Influence of temperature on PVA adhesives and bond strength

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

What effects does temperature have on PVA adhesives

We can better understand the influence of extreme temperatures or large temperature variations by analyzing the nature of these adhesives. PVA adhesives are aqueous dispersions. This means that 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 (bind to each other) to form the adhesive with bonding properties.

The distance between clusters decreases for several reasons, one of which is the decreasing amount of water. This is where temperature comes in. When it's too low, the water starts to freeze, solid crystals form, and the amount of liquid decreases. The reactive groups begin to close together and end up reacting. When the temperature is too high (hot summer days), the result is the same. The water evaporates, the groups get closer and bind together.

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

Storage and transportation of adhesives

In winter and summer, the biggest problems with adhesives arise from transportation 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 you need to be careful when storing adhesives. Don't think that it's not -10°C in the storage, so the adhesive cannot freeze. Ice crystals start to form in the water as soon as the temperature approaches 0°C. The optimum temperature range for storage is between +5°C and +35°C. Transportation can also cause problems. Unless heated machines are used, low temperatures can affect the adhesive's bonding properties. Especially if the adhesive is left overnight in an unheated machine.

Because such problems occur despite warnings about transportation and storage, manufacturers of PVA adhesives 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 in Târgu Mureș. The adhesives are designed to withstand 3-5 freeze-thaw cycles. After thawing, the viscosity is slightly increased and the adhesive needs to be slightly agitated. Do not add water more than 10% because the adhesive properties decrease.

PVA adhesives cannot withstand such freeze-thaw cycles indefinitely, so they must be transported and stored safely and their freezing must be only accidental. If the problem persists, at some point the threshold is reached where the reaction is no longer reversible and the adhesive is totally compromised.

Temperature at work

While working, temperature plays a very important role, both in achieving a quality bond and in modification of pressing time. The temperature in the workroom is important, but also the temperature of the wood, adhesive or wood by-products that are glued. All should have similar values, close to the values recommended by the manufacturers in the data sheets.

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

Before you start gluing, make sure that the wood and glue have been brought from the warehouse to room 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 condensation of moisture in the air on a glass of cold water in summer. It is another reason to wait for the wood to reach room temperature and thus re-equilibrate its moisture.

The adhesive must also be brought to room temperature before starting work. Otherwise it will be more viscous and will penetrate less into the wood, decreasing the bond strength. 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 a decrease in the bond strength.

Adhesive film formation and bond strength

An important parameter in the bonding process is minimum required clamping temperature (MTCT) and is the temperature at which the adhesive can still form a continuous film. At lower temperatures, the film will no longer form and instead of a continuous film, a white powdery, tack-free coating will result. The temperature at which this occurs is also called bleaching point andi is between 3 and 5°C.

Sometimes poor bonding due to failure to observe the minimum temperature is not noticeable. It appears that the film is formed and the bonding has taken place. However, if the glued parts are sectioned, you will see the white powder formed in the wood. Testing the breaking strength of the piece, it will be much lower than normal (min. 10 N/mm²).

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

Very high temperatures also create problems. If the temperature in the hall is very high, the adhesive forms a surface 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 bond decreases. During very hot periods, in workshops where the temperature rises very high, it is recommended to have a shorter open-phase time and to apply the adhesive to both objects that are being glued together.

No matter what gimmicks and higher quality products manufacturers are thinking up, stick to optimal working conditions as much as possible. Because temperature also influences the finish, not just the adhesives, and it will be difficult to maintain high production quality.