HEAT STABLE THERMOPLASTIC RESIN COMPOSITION

Abstract: A thermoplastic resin composition having improved heat stability which comprises a thermoplastic resin which contains at least 5 % by weight of an α-alkyl-substituted aromatic copolymer comprising α-alkyl-substituted aromatic vinyl compound and at least one other comonomer copolyme­ rizable therewith, wherein the composition contains a metal-­ deactivating agent which has a molecular weight of not more than 2,000 and at least one group of the formula: or an organometallic colorant containing free metal in an amount of not more than 2,000 ppm, from which a molded arti­ cle having no or less silver streaks can be produced.

Description: The present invention relates to a heat stable thermoplastic resin composition. More particularly, it relates to a thermoplastic resin having improved heat stability which can be molded even at high temperatures to give a molded article having few or no silver streaks on the article. It is known that α-methylstyrene base copolymer with good heat resistance is produced by replacing a part or the whole of styrene in a styrene base resin such as a styrene-acrylonitrile copolymer with α-methylstyrene and that blend of the α-methylstyrene base copolymer with other resin such as an ABS graft polymer provides a resin composition which has improved heat resistance. The α-methylstyrene base copolymer and the blend of said copolymer with the ABS graft polymer are practically used. Such copolymer or blend may be colored by an inorganic or organic colorant. As an α-methylstyrene content in the copolymer or the blend increases, their heat resistance is improved but their processability is deteriorated. Therefore, the copolymer or the blend having the higher α-methylstyrene content should be processed at a higher temperature. But, at the higher processing temperature, silver streaks often appear on the surface of the molded article and do not disappear in subsequent fabrication steps such as coating. When the processing temperature is lowered to prevent the formation of silver streaks, processability is decreased and problems such as short molding are caused. JP-A-51 014 885 as quoted in Data Base CA 85(4):22323q or Patent Abstracts of Japan, Vol. 8, No. 145 (C-232) [1582], both describe resin compositions which contain compounds having at least one -CO-NH- group. These compounds are used as metal deactivators to prevent heavy metal-catalyzed thermal degradation of the polymer. It is known that, in general, the silver streaks appear more frequently when the resin contains a larger amount of volatile components. To remove the volatile components, the resin is intensively dried before molding. Since the presence of volatile components in the resin is not the sole cause for the silver streak and there may be some unknown causes, intensive drying of the resin does not necessarily prevent the formation of silver streaks. Addition of an antioxidant cannot prevent the formation of silver streaks either. In some cases, a colorant is added to the α-methylstyrene base copolymer and its resin composition before or during molding as in the case of other thermoplastic resin. Particularly, the addition of an organometallic colorant causes more frequently the silver streaks than other colorants. One object of the present invention is to provide a thermoplastic resin composition comprising an α-alkyl-substituted aromatic copolymer which has good heat stability. Another object of the present invention is to provide a thermoplastic resin composition comprising an α-alkyl-substituted aromatic copolymer which gives a molded article having few or no silver streak on its surface. According to one aspect of the present invention, there is provided a thermoplastic resin composition according to claim 1. Further embodiments can be found in the dependent claims. Generally, the α-alkyl-substituted aromatic vinyl compound contained in the α-alkyl-substituted aromatic copolymer used in the present invention is represented by the formula: wherein R₁ is C₁-C₃ alkyl and R₂ is hydrogen, halogen or C₁-C₃ alkyl. Specific example of the vinyl