Process for producing a thermoplastic elastomer

Abstract: A thermoplastic elastomer exhibiting scarce tendency of depositing gummy crust around the extrusion die upon extrusion molding articles therefrom and a superior oil-resistance, which comprises (A) a crystalline polyolefin resin and (B) a cross-linked rubber component produced by a specific melt-kneading technique and which has a certain high level of gel content, a high oil-resistance and a certain reduced rate of deposition of gummy crust on the extrusion die.

Description: FIELD OF THE INVENTION The present invention relates to a process for producing a thermoplastic elastomer. The thermoplastic elastomer comprises a crystalline polyolefin resin and a cross-linked rubber component. The elastomer obtainable by the process of the present invention exhibits improved resistance to oils and has a low gummy crust deposition rate on surfaces of extrusion dyes when used in the production of moulded products. BACKGROUND OF THE INVENTION Due to light weighing and ease in recycling, thermoplastic elastomers based on polyolefin have found their wide applications for use as energy-sparing and resources-sparing elastomers, in particular, as substitute for vulcanized rubber for parts and elements in, such as automobiles, industrial machines and electric and electronic devices, and for architectural materials. However, thermoplastic polyolefinic elastomers of the prior art are inferior in the resistance to oils and, in particular, suffer from a problem that they may swell upon contact with non-polar solvents, such as aromatic organic solvents, gasoline and mineral oils, resulting in limitation of the application range. The conventional thermoplastic elastomers exhibit more tendency to cause deposition of gummy crust at around extrusion die upon extrusion molding of such elastomers into articles than other commonly employed resins, whereby the molded articles may suffer from deterioration of appearance due to adhesion of the separated gummy crust. US 5,552,482 relates to a thermoplastic elastomer composition which comprises (I) a dynamically cross-linked thermoplastic elastomer prepared from a composition comprising a crystalline polyolefin and amorphous rubber, and (II) a non-cross-linked polymer which is a heterophasic olefin polymer, a crystalline 1-butene homopolymer or an amorphous ethylene/propylene or ethylene/butalene copolymer. The thermoplastic elastomer composition is prepared by dynamically curing the components of (I) in the presence of a curing agent and subsequently adding to the cross-linked mixture the non-cross-linked polymer (II). GB 2,007,683 relates to a thermoplastic elastomer composition comprising a crystalline polyolefin and a rubber which is cured with a phenolic curative. The thermoplastic elastomer composition is produced by mixing the polyolefin resin and the rubber, heating the mixture to a temperature sufficient to soften the polyolefin resin and adding the phenolic curative once the molten resin and rubber have become intimately mixed. The mixture is then heated and masticated at an appropriate curing temperature. SUMMARY OF THE INVENTION An object of the present invention is to provide a process for producing a thermoplastic elastomer based on polyolefin, which exhibits an improved resistance to oils and has scarce tendency to cause deposition of gummy crust onto extrusion die upon extrusion molding of articles therefrom. According to a first aspect, the present invention provides a process for producing a thermoplastic elastomer, comprising the steps of: (i) melt-kneading a crystalline polyolefin resin (A 1 ) and a rubber component (B 1 ) in the presence of a cross-linking agent (D) in a first extruder; (ii) kneading a crystalline polyolefin resin (A 2 ) and/or a rubber (B 2 ) in a second extruder and supplying the resulting kneaded mass to the first extruder at a position downstream of the midpoint of the barrel of the first extruder where the cross-linking reaction between A 1 and B 1 has been completed; and (iii) melt-kneading the kneaded mass obtained in step (ii) together with the cross-linked melt-kneaded mass obtained in step (i) in the first extruder, wherein (A 1 ) and (A 2 ) may be the same or different and (B 1 ) and (B 2 ) may be the same or different. Preferably, the (A 1 ):(B 1 ) weight ratio is 5:95-70:30 in step (i). Preferably, 30-100 parts by weight of (A 2 ) and/or (B 2 ) are supplied to the first extruder in step (ii) per 100 parts by weight of (A 1 ) and (B 1 ) present in the first extruder. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is aperspective view of an apparatus for realizing the process for producing the thermo-plastic elastomer according to the present invention. In the context of this specification, the word "gummy crust" means a deposit occuring and adhering on the extrusion die during extrusion molding of the thermoplastic elastomer into molded articles on an extrusion molding machine and exhibiting different color and material properties from the raw material to be molded. For the gummy crust, there are yellow sticky one and black particulate one, which are believed to be degradation products of the starting polymer, though the mechanism of formation thereof is unknown. The thermoplastic elastomer obtainable by the process of the present invention comprises a crystalline polyolefin resin (A) and a cross-linked rubber component (B) and has a gel content a), as defined below, of at least 20 % by weight, preferably at least 30 % by weight, a weight change range Δ W b) before and after the soaking, as described above, of 80 % by weight or less, preferably 60 % by weight or less, and a gummy crust