Porcelain enamel is produced from natural, inorganic, raw materials. It is not only an exceptionally durable finish, it is also 100 percent recyclable and as such an inherently environmentally-friendly product. A blend of minerals is melted in much the same manner as common window or container glass; this is the beginning of the process. Once all of the ingredients are fully melted—at temperatures over 2000 degrees Fahrenheit—the molten mixture is poured from a smelter and quenched between water-cooled rollers. This process is known as “fritting.” The quick-cooled ribbon of glass is then shattered into flakes, or frit. The chemical composition of the frit/glass is tailored to the end-product performance requirements and metal substrate used.
For the frit formulator, there are a few key things the enamel coating must accomplish. For a ground coat, the first layer of glass on the metal, the porcelain enamel glass must 1) adhere to the metal (bond), 2) match the thermal expansion of the metal substrate, and 3) have a smooth, defect-free surface. For the second layer, the cover coat, the enamel must 1) have the desired surface color and texture, and 2) have the appropriate chemical durability for its intended use. In special cases, a single coat of enamel must accomplish all of these things, and in other cases, three enamel layers are required. Table 1 shows the common chemical compositional range of both ground coat and cover coat enamels, along with the basic function of the chemical constituents.
Porcelain Enamel Preparation
To prepare the frit for application to the substrate, it is combined with other materials, which may include refractories, pigments, opacifiers, clays, and surfactants. These ingredients are mixed as a dry powder or an aqueous suspension to be applied to the metal substrate. The frit and raw materials are usually ball-milled to accomplish both complete mixing and proper particle size reduction and distribution. Today, most enamel applicators purchase either electrostatic dry powder enamel which is ready to spray or ready-to-use (RTU) powder where water is added to make the liquid enamel suspension.
Table 1: Frit Compositional Ranges
Powder Enamel Application
With the invention of powder coating about 60 years ago, a lot of work began in the coatings and finishing industries to leverage this new technology. The porcelain enamel industry was an early adopter of electrostatic powder coating, but a lot of development was required from both the material and equipment aspects. By the 1980s, porcelain enamel powder coating was taking place in many factories, primarily for flatware but also in several lines for cavities, like ovens and dishwashers.
The physics of powder coating of porcelain enamel powder is the same as for traditional powder paint, although there are some differences in behavior, etc. One of the most noticeable is the transfer efficiency; porcelain enamel powder is about 40 percent efficient, so much more powder travels through the recovery system in comparison with traditional powder paint. Additionally, porcelain enamel powders are often applied in a thicker film; usually six to eight mils. Almost all of the electrostatic dry powder porcelain enamel application is for major appliances; for exterior parts it is usually a two coat/one fire process, and for interior (non-appearance) parts it is a one coat/one fire process. In two coat/one fire processing, the first coat is a special ground coat enamel (designed to adhere to the steel) which is applied to a thickness of one to two mils, directly followed in a second application booth with a cover coat (for the final color of the part) at a thickness of five to seven mils. Both layers are melted/cured at about 1500 degrees Fahrenheit in a single pass through the furnace. Firing times are typically dependent on the thickness of the metal; for successful enamel coatings the time at peak temperature is two to three minutes.
For electrostatic powder enameling, the benefits are similar to powder paint systems. They include full recovery of overspray, coating thickness control, film uniformity, better edge coverage, gloss, and smoothness of finish after firing, plus the advantages of the automation and control of today’s powder application systems. The primary limitation for electrostatic powder enameling is the choice of colors; this is because the color must be smelted into the frit. The use of frit plus pigment is problematic due to differences in particle size, density, and electrical charging parameters – thus, recycling is difficult since the various constituents apply at different rates. Color change requires either multiple booths or significant clean-up time.
We have seen recent growth in the combination of robotics with powder application, which is improving production repeatability and efficiencies as well as reducing costs. R&D on electrostatic powder enamel chemical formulations and grinding technologies is also continuing with the objectives of improving application parameters and reducing defects.
Wet Enamel Application
For wet porcelain enamel systems, the typical application includes spraying or flow-coating. However, there are other application methods for specific end products such as roll coating of a continuous strip for whiteboards and electrocoating or electrophoresis which provides a very smooth uniform surface, along with best edge and hole coverage, when both sides of a substrate are enameled. Decorating enameled products/surfaces employs a variety of other processes including screen printing, direct emulsion imaging, digital ink-jet printing, transfers (i.e., water slide decals), and hand painting.
This is the step in the process where two materials, glass and steel, become one. Once coated, the parts must be fired at about 1500 degrees Fahrenheit to properly fuse the glass coating and establish the strong chemical bond to the steel or other metallic substrate.
Currently there are a number of developments underway in multiple areas, including new uses for porcelain enamel, improved frits and materials, as well as better enamel application systems and equipment.
One promising new use of porcelain enamel is a reactive glass coating for reinforcing steel in concrete where the coating actually bonds to the concrete. For smooth steel parts like fibers, the pullout strength improvements are as great as five times that of uncoated steel. For deformed rebar, the improvement in pull out strength is about 15 percent, which can be significant in areas of rebar crowding where development length is an issue. Almost equally important, the porcelain enamel coating also provides a strong barrier to corrosion. The electrostatic powder coating process is showing promise in this developing area. Additionally, proprietary developments are underway for several markets: renewable energy, oil and gas extraction/transportation, and high temperature coatings for metals.
In the area of new enamel materials, research and development continues on new colors and surface finishes and effects. This includes metallic colors, simulation of polished metal surfaces, textured surfaces (either the enamel or the steel substrate), perforated metal, and applications of porcelain enamel in public art projects.
For application systems, development continues on improving powder transfer efficiency, better enamel powder delivery, and new control automation technologies. Additional work is being conducted on the integration of robotics into both powder and wet porcelain enamel application systems.
With roots dating back as many as 4,000 years, porcelain enamel has a long and evolving history. From early applications found in artifacts like jewelry and religious iconography to industrial applications dating back to the late 1800s with the enameling of cast iron, this technology has continued to reinvent itself to meet the needs of modern manufacturing. With an array of developments underway, no doubt porcelain enamel will continue to be a relevant coating technology for many years to come.