Composites Science and Technology 63 (2003) 1287–1296 www.elsevier.com/locate/compscitech Biodegradable composites based on l-polylactide and jute fibres David Placketta,*, Tom Løgstrup Andersenb, Walther Batsberg Pedersenc, Lotte Nielsenc a Danish Polymer Centre, Technical University of Denmark, Building 423, Lyngby 2800, Denmark b Materials Research Department, Risø National Laboratory, 4000 Roskilde, Denmark c Danish Polymer Centre, Risø National Laboratory, 4000 Roskilde, Denmark Accepted 21 February 2003 Abstract Biodegradable polymers can potentially be combined with plant fibres to produce biodegradable composite materials. In our research, a commercial l-polylactide was converted to film and then used in combination with jute fibre mats to generate composites by a film stacking technique. Composite tensile properties were determined and tensile specimen fracture surfaces were examined using environmental scanning electron microscopy. Degradation of the polylactide during the process was investigated using size exclusion chromatography. The tensile properties of composites produced at temperatures in the 180–220 C range were significantly higher than those of polylactide alone. Composite samples failed in a brittle fashion under tensile load and showed little sign of fibre pull-out. Examination of composite fracture surfaces using electron microscopy showed voids occurring between the jute fibre bundles and the polylactide matrix in some cases. Size exclusion chromatography revealed that only minor changes in the molecular weight distribution of the polylactide occurred during the process. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Polylactide; A. Fibres; B. Mechanical properties; Electron microscopy; Size exclusion chromatography 1. Introduction Biodegradable polymers have been the subject of research and development since the early 1970s and growing pressure on the world’s resources as well as concerns about disposal of plastics led to intensified interest and commercial activity in the 1990s [1]. Biodegradable polymers, or biopolymers as they are sometimes known, are now attracting interest for a wide range of applications from special high-performance products in medicine and surgical applications through to commodity uses in films and packaging [2]. In terms of volume applications, the biopolyester polylactide or PLA is arguably the biodegradable polymer that has the greatest commercial potential. PLA is a polymer of lactic acid and is commonly produced by ring-opening polymerisation of the cyclic lactide dimer [3]. PLA may also be produced by direct polycondensation of lactic acid and although this route usually yields fairly low molecular weight species, special procedures can overcome this problem [4]. Lactic acid, the starting material for PLA synthesis, can be produced by fer* Corresponding author. Fax: +45-4588-2161. E-mail address: [email protected] (D. Plackett). mentation from a number of different renewable resources and, for example, a large PLA production facility based on use of corn starch-derived lactic acid and with an annual production capacity of 140,000 metric tons is currently being established in North America by Cargill Dow LLC, a joint venture between Cargill Corporation and Dow. A recent review article lists a number of other current manufacturers of PLA [5]. The commercially attractive features of PLA, for example as a future commodity packaging material, include its production from renewable resources as well as its good mechanical properties. After use, PLA polymers can be recycled or alternatively disposed of by incineration or by landfilling. Landfill disposal effectively closes the loop in returning the polymer to the soil where it biodegrades. Obstacles in the path of wider application of PLA products include the present price of the polymer relative to commodity thermoplastics and its brittle character in thicker materials. In addition, a recent article suggested that biodegradable polymers are not really as ‘‘green’’ as is generally assumed if total energy costs are considered [6]. However, PLA is still in its relative infancy in terms of development for commercial applications and considerable progress in overcoming 0266-3538/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0266-3538(03)00100-3 1288 D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296 perceived problems with its use and parallel decreases in bulk prices might be anticipated in the coming years. For example, Bogaert and Coszach [7] suggest that the sales price for PLA could decrease significantly over the period 2001–2008 while market potential could increase by a factor of 10. The use of natural fibres to reinforce thermoplastics such as polypropylene and polyethylene as an alternative to synthetic or glass fibres has been and continues to be the subject of research and