THERMOPLASTIC ELASTOMER COMPOSITION

Abstract: There is provided a thermoplastic elastomer composi­ tion having excellent surface properties such as adhesion, paintability and printability together with impact resistance. The composition comprises (A) 20 - 80% by weight of a poly­ propylene resin, (8) 5 - 38 % by weight of an ethylene copolymer comprising 82 - 98.7 mol% of ethylene units, 1 - 15 mol% of ester units selected from alkyl acrylate and alkyl methacrylate units wherein the alkyl groups are of 1 - 8 carbon atoms and 0.3 - 3 mol% of unsaturated dicarboxylic acid anhydride units and (C) 5 - 70 % by weight of at least one of an ethylene-propylene copolymer rubber and an ethylene-­ propylene-non-conjugated diene copolymer rubber which contain 50 - 85 mol% of ethylene units and have a Mooney viscosity (ML₁+₄ 100°C) of 20 - 100 and an iodine value of 20 or less.

Description: BACKGROUND OF THE INVENTION This invention relates to a thermoplastic elastomer composition. Thermoplastic elastomers are known materials and recently their uses are being steadily increased including parts of automobiles, household appliances, etc. because of their low hardness and flexible properties and their moldability by the methods such as extrusion molding, injection molding, etc. like the conventional thermoplastic resins. Especially, olefinic thermoplastic elastomers have low specific gravity, heat resistance and superior balance in properties in addition to the above properties and hence are in increased demand especially in the field of automobile parts also due to the trend of lightening of automobiles. Recently, they have been found to have their uses also in the field of injection molding of large articles such as bumpers of automobiles. There are movements to heighten fashionability by painting the surface of bumpers in the same colors as the bodies. Furthermore, printing is often made on household appliances to increase decorative effects. This invention relates to a thermoplastic elastomer composition capable of providing molded products improved in surface processabilities such as adhesiveness, paintability and printability together with impact resistance and heat resistance and contemplates use for parts of automobiles and household appliances as mentioned above. Hitherto, compositions comprising olefinic copolymer rubbers or olefinic plastics have been well known as olefinic thermoplastic elastomers. However, owing to their non-polarity, in many cases, surface processing for improvement of adhesion, paintability, etc. are difficult. In order to solve these problems, a medium called "primer", typical composition of which comprises halogenated polypropylene and toluene, is used as a layer between olefinic thermoplastic elastomer and adhesives and paint to further improve the compatibility therebetween. However, due to expensiveness of the primer per se and addition of one extra step, the cost of final products increases and thus further improvement has been demanded. For improvement of adhesion, modification of the surface of polypropylene molded products by physical or chemical etching has been studied and employed. However, the step of this method is more complex than the primer coating step and besides modification of the molded products may be caused depending on the etching conditions and improvement is insufficient. For improvement of paintability, there has been employed a method of imparting polarity to the surface of molded products by irradiation with plasma in place of coating with primer. However, this method has not yet provided satisfactory results in that the plasma irradiation step is carried out in batch system and treatment becomes ununiform for molded products of some shapes. SUMMARY OF THE INVENTION Accordingly, one object of this invention is to provide a thermoplastic elastomer composition free of the above defects and remarkably superior in adhesion, paintability and printability. Another object of this invention is to provide a molded product excellent in surface workability and mechanical characteristics which comprises said thermoplastic elastomer composition. DESCRIPTION OF THE INVENTION As a result of the inventors' intensive researches to attain the above objects, it has been found that a composition which comprises a polypropylene resin, an ethylene copolymer comprising an ethylene