CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of international patent application PCT/EP2011/058013, filed May 18, 2011, designating the United States and claiming priority from European application 10168647.5, filed Jul. 7, 2010, and the entire content of both applications is incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a rubber mixture, particularly for pneumatic tires, and for belts, drive-belts, and hoses.

BACKGROUND OF THE INVENTION

It is the rubber constitution of the tread that to a large extent determines the traveling properties of a tire, especially of a pneumatic tire. Similarly, the rubber mixtures which are used in drive-belts, hoses, and other belts, particularly at those places subject to high mechanical loading, are substantially responsible for stability and longevity of these rubber products. Accordingly, the requirements imposed on these rubber mixtures for pneumatic tires and for belts, drive-belts, and hoses are very exacting.

In order to improve the traveling properties, the treadâ€”for exampleâ€”of a pneumatic tire is often divided into two parts: firstly, into the upper tread part, which is in direct contact with the roadway and referred to as the cap, and secondly into the underlying lower tread part, which is also referred to as the base.

The base here has a number of functions to fulfill. The use of a base is supposed to reduce the rolling resistance of the tire, and so the mixture used must possess a low hysteresis. At the same time, the rubber mixture of the base must exhibit sufficiently high tack during the tire manufacturing operation, so that the tread remains adhering to the tire carcass. For a variety of reasons, many rubber mixtures for the cap use a relatively high amount of silica, which means in turn that the electrical conductivity of the upper tread part is only very low, or is zero. In that case it is necessary to ensure the electrical conductivity of the tread through the use of a â€œcarbon center beamâ€ , that is, of a conductive path which pervades the cap and is composed of an electrically conductive rubber mixture containing carbon blackâ€”this entails additional production cost and complexity. A further requirement is often for high stiffness, in order to improve the handling qualities. And, in addition to all of these requirements, the structural durability must be ensured.

It is known, that the requirements identified above, such as low hysteresis, sufficient tack, electrical conductivity, high stiffness, and structural durability, are in conflict with one another and that usually only an unsatisfactory compromise can be foundâ€”that is, any improvement in respect of one requirement is accompanied by a deterioration in at least one further requirement. For example, the â€œlow hysteresisâ€ requirement demands a rubber mixture with a low degree of filling and a high degree of crosslinking, but this leads to a poor electrical conductivity and a low structural durability.

There is also a conflict of objectives between the crack resistance (structural durability), the stiffness (handling), and the hysteresis (rolling resistance).

Such requirements are also found with industrial rubber products, such as belts and drive-belts.

EP 1 589 068 A1 discloses rubber mixtures for the tread base that comprise a combination of 5 to 50 phr of butadiene rubber and 50 to 95 phr of polyisoprene as a rubber component. The rubber mixture comprises an activated carbon black as its sole filler component, preferably in amounts of 55 to 75 phr. The rubber mixture has high flexibility in conjunction with high stiffness, in order thus to improve the handling qualities.

For the purpose of improving the chipping and chunking characteristics of a pneumatic tire tread, the rubber mixture described in U.S. Pat. No. 7,902,285 comprises 5 to 80 phr of a mineral oil plasticizer and 5 to 30 phr of a resin having a defined molecular weight and softening point, plus 5 to 100 phr of a defined carbon black. A rubber component which is used in high amounts here is styrene-butadiene rubber.

United States patent application publication 2011/0071245 describes a rubber mixture, particularly for the base of a tread, which is distinguished by improved heat buildup and improved abrasion characteristics. The rubber mixtures described therein contain only 20 to 40 phr of a carbon black.

SUMMARY OF THE INVENTION

The object on which the invention is based, therefore, is to provide a rubber mixture, more particularly for pneumatic tires with a cap/base tread construction, that is able to resolve the conflict of objectives between lower hysteresis and higher stiffness, while retaining a relatively high-level crack resistance, and hence to allow the use of rubber mixtures with low hysteresis, particularly for the base of a pneumatic tire, without adversely affecting the tire production operation or the other properties of the tire.

