Effects of Preheating Temperature of the Mould on the Properties of Rapid-curing Carbon Fibre Composites Fabricated by Vacuum-

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1 Effects of Preheating Temperature of the Mould on the Properties of Rapid-curing Carbon Fibre Composites Fabricated by Vacuum- Effects of Preheating Temperature of the Mould on the Properties of Rapid-curing Carbon Fibre Composites Fabricated by Vacuum- Kaomin Zhang, Yizhuo Gu*, Shaokai Wang, Mi Li and Zuoguang Zhang Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing, , P.R. China Received: 26 July 2014, Accepted: 15 October 2014 SUMMARY Shortening the curing time is critical to improve the processing efficiency of composites. In this paper, unidirectional carbon fibre-reinforced rapid curing epoxy composite laminates were fabricated by vacuumassisted resin injection moulding (VARIM). Different rapid curing processes (namely P80, P85, and P90) were employed, which preheating temperatures were 80, 85 and 90 C, respectively. Different preheating temperatures of mould and fibre preform were conducted, to demonstrate the effects on the properties and processing time of the composites. In addition, a slow curing process (N80) was investigated to verify the effect of rapid curing on the mechanical properties of the composites. The relationships among preheating temperature, curing cycle and properties of the composites were analyzed. The results showed that the preheating temperature had obvious effects on the total cycle time and properties of the composites. The cycle time of the P80 process was 947 s, whereas that of the P90 process was 702 s. However, nonuniformity of the mechanical properties along the resin flow direction was more obvious for the P90 process, and more voids formed in the resin outlet region. Composites fabricated by the P80 process showed comparable mechanical properties and processing quality to those from the N80 process. It is suggested that a rapid curing process with proper preheating temperature is acceptable to improve the processing efficiency. Keywords: Carbon fibre composite, Epoxy resin, Preheating, VARIM, Rapid cure 1. INTRODUCTION Advanced composites have rapidly developed since the invention of carbon fibre. Two important considerations in the production of advanced composites, in general, are manufacturing time and cost of the composites. Vacuum-assisted resin injection moulding (VARIM) is a widely used low-cost liquid composite moulding (LCM) process for large structures, including boat hull, wind turbine blade and automobile chassis components 1,2. However, its long cycle time makes it only suitable for low and medium volumes of production, and the application of VARIM for high volumes of production is restricted. Smithers Information Ltd., 2014 In the process of VARIM, layers of fibrous reinforcement are laid on a one-sided mould to create a preform, and then thermoset resin injected into a mould cavity covered by a vacuum bag impregnates the reinforcement (resin injection stage); this is followed by resin curing (curing stage) and cooling of composite part (cooling stage). Hence a reduction of cycle time of VARIM process should be realized from the decreases of filling time, curing time and cooling time. In the resin injection stage, the successful application of the VARIM process depends on low resin viscosity so as to achieve uniform and complete fibre saturation 3-5. Low resin viscosity increases the flow rate and consequently Corresponding author: Yizhuo Gu, benniegu@buaa.edu.cn, Tel: reduces the infusion time 6. In addition, low viscosity improves the wetting of fibres by the resin matrix which has been demonstrated to increase fibreresin adhesion and enhance mechanical properties 7-9. The temperature of the resin in the injection stage is a critical processing parameter to influence the viscosity of the resin system. Preheating the mould and the dry fibre preform can facilitate resin flow as a consequence of viscosity reduction at elevated temperature during the process of resin injection 10. Additionally, a proper preheating process for the mould and fibre preform is useful to reduce heating time and promote the curing of resin system, and hence to shorten the time of curing stage. However, an improper resin or mould preheating temperature might result in poor wetting of the fibre preform and produce voids in cured composite. Polymers & Polymer Composites, Vol. 22, No. 9,

