Rubber compositions

Abstract: Rubber compositions having excellent adhesion to metals, particularly suitable for steel cord coating rubbers, having a high vulcanizing rate and little offensive smell in an unvulcanized state, comprise 100 parts by weight of a rubber and 0.01-1 part by weight as a cobalt element content of at least one cobalt oxyketone complex represented by the following general formula: wherein R₁-R₄ are the same or different and each are a C₁₋₁₈ alkyl, C₅₋₁₂ cycloalkyl, C₆₋₁₄ aryl, C₆₋₁₄ aryl-C₁₋₄ alkyl, C₁₋₁₈ alkoxyl, C₅₋₁₂ cycloalkoxyl, C₆₋₁₄ aryloxyl, or C₆₋₁₄ aryl-C₁₋₄ alkoxyl group, or C₂₋₁₈ hydrocarbon group having at least one double bond.

Description: The present invention relates to rubber compositions having good metal adhesive properties, and more particularly to rubber compositions suitable for adhering to metal which exhibit excellent adhesion when used as a steel cord coating rubber. Recently, remarkable developments have been attained in so-called "steel tires" wherein steel cords are used for belts or carcass plies thereof, while various studies have been made of adhesion between steel cord and rubber. In general, methods for bonding a steel cord with a rubber can be roughly classified into two groups; one of which is known as an RHS bonding system which utilizes resorcinol/hexamethylene tetramine/silica, and the other wherein metal salts, particularly cobalt salts of organic acids, are compounded with the rubber. Specifically the latter, inter alia, has been widely utilized and rubber compositions compounded with metal salts have heretofore been proposed as steel cord coating rubbers. For example, in JP-B-56-39,828, JP-A-54-52,188, JP-B-50-33,905, 49-20,072, 52-26,276, 54-31,511, and JP-A-58-89,631, there are disclosed rubber compositions wherein cobalt naphthenate, cobalt stearate, cobalt oleate, cobalt tallate, cobalt resinate or the like is compounded as an adhesion promoter. Further, JP-A-54-47,778 and 62-146,936 discloses a rubber composition compounded with cobalt acetylacetonate. Meanwhile, demands for safety, high speed travelling properties, durability, etc. of automotive tires have recently been increasing so that the above conventional techniques do not always provide sufficiently satisfactory adhesion. That is, in tires reinforced with steel cords, heat generated during running induces deterioration of bonding force between the steel cords and rubber, causing failure of the tires. However, according to the conventional techniques, a satisfactory thermoresistant adhesion is not obtainable. Alternatively, from the standpoint of rationalization and energy saving in manufacturing processes, a rapid vulcanization is required. Therefore, adhesion promoters are required for promoting adhesion of rubbers to steel cords not only to improve the thermoresistant adhesion but also to augment initial bonding strength. However, adhesion promoters which satisfactorily meet these required characteristics have not yet been proposed. The cobalt acetylacetonate compounded rubber compositions described in JP-B-56-19,820 can provide relatively high bonding strength. However, these rubber compositions have such drawbacks as a low vulcanizing rate and an offensive smell of unvulcanized rubber compositions. As a result of researches to find an adhesion promoter suitable to be compounded with rubber compositions which exhibit a significant improvement in thermoresistant adhesion and initial adhesion of the rubber compositions, as compared with conventional cobalt naphthenate, etc. and which can also provide the resulting rubber compositions with a high vulcanizing rate and low emission of offensive smell, as compared with cobalt acetylacetonate, the present inventors have found that by utilizing specified cobalt oxyketone complexes notable improvements can be achieved both in thermoresistant adhesion and initial adhesion of rubber compositions and that the resulting rubber compositions have a high vulcanizing rate and little emission of offensive smells. As a result, the present invention has been accomplished. Accordingly, the present invention aims to provide rubber compositions suitable for adhering to metals which have improved both thermoresistant adhesion and initial adhesion as well as high vulcanizing rate and substantially no offensive smell in an unvulcanized state. The present invention provide a rubber composition comprising 100 parts by weight of a rubber and 0.01-1 part by weight as a cobalt element content of at least one cobalt oxyketone complex compounded in said rubber, which complex is represented by the following general formula (I): wherein R₁, R₂, R₃ and R₄ are the same or different C₁-C₁₈ alkyl, C₅-C₁₂ cycloalkyl, C₆-C₁₄ aryl, C₆-C₁₄ aryl-C₁-C₄ alkyl, C₁-C₁₈ alkoxyl, C₅-C₁₂ cycloalkoxyl, C₆-C₁₄ aryloxyl, C₆-C₁₄ aryl-C₁-C₄ alkoxyl groups, or C₂-C₁₈ hydrocarbon groups having at least one double bond, and wherein the total number of carbon atoms in R₁, R₂, R₃ and R₄ is at least 5. In a preferred embodiment of the rubber compositions according to the present invention, R₂ and R₄ in the formula (I) are both methyl groups, and R₁ and R₃ are the same or different groups represented by the following formulae (II) and (III), respectively: wherein R₅-R₁₄ are the same or different, and are each a hydrogen atom, or C₁-C₁₈ alkyl, C₅-C₁₂ cycloalkyl, C₆-C₁₄ aryl, C₆-C₁₄ aryl-C₁-C₄ alkyl, C₁-C₁₈ alkoxyl, C₅-C₁₂ cycloalkoxyl, C₆-C₁₄ aryloxyl, C₆-C₁₄ aryl-C₁-C₄ alkoxyl groups, or C₂-C₁₈ hydrocarbon groups having at least one double bond. The present invention will be explained in more detail hereinafter. Rubbers composing the rubber compositions of the present invention are sulphur-vulcanizable. Mention may be made of natural rubbers, polyisoprene rubbers, polybutadiene rubbers, styrene/butadiene copolymer rubbers, isoprene/isobutylene copolymer rubbers, halides thereof, ethylene/propylene/diene terpolymer rubbers (EPDM), butadiene/propylene copolymer rubbers, butadiene/ethylene copolymer rubbers, butadiene/isoprene copolymer rubbers, polypentenamer and the like, or blends thereof. The natural rubbers, polyisoprene rubbers, polybutadiene rubbers, styrene/butadiene copolymer rubbers and blends thereof, among the others, are particularly preferred because of high versatility. In the present invention, the cobalt oxyketone complexes to be compounded with such rubbers are represented by the above general formula (I). As embodiments of R₁, R₂, R₃ and R₄ in the above formula (I), mention may be made of methyl, ethyl, propyl, isopropyl, n-butyl, tert-butyl, n-pentyl, isopentyl, sec-pentyl, neopentyl, n-hexyl, isohexyl, sec-hexyl, cyclohexyl, n-heptyl, isoheptyl, tert-heptyl, octyl, 2-ethylhexyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, methylcyclohexyl, cyclohexylmethyl, naphthyl, anthracenyl, naphthylmethyl, cycloheptyl, cyclooctyl, phenyl, benzyl, cyclononyl, cyclodecyl, cycloundecyl, cyclododecyl, 9-octadecenyl, methylphenyl and the like. Particularly, among the others, combinations such that the total number of carbon atoms of R₁, R₂, R₃ and R₄ is at least 10 are preferable. Further, in the above-described formulae (II) and (III) representing R₁ and R₃, respectively, the substituent groups, R₅∼R₁₄, can be hydrogen atom in addition to the groups same as the above. These cobalt oxyketone complexes can be obtained, for example, by adding an oxyketone compound to sodium hydroxide aqueous solution followed by a further reaction with cobalt chloride, in accordance with the following reaction formulae: wherein definitions of R₁, R₂, R₃ and R₄ are same as those of the general formula (I). Further, the preferable cobalt oxyketone complexes represented by the above formula (I) wherein R₁ and R₃ are substituted with the groups represented by the aforementioned formulae (II) and (III), respectively, and R₂ and R₄ are both methyl group, also can be obtained in accordance with the following reaction formulae similar to the above: wherein definitions of R₅∼R₁₄ are same as those of the general formulae (II) and (III). In the present invention, the cobalt oxyketone complexes are compounded in an amount of 0.01∼1 part as a cobalt element content with 100 parts of rubber by weight. If the amount is less than 0.01 part by weight, sufficient effects can not be expected, while compounding in excess of 1 part by weight is not preferable not only for possibly causing deterioration of physical properties of the rubber but also from an economical point of view. In the present invention, the cobalt element content is preferred particularly to be 0.05∼0.5 part by weight. Additionally, the rubber compositions of the present invention may contain, according to purposes, various compounding additives beginning with sulphur as a vulcanizing agent, in addition to the above-described cobalt oxyketone complexes. The sulphur as a vulcanizing agent is preferred to be in an amount of about 0.5∼10 parts per 100 parts of rubber, by weight. As other compounding additives, those usually employed in the rubber industry, for example, reinforcements or fillers, such as carbon black, silica, calcium carbonate or the like; softeners such as aromatic oils; vulcanizing accelerators; vulcanizing accelerating aids; age resistant agents; and the like, can be arbitrarily compounded, if required, in a range of usual amount of compounding. Such rubber compositions according to the present invention are suitably