Multi-Strand Cable with Two Multi-Strand Layers

The invention relates to cords and to a tyre comprising these cords.

Cords of structure 1×N are known from the prior art, as described in document WO2016/131862. These cords comprise a single layer of N=4 strands wound in a helix at a pitch 20 mm. Each strand comprises, for the one part, an internal layer of 3 internal threads wound in a helix at a pitch p1=6.7 mm and an external layer of 8 external threads wound in a helix around the internal layer at a pitch p3=10 mm. The structural elongation of the cord is 2.8% and the diameter of the cord is 3.8 mm, the linear mass being 36.4 g/m, and the endurance criterion equal to 3635 N×m/g.

These cords have the advantage of having a relatively high structural elongation but there is scope for improvement of the endurance criterion in order to increase the endurance performance of the reinforcers while at the same time reducing the shear in the polymer matrix.

Nowadays, there is an emerging need to develop new cords for application in crown plies, particularly zero-degree plies such as hooping crown plies. The purpose of these plies is to hoop the tyre in order both to reduce shear at the edge of the ply and to reduce the stiffness of the crown block at the centre with respect to attack.

The object of the invention is a cord that offers sufficient flexibility and structural elongation to allow the building of the tyre and reduce the stiffness of the crown block, with an improved endurance criterion to withstand cyclic tensile stress loadings.

To this end, one subject of the invention is a multi-strand cord with two layers of multi-strand elements, wherein the cord comprises:

in MPa·mm is the maximum bending stress per unit curvature experienced by the internal and external threads of the internal and external strands, where di and di′ are the diameter of the metal threads and i and i′ range from 1 to 3 and where Msteel=200 000 MPa;

Thanks to this multi-strand cord configuration with two layers of multi-strand elements, the cord according to the invention makes it possible to obtain a cord with a sufficient mass of metal while at the same time keeping the threads slender to make it possible to achieve improved endurance performance and thus improve the trade-off between shear in the polymer matrix and endurance performance of the cord and improve resistance to splitting.

On the one hand, by virtue of its relatively low endurance criterion, the cord according to the invention makes it possible to reduce the levels of stress in the cord subjected to tensile stress loading and therefore to extend the life of the tyre. Specifically, the inventors behind the invention have discovered that the first determining criterion for improving the endurance performance of a cord in a corrosive environment was not only the force at break, as is widely taught in the prior art, but also the endurance criterion, which is represented in the present application by an indicator equal to a combination of bending stress, cord diameter and mass of metal in the cord:

By definition, the diameter of the cord is the diameter of the smallest circle inside which the cord without the wrapper can be circumscribed.

The structural elongation As, which is a parameter well known to a person skilled in the art, is determined for example by applying the standard ASTM D2969-04 of 2014 to a cord tested so as to obtain a force-elongation curve. As is derived from the curve obtained as being the elongation, in %, corresponding to the point of intersection between the tangent to the elastic portion of the force-elongation curve and the elongation axis of the force-elongation curve. It will be recalled that a force-elongation curve comprises, in the direction of increasing elongations, a structural portion, an elastic portion and a plastic portion. The structural portion corresponds to a structural elongation of the cord that results from the moving-together of the different strands and metal threads that make up the cord. The elastic portion corresponds to an elastic elongation that results from the construction of the cord, in particular of the angles of the various layers and of the diameters of the metal threads. The plastic portion corresponds to the plastic elongation that results from the plasticity (irreversible deformation beyond the elastic limit) of the metal threads.

In the invention, the cord comprises two layers of multi-strand elements, which means to say that it comprises an assembly made up of a layer of Y>1 multi-strand elements, wound around a single layer of multi-strand elements, neither more nor less, which means to say that the assembly has two layers of multi-strand elements, not one, not three, but only two.

In the invention, the multi-strand element has one layer of strands, meaning that it comprises an assembly made up of one layer of strands, neither more nor less, meaning that the assembly has one layer of strands, not zero, not two, but only one.

In one embodiment, the internal multi-strand element of the cord is surrounded by a polymer composition and then by the external layer.

Advantageously, each strand has cylindrical layers.

Advantageously, each strand in the multi-strand element has two layers, meaning that it comprises an assembly made up of two layers of metal threads, neither more nor less, meaning that the assembly has two layers of metal threads, not one, not three, but only two. The external layer of each strand is wound around the internal layer of this strand in contact with the internal layer of this strand.

Highly advantageously, each strand of the internal layer and each strand of the external layer have cylindrical layers. It will be recalled that such cylindrical layers are obtained when the various layers of strands are wound at different pitches and/or when the directions of winding of these layers differ from one layer to the other. A strand with cylindrical layers is very highly penetrable, unlike a strand with compact layers in which the pitches of all the layers are the same and the directions of winding of all the layers are the same, and which exhibits far lower penetrability.

Advantageously, each strand of the internal layer and each strand of the external layer are desaturated, which means that there is enough space between the threads of the external layer to allow an elastomer compound to impregnate each strand.

For preference, the strands do not undergo pre-shaping.

Advantageously, the cord as defined above is bare, meaning that it does not have any polymer composition; in particular the cord does not have any elastomer composition.

