US3472665A - Electroless coating of cobalt and nickel - Google Patents

Description

United States Patent 3,472,665 ELECTROLESS COATING OF COBALT AND NICKEL Elton D. Prueter, Saginaw, and Wilhelm E. Walles, Midland,-Mich., assignors to The Dow Chemical Company,

Midland, Mich., a corporation of Delaware No Drawing. Continuation-impart of application Ser. No. 590,516, Oct. 31, 1966, which is a continuation-in-part of application Ser. No. 297,959, July 26, 1963. This application June 2, 1967, Ser. No. 643,059

Int. Cl. C09d /10; C03c 25/02 US. Cl. 106-1 16 Claims ABSTRACT OF THE DISCLOSURE A continuous film of cobalt or nickel can be applied to solid metallic and nonmetallic surfaces by immersion in a solution of a cobalt or nickel salt in a liquid 2-oxazolidinone at a temperature above about 200 C.

This application is a continuation-in-part of US. patent application Ser. No. 590,516, filed Oct. 31, 1966, and now abandoned, which was in t rn a continuation-in-part of US. Patent application Ser. No. 297,959, filed July 26, 1963, and now abandoned.

BACKGROUND STATEMENT OF THE INVENTION A new process has been discovered for depositing a continuous cobalt or nickel film on a solid substrate by contacting. (a) the solid substrate with a solution of (b) a cobalt or nickel salt and (c) a liquid 2-oxazolidinone at a temperature above about 200 C. for a time sufiicient to deposit a continuous cobalt or nickel film thereupon. More specifically, the electroless coating bath is a solution of about 0.01-50 weight percent of cobalt or nickel salt in a liquid 2-oxazolidinone of the formula:

where each R individually is H, C -C alkyl or C -C hydroxyalkyl and where together the R groups contain not more than 9 carbons. Preferably, it is a solution of about l-2-0 weight percent of the cobalt or nickel salt in S-methyl-Z-oxazolidinone heated at a temperature of about 2'00'-300 C. The resulting cobalt or nickel coating on a clean substrate is a very uniform, continuous and tightly adhering film.

The process is elegant in its simplicity. It avoids difficult high temperature and vacuum techniques as well as complex chemical or electrolytic reduction systems. The process is highly specific for coating with cobalt or nickel. Yet, at the same time, it is applicable to .a wide variety of metallic and nonmetallic solid substrates. For example, applied to a non-conductor, it gives effective surface elec- 3,472,665 Patented Oct. 14, 1969 trical conductivity. Applied to an etched magnesium plate, it provides a superior surface for chrome plating. It is particularly suitable for thin coatings, e.g., about 01-50 IIllCI'OIlS.

GENERAL DESCRIPTION Substrates to be coated with cobalt or nickel by this process must be solid and essentially inert under the process conditions. Porous materials which swell in contact with the plating bath are usually poor substrates. In general rigid, non-porous materials are most suitable. Thermoplastic materials can be used provided the softening point is at least 10 C. higher than the required bath temperature.

A great variety of metallic and nonmetallic substrates can be used. Thin, shiny layers of cobalt or nickel can be applied to such metals as gold, platinum, silver, copper, lead, tin, antimony, bismuth, magnesium and aluminum as well as to such alloys as bronze, brass and steel. Note that the process will deposit cobalt or nickel on a metal higher in the electrochemical series. Suitable inorganic substrates include asbestos, glass, titanium phosphate, titanium dioxide, silicon carbide, alumina, carbon black, calcium carbonate, calcium metasilicate, calcium silicate, kaolin, talc, silica, diatomaceous earth, quartz and the like in crystalline or amorphous form. Other electrically nonconductive substrates include solid thermoset resinous polymers such as melamine-formaldehyde resins, phenolfurfural resin, polyester and epoxy resins, silicone resins as well as irradiated polyethylene and polypropylene. Filled hydraulic cements such as transite and filled phenolformaldehyde or urea-formaldehyde resin also can be used.

The actual shape of the substrate is not critical so long as the surface to be coated can be contacted with the 2- oxazolidinone bath. The process can be used with substrates which are regular or irregular, crystalline or amorphous, a film, foil, fiber or wire, etc. It is particularly suited for mirror coating glass and metallic surfaces and for coating inorganic crystal needles or whiskers.

Cobalt and nickel salts suitable for use in this process should have a solubility of at least 0.01 weight percent in the 2-oxazolidinone bath and preferably are in essentially anhydrous form. Inorganic mineral acid salts such as cobaltous chloride, bromide and iodide, nickel chloride, bromide and iodide, cobaltous sulfate, nickel sulfate and the like are operable. However, salts of weak organic acids such as cobaltic and cobaltous acetylacetonate, nickel acetylacetonate, cobaltous acetate, nickel acetate, cobaltous formate, nickel formate, cobaltous propionate and nickel propionate are preferred, particularly cobalt and nickel salts of C C carboxylic acids.

