Submitted to: Journal of Applied Polymer Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/23/2007
Publication Date: 1/15/2008
Citation: Mohamed, A., Finkenstadt, V.L., Palmquist, D.E. 2008. Thermal properties of extruded-injection molded poly (lactic acid) and bio-based composites. Journal of Applied Polymer Science. 107(2):898-908. 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.
Technical Abstract: In order to determine the degree of compatibility between PLA and different biomaterials (fibers), PLA was compounded with sugar beet pulp and apple fibers. Fibers were added at 85:15 and 70:30 PLA:Fiber. The composites were blended by extrusion followed by injection molding. Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) were used to analyze the extruded (EX) and the extruded-injection molded (EXIM) composites. After melting in the DSC sealed pans, composites were cooled by immersion in liquid nitrogen and aged (stored) at room temperature for 0, 7, 15, and 30 days. After storage, samples were heated from 25 to 180 degrees C at 10 degrees C/min. The neat PLA showed a glass transition (Tg) at 59 degrees C with 0.464 delta Cp. The glass transition was followed by crystallization and melting transitions. Enthalpic Relaxation (ER) of PLA and composites steadily increased as a function of storage time. Although the presence of fiber has little effect on ER, injection molding reduced ER. The percentage crystallinity of neat unprocessed PLA dropped by 95% after EX and by 80% for the EXIM. The degradation Activation Energy (Ea) of neat PLA exhibited a significant drop in nitrogen while increased in air. Overall, injection molding appeared to reduce Ea for all composites. Sugar beet pulp (SBP) significantly reduced Ea values in nitrogen environment. In air environment, both SBP and apple fibers increased Ea values. Enzymatic degradation of the composites showed higher degradation rate for the EX samples versus EXIM, while apple composites exhibited higher weight loss. The TGA data showed that neat unprocessed and EX PLA degradation followed a one-step mechanism, while EXIM showed two-steps degradation. Higher fiber content resulted in up to three-steps degradation mechanisms.