Surface characterisation of radiata pine high-temperature TMP fibres by X-ray photo-electron spectroscopy

In wood-based panel products, such as medium density fibreboard (MDF), fibre properties have a major influence on final panel performance. In particular, surface chemical properties of the fibres may critically influence bonding between the fibre and adhesive and so hold the key to improving bonding effectiveness. This study examined fibres of radiata pine (Pinus radiata D. Don) high temperature thermomechanical pulp (TMP, used for MDF manufacture. The surface compositions of commercial and pilot plant generated fibres were determined by X-ray photoelectron spectroscopy (XPS). XPS has been previously used to examine chemical pulp and its use was extended to investigate the surface composition of both normal high-temperature TMP and extracted high-temperature TMP fibres (i.e. fibres from which wood extractives and other compounds had been removed). In addition, the bulk chemical compositions of the high-temperature TMP fibres were obtained. For high-temperature TMP fibre, the surface composition was 23-58 % extractives, 31-44 % lignin, and 11-33 % wood polysaccharides. When these values are compared with those found in radiata pine sapwood (2 % extractives, 26-28 % lignin, and 64-66 % carbohydrate) it can be concluded that wood extractives are highly concentrated on the fibre surface. The presence of elevated levels of lignin on the surface of the fibres supports the theory that cleavage of adjacent tracheids during refining primarily occurs at the middle lamella-primary wall boundary. Extraction of high-temperature TMP fibre (either by an organic solvent or liquid CO2) removed the wood extractives and exposed underlying carbohydrate (cellulose and hemicellulose). Sequential extraction of high-temperature TMP fibre (first by an organic solvent and then with water) removed both wood extractives and water-soluble carbohydrate, leaving a surface which was approximately 60-80 % lignin. The presence of lignin and extractives on the fibre surface increases the surface hydrophobicity and is likely to influence bonding between the water-based UF adhesive and the fibre.