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Composites Science and Technology

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Interfacially reinforced unsaturated polyester carbon fiber composites with avinyl ester-carbon nanotubes sizing agent

Zijian Wua,b,∗, Hongyu Cuib, Lei Chenc, Dawei Jiangd, Ling Wenga,b, Yingyi Mab, Xuejiao Lib,Xiaohong Zhanga,b, Hu Liue,f, Ning Wangg, Jiaoxia Zhangf,h, Yong Maf,i, Mingyan Zhanga,b,Yudong Huangc,∗∗, Zhanhu Guof,∗∗∗

a Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin, 150040, Chinab College of Material Science and Engineering, Harbin University of Science and Technology, Harbin, 150040, Chinac School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin, 150001, ChinadHeilongjiang Key Laboratory of Molecular Design and Preparation of Flame Retarded Materials, Northeast Forestry University, Harbin, 150040, ChinaeNational Engineering Research Center for Advanced Polymer Processing Technology, Zhengzhou University, Zhengzhou, 450002, Chinaf Integrated Composites Lab (ICL), Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, TN, 37996, USAg State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou, 570228, Chinah School of Material Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang, Jiangsu, 212003, Chinai College of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China

A R T I C L E I N F O

Keywords:A. Carbon fibersA. CoatingB. InterphaseB. Mechanical properties

A B S T R A C T

A amino-functionalized carbon nanotubes (CNTs)-containing sizing agent was prepared for improving the in-terface bonding and impact toughness of carbon fibers (CFs) reinforced unsaturated polyester (UP) composites.More reactive groups and better interfacial compatibility with UP make vinyl resin M7270 a more suitablepolymer for preparing sizing agent for CF/UP composites when compared to epoxy, MR13006 and R806. Thesurface characteristics of CFs and the interfacial properties of the composites before and after surface mod-ification were investigated. The observed uniformly dispersed CNTs-containing sizing agent on the CF surfaceobviously increased the surface roughness. The amount of polar functional groups and the wettability of CFswere significantly enhanced after the coating treatment. The interlaminar shear strength (ILSS) and impacttoughness were enhanced by 32.3 and 55.2%, respectively. The sizing agent effectively enhanced the interfacialadhesion by improving the surface energy, and increasing chemical bonding and mechanical interlocking.

1. Introduction

Based on radical polymerization, unsaturated polyester (UP) has abetter curing efficiency than epoxy. Some UP can finish curing within1min, such as MR13006 from Ashland. In order to have a higher pro-duction rate per each used mold, the curing efficiency becomes moreimportant for manufacturing enterprises [1,2]. Meanwhile, the me-chanical properties of CF/UP composites are preferred to be close to thelevel of CF/epoxy composites. The ultimate performance of compositesis largely dependent on the interfacial properties [3]. However, theinterfacial compatibility between CFs and UP is relatively poor, whichleads to a lower interlaminar shear strength (ILSS) and impact tough-ness [4].

Polymer nanocomposites are promising in many fields from the

viewpoint of their inherent excellent properties [5–18]. In the field ofcarbon fiber composites, the interphase containing nanomaterials (e.g.,CNTs) is helpful to transfer stress and creates mechanical interlockingbetween CFs and resin [19]. Various kinds of interface treatment ap-proaches have been employed to introduce nanomaterials into the in-terface of composites, such as chemical grafting [20,21], coating withsizing agent [22], chemical vapor deposition (CVD) [23], electro-chemical deposition(ECD) [24] and electrophoretic deposition (EPD)[23,24]. Compared to other approaches, coating with sizing agent takesadvantage of dispersing nanomaterials in resins and introducing nodamage to the fibers [25]. For an enhanced interfacial compatibility,the first priority for preparing a sizing agent is to select a suitable mainpolymer [26]. Epoxy is widely applied to prepare commercial sizingagent for most CF enterprises. It has been proved that epoxy sizing

https://doi.org/10.1016/j.compscitech.2018.05.051Received 1 February 2018; Received in revised form 28 April 2018; Accepted 30 May 2018

∗ Corresponding author. Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin, 150040, China.∗∗ Corresponding author.∗∗∗ Corresponding author.E-mail addresses: zijian.wu@hrbust.edu.cn (Z. Wu), ydhuang.hit1@aliyun.com (Y. Huang), zguo10@utk.edu (Z. Guo).

