Polyester reinforced rubber

Abstract: It is often desirable to increase the modulus of rubbers utilized in a wide variety of applications. This invention discloses a technique for modifying rubbers to improve their modulus without sacrificing other properties. By utilizing the process of this invention, rubber compositions having increased modulus are prepared by grafting polyester onto rubber. This can be accomplished by reacting a polydiene rubber with a thioacid or maleic anhydride and subsequently with the polyester. It is also possible to prepare such polyester modified rubber compositions by reacting an epoxidized rubber with the polyester.

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 subject invention discloses a technique for modifying a rubber so as to greatly increase its modulus. This results in the rubber being better suited for applications where a high degree of stiffness is desirable. However, this modification procedure does not generally sacrifice other desirable characteristics of the rubber. For instance, the rubber compositions of this invention have an increased modulus but do not have an increased degree of hysteresis. The rubber compositions of this invention are prepared by reacting maleic anhydride with the rubber and at least one polyester. This procedure grafts polymer chains which are comprised of polyester onto the polymer chains of the rubber. The subject invention specifically discloses a process for preparing a rubber composition which comprises reacting 0.1 to 20 parts per 100 parts of rubber by weight of maleic anhydride with at least one polydiene rubber and 20 to 100 parts per 100 parts of rubber by weight of at least one polyester having a melting point of less than 240°C. It is generally preferred to react the rubber with the maleic anhydride at a temperature which is within the range of 180°C to 200°C to form a rubber/maleic anhydride adduct and to subsequently react the rubber/maleic anhydride adduct with the polyester at a temperature which is at least as high as the melting point of the polyester. Detailed Description of the Invention The process of this invention can be utilized to modify virtually any type of rubbery elastomer which contains double bonds. 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 12 carbon atoms and preferably contain from 4 to 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, 2-vinylnaphthalene, α-methylstyrene, 4-phenylstyrene, 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. The technique of this invention is particularly well suited for utilization in modifying natural rubber, synthetic polyisoprene, and cis-1,4-polybutadiene. As a general rule, the polyesters utilized in modifying the rubbers of this invention will have a melting point of less than 240°C. In most cases it is preferable for the polyester to have a melting point of less than 200°C. The polyester utilized to modify the rubber, will typically be a polyester elastomer. Virtually any type of polyester elastomer having a melting point of less than 240°C can be used. Such polyester elastomers are widely available commercially. For instance, E. I. du Pont de Nemours & Company sells suitable polyester elastomers under the trademark Hytrel™. Dupont Hytrel™ 5555 has been determined to be highly suitable for use as a polyester elastomer in the high modulus rubber compositions of this invention. The polyester elastomers used in the high modulus rubber compositions of this invention will normally contain both polyether and polyester segments. For example, such a polyester elastomer is comprised of the reaction product of (a) terephthalic acid or a dialkyl ester thereof, (b) a dimer acid, (c) a poly(tetramethylene oxide) glycol and (d) 1,4-butane diol. Polyester elastomers of this general type are described in greater detail in U.S.-A- 4,254,001, Similar polyester elastomers which additionally contain chain branching agents and ionic compounds are described in U.S.-A- 4,383,106 and U.S.-A- 4,390,687. Patents U.S.-A-2,623,031, 3,023,192, 3,651,014, 3,763,109, 3,766,146, 3,896,078, 4,013,624 and 4,264,761, also describe polyester elastomers and techniques that can be utilized in their preparation. The modified rubber compositions of this invention can also be prepared by simply reacting the maleic anhydride with the polydiene rubber and the polyester. This can be accomplished by simply mixing the maleic anhydride homogeneously throughout a blend of the polydiene rubber and the polyester and heating the maleic anhydride/polyester/polydiene rubber blend. The double bond in the maleic anhydride will react with the double bonds present in the polydiene rubber and the anhydride groups in the maleic anhydride will react with hydroxy groups and carboxyl groups present in the polyester. However, it is preferred for the polyester to have predominately hydroxyl groups at ends of its polymer chains since hydroxyl groups are much more reactive than carboxyl groups with anhydrides. This reaction causes polyester chains to be grafted onto the backbone of the