deposition rate c), as described above, of 30 mg or less, preferably 20 mg or less. The gel content a), the weight change range Δ W b) before and after soaking in paraffin oil and the gummy crust deposition rate c) are determined as follows: <<Method for Determining the Gel Content a), namely, Cyclohexane-insoluble Matter>> About 100 mg of a sheet sample of thermoplastic elastomer are weighed precisely and the sheet is cut into grains of a size of 0.5 mm × 0.5 mm × 0.5 mm. The resulting grains are then soaked in 30 ml of cyclohexane in a closed vessel at a temperature of 23 °C for 48 hours. The granular sample after soaking is taken out of the vessel onto a filter paper and dried at room temperature over a period of 72 hours or more until a constant weight will be reached. The weight value calculated by subtracting the weight of other cyclohexane-insoluble matter than that of the polymer components (namely, fibrous filler, filling materials, pigments and so on) from the weight of the dried sample is taken as the "corrected final weight (Y)". On the other hand, the weight value calculated by subtracting the sum of the weight of other cyclohexane-soluble matter than that of the polymer components (namely, for example, softening agent and so on) plus the weight of other cyclohexane-insoluble matter than that of the polymer components (namely, fibrous filler, filling materials, pigments and so on) from the weight of the dried sample is taken as the "corrected original weight (X)". Here, the gel content a), i.e. the proportion of cyclohexane-insoluble matter in the thermoplastic elastomer, can be calculated by the following equation (1): Gel content (wt. %) = Correct. final weight (Y) Correct. original weight (X) × 100 <<Method for Determining the Weight Change Range Δ W b) before and after the Soaking>> A rectangular plate of a size of 150 mm × 120 mm × 2 mm is formed by injection molding from the thermoplastic elastomer, from which test pieces each in a form of square plate of a size of 20 mm × 20 mm × 2 mm are cut out. Each test piece which has been weighed preliminarily is soaked in paraffin oil of 50 °C for 24 hours, whereupon the resulting soaked test piece is weighed again to determine the weight change range Δ W b) before and after the soaking. <<Method for the Determination of Gummy Cruster Deposition Rate c)>> Using a monoaxial extruder with a full-flight extrusion screw having a diameter of 50 mm, an L/D ratio of 28 and a compression ratio of 4.0 equipped with an extrusion die having an extrusion aperture of 25 mm × 1 mm, the thermoplastic elastomer is extruded into as ribbon at an extrusion rate of 20 m/min. under a gradient temperature elevation from the extruder supply inlet to the die outlet of 160 - 210°C . The gummy crusts deposited around the extrusion die within an interval of 10 minutes during the extrusion are collected and weighed. Here, the above gradient temperature elevation is realized under the condition of the settled temperatures for the kneading zones C1 to C4 in the extrusion barrel, for the extruder head (H) and for the die (D), respectively, as follows: Set temperature (°C ) for C1/C2/C3/C4/H/D = 160/170/180/190/200/210 The thermoplastic elastomer obtainable by the process of the present invention is a resin composition comprising a crystalline polyolefin resin (A) and a cross-linked rubber component (B), wherein a composition containing, on the one hand, a cross-linked product (C) in which a rubber component (B 1 ) is cross-linked in a state mixed with a crystalline polyolefin resin (A 1 ) and, on the other hand, a crystalline polyolefin resin (A 2 ) and/or a rubber component (B 2 ) is preferred. As the crystalline polyolefin resin (A 1 ) constituting the cross-linked product (C), there may be exemplified homopolymers and copolymers of α -olefins having 2 - 20 carbon atoms, preferably 2 - 10 carbon atoms, containing these principal components usually in an amount of at least 85 mole %, preferably at least 90 mole %, such as ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, 4-methyl-1-pentene and 3-methyl-1-pentene. As the crystalline polyolefin resin (A 1 ), there may be exemplified favorably polyolefin resins of homopolymers of ethylene, propylene and 1-butene and copolymers constituted mainly of olefin comonomers, such as ethylene, propylene and 1-butene, wherein special preference is given to polypropylene. As the polypropylene, a propylene homopolymer or a copolymer of propylene having a propylene content in the range of 85 - 100 mole %, preferably in the range of 90 - 100 mole %, with other α -olefin comonomer than propylene having 2 - 10 carbon atoms. The crystalline polyolefin resin (A 1 ) constituting the cross-linked product (C) may consist of one single species of resin or of two or more species of polyolefin resins. The crystalline polyolefin (A 1 ) constituting the cross-linked product (C) may favorably be those which have a melt flow rate (MFR, determined according to ASTM D 1238-65T at 230 °C under a load of 2.16 kg) in the range of 0.1 - 100 g/10 min., preferably of 0.3 - 50 g/10 min. The proportion of the crystalline polyolefin resin (A 1 ) constituting the cross-linked product (C) may favorably be in the range of 5 - 70 parts by weight, preferably 10 - 50 parts by weight, per 100 parts by weight of the sum of the components (A 1 ) and (B 1 ) constituting the cross-linked product (C). In the content of the crystalline polyolefinic resin (A 1 ) within the above range, the thermoplastic elastomer has superior elasticity and moldability. Concrete examples of the rubber component (B 1 ) constituting the cross-linked product (C) include olefinic copolymer rubbers