equilibrium reactions between PLLA, L-lactic acid, and L-lactide [35].

It has been reported that to successfully achieve high molecular weight PLA, the

condensed water must be removed to a level of 1 ppm from the polymerization reaction

without the evaporation of lactide from the system [35]. Therefore, the best reaction

conditions to maintain water removal may consist of: 1) a reaction temperature of 180 to

200oC; 2) a low pressure of below 5 torr; and 3) a long reaction time (30-40 h) in

presence of a catalyst [32, 35, 36]. It has been also suggested that the use of a catalyst,

which has the ability of activating the dehydration reaction and deactivating the

formation of lactide, can substantially improve the polycondensation and the production

of high molecular weight PLA [36]. Some efforts to increase the molecular weight of

PLA by changing the polycondensation reaction conditions were also studied. For

instance, it was found that by controlling the decompression and esterification rate, high

molecular weight PLA could be synthesized. Decompressing the polymerization reactor

for 7 h as well as increasing the esterification time from 3 to 7 h was shown to result in

the production of PLLA with molecular weights ranging from 30 to 120 kDa [36, 41].

For the purpose of increasing the molecular weight of PLA in polycondensation, three

methods have been used including solution, solid/melt, and stereo-block

polycondensation.

19

Solution polycondensation (Figure 2.7) was first developed in 1995 by Ajioka et al.,

when they successfully synthesized PLLA with high molecular weight of 100 kDa with

the use of diphenyl ether as an azeotropic solvent [36]. In this reaction technique, the

reflux of the high boiling solvent at reduced pressure leads to the azeotropic distillation of

the condensed water and to making it be adsorbed on molecular sieves. This method is

mostly used in the co-polycondensation of L-lactic acid and other monomers such as

hydroxyl-acids and diol/dicarboxylic acid combinations [35]. However, solution

polycondensation has some drawbacks such as multiple reactions and complex process

control, which directly leads to high PLA cost, difficulties in removing the solvent from

the final product, and ecological problem and concept of the solution polycondensation

due to the use of organic solvents [36].

Figure 2.7 Diagram of the azeotropic dehydration polycondensation of LA [36].

Melt/solid polycondensation of L-lactic acid was investigated to eliminate the need for a

solvent, which is necessary in solution polycondensation. Melt/solid polycondensation

involves a continuous dehydration of oligo(L-lactic acid) in the presence of different

ionic metal catalysts. However, this method has its own disadvantages such as

discoloration of the polymer and depolymerization of the product to L-lactide.

Nevertheless, it was shown that by the incorporation of a bi-component catalyst system

containing tin dichloride hydrate and p-toluenesulfonic acid (TSA), the aforementioned

20

drawbacks could be eliminated [42]. Moreover, the molecular weight of the final product

could be increased over 100 kDa. In addition to the bi-component catalyst systems

mentioned above, other catalysts systems including Sn(II)-Ge(IV) and Sn(II)-Si(IV)

could also effectively increase the molecular weight of PLA. Another melt/solid

polycondensation of L-lactic acid can be used where the bi-component catalysts are based

on Sn(II) and TSA. This polycondensation technique has three main steps; 1) an

oligomeric PLLA with a degree of polymerization of around 8 is prepared by thermal

dehydration of L-lactic acid, 2) thereafter, the oligomeric PLLA is mixed with the bi-

component catalyst to further undergo the melt-polycondensation at 180oC for 5 h, and 3)

the melt-polycondensate undergoes a heat-treatment procedure at 105oC for

crystallization and then subjected to solid-state polycondensation (SSP) at 140-150oC for

chain extension. Throughout this method and within a short reaction time, the molecular

weight of the SSP polymer increases to more than 500 kDa [35].

In stereo-block polycondensation, stereo-block polylactic acid (sb-PLA) is produced.

Through this method, the melt polycondensation of L- and D-lactic acids results in the

formation of PLLA and PDLA with medium molecular weights (20-40 kDa).

Subsequently, a stereocomplex formation is obtained by simple melt blending of both

polymers in a 1:1 ratio. Finally, the stereocomplexed mixture undergoes a solid-state

polycondensation where chain extension leads to significant molecular weight increase

(80-100 kDa) of the final product. The molecular weight of the polymer strongly depends

on the lactide/oligomer content in the melt-blend, which is controlled by the melt

blending conditions. It was shown that by changing the PLLA:PDLA ratio from 1:1, the

molecular weight of the final product increases to above 100 kDa [35]. Moreover, by

alteration of the PLLA and PDLA composition and the length of the homopolymer,

various types of sb-PLA with a wide range of properties and applicability can be obtained

[35, 43].

2.3.2 Ring-opening Ring-opening (ROP) of lactide is the most commercially available polymerization

method used in the production of polylactic acid. Before the introduction of the lactide

purification technique by DuPont in 1954, ring opening of lactide resulted in the

21

synthesis of low molecular weight polymers [36]. In this process, ring opening of lactide

(lactic acid cyclic dimer) takes place in the presence of a catalyst.

Ring-opening polymerization of polylactic acid is composed of three steps including

polycondensation, depolymerization, and ring opening polymerization. Catalytic ring

opening polymerization offers the control over the molecular weight of the polymerized

polylactic acid. The sequence of D- and L-lactic acid units and their ratio in the polymer

are governed by the control of residence time, temperatures, catalyst type, and

concentration. Ring opening of lactide can be performed in bulk, melt, or in solution by

the use of cationic, anionic, and coordination-insertion mechanisms based on the catalyst

[44].

In the ring opening method, the polymerization equilibrium is thermodynamically

directed, while the initiator governs the polymerization, stereoselectivity, and rate. The

most commonly used initiators are metal alkoxide or amide coordination compounds,

which have been used in the ring opening of polylactic acid due to their tolerance,

selectivity, rate, and lack of side reactions. Amongst all the initiators, stannous octoate is

preferable as a result of its high reaction rate, high conversion rate, and high molecular

weights especially under mild polymerization conditions [44]. However, for some

biomedical applications, the presence of metal residues are not acceptable. Therefore, low

toxicity organocatalytic or enzyme initiating systems are used [36]. Moreover, ring-

opening polymerization has the ability to control the synthesis of block, graft, and star

polymers. Therefore, it is a versatile and commercially benign method compared to

condensation polymerization [35, 36].

22

Chapter 3. PLA modification

3.1 Totally biodegradable PLA composites

The last two chapters presented an overview on biodegradable polymers from natural

resources with a focus on polylactic acid as one of the most promising biodegradable and

biocompatible polymers. Due to good mechanical and structural characteristics and good

thermal properties, PLA has found various applications in automotive, construction, and

packaging industries. Polylactic acid has similar mechanical and thermal properties as

conventional polymers such as polypropylene (PP). However, this totally biodegradable

and environmentally benign polymer suffers from some disadvantages including

brittleness, poor melt strength, low heat deflection temperature (HDT), slow

crystallization rate, and narrow processing window [45, 46]. Therefore, PLA

modification with the help of natural fiber reinforcements has been considered to

overcome some of the aforementioned drawbacks. In addition, replacement of

conventional and long-lasting petroleum-based plastics increased. Amongst the numerous

natural resource-based materials, biodegradable polymers reinforced with natural fibers

have attracted much attention due to their full biodegradability and environmental

friendliness. Furthermore, the reinforcement of biobased PLA with natural fibers offers

the possibility of manufacturing totally bio-renewable and biodegradable composites

[46].

Natural fibers have been used as reinforcements for plastics due to their flexibility during

processing, low cost, and high specific stiffness [45]. Table 3.1 presents some mechanical

properties of commercially used natural fibers in comparison with well-known E-glass

fiber. Generally, natural fibers are distinguished from each other based on their source

and type. For instance, two types of plants can be used to produce natural fibers defined

as primary and secondary. Primary plants are grown for their fiber content, but secondary

plants are those obtained as by-products of other manufacturing processes. Hemp, kenaf,

and sisal are examples of primary plants, while pineapple, coir, and oil palm are

considered as secondary plants. Another classification based on fiber type can be made

where six categories are defined: bast fibers (flax, jute, hemp, and kenaf), leaf fibers

23

(sisal and pineapple), seed fibers (coir and cotton), core fibers (kenaf and hemp), grass

and reed fibers (wheat and rice), and all other types (wood and roots) [45].

The overall characteristics of natural fiber reinforced biodegradable polymers depend on

various factors including fiber type, fiber modification, environmental growth conditions

(source), and processing methods [45]. Several factors such as chemical composition, cell

dimensions, microfibrillar angle, defects, structure, physical properties, mechanical

properties, and fiber-matrix interaction are known to influence the composite

performance. Among natural fibers, cellulosic fibers have attracted much attention in

recent years as PLA reinforcements. Cellulosic natural fibers are considered as a

composite of cellulose, hemicellulose, and lignin. In other words, unidirectional cellulose

microfibrils as reinforcements are surrounded by a matrix blend of hemicellulose and

lignin (Figure 3.1) [48]. Cellulose is the most crucial component of natural fibers such as

flax, hemp, cotton, jute, wood, etc. It is reported to have an E-modulus of 140 GPa [47].

The amount of cellulose and the microfibrils angle control the mechanical properties of

natural fibers. Furthermore, other factors including fiber diameter, length, and aspect

ratio (fiber length/diameter, L/D) influence the mechanical properties of natural fibers. It

was shown that increasing fiber diameter has a negative effect on the mechanical

properties of the composites (lower mechanical performances) due to reduction in the

aspect ratio of the fiber [47]. Finally, the mechanical properties of natural fiber-reinforced

composites rely on fiber-matrix adhesion, which is in direct relation with processing

conditions. Therefore, some studies investigated the effect of fiber modifications and

processing conditions on the mechanical properties of natural fiber-reinforced composites

(NFC) [47, 49, 50]. For instance, the use of a two-step extrusion process followed by

injection molding for the preparation of PLA based composites was suggested [47].

Figure 3.1 Schematic representation of a lignocellulosic fiber [48].

24

Table 3.1 Comparison of the mechanical properties between various natural fibers and E-glass fiber [47].

Fiber Type Density (g/cm3)

E-Modulus (GPa)

Specific Modulus (E-Modulus/Density)

E-glass 2.55 73 29 Hemp 1.48 70 47 Flax 1.4 70 45 Jute 1.46 20 14 Sisal 1.33 38 29 Coir 1.25 6 5

Cotton 1.51 12 8

Different types of natural fibers have been selected to reinforce PLA: flax, wood, kenaf,

etc. Several studies have been conducted to describe the effect of each fiber on the

mechanical, thermal, and physical properties of PLA composites. The use of flax and

abaca fibers was shown to result in improved mechanical properties of the composites, as

well as increasing crystallinity (transcrystallinity around flax fiber) which directly

influenced the tensile and shear properties of the composites. For example, it was shown

that the strength of flax fiber-reinforced PLA composites is about 50% higher than its

polypropylene as the matrix [45]. Other studies are described next.

Serizawa et al. [51] studied the effect of kenaf fibers on the mechanical and thermal

properties of PLA composites. The addition of kenaf fiber led to flexural modulus

increase from 4.5 to 7.6 GPa for neat PLA and PLA/20 wt.% kenaf fiber composite,

respectively. However, the flexural and impact strengths decreased proportionally to

kenaf content. For example, flexural strength decreased from 132 to 93 MPa for neat

PLA and PLA/20 wt.% kenaf fiber composite, respectively; while the impact strength

decreased from 4.4 to 3.1 kJ/m2 at 0 and 20 wt.% kenaf, respectively. Furthermore, the

crystallization rate was improved by fiber addition. The authors related this improvement

to the effect of cellulose on PLA as a crystal-nucleating agent facilitating polymer

crystallization.

Kaewpirom and Worrarat produced composites of PLA and Pattawia pineapple leaf fiber

(PALF) via extrusion followed by injection molding [52]. PALF fibers had a length of 1-

25

3 mm and were used at 10-50 wt.% concentrations. The results of tensile properties of the

composites revealed that tensile strength increased from 7 MPa to a maximum of about

18 MPa for neat PLA and PLA/50 wt.% PALF fiber composite, respectively. Tensile

modulus was also improved by fiber addition and reached a maximum of about 4.3 GPa

from 2.8 GPa for PLA/50 wt.% PALF composite and neat PLA, respectively. Elongation

at break increased from 0.3 to an optimum amount of 0.9% by increasing fiber content

from 0 to 40 wt.%. Scanning electron micrographs of the fractured specimens showed

good dispersion of PALF fibers in the PLA matrix for 10 wt.% PALF. However, by

further increasing the fiber concentration to 40 and 50 wt.%, fiber aggregation and the

formation of voids became clear. The results of thermogravimetric analysis (TGA)

showed that all the PLA/PALF composites were thermally stable up to 225oC.

Nevertheless, the composites with lower concentrations of PALF presented higher

thermal stability (close to 300oC).

Awal et al. [53] prepared PLA biocomposites reinforced with wood fibers (20 wt.%)

through extrusion and injection molding. The results of thermogravimetric analysis

revealed a 5% weight loss at 274oC for PLA/WF composites which is between the 5%

decomposition temperature of neat PLA and WF at 304oC and 208oC, respectively. The

maximum weight loss of the composites was around 450oC which was significantly

higher than 375 and 350oC for neat PLA and WF, respectively. The authors also

performed rheological analysis on the PLA/WF composites. Shear viscosity of PLA and

PLA composites showed shear rate dependent viscosity (shear-thinning) at constant

temperature. In addition, PLA/WF composites showed a non-Newtonian behavior which

is a favorable characteristic in polymer processing. Tensile modulus of the PLA/20 wt.%

WF composites increased by about 29% compared to neat PLA. However, the flexural

strength of the composites decreased from 114.1 to 93.4 MPa for neat PLA and PLA/20

wt.% WF, respectively. In contrast, the flexural modulus of the composites showed an

increasing trend from 3.13 to 3.85 GPa for neat PLA and PLA/20 wt.% WF composites,

respectively. The authors related the decrease in flexural strength to poor interaction

between wood flour and PLA. Notched impact strength of the composites with 20 wt.%

WF was also improved and increased from 23 J/m for neat PLA to 24 J/m.

26

Ho et al. [54] studied the effect of bamboo charcoal (BC) particles with various contents

(0, 2.5, 5, 7.5, and 10%). The mechanical and thermal properties of the composites were

studied for samples prepared via extrusion and injection molding. It was shown that the

mechanical properties of the composites significantly improved with BC addition. The

maximum tensile strength, flexural strength, and ductility index (the ratio of the total

deformation at maximum load to the elastic limit deformation) of the composites

increased by about 43, 99, and 52%, respectively compared to neat PLA. These

improvements in mechanical properties were attributed to uniform BC distribution, as

well as high BC aspect ratio and surface area. By comparison of the thermal properties of

neat PLA and PLA reinforced with 7.5 wt.% BC, it was concluded that the glass

transition and melting temperatures decreased from 67.3 to 63.9 and 159.8 to 156.7oC,

respectively. The degree of crystallinity was also reduced from 49.5 to 13.9% for neat

PLA and PLA reinforced with 7.5 wt.% BC, respectively. Scanning electron microscopy

was used to confirm good BC dispersion at contents (below 7.5%).

