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Improved tensile strength and thermal stability of thermoplastic carbon fiber fabric composites by heat induced crystallization of in situ polymerizable cyclic butylene terephthalate oligomers.

INTRODUCTION

Worldwide interest in energy reduction is increasing because of global issues, such as global warming, carbon emission, and inflation of oil prices, and weight savings in automobiles represent one of the most important solutions to the energy problem. A weight savings of 30% of the total weight of an automobile could be achieved by replacing metal or ceramic automotive parts with carbon fiber reinforced composite (CFRC) because the mechanical properties of the CFRC are equivalent to those of metals or ceramics according to Toray's report [1], For electric automobiles, whose production rate is expected to grow rapidly in the world market over the next 10 years, weight savings in the body, frame, and parts through the use of CFRC are required because of the use of heavy electrolytes in the secondary batteries. The usage of continuous CFRCs for automotive materials is, therefore, inevitable to satisfy the required properties of automotive materials [1, 2]. However, there is a critical economic challenge related to the cost of carbon fibers, whose manufacturing process includes stabilization and carbonization steps that require a large consumption of energy [1].

Another important problem is the slow production of carbon fiber fabric reinforced composite (CFFRC). CFFRC can be manufactured with various processing methods based on the properties of the raw materials as well as the use, size, and appearance of the composites. The typical processing methods include resin transfer molding (RTM) and injection molding (IM) for thermoset and thermoplastic polymer matrices, respectively. During RTM, low viscosity thermoset resins are injected after the carbon fiber fabric (CFF) is situated in the metal mold appropriately, followed by curing of thermoset resins [3, 4], Although the thermal stability and mechanical properties of the thermosetting CFFRC produced by RTM are excellent because the cured thermoset resins are never remelted, a long curing time is required and recycling is not permitted. For IM, molten thermoplastic resins are injected after the CFF is appropriately located in the metal mold, and then, the injected resins are rapidly cooled [5, 6], This processing provides the advantage of easy shaping for various applications and recycling of the produced thermoplastic CFFRC. However, filling of the resin into the CFF is imperfect because of low processability caused by the rapid increase in viscosity as the molten thermoplastic resin cools [7, 8].

A manufacturing process using low viscosity and polymerizable thermoplastic resins has been proposed to simultaneously complement the typical manufacturing processes of thermoset and thermoplastic CFFRCs. When polymerizable thermoplastic resins, such as cyclic butylene terephthalate (CBT), polyamide 12, and polyamide 6, are heated, they become liquid-like with low viscosities and then the CFF is impregnated by the hot resin [9-14], Thermoplastic CFFRCs are produced as the resins are polymerized by continuous heating. However, nonuniform properties of CFFRCs with polyamide matrix are possible because of the heat generation during polymerization. For the CBT resin, there is no heat generation during polymerization because the polymerization of the resin is entropically driven [13, 14],

Various processing methods were applied to fabricate fiber reinforced composites with the CBT resin [13-16]. Glass fiber fabric reinforced composites with the CBT resin were fabricated by RTM [13, 14] and compression molding [15]. A modified film stacking technique was applied to fabricate CFFRCs with the CBT resin [16], In this study, a fast manufacturing processing using the CBT resin was developed to compensate for the disadvantages of the typical processes, and thermoplastic CFFRCs were prepared using the fast manufacturing process with the CBT resin. In addition, the structure and properties of the composites prepared by thermal annealing were investigated to develop appropriate post processing of the CFFRCs for fast production and lowering the processing cost as illustrated in Fig. 1.

EXPERIMENTAL

Materials

The used CBT resin (CBT 160) was supplied by Cyclics Corporation (NY). The melt range of the resin was 130-150[degrees]C because the number of butyl groups in the oligomer mixture that comprised the resin varies from 2 to 7. The initial molten oligoesters have a low viscosity of ~20 cps. Because a tin-based catalyst is included in the CBT 160 resin, the viscosity of the resin increases rapidly as the entropically driven ring-opening polymerization of cyclic oligoesters proceeds at temperatures higher than 160[degrees]C. Polymerized CBT (pCBT), whose structure is similar to polybutyleneterephthalate, is formed after the polymerization of the oligoesters, and the density of the pCBT is 1.3 g/[cm.sup.3]. The CFF (CF-3327EPC, Hankuk Carbon, Miryang, Korea) was used for reinforcement, and composed of T-300 grade CF with a density of 1.82 g/ [cm.sup.3] (205 g/[m.sup.2]) and a thickness of the CF is ~0.27 mm.

