Metal wire with two-layered coating for reinforcing articles made of elastomeric material and such reinforced articles

Abstract: The invention relates to a metal wire for reinforcing articles made of elastomeric material, especially vehicle tyres. The wire in question has a coating formed by two concentric layers of zinc-nickel or zinc-cobalt alloys, the radially more internal layer of which has a zinc content greater than 90% whilst the radially more external layer has a nickel or cobalt content greater than 50%.

Description: The present invention relates to metal wires with a double coating layer, in particular, steel wires for reinforcing articles made of elastomeric material, as well as to articles made of elastomeric material, especially tyres for vehicles, reinforced with the said wires. It is known that to protect metal wires incorporated in elastomeric material from corrosion and to obtain good adhesion of such wires to the elastomeric material the wires are coated with a metal alloy. An alloy which has been in use for a long time for coating metal wires for reinforcing articles made of elastomeric material is an alloy of copper and zinc and more specifically an alloy containing approximately 60 - 70% copper and approximately 40 - 30% zinc. As already known, metal wires coated with a layer of alloy as referred to above show deterioration in adhesion to elastomeric material over a period of time and are not satisfactorily protected against corrosion. To overcome both disadvantages, the use of other types of alloy for coating wires has been proposed. In particular, the use of ternary alloys of brass (Cu-Zn) with added nickel or cobalt or other metals as the third element of the alloy, for forming the coating layer of the wires has been proposed. With alloys of this type, some improvement is obtained in the problems of reducing deterioration in adhesion over a period of time to elastomeric material and protection of the wires against corrosion, but such improvement is not sufficient to achieve a significant improvement of quality level of articles made of elastomeric material reinforced in this way, especially tyres. A different solution which is satisfactory for both problems is that described in the European patent 283,738 held by the same Applicant. That solution consists in coating the metal wires with two concentric, superimposed layers, both made from alloys selected from a zinc-nickel alloy and a zinc-cobalt alloy, of which the radially more internal layer contains zinc in a quantity between 60% and 90% inclusive, whilst the radially more external layer contains nickel or cobalt in a quantity between 60% and 80% inclusive. The metal wires in accordance with the above-mentioned European patent whilst solving both the problem of achieving their protection against corrosion as well as that of adhesion to elastomeric material with acceptable deterioration over a period of time, still present certain problems for the purposes of their production on an industrial scale. Such unsatisfactory industrial production occurs during the drawing phase which wires undergo after formation of the coating and consists in excessively frequent breaking of the said wires during wire-drawing, as well as excessive wear on drawplates. In accordance with the present invention it has been found that it is possible to produce metal wires for reinforcing articles made of elastomeric materials and to produce articles made of elastomeric materials reinforced with the said wires which preserve the good properties of satisfactory adhesion over a period of time to elastomeric material and the resistance to corrosion of metal wires which has already been satisfactorily achieved in accordance with European patent 283.738, and that furthermore such wires do not have the disadvantages for wire-drawing presented by the latter, and this, surprisingly, by adjusting the quantity of nickel or cobalt in the radially internal layer. In its first aspect the present invention relates to a metal wire, for reinforcing articles made of elastomeric material, which is provided with a coating comprising two concentric layers arranged radially one on top of the other, said layers being made of an alloy selected from a zinc-cobalt alloy and a zinc-nickel alloy, characterised in that the alloy from which the radially inner layer of the coating is made has a zinc content from 95% to 98% and the alloy from which the radially outer layer of the coating is made has a nickel content greater than 50%. In a further aspect, the present invention relates to an article and a motor-vehicle tyre comprising reinforcement metal wires with the above mentioned construction. The present invention will be better understood from the following description, given by way of examples, of some forms of realization of the invention. The naked metal wire used in the present invention is an ordinary metal wire, in particular made of steel, well known in rubber technology and in particular in the technology of tyres. On the surface of this naked wire, a first layer of coating is formed from a metal alloy selected from a special zinc-cobalt alloy or zinc-plated cobalt alloy and a