In conclusion, the results obtained from recent studies on PLA composites reinforced

with natural fibers prove the successful manufacturing of totally biodegradable and

renewable composites with reasonable mechanical, thermal, and rheological properties

which can compete with their petroleum-based counterparts.

3.2 PLA composite foams

Widespread applicability of natural fiber-reinforced composites due to their low cost,

higher modulus, and higher crystallinity is not negligible. Nevertheless, some drawbacks

associated with the use of cellulosic fibers restricted some industrial sectors. The

aforementioned disadvantages including increased brittleness, decreased impact strength,

and enhanced overall weight can be partially eliminated by a process known as foaming.

Foamed composite materials have specific characteristics including low cost, light

weight, high surface area, and low thermal conductivity [55]. Furthermore, foaming leads

to weight reduction.

27

Various blowing agents are used to foam polymers, which are classified as chemical and

physical. By the addition of chemical blowing agents (CBA), foam cells are produced

under the effect of temperature/pressure release. Chemical blowing agents are also

divided into two categories: exothermic (azodicarbonamide) and endothermic (sodium

bicarbonate). Physical blowing agents (PBA) consist of compressed gases (nitrogen (N2)

and carbon dioxide (CO2)) or liquids (water, low boiling point alcohols and light

hydrocarbons). These gases dissolve and are homogenized inside the polymer matrix

which further by the help of a thermodynamic instability such as pressure drop and/or

increased temperature, foam cells are nucleated [56]. Physical foaming of polymers and

composites, leads to the formation of cellular structures with cell sizes smaller than 10

µm and cell densities greater than 109 cells/cm3, which are also known as microcellular

foams [57]. As opposed to chemical blowing agents, physical blowing agents have no

decomposition temperature, thus lower processing temperatures are required to foam

polymers resulting in more cost effective foam production. Low processing temperature

also prevents the degradation of both polymer and fibers [58].

Three main techniques can be used to foam PLA/lignocellulosic fiber-reinforced

composites: batch, injection molding and extrusion. Through batch process, usually

physical blowing agents (supercritical gas such as CO2) are used to form the cellular

structure inside the polymer matrix. However, via injection molding and extrusion, both

chemical and physical blowing agents are available, but CBA are generally used for high

density foams, while PBA are used for lower densities. Various parameters govern the

properties of foamed PLA/lignocellulosic fiber-reinforced composites including melt

viscosity, process conditions (pressure, temperature, flow rate, equipment design),

lignocellulosic fiber type and content, as well as fiber treatment (interfacial properties)

[59].

Compared to other techniques, injection molding has some advantages such as low

material costs, high dimensional stability, greater energy efficiency, and shorter cycle

time. Moreover, foam injection molding results in better mechanical properties of the

final product (improved fatigue life, impact strength, and toughness) [60]. Both chemical

and physical blowing agents can be used in injection molding. In the former,

PLA/lignocellulosic fiber reinforced pellets and CBA are dry-blended prior to being

28

injection molded to the desired shapes. However, for physical blowing agents,

supercritical gases such as carbon dioxide (CO2) and nitrogen (N2) are generally used.

The solubility of nitrogen is lower than carbon dioxide, but cell nucleation is higher in the

former [60].

Blowing agent addition also leads to lower melt viscosity of the polymer due to a

plasticizing effect. Therefore, processing can be done at lower temperature reducing

energy use and processing costs. Lower temperature is also advantageous for temperature

sensitive biopolymers such as PLA and natural fibers to limit degradation.

Although foam injection molding has been widely used in the production of PLA parts

for various applications, limited works focused on the manufacture of

PLA/lignocellulosic fiber composite foams.

It is believed that natural fibers (lignocellulosic fibers) can improve the foam cell

morphology by modification of polymer melt viscosity. For instance, the addition of

lignocellulosic fibers is known to result in lower cell size (higher viscosity) and increased

foam cell density (heterogeneous nucleation effect) [59, 61, 62, 63, 64, 65, 66]. In

addition, lignocellulosic reinforcement of foamed PLA composites led to increased

specific tensile and flexural moduli. Moreover, changes in PLA crystallinity induced by

fiber addition can potentially alter the foam cell characteristics of PLA composites [67].

Pilla et al. [67] produced foamed flax fiber reinforced PLA at three different flax

concentrations (1, 10, and 20% wt.) through a microcellular injection molding process.

The effect of silane treatment was also investigated on the morphological, thermal, and

mechanical properties of foamed composites. SEM images showed that the average cell

diameter was larger in neat PLA compared to PLA/flax composites. They concluded that

the addition of flax fiber decreased cell size but increased cell nucleation, which limited

the amount of supercritical fluid (N2) required for cell growth. Furthermore, the presence

of flax fibers resulted in increased viscosity inducing strain-hardening which hindered

cell growth and cell coalescence resulting in lower cell size. Compared to an average cell

size of 8.4 µm for neat PLA, the cell sizes of PLA/flax composites decreased by about

11, 47, and 67% at 1, 10, and 20% wt. flax fiber, respectively. Increasing fiber

concentrations increased cell density from approximately 2.5x107 cells/cm3 for neat PLA

to 7x107, 1.4x108 and 2.2x108 cells/cm3 at flax concentrations of 1, 10, and 20% wt.,

29

respectively. This behavior was again associated to heterogeneous cell nucleation leading

to more uniformity in cell nucleation and cell growth. Moreover, the morphological

analyses revealed that the silane treatment of flax fiber did not affected cell size and cell

density of the foamed composites. Results of differential scanning calorimetry (DSC) for

the first heating cycle showed that the addition of flax fiber enhanced the degree of

crystallinity from 8% for neat PLA to 9, 16, and 27% at 1, 10, and 20% wt. flax content,

respectively. The second heating cycle also resulted in increased degree of crystallinity

from 8% for neat PLA to 11, 21, and 31% at 1, 10, and 20% wt. fiber concentration,

respectively. Flax fibers were believed to act as crystallization nucleating agents, thereby

the degree of crystallinity increased with fiber addition, but the silane treatment did not

show any significant effect on foam crystallinity. The weight reduction of neat PLA,

PLA/1% flax/1% silane, PLA/10% flax/0% silane, PLA/1% flax/1% silane, and

PLA/20% flax/1% silane was found to be similar (between 13 and 19%). Stress-strain

curve for neat PLA and PLA/1% flax/1% silane showed necking. On the other hand, the

other foamed samples presented a brittle mode of fracture. This is due to higher fiber

loadings and the presence of a few large cells acting as stress concentration sites. The

specific toughness and strain at break of foamed PLA/flax fiber composites remained

unchanged (and similar to neat foamed PLA) at about 0.001 MPa/(kg/m3) and 4%,

respectively. On the contrary, the specific tensile modulus increased by about 3, 10, and

22% for the foamed composites at 1, 10, and 20% fiber loadings, respectively. Higher

specific modulus for the composites is due to the higher modulus of flax fibers compared

to PLA and to the restraining effect of fillers on polymer chain movements leading to

increased stiffness. No significant effect of silane was reported on the specific toughness,

strain at break, and specific tensile modulus of foamed composites. Overall, specific

toughness, strain at break, and specific tensile strength of the foamed composites were

lower than their solid counterparts. However, for specific tensile modulus, no significant

difference was seen for solid and foamed PLA composites. Specific tensile strength of

foamed composites with 1, 10, and 20% flax fiber decreased by about 4, 15, and 5%,

respectively. The authors related this decrease in specific tensile strength to the presence

of large voids in the composites acting as stress concentration sites leading to lower

mechanical properties. Unlike the unfoamed composites, the effect of silane treatment

30

was more pronounced in foamed samples; i.e. the specific tensile strength was improved

by a factor of 8 compared to the untreated fiber composites. The dynamic mechanical

analysis showed that besides PLA/1% flax/1% silane, increasing flax concentration to 10

and 20%, increased the storage modulus compared to the neat foamed PLA. Moreover,

the storage modulus of the foamed samples was higher than their solid counterparts due

to the presence of foam cells. Based on tan δ curves, they reported that flax fiber addition

did not change the glass transition temperature of PLA. However, the glass transition of

the foamed composites was around 2-3oC lower than for unfoamed samples. Silane

treatment did not affect the glass transition. The area under the tan δ curve decreased by

about 3, 29, and 58% at 1, 10, and 20% wt. fiber content showing that the damping

energy is lower in foamed composites than for neat PLA foam. Moreover, the area under

the tan δ curve of foamed specimens was lower than their unfoamed counterparts which

is in agreement with the results for toughness and strain at break. This behavior can be

associated to the formation of large voids.

Ding et al. [63] prepared foamed composites of PLA/PEG (polyethylene glycol as the

lubricating agent) reinforced with two types of natural fibers including northern bleached

softwood Kraft (NBSK) and black spruce medium density fiberboard (MDF) through

microcellular injection molding. The scanning electron microscopy images showed

elongated cells in regions near the skin layer which were formed due to low mold

temperature and high shear stress induced by melt flow during mold cavity filling.

Furthermore, SEM images revealed that the addition of cellulosic fibers resulted in the

formation of finer cells with more uniform structures. Approximately 90% of the foam

cells had cell sizes lower than 20 µm. In addition, cell density increased from 1.7 x 107 to

6.5x107 and 1.8x108 cells/cm3 for PLA/PEG, PLA/25 phr NBSK, and PLA/25 phr MDF

composites, respectively. The degree of crystallinity was also increased from 7% to 17%,

and 16% for PLA/PEG, PLA/25 phr NBSK, and PLA/25 phr MDF composites,

respectively. The increase in the degree of crystallinity was attributed to the practical

effect of cellulose fibers in aligning (orientation) PLA molecules during microcellular

foam injection molding.

Zafar et al. [55] investigated the effect of willow fiber addition (20 and 30 wt.%) on the

morphological, thermal, and mechanical properties of unfoamed and foamed composites

31

(microcellular) based on PLA. The results for cell morphology revealed that willow fiber

addition decreased the cell size from 33.7 to 20.6, and 18.1 µm for neat PLA and PLA/20

wt.% willow fiber, and PLA/30 wt.% willow fiber, respectively. Cell density was also

affected by willow fiber reinforcement and showed an increasing trend from 5.0x106 to

6.8x106 cells/cm3 for neat PLA, and PLA/30 wt.% willow fiber, respectively. For

unfoamed composites, specific flexural and tensile strengths decreased with the addition

of willow fiber and reached a minimum of 78 and 43 MPa/(g/cm3) for a willow fiber

content of 30 wt.%, respectively. Similar decreasing trend was also reported for foamed

composites. The specific flexural strength of foamed composites decreased from 95 to 68

MPa/(g/cm3) for neat PLA and PLA/30 wt.% willow fiber, respectively. The specific

tensile strength of foamed composites also decreased from 46 to 40 MPa/(g/cm3) for neat

PLA and PLA/30 wt.% willow fiber, respectively. Compared to unfoamed composites,

all the foamed counterparts showed lower mechanical properties. The authors related this

decrease to stress concentration around foam cells decreasing the strength of foamed

composites. On the contrary, the specific flexural modulus of the unfoamed composites

increased with fiber addition. The specific flexural modulus increased by 11 and 30% for

PLA/20 wt.% willow fiber and PLA/30 wt.% willow fiber, respectively, while the

specific tensile modulus of PLA/20 wt.% willow fiber and PLA/30 wt.% willow fiber

increased about 21 and 41%, respectively. The flexural modulus of foamed composites

increased from 3.4 to the maximum of 4.4 GPa/(g/cm3) for neat PLA and PLA/30 wt.%

willow fiber respectively, while the specific tensile modulus increased from 2.4 to the

maximum of 3.0 GPa/(g/cm3), respectively. Results of specific notched impact strength

also showed that with the addition of willow fiber, the specific impact strength decreased

from 18 to the minimum of 10 Jm-1/(g/cm3) for neat PLA and PLA/30 wt.% willow fiber,

respectively. However, for foamed composites the specific notched impact strength

increased by about 16% and 45% for PLA/20 wt.% willow fiber and PLA/30 wt.%

willow fiber, respectively. The degree of crystallinity increased with both foaming and

willow fiber addition. The degree of crystallinity increased from 23% to the maximum of

28% for unfoamed neat PLA and unfoamed PLA/30 wt.% willow fiber, respectively. In

addition, for foamed composites, the degree of crystallinity increased from 27% to the

maximum of 33% for neat PLA and PLA/30 wt.% willow fiber, respectively. Comparing

32

the results of thermogravimetric analysis showed that the foamed samples had lower

thermal stability compared to their unfoamed counterparts, which contributed to lower

interfacial bonding between the matrix and the fibers during foaming. For instance, the

decomposition temperature decreased from 363 to the minimum of 344oC for neat PLA

and PLA/30 wt.% willow fiber, respectively; while this temperature decreased from 364

to the minimum of 337oC for foamed neat PLA and foamed PLA/30 wt.% willow fiber,

respectively.

3.3 Thesis Objectives The use of biodegradable and natural resource-based plastics in various industrial sectors

such as automotive, packaging, construction, etc., has been significantly enhanced due to

increasing environmental concerns over petroleum-based plastics. Among different

natural resource-based polymers, polylactic acid (PLA) has attracted much attention due

to its competing mechanical properties, which are similar to the properties of widely used

petroleum-based plastics (polypropylene). However, PLA suffers from some limitations

including brittleness and low melt strength. Therefore, PLA reinforcement with natural

fibers is considered as a solution to the aforementioned drawbacks. Furthermore, the

reinforcement of PLA with natural fibers contributes to the manufacture of totally

biodegradable and environmentally friendly composites offering good mechanical and

thermal properties. However, natural fiber-reinforced PLA composites have some

disadvantages such as low impact strength and higher weight. As a result, creating a

cellular structure (foaming) inside the polymer matrix of the composites can be beneficial

in terms of improving impact strength and decreasing the overall weight of the

composites.

In this thesis, a focus is made on the reinforcement of PLA with two types of natural

fibers: wood flour and flax fibers. Foamed PLA and PLA/wood flour composites were

also manufactured. So the main objectives of this study are:

1. To study the reinforcement of PLA with two types of natural fibers including

wood flour and flax fiber for comparison purposes.

33

2. The preparation and characterization of PLA/wood flour and flax fiber composites

without the use of any coupling agents through injection molding.

3. The evaluation of morphological, mechanical, thermal, and rheological properties

of PLA/wood flour and flax fiber composites.

4. The possibility to foam PLA and PLA composites through injection molding with

the use of a chemical blowing agent (azodicarbonamide).

5. The investigation of the injection molding processing conditions (mold

temperature, shot size, etc.) on the morphological, mechanical, and thermal

properties of foamed PLA.

6. To study the effect of wood flour reinforcement on the morphological,

mechanical, and thermal properties of foamed PLA/wood flour composites.

