Rapid Curing Epoxy Resin and its Application in Carbon Fibre Composite Fabricated Using VARTM Moulding

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1 Rapid Curing Epoxy Resin and its Application in Carbon Fibre Composite Fabricated Using VARTM Moulding Rapid Curing Epoxy Resin and its Application in Carbon Fibre Composite Fabricated Using VARTM Moulding C.R. Wang, Y.Z. Gu 1*, K.M. Zhang, M. Li, and Z.G. Zhang Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing, , P.R. China Received: 6 October 2012, Accepted: 13 December 2012 SUMMARY A rapid curing resin system made from the diglycidyl ether of bisphenol A epoxy, modified imidazole and modified aliphatic amine was developed. The curing behaviour and following characteristic of the resin were evaluated by differential scanning calorimetric and rheological analysis, respectively. Carbon fibre-reinforced polymer composite (CFRP) laminates using this resin system were manufactured through vacuum-assisted resin transfer moulding (VARTM) under short-term curing schedule. Subsequently, studies were performed on the processing quality, mechanical properties and heat resistance of the composites. The effects of post-cure duration on the interfacial bonding and the properties of the composites were investigated. The results showed that the curing time of a CFRP laminate using the studied resin under 120 C could be controlled within 13 min with more than 95% curing degree and few defects. There were variations of the mechanical and thermal properties of the composites with different post-cure durations. The optimum properties were obtained after a post-cure process of 60 min, and extended post-cure treatment were detrimental. The corresponding mechanism was discussed from the viewpoint of interfacial bonding between carbon fibre and epoxy matrix. Keywords: Epoxy resin, Carbon fibre composite, Rapid curing, Post-cure, Interfacial bonding 1. Introduction As global warming and pollution are becoming serious nowadays, the world-wide demand for decreasing fuel consumption of automotive has gained increasing attention during the last few years. Against this background, the introduction of lightweight materials to reduce fuel consumption will make a great contribution to the future development of the automotive industry. Compared with other lightweight materials, the application of carbon fibre-reinforced polymer composites (CFRP) offers a great potential to realize lightweight concepts owing to its low density, excellent mechanical performances, corrosion resistance and fatigue durability. However, the conventional production time of CFRP (several hours Smithers Rapra Technology, 2013 per part) is far beyond the targeted manufacturing cycle (several minutes per part) of automotive field, and thus it has significantly restricted the application of CFRP in the automotive industry 1-3. For fibre-reinforced thermoset matrix composites, e.g. epoxy resin composite, the three dimensional crosslinking structures, which contribute to the mechanical properties and thermal characteristics, form through curing reaction. Completing the curing reaction, however, generally requires a long time that is the major part of the whole production cycle. Therefore, developing a rapid curing resin system and its corresponding short-term process of production is necessary to realize a wider application of CFRP * Corresponding author: Y. Z. Gu, School of Materials Science and Engineering, Beihang University, 37 Xueyuan Road, Haidian District, Beijing, China benniegu@buaa.edu.cn Tel: Fax: in the automotive industry. Recently, some researches have been done on the development of rapid curing thermoset composite systems. For example, Kame et al. developed a rapid curing epoxy resin system composed of 2-methylimidazole and an alcoholic type chain transfer agent. Using this resin system, CFRP panel (350 mm 700 mm 2 mm) was successfully fabricated in 10 min by an isothermal resin transfer moulding (RTM) process at 105 C 4,5. Resin 05127/curing agent 05443, the product of Hexion Specialty Chemicals, has been used in high-volume RTM in particular for truck applications with curing time of 23 min 6. Mitsubishi Rayon Co. Ltd has developed a fast curing resin formulation for prepreg compression moulding (PCM), the minimum curing time of which is 5 min at 140 C 7. In addition, non-isothermal RTM process in which dual-initiator ethylenic resin system is injected into the preheated mould has been utilized to obtain a reduction of cycle time 8,9. Polymers & Polymer Composites, Vol. 21, No. 5,

