Metallic surfaces, special effect colors, and reflective finishes have increasingly found their way into product design. The metallic trend shows no sign of abating. And the growing demand for stainless steel and copper has caused dramatic increases in base metal prices.
For example, 304 stainless steel typically used for appliances contains 8 to 10.5% nickel. According to USGS Minerals Information, about two-thirds of all nickel production was earmarked for stainless steel in 2005. And nickel prices were the highest since 1989. Cost data from the London Metals Exchange shows an even steeper price jump with nickel prices nearly quadrupling from $5.40/lb in December 2005 to $22.78/lb in May of this year. Copper prices have risen sharply as well.
Designers of appliances have taken these sharp cost increases on the chin. But that's about to change. Economical materials such as porcelain-enamel coatings have emerged. These engineered coatings give trendy metallic looks with a substantially lower sticker price.
Cost savings per square foot for a typical kitchen range serves as an example. With porcelain-enamel coatings called Evolution, designers can fabricate the range from 24-gauge plain steel instead of 300 Series stainless. The coating would go on as a 0.002-in. (2-mil) ground coat and a 0.004-in. (4-mil) cover coat. Using general market prices from May '07, the metallic-look porcelain enamel would have cut costs by nearly 60%. The porcelain coating also withstands scratches that would relegate a significant percentage of stainless-steel parts to the scrap heap.
Unlike their strictly metal counterparts, porcelain-enamel coatings don't discolor from heat and resist stains, scratches, and chemical cleaners. Fingerprints also easily come off. In addition, metallic-look coatings for refrigerators, ranges, cooktops, sinks, and plumbing can also give cabinetry matching or contrasting hues. And the coating resists heat well enough to be used on exterior surfaces of cookware, barbecue grills, and fire bowls that must take not only heat from flames, but weather and corrosion.
The coatings can help differentiate products because they have a wide range of custom metallic color possibilities. Metallic pigments and metal-flake sparkle effects can combine with most enamel colors. Brushed, satin, or highly reflective finishes are also possible.
Porcelain enamels are glassy coating materials that protect substrates while sprucing up their looks. They bond to metals (typically carbon steel, stainless steel, cast iron, or aluminum) at temperatures from 1,000 to 1,600°F (538 to 871°C).
There are two general types: ground coats and cover coats. Ground coats contain adherence-promoting oxides and are used for oven cavities, stove grates, hot water tanks, and dual-purpose finish coats. A pyrolytic ground coat is an extremely heat-resistant enamel for self-cleaning oven cavities exposed to operating temperatures of about 1,000°F.
Cover coats provide additional chemical, physical, or cosmetic properties. They need a ground coat as a primer layer. Aluminum enamels can be considered cover coats. They contain low-temperature glasses that fuse directly onto aluminum and commonly serve on cookware. Performance benefits of porcelain enamels include:
Sanitary qualities — The hard, dense surface is an excellent barrier to odor and bacteria.
Easy cleaning — The hardness and abrasion resistance makes for easy cleaning with mild cleaning solutions. High gloss and surface lubricity also lets graffiti wipe off.
Scratch and abrasion resistance – Hardness also gives substantially more resistance to scrapes than the hardest organic coating.
Chemical and corrosion resistance — Surfaces stand up to acids, alkalis, water, solvents, oils, and UV light, as well as to corrosive industrial atmospheres, salt, air, gas, smoke, and soil.
Flameproof — Heat doesn't change physical or chemical properties or appearance.
Color stability — Coating colors are used as physical color standards for the ink, plastics, and textile industries. Glossy, acid-resistant porcelain enamel has shown no change in color or gloss after 15 yr of exposure to weather. And it will not peel, blister, or delaminate from the metal surface if applied correctly.
Environmentally friendly — The application process requires no solvents.
Performance versus stainless steel
Porcelain enamel has long been used as an exterior finish. Several tests show how it compares to stainless steel. The tests gauge hardness, abrasion resistance, impact resistance, room-temperature stain resistance, heat resistance, and cleanability.