compound are α-methylstyrene, α-methyl-p-chlorostyrene, α-methyl-p-methylstyrene and the like. These vinyl compounds may be used individually or as a mixture of two or more of them. In view of heat resistance and a property balance of the copolymer, α-methylstyrene is preferred. As other compounds copolymerizable with the α-alkyl-substituted aromatic vinyl compound, exemplified are aromatic vinyl compounds except the α-alkyl-substituted aromatic vinyl compound, vinyl cyanide, unsaturated carboxylic acids and their alkyl esters or anhydrides, maleimide and its N-alkyl or N-aryl substituted derivatives and rubbery polymers. These compounds may be used individually or as a mixture of two or more of them. Specific examples of the aromatic vinyl compound except the α-alkyl-substituted one are styrene, p-methylstyrene, nuclear halogen-substituted styrene and mixtures thereof. Among them, styrene is preferred. Specific examples of the vinyl cyanide are acrylonitrile, methacrylonitrile, maleonitrile, fumaronitrile and mixtures thereof. Among them, acrylonitrile is preferred. Specific examples of the unsaturated carboxylic acid and its alkyl ester or anhydride are acrylic acid, methacrylic acid, fumaric acid, itaconic acid, maleic acid, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate and maleic anhydride. Among them, methyl methacrylate and maleic anhydride are preferred. Specific examples of maleimide and its N-alkyl or N-aryl substituted derivative are maleimide, methylmaleimide, ethylmaleimide, N-phenylmaleimide and O-chloro-N-phenylmaleimide. Among them, N-phenylmaleimide is preferred. Specific examples of the rubbery polymer are polybutadiene, butadiene-styrene copolymer, butadiene-acrylonitrile copolymer, ethylene-propylene copolymer, ethylene-propyrene-non-conjugated diene copolymer, polybutyl acrylate, ethylene-vinyl acetate copolymer and chlorinated polyethylene. The ratio of the α-alkyl-substituted aromatic vinyl compound and other comonomer in the α-alkyl-substituted aromatic copolymer is not limited. In view of the heat resistance and the property balance of the composition, a preferred content of the α-alkyl-substituted aromatic vinyl compound is from 5 to 95 % by weight and that of other compounds is from 95 to 5 % by weight. Among the other compounds copolymerizable with the α-alkyl-substituted aromatic vinyl compound, styrene, acrylonitrile, methyl methacrylate, maleic anhydride, methacrylic acid, N-phenylmaleimide and polybutadiene are preferred. Specific examples of the α-alkyl-substituted aromatic copolymer are α-methylstyrene-acrylonitrile copolymer, α-methylstyrene-methyl methacrylate-(acrylonitrile or styrene) copolymer, α-methylstyrene-maleic anhydride-(acrylonitrile or styrene) copolymer, α-methylstyrene-N-phenylmaleimide-(acrylonitrile or styrene) copolymer, α-methylstyrene-polybutadiene-(acrylonitrile or styrene) copolymer and the like. There is no limitation on the structure of the α-alkyl-substituted aromatic copolymer. In view of the formation of silver streaks, heat resistance and processability of the composition, the copolymer preferably has an intrinsic viscosity of 0.3 to 1.0 (at 30°C in dimethylformamide). The α-alkyl-substituted aromatic copolymer may be prepared by any of conventional polymerization methods suitable for polymerization of the α-alkyl-substituted aromatic vinyl compound, for example, emulsion polymerization, suspension polymerization, solution polymerization, bulk polymerization and combinations thereof. Examples of other polymer which may be contained in the thermoplastic resin composition of the present invention are a polymer comprising at least one monomer of the above described monomers such as the aromatic vinyl compounds except the α-alkyl-substituted aromatic vinyl compound, vinyl cyanide, the unsaturated carboxylic acids and their alkyl esters or anhydrides and maleimide and its N-alkyl or N-aryl substituted derivatives; and conventional resins such as polycarbonate, polyphenyleneoxide, polysulfone, polyester, polyamide, polyacetal and the like. These other polymers may be used individually or as a mixture of two or