Porcelain enamel for aluminized steel

Abstract: The present invention provides a composition for use in forming an adherent, brilliant white, chemically and mechanically resistant porcelain enamel coating on aluminized steel. The composition includes a self-opacifying glass frit component which includes high levels of titanium dioxide, lithium oxide, sodium oxide, and potassium oxide. The composition is able to produce a brilliant white finish on aluminized steel in one coat without the addition of titanium oxide during milling.

Description: 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. Patent 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. Patent 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 800 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. Document US 5,650,364 describes compositions for self-opacifying enamel frits for the enameling of Aluminum or Aluminum alloys, which contain rather important amounts of V 2 O 5 which is a comparatively expensive material. Document US 3,383,225 describes acid-resistant enamels which may be coloured with oxides such as NiO or CoO, or which may contain SnO 2 , ZrO 2 and TiO 2 as opacifying Oxides added to a frit. Most of the frit compositions disclosed therein contain harmful Oxides such as CdO and PbO. Document US 2,930,713 describes various vitreous enamel frits destined for use in the enamelling of steel, not alumina. The document further describes the application of a ground coat followed by the application of a cover coat. Disclosed ground an cover coats mostly contain toxic PbO and/or CdO. Document US 2,975,070 describes vitreous enamels which contain toxic PbO. Document EP 453 897 A describes enamels which are made of mixtures of two different frits A and B. The enamels are destined to be employed as self-cleaning surfaces on the inside of e.g. backing ovens. Being composed of both frit A and B, and with the latter containing nearly 20% of Fe 2 O 3 , these enamels will at most have a yellowish color. Swiss Patent CH 414 073 describes frits for the production of acid resistant enamels which contain CdO, PbO (both toxic) and/or coloring transition metal oxides. Also described therein is the use of carcinogenic K 2 Cr 2 O 7 as an additive so as to assure a smooth surface of such enamels. Document FR 2 134 417 describes frits for enamels made of different Metal Oxides. It does not teach the use of Sb 2 O 3 in such frits. Document FR 2 565 224 describes acid resistant enamel coatings on alumina containing a certain amount of SiO 2 , which according to the document helps in raising the acid-resistance, but necessitates a relatively high amount of added V 2 O 5 to lower the burning temperature below 560°C, which makes the procedure expensive. Alternatively, the use of Fe 2 O 3 is described, which in turn leads to yellowish enamels. Document US 2,986,530 describes frits destined for the production of electroluminescent vitreous enamels, in which finely divided Zinc particles are incorporated. In document JP 61 091040 A, several frits with a relatively high amount of SiO 2 are disclosed. As described above, that leads to high burning temperatures (according to FR 2 565 224), causing problems when used on alumina or aluminised steel. Document US 5,266,357 describes an enamel suitable for the enameling of Aluminum containing 8.5% by weight of V 2 O 5 , which is an important amount of that expensive material. The document further describes the application of a second enamel coat leading to a white enamel. 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 ofthe 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: Oxide Weight% More Preferred 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 Patent 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. Patent 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 may be milled in conjunction with other mill additions such as sodium molybdate, molybdenum trioxide (molybdic acid hydride/ammonium polymolybdate), sodium silicate, quartz, bentonite, etc. to produce a suitable composition. Also, it will be appreciated that depending upon the particular application one or more pigments or coloring agents may be included in the enamel composition. As is well-known in the art, there is a wide range of other acceptable mill agents or components that may also be utilized in the present invention to produce the desired resultant product. However, it will be appreciated that the use of certain additions may have an undesirable effect on the properties of the resultant enamel. Generally, it is best to avoid additions that contain any form of chlorine. The resultant product preferably provides a coarseness of 1% by weight through a 325 mesh sieve (45 microns). However, it will be appreciated the milling fineness may be altered to meet the requirements of the specific application process being utilized or the desired final properties of the enamel. Whether applied by wet or by dry processes, the coating composition of the present invention may be applied at a rate of from about 250 to about 450 grams per square meter so as to provide a coating thickness of from about 2 to about 8 mils subsequent to firing. The coating thickness is not critical, and can be varied based upon the particular application or need or opacity required. Articles that have been coated with the coating composition of the present invention may be fired in a conventional manner using conventional firing equipment. Firing is generally conducted in an air convection furnace at a temperature of from about 560°C to about 600°C for a period of from about 6 to about 15 minutes. However, it will be appreciated that depending upon the specific application and composition of the glass component, a longer or shorter firing period could be used. It will be appreciated that products coated with enamel compositions made in accordance with the present invention have a brilliant white opaque finish. The fired appearance of the final coating is smooth and uniform and substantially free of ripples or curtaining. Additionally, the enamel coatings made in accordance with the present invention are color stable, resistant to chemical attack (acid resistance), and display good mechanical resistance properties. Further, it will be appreciated that the enamels of the present invention may also be used on aluminum, of course, the thermal expansion of the enamel system may need adjustment to form a suitable bond on aluminum. The following examples are intended only to illustrate the invention and should not be construed as imposing limitations upon the claims: Example I Glass frits A, B and C, having the following compositions, were prepared using conventional glass melting techniques: Weight Percent Oxide A B C SiO 2 29.68 29.61 26.47 TiO 2 27.72 27.64 27.89 Na 2 O 11.12 11.10 15.25 B 2 O 3 13.44 13.41 11.75 K 2 O 8.22 8.19 7.00 LiO 2 4.93 5.47 5.86 Sb 2 O 3 2.94 2.93 3.30 P 2 O 5 0.00 0.00 2.48 BaO 1.10 0.00 0.00 MgO 0.85 0.85 0.00 Al 2 O 3 0.00 0.80 0.00 Example II Porcelain enamel coating compositions were prepared using the glass frits A, B and C of Example I to provide compositions I, II and III. In each case, the components as shown below were ground in a ball mill in a conventional manner to provide a coarseness of 1% by weight through a 325 mesh sieve (45 microns). Weight Percent Component I II III Frit A 100 -- -- Frit B -- 100 -- Frit C -- -- 100 Sodium molybdate 0.5 0.5 -- Molybdenum trioxide 1 1 -- Sodium silicate 2.5 2.5 -- Water (ml. per 100 g. frit) 45 45 -- Methylhydrogenpolysiloxane) -- -- 0.2 Dimethylpolysiloxane) -- -- 0.03 The resulting coating compositions I and II were applied to aluminized steel plates using a conventional wet spray coating technique at a rate of 350 grams per square meter and then dried. The resulting coating composition III was applied to an aluminized steel plat using a conventional dry application technique (standard corona discharge gun at 70Kv) also at a rate of 350 grams per square meter. Each of the three enamel coated aluminized steel plates were then fired in an air convection furnace at 580°C for 12 minutes. In all three cases, an adherent, crystallized, brilliant white, chemically and mechanically resistant, porcelain enamel finish was obtained. While the invention has been explained in relation to its preferred embodiments, it is to be understood that various modifications thereof will become apparent to those skilled in the art upon reading this specification. Therefore, it is understood that the invention disclosed herein is intended to cover such modification as fall within the scope of the appended claims.

