Effect of the interface structure on the morphology and the mechanical, thermal, and flammability properties of polypropylene/poly(phenylene ether)/magnesium hydroxide composites

In the present study the highly flammable nature of isotactic polypropylene (iPP) is suppressed by incorporating the inorganic flame retardant filler, magnesium hydroxide (MH), and the flame-retardant polymer, poly(2,6-dimethyl-1,4-phenylene ether) (PPE). In the iPP/PPE/MH composite, the MH is selectively dispersed in the PPE domain with the average domain size about 1.5 μm. However, upon addition of polystyrene-block-poly(ethylene-co-butylene)-block-polystyrene (SEBS), the MH moves to the SEBS phase located at the interface between iPP and PPE, whereas the domain size of PPE was drastically reduced to less than 0.3 μm and the domains tended to form aggregates. It was demonstrated that interface modification by dodecanoic acid as a surface treatment reagent for MH and by SEBS as a compatibilizer between iPP and PPE significantly improves the macroscopic mechanical and thermal properties of the composites in a synergetic manner. Cone calorimetry tests revealed that incorporation of PPE and SEBS drastically reduces the peak heat release rate and that PPE facilitates char formation, which serves as a physical barrier for heat flux from the flame to the polymer surface, as well as a diffusion barrier for gas transport to the flame. The idealized mechanism of flame retardancy is also proposed for the iPP/PPE/MH composites.