Molecular Weight and Degree of Conversion

Gel permeation chromatography (GPC, HLC-8320 GPC EcoSEC, Tosoh, Tokyo, Japan) was used to measure the molecular weight and conversion of the resin from oligomers to polymers. The GPC measurements were performed using a mixture containing 98% chloroform and 2% hexafluoro-2-isopropanol as the solvent with a solvent flow rate of 0.8 mL/min at a temperature of 20[degrees]C. Two Waters PL HFIP-gel columns were used, and the chromatograph was connected to a Waters 484 UV detector working at 254 nm. A universal calibration was carried out using various polystyrene standards to relate the retention time to the molecular weight. For sample preparation, 2 mg polymer resin was dissolved in 80 pL HFIP, and the HFIP solution was diluted in 4 mL chloroform after complete dissolution of the matrix.

The degree of oligomer conversion was measured by comparing the quantity of oligomers remaining to the quantities of polymers. The degree of conversion was calculated according to Eq. 1:

[alpha] = 1 - [A.sub.oil]/[A.sub.tot] (1)

in which [A.sub.oil] is the area below the oligomer peaks of the retention time curve, and [A.sub.tot] is the total area below the retention time curve.

Thermal Behavior

The melting and crystallization temperatures of the CBT resin were measured using a differential scanning calorimeter (Q20, TA Instrument, DE) under nitrogen in the temperature range of 50-250[degrees]C. The samples were heated from 50 to 250[degrees]C and then maintained at 250[degrees]C for 5 min. The samples were cooled from 250 to 50[degrees]C after the holding step. The melting temperature of pCBT was also investigated with regard to the annealing temperature and time under the same measurement conditions. All of the heating and cooling processes were performed using a rate of 10[degrees]C/min. The thermal decomposition temperature of the CBT and pCBT resins was investigated using the thermo gravimetric analysis (TGA; Q50, TA Instrument) with regard to annealing temperature and time under air atmosphere. The measurement was performed in the temperature range of 40-400[degrees]C using a heating rate of 10[degrees]C/min. In addition, the thermal decomposition of the CFFRC samples was evaluated by TGA under air atmosphere. The measurement was performed in the temperature range of 40-900[degrees]C using a heating rate of 10[degrees]C/min.

Crystalline Structure

Wide-angle X-ray diffraction (WAXD) measurements were carried out using an X-ray diffractometer (M18XHF-SRA, MAC Science, Yokohama, Japan) using Ni-filtered Cu K[alpha] X-rays ([lambda] = 0.1542 nm) to investigate the crystalline structure of the CBT and pCBT resins with regard to annealing temperature and time, and the diffraction intensity was recorded by continuous scanning at a rate of 0.02[degrees] [s.sup.-1] over the range 10[degrees] < 2[theta] < 40[degrees] ([theta] = Bragg angle).

Sample Preparation

The materials were dried prior to processing for 12 h at 110[degrees]C to remove moisture that can interfere with polymerization of the CBT resin. As shown in Fig. 1, sufficient CBT powders were sprinkled on the steel mold, and then the CFF was laminated on the sprinkled CBT powder. Enough CBT powders were sprinkled again on the laminated CFF. The target thickness of the specimens was matched by repeating the process, and then the CFFRC samples were prepared using a hot press (D3P-20J, Dae Heung Science, Incheon, Korea). A compressive pressure of 1 MPa was applied at 250[degrees]C for 2 min because the oligomers are almost converted into polymers at the processing conditions. Because the viscosity of the CBT resin during the first thermal process is as low as ~20 cps, which results in extra CBT resin flowing out, it is expected that the material content in the CFFRCs is uniform, regardless of the amount of sprinkled CBT powders. The prepared CFFRC specimens were cut according to the ASTM D3039 specimen standards.

Mechanical Property

The tensile test was performed according to ASTM D3039 using a universal testing machine (WL2100, With Lab, Anyang, Korea) with a crosshead speed of 5 mm/ min at room temperature.

Morphological Properties

Cross sections of the CFFRC tensile specimens were polished using a sample polishing machine (TegraPol-15, Ballerup, Denmark), and the cross section of the CFFRC tensile specimens was observed using an optical microscope (OM, BX51, Olympus, Tokyo, Japan). In addition, the fracture surface of the CFFRC tensile specimens was observed by field emission scanning electron microscopy (FE-SEM, Hitachi S-4700, Hitachi High Technologies Corp., Tokyo, Japan). The objectives were coated with platinum under vacuum for 200 s using a sputter coating machine (Ion Sputter E-1030, Hitachi High Technologies Corp.). Then, FE-SEM observation was performed at 15.0 kV.

