Polyacrylonitrile/acrylamide-based carbon fibers prepared using a solvent-free coagulation process: fiber properties and its structure evolution during stabilization and carbonization.
Polyacrylonitrile (PAN)-based carbon fibers are used as important reinforcement materials in many applications due to its high strength and modulus, high thermal capabilities, and also its light weight (1).(2). It is well known that the final properties of carbon fibers are determined by the nature of the precursor fibers (3-6). Precursor fibers play a main role in carbon fiber quality as carbon liber inherits the characteristic of precursor fibers. Numerous studies reported that addition of acid monomers, i.e.. itaconic acid (IA) and methacrylic acid (MA) in precursor fibers are beneficial in the stabilization of the fibers, by lowering the time of stabilization and extending the thermal degradation (5-7). However, Wangxi et al. (4) discovered that PAN fibers containing acrylamide (AM), an alkaline monomer, is more effective in promoting stabilization process as compared with I A. Nevertheless, Wangxi and coworkers did not discover the optimum composition of AM in the PAN/AM fibers.
The optimum amount of comonomers should be used to obtain the best quality of carbon fibers (7). However, there are still few reports on the effects of AM loading in the overall mechanical and thermal properties of PAN/ AM-based carbon fibers. In this study, solvent-free coagulation bath was used during the fabrication process of PAN/AM fibers. The organic solvents such as dimelhylformamide (DMF) and dimethylacetamide (DMAc) used in the conventional coagulation bath could be hazardous, because it could cause cancer for a long period of exposure (8). Thus, the development of PAN/AiM-based carbon liber using the solvent free coagulation process is important and beneficial as it is environmentally friendly. However, the production of a high quality carbon fiber precursor via solvent free coagulation process is fairly new and a lot of information is yet to be discovered. This process may involve many steps that must be controlled and optimized. Therefore, the objectives of this study are to investigate the manipulation of AM content in PAN fiber and to study its mechanical and thermal properties, to provide an avenue to further enhance the performance of PAN/AM-based carbon fibers using solvent-free coagulation bath in the future.
Materials and Fabrication of PAN Fibers
PAN in powder form was obtained from Aldrich. Meanwhile, AM purchased from Sigma Aldrich, Germany, was used as an additive. DMF purchased from Merck, Germany, was used as solvent. The method to prepare uniform spinning dope of PAN and AM in DMF was reported earlier (8), (9). Eighteen weight percent of PAN/DMF solution was prepared, into which different amounts of AM (2%, 5%, and 7%) were added. The slurry was healed continuously at 70 [degrees]C until a highly viscous dope solution became ready for spinning process. Then, the homogeneous dope was degassed in ultrasonic bath (Branson Ultrasonics) for 24 h to remove the gas bubble present in the dope. The dry-jet-wet spinning technique was used to produce PAN fibers. The high interest regarding environment issues has resulted in the development of PAN liber fabrication in solvent-free coagulation process using 100% tap water in the coagulation bath (8), (9). The spinning conditions used in this study are listed in Table 1. The PAN/AM fibers fabricated have a round shape cross-section with the fiber diameter approximately 100 [um] m. The unique feature of the spinning process is to use 100% tap water in the coagulation bath, which has been guaranteed to be environmentally friendly and economical 18, 9]. The schematic diagram of the spinning equipment is shown in Fig. 1.
TABLE 1. Dry-wet spinning process specifications. Spinning dope PAN/AM/DMF AM wt% in polymer 0, 2, 5, 7 wt% Polymer concentration 18 wt% Dope temperature Ambient temperature Spinneret diameter 350 [micro]m Air gap distance 1 cm No of holes for the spinneret 10 holes Dope extrusion rate 0.019 [cm.sup.3] [s.sup.-1] Residence time 18 s Coagulation bath composition 100% tap water Coagulation bath temperature 17 [degrees]C
In the first step of the pyrolysis process, which is better known as stabilization, the fibers are treated under tension in an oxidizing atmosphere. Stabilization process was performed in an air flow condition by heating at a heating rate of 2 [degrees]C/min until 275 [degrees]C, before constant heating was carried out for 30 min. This important step is necessary in preparing the fibers so that they can withstand higher temperatures without decomposing during the carbonization treatment, by further orienting and then cross-linking the molecules (10). The quality of the resulting carbon fibers depends strongly on the degree of stabilization (11). Figure 2 illustrates Carbolite (Model CTF 12/65/550) wire wound tube furnace with Eurotherm 2416CC temperature control system that was used in all pyrolysis process in this study. The carbonization process was performed under tension in a high-purity nitrogen stream with a flow rate of 0.2 L/min at a heating rate of 3[degrees] C/min until 800[degrees] C was reached. Then the temperature was kept at 800 [degrees]C for 30 rain, which is known as the soaking time. The pyrolysis temperature and other heat treatment parameters of this study are as summarized in Table 2.