Composites Science and Technology 164 (2018) 195–203

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agent can effectively improve the interfacial properties of carbon fiberreinforced epoxy composites. However, the reinforced effect of epoxysizing agent has not been studied for the UP matrix, and a proper sizingagent is needed.

In this study, four kinds of polymers were used to prepare sizingagents according to their wide applications for sizing agent preparationor similar chemical structure with UP, including widely used epoxy, UP(MR13006), and two kinds of vinyl ester (M7270 and R806) to disclosethe effects of polymer structure on the interfacial properties of CF/UPcomposites. A comparison study indicated that M7270 was the mosteffective for interfacial property enhancement. The massive polargroups (hydroxyl groups and epoxy groups) and relative low averagemolecular mass (< 1000 g/mol) of M7270 warrant a good spreadabilityon the fiber surface. The unsaturated double bonds of M7270 make iteasier to participate in the curing reaction with UP matrix. CNT isconsidered to be a promising reinforcing material because of its uniquemechanical and physical properties [27–32]. Therefore, a novel sizingagent containing amine functionalized CNTs (CNTs-EDA) and vinylresin M7270 was prepared for enhancing the interface performance ofCF/UP. The effects of this novel sizing agent on surface characteristicsof CFs were studied by XPS, FTIR, TGA and SEM, and the interfacialadhesion of final CF/UP was discussed with ILSS, impact test and AFMin force modulation mode (FMAFM).

2. Experimental

2.1. Materials

Un-sizing CFs (raw CFs) were obtained from Guangwei Co., China(12 K, diameter: 7 μm). CNTs (diameter: 8–15 nm and length: 0.5–2 μm)were obtained from Chinese Academy of Sciences, Chengdu. MR13006(Volume shrinkage: 8.5%) and low shrinkage agent LP4016 (vinylacetate polymer) were obtained from Ashland China. Vinyl ester M7270(Volume shrinkage: 4.6%), vinyl ester R806 (Volume shrinkage: 7.5%)and corresponding imidazole curing agents were purchased fromSHOWA HIPOLYMER Co., China. Water-soluble EP (volume shrinkage:2%, solid content 50%) and corresponding amine hardendner werepurchased by Guangwei Co., China. Tert-butyl peroxybenzoate (TBPB)from ACROS ORGANICS was used as an initiator for the CF/UP system.Ethylenediamine (EDA), SOCl2, acetone, DMF, THF, H2SO4 (98%),

HNO3 (68%) and ammonium bicarbonate were purchased fromChangzheng Co., China. Fig. 1a shows the molecular formulas of fourkinds of main polymers for preparing sizing agents.

2.2. Preparation of sizing agents containing different main polymers

2.2.1. Sizing agents containing water-soluble EP4 g water-soluble EP, and amine curing agent (1 wt% water-soluble

EP) were dispersed in deionized water in a sonication bath for 15min toprepare 200 g sizing agent, the content of main polymer in sizing agentwas 1wt%.

2.2.2. Sizing agents containing MR1300610.5 g MR13006, 4.5 g LP4016, and 0.15 g TBPB were mixed uni-

formly and 2 g such mixture was dispersed in acetone in a sonicationbath for 15min to prepare 200 g sizing agent, the content of mainpolymer in sizing agent was 1 wt% as well.

2.2.3. Sizing agents containing R80615 g R806 and 0.3 g TBPB were mixed uniformly and 2 g such

mixture was dispersed in acetone in a sonication bath for 15min toprepare 200 g sizing agent, the content of main polymer in sizing agentwas 1wt%.

2.2.4. Sizing agents containing M727015 g M7270, 0.3 g imidazole curing agents and 0.3 g TBPB were

mixed uniformly, and 2 g such mixture (M7270 mixture, for short) wasdispersed in acetone in a sonication bath for 15min to prepare 200 gsizing agent, the content of main polymer in sizing agent was 1 wt%.