Claims: A porcelain enamel coating composition for use on an aluminum surface having a glass component comprising by weight from about 27% to about 33% SiO 2 , from about 25% to about 31% TiO 2 , from about 10% to about 15% Na 2 O, from about 8% to about 15%B 2 O 3 , from about 5% to about 12% K 2 O, from about 4% to about 8% Li 2 O, from about 2% to about 6% Sb 2 O 3 , from about 0% to about 2% MgO, from about 0% to about 2% Al 2 O 3 , from about 0% to about 3% BaO, and from about 0% to about 5% V 2 O 5 . A porcelain enamel coating composition as in claim 1 wherein said glass component comprises from about 85% to about 100% of said composition by weight, and mill additions comprise from about 0% to about 15% by weight of said composition. A section of aluminized steel having disposed thereon a porcelain enamel coating layer formed by applying a porcelain enamel coating composition as set forth in claim 1 to said section of aluminized steel. A section of aluminized steel having disposed thereon a porcelain enamel coating layer as in claim 3 wherein said porcelain enamel coating layer has a thickness of about 2 to about 8 mils. A section of aluminized steel having disposed thereon a porcelain enamel coating layer formed by applying a porcelain enamel coating composition as set forth in claim 1 to said section of aluminized steel and then firing said section of aluminized steel at a temperature of from about 560°C to about 600°C. A method of forming a white crystallized enamel coated section of aluminized steel comprising the steps of: (i) providing a section of aluminized steel; (ii) applying an enamel composition as set forth in claim 1; and (iii) firing said enamel composition.

Inventors: ARONICA ALAIN, SOUCHARD THIERRY, ARONICA, ALAIN, SOUCHARD, THIERRY
Applicants: FERRO FRANCE S A R L, FERRO FRANCE S.A.R.L.
Application year: 1998
Main classification: C03C8
IPC codes: C23D5, C03C8