The third critical element is the liquid 2-oxazolidinone which serves as a reaction medium and also is intimately involved in providing the essential cobalt or nickel atoms for the coating process. Suitable 2-oxazolidinones can be made, for example, by the process of Walles, US. Patent 3,179,667. These liquids have the general formula:

where each R individually is H, C -C alkyl or C -C hydroxyalkyl and where together the R groups contain not more than 9 carbons. Typical of the hydroxyalkyl -C H (OH) and -C H (OH) Among such solvents 5-methyl-2-oxazolidinone is preferred.

In practice a solution containing 0.01-50 weight percent, preferably about 1-20 weight percent, of the cobalt or nickel salt in the 2-oxazolidinone is normally used as the coating bath. Although minor amounts of water can be tolerated, the system is preferably essentially anhydrous because of the operating temperature above about 200 C. A coating temperature of about 200-300 C. and preferably about 225 275 C. is normally used. At lower temperatures deposition of cobalt or nickel is too slow. To obtain maximum coating adhesion, the substrate surface should be clean and dry before immersion in the coating bath. Eflicient agitation of the bath is required to assure optimum surface contact.

The cobalt or nickel coating is deposited very uniformly. Its thickness is a function primarily of the contact time, bath temperature and reagent concentration. Since the coating continues as long as cobalt or nickel atoms are provided, there is no real limit on the coating thickness. But in practice the process is best suited for thin 0.1-50 micron coatings. Typically a 0.5-5.0 micron nickel coating can be applied in 1-2 hours using a l0% solution of nickel acetylacetonate at 230260 C.

The following examples further illustrate this invention. Unless otherwise indicated, all parts and percentages are by weight.

Example l.Cobalt films on glass (A) A mixture of 0.78 g. (2.0 mmoles) of cobaltic acetylacetonate and 20 g. (200 mmoles) of 5-methyl-2- oxazolidinone charged to a thoroughly cleaned glass test tube and then heated to 250 C. After 1.5 hr. at 250 C., the solution was removed and a continuous cobalt film was found on the inside wall of the test tube.

(B) Similar experiments were carried out using 1 and 2 percent solutions of various cobalt salts and 2-oxazolidinones as shown in Table 1. In each case a continuous coating of cobalt was deposited on the inner wall of the test tube.

TABLE 1 2-oxazolidi- Run Cobalt salt none Conditions 113-1- cobaltous acetate.-. 5-methyl. 250 0., 2 hrs. 113-2 Cobaltous formate do. 250 0., 2 hrs. 113-3 Cobaltous acetate. 5 ethyl. 5 0. 113-4 Cobaltous chloride 5-methy 113-5- Cobaltic acetylacetonate do Example 2.Nickel films on glass (A) A nickel coating bath was prepared by dissolving 0.26 parts of nickel acetylacetonate in parts of 5- methyl-Z-oxazolidinDne. A nickel mirror formed on a clean glass surface immersed for 3 hrs. in this bath at 250-255 C. However, the nickel mirror was not so highly adherent to the glass as a cobalt mirror made concurrently using a bath containing 0.40 parts of cobaltic acetylacetonate in 10 parts of 5-methyl-2-oxazolidone.

(B) The procedure of Example 2A was repeated with a test tube rigorously cleaned with bichromate/sulfuric acid, rinsed with water and finally isopropanol. A better adhering Ni-mirror was obtained.

Example 3.Electroconductive cobalt coatings (A) A piece of commercial Transite asbestos-filled cement building panel A thick was coated with cobalt by immersion in a solution of 5 parts cobaltic acetylacetonate in 100 parts 5-methyl-2-oxazolidinone for 2.5 hrs. at 245 -250 C. The coated Transite strip had a grayish metallic appearance with a shiny metallic luster in some places.

The surface electrical resistance of the Transite panel was measured on several sides with an ohm meter and surface electrodes mounted 4.5 cm. apart with the following results:

Ohms Original Transite 20 l0 Co coated Transite 8-28 The surface resistance of the coated Transite is low enough for electrolytic deposition of other metals on top of the cobalt.

(B) The procedure of Example 2A was repeated with a plastic bottle cap made of a urea-formaldehyde resin filled with carbon black. A gray cobalt coating on the cap was obtained having the following surface resistance between electrodes 0.8 cm. apart:

Ohms Original resin 20X10 Co coated resin 10-30 The films of cobalt and nickel deposited on glass and other inert substrates by the processes of this invention are not affected by acetone, water nor prolonged exposure to air. Mirrors thereof do not transmit but do reflect light. They are particularly useful in reflecting special wavelengths of light.

