Method of treating surface of molded article of thermoplastic elastomer composition

Abstract: A method for treating the surface of a molded article, which comprises a combination of the steps of: providing a molded article made of a thermoplastic elastomer composition obtained by a thermal treatment of a mixture in the presence of an organic peroxide, which mixture is comprised of (A) an oil-extended olefinic copolymer rubber of an olefinic copolymer rubber having a Mooney viscosity (ML₁+₄100°C) of 90-200 and a mineral oil type softening agent, (B) an olefinic plastic, and (C) a bismaleimide compound, the ratio by weight of (A) the oil-extended olefinic copolymer rubber to (B) the olefinic plastic, A/B, being 24/76 - 80/20, the content of (C) the bismaleimide compound being 0.1-3 parts by weight based on 100 parts by weight of the total amount of (A) the oil-extended olefinic copolymer rubber and (B) the olefinic plastic, contacting the surface of the molded article with a UV-rays absorbing liquid, and irradiating the surface of the molded article with UV-rays having main wave lengths of 300 nm, or less. According to the surface treatment of the present invention, the olefinic thermoplastic elastomer to which paints can hardly be applied hitherto shows good paintability and printability in addition to the inherent useful properties. Paint-coated molded articles obtained by the method of this invention find wide utility in the fields of various industries, especially in the field of automobile industry as exterior or interior parts.

Description: The present invention relates to methods for treating and painting the surface of a molded article made of a thermoplastic elastomer composition. More particularly, the present invention relates to methods for treating and painting the surface of a molded article made of a thermoplastic elastomer composition derived from a mixture of an oil-extended olefinic copolymer rubber, an olefinic plastic and a bismaleimide compound in a specific proportion. Thermoplastic elastomers are known materials which are steadily finding in recent years increased applications chiefly in the industrial field of motor vehicles and domestic electric appliances, utilizing their non-rigid soft property and such a characteristic that they are moldable by way of extrusion molding or injection molding utilizable for conventional thermoplastic resins. Among these thermoplastic elastomers, olefinic thermoplastic elastomers possess, in addition to the above mentioned beneficial properties, a low specific gravity, heat-resisting property and a good balance of other physical properties. Accordingly, they are especially useful in the field of auto parts with the tendency of rendering automobiles lightweight. In recent years, they begin to be utilized even as bumpers of automobiles where a large scale injection molding technique is demanded. In bumpers as exterior parts, a movement is seen to make them fashionable by applying a paint thereonto in combination with the body color. Also, printing of patterns is frequently applied also to domestic electric appliances to enhence their ornamental value. From the past, compositions comprising olefinic copolymer rubbers and olefinic plastic substances are known as the olefinic thermoplastic elastomers. As they are devoid of polarity, however, a surface treatment for enhancing adhesiveness or paintability of the elastomers is often difficult. In order to improve these properties, a single layer of a primer comprising, for example, a chlorinated polypropylene is generally interposed between the olefinic thermoplastic elasomer and an adhesive or a paint thereby enhancing compatibility between the paint and the elastomer. However, such primer itself is expensive and the step for forming a primer layer is added to the treatment, thus making the cost of the final product higher. Accordingly, an improvement is desired in this respect. In order to improve adhesiveness of the elastomers, for example, a surface treatment including modification of the surface by etching in a physical or chemical manner is discussed in the prior art and actually applied to shaped articles made of polypropylene. In this method, however, the effect of improvement is insufficient and some problems are raised in that the treatment is complicated more than the step for coating a primer and in that deformation of the shaped article may take place depending on the condition of etching. Further, a method for improving paintability is now