Surface modification of fly ash – characterisation and evaluation as reinforcing filler in polyisoprene

Fly ash, used in conjunction with bis(triethoxysilylpropyl)tetrasulphane (TESPT) as coupling agent, has been investigated as a reinforcing filler for polyisoprene compounds. The TESPT adsorption isotherm on silica showed that in forming a monolayer, the molecule assumes an inverted U conformation on the surface. With silica, half a monolayer of TESPT gave the maximum bound rubber formation. Fly ash particles were modified by acid and base treatment and characterised by electron microscopy and surface area determination. Little bound rubber formed on any of the modified fly ash particles, the green strength of compounds with 30 phr filler was poor, and limited reinforcement was obtained only with the acid–base modified material. This filler had a large surface area (146 m² g^-1 ), but even for this filler, TESPT addition led to little bound rubber formation, possibly because of the lack of hydroxy groups on the surface.