CA2062997A1 - Fibrillated ptfe modified rubber - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Low strain modulus of a sulfur cured rubber is improved by incorporating a fibrillated polytetrafluoroethylene (PTFE) into uncured rubber.
Other compounding ingredients may be conventional and in conventional amounts. The amount of fibrillated PTFE is typically from about 0.5 to about 8 phr; the amount required will vary from grade to grade of PTFE.
The resulting vulcanized rubber is particularly useful as a tire tread stock, the vulcanized rubber has a higher low strain modulus (e.g., modulus taken at either 50% or 100% elongation) than does otherwise similar rubber containing no PTFE.

Description

2~ ir ~ ~ J7 FIBRILLATED PTFE MODIFIED RUBBER

TECHNICAL FIELD
This invention relates to rubber having improved low strain modulus and more particularly to rubber containing a low strain modulus improving additive.

BACKGROUND OF THE INVENTION
It is sometimes desirable to increase the modulus of rubber compounds. For instance, it is generally desirable to increase the modulus of rubber compounds which are utilized in tire tread base compositions and in tire wire coat compounds. A higher degree of stiffness in such rubber compositions is conventionally attained by incorporating larger amounts of fillers, such as carbon black, into the rubber compounds and/or by increasing the state of cure of such compounds.
Unfortunately, both of these techniques lead to undesirable results. For instance, the incorporation of additional carbon black into rubber compounds typically leads to high levels of hysteresis.
Accordingly, the utilization of such compounds in tires results in excessive heat build-up and poor cut growth characteristics. The utilization of high amounts of sulfur to attain a high state of cure typically leads to poor aging resistance. Furthermore, it is highly impractical to reach high levels of stiffness by increased state of cure alone. For these reasons, it is not possible to attain the desired degree of stiffness in tire tread base compounds by simply adding higher levels of fillers or curatives.
Published European patent application Publication No. 0 106 180, published April 25, 1984 discloses reinforced perfluoro elastomers with high multidirectional tear strength which includes fibrillated polytetrafluoroethylene (PTFE) in amounts of 1 to 40 phr, preferably 2 to 20 phr (parts of PTFE
per 100 parts of the perfluoro elastomer), in addition to conventional fillers and reinforcing agents including carbon black. In order to avoid directionality in tear strength and to thereby achieve multidirectional tear strength, according to the published application (particularly page 5, line 31 to page 6, line 8 thereof) one can mill a blend of a perfluoro elastomer, fibrillatable PTFE and compounding ingredients; pulverize the milled materials, particularly at cryogenic temperatures; place the resulting particles into a mold and press cure into desired shape, preferably under vacuum, followed by standard post-curing of the press cured article.
Published European patent application Publication No. 0 225 792, published June 16, 1987, discloses a blend of a cured fluoroelastomer and a thermoplastic copolvmer of tetrafluoroethylene (TFE) which is present as generally spherical particles having a particle size of less than 10 microns. The TFE copolymer typically contains 2 - 50 mole percent, preferably 3 - 25 mole percent, of a perfluorinated comonomer and 98 - 50 mole percent, preferably 97 - 75 mole percent, TFE. The amount of TFE copolvmer is from 2 to 50 parts by weight, preferably 5 - 30 parts by weight, of TFE
copolymer per 100 parts by weight of fluoroelastomer.
U. S. Patent Nos. 4,507,439 and 4,596,855 (which is a continuation-in-part of patent 4,507,439) of Stewart, both assigned to E. I. du Pont de Nemours and Company, disclose elastomeric compositions comprising an elastomeric matrix having dispersed therein about 3 - 30 phr of powdered PTFE which has been treated with about 50 - ~20 percent of sodium naphthalene addition compound, the percentage denoting the percentage of the 2~i$7 theoretical amount required for the alkaline metal to react with all the fluorine atoms on the surface of the PTFE powder. According to these patents (example 2 of both patents) "cured elastomer slabs containing untreated PTFE powder had rough surfaces, were distorted and contained visible particles of agglomerated PTFE powder and PTFE fibrils", while the slabs containing treated PTFE powder were "smooth and undistorted, without visible agglomeration". The elastomer in example 2 of each patent was a fluoroelastomer.

SUMMARY OF THE INVENTION
This invention according to one aspect provides a vulcanizable rubber composition comprising a non-fluorinated diene rubber or mixture thereof, and an amount of low strain modulus-improving fibrillated polytetrafluoroethylene sufficient to improve the low strain modulus of said composition.
This invention according to another aspect provides a vulcanizate of the foregoing composition.
This invention according to another aspect provides a rubber article and in particular a tire comprising the aforesaid vulcanizate.
This invention according to still another aspect provides a process for preparing a rubber composition having improved low strain modulus which comprises (a) incorporating into an uncured non-fluorinated diene rubber or mixture thereof (1) a low strain modulus improving amount of a fibrillatable polytetrafluoroethlyene which is effective to improve low strain modulus and (2) a curing agent or mixture thereof, and (b) curing said composition.

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, ~ ~J7 BRIEF DESCRIPTION OF THE DRAWING
The sole figure of drawing is a cross-sectional view of a vulcanized radial tire having one or more components whose composition is in accordance with this invention.

DETAILED DBSCRIPTION OF THE INVENTI~N

The process of this invention can be utilized to modify virtually any type of rubbery elastomer which contains double bonds and which does not contain fluorine. The elastomer preferably contains no halogen of any kind. The rubbers which are modified in accordance with this invention typically contain repeat units which are derived from diene monomers, such as conjugated diene monomers and/or nonconjugated diene monomers. Such conjugated and nonconjugated diene monomers typically contain from 4 to about 12 carbon atoms and preferably contains from 4 to about 8 carbon atoms. Some representative examples of suitable diene monomers include 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 2-methyl-1,3-pentadiene, 3,4-dimethyl-1,3-hexadiene, 4,5-diethyl-1,3-octadiene, phenyl-1,3-butadiene and the like. The polydiene rubber can also contain various vinyl aromatic monomers, such as styrene, 1-vinylnaphthalene, 1-vinylnaphthalene, alpha-methylstyrene, 4-phenylstryrene, 3-methylstyrene, and the like. Some representative examples of polydiene rubbers that can be modified by utilizing the procedure of this invention include polybutadiene, styrene-butadiene rubber (SBR), synthetic polyisoprene, natural rubber, isoprene-butadiene rubber, isoprene-butadiene-styrene rubber, nitrile rubber, carboxylated nitrile rubber, and EPDM rubber. Preferred rubbers are hydrocarbon rubbers. The technique of this invention a ~J J 7 is particularly well suited for utilization in modifying natural rubber, synthetic cis-l-4,polyisoprene, and cis-1,3-polybutadiene (i.e., conjugated diolefins) and blends or mixtures thereof.
The polytetrafluoroethylene (PTFE) which is used to improve the low strain modulus of rubber must be a fibrillatable PTFE, i.e., a PTFE which fibrillates upon compounding into uncured rubber followed by application of shear. Such shear is applied during conventional mixing or milling of the compounding ingredients.
Non-fibrillatable grades of PTFE will not give the desired improvement (or any noticeable improvement) in low strain modulus. Also, not all fibrillatable grades of PTFE are equally effective, and some grades do not offer the desired improvement or any observable improvement in low strain modulus.
The fibrillatable PTFE grades used according to this invention are homopolymers having a molecular weight in excess of 1,000,000, preferably in excess of 2,000,000. Either a single grade or a mixture of `
fibrillatable PTFE grades can be used.
A PTFE which has been found to be singularly useful in improving low strain modulus is "Teflon" 60, which is made and sold by E.I. duPont de Nemours & Co., Wilmington, Delaware, USA. ("Teflon" is a registered trademark of E. I. duPont de Nemours and Co.) The grade of PTFE is characterized by dimensions on the order of .16 ~ x 32 ~ (~ = microns)(representing a representative diameter and a representative length, respectively) which corresponds to an aspect ratio of about 200 after fibrillation. Another PTFE which also gives some improvement in low strain modulus is "Teflon" 6C, which is also made and sold by du Pont.
This grade was found to be less effective than "Teflon"

