Classification of adhesives according to moisture resistance

Adhesives are very important auxiliary materials in the wood industry. In contrast to varnishes and paints, which are visible and can be presented by spectacular examples, the qualities of adhesives are revealed when it comes to the durability of furniture and other wooden/wood-derived objects. This is why I believe that information about adhesives, their characteristics, how to choose them according to the environment where the glued object is to be used, is important.

Adhesives are natural or synthetic materials that create strong bonds between two pieces of the same or different materials. In the past, natural adhesives of animal or plant origin were used. I'm sure many people remember the bone glue that was made at Mar Marascans. I still remember the mountains of bones (and the smell) that I used to "admire" for 30 minutes while the train from Iași stopped in Mărășești station to change the locomotive. Bone glue is not the only natural adhesive. Fish glue, casein glue, animal blood glue and starch-based glues are just a few examples.

In the wood industry, adhesives have a variety of uses. They are used in:

• production of wood-based panels
• gluing furniture components
• veneering of panels
• production of solid wood panels and laminated wood

In all these areas, synthetic adhesives are used, natural ones having long been abandoned because they are unable to keep up with industry requirements.

There are various types of adhesives used for wood, the most important being ureo-formaldehyde (urelit), vinyl (polyvinyl acetate PVA), polyurethane, epoxy or chloroprene. The most important thing is the strength of the bond. This is determined by laboratory tests, and a good joint is one that does not split at the joint line but in the glued material (the wood cracks).

Adhesives are used to bond all kinds of objects, hence their characteristics. These objects may be used indoors or outdoors, they may be subject to different forces, they may be finished objects or objects to be processed. The choice of adhesive must be made according to these uses. But even within each category there are classifications. One of the most important is the adhesives' resistance to water and moisture, which is very important for wooden objects used outdoors or in damp environments. From this point of view adhesives are divided into 4 classes:

• D1 - for bonding softwood used indoors in non-moisture exposed environments;
• D2 - for bonding all types of wood used indoors in non-moisture exposed environments;
• D3 - for interior construction exposed to damp or damp, but short term exterior construction in mild climates. It is the adhesive used for joining steps, doors or windows in protected areas.
• D4 - for joints requiring increased moisture resistance. It is used for indoor joints in areas exposed to moisture (bathroom, kitchen) or outdoor joints in exposed areas (garden furniture). It is a very suitable adhesive for solid wood panels, laminated wood, doors and windows. [Details of a D4 adhesive]

As I said, the strength of these joints can be tested. There is laboratory equipment that subjects the joint in question to forces until the object fails. The breaking strength is measured in N/mmp and there is a minimum value for each class of adhesives set by international standards. When testing adhesive strength in wet conditions, the joint is kept in water for hours or even days before testing. The immersion time, strength level and test method are standardized.

Often, to find out the strength of joints, those interested do empirical but very conclusive tests. It's similar to coin tests to see adhesion of the varnish to the substrate. I was talking at one point with Mr Matefy Csaba from Szolvegy, distributor of Tiszabond adhesives, about how he tested an adhesive. It all started with a discussion about the situation in the wood industry and how they always have to find solutions to help manufacturers keep within their price range. Wanting to help out a partner, he thought of replacing the one-component adhesive they were using with a two-component one that was used in the past and was cheaper. But he needed some strength in the joint, so Mr. Csaba glued two pieces of wood together, took the board home and boiled it. He should have tested it after 3 hours in the water, but to be on the safe side he boiled it for 8 hours. The joint held together, it didn't fall apart, so he exchanged the product to his partner with a good heart.

The quality of bonding is not only about its resistance in different environments. There are many other requirements to be met, related to elasticity, transparency, sandability, formaldehyde emission or VOCs. But all this is covered in other articles.