METHOD FOR PROVIDING A CONDUCTIVE SURFACE ON A NON-CONDUCTIVE POLYMERIC SURFACE

Description

FIELD OF INVENTION

The disclosure relates to a method for providing a conductive surface on a non-conductive surface, in particular a polymeric surface. In particular the method relates to attaching silver ions to a polymeric surface to facilitate the adhesion of a metallic layer to the polymeric surface.

BACKGROUND

Modern engineered polymers are applied in various industries throughout the world and electroplating is a common approach to finish polymer parts for chemical and wear protection, improved surface hardness, electrical conductivity, EMI shielding or decorative appearance. However, polymeric substrates are non-conductive and cannot be electroplated directly, thus pre-treatments are required prior to electroplating.

The conventional approach to metalize a non-conductive polymer substrate comprises four main steps, including etching, catalysation, electroless plating and finishing with electroplated metal of choice.

Typically etching creates a rough hydrophilic surface comprising micro/nano pores. This step not only prepares the polymer substrates for subsequent wet processes, but also essential to create pores which promote coating adhesion using mechanical interlock. The widely used etchant contains chromium trioxide (a hexavalent chromium species) and sulphuric acid. However, hexavalent chromium has numerous health and environmental concerns and requires complicated treatments prior to disposal. Therefore, a non-chromic etchant is preferred and there are numerous patents protecting the formulation and processes for non-chromic etches. Permanganate is a strong oxidizing agent and non-toxic, thus is an ideal alternative to chromic acid etchant. However, the acidic environment, required to ensure the solution oxidizing ability and etching performance, readily decomposes permanganate. This instability of permanganate reduces its performance and hinders wide application. U.S. Pat. No. 9,657,226 discloses an acidic permanganate etch together with approaches to stabilise the etch. Despite many improvements, all non-chromic acid etches in the art, still significantly rely on the creation of a densely porous polymer surface to develop a mechanical interlock between the metallic surface and the polymeric substrate.

Following etching, the polymer substrate is catalysed for subsequent electroless plating. Once again, the art, contains numerous catalysation processes and methods, where typically palladium, is adopted as a catalyst. Palladium is expensive and conventional processes frequently adopt the three sub-steps of sensitization, catalysation and activation. Even though the catalysing process was improved to a two-step process by using colloidal palladium, rinsing is still required between steps and the unstable catalysation bath requires careful maintenance, making the process less sustainable. Silver (Ag) is a well-known, less expensive, alternative to palladium (Pd), but the existing art conventionally requires a Sn sensitization step prior to Ag activation. There is mention in the art of adding a silver ionic catalyst to the etching step, such as proposed in EP09013498B1, however this process still requires a subsequent chemical step to activate the catalyst.

WO2018208177A1 proposes a method to directly introduce silver nanoparticles into polymers using the polymer chemical energy to reduce the silver ions to silver metal in the absence of an external reducing agent. This method primarily pertains to polymer precursors and does not provide a method to reactivate polymerised materials.

More recently, a new generation of polymer plating has been developed (disclosed in US20210238748A1). This process modifies the surface chemistry of the polymer by chemically attaching a further long chain polymer to the surface, the chemical attachment and dense vertically aligned long chains promotes coating adhesion through mechanical interlock without etching the polymer surface. The disclosed process still requires catalysation to support electroless metal deposition.

Currently the state of the art for plating on polymeric substrates are toxic, expensive, energy intensive, time consuming and typically specific to a narrow range of polymer substrates. Thus, there is a need in the art for a fast low temperature sustainable polymeric plating process to develop an adhesive metallic coating.

It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in New Zealand or in any other country.

SUMMARY OF THE INVENTION

In a first aspect, the present disclosure provides a conductive surface on a nonconductive substrate comprising the steps of:

• • a. providing a nonconductive substrate;
  • b. conditioning the nonconductive substrate with a hydrophilizing agent to render the substrate hydrophilic;
  • c. contacting the nonconductive substrate with an aqueous solution comprising a metal ion;
  • d. covalently bonding the metal ion to the conditioned nonconducting substrate by the application of energy to form an activated substrate surface comprising metal ion seeds; and
  • e. contacting the activated substrate surface with an electroless plating solution comprising a plating metal salt and a reducing agent, the reducing agent being suitable for reducing both the plating metal salt and the metal ion seeds on the activated substrate surface to thereby form a conductive surface on the non-conductive substrate.

In one example, the nonconductive substrate is a polymeric substrate.

In one example, the conditioned chemical bonds on the surface of the nonconducting substrate includes a percentage of carbon-carbon double bonds of between about 10 to about 50% of the carbon backbone of the substrate.

In one example, the hydrophilizing agent is a solution comprising one permanganate salt, one acid and one complexing agent sourced from phosphorus containing species, having a phosphorus containing anion group of between about 0.01 and about 1 mol/L.

