Recyclable biobased self-blown non-isocyanate polyurethane foams: Influence of blowing agent structure and concentration

Polyhydroxyurethanes (PHUs), made by the aminolysis of cyclic carbonates, are non-isocyanate polyurethane materials with promising potential as benign alternatives to isocyanate-based polyurethanes. Recently, thiols were shown to react with cyclic carbonates to liberate CO₂, presenting a simple pathway to produce self-blowing PHU foams. We recently developed a rheology-guided, rapid preparation method for self-blowing PHU foams, allowing for dramatic reduction of reaction times to achieve foams. This method unlocks the potential to employ less reactive monomers, such as biobased monomers that are important for sustainability. Here, we have studied the effects of blowing agent concentration and thiol functionality to understand the structure-property relationships in PHU foams. We demonstrate that morphology depends on thiol concentration but is independent of the thiol functionality. In contrast, compressive mechanical properties depend greatly on both concentration and functionality of blowing agents. To address the sustainability challenges of network foams, the PHU foams can be melt-reprocessed into bulk films with full recovery of crosslink density. We further elaborate the dynamic behaviors of these dynamic network foams via stress relaxation, observing tunable relaxation times and activation energies that depend on structure and concentration of the thiols. Lastly, we show excellent elevated-temperature creep resistance of the bulk PHU films, highlighting potential applications of the recycled foams as elastomers.