Flame retardant mechanism of organo-bentonite in polypropylene

Effects of organo-bentonite on the flame retardancy, mechanical and rheological properties of intumescent flame retardant (IFR) polypropylene (PP) were investigated. The X-ray diffraction and transmission electron microscopy measurements indicated that the organo-clay particles were partially exfoliated. Thermogravimetric analysis showed that incorporating 1.8 mass% organo-clay into PP/IFR considerably enhanced the thermal stability and increased the char residue by ca. 7 mass% relative to the sample without organo-clay. Limiting oxygen index (LOI) tests revealed that incorporation of organo-clay led to a further increase in LOI value of intumescent retarded PP samples. The cone calorimeter measurements demonstrated that the organo-clay cannot only further reduce the first peak heat release rate (PHRR1), especially for the reduction in smoke level, but delay the time to the secondary combustion (PHRR2) of intumescent flame retardant PP samples relative to PP/IFR. Unlike the plainland-like structure of residue of PP/IFR, the char residue of PP/IFR/organo-bentonite displayed a folded structure or mountain terrain-like morphology which was due to the deposition of char residue generated by IFR on the surface of clay mineral layers. This interesting morphology of residue was responsible for the enhanced thermal stability, improved flame retardancy, and higher char yields. The rheological behavior of the flame retarded PP samples suggested that the organo-clay can strengthen the three-dimensional network structure formed by the IFR particles in the flame retarded samples. The presence of organo-clay can somewhat enhance the tensile strength and Izod impact strength of PP/IFR composites. Larger organo-clay amounts decreased the above mechanical properties.