2.3. Preparation of CNTs-EDA

The CNTs used in this study were dispersed and oxidized in themixed acids (V(HNO3):V(H2SO4)= 1.2:1) at 65 °C for 8 h. After treatment,the CNTs were filtered, washed with deionized water to reach pH 7 anddried at 60 °C for 8 h to get carboxyl functionalized CNTs (CNTs-COOH). 2 g CNTs-COOH, 150mL SOCl2 and 10mL DMF were mixeduniformly in an 80 °C oil bath for 40 h to get the acyl chloride func-tionalized CNTs (CNTs-COCl). Then the CNTs-COCl was reacted with100mL EDA at 60 °C for 36 h to produce amine functionalized CNTs

Fig. 1. SEM images of (a) CF coated with M7270, (b) CF coated with R806, (c) CF coated with MR13006, (d) CF coated with epoxy; (e) ILSS and (f) the load-displacement curves of CFs composites.

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(CNTs-EDA) [33], as verified by FTIR and XPS spectra of CNTs, Fig. 4.

2.4. Preparation of sizing agents containing CNTs-EDA and M7270

1 g CNTs-EDA and 2 g M7270 mixture were dispersed in acetone ina sonication bath for 15min to prepare 200 g sizing agent (M7270-CNTs-EDA), the content of main polymer M7270 in sizing agent was1 wt% and CNTs-EDA content was 0.5 wt%.

2.5. Sizing treatment and the preparation of composites

The CFs were treated with a self-made anodic oxidation equipmentfor 4min in 5% ammonium bicarbonate solution with an electric cur-rent density of 300mA/g. After being washed and dried, the CFs werecoated with M7270-CNTs-EDA. The resin system was prepared bymixing MR13006, low shrinkage agent LP4016 and initiator TBPB in aratio of 105:45:1. The prepreg containing the surface modified CFs andthe resin system was transferred into a mold for curing process. Suchprepreg was maintained at 80 °C for half an hour without pressurizingand then 140 °C for 1 h under 10MPa. The resin content of the finalproduct should be restricted to 35 ± 1wt% as suggested for Ashlandcompany [1,2,4].

2.6. Characterizations

The fiber surface morphology was observed with SEM (FE-SEM,JEOL JSM-6335 F, Japan). FT-IR spectroscopy (Bruker Optics, Vertex70, Germany) was carried out to characterize the surface group com-position of CNTs. XPS (Thermo Scientific, USA) was employed to in-vestigate the surface chemical composition of CNTs and CFs using amonochromated Al Ka source (1486.6 eV) at a base pressure of2× 10−9 mbar. Dynamic contact angle (DCA) was measured by a CAmeter (DCAT21, Data-Physics Instrument, Germany). Deionized water(γd= 21.8 mJ/m2, γ=72.8mJ/m2) and diiodomethane

(γd= 50.8 mJ/m2, γ=50.8mJ/m2) were employed as test medium.CA was calculated by mass variation when CF was immersed to the testmedium based on Wilhelmy's Equation [2]. AFM in force modulationmode (Solver-P47H, NT-MDT, Russia) was carried out to characterizethe relative modulus of composite interphase. The spring constant andthe resonant frequency of the FMAFM tip were 1–5 Nm−1 and70–150 kHz, respectively. For interface characterization, the compositespecimens were polished perpendicularly to the fiber axis using in-creasingly finer sand papers and finally polished with a Cr2O3 suspen-sion of an average grain size of 50 nm, and cleaned with water in ul-trasonic washer and dried [34]. Based on ASTM D2344, the ILSS testswere carried out on a mechanical testing machine (5569, Instron, USA)at a cross-head speed of 2mm/min. The specimen dimensions for ILSStest were 20mm×6.5mm×2mm. An impact test system (9250HV,Instron, USA) was used to characterize the impact toughness of com-posites with impact span of 40mm. The specimen dimensions for im-pact test were 55mm×6.5mm×2mm. The drop velocity and dropweight were of 1ms-1 and 3 kg, respectively. The recorded value of ILSSand impact toughness for each group of specimens was averaged fromthe data of 5 successful measurements.

