Polypropylene-base resin composition

Abstract: A polypropylene-base resin composition comprising a polypropylene-base resin and carbon black in an amount of 0.01 - 0.6 wt.% based on the polypropylene-base resin and being suitable for use in the production of molded articles having excellent paintability.

Description: This invention relates to a polypropylene-base resin composition, and particularly to a polypropylene-base resin composition suitable for use in the production of molded articles having excellent paintability. This invention also relates to a molded polypropylene-base resin article, especially, a car bumper, which has excellent paintability. The polypropylene component is a crystalline ethylene-propylene block copolymer. Polypropylene-base resins are excellent in weight reduction, durability, safety, moldability, economy and the like and their molded articles are hence widely used as various industrial elements, parts, components and so on typified by car parts and components such as bumpers. Many of such molded articles are colored in black or similar dark colors in view of their color tones, weatherability, etc. Reflecting recent needs on the market, there is an ever-increasing demand for painting such molded articles to impart high-quality appearance thereto from the aesthetic consideration. Accordingly, it is now indispensable to impart paintability to them. These days, molded polypropylene-base resin articles are in many instances painted by treating their surfaces with a halogen-containing organic solvent such as trichloroethylene or trichloroethane, followed by their primer coating and topcoating [Mod. Plas., 39 (No. 9) 153 (1962)]. With a view toward solving the problem of environmental pollution due to such halogen-containing organic solvents and reducing the manufacturing cost by simplifying their painting process, it has been started using a new process in which subsequent to plasma surface treatments of molded article, their topcoating are carried out [J. Appl. Poly. Sci., 11 1461 (1967) and U.S. Patent No. 4,465,715]. Most polypropylene-base resins have already been colored with inorganic or organic colorants or mixtures thereof prior to their painting, no matter whether they are painted at their entire surfaces or at their limited surface areas. It is a common practice to incorporate carbon black or a black or blackish pigment consisting primarily of carbon black, particularly, in elements, parts, components and the like which are used outdoors, in view of the manner of their use and with a view to making improvements to their weatherability. Under the circumstances, carbon black is incorporated in large proportions in polypropylene-base resins. For example, FR-A-2282447 discloses an injection-mouldable and electrostatically paintable composition of 100 parts of an ethylene-higher α-olefin copolymer rubber; 20-100 parts of a crystalline propylene polymer; and carbon black in an amount of at least 15 and preferably 25-40 parts based on the total weight of polymer. It has however been found that the paintability of a molded article of a polypropylene-base resin added with a large amount of carbon black is considerably reduced when a paint is applied thereto subsequent to its plasma treatment. According to the invention there is provided a polypropylene-base resin composition suitable for use in the production of molded articles, comprising a) 100 parts by weight of a crystalline ethylene-propylene block copolymer; b) 7-60 parts by weight of a thermoplastic elastomer; c) 3-30 parts by weight of an inorganic filler; and d) 0.01-0.6 wt. % based on the total weight of the copolymer (a), elastomer (b) and filler (c) of carbon black. Such a composition may have excellent paintability. It may be suitable for producing bumpers and many molded articles. Illustrative examples of the thermoplastic elastomer may include ethylene-propylene rubbers, ethylene-propylene-diene rubbers, styrene-butadiene rubbers, and isobutylene-isoprene rubbers. Use may be made of an inorganic filler which is either powdery or fibrous. As its illustrative examples, may be mentioned calcium oxide, magnesium oxide, calcium hydroxide, magnesium hydroxide, aluminum hydroxide, magnesium carbonate, calcium silicate, magnesium silicate, calcium sulfate, magnesium sulfate, calcium carbonate, barium sulfate, calcium sulfite, potassium titanate, talc, clay, mica, glass powder, basic magnesium carbonate, dolomite, and wollastonite. Among them, calcium carbonate, barium sulfate and talc are particularly preferred. These various kinds of inorganic fillers may be employed subsequent to their surface treatments with various kinds of coupling agents, surfactants or the like. These surface treatments contribute to the improvements of the stiffness and impact resistance of the resulting resin composition. The average particle size of the inorganic filler may be 6 µm or smaller, or preferably 5 µm or smaller, and particles having particle sizes of 10 µm or greater amount to 5 wt.