Surface-modified ammonium polyphosphate for flame-retardant and reinforced polyurethane composites

Abstract

Seeking a facile strategy to synthesize efficient, inexpensive and robust flame retardants which endow the polyurethane (PU) composites with desirable flame retardancy and mechanical property remains compelling but challenging. Here, we report the surface modification of commercial ammonium polyphosphate (APP) by 3-amino-1,2-propanediol in water solution to construct a novel organic–inorganic hybrid. The functionalized APP (FAPP) with multi-hydroxyl, which acts as both intumescent flame retardants and crosslinking agent, was covalently introduced into solvent-free PU composite via a thermo-curing process. Interestingly, FAPP exhibits good compatibility and strong interfacial interactions within PU matrix, which in turn significantly enhances the tensile strength (~40%) of the resultant PU composite, but without compromising the flexibility. With the incorporation of 15 wt% FAPP, the PU composite reaches a limiting oxygen index of 27.2%, and passes UL-94 V-0 rating. Meanwhile, the peak heat release rate and total heat release are respectively decreased by 64.5% and 37.8% over that of pristine PU, which is far superior to original APP. The high flame-retardant efficiency primarily results from the formation of compact and intumescent char layers in condensed phase. Accordingly, our practice may be extendable to the implementation in other high-performance flame-retardant polymer-matrix composites. • An eco-friendly and cheap organic–inorganic hybrid (FAPP) was synthesized in water solution. • FAPP was covalently conjugated into solvent-free PU by thermo-curing. • FAPP endows PU with excellent fire safety through promoting char formation. • FAPP reinforces the tensile strength of PU while maintains the extensibility.