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KTA-TATOR, INC.
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Phone: 412.788.1300
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KTA Challenge #5
Answer - Knife Test


Photograph 1 - Challenge microphotograph at 200x	Photograph 2 - X-cut made through the coating for adhesion testing in accordance with ASTM D3359	Photograph 3 - Adhesion test area after application and removal of pressure sensitive tape (detached side of finish coat is facing up)

Challenge 5 (Photograph 1) is a digital microphotograph (200x, taken in the KTA laboratory) of one of the incisions made through a coating system when conducting an adhesion test in accordance with ASTM D3359, "Measuring Adhesion by Tape Test." The strip in the center of the microphotograph is where the knife blade penetrated a red primer to the steel. The gray on either side of the red is the finish coat. Portions of the finish coat disbonded cleanly from the primer when cutting, exposing the top surface of the red primer on both sides of the cut.

Two methods of adhesion testing are described in ASTM D3359: Method A, X-Cut Tape Test and Method B, Cross-Cut Tape Test. As shown in Photograph 2, Method A has been used. An "X" has been scribed through the coating to the substrate. The next step in the test procedure is to apply a special pressure sensitive tape to the scribe, rub it firmly with an eraser to assure that good contact has been made between the tape and the coating, then rapidly remove the tape by pulling it back on itself at an angle as close to 180º as possible. The amount of coating removed by the tape is evaluated using an ASTM rating scale from 0A (removal of coating beyond the "X") to 5A (no peeling or coating removal). The adhesion between coats in the Challenge is 0A (Photograph 3), as the coating was removed far beyond the area of the "X." This indicates that the adhesion between coats is very poor.

Tape testing is a quick, inexpensive method for evaluating the adhesion of coatings. One common purpose for conducting tape adhesion testing is for determining whether an existing coating is a candidate for overcoating. In the case of the coating system in the Challenge, if an overcoat had been applied, future detachment between the existing coats could be expected. The curing stresses and weight of the new coating alone could peel the existing finish from the primer. Even if the existing system withstood the stresses of the initial application, weathering exposure (wind, rain, freeze/thaw conditions) would inevitably lead to detachment in the future. Conducting a series of adhesion tests prior to overcoating can help to prevent such disastrous consequences.

KTA distributes Tape Adhesion Test Kits in addition to instruments used for other types of adhesion testing (e.g., tensile adhesion testing). The KTA laboratory or KTA field personnel can conduct adhesion tests for you, or if you choose to conduct the tests yourself, we can assist in interpreting the results.

This Challenge proved to be too difficult for anyone to guess correctly. Responses included an unfinished weld on the corner of a steel box, a freeze seal application, a coated panel held against a mirror, and exfoliated metal, but the far majority felt it was the scribe created by a Tooke Gage (a microscopic technique for measuring coating thickness that requires scribing the coating with a special cutting tip). Because there were no correct answers, everyone was eligible for the prize. The winner, who happened to be one of the Tooke Gage respondents, is John Schroeder. Mr. Schroeder works at High Steel Structures in Lancaster Pennsylvania.