3. Results and discussion

3.1. Surface topography and chemical composition of CFs

Fig. 1a–d shows the topography and oxygen element linear dis-tribution of CFs. The sizing agents containing M7270, R806 and water-soluble EP exhibited good spreadability on the CF surface. Compared toother three samples, the interfacial compatibility of CFs with the sizingagent containing MR13006 is worse. As shown in Fig. 1c, the dis-tribution curve of oxygen element had a larger fluctuation, showingthat the MR13006 sizing agent cannot be distributed uniformly on theCF surface. Moreover, the dark color section in Fig. 1c corresponds tothe high oxygen element content portion of the distribution curve,

Fig. 2. (a) Structure schematic of different polymers; C1s spectra of XPS: (b) CF coated with epoxy, (c) CF coated with MR13006, (d) CF coated with R806, (e) CFcoated with M7270.

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indicating the aggregation of MR13006 with poor conductivity.Fig. 2b–e shows the C1s XPS spectra of the CFs coated with different

sizing agents. The C1s spectrum of the CFs coated with water-soluble EPdemonstrated 57.4% CeC, 41.9% CeO and 0.7% OeC]O bonds, theCeO content is much higher than that of OeC]O bond. Similarly, thecontent of CeO bond on the CFs coated with M7270 and R806 is alsorelatively high, 31.2% and 30.5% respectively. In contrast, the C1sspectrum of the CFs coated with MR13006 consisted of 75.5%CeC,13.9%CeO and 10.6%OeC]O bonds, the content of OeC]O bond isthe highest among all the samples, and the CeO bonds are relativelyless. As shown in Fig. 2a, each repetitive structure unit of the epoxymolecular includes one hydroxyl group, and two end epoxy groups. NoOeC]O bonds have been found in the molecular structure and corre-spond to the XPS C1s spectra of the CFs. Vinyl ester is the reactionproduct of epoxy and unsaturated monoacids, thus M7270 and R806have remarkable structure features of the epoxy, possessing relativelyhigh quantity of CeO bonds.

SEM images (Fig. 1a–d) reveals that the epoxy sizing agent has thebest uniformity on the fiber surface, which is attributed to plenty ofhydroxyl groups of epoxy polymer chains. It should be noted that thereis no ester group in epoxy molecular structure, confirming that hy-droxyl groups have a better interfacial compatibility with carbon fibersurface treated with anodic oxidation than ester groups. This is thereason for choosing epoxy to prepare the sizing agent in most com-mercial carbon fiber production enterprises.

M7270 and R806 keep the structural features of epoxy backbone,which are characterized by multiple hydroxyl groups. Therefore, com-pare to MR13006, M7270 and R806 have better spreading property onthe fiber surface (Fig. 1a–c). As shown in Fig. 2a, both ends of R806molecular chains are vinyl groups. For M7270, one end is still vinylgroup, but the other end of molecular chains is substituted with epoxygroups, which is beneficial to improve the interfacial compatibility.Note that MR13006 with the highest oxygen content shows the lowestCeO content and the OeC]O content of MR13006 is the highestamong four kinds of main polymers. As shown in Fig. 1c, the MR13006is hard to be distributed uniformly on the fiber surface, indicating thatthe CeO groups have a better compatibility for carbon fiber surfacethan the OeC]O groups. From the structure schematic of MR13006(Fig. 2a), it can be seen that the hydroxyl groups appear only at theends of molecular chain and the repeating units of molecular chaincontain a large amount of ester groups and carbonyl groups, whichagree well with the fitted curves of XPS C1s. Moreover, the length of themolecular chain for different polymer is one important factor to influ-ence the surface coating. M7270, R806 and the epoxy used in this paperhave a similar number average molecular mass (< 1000 g/mol).However, the molecular mass of MR13006 is about 3000 g/mol, whichmeans that a longer molecular chain cannot get inside the tiny groovesand voids.