% or less. If the average particle size of the inorganic filler exceeds 6 µm, the resulting polypropylene-base resin composition is reduced in impact resistance. The term "average particle size" as used herein means a Nussenstein's Particle size as measured in accordance with the light transmission method and is a value determined as the particle size at 50% in the corresponding cumulative particle size distribution (which is generally called "D₅₀"). As exemplary carbon black materials useful in the practice of this invention, may be mentioned furnace black, thermal black, channel black, acetylene black and the like which are all routinely employed to color polypropylene-base resins in black. In view of color tone, heat resistance, weatherability and the like, those having an average particle size of 5 - 40 mµ and a specific surface area of 50 - 1100 m²/g are employed preferably. Among the above-exemplified carbon black materials, furnace black and channel black are preferred. Needless to say, two or more of these carbon black materials may be employed in combination. It is preferable to use carbon black the average particle size of which falls within the above-specified range. If the average particle size is too small, the particles undergo secondary coagulation and their dispersibility is hence lowered. Besides, such very fine carbon black is more costly. If the average particle size becomes excessively large, the color tone is rendered poorer and the weatherability is reduced. It is hence not preferred to use carbon black having an average particle size outside the above-described range. As the value of the average particle size of carbon black, there is used a value measured directly by an electron microscope or that calculated on the basis of a specific surface area, which is in turn determined depending on the amount of adsorbed nitrogen, iodine or the like, while supposing that the particles are spherical. If the sum of the thermoplastic elastomer and inorganic filler, which are incorporated in the propylene-base resin to make up the above-described polypropylene-base resin, were to exceed 100 parts by weight per 100 parts of the block copolymer, the balance of mechanical and physical properties such as stiffness and impact resistance and paintability of the resulting polypropylene-base resin composition would be reduced. Therefore the permitted amounts of thermoplastic elastomer and inorganic filler are defined so that the sum cannot exceed the above-mentioned limit. The above-mentioned carbon black is added in an amount of 0.01 - 0.6 parts by weight or preferably 0.05 - 0.5 parts by weight, both per 100 parts by weight of the block copolymer. Any amounts smaller than 0.01 parts by weight lead to reduced weatherability. Although a conventional heat-resistance/weatherability stabilizer may be employed to improve the weather-ability, such a stabilizer must be used in a considerably high proportion. Thus, the price of the resultant resin composition becomes very high. Moreover, the stabilizer comes up to the surface of a molded article subsequent to its molding, leading to such drawbacks that its paintability and external appearance are reduced. It is hence impractical to rely upon such a stabilizer. On the other hand, addition of the carbon black in any amount greater than 0.6 parts by weight is not preferred because the peeling strength of a paint film applied subsequent to a plasma treatment is reduced to a significant extent and blister is rendered more susceptible to occur in a warm water resistance test. One or more of antioxidants, thermal stabilizers, ultraviolet absorbents, flame retardants, nucleating agents, organic or inorganic pigments and/or other such materials which are usually added to polyolefins, may also be added to the block copolymer, so long as their contents are limited to such levels that advantageous effects of the present invention are not impaired substantially. The crystalline ethylene-propylene block copolymer a) may preferably contain 7 - 30 wt.% of ethylene and 65 wt.% or more of matter insoluble in boiling n-heptane in view of the impact resistance and modulus of elasticity in flexure of a molded article to be obtained. As the thermoplastic elastomer b), it is particularly preferred to use an ethylene-propylene rubber and/or an ethylene-propylene-diene rubber, each of which has a propylene content of 20 - 90 wt.