How the drying time of aracite adhesives can be influenced

Whether it's adhesives for big industry or those used in carpentry workshops, the time the object stays in the press or clamped in the clamps is very important. It determines the entire production flow or the time in which an order can be delivered. That's why it's important to know what factors influence the reaction of adhesives and how to make the bonding as strong as possible. Although the generic term is 'air-bonded' or 'air-bonded' adhesives, the PVA adhesivesThey can be of several types and must be chosen according to the characteristics of the production, the existing equipment and the usual conditions in the factory or workshop.

Factors influencing the reaction time of polyvinyl acrylic PVA adhesives

PVA adhesives, also known as aracite adhesives, are aqueous dispersions. In order to bond, the water must evaporate and the molecules must come very close together and react to form the adhesive film. This is the theoretical explanation for the bonding. In real life, however, there are a number of variables that influence both the reaction speed of the adhesive and the strength of the bond. This is why the manufacturer gives an indicative drying time for the adhesive under certain environmental conditions in the hall or workshop where the bonding is carried out.

As it is about water evaporation, the factors that influence the soldering time are temperature, humidity and air circulation in the premises. Water will evaporate more quickly at high temperatures and low humidity and if the air is moving in the workspace. These are general rules for products with water as a solvent, and also apply to drying other materials (e.g. varnishes or paints).

With water as a solvent, the adhesive will be sensitive to low temperatures. Therefore it should not be transported or stored at temperatures below 5°C. Freezing leads to loss of quality and the adhesive will not hold. However, there are exceptions. Adhesive TISZABOND D3D produced and distributed by Szolvegy from Târgu Mureș, withstands repeated freeze-thaw cycles without losing its qualities.

Another factor influencing drying time is wood moisture or wood derivatives. The better the wood is dried, i.e. the moisture content is in the range 8-12% (for wood used indoors), the faster it will stick. Some of the water will be naturally absorbed into the wood which will cause the molecules to come together and trigger the polymerisation reaction that leads to the formation of the adhesive film. Another advantage of using a wood with low humidity is the strength of the glue between the two elements because, along with the water, the glue enters the wood.

Layer thickness deposited on the elements to be glued is another factor influencing the drying time. Water evaporates more slowly from a thick layer, prolonging the waiting time for the open phase of soldering.

Soldering process

Bonding is basically done in 3 stages:

• application of adhesive and open phase,
• the reaction itself, when the elements are in the press (or clamped),
• finalising the reaction.

Fopen area of soldering is the time between when the adhesive has already been applied and when the items are pressed or clamped. Water evaporates much faster at this stage and it is not recommended that immediately after the adhesive has been applied, the elements are put in the press as this would greatly prolong the reaction time.

The actual bonding takes place in the press, when the pieces are "forced" to stand next to each other. Much of the water has already been removed in the previous phase and now the reaction between the adhesive molecules takes place. When they are next to each other the reaction will start regardless of the conditions. But the process is accelerated by the temperature. Unlike cold pressing which can take up to 2 hours, the time needed for bonding at high temperatures is in the order of minutes.

In finalisation phase continuous reaction deep in the solder film. The reaction starts at the surface and continues inwards. When the parts are removed from the press, the film has formed between them, but if they are immediately subjected to testing or machining it is possible that the solder may come apart. Manufacturers therefore recommend that after removal from the press, the parts are left for a while before processing.

Special types of PVA adhesives

Because time is an important component in production, both manufacturers of adhesives and manufacturers of presses and other such machinery are trying to find solutions to reduce it as much as possible. This results in adhesives that need a shorter open phase time or have a faster reaction speed.

For a faster reaction there are hot presses or CIF high frequency current presses. Special adhesives must be used for CIF presses. Szolvegy recommends the following for those with such presses TISZABOND D2 Rapid 2150a PVA adhesive that forms the film in 5 minutes. The adhesive is recommended for bonding furniture items that do not stand in high humidity environments.

Short working time is important, but it should not take precedence over quality. As much as we want certain processes to be shortened, it must be understood that quality bonding requires, in addition to a good adhesive, compliance with the right technologies and the right conditions, both in the work area and in the storage area. The elements being bonded also influence the process. Their temperature and humidity are just as important as those of the working environment. Proper water evaporation in the open phase, for example, cannot be achieved if the wood is brought directly from unheated storage without the necessary acclimatisation time. If you want a shorter gluing time and resistant glues you have to act on elements that do not influence the final quality of the product.