Composition and tire with tread containing calcium carbonate

The invention relates to a rubber composition containing relatively low levels of carbon black and/or silica reinforcement together with a particulate calcium carbonate and selected modifiers. The invention particularly relates to a tire having a component, particularly a tread, of such composition.

FIELD
 The invention relates to a rubber composition containing relatively low
 levels of carbon black and/or silica reinforcement together with a
 particulate calcium carbonate and selected modifier(s). The invention
 particularly relates to a tire with a component thereof, particularly a
 tire tread.
 BACKGROUND
 Rubber compositions are typically used for tire treads which may be
 optimized for various rubber composition properties to promote one or more
 of three tire properties; namely, traction, rolling resistance and
 treadwear.
 In this regard, typically desirable physical properties for tire tread
 rubber compositions may include, for example, hysteresis, hardness and
 modulus. Some properties are normally indicated by their rebound
 properties, tangent delta (Tan. delta) at 0.degree. C., and abrasion
 resistance. Hysteresis is conventionally related to Hot Rebound values.
 Such physical properties are well known to those having skill in the
 rubber compounding art and, in general, are considered to be somewhat
 predictive of tire tread performance.
 More specifically, it is sometimes desired that a tire have a relatively
 low rolling resistance to enhance a vehicle's fuel economy. A lower
 hysteresis, usually evidenced by a higher hot rebound value, of a cured,
 or vulcanized, rubber composition is predictive of reduced heat build-up
 for the rubber composition and, therefore, of its utility for a relatively
 low rolling rubber composition for use as a tire tread.
 It is often an accepted practice to reduce particulate reinforcement of a
 rubber composition to a relatively low level to reduce its hysteresis and
 thus, a predictive reduction in rolling resistance for a tire tread
 application. For example, where it might be desired to reduce a tire's
 rolling resistance by reducing hysteresis increasing hot rebound values of
 a rubber composition for a tire tread application, particulate
 reinforcement such as carbon black and/or silica might conventionally be
 reduced from a somewhat normal range of about 55 to about 80 phr to a
 reduced amount of such reinforcement in a range of about 30 to about 50,
 and particularly about 30 to about 45, phr. Such reduction in particulate
 reinforcement for a tire tread rubber for reduction in a tire's rolling
 resistance is well known to those having skill in such art.
 However, commensurate with rolling resistance of the tire tread composition
 reduction by reducing reinforcing filler content, with other aspects of
 the rubber composition being essentially unchanged, the tire tread wear
 resistance often decreases, as may be evidenced by an increase in the
 tire's treadwear, and there is usually a degree of loss in tire tread
 traction. In one aspect, a tire's increase in treadwear can sometimes be
 predicted or somewhat correlated, reduction in the cured rubber
 composition's abrasion resistance.
 Accordingly, it is desired herein to provide a tire tread rubber
 composition with reduced hysteresis (increased hot rebound values) while
 substantially maintaining an acceptable abrasion resistance, and usually
 an acceptable tire tread traction.
 It is desired to provide such rubber composition with low levels of carbon
 black and/or silica reinforcement.
 It is recognized herein that calcium carbonate has sometimes been used as a
 relatively low cost filler and extender for various polyolefin resins and
 some elastomers. However, it is believed herein that it has not been used
 in tire treads with a relatively low carbon black content together with
 specified modifier(s).
 In the description of this invention, the term "phr," where used herein,
 and according to conventional practice, refers to "parts of a respective
 material per 100 parts by weight of rubber or elastomer".
 In the description of this invention, the terms "rubber" and "elastomer,"
 if used herein, may be used interchangeably, unless otherwise prescribed.
 The terms "rubber composition," "compounded rubber" and "rubber compound,"
 if used herein, are used interchangeably to refer to "rubber which has
 been blended or mixed with various ingredients and materials" and such
 terms are well known to those having skill in the rubber mixing or rubber
 compounding art.