RESULTS AND DISCUSSION

Fig. 2 shows the processing time for 90% conversion from CBT to pCBT at processing temperatures of 170, 190, 200, 210, 230, and 250[degrees]C. The conversion from CBT to pCBT was measured using GPC analysis as shown in the inset image, and the data were appropriately fitted. The time required for 90% conversion obtained from the fitted data was used as the value of the y-axis, and the final form of the figure was determined by creating a log-log plot. A compression temperature of 250[degrees]C was selected because the full conversion time at 250[degrees]C was within a few minutes of the processing time required by the currently used processing in the automobile industry. Higher temperature than 250[degrees]C is beneficial in the aspect of processing time but the CBT resin could thermally decompose at higher temperature than 250[degrees]C. A compression time of 2 min was selected to achieve nearly 100% conversion.

Longer processing time than 2 min at 250[degrees]C can be applied to improve the degree of conversion. However, this strategy results in the significant reduction of production capacity of equipment, that is, if processing conditions at 250[degrees]C for 4 min are applied, the production capacity drops by half. Conversely, thermal annealing can improve the degree of conversion without loss of the production capacity of equipment because the thermal annealing can be applied using only typical ovens. Therefore, it is helpful to investigate appropriate conditions for the thermal annealing.

A differential scanning calorimetry (DSC) thermogram of the CBT resins is shown in Fig. 3 as a function of the annealing temperature. The melting temperature and heat of fusion of the CBT and pCBT resins are summarized in Table 1. The melting peak of the pristine CBT 160 resin was observed in the temperature range of 130-150[degrees]C. The broad exothermic peak at ~180-200[degrees]C was attributed to crystallization of inconsistent CBT chains arising from incomplete polymerization because the ring-opening polymerization of the cyclic oligoesters proceeded with the incorporated catalyst after the first melting, and the entropically driven polymerization is a nonexothermic reaction. The endothermic peak at ~225[degrees]C was a melting peak corresponding to pCBT. The endothermic peak in the temperature range of 130-150[degrees]C was not observed in the thermogram of specimens that were compression molded at 250[degrees]C for 2 min because of conversion of the oligoesters into pCBT. Comparison of the melting peak of specimens annealed at 160[degrees]C for 2 h after compression molding with that of unannealed specimens revealed that the melting temperature did not vary and the heat of fusion increased slightly, which indicates that the crystallinity was increased slightly by the annealing at 160[degrees]C. A double melting peak appeared in the thermogram of the specimen annealed at 180[degrees]C for 2 h.

When the annealing temperature of 190[degrees]C was applied for 2 h, the melting temperature decreased and the heat of fusion substantially increased, which indicates that this annealing temperature was effective in increasing the crystallinity of pCBT prepared under high speed processing. In the specimens annealed at 200[degrees]C for 2 h, both the melting temperature and heat of fusion increased. In fact, the heat of fusion was the highest among samples prepared using the considered annealing temperatures. The improvement in crystallinity was achieved by annealing at 200[degrees]C for 2 h. The highest melting temperature was obtained by annealing at 210[degrees]C for 2 h; a nearly perfect crystal was formed at this annealing temperature. However, the heat of fusion and crystallinity were reduced because of the fusion of some crystallites with low melting temperatures. Therefore, an annealing temperature below 180[degrees]C is not appropriate because the annealing effect was negligible or a double melting behavior was observed, which resulted in an adverse effect on mechanical properties. An annealing temperature higher than 210[degrees]C was also not favorable because of the fusion of crystallites with low melting temperatures. An annealing temperature of 200[degrees]C was the optimal annealing temperature because of the improvement in the crystallinity.

A WAXD profile and the characteristic crystalline reflections of the CBT and pCBT specimens, which are shown in Fig. 4, were obtained to investigate the development of the crystalline structure of the pCBT specimens as a function of the annealing temperature and to determine the cause of the double melting peak of the pCBT specimen annealed at 180[degrees]C. First, the crystalline peak of CBT 160 indicated that the resin was composed of crystalline oligomers. The WAXD pattern of the pCBT specimen compression molded at 250[degrees]C for 2 min was different from that of the pristine CBT resin, which means that the molten CBT oligomers were polymerized and crystallized. The characteristic crystalline peaks were observed at 20=14.8[degrees] and 24.2[degrees] in the WAXD pattern of the pCBT specimen compression molded at 250[degrees]C for 2 min, but these peaks were not observed in the WAXD pattern of the annealed pCBT specimens after compression molding at 250[degrees]C for 2 min. The peak positions were consistent with the peaks of the CBT oligomers with the highest and second highest intensity. This result indicates that the CBT oligomers remained in the pCBT specimen that was compression molded at 250[degrees]C for 2 min, and the mechanical properties and thermal stability of the specimens were improved by annealing.

