Adhesion, essential to adequate coating

It is no secret that good adhesion is a basic requirement for taking full advantage of the functionalities of a coating or paint. After all, flaking paint layers needing to be repaired imply high costs for companies and governments. The adhesion between paint and surface is decisive for a high-quality finish.

Phenomena such as flaking occur only after a few years and it is very difficult to pinpoint the exact cause of this premature failure. This often results in a lengthy procedure to find out exactly what caused it: the quality of the paint, the quality of the pre-treatment, the conditions during the application, layer thickness, parameters of the drying process, etc.

Adhesion = interaction

The right choice of paint in function of the application is a crucial phase in a project. It is often based on prior knowledge from other assignments or is determined experimentally on the basis of the results from a preliminary study.

However, the selection of the paint does not guarantee a quality finish, because it is also affected by the interaction between the paint and the surface to be coated. And this interaction is in fact crucial in a properly applied coating process, while it is also affected by factors such as the presence of contaminants on the surface, the crystallinity of plastics, the oxide film deposited on metals ... 

A term commonly used to refer to the interaction at the substrate coating interface is the surface wetting or degree of wetting, or also the liquid film that can be formed by the paint on the surface to be coated.

Processes regularly used as preparation for a coating process in industrial applications include surface cleaning, pickling and passivation, blasting, surface activation by processes such as plasma and corona or the use of conversion or primer layers.


(Source : https://coatings.specialchem.com)

These processes often contribute to increasing the surface energy of a surface or to reducing the surface tension of the coating. The properties that have an effect on wetting can perfectly be analysed by measurements such as XRF, FTIR, surface energy measurements, contact angle measurements ...  However, these results can only be used to make a prediction and do not give a definite answer about the adhesive strength.

The role applying the coat correctly

Spray coatings are atomised into small droplets, each of which has its own interaction with the substrate, while in the case of squeegee coating, lock or roll coating, the coating is pressed onto the surface. It is therefore possible that a coating can be applied by means of a rolling process, but cannot be used in a spray application.

The conditions under which a coating is applied also affect the wetting and the adhesion of the coating layer. When working outdoors, the dew point or the point at which condensation forms must be taken into account. Care must also be taken to ensure that the times between preparation and lacquering and drying times are respected.

Measuring is knowing

Adhesion strength control is in many cases part of coating tests within a quality control system, but because of the destructive nature of the tests, they are often overlooked at the time of delivery. For large orders, the production of test panels onsite and supervision by a recognised coating inspector is certainly recommended.

There are several tests to check the adhesion, but make sure to be always alert during the performance of these tests, so you can make a correct assessment.

The cross-cut test or tape adhesion test is the most commonly used test because of its simplicity, but its evaluation is based on a relative scale that indicates an adhesion value from 0 to 5. The test is often performed incorrectly because no account is taken of the prescribed distance between the blades which determine the cutting pattern. This distance depends on the layer thickness of the total painting system. For a layer thickness of up to 60 µm a grid comb with 1 mm spacing between the parallel blades is used, for layer thickness 60-120 µm this is 2 mm and greater layer thicknesses of up to 250 µm require 3 mm spacing.

Also, the scratches have to be made all the way down to the substrate, which requires a lot of strength from the operator for thicker layers.

Furthermore, a standardised tape with a known adhesive strength should be used to try to peel off the coating. However, certain anti-stick, anti-graffiti and easy clean coatings keep the tape from adhering properly, resulting in a poorer transfer of forces which can lead to false positives.

The dolly pull-off test is a second way to test the adhesion of the coating. The test consists of applying a ‘dolly’ to the coating with a glue and then measuring the force required to pull the coating off the substrate with perpendicular traction (‘pull-off’). In this test it is important to choose the right type of glue to attach the dolly and also to make sure that the test surface is flat enough. The adhesion is expressed as a force per unit area.

Depending on the application, the adhesion will often be determined under the influence of a load, for example by means of an impact test, cupping test or flexibility test.

More information about these tests can be found here.

Conclusions

Good adhesion between substrate and coating system can only be achieved by selecting the right coating, using the correct application method and a good understanding of the interactions that contribute to a good adhesion.

Sirris Smart Coating Lab will be glad to guide you in the choice of the correct coating, reformulation of the coatings, demonstration of pre-treatment techniques such as plasma and corona treatment and the performance of wetting and adhesion tests to determine substrate-coating interactions.