The questions in this article come from you. Those who posted them in the comments section at the end of the articles in the magazinedinlemn.ro received the answers at the time. They are questions about small mistakes made in production, whether in haste or carelessness, about unclear working procedures, special properties and even special uses of adhesives. And because not everyone goes through the questions part - I know this because I get the same question several times - I've collected some of them here. They're not all adhesive-related questions, I've picked the ones that I think will help you, especially those in production, but also some more specific questions. I hope you find this approach helpful.
How will the strength of the aracite solder be affected if I forget to add the hardener?
PVA adhesives in the D2 or D3 moisture resistance class can have hardener added up to 5%, depending on the manufacturer's specifications, making them more resistant to moisture. Without this catalyst, the bonding properties do not disappear and the adhesive is just as resistant to mechanical action but without moisture resistance. In other words, the bond strength of PVA adhesives will not be affected as long as the bonded objects are only used in dry indoor environments. The superior cross-linking given by the hardener gives the adhesive strength in damp indoor environments - bathroom, kitchen - or outdoors.
What is the lifetime of the adhesive in a laminated wood beam?
The adhesives used for laminated beams are polyurethane, melamine or phenol-resorcinol. A very strong bond line is formed after polymerisation. In practice, the result is a highly durable plastic material that is stronger than wood. The wood will certainly rot before the adhesive deteriorates. If the gluing is done correctly, the adhesive has absorbed sufficiently into the wood, all the gluing instructions have been followed, any problems will not arise from the adhesive. Further information about polyurethane adhesives can be found at here.
How long can the PVA adhesive stay open?
How long the manufacturer recommends. If you do not have this recommendation, make sure that the adhesive film is wet and continuous when the parts are in contact. This open phase time is influenced by the environmental conditions in the room where the bonding is done and the humidity of the wood. Working temperature, humidity in the atmosphere, air currents, very low or very high humidity wood can influence the open phase time. Great care should be taken with extreme conditions. In summer at very high temperatures and in winter at low temperatures, plows with special properties may be needed. An adhesive that can be used even at 0°C is, for example, TISZABOND D3 WINTER. It's hard to believe, but the glue was made specifically for working with it in halls where the temperature inside reaches 0°C in winter (see the video at the end).
What do I do if the aracite has thickened?
The adhesive thickens if the temperature in the work/storage area drops or if the curing reaction has started in the container for various reasons: out of warranty, very high storage temperature, unsealing of the packaging and exposure of the product to factors that trigger the reaction. If the temperature drop is within the limits recommended by the manufacturer, the adhesive is not affected and can be used. If the temperature reaches the freezing limit, the adhesive freezes and its properties are affected.
Although it is an aqueous dispersion, there are also aracite adhesives that can be used after repeated freeze-thaw cycles. It is possible that after such a cycle the viscosity of the adhesive will increase. It just needs to be mixed well and can be used without problems. About such an adhesive resistant to repeated freeze-thaw cycles I wrote here. It's a TISZABOND adhesive from the company Szolvegy from Târgu Mureș.
How is the strength of a solder tested?
The qualities of an adhesive are specified in the product data sheet and must comply with certain standards. For example, PVA adhesives used for furniture and non-structural fittings must meet the requirements of the European standard EN204. Resistance tests are also standardised. Adhesives are tested for tensile, bending or twisting. In all these cases, the wood must yield before the glue is applied for the glue to be declared good. There are also specific standards, such as IKEA's tougher than normal standards, where the wooden object (bed, chair, table, etc.) is tested so that it can demonstrate resistance to different cycles.
The adhesives used to obtain structural elements for houses or wooden structures (laminated beams, CLT) have much higher requirements. Standards and strength tests are specific to each type of adhesive used to make these elements (polyurethane, melamine, phenol-resorcinol, etc.).
How important is the moisture content of the wood in the case of ordinary gluing?
In the case of wood glue (aracite type), it is very important because the glue is absorbed into the wood. The higher the moisture content of the wood, the lower the absorption. This leads to a decrease in glue strength. If the wood is very dry, however, the absorption into the wood can be very high, decreasing the amount left on the surface or absorbing it completely in places where the deposited layer is thinner. In this case the result is also a decrease in the strength of the solder. The moisture content of wood used for interior projects should be in the range 8-12%, and for exterior projects around 14%.
However, there are adhesives, such as polyurethane or epoxy, that work even with high wood moisture.
Why is the elasticity of an adhesive important if I want a rigid bond?
Wood is a resilient material and a rigid adhesive can limit its movement. In addition, after gluing, some of the hardened adhesive remains on the surface and must be removed mechanically. For example, panels obtained by gluing lumber, they need to be sanded, calibrated, straightened or roughened. An elastic adhesive will be less aggressive to the grinding wheels, abrasive blades or cutting blades, prolonging their life.
Can a film of PVA adhesive, applied to the surface of a wooden object permanently placed outdoors, provide a protective water-repellent barrier in a mountain climate environment?
The film will not stand the test of time and will bleach. PVA D3 and D4 adhesives are moisture resistant when two pieces of wood are put together. The adhesive then has reduced direct contact with water, sun and weather. If the adhesive is applied to wood, the surface in contact with the outside is very large and the adhesive does not have the necessary characteristics to make it resistant to the action of environmental factors.
Sometimes, problems with sticking in production arise because of the speed at which work is done. Other times it is due to specific working conditions or product characteristics. To reduce them, it is advisable to maintain a continuous relationship with the adhesive supplier. An honest partnership will always lead to viable solutions and problem solving. I recommend watching the company interview below to see how important such collaboration is. Just one example: simply customising labels has led to the elimination of defects that have arisen from using the wrong adhesive.
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