Self-Blown Non-Isocyanate Polyurethane Foams with Disulfide-based Covalent Adaptable Networks: Enhanced Reprocessability And Adhesive Properties
Emeline Gillissen (1,3), Victor Lechuga-Islas (1,3), Maxime Bourguignon (1), Bruno Grignard (1,2), Christophe Detrembleur (1,3)
1) Center for Education and Research on Macromolecules (CERM), CESAM Research Unit, University of Liege, Sart-Tilman B6a, 4000 Liege, Belgium
2) FRITCO2T Platform, University of Liege, Sart-Tilman B6a, 4000 Liege, Belgium
3) WEL Research Institute, avenue Pasteur, 6, 1300 Wavre, Belgium
Conventional polyurethane (PU) foams are thermoset materials that are profusely exploited in our society. However, they are produced from toxic isocyanates and are difficult to recycle. CO2 self-blowing polyhydroxyurethane (PHU) foams represent a promising alternative. Their production from the polyaddition between polycyclic carbonates and polyamines enables the introduction of hydroxyurethane linkages, which are dynamic through transcarbamoylation reactions. This intrinsic dynamicity within a thermoset material characterizes PHU foams as covalent adaptable networks (CANs), a crucial asset for their recyclability. However, as transcarbamoylation reactions are slow and require high temperatures (~160 °C), initial hot-pressing tests remained challenging. Therefore, we elaborated new PHU foams that feature dual dynamic bonds, through the incorporation of dynamic disulfide bonds via the use of cystamine in the formulation. We postulated that these additional dynamic bonds would facilitate the reprocessing of the material through a momentary de-crosslinking under the heat stimulus. This was attested through the fast and easy (5 min, 120 °C, < 1 ton) reprocessing of cystamine-based PHU foams into films. The dynamicity of disulfide bonds was further studied through stress-relaxation experiments. This highlighted a clear correlation between the relaxation of the network and both the disulfide-bond concentration and the relaxation temperature. More importantly, this study enabled the discovery of a water-dependant relaxation of our cystamine-based samples, with significantly reduced relaxation times in presence of water. Finally, we established that our samples could be used as adhesives, particularly for stainless steel, with performance similar to commercially available glues, higher creep resistance, and unique reusability after breakage.