This object is achieved by means of a rubber mixture having the following constitution:

60 to 85 phr of at least one natural or synthetic polyisoprene and
15 to 40 phr of at least one butadiene rubber and/or of at least one styrene-butadiene rubber, the styrene-butadiene rubber having been solution-polymerized and possessing a glass transition temperature of less than or equal to âˆ’55Â° C., and
5 to 15 phr of at least one plasticizer oil and
15 to 75 phr of at least one silica and
2 to 10 phr of at least one tackifier resin, and
further additives.

The unit phr (parts per hundred parts of rubber by weight) used in this specification is the usual quantity unit in the rubber industry for mixture formulas. The metering of the parts by weight of the individual substances here is always based on 100 parts by weight of the total mass of all the rubbers present in the mixture.

Surprisingly it has been found that through the combination of 60 to 85 phr of a natural and/or synthetic polyisoprene and of 15 to 40 phr of a butadiene rubber and/or of a styrene-butadiene rubber, the styrene-butadiene rubber having been solution-polymerized and possessing a glass transition temperature of less than or equal to âˆ’55Â° C., and of 15 to 75 phr of a silica, rubber mixtures are made possible that have a relatively low hysteresis, especially for the base of a pneumatic tire, without any adverse effect on the tire production operation or the other tire properties. This is true not only in respect of the vehicle tread, and especially the base, but also in respect of further, internal tire components. The rubber mixtures for the further internal tire components are summarized below and, as is usual in tire technology, are also referred to as body compounds or body mixtures.

The term â€œbody mixtureâ€ essentially comprises sidewall, inner liner, apex, belt, shoulder, belt profile, squeegee, carcass, bead reinforcement, other reinforcement inserts, and/or solid tire.

The rubber mixture of the invention finds further application in the development of mixtures for drive-belts, other belts, and hoses, since there as well requirements are imposed with regard to low hysteresis, sufficient tack, electrical conductivity, and structural durability.

The rubber mixture comprises 60 to 85 phr, preferably 65 to 80 phr, of at least one natural or synthetic polyisoprene and 15 to 40 phr, preferably 10 to 35 phr, of at least one butadiene rubber and/or of at least one styrene-butadiene rubber, the styrene-butadiene rubber having been solution-polymerized and possessing a glass transition temperature, T_g, of less than or equal to âˆ’55Â° C.

The styrene-butadiene rubber can be functionalized 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. There are, however, also further functionalizations, known to the skilled person, and also referred to as modification.

The rubber mixture may further comprise 0 to 5 phr, preferably 0 to 2 phr, of a further polar or apolar rubber. The polar or apolar rubber is selected in this case from the group consisting of emulsion-polymerized styrene-butadiene rubber and/or liquid rubbers and/or halobutyl rubber and/or polynorbornene and/or isoprene-isobutylene copolymer and/or ethylene-propylene-diene rubber and/or nitrile rubber and/or chloroprene rubber and/or acrylate rubber and/or fluoro rubber and/or silicone rubber and/or polysulfide rubber and/or epichlorohydrin rubber and/or styrene-isoprene-butadiene terpolymer. More particularly, styrene-isoprene-butadiene terpolymer, butyl rubber, halobutyl rubber or ethylene-propylene-diene rubber are employed in producing industrial rubber products.

The rubber mixture of the invention comprises 15 to 75 phr, preferably 20 to 60 phr, more preferably 20 to 50 phr, of silica. The total amount of silica here is attached to the polymer matrix, in a particularly advantageous embodiment, by a coupling agent, preferably silane.

The silicas used in the tire industry are generally precipitated silicas, which are characterized in particular according to their surface area. Characterization here takes place by specification of the nitrogen surface area (BET) in accordance with DIN 66131 and DIN 66132, as a measure of the internal and external surface area of the filler, in m²/g, and of the CTAB surface area to ASTM D 3765, as a measure of the external, surface area, which is often considered to be the rubber-active surface area, in m²/g.

Used in accordance with the invention are silicas having a nitrogen surface area of between 120 and 300 m²/g, preferably between 150 and 250 m²/g, and a CTAB surface area of between 120 and 230 m²/g, preferably between 140 and 200 m²/g.

