A study of the flammability reduction mechanism of polystyrene-layered silicate nanocomposite: layered silicate reinforced carbonaceous char

Polymer-layered silicate nanocomposites (PLSNs) have been shown to have significantly reduced flammability. Based upon numerous studies to date, the proposed mechanism by which flammability reduction occurs is through formation of a silicate-reinforced char layer that protects the underlying polymer. To further understand this condensed phase phenomena, polystyrene (PS) PLSN was studied during various states of decomposition. The materials were exposed to non-oxidizing pyrolysis conditions using the National Institute of Standards and Technology (NIST) gasification apparatus. Samples were exposed for a range of pyrolysis times. The resulting partially decomposed PS PLSNs were analyzed by wide angle X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Sampling was done at different depths in each sample. The results show that as the PLSN was heated, the PS in the layered silicate galleries decomposed and volatilized, resulting in a collapse of the layer spacing, which yielded a stable silicate-carbon char layer. This layered silicate or clay appears to catalyze formation of carbonaceous char which yields a clay reinforced carbon char layer with a d-spacing of 1.3 nm; this unique structure forms quickly under pyrolysis conditions, and is stable even after extended pyrolysis times. TGA data suggest that there are two types of carbonaceous materials formed in the carbon phase of the clay-carbon char; a carbon that can be decomposed and volatilized by heating in nitrogen, and a second fraction of material which requires more aggressive conditions, that is, heating (>700 degrees C) in the presence of air. Copyright (C) 2006 John Wiley & Sons, Ltd.