Rubber composition having high tenacity, high modulus and good cut growth resistance

Abstract: It is desirable to increase the modulus of rubbers utilized in a wide variety of applications. For example, it is typically desirable to increase the modulus of rubber compounds which are utilized in tire tread base compositions and in tire wire coat compounds. Unfortunately, most conventional techniques for increasing the modulus of a rubber compound are detrimental to other rubber properties, such as hysteresis and cut growth resistance. By utilizing the techniques of this invention rubber compounds can be prepared which exhibit high tensile strength, high modulus, and good cut growth resistance. The subject invention specifically relates to a process for preparing a highly dispersed blend of a polyester in an elastomer which comprises (1) blending a solution of a rubber in an organic solvent with a solution of a polyester in an organic solvent to produce a dispersion containing said rubber and said polyester, and (2) recovering the highly dispersed blend from the dispersion. This process can be used to increase the modulus of virtually any type of rubber including cis-1,4-polybutadiene, styrene-butadiene rubber (SBR) polyisoprene, isoprene-butadiene rubber, isoprene-butadiene-styrene rubber (SIBR), nitrile rubber, and the like.

Description: 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. Summary of the Invention The elastomeric compositions of this invention are prepared by dispersing polyethylene terephthalate (PET) throughout a rubber. This is done by blending a solution containing the rubber with a solution containing the polyester and then subsequently coagulating the dispersion to recover a highly dispersed blend of the PET in the rubber. This highly dispersed blend is an elastomeric composition which exhibits high modulus, high green strength and good cut growth resistance. This process can be employed to increase the modulus of polyisoprene, high cis-1,4-polybutadiene and styrene-butadiene rubber (SBR). The subject invention specifically discloses a process for preparing an elastomeric composition having a high modulus and high green strength which comprises (1) blending a solution of a rubber with a solution of a polyester to produce a rubber/polyester dispersion, and (2) coagulating the rubber/polyester dispersion to recover said elastomeric composition. The subject invention also reveals a process for preparing a highly dispersed blend of a polyester in an elastomer which comprises (1) blending a solution of a rubber in an organic solvent with a solution of a polyester in an organic solvent to produce a dispersion containing said rubber and said polyester, and (2) recovering the highly dispersed blend from the solution. Detailed Description of the Invention The process of this invention can be utilized to increase the modulus of virtually any type of rubbery elastomer. The rubbers which are modified in accordance with this invention are polyisoprene, SBR and cis-1,4-polybutadiene. In making the highly dispersed blends of this invention a solution of the rubber is thoroughly mixed with a solution of the PET. The rubber solution (rubber cement) can be prepared by dissolving a dry rubber in a suitable organic solvent. However, if the rubber is prepared by solution polymerization there is no need to recover it from the solvent in which the polymerization was carried out before blending it with the polyester solution. Such solution rubbers can be prepared by any standard solution polymerization technique. The organic solvent utilized in the rubber solution will typically be a saturated aliphatic hydrocarbon or an aromatic hydrocarbon. Such organic solvents will normally contain from 5 to 12 carbon atoms per molecule and will be liquids at room temperature. Some representative examples of suitable aromatic solvents include benzene, toluene, xylenes, ethylbenzene, diethylbenzene, isobutylbenzene, and the like. Some representative examples of suitable aliphatic solvents include n-hexane, cyclohexane, methylcyclohexane, isohexanes, n-heptane, n-octane, isooctanes, n-decane, 2,2-dimethylbutane, petroleum ether, kerosene, petroleum spirits and petroleum naphtha. Halogenated hydrocarbons, such as chloroform, methylene chloride, tetrachloroethane and the like, can also be employed as the organic solvent. The rubber cements