Influence of morphology on the flame retardancy of polystyrene/nylon-6/ammonium polyphosphate/clay blends

Ammonium polyphosphate (APP) and clay were utilized to flame retard polystyrene/nylon 6 (PS/PA6) blends. X-ray diffraction and transmission electron microscopy (TEM) results showed that clay formed exfoliated structures in PS/PA6/APP/clay. The results of Fourier transform infrared spectra and TEM indicated that APP and clay were exclusively dispersed in the PA6 phase. The influences of the distribution of APP and clay in PS/PA6 blends and the continuity of the (PA6 + APP) phase on flame retardancy were investigated. The flame-retardant properties were evaluated by limiting oxygen index (LOI), vertical flammability test, and cone calorimeter tests. For blends with a continuous PA6 + APP phase, the decrease of PA6 content caused an increase in LOI values from 26 to 34% and a remarkable reduction of the heat release rate. Results of thermogravimetric analysis indicated that the localization of APP and clay in the PA6 phase caused an increase in APP concentration in the PA6 phase with decreasing PA6 content, resulting in the rapid formation of intumescent charred layer, which augmented the flame retardancy. The transformation of (PA6 + APP) phase morphology from a continuous state to a discontinuous state at a PA6 content of below 40% (w/w) was observed. Meanwhile, the flame-retardant properties were decreased as the PA6 content decreases. The discontinuous intumescent charred layer thus formed and the inhibiting action of clay on the residue char from expanding could be responsible for the deterioration of flame-retardant properties, which were also confirmed by scanning electron microscopy and the aspect of intumescent charred layer.