US2975070A - Vitreous enamel - Google Patents

Description

United States PatentO 2,975,010 VITREOUS ENAMEL Burton C. Bricker, Wilmington, DeL, and Lewis C. Hoffman, Scotch Plains, N..l., assignors to E. I. du Pont de Nemonrs and Company, Wilmington, Del., :1 corporation of Delaware No Drawing. Filed Dec. 11, 1958, Ser. No. 779,525

1 Claim. (Cl. 106-49) Patented Mar. 14, 1961 tent of such a vitreous composition, it is necessary to increase the alkali metal oxide content to maintain a maturing temperature of below 1000 F. But, upon increasing the alkali metal oxide content beyond a limit, the frit becomes too corrosive to the aluminum or aluminum alloy during coating and firing operations; Again,

the presence of slightly too much K will produce a difficulties have arisen in the consistent and uniformformation of hard, glossy, vitreous enamels of good adherence and resistance to the weathering influences under atmospheric conditions in all climates.

'Itjis an object of this invention to produce an improved vitreous enamel coating for aluminum and aluminum-rich alloys.

It is another object to produce a vitreous enamel coating for aluminum and aluminum-rich alloys that will have an improved color stability.

It is a further object to produce a vitreous enamel coating for aluminum and aluminum-rich alloys that will have an improved weather resistance.

It is a still further object to produce a vitreous enamel coating for aluminum and aluminum-rich alloys that will have an improved opaqueness with a given content of opacifying agent.

Other objects of the invention will appear hereinafter.

It has now been found that anew and improved series of aluminum enamels can be prepared which, in comparison to previously known aluminum enamels, have a greatly reduced tendency to form the yellow devitrification product comprised of lead titanate, have areduced tendency to dissolve opacifying agents, have up to 50% greater. weathering or chemical resistance, are easily applied to aluminum without extensive pretreatment, do not objectionably corrode aluminum during application and firing, and have otherwise the desirable gloss and other properties of previously known aluminum enamels, for example, the enamels described and claimed in Deyrup, US. Patent No. 2,467,114; 3 In order to obtain the requisite chemical resistance it is desirable that the enamel composition containat least about 19 mol percent oftitanium dioxide, The presence of more than about 8% lead oxide in a vitreous composition containing more than about 19% titanium dioxide is veryundesirable' since the resulting frit may contain a considerable amount oflead titanate, which is subject to. highly objectionable yellow devitrification upon slight over-firing. l n redu cing the lead oxide con- Silicon oxide (SiO 33-37 Titanium oxide (TiO ,19-25v Sodium oxide (NagO) .."2 1- 3 0 I Lithium oxide (Li O) 0-6 Potassium oxide (K 0) 0-10 frit that is insufiiciently acid or weather resistant. Too much Li O causes the frit to dissolve too great a quantity of opacifying agents, for example, Ti0 or SnO As a consequence, it is exceedingly diflicult to formulate a satisfactory enamel for purposes of enamelling aluminum or aluminum-rich alloys. In the enamel of this invention a slight quantity of Sb O is useful to mitigate to some extent the corrosive effects of alkali metal oxides,

and BaO contributes thereto to maintain a low melting range and impart a corrosion resistance to the" final enamels.

The new and improved vitreous enamels of this invention have a firing or, maturing temperaturebetweenn920 F. and 1000 F. and contain the following constituents within the following ranges and proportions:

I Mole percent Lead oxide (PbO) Antimony oxide (Sb O 0-1 Barium oxide' (BaO) 0-6 Boron oxide (B 0 01 Cadmium oxide (CdO) 0-2 The totalv alkali metal oxide content (Li- O-|-Na O'+K O) of the enamel composition must be between 33 and 37 mole percent, and the ratio of the silica content plus twice the titanium oxide content of the composition, in mole percent, to the alkali metal oxide content plus the lead oxide content, in mole percent, must be between 1.8 and 2.4, i.e.

2 Na O+ .8 and 2.4

Small percentages of'other constituents than those above mentioned may be contained in the vitreous For example, such enamels" may contain up to about 2% of A1 0 4% of SrO or I CaO, 1% of C00, 0.1% of MnO, 0.1% of NiO, 3% of. ZnO or 4% of 210 however at least of content S enamels of this invention.

of these enamels must consist of the above-named ingredients within the ranges stated. v

The enamel constituents are expressed in mole per cent as defining the invention more accurately than could be done by weight percentages, however, for ready 'comparison with compositions expressed in weightpercerit ages, the examples will be set forth in both weight and 1 -mole percentages.