unit, an ester unit selected from an alkyl acrylate and an alkyl methacrylate and unsaturated dicarboxylic acid anhydride unit and an ethylene-propylene or ethylene-propylene-non-conjugated diene copolymer rubber is free from the above defects and is excellent in adhesion and paintability. Thus, this invention has been accomplished. That is, this invention relates to a thermoplastic elastomer composition which comprises: (A) 20 - 80 % by weight of a polypropylene resin, (B) 5 - 38 % by weight of an ethylene copolymer having 82 - 98.7 mol % of an ethylene unit, 1 - 15 mol % of an ester unit selected from an alkyl acrylate and alkyl methacrylate having an alkyl group of 1 - 8 carbon atoms and 0.3 - 3 mol % of an unsaturated dicarboxylic acid anhydride unit and (C) 5 - 70 % by weight of an ethylene-propylene and/or ethylene-propylene-non-conjugated diene copolymer rubber containing 50 - 85 mol % of an ethylene unit and having a Mooney viscosity (ML₁+₄ 100°C) of 20 - 100 and an iodine value of 20 or less. Construction of this invention is specifically explained below. (A) Polypropylene resins As the polypropylene resins, there may be used, alone or in combination of two or more, crystalline polypropylenes, crystalline polypropylene-ethylene block or random copolymers and crystalline polypropylene-α-olefin copolymers (carbon number of the α-olefin is 4 - 10). Melt index (according to JIS K6758) of said polypropylene resins is generally 0.5 - 100, preferably 1 - 50. (B) Ethylene copolymer The ethylene copolymers used in this invention are those which comprise 82 - 98.7 mol % of an ethylene unit, 1 - 15 mol % of an ester unit selected from alkyl acrylate and alkyl methacrylate having 1 - 8 carbon atoms and 0.3 - 3 mol % of an unsaturated dicarboxylic acid anhydride unit. As examples of monomers having ester unit contained in the ethylene copolymers, mention may be made of methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, 2-ethylhexyl methacrylate and butyl methacrylate (which are called "unsaturated ester compounds" hereinafter). Amount of the ester unit contained in the ethylene copolymers is generally 1 - 15 mol %, preferably 3 - 12 mol %. When the amount is outside this range, the resulting thermoplastic elastomer composition is inferior in compatibility with polypropylene resins or is poor in improvement in mechanical characteristics such as impact value. As examples of monomers,having unsaturated dicarboxylic acid anhydride unit, mention may be made of maleic anhydride, citraconic anhydride, itaconic anhydride and tetrahydrophthalic anhydride, and especially preferred is maleic anhydride. Amount of the unsaturated dicarboxylic acid anhydride in the ethylene copolymer is generally 0.3 - 3 mol %, preferably 0.5 - 2.8 mol %. If less than the lower limit, polar groups which participate in the surface processing are deficient, giving insufficient improvement and when more than the upper limit, the polarity is too strong and the ethylene copolymer becomes non-compatible with polypropylene resins. Method for production of the ethylene copolymers has no special limitation and there may be employed the known methods such as solution polymerization, emulsion polymerization and high-pressure bulk polymerization. The most commonly used is a high-pressure polymerization method which carries out copolymerization with ethylene in the presence of a radical forming agent under 51 - 304 MPa (500 - 3,000 atm) and at 100 - 300°C in the presence or absence of a suitable solvent or chain transfer agent. According to such high-pressure polymerization method, it is suitable to subject a solution of said unsaturated dicarboxylic acid anhydride in said unsaturated ester compound to pressurizing by a high-pressure pump and directly introduce it into a tank-like or tubular reactor. Melt index (according to JIS K6760) of thus obtained ethylene copolymers is suitably 0.5 - 300, preferably 1 - 200. When the melt index is less than the lower limit, the ethylene copolymer is inferior in compatibility with polypropylene resins and when more than the upper limit, the resulting thermoplastic elastomer composition is inferior in heat resistance and surface hardness. (C) Ethylene-propylene copolymer rubber and ethylene-propylene-non-conjugated diene copolymer rubber. The ethylene-propylene copolymer rubber and/or ethylene-propylene-non-conjugated