Abstract:
This study evaluated the capability of ultrafine fly ash (untreated and physico-chemically modified) when utilized as filler in cis-1,4-polyisoprene rubber. Physico-chemical modification of the ultrafine coal fly ash was achieved using two techniques originally intended for aluminium extraction from coal fly ash. These involved (1) thermochemical treatment using ammonium sulphate (TCT-FA), followed by controlled aqueous dissolution (TCT-L) and (2) acid leaching by a sulphuric acid reflux method (SAL). In addition, chemical modification of ultrafine fly ash and physico-chemically modified (TCT-L and SAL) ultrafine fly ash using a silane coupling agent (Si-69) was investigated. The chemical and physical properties of the untreated and modified fly ash were characterized in order to effectively track the changes brought about by the chemical and physical modifications applied. Rubber composites containing fly ash, carbon black and fly ash-carbon black hybrids were prepared and their mechanical properties were studied in order to assess the viability of untreated and modified fly ash as filler in rubber. Several analytical techniques such as XRD, XRF, SEM, TGA, FTIR, BET, zeta potential and PSD analysis were utilised to characterise the fly ash samples and ICP-MS was used to determine the extraction efficiencies of the elements from fly ash during the chemical modification process. As evidenced by contact angle measurements silane treatment was only successful on the untreated coal fly ash (UFA) and not on the TCT and SAL sample.
The thermochemical treatment and aqueous dissolution treatment procedure was successful in increasing the surface area and surface roughness, and decreasing the particle-particle agglomeration of the fly ash sample. Sulphuric acid treatment decreased the particle-particle agglomeration, however, an increase in surface roughness was not observed via FE-SEM measurements. The increase in specific surface of this sample, as recorded by BET, might have been a result of increase in particle porosity instead. The zeta potential of the samples changed from -26.7 mV in untreated fly ash (UFA) to +16.3 mV and -0.5 mV for TCT-L and SAL, respectively. Silane treatment was only successful on the UFA and not on the TCT and SAL sample as evidenced by contact angle measurements. Incorporation of ultrafine fly ash in cis-1,4-polyisoprene vulcanizates resulted in marginal improvement in the cure and mechanical properties when compared to the neat rubber vulcanizate. The fly ash samples were not significantly reinforcing and the properties they imparted were inferior to the least reinforcing carbon black. Silane treatment of the fly ash improved the mechanical properties of the vulcanizates: in-situ silane treated samples produced better results than the pre-treated samples. TCT-filled vulcanizates performed better than the UFA- and SAL- filled vulcanizates.