2 C.R. Wang, Y.Z. Gu, K.M. Zhang, M. Li, and Z.G. Zhang For composite rapid curing technology, dramatically shortened resin gel time and an explosion of curing exothermal heat would result in some problems of processing qualities, such as poor fibre wetting, non-uniform cure and high residual stress 10,11. Thus, in order to manufacture high performance composite parts using short-term process, it is crucial to understand the effects of cure schedule on the processing qualities and to optimize resin processability. Moreover, the post-cure process is often adopted after the moulding and demoulding stages of composite so as to improve the curing degree and curing uniformity. A post-cure process that does not occupy processing mould can be carried out for batch quantity. Thus by applying post-cure process to the rapid curing resin, the performances and the manufacturing efficiency of composite parts are expected to be achieved simultaneously. In this paper, a rapid curing epoxy system composed of diglycidyl ether of bisphenol A (DGEBA), modified imidazole and modified aliphatic amine was developed. The curing behaviour and flowing characteristic of the resin were analyzed. CFRP laminates using this resin system were fabricated through vacuum assisted resin transfer moulding (VARTM) methods under short-term cure schedule. The following studies were performed on the mechanical properties and heat resistance of the composites, and the effects of different post-cure duration on the interfacial bonding and the properties of composite laminates were further investigated. 2. Experimental 2.1 Materials The epoxy resin used in this study was DGEBA E51 (supplied by Bluestar New Chemical Materials Co. Ltd). The cure agents included modified aliphatic amine (homemade) and modified imidazole (supplied by Bluestar New Chemical Materials Co. Ltd) with a low viscosity that could improve the flowing characteristic for the mixture with epoxy resin. The reinforcement was T700 unidirectional carbon fibre fabric (supplied by Jiangsu Tianniao High Technology Co. Ltd) with areal density of 200 g/m Preparation of Neat Resin The formulations of the resin system are coded as X-Y, where X refers to the curing agents and Y refers the weight ratio between imidazole and aliphatic amine. Three formulations based on 100 parts by weight of DGEBA E51 were used in this work, including imidazole-9/0, imidazole/aliphatic amine-8/2 and imidazole/aliphatic amine-6/6. Resin samples were prepared through casting methods. After blending the epoxy resin and cure agents homogeneously, the resin system was degassed for 10 min at room temperature using a vacuum oven and then cast into a mould. Then the resin system was cured at 120 C for 10 min in an oven. After the completion of the curing process, the samples were naturally cooled down to room temperature. 2.3 Preparation of Composites The composite laminates were prepared through VARTM method, as illustrated in Figure 1. Nine layers of T700 unidirectional carbon fibre fabrics (300 mm 300 mm 2 mm) were stacked in the longitudinal direction on the mould to form a laminate with 2 mm thickness. The peeling ply, highly permeable medium and distribution mesh were laid over the surface of the fabric successively. The distribution mesh was terminated at a gap 30 mm from the end of the fabric preform, preventing resin race-tracking as it flows through the medium and the fibre preform. In order to monitor the temperature history inside the laminates during the curing process, a K-type thermocouple was embedded in the preform. The short-term curing cycle contained three stages: injection stage, curing stage and cooling stage. In the injection Figure 1. Schematic diagram of VARTM process 316 Polymers & Polymer Composites, Vol. 21, No. 5, 2013