TABLE 2. Pyrolysis Temperature and other heat treatment parameters. Pyrolysis Pyrolysis Type of Heating Gas Soaking process temperature gas rate flowrate time ([degrees] ([degrees] (L/min) (min) C) C/min) Oxidation 275 Compressed 2 0.2 30 stabilization air Carbonization 800 [N.sub.2] 3 0.2 30 Carbonization 800 [N.sub.2] 3 0.2 30
Several characterization methods were used to study the PAN-based carbon fibers fabricated using solvent free coagulation process. Differential Scanning Calorimetry (DSC) was used to measure the heat evaluated by the exothermic reaction of the PAN fibers. DSC was carried out by a Mettler Toledo DSC model DSC822. Sample was first dried in an oven at 60 [degrees]C overnight to remove any moisture/solvent. The samples were then heated at programmed temperatures from 100--400[degrees]C at a healing rate of 10 [degrees]C/min under nitrogen atmosphere. Powdered samples (~2-5 mg each) were weighed and encapsulated in flat-bottomed aluminium pans of 45 [um]L volume with crimped-on lids. Evolved heat was determined following calibration with indium (28.4 J [g.sup.-1]) using the integration of the areas under the endothermal peaks and corrected base lines. The tensile test of PAN fibers was performed using tensile testing machine (LRX2.5 KN LLOYD Instrument with a load cell of IN, in accordance with ASTM D 3379 (25 mm gauge length was used for each PAN fiber and crosshead speed was 5 mm/min. The data was then interfaced with LRX Mechanical Testing Software. Vario elemental analyzer III was used to investigate the elements contained in the fiber in weight percentage. Wide angle X-ray Diffraction (XRD) Rigaku Rint 1200 with nickel-filtered Cu K[alpha] radiation (I = 0.154056 nm) at 40 kV and 20 mA was performed on PAN fibers to observe its crystalline related properties. The diffractogram was scanned with a scanning rate of 2 [degrees] mi[n.sup.-1] in a 2 [theta] range of 5--40[degrees] at room temperature. The data was corrected for Lorentz and polarization effects and finally normalized to a convenient standard area. The FTIR Nicolet Magna-IR560, of potassium bromide (KBr) type, was used to identify and classify the chemical structure transformation of PAN/AM fibers prepared under different heat treatment stages. The IR absorption spectra were obtained at room temperature in a range from 3000 to 500 c[m.sup.-1] with a resolution of 4 c[m.sup.-1] and averaged over 16 scans per sample.
RESULTS AND DISCUSSION
Modification on Conventional Coagulation Bath in Dry-Jet Wet Spinning Process
Conventionally, the coagulation bath contains solvent and nonsolvent. The common nonsolvenl used is water. Meanwhile, solvents frequently used for controlling the mass transfer in the coagulation bath are organic solvents such as DM Ac and DMF (3). The organic solvent in the coagulation bath acts as a resistance to the diffusion of solvent from inside the as-spun liber into the coagulation bath and sequentially reduces the possibilities of instantaneous coagulation. However, the huge amount of solvent used in the coagulation bath could be harmful to humans as it is carcinogenic. Long exposure toward the organic solvents would increase the chances of getting cancer. Therefore, to reduce the risk, a solvent-free coagulation process was considered in this work. To compensate the absence of solvent in the coagulation bath, the temperature in the coagulation bath was lowered to 17[degrees]C (8), The low temperature of coagulation bath enabled the domination of solvent outflow from the PAN fibers over nonsolvent inflow from the coagulation bath, resulting in an improvement in the mechanical properties of PAN fibers.
