The present invention relates to a copper or copper alloy plated wire, a method for manufacturing a copper or copper alloy plated wire and to a motor vehicle tyre comprising the copper or copper alloy plated wire.

Tyre bead wire is typically manufactured by a immersing the steel wire in an acidic pickling solution to remove oxides from the steel wire surface, pre- drawing the steel wire, patenting the steel wire and finish drawing the steel wire to further reduce the cross-section on the steel wire. The finish drawn wire is then immersed in an alkaline solution to remove residual lubricant resulting from the drawing process before the wire is immersed in an acidic pickling solution to remove oxides from the steel wire surface and to activate the surface for subsequent deposition. The pickled wire is then immersed in a copper, brass or bronze plating solution before the plated steel wire is transferred to an oven for drying.

Such wires are used to reinforce pneumatic tyres such as those found on motor vehicles, e.g. cars, buses and motor cycles. The plated wire is embedded in a rubber compound and when cured, copper metal from the plating layer chemically reacts with sulphur in the rubber to form copper sulphide, which anchors the rubber compound to the plated steel wire.

If the plated steel wire is exposed to air and is left untreated, oxides of copper form on the surface of the copper, brass or bronze plating layer, for instance, cuprous oxide (Cu ₂ 0) and cupric oxide (CuO). However, due to the presence of cupric oxide at the plating layer surface, a reduction in adhesion strength is obtained between the plated steel wire and rubber compounds from the tyre.

It is an object of the present invention to minimise the formation of cupric oxide at surface of a copper, brass or bronze plated wire.

It is an object of the present invention to increase the adhesion between a tyre bead wire and rubber compounds such as those found in pneumatic tyres for motor vehicles.

The inventors found that when the copper or copper alloy plated wire is cured, copper atoms diffuse to the surface of the plating layer and that a proportion of those copper atoms are oxidised to form Cu ⁺ ions. In accordance with reaction (I) below, the Cu ⁺ ions then react with sulphur atoms present in the rubber compound to form a non-stoichiometric dendritic network (Cu _x S) that adheres the rubber compound to the plated wire. S + 2e ^" —► S ^2" + Cu ⁺ —► Cu _x S (I)

It is now understood that the formation of the Cu _x S dendritic network is largely dependent on the availability of Cu ⁺ ions and copper metal at the plating layer surface. It was therefore proposed to increase the concentration of Cu ⁺ ions at the plating layer surface in order to (i) promote the formation of the dendritic network and (ii) increase the adhesion strength between the rubber and the plated wire.

It was found that the concentration of Cu ⁺ ions at the copper or copper alloy plating layer surface could be increased by controlling the content of cuprous oxide in the oxide layer formed on the surface of the plating layer. In this respect the inventors found that an oxide layer comprising a high content of an electron deficient p-type oxide such as cuprous oxide increased the concentration of Cu ⁺ ions at the plated wire surface since a greater proportion of copper metal is oxidised to Cu ⁺ as it diffuses through the oxide layer and into to the rubber compound of the pneumatic tyre. In addition, by increasing the content of cuprous oxide in the oxide layer, the formation of an electron rich n-type oxide such as cupric oxide is suppressed. This has the further advantage of avoiding or at least reducing the undesirable reduction of Cu ⁺ ions to copper metal, which would reduce the concentration of Cu ⁺ ions at the plated wire surface and consequently cause a reduction in adhesion strength between the plated wire and the rubber.

It was found that very good adhesion between the plated wire and rubber compounds in pneumatic tyres could be obtained when the cuprous oxide content was at least 70%.

Accordingly, a first aspect of the invention relates to a plated wire for reinforcing motor vehicle tyres, which comprises a wire, a copper or copper alloy plating layer on the wire and an oxide layer formed on the surface of the copper or copper alloy plating layer, wherein the oxide layer comprises at least 70% cuprous oxide (Cu ₂ 0).

In a preferred embodiment the oxide layer comprises 75-85 % Cu ₂ 0. It was found that further improvements in adhesion strength between the plated wire and the rubber compound of the pneumatic tyre could be obtained when the oxide layer comprised 75-85 % Cu ₂ 0.

In a preferred embodiment the oxide layer comprises less than 5 wt%

CuO. When the oxide layer comprised less than 5 wt% of CuO, a high concentration of Cu ⁺ ions at the copper alloy plating layer surface was obtained. This has been attributed to a significant proportion of the Cu ⁺ ions that were formed during the diffusion of copper metal through the oxide layer, not being reduced back to copper metal by CuO. When the oxide layer contained more than 5 % of CuO, the concentration of Cu ⁺ ions reduced to an extent that reduced adhesion between the plated wire and the rubber compound was observed.