utilized in the practice of this invention will typically contain from 5 weight percent to 35 weight percent rubber, based upon the total weight of the rubber cement. It is typically preferred for the solution of the rubber to contain from 10 weight percent to 30 weight percent rubber. It will generally be more preferred for the rubber cement to contain from 15 weight percent to 25 weight percent rubber. PET which is useful in making the high modulus elastomeric compositions of this invention can be synthesized utilizing conventional polymerization techniques which are well known in the art. Thus, conventional temperatures, catalysts, amounts of catalysts, stabilizers and the like, are used in manners well known in the literature and art. For instance, a two step process can be utilized in preparing such polyesters. In cases where diesters are utilized as the diacid component in the first step, known as the transesterification stage, the diacid component and the diol component can be heated to a temperature which is within the range of 150°C to 240°C under an inert gas atmosphere, such as nitrogen or a Noble gas. For economic reasons, nitrogen will usually be utilized. This polymerization reaction can be carried out in the presence of appropriate catalysts, such as titanium alkoxides, tetraalkyl titanium compounds, or zinc acetates. In cases where diacids are utilized as the-diacid component, the diacid component and the diol component are heated to a temperature which is within the range of 240°C to 290°C at a pressure of 10 psi (6.9 x 10 4 Pa) to 70 psi (4.8 x 10 5 Pa). A catalyst is normally not utilized in such esterification steps. In the second step, the polycondensation reaction can be carried out under a reduced pressure of less than about 0.5 mm of mercury (66.7 Pascals) at a temperature which is typically in the range of 230°C to 300°C. The polymerization time required will vary with the amount and type of catalyst used as well as the polymerization temperature utilized. The extent of the polycondensation will also depend somewhat on the desired molecular weight of the copolyester being synthesized. It is desirable for the polyester to have an IV (intrinsic viscosity) of at least 0.4 dl/g in a 60:40 phenol:tetrachloroethane solvent system at a temperature of 30°C. It is normally preferred for the polyester to have an IV which is within the range of 0.6 dl/g to 6.0 dl/g. It is generally most preferred for the polyester to have an IV which is within the range of 1.0 dl/g to 3.0 dl/g. A solution of the PET is prepared by simply dissolving the PET in a suitable solvent. The solvent selected will vary with the type and molecular weight of the polyester chosen. Normally a wide variety of solvents or solvent systems capable of dissolving the polyester at room temperature or at a slightly elevated temperature can be employed. Some representative examples of suitable solvents in which PET can be dissolved include nitro-benzene, trifluoroacetic acid, acetonapthone, hexafluoroacetone hexafluoroisopropanol, meta-cresol, trifluoroacetic acid/methylene chloride mixed solvent systems, nitro-benzene/tetrachloroethane mixed solvent systems, hexafluoroisopropanol/chloroform mixed solvent systems, and tetrachloroethane/phenol mixed solvent systems. Trifluoroacetic acid/methylene chloride mixed solvent systems which contain from about 25 weight percent to about 75 weight percent trifluoroacetic acid and from about 25 weight percent to about 75 weight percent methylene chloride are preferred. It is generally more preferred for such solvent systems to contain from about 40 to about 60 weight percent trifluoroacetic acid and from about 40 to about 60 weight percent methylene chloride. The concentration of the PET solution will vary with the type of PET, the PET molecular weight, the PET crystallinity, and with the solvent system being employed. For example, the maximum concentration of PET which can be dissolved in a solvent system containing 50 weight percent trifluoroacetic acid and 50 weight percent methylene chloride decreases from about 40% at an IV of 0.6 dl/g, to 30% at an IV of 1.0 dl/g, to 25% at an IV of 2.0 dl/g, to about 15% at an IV of 4.5 dl/g. In making the elastomeric compositions of this invention a solution of the rubber is simply blended with a solution of PET. The solutions are thoroughly mixed together to attain an