Stressing the Achilles Heel

Every coating has its own Achilles heel — that is, a point of weakness. The ideal test wonld accelerate all stresses to the same extent. It would then be possible to compare coatings with different aging mechanisms — different Achilles heels — to each other. 

Unfortnnately, it is not possible to accentuate all stresses evenly. Furthermore, it is not possible to accentnate all weathering factors and still maintain the balance between them that exists in the held. When we increase the percentage of time with UV load, for example, we change the ratio of light and dark and move a step away from the real dinmal cycle seen in the held. 

Becanse it is not possible to evenly accelerate all aging factors, the best testing tries to imitate an expected failure mechanism. Each test accentuates one or a few stresses that are rate-controlhng for a mechanism. By choosing the right test, it is possible to thus probe for certain expected weaknesses in the coating/substrate system. The trick, of course, is to correctly estimate the failure mechanism for a particular application, and thus pick the most suitable test. 

and Strom, G.,SAETechnical Paper Series, 932338, Society of Automotive Engineers, Warrendale, PA, 1993. 

Forsgren, A. and Appelgren, C., Performance of organic coatings at various field stations after 5 years exposure, SCI Rapport 2001 5E, Swedish Corrosion Institute, Stockholm, 2001. 

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