COMPOSITION FOR VULCANIZED RUBBER AND VULCANIZED RUBBER OBTAINED THEREFROM

Abstract: A composition for vulcanized rubber of the invention comprises a conjugated diene rubber, an ethylene- alpha -olefine copolymer rubber and a styrenic oligomer. It provides a vulcanized rubber having excellent mechanical strengths as well as excellent weatherability and heat-aging resistance.

Description: The present invention relates to a vulcanizable rubber composition comprising a conjugated diene rubber and an ethylene-alpha-olefin copolymer rubber and capable of providing a vulcanized rubber which has excellent mechanical strength, weather resistance and heat aging characteristics. The weather resistance and heat aging characteristics of vulcanized conjugated diene rubbers, such as natural rubbers (NR), isoprene rubbers (IR), styrene-butadiene rubbers (SBR) and butadiene rubbers, are poor. Various attempts have been made to improve the weather resistance and heat aging characteristics of vulcanized conjugated diene rubbers by blending an ethylene-alpha-olefin copolymer rubber (EPDM) which has excellent weather resistance and heat aging characteristics, with a conjugated diene rubber and vulcanizing the resulting blend. However, there has been a problem in that the mutual vulcanization of the conjugated diene rubber and the EPDM becomes insufficient because of a slow vulcanization rate of the latter, thereby mechanical strength of the resulting vulcanized rubber becomes poor. The invention seeks to provide a vulcanizable rubber composition capable of providing a vulcanized rubber which not only has excellent weather resistance and heat aging characteristics but also excellent mechanical strength. According to the invention there is provided a vulcanizable rubber composition which comprises (A) a conjugated diene rubber, (B) an ethylene-alpha-olefin copolymer rubber having an iodine value not exceeding 50 and (C) an oligomer of at least one styrenic monomer, the oligomer (C) having a number average molecular weight Mn of 400 to 2000. The invention also provides a vulcanized rubber obtained by vulcanizing the vulcanizable rubber composition. Conjugated diene rubbers (A) Conjugated diene rubbers (A) which can be used in the compositions of the invention including homopolymers of conjugated diene compounds or copolymers primarily comprised of polymerized units of conjugated diene compounds, and may be natural or synthetic rubbers. Suitable conjugated dienes include butadiene, isoprene and chloroprene. Suitable comonomers which may be copolymerised with such conjugated dienes are vinyl compounds such as styrene and acrylonitrile. Examples of preferred conjugated diene rubbers, which can be used herein, include, for example, natural rubbers (NR), butadiene rubbers (BR), isoprene rubbers (IR), styrene-butadiene rubbers (SBR), acrylonitrile-butadiene rubbers (NBR) and chloroprene rubbers (CR). Ethylene-alpha-olefin copolymer rubbers (B) Ethylene-alpha-olefin copolymer rubbers (B) are rubbery copolymers of ethylene and at least one alpha-olefin, or rubbery copolymers of ethylene, at least one alpha-olefin and at least one polyene. Suitable alpha-olefins which are copolymerized with ethylene to provide the ethylene-alpha-olefin copolymer rubbers (B), are those having from 3 to 10 carbon atoms. As the alpha-olefin, propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene and 1-decene can be used alone or in combination. Of these, propylene and 1-butene are particularly preferred. The ethylene-alpha-olefin copolymer rubber (B) used herein may further contain units derived from at least one polyene compound in an amount such that an iodine value of the rubber (B) does not exceed 50. Examples of suitable polyene compounds include, for example, acyclic non-conjugated diene compounds such as 1,4-hexadiene, 1,6-octadiene, 2-methyl-1,5-hexadiene, 6-methyl-1,5-heptadiene and 7-methyl-1,6-octadiene; cyclic non-conjugated diene compounds such as cyclohexadiene, dicyclopentadiene, methyltetrahydroindene, 5-vinylnorbornene, 5-ethylidene-2-norbornene, 5-methylene-2-norbornene, 5-isopropylidene-2-norbornene and 6-chloromethyl-5-isopropenyl-2-norbornene; and triene compounds such as 2,3-diisopropylidene-5-norbornene, 2-ethylidene-3-isopropylidene-5-norbornene, 2-propenyl-2,2-norbornadiene, 1,3,7-octatriene and 1,4,9-decatriene. Of these, cyclic non-conjugated dienes, in