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Composites Science and Technology 164 (2018) 195–203

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Composites Science and Technology

journal homepage: www.elsevier.com/locate/compscitech

Interfacially reinforced unsaturated fiber composites with a T vinyl -carbon nanotubes 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 and Its Application, Ministry of , Harbin University of Science and Technology, Harbin, 150040, China b College of Material Science and Engineering, Harbin University of Science and Technology, Harbin, 150040, China c School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin, 150001, China d Heilongjiang Key Laboratory of Molecular Design and Preparation of Flame Retarded Materials, Northeast University, Harbin, 150040, China e National Engineering Research Center for Advanced Processing Technology, Zhengzhou University, Zhengzhou, 450002, China f Integrated Composites Lab (ICL), Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, TN, 37996, USA g State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou, 570228, China h School of Material Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang, Jiangsu, 212003, China i College of and Engineering, Shandong University of Science and Technology, Qingdao 266590, China

ARTICLE INFO ABSTRACT

Keywords: A amino-functionalized carbon nanotubes (CNTs)-containing sizing agent was prepared for improving the in- A. Carbon fibers terface bonding and impact toughness of carbon fibers (CFs) reinforced unsaturated polyester (UP) composites. A. Coating More reactive groups and better interfacial compatibility with UP make vinyl M7270 a more suitable B. Interphase polymer for preparing sizing agent for CF/UP composites when compared to , MR13006 and R806. The B. Mechanical properties surface 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 surface obviously increased the surface roughness. The amount of polar functional groups and the wettability of CFs were significantly enhanced after the coating treatment. The interlaminar shear strength (ILSS) and impact toughness were enhanced by 32.3 and 55.2%, respectively. The sizing agent effectively enhanced the interfacial adhesion by improving the surface energy, and increasing chemical bonding and mechanical interlocking.

1. Introduction viewpoint of their inherent excellent properties [5–18]. In the field of carbon fiber composites, the interphase containing nanomaterials (e.g., Based on radical , unsaturated polyester (UP) has a CNTs) is helpful to transfer stress and creates mechanical interlocking better curing efficiency than epoxy. Some UP can finish curing within between CFs and resin [19]. Various kinds of interface treatment ap- 1 min, such as MR13006 from Ashland. In order to have a higher pro- proaches have been employed to introduce nanomaterials into the in- duction rate per each used mold, the curing efficiency becomes more terface of composites, such as chemical grafting [20,21], coating with important for enterprises [1,2]. Meanwhile, the me- sizing agent [22], chemical vapor deposition (CVD) [23], electro- chanical properties of CF/UP composites are preferred to be close to the chemical deposition(ECD) [24] and electrophoretic deposition (EPD) level of CF/epoxy composites. The ultimate of composites [23,24]. Compared to other approaches, coating with sizing agent takes is largely dependent on the interfacial properties [3]. However, the advantage of dispersing nanomaterials in and introducing no interfacial compatibility between CFs and UP is relatively poor, which damage to the fibers [25]. For an enhanced interfacial compatibility, leads to a lower interlaminar shear strength (ILSS) and impact tough- the first priority for preparing a sizing agent is to select a suitable main ness [4]. polymer [26]. Epoxy is widely applied to prepare commercial sizing Polymer nanocomposites are promising in many fields from the agent for most CF enterprises. It has been proved that epoxy sizing

∗ 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: [email protected] (Z. Wu), [email protected] (Y. Huang), [email protected] (Z. Guo). https://doi.org/10.1016/j.compscitech.2018.05.051 Received 1 February 2018; Received in revised form 28 April 2018; Accepted 30 May 2018 Available online 31 May 2018 0266-3538/ © 2018 Elsevier Ltd. All rights reserved. Z. Wu et al. Composites Science and Technology 164 (2018) 195–203

