Effects of phosphorus-nitrogen containing flame retardants on the compostability of poly(lactic acid)

• PLA with P-N containing FRs are compostable under industrial composting conditions • Physical disintegration occurs within 25–28 days for all FR-PLA formulations • Polyphosphate FRs influence PLA degradation kinetics, but not hinder biodegradation • Phytotoxicity testing indicates limited adverse effects of FR on compost quality • P-N FRs support fire-safe, circular-economy–aligned PLA biocomposites Poly(lactic acid) (PLA), a versatile bio-based polyester, is valued for its industrial compostability. However, the incorporation of flame retardants (FRs) can alter its degradation behavior. In this study, PLA composites containing 15% of commonly used phosphorus- and nitrogen containing flame retardants—ammonium polyphosphate (APP), microencapsulated ammonium polyphosphate (MC-APP), and melamine polyphosphate (MPP)—were prepared and subjected to controlled aerobic composting to evaluate their disintegration and degradation behavior. Post-composting analyses using gel permeation chromatography, Fourier-transform infrared spectroscopy, thermogravimetric analysis, and differential scanning calorimetry revealed that the additives influenced the degradation kinetics but did not compromise compostability. All flame retarded PLA composites achieved complete disintegration within 21–28 days under industrial composting conditions. APP was found to leach out rapidly under composting conditions, resulting in only marginal effects on the biodegradation process, whereas the less hydrolysis-sensitive MC-APP and MPP particles were found to slightly accelerate the bio-fragmentation of the PLA’s molecular chains. Ecotoxicological evaluation of the compost, using Sinapis alba (white mustard) root elongation as an indicator, showed that the phytotoxicity of the examined polyphosphate flame retardants is influenced not only by their chemical structure but primarily by their solubility. Based on the EC 20 values, the mature compost derived from flame-retarded PLA composites is expected to exhibit only limited adverse effects on compost quality under industrial composting conditions. Overall, the results demonstrate that PLA composites containing polyphosphate-type flame retardants maintain industrial compostability, with only minor differences in degradation rate, mechanism, and compost quality compared to neat PLA.