GB2169925A - Process for providing a metal coating on a polymer surface - Google Patents

Abstract

An adherent electrically conductive surface layer is electrolessly deposited on a non-conductive generally rigid substrate e.g. for EMI/RFI shielding by first applying to the surface a polymer lacquer layer of e.g. 2 to 20 microns thickness containing particles of metal such as Ni, Cu, Ag, Al, Fe, Co, Pd, Ti, Zn, Mg, An, Pt or alloys thereof (e.g. 0.2-10 microns), allowing it to convert to an adherent solid surface layer; if necessary treating this surface layer to ensure that the outermost surface particles serve as good electroless initiation sites; and subsequent electroless deposition e.g. of a Cu, Ni, Ni/P alloy, or Ni/B alloy layer of e.g. 0.5-10 microns thick which can itself by electrolytically or electrolessly overplated if desired. The polyester layer may be formed by applying a polymeric material dispersed in an organic solvent, although it may be applied as a polymerisable material. Numerous examples of suitable materials are provided.

Description

SPECIFICATION Process for Providing a Metal Coating on a Polymer Surface This invention relates to the electroless deposition of a metal coating on the surface of a polymeric material and to articles of polymeric material thereby coated on at least one face.

While the invention is generally applicabie over the whole area of decorative and/or protective coatings on polymers (e.g. against corrosion, or other attack, on the substrate) as achieved by electroless coatings it has its origin in and is of primary utility for the provision of coatings for EMI/RFI shielding purposes.

Electrical or electronic equipment can, in addition to its intended function, generate electromagnetic interference or EMI, the frequency of which can extend from DC to 1 GH3 and the higher frequencies of which are often referred to as radio-frequency interference or RFI. If the equipment were totally located within a conductive ferrous metal box, with no apertures or windows, both components of the interfering wave, the E vector (magnetic field, dominating near the source) and the H vector (electric field, dominating further from the source) would be effectively attenuated, by providing a preferred path for the magnetic flux x thereby containing it within a specific region and by absorbing or reflecting the electric field portion and leading any current induced away to earth.

Similarly, the provision of such a box would shield the enclosed equipment from external EMI/RFI.

In the early days of electrical or electronic assembly, this shielding requirement was often at least approximately dealt with by using conventional ferrous metal housing cabinets.

Nowadays, however, housings for electrical or electronic equipment are conventionally made of polymer, essentially transparent to EMI/RFI waves, and have more complex shapes and orifices in the moulded shell. A wide range of specifications for shielding, depending on the type of equipment, its electromagnetic environment in use, whether it is domestic or industrial, its energy requirements, etc has been required. It is not possible to generalise compietely, but normally at least 20dB shielding will be required, and 20--80dB is a preferred extent of attenuation; 10dB represents a 90% signal strength reduction, 20dB a 99% reduction and so on logarithmically.

To achieve such shielding a number of methods have been proposed, since the geometry of the system, strength of EMI/RFI signal, attenuation required, and price/effectiveness constraints are widely variable.

One approach is to alter the electrical properties of the polymer housing itself by moulding it from a polymer containing suitable conductive metal particles as a filler. Although such materials are transparent to low-frequency magnetic fields, this is not a serious drawback for equipment enclosures (as distinct from component casings) since the distance from the internal components, themselves probably already magneticaliy shielded, reduces the effects to far-field magnetic effects. Nonetheless there are drawbacks to the practical use of such filled polymers, since there is excessive mould wear as the material is formed and since moreover there is non-uniform particle distribution especially in thin or complex mould sections.

Another approach therefore is to apply a conductive layer to the moulded polymer article.

Metal foil tape is a useful shield for small-scale prototype or development work but is labourintensive to cut to shape and apply without leaving linear gaps allowing EMI/RFI to escape.

Molten metal spraying, e.g. with zinc droplets generated by an electric arc and propelled by a gas gives a durable, conductive coating of high attenuation, but is again expensive and labour intensive and is unpleasantforthe operative. More modern techniques for applying a metal layer e.g.

vacuum metallizing (evaporation on to the polymer surface) or ion plating (dislodgement of metal atoms from a target by excited argon ions, followed by build-up on the polymer) have therefore been suggested and give useful products. However, the first mentioned needs a large metalliser and thus high prime cost, and is sensitive to contamination.