more polymers. In view of the property balance of the resin composition, preferred other polymers are a copolymer of the aromatic vinyl compound with vinyl cyanide and/or the alkyl ester of unsaturated carboxylic acid, a graft polymer of the rubbery polymer with an aromatic vinyl compound and at least one of vinyl cyanide and the alkyl ester of unsaturated carboxylic acid, and a mixture of the former copolymer and the latter graft polymer. The amount of α-alkyl-substituted aromatic copolymer is preferably at least 5 % by weight, more preferably from 5 to 95 % by weight based on the whole weight of the thermoplastic resin. According to the present invention, the content of active free metal in the resin composition is reduced by the addition of the metal-deactivating agent selected from: wherein R₁ and R₂ are the same or different and hydrogen, C₁-C₂₀ alkyl, hydroxyl alkyl, aryl-substituted alkyl, hydroxyaryl-substituted alkyl, aryl, hydroxyaryl, alkyl-substituted aryl or alkyl-substituted hydroxyaryl; R₃ and R₄ are the same or different and C₁-C₂₀ alkylene, hydroxyalkylene, aryl-substituted alkylene or hydroxyaryl-substituted alkylene; and k is a number of 0 to 20. Specific examples of the metal-deactivating agent are the following compounds: When the metal-deactivating agent has a molecular weight larger than 2,000, its dispersibility in the resin is poor, so that the formation of silver streak cannot be effectively suppressed. The metal-deactivating agent of the present invention is used in an amount of 0.001 to 5.0 parts by weight, preferably 0.05 to 1.0 parts by weight per 100 parts of the thermoplastic resin. When the amount of the metal-deactivating agent is less than 0.001 parts by weight, the formation of silver streak at high molding temperature is not prevented, and when it is more than 5.0 parts by weight, the final resin composition has decreased heat resistance and stiffness although the formation of silver streak is prevented. The thermoplastic resin composition of the present invention may be prepared by any of conventional mixing or kneading methods, for example, by using a Banbury mixer, a single or twin screw extruder, and the like. In addition to the above essential components, the thermoplastic resin composition of the present invention may contain any conventional additive which is added to the thermoplastic resin. Examples of such additives are an inorganic colorant, an organic colorant, an inorganic filler, an antioxidant, a plasticizer, an antistatic agent, an ultraviolet light absorber, a flame retardant, a blowing agent, a lubricant and the like. The present invention will be illustrated by following Examples. In Examples, "parts" and "%" are by weight unless otherwise indicated. Example 1 and Comparative Example 1 Preparation of α-alkyl-substituted aromatic copolymer In a reactor an interior space of which had been replaced with nitrogen, 30 % of a monomer mixture (I) shown in Table 1 was added and reacted at 70°C for 1 hour and then the rest of the monomer mixture (I) was continuously added over 3 hours to continue polymerization. After exothermic heat due to polymerization was not generated any more, a monomer mixture (II) was continuously added over 1 hour at 70°C. After the completion of polymerization, the reaction mixture was salted out, and the product was dried to obtain an α-alkyl-substituted aromatic copolymer (a-1 to a-3). Copolymer a-1 a-2 a-3 Monomer mixture (I) α-Methylstyrene 70 80 30 Acrylonitrile 5 8 3 t-Dodecylmercaptan 0.05 0.05 0.05 Sodium laryl sulfate 2 2 2 Potassium persulfate 0.5 0.5 0.5 Deionized water 120 120 120 Monomer mixture (II) α-Methylstyrene 0 0 10 Styrene 0 0 10 Acrylonitrile 25 12 27 Methyl methacrylate 0 0 10 N-phenylmaleimide 0 0 10 Intrinsic viscosity 0.60 0.48 0.55 Preparation of ABS graft polymer In a reactor an interior space of which had been replaced with nitrogen, a composition shown in Table 2 was polymerized at 65°C for 6 hours. The resulting polymer latex was salted out with potassium chloride and dried. Component Parts Polybutadiene latex 40 (solid components) Styrene 40 Acrylonitrile 20 t-Dodecylmercaptan 