Porcelain enamel for aluminized steel

Details Porcelain enamel for aluminized steel Porcelain enamel for aluminized steel

2000-11-22

2002-11-05

Fiorilla, Christopher A. (Department: 1731)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S017000

Reexamination Certificate

active

06475939

ABSTRACT:

FIELD OF INVENTION
The present invention concerns chemical compositions for use in forming porcelain enamel coatings on aluminized steel. More particularly, the present invention concerns compositions for use in forming adherent, brilliant white, chemically and mechanically resistant, porcelain enamel coatings on aluminized steel.
BACKGROUND OF INVENTION
Aluminized steel is widely used in automotive, appliance, building material, cookware, and other applications where rust resistance is required. In many of these applications, it is highly desirable to apply an adherent, bright white, chemically and mechanically resistant, porcelain enamel coating to the aluminized steel.
There are many conventional white porcelain enamel compositions known in the art for use on steel. However, these conventional porcelain enamel compositions, which are typically fired in 780° to 950° C. range, are not suitable for use on aluminized steel because they mature at temperatures exceeding the melting point of aluminum (660° C.).
In an effort to apply porcelain enamel coatings to aluminized steel, resort was made in the past to special enamels which had been developed for use on aluminum and alloys of aluminum. However, use of these special enamels, which could be fired in the 530° to 570° C. range, was not particularly successful due in part to incompatible thermal expansion properties as between the special enamels and the aluminized steel. The linear coefficient of thermal expansion of aluminum, for which these special enamels were developed, is nearly twice that of aluminized steel.
Examples of prior art patents include U.S. Pat. No. 3,061,449, issued to Hoffman, which discloses a porcelain enamel composition for use on aluminized steel which contains significant levels of PbO. Although this enamel composition exhibited compatible thermal expansion properties for use on aluminized steel, it did not produce light colored opaque enamel surfaces. Pigments or other coloring agents had to be added to the composition according to Hoffman during milling in order to achieve a light color. It is generally desirable to achieve opacity in an enamel by having the glass frit component form opaque crystals on firing (self-opacifying), rather than by adding pigments or other coloring agents to the enamel system during milling, because self-opacifying enamels generally have superior luster, hiding power, and whiteness properties.
U.S. Pat. No. 5,266,357, issued to Preuss et al., discloses a process whereby a porcelain enamel coating can be applied to aluminized steel by: (I) applying an enamel slip composition containing a low-temperature enamel frit (passing into glass flux at less than 600° C.), a mill addition of high-melting inorganic oxidic powder, and a mill addition of copper or iron oxides; (II) drying the slip coating; and (III) firing at temperatures in the 8000 to 850° C. range. Although enamels applied to aluminized steel according to this process displayed improved adhesion and resistance properties as compared to the art at the time, light colored opaque enamel surfaces could not be produced by this process, thus requiring an additional second application or top coat of a white enamel composition in order to achieve a light colored enamel surface.
Accordingly, a composition is needed for use in forming an adherent, brilliant white, chemically and mechanically resistant porcelain enamel coating on aluminized steel. Such a composition should include a self-opacifying glass component, and should avoid use of toxic materials such as, for example, PbO, and should be able to be applied in only one coating layer.
SUMMARY OF INVENTION
The present invention is directed to a chemical composition for use in forming an adherent, brilliant white, chemically and mechanically resistant, porcelain enamel coating on aluminized steel. The composition according to the present invention includes a novel glass component and optional mill additions. The glass component includes by weight of from about 20 to about 40% SiO
2
, from about 20 to about 40% TiO
2
, from about 8 to about 20% Na
2
O, from about 0 to about 20% B
2
O
3
, from about 2 to about 15% K
2
O, from about 1 to about 11% Li
2
O, from about 0 to about 8% Sb
2
O
3
, from about 0 to about 5% P
2
O
5
, from about 0 to about 4% BaO, from about 0 to about 3% MgO, and from about 0 to about 3% Al
2
O
3
. The glass component may comprise one or more glass frits so as to provide the overall stated composition. The glass component is capable of self-opacifying on firing at temperatures well below the melting point of aluminum.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.
DETAILED DESCRIPTION
The novel enamel composition of the present invention is particularly well-suited for use on aluminized steel such as that typically used in automotive, appliance, sanitary ware, building material, cookware, and other applications where rust resistance is required. Other uses will be apparent to those skilled in the art.
The novel porcelain enamel composition of the present invention preferably comprises from about 85% to about 100% by weight of the glass component. The glass component of the enamel composition may comprise one or more glass frits. Preferably, the glass component provides a composition as follows:
More Preferred
Oxide
Weight %
Weight %
SiO
2
20-40
27-33
TiO
2
20-40
25-31
Na
2
O
 8-20
10-15
B
2
O
3
 0-20
 8-15
K
2
O
 2-15
 5-12
Li
2
O
 1-11
4-8
Sb
2
O
3
0-8
2-6
P
2
O
5
0-5
0-3
BaO
0-4
0-3
MgO
0-3
0-2
Al
2
O
3
0-3
0-2
SrO
0-3
0-2
MoO
3
0-3
0-2
ZnO
0-3
0-2
SnO
0-3
0-2
V
2
O
5
 0-10
0-5
The glass frit or frits that comprise the glass component may be prepared utilizing conventional glass melting techniques. A conventional ceramic refractory, fused silica, or platinum crucible may be used to prepare the glass frit. Typically, selected oxides are smelted at around 1250° C. for 30 minutes. The molten glass formed in the crucible is then converted to glass frit using water-cooled rollers and milling equipment. It will be appreciated that the step of producing the glass frit is not per se critical and any of the various techniques well-known to those skilled in the art can be employed.
The enamel composition according to the present invention is suitable for application to an aluminized steel substrate using conventional techniques such as, for example, either wet or dry application processes. Suitable wet application processes include wet spraying, wet electrostatic application, dipping and flow coating, which are well-known, and electrophoretic application. The disclosures of U.S. Pat. Nos. 5,002,903, 4,085,021 and 3,841,986 are incorporated herein by reference for their teachings relative to the electrophoretic application of porcelain enamel coating compositions. Dry application processes, such as for example a standard corona charging gun, are also well-known. The disclosures of U.S. Pat. Nos. 4,110,487 and 3,928,668 are hereby incorporated by reference for their teachings relative to the dry or electrostatic application of porcelain enamel coating compositions. When employing a wet application or technique, it is generally preferred that the aluminized steel be degreased prior to the application of the enamel.
It will be appreciated that the glass frit component may be used in combination with various mill additions. It will also be appreciated that the mill additions will vary depending upon the specific application conditions being utilized. For dry applications, the glass component may be used in combination with one or more conventional organopolysiloxanes. Such organopolysiloxanes are commonly employed in enamel compositions that are designed for dry application. For wet application processes, the glass frits m

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