% and a Mooney viscosity of 15 - 100 at 100°C. If the content of the thermoplastic elastomer is less than 7 parts by weight in the above polypropylene-base resin, a molded article, especially, a bumper or the like from a resin composition obtained by mixing carbon black with the polypropylene-base resin is reduced in low-temperature impact resistance when it is painted subsequent to its plasma treatment. On the other hand, any amount greater than 60 parts by weight renders the surface layer of a molded article, especially, a bumper or the like more susceptible to peeling, thereby reducing the peeling structure of its paint film and moreover, resulting in a reduction to the stiffness of the molded article. It is thus not preferable to incorporate the thermoplastic elastomer in any amounts outside the above-specified range. If the inorganic filler is incorporated in any amount smaller than 3 parts by weight, an article molded from the resin composition is reduced in stiffness and has poor utility, especially, as a bumper. If the content of the inorganic filler exceeds 30 parts by weight, the warm water resistance and moisture resistance of an applied paint film are reduced and blister is susceptible to occur between the resin layer and paint film and in addition, the low-temperature impact resistance of the molded article is also reduced, all, due to the water contained in the inorganic filler. It is hence not preferred to incorporate the inorganic filler in any amounts outside the above-specified range. In the present invention, the resin composition containing the block copolymer and carbon black may be mixed, e.g. by using a Henschel mixer or the like which is commonly employed in the present field of art. The resultant mixture may be kneaded by a single-screw extruder, a twin-screw extruder, rolls, or a Banbury mixer, followed by its pelletization. The thus-obtained resin composition may then be formed into a desired molded article by a usual molding process such as injection molding, extrusion molding or compression molding. The resultant molded article may be subjected to a surface treatment with plasma prior to its painting. This plasma surface treatment may be conducted by bringing plasma, which has been obtained by exciting oxygen, nitrogen or another inert gas, or a mixed gas thereof, e.g. with a high-frequency wave discharge or microwave discharge, into contact with the surface of the molded article. Although subject to the type of a gas to be employed, the following treatment conditions may generally be employed - pressure: 13-66 Nm⁻² (0.1-5 Torr); and treatment time: about 5-600 seconds. It may of course be necessary to increase the treatment time or to change the shape of the treatment tank in accordance with the shape of each molded article. Certain Examples and Comparative Examples will hereinafter be given to describe the present invention and its effects specifically. Examples 1-3 To 100 parts by weight of an ethylene-propylene block copolymer (PP-E) having an ethylene content of 8% by weight, the melt flow index of which was 3.8 g/10 min, were added 0.05 part by weight of 2,6-di-tert-butyl-4-methylphenol, 0.1 part by weight of calcium stearate, 0.2 part by weight of tetrakis-[methylene-3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate]methane, 0.4 part by weight of bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate and 0.2 part by weight of 2-(3-tert-butyl-5-methyl-2-hydroxyphenyl)-5-chlorobenzotriazole as well as furnace black having an average particle size of 24 µm and a specific surface area of 180 m²/g (Carbon Black E); EPR-A; and talc and/or calcium carbonate in an amount shown in Table 1. After mixing the thus-proportioned mixtures separately in a Henschel mixer, they were separately pelletized by an extruder having a barrel diameter of 40 mm and maintained at 230°C. The thus-obtained resin compositions were separately injection-molded into test pieces (80 x 160 x 2 mm plates). After subjecting each of these test pieces to a plasma treatment under conditions defined below, their paintability was evaluated. For this, a two-pack topcoating urethane paint was applied to a thickness of ca. 100 µm. The thus-painted test pieces were dried at 90°C for 40 minutes and then allowed to stand at room temperature for 48 hours, thereby obtaining painted test pieces. Cuts were then formed at a distance of 1.0 cm by a cutter in the paint film of each of the painted test pieces. The 180° peeling strength of each paint film was measured by meals of an Instron tensile testing machine. Furthermore, these painted test pieces were dipped in warm water of 40°C and the quality of paint films were visually observed 240 hours later to evaluate their paintability. Results are summarized in Table 1. Conditions for Plasma Treatment 1) Apparatus: Microwave plasma treatment apparatus (Model: TMZ-2026M; manufactured by Toshiba Corporation). 2) Atmosphere: Oxygen. 3) Time: 30 seconds. 