Performance of an intumescent-flame-retardant master batch synthesized by twin-screw reactive extrusion: effect of the polypropylene carrier resin

A novel way of preparing an intumescent flame retardant (IFR) and its master batch by the reactive extrusion of melamine phosphate and pentaerythritol, together with a polypropylene (PP) carrier, in a twin-screw extruder has been established. The effect of the PP carrier resin on the flame retardancy and water resistance of the intumescent-flame-retarding PP materials was investigated by using the limited oxygen index (LOI), UL94 test, elemental analysis (EA), thermogravimetric analysis (TGA) and water solubility testing, respectively. In addition, the mechanism of the carrier effect was analysed by melt flow index (MFI), scanning electron microscopy (SEM), and TGA. The experimental results show that with increasing content of carrier resin (PP) in the IFR master batch from 0 to 15 wt%, the flame retardancy LOI of the PP/IFR (25 wt% loading) blend increased from 29.5 to 32, and the UL94 level at 1.6mm thickness was improved from failure to a V-0 rating. In addition, the water resistance of a flame-retarded composite at 25 wt% IFR loading was greatly improved, i.e. after treatment with hot water at 70degreesC for 168 h, the reducing rate of element N in the IFR/PP blend decreased from 18.3% with 5 wt% carrier resin to 12.9% with 15 wt% carrier resin, and the LOI reduction rate decreased from 15.3% without any carrier resin to 10.9% with 15 wt% carrier resin, while the flame-retarded PP with an IFR master batch containing 10-15 wt% carrier resin maintained its UL94 V-0 rating at 3.2 mm thickness. The improvement in flame retardancy was attributed to the improved flame-retardant (FR) dispersion due to the carrier resin, while the improved water resistance was explained by the lowered water solubility of the IFR, the improved FR dispersion and the improved compatibility of the FR with the matrix resin due to the carrier resin. Reactive extrusion is a novel way to obtain pelletissed IFRs without powder pollution and their corresponding flame-retarding materials with a number of desired properties. (C) 2004 Society of Chemical Industry.