special zinc-nickel alloy. The special metal alloys of zinc-cobalt and zinc-nickel suitable for forming the first coating layer of a wire in accordance with the invention have the characteristic that their zinc content is at least 95%. If cobalt is present in the metal alloy used to form the first coating layer, it is considered that a reduction of the percentage of this element relative to the zinc will favour "drawability" of the wire. In accordance with the above, the cobalt content should preferably be less than 5% and even more preferably, not greater than 2%. Forming of the first coating layer of the wire can be achieved by any known method whatsoever and especially by one of the two methods described in European patent application no. 283.738. For example, with the method of simultaneous deposition of the two metals zinc and nickel by means of a galvanic bath, in which the quantities of the two metals for forming the first coating layer on a wire in accordance with the invention are as previously indicated, the composition of the said bath and the operating methods for the deposition are as follows: bath temperature between 10 and 30°C inclusive pH of the bath between 4 and 8 inclusive current density (in amperes/dm2) between 5 and 30 inclusive hexahydrate of nickel sulphate 170 g/1 zinc sulphate 10 g/l borax 10 g/l sal ammoniac 14 g/l lauryl sodium sulphate 1 g/l Still by way of example, if the first coating layer on a wire in accordance with the invention is a zinc-cobalt alloy in the special percentages stated above, the galvanic bath and the operative methods for the same are as follows: bath temperature 55°C pH of the bath 2 ± 0.1 current density (in amperes/dm2) between 60 and 80 inclusive heptahydrate of cobalt sulphate 120 g/l heptahydrate of zinc sulphate 620 g/1 sodium sulphate 75 g/l The preferred thickness for the first coating layer during the forming of the coating itself and before the drawing to which the wire with completed coating will be subjected is between 0.25 and 2 microns inclusive. Checking such thickness is carried out by means of quantitative analysis of the usual sort. Each thickness value stated in this text should be checked by such a method. On top of the first layer, a wire in accordance with the invention exhibits a second coating layer formed from an alloy selected from nickel-zinc and cobalt-zinc alloys having the characteristics that their nickel or cobalt content is greater than 50%; the nickel content should preferably be between 70% and 100% inclusive, or alternatively, the cobalt content should be between 70% and 90%. This second coating layer is also formed in the way indicated by European patent no. 283.738 by means of simultaneous deposition of the metal components in a galvanic bath. For example, a galvanic bath for forming the second layer of a wire in accordance with the invention, when the said second layer is an alloy of zinc and nickel in the preferred percentages stated above, the operating conditions of the galvanic bath will be as follows: bath temperature 60°C pH 2.5 ± 0.1 current density (amperes/dm2) between 25 and 35 hexahydrate of nickel sulphate 550 g/l zinc sulphate 20 g/l boric acid 35 g/l sodium sulphate 35 g/l Again as an example, if the second coating layer consists of an alloy of zinc and cobalt in the preferred percentages indicated above, the galvanic bath used will be as follows: bath temperature 55°C pH 2 ± 0.1 current density (amperes/dm 2 ) between 60 and 80 inclusive zinc sulphate 620 g/l cobalt sulphate 125 g/l sodium sulphate 75 g/l The preferred thickness for the second layer during forming of the coating and before the drawing to which the wire with completed coating will be subjected is between 0.25 and 2 microns inclusive. The total coating of the wire on leaving the coating bath will therefore be between 0.5 and 4 microns inclusive. The wire according to the invention, complete with two concentric, superimposed coating layers having the characteristics stated above has undergone the usual wire- drawing operations by passing it through the usual drawplates in order to reduce its overall external diameter to the customary values making it suitable for use as a reinforcement for articles made of elastomeric material. Especially for use in the field of vehicle tyres, the wire- drawing operations are carried out in successive passes until the initial diameter of the wire (variable between 0.70 and 3.70 mm) is reduced to values between 0.12 mm and 0.30 mm inclusive, enabling values for the first coating layer between 0.15 and 0.40 microns inclusive to be obtained and between 0.05 and 0.20 microns inclusive for the second coating layer, the total thickness of the coating finally being between 0.20 and 0.60 microns inclusive. Here is to be noted that, after the wire drawing steps, due to the very small thickness involved with the coating layers and due to the very strong mechanical stresses acting on the wire during said drawing steps, the radially outer layer may lose its continuity, assuming a particular typical blotchy effect which we may call "leopard skin" appearance. Stated in other words, the radially outer coating layer may appear to