IMPROVEMENTS TO METAL WIRES USED FOR REINFORCING ELASTOMERIC MATERIAL

Abstract: The invention concerns a metal wire, used to produce reinforcing elements for articles of elastomeric material, which is provided with a coating able to ensure at the same time a good resistance to corrosion, good values of initial adhesion and a low decay, due to ageing, of the bond with the elastomeric material, said coating being formed by two concentric layers of a Zn/Ni alloy, where the radially inner layer contains Zn in an amount ranging between 60% and 90% on the layer total and the radially outer layer contains Ni in an amount ranging between 60% and 80% on the layer total.

Description: The present invention relates to the metal wires provided with an anti-corrosion coating able to ensure a very good adhesion with elastomeric materials and, in particular, it refers to the fine steel wires used to obtain the well known cords used as reinforcement in articles of elastomeric material, as for instance pneumatic tyres, conveyor belts, driving belts and the like. It is known that these wires must possess contemporaneously characteristcs of good resistance to corrosion and good adhesion to the elastomeric material, qualities which a naked steel wire does not possess in any degree. As a result, the wire must be covered with a suitable coating. After having checked the results obtainable with single metal coatings, scientists some time ago adopted for the above mentioned wires a brass coating, namely a copper and zinc (Cu/Zn) alloy, containing indicatively 70% copper and 30% zinc, which in the past gave in general good results as regards adhesion but showed insufficient protection characteristics in respect of corrosion. In fact, during the vulcanization process, the copper in the alloy reacts with the sulphur present in the compound, originating a thin layer of copper sulphide which forms an interface between the compound and the wire, providing a good initial adhesion of the compound to the brass coating surrounding said wire. However, with ageing, the rubber/metal bonding, at its strongest immediately after the vulcanization cycle, decays considerably; in fact, not only there is a deterioration in the characteristics of the layer of copper sulphide, but also in those of the underlying brass layer, owing to the continual action of corrosion. Efforts to improve this situation have led to the adoption of alloy other than brass, for example ternary alloys, as disclosed in DE 2 856 325, or to the substitution either of the copper or of the zinc with alternative metals. Of all the alloys tested, those composed of zinc and nickel or of zinc and cobalt have proved to be of particular interest. In fact, substituting copper with nickel or cobalt reduces degradation of adhesion through ageing; however, the problem of increasing the maximum level of adhesion and at the same time the resistance to corrosion remains unsolved. In terms of alloys composition, these two characteristics are in fact contradictory: increasing the percentage of a metal (for example Ni) improves one characteristic (in this case, adhesion to the elastomeric material), but causes deterioration of the other, so that research can only lead to a better compromise between the different requirements. Chemical Abstracts, vol. 99, No. 12, September 1983, page 493, Abstract No. 95821z discloses depositing two subsequent layers of Zn/Ni alloys over sheet steel, in case with machined surfaces, wherein the Ni percentage is lower than or equal to 20% in both layers, and the content difference between the layers is at most of 18%. The process aims to improve the adherence. The Applicant have now devised a new type of coating which overcomes the above mentioned difficulties and at the same time managed to improve both of the desired characteristics. Accordingly, the present invention consists of a metal wire for the reinforcement of articles of elastomeric material, provided with a corrosion-resistant coating and adapted to adhere to said elastomeric material, the coating being substantially composed of metal alloys of two metals, the first metal being zinc, the second being selected between nickel and cobalt, characterized by that said coating comprises two concentric layers, placed radially one above the other, the radially inner layer containing from 60% to 90% (wt) of zinc, and the radially outer layer containing from 60% to 80% (wt) of said second metal, the difference of the percentage of said second metal between the inner and the outer layer being no less than 20% (wt). As indicated, the radially innermost layer will contain Zn percentage ranging between 60% and 90%, whilst the difference between the percentages of Ni or Co concentration in the two superimposed layers will be at least 20%; obviously, the difference between this value and 100, in both layers, will be supplied by the other metal of the alloy. The present invention will now be explained in the detailed description given below with reference to preferred embodiments using Nickel without prejudice as for what concerns the substitution of nickel with cobalt. The metal wire forming the subject of the invention is the usual metal wire well known in rubber technology, in particular tyre technology. It is drawn to a diameter of btween 0.12 and 0.50 mm and specially used, in strands, to form the cords employed as reinforcing elements, particularly in rubberized fabrics. The above mentioned wire, whilst still at a diameter greater than that of its final use, is coated with a first protective layer, composed of a nickel