The second needs a first chrome primer coat as well as expensive equipment.

As a different type of approach, deposition of metal from solution has been tried. Electroplating onto polymer is a known technology, but is inconvenient in that it invariably involves a prior deposition from an electroless type of process on to the polymer surface. Electroless plating has advantages in that it deposits a thin but uniform coating, is relatively cheaper, and can be used as a batch process. However, it does cover both sides of the substrate part or panel so that in addition to the sequence of steps required for such plating one side-the eventual outside-must thereafter be repainted or decoratively plated, which is a procedural expense and disadvantage.

Moreover, electroless deposition as practiced in the prior art is a rather eiaborate process. It involves the provision, at the surface of a polymer, of nuclei, or a continuous layer, of a metal capable of initiating electroless deposition. This is conventionally done by (a) immersion in stannous chloride, rinsing, and applying a solution of a silver or palladium salt (or vice versa) to obtain a coating rather as a mirror is silvered or (b) immersion in a colloidal solution of palladium or copper, causing some of the colloid to be adsorbed on the surface of the polymer, and thereafter stripping the protective colloid.

In practice, if the polymer surface is smooth when the above processes are carried out an adherent metal depositif is not achieved. Thus it is in practice always necessary to etch the polymer surface first.

Different polymers require different etching techniques: ABS copolymer needs chromosulphuric acid, while others are even more difficult, or impossible to etch.

Of recent years, therefore, in the field of EMI/RFI shielding it has been proposed to provide and utilise a conductive layer in the form of an applied paint or lacquer e.g. a vehicle of a dissolved polymer containing dispersed therein particles of conductive metal. Examples of such paints are silver polyacrylic, silver/polyurethane, nickel polyurethane, nickel/polyacrylic, copper/acrylic or graphite/polyacrylic (with graphite being considered as a conductive metal for this purpose). The silverbased lacquers are the most expensive and give best conductivity. Generally the lacquers adhere well to the substrate, but they do have a tendency to be vulnerable to scratches, giving effectively a slot antenna for the escaping signal.Silver apart, however, they tend to be the cheapest and most cost-effective coatings, and can readily be applied to one side only of a panel by brushing, roller, or spraying.

The present invention is related to two of the above methods of EMI/RFI shielding discussed above, namely, the initial use of a metal-loaded or like paint, whether conductive or not, and the subsequent use of electroless plating.

Electroless deposits have superior characteristics for shielding applications compared to conductive paints for the following reasons:- (a) They are generally more conductive as deposited.

(b) Since an essentially continuous coherent metal layer is obtained, they retain their conductivity much better in service than conductive paints because the latter rely upon numerous contacts between individual conductive particles and the electrical contact can deteriorate due to a build up of oxide of other corrosion products in service.

Although these particluar advantages of electrolessplating have been evident for some considerable time to those skilled in the art there has been only relatively little utilisation of electroless plating for EMI/RFI suppression. It would appear that the procedural disadvantages listed above have in practical terms hitherto outweighed such advantages.

We have now, however, unexpectedly found that a metal-containing lacquer can be formulated to provide the following characteristics and thereby enable a subsequent electroless deposit to function as a useful shielding coating. The characteristics are: (i) active nuclei in an applied layer of lacquer (acting as initiating sites for electroless deposition) are sufficiently close together that a subsequent electroless deposition is effectively a continuous layer of metal deposit, (ii) any film of lacquer overlying these nuclei either does not inactivate the initiation of electroless plating, or else can readily be removed (prior to such plating) without damage to the structure of the whole'lacquer layer, and (iii) the lacquer is adhesive, coherent and strong so as to ensure a good bond between the substrate and the eventual electroless deposit.

