The invention relates to a steel wire with a substantially k rectangular cross-section for the reinforcement of elastomers

5 and in particular to a bead wire for reinforcing a vehicle tire bead.

Bead wire with a round, rectangular or square cross-section is known. In general bead wires have a thin metal coating

10 such as copper, bronze or brass on their surface to promote the adhesion to the elastomers which they have to reinforce. During the manufacture of bead wire, the last step is general¬ ly the coating step with the thin copper or brass layer. This is different from the manufacturing process for brass coated

15 wire for tire cord used to reinforce the belt and/or carcass portions of a tire. Wire for tire cord is generally drawn after the coating is applied to the wire surface. In this way the coated wire obtains a rather smooth surface. This is not always true for bead wire which, even after coating with a

20 thin layer of copper or brass may retain a rather rough sur¬ face. It is believed that surface roughness in some instances may promote the anchoring of the surrounding rubber to the bead wire surface. In this way efforts have been made in the past to provide an extra rough surface by deep etching a

25 steel wire for rubber reinforcement before coating it with copper. This etching process however is a slow process and the deep etching generally adversely affects a durable bond of the copper to ferrous oxyde or carbide grains which become exposed at the wire surface due to said deep etching.

30

European patent application No. 89201852 of applicant (to be published under No.351.909) deals in particular with a round steel wire having a micro roughened surface and a metal

coating layer for promoting the adhesion to elastomers. The round wire is characterized by the fact that its bare steel surface (i.e. after removal of the coating layer) has a brightness range meeting the relation 37 < Ld ¹ ' < 55 for w.i e diameters d between 0,8 and 2 mm.

According to one aspect of the present invention there is pro¬ vided a substantially rectangular steel wire for reinforce¬ ment of elastomers, having a micro-roughened surface to which has been applied a metal coating layer whereby to increase the adhesion capacity to elastomers, wherein the micro-roughening is such that upon removal of the coating layer the flat surface section of the wire would have a brightness factor L (as herein defined) in the range of 40 to ⁵⁵ -

According to another aspect of the present invention there is provided a process to improve the adhesion capacity of a sub¬ stantially rectangular steel wire to elastomers comprising continuously and controllably micro-roughening the surface of said steel wire by etching the wire surface during a pickling treatment and before application of an adhesion promoting metal coating layer to said surface, wherein the micro-roughening is such that upon removal of the coating layer the flat surface section of the wire would have a brightness factor L (as herein defined) in the range of 40 to 55.

The term "substantially" rectangular is intended to include either a rectangular or square cross-section or such a sec¬ tion with rounded off corners. Likewise the term "substan¬ tially rectangular cross-section" is also intended to cover wire cross-sections as shown e.g. in the following publica-

tions : FR 2.354.152, GB 2.119.323 (figures 4 and 5), EP

0288985 (figures 1 to 5), DE 2.410.750 (figures 3 to 5),DE

2.810.848 (figure 4), DE 1.918.224 (figures 4 and 9 to 17), FR 2.390.300 (figure 5), FR 2.184.664 (figure 1) and variants thereof.

The L-value as here defined is determined by means of the Chro ameter II-Reflectance (Minolta). The L-value is deter¬ mined on the bare steel wire, i.e. either before a copper, bronze or brass coating is applied to the wire surface or after said coating has been removed from a coated wire. The L-value could in principle also be measured on the coated wire. However to avoid influences of coating color (shades of yellow, red or brown) on the L-value, applicant has choosen to measure only on bare wires (after removal of the metal coating as described further). Possible colour effects are thus isolated from natural brightness (or lack of bright¬ ness). Reference L-values are first determined of a standard white substrate accompanying the Chromameter in view of cali- bration of said measuring instrument. With the standard CIE illuminant C (6774 K) the reference L-value used by applicant is 97,30 on the white plate. For the purpose of measuring the brightness L, a series of wires are arranged side by side against each other on a platform just below the measuring tip of the measuring head. The measuring area at said tip is a circle with a diameter of 8 mm.

In the embodiments of the invention, using such a method of testing it has been found that rectangular and surface roughened bare bead wires with a width of 3 mm and height of 1.5 mm were found to offer, after applying a copper, brass or bronze coating of about 0,5 to 2 g of coating metal per m ² of wire, a favourable adhesion behaviour when the L-value for their flat faces was situated between about 40 and 55 for the

bare wires (i.e. after removal of the coating with a persul- fate solution) .

