What is a pull-off adhesion test?

A more quantitative adhesion test is the pull-off test, in which a loading fixture (often called a dolly or peg) is secured to the coating with an adhesive. By using a portable pull-off adhesion Tester, such as the PosiTest AT, apply increasing loads until the cart is pulled. The force required to pull the cart or the force experienced by the cart yields the tensile strength in pounds per square inch (psi) or megapascals (MPa). Failure will occur along the weakest plane within the system consisting of dolly, adhesive, coating system and substrate and will be exposed through the fracture surface.

This test method maximizes tensile stress when compared to shear stress applied by other methods such as scratching or knife sticking and results may not be comparable. Also, pull-off strength measurements depend on the instrumentation used in the test. Results obtained with different equipment or with the same coating on substrates with different stiffnesses may not be comparable.

Portable pull-off adhesion Testers operate using mechanical (hand twist), hydraulic (oil) or pneumatic (air) pressure. Depending on their ability to ensure vertical pull-off force, they are classified as fixed-aligned or self-aligned. Good repeatability is obtained when the pull-off force is perpendicular to the surface being measured.

The availability of a full line of pull-off adhesion Tester models facilitates the measurement of coating adhesion on virtually any rigid substrate. For example, a 20mm dolly may be an excellent choice for typical coating bond strengths on metal, plastic and wood, while a 50mm dolly is better suited for lower bond strength coatings on masonry substrates such as concrete. Different trolley sizes are available to meet special measurement needs. For example, reducing the dolly size from 20 mm to 10 mm increased the gage's pull-off range by 4 times, allowing some manufacturers to accurately measure bond strengths in excess of 12,000 psi (82 MPa). Inexpensive carts eliminate the need for re-use, greatly simplifying the preparation process, with the added benefit of retaining physical samples for future reference or as evidence of pull-off test results.

The PosiTest AT Adhesion Tester is DeFelsko's solution for accurate coating adhesion testing. Pull-off adhesion Testers, available in manual (PosiTest AT-M) and automatic (PosiTest AT-A) models, measure the adhesion of coatings to metal, wood, concrete and other rigid substrates.