US2498485A - Method of tinning a steel shell bearing - Google Patents

Description

Patented Feb. 21, 1950 METHOD OF TINNING A STEEL SHELL BEARING John H. Clawson, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York No Drawing. Application August 7, 1946, Serial No. 688,965

1 Claim.

This invention is concerned with a method of partially tinning metallic articles. More particularly, the invention is concerned With a method of partially tinning a metallic article which comprises shielding the portion of the article desired to be kept free of the tin plating with an organosubstituted polysiloxane resin, and thereafter dipping the shielded article in the tinning bath.

Several methods have been employed to mask a portion of a metallic article intended to be tinplated so as to prevent that portion of the article from being plated when the article is later dipped in the tin plating bath. One method comprises coating the portion of the article desired to be protected from the tinning bath with an alkali silicate, more specifically sodium silicate, prior to the plating operation. However, there are several disadvantages which accompany the use of sodium silicate as the masking (or rejecting) medium for the metallic article. Since the temperature of the tinning bath may be from about 300 to 500 C. or higher, depending, e. g., on the exact constitution of the bath, it often happens that when any portion of the sodium silicate becomes immersed in the bath, small pieces of the sodium silicate will chip oif and fall into the tinning bath thereby contaminating it. This is especially true because of the brittleness and poor coeflicient of expansion of sodium silicate which, when applied as a coating on a metallic article having a greater coefficient of expansion, causes the sodium silicate to chip away when the metal is caused to expand by any extreme temperature change as is usually the case when a cold metal is dipped in a hot tinning bath.

The use of sodium silicate is also disadvantageous because of the poor acid and moisture resistance of the sodium silicate. After masking the metallic article with the sodium silicate, a length of time may often elapse before the article is actually tinned. During this period the sodium silicate, because of its extreme hygroscopicity, tends to absorb moisture unless extreme precautions are taken. When later the article is dipped in the tinning bath, sputtering caused by the moisture contained in the sodium silicate results in flaking away of pieces of the film, thereby exposing unprotected portions of the article which it is desired to keep free from tinning.

It is often necessary, and usually desirable, to immerse the metallic article in an acid pickling solution prior to the tinning operation, especially if any length of time has elapsed after a previous pickling operation. Such an acid treatment yields a clean surface on which the tin can be Ill-) plated. Sodium silicate, because of its poor resistance to acids, especially to dilute acid solutions, would obviously be at a disadvantage if it were necessary to dip the entire sodium silicatecoated metallic article into the acid pickling solution prior to the actual tinning operation.

Ceramic coatings have also been employed as masking materials for preventing the deposition of tin plate on portions of an article desired to be tinned by dipping in a tinning bath. However, the use of ceramic coatings as shielding materials also presents many difficulties. In order to get a sufliciently protective coating, it is necessary to put a thick coating on the article desired to be masked. If the dimensions of the article are to be kept within close tolerances, the use of thick masking coatings is obviously a disadvantage. In addition, the ceramic coatings are often quite porous, thereby permitting absorption of such liquid materials as, for example, pickling solutions, fiuxing solutions, etc, usually employed in the preparation of the article for dipping in the tinning bath. On occasion, because of the porosity of the ceramic coating, minute portions of the tinning bath have found their way onto those portions of the article desired to be kept free of the tin plating. Moreover, ceramic coatings as masking media are susceptible to the same objection as regards their moisture pick-up and their brittleness as was pointed out above in connection with the same defect inherent in the properties of sodium silicate.

I have now discovered that the above-discussed defects can be satisfactorily obviated if an organo-substituted polysiloxane resin (for brevity hereinafter referred to as the polysiloxane resin) is employed as the masking material for the metallic article. These polysiloxane resins, more particularly, hydrocarbon-substituted polysiloxane resins, have outstanding heat-resistance and heat-stability at elevated temperatures. At temperatures encountered in the tinning baths (from about 300 to 500 C. or more), the polysiloxane resins are admirably suited for the stated purpose, since they are able to withstand the elevated temperatures for the short periods of time within which any part of the shielded portion of the metallic article may happen to be immersed in the molten tinning bath. In addition, the polysiloxane resins have excellent moistureand acidresistance which renders them unaffected by any moisture medium or by the pickling baths ordinarily employed for that purpose. Due to the flexibility which can be obtained with the polysiloxane resins, there is not as great a danger 3 that the resinous coating will chip from the coated article under the extreme temperature changes, thereby to contaminate the tinning bath.

The term tinning bath or other expressions using the term tinning is intended to include within its meaning molten metal baths wherein at least a preponderant amount of the molten materials comprises tin. Examples of materials that may be used in such baths are tin or alloys of tin which melt at a temperature of the order of 300 to 500 C. Examples of alloys corresponding to the above description are (in addition to pure tin) alloys comprising tin and lead, alloys comprising tin and antimony, alloys comprising tin and cadmium, alloys comprising tin and copper, alloys comprising tin and zinc, alloys comprising tin and aluminum, etc.