60; in other words, greater loadings of "Teflon" 6C are required to give equivalent increases in 50 percent modulus then are required if "Teflon" 60 is used. Both of these are fibrillated of (fibrillating) grades of PTFE. They are sold as finely divided powders.
A third grade, "Teflon" DR, was tested and found to be ineffective. "Teflon" DR is believed to be a non-fibrillatable grade of PTFE. "Teflon" DR is believed to be the same as "Teflon" MF-1500, which is sold commercially by duPont.
The entire PTFE content of elastomer compositions of this invention should be in the form of one or more fibrillating grades. It is believed that the degree of fibrillation of PTFE is correlated with its effectiveness as a low strain improving PTFE additive, although this has not been definitively established.
In other words, it is believed that "Teflon" 60 fibrillates to a greater degree than "Teflon" 6C and that "Teflon" DR does not fibrillate.
Presence or absence of fibrillation can be observed by compounding three parts by weight (phr) of the candidate PTFE grade in 100 parts of elastomer and observing the compound under a scanning electron microscope (SEM) at a magnification of 25,000x. Fibril structure is observable if a fibrillatable grade of PTFE is used but not if a non-fibrillating grade of PTFE is used. Samples of elastomer matrix compounded with either "Teflon" 60 and "Teflon" 6C at a loading of 3 phr were observed under a scanning electron microscope (SEM) to have fibril structure, while the same elastomer compounded with 3 phr of "Teflon" DR had no observable fibril structure, and in fact the "Teflon" DR could not be observed under the microscope.
It is believed that observation of a test sample compounded in this manner is both a reliable indicator of fibrillation and the best indicator, other than actual testing of a compounded test sample for 50 percent modulus, of suitability of a given grade of PTFE for the purposes of this invention.
Another test procedure which can be used for screening particulate PTFE materials for suitability is the Mooney viscosity measurement. Elastomers reinforced with 3 phr of PTFE (only one grade being present in each sample) gave Mooney ML tMooney large) viscosities at 100C follows: "Tefloni' 60, 81.5;
"Teflon" 6C, 81.5; "Teflon" DR 74.5; control (no "Teflon") 73. These results were obtained with samples containing 100 phr of raw rubber, 3 phr of "Teflon", and no other compounding ingredients. The Mooney viscosity measurement is a relatively easy test to run and appears to correlate fairly well with actual performance. Suitable grades of PTFE, when compounded according to this test procedure, gives Mooney ML
viscosities in excess of about 78 at 100C.
The suitable grades of PTFE also appear to have average molecular weights (believed to be weight average) in excess of about 1,000,000 and usually in excess of 2,000,000 while unsuitable grades appear to have molecular weights of less than about 1,000,000.
For example, "Teflon" 6C and "Teflon" 60 are believed to have average molecular weights of about 5,000,000, while "Teflon" 1500 is reported to have an average molecular weight of about 500,000. However, molecular weight is relatively difficult to determine and so is not particularly a desirable screening tool.
The PTFE used should be a homopolymer of tetrafluoroethylene (TFE). "Teflon" 1500 (not suitable) is reported to be a copolymer of TFE and HFP
(hexafluoropropylene).

,~'r2~1;1 "Low strain modulus" herein refers to modulus values at strains (or elongations) of 150% or less, and is measured herein by modulus measurements taken at 50%
elongation and 100% elongation.
The amount of fibrillated PTFE required for effective low strain modulus improvement is generally in the range of about 0.5 phr to about 7 phr. The required amount will vary from one grade of PTFE to another. For example, the most effective PTFE grade 10 tested, Teflon 60 should be added in amounts ranging from about 0.5 phr to about 4 phr. On the other hand, the amount of Teflon 6C was found to be in the range from about 2 phr to about 7 phr. In any case, the amount of any given grade of fibrillated PTFE can be 15 readily determined by preparation of a standard laboratory recipe, such as the recipe given in the examples below, and testing of the cured compound in a standard tensile tester, as will also be illustrated below.
Throughout the specification, including the claims, the term, "phr", denotes parts by weight per 100 parts of rubber. Also, all amounts are given in parts by weight unless the contrary is expressed.
The vulcanizable rubber composition of the present 25 invention contains a curing agent or mixture thereof.
This curing agent may be either a sulfur vulcaniæing agent or mixture thereof, or a peroxide curing agent.
For most purposes, including preparation for tire tread stock, a sulfur vulcanizing package, i.e., a sulfur 30 vulcanizing agent or mixture thereof, is preferred.
The amount of sulfur vulcanizing agent or mixture thereof will vary depending on the type of rubber and the particular type of sulfur vulcanizing agent that is used. Generally speaking, the amount of sulfur 35 vulcanizing agent ranges from about 0.1 to about 10 phr "3''~.~

