Q: Concerning Evaluation of the Pull-Off Adhesion for Paints and Coatings. Currently, the crosscut test method is being used; however, this evaluation method is subjective, and results may be variable. Is there any better method for the result quantification?
A: ISO 4624 (2002) introduces Pull-off test method. With the pull-off method, the adhesion of the coating is quantified as a force value, making it possible to evaluate adhesion numerically.
The pull-off method is a test method in which a dedicated dolly: cylindrical measuring probe,is adhered to a flat painted plate with an adhesive, and after hardening, the dollyis pulled off to measure and quantify adhesion (the tensile force required to break). It is possible to quantify and evaluate adhesion, including those with the coating that would be classified as Class 0 (no peeling) in a crosscut test.
*If the coating adhesion is stronger than the staying force of the adhesive, the adhesion measurement is unable. The result will be that the coating adhesion is greater than the peak value of the test (= measured staying power).
This process: Pull-Off Adhesion evaluation Steps 1–3 below explain the basic test method. For conducting tests that comply with the specific standards, please follow the requirements stated in each standard.
● Step 1: Use an adhesive to attach the dolly to a flat painted board.
Apply the adhesive evenly to the bottom of the dolly and adhere to the paint. After adhering, leave it in a stable condition until the adhesive is fully hardened.
● Step 2: Cut the coating along the dolly
Once the adhesive has hardened, make a circular cut in the coating along the dolly. Make the cut, making sure that the cut coating reaches the painted board surface. NOTE: Do not apply lateral force to the dolly when cutting.
● Step 3: Pull up the dolly and measure the tensile force required to break
The dolly (fitted on the measuring shaft of the force gauge) is pulled up vertically, and the circular cut paint coating is peeled off to measure the coating adhesion strength (Pull-off Adhesion strength).
A force gauge is used to pull the dolly up vertically and measure the paint strength when the paint-coating breaks. Measurements are generally taken multiple times, and the average peak value is calculated. ISO 4624 (2002) recommends repeating the same test 6 or more times on the same sample. Measurement results are recorded as breaking force (N) or breaking strength (MPa)
Force Measuring Instrument: Measuring instrument used to quantify tensile / compressive, peel forces, etc. Used in combination with the Test Stand with the varieties of optional attachments as desktop testing equipment. In paints and coatings adhesion tests (pull-off method), if force values frequently exceed 200N, using a motorized test stand is recommended. Additionally, to accurately measure instantaneous peak values (maximum force values), force gauges require a high sampling rate.
This Pull-off Adhesion strength test method for the adhesion of the coatings is also applicable to the testing of the plating and other similar samples. Although the test requires the preparation of flat test samples and a motorized test stand, it is possible to evaluate subtle differences in adhesion that are not achieved from crosscut tests. Furthermore, for coatings that are all classified as Class 0 (no peeling) in the crosscut test, as long as the strength of the follow-on coating remains, adhesion can be quantified and evaluated to determine the merits.
This concludes the introduction to the Pull-Off Adhesion evaluation for Paints and Coatings, the evaluating methods for the adhesion of coatings. The pull-off method has the advantage for quantitative evaluation with numerical values, and the crosscut method is easier to implement. We hope this helps in considering which method is appropriate for the requirements.
There are some useful examples and case study videos introduced on the IMADA website based on coating adhesion tests related to ISO 4624 (2002). The Force Channel also provides useful information and other options on force measurements, including introductions to measurement examples and case studies. Please refer to the details for your reference.