polydiene rubber. This reaction can be depicted as follows: wherein R represents chains of the rubber and wherein Y represents polymer chains of the polyester. The free carboxyl groups generated by such reactions can theoretically further react with hydroxyl groups of other polyester chains to produce: wherein two polyester chains are grafted onto the rubber by only one maleic anhydride molecule. The maleic anhydride, polyester, and polydiene rubber can be simultaneously mixed together and heated to graft the polyester onto the rubber. In another scenario, the maleic anhydride can be reacted with the rubber to produce a rubber/maleic anhydride adduct and the rubber/maleic anhydride adduct can then be subsequently reacted with the polyester to produce the modified rubber of this invention. In still another technique of this invention, the maleic anhydride is first reacted with the polyester to produce a polyester/maleic anhydride adduct and the polyester/maleic anhydride adduct is subsequently reacted with the rubber. It is desirable to prereact the maleic anhydride with the polyester in a first reaction step and to subsequently react the reaction product with the rubber because the rubber is subjected to elevated temperatures for a shorter duration of time and accordingly less degradation occurs. The reaction between the maleic anhydride and the polyester will typically be carried out at a temperature which is within the range of 150°C to 300°C. It will preferably be carried out at a temperature which is within the range of 165°C to 250°C and will more preferably be conducted at a temperature which is within the range of 180°C to 200°C. However, such reactions between the maleic anhydride and the polyester will be carried out at a temperature which is at least as high as the melting point of the polyester. Reactions between rubber/maleic anhydride adducts and polyester will also be conducted within these temperature ranges. The reactions between rubber and maleic anhydride and the reactions between rubber and polyester/maleic anhydride adducts will typically be carried out at a temperature which is within the range of 150°C to 300°C. Such reactions will more typically be conducted at temperatures which are within the range of 165°C to 250°C. The preferred temperature for such reactions depends upon the rubber being utilized and the reaction mechanism. For instance, most polydiene rubbers, such as high cis-1,4-polybutadiene, medium vinyl polybutadiene, SBR, synthetic polyisoprene, natural rubber, isoprene-butadiene rubber, nitrile rubber and isoprene-butadiene-styrene rubber, will react with maleic anhydride or polyester/maleic anhydride adducts at temperatures of greater than 180°C by an electrocyclic reaction without the need to utilize a catalyst. The most preferred temperature for conducting such reactions is accordingly from 180°C to 200°C. In cases where the polyester has a melting point of above 200°C then the preferred reaction temperature is slightly above the melting point of the polyester. Such reactions can be carried out at lower temperatures by a free radical mechanism. However, free radicals lead to gel formation and such procedures are generally not desirable. In fact, it is beneficial to conduct such reactions which are carried out utilizing electrocyclic reaction mechanisms in the presence of free radical trapping antioxidants to inhibit gel formation. Some representative examples of free radical trapping antioxidants which can be used include (1) the reaction product of p-nonyl phenol, formaldehyde, and dodecane thiol-1 (Wingstay® K) and (2) 2-(2-hydroxy-3-t-butyl-5-methylbenzyl)4-methyl-6-t-butyl phenyl methacrylate. EPDM rubber will generally not react with maleic anhydride or polyester/maleic anhydride adducts at commercially acceptable reaction rates by electrocyclic reaction mechanisms. For this reason, it is desirable to utilize a free radical reaction in reacting EPDM with maleic anhydride or polyester/maleic anhydride adducts. Such reactions are generally conducted at a temperature which is within the range of 180°C to 210°C. Such reactions are conducted in the presence of one or more free radical catalysts. Some representative examples of free radical initiators which can be used include various peroxygen compounds such as potassium persulfate, ammonium persulfate, benzoyl peroxide, hydrogen peroxide, di-t-butyl peroxide, dicumyl peroxide, 2,4-dichlorobenzoyl peroxide, decanoyl peroxide, lauroyl peroxide, cumene hydroperoxide, p-menthane hydroperoxide, t-butyl hydroperoxide, acetyl acetone peroxide, methyl ethyl ketone peroxide, succinic acid peroxide, dicetyl peroxydicarbonate, t-butyl peroxyacetate, t-butyl peroxymaleic acid, t-butyl peroxybenzoate, acetyl cyclohexyl sulfonyl peroxide and the like; the various azo compounds such as 2-t-butylazo-2-cyanopropane, dimethyl azodiisobutyrate, azodiisobutyronitrile, 2-t-butylazo-1-cyanocyclohexane, 1-t-amylazo-1-cyanocyclohexane, and the like; the various alkyl perketals, such as 2,2-bis-(t-butylperoxy)butane, ethyl 