In one aspectthe present invention consists of a process for providing on a surface of a generally rigid electrically non-conductive substrate an electrically conductive surface layer, which process comprises:- (a) applying to the surface at least one layer of a polymeric or polymerisable material in liquid form having suspended therein particles exhibiting or capable of exhibiting a metallic outer particle surface, and allowing the or each such applied layer to convert to an adherent surface layer, (b) carrying out any treatment necessary to ensure that particles at the outermost surface of the adherent surface layer serve as initial sites for electroless plating, and (c) contacting the said outermost surface with an electroless plating solution until a coherent electroless deposited electrically conductive surface layer is built up.

Such electrically conductive surface layer constitutes protective EMI/RFI shielding when used in the relevant environment.

The technique as described above is of considerable advantage generally in the field of electroless plating since it opens the door to such plating on virtually any polymer, it being generally speaking much easierto find a lacquer (i.e. liquid material as discussed above) to adhere to a "difficult" polymer such as polyvinylchloride than to etch such a polymer.

It may therefore be advantageous to use a separate lacquer (which may or may not contain metal particles) as a primer to improve adhesion between the substrate and the metal-particiecontaining lacquer.

It is important to realise that although the lacquer or like coating applied in the practice of this invention may be conductive, it is not necessarily so, and indeed it may now be possible in some formulations to provide other than conductive coatings, which yet form useful lacquers in the practice of the invention.

Thus, for example, it is possible to use lacquers containing aluminium particles. These are generally electrically non-conductive since the particles are coated with non-conductors such as Awl203. Also, even lacquers loaded with metals which are conductive per se do not have to be formulated to give conductive lacquers yet can still be utilised in the present invention.

While the Applicants do not wish to be limited by any hypothesis as to the mode of action of their invention, it seems possible that the individual metal particles, not necessarily themselves in electrically conductive contact, initiate metal deposition from exposed portions at the polymer surface such that the depositing metal grows laterally as well as at right angles to the surface, eventually joining up to become an effectively continuous metal layer anchored to the polymer substrate by a metallic bond via the particles embedded in the lacquer.

It is a valuable feature of the invention that only one surface need be treated with the liquid particlecontaining material, whereby only one surface receives an electroless deposit and final repainting is therefore unnecessary.

It is envisaged that this invention will mostly be applicable to application of substantial layers to rigid polymer mouldings e.g. the base or side walls of a computer housing, television cabinet, or the like. As discussed below, it can generally be distinguished from inventions involving coated tapes or coated flexible printed circuit substrates.

Application of the liquid material can be by brushing, spraying, roller applicatior, or any technique preferably offering the option of selective one-side application.

The liquid material is usually a polymeric material dissolved in, or dispersed in, an organic solvent (or mixture of solvents) or water, although it may be polymerisable e.g. from the monomeric or oligomeric state by radiation curing.

Typical polymer vehicles may be: 1. Film forming by evaporation of the solvent of water.

Examples are acrylic homopolymers or copolymers, polyurethanes, polyamides, polyesters, alkyds, ethylene copolymers with acrylates or vinyl acetate, chlorinated or unchlorinated, homopolymers or copolymers of vinyl chloride, vinyl acetate or vinyl proprionate, cyclised or chlorinated rubber, nitrocellulose, ethyl or ethyl/hydroxyethyl-ceUulose, petroleum hydrocarbon resins, coumarone-indene resins, terpene resins, ketone resins, polyvinyl acetals, cellulose acetobutyrate, cellulose acetoproprionate, shellac, sandarac, and other natural resins singly or in combination, or 2. Film forming by evaporation of the solvent or water and subsequent crosslinking through reactive groups; or by radiation curing of a liquid resin.

Examples are phenolics, epoxies, alkyds, polyesters, acrylics, hydroxylated copolymers of vinyl chloride and vinyl acetate, amine or amide materials such as polyamides, urea formaldehyde, melamine formaldehyde, hexamethoxy methyl melamine, benzoguanamine-formaldehyde, isocyanates.

The applied layer or layers can be of widely variable total thickness. From 2 to 20 microns is preferred, preferably 5--15 microns e.g. 5 microns preferably applied as one layer, but possibly two or more.

