Thermal analyses of poly(lactic acid) PLA and micro-ground paper blends

There is a continuing demand for bio-sustainable and readily recyclable polymers for hot food packaging and hot beverage containers. An important candidate for these applications is poly(lactic acid), PLA, which possesses insufficient heat deflection temperature (HDT) performance, short of being fully crystallized. In order to address this issue, correlations were established for the observed enhancement in HDT with PLA resin after annealing with both thermal properties and the crystallinity of PLA (Ingeo™ 4032D) and extrusion melt blends thereof. The kinetic parameters of crystallization (reaction order, n and rate constant, k (s−1)) were obtained after fitting the Avrami equation. The reaction order for all of the systems had an n of approximately 3. Models derived from the Avrami equation with n ~ 3 are consistent with systems that undergo heterogeneous nucleation and subsequent spherulitic crystal growth. The addition of paper and talc did not change the model but did change the rate of crystallization. An increased quantity of cellulosic filler led to an increased rate of crystallization. The smaller particle sized fillers offered the greatest improvement in the rate of crystallization. However, the smaller-sized inorganic fillers did not appear to further improve HDT above that of the annealed blends of PLA/paper. The melting temperature for the PLA and all of the blends thereof formed containing paper from different sources of talc had identical melting temperatures (Tm). The variations in the HDTs of the composites were not found to be mirrored in the Tm. Annealing typically raised the crystallinity from approximately 10–15 to 30–40%. In situ determinations of enhanced moduli via annealing were performed using solid torsion bar dynamic mechanical spectroscopy, the results of which support the findings obtained via DSC. The annealed PLA/paper blends were found to be superior overall to the PLA/talc blends and are, importantly, sustainable.