Tape (Peel) Adhesion Tips and Tricks:

NOTE: The following comments are the opinion of the author based on experience using the following equipment in both a laboratory and field environments. Any comments are welcome and can be forwarded to me at tswan@cci-inspection.com

There are two basic types of adhesion tests, Peel and Tensile. For additional information on Tensile Adhesion Testing, click on the Tensile Adhesion Testing Tips and Tricks on the menu.

The Tape (Peel) Adhesion Test will determine if the bond strength of a coating on a substrate is “Generally Adequate” and was designed specifically for use on steel substrates. Use on other substrates may not be consistent. It is not intended to determine high levels of adhesion.

Adhesion tests are DESTRUCTIVE and generally should only be conducted if a problem is suspected.

Tape Adhesion is addressed in ASTM D 3359, “Standard Test Methods for Measuring Adhesion by Tape Test” and ISO 2409, “Paints and varnishes - cross-cut test”.

There are two types of Tape Adhesion tests, Method A – X Cut and Method B- Cross Cut. ASTM D 3359 provides the following guidelines:

Coating Thickness Type Test Spacing

Under ISO 2409 for coatings 5-10 mils it allows for Method B with a 3 mil spacing.

WHEN IS ADHESION TESTING APPROPRIATE?

There are several good reasons for testing adhesion.

• There are questions about surface preparation, intercoat cleanliness or overcoat window times.
• When you want to overcoat an existing coating
• When there is a premature coatings failure and you want to determine the cause.

What Tape Should I USE?

When D3359 was developed, Permacel 99 tape was the specified tape. Permacel no longer certifies its tape to the original properties and ASTM no longer requires this specific tape, however, since no replacement is specified it is still largely used for this test. (Elcometer Part Number K0001539M001).

Any tape agreeable to all parties involved is acceptable but keep in mind Permacel 99 tape is closer to drafting tape than duct tape.

When performing the ISO test, the method requires ISO Tape be used (Elcometer Part Number K0001539M002).

How Accurate is this test?

There are many things that can affect the accuracy of the test and for that reason grading is done on a 0 to 5 scale to avoid the appearance of greater accuracy. Some of the items that can effect the accuracy of the test are:

• Type of Tape Used
• Is the coating the thickness it is supposed to be.
• How well the tape adheres to the coating
• How well the tape is applied to the coating
• Angle of pull of the tape
• Rate of Pull of the tape
• Squareness of knife to the surface when scoring
• NOTE: Flexible panels need to be well supported
• Sharpness of the blade
• Accuracy of the spacing
• Pressure applied during the cut
• Age of the tape
• Temperature and humidity during the test

When run properly and consistently, generally repeatable results can be achieved within one to two units.

Rating Area Removed

Now that you ran the tests and got numbers, what do you do with them. Part of the answer goes back to the reason the test was run in the first place.

Make sure you know what the coating thickness IS not just what it is SUPPOSED TO BE. If you use a 2 mm spacing on a coating reported to be 4 to 5 mils and it turns out to be 11 to 12 mils, it will probably fail the test.

Reason 1: We suspected a problem with the application:

There is no way to give an exact answer, but for a new coating system I would expect a 4 or 5 with 0, 1 or 2 indicating a possible problem. A 3 is a little more “iffy” to judge. Was it closer to a 4 or to a 2. Is the DFT in the proper range for the test. Is the break between coats or at the substrate.

This test is not a substitute for inspection or experience but is only one tool to be used in the evaluation of the application.

Reason 2: We need to overcoat an existing coating, will we have a problem.

Adhesion testing is only one of the criteria used to determine if a coating can be overcoated. As new coatings cure, they exert forces on the existing coatings. The greater the adhesiveness of the existing coating, the more curing stresses they can adsorb without failing. Generally with proper surface preparation, if all other factors, (condition of substrate, number of coats, current DFT) are within reason, then after running a test patch, conditions 3, 4 and 5 are probably OK to overcoat. If you are not sure, consult your coating supplier or a independent third party.

What is best – Single or Multi-blade?

For an X Cut you need a single blade and a straight edge. It is best if you have one that keeps the blade perpendicular to the score line.

You can also use a single blade for the crosshatch test, however, to do it accurately requires use of a cross-hatch template (Elcometer SP3000). The advantage of this method is box cutters are inexpensive and blades are readily available.

Multiple cut blades (Elcometer 107) ensure proper spacing and are much easier to use. They must be used on nonflexible, smooth surfaces to get a good cut with all blades. If all the blades are not cutting evenly, use a piece of smoked glass, (use a candle or match to blacken glass with soot) and lightly score through the smoke with the blade. All blades should tough the surface or the blade should be replaced. If the blade is good, check the item being tested for flexibility, flatness or smoothness. The blades for the Elcometer 107 have 4 cutting surfaces. Start in the No. 1 position and rotate until the No.4 blade is used.

Test Tips:

• Tape should be pulled at a 180^o Angle.
• Make sure blades are sharp (preferably new).
• Keep blades square to the surface.
• Cut in an as even a motion as possible.
• Make sure tape is new or has been stored properly.
• Make sure surface is clean and dry before applying tape.
• Rub tape onto surface using pencil eraser.
• Pull tape within 1 to 2 minutes; remember the bond increases with time.
• Run two more confirming test in same area.
• Keep the tape as a record of the test
• It is often easier to read the tape than the panel.
• For X-cut tests use a ruler to measure failure
• For X-Cut use a straight edge.
• Report Number of Tests, Mean, Range, where failure occurred, type of test used, environmental conditions, location of tests
• Keep a copy of ASTM D 3359 (or ISO 2409) available