diene copolymer rubber used in this invention contain 50 - 85 mol % of an ethylene unit and have a Mooney viscosity (ML₁+₄ 100°C) of 20 - 100 and an iodine value of 20 or less. The ethylene-propylene copolymer rubber and/or ethylene-propylene-non-conjugated diene copolymer rubber may be partially crosslinked with an organic peroxide. Said organic peroxides include 2,5-dimethyl 2,5-di(t-butylperoxy)hexane, 2,5-dimethyl 2,5-di(t-butylperoxy)hexyne-3,1,3-bis(t-butylperoxyisopropyl)benzene, 1,1-di(t-butylperoxy)3,5,5-trimethylcyclohexane, 2,5-dimethyl 2,5-di(peroxybenzoyl)-hexyne-3 and dicumyl peroxide. If necessary, crosslinking aids such as bismaleimide compounds and divinyl benzene may further added and mixed. When the ethylene unit is less than 50 mol %, impact strength decreases and in the case of partially crosslinked rubber, decomposition with the organic peroxide proceeds too much, causing reduction of mechanical properties and bad appearance of molded products. When ethylene content is more than 85 mol %, the copolymer rubber becomes crystalline to result in reduction of impact resistance. When the Mooney viscosity at 100°C (ML₁+₄ 100°C) is less than 20, mechanical properties are inferior and when more than 100, fluidity is poor and appearance of injection molded products becomes inferior. Preferred range is 30 - 90. When iodine value is more than 20, this is economically not advantageous and in the case of partially crosslinked copolymer rubber, crosslinking with the organic peroxide proceeds too much and stones often occur. As the non-conjugated dienes, mention may be made of, for example, methyltetrahydroindene, 5-vinylnorbornene, 5-ethylidene-2-norbornene, dicyclopentadiene, 1,4-hexadiene and 1,6-octadiene. (D) Compositional proportions The proportion of the ethylene copolymer which is component (B) in the thermoplastic elastomer composition of this invention is 5 - 38 % by weight, especially 10 - 20 % by weight. When the proportion of the ethylene copolymer (B) is less than 5 % by weight, there are recognized substantially no effects of improvements in surface processabilities such as adhesion, paintability and printability and when more than 38 % by weight, properties of the resulting compositions such as stiffness and heat resistance become inferior and no gain effects are seen in improvement of surface processability. The proportion of the ethylene-propylene copolymer rubber and/or ethylene-propylene-non-conjugated diene copolymer rubber which are component (C) in the thermoplastic elastomer composition of this invention is 5 - 70 % by weight, especially 10 - 70 % by weight. When less than 5 % by weight, mechanical properties such as impact strength are inferior and when more than 70 % by weight, thermoplastic properties are developed with difficulty. (E) Mixing method and molding method In preparation of the thermoplastic elastomer composition of this invention, there may be additionally incorporated, within the range not to substantially damage the characteristics of the composition, various styrene elastomers such as styrene-butadiene random copolymer, styrene-butadiene block copolymer and its hydrogenated product, various polyolefin elastomers such as non-crystalline ethylene-propylene copolymer and ethylene-propylene-dicyclopentadiene copolymer, and various functional group-containing polyolefins such as maleic anhydride graft modified polypropylene and glycidyl group-containing ethylene copolymer. Furthermore, if necessary, there may also be added process oils such as paraffinic oils, aromatic oils and naphthenic oils for improvement of softening property, extendability and processability of the rubbers. Moreover, various additives such as antioxidant, weathering agent, antistatic agent, foaming agent and colorant may also be added in a suitable amount. The composition of this invention is obtained by uniformly mixing the above components in predetermined mixing ratios. The uniform mixing can be carried out by mechanical melt kneading with common kneaders for thermoplastic resins. The kneaders used include single-screw extruder, twin-screw extruder Banbury mixer and twin roll. The mixing operation of the components may be carried out simultaneously or dividedly. (F) Characteristics and uses The thermoplastic elastomer compositions of this invention possess excellent adhesion, paintability