0.3 Potassium persulfate 0.3 Disproportionated potassium rosinate 2.0 Water Up to 150 AS copolymer Styrene-acrylonitrile copolymer "Sebian® N-JD" (acrylonitrile content, 27 %; intrinsic viscosity, 0.62 in dimethylformaide at 30°C). Polycarbonate Polycarbonate "Panlite® l-1250" (poly-4,4'-dioxydiphenyl-2,2-propane carbonate). Metal-deactivating agent Colorant Phthalocyanine Blue "Sumitone® Cyanine Blue GH", which contains 2,850 ppm of free metal. Antioxidant (a) n-Octadecyl-3-(4'-hydroxy-3',5'-di-tert-butylphenyl) propionate "Irganox® 1076". (b) 1,3,5-Trimethyl-2,4,6-tris-(3.5-d-tert-butyl-4-hydroxybenzyl)benzene "Irganox® 1330". (c) Phenyl-β-naphthylamine The α-alkyl-substituted aromatic copolymer (a-1 to a-3), the ABS graft polymer, the AS copolymer, polycarbonate, the metal-deactivating agent (B-1 to B-12), the colorant and the antioxidant (1 to 3) were mixed by the Banbury mixer and then kneaded and pelletized by an extruder. The pellets were dried at 110°C for 4 hours and molded under the below described conditions to prepare six sample plates (from second to seventh shots) (each 150 mm x 70 mm x 3 mm). The formation of silver streaks was visually observed and evaluated according to the following criteria: O: No silver streak X: Silver streaks formed XX: A large number of silver streaks formed Molding conditions A 0.059 kg (3.5 ounce) injection molding machine FS-75 (Nissei Jushi). Resin temperature: 280°C (preset temp.: 270°C) 290°C (preset temp.: 280°C) Detention time: 3 or 10 minutes Back pressure: 0.49 MPa (5 kg/cm²) Injection rate: 8/10 Screw rotation rate: 100 rpm Mold temperature: 75°C Molding cycle: 8 sec. (injection)/ 15 sec. (cooling)/ 5 sec. (removing) The composition of each thermoplastic resin composition and evaluation results of silver streaks are shown in Table 3. A sample plate made from each of two thermoplastic resin compositions one of which contained no metal-deactivating agent and the other of which contained the metal-deactivating agent according to the present invention was examined on heat deflection temperature (0.64 cm (¼"), 1.82 MPa (264 psi), no annealing), processability (230°C, 5.88 MPa (60 kg/cm²)) and notched Izod impact strength (0.64 cm (¼"), 23°C). It was confirmed that there was no material difference between the two compositions.

Claims: A thermoplastic resin composition which comprises a thermoplastic resin which contains at least 5 % by weight of an α-alkyl-substituted aromatic copolymer comprising repeating units derived from an α-alkyl-substituted aromatic vinyl compound and at least one other comonomer copolymerizable therewith, wherein the composition contains from 0.001 to 5.0 parts by weight per 100 parts by weight of the thermoplastic resin of a metal-deactivating agent which has a molecular weight of not more than 2,000 selected from the group consisting of the compounds of the formulae: wherein R₁ and R₂ are the same or different and hydrogen, C₁-C₂₀ alkyl, hydroxyl alkyl, aryl-substituted alkyl, hydroxyaryl-substituted alkyl, aryl, hydroxyaryl, alkyl-substituted aryl or alkyl-substituted hydroxyaryl; R₃ and R₄ are the same or different and C₁-C₂₀ alkylene, hydroxyalkylene, aryl-substituted alkylene or hydroxyaryl-substituted alkylene; and k is a number of 0 to 20. The thermoplastic resin composition according to claim 1, wherein the amount of the α-alkyl-substituted aromatic vinyl compound in the α-alkyl-substituted aromatic copolymer is 5 to 95 % by weight. The thermoplastic resin composition according to claim 1, wherein the thermoplastic resin comprises the α-alkyl-substituted aromatic copolymer and at least one other thermoplastic polymer selected from the group consisting of a copolymer of an aromatic vinyl compound with vinyl cyanide and/or an alkyl ester of unsaturated carboxylic acid and a graft polymer of a rubbery polymer with an aromatic vinyl compound and at least one of vinyl cyanide and the alkyl ester of unsaturated carboxylic acid.

Inventors: HASHIMOTO, KENJI, MINEMATSU, HIROYUKI, TAKAHASHI, SYUZI
Applicants: SUMITOMO DOW LIMITED
Application year: 1987
Main classification: C08K5
IPC codes: C08K5, C09B67