A new mechanism of flame retardancy for polyurethanes

Abstract

A chemical model is outlined which gives a simple description of flame-retardant efficiency of organic bromides in flexible polyurethane foam systems. The mechanism suggests dehydrobromination by an interplay of SN1 and SN2 reactions in intramolecular and intermolecular processes. Evidence is presented to demonstrate that flame-retardant efficiency is directly related to the facility of dehydrohalogenation by an intramolecular process in which cyclic urethanes (five to ten membered rings) are formed by either SN1 and/or SN2 reactions. The requirement for a nucleophile, such as carbamate nitrogen, in halogen systems is suggested. In other mechanistic aspects, it was shown that flame retardancy passes through a maximum as a function of bromide content in the foam. It is believed that this behavior reflects certain undefined combustion aspects in the solid phase. It is also shown that unlike additive flame retardants, the TGA's of neither the starting bromine-containing alcohols nor their model N-phenylcarbamates correlate with the weight loss curves of the control foam. Instead, the TGA's of both flame retardant and nonflame-retarded foams from the reactive bromide alcohols all fall in about the decomposition range of the control foam. As a practical consequence of the results reported herein, it is now possible to correlate flame retardancy and structure as well as formulate new flame retardants with greater assurance of success.