Plastics Materials Having Electrodeposited Metal Coatings

Abstract

It is known to coat plastics materials, especially thermoplastics such as ABS, with metal by electrodeposition, typically with chromium on top of nickel. Conventional methods are often unsatisfactory when applied to thermosetting resin-based articles, the metal coating peeling or flaking off when the articles are subjected to repeated changes in temperature. It has now been found that copper or nickel coatings adhere well to surfaces, prepared in a conventional manner, of thermosetting plastics as well as of thermoplasts, provided that the coatings are electrodeposited in a manner such that they have an average stress in tension of at least 2,000 kg./sq. cm. E.g., a molding prepared from "Araldite" MY 750, HT 972, calcined china clay and glass fibers, is prepared for electroplating by A. etching with chromic acid B. depositing a film of palladium through treatment with stannous chloride and palladium chloride solutions C. depositing a film of copper or nickel by reduction of copper sulphate or nickel sulphate solutions. Next, a nickel coating is electrodeposited under conditions such that it had an average stress in tension of 2,440 to 6,460 kg./sq. cm.: this coating being dull, a bright, lightly stressed nickel coating was electrodeposited before the final layer, of chromium, was applied.

Description

This invention relates to coating plastics materials with metal by electroplating and to plastics materials so coated.

Increasingly, articles of plastics material are being electroplated with metals, particularly with chromium deposited on nickel. Before being plated, the surfaces of the article have to be treated to render them electrically conductive and also to modify them so that the electrodeposited metal adheres well thereto. Methods of achieving this are well known (see e.g. H. Narcus "Metallising of Plastics," Reinhold, 1960).

First, the article is preconditioned, that is to say, its surfaces are etched with a substance to promote bonding with the coatings applied later. The substance used depends on the type of plastics material to be etched; commonly, a mixture of potassium dichromate, sulfuric acid and water, sodium-naphthalene-tetrahydrofuran complexes, a mixture of sulfuric and phosphoric acids, nitric acid, or organic solvents (such as acetone, generally with hydroquinone and pyrocatechol) are used. This chemical treatment may be supplemented with, or replaced by, a mechanical cleansing operation, the surfaces being abraded by tumbling the article in an abrasive powder (sometimes with water) or by "vapor blasting" with very fine particles of abrasive material in a jet of air and water. Next, in the so-called "sensitizing" and "activating" stages, an electrically-conducting layer of metal is applied to the article, sometimes by means of paints containing metal powders, by spraying on metal powders or by vacuum sputtering, but usually by depositing the metal from a solution of its salt by chemical reduction. A film of palladium or silver is deposited by immersing the article in a solution of stannous chloride or other source of stannous ions, and then in a solution of, e.g. palladium chloride or silver nitrate. Sometimes the silver nitrate is used in aqueous ammoniacal solution in the presence of an organic reducing agent such as an aldehyde, with or without prior treatment with stannous chloride. Next, copper or nickel is applied by electroless deposition, then nickel is applied by electroplating, followed by, if required, chromium. Instead of applying nickel by electroplating, silver, palladium, or gold may be so deposited. In certain circumstances, where the surfaces of the plastics article have been suitably treated, the electroless deposition of copper or nickel can be dispensed with, the article being electroplated with copper instead. Conventionally, the electroplated layers applied to plastics articles are only lightly stressed in tension, this stress being not more than about 1,500 kg./sq.cm.

Thermoplastics materials which have been electroplated include chiefly acrylonitrile-butadiene-styrene copolymers (ABS), but also polyolefines, polymethacrylates and polycarbonates. For some purposes it is desirable to use thermoset plastic materials which, unlike many thermoplastics materials, do not soften and flow at moderately high temperatures. However, methods which, when applied to thermoplastics materials, give rise to electroplated nickel or copper having adequate adhesion to the substrate, are often unsatisfactory when applied to thermoset plastics materials, the electroplated nickel, copper, or certain other metals adhering insufficiently to withstand the dimensional changes that occur on repeated heating and cooling, while if the coating is accidentally perforated the electroplated metal around the perforation peels off. Because the electroplated nickel or copper fails to adhere, the overlying chromium plating readily peels off with it.

Recently it has been suggested that metal coatings which are electrolytically deposited on plastics materials, particularly poly-(oxymethylenes), adhere firmly and withstand repeated large changes in temperature provided that these metal coatings have a ductility and a tensile strength (i.e. the minimum force applied in tension necessary to rupture the film) which are above certain values, and that the coated articles have a compressive stress within a specified range.

