The invention relates to a steel wire for the reinforcement of elastomers, in particular 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 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 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 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 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.

It is now an object of the present invention to avoid said drawbacks and to produce a wire with a controllable micro- roughening at its surface in view of maximal increase of

the adhesion capacity to elastomers. The microroughening effect according to the invention is substantially different from and much finer than the rather coarse roughness of a wire surface obtained eg. by wire drawing. In fact the micro- roughness is superposed on the base of and generally follows the pattern of the coarser irregularities of a conventional wire surface.

The surface conditioning treatment according to the invention offers further a fast, continuous and controllable process which avoids the drawbacks of deep etching and which produces said typical icrosurface roughening with the effect of a drastically improved adhesion performance to rubbers. So far the mechanism of the improved adhesion behaviour is not fully understood. According to the invention the bare circular steel wire (i.e. from which an adhesion promoting coating layer has been removed), eg. a bead wire with a diameter d between 0,8 mm and 2 mm, (particularly between about 0,89 and about 1,60 mm) will offer a brightness factor (L-value as defined below) meeting the relation 37 < L.d-/ ³ < 55 and preferably 38 < L.d ¹ / ³ < 50.

The L-value is determined and defined by means of the Chroma- meter II-Reflectance (Minolta).' The L-value is determined 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. How¬ ever to avoid influences of coating color (shades of yellow, red or brown) on the L-value, applicant has choosen to mea¬ sure only on bare wires (after removal of the metal coating as described further). Possible color effects are thus iso¬ lated from natural brightness (or lack of brightness). Refe¬ rence L-values are first determined of a standard white sub¬ strate accompanying the Chromameter in view of calibration

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. Surface roughened bare bead wires with a diameter d of 0,96 mm were found to offer, after applying a copper or bronze coating of about 0,5 to 2 g of coating metal per vr of wire, a favourable adhesion behaviour - when their L-value was situated between about 40 and 50 for the bare wires (i.e. after removal of the coating with a persulfate solution as indicated in the example below.). For a similarly roughened (and coated) bead wire with a diameter d of 1,55 mm, L-values between 33 and 45 on the bare wires showed a considerably improved adhesion level. The specific adhesion values are given in the examples below. For the purpose of measuring the brightness L, a series of wires are arranged side by side against each other on a plat¬ form just below the measuring tip of the measuring head. The measuring area at said tip is a circle with a diameter of 8 mm.

Wires with a rectangular cross section, treated according to the invention and similarly examined in a side by side arrangement under the measuring head of the Chromameter II generally offer a larger L-value for a comparative micro-surface roughening with the result of a similar adhe- sion capacity increase. This can be explained by the fact that the light reflection pattern (as input for the regis¬ tered L-value) of said rectangular wires is less scattered than for circular wires

The invention provides also some processing possibilities to attain the specific micro-roughening effect on the wire sur¬ face in a controllable way.

In general, the roughening effect can be produced by any suitably controlled continuous etching process during

pickling such as an electrolytic pickling in a sulphate-ion containing bath before the 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 versus that on conventionally pickled wires. This means that during the etching process, Fe from the wire surface is preferentially dissolved versus 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 bright¬ ness. 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 eg., 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 appea- ranee. 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 and

providing evidence of the improved adhesion behaviour and per¬ formance.

A number of conventionally drawn circular bead wires (steel wires with a carbon content of at least 0,6 %) with a diame¬ ter d of 0,96 mm (set A) resp. 1,55 mm (set B) were conti¬ nuously passed through a specially designed pickling installa¬ tion in view of producing a micro-roughening effect at their surfaces. The wires were subsequently conventionally electro- less plated in a CuSO^bath with a copper coating (coating weight about 0,3 to 0,7 g Cu per kg of wire). Both the uncoated and the coated wires showed a dull and dark shaded appearance in comparison to non roughened wires. The L-value of the thus treated bare wires was determined as described above.

The wire-sets treated according to the invention were embed¬ ded in a number of rubber compounds and vulcanised. Their adhesion capacity was further determined according to a con- ventional pull out test and compared with the adhesion beha¬ viour of conventional bead wires of the same diameter and com¬ position and without having been subjected to the micro-surface roughening treatment according to the inven¬ tion.

The electrolytic pickling line for set A comprised, down¬ stream a conventional HC1-pickling bath (about 50'C and with about 200 g/1 HC1), four consecutive cells wherein the wire was alternatively polarised as anode respectively as cathode. The electrolyte was a solution of 200 g Na ₂ S0 ₄ per 1. at 40 ^* C. The treatment time at a current density of 100 A/dm ² of the wire in the electrolytic pickling line was about 2 sec. After rinsing and electroless plating with copper, the wires were properly embedded and vulcanised in different rubber compounds (Cp 1 to Cp 5) and the adhesion value (N)

was determined. The results are given in table 1 stating also the results for the corresponding L-value for a number of said pickled wires (sets A and B) from which the copper coating was removed and according to the invention (indicated "Inv."). Each time the comparison with a conventional (non roughened) bead wire is given (reference samples identified as "Ref") after similar removal of its copper coating.

The removal of the Cu-coating is carried out in a conven- tional way with an ammonium persulphate-solution whereafter the bare wire is rinsed and dried. Removal of the metal coating will of course always be carried out with agents which do not dissolve or attack the steel wire itself.

Table 1

It is clear from table 1 that the adhesion performance (N) increases substantially when comparing for the same rubber compound the Reference wire (Ref) with the Invention wire (Inv). The registered improvement is on the average over 30 %.

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.