METHOD OF METAL PLATING OF POLYMER-CONTAINING SUBSTRATES

Abstract

Method of forming a metallic plating (9) on a substrate (1), comprising the steps of: —providing a substrate (1) comprising a hydrocarbon-based polymer containing C—C and either or both of C—H and N—H bonds; —covalently bonding an azide-containing primer compound (3) to said substrate (1) by C—H and/or N—H insertion, said primer compound (3) comprising molecules each having at plurality of C—H and/or C—N insertion sites; —in the absence of in-situ polymerisation, covalently bonding a pre-synthesised chelating polymer (5) to said primer compound (3) by C—H and/or N—H insertion, said chelating polymer (5) being capable of forming ligand bonds with metal atoms or ions; —dispersing a plating catalyst (7) in said pre-synthesised polymer (5); —forming said metallic plating (9) on said pre-synthesised polymer (5) by means of electroless plating or electroplating

Description

TECHNICAL FIELD

The present invention relates to the technical field of metal plating of non-metallic substrates. More particularly, it relates to a method particularly suitable for plating particularly smooth polymer-containing substrates with a metal such as copper, palladium, nickel, silver, nickel phosphorous (Ni—P), nickel boron (Ni—B), cupronickel or other metal.

STATE OF THE ART

Metal plating is often applied to non-metallic substrates made of e.g. polymer, glass, ceramic or similar for various reasons. For instance, it can be applied in order to create mirrors, for aesthetic reasons, to form electrical circuit tracks as interconnects, as antennas, as inductors, and so on.

Typical electroless (autocatalytic) and electrolytic plating processes require a certain roughness of the substrate in order to achieve a good adhesion between the metal plating and the substrate, without which the metal plating peels off extremely easily.

A particularly simple electroless precious metal (e.g. silver) plating process is described in document CH710579. Firstly, the surface is activated by wetting it with a suspension of precious metal nanoparticles in an organic solvent. This results in the deposition of the nanoparticles on the substrate, which then catalyse the autocatalytic deposition of the precious metal when the activated substrate is immersed in an aqueous solution of precious metal salt.

Another simple process is disclosed in document EP0250867. In this document, a pre-synthesised chelating polymer is coated directly onto a polymer substrate and then electrolessly-plated in the presence of a metal ion catalyst. The adhesion of chelating polymer to the substrate is poor, since it is only attached thereto by relatively weak physical adsorption, namely Van der Waal's forces, and hence the adhesion of the resulting plating to the substrate is also by definition poor.

In order to improve the adhesion of the metal layer to the substrate, this latter often needs to be roughened by either mechanical or chemical roughening, e.g. by means of etching (such as deep reactive ion etching, plasma etching, chemical etching or similar) or by mechanical means (sanding, bead blasting etc.) if the substrate is particularly smooth.

Document EP2626488 describes a method of plating a substrate which eliminates the need for roughening the substrate. In this process, a primer compound incorporating triazine compounds is applied to the surface and is light-cured. This primer is formed by means of a very complex synthesis of a very specific molecule comprising azide, triazine and hydroxyl functions in order that it both covalently bonds to the substrate and serves as an anchor for the metallic plating. The primer-activated surface is then treated with a metal catalyst solution, and electroless plating is subsequently carried out in a solution of metal salt as above. However, the synthesis of the primer compound is complex and proprietary, which is uneconomic and difficult to carry out in small manufacturing facilities, and the resulting compound is unstable and reacts far too easily.

Document WO2017060656 describes a process for grafting a polymeric thin film onto a substrate, the film being polymerised in situ and subsequently metallised. Since the polymer is synthesised in situ, it requires specific monomers and is a complex process requiring special equipment for graft polymerisation. Furthermore, the adhesion of the metal layer to the substrate does not appear to be adequate when the surface of the substrate is extremely smooth, and hence surface roughening appears necessary.

Documents EP3296845 and EP3200023 both describe highly complex processes which first involve polymerisation in situ before application of high molecular weight polyacrylic acid. Furthermore, document US2010/310800 again discloses a process in which a chelating polymer is polymerised in-situ. The methods of these documents hence require proprietary chemistry and are hence expensive and difficult to carry out.

An aim of the present invention is thus to propose a method of forming a metal plating on a polymer-containing substrate in which the deficiencies of the prior art are at least partially overcome.

