Thermal, Thermomechanical, and Fire Behaviour of Casein-Modified Unsaturated Polyester Resin Biocomposites

The increasing demand for sustainable engineering thermosets, such as unsaturated polyester resins (UPR), together with stricter fire safety requirements, promotes the development of environmentally friendly polymer composites. In this study, UPR synthesized from recycled post-consumer poly(ethylene terephthalate) was modified with casein particles to investigate their effects on thermal, thermomechanical, and fire-related properties. The obtained biocomposites were characterized by thermal analysis, thermomechanical testing, and flammability evaluation. Casein incorporation affected the degradation behaviour of UPR and promoted residue formation at elevated temperatures. Cone calorimetry demonstrated a reduction in peak heat release rate from 600 to 407 kW·m−2 and total heat release from 86 to 65 MJ·m−2 for the composite containing 15 wt.% casein. The maximum average rate of heat emission decreased from 345 to 255 kW·m−2, while combustion residue increased from 0.1 to 3.5%, confirming enhanced char formation. Fire performance also improved, with the Fire Potential Index increasing from 0.053 to 0.071 m2·s·kW−1 and the Fire Retardancy Index reaching 1.77 for UPR+C15. The heat deflection temperature increased from 56.9 to 71.3 °C, whereas the Vicat softening temperature decreased slightly. Overall, casein is a promising bio-based additive for tailoring the properties of sustainable UPR composites.