Soft Foam Polyurethane Recycling and Foaming in Reactive Extrusion Using Partial Aminolysis and a Latent Cross-Linking Chemistry

Abstract

Polyurethane (PU) foams are widely used as thermosets and yet poorly recycled due to their generally cross-linked nature and the difficulty in recovering their isolated monomers from chemical recycling processes. A mechanochemical recycling process was demonstrated in three steps in an internal mixer: partial aminolysis, cross-linking, and foaming. Partial aminolysis was successfully demonstrated using a cycloaliphatic diamine and yielded a melt-processable PU with gel content (GC) of 54.4% using as low as 6 wt % of diamine from a pristine PU with 94.1% GC. The material was then efficiently cross-linked by blending it with a blocked isocyanate based on 4,4′-methylenebis (phenyl isocyanate) at 100 °C and then triggering its reactivity at 160 °C to yield a material with GC of 84.7%. The blocked isocyanate unblocking kinetics were assessed using rheometry and revealed partial reactivity even at 100 °C. The mechanical properties of the cross-linked recycled PU exhibited significantly improved properties as a consequence of network reformation. Finally, the concomitant use of a chemical blowing agent with blocked isocyanate allowed to yield a cross-linked recycled PU foam as a final step of mechanical processing. The strong reactivity of the blocked isocyanate appeared to limit the foaming marginally. Finally, the cross-linking and foaming process was performed successfully in one step in a single-screw extruder. This study demonstrates the potential of mechanical and chemical hybrid processes as short-loop recycling processes for thermosets.