METHOD FOR IN-MOLD POLYMERIZATION OF UNSATURATED COMPOUNDS

Abstract: An unsaturated compound having a vinyl or vinylidene group is rapidly in-mold polymerized by mixing the unsatura­ ted compound containing a polymerization initiator and the unsaturated compound containing a polymerisation accelera­ tor, pouring the mixture into a mold cavity and polymerizing the monomer in a period of time not longer than 30 minutes.

Description: The present invention relates to a method for in-mold polymerization of an unsaturated compound containing a vinyl or vinylidene group (hereinafter referred to as "vinyl type monomer"). More particularly, it relates to a method for in-mold polymerization of a vinyl type monomer comprising mixing the vinyl type monomer, optionally containing the polymer thereof, with a polymerization initiator and a polymerizing accelerator in a specific manner and rapidly polymerizing and molding it in a mold to efficiently produce a molded article. In general, a vinyl or vinylidene polymer (hereinafter referred to as "vinyl type polymer) is molded by a melt molding, for example, injection molding comprising melting a polymer in the form of powder, beads or pellets at a high temperature and injecting the molten polymer into a mold. For producing a sheet material of polymethyl methacrylate, a cell casting method is known which comprises pouring methyl methacrylate, optionally containing its polymer, into a cell made of glass plates and gradually polymerizing the methyl methacrylate over a long period of time, for example, several hours and a continuous casting method comprising casting an unsaturated monomer comprising methyl methacrylate containing its polymer on an endless metal belt and polymerizing it in a comparatively short period of time (cf. Japanese Patent Publication Nos. 30985/1977 and 35393/1977). When the vinyl type polymer is melt molded, for example, by injection molding, it should be molded in a narrow temperature range in which the polymer is molten but not thermally decomposed since the polymer tends to be thermally decomposed at the high temperature and pressure required for melt molding. Therefore, the melt molding of the vinyl type polymer requires a large amount of energy and a large scale complicated apparatus. In addition, even at a high temperature, the molten vinyl type polymer has very high viscosity so that great skill is required to transfer surface images of the mold to the molded article precisely. Cell casting is not generally employed except for molding the plate form article since the polymerization rate in this method cannot be increased, resulting in poor productivity. Although continuous casting has good productivity, it requires an expensive apparatus and can produce only a plate form article. In these conventional methods, all the components including the monomer and the polymerization initiator are mixed together and poured into a reaction cavity to polymerize the monomer. In such a case, if a polymerization initiator providing a high polymerization rate is used, the polymerization reaction does not proceed homogenously. Also, there is disclosed in US-A-2 744 886 the in-mold polymerization of acrylate esters with a diacyl peroxide polymerization initiator and a tertiary amine activator in the presence of thiophenol promoter. It would be desirable to provide a method for efficiently producing a precise molded article of the vinyl type polymer by using a comparatively small amount of energy and a comparatively simple apparatus. It would also be desirable to provide a method for rapid in-mold polymerization of the vinyl type monomer. The present invention provides a method for the production of a molded article by in-mold polymerization of an unsaturated compound containing a vinyl or vinylidene group which method comprises continuously mixing two monomer portions, one of the two monomer portions containing the unsaturated compound and a polymerization initiator and the other portion containing the unsaturated compound and a polymerization accelerator, said mixing being such as to provide a homogeneous mixture, continuously pouring the mixture into a mold cavity, and polymerizing the monomer over a period of time not longer than 15 minutes. Examples of the vinyl type monomer to be polymerized by the method of the invention are methacrylate or acrylate