There are also disclosed herein some processing possibilities to attain the specific micro-roughening effect on the wire surface in a controlled way.

In general, the roughening effect can be produced by any suitably controlled continuous etching process during pick- ling such as an electrolytic pickling in a sulphate-ion containing bath before plating with Cu (from a CuSO^-bath), brass, or bronze having a tin content of between about 0,5 and 16 % weight. An alternative continuous process relates to the addition of a sufficient amount of extra and sufficiently oxydising agents in a conventional pickling bath (eg. HC1) before the wire is plated with copper, brass or bronze. Said oxydising agents should of course be able to oxydise iron and will not form a metal deposit on the wire surface. A further suitable method relates to electrolytic pickling in HC1 , eg. with alternate current. Still another advantageous method relates to the addition of some oxidising agents to the plating bath. By these treatments, the oxygen and carbon content at the wire surfaces are generally increased as compared to on conventionally pickled wires. This means that during the etching process, Fe from the wire surface is preferentially dissolved as opposed to the iron carbides (and possibly iron oxydes) which may generally be present at the wire surfaces. This dissolution creates micro-cavities and pores into the wire surface whereby it loses its brightness. This results in a decrease of the L-value as described above. As a result of this microroughening on the bare wire a lower L-value (brightness) is also observed on the coated wire, i.e. after applying a conventional adhesion promoting metal coating layer such as e.g. copper than for a wire with a

similar coating but non pre-etched (roughened) according to the invention. The thin coating will generally follow the uneveness (micro-cavities) of the wire surface which will result in a rather dark and dull shaded wire surface appearance. The metal coating layer may be copper or bronze with eg. 0,5 to 3 % weight Sn or brass with a copper content between 60 and 72 % weight.

An embodiment of the invention will now be described by way of example in view of clarifying some details thereof.

In tests a number of rectangular steel wires with a thickness of 1,5 mm and a width of 3 mm were continuously passed through a pickling installation (HC1) in view of producing a micro-roughening effect at their surfaces. A typical oxy- dizing agent for the ferrous ions (Fe ^2"1" ) was added. After this etching treatment the wires were rinsed and dried. The wires were subsequently conventionally plated with brass. Both the uncoated and the coated wires showed a dull and dark shaded appearance in comparison to non roughened wires. The brightness L-value of the thus treated bare wires was deter¬ mined as described above.

Samples of the rectangular wires were compared in three situa- tions. In a first situation the L-values were determined on the bare wire surfaces (before application of a brass coating). These wires were not micro-roughened (etched) according to the invention and L ranged from 61 to 63. In a second situation the wires were again not etched according to the invention, were coated with brass and the brass was then removed by dissolution. Here L-values ranged from 61 to 67. Finally, in a third situation the wires were etched according to the invention (HC1 pickling plus oxidation) and plated with brass. The conventional brass layer was then removed (in

an ammonium persulphate solution) whereafter the bare wire was rinsed and dried. The L-value for the five samples ranged in this case from 37 to 55. The average was L = 47.4.

The rectangular wire, treated in accordance with the present invention exhibits comparable increases in adhesion capacity to the round wires in EP 351.909 but the treatment is such as to give slightly higher L-values.

It is assumed that the micro roughened wire surface is compa¬ rable to a dispersion of copper coated microcavities and pores between oxyde and carbide grains (created by controlled etching). The copper will generally be better bonded in pores than on said grains. Upon vulcanisation of rubber to said sur- face (and after ageing) copper sulfide particles may be formed which are responsible for deterioration of the adhe¬ sion performance. Presumably however, these sulfide particles remain better anchored in the pores and cavities thereby refraining deterioration of the adhesion capacity.

When the steel wire according to the invention is a bead wire, then the invention of course relates also to a vehicle tire having its bead area reinforced with said wire. In prin¬ ciple any elastomeric body can be reinforced with steel wires according to the invention.

The surface conditioning treatment according to the invention and as described in said EP 89201852 offers a fast, con¬ tinuous and controllable process which avoids the drawbacks of deep etching and which produces said typical microsurface roughening with the effect of an improved adhesion perfor¬ mance to rubbers. So far the mechanism of the improved adhe¬ sion behaviour is not fully understood.

Thus at least in its preferred embodiments the present inven¬ tion provides a substantially rectangular wire with a control¬ lable microroughening at its surface which maximises its ability to adhere to elastomers.