Halogen-free, low flammability polyurethanes derived from deoxybenzoin-based monomers

Abstract

Two novel deoxybenzoin-containing monomers, a diisocyanate and a diol, were synthesized and used to prepare polyurethane materials and foams. 4,4′-Diaminodeoxybenzoin was prepared by Ullmann coupling of 4,4′-dibromobenzil with tert-butyl carbamate. Reduction of one of the carbonyl groups, and t-Boc deprotection, gave the aromatic diamine, which was converted to 4,4′-deoxybenzoin diisocyanate (DBDI) with triphosgene. DBDI was then used to prepare two different polyurethanes, one by reaction with 1,3-propanediol, and the other by reaction with deoxybenzoin diol. The polyurethanes obtained from these reactions displayed low heat release capacity (HRC) values relative to conventional polyurethanes prepared from 4,4′-methyldiphenyl diisocyanate (MDI) and 2,4-toluene diisocyanate (TDI). Polyurethanes prepared in this study displayed HRC values as low as ∼130 J g⁻¹ K⁻¹, and char yields as high as ∼40%. In addition, deoxybenzoin-based oligomers containing hydroxyl end groups were also synthesized and used to prepare polyurethane foams that displayed markedly lower HRC values, and higher char yields, than conventional polyurethane foams, which burn readily and drip. Taken together, the integration of deoxybenzoin moieties into the diisocyanate or polyol component of the polymer structure is found to lower heat release rates and reduce flammability in the absence of any added flame retardant, such as halogenated compounds that represent health and environmental hazards.