DISCLOSURE OF THE INVENTION

More specifically, the invention relates to a method of forming a metallic plating on a substrate, as defined in claim 1. The method according to the invention comprises the steps of:

• • providing a substrate comprising, on at least one surface thereof, a hydrocarbon-based polymer containing C—C (carbon-carbon) and either or both of C—H (carbon-hydrogen) and N—H (nitrogen-hydrogen) bonds, such as for instance, polycarbonate (PC), poly methyl methacrylate (PMMA), polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), polyamide, thermoplastic polyurethane (TPU), polyether ether ketone (PEEK), an epoxy-based polymer, an acrylate-based polymer, 3D-printed resins, polymer-based photoresists, printed circuit board (PCB) substrate materials, a glass-reinforced epoxy laminate material such as FR4, or any other suitable polymer. The substrate may be formed by extrusion, hot moulding, injection moulding, sintering by laser or by heat and pressure, LIGA, stereolithography or other form of 3D printing, photolithography, or any other process suitable for the material in question, or by depositing said polymer as a layer on another material such as glass, metal, ceramic, silicon or similar, this polymer layer being the “substrate” in the sense of the invention;
  • covalently bonding an azide-containing primer compound to said substrate by at least one of C—H or N—H insertion (as appropriate) on said polymer, that is to say by inserting a carbon or nitrogen atom (as appropriate) of the primer compound into a C—H or N—H bond (as appropriate) of the polymer, said primer compound comprising molecules each having a plurality of C—H and/or N—H insertion sites (i.e. at least two C—H and/or N—H insertion sites), i.e. two or more azide groups, which are capable of C—H or C—N insertion into a C—H or C—N bond of the polymer molecule. The primer compound may for instance be provided in aqueous or alcohol-based solution.
  • in the absence of in-situ polymerisation (hence in the absence of a polymerisation-promoting agent such as a polymerisation initiator), and indeed without any intermediate step involving in-situ polymerisation, covalently bonding a pre-synthesised chelating polymer directly to said primer compound by at least one of C—H or NH insertion, said polymer comprising for instance at least one of polyacrylic acid, nitrogen-bearing polymers such as polyethyleneimine, polyvinylpyridine, polyvinylpyrolidone, polyacrylic acid, polyethylamine, poly(2-vinyl pyridine) (P2VP), poly(4-vinyl pyridine) (P4VP), polyaminophenylene (PAP) and so on. A chelating polymer is a polymer which can form ligand bonds (i.e. by definition ionic and/or coordinate covalent bonds) with metal atoms or ions, and to this end a logarithmic chelation stability constant log₁₀ β of at least 5.0 is advantageous, and polymers which can form multiple ligand bonds with individual metal atoms are also particularly suitable;
  • dispersing a plating catalyst in said pre-synthesised polymer, said catalyst being chosen to promote either electroless plating or electroplating of a metal. The catalyst can e.g. be applied in aqueous solution or as a suspension of catalyst particles;
  • forming said metallic plating on said pre-synthesised polymer by means of electroless plating or electroplating. The metal in question may comprise copper, nickel, Ni—P, Ni—B, silver, palladium or other suitable metal or mixture (alloy) of metals, and may be applied in a plating bath containing a solution of an appropriate metal salt, with or without application of electrical current as appropriate.

The azide-containing primer compound may contain at least two, preferably at least ten, further preferably at least forty azide groups situated on side chains of a core polymeric chain.

As a result, extremely good adhesion between the metal plating layer and the substrate can be achieved using simple, commercially-available chemistry which is stable at room temperature and without recourse to special synthesis processes or special equipment, such as would be required for growing polymers in-situ on the substrate from their constituent monomers. Indeed, the method has a minimum of steps and is hence very economical. The plating is formed around the individual molecules of polymer and in intimate contact therewith, and since the polymer molecules are indirectly covalently bonded to the substrate via the primer compound with at least one C—H or C—N insertion between the primer and the substrate on the one hand, and with at least one C—H or C—N insertion between the primer and the chelating polymer on the other hand, the adhesion is excellent since covalent bonds are the strongest type of chemical bonds.

The use of using such azides as primers gave a surprising and unexpected technical effect. In general, they are not generally known as adhesion promoters/primers/coupling agents in the specific context of the method of the present invention, and present many advantages over typical compounds based on silanes, isocyanates, acrylates or hydroxyls, which would be the obvious choices to test. In particular, the azides in question are generally stable at room temperature and hence do not need to by synthesised and used immediately, and they are stable under the extreme pH conditions. This is particularly important in the case of certain plating processes which are carried out at a pH of 1-3 (highly acidic) or 12-14 (highly alkaline), and which the bonds formed with conventional primers cannot survive—this azide primer pH insensitivity is not generally known, and hence the compatibility with such extreme-pH plating processes is extremely surprising. As a result, the method of the present invention allows plating over an extremely wide pH range, not only near neutral pH.

Advantageously, the catalyst may comprise metal ions and/or metal nanoparticles. In the case of ions, these may be applied in the form of an aqueous solution of an appropriate metal salt (such as a palladium, silver, platinum, copper or nickel salt). Alternatively, the catalyst may be formed from metal ions applied in solution and then subsequently reduced to metal in situ with a suitable reducing agent. In in the case of metal nanoparticles, these may be applied as a suspension of palladium, silver, nickel, copper or similar nanoparticles in water or an organic solvent.

In a particular variant, the primer compound and said pre-synthesised polymer are applied to said substrate simultaneously, e.g. in a mixed aqueous or alcohol-based solution.

