Submitted to: North American Thermal Analysis Society Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 8/17/2008
Publication Date: 8/17/2008
Citation: Mohamed, A., Finkenstadt, V.L., Palmquist, D.E., Rayas-Duarte, P. 2008. Thermal properties of extruded injection-molded poly (lactic acid) and milkweed composites: degradation kinetics and enthalpic relaxation [abstract]. North American Thermal Analysis Society Meeting. p. 76.
Technical Abstract: Currently, most polymer composites utilize petroleum-based materials that are non-degradable and difficult to recycle or incur substantial cost for disposal. Green composites can be used in nondurable limited applications. In order to determine the degree of compatibility between Poly (lactic Acid) (PLA) and different biomaterials, PLA was compounded with milkweed fiber. Milkweed is a new oil seed crop. After oil extraction, the remaining cake retained approximately 10% residual oil and 47% protein. The pressed seed cake (10% moisture) was ground and passed through a 300 micrometers screen. The fiber was added at 85:15 and 70:30 PLA:Fiber. The composites were blended by extrusion (EX) followed by injection molding (EXIM). Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) were used to analyze the EX and the EXIM composites. The effect of the fiber on the enthalpic relaxation (ER) of PLA was determined by aging. 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 room temperature to 180 deg C at 10 deg C/min. The pure PLA showed a glass transition (Tg) at 59 deg C and the corresponding delta Cp was 0.464 J/g/deg C. The PLA glass transition was followed by crystallization and melting transitions. The ER of neat 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, as a result of EX, by 95% and by 80% for the EXIM. The degradation was done in air and nitrogen environment. The degradation Activation Energy (Ea) of neat PLA exhibited a significant drop in nitrogen environment, while increased in air, indicating PLA resistant to degradation in the presence of oxygen. Overall, injection molding appeared to reduce Ea of the composite. Milkweed significantly reduced Ea values in a nitrogen environment, while in air Ea exhibited increased values. Enzymatic degradation of the composites showed a higher degradation rate for the EX samples versus EXIM, while 30% milkweed exhibited higher weight loss compared to the 15%.