Teaching an Old Standard New Tricks
ASTM D 3359, commonly called the crosscut adhesion test, is one of the most used standards in the ASTM library. Despite the fact that the original standard is over 30 years old, this test method is still extensively used for the qualitative assessment of a coating’s adhesion in industries ranging from automotive to manufacturing.
In Method A, a razor blade is used to cut an X in the coating. In Method B, a multi-toothed blade is used to score parallel lines through the coating to the substrate. A second series of parallel lines are scored through the coating at 90° to the first set to create a cross-hatch grid pattern. After the cuts have been made using either method, a 3-in. strip of tape is applied to the grid and subsequently removed. This strip is then examined to estimate how much, if any, of the coating was removed from the substrate. For Method B, the tape is then compared to an X-cut classification chart to determine the coating’s classification rating (see Figure 1).
Tape BackgroundThe strength and consistency of the tape used can dramatically impact the outcome of the test. If a user performed the test with Scotch tape, a given coating would probably yield 5B (unless the coating was very weak). If duct tape was used to perform the test, however, it might yield 0B.
While certain applications may require a special tape, this is acceptable in the standard as long as the tape used is agreed upon between all interested parties. However, it is important to use the same tape for the life of the testing program. If a user were to change tapes midway through the program, the results obtained with the new tape could not be compared to previous tests.
Over the years, the tape that has been suggested for use with ASTM D 3359 has changed several times. One of the original tapes suggested for use with the standard was a 3M product that was later removed from the standard at 3M’s request. The company had informed the committee that its tapes were manufactured to a minimum adhesion value but could actually have a much higher strength. The lack of consistency in adhesion would make it a poor tape to use with the standard.
Some time later, ASTM was informed that Permacel’s P-99 tape was being used extensively by the testing community. Until its recent discontinuation by the manufacturer, this tape was suggested for use with the standard for many years.
Developing a SolutionA solution for current users of the standard has been developed by the SEMicro Divison of M.E. Taylor Engineering Inc. The company first obtained several samples of Permacel P-99. Although the adhesive strength of the tape was published in Permacel’s technical data sheet at 52 oz/in., the test method that obtained the published value and the dwell time of the tape were not listed. The dwell time is important because users of ASTM D 3359 are required to remove the tape in 90 ± 30 seconds from application. In many cases, pressure-sensitive adhesives become stronger the longer they are left applied.
The rolls of P-99 tape were sent to an independent third party, Chemsultants International of Mentor, Ohio, for testing. Chemsultants performed 10 different peel tests on the tape and reported an average strength of 61.0 oz/in., with a standard deviation of 1.3. (Note the difference from the published value of 52 oz/in., which was most likely due to the tests’ dwell time.)
The next step in the process involved contacting an adhesive formulator and requesting a product with a synthetic rubber adhesive similar to the Permacel product and with an adhesive strength of as close to 61.0 oz/in. as possible. After substantial research, it was determined that no off-the-shelf product had an adhesive value within 20% of the adhesive value of the Permacel product when used with a 90-second dwell time. A difference of 20% is unacceptable, as this could result in a different classification rating from the Permacel product for the same test.
SEMicro developed a new product, called CHT tape, to take the place of Permacel P-99. To keep the comparisons consistent, this tape was also sent to Chemsultants for testing. The results show that its adhesive strength average is 58.6 oz/in. with a 1.7 standard deviation, representing a 3.9% difference to the Permacel product and providing users with consistency for the life of their quality program.
The standard 60-yard rolls of Permacel P-99 were rarely fully used by testers before the shelf life of the product required disposal. (The effect of shelf life on the adhesion of the product cannot be overlooked; an expired roll was also sent to Chemsultants for testing and averaged 64.4 oz/in., with a standard deviation of 3.9.) In addition, the product included no liner and was wound on itself, so laps of the tape were required to be removed and discarded at the beginning of the testing day.
The new version of the tape is sold in booklets of 50 test strips that measure 3.5 in. A blank 0.5-in. tab on the end of each strip makes them easy to remove from the booklet, apply to the coating and subsequently remove without the user having to pick at the tape edge (see sidebar). The booklets are also lighter and easier to carry, less costly to ship, and easier to store since they take up less space. Each strip is on a liner, so no tape needs to be discarded and all of the tape can be used. The booklets are also about half the cost of a roll of tape and include a copy of the ASTM classification rating system.
Improved TestingAlthough ASTM D 3359 is qualitative in nature and not meant to be extremely accurate, it is still very popular in the adhesion testing community because it is fast and easy to perform. Since the test is used in some industries where consistent quality is important (e.g., automotive), a proper alternative must be available for those who had used Permacel’s P-99 product. The CHT product from the SEMicro Division was developed in a scientific manner and made many improvements along the way.
For more information, contact the SEMicro Division of M.E. Taylor Engineering Inc. at 15817 Crabbs Branch Way, Rockville, MD 20855; phone (301) 975-9798; fax (301) 975-9653; email firstname.lastname@example.org; or visit www.adhesiontesting.com.