be a plurality of stains, or large spots partially embedded into the radially inner coating layer. Determination on the drawn wire of the existence of the two layers can suitably be carried out by X-ray diffraction techniques and/or by the Auger Electronic Spectroscopy (AES) method. The wires in accordance with the invention solve the problem described previously, as demonstrated by the experimental tests reported below. For these experimental tests, three special wires in accordance with the invention are used, indicated in this text as wires A, B, C, and a special, already known wire according to European patent 283.738, indicated in this text as wire N1. All the special wires mentioned above are made of C 70 steel (an ordinary steel for wires destined for use in the field of tyres, having a carbon content up to 0.70%) and have identical dimensions and structure; to be more precise, all the wires, before drawing, have an overall external diameter of 1.40 mm and are provided with a coating formed by two concentric, superimposed layers, each of which has a thickness not greater than 2 microns. The characteristics of the above-mentioned wires are indicated in Table 1: Wire Name A B C N1 First layer 95% Zn 98% Zn 98% Zn 70% Zn 5% Co 2% Co 2% Co 30% Co Second layer 80% Ni 80% Ni 90% Ni 80% Ni 20% Zn 20% Zn 10% Zn 20% Zn All the special wires described above have undergone wire- drawing to reduce them to a final diameter of 0.25 mm, to which for the coating layers mean thickness values of 0.35 microns for the internal layer and 0.15 microns for the external layer correspond. For this purpose, 19 drawplates made of type V 32 tungsten carbide were used, essentially already known, in a bath of a common lubricant (VSV 351/IL oil marketed by Zeller Gmelin), the results obtained being reported in Table 2. Table 2 gives in particular, mean percentage values of losses in weight of the coating caused by the said wire-drawing, mean percentage values in weight of drawn wire after which it is necessary to change the drawplates, and mean percentage values in weight of wire drawn for each wire breakage, values of 100 having been assumed for N1 wire. Wire Name A, B, C N1 Loss of coating in weight 20 - 30 % minimum of 40% Quantity of wire produced per drawplate 300 100 Quantity of wire produced for each wire breakage 400 100 The results of the experimental tests shown in Table 2 clearly demonstrate that with the wires in accordance with the invention, both the problem of reducing wear on the drawplates during wire-drawing carried out after completion of coating the said wires and that of reducing the frequency of breakages of wires during wire-drawing are solved. Further experimental tests have been carried out on small cords made from already known wires in accordance with European patent 283.738 as well as wires in accordance with the present invention in order to determine the values of adhesion to elastomeric material and deterioration in time of adhesion between the small cords and the elastomeric material. For these further experimental tests, a mix was used in accordance with the recipe given in Table 3. Components of the Mix Quantity in Weight - Natural rubber 100 - Zinc oxide 8 - 6PPD antiozonant 2 TMQ antioxidant 1 - Complex cobalt salt (Manobond 680C) 0.5 - HAF lampblack 50 - Accelerator - Santocure MCR 1 - Sulphur 6 For these latter tests, test pieces were prepared in accordance with ASTM D2229-80, which specifies vulcanization for a period of 30 minutes at 151 °C of small, prismatic blocks of elastomeric material in which small cords have been incorporated (made by combining 5 wires 0.25 mm in diameter) with approximately 12 mm gaps between each other and embedded 12.5 mm deep in the elastomeric material. The test according to which adhesion of rubber to metal is measured consists of determining the force necessary to draw out the individual cords from the prismatic block containing them and determining the degree of covering with elastomeric material of the cords extracted by force from the block. To measure the deterioration over a period of time of adhesion, the test is again the one previously described, but the withdrawing test on the cords is carried out after ageing treatment of the test piece consisting in storing it for a pre-fixed period of 4 days in an environment at 65°C with 90% relative humidity. With the mix and the small cords described above, the same number of test pieces were prepared as the number of small cords formed from the wires covered with the different types of coatings previously described. The experimental results obtained are reproduced in Table 4. Test pieces made of mix containing small cords A, B, C N1 Mean value of force effecting withdrawal of a small cord from a test piece not subjected to ageing 537 N 414 N Mean value of force effecting withdrawal of a small cord from a test piece subjected to ageing 450 N 350 N Index of cover of small cord extracted from test piece not subjected to ageing 4 4 Index of cover of small cord extracted from test piece subjected to ageing 3 2 In this text the numerical values of the cover indices have the following meaning: Cover index 4 signifies small