The particles used can be metal particles, fibres or flakes; or can be particles, fibres or flakes which are metal-coated; or they can be such particles generally which are convertible to exhibit a metal surface layer before or during the electroless plating stage. They will usually comprise from 1% to 50% v/v of the mixture, more preferably 10% to 40% v/v and often about 30% v/v. The particles may consist of, generally comprise, or be coated with a single member or an alloy having as a major component the metals in the list nickel, copper, silver, aluminium, iron, cobalt, palladium, titanium, zinc magnesium, gold or platinum; or have at least a surface convertible to such metal before or during electroless plating. As discussed above, the amount and nature of such particles may be but are not necessarily such as to render the mixture electrically conductive.The criterion appears to be that the initial sites for plating are close enough together at the surface. The particle size may be from 0.1 to 50 microns, usually about 0.2 to 10 e.g. 0.5 to 5. It is of some advantage if particles protrude from the layer or congregate at the outer surface: but particles which congregate at the substrate interface are not preferred since they will be generally less accessible as initiation sites for electroless platir g.

In some cases the applied layers, av;ng cured or dried, and the particle type and loading will be such that electroless plating can take place without an intermediary stage. In other cases, treatment with a suitable acid can be used to render the surface effective, all those listed metals except aluminium being possible instances of such a step. It is important to note that the acid may affect not only the metal but any adherent film of polymer thereover: thus, nitric acid is preferred to hydrochloric acid for copper, nickel or silver particles. In yet other cases, especially if the metal is aluminium, a different type of process involving the use of zinc in an alkali metal hydroxide may be valuable as rendering the surface effective for subsequent electroless plating.

The electroless plating may be of a type well known in itself, depositing copper, nickel, nickel/ phosphorus alloy, nickel/boron alloy, etc. Some of these give stronger magnetic shielding than others.

Additionally, the electroless deposit can be overplated, electrolessly or electrolytically, with nickel, iron, cobalt or alloys predominating therein.

Typical electroless layer thicknesses are 0.5 to 10 microns e.g. 0.5 to 2.5 for nickel and 1.0 to 5.0 for copper.

The present invention, as set forth above, is to.be distinguished on the one hand from the inventions of Schneble et al (USP3226256 and 3347724) and on the other from the invention of Diebold et al (USP 3332860). Schneble is concerned with a specific non-conductive paint applied to small regions i.e. as a printed circuit, on a flexible substrate. Even if conductive particles can be used, they must be widely disseminated in the point so that it is non-conductive; else the whole point of his invention is lost. This counterindicates the use of paint as in the present invention, applied over the whole of a surface for shielding purposes. Diebold, on the other hand is concerned with very small scale flexible substrates in tape or like form, with very thin support layers for very fine iron particles which are subsequently made even finer.The technology is that of a "displacement" process rather than that of an electroless deposition property so called, so is in practice different from that of the present invention and the end product appears more suitable for a component shield orwrapthan a housing panel with its different EMI/RFI environment and physical requirements.

The invention will be further described with reference to the following Examples.

EXAMPLE 1 Methyl butyl methacrylate copolymer as available from Cole Polymers Ltd under the Trade Mark COLACRYL 1202, comprising 50% w/w non-volatile components was dissolved in xylene to a sprayable viscosity at room temperature of 2025 seconds in a Ford 4flow-cup. To this was added 50% w./v/ i.e.

500 g/l of nickel flake pigment known under the Trade Mark of NOVAMET Flake Pigment Nickel HCT, as available from INCO. The mixture was sprayed to a total thickness of about 5 microns. The dried surface was treated with nitric acid, 20% wlv, at 20"C. and plated with copper to a 2-micron thickness from an electroless solution containing free NaOH 7--9g/i, formaldehyde 2-3 ml/l and copper sulphate 10g/l, the solution also containing stabilizers and the compound known under the Trade Mark QUADROL, a derivative of 1,2diaminoethane available from BASF.

EXAMPLES 2 -5 Example 1 was repeated except that a different electroless coating was utilised, as shown in each case, for electroless nickel coatings.

