Smoke and Odor

Q: We recently started to run polyester urethane powders in our system. Whenever we run these powders, we get a lot of smoke and odor in the building. We don’t have this problem with epoxies and hybrids. What’s causing the excess smoke from the oven? Is it harmful? How can we stop it? C.B.

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A: The polyester urethane powder has an emissive component that evolves from the film during the cure process. The coatings have a blocking agent such as ?-caprolactam that reacts as it reaches high temperature. As the material evolves from the coating, it can create some smoke and odor. A typical epoxy or hybrid doesn’t contain this element and does not create the same level of emission during cure. The toxicology of ?-caprolactam has been extensively investigated. Studies show it’s not carcinogenic, teratogenic or mutagenic. Since the path to exposure is so remote, the industry does not regard it as a health hazard. You can find out much more on that subject in the Powder Coating Institute’s Technical Brief #18.

What can you do? Your problem is that you don’t have enough exhaust or a proper design for containment of gases in your oven. The exhaust rate has to be high enough to remove the gases coming off the powder and the by-products of combustion. That may require exhaust rates of six to eight turnovers per hour. You can read some examples in the Chemical Coaters Association International book on Powder Coating.

As for the containment at the entrance and exit of the oven, the most important feature is how long the vestibule is. An air curtain or heat-relief hood is less effective if the vestibule is too short. The containment device has less impact if the heated area is too close to the opening and the gases and heat escape containment. The appropriate length of the vestibule varies depending on the size of the opening and some other variables, but typically it should be a minimum of 5 ft for smaller openings and possibly as long as 10 ft for larger openings. Get your exhaust up to the correct level and make sure you have some good containment at the entrance and exit of the oven.