PORCELAIN ENAMELS WITH A METALLIC APPEARANCE
Charles Baldwin and Louis Gazo
Ferro Corporation, 4150 E. 56th Street, Cleveland, OH 44105 USA

Introduction
As a fused glass coating, porcelain enamel has long been used to protect metal substrates from chemical attack, abrasion, and other corrosive and mechanical damage while also serving a decorative function. While a wide color and gloss range has been possible using traditional oxide pigments and other additives, it has not been possible to formulate enamel with an appearance similar to uncoated aluminum or stainless steel. Unfortunately for frit suppliers, this has left stainless steel as a major viable alternative to an enameled surface.
Generally, the only vitreous metallic finishes have been achieved using pearlescent pigments.
These are flakes of mica coated with titanium dioxide or some other type of surface modification.
While these can provide a sparkle appearance, they do not have the opacity of metallic pigments.
Organic fluoropolymer and silicone coatings with a metallic appearance are commercially available, but, relative to porcelain enamels, these have poor mechanical and thermal properties.
Using new additive technology, enamels with a true metallic appearance have been developed that can be applied to aluminum, aluminized steel, or steel. These have the performance of vitreous enamel while having an aesthetically pleasing bright metallic finish. The metallic look can be used to break up the uniform shades of black and white that have been very common on porcelainized surfaces.

Metallic pigments
The perceived color of metal arises from the wavelength distribution of incident radiation. Metals are opaque because incident radiation within the range of visible light excites electrons into unoccupied states above the Fermi energy. The energy of a photon then emitted by an electron as it moves from a high to low energy state equals that of the original absorbed electron. A bright silvery appearance with incident white light shows that the metal is highly reflective over the entire visible spectrum [1]. Because of the band gap in the electronic structure of oxide pigments and glasses, the energy of the emitted photons does not necessarily equal that of absorbed photons.
Thus, the oxides are colored and a silvery metallic appearance is difficult to obtain in practice.
Metallic pigments are a small, useful class of inorganic colorants made up of fine particles of ductile metals. Metal pigments are especially good for providing a metallic appearance because the opacity, metallic color, sheen, and density of the bulk metal are retained [2].
Metallic pigments originated with the beaten gold foil used decoratively by the Egyptians alongside the first examples of porcelain enamel. Over the years, the foil was beaten thinner and thinner and eventually into gold powder used in artwork and inks. The high cost of gold lead to a search for substitutes. In the nineteenth century, Sir Henry Bessemer invented a mechanical stamping process using steel hammers on steel anvils to make metal foils. At the same time, advances in aluminium smelting decreased the cost of aluminum. The explosion risk associated with dry communitation of aluminum was solved in the 1920’s with wet ball milling. Since the 1970’s, there has been an upswing in the use of metallic products. New finishes with a metallic flair are expected to be in demand on automobiles through 2007 – 2008 [3].

Table 1. Properties of metallic pigments
Metal
Color
Brilliance
Density (g/cc)
Corrosion
Resistance
Tm (ºC) [4]
Aluminum
White
High
2.7
Moderate
646 – 657
Nickel
Off-white
Medium
8.9
Good
1454
Stainless Steel
Off-white
Medium
~8.0
Good
1370 - 1400

Table 1 shows selected properties of three types of metallic pigments. The most common metallic worldwide, aluminum pigments have long been used in paints and inks. Aluminum is particularly attractive because of the silvery color, low density, and relatively low cost. However, to be used inaqueous coatings, considerable work has been invested in the development of encapsulation technologies to prevent corrosion and out gassing caused by the water and alkaline pH values [5].
Nickel provides a slightly yellow color with a rich luster. For stainless steel, alloy 316 is the most commonly used, and stainless steel pigments offer advantages over aluminum pigments. When added to an epoxy coating, an increase in the abrasion and corrosion resistance was observed in addition to the decorative effect [6]. Several other pigments including aluminum-bronze, copper, zinc, iron, silver, and titanium have been used to make metallic organic coatings.
The two main types of metallic pigments are leafing and non-leafing grades. Leafing pigments align horizontally at the coating surface to produce a dense, scale-like layer. Non-leafing pigments do not orient at the surface and create a random, “sparkle” effect. Specifically, sparkle is the reflection of light in a non-uniform manner. Whether or not a metallic colorant will leaf depends on the hydrophobocity of the fatty acid with which it was milled by the pigment supplier [7].

