Adhesion, Part II.

Last month, I discussed adhesion from the standpoint of requirements for good adhesion and the causes of adhesion failures. This month, I am turning to adhesion testing. Unfortunately, there is no adhesion test that is truly adequate for measurements with coatings. Any test can tell bad adhesion from good, but no practical test of which I am aware has sufficient accuracy or precision to consistently distinguish between small to moderate differences in adhesion. Such capability is needed if continuous improvement in adhesion is to be accomplished. With care and craftiness, existing adhesion tests can be useful, but they must be coupled with results of field observations and performance testing (weathering, etc.) to get a true picture of adhesion.

Common methods include the tape test, scrape or knife adhesion, direct pull-off, peel strength, and various forms of impact testing. Many other tests exist, most of which are not practical for routine testing. The simplest and probably the most popular test is cutting and taping (ASTM D 3359, ISO 2409), where an X-cut or crosshatch (grid) pattern of cuts is made in the coating. Pressure-sensitive tape is applied over the cut area, and then pulled away, removing any material that does not adhere. The resistance (or the lack of it) to removal is the measure of adhesion. This test works very well for the ranking of panels prepared and tested at the same time, but absolute values from one series of tests to another never should be compared.

Scraping a coating is another way to measure adhesion. There is a simple qualitative test that involves taking a knife and scraping or picking at different painted areas to see how easy it is to remove the coating. A more quantitative test uses an instrument called a balanced beam tester (ASTM D 2197) to measure resistance to scraping with a weighted metal stylus or loop.

In a pull-off test (ASTM D 4541 and D 5179, ISO 4624), a cylindrical or hour glass-shaped plug or "dolly" (usually 0.5 to 2.0 cm in diameter) is glued to the surface of the coating on a panel. A portable device or a tensile tester is used to measure the force needed to pull the plug and coating away from the substrate or an underlying coating. Although this would appear to be an excellent test, interlaboratory testing by ASTM Committee D01 has shown otherwise. The precision is not good. Problems include failure of the glued joint, weakening of the coating by the glue or its solvents, the difficulty of doing an absolutely straight pull, etc.

A standard adhesion test for many materials is to measure the force that it takes to peel one layer from another. For coatings, it is necessary to place a piece of Teflon[R] tape or Kevlar[R] film at one end of the panel before the coating is applied. After application and cure, the tape or film is removed to leave a flap or tab. The tab is then pulled with a tensile tester at an angle of 90[degrees] or it is looped back over the panel and pulled parallel to the panel (180[degrees] testing) until the coating disbonds or breaks. Unfortunately, this test also has drawbacks as flaws in the film being pulled, whether it stretches or breaks and the pulling velocity all affect the results. Most peel strength data on coatings are measures of a combination of viscoelastic and adhesion properties.

Sometimes, the only really useful test is some form of impact testing. For example, observations of automotive coatings panels that have been chipped by being struck by stones or metal shot can provide valuable information about where adhesion failures occur and how bad they are. However, results also are dependent on the ability of the coating layers to absorb energy. If you want to reduce chip by improving the adhesion between layers and and/or changing the locus of failure, there is no adequate independent test to which you can turn.

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"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences, and/or photos of coatings defects to Cliff Schoff, c/o "Coatings Clinic," CoatingsTech, 492 Norristown Rd., Blue Bell, PA 19422; or email [e-mail].