Advantageously, said pre-synthesised polymer may be covalently bonded to said primer by depositing said pre-synthesised polymer on said substrate and subjecting it to UV radiation or heat. This step is particularly applicable in the case in which simply contacting a solution of the polymer with the primed substrate is insufficient to form sufficient covalent bonds for strength. This UV or heat activation hence causes such bonds to form.

Advantageously, said polymer has a molecular weight of at least 100,000 Daltons, preferably at least 500,000 Daltons, further preferably at least 750,000 Daltons, even further preferably at least 1,000,000 Daltons. This ensures that the polymer molecules are sufficiently long that many of them are bonded at multiple points along their lengths to the substrate by means of the primer compound. Once the metal plating has been formed intimately around the polymer molecules, it thus forms a composite at least in a part of its thickness adjacent to the substrate which is securely anchored thereto by means of covalent bonds.

It should be noted that the primer advantageously does not contain one or more (or indeed any) of: a triazine ring, an alkylsilane group, a silanol group, a maleimide group, or a complex compound based on aluminium or titanium. Furthermore, the primer does not comprise dextran, nor does it comprise a methacrylate or acrylate.

The aim of the invention is likewise attained by a product comprising a substrate as described above, and a metallic plating provided on a surface of said substrate, wherein said metallic plating is formed on said substrate by means of one of the methods described above. This metallic plating may, for instance, form an antenna.

The metallic plating thus formed has excellent adhesion to the substrate for the reasons discussed above.

Advantageously, said metallic plating forms a composite between its constituent metal and said polymer in at least a part of its thickness, which ensures excellent adhesion.

Advantageously, said metallic plating comprises at least one of copper, nickel, silver, nickel phosphorous, nickel boron, cupronickel, platinum or palladium.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details of the invention will appear more clearly upon reading the description below, in connection with the following figures which illustrate:

FIG. 1: a schematic (not to scale) representation of a method of plating according to the invention;

FIG. 2: a schematic (not to scale) representation of a variant of a method of plating according to the invention;

FIG. 3: a schematic (not to scale) representation of how polymer molecules are bonded to the primer used in the method of FIG. 1; and

FIG. 4: a schematic view of a product wherein the plating forms an antenna.

EMBODIMENTS OF THE INVENTION

FIG. 1 illustrates a first embodiment of a method according to the invention.

In step 101, a substrate 1 is provided. This substrate 1 comprises a polymer intended to receive a metallic plating on at least one surface thereof. This polymer may be, for instance, polycarbonate (PC), poly methyl methacrylate (PMMA), polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), polyamide, thermoplastic polyurethane (TPU), polyether ether ketone (PEEK), an epoxy-based polymer, an acrylate-based polymer, 3D-printed resins, polymer-based photoresists, printed circuit board (PCB) substrate materials, a glass-reinforced epoxy laminate material such as FR4, or any other suitable polymer. It may be fibre reinforced by means of glass fibres, carbon fibres, basalt fibres, natural fibres (hemp, cotton, etc.) and so on, and may contain other fillers. Furthermore, the substrate 1 may be monolithic, or may be made of another material (glass, metal, ceramic, silicon, silicon oxide, silicon nitride, silicon carbide or similar) provided with a polymeric layer on its surface.

Such a substrate may for instance be a printed circuit board (PCB) substrate, a housing for an electrical device such as a mobile telephone, smartphone, tablet computer, laptop computer, or any other electronic or mechanical device. It may be produced by moulding, 3D printing (additive manufacture), photolithography, stereolithography, LIGA, hot pressing, sintering (by laser or pressure), or any other convenient process appropriate for the material in question. It may have a simple, planar shape, or may have a complicated three-dimensional form.

Although the method of the invention is applicable to surfaces of any roughness, it is particularly suited for plating extremely smooth surfaces such as those with an R_a roughness of 10 nm or less, more particularly 10 nm or less, 7 nm or less, or even 5 nm or less, as measured according to the ISO 4287:1997 standard. However, surfaces deliberately roughened during manufacture of the substrate, or in a separate chemical etching, plasma etching or mechanical roughening (e.g. sanding, bead blasting or similar) step are not excluded from the scope of application of the present invention. Furthermore, the method can be used with surfaces which are very difficult to etch chemically using industry-standard etchants such as a chromic etch, permanganate etch, acid etch, basic etch, etc. An example of a difficult-to-etch surface is Prototherm®, an epoxy-based polymer manufactured by Somos. Also, the surface may be activated e.g. by exposure to ultraviolet light, heat and/or plasma under atmospheric or (partial) vacuum conditions.