which are elastic amorphous random copolymers having at least 50 mole % of an α-olefin of 2 - 20 carbon atoms, butadiene rubber, isoprene rubber, styrene/ethylene copolymer rubber, styrene/butadiene copolymer rubber and hydrogenation product thereof, styrene/isoprene copolymer rubber and hydrogenation product thereof, nitril rubber, natural rubber, butyl rubber, polyisobutylene and acryl rubber. As the α -olefin of 2 - 20 carbon atoms, there may be exemplified ethylene, propylene, 1-butene 4-methyl-1-pentene, 1-hexene and 1-octene. Among them, preference is given to ethylene and propylene. For the rubber component (B 1 ), rubbers based on olefin, such as copolymers of two or more α-olefins having the principal constituents usually in a proportion of less than 85 mole %, preferably less than 82 mole %, and α -olefin/non-conjugated diene copolymers constituted of at least two α -olefins and a non-conjugated diene, may preferably be employed, in particular, ethylene/α -olefin/non-conjugated diene copolymer rubbers (with ethylene/α -olefin mole ratios in the range of about 90/10 to 50/50) are preferred. As the ethylene/α -olefin/non-conjugated diene copolymer rubber mentioned above, ethylene/propylene/non-conjugated diene copolymer rubbers are preferred. For the non-conjugated diene, there may be enumerated, for example, dicyclopentadiene, 1,4-hexadiene, cyclooctadiene, methylene norbornene and ethylidene norbornene. The ethylene/α -olefin/non-conjugated diene copolymer rubber in which such a non-conjugated diene is employed as a comonomer may favorably have an iodine value of 35 or less, preferably 25 or less. The rubber component (B 1 ) constituting the cross-linked product (C) may favorably have a melt flow rate (MFR) in the range of 0.1 - 5 g/10 min., preferably 0.1 - 2 g/10 min. (determined according to ASTM D 1238-65T, at 230°C under a load of 2.16 kg). The rubber component (B 1 ) constituting the cross-linked product (C) may be composed of one single polymer species or of two or more polymer species. The proportion of the rubber component (B 1 ) constituting the cross-linked product (C) according to the present invention may favorably amount to 30 - 95 parts by weight, preferably 90 - 50 parts by weight, per 100 parts by weight of the sum of the components (A 1 ) and (B 1 ) constituting the cross-linked product (C). In such a proportion of the rubber component (B 1 ), the thermoplastic elastomer according to the present invention exhibits a high elasticity and a superior moldability. For the cross-linked product (C), a cross linking agent (D) used commonly in thermosetting rubbers may be employed, such as an organic peroxide, sulfur, phenol resin, amino resin, quinone and its derivative, amine compound, azo compound, epoxy compound or isocyanate. Among them, preference is given to organic peroxide. Concrete examples of organic peroxide include dicumyl peroxide, di-tert-butyl peroxide, 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane, 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexyne-3, 1,3-bis(tert-butylperoxyisopropyl)benzene, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, n-butyl-4,4-bis(tert-butylperoxy)valerate, benzoyl peroxide, p-chlorobenzoyl peroxide, 2,4-di-chlorobenzoyl peroxide, tert-butylperoxybenzoate, tert-butylperoxyisopropyl carbonate, diacetyl peroxide, lauroyl peroxide and tert-butylcumyl peroxide. Among them, 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane, 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexyne-3, 1,3-bis(tert-butylperoxyisopropyl)benzen, 1,1-bis-(tert-butylperoxy)-3,3,5-trimethylcyclohexane and n-butyl-4,4-bis(tert-butylperoxy)valerate are favorable in view of odor and stability to scorch. Preference is given especially to 1,3-bis(tert-butylperoxyisopropyl) benzene and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane. The organic peroxide may favorably be employed in an amount in the range of 0.1 - 2 parts by weight, preferably 0.2 - 1.5 parts by weight, per 100 parts by weight of the sum of the components (A 1 ) and (B 1 ) constituting the cross-linked product (C). When the amount of the organic peroxide used is in the above range, the resulting thermoplastic elastomer will have properties well balanced among the rubbery elasticity, moldability, heat resistance and appearance of the molded article. Upon the cross linking of the rubber component (B) using the organic peroxide, a cross linking assistant (E) may be used. Concrete examples of the cross linking assistant (E) include divinyl compounds, such as divinylbenzene and the like; oximes, such as p-quinone dioxime and p,p'-dibenzoylquinone dioxime; nitroso compounds, such as N-methyl-N-4-dinitrosoaniline and nitrosobenzene; maleimides, such as trimethylolpropane-N,N'-m-phenylenedimaleimide; and others, such as sulfur, diphenylguanidine and so on. Other compounds may also be used for the cross linking assistant (E), for example, polyfunctional methacrylate monomers, such as ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, trimethylolpropane trimethacrylate and allyl methacrylate; polyfunctional vinyl monomers, such as vinyl butyrate and vinyl stearate; and polyfunctional allyl monomers, such as triallyl cyanurate and so on. By performing the cross linking reaction using the organic peroxide in the presence of a cross linking assistant (E) mentioned above, a uniform and mild cross linking reaction may be expected. Among the above-indicated cross linking assistants (E), divinylbenzene is at the most preferred. Divinylbenzene is easy in handling, exhibits a better compatibility with the crystalline polyolefin resin (A 1 ) and with the rubber component (B 1 ) and acts as a