In a preferred embodiment the oxide layer comprises less than 3 % CuO, which led to a further increase in the concentration of Cu ⁺ ions at the copper or copper alloy plating layer surface and consequently, further improvements in adhesion strength between the plated wire and the rubber compound.

Preferably, the CuO content is kept as low as possible, e.g. a content of less than 1 % CuO.

In a preferred embodiment the copper alloy plating layer comprises tin, preferably less than 3 wt% tin. Copper alloys comprising tin are referred to as tin bronze or more generally as bronze. In an embodiment the copper alloy plating layer is a tin bronze plating layer. A tin bronze layer is generally known as a bronze plating layer. Plating layers comprising copper and tin are particularly preferred since the presence of tin improves the corrosion protective properties of the plated wire relative wires provided with a copper plating layer. Bronze plating layers are particularly preferred since the presence of tin improves the corrosion protective properties of the plated wire relative wires provided with a copper plating layer. By limiting the tin content to less than 3 wt%, the oxide layer will predominantly comprise oxides of copper, particularly Cu ₂ 0, and a only a small amount of tin oxide. Due to the relatively high content of Cu ₂ 0 in the oxide layer, improvements in adhesion can be obtained together with an improvement in the corrosion protective properties of the plated wire.

In a preferred embodiment the copper alloy plating layer comprises zinc. In a preferred embodiment the wire comprises steel, preferably carbon steel. Further, it is preferable that the composition of the steel wire comprises in wt % : 0.70 - 0.85 % C, 0.20 - 0.30% Si, 0.50 - 0.60% Mn, 0.008 - 0.015% S and 0.008 - 0.015% P, the remainder being iron and unavoidable impurities. The steel wire may additionally comprise trace amounts of Cu, Cr and Ni, either individually or in combination. Steel wires having a composition falling within the above compositional range exhibit very good mechanical properties that enable them to be used as tyre bead wire. Moreover, very good adhesion exists between the steel wire and the copper or copper alloy plating layer, which extends the working lifetime of the plated wire.

In a preferred embodiment the wire has a diameter of 0.96 - 1.83mm, making the plated wire suitable for a variety of applications. For instance, the plated wire may be used in truck tyres when the wire has a diameter of approximately 1.60 mm. A second aspect of the invention relates to a method for manufacturing a plated wire, which comprises the steps of:

a) providing a copper or copper alloy plated wire;

b) treating the copper or copper alloy plated wire with an alkaline solution; c) removing the copper or copper alloy plated wire from the alkaline solution, and

d) subjecting the copper or copper alloy plated wire to a heat treatment to preferentially form Cu ₂ 0 at the surface of the copper alloy plating layer. The inventors found that during the manufacture of the plated wire, the content of Cu ₂ 0 and CuO in the oxide layer that is formed on the plating layer could be controlled by subjecting the plated wire to a chemical treatment and to a thermal treatment. This is in contrast to conventional manufacturing processes where the plated wire is subjected to a heat treatment only. The effect of subjecting the plated wire to a chemical treatment and to a heat treatment is that the content of Cu ₂ 0 in the oxide later could be increased to at least 70% and the content of CuO in the oxide layer could be kept to a minimum, i.e. the oxide layer comprised only trace amounts of CuO.

In a preferred embodiment the alkaline solution comprises NaOH. Oxide layers comprising a high content of Cu ₂ 0, i.e. at least 70% Cu ₂ 0, and a low content of CuO could be obtained when the alkaline solution comprised NaOH. Alternatively, the alkaline solution may comprise KOH, an ammonium based salt or a buffered solution such as Tri buffer.

By treating the copper or copper alloy plated wire with an alkaline solution, OH ^" ions are adsorbed onto the surface of the copper or copper alloy plating layer. In accordance with the half cell oxidation reaction (II), the adsorbed OH ^" ions and copper react to form Cu ₂ 0.

2 Cu-OH —► Cu ₂ 0 + H ₂ 0 (II) However, in order for the reaction to occur spontaneously, the reaction needs to couple with a reduction reaction or an additional source of energy needs to be provided. In this instance, the heat treatment that occurs at the end of the plated wire manufacturing process can be used as the additional energy source. Making use of the heat treatment at the end of the plated wire manufacturing process also has the advantage that any Cu ₂ 0 that is formed is annealed to the surface, which increases the coherence and ductility of the oxide layer and further increases the adhesion between the oxide layer and the copper or copper alloy. In a preferred embodiment the pH of the alkaline solution is between pH 8 and pH 13. The inventors found that a reduction in the amount of Cu ₂ 0 present in the oxide layer was observed when the pH of the alkaline solution was kept below pH 8. In addition, an increase in the amount of CuO present in the oxide layer increased when the pH of the alkaline solution exceeded pH 13.

