Intermediate Velocity Impact Response of Carbon/Epoxy Composites with Polycarbonate Facing

Intermediate Velocity Impact Response of Carbon/Epoxy Composites with Polycarbonate Facing Intermediate Velocity Impact Response of Carbon/Epoxy Composites with Polycarbonate Facing U.K.Vaidya 1*, C.A. Ulven 1, M.V. Hosur 2, J. Alexander 2 and L. Liudahl 3 1 Department of Materials and Science Engineering, University of Alabama at Birmingham, Birmingham, AL 35294, USA 2 Center for Advanced Materials, Tuskegee University, Tuskegee, AL 36088, USA 3 Department of Mathematics, North Dakota State University, Fargo, ND 58103, USA Received: 21st October 2002; Accepted: 28th February 2003 SUMMARY Woven fabric composites are increasingly being used in composite structures for applications in the aircraft, marine, and automotive industries. With emerging low-cost processing techniques for composite materials, the role of fabric architectures in sustaining low, intermediate, and high velocity impact loads is a subject of interest. An example of a low-cost process is the out-of-autoclave, vacuum assisted resin transfer molding (VARTM) technique. The present study evaluates the intermediate velocity impact response of two commonly used structural carbon fabric laminates produced from plain and 2/2 twill woven fabrics, processed using VARTM. A series of impact tests have been performed on the all plain, all twill and hybrid plain-twill weave carbon/ epoxy laminates. All laminates studied were covered with a polycarbonate facing in order to enhance the impact resistance of the carbon/epoxy laminates. The perforation mechanism, ballistic limit, and damage evolution of each laminate has been studied. The results from the experiments are reported. 1. INTRODUCTION Fiber reinforced polymer matrix composites have been widely used in aerospace, armour vehicles, and marine structures because of their high specific strength and stiffness [1]. In general, low, intermediate, and high velocity impact loading is of concern in polymeric composites. While several studies have addressed the low velocity impact of composites [2], studies on intermediate and high velocity impact are of equal interest [3-7]. With emerging low processing methods such as vacuum assisted resin transfer molding (VARTM), the process-performance relationships to various loading threats needs attention. The structures of interest in defence applications are commonly composed of woven fabric carbon/graphite, Kevlar, and glass fiber reinforced polymer matrix composites. Among these, carbon/ * author to whom correspondence should be addressed E-mail: uvaidya@uab.edu graphite fiber composites are susceptible to impact damage because of their poor impact resistance, and are the focus of the present work. The use of woven fabrics in composite structures is increasing. Because of the interlacing of the fibre bundles, woven composites possess high ratios of strain to failure in tension, compression, and impact loads [8]. Amongst various architectures, the plain weave and 2/2 twill weave fabrics, represented in Figure 1, are of interest in composite structures. The plain weave follows one over and one under interlacing of the tows and is the most fundamental fabric architecture that has been studied by various researchers [8-10]. It provides balanced properties in the 0 and 90 directions unlike unidirectional laminates. The 2/2 twill woven fabric exhibits balanced bi-directional properties in the fabric plane, and higher specific strength and stiffness as well as greater dimensional stability as compared with unidirectional tape composites. Several studies report the determination of in-plane properties of woven fabric composites [8-10]. Polymers & Polymer Composites, Vol. 11, No. 6, 2003 421