development. The potential advantages of using natural fibres have been well documented and are generally based on environmental friendliness as well as health and safety factors [8]. Challenges to the use of natural fibres include their tendency to absorb moisture and their susceptibility to thermal degradation at processing temperatures over 200 C, although technical solutions to these problems are feasible. Wider adoption of natural fibres in polymer reinforcement would also be assisted by the development of international standards as well as methods to produce agro-fibres in a way that gives consistent material from season to season and from year to year. A number of researchers and research groups have already identified the possibilities for new fully biodegradable composite products through combining biodegradable polymers with natural fibre reinforcement [9–11]. The review paper by Mohanty et al. [5] also provides a useful reference point and contains a summary of literature pertaining to biofibre-reinforced biodegradable polymers. As also pointed out by Keller et al. [12], the application of fibre reinforcement to biopolymers such as PLA should produce composites with suitable mechanical properties for lightweight construction materials. Given the increasing commercial interest in natural fibre-reinforced polymer composites, for example in the building products and automotive sectors in particular, and demands for materials that can either be recycled or can safely biodegrade in the environment at the end of their service life, the development of fully biodegradable composites for a wide range of applications could be an interesting next step in the evolution of natural fibre composite materials. This paper describes research in which composites were prepared from a commercial PLA in combination with jute fibres. Jute (Corchorus capsularis L.) is an important tropical crop and grows in India, Bangladesh, China, Thailand, Nepal and Indonesia. In our research, jute in the form of a non-woven mat was used as a model system for what might be feasible with a wide range of plant fibres. The PLA/jute composites were characterised in terms of mechanical properties and also examined microscopically. In addition, the possible degradation of the PLA polymer at high temperature during processing was investigated using size exclusion chromatography (SEC). 2. Experimental 2.1. Materials and processing The PLA polymer used in this work was a commercial l-polylactide granulate identified as L5000 from Biomer of Krailling, Germany. Jute fibres were used in the form of a non-woven mat of basis weight 300 g/m2 purchased from JB Plant Fibres Ltd of the UK. The PLA granulate was used as received and was converted into a film approximately 0.2 mm in thickness using a Haake single-screw extruder. Temperatures in the three extruder barrel zones were set to 160, 180 and 190 C and the temperature in the die was set at 190 C. The PLA film was collected on rolls and stored in the laboratory under ambient conditions prior to use. Composites containing about 40% jute fibre by weight were produced using a film-stacking procedure as shown schematically in Fig. 1. Lay-ups (300 mm120 mm) were prepared in which sections of jute fibre mat were stacked up with several PLA film layers on either side. For this procedure, jute mat sections were cut with the length direction at 90 to the direction of the mat roll. This was because previous experience had shown that mat properties varied with direction and that sections cut at 90o provided composites with higher tensile properties than sections cut in the direction of the roll. The lay-ups were subjected to rapid press consolidation using a process involving the following steps: (1) precompression, (2) contact heating under vacuum, (3) rapid transfer to a press for consolidating and cooling and (4) removal of the finished part from the press. In a typical experiment, a lay-up of PLA films and jute fibre mat sections was placed between two Teflon sheets in a Fig. 1. Schematic of process for fabricating PLA/jute composites. D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296 metal frame, pre-pressed for 15 s at 3.3 MPa and then rapidly conveyed into the heating stage. The heating section was sealed automatically as soon as the lay-up had been transferred and pressure in the section was then reduced to approximately 400 Pa (four millibar) within 1–2 min. In this stage, the pre-compressed layups were heated by contact with the platens for a total time of either 3 or 10 min with the platen temperatures set at 180, 190, 200, 210 or 220 C. At the end of the heating stage the frame holding the lay-up was rapidly transferred back to the press where the assembly was consolidated under a pressure of 3.3 MPa at 60 C for 1 min. The quality of the 2 mm-thick pressed composites was examined by cross-cutting