4) Gas pressure: 132 Nm⁻² (1.0 Torr.) 5) Gas flow rate: 480 cc/min. 6) Microwave output power: 1500 W. Evaluation Standard of Paint Film Quality ○ : No visible blister in paint film. ▵: Some blister in paint film. X: Considerable blister in paint film. Results are shown in Table 1. Comparative Examples 1 - 5: Tests were conducted in the same manner as in Example 1 except that the amounts of Carbon Black E or F were changed as shown in Table 1 based on 100 parts by weight of their respective polypropylene-base resins. Results are shown in Table 1. PP-D is a propylene homopolymer whose melt flow index is 4.0 Carbon black F is channel black having an average particle size of 18 µm and a specific surface area of 210 m²/g. EPR-A is an ethylene-propylene rubber with a propylene content of 26 wt % and a Mooney viscosity of 24 at 100°C. Examples 4 - 6: To 100 parts by weight in total of 75 parts by weight of a crystalline ethylene-propylene block copolymer containing 12.5 wt.% of ethylene and 94.2 wt.% of matter insoluble in boiling n-heptane and having a melt flow index of 13.5 g/10 min (hereinafter called "PP-F") , 20 parts by weight of an ethylene-propylene rubber having a propylene content of 59 wt.% and a Mooney viscosity of 27 at 100°C (hereinafter called "EPR-C") and 5 parts by weight of talc having an average particle size of 1.1 µm, were added 0.05 part by weight of 2,6-di-tert-butyl-4-methylphenol, 0.1 part by weight of calcium stearate, 0.2 part by weight of tetrakis-[methylene-3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate]methane, 0.4 part by weight of bis(2,2,6,6-tetramethyl-4-piperidyl)-sebacate and 0.2 part by weight of 2-(3-tert-butyl-5-methyl-2-hydroxyphenyl)-5-chlorobenzotriazole as well as furnace black having an average particle size of 22 mµ and a specific surface area of 200 m²/g (Carbon Black G) or channel black having an average particle size of 16 mµ and a specific surface area of 220 m²/g (Carbon Black H) in an amount shown in Table 2. After mixing the thus-proportioned mixtures separately in a Henschel mixer, they were separately pelletized by an extruder having a barrel diameter of 40 mm and maintained at 230°C. The thus-obtained resin compositions were separately injection-molded to obtain test pieces (80 x 160 x 2 mm plates). After subjecting each of these test pieces to a plasma treatment in the same manner as in Example, 1, their paintability were evaluated. Results are shown in Table 2. Example 7: A test was conducted in the same manner as in Example 4 except that calcium carbonate having an average particle size of 1.9 µm was used in place of the talc. Results are shown in Table 2. Example 8: A test was conducted in the same manner as in Example 4 except that the same EPR-A as that employed in Example 1 was used in place of EPR-C. Results are shown in Table 2. Example 9: A test was conducted in the same manner as in Example 4 except that ethylene-propylene-diene rubber (hereinafter called "EPDM") having a propylene content of 27 wt.% and a Mooney viscosity of 88 at 100°C and containing ethylidenenorbornane as a third component was used in place of EPR-C in an amount shown in Table 2. Results are shown in Table 2. Examples 10 & 11: Tests were conducted in the same manner as in Example 4 except that PP-F, EPR-C and talc were used in amounts shown in Table 2. Results are shown in Table 2. Example 12: A test was conducted in the same manner as in Example 4 except that a crystalline ethylene-propylene block copolymer containing 14 wt.% of ethylene and 85 wt.% of matter insoluble in boiling n-heptane and having a melt flow index of 3.5 g/10 min (PP-G) was used in place of EPR-C in an amount shown in Table 2. Results are shown in Table 2. Comparative Examples 6 - 10: Tests were conducted in the same manner as in Examples 4, 5 and 7 to 9 except that Carbon Black A or B was added in amounts shown in Table 3. Results are shown in Table 3. Comparative Examples 11 to 13 (Comparative Tests as Bumpers): PP-F, EPR-C and talc were mixed in amounts shown in Table 4. The resultant mixtures were separately pelletized in the same manner as in Example 4. The thus-obtained pellet samples were separately molded by an injection-molding machine into prescribed test pieces. Physical properties of the test pieces were then measured. Since the balance of their physical properties, i.e., stiffness and impact resistance as resin compositions for bumpers were inferior compared with the test piece obtained in Example 36, their paintability were not evaluated. Measurement results of their physical properties are shown in Table 4, in comparison with the corrsponding data of the test piece obtained in Example 4. In Table 4, each melt flow index (MI), modulus of elasticity in flexure and Izod impact strength was measured in accordance with ASTM D-1238, ASTM D-790 and ASTM D-256, respectively.

