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November 26 - December 2, 2012

When should the adhesion of an applied coating or lining multi-coat system be tested?

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Selected Answers

From Manpreet Singh of Spiecapag Ausralia PTY LTD on December 17, 2012:
     If the client's specification calls for adhesion testing, the paint system should be simulated on a test specimen of the same material class,  100mm square and 6mm thick. ISO 4624 describes the method of performing the adhesion test. Acceptance criteria, unless specified by end user, shall be a minimum of 7MPA at 23  C, and at 65 C, no more than 40% decrease from pull off at 23  C.

From Bryant Chandler of Greenman-Pedersen, Inc. on November 28, 2012:
     Adhesion testing on coatings must be done after the proper cure time at temperature. This enables the coating to develop the full physical properties. If the coating is tested prematurely, the results will often times not meet the specified minimum requirement. The test may or may not be destructive, depending on 1) the thickness of the coating/substrate, and 2) whether or not the test is continued until coating disbondment. As called for in ASTM D 7234 (concrete testing), scoring around the dolly down to the substrate will definitely require a repair even if the test does not go to failure and stops a the minimum test value, or a thick coating system (> 30 - 40 mils) on a metallic substrate may require scoring if called for in the specification. If the test can be stopped at the minimum value specified without causing coating failure, than the dolly can be removed, often times by striking the dolly with a sharp blow from the side or carefully inserting a sharp 5:1 tool (putty knife) at the glue line and shearing off the dolly. Repairing the top surface may be required but is much better than haveing to repair the total coating system. When testing thermal sprayed coatings, always perform the adhesion tests prior to application of the seal coat. Tests performed after the seal coat application will result in test values that are 2 - 3 times the value of virgin thermal sprayed coating.

     Adhesion testing should be done to ensure the durability of the coating applied due to wear and tear of surroundings.

From Daniel Liu of APC on February 8, 2013:
     First, it will be up the specification to decide if an adhesion test is required, including the information of acceptance criteria and tester type. From my experience in the field of tank coating, this test is normally not required in the specification, because it is a destructive method. As you know, the more a repair area is made, the more weak point it creates. However, it is quite necessary to make a proper adhesion test recommended by the paint maker when the application has or may have deviated from the specification, such as passing the overcoating interval, lost control of humidity, wrong mixing ratio of plural component coatings, and so on. Adhesion is quite important for tank coatings that are to be be immersed in liquid. But it does not mean the whole coating system is conclusively qualified for service even if the adhesion test is passed. This is only a reference.

From Tom Swan of M-TEST on December 4, 2012:
     The one thing no one has mentioned is if the specification calls for an adhesion test, it should specify failure criteria as well as the pull tester used to test the coating. All pull testers pull at different rates, and when I discuss pull tests with most people, they have no idea what the pass / fail criteria is or which adhesion tester to use. If you want to use adhesion testing for pass/fail testing, the specifications should specify the minimum pull required and what tester to use. Also note, if you stop the test before it fails, there is no guarantee that the pull fixture will not take off the coating when you try to remove it,even if the coating did not fail, and it does not mean the adhesion or coating integrity was not affected by the pull.

From Karen Fischer of Amstar of WNY on November 28, 2012:
     Adhesion testing should be performed for one of two basic reasons: (1) if the specification calls for it as a qualifying test for acceptance of the coating system, and (2) if there is a failure or suspected failure in the coating system (material and/or methods) that cannot be evaluated (or fully evaluated) by non-destructive methods. One must always keep in mind that an adhesion test is a destructive test and that the resulting test area now becomes a repair that could, itself, fail. This is especially important to keep in mind in the case of linings or any system that will be under immersion service, mechanical service or a chemical/harsh environmental exposure. Since it may be necessary to perform adhesion testing in multiple areas (depending on the size of the suspected areas), there will be multiple repairs. Destructive testing should always be the last, not first method employed when evaluating for suspected or obvious failure.

From Atanas Cholakov of ACT on January 21, 2013:
     Adhesion should be tested after the complete cure of the coating system. Info on curing can be acquired from the paint supplier's tech representative.  In the material's product data sheet, curing is highlighted in a table in accordance with different ambient temperatures and other conditions.

From A. Damer Agha Alkla of NATIONAL PAINTS FACTORIES CO. on December 3, 2012:
     After getting full curing for your applied system, you may do the pull-off test to confirm the quality of adhesion of the coatings.

From trevor neale of TF Warren Group on November 27, 2012:
Critical service specifications typically call for adhesion testing, so assume this question relates to field painting where weather and other delays are often unavoidable and formal adhesion testing is not part if the job/project specification. If there is any suspicion that adhesion may be compromised, then the appropriate form of adhesion testing is recommended to ensure the complete system integrity. This is simply a CYA procedure to avoid possible conflicts,or worse, premature failures.

From James Albertoni of CA Department of Water Resources on November 26, 2012:
     Some good instances where a multi-coat system should be tested for adhesion include if the re-coat window is missed, if the topcoat is not specifically recommended by the manufacturer to be comptaible with the basecoat, if the basecoat and topcoat are from two different manufacturers, or if it is suspected one of the coats was mixed slightly off ratio. Most importantly, it should always be tested for adhesion if the spec calls for it.

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Tagged categories: Adhesion; Performance testing

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