In one example, the conditioning step with the hydrophilization agent is performed between about 5 and about 20 minutes.

In one example, the aqueous solution comprising a metal ion further includes a complexing agent, and optionally a surfactant.

In one example, the complexing agent may be an ammonium ion.

In one example, the surfactant, if present, may be polyvinylpyrrolidone (PVP).

In one example, the aqueous solution comprising a metal ion is maintained at a temperature of between about 10 and about 40 degrees Celsius.

In one example, the non-conducting substrate is contacted with the aqueous solution comprising a metal ion.

In one example, the contact time between the substrate and aqueous solution is between about 1 and about 10 minutes.

In one example, the aqueous solution comprising a metal ion is sprayed on the non-conducting substrate.

In one example, the aqueous solution comprising a metal ion comprises one or more of AgNO₃, CuSO₄, NiSO₄, or CuCl₂ in a concentration between about 0.005 and about 0.1 mol/L.

In one example, energy source is heat and the temperature is between about 60 and about 90 degrees Celsius.

In one example, the energy source is applied as a hot air stream or a hot air environment.

In one example, the energy source is applied for a contact time of between about 2 and about 10 minutes.

In one example, the metal is Ag, and the electroless plating solution comprises electroless nickel boron and the reducing agent amine borane.

In one example, the metal is Ag and the electroless plating solution comprises copper and the reducing agent is selected from formaldehyde or amine borane.

In another aspect, the present disclosure provides a conductive metal surface comprising a hydrophilic conditioned nonconductive substrate, activated metal seeds, an electroless metal deposited metal surface, and an electroplated metal surface.

In one example, the density of activated metal seeds on the nonconductive substrate is sufficient to produce a bonding strength of 10 N/cm or higher when tested according to ASTM B533.

The conductive metal surface comprising a hydrophilic conditioned nonconductive substrate, activated metal seeds, an electroless metal deposited metal surface, and an electroplated metal surface, produced according to any one of the aspects and examples disclosed herein.

According to the aspects illustrated herein, a method to metalize a chemically conditioned substrate is provided. In one step of the process the substrate is hydrophilized using metal-absorbing functional groups. The substrate may be selected from one or more long chained polymers containing double bonded carbon atoms.

In one step, the conditioning step may further include or comprise destressing a polymeric substrate. The destress process may include a heat-treatment step. Alternatively, the destressing step may include immersion in an organic solvent such as but not limited to acetic acid or acetone.

The conditioning step may comprise or include hydrophilizing the destressed substrate using a solution, the composition of which is selected based on the nature of the substrate. In one example the hydrophilizer may comprise one stable potassium permanganate salt, one acid and one phosphorous source containing phosphates or pyro phosphates. The conditioning step may operate at temperatures between about 25 and about 45 degrees Celsius.

In one example the conditioning step may include a manganese dioxide removal step using an oxalic and/or sulphuric acid solution by way of example.

In one embodiment the polymeric substrate is contacted for between about 1 and about 10 minutes using metal ions either by immersion or spraying the metal-containing solution comprising monovalent or divalent ions. In one example the catalyst ions may require an activation step.

In a preferred example the metal ion source is an aqueous silver nitrate solution. The functional group(s) introduced during the conditioning step enable the absorption of metal ions. Without wanting to be bound by any particular theory, the hydrogen atoms on the conditioned polymeric surface are likely exchanged with the metal ions during this step. In a later step, the influence of external energy activates the metal ions and strengthens the bond between the polymeric surface and the metal ions. After a period of time, preferably between about 1 and about 10 minutes, the polymer chains reorient the hydroxyl groups and couple the metal seeds to the polymer surface.

Alternatively, the catalyst bath may include a surfactant. Inclusion of surfactant enables the uniform seeding of the polymer surface with the metal ions.

A one or two-step activation process may be employed to promote the reorientation of the metal seeded hydroxyl groups on the polymer surface. The preferred example adopts a low velocity hot air flow. An alternate activation step involves a two-step process, which comprises low velocity air drying followed by heat treatment in an oven.

Commercially available electroless and electrolytic plating baths produce one or more layers of conductive metal coatings compatible with the metal seeds on the conditioned polymer surface. In one embodiment the metallized polymer is post processed to increase the adhesion between metal coating and the polymer substrate by means of covalent bonds. The post processing step comprises either heat treatment for a duration between about 1 to about 5 hours or aging in ambient conditions for between about 5 and about 10 days.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. In the present specification and claims, the word “comprising” and its derivatives including “comprises” and “comprise” include each of the stated integers but does not exclude the inclusion of one or more further integers.