It has now been found that electrolytically deposited nickel, copper, silver, palladium, or gold adhere well to plastics materials, especially thermoset plastics materials, provided that the electrolytically deposited layer is highly stressed in tension.

Stress in tension is associated with a metal coating being deposited in a condition such that, providing the substrate can be distorted, the coating contracts, whereas compressive stress is associated with the contrary condition, the coating having a tendency to expand.

The present invention provides an article of plastics material coated with an adherent, highly stressed, electrolytically deposited film of copper, nickel, silver, palladium or gold, which film has an average stress in tension of at least 2,000 kg./sq.cm. and preferably of between 2,300 kg./sq.cm. and 10,000 kg./sq.cm.

The highly stressed film may be of copper, and advantageously immediately overlie a film, applied by electroless deposition, of palladium or silver. Or the highly stressed film may be of silver, palladium, or gold, and especially nickel; such a film is conveniently applied onto a film, applied by electroless deposition, of nickel or copper. It is generally more convenient that such a film is of nickel, since electroless copper baths tend to be less stable than are electroless nickel baths. These films of nickel or copper, applied by electroless deposition, in turn preferably immediately overlie a film, applied by electroless deposition, of palladium or silver.

Highly stressed films are sometimes dull. In such cases a lightly stressed, bright film of nickel, copper, silver, palladium, or gold, i.e. one having an average stress in tension of not more than 1,500 kg./sq.cm., preferably between 1,050 kg./sq.cm. and 1,400 kg./sq.cm., may be electrolytically deposited onto such films. However, the average thickness of this lightly stressed film, together with any subsequent, electrolytically deposited films of metal, should not greatly exceed, say ten times, that of the highly stressed film, or the advantages of the invention may be forfeited.

When the lightly stressed film is of nickel, a film of cadmium, tin, silver, gold, lead, or particularly chromium, may be electrolytically deposited thereon.

The nickel, copper, silver, palladium or gold may be deposited in the two conditions, viz highly stressed and lightly stressed, by using plating baths of differing composition and/or pH. For example, nickel baths usually contain nickel chloride and nickel sulfate, and different ratios of chloride to sulfate ions favor formation of nickel in a more highly, or less highly, stressed condition : other things being equal, when chloride ions preponderate, the nickel is more highly stressed, and when sulfate ions are in the majority, the nickel is less highly stressed. Use of a less acidic plating solution (typically having a pH of between 5 and 6.8) also favors formation of a more highly stressed layer than does use of one having a pH below 5.

Various methods are available for determining the stress in tension (see e.g., Chapter 14, by Kushner, in "Electroplater's Process Control Handbook" edited by D. Gardner Foulke and Francis D. Crane, published by Reinhold). A convenient method is that of Graham and Soderberg, which is described by Kushner. In this method, thin shim steel about 7 cm. long and 1 cm. wide is bent into an arc of known radius of curvature, and the reverse (inner) surface is coated with a lacquer to prevent the metal being deposited on that surface. The shim steel is firmly clamped to a sheet steel base about 3 mm. thick to prevent bending during plating, and nickel, copper, silver, palladium, or gold is then electrodeposited. The plated shim steel is removed, and the change in radius of curvature due to the tensile stress of the film of electrodeposited metal is measured. The average stress in tension, S.sub.a, can be found by means of the relationship

where t is the thickness of the base, d is the thickness of the deposit, E is Young's modulus for the base, and r.sub.b and r.sub.a are the radii of curvature of the shim steel strip before and after plating.

For ease of electroplating the plastics material should contain a filler; the filler should, of course, be inert, i.e. resistant to attack by the various solutions employed. It is further desirable that the filler be present in a substantial amount, the plastics material containing at least 20 percent, and better at least 50 percent, by volume of the filler. Suitable fillers include china clay, which may be calcined (molochite) and titanium dioxide, but glass fibers or fibers of a textile such as a polyester are particularly preferred.

The plastics article should be etched, preferably by chemical means because the surface finish of mechanically etched plastics material may be poor unless a very thick layer of metal is applied by electroplating : to apply such layers is uneconomic, and further, they do not adhere as well as those of conventional thickness. Chromic acid mixtures (potassium dichromate-sulphuric acid-water) are usually suitable.

