Muhammad Ajmal, Muhammad Asad, Shao Yuchao, Wenjing Lu
Rigid polylactic acid (PLA)/poly(butylene adipate-co-terephthalate) (PBAT) mineral-filled composites dominate certified industrial-compostable cutlery, yet the roles of composting conditions, abiotic processes, and biotic activity in their degradation and residual chemistry remain unresolved. Three certified products, a flexible PBAT-dominant film (FBP), a medium-rigid PLA-rich product (MRBP), and a rigid PLA/PBAT composite (RBP), were screened under ISO 20200 at 25, 37, 58, and 75 °C in a controlled industrial-composting simulation. At the regulated 58 °C setpoint, pseudo-first-order fits identified RBP as the slowest of the three materials (k = 0.0246 d⁻1; t90 ≈ 3.4 months); RBP was therefore selected for mechanistic study. Compost physicochemical trends and bacterial/fungal amplicon profiles indicated thermophilic process turnover and community succession, while matched 58 °C abiotic treatments spanning dry heat, intact hydrothermal pieces, and fragments with or without nutrient amendment isolated the thermal-hydrolytic contribution. Day-60 abiotic mass loss reached only 25.6-34.7% across hydrothermal RBP treatments, whereas thermophilic composting produced ≈57.4% mass loss with deeper organic depletion, indicating that heat and moisture alone do not reproduce the compost outcome. Infrared and thermogravimetric analyses of Day-60 micro-residues (1-2 mm and <1 mm) showed retained polyester and mineral-filler signatures, downshifted decomposition onsets, and high non-volatile residue, identifying polymer-compost hybrid particles rather than compost-assimilated material. The results attribute rigid-composite disintegration to a thermal-hydrolytic baseline augmented by a compost-environment contribution (most plausibly biotic, though inferred rather than directly measured) and underscore the need to couple sieve-based disintegration metrics with residue chemistry when assessing end-of-life outcomes.