Effects of shell thickness of polystyrene-encapsulated Mg(OH)2 on flammability and rheological properties of high-impact polystyrene composites

Polystyrene (PS)-encapsulated magnesium hydroxide (Mg(OH)(2)) particles with various shell thicknesses were successfully prepared using a method of in situ polymerization of styrene in a high-speed mixer. High-impact polystyrene (HIPS)/Mg(OH)(2) composites were prepared by melt blending. They were characterized using cone calorimetry, horizontal burning rate, rheology and scanning electron microscopy in order to investigate the effects of the shell thickness of the PS-encapsulated Mg(OH)(2) on the flame retardancy and rheological properties of the resulting composites. Rheological tests showed that the composites containing encapsulated Mg(OH)(2) had a stronger solid-like response at low frequency than that of the sample containing untreated Mg(OH)(2). However, with PS/Mg(OH)(2) ratio increasing up to 6.0 wt% and above, the dynamic viscosity, loss modulus and storage modulus of HIPS/Mg(OH)(2) composites decreased. The optimum PS/Mg(OH)(2) ratio, 4.5 wt%, was determined using a new 'crossover point' rheological method. The combustion tests showed that compared to the composites containing untreated Mg(OH)(2), the fire retardancy of the composites containing PS-encapsulated Mg(OH)(2) was improved significantly. Also, there appeared to be a critical PS/Mg(OH)(2) ratio, namely 6.0 wt%, for optimum flame-retarding properties. However with the continuous increase of PS/Mg(OH)(2) ratio, the fire resistance of the composites declined somewhat, which can be explained by acceleration of combustion of the composites due to the introduction of free PS chains of low molecular weight on the surface of Mg(OH)(2). (c) 2007 Society of Chemical Industry.