Biocomposite Films Based on Biopolysaccharide and Hydrothermal Carbonised Pine Sawdust Waste: Properties, Performance, and Degradation Behaviour in Different Environments

Pine sawdust waste was converted by hydrothermal carbonisation into carbon-rich particles (HTC-WS) and incorporated into chitosan to produce a sustainable biocomposite film (Chit_HTC-WS). Chitosan–microcrystalline cellulose film (Chit_MCC) and neat chitosan (Chit) were also prepared for comparison. The films were characterised by structural, mechanical, thermal, surface, UV resistance, and degradation properties in freshwater, simulated seawater, and soil burial conditions. The results obtained highlight that HTCWS provides UV blocking and reduces transparency in the visible range, similarly to MCC. Despite their different chemical natures and elemental compositions, both HTCWS and MCC can reduce chitosan rigidity, increasing the ductility of the film, albeit less pronouncedly, compared to the neat chitosan film. Both HTC-WS and MCC can slightly improve the photo-oxidation resistance of chitosan-based films due to their ability to absorb harmful UVB rays. Neat Chit shows rapid degradation in soil burial (up to disappearance within ca. 15 days), whereas Chit_MCC and, even more so, Chit_HTC-WS, show significantly less degradation; specifically, Chit_HTC-WS does not disappear even after 96 days, exhibiting weight loss of ca. 30%, whereas Chit_MCC disappears after 96 days. Interestingly, chitosan shows different hydrolytic degradation in freshwater and simulated seawater, and as expected, the presence of salts slows down its hydrolysis. The presence of MCC and HTC-WS reduces the hydrolytic degradation of chitosan-based films,