3 Rapid Curing Epoxy Resin and its Application in Carbon Fibre Composite Fabricated Using VARTM Moulding stage, vacuum pressure (0.098 MPa) via vacuum pump was applied to the preform to make the resin flow into the preform. The injection stage finished after 5 min, and then the injection pipe was shut. In the curing stage, the assembly was put into an oven at 120 C. The curing time (t cure ) was defined as the time required for the temperature of laminates reaching the plateau from the beginning of heating. Finally, the assembly was removed out of the oven and was then naturally cooled down to room temperature. The fibre volume fraction (V f ) of the composite was estimated according to the following equation: V = FAW N f ρ f h (1) where FAW is the fibre areal density (200 g/m 2 ), N is the number of plies, ρ f is fibre density (1.8 g/m 3 ), and h is specimen thickness. In the present study, the average specimen thickness was 2.1 mm, giving a V f of 48%. In addition, a set of post-cure treatments with different duration at 120 C was applied after the VARTM process mentioned above to investigate the effects of post-cure process on the mechanical and thermal properties of the composite laminates. 2.4 Testing Method Differential scanning calorimetric analysis. Differential scanning calorimetry (DSC) was performed on a thermal analyzer (Mettler Toledo) to study the curing behaviour of the epoxy resin. Dynamic scanning was conducted from 25 C to 200 C with 10 C/min, and isothermal scanning was carried out at 100 C, 110 C, 120 C and 130 C. The curing degree was calculated as follows: α DSC = H t H T (2) where H t is the heat released at time t, obtained by integration of the calorimetric signal up to this time, and H T is the total heat of reaction associated with the complete cure. Rheological analysis. Rheological measurements were carried out in parallel plate mode with a Gemini rheometer (Bohlin Instruments) to obtain resin complex viscosity during the curing process. Data were generated with the disc oscillating at 1.0 H Z. The dynamic viscosities versus temperature were determined from 25 C to 200 C (at the heating rate of 5 C /min). Isothermal tests were performed at 30 C, 60 C and 80 C, respectively. Dynamic mechanical thermal analysis. Dynamic mechanical thermal analysis (DMTA) was carried out in the three-point bending mode on a DMA Q800 (TA Instruments) to obtain glass transition temperatures (T g ) of cured composites. The composite samples (30 mm 8 mm 2 mm) were heated from 25 C to 200 C at 2 C/min and the frequency of the applied oscillating stress was 1 H Z. Optical microscopy. The composite samples were cut from the laminates at a position more than 15 mm away from the edge and were then potted using epoxy resin. The cross sections of samples were wet ground with successively finer silicon sandpaper from 300 to 2000 grit, and then were wet polished using chromium oxide to obtain a smooth surface. Finally, an optical microscope (Olympus BX51M) was used to observe the polished sections for evaluating defects and fibre distribution inside the composite laminates. Mechanical tests. Both flexural tests and interlaminar shear test were completed using an Instron 5565 universal testing apparatus equipped with a 5 kn load cell. The flexural properties of the resin cast samples (80 mm 10 mm 3.5 mm) and the composite laminate samples (80 mm 12.5 mm 2 mm) were measured in accordance with GB/T and GB/T , respectively. For each resin cast and composite laminate, five samples were tested at a load rate of 2 mm/min. Interlaminar shear strength (ILSS) of the composite laminate samples (20 mm 10 mm 2 mm) was measured according to JC/T For each composite laminate, five samples were tested at a constant load rate of 1 mm/min. Scanning electron microscopy. A CamScan-Apollo 300 scanning electron microscope (SEM) was used to examine the fracture surface of gold-coated composite samples after flexural test. 3. Results and Discussion 3.1 Curing Behaviour of Epoxy Resin System Two types of curing agents (modified imidazole and modified aliphatic amine) were used in the present work. Imidazole, as a kind of tertiary amines, exhibits benefits in the reaction rate, mechanical property and pot life etc. It is generally believed that the curing mechanism for the cure of epoxy resins with imidazole involves two-step reactions. The first step in the curing process is the formation of epoxide/imidazole adducts. Then these adducts initiate the etherification reaction which crosslinks the resin 12. It would be expected that imidazole with chaingrowth polymerization mechanism would serve as an effective curing agent for epoxy resin. However, for short-term curing the resin only containing imidazole yields too much heat, easily resulting in uncontrollable process. According to the study of Rozenberg, the mixture of amines, especially of primary and tertiary ones, is often used to modify technological Polymers & Polymer Composites, Vol. 21, No. 5,