Metallic enamel formulation
Blending commercially available metallic pigments with a low-temperature enamel system made metallic enamel. To coat steel parts, a low-temperature frit with a linear thermal expansion of about 14 x 10-6/°C was milled in a pigment-free formulation suitable for application to aluminized steel.
The mill was emptied, a proprietary wetting agent was added, and a few percent of the metallic pigments was blended-in. The basic recipe is shown in Table 2. To coat aluminum parts, clearenamel with a suitable low-temperature, higher-expansion, about 16 x 10-6/°C, frit was used.

Table 2. Metallic enamel formulation
Raw Material
Amount
Enamel Slip
100
Pearlescent Pigment
0 – 5 %
Metallic Pigment
0 – 5 %
Wetting Agent
5 drops/L enamel slip

Fig.1 were then enameled by first applying an electrostatic base-coat typically used for 2C1F applications. This was fired, the metallic enamel was applied using wet spray methods, and then the part was fired at 538 °C for sufficient time for the enamel to show gloss. The drip pan, lantern hood, and grill all showed a silvery, metallic appearance not unlikebare-stainlesssteel.

Fig. 1 Three examples of parts coated with metallic enamel: (a) drip pan, (b) grill, (c) lantern hood.

 

Testing
The acid resistance of the metallic enamels was determined by the frit used in the clear enamel milling. The spot acid resistance was tested using ASTM C 282-99 “Standard Test Method for Acid Resistance of Porcelain Enamels (Citric Acid Spot Test).” This procedure is equivalent to PEI T-21 “Test for Acid Resistance of Porcelain Enamels (Citric Acid Spot Test).” Several drops of a 10 % aqueous solution of anhydrous citric acid were placed on the panel underneath a watch glass for 15 minutes. The degree of etching was rated from C to AA with AA being the best possible rating. When the medium expansion frit was used, an acid resistance rating of A was obtained.

To verify that the low-temperature enamel was well bonded to the ground coat, a drip pan like the one shown in was thermally shocked. The part was held at 316 °C for 30 minutes and then immersed in room temperature 21 °C water. After three cycles, none of the metallic coating spalled or flaked off the part, indicating good bond, which was then confirmed with impact testing.
The abrasion resistance of the metallic enamels was compared to organic coatings and uncoated stainless steel using the taber abrasion test described in ASTM D 4060-95 “Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser.” For this test, 10 cm by 10 cm square panels with center holes were abraded on a Taber abraser. The panel rotated on the abrasion machine for a set number of cycles under weighted abrasive wheels. Every 1000 cycles, the wheels were re-surfaced using silicon carbide paper disks, and a total of 2000 cycles were run.
All of the panels were tested with the most abrasive CS-17 wheels. The panels were weighed before and after the test. Metallic coatings made with either stainless steel pigment or aluminum pigment were compared to conventional aluminum enamel, bare stainless steel, and two commercially available high-temperature organic coatings. The organics were a silicone-polyester used on griddles and bake ware and a PTFE-type coating. The PTFE coating was from a high-end piece of cookware and was a three-layer coating with a base coat, a filled interlayer for abrasion resistance, and a fluoropolymer-rich surface layer for hydrophobocity.

Fig. 2 Comparative taber abrasion test results.