Stainless steel is defined as alloy steel containing more than 10% chromium. A passive chromium-oxide layer forms on the surface and resists corrosion. Stainless steel is classified according to the iron phase present in the alloy. Steels of various classifications have properties that suit them for particular uses. The austenitic 304 alloy is widely used in appliances.
For the comparison tests, two porcelain enamels were run against 304 brushed stainless steel: A cover coat (Ferro PC168C on PL52 ground coat) that is typically used on cooktops; and an aluminum enamel (Ferro GL4317) that can be used on cookware. The metallic-look cover-coat enamels were in development at the time of testing. However, their formulation and performance closely resembles that of production cover coats. Metallic-look colors are also available in aluminum-enamel formulations.
Hardness of typical porcelain enamels runs between 5 and 6 on the Mohs scale. This is relatively greater than the 88 HRB
hardness of 304 brushed stainless steel. The ASTM D 33630-00 Standard Test Method for Film Hardness by Pencil Test covers the hardness of organic paints. This test assesses the force required to gouge a coating with a drawing lead of calibrated hardness. Both enamels were off the scale and could not be scratched with the hardest 9H pencil, while a softer 5H pencil scratched the stainless sample.
The ASTM D 4060-95 Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser evaluates abrasion resistance. Panels experienced 2,000 cycles under the hardest CS-17 wheels with a 1-kg load. The technicians measured weight loss. Stainless steel lost about as much weight as the enamels but was visually scratched up and damaged. Chipping has been an issue for the enameling industry. But there's now a better understanding of frit formulations and metal-design principles to minimize the chance for chipping.
Impacts will deform and possibly dent stainless steel, but the metal won't be seriously compromised. However, stainless has a reputation for scratching easily. Furthermore, a porcelain-enamel finish readily covers stamping defects and scratches in the metal, which would otherwise make stainless steel high-cost scrap.
Six foodstuffs and three typical household cleaners served as a test for stain resistance. The foods were barbecue sauce (Bullseye Original), Heinz Ketchup, RealLemon lemon juice, Heinz Worcestershire Sauce, Twinings Earl Grey tea, and Heinz vinegar. The cleaners were Easy Off, Formula 409, and ammonia (Topco Top Crest). Easy Off, containing approximately 5% NaOH, is quite alkaline. Formula 409 contains a mixture of surfactants and a likely smaller amount of NaOH in 2-propanol.
Tests with foods used soils placed under watch glasses for 48 hr. Test duration for cleaners was 24 hr. A value of zero was assigned for no stain and a one was assigned for a permanent stain. The total number of stains was then added up to generate a ranking for the surface.
All six soils left light marks on the 304 stainless steel. In contrast, the aluminum enamel had no stains, and the cover coat was stained only by vinegar. The stainless was attacked by both the Formula 409 and ammonia. Neither stained the cover coat and the aluminum enamel was stained by ammonia.
Heat-resistance tests used the Lab color of panels measured by a DataColor International Spectraflash SF 450 color machine. This machine measures the brightness, the amount of red or green, and the amount of yellow or blue. For consistency, the stainless-steel panel was always measured with the same orientation to the brushing. The panels saw 750°F (399°C) for up to 72 hr. Every 24 hr, panels came out of the furnace and were measured for color after cooling.
The stainless steel rapidly changed color during testing after only 24 hr. Visually, it sharply shifted to yellow and got darker. The cover coat showed little or no change in color readings. The aluminum enamel shifted slightly brighter and more yellow, probably because test temperature was close to the firing temperature of the coating.
A modified version of the European FAN (Facile Á Nettoyer, literally Easy-to-Clean) test gauged clean up. Seven steel rings were glued to the surface of each panel and filled with grape jam, egg beaters, ketchup, salted reconstituted milk, lemon juice, olive oil, and gravy. Test panels then baked at 450°F (232°C) for 1 hr, which decomposed the glue so the rings could be removed. A point system ranked cleanability of each soil for ease of removal and staining, with a maximum score of 42 for a completely wipe-clean surface and a minimum of 0 for a material to which all of the burned-on residue strongly adhered.