3.2. ILSS of CF/UP composites

Fig. 1e and f shows the ILSS and load displacement curves of CF/UPcomposites before and after coating. The load is approximately linear tothe displacement in the initial period of loading. With further in-creasing the displacement, the load declines sharply from peak value,which is probably due to the slipping between the fibers and the UPresin or the yielding of UP. Anodic oxidation is an effective means toincrease the roughness and reactive activity of fiber surface, which ishelpful to enhance the interface adhesion and interlocking between CFand UP. It was discovered that the fiber after anodic oxidation couldmake ILSS increase to 50.3MPa from the original 47.7 MPa. With thebenefit of massive hydroxyl groups, the sizing agent containingM7270、R806 and epoxy can distribute uniformly on the CF surface.Based on previous analysis, the sizing agent containing MR13006 hasthe lowest hydroxyl group content, which is hard to obtain a goodwettability on the fiber surface. Moreover, the molecular weight and

viscosity of MR13006 are much higher than those of other three mainpolymers for preparing sizing agent, which is an obstacle for shortrange contact between the molecular chain and the fiber surface. Theinterface layer with the gradient variation of mechanical properties isbeneficial to transfer and disperse stress. However, the exorbitant mo-lecular weight is not helpful to form such gradient interface layer [35].The matrix resin is composed of MR13006 (molecular weight: 3000 g/mol) and LP4016 (molecular weight: 40000 g/mol). The exorbitantmolecular weight of resin mixture will limit the movement of polymermolecular chains from the interface region to the matrix resin region.The ILSS of CF/UP composites treated with MR13006 sizing agent isjust 52.3MPa.

The relatively low molecular weight of R806 and M7270 makesthem more easily form the interface layer with the gradient mechanicalproperties. Owing to good wettability, the ILSS of R806 and M7270sizing agent was 57.4 and 58.8MPa, respectively. The good interfacebonding stems from the molecular structure and physicochemicalproperty of polymer themselves. A large amount of hydroxyl groupsand relatively low molecular weight of R806 and M7270 assure thegood wettability of sizing agent on the fiber surface. Unsaturateddouble bonds coexist in vinyl resin, UP resin and styrene solvent, andsimilar chemical structure is beneficial to enhance the interface com-patibility. More importantly, unsaturated bonds can participate in thecuring process of UP matrix through the reaction of radical poly-merization, and generate solid interphase linked with covalent bonds.During curing processes, high volume shrinkage of sizing agent polymercould lead to relatively high stress in the interface region and led to theinterface bonding failure. Compared to R806, M7270 has a relativelylower volume shrinkage (M7270:4.6% vs R806:7.5%), which helps toimprove the ILSS of composites. Compared to the unsaturated doublebonds of R806, the epoxy end groups of M7270 react more easily withthe fiber surface treated by anodic oxidation. It is worth noting that theepoxy sizing agent with the lowest volume shrinkage (2%) has theworst ILSS. Epoxy is widely applied to prepare commercial sizing agentfor most CF enterprises. Epoxy sizing agent can effectively improve theinterface compatibility for the fibers reinforced composites, especiallyfor the carbon fibers reinforced epoxy systems. But obviously, vinylresin sizing agents have a better interface compatibility for the CF/UPcomposites.

3.3. The design of sizing agents containing CNTs-EDA and M7270

M7270 is chosen as the main polymer to prepare sizing agent con-taining CNTs-EDA for the CF/UP composites. Introducing nanoparticlesto the interface of CF reinforced polymer composites has been proved tobe an effective way to toughen and reinforce composites [36]. Fig. 3shows the chemical reaction diagram for M7270-CNTs-EDA in the CF/UP composites. As shown in Fig. 3, a large number of oxygen containinggroups were introduced to the CF surface after the anodic oxidationtreatment, which benefited their reaction with M7270, MR13006 andCNTs-EDA via chemical or physical bonding. In order to shorten thecuring time, high temperature curing agent TBPB was chosen forMR13006 curing at 140 °C as recommended by the Ashland Company.High temperature solidification is helpful to promote the interactions orto create chemical bonding between polar groups. The amino groups ofCNTs-EDA can react with epoxy groups of M7270 at 140 °C, which canenhance the stability of CNTs in the interface region. M7270 molecularchain acts as a bridge between CF and UP, whose one end is connectedto the fiber surface by hydroxyl groups and the other end is involved inthe curing process by unsaturated double bonds [37].