In step 102, the substrate 1 is treated with a primer compound 3 so as to form a coating thereupon, this primer compound being azide based and containing molecules each comprising a plurality of C—H and/or N—H insertion sites which can hence each covalently bond to at least one polymer molecule of the substrate 1 and to at least one polymer molecule of the pre-synthesised chelating polymer 5 (see below). Such primers have a polymer backbone which may be neutral or may contain various bonding groups such as amino groups, silane groups, thiol groups, phosphonates and so on, and which further comprise a plurality of C—H or N—H insertion groups, specifically two or more of azide groups situated on sidechains of the core polymeric chain constituting said polymer backbone. Ideally, the number of azide groups is at least ten, further preferably at least forty.

Specifically, such primers may incorporate for instance the compounds disclosed in EP2236524. To this end, this document is herein incorporated by reference in its entirety, but other azide-containing primers are of course possible. It should be noted that the discovery that the adhesion promoter of this document was particularly suitable for use as a primer in the specific plating process of the invention was entirely unexpected, since neither in this document, nor in the datasheet for the corresponding commercial product AziGrip4 (from Susos®), is there any suggestion at all that it would have the above-mentioned advantageous technical effects in this role. The standard primers to be experimented with are based on silanes, isocyanates, thiols, acrylates or hydroxyls. However, these are typically pH sensitive and are hence not suitable for plating at extremely low pH (1-3; typically associated with electroplating) or extremely high pH (12-14; typically associated with electroless plating) since they decompose under such conditions. Indeed, the pH insensitivity of azide-containing primers is not generally noted, making the unexpected technical effect of the invention all the more surprising.

The primer 3 is directly applied in a layer to a surface of the substrate 1 which is intended to be plated, for instance by dip-coating, drop-coating, spraying, bar-coating, brushing or any other method. To this end, the primer may be dissolved in a solvent such as toluene, chloroform, dichloromethane, an alcohol, particularly methanol, ethanol, isopropanol or a mixture of two or more thereof. It should be noted that the primer 3 is not synthesised in situ, nor is it synthesised immediately before use, since it is stable at room temperature.

The solvent is then allowed to evaporate under ambient conditions or in an oven set at a moderate temperature such as 20° C. to 100° C., leaving the layer of primer 3 on the surface of the substrate 1.

In step 103, a layer of pre-synthesised chelating polymer 5 is coated directly on the primer layer 3, in the absence of any preceding in-situ polymerisation-based step, and without the presence of a polymerisation-promoting agent such as a polymerisation initiator. Depending on the chosen polymer, this may be applied as a solution of the polymer in water or in an appropriate organic solvent. By “pre-synthesised”, it is meant that the polymer in question is applied in already-polymerised form, and is hence not synthesised in situ during execution of the method by means of monomers being joined together by a polymerisation agent such as a catalyst. In other words, the polymer molecules retain the same molecular weights and do not grow when applied to the substrate 1. The chelating polymer 5 is also bonded to the primer 3 by C—H or N—H insertion in a similar manner to the bonding between the primer 3 and the polymer of the substrate 1, C—H or N—H-insertable groups of the primer 3 again being inserted into C—H or N—H bonds of the chelating polymer 5.

The polymer in question is chosen for its interaction properties with metal ions and/or nonvalent metal particles. Examples of such polymers include polyacrylic acid, nitrogen-bearing polymers such as polyethyleneimine, polyvinylpyridine, polyvinylpyrolidone, polyacrylic acid, polyethylamine, poly(2-vinyl pyridine) (P2VP), poly(4-vinyl pyridine) (P4VP), polyaminophenylene (PAP) and so on. The important point is that the polymers in question can form ligand bonds with the catalysts mentioned in respect of step 104 below, and are hence chelating polymers, i.e. polymers which will bind to metals and/or to metal ions. Ligand bonds are by definition ionic and/or coordinate covalent bonds formed between the chelating polymer 5 and a metal atom or metal ion, as is generally known and understood from the relevant scientific literature, and all the example polymers given above are capable of forming such bonds. A logarithmic chelation stability constant log₁₀ β of at least 5.0 is advantageous (and is fulfilled by all the above-mentioned polymers), and polymers capable of forming polydentate ligands are preferred in order to bond strongly to the metal ions. Ideally, the polymer molecules in question will have a high molecular weight, such as at least 100,000 Daltons, preferably 500,000 Daltons, preferably at least 750,000 Daltons. The larger the molecular weight of the polymer, the more chelation sites are present, leading to attachment of a greater number of metal ions to the polymer. Multiple layers of lower molecular weight chelating polymers are also possible, as are certain other lower molecular weight polymer molecules such as dendrimers (e.g. polyamidoamine, also known as PAMAM) and metal chelating hyperbranched polymers. The publication “Metal chelate dendrimer, star and hyperbranched polymers”, Uflyand Igor and Zhinzhilo Vladimir, LAP Lambert Academic Publishing, EAN 9786202013246 contains many examples of both of these types of polymers.

The polymer solution may be coated onto the primer layer 3 by dip-coating, drop-coating, spraying, bar-coating, brushing or any other method of thin film coating of the polymer in aqueous or alcohol-based solution.