Conversion Process of PAN!AM Fibers to Carbon Fibers
In this study, PAN/AM fibers were pyrolyzed by two heat treatment steps, i.e., oxidation stabilization and carbonization. Oxidation stabilization is an essential process because it allows the subsequent polymer degradation reactions to prepare the fibers for subsequent process without collapse of the fiber or loss of orientation (11), (12). During stabilization, the PAN structure underwent cyclization reaction that converted the triple bond structure C [equivalent to] N to double bond structure C[equivalent to]N, resulting in a cyclic pyridine ring consisting of a six-membered ring structure composed of five carbon atoms and a nitrogen atom. The cyclization mechanism of PAN homopolymer is as shown in Fig. 3. The addition of AM, an alkaline monomer helped initiate the stabilization reaction and facilitate the process of cyclization. Thus, cyclization occurred at lower temperatures and in broader thermal ranges as proved by DSC experiments that are discussed in the "Effects of AM Loading to Thermal Properties of PAN-Based Carbon Fiber" section. The possible reaction of AM that facilitated the conversion process of PAN/AM fiber can be best described by Fig. 4. Cyclization reaction occurred when the carbon atoms in nitriles from PAN were attacked by the hetero atoms from AM. Meanwhile, during the soaking period, the noncarbon elements within the fibers were removed as volatile gases in the form of methane, hydrogen, hydrogen cyanide, water, carbon monoxide, carbon dioxide, ammonia, and various other gases (3). A low heating rate of 3[degrees]C/min was used during carbonization, which reduced the instantaneous mass and heat transfer during exothermic reaction (13). Thus, it helps in lowering the possibility of damaging the structure of PAN/AM carbon fibers derived from the precursor fiber in this study.
Effects of AM Loading to Mechanical Properties of PAN-Based Carbon fiber
There are various types of comonomers used such as methacrylate, AM, aminoethyl-2-methyl propenoale, I A, acrylic acid, MA, vinyl bromide, etc. (4), (5), (14), (15). These comonomers were blended with pure PAN during dope preparation to form a homogeneous PAN copolymer. The use of comonomers partially disrupted the nitrile-nitrile interactions, allowing for better chain alignment and acting as an initiator in the formation of the ladder polymer (16). PAN fibers containing acidic comonomers are the most commonly used precursor for carbon fibers because of its excellent performance. For example, IA received great attention in recent years because it is generally believed to promote the process of stabilization (17), (18). Thus, the nitriles oligomerization is apparently relaxed and initiated at lower temperature. However, Wangxi et al. (4) discovered that PAN fibers containing AM is more effective in promoting stabilization process, when compared with I As. In addition, Sivy et al. (19) reported that AM units acted as the initiation sites, for nitriles oligomerization and the oligomerization occurred through an ionic mechanic pathway, leading to a relaxed exothermic reaction. However, there is a limitation on the consumption of comonomers whereby an optimum amount of comonomers should be used to acquire the best quality of carbon fibers (7). An excessive amount of comonomer causes a great weight loss, which then reduces the mechanical strength of the carbon fibers.
In this study, 2-7 wt% of AM was added to investigate the effects of the comonomer loading on the mechanical and thermal properties of PAN-based carbon fibers. Acrylic fibers spun from homopolymer PAN suffer from poor hygroscopicily and low dye pickup because of its high degree of ordering, lack of segmental mobility as a result of a compact structure and a high [T.sub.g] (3), (20). The incorporation of AM into PAN during dope preparation is necessary as it enhances the solubility and thermal properties of PAN. Young's Modulus or also known as Elastic Modulus is a ratio of stress over strain and is widely used to determine the mechanical strength of materials. The Young's modulus of PAN precursor fiber is the best parameter representing the mechanical performance of carbon liber as there is a direct correlation between the Young's Modulus of the primary precursor and the resulting carbon fiber as revealed by Chari et al. (21).
The Young's Modulus of PAN liber precursor and PAN-based carbon fiber together with its ratio is shown in Table 3. The average of at least five readings was taken for the measurement of the Young's Modulus of the PAN-based carbon fiber precursor with different weight percentage amount of AM. Error bars for the Young's modulus values corresponding with the standard deviation of measurements were calculated from at least live fibers. Typically, the error bar values were very small, at around 0.03 GPa for all readings, and can be concluded as being not significant. It was found that the Young's modulus increased with the addition of acrlyamide up to 5% AM. The reason of these increments was because as the AM was added to the dope solution of PAN fibers, the solubility increased and turned the solution into a finer fluidity, which allowed the spinning process to run smoothly and the as-spun fibers to undergo a stretching process without failure. Consequently, the polymer chains were easily aligned, hence producing the fibers with higher crystalline region and higher mechanical properties. Generally, it is believed that the belter the orientation of the crystallites, the higher is the value of the ultimate Young's modulus (11). Thus, the Young's modulus of PAN fiber was increased from 3.03 GPa for PAN homopolymer up to 5.54 GPa for PAN containing 5% AM.
