Thermal stability and fire retardant performance of photo-oxidized nanocomposites of polypropylene-graft-maleic anhydride/clay

Polypropylene-graft-maleic anhydride (PPgMA)/montmorillonite nanocomposites have been prepared using direct melt intercalation techniques (extrusion and injection molding). Characterization of the nanocomposites, performed with X-ray diffraction and transmission electronic microscope techniques, suggested an importance of the mode of preparation and the type of organic modification that was used in the montmorillonite on the dispersability of clay in the polymer matrix. Injected mold nanocomposites were generally exfoliated in opposite to extruded samples that presented different structures (exfoliated, immiscible). Thermal degradation and fire behaviour of the corresponding nanocomposites displayed an improvement in comparison to pure PPgMA; this improvement was more important for injected mold specimen than for the extruded ones. It was ascribed to the presence of clay layers which act as diffusion barriers and reduced the mass loss; results that got the support of Limiting Oxygen Index measurements. A deterioration of these properties was observed on UV-irradiated nanocomposites. This was ascribed to the generation of weak points issue from the photo-oxidation reactions that take place mainly on the sample surface as shown in the oxidation profile. The oxygen starvation and reduced radical mobility, due to the presence of clay platelets, are blamed to be responsible for this oxidation profile. On the other hand, pure PPgMA showed an outstanding improvement of the fire behaviour with ageing time. Improvement that was attributed to cross-linking reactions which create a compact structure that is less inclined to volatilize.