1. Field of the Invention
The present invention relates to steel cords for reinforcing vulcanizable elastomeric materials such as rubber tires, belts and the like. More particularly, it relates to multiwire layered steel cord constructions, especially compact cords of elevated strengthening power, which cords are in addition less prone to erratic filament breakage and to premature cord failures as a result of frequently occurring corrosion and fatigue effects in service. Such improved steel cords can be employed to reinforce a variety of rubber products, but are particularly intended for use as high-duty tire reinforcements and more in particular for obtaining a lasting tire life, e.g. in a truck tire exposed to harsh driving conditions.
2. Background Art
In general, steel cords for use in elastomer systems are prepared by twisting together a plurality of metallic filaments to form cord structures which may widely diverge with respect to construction type, ranging from simple cords (3.times.1, 4.times.1, 2+2, etc.) to more elaborate multi-strand and mixed multiwire/multistrand structures (7.times.7, 1.times.3+5.times.7+1, 7.times.4, etc.). In tire applications filaments are usually high-carbon (0.7-0.8% C) steel wires of diameter 0.1 to 0.4 mm with a tensile strength of about 2500-2700 N/mm.sup.2 and all the wires have a rubber adherable surface coating, in most cases a copper-zinc alloy (brass) comprising more than 55% Cu by weight, more generally a thin brass layer of a thickness below 0.50 .mu.m containing from 60 to 75% Cu and 40 to 25% Zn in weight.
Besides the more common cables including several strands, the industry has developed more recently layered cords for tire service. Said cords may be prepared by stranding together a plurality of brassed wires either in one operation or in successive cabling steps to form a desired multilayer cable structure (e.g. a 3+9+15 construction assembled from 27 wires). By wire layer is meant a twisted assembly of wires in tubular form around a core, which layer has a thickness of substantially one wire diameter.
In cross-section said cords show a more or less closely packed arrangement of wires which form a regular pattern of successive multiwire layers around a core, which core may be comprised of one or more wires or of a central strand. Said consecutively-arranged wire layers may vary in geometrical form (e.g. concentric rings, polygonal arrangement, etc.) and in packing density (compactness) according to cabling method, twisting pitch, core shape and to wire diameters and number of wires per layer.
Such layered cords have certain advantages over the more traditional multistrand cords. There are in fact two main reasons for their introduction: first they occupy less volume for a comparable cord breaking strength (the structure of multistrand cords contains more open space) and secondly, the cord wires are less sensitive to fretting damage because of predominantly linear wire contacts as opposed to the normally present wire cross-overs in multistrand cords.
Although considerable progress has been made in the last 10 years in the art of steel cord making and rubber/steel cord-composite manufacturing and in bonding vulcanized rubber to brass-coated steel wires, there are still major deficiencies in the industry. These include separation of tire rubber from the steel wires after relatively short periods of roadwear (rapid ageing accelerated by corrosion), poor adhesion retention, premature breakage of wires and cords.
Said problems may become even more severe with multilayer cord reinforcements (in particular with compact high-tensile cords) employed in exacting tire environments (heavy driving loads, moisture penetration, heat and humidity ageing, corrosion by moisture and road salts and the like). Under these circumstances premature cord failures and unreliable tire service life are increasingly observed. This underlines the fact that a thin brass layer (usually from 0.10 to 0.40 .mu.m) cannot provide an adequate protection of the steel wire substrate against incidental corrosion attack and hydrogen embrittlement effects. Such an increase in brass layer thickness above 0.40 .mu.m (to afford a better protection) is generally not recommended for reasons of cost and adhesion (especially poor adhesion retention after ageing) cord manufacturers and tire builders have made numerous trials on a lot of cord and coating aspects to develop improvements to solve this persistent problem. Despite all these efforts on adapted cord structures, ternary brass coatings, rubber ingredients and other improvements, the failure problem, especially in the use of compact layered cords, was not satisfactorily solved.
From our improvement efforts and extensive testing, we could separate two important problem areas intimately related to the phenomenon of unsufficient bond and cord durability when applying high-performance compact multilayer cords in exacting tire environments.
A first (general) problem area remains the gradual decrease of the cord/rubber bond stability during normal tire service and further the accelerated adhesion loss due to heat and humidity ageing and to corrosion (moisture+road salts) effects. In this respect we noticed that layered cords, especially the compact versions, are relatively more sensitive to early rubber debonding and separation of rubber plies.
A second problem area is unexpected brittle breakage of wires and premature cord failures. This mainly reflects the fact that most conventional brassed cords have a poor resistance to incidental overstressing and to hydrogen embrittlement and lack sufficient corrosion fatigue endurance. When using high-tensile cords composed of wires with a strength in excess of 2700 N/mm.sup.2, (and especially above 3000 N/mm.sup.2), the increase in erratic wire breaks may become untolerable. This seriously restricts the fitness for tire use of high-performance steel cords.
This erratic situation is particularly frustrating in the use of more expensive cord constructions of high specific strength (i.e. layered compact cords consisting of an elevated number of wires, in casu high-tensile wires). Furthermore, it may considerably hamper the technical advancement in developing more economical weight-saving high-strength steel cord/rubber composites, such as desired for high-duty truck tires.
Our efforts to provide an improved multilayer cord which does not suffer from the major deficiencies of prior art compact and layered cords were primarily devoted to afford an adequate solution to the premature wire and cord failure problem. Indeed, the teachings of our investigations and in-depth failure analysis revealed that lack of corrosion fatigue endurance and resistance to hydrogen embrittlement of embedded wires and cords may affect tire life and performance in a more radical and catastrophic way than progressive ageing and adhesion degradation. In this respect it has been clearly established that unexpectedly early wire and cord fractures are the dominant deficiency restricting the use of high-strength multilayer cord as a high-duty tire reinforcement in corrosion fatigue conditions. More in particular, it has been surprisingly found that sudden wire breakage occuring preferentially in the subsurface cord layer is the major cause of early cord failure and related shortening of cord/rubber composite life. Thus, the prevention of a localized filament breaking phenomenon appeared to be the key to a novel and effective solution to the frequently observed poor or erratic durability of rubber vulcanizates reinforced with compact layered cords.