NiSO47H2O 33 gil Sodium hypophosphite 20 gil Mali acid 18 gil Pb++ 0.003 gil pH 5.5 Temperature 86 EXAMPLE 3 NiCl26H2O 30 g./l Sodium hypophosphite 10 g./l Ammonium citrate 65 g./l Ammonium chloride 50 gil pH 9 Temperature 86 EXAMPLE 4 NiCl26H2O 30 gil Sodium succinate 20 gil Diethylamine borane 3 g/l Methanol 40 gil Sodium acetate 20 g/l Sodium citrate 10 g/l pH 5.6 Temperature 60 EXAMPLE 5 NiCl26H2O 30 g/l Ethylenediamine 60 g/l Sodium borohydrate 0.6 g/l Thallium nitrate 0.07 g/l Sodium hydroxide 40 g/l pH 12.5 Temperature 86 EXAMPLE 6 Copper sulphate (CuS04 5H2O) 10 g/l Ethylene diamine tetra acetic acid (disodium salt) 25 g/l Rochelle salt (Potassium sodium tartrate) 10 gel Sodium Hydroxide 10 g/l Telluric acid 0.2 gIl (Sodium Salt of 2-ethyl hexyl sulphate (40% active) known undertheTrade Marks of "TERGITOL 08" 10 mill Formaldehyde solution (37% W/W) 10 mln Temperature 45"C EXAMPLE 7 A lacquer was prepared by mixing 42 parts by weight of COLACRYL 1202 28 parts by weight, xylene (80% paraisomer, balance mainly ortho) and 28 parts by weight propan-2-ol to form a homogenous mixture.

To 100 parts by weight of the above lacquer was added 50 a parts by weight of NOVAMET HCT, with stirring until the NOVAMET powder was completely wetted and a uniform dispersion obtained.

(COLACRYL 1202 is obtainable from Cole Polymers Ltd, 686 Mitcham Road, Croydon CR9 3BG and NOVAMET HCT is a finely divided nickel powder obtainable from Hart Coating Technology, P.O. Box 10, Brierly Hill, West Midlands DY5 2RQ).

A flat sheet of filled ABS plastic approximately 45 cmx45 cm was sprayed on one side with this lacquer, using an air-assisted gravity-fed spray gun at an air pressure of 2 bar to give a dry film thickness in the range 10-15 micrometers.

Precautions were taken throughout to keep the nickel powder suspended, by stirring the lacquer just prior to spraying. The lacquer film was then allowed to dry at room temperature for one hour and then further dried in a hot air circulating oven for 30 mins at 80"C.

After removal from the oven, the coated sheet was allowed to cool to ambient temperature and then immersed for four minutes at 20"C. in an aqueous solution containing: 100 ml/litre Nitric Acid (59 /O W/W) 20 ml/litre Pure Sulphuric Acid (96% W/W) 20 ml/litre Hydrochloric Acid (S.G. 1.14) 10 g/litre Ferric Chloride.

It was then rinsed thoroughly in cold water and immersed in an electroless copper solution of the alkaline formaldehyde reduced type (HTH Electroless Copper as supplied by W. Canning Materials Ltd) at 50 C forS0 minutes.

The plated sheet was removed from this solution, rinsed thoroughly in cold water, and immersed in an electroless nickel solution of the alkaline sodium hypophosphite reduced type (NIPLAS, as supplied by W. Canning Materials Ltd), for 15 mins at 50"C.

(Plating in the electroless nickel solution was initiated by touching the copper plating surface with a piece of mild steel which was itself plated with nickel). As a result of both electroless depositions, average thicknesses of 2 micrometers of copper and 2 micrometers of nickel/phosphor9us alloy were obtained.

The specimen was again rinsed thoroughly, with distilled water, and dried with hot air.

Test results obtained were as follows: The surface resistance was measured on the treated side and found to be 28 milliohms per square. After subjecting the coated sheet to 21 days damp heat to BS 2011 Part 2.1 Ca, the surface resistance was found to be 30 milliohms per square.

