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Runny Powder Coating

We manufacture a range of outdoor lighting fixtures and have experienced paint degradation on some of them. When painted with silver or white polyester powder coating, the finish will degrade over a period of months and sometimes look as if the paint has run.

The fixture is cast out of LM6 aluminium and then aluchromed. We paint them ourselves with a polyester powder coating in a variety of colours but mainly black. They do get hot when in use and can reach 280 °C. The problem will not effect all of a batch so there must be some inconsistency in a process somewhere that is causing it. It never seems to happen on black fixtures, but that may be because they cool better.

I am wondering if it is a reaction with the aluchroming at high temperatures and that we should not bother with it as LM6 is supposed to be highly corrosion resistant. It could also be a reaction to salt spray or chlorine as these fixtures are often fitted around swimming pools on cruise ships.



Has anyone any ideas please?

John Hickinbotham
- Worcester, Worcestershire, United Kingdom
2006

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Dear sir.
Your problem is U.V. light this is chaulking the coating since black power is more resistence then white powder. I suggest you use TGC pure polyester powder from reputed manufacturer. After coating the parts should not expose to temperature more then 150 °C .

Zainuddin Nuruddinz
Microplast Systems - Karachi, Pakistan
2006

Thanks for your reply Zainuddin, this is not UV powdering, there is no chalky surface, the surface is glazed as if it was gloss paint.

John Hickinbotham
- Worcester, Wrocestershire, United Kingdom
2006

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Hi John,

Firstly, the "dripping" is not an effect of salt spray and it is doubtful that it is chlorine. The problem is almost certainly with your process. It looks like the part that you are coating has not been cured correctly and when the part is exposed to the heat you talk about the powder melts and flows before fully curing. The reason this does not happen with the black powder is that the black powder will have a different chemistry and pigment volume concentration (PVC). Also, in the initial cure schedule, it will absorb more heat and cure more efficiently.

Hope this helps,

Drew Devlin
- North Lincolnshire
2006