The knowing three (3) tables disclos 'ten spe,

examples of compositions prepared in accordance with: theinvention. Table-I discloses the compositions in mole percent and includes the aforesaid mole percent ratios, Table II in weightpercent, and Table III inw eight percent of the batch ingredients from whichjthevitre igs enamel hit is prepared.

TABLE I Mole percent of melted compositions I Example N 1 2 3 4 5 6 7 8 9 10 5.6 4.4 4.4 4.3 4.4 4.3 4.3 4.3 4.3 4.2 4.3 3.2 3.2 3.2 5.1 50- 5.0 5.0 9.5 2.9 0.6 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.4 0.1 0.2 0.2 0.2 3.5 1.9 1 1.5 19 12 4.3 49 5.5 4.0 3.3 Mole Percent Ratio: f 1 n h R,0+Pbo TABLE 11 Weight percent of melted compositions Example N0 1 2 3 4 6 6 7 8 9 10 TABLE III Batches in weight percent Example N0 1 2 3 4 6 6 7 8 9 10 PbO, Litharge 14.3 13.0 13.1 12.2 12.2 7.1 6.3 4.3 4.7 4.3 $102, Potters Fllnt 22.5 22.3 22.5 23.2 22.4 23.0 22.1 21.3 22.1 21.5 T104..- 16.1 20.1 20.2 20.1 21.0 20.3 21.5 20.4 19.3 20.1 25.5 30.7 30.9 30.7 27.7 27.3 27.3 26.9 24.7 34.0 4.1 3.4- 3.4 3.4 3.4 3.4 3.4 3.3 3.2 3.3 7.1 5.0 5.0 5.0 7.7 7.6 7.6 7.5 13.3 4.5 1.3 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.3 1.4 1.3 2.2 2.1 22 41 42 96 104 11.7 3.3 3.2

In preparing these frits, the batch is weighed, mixed, metaborate as described in US. Patent 2,653,877 should heated in a crucible until the mix is completely melted to be added. a homogeneous glass. Crucible temperatures may vary The dispersion or suspension of the powdered enamel in within wide limits, but 900 C. to 1200 C. is a particua suitable vehicle may be applied to the aluminum by the larly suitable range. The molten glass is fritted by runwell-known procedures of spraying, dipping, stencilling, ning into water winch shatters it into small pieces formbrushing, etc. As an example of a convenient vehicle for ing the frit. The frit is advantageously freed of water by use in the spray application, one composed of methyl low-temperature drying, for example, at a temperature less cellulose 0.75%, sodium chloride 1%, octyl alcohol than 50 C. 0.05%, and water 98.2% (all parts by weight) may be The frit is then placed, :for grinding, in a ball mill or used; or commercial water glass (sodium silicate solution) other mill, with water or other suitable liquid, or in some 0.4%, sodium fluosilicate 0.4%, and water 99.2%. A cases it may be dry-ground. Dry grinding maybe presuitable vehicle for applying the vitreous enamel by the ferred if the frit is to be stored for long periods because squeegee-stencil application procedure is one composed it obviates any attack of water on the comniinuted frit of ethyl cellulose (25 c.p.s. viscosity grade) 4%, soya and eliminates the operations of filtering and drying. If lecithin 0.25%, and terpineol 95.75%. When using aquethe frit is to be used directly in enamel, it is advantaous vehicles for application, it is frequently desirable to geously ground in water which serves as suspending me-. include in the vehicle corrosion inhibitors to prevent the dium for application to the metal articles. During the possible corrosion of the metal article by the enamel grinding, it is convenient to include with the frit white suspension, since such corrosion evolves hydrogen which pigments (opacifying agents) such as titanium dioxide, may mechanically break up the enamel film before firing. or coloring pigments such as cobalt aluminuate' blue, in Sodium silicate and fiuosilicate may be used for this puramounts up to 10% by weight of the total enamel compose, as in the vehicle cited above; or a small amount of positions. These are, of course, omitted if a clear, colorsoluble chromate such as 0.1% byweight of potassium less, vitreous coating is desired. It is advantageous also chromate may be incorporated in the aqueous vehicle. to include suitable dispersing and bonding agents as addi- It is desirable to apply the enamel to the surface of the tions to the frit in the grinding mill. 1f the enamel is aluminum in any manner which will obtain a uniform and subject to tearing when applied and fired; anti-tearing smooth coating before firing. The thickness of the coat agents, such as the alkali metal silicates and alkali metal may be varied within wide limits according to the type of effect desired, although 15 to 30 grams (based on dry weight of enamel) per square foot per coat is generally preferred, several coats with separate firings being applied if desired. It is not generally necessary to apply any preliminary treatment to the surface of the aluminum or alloy other than removal of grease or excessive amounts of corrosion products except as described below. Usually these enamels may be applied to any aluminum or aluminum alloy surfaces without preliminary cleaning, provided the surface to be enameled is not contaminated. Cast metal articles to be enameled should be made with proper care to ensure dense castings free of undue porosity. In general, articles for enameling should be designed in accordance with the well-known principles obtained in other metal-enameling arts, particularly to avoid excessively sharp edges and corners in design. If such sharp edges, corners or burrs are present, they should be lightly rounded oif by filing, grinding or other suitable means.