Example 4.Cobalt coatings on metal surfaces To explore the deposition of cobalt on metal surfaces, a solution of 8 parts cobaltic acetylacetonate in 100 parts 5-methyl-2-oxazolidinone was used as a standard coating bath with a contact time of 2 hrs. at 245'-265 C. The surfaces of the test pieces were cleaned as necessary with steel wool and then wiped with a soft paper. After treatment the pieces were washed thoroughly, dried and the coating thickness measured by X-ray fluorescence. Typical results are given in Table 2.

TABLE 2.COBAL'1 COATING OF VARIOUS METALS Coat thickness Base metal (micron) Remarks 0. Shiny.

1. 6 Do. 1. 3 Shiny and dark areas. Lead (foil) 0. 40 Black. Copper (strip) 0. Shiny. Brass (rod) 1. 5 Do. Aluminum (strip). 0. G8 Dull. Antimony (chunk)- 0. 01 Tarnishcd.

Example 5.-Nickel coatings on metal surfaces The general procedure of Example 4 was followed using a solution of 6 parts nickel acetylacetonate in parts of 5-methyl-2-oxazolidinone with a contact time of 55 min. at 234235 C. Typical results are given in Table 3.

TABLE 3.NICKEL COATING OF "ARIOUS METALS 1 Tln (M.P. 232 C.) was coated at 220 C. for 2 hrs.

Similar attempts to deposit iron, chromium, zinc and aluminum on copper from a solution of the corresponding acetylacetonate in 5-methyl-2-oxazolidinone were unsuccessful even at a temperature close to the HP. of the 5-methyl-2-oxazolidinone, about 270 C.

Example 6.Nickel coatings on inorganic crystals (A) About 140 parts of S-methyl-Z-oxazolidinone was heated under a nitrogen atmosphere to about 115 C. Then 15.0 parts of nickel acetylacetonate and 15.0 parts of titanium phosphate crystal needles were added. The transparent titanium phosphate crystals had the composition 2TiO 'P O and were about 15-30 microns in length and 1-3 microns in thickness. The well stirred mixture was heated for 1 hour at 235255 C. Then, the crystals were recovered by filtration and washed by slurrying with water and then with acetone to give 17.0 parts of dry black crystalline powder. The product contained 14.9% Ni, 27% Ti and 17% P. The presence of nickel in elementary form was confirmed by X-ray analysis. Microscopic examination clearly reveals a veryuniform, nontransparent nickel coating, without visible open spots.

(B) In another experiment an extremely thin coating of nickel was applied to the titanium phosphate by reducing the contact time. The resulting coated crystals contained only 2.1% Ni.

(C) In a similar manner nickel coatings have also been deposited on crystalline needles of titanium dioxide and barium titanate. The process is also applicable to crystals of silicon carbide, carbon and alumina, for example, and to coating with cobalt as well as nickel.

We claim:

1. A method for depositing a continuous film of cobalt or nickel on a solid substrate which comprises contacting (a) the solid substrate with a nonaqueous solution of (b) about 0.01-50 weight percent of a cobalt or nickel salt and (c) a liquid 2-oxazolidinone of the formula:

where each R individually is H, C -C alkyl or C -C hydroxyalkyl and where together the R groups contain not more than 9 carbons, at a temperature above about 200 C. for a time sufficient to deposit a continuous cobalt or nickel film thereon.

2. The process of claim 1 where the solid substrate is a metal.

3. The process of claim 1 where the solid substrate is an inorganic crystal.

4. The process of claim 1 where the solid substrate is a nonconductive solid resin.

5. The method of claim 1 where the cobalt or nickel salt is a salt of a mineral acid.

6. The method of claim 1 where the cobalt or nickel salt is the salt of a C C carboxylic acid.

7. The method of claim 1 where the salt is cobalt or nickel acetylacetonate.

8. The method of claim 1 where the 2-oxazolidinone is S-methyl-Z-oxazolidinone.

9. The method of claim 1 wherein the deposition temperature is about 2 00 300 C.

10. A solution for an electroless coating of substrates with cobalt or nickel comprising about 0.0lweight percent of a cobalt or nickel salt in a liquid 2-oxazolidinone of the formula:

where each R individually is H, C -C alkyl or C -C hydroxyalky-l and where together the R groups contain not more than 9 carbons.

11. The solution of claim 10 where the salt is an acetylacetonate and the liquid is 5-methyl-2-oxazolidinone.

12. A solid substrate coated with a continuous film of cobalt or nickel by the process of claim 1.

13. The product of claim 12 where the substrate is ametal.

14. The product of claim 12 where the substrate is an inorganic crystal.

15. The product of claim 12 where the substrate is a nonconductive solid resin.

16. The product of claim 12 where the coating has a thickness of about 0.1-50 microns.

References Cited UNITED STATES PATENTS 3,250,784 5/ 1966 Gensheimer et a1. 3,294,578 12/1966 Popeck.

JULIUS FROME, Primary Examiner L. HAYES, Assistant Examiner U.S. C1. X.R.