Add chemical hardeners to ready-for-use, one-component white glues to achieve adhesives with a higher classification. For larger containers, the use of a classic drilling machine with standard mixer attachment has proven useful
Dispersion adhesives (white glues) have been used in workshops for more than 50 years. What has to be considered for a successful application? Edging specialist Ostermann answers the most frequently asked questions …
The Ostermann range comprises a large choice of different adhesives. The many joiners, carpenters and cabinetmakers in the technical sales consultancy are frequently confronted with all kinds of questions regarding practical application. Their expert knowledge, combined with close co-operation with suppliers like Henkel and Jowat, leaves no carpentry question unanswered.
White glues have been a classic in workshop applications. Still, there are some important details to be considered for the application
Why do bonding defects mostly occur?
There are several process steps that need to be considered in order to obtain a perfect result with dispersion glues. These are indicated on the corresponding technical data sheets. If the different steps are not properly executed, bonding defects may occur.
One mistake often made is to exceed the open time. If it is exceeded – for example, if the glue application on large workpieces takes too long – the bond between the two substrates will not be as strong. Another source of error is the minimum pressure time.
Although it is indicated on the technical data sheet of the particular product, it is also influenced by the type of wood and the pressing temperature. A longer pressing time is needed for hard-textured woods as well as for resinous or oleaginous woods.
If the boards the adhesive is applied to are too cold, there will be a so-called chalk effect. The drying adhesive is crumbly, there are white spots and and there is no bonding effect
How can the surrounding temperature lead to bonding defects?
The open time indicated on technical data sheets is subject to standardised conditions according to DIN EN 16556.
The required time is longer when the room temperature is low. With temperatures above 30°C, the open time is significantly shorter. A 10°C higher temperature reduces the open time by approximately one third.
If the boards the adhesive is applied to is too cold, there is a so-called chalk effect. The drying adhesive gets crumbly, there are white spots and there is no bonding effect.
Are dispersion glues water-resistant?
Dispersion glues are classified under standardised conditions into four different groups, according to their water-resistance (D1 to D4). D1 is used for up to 15% wood moisture, D2 for up to 18%, D3 for short-time influence of humidity (for bathroom furniture, for example) and D4 for frequent and longer influence of water (in swimming pools. for instance).
The standard classifications in Europe are D2 to D4. For applications in exposed exterior areas, use dispersion adhesives classified as D4. Still, the additional use of an adequate surface coating is indispensable in order to protect the furniture.
The dark spots on the surface of the wood have been caused by iron content within the adhesive
What is to be taken into account when using hardeners?
To get a homogenous mix, stir the dispersion and the hardener in smaller containers for at least five minutes. For larger containers, the proven method is to use a classic drilling machine with a standard mixer attachment. In the latter case, the stirring time is extended (a rule of thumb – for every 10kg, stir adhesive for 5-10 minutes).
Do not use too much power, though. It may destroy the glue mix and significantly reduce the pot life. For machine stirring, pay attention to the rotary direction of the drilling machine. Usually, it should be adjusted to clockwise rotation. For longer pot lives, always freshly stir the adhesive.
Why role does the pH value play?
Dispersion glues can change the colour of the wood. The reason for this is the acidity of the adhesives, which can lead to discolouration, especially with tannic woods.
White glues classified as D1 or D2 are pH-neutral and do not cause discolouration. Classic D3 dispersion glues are in the acidic range, with a pH value of 2.5-3.5. Some wood types are particularly susceptible to discolouration. When processing, for example, cherry tree or maple, but also thin veneers, the acidic pH value can cause colour changes in the wood.
If and how quickly a particular wood will discolour also depends both on the location and the region in which the natural wood product originates. Another factor is the exposure of the finished workpiece to heat and UV radiation.
Therefore, for approximately 10 years, there have been pH-neutral D3 glues with a pH value of 5.0-7.0.
Why must the adhesives not come into contact with iron?
If the acidic dispersion comes into contact with iron and is then applied to the wood, a classic blue discolouration in the wood will occur. Therefore, all components that come into contact with the adhesive should be made of stainless steel (V2A/V4A) or plastic. When using a drilling machine and a blender, the blender must not show any defects in the coating (no bare metal), otherwise discolourations might occur.