JP7473797B2 - Rubber composition for tires - Google Patents

Description

The present invention relates to a rubber composition for tires that has improved heat aging resistance and water-resistant adhesion.

In recent years, rubber materials with excellent heat aging resistance have been used as tire components to improve the long-term durability of tires. For example, Patent Document 1 proposes that a rubber composition containing a cyclic polysulfide as a vulcanizing agent improves heat aging resistance and improves mechanical properties such as tensile elongation at break. Here, rubber materials that require adhesion to metals may contain a large amount of sulfur, and in such cases, physical property changes due to aging tend to be significant. Therefore, it is thought that heat aging resistance can be improved by adding a cyclic polysulfide as a vulcanizing agent to a rubber material that requires adhesion to metals.

However, rubber compositions containing cyclic polysulfides have the problem that they do not provide sufficient adhesion to metals, particularly water-resistant adhesion. For this reason, it has been difficult to compound cyclic polysulfides in components that require adhesion to metal cords, such as belt coats, carcass coats, belt edge cushions, belt cushions, undertreads, and tie rubbers, or components adjacent to these components. Therefore, there is a demand to improve the heat aging resistance and heat aging resistance of tire rubber compositions that coat metal cords or that are adjacent to the rubber that coats them, beyond conventional levels.

JP 2006-193679 A

The object of the present invention is to provide a rubber composition for tires that has improved heat aging resistance and water-resistant adhesion beyond conventional levels.

（式（Ｉ）中、Ｒは置換もしくは非置換の炭素数２～２０のアルキレン基、置換もしくは非置換の炭素数２～２０のオキシアルキレン基、または芳香族基を含むアルキレン基を示し、ｘは平均２～６の数、ｎは１～２０の整数を表す。） The rubber composition for a tire of the present invention that achieves the above-mentioned object contains 100 parts by mass of a sulfur-vulcanizable rubber containing 50% by mass or more of natural rubber, 0.05 to 1 part by mass of an organic cobalt acid or cobalt complex (expressed as cobalt), 1 to 8 parts by mass of a phenol derivative, 1 to 15 parts by mass of a cyclic polysulfide represented by the following formula (I), sulfur, and optionally a polyvalent methylolmelamine, wherein when the blending amount of the sulfur is a parts by mass, the blending amount of the cyclic polysulfide is b parts by mass, the blending amount of the phenol derivative is c parts by mass, and the blending amount of the polyvalent methylolmelamine is d parts by mass, the following relationship formulas (1) to (3) are satisfied, and the phenol derivative is a resorcinol resin, a cresol resin, or parahydroxybenzoic acid .
5≦a+(2b/3)≦11 (1)
d≦c/4 (2)

(In formula (I), R represents a substituted or unsubstituted alkylene group having 2 to 20 carbon atoms, a substituted or unsubstituted oxyalkylene group having 2 to 20 carbon atoms, or an alkylene group containing an aromatic group, x represents an average number of 2 to 6, and n represents an integer of 1 to 20.)

The rubber composition for tires of the present invention contains 100 parts by mass of a sulfur-vulcanizable rubber containing 50% by mass or more of natural rubber, 0.05 to 1 part by mass of an organic cobalt acid or cobalt complex (cobalt), 1 to 8 parts by mass of a phenol derivative, 1 to 15 parts by mass of a cyclic polysulfide represented by the above formula (I), sulfur, and optionally polymethylolmelamine, wherein the blending amount a parts by mass of the sulfur, the blending amount b parts by mass of the cyclic polysulfide, the blending amount c parts by mass of the phenol derivative, and the blending amount d parts by mass of the polymethylolmelamine satisfy the following relationship:
5≦a+(2b/3)≦11 (1)
d≦c/4 (2)
Since the above requirements are satisfied, it is possible to improve the heat aging resistance and the water-resistant adhesion to the metal cord to levels higher than those of the conventional level.

The phenol derivative may be resorcinol resin, cresol resin, or parahydroxybenzoic acid, which provides excellent water-resistant adhesion to the metal cord.

A tire formed with at least one of the following rubber compositions using the rubber composition: belt coat, carcass coat, belt edge cushion, belt cushion, undertread, and tie rubber has excellent heat aging resistance and excellent water-resistant adhesion to the metal cord, improving tire durability beyond conventional levels.

In the present invention, the rubber composition for tires contains a sulfur-vulcanizable rubber as a rubber component. The sulfur-vulcanizable rubber contains 50% by mass or more, preferably 80 to 100% by mass, of natural rubber. By containing 50% by mass or more of natural rubber, it is possible to impart metal adhesion to the rubber composition and increase the elastic modulus. The rubber composition for tires can contain sulfur-vulcanizable rubber other than natural rubber, such as diene rubbers such as polyisoprene rubber, polybutadiene rubber, styrene-butadiene rubber, and chloroprene rubber, and partially hydrogenated products thereof, which can be contained alone or as an arbitrary blend together with natural rubber.

