Reprocessing Thermoset Polyurethane Foams Using Their Residual Polymerization Catalysts
Subeen Kim, Lillian M. Felsenthal, Oliver Sala, Oliver Welz, Maike Bergeler, Alaaeddin Alsbaiee, William R. Dichtel
Abstract
Reprocessing thermoset polyurethane (PU) using carbamate exchange reactions is a novel approach to recycling PU foam waste. Previous works on PU reprocessing have required carbamate exchange catalysts to be added during the recycling process. However, adding new exchange catalysts increases processing costs, raises concerns about health or environmental damage from catalyst leaching, and would require the reformulation of established commercial products. However, PU foams contain residual synthesis catalysts that might also catalyze carbamate exchange such that existing commercial products might be reprocessable without further modification. We selected 1,4-diazabicyclo[2.2.2]octane (DABCO), the most widely used PU gelling catalyst, as a representative tertiary amine catalyst and evaluated its catalytic activity using model compound studies. Crossover experiments showed that the dominant mechanism of carbamate exchange varies, depending on the concentration of the free alcohols in the system: DABCO catalyzes both dissociative and associative carbamate exchange, but excess alcohol groups inhibit catalysis of DABCO. The catalytic activity of DABCO in the reprocessing environment was examined through crossover between linear polymers in a twin-screw extruder. Microcompounding accelerated bond exchange compared to the small molecule study due to the high shear force. Model thermoset PU films synthesized with DABCO displayed faster stress relaxation compared to a catalyst-free film. Three commercial thermosetting PU foams were successfully reprocessed into PU films at elevated temperatures using the remaining DABCO or other tertiary amine catalysts incorporated in the PU foams during synthesis. The resulting films maintained the glass transition temperature of the original foam, and the tensile properties of PU films resemble those of the feedstock PU foams (rigid or soft). Appliance PU foams were reprocessed while adding glass fibers to produce composites with a tunable elastic modulus. This study shows the promise of recycling thermosetting PU waste using residual catalysts in PU products without altering their commercial formulations.