Rubber composition

Abstract: A vulcanized rubber composition comprises (a) 100 parts by weight of a synthetic rubber and (b) from 0.5 to 10 parts by weight of an alkylene oxide adduct of a saponified ethylene-saturated carboxylic acid vinyl ester copolymer. The composition can be vulcanized to provide a vulcanized rubber composition which retains an excellent appearance over a prolonged period of time because of its freedom from blooming; has excellent antistatic performance due to its electrical conductivity; can be produced in various colors including white; and also has excellent tensile strength and specific gravity.

Description: This invention relates to a vulcanizable rubber composition that can be vulcanized to provide a vulcanized rubber composition which retains an excellent appearance over a prolonged period of time because of freedom from blooming; which has excellent antistatic performance due to its electrical conductivity; which can be produced in various colors including white; and which also has excellent properties in terms of tensile strength and specific gravity. Because of its excellent properties described above, the resultant vulcanized rubber composition can be utilized in a wide range of fields: for example, in applications which take advantage mainly of its excellent appearance, such as rubber automotive parts (e.g., automotive interior trim parts made of rubber, weather strips, window frames, various hoses, tubes, brake parts, rubber vibration insulators, automotive rubber body parts, and sidewalls of tires, rubber building components (e.g. waterproof sheets and gaskets), general-purpose industrial components made of rubber (e.g. linings, belts, sealing materials, packings, rubber rolls, O-rings, diaphragms, electrical wires, sealants, hose linings, and various boots), and rubber parts for helmets; and also in applications which take advantage mainly of the antistatic characteristics and whiteness of the vulcanized rubber composition, such as paper-feed belts or rolls for use in, e.g., copying machines, printers, facsimile machines, optical character readers, cash dispensers, money exchange machines, computers and automatic vending machines. It is known that vulcanized rubber compositions generally suffer from the so-called blooming phenomenon, in which vulcanization accelerators migrate to the surface and deposit thereon with the lapse of time, so that the appearance of the vulcanized rubber compositions deteriorates. A known method for inhibiting such blooming is to incorporate a non-reactive phenol-formaldehyde resin or polyethylene glycol) (see JP-B-59-15345). (The term "JP-B" as used herein means an "examined Japanese patent publication".) Although this conventional technique is effective in retarding the progress of blooming to some degree, it has been ineffective in preventing the occurrence of blooming over a sufficiently long period of time. On the other hand, as a technique for imparting electrical conductivity and antistatic performance to vulcanized rubbers, incorporation of carbon black or a metal powder into such rubbers is known. However, the use of carbon black has entailed the problem that, since the resulting rubber inevitably assumes a black color, it cannot be used in applications requiring tints other than black, e.g., such applications as paper-feed belts or rollers, which are strictly required not to cause black fouling. Further, the use of a metal powder has been unsatisfactory in that the resulting rubber has reduced tensile strength and increased specific gravity. Accordingly, an aim of the present invention is to provide a vulcanizable rubber composition that can be vulcanized to provide a composition which retains an excellent appearance over a prolonged period of time because of freedom from blooming; which has excellent antistatic performance due to its electrical conductivity; which can be produced in various colors including white; and which also has excellent properties in terms of tensile strength and specific gravity. According to the present invention there is provided a rubber composition which comprises (a) 100 parts by weight of a synthetic rubber and (b) from 0.5 to 10 parts by weight of an alkylene