being adopted wherein the surface of shaped articles is oxidatively polarized by plasma irradiation in place of coating a primer. However, a problem is also involved in this method; the step for plasma irradiation is batchwise and the treatment condition is not even depending on the shape of the articles. In EP-A-0285273 a method is disclosed for increasing the surface energy of a synthetic organic polymer, whereby the shaped body is more susceptible to be coated, printed, bonded or dyed, which comprises contacting the surface of the body with a UV-ray-absorbing organic liquid at a temperature and for a time sufficient to swell the surface, and irradiating the swollen surface with UV-rays. A similar method for treating the surface of a polyolefin product is disclosed in EP-A-0344756. In EP-A-0338860 there is disclosed a thermoplastic elastomer composition of excellent injection mouldability which comprises a mixture (F) comprising 100 parts by weight of a mixture (C) essentially consisting of 24-80 parts by weight of an oil-extended olefinic copolymer rubber (A) and 20-76 parts by weight of an olefinic plastic B; said oil-extended olefinic copolymer rubber (A) comprising 100 parts by weight of an olefinic copolymer rubber (a) having a Mooney viscosity (ML₁+₄ 100°C) of 90-200 and a propylene content of 10-60% by weight and 20-60 parts by weight of a mineral oil softener (b), and 0.1-3 parts by weight of a bismaleimide compound (D), said mixture (F) being dynamically heat treated in the presence of an organic peroxide (E). As a result of extensive research made by the present inventors to develop a new method for treating and painting the surface of a moulded article made of a thermoplastic elastomer composition of the kind taught in EP-A-0338860 but which overcomes drawbacks as seen in the prior art, it has now been found that the thermoplastic elastomer composition can be treated at the surface thereof by contacting with a vapor of 1,1,1-trichloroethane and then irradiating it with UV-rays of a specific wave length. The present invention has been accomplished on the basis of the above finding. Thus, in accordance with the present invention, there is provided a method for treating the surface of a moulded article made of a thermoplastic elastomer composition, which comprises the steps of: providing a shaped article made of a thermoplastic elastomer obtained by a thermal treatment of a mixture in the presence of an organic peroxide, which mixture is comprised of (A) an oil-extended olefinic copolymer rubber of an olefinic copolymer having a Mooney viscosity (ML₁+₄ 100°C) of 90-200 incorporated with a softening agent of a mineral oil series, (B) an olefinic plastic substance, and (C) a bismaleimide compound, the ratio by weight of (A) the oil-extended olefinic copolymer rubber to (B) the olefinic plastic substance, A/B, being 24/76 - 80/20, the content of (C) the bismaleimide compound being 0.1-3 parts by weight based on 100 parts by weight of the total amount of (A) the oil-extended olefinic copolymer rubber and (B) the olefinic plastic substance, contacting the surface of the moulded article with a vapor of 1,1,1-trichloroethane, irradiating the surface of the moulded article with UV-rays having main wave lengths of 300 nm or less, and coating the UV-rays irradiated surface with a polyurethane type paint. Below is a detailed description of the method of the present invention. (1) Starting material: Suitable olefinic copolymer rubbers (a) used for the production of the thermoplastic elastomer composition include, random amorphous elastic copolymers containing an olefin as a predominant component, such as ethylene-propylene copolymer rubber, ethylene-propylene-non-conjugated diene copolymer rubber, ethylene-butene-non-conjugated copolymer rubber and propylene-butadiene copolymer rubber. In the present invention, an olefinic copolymer rubber containing 10-60% by weight of propylene is especially preferable. Also especially preferable in the present invention is ethylene-propylene-diene copolymer rubber. Illustrative of the preferable non-conjugated dienes are, for example, dicyclopentadiene, 1,4-hexadiene, cyclooctadiene, ethylene-norbornene and ethylidene-norbornene. In particular, ethylidene-norbornene is most preferable. In such ethylene-propylene-ethylidene-norbornene copolymer rubber (referred to hereinafter simply as EPDM), the content of propylene preferably is 10-60% by weight, more preferably 20-40% by weight while the content of ethylidene-norbornene