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 H-1 and solid-s tate C-13 NMR to characterise the condensed-phase residues obtained upon th e degradation under inert and oxidative conditions of urethane-modified pol yisocyanurate foams based on polypropylene glycol (PPG) and 4,3'-diisocyana to diphenylenemethane (MDI). In particular, we examine the relationship bet ween chain mobility and volatile loss and relate this to the flammability o f these materials as characterised by limiting oxygen index (LOI) measureme nts. Differential scanning calorimetry (DSC), thermogravimetry (TGA) and py rolysis experiments reveal that the biggest difference in the behaviour of the foams is under inert rather than oxidative conditions. It is thus concl uded that the difference in the observed flammability of the samples derive s from differences in the volatile release profiles upon degradation in an essentially inert environment. Both DSC and high temperature H-1 NMR result s clearly indicate that there are two major scission processes occurring wi thin the polymers. The lower temperature process is due to the scission of the urethane links, whilst a higher temperature process that becomes increa singly significant as the isocyanurate content of the polymer increases, is due to the scission of the isocyanurate linkages. In addition, C-13 NMR da ta 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 conj unction of three factors, all associated with the evolution of PPG or PPG f ragments. 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 isocyanur ate linked foams restricts the diffusion of volatile species formed by and subsequent to the scission of any urethane bonds or the glycol backbone. (C ) 2000 Elsevier Science Ltd. All rights reserved.