oxide adduct of a saponified ethylene-saturated carboxylic acid vinyl ester copolymer. The synthetic rubber (a) which may be used in the rubber composition of the present invention is not particularly restricted, and may be any of a wide variety of synthetic rubbers. Examples thereof include ethylene-α-olefin copolymer rubbers, styrene-butadiene copolymer rubbers (SBR), chloroprene rubbers (CR), acrylonitrile rubbers (NBR), and isoprene-isobutylene copolymer rubbers (IIR). The ethylene-α-olefin copolymer rubbers herein are copolymers of ethylene and at least one α-olefin (EPM) or copolymers of ethylene, at least one α-olefin, and at least one nonconjugated diene (EPDM). Examples of the α-olefin include propylene, 1-butene, 1-pentene, and 1-hexene. Examples of the nonconjugated diene include dicyclopentadiene, ethylidene norbornene, 1,4-hexadiene, and methyltetrahydroindene. The alkylene oxide adduct (b) of a saponified ethylene-saturated carboxylic acid vinyl ester copolymer (hereinafter, often referred to as "adduct (b)") which is used in the rubber composition of the present invention is an adduct obtained by the addition of an alkylene oxide to a saponification product of a copolymer comprising ethylene and a saturated carboxylic acid vinyl ester. As the saturated carboxylic acid vinyl ester, any of a wide variety of such esters may be used without any particular limitation. Examples thereof include vinyl acetate, vinyl propionate, and vinyl butyrate, with vinyl acetate being preferred. As the alkylene oxide, any of various alkylene oxides may be used without any particular limitation. Examples thereof include ethylene oxide, propylene oxide, and butylene oxide, with ethylene oxide being preferred. The ethylene-saturated carboxylic acid vinyl ester copolymer is not particularly limited with respect to the ethylene content and number average molecular weight. Preferably, however, the ethylene content and number average molecular weight of the copolymer are from 1 to 90% by weight and from 1,000 to 20,000, respectively. The saponification degree of the saponified ethylene-saturated carboxylic acid vinyl ester copolymer is not particularly limited, but is preferably in the range of from 30 to 100%, more preferably from 50 to 100%. The amount of the alkylene oxide added to the saponified ethylene-saturated carboxylic acid vinyl ester copolymer is not particularly limited, but is preferably in the range of from 20 to 1,000 parts by weight per 100 parts by weight of the saponified copolymer. The adduct (b) which is used in the rubber composition of the present invention can, for example, be obtained as follows. First, an ethylene-saturated carboxylic acid vinyl ester copolymer is heated in a solvent in the presence of an alkali, thereby to obtain a saponified ethylene-saturated carboxylic acid vinyl ester copolymer. This saponified copolymer is then reacted with an alkylene oxide by heating in the presence of an alkali. From the resulting reaction mixture, the adduct (b) to be used in the present invention is separated and obtained. In the rubber composition of the present invention, the above-described adduct (b) is contained in an amount of from 0.5 to 10 parts by weight, preferably from 1 to 5 parts by weight, per 100 parts by weight of the above-described synthetic rubber (a). If the amount of adduct (b) is below 0.5 part by weight, the effect of inhibiting the occurrence of blooming in the resulting vulcanized rubber composition and the effect of imparting antistatic performance to the vulcanized rubber are insufficient. On the other hand, an adduct (b) amount exceeding 10 parts by weight poses practical problems such as a decrease in the strength of the resulting vulcanized rubber composition, although the effect of inhibiting the occurrence of blooming and the effect of imparting antistatic performance are maintained. From the rubber composition of the present invention, a vulcanized rubber composition can, for example, be obtained by the following method. That is, synthetic rubber (a), adduct (b), and optionally, other compounding additives which are ordinarily used in the rubber industry, such as an anti-aging agent and a processing aid, are kneaded by means of