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22 POWDER COATING, February 2018 The quality of cure When I ask customers what they know about powder coating, the answers I get almost always relate to the durability of the coating. Words such as hard, strong, tough, and durable are used freely. In all these cases, the cus- tomers do not realize that they are speaking about one of the most impres- sive things about powder coating—the cross-link. This article is an introduction to the uniqueness of the cure mechanism in powder coating, the challenge faced by the custom coater when it comes to unique fabrications, tools that all powder coaters should be aware of, and why this matters to your customers. Please note that for the purpose of this article, the type of curing oven being referenced is a natural gas fired convection batch oven. Defining terms Cure? Convertible? Cross-link? Poly- merization? Evaporation? To better understand the concepts presented in this article, it is worthwhile to take a moment to define some key terms. In the coatings world, cure refers to the process of a coating transitioning from a liquid state to a solid state. Assume that you have a bedframe in your home that has been coated with a lacquer. This lac- quer would have cured through the evaporation of the solvents. The coating attained its desired properties of appear- ance and hardness as the solvent base evaporated. The process of evaporation is referred to as the curing mechanism. This is an example of a nonconvertible coating: It can be removed after it has dried by use of the same solvent base it was suspended in. In other words, its properties are reversible. There are a variety of chemistries available to liquid and powder coaters that are noncon- vertible, also known as thermoplastic. Now let's consider the same bedframe as being powder coated. Assuming that it was properly pretreated, racked, and coated, we will examine the curing process. The coating began in powder form, melted into a liquid state (this is often referred to as the gel) while in the curing oven, and then, upon removal and cooling, settled into its final solid state. The technical explanation for the cure is that it underwent polymeriza- tion through forced fusion cure (the curing mechanism). More simply, when we talk about cross-linking, this is what we are referring to. Through the heat of the oven, the substrate tempera- ture was elevated to the point where a chemical reaction took place, resulting in the hardened powder coating. Once the cross-linking has occurred, the coating will not change back to its original state (although it can be soft- ened and removed through the use of aggressive industrial grade paint strip- pers). This type of coating is known as a convertible coating due to the transfor- mation that takes place at the chemical level in the powder coating. The net result of this process is that powder coat- ing is recognized as being a hard, strong, tough, durable coating. Cure windows Sounds simple enough doesn't it? The equation is simple: Temperature + Time = Cure A key ingredient that was left out of that overview of key terms was temperature + time. For the thermoset powder coat- ing to properly cross-link, the substrate (in this case a heavy steel bedframe) needs to be held at or above a certain temperature for a defined period of time. If our polyester powder coating requires 15 minutes at 356°F, this does NOT mean that you set the oven to 356°F and put the bedframe in for 15 minutes—you might wet out the coat- ing and it might look cured, but it will not have any of the mechanical or chemical resistance properties of a prop- erly cured coating. This combination of time and temper- ature is commonly referred to as the cure window. Every powder manufac- turer should be able to provide you with a technical data sheet (TDS) that out- lines the cure window for their product. As the custom powder coater, it is our responsibility to determine how long the bedframe needs to be in the oven to bring it up to the required temperature and then hold it there. We will base our cure parameters on the example cure schedule shown in Figure 1. Assume that the bedframe is made from 2-inch by 2-inch hollow structural steel (HSS) with 1/4-inch wall thickness. It weighs approximately 200 pounds and is 6 feet by 8 feet by 3.5 feet. As custom coaters, we likely have an idea of the time and temperature required to cure this product from past experience. As it stands Chris McKinnon Aegis Industrial Finishing Coater's Corner Coater's Corner Cure schedule showing time and temperature Figure 1