essentially homogenous rubber/PET dispersion. The elastomeric composition is subsequently recovered from the rubber/PET dispersion. The elastomeric composition recover is a highly dispersed blend of PET in the elastomer. Because the polyester is extremely well dispersed throughout the rubber the elastomeric composition exhibits high modulus, high green strength, and a high level of resistance to cut growth. The elastomeric composition is recovered from the rubber/PET dispersion by removal of the organic solvent. This can be accomplished by evaporation, coagulation, or any other suitable means. In most commercial operations, it will be preferred to recover the elastomeric composition from the rubber/PET dispersion by coagulation. In such a coagulation process, the rubber/PET dispersion can be coagulated by simply pouring it into an organic solvent which is capable of dissolving the organic solvent system in which the rubber and PET is dispersed but which is not a good solvent for the rubber or PET. In some cases it is preferable to add a suitable coagulating agent to the rubber/PET dispersion to induce coagulation. Alcohols and ketones can typically be utilized in such coagulation procedures. Some representative examples of alcohols which can be utilized include methanol, ethanol, isopropyl alcohol, n-propyl alcohol, cyclohexanol, n-butyl alcohol, isobutyl alcohol, s-butyl alcohol, t-butyl alcohol, and n-pentanol. Some representative examples of ketones which can be used in the coagulation procedure include acetone, methylethylketone, methylphenolketone and cyclohexanone. After coagulation the elastomeric composition can be recovered from the organic solvents by decantation or filtration. It is often desirable to squeeze residual organic solvents from the elastomeric composition by mechanical means, such as squeeze rollers. Additional residual organic solvent can subsequently be removed by evaporation. The evaporation of residual solvent can be accelerated by heating the elastomeric composition to a slightly elevated temperature. This temperature will generally be within the range of about 25°C to about 125°C. Higher temperatures can lead to polymer degradation and are accordingly undesirable. It is also generally beneficial to increase the surface area of the elastomeric composition to facilitate the evaporation of residual organic solvent. It is normally desirable to utilize vacuum to accelerate the rate at which the residual organic solvent evaporates from the rubber/PET blend. In practicing this invention the PET solution will be blended with the rubber solution in amounts whereby the highly dispersed blend of PET in the elastomer will contain from 1 phr to 55 phr (parts per hundred parts of rubber) of PET. In most cases, it will be preferred for the elastomeric composition to contain from about 2 phr to about 40 phr of PET. It will typically be more preferred for the elastomeric composition to contain from about 5 phr to about 20 phr of PET. This invention is illustrated by the following working examples which are presented merely for the purpose of illustration and are not to be regarded as limiting the scope of the invention or the manner in which it can be practiced. Unless specifically indicated otherwise, parts and percentages are given by weight. Example 1 In this experiment a high modulus elastomeric composition was prepared by the technique of this invention. In the procedure utilized 500 grams of a 20 percent solution of synthetic polyisoprene (Natsyn®) in hexane was diluted with 1,800 ml of methylene chloride. Then 80 ml of a polyethylene terephthalate in a mixed solvent system containing 50 percent trifluoroacetic acid and 50 percent methylene chloride was added with rapid stirring. The polyethylene terephthalate solution contained 10 grams of polymer which had an intrinsic viscosity of 1.9. The resulting dispersion was poured into excess isopropanol and an opaque elastomer precipitated. The elastomer which was recovered was washed with additional isopropanol and subsequently dried in a vacuum oven at 40°C for 16 hours. The elastomeric composition recovered was then compression molded at 100°C into 4" x 4" x 1/8" (10.2 cm x 10.2 cm x 3.2 mm) sheets. The sheets were cut into "dumbbell" test strips which were 1/8" (3.2 mm) wide. The test strips were then evaluated in an Instron tensile machine at 20" (51.8 cm) per