agent can effectively improve the interfacial properties of carbon fiber HNO3 (68%) and ammonium bicarbonate were purchased from reinforced epoxy composites. However, the reinforced effect of epoxy Changzheng Co., China. Fig. 1a shows the molecular formulas of four sizing agent has not been studied for the UP matrix, and a proper sizing kinds of main for preparing sizing agents. agent is needed. In this study, four kinds of polymers were used to prepare sizing 2.2. Preparation of sizing agents containing different main polymers agents according to their wide applications for sizing agent preparation or similar chemical structure with UP, including widely used epoxy, UP 2.2.1. Sizing agents containing water-soluble EP (MR13006), and two kinds of vinyl ester (M7270 and R806) to disclose 4 g water-soluble EP, and curing agent (1 wt% water-soluble the effects of polymer structure on the interfacial properties of CF/UP EP) were dispersed in deionized water in a sonication bath for 15 min to composites. A comparison study indicated that M7270 was the most prepare 200 g sizing agent, the content of main polymer in sizing agent effective for interfacial property enhancement. The massive polar was 1 wt%. groups (hydroxyl groups and epoxy groups) and relative low average molecular mass (< 1000 g/mol) of M7270 warrant a good spreadability 2.2.2. Sizing agents containing MR13006 on the fiber surface. The unsaturated double bonds of M7270 make it 10.5 g MR13006, 4.5 g LP4016, and 0.15 g TBPB were mixed uni- easier to participate in the curing reaction with UP matrix. CNT is formly and 2 g such mixture was dispersed in acetone in a sonication considered to be a promising reinforcing material because of its unique bath for 15 min to prepare 200 g sizing agent, the content of main mechanical and physical properties [27–32]. Therefore, a novel sizing polymer in sizing agent was 1 wt% as well. agent containing amine functionalized CNTs (CNTs-EDA) and vinyl resin M7270 was prepared for enhancing the interface performance of 2.2.3. Sizing agents containing R806 CF/UP. The effects of this novel sizing agent on surface characteristics 15 g R806 and 0.3 g TBPB were mixed uniformly and 2 g such of CFs were studied by XPS, FTIR, TGA and SEM, and the interfacial mixture was dispersed in acetone in a sonication bath for 15 min to adhesion of final CF/UP was discussed with ILSS, impact test and AFM prepare 200 g sizing agent, the content of main polymer in sizing agent in force modulation mode (FMAFM). was 1 wt%.

2. Experimental 2.2.4. Sizing agents containing M7270 15 g M7270, 0.3 g imidazole curing agents and 0.3 g TBPB were 2.1. Materials mixed uniformly, and 2 g such mixture (M7270 mixture, for short) was dispersed in acetone in a sonication bath for 15 min to prepare 200 g Un-sizing CFs (raw CFs) were obtained from Guangwei Co., China sizing agent, the content of main polymer in sizing agent was 1 wt%. (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 2.3. Preparation of CNTs-EDA (Volume shrinkage: 8.5%) and low shrinkage agent LP4016 (vinyl acetate polymer) were obtained from Ashland China. Vinyl ester M7270 The CNTs used in this study were dispersed and oxidized in the

(Volume shrinkage: 4.6%), vinyl ester R806 (Volume shrinkage: 7.5%) mixed acids (V(HNO3):V(H2SO4) = 1.2:1) at 65 °C for 8 h. After treatment, and corresponding imidazole curing agents were purchased from the CNTs were filtered, washed with deionized water to reach pH 7 and SHOWA HIPOLYMER Co., China. Water-soluble EP (volume shrinkage: dried at 60 °C for 8 h to get carboxyl functionalized CNTs (CNTs-

2%, content 50%) and corresponding amine hardendner were COOH). 2 g CNTs-COOH, 150 mL SOCl2 and 10 mL DMF were mixed purchased by Guangwei Co., China. Tert-butyl peroxybenzoate (TBPB) uniformly in an 80 °C oil bath for 40 h to get the func- from ACROS ORGANICS was used as an initiator for the CF/UP system. tionalized CNTs (CNTs-COCl). Then the CNTs-COCl was reacted with

Ethylenediamine (EDA), SOCl2, acetone, DMF, THF, H2SO4 (98%), 100 mL 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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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) CF coated with M7270.