with the range of from about 0.5 to about 7 being preferred.
Examples of suitable sulfur vulcanizing agents include elemental sulfur and sulfur donating vulcanizing agents, for example, an amine disulfide, a polymeric polysulfide or a sulfur olefin adduct.
The amount of peroxide curing agent or mixture thereof, when used, is also typically within the range of about 0.1 to about 10 phr with an amount from about 1 to about 5 phr being preferred.
In addition to the above, other rubber additives may be incorporated in the sulfur vulcanizable material. The additives commonly used in rubber vulcanizates are, for example, carbon black, silica, tackifier resins, processing aids, antioxidants, antiozonants, stearic acid, activators, waxes, oils and peptizing agents.
Other additives not mentioned above may be included, and certain additives mentioned above may be omitted depending on the intended use of the material.
For example, an adhesion promoter or mixture thereof may be included in fabric-reinforced tire components such as carcasses and reinforcing belts, and in fabric reinforced articles such as rubber hoses and conveyor belts, for promoting adhesion of the vulcanized rubber stock to the cords of reinforcing material (typically brass coated steel, polyester or nylon).
Additives are typically present in conventional amounts. For example, the vulcanizable rubber formulation may contain: carbon black, from about 20 to about 100 phr; silica, from about 5 phr to about 25 phr; tackifier resins from about 0 phr to about 20 phr;
processing aids from about 1 phr to about 10 phr;
antioxidants from about 1 phr to about 10 phr;
antiozonants from about 1 phr to about 10 phr; stearic acid from about 0.1 phr to about 4 phr; zinc oxide from about 2 phr to 10 phr; waxes from about 1 phr to 5 phr;
oils from about 5 phr to 30 phr; peptizers from about 0.1 phr to 1 phr; adhesion promoter (when present) from about 1 to about 10 phr; and retarder from 0.05 phr to 1.0 phr. The presence and relative amounts of the above additives are not an aspect of the present invention and can be added at any desired level for a particular application.
Accelerators may be used to control the time and/or temperature required for vulcanization and to improve the properties of the vulcanizate. In some instances, a single accelerator system may be used, i.e., primary accelerator. Conventionally, a primary accelerator is used in amounts ranging from about 0.5 phr to about 2.0 phr. Combinations of two or more accelerators may also be used at appropriate levels to accelerate vulcanization. Such combinations are known to be synergistic under appropriate conditions and one of ordinary skill in the art would recognize when their use would be advantageous and at what levels.
Suitable types of accelerators that may be used include amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates and xanthates. Preferably, the primary accelerator is a sulfenamide. If a secondary accelerator is used, the secondary accelerator is preferably a guanidine, dithiocarbamate or thiuram compound.
Except for a fibrillatable PTFE, additives and amounts thereof may be and typically are conventional.
The uncured or vulcanizable rubber compositions of this invention are essentially water free, i.e., they contain no added water, and the only water in these compositions (or compounds as they are known in the rubber industry) is whatever water is associated with S~7 either the additives or the elastomer matrix stock.
The total amount of additives is usually not over about 200 phr, since higher overall additive levels will typically result in a vulcanized rubber which is too stiff.
Conventional rubber compounding techniques can be used to form compositions according to this invention. For example, rubber and desired additives (typically all except the accelerators and optionally zinc oxide) can be mixed together in a first mixing stage to form a masterbatch, and the accelerator(s) and zinc oxide (if not added previously) can be added in a second mixing stage to form a production mix, which is formed into the desired uncured rubber article or tire component.
Vulcanization of the compounded rubbers of the present invention may be conducted at conventional temperatures used for vulcanizable materials. For example, temperatures may range from about 100C to 200C. Preferably, the vulcanization is conducted at temperatures ranging from about 110C to 180C. ~ny of th~ usual vulcanization processes may be used, such as heating in a press mold, heating with superheated steam or hot air or at a salt bath.
Vulcanized rubber compositions of this invention are particularly useful as the rubber of tire treads.
~ther tire components, such as sidewalls, carcasses, reinforcing belts, interfacial material between wire coats (i.e. reinforcing belts), tread bases and apexes and other rubber articles such as hoses, conveyor belts, drive belts, treads for off-road vehicles such as tractors, and the like, can be prepared from rubber compositions of this invention. Compositions of this invention in general are useful as the rubber or elastomer ingredient of rubber articles and tire ~r~