2 Kaomin Zhang, Yizhuo Gu, Shaokai Wang, Mi Li and Zuoguang Zhang In order to improve the properties of the composite laminates, Wen-Bin et al. 11 experimentally investigated the effect of preheating temperature during resin transfer moulding (RTM) process on the properties of composites. The results showed that the flexural strength decreased as the preheating temperature increased with high injection pressure. Kedari et al. experimentally studied the effect of mould temperature and inlet pressure on void content and fibre volume fraction of polyester/e- Glass fibre composites fabricated by vacuum-aided resin transfer moulding (VARTM) 12. The results indicated that a high mould temperature combined with a low inlet pressure can produce VARTM parts with high fibre volume fraction and low void content. In order to minimize cycle time, Shojaei et al. examined the role of initiators with different reactivities on the process cycle of RTM using numerical simulation 13. The results revealed that a temperature distribution was generated along the resin flow direction for the preheating mould. It is detrimental to the uniformity of curing degree and mechanical properties. In summary, preheating process is an important factor to change processing efficiency for LCM process and the properties of composites, so it should be designed carefully to obtain both short cycle time and good-quality composites. In our previous study, a rapid curing epoxy resin system, diglycidylether of bisphenol-a (DGEBA) epoxy resin/modified imidazole/aliphatic amine, was developed for VARIM process, which exhibited less than 5 min curing time under 120 C 14. This resin system is used in the production of civilian products, so shortening the processing cycle is critical. Compared with polyester and general epoxy resin, the chemorheology of the rapid-curing resin is more complicated with a preheated mould and fibre preform. Under the preheating conditions, the viscosity of the resin system and the heating time can be reduced. Additionally, because of the characteristics of rapid-curing resin, preheating process might result in obvious crosslinking reactions during the resin injection stage, and affect the impregnation of the fibre and the temperature distribution. In order to further improve the processing efficiency of rapid-curing VARIM, the processing conditions, especially the preheating temperature of the mould and preform, should be studied and optimized. The intention of this study was to investigate the effects of preheating and curing temperature of VARIM on the properties of carbon fibre/ rapid curing epoxy resin composites, including void content, fibre volume fraction, curing degree and mechanical properties of the composites. For this purpose, four kinds of processing cycles with different preheating and curing temperatures were adopted. The uniformity of temperature and composite properties along the resin flow direction were investigated. The effects of rapid cure on the properties of the composites was analyzed. 2. MATERIALS AND METHODS 2.1 Material The epoxy resin used in this study was DGEBA E51 (supplied by Bluestar New Chemical Materials Co. Ltd). The cure agents included a modified Figure 1. Schematic diagram of VARIM process aliphatic amine (home-made) and a modified imidazole (supplied by Bluestar New Chemical Materials Co. Ltd). The weight ratio of epoxy/ imidazole/aliphatic amine was 100:6:6 (labelled as EIA). The reinforcement was T700SC unidirectional carbon fibre fabric with an areal density of 200 g/m 2 (supplied by Jiangsu Tianniao High Technology Co. Ltd). 2.2 Composites Fabrication The composite laminates were prepared by the VARIM method, as illustrated in Figure 1. Nine layers of T700 unidirectional carbon fibre fabrics (320 mm 160 mm 2 mm) were stacked in the longitudinal direction on a 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 flowed through the medium and the fibre preform. In order to monitor the temperature history inside the laminates during the curing process, three thermocouples were protruded into the fabrics at the middle of the stack, as shown in Figure 2. Temperatures T 1, T 2 and T 3 represent the temperatures of locations I, II and III respectively. Additionally, the start and completion of resin injection can be deduced from the decrease and increase of T 1, and the change of T 3 is also an assessment of the completion of resin injection. 826 Polymers & Polymer Composites, Vol. 22, No. 9, 2014

