Analytical Assessment of Biodegradation Claims in Post-Consumer Polymer–Dextrin Composites: FTIR Identification, Phase Balance Constraints, and Removable Inventory Kinetics

Abstract

Gravimetric mass loss in polymer–biogenic filler systems is often reported as polymer biodegradation without establishing which phase of the material is lost. This study proposes and applies an analytical framework that combines FTIR identification of the starting material, a phase balance ceiling, and removable inventory kinetics to assess such claims. Post-consumer cup plastic was blended with pyroconverted cassava (Manihot esculenta) peel dextrin in four matrix/dextrin/turpentine formulations (70/10/20 to 30/50/20% w/w) and exposed for 30 days to Aspergillus niger and A. fumigatus in Rivalier moist chambers with Sabouraud dextrose broth. FTIR revealed that the material sold as polypropylene contained polystyrene: the matrix ranged from essentially polystyrene (formulation B) to polypropylene-rich (C and D), as shown by the aromatic 755/697 cm−1 doublet and a PS/PP spectral index spanning more than one order of magnitude. Mass loss profiles followed a first-order removable inventory model with rate constants κ of 9.9×10−3 to 67.8×10−3d−1 (nominal residual solvent scenario) and inventory half-lives of 10–70 d. The removal rate increased with dextrin content (ρ=0.80 for both species) with a reproducible reversal between 30 and 40% dextrin that survives explicit bounding of the specimen surface-to-volume covariate. Five of the eight conditions remained below the removable fraction ceiling even on a dry basis; the other three required residual solvent fractions of only 1.2, 1.9, and 11.9% w/w, below the 20% nominal loading, so that no observation requires degradation of the PP/PS matrix. A greenness assessment of the framework with AGREE (0.62), MoGAPI (77%), and AGSA (66.7%) places it above the confirmatory methods it is intended to gate (respirometry, high-temperature GPC), with low throughput, manual operation, and the biosafety of the fungal assay as its main remaining limitations. The framework provides a reproducible, low-burden basis for separating selective removal of biodegradable or volatile phases from evidence that can legitimately support polymer biodegradation claims.