The amount of the silane that is advantageously used is 0.5 to 10 phr, preferably 1.0 to 6 phr, more preferably 1.5 to 4 phr. Silane coupling agents which may be used here are all of the silane coupling agents known to the skilled person for use in rubber mixtures. It is also possible, however, for the silica not to be attachedâ€”in other words, for no coupling agent to be used.

Particularly for the use of the rubber mixture as a base of the tread of a tire, sufficient tack on the part of the unvulcanized mixture is of great importance, so that the tread remains adhering during the production operation. For this purpose the rubber mixture must contain at least 2 to 10 phr of a tackifier resin. Tackifier resins used are natural or synthetic resins, such as hydrocarbon resins, which act as tackifiers. The hydrocarbon resins may be phenolic, aromatic or aliphatic. The tackifier resins are preferably selected from the group consisting of rosins and their esters, terpene-phenolic resins, alkyne-phenolic resins, phenolic resins, and coumarone-indene resins, with phenolic resins being especially suitable for the present invention.

The rubber mixture disclosed herein further comprises 5 to 15 phr, preferably 5 to 10 phr, of at least one plasticizer oil, the plasticizer oil preferably being a mineral oil selected from the group consisting of DAE (distilled aromatic extracts) and/or RAE (residual aromatic extract) and/or TDAE (treated destillated aromatic extracts) and/or MES (mild extracted solvents) and/or naphthenic oils. It is advantageous to add a plasticizer oil to the rubber mixture for the base of a tread, since in the finished tire, plasticizers migrate generally in accordance with the concentration gradient, and the migration can be limited by means of the stated measure. A positive influence on the rolling resistance characteristics has been observed when the plasticizer oil has a relatively low glass transition temperature (T_g). It is therefore extremely preferred to use MES, very preferred to use TDAE, and preferred to use RAE.

Within the rubber mixture there may also be 0 to 5 phr of at least one further, additional plasticizer. This further plasticizer may be a synthetic plasticizer and/or a fatty acid and/or a fatty acid derivative and/or a resin and/or a factice and/or a vegetable oil or a BTL (biomass-to-liquid) oil.

In one particularly preferred embodiment, the rubber mixture comprises 20 to 60 phr, preferably 45 to 60 phr, of at least one carbon black. It is preferred here if the carbon black possesses an iodine number to ASTM D 1510 of between 60 and 300 g/kg, preferably between 80 and 130 g/kg, and a DBP number to ASTM D 2414 of between 80 and 200 cm³/100 g, preferably between 100 and 140 cm³/100 g. The iodine number to ASTM D 1510 is also referred to as the iodine absorption number. The DBF number to ASTM D 2414 defines the specific absorption volume of a carbon black or of a light-colored filler by means of dibutyl phthalate. The use of a carbon black with these qualities has advantages in terms of the abrasion behavior of the overall tread, since small amounts of the base carbon black get into the cap rubber mixture as a result of the return process during the tire production operation. Tire tests have shown that even such small amounts mean that there is a distinct deterioration in abrasion when carbon blacks are used whose iodine and DBP numbers deviate from those specified above.

The total amount of fillers, that is, essentially the amount of silica and carbon black together, ought to be between 50 and 80 phr in order to maintain the degree of filling of the mixture at a good level and hence not to adversely affect the required properties.

The rubber mixture further comprises, preferably, 0.1 to 10 phr, more preferably 0.2 to 8 phr, very preferably 0.2 to 4 phr, of zinc oxide. It is usual to add zinc oxide as an activator, usually in combination with fatty acids (for example, stearic acid), to a rubber mixture for sulfur crosslinking with vulcanization accelerators. The sulfur is then activated for vulcanization by formation of a complex. The zinc oxide typically used in this case has a BET surface area generally of less than 10 m²/g. However, so-called nano-zinc oxide, with a BET surface area of 10 to 60 m²/g, can also be used.

The rubber mixture additionally comprises further additives. Further additives include, substantially, the crosslinking system (crosslinkers, accelerators, and retardants), ozone inhibitors, aging inhibitors, masticating assistants, and further activators.

The proportion of the total amount of further additives is 2 to 50 phr, preferably 5 to 20 phr.