particular dicyclopentadiene and 5-ethylidene-2-norbornene are preferred. In the ethylene-alpha-olefin copolymer rubber (B) used herein the molar ratio of ethylene to alpha-olefin (ethylene-alpha-olefin) is normally from 50/50 to 95/5 from the view points of strength and flexibility of the final vulcanized product. An optimum molar ratio of ethylene to alpha-olefin depends upon the particular alpha-olefin employed. For instance, in the case of propylene it falls normally within the range from 50/50 to 90/10 and in particular from 60/40 to 87/13, while in the case of an alpha-olefin having at least 4 carbon atoms it falls normally within the range from 80/20 to 95/5 and in particular from 85/15 to 95/5. The vulcanizable rubber composition according to the invention contains from 5 to 95, preferably from 40 to 90, parts by weight of the conjugated diene rubber (A) and from 95 to 5, preferably from 60 to 10, parts by weight of the ethylene-alpha-olefin copolymer rubber (B), the combined weight of the (A) and (B) being 100 parts. Styrene oligomers (C) The term "styrene oligomer" used herein means oligomers derived from at least one styrenic monomer, the oligomer having a number average molecular weight Mn of 400 to 2000, preferably from 500 to 1500. Thus, styrene oligomers (C) which can be used in the compositions of the invention include homopolymers and copolymers of styrenic monomers such as styrene, alpha-methylstyrene, vinyltoluenes and isopropenyltoluenes (IPT). Furthermore, in the practice of the invention use can be made as the styrene oligomer (C) of oligomers prepared by copolymerization of at least 80 % by weight of at least one styrenic monomer with up to 20 % by weight of a so-called C₅ petroleum fraction which is obtained by pyrolysis of naphtha and comprises isoprene and cyclopentadiene. The vulcanizable rubber composition according to the invention contains the styrene oligomer (C) in an amount of from 1 to 50 parts by weight, preferably from 5 to 40 parts by weight, based on 100 parts of the combined weight of the conjugated diene rubber (A) and the ethylene-alpha-olefin copolymer rubber (B). Besides the above-mentioned components (A), (B) and (C), the vulcanizable rubber composition according to the invention may contain softeners, rubber reinforcing agents, fillers, vulcanizing agents, vulcanization accelerators, auxiliary vulcanizing agents and other additives so far as the purpose of the invention is not hindered. The vulcanizable composition according to the invention can be processed to provide a vulcanized rubber in the same manner as in conventional processes for vulcanizing rubber. More particularly, the composition according to the invention is compounded with suitable additives in the manner as hereinafter described to provide an unvulcanized rubber compound, which is formed to a desired article and then vulcanized. Depending upon the intended use and desired properties of the vulcanized rubber product, the particular proportions of the components (A), (B) and (C), kinds and loadings of any softening agent, rubber reinforcing agent and filler, kinds and loadings of compounds constituting the vulcanization system including the vulcanizing agent, vulcanization accelerator and auxiliary vulcanizing agent as well as vulcanization process and conditions are appropriately selected. As the softening agent, use can be made of those normally used in rubber industry, including, for example, petroleum softening agents such as process oil, lubricating oil, paraffin, fluid paraffin, petroleum asphalt and vaseline; coal tar softening agents such as coal tar and coal tar pitch; fatty oil softening agents such as castor oil, linseed oil, rapeseed oil and coconut oil; tall oil; factice; waxes such as beeswax, carnauba wax and laurin; fatty acids and their salts such as ricinoleic acid, palmitic acid, barium stearate, calcium stearate and zinc laurate; and synthetic polymeric substances such as petroleum resins, atactic polypropylene and coumarone-indene resins. Of these, petroleum softening agents, in particular, process oil is preferred. As the rubber reinforcing agent, there can be used fine particulate silicic acid and various carbon blacks such as SRF, GPF, FEF, HAF, ISAF, SAF, FT and MT. In preparing a vulcanized rubber according to the invention sulfur compounds and organic peroxides are used as the vulcanizing agent. Examples of the