U.K.Vaidya, C. Ulven, M.V. Hosur, J. Alexander and L. Liudahl Figure 1. a) Plain weave fabric; b) 2/2 Twill weave fabric (a) (b) Recent studies [11] have demonstrated the utility of using a sacrificial tough polycarbonate layer bonded to carbon fiber composite to improve their impact resistance. Polycarbonate is a tough, dimensionally stable, low density (1.20 g/cm 3 ), transparent thermoplastic and is a suitable candidate material for many lightweight applications that demand high performance under repeated blows, shattering and spalling [11-12]. It is characterized as the highest impact resistant thermoplastic material (impact strength: 600-850 J/m) within a temperature range of 5 C to 140 C. The use of polycarbonate as a facing for carbon/epoxy laminates is attractive, as the laminate can be designed for lower thickness, yet could sustain desired impact loading, thereby translating to cost savings. Polycarbonate can be used as a sacrificial layer by removing and replacing impact damage contained within the polycarbonate layer. The present work aims to investigate to what extent the crimp / undulation of woven fabric influences the impact performance of the composite. A common consideration in this study is the presence of a sacrificial polycarbonate facing to the carbon/epoxy laminate. The facing shares a large portion of the initial impact load and damages upon impact. The ballistic limit is 20% lower in similar composite panels without polycarbonate facing [13]. Previous studies have reported that the in-plane properties of woven composites [1, 8-10] are reduced by the crimp of the weave. The differences in the plain and 2/2 twill weave architectures is significant in terms of crimp / undulation of the interlacing yarn. Experimental studies pertaining to the influence of intermediate velocity transverse impact loading of plain and twill weave fabric composites bonded to a polycarbonate sheet are reported in this paper. The influence of composite hybridizing with plain and twill woven fabric has also been considered. The rationale for hybridizing plies is twofold. Hybridization may either be the design intent to vary in-plane elastic properties, or a consequence of stacking variations that occur during processing, which may result in less than an ideal stacking condition, thereby resembling a hybridized weave. Two sets of carbon/epoxy composite panels were produced in order to focus on both the ballistic limit variations and damage growth characteristics due to the different weave architectures and hybrids of these weave architectures. 2. SPECIMEN FABRICATION The first set of composite panels was fabricated using plain (3K yarn, 12.5x12.5 count, 194.88 g/m 2, 0.2286 mm, tensile strength 1.17 GPa, modulus 230 GPa) and 2/2 twill (3K yarn, 12.5x12.5 count, 194.88 g/m 2, 0.3175 mm, tensile strength 1.17 GPa, modulus 230 GPa) woven T300B-40B-3K-Toray carbon fabric and Applied Poleramic SC-14 epoxy resin through a vacuum assisted resin transfer molding (VARTM) process [11]. The gel time of the resin was approximately three hours and the curing was completed in twelve hours. Three types of panels were fabricated, each containing seven layers of carbon fabric. They included: a) all seven plain weave plies, b) all seven twill weave plies, and c) hybrid lay-up with four twill and three plain weave plies. The nomenclature for the panels is provided in Table 1. The second set of composite panels was fabricated using eight layers of the same carbon fabric specified above with Vantico 8640 epoxy resin processed as 422 Polymers & Polymer Composites, Vol. 11, No. 6, 2003

Intermediate Velocity Impact Response of Carbon/Epoxy Composites with Polycarbonate Facing Table 1. Specimen nomenclature, first set PW7#: TW7#: PWT7#A: PWT7#B: Plain Weave, 7 layers, # of Sample Twill Weave, 7 layers, # of Sample Plain & Twill Weave Hybrid, 7 layers, # Sample, Plain Weave on back Plain & Twill Weave Hybrid, 7 layers, # Sample, Twill Weave on back Table 2. Specimen nomenclature, second set All All T: P: Twill Weave Plain Weave # T+#P-#: # Twill layers + # Plain layers, # of Sample Table 3. Details of samples and projectile Polycarbonate Carbon laminate: Total Mass sheet: mass of polycarbonate bonded to carbon plies: of fragment simulating projectile: 3 Average mass 43 g, volume 38.5 cm 3 Average mass 35 g, volume 25.6 cm 78 g 16 g, 12.7 mm diameter above. The gel time of the resin was approximately eight hours and the curing was completed in sixteen hours. A spectrum of panels was fabricated, each containing eight layers of carbon fabric. They included: a) all plain weave plies, b) all twill weave plies, and c) hybrids of varying twill and plain weave plies. The nomenclature of the panels is provided in Table 2. Each panel was bonded to a polycarbonate sheet (Clear PC (Lexan ), Supplier Precision Plastic and Punch Co) of 2.5 mm thickness. The polycarbonate sheet was bonded to the composite on one side using 3M spray adhesive (Super 77). The average thickness of the plain weave laminate was 1.6 mm thick, for twill and hybrid weave it was 1.7 mm. The total thickness of the laminate and polycarbonate was 4.1 mm for the plain weave-polycarbonate, and 4.2 mm for the twill and hybrid weave-polycarbonate. 3. EXPERIMENTAL The intermediate velocity impact tests were performed using a gas-gun test set up. A 16 g, 50-calibre (12.7 mm diameter) fragment simulating projectile (FSP) was used. The gun consisted of a 3 m barrel, a firing chamber, and a capture chamber. The sample was placed in the capture chamber. Sabo-assisted projectiles could be launched to velocities of 200 m/ sec. A sabot stripper plate mounted in front of the muzzle was used to separate the projectile from the launching sabot before impacting the target. Samples of dimension 101.6 mm x 101.6 mm (4" x 4") were used. The sample was mounted in a simply supported boundary condition along its four edges, sandwiched between rollers on two rigid aluminum plates. Two chronographs (Model ProChrono Digital) were mounted with clamps to the bottom of the capture chamber with a transparent optical window to record the incident and residual velocity of the projectile. Varying the pressure of gas in the firing chamber varied the impact velocity. Tests were conducted on the samples described: a) all plain weave, b) all twill weave, and c) hybrids of varying plain-twill weave laminas. In all the tests, the polycarbonate sheet faced the impact side. At least three samples were tested in each category to ensure repeatability. Table 3 provides information on the projectile, the mass of the constituent laminate, and that of polycarbonate. Polymers & Polymer Composites, Vol. 11, No. 6, 2003 423

