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The five common theories of adhesion are mechanical interlocking, diffusion, electronic, specific adhesion and covalent chemical bonding. The specific adhesion theory, which explains adhesion by intermolecular and interatomic forces between molecules and atoms, such as van der Waals bonds, hydrogen bonds and electrostatic interactions, is the theory most applicable to wood-polymer adhesion. The action of water on UF-cellulose bonds is firstly to displace the UF adhesive from adsorption sites on the cellulose, although water breaks down the amino-plastic bond within the UF adhesive itself. PF resins have a higher adhesion energy than water and are not as easily displaced from the cellulose adsorption sites. Because of this, wood composites bonded with PF resins are more water resistant than those containing UF resins.
Diffusion is not likely to be a major contributing factor to wood-polymer adhesion as thermosetting polymers have highly cross-linked chains and the cellulose has crystalline structures which will not diffuse with each other. However, in fibreboards made with natural lignin resins, pressed at high moisture contents, the glass transition temperature of the lignin decreases and interdiffusion between the lignin from different fibres probably plays a major role in holding the fibres together. Fibreboards bonded with lignin based adhesives are beyond the scope of this study.
MUF and PUF resins incorporate the better adhesive qualities and moisture resistance of MF and PF resins, with the cheap cost of UF resins.
MF impregnated paper laminates are commonly used as surface overlays for MDF. They are inert to most substances, do not stain, are extremely tough, can improve the sorption characteristics and have an almost unlimited range of colours. This thesis examined MDF in its raw form and none of the board samples used were laminated.