and zinc alloy, which contains a proportion of zinc variable between 60% and 90%, whilst the remaining proportion is nickel; said layer, obviously, will contain other metals, but only in the form of impurities, namely in substantially irrevelant amounts, measured from very small percentages of concentrations. The deposition of the above mentioned alloy, effected in a galvanising bath using standard methods, well known to the scientists, can be carried out in two different ways. In the first method, zinc and nickel are deposited on the wire separately and one after the other; subsequently, an appropriate thermal treatment, i.e. heating of the wire, originates the reciprocal diffusion of the two metals, leading to the formation of the Zn/Ni alloy. The preferred percentages of the two metals in this first layer, whose function is to protect the wire from corrosion, is of the order of 70% Zn and 30% Ni. Alternatively, the two metals may be deposited in the desired percentages, contemporaneuously, using a single galvanising bath, in this way obtaining a wire already coated with the desired alloy. One possible operating method is to use a galvanising bath, the composition of which is given below, the bath being effected in the following conditions: The preferred thickness of this first layer,independently of the manner in which it is obtained,is between 0.25 and 2 µm. Thus treated, the wire is then coated with a second layer, also in Zn/Ni alloy, but with the percentages of metals substantially the inverse of those of the first layer, and more precisely having a nickel content of between 60% and 80%, the remaining percentage on the total of 100% being made up with zinc, not considering the presence of the already mentioned impurities. In any case the difference between the quantity of nickel contained in the two radially superimposed layers (in %) is not less than 20%. Obviously, as regards the second layer, the deposit of the two metals cannot be effected in two separate and subsequent phases as indicated above, but must be carried out in a single phase, i.e. by co-deposition of the two metals; in the contrary case, the thermal treatment necessary for the creation of the aloy by the diffusion of the two metals would affect also the radially inner layer, already deposited, originating the diffusion of all metals forming the coating and the consequent creation of a substantially homogeneous single layer, going against the teaching of the present invention. For deposition in a single phase, one possible operating method is to use a bath of the composition given below, the bath being effected in the following conditions: The preferred thickness of this second layer is between 0.25 and 2 µm; moreover, the thicknesses of the two layers are combined together, so that the final thickness of the coating is preferably ranging between 0.50 and 4 µm. It is evident that the thickness of each layer of the coating and that of the coating in its whole is that found on removal from the galvanising bath, i.e. prior to any operation which may alter said value, in particular prior to any drawing. After the two layers of coating have been deposited in the manner and with the characteristics desired, the diameter of the wire is brought to its final value by being drawn one or more times. In this way a wire is made, provided with two radially superimposed coaxial layers in Zn/Ni alloy, each having a different function, that of the radially innermost layer being resistance to corrosion and that of the radially outermost layer being improved adhesion to the elastomeric material. The wire has demonstrated a higher quality level than any other known wire, in terms of both the required characteristics. These superior qualities of the wire according to the invention in comparison with the already known wires are clearly brought into evidence by the tests carried out by the Applicant, which are described in detail herebelow. EXAMPLE I A wire obtained as described above by two successive co-depositions of Zn and Ni from galvanising baths and by a further drawing to bring it to the final diameter of 0.175 mm, provided with a coating (coating 1) formed by a radially inner layer, 0.1 µm in thickness, constituted by 70% of Zn and 30% of Ni, and by a radially external layer, 0.1 µm in thickness, constituted by 30% of Zn and 70% of Ni, was suspended in a climatic chamber at 45°C and at 90% relative humidity (R.H). The commonly used wire, having the same diameter, provided with a coating (coating 2) constitued by a layer, 0.2 µm in thickness, of an alloy containing 35% Zn and 65% Cu, obtained according to the conventional techniques, was suspended in the same environment. The tensile strenghts (T.S.) of the wires was measured after different times of stay in these conditions, intended to facilitate a quick corrosion with formation of rust. The per cent decay of the tensile strenghts (T.S.