employed in adhesion particularly to metals, and preferably used as a coating composition for steel belts, steel breakers, steel carcass plies or the like, of tires. In addition, they are applicable in industrial articles other than tires, for example, conveyer belts, rubber vibration isolators, or the like. The present invention will be explained in more detail hereinafter with reference to manufacturing examples of cobalt oxyketone complexes, examples and comparative examples. Additionally, in the examples, evaluation methods of vulcanizing properties, initial adhesion, thermoresistant adhesion and smell of unvulcanized rubbers are as follows: Vulcanizing property Using unvulcanized rubber compositions, a vulcanizing rate T (90) was determined in accordance with ASTM D 2084. Initial adhesion A steel cord/rubber composite consisting of an unvulcanized rubber embedding a steel cord of 1mm x 5mm x 0.23mm with brass-plated surfaces was vulcanized at 145°C for 20 minutes. Then, a test of peeling the steel cord off the embedding rubber was conducted in accordance with JIS K 6301, peeling test. The adhesion was evaluated by the amount of rubber remaining on the cord. A condition wherein the cord was completely covered by the rubber was denoted as 100, and a condition wherein no rubber remained as adhered was denoted as 0. Thermoresistant adhesion A steel cord/rubber composite similar to that used in the initial adhesion test was left to stand for 7 days in an oven at 120°C. Then the adhesion was evaluated in the same manner as that for the initial adhesion, except that the vulcanizing time of the composite was 40 minutes. Smell of unvulcanized rubber The smell was evaluated during a kneading operation with a Banbury mixer. ○ : a smell, not particularly offensive. ▵ : a smell, offensive but tolerable. X : a smell, offensive and intolerable. Manufacturing Example 1 (Synthesis of 2,4-octanedione cobalt complex) Four grams (0.1 mol) of sodium hydroxide were dissolved in 150 cc of water. To this solution, 14.2 g (0.1 mol) of 2,4-octanedione were added slowly while stirring. After the dissolution had been completed, the obtained solution was dripped taking 10 minutes while vigorously stirring into a cobalt (II) chloride aqueous solution which had been prepared in advance by dissolving 11.9 g (0.05 mol) of hexahydrated cobalt (II) chloride in 200 cc of water. The produced rose-red-colored precipitate was separated by filtration and washed with water until the washing water became substantially colorless, followed by vacuum drying to obtain 15.1 g of the object: 2,4-octanedione cobalt complex. Manufacturing Examples 2∼6 In the same manner as Manufacturing Example 1 except that 5,7-undecanedione (Manufacturing Example 2), 2,4-undecanedione (Manufacturing Example 3), 2,4-pentadecanedione (Manufacturing Example 4), 2,4-heneicosanedione (Manufacturing Example 5) and 1-phenyl-1,3-butanedione (Manufacturing Example 6) were used in lieu of 2,4-octanedione, cobalt complexes of 5,7-undecanedione, 2,4-undecanedione, 2,4-pentadecanedione, 2,4-heneicosanedione and 1-phenyl-1,3-butanedione were synthesized, respectively. Examples 1∼6, and Comparative Examples 1∼4 Eighty parts by weight of a natural rubber, 20 parts by weight of a synthetic polyisoprene rubber (IR 2200), 50 parts by weight of carbon black, 8 parts by weight of zinc white, 3 parts by weight of sulphur, 0.5 part by weight of a vulcanization accelerator (sulfeneamide) were compounded with 0.2 part by weight as a cobalt element content of various cobalt oxyketone complexes shown in Table 1 (Examples 1∼6) and the mixture were kneaded in a Banbury mixer to prepare respective rubber compositions. With respect to the obtained rubber compositions, vulcanizing properties, initial adhesion, thermoresistant adhesion were evaluated. Additionally, for comparative purpose, the same evaluation was made of those compounded with hitherto known cobalt naphthenate (Comparative Example 1), cobalt stearate (Comparative Example 2), Manobond C (Comparative Example 3), cobalt acetylacetonate (Comparative Example 4), respectively, in lieu of the cobalt oxyketone complex. The result is shown in Table 1. As is clear from Table 1, the rubber compositions according to the present invention with which the cobalt oxyketone complex was compounded were high in vulcanizing rate and excellent both in initial adhesion and thermoresistant adhesion. Manufacturing Example 7 (Synthesis of acetoacetanilide cobalt complex) Four grams (0.1 mol) of sodium hydroxide were dissolved in 150 cc of water. To this solution, 17.7 g (0.1 mol) of acetoacetanilide were added while stirring. After the dissolution had been completed, the obtained solution was