dispersing agent for the organic peroxide to solubilize the organic peroxide, so that a uniform cross linking can be realized by the heat treatment, whereby a thermoplastic elastomer having a better balance between the flowability and the material properties can be obtained. It is favorable to use the cross linking assistant (E) in an amount in the range of 0.1 - 3 %, preferably 0.2 - 2 %, based on the weight of the entire mass to be subjected to cross linking treatment. In case the amount of the cross linking assistant (E) blended in the starting composition to be subjected to cross linking treatment is in the above range, the resulting thermoplastic elastomer will not suffer from alteration in the material properties upon molding thereof due to thermal hysteresis, since no remainder of the cross linking assistant (E) as unreacted monomer is present in the thermoplastic elastomer, and, in addition, the resulting thermoplastic elastomer exhibits a superior flowability. The crystalline polyolefin resin (A 2 ) to be blended with the cross-linked product (C) may either be identical with or different from the crystalline polyolefin resin (A 1 ) constituting the cross-linked product (C). Thus, those exemplified previously for the crystalline polyolefin resin (A 1 ) can be employed for the crystalline polyolefin resin (A 2 ). The crystalline polyolefin resin (A 2 ) may consist of one single polymer species or of two or more polymer species. As the crystalline polyolefin resin (A 2 ) to be blended with the cross-linked product (C), polypropylene or a low density linear polyethylene is preferred, wherein a particular preference is given to a polypropylene having an MFR of 10 g/10 min. or lower. By using such a polypropylene product, a thermoplastic elastomer which causes scarce occurrence of deposition of gummy crust upon extrusion molding thereof and which exhibits an increased oil-resistance can be obtained. The rubber component (B 2 ) to be blended with the cross-linked product (C) may be identical with or different from the rubber component (B 1 ) constituting the cross-linked product (C). Thus, those exemplified previously for the rubber component (B 1 ) can be used as the rubber component (B 2 ). The rubber component (B 2 ) to be blended with the cross-linked product (C) may consist of one single polymer species or of two or more polymer species. As the rubber component (B 2 ) to be blended with the cross-linked product (C), it is preferable to use a rubber which is the same as the rubber component (B 1 ) constituting the cross-linked product (C) or is well compatible therewith and/or an olefinic rubber which is an amorphous random elastic copolymer having a content of an α -olefin of 2 - 20 carbon atoms of at least 50 mole %. As the amorphous random elastic copolymer, there may be enumerated α -olefin copolymers composed of two or more α -olefins and α -olefin/non-conjugated diene copolymers composed of two or more α -olefins and a non-conjugated diene. Concrete examples for such an amorphous random elastic copolymer include: 1) Ethylene/α -olefin copolymer rubbers with mole ratios of ethylene/α -olefin in the range of about 90/10 - 50/50 2) Ethylene/α -olefin/non-conjugated diene copolymer rubbers with mole ratios of ethylene/α -olefin in the range of about 90/10 - 50/50 3) Propylene /α -olefin copolymer rubbers with mole ratios of propylene/α -olefin in the range of about 90/10 - 50/50 4) Butene/α -olefin copolymer rubbers with mole ratios in the range of about 90/10 - 50/50 As the above-mentioned α -olefin, there may be exemplified ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and 1-octene. As the non-conjugated diene mentioned above, there may be exemplified dicyclopentadiene, 1,4-hexadiene, cyclooctadiene, methylenenorbornene and ethylidenenorbornene. The ethylene/α -olefin/non-conjugated diene copolymer rubber given in above 2) in which a non-conjugated diene such as given above is copolymerized may preferably have an iodine value of 25 or lower. For the olefinic rubbers given in above 1) to 4), those which have an MFR in the range of 0.1 - 10 g/10 min., preferably 0.2 - 5 g/10 min., are preferred. Among the olefinic rubbers mentioned above, special preference is given to ethylene/propylene copolymer rubbers and, in particular, those in which the ethylene/propylene mole ratio is in the range of 30/70 - 90/10 are preferred. The amount of the crystalline polyolefin resin (A 2 ) and/or the rubber component (B 2 ) to be incorporated in the cross-linked product (C) according to the present invention may favorably be in the range of 3 - 100 parts by weight, preferably 5 - 70 parts by weight, per .100 parts by weight of the sum of (A 1 ) and (B 1 ) constituting the cross-linked product (C). Either one or both of the crystalline polyolefinic resin (A 2 ) and the rubber component (B 2 ) may be incorporated in the cross-linked product (C). In the thermoplastic elastomer composition obtainable by the process of the present invention, other ingredients than the crystalline polyolefin resin (A 2 ) and the rubber component (B 2 ) may be incorporated, such as softening agent (F), inorganic filler (G) and so on, as starting ingredients for the thermoplastic elastomer. When, in particular, a softening agent (F) is employed, a thermoplastic elastomer exhibiting higher elasticity and superior moldability can be obtained. Concrete examples of the softening agent (F) include petroleum products, such as process oils, lubricating oils, paraffin, liquid paraffin, polyethylene wax, polypropylene wax, petroleum asphalt and Vaseline; coal tar materials, such as coal tar and coal tar pitch; fatty oils, such as castor oil, linseed