TABLE 3. The Young's modulus of PAN-based carbon fiber and precursor PAN fiber. The Young The Young Ratio of the modulus Young modulus modulus of of PAN-based of AM(wt%) PAN fiber (GPa), carbon fiber carbon fiber lo (GPa) . precursor [ E.sub.p ] [ E.sun.c ] fiber ([ E.sub.c ] /[E.sub.p ]) 0% AM 3.03 [ + or -] 18 [ + or - ] 5.94 0.0333 0.3 2% AM 4.17 [ + or -] 23 [ + or - ] 5.52 0.0287 0.3 5% AM 5.54 [ + or -] 35 [ + or - ] 6.32 0.0339 0.3 7% AM 5.27 [ + or -] 30 [ + or - ] 5.69 0.0273 0.3
However, the increasing trend of the Young's modulus did not continue on reaching its optimum point at 5% AM. As the AM loading was further increased to 7%, the Young's modulus decreased from 5.54 to 5.27 GPa. The excess of incorporation hydrophilic moieties, 1% AM that had an ionic charge increased the hydrophilicity of PAN fibers. With ihe addition of hydrophilicity, the inflow of nonsolvenl toward the (ibers became too rapid and dominated the solvent outflow from PAN fibers, resulting in the formation of more voids. Micro pores and voids are basically empty spaces that exist in polymer matrix of the PAN fibers, and they are normally filled with nonsolvent. Thus, the process of mass transfer to turn the as-spun fiber into a rigid structure was disturbed by the excess of AM. This result proves that there is a limitation on the consumption of comonomers whereby in this work, 5% AM is the optimum amount of comonomers.
Meanwhile, the ratio of carbon fiber to precursor modulus was calculated to confirm its correlation between the Young's modulus of the primary precursor and the resulting carbon liber. The ratios of all fibers were constantly about 5.5 to 6.5, which validated the performance of resulting carbon fibers as being significantly dependent on the mechanical properties of precursor fiber. The Young modulus of resulting carbon fibers for homopolymer PAN depicted the lowest value at 18 GPa. most probably due to fiber shrinkage during heat treatment that led to poor mechanical properties.
Effects of AM Loading to Thermal Properties of PAN-Based Carbon Fiber
The effect of addition of AM on the thermal properties of the PAN precursor was investigated by DSC experiments. The pyrolysis of homopolymer PAN is uncontrollable because of difficulties involved in the initial oxidation stage. The difficulty arises due to the sudden and rapid evolution of heat, coupled with a relatively high initiation temperature. This rapid surge of heat can cause chain scission, resulting in poor carbon liber properties (10). Referring to the proposed mechanism of homopolymer PAN in Fig. 3, the acrylonitrile molecule has a highly polar nitrile group that may cause a polymerization via free radical cyclization. The dipole-dipole interaction may also cause fiber shrinkage upon heat treatment. This shrinkage is believed to have the ability to disrupt the fiber orientation, ultimately leading to poor mechanical properties. The shrinkage behavior corresponds with the Young modulus's result obtained for carbon fibers derived from homopolymer PAN as depicted in Table 3.
