Study on UV Aging of Thermoplastic Polyurethane and Its Crosslinked Product

To elucidate the formation of crosslinked products and their influence on material degradation, thermoplastic polyurethane (TPU) films were subjected to accelerated UV aging for various durations. Post-aging, the samples underwent Soxhlet extraction with tetrahydrofuran (THF), yielding an insoluble fraction-operationally defined as the crosslinked product-and a soluble uncrosslinked fraction. The mechanical properties, molecular weight distribution, swelling behavior, thermal properties, and chemical structure were analyzed. As UV aging progressed, both tensile strength and elongation at break deteriorated markedly. Concurrently, GPC analysis revealed a continuous decrease in molecular weight and a broadening of the molecular weight distribution, confirming that chain scission was the dominant degradation pathway. An insoluble network-like residue, defined as the crosslinked product, first appeared after 12 h of aging, with its content increasing to 22.9% after 300 h. Swelling tests showed that the crosslinked product had a high gel fraction, and its swelling ratio decreased from 196.8% to 157.4%, indicating the formation of a stable and increasingly dense network. DSC and TG results revealed restricted segmental motion, altered thermal transition behavior, and enhanced char-forming ability. The glass transition temperature of the crosslinked product exceeded that of the pristine TPU film. FTIR analysis showed variations in the -NH₂, C=O, C-O, and C-O-C bands, confirming structural evolution within both hard and soft segments. In summary, UV aging of TPU involves a complex interplay among chain scission, degradation of soft segments, rearrangement of hard segments, evolution of hydrogen bonds, and radical-induced crosslinking. Crucially, the crosslinked network formed during aging plays a pivotal role in determining the macroscopic structural, thermal, and mechanical properties of the polymer.