34

Chapter 4. Biocomposites of Flax Fiber and Polylactic Acid:

Processing and Properties

Résumé Dans ce projet, on étudie l'effet de la concentration de la fibre de lin (15, 25 et 40% en

poids) sur les propriétés mécanique, morphologique, rhéologique et thermique d'acide

polylactique (PLA). Dans un premier temps, aucun agent de couplage n’a été utilisé pour

produire des composites entièrement biodégradables et biosourcés. En particulier, des

tests en flexion ont été réalisés sur les composites pour évaluer leurs propriétés

mécaniques, ainsi que la densité, la calorimétrie différentielle à balayage (DSC),

l’analyse thermogravimétrique (TGA) et des essais rhéologiques. Tout d'abord, les

images de microscopie électronique à balayage (MEB) montrent une bonne dispersion

des fibres de lin dans la matrice PLA avec un contact adéquat entre les deux phases

menant à un bon transfert de contraintes. Les résultats montrent que l'ajout de 40% en

poids de fibres de lin a entraîné une augmentation de 142% du module de flexion. On a

constaté que l’ajout de la fibre de lin a diminué significativement les température de

transition vitreuse, de cristallisation et de fusion du PLA.

H. Teymoorzadeh, and D. Rodrigue, Biocomposites of flax fiber and polylactic acid:

processing and properties. J. Renew. Mater., 2(4), 270-277 (2014).

35

Abstract This work investigates the effect of flax fiber addition (15, 25, and 40 wt.%) on the

mechanical, morphological, rheological, and thermal properties of polylactic acid (PLA).

As a first step, no coupling agent was used to produce fully biodegradable and bio-based

composites. In particular, flexural tests were performed on the composites to evaluate

their mechanical properties, while density, differential scanning calorimetry (DSC),

thermogravimetric analysis (TGA), and rheological tests were also carried out. First,

scanning electron microscopy images (SEM) show good flax fiber dispersion in the PLA

matrix along with good contact between both phases leading to improved stress transfer.

Based on the results obtained, the addition of 40 wt.% flax fiber resulted in a 142%

increase in flexural modulus. It was also found that flax fiber significantly decreased the

glass transition, crystallization, and melting temperatures of PLA.

Keywords: Polylactic acid; Flax fiber; Composites; Processing; Properties.

36

4.1 Introduction Recently, the use of bio-fibers as substitutes of glass and carbon fibers in the production

of reinforced polymer composites has been extensively increased due to ever increasing

environmental concerns. Bio-fibers are abundant, cost-effective, low density with good

specific mechanical properties comparable to glass and carbon fibers [6, 49]. One of the

most widely used bio-fibers is flax (Linum Usitatissimum) and Canada is the largest

producer and distributer of flax in the world since 1994. In 2005-2006, Canada produced

approximately 1.035 million tons of flax.

Flax is known to have superior mechanical properties compared to other bio-fibers such

as kenaf, sisal, jute, hemp, etc. Among all the natural fibers, flax has the best

performance considering its low cost, low weight, and its high strength and stiffness. For

instance, maximum tensile strength of flax, hemp, kenaf, and jute are 2000, 900, 930, and

800 MPa, respectively. In addition, the specific modulus of flax is about 45 (GPa cm3/g),

which is significantly higher than 29 (GPa cm3/g) for E-glass [68].

Most recently, the use of bio-based and biodegradable polymers reinforced with natural

bio-fibers in the production of “green” composites increased due to environmental

concerns. The use of bio-fibers as reinforcement in biodegradable polymers such as

polylactic acid (PLA) and polyhydroxyalkanoates (PHA) has been established as new

pathways toward the production of more sustainable materials with less or negligible

hazardous effects on the environment. Among the different types of biodegradable

polymers, PLA is considered as the best example due to its vast application range.

Overall, biodegradable and bio-compostable polymers can be used in packaging and

37

agriculture, but PLA applications are even wider: electronic, automotive, and

construction industries [69]. The mechanical and physical properties of PLA are also

acceptable compared to petroleum-based and other biodegradable polymers. For instance,

compared to polypropylene (PP), PLA has higher tensile strength and tensile modulus

with a similar melting temperature of 170 oC [70, 71]. In addition, tensile strength, tensile

modulus, impact resistance, and barrier properties of PLA are comparable to

polyethylene terephthalate (PET), the main polyester thermoplastic resin from petroleum-

based monomers [72].

Although PLA has good mechanical and physical characteristics, it also has some

disadvantages such as brittleness, sensitivity to high temperature/humidity, low impact

strength, and high cost [73]. As a solution to these problems, addition of bio-fibers is well

known to positively improve the weak thermal stability and mechanical properties of

PLA [49, 74]. Several investigations have been reported on the mechanical, thermal, and

physical characteristics of various types of PLA composites reinforced with different bio-

fibers. Some studies suggested that these composites have comparable or even higher

properties than conventional composites such as PP/natural fiber reinforced composites

[49, 70, 75, 76]. Meanwhile, several studies showed that for bio-fiber reinforced

composites based on PLA, the mechanical properties (tensile and flexural moduli)

increased due to the presence of natural fibers [77, 78]. The reason behind these

improvements is the polar nature of PLA, which results in good interaction with bio-

fibers leading to enhanced mechanical properties [71]. It is also claimed that interfacial

adhesion between PLA and bio-fibers is rather strong by nature [69, 79].

38

Some studies on the mechanical and physical properties of flax/PLA composites have

been published. Oksman et al. [49] reported that the introduction of 30 wt.% flax fiber in

PLA increased the tensile modulus from 3.4 to 8.3 GPa. However, increasing fiber

content from 30 to 40 wt.% did not further improve the modulus of the composites,

which dropped to 7.3 GPa. They also reported that the glass transition temperature of

their PLA/flax composites increased from 50 oC for neat PLA to 60 oC for PLA/flax

composites. Bax and Mussig studied the mechanical properties of PLA/flax composites at

different fiber contents (10 to 30 wt.%) [80]. It was shown that flax fiber increased tensile

strength from 43 MPa at 10 wt.% flax to 54 MPa at 30 wt.% flax. Tensile modulus and

impact strength were enhanced and their maximum values were 6.3 GPa and 72.2 kJ/m2,

respectively. Increasing fiber content from 10 wt.% to 30 wt.%, increased both tensile

strength and modulus from 42.7 MPa and 3.9 GPa to 72.2 MPa and 6.3 GPa,

respectively. Arias et al. [81] prepared composites of PLA/flax and evaluate their

mechanical and thermal properties. Their results showed that the addition of 20 wt.% of

flax fiber increased the Young’s modulus by 50%. Results of differential scanning

calorimetry revealed that the glass transition temperature did not change after flax fiber

addition and remained close to 60 oC. Yuan et al. [82] showed that the interaction

between flax fiber and PLA seemed to be efficient as a result of good flax fibers wetting

by PLA. Flax fiber addition from 30 to 40 wt.% increased both flexural strength and

modulus from 28.7 MPa to 37.0 MPa and from 3.2 to 4.7 GPa, respectively.

Since there is limited published work on the manufacture of PLA/flax fiber composites

via injection molding, the aim of this work is to produce PLA composites by this process

and to further investigate the effect of flax fiber on the mechanical, and thermal

39

properties of PLA/flax fiber composites. As a first step, no coupling agent or surface

treatments were used to produce the composites. The results are completed with

rheological measurements to study the effect of flax on PLA in the melt state.

4.2 Materials and methods The matrix used in this study is PLA grade 3251D from NatureWorks LLC. This polymer

has a MFI of 80 g/10 min (190 oC/2.16 kg), a density of 1.24 g/cm3, a processing melting

temperature of 188-210 oC and a crystalline melting temperature of 155-170 oC. Flax

fibers obtained from Biolin research Inc. (Saskatoon, SK, Canada) were used and sieved

between 125 and 250 microns. Three different flax contents were used: 15, 25, and 40

wt.%.

4.2.1 Composite preparation Both PLA pellets and flax fibers were dried in an oven at 80 oC for 24 h prior to

extrusion. Compounding was performed in a HAAKE co-rotating twin-screw extruder

(Polylab OS) with a flow rate of 500 g/h and a circular die diameter of 3 mm at 100 rpm.

The materials were then pelletized and dried in an oven at 80 oC for 24 h before being

injection molded. The injection molding machine was a NISSEI (model PS 60E9ASE)

with a mold temperature of 30 oC. The materials were molded into rectangular bars

having dimensions of 115 mm in length, 25 mm in width, and 3.1 mm in thickness.

The temperature profiles for extrusion and injection molding are shown in Table 4.1. For

characterization, the samples were cut directly from the molded parts.

4.2.2 Mechanical characterization

40

Flexural testing (three-point bending with a span of 50 mm) was performed on specimens

with dimensions of 60.0 x 12.5 x 3.1 mm3 on an Instron universal tester model 5565 with

a speed of 2 mm/min. At least five samples were tested to get an average.

Table 4.1 Extrusion and injection molding temperature profile used.

Extrusion Temperature

Profile (oC)

Injection Molding Temperature

Profile (oC)

T1: 195 T2: 200 T3: 200 T4: 175 T5: 150 T6: 140

T1: 195 T2: 200 T3: 195 T4: 190

- -

4.2.3 Thermal properties The glass transition (Tg), crystallization (Tc), and melting (Tm) temperatures were

measured by differential scanning calorimetry (DSC). The tests were performed using a

METTLER DSC 7. Samples of approximately 10 mg were analyzed in aluminum pans by

heating from 25 to 200 oC at a rate of 10 oC/min under nitrogen. To obtain the

decomposition temperature of the composites, thermogravimetric analysis (TGA) was

performed using a TA Instruments model Q5000 IR. Between 1 and 5 mg of composite

was heated from 50 to 600 oC at a heating rate of 10 oC/min under nitrogen.

4.2.4 Scanning electron microscopy (SEM) Fractured surfaces of the composites (from notched Izod impact tests) were selected to

investigate the PLA-flax fiber interface. Sample surfaces were studied with a JEOL JSM-

840A scanning electron microscope (SEM) with a high vacuum gun and a voltage of 15

41

kV. Samples were first sputter coated with a thin layer of gold.

4.2.5 Density Density of the composites was measured using a gas (nitrogen) pycnometer ULTRAPYC

1200e (Quantachrome instruments).

4.2.6 Rheology The melt rheological properties of PLA and PLA/flax fiber bio-composites were

measured using a Rheometric Scientific ARES rheometer. Dynamic oscillatory tests were

performed using a 25 mm parallel plate geometry with gaps of about 2 mm. The tests

were performed under nitrogen at two temperatures spanning the range of processing

conditions (170 and 190 oC). First, strain sweep tests were made to determine the effect

of deformation. Then, frequency sweeps between 0.1 and 400 rad/s were performed in the

linear viscoelastic zone of each material.

4.3 Results and discussion 4.3.1 Scanning electron microscopy (SEM) Figures 4.1 (a-f) show SEM images of PLA/flax fiber composites with different fiber

contents and magnifications. As it can be seen in images (a, c, and e), good dispersion of

flax fiber in PLA is obtained. SEM images at higher magnification (b, d, and f) show the

interfacial contact between flax fibers and PLA. It is revealed that flax fiber surface is

well coated with PLA as lack of any gaps between both phases seems to imply good

interaction and adhesion [83]. As reported earlier, the polar and hydrophilic nature of

both flax and PLA is believed to be the reason for good dispersion and uniformity inside

the composites [80, 84, 85].

42

Figure 4.1 SEM images of: a) PLA/15% flax (x25), b) PLA/15% flax (x250), c) PLA/25% flax (x25), d) PLA/25% flax (x250), e) PLA/40% flax (x25), f) PLA/40% flax (x250).

43

Table 4.2 Density of the materials.

Sample Flax content (wt.%)

Density (g/cm3)

Flax fiber reinforced PLA

Neat PLA

Flax fiber

15

25

40 - -

1.332 (0.001)

1.343

(0.001)

1.410 (0.001)

1.300

(0.010)

1.500 (0.010)

4.3.2 Density Density of the PLA/flax fiber composites is presented in Table 4.2. It is shown that as a

result of flax fiber addition (density of 1.50 g/cm3), the density of PLA/flax fiber

composites increased from 1.30 (neat PLA) to 1.41 g/cm3 for 40 wt.% flax fiber

reinforced PLA composites. Increasing flax fiber content from 15 to 25, and 40 wt.%

resulted in density increase of about 2.3, 3.1, and 8.5%, respectively. Teymoorzadeh and

Rodrigue have also reported similar increases for PLA/wood flour composites [83].

4.3.3 Mechanical properties Figure 4.2 shows the flexural modulus of PLA/flax composites. The addition of flax

significantly increased the flexural modulus of the composites. As it can be seen, the

flexural modulus increased from 2.5 GPa for neat PLA (0 wt.% flax fiber) to 6.0 GPa at

40 wt.% flax fiber. This value is much higher than the 4.7 GPa reported by Yuan et al.

[82]. Actually, increasing flax content from 15 to 25, and 40 wt.% increased the flexural

44

modulus by 38, 96, and 142%, respectively. In a similar study [83], a flexural modulus

increase from 2.4 GPa for neat PLA to 5.9 GPa at 40 wt.% wood flour was reported.

4.3.4 Thermal Properties Figure 4.3 presents the DSC plots for PLA and PLA/flax composites. As it can be seen,

addition of flax to PLA decreased glass transition (Tg), crystallization (Tc), and melting

(Tm) temperatures. In a work by Lee et al. [86], a slight decrease in Tg, Tc, and Tm, was

also reported for PLA/wood flour composites. They claimed that in the case of glass

transition temperature, this value is dependent on molecular characteristics, composition,

and compatibility of the components in the amorphous matrix. The results for Tg are also

in agreement with studies earlier performed on PLA/flax composites [49, 81]. Table 4.3

shows DSC and TGA results for neat PLA and PLA/flax composites. It is shown that by

increasing flax content from 0 to 15, and 25 wt.%, Tg decreased by about 3 and 2 oC,

respectively. Furthermore, as fiber content increased from 25 to 40 wt.%, the Tg value

remained unchanged. In the case of Tc and Tm, increasing flax content changed these

values by about 2 to 6 oC. For instance, the Tc value as a function of fiber content

decreased from 99 oC to 93 oC. As it can be seen in Table 4.3, Tm decreased from 170 to

166 oC. Lee et al. [86] claimed that the presence of voids within the PLA structure is the

reason for the reduction of melting temperature. Moreover, the authors also related the

decrease in melting temperature to the effect of flax fiber acting as a nucleating agent.

Results of TGA analysis show an increase in decomposition temperature (Td) from 282

oC for neat PLA to 340 oC for the composites with 40 wt.% flax fiber.

