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February 13 - February 17, 2017

Why would a pull-off adhesion test give a high value (no failure) for a solventless epoxy that later showed poor adhesion in service?

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Selected Answers

Because the test sample was prepared according to manufacturer's recommendation and in certain environmental conditions as required. The test sample has a small area to apply and monitor epoxy application; but on the job, the applicator may not have met the required environmental conditions, improperly mixed material, and added  thinner to make easy application or evenness in appearance, any of which may may have caused poor adhesion in service.

From simon daly of Hempel A/S on February 14, 2017:
The answers relating to application issues are, of course, very valid; however, there are other fundamentals to consider even if application is applied in approximately the same way as when the high values were achieved. Water absorption and subsequent permeation to the substrate may result in deterioration of some types of adhesive bonds to the substrate, reducing adhesion and ultimately (but not always) the development of blistering. The question does not state where the adhesion is subsequently reduced, whether to the substrate of inter layer; however, the presence of weak intercoat layers (such as those caused by amine bloom) may not necessarily reveal adhesive failure initially but when solubilized by water in the film may show dramatically reduced adhesion. The period of time over which this occurs may be influenced by a number of factors, temperature and thermo-osmosis (the so called "cold-wall" effect) being just two.

From Steve Brunner of WPC Technologies on February 13, 2017:
Often, the high value was done in the lab under ideal conditions and proper mix ratio. Poor adhesion in service can have many variables, including improper mix ratios; coating applied too thick; poor surface preparation; and,  if using heated equipment, the setting was too hot. The best way to obtain adhesion in service similar to that seen in with the test is to read and the manufacturer's application guides.

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Tagged categories: Adhesion; Epoxy; Quality control; Testing + Evaluation

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