oil, rapeseed oil, soybean oil and coconut oil; waxes, such as tall oil, carnauba wax and lanolin; fatty acids, such as ricinoleic acid, palmitic acid, stearic acid, 12-hydroxy-stearic acid, montanic acid, oleic acid and erucic acid and metal salts of them; synthetic high molecular substances, such as petroleum resin, cumarone-indene resin and atactic polypropylene; plasticizers based on ester, such as dioctyl phthalate, dioctyl adipate and dioctyl sebacate; and others, such as microcrystalline wax, liquid polybutadiene and modification and hydrogenation products thereof. The amount of the softening agent (F) to be incorporated in the elastomer composition obtainable by the process of the present invention may favorably be in the range of 10 - 300 parts by weight, preferably 20 - 200 parts by weight, per 100 parts by weight of the rubber component (B). Concrete examples of the inorganic filler (G) include glass fiber, potassium titanate fiber, carbon fiber, calcium carbonate, calcium silicate, clays, kaolin, talc, silica, diatomaceous earth, powdery mica, asbestos, alumina, barium sulfate, aluminum sulfate, calcium sulfate, magnesium carbonate, molybdenum disulfide, graphite, glass beads and Shirasu baloons. The amount of the inorganic filler (G) to be incorporated in the elastomer composition may favorably be in the range of 1 - 100 parts by weight, preferably 2 - 50 parts by weight, per 100 parts by weight of the sum of the crystalline polyolefin resin (A) and the rubber component (B). The thermoplastic elastomer may contain, if necessary, various additives, such as thermal stabilizer, antioxidant, anti-weathering agent, antistatic agent, colorant and lubricant, in an amount not obstructing the object of the invention. The thermoplastic elastomer is a resin composition comprising a cross-linked product (C) in which the rubber component (B 1 ) in a state mixed with the crystalline polyolefin resin (A 1 ) is cross-linked, a further crystalline polyolefin resin (A 2 ) and/or a further rubber component (B 2 ) as well as optional additives. The cross-linked product (C) is present in the elastomer composition in a form of gelled product having cross-linked rubber component (B 1 ) and embracing the crystalline polyolefin resin (A 1 ) and contains a gelled component which is insoluble in cyclohexane at 23°C . The crystalline polyolefin resins (A 1 , A 2 ) and other constituents and ingredients are uniformly dispersed in the body of the cross-linked product (C) as a uniform mixture. The thermoplastic elastomer can be obtained by the process of the present invention, which comprises the steps of melt-kneading the crystalline polyolefin resin (A 1 ) together with the rubber component (B 1 ) in a first extruder in the presence of the cross linking agent (D), supplying a kneaded mass of the crystalline polyolefin resin (A 2 ) and/or the rubber component (B 2 extruded from a second extruder to the first extruder at a position within the range of the exit-side half of the first extruder and melt-kneading the so-supplied kneaded mass (A 2 and/or B 2 ) therein together with the kneaded mass which has been subjected to the melt-kneading in the first extruder and in which the cross linking reaction has substantially been completed. Now, the description shall be directed to the production process itself. For the first extruder to be employed in the production process, every voluntary extruder permitting melt-kneading of the crystalline polyolefin resin (A 1 ) and the rubber component (B 1 ) in the presence of a cross linking agent (D) can be used without any special limitation. As the first extruder, there may be employed, for example, a monoaxial or biaxial extruder, wherein preference is given to the use of a biaxial extruder, especially one which has an L/D ratio, namely, the ratio of the effective length L to the outer diameter D of the screw, of 35 or higher, preferably in the range of 40 - 60, more preferably in the range of 44 - 56. For such a biaxial extruder, every voluntary one can be employed, for example, one in which both the screws rotate in the same rotational direction or in the opposite direction or one in which both the screws are held in engagement with each other or held without engagement with each other. Among them, those in which both screws rotate in the same direction under engagement with each other are more preferred. Concrete examples of such a biaxial extruder include ZSK (trademark) of the firm Warner, TEM (trademark) of Toshiba Machine Co., Ltd., TEX (trademark) of The Japan Steel Works, Ltd., GT (trademark) of Ikegai Iron Works, Ltd. and KTX (trademark) of Kobe Steel, Ltd. The first extruder is provided with a fitting for connecting the exit of the second extruder to the first extruder at a position within the range of the exit-side half of the first extruder barrel, in order to introduce the kneaded mass in the second exteruder into the first extruder midway the kneading path therein. The second extruder may either be monoaxial or biaxial, wherein preference is given to a biaxial one in case where two or more species of polymers have to be supplied to the first extruder. The position of introduction of the kneaded mass from the second extruder into the first extruder may be within the range of the exit-side half of the kneading path in the first extruder and may preferably within the range downstream the 3/5 of the full length of the barrel measured from the die-side end thereof. Usually, the first extruder is provided with supply means for supplying the crystalline polyolefin resin (A 1 ) and the rubber component (B 1 ) and others, such as feed hopper, metering feeder and the like. In the