Submitted to: Journal of Applied Polymer Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/30/2007
Publication Date: 9/30/2008
Citation: Mohamed, A., Finkenstadt, V.L., Rayas-Duarte, P., Palmquist, D.E., Gordon, S.H. 2008. Thermal Properties of Extruded and Injection-Molded Poly (Lactic Acid)-Based Cuphea and Lesqueralla Bio-Composites.. Journal of Applied Polymer Science. 111(1)114-116. Interpretive Summary: Despite the convenience and the practicality of petroleum-based polymers used for food and other consumer goods packing, there is evidence for ecological disturbance. The development and use of biodegradable plastics in packaging for environmental protection has been stimulated by public concerns and interest. Most polymer composites are difficult to recycle or incur substantial cost for disposal. Green composites use agricultural-based polymers and biodegradable plant-based fillers. Preparation of beneficial polymer composites is possible only when the biodegradable polymers are compatible with the bio-fillers. Compatibility can be determined by measuring the degree of intermolecular interactions between the biodegradable polymers and bio-fillers in the bio-composites. In this work, the degree of interaction in polymer bioblends containing natural biodegradable polylactic acid and sugar beet pulp or apple fibers were investigated using Thermal Analysis. The study included aging properties after storage for up to one month. The degradation mechanism of the composites was also determined. The current study will enable us to introduce these blends for consideration by the packaging industry. The blends will also reduce the cost of poly (lactic acid) use and increase the agriculture by-products value of alternate crops.
Technical Abstract: The degree of compatibility between poly (lactic acid) (PLA) and different bio-fillers was examined using thermal methods. The biofillers were fibers extracted from cuphea and lesquerella seeds. Bio-composites of PLA:Fiber were prepared at 85:15, 70:30 and 100:00 and blended by extrusion and then injection molded. Thermal properties of the extruded (EX) and the extruded-injection molded (EXIM) composites were examined using Deferential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). Composites analysis using DSC provided information essential for determining: 1) glass transition (Tg); 2) crystallization and melting temperatures and delta H; 3) percent crystallinity; and 4) Enthalpic Relaxation (ER). TGA was used to measure the mechanism of the thermal decomposition of the composites. The effect of fiber composition, processing conditions, and aging time on the thermal properties of the bio-composites was used to examine the relative compatibility of the fibers with PLA. The data showed that ER increased steadily as a function of aging. Due to the difference in protein content between cuphea and lesquerella, the two fibers influenced the Tg (temperature and delta Cp) differently. The percent crystallinity of neat PLA was significantly reduced by EX or EXIM, while the enzymatic degradation showed that EX composites are more biodegradable than EXIM. The TGA profiles indicated a multi-step degradation especially in air. The largest value of q, which indicated that, intermolecular interaction was strongest between lesquerella and PLA extruded materials, compared to the other three blends, possibly due to the higher protein content. Results also indicated that intermolecular interaction was least strong (q = -33.9156) in the EXIM PLA-cuphea. Although the q values of both cuphea composites were significantly lower than those of the lesquerella samples, this greater effect of extrusion over injection molding on intermolecular interaction was also seen in the q value of the EX and EXIM samples.