The characteristic peaks in the WAXD pattern of the pCBT specimens annealed at 180[degrees]C after compression molding at 250[degrees]C for 2 min were nearly identical to those of other annealed pCBT specimens, except for the pCBT specimens annealed at 210[degrees]C. Because the crystalline structure of the pCBT specimens annealed at 180[degrees]C was not different from that of the others, the double melting peak in the DSC thermogram of the pCBT specimens annealed at 180[degrees]C may have resulted from the melting and recrystallization of imperfect crystals [17]. The characteristic peaks in the WAXD pattern of the pCBT specimens annealed at 210[degrees]C were slightly shifted to a higher Bragg angle because of the more perfect crystalline structure of the pCBT specimens annealed at 210[degrees]C.

The thermal stability of the CBT and pCBT specimens was measured using TGA with different annealing temperatures, and the thermogram of the specimens shown in Fig. 5a. As shown in Fig. 5b, the initial thermal decomposition temperature of the pCBT specimen was shifted to a higher temperature relative to the annealing temperature. However, the initial thermal decomposition temperature of the pCBT specimens annealed at 210[degrees]C decreased. In addition, the weight residue of the pCBT specimens annealed at 180[degrees]C, 190[degrees]C, and especially 200[degrees]C was higher than that of the unannealed specimens shown in Fig. 5c. These findings are attributed to the decomposition of the remaining oligoesters at a lower temperature as a result of the high density of functional groups, which represent the initial decomposition site. In addition, the annealing temperature of 200[degrees]C is the best annealing temperature to convert the remaining oligomers after high-speed compression molding at 250[degrees]C for 2 min into the polymerized pCBT molecules. Therefore, an annealing temperature of 200[degrees]C was also optimal to improve the thermal stability of pCBT.

The compression molded pCBT specimens were annealed at 200[degrees]C to optimize the annealing time. The thermal behavior of the pCBT specimens is shown in Fig. 6, and both the melting temperature and heat of fusion of the pCBT resins are summarized in Table 2 with respect to annealing time. The melting temperature did not significantly change but the heat of fusion increased for annealing times of 30, 60, and 90 min, which indicates an increase in crystallinity. The melting temperature of the pCBT specimen annealed at 200[degrees]C for 120 min increased, and the heat of fusion of this specimen was the largest. However, the melting temperature, heat of fusion, and crystallinity of the pCBT specimen annealed at 200[degrees]C for 150 min decreased. Therefore, 120 min was the most appropriate annealing time to improve the crystallinity of pCBT.

The WAXD profile and characteristic crystalline reflections of the pCBT specimens are shown in Fig. 7 with respect to annealing time to demonstrate the development of the crystalline structure of the pCBT specimens. The characteristic crystalline peak of pCBT specimen annealed at 200[degrees]C for 120 min was sharper, which confirms the development of more perfect crystalline structure. The thermal stability of pCBT was measured using TGA for different annealing times, and the thermogram of the annealed pCBT specimens is shown in Fig. 8a. The initial thermal decomposition temperature of the pCBT specimen was shifted to a higher temperature with respect to the annealing time, as shown in Fig. 8b. In addition, as shown in Fig. 8c, the weight residue of all of the pCBT specimens annealed at 200[degrees]C was higher than that of the unannealed specimens. In particular, the initial decomposition temperature and weight residue of the pCBT specimens annealed at 200[degrees]C for 120 min increased substantially. Therefore, an annealing time of 120 min was also the optimal condition for improving the thermal stability of the pCBT.

The pores inside the CFFRC specimen, the CF content, and the properties of the interface between the reinforced CF and the used resin as well as the mechanical properties of the raw materials can affect the mechanical properties of the CFFRC. A cross section of the CFFRC specimen compression molded at 250[degrees]C for 2 min was observed by OM and FE-SEM to evaluate the impregnation of the CFF with the CBT resin. The images were obtained after the cross section of the CFFRC tensile specimen was polished and are shown in Fig. 9. The CFF layers and the resin within the CFF were observed, and the CF inside the CFF remained in its initial form, which is arrayed by alternate turns in the horizontal and vertical directions. Vertically aligned CFs were also observed in the FE-SEM images. The inside of the employed CFF was filled with the resin and had no voids, which indicates that the CFFRC was prepared without pores, even though the applied processing speed was high.

