Adhesion / adhesive strength

Cross-cut and cross-cut adhesion tests

A cross-cut adhesion test is a method of determining the resistance of coatings to delamination from substrates. It is usually performed with a handheld device that allows the user to cut a grid pattern into the coatings to the point where the device meets the substrate. The user can then observe the properties of the coating to see how well it adheres to the substrate after cutting.

A cut pattern with no peeling or minimal disturbance of the colour or coating indicates that it is adhered to the substrate. The inspector can then apply and remove a pressure sensitive tape over the cuts to measure the extent of separation from the substrate.

ASTM standards for a cross-cut adhesion test

The ATSM standard for cross-cut adhesion testing is ASTM D3359. This is the standard test definition for evaluating adhesion by the tape test when testing films on metallic surfaces. Due to the grid-like cuts made in the films being tested, the test is sometimes referred to as a cross-hatch test.

There are two methods:

Method A, in which a lattice pattern is used, and

Method B, in which the cross-cut pattern is used.

Knife tests

In this simple test, the coating is scratched with a utility knife. It determines whether the adhesion of a coating to a substrate or to another coating (in the case of multi-layer systems) is generally sufficient. The performance is based on both the degree of difficulty in removing the coating from the substrate and the size of the coating removed.

Using the knife and the cutting guide, an X-cut is made through the coating down to the substrate. At the apex, the tip of the knife is used to lift the coating from the substrate or from the coating underneath.

This is a very subjective test, the value of which depends on the experience of the tester. A coating that has a high level of cohesive strength may appear to have poorer adhesion than a coating that is brittle and therefore breaks easily when tested. There is no known correlation to other adhesion tests (peel test, tape, etc.).

A standard method for the application and performance of this test is available in ASTM D6677-Standard Test Method for Evaluating Adhesion by Knife.

Pull-off adhesion tests

A more quantitative test of adhesion is the peel test, in which a loading device, usually called a dolly or stub, is attached to a coating with an adhesive.

Using a tensile strength tester, an increasing load is applied until the dolly is pulled off. The force required to pull the dolly off, or the force the dolly can withstand, gives the tensile strength in pounds per square inch (psi) or megapascals (MPa).