3 Effects of Preheating Temperature of the Mould on the Properties of Rapid-curing Carbon Fibre Composites Fabricated by Vacuum- Three different preheating temperatures of the mould and fibre preform were used to study the effects of preheating temperature on processing cycle and composite properties, including 80, 85 and 90 C. For clarity of the discussion, the samples processed at different preheating temperatures of the mould and fibre preform were referred to as P80, P85 and P90. In the case of the P80 process, before resin injection, the assembly was put into an oven which had been preheated to 90 C. Then the assembly was elevated to 80 C, followed by the resin injection stage at which the resin under 30 C was injected into the preheated fibre preform, and then the oven was elevated to 120 C to complete the curing stage. After the temperature of laminate came to a peak and began to decrease, the assembly was taken out of the oven to cool down naturally. P85 and P90 processes were the same as the P80 process, except the preheating temperature of mould and fibre preform. In addition, in order to investigate the effects of the rapid curing process on the properties of composites, a comparatively slow curing process was utilized. The preheating of the assembly and resin injection of the slow curing process were similar to those of the P80 process, and the only difference was that the assembly was cured at 80 C for 2 h followed by 120 C for 0.5 h at the curing stage. After that, the assembly was taken out of the oven to cool down, and the sample was labelled as N80. All the laminates were naturally cooled down to 60 C and then demoulded. For the laminates fabricated by P80, P85 and P90 processes, the properties of the composites at different locations shown in Figure 2 were studied. For the laminate fabricated by N80 process, the properties of the composites studied in this paper were the average value of samples cut from locations I, II and III. 3. TESTING PROCEDURES 3.1 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. Approximately 6-7 mg epoxy resin mixed with curing agent was subjected to a scan rate of 10 C/min from 25 C to 250 C. The objective was to calculate the total heat of reaction of the resin system (H T ), which was found to be approximately J/g. The degree of cure of the composites was calculated as follows: α = H T H r HT H r is the residual heat of the composites obtained from a dynamic scanning of the composites. 3.2 Rheological Analysis Rheological measurements were carried out in parallel plate mode with a Gemini rheometer (Bohlin Instruments) to obtain the resin complex viscosity. The data were generated with the disc oscillating at 1.0 Hz. The dynamic viscosities versus temperature were determined from 25 C to 200 C (at a heating rate of 5 C /min). Isothermal tests were performed at 30, 60, 80, 85 and 90 C, respectively. 3.3 Temperature Monitoring of the Curing Process of Resin Casting Resin castings of dimensions 100 mm 10 mm 10 mm were fabricated. The resin system EIA was degassed in a vacuum oven for 15 min under 30 C before casting into the mould, and then the mould was put into an oven which had been elevated to 120 C to cure the resin. A K type thermocouple was protruded into the geometrical centre of the casting as shown in Figure 3, and the temperature of the resin casting during moulding process was monitored. 3.4 Void Content and Fibre Volume Fraction The composite samples were cut from the laminates at the positions more than 15 mm away from the edge and were then potted using epoxy resin. Fifteen specimens were cut from the laminates fabricated by preheating processes with each three specimens from every location displayed in Figure 2. The cross-sections of the samples were wet ground with successively finer silica sandpaper from 300 to 2000 grit, and then were wet-polished with chromium oxide to obtain smooth surfaces. Finally, the Figure 2. Schematic diagram of thermocouple location in the laminate:(a) vertical view; (b) front view Polymers & Polymer Composites, Vol. 22, No. 9,

4 Kaomin Zhang, Yizhuo Gu, Shaokai Wang, Mi Li and Zuoguang Zhang polished samples were viewed at 200 magnification and 500 magnification using an optical microscope (Olympus BX51M) to calculate the void content and fibre volume fraction. The images were obtained through the entire specimen cross-section thickness. Image software package Sisc-Ics8 was used to process the digital pictures. 