Adhesion is the ability for a coating such as paints and varnishes to adhere to a substrate termed as ‘substrate adhesion’ or the ability for a coating to adhere to multiple coats in the system which is termed ‘inter-coat adhesion’. For coatings to perform satisfactorily, they must adhere to the substrate on which they are applied including metal, concrete, wood, and more.
In accordance with ASTM D4541/D7234, ISO 4624, and others, adhesion testers evaluate the adhesion (pull-off strength) of a coating by determining the greatest tensile pull-off force that it can withstand before detaching. Breaking points, demonstrated by fractured surfaces, occur along the weakest plane within the system consisting of the dolly (loading fixture, pull stub), glue, coating layer(s), and the substrate.
The PosiTest AT series measures the force required to pull a specified test diameter of coating away from its substrate using hydraulic pressure.
The goal of adhesion testing is to produce a coating failure. Coating adhesion is an indicator of how well the surface was prepared and how well the coating has bonded to the surface and/or additional coating layers. Testing adhesion offers a quantifiable method of determining if a paint or coating system is fit-for-purpose and ready to meet the quality demands of job specifications.
Basic steps for pull-off adhesion test:
Dolly and coating preparation — clean and abrade the dolly and the coating
Glue and dolly application — prepare the glue, apply it to the dolly, and place the dolly on the coated surface. Allow the glue to cure properly.
Test area isolation — isolate the test area of the coating from the area surrounding the dolly by cutting or drilling (optional).
Pull-off test — perform the test
Analysis of the test results — examine and evaluate the dolly and coating to determine the nature of the coating fracture
To learn more about the pull-off adhesion test procedure, watch the How to Measure Adhesion Using the PosiTest AT Adhesion Tester video:
Upon completion of a pull-off test, the dolly and coated surface should be examined. In addition to pull-off force, many National and International standards such as ASTM D4541 and ISO 4624 require the nature of fracture to be recorded.
Cohesive fracture:
fracture occurs within a coating layer (same coating on dolly face and coated surface).
Adhesive fracture:
fracture occurs at the interface between layers (coating on dolly face differs from surface).
Glue failure: visible separation of the glue from itself, the coating, or dolly (no coating visible on the dolly face).
The PosiTest AT Verifier is ideal for checking the accuracy and operation of adhesion testers and is an important component in fulfilling both ISO and in-house quality control requirements. The PosiTest AT Verifier and included documents allow the user to certify the accuracy of PosiTest Adhesion Testers.
For coatings to perform satisfactorily, they must adhere to the substrate on which they are applied. A variety of recognized methods can be used to determine how well a coating is bonded to the substrate. Commonly used measuring techniques are performed with mechanical adhesion testers or with a knife.
Review the brief overview of coating adhesion test methods below and find more information in our article, “Test Methods for Coating Adhesion”
A Tensile Pull-Off Adhesion Test is a more quantitative test where a loading fixture (dolly or stub) is affixed by an adhesive to a coating. A load is increasingly applied to a portable pull-off adhesion tester, such as the PosiTest AT series, until the dolly is pulled off of the surface. The force required to pull the dolly off, or the force the dolly withstood, yields the tensile strength in mega Pascals (MPa) or pounds per square inch (psi).
A Push Off Adhesion Test is similar to a pull-off test, except that the adhesion tester measures the force required to push the dolly (stub) off of the surface. Each dolly contains a center hole through which a pin is pushed using manual or hydraulic pressure until the dolly is pushed off of the surface.
A subjective test where an ‘X’ with a 30 to 45 angle between the legs is cut into a coating system using a utility knife and metal straight edge. The knife is then used to attempt to lift the coating from the substrate or another coating layer below. ASTM D6677 Standard Test Method for Evaluating Adhesion by Knife describes this test method.
ASTM D3359 describes two methods for testing adhesion by means of applying pressure-sensitive tape over the cuts made to the film. These tests were originally designed to test the adhesion of coatings to steel but have since been used on other hard substrates and even softer substrates like wood or plaster.
Method A (X-cut) for coating systems over 5 mils (125 µm)
Method B (Cross-cut) for coating systems less than 5 mils (125 µm)
The X-cut tape test is primarily intended for use at job sites. Using a sharp razor blade, scalpel, knife, or other cutting device — an X-cut is made through the coating down to the substrate. A steel or other hard metal straightedge is used to ensure straight cuts. Pressure-sensitive tape is placed on the center of the intersection of the cuts and then removed rapidly. The X-cut area is then inspected for removal of coating from the substrate or underlying coating and rated.
To read more about ASTM D3359 Method A and other adhesion test methods, find our article, “Test Methods for Coating Adhesion”
A cross hatch test (cross cut) is fast and simple to perform with an inexpensive cross-hatch cutter test kit. A lattice pattern is cut into the finish film down to the substrate, using a cross-hatch cutter. The test area is then brushed diagonally five times in each direction to remove any loose film finish particles.
A special tape for testing adhesion is then firmly applied over the cross-hatch test area and removed quickly by pulling the tape back off of the test area to reveal the amount of coating lifted off by the test tape. The cross-hatched test area is then visually compared to the ASTM D3359 standard.
To read more about ASTM D3359 Method B and other adhesion test methods, find our article, “Test Methods for Coating Adhesion”
The scrape test utilizes a weighted balance beam with a rounded stylus or loop attached to the beam. The painted panels are pushed underneath the stylus and the beam is loaded with increasing amounts of weight until the cured film is ‘scraped loose’ from the substrate. The adhesion of the film is reported in the weight of kilograms required to remove the film. Depending on the coating and the substrate to be tested, ASTM D2197, D2248, D2454, and D5178 standard test methods may apply.
This test method determines the tensile strength of concrete near to the prepared surface, which can be used as an indicator of the adequacy of surface preparation before applying a repair or an overlay material.
Refer to ASTM C1583 for a complete description of the test standard.
This standard covers procedures for assessing the adhesion of relatively ductile coating films to metallic substrates by applying and removing pressure-sensitive tape over the cuts made to the film.
Refer to ASTM D3359 for a complete description of the test standard.
This test method details procedures for evaluating the pull-off adhesion strength of coatings applied to concrete using portable pull-off adhesion testers.
Refer to ASTM D7234 for a complete description of the test standard.
This test method details procedures for evaluating the pull-off adhesion strength of coatings applied to metal substrates using portable pull-off adhesion testers.
Refer to ASTM D4541 for a complete description of the test standard.
The knife test is a test procedure for assessing the adhesion of paint and coatings to a substrate or another coating by use of a knife.
Refer to ASTM D6677 for a complete description of the test standard.
A test where a series of weights are added to a hardened stylus in 0.5 kg increments as the test sample is moved (scraped) against the stationary stylus.
Refer to ASTM D2197 for a complete description of the test standard.
An International Standard which describes methods for determining the adhesion of paint and other coatings to a substrate by use of a pull-off adhesion test.
Refer to ISO 4624 for a complete description of the test standard.
This European Standard specifies the methods for determining characteristics for adhesives used in internal and external installation of ceramic tiles. This standard calls for a 50mm x 50mm square dolly.
Refer to BS ES 12004 for a complete description of the test standard.
This European standard specifies a method for measuring the bond strength of concrete surfaces, concrete repairs, and overlay materials including grouts and mortars used in the repair of concrete.
Refer to BS EN 1542 for a complete description of the test standard.