A metal thread is understood to be a metal monofilament comprising a core made up predominantly (that is to say more than 50% of its weight) or entirely (100% of its weight) of a metal material, for example a carbon steel. The metal thread may advantageously comprise a layer of a metal coating covering the core, the metal coating being chosen from zinc, copper, tin and alloys of these metals, for example brass. Each thread is preferably made of pearlitic or ferritic-pearlitic carbon steel.

The values of the features described in the present application for the bare cord are measured on or determined from cords directly after they have been manufactured, that is to say before any step of embedding in a polymer matrix, in particular an elastomer matrix.

In the present application, any range of values denoted by the expression “between a and b” represents the range of values from more than a to less than b (that is to say excluding the end points a and b), whereas any range of values denoted by the expression “from a to b” means the range of values from the end point “a” as far as the end point “b”, namely including the strict end points “a” and “b”.

Advantageously, As≥1.5% and preferably As≥2.0%.

Another subject of the invention is a cord extracted from a polymer matrix, wherein the extracted cord comprises:

in MPa·mm is the maximum bending stress per unit curvature experienced by the internal and external threads of the internal and external strands, where di and di′ are the diameter of the metal threads and i and i′ range from 1 to 3 and where Msteel=200 000 MPa;

Preferably, the polymer matrix is an elastomer matrix.

The polymer matrix, preferably elastomer matrix, is based on a polymer, preferably elastomer, composition.

A polymer matrix is understood to be a matrix comprising at least one polymer. The polymer matrix is thus based on a polymer composition.

What is meant by an elastomer matrix is a matrix containing at least one elastomer. The preferred elastomer matrix is thus based on the elastomer composition.

The expression “based on” should be understood as meaning that the composition comprises the compound and/or the product of the in situ reaction of the various constituents used, some of these constituents being able to react and/or being intended to react with one another, at least partially, during the various phases of manufacture of the composition; the composition thus being able to be in the fully or partially crosslinked state or in the non-crosslinked state.

A polymer composition is understood as meaning that the composition comprises at least one polymer. Preferably, such a polymer may be a thermoplastic, for example a polyester or a polyamide, a thermosetting polymer, an elastomer, for example natural rubber, a thermoplastic elastomer or a combination of these polymers.

An elastomer composition is understood as meaning that the composition comprises at least one elastomer and at least one other component. Preferably, the composition comprising at least one elastomer and at least one other component comprises an elastomer, a crosslinking system and a filler. The compositions that can be used for these plies are conventional compositions for the skim coating of filamentary reinforcing elements and comprise a diene elastomer, for example natural rubber, a reinforcing filler, for example carbon black and/or silica, a crosslinking system, for example a vulcanizing system, preferably comprising sulfur, stearic acid and zinc oxide, and optionally a vulcanization accelerant and/or retarder and/or various additives. The adhesion between the metal threads and the matrix in which they are embedded is afforded for example by a metal coating, for example a layer of brass.

The values of the features described in the present application for the extracted cord are measured on or determined from cords extracted from a polymer matrix, in particular an elastomer matrix, for example of a tyre. Thus, for example on a tyre, the strip of material radially on the outside of the cord that is to be extracted is removed in order to be able to see the cord that is to be extracted radially flush with the polymer matrix. This removal can be done by stripping using cutters and grippers, or else by planing. Next, the end of the cord that is to be extracted is disengaged using a knife. The cord is then pulled so as to extract it from the matrix, applying a relatively shallow angle in order not to plasticize the cord that is to be extracted. The extracted cords are then carefully cleaned, for example using a knife, so as to detach any remains of polymer matrix locally adhering to the cord, while taking care not to damage the surface of the metal threads.

In order to determine the linear mass of the extracted cord, a cross section of the cord in the elastomer matrix is taken and the surface area of steel is determined using image processing and multiplied by the density of the steel.

In order to measure the linear mass of the extracted cord it is also possible, after the operation described hereinabove, to weigh one metre of cleaned cord in order to determine, over 10 measurements, the mean linear mass of cleaned cord.

The advantageous features described below apply both to the bare cord and to the cord extracted from a polymer matrix.

Advantageously, the criterion Vis greater than or equal to 1000 N×m/g and preferably greater than or equal to 1500 N×m/g.

Advantageously, the criterion Vis less than or equal to 2500 N×m/g.

Advantageously, M ranges from 30 to 150 g/m, and preferably from 40 to 120 g/m.

As a preference, the cord has a cord diameter such that D ranges from 3 mm to 9 mm, preferably from 4 mm to 7 mm.

By definition, the diameter of a strand is the diameter of the smallest circle inside which the strand can be circumscribed.

For preference, the diameters of the metal threads range, independently of one another, from 0.15 mm to 0.50 mm, preferably from 0.18 mm to 0.35 mm and more preferably from 0.20 mm to 0.30 mm.

For preference, the threads of the one same layer of a predetermined strand all have substantially the same diameter. Advantageously, the external strands all have substantially the same diameter. What is meant by “substantially the same diameter” is that the threads or the strands have the same diameter to within the industrial tolerances.

Advantageously, Y is equal to 6, 7, 8, 9 or 10, preferably Y=6, 7 or 8 and more preferentially Y=6.

Advantageously, K=2, 3 or 4, preferably K=3 or 4.

Advantageously, L=2, 3 or 4 and preferably L=3 or 4.

In a first embodiment, each strand of the internal layer has two layers.