3.5 Dynamic Mechanical Thermal Analysis Dynamic mechanical thermal analysis (DMA Q800) was performed in a three-point bending mode at a frequency of 1 Hz with a strain of 0.04%. The composite samples (30 mm 8 mm 2 mm) were heated from 25 C to 200 C at 5 C /min. The storage modulus (E ) of the composite specimen was measured as a function of temperature. During the glass transition, the storage modulus of the composite material was significantly reduced, and the glass transition temperature (Tg) was determined to be the intersection of two tangent lines from the storage modulus, based on ASTM: D e Three-Point Flexure Testing Three-point flexure tests were conducted, based on ASTM D Samples with dimensions 80 mm 12.5 mm 2 mm were prepared from the laminate with original surfaces. All these coupons were conditioned according to the ASTM D5229/D5229M-12 standards prior to testing (65% relative humidity, 20 C for at least 48 h). A span- to-depth ratio of 32:1 was chosen such that the failure occurred in the outer fibres and was due only to the bending moment. The rate of the cross-head was 2 mm/min, and the load as a function of displacement was recorded. At least six specimens were tested for every laminate and location. 3.7 Short Beam Shear Test Short beam shear (SBS) tests were conducted according to ISO The inter-laminar shear strength (ILSS) of the composites was measured at a crosshead speed of 1 mm/min. ILSS of the composites was determined with a span-to-thickness ratio of 5 and length-to-thickness ratio of 10. A minimum of 6 specimens were tested for every laminate and location. 3.8 Scanning Electron Microscopy A CamScan-Apollo 300 scanning electron microscope (SEM) was carried out to examine the fracture surfaces of Figure 3. Schematic diagram of position of the thermocouple in resin casting gold-coated composite samples after the short beam shear test. 4. RESULTS AND DISCUSSION 4.1 Thermal History of Different Processes According to previous studies, the matrix viscosity of a LCM process should be lower than 1.0 Pa.s, and preferably lower than 0.5 Pa.s The complex viscosity profiles of the epoxy resin system containing imidazole/aliphatic amine at constant temperatures are plotted in Figure 3. As shown in Figure 4, the times of the viscosity rise to 0.5 Pa.s at 60, 80, 85 and 90 C were 424, 243, 193 and 130 s, respectively. The viscosity of the resin at 30 C was higher than 0.5 Pa.s, which is detrimental to resin impregnation with fibre preform. In our previous study 18, the resin injection time at 80 C was about 130 s. Injecting resin at preheating temperatures of 85 and 90 C is theoretically practicable according to the results shown in Figure 4, but the impregnation, temperature distribution and curing process should be noticed 19,20. Figure 5 shows the thermal history of the casting of the resin system EIA. It is shown that before 95 C, the average heating rate of the casting is 14 C/min. However, when the temperature arises to about 95 C, the average heating rate is 52 C/min. Figure 6 shows the DSC curve of the resin system EIA. The results show that the initial reaction temperature of EIA system is lower than 60 C. The lower initial reaction temperature of EIA system is attributed to the existence of aliphatic amine which is the low temperature curing agent. The heat generated by the reaction of epoxy and aliphatic amine increases the heating rate of the cast, and this is the reason for the high heating rate of 14 C/min under 95 C. The obviously high heating rate above 95 C is attributed to the rapid reaction of epoxy and imidazole. Hence, the 828 Polymers & Polymer Composites, Vol. 22, No. 9, 2014

5 Effects of Preheating Temperature of the Mould on the Properties of Rapid-curing Carbon Fibre Composites Fabricated by Vacuum- time difference of heating up to 95 C (labelled as t 95 ) among T 1, T 2 and T 3 (the temperatures at locations I, II and III as shown in Figure 2) was utilized to characterize the uniformities of temperature and cure state along the resin flow direction during composite processing. Figure 7 shows the thermal history of the composites fabricated by different processes. The labels t fill, t cure, and t cool in Figure 7 refer to the filling time, curing time and cooling time, respectively. T o represents the temperature of the oven. In the resin injection stage, the temperature of T 1 is lower than T 2 and T 3 especially for P80 process, and the lower value of T 1 is attributed to the lower initial temperature of the resin. The reaction heat of the resin and the preheating of the mould and fibre preform result in higher values of T 2 and T 3 in the resin injection stage. Compared to the P80 process, the values of T 1 of the P85 and P90 processes in the resin injection stage are slightly lower than those of T 2 and T 3. Higher preheating temperature results in higher resin temperature and promotes the crosslinking reaction of the epoxy resin. Additionally, a higher reaction degree of resin system in resin injection stage reduces the heat of reaction in the curing