What is Adhesion Testing?

Adhesion testing is used to determine the extent to which coatings and paintings are bonded to the surface measurement equipment to which they are applied. Failing to stick and adhere to the substrate may lead to serious operational problems that can result in economic losses.

So, what are the methods used to perform adhesion testing? Well, there are several recognised and useful methods for determining how well a painting or coating bonds with the substrate to which it is applied. Some of the common testing techniques are done using a knife or a pull-off adhesion tester.

It is important that after conducting the test to record whether the bond failure was adhesive (that is a failure at the surface of the substrate) or cohesive (meaning failure that occurs within the substrate or coating).

Adhesion Testing Techniques

Pull-Off Test

This testing is done through sticking a dolly or stub item to the wet film thickness material and then trying to pull it off using a pull-off adhesion tester. In this method, a load is usually applied in an increasing manner onto the surface until the stub or dolly gets pulled off.

The force needed to detach the dolly off or the amount of force the dolly is able to withstand is referred to as tensile strength. In this technique, failure will be realised along the weakest section of the adhesive, painting system, dolly, and substrate, and it will be visible through the fracture surface.

Key Advantages of the Pull-Off Test Method

• It is used with any type of coating on flat surfaces
• It determines the amount of pressure required to detach the dolly
• It uses tensile load rather than shear load
• The testing can be conducted on-site
• It is a quantitative testing method
• Applies ISO 4624 and ASTM D4541 standards of testing

Knife Test

This is an easy test and needs the use of a knife to pick at the painting. It determines if the adhesion of a painting/coating onto a substrate or a different painting (for multi-paint systems) is generally sufficient. How well the method works is determined by the level of difficulty when removing the painting/coating from the substrate as well as the size of the removed painting.

Tape Test

This method is commonly used to perform tests on metal substrate surface. In the technique, a pressure sensitive tape gets applied and detached or pulled off over cuts created in the painting or coating. The tape test is categorized into two; the X-cut tape test as well as the cross hatch tape test.

Two cuts are created into the coating and into the subtrate where they intersect to form an X. Then a tape is positioned at the middle of the meeting point of the cuts and then removed quickly. After that the X-cut section is checked for removal of the coating from the surface of the substrate and then recorded and rated.

Scrape Test

The method is performed using the balance-beam scrape-adhesion tester that is used to scrape the adhesive material from the surface of the substrate. A rounded stylus is used to apply pressure in an increasing way until the adhesive is scraped off.

Key Advantages of the Scrape Test Method

• It cannot be conducted on-site, particularly on-premises; it is only done in a laboratory setting.
• Applies ASTM D2197 standards of testing
• It may be utilized for lacquers, paints, varnishes, and coatings
• May only be done on flat surfaces
• It is a quantitative testing method

Cross-Cut Test

This method is achieved through cutting into the adhesive material (all through onto the substrate) in one of two patterns using a knife, while you apply a pressure-sensitive tape on top of the section, then you quickly pull it off.

Key Advantages of the Cross-Cut Test Method

• It is speedy in nature
• It is a qualitative testing method
• The results can be easily noticed as they are judged on a scale of 0 to 5.
• Can be done using a knife and a PSA tape which are easy to access
• Reliable as it is designed for paints and coatings

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