Our solution for taking curing shrinkage into account during bonding

The most common causes of stresses in bond lines during adhesive curing are the volume reduction of the adhesive due to curing and different thermal expansions of the adhesive and the joined parts or of two different joined parts. When designing adhesive bonds, the question of volume reduction, also known as curing shrinkage, is crucial. It is important to determine the material value and understand how much volume reduction actually causes stress.

 

Stress build-up during adhesive curing

The volume reduction of the adhesive during curing is linear with the conversion of the curing reaction. Therefore, the volume reduction is greatest at the beginning of the reaction, then decreases and asymptotically approaches a value.

 

The gel point

In the uncured state, the adhesive is a viscoelastic liquid—the viscous properties predominate over the solid properties; the loss modulus is greater than the storage modulus. When a force is applied to such a sample, the adhesive flows. It is not capable of building up stresses or transmitting significant forces.

This changes at the gel point. The sample has now reacted to such an extent that a loose spatial network has formed throughout the sample. After passing the gel point, the storage module is greater than the loss module. The solid properties now dominate the sample. If a force is applied to a viscoelastic solid, the body is deformed. A change in the volume of the adhesive now leads to stresses.

 

Consideration of the relaxation behavior

Another important property of viscoelastic solids that must be considered is relaxation. Movements at the molecular level enable viscoelastic solids to dissipate applied stresses. This relaxation capacity changes during the curing process. It decreases with increasing network density. Stresses that can occur due to a change in volume during adhesive curing are reduced by the relaxation capacity of the adhesive.

 

Induced stresses during thermal post-curing due to different thermal expansion coefficients

If the adhesive curing process includes a temperature change, stresses also occur in the bonded joint due to the different thermal expansion coefficients of the adhesive layer and the joined part. The temperature difference between the glass transition temperature and room temperature is decisive here, as virtually no stresses can build up above the glass transition temperature due to the temperature-dependent low modulus of elasticity and the instantaneous stress relaxation in the adhesive.

 

Possibility of determining the volume change

There are various ways of measuring the volume change during adhesive curing; for example, a mercury dilatometer can be used. The crucial question, however, is how large the proportion of the volume change is that leads to stresses in the adhesive joint.

The shrinkage bending test was developed to measure this proportion and the resulting stress. The measurement is taken indirectly via the deflection of a substrate coated with adhesive and application of linear beam theory. Since deflection can only be measured when the adhesive can transfer forces, this method only records the stress-effective shrinkage proportion.

In the case of thermal curing or thermal post-curing, the proportion of shrinkage based on different coefficients of thermal expansion can be separated from the volume change caused by curing. It may turn out that the different thermal expansion is the main contributor to the stress, while the contribution of the volume change caused by curing is small.

The question of the relevant volume reduction for stress formation in bond lines is complex. It is not the entire volume change of the adhesive that leads to stresses in the adhesive joint. Initially, the adhesive is still fluid and cannot transfer forces. If stresses are calculated based on the volume reduction after passing the gel point, the values will be too high because the relaxation capacity of the adhesive is not taken into account. The shrinkage bending test is an effective means of determining the contribution of volume change to stress. It can be seen that stresses during cooling after curing often have a greater influence than the stresses that arise during curing.

Our experts have extensive knowledge in this area and will be happy to advise you!