DOI 10.1007/s11029-018-9700-5 Mechanics of Composite Materials, Vol. 53, No. 6, January, 2018 (Russian Original Vol. 53, No. 6, November-December, 2017)
Mechanical reinforcement of epoxy composites with carbon fibers and HDPE
R. He,1 Q. Chang,1 X. Huang,2 and J. Li3*
Keywords: composite, carbon fiber, HDPE, reinforcement Silanized carbon fibers (CFs) and a high-density polyethylene with amino terminal groups (HDPE) were introduced into epoxy resins to fabricate high-performance composites. A. mechanical characterization of the composites was performed to investigate the effect of CFs in cured epoxy/HDPE systems. The composites revealed a noticeable improvement in the tensile strength, elongation at break, flexural strength, and impact strength in comparison with those of neat epoxy and cured epoxy/HDPE systems. SEM micrographs showed that the toughening effect could be explained by yield deformations, phase separation, and microcracking.
1. Introduction Carbon fibers (CFs) are widely used as reinforcing fillers in polymer composites, especially in structural ones [1-8]. CFs with their good mechanical properties and high length-to-diameter ratios show high reinforcing effects in epoxy composites [2, 6, 9, 10]. Even a very low concentration of CFs in these composites often leads to exceptional mechanical properties [2, 5, 9, 11-13], while the polymer imparts a low density to them and simplifies their processing [14]. However, the full development of CF reinforcement has not been entirely successful in polymer composites due to the too poor compatibility and weak interaction between CFs and polymer matrixes [1]. The thermoplastic polymer HDPE has a lower melting and solution viscosities than linear polymers with the same molar mass. A significant advantage of HDPE is the high density of terminal functional groups in its framework [15], which leads to an excellent compatibility and interaction between HDPE and other polymer matrixes. Moreover, HDPE has a great amount of School of Mechanical and Electrical Engineering, Ningbo Dahongying University, No. 899 Xueyuan Road, Haishu District, Ningbo, ZheJiang Province, China; 2 Ningbo Jinke automation equipment Co., Ltd, No. 289 Jiangbin Road, Beilun District,Ningbo, ZheJiang Province, China; 3 College of Engineering, Shanghai Second Polytechnic University, Shanghai 201209, P.R. China * Corresponding author; e-mail:
[email protected] 1
Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 53, No. 6, pp. 1083-1092 , NovemberDecemer, 2017. Original article submitted February 3, 2016; revision submitted August 7, 2017. 0191-5665/18/5306-0753 © 2018 Springer Science+Business Media, LLC
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free volumes and free spaces in its structure and therefore can improve the impact resistance of thermosetting polymers [16, 17]. Furthermore, its unique globular structure can reduce the shrinkage of some thermosetting polymers, especially epoxy resins, due to stretching of its flexible chains when their terminal groups interact with other functional groups of polymer matrixes [17-19]. Because of these favorable characteristics, HDPE is widely used as a modifier for epoxy resins [18]. However, the introduction of HDPE into epoxy composites may affect some of their mechanical properties, such as the strength, modulus, and stiffness, owing to the introduction of the flexible chains present in the structure of HDPE [19, 20]. In the literature, there are some works dealing with the modification of fibers with HDPE, see e.g., [21]. But investigations focused on the use of fibers as reinforcing fillers for epoxy resins modified by HDPE are only few [22]. The present work considers the fiber reinforcement in epoxy/HDPE composites. The advantages of this synergetic reinforcement route of epoxy composites lie in the fact that the functional groups at the terminal chains of HDPE not only allow a chemical interaction with epoxy matrixes, but also improve the compatibility between fibers and the matrixes. The introduction of HDPE can increase the toughness of the composites, and the presence of fibers can improve their thermal and mechanical properties. 2 Experimental 2.1. Materials An aqueous H2O2 (30 wt.%), methyl acrylate, diethylenetriamine, methanol, and N, N-dimethylacetamide were obtained from the Shanghai Huashi Chemical Reagent Company, Shanghai, China. γ-aminopropyl triethoxysilane (γ-APS) was purchased from the Zhenghua Chemical Reagent Co., Ltd, Shanghai, China. Diglycidyl ether of bisphenol-A epoxy resin E-51, with an epoxide equivalent of 185-208 g/eq was purchased from the Shanghai Xinxing Adhesive Co., Ltd, Shanghai, China. Polyamide 650 with an amine value of 220 ± 20 mgKOH/g was supplied by the Shanghai Resin Factory, Shanghai, China. CFs, with a diameter of 10 μm and an average length of 30 μm, were supplied by the Wise CO., LTD, Shanghai, China. 