dripped taking 10 minutes while vigorously stirring into a cobalt (II) chloride aqueous solution which had been prepared in advance by dissolving 11.9 g (0.04 mol) of hexahydrated cobalt (II) chloride in 20 cc of water. Then, the produced precipitate was separated by filtration and washed with 200 cc of water, followed by vacuum drying to obtain 19.0 g of the object: acetoacetanilide cobalt complex. Manufacturing Examples 8∼11 In the same manner as Manufacturing Example 7 except that acetoacetyl p-toluidide (Manufacturing Example 8), acetoacetyl m-xylidide (Manufacturing Example 9), acetoacetyl o-anisidide (Manufacturing Example 10) and acetoacetyl o-chloroanilide (Manufacturing Example 11) were used in lieu of acetoacetanilide, cobalt complexes of acetoacetyl p-toluidide, acetoacetyl m-xylidide, acetoacetyl o-anisidide and acetoacetyl o-chloroanilide were synthesized, respectively. Examples 7∼11, and Comparative Examples 5∼8 Eighty parts by weight of a natural rubber, 20 parts by weight of a synthetic polyisoprene rubber (IR 2200), 50 parts by weight of carbon black, 8 parts by weight of zinc white, 3 parts by weight of sulphur, 0.5 part by weight of a vulcanization accelerator (sulfenamide) were compounded with 0.2 part by weight as a cobalt element content of various cobalt oxyketone complexes shown in Table 2 (Manufacturing Examples 7∼11) and the mixtures were kneaded in a Banbury mixer to prepare respective rubber compositions. With respect to the obtained rubber compositions, initial adhesion, thermoresistant adhesion and smells of unvulcanized rubbers were evaluated. Additionally, for comparative purpose, the same evaluation was made of those compounded with hitherto known cobalt naphthenate (Comparative Example 5), cobalt stearate (Comparative Example 6), Manobond C (Comparative Example 7), cobalt acetylacetonate (Comparative Example 8), respectively, in lieu of the cobalt oxyketone complex. The result is shown in Table 2. As is clear from Table 2, the rubber compositions according to the present invention with which the cobalt oxyketone complex was compounded had no offensive smells in an unvulcanized state and were excellent both in initial adhesion and thermoresistant adhesion. As explained in detail above, the rubber compositions according to the present invention which comprise a rubber and a specified cobalt oxyketone complex compounded therewith in a specified amount, are extremely high in vulcanizing rate and remarkably improved in both initial adhesion and thermoresistant adhesion. Accordingly, the rubber compositions according to the present invention are very effective for adhering to articles to be exposed to high temperature, and further allow the articles to be produced effectively by virtue of the excellent initial adhesion and vulcanizing characteristics. Such rubber compositions of the present invention are very useful particularly as a rubber for adhering to steel cords.

Claims: A rubber composition comprising 100 parts by weight of a rubber and 0.01-1 part by weight as a cobalt element content of at least one cobalt oxyketone complex compounded with said rubber, which complex is represented by the following general formula: wherein R₁, R₂, R₃ and R₄, which may be the same or different, are each a C₁-C₁₈ alkyl, C₅-C₁₂ cycloalkyl, C₆-C₁₄ aryl, C₆-C₁₄ aryl-C₁-C₄ alkyl, C₁-C₁₈ alkoxyl, C₅-C₁₂ cycloalkoxyl, C₆-C₁₄ aryloxyl, or C₆-C₁₄ aryl-C₁-C₄ alkoxyl groups, or C₂-C₁₈ hydrocarbon groups having at least one double bond, and wherein the total number of carbon atoms contained in R₁, R₂, R₃ and R₄ is at least 5. A rubber composition as claimed in claim 1, characterized in that said cobalt oxyketone complex is compounded in an amount of 0.05-0.5 part by weight as a cobalt element content. A rubber composition comprising 100 parts by weight of a rubber and 0.01-1 part by weight as a cobalt element content of at least one cobalt oxyketone complex compounded with said rubber, which complex is represented by the following general formula: wherein R₅-R₁₄, which may be the same or different, are each a hydrogen atom, or a C₁-C₁₈ alkyl, C₅-C₁₂ cycloalkyl, C₆-C₁₄ aryl, C₆-C₁₄ aryl-C₁-C₄ alkyl, C₁-C₁₈ alkoxyl, C₅-C₁₂ cycloalkoxyl, C₆-C₁₄ aryloxyl, or C₆-C₁₄ aryl-C₁-C₄ alkoxyl group, or C₂-C₁₈ hydrocarbon group having at least one double bond. A rubber composition as claimed in claim 3, characterized in that said cobalt oxyketone complex is compounded in an amount of 0.05-0.5 part by weight as a cobalt element content.

Inventors: KINOSHITA TAKESHI, KUMAMOTO YOSHIYUKI, MORITA KOICHI, KINOSHITA, TAKESHI, KUMAMOTO, YOSHIYUKI, MORITA, KOICHI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1988
Main classification: C08K5
IPC codes: C08K5