Figure 5 shows the trend of exothermic curve in various amounts of the comonomers in PAN fibers and also the homopolymer PAN for the purpose of comparison. The exothermic regime of AM containing PAN precursor was much broader and the cyclization reaction started at lower temperatures, when compared with the PAN homopolymer precursor. The exothermic process is the result of thermal stabilizing processes, and the cyclization of nitrile group contributed to the evolution of large amount of heat released from the PAN fiber samples (3), (10). Figure 5 depicts that the presence of AM reduced the stabilization initiation temperature and facilitated the process, which made the cyclization occur in a broader thermal range. For the homopolymer PAN, a very sharp peak was obtained with initiation temperature at 317.7 [degrees]C, showing sudden and rapid energy release occurring at a high initiation stabilization temperature. It was also found that the amount of energy released was also very large, when compared with the other samples containing AM. With the increase in AM content from 2 wt% to 5 wt%, the exothermic peak became broader and shifted to lower initiation temperature at 290.8 [degrees]C and 271.7 [degrees]C, respectively. It proves that AM unit acts as initiation sites of nitriles and promotes the nitriles cyclization. This result was also in accordance with the reported work by previous researchers, which claimed that comonomers often act as an initiator for the stabilization reaction and facilitate the process, thus causing cyclization to occur at lower temperatures and in broader thermal ranges (3), (10), (16)
AM has amide groups that can form cyclic structures with adjacent nitrile groups of PAN. These cyclic structures propagate to the larger structure through nitrile oli-gomerization and dehydrogenation and subsequently slow down the propagation steps (22). By slowing down the propagation steps, sudden and rapid energy release can be controlled, which prevents the structure of PAN fibers from damage. However, the trend of better exothermic reaction did not continue after the optimum loading of 5% AM. As the AM loading was further increased to 7%, the exothermic peak became sharper than PAN containing 5% AM. It was also found that initiation stabilization temperature of PAN with 7% AM was slightly higher than AM 5% at 279.3 [degrees]C. This is in accordance with the result of mechanical properties represented by the Young modulus, i.e., the Young modulus of PAN/5% AM was better than other precursors. Thus, 5% AM seems to be the optimum amount of the comonomer in the PAN precursor fibers.
Elemental and Microstntcture Analysis of PAN-Based Carbon Fiber
As mentioned in ihe "Pyrolysis Process" section, pyrolysis is a conversion process that is compulsory in converting PAN fiber precursor into carbon fiber. Oxidative stabilization forms the ladder structure in enabling the fibers to undergo further pyrolysis process at higher temperatures. Meanwhile, CF are the fibers that have undergone heat treatment at 800[degrees]C in an inert condition to remove the noncarbon elements as volatile gases. Because 5% AM is the optimum amount of comonomer in the PAN precursor, the pyrolysis process was further done to this type of fiber. From the elemental analysis results with Vario Elemental Analyzer as depicted in Fig. 6, the stabilized liber showed low contents of oxygen and hydrogen due to effective dehydrogenation
Elemental percentage of various elements of PAN/AM 5% fiber Nitrogen Carbon Oxygen Hydrogen Stabilized Fiber 20.57 62.35 10.78 6.30 Carbonized Fiber(30 minutes] 13.42 78.09 6.82 1.67 Carbonized Fiber(60 minutes) 12.25 85.91 1.23 0.61 FIG. 6. Elemental percentage of various elements of PAN/AM 5% fiber. Note: Table made from bar graph.
During oxidative stabilization, comonomer AM, which has amide groups, formed cyclic structures with the adjacent nitrile group of PAN. These cyclic structures of amide groups propagated to the larger structure through nitrile oligomerization and dehydrogenation while the amide groups disappeared and the molecules annularly cross-linked (22), (23). The possible reaction of amide group with the adjacent nitrile group of PAN is best described as in Fig. 4. Thus, -C-C-C-C-N group decreased and aromatic ring-CN was increased. Correspondingly, low content of hydrogen and oxygen was achieved. On carbonization of the stabilized fibers, the corresponding carbon yield during soaking limes of 30 and 60 min was increased from 78.09% to 85.91%. The carbon content steadily increased with prolonged heat treatment, but the nitrogen content of carbonized fibers negatively correlated with the carbonization lime. This result demonstrates that nitrogen continued to be eliminated from the stabilized fibers during carbonization. The formation of new carbon basal planes and the evacuation of a large amount of nitrogen were continuous during carbonization, thus increasing the carbon content (12). Hence, the evolution of elemental composition has a good correlation with the burn-off, suggesting that the dominant change in the fibers during heat treatment was the loss of noncarbon elements (24).
