Heat-resistant Microporous Star-shaped Casting Polyurethane for Airless Electric Bicycle Tires

Airless tires are essential for enhancing the safety, reliability, and convenience of maintenance of electric bicycles. Polyurethane (PU) is considered a promising candidate for such applications owing to its versatile properties. However, their use is limited by insufficient heat resistance and excessive dynamic heat generation under cyclic loading. In this study, star-shaped trifunctional polypropylene glycerol (PPG3) was incorporated into conventional poly(tetramethylene glycol) (PTMG) and 4,4′-methylenediphenyl diisocyanate (MDI)-based systems to construct microporous star-shaped casting polyurethanes (SCPU), with water serving as a green foaming agent. Unlike conventional small-molecule trifunctional crosslinkers that create junctions within hard segment domains, PPG3 introduces long flexible arms between the hard segments, anchoring the crosslinking points at its molecular core. The large steric hindrance of PPG3 effectively suppresses soft segment crystallization and lowers the degree of microphase separation, whereas the crosslinked network restricts chain mobility, thereby reducing dynamic heat generation. These structural features also enhance the heat resistance, yielding a softening temperature of 183 °C, which is 30.9% higher than that of polyurethane without PPG3. When applied to airless tires by casting SCPU into rubber treads, the fabricated hybrid airless tires achieved a rolling distance of over 3000 km under a load of 65 kg at 25 km/h without structural failure, satisfying practical performance requirements. This strategy offers a simple, solvent-free, and environmentally friendly process, underscoring the potential of SCPU for scalable production of high-performance airless tires.