4 C.R. Wang, Y.Z. Gu, K.M. Zhang, M. Li, and Z.G. Zhang Figure 2. Dynamic DSC at 10 C/min Figure 3. Temperature history of the resin cast during curing process (175.6 kj/mol). It shows that the blending of the two kinds of cure agents, comparing with imidazole, gives a gentler curing exothermal rate and triggers the ring-open reaction of epoxy at lower temperature. The condensation between modified aliphatic amine and epoxide groups not only accelerates the initial stage of the polymerization, but also strengthens autocatalysis of imidazole at higher conversion, resulted from high content of hydroxyl group formed during the curing process The temperature profile of the resin cast during the curing process is presented in Figure 3. The temperature of imidazole-9/0 formulation increases slowly before reaching 100 C and then exhibits a sharp exothermal peak with a peak temperature of over 200 C. On the contrary, after adding aliphatic amine, the exothermal peak became flatter with a significant decreased peak value of temperature. This phenomenon is in accordance with the trend shown in Figure 2, indicating the efficiency of the method of blending two curing agents. Moreover, it can be observed that for the formulation with higher content of aliphatic amine (imidazole/ aliphatic amine-6/6 formulation), the temperature increases more rapidly and the time required to complete cure stage is shortened significantly. and processing properties of epoxyamine compositions 13. Therefore, we decreased the amount of imidazole and added a certain amount of aliphatic amine to simultaneously guarantee the rapid curing reaction and moderate releasing heat. The blending of imidazole and aliphatic amine at an optimum ratio might be an effective curing agent for epoxy resin with high curing rate and controllable processing properties. Dynamic scanning DSC was performed to investigate the exothermal behaviour of imidazole-9/0, imidazole/aliphatic amine-8/2 and imidazole/aliphatic amine-6/6 formulations. As shown in Figure 2, the exothermal peak of imidazole-9/0 formulation is sharp and narrow with a total reaction heat of kj /mol, while the exothermal peak of imidazole/aliphatic amine-6/6 formulation is broader and flatter with a slightly increased heat of reaction Furthermore, in order to investigate the curing behaviour of imidazole/aliphatic amine-6/6 formulation, isothermal DSC at different temperatures was carried out to measure the curing degree with increasing time, which is depicted in Figure 4. As shown in Figure 4, the curing rate increases significantly as the temperature increases from 100 C to 120 C, whereas there is no dramatic variation between 120 C and 130 C. The time required to achieve 95% degree of cure at 120 C is 4.6 min, suggesting that the blending of imidazole and aliphatic amine at the weight ratio of 6/6 is an effective rapid curing agents for epoxy resin, which is chosen as the rapid curing agent in the 318 Polymers & Polymer Composites, Vol. 21, No. 5, 2013

5 Rapid Curing Epoxy Resin and its Application in Carbon Fibre Composite Fabricated Using VARTM Moulding following study. Meanwhile, 120 C is chosen as the cure temperature of the studied resin system. Figure 4. Isothermal DSC at different temperatures for imidazole/aliphatic amine-6/6 formulation 3.2 Rheological Behaviour of Epoxy Resin System Viscosity is an important parameter for Liquid composite moulding (LCM) process. The ideal viscosity for injection process should be less than 1.0 Pa.s. The complex viscosity profiles of imidazole/ aliphatic amine-6/6 formulation at both dynamic temperatures and constant temperatures are plotted in Figure 5 and Figure 6, respectively. As shown in Figure 5, the viscosity is lower than 1.0 Pa.s in the temperature range of C. Above 90 C, the beginning of cross-link reactions leads to resin gel with an associated significant increase in viscosity. For isothermal conditions (Figure 6), the gel times for 30, 60 and 80 C are 15, 8 and 4 min, respectively. Therefore, the rheological behaviour of the resin system at room temperature and higher temperature can meet the requirements on resin flowability for injection during rapid curing process. 3.3 Properties of Composite Laminates The temperature profile inside the composite laminates during the curing process (after resin injection stage) is presented in Figure 7. As the assembly is heated in the isothermal condition of 120 o C (indicated by the cure stage in Figure 7), the measured temperature increases rapidly, accompanied by the curing reaction and exothermal heat. The temperature reaches 130 C after 10 min, exhibiting a moderate temperature overshooting which results from a rapid exothermal reaction. When the temperature goes down to around 120 C and remains unchanged for 30 s, the composite laminates are cooled down in the conditions of ambient air (indicated by the cooling stage in Figure 7), and the temperature of the laminates decreases rapidly from 120 C to 50 C within 15 min. It is found that the curing Figure 5. Viscosity of imidazole/ aliphatic amine-6/6 formulation at 5 C/min Figure 6. Viscosity of imidazole/ aliphatic amine-6/6 formulation at different temperatures Polymers & Polymer Composites, Vol. 21, No. 5,