(CNTs-EDA) [33], as verified by FTIR and XPS spectra of CNTs, Fig. 4. (γd = 50.8 mJ/m2, γ = 50.8 mJ/m2) were employed as test medium. CA was calculated by mass variation when CF was immersed to the test 2.4. Preparation of sizing agents containing CNTs-EDA and M7270 medium based on Wilhelmy's Equation [2]. AFM in force modulation mode (Solver-P47H, NT-MDT, Russia) was carried out to characterize 1 g CNTs-EDA and 2 g M7270 mixture were dispersed in acetone in the relative modulus of composite interphase. The spring constant and − a sonication bath for 15 min to prepare 200 g sizing agent (M7270- the resonant frequency of the FMAFM tip were 1–5Nm1 and CNTs-EDA), the content of main polymer M7270 in sizing agent was 70–150 kHz, respectively. For interface characterization, the composite 1 wt% and CNTs-EDA content was 0.5 wt%. specimens were polished perpendicularly to the fiber axis using in- creasingly finer sand and finally polished with a Cr2O3 suspen- 2.5. Sizing treatment and the preparation of composites 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 tests The CFs were treated with a self-made anodic oxidation equipment were carried out on a mechanical testing machine (5569, Instron, USA) for 4 min in 5% ammonium bicarbonate solution with an electric cur- at a cross-head speed of 2 mm/min. The specimen dimensions for ILSS rent density of 300 mA/g. After being washed and dried, the CFs were test were 20 mm × 6.5 mm × 2 mm. An impact test system (9250HV, coated with M7270-CNTs-EDA. The resin system was prepared by Instron, USA) was used to characterize the impact toughness of com- mixing MR13006, low shrinkage agent LP4016 and initiator TBPB in a posites with impact span of 40 mm. The specimen dimensions for im- ratio of 105:45:1. The prepreg containing the surface modified CFs and pact test were 55 mm × 6.5 mm × 2 mm. The drop velocity and drop -1 the resin system was transferred into a mold for curing process. Such weight were of 1 ms and 3 kg, respectively. The recorded value of ILSS prepreg was maintained at 80 °C for half an hour without pressurizing and impact toughness for each group of specimens was averaged from and then 140 °C for 1 h under 10 MPa. The resin content of the final the data of 5 successful measurements. product should be restricted to 35 ± 1 wt% as suggested for Ashland company [1,2,4]. 3. Results and discussion

2.6. Characterizations 3.1. Surface topography and chemical composition of CFs

The fiber surface morphology was observed with SEM (FE-SEM, Fig. 1a–d shows the topography and element linear dis- JEOL JSM-6335 F, Japan). FT-IR spectroscopy (Bruker Optics, Vertex tribution of CFs. The sizing agents containing M7270, R806 and water- 70, Germany) was carried out to characterize the surface group com- soluble EP exhibited good spreadability on the CF surface. Compared to position of CNTs. XPS (Thermo Scientific, USA) was employed to in- other three samples, the interfacial compatibility of CFs with the sizing vestigate the surface chemical composition of CNTs and CFs using a agent containing MR13006 is worse. As shown in Fig. 1c, the dis- monochromated Al Ka source (1486.6 eV) at a base pressure of tribution curve of oxygen element had a larger fluctuation, showing − 2×10 9 mbar. Dynamic contact angle (DCA) was measured by a CA that the MR13006 sizing agent cannot be distributed uniformly on the meter (DCAT21, Data-Physics Instrument, Germany). Deionized water CF surface. Moreover, the dark color section in Fig. 1c corresponds to (γd = 21.8 mJ/m2, γ = 72.8 mJ/m2) and diiodomethane the high oxygen element content portion of the distribution curve,

