Wood adhesives

Wood joints generally should be close contact with a gap of less than 0.15 mm, but so-called gap-filling adhesives satisfactorily bond up to 1.3 mm. Polyvinyl acetate (PVA) wood glues are widely used for most on-site work and in the factory assembly of mortice and tenon joints for doors, windows and furniture. The white emulsion sets to a colourless translucent thermoplastic film, giving a bond of similar strength to the timber itself, but insufficient for bonding load-bearing structural members. Components should be clamped in position for up to 12 hours to ensure maximum bonding, although increasing the temperature may reduce this. Waterproof PVA adhesives which partially cross-link on curing are suitable for protected external use but not immersion in water. PVA adhesives generally retain their strength up to 60°C and do not discolour the timber, except by contact with ferrous metals.

The thermosetting wood resins are mainly two-component systems based on phenolic compounds such as urea, melamine, resorcinol or phenol which cure with formaldehyde to produce load-bearing adhesives (BS EN 301: 1992). Most formulations require the mixing of the resin and hardener, but a premixed dry powder to which water is added is also available. Structural resin-based adhesives are designated for exterior (Type 1) or protected (Type 2) use. Melamine formaldehyde adhesives will not resist prolonged exposure to weathering. Urea formaldehyde adhesives are generally moisture-resistant or for interior use only. Certain timber fire-retardant and preservative treatments reduce the efficiency of timber adhesives, although generally those based on phenol formaldehyde/resorcinol formaldehyde are unaffected.

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