Bonding Experience: Tips on Adhesion
A well-adhered coating system is a lot like a good relationship: Both rely on a strong bond to avoid falling apart.
How well the layers of a coating system adhere to one another and to the substrate is the focus of "Adhesion Testing," the newest addition to the PaintSquare Video Learning Center series.
Created and narrated by experts at KTA-Tator Inc., the Video Learning series covers a wide range of topics in protective coatings work, from quality-control and health and safety, to basics on the proper use of coatings instruments.
From Lead Detection to Ventilation
Previous titles in the series include:
The series of short video segments includes "Corrosion Assessment."
Future topics in the series will include holiday detection, steel surfaces and moisture testing.
Each installment is available free in the PaintSquare Video Learning Center.
Measuring the Bond
In this video (4:24), KTA's William Corbett discusses how to properly perform adhesion testing. Such testing may be required after a coating system has been applied and cured—or, years later, to determine a system's condition or whether it can be successfully overcoated.
Adhesion testing measures the bond of the layers to one another and to the substrate, Corbett explains.
The inner strength of each coating layer, or cohesion strength, is also tested.
Because adhesion and cohesion are highly variable, there is no industry-wide standard for minimum adhesion or cohesion strength.
However, Corbett notes, project specification may require a minimum adhesion value.
Adhesion Testing Methods
Two types of adhesion can be assessed: shear adhesion and tensile, or pull-off, adhesion.
Adhesion testing may be required for a coating system after it has been applied and cured or several years later, to determine the coating's condition for overcoating.
Shear adhesion can be tested using special tape or a knife. These test methods are addressed in ASTM D3359, Standard Test Method for Measuring Adhesion by Tape Test, and ASTM D6677, Standard Test Method for Evaluating Adhesion by Knife.
Tensile adhesion testing requires an instrument with loading fixtures and is addressed in ASTM D4541, Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers.
Since adhesion tests are destructive, they should not be conducted unless required by the project specification, Corbett warns.