Recycling self-blowing non-isocyanate polyurethane (NIPU) foams

Abstract

CO2 self-blowing polyhydroxyurethane (PHU) foams are emerging alternatives to conventional polyurethane (PU) foams made of harmful isocyanates. It is thus crucial to assess their recyclability. Therefore, this master thesis aims to elaborate a foam-to-foam recycling methodology of end-of-life PHUs leveraging from their inherent reprocessability, even for thermosets thanks to the dynamicity of hydroxyurethane linkages, and using supercritical CO2 (scCO2) as physical blowing agent. Initial experiments highlighted significant challenges in reprocessing PHUs into thick films and their refoaming due to the high density and crosslinking density of reprocessed material. Thus, two strategies were developed to overcome these hurdles. First, grinded PHU foams have been exploited as fillers within PCL, a polymer easily foamed by scCO2. Such approach rendered PCL foams, with PHU acting as a structural reinforcement component. The second strategy focused on the engineering of new PHU formulations delivering samples featuring dual dynamic bonds (disulfide and hydroxyurethane). We postulated that the presence of disulfides, introduced within PHUs by utilizing biobased cystamine, should facilitate both the reprocessing of PHU foams into films as well as their scCO2 refoaming via temporary de-crosslinking. The dynamicity of the disulfide linkages was further studied by stress-relaxation studies which highlighted a correlation between the relaxation of the material and the temperature/cystamine content, alongside with the discovery of a water-dependent relaxation. However, while disulfide bonds facilitated the reprocessing of PHUs into films, their refoaming remained a major obstacle probably due to insufficient de crosslinking of PHUs during the impregnation and/or expansion step but also due to low CO2 solubility within the PHU matrix prior refoaming.