Problems when gluing wood, causes and how to solve them

Problems with wood gluing are not necessarily the fault of the adhesive. There are many other factors to take into account, including storage and room temperature, wood substrate preparation, moisture content, amount of glue applied, setting time. The parameters to be respected as well as the way of working are always specified in the product data sheets and for a good quality of the glueing, they must all be taken into account. As I always say, no matter how good the products are, they don't work wonders. We spoke to the company's representatives Szolvegy from Târgu Mureș, manufacturer and distributor of adhesives TISZABOND, to see how glueing problems can occur even when using quality adhesives. So let's see how gluing is influenced by working parameters, wood qualities and environmental factors and what problems can occur when gluing wood.

PVA-based dispersion adhesives (polyvinyl acetate) have a liquid base component and finely divided solid components floating in the base component, together forming the dispersion. During the drying of the adhesive, when it transforms from liquid to solid, water is removed with the effect of bringing the granules together and then fusing, with film formation. This is how the setting is achieved and the process is irreversible (no return to the original liquid form). If this particle binding process is not complete, the adhesive will not provide the desired bond. Solidification of the adhesive only takes place under optimum conditions, which are always specified in the product data sheet.

Minimum film forming temperature

One of the most important parameters influencing bonding isminimum temperature required for gluing. PVA adhesives are water soluble, so water is the main liquid component. This type of adhesive freezes at temperatures below 0°C and the adhesive breaks down. Consequently, a temperature above 0°C is required for the adhesive film to solidify.

Minimum Film Forming Temperature (MFT) is the temperature at which the adhesive can still create a continuous film. At temperatures below this the film does not form. The result of drying will be a powdery, tackless, white coating that peels off the wood. The temperature at which this phenomenon occurs is also called "bleaching point". This information is always given in the product data sheet and is around 5°C.

There are various reasons why the adhesive film does not reach the required minimum temperature. Here are some of them:

• the adhesive has a low temperature, below the minimum film forming temperature. The reason may be that the adhesive is stored in an inappropriate place. For example, the storage room is not heated at the end of the shift (during the night) or on weekends. During this time the temperature drops very low. The temperature of the adhesive will also drop and at the start of the job, if it is not allowed to reach the required temperature (minimum film forming temperature), there will be problems with curing.
• the temperature of the wood material to be glued is too low. This may be because the wood stored in the open air or in a warehouse has not had sufficient time to reach the minimum temperature for the formation of a film before processing begins. In such glued pieces, because the entire cross-sectional area has not been heated, the centre of the gluing plane 'cools' below the minimum film forming temperature. Even if the grip on the edges will be good, only a powdery layer forms inside and the adhesive film loosens.
• reduced temperature in the workspace.

How to avoid problems caused by low temperatures. Eliminating the causes

Adhesive must be stored in heated premises.Cooling below the minimum film forming temperature should be avoided. This means that the adhesive should never be stored in places where the temperature is below 5°C, even if it is outside the actual working time (outside shifts, at weekends).

Before applying the adhesive wood material must be heated so that the temperature of the wood over the entire cross-sectional area exceeds the minimum temperature for film formation. It is recommended that the wood material be stored a few days before processing in suitably heated rooms and that the piles/stacks be so divided and arranged that the warm air from the heat source has access to as large an area of wood as possible. If the stacks are not properly opened the air will not reach the middle planks which will not heat up. Let's not forget that wood is a very good insulator.

The workshop where the gluing is done must be heated to a temperature compatible with the gluing process. Glues cannot be made in unheated rooms or where the heating has been on for a short time and the temperature inside is very low, preventing the wood and the adhesive from exceeding the minimum film forming temperature.

Removing water from the adhesive to form the film

Another parameter that influences the quality of the glue is the removal of water. The absorption of PVA adhesives in dispersion is achieved by evaporation and/or absorption of water into the wood pieces. Water removal from the adhesive is done in two ways:

By evaporation. The characteristics of adhesives and their working mode are usually set by manufacturers at 20°C and a relative air humidity of 50-60%. If the conditions of use in the factory differ from the above, the parameters of use of the adhesives (open time, release time, rest time, etc.) must also be changed. If the deviation from the optimum values is very large the times mentioned above are modified.

Increasing the temperature will lead to faster drying of the adhesive resulting in shorter processing times. On the contrary, if the temperature decreases the processing times increase. The variation of air humidity in the working area also influences water removal. If the humidity decreases, the adhesive dries faster and processing times decrease, and if the humidity increases, processing times increase. Temperature and air humidity variations are especially important in winter and summer because they reach values that affect the bonding process.

By diffusion of water into the wood. The moisture content of the wood influences the curing time of the adhesive. If the wood is drier, the water in the adhesive absorbs more quickly, while in wetter wood the water in the adhesive absorbs more slowly. If the wood is too dry, the glue will be absorbed deep into the wood, leaving an insufficient layer of glue on the gluing surface and a poor bond. And if the humidity of the wood is too high, the adhesive layer will dry more slowly and the technological times (press time, rest time) will increase greatly. In extreme cases, when the wood is very wet, curing may not even take place.

How to avoid problems caused by water disposal. Explanation of different behaviour

With the change in the external environment parameters (temperature, humidity) it is also necessary to modify the technological times accordingly. Especially during the change of seasons, in spring when there are big changes in temperature from one day to the next, or during hot summer periods.

The moisture content of the wood is very important because it significantly influences the workability, the gluability. Freshly cut wood contains a large amount of water, which must be removed before processing. This is done by drying. After drying, the wood will change its moisture level depending on the external environmental conditions, until it reaches an equilibrium with the external moisture in the given climate zone. So, for a given temperature and relative humidity, a certain level of wood moisture corresponds.

Wood should be dried according to its use, inside or outside. Wood used indoors (furniture, panelling, decorative items) should have moisture content in the range 8-12%, while wood used outdoors (exterior cladding, pergolas, fences) the 14-18% will dry out.

It is recommended that during drying a moisture balance is reached whose value is close to the moisture value during processing. This saves energy and greatly reduces subsequent dimensional changes and deformation of the finished product.

Another consequence of the variation of the element's humidity is its dimensional variation and sometimes, deformation.. Dimensional variation of wood is usually manifested by shrinkage or swelling. Because of these changes the solder is subject to additional stresses. In addition, wood is an anisotropic material with different properties depending on the direction along, tangential or perpendicular to the grain. This makes its variation different depending on the type of cut. Thus, pieces that look almost identical but are different in terms of cut (along, perpendicular or tangential to the grain) will have different changes when the moisture varies. The result will be that some pieces will curve and others will not, or the curvature will be different even if the pieces are identical.

Determining when the problem occurred is very important

In order to determine the causes of problems with wood gluing, it is very important to see whether the problem occurred during or after gluing, as well as the time that elapsed before it occurred. In some cases quality problems occur during or immediately after gluing. They are easier to notice and to fix. But if they occur days or even weeks later, the causes can be multiple and it can take time to pinpoint the exact factor that triggered the problem. The wood used may play a role. If the gluing is not done perfectly, following all the indications given by the manufacturer in the product data sheet, the dimensional variations of the wood caused by changes in external parameters generate additional pressures that stress the gluing and can lead to its destruction.

Szolvegy professionally addresses any problem that may arise in the wood gluing process. The company's specialists focus first and foremost on finding the cause so that problems can be eliminated and the risk of their recurrence removed altogether. The combination of the professionalism of the technicians, quality adhesives and a very good relationship with the partners, based on collaboration and honesty, make "for a secure bond use TISZABOND" adhesivesnot just a company slogan.