Biobased, Reprocessable Non-isocyanate Polythiourethane Networks with Thionourethane and Disulfide Cross-Links: Comparison with Polyhydroxyurethane Network Analogues

Abstract

Here, we report the first study of the reprocessability and properties of non-isocyanate polythiourethane (NIPTU) networks in which the cross-links are of two types, thionourethane and disulfide, the latter obtained by auto-oxidation of pendant thiol groups. Our two NIPTU networks are biowaste based because starting materials can be derived from cashew nutshells or rice husks. We use dynamic mechanical analysis, tensile testing, and water sorption studies to compare our NIPTU networks with structurally analogous polyhydroxyurethane (PHU) networks, which are non-isocyanate polyurethanes (PUs). With significant advantages in reactivity, mechanical properties, and cross-link density, NIPTU networks can be favorable alternatives to PHU networks. Our biobased NIPTU networks also exhibit better water resistance, with a factor of ∼3 reductions in water sorption relative to their PHU analogues. Due to their dynamic covalent cross-links, our NIPTUs show excellent reprocessability with complete recovery of cross-link density after multiple reprocessing steps as well as potential as self-healing polymers. Although their creep viscosity activation energies differ by a factor of 2, both NIPTU networks exhibit excellent creep resistance up to 80–100 °C. Thus, with elevated-temperature creep resistance and significant advantages in reactivity, mechanical properties, and water resistance, in many applications, e.g., water-resistant coatings, biobased NIPTU networks are favored as non-isocyanate substitutes for PU networks.

Reprocessable, Bio-Based, Self-Blowing Non-Isocyanate Polyurethane Network Foams from Cashew Nutshell Liquid

“…We attempted to post-cure our materials at 120 °C for 4 h, but it only led to a marginal increase in cyclic carbonate conversion. Indeed, a previous study by Chen et al employing NC-514 to synthesize bulk PHU networks also shows incomplete conversion even after extended post-curing at 80 °C for 3 days . Nevertheless, swelling results show a sufficient cross-linking degree in the foam materials.…”

“…Indeed, a previous study by Chen et al employing NC-514 to synthesize bulk PHU networks also shows incomplete conversion even after extended post-curing at 80 °C for 3 days. 48 Nevertheless, swelling results show a sufficient cross-linking degree in the foam materials. With the successful syntheses of NC-514 foams in this study and nonbio-based foams in our previous study, 28 we have demonstrated that our rheological strategy applies in various situations and for different monomer structures to construct NIPU foams and that it shows promise for significantly accelerating the discovery of NIPU foam materials.…”

“…NC-514-CC was synthesized following the procedure described in ref . (See Figures S4 and S6 in the Supporting Information for more details.)…”

“…We observed increasing hysteresis loss with increasing thiol content (Table S4), consistent with previous works that reported more energy loss with more thiol linkages. 48 Furthermore, high hysteresis loss (>40%) signifies pronounced energy absorption, suitable for memory foam mattress applications, whereas low hysteresis loss (<30%) contributes a more resilient or "bouncy" feel to the foams. 61 Importantly, the tunability of hysteresis properties shows promise for NC-514 foams to be applied in different market segments of flexible PU foams.…”

“…63,64 Conversely, NIPU materials (both nonisocyanate PTU (NIPTU) and PHU) retain excellent reprocessing under a stoichiometric balance of monomers. 14,44,48 However, NIPTU networks have not been shown to provide for self-blowing foam production.…”