A model experiment was done to clear the formation mechanism of protective layers during combustion of polypropylene (PP)/organically modified montmorillonite (OMMT) nanocomposites. The investigation was focused on the effects of annealing temperature on the structural changes and protective layer formation. The decomposition of OMMT and degradation of PP/OMMT nanocomposites were characterized by means of thermogravimetric analysis (TGA). The structural evolution and composition change in the surface region of PP/OMMT nanocomposites during heating were monitored by means of X-ray photoelectron spectroscopy (XPS)

ATR-FTIR and field emission scanning electron microscopy (FESEM). The results showed that the formation of the carbonaceous silicate barrier in the surface region of PP/OMMT nanocomposites resulted from the following three processes: (1) The formation of strong acid sites on the MMT sheets

which could promote the degradation of PP and the carbonization of its degradation products; (2) The gases and gas bubbles formed by decomposition of the surfactant and degradation of PP

which pushed the molten sample to the surface; (3) The degradation of PP and the carbonization of the degradation products

which led to accumulation of MMT sheets tightly linked by the char in the surface region.