Patent Description:
Tires with reduced rolling resistance are desirable for the improved fuel economy. Other desirable performance attributes for tires include reduced heat buildup in the tire tread during use to promote tire tread durability. To promote one or more of these desirable properties, the hysteretic property of the tire rubber is modified. Fillers in the tire composition are known to influence hysteresis of the rubber. For example, a reduction in hysteresis loss of tire rubber may be achieved by changing a proportion of reinforcing carbon black filler in the tire rubber. This modification may be achieved by replacement with other fillers, for example, an increase in precipitated silica filler content may balance the reduction in carbon black. These modifications ideally promote a reduction in the rubber's hysteresis losses.

There are problems, however, with filler modifications. Typically, these problems manifest themselves in tradeoffs between the desirable performance properties. Improvement in one attribute causes a lessening of one or more other beneficial attributes. As an example, a significant reduction in rubber reinforcing carbon black content of the tread rubber also produces a significant reduction in the thermal and electrical conductivity of the rubber. This is particularly apparent as the rubber reinforcing carbon black content falls below what is known as its percolation point. A tread rubber composition with reduced rubber reinforcing carbon black content but having substantial thermal conductivity and electrical conductivity for the tire tread rubber composition is desirable.

Carbon nanotubes are known to improve thermal and electrical conductivity of tire treads. There are, however, problems with use of carbon nanotubes in the tread rubber. For one, dispersion of carbon nanotubes during formulation and mixing of the composition is problematic. Poorly dispersed fillers can have potentially negative effects on tread performance and may prohibit inclusion of other components.

While current rubber compositions are commercially successful, there is a need to fabricate tire treads to achieve simultaneously high levels of durability, low heat buildup, and low rolling resistance to achieve more efficient energy use together with improved tire life in original and replacement tire treads.

The invention relates to a tire in accordance with claim <NUM> and to a method in accordance with claim <NUM>.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with detailed description given below, explain the invention. <FIG> is a cross-sectional view of a pneumatic tire in accordance with one embodiment of the invention.

In the description of this invention, the terms "rubber" and "elastomer" are used interchangeably, unless otherwise stated. In addition, the term "phr" refers to parts of a respective material per hundred parts by weight of rubber or elastomer.

"Pneumatic tire" refers to a laminated mechanical device of generally toroidal shape (usually an open-torus) having bead cores and a tread and made of rubber, chemicals, fabric, and steel, or other materials. When mounted on the wheel of a vehicle or aircraft or similar application, the tread provides traction, and the tire supports the vehicle load. "Carcass" means the tire structure apart from the belt structure, tread, undertread, and sidewall rubber over the plies, but including the bead cores. "Sidewall" refers to a portion of the outside surface of a tire between the tread and the bead. "Tread" refers to a molded rubber component which, when bonded to a tire casing, includes that portion of the tire that contacts the road when the tire is normally inflated and used under normal load.

"Axial" and "axially" refer to directions that are parallel to an axis of rotation of the tire. "Radial" and "radially" refer to directions toward or away from the axis of rotation of a tire. "Circumferential" or "circumferentially" refers to a portion of the tire at or near the farthest radial distance from the axis of rotation. "Equatorial Plane" (or "EP") refers to a plane perpendicular to the tire's axis of rotation and passing through the center of its tread.

The <FIG> shows a simplified cross-section of a pneumatic tire <NUM> that has an improved lifespan and capacity for retreading. The tire <NUM> includes tread portion <NUM> from which a pair of sidewalls <NUM> extend and are connected to the tread portion <NUM> by shoulder regions <NUM>. The tread <NUM> is adapted to be ground contacting when the tire <NUM> is in use. The shoulder regions <NUM> extend predominantly axially outwardly from the tread portion <NUM>. The sidewalls <NUM> extend predominantly radially inwardly from the shoulder regions <NUM>.

A carcass <NUM> of the tire <NUM> includes one or more continuous radial plies <NUM> extending from side to side. One ply is shown in the <FIG>. The carcass <NUM> is located radially inwardly from the tread portion <NUM> and axially inwardly from the sidewalls <NUM>. The carcass <NUM> acts as a supporting structure for components located axially or radially outwardly including, for example, the tread portion <NUM> and sidewalls <NUM>. The one or more radial plies <NUM> may comprise cords or reinforcing wires of, for example, steel, nylon, polyester, rayon, glass, etc., embedded in a rubber matrix. Carcass <NUM> of the tire has a pair of axially spaced bead wires <NUM> around which are wrapped the distal ends of the radial plies <NUM>. The bead wires <NUM> may comprise, for example, substantially inextensible coils made of round metal filaments.