production process, the first extruder is first supplied with the crystalline polyolefin resin (A 1 ) and the rubber component (B 1 ), whereupon they are melt-kneaded therein in the presence of a cross linking agent (D) to effect cross linking of them. For supplying the cross linking agent (D) and the cross linking assistant (E), there may be employed, for example, techniques in which they are blended preliminarily with the pelletized starting polymer products to be subjected to the cross linking reaction and the resulting mixture is fed to the first extruder; or in which they are supplied to the first extruder, separately from the polymer materials, via an opening arranged in the extruder barrel thereof upstream from the fitting for connection path from the second extruder. In the case of the latter, the opening of the extruder barrel may preferably be arranged on the barrel at a position upstream from the 3/5 position of the full length of the barrel measured from the feed hopper-side end of the barrel. The cross linking reaction should have been completed substantially before the new starting polymer materials are supplied, namely, before the kneaded mass has reached the position of the fitting for the connection path from the second extruder. Therefore, it is favorable to settle the temperature in the portion where the cross linking reaction takes place, such as for example, in either one barrel zone in the range from the position at which the cross linking agent (D) is supplied to the first extruder to the position at which the fitting for the connection path from the second extruder is disposed therein, at a temperature which is by at least 20 °C , preferably at least 30°C , higher than the one minute half-life temperature of the cross linking agent (D). In the production process, the cross-linked product (C) in which the cross linking reaction has substantially been completed is melt-kneaded together with the non-cross-linked polymer substances within the region in the first extruder downstream the location where the kneaded mass from the second extruder is introduced thereinto to thereby build up an elastomeric composition in which the cross-linked product (C) and the non-cross-linked polymer molecules are present in a uniformly dispersed state. As detailed above, the thermoplastic elastomer obtainable by the process of the present invention exhibits an improved oil-resistant property with scarce tendency of depositing gummy crust on the surfaces around the extrusion die upon extrusion molding thereof. The thermoplastic elastomer according to the present invention can find wide applications in using as an energy-sparing and resources-economizing elastomer, in particular, as a substitute for vulcanized rubber, for example, for parts and elements of automobiles and industrial machines, for electric and electronic devices and for architectural materials. According to the production process, a thermoplastic elastomer exhibiting an improved resistance to oils with scarce tendency of depositing gummy crust around the extrusion die upon extrusion molding thereof can be produced efficiently, by performing the process steps comprising melt-kneading a crystalline polyolefin resin and a rubber component in the presence of a cross linking agent in a first extruder, supplying a kneaded mass of a crystalline polyolefin resin and/or a rubber component extruded from a second extruder to the first extruder at a position within the range of the exit-side half of the first extruder and melt-kneading the so-supplied kneaded mass therein together with the kneaded mass which has been subjected to the melt-kneading in the first extruder and in which the cross linking reaction has substantially been completed. THE BEST MODE FOR EMBODYING THE INVENTION Now, the present invention is described by embodiments shown in the Drawings appended. Fig. 1 shows an apparatus to be used for effecting the production process of the present invention in a perspective view. This apparatus comprises a first extruder 1 and a second extruder 2. The first extruder 1 is constituted of a biaxial extruder having an L/D ratio of 35 or higher equipped with a feed hopper 5 and an extrusion die 6, wherein the extrusion outlet of the second extruder 2 is connected to the first extruder 1 by a fitting 9 at the location of 3/10 of the full length of the extruder barrel 8 of the first extruder 1 measured from the die-side end thereof. The second extruder 2 is constituted of a biaxial extruder constructed so as to effect a preliminary kneading of the other portion of the starting materials and to introduce the so-kneaded mixture into the melt-kneaded mass in the first extruder 1 at a proper location thereof via a connection path 16 fitted by the fitting 9. For producing the thermoplastic elastomer using the apparatus of Fig. 1, the crystalline polyolefin resin (A,), the rubber component (B,), the cross linking agent (D) and additives to be incorporated on requirement are supplied to the first extruder 1 via the feed hopper 5, in order to process them in the first extruder by melt-kneading them to cause cross linking of the kneaded mass. Here, the type of the extruder, settled temperature for the melt-kneading, each specific cross linking agent (D) and the amount thereof to be used are selected adequatly in such a manner that the cross linking reaction should have substantially been completed before the extruded kneaded mass from the second extruder 2 is introduced hereinto. In the apparatus of Fig. 1, the substantial completion of the cross linking reaction of the kneaded mass in the first extruder before the introduction of the extruded kneaded mass from the second