Synthesis and characterization of expandable graphite-poly(methyl methacrylate) composite particles and their application to flame retardation of rigid polyurethane foams

The expandable graphite (EG) is well proved to be a good intumescent flame retardant for rigid polyurethane foam (RPUF), however, as it is pulverized into fine particles (pEG) for the purpose of improving the mechanical properties of the foam composite, the flame-retardant properties of pEG-filled RPUF (pEG/RPUF) are deteriorated. To improve both the mechanical properties and flame-retardant performance of pEG/RPUF composite, the pEG particles were encapsulated with a layer of polymer, poly(methyl methacrylate) (PMMA). The Fourier transform infrared spectroscopy (FTIR) examination, thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) observation well demonstrated that the pEG-PMMA particles were successfully synthesized via emulsion polymerization, with 22.09 wt% PMMA. In contrast to the pEG, the addition of 10 wt% of pEG-PMMA particles into RPUF led to a considerable increase of the compressive strength and modulus and flame retardancy (limiting oxygen index, horizontal and vertical burning rates). The improvement of mechanical properties and flame-retardant behavior of pEG-PMMA particles filled RPUF was attributed to the desirable dispersion of pEG in PU matrix without destroying the integrality of the RPUF cell system, the good interfacial adhesion between PMMA and RPUF, and sealing the fine EG particles without losing oxidant, hence, to increase their expanded volume as exposed to fire.