It will be appreciated that reference herein to “preferred” or “preferably” is intended as exemplary only. The claims as filed and attached with this specification are hereby incorporated by reference into the text of the present description.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: is a flow diagram of the method to metallise nonconductive surfaces

FIG. 2: shows the results of ASTM D3359 cross hatch testing on various substrates.

FIG. 3: shows the results of a peel test performed according to ASTM B533 on a metallic surface deposited according to aspects of the invention

FIG. 4: shows the ASTM D3359 cross hatch testing on ABS treated with various activation enhancement treatments.

FIG. 5: is an XPS analysis of an activated ABS surface showing the chemical bonding information between the polymer and the metal seed

FIG. 6: is an SEM of conditioned ABS (left) and PPS (right) polymers according to aspects of the invention

FIG. 7: is an SEM image of a traditionally etched polymer substrate

FIG. 8 is an XPS analysis of an activate PPS surface.

FIG. 9: is a surface and cross-section image of a PPS-CF substrate showing metallic adhesion developed through the disclosed process.

Reference will now be made in detail to the present preferred examples of the disclosure, examples of which are illustrated in the accompanying drawings.

Definitions

In each instance herein, in descriptions and examples of the present invention, the terms “comprising”, “including”, etc., are to be read expansively, without limitation. Thus, unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like are to be construed in an inclusive sense as to opposed to an exclusive sense, that is to say in the sense of “including but not limited to”.

The term “about” or “approximately” usually means within 20%, more preferably within 10%, and most preferably still within 5% of a given value or range. Alternatively, the term “about” means within a log (i.e., an order of magnitude) preferably within a factor of two of a given value.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an example method for producing a metallic coating on a non-conductive polymer substrate.

At block 102, the method begins.

At block 104, the method conditions a polymer substrate. The conditioning step prepares, for example but without limitation, a polymer substrate surface for bonding to a metal ion. The conditioning step creates a hydrophilic surface on a polymer substrate and induces functional groups that easily absorb metal ions. The polymeric substrate may be either a single polymer or co-polymer or be reinforced with glass fibres, carbon fibres or other reinforcing materials known in the art. The preferred polymer chain contains easily oxidized functional groups, such as double bonded carbon as seen in hydrocarbons containing vinyl groups or benzyl rings and sulphides. Preferred structures are shown below. Polyesters and polymers that only contain C-C single bonds are less preferred. However, blends of those with preferred polymers may be selected. The polymer substrate may be selected from Acrylonitrile Butadiene Styrene (ABS), polyamide (PA), Polyphenylene Sulphide (PPS), phenolic resin or blends of these polymers.

The polymeric substrate may be produced by injection moulding, 3D printing, or other such methods that create a desirable finish product shape. The polymeric substrate may be solid or containing blind holes, open holes or channels or other features suitable for the final application. In one example, the conditioning step may comprise or include destressing a substrate. The destressing process is selected based on the polymeric substrate shape, its composition and processing conditions. The destress process may include heat treatment between 60 and 75 degrees Celsius for 0 to 2 hours preferably. The heat-treatment may be conducted in oven or in hot water. In one example, the destress process may include immersion in an organic solvent between 15 and 30 degrees Celsius for 0.5 to 30 minutes. An example of such organic solvent may be acetic acid. In another example an organic solvent may be 25% acetone.

In one example, the conditioning step may comprise hydrophilizing a polymeric substrate following destressing. The hydrophilizer bath composition is selected based on the polymeric substrate chemistry, since the hydrophilizer selectively breaks the polymer chains and attaches active functional groups to the polymer chains. These active functional groups enhance the hydrophilicity of the polymeric substrate. The hydrophilic groups possess an affinity for metal ions, such that metal ions are absorbed and bonded to the polymer chains. Hydrophilic functional groups may be selected from polar groups such as hydroxyl, sulfonic and carbonyl groups among others known in the art.

In one example, the polymeric substrate is ABS and the conditioning step includes a hydrophilization step and the hydrophilizer solution comprises at least one permanganate salt, one acid and one source of phosphorous containing species selected from phosphate and pyrophosphate. The permanganate salt may be selected from those well known in the art that dissolves in an aqueous solution, preferably potassium permanganate or sodium permanganate. The concentration of the permanganate salt is between 10 g/L and 200 g/L or the maximum solubility of the particular permanganate salts. The acid may be inorganic acid or organic acid. The inorganic acid may be sulphuric acid, nitric acid or phosphoric acid. The organic acid may be acetic acid. The concentration of acid is between 3.8 and 10 mol/L for a monobasic acid. The phosphorous containing species may be phosphate salt, hydrogen phosphate salt, dihydrogen phosphate salt or pyrophosphate salt. The concentration of phosphorous containing species is between 0.05 and 1 mol/L. The conditioning hydrophilizer may operate between 25 and 45 degrees Celsius and for 5 to 20 minutes.

