EP2184401A1

EP2184401A1 - Cord, process for producing the same, and composite of cord with rubber

Info

Publication number: EP2184401A1
Application number: EP08791298A
Authority: EP
Inventors: Mitsuhiro Nishimura
Original assignee: Bridgestone Corp
Current assignee: Bridgestone Corp
Priority date: 2007-07-17
Filing date: 2008-07-17
Publication date: 2010-05-12
Also published as: CN101802297A; JPWO2009011397A1; EP2184401A4; WO2009011397A1; US20100170215A1

Abstract

A cord obtained by twisting a plurality of strands each composed of a plurality of filaments is provided. A circumferential surface of the filament is coated with unvulcanized rubber to improve rubber intrusion, thereby improving corrosion resistance and suppressing twisting-loss.

Description

TECHNICAL FIELD

The present invention relates to a cord used for reinforcing a rubber product such as a tire and an industrial belt, method of producing same, and a rubber-cord composite body.

RELATED ART

A high strength is demanded for a cord used for reinforcing a tire, especially a tire for a construction vehicle, so that a cord made of a plurality of strands produced by twisting a plurality of filaments, which strands are further twisted to form a so-called multiple-twisting structure, is used for this purpose.
In the production of the cord having a multiple-twisting structure, however, a strand is made from a plurality of filaments and then a plurality of strands are twisted to give the cord. Thus, it has more filaments and a more complicated twisting structure than a cord having a single twisting structure used for reinforcing, for example, a passenger vehicle. As a result, a tensile strength of the cord becomes smaller than the sum of tensile strengths of the cords, which increases twisting loss.
In a field of a wire rope used in a cargo handling machinery represented by a crane, for example, due to the large number of filaments constituting a strand and an indentation produced by an action of a mesh of respective strands constituting the wire rope, the twisting loss increases. Therefore, a technique that suppresses the twisting loss by filling an elastomer or a thermal plastic resin between an inside of a core and side strands has been proposed.

Patent Document 1: JP 2992783 B
Patent Document 2: JP 8158275 A

DISCLOSURE OF THE INVENTION

Although the above-mentioned technique can suppress the twisting loss, it is demanded particularly for a cord used for reinforcing a tire to ensure corrosion resistance as a reinforcing material. That is, when the tire suffers an external injury, water intrudes from the external injury into the tire to reach the cords, which may corrode the cords. If any spaces exist inside the cords, the water disperses through the spaces in the axial direction of the cord and the corrosion area expands along the cord. This may cause a malfunction due to a separation with the corrosion area being as the origination of the separation. A characteristic capable of suppressing the intrusion of water into the inside of the cord, i.e. corrosion resistance, is, therefore, demanded particularly for a cord for reinforcing a tire in order to avoid such dispersion of the corrosion.
The corrosion resistance is highly desired for the cord having the above-mentioned multiple-twisting structure. In order to improve the corrosion resistance of the cord having such multiple-twisting structure, it is necessary to allow the rubber to sufficiently intrude into the inside of the strands constituting the cord.
In order to improve the ability of the rubber to intrude into the strands constituting the cord, a commonly used means is that sufficient spaces extending from the outside of the strands to the inside of the strands are provided to allow the rubber to sufficiently intrude into the inside of the strand during a vulcanization step of a tire manufacturing process.
The cord having the multiple-twisting structure is, however, formed by twisting a plurality of filaments to obtain a strand and further twisting a plurality of the strands. The structure of the cord, thus, becomes complicated and it is difficult to provide spaces for allowing the rubber to sufficiently intrude into the inside of the strands. As a result, the rubber cannot sufficiently intrude into the inside of the strands during the vulcanization step of the tire manufacturing process, so that improvement of the corrosion resistance is difficult.
The object of the present invention is, therefore, to provide a cord structure capable of suppressing twisting loss and improving corrosion resistance particularly of a cord having a complicated multiple-twisting structure by allowing rubber to sufficiently intrude into the inside of strands constituting the cord.
The present inventor had dedicated to study a means for improving corrosion resistance of a cord having a complicated multiple-twisting structure and found it effective for allowing the rubber to sufficiently intrude into the inside of the strands during the vulcanization step of the tire manufacturing process to coat the filaments used for the production of the strands with unvulcanized rubber prior to twisting a plurality of the strands to form a cord. The present invention is completed in this way.
The gist of the present invention is as follows:

(1) A cord including a plurality of strands that are twisted together, each strand having a core composed of a filament or a plurality of filaments that are twisted together, and at least one layer of sheaths arranged around the core, each sheath being composed of a plurality of filaments that are twisted together, wherein at least a circumferential surface of the core of the strand is coated with unvulcanized rubber.