sulfur compounds which can be used as the vulcanizing agent include, for example, sulfur, sulfur monochloride, sulfur dichloride, morpholine disulfide, alkylphenol disulfide, tetramethylthiuronium disulfide and selenium dimethyldithiocarbamate. Of these, sulfur is preferred. The sulfur compounds are used in an amount of from 0.1 to 10 parts by weight, preferably from 0.5 to 5 parts by weight, based on 100 parts of the combined weight of the conjugated diene rubber (A) and ethylene-alpha-olefin copolymer rubber (B). Examples of the organic peroxides which can be used as the vulcanizing agent include, for example, dicumyl peroxide, 2,5-dimethyl-2,5-di-(tert.-butylperoxy)hexane, 2,5-dimethyl-2,5-di-(benzoylperoxy)hexane, 2,5,-dimethyl-2,5-di-(tert.-butylperoxy)hexyne-3, di-tert.-butyl peroxide, di-tert.-butylperoxy-3,3,5-trimethylcyclohexane and tert.-butyl hydroperoxide. Of these, dicumyl peroxide, di-tert.-butyl peroxide and di-tert.-butylperoxy-3,3,5-trimethylcyclohexane are preferred. When a sulfur compound is used as the vulcanizing agent in preparing a vulcanized rubber according to the invention, a vulcanization accelerator is preferably used. Examples of the vulcanization accelerators which can be used include, for example, thiazole compounds such as N-cyclohexyl-2-benzothiazole-sulfenamide, N-oxydiethylene-2-benzothiazole-sulfenamide, N,N-diisopropyl-2-benzothiazole-sulfenamide, 2-mercaptobenzothiazole, 2-(2,4-dinitrophenyl) mercaptobenzothiazole, 2-(2,6-diethyl-4-morpholinothio) benzothiazole and dibenzothiazole disulfide; guanidine compounds such as diphenylguanidine, triphenylguanidine, di-o-tolylguanidine, o-tolyl biguanide and diphenylguanidine phthalate; aldehyde-amine and aldehyde-ammonia reaction products such as acetaldehyde-aniline reaction product, butylaldehyde-aniline condensation product, hexamethylenetetramine and acetaldehyde-ammonia reaction product; imidazoline compounds such as 2-mercaptoimidazoline; thiourea compounds such as thiocarbanilide, diethylthiourea, dibutylthiourea, trimethylthiourea and di-o-tolylthiourea; thiuram compounds such as tetramethylthiuram monosulfide, tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide and pentamethylenethiuram tetrasulfide; salts of dithio acids such as zinc dimethyldithiocarbamate, zinc diethyldithiocarbamate, zinc di-n-butyldithiocarbamate, zinc ethylphenyldithiocarbamate, zinc butylphenyldithiocarbamate, sodium dimethyldithiocarbamate, selenium dimethyldithiocarbamate and tellurium diethyldithiocarbamate; xanthate compounds such as zinc dibutylxanthate; and zinc flower. The vulcanization accelerator is suitably used in an amount of from 0.1 to 20 parts by weight, preferably from 0.2 to 10 parts by weight, based on 100 parts of the combined weight of the conjugated diene rubber (A) and ethylene-alpha-olefin copolymer rubber (B). When an organic peroxide is used as the vulcanizing agent in preparing a vulcanized rubber according to the invention, an auxiliary vulcanizing agent is preferably used. Examples of the auxiliary vulcanizing agents which can be used include, for example, sulfur, quininedioxime compounds such as p-benzoquinonedioxime, methacrylate compounds such as polyethyleneglycol dimethacrylate, allyl compounds such as diallyl phthalate and triallyl cyanurate, maleimide compounds and divinylbenzene. The unvulcanized rubber compound may be prepared as follows. The components (A), (B) and (C) and any filler and softening agent are mixed and kneaded at a temperature of from about 80 to 170 °C. for a period of from 3 to 10 minutes using a mixer such as a Banbury mixer. The resulting mixture is then further mixed and kneaded together with an added vulcanizing agent, and any added vulcanization accelerator and auxiliary vulcanizing agent, when used, at a temperature of about 40 to 80 °C. for a period of 5 to 30 minutes, using rolls such as open rolls. The resulting composition is finally extruded to provide the unvulcanized rubber compound in the form of a ribbon or a sheet. The rubber compound so prepared is then formed into a desired shape by means of an extruder, calender rolls or press, and heated at a temperature of from 150 to 270 °C. for a period of 1 to 30 minutes to provide a vulcanized rubber product at the same time as or subsequent to the forming. In the latter case, a separate vulcanizing vessel is used. Upon vulcanization a mold may or may not be