components which are subject to stress. The increased low strain modulus of these compositions reduces the amount of strain or deformation due to such stress.
For further understanding of this invention, reference is made to the accompanying drawing.
In the drawing, components of the tire 1 as shown include a ground-contacting tread 2 and a pair of sidewalls 3 which abut the tread 2 in the shoulder regions 4. A fabric-reinforced rubber carcass 5 of generally toroidal shape and consisting of one or more plies supports the tread and sidewalls. A
circumferential fabric-reinforced belt 6 of one or more plies is positioned between the carcass 5 and the tread 2.
Tire 1 also includes a pair of spaced circumfer-entially extending bundled wire beads 7 wh~ch are substantially inextensible. Carcass 5 extends from one bead 7 to the other and the side edges may be wrapped around the beads as shown. Tire 1 may also include a pair of stiff apex components 8 of triangular cross section in the region of beads 7 and a pair of stiff chafer components 9 which are positioned in the bead region, basically between the respective beads 7 and the rim on which the tire is to be mounted. The apex components 8 and chafer components 9 add dimensional stability to the tire by resisting forces imparted to it during cornering.
The structure of tire 1 may be conventional. Tire 1 illustrated in the drawing is of conventional structure and has been simplified in the interest of clarity by omitting parts which are not required for an understanding of this invention.
One or more components of a tire 1 can be formed of a vulcanized rubber composition (or compound) in accordance with this invention. The remaining 2~ 6~37 components can be of conventional composition(s). In a preferred embodiment, tread 2 or apex components 8, or both, are composed of a composition in accordance with this invention. Another tire component which is advantageously formed of a composition in accordance with this invention includes reinforcing belt 5. In general, any tire component that is subjected to stress is advantageously formed of a composition of this in~ention.
Vulcanized compositions of this invention have increased low strain modulus, i.e., modulus as measured at either 50% or 100% elongation, compared to the corresponding values in control compositions. In other words, compositions of this invention are stiffer than control compositions having the same carbon black or silica content. A control composition shall be understood as a vulcanized rubber having the same ingredients in the same amounts, except for the absence of fibrillated PTFE, as a vulcanized composition of this invention, and prepared and cured in the same way.
Typically, 300% modulus of compositions of this invention is also higher than in corresponding control compositions. Tensile strength and elongation at break are typically or slightly but not significantly diminished compared to the control. There is ordinarily no significant effect on tan delta at either 0C or 60C. As is known in the art, tan delta at 0C
is taken as a measure of the skid resistance of a given rubher compound, while tan delta at 60C is taken as a measure of the heat buildup in a tire, low heat buildup being correlated with low rolling resistance. Tan delta is, basically, a measure of the ratio of loss modulus (E") to storage modulus (E'). Storage modulus is considered a measure of a rubber's ability to dissipate energy and loss modulus is considered a 2~ 7 measure of the rubber's ability to absorb energy. `-Thus, in effect, tan delta is a measure of a rubber compound's viscoelastic character and has been observed to relate to tire performance.
Tear strength of compositions according to this invention is acceptable for the uses disclosed herein but may be slightly to somewhat lower than the tear strength of control compositions, since most materials which increase tear strength lower modulus, and conversely most materials which increase modulus lower tear strength. (Tear strength of compositions herein was not measured).
The practice of this invention is illustrated by reference to the following examples, which are intended to be representative rather than restrictive of the scope of the invention. Unless otherwise indicated, all parts and percentages are by weight.