a small section from each trial panel, wetting the cross-section and looking for indications that fibres had not been wetted. Fibre wetting appeared to be inadequate when processing at 180 C for 3 min and therefore a heating time of 10 min was found to be necessary in this case. Heating times of 3 min were used at all other process temperatures. Reference sample panels without fibre reinforcement were prepared in a similar manner by laying up a sufficient number of PLA films without inclusion of fibre mat sections and using a heating time of 10 min at 190 C. In order to determine the influence of jute fibre moisture content on PLA during processing, a number of mat sections were vacuum oven dried overnight and then used to make composites at process temperatures of 200, 210 and 220 C. Samples for characterisation studies including tensile and impact property tests and analysis by size exclusion chromatography were cut from each panel. 2.2. Tensile tests A total of three or four specimens for tensile testing were prepared by cutting rectangular samples from each test panel and then converting these to the final dogbone shape using a computer-controlled routing machine. Specimens measured 180 mm in length, 25 mm in maximum width and 15 mm in the narrowest section. The test specimens were placed in a conditioning room at 65% relative humidity and 23 C and weighed at regular intervals until equilibrium had been reached. Following conditioning, each dogbone specimen was measured for width and thickness at the mid-point and at locations 10 mm either side of the mid-point. These measurements were used to calculate initial sample cross-section. Samples were then subjected to tensile testing according to ASTM D 638-99 [13] using an Instron 6025 100 kN test machine. The machine was operated at a crosshead speed of 2 mm/min. The longitudinal strain of each specimen was recorded with two back-to-back extensometers. The average strain was recorded and stress was calculated as load divided by 1289 initial cross-section. Results were expressed as plots of stress (MPa) against strain (%). The ultimate tensile strength or tensile strength at yield, the tensile stiffness, as determined by the slope of the tangent to the stress– strain curve drawn through the origin, and elongation at maximum stress were determined from the stress– strain plots. 2.3. Impact tests The impact resistance of a PLA/jute composite in comparison to a pure PLA material was determined on unnotched Izod test specimens by a procedure outlined in ASTM standard D 256-97 [14] and using a Zwick test machine with a pendulum of 0.46 J energy. Samples were conditioned at 65% relative humidity and 23 C until equilibrium moisture content before testing. A total of 15 samples of PLA and 15 of PLA/ jute composite were tested to determine mean impact resistance. The PLA/jute composites had been prepared using a heating time of 3 min at 200 C. The reference PLA panels were prepared using a heating time of 10 min at 190 C because other time/temperature combinations resulted in unacceptable air pocket entrapment within the panels. 2.4. Electron microscopy The fracture surfaces of selected tensile test specimens were examined using an ElectroScan E3 environmental scanning electron microscope (ESEM). Samples for examination were obtained by cutting sections about 2– 3 mm in length from just below the fracture zone. Tweezers were then used to place sections with fracture surfaces facing upwards and these were fixed on to a sample holder by embedding in a layer of wax. The sample holder was then placed in the ESEM vacuum chamber and the pressure in the chamber reduced to approximately 470 Pa (3.5 Torr). A fine mist of water was sprayed into the ESEM vacuum chamber in order to enhance sample surface conductivity and to optimise the clarity of the image. The instrument was operated at 20 kV and various sample surfaces were scanned to obtain an impression of fibre fracture, fibre distribution and the appearance of the fibre/polymer interface. 2.5. Size exclusion chromatography Small samples of PLA granulate, PLA film, PLA panels or PLA/jute panels, measuring a few mm on either side, were prepared for SEC analysis by dissolving in tetrahydrofuran (THF) overnight at room temperature. The samples were then heated at 65 C for 6 h to obtain complete dissolution of PLA. Before injection, the samples were filtered through a 0.2 mm Millipore filter (FGLP type). A sample of jute fibre mat was subjected to 1290 D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296 the same treatment in order to determine whether jute fibre extracts might interfere with the analyses. A separation according to molecular size was obtained by using a pre-column and a HT6E column from Waters Associates, USA. The eluate from the columns was monitored using both RALS (light scattering) and RI (refractive