of a C₁-C₂₅ straight, alicyclic or aromatic alcohol or phenol (e.g., methyl methacrylate, ethyl methacrylate, butyl methacrylate, phenyl methacrylate, benzyl methacrylate, cyclohexyl methacrylate, bornyl methacrylate, isobornyl methacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, phenyl acrylate, benzyl acrylate, cyclohexyl acrylate, bornyl acrylate and isobornyl acrylate); vinylnitriles (e.g., acrylonitrile, methacrylonitrile and crotonitrile); vinyl esters (e.g., vinyl acetate, vinyl propionate and vinyl benzoate); methacrylic acid; and acrylic acid. Of these, the methacrylates are preferred because of their good reactivity. Although the monomer may be used alone, it can be used as a mixture with at least one other monomer to improve the characteristics of the polymer produced and/or increase the reactivity of the monomers. In addition, at least one other copolymerizable monomer such as styrene, α-methylstyrene, p-methylstyrene or maleic anhydride may be used together with the above monomer(s). Although the monomer as such can be polymerized, it is preferred to polymerize the monomer containing its polymer, a so-called "syrup", to improve the polymerization activity and ease of handling. The content of the polymer in the syrup is usually from 3 to 40 % by weight. When the polymer content in the syrup is larger than 40 % by weight, the syrup has too large a viscosity so that the handling of the syrup is made more difficult. The syrup may be prepared by a conventional method. For example, the polymerization of the monomer is initiated by the addition of a polymerization initiator and terminated when conversion reaches a certain level, or an adequate amount of the polymer is dissolved in the monomer. As the polymerization initiator, a radical polymerization initiator is preferably used, although other initiators may be used. Of these, the peroxide type radical initiators are preferred. In view of stability of the monomer containing the polymerization initiator and optionally the polymer, an initiator is preferred which has a half life of 10 hours at a temperature not lower than 50°C and preferably not lower than 60°C. Examples of the radical polymerization initiator are dialkyl peroxides (e.g., di-t-butyl peroxide, t-butyl cumyl peroxide and dicumyl peroxide); diacyl peroxide (e.g., lauroyl peroxide, benzoyl peroxide and succinic acid peroxide); peroxy esters (e.g., t-butyl peroxyacetate, t-butyl peroxybenzoate, t-butyl peroxypivalate and t-butyl peroxylaurate); hydroperoxides (e.g., t-butyl hydroperoxide and cumene hydroperoxide); and ketone peroxides (e.g., methyl ethyl ketone peroxide), and mixtures thereof. The amount of the initiator is from 0.05 to 5 parts by weight, preferable from 0.5 to 2 parts by weight per 100 parts by weight of the monomer optionally containing the polymer. When the amount of the initiator is less than 0.05 parts by weight, the polymerization does not proceed completely. When it is more than 5 parts by weight, control of the polymerization is difficult and the polymer produced has inferior weather and heat resistances. The polymerization accelerator, i.e. a compound which decomposes the polymerization initiator and accelerates the formation of radicals, is preferably a reductant. As the reductant, a compound may be used which is soluble in the vinyl type monomer, for example, mono-, di-, tri- or tetra-valent sulfuric compounds such as sulfinates (e.g., methyl p-toluenesulfinate and ethyl p-toluenesulfinates), thioureas (e.g., tetramethylthiourea and dibutylthiourea), mercaptans (e.g., laurylmercaptan, t-dodecylmercaptan, octylmercaptan, butylmercaptan, 2-ethylhexylmercaptan, 2-ethylhexyl thioglycolate, pentaerythritol tetrathioglycolate, glycol dimercaptoacetate, 2-mercaptoethanol and its esters, β-mercaptopropionic acid and its esters); tertiary amines (e.g., N,N-dimethylaniline, N,N-dimethyl-p-toluidine and N,N-di-(2-hydroxypropyl)-p-toluidine); or trivalent phosphorous compounds such as phosphites (e.g., trimethyl phosphite, triethyl phosphite and tri-n-butyl phosphite). The reductant is used in an amount of 0.05 to 5 parts by weight, preferably 0.5 to 2 parts by weight per 100 parts by weight of the monomer optionally containing the polymer. When the amount used is less than 0.05 parts by weight, the polymerization rate is not sufficiently increased. When