 SUMMARY AND DESCRIPTION OF THE INVENTION
 In accordance with this invention, a rubber composition is provided, and
 particularly a tire with a component of such composition, including a tire
 tread, which comprises (A) 100 parts by weight of elastomer consisting
 essentially of (1) at least one diene-based elastomer, or (2) a
 combination of diene-based elastomer and an epoxidized diene-based
 elastomer, preferably as a cis 1,4-polyisoprene natural rubber, (B) about
 15 to about 45, alternatively about 25 to about 40, phr of particulate
 reinforcement selected from carbon black and/or precipitated silica, said
 silica having silanol groups on the surface thereof, (C) about 5 to about
 50, alternatively about 10 to about 30, phr of particulate calcium
 carbonate and (D) at least one modifier comprised of at least one of
 dithiodipropionic acid, nicotinamide and bis-3(trialkoxysilylalkyl)
 polysulfide having an average of from 2.1 to about 4 sulfur atoms in its
 polysulfide bridge and where such alkyl groups contain from two to four
 carbon atoms.
 In one aspect of the invention, wherein said elastomers are about (a) 5 to
 about 30 phr of epoxidized diene-based rubber, preferably epoxidized
 natural cis 1,4-polyisoprene rubber, containing about 20 to about 50
 percent epoxidization and (b) about 95 to about 70 phr of said diene based
 rubber.
 In one aspect, the said particulate reinforcement for the rubber
 composition may be composed of about 15 to about 30 phr of such carbon
 black without precipitated silica or about 13 to about 35 phr of
 precipitated silica and about 2 to about 20 phr of carbon black.
 For the purposes of this invention, a significant aspect is the relatively
 low concentration of carbon black reinforcement in combination with the
 use of calcium carbonate and modifiers).
 The modifier may be dithiodipropionic acid.
 The modifier may be nicotinamide.
 The modifier may be a bis-3(trialkoxysilylalkyl) polysulfide having from 2
 to about 8, with an average of about 2.1 to about 4, sulfur atoms in its
 polysulfidic bridge. Usually an average of from 2.1 to about 2.5 or from
 about 3.2 to about 4 sulfur atoms are preferred for the polysulfidic
 bridge. Usually the trialkoxy groups are selected from trimethoxy and
 triethoxy groups and the remaining alkyl groups are selected from ethyl,
 propyl and butyl groups. Thus, such modifier might, for example, be
 selected from bis-1(trimethoxysilylpropyl) polysulfide and
 bis-3(triethoxysilylpropyl) polysulfide.
 While it is recognized that such modifiers might also sometimes be known as
 silica couplers having a moiety reactive with silanol groups contained on
 the surface of the silica and another moiety, usually a polysulfide,
 interactive with one or more of said diene-based elastomers, in this case
 it appears that they serve to interact with the calcium carbonates.
 Usually the modifier is present as a weight ratio of modifier to calcium
 carbonate in a range of about 1/100 to about 1/1, alternatively in a range
 of about 1/50 to about 1/5 or in a more limited range of about 1/20 to
 about 1/10.
 Other forms of silica, including fumed silica which is different from
 precipitated silica, are not preferred.
 Usually the calcium carbonate has a particle size average diameter in a
 range of about 50 to about 14,000 nanometers (nm).
 In practice, the surface of the particulate calcium carbonate can be
 coated, if desired, to aid in its dispersion in a rubber composition. Such
 coating may be, for example, in a form of a polyolefinic material,
 particularly a saturated polyolefinic material.
 FURTHER DETAILED DESCRIPTION
 It is considered that this invention is particularly applicable where it is
 desired to endeavor to minimize tire tread rolling resistance and, namely
 to provide higher hot rebound values for a tire tread elastomer
 composition, but where it is also desired to substantially retain rubber
 composition physical properties such as, for example, DIN abrasion which
 may relate to treadwear, stiffness, tensile strength and modulus and/or
 tread traction.
 In the practice of this invention, it is believed to be a very substantial
 departure from past practice for rubber tire treads to utilize relatively
 low levels of reinforcing pigments such as carbon black and/or silica and
 to correspondingly use calcium carbonate as a relatively non-reinforcing
 filler, together with modifier(s), to increase the overall filler level to
 a range of about 30 to about 65 phr for the total amount of the carbon
 black and/or silica, as the case may be, and calcium carbonate, with the
 amount of carbon black and optional silica being restricted to a maximum
 of about 45 phr.
