US 3681019 A
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United States Patent Office 3,681,019 Patented Aug. 1, 1972 3,681,019 COATED SUBSTRATE OR ARTICLE HAVING A LOW FRICTION SURFACE RESISTANT T O DEWETTING AT ELEVATED TEMPERATURES AND PROCESS OF FORMING Sheldon W. Dean, Hamden, Conn., assignor to Olin Corporation No Drawing. Filed Feb. 1, 1971, Ser. No. 111,724 Int. Cl. B44d 1/34; C23c 3/00 U.S..Cl. 29195 11 Claims ABSTRACT OF THE DISCLOSURE A substrate or article having a low friction surface resistant to dewetting at elevated temperatures and a process of making same. The low friction surface comprises two layers. The first layer comprises a metal selected from zinc, copper, cadmium, bismuth, tin, antimony, lead, iron and their alloys. The second layer comprises a metal soap. Preferably, the first layer is zinc and the second layer is a sodium soap and the substrate or article is a lamp base of aluminum or an aluminum alloy.
BACKGROUND OF THE INVENTION This invention relates to the formation of a low friction coating on the surface of an aluminum or aluminum alloy substrate or article. More specifically, it relates to a coating used on lamp bases during the manufacture of light bulbs to prevent sticking of the lamp base in the assembly fixture.
In the manufacture of conventional light bulbs, it is necessary to bond the glass envelope to the lamp base by means of a plastic material which is placed into the lamp base and cured therein at high temperatures generally in excess of 400 to 600 F. depending upon the type of bulb being manufactured. During this process, the lamp base is screwed into a holding fixture or jig. After the processing is terminated and the lamp base and its contents are cooled, it must be unscrewed from the holding fixture. If the base which is usually of aluminum or an aluminum alloy galls or is subject to excessively high friction during the unscrewing process, the bulb is prone to breakage with resultant losses and delays in the manufacturing process.
The conventional approach to this problem has been to dip the lamp base prior to insertion in the holding fixture in a soap solution such as a mixture of sodium salts and fatty acids dissolved in water. Sodium soaps should be excellent high temperature lubricants because they do not generally melt or decompose at the temperature encountered during processing. However, they are hydroscopic and can dissolve in the moisture they pick up forming a solution which forms a fluid film. The aluminum surface adjacent the sodium soap coating tends to react with the fatty acid ion in the solution to form a thin film of aluminum soap on the metal surface. This in turn causes the sodium soap solution to dewet the aluminum surface and form discrete droplets. These droplets are not as efiicient for lubrication as the original soap film and as a result the lamp bases tend to stick and also have a nonuniform spotty appearance.
In companion application, Ser. No. 109,197, filed Jan. 25, 1971, there is disclosed a low friction surface comprising three layers for use on metal bases of aluminum or aluminum alloys. The first layer adjacent the metal base of aluminum or aluminum alloy comprises a metal selected from zinc, cadmium, bismuth, tin, antimony, lead, iron, copper and copper alloys. The second layer formed over the first layer comprises a metal soap or salt of the metal of the first layer and the third layer comprises a fatty acid. The second layer is generally formed by reacting in part the first and third layers. Therefore, under extreme pressure, if the metal soap second layer is scraped away more metal soap is formed by reaction of the remaining first and third layers. This low friction surface may be useful in preventing the dewetting referred to above but it is primarily useful for its low friction and anti-galling properties under conditions of extreme pres sure.
SUMMARY OF THE INVENTION In accordance with this invention, a low friction surface coating has been found which is not prone to dewetting and is, therefore, useful in applications such as the manufacture of light bulbs described above.
This invention comprises a low friction surface having two layers. The first layer comprises a metal selected from zinc, copper, cadmium, bismuth, tin, antimony, lead, iron and their alloys. The second layer comprises a metal soap of a metal other than the metal in the first layer.
This coating provides good low friction qualities to prevent sticking, for example, of a lamp base in a holding fixture and, further, it is not prone to dewetting as in the case when the metal layer is not employed.