The process of the present invention may be applied to thermoplasts, such as a polyolefine, a polymethacrylate, a polycarbonate, an acrylonitrile - butadiene-styrene copolymer, or a polysulfone, but it is particularly suitable for plating cured thermoset plastics materials, especially such materials which are subjected in use to temperature of at least 80.degree. C. and which accordingly, containing filler if necessary, have a heat distortion point (measured according to British Standards Specification 2782 Method 102G) of at least 80.degree. C. The thermoset plastics material may be, for example, a cured epoxy resin, i.e. obtained by curing a substance containing on average more than one 1,2-epoxide group per molecule such as a polyglycidyl ether or a polyglycidyl ester; a cured aminoplast, such as a melamine-formaldehyde resin; or a cured phenoplast, such as phenol-formaldehyde resin. The filled thermoset plastics material may be a glass fiber laminate. Such laminates, electroplated with silver, palladium, or gold, are useful for printed circuits.

The following examples illustrate the invention.

EXAMPLE I

The plastics articles employed were prepared from an epoxy resin moulding composition, the ingredients of which were:

Parts by weight __________________________________________________________________________ a polyglycidyl ether of bisphenol A, epoxy content 5-5.2 equiv./kg. 23.3 bis(4-aminophenyl)methane, (curing agent) 6.3 "superfine molochite" (filler) 60 modified Montan wax, available from Hoechst under the designation "OP Wax" (release agent) 0.33 glass fibers silane-treated (filler) 10 __________________________________________________________________________

In place of the OP wax, other release agents such as calcium stearate, glycerol monostearate, stearic acid, or carnauba wax could be used. The articles were formed by transfer molding, the molding time being 3 minutes and the temperature 165.degree. C. Radio frequency preheating was used.

At the conclusion of each of the following stages, the articles were rinsed with distilled water.

Etching

Moldings so prepared were etched by immersion for 10 to 20 minutes at room temperature, or better at 65.degree. C., in a mixture of 11g. of potassium dichromate, 250 ml. of distilled water and 750 ml. of concentrated sulfuric acid.

Sensitizing

The etched mouldings were immersed for 2 to 4 minutes at room temperature in a solution of 10 g. of stannous chloride (SnCl.sub.2 .sup.. 2H.sub.2 O) and 40 ml. of concentrated hydrochloric acid in 1 liter of distilled water, and then for 3 to 5 minutes, at room temperature or at 35.degree. C., in a solution of 1 g. of palladium chloride (PdCl.sub.2 .sup.. 2.sub.2 O) and 10 ml. of concentrated hydrochloric acid in 4 liters of distilled water.

Electroless copper or nickel coating

The next stage was coating with either copper or nickel.

To coat the articles with copper, the following solutions were prepared, and mixed in equal volumes when required for use ##SPC1##

the articles being immersed therein for 5 to 10 minutes at room temperature.

To coat the articles with nickel, they were immersed for 4 to 6 minutes at 65.degree. to 85.degree. C. in a solution containing 35 g. of nickel sulfate (NiSO.sub.4 .sup.. 6H.sub.2 O), 10 g. of sodium citrate (Na.sub.3 C.sub.6 H.sub.5 O.sub.7 .sup.. 2H.sub.2 O), 10 g. of sodium acetate (NaC.sub.2 H.sub.3 O.sub.2.sup.. 3H.sub.2 O), 15 g. of sodium hypophosphite (NaH.sub.2 PO.sub.2.sup.. H.sub.2 O), 20 g. of magnesium sulfate (MgSO.sub.4.sup.. 7H.sub.2 O) and 1 liter of distilled water.

Highly stressed electroplated nickel coating

The articles were then electroplated with nickel from a solution held at 30.degree. to 35.degree. C. and containing, per liter of distilled water, 300 g. of nickel sulfate (NiSO.sub.4.sup.. 6H.sub.2 O), 64 g. of nickel chloride (NiCl.sub.2.sup.. 6H.sub.2 O), 32 g. of boric acid, 18 g. of sodium formate and 8 g. of cobalt sulfate (CoSO.sub.4.sup.. 7H.sub.2 O), the current density being 1 to 2 amps/sq. dcm. The pH of this solution was 5.3; in some experiments the pH was adjusted before plating to 5.6 or 6.1 by adding nickel carbonate or to 4.2 by adding concentrated sulfuric acid. The coating was on average 0.02 mm. thick. As determined by the Graham-Soderberg method, the tensile stresses of nickel plated at the different pH values were:

pH tensile stress __________________________________________________________________________ (kg./sq.cm.) 4.2 1260 5.3 2440 5.6 3780 6.1 6460 __________________________________________________________________________

At the lowest pH, 4.2, the tensile stress of the nickel was insufficient and the nickel had inadequate adhesion.