(2) The cord according to the above-item (1), wherein the strand is produced by wrapping a plurality of filaments in an identical direction at an identical pitch.
(3) The cord according to the above-item (1) or (2), wherein the cord is obtained by wrapping a plurality of sheath strands composed of a plurality of filaments around a core strand composed of a plurality of filaments.
(4) The cord according to the above-item (3), wherein a circumferential surface of the core strand is coated with unvulcanized rubber.
(5) The cord according to the above-item (3) or (4), wherein six strands are arranged around a sheath strand.
(6) A rubber-cord composite body, wherein a plurality of the cord according to any one of the above-items (1) to (5) are arranged in parallel with each other and coated with rubber.
(7) A method of producing a cord including a plurality of strands that are twisted together, each strand having a core composed of a filament or a plurality of filaments that are twisted together, and at least one layer of sheaths around the core composed of a plurality of filaments that are twisted together, wherein at least a circumferential surface of the core of the strand is coated with unvulcanized rubber prior to form the cord.
(8) The method according to the above-item (7), wherein a plurality of sheath strands are twisted around a core strand, and a circumferential surface of the core strand is coated with unvulcanized rubber.
According to the present invention, the filaments constituting the strand are preliminarily coated with unvulcanized rubber, so that the unvulcanized rubber is allowed to flow and thus be distributed into the space between the filaments in the strand. The rubber, therefore, can sufficiently intrude into the inside of the strand, which enables to provide a cord having improved corrosion resistance and suppressed twisting reduction.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a sectional view of a cord according to the present invention;
Fig. 2 is a sectional view of a cord according to the present invention;
Fig. 3 is a diagram showing a method of producing a cord according to the present invention;
Fig. 4 is a diagram showing a method of producing a cord according to the present invention;
Fig. 5 is a sectional view of a cord according to Comparative Example;
Fig. 6 is a sectional view of a cord according to the present invention;
Fig. 7 is a sectional view of a cord according to the present invention;
Fig. 8 is a sectional view of a cord of Comparative example;
Fig. 9 is a sectional view of a cord according to the present invention;
Fig. 10 is a sectional view of a cord according to the present invention;
Fig. 11 is a sectional view of a cord according to Comparative Example;
Fig. 12 is a sectional view of a cord according to the present invention;
Fig. 13 is a sectional view of a cord according to the present invention;
Fig. 14 is a sectional view of a cord according to Comparative Example;
Fig. 15 is a sectional view of a cord of Conventional Example;
Fig. 16 is a sectional view of a cord of Conventional Example;
Fig. 17 is a sectional view of a cord according to Conventional Example;
Fig. 18is a sectional view of a cord according to Conventional Example;

REFERENCE SYMBOLS

1: cord
2: core strand
3: sheath strand
4: core filament
4a: sheath filament
4b: sheath filament
5: core
6: outermost sheath layer
6: first sheath
7: unvulcanized rubber
8: unvulcanized rubber-coating equipment
9: wire collector
10: twisting machine
11: unwind reel

