Q: One of my customers is facing an unknown problem when they use a polyester powder coating. The coated pieces are aluminum profiles, and their application is vertical.
They are seeing an unknown material looks like snow, is mostly found inside the baking oven, and causes defects on the coated surface. It seems to happen with the powder we supply under usual conditions in the customer’s oven, in my opinion. Can you advise from your experience what it is and how it happens?
A: Thank you for your question. I have seen this problem before. It can be summed up in one word: benzoin.
Nearly all powder coating formulas use a degassing agent to assist in the film formation of the coating while it is melting in the oven. Degassing is needed because, as the particles coalesce, air pockets form in between the particles (I like to call it interstitial air). Incorporating benzoin allows the bubbles to escape during this melt phase.
Sounds great so far, right? Part of the mechanism of degassing involves the volatilization of the benzoin, as its melting point is 132°C and it tends to sublime above this temperature. How do we know this? I have taken samples of the snowflake-like residue found in powder cure ovens and had our analytical lab characterize it. What they found was the majority of the residue is benzoin.
How do you minimize/stop this problem? My experience with finding high concentrations of benzoin in a cure oven usually indicated a very “tight” oven. By that, I mean an oven with very little exhaust. This is a common issue; some oven designers think that since powder coatings are promoted as having no VOCs (volatile organic compounds), they don’t have any volatiles. This is not the case.
VOCs refer to regulated organic compounds that present a deleterious effect on the atmosphere. Powder volatiles do not, but they still exist. Hence, the solution to your problem is to 1) thoroughly clean/vacuum your oven and 2) provide more exhaust to the oven.
This should eliminate the problem. Oh, and one other thing: have a trusted analytical laboratory analyze the residue with infrared spectroscopy. Benzoin absorption bands are unmistakable.