Permanganate is a strong oxidizing agent, and its oxidizing power is further enhanced in an acidic environment or increases with decreasing pH of a solution. Compared with conventional chromic etch, permanganate is environmentally friendly and does not have health concerns, while maintaining sufficient oxidizing power. However, permanganate readily decomposes, especially in acidic solutions, to form divalent manganese ions and manganese dioxide. Divalent manganese ions further catalyse and therefore speed up the decomposition of permanganate. Complexing divalent manganese ions with stabilizing agents reduces their influence on permanganate anions, thus suppressing the decomposition of permanganate. In an oxidizing environment, inorganic species are preferred over organic species, as organic species may be destroyed by permanganate. In this case, phosphates and pyrophosphates are preferred complexing agents. In the presence of complexing agent, the lifetime of permanganate hydrophilizer could be extended.

In one example, the conditioning step includes a method to remove the residual manganese dioxide produced in the previous step. In one example, the substrate is immersed in a solution that comprises oxalic acid between 5 and 20 g/L and sulphuric acid between 5 and 15 percent by volume. The manganese dioxide removal step may be conducted by soaking or by sonication. In one example, the manganese dioxide removal solution operates between 30 and 80 degrees Celsius, preferably between 50 and 60 degrees Celsius, for between 0.5 to 10 minutes, preferably between 2 and 5 minutes. In one example, the removal step operates in sonication for 1 to 5 minutes, preferably 3 minutes.

At block 106, the method comprises contacting the conditioned polymer surface with a solution containing metal ions. In one example the substrate is immersed in a bath to contact the solution, in an alternative example the solution is sprayed on the substrate. In principle the metal ions can be any monovalent or divalent metal ions suitable for catalysing electroless reduction of a coating metal, such metal ions include Ag⁺, Ni⁺, Ni²⁺, Cu⁺, Cu²⁺, Co⁺, Co²⁺ amongst others. Monovalent ions are preferred over divalent ions as hydrophilic hydroxyl groups introduced on the polymer surface can reduce a monovalent ion. Divalent ions may require a further step of addition of a reduction chemical to complete the reduction. The reduction chemical will depend on the selected ion. The preferred monovalent ion is silver.

The solution containing the metal ion source is aqueous and the Ag ion source may be selected from an Ag salt that is water soluble. In a preferred example the Ag ion source is silver nitrate and the concentration in the bath is between 0.005 and 0.1 mol/L, preferably between 0.01 and 0.05 mol/L. The preferred concentration of the silver ion source is determined by the plating rate of the following electroless deposition step. In an alternative example the silver ion is complexed, and the complexing agent is ammonium ion, complexing the silver ion source helps prolong the life of the metal ion containing solution. In an alternative example, the silver ion is stabilized with Polyvinylpyrrolidone (PVP), to minimize the agglomeration of silver nanoparticles when activated.

While not wishing to be bound by theory, the inventors understand that polar functional groups, such as hydroxyl or carboxyl, are introduced onto polymer chains during the conditioning step. When contacted with a solution containing monovalent metal ions, the metal ions are absorbed on the hydrophilic polymer surface. In a later step, under the influence of an external energy source, hydrogen anions, from the functional groups exchange places with the monovalent metal ions, which are reduced. At other places on the polymer surface monovalent metal ions are reduced but not being attached to the surface, do not contribute to the coating adhesion. At the same time, water molecules attached to the polymer chain or trapped in the polymer matrix are driven off, so that the bonding strength between metal atoms and polymer improves. Over time, the polymer chains reorientate the outwardly pointing hydroxyl groups to minimize the surface energy, which tightly couples the attached metal particles to polymer surface

In a preferred example the metal ion bath is at room temperature, alternatively the catalyst ion bath can be at any temperature between 10 degrees Celsius and 40 degrees Celsius. The temperature being selected to maximize the lifetime of the metal ion solution and optimise the reaction between the metal ions and the hydrophilic groups on the substrate. Sufficient time for the metal ions to be absorbed onto the hydrophilic groups is between 1 to 10 minutes, preferable 5 minutes. Excessive contact times between the substrate and metal ion solution absorbs too much metal ion on the surface which may cause an excessive reaction rate in the subsequent electroless deposition step.

In one example, the substrate is ABS and metal ion bath comprises 2 g/L silver nitrate and the contacting process operates at room temperature for 10 minutes.

In an alternative example, the metal ion bath comprises 8 g/L silver nitrate and a liquid polymer/surfactant, such as, 40 g/L of Polyvinylpyrrolidone, to enable a more uniform seeding of the metal ions on the polymer surface.

In an alternative example, the metal ion bath comprises 0.3 mol/L cuprous chloride with 5.2 mol/L hydrochloric acid to increase the solubility of cuprous chloride.

