Patent Application
20110259492
There has been an increasing demand to develop tires with a high level of handling performance, good stability and steering response when changing lanes, avoiding obstacles on the road and cornering. Improved road grip without compromising stability is critical for vehicles traveling at high speed. However, higher tire operating temperatures are encountered at high speeds than are experienced during normal driving and the hot rubber in the tire becomes more pliable which reduces the handling stability of the tire, a so-called “borderline” use of said tire.
A widely adopted method to improve stability, particularly road gripping properties, is to increase the hysteresis loss of tread rubber compositions. A large hysteresis loss during the deformation of tread is used for increasing a friction force between the tread and road surface. However, a significant increase of heat buildup will occur during the running of the tires as the hysteresis loss of the tread rubber becomes large, causing wear resistance of the tread rubber to deteriorate rapidly. On the other hand, it is believed that controllability is significantly influenced by hardness (which is closely related to cornering stiffness of a tire) and breaking strength of rubber compositions. In order to enhance controllability, especially steering response, it is necessary to increase the stiffness of the tire compound in general and the tread in particular, which in most cases results in lower hysteresis loss. It is very difficult to achieve both of these desired properties by conventional compounding techniques. There is therefore a need for a tread with improved cornering stiffness.
The present invention is directed to a pneumatic tire comprising a tread, the tread comprising a ground contacting rubber composition comprising a diene based elastomer and short fibers, wherein the short fibers extend lengthwise in a substantially axial direction of the tire.
There is disclosed a pneumatic tire comprising a tread, the tread comprising a ground contacting rubber composition comprising a diene based elastomer and short fibers, wherein the short fibers extend lengthwise in a substantially axial direction of the tire.
With reference now to
The tread 52 is constructed from a tread stock, which may be produced by calendaring or injection molding of rubber compound containing short fibers, see for example U.S. Pat. Nos. 4,871,004; 6,106,752; 6,387,313; and 6,899,782. In the case of calendaring for example, a rubber compound containing short fibers may be calendared into thin sheets wherein the fibers orient such that their length dimension is extended substantially in the mill direction, that is, in the direction of forward propagation of the sheet through the calendar. Calendared sheets so produced can then be stacked to produced tread stock of the desired tread thickness. In fabricating the tread stock, the calendared sheets may be positioned so that the oriented fibers are disposed at a desired orientation with respect to the axial or circumferential directions of the tire in which the tread stock is used. In this context, when referring to the orientation of the short fibers as being oriented in a “substantially axial direction of the tire,” it is meant herein that the tread is constructed from tread stock wherein the rubber compound comprising the short fibers is oriented with its mill direction parallel to the axial direction of the tire. The tread is thereby anisotropic, showing directionality of physical and performance properties due to the directionality of the short fibers.
With reference now to
Extension length 110 and angle of extension θ may be determined for example by a least squares regression to determine a best fit line through a microscopic image of a dispersed fiber with reference to appropriate dimensional axes. In one embodiment, the extension length of at least 90 percent of the short fibers is within 30 degrees of a line drawn parallel to the axial direction of the tire, i.e. θ≦30 degrees. In one embodiment, the extension length of at least 90 percent of the short fibers is within 15 degrees of a line drawn parallel to the axial direction of the tire, i.e. θ≦15 degrees.
Short fibers 102 may be disposed in rubber compound across the axial width of tread 52, or in one or more distinctly defined zones of the tread. In this way, the beneficial effect of the oriented fibers may be realized with minimal use of fibers. With reference now to
The rubber composition such as that used in tread stock 100 and tread 52 includes short fibers. Suitable short fibers include any textile fibers as are known in the art. In one embodiment, the short fibers are selected from the group consisting of polyaramid fibers, polyester fibers, polyamide fibers, polyketone fibers, polybisoxazole fibers, rayon fibers, and metal fibers. In one embodiment, the short fibers are polyaramid fibers. In one embodiment, the short fibers are fibrillated polyaramid fibers.
In one embodiment, short fibers have a length ranging from 0.1 to 10 mm. In one embodiment, the short fibers have a thickness ranging from 1 to 20 microns.
In one embodiment, the short fibers are present in the rubber composition in an amount ranging from 0.5 to 30 phr. In one embodiment, the short fibers are present in the rubber composition in an amount ranging from 5 to 15 phr. The short fibers may be used as the raw fiber or pre-mixed with an elastomer as a masterbatch.