 The carbon blacks used may be conventional rubber reinforcing carbon
 blacks. Such carbon blacks may, for example have a DBP (dibutylphthalate)
 adsorption value in a range of about 70 to about 150, usually about 80 to
 about 130, cc/100 gm and a corresponding Iodine adsorption value in a
 range of about 40 to about 140, usually about 60 to about 125, g/kg.
 Representative of various carbon blacks, for example, that fall within the
 above ranges include, for example, N121, N220, N234. The aforesaid N121
 and N220 carbon blacks reportedly have a DBP value of about 130 and 114,
 respectively, and an Iodine adsorption value of about 120 and 121,
 respectively. If desired, a combination of high reinforcing and relatively
 low reinforcing carbon blacks might be used such as, for example, a
 combination of N110 and/or N220 high reinforcing carbon black with N550
 and/or N660 relatively low reinforcing carbon black wherein about 60 to
 about 80 weight percent of the carbon black is the high reinforcing carbon
 black.
 It is one important aspect of this invention that the addition of the
 calcium carbonate filler, and associated modifier(s), might be used
 exclusive of, or in the absence of silica or possibility only a relatively
 small amount of silica such as, for example about 10 to about 15 or 20 phr
 of silica to endeavor to achieve a reduction of rebound values for a
 rubber composition intended for use as a tire tread (prediction of less
 rolling resistance), while still maintaining a satisfactory DIN abrasion
 value for the cured rubber composition, as compared to using relatively
 low levels of carbon black alone for the rubber reinforcement.
 While it has been contemplated that addition of substantial and, therefore,
 quantitative amounts of precipitated silica, together with a silica
 coupler, as compared to use of a carbon black reinforcement, to a rubber
 composition may enhance, or promote, an increase in rebound values and,
 therefore, hysteresis which is predictive of a reduction in rolling
 resistance for a tire tread, the significance of this invention is that,
 for many rubber compositions contemplated for use as a tire tread, the
 quantitative use of silica reinforcement may not be necessary where
 relatively low tire tread rolling resistance is a significant
 consideration for the tire tread.
 This is accomplished by replacing at least a portion of the silica with
 calcium carbonate but requiring one of more of said modifiers to also be
 present.
 Thus a reduced amount of silica content in the tread rubber composition
 might be used and still achieve the rubber composition's rebound and DIN
 abrasion values.
 A significance of this discovery, at least in one sense, is even greater
 when taking into consideration that (i) the silica is considerably more
 expensive than the carbon black and calcium carbonate and, further, that
 (ii) considerably more mixing, and thus greater rubber processing, time is
 normally required when using relatively high concentrations of silica
 reinforcement for the tire tread rubber composition.
 Thus, it is contemplated for this invention that use of the calcium
 carbonate, together with the modifier(s) may enhance the rubber
 composition processing, namely by reducing the mixing timer, as compared
 to a rubber composition containing using a silica concentration of 60 phr
 or greater.
 It is understood that the DBP (dibutylphthalate) adsorption values may be
 determined by ASTM test D2414 and the Iodine values may be determined by
 ASTM test D1510.
 For example, the Iodine value, or number, for carbon black is a measure of
 its surface area and is expressed in units of g/kg. A higher Iodine value
 is indicative of smaller particle size which, in turn, is indicative of
 higher surface area for the carbon black and typically a higher
 reinforcing carbon black for elastomers.
 The DBP (dibutylphthalate) adsorption value for carbon black is a measure
 of its structure, or aggregate size and is higher DBP adsorption value
 indicative of larger aggregates which, in turn, is indicative of higher
 structure for the carbon black.
 Iodine values and DBP values together with representative ASTM designated
 N-numbers may be found, for example, in The Vanderbilt Rubber Handbook,
 Thirteenth Edition (1990), page 417.