45

4.3.5 Rheology As it is the case for most filled polymers, the linear viscoelastic range (onset of strain

dependence) is shorter. Based on the results obtained, strain amplitude for frequency tests

were selected as 15, 4, 0.7, and 0.02% for 0, 15, 25, and 40 wt.% flax composites,

respectively. Based on these values, Figures 4.4 and 4.5 present the dynamic viscosity

curves for neat PLA and PLA composites at two temperatures (170 and 190 oC). As

expected, a shear-thinning behavior was obtained as viscosity decreases with increasing

frequency. Also, dynamic viscosity increased with flax content and decreased with

increasing temperature. By increasing the flax fiber content from 0 to 40 wt.%, the

dynamic viscosity at 100 rad/s increased from 0.3 to 1.9 kPa.s at 170 oC, while the

increase is from 0.1 to 1.7 kPa.s at 190 oC. The dynamic viscosity at 100 rad/s for 40

wt.% flax at 170 oC was around 1.9 kPa.s, which decreased to 1.6 kPa.s by increasing

temperature at 190 oC. Shumigin et al. [88] pointed out that for PLA filled with cellulose

fibers, the dynamic viscosity increased compared to the neat polymer matrix as a result of

cellulosic fiber addition. They showed that the zero shear viscosities (η0*) for PLA

reinforced with 10 wt.% cellulose fiber was about 16% higher for neat PLA. They also

claimed that viscosity increase is related to concentration, particle size, distribution, and

filler shape. It is obvious that the presence of rigid fillers in the matrix restrict polymer

chain mobility.

46

Figure 4.2 Effect of flax fiber content on the flexural modulus of PLA.

Figure 4.3 DSC thermographs for PLA and PLA/flax fiber composites.

47

Table 4.3 Effect of flax content on the thermal properties of PLA.

Sample No.

Flax content (wt.%)

Decomposition Temperature

(oC)

Glass Transition

(oC)

Crystallization Temperature

(oC)

Melting Temperature

(oC)

1 2 3 4

0

15

25

40

282

332

332

340

62

59

60

60

99

95

95

93

170

166

168

167


48


Moreover, they showed that as cellulose fiber content increased from 0 to 10 wt.%,

dynamic viscosity increased from 1.7 kPa.s for neat PLA to 2.1 kPa.s for PLA polymer

filled with 10 wt.% cellulose fiber.

On the other hand, Figures 4.6 and 4.7 show the storage modulus as a function of

frequency and fiber content. Based on these results, the storage modulus of PLA

increased with flax fiber addition and decreased with temperature. It is believed that the

presence of rigid flax fibers within the PLA structure results in improved stress transfer

from the polymer matrix to the flax fibers and hinders polymer chain deformation. It can

be seen that by increasing the flax fiber content from 0 to 40 wt.% at 170 oC, the storage

49

modulus at 100 rad/s increased from 8 to 78 kPa. At 190 oC, the storage modulus at 100

rad/s increased from 1.5 to 360 kPa as flax fiber content increased from 0 to 40 wt.%.

Moreover, it is noticeable that the slope of the elastic moduli curve at low frequency

decreases as flax fiber content increases [87]. As flax fiber content increases, the

presence of a plateau at low frequency represents an apparent yield stress, which can also

be detected in viscosity curves by the disappearance of the Newtonian plateau between

15 and 25 wt.% flax.

Figure 4.6 Storage modulus as a function of frequency and flax fiber content (170 oC).

50

Figure 4.7 Storage modulus as a function of frequency and flax fiber content (190 oC).

Based on the results obtained from rheological properties of PLA and PLA/flax

composites, it is highly noticeable that mechanical properties increase of PLA/flax

composites is similar as for rheological properties. As the rheological studies revealed,

the storage modulus of PLA and PLA composites increased due to flax fiber addition.

Therefore, one could relate both types of properties together.

51

4.4 Conclusion Composites based on PLA and flax fibers were prepared to investigate the effect of flax

fiber content (0-40 wt.%) on the mechanical, morphological, rheological, and thermal

properties of bio-composites. In particular, injection molding was used to prepare the

samples.

First, based on SEM images, good flax fiber dispersion within the PLA matrix was

observed. Moreover, good adhesion between flax fibers and PLA without the use of any

coupling agent was shown. Second, mechanical characterization tests revealed that flax

fibers significantly improved the flexural modulus of PLA/flax composites. Based on the

results obtained, PLA flexural modulus increased from 2.4 to 6.0 GPa at 40 wt.% flax.

Third, thermogravimetric analysis showed an increase in the decomposition temperature

of the composites from 282 oC for neat PLA to 340 oC at 40 wt.% flax fiber. DSC results

revealed a reduction in glass transition, crystallization, and melting temperatures. Glass

transition temperature decreased from 62 to 59 oC for neat PLA and PLA with 15 wt.%

flax fiber, respectively. There was also a reduction in melting temperature from 170 oC

for neat PLA to 166 oC for PLA with 40 wt.% flax fiber. Finally, rheological

characterizations showed a non-Newtonian behavior for PLA and its composites.

Dynamic viscosity and storage modulus of PLA can substantially increase as a result of

flax fiber addition.

Based on the results obtained, reinforcing PLA with flax fibers without any coupling

agent or surface modification resulted in composites with acceptable mechanical,

physical, and thermal properties comparable to their conventional counterparts.

52

Acknowledgement This study was financially supported by FRQNT (Fonds de Recherche Nature et

Technologie du Québec). Technical help form Mr. Yann Giroux was also highly

appreciated.

53

Chapter 5. Biocomposites of Wood Flour and Polylactic Acid:

Processing and Properties

Résumé Dans ce travail, on étudie l'influence de la concentration en farine d'érable (15, 25 et 40%

en poids) sur les propriétés mécanique, physique et morphologique de l'acide

polylactique (PLA). En particulier, différents tests ont été effectués (flexion et résistance

aux chocs) pour étudier les propriétés mécaniques, tandis que d'autres caractérisations

comme la densité, la dureté, l'analyse mécanique dynamique (DMA), la calorimétrie

différentielle à balayage (DSC) et l'analyse thermogravimétrique (TGA) ont été

effectuées afin d’évaluer les caractéristiques des biocomposites. Les résultats montrent

une légère augmentation de la résistance aux chocs et des propriétés en flexion des

biocomposites avec l’ajout de farine de bois. L’utilisation de 40% en poids de farine de

bois conduit à une augmentation de la résistance au choc et du module de flexion jusqu'à

un maximum de 26,2 J/m et 5,9 GPa respectivement. Les propriétés thermiques des

matériaux composites à base de PLA ont également été influencées par l’ajout de farine

de bois. Les températures de transition vitreuse, de cristallisation et de fusion ont diminué

à des valeurs minimales de 55,3, 94,1 et 164,8°C. On a constaté que sans l'utilisation d'un

traitement de surface ou d'un agent de couplage, une bonne adhésion entre la farine de

bois et le PLA a été réalisée grâce à leur nature polaire.

H. Teymoorzadeh and D. Rodrigue, Biocomposites of wood flour and polylactic acid:

processing and properties. J. Biobased Mater. Bioenergy, 9(2), 252-257 (2015).

54

Abstract In this work, the effect of maple wood flour content (15, 25, and 40 wt.%) on the

mechanical, physical, and morphological properties of reinforced polylactic acid (PLA)

composites was studied. In particular, flexural and impact strengths were evaluated as

mechanical properties, while other characterizations included density, hardness, dynamic

mechanical analysis (DMA), differential scanning calorimetry (DSC), and

thermogravimetric analysis (TGA) were measured to better investigate the characteristics

of these bio-composites. The results showed that wood flour addition substantially

increased the flexural properties and impact strength of bio-composites. The results

revealed that at 40 wt.% wood flour, flexural properties and impact strength increased to

a maximum of 5.9 GPa and 26.2 J/m, respectively. Thermal properties of PLA

composites were also affected by wood flour addition. Glass transition, crystallization

and melting temperatures decreased to minimum values of 55.3, 94.1, and 164.8 oC. It

was also found that without the use of any coupling agent or wood surface treatment,

good adhesion between wood flour and PLA was possible due to their polar nature.

Keywords: Biocomposites; Wood flour; Polylactic acid; Processing; Properties.

55

5.1 Introduction Environmental concerns and new regulations aiming at increasing the use of more

environmentally friendly materials have been acting as driving forces in the search for

biodegradable materials with less negative effects on the environment [49]. Due to

numerous applications of petroleum-based polymers in automotive, construction, and

furniture industries, many researchers showed interest in finding suitable biodegradable

substitutes for these materials, which have been used for decades. The so-called bio-

polymers offer environmental benefits such as biodegradability, less greenhouse gas

emission, and renewability of the monomers [77].

Polylactic acid (PLA) is aliphatic thermoplastic polyester derived from the fermentation

of renewable resources such as sugar feedstocks. PLA degradation leads to the

production of CO2 and H2O. It is today one of the most versatile biodegradable polymer

being an acceptable replacement for long-lasting petroleum-based plastics such as

polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), etc. [74]. PLA has

good mechanical performance with tensile strength and modulus around 62 MPa and 2.7

GPa, respectively which are significantly higher than 36 MPa and 1.2 GPa for neat

polypropylene as the most commonly used polymer in the industry [70]. PLA has also a

melting temperature of 170 oC, which is similar to polypropylene [71]. However, there

are some disadvantages such as low thermal stability (around 210 oC) as well as

brittleness; i.e. limited elongation at break and impact strength [70]. Several studies

aiming at improving PLA thermal and mechanical performance were published and it

was claimed that reinforcing PLA with fibers is considered as one possible way to

overcome the aforementioned disadvantages [49, 75]. Several types of fibers can be used

to reinforce PLA including synthetic (glass and carbon) and natural (wood, hemp, flax,

etc.). Natural fibers offer many advantages over synthetic fibers such as biodegradability

and recyclability, relatively higher specific strength and stiffness, as well as lower cost

[49, 86]. Furthermore, composites reinforced with natural fibers, compared to synthetic

fibers, have lower density, which is beneficial when considering lower energy

consumption during processing and lower part weight for specific applications like

automotive and packaging [81].

56

Several works investigated the effects of natural fiber reinforcement on the mechanical

and physical characteristics of PLA. Some studies suggested that the mechanical

properties of natural fiber reinforced PLA composites are comparable, or in some cases

higher than their conventional counterparts such as PP/natural fiber reinforced

composites [49, 70, 76]. Several studies showed that for composites of PLA and natural

fibers, mechanical properties, namely tensile and flexural moduli, increased due to

natural fiber addition [70, 71, 74-79, 81, 86, 88-94]. Some believe that the polar structure

of PLA is expected to provide improved fiber-matrix bonding resulting in higher

composite properties [71]. Peltola et al. [79] stated that the interfacial adhesion between

PLA and natural fibers (lignocellulosic fibers) is strong by nature, leading to good

mechanical properties of their composites. Nishino et al. [71] prepared composites of

kenaf and PLA. Based on scanning electron microscopy images, good adhesion between

kenaf and PLA was presented. Ganster and Fink [92] reinforced a series of polymers

including polyethylene, polypropylene, high impact polystyrene (HIPS), and PLA with

spun cellulosic fibers including cordenka 700, enka viscose, viscose silver, newcell,

tencel silver and carbamate. It was shown that in the case of biodegradable PLA, unlike

the other polymers, excellent mechanical properties were obtained without the use of any

coupling agents.

Due to limited published works on PLA and its reinforced composites with natural fibers

such as wood flour and processing via injection molding, this paper provides an in-depth

analysis of the mechanical and thermal properties of this polymer and its wood flour

reinforced composites. As a first step, no coupling agent or wood surface treatment was

used to produce a fully bio-based and biodegradable composite. The samples were

injection molded to evaluate their properties.

5.2 Materials and methods The matrix used in this study is PLA grade 3251D from NatureWorks LLC. This polymer

has a MFI of 80 g/10 min (190 oC/2.16 kg), density of 1.24 g/cm3, melting temperature of

188-210 oC and crystalline melting temperature of 155-170 oC. Maple wood flour

obtained from the department of wood science and forestry (Université Laval) was used

57

and sieved between 125 and 250 microns. Three wood flour contents were used (15, 25

and 40 wt.%) to prepare the bio-composites. Higher wood contents were investigated, but

were found almost impossible to process through extrusion/injection due to high

viscosity.

5.2.1 Sample preparation Both PLA pellets and wood flour were dried in an oven at 80 oC for 24 h prior to

extrusion. Compounding of PLA and wood flour was undertaken in a HAAKE co-

rotating twin-screw extruder (flow rate of 0.1 kg/h to 2 kg/h and die diameter of 3 mm) at

100 rpm. Two types of feeders, one for PLA and the other for wood flour, were used.

Series of calibration tests were done to achieve proper PLA and wood flour flow rates

according to the desired fiber contents in the composites. The extrusion temperature

profile is given in Table 1. The materials were then pelletized and dried in an oven at 80 oC for 24 h before being injection molded on a NISSEI machine with a mold temperature

of 30 oC. The injection pressure and injection speed for neat PLA and PLA composites

are reported in Table 5.2. The materials were molded into rectangular bars having

dimensions of 115 mm in length, 25 mm in width, and 3.1 mm in thickness. Temperature

profile for injection molding of the samples is shown in Table 5.1. For characterization,

the samples were cut directly from the injection molded parts.

Table 5.1 Extrusion and injection molding temperature profiles.

Sample Extrusion Temperature

Profile (oC)

Injection Molding

Temperature Profile

(oC)

Neat PLA and

Wood flour reinforced

PLA

Z1: 195 Z2: 200 Z3: 200 Z4: 175 Z5: 150 Z6: 140

T1: 195 T2: 200 T3: 195 T4: 190

58

5.2.2 Mechanical characterization Flexural testing (three-point bending with a span of 50 mm) was performed on specimens

with dimensions of 60.0 x 12.5 x 3.1 mm3 on an Instron universal tester model 5565 with

a speed of 2 mm/min based on ASTM D790. Samples for impact strength were cut to

dimensions of 64.0 x 10.1 x 3.1 mm3. Notched Izod impact strength test was performed

on a Tinius Olsen Model Impact 104 following ASTM D256. Shore D hardness of the

composites was measured using a PTC instrument model 307L according to ASTM

D2240. At least five samples were tested for each mechanical characterization.

5.2.3 Thermal properties The glass transition (Tg), crystallization (Tc), and melting temperatures (Tm), were

measured by differential scanning calorimetry (DSC). The tests were performed using a

METTLER DSC 7.

Table 5.2 Injection molding pressure and speed.

Sample Injection pressure (kg/cm2)

Injection speed (cm3/s)

Neat PLA 752 98

PLA/15% WF 903 98

PLA/25% WF 903 98

PLA/40% WF 1053 98

59

a b

c d

e f

Figure 5.1 SEM images of a) PLA/15% WF (x25), b) PLA/15% WF (x250), c) PLA/25% WF (x25), d) PLA/25% WF (x250), e) PLA/40% WF (x25), and f) PLA/40% WF (x250).

60

Samples of approximately 10 mg were analyzed in aluminum pans by heating from 25 to

200 oC at a rate of 10 oC/min under nitrogen. To obtain the decomposition temperature of

the composites, thermogravimetric analysis (TGA) was performed using a TA

INSTRUMENTS model Q5000 IR. Between 1 and 5 mg of composite was heated from

50 to 600 oC at a heating rate of 10 oC/min under nitrogen.

5.2.4 Dynamic mechanical analysis (DMA) Dynamic mechanical analysis was performed on a TA INSTRUMENTS RSA3 equipped

with a three-point bending fixture. The temperature was increased from 35 to 130 oC with

a heating rate of 1.5 oC/min and using a frequency of 1 Hz under the auto-tension mode

(0.02% deformation). Test specimen dimensions were 40.0 x 12.0 x 3.1 mm3.

