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issue: December 2006 APPLIANCE Magazine

Porcelain Enamel
Advanced Coatings for the Home of Tomorrow


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by Charles Baldwin, Brad Devine, Dave Fedak, and William Warner, Ferro Corporation

As mature products, porcelain enamel and major appliances require technological advances for added sales and profitability. Three new porcelain enamel products and their most suitable uses are reviewed.

Product Life Cycle

Figure 1 shows a plot of product life cycle versus sales and profits. [1] There are four stages. First, during the introduction stage, there is typically a product development phase where profits may be negative as research and development costs are recovered by initial sales. The product should be carefully monitored and can be withdrawn if growth is not reached in an acceptable timeframe. Second, if the product sells successfully, there is a stage of rapid growth as orders quickly increase. Output surges, and the price can be set high if there is little competition. Portable digital music players such as the iPod are at this stage. Maturity, the third stage, is the most common and the one that describes both porcelain enamels and major appliances. Marketing and finance are the most common activities during this stage. Competition can be fierce, and research and development activities are focused on incremental improvements or increasing efficiency. Finally, in the decline stage, perhaps from technological obsolescence, the market begins to shrink, as do sales and profits. The choice may be made to discontinue the product. VCRs and typewriters would be in the declining stage.
While porcelain enamel usage remains strong on major appliances and in related industries (plumbing ware, barbeques, cookware, etc,), a look at the major appliance milestones listed in Table 1 suggests a need for innovation.

Year Milestone   
Mid-1850s
First gas range   
1893 First powered dishwasher
1924 First white enameled range   
1963 First self-cleaning ranges   

Table 1. Appliance milestones [3]   


Furthermore, the percentage of surface area of a range that is porcelain enameled has fallen quite a bit since the first white enameled oven came off the line in 1924. A significant portion is powder-paint coated. Alternatively, stainless steel is used. Refrigerators, dishwashers and clothes dryers are rarely porcelain enameled today.
Two new features engineered into porcelain enamel are enhanced cleanability and metallic appearances. For cleanability, wipe-clean release was created for temperatures below 600°F (316°C), and alternatives to self-clean (and continuous clean) porcelains were developed for temperatures higher than 600°F (316°C). To modernize appearance, metallic enamels have been formulated that capture the appearance of stainless steel while retaining the mechanical and thermal durability of the porcelain.


Figure 1. Product life cycle [2]

Wipe-Clean Release Enamel

Non-stick enamel combines the cleanability of the organic non-sticks with the durability of vitreous enamel. It offers the scratch resistance of enamel and the cleanability of PTFE (polytetrafluoroethylene). This is a patented technology that bridges the gap between the organic non-sticks and low-temperature porcelain enamel.
For features and benefits, the improved cleanability saves consumers time and effort. Because the coating is difficult to scratch and heat resistant, it retains its original appearance longer than PTFE. Its ceramic nature also suggests environmental friendliness.


Figure 2. Cleanability test results

Cleanability was evaluated using the Easy-to-Clean (ETC) test, developed by Ferro-France, by cooking Egg Beaters®, ketchup, salted whole milk, lemon juice, and gravy separately onto the coating at 450°F (232°C) for 1 hour and assessing the force needed to remove each soil. A score of 5 was received if the soil was easily removed, and a score of 1 was received if it could not be removed, for a maximum possible score of 25. The results are shown in Figure 2. The cleanability of traditional enamel exceeded that of stainless. In turn, the coating matched PTFE and out-performed enamel.
The hardness was measured using ASTM D 3363-00 “Standard Test Method for Film Hardness by Pencil Test.” Different calibrated hardness pencils are worked through from the hardest to the softest and the one that will not rupture or gouge the coating is reported. The range, from softest to hardest, is: 6B - 5B - 4B - 3B - 2B - B - HB - F - H - 2H - 3H - 4H - 5H - 6H – 7H – 8H – 9H
Figure 3 shows the results of pencil hardness testing conducted on a PTFE coating and the non-stick enamel at room temperature and in a pan left on a high flame on a gas range for 30 minutes. Note that the PTFE coating softened considerably. Therefore, during cooking, any metal utensils would be more likely to severely damage the coating.