197 Z. Wu et al. Composites Science and Technology 164 (2018) 195–203 indicating the aggregation of MR13006 with poor conductivity. viscosity of MR13006 are much higher than those of other three main Fig. 2b–e shows the C1s XPS spectra of the CFs coated with different polymers for preparing sizing agent, which is an obstacle for short sizing agents. The C1s spectrum of the CFs coated with water-soluble EP range contact between the molecular chain and the fiber surface. The demonstrated 57.4% CeC, 41.9% CeO and 0.7% OeC]O bonds, the interface layer with the gradient variation of mechanical properties is CeO content is much higher than that of OeC]O bond. Similarly, the beneficial to transfer and disperse stress. However, the exorbitant mo- content of CeO bond on the CFs coated with M7270 and R806 is also lecular weight is not helpful to form such gradient interface layer [35]. relatively high, 31.2% and 30.5% respectively. In contrast, the C1s The matrix resin is composed of MR13006 (molecular weight: 3000 g/ spectrum of the CFs coated with MR13006 consisted of 75.5%CeC, mol) and LP4016 (molecular weight: 40000 g/mol). The exorbitant 13.9%CeO and 10.6%OeC]O bonds, the content of OeC]O bond is molecular weight of resin mixture will limit the movement of polymer the highest among all the samples, and the CeO bonds are relatively molecular chains from the interface region to the matrix resin region. less. As shown in Fig. 2a, each repetitive structure unit of the epoxy The ILSS of CF/UP composites treated with MR13006 sizing agent is molecular includes one hydroxyl group, and two end epoxy groups. No just 52.3 MPa. OeC]O bonds have been found in the molecular structure and corre- The relatively low molecular weight of R806 and M7270 makes spond to the XPS C1s spectra of the CFs. Vinyl ester is the reaction them more easily form the interface layer with the gradient mechanical product of epoxy and unsaturated monoacids, thus M7270 and R806 properties. Owing to good wettability, the ILSS of R806 and M7270 have remarkable structure features of the epoxy, possessing relatively sizing agent was 57.4 and 58.8 MPa, respectively. The good interface high quantity of CeO bonds. bonding stems from the molecular structure and physicochemical SEM images (Fig. 1a–d) reveals that the epoxy sizing agent has the property of polymer themselves. A large amount of hydroxyl groups best uniformity on the fiber surface, which is attributed to plenty of and relatively low molecular weight of R806 and M7270 assure the hydroxyl groups of epoxy polymer chains. It should be noted that there good wettability of sizing agent on the fiber surface. Unsaturated is no ester group in epoxy molecular structure, confirming that hy- double bonds coexist in vinyl resin, UP resin and solvent, and droxyl groups have a better interfacial compatibility with carbon fiber similar chemical structure is beneficial to enhance the interface com- surface treated with anodic oxidation than ester groups. This is the patibility. More importantly, unsaturated bonds can participate in the reason for choosing epoxy to prepare the sizing agent in most com- curing process of UP matrix through the reaction of radical poly- mercial carbon fiber production enterprises. merization, and generate solid interphase linked with covalent bonds. M7270 and R806 keep the structural features of epoxy backbone, During curing processes, high volume shrinkage of sizing agent polymer which are characterized by multiple hydroxyl groups. Therefore, com- could lead to relatively high stress in the interface region and led to the pare to MR13006, M7270 and R806 have better spreading property on interface bonding failure. Compared to R806, M7270 has a relatively the fiber surface (Fig. 1a–c). As shown in Fig. 2a, both ends of R806 lower volume shrinkage (M7270:4.6% vs R806:7.5%), which helps to molecular chains are vinyl groups. For M7270, one end is still vinyl improve the ILSS of composites. Compared to the unsaturated double group, but the other end of molecular chains is substituted with epoxy bonds of R806, the epoxy end groups of M7270 react more easily with groups, which is beneficial to improve the interfacial compatibility. the fiber surface treated by anodic oxidation. It is worth noting that the Note that MR13006 with the highest oxygen content shows the lowest epoxy sizing agent with the lowest volume shrinkage (2%) has the CeO content and the OeC ]O content of MR13006 is the highest worst ILSS. Epoxy is widely applied to prepare commercial sizing agent among four kinds of main polymers. As shown in Fig. 1c, the MR13006 for most CF enterprises. Epoxy sizing agent can effectively improve the is hard to be distributed uniformly on the fiber surface, indicating that interface compatibility for the fibers reinforced composites, especially the CeO groups have a better compatibility for carbon fiber surface for the carbon fibers reinforced epoxy systems. But obviously, vinyl than the OeC]O groups. From the structure schematic of MR13006 resin sizing agents have a better interface compatibility for the CF/UP (Fig. 2a), it can be seen that the hydroxyl groups appear only at the composites. ends of molecular chain and the repeating units of molecular chain contain a large amount of ester groups and carbonyl groups, which 3.3. The design of sizing agents containing CNTs-EDA and M7270 agree well with the fitted curves of XPS C1s. Moreover, the length of the molecular chain for different polymer is one important factor to influ- M7270 is chosen as the main polymer to prepare sizing agent con- ence the surface coating. M7270, R806 and the epoxy used in this taining CNTs-EDA for the CF/UP composites. Introducing nanoparticles have a similar number average molecular mass (< 1000 g/mol). to the interface of CF reinforced polymer composites has been proved to However, the molecular mass of MR13006 is about 3000 g/mol, which be an effective way to toughen and reinforce composites [36]. Fig. 3 means that a longer molecular chain cannot get inside the tiny grooves shows the chemical reaction diagram for M7270-CNTs-EDA in the CF/ and voids. UP composites. As shown in Fig. 3, a large number of oxygen containing groups were introduced to the CF surface after the anodic oxidation 3.2. ILSS of CF/UP composites treatment, which benefited their reaction with M7270, MR13006 and CNTs-EDA via chemical or physical bonding. In order to shorten the Fig. 1e and f shows the ILSS and load displacement curves of CF/UP curing time, high temperature curing agent TBPB was chosen for composites before and after coating. The load is approximately linear to MR13006 curing at 140 °C as recommended by the Ashland Company. the displacement in the initial period of loading. With further in- High temperature solidification is helpful to promote the interactions or creasing the displacement, the load declines sharply from peak value, to create chemical bonding between polar groups. The amino groups of which is probably due to the slipping between the fibers and the UP CNTs-EDA can react with epoxy groups of M7270 at 140 °C, which can resin or the yielding of UP. Anodic oxidation is an effective means to enhance the stability of CNTs in the interface region. M7270 molecular increase the roughness and reactive activity of fiber surface, which is chain acts as a bridge between CF and UP, whose one end is connected helpful to enhance the interface adhesion and interlocking between CF to the fiber surface by hydroxyl groups and the other end is involved in and UP. It was discovered that the fiber after anodic oxidation could the curing process by unsaturated double bonds [37]. make ILSS increase to 50.3 MPa from the original 47.7 MPa. With the benefit of massive hydroxyl groups, the sizing agent containing 3.4. The amino-functionalization CNTs M7270、R806 and epoxy can distribute uniformly on the CF surface. Based on previous analysis, the sizing agent containing MR13006 has Fig. 4a&b shows the SEM images of raw CNTs and CNTs-EDA, re- the lowest hydroxyl group content, which is hard to obtain a good spectively. CNTs-EDA has a better dispersity when compared to raw wettability on the fiber surface. Moreover, the molecular weight and CNTs and a layer of organic coating attached on the CNT surface, which