The solvent is then allowed to evaporate under ambient conditions or in an oven set at a moderate temperature such as 20° C. to 100° C., leaving a layer of polymer 5 attached to the primer 3.

Subsequently, in step 103a, if required, the polymer 5 is reacted with the primer 3 so as to create covalent bonds between these two substances by means of e.g. UV light (such as UVC light), heating to a sufficient temperature (such as to above 120° C., preferably to above 125° C.), or by any other convenient method such as exposure to plasma, local heating with ultrasound or similar. If the polymer 5 has already sufficiently bonded to the primer 3 in step 103, e.g. during the evaporation of the solvent, this step is unnecessary.

Subsequent to this bonding step, and if required, excess polymer which has not bonded to the primer 3 can be rinsed off with an appropriate solvent and optionally also by sonication to agitate the unbonded polymer, and the substrate can then be dried again if required, for instance in the case in which the solvent used for rinsing is incompatible with the next step.

In step 104, a plating catalyst 7 is distributed throughout the polymer 5. This catalyst, which is chosen for its promotion of electroless plating and/or electroplating, may be formed of metallic ions, metallic nanoparticles, or similar. In the case of metallic ions, these may be Pd²⁺ ions, Cu²⁺ ions, Pt²⁺ ions, or any other suitable ²⁺ valence metal ions provided as a solution of an appropriate metal salt. These ions are complexed by the molecules of polymer 5 and thus are anchored thereon. The catalyst 7 is applied by dip-coating, drop-coating, spraying, bar-coating, brushing or any other method of thin film coating of the catalyst 7 in aqueous solution in the case of ions, or as a suspension of metallic nanoparticles in water or an organic solvent such as ethanol. As a result, the ions or metallic nanoparticles diffuse into the polymer layer 5 and attach to the polymer molecules by ligand bonding, i.e. by ionic bonding and/or coordinate covalent bonding as appropriate, as mentioned above. Optionally, the in the case of metal ions, these may be reduced to zero-valence metal atoms by means of a redox reaction with another appropriate reducing agent such as formaldehyde or a salt such as sodium hypophosphite, sodium borohydride, ascorbic acid, dimethylamine borane, boric acid with an addition of acetic or sulphuric acid, or other suitable salt applied in a bath, by spray coating, bar coating, dip coating, drop coating, brushing, or any other suitable method. The excess reducing agent can then be removed by rinsing in water, with or without sonication. This reduction results in catalyst metal being intimately and strongly attached to the polymer layer 5

In step 105, the polymer-grafted, catalyst-containing substrate 1 is then subject to an electroless (autocatalytic) plating of copper, nickel, nickel-phosphorous, nickel boron, palladium, silver, cupronickel, platinum or other suitable metal in a plating bath comprising a suitable salt of the metal in question. Since the solution of metal salt penetrates through the polymer layer 5, the layer of metallic plating 9 which is formed reaches the primer 3 layer. Alternatively, the plating 9 can be applied by electroplating in a suitable electroplating bath, to the same effect.

Since the plating is formed not only on the surface of the polymer layer but is formed right throughout the polymer layer 5, including down to the primer layer 3, it is firmly anchored to the polymer molecules. These latter are themselves covalently bonded to the primer 3, which is itself covalently bonded to the substrate 1.

As a result, since the metal plating is intimately intermingled with the tangle of polymer molecules 5 which are covalently bonded to the substrate 1 via the primer 3, the adhesion between the metal plating 9 and the substrate 1 is significantly improved. This has been proven by the results of experimental tests, the results of which have been reproduced below.

Another way of looking at the resulting structure is that the portion of the plating layer 9 immediately adjacent to the primer 3 and substrate 1 is a metal-polymer composite with the metal filling up essentially all the space between the individual polymer molecules, these molecules being covalently bonded to the primer 3 and hence indirectly to the substrate 1. The portion of the metal plating 9 which grows beyond the extent of the polymer 5 molecules is monolithic. However, if the metal layer 9 is only as thick as the polymer layer 5, the entire plating is hence a metal-polymer composite.

The resulting plated substrate 1 forms at least part of a product 11, the plating forming e.g. the tracks of an electrical circuit, an antenna (such as a GHz or THz antenna in a wireless electronic device), a mirror, or any other product 11.

FIG. 2 illustrates schematically a variation on the method of FIG. 1, which differs from this latter in the following particulars.

Steps 101 and 102 are as in FIG. 1. However, steps 103 and 104 of FIG. 1 are combined in a single step 106, in which a mixture of polymer 5 and primer 3 in an appropriate solvent is coated onto the substrate 1 by dip-coating, drop-coating, spraying, bar-coating, brushing or any other method of thin film coating of the polymer in aqueous or alcohol solution.