3.4. The amino-functionalization CNTs

Fig. 4a&b shows the SEM images of raw CNTs and CNTs-EDA, re-spectively. CNTs-EDA has a better dispersity when compared to rawCNTs and a layer of organic coating attached on the CNT surface, which

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Fig. 3. The chemical reaction diagram for M7270-CNTs-EDA in the CF/UP composites.

Fig. 4. SEM images of (a) raw CNTs and (b) CNTs-EDA; (c) FTIR of CNTs; (d) TG curves of CNTs; XPS spectra of (e) C1s, (f) N1s and (g) O1s peaks of CNTs-EDA.

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is attributed to the introduced ammonium. Fig. 4c shows the infraredspectra of CNTs before and after surface functionalization. The infraredabsorption peaks of CNTseCOOHeA at 3438, 1726, 1384 and1097 cm−1 correspond to the stretching vibration of eOH, C]O,eOSO3H and eCeO, respectively. Compared to raw CNTs, the peakintensity of the eOH stretching vibration at 3438 cm−1 is significantlyincreased, suggesting that the CNTs have been oxidized during the acidtreatment [34]. For CNTs-EDA, new characteristic peak at 1580 cm−1 isassigned to the deformation vibration of NeH, the peak at 1180 cm−1

corresponds to the stretching vibration of CeN, and the peak at800 cm−1 is assigned to the out-of-plane bending of NH2, indicatingthat amino groups are grafted onto the CNTs through covalent bonding[38].

Such surface treatment could be further assessed by XPS, as shownin Fig. 4e, f&g. From the C1s spectra, the characteristic peak at 284.4 eVcorresponds to the C]C of graphitized carbon. The peak at 285.2、286.4 and 288.4 eV were assigned to the CeN, CeO and NeC]O, re-spectively, indicating that EDA was covalently linked with CNTs surfaceby the amide bonds [39]. For the N1s spectra, the binding energy peaksat 399.6 and 402.2 eV were ascribed to the CeN and ammonium, re-spectively. Moreover, the appearance of NeC]O peak in O1s spectrafurther verifies the amination of CNTs.

In view of the thermal instability of absolute majority organicgroups, TGA is an effective way to provide relative information aboutfunctional modification of CNTs-EDA. Fig. 4d shows the TGA curves ofraw CNTs, CNTseCOOHeA and CNTs-EDA, respectively. Raw CNTs arerelatively thermal stable in the range of 0–680 °C, but CNTseCOOHeAhave a 13% weight loss in this temperature range. Compared toCNTseCOOHeA, CNTs-EDA shows the phenomenon of multilevelthermal degradation, which was attributed to more kinds of functionalgroups of CNTs-EDA. However, the thermal weight loss curves ofCNTseCOOHeA and CNTs-EDA overlap in the range of 0–200 °C,which is likely due to the volatilization of water during the process ofamination and carboxylation. With further increasing the temperature,the groups were eliminated by thermal degradation. Therefore, theamidogen content of CNTs-EDA could be estimated by TGA curves ofCNTs, which was approximately 6wt%.

3.5. The effect of M7270-CNTs-EDA on surface properties of CFs

Fig. 5a&b shows the surface morphology of CFs coated with M7270sizing agent containing raw CNTs and CNTs-EDA, respectively. CNTs-EDA has a better dispersion in sizing agent than raw CNTs and no ob-vious aggregations were observed in Fig. 5b. The surface roughness(Ra) of the fibers treated with M7270-CNTs-EDA was obviously in-creased in comparison with those treated with M7270 sizing agentcontaining raw CNTs. The variation of surface polar groups influencesthe surface free energy and its components. The higher surface energymeans that the matrix can be uniformly dispersed on the fiber surface,which is beneficial to enhancing the interfacial compatibility of com-posites [40]. DCA tests were carried out to examine the effects ofM7270-CNTs-EDA on the surface energy and the results are summar-ized in Fig. 5c&d. After treated with M7270-CNTs-EDA, the CA of CFs inboth polar and non-polar test mediums declined apparently. Therefore,its polar component and dispersion component of the surface energy allincreased significantly, which is mainly attributed to the increasedquantity of active functional groups. Note that the contact angles inwater for CFs treated with pure M7270 sizing agent and M7270-CNTs-EDA are nearly the same, which means that the introducing of CNTs-EDA has almost no influence on the wettability of fiber surface andnearly all CNTs-EDA are coated with M7270 resin. The increased dis-persion component was due to the increased roughness caused by CNTs-EDA on the fiber surface and the differences of graphite structure be-tween CFs and CNTs-EDA. Moreover, the increased polar component ofthe sizing agent treated CFs is attributed to the polar groups in M7270molecular chains and the eNH2 groups of CNTs-EDA.

