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Humidity failures

We are epoxy powder coating machined automotive brake drums and are having adhesion failures at the scribe after 96 hour humidity. We have tried various pretreat methods and different powders with the same results. If we use liquid coating we don't have the problem. Any clues about what could be happening would be appreciated.

Rick Smith
- Waterloo, Ont., Canada
2002

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Hi Rick,

There are numerous reasons why this may be the case, some of which you have mentioned.

One of the main reasons for this, excluding pre-treatment, is that the powders are not cured properly. The curing schedules quoted for powder coating are based on metal temperature as well as just air temperature. If you are using a thick gauge metal it will take longer to heat up thus reducing the heat transfer to the powder coating. Try a solvent rub on the powder to see if it has cured properly.

There are a few ways you can combat under cure, pre-heat the substrate then powder coat, leave the objects in the oven longer or raise the temperature of your oven. These solutions also have drawbacks. If you pre-heat the object the energy cost rise and if you leave it in the oven for longer or raise the temperature it may affect the colour and gloss level. Try a few times and different methods testing each for sufficient cure each time.

Hope this helps,

Drew Devlin
- Lincolnshire
2002