an internal kneading machine, open rolls, or an extruder. Into the thus-obtained composition are incorporated a vulcanizing agent, and a vulcanization accelerator, thereby to obtain an unvulcanized composition. By vulcanizing this unvulcanized composition in an ordinary way, a vulcanized rubber composition can be obtained. If desired, various additives known in the rubber-processing industry may be incorporated in the rubber composition of the present invention. Examples of such additives include carbon black, mineral fillers, e.g., talc, clay, calcium carbonate, and silica, and a process oil as a softener. The vulcanizing agent to be added to the rubber composition of the present invention may be either a sulfur compound or an organic peroxide. Examples of the sulfur compound include sulfur, sulfur chloride, sulfur dichloride, morpholine disulfide, alkylphenol disulfides, tetramethylthiuram disulfide, and selenium dimethyldithiocarbamate. Of these, sulfur is preferably used. Examples of the organic peroxide include dicumyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3, di-t-butyl peroxide, di-t-butylperoxy-3,3,5-trimethylcyclohexane, and t-butyl hydroperoxide. Of these, dicumyl peroxide, di-t-butyl peroxide, and di-t-butylperoxy-3,3,5-trimethylcyclohexane are preferably used. In the case where a sulfur compound is employed as the vulcanizing agent, it is preferable to use a vulcanization accelerator in combination. Examples of the vulcanization accelerator include 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-dibenzothiazyl disulfide; guanidine compounds such as diphenylguanidine, triphenylguanidine, di-o-tolylguanidine, o-tolyl biguanide, and diphenylguanidine phthalate; aldehyde-amine or aldehyde-ammonia compounds such as reaction products of acetaldehyde with aniline, butyraldehyde-aniline condensates, hexamethylenetetramine, and acetaldehyde ammonia; 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; dithioic acid salt compounds 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 oxide. In the case where an organic peroxide is employed as the vulcanizing agent, it is preferable to use a vulcanization auxiliary in combination. Examples of the vulcanization auxiliary include quinone dioxime compounds such as p-quinone dioxime; methacrylate compounds such as polyethylene glycol dimethacrylate; allyl compounds such as diallyl phthalate and triallyl cyanurate; maleimide compounds; and divinylbenzene. Although in the present invention, vulcanized rubber compositions which assume tints other than black and have antistatic performance can be obtained without using carbon black, incorporation of carbon black in the rubber composition of the present invention to give a black rubber may be conducted depending upon the application. Such a black rubber composition is also within the scope of the present invention. The present invention will be explained in more detail with reference to the following Examples, which should not be construed to be limiting the scope of the invention. PREPARATIVE EXAMPLE 1 Into a 1-liter stainless steel autoclave equipped with a stirrer, a gas phase discharge line, and a feed line were charged 100 g of an ethylene-vinyl acetate copolymer having a vinyl acetate content of 31% by weight, a number average molecular weight of 1,800, and a softening point of 30°C, 200 g of methyl alcohol, and 0.4 g of sodium hydroxide. The mixture was allowed to react for 2 hours with stirring by heating at a temperature of 65°C while the gas phase discharge line was kept open. Thereafter, the temperature was raised to 140°C over a period of 1 hour thereby to remove all the volatile substances from the system. The thus-obtained saponification product was in a wax-like state with a melting point of 55°C and had a saponification degree of 60%. Subsequently, 0.17 g of potassium hydroxide was added to the saponification product. The mixture was heated to 180°C, and ethylene oxide was introduced until the pressure became 2 kg/cm². After a pressure decrease due to the addition reaction of ethylene oxide was observed, ethylene oxide was further introduced intermittently so as to keep the pressure at 2 kg/cm². Thus, ethylene oxide was fed in a total amount