index) detectors from Viscotek Corporation, USA. Data was handled by means of Trisec1 software supplied by Viscotek Corporation. Separations were performed at room temperature using THF as eluent at a flow rate of 1 ml/min. Four samples were analysed for each type of material. Analytical results were presented in the form of cumulative weight fraction or the differential of the cumulative weight fraction as a function of log (molecular weight). almost double the tensile strength and more than double the tensile stiffness of the polylactide by incorporating jute fibres and using a heating temperature of 210 C. A statistically equivalent result was obtained when the temperature in the heating stage was raised to 220 C, which is well above the recommended processing temperature for the polylactide. The tensile strength and stiffness values for the 40% jute/PLA composites are similar to the values obtained for 40% fibreglass/polyropylene composites [15]. It is possible that the reduced viscosity of the polylactide at the higher temperatures and hence better flow properties may have helped to improve fibre wetting and therefore led to the trend of increasing tensile strength and stiffness as a function of heating temperature, although other factors such as changes in PLA crystallinity, composite porosity and jute fibre surface chemistry may also have contributed. The increase in tensile stiffness with increasing process temperature was not as marked as was the case with the tensile strength. Average strain at maximum stress was slightly lower in the composites when compared with the pure PLA matrix but showed little if any variation with processing conditions. The increase in tensile strength and stiffness with addition of jute fibres to PLA suggests some fibre/polymer surface compatibility and good stress transfer between the fibres and the matrix. A decrease in strain at maximum stress with agro-fibre addition is typical of uncoupled thermoplastic composites and is also seen in coupled systems, although the decrease in failure strain with increasing fibre amount is not as severe in these cases. 3. Results and discussion 3.1. Tensile tests The results of typical weight measurements on dogbone samples during pre-test conditioning are shown in Fig. 2 and indicate that conditioning to equilibrium at 65% relative humidity and 23 C took between two and three weeks and resulted in a final moisture uptake of about 2% on the basis of original sample weight. Moisture uptake by PLA panels without jute fibre reinforcement was virtually zero over the same period. Mean tensile strength, tensile stiffness and elongation values are shown in Table 1. The results, as summarised for example in Fig. 3, indicate that it was possible to 3.2. Impact resistance The results of unnotched Izod impact tests are shown in Table 2 and indicate that there was no statistical difference in mean impact resistance between the PLA processed at 190 C and the PLA/jute composite processed at 200 C. The literature indicates that the impact strength of composites is very sensitive to the choice of fibre and matrix. For example, Chuai et al. [16] found that reinforcement of polypropylene with untreated softwood fibres led to a significant decrease in unnotched Fig. 2. Weight changes in PLA/jute (fabricated at 210 C) at 65% relative humidity and 23 C. Table 1 Tensile test results for PLA and PLA/jute composites Sample type Process temperature ( C) Heating time (min) Tensilea strength (MPa) Tensilea stiffness (GPa) Elongationa at max. stress (%) PLA PLA/juteb PLA/juteb PLA/juteb PLA/juteb PLA/juteb 190 180 190 200 210 220 10 10 3 3 3 3 55 72.7 89.3 93.5 100.5 98.5 3.5 8.1 8.5 8.7 9.4 9.5 2.1 1.5 1.8 1.6 1.6 1.5 a b Values in parentheses are standard deviations. Composites containing approximately 40% jute on weight basis. (0.5) (2.3) (3.6) (1.1) (0.4) (3.3) (0.1) (0.4) (0.6) (0.0) (0.2) (0.5) (0.1) (0.0) (0.1) (0.1) (0.1) (0.1) 1291 D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296 Izod impact strength with increasing fibre content; however, fibre treatment to improve surface compatibility with polypropylene led to either a much smaller decrease in impact strength or even an increase depending upon the compatibilising treatment that was used. Wan et al. [17] studied carbon fibre-reinforced PLA composites and discovered that unnotched impact strength increased with fibre reinforcement up to a maximum value at 30% fibre volume fraction. As noted by Sanadi et al. [15], the unnotched impact resistance of thermoplastics generally decreases in the presence of agro-fibres. The reason for this is that the addition of fibres creates regions of stress concentration that require less energy to initiate a crack. Therefore, it is a positive result from our research that unnotched impact resistance does not decrease while composite tensile strength and