The rubber composition may be made with rubbers or elastomers containing olefinic unsaturation. The phrases “rubber or elastomer containing olefinic unsaturation” or “diene based elastomer” are intended to include both natural rubber and its various raw and reclaim forms as well as various synthetic rubbers. In the description of this invention, the terms “rubber” and “elastomer” may be used interchangeably, unless otherwise prescribed. The terms “rubber composition,” “compounded rubber” and “rubber compound” 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. Representative synthetic polymers are the homopolymerization products of butadiene and its homologues and derivatives; for example, methylbutadiene, dimethylbutadiene and pentadiene as well as copolymers such as those formed from butadiene or its homologues or derivatives with other unsaturated monomers. Among the latter are acetylenes, for example, vinyl acetylene; olefins, for example, isobutylene, which copolymerizes with isoprene to form butyl rubber; vinyl compounds, for example, acrylic acid, acrylonitrile (which polymerize with butadiene to form NBR), methacrylic acid and styrene, the latter compound polymerizing with butadiene to form SBR, as well as vinyl esters and various unsaturated aldehydes, ketones and ethers, e.g., acrolein, methyl isopropenyl ketone and vinylethyl ether. Specific examples of synthetic rubbers include neoprene (polychloroprene), polybutadiene (including cis-1,4-polybutadiene), polyisoprene (including cis-1,4-polyisoprene), butyl rubber, halobutyl rubber such as chlorobutyl rubber or bromobutyl rubber, styrene/isoprene/butadiene rubber, copolymers of 1,3-butadiene or isoprene with monomers such as styrene, acrylonitrile and methyl methacrylate, as well as ethylene/propylene terpolymers, also known as ethylene/propylene/diene monomer (EPDM), and in particular, ethylene/propylene/dicyclopentadiene terpolymers. Additional examples of rubbers which may be used include alkoxy-silyl end functionalized solution polymerized polymers (SBR, PBR, IBR and SIBR), silicon-coupled and tin-coupled star-branched polymers. The preferred rubber or elastomers are polyisoprene (natural or synthetic), polybutadiene and SBR.
In one aspect the rubber is preferably of at least two of diene based rubbers. For example, a combination of two or more rubbers is preferred such as cis 1,4-polyisoprene rubber (natural or synthetic, although natural is preferred), 3,4-polyisoprene rubber, styrene/isoprene/butadiene rubber, emulsion and solution polymerization derived styrene/butadiene rubbers, c is 1,4-polybutadiene rubbers and emulsion polymerization prepared butadiene/acrylonitrile copolymers.
In one aspect of this invention, an emulsion polymerization derived styrene/butadiene (E-SBR) might be used having a relatively conventional styrene content of about 20 to about 28 percent bound styrene or, for some applications, an E-SBR having a medium to relatively high bound styrene content, namely, a bound styrene content of about 28 to about 45 percent.
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, as E-SBAR, in amounts, for example, of about 2 to about 30 weight percent bound acrylonitrile in the terpolymer.
Emulsion polymerization prepared styrene/butadiene/acrylonitrile copolymer rubbers containing about 2 to about 40 weight percent bound acrylonitrile in the copolymer are also contemplated as diene based rubbers for use in this invention.
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.
In one embodiment, c is 1,4-polybutadiene rubber (BR) may be used. 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.
The term “phr” as used herein, and according to conventional practice, refers to “parts by weight of a respective material per 100 parts by weight of rubber, or elastomer.”
The rubber composition may also include up to 70 phr of processing oil. Processing oil may be included in the rubber composition as extending oil typically used to extend elastomers. Processing oil may also be included in the rubber composition by addition of the oil directly during rubber compounding. The processing oil used may include both extending oil present in the elastomers, and process oil added during compounding. Suitable process oils include various oils as are known in the art, including aromatic, paraffinic, naphthenic, vegetable oils, and low PCA oils, such as MES, TDAE, SRAE and heavy naphthenic oils. Suitable low PCA oils include those having a polycyclic aromatic content of less than 3 percent by weight as determined by the IP346 method. Procedures for the IP346 method may be found in Standard Methods for Analysis & Testing of Petroleum and Related Products and British Standard 2000 Parts, 2003, 62nd edition, published by the Institute of Petroleum, United Kingdom.
The rubber composition may include from about 10 to about 150 phr of silica. In another embodiment, from 20 to 80 phr of silica may be used.
The commonly employed siliceous pigments which may be used in the rubber compound include conventional pyrogenic and precipitated siliceous pigments (silica). In one embodiment, precipitated silica is used. The conventional siliceous pigments employed in this invention are precipitated silicas such as, for example, those obtained by the acidification of a soluble silicate, e.g., sodium silicate.
Such conventional silicas might be characterized, for example, by having a BET surface area, as measured using nitrogen gas. In one embodiment, the BET surface area may be in the range of about 40 to about 600 square meters per gram. In another embodiment, the BET surface area may be in a range of about 80 to about 300 square meters per gram. The BET method of measuring surface area is described in the Journal of the American Chemical Society, Volume 60, Page 304 (1930).
The conventional silica may also be characterized by having a dibutylphthalate (DBP) absorption value in a range of about 100 to about 400, alternatively about 150 to about 300.
