Sustainable CO2 Utilization as a Blowing Agent in Thermoset PHU Foam Production with Humidity-Responsive Shape Memory

Polyurethane (PU) foams are essential for energy-efficient insulation but are problematic due to the use of harmful isocyanates. Nonisocyanate polyurethanes (NIPUs) offer a safer, more sustainable alternative, aligning with EU regulations and climate goals. In this work, we report an eco-friendly method for producing thermosetting NIPU foams with tailored properties and humidity-responsive shape memory using supercritical CO₂ as a physical blowing agent. This innovative approach not only replaces current flammable, greenhouse-gas-emitting agents with high global warming potential but also revalorizes CO₂ in the manufacturing and synthesis process. The method involves CO₂ pressure-induced absorption, temperature-induced desorption, and curing of five-membered cyclic carbonate/amine resins. At elevated temperatures, simultaneous CO₂ release and NIPU cross-linking drive cellular structure formation. We studied the effects of curing agents, foaming/curing temperatures, and the impact of stabilizers on the final foam properties. The resulting foams demonstrated tunable densities (270–451 kg/m³), compression moduli (16–350 kPa), and cell sizes (0.33–0.99 mm). Notably, these NIPU foams also exhibited humidity-triggered shape memory behavior, which can greatly expand their functionality. This process ensures a controlled and sustainable approach to fabricating NIPU thermoset foams and represents a transformative step forward in the development of greener PU-based materials.