5.2.5 Scanning electron microscopy (SEM) Fractured surfaces of the composites (from notched Izod impact tests) were selected to

investigate the PLA-wood interface. Sample surfaces were studied with a JEOL JSM-

840A scanning electron microscope (SEM) with a high vacuum gun and voltage of 15

kV. Samples were sputter coated with a thin layer of gold and were viewed perpendicular

to the fractured surface (cross-section).

61

Table 5.3 Density of neat PLA, wood flour, and PLA/wood flour composites.

Sample

(wt.%)

Density (g/cm3)

(Standard deviation)

Wood flour reinforced PLA

15

25

40

1.33

(0.01)

1.37 (0.01)

1.38

(0.01)

Neat PLA

-

1.30 (0.01)

Wood flour

-

1.39

(0.01)

Figure 5.2 Flexural modulus of neat PLA and PLA composites.

62

5.2.6 Density Density of the composites was measured using a gas (nitrogen) pycnometer ULTRAPYC

1200e instrument from 5 to 6 mg of each sample. For the determination of void content,

ASTM D-2734 was used to compare the theoretical and experimental density of the

composites.

5.3 Results and discussion 5.3.1 Scanning electron microscopy (SEM) Figures 5.1 (a-f) show SEM images of the composites with different wood flour contents.

Good dispersion of wood flour in the polymer matrix is revealed in the images (a, c, and

e). As it can be seen, by increasing wood flour content from 15 to 40 wt.%, composites

with better wood dispersion and more uniform wood distribution are produced. SEM

images with higher magnification (b, d, and f) show the interface between PLA and wood

flour. Based on these SEM images, wood surface is well coated with PLA. Lack of any

gaps between the interface of wood flour and the matrix indicates good interaction and

adhesion of wood to PLA. As stated earlier, the polar structure of PLA is expected to

provide good interaction with wood particles [95].

Figure 5.3 Notched Izod impact strength of neat PLA and PLA composites.

63

Figure 5.4 Shore D hardness of neat PLA and PLA composites.

It is also believed that the interfacial adhesion between PLA and natural fibers

(lignocellulosic fibers) is strong by nature [79], therefore better uniformity (distribution

and dispersion) can be achieved.

5.3.2 Density Density results of the composites are summarized in Table 5.3. As it can be seen, wood

flour addition (density of 1.39 g/cm3) increases the density of PLA composites. Density

increased by about 2.3, 5.3 and 6.1% for wood content of 15, 25 and 40 wt.%,

respectively. Based on the experimental results and within experimental uncertainty, no

significant void content was measured as observed in the SEM images (see Figure 5.1).

5.3.3 Mechanical properties Results of flexural modulus, impact strength, and hardness are shown in Figures 5.2 to

5.4. It is found that wood flour addition improved the flexural modulus of PLA

composites. This shows that the incorporation of wood into the matrix provides

successful reinforcement. As it can be seen, flexural modulus of the composites increased

by about 54, 79 and 145% by increasing wood contents from 15 to 25 and 40 wt.%,

respectively. This proportional increase is in agreement with previous studies where it

was reported that flexural modulus increased with the addition of wood flour in PLA [70,

75, 88]. Since wood is a material with a high strength to weight ratio, therefore its use as

64

reinforcement in PLA leads to higher flexural modulus [74]. The highest flexural

modulus in this study appeared at 40 wt.% wood flour content (5.9 GPa) which is

significantly higher than 4 GPa reported by Liu et al. [74]. This large increase in flexural

modulus from 2.4 GPa to the maximum of 5.9 GPa suggests effective stress transfer

between the polymer matrix and wood [70].

Table 5.4 Dynamic mechanical analysis of neat PLA and PLA/wood flour composites.

Sample tan δ

(-)

PLA

0.12

PLA/15% WF 0.30

PLA/25% WF

PLA/40% WF

0.70

0.68

65

Figure 5.5 Dynamic storage modulus of neat PLA and PLA/WF composites.

Impact strength was also improved by wood reinforcement. The impact strength of neat

PLA was about 16 J/m, which was significantly increased to a maximum of 26.2 J/m

(64% increase) at 40 wt.% wood flour, showing that wood flour can improve the impact

strength of PLA. It is also shown that the impact strength increased by about 59% and

62% for 15 and 25 wt.% wood, respectively. The results of surface hardness are given in

figure 5.4. As it can be seen, wood addition increased the shore D hardness of PLA from

77.8 to the maximum of 89.8 at 40 wt.% wood flour (6% increase). The same increasing

trend in shore D hardness of starch grafted PP/kenaf fibers was reported in a study by

Hamma et al. [84]. They concluded that this behavior is related to decreased flexibility

and increased stiffness resulting from wood addition. Mishra et al. [85] also reported an

increase in shore D hardness of unsaturated polyester reinforced with up to 45 wt.% sisal,

hemp, and banana fibers. They related this increase to good fiber dispersion into the

66

matrix and also to a strong interfacial bonding between the fibers and the polymer matrix.

5.3.4 Dynamic mechanical properties Figure 5.5 presents the dynamic storage modulus of neat PLA and PLA composites as a

function of temperature from 35 to 130 oC. It is revealed that the storage modulus of the

PLA-wood flour composites (Tg around 57 oC) is higher than neat PLA. Huda et al. [70]

stated that this increase in storage modulus is due to the reinforcement effect of wood

flour leading to efficient stress transfer from the matrix to wood particles. They also

observed an increase in the storage modulus from 8.5 to 11.3 GPa by increasing wood

fiber content from 20 to 40 wt.%. Based on the data of Table 5.4, increasing wood flour

content from 0 to 15, 25, and 40 wt.%, proportionally increased the storage modulus by

about 49, 65, and 133%. As shown earlier, Tg of the composites compared to neat PLA

decreased as a result of wood flour addition. It is shown that increasing wood flour

content from 0 to 40 wt.% slightly decreases the glass transition temperature from 54.8 to

53.0 oC. As reported by Oksman et al. [49] and observed in Figure 5, the storage modulus

of PLA/WF composites increased at around 78 oC. This increase can be associated to

cold crystallization of the semi-crystalline PLA matrix. In addition to storage modulus,

the value for tan δ at the glass transition temperature (around 57 oC) is presented in Table

4. Tan d is known to be the ratio of the storage modulus to the loss modulus, which is a

measure of the loss energy in relation to the recoverable energy [70]. Tan δ was found to

range between 0.1 for neat PLA to the maximum value of 0.7 for the PLA composite

reinforced with 25 wt.% wood flour.

67

Table 5.5 Thermal properties of neat PLA and PLA/wood flour composites.

Sample

wt.%

Decomposition

temperature (oC)

Glass

transition (oC)

Crystallization

temperature (oC)

Melting

temperature (oC)

Wood flour

reinforced PLA

15

25

40

335.4

335.2

342.7

56.9

57.6

55.3

94.6

91.6

-

168.0

164.8

165.9

Neat PLA - 282.3 63.9 99.0 170.4

5.3.5 Thermal properties Results of DSC and TGA analyses are summarized in Table 5.5. The decomposition

temperature of neat PLA was about 282.3 oC, which increased to 342.7 oC for PLA

composites reinforced with 40 wt.% wood flour. It is shown that by increasing wood

flour content from 15, to 25 and 40 wt.%, the decomposition temperature increased from

335.4 to 342.7 oC. Results of glass transition (Tg), crystallization (Tc) and melting (Tm)

temperatures are also given in Table 5. Based on these results, Tg, Tc, and Tm decreased

with wood flour addition. This reduction was also found by Lee et al. [86] where they

reported a slight decrease in Tg, Tc, and Tm of about 2 to 4 oC for the composites of

PLA/wood flour. They stated that in the case of Tg, this value is dependent on molecular

characteristics, composition, and compatibility of the components in the amorphous

matrix. The reduction in Tg, Tc, and Tm is found to be proportional to wood content. As it

is shown in Table 5, the melting temperature (Tm) decreased by about 2.4, 5.6, and 4.5 oC

for composites reinforced with 15, 25, and 40 wt.% wood flour.

68

5.4 Conclusion This study investigated the effect of wood flour content on the mechanical, physical and

thermal properties of polylactic acid (PLA) based bio-composites. First, scanning

electron microscopy showed good interaction between wood and PLA without addition

of coupling agents or wood surface treatment. Second, the mechanical tests showed that

wood addition significantly increased flexural modulus, impact strength and hardness of

PLA polymers. Based on the results from mechanical tests, flexural modulus increased

from 2.4 GPa for neat PLA to 5.9 GPa for composites reinforced with 40 wt.% wood

flour. Impact strength and shore D hardness also increased to the maximum values of

26.2 J/m and 89.8, respectively at 40 wt.% wood flour content. It was also shown that

increasing wood flour content from 15 to 40 wt.% enhanced the flexural modulus and

shore D hardness of the composites, therefore the best wood flour content for achieving

acceptable mechanical characteristics of composites is believed to be around 40 wt.%.

However, more studies would be needed to clarify if higher wood contents can be

produced with higher mechanical properties. Third, thermogravimetric analysis showed

that the decomposition temperature of PLA composites was increased to 342.7 oC

compared to neat PLA with a decomposition temperature of 282.3 oC. Results of the

thermal properties of PLA/wood flour composites also revealed that the presence of wood

decreased the glass transition, crystallinity, and melting temperature of the composites.

Glass transition temperature was decreased from 63.9 oC for neat PLA to the minimum of

53.3 oC for composites reinforced with 40 wt.% wood flour. Melting temperature was

also decreased from 170 oC for neat PLA to 165.9 oC for PLA composites reinforced with

40 wt.% wood flour. Based on the results of this study, fully biodegradable and bio-based

composites of PLA and wood flour with acceptable mechanical and thermal properties

were manufactured.

Acknowledgment This study was financially supported by FRQNT (Fonds de Recherche Nature et

Technologie du Québec). Technical help form Mr. Yann Giroux was also highly

appreciated.

69

Chapter 6. Morphological, Mechanical, and Thermal

Properties of Injection Molded PLA Foams/Composites based

on Wood Flour

Résumé Dans ce travail, le moulage par injection a été utilisé pour produire des mousses d'acide

polylactique (PLA) en utilisant l'azodicarbonamide comme agent moussant chimique afin

d’étudier l'effet de la concentration de farine de bois (15, 25 et 40% en poids) sur la

morphologie (microscopie électronique à balayage), la densité (pycnométrie à gaz), ainsi

que les propriétés mécaniques (traction, flexion et impact) et les propriétés thermiques

(calorimétrie différentielle à balayage). En particulier, la réduction de la masse

volumique a été contrôlée par la quantité de matière (masse) injectée. Les résultats ont

montré que les propriétés du PLA ont augmenté avec la concentration en bois, mais ils

ont diminué avec la réduction de la densité. Néanmoins, le module de flexion spécifique

(par unité de poids) augmente toujours avec le moussage. Le moussage a aussi augmenté

de manière significative la cristallinité du PLA.

H. Teymoorzadeh, and D. Rodrigue, Morphological, Mechanical, and Thermal Properties

of Injection Molded PLA Foams/Composites based on Wood Flour, J. Cell. Plast.,

Submitted (2015).

70

Abstract

In this work, injection molding was used to produce polylactic acid (PLA) foams using

azodicarbonamide as a chemical foaming agent and to study the effect of wood flour

concentration (15, 25, and 40% wt.) on morphology (scanning electron microscopy),

density (gas pycnometry), as well as mechanical (tensile, flexural, and impact) and

thermal (differential scanning calorimetry) properties. In particular, density reduction was

controlled by the amount of material injected (shot size). The results showed that PLA

properties increased with wood content, but decreased with density reduction.

Nevertheless, specific flexural modulus (per unit weight) always increased with foaming.

Foaming was also shown to significantly increase PLA crystallinity.

Keywords: Polylactic Acid, Wood Flour, Injection Molding, Composites, Foams.

71

6.1 Introduction

Polylactic acid (PLA) is a biodegradable and biocompatible thermoplastic polyester from

biomass feedstocks (e.g. corn and sugarcane). Recently, due to increasing awareness

concerning the effects of petroleum-based plastics on the environment, the use of PLA

has significantly increased as a green substitute for conventional plastics. Owing to

reasonable tensile strength (around 62 MPa) and tensile modulus (around 2.7 GPa),

which are superior to commonly used polyethylene and polypropylene, PLA found

various applications in packaging, biomedical, transportation, and structural sectors [69,

70]. However, PLA has some major drawbacks such as brittleness and sensitivity to high

temperature and humidity, which significantly limit its applicability [96]. To overcome

these disadvantages, reinforcement with low cost natural fibers has been considered.

Nevertheless, adding reinforcement generally leads to higher density, as well as reduced

elasticity and impact strength [57]. This is why foaming was investigated since a cellular

structure inside PLA can potentially overcome the aforementioned disadvantages.

Foaming also offers other advantages since less material is used and greater dimensional

stability is obtained [97, 98]. Foaming can also improve some processing limitations

including poor melt strength, slow crystallization kinetics, and processing capability [97,

99, 100].

Foam injection molding (FIM) is a process where a physical or chemical blowing agent is

introduced into a polymer melt to create a single-phase polymer-gas solution [101, 102].

In chemical foaming, a chemical compound is used to generate gases via thermal

decomposition. Upon injection, the polymer-gas mixture enters the mold and cells are

nucleated due to pressure release [103]. Since injection molding is very versatile,

foaming thermoplastics has attracted much attention offering advantages such as shorter

cycle times and lower injection pressure, as well as less energy and raw materials

consumption [102].

Several studies investigated the effect of various processing conditions on the final

properties of injection molded PLA foams [97, 99, 104, 105, 106]. One approach to

control density reduction is the shot size; i.e. the amount of partial filling of the mold

72

cavity allowing the PLA/foaming agent mixture to expand. Compared to conventional

polymers, short shot injection of PLA foams is challenging due to weak PLA melt

strength and therefore no well-established studies have been done on this subject [100].