%) can be considered as a measure of the entity of the corrosion process. Said percent decay was calculated by the formula: TS% = (initial TS - final TS)/ initial TS x 100 The following results were obtained: DAYS OF STAY IN THE CLIMATIC CHAMBER TS% coating 1 coating 2 15 1% 38% 30 10% 65% From these results, it is evident that in the wires coated according to the invention the resistance to corrosion is considerably higher. EXAMPLE II The same test described in Example I was carried out on wires having a diameter of 0.25 mm and provided with a coating, 0,25 µm in thickness, which - in the wire produced according to the present invention - was formed by two concentric layers of 0.125 µm each. The following results were obtained: DAYS OF STAY IN THE CLIMATIC CHAMBER TS% Coating 1 Coating 2 15 0% 17% 30 5% 38% Also in this case a high resistance to corrosion is shown by the wires obtained according to the invention. EXAMPLE III This test concerned the adhesion of 7x4x0.175 cords, obtained with the wires described in Example I, to rubber compounds after vulcanization and the persistance of the adhesion value after ageing at high temperature and in the presence of relvant values of relative humidity (R.H.) The compounds tested had the following composition (in parts on 100 rubber parts): The adhesion was tested by the method of measuring the force neccessary to extract the cord from a small cylinder of vulcanized rubber which is described in Kautschuk und Gummi Kunststoffe, Vol. 5, 1969, pages 228-232. The specimens were cured for 30′ at 151°C: the pulling force was measured by an electronic dynamometer. The thickness of the rubber layer covering the cord extracted from the specimen, evaluated by comparison with standard specimens and expressed by a "coating index", indicated the percentage of the cord surface which was still well rubberized. When the value of the "coating index" was high, the detachment of the cord from the compound occurred by insufficient cohesion of the latter and not by insufficient adhesion of the rubber/metal interface. The specimens were aged in a climatic chamber at 65°C and at 90% relative humidity. The obtained results are reported in the following table, where PF indicates the value of the pulling force (measured in Newton) and CI the value of the coating index. The results bring unquestionaly into evidence the improvement in the adhesion persistence under ageing ascertained in the cords built up with the wires coated in accordance with the invention. EXAMPLE IV The tests were carried out as described in Example III, but on 2+2x0.25 cords built up with the wires described in Example II. The obtained results, expressed with the same indications as in the preceding Table, are reported below: Also in connection with this cord the results demonstrate the considerable improvements in the resistance of ageing of the bond when use is made of the wires coated with the double layer of Ni/Zn alloy in accordance with the teaching of the present invention. The Applicant deem that the very good results obtained may depend on the reasons indicated herebelow which anyhow are not to be regarded as a limitation of any kind or scope in respect of the present invention. First of all, the Zn/Ni alloy (in a wide field of variable reciprocal percentages) forms coatings much more compact than those obtainable with brass and non-porous; further, the process of dezincification is slowed significantly in comparison with Cu/Zu alloys. The nickel forms fine and compact layers of sulphide which adhere very well both to the rubber and to the underlying anti-corrosive layer containing a high Zn percentage; further, the alloys according to the invention are very ductile, thus offering greater advantages as regards wire drawability. Moreover, the Zn/Ni alloy tends to be self-levelling, having as a result highly smooth surfaces, even when deposited on uneven surfaces. At last the Applicant would like to emphasize that a good behaviour, in part with substantially analogous results, can be obtained substituting the zinc with aluminium, in this case, however, encountering difficulties in the electro-deposition of this metal; as regards nickel, the Applicant have already pointed out that it can be replaced by cobalt so that, in the present specification, any reference to nickel represents an equivalent reference to cobalt; consequently, the invention is not restricted to the use of the Zb/Ni alloy, but comprises the use of the Zn/Co alloy.

Claims: A metal wire for the reinforcement of articles of elastomeric material, provided with a corrosion-resistant coating and adapted to adhere to said elastomeric material, the coating being substantially composed of metal alloys of two metals, the first metal being zinc, the second being selected between nickel and cobalt, characterized by that said coating comprises two concentric layers, placed radially one above the other, the radially inner layer containing from 60% to 90% (wt) of zinc, and the radially outer layer containing from 60% to 80% (wt) of said second metal, the difference of the percentage of said second metal between the inner and the outer layer being no less than 20% (wt). A metal wire as claimed in claim 1, characterized by the fact that the total thickness of the coating is not greater than 4 µm. A metal wire as claimed in claim 1, characterized by the fact that the thickness of the radially inner layer is comprised between 0.25 and 2 µm. A metal wire as claimed in claim 1, characterized by the fact that the thickness of the radially outer layer is comprised between 0.25 and 2 µm. A metal wire as claimed in claim 1, characterized by the fact that said second metal is nickel and the radially inner layer is obtained by electro-deposition of Zn and Ni in two successive phases and by subsequent formation of the alloy by diffusion due to thermal treatment. A metal wire as claimed in claim 1, characterized by the fact that said second metal is nickel and the wire is coated by said two layers prior to being drawn to its final diameter.

Inventors: VAN OOIJ, WIM J.
Applicants: PIRELLI COORDINAMENTO PNEUMATICI SOCIETA PER AZIONI
Application year: 1988
Main classification: C25D5
IPC codes: C08J5, B29D30, C25D7, D07B1, B23K20, C25D5