Surface characterisation and mechanisms of adhesion in wood fibre - polypropylene composites

The esterification reaction between wood fibres and maleated polypropylenes, and wood fibre-polypropylene interfacial bond strength were investigated. The modification techniques used in this study involved bulk treatment in a thermokinetic reactive processor and solution coating in xylene (solvent). X-ray Photoelectron spectroscopy and Fourier Transform Infrared Spectroscopy were used to characterise the wood fibres surface before and after treatment and to elucidate the nature of the link between wood fibres and maleated polypropylenes. Inverse Gas Chromatography, Wood Veneer Lap Shear Test and Wood Veneer Pull-out Test were used to estimate the interfacial bond strength between wood fibre and polypropylene matrix. Confocal Imaging System and Scanning Electron Microscopy were employed to monitor the morphology at the interface, dispersion and orientation of wood fibres in polypropylene matrix. Mechanical properties of wood fibre-polypropylene composites were determined. Although maleated polypropylene and bleached Kraft cellulose were found to chemically react by esterification in solution, no evidence of esterification reaction between lignocellulosic materials (Thermomechanical pulp, recycled newsprint, wood flour and ALCELL lignin) was observed either with bulk or solution treatment. The correlation between esterification results and mechanical properties of the composites indicates that there is no conclusive or direct evidence of the effect of ester links on the mechanical properties of the composites. The comparison of wood-polypropylene interfacial bond strength and mechanical properties of wood fibre-polypropylene composites shows that the dispersion and orientation of wood fibres in polypropylene matrix, mechanical interlocking of polypropylene with the irregularities of wood fibres and interdiffusion or entanglements of polypropylene chains at the interface following maleated polypropylene treatment are of direct importance to the mechanical properties of the composites. Although the dispersion forces are only of indirect significance in determining the strength properties of the composites, they are the required mechanisms for achieving a meaningful interfacial bond strength as it determines the wettability and penetration of wood fibres by polypropylene matrix.