Characterization of the surface and physical properties of South African coal fly ash modified by sodium lauryl sulphate (SLS) for applications in PVC composites

The application of coal fly ash as mineral filler in polymers is hampered by the aggregation of the ash particles and a lack of interaction between the ash and polymer. Therefore surface treatment is usually performed on mineral fillers to enhance workability and compatibility between the polymer and filler. In this study, a commercially available South African coal fly ash sample was surface modified under a variety of conditions with an anionic surfactant, sodium lauryl sulphate (SLS), with the aim to alter the surface properties of the ash. The properties of the modified products were compared to those of the untreated samples by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA-IR), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray fluorescence spectroscopy (XRF). A small shift of the SO stretch vibration in FTIR spectra of SLS treated fly ash is indicative of interaction between fly ash and SLS. Although the overall chemical composition of the SLS modified coal fly ash sample was not altered extensively, significant changes could be observed in its physical properties. Contact angle measurements indicated that the hydrophilic surface of untreated fly ash was rendered hydrophobic after SLS treatment. SEM and TEM results showed agglomerates on the surface of most of the fly ash spheres. Feasibility tests of using fly ash samples as filler in PVC indicated that SLS treated fly ash can successfully replace CaCO3 as filler in PVC under conditions of low filler loadings.