is preferably 1-30% by weight, more preferably 3-20% by weight. If the content of propylene is less than 10% by weight, the resultant copolymer rubbers will become poor in flexibility. On the other hand, if the content exceeds 60% by weight, the resultant copolymer rubbers will be deteriorated in mechanical properties. The olefinic copolymer rubber used in the present invention has a Mooney viscosity (ML1+4100°C) within the range of 90-200, preferably 120-170. If the Mooney viscosity is less than 90, the resultant composition will become poor in tensile properties which is necessary for impact-strength at a low temperature. On the other hand, if the Mooney viscosity exceeds 200, the resultant article obtained by injection molding will have gel-like lumps on its surface so that its tensile strength and impact-strength at a low temperature will be deteriorated. Generally in the case of ethylene-propylene-non-conjugated diene copolymer rubbers, the content of propylene is preferably 10-60% by weight. If the content of propylene is less than 10% by weight, the content of ethylene in the resultant copolymer rubber will correspondingly be larger so that it will show crystallinity and poorer heat-resistance as the result. On the other hand, if the content of propylene exceeds 60% by weight, the decomposition reaction of the composition by an organic peroxide will excessively proceed. The iodine value (the degree of unsaturation) of the olefinic copolymer rubber used in this invention is preferably less than 16, and within this range, a partially crosslinked composition well balanced in processability and rubbery properties can be obtained. The mineral oil type softening agent (b) used in this invention is a process oil comprised of petroleum fractions of high boiling points belonging to paraffin type, naphthene type and aromatic type. This softening agent is used for the purpose of facilitating the processing in thermal treatments of olefinic copolymer rubbers or olefinic plastics, assisting dispersion of carbon black or the like, enhancing processability of the resultant thermoplastic elastomer composition, and reducing hardness to increase flexibility and elasticity. The oil-extended olefinic copolymer rubber (A) in the present invention is manufactured by incorporating the above mentioned olefinic coppolymer rubber (a) with the mineral oil type softening agent (b). The content of the mineral oil type softening agent (b) preferably is 20-60 parts by weight based on 100 parts by weight of the olefinic copolymer rubber (a). If the softening agent of a mineral oil series (b) is less than 20 parts by weight, the resultant composition will become poor in processability on injection molding so that a flow mark will be generated in the resultant molded article or distortion of the article near the gate will be larger. On the other hand, if the content of the softening agent exceeds 60 parts by weight, the resultant molded article will be deteriorated in tensile strength and impact-strength at a low temperature and will extremely be damaged in its appearance as gel-like lumps are formed on the surface of article obtained by injection molding. The Mooney viscosity of ethylene-propylene-ethylidene-norbornene copolymer rubber preferred among the oil-extended olefinic copolymer rubbers is calculated by the following equation: log ( ML 1 ML 2 ) = 0.0066 (ΔPHR) wherein ML1 stands for the viscosity of EPDM, ML2 for the viscosity of the oil-extended EPDM and ΔPHR for the amount of extended oil per 100 parts by weight of EPDM. Illustrative of the olefinic plastic (B) which may be used in this invention are, for example, isotactic polypropylene, a copolymer of propylene and a small amount of other α-olefin such as propylene-ethylene copolymer, propylene-butene-1 copolymer, propylene-hexene-1 copolymer and propylene-4-methyl-1-pentene copolymer. It is preferred that the olefinic plastic is previously mixed with an organic peroxide and subjected to thermal decomposition to decrease the molecular weight of the substance thereby increasing its flow property. In case the olefinic plastic is subjected to thermal decomposition in the presence of an organic peroxide, an olefinic plastic having a melt index (M₀; measured according to JIS K7210, 230°C) of 1-10 g/10 minutes, preferably 2-6 g/10 minutes is used before the thermal decomposition and is modified by thermal decomposition conducted in the presence of an organic peroxide to have a melt index (M₁) of 20-50 g/10 minutes, preferably 