Influence of temperature on PVA adhesives and bond strength

All adhesives, regardless of their nature, are influenced by the environmental conditions in the workplace. Humidity, dust, temperature can influence the bonding, lowering its strength. Of all the factors, temperature has the strongest effects. Both the storage temperature and the temperature of the hall or the wood to be glued are important. PVA adhesives - aracite type - are among those on which temperature has a determining influence, both in the storage and in the working phase. Extreme temperatures result in poor quality gluing or loss of material.

What effects does temperature have on PVA adhesives

The influence of temperature extremes or large temperature variations is better understood by looking at the nature of these adhesives. PVA adhesives are aqueous dispersions. That is, they are reactive molecules (groups) that float in water far enough apart that they do not react with each other. When the distance decreases, the molecules move closer together and react (bond with each other), forming the adhesive with bonding properties.

The distance between groups is decreasing for several reasons, one of which is the decreasing amount of water. This is also where temperature comes in. When it is too low, the water starts to freeze, solid crystals form and the amount of liquid reduces. The reactive groups start to come together and react. When the temperature is too high (hot summer days), the result is the same. The water evaporates, the groups come together and bind.

In both cases the reaction is irreversible because another product is formed, different from the initial reactive groups, which no longer has the ability to react to bind the objects. The new product no longer breaks down when the amount of water increases again and so the adhesive is lost.

Storage and transport of adhesives

During winter and summer periods, the biggest problems with adhesives arise from transport and storage. Very high or very low temperatures often cause the reactive groups to react with each other, and the quality of the bond suffers.

Especially in winter, care should be taken when storing adhesives. Don't think that it's not -10°C in the warehouse, so the adhesive can't freeze. Ice crystals start to appear in the water as soon as the temperature approaches 0°C. The optimal temperature range for storage is between +5°C and +35°C. Problems can also occur during transport. If heated machines are not used, low temperatures can affect the adhesive's bonding properties. Especially if the adhesive stays overnight in the unheated machine.

Because such problems occur despite warnings about transport and storage, PVA adhesive manufacturers are trying to find solutions to avoid them. There are adhesives that withstand repeated freeze-thaw cycles, such as TISZABOND D3D adhesives distributed by the company Szolvegy from Târgu Mureș. The adhesives are so formulated that they can withstand 3-5 freeze-thaw cycles. After thawing, the viscosity is slightly increased and the adhesive has to be slightly agitated. Do not add more than 10% water as this decreases the adhesive properties.

PVA adhesives cannot withstand such freeze-thaw cycles indefinitely, so they must be transported and stored safely and freeze only accidentally. If the problem persists, there comes a point when the reaction is no longer reversible and the adhesive is totally compromised.

Temperature during work

During the work the temperature plays a very important role, both in obtaining a quality bond and in modification of pressing time. The temperature in the workroom is important, as is the temperature of the wood, glue or wood by-products being glued. All should have similar values, close to the values recommended by the manufacturers in the technical data sheets.

The optimal temperature range for the workroom is considered to be 18-20°C. Don't imagine that working at 15°C or 28°C will cause you problems. It is only the optimum temperature that all the properties in the data sheet refer to. In real life, the temperature in workshops can drop as low as 5°C or rise to 40°C. It is best to avoid gluing at such temperatures because problems with glue strength and bleaching of the glue line can occur.

Before you start gluing make sure that the wood and glue have been brought from storage and have reached ambient temperature. Cold wood brought into the heated hall may have a temporary increase in surface moisture due to moisture in the atmosphere condensing on contact with the wood. The phenomenon is similar to the condensation of moisture from the air on a glass of cold water in summer. It is another reason to wait for the wood to reach the temperature of the wood and thus rebalance its moisture.

The adhesive must also be brought to room temperature before work begins. Otherwise it will be more viscous and will penetrate less into the wood, reducing the strength of the glue. The tendency will be to dilute it, thus reducing the amount of active substance and increasing the open-phase evaporation time. Decreasing the amount of active substance leads to decreased glue strength.

Adhesive film formation and bond strength

An important parameter in the bonding process is minimum required temperature for gluing (MRT) and represents the temperature at which the adhesive can still form a continuous film. At lower temperatures, the film no longer forms and instead of a continuous film, a white powdery layer without adhesion will result. The temperature at which this phenomenon occurs is also called bleaching point andi is between 3 and 5°C.

Sometimes poor soldering due to failure to observe the minimum temperature is not noticeable. It looks like the film is formed and the soldering has taken place. However, if the glued parts are cut, the white dust formed in the wood is visible. When testing the breaking strength of the piece, it will be much lower than normal (min.10 N/mm²).

This seemingly good bond is a problem because it is hard to discover. That's why manufacturers try to lower the minimum temperature. TISZABOND 3011/D3D winter adhesive from Szolvegy is such an adhesive. The minimum film-forming temperature is 0°C thus avoiding weak adhesion at low temperatures.

Very high temperatures also create problems. If the temperature in the hall is very high, the adhesive forms a shallow puddle very quickly in the open phase. The water inside the film will evaporate more slowly and when the pieces are pressed together, the glue does not penetrate the wood well and the strength of the glue decreases. In very hot periods, in workshops where the temperature rises very high, a shorter open phase time is recommended and the adhesive should be applied to both gluing objects.

Regardless of the artifice and superior quality products that manufacturers think of, respect as much as possible the optimal working conditions. Because temperature also influences the finish, not just the adhesives, and it will be difficult to maintain high production quality.