（式（Ｉ）中、Ｒは置換もしくは非置換の炭素数２～２０のアルキレン基、置換もしくは非置換の炭素数２～２０のオキシアルキレン基、または芳香族基を含むアルキレン基を示し、ｘは平均２～６の数、ｎは１～２０の整数を表す。） The rubber composition for tires can improve heat aging resistance and mechanical properties of the rubber composition by blending the cyclic polysulfide represented by the following formula (I) and sulfur, thereby improving tire durability and abrasion resistance to levels higher than those of conventional rubber compositions.

(In formula (I), R represents a substituted or unsubstituted alkylene group having 2 to 20 carbon atoms, a substituted or unsubstituted oxyalkylene group having 2 to 20 carbon atoms, or an alkylene group containing an aromatic group, x represents an average number of 2 to 6, and n represents an integer of 1 to 20.)

In formula (I), R represents a substituted or unsubstituted alkylene group having 2 to 20 carbon atoms, a substituted or unsubstituted oxyalkylene group having 2 to 20 carbon atoms, or an alkylene group containing an aromatic group, and preferably represents a substituted or unsubstituted alkylene group having 4 to 8 carbon atoms, a substituted or unsubstituted oxyalkylene group having 4 to 8 carbon atoms, or an alkylene group containing an aromatic group.

Examples of alkylene groups include linear or branched alkylene groups such as ethylene, propylene, butylene, pentylene, hexylene, octylene, nonylene, decylene, and 1,2-propylene, and these alkylene groups may be substituted with a substituent such as a phenyl group or a benzyl group.

Examples of the oxyalkylene group include oxyalkylene groups optionally bonded with groups (CH ₂ CH ₂ O) _p and (CH ₂ ) _q (wherein p is an integer of 1 to 5, and q is an integer of 0 to 2). Preferred oxyalkylene groups are:
_{-CH2CH2OCH2CH2- , - ( CH2CH2O ) 2CH2CH2- , ​}
( _-CH2CH2O ) _{3CH - CH2-} , - _{( CH2CH2O ) 4CH2CH2- ,}
- _{( CH2CH2O ) 5CH2CH2- ,} - _{( CH2CH2O} ) _{2CH2- ,}
An example is --CH ₂ CH ₂ OCH ₂ OCH ₂ CH ₂ --.

Examples of substituents for the alkylene group and oxyalkylene group include a phenyl group, a benzyl group, a methyl group, an epoxy group, an isocyanate group, a vinyl group, and a silyl group.

In formula (I), x is an average number of 2 to 6, preferably an integer of 3 to 5, and more preferably an integer of 3.5 to 4.5. Furthermore, n is an integer of 1 to 20, preferably an integer of 1 to 15, more preferably an integer of 1 to 10, and even more preferably an integer of 1 to 5. Such cyclic polysulfides can be produced by ordinary methods, such as the production method described in JP-A-2007-92086.

The cyclic polysulfide represented by formula (I) is compounded in an amount of 1 to 15 parts by mass, preferably 3 to 10 parts by mass, and more preferably 4 to 7 parts by mass, per 100 parts by mass of the sulfur-vulcanizable rubber. By making the compounding amount b of the cyclic polysulfide 1 part by mass or more, it is possible to improve heat resistance. Also, by making the compounding amount b of the cyclic polysulfide 15 parts by mass or less, it is possible to impart water-resistant adhesion to the rubber composition.

The rubber composition for tires contains a cyclic polysulfide represented by formula (I) and sulfur. When the amount of sulfur is a parts by mass and the amount of cyclic polysulfide is b parts by mass relative to 100 parts by mass of the sulfur-vulcanizable rubber, the following relational formula (1) is satisfied.
5≦a+(2b/3)≦11 (1)
The amount of sulfur (a parts by mass) is determined to satisfy the relational expression (1) with the amount of cyclic polysulfide (b parts by mass). By making the amount [a+(2b/3)] 5 or more, the rubber composition can be given initial adhesion and water-resistant adhesion. By making it 11 or less, hardening due to heat aging can be suppressed. The value [a+(2b/3)] is preferably 6 or more and 10 or less, more preferably 7 or more and 9 or less.

In the present invention, the rubber composition for tires can ensure adhesion and water-resistant adhesion to metal cords by blending an organic cobalt acid or cobalt complex. The organic cobalt acid or cobalt complex is blended in an amount of 0.05 to 1 part by mass, preferably 0.1 to 0.5 parts by mass, and more preferably 0.1 to 0.2 parts by mass, of cobalt per 100 parts by mass of sulfur-vulcanizable rubber. By using an organic cobalt acid or cobalt complex in an amount of 0.05 part by mass or more in terms of cobalt, adhesion and water-resistant adhesion between the rubber composition for tires and metal cords can be ensured. Furthermore, by using cobalt in an amount of 1 part by mass or less, adhesion and water-resistant adhesion can be ensured.