tensile modulus increase when jute is used to reinforce PLA. 3.3. Electron microscopy ESEM photomicrographs of PLA/jute tensile specimen fracture surfaces are shown in Figs. 4–7. The brittle nature of the jute fibre fracture is clearly seen and is quite different in appearance from cases where, for example, jute is embedded in polypropylene and much more fibre pull-out generally occurs. As with the tensile test results, this is suggestive of a better interface between fibre and polymer but Figs. 6 and 7 in particular also show that gaps in the fibre/polymer interface can arise either as a result of manufacturing or during Table 2 Impact resistance test results for PLA and PLA/jute composites Fig. 3. Tensile strength of PLA fabricated at 190 C, PLA/jute fabricated at 190 C and PLA/jute fabricated at 210 C. Sample type Process temperature ( C) Heating time (min) Mean impact resistance (kJ/m2)a PLA PLA/juteb 190 200 10 3 15.4 (1.9) 14.3 (2.1) a b Values in parentheses are standard deviations. Composites containing 40% jute on weight basis. Fig. 4. ESEM photomicrograph of a PLA/jute composite tensile fracture surface showing brittle failure of fibres (PLA/jute composite fabricated from undried jute at 210 C). 1292 D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296 the process of tensile testing. The brittle nature of PLA and its manner of breakage, leaving plate-like areas on the fracture surface, is shown in Figs. 5 and 6. The typical surface zone of these composites, largely consisting of polymer in the first 50 to 100 microns, is also shown at the top of Figs. 6 and 7. 3.4. Size exclusion chromatography The results of SEC analyses of THF solution extracts from PLA or PLA/jute composites are shown in Figs. 8 and 9. Analysis of THF extracts from jute fibres alone showed no contributions that would potentially interfere with the determination of PLA molecular weight distributions. Fig. 8 shows cumulative weight fraction and the differential of the cumulative weight fraction as a function of log(molecular weight) for the PLA granulate, PLA film and PLA/jute composites processed between 180 and 220 C. A trend to decreasing weightaverage molecular weight is shown going from PLA granulate through PLA film to the PLA/jute compo- Table 3 SEC analyses of PLA and PLA/jute composites Sample type Process temperature ( C) Weight-average molecular weight (Mw)a Polydispersity (Pd)a PLA granulate PLA film PLA/juteb PLA/juteb PLA/juteb PLA/juteb PLA/juteb PLAc PLAc PLA/dried juted PLA/dried juted PLA/dried juted N.A.e N.A.e 180 190 200 210 220 200 220 200 210 220 274,850 263,500 252,850 252,550 247,725 244,150 238,525 239,700 219,300 252,700 251,400 244,650 2.57 2.35 2.32 2.34 2.36 2.34 2.33 2.43 2.37 2.22 2.32 2.34 a b c d e Mean of four analyses. Jute mats used in air-dried form. PLA converted to test plates without inclusion of fibre mats. Vacuum oven-dried jute. N.A.=not applicable. Fig. 5. ESEM photomicrograph of a PLA/jute tensile fracture showing broken jute fibre bundles and the plate-like fracture of the PLA matrix (PLA/jute composite fabricated from undried jute at 210 C). D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296 1293 Fig. 6. ESEM photomicrograph of a PLA/jute tensile fracture surface showing void spaces between jute fibre bundles and the PLA matrix (PLA/ jute composite fabricated from undried jute at 220 C). sites. Although differences in weight-average molecular weight of 10,000 or less are probably not significant in these experiments, there is clearly a trend towards decreasing weight-average molecular weight with increasing process temperature. Fig. 9 illustrates this more clearly when the cumulative weight fraction is plotted against log (molecular weight) over a narrower range on both axes. The overall results in terms of weight-average molecular weight and polydispersity [weight-average molecular weight (Mw)/number-average molecular weight (Mn)] are shown in Table 3. The narrower the molecular weight range, the closer are the values of Mw and Mn, and the ratio of Mw/Mn may thus be used as an indication of the breadth of the molecular weight range in a polymer sample. The trend of decreasing weight-average molecular weight progressing from raw PLA granulate through PLA film to PLA composites incorporating air-dried jute is also clearly shown in Table 3. Although the Mw values appear higher in cases where vacuum oven-dried jute has been used, it is unlikely that these are real differences in statistical terms. The conclusion that the presence of moisture in the fibres during press consolidation causes only minor degradation of PLA is also reinforced by the relatively constant polydispersity values shown in Table 3. The stability of PLA during the process may be a result of processing under vacuum. Observations during the heating stage of the process indicated that moisture is removed quite rapidly and therefore the time during which PLA might react with water at high temperature would be quite short. 