Lightly stressed nickel coating

This was applied from a conventional proprietary formulation available under the designation "Silvercrown Supersonic Bright Nickel" from Silvercrown Limited, Slough, Bucks., England. The bath, which was kept at 37.degree. to 43.degree. C., had a pH in the range 3.5 to 4.5, and the current density was 2.7 to 4.3 amps./sq. dcm. This coating also has an average thickness of 0.02 mm. In place of "Silvercrown Supersonic Bright Nickel" there was also used successfully:

"EFCO Bright Nickel" (a proprietary formulation available from Electro-Chemical Engineering Co. Ltd., Woking, Surrey, England) the bath being kept at 45.degree. to 55.degree. C. and having a pH of 3.9, and the current density being 3.2 to 5.4 amps./sq. dcm.; or

"Canning Super Gleamax Bright Nickel" (a proprietary formulation available from W. Canning and Co. Ltd., Birmingham 18, England), the bath, which had a pH of 3.9 to 4.5, being kept at 45.degree. to 50.degree. C. and the current density employed being 4.3 amps./sq. dcm.

Chromium-plating

The nickel-plated articles were finally plated with chromium. There was used a proprietary formulation, "Silercrown Bright Chromium," available from Silvercrown Limited, Slough, Bucks., England; the bath was kept at 38.degree. to 42.degree. C., the current density was 11 to 13 amps./sq. dcm., and the maximum voltage at strike was 5. The plating time was 2 minutes. In place of "Silvercrown Bright Chrome Solution" there was also used a bath containing 250 g. per liter of chromic oxide (CrO.sub.3) and 2 g. per liter of concentrated sulfuric acid : this bath was kept at 50.degree. C. and the current density was 10 to 20 amps./sq. dcm.

For purposes of comparison, moldings prepared from the same epoxy resin formulation were plated as described above except that the electrodeposition of the nickel was effected under conventional conditions.

The bath was similar to that employed to deposit the highly stressed coating, but it contained 50 g. of sodium formate per liter of distilled water instead of 18 g., and no cobalt sulfate; its pH was 4.5 and its temperature 40.degree. C., while the current density was 2.7 to 3.2 amps./sq. dcm. The nickel so deposited had essentially the same average thickness as the combined average thicknesses of the two layers of nickel electrodeposited as described above, i.e. an average of 0.05 mm. compared with the average total of 0.04 mm. for the two layers electrodeposited in accordance with this invention. The tensile stress of the single, conventionally electrodeposited layer was 1,400 kg./sq.cm. Chromium was then applied by electroplating in a conventional manner.

The Jaquet peel test could not be carried out with the chromium-plated moldings having coatings of nickel electrodeposited as described in accordance with the process of this invention; the adhesion was so great that the coating could not be pried off. On the other hand, the chromium could readily be peeled off from the castings prepared in the conventional manner.

The coatings produced as described by the process of this invention were distinguished by their resistance to repeated extreme changes in temperature. Samples of the chromium-plated moldings were held at -72.degree. C. or at -85.degree. C. for 1 hour, then immediately placed in an oven heated to 145.degree. C. for 1 hour : next, the samples were immediately cooled to -72.degree. C. or -85.degree. C. and the process repeated. After six, or even eight, cycles of cooling and heating, the coatings still adhered to the underlying plastics material, whereas the chromium on moldings plated in a conventional manner could be peeled off before fewer than six such cycles had been completed.

EXAMPLE II

In other experiments there were used:

a. moldings of a thermoplastic resin, viz. a polysulfone available from Union Carbide Corporation under the designation "Polysulfone P 1700," containing 10 percent by weight of molochite,

b. moldings prepared from an asbestos- filled phenol-formaldehyde resin, and

c. moldings prepared from a mineral- filled melamine-formaldehyde resin.

These moldings were electroplated with nickel in accordance with the invention, and then with chromium, as described above. The chromium plating remained intact despite the articles being subjected to four cycles of cooling to -60.degree. C. for 1 hour and then heating at 80.degree. C. for 1 hour, with intervals of only 15 minutes between each heating or cooling stage.

EXAMPLE III

A laminate was prepared by a wet lay-up technique from eight layers of glasscloth, and as the thermoset material, a mixture of 100 parts by weight of an epoxide resin, a polyglycidyl ether of bisphenol A (i.e. 2,2-bis(4-hydroxyphenyl)propane) having a 1,2-epoxide content of 5.2 equivalents per kilogram, and 27 parts by weight of 4,4'-diaminodiphenylmethane. The assembly was heated at 150.degree. C. for 1 hour under a pressure of 7 kg./sq.cm., and the resin was post-cured at 180.degree. C. for 3 hours.