U.K.Vaidya, C. Ulven, M.V. Hosur, J. Alexander and L. Liudahl The impact tests were designed to investigate the damage evolution below, at, and beyond the ballistic limit. The ballistic limit velocity denoted by V BL is considered as the velocity at which the energy absorption was maximized. At V BL, the projectile remains embedded in the panel, and in some instances penetrates fully. The projected damage in the sample was at a maximum at this condition. For tests below the ballistic limit, the projectile rebounded from the panel and was recovered from the impact side. For beyond ballistic limit tests, penetration was complete and the damage zone was smaller. For the test velocities adopted here, the residual velocity was appreciable only for few cases as reported in Table 6. 4. RESULTS AND DISCUSSION All of the samples exhibited transverse and longitudinal cracking patterns along with back face bulging, the extent dependent on the type of weave and the amount of each type of weave within the laminate. These features were quantified by measuring the parameters illustrated in Figure 2 with respect to the back face. Because of the variation in crack lengths from the center of penetration within each specimen, an average of half of the total transverse and longitudinal crack lengths was measured and reported. These measurements provided information about the principal dimensions of the impact damage zone, such as entrance and exit areas for perforation and the damage profile (shape, size and location). Besides measuring the damage evolution and failure modes of each target, the incident velocity was measured for each projectile in both sets of samples. Table 4 summarizes the damage observations and V BL results for the first set of samples. Table 5 summarizes the damage observations for the second set of samples. The V BL results were not evaluated for the second set of samples because of the use of a lower impact resistant resin system. For all samples in both sets, the polycarbonate facing, due to its thickness, exhibited localized perforation in conjunction with plastic deformation at the point of impact and a damage zone equivalent to the diameter of the projectile as shown in Figure 3. The thickness Figure 2. Quantitative measurements on back face a) back view, and b) side view Table 4. Visible % back face damage* at VBL for the first set of samples Specimen Transverse (%) Longitudinal (%) Bulge (%) V L B ) (m/s PW7 28. 4 22. 5 168. 3 100 TW7 26. 5 24 54. 8 120 PWT7-A 27. 9 15. 7 114. 3 98 PWT7-B 29. 4 26. 5 54 105 *Based off 101.6 mm x 101.6 mm (4 in x 4 in) simply supported sample 424 Polymers & Polymer Composites, Vol. 11, No. 6, 2003

