Salt Spray/QUV Test Results

Question:

I have always enjoyed reading your magazine and it has helped guide me through the transition from solvent based paints to waterborne paint and finally into powder coated finishes.

We are testing clear powder coated acrylics on solid brass with an average mil thickness of 2.5. The substrates tested pass with acceptable performance 2,000 hours salt spray and QUV without the brass tarnishing or pitting. We are looking to place a warranty on our products but are not sure how long 2, 000 hours of testing will last in the field. Is there a cross reference that corresponds the hours tested for salt spray/ QUV to the years the product can be expected to last in the field? For example, 1,000 hours acceptable salt spray/QUV will last one year in the field or 2,000 hours will last five years. Please advise if you have any knowledge on this issue. M.H.

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Answer:

Unfortunately, I have neither references that relate hours in salt spray or QUV to actual hours of real time exposure, nor do I know how long 2,000 hours of testing relates to how long a coating will last in the field. One reason is the vast difference in exposure conditions from one locale to another. Another reason is the different effects accelerated tests have on different coating systems. Furthermore, there are also surprises. For example, in the 1970s, my company monitored the performance of our standard and a candidate finish system on pole-mounted distribution transformers. We were surprised to find that the finish system held up better in the aggressive industrial atmosphere of the eastern United States than it did in the benign rural Midwest. Go figure!

During nearly 50 years of testing industrial finishes, I also noticed that failure modes after exposure to accelerated testing did not always agree with those of actual exposures. The actual exposures were conducted by testing painted panels on factory roofs throughout the country and by Florida exposure.

Your may want to ask the suppliers of accelerated testing equipment if they have data relating test results to real time exposure. On the other hand, you can make your own judgement of life expectancy of specific finish systems with relation to accelerated tests if you have actual real time exposure data. Unfortunately, most people don't want to wait that long.