The organic acid of the cobalt organic acid may be, for example, a saturated or unsaturated fatty acid having 6 to 24 carbon atoms, and may be either linear or branched. The organic acid may be, for example, stearic acid, oleic acid, lauric acid, linoleic acid, neodecanoic acid, rosin acid, palmitic acid, etc. The organic acid of cobalt or a cobalt complex may be, for example, cobalt stearate, cobalt palmitate, cobalt octenoate, cobalt naphthenate, cobalt neodecanoate, cobalt trineodecanoate borate, cobalt rosinate, etc.

In the present invention, the rubber composition for tires can further improve adhesion to metal cords and water-resistant adhesion by blending a phenol derivative. The phenol derivative is blended in an amount of 1 to 8 parts by mass, preferably 2 to 6 parts by mass, and more preferably 2 to 4 parts by mass, per 100 parts by mass of sulfur-vulcanizable rubber. By using 1 part by mass or more of the phenol derivative, the initial adhesion to metal cords and water-resistant adhesion can be further improved. Also, by using 8 parts by mass or less, both water-resistant adhesion and heat aging resistance can be achieved.

Examples of phenol derivatives include phenol resin, resorcinol resin, cresol resin, cashew-modified phenol resin, and parahydroxybenzoic acid. Resorcinol resin, cresol resin, and parahydroxybenzoic acid are preferred.

The rubber composition for tires may optionally contain polymethylolmelamine. By containing polymethylolmelamine, the rubber hardness of the rubber composition can be increased. The amount of polymethylolmelamine is determined in relation to the amount of phenol derivative. When the amount of phenol derivative is c parts by mass and the amount of polymethylolmelamine is d parts by mass relative to 100 parts by mass of sulfur vulcanizable rubber, the following relational formula (2) is satisfied.
d≦c/4 (2)
By making the amount of polyvalent methylol melamine d parts by mass equal to or less than one-fourth of the amount of phenol derivative c parts by mass, it is possible to improve heat aging resistance and water-resistant adhesion to metal cords. The amount of polyvalent methylol melamine d is preferably c/8 or less, more preferably c/10 or less. The amount of polyvalent methylol melamine d may be 0. In other words, it is not necessary to have a curing agent for a phenol derivative such as polyvalent methylol melamine.

The rubber composition for tires may contain reinforcing fillers such as carbon black, silica, clay, aluminum hydroxide, and calcium carbonate. The rubber composition for tires may contain various compounding agents that are generally used in rubber compositions, such as vulcanization or crosslinking agents, vulcanization accelerators, antioxidants, plasticizers, processing aids, liquid polymers, thermosetting resins, and thermoplastic resins. These compounding agents can be kneaded in a general manner to form a rubber composition, which can then be used for vulcanization or crosslinking. The compounding amounts of these compounding agents may be conventional general compounding amounts, as long as they do not violate the object of the present invention. The rubber composition may be produced by mixing the above-mentioned components using a known rubber kneading machine, such as a Banbury mixer, kneader, roll, etc.

The rubber composition for tires of the present invention can be suitably used for belt coats, carcass coats, belt edge cushions, belt cushions, undertreads, tie rubbers, etc. of pneumatic tires. Tires using the rubber composition of the present invention have excellent heat aging resistance and excellent water-resistant adhesion to metal cords, so tire durability can be improved beyond conventional levels.

The present invention will be further explained below with reference to examples, but the scope of the present invention is not limited to these examples.

25 types of rubber compositions (Examples 1-11, Standard Example, Comparative Examples 1-13) were prepared by mixing the ingredients except for sulfur, vulcanization accelerator, polymethylolmelamine, and cyclic polysulfide in a 1.8 L closed mixer at 160°C for 5 minutes, releasing the master batch, adding sulfur, vulcanization accelerator, polymethylolmelamine, and cyclic polysulfide, and kneading with an open roll, with the common compounding ingredients in Table 4 expressed in parts by mass relative to 100 parts by mass of natural rubber shown in Tables 1-3. The heat aging resistance and water-resistant adhesion of the obtained rubber compositions were evaluated by the following methods.

Heat aging resistance The rubber composition was vulcanized at 160°C for 20 minutes using a 2mm thick mold, and the resulting vulcanized rubber sheet was punched out with a JIS No. 3 dumbbell to obtain a sample. The sample was heat aged for 5 days in a gear oven set at 80°C, and then the breaking elongation was measured together with an unaged sample using a tensile tester according to JIS K6251. The ratio of the breaking elongation (Eb) of the unaged sample to the breaking elongation (Ea) after aging was taken, and Ea/Eb was used as an index of heat aging resistance. The index of the standard example was expressed as 100. When the index was 105 or less, the heat aging resistance was considered to be poor.