of 170 g over a period of 1 hour. At the time when the pressure had decreased to 0.4 kg/cm², the temperature was lowered to 100°C, and the reaction product was taken out. As a result, 250 g of an adduct was obtained, which had a melting point of 70°C and a hydroxyl value of 100 mg-KOH/g. In this adduct, the amount of ethylene oxide added to was 200 parts by weight per 100 parts by weight of the saponification product. PREPARATIVE EXAMPLE 2 Into a 700-liter stainless steel autoclave equipped with a stirrer, a gas phase discharge line, and a feed line were charged 160 kg of an ethylene-vinyl acetate copolymer having a vinyl acetate content of 31% by weight, a number average molecular weight of 1,800, and a softening point of 30°C, 320 kg of methyl alcohol, and 0.96 kg of sodium hydroxide. The mixture was allowed to react for 2 hours with stirring by heating at a temperature of 65°C while the gas phase discharge line was kept open. Thereafter, the temperature was raised to 142°C over a period of 1 hour thereby to remove all the volatile substances from the system. The thus-obtained saponification product was in a wax-like state with a melting point of 83°C and had a saponification degree of 90%. Subsequently, 0.27 kg of potassium hydroxide was added to the saponification product. The mixture was heated to 180°C and ethylene oxide was introduced until the ethylene oxide pressure in the autoclave became 2 kg/cm². After a pressure decrease due to the addition reaction of ethylene oxide was observed, ethylene oxide was further introduced intermittently so as to keep the pressure at 2 kg/cm². Thus, ethylene oxide was fed in a total amount of 272 kg over a period of 1 hour. At the time when the pressure had decreased to 0.4 kg/cm², the temperature was lowered to 100°C, and the reaction product was taken out. As a result, 408 kg of an adduct was obtained, which had a melting point of 51°C and a hydroxyl value of 110 mg-KOH/g. In this adduct, the amount of ethylene oxide added was 200 parts by weight per 100 parts by weight of the saponification product. PREPARATIVE EXAMPLE 3 Into a 1-liter stainless steel autoclave equipped with a stirrer, a gas phase discharge line, and a feed line were charged 100 g of an ethylene-vinyl acetate copolymer having a vinyl acetate content of 16% by weight, a number average molecular weight of 2,500, and a softening point of 72°C, 200 g of methyl alcohol, and 0.6 g of sodium hydroxide. The mixture was allowed to react for 2 hours with stirring by heating at a temperature of 65°C while the gas phase discharge line was kept open. Thereafter, the temperature was raised to 140°C over a period of 1 hour thereby to remove all the volatile substances from the system. The thus-obtained saponification product was in a wax-like state with a melting point of 90°C and had a saponification degree of 90%. Subsequently, 0.17 g of potassium hydroxide was added to the saponification product. The mixture was heated to 180°C, and ethylene oxide was introduced until the pressure became 2 kg/cm². After a pressure decrease due to the addition reaction of ethylene oxide was observed, ethylene oxide was further introduced intermittently so as to keep the pressure at 2 kg/cm². Thus, ethylene oxide was fed in a total amount of 170 g over a period of 1 hour. At the time when the pressure had decreased to 0.4 kg/cm², the temperature was lowered to 100°C and the reaction product was taken out. As a result, 250 g of an adduct was obtained, which had a melting point of 52°C and a hydroxyl value of 54 mg-KOH/g. In this adduct, the amount of ethylene oxide added was 200 parts by weight per 100 parts by weight of the saponification product. EXAMPLES 1 TO 3 AND COMPARATIVE EXAMPLES 1 AND 2 For each example, ingredients as shown in Section (I) in Table 1 were kneaded with a Banbury mixer for 4 minutes at a temperature of 70 to 130°C. After the kneaded compound was cooled to room temperature, ingredients as shown in Section (II) in Table 1 were incorporated into the compound by means of open rolls at a temperature of 40 to 60°C. The thus-obtained composition was measured for Mooney viscosity (ML₁+₄ 100°C) in accordance with JIS K-6300. The composition was then vulcanized by means of