6 C.R. Wang, Y.Z. Gu, K.M. Zhang, M. Li, and Z.G. Zhang Figure 7. Temperature history of the composite during curing process after resin injection stage Figure 8. Optical microscope cross-section images of the cured composite laminate: (a) 50 (b) 25 degree of the composite laminates after the short-term curing process is about 95.2% (calculated by Eq. (2)), indicating a high crosslink density of the matrix in composites. The time of the curing process with a heat source is within 13 min. Considering the resin injection stage (spending 5 min), the composite laminate is fabricated in 18 min. By adopting a preheated mould and fabric as well as improving heating efficiency, the cycle time is expected to be further reduced 8,9. In order to evaluate the processing quality of composite laminate fabricated under short-term curing cycle, the micrographs of the cross sections of composite laminate samples were obtained using optical microscope. As shown in Figure 8, although it is clear to see some resin-rich regions formed due to the large gap among the fibre bundles, few voids and delamination can be observed, indicating excellent wettability of the epoxy resin on the fibre. It can be concluded that using the studied epoxy resin system and the designed VARTM process, the composite can be rapidly fabricated in 18 min with good processing quality. Figure 9. The effect of post-cure time on the flexural modulus of the composite laminate 3.4 Effect of Post-cure Process on Properties of Composites In the consideration of increasing curing degree and releasing curing residual stress, the post-cure process at 120 C after the rapid curing process was carried out to improve the properties of composites. The flexural properties of the composite laminate samples fabricated using different post-cure time are shown in Figure 9 and Figure 10. It can be seen that a short post-cure time, up to 30 min, produces a slight improvement in the flexural modulus of about 8% (Figure 9) and increases the flexural strength of about 10.3% (Figure 10). However, a prolonged treatment (more than 60 min) produces a visible drop of flexural strength (Figure 10). The phenomenon demonstrates that the post-cure treatment can improve the 320 Polymers & Polymer Composites, Vol. 21, No. 5, 2013

7 Rapid Curing Epoxy Resin and its Application in Carbon Fibre Composite Fabricated Using VARTM Moulding flexural properties of composites to a certain degree, however, that an inappropriate post-cure duration may do a disservice to the mechanical properties. It has been reported that, for thermosetbased fibre composites, the post-cure process has potential effects on two aspects 17,18. The first is through the improvement of crosslinking density of the matrix, strengthening the mechanical properties of the resin matrix and its corresponding composite. The second is through the establishment of more covalent bonds across the interface, resulting in an increase in the interfacial bonding strength. Therefore, in order to explore the mechanism of post-cure process, the ILSS of the composite samples with different pot-cure time was measured, and the results are depicted in Figure 10. Similar to the change trend of flexural strength, the ILSS experiences an improvement as a result of the extension in post-cure duration up to 60 min, but undergoes a reduction with 90 min post-cure. The phenomenon is attributed to the variations in fibre/matrix interfacial bonding strength. The interfacial property of composite seems to be the optimum with a post-cure duration of 60 min, and a prolonged postcure process weakens the interfacial bonding strength. Similar results were also reported in previous studies that shows the weakening of the fibre/ matrix interface after the extended postcure treatment. The fracture surfaces of composite samples subjected to flexural testing