Veneer plywood board (Multiplex), OSB, MDF and HDF boards, laminated board, chipboard.
Wood is a natural resource that is produced through the lignification of certain plants. It is primarily the trunk that is utilized, but also roots and branches can be further processed.
What does wood consist of?
Wood consists of cellulose, lignin and wood type specific ingredients (resins, fats, oils, wax, tannic acid and others). Wood is a hygroscopic material, which means that it is naturally able to absorb and release water as a result of changes in ambient humidity and temperature.
Which properties does wood have?
Due to its hygroscopic nature, wood shrinks or swells, contracts or expands as its moisture content changes. In short: it moves or "works". A special characteristic of wood is that it can swell and shrink to a different extent in all three dimensions (= anisotropy).
To minimize the movement of wood as much as possible, the following moisture values can be recommended based on many years of practical experience:
For outdoor applications 12-15 % wood moisture
For indoor applications 8-12 % wood moisture
During their installation, wooden materials should have the same moisture content as can be expected during the later use.
The structure and the ingredients of different wood species have a major influence on their workability and bondability. Of course, there are also wood species that are impossible or only difficult to bond. These include a great number of tropical woods as well as some European species (e.g. larch wood). Even within a single wood species, structure and ingredients can vary considerably. Among the influencing factors is the growing region, the dryness of the trunk and tree diseases.
Ingredients can cause discoloration when getting into contact with adhesives, paints and tools. If in doubt, it is therefore always recommended to carry out preliminary bonding tests.
Are the properties of wood changed by treatment?
There are different methods of wood modification through which either the visual appearance or the properties of the wood can be changed. Wood modification can be done physically or chemically and will invariably influence the bondability of the material. We therefore recommend using only high-quality adhesives and wood glues.
Here, you will find an overview of the most commonly used methods of wood modification:
The water absorption capacity of wood can also be reduced by thermal treatment. This additionally prevents infestation by insects and fungal attack. However, thermal treatment has a negative effect on the strength of the wood, thus limiting the number of possible applications. For the time being, no bonding recommendation can be given for this material.
Due to a chemical reaction between ammonia gas and wood compounds, fumed wood will have a darker colour. This can be a mere surface colouring, but also a wood fibre saturation to achieve greater colour intensity over the entire depth of the wood. Since this method may cause ammonia residues to remain in the wood, the bonding of fume-treated wood must be regarded as problematical. Ensuring sufficient airing time for the wood after fuming may have a positive effect on its bondability.
When buying hardwood or veneers, the wood supplier should be asked about the pre-treatment of the wood. Sometimes, wood is treated with steam in order to compensate for differences in colour. During this process, the natural wood ingredients may be concentrated near the surface and thus affect the material's bondability.
During this process, the wood (usually spruce or cedar) is treated with acetic acid. This reduces the water absorption capacity and the movement (swelling/shrinkage) of the wood. The influence of acetic acid residues on the bondability of wood is not known and long-term experience is still missing.
Other surface treatments
Wood glues have been developed for joining untreated wooden surfaces with each other. The properties warranted for these bonds are based on extensive testing and many years of practical experience. Any method of pre-treatment may influence the bondability of the wood. If in doubt, the user should carry out a sufficient number of preliminary bonding tests.