Biobased, reprocessable, self-blown non-isocyanate polyurethane foams: Influence of blowing agent structure and functionality

The ever-increasing scrutiny of polymer environmental issues calls for advancements in polymers derived from sustainable sources, which can include biobased precursors. However, biobased polymers often exhibit properties inferior to petrochemical-based materials. Here, we address this issue relevant in the development of polyhydroxyurethane (PHU) foams made from the tandem aminolysis-decarboxylation of dimer-acid-derived cyclic carbonates by probing the effects of thiol structure and functionality on foam properties. Incorporating difunctional, trifunctional, and tetrafunctional thiols, we observed that the foam morphology is independent of thiol structure. Using dynamic mechanical analysis, compressive testing, and hysteresis testing, we demonstrated enhancements of mechanical properties due to increase in cross-link density. Additionally, with the glass transition temperature well below room temperature, the foams are categorized as high-resiliency flexible foams. Then, we reprocessed the foams into bulk materials via hydroxyurethane dynamic chemistry, showing full recovery of cross-link density and demonstrating added sustainability and recyclability benefits of these materials. Moreover, the utility of the reprocessed films as dimensionally stable elastomers is demonstrated by the elevated-temperature creep arrest. Notably, cyclic carbonates derived from dimer acid also contain ester groups that can exchange during reprocessing. We performed stress relaxation experiments and demonstrated that transesterification plays a key role in tuning the relaxation phenomena. This study elucidates the modular control of PHU foam mechanical properties and demonstrates the tunable design and relaxation behaviors of biobased PHU foams.