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter a cord according to the present invention is discussed in detail with reference to the attached drawings. A cross-section of the cord according to the present invention is shown in Fig. 1. The cord 1 shown in the figure has a core strand 2 and six sheath strands 3 twisted around the core strand 2. In this embodiment, the core strand 2 and the sheath strands 3 have the same configuration. That is, each strand has a 3+9 structure in which a sheath 6 consisting of nine sheath filaments 4a is twisted around a core 5 consisting of three core filaments 5. It is essential that at least a circumferential surface of the core 5 constituting the core strand 2 and sheath strand 3 is coated with unvulcanized rubber 7.
This is because the unvulcanized rubber 7 coating the surroundings of the core 5 constituting the core strand 2 and the sheath strand 3 fill inside of the each strand to prevent water intruding through an external injury of the tire from reaching the inside of the cord. Thus, the corrosion resistance can be improved. Further, as a result of preventing the contacts between the filaments, the twisting loss can also be suppressed.
It is noted that the core strand 2 and the sheath strand 3 do not necessarily have the shown structure as far as they are formed by twisting a plurality of filaments. It is, however, preferable that each strand has a compact structure in which the plurality of filaments are twisted in the same direction with the same pitch.
This is because the compact structure allows a line contact between the filaments in the cord and avoids a point contact between the filaments to suppress fretting wear, so that the corrosion resistance is improved. Further, the compact structure enables to complete the twisting of the strands in a single step in the process of producing the strand, which improves the productivity.
In the above-mentioned cord shown in Fig. 1, a circumferential surface of the core strand 2 is preferably coated with the unvulcanized rubber 7, as shown in Fig. 2.
That is, in addition to coating at least the circumferential surface of the core 5 of the each strand with the unvulcanized rubber 7, the circumferential surface of the core strand 1 is coated with the unvulcanized rubber to more certainly avoid a contact between the core strand 2 and the sheath strand 3, so that the twisting reduction can be further suppressed. Moreover, the inside of the cord is filled with the unvulcanized rubber, which improves the corrosion resistance of the cord.
The cord formed by twisting six sheath strands around a core strand as shown in Figs. 1 and 2 is a typical multiple-twisting structure cord, and has an advantage that overall balance of the cord can be easily maintained when strands with the same structure are twisted together.
It is noted that the shown cord consists of filaments with the same diameter, but filaments with different diameters may be used in combination.
Furthermore, although not shown, a wrapping cord or a wrapping filament may be used if the strand needs to be constrained.
A number of the above-mentioned cords are arranged in parallel to give a composite body of rubberized cords and rubber used for reinforcing a tire. More specifically, the rubber-cord composite body is applied to a tire as a belt of a ply of a carcass to reinforce the tire.
Next, a method of producing a cord according to the present invention is discussed in detail with reference to an apparatus for producing a cord shown Fig. 3. The apparatus for producing a cord shown in Fig. 3 is provided with a wire-bundler 9 which is equipped with a given number of unwind reels 11 for unwinding respective core filaments 4 and a given number of unwind reels 11 for unwinding respective sheath filament 4a and which bundles the filaments unwound from the respective unwind reels 11, and a twisting device 10 for twisting the bundled filaments together. An unvulcanized rubber-coater 8 for coating the core filament 4 with unvulcanized rubber is disposed between the unwind reels 11 and the wire-bundler 9. In order to produce a cord by means of the twisting device, filaments 4 to be a core and filaments 4a to be a sheath are firstly fed from the unwind reels 11 to the unvulcanized rubber-coater 8 where circumferential surfaces of the filaments 4 are coated with unvulcanized rubber. Thereafter, the core filaments 4 and the sheath filaments 4a are gathered and twisted by the wire-bundler 9 to form a core strand 2. Six sheath strands 3 are produced in the same manner, and the core strands 2 and the sheath strand 3 are twisted together by means of the twisting device 10 shown in Fig. 3 to produce a cord 1.
In this regard, it is essential for producing a cord according to the present invention that the unvulcanized rubber-coater 8 is disposed between the unwind reels 11 and the wire-bundlers 9 to coat at least the circumferential surface of the core of the strand with unvulcanized rubber.
This is because arranging the unvulcanized rubber-coater between the unwind step and the wind step of the production of the cord enables to produce a cord in a series of cord producing steps as in a conventional manner, so that the present method can be a remarkably effective for improving productivity and ensuring an operation space.
Further, as shown in Fig. 4, it is preferable that a manufacturing apparatus similar to that shown in Fig. 3 in which an unvulcanized rubber-coater 8 is disposed between the unwind reels 11 and the wire-bundler 9 whereby a circumferential surface of a core strand 2 is further coated with unvulcanized rubber 7 and then a plurality of sheath strands 3 are twisted around the core strand 2 to produce a cord 1.
This is because the unvulcanized rubber coating the circumferential surface of the core strand 2 serves as a cushion material. Thus, when the core strand 2 and the sheath strands 3 bear the tensile strength while they are twisted together, tightening of twist due to contacts between strands can be reduced. As a result a cord having less twisting loss can be obtained.
The strand constituting the cord according to the present invention is twisted by a twist buncher, so that the strand having, for example, a 3+9 structure or a 1+6+12 structure can be formed into a so-called compact structure of 12 cc or 19 cc to reduce the number of production steps and to improve productivity of the strands.