The rubber mixture is vulcanized in the presence of sulfur or sulfur donors; certain sulfur donors may also act as vulcanization accelerators. Sulfur or sulfur donors is or are added to the rubber mixture in the last mixing step, in the amounts customary to the skilled person (0.4 to 4 phr; sulfur preferably in amounts of 1.5 to 2.5 phr). To control the required time and/or temperature of the vulcanization and in order to improve the properties of the vulcanizate, the rubber mixture may comprise vulcanization modifiers such as vulcanization accelerators, vulcanization retardants, which in accordance with the invention are present in the above-described additives, and vulcanization activators, as described above.

The rubber mixture disclosed herein is produced by the method customary in the rubber industry, which involves first, in one or more mixing stages, preparing a basic mixture including all of the ingredients apart from the vulcanizing system (sulfur and vulcanization modifiers). Addition of the vulcanizing system in a final mixing stage produces the completed mixture. The completed mixture is processed further by an extrusion operation, for example, and brought into the appropriate form.

A further object of this disclosure is that of using the above-described rubber mixture for producing pneumatic tires, more particular for producing the base of the tread of a tire and/or a body mixture of a tire, and for producing drive-belts, other belts, and hoses.

For use in pneumatic tires, the mixture is preferably brought into the form of a tread and is applied in a known manner during the production of the green tire. Alternatively the tread, in the form of a narrow strip of rubber mixture, can be wound onto a green tire. If the tread is in two parts, as described at the outset, then the rubber mixture is employed preferably as the mixture for the base.

Production of the rubber mixture disclosed herein for use as a body mixture in vehicle tires takes place as already described for the tread. The difference lies in the shaping after the extrusion operation. The resultant forms of the rubber mixture disclosed herein for one or more different body mixtures are then used to construct a green tire. For the use of the rubber mixture in drive-belts and other belts, more particularly in conveyor belts, the extruded mixture is brought into the appropriate form, and, during this procedure or afterward, is frequently provided with strength elements, examples being synthetic fibers or steel cords. This usually produces a multilayer construction, consisting of one and/or a plurality of layers of rubber mixture, one and/or a plurality of layers of the same and/or different strength elements, and one and/or a plurality of further layers of the same and/or of another rubber mixture. A sufficient tack is also relevant here, for example, so that a firmly adhering assembly can be formed between the individual layers or, where appropriate, between the rubber mixture and the strength elements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The invention will now be elucidated in more detail by means of comparative examples and working examples, which are summarized in Table 1. Inventive mixtures begin with â€œ1â€ , while the comparative mixtures are labeled with â€œCâ€ .

For all of the mixing examples present in the table, the quantity figures indicated are parts by weight, and are based on 100 parts by weight of total rubber (phr).

Mixtures were produced under usual conditions in two stages in a laboratory tangential mixer. Test specimens were produced from all of the mixtures by vulcanization, and these test specimens were used for determining physical properties typical for the rubber industry. The test methods employed for the above-described tests on test specimens were as follows:

- Shore A hardness at room temperature (RT) and 70Â° C. in accordance with DIN 53 505
- Rebound resilience at room temperature (RT) and 70Â° C. in accordance with DIN 53 512
- Stress values (modulus) at 200% elongation at room temperature in accordance with DIN 53 504
- Tensile strength at room temperature (RT) in accordance with DIN 53 504
- Graves tear resistance at room temperature (RT) in accordance with DIN 53 515
- Energy at break at room temperature (RT) in accordance with DIN 53 448
- Elongation at break at room temperature (RT) in accordance with DIN 53 504
- Dynamic storage modulus Eâ€² at 55Â° C. in accordance with DIN 53 513 at 8% elongation
- Volume resistance in accordance with standard DIN IEC 93

