Silane coupling agents have been used traditionally in the past in the development of conventional polymer composites reinforced with glass fibres. Silane is a class of silicon hydride with a chemical formula SiH4. Silane coupling agents have the potential to reduce the incidence of hydroxyl groups in the fibre-matrix interface. In the presence of moisture, hydrolysable alkoxy groups result in the formation of silanols.
Silanols react with hydroxyl groups of the fibre, forming a stable, covalently bonded structure with the cell wall. As a result, the hydrocarbon chains provided by the reaction of the silane produce a cross-linked network due to covalent bonding between fibre and polymer matrix. This results in a hydrophobic surface in the fibre, which in turn increases the compatibility with the polymer matrix.
As mentioned earlier silane coupling agents have been effective for the treatment of glass fibres for the reinforcement of polypropylene. Silane coupling agents have also been found to be useful for the pre-treatment of natural fibres in the development of polymer composites. Wu et. al. demonstrated that wood fibre/polypropylene composites containing fibres pre-treated with a vinyl-tri methoxy silane significantly improved the tensile properties. It was discovered that the significant improvement in tensile properties was directly related to a strong interfacial bond caused by the acid/water condition used in the fibre pre-treatment .
In a study by Bengtsson et al. the use of silane technology in crosslinking polyethylene-wood flour composites was investigated . Composites of polyethylene with wood-flour were reacted in-situ with silanes using a twin screw extruder.
The composites showed improvements in toughness and creep properties and the likely explanation for this improvement was that part of the silane was grafted onto polyethylene and wood, which resulted in a cross-linked network structure in the polymer with chemical bonds occurring at the surface of wood. X-ray microanalysis showed that most of the silane was found within close proximity to the wood-flour.
It is known that silanes can interact with cellulose through either free radical or condensation reaction but also through covalent bonding by the reaction of silanol groups and free hydroxyl groups at the surface of wood, however the exact mechanism could not be ascertained. In a study by González et al, focused on the development of PLA based composites incorporating untreated and silane treated sisal and kraft cellulose fibres.
The tensile properties of the resulting composites did not present any major statistical difference between composites with untreated cellulose fibres and silane treated cellulose fibres, which suggested that silane treatment of the cellulose fibres did not contribute to further optimisation in the reinforcing affect of the cellulose fibres. The analysis of the high resolution C1s spectra (XPS) indicates that for C1 (C-C, C-H), the percentage of lignin in the intreated sisal fibres was higher, in comparison with kraft fibres. But after modification with silanes, the C1 signal decreases for sisal fibres, which shows that attempted grafting with the silane has resulted in removal of lignin and exposed further cellulose. The higher C1 signal reported for kraft fibres suggested some grafting with silane as a result of the contribution from the alkyl chain of the attached silanol, but no further characterisation was provided to support grafting of silanes to kraft fibres.