stage, and thus a relatively uniform temperature distribution in the curing stage can be seen in Figures 7b and c. Hence, the P80 process has more uneven temperatures than the P85 and P90 processes; i.e. high preheating temperature of the mould and preforms can reduce the temperature difference along the resin flow direction. Figure 4. Isothermal viscosity of the rapid curing epoxy resin Figure 5. Thermal history of the casting of resin system EIA Figure 6. Dynamic DSC curve of resin system EIA The t fill of the P80, P85 and P90 processes were 130, 130 and 106 s, respectively. The P90 process possessed the shortest resin injection time. This is attributed to its having the lowest resin viscosity in the filling stage. The curing times of the P80, P85 and P90 processes were 523, 419 and 313 s, and the total cycle times were 947, 829 and 702 s, respectively. Thus, high preheating Polymers & Polymer Composites, Vol. 22, No. 9,

6 Kaomin Zhang, Yizhuo Gu, Shaokai Wang, Mi Li and Zuoguang Zhang Figure 7. Thermal history of different processes: (a) P80 process; (b) P85 process; (c) P90 process; (d) N80 process (a) (b) (c) (d) temperature is useful to shorten the total cycle time. Compared with the P80 process, as above-mentioned, the resin was elevated to higher temperature at the terminal point of filling stage for the P85 and P90 processes. The higher temperature promotes the crosslinking reaction of the epoxy resin during the filling stage, and the heat of the reaction further increases the temperature of the composites, resulting in shorter heating time in the curing stage. There are almost the same cooling times for the different processes, because the cooling process and temperatures at the terminal point of curing stage are similar. Just like the P80 process, for the N80 process, the resin flowed into the fibre preform which had been preheated to 80 C. The cycle time of the N80 process is 2.5 h which is obviously longer than that of the P80 process. Compared with rapid curing of P80 process, attributed to the slow heating rate of N80 process, the time difference of heating up to 95 C for the N80 process is shorter than that of P80. Additionally, more temperature uniformity along the resin flow direction during the total cycle time is obtained for N80 process, and this is useful to improve the uniformity of curing degree. In addition, the reduction of residual stresses will be expected for the N80 slow cure 21, Processing Quality of Composites With Different Processes To understand how the injection and curing processes affected the processing quality of the composites, glass transition temperature, curing degree, void content and fibre volume fraction of the composites were experimentally investigated. The results are shown in Tables 1, 2, 3 and 4. The effect of cure cycle on the glass transition temperature is shown in Table 1. The average glass transition temperatures of the composites fabricated by rapid curing processes are similar to each other, and the different preheating temperatures 830 Polymers & Polymer Composites, Vol. 22, No. 9, 2014

7 Effects of Preheating Temperature of the Mould on the Properties of Rapid-curing Carbon Fibre Composites Fabricated by Vacuum- Table 1. Glass transition temperature (T g ) of composites fabricated by the different processes Curing process T g of location I ( C) have no considerable effect on the glass transition temperature. Compared with rapid curing process, the glass transition temperature of the composites fabricated by N80 process is lower. This may be attributed to lower curing temperature (80 C) at the first curing stage of N80 process, even though it is followed with a higher post curing temperature stage (120 C). The effect of cure cycle on curing degree is shown in Table 2. It can be seen that there are no significant changes in curing degree of the composites prepared by different cure cycles. T g of location II ( C) T g of location III ( C) Average ( C) P P P N Table 2. Curing degree of composites fabricated by the different processes Curing process Curing degree of location I Curing degree of location II Curing degree of location III Average P P P N Table 3. Void content of composites fabricated by the different processes Curing process Void content of location I Void content of location II Void content of location III Average P ± ± ± ±0.04 P ± ± ± ±0.035 P ± ± ± ±0.08 Table 4. Fibre volume fraction of composites fabricated by the different processes Curing process Fibre volume fraction of location I Fibre volume fraction of location II Fibre volume fraction of location III Average P ± ± ± ±0.50 P ± ± ± ±1.0 P ± ± ± ±1.1 However, there is a slight decrease in the curing degree of the composites fabricated by the N80 process. Compared with modified imidazole, in the first cure step of 80 C, the reaction of modified aliphatic amine and epoxy resin is more easily started. Longer curing time under 80 C causes more epoxy groups to be consumed by modified aliphatic amine before the initiation of the reaction of epoxy group and imidazole, resulting in fewer epoxy groups being consumed by modified imidazole at high curing temperature. Additionally, for the chain growth polymerization mechanism of imidazole and epoxy resin, chain growth is more difficult at the later curing stage at which the reaction rate is diffusion controlled 23. It is believed that the different crosslink structure of the resin system obtained in the N80 process is responsible for the lower glass transition temperature and curing degree. The void contents and fibre volume fractions of the laminates are shown in Tables 3 and 4, respectively. As shown in Table 3, the average void contents of the composites fabricated by P80, P85 and P90 processes were 0.39%, 0.57% and 0.86%, respectively. Void content of the composites increased with increasing preheating temperature. Along the resin flow direction, the void content was lowest in location I which was nearest to the inlet, and the highest void content was found in location III which was nearest to the outlet. The nonuniformity of void content distribution along resin flow direction was especially obvious for the composite fabricated by the P90 process. For the P90 process, the viscosity of the resin at flow front is higher than those of P80 and P85 processes at the later part of the filling stage. The higher viscosity in flow front is not only detrimental to the impregnation of resin with fibre, but also blocks full resin flow from inlet to outlet. This is supposed to be the reason for higher void content in location III of the composites fabricated by the P90 and P85 processes. In addition, the fibre volume fractions of the composites fabricated by the P80, P85 and P90 processes are 48.6, 49.4 and 50.3%, respectively, as shown in Table 4. For VARIM processing, the fibre volume fraction is mainly affected by applied pressure, compression characteristic of fibre preform and the flow path of the resin. These parameters of the P80, P85 and P90 processes are basically the same and defects are not obvious, hence no obvious difference is found Polymers & Polymer Composites, Vol. 22, No. 9,

8 Kaomin Zhang, Yizhuo Gu, Shaokai Wang, Mi Li and Zuoguang Zhang in fibre volume fraction for composites fabricated by the different processes. 4.3 Mechanical Properties of Composites With Different Processes Figure 8 shows the flexural properties of composites fabricated by the different processes. Nonuniformity of flexural strength along the resin flow direction is found, and no obvious change in the modulus at different locations is found. The flexural strength of the composite fabricated by the P80 process increases from resin inlet to outlet. However, the flexural strength of composites fabricated by the P85 and P90 processes decreases from resin inlet to outlet. Additionally, at location I, the flexural strength of the composite fabricated by the P80 process is slightly lower than those for the P85 and P90 processes. Similar to flexural strength, the ILSS of the composite fabricated by the P80 process increases from resin inlet to resin outlet, while the ILSS of the composite fabricated by the P90 process decreases from inlet to outlet. In addition, samples from the P90 process, which has a longer t 95 than the P80 and P85 processes, show the most significant difference of mechanical properties between different locations. In order to further clarify the effects of preheating temperature on the mechanical properties and impregnation quality, the micrographs of the fracture surface after shortbeam shear testing were obtained by SEM. The results are shown in Figure 9. For the samples from the P85 and P90 processes, clearly poor impregnation and clean fibre surface are observed at location II and location III. Compared with the P80 process, the temperature distributions of the P85 and P90 processes are more uniform, resulting in small residual stresses. Moreover, the lower mechanical properties at location II and location III for P85 and P90 processes are mainly attributed to poor impregnation Figure 8. Comparison of the mechanical properties of composites processed by the P80 and N80 processes:(a) flexural strength; (b) flexural modulus; (c) interlaminar shear strength (a) (b) (c) 832 Polymers & Polymer Composites, Vol. 22, No. 9, 2014