The taber abrasion results are shown in Fig. 2. Because it is not thought to contain rereinforcement, the silicone-polyester showed the most weight loss followed by the PTFE coating.
The metallic enamels showed similar weight loss to a traditional aluminum enamel and to bare stainless steel.
The Pencil Test evaluated the scratch resistance of the coatings.” The following hardness scale is assigned to a set of drawing leads where 6B is the softest and 9H the hardest:

6B – 5B – 4B – 3B – 2B – B – HB- F – H – 2H – 3H – 4H – 5H – 6H – 7H – 8H – 9H

The test plate was placed on a firm, level horizontal surface. Starting with the hardest lead, the pencil was held firmly against the panel at a 45° angle and pushed firmly away from the test operator. Sufficient force was exerted to either crumble the lead or to cut through the coating.

This was repeated going down the scale until a pencil was found that would not cut through the film or scratch the surface for a distance of at least 3 mm.

Table 3. Pencil hardness of coatings
Coating Pencil
Hardness
Bare Stainless Steel
5H
Silicone
HB
3C PTFE
7H
Aluminum Enamel
> 9H
Metallic w/ Aluminum
> 9H
Metallic w/ Stainless
> 9H

Table 3 shows the hardest pencils that would not rupture or scratch the surface. The metallic enamels and conventional aluminum enamel could not be scratched with the hardest 9H lead.
Because of the reinforcement, the PTFE had a hardness of 7H. Stainless steel could be scratched with a 5H pencil, and the silicone-polyester was the softest and could be scratched with an HB lead.
The heat resistance of the metallic enamels was compared to silicone-polyester paints by exposing coated panels to 400 °C for two 50-hour intervals. The L, a, b coloring parameters were measured initially, at 50 hours, and at 100 hours. The color change ΔE was calculated using Equation 1.

In Equation 1, Li, ai, and bi are the L, a, b coloring parameters at 0 hours and Lf, af, and bf are the L, a, b coloring parameters at 50 or 100 hours.

Fig. 3 Change in color versus hours at 400 °C.

Fig. 3 shows ΔE versus hours at 400 °C for two silicone-polyester coatings and two metallic aluminum enamels. The first silicone completely changed color. Even though the second was colored with an inorganic oxide, it still significantly changed color and was severely degraded after 100 hours. The metallic enamels showed very low values for ΔE and retained their original appearance after 100 hours at 400 °C.
The L, a, b coloring parameters of a stainless steel 304-alloy coupon was measured before and after exposure to 400 °C for one hour. Initially, L = 51.09, a = -0.14, and b = 0.51. Afterwards, L = 40.24, a = 3.10, and b = 7.14 with ΔE = 13.12. With ΔE < 1 after 100 hours at 400 °C, the metallic enamels showed superior color stability when exposed to heat.

Summary
Blending metallic pigments used in the paint industry into clear enamel systems made metallic enamels. Aluminum, aluminized steel, and enameling steel were coated. The metallic enamels were more abrasion and scratch resistant than the most heat-resistant organics currently used to make metallic paint. The color and gloss stability of two of the metallic enamels was superior to a silicone-polyester coating and to bare stainless steel. Therefore, metallic enamels can potentially replace bare stainless steel surfaces or paints to offer a more durable, longer-lasting finish.

References
[1] W. D. Callister: J. Materials Science and Engineering: An Introduction (New York: Wiley, 1990, 713 -714).
[2] Ian Wheeler: Metallic Pigments in Polymers, Shawbury, UK, Rapra Technology Limited, 1999, p.3.
[3] C. C. Esposito: Metallic Pigments: New Finishes on the Rise, Coatings World 2002.
[4] W. D. Callister, J. Materials Science and Engineering: An Introduction (New York: Wiley, 1990, 738 -739).
[5] B. Müller: Reactive & Functional Polymers Vol. 39 (1999) pp.165–177.
[6] M. Selvaraj: Anti-Corrosion Methods and Materials Vol. 44 (1997) pp.13–19.
[7] M. Davies, Industrial Paint & Powder Vol. 76 (2000) pp.38–40.

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