extruder can easily be attained by a proper selection of the above-mentioned conditions, since the connection path 16 for introducing the kneaded mass from the second extruder 2 is disposed on the extrusion barrel 8 of the first extruder 1 at a position within the range of downstream-side half thereof. The second extruder 2 is supplied, via the feed hopper 5, with the crystalline polyolefin resin (A 2 ) and/or the rubber component (B 2 ) as well as other additives to be incorporated on requirement, in order to effect preliminary kneading of them, before the resulting kneaded mass extruded therefrom is transferred to the first extruder 1 via the connection path 16 at a position at which the cross linking reaction of the melt-kneaded mass in the first extruder 1 has substantially been completed, so as to subject the so-transferred kneaded mass to the melt-kneading in the first extruder 1 together with the melt kneaded mass existing in the first ectruder 1. The melt-kneaded mass which has been cross-linked in the first extruder 1 and the additionally introduced kneaded mass of the starting materials from the second extruder 2 are subjected to a further melt-kneading in the range in the first extruder 1 downstream from the position 9 of introduction of the kneaded mass from the second extruder 2, whereby the contemplated thermoplastic elastomer can be obtained, which is extruded from the extrusion die 6. While the apparatus of Fig. 1 uses a feed hopper 5 for supplying the cross linking agent (D) and the optionaly added additives, it is possible to arrange an opening in the extruder barrel 8 at a position between the feed hopper 5 and the connection path 16 for exclusive use for supplying these ingredients. It is permissible to use other type of extruder or a mixer kneader, such as a monoaxial extruder or a high speed continuous kneader mixer, as the second extruder 2. EXAMPLES Below, the present invention will further be described in more detail by way of Examples, wherein these Examples should not be regarded as limiting the present invention in any sense. In the description of Examples, the gel content, the oil-resistance expressed by Δ W and the amount of deposition of gummy crust are recited each in a value obtained by the procedures explained previously. The feed components employed for these Examples are as follows: <<The Crystalline Polyolefin Resin (A)>> (A-1) A homopolymer of propylene with an MFR of 15 g/10 min. (A-2) A homopolymer of propylene with an MFR of 1.0 g/10min. (A-3) A homopolymer of 1-butene with an MFR of 0.7 g/10 min. (A-4) A random copolymer of ethylene/4-methyl-1-pentene having a density of 0.920 g/cm 3 , an ethylene content of 97 mole % and an MFR of 3.6 g/10 min. (A-5) A homopolymer of propylene having an MFR of 25 g/10 min. (A-6) A homopolymer of propylene having an MFR of 0.5 g/10 min. (A-7) A homopolymer of 1-butene having an MFR of 1.5 g/10 min. (A-8) A random copolymer of ethylene/4-methyl-1-pentene having a density of 0.920 g/cm 3 , an ethylene content of 97 mole % and an MFR of 2.0 g/10 min. <<The Rubber Component (B)>> (B-1) An oil-extended product extended with 40 parts by weight of an oil (extender oil: DYNAPROCESS OIL PW-380 (trademark) of Idemitsu Kosan Co., Ltd,) per 100 parts by weight of a copolymer rubber of ethylene/propylene/5-ethylidene-2-norbornene having an ethylene content, based on the total sum of ethylene and propylene, of 80 mole %, an iodine value of 12 and an MFR of 0.2 g/10 min. (B-2) A butyl rubber with an unsaturation degree of 0.7 mole % and an MFR of 0.6 g/10 min. (B-3) A hydrogenation product of styrene/isoprene block copolymer having a styrene content of 30 % by weight, an MFR of 2.0 g/10 min. and a hydrogenation degree of 90 % (B-4) An ethylene/1-butene copolymer with an ethylene content of 81 mole % and an MFR of 0.5 g/10 min. (B-5) An ethylene/propylene copolymer with an ethylene content of 41 mole % and an MFR of 0.4 g/10 min. (B-6) An oil-extended product extended with 40 parts by weight of an oil (extender oil: DYNAPROCESS OIL PW-380 (trademark) of Idemitsu Kosan Co., Ltd.) per 100 parts by weight of an ethylene/propylene/5-ethylidene-2-norbornene copolymer rubber having an ethylene content, based on the total sum of ethylene and propylene, of 78 mole %, an iodine value of 14 and an MFR of 0.2 g/10 min. (B-7) An ethylene/1-butene copolymer having an ethylene content of 81 mole % and an MFR of 0.7 g/10 min. <<The Cross Linking Agent (D)>> (D-1) 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane with a one minute half-life temperature of 179°C <<The Cross Linking Assistant (E)>> (E-1) Divinylbenzene <<The Softening Agent (F)>> (F-1) A paraffinic process oil (DYNAPROCESS OIL PW-380 (trademark) of Idemitsu Kosan Co., Ltd.) Example 1 Thermoplastic elastomers were produced using an apparatus corresponding to that sown in Fig. 1. Thus, 20 parts by weight of a pelletized product of the feed component (A-5) as the crystalline polyolefin resin (A 1 ), 80 parts by weight of a pelletized product of the feed component (B-6) as the rubber component (B,), 0.3 part by weight of the cross linking agent (D-1) and 0.4 parts by weight of the cross linking assistant (E-1) were blended on a Henschel mixer and the resulting mixture was supplied to the first extruder 1 (a biaxial extruder having a screw diameter of 53 mm, an L/D ratio of 45 and temperature-controllable 12 zones in the extruder barrel 8) via the feed hopper 5, in order to cause cross linking reaction of the polymer materials supplied thereto. The shearing velocity therein was 2.500 sec. -1 . The temperatures of the 12 zones (C1 to C12 arranged in order from the feed hopper-side) in the extruder