In a preferred embodiment the pH of the alkaline solution is between pH

10 and pH 13. It was found that the Cu ₂ 0 content in the oxide layer could be increased by increasing the pH of the alkaline solution to at least pH 10, and thereafter subjecting the plated wire to the heat treatment.

In a preferred embodiment the pH of the alkaline solution is between pH

11 and pH 13. The Cu ₂ 0 content in the oxide layer could be increased still further by increasing the pH of the alkaline solution to at least pH 11 and then subjecting the plated wire to the heat treatment.

In a preferred embodiment the copper or copper alloy plated wire is subjected to a heat treatment between 100°C and 250°C. It is preferred not to subject the plated wire to a heat treatment below 100°C since the Cu ₂ 0 content in the oxide layer is too low to afford any significant increase in Cu ⁺ ion generation. It is also preferred not to subject the plated wire to a heat treatment above 250°C since this may increase the content of CuO in the oxide layer, which may reduce the availability of Cu ⁺ ions at the plated wire surface and consequently reduce the adhesion strength between the plated wire and the rubber compound of the tyre bead.

In a preferred embodiment the copper or copper alloy plated wire is subjected to a heat treatment between 100°C and 200°C. By subjecting the copper alloy plated wire to a chemical treatment and then a heat treatment between 100°C and 200°C, the oxide layer formed on the surface of the copper or copper alloy plating layer contained 73% - 85% Cu ₂ 0 and less than 1% CuO. Thus, when copper atoms diffuse through the oxide layer during curing, a large proportion of the copper atoms are oxidised by Cu ₂ 0 to form Cu ⁺ ions, which promotes the formation of the Cu _x S dendritic network. Since the oxide layer has a CuO content of less than 1%, only a small proportion of Cu ⁺ ions will be chemically reduced back to copper metal, which would inhibit dendrite formation.

In a preferred embodiment the copper or copper alloy plated wire is subjected to a heat treatment between 150°C and 200°C. The inventors found that the content of Cu ₂ 0 in the oxide layer could be increased by increasing the temperature of the heat treatment. With this effect in mind as well as a desire to minimise energy consumption during the manufacture of plated wire for tyre beads, it is preferred to subject the plated wire to a heat treatment of at least 150 C since a good balance between Cu ₂ 0 content in the oxide layer and energy consumption is obtained.

A third aspect of the invention relates to a motor vehicle tyre comprising the plated wire according to the first aspect of the invention or the plated wire manufactured according to the second aspect of the invention. It was found that very good adhesion between the plated wire of the invention and rubber compounds of a motor vehicle tyre could be obtained. Accordingly, the plated wire according to the first aspect of the invention and the plated wire manufactured according to the second aspect of the invention are very suitable for use as a tyre bead wire in motor vehicle tyres.

In a preferred embodiment the motor vehicle tyre comprises a non- stoichiometric dendritic network between the plated wire and a rubber compound of the motor vehicle tyre.

In a preferred embodiment the rubber compound comprises natural rubber. Alternatively, the rubber compound comprises synthetic rubber. It was found that very good adhesion could be obtained irrespective of whether the rubber compound comprised natural rubber or synthetic rubber. Preferred synthetic rubber compounds comprise styrene-butadiene rubbers (SBR), nitrile- butadiene rubbers (NBR) and ethylene propylene diene monomer rubbers (EPDM).

In a preferred embodiment the plated wire of the invention is used for reinforcing rubber hoses, preferably high pressure rubber hoses. The plated wire is also suitable for use in conveyor belts.

The invention will be now be elucidated by way of example. These examples are intended to enable those skilled in the art to practice the invention and do not in anyway limit the scope of the invention as defined by the claims.

According to an example of the invention a steel wire was provided which comprised in wt% : 0.75 % C, 0.25 % Si, 0.53% Mn, 0.012 % S and 0.010 % P, trace amounts of Cu, Cr and Ni, the remainder being iron and unavoidable impurities. The steel wire having a cross-section of 0.96 - 1.83mm was subsequently immersed in an alkaline solution to remove residual lubricant resulting from the wire drawing process. Then the wire was immersed in an acidic pickling solution to remove oxides from the steel wire surface and to activate the surface for subsequent deposition of a copper alloy layer. The pickling solution comprised 60 gl ^"1 H ₂ S0 ₄ . The cleaned wire was then immersed in a plating solution, which contained H ₂ S0 ₄ (25 gl ^"1 ), CuS0 ₄ (35 gl ^"1 ) and SnS0 ₄ (0.1 gl ^"1 ), and was thereafter treated with a NaOH (pH 8 - pH 13) solution for 1 second before the plated steel wire was subjected to a heat treatment. The copper alloy plated steel wire obtained thusly is copper-tin alloy plated steel wire, e.g. bronze plated steel wire.