Claims: A polypropylene-base resin composition suitable for use in the production of molded articles, comprising a) 100 parts by weight of a crystalline ethylene-propylene block copolymer; b) 7 - 60 parts by weight of a thermoplastic elastomer; c) 3 - 30 parts by weight of an inorganic filler; and d) 0.01 - 0.6 wt. % based on the total weight of the copolymer (a), elastomer (b) and filler (c) of carbon black. A resin composition as claimed in claim 1, wherein the carbon black has an average particle size of 5 - 40mµ and a specific surface area of 50 - 1,100 m²/g. A resin composition as claimed in claim 1 or 2 which is suitable for use in the production of bumpers. A resin composition as claimed in any preceding claim wherein the ethylene-propylene block copolymer contains 7 - 30 wt.% of ethylene and at least 65 wt.% of matter insoluble in boiling n-heptane. A resin composition as claimed in any preceding claim wherein the thermoplastic elastomer is an ethylene-propylene rubber, ethylene-propylene-diene rubber, styrene-butadiene rubber or isobutylene-isoprene rubber. A resin composition as claimed in any of claims 1-4 wherein the thermoplastic elastomer is an ethylene-propylene rubber or ethylene-propylene-diene rubber having a propylene content of 20 - 90 wt.% and a Mooney viscosity of 15 - 100 at 100°C. A resin composition as claimed in any preceding claim wherein the inorganic filler is calcium oxide, magnesium oxide, calcium hydroxide, magnesium hydroxide, aluminum hydroxide, magnesium carbonate, calcium silicate, magnesium silicate, calcium sulfate, magnesium sulfate, calcium carbonate, barium sulfate, calcium sulfite, potassium titanate, talc, clay, mica, glass powder, basic magnesium carbonate, dolomite or wollastonite, which has an average particle size of 6µm or smaller and contains 5 wt.% or less of particles having particle sizes of 10 µm or greater. A resin composition as claimed in claim 7 wherein the inorganic filler is calcium carbonate, barium sulfate or talc. A molded polypropylene-base resin article, characterised in that said article has been obtained by subjecting, to a plasma treatment, a first article molded from the resin composition claimed in any preceding claim and having the same shape as said first article; and in that it is paintable. A molded article according to claim 9 which is a bumper.

Inventors: ABE MASARU, KAWAI YOICHI, MAKI MASAMI, SEKIGUCHI KATSUMI, YOKOTE SACHIO, ABE, MASARU, KAWAI, YOICHI, MAKI, MASAMI, SEKIGUCHI, KATSUMI, YOKOTE, SACHIO
Applicants: MITSUI TOATSU CHEMICALS, MITSUI TOATSU CHEMICALS, INC.
Application year: 1986
Main classification: C08L23
IPC codes: C08K3, C08L23, C08L53, C08L9