3.6. The effect of M7270-CNTs-EDA on interfacial bonding of CF/UPcomposites

The ILSS and load-displacement curves were shown in Fig. 6a and b.As shown in Fig. 6a, the introducing of CNTs-EDA obviously improvedthe interface bonding of the CF/UP composites. The ILSS increased from47.7MPa for the raw CFs to 63.1 MPa for the CFs treated with M7270-CNTs-EDA by 32.3%. After treated with M7270-CNTs-EDA, the polarfunctional groups of sizing agent played an important role in improvingthe interfacial bonding between the CFs and the UP matrix. The polargroups could increase the wettability of CFs for UP matrix andstrengthen the interactions of molecular in the interface region. Fur-thermore, the unsaturated double bonds of M7270 could participate inthe curing process of UP matrix through radical polymerization. Com-pared to the CFs treated with M7270 sizing agent without CNTs, theILSS of CFs treated with M7270-CNTs-EDA still increased by 7.3%. Thefurther enhancement in ILSS was partly due to the anchoring effect ofCNTs in sizing agent for the UP resin. The enhanced mechanical in-terlocking and the local mechanical properties of interface region arebeneficial to transfer the load. Furthermore, the interface layer withgradient mechanical properties caused by CNTs-EDA in sizing agent isnot only beneficial to transfer and disperse stress, but also for elim-inating the internal stress between CFs and UP matrix when curing isdone [41].

Fig. 6c shows the impact properties of CF/UP composites before andafter interface modification. The initial and propagative energy of theraw CFs composites were 0.82 and 0.23 J, respectively. The M7270-CNTs-EDA treatment obviously increased the impact toughness of theCF/UP composites. The initial and propagative energy of the compo-sites treated with M7270-CNTs-EDA increased to 1.04 and 0.59 J, re-spectively.

Strong interface bonding is not only beneficial to enhance the ILSS,but can also influence other properties of composites, such as wet andheat ageing resistance property, acid-base resistance property andconductance. However, strong interface bonding leads to stress con-centration and goes against the movement of polymer molecular chainsin the interface region, which result in the decrease of the impactproperty [34]. Whereas, by reasonably designing the interface mod-ification, both ILSS and impact property can be enhanced at the sametime.

It is worth noting that the pure M7270 sizing agent without CNTs isbeneficial for increasing the ILSS of CF/UP composites, but doing littleto the toughness improvement, which indicate that CNTs are key factorfor toughness enhancement. Introducing CNTs to vinyl ester sizingagent can structure the rough surface of CFs, which restricts themovement of molecules in the interphase and improves the mechanicalinterlocking between CFs and UP resins. When the impact load wasapplied, the crack propagating orientation will be changed by CNTs andcreates more tiny cracks in the bonding interface, which are helpful todissipate the impact energy [34]. Furthermore, the CNTs containinginterphase with proper modulus between CFs and UP resin is alsohelpful to prevent the cracks from direct contacting the fiber surfaceand reduces the stress concentration in the interface region.

Fig. 6d & e shows the impact fracture surfaces of the compositestreated with and without M7270-CNTs-EDA. As shown in Fig. 6d,plenty of raw fibers were pulled out from the UP resin and lots of voidsappeared in the UP resin, indicating a weak interfacial bonding be-tween CFs and the UP matrix. After treated with M7270-CNTs-EDA, asmall number of fibers were pulled out and the pullout length of thefiber became shorter. Moreover, the UP and CF debris were found to beuniformly distributed throughout the cross section, which was attrib-uted to the enhanced roughness and surface energy of the fibers, furtherconfirming an enhanced interphase of the CFs and UP resin.