The tensile strength of pCBT and CFFRCs is shown in Fig. 10a. The tensile strength of the CFFRC compression molded at 250[degrees]C for 2 min and the CFFRC annealed at 200[degrees]C for 120 min after compression molding at 250[degrees]C for 2 min was improved by ~550 and 625%, respectively, relative to the pCBT matrix. The CF content in the CFFRC specimen was evaluated using TGA and determined to be ~76 wt% (70 vol%), based on the density of the materials, as shown in Fig. 10b. The fracture surface of the CFFRC tensile specimen compression molded at 250[degrees] for 2 min was observed by FE-SEM and is shown in Fig. 11. Complex fractures, such as fiber breakage, interface debonding, and delamination, were observed, which indicates excellent interface adhesion between the reinforced CFF and the matrix. Hence, applied external forces can be effectively transferred to the CFF reinforcement because of this excellent interface adhesion. The superior tensile strength of the CFFRC prepared under high-speed processing was achieved because the high CF content was obtained from the low viscosity and high impregnation characteristics of the used CBT resin. As expected, the thermal stability of the CFFRC annealed at 250[degrees]C for 120 min was improved by ~10[degrees]C. Therefore, the high-speed processing method for the thermoplastic CFFRC has the potential for use in automatic mass production of automotive parts, and the tensile strength and thermal stability of the molded CFFRC can be improved by the appropriate annealing.

CONCLUSIONS

A novel fast manufacturing process was suggested using a low viscosity polymerizable CBT resin for fabricating CFFRCs. Because the CFFRC was prepared without pores though high-speed processing, the tensile strength of the CFFRC compression molded at 250[degrees]C for 2 min was 440 MPa, which represents an improvement of ~550% relative to the pCBT matrix. Structure and properties of the composites were altered by thermal annealing to develop appropriate post processing of the CFFRCs for fast production and cost reduction. The mechanical properties and thermal stability of the specimens were improved by thermal annealing as the remaining CBT oligomers were polymerized further. Annealing at 200[degrees]C for 120 min was the best annealing condition for improving the mechanical properties and thermal stability of pCBT because of improvement in crystallinity under these conditions. The tensile strength of the CFFRC annealed at 200[degrees]C for 120 min after compression molding at 250[degrees]C for 2 min was 500 MPa, which represents an improvement of ~625% relative to the pCBT matrix. As expected, the thermal stability of the CFFRC annealed at 250[degrees]C for 120 min was improved by ~10[degrees]C.

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Sung Ho Kim, (1) Ye Ji Noh, (2) Young Woung Ko, (2) Seong Yun Kim, (2) Jae Ryoun Youn (3)

(1) Next Generation Product-prestige Solution Team, LG PRI Production Engineering Research Institute, LG-ro 222, Jinwi-myeon, Pyeongtaek-si, Gyeonggi-do 451-713, Korea

(2) Carbon Convergence Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), Eunha-ri San 101, Bongdong-eup, Wanju-gun, Jeollabukdo 565-905, Korea

(3) Department of Materials Science and Engineering, Research Institute of Advanced Materials (RIAM), Seoul National University, Daehak-Dong, Gwanak-Gu, Seoul 151-744, Korea

Correspondence to: Seong Yun Kim; e-mail:sykim82@kist.re.kr Jae Ryoun Youn; e-mail: jaeryoun@snu.ac.kr

Contract grant sponsor: Ministry of Education, Science and Technology (Korea Institute of Science and Technology [KIST] Institutional Program and the Basic Science Research Program through the National Research Foundation of Korea [NRF]); contract grant number: R11-2005-065.

DOI 10.1002/pen.23765

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Melting temperature and heat of fusion of the resins
with respect to annealing temperature.

Annealing
temperature      Annealing      [T.sub.m]     [DELTA][H.sub.m]
([degrees]C)    time (min)    ([degrees]C)          (J/g)

Pristine            120            228              16.8
Unannealed                         224              30.9
160                                224              39.3
180                                225              58.0
190                                220              63.4
200                                229              70.4
210                                233              59.3

TABLE 2. Melting temperature and heat of fusion of the
resins with respect to annealing time.

                Annealing
               temperature      [T.sub.m]     [DELTA][H.sub.m]
Annealing     ([degrees]C)    ([degrees]C)         (J/g)

Unannealed         200             224              30.9
30                                 223              58.0
60                                 226              52.4
90                                 222              62.0
120                                229              70.4
150                                224              63.8
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Author:Kim, Sung Ho; Noh, Ye Ji; Ko, Young Woung; Kim, Seong Yun; Youn, Jae Ryoun
Publication:Polymer Engineering and Science
Article Type:Report
Date:Sep 1, 2014
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