TABLE 1UnitC1C2C3C4C5I1I2I3I4I5I6ConstituentsPolyisoprene^aphr10040707070807060707070BR^bphrâ€”60â€”3030203040â€”30â€”SSBR^cphrâ€”â€”â€”â€”â€”â€”â€”â€”30â€”30SSBR^dphrâ€”â€”30â€”â€”â€”â€”â€”â€”â€”â€”Silica^ephr202020â€”10202020205050Silane^fphr1.51.51.5â€”0.751.51.51.51.53.73.7Carbon black, N339phr5050506555505050502020Plasticizer oil^gphr55555555577Tackifier resin^hphr44444444444Further additivesⁱphr7777777777.57.5Acclerator^jphr3333333333.33.3Sulfurphr2222222222.12.1PropertyHardness RTShA6872697170707071706968Hardness 70Â° C.ShA6367646665666666656565Rebound RT%4449414546464647474848Rebound 70Â° C.%5862585861606161616365Modulus 200%MPa109.99.811.410.210.11010.110.28.59.6Eâ€² 55Â° C., 8%MPa6.16.56.67.46.66.36.56.46.55.95.9Tensile strengthMPa2219211920212120222121Elongation at break%420350390320370410390380400480480Energy at breakMJ/m3.93.33.92.73.13.73.73.53.64.23.3Tear resistanceN/mm7052695767696561656755Volume resistanceÎ© * cm3E+044E+045E+043E+022E+032E+049E+031E+042E+041E+143E+14^aSMR 10^bHigh-cis polybutadiene, cis fraction â‰§ 95% by weight^cSSBR,NS612 from Nippon Zeon, T_gâˆ’65Â° C., OH-group-modified^dSSBR NS210 from Nippon Zeon, T_gâˆ’45Â° C.^eVN3 from Evonik^fTESPD^gMES^hPhenolic resin, Koresinâ€‰Â® from BASFⁱAging inhibitors, waxes, optionally DPG^jsulfenamide accelerator, CBSZnO, stearic acid, and optionally further processing assistants were used in the customary amounts.

Investigations have shown that, for the rubber mixture for the base of a pneumatic tire tread, the rebound value at 70Â° C. is correlated with the rolling resistance characteristics, and the hardness at RT is correlated with the handling. Established parameters for the structural durability are the energy at break and the tear resistance, as physical mixture parameters for characterizing the crack resistance. From Table 1 it can be seen that there is a correlation between the energy at break, the stress value at 200% elongation, and the value for the rebound at RT. The higher the value for the rebound at RT or the stress value at 200% elongation, the lower the value for the energy at break. The object of the present disclosure can be summarized in abbreviated form as the necessity to find a rubber mixture having a low modulus but a high hardness and rebound.

Surprisingly, and as evident from Table 1, the use of BR and/or of an above-defined SSBR has proven advantageous. This was surprising because the use of BR typically has an adverse effect on the tensile properties. Amazingly, from the mixture C1, which contains only polyisoprene, there are virtually no advantages apparent in respect of the tensile properties, and in fact there are disadvantages in rebound and in hardness at RT. The use of BR and/or of an above-defined SSBR results, surprisingly, in a shifting of this conflict, since in the corresponding mixtures there are increases in hardness at RT and in rebound, without at the same time a rise in the modulus at 200% elongation. It has emerged that there is an optimum range for BR and/or for an above-defined SSBR at between 15 and 40 phr. Below this range, the effect is too small, and above it the crack resistance (see tear resistance value) becomes too low (mixtures C1, C2, and I1 to I3). SSBRs with a low T_g, and preferably in modified form, exhibit an effect similar to that of BR (mixtures C3 and I2, I4, I5, and I6). A further measure for lifting this conflict of objectives to a higher level is the (partial) replacement of carbon black by silica, which is preferably attached by a coupling agent. This (partial) replacement produces a lowering in the modulus in conjunction with consistent hardness and, surprisingly, reduced hysteresis (mixtures C4 C5 and I2, I5, and I6).

In order not to go below the electrical conductivity prescribed for the tire according to Guideline 110 of the Wirtschaftsverband der deutschen Kautschukindustrie e.V. [German rubber industry association], the electrical volume resistance, particularly for the base mixture, must not be below 1E+08 Î©*cm, more preferably 1E+05 Î©*cm. From Table 1 it is apparent that the amount of carbon black should not, therefore, be below 45 phr (mixtures I5 and I6). Otherwise, the required electrical conductivity must be ensured by means of other measures or components.

In summary, it can be ascertained from Table 1 that a rubber mixture of the invention results in a significantly improved rubber mixture in terms of low hysteresis with high stiffness and good tensile strength, as is required especially for the rubber-mixture for the base of a two-part tread.

It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.