In limited works, it was shown that short shot injection molding might affect the quality

of foam cell morphology and foam cell orientation/deformation. For instance, Ameli et

al. [97] investigated two series of foam injection molding processes as high-pressure and

low-pressure via the Mucell technology. At low-pressure, only 70% of the cavity volume

was filled, while at high-pressure the cavity was fully filled and foaming occurred with a

mold opening technique. The authors also reported cell elongation in the former due to

deformation in the flow direction. Pantani et al. [105] also used the short shot technique

and filled about 80% of the cavity. Similarly, elongated cells were reported via

microscopy analysis. In another study, Ameli et al. [104] foamed PLA samples with short

shot and full shot injection molding. Cell density of short shot foamed samples was

around 3x106 cells/cm3, which was higher than 1.5x106 cells/cm3 for full shot foamed

PLA. In addition, cell size decreased from 224 to 94 microns for full shot and short shot

foamed PLA samples, respectively. It was also shown that cell morphology via short shot

was improved by the addition of talc and nanoclay as nucleating agents. Ameli et al. [99]

prepared PLA composites based on talc through short shot injection molding. It was

concluded that the addition of 5% wt. of talc resulted in improved foamability and better

foam cell morphology. Previous works on foam injection molding of PLA suggested that

foam cell morphology and mechanical properties could be improved by the use of

particles such as nanoclay, talc, and natural fibers [67, 97, 99, 100, 104, 105, 107]. In a

study by Pilla et al., [67] PLA foams and composites based on flax fiber were produced

through microcellular injection molding. They showed that by increasing flax content

from 0 to 20% wt., cell size decreased from 8.4 to 2.8 µm, and cell density increased

from 3x107 to 2.4x108 cells/cm3. The degree of crystallinity was also increased from 8%

for neat foamed PLA to 31% for PLA/20% wt. flax fiber composites. It was also reported

that microcellular foaming decreased the specific toughness from 0.004 MPa/(kg/m3) for

neat solid PLA to 0.001 MPa/(kg/m3) for PLA reinforced with 1% wt. flax fiber. Strain at

break also decreased from 11% for unfoamed neat PLA to 3% for PLA composites with

1% wt. flax fiber.

73

In this study, the effect of wood flour addition on the morphological, mechanical, and

thermal properties of PLA foams is investigated for samples produced via short shot

injection molding. Moreover, considering the limited amount of published work on foam

injection molding of PLA/natural fiber composites, data obtained through this study

would be helpful for future investigations.

6.2 Materials and methods

PLA grade 3251D from NatureWorks LLC (USA) was used as the matrix. This PLA was

supplied in pellets with a MFI of 80 g/10 min (190 oC/2.16 kg) and a density of 1.30

g/cm3. Maple wood flour WF (grade H200) was purchased from P.W.I. Industries Inc.

(St-Hyacinthe, Canada) and was mechanically sieved to keep only particles between 125

and 250 microns. Three different concentrations (15, 25, and 40% wt.) of wood flour

were selected. It is worth mentioning that higher wood content were tested, but no good

samples were obtained due to the high viscosity of the compounds in the melt state.

Activated azodicarbonamide (Celogen 754A from Lion Copolymer, USA) was used as

the exothermic chemical foaming agent (CBA). This foaming agent has a decomposition

temperature range of 165-180 oC and a gas yield of 200 cm3/g.

6.2.1 Sample Preparation

Both PLA pellets and wood flour were kept in a forced air oven overnight at 80 oC to

remove moisture prior to processing. Firstly, PLA and wood were compounded in a

Haake Rheomex (OS PTW16) co-rotating twin-screw extruder (flow rate of 500-800 g/h

and die diameter of 3 mm) at 100 rpm. The temperature profile for neat PLA and

PLA/wood composites is given in Table 6.1. The extruded materials were then pelletized

and dried in an oven at 80 oC for 24 h before being molded on a Nissei injection molding

machine (PS60E9ASE) with a mold temperature of 30 oC. Based on preliminary testing,

0.5% wt. of CBA was found to be the minimum amount of CBA for good PLA and

PLA/wood flour foaming and this value was selected for the experimental plan. The

foaming agent was dry-blended with the PLA pellets before being introduced into the

74

injection molding machine feed hopper. To obtain foamed parts with different densities,

the mold cavity was partially filled. Preliminary results showed that between 31 and 43%

of the total volume shot size of the injection molding machine (114 cm3) produced good

foamed samples; i.e. homogeneous structures. Therefore, five shot sizes were selected as

reported in Table 6.2. Following preliminary tests, shot sizes of 71% and 86% were used

for neat PLA and foamed/unfoamed PLA/wood flour composites, respectively. The mold

cavities directly produced the samples into the desired geometries for mechanical testing

(tensile, flexural, and impact). The temperature profile for injection molding is also

reported in Table 6.1. Other parameters are reported in Table 6.3 with sample coding.

Table 6.1 Extrusion and injection molding temperature profile.

Extrusion

temperature

profile

(oC)

Injection molding

temperature

profile

(oC)

T1: 195

T2: 200

T3: 200

T4: 175

T5: 150

T6: 140

T1: 195

T2: 200

T3: 195

T4: 190

-

-

75

Table 6.2 Shot sizes of unfoamed and foamed PLA samples with coding.

Sample Shot size (%)

Neat PLA 71

SS1 31

SS2 33

SS3 36

SS4 38

SS5 43

WP1, WP2, WP3 86

FWP1, FWP2, FWP3 86

* SS: Shot Size, WP1: PLA/15% WF, WP2: PLA/25% WF, WP3: PLA/40% WF, FWP1: foamed

PLA/15% WF, FWP2: foamed PLA/25% WF, FWP3: foamed PLA/40% WF.

Table 6.3 Injection molding pressure.

Sample Pressure (kPa)

Neat PLA 73746

SS1, SS2, SS3, SS4, SS5 9120

WP1, WP2, WP3 88554-103264

FWP1, FWP2, FWP3 95909

* SS: Shot Size, WP1: PLA/15% WF, WP2: PLA/25% WF, WP3: PLA/40% WF, FWP1: foamed

PLA/15% WF, FWP2: foamed PLA/25% WF, FWP3: foamed PLA/40% WF.

76

6.2.2 Scanning Electron Microscopy (SEM)

A JEOL JSM-840A scanning electron microscope (SEM) with a high vacuum gun and a

voltage of 15 kV was used to evaluate the morphology. The samples were first fractured

in liquid nitrogen and then sputter coated with a thin layer of gold before the exposed

surface was examined at different magnification.

6.2.3 Foam Cell Characterization

Foam cell diameter, skin thickness, and cell-population density were measured via an

image analysis technique. The images obtained via SEM were analyzed by the Image-Pro

Plus software. For cell diameter, the software can directly obtain the average diameter of

the cells in the micrograph. Skin thickness is also measured based on the average distance

from the top surface of the sample to the first foam cell. For cell-population density (Nf),

the value was obtained as:

𝑵𝑵𝒇𝒇 =𝒏𝒏𝑨𝑨

𝟑𝟑𝟐𝟐 (6.1)

where n and A are the number of cells and the area of the micrograph, respectively [108].

6.2.4 Mechanical Characterization

Tensile (type IV dog-bone with 3.1 mm thickness) and flexural (60 x 12.5 x 3.1 mm3)

tests were carried out on an Instron universal tester model 5565 at room temperature

based on ASTM D638 and D790, respectively. Notched Izod impact strength (64 x 10.1 x

3.1 mm3) was performed on a Tinius Olsen Model Impact 104 at room temperature

according to ASTM D256. At least five samples were tested to report the average and

standard deviation.

77

6.2.5 Thermal Properties

Thermal properties (temperatures and crystallinity) were measured by differential

scanning calorimetry (DSC). The tests were performed on a Perkin Elmer DSC 7.

Samples of approximately 5-6 mg were analyzed in aluminum pans by heating from 25 to

200 oC at a heating rate of 10 oC/min under nitrogen. In order to measure crystallinity

(𝜒𝜒 ), the following equation was used:

𝝌𝝌𝒄𝒄(%) = ∆𝑯𝑯𝒎𝒎∆𝑯𝑯𝒎𝒎𝟎𝟎

× 𝟏𝟏𝟏𝟏𝟏𝟏𝒘𝒘

(6.2)

where Δ𝐻𝐻 is the enthalpy for melting, Δ𝐻𝐻 is the enthalpy of melting for a 100%

crystalline PLA (93.7 J/g) [67], and w is the weight fraction of PLA in each sample.

6.2.6 Density

Density of the foamed and unfoamed samples was measured using a gas (nitrogen)

pycnometer ULTRAPYC 1200e instrument. At least five samples were tested for the

average and standard deviation.

6.3 Results and Discussion

6.3.1 Density

The results for density of foamed PLA samples are shown in Table 6.4. As seen, for a

shot size of 31%, density decreased from 1.30 g/cm3 for neat PLA to 0.97 g/cm3, which is

about 25% reduction. By further increasing the shot size to the maximum of 43%, density

decreased by only 15-17%. As density is mainly controlled by the shot size (partial mold

filling at fixed CBA content), better foam cell growth is achieved at the smallest shot size

of 31%, giving cells the possibility to grow more freely with more space given by

injecting less material inside the mold. The results for density of PLA composites are also

presented in Table 4. As shown, the lowest density for foamed PLA composites is

78

obtained for foamed PLA/15% WF, as density decreased from 1.30 g/cm3 for neat PLA

to 1.20 g/cm3 (approximately 8% density reduction). As wood flour concentration is

further increased, density reduction decreased to 6 and 3% for PLA/25% WF and

PLA/40% wt. WF, respectively. For unfoamed PLA composites, as expected, density

increased from 1.30 g/cm3 to 1.31, 1.32 and 1.34 g/cm3 at 15%, 25% and 40% WF,

respectively. This density increase for the composites is similar to our previous studies

since the density of the wood flour (1.39 g/cm3) is higher than neat PLA (1.30 g/cm3) [83,

109]. Comparing the density between foamed and unfoamed PLA composites revealed

that for the range of wood flour concentrations tested, foaming decreased the density. For

instance, compared to their unfoamed counterparts at similar wood flour concentrations,

density of the foamed composites decreased by around 6-7%. Overall, comparing with

the average density of neat foamed PLA, wood flour reinforced PLA samples had higher

densities by about 13, 15, and 19% with wood flour content of 15, 25 and 40% WF,

respectively.

Table 6.4 Density (±0.01 g/cm3) of the samples produced.

Sample Density

(g/cm3)

Neat PLA 1.30

SS1 0.97

SS2 1.08

SS3 1.10

SS4 1.08

SS5 1.09

WP1 1.31

WP2 1.32

WP3 1.34

FWP1 1.20

FWP2 1.22

FWP3 1.26

79

6.3.2 Foam cell morphology

Figure 6.1 shows typical SEM images for foamed PLA and PLA composites at two

different magnifications (x25 and x200). For the foamed samples, skins on both sides of

the parts were produced (a compact layer without any foam cell) enclosing a foam core.

The presence of large cells in the core is probably due to cell coalescence because of the

slow crystallization rate and low melt strength of PLA. Based on the SEM images at

higher magnification, it is revealed that the number of cells per unit area increased as a

result of increasing shot size. Moreover, foaming neat PLA by increasing shot sizes from

31 to 43% resulted in the production of fine-cell structures with smaller diameters. On the

contrary, the addition of wood flour significantly altered the cell morphology: a narrower

cell size distribution is observed for PLA/wood flour composites compared to their neat

counterparts [105]. As presented in the following section, the addition of wood flour

reduced the foam cell diameter from 70 𝜇𝜇m for foamed PLA to the minimum of 38 𝜇𝜇m

for foamed PLA/40% WF. In addition, more uniform cell morphology is seen for foamed

PLA/15% WF samples compared to 25% and 40% WF foamed PLA. Based on SEM

images, it is revealed that as wood flour content increased, the cellular structure of the

composites started to deteriorate. In other words, at concentrations higher than 15% wt.,

cell coalescence occurs leading to the formation of irregular-shaped foam cells [65].

80

(a)

(b)

(c)

(d)

(e)

(f)

81

(g)

(h)

(i)

(j)

(k)

(l)

82

(m)

(n)

(o)

(p)

Figure 6.1 SEM images at different magnification of the foamed samples: a,b) SS1, c,d) SS2, e,f) SS3, g,h)

SS4, i,j) SS5, k,l) foamed PLA/15% WF, m,n) foamed PLA/25% WF, and o,p) foamed PLA/40% WF (see

Table 6.2 for the definition).

6.3.3 Cell Diameter

Figure 6.2 shows the results for the cell diameter of the foamed samples. As shown, cell

diameter decreased by about 54% with increasing shot size from 31 to 43%. SEM images

also show the decreasing trend in cell diameter. Moreover, wood flour concentration

altered the cell diameters. For instance, the average cell diameter of 15% WF composites

decreased by about 9% compared to neat PLA foam at a shot size of 31%. However, at

25 and 40% WF, the average cell diameter significantly decreased from 70 to 60 and 38

𝜇𝜇m for neat foamed PLA and PLA composites at 25 and 40% WF, respectively. In

83

addition, increasing wood flour concentration from 15 to 40% wt. decreased the average

cell diameter from 100 to 38 𝜇𝜇m (60% lower), respectively. As fillers and reinforcements

are believed to act as heterogeneous nucleating sites for cell nucleation, thereby

composites reinforced with various synthetic or natural particles show smaller cell sizes

compared to neat foams [66, 67, 110]. Moreover, wood flour increases the melt viscosity

and induces PLA strain hardening which inhibits cell growth leading to the formation of

smaller cell sizes [67]. In the literature, similar decreasing trends in cell sizes were

observed for foamed PLA composites and nanocomposites prepared via injection

molding [67, 111, 112, 113, 114].

Figure 6.2 Cell diameter of the samples produced (see Table 6.2 for the definition).

84

6.3.4 Cell-Population Density

Figure 6.3 reports on the cell-population density of the foamed samples. As it can be

seen, both shot size and wood flour content influenced the cell density of foamed PLA.

By increasing the shot size from 31 to 38%, cell density increased from 0.5x107 to

3.7x107 cells/cm3. However, by further increasing the shot size from 38 to 43%, cell

density decreased to 2.7x107 cells/cm3. It can be concluded that the optimum shot size is

38% to obtain the highest cell-population density for the conditions studied. Moreover,

wood flour addition significantly decreased cell density from 3.7x107 cells/cm3 for

foamed PLA with a shot size of 38% to 0.3x107 cells/cm3 for foamed PLA/15% WF. It is

believed that the addition of wood flour proportionally decreases the volume fraction of

the polymer matrix resulting in less polymer being available to dissolve the foaming

agent. Therefore the formation of foam cells is more limited in space. In addition, the

presence of wood flour leads to heterogeneous cell nucleation at the interface between

WF and PLA leading to a reduction in overall nucleation rate followed by a decrease in

cell nucleation resulting in lower cell-population density [65]. Increased PLA viscosity

due to wood flour addition also limits foam cell growth and foaming agent diffusion.

Accordingly, a lower amount of cells are created within the polymer matrix leading to

lower cell-population density [115]. It is also shown that by increasing wood flour

concentration from 15 to 25% WF, cell density increased from 0.3x107 to 0.5x107

cells/cm3, respectively. But further increase of wood flour from 25 to 40% WF led to a

23% decrease in cell density (0.4x107 cells/cm3).

85

Figure 6.3 Cell-population density of the samples produced (see Table 6.2 for the definition).

6.3.5 Skin thickness

Figure 6.4 presents the results for skin thickness obtained from the cross-section of each

sample containing both solid faces and foam cores as shown in the SEM images (figure

6.1). For foamed PLA, as the shot size increases from 31% to 36%, skin thickness

decreases from 270 to 50 µm, but there is no significant difference between the skin

thickness at higher shot sizes (36, 38, and 43%) and they are all being closed to 50 µm.

