Accelerated Aging Tests

Question:

I recently experienced a failure on an accelerated aging test of a polyurethane coating system. The test consisted of 24 days of humidity (90% relative humidity at 53° C), followed by 25 temperature cycles (+71– -45°C), followed by a repeat of the 24 day humidity exposure, followed by a repeat of the 25 temperature cycles. Damaged coating was observed after the second humidity exposure, and the coating failed after the second series of temperature cycling. Failure occurred between the topcoat (polyurethane per MIL-PRF-85285, Type I) and the coating beneath it, a polyurethane plural coating. Our paint experts think that the parameters of the aging test, specifically the +71°C of the temperature cycling, was too aggressive. They are concerned that it exceeds the glass transition temperature of the coatings. We’re trying to determine what the glass transition temperature of each coating is and may run the test again but adjust the temperature exposure accordingly so that it does not exceed the glass transition temperature. Is there something we should be looking at besides glass transition temperature? Is there some other critical property that affects the way the coatings respond to accelerated aging tests?

Answer:

Accelerated aging tests of organic coatings is difficult, for the coatings as well as the interpreter. These tests are usually designed by a committee, often resulting in; “if -40°C is a good lower limit, -45°C is better.” A coating’s physical properties such as its Glass Transition Temperature (T(g)) can play a significant part in test results.

The problem is not the T(g) of the polyurethane coatings. Your determination will show it is higher than 71°C (159.8°F). In fact, the continuous service temperature for polyurethane coatings is 121.11°C (250°F). Your problem may be caused by cracking and film delamination at or near the low temperature point, -45°C(-49°F), of the cycle. When I had a real job, we used to run a thermocycling test of -40° C- +100°C (-40°F-+212°F). Cracking failures of coatings usually occurred as the temperature was falling.

The fact that the topcoat separated from the primer indicates an inter-coat adhesion problem, since the coefficients of linear expansion for both coating should be the same. I suggest you have your paint experts look into the cause of the adhesion failure.