Engineering Porcelain Enamel
A Novel Non-Stick Porcelain Enamel

by Charles Baldwin, Alain Aronica, Brad Devine, Graham Rose,Ferro Corporation

To fulfill the industry’s long-held desire for a truly easy-to-clean surface, a new ceramic-based, non-stick coating for household and commercial cooking appliances has been developed that has excellent scratch, abrasion, and heat resistance, as well as superb cleanability.

Because porcelain enamel is a glass-based coating, it is much more scratch-, abrasion-, and heat-resistant than organic paints. However, burned-on food residue forms hydrogen bonds and strongly adheres to enamel. The cleanability of enamel can be improved by maximizing the acid resistance, catalyzing the transformation of burned-on residue into ash, pyrolyzing the residue into ash, or applying an easy-to-clean top-coat to the enamel [1,2]. However, none of these have the cleanability of organic non-stick coatings.

The two major families of organic non-stick coatings are those composed of either fluoropolymers such as polytetrafluoroethylene (PTFE) or of silicone-polyesters. PTFE-based coatings have been widely used on small appliances, cooktop grills, cookware, and bakeware. Middle-market PTFE coatings are two-ply with a binder-containing base coat and a fluoropolymer-rich finish coat. The most durable PTFE coatings for high-end cookware are three-coat systems with a ceramic oxide-containing intermediate coat for scratch resistance [3]. Silicone-polyesters are widely used on bakeware as well as the exterior of cookware in a variety of colors. These two families of organic materials have a low surface energy, which prevents the adhesion of burned-on foods. Thus, for example, only water and paper towel can be used to remove residue with the application of minimal force. However, both types of coating are soft and easily scratched or gouged. The abrasion resistance of the PTFE coatings tends to be much less than porcelain enamel. Additionally, there are two health and environmental concerns with PTFE. The first involves a surfactant used in the manufacture of fluoropolymer resins, but this material is not present in cured PTFE coatings [4]. The second is the emission of toxic by-products from PTFE during thermal decomposition, which can occur if, for example, cookware is overheated [5].

RealEase™ is a ceramic-based, non-stick coating developed by Ferro. This new coating technology marries the cleanability of the organic non-sticks to the durability of vitreous enamel. In a single coat, it offers the scratch resistance of enamel and the cleanability of PTFE. It is a patented technology that stands to bridge the gap between the organic non-sticks and low-temperature porcelain enamel.

Processing

The new coating technology is applied to degreased and roughened aluminum, aluminized steel, brass, or copper. Unlike conventional enamel, it can be applied to die-cast aluminum. Roughening can be done with sandblasting or acid etching. It can be applied to mild steel, stainless steel, cast iron, glass, or ceramics after the application of special patented hard-bases. Hard-bases consist of an enamel base coat with a rough surface [6].

This new coating technology is supplied as a wet, ready-to-use (RTU) system. The RTU slip requires no adjustment of set, gravity, or color. Like enamels and unlike PTFE, the overspray can be reclaimed and re-used at 30 percent. The coating is dried at about 125ºF (52ºC). Once dried, it may be screen printed with other non-stick enamel colors and fired in a single process. The coating fires at less than 1,000ºF (538ºC). The time varies with the metal thickness and thermal conductivity, and a convection-type oven is preferred. Volatile emissions during curing are 70 to 80 percent less than PTFE.

Fired Properties

Typically, the non-stick enamel is about 2-mils (50-60 µm) thick without hard-base and as low as 1.5-mils (40 µm) with hard-base. The hard-base is typically 1.5-mil (40-µm) thick and requires an additional fire.

This new coating technology can be supplied in many colors, except bright white. Generally, the color palette is similar to that for aluminum enamels. The new coating has a satin finish with a 60-degree gloss of 2-10, depending on firing conditions. Possible effects are mottles, stipples, shadow application, and recently developed metallic colors.

The new coating technology has lower surface energy than conventional enamel and is, therefore, hydrophobic and easy-to-clean. The balance of forces arising from a sessile drop of liquid water (l) on the coating surface (s) under a vapor (v) is schematically shown in Figure 1 and is described with Young’s equation, which is:

where:
	is the energy per unit area of the appropriate interface, and
	is the contact angle between the liquid and the substrate.