Accordingly, it is a principal object of this invention to provide a substrate or article having a low friction surface comprising two layers, the first layer being a metal selected from zinc, copper, cadmium, bismuth, tin, antimony, lead, iron and their alloys and the second layer being a metal soap of a metal other than the metal in the first layer.
It is another object of this invention to provide a substrate or article as above wherein the metal soap has a melting point in excess of 400 F. and, preferably, in excess of 600 F.
It is a further object of this invention to provide a metal substrate or article as above wherein the substrate or article is a lamp base.
It is a further object of this invention to provide a process of coating a metal substrate or article with two layers, the first layer being a metal selected from zinc, copper, cadmium, bismuth, tin, antimony, lead, iron and their alloys and the second layer being a metal soap of a metal other than the metal in the first layer.
It is a further object of this invention to provide a process as above wherein the metal substrate or article is a lamp base.
Other objects and advantages will become apparent to those skilled in the art from the following description.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The instant invention is directed to a low friction surface which minimizes the problem of dewetting of a metal soap coating on a metal base when it is held at elevated temperatures as previously described. The low friction surface of this invention comprises two layers on a substrate or article which will be referred to generically hereinafter as a metal base.
The first layer adjacent a surface of the metal base comprises a metal selected from zinc, copper, cadmium, bismuth, tin, antimony, lead, iron and their alloys. The second layer which is formed over the first layer comprises a metal soap of a metal other than the metal comprising the first layer.
The metal soap must be one which will not melt or decompose at temperatures encountered during processing of the metal substrate or article. Generally, the metal soap should melt or decompose at temperatures in excess of 400 F. and, preferably, in excess of 600 F. A metal soap of sodium is preferred for this purpose; however, other soaps such as potassium soaps which meet the above temperature requirements could be used. The metal soap is preferably sodium stearate. a
The process of this invention comprises forming on a surface of a metal base a layer of metal selected from zinc, copper, cadmium, bismuth, tin, antimony, lead, iron and their alloys followed by forming a layer of a metal soap of a metal other than that in the metal layer over the metal layer.
As above, the metal soap should have a melting point in excess of 400 F. and, preferably, in excess of 600 F. The metal soap is preferably a sodium soap and, more preferably, sodium stearate. The process of this invention is particularly applicable to the coating of lamp bases to prevent them fro-m sticking to holding fixtures during fabrication of ordinary light bulbs.
The metal layer may be formed by any conventional technique such as immersion plating, electroplating, vapor deposition or other suitable process. However, it is preferred to form the metal layer by immersion plating since this results generally in a particulate deposit which enhances the non-dewetting properties of this invention. Particulate deposits are deposits which under sufficient magnification appear to be made up of particles.
The metal soap layer is generally formed over the metal layer by immersing the metal base with the metal layer in an aqueous solution of the metal soap. Any excess metal soap is removed by conventional wiping techniques.
It has, been found that the low friction surface of this invention when applied to the manufacture of light bulbs provides adequate anti-sticking properties and less tend ency to dewet and form beads, than when a metal soap is used alone. The following description is believed to be the mechanism which results in the aforenoted improvement. This description is presented, however, for purposes of example and is not meant to be limitative of the invention as claimed herein.
It is well known that as the contact angle formed between a coating and a substrate increases, the wetting tendencies of the coating decrease. The contact angle formed between the coating of this invention and the metal base is a function of many factors as expressed by the Wenzel modification of Youngs equation:
'y -=the solid vapor surface tension 7 =th6 solid liquid interfacial tension y =the liquid-vapor surface tension r=the ratio or true to apparent surface area =the contact angle measured through the liquid phase..
r the ratio of true to apparent surface area requires further definition. The apparent surface area is determined by conventional geometric measurement of the surface area. The true surface area, however, also takes into account the texture of the surface, for example, the roughness of the surface. Therefore, for an absolutely smooth surface, the apparent and true surface area would be equal. However, if the surface is roughened or if the texture is not absolutely smooth, there is an increase in actual or true surface area.