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Example 1 Rubber compounds containing fibrillated PTFE, having the compositions shown in Table I below, were prepared as follows:
Synthetic polyisoprene (i.e., a high cis-1,4-polyisoprene), a PTFE (except in control run lE), thiodiphenylamine (a high temperature antidegradant) and diaryl-p-phenylenediamine (an antioxidant) were charge to a 300 gram sized Brabender mixer and mixed in this mixer for 5 minutes at 70 rpm. The temperature was initially 110C, rising to 150C. This resulted in a "Teflon"/polyisoprene mixture (or premix).
Compounding of the "Teflon"/polyisoprene mixture was carried out in two stages, i.e., a first or "nonproductive" stage, in which the above premix and additional compounding ingredients, as shown in Table I
below, were mixed in a 60 gram Brabender mixer for 5.5 minutes at 120C; and a second or "productive" stage, in which the first stage, or nonproductive mix was compounded with further compounding ingredients, also shown in Table I below, in a 60 gram Brabender mixer for 3 minutes for 50C. The term, "productive stage", denotes the final compounding stage, which results in formation of a product; the term, "nonproductive stage"
denotes an earlier mixing stage, resulting in the formation of an intermediate which is further compounded.
Two grades of PTFE, i.e., "Teflon" 60 and "Teflon"
DR, both made by E.I. du Pont de Nemours & Company, were used in this example.
Recipes in Table I below are given in parts by weight. Amounts were chosen so that the sum of polyisoprene and PTFE contents in each run was 100 parts by weight.

2~ 7 A sample of uncured compound lA was examined by SEM (scanning electron microscopy) and TEM
(transmission electron microscopy). SEM examination revealed fibrils which were estimated to be about 32 x 0.16 ~ in size. There was no sign of fibrils by TEM, and what was believed to be PTFE showed no distinct boundaries and merged into the rubber/carbon black background.
The rubber compounds were cured at 150C for 25 minutes. Physical testing is described in Example 3.

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18 ;~ 6;~37 Example 2 Rubber compounds having the compositions shown in Table II were compounded and cured as described in Example 1. It will be noted that the compositions in runs lA and 2F are the same. The test samples in run 2F
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Z~ i7 This Example describes physical testing of samples of the compounds prepared in Examples 1 and 2. Modulus and tensile and elongation measurements were made in an Instron tensile tester model number 1122. Modulus and tensile measurements are measured in megapascals (MPa). The tensile measurements represent tensile strength at break. Tan delta measurements were made in a Rheovibron dynamic viscoelastometer, Model No. DDV~
II-C, made by Toyo Measuring Instrument Company, Ltd.
Results are shown in Table III.
In Table III, "M50", "M100" and "M300" denote 50%
modulus, 100% modulus and 300% modulus respectively.

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As can be seen from Table III, "Teflon'l 60 gave appreciable increase in 50 percent modulus and 100 percent modulus as compared to the control (run lE) at both 1 phr and 3 phr loadings. "Teflon" 6C also gave improved 50 percent modulus and 100 percent modulus as compared to the control, particularly at 3 phr and 6 phr loadings, but it will be observed that the amount of "Teflon" 6C was appreciably greater than the amount of "Teflon 60" required for comparable increase in 50 percent modulus and 100 percent modulus. "Teflon" DR, on the other hand, appears to be ineffective.
Example 4 Samples A, B and C, which were synthetic cis-1,4-polyisoprene reinforced with 3 phr of "Teflon" 60, "Teflon" 6C and "Teflon" DR, respectively, were prepared. Also prepared was a control sample ("ccntrol") which contained no Teflon reinforcing agent but was also cis-1,4-polyisoprene~ The other compounding ingredients and the processing methods were the same for all samples and are as described in Example 1 except that the mixing time in the Brabender mixer was 20 minutes at 300F (about 150C ).
The rubber compounds were cured at 300F (about 150C) for 20 minutes. New samples of each composition were prepared and tested. Modulus, tensile and elongation measurements were made on an Instron tensile tester, model number 1122. Results (mean values for two samples) are shown in Table IV below.
In Table IV, "MPA" denotes megapascals.
Sample A ("Teflon" 60) showed considerable improvement in 50 percent modulus and 100 percent modulus (which together indicate low strain modulus) as compared to the control. Sample B ("Teflon" 6C) showed marked improvement in 50 percent and 100 percent modulus, but less than that of Sample A. Sample C