 In the practice of this invention, as hereinbefore pointed out, the rubber
 composition is comprised of (i) at least one diene-based elastomer, or
 rubber or (ii) a combination of at least one diene-based elastomer with an
 epoxidized diene-based elastomer such as epoxidized natural rubber.
 Such diene-based elastomers are typically selected from homopolymers and
 copolymers of conjugated dienes and copolymers of conjugated diene(s) and
 vinyl aromatic compound such as, for example, styrene and
 alpha-methylstyrene. Such dienes may, for example, be selected from
 isoprene and 1,3-butadiene and such vinyl aromatic compounds may be
 selected from styrene and alpha-methylstyrene. Such elastomer, or rubber,
 may be selected, for example, from at least one of cis 1,4-polyisoprene
 rubber (natural and/or synthetic, and preferably natural rubber),
 3,4-polyisoprene rubber, styrene/butadiene copolymer rubbers,
 isoprene/butadiene copolymer rubbers, styrene/isoprene copolymer rubbers,
 styrene/isoprene/butadiene terpolymer rubbers, cis 1,4-polybutadiene
 rubber, trans 1,4-polybutadiene rubber (70-95 percent trans), low vinyl
 polybutadiene rubber (10-30 percent vinyl), high vinyl polybutadiene
 rubber (30-90 percent vinyl).
 In one aspect, the rubber is often comprised of at least two diene-based
 elastomers in which one of the elastomers is cis 1,4-polybutadiene. Such
 combination of elastomers may also be used together with an epoxidized
 diene-based elastomer such as epoxidized natural rubber. For example, such
 combination of two or more diene-based rubbers may be cis 1,4-polyisoprene
 rubber (natural or synthetic, although natural is usually preferred),
 3,4-polyisoprene rubber, isoprene/butadiene copolymer rubber,
 styrene/isoprene/butadiene rubber, emulsion and solution polymerization
 derived styrene/butadiene rubbers, cis 1,4-polybutadiene rubbers, medium
 vinyl polybutadiene rubbers (30-55 percent vinyl), high vinyl
 polybutadiene rubbers (55-90 percent vinyl) and emulsion polymerization
 prepared butadiene/acrylonitrile copolymers.
 The epoxidized natural rubber is known to those having a skill in the art
 and may be described as a modified form of natural cis 1,4-polyisoprene
 rubber in which some of its unsaturation is replaced by epoxidized groups.
 Epoxidized natural rubber which may be used in this invention may have a
 level of epoxidized modification ranging from about 15 to about 85,
 preferably about 20 to about 50, mole percent.
 A particularly preferred level epoxidized level for the natural rubber is
 about 25 mole percent. As is known to those having skill in such art,
 epoxidized natural rubber can be obtained by epoxidizing natural rubber
 latex. Such epoxidized natural rubber may be obtained from Malaysian
 rubber producers under a designation, for example, of ENR 25 (25 percent
 epoxidized level) and ENR 50 (50 percent epoxidized level).
 In one aspect of this invention, an emulsion polymerization derived
 styrene/butadiene (E-SBR) might be used as a diene-based elastomer having
 a relatively conventional styrene content of about 20 to about 28 percent
 bound styrene or, for some applications, an E-SBR having a relatively high
 bound styrene content, namely a bound styrene content of about 30 to about
 45 percent.
 The relatively high styrene content of about 30 to about 45 for the E-SBR
 can be considered beneficial for a purpose of enhancing traction, or skid
 resistance, of the tire tread. The presence of the E-SBR itself is
 considered beneficial for a purpose of enhancing processability of the
 uncured elastomer composition mixture, especially in comparison to a
 utilization of a solution polymerization prepared SBR (S-SBR).
 By emulsion polymerization prepared E-SBR, it is meant that styrene and
 1,3-butadiene are copolymerized as an aqueous emulsion. Such are well
 known to those skilled in such art. The bound styrene content can vary,
 for example, from about 5 to about 50 percent.
 In one aspect, the E-SBR may also contain acrylonitrile to form a
 terpolymer rubber in amounts, for example, of about 2 to about 30 weight
 percent bound acrylonitrile in the terpolymer.