The score for stainless steel was about 9, compared to 14 for the aluminum enamel and about 12 for the cover coat. For stainless, the baked-on egg, olive oil, and gravy left stains that were not present on the enamels. Grape jam wouldn't come off any of the materials.
American Trim, (814) 833-7758, amtrim.com
Ferro Corp., (216) 641-8580, ferro.com
Porcelain Industries, (615) 446-7400, porcelain-industries.com
Performance data for the stainless-style cover coat color
Metallic-look porcelain enamel coatings meet or exceed OEM performance requirements for laundry and cooktop applications.
How to coat with porcelain enamel
Evolution cover coats come as powders that get reconstituted with water (100:47) in a high-shear blender. The resulting enamel slip is put through a 60 to 100 mesh screen and applied with a wet spray gun onto a suitable metal substrate. It goes on top of a fired-enamel ground coat that can be applied either electrostatically or by wet spray. The cover-coated part is fired at temperatures from 1,480 to 1,570°F (804 to 854°C) for 2 to 3 min. The furnace must be at the proper firing temperature from the beginning, not ramping up. It's also important that parts be correctly spaced on hangers. Parts should be designed to hang with the longest dimension down to reduce warping.
All forming and assembly of the product takes place before finishing. There are no post-finishing steps, although panel controls or other decorations are sometimes added by screen printing. The fired thickness of the cover and ground coats is 5 to 6 mils (125 to 150 μm).
While most laundry and range OEMs have porcelain enamel coating lines, others make use of job shops such as American Trim, Erie, Pa. and Porcelain Industries, Dickson, Tenn. Application costs typically are competitive with those of powder coating.
Substrates should be enameling grade, low-carbon steel that can withstand the process firing temperatures. Use of hot rolled or high-carbon steel generally results in a poor finish. Most major steel suppliers carry enameling steel.
To prevent warping during firing, steel should be at least 14 to 24 gauge, thicker for large parts. Any attachments should be made of the same material and two gauges lower than the base part. Flanges should also be used to make parts more rigid. Flanges on panels of 10 to 12 ft (3.05 to 3.66 m) should be at least 0.50 in. (12.7 mm) with a depth of 0.75 in. (19.05 mm). Try to avoid cut-outs and notches in flanges. They concentrate stresses on the flat area being reinforced, leading to hairline defects, cracks, and chipping. In place of flanges, long edges may be rolled to add rigidity. Open rolling is recommended to help clean the part, and rolls should have a minimum radius of 0.19 in. (4.82 mm) to prevent spalling of the coating.
In general, enamels tend to pull away from sharp edges and corners. Rounded corners let the coating adhere better and minimize chipping. For two-coat enamels, the radius should be at least 0.19 in. (4.82 mm). Less than that could lead to mechanical failure in the fired coating. Embossing in a panel should also observe the minimum radius.
If the ground coat is to be applied electrostatically, it's important to avoid creating Faraday cage areas or corners that can keep the powder enamel from sticking. Observing minimum radius requirements and making tabs less than 0.50 in. long will prevent Faraday cage defects.
Screw or bolt holes must be large enough to avoid formation of enamel bridges. Hole dimensions should be 0.062 in. (15.75 mm) larger than the bolt to compensate for enamel buildup, or the hole may need reaming. Where appearance is critical, the holes should be dimpled to reduce or hide sharp edge burn-off.
More detailed information, including guidelines for one-coat aluminum enamels, is available in the technical manual PEI-101: Design & Fabrication of Metal of Porcelain Enamel. This and other technical manuals, coaters, and other suppliers can be found on the Porcelain Enamel Institute Web site (www.porcelainenamel.com).
Edited by Jean M. Hoffman