Intermediate Velocity Impact Response of Carbon/Epoxy Composites with Polycarbonate Facing Table 5. Visible % back face damage* at VBL for the second set of samples S ample A vg. Trans. Crack (%) A vg. Long. Crack (%) Avg. Bulge (%) 7P+1T-C2 24.97 20.67 13.22 6P+2T-C2 23.43 22.16 12.46 5P+3T-C2 25.86 26.28 11.06 4P+4T-C2 25.36 23.88 9. 3 3P+5T-D2 29.69 25.54 10.91 2P+6T-C2 36.27 32.52 10.57 1P+7T-D2 32.54 29.76 8.47 All T 29.08 34.47 10.08 1P+7T-A1 37.14 34.38 9. 9 2P+6T-B1 39.28 32.92 9. 8 3P+5T-A1 31.34 32.73 9.63 4P+4T-A1 30.18 31.46 11.61 5P+3T-A1 28.13 28. 1 9.58 6P+2T-B1 26.48 25.29 9.45 7P+1T-A1 23. 2 19.41 9.43 All P 25.08 27.25 13.63 *Based off 101.6 mm x 101.6 mm (4 in x 4 in) simply supported sample Figure 3. Polycarbonate with localized perforation in conjunction with plastic deformation due to impact of the polycarbonate was not sufficient to cause radial cracking or shattering. Depending on the thickness of the facing, polycarbonate can exhibit brittle or ductile impact failure. The role of the polycarbonate in all the tests was to reduce the velocity of the projectile by means of the mechanism of localized melting. As reported in Table 4 for the first set of samples, the extent of transverse cracking of the back face was within 10% for the all plain (PW7), all twill (TW7), hybrid with plain weave on the back face (PWT7-A) and hybrid with twill weave on the back face (PWT7- B). However, the extent of longitudinal cracking was Polymers & Polymer Composites, Vol. 11, No. 6, 2003 425

U.K.Vaidya, C. Ulven, M.V. Hosur, J. Alexander and L. Liudahl approximately 68% higher for the PWT7-B samples than for the PWT7-A samples. In addition, the all TW7 exhibited 6% higher longitudinal cracking than the PW7 samples. In all cases for the first set of samples, the PW7 and PWT7-A samples exhibited a significant bulge on the back face on account of the formation of a fiber shear zone. The bulge was measured to be 168% for the PW7 samples (211% larger than the TW7), 114% for the PWT7-A samples (111% larger than the PWT7-B), and only 54% for both TW7 and PWT7-B samples. The energy absorption mechanism in the plain weave dominant laminates was observed to be related to fiber shearing and therefore a higher amount of bulging occurred, and reduced longitudinal cracking. In contrast, the twill weave dominant laminates exhibited higher back face tensile cracking and less fiber shearing, and therefore reduced bulging of the back face. Figures 4 a-d represent the failure modes for the PW7, TW7, PWT7-A and PWT7-B laminates for just below the impact limit condition. Below ballistic limit, the damage incurred by the twill weave laminates was smaller than that of the plain weave (Figure 4a vs. 4b). The hybrid laminates appear to exhibit similar damage states (Figure 4c vs. 4b). The hybrid laminate with twill weave on the back face exhibited the largest amount of in-plane cracking. However, the back face bulge of the hybrid laminate with plain weave on the back face was significantly larger as a result of fiber shearing. The back face damage propagation is higher in the hybrid laminates than in the all plain and all twill laminates because of the competing failure mechanisms (in-plane cracking/delamination vs. through the thickness shearing) in the hybrid laminates. Figure 5 a-d illustrates the same category of samples at ballistic limit. At the ballistic limit the longitudinal cracking of the twill weave laminates was higher than that for the plain weave. This was attributed to fiber shearing in the plain weave laminates. For the 2/2 twill weave, the undulations run over two tows and this small variation (of straightness of the 2/2 twill weave yarns) changes the failure mode to a tensile fiber fracture mode (with less shear bulging). Figures 6a-b compare an idealized stacking of the plain and twill weave. Figure 4. Failure mode below ballistic limit. a) all plain, b) all twill, c) hybrid, plain on back, d) hybrid, twill on back (a) (b) (c) (d) 426 Polymers & Polymer Composites, Vol. 11, No. 6, 2003

Intermediate Velocity Impact Response of Carbon/Epoxy Composites with Polycarbonate Facing Figure 5. Failure mode at ballistic limit. a) all plain, b) all twill, c) hybrid, plain on back, d) hybrid, twill on back (a) (b) (c) (d) Figure 6. Undulations in a) Plain weave, b) 2/2 Twill Weave The alternating over-under undulations of the all plain weave (PW7) resulted in a 20% lower ballistic limit than the all twill weave (TW7) laminates. For the hybrid weave samples, the ballistic limit was 7% higher for the PWT7-B than the PWT7-A laminates, i.e. when the twill weave was on the back face. Figure 7 demonstrates the bounds of energy absorbed vs. velocity at ballistic limit for the different sample types. The hybrid samples, as expected, exhibited a ballistic limit characteristic in between that of the all plain and all twill weave. The lowest value of the ballistic limit was obtained from the hybrid with a plain weave on the back face (PWT7- A) panels. This was perhaps because of the limited role of the twill weave layers which were closer to the compression zone (impact side), and lesser plain Polymers & Polymer Composites, Vol. 11, No. 6, 2003 427

