Steel cord for use as a reinforcing element in a belt layer of a vehicle pneumatic tire and method for manufacturing a steel cord

Abstract

Steel cord (1) for use as a reinforcing element in a belt layer of a vehicle pneumatic tire with at least two flat steel filaments (2) stacked on top of each other in planar contact and running parallel to each other, characterized in that the steel cord (1) has an outer pre-crosslinked rubber layer (3) with a layer thickness of 0.1 µm to 300 µm, which encases the stack of steel filaments (2).

Description

The invention relates to a steel cord for use as a reinforcing element in a belt layer of a vehicle tire, comprising at least two flat steel filaments stacked on top of each other in planar contact and running parallel to each other. The invention further relates to a method for producing a steel cord for use as a reinforcing element in a belt layer of a vehicle tire. The reinforcing elements in the belt plies of vehicle pneumatic tires are typically steel cords consisting of interconnected, circular-section steel filaments. The use of monofilaments with round or elliptical cross-sections, or of steel cords made of interconnected, elliptical-section steel filaments, is also known. A steel cord of the type mentioned above is known, for example, from WO 2012 / 168007 A1. This steel cord consists of a number of flat steel filaments, in particular two to nine steel filaments, which have wide, parallel outer surfaces that are significantly wider than the thickness of the steel filaments, allowing the steel filaments to be stacked on top of each other in planar contact. With multiple steel filaments, the width of the steel filaments increases from the outer to the inner ones.The stacked steel filaments are spirally wound around a thin steel helix to ensure their cohesion. These steel cords are installed in the belt plies of pneumatic tires in such a way that the flat outer surfaces run parallel to the belt ply plane. Each flat steel filament already possesses high lateral stiffness; therefore, a steel cord with several stacked flat steel filaments also has high lateral stiffness, which is particularly advantageous for the handling characteristics of the pneumatic tire. The spiral winding of the stacked flat steel filaments is relatively complex, and care must be taken to prevent the individual steel filaments from shifting unintentionally against each other during the winding process. Furthermore, the helix / filament intersections are highly stressed friction points where the material is prone to excessive fretting, i.e., wear and tear.Material abrasion is prone. The invention is therefore based on the objective of ensuring particularly good cohesion of the steel filaments in a steel cord of the type mentioned above and the dimensional stability of the steel cord. The invention is also based on the objective of providing a method for producing a corresponding steel cord with high lateral stiffness. The problem is solved according to the invention with a steel cord which has an outer pre-crosslinked rubber layer with a layer thickness of 0.1 µm to 300 µm, which encases the stack of steel filaments. An alternative method according to the invention is characterized by the following steps: a) Providing several stacked, parallel, and planar steel filaments in contact with each other, b) Passing the stacked steel filaments through an immersion bath of a rubber solution, whereby the stack of steel filaments is completely wetted on the outside with the rubber solution, c) Expelling the wetted stack from the immersion bath, passing it by a vacuum system for the extraction of excess rubber solution and conveying it to a drying device in which the solvent of the rubber solution is evaporated, d) Conveying the coated stack to an irradiation device by means of which pre-crosslinking of the outer rubber layer takes place, e) Conveying the steel cord produced in this way, provided with a thin rubber layer, to a winding spool. Another alternative to the process is characterized by the following steps: a) providing several stacked, parallel, flat steel filaments in contact with each other, b) overmolding the stack of flat steel filaments with a rubber layer in an extruder with an adhesive compound, c) conveying the rubber-coated stack to an irradiation device, by means of which pre-crosslinking of the rubber layer takes place, d) conveying the steel cord produced in this way, provided with a thin rubber layer, to a winding spool. A particular advantage of the invention lies in the fact that the steel cord, consisting of flat filaments, is provided with a thin layer of rubber, which ensures excellent adhesion of the flat filaments and effectively prevents the individual steel filaments, which are not stranded together, from shifting against each other. Since the thin rubber layer is pre-cured, its tackiness is so low that the steel cord can be easily wound onto and unwound from spools without damaging the rubber layer. Both the dipping process and the overmolding process in an extruder are advantageous and easily implemented methods for coating the stack of steel filaments with a layer of rubber. In a preferred embodiment of the steel cord, the rubber layer fills the spaces between the end sections of the stacked steel filaments. This measure promotes good cohesion of the individual flat steel filaments. According to a preferred embodiment of the invention, the thickness of the outer rubber layer of the steel cord according to the invention is 50 µm to 200 µm. Particularly preferred are embodiments in which the thickness of the steel filaments is 0.1 mm to 0.5 mm, in particular 0.2 mm to 0.4 mm. The width of the steel filaments is preferably 1.5 to 3 times, in particular 2 to 2.5 times, the thickness of the steel filaments. In both alternative methods according to the invention, it is advantageous if the irradiation and pre-crosslinking of the outer rubber layer is carried out using an electron beam process. This allows the pre-crosslinking of the rubber layer to be performed at a relatively high process speed. The immersion bath process offers several advantageous improvements. For example, the drying process of the drying device can be carried out in a heated chamber. This allows the solvent to be removed from the rubber solution at a high process rate. In a further advantageous embodiment of the immersion bath process according to the invention, the rubber solution comprises nonpolar polymers with double bonds that are crosslinkable with sulfur. The rubber compound can also include an organic, nonpolar solvent. In this way, the solvent is optimally matched to the nonpolar polymers in the rubber solution. If, according to a further feature of the invention, a rubber solution with a polymer content of a maximum of 40 wt.