The variety of wood species used in the production of Parquet and furniture is bewildering. Certain species are known to cause adhesion problems when UV coated. In the furniture and especially the parquet industry, where 95% or more, of pre-finished parquet is UV coated, this means that special attention must be given to a small number of problematic woods, that may only form a small proportion of the total production. To make matters worse, there is variation between batches and geographical sources of these woods. Further adding to confusion, there are multiple names for some species. Adhesion is normally measured by scotch tape crosshatch test, or by the key or coin test. The most important is generally the coin test, which is particularly revealing in the case of multiple layers of coating, typical in both of these industries.
Various reasons and mechanisms have been put forward to explain adhesion problems on wood.
• Oily residues within the wood exude to the surface before and after coating and curing. The residue at the interface of wood and coating forms a physical barrier. The UV curing process (IR is often 60% of output) heats the wood and also draws out oleophilic residue. • Wood, which has been stored at a controlled humidity can have water-soluble (hydrophilic) residues on the surface. These salts etc. could act as above.
• Apart from oils and salts, present in fairly large quantities and acting as a barrier, there is a bewildering
list of materials that are present in much smaller quantities. Each wood has it's own list of terpenes.
Acids, aldehydes, ketones, and alcohols are also present in many woods. Some of these (e.g.
quinones) could have a significant hindering effect on the UV reaction at the surface. This area alone could make a very interesting paper in it's own right.
• Some wood is very dense. That fact, combined with a tendency for the industry to use higher viscosity
coatings, means that there may be another physical problem in some cases. There may be very limited penetration of the surface; therefore there is reduced adhesion.
• Dimensional stability of wood varies enormously between species. Water content and seasonal
variation can lead to big changes.
All of these possibly contribute to varying degrees in individual cases.