After application of the enamel to the metal, the composition is fired in a suitable furnace in order to fuse the powdered enamel to a continuous glassy coating. The fining conditions are not critical, except that the article must be brought to a temperature high enough to melt the enamel but not high enough to injure the metal article. The temperature necessary depends to some extent on the particular aluminum alloy being coated and on the particular enamel composition. However, it is not necessary to employ a temperature above 1000 F. and in some cases the enamel may be satisfactorily melted as low as 920 F. It is merely necessary to keep the article at the selected temperature for a few minutes to permit the enamel to fuse completely to a uniform, adherent, vitreous layer. Several coats may be applied, if desired. The firing, of course, should be effected in a furnace free of sulphate or chloride, or other objectionable fumes, which might impair the desired gloss. In some cases, the enameling may be advantageously combined with the heat treating of heat-treatable alloys. Long heat-treating periods, even of several hours duration, generally do not adversely affect these enamels. When thin sheet stock is enameled, particularly of soft metals in this group such as pure aluminum sheet, it may be found to warp on cooling because of the differential contraction of metal and enamel during cooling. This may be obviated by enameling both sides of the sheet. Or, if desired, we find that, surprisingly, such warped sheet may be mechanically bent back to its original shape Without any adverse effect on the enamel whatever. Or, if desired, warping difliculties may be minimized by use of the stronger alloys in fabricating articles for enameling.

Preferably, the aluminum or aluminum-rich alloy, prior to application of the enamels of this invention, is pretreated by the chromate treatment of Deyrup et al. as disclosed in U.S. Patent 2,544,139. Contrary to the method disclosed in that patent, the chromate-treated aluminum or aluminum alloy surfaces do not, in the case of some alloys, have to be pre-fired. Samples of an aluminum alloy sheet stock composed of aluminum alloy (containing by weight 0.25% copper, 0.6% silicon, 1% magnesium, and 0.25% chromium) is dipped into a bath consisting of and having a temperature of 50 C. After 5 minutes, the samples are Withdrawn, rinsed with water, and dried at room temperature.

Each of these samples was coated with one of the specific examples of vitreous enamel disclosed in the above tables. Before coating, each enamel was provided 7 with a mill addition consisting of 10% by Weight of Ti0 pigment and 7% by weight of a mixture of an antitearing agent consisting of The coated samples were fired for 10 minutes at a temperature of 960 F. and had the following properties Gloss: gloss/mil measured with a photovolt glossmeter relative to a black glass standard angle of incidence 45 -80 Opacity: Reflectance percent measured with a photovolt reflectometer, green incident light relative to MgO standard Corrosion resistance: Rating P.E.I. test, exposure to 10% citric acid for 15 minutes All percentages referred to throughout the specification and claims, unless otherwise specifically stated, refer to mole percentages.

Since it is obvious that many changes and modifica tions can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that this invention is not to be limited to said details except as set forth in the appended claim.

We claim:

A vitreous enamel frit, suitable for use as a protective coating on the surface of aluminum and aluminumrich alloys, having a firing and maturing temperature below 1000 F., said frit consisting of the following, in mole percentages:

all of said oxides being melted into said enamel composition.

References Cited in the file of this patent UNITED STATES PATENTS 2,492,523 Coffeen et al. Dec. 27, 1949 2,642,364 Beatty et al June 16, 1953 2,909,438 Kautz Oct. 20, 1959 FOREIGN PATENTS 665,903 Great Britain Jan. 30, 1952