In an alternative example, the catalyst metal bath comprises cupric ions between 0.1 and 0.2 mol/L. The cupric ion maybe sourced from copper sulphate, copper acetate or any water-soluble cupric salts known in the art.

At block 108, the method enhances the metal ion on the substrate. The enhancement step reduces and fixes the metal ions to a polymer substrate. While reduction of the metal ion may occur naturally over time the process may be accelerated by external energy source, such as heat. The enhancement process may comprise either one or two steps. In a preferred example the enhancement step adopts a low velocity hot air flow at temperatures between 60 and 90 degrees Celsius for 2 to 10 minutes, preferable a flow rate of 15 m/s and temperature of 80 degrees Celsius and 3 minutes. In an alternative example a two-step activation comprises drying in a low velocity compressed air flow at room temperature, followed by an oven heat treatment between 60 and 90 degrees Celsius for 5 to 10 minutes. In one example, the activated substrate is stored for 1 hour before next step to further integrate the reduced Ag nanoparticles into the polymer substrate.

At block 110, the method deposits a conductive surface on a polymer substrate from an electroless plating bath, that is compatible with metal catalysation seeds. The electroless plating bath may be selected from commercial nickel or copper plating baths to produce a nickel or copper conductive layer. In one example, the electroless plating bath comprises nickel sulphate hexahydrate 50 g/L, sodium pyrophosphate 100 g/L and dimethylamine borane 3 g/L and operates between 25 and 40 degrees Celsius and the polymer substrate is fully covered with a conductive layer in 30 minutes.

At block 112, the method electroplates the substrate with one or more layers of surface metal selected according to the required component functionality. The metal layer may comprise a single metal or metal alloy layer, or multiple metal or metal alloy layers. The metal coating maybe selected from nickel with various finish of matt to full bright, copper, tin, cobalt, and nickel-zinc alloy among others.

At block 114 the method post treats the metallised polymer surface to increase adhesion between the metal coating and the substrate. Post-treatments may comprise heat treatment between about 40 and about 70 degrees Celsius for about 1 to about 3 hours or aging at ambient conditions for about 1 week, or combination of these two steps. Post treatment further improve the coating adhesion by eliminating the moisture trapped in polymer substrate.

The adhesion between the polymer substrate and the metal surface is developed by covalent bonds between the metal atoms and the polymer surface. These covalent bonds are initially created by the hydroxyl or carboxyl groups introduced on the polymer surface by the conditioning step. The conditioning step introduces the new functional groups by breaking the bonds on one or more surface polymers and attaching functional groups to the bone sites. The metal ion is reduced by new functional groups as shown in the equation below.

In a subsequent step the initial bonds of the catalyst atoms to the substrate may be improved by the rearrangement of the polymer chains to better integrate the metal nano particles. This process is shown in the equation below. The process occurs naturally over time or may be accelerated by the application of heat. In one example the metal coated substrate is heat treated at a temperature between about 40 degrees Celsius and about 100 degrees Celsius, preferably about 60 degrees Celsius for a period between about 10 minutes and about 2 hours, preferably about 30 minutes. In an alternative example the metallized polymer substrate is aged naturally for a period between about 12 hours and about 72 hours, preferably about 24 hours, at room temperature.

EXAMPLES

The following examples point out specific operating conditions and illustrate the practice of the disclosure. However, these examples are not to be considered as limiting the scope of the disclosure. The examples are selected to specifically illustrate aspects of the polymer surface activation, attachment, to achieve adherent metallic coatings on the metal seed.

Example 1 Conditioning Processes

Conditioning depends on the polymer substrate, the examples provided here discuss the conditioning according to aspects of the invention for a variety of commercially important substrates.

Conditioning disrupts the polymer chains on the surface of the substrate and introduces hydrophilic functional groups onto the surface of the substrate. While conditioning may also etch the surface, the development of a very rough surface is not required for coating adhesion. The conditioning step comprise an optional destress step and a hydrophilization step. A MnO₂ removal step may be required if hydrophilizer solution is permanganate based. The effectiveness of a conditioning process can be measured by wettability test. A polymer surface is considered as sufficiently conditioned if its contact angle with water is less than 10°. Table 1 summarizes the conditions parameters for various polymer substrates.