The XRD analysis was done to confirm the growth of aromatic structure in performing new carbon basal plane upon carbonization. Figure 7 shows XRD patterns of the as-spun PAN fiber (PF), the stabilized fibers (SF), and also the carbonized fibers (CF) with 60 min carbonization time. Clearly, the strongest diffraction peak of PF occurred at 2 [theta] = 16.9[degrees] and 28.8[degrees], reflecting the ordered crystal structures made up of PAN linear macromolecules. This result is in agreement with other reports from previous researchers using conventional coagulation bath (25-27). Whilst for the SF, the diffraction pattern was clearly different from the PF due to oxidation, dehydrogenation and cyclization during oxidative stabilization. A weak peak was displayed at 2 [theta] = 16.9, corresponding with the structure of remnant linear segments whilst another broad peak was displayed at 2 [theta] = 25.0[degrees] relative to the ladder polymer structure. The appearance of these two broad and weak peaks at about 16.9[degrees] and 25.0[degrees] suggests that crystallization of the as-spun PAN fibers occurred to a certain extent during the cyclization process while temperature was increased from room temperature to stabilization temperature (25). The XRD patterns of the CF exhibited almost no sharp diffraction lines, which indicate that they were all amorphous (25), (26). However, one broad diffraction peak appeared at 2 [theta] of about 25.5[degrees]. This broad peak is very close to the diffraction peak characteristic of the graphitic structure, which happened to appear at 26 of about 26.5[degrees] (25).
FTIR analysis is a well known and useful method for comparing qualitatively either vibrating absorption spectra of fibers or relative intensities of the respective band (24), (26), Figure 8 shows the structural transformation of PAN/ AM fibers prepared under different heat treatment stages. For PAN/AM precursor fibers (PF), there were some well-known characteristic absorption peaks observed such as C [equivalent to] N stretching vibration at 2245 c[m.sup.-1], C-[H.sub.2] bending vibration at 1450 c[m.sup.-1] and 1064 c[m.sup.-1] and C-[H.sub.2] stretching vibration, at 2931 c[m.sup.-1]. There was also a strong peak representing AM, such as an amide characteristic absorbance peak in 1625 c[m.sup.-1]
After stabilization, PF changed into SF with the most prominent structural changes being the disappearance of peak at 2245 c[m.sup.-1], attributed to C = N of vibration peaks of C [equivalent to] N in IR spectrum. Some overlapping peaks appeared between 1000-1700 c[m.sup.-1] bands, which are possibly conjugated (C=C, C=N, C=0, N-H mixed) associated with heteroatomic ring systems. Another important structural change is the disappearance of amide band at 1625 c[m.sup.-1] for stabilized fibers and carbon fibers. It is due to the amide band diminishing during dehydrogenation and cyclization. C-[H.sub.2] absorption peaks also decreased significantly as a result of effective dehydrogenation. While heating the SF to 800[degrees]C in inert condition with heating time of 60 min to transform SF to carbonized fiber (CF), C[H.sub.2] absorption bands at 2931, 1360, 1051 c[m.sup.-1] were gradually reduced. The FTIR spectra of PAN/AM PF, SF, and CF confirmed that heat treatment had changed the structure of the fibers accordingly.
PAN/AM precursor fibers were successfully fabricated via environmentally friendly solvent-free coagulation process. From the DSC and tensile testing result obtained, it showed that the presence of AM in the PAN fibers enables the production of carbon fiber precursor with good mechanical and thermal properties. The fibers fabricated from a dope containing 18 wt% PAN and 5 wt% AM had the highest Young Modulus and also exhibited superior process, thus 5 wt% AM is the optimum amount of comonomer for PAN precursor fibers. The carbon content of fibers increased with prolonged heat treatment time and increased temperature, but the noncarbon elements were negatively correlated with the pyrolysis time. Meanwhile, the XRD patterns and FTIR spectra of the fibers indicated that precursor PAN/AM fiber had successfully transformed into carbon fibers before the heal treatment process.
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N. Yusof, (1), (2) A.F. Ismail (1),(2)
(1.) Advanced Membrane Research Technology Centre (AMTEC), Universiti Teknologi Malaysia, 81310 Skudai, Johor Bahru, Malaysia
(2.) Department of Gas, Faculty of Petroleum and Renewable Energy Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor Bahru, Malaysia
Correspondence to; A.F. Ismail: e-mail: firstname.lastname@example.org
Contract grant sponsors: Ministry of Higher Education Malaysia. Universiti Teknologi Malaysia.
Published online in Wiley Online Library (wileyonlinelibrary.com).
[c] 2011 Society of Plastics Engineers
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|Author:||Yusof, N.; Ismail, A.F.|
|Publication:||Polymer Engineering and Science|
|Date:||Feb 1, 2012|
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