The present invention relates to an adhesive, the use thereof, a method for producing the same, an engineered wood board, and a method for producing the engineered wood board.
Adhesive are a decisive constituent in the industrial production of a multitude of products, for example in the production of wood-based materials. Adhesives can be classified according to different criteria based on the chemical basis of the adhesives or their setting mechanism. Proceeding from the production method of the adhesives, a total of three superordinate classes of adhesives are defined: Polymerization adhesives, polyaddition adhesives, and polycondensation adhesives. Further divisions of the adhesives with respect to their physical and/or chemical properties, such as e.g. hot-melt adhesives or solvent adhesives, also are possible.
The polymerization adhesives are produced by reaction of monomers including a carbon-carbon double bond after activation. The activation of the starting materials can be effected by suitable catalysts or radicals and/or in the presence of radiation, for example UV radiation or electron beam. Typical polymerization adhesives are to be assigned e.g. to the group of acrylate adhesives.
A polycondensation adhesive, on the other hand, can be obtained by reaction of two monomer molecules by splitting off a simple molecule, such as water, acid or alcohol. The polymeric reaction product thus is present together with a by-product obtained during the reaction, so that corresponding measures are required during the processing of these adhesives. The most important polycondensates for use as adhesives include polyamides, polyesters and silicones or formaldehyde condensates, wherein reference should be made here in particular to the phenol-formaldehyde adhesives (PF), cresol/resorcinol-formaldehyde adhesives, urea-formaldehyde-resin adhesives (UF) or melamine-formaldehyde adhesives (MF).
The production of the polyaddition adhesive is based on the addition of various reactive monomer molecules by simultaneous migration of a hydrogen atom from the one component to the other component. Typical representatives include epoxy resin adhesives, polyurethanes or polycyanurates.
For the production of wood-based materials such as engineered wood boards, wood chipping products are coated with the adhesive and compressed to molded articles by applying pressure and temperature.
The type of adhesive used is substantially influenced by the size and quality of the wood fibers and/or wood chips used.
For example in the production of wood particle boards and wood fiber boards, such as e.g. MDF and HDF boards, which are produced from wood fibers in a dry process, there are frequently used polycondensation adhesives, in particular in the form of urea-formaldehyde resins. The particular advantage in the use of formaldehyde resins as adhesives consists in their high availability, the low costs as well as an easy manufacturability and handleability. Since the formaldehyde resins usually are produced by reaction with an excess of formaldehyde, these excesses of formaldehyde also are detectable in the intermediate and/or end products. Since formaldehyde, however, is classified as carcinogenic, the use of formaldehyde resins thus turns out to be disadvantageous in particular for the production of engineered wood boards for use indoors. In addition, the resins condensed-out have a low water stability.
In the production of boards from oriented wood chips, so-called OSB boards, on the other hand, polyaddition adhesives containing urethanes, e.g. on the basis of diphenyl methane diisocyanate adhesives, are used more and more. The complete chemical reaction without disturbing excesses and the high adhesive force are regarded as particular advantages of polyurethane adhesives. What is regarded as particularly disadvantageous, on the other hand, is the limited availability at a high price and the affinity to metal, so that metallic tools and plant sections must particularly be protected against direct contact. It is also disadvantageous that e.g. PMDI reacts already with the water from the room air humidity.
It is known that with a joint use of polycondensation adhesives and polyaddition adhesives incompatibilities will occur between the two adhesive systems, which will lead to comparatively poor technological properties. This problem is particularly pronounced at the interfaces of layers with different glue systems.