The flammability of urethane-modified polyisocyanurates and its relationship to thermal degradation chemistry

In this paper we report the use of both in-situ solid-state 1H and solid-state 13C NMR to characterise the condensed-phase residues obtained upon the degradation under inert and oxidative conditions of urethane-modified polyisocyanurate foams based on polypropylene glycol (PPG) and 4,4′-diisocyanato diphenylenemethane (MDI). In particular, we examine the relationship between chain mobility and volatile loss and relate this to the flammability of these materials as characterised by limiting oxygen index (LOI) measurements. Differential scanning calorimetry (DSC), thermogravimetry (TGA) and pyrolysis experiments reveal that the biggest difference in the behaviour of the foams is under inert rather than oxidative conditions. It is thus concluded that the difference in the observed flammability of the samples derives from differences in the volatile release profiles upon degradation in an essentially inert environment. Both DSC and high temperature 1H NMR results clearly indicate that there are two major scission processes occurring within the polymers. The lower temperature process is due to the scission of the urethane links, whilst a higher temperature process that becomes increasingly significant as the isocyanurate content of the polymer increases, is due to the scission of the isocyanurate linkages. In addition, 13C NMR data on the residues clearly show that PPG is lost preferentially from those materials with the highest urethane:isocyanurate ratio. The different fire performance of the four foams under study here is thus ascribed to the conjunction of three factors, all associated with the evolution of PPG or PPG fragments. First, the lower thermal stability of the urethane links leads to facile depolymerisation to yield free PPG from those foams where urethane dominates over isocyanurate linkages. Second, the lower molar mass PPG from these foams is more volatile than that in the isocyanurate dominated foams. Third, the more rigid cross-linked network of the predominately isocyanurate linked foams restricts the diffusion of volatile species formed by and subsequent to the scission of any urethane bonds or the glycol backbone.