used. When a mold is not used, the steps of forming and vulcanization are usually carried out continuously. The heating in the vulcanizing vessel may be carried out by means of hot air, a fluidized bed of glass beads, UHF (ultra-short high frequency) or steam. The vulcanizable rubber composition according to the invention provides a vulcanized rubber which has excellent weather resistance and heat aging characteristics and also excellent mechanical strength. Examples The invention will now be described by reference to the following examples. The number average molecular weight Mn of a styrene oligomer referred to in the examples was determined from a chromatograph pattern of a given sample obtained under conditions including a solvent of THF and a temperature of 40 °C. using an HLC 802R type high speed liquid chromatography supplied by TOYO Soda K. K. with columns: G4000H₈, G3000H₈ and two G2000H₈, all manufactured by the same company. The corelation diagram calibration curve was prepared using monodispersed polystyrenes. Example 1 A 4.3 liter Banbury mixer (OOC type, supplied by KOBE Steel Works K. K.) was charged with 70 parts by weight of SBR as the diene copolymer rubber (A) having a styrene content of 24 % by weight, and a Mooney viscosity (ML₁+₄, 100 °C.) of 52; 30 parts by weight of EPT as the ethylene-alpha-olefin copolymer rubber (B) having an ethylene/propylene molar ratio of 66/34, ethylidenenorbornene as a diene component, an iodine value of 22 and a Mooney viscosity (ML₁+₄, 100 °C.) of 85; 10 parts by weight of a copolymer of isopropenyltoluene (IPT) and C₅ petroleum fraction as the styrene oligomer (C) having an IPT content of 95 % by weight and a number average molecular weight Mn of about 750; 5 parts by weight of zinc flower; 1 part by weight of stearic acid; 55 parts by weight of FEF carbon black ("SEAST" supplied by TOKAI Carbon K. K.) and 15 parts by weight of silica ("TOKUSIL GU" supplied by TOKUYAMA Soda K. K.) as the rubber reinforcing agents; and 25 parts by weight of a paraffin oil ("DIANA PROCESS PW-90" as the softening agent, and the resulting mixture was mixed at a temperature of 130 °C. for a period of 6 minutes to provide a kneaded mixture. The mixture so obtained was further kneaded together with 1.5 parts by weight of sulfur as the vulcanizing agent and 1.5 parts by weight of N-cyclohexyl-2-benzothiazole-sulfenamide (CBS supplied by OHUCHI SHINKOH Chemicals K. K.) as the vulcanization accelerator for a period of 10 minutes with open rolls, each roll having a diameter of 20.3 cm (8 inches) and a length of 51 cm (20 inches), maintained at a temperature of 60 °C. to provide a rubber compound. The rubber compound thus obtained was pressed and heated to provide a vulcanized rubber sheet using a vulcanizing press under conditions including a plate temperature of 160 °C., a pressing pressure of 100 kg/cm² and a pressing time of 30 minutes. The vulcanized rubber sheet so prepared was conditioned in a constant temperature room maintained at 25 °C. for a period of 24 hours and thereafter subjected to the following hardness and tensile tests. Hardness test Hardness (JIS A) is measured on a stack of 6 vulcanized sheets, using a JIS A hardness tester in accordance with a method described in JIS K 6301 (Methods for testing physical properties of vulcanized rubber), paragraph 5-2 (Spring hardness test). Tensile test Tensile stress at break T B and tensile elongation at break E B are determined on a No. 3 dumbbell specimen described in JIS K 6301 punched from a vulcanized sheet, in accordance with a method described in paragraph 3 of JIS K 6301 at a temperature of 25 °C. with a tensile rate of 500 mm/min. The results are shown in Table 1. Example 2 A vulcanized rubber sheet was prepared and subjected to the hardness and tensile tests as in Example 1, except that the copolymer of isopropenyltoluene (IPT) and C₅ petroleum fraction was used in an amount of 20 parts by weight and the paraffin oil was used in an amount of 15 parts by weight. The results are shown in Table 1. Comparative Example 1 A vulcanized rubber sheet was prepared and subjected to the hardness and tensile tests as in Example 1, except that the copolymer of isopropenyltoluene (IPT) and C₅ petroleum fraction was not used and the paraffin oil was used in an amount of 35 parts by weight for a purpose of achieving the substantially same hardness as in Example 1. The