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New life for old coatings

Article-New life for old coatings

New life for old coatings

Old coatings may wear away, but they don't have to die. Instead, some traditional coating methods are finding a successful new life in current applications.

In one case described below, an anodic epoxy electrocoat's low processing temperature made it a natural fit for engine and drive-train components in need of some wear resistance. Another application saw older liquid coatings help smooth a bicycle company's transition to more modern powder coatings. In the decorative plating realm, plastic materials development promises to bring chrome plating to new heights--new high temperatures, that is. And two well-known functional metal coatings have together solved some of the problems with finishing powder-injection molded parts.

Anodic gets some respect. Ask electrocoaters about the state of their art, and the answer probably won't involve an anodic process--which deposits negatively charged paint on positively charged substrates. "Anodics are not thought to be as robust as cathodic electrocoats, so people tend to ignore them," explains Lyle Gilbert of MetoKote Corp. (Lima, OH), one of the country's top electrocoating houses. They get so little respect in finishing circles that Gilbert even calls them the "Rodney Dangerfield" of electrocoats.

But at MetoKote, where cathodic electrocoats are the norm, one anodic epoxy coating is held in high esteem. Gilbert reports that the company has found successful applications for a low-cure anodic coating from PPG Industries (Pittsburgh, PA). Called P-150, this anodic epoxy cures at temperatures as low as 180 to 220F versus 360 to 400F for "high-cure" thermosets.

MetoKote first used the low-cure coating on a General Motors steering gear that had run into problems with other finishing methods. "Liquid paint on the assembled gear didn't provide a coating with the required physical properties," Gilbert recalls. Another attempted coating method, individually electrocoating parts before machining and assembly, left unprotected areas on the finished components. According to Gilbert, switching to the low-cure electrocoat immediately solved the performance and coverage issues. What's more, the low-cure temperature didn't damage the gear's internal seals and bearings, which would have caused grease to drain out--an ever-present risk with high-cure temperatures. And as a final benefit, the low-cure temperature enabled MetoKote to mask parts of the gear with an inexpensive vinyl material instead of more costly high-temperature masking materials.

Low temperatures and improved masking capabilities make this anodic coating a good fit in other applications too, according to Gilbert. MetoKote has used it on a Navistar drive shaft assembly that contains grease and rubber components. "Other good applications are parts with rubber bonded on some portion of them," says Gilbert, citing clutch pulleys for automotive air conditioners, rubber bonded auto body mounts, and bonded computer magnets. Gilbert speculates that the process could even be used to coat an assembled engine--with the oil in it. He also considers it a candidate for large castings or assemblies that would take too long to cool if brought up to "high-cure" temperatures.

Of course, for parts without heat or masking issues, MetoKote still recommends cathodic electrocoats, which simply provide far-better corrosion protection. "The low-cure might give you 336 hours of salt-spray resistance versus more than 1,000 hours with a cathodic ecoat," Gilbert says.

A liquid transition. In the color-obsessed bicycle industry, changing finishing systems could mean a rough ride. Huffy Bicycle (Miamisburg, OH), however, has kept the bumps to a minimum by combining its newer powder coat finishes with its traditional liquid coatings.

Environmental friendliness and superior physical properties--especially weatherability, durability, and impact resistance--play an important role in the on-going shift to powder coatings, according to Kent Smith, a former corporate finishing engineer who now runs operations at the company's manufacturing facility in Nuevo Laredo, Mexico. But the combination of liquid and powder coating technologies springs more from an often-overlooked engineering goal that never takes a back seat for Huffy: maintaining a large color and visual effects pallet. Drawing a comparison between the bicycle and fashion industries, Smith says, "Both need a wide range of colors in order to grab people."

In its current hybrid system for finishing bicycle frames, Huffy most often applies a liquid topcoat over a powder base coat, Smith reports. The company has tried other combinations as well, including ones with powder topcoats. "We've used just about every combination you can come up with," he says.

While he declines to reveal just how many colors Huffy works with, Smith notes that it's common to spray a bicycle frame with two or three different paints to achieve a given color. Add to color the need for effects, such as fades, splatter, or dots. With seemingly endless aesthetic possibilities, Huffy engineers had doubts that the company's color requirements could be met solely by powder coating--unless it was combined with liquid coating. Together, the two technologies not only offer ample color possibilities, but also a chance to gain some experience with powder finishing.