2.2. Fabrication of epoxy/HDPE/CF composites The carbon fibers were hydroxylated and silanized with H2O2 and γ-aminopropyl triethoxysilane, respectively. After drying in vacuum, they were introduced into the mixture of epoxy resin and HDPE (HDPE/epoxy, 20/100, wt/wt) and stirred for 10 min. Then, a polyamide curing agent (epoxy/polyamide, 100/120, wt/wt) was added to the blend, stirred for 10 min, poured into a stainless steel mold, and cured in an oven at 60°C for 48 h. 2.3. Characterization Infrared spectroscopy (IR) was carried out on a Tensor 37 attenuated total internal reflectance Fourier transform infrared spectroscope (ATR-IR, Germany, Bruker) within the range of 400-4000 cm−1. A thermal gravimetric analysis (TGA) was performed using a SHIMADZU DTG 60 TG/DTA simultaneous measuring instrument (Japan, SHIMADZU) in a nitrogen atmosphere at a heating rate of 20°C min−1 from 30 to 800°C. The tensile strength was examined on an SHIMADZU AG-X plus test machine (Japan, SHIMADZU) on dogbone-type specimens at a crosshead speed of 10 mm min−1. The flexural tests were run on a UTM4000 electromechanical universal testing machine (China, SANS) on specimens of dimensions 100 × 15 × 4 mm. The unnotched impact strength was measured on a ZBC 50 pendulum impact testing machine (China, New SANS) according to the National Standard of China (GB/T 2567-2008). All of the results presented are average of five specimens. The impact morphology of the composites was examined using a field emission scanning electron microscope (FESEM, Japan, Hitachi SU8000) at an acceleration voltage of 3.0 kV. 754
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Fig. 1. Attenuated total reflection-infrared intensity (ATR-IR) spectrum: absorptance T vs. wave number k. 3. Results and Discussions The ATR-IR spectrum, shown in Fig. 1a, provided preliminary evidence to confirm a successful synthesis of epoxy/ HDPE. For instance, the bands at 774.2 and 1552.8 cm−1 are ascribed to the out-of-plane and in-plane, respectively, bending vibrations of N–H. The peak at 1121.1 cm−1 is assigned to the stretching vibration of C–H. The absorption at 1277.8 cm−1 is attributed to the stretching vibration of C–O–C. The peak at 1358.9 cm−1 is ascribed to the stretching vibration of C–N. The band at 1464.9 cm−1 is attributed to the bending vibration of –CH2. The absorption at 1647.5 cm−1 corresponds to the stretching of C=O. The peaks at 2928.3 and 2834.3 cm−1 are attributed to the asymmetric and symmetric, respectively, stretching vibrations of C–H. The absorptions at 3280.9 cm−1 is assigned to the stretching vibration of N–H of the primary amino groups and imino groups. The ATR-IR spectra of epoxy/HDPE and a composite with 2 wt.% CFs are shown in Fig. 1b. The band at 1086.8 cm−1 is assigned to the asymmetric stretching vibration of C–O–C. The bands at 915.8 and 841.4 cm−1 confirm the presence of oxirane groups and are attributed to the vibration of C–O and C–O–C, respectively. The bands at 2943.5, 2854.5, 1647.7, 1582.7, 1416.2, and 1332.5 cm−1 indicate a reaction between HDPE and epoxy matrixes. The spectrum of the composite with 2 wt.% CFs shows a weak band at 1610.6 cm−1, which is attributed to the bending vibration of –NH2. The absorption peaks at 1181.4,1110.2, and 1036.7 cm−1 are ascribed to the stretching vibrations of C–O–Si, the asymmetric stretching vibration of Si–O–Si, and the symmetric stretching vibration of Si–O, respectively. The presence of the band at 1513.7 cm−1 is assigned to the bending vibration of C–H, which may be another piece of evidence of good compatibility between CFs and epoxy matrixes in the present of HDPE. In Fig. 2a, the tensile strengths of neat epoxy and five composites with 1, 2, 3, 4, and 5 wt.% CFs are illustrated. As is seen, the maximum increase in strength was achieved for the composite with 2 wt.% CFs, namely, 31.1% compared with that of neat epoxy. A further addition of CFs decreased the tensile strength gradually, which can be related to the varying dispersion morphology of CFs in the epoxy matrixes. By increasing the content of CFs, the probability of CF aggregation was intensified, which decreased the strength. The elongations at break of each formulation are shown in Fig. 2b. It is seen that the maximum elongation, equal to 5.24%, exhibited the formulation with 2 wt.% CFs, which was by 115.6% higher than that of neat epoxy. These results demonstrate a notable synergic effect of HDPE and CFs on the tensile properties of epoxy composites. The effect of CF content on the flexural behavior of epoxy/HDPE composite was also investigated. For the modified composites, the flexural strength slightly increased with content of CFs, as shown in Fig. 2c. It is worth noting that the flexural strength reached it maximum for the composite with 2 wt.% CFs, which was by about 17.2% higher than that of neat epoxy. A further growth in the content of CFs decreased the strength, which was caused by the aggregation of CFs. The impact strength is the ability of a material to withstand suddenly applied loads. It measures the impact energy required to fracture a specimen. In order for a material or object to have a higher impact strength, stresses in it have to be distributed evenly throughout. The impact strengths of neat epoxy and the epoxy composites investigated are shown in Fig. 2d. As is seen, the impact strengths 755
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Fig. 2. Tensile strengths st (a) strain at break eb (b), flexural strength sf (c) and impact strengths Eimp of neat epoxy and CF/epoxy composites with various weight fractions of CFs.