This can be related to an increased foam core region due to more effective foaming (cell

nucleation and cell growth) as described above. Skin thickness of the foamed PLA/wood

flour composites is also shown in Figure 6.4. It is clear that the presence of wood flour

significantly increased the skin thickness of foamed PLA: from 270 µm for neat PLA

86

foam with a 31% shot size up to 600 µm for foamed PLA/15% WF composite. Since the

addition of WF increases the PLA matrix viscosity, this hinders foam cell growth and gas

diffusion of the foaming agent in the core region. Therefore, thinner foam core and

thicker unfoamed skin layer is obtained [115]. However, as shown in Figure 6.4, by

increasing wood flour concentration from 15 to 40% WF, skin thickness decreased from

600 to 250 µm. This trend can be attributed to more effective nucleating effect of wood

flour beyond 15% WF, which facilitates the formation of foam cells within the core

region resulting in thinner skin layer.

Figure 6.4 Skin thickness of foamed PLA and foamed PLA/WF composites (see Table 6.2 for the

definition).

6.3.6 Mechanical Properties

6.3.6.1 Shot size

The results for specific mechanical properties of foamed and unfoamed PLA are

summarized in Table 6.5. As it can be seen, there are variations in specific tensile

modulus and specific tensile strengths as shot size changes from 31% to 43%. Generally,

87

foaming PLA with all the shot sizes resulted in reduced specific tensile strengths and

modulus. As it is shown, specific tensile strength of PLA decreased from 977 MPa/(g.cm-

3) for unfoamed neat PLA to an average of 769 MPa/(g.cm-3) for foamed neat PLA

samples (21% reduction). The best result for the elongation at break of foamed PLA was

obtained for a shot size of 38% showing only a 17% decrease compared to other shot

sizes. Ameli et al. [99] reported a similar decreasing trend in specific tensile properties,

while Peinado et al. [100] also reported lower mechanical properties for foamed PLA as a

result of foaming agent addition. This decrease in mechanical properties is due to the

presence of voids, which provides stress concentration regions within the polymer matrix

resulting in reduced mechanical properties of foamed polymers [67]. On the contrary,

specific flexural properties showed a different trend. Based on Table 6.5, foaming PLA

significantly increased the specific flexural modulus. The specific flexural modulus of

foamed PLA with a 31% shot size increased by about 35% from 2364 MPa/(g.cm-3) to

3200 MPa/(g.cm-3). For the specific flexural strength however, foamed PLA showed

lower values compared to unfoamed PLA. Overall, compared to neat PLA, the average

specific flexural modulus increased by about 22% and the specific flexural strength

decreased by around 48%. Moreover, by increasing the shot size from 31% to 43%, the

specific flexural strength increased by about 37%. Specific impact strength was also

decreased from 28 to 19 J.m-1/g.cm-3 for a shot size of 38% (32% reduction). On average,

foaming decreased the specific impact strength of PLA by about 25%. It was also

reported in a study by Ameli et al. [97] that short shot injection molded PLA foams

showed lower specific impact strengths. They stated that foam cells acted as crack

initiators resulting in lower impact strength. There is no significant difference in the

specific impact strength of foamed PLA samples with different shot sizes.

88

Table 6.5 Specific mechanical properties of foamed and unfoamed PLA with different shot sizes (see Table

6.2 for the definition).

Sample Tensile

modulus

(MPa/(g.cm-3))

Tensile

strength

(MPa/(g.cm-3))

Elongation

at break

(%)

Flexural

modulus

(MPa/(g.cm-3))

Flexural

strength

(MPa/(g.cm-3))

Impact

strength

(J.m-1/(g.cm-3))

Neat PLA

977 (48)

50 (1)

6 (1)

2364 (74)

68 (1)

28 (3)

SS1 808 (37)

26 (2)

4 (1)

3200 (92)

30 (4)

23 (3)

SS2 777 (32)

27 (1)

4 (1)

2965 (66)

31 (2)

21 (2)

SS3 728 (45)

24 (2)

4 (1)

2914 (56)

32 (2)

22 (1)

SS4 717 (34)

30 (3)

5 (1)

2497 (82)

37 (2)

19 (1)

SS5 737 (32)

29 (3)

4 (1)

2782 (54)

41 (3)

22 (2)

*values in parenthesis are standard deviations.

6.3.6.2 Wood Flour Reinforcement

Table 6.6 presents the results for the mechanical properties of foamed and unfoamed

wood flour reinforced PLA. Wood flour effectively enhanced the specific tensile modulus

from 977 MPa/(g.cm-3) for neat PLA to 1389 MPa/(g.cm-3) and 1394 MPa/(g.cm-3) for

the foamed composites at 25% and 40% WF, respectively. This increase is attributed to

the higher modulus of wood flour (11 GPa) compared to PLA (3.5 GPa) and its

restraining effect on PLA chain mobility which increases stiffness [67]. The same

increase in tensile modulus is also observed for unfoamed PLA/wood flour composites

which increased from 977 MPa/(g.cm-3) to 1492 MPa/(g.cm-3) at 40% WF. Overall, due

to the presence of voids (foam cells) in the foamed composites, which provides stress

89

concentration regions under tensile loads, the average tensile modulus and strengths of

the foamed composites are 13% and 28% lower than their unfoamed counterparts,

respectively. It is noticeable that the elongation at break of PLA decreased from 6% to

3% and 1% for unfoamed and foamed PLA/wood flour composites, respectively. The

presence of rigid wood flour particles restricts the motion of PLA molecules (lower

deformation) resulting in lower elongation at break of PLA. The average elongation at

break of the unfoamed composites is about 160% higher than foamed samples. The

presence of both fillers and foam cells significantly decreased the elongation at break of

PLA. On the other hand, specific flexural modulus was improved from 2364 MPa/(g.cm-

3) to 5089 MPa/(g.cm-3) for foamed PLA/25% wt. WF composites. Similar increases in

flexural properties (strength and modulus) were also reported by Ameli et al. [97] High

strength to weight ratio of wood flour is associated with increased flexural modulus of the

foamed composites as reported in the literature [83]. Moreover, the average specific

flexural modulus of the foamed composites was 2% higher than unfoamed samples. At

constant wood concentration of 15% and 25% WF, the specific flexural modulus

increased by about 5%, but 40% WF is believed to be the limit to achieve good flexural

properties in foamed composites. Overall, the incorporation of a foaming agent and wood

flour resulted in higher specific flexural modulus while maintaining low density [100].

Impact strength of PLA was affected by both wood flour and foaming. Based on the data

of Table 6, impact strength of neat PLA decreased by about 15% and 3% for unfoamed

and foamed PLA composites, respectively. This is in agreement with the findings of Pilla

et al. [67] which reported reduced toughness of PLA/flax composites compared to neat

PLA. They related this toughness decrease to the stress concentration behavior of flax

fibers as well as to the rigidity and brittleness of flax fibers. It was also reported that

foaming significantly increased the impact strength of composites from 23 to 27 (J.m-

1/(g.cm-3)) (17% increase). In addition, mechanical properties improvement of the

unfoamed and foamed PLA/wood flour composites can be related to the higher stiffness

of wood flour compared to PLA, as well as the restricting effect of wood flour on PLA

chain mobility leading to higher composite stiffness [67].

Compared to neat foamed PLA, specific impact strength, specific flexural strength, and

specific tensile modulus of the foamed PLA composites increased by about 26%, 5%, and

90

64% respectively, at different wood flour concentrations. The increased impact strength

of the foamed composites can be attributed to the reinforcing effect of wood flour and the

presence of thicker skin layers in the foamed composites. As shown earlier, the skin

thickness of PLA increased from an average of 104 µm for neat foamed PLA to an

average of 383 µm for PLA composites [97]. Compared to the neat foamed samples,

foamed composites showed higher specific flexural modulus, which could be associated

to improved morphology (cell diameter) with wood addition [97]. As discussed earlier,

by increasing WF concentration, smaller cells were obtained which significantly

improved the uniformity of the morphology of foamed composites compared to their neat

foam counterparts. The maximum specific flexural modulus for the neat foamed PLA

samples was about 3200 MPa/(g.cm-3), which increased by about 60% to reach 5089

MPa/(g.cm-3) for PLA/25% WF. Furthermore, elongation at break of the foamed

composites is lower than neat foamed PLA. It is believed that in this case, wood flour

particles can also act as nucleating agents to increase crystallization rates. Therefore, a

more crystalline structure results in lower elongation at break of the composites [97].

This effect is described in the next section.

91

Table 6.6 Specific mechanical properties of PLA and PLA/wood flour composites (see Table 6.2 for the

definition).

Sample Tensile

modulus

(MPa/(g.cm-3))

Tensile

strength

(MPa/(g.cm-3))

Elongation

at break

(%)

Flexural

modulus

(MPa/(g.cm-3))

Flexural

strength

(MPa/(g.cm-3))

Impact

strength

(J.m-1/(g.cm-3)

Neat PLA

977 (48)

50 (1)

6 (1)

2364 (92)

68 (1)

28 (3)

WP1 1174 (42)

46 (4)

4 (1)

3519 (87)

65 (1)

26 (1)

WP2 1426 (37)

34 (4)

2 (1)

4854 (91)

40 (3)

20 (1)

WP3 1492 (45)

36 (4)

2 (1)

5084 (67)

35 (2)

25 (1)

FWP1 782 (42)

27 (2)

4 (1)

3700 (73)

43 (2)

25 (2)

FWP2 1389 (44)

29 (4)

3 (1)

5089 (66)

34 (1)

29 (1)

FWP3 1394 (36)

28 (2)

2 (1)

4960 (52)

27 (2)

24 (2)

*values in parenthesis are standard deviations.

6.3.7 Thermal Properties

6.3.7.1 Shot size

Based on the results presented in Table 6.7, melting temperature was not affected by

foaming and shot size. On the other hand, foaming and shot size affected crystallinity. As

reported in Table 6.7, foaming led to a maximum of 19% increase in crystallinity for

samples foamed at lower shot sizes (31 and 38%). This crystallinity increase can be

associated to the plasticizing effect of the blowing gas during injection molding, which

facilitated PLA crystallization kinetics via higher polymer chain mobility [104]. It is also

92

noticeable that by increasing shot size from 38% to 43%, crystallinity decreased from

49% to 42% indicating again that the optimum condition is a shot size of 38%.

6.3.7.2 Wood Flour

As seen in Table 6.7, wood flour did not change the melting temperature without

foaming. However, crystallinity was affected by wood flour addition. Based on these

results, it can be seen that crystallinity increased by 7% and 27% for PLA composites

with 15 and 25% WF, respectively. A 12% decrease in crystallinity was observed for

PLA/40% WF. It can be assumed that at 40% WF, PLA molecules mobility is highly

hindered by the presence of a high number of wood particles. Therefore lower

crystallization kinetics is achieved resulting in lower crystallinity of the aforementioned

composites. Foamed PLA composites showed lower melting temperature compared to

unfoamed/foamed neat PLA and their unfoamed counterparts. Crystallinity also

decreased from 41% for unfoamed PLA to 33% for foamed PLA/40% WF. Compared to

foamed PLA, crystallinity also had a decreasing trend from 49% for foamed PLA with

shot sizes of 31 and 38% to 33% for foamed PLA/40% WF. The presence of hydroxyl

groups in wood flour and carboxylic acid functional groups in PLA can lead to the

formation of ester bonds. As a result, crystallinity of wood flour reinforced PLA foams is

hindered due to the formation of rigid structures leading to lower crystallinity degree [67,

105]. Therefore, foaming and wood flour addition both decreased PLA crystallinity. On

average, the crystallinity of the foamed composites is about 20% lower than neat foamed

samples. This is related to the hindering effect of fillers on the crystallization kinetics of

polymer molecules. Moreover, the average crystallinity of foamed PLA composites is

16% lower than their unfoamed counterparts due both effects of fillers and foam cells

restraining PLA molecular chain mobility.

93

Table 6.7 Thermal properties of the sample produced (see Table 6.2 for the definition).

Sample Melting Temperature (oC) Crystallinity (%)

Neat PLA 166.8 41

SS1 166.9 49

SS2 166.5 47

SS3 166.5 48

SS4 166.2 49

SS5 166.8 42

WP1 166.6 44

WP2 166.5 52

WP3 166.3 36

FWP1 165.5 37

FWP2 165.8 42

FWP3 165.6 33

6.4 Conclusion

In this study the effects of injection molding shot size and wood flour concentration on

the morphological, mechanical, and thermal properties of PLA have been investigated.

Firstly, density decreased by about 25% for foamed PLA with a shot size of 31%. Foam

cell density increased to 3.7x107 cells/cm3 for a shot size of 38%. Foaming also resulted

in a 35% increase in specific flexural modulus of the foamed samples with a shot size of

31%. Second, reinforcing foamed PLA with wood flour resulted in a 92% decrease in

cell-population density of the composites compared to neat foamed PLA samples. It was

also shown that foaming decreased the density of composites by around 7% with various

concentrations of wood flour compared to their unfoamed counterparts. At 25% WF, the

specific flexural modulus increased by about 60% compared to the neat foamed PLA

with a shot size of 31%. Results from differential scanning calorimetry (DSC) also

showed that foaming increased the crystallinity of PLA composites. On the other hand,

wood flour concentration did not change the melting temperature of the unfoamed PLA

94

composites, but increased the crystallinity at 15% and 25% WF. Overall, foaming and

wood flour reinforcement resulted in lower crystallinity at wood flour concentrations of

15% and 40% WF.

Finally, successful foaming of PLA with a chemical foaming agent through injection

molding was achieved, and wood flour addition was shown to improve the specific

mechanical properties of foamed PLA.

Acknowledgements

This study was financially supported by FRQNT (Fonds de Recherche Nature et

Technologie du Québec) and NSERC (National Science and Engineering Research

Council of Canada). Technical help form Mr. Yann Giroux was also highly appreciated.

95

Chapter 7. Conclusions and recommendations

7.1 Conclusion The aim of this thesis was to produce first a fully biodegradable/biosourced polymer

composites. In particular, PLA was reinforced with two different natural fibers including

wood flour and flax fibers. Then, the second objective was to evaluate the foamability of

the PLA/wood flour composites through injection molding. For this part, an exothermic

chemical foaming agent was selected (azodicarbonamide).

In the first step, PLA was mixed with flax fiber via extrusion and further processed by

injection molding to manufacture the final parts. The effect of flax fiber content (15, 25,

and 40% wt.) on the morphological, mechanical, thermal, and rheological properties of

the composites was evaluated. The interfacial adhesion between PLA and flax fibers was

shown to be very good as characterized by both scanning electron microscopy images

and mechanical properties. The flexural modulus of PLA was shown to increase from 2.4

to 6.0 GPa at a flax fiber concentration of 40% wt. Also, the glass transition, melting, and

crystallization temperatures decreased as a result of flax fiber addition. These results can

be associated to heterogeneous nucleation effects of the solid particles in the matrix.

Furthermore, the decomposition temperature of PLA seemed to be improved by the

addition of flax fibers. The rheological studies also revealed a shear-thinning behavior

which is typical for most thermoplastics and their composites. The dynamic viscosity

increased with flax fiber content and decreased with increasing temperature as expected.