and printability with maintaining mechanical strength, heat resistance, impact strength, surface gloss, appearance, weather resistance similar to or better than those of olefinic thermoplastic elastomers. The thermoplastic elastomer compositions according to this invention exhibit the excellent effects as mentioned above and hence can be utilized in the fields where adhesion, painting and printing are required. The composition according to this invention can be molded by generally employed molding machines for thermoplastic resins and by the methods used for molding of thermoplastic resins such as injection molding, extrusion molding, blow molding and calender molding. Typical examples of uses are enumerated below: (1) Exterior trims of automobiles such as solft bumpers, corner bumpers, side shields, overriders, bumper moles and side moles. (2) Interior trims of automobiles such as ceiling sheets, seat belt covers and interior mats. (3) Various hose parts of household appliances (washing machines, bedclothes drier) The following nonlimiting examples and comparative examples further explain this invention. (1) Preparation of ethylene copolymer sample: Ethylene containing a small amount of propane as a chain transfer agent and a solution prepared by dissolving maleic anhydride in various unsaturated ester compounds shown in Table 1 are continuously fed under pressure to a 1.2 ℓ reactor equipped with a stirrer and copolymerization was carried out under a pressure 0.16 kg/m² of (1,600 kg/cm²) and at a rate of 1.5 - 3.2 kg/hr with controlling the polymerization temperature to 180 °C to obtain the following various samples. (2) Methods for measurement of properties: A plate of 150 mm x 90 mm x 2 mm was made by molding by screw in-line type injection molding machine (99 g [3.5 ounces]) manufactured by Nissei Jushi Kogyo Co. and this was used as a test piece for measurement of initial adhesion degree of coating and adhesion strength-1 referred to below. 1. Initial adhesion degree of coating: The same plate as prepared hereinabove was subjected to surface cleaning in 1,1,1-trichloroethane vapor (74°C) for 30 seconds and dried at room temperature and thereafter, directly spray coated with an urethane paint (Flexthane #101 manufactured by Nippon Bee Chemical Co.) and baked in an oven of 120°C for 30 minutes. Thus formed film of the above sample was cut lengthwise and crosswise by a razor to form one hundred squares (10 x 10) of 2 mm x 2 mm and thereon was applied adhesive tape (manufactured by Nichiban Co.) of 24 mm wide under pressure by fingers. Then, the tape was rapidly peeled off by taking one end of the tape and the number of the squares left on the plate was counted and was taken as percentage of retention (%) for evaluation. 2. Adhesion strength (i) Adhesion strength - 1 The same plate as used for the initial adhesion 1 was coated with about 150 g/m² of a mixture of 97 % by weight of G250 (Registered Trade Mark) manufactured by Cemedine Co. and 3 % by weight of Desmodur (Registered Trade Mark) as an adhesive by an air spray gun and heated at 80°C for 2 minutes. Thereto was allowed to adhere PPX (Registered Trade Mark) manufactured by Toray Industries Inc. heated at 110°C for 2 minutes as a surface skin under a pressure 2. 10⁻⁵kg/m² (0.2 kg/cm²). This was left to stand in an atmosphere of 23°C and 65 ± 5 % RH for 7 days and then, subjected to 180° peeling test to measure the strength. (ii) Adhesion strength - 2 Using the above screw in-line type injection molding machine of Nissei Jushi Kogyo Co., a sample was made by inserting a metal plate of 2 mm thick into a mold of 4 mm thick at a molding temperature of 220°C and under an injection pressure of 0.065 kg/m² (650 kg/cm²) and this sample was used for test. The adhesion strength was 180° peeling strength measured at 23°C and at a speed of 50 mm/min. 3. Hardness (Shore A) Shore A hardness was measured at 23°C according to ASTM D2240. 