Submitted to: Journal of Applied Polymer Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/9/2008
Publication Date: 10/2/2008
Citation: Mohamed, A., Finkenstadt, V.L., Palmquist, D.E., Rayas-Duarte, P. 2009. Thermal Properties of Extruded Injection-Molded Poly (lactic acid) and Milkweed Composites: Degradation Kinetics and Enthalpic Relaxation. Journal of Applied Polymer Science. 111:175-184. 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 bio-blends containing natural biodegradable poly-lactic acid and milkweed by-product 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: In order to determine the degree of compatibility between Poly (lactic Acid) (PLA) and different biomaterials, PLA was compounded with milkweed fiber, a new crop oil seed. 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 (IM). Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) were used to analyze the 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 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 'Cp was 0.464 J/g/deg C followed by crystallization and melting transitions. The enthalpic relaxation (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 it. The percentage crystallinity of neat unprocessed PLA dropped by 95% and 80% for the EX and IM respectively. The degradation Activation Energy (Ea) of neat PLA exhibited a significant drop in nitrogen environment, while increased in air, indicating PLA resistant to heat degradation in the presence of oxygen. Overall, injection molding appeared to reduce Ea of the composites, while milkweed significantly reduced Ea values in nitrogen environment. Enzymatic degradation of the composites revealed higher degradation rate for the EX samples versus IM, while 30% milkweed exhibited higher weight loss compared to the 15%.