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Fig. 5. SEM images of CFs coated with (a) M7270-raw CNTs and (b) M7270-CNTs-EDA; (c) Contact angle and (d) Surface energy of CFs.

Fig. 6. (a) ILSS, (b) Load-displacement curves and (c) Impact toughness of CFs composites; Impact fracture morphology of the composites (d) before and (e) aftersizing treatment.

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3.7. The effect of M7270-CNTs-EDA on interphase modulus of CF/UPcomposites

FMAFM was carried out to evaluate the relative stiffness of inter-phase of CF/UP composites. The amplitude of the FMAFM tip underscanning approximate corresponds to the relative modulus of the testspecimens. In short, a smaller amplitude of FMAFM tip means a higherrelative modulus of test specimen, and vice versa [34]. The corre-sponding working principle diagram of FMAFM is shown in Fig. 7a.Fig. 7b & c shows the force modulation images of the CF/UP compo-sites. Compared to the image of composites before the sizing treatment(Fig. 7b), the force modulation image of composite treated with M7270-CNTs-EDA (Fig. 7c), shows obvious color contrast among the CF, in-terface region and the UP, indicating a gradient variation of modulusfor these three phases. The appearance of interface region could also beconfirmed by the MAG-position curves of the cross section. As shown inFig. 7e, there is an obvious modulus transition region corresponding tothe interface region. By contrast, only two phases could be observed forraw CFs and UP in Fig. 7b & d and the relative hardness suddenly

changes from the fiber section to the UP section without any transitionarea. The color of interphase in Fig. 7c is a little lighter than CFs and alittle darker UP region, which means that the interphase modulus isintermediate between the reinforcement and UP resin. The interphasewith moderate modulus can provide a cushion when the stress wastransferred from UP to CFs.

4. Conclusions

More reactive groups and better interfacial compatibility with UPmake M7270 a more suitable polymer for preparing sizing agent for CF/UP composites when compared to epoxy, MR13006 and R806. M7270-CNTs-EDA was prepared to increase the interfacial properties of CF/UPcomposites. After sizing treatment, the roughness, polar functionalgroups and wettability of CF surface were improved obviously, and theILSS and impact toughness were improved by 32.3 and 55.2%, re-spectively. The M7270-CNTs-EDA effectively increased the interfacecompatibility of composites not only by using the reactive groups ofM7270 to improve the surface energy and react with UP matrix during

Fig. 7. (a) Working principle diagram of FMAFM; (b, c) FMAFM images and (d, e) MAG-position curves of composites before and after sizing treatment.

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the curing process, but also by improving the mechanical interlockingby CNTs-EDA. Furthermore, the CNTs-interphase with a gradientmodulus is beneficial to transfer and disperse load.

Acknowledgement

Supported by Key Laboratory of Engineering Dielectrics and ItsApplication (HUST), Ministry of Education, University Nursing Programfor Young Scholars with Creative Talents in Heilongjiang Province, theNSFC (No.51677045, 51577045, 21604019), Heilongjiang NaturalScience Foundation (QC2017038), China Postdoctoral ScienceFoundation (No.2017M610212 & 2016M601402), and HeilongjiangPostdoctoral Fund (No.LBH-Z16089, LBH-Z16004).

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.compscitech.2018.05.051.

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Interfacially reinforced unsaturated polyester carbon fiber composites with a vinyl ester-carbon nanotubes sizing agentIntroductionExperimentalMaterialsPreparation of sizing agents containing different main polymersSizing agents containing water-soluble EPSizing agents containing MR13006Sizing agents containing R806Sizing agents containing M7270

Preparation of CNTs-EDAPreparation of sizing agents containing CNTs-EDA and M7270Sizing treatment and the preparation of compositesCharacterizations

Results and discussionSurface topography and chemical composition of CFsILSS of CF/UP compositesThe design of sizing agents containing CNTs-EDA and M7270The amino-functionalization CNTsThe effect of M7270-CNTs-EDA on surface properties of CFsThe effect of M7270-CNTs-EDA on interfacial bonding of CF/UP compositesThe effect of M7270-CNTs-EDA on interphase modulus of CF/UP composites

ConclusionsAcknowledgementSupplementary dataReferences