are shown in Figure 11. It can be seen from Figure 11 that the fracture surface of the composites produced with 60 min post-cure (Figure 11b) exhibits extensive resin patches covering the fibre surface. It suggests that the crack favour a path through the resin matrix which can be considered as an indication of a strong interfacial bonding. On the other hand, as regards the samples without post cure and with 120 min post-cure, the micrographs in Figure 11a and Figure 11c show clean surface, corresponding to poor interfacial adhesion of the composites. All these facts demonstrate that the composite laminates achieve optimum fibre/matrix interfacial bonding under post-cure duration of 60 min. The effects of post-cure time on the flexural modulus and the flexural strength of resin cast samples are presented in Figure 12. As shown in Figure 12, the flexural modulus and the flexural strength of the samples cured under the short-term process are 2.81 GPa and 114 MPa, respectively. After the post-cure treatment with different time, no obvious change in the flexural properties of post cured samples can be concluded, suggesting that the post-cure process has no Figure 10. The effect of post-cure time on the flexural strength and the ILSS of the composite laminate Furthermore, an SEM examination was carried out to search for the evidence Figure 11. SEM micrographs from the fracture surface after flexural test under different post-cure duration: (a) 0 min (b) 60 min (c) 120 min Polymers & Polymer Composites, Vol. 21, No. 5,

8 C.R. Wang, Y.Z. Gu, K.M. Zhang, M. Li, and Z.G. Zhang Figure 12. The effect of post-cure time on the flexural property of neat resin volume, resulting in an increase in the T g 23. However, with the prolongation of post-cure duration with 120 min, there is a weakening phenomenon of the interfacial bonding, which might increase the mobility of local segment, and thereby decreases the T g. Therefore, the effect of post cure on the interfacial bonding is believed to be the main factor that dominates the mechanical properties of the rapid curing CFRP material with different post-cure duration. Moreover, adopting optimum post-cure process is an effective way to improve the performance of rapid curing thermoset composite. 4. Conclusion Figure 13. The effect of post-cure time on the glass transition temperature of the composite laminate A rapid curing epoxy resin system, DGEBA epoxy resin/modified imidazole/aliphatic amine, has been developed for the VARTM process, which exhibits short curing time and good flowing characteristics. The curing time of a carbon fibre composite laminate using this resin at 120 C could be controlled within 13 min, reaching a high curing degree of more than 95%. Few defects were observed inside the composite laminate, indicating good wettability between the fibre and the resin. significant effect on the mechanical properties of the resin matrix. Given that the post-cure treatment might increase the cross-link density of the matrix, dynamic mechanical analysis of the composites was performed to study the variation of T g for different post-cure duration. As presented in Figure 13, the T g of the composite slightly increases from 98 C to 101 C with a short postcure treatment up to 60 min, whereas decreases to 93 C for 120 min postcure duration. The variation of T g may also be ascribed to the change in fibre/ matrix interfacial bonding strength rather than the change of cross-density. Through the replacement of van der Waals interactions by the covalent bonds across the interface, the postcure duration of 60 min is supposed to decrease the mobility of segment by invoking the decrease of the free In addition, it was found that the postcure process had negligible effect on the flexural properties of the neat resin. However, there were variations of the mechanical properties of the composites with different post-cure duration. The maximum flexural strength, ILSS and glass transition temperature of composites were obtained after a post-cure process of 60 min, and a prolonged post-cure process caused a decrease in these properties. It is believed that the major reasons for the change in composite properties were the improvement in fibre/matrix interfacial bonding with moderate post-cure treatment and the degradation of interfacial bonding with excessive post-cure. Therefore, adopting optimum postcure process is an effective way to 322 Polymers & Polymer Composites, Vol. 21, No. 5, 2013

