Thermal properties of extruded injection‐molded poly(lactic acid) and milkweed composites: Degradation kinetics and enthalpic relaxation

To determine the degree of compatibility between poly(lactic acid) (PLA) and different biomateri- als, PLA was compounded with milkweed fiber, a new crop oil seed. After oil extraction, milkweed remaining cake retained approximately 10% residual oil, 47% pro- tein, and 10% moisture. The fiber (300 lm) was added at 85 : 15 and 70 : 30 PLA : Fiber and blended by extrusion (EX) followed by injection molding (IM). Differential scanning calorimetry (DSC) and thermogravimetric analy- sis (TGA) were used for testing 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 � Ca t 10 � C/min. The pure PLA showed a glass transition (Tg) at 60.3 � C and the corresponding DCp was 0.464 J/g/ � 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 had little effect on ER, IM 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 exhib- ited a significant drop in nitrogen environment, whereas increased in air, indicating PLA resistant to heat degrada- tion in the presence of oxygen. Overall, IM appeared to decrease Ea of the composites, whereas milkweed signifi- cantly reduced Ea values in nitrogen environment. Enzy- matic degradation of the composites revealed higher degradation rate for the EX samples versus IM, whereas 30% milkweed exhibited higher weight loss compared to the 15%. The degradation mechanism was observed by looking at the percent conversion as a function of Ea from the TGA data, where multisteps degradation occurred mostly in air. V C 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 175-184, 2009