Cuts were made through the deposits, in a square-grid configuration at 2 millimetre spacing, eleven parallel cuts being made followed by eleven parallel cuts at right angles.

Tape 25 millimetres wide, having an adhesive strength of 1 kgf per 25 millimeters, was then applied over the whole area of the grid, and pressed down to ensure full adhesion. This tape was then rapidly peeled off from one end without jerking with the direction of pull parallel to the surface.

None of the squares in the grid suffered detachment of the coating.

EXAMPLE 8 A drawback for the lacquer formulation of Example 7 is that there is a tendency of the added nickel powder to settle, necessitating frequent agitation of the lacquer just prior to, and at frequent intervals during, spraying. An improvement which substantially overcomes this shortcoming is the addition of BENTONE SD2 (obtainable from Steetly Minerals Ltd. P.O. Box. 2, Galeford Hill, Worksop, Notts. S81 8AF).

An example of a lacquer formulation containing this is: 198 parts by weight COLACRYL 1202 99 parts by weight xylene (as in Example 7) 198 parts by weight propan-2-ol 5 parts by weight BENTONE SD2.

Such aformulationwasthroughly mixed and 100 parts by weight thereof was mixed with 50 parts by weight NOVAMET HCT. The resulting composition was sprayed and plated, as in the previous Example 7, with similar results. However, the lacquer only required to be agitated at intervals of twenty minutes to keep the nickel powder in suspension.

Moreover, whereas any nickel powder which settled out of the lacquer in Example 7 was very difficult to redisperse after storage for several days, this redispersal was relatively simple with the lacquer which contained the BENTONE.

EXAMPLE 9 The lacquers of Examples 7 and 8 were also used to treat the surfaces of PVC (clear), foamed PVC and Polycarbonate, substantially similar results were obtained.

EXAMPLE 10 When the procedures of Example 9 were used to treat a particular sample of PVC which was dyed red, poor adhesion of the lacquers to the substrate was found. Accordingly, in a repeat spraying procedure, the polymer surface was first wiped over with a cloth wetted with butanone (ethyl methyl ketone).

Good results were then obtained. This 'solvent wipe' was also found beneficial when treating glass-filled polyphenylene oxide available under the trade name of NORYL.

EXAMPLE 11 This example utilises aluminium powder.

60 parts by weight COLACRYL 1202, 30 parts by weight of xylene (as in Example 7) 25 parts by weight aluminium powder (SPARKLE SILVER 550p as sold by Silverline, Ban heat Industrial Estate, Laven, Fife, Scotland) were stirred to dispense the aluminium and sprayed as in Example 7 (on ABS).

The sheet was dried allowed to dry at ambient temperature. No hot air drying was used. The panel was then immersed in nitric acid (10% VN of 59% WIW nitric acid) at 20"C for 2 minutes, rinsed in cold water, immersed in a modified alloy zincate solution (BONDAL as sold by W. Canning Materials Ltd) at 25"C for 3 minutes, rinsed thoroughly and then immersed in the electroless nickel solution as in the first example for 30 minutes. A deposit of nickel/ phosphorus alloy of 3 micrometers thickness was obtained.

The adhesion in this case, was slightly inferior to that obtained in the Example 7 but is sufficient for some purposes.

EXAMPLE 12 In this Example silver-coated glass sphere (ballotini) are used as the initiating metal.

A lacquer was made up from 42 parts by weight COLACRYL 1202 28 parts by weight Xylene (Example 7) 28 parts by weight propan-2-ol To 100 parts by weight of this were added 50 parts by weight of silver coated ballotini 8500 S2 (as supplied by Croxton & Garry Ltd, Curtis Road, Dorking, Surrey RH4 1XA). The mixture was stirred until a uniform dispersion was obtained.

This was sprayed according to the method described in Example 7. After spraying, the lacquer was allowed to dry in air at ambient temperature.

No hot air drying was used.

The panel was immersed in Nitric Acid (10% VN of 59% WIW Nitric Acid) for30 seconds at200C.