it is more than 5 parts by weight, the polymer produced is coloured and/or its strength is decreased. As an auxiliary component for the polymerization accelerator, a heavy metal containing compound and a hydrogen halogenide salt of an amine or a quarternary ammonium halogenide may be used. These compounds further accelerate the polymerization reaction. The heavy metal containing compound, may be, for example, a compound of at least one metal which is iron, copper or cobalt which compound is soluble in the vinyl type monomer. Specific examples are naphthenates and acetylacetonates of these metals. The amount of the heavy metal containing compound to be added is 0.005 to 10 ppm, preferable 0.1 to 5 ppm in terms of the metal amount per weight of the vinyl type monomer. When the amount is less than 0.005 ppm, the polymerization is not effectively accelerated. When it is larger than 10 ppm, the polymerization initiator is inactivated so that the polymerization is inhibited. The hydrogen halogenide salt of an amine or the quarternary ammonium halogenide, may be, for example, hydrogen chloride salts or hydrogen bromide salts of an amine (e.g., n-amylamine, n-hexylamine, n-octylamine, n-decylamine, laurylamine, palmitylamine, dibutylamine, tributylamine, N,N-dimethylbenzylamine, N,N-dimethyl-p-toluidine, phenethyldibutylamine, N,N,N',N'-tetramethylhexamethylenediamine, N,N,N',N'-tetramethylpropyldiamine, N,N-diethylbenzylamine, N,N-dibutylbenzylamine and phenethyldiethylamine); and tetramethylammonium chloride, benzyltrimethylammonium chloride, benzyltriethylammonium chloride, benzyltributylammonium chloride, trioctylmethylammonium chloride and β-phenethyldibutylethoxycarbonylmethylammonium chloride. These compounds may be used in an amount of 0.005 to 1 parts by weight, preferably 0.01 to 0.5 parts by weight per 100 parts by weight of the vinyl monomer optionally containing the polymer. When the amount is less than 0.005 parts by weight or larger than 1 parts by weight, the polymerization is not effectively accelerated. These compounds may be used as such when they are soluble in the monomer, or in the form of a solution in a suitable solvent (e.g., isopropanol, dimethylphthalate, diethylene glycol and the like) when it is insoluble in the monomer. According to the present invention, the monomer containing the polymerization initiator and optionally the corresponding polymer and the monomer containing the polymerization accelerator are mixed to initiate the polymerization. To carry out and complete the polymerization of the vinyl type monomer rapidly, the initiator and other additives with high polymerization activities are used. Therefore, when the initiator and other additives are added individually to the same portion of the monomer, it is sometimes very difficult to mix them uniformly within a short time and to carry out the polymerization reaction homogeneously. According to the method of the present invention, the monomer and the additive components including the polymerization initiator are homogeneously mixed in a short time so that the reaction proceeds homogeneously. The auxiliary components for the polymerization accelerator may be added to either one of the two monomer portions. The mixing of the two monomer portions is preferably carried out at a temperature not higher than 50°C for 10 minutes or less. When the mixing temperature is higher than 50°C, the polymerization is unfavorably partially initiated and proceeds during mixing. When the mixing time is too long, the polymerization proceeds during mixing and pouring. As far as the monomer portions are thoroughly mixed, the shorter mixing time is the better. The monomer portions are continuously mixed and poured into the mold cavity, since the monomer and the additives are homogeneously mixed and a large article can be produced. In any case, a time from the mixing of the monomer to pouring it into the mold cavity should be as short as possible. The mixing can be carried out by a per se conventional method, for example, by means of a collision type mixer, a dynamic mixer or a static mixer. In comparison with the conventional molding method in which a molten resin is poured in the mold cavity, the method of the present invention requires a comparatively low injection pressure since the vinyl type monomer which is used as a molding material