The conventional silica might be expected to have an average ultimate particle size, for example, in the range of 0.01 to 0.05 micron as determined by the electron microscope, although the silica particles may be even smaller, or possibly larger, in size.
Various commercially available silicas may be used, such as, only for example herein, and without limitation, silicas commercially available from PPG Industries under the Hi-Sil trademark with designations 210, 243, etc; silicas available from Rhodia, with, for example, designations of Z1165MP and Z165GR and silicas available from Degussa AG with, for example, designations VN2 and VN3, etc.
Commonly employed carbon blacks can be used as a conventional filler in an amount ranging from 10 to 150 phr. In another embodiment, from 20 to 80 phr of carbon black may be used. Representative examples of such carbon blacks include N110, N121, N134, N220, N231, N234, N242, N293, N299, N315, N326, N330, N332, N339, N343, N347, N351, N358, N375, N539, N550, N582, N630, N642, N650, N683, N754, N762, N765, N774, N787, N907, N908, N990 and N991. These carbon blacks have iodine absorptions ranging from 9 to 145 g/kg and DBP number ranging from 34 to 150 cm3/100 g.
Other fillers may be used in the rubber composition including, but not limited to, particulate fillers including ultra high molecular weight polyethylene (UHMWPE), crosslinked particulate polymer gels including but not limited to those disclosed in U.S. Pat. No. 6,242,534; 6,207,757; 6,133,364; 6,372,857; 5,395,891; or 6,127,488, and plasticized starch composite filler including but not limited to that disclosed in U.S. Pat. No. 5,672,639. Such other fillers may be used in an amount ranging from 1 to 30 phr.
In one embodiment the rubber composition may contain a conventional sulfur containing organosilicon compound. Examples of suitable sulfur containing organosilicon compounds are of the formula:
Z-Alk-Sn-Alk-Z I
in which Z is selected from the group consisting of
where R1 is an alkyl group of 1 to 4 carbon atoms, cyclohexyl or phenyl; R2 is alkoxy of 1 to 8 carbon atoms, or cycloalkoxy of 5 to 8 carbon atoms; Alk is a divalent hydrocarbon of 1 to 18 carbon atoms and n is an integer of 2 to 8.
In one embodiment, the sulfur containing organosilicon compounds are the 3,3′-bis(trimethoxy or triethoxy silylpropyl)polysulfides. In one embodiment, the sulfur containing organosilicon compounds are 3,3′-bis(triethoxysilylpropyl)disulfide and/or 3,3′-bis(triethoxysilylpropyl)tetrasulfide. Therefore, as to formula I, Z may be
where R2 is an alkoxy of 2 to 4 carbon atoms, alternatively 2 carbon atoms; alk is a divalent hydrocarbon of 2 to 4 carbon atoms, alternatively with 3 carbon atoms; and n is an integer of from 2 to 5, alternatively 2 or 4.
In another embodiment, suitable sulfur containing organosilicon compounds include compounds disclosed in U.S. Pat. No. 6,608,125. In one embodiment, the sulfur containing organosilicon compounds includes 3-(octanoylthio)-1-propyltriethoxysilane, CH3(CH2)6C(═O)—S—CH2CH2CH2Si(OCH2CH3)3, which is available commercially as NXT™ from Momentive Performance Materials.
In another embodiment, suitable sulfur containing organosilicon compounds include those disclosed in U.S. Patent Publication No. 2003/0130535. In one embodiment, the sulfur containing organosilicon compound is Si-363 from Degussa.
The amount of the sulfur containing organosilicon compound in a rubber composition will vary depending on the level of other additives that are used. Generally speaking, the amount of the compound will range from 0.5 to 20 phr. In one embodiment, the amount will range from 1 to 10 phr.
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, sulfur donors, curing aids, such as activators and retarders and processing additives, such as oils, resins including tackifying resins and plasticizers, fillers, pigments, fatty acid, zinc oxide, waxes, antioxidants and antiozonants and peptizing agents. 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. Representative examples of sulfur donors include elemental sulfur (free sulfur), an amine disulfide, polymeric polysulfide and sulfur olefin adducts. In one embodiment, the sulfur-vulcanizing agent is elemental sulfur. The sulfur-vulcanizing agent may be used in an amount ranging from 0.5 to 8 phr, alternatively with a range of from 1.5 to 6 phr. Typical amounts of tackifier resins, if used, 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. 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 through 346. Typical amounts of antiozonants comprise about 1 to 5 phr. Typical amounts of fatty acids, if used, which can include 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.