198 Z. Wu et al. Composites Science and Technology 164 (2018) 195–203

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.

199 Z. Wu et al. Composites Science and Technology 164 (2018) 195–203 is attributed to the introduced ammonium. Fig. 4c shows the infrared 3.6. The effect of M7270-CNTs-EDA on interfacial bonding of CF/UP spectra of CNTs before and after surface functionalization. The infrared composites absorption peaks of CNTseCOOHeA at 3438, 1726, 1384 and − 1097 cm 1 correspond to the stretching vibration of eOH, C]O, The ILSS and load-displacement curves were shown in Fig. 6a and b. eOSO3H and eCeO, respectively. Compared to raw CNTs, the peak As shown in Fig. 6a, the introducing of CNTs-EDA obviously improved − intensity of the eOH stretching vibration at 3438 cm 1 is significantly the interface bonding of the CF/UP composites. The ILSS increased from increased, suggesting that the CNTs have been oxidized during the acid 47.7 MPa for the raw CFs to 63.1 MPa for the CFs treated with M7270- − treatment [34]. For CNTs-EDA, new characteristic peak at 1580 cm 1 is CNTs-EDA by 32.3%. After treated with M7270-CNTs-EDA, the polar − assigned to the deformation vibration of NeH, the peak at 1180 cm 1 functional groups of sizing agent played an important role in improving corresponds to the stretching vibration of CeN, and the peak at the interfacial bonding between the CFs and the UP matrix. The polar −1 800 cm is assigned to the out-of-plane bending of NH2, indicating groups could increase the wettability of CFs for UP matrix and that amino groups are grafted onto the CNTs through covalent bonding strengthen the interactions of molecular in the interface region. Fur- [38]. thermore, the unsaturated double bonds of M7270 could participate in Such surface treatment could be further assessed by XPS, as shown the curing process of UP matrix through radical polymerization. Com- in Fig. 4e, f&g. From the C1s spectra, the characteristic peak at 284.4 eV pared to the CFs treated with M7270 sizing agent without CNTs, the corresponds to the C]C of graphitized carbon. The peak at 285.2、 ILSS of CFs treated with M7270-CNTs-EDA still increased by 7.3%. The 286.4 and 288.4 eV were assigned to the CeN, CeO and NeC]O, re- further enhancement in ILSS was partly due to the anchoring effect of spectively, indicating that EDA was covalently linked with CNTs surface CNTs in sizing agent for the UP resin. The enhanced mechanical in- by the amide bonds [39]. For the N1s spectra, the binding energy peaks terlocking and the local mechanical properties of interface region are at 399.6 and 402.2 eV were ascribed to the CeN and ammonium, re- beneficial to transfer the load. Furthermore, the interface layer with spectively. Moreover, the appearance of NeC]O peak in O1s spectra gradient mechanical properties caused by CNTs-EDA in sizing agent is further verifies the amination of CNTs. not only beneficial to transfer and disperse stress, but also for elim- In view of the thermal instability of absolute majority organic inating the internal stress between CFs and UP matrix when curing is groups, TGA is an effective way to provide relative information about done [41]. functional modification of CNTs-EDA. Fig. 4d shows the TGA curves of Fig. 6c shows the impact properties of CF/UP composites before and raw CNTs, CNTseCOOHeA and CNTs-EDA, respectively. Raw CNTs are after interface modification. The initial and propagative energy of the relatively thermal stable in the range of 0–680 °C, but CNTseCOOHeA raw CFs composites were 0.82 and 0.23 J, respectively. The M7270- have a 13% weight loss in this temperature range. Compared to CNTs-EDA treatment obviously increased the impact toughness of the CNTseCOOHeA, CNTs-EDA shows the phenomenon of multilevel CF/UP composites. The initial and propagative energy of the compo- thermal degradation, which was attributed to more kinds of functional sites treated with M7270-CNTs-EDA increased to 1.04 and 0.59 J, re- groups of CNTs-EDA. However, the thermal weight loss curves of spectively. CNTseCOOHeA and CNTs-EDA overlap in the range of 0–200 °C, Strong interface bonding is not only beneficial to enhance