Various organo-substituted polysiloxane resins, e. g., hydrocarbon-substituted polysiloxane resins, may be employed in the practice of my invention. These include hydrocarbon-substituted polysiloxane resins containing an average of from about 0.5 to less than 2 hydrocarbon groups per silicon atom. High temperature-resistant thermoplastic polysiloxane resins are usually obtained when the polysiloxane resin contains an average of from about 1.5 or 1.6 to less than 2,

-e. g., 1.6 to 1.9, hydrocarbongroups (e. g., methyl,

ethyl, phenyl, or total methyl and phenyl radicals, etc.) per silicon atom. It will of course be understood by those skilled in the art that i it is possible, by the application of sufficient heat, to convert so-called thermoplastic polysiloxane resins to a state whereby their characteristics and properties resemble those of heat-converted or thermoset polysiloxane resins.

In addition to the aforementioned polysiloxane resins, I may also employ the usual heat-hardenable or potentially thermosetting hydrocarbonsubstituted polysiloxane resins, e. g., polysiloxane resins containing an average of from about 0.5 to 1.3 or 1.4 hydrocarbon groups (e. g., methyl, ethyl, phenyl, or total methyl and phenyl radicals, etc.) per silicon atom. I prefer to employ as the shielding or masking medium a hydrocarbon-substituted polysiloxane resin containing an average of from about 1.0 to 1.8 hydrocarbon groups per silicon atom. More specific directions for the preparation of the aforementioned hydrocarbon-substituted polysiloxane resins may be found in Rochow U. S. Patents 2,258,218 to 2,258,222, assigned to the same assignee as the present invention. It will also be apparent that the hydrocarbon-substituted polysiloxane resins employed in this invention may be further modified by the admixture with them of thermosetting resins such as phenolic, urea and melaminealdehyde' condensation products, unsaturated alkyd resins, etc.

Various methods well known in the art may be e employed to practice my invention. The portion of the article desired to be protected from 1 the tinning plate may be dipped in, sprayed, coated or brushedwith the particular polysiloxane resin (or a solution of the polysiloxane resin) employed as the masking medium. The shielded article is then preferably heated at an elevated temperature of the order of 150 to 300 C. for approximately 10 to 30 minutes. The temperatures and times of heating are not critical.

The main object in these steps is to drive off any solvent which may be present and to advance the polysiloxane resin to a more advanced state of cure. This heat-treatment willordinarily convert heat-convertible pol-ysiloxane resins to the substantially infusible and insoluble state, thus increasing their heat-resistance at more elevated temperatures.

It is customary to dip the masked article, prior to the plating step, in an acid picklin solution. The article may then be dipped for a short time in fiuxing solutions usually employed immediately preceding the tin-plating step. The shielded article is thereafter dipped in the specific type of molten tinning bath employed for the occasion in accordance with methods now well known in the art.

Inorder that those skilled in the art may better understand how the present invention may be practiced, the following example is given by Way of illustration and not by way of limitation.

Example A steel shell bearing was coated on the outside by brushing on a hydrocarbon-substituted polysiloxane resin containing an average of about 1.7 total methyl and phenyl radicals per silicon atom (dissolved in toluene to about '70 per cent solids). The coated bearing was then heated in a 150 C. oven for about 20 minutes to advance the resin to the substantially infusible and insoluble state. The bearing was then placed in a muriatic acid pickling bath for about 1 minute and thereafter in a flux (a bath used to prepare the article for the tin plating operation) comprising zinc chloride dissolved in Water. The bearing was immersed in a tinning bath containing pure molten tin for a few seconds (temperature of the bath was about 400 C.). After exposure to air for a short time, it was observed that the hearing was satisfactorily coated with the tin plate on th inside and that no tin at all adhered to the surface of the bearing which had previously been coated withthe .polysiloxane resin. Neither the pickling solution, nor the flux solution, nor the tinning bath had perceptibly affected the polysiloxane coating on the outside of the bearing. This result followed 'even though the thickness of the polysiloxane coating was less than 0.002 inch thick.

The use of a methyl-substituted polysiloxane resin containing an average of about 1.5 methyl groups per silicon atom in place of the methyland phenol-substituted polysiloxane resin employed in the above example afforded the same protection against plating by the tin.

It is to be understood that this invention is not intended to be restricted to the particular polysiloxane resin nor tinning bath employed in the above example, nor to the specific article masked and tinned. Other polysiloxane resins of the type previously described, including methyl-, ethyl-, propyl-, phenyl-, benzyl-, tolyltotal methyland phenyl-, etc; substituted polysiloxane resins may also be employed. This invention is especially applicable in the masking and tin plating of bearings, boxes, sheets of metal, i. 'e., metal to which tin plating Will adhere, decorative tinning, etc. After the tin plating operation, the polysiloxane resin may easily be removed from the plated object by abrasion or by various chemical means. However, by virtue of the fact that the polysiloxane resin can be present in such a thin layer (and still act as an effective masking agent), the polysiloxane coating may be allowed to remain on the metallic article. i

What I claim as new and desire to secure by Letters Patent of the United States is:

The method of partially tinning a steel shell bearing which comprises (1) applying to the outside of the bearing a thin coating of a methyl and phenyl-substituted polysiloxane containing an average of about 1.7 total methyl and phenyl groups per silicon atom, (2) heating the coated bearing at a temperature of about 150 C. to advance the resinous coating to the infusible and insoluble state, (3) placing the bearing in an acid pickling bath and then in a flux bath comprising water and zinc chloride, and (4) clipping the treated bearing in a molten tinning bath maintained at a temperature of around 400 C. to obtain an article tinned. on the inside and completely free of tin on the outside.

JOHN H. CLAWSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS