Formulation and Burning Behaviour of Fire Retardant Polyisoprene Rubbers

Abstract

This research aimed to develop new fire retardant rubber formulations, by surveying the existing knowledge base for fire retardant approaches for polyisoprene rubber, characterising unmodified compounds, formulating and studying fire retarded compounds for use within suspension and anti-vibration mounting systems. Materials have been prepared on a bench scale and evaluated for physical properties. Thermal decomposition has been studied using Thermogravimetric Analysis (TGA) in both air and nitrogen. Burning behaviour has been studied using a horizontal burning rate method, Limiting Oxygen Index (LOI) and the cone calorimeter.
Basic rubber mixtures were prepared to investigate the interactions between the polymer and additives, under TGA conditions. Zinc oxide was found to have little effect on the polymer decomposition, while silica reduced thermal stability. When decomposed in air, increasing levels of carbon black reduce the rate of mass loss in the polymer. Comparing formulations with different cross-linking types, sulphur without cross-linking increases the heat release in a cone calorimeter; but when low levels of sulphur form efficient cross-links, heat release is suppressed, as also occurs with organic peroxide cross-linking.
The effect of carbon black on burning behaviour was compared with inorganic fillers. Within the cone calorimeter, material containing carbon black formed a char-like residue which provides some reduction in the rate heat release, and did not contribute to the fuel load. The use of inorganic fillers yielded more rapid burning behaviour. Any level of carbon black addition gives a reduction in the rates of heat, smoke, CO, and CO2 release, confirming that carbon black had a stabilising effect.
Intumescent formulations were prepared using ammonium polyphosphate (APP), pentaerythritol and melamine, and separately using expandable graphite (EG). Within the cone calorimeter both systems yielded a significant reduction in the first peak of heat release rate, but a much higher second peak than for the unmodified compound. This second peak value is associated with the significant levels of intumescence observed. The use of EG gave a greater level of fire retardance compared to the APP formulation.
Hydrated fillers, and blends thereof, were investigated; an equal blend of aluminium hydroxide (ATH) and magnesium hydroxide (MH) was found to yield the lowest peak release rate on the cone calorimeter. Hydrated fillers were investigated with synergists proposed in the literature. Little benefit was noted for these additives when used as partial replacements for the filler.