has lower viscosity than the molten resin. Therefore, the volume of the mold cavity can vary from a small one to a large one. The mold may be made of metal, resin, elastomer or wood. Among them, a metal mold is preferred due to its preciseness and durability. The temperature of the mold can be controlled by a heater embedded in the mold wall, air cooling, or a jacket or a conduit provided with in the mold wall through which a heat transfer medium such as water or oil is passed. The polymerization time in the method of the present invention varies with the kinds and amounts of the polymerization initiator and/or other additives, the mold temperature and is not longer than 15 minutes, preferably not longer than 10 minutes. The method of the present invention may be used to produce any molded article made of the vinyl type polymer. That is, by changing the shape and size of the mold cavity, any molded article can be produced by the method of the present invention. Particularly, the method of the present invention is suitable for producing optical parts since the molded article has significantly less internal strain. The optical parts include conventional lenses, Fresnel lenses, optical recording disc media, prisms and millers. Among them, the Fresnel lenses which is required to have a precise shape are preferably produced by the method of the present invention, since the precise image of the mold can be transferred to the produced polymer according to the present invention. Since a large size article also can be molded by the method of the present invention, automobile parts such as a windscreen, a sun roof, a side window and rear window, a sign board, a water tank panel and a bathtub are produced. According to the present invention, the vinyl type polymer can be molded at a much lower temperature and pressure than in conventional methods, and the surface image of the mold cavity can be precisely transferred to the molded article more easily than by conventional methods. In addition, the molded article of the vinyl type polymer can be produced in a shorter time than by a conventional cast polymerization method. EXAMPLES Preferred embodiments of the present invention will be illustrated by following examples, in which a haze value and Rockwell hardness are measured according to ASTM D 790 and ASTM D 785, respectively. Example 1 Preparation of a Raw Syrup To a stainless steel vessel, methyl methacrylate resin (SUMIPEX (trade mark) B MH manufactured by Sumitomo Chemical) (2 kg) and methyl methacrylate monomer (8 kg) were added and stirred for 24 hours to prepare a raw syrup, which was a viscous liquid having a viscosity of 0.1 Pas (one poise). Preparation of Syrup A In the raw syrup (1,000 g) charged in a 2 liter stainless steel vessel, an 8 % by weight solution of dibutylamine hydrogen chloride in diethyleneglycol (12.5 g) and t-butyl peroxybenzoate (10 g) were added and dissolved to prepare Syrup A. Preparation of Syrup B In the raw syrup (1,000 g) charged in a 2 liter stainless steel vessel, 2-mercaptoethanol (4 g) and a 1 % by weight solution of copper naphthenate (Cu content, 10 %) in methyl methacrylate (0.4 g) were mixed and dissolved to prepare Syrup B. When Syrups A and B were stored at a room temperature (25°C), their viscosities were not substantially changed and remained in a usable range after one day. Syrup A (300 g) and Syrup B (300 g) were pumped in a dynamic mixer and mixed at a room temperature (25°C) with an average mixing time of 30 seconds. Then, the mixture was poured into a mold (350 x 350 x 3 mm) kept at 55°C. After 6.8 minutes from the completion of pouring, the polymerization temperature became maximum. After 8 minutes, the molded polymer was removed from the mold to obtain a molded article, which had haze value of 1.0 % and Rockwell hardness of 97 (M scale) as well as good gloss. Example 2 Syrup A (300 g) and Syrup B (300 g) prepared in Example 1 were separately supplied from two tanks at the same rate to the mixer with an average mixing time of 2 minutes. The mixture was immediately poured into a mold (350 x 350 x 4 mm) kept at 85°C. After 2 minutes from the completion of pouring, the mold containing the mixture was cooled to obtain polymethyl methacrylate, which was homogeneous, transparent and had good surface hardness and gloss. Example 3 Preparation of a Raw Syrup To a reactor, methyl methacrylate (560 g), methyl acrylate (140 g) and azobisisobutylonitrile (0.07 g) were added and partially polymerized with gentle stirring at 80°C. When the viscosity of the reaction mixture reached about 0.1 Pas (one poise), the vessel was cooled with iced water to obtain a raw syrup having a viscosity of 0.2 Pas (2 poise). Preparation of Syrup A' In the raw syrup (300 g) charged in an egg plant type 500 ml glass flask, ethyleneglycol dimethacrylate (9 g) and benzoyl peroxide (12 g) were added and dissolved to obtain Syrup A'. Preparation of Syrup B' In the raw syrup (300 g) charged in an egg plant type 500 ml glass flask, ethyleneglycol dimethacrylate (9 g), 2-ethylhexyl thioglycolate (0.3 g) and N,N-dimethylaniline (6 g) were added and dissolved to obtain Syrup B'. Syrups A' and B' were charged and mixed in the mixer with an average mixing time of 30 seconds. Then, the mixture was poured into a mold (350 x 350 x 4 mm) kept at 80°C. After 4 minutes from the completion of the pouring, the polymerization temperature became maximum. After 8 minutes from the completion of the pouring, the polymer was removed from the mold to obtain a molded article, which had no bubble, good gloss, haze value of 1.2 % and Rockwell hardness of 97 (M scale). Comparative Example 1 The following components were thoroughly mixed: Raw syrup of Example 1 600 g t-Butyl peroxybenzoate 3 g Dibutylamine hydrogen chloride 0.3 g Diethyleneglycol 3 g 2-Mercaptoethanol 1.2 g Copper naphthenate (Cu²⁺, 10 %) 0.001 g The mixture was poured in a mold (350 x 350 x 3 mm) kept at 85°C. The polymer produced had unhomogeneous transparency and many bubbles therein. The residual mixture had poor storability so that it was polymerized at a room temperature (25°C) after 60 minutes with vigorous heat development.

Claims: A method for the production of a molded article by in-mold polymerization of an unsaturated compound containing a vinyl or vinylidene group which method comprises continuously mixing two monomer portions, one of the two monomer portions containing the unsaturated compound and a polymerization initiator and the other portion containing the unsaturated compound and a polymerization accelerator, said mixing being such as to provide a homogeneous mixture, continuously pouring the mixture into a mold cavity, and polymerizing the monomer over a period of time not longer than 15 minutes. A method as claimed in claim 1 wherein the period of time is not longer than 10 minutes. A method as claimed in claim 1 or claim 2 wherein the unsaturated compound comprises at least one methacrylate or acrylate. A method as claimed in claim 1, claim 2 or claim 3 wherein the mold is maintained at a temperature in the range of from 50 to 150 o C. A method as claimed in any one of the preceding claims wherein the polymerization initiator is a peroxide-type radical polymerization initiator which is used in an amount of 0.05 to 5 parts by weight per 100 parts of the monomer, and the polymerization accelerator is a reductant which is used in an amount of 0.05 to 2 parts by weight per 100 parts by weight of the monomer. A method as claimed in any one of the preceding claims wherein the unsaturated compound contains its polymer in an amount of from 3 to 40% by weight based on the weight of the syrup, i.e. the monomer containing the polymer. A method as claimed in any one of the preceding claims wherein the unsaturated compound containing the polymerization accelerator additionally contains a heavy metal compound. A method as claimed in any one of the preceding claims wherein the unsaturated compound containing the polymerization accelerator additionally contains a hydrogen halogenide salt of an amine or a quaternary ammonium halogenide. A method as claimed in any one of the preceding claims wherein the mold is a mold to produce an optical part, which is preferably a Fresnel lens. A method as claimed in any one of claims 1 to 8 wherein the mold is a mold to produce a large size article selected from an automobile part, a signboard, a water tank panel and a bath tub.

Inventors: KATO, YASUYUKI, MORITANI, MASAHIKO, YASUNORI- YUKIO, YUYAMA, MASAHIRO
Applicants: SUMITOMO CHEMICAL COMPANY, LIMITED
Application year: 1986
Main classification: C08F2
IPC codes: C08F2, B29K33, B29L11, C08F18, B29C39, B29C67, C08F20, C08G63, C08K5, C08L33, C08F4