Figure 3. Pencil hardness test results


With superior cleanability, scratch resistance and heat resistance, potential applications for the non-stick enamel material up to 500°F to 600°F (260°C to 316°C) can be considered. For domestic uses, these include cookware and bakeware (aluminum, aluminized steel, stainless steel, or ceramic), small appliances, toaster and microwave ovens, simmer plates, and outdoor/backyard grills/griddles. Because this coating is certified as safe for use in restaurant kitchens, it is also suitable for commercial kitchenware, cookware, bakeware, and appliances.

SteamClean Enamel

Oven cleaning technology currently consists of three types: (1) self-cleaning pyrolytic ground coat, (2) non-self-cleaning ground coat and (3) catalytic continuous clean enamels.
For the self-cleaning enamel, food residue is reduced to ash by exposure to temperatures between about 900°F and 1,000°F (482°C and 538°C). This is the leader in the North American market because of minimal consumer interface with the oven to remove the ash after the cleaning cycle.  However, there are several concerns surrounding its use. First, high heat is required, necessitating extra insulation and safety interlocks. Second, there are concerns about the possible release of toxic fumes. [4] Third, the insulation also makes the addition of advanced electronics (e.g., to integrate the range into a home network) more difficult. Finally, to meet the requirements of surviving multiple clean cycles, the enamel generally contains hard, chemically resistant frits that, without the high-temperature exposure, have poor release properties on their own.
Non-self-clean enamel requires significant effort by the consumer and/or harsh alkaline saponifying cleansers (e.g., Easy-Off®) that have a pH of approximately 14.[5]
Catalytic continuous clean enamels contain specially formulated glasses, which have high levels of metal oxides. When fired out with a porous microstructure, this enabled the reduction to ash at normal cooking temperatures. This type of enamel has largely fallen out of use in North America.
Steam clean enamel is a porcelain enamel with a patented formulation that allows baked-on food residues to be released with exposure to moisture (either as water or steam). As such, it addresses many of the problems with the self-cleaning enamel. It has the mechanical durability and thermal resistance of traditional porcelain enamel, can be applied in a single fire to steel and fires out between 1,470°F and 1,570°F (799°C and 854°C). 



Figure 4. (a) Plates with baked-on food before and (b) after exposure to warm soapy water for 15 minutes


Figure 4 shows the result of a cleanability test. A self-cleaning ground coat panel is on the left and a steam clean enamel-coated panel on the right. Clockwise from the upper left, the soils were a 50:50 egg/oil blend, French vanilla cake mix, cherry preserves, and Hollandaise sauce. These were baked on at 450°F (232°C) for 1 hour in a gas oven. The panels were then soaked in warm soapy water for 15 minutes. The soils were all released from the steam clean-enameled plate. On the self-cleaning enamel, the cherry preserves could not be removed even with vigorous scrubbing and abrasive cleaners.
To obtain quantitative data, the easy-to-clean test was run on alloy 304 stainless steel, a plate of Ceran glass cooktop material, a typical titanium-opacified cover coat, self-cleaning ground coat, steam clean enamel, and non-stick enamel. Results are shown in Figure 5. After the moisture exposure, the release properties of the new technologies significantly exceeded those of a pyrolytic ground coat.