3,3-bis(1-butylperoxy)butyrate, 1,1-di-(t-butylperoxy) cyclohexane, and the like. Mercapto benzothiazyl disulfide is a preferred free radical catalyst. The maleic anhydride is mixed throughout the rubber and/or the polyester utilizing conventional mixing procedures. For example, the mixing can be carried out in a Banbury mixer or a mill mixer. Normally, such blends will be prepared by mixing from 20 phr to 100 phr of the polyester throughout the polydiene rubber. The polyester can, of course, be prereacted with the maleic anhydride prior to mixing it with the rubber. In most cases it will be preferred to utilize from 20 phr to 80 phr of polyester in the blend. It will be typically more preferred to utilize from 35 phr to 65 phr of polyester in the blend. The amount of maleic anhydride utilized will typically be within the range of 0.1 phr to 20 phr. In most cases it will be preferred to utilize from 0.5 phr to 3 phr of maleic anhydride. It is generally most preferred to utilize from 0.8 phr to 2 phr of the maleic anhydride. 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. Examples 1-11 In this series of experiments, high cis-1,4-polyisoprene was modified with a variety of polyester elastomers in accordance with the process of this invention. The modification procedure was carried out in a 280 gram Brabender preparative mixer which was electrically heated. In this series of modification procedures, the cis-1,4-polyisoprene was mixed with 25 phr of the polyester elastomer specified in Table I, 0.75 phr of maleic anhydride, 0.5 phr of phenothiazine (a high temperature antidegradant) and 0.5 phr of a stabilizer. The maleic anhydride, antidegradant, and stabilizer were added with the rubber at the beginning of the mix cycle. After three minutes, when the temperature was up to about 190°C, the polyester elastomer was added with mixing being continued for an additional 4.5 minutes at the same temperature or a higher temperature, if it was necessary to melt the polyester elastomer. The maximum mixing temperature utilized is specified in Table I. The rubber compositions made were compounded with 45 phr of carbon black, 9 phr of an oil, 2 phr of N-phenyl-N'-(1,3-dimethylbutyl)-p-phenylenediamine, 1 phr of diaryl-p-phenylene diamine, 1 phr of a wax, 3 phr of stearic acid, 3 phr of zinc oxide, 0.8 phr of N-oxydiethylene benzothiazole-2-sulfenamide, 0.4 phr of diphenyl guanidine, and 1.6 phr of insoluble sulfur. The rubber compositions were then cured at 300°F (149°C) for 25 minutes. The 50% modulus, 100% modulus, tensile strength and elongation of the cured rubber samples made are reported in Table I. Table I clearly shows that the polyester modified rubbers of this invention have significantly higher modulus than does unmodified high cis-1,4-polyisoprene (see Example 11 which was done as a control). A particularly high modulus was achieved in Example 7 which utilized Hytrel™ 5555 as the polyester elastomer for modifying the rubber. The modified rubber made in Example 7 was also determined to have a tan delta at 0° of 0.108 and a tan delta at 60° of 0.102. The control (Example 11) had a tan delta at 0° of 0.161 and a tan delta at 60° of 0.078. This series of experiments clearly shows that rubbers can be modified in accordance with this invention utilizing polyester elastomers to attain high levels of stiffness without increasing the state of cure. 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 this art that various changes and modifications can be made therein without departing from the scope of the subject invention.

Claims: A process for preparing a rubber composition which is characterized by reacting 0.1 to 20 parts per 100 parts of rubber by weight of maleic anhydride with at least one polydiene rubber and 20 to 100 parts per 100 parts of rubber by weight of at least one polyester having a melting point of less than 240°C. A process as specified in claim 1 characterized in that said polyester is a polyester elastomer. A process as specified in claim 1 or 2 characterized in that the process is carried out by reacting the polyester with the maleic anhydride to produce a polyester/maleic anhydride adduct and subsequently reacting the polyester/maleic anhydride adduct with the rubber. A process as specified in claim 1, 2, or 3 characterized by being carried out in the presence of at least one free radical trapping antioxidant. A process as specified in claim 1, 2, 3 or 4 characterized in that said polydiene rubber is selected from the group consisting of high cis-1,4-polybutadiene, medium vinyl polybutadiene, SBR, synthetic polyisoprene, natural rubber and isoprene-butadiene rubber; and in that from 0.5 to 3 parts per 100 parts of rubber by weight of the maleic anhydride is utilized.

Inventors: BAUER RICHARD GEORGE, BURLETT DONALD JAMES, PYKE JAMES BOLTON, BAUER, RICHARD GEORGE, BURLETT, DONALD JAMES, PYKE, JAMES BOLTON
Applicants: GOODYEAR TIRE & RUBBER, THE GOODYEAR TIRE & RUBBER COMPANY
Application year: 1990
Main classification: C08G81
IPC codes: C08L9, C08F279, C08L21, C08G81, C08C19