Testing Adhesions

2/5/2013 10:00:00 AM

Article by David Jackson

Adhesion of the coatings to each other and to the wood surface is critical because even the most durable coatings will not hold up without proper adhesion. Testing to verify that the processes and finishes you use together have good adhesion is essential to provide not only an aesthetically pleasing but a durable finish to your customers.

There are many ways to test adhesion. Two of the most common are cross hatch adhesion and scrape adhesion which is similar to the common nickel scrape test.

Cross hatch adhesion testing is normally comprised of making six straight cuts and six perpendicular cuts to produce a cross hatch. Tape is applied and pressed down on the cross hatch area, then removed. This will determine if the coatings and substrate adhere to each other better than the topcoat to the tape. The by-the-book method is ASTM D3359. Specific tape produced for this test is required to meet the ASTM criteria and special cutting tools can be used to produce six cuts simultaneously at the prescribed width between cuts.

This test can be approximated using razor blades and masking tape. Just remember that your results may not correspond with the ASTM results and should be considered against a standard panel to determine any variance. Make six parallel cuts, approximately 2 mm apart, just deep enough to get through the coating and about 1 in. in length. Use a straight edge if necessary to keep lines parallel. Then make six more parallel cuts approximately 2 mm apart perpendicular to the original cuts about 1 in. long. This should produce a cross hatch area of twenty five squares, five by five. Brush off any chips or scrapings. Use approximately 3 in. of tape and rub firmly using a pencil eraser. Pull the tape off, going back across itself. The rating scale is based on 6 categories.

5B    0 percent removed
4B   less than 5 percent removed
3B    5-15 percent removed
2B    15-35 percent removed
1B   35-65 percent removed
0B   more than 65% removed

Keep in mind that by not following the ASTM method your results may vary. Still, this can be a useful test as it can show major variances in adhesion. Concern should be raised if adhesion gets down into the 2B or lower range, depending on the finishes used. Consult with your finish supplier for more information.

Another common adhesion test is a scrape adhesion test. In this test the surface of the finish is pressed hard enough to dent down to the substrate, and then the item used to make the dent is scraped along the wood for an inch or two creating a small gully. This test determines the finishes’ ability to adhere to the surface. The common instrument from the past to produce consistent results is the Model 1001 adhesion tester. However, a nickel in hand has been used countless times. Note: The adhesion tester Model 1001 is difficult to locate since the company that produced it has changed ownership and name. Please comment to this blog if you have information regarding the Model 1001 or a new version. Thank you.

Many variations of the nickel test are around with the nickel being held differently in different variations. My favorite is to hold the nickel between thumb and forefinger. Then press the nickel into the finish enough to cause a dent and move the nickel sideways with the grain, against the grain, across the grain one way and across the grain the other way forming a tic-tac-toe lattice. Each dented line created should be approximately 2 in. in length.

When using a nickel by hand there are many variables. Some include: the way the nickel is held; how worn is the nickel; how much pressure is used; and what angle is the nickel against the finished panel. To limit the variables the tests should be conducted using the same nickel, by the same person attempting to duplicate the process for each test.

Ideal results will show that the finish is flexible enough and has adhesion to not crack, whiten, or chip off. Consult with your finish supplier for more information.

These tests should be conducted on cured finish panels. Typically that means 10-14 days air dry, but may vary depending on your specific curing process and type of finish used. Consult with your finish supplier for appropriate cure times and conditions for test panels.

Based on many adhesion failure analyses, most adhesion failures would have been preventable if a test panel was finished to determine suitability of the processes and finishes used.