30-40 g/10 minutes. The ratio of the melt indices before and after the thermal decomposition (M₁/M₀) is 2-50. If the degree of thermal decomposition (M₁/M₀) of the olefinic plastic with an organic peroxide is less than 2, the resultant composition will be inferior in its flow property, or undesirable phenomenona such as flow marks will be observed on the surface of the resultant molded article by injection molding. On the other hand, if the degree of thermal decomposition (M₁/M₀) exceeds 50, the resultant article will be inferior in tensile characteristics necessary for impact-strength at a low temperature. The value of M1/M0 is preferably 3-40, more preferably 5-20. Examples of the bismaleimide compound (C) used in this invention include N,N'-m-phenylene-bismaleimide and toluene-bis-maleimide. As N,N'-m-phenylene-bismaleimide are used, for example, commercially available HVA-2 (DuPont, U.S.A.) and Soxinol BM (Sumitomo Chem., Japan) Illustrative of the organic peroxide (D) used in the present invention are, for example, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexine-3, 1,3-bis(tert-butylperoxyisopropyl)benzene, 1,1-di(tert-butylperoxy)-3,5,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(peroxybenzoyl)hexine-3, and dicumyl peroxide. These compounds are used singly or may be used, if necessary, in mixture of at least two. (2) Proportion of the starting materials The ratio by weight of the oil-extended olefinic copolymer rubber (A) to the olefinic plastic (B) in this invention is within the range of 24/76 - 80/20. If the ratio is smaller than 24/76, the crosslinking reaction by the organic peroxide will become insufficient and the copolymer rubber will be devoid of the flexibility and low hardness which are required for the thermoplastic elastomer composition. On the other hand, if the ratio exceeds 80/20, the appearance of the resultant injection-molded article, which is regarded as important for exterior parts for automobiles will be damaged, and the article will be deteriorated in heat-resistance. The proportion of the bismaleimide compound (C) is in an amount of 0.1-3 parts by weight based on 100 parts by weight of the total amount (A+B) of the oil-extended olefinic copolymer rubber (A) and the olefinic plastic (B). If the proportion of the bismaleimide compound is less than 0.1 part by weight, the improvement in the physical properties such as tensile property will be insufficient. On the other hand, if the proportion exceeds 3 parts by weight, flow marks will be generated to deteriorate the appearance of the resultant molded article. The proportion of the organic peroxide is generally 0.01-1.5 parts by weight, preferably 0.01-1 part by weight based on 100 parts by weight of the total amount (A+B+C) of the oil-extended olefinic copolymer rubber (A), the olefinic plastic (B) and the bismaleimide compound (C). (3) Production of the thermoplastic elastomer composition: The production of the thermoplastic elastomer composition in the present invention is attained by mixing the oil-extended olefinic copolymer rubber (A), the olefinic plastic (B) and the bismaleimide compound (C) together and kneading the mixture under fusion. The kneading temperature in this case is within the range of 170-280°C. Utilizable as the kneading apparatus are known conventional ones including a mixing roll, a Bambury mixer, an extruder, a kneader and a continuous mixer. In the kneading under fusion, it is important to disperse the bismaleimide compound homogeneously into the mixture. It is desirable to conduct kneading of these ingredients in the atmosphere of an inert gas such as nitrogen or carbon dioxide. The resultant mixture is then subjected to a thermal treatment in the presence of an organic peroxide. This thermal treatment is carried out dynamically as in the above kneading by the aid of a conventional kneading apparatus. The temperature for the thermal treatment is within the range of 180-300°C, preferably 180-270°C. A thermoplastic elastomer composition possessing beneficial properties for molding materials is obtained by this thermal treatment. The thermoplastic elastomer composition used in this invention may be incorporated, if necessary, with a filler such as carbon black, a lubricant, a pigment, a stabilizer, a UV-absorbing agent and the like additives or modifiers. (4) The surface treatment and painting of molded articles made of the thermoplastic elastomer composition: In accordance with the method of this invention, the thermoplastic elastomer