Water-resistant adhesion: Test was performed according to ASTM D2229. A 3+9+15 brass-plated wire was embedded in the rubber composition, and a specific mold was used to vulcanize the rubber at 160°C for 20 minutes to obtain a sample. The sample was placed in a constant temperature and humidity chamber set at 70°C and 96% humidity for 2 weeks, after which a pull-out test was performed, and the coverage of the rubber on the wire surface was visually determined and used as an index. The value was expressed as an index with the index of the standard example being 100. An index of 95 or less was deemed to indicate poor water-resistant adhesion.

The types of raw materials used in Tables 1 to 3 are shown below.
-NR: Natural rubber, RSS#3
Organic acid cobalt: neodecanoic acid cobalt borate, cobalt content 22% by mass, DICNATE NBC-II manufactured by DIC Corporation
- Sulfur: FLEXSYS Crystex HS OT20
Cyclic polysulfide: THIOPLAST CPS 200 manufactured by Akzo Nobel, in which R is --CH ₂ CH ₂ OCH ₂ OCH ₂ CH ₂ --, x (average) is 4, and n is 10.
-Resorcinol resin: TECHNIC B18S manufactured by TECHNO WAXCHEM
- Cresol resin: Sumikanol 610 manufactured by Taoka Chemical Co., Ltd.
- Parahydroxybenzoic acid: Parahydroxybenzoic acid manufactured by Ueno Pharmaceutical Co., Ltd. - HMMM: Hexamethoxymethylmelamine, Cyrez 964RPC manufactured by Allnex

The types of raw materials used in Table 4 are shown below.
- Carbon black: Tokai Carbon Co., Ltd., Seast 300
- Anti-aging agent: LANXESS VULKANOX 4020
- Zinc oxide: 3 types of zinc oxide manufactured by Seido Chemical Industry Co., Ltd. - Vulcanization accelerator: Noccela DZ-G manufactured by Ouchi Shinko Chemical Co., Ltd.

As is clear from Tables 1 to 3, it was confirmed that the rubber compositions for tires of Examples 1 to 11 had improved heat aging resistance and water-resistant adhesion beyond conventional levels.

As is clear from Table 1, the rubber compositions of Comparative Examples 1 and 2 have poor water-resistant adhesion because the blended amount of the phenol derivative is less than 1 part by mass.
The rubber composition of Comparative Example 3 has poor heat aging resistance because the blended amount of the phenol derivative exceeds 8 parts by mass.

As is clear from Table 2, the rubber compositions of Comparative Examples 4 and 5, in which the amount of cyclic polysulfide blended was less than 1 part by mass, showed no improvement in heat aging resistance.
The rubber compositions of Comparative Examples 6 and 7, which do not contain sulfur, are inferior in water-resistant adhesion.
In the rubber compositions of Comparative Examples 8 and 9, the amount d of polymethylolmelamine exceeds 1/4 of the amount c of the phenol derivative, and therefore the heat aging resistance is inferior to that in the case where the amount does not exceed this value.

As is clear from Table 3, the rubber compositions of Comparative Examples 10 to 13 have poor heat resistance because they do not contain cyclic polysulfide.

Claims (2)

A rubber composition for tires comprising 100 parts by mass of a sulfur-vulcanizable rubber containing 50% by mass or more of natural rubber, 0.05 to 1 part by mass of an organic cobalt acid or cobalt complex (expressed as cobalt), 1 to 8 parts by mass of a phenol derivative, 1 to 15 parts by mass of a cyclic polysulfide represented by the following formula (I), sulfur, and optionally a polyvalent methylolmelamine, wherein the following relationship formulas (1) and (2) are satisfied when the blending amount of the sulfur is a parts by mass, the blending amount of the cyclic polysulfide is b parts by mass, the blending amount of the phenol derivative is c parts by mass, and the blending amount of the polyvalent methylolmelamine is d parts by mass, and the phenol derivative is a resorcinol resin, a cresol resin, or a parahydroxybenzoic acid .
5≦a+(2b/3)≦11 (1)
d≦c/4 (2)

(In formula (I), R represents a substituted or unsubstituted alkylene group having 2 to 20 carbon atoms, a substituted or unsubstituted oxyalkylene group having 2 to 20 carbon atoms, or an alkylene group containing an aromatic group, x represents an average number of 2 to 6, and n represents an integer of 1 to 20.) A tire comprising at least one selected from a belt coat, a carcass coat, a belt edge cushion, a belt cushion, an undertread, and a tie rubber formed with the rubber composition according to claim 1 .