4. Izod impact strength: This was measured at -40°C according to JIS K7110. The test piece for the measurement was made by molding using a screw in-line type injection molding machine [283 g (10 ounces)] manufactured by Sumitomo Heavy Industries Ltd. Example 1 A mixture of Sumitomo Noblen® AW564 as a polypropylene resin, an ethylene copolymer C (as shown in Table 1) and an ethylene-propylene copolymer rubber (called EPR-1) having a Mooney viscosity of 90 at 100°C and an ethylene content of 69 wt% in the proportion as shown in Table 2 and 0.05 wt part (based on 100 wt parts of said mixture) of Irganox® 1010 (manufactured by Ciba Geigy Co.) were melt kneaded by Banbury mixer at 180°C for 5 minutes. Thus obtained elastomer composition was subjected to the evaluation tests. Example 2 A mixture of Sumitomo Noblen® AW564 and an ethylene-propylene copolymer rubber (called EPDM-1) having a Mooney viscosity of 86 at 100°C, an ethylene content of 50 wt% and an iodine value of 10 in the proportion as shown in Table 2 and 0.4 wt part (based on 100 wt parts of said mixture) of 1,3-bis(t-butyl-peroxyisopropyl)benzene as a crosslinking agent were melt kneaded by a Banbury mixer at 180°C for 5 minutes. To thus obtained elastomer composition was further added the ethylene copolymer C at the proportion as shown in Table 2 and these were again kneaded by a Banbury mixer. Thus obtained final elastomer composition was subjected to the evaluation tests. Examples 3-6 and Comparative Examples 1-4 Examples 1 and 2 were repeated except that kinds and amounts of ethylene copolymer were changed as shown in Table 2 and thus obtained elastomer compositions were subjected to the evaluation tests. Results of Examples 1-6 and Comparative Examples 1-4 are shown in Table 2. As is clear from Table 2, the compositions of examples of this invention had superior balances of various properties to those of comparative examples which lacked the requirements of this invention. As explained hereabove, this invention can provide thermoplastic elastomer compositions having excellent surface processabilities such as adhesion, paintability and printability together with high impact strength and molded products obtained therefrom.

Claims: 1. A thermoplastic elastomer composition which comprises (A) 20 - 80 % by weight of a polypropylene resin, (B) 5 - 38 % by weight of an ethylene copolymer comprising 82 - 98.7 mol% of ethylene units, 1 - 15 mol% of ester units selected from alkyl acrylate and alkyl methacrylate units wherein the alkyl groups are of 1 - 8 carbon atoms and 0.3 - 3 mol% of unsaturated dicarboxylic acid anhydride units and (C) 5 - 70 % by weight of at least one of an ethylene-propylene copolymer rubber and an ethylene-propylene-non-conjugated diene copolymer rubber which contain 50 - 85 mol% of ethylene units and have a Mooney viscosity (ML₁+₄ 100°C) of 20 - 100 and an iodine value of 20 or less. 2. A thermoplastic elastomer composition according to claim 1 wherein the polypropylene resin (A) is at least one of crystalline polypropylene, crystalline propylene-ethylene block and random copolymers and crystalline propylene-α-olefin copolymers. 3. A thermoplastic elastomer composition according to claim 1 or 2 wherein the polypropylene resin (A) has a melt index (JIS K6758) of 0.5 - 100. 4. A thermoplastic elastomer composition according to claim 1, 2 or 3 wherein the ester units contained in the ethylene copolymer (B) are derived from a monomer selected from methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, 2-ethylhexyl methacrylate and butyl methacrylate. 5. A thermoplastic elastomer composition according to any one of the preceding claims wherein the unsaturated dicarboxylic acid anhydride units in the ethylene copolymer (B) are derived from a monomer selected from maleic anhydride, citraconic anhydride, itaconic anhydride and tetrahydrophthalic anhydride. 6. A thermoplastic elastomer composition according to any one of the preceding claims wherein the ethylene copolymer (B) has a melt index (JIS K6760) of 0.5 - 300. 7. A thermoplstic elastomer composition according to any one of the preceding claims wherein the rubber (C) is crosslinked with an organic peroxide. 8. A thermoplastic elastomer composition according to any one of the preceding claims wherein rubber (C) is derived from a non-conjugated diene selected from methyltetrahydroindene, 5-vinyl-norbornene, 5-ethylidene-2-norbornene, dicyclopentadiene, 1,4-hexadiene and 1,6-octadiene. 9. A molded product prepared from a composition as claimed in any one of the preceding claims.

Inventors: DAIMON, AKIO, MITSUNO, TATSUYUKI, SHINONAGA, HIDEO
Applicants: SUMITOMO CHEMICAL COMPANY, LIMITED
Application year: 1987
Main classification: C08L23
IPC codes: C08L33, C08L7, C08L21, C08L101, C08L77, C08L51, C08L23