 The solution polymerization prepared SBR (S-SBR) typically has a bound
 styrene content in a range of about 5 to about 50, preferably about 9 to
 about 36, percent. The S-SBR can be conveniently prepared, for example, by
 organo lithium catalyzation in the presence of an organic hydrocarbon
 solvent.
 A purpose of using S-SBR is for improved tire rolling resistance as a
 result of lower hysteresis when it is used in a tire tread composition.
 The 3,4-polyisoprene rubber (3,4-PI) is considered beneficial for a purpose
 of enhancing the tire's traction when it is used in a tire tread
 composition.
 The 3,4-PI and use thereof is more fully described in U.S. Pat. No.
 5,087,668. The Tg refers to the glass transition temperature which can
 conveniently be determined by a differential scanning calorimeter at a
 heating rate of 10.degree. C. per minute.
 The cis 1,4-polybutadiene rubber (BR) is considered to be beneficial for a
 purpose of enhancing the tire tread's wear, or treadwear.
 Such BR can be prepared, for example, by organic solution polymerization of
 1,3-butadiene.
 The BR may be conveniently characterized, for example, by having at least a
 90 percent cis 1,4-content.
 The cis 1,4-polyisoprene and cis 1,4-polyisoprene natural rubber are well
 known to those having skill in the rubber art.
 A styrene/isoprene/butadiene terpolymer elastomer (SIBR) may be used in
 rubber composition of this invention. Representative examples of various
 SIBR's may be found, for example, in U.S. Pat. Nos. 5,137,998, 5,159,020
 and 5,272,220.
 It is readily understood by those having skill in the art that the rubber
 composition would be compounded by methods generally known in the rubber
 compounding art, such as mixing the various sulfur-vulcanizable
 constituent rubbers with various commonly used additive materials such as,
 for example, curing aids, such as sulfur, activators, retarders and
 accelerators, processing additives, such as oils, resins including
 tackifying resins, coupling agent, and plasticizers, fillers, pigments,
 fatty acid, zinc oxide, waxes, antioxidants and antiozonants, peptizing
 agents and reinforcing materials such as, for example, carbon black. As
 known to those skilled in the art, depending on the intended use of the
 sulfur-vulcanizable and sulfur-vulcanized material (rubbers), the
 additives mentioned above are selected and commonly used in conventional
 amounts.
 The composition of the present invention may contain conventional amounts
 of known rubber chemicals.
 Typical amounts of tackifier resins, if used, may comprise about 0.5 to
 about 10 phr, usually about 1 to about 5 phr. Typical amounts of
 processing aids comprise about 1 to about 50 phr. Such processing aids can
 include, for example, aromatic, napthenic, and/or paraffinic processing
 oils. Typical amounts of antioxidants comprise about 1 to about 5 phr.
 Representative antioxidants may be, for example,
 diphenyl-p-phenylenediamine and others such as, for example, those
 disclosed in The Vanderbilt Rubber Handbook (1978), pages 344-346. Typical
 amounts of antiozonants comprise about 1 to 5 phr. Typical amounts of
 fatty acids, if used, which are usually comprised primarily of stearic
 acid, comprise about 0.5 to about 3 phr. Typical amounts of zinc oxide
 comprise about 2 to about 5 phr. Typical amounts of waxes comprise about 1
 to about 5 phr. Often microcrystalline waxes are used. Typical amounts of
 peptizers comprise about 0.1 to about 1 phr. Typical peptizers may be, for
 example, pentachlorothiophenol and dibenzamidodiphenyl disulfide.
 The vulcanization of the rubber composition is conducted in the presence of
 a sulfur-vulcanizing agent. Examples of suitable sulfur-vulcanizing agents
 include elemental sulfur (free sulfur) or sulfur-donating vulcanizing
 agents, for example, an amine disulfide, polymeric polysulfide or sulfur
 olefin adducts. Preferably, the sulfur-vulcanizing agent is elemental
 sulfur. As known to those skilled in the art, sulfur-vulcanizing agents
 are used in an amount ranging from about 0.5 to about 4 phr, or even, in
 some circumstances, up to about 8 phr, with a range of from about 1.5 to
 about 2.5, sometimes from about 2 to about 2.5, being preferred.