9 Effects of Preheating Temperature of the Mould on the Properties of Rapid-curing Carbon Fibre Composites Fabricated by Vacuum- Figure 9. SEM micrographs of the fracture surface after short beam shear test for the samples from different cure cycles of the resin with fibre, as shown in Figure 9. For the samples from the P80 process, good impregnation and rough fibre surface are observed at the three locations. Compared with the P85 and P90 processes, there is a more inhomogeneous temperature distribution along the resin flow direction and higher peak value at location I, as shown in Figure 7a. This might also result in high thermal residual stress, which is detrimental to the mechanical properties of composites 17,24. This may be the reason for the lower mechanical properties at location I of the composites fabricated by the P80 process. In order to illustrate the effect of rapid curing on the mechanical properties of the composites, the mechanical properties of the composites fabricated by the P80 and N80 processes were comparably analyzed. Compared with the P80 process, the samples fabricated by the N80 process experienced more uniform thermal history along the resin flow direction; hence more uniform mechanical properties could be expected. As shown in Figure 10, the flexural strength and ILSS of the samples fabricated by the N80 process varied slightly along the resin flow direction, and the mechanical properties of the N80 and P80 were similar. This results show the importance of controlling thermal history in rapid curing VARIM for the stabilisation of composite properties. In summary, the preheating temperature has obvious effects on the total processing time and properties of the composites fabricated by rapidcuring VARIM processes. A high preheating temperature is useful to reduce the total cycle time. However, nonuniformity of the properties along the resin flow direction is obtained. Composites fabricated by the rapid and the slow curing processes have comparable processing quality and mechanical properties. In this paper, considering both cycle time and properties of the composites, the P80 process is considered the best rapidcuring condition. Polymers & Polymer Composites, Vol. 22, No. 9,

10 Kaomin Zhang, Yizhuo Gu, Shaokai Wang, Mi Li and Zuoguang Zhang Figure 10. Comparison of mechanical properties of the composites prepared by the P80 and N80 processes:(a) flexural strength; (b) flexural modulus; (c) Interlaminar shear strength (a) (b) (c) 5. CONCLUSIONs The work reported the use of different preheating temperature processes to fabricate unidirectional carbon fibre-reinforced epoxy resin matrix composite laminates by means of rapidcuring VARIM. The relationships among preheating temperature, cycle time and properties of the composites were discussed, and the effects of rapidcuring processes on the mechanical properties were analyzed. It was found that, due to the reduction of curing time, a high preheating temperature was useful to shorten the total cycle time. However, too high a preheating temperature was detrimental to the mechanical properties and yielded nonuniform properties along the resin flow direction. In the case of 80 C preheating temperature, the flexural strength of the composites fabricated was increased from resin inlet to outlet. However, when the preheating temperatures were 85 and 90 C, the flexural strength of the composites decreased from resin inlet to outlet. When a higher preheating temperature was employed, poor resin and fibre impregnation close to the resin outlet were responsible for the lower mechanical properties. Moreover, the lower mechanical properties of the composites close to the resin inlet were attributed to the higher thermal residual stress when preheating temperature was 80 C. The slow curing process of the composites led to similar mechanical properties to the rapid curing process when proper processing parameters were utilized, and the cycle time was shortened from 2.5 h to less than 16 min. These results indicate that a proper preheating temperature of the mould and the fibre preform is essential for quality control and cycle time reduction. ACKNOWLEDGEMENT This work was supported by funding from the Basic Scientific Research Program of Beihang University [Project No. YWF-14-CLXY-001]. 834 Polymers & Polymer Composites, Vol. 22, No. 9, 2014

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12 Kaomin Zhang, Yizhuo Gu, Shaokai Wang, Mi Li and Zuoguang Zhang 836 Polymers & Polymer Composites, Vol. 22, No. 9, 2014