Two-coat powder process fights corrosion

Figure 1
Hartford Finishing, Hartford, Wis., believes that it has a two-coat powder coating process that will provide long-term coverage on sharp edges while still maintaining the overall durability that manufacturers expect from the powder coating process.

Heavy-equipment manufacturers desire a final finish on their products that looks good and lasts a long time. Unfortunately, that’s not been their experience.

A crane may not require a Class A surface finish that is found on a Corvette, but it does require a coating that remains durable over the lifetime of the heavy equipment’s life cycle, not just in the first few years that it’s introduced to the field. If the coating remains consistent, the heavy-equipment manufacturer doesn’t have to worry about corrosion claims from customers, which can lead to millions in repairs each year.

As a result, these manufacturers of big equipment have pushed their coating suppliers to figure out a way to produce durable coatings that will maintain corrosion protection even over sharp edges. E-coat, or electrocoating, the process of submerging a metal workpiece in an electrified tank of paint and water to attract paint onto the surface, was considered a contending technology to offer strong corrosion protection, but the heavy-equipment manufacturers learned that the automakers that use the e-coat process also use hems and other metal forming tricks to minimize sharp edges on parts; it’s too difficult to form those types of edges with plate, so e-coat wasn’t going to do the trick. That led those same OEMs to push for a powder-on-powder process, in which a powder coat primer was placed on the workpiece, slightly cured, and then topped with a powder topcoat. That approach showed promise.

Hartford Finishing, Hartford, Wis., thinks it has found a way to offer that powder-on-powder approach (see Figure 1) in a much more consistent and affordable way.

Building a New Line

The powder-on-powder process is a good one, but the way that many custom coaters are set up, it also can be an expensive one. They don’t have separate powder booths because their clean-room setups are designed for one powder booth, and they have to send parts through the line twice—once for the primer coat to be applied and slightly cured and another time for the topcoat to be applied and fully cured, which can be a considerable expense for the custom coater. Additionally, because the custom coater is applying two kinds of powders in the same booth, it can’t reclaim and reuse any overspray; it’s all spray-to-waste.

Hartford Finishing recognized the shortcomings and committed to building a new line as part of a new 110,000-square-foot facility. The facility opened in January 2014, and the two-coat powder line, which takes up about 35,000 sq. ft., was up and running shortly thereafter.

“We saw the benefit of being able to do the primer and topcoat in one continuous line with the addition of a gel oven. So the product today would basically go through a wash system, go into a clean room where the primer is applied, exit the clean room, go into a gel oven, work its way back into the clean room where the topcoat is applied, and then gow into an oven where it is fully cured,” said Brian Wendorff, Hartford Finishing’s vice president.

This line has elements that are familiar to any custom coater or metal fabricator, but some key aspects stand out.

The pretreatment process is an extensive one (see Figure 2). The 12-stage system includes a manual pressure wash, alkaline cleaner, ferrous cleaner, two rinses, sulfuric acid to remove laser scale, a neutralizing rinse, a reverse-osmosis (RO) rinse, an iron phosphate cleaner, two additional RO rinses, and a seal rinse. The pretreatment process can accommodate a 48-in. by 72-in. by 14-ft. part.

The gel oven is used to cure the primer coat to about 75 percent. The gas-fired oven is arguably the most critical piece of the complete two-coat powder system, according to Wendorff.

Figure 2
Hartford Finishing’s new building gave it the room to invest in a robust, 12-stage pretreatment system.

“We have learned with different primers how we need to manipulate that to get the best adhesion and edge coverage. That does become part of the recipe for each job,” he said.

Hartford Finishing has two Nordson quick-color-change booths with automated guns in the clean room, where the powder primer and topcoat are applied separately. Manual powder application takes place as well to touch up hard-to-reach areas where automated guns may not provide complete coverage.

The curing oven that follows topcoat application is designed with three zones. Curing time can be from 45 to 75 minutes. A powder coating recipe can be adjusted so that the final curing cycle promotes the greatest amount of cross-linking between the topcoat and the primer.

The arrangement of two separate powder application booths and inline ovens allows Hartford Finishing to use different powder coating chemistries. That means an epoxy primer coat can be used for impact and corrosion resistance and then a urethane or a polyester topcoat can be used for UV stability. When other custom coaters were working with a powder-on-powder application with just one booth and had to move parts around the line twice for complete coverage, they typically had to use primer and topcoat powders with the same chemistry to ensure cross-linking, which shortchanged the final coating’s durability.

Wendorff said that Hartford Finishing’s two-coat process is providing a 22 percent increase in edge coverage when compared to e-coat and powder topcoat combination processes. Also, testing has revealed that the two-coating system provides corrosion protection up to 4,000 hours of salt spray exposure.

“We are very confident that our design is very successful,” Wendorff said. “The salt spray hours that we are achieving and the edge protection are very good.”

The next step is reducing the amount of wastewater the new powder coating process produces. After working with its own wastewater team, pretreatment chemical suppliers, and the Wisconsin Department of Natural Resources, the company is reusing about 20 percent of the system’s wastewater. Further investigation into the finishing process leads company management to believe that at least 75 percent of the wastewater may be able to be recycled for use in the system.

Two—whether it’s two powder coat applications or a two-time usage of the same water—appears to be the magic number.