The solvent is then allowed to evaporate under ambient conditions or in an oven set at a moderate temperature such as 20° C. to 100° C., leaving a layer of intermixed polymer 5 and primer 3 on the substrate 1, the primer molecules binding the polymer molecules to the substrate as before, but also with residual primer 3 distributed amongst the polymer 5. The block illustrating the primer 3 in this particular case is purely schematic and does not indicate a solid layer of primer 3, but rather the extent of its distribution throughout the polymer 5 once the solvent has evaporated. For this reason it has been illustrated with dashed lines.

If required, a step of irradiation or heating 106a can be carried out in identical fashion to step 103a described above.

Subsequently, steps 104 and 105 are carried out as above, with the metal plating 9 being formed through the intermingled polymer/primer layer, right down to the surface of the substrate 1, with the same effects as in the method of FIG. 1. Again, the extent of the dispersed primer compound 3 is illustrated with a dashed line, however this is again not a solid layer. In essence, the thickness between the dashed line and the surface of the substrate 1 comprises not only polymer 5 and metal 9, but also primer 3 dispersed therein.

FIG. 3 illustrates schematically an important aspect of the result of the method of the present invention. Since the polymer 5 used in the method of the invention is pre-synthesised, each molecule of polymer is covalently bonded to the primer 3 at one or more points along its length, indicated schematically by “X” symbols. For instance, polymer molecule 5a is covalently bonded to the primer 3 at one of its extremities. Polymer molecules 5b and 5e are covalently bonded to the primer 3 each at a single point along their lengths near one of their respective extremities. Polymer molecule 5c is covalently bonded at a plurality of points along its length, of which two are visible in the figure. Polymer molecule 5d is likewise covalently bonded to the primer 3 at a number of points. In the alternative case of a method according to FIG. 2, the only difference is that there is no discrete layer of primer compound 3, the individual molecules of primer compound being intermingled with the polymer 5.

This is distinct from a polymer synthesised in situ such as that disclosed in WO2017060656, which is only bonded to the underlying layer by a single extremity from which each individual molecule is grown. The bonding arrangement of the invention significantly increases the bonding strength between polymer 5 and primer 3 (and hence substrate 1) since many, if not most, polymer molecules will be bonded at multiple points and the angle made between the polymer molecule and the normal to the underlying layer will be typically greater. Since the metal plating 9 is intermingled with the polymer 5, its adhesion strength to the substrate will be significantly greater due to the larger number of covalent bonds present between the polymer and the primer (and hence with the substrate) in a given surface area.

Experimental tests carried out according to the strictest appropriate testing procedure, namely ASTM standard D3359-09, on a polymer substrate of surface roughness R_a<5 nm using conventional plating methods and plated according to the invention showed an adhesion improvement from grade 0B (the lowest possible grade) to grade 5B (the highest possible grade).

As can be seen from the foregoing, the method of the invention is carried out entirely without any step of in-situ polymerisation, i.e. no polymer is grown from monomers or other polymer precursors on the substrate 1 or on the primer 3. The chelating polymer 5 is simply applied fully-formed thereupon, and retains its initial molecular weight. Indeed, the entire method can be carried out using readily-available off-the-shelf chemistry.

FIG. 4 illustrates schematically a product 1 in which the metal plating 9 is configured on the substrate 1 as an antenna, such as a GHz or THz antenna. The substrate 1 may be a printed circuit board, a casing for an electronic device, or similar, and the plating tracks may be structured by masking pre-deposition, or masking and subsequently etching the plating 9 after deposition by any convenient means.

Several concrete examples of tests were carried out, the details of which are as follows. For information, each example follows the method of FIG. 1.

Example 1

The substrate 1 is produced in step 101 by stereolithography from SOMOS® Prototherm 12120 epoxy-based resin without fillers, which is commercially available and has an unchanging formulation, ensuring reproducibility for the skilled person. The surface roughness of the substrate as measured by atomic force microscopy was between 3 nm and 7 nm, RMS (root mean square) value. After an atmospheric plasma surface activation, in step 102 the substrate 1 was drop-coated in the commercially-available azide-based polymeric adhesion promoter Azigrip4 (Susos®) (specifically product reference HVE256-3-1) which constitutes the primer 3, and the solvent is allowed to evaporate during a period of 30 minutes. This results in a N atom of the primer 3 inserting into a C—C or C—H bond of the polymer of the substrate 1.

In step 103, the substrate 1 was drop-coated in a 5 mg/mL aqueous solution of polyacrylic acid of molecular weight MW=3'000'000 Daltons, which constitutes the chelating polymer 5. The water was allowed to evaporate in an oven set at 40° C. for 2 h. The substrate 1 was then placed under UVC exposure in step 103a to ensure covalent grafting of the polymer 5 onto the substrate 1. The non-grafted polymer 5 was removed by sonication in water.

In step 104, the substrate 1 was immersed in a 5 mM solution of tetraaminepalladium (II) chloride monohydrate in water for 5 minutes. The substrate 1 was rinsed in water then placed in a 1M solution of sodium hypophosphite for 2 minutes in order to reduce the Pd²⁺ ions to Pd metal, which constitutes the plating catalyst 7. The substrate 1 was again rinsed in water, with sonication, so as to remove excess Pd metal and residual Pd²⁺ ions (if present).