of the modified composites were higher than those of the neat epoxy and untreated thermosets with the various content of CFs used; 2 wt.% CFs provided the best impact strength. The impact strength of the 2 wt.% CFs modified composite was 13.375 kJ/m2 — by 74.4% higher than that of neat epoxy. From Fig. 2, it is evident that 2 wt.% was the optimum percentage of CFs in epoxy/HDPE composites to obtain their maximum high mechanical characteristics, which agrees with other literature data for epoxy/fiber composites. The mechanical properties of such composites depend mainly on the properties of reinforcing fillers and matrixes and the compatibility and interaction between them. The most important factor to obtain a good fiber-reinforced composite is adhesion between the matrix and fibers. The tendency of carbon fibers to moisture absorption is high due to the presence of hydroxyl and amino groups, which are introduced by hydroxylation and silylanization, and this results in poor wetability and a weak interfacial bond between the fibers and epoxy matrixes. Therefore, in order to develop composites with good mechanical performances, it is necessary to impart hydrophobicity to carbon fibers. Introducing HDPE into epoxy matrixes can enhance the compatibility between CFs and epoxy matrixes. This is explained by the high density of the functional terminal groups in HDPE, which improves the stress transfer to carbon fibers and, consequently, promotes their reinforcement efficiency. Furthermore, the introduction of HDPE increases the ductility of composite and hinders crack initiation and propagation. The SEM micrographs of impact fracture surfaces of neat epoxy, epoxy/HDPE, and a composite with 2 wt.% CFs are shown in Fig. 3. The neat epoxy (Fig. 3a) had a typical smooth, glassy surface and a low crack resistance. The fracture surface of epoxy/HDPE composite exhibited shearing deformations and secondary phases (Fig. 3b). The rough and coarse fracture surface of the HDPE-modified material suggests that the modified material has failed more plastically than the neat epoxy. Compared with the cured epoxy/HDPE system, the fracture surface of the composite with 2 wt.% CFs showed more shearing deformations and phase separations (Fig. 3c). The HDPE phases appeared as spherical particles with average diameters in the range of 1-15 μm, which were embedded and well dispersed in the epoxy matrices. This pointed to the excellent compatibility between the HDPE and epoxy matrixes. It is interesting to note that there were several microcracks in the HDPE 756
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Fig. 3. SEM micrographs of impact fracture surfaces of neat epoxy (a), epoxy/HDPE (b), and a composite with 2 wt.% CFs (c). separation phases in the 2 wt.% CFs composite, whereas there were no cracks/microcracks in the second phase of the epoxy/ HDPE system. Microcracks, which may be attributed to the synergic effect of HDPE and CFs, can absorb energy and thus increase the impact resistance. On the whole, the incorporation of CFs into epoxy/HDPE composites can lead to considerable yield deformations, phase separations, and microcracks, and as a result, can rise the toughness of epoxy composites. 4. Conclusions Epoxy-based composites were fabricated introducing small amounts of carbon fibers (CFs) into an epoxy resin modified by hyperbranched polymers with amino terminal groups (HDPE). Their mechanical properties were found to be noticeably better than those of the neat epoxy and cured epoxy/HDPE systems. The highest tensile strength exhibited the composite with 2 wt.% CFs. The flexural strength slightly increased with content of CFs. The impact strength of the modified composite was higher than that of neat epoxy and untreated thermosets. The composite with 2 wt.% CFs had the highest impact strength. Acknowledgments. Thanks for the support of the National Natural Science Foundation of China (Grant No. 51405284) and School-level key disciplines materials science and engineering XXKZD1601. References 1. J. Li, “Interfacial studies on the O3-modified carbon-fiber-reinforced polyamide 6 composites,” Appl. Surface Sci., 255, Iss. 5, Pt 2, 2822-2824 (2008). 2. Wei Song, Aijuan Gu, Guozheng Liang, and Li Yuan, “Effect of the surface roughness on interfacial properties of carbon fibers reinforced epoxy resin composites,” Appl. Surface Sci., 257, Iss. 9, 4069-4074 (2011). 3. Jayashree Bijwe and Rekha Rattan, “Influence of weave of carbon fabric in polyetherimide composites in various wear situations,” Wear, 263, Issues 7-12, 984-991 (2007). 4. Mohit Sharma, Jayashree Bijwe, and Peter Mitschang, “Wear performance of PEEK-carbon fabric composites with strengthened fiber–matrix interface,” Wear, 271, Issues 9-10, 2261-2268 (2011). 5. Mohit Sharma and Jayashree Bijwe, “Influence of fiber-matrix adhesion and operating parameters on sliding wear performance of carbon fabric polyethersulphone composites,” Wear, 271, Issues 11-12, 2919-2927 (2011). 6. Xuezhong Zhang, Yudong Huang, and Tianyu Wang, “Plasma activation of carbon fibres for polyarylacetylene composites,” Surface and Coatings Technology, 201, Issues 9-11, 4965-4968 (2007). 757
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