% is used, a very thin rubber layer can be produced on the stack of steel filaments. Furthermore, it is advantageous if the rubber solution does not contain any fillers, as these are generally insoluble and would result in an inhomogeneous surface coating. Further features, advantages and details of the invention will now be described in more detail with reference to the single figure, Fig. 1, which schematically represents a cross-section of an embodiment of a steel cord according to the invention. Fig. 1 shows the cross-section of a steel cord, which is particularly suitable as a reinforcing element in the belt layers of pneumatic vehicle tires. The steel cord 1 has a number, in the example shown three, of steel filaments 2 with an elongated, flat cross-section. Such steel filaments are therefore commonly referred to as flat steel filaments. The three steel filaments 2 are positioned or stacked on top of each other and thus not stranded together. In the embodiment shown in Fig. 1, the steel cord 1 has three identical flat steel filaments 2, whose thickness d is 0.1 mm to 0.5 mm, preferably 0.2 mm to 0.4 mm, and whose width b is 1.5 to 3 times, preferably 2 to 2.5 times, the thickness d. The outer surfaces 2a of each steel filament 2, which are parallel to the width, are flat, so that the stacked steel filaments 2 lie flat against each other at these outer surfaces 2a. The number of steel filaments 2 in the steel cord 1 can be between one and five, preferably between two and four. The steel cord 1 can also have steel filaments 2 of different dimensions, wherein the outer steel filaments 2 have a smaller width b than the inner filaments 2. For example, when using five steel filaments 2, the width b of the steel filaments 2 decreases gradually from the outermost to the innermost. The stack of steel filaments 2 has a thickness D that does not exceed the greatest width b. The steel cords 1 are positioned in a belt layer of a vehicle tire such that the outer surfaces 2a of the steel filaments 2 are oriented in the axial direction. The stack of steel filaments 2 is coated on the outside with a thin layer of rubber 3, which ensures the cohesion of the stacked steel filaments 2 and also fills the local gaps between the lateral edge sections of the stacked steel filaments 2. To produce the rubber layer 3, the "endless" stack of steel filaments 2 is immersed in a bath containing a rubber solution and completely wetted with the rubber solution. After being removed from the immersion bath, the rubber solution coats the stack in the form of a thin layer of material. In the next step, excess rubber solution is extracted by passing the stack through a vacuum system. In a drying device, which, for example, includes a heating chamber through which the coated stack is passed, the solvent evaporates from the rubber solution under heat.The coated stack is then transferred to an irradiation device to pre-crosslink the rubber layer 3, thereby reducing its stickiness. The rubberized steel cord 1 produced in this way is then transferred to a winding spool, where it is wound and temporarily stored. The thickness of the pre-crosslinked rubber layer 3 is between 0.1 µm and 300 µm, preferably between 50 µm and 200 µm. The rubber solution of the dipping bath can be based on at least one polar or at least one nonpolar, sulfur-crosslinkable polymer selected from the group consisting of natural polyisoprene and / or synthetic polyisoprene and / or butadiene rubber and / or styrene-butadiene rubber and / or solution-polymerized styrene-butadiene rubber and / or emulsion-polymerized styrene-butadiene rubber and / or liquid rubbers and / or isoprene-isobutylene copolymer and / or ethylene-propylene-diene rubber and / or acrylate rubber and / or styrene-isoprene-butadiene terpolymer and / or hydrogenated acrylonitrile butadiene rubber and / or isoprene-butadiene copolymer and / or hydrogenated styrene-butadiene rubber. The polymer content of the rubber solution is a maximum of 40% by weight. The rubber, in particular the styrene-butadiene rubber, can be modified with hydroxyl groups and / or epoxy groups and / or siloxane groups and / or amino groups and / or aminosiloxane and / or carboxyl groups and / or phthalocyanine groups. Other modifications known to those skilled in the art, also referred to as functionalizations, are also possible. In particular, the butadiene rubber can be wholly or partially hydrogenated. The rubber solution further contains at least one organic, nonpolar solvent in which the polymer is completely soluble. Pure substances, such as tetrahydrofuran or cyclohexane, as well as mixtures of hydrocarbons, such as petroleum ether with a boiling range between 60°C and 100°C, can be used. The rubber solution can, for example, comprise the following components: - Petroleum ether, 95 wt.% - Polyisoprene, 5 wt.% - Optional: Antioxidant One of the antioxidants commonly used in the rubber industry can be added as an aging inhibitor, for example, an antioxidant from the paraphenylenediamine group (6PPD, DTPD). A typical concentration is, for example, 5% by weight based on the polymer; this equates to 0.0025% by weight of antioxidant in the solution. In an alternative embodiment of the invention, the stack of flat steel filaments 2 is overmolded in an extruder with an adhesive compound known to those skilled in the art and thus coated with a rubber compound whose layer thickness is from 0.1 µm to 300 µm, preferably from 50 µm to 200 µm. The adhesive compound contains an adhesive system known per se, which ensures the adhesion of the rubber layer 3 to the brass surface of the filaments 2. The steel filaments 2 coated with the adhesive compound are then pre-crosslinked as described above, and the steel cord 1 thus produced is wound up. Reference number list 1 Steel cord 2 Steel filament 2a Outer surface 3 Rubber layer b Width d Strength D Thickness