In step 105, the coated substrate 1 was placed in an electroless copper bath from Dow (Circuposit™ 3350) at a temperature of 45° C. for 10 minutes in order to form the metallic plating 9. This particular bath is a copper chloride bath containing EDTA as a stabiliser, formaldehyde as a reducing agent, organic stabilisers and a high pH of 12-14.

After plating, the adhesion between the copper plating 9 and the substrate 1 was tested using the normalised test ASTM standard D3359-09 and improved from grade 0B when plated without the presence of the grafted polymer 5 to grade 5B when plated with the grafted polymer 5.

Example 2

In step 101, the substrate 1 was produced by stereolithography, again from SOMOS Prototherm 12120 resin. After an atmospheric plasma surface activation, in step 102 the substrate 1 was dip-coated the commercial azide-based adhesion promoter Azigrip4 (Susos) at 100 mm/minutes and the solvent was allowed to evaporate for 5 minutes. Again, the Azigrip4 constitutes the primer 3.

In step 103 the substrate 1 was similarly dip-coated in a 5 mg/mL solution of polyacrylic acid of molecular weight MW=3'000'000 Daltons in water, the polyacrylic acid constituting the chelating polymer 5. The water was then allowed to evaporate in an oven set at 40° C. for 2 h.

In step 103a the substrate was placed under UVC exposure to ensure covalent grafting of the polymer 5 onto the substrate 1. The non-grafted polymer was then removed by sonication in water.

In step 104, the coated substrate 1 was immersed in a 3 mM solution of palladium (II) chloride and 0.1M solution of NaCl in water for 5 minutes. The substrate 1 was then rinsed in water then placed in a 1M solution of sodium hypophosphite for 2 minutes to reduce the Pd²⁺ ions to Pd metal, which constitutes the plating catalyst 7. The substrate 1 was then rinsed in water, with sonication so as to remove excess Pd metal and residual Pd²⁺ ions (if present).

In step 105, the substrate 1 was placed in an electroless copper bath from Dow (Circuposit™ 3350) at a temperature of 45° C. for 10 minutes to form the metallic plating 9.

After plating, the adhesion of the metal plating 9 to the substrate 1 was tested using the normalized test ASTM standard D3359-09 and the test results improved from grade 0B when plated without the presence of the grafted polymer 5 to grade 5B when plated with the grafted polymer 5.

Example 3

In step 101, a commercial extruded polyethylene sheet of 1 mm thickness and dimensions of 25 mm by 75 mm was provided as substrate 1. After an atmospheric plasma surface activation, in step 102 the substrate 1 was dip-coated in the commercial adhesion promoter Azigrip4 (Susos) at 100 mm/minute and the solvent was allowed to evaporate for 5 minutes. Again, the Azigrip 4 constitutes the primer compound 3.

In step 103, the substrate 1 was dip-coated in a 5 mg/mL solution of polyacrylic acid of molecular weight 3'000'000 Daltons in water, the polyacrylic acid constituting the chelating polymer 5. The water was then allowed to evaporate in an oven set at 40° C. for 2 h.

In step 103a, the substrate was placed under UVC exposure to ensure covalent grafting of the polymer 5 onto the substrate. The non-grafted polymer was then removed by sonication in water.

In step 104, the substrate 1 was placed in a 5 mM solution of tetraaminepalladium (II) chloride monohydrate in water for 5 minutes. The substrate was rinsed in water and was then placed in a 1M solution of sodium hypophosphite for 2 minutes to reduce the Pd²⁺ ions to Pd metal and thereby to form the plating catalyst 7. The substrate was then rinsed in water, with sonication, so as to remove excess Pd metal and residual Pd²⁺ ions (if present).

In step 105, the substrate was placed in an electroless copper bath from Dow (Circuposit™ 3350) at a temperature of 45° C. for 10 minutes, so as to form the metallic plating 9.

After plating, the adhesion of the copper plating 9 was tested using the normalized test ASTM standard D3359-09 and was measured to be grade 4B with the grafted polymer.

Example 4

In step 101, the substrate was produced by stereolithography, again from SOMOS Prototherm 12120 resin. After an atmospheric plasma surface activation, the substrate was drop-coated in step 102 in the commercial adhesion promoter Azigrip4 (Susos) as before, and the solvent was allowed to evaporate for 30 minutes in air.

In step 103, the substrate was drop-coated in 25% solution of polyethyleneimine (PEI) of molecular weight 750'000 Daltons in water, the PEI constituting the chelating polymer 5. The water was allowed to evaporate in an oven set at 40° C. for 2 h.

In step 103a, the substrate was placed under UVC exposure to ensure covalent grafting of the polymer 5 onto the substrate. The non-grafted polymer was removed by sonication in water.