Figure 5. Cleanability results for appliance finishes

Metallic Enamels

Previously, a comparison of the mechanical, chemical and thermal resistance of porcelain enamel and stainless steel concluded that while the appearance of stainless steel is very popular, porcelain enamel is more suitable for an appliance finish.
While stainless steel has a pencil hardness of 5H and a Rockwell Hardness of 88 HRB (roughly equivalent to a Mohs hardness of 4), the pencil hardness of sheet steel porcelain is so hard it is off the scale. It also has a Mohs hardness of 5 to 6. Furthermore, stainless-steel plates have shown to significantly turn yellow and darken after exposure to 750°F (399°C), while the enamel did not show any signs of discoloration.[6]
To combine the durability of porcelain enamel and appearance of the metal, a stainless-steel colored enamel was developed. The color is described as “stainless style.”
Table 2 shows results of comparative testing between a conventional titanium-opacified cover coat and the stainless-style cover coat. First, the materials were tested according to ASTM D 4060-95 “Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser” and then ASTM C 282-54 “Resistance of Porcelain Enameled Utensils to Boiling Acid.” The test as preformed for 6 hours with a 6-percent citric acid aqueous solution and a 5-percent tetrasodium pyrophosphate solution. Third, the fired enamels were evaluated per ASTM C 282-99 “Standard Test Method for Acid Resistance of Porcelain Enamels (Citric Acid Spot Test).” Lastly, the panels were exposed to three heating cycles at 600°F (316°C) for 30 minutes, followed by immersion into room temperature water to test thermal shock resistance. With such properties, the metallic enamels can be used for a wide range of applications, including surfaces currently coated with enamels firing in the 1,500°F range such as cooktops, ranges, sanitary ware, refrigerator exteriors, steel or cast-iron cookware and bakeware, car mufflers, and barbecue grills.



Table 2. Comparative test results between a white cover coat and
the stainless-style cover coat

 
The rising cost of nickel is also making stainless steel more expensive to use in appliances. There has been a the 575-percent rise in the cost of nickel between 2001 and 2006.[7, 8] Stainless-steel alloy 304, which is the one most used in major appliances, is 8 percent to 10 percent nickel.[9] Switching from stainless steel to metallic enamel has been reported to offer up to a 50 percent cost savings while at the same time improving scratch resistance, heat resistance and cleanability.

Summary

Three new porcelain enamel technologies were reviewed: non-stick enamel, steam clean enamel and stainless steel-colored cover coat. First, with a firing temperature and cleanability similar to PTFE coatings but superior mechanical and thermal resistance, non-stick enamel would be most suitable for use on pots, pans, bakeware, griddles, and small appliances. Because it is difficult to scratch the material and it has a lower tendency to discolor on exposure to heat, the original appearance would be maintained longer than PTFE. Second, steam clean enamel offers an oven-cleaning mechanism free from the necessity of high heat and fumes. Third, metallic enamels would permit enamel to have new appearances, reduce costs and be suitable for high-end products. These three new products create the potential to move to the growth portion of the product life cycle curve.


References

[1]“Product Life Cycle Management,” Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/wiki/Product_life_cycle_management; June 12, 2006.
[2] “Marketing – Products – Product Life Cycle,” http://www.tutor2u.net/business/marketing/products_lifecycle.asp (June 12, 2006)
[3] “Appliance Milestones,” http://www.aham.org/consumer/ht/action/GetDocumentAction/id/1408 (June 25, 2006)
[4] “Tips from eHow Users on How to Clean an Oven,” http://www.ehow.com/tips_2017.html; July 31, 2006
[5] Reckitt & Colman Inc. 1993.
Low Temperature Non-Caustic Oven Cleaning Composition.
United States Patent 5,380,454.
1993-07-09.
[6] Dave Fedak and Charles Baldwin, “A Comparison of Enameled and Stainless Steel Surfaces,” Proceedings of the 67th Porcelain Enamel Institute Technical Forum, 45 – 53 (2005).
[7] “Nickel,” USGS Minerals Information: Nickel, http://minerals.usgs.gov/minerals/pubs/commodity/nickel/nickemcs06.pdf
(24 October 2006).
[8] “Nickel,” London Metal Exchange: Primary Nickel, http://www.lme.co.uk/nickel.asp
(24 October 2006).
[9] ASM Committee on Wrought Stainless Steels, “Wrought Stainless Steels” in Metals Handbook Ninth Edition Volume 3: Properties and Selection: Stainless Steels, Tool Materials, and Special-Purpose Metals, edited by William H. Cubberly et al., (American Society for Metals: Metals Park, OH, 198), p. 5.


This is an edited version of a paper presented at the 68th Annual Porcelain Enamel Institute (PEI) Technical Forum, held in May 2006.

 

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