Comprehending Pull-Off Adhesion Testing

From JPCL, December 2020

By Rob Francis, R A Francis Consulting Services Pty Ltd

Pull-off adhesion testing is increasingly being specified and used by coating inspectors. Manufacturers have produced new equipment models and more papers are being published on the subject, but is this increased interest based on the testing’s ability to provide useful information on coating system performance, or is this a case of a test method being more attractive because it can produce a number—even if that number has little meaning? This article will look closely at pull-off testing—specifically, what is measured and what the results mean....

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Comment from Wilfred Offor, (12/21/2020, 4:33 AM)

Excellent article or topic. However, there seem to be confusion on the One MPa is equal to 14 psi. Using the equivalent stated in this article cannot relate with the example shown in this article --- For example, if one operator gets a result of 7 MPa (1,000 psi) with a Type II gauge under given conditions, then a second result under different conditions needs to be less than (7- [7 x .65]) = 2.5 MPa (360 psi) or more than (7+ [7 x .65]) = 11.6 MPa (1,680 psi) for the change to be considered significant. Sir, please can the author explain more on this example or conversion value. Probably, I am misunderstanding the author. Thanks

Comment from Rob Francis, (12/22/2020, 1:38 AM)

Wifred. The conversion is correct. The Type II is only +/-65% accurate so a second result has to be outside this range to be classed as significantly different. That is what the calculation is trying to show. I admit it is rather complex. There is further discussion in Reference 2.

Comment from Alex Yague, (12/22/2020, 5:30 AM)

Hi Rob. Very interesting article. It made me think about that we need to be very cautious when comparing Pull-off values and how we interpret the results. I have one comment when the article says that "One MPa is equal to 14 psi", I think it should be 145 psi. Thank you

Comment from Wilfred Offor, (12/26/2020, 3:20 PM)

Dear Dr Francis. Thanks for your response. Just as Alex Yague commented on 'One MPa is equal to 14psi" as stated in your article was my concern in your worked example.

Comment from wenbo chen, (1/19/2021, 3:30 AM)

Can the Self-aligning Adhesion Tester Type V - manual version be considered to be in compliance with NORSOK M501 which specified that test equipment with an automatic centred pulling force shall be used.

Comment from gavin bowman, (1/19/2021, 6:41 AM)

Well written and interesting article

Comment from Mika Mack, (1/19/2021, 11:35 AM)

My question is in regards to concrete and micro fracturing. When to score , trying to hand score is tediously time consuming, yet with a guide. No fracturing. Scoring with a template and mechanical means smooth, yet variability in the speed of the drill can fracture the concrete and/or the coating. Resulting in poor values. not a true representation of substrate or the coatings performance. In the data presented, were all the pull values to failure or refusal? And the failure modes comparable across the different types of gages? Thank you for you quantifying article")

Comment from Rob Francis, (1/20/2021, 2:24 AM)

Apologies for the error. One MPa is 145psi. Please correct in your copy. Wenbo - I would consider the NORSOK term " automatically centred pulling force" to mean the same as "self-aligning". But with normative standards you should really get a ruling from the Standards body. Mika, I don't have experience with testing coatings on concrete.

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