In accordance with this invention, the metal coating is preferably in the form of a particulate deposit as aforcnoted, which results in an increased true surface area as compared to apparent surface area. Therefore, using the Wenzel equation, one can see that the r factor for the metal layer of this invention is greater than for the uncoated aluminum surface of the metal base which would be comparatively smooth. Thus, the contact angle 0 is reduced using the metal layer as compared to the metal base without the metal layer.
Consequently, there is less tendency for the metal soap to dewet the metal layer of this invention as compared to the metal base without the metal layer.
As noted in the Background of the Invention, the prior art process resulted in the formation of an aluminum soap film, generally aluminum stearate, between the aluminum lamp base surface and the sodium soap coating. The aluminum stearate filmis substantially insoluble in the sodium soap coating, and therefore tends to decrease the effective '7 value. relative to This results in anincrease in the contact angle and a consequent tendency for the sodium soap layer to. dewet the lamp base surface.
In accordance with this invention, any soap film which might form between the metal layer and the metal soap layer is not as insoluble in the, metal soap layer and, therefore, is not as likely to form a compact film over the metal layer surface. This tends to increase the effective 78 value relative to 7 and, thereby, reduces the contact angle between the metal layer and the metal soap layer thereby substantially reducing thetendency of the metal soap layer to dewet the metal layer.
Therefore, the inventive low friction surface of this invention is doubly advantageous in preventing dewetting of the metal soap layer. First, there is an increase in the ratio of true surface area to apparent surface area by virtue of the fact that a particulate deposit is generally employed. Second, a metal soap of the metal layer which forms would be more soluble than aluminum soap in the metal soap layer, thereby reducing the tendency to form a film. Both of these factors reduce the contact angle and, therefore, the tendency to dewet.
In accordance with this invention it is preferred to use an immersion plating step to form the metal layer since this results in a particulate deposit which as aforenoted results in a significantly larger true surface area and an increase in the r factor. Therefore, the use of an immersion plating step to deposit metals such as zinc, copper, cadmium, bismuth, tin, antimony, lead, iron and their alloys is effective in preventing dewetting in metal soap applications. Furthermore, the inventive coating comprising a particulate metal deposit plus a metal soap layer has significantly better frictional properties than bare aluminum.
The following examples will serve to illustrate the invention.
EXAMPLE I A series of aluminum lamp bases were treated as follows:
(1) Deoxidize in 20% H 80 for 30 seconds.
(2) Rinse for 1 minute in tap water.
(3) Immersion plate (see below for details).
(4) Rinse 1 minute in tap water.
(5 Immerse in sodium soap solution (Amber Soap,
10 g.p.l., '160" F., 30 seconds).
(6) Dry in oven at 210 F.
Two immersion plating systems were used: Cuprate and zincate. The cuprate bath was prepared as follows:
grams CuSO -5H O dissolved in 500 ml. distilled water 100 grams 70% ethylene diamine added.
The bath was then made up with distilled water to I later and used at room temperature. A two minute immersion was used with this bath.
The zincate bath was prepared as follows:
A liter beaker was used and 350 grams of sodium hydroxide was added to the beaker. About 500 mls. of distilled water was added to dissolve the sodium hydroxide. Then 100 grams of zinc oxide (ZNO) was added to this solution and it was stirred until solution was complete. In a separate container 10 grams of Rochelle salt (NaKC H O was dissolved in 50 mls. of distilled water and then 1 g. of ferric chloride (FeCl 6H O) was added and stirred until dissolved. This solution was then added to the sodium hydroxide-zinc oxide solution previously prepared and the entire solution was adjusted to a volume of 1 l. by adding distilled water. This zincate solution was then allowed to cool to room temperature for use. A one minute immersion was used in this bath.
The specimens prepared in this way were heated to 400 F. or 600 F. Neither treatment showed any evidence of dewetting.
This example serves to show the improvement obtained with the low friction surface of this invention with respect to the dewetting of the metal soap layer.
EXAMPLE II In the previous example, the immersion plating steps involved plating copper or zinc. The following tests were run to demonstrate immersion plating with a copper alloy such as brass and with cadmium. The general processing sequence was as follows:
(1) Rack a number of flat aluminum panels.