After thorough rinsing the panel was immersed first in an electroless Copper solution and then in an electroless Nickel solution ali as described in Example 7.

The performance of the coated sheet was as reported for that of Example 7.

Modifications may be made in the invention as defined and exemplified above. In particular, although the invention has its origin in the technology of shielding against EMI/RFI, the discovery of a system in which there is a good polymer-to-lacquer bond, a good lacquer-to-particle bond and a good particle initiation site formation and subsequent bonding radically simplifies and extends the techniques of electroless plating generally.

Claims (23)

1. A process for providing on a surface of a generally rigid electrically non-conductive substrate an electrically conductive surface layer, which process comprises:- (a) applying to the surface at least one layer of a polymeric or polymerisable material in liquid form having suspended therein particles exhibiting or capable of exhibiting a metallic outer particle surface, and allowing the or each such applied layer to convert to an adherent surface layer, (b) carrying out any treatment necessary to ensure that particles at the outermost surface of the adherent surface layer serve as initial sites for electroless plating-and (c) contacting the said outermost surface with an electroless plating solution until a coherent electroless deposited electrically conductive surface shielding layer is build up.

2. A process as claimed in claim 1 in which the particles are metal particles with a metallic surface.

3. A process as claimed in claim 1, in which the particles comprise a metallic surface over an inert core.

4. A process as claimed in claim 1 in which the particles comprise a surface convertible to a metallic surface before or during electroless plating.

5. A process as claimed in any one of claims 1 to 4 in which the applied layer is from 2 to 20 microns in thickness.

6. A process as claimed in any one preceding claim in which the particles range in transverse dimension from 0.2 to 10 microns.

7. A process as claimed in any one preceding claim in which the suspension sprayed contains from 10 to 40% v/v of the said particles.

8. A process as claimed in any one preceding claim in which the particles, or the metallic surfaces derived therefrom, comprise a metal chosen from or a metal alloy having as a major component nickel, copper, silver, iron, cobalt, palladium, titanium, zinc, magnesium, gold or platinum.

9. A process as claimed in claim 8 in which the particles at the outermost surface of the adherent surface layer are treated with an acid priorto electroless plating.

10. A process as claimed in claim 9, in which the particles comprise copper, nickel or silver values and the acid is nitric acid.

11. A process as claimed in any one preceding claim, in which the particles, orthe metallic surfaces derived therefrom, comprise aluminium.

12. A process as claimed in claim 11 in which the particles at the outermost surface of the adherent surface layer are contacted with a solution containing zinc in an alkali metal hydroxide prior to electroless plating.

13. A process as claimed in any one preceding claim in which the electroless plating is such as to deposit copper, nickel, nickel-phosphorus alloy or nickel-boron alloy.

14. A process as claimed in claim 13 in which the electroless deposition is in the form of a layerfrom 0.5 to 10.0 microns thick.

15. A process as claimed in any one preceding claim in which a subsequent layer is deposited electrolytically or electrolessly over the electroless deposited layer.

16.A process as claimed in claim 15 in which the electroless layer is copper and nickel or a nickel alloy is deposited thereon.

17. A process as claimed in any one preceding claim in which the polymer or polymerisable material is dissolved or dispersed in water or an organic solvent and forms a polymeric film upon evaporation of the solvent of dispersant.

18. A process as claimed in any of claims 1 to 16 in which the polymer or polymerisable material is dissolved or dispersed in water or an organic solvent and subsequently cures to a polymeric film after evaporation of the solvent or dispersant.

19. A process as claimed in claim 18 in which the polymer or polymerisable material is a liquid resin and is subsequently cured to a polymeric film by radiation curing.

20. A process as claimed in any one preceding claim in which the polymeric coating is applied by spraying, dipping, roller-coated or printing.

21. A process as claimed in claim 1 and substantially as herein described with reference to any of the Examples.

22. An article having at least one generally rigid electrically non-conductive surface provided with an electrically conductive surface layer by the process as claimed in any one preceding claim.

23. An article as claimed in claim 22 which is EMl/RFl-shielded by the said conductive surface layer.