U.K.Vaidya, C. Ulven, M.V. Hosur, J. Alexander and L. Liudahl Figure 7. Absorbed energy vs. ballistic limit velocity for constituent and hybrid samples weave plies subjected to shear during the passing of the projectile through the back side layers. This was also noted by the comparison of PW7 to PWT7-A. In comparison to the PW7, the shear bulge is lower for the hybrid PWT7-A laminate. Table 6 summarizes the impact test data. Out of those samples that exhibited perforation, only the PWT7-A sample had its residual velocity recorded. For all other samples, the residual velocity was not appreciable enough to give an exit velocity reading. The projectile was recovered at the backside of the panel, indicating it had slowed sufficiently to drop off after complete penetration. The incident energy for the samples is reported in Table 6 as well. For the ballistic limit velocities, this may be treated as the absorbed energy in the sample, neglecting energy loss at the supports. The TW7 samples exhibited the highest energy absorption at 115 J. Compared to 80 J for the PW7 samples, the TW7 samples generated a 35 J increase in energy absorption. Hybridization did not provide significant benefits in terms of energy absorbed. The PWT7-B sample exhibited higher energy than the corresponding PWT7-A sample. The influence of architecture and hybridization in the absorption of energy due to a intermediate velocity impact event was characterized in the first set of samples. However, the influence of varying the placement of either weave architectures within the hybrid samples needed to be further explored. The second set of samples depicts the extent of transverse and longitudinal cracking along with bulging at the ballistic limit as the number of layers with either type of architecture was increased within the hybrid laminates. Figures 8 & 9 illustrate the effect of increasing both the number of twill woven plies and the number of plain woven plies with respect to back face crack growth at ballistic limit. As the number of plainwoven plies is increased from the side of the hybrid adhered to the PC, the transverse and longitudinal crack growth gradually decreases. This is inversely proportional to the samples in which the transverse and longitudinal crack growth gradually increases as the number of twill woven plies increases from the side of the hybrid adhered to the PC (Figures 10-12). This indicates that the number of one type of weave architecture has more influence on the amount of back face cracking within a hybrid 428 Polymers & Polymer Composites, Vol. 11, No. 6, 2003

Intermediate Velocity Impact Response of Carbon/Epoxy Composites with Polycarbonate Facing Table 6. Ballistic test data for the first set of samples S ample V in (m/s) V out (m/s) Energy (J) PW71 85 0 58 PW74 89 0 63 PW72 100 0 80 PW73 122-119 TW71 109 0 95 TW74 120 0 115 TW72 131-137 PWT71A 88 0 62 PWT77A 98 0 77 PWT78A 102-83 PWT73A 128 63 131 PWT76B 82 0 54 PWT72B 83 0 55 PWT75B 105 0 88 PWT74B 127-129 Note: Bold values represent 'at ballistic limit'; No values on Vout represent penetration, but no sufficient residual velocity to obtain a reading on the exit Figure 8. % Transverse crack growth vs. increasing lamina for eight layer hybrid samples Polymers & Polymer Composites, Vol. 11, No. 6, 2003 429

U.K.Vaidya, C. Ulven, M.V. Hosur, J. Alexander and L. Liudahl Figure 9. % Longitudinal crack growth vs. increasing lamina for eight layer hybrid samples Figure 10. Back face damage growth for eight layer plain woven sample Figure 11. Back face damage growth for eight layer (4 twill, 4plain) hybrid samples 430 Polymers & Polymer Composites, Vol. 11, No. 6, 2003

