Thermal degradation behavior and fire performance of halogen-free flame-retardant high impact polystyrene containing magnesium hydroxide and microencapsulated red phosphorus

The thermal degradation behavior and fire performance of a series of halogen-free flame-retardant high impact polystyrene (HIPS) composites containing magnesium hydroxide (MH) and microencapsulated red phosphorus (MRP) were studied by various means. It is shown that the oxidation of MRP has much influence on the thermal degradation behavior of the composite. This oxidative reaction releases heat and causes mass gain when the composite is degraded in air. No exothermic peak and mass gain peak appear in nitrogen under identical conditions. The HIPS composite containing both MH and MRP generate a smooth, compact, continuous and stable charred residue layer upon degradation in air and in flame. The residue is mainly composed of amorphous magnesium phosphate and black carbon. At the same loading level of 50 wt%, the composite filled by either MH or MRP individually burns out, drips vigorously and fails to pass the UL-94 test. However, the HIPS/MH/MRP composite self-extinguishes quickly and reaches the V-0 rating with ease. There is appreciable synergism between MH and MRP on the flame retardancy of HIPS. The combination of both flame retardants enhances the charring capacity and fire retardancy of the composite and decreases the loading of flame retardants remarkably. The flame retardant effect occurs mainly in the condensed phase.