In one embodiment, the tire <NUM> further includes an optional inner liner (or air barrier layer) <NUM> disposed radially inwardly from the carcass <NUM>. The optional rubber tire inner liner <NUM> may be any known rubber inner liner for use in pneumatic tires <NUM>. In one example, the rubber inner liner <NUM> can be a non-butyl general purpose rubber (GPR) or a rubber composition disclosed herein. In another example, the rubber inner liner <NUM> can be a sulfur curative-containing halobutyl rubber composition of a halobutyl rubber such as for example chlorobutyl rubber or bromobutyl rubber. Such tire halobutyl rubber based inner liner layer may also contain one or more sulfur curable diene-based elastomers such as, for example, cis <NUM>,<NUM>-polyisoprene natural rubber, cis <NUM>,<NUM>-polybutadiene rubber and styrene/ butadiene rubber, or mixtures thereof. Rubber inner liner <NUM> is typically prepared by conventional calendaring or milling techniques such as to form a strip of uncured compounded rubber of appropriate width. When the tire <NUM> is cured, the rubber inner liner <NUM> becomes an integral, co-cured, part of the tire <NUM>. Tire inner liners, like that of rubber inner liner <NUM>, and their methods of preparation are well known to those having skill in such art.

With continued reference to the <FIG>, the tire <NUM> further includes at least one fiber-reinforced rubber layer <NUM>, which can be supported by the carcass <NUM> and interposed between the tread portion <NUM> and the carcass <NUM>. The fiber-reinforced rubber layer <NUM> can define a barrier/barrier layer and includes a rubber compound that is reinforced with one or more types of fiber and has one or more chemical compounds that provide desirable barrier properties (e.g., abrasion resistance properties), thereby helping to prevent wear and/or tear from extending beyond the tire tread <NUM> and into its underlying layer(s), such as the carcass <NUM>, and desirably increasing the overall lifespan of the tire <NUM>.

In accordance with one embodiment of the invention, one or more portions of the tire <NUM> consist of or comprise a cured rubber of a rubber composition disclosed herein. By way of preferred example, the cured rubber forms the tread <NUM> and/or the sidewall <NUM> of the tire <NUM>.

In one embodiment, the rubber composition includes <NUM> phr of an elastomer including at least one diene-based elastomer modified by one or more additives and in which one or more particulate fillers are dispersed. The fillers include carbon nanotubes, described below, and may include other particulate fillers. As such, the rubber composition may include a combination of fillers of different compositions and amounts. By way of example, the filler may include a mixture of particulates of carbon black, silica, and carbon nanotubes. In one embodiment, the filler consists of carbon black particulates, silica particulates, and carbon nanotubes and combinations thereof. In embodiments of the invention, the filler of the rubber composition is limited to a predetermined maximum amount. By way of example, the filler may not exceed <NUM> phr. By way of additional example, the filler may be present in a range of <NUM> phr to <NUM> phr. Advantageously, embodiments of the rubber composition may be utilized in larger tires, such as OTR tires, with improved tear and crack propagation resistance. Generally, embodiments of the rubber composition exhibit improved thermal conductivity. This improvement is believed to also improve cure times and cure efficiency.

More specifically, in embodiments of the invention, various rubbers, including mixtures thereof, can be used as the rubber component of the rubber composition. Various diene-based elastomers may be used for the rubber composition. For example, polymers and copolymers of at least one monomer comprised of at least one of isoprene and <NUM>,<NUM>-butadiene and from styrene copolymerized with at least one of isoprene and <NUM>,<NUM>-butadiene. Representative conjugated diene-based elastomers are, for example, at least one of cis <NUM>,<NUM>-polyisoprene (natural and synthetic), cis <NUM>,<NUM>-polybutadiene, styrene/butadiene copolymers (aqueous emulsion polymerization prepared and organic solvent solution polymerization prepared) (e.g., ESBR and SSBR, respectively), medium vinyl polybutadiene having a vinyl <NUM>,<NUM>-content in a range of <NUM> to <NUM> percent, isoprene/butadiene copolymers, and styrene/isoprene/butadiene terpolymers or combinations of the above. By way of example only, an <NUM> phr to <NUM> phr ratio of natural rubber to ESBR may be utilized. Tin coupled elastomers may also be used, for example, tin coupled organic solution polymerization prepared styrene/butadiene co-polymers, isoprene/butadiene copolymers, styrene/isoprene copolymers, polybutadiene and styrene/isoprene/butadiene terpolymers. In one embodiment, the conjugated diene-based elastomer may be an elastomer, such as a styrene/butadiene copolymer containing at least one functional group reactive with hydroxyl groups on a precipitated silica. For example, the functional group may include at least one of siloxy, amine, imine, and thiol groups, for example, comprised of a siloxy and least one of amine and thiol groups.