 Accelerators are used to control the time and/or temperature required for
 vulcanization and to improve the properties of the vulcanizate. In one
 embodiment, a single accelerator system may be used, i.e., primary
 accelerator. Conventionally and preferably, a primary accelerator(s) is
 used in total amounts ranging from about 0.5 to about 4, preferably about
 0.8 to about 2, phr. In another embodiment, combinations of a primary and
 a secondary accelerator might be used with the secondary accelerator being
 used in amounts of about 0.05 to about 5 phr in order to activate and to
 improve the properties of the vulcanizate. Combinations of these
 accelerators might be expected to produce a synergistic effect on the
 final properties and are somewhat better than those produced by use of
 either accelerator alone. In addition, delayed action accelerators may be
 used which are not affected by normal processing temperatures but produce
 a satisfactory cure at ordinary vulcanization temperatures. Vulcanization
 retarders might also be used. Suitable types of accelerators that may be
 used in the present invention are amines, disulfides, guanidines,
 thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates and
 xanthates. Preferably, the primary accelerator is a sulfenamide. If a
 second accelerator is used, the secondary accelerator is preferably a
 guanidine, dithiocarbamate or thiuram compound.
 The presence and relative amounts of most of the above additives are not
 considered to be an aspect of the present invention which is more
 primarily directed to the utilization of relatively low levels of
 reinforcing pigments as carbon black and/or silica together with a
 particulate calcium carbonate filler in rubber compositions for tire
 treads.
 The rubber composition may be prepared, for example, by thermomechanically
 working and mixing the diene-based rubber, epoxidized natural rubber if
 used, calcium carbonate, modifier(s), carbon black and/or silica and other
 rubber compounding ingredients, exclusive of the rubber curatives, in at
 least one sequential mixing step with at least one mechanical mixer,
 usually referred to as "non-productive" mix stage(s), to a temperature in
 a range of about 160.degree. C. to about 190.degree. for a sufficient
 duration of time, usually within about 4 to about 8 minutes, followed by a
 final mix stage in which the curatives, such as sulfur and accelerators,
 are added and mixed therewith for about 1 to about 4 minutes to a
 temperature within a range of about 90.degree. C. to about 125.degree. C.
 The terms "non-productive" and "productive" mix stages are well known to
 those having skill in the rubber mixing art.
 It is to be appreciated that the rubber composition is conventionally
 cooled to a temperature below about 40.degree. C. between the aforesaid
 mix stages.
 It is to be further appreciated that the aforesaid duration of time for the
 required temperature maintenance for the mixing process(es) during the
 non-productive mix stages can be accomplished, for example, by (i)
 adjusting the motor speed of the mixer, namely reducing the motor speed
 after the desired temperature of the rubber composition is reached, in a
 variable speed mixer or by (ii) utilizing two or more mix stages
 sufficient to satisfy the duration requirement for the aforesaid maximum
 mixing temperature maintenance.
 Vulcanization of the rubber composition of the present invention is
 generally carried out at conventional temperatures ranging from
 100.degree. C. to 200.degree. C. Preferably, the vulcanization is
 conducted at temperatures ranging from 110.degree. C. to 180.degree. C.
 Any of the usual vulcanization processes may be used such as heating in a
 press or mold, heating with superheated steam or hot air or in a salt
 bath.
 Upon vulcanization of the sulfur-vulcanized composition, the rubber
 composition of this invention can be used for various purposes. For
 example, the sulfur-vulcanized rubber composition may be in the form of a
 tread for a pneumatic tire which is the subject of this invention. Such
 tires can be built, shaped, molded and cured by various methods which are
 known and will be readily apparent to those having skill in such art. As
 can be appreciated, the tire may be a passenger tire, aircraft tire, truck
 tire and the like. Preferably, the tire is a passenger tire. The tire may
 also be a radial or bias, with a radial tire being preferred.