composition is molded in any desired shape, and the surface of the molded article is brought into contact with the vapor of 1,1,1-trichloroethane and then irradiated with UV-rays. By this surface treatment, the surface of the molded article is rendered active so that the surface is excellentlly adapted for painting, adhesion and printing. The molded article of the thermoplastic elastomer may be in any desired shape such as films, plates, fibers, etc. The surface of the molded article is then irradiated with UV-rays having wave lengths of 300 nm or less, in particular wave lengths of 254 nm and 185 nm as main active wave lengths. It is preferred to maintain the UV-rays at a higher strength. The time for contacting the surface of the molded article with the vapor of 1,1,1-trichloroethane is within the period from 5 seconds to 10 minutes, preferably within the period from 5 seconds to 5 minutes. If the contact time is shorter than 5 seconds, the improving effect on acceptability to painting, adhesion and printing on the surface of the molded article will be insufficient. On the other hand, if the contact time is longer than 10 minutes, it will bring about economical disadvantage and will give an adverse effect as deterioration takes place on the surface. The time for irradiation of UV-rays is within the period from 20 seconds to 10 minutes, preferably within the period from 30 seconds to 5 minutes. If the irradiation time is shorter than 20 seconds, the improving effect on acceptability to painting, adhesion and printing on the surface of the molded article will be insufficient. On the other hand, if the irradiation time is longer than 10 minutes, it will bring about economical disadvantage and will give an adverse effect as deterioration of the surface takes place. A paint is then applied according to a known conventional method onto the molded article thus subjected to the surface treatment. In this case, a polyurethane paint is used. In particular, polyurethane paints possess flexibility and so widely utilized for parts of automobiles and bicycles. Such polyurethane paints include paints having a polyurethane structure, such as acrylic polyurethane paints, polyester-polyurethane paints and modified polyurethane paints. Paint-coated molded articles of the thermoplastic elastomer composition can widely be utilized in the following typical fields: (1) exterior parts of automobile such as bumper, bumper corner and bumper air dam skirt, etc. (2) exterior parts with ornamental function of automobile, such as side mole, steering wheel, splashboard, etc. (3) electric appliances such as connector, cap plug, etc. The present invention will be illustrated in more detail by way of an Example and Comparative Example. (Testing Methods in Examples) (1) Method for preparing test piece: An elastomer composition sample was molded to prepare a flat panel of 100 mm in width, 400 mm in length and 3 mm in thickness by the aid of an injection molding machine (Neomat injection molding machine manufactured by Sumitomo Heavy Machinery Indus. Co.Ltd., Japan). This flat panel was used as sample in each test. (2) Method for measuring physical properties: (1) Initial adhesiveness of paint: Test pieces for measurement were molded by using a 13 oz screw in line type injection molding machine (manufactured by Sumitomo Heavy Machinery Co., Ltd., Japan) and coated. The coating of the sample was cut with a razor into 100 (10 in longitudinal direction and 10 in lateral direction) small squres having 2 mm in size. A cellophane tape of 24 mm in width (manufactured by Nichiban Co., Ltd., Japan) was applied onto the coating by pressing with thumb and then peeled off quickly whereby the number of cut squares remaining on the sample was counted to obtain the ratio (%) of the residual cut squares. The initial adhesiveness was evaluated by this ratio. Example 1 In a Bambury mixer were placed 48 parts by weight of an oil-extended rubber (referred to hereinafter simply as oil-ex EPDM-1) prepared by incorporating 100 parts by weight of an ethylene-propylene-ethylidene-norbornene copolymer rubber having a Mooney viscosity (ML₁+₄100°C) of 143, a propylene content of 25% by weight and an ethylidene-norbornene content of 5% by weight with 41 parts by weight of a paraffinic process oil, 52 parts by weight of polypropylene (referred to hereinafter simply as PP-1) having a melt index (JIS K-7210, 230°C, 2.16 kg) changed from 4g/10 minutes to 30g/10 minutes (M₁/M₀=7.5) by thermal decomposition and 0.4 parts by