Decomposition mechanism of fire retarded ethylene vinyl acetate elastomer (EVA) containing aluminum trihydroxide and melamine

Abstract

The objective of this study is to evaluate the influence of aluminum trihydroxide (ATH) and melamine (MEL) mixture on the flame retardant and smoke release properties of ethylene vinyl acetate (vinyl acetate content of 60 wt%, EVA). The fire retardant properties were evaluated by mass loss calorimetry (MLC), limiting oxygen index (LOI) and UL-94. A home-made smoke test was developed to evaluate the smoke release of the material in a given scenario. It was found that the addition of ATH to EVA leads to improved fire retardant properties and reduced smoke release. Partial substitution of ATH by MEL at different ratios resulted in better results in MLC for a ratio of ATH:MEL of 5:1 and 10:1. Moreover, a lower quantity of smoke was released for EVA–ATH–MEL materials during material decomposition.A ratio of ATH:MEL of 5:1 was found to be the most promising. It was also shown that EVA–ATH–MEL ignited at shorter time in MLC test but longer ignition time in smoke test in comparison to EVA–ATH which is due to different test conditions. To understand the role of melamine on material properties, the degradation pathway of EVA–ATH–MEL was analyzed in details. The dispersion of the additives was examined by scanning electron microscopy (SEM) showing that all additives are homogenously dispersed in the matrix. In the next step the pyrolytic decomposition was investigated using thermogravimetric analysis (TGA) and pyrolysis-gas chromatography–mass spectrometry (py-GCMS). The decomposition of EVA–ATH–MEL was then analyzed in the gas phase using MLC coupled with a Fourier transform infrared spectrometer (MLC–FTIR) and in the condensed phase using solid state nuclear magnetic resonance (NMR) of 13C and 27Al. It was concluded that the additives protected the EVA both by a gas and condensed phase mechanism. The endothermic decomposition of ATH has a cooling effect and dilutes the fuel through release of water in the gas phase. Moreover, in the condensed phase, EVA–ATH–MEL is protected through formation of a char acting as barrier slowing down heat and mass transfer compared to EVA–ATH. Ignition at shorter times of EVA–ATH–MEL in comparison to EVA–ATH was found to be due to higher amount of flammable gases in the gas phase before ignition.