In embodiments of the invention, the rubber composition includes carbon nanotubes in an amount of at least <NUM> phr up to <NUM> phr. In one embodiment, the carbon nanotubes are present at <NUM> phr or in a range of from <NUM> to <NUM> phr. Carbon nanotubes according to embodiments of the invention are generally parallel, multi-walled carbon nanotubes.

Preferably, the carbon nanotubes have a nested structure of from <NUM> walls to <NUM> walls.

Carbon nanotubes are very small diameter fibers with high aspect ratio. Individual fibers preferably have an average diameter in a range of from <NUM> to <NUM> and average diameters in a range from <NUM> to <NUM> or in a range from <NUM> to <NUM>. Individual fibers preferably have an average length of between <NUM> and <NUM>,<NUM>. In a preferred embodiment, the carbon nanotubes may have an average length less than <NUM>,<NUM>, for example, <NUM>. The length to diameter ratio of the carbon nanotubes is at least <NUM>, meaning the length is at least <NUM> times greater than the diameter.

The carbon nanotubes may be produced from high purity, low molecular weight hydrocarbons in a continuous, gas phase, catalyzed reaction or by other means/methods. As produced, the carbon nanotubes may be intertwined together in agglomerates. These tangled agglomerates of carbon nanotubes may be untangled and/or dispersed prior to use. Preferably, one or more surfaces of the carbon nanotubes may be modified during or following their synthesis. In one embodiment, modification includes adding oxygen or oxygenated functional groups to the surface of the carbon nanotubes. This may be achieved by acid treating the carbon nanotubes so that they may be functionalized with OH and COOH groups. To that end, modification of the carbon nanotubes may include treatment in an oxygen-containing environment. Carbon of the nanotube atomic structure may bond with oxygen via a reaction with oxygen or an oxygen-containing reactant. The oxygen or oxygen-containing reactant may bond to the surface of the as-formed carbon nanotube or may diffuse into the structure of the tube wall. The amount of oxygen in the carbon nanotubes following modification ranges from <NUM> wt. % to <NUM> wt. % or from <NUM> wt. % to <NUM> wt. Usable carbon nanotubes are commercially available from Molecular Rebar Design, LLC of Austin, Texas as MRO Gen II in naphthenic oil.

Other fillers may be mixed with the rubber composition. By way of example, other fillers may include silica filler. This preferably includes precipitated silica particulates, and fumed (pyrogenic) silica particulates in which aggregates of precipitated silicas may be present. Silica is present in a measurable amount (e.g., at least <NUM> phr) but at most <NUM> phr. For example, from <NUM> phr to less than <NUM> phr. The silica may be synthetic, amorphous, and precipitated silica that is highly dispersible with a CTAB (i.e., ASTM D6845) of at least <NUM><NUM>/g.

Advantageously, silica content may be reduced and also uncoupled silica (i.e., no silane) can be used for improved tear strength in combination with carbon nanotubes.

Accordingly, the rubber composition used in the invention is preferably silane free.

Generally, tires with relatively high silica content (e.g., > <NUM> phr silica) are stiffer and so have a reduced rolling resistance imparted by the high silica content. In embodiments of the invention, silica content is relatively low, but rolling resistance is also relatively low and thermal conductivity is relatively high. This is a beneficial combination thought to be provided by inclusion of the carbon nanotubes. Thus, it is believed that this combination provides improved stiffness, maintains hysteresis, provides crack growth resistance, maintains bulk tear resistance, and improves abrasion resistance while also improving thermal conductivity.

If included, silica fillers may include aggregates obtained by the acidification of a soluble silicate, e.g., sodium silicate and may include co-precipitated silica and a minor amount of aluminum. The silica particles might usually be characterized, for example, by having a BET surface area, as measured using nitrogen gas, preferably in the range of <NUM><NUM> per gram to <NUM><NUM> per gram, and more usually in a range of <NUM><NUM> per gram to <NUM><NUM> per gram. The silica particles may also be characterized by having a dibutylphthalate (DBP) absorption value in a range of <NUM> cc/<NUM> to <NUM> cc/<NUM>, and more usually <NUM> to <NUM> cc/<NUM> (according to ASTM D2414). Various commercially available precipitated silicas usable, for example, include silicas from Solvay under the Premium SW brand; PPG Industries under the Hi-Sil trademark including Hi-Sil <NUM>, Hi-Sil <NUM>, and Agilon <NUM>-D; silicas sold under the marks Zeosil 1165MP and Zeosil 165GR from Rhodia; silicas from Degussa AG designated VN2 and VN3, as well as other grades of silica, particularly precipitated silicas, which can be used for elastomer reinforcement. Coupling agents may be used if desired to aid in coupling the silica (e.g., precipitated silica with hydroxyl groups on its surface).