It was also shown that the storage modulus of PLA increased with flax fiber addition and

decreased with temperature. According to the results obtained, reinforcement of PLA

with flax fibers without the use of any coupling agent/fiber surface treatment was

possible leading to the manufacture of 100% biocomposites with acceptable mechanical,

physical, and thermal properties which can compete with their petroleum-based

counterparts.

96

In the second step, wood flour (WF) was selected to reinforce PLA. Compounding of

PLA and WF was carried out in a twin-screw extruder followed by injection molding to

obtain the test specimens. A complete series of morphological, mechanical, thermal, and

dynamic mechanical analysis were performed to get a complete evaluation of WF

addition (15, 25, and 40% wt.) on the aforementioned properties. Scanning electron

microscopy images showed good wood dispersion along with acceptable interfacial

adhesion between both phases. Wood flour addition substantially improved both flexural

modulus and impact strength of PLA. Thermal properties of PLA were also modified due

to wood four addition. For instance, the glass transition, melting, and crystallization

temperatures decreased due to wood flour addition. Based on the dynamic mechanical

analysis, the storage modulus of PLA composites is higher than neat PLA. It was also

shown that, by increasing WF content from 15 to 40% wt., the storage modulus was

proportionally enhanced. Finally, it was possible to produce totally biodegradable PLA

composites without the use of any wood surface treatments and/or coupling agent, while

their properties were similar to their conventional counterparts; i.e. when compared to

polyethylene and polypropylene.

Finally, through the last step of this study, composites of PLA and wood flour were

selected for the purpose of foaming. Foaming was carried out with the use of an

exothermic foaming agent (azodicarbonamide) via injection molding. Injection foaming

proceeded after mixing PLA and wood flour by extrusion. In this case, the shot size,

which is the amount of materials injected into the mold (31, 33, 36, 38, and 43% of the

machine capacity) and wood flour content (15, 25, and 40% wt.) were varied. The

characterization included mechanical and thermal properties. A 25% reduction in density

of the neat PLA was achieved for a shot size of 31%, while the maximum foam cell

density (3.7x107 cell/cm3) was reported for a shot size of 38%. It was also shown that

foaming resulted in a 35% increase in flexural modulus of neat PLA foam with a shot

size of 31%. Second, reinforcing foamed PLA with wood flour resulted in a 92%

decrease in cell-population density of the composites compared to neat foam PLA

samples. It was also shown that foaming decreased the density of the composites by

around 7% with various concentrations of wood flour compared to their unfoamed

97

counterparts. In addition, for a wood flour content of 25% wt., the flexural modulus

increased by 60% compared to the neat PLA foam with a shot size of 31%. Foaming was

found to increase PLA crystallinity, probably due to the elongational deformation rates

around the growing bubbles leading to more ordering at a microscopic level (higher

degree of polymer molecule orientation). But, the combination of foaming and wood

flour reinforcement (at 15 and 40% wt.) led to lower crystallinity. This result can be

associated to steric hindrance effects of rigid particles destroying the local order between

gas cells. Finally, successful foaming of PLA with a chemical foaming agent through

injection molding was achieved, and wood flour addition was shown to improve the

specific mechanical properties of PLA foams.

7.2 Recommendations The focus of the current research was on the manufacturing and properties of 100%

biodegradable/biosourced composites and foams. Based on the promising results from

morphological, mechanical, thermal, and rheological studies, further investigation would

be of interest to complete the research on PLA composites and composite foams

reinforced with natural fibers. Therefore a series of suggestions for future work are

presented as follows:

1. As mentioned earlier, the manufacture of PLA/wood flour and flax fiber was

shown to result in acceptable interfacial adhesion between the polymer matrix and

the reinforcement phases. However, the use of a coupling agent such as silane or

maleated PLA can also be of interest to evaluate the effect of coupling agent

addition and its content on the properties of the composites.

2. It was reported that both wood flour and flax fiber led to increased PLA

mechanical properties. Therefore, the use other natural fibers such as hemp, sisal,

jute, etc. as well as producing hybrid composites (blending different fibers) may

further enhance the flexural, tensile, and impact properties of these composites.

3. The foaming process was carried out by injection molding using an exothermic

foaming agent. Correspondingly, selecting an endothermic foaming agent (e.g.

sodium bicarbonate), making a blend of both exothermic and endothermic

98

foaming agents or using a physical foaming agent (e.g. N2 or CO2) would be of

interest to investigate the effect of each type of foaming agent on the

morphological, mechanical, and thermal properties of foamed PLA and PLA

composites.

4. The main purpose of foaming PLA and its composites was to achieve the lowest

density possible. However, having wider ranges of density and investigating this

effect on the properties of foamed PLA and foamed PLA composites based on

lignocellulosic fibers could lead to a deeper understanding of their foaming

behavior. Performing modeling of the mechanical properties with respect to the

density of foamed PLA and foamed composites would be helpful for process

optimization and design purposes. Comparison between the models and the

experimental data (validation) can be another possibility to better describe the

foaming behavior of PLA and its composites.

5. Modification of the processing conditions in injection molding (e.g. pressure,

speed, mold temperature, time, etc.) towards the manufacturing of better foam

morphologies would also be of interest, as well as to investigate their effect on the

morphological, thermal, and mechanical properties of foamed PLA and foamed

PLA composites.

6. Studying the biodegradability of the reinforced foamed and unfoamed composites,

as well as to evaluate the mechanical and thermal stability of these materials over

a specified time period would complete the characterization in terms of stability

and biodegradability of these materials with respect to their final application.

7. Investigating the effect of different PLA grades (various ratio of L- to D- lactides/

L- to D- lactic acid) on the thermal and mechanical properties of both unfoamed

and foamed composites would widen our understanding of the relations between

molecular properties and macroscopic behaviors of PLA.

8. Finally, it can be possible to produce blends of PLA with other biodegradable

polymers (PHA, PHB, PCL, etc.) and study the effect of PLA content on the

properties of these blends as matrices. This would include the neat matrix, as well

as the foams, composites, and composite foams.

99

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Appendix (A): Injection molding condition

A.1 PLA/wood flour and flax fiber composites The injection molding conditions for the production of PLA composites reinforced by

both flax fiber and wood flour was as follows:

Injection time: 4 s

Curing time: 45 s

Cycle start time: 0.5 s

PB: 10 MPa VS: 30% SM: 41 mm SD: 3 mm

V1: 80 mm/s V2: 40 mm/s V3: 60 mm/s V4: 20 mm/s

P1: 65 MPa P2: 60 MPa P3: 60 MPa

S1: 26 mm S2: 24 mm S3: 8 mm S4: 5 mm

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A.2 Neat foamed PLA The injection molding conditions for the production of neat foamed PLA samples was as

follows:

Injection time: 4 s

Curing time: 30 s


PB: 10 MPa VS: 40% SM: 13-18 mm SD: 3 mm


P1: 50 MPa P2: 50 MPa P3: 35 MPa

S1: 21 mm S2: 18 mm S3: 7 mm S4: 5 mm

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A.3 Foamed PLA/wood flour composites The injection molding conditions for the production of foamed composites of PLA and

wood flour was as follows:

Injection time: 4 s

Curing time: 30 s


PB: 10 MPa VS: 40% SM: 22 mm SD: 3 mm


P1: 50 MPa P2: 50 MPa P3: 35 MPa

S1: 21 mm S2: 18 mm S3: 7 mm S4: 5 mm

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Appendix (B): Comparison of mechanical properties between commercialized composites and PLA based composites reinforced with wood flour and flax fiber

B.1 Introduction Plastic-based composites reinforced with natural fibers have gained much interest due to

their low environmental impact, low cost and their various applications [1]. The

properties of the aforementioned composites have been at the center of attention to

develop functional materials with a wide range of applications from construction to

automotive, and packaging industries. Among the plastics, those originating from

petroleum-based sources have been used for a long time. Polypropylene (PP),

polyethylene (PE), and polyvinyl chloride (PVC) are examples of petroleum-based

plastics. Considering the processing temperature that natural fibers can withstand (not

higher than 200 oC), the aforementioned plastics have been selected for the production of

natural fiber reinforced composites as they soften below that critical temperature [2].

However, their important environmental effects led to their substitution by renewable-

based plastics such as polylactic acid (PLA) [3]. Several investigations have been carried

out on the properties of PLA-natural fiber reinforced composites. A concise review on

this subject can be found in chapter 3. However, throughout this appendix, a comparison

of the properties of our PLA-natural fiber reinforced composites with conventional

polymer composites (e.g. PE and PP) reinforced with natural fibers is presented to better

describe how far renewable-based plastics and their composites have come to replace

commercially available materials.

B.2 Discussion Polyethylene has been used as the main matrix for wood flour reinforced composites due

to its easy availability, easy processing, and acceptable mechanical properties [4]. Since

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polyethylene is hydrophobic and wood flour is hydrophilic, most studies focused on the

evaluation of their composite properties under the effect of coupling agents. Taking into

account that in our study no coupling agent was used, the comparison may not be fair.

Therefore, only the works in which had no additional coupling agents in their composite

structures were selected.

In a study by Xiong et al. [5], it was found that the addition of wood flour increased both

crystallization temperature (Tc) and degree of crystallinity since wood flour acted as a

nucleating agent favoring the crystallization of high-density polyethylene (HDPE). For

instance, crystallization temperature increased from 117 oC for neat HDPE to a maximum

of 121 oC at 70 phr of wood flour. Similarly, the degree of crystallinity increased from

77.5% to a maximum of 86% between HDPE and 50 phr of wood flour, respectively. In

our study however, the PLA degree of crystallinity (41%) is lower than the degree of

crystallinity of HDPE (77.5%). Furthermore, wood flour reinforcement led to an increase

of the degree of crystallinity from 41% to 52% at 0 and 25 wt.% of wood flour content,

respectively. As shown in Figure B.1, it is also evident that the addition of wood flour

slightly decreased the melting temperature from 134.7 to a minimum of 133.3 oC for neat

HDPE and the composites with 50 phr of wood flour, respectively. The same decreasing

trend was also reported in our study where the addition of 25 wt.% of wood flour resulted

in the reduction of the melting temperature from 170.4 to 164.8 oC at 25 wt.% of wood

flour content, respectively.

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Figure B.1 DSC thermogram of HDPE/wood flour composites at different wood flour content: a) heating and b) cooling [5].

In another study by Marcovich and Villar [6], the melting temperature remained

unchanged at around 122.2 oC as a result of wood flour addition for 30 and 40 wt.%

compared to neat polyethylene. On the contrary, the degree of crystallinity decreased

from 38.5% to 28.1%, and 24.5% at 0, 30, and 40 wt.% of wood flour. Table B.1 presents

the mechanical properties of some conventional plastics reinforced with natural fibers as

well as the properties reported in our study. It is worth mentioning that the natural fiber

contents used in all the studies were covering the range of 15 to 45 wt.%. It can be seen

that tensile strength and flexural modulus are both higher in PLA/wood flour or flax

composites compared to those of PE reinforced with wood flour or flax fibers. This

difference could be related to better processing of PLA and wood flour/flax fiber

dispersion through different stages such as extrusion and injection molding, which

contributed to higher aspect ratio of the natural fibers resulting in better structural

115

performance and stress transfer between the PLA matrix and wood flour reinforcement. It

is also known that the type of natural fiber (e.g. species) and its particle size can also

influence the mechanical properties of the final composites [7]. On the contrary, the

notched Izod impact strength in our study is lower than for PE/wood flour.

Table B.1 Comparison of mechanical properties between PE/wood flour and PLA/wood flour composite.

Matrix Wood Flour

Content (%)

Flax Fiber

Content (%)

Tensile Strength (MPa)

Tensile Modulus

(GPa)

Flexural Strength (MPa)

Flexural Modulus

(GPa)

Notched Izod

Impact Strength (kJ/m2)

Charpy Impact

Strength (kJ/m2)

PE [8] - 20 - 25 2.4 14.5 - PE [5] 50 - 30 - 25.5 - 3 - PE [6] 30 & 40 - - 2.8a &

3.5b - - 0.03a &

0.02b -

PE [7] 25-45 - 30 2 40 2.3 - - PP [9] - 30 47 12

PLA [10] 15, 25 & 40

- 60c, 45d & 48e

1.4c, 1.8d & 2e

- 3.8c, 4d & 5,9e

0.02 -

PLA [11] - 15, 25 & 40

- - - 3.8c, 4.1d & 6e

0.02 -

a) 30 wt% WF, b) 40 wt% WF, c) 15 wt% WF, d) 25 wt% WF, and e) 40 wt% WF.

Xiong et al. [5] also evaluated the shear viscosity of PE/wood flour composites. As

presented in Figure B.2, the shear viscosity was dependent on the wood flour content. In

other words, by increasing the wood flour content from 10 to 70 phr, shear viscosity

increased from 2000 to 50000 Pa.s. Compared to neat PE, the shear viscosity increased

due to wood flour addition. Similar increasing trend was observed in our study where the

shear viscosity increased with flax fiber addition and the increase was also proportional

to flax fiber content from 15 to 45 wt.%.

116

Figure B.2 Shear viscosity as a function of wood flour content [5].

B.3 Reference [1] K.L. Pickering, M.G. Auran Efendy, T.M. Le, A review of recent developments in

natural fiber composites and their mechanical performance. Comp. A, 83, 98-112

(2016).

[2] P.A. Dos Santos, J.C. Giriolli, J. Amarasekera, G. Moraes, Natural fibers plastic

composites for automotive applications. 8th Annual automotive Composites

Conference and Exhibition, Troy, MI, USA, 492-500 (2008).

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(PLA) composites. Compos. Sci. Technol. 63, 1317-1324 (2003).

[4] M. Kaseem, K. Hamad, F. Deri, Y.G. Ko, Properties of polyethylene/wood

composites: a review of recent works. Polymer Science Ser. A, 57, 689-703 (2015).

[5] C. Xiong, R. Qi, Y. Wang, Wood-thermoplastic composites from wood flour and

high-density polyethylene. J. Appl. Polym. Sci., 114, 1160-1168 (2009).

117

[6] N.E. Marcovich, M.A. Villar, Thermal and mechanical characterization of linear low-

density polyethylene/wood flour composites, J. Appl. Polym. Sci., 90, 2775-2784

(2003).

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physical and mechanical properties of wood plastic composites. Comp. A, 40, 1975-

1981 (2009).

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performance of recycled polyethylene/wood flour composites by synergistic

compatibilization at multi-scale interfaces. Comp. A, 64, 90-98 (2014).

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optimization of the coupling agent efficiency. Compos. Interfaces, 7, 59-75 (2000).

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processing and properties. J. Biobased Mater. Bioener., 9, 252-257 (2015).

[11] H. Teymoorzadeh, D. Rodrigue, Biocomposites of flax fiber and polylactic acid:

processing and properties. J. Renew. Mater., 2, 270-277 (2014).

118

Appendix (C): DSC thermogram of the samples

Figure C.1 Foamed neat PLA samples.

Figure C.2 Unfoamed PLA/wood flour composites.

119

Figure C.3 Foamed PLA/wood flour composites.

Documents

Thèse Hedieh Teymoorzadeh Doctorat en génie chimique