Experiments were carried out to investigate the effect of temperature and solution pH on the composition of the oxide layer that is formed on the surface of the bronze plated steel wire. The experimental conditions are shown in Table 1. X-ray photoelectron spectroscopy (XPS) was used to identify and quantify the amount of Cu, Cu ₂ 0 and CuO in the oxide layer. XPS spectra were recorded on a Thermo VG Scientific XPS with an Alpha 110 hemispherical analyser and a dual anode (aluminium) source. The measured spot size was 5mm in diameter for large area analysis. Lenses within the analyser were also set for large area analysis. The high voltage was 12.00 kV and the beam current was 6.67 mA to give a power of 80 W. Survey scans were collected between 1100 to 0 eV at intervals of 0.5 eV at a pass energy of 50 eV. 10 repeat scans were collected and averaged with a dwell time per point of 20 mS. High resolution scans were collected over a binding energy range appropriate to the peaks of interest at intervals of 0.05 eV, at a pass energy of 20 eV, and again 10 repeat scans were collected and averaged with a dwell time per point of 20 mS. The results of the experiments are shown in Table 1.

Using comparative example CI as a reference (100°C heat treatment only), an increase in Cu ₂ 0 content of approximately 1% was observed when the plated wire was treated with an alkaline solution (pH 8) and then subjected to a 100°C heat treatment (El). However, as the pH was increased to pH 10 (E3), the Cu ₂ 0 content in the oxide layer drops to 68.2%, representing an increase of only 0.5 % relative to comparative example CI. When the plated wire was treated with an alkaline solution (pH 12) and then heated to 100°C (E7), the Cu ₂ 0 content in the oxide layer was 73%, an increase of approximately 4 % relative to CI .

Using comparative example C2 as a reference (200°C heat treatment only), an increase in Cu ₂ 0 content of approximately 6% was observed when the plated wire was treated with a pH 8 alkaline solution and heated at 200°C (E2). As the pH of the alkaline solution was increased to pH 10 (E4), the Cu ₂ 0 content in the oxide layer increased to 78 %, representing an increase of nearly 10% relative to C2. It was found that the content of Cu ₂ 0 in the oxide layer could be increased to 85% by treating the plated wire with an alkaline solution having a pH of 11 or 12 and then subjecting the plated wire to a heat treatment of 200°C (E5 and E8). Relative to C2, this represents an increase in Cu ₂ 0 content in the oxide layer of 16.5 %. Similarly, a Cu ₂ 0 content of 84 % was obtained when the pH was of the alkaline solution was increased to pH 13 (E10). It was also found that if the temperature of the heat treatment was too high, e.g. 300°C , then the oxide layer comprised an undesirable amount of CuO (examples 6, 9 and 11), which would be detrimental to the formation of the dendritic network and the adhesion between the plated wire and the rubber compound of the tyre bead.

The above results show that the formation of Cu ₂ 0 or CuO depends on (i) the pH of the alkaline solution and (ii) the temperature of the heat treatment the copper or copper alloy plated wire is subjected to. Further, it is understood that if the temperature of the heat treatment is too high, e.g. 300°C, then CuO is formed in preference to Cu ₂ 0. Similarly, it is understood that CuO is preferentially formed if the pH of the alkaline solution is too high, e.g. > pH 13.

Table 1

At% of copper species

Experimental

Example

conditions Cu

Cu ₂ 0 CuO metal

No solution treatment, 100°C,

CI 32.2 67.8 traces

Air

No solution treatment, 200°C,

C2 27.9 68.5 3.6 Air

El pH = 8 (Tris), 100°C 31.3 68.7 traces

E2 pH = 8 (Tris), 200°C 25.3 74.7 traces

E3 pH = 10 (NaOH), 100°C 32 68.2 traces

E4 pH = 10 (NaOH), 200°C 22 78 traces

E5 pH = 11 (NaOH), 200°C 15 85 traces

E6 pH = 11 (NaOH), 300°C traces 69 31

E7 pH = 12 (NaOH), 100°C 27 73 traces

E8 pH = 12 (NaOH), 200°C 15 85 traces

E9 pH = 12 (NaOH), 300°C traces 79 21

E10 pH = 13 (NaOH), 200°C 16 84 traces

El l pH = 13 (NaOH), 300°C traces 68 32