Morphological features and flammability of MDI/HMDI-based segmented polyurethanes containing 3-chloro-1,2-propanediol in the main chain

Abstract

The morphology and flammability behaviour of a series of segmented nanostructured polyurethane elastomers based on 4,4′-diphenylmethane diisocyanate (MDI)/1,6-hexamethylenediisocyanate (HMDI), polyoxypropylenediol and low-molecular chain extenders: 1,2-propanediol (PD) or 3-chloro-l,2-propanediol (CPD), have been investigated. The samples were characterised by gel permeation chromatography (GPC); morphology features were analysed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Limited Oxygen Index (LOI) and a thermovision camera (TC) have been used to investigate the burning process. It was found that introduction of 3-chloro-1,2-propanediol into the polymer structure changes both the morphology and the flammability behaviour. A possible mode of stabilising action of this internal flame retardant is discussed in terms of relatively rough structure of polyurethanes containing CPD that may play an important role by inhibiting polymer-specific condensed phase-reactions. Some cooling and fuel-diluting effects of the evaporated CO 2 or crystallized water may push the heat- and light-emitting zone of the flame further away from the pyrolysing polymer surface. This increased distance would cut down heat transfer. On the other hand, the rate of evolution of volatile products from the degrading polymer is probably controlled by diffusion and the primary products of degradation may undergo secondary reactions while they are retained in the hot zone.