(2) Non-etch clean minutes 55 C.
( 3 Rinse.
(4) Nitric acid solution clip (1 vol. HNO to 1 vol. water) room temperature 5 minutes.
(6) Blow dry.
(7) Immersion plate.
The immersion plating baths used in Step 7 were as follows:
( 1) Cadmium:
(A) Composition: in distilled water CdSO -3.8 g.p.l. 48% HF-110 mL/l. (B) Temperature-room. (C) Time-30 seconds. 2) Brass:
(A) Composition: in distilled water ZnO-IIO g.p.l. NaOH-4l5 g.p.l. CuCN-13 g.p.l. NaCN-22 g.p.l. PbCO -0.135 g.p.l. (B) Solution filtered after make up (C) Temperature-45 C. (D) T ime-3 minutes.
The resultant coatings were adherent and suitable for use in accordance with this invention.
The immersion plating baths described above in Examples I and II are conventional and do not form a part of the instant invention. Similarly, immersion plating baths for the other metals which may be used as the metal layer are known in the art. The zincate bath in particular has been described in ASTM Specification: 253-68 and in U.S. Pat. Nos. 3,650,886 and 2,676,916. The zincate bath is preferred since it operates efiectively for two reasons. First, it is strong alkaline and, therefore, has a strong solubilizing effect on all aluminum oxides.
Secondly, it contains zinc which is more noble than aluminum and which, therefore, plates out and protects the base metal by replacement. Any system containing these two properties would be potential as an immersion plating step.
As aforenoted, other metals such cadmium, bismuth, tin, antimony, lead, iron, copper and their alloys would also work for this purpose. However, zinc is preferred because it is active to aluminum in most corrosion situations so that it would not accelerate corrosion of alumimum and in fact it has been shown to improve the corrosion resistance of aluminum.
It is to be understood that the invention is not limited to the illustrations described and shown herein, which are deemed to be merely illustrative of the best modes of carrying out the invention, and which are suitable of modification of form, size, arrangement of parts and details of operation. The invention rather is intended to encompass all such modifications which are within its spirit and scope as defined by the claims.
What is claimed is:
1. A coated substrate or article comprising:
a metal base of aluminum or an aluminum alloy;
a first layer on a surface of said base consisting essentially of a metal selected from the group consisting of zinc, copper, cadmium, bismuth, tin, antimony, lead, iron and their alloys; and
a second layer over said first layer consisting essentially of a metal soap having a melting point of at least 400 F.
2. A substrate or article as in claim 1 wherein said metal soap has a melting point of at least 600 F.
3. A substrate or article as in claim 1 wherein said first layer comprises zinc.
4. A substrate or article as in claim 1 wherein said metal soap layer comprises a sodium soap.
5. A substrate or article as in claim 4 wherein said substrate or article comprises a lamp base, and wherein said first layer comprises zinc.
6. A process of forming a coated substrate or article comprising:
providing a metal base of aluminum or an aluminum alloy;
forming on a surface of said metal base a layer of a metal selected from the group consisting of zinc, copper, cadmium, bismuth, tin, antimony, lead, iron and their alloys; and
forming on said metal layer a layer of a metal soap by immersing said metal base with said metal layer in an aqueous solution of said metal soap, said metal soap having a melting point of at least 400 7. A process as in claim 6 wherein said metal soap has a melting point of at least 600 F.
8. A process as in claim 6 wherein said metal layer comprises zinc.
9. A process as in claim 6 wherein said metal soap is a sodium soap.
10. A process as in claim '9 wherein said article or substrate comprises a lamp base, and wherein said metal layer comprises zinc.
11. A process as in claim 9 wherein said metal layer is formed by immersion plating.
References Cited UNITED STATES PATENTS 3,032,435 5/1962 Michel 117-89 X 3,556,867 1/1971 Glasson 117-134 X 2,580,773 1/1952 Heiman 117-71 M 2,098,256 11/1937 Parker 117-134 2,963,391 12/1960 Kubie 117-134 X RALPH HUSACK, Primary Examiner U.S. Cl. X.R.