Beyond keeping its color choices wide open, the technology combo also resulted in some unexpected benefits. For one, it produces a better depth-of-image than a liquid coating could on its own. "With the combined powder and liquid coatings, the parts have an intense 'wet' appearance, an effect that is highly desirable in the bike industry," says Smith. The combination coatings also reduced the "pigment slide" defects sometimes found in highly pigmented liquids. And finally, the combination technology helped reduce costs, a benefit Smith attributes to fewer paint rejects, lower labor costs, and reduced coating material waste.

"Ultimately we'd like to be all powder," Smith says, "but until powder can meet all our color requirements, liquid still has a place in our operation."

While many protective coating jobs normally employ a cathodic electrocoat, which does a superior job of fighting corrosion, this anodic epoxy electrocoat has an advantage in that it can bring adequate performance to applications that can't tolerate high cure temperatures.

Better plating plastics. For years, chrome plating has dressed up plastic components. On automotive exteriors, it's ubiquitous. And plated plastics have started to turn up inside the car. For example, Siegel-Robert Inc. (St. Louis), one of the world's largest electroplaters of plastics, already produces a coinjection molded chrome-plated interior door handle and will do "several more" for the 2001 model year, reports Mike Honigfort, vice president of design and methods engineering.

While ABS has long been the material of choice for chrome plating, demanding applications have created a need for more robust materials. "There's always a need for high temperature, high-impact plateable materials," says David Rudder, director of Siegel-Robert's technology center. "Some difficult applications require something stronger than ABS," he says. In recent years, that need for better properties has been filled by PC/ABS blends. With some adjustments to molding parameters and existing ABS plating chemistry, some platers can now accommodate blends with PC content in the neighborhood of 40%, says Honigfort.

The push for properties has also resulted in plating materials suitable for higher heat environments. GE Plastics, for example, recently introduced a plateable version of its Noryl PPO suitable for plating in ABS chemistry--although with the addition of a pre-etching step. The first of these grades offered a heat deflection temperature (HDT) of 235F, compared to roughly 210F for PC/ABS. A new grade, which is now available in sampling quantities, widens the temperature gap further with an HDT of 275F. Kristie Dolan, project leader for Noryl, predicts that new grade, called PN 275, will serve as a good drop-in replacement for some automotive parts currently produced in ABS or PC/ABS, including wheel covers and front grilles. According to Dolan, wheel covers are perhaps the most promising of these because they have to endure higher temperatures than in the past. "Today's braking systems get hotter and hotter," she says, noting that some heat specifications have crept up to 250F from 200F.

A coating for porous PIM parts. Functional chrome finishes for steel have a long history. So do fluoropolymer coatings such as PTFE. Put them together, though, and you end up with a coating especially well suited to the "porous microstructure" of powder-injection-molded parts, according to J. Kelly Mowry, president of Gull Industries, a metal finishing firm in Houston. This "duplex coating," which Gull calls Gullon and has applied to a variety of powdered metal parts, combines a 0.00035-inch-thick layer of wear-and corrosion-resistant chrome with a similarly thin layer of lubricating fluoropolymer.

Recently the coating found its way into a gas-chromatography machine after a redesign added a metal-injection-molded coupling and drive for the valve that feeds samples to the detector. The maker of the chromatograph, Valco Instruments (Houston, TX), had long manufactured the valve body component from heat-treated stainless, which it would then chrome plate. "So we had been plating all along," explains Scott Hall of Valco. But the switch to a powder-molded stainless steel, for all its manufacturing advantages, resulted in parts that without coating would produce wear-induced residue, then gall, and eventually need replacement.

Hall describes the resulting black, powdered residue as "a nuisance problem" since the residue stays within the valve housing rather than entering the flow path of the samples. Yet he points out that the analytical chemists tend to have an understandable aversion to residue that they didn't put in the machine for analysis. That same intolerance of impurities--as well as operating temperature up to 300Cforced Valco to rule out liquid lubricants as a solution to its wear and seizing problems. "You don't want any corrosion or impurities at all in a device that detects in parts per billion," Hall says. The combination chrome finish and fluoropolymer coating was the solution.

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