Intermediate Velocity Impact Response of Carbon/Epoxy Composites with Polycarbonate Facing Figure 12. Back face damage growth for eight layer twill woven sample laminate than the placement of the weave architecture in question. The effect of bulging in the hybrid laminates was on average increased as a result of the amount of plain woven plies and decreased because of the number of twill woven plies (Figures 10-12). This is explained by the fact that as the number of plain woven plies increases, regardless of position, the shear field is increased and shear failure dominates. Variations in the average bulging effects were caused by measurement errors due to larger or smaller amounts of PC melting at the PC / laminate interface. A multivariate analysis of variance [14] between transverse cracking, longitudinal cracking, and back face bulging data was used to describe the relationship between the quantity of each weave and the placement within the hybrid composite panels to the type of damage that occurs during an impact event. For this analysis, two null hypotheses had to be made: 1. Twill or plain weaves placed first in the stacking sequence does not significantly change the damage propagation. 2. The amount of twill or plain woven layers in the laminate does not significantly change the damage propagation. Using the Wilks lambda test statistic and checking the rejection region, the first calculation determined a P-value of 0.622. A P-value of 0.622 confirmed that the first null hypothesis was correct. This result verified that twill or plain woven fabric placed first in the stacking order does not significantly change the damage propagation. However, in the second calculation a P-value of 0.0464 was determined. A P- value of 0.0464 confirmed that the second null hypothesis was incorrect. This result verified that the amount of twill or plain woven layers in the composite laminate does significantly change the amount of damage propagation. 5. CONCLUSIONS 1. Woven carbon fabric composites of plain, 2/2 twill, and hybrid weaves manufactured by the VARTM process and bonded to polycarbonate facing were subjected to intermediate velocity impact under simply supported boundary conditions. The role of the sacrificial polycarbonate facing was to reduce the incident velocity of impact to the panel and withstand low velocity impact. 2. The twill weave panels exhibited 20% higher ballistic limit than the plain weave. The hybrid weave exhibited impact response in between that of the plain and satin weave and was sensitive to the positioning of the weave with respect to the impact direction. 3. The progression of damage and damage modes were found to be sensitive to the number of plies and the type of woven fabric. The prominent damage modes were tensile side fracture of the plies for 2/2 twill weave dominant samples, and fiber shear fracture for the plain weave dominate samples. 4. A multivariate statistical analysis revealed that twill or plain woven fabric placed first in the stacking sequence does not significantly change the damage propagation, but the number of twill or plain woven layers in the composite laminate does significantly change the amount of damage propagation. Polymers & Polymer Composites, Vol. 11, No. 6, 2003 431

U.K.Vaidya, C. Ulven, M.V. Hosur, J. Alexander and L. Liudahl 6. REFERENCES 1. P.K. Mallick, Fiber Reinforced Composites, Second Edition, Marcel Dekker Inc., New York, NY, USA (1993). 2. S. Abrate, Impact on Composite Structures, Cambridge University Press, New York, NY, USA (1998). 3. J.A. Zukas and D.R. Scheffler, International Journal of Solids and Structures, 38, (2001) 3321-3328. 4. B. Wang and G. Lu., Journal of Materials Processing Technology, 57, (1996) 141-145. 5. J.G. Herrington and B.P. Rajagopalan, International Journal of Impact Engineering, 11, 1, (1991) 33-40. 6. M.L. Wilkins, International Journal of Engineering Science, 16, (1978) 793-807. 7. S.J. Bless and D.R. Hartman, 21 st International SAMPE Technical Conference, Atlantic City, NJ, 852-866 (1989). 8. S. Ng, P. Tse and K. Lau, Composites Part B, 29B, 735-744 (1998). 9. D. Scida, Z. Aboura, M.L. Benzeggagh and E. Bocherens, Composites Science and Technology, 57, (1997) 1727-1740. 10. N.K. Naik and V.K Ganesh, Composites Science and Technology, 45, (1992) 35-152. 11. U.K. Vaidya, M. Kulkarni, A. Haque, M.V. Hosur, and R. Kulkarni, Materials Technology, 15, 3, September (2000) 202-214. 12. S.C. Wright, N.A. Fleck, and W.J. Stronge, International Journal of Impact Engineering, 13, 1, February (1993) 1-20. 13. C.A. Ulven, U.K. Vaidya, M.V. Hosur, and J. Alexander, 33 rd International SAMPE Technical Conference, Seattle, WA, 37-45, (November 2001). 14. J.H. Bray and S.C. Maxwell, Multivariate Analysis of Variance, Sage Publications, Beverly Hills, CA, USA (1985). 7. ACKNOWLEDGEMENT This work was supported by the Air Force Research Laboratory (AFRL) Grant F33615-99-C-3608. The authors would like to express their sincere appreciation to Dr. Arnold Mayer, Technical Monitor at AFRL. 432 Polymers & Polymer Composites, Vol. 11, No. 6, 2003