If:

the surface will be wetted to decrease the area of the higher energy s/v interface; this is the situation with conventional enamel.

If:

balling up of the water will occur to reduce the area of the higher energy s/l interface.

Typically, is only 60 degrees for conventional enamel, about 120 degrees for PTFE, and about 110 degrees for the new coating technology. Droplets of water wet and spread on a dual-purpose black ground coat. On non-stick enamel, they are repelled by the surface and form beads.

The cleanability has been assessed by the European FAN (Facile à Nettoyer, translated to Easy-to-Clean) test. First, five steel rings are glued to the coating surface. Second, salted whole milk, gravy, lemon juice, egg yolk, and ketchup are placed in each ring. Third, the test panel is baked at 482°F (250°C) for 30 min. Then, the glue thermally decomposes, which allows the ring to be removed. Finally, the cleaning is rated using the fan system.

The results for the new coating technology, PTFE, stainless steel, and a typical porcelain enamel are shown in Figure 2. The new non-stick enamel achieves a perfect score and shows cleanability equal to PTFE and much better than stainless steel or traditional enamel.

The new coating technology exhibits good acid and alkali resistance. It achieves a rating of AA on the PEI T-21 citric acid spot resistance test. It is UV-stable and unaffected by solvents. It has excellent scratch and abrasion resistance.

Scratch resistance is evaluated using ASTMD3363-00 “Standard Test Method for Film Hardness by Pencil Test.” The force required to gouge a coating with a drawing lead of calibrated hardness is assessed. The new coating shows a rating of 8H compared to an average of 4H for PTFE coatings. Results are shown in Figure 3.

Abrasion resistance is measured using ASTMD4060-95 “Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser.” The weight loss before and after 2,000 cycles under the most abrasive CS-17 wheels under a 1-kg load was measured and normalized to the coating thickness. Results are shown in Figure 4. On average, the new coating technology was less damaged by abrasion than PTFE and significantly less than silicone-polyesters.

The new coating technology has more heat resistance than the organic non-stick coatings. It can be used in service at 600° F (315°C) for extended periods of time. Figure 5 shows the color stability of non-stick enamel compared to a high-temperature silicone-polyester paint. After 100 hours at either 662°F (350ºC) or 752ºF (400ºC), non-stick enamel showed little color change versus a ?E as high as 51.17 for the silicone-polyester after 100 hours at 752º F (400ºC). PTFE shows similar degradation at elevated temperatures.

Potential applications for non-stick enamel include the interior or exterior of all types of cookware, including ceramic bakeware. The new coating is listed with NSF International as compliant with Standard 51. It is certified as safe for food contact (acidic, aqueous, < 8-percent alcohol beverages, dairy, dry solids, and other) up to 750°F (398ºC). With superior heat resistance, it is very promising for heavy-duty use environments such as commercial kitchen restaurants. It would be very suitable for small appliances such as toaster ovens, waffle irons, and microwaves. For large cooking appliances, it is an alternative to silicone-polyesters and PTFE on grills, griddles, and simmer plates.

In summary, the new coating technology is the first true, non-stick porcelain enamel. It is water-based and applied as a single coat. Compared to silicone-polyesters and PTFE, it is much more scratch-, abrasion-, and heat-resistant.

References

[1] H. Berkenkoetter et al., “Method of making a temperature and scratch-resistant anti-sticking coating,” USP 6372290, April 16, 2002.

[2] “U-Jin’s Coating System”, U-Jin Porcelain Enamel Limited 2001. (12 April 2004)

[3] P. Thomas, “The Use of Fluoropolymers for Non-Stick Coating Utensils,” Surface Coatings International 12, 1998.

[4] “PFOA-Facts.com” The Society of the Plastics Industry, Inc. (SPI) (14 September 2004).

[5] David A. Ellis et al. “Themolysis of Fluropolymers as a Potential Source of Halogenated Organic Acids in the Environment,” Nature 412, 2001.

[6] William D. Faust, “Ceramic Substrate for Non-Stick Coatings,” Proceedings of the Porcelain Enamel Institute Technical Forum, 63, 2001

This is an edited version of a paper delivered at the Porcelain Enamel Institute (PEI) Technical Forum, held April 27-29, 2004 in Nashville, TN, U.S.