minute to determine their tensile modulus and tensile strength. Control samples which consisted totally of the synthetic polyisoprene were also evaluated. The results of this tensile testing are as follows. Tensile Modulus (Pa) Tensile Strength (Pa) 50% 100% Polyisoprene (control) 6.3x10 4 1.2x10 5 1.6x10 5 Polyisoprene/PET Blend 1.2x10 5 2.0x10 5 3.7x10 5 As can be seen the incorporation of 10 phr of PET into the synthetic polyisoprene by the technique of this invention very substantially increased both tensile modulus and tensile strength. More specifically, at 50 percent and 100 percent elongation the tensile modulus of the samples was increased by 91 percent and 67 percent, respectively. The tensile strength of the polyisoprene was increased by 131 percent. This example clearly shows that the technique of this invention can be utilized to greatly improve the tensile modulus and tensile strength of rubbers. Example 2 In this experiment a high modulus elastomeric composition was made using the procedure described in Example 1 except that only 8.6 grams of PET was in the polyester solution. Thus, the blend recovered contained 8.6 phr of the PET. The blend was compounded with sulfur, accelerators, and other standard rubber chemicals and cured using typical vulcanization procedures. The vulcanizate was then evaluated in an Instron tensile machine to determine its modulus, elongation at break, and break strength. It was determined that the cured blend made in this experiment had a tensile modulus which was 57 percent, 95 percent, and 27 percent greater than a cured polyisoprene control at elongations at 50 percent, 100 percent and 300 percent, respectively. The elongation at break and break strength of the cured blend were 6 percent and 7 percent, respectively, less than those of the cured polyisoprene control. The actual results of this tensile testing are as follows: Tensile Modulus (Pa) Elongation at Break Break Strength 50% 100% 300% Cured 7.5 1.2 5.6 Polyisoprene x x x 604% 1.9 x 10 7 (Control) 10 5 10 6 10 6 Cured 1.2 2.4 7.0 Polyisoprene/ x x x 570% 1.8 x 10 7 PET Blend 10 6 10 6 10 6

Claims: A process for preparing a high green strength highly dispersed blend of polyethylene terephthalate in an elastomer which is characterized by (1) blending a solution of a rubber selected from the group consisting of polyisoprene, high-cis-1,4-polybutadiene, and styrene-butadiene rubber in an organic solvent with a solution of polyethylene terephthalate (PET) in an organic solvent to produce a dispersion containing said rubber and from 1 to 55 parts per hundred parts of rubber of said polyethylene terephthalate, and (2) recovering the highly dispersed blend from the dispersion. A process as specified in claim 1 wherein the highly dispersed blend is recovered by coagulation of the dispersion. A process as specified in claim 1 or 2 characterized in that the rubber is polyisoprene. A process as specified in any of the preceding claims characterized in that the dispersion containing the rubber and PET contains from 2 to 40 parts per hundred parts of rubber of PET. A process as specified in claim 2 characterized in that the coagulation is carried out by mixing the rubber/polyethylene terephthalate dispersion with at least one member selected from the group consisting of alcohols and ketones. A process as specified in any of the preceding claims characterized in that the dispersion containing the rubber and the polyethylene terephthalate contains from 5 to 20 parts per hundred part of rubber of the polyethylene terephthalate. A process as specified in any of the preceding claims characterized in that the polyethylene terephthalate has an intrinsic viscosity as measured in a 60:40 phenol:tetrachloroethane solvent system at a temperature of 30°C which is within the range of 0.6 dl/g to 6.0 dl/g. A process as specified in any of the preceding claims characterized in that the polyethylene terephthalate has an intrinsic viscosity as measured in a 60:40 phenol:tetrachloroethane solvent system at a temperature of 30°C which is within the range of 1.0 dl/g to 3.0 dl/g.

Inventors: CALLANDER DOUGLAS DAVID, COHN GERALD, TUNG WILLIAM C T, CALLANDER, DOUGLAS DAVID, COHN, GERALD, TUNG, WILLIAM C.T.
Applicants: GOODYEAR TIRE & RUBBER, THE GOODYEAR TIRE & RUBBER COMPANY
Application year: 1991
Main classification: C08J3
IPC codes: C08L67, C08J3, C08L9, C08L21