EXAMPLES

Various cords are manufactured under the specifications shown in Tables 1 and 2. The structures of the cords are shown in Figs. 1, 2 and 5-18. It is noted that each of Figs. 15-18 shows a sectional view of a conventional cord and each of Figs. 1, 2 and 5-14 shows a sectional view of a cord according to the present invention.
A ply in which a plurality cords according to the present invention are arranged in parallel with each other and embedded in a rubber sheet with a given space therebetween is applied to a belt and a test tire is built with using this belt. The test tire is examined in relation to a ratio (%) of rubber intruding inside of a core strand of a cord used for reinforcement, a ratio (%) of rubber intruding inside of a sheath strand, a ratio (%) of rubber intruding between the core strand and the sheath strand, and twisting loss (%). In addition, the number of steps is also examined in the method of producing a cord according to the present invention.
In order to measure the ratio (%) of rubber intruding inside of a core strand, the ration (%) of rubber intruding inside of a sheath strand and the ratio (%) of rubber intruding between the core strand and the sheath strand, the test tire is dissected and a cord is isolated and separated into strands. The circumferential surface of the core strand is observed from four directions with a magnifying glass to measure an area on the circumferential surface covered with the rubber by means of an image processing and analyzing device. Then, if each strand has two layered twisting structure, the sheathes of each strand are removed and the circumferential surface of the core is observed from four directions with the magnifying glass to measure an area on the circumferential surface covered with the rubber by means of the image processing and analyzing device. If each strand has three layered twisting structure, the sheathes in the outermost layer of each strand are removed, the circumferential surface of the first sheath 6a located in the middle of the core and the outermost sheath is observed from four directions with the magnifying glass to measure an area on the circumferential surface covered with the rubber by means of the image processing and analyzing device, thereafter the first sheath is removed, and the circumferential surface of the core is observed from four directions to measure an area on the circumferential surface covered with the rubber by means of the image processing and analyzing device. A cord without an unvulcanized rubber coating is subjected to a similar image processing to measure the surface area of each strand.
The value thus measured is substitutes in the following equations (1), (2) and (3) to give the rubber intrusion ratio (%) of the core strand, the rubber intrusion ratio (%) of the sheath strand and the rubber intrusion ratio (%) between the core strand and the sheath strand. The results are also shown in Tables 1 and 2. $rubber intrusion ratio between the core strand and the sheath strand = (the area on the circumferential surface of the core strand covered with rubber / the surface area of the circumference of the core strand) * 100$
$rubber intrusion ratio of inside of each strand (two - layered twisting structure) = (the area on the circumferential surface of the core covered with rubber / the surface area$
$of the circumference of the core) * 100$
$rubber intrusion ratio of inside of the core strand and inside of the sheath strand (three - layered twist structure) = [(the area on the circumferential surface of the first sheath / the surface area on the circumference of the first sheath) * the number of filaments in the first sheath + (the area on the circumferential surface of the core covered with the rubber / the surface area of the circumference of the core) * the number of filaments in the core] * 100 / the number of filaments in the first sheath and the core$
The difference between the sum of the strengths of the filaments constituting the cord and the strength of the cord measured by subjecting the cord isolated from the dissected test tire to a tensile testing compliant with JIS Z 2241 is calculated, and the twisting loss is computed as a ratio of the calculated difference to the sum of the strengths of the filaments constituting the cord. The results are also shown in Tables 1 and 2.
The number of twisting steps is calculated by the following equation (4). The results are also shown in Tables 1 and 2. It is noted that the number of multiple-twisting step is always 1 in the present invention. $the number of twisting steps = the number of strand - twisting steps + the number of multiple - twisting steps$
As shown in Tables 1 and 2, in comparison with the conventional examples 1-4 and the comparative examples 1-4, the inventive examples 1-8 in which inside of each strand is coated with rubber have remarkably improved rubber intrusion ratio of inside of each strand. Thus, the corrosion resistance and the effect of suppressing fretting wear are enhanced.
In order to facilitate the rubber intrusion between the core strand and the sheath strand, as can be seen from the results of the inventive example 1-8, it is effective to coat the circumferential surface of the core strand with rubber and then twist the sheath strands together.
With respect to reducing the contact pressure due to the tightening of twist between the core strand and the sheath strand, attention should be drawn to the fact that the inventive examples 2, 4, 6 and 8 in which the circumferential surface of the core strand is coated with unvulcanized rubber can greatly suppress the twisting loss as compared to the conventional examples 1-4 since the unvulcanized rubber serves as a cushion material. Further, the rubber intrusion between the core strand and the sheath strand is improved, so that it is possible to concurrently improve the ratio of exerting strength and the corrosion resistance of the entire cord. The ratio of exerting strength as used herein is defined as a ratio of actual strength of the cord with respect to a value calculated from a steel material, a strand diameter and the number of element strands.

Claims

A cord including a plurality of strands that are twisted together, each strand having a core composed of a filament or a plurality of filaments that are twisted together, and at least one layer of sheaths arranged around the core, each sheath being composed of a plurality of filaments that are twisted together, wherein at least a circumferential surface of the core of the strand is coated with unvulcanized rubber.
The cord according to claim 1, wherein the strand is produced by wrapping a plurality of filaments in an identical direction at an identical pitch.
The cord according to claim 1 or 2, wherein the cord is obtained by wrapping a plurality of sheath strands composed of a plurality of filaments around a core strand composed of a plurality of filaments.
The cord according to claim 3, wherein a circumferential surface of the core strand is coated with unvulcanized rubber.
The cord according to claim 3 or 4, wherein the cord has six sheath strands around one core strand.
A rubber-cord composite body, wherein a plurality of the cord according to any one of claims 1-5 are arranged in parallel with each other and coated with rubber.
A method of producing a cord including a plurality of strands that are twisted together, each strand having a core composed of a filament or a plurality of filaments that are twisted together, and at least one layer of sheaths around the core composed of a plurality of filaments that are twisted together, wherein at least a circumferential surface of the core of the strand is coated with unvulcanized rubber prior to form the cord.
The method according to claim 7, wherein a plurality of sheath strands are twisted around a core strand, and a circumferential surface of the core strand is coated with unvulcanized rubber.