Outgassing in Powder Coatings

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Original article:

 
Outgassing describes the phenomena of entrapped gasses being released through a powder coating during the cure process. When this gas passes through the coating, it creates pinholes. These pinholes can provide a path for moisture or corrosive materials through the coating and to the substrate, causing coating failure. Furthermore, these holes can be unsightly on high quality appearance parts.

Causes of Outgassing 
Outgassing can be attributed to the substrate material, a surface contaminant, or the powder itself. Following are the specifics:

1. Castings: Outgassing can occur when a powder coating is applied over a cast metal surface (iron, steel, aluminum, brass, etc.). Gasses are entrapped into the casting material during the pouring process of both sand and die type castings. These gasses can be anything from entrapped air to gas formed during the cooling of the molten metal. The quality of the metal used and the care taken in the pouring process will directly affect the amount of gas that is entrapped. For instance, higher quality castings using higher quality metals have lower amounts of entrapped gas.

2. Galvanized (Zinc Coated) Substrates: Outgassing can occur when a powder coating is applied to a substrate that has a zinc coated surface (i.e. galvanized steel). As with castings, the process of applying the zinc to the steel substrate produces gasses that can be entrapped within this surface coating. This is especially true when “hot-dip” galvanizing is used. However, this does not occur when galvannealed steels are used, since the surface has been annealed to relieve the stresses and release the entrapped gasses.

3. Surface Contamination: Outgassing can occur when a powder coating is applied over a substrate that has a contaminant on the surface. This contaminant can be oil, grease, mold release, or any other material that can vaporize through the powder coating during the curing process. Although this situation is normally considered improper cleaning of the substrate and not an outgassing problem, the defect on the coated surface is the same.

4. Coating Thickness: Outgassing can occur when some powder coatings are applied in a heavy film thickness in a single coat. In this case, gasses given off during the cure process are released through the outer surface of this coating causing pinholes in the surface. Some coating are more susceptible to this problems than others, like Primids and emissive Polyurethanes.

How to Eliminate the Effects of Outgassing
Following are the different methods that have been proven to eliminate this problem:

1. Preheating the Part: This method is the most popular to eliminate the problem of outgassing. The part to be coated is preheated above the cure temperature for at least the same amount of time to cure the powder to allow the entrapped gas to be released prior to applying the powder coating. This solution may not eliminate all the outgassing if the part has an enormous amount of entrapped gasses, where the gas just keeps being released, no matter how much or how often you preheat the part.

2. Seal the Part Surface: This method requires the application of a material under pressure that is used to seal the entrapped gasses within the substrate, therefore, eliminating the outgassing from occurring entirely. Search for companies that specialize in impregnating/sealing casting technologies to obtain more information.

3. Change the Curing Technology: A change in curing technology to IR or IR/UV can eliminate the outgassing problem since the only the part surface is heated to cure the powder coating. In this case, the part substrate is not entirely heated, a necessity to release of the entrapped gasses.

4. Powder Formulation: The coating powder used for your specific application can be changed by your powder supplier to have enhanced flow characteristics. This means that the powder will stay in a liquid form for a prolonged period of time during the cure process. This allows the entrapped gasses in the substrate to escape when the coating is still liquid and flow over the pinhole, creating a smooth and hole-free surface.

5. Change or Improve the Substrate: Substituting the casting material with one that has less gassing issues can be a graceful solution. Working with your casting supplier to add vents or chills in particularly problematic areas is another area that can improve the substrate or eliminate the outgassing.

6. Eliminate the Contamination: Parts that have surface contamination are best corrected by eliminating of the contaminant. Identify the contaminant and remove it prior to powder coating and this problem will go away.

7. Control the Coating Film Thickness: If the outgassing problem is caused by excessive film build on the part, then the easiest way of correcting the problem is to reduce the film thickness. If a heavy film thickness is required for the application, then select a different coating material or apply the coating using two thinner coats.

Now that you know what causes outgassing and some methods to eliminate it, you are better equipped to prevent coating failure and poor coating appearance.

Nick Liberto, P.E., is president of Powder Coating Consultants, division of Ninan Inc., an independent technical consulting firm in Bridgeport, Conn. He can be reached at pcc@powdercoat.com coat.com