a hot press at 160°C for 10 minutes to obtain a vulcanized sheet having a thickness of 2 mm. This vulcanized sheet was subjected to a tensile test in accordance with JIS K-6301. The vulcanized sheet was also allowed to stand at room temperature for 20 days, and its appearance was then examined to evaluate the degree of blooming. The results obtained are summarized in Table 1. EXAMPLES 4 TO 6 AND COMPARATIVE EXAMPLES 3 TO 5 Using a mixture of two kinds of rubbers as a synthetic rubber, rubber compositions were prepared in the same manner as in Examples 1 to 3 except that ingredients as shown in Table 2 were used in place of those shown in Table 1. From the thus-prepared rubber compositions, vulcanized sheets were obtained and evaluated in the same manner as in Examples 1 to 3 except that the vulcanized sheets were allowed to stand in a room for 15 days. The results obtained are summarized in Table 2. EXAMPLES 7 TO 10 AND COMPARATIVE EXAMPLES 6 TO 9 Using various kinds of rubbers as a synthetic rubber, rubber compositions were prepared in the same manner as in Examples 1 to 3 except that ingredients as shown in Table 3 were used in place of those shown in Table 1. From the thus-prepared rubber compositions, vulcanized sheets were obtained and evaluated in the same manner as in Examples 1 to 3 except that the vulcanization time was varied as shown in Table 3 and that the vulcanized sheets were allowed to stand in a room for 15 days. The results obtained are summarized in Table 3. EXAMPLES 11 TO 13 AND COMPARATIVE EXAMPLE 10 For each example, ingredients as shown in Section (I) in Table 4 were kneaded with a Banbury mixer for 4 minutes at a temperature of 70 to 130°C. After the kneaded compound was cooled to room temperature, ingredients as shown in Section (II) in Table 4 were incorporated into the compound by means of open rolls at a temperature of 40 to 60°C. The thus-obtained composition was measured for Mooney viscosity (ML₁+₄ 100°C) in accordance with JIS K-6300. The composition was then vulcanized by means of a hot press at 160°C for 20 minutes to obtain a vulcanized sheet having a thickness of 2 mm. This vulcanized sheet was subjected to a tensile test (JIS K-6301) and further measured for compression set and elongation set (JIS K-6301) and for volume resistivity (JIS K-6911). The results obtained are summarized in Table 4. Notes other than the above are the same as those described under Table 1. Notes other than the above are the same as those described under Tables 1 and 2.

Claims: A rubber composition comprising synthetic rubber characterised in that the composition contains (a) 100 parts by weight of a synthetic rubber and (b) from 0.5 to 10 parts by weight of an alkylene oxide adduct of a saponified ethylene-saturated carboxylic acid vinyl ester copolymer. A rubber composition as claimed in claim 1, in which said synthetic rubber (a) is an ethylene-α-olefin copolymer rubber, a styrene-butadiene copolymer rubber, a chloroprene rubber, an acrylonitrile rubber or an isoprene-isobutylene copolymer rubber. A rubber composition as claimed in claim 1 or 2, in which the saturated carboxylic acid vinyl ester in said alkylene oxide adduct (b) is vinyl acetate. A rubber composition as claimed in claim 1, 2 or 3, in which the alkylene oxide in said alkylene oxide adduct (b) is ethylene oxide. A rubber composition as claimed in claim 1 or 2, in which said alkylene oxide adduct (b) is an addition product obtained by adding from 20 to 1,000 parts by weight of an alkylene oxide to 100 parts by weight of a saponified ethylene-saturated carboxylic acid vinyl ester copolymer having an ethylene content of from 1 to 90% by weight, a number average molecular weight of from 1,000 to 20,000, and a saponification degree of from 30 to 100%. An article produced by vulcanizing a rubber composition as claimed in any preceding claim.

Inventors: JUNICHI KOSHIBA, KEISAKU YAMAMOTO, KIYOSHI IKEDA, NOBUHIRO NATSUYAMA, SUMIO HARA, YOSHIO TANIMOTO, JUNICHI, KOSHIBA, KEISAKU, YAMAMOTO, KIYOSHI, IKEDA, NOBUHIRO, NATSUYAMA, SUMIO, HARA, YOSHIO, TANIMOTO
Applicants: SUMITOMO CHEMICAL CO, SUMITOMO CHEMICAL COMPANY, LIMITED
Application year: 1991
Main classification: C08L21
IPC codes: C08L21, C08L23