Optimizing Sustainable Poly(lactic acid) (PLA) Composites: A Comprehensive Study on the Impact of Thermoplastic Starch (TPS) and α-Cellulose Ratios

Poly(lactic acid) (PLA) composites were developed by a melting extrusion technique with different thermoplastic starch (TPS) and α-cellulose ratios (30:0–15:15), while keeping constant PLA content and PLA-grafted glycidyl methacrylate (PLA-g-GMA) compatibilizer content of 60 and 10 wt %, respectively. PLA-g-GMA, which had a titrated epoxy content equivalent to 12.72%, improved interfacial adhesion by causing a nucleophilic ring-opening reaction between its epoxy functional groups and the hydroxyl functionality of hydrophilic fillers (TPS or α-cellulose) to form covalent ether linkages. Although homogeneous dispersion was achieved by this mechanism at low levels of α-cellulose loading, concentrations ≥10 wt % resulted in the reaggregation of the filler and worse morphological defects. Performance analysis showed considerable “trade-offs”: α-cellulose-free composite (PLA/T30/C0) maintained tensile strength between composites and neat PLA, while much higher increases in elongation at break value were observed (3.29 ± 0.21%); the reaggregation at higher α-cellulose loadings led inversely to a significant drop in both tensile strength and brittle behavior of biocomposites. Thermally, the hydrophilic fillers played the roles of heterogeneous nucleating agents, greatly enhancing crystallinity degree (Xc up to 22.7%) but also causing residual moisture, inducing hydrolytic degradation and lowering slightly thermal stability (Tmax) as well as giving rise to rheological instability due to a high MFI increase up to 58% upon moderate loadings. The important features are that the addition of hydrophilic materials such as cellulose significantly reduced the barrier property to water vapor, which is 7.1 times higher than that of neat PLA. In addition, the migration investigation conducted on the best PLA/T30/C0 formula revealed that it exceeded the maximum regulatory limit (30 mg/dm3) for total dissolved substances in distilled water (30.2 mg/dm3) and 20% ethanol simulants (38.2 mg/dm3). These poor barrier and migration properties of the composite are such that it cannot be commercialized as a high-moisture or alcohol-containing food, despite having been tested as compliant with fatty foods. Thus, although the increased ductility of the TPS-only composite and its susceptibility to hydrolytic degradation resulted from high water absorption offer advantages for applications in which fast deterioration under natural conditions is required, contributing to sustainable waste management (e.g., horticultural pots), it is not well suited for food packaging where protection against moisture is required.