cord completely covered with elastomeric material; Cover index 3 signifies small cord 2/3rd covered with elastomeric material; Cover index 2 signifies small cord 1/3rd covered with elastomeric material; Cover index 1 signifies small cord with no cover of elastomeric material at all Further experimental tests have been carried out with small cords made from wires in accordance with the invention, incorporated in vulcanized elastomeric material and with small cords made from N2 wires brass-plated with 67.5% copper and 32.5% zinc in order to ascertain the existing differences between the characteristics of adhesion of the cords to the elastomeric material and with regard to the presence of corrosion on the wires forming the cords. The small cords mentioned above are again small cords of 5 x 0.25 formation (i.e. consisting of a strand of 5 wires 0.25 mm in diameter) and differ from each other only in the different types of wires from which they are made; these cords will therefore be marked in this text with the same index as that used to mark the wires from which they are made. Size 165 R 13 tyres with P4 tread were prepared, using the mix described above to form breakers, as well as small cords A, B, C in accordance with the invention, and the small, brass-plated cord N2 indicated above. 20 cm wide sections were cut from the tyres and subjected to ageing treatment in a saline mist chamber. The treatment conditions to which the test pieces were subjected are as follows: - exposure times 20 and 40 days - saline solution 5% aqueous solution of sodium chloride - mist density 1.5 cc/h on an area 80 cm 2 - chamber internal temperature 45°C After ageing in this way, the sections were treated by removing the elastomeric material forming the tread in order to recover the layer of breaker structure containing the small cords (the subject of the investigation) and hence to note the adhesion characteristics of the small cords to the elastomeric material in which they were incorporated and the presence of corrosion in the wires forming the small cords. Similar data was recorded on sections not subjected to ageing in the saline mist chamber, i.e. taken from vulcanized tyres as such. Determination of adhesion values was carried out by tearing three small cords off the strips of breaker fabric, each cord separated from the adjacent strips by one small cord, keeping the tear-off angle constantly at 90 degrees. The data recorded are reproduced Table 5. adhesion degree of cover traces of corrosion Tyres with breaker with cords A, B, C not treated in saline mist chamber 11 N 4 no Tyres with breaker with cords A, B, C, treated for 20 days in saline mist chamber 9 N 4 no Tyres with breaker with cords A, B, C, treated for 40 days in saline mist chamber 6 N 3 no Tyres with breaker with N2 cords not treated in saline mist chamber 8 N 4 no Tyres with breaker with N2 cords treated for 20 days in saline mist chamber 6 N 3 no Tyres with breaker with N2 cords treated for 40 days in saline mist chamber 3 N 1 traces The differences between the adhesion values reported above for the withdrawal tests from the block (ASTM D2229-80) and those just reported for the tear-off test from the strip of breaker fabric are due to the different types of tests carried out. At all events, from all the test results, the small cords made from wires in accordance with the present invention, after incorporation in elastomeric material, show better adhesion results and better resistance to corrosion than those shown by the small cords made from brass-plated wires.

Claims: Metal wire for reinforcement of articles made of elastomeric material, provided with a coating comprising two concentric layers arranged radially one on the top of the other, said layers being made of an alloy selected from a zinc-cobalt alloy and a zinc-nickel alloy, characterized in that the alloy from which the radially inner layer of the coating is made has a zinc content from 95% to 98%, and the alloy from which the radially outer layer of the coating is made has a nickel content greater than 50%. Metal wire in accordance with claim 1, characterized in that the radially inner layer of the coating is made of a zinc-cobalt alloy. Metal wire in accordance with claim 1 characterized in that the radially inner layer of the coating is made of a zinc-cobalt alloy and the radially outer layer of the coating is made from a zinc-nickel alloy with a nickel content of at least 70%. Metal wire in accordance with claim 1 characterized in that the radially inner layer of the coating is made of a zinc-cobalt alloy and the radially outer layer of the coating is made from a zinc-cobalt alloy with a cobalt content of 70% to 90%. Article comprising a cross-linked elastomeric material blend incorporating metal reinforcement wires as claimed in any of claims 1 to 4. Motor vehicle tyre comprising metal reinforcement wires as claimed in any of claims 1 to 4.

Inventors: GIRIDHAR JAYARAMACHANDRAN, VAN OOIJ WIM J, GIRIDHAR, JAYARAMACHANDRAN, VAN OOIJ, WIM J.
Applicants: PIRELLI, PIRELLI COORDINAMENTO PNEUMATICI S.P.A.
Application year: 1992
Main classification: B32B15
IPC codes: C25D5, D07B1, D02G3, C25D7, B32B15, C23C28