9 Rapid Curing Epoxy Resin and its Application in Carbon Fibre Composite Fabricated Using VARTM Moulding improve the performance of rapid curing thermoset composite, and the interfacial properties should be carefully evaluated for manufacturing rapid curing CFRP. Acknowledgements This work was supported by funding from the National 973 Program of China [Project No. 2010CB631100] and the National 863 Program of China [Project No. 2009AA0345]. References 1. Gutowski T.G.., Advanced Composites Manufacturing. 1st ed. Wiley (1997). 2. Harper A., MIT offers economic route to high volume RTM. Reinf Plast., 42, (1998) Verrey J., Wakeman M.D., Michaud V., and Manson J-AE., Manufacturing cost comparison of thermoplastic and thermoset RTM for an automotive floor pan. Compos. Part A., 37(1), (2006) Kamae T., Oosedo H., Tanaka G., and Iwasawa S., Epoxy resin composition, process for producing fiber-reinforced composite materials and fiber-reinforced composite materials. Patent B2, USA (2006). 5. Kamae T., Tanaka G., and Oosedo H., A rapid cure epoxy resin system for a RTM process. In, Mallick PK Proceedings of the Twelfth U.S.- Japan Conference on Composite Materials, pp DEStech Publications Inc (2006). 6. Reichwein H.G., Langemeier P., Hasson T., and Schendzielorz M., Light, strong and economical epoxy fiber reinforced structures for automotive mass production. SPE Automotive Composites Conference. Troy, United States (2010). 7. Mitsubishi Rayon Co LTD., Introduction of new large tow carbon fiber products and PCM technology. (2011). 8. Shojaei A., Farrahinia H., and Pishvaie S.M.R., Effect of system of initiators on the process cycle of nonisothermal resin transfer molding Numerical investigation. Compos. Part A. 41(1), (2010) Blanchard P.J. and Rudd C.D., Cycle time reduction in resin transfer moulding by phased catalyst injection. Compos. Sci. Technol. 56(2), (1996) White S.R. and Hahn H.T., Process modeling of composite materials, Residual stress development during cure. Part I. Model formulation. J. Compos. Mater. 26(16), (1992) White S.R. and Hahn H.T., Process modeling of composite materials, Residual stress development during cure. Part II. Experimental validation. J. Compos. Mater. 26(16), (1992) Heise M.S. and Martin G.C., Analysis of the cure kinetics of epoxy/imidazole resin systems. J Applied Polym Science, 39(3) (1990) Rozenberg B.A., Kinetics, thermodynamics and mechanism of reactions of epoxy oligomers with amines. Adv. Polym. Sci. 75, (1986) Mutin I.I., Kushch P.P., Komarov B.A., Arutyunyan Kh.A., Smirnov Yu.N., Davtyan S.P., and Rozenberg B.A., Mutual influence of polymerization and polycondensation reactions in the curing of epoxide oligomers by amines. Polym. Sci. USSR. 22(8), (1980) Fernández-Francos X., Cook W.D., Serra Ā., Ramis X., Liang G.G., and Salla J.M., Crosslinking of mixtures of DGEBA with 1, 6-dioxaspiro [4, 4] nonan-2, 7-dione initiated by tertiary amines. Part IV. Effect of hydroxyl groups on initiation and curing kinetics. Polym. 51(1), (2010) Foix D., Jiménez-Piqué E., Ramis X., and Serra Ā., DGEBA thermosets modified with an amphiphilic star polymer. Study on the effect of the initiator on the curing process and morphology. Polym. 52(22), (2011) Lindsey K.A., Rudd C.D., and Fraser I.M., Effects of post-cure on the interfacial properties of glass fibreurethane methacrylate composites. J. Mater. Sci. Lett. 12(12) (1993) Gerard J.F., Galy J., Pascault J.P., Cukierman S., and Halary J.L., Viscoelastic response of model epoxy networks in the glass transition region. Polym. Eng. Sci. 31(8) (1991) Takao O.T, Matsuoka T., and Sakaguchi K., Effect of post-cure time on mechanical properties of plain-woven glass fabric composites. J. Soc. Mater. Sci. 55(10) (2006) Tucker R., Compston P., and Jar P.Y.B., The effect of post-cure duration on the mode I interlaminar fracture toughness of glass-fibre reinforced vinyl ester. Compos Part A. 32(1) (2001) Ota T. and Matsuoka T., Effect of post-cure condition on interfacial properties of glass fiber/vinyl ester composites. In, High Performance Structures and Materials IV. WIT Press (2008). 22. Ogi K., Influence of thermal history on transverse cracking in a carbon fiber reinforced epoxy composite. Adv. Compos. Mater. 11(3) (2002) Mark J. and Naqi K., Physical properties of polymers. 3rd ed. Cambridge University Press (2004). Polymers & Polymer Composites, Vol. 21, No. 5,

10 C.R. Wang, Y.Z. Gu, K.M. Zhang, M. Li, and Z.G. Zhang 324 Polymers & Polymer Composites, Vol. 21, No. 5, 2013