1294 D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296 Fig. 7. ESEM photomicrograph of a PLA/jute composite tensile fracture surface showing void spaces between fibre bundles and the PLA matrix (PLA/jute composite fabricated from vacuum oven-dried jute at 210 C). Fig. 8. SEC analytical results expressed as cumulative weight fraction (Wf) (curve envelope 1) or d(Wf/logMw) (curve envelope 2) as a function of log molecular weight of PLA. The outer curves on each envelope show results for PLA granulate and PLA/jute (220 C) respectively. D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296 1295 Fig. 9. SEC analytical results showing cumulative weight fraction (wf) 10 as a function of log molecular weight of PLA. The outer lines on the envelope show results for PLA granulate and PLA/jute (220 C), respectively. 4. Conclusions 1. The tensile strength and stiffness of PLA can be approximately doubled when jute fibre reinforcement is used on a 40 wt.% basis. However, impact resistance as measured by an unnotched Izod test is not increased. 2. The increase in tensile strength in particular is dependent on the temperature in the heating stage of the process, showing a maximum in the range 210–220 C. 3. Electron microscopy observations show brittle failure of jute fibres under tension and void spaces between fibre and polymer matrix indicating that the strength of the jute/PLA interface could be improved. 4. Limited PLA degradation occurs during rapid press consolidation under vacuum and this might be attributable to rapid removal of air and water during the heating stage. Acknowledgements The assistance of the late Palle Vagn Jensen of Risø National Laboratory, Materials Research Department in obtaining ESEM photomicrographs of PLA/jute tensile specimen fracture surfaces is gratefully acknowledged. References [1] Chandra R, Rustgi R. Biodegradable polymers. Prog Polym Sci 1998;23:1273–335. [2] Tournois B. Recent developments in application of raw materials in bioplastics. In: Derksen JTP, Mogendorff J, Mangan C, Daskaleros T, editors. Proceedings of the symposium on Renewable Bioproducts—Industrial Outlets and Research for the 21st century. Wageningen, Holland, June 1997. European Commission Publication EUR 18034; 1997. [3] Jacobsen S, Fritz H-G, Degee P, Dubois P, Jerome R. New developments in the ring opening polymerization of polylactide. Industrial Crops and Products 2000;11:265–75. [4] Enomoto K, Ajioka K, Yamaguchi A. US Patent 5310865. 1995. [5] Mohanty AK, Misra M, Hinrichsen G. Biofibres, biodegradable polymers and biocomposites. Macromol Mater Eng 2000;276/ 277:1–24. [6] Gerngross T, Slater SC. How green are green plastics? Scientific American 2000;August:24–9. [7] Bogaert J-C, Coszach P. Poly (lactic acids): a potential solution to plastic waste dilemma. Macromol Symp 2000;153:287–303. [8] Eichhorn SJ, Baillie CA, Zafeiropoulos N, Mwaikambo LY, Ansell MP, Dufresne A, et al. Review—current international research into cellulosic fibres and composites. Journal of Materials Science 2001;36:2107–31. [9] Herrmann AS, Nickel J, Riedel U. Construction materials based upon biologically renewable resources—from components to finished parts. Polymer Degradation and Stability 1998;59:251–61. [10] Riedel U, Nickel J. Natural fibre-reinforced biopolymers as construction materials—new discoveries. Die Angewandte Makromolekulare Chemie 1999;272:34–40. [11] Jiang L, Hinrichsen G. Flax and cotton fiber reinforced biodegradable polyester amide composites, 2. Die Angewandte Makromolekulare Chemie 1999;268:18–21. [12] Keller A, Bruggmann D, Neff A, Muller B, Wintermantel E. 1296 D. Plackett et al. / Composites Science and Technology 63 (2003) 1287–1296 Degradation kinetics of biodegradable fiber composites. Journal of Polymers and the Environment 2000;8(2):91–6. [13] ASTM Standard D 638-99. Standard test method for tensile properties of plastics. West Conshohocken, PA: American Society for Testing and Materials; 1999. [14] ASTM Standard D 256-97. Standard test methods for determining the Izod pendulum impact resistance of plastics. West Conshohocken, PA: American Society for Testing and Materials; 1997. [15] Sanadi AR, Caulfield DF, Jacobsen RE. Agro-fiber thermo- plastic composites. In: Rowell RM, Young RA, Rowell JK, editors. Paper and composites from agro-based resources. New York: CRC Press; 1997 [Chapter 12].. [16] Chuai C, Almdal K, Poulsen L, Plackett D. Conifer fibers as reinforcing materials for polypropylene-based composites. Journal of Applied Polymer Science 2001;80:2833–41. [17] Wan YZ, Wang YL, Li QY, Dong XH. Influence of surface treatment of carbon fibers on interfacial adhesion strength and mechanical properties of PLA-based composites. Journal of Applied Polymer Science 2001;80:367–76.