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. The primary accelerator(s) may be used in total amounts ranging from about 0.5 to about 4, alternatively about 0.8 to about 1.5, phr. In another embodiment, combinations of a primary and a secondary accelerator might be used with the secondary accelerator being used in smaller amounts, such as from about 0.05 to about 3 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. In one embodiment, the primary accelerator is a sulfenamide. If a second accelerator is used, the secondary accelerator may be a guanidine, dithiocarbamate or thiuram compound.
The mixing of the rubber composition can be accomplished by methods known to those having skill in the rubber mixing art. For example, the ingredients are typically mixed in at least two stages, namely, at least one non-productive stage followed by a productive mix stage. The final curatives including sulfur-vulcanizing agents are typically mixed in the final stage which is conventionally called the “productive” mix stage in which the mixing typically occurs at a temperature, or ultimate temperature, lower than the mix temperature(s) than the preceding non-productive mix stage(s). The terms “non-productive” and “productive” mix stages are well known to those having skill in the rubber mixing art. The rubber composition may be subjected to a thermomechanical mixing step. The thermomechanical mixing step generally comprises a mechanical working in a mixer or extruder for a period of time suitable in order to produce a rubber temperature between 140° C. and 190° C. The appropriate duration of the thermomechanical working varies as a function of the operating conditions, and the volume and nature of the components. For example, the thermomechanical working may be from 1 to 20 minutes.
The pneumatic tire of the present invention may be a race tire, passenger tire, aircraft tire, agricultural, earthmover, off-the-road, truck tire, and the like. In one embodiment, the tire is a passenger or truck tire. The tire may also be a radial or bias.
Vulcanization of the pneumatic tire of the present invention is generally carried out at conventional temperatures ranging from about 100° C. to 200° C. In one embodiment, the vulcanization is conducted at temperatures ranging from about 110° C. to 180° 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. 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.
The invention is further illustrated by the following nonlimiting example.
In this example, the effect of orienting short fibers in the tread compound of a tire is illustrated. The unexpected effect of a surprising increase in cornering power in a tire with laterally oriented short fiber as compared with a tire with circumferentially oriented or randomly oriented tires is shown.
Otherwise identical tires (205/55R16) were constructed with three different short fiber orientations in the tread. Tire A was a control tire with fibers randomly distributed in the tread compound. Tire B was a comparative tire with fibers oriented substantially parallel to the circumferential direction of the tire. Tire C was representative of the present invention and had fibers oriented substantially perpendicular to the circumferential direction of the tire and substantially parallel to the axial (lateral) direction of the tire.
Tread compound for all three tires was mixed including 7 phr of chopped aramid fibers (Kevlar® pulp) in a compound of 30 phr polybutadiene, 23.5 phr natural rubber, and 46.5 phr styrene-butadiene rubber. The rubber mixing following procedures as are known in the art, with a multi-step mix procedure including non-productive and productive mix steps. Standard amounts of curatives, processing aids, antidegradants, and fillers were also used.
For control tire A, the tread compound was extruded to form the tread stock. The extruded tread stock with randomly oriented fibers was then used to construct tire A.
For tires B and C, the tread compound was calendared to a thickness of 1.63 mm with a small mill gap to increase fiber alignment in the compound. The calendared sheet was then cut into appropriately sized sections and the sections stacked four high to construct each tread. For tire B, each calendared sheet was oriented with the mill direction parallel to the circumferential direction of the tire. For tire C, each calendared sheet was oriented with the mill direction parallel to the axial direction of the tire (i.e., perpendicular to the circumferential direction of the tire). The stacked calendared sheets were then used to construct tires B and C. All tires were cured in a tire mold following standard curing protocol.
Microscopic inspection of microtomed sections of cured tread samples from each tire confirmed the orientation of fibers in tire B substantially parallel to the circumferential direction of tire B and in tire C substantially parallel to the axial direction of tire C, and a lack of definitive orientation of fibers in tire A.
The tires were tested for cornering power at speeds of 7 km/hr and 50 km/hr on an MTS Flat-Trac® dynamic force and moment testing machine. Results are given in Table 1.
As seen in Table 1, Tire C with short fibers oriented substantially parallel to the axial (lateral) tire direction of the tread showed an unexpectedly higher cornering power as compared with Tire B with short fibers oriented substantially parallel to the circumferential direction of the tire direction of the tread. Significantly, at a tire speed of 7 km/hr Tire C showed a 6.6 to 9.1 percent increase in cornering power versus control, as compared with a 0.2 to 0.5 percent increase for Tire B. Similarly, at a tire speed of 50 km/hr Tire C showed a 3.4 to 6.6 percent increase in cornering power versus control, as compared with a −0.5 to 0.3 percent change for Tire B.
In one embodiment, then, the tire has a cornering power greater than an otherwise identical tire with the short fibers randomly oriented. In one embodiment, then, the tire has a cornering power at least five percent greater than an otherwise identical tire with the short fibers randomly oriented.