In one embodiment, the rubber composition includes a resin in an amount from <NUM> phr up to <NUM> phr. An exemplary resin is a modified gum rosin with a softening point of <NUM> and an acid number of <NUM>. Other acceptable or additional resins that can be included are heat reactive hydrocarbon resins derived from the polymerization of the unsaturated constituents of the petroleum C9 fraction with softening points of <NUM>, and mixtures of alkylated naphthenic and aromatic resins with softening points of <NUM>.

In a preferred embodiment of the invention, the rubber composition includes two or more different resins in an amount from <NUM> phr up to <NUM> phr each.

Advantageously, the amount of resin in the rubber composition may be increased because the rubber composition has increased thermal conductivity. This relative operating temperature reduction permits higher amounts of resin which improves the tear resistance of the tire of the rubber composition. Generally, it is believed that higher loading of resin negatively effects compound hysteresis. However, the resin, depending on the type, may improve elongation and tensile properties and improve incorporation and dispersion of fillers.

The rubber composition may include antioxidants, for example in an amount from <NUM> to <NUM> phr. Representative antioxidants may be, for example, diphenyl-p-phenylenediamine and others or mixtures of those antioxidants, for example, those disclosed in <NPL>. Fatty acids may also be added to the rubber composition. These can include stearic acid and combinations of stearic acid with one or more of palmitic acid oleic acid and may comprise, for example, from <NUM> to <NUM> phr. Zinc oxide is also added to the rubber composition and may be present in an amount from <NUM> to <NUM> phr. Peptizers, where used, may be present in an amount from <NUM> to <NUM> phr.

In an exemplary embodiment, carbon black particulates are added to the rubber composition from <NUM> phr to <NUM> phr. By way of further example, the rubber composition may be from <NUM> phr to <NUM> phr carbon black. Embodiments of the invention have <NUM> phr or less of total filler loading, where total filler loading is the amount of carbon nanotubes, silica, carbon black, and other solid filler particulates added to the rubber composition during mixing and prior to curing. By way of additional examples, embodiments of the invention may have a total filler loading of between <NUM> phr and <NUM> phr and of between <NUM> phr to <NUM> phr. In one embodiment of the invention, the rubber composition includes an oil, for example, in an amount from <NUM> phr to <NUM> phr. Oils can include, for example, aromatic, naphthenic, paraffinic oils, and/or vegetable oils. By way of example, the oil may be present in an amount of <NUM> phr and be included based on a dispersion of the carbon nanotubes. The dispersion of the carbon nanotubes of a given amount includes <NUM> wt.

Preferably, other than the oil from the dispersion of the carbon nanotubes, no other oils are included in the rubber composition.

In one embodiment, the rubber composition may include a wax. Typical amounts of wax are from <NUM> phr to <NUM> phr. Exemplary wax includes a microcrystalline wax and/or a refined paraffin wax.

Vulcanization is conducted in the presence of a sulfur-vulcanizing agent. The sulfur-vulcanizing agents may be used, for example, in an amount ranging from <NUM> to <NUM> phr. 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.

Sulfur vulcanization accelerators may be 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. A primary accelerator may be added in total amounts ranging, for example, from <NUM> phr to <NUM> phr, and by way of additional example, from <NUM> phr to <NUM> phr. In one embodiment, a secondary accelerator may be used with the primary accelerator. The secondary accelerator may be added in smaller amounts than the primary accelerator, for example, from <NUM> to <NUM> phr. This may activate and improve the properties of the vulcanizate. The primary accelerator is a sulfenamide. If a secondary accelerator is used, the secondary accelerator may be, for example, a guanidine, dithiocarbamate or thiuram compound. In addition, delayed action accelerators may be used, for example, which are not affected by normal processing temperatures but produce a satisfactory cure at ordinary vulcanization temperatures. Vulcanization retarders might also be used, where desired or appropriate. Suitable types of accelerators include, for example, amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates, and xanthates.

A dispersion of the carbon nanotubes is prepared prior to adding the dispersed carbon nanotubes to the rubber composition. A selected amount of the carbon nanotubes is added to a carrier. Surfaces of the carbon nanotubes are oxygenated to a preselected level prior to dispersion. One exemplary carrier is an oil disclosed above.

Claim 1:
A pneumatic tire having a component comprising a rubber composition, the rubber composition comprising <NUM> phr of at least one diene-based elastomer and a filler comprising <NUM> phr to <NUM> phr of carbon nanotubes having of from <NUM> wt.% to <NUM> wt.% oxygen content and a measurable amount but at most <NUM> phr silica particles.