the ILSS, which is likely due to the volatilization of water during the process of but can also influence other properties of composites, such as wet and amination and carboxylation. With further increasing the temperature, heat ageing resistance property, acid-base resistance property and the groups were eliminated by thermal degradation. Therefore, the conductance. However, strong interface bonding leads to stress con- amidogen content of CNTs-EDA could be estimated by TGA curves of centration and goes against the movement of polymer molecular chains CNTs, which was approximately 6 wt%. in the interface region, which result in the decrease of the impact property [34]. Whereas, by reasonably designing the interface mod- 3.5. The effect of M7270-CNTs-EDA on surface properties of CFs ification, both ILSS and impact property can be enhanced at the same time. Fig. 5a&b shows the surface morphology of CFs coated with M7270 It is worth noting that the pure M7270 sizing agent without CNTs is sizing agent containing raw CNTs and CNTs-EDA, respectively. CNTs- beneficial for increasing the ILSS of CF/UP composites, but doing little EDA has a better dispersion in sizing agent than raw CNTs and no ob- to the toughness improvement, which indicate that CNTs are key factor vious aggregations were observed in Fig. 5b. The surface roughness for toughness enhancement. Introducing CNTs to vinyl ester sizing (Ra) of the fibers treated with M7270-CNTs-EDA was obviously in- agent can structure the rough surface of CFs, which restricts the creased in comparison with those treated with M7270 sizing agent movement of molecules in the interphase and improves the mechanical containing raw CNTs. The variation of surface polar groups influences interlocking between CFs and UP resins. When the impact load was the surface free energy and its components. The higher surface energy applied, the crack propagating orientation will be changed by CNTs and means that the matrix can be uniformly dispersed on the fiber surface, creates more tiny cracks in the bonding interface, which are helpful to which is beneficial to enhancing the interfacial compatibility of com- dissipate the impact energy [34]. Furthermore, the CNTs containing posites [40]. DCA tests were carried out to examine the effects of interphase with proper modulus between CFs and UP resin is also M7270-CNTs-EDA on the surface energy and the results are summar- helpful to prevent the cracks from direct contacting the fiber surface ized in Fig. 5c&d. After treated with M7270-CNTs-EDA, the CA of CFs in and reduces the stress concentration in the interface region. both polar and non-polar test mediums declined apparently. Therefore, Fig. 6d & e shows the impact fracture surfaces of the composites its polar component and dispersion component of the surface energy all treated with and without M7270-CNTs-EDA. As shown in Fig. 6d, increased significantly, which is mainly attributed to the increased plenty of raw fibers were pulled out from the UP resin and lots of voids quantity of active functional groups. Note that the contact angles in appeared in the UP resin, indicating a weak interfacial bonding be- water for CFs treated with pure M7270 sizing agent and M7270-CNTs- tween CFs and the UP matrix. After treated with M7270-CNTs-EDA, a EDA are nearly the same, which means that the introducing of CNTs- small number of fibers were pulled out and the pullout length of the EDA has almost no influence on the wettability of fiber surface and fiber became shorter. Moreover, the UP and CF debris were found to be nearly all CNTs-EDA are coated with M7270 resin. The increased dis- uniformly distributed throughout the cross section, which was attrib- persion component was due to the increased roughness caused by CNTs- uted to the enhanced roughness and surface energy of the fi bers, further EDA on the fiber surface and the differences of structure be- confirming an enhanced interphase of the CFs and UP resin. tween CFs and CNTs-EDA. Moreover, the increased polar component of the sizing agent treated CFs is attributed to the polar groups in M7270 molecular chains and the eNH2 groups of CNTs-EDA.