weight of N,N'-m-phenylene-bismaleimide (referred to hereinafter simply as BM). The mixture was kneaded under fusion at 180°C and for 8 minutes in nitrogen atmosphere. The kneaded mixture was then passed through rolls and a sheet cutter to form pellets (referred to hereinafter simply as CP-1). Next, CP-1 and 0.07 parts by weight of 1,3-bis(tert-butylperoxyisopropyl)benzene (referred to hereinafter simply as PO-1) were mixed together in a tumbler blender, and the pellets were extruded at 240°C by the aid of a 65 mm extruder in nitrogen atmosphere to obtain pellets of the thermoplastic elastomer composition. These pellets were molded by the above mentioned injection molding machine and the resultant test plate was contacted with a vapor of 1,1,1-trichloroethane (74°C) for 30 second. The test plate was placed in front of a UV-rays irradiating apparatus equipped with a low pressure mercury lamp (200W made of synthetic quartz) and irradiated for 1 minute and 30 seconds with UV-rays having main active wave lengths of 254 nm and 185 nm in the air while maintaining the distance between the light source and the test plate at about 15 cm whereby the surface treatment of the test plate was carried out. The surface of the test plate thus treated was then spray coated with an acrylic polyurethane paint R271 (manufactured by Nippon Bee Chemical Co., Ltd., Japan), baked for 30 minutes at 90°C and dried. The initial adhesiveness of the resultant paint-coated article was measured and found to be 100%. Comparative Example 1 A paint-coated article was manufactured in the same manner as described in Example 1 except that the contact of the test plate with 1,1,1-trichlorethane vapor was not carried out. The initial adhesiveness of the paint-coated article was measured and determined to be 0%. Thus, it is shown that according to the invention a paint-coated molded article with excellent adhesiveness to the paint can be obtained.

Claims: A method for treating and painting the surface of a molded article, which comprises the steps of: providing a molded article made of a thermoplastic elastomer composition obtained by a thermal treatment of a mixture in the presence of an organic peroxide, which mixture is comprised of (A) an oil-extended olefinic copolymer rubber comprising an olefinic copolymer rubber having a Mooney viscosity (ML₁+₄100°C) of 90-200 and a mineral oil type softening agent, (B) an olefinic plastic, and (C) a bismaleimide compound, the ratio by weight of (A) the oil-extended olefinic copolymer rubber to (B) the olefinic plastic, A/B, being 24/76 - 80/20, the content of (C) the bismaleimide compound being 0.1-3 parts by weight based on 100 parts by weight of the total amount of (A) the oil-extended olefinic copolymer rubber and (B) the olefinic plastic, contacting the surface of the molded article with a vapor of 1,1,1-trichloroethane, irradiating the surface of the molded article with UV-rays having main wave lengths of 300 nm or less, and cotaing the UV-rays irradiated surface with a polyurethane type point. A method according to claim 1, wherein the content of the mineral oil type softening agent in (A) the oil-extended olefinic copolymer rubber is 20-60 parts by weight based on 100 parts by weight of the olefinic copolymer rubber. A method according to claim 1 or claim 2, wherein the olefinic copolymer rubber is ethylene-propylene-non-conjugated-diene copolymer rubber. A method according to claim 3, wherein the content of propylene in the ethylene-propylene-non-conjugated-diene copolymer rubber is 10-60% by weight. A method according to any preceding claim, wherein (B) the olefinic plastic is previously thermally decomposed with an organic peroxide and has a melt index before the thermal decomposition (M0) of 1-10 g/10 minutes and a melt index after the thermal decomposition (M1) of 20-50 g/10 minutes, the ratio of M1/M0 being 2-50. A method according to any preceding claim, wherein (B) the olefinic plastic is isotactic polypropylene or propylene-olefin copolymer. A method according to any preceding claim, wherein (C) the bismaleimide compound is N,N'-m-phenylene-bismaleimide.

Inventors: KATOH KOHICHIROH, SHINONAGA HIDEO, SOGABE SATORU, SUZUKI KYOJI, KATOH, KOHICHIROH, SHINONAGA, HIDEO, SOGABE, SATORU, SUZUKI, KYOJI
Applicants: AGENCY IND SCIENCE TECHN, SUMITOMO CHEMICAL CO, DIRECTOR-GENERAL OF THE AGENCY OF INDUSTRIAL SCIENCE AND TECHNOLOGY, SUMITOMO CHEMICAL COMPANY, LIMITED
Application year: 1991
Main classification: C08J7
IPC codes: C08K5, C08J7, C08L23