Jr ~ ~ 7 ("Teflon" DR), on the other hand, showed no significant difference from the control in 50 percent modulus and 100 percent modulus. All "Teflon" loadings in samples A, B and C were 3 phr. This shows that "Teflon" 6C is a good low strain modulus improving agent and "Teflon"
DR is ineffective as a low strain modulus improving agent.

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~ ~C~7 While certain representative embodiments and details have been shown for the purpose of illustrating the subject invention, it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention.

Claims (20)

1. A vulcanizable rubber composition comprising a non-fluorinated diene rubber or mixture thereof, and an amount of low strain modulus-improving fibrillated polytetrafluoroethylene sufficient to improve the low strain modulus of said composition.

2. A vulcanizable rubber composition as claimed in claim 1 wherein the amount of said fibrillated polytetrafluoroethylene is from about 0.5 to about 8 parts by weight per 100 of rubber.

3. A vulcanizable rubber composition as claimed in claim 1 wherein said fibrillated polytetrafluoroethylene has an average molecular weight of at least about 1,000,000.

4. A vulcanizable rubber composition as claimed in claim 1 wherein said fibrillated polytetrafluoroethylene has an average molecular weight of at least about 2, 000, 000 .

5. A vulcanizable rubber composition as claimed in claim 1, further including a curing agent or mixture thereof.

6. A vulcanizable rubber composition as claimed in claim 5 wherein said curing agent or mixture thereof is a sulfur vulcanizing agent or mixture thereof.

7. A vulcanizable rubber composition as claimed in claim 1 wherein said diene rubber is a polymer of a conjugated diene or mixture thereof.

8. A rubber composition as claimed in claim 7 wherein said conjugated diene is cis-1,4-polyisoprene.

9. A vulcanizate of the composition of claim 1.

10. A vulcanizate as claimed in claim 9, said vulcanizate having an appreciably greater 50 percent modulus than that of an otherwise similar vulcanizate which does not contain polytetrafluoroethylene.

11. A vulcanizate as claimed in claim 10, said vulcanizate having a 50 percent modulus which is at least 10 percent higher than that of said otherwise similar vulcanizate which does not contain said polytetraflouroethylene.

12. A rubber article comprising the vulcanizate of claim 9.

13. A rubber article as claimed in claim 12, said article being a tire.

14. A tire as claimed in claim 13, said tire comprising a plurality of components including a tread and a reinforcing belt, the rubber of at least one of said components consisting essentially of said vulcanizate.

15. A tire as claimed in claim 13 said tire comprising a plurality of components including a tread, wherein the rubber of said tread consists essentially of said vulcanizate.

16. A process for preparing a rubber composition having improved low strain modulus which comprises:

(a) incorporating into an uncured diene rubber or mixture thereof (1) a low strain modulus improving amount of a fibrillated polytetrafluoroethylene which is effective to improve low strain modulus and (2) a curing agent or mixture thereof, and (b) curing said composition.

17. A process as claimed in claim 16 wherein the amount of said fibrillated polytetrafluoroethylene is from about 0.5 to about 8 parts by weight per 100 of rubber.

18. A process as claimed in claim 16 wherein said fibrillated polytetrafluoroethylene has a molecular weight of at lest about 1,000,000.

19. A process as claimed in claim 16 wherein said fibrillated polytetrafluoroethylene has a molecular weight of at least about 2,000,000.

20. A process as claimed in claim 16 wherein said curing agent or mixture thereof is a sulfur curing agent or mixture thereof.