1. Field of the Invention
The present invention relates to the use of isocyanate-terminated prepolymers as wood adhesives.
2. Description of the Prior Art
Adhesives based on phenol- or resorcinol- or melamine formaldehyde condensates are known and are used to bond wood, in particular in highly loaded wooden structures with long lifetimes (e.g. EP-A 0 879 270). The reaction of the aqueous hardener mix with the melamine resin is a polycondensation reaction, in which water is released as the polymer is built up. In order for the reaction to be reproducible, the water content of the wood must therefore meet strict requirements.
In the case of highly loaded wooden structures such as load-bearing components, great demands are made on the mechanical strength of the components. For example, the bond should remain sufficiently strong even after many years of weathering.
The general usefulness of adhesives is determined by the fulfilment of standards, such as DIN EN 204, loading group D4 or WATT 91 (Wood Adhesives Temperature Test).
The bonding of wood with polyurethane adhesives both as a one-component and two-component system is known and is described in the journal “Ahäsion—kleben&dichten”, [Adhesion—bonding and sealing] 41, 1-2/97, pg.37–38 (1997). One-component systems are suitable in particular for wood substrates, as the natural moisture content of the wood ensures that sufficient water is available as a reactant. Thus, there is no need to check the water content precisely or, as is usual with many water-impermeable substrates, to mist the surface of the substrate with water to ensure a complete reaction.
However, a disadvantage of both one- and two-component systems is that when low viscosity polyisocyanates are used they migrate into the wood during the compression process. Although this produces a good bond between adhesive and wood, too great a degree of migration results in an uncontrolled shift in the NCO/OH ratio, which is important for the polymer structure. The resulting lack of polyisocyanate in the adhesive in the bonded joint may, in the extreme, lead to faulty bonds.
To ensure a sufficient concentration of adhesive in the bonded joint, higher viscosity systems can be used, but these may pose problems for application. Another possibility is to add fillers. Suitable fillers include organic fillers. Mineral fillers are less suitable because they have a strong tendency to form sediment and may produce increased abrasion in the complex pumping and dosing systems if fed automatically.
Wood dust, cellulose fibers, and also plastic fibers may be used as organic fillers. Plastic fibers in the form of microshort-fibers also act as a thixotropy additive. Particularly suitable fillers are dispersions produced in situ of polyureas and/or polyhydrazodicarbonamides in polyethers having hydroxyl groups, which are generally used to produce polyurethane foams with increased compressive stress values (e.g. DE-OS 25 13 815).
For flexible bonds with excellent cohesive strength, both one-component and two-component polyurethane adhesives based on organic polyisocyanates and dispersions of polymers in organic hydroxyl compounds are used, for example as described in DE-OS 27 19 720.
A substantial disadvantage of bonded wooden structures is the susceptibility of the adhesive to moisture, which weakens the adhesive layer, thus restricting the use of bonded wooden structures in continuously damp environments. The result of this is that extensive adherent failure can often be detected in the wood in its dry state, while cohesive failure is observed after lengthy immersion in water or in the boiling water test.
An object of the present invention is to provide a one-component wood adhesive, in particular for bonding load-bearing components, based on isocyanate-terminated prepolymers, which has improved water resistance.
It was surprisingly found that fillers based on polyaddition products of, for example, toluylene diisocyanates and hydrazine hydrate (polyhydrazodi-carboxylic acid amides) are beneficial to the wet resistance of wood. Since hydrazodicarboxylic acid amide structures are suitable for the formation of hydrogen bridges, a certain amount of hydrophilicity and thus poorer wet resistance would have been expected.