barrel 8 and of the die 6 (D) were settled as follows: Set temperatures (°C ) for C1/C2/C3/C4/C5/C6/C7/C8/C9/C10/C11/C12/D = 160/160/170/180/200/220/230/230/200/200/200/200/200 A preliminarily mixed mass of the homopolymer of propylene (A-6) as the crystalline polyolefin resin (A 2 ) was supplied from the second extruder 2 (a biaxial extruder having a screw diameter of 30 mm, an L/D ratio of 32 and an evenly settled extruder internal temperature of 220°C) to the first extruder 1 via the connection path 16 at a position of 4/13 of the full length of the barrel 8 measured from the die-side end of the barrel 8 (namely, downstream from the position of 1/2 of the barrel full length). The supply rate of the propylene homopolymer was chosen to be 20 parts by weight per 100 parts by weight of the polymer materials supplied to the first extruder1 via the feed hopper 5. The so-obtained thermoplastic elastomer was tested in the manner set out below, wherein the test results as recited in Table 1 were obtained. A pelletized product of the so-obtained thermoplastic elastomer was processed into a rectangular sheet of a size of 150 mm × 120 mm × 2 mm by injection molding, from which a test piece of a size of 20 mm × 20 mm × 2 mm was cut out in order to estimate the oil-resistance (degree of swell) by an oil-swell test. The oil-swell test was carried out by immersing the test piece, which had been weighed preliminarily, in a paraffin oil at 50°C for 24 hours and the so-treated test piece was weighed, in order to determine the weight difference Δ W before and after the oil immersion. The test results are recited in Table 1. On the other hand, the thermoplastic elastomer obtained as above was processed on a monoaxial extruder having a full flight screw of a diameter of 50 mm, an L/D ratio of 28 and a compression ratio of 4.0 by extruding through a die having an extrusion aperture of 25 mm × 1 mm under a gradient temperature elevation from 160°C to 210 °C from the supply inlet to the die outlet of the monoaxial extruder at an extrusion rate of 20 m/min. to produce a ribbon, wherein the amount of gummy crust deposited around the die within an interval of 10 minutes was observed. Here, the gradient temperature elevation was realized under the following temperature conditions for the barrel zones (C1) to (C4), the head (H) and the die (D): Set temperatures (°C ) for C1/C2/C3/C4/H/D = 160/170/180/190/200/210 The gel content was also determined using the equation (1) given previously. The results are recited in Table 1. Examples 2 - 8 and Comparative Examples 1-3 In the same manner as in Example 1 , thermoplastic elastomers were produced with varying blending proportions of the feed components as given in Table 1, for which tests were carried out in the manner set out above. The test results are given in Table 1. Example 9 A thermoplastic elastomer was produced and tested in the same manner as in Example 1 except that a liquid mixture of the feed components of (D-1), (E-1) and (F-1) in the proportion as given in Table 1 was supplied to the first extruder by pumping it into a barrel opening disposed on the barrel 8 of the first extruder at a position of 10/13 of the full length of the barrel 8 measured from the die-side end of the barrel 8. The test results are recited also in Table 1. Comparative Example 4 A thermoplastic elastomer was produced and tested in the same manner as in Example 1, except that the junction position of the connection path from the second extruder 2 (a biaxial extruder having a screw diameter of 30 mm, an L/D ratio of 32 and an evenly settled extruder internal temperature of 220°C ) on the extruder barrel 8 of the first extruder 1 was settled at a position of 9/13 of the full length of the barrel 8 measured from the die-side end of the barrel 8 (namely, upstream from the position of 1/2 of the length of the barrel 8) and that the blending proportion of the feed components was changed to that given in Table 1. The test results are also given in Table 1. From the results of Table 1, it is recognized that the thermoplastic elastomer according to the present invention is superior in the resistance to oils with reduced deposition of gummy crust around the extrusion die.

Claims: A process for producing a thermoplastic elastomer comprising the steps of: (i) melt-kneading a crystalline polyolefin resin (A 1 ) and a rubber component (B 1 ) in the presence of a cross-linking agent (D) in a first extruder; (ii) kneading a crystalline polyolefin resin (A 2 ) and/or a rubber (B 2 ) in a second extruder and supplying the resulting kneaded mass extruded from the second extruder to the first extruder at a position downstream of the midpoint of the barrel of the first extruder by which point the cross-linking reaction between A 1 and B 1 has been completed; and (iii) melt-kneading the kneaded mass obtained in step (ii) together with the cross-linked melt-kneaded mass obtained in step (i) in the first extruder, wherein (A 1 ) and (A 2 ) may be the same or different and (B 1 ) and (B 2 ) may be the same or different. A process according to Claim 1, wherein the (A 1 ):(B 1 ) weight ratio is 5:95-70:30 in step (i). A process according to Claim 1 or Claim 2, wherein 3-100 parts by weight of (A 2 ) and/or (B 2 ) are supplied to the first extruder in step (ii) per 100 parts by weight of (A 1 ) and (B 1 ) supplied to the first extruder.

Inventors: ITOH YUICHI, UCHIYAMA AKIRA, ITOH, YUICHI, UCHIYAMA, AKIRA, Itoh, Yuichi c/o Mitsui Chemicals, Inc., Uchiyama, Akira c/o Mitsui Chemicals, Inc.
Applicants: MITSUI CHEMICALS INC, MITSUI CHEMICALS, INC.
Application year: 1998
Main classification: C08L23
IPC codes: C08L15, C08J3, C08L23, C08L21