In step 104, the substrate was placed in a 5 mM solution of tetraaminepalladium (II) chloride monohydrate in water for 5 minutes. The substrate 1 was rinsed in water and then placed in a 1M solution of sodium hypophosphite for 2 minutes in order to reduce the Pd²⁺ ions to Pd metal to form the plating catalyst 7. The substrate was then rinsed in water, with sonication, so as to remove excess Pd metal and residual Pd²⁺ ions (if present).

In step 105, the substrate was placed in an electroless copper bath from Dow (Circuposit™ 3350) at a temperature of 45° C. for 10 minutes in order to deposit the metallic plating 9.

After plating, the adhesion between the plating layer 9 and the substrate 1 was tested using the normalized test ASTM standard D3359-09 and measured to be grade 5B with the grafted polymer.

As can be seen from the foregoing, the invention achieves the desired results of providing a metallic plating 9 on a polymer-containing substrate 1 with excellent adhesion. Furthermore, this is achieved using commercially-available products, and without resorting to complex processes or complex, bespoke chemistry. The invention is thus simple and economic to carry out on standard, existing equipment.

Although the invention has been described in terms of specific embodiments, variations thereto are possible without departing from its scope as defined in the appended claims. It is particularly noted that, although the tests were carried out forming electroless copper platings, nickel, palladium and other metallic platings such as platinum, silver, cupronickel, Ni—B and Ni—P are also possible using the same principles. Furthermore, electroplating may be carried out as an alternative to electroless plating.

Claims

1.-15. (canceled)

16. A method of forming a metallic plating on a substrate, comprising the steps of:

providing a substrate comprising a hydrocarbon-based polymer containing C—C and either or both of C—H and N—H bonds;

covalently bonding an azide-containing primer compound to said substrate by C—H and/or N—H insertion, said primer compound comprising molecules each having at plurality of C—H and/or C—N insertion sites;

in the absence of in-situ polymerisation, covalently bonding a pre-synthesised chelating polymer to said primer compound by C—H and/or N—H insertion, said chelating polymer being capable of forming ligand bonds with metal atoms or ions;

dispersing a plating catalyst in said pre-synthesised polymer;

forming said metallic plating on said pre-synthesised polymer by means of electroless plating or electroplating.

17. The method according to claim 16, wherein said azide-containing primer compound contains at least two, preferably at least ten, further preferably at least forty azide groups situated on side chains of a core polymeric chain.

18. The method according to claim 16, wherein said catalyst comprises metal ions and/or metal nanoparticles.

19. The method according to claim 16, wherein said catalyst is formed from metal ions which are subsequently reduced to metal by a reducing agent.

20. The method according to claim 18, wherein said ions are palladium ions, platinum ions, nickel ions, silver ions or copper ions.

21. The method according to claim 19, wherein said ions are palladium ions, platinum ions, nickel ions, silver ions or copper ions.

22. The method according to claim 16, wherein said primer compound and said pre-synthesised polymer are applied to said substrate simultaneously.

23. The method according to claim 16, wherein said pre-synthesised polymer is deposited on said substrate and is subjected to UV radiation or heat so as to cause the pre-synthesised polymer to bond covalently to said primer compound.

24. The method according to claim 16, wherein said metallic plating is applied by means of electroless plating or electroplating.

25. The method according to claim 24, wherein said electroless plating or electroplating is carried out in a pH range of 1-3 or 12-14.

26. The method according to claim 16, wherein said polymer has a molecular weight of at least 100,000 Daltons.

27. The method according to claim 26, wherein said polymer has a molecular weight of at least 500,000 Daltons.

28. The method according to claim 27, wherein said polymer has a molecular weight of at least 750,000 Daltons.

29. The method according to claim 28, wherein said polymer has a molecular weight of at least 1,000,000 Daltons.

30. The method according to claim 16, wherein said method does not include any in-situ polymerisation step.

31. The method according to claim 16, wherein said primer does not contain one or more of the following:

a triazine ring;

an alkylsilane group;

a silanol group;

a maleimide group;

a complex compound based on aluminium or titanium;

dextran.

32. The method according to claim 16, wherein said primer does not contain any of the following:

a triazine ring;

an alkylsilane group;

a silanol group;

a maleimide group;

a complex compound based on aluminium or titanium;

dextran.

33. A product comprising: wherein said metallic plating is formed on said substrate by the method of claim 16.

a substrate comprising a hydrocarbon-based polymer containing C—C and C—H or N—H bonds;

a metallic plating provided on a surface of said substrate;

34. A product according to claim 33, wherein said metallic plating is a composite between its constituent metal and said polymer in at least a part of its thickness.

35. A product according to claim 33, wherein said metallic plating comprises at least one of copper, nickel, silver, nickel phosphorous, nickel boron, cupronickel, platinum or palladium.

36. A product according to claim 33, wherein said metallic plating forms an antenna.