200 Z. Wu et al. Composites Science and Technology 164 (2018) 195–203

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) after sizing treatment.

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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.

3.7. The effect of M7270-CNTs-EDA on interphase modulus of CF/UP changes from the fiber section to the UP section without any transition composites area. The color of interphase in Fig. 7c is a little lighter than CFs and a little darker UP region, which means that the interphase modulus is FMAFM was carried out to evaluate the relative stiffness of inter- intermediate between the reinforcement and UP resin. The interphase phase of CF/UP composites. The amplitude of the FMAFM tip under with moderate modulus can provide a cushion when the stress was scanning approximate corresponds to the relative modulus of the test transferred from UP to CFs. specimens. In short, a smaller amplitude of FMAFM tip means a higher relative modulus of test specimen, and vice versa [34]. The corre- sponding working principle diagram of FMAFM is shown in Fig. 7a. 4. Conclusions Fig. 7b & c shows the force modulation images of the CF/UP compo- sites. Compared to the image of composites before the sizing treatment More reactive groups and better interfacial compatibility with UP (Fig. 7b), the force modulation image of composite treated with M7270- make M7270 a more suitable polymer for preparing sizing agent for CF/ CNTs-EDA (Fig. 7c), shows obvious color contrast among the CF, in- UP composites when compared to epoxy, MR13006 and R806. M7270- terface region and the UP, indicating a gradient variation of modulus CNTs-EDA was prepared to increase the interfacial properties of CF/UP for these three phases. The appearance of interface region could also be composites. After sizing treatment, the roughness, polar functional confirmed by the MAG-position curves of the cross section. As shown in groups and wettability of CF surface were improved obviously, and the Fig. 7e, there is an obvious modulus transition region corresponding to ILSS and impact toughness were improved by 32.3 and 55.2%, re- the interface region. By contrast, only two phases could be observed for spectively. The M7270-CNTs-EDA effectively increased the interface raw CFs and UP in Fig. 7b & d and the relative hardness suddenly compatibility of composites not only by using the reactive groups of M7270 to improve the surface energy and react with UP matrix during

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