Biodegradation of PLA in a simulated marine environment as affected by mixing with starch

PLA degradation at the low temperatures of the marine environment is highly limited, whereas thermoplastic starch (TPS) can be blended with this polymer to modulate its properties and promote biodegradation. In this study, the effect of PLA‒TPS blending at different ratios (1:4, 1:1, 4:1, and 1:0) on the film's properties and biodegradation/disintegration behavior under laboratory conditions simulating the pelagic zone was analyzed. Films were prepared by melt‒blending and compression molding, and their mechanical, barrier, optical, and thermal properties were characterized. Biodegradation was evaluated by respirometric analyses following ISO 23977-1, and disintegration was monitored through the weight‒loss, while changes in microstructure, thermal behavior, and molecular size distribution of the residual film were also characterized. The properties of PLA‒TPS blend films were mainly affected by the properties of the matrix continuous phase (TPS, at low PLA ratio, and vice versa). Mechanical resistance, elastic modulus, oxygen permeability, and water vapor barrier capacity decreased in the blend films as the TPS ratio rose (58, 19, 35, and 600%, respectively, at the lowest starch ratio). The increase in the TPS ratio improved the biodegradation and disintegration of the blends. Biodegradation of the film's small particles after 160 exposure days ranged from 69% in neat PLA to 100% in samples with PLA‒TPS ratios of 1:4 and 1:1. However, disintegration percentages of larger film samples ranged from 0 (for neat PLA) to 59% (for the blend of 1:4 ratio). Degradation and water solubilization of the TPS fraction generated internal pores that increased the films' internal surface area, facilitating water access to the PLA phase and promoting PLA hydrolysis and subsequent bio-assimilation of monomers. Conversely, starch degradation was delayed by polymer retrogradation/recrystallisation and by the encapsulating effect of PLA, which prevented starch from being exposed to the aqueous medium and microbial action.