TABLE 1
Conditioning parameters for various polymer substrates

Hydrophilization

MnO2 removal

Substrate	Destress	Composition	Temperature	Time	Composition	Process
ABS	Acetic	KMnO4	35° C.	10	Oxalic acid	Soak,
	acid	20 g/L +		minutes	10 g/L +	50° C.,
		H2SO4			H2SO4	3 minutes
		20 vol %			10 vol %
PPS	N/A	KMnO4	25° C.	30	Oxalic acid	Sonication,
		20 g/L +		minutes	10 g/L +	50° C.,
		H2SO4			H2SO4	3 minutes
		40 vol %			10 vol %
PETG	N/A	KMnO4	25° C.	30	Oxalic acid	Soak,
		20 g/L +		minutes	10 g/L +	50° C.,
		H2SO4			H2SO4	2 minutes
		40 vol %			10 vol %
PA	N/A	H2SO4	20° C.	20	N/A	N/A
		20 vol %		seconds
Paper	N/A	KMnO4	35° C.	10	Oxalic acid	Soak,
reinforced		20 g/L +		minutes	10 g/L +	50° C.,
phenolic		H2SO4			H2SO4	5 minutes
resin		20 vol %			10 vol %

Example 2: Metal Seed Implantation and First Metalisation

The selection of metal ions and the activation process depends on the functional groups generated on the polymer by the conditioning step and the selected electroless coating bath for the subsequent first metallisation step. The examples provided here discuss the metal seeds implantation according to aspects of the invention.

In one example, conditioned ABS coupon was activated with Ag and plated with electroless Ni bath. ABS coupon is firstly conditioned in a solution comprising KMnO₄ 20 g/L and H₂SO₄ 20% by volume. The conditioning step was carried out at about 35° C. for about 10 minutes. The residual MnO₂ produced from the conditioning step was then removed in a solution comprising 10 g/L oxalic acid and 10% sulphuric acid, operating at about 50° C. for about 2 minutes. The ABS coupon was then rinsed with deionized water and immersed in a solution containing 10 g/L of silver nitrate for 10 minutes under room temperature. Following immersion, the ABS coupon was treated in a hot air stream at about 70° C. till the surface dries, usually taking about 2 minutes. finally, the ABS coupon was immersed in an electroless Ni-B bath, comprising 50 g/L nickel sulphate hexahydrate, 100 g/L sodium pyrophosphate and 3 g/L of Dimethylamine Borane. Electroless Ni-B operates at about 30° C. for about 30 minutes. The ABS coupon was fully plated with a conductive Ni layer and ready for subsequent electroplating with the metal of choice.

In another example, conditioned ABS was activated with Cu and plated with electroless Cu bath. The ABS coupon was conditioned from the process detailed above. The ABS coupon was then immersed in a solution containing cuprous chloride and chloric acid for about 10 minutes. The ABS coupon was then enhanced in hot-air stream and plated with electroless Ni, as detailed above.

In another example, the PPS coupon was activated with Cu and plated with electroless Ni bath. The PPS coupon was firstly conditioned in a solution comprising 20 g/L KMnO₄ and 40% H₂SO₄. The conditioning operates at about 27° C. for about 30 min. The residual MnO₂ produced from the conditioning step was then removed in a solution comprising 10 g/L oxalic acid and 10% sulphuric acid, operating at about 50° C. with sonication for about 2 minutes. Following rinse with deionized water, the PPS coupon was then immersed in a solution containing cuprous ions. The cuprous solution was prepared by mixing ascorbic acid solution with copper sulphate solution and contains 0.125 mol/L Cu and 0.002 mol/L ascorbic acid. The PPS coupon was then rinsed with deionized water and plated with electroless Cu comprising 50 mmol/L EDTA, 50 mmol/L copper chloride, 0.1 mol/L boric acid and 0.1 mol/L of DMAB. Table 2 summarizes the activation parameters using various metal ion sources.

TABLE 2
Activation parameters for various metal ions

Activation

Seed metal

Energy

temper-

Composition	Temperature	Duration	source	ature	Duration
10 g/L	20° C.	10	Hot air	70° C.	3
AgNO3		minutes	stream		minutes
0.3 mol/L	20° C.	10	Hot air	70° C.	3
CuCl and 5.2		minutes	stream		minutes
mol/L HCl
0.2 mol/L	20° C.	10	Ascorbic	20° C.	1
CuSO4		minutes	acid		minutes

Example 3: Coating Process Performance

The objective of the disclosed methods was to create an adherent metallic coating on a polymer surface. The two commonly adopted measurements for coating adhesion are quantitative peel testing and qualitative cross hatch testing. Peel testing was performed according to ASTM B533 with results continuously recorded as newtons/cm during the peel and presented as peak, average and standard deviation for the test. Cross hatch testing was performed according to ASTM D3359 with qualitative results recorded as 0B to 5B with 5B being the best adhesion with 0% coating removal.

In the following examples the process to develop adherent metallic coatings on a number of commercially important polymer surfaces according to certain aspects of the invention is described.

In one example, ABS coupon was plated with electroless Ni according to the process detailed in Example 2 and then electroplated with bright Ni. The bright Ni bath comprised 270 g/L nickel sulphate, 60 g/L nickel chloride, 40 g/L boric acid and commercially available additives. Bright nickel was plated at about 4 A/dm² for about 20 minutes to create about 16 μm thick of nickel coating. Cross hatch test following ASTM D3359 demonstrated excellent coating adhesion, was classified as 5B (FIG. 201).