results are shown in Table 1. Example 3 A vulcanized rubber sheet was prepared and subjected to the hardness and tensile tests as in Example 2, except that the 20 parts by weight of the copolymer of isopropenyltoluene (IPT) and C₅ petroleum fraction was replaced with 20 parts by weight of a styrene oligomer ("Highmer-ST-95" supplied by SANYO Chemical Industries K. K.) The results are shown in Table 1. Properties of vulcanized rubber Example 1 Example 2 Comp. Ex. 1 Example 3 Hardness(JIS A) 56 57 56 65 Tensile stress at break TB [kgf/cm²] 125 138 93 130 Elongation at break EB[%] 440 540 410 450 Example 4 A 4.3 liter Banbury mixer (OOC type, supplied by KOBE Steel Works K. K.) was charged with 70 parts by weight of NBR as the diene copolymer rubber (A) having an acrylonitrile content of 33 % by weight, and a Mooney viscosity (ML₁+₄, 100 °C.) of 78; 42 parts by weight of EPT as the ethylene-alpha-olefin copolymer rubber (B) having an ethylene/propylene molar ratio of 78/22, ethylidenenorbornene as a diene component, an iodine value of 13, a Mooney viscosity (ML₁+₄, 100 °C.) of 70 and containing 40 % by weight of an oil extender; 20 parts by weight, that is 17.8 parts by weight based on 100 parts of the combined weight of the (A) and (B), of a copolymer of isopropenyltoluene (IPT) and C₅ petroleum fraction as the styrene oligomer (C) having an IPT content of 95 % by weight and a number average molecular weight Mn of about 750; 5 parts by weight of zinc flower; 1 part by weight of stearic acid; 65 parts by weight of ISAF carbon black ("SHIEST 600" supplied by TOKAI Carbon K. K.) as the rubber reinforcing agent; and 5 parts by weight of a paraffin oil ("DIANA PROCESS PW-90") as the softening agent, and the resulting mixture was mixed at a temperature of 130 °C. for a period of 6 minutes to provide a kneaded mixture. The mixture so obtained was further kneaded together with 1.5 parts by weight of sulfur as the vulcanizing agent and 1.5 parts by weight of N-cyclohexyl-2-benzothiazole-sulfenamide (CBS supplied by OHUCHI SHINKOH Chemicals K. K.) as the vulcanization accelerator for a period of 10 minutes with open rolls, each roll having a diameter of 20.3 cm (8 inches) and a length of 51 cm (20 inches), maintained at a temperature of 60 °C. to provide a rubber compound. The rubber compound was pressed and heated to provide a vulcanized rubber sheet using a vulcanizing press under conditions including a plate temperature of 160 °C., a pressing pressure of 100 kg/cm² and a pressing time of 30 minutes. The vulcanized rubber sheet so prepared was conditioned in a constant temperature room maintained at 25 °C. for a period of 24 hours and thereafter subjected to the above-mentioned hardness and tensile tests. The results are shown in Table 2. Properties of vulcanized rubber Example 4 Hardness (JIS A) 72 Tensile stress at break T B [kgf/cm²] 121 Elongation at break E B [%] 330

Claims: A vulcanizable rubber composition which comprises (A) a conjugated diene rubber, (B) an ethylene-alpha-olefin copolymer rubber having an iodine value not exceeding 50 and (C) an oligomer of at least one styrenic monomer, the oligomer (C) having a number average molecular weight Mn of 400 to 2000. A vulcanizable rubber composition according to claim 1 which comprises 5 to 95 parts by weight of (A) and 95 to 5 parts by weight of (B), the combined weight of (A) and (B) being 100 parts, and from 1 to 50 parts by weight of (C) based on 100 parts of the combined weight of (A) and (B). A vulcanizable rubber composition according to claim 2 which comprises 40 to 90 parts by weight of (A) and 60 to 10 parts by weight of (B), the combined weight of (A) and (B) being 100 parts. A vulcanizable rubber composition according to claim any one of the preceding claims wherein the oligomer (C) has a number average molecular weight Mn of from 500 to 1500. A vulcanized rubber obtained by vulcanizing a vulcanizable rubber composition as claimed in any one of claims 1 to 4.

Inventors: MISHIMA TAKASHI MITSUI PETROCH, NONAKA SHUICHI MITSUI PETROCHE, UEKI NORIHITO MITSUI PETROCHEM, MISHIMA, TAKASHI MITSUI PETROCHEMICAL INDUSTRIES,, NONAKA, SHUICHI MITSUI PETROCHEMICAL INDUSTRIES,, UEKI, NORIHITO MITSUI PETROCHEMICAL INDUSTRIES,
Applicants: MITSUI PETROCHEMICAL IND, MITSUI PETROCHEMICAL INDUSTRIES, LTD.
Application year: 1989
Main classification: C08L9
IPC codes: C08L23, C08L9, C08L21, C08K5, C08L25