Dealing with Outgassing—Root Causes and Mitigation Techniques
Posted on Wednesday, May 12, 2021
By Kevin Biller
Outgassing defects in powder coatings can be an unexpected and sporadic menace that seemingly appear out of nowhere. Finding a fix can be elusive unless you understand the root causes of outgassing. Let’s take a look at which substrates are most prone to outgassing, mitigation strategies, and how to differentiate between simple gassing and full-fledged outgassing.
Simple Gassing vs. Outgassing—Which is Which?
Gassing can be defined as the evolution of volatiles emanating from within the powder coating itself. On the other hand, outgassing refers to the macro emission of gas(es) expelled from a substrate that travel from the substrate through a molten powder coating film. Let’s talk about gassing first.
Powder coatings, by their nature, emit volatiles in a few different ways. Powder particles coalesce during film formation as it melts and gels in an oven. As the particles fuse together, interstitial air between them pushes through the film and out into the ambient oven environment. In addition to the expulsion of this interstitial air, a small amount of moisture also evolves. For this reason, powder coatings are formulated with “degassing” agents to expel this air and moisture from the film quickly and completely before it hardens. Some powder coatings also emit “volatiles of cure” based on their chemistry. Polyester HAA-based powders evolve water as a by-product of cure. In addition, most polyurethane powder coatings emit є-caprolactam, a chemical blocking agent. Degassing agents in the formula minimize defects caused by volatiles of cure. Nevertheless, thick films, generally 5.0 mils (100 microns) and greater, can exhibit “pinholing” due to the emission of cure volatiles. Pinholes are defined as circular micro-voids in the coating surface. Upon close inspection they are visible to the naked eye and under low magnification they appear as perfectly concentric holes in the coating surface. Therefore, film thickness control is rather critical when using these types of powder coatings. It should be kept under 5.0 mils and preferably less than 4.0 mils.
Volatiles from Substrates
Now that we know how to define gassing, let’s take a look at outgassing. These gases originating from substrates can
cause defects as well. Specifically, gases can evolve from hot dipped galvanized steel and cast metal parts. The evolution from these substrates during powder cure can cause defects ranging from pinholes to large, unsightly blisters. Both types of substrates can be wildly inconsistent depending on age and how they have been processed. We’ll tackle galvanizing first.
Ferrous substrates such as steel are sometimes coated with zinc to protect the iron from oxidizing into ferrous oxide (Fe2O3-nH2O), commonly known as rust. This can be a hot-dipped process involving deposition by immersing the part into a molten bath of zinc mixed with a few additives such as magnesium and aluminum. The galvanized item is then removed from the tank and cooled, oftentimes by a water quench. The rate of cooling affects the “spangle,” or size of crystallites, which can vary from less than a millimeter to a few centimeters. Highly spangled galvanizing is more problematic for outgassing and powder topcoat adhesion. Water can be entrained into the zinc coating from the quenching process as well as from a humid ambient environment. In addition, aqueous solutions commonly used to clean and pretreat galvanized surfaces prior to powder coating can be retained in the zinc surface and become a source of outgassing. Some experts also speculate that hydrogen can be emitted from galvanized surfaces exposed to elevated temperatures. These emissions can cause outgassing defects as well.
To eliminate the potential presence of water in the grain of a galvanized surface, parts should always be prebaked prior to applying a powder coating. A temperature 20 to 30 degrees Fahrenheit higher than the recommended bake temperature is usually sufficient to drive off any volatiles imbedded into the galvanizing. It is critical to allow the metal to reach this elevated temperature and not just the oven air. Therefore, ensure that the parts reach this temperature before retrieving them from the oven. Allow the parts to cool within 25 degree Fahrenheit of the ambient temperature before applying the powder coating. This process is described in ASTM D7803 and should eliminate pinholing due to volatiles associated with a galvanized surface.
A third technique is to use a powder coating designed to mitigate the expulsion of gas during the powder coating curing process. Basically, we formulators incorporate a waxy substance into the formula that allows for a soft, fluid passageway for the evolving gas to escape. Because the additive is soft and fluid in the oven, it tends to collapse and refill the void made by the volatile gas. Powder suppliers call these “OGF” or outgassing forgiving powder coating products. In addition, some powder manufacturers supply an OGF additive that can be incorporated on-site in your finishing shop.
Cast Metal Parts
One of the most common root causes of outgassing defects is the application of a powder coating over a cast metal part. The metal casting process is not new and goes back thousands of years. Still, it is commonly used to produce
parts of intricate shapes as well as heavy items that would be otherwise difficult to fabricate. Molten metal is poured into a negative mold through a narrow opening called a sprue. The mold and casting are allowed to cool and the cast metal parts are extracted. Steel, aluminum, and zinc, as well as magnesium alloy castings, are commonly used to fabricate products serving the automotive, aerospace, ACE (agricultural, construction, and earthmoving), appliance, medical, pipeline, and many more markets.
Because the molten metal is essentially poured into a cavity inside the mold, porosity can develop within the structure of the metal. Depending on the quality of the casting process, this porosity oftentimes extends to the outer surface of the casting. These pores can harbor moisture, cleaning and pretreatment chemicals, and air. If powder coated, these contaminants evolve during the curing process and can cause large blisters. Eliminating these defects can take many paths.
Contacting your cast metal parts supplier is the place to start. There are actions that can be taken to reduce or eliminate porosity in metal castings. Alloy composition, metal temperature, quality of the molds, and process steps such as vibration and speed of the pour can be investigated to reduce porosity. In some cases, nitrogen can be incorporated into the casting process to minimize porosity. If you don’t have the leverage to influence a metal caster’s process, then other mitigation strategies can be contemplated.
The best in-plant solution is to prebake the cast metal parts prior to applying powder coating. The goal is to drive out all potential volatiles before the powder is applied. This can be accomplished by heating the parts to a temperature above the recommended bake temperature. Typically, a temperature approximately 25 degrees Fahrenheit higher than the powder bake temperature will accomplish this. Don’t assume that the oven air temperature is equal to the part temperature; make sure that the part reaches that the target temperature. This can often be accomplished with the dry-off oven at the end of the pretreatment process. Apply the powder coating while the part is still warm—around 25 to 30 degrees Fahrenheit above ambient temperature. Avoid allowing the parts to cool and sit too long as they will eventually reabsorb ambient moisture and entrain air.
A third mitigation technique is to use an OGF (outgas forgiving) powder coating as described above in the galvanized parts section. These are designed to mitigate the expulsion of gas during the powder coating curing process. OGF powders contain a waxy substance that allows the evolving gas to escape without creating a surface defect such a pinhole or blister. You can also consider using an outgassing prevention additive that can be incorporated on-site to a powder coating.
Using a low temperature cure powder is another, albeit partial, mitigation technique. Curing a powder coating at a lower temperature would reduce the tendency for volatiles to evolve from a porous casting. You may want to consider a using a powder coating that can cure at 325 degrees Fahrenheit instead of 375 to 400 degrees Fahrenheit. This
is an incomplete solution, although it should reduce the incidence of the outgassing defects.
Experiencing outgassing defects on your finishing line can be frustrating. Recognizing the potential for outgassing defects, especially with galvanized and cast metal parts, can save you a lot of headaches. When encountering these types of parts for the first time it is a good idea to spray just a few of them before running the risk of an entire oven full of defective parts. Make adjustments in process and materials before you commit to a high volume of parts.
Kevin Biller is technical editor of Powder Coated Tough and president of The Powder Coating Research Group.