In one example, ABS coupon was plated with electroless Ni according to the process detailed in Example 2 and then electroplated with bright Cu. The bright Cu bath comprises 195 g/L copper sulphate, 75 g/L sulphuric acid and commercially available additives. Bright Cu plating operates at about 4 A/dm² for about 100 minutes to create about 44 μm of copper coating. The copper plated ABS coupon was conditioned at about 70° C. for about 1 hour prior to peel strength test following ASTM B533 (FIG. 301). The average peel strength of 19.2 N/cm demonstrated excellent coating adhesion (FIG. 302).

In another example, the PPS coupon reinforced with glass fibre (PPS-GF) was firstly conditioned in a solution comprising KMnO₄ 20 g/L and H₂SO₄ 40% by volume. The conditioning operated at about 27° C. for about 30 minutes. The residual MnO₂ produced from the conditioning step was then removed in a solution comprising 10 g/L oxalic acid and 10% sulphuric acid, operating at about 50° C. with sonication for about 2 minutes. Following rinsing with deionized water, the PPS-GF coupon was then rinsed with deionized water and immersed in a solution containing 2 g/L of silver nitrate for about 10 minutes under room temperature. Following immersion, the PPS-GF coupon was treated in a hot air stream at about 70° C. till the surface dried. Finally, the PPS-GF coupon was plated with electroless Ni and electrolytic Ni of about 10 μm. The Ni coating pass cross hatch test following ASTM D3359, was classified as 5B (FIG. 202).

In another example, the PETG coupon is firstly conditioned in a solution comprising KMnO₄ 20 g/L and H₂SO₄ 40% by volume. The conditioning operates at about 27° C. for about 1 hour. The residual MnO₂ produced from the conditioning step was then removed in a solution comprising 10 g/L oxalic acid and 10% sulphuric acid, operating at about 50° C. for about 2 minutes. The PETG coupon was then rinsed with deionized water and immersed in a solution containing 2 g/L of silver nitrate for about 10 minutes under room temperature. Following immersion, the PETG coupon was treated in a hot air stream at about 70° C. till the surface dried. Finally, the PPS coupon was plated with electroless Ni. The Ni coating pass cross hatch test following ASTM D3359, was classified as 5B (FIG. 203).

In another example, the PA6 coupon was firstly conditioned in a solution comprising 20% sulphuric acid, at about 20 degrees Celsius for about 20 seconds. Following rinsing with deionized water, the conditioned PA6 coupon was then immersed in a solution containing 2 g/L of silver nitrate for about 10 minutes at room temperature. The PA6 coupon was then activated in a hot air stream at about 70 degrees Celsius till the surface dried. Finally, the PA6 coupon was plated with electroless and electrolytic Ni of about 10 μm. The Ni coating adhesion was classified as 5B, following ASTM D3359.

In another example, the paper reinforced phenolic resin coupon was firstly conditioned in a solution comprising 20 g/L potassium permanganate and 20% sulphuric acid at about 35 degrees Celsius for about 15 minutes. The residual MnO₂ were removed in a solution comprising 10 g/L oxalic acid and 10% sulphuric acid, at about 50 degrees Celsius for about 2 minutes. Following rinsing in deionized water, the conditioned paper reinforced phenolic resin was immersed in a solution containing 2 g/L silver nitrate for about 5 min, at room temperature. The paper reinforced phenolic resin was then treated in hot air stream at about 70 degrees Celsius for about 2 minutes. The activated paper reinforced phenolic resin was finally plated with electroless and electrolytic Ni of about 10 μm. The Ni coating adhesion was classified as 5B, following ASTM D3359. Table 3 summarizes the coating adhesion results of the various polymeric substrates.

TABLE 3
Coating adhesion of various metallized polymer substrates

Activation Step

Adhesion

Composition

Enhancement

Metallization

Result

Substrate	Time	Temp/Time	Step	D3359	B533
ABS	10 g/L AgNO3,	70° C. air stream	Electroless Ni and	5B	N/A
	10 minutes	3 minutes	electrolytic Ni
ABS	10 g/L AgNO3,	70° C. air stream	Electroless Ni and	5B	19.2
	10 minutes	3 minutes	electrolytic Cu		N/cm
PPS-GF	2 g/L AgNO3	70° C. air stream	Electroless Ni and	5B	N/A
	5 minutes	3 minutes	electrolytic Ni
PETG	2 g/L AgNO3,	70° C. air stream	Electroless Ni	5B	N/A
	10 minutes	3 minutes
PA	2 g/L AgNO3,	70° C. air stream	Electroless Ni and	5B	N/A
	10 minutes	3 minutes	electrolytic Ni
PRP	2 g/L AgNO3	70° C. air stream	Electroless Ni and	5B	N/A
	5 minutes	3 minutes	electrolytic Ni

Example 4: Activation Performance

A method to activate a polymeric surface for electroless plating by an external heat source is described herein. The following examples demonstrate how the heat treatment develops coating adhesion.