Claims

Steel cord (1) for use as a reinforcing element in a belt layer of a vehicle pneumatic tire with at least two flat steel filaments (2) stacked on top of each other in planar contact and running parallel to each other, characterized in that the steel cord (1) has an outer pre-crosslinked rubber layer (3) with a layer thickness of 0.1 µm to 300 µm, which encases the stack of steel filaments (2). Steel cord (1) according to claim 1, characterized in that the rubber layer (3) fills the spaces between the end sections of the stacked steel filaments (2). Steel cord (1) according to claim 1 or 2, characterized in that the thickness of the rubber layer (3) is 50 µm to 200 µm. Steel cord (1) according to claim 1 or 2, characterized in that the thickness (d) of the steel filaments (2) is 0.1 mm to 0.5 mm, preferably 0.2 mm to 0.4 mm. Steel cord (1) according to one of claims 1 to 3, characterized in that the width (b) of the steel filaments (2) is 1.5 to 3 times, preferably 2 to 2.5 times, their thickness (d). A method for producing a steel cord (1) for use as a reinforcing element in a belt layer of a vehicle pneumatic tire, comprising the following steps: a) providing several stacked, parallel, flat steel filaments (2) in contact with each other, b) passing the stacked steel filaments (2) through an immersion bath of a rubber solution, whereby the stack of steel filaments (2) is completely wetted on the outside with the rubber solution, c) removing the wetted stack from the immersion bath, passing it through a vacuum system for the extraction of excess rubber solution, and conveying it to a drying device in which the solvent of the rubber solution is evaporated, d) conveying the coated stack to an irradiation device by means of which pre-crosslinking of the outer rubber layer (3) takes place, e) conveying the thus produced,a steel cord (1) provided with a thin rubber layer (3) to form a winding spool. Method according to claim 6, characterized in that in step c) the drying device comprises a heated chamber. Method according to claim 6 or 7, characterized in that in step d) the irradiation is carried out using an electron beam method. Method according to one of claims 6 to 8, characterized in that the rubber solution comprises nonpolar polymers with double bonds which can be crosslinked with sulfur. Method according to one of claims 6 to 9, characterized in that the rubber solution comprises an organic nonpolar solvent. Method according to one of claims 6 to 10, characterized in that the rubber solution has a polymer content of a maximum of 40 wt.%. Method according to one of claims 6 to 11, characterized in that the rubber solution contains no fillers. Method according to one of claims 6 to 12, characterized in that the rubber solution comprises an anti-aging agent. Method for producing a steel cord (1) for use as a reinforcing element in a belt layer of a vehicle pneumatic tire, comprising the following steps: a) providing several stacked, parallel, flat steel filaments (2) in contact with each other, b) coating the stack of flat steel filaments (2) with a rubber layer (3) by overmolding the stack of flat steel filaments (2) in an extruder with an adhesive compound, c) conveying the stack coated with the rubber layer (3) to an irradiation device by means of which pre-crosslinking of the rubber layer (3) takes place, d) conveying the steel cord (1) produced in this way and provided with a thin rubber layer (3) to a winding spool. Method according to claim 14, characterized in that in step d) the irradiation is carried out using an electron beam method.

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