Laminates were also made by a "prepreg" technique : layers of glasscloth were impregnated with a solution in ethyl methyl ketone of the same mixture of epoxide resin and curing agent, and the mixture was advanced to a B-stage resin by heating for 30 minutes at 100.degree. C. The resin was cured by heating as before, i.e. for 1 hour at 150.degree. C. under a pressure of 7 kg./sq.cm. and then for 3 hours at 180.degree. C.

The laminates were etched, sensitized and coated with nickel by electroless deposition as described in Example I. Next, a highly-stressed coating of copper was applied from a bath containing, per liter, 19 g. of cuprous cyanide, 45 g. of sodium cyanide, and 4 g. of sodium hydroxide. The bath was kept at 40.degree. to 45.degree. C. and the current density was 2.15 amps/sq.dcm. A highly adherent copper film, having a stress in tension of more than 2,000 kg./sq.cm., was obtained.

Coatings of silver, likewise showing good adhesion and being highly stressed in tension, were electrodeposited on similar glasscloth laminates bearing nickel applied by electroless deposition.

Claims

We claim:

1. An article of plastics material bearing thereon an adherent, highly stressed, electrolytically deposited film of a metal selected from the group consisting of copper, nickel, silver, palladium and gold, which film has an average stress in tension of at least 2,000 kg./sq.cm.

2. An article as claimed in claim 1, wherein the said film has an average stress in tension of at most 10,000 kg./sq.cm.

3. An article as claimed in claim 1, wherein the said film is of a metal selected from the group consisting of copper and nickel.

4. An article as claimed in claim 3, wherein said film is of copper and immediately overlies a film, applied by electroless deposition, of a metal selected from the group consisting of palladium and silver.

5. An article as claimed in claim 3, wherein said film is of nickel immediately overlies a film, applied by electroless deposition, of a metal selected from the group consisting of nickel and copper.

6. An article as claimed in claim 5, wherein said film, of nickel or copper, immediately overlies a film, applied by electroless deposition, of palladium or silver.

7. An article according to claim 6, wherein said film of palladium or silver is applied by electroless deposition upon a chemically etched surface of the plastics material.

8. An article according to claim 1, wherein said highly stressed film of a metal selected from the group consisting of copper and nickel immediately underlies a lightly stressed, electrolytically deposited film of a metal selected from the group consisting of nickel and copper having an average stress in tension of not more than 1,500 kg./sq.cm., the average thickness of said lightly stressed film, together with any subsequent, electrolytically deposited, films of metal, does not exceed ten times that of the said highly stressed film.

9. An article according to claim 8, wherein said lightly stressed film of copper, or nickel, has an average stress in tension of between 1,050 kg./sq.cm. and 1,400 kg./sq.cm.

10. An article according to claim 8, wherein the said lightly stressed film is of nickel and immediately underlies an electrolytically deposited film of chromium, cadmium, tin, silver, gold or lead.

11. An article according to claim 10, wherein the said lightly stressed film of nickel immediately underlies a film of chromium.

12. An article according to claim 1, wherein the plastics material contains an inert filler.

13. An article according to claim 12, wherein the plastics material contains at least 20 percent by volume of filler.

14. An article according to claim 13, wherein the plastics material contains at least 50 percent by volume of filler.

15. An article according to claim 12, wherein the filler is selected from the group consisting of glass fibers and textile fibers.

16. An article as claimed in claim 1, wherein the plastics material is a thermoset material.

17. An article as claimed in claim 16, wherein the thermoset plastics material is a glass fiber laminate.

18. An article as claimed in claim 1, wherein the plastics material, unfilled or containing inert filler, has a heat distortion point (measured according to British Standards Specification 2782, Method 102 G) of at least 80.degree. C.

19. An article as claimed in claim 1 wherein the said plastics material contains an inert filler and has a heat distortion point (measured according to British Standards Specification 2782, Method 102 G) of at least 80.degree. C.

20. An article as claimed in claim 1 wherein the plastics material is selected from the group consisting of cured epoxy resins, aminoplasts, and phenoplasts.

21. An article of plastics material bearing thereon an adherent, highly stressed, electrolytically deposited film of a metal selected from the group consisting of copper and nickel, which film has an average stress in tension of at least 2,000 kg./sq.cm.