The ABS coupons in this example were conditioned as detailed in Table 3 and immersed in a solution containing 10 g/L silver nitrate, at room temperature for about 10 minutes.

In one example, the ABS coupon was then left at ambient conditions until dry and then plated with electroless and electrolytic Ni. Cross hatch testing following ASTM reveals coating adhesion, classified as 1B (FIG. 401).

In one example, the ABS coupon was activated in a hot air stream at about 70 C and left for about 1 hour prior to electroless and electrolytic Ni plating. The coating had excellent adhesion, classified as 5B (FIG. 402).

In one example, excess Ag solution was blown off the ABS coupon using cold air, and the coupon was heat treated in an oven at about 60° C. for about 5 min prior to electroless and electrolytic Ni plating. The coating had excellent adhesion, classified as 5B (FIG. 403).

In one example, excess Ag solution was blown off the ABS coupon using cold air, and the coupon was heat treated in an oven at about 40° C. for about 5 min prior to electroless and electrolytic Ni plating. The coating exhibited good adhesion, classified as 4B (FIG. 404).

The examples above demonstrated that various forms of heat treatment significantly improve the adhesion of a metal coatings to ABS substrates. XPS analysis only detected COO-Ag bond (FIG. 5, 504, FIG. 5 505 and FIG. 5. 506) on the ABS surfaces activated using heat treatment. ABS surfaces conditioned according to aspects of the disclosure only exhibit widely dispersed relatively deep pores (603), depicted in the SEM image (FIG. 601). Comparatively a polymeric surface etched following the existing art exhibits a dense deep porous structure FIG. 7, 701, required to develop a mechanical interlock (J. S. Seo, et al., Surfaces and Interfaces, Volume 26, 2021).

Therefore, it may be deduced that the chemical bond generated in the heat treatment step r dominate the coating adhesion mechanism.

Example 5: Metal Bonding to Sulphonic Functional Group on PPS Substrates

PPS is traditionally a very difficult substrate on which to develop adherent metallic coatings and frequently mechanical etching provides the only viable approach. The example demonstrated that hydrophilization in the permanganate, sulphuric acid-based solution effectively developed sulphonic functional groups to which silver was covalently bonded after heat activation.

FIG. 8, 801 shows the aromatic polymer signature corresponding to the benzyl ring (803) of the hydrophilized substrate. FIG. 8, 802 shows that the metal seeds are bound to the PPS polymeric surface by covalent bonds between Ag and (—C₆H₄S—) functional group (804).

FIG. 8, 805 provides concentration data of the metal seeds on the polymeric surface and at a depth of 15 nm from the surface. The increased concentration of metal seeds at the sub-surface demonstrates that the enhancement process described in the invention, i.e., reorientation of polar groups and associated movement of metal seeds into the polymeric sub-surface.

The foregoing description is by way of example only and may be varied considerably without departing from the scope of the present disclosure. For example, a wide variety of hydrophilization baths and processes will produce identical polar groups on polymeric surfaces, many monovalent and divalent metallic ion species will effectively bond to active polar groups; and a variety of external energy sources will serve to activate the bond between the polar groups and the metallic ion species.

Example 6: Adhesion Test on PPS-CF Substrates

As stated in Example 5, PPS-CF (carbon fiber) substrates are inherently difficult surfaces on which to develop adherent metallic coatings. The previous example demonstrated that hydrophilization in permanganate, sulphuric-acid based solution effectively developed sulphonic functional groups to which silver was covalently bonded after heat activation. Here we demonstrate adherent plating on such an activated surface.

The activated PPS-CF coupon was then immersed in an electroless Ni plating bath. The resultant coating (FIG. 9) was about 9 μm thick (901) and exhibited excellent adhesion (902). The image, 901, demonstrates that the Ni coating is adherent not only to the PPS matrix but also to the carbon fiber fillers in the composite.

The cross-hatch test was performed according to ASTM D3359 with qualitative results recorded as 0B to 5B with 5B being the best adhesion with 0% coating removal.

The features described with respect to one example may be applied to other examples or combined with or interchanged with the features of other examples, as appropriate, without departing from the scope of the present disclosure.

The present disclosure in a preferred form provides the advantages of greater adhesion, more energy efficiency with shorter processing times; and provides an inherently safer process due to lower processing temperatures and elimination of toxic chemicals than are often associated with methods in the prior art.

Other examples of the disclosure will be apparent to those skilled in the art from consideration of the specification and practice of forms of the examples disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the disclosure being indicated by the following claims.