Process for preparing styrenic resin

Abstract: A process for preparing a styrenic resin is herein disclosed which comprises the step of volatilizing·extruding a styrenic polymer (SPS) powder having a syndiotactic configuration in the presence of water, an alcohol or an antioxidant by the use of an extruder to obtain pellets. According to this process, the operation of volatilization·extrusion is done in the presence of water, the alcohol or the antioxidant to stably and efficiently prepare the SPS pellets in which yellowing is inhibited, and the operation of the volatilization·extrusion is carried out in the presence of water to stably and efficiently prepare the SPS pellets containing a less amount of a remaining volatile component such as an unreacted monomer.

Description: Field of the Invention The present invention relates to a process for preparing a styrenic resin, and more specifically, it relates to a method for effectively inhibiting the yellowing of pellets of a styrenic polymer powder having a syndiotactic configuration in the case that the styrenic polymer powder is volatilized and extruded by the use of an extruder to prepare the pellets. Background Art A styrenic polymer (hereinafter abbreviated to "SPS" sometimes) having a syndiotactic configuration obtained by improving a styrenic polymer having an atactic configuration is excellent in heat resistance and chemical resistance. The group of the present inventors has already succeeded in the development of a styrenic polymer having a high syndiotacticity, and this group has further disclosed that the SPS can be obtained by the use of a catalyst comprising a titanium compound, a catalytic product (an alkylaluminoxane) of an organic aluminum compound and a condensing agent, or a coordinated complex compound comprising a cation and an anion in which a plurality of groups are bonded to a metal (Japanese Patent Application Laid-open Nos. 187708/1987 and 249503/1992). The above-mentioned styrenic polymer is prepared by a non-solvent polymerization method or a slurry polymerization method, but in the case that the above-mentioned styrenic polymer is prepared by such a method, it is advantageous from the viewpoints of catalytic activity and productivity that styrene is highly concentrated and then polymerized. However, this preparation is carried out by the use of a catalyst containing a metal such as Al, Ti or Fe, and therefore a large amount of a metallic component remains in the obtained polymer, depending upon the kind of selected catalyst. This remaining metallic component causes the deterioration of the molecular weight and color tone of the polymer. Accordingly, it has been desired to decrease the remaining catalytic component in the obtained polymer product by a deashing treatment (the removal of the remaining catalyst). As such deashing treatment method, there is a method which comprises adding a deashing agent, a method which comprises swelling the polymer with a swelling agent, and a method which comprises raising a treatment temperature in each of these methods to improve the deashing efficiency. However, these methods have some problems. For example, the deashing efficiency is poor, washing is required, and a stable operation is difficult to perform. That is to say, in the method which comprises adding the deashing agent for the treatment, a step for removing the deashing agent is essential, and for the sake of the removal, a large amount of a solvent must be used, which makes the preparation process intricate and increases the cost of facilities. In the method which comprises adding the swelling agent for the treatment, deactivation and deashing can be accomplished under relatively mild conditions, but the polymer may dissolve or cohere, depending upon the composition and conversion of the polymer, the composition of the treatment solution and the treatment temperature. Owing to this fact, a stable operation cannot be accomplished. In addition, since the polymer swells, waste liquid containing the remaining catalyst solution gets into the polymer, and thus the amount of the metal cannot be decreased to a certain level or less. Therefore, it has been desired to develop a method for stably and easily preparing the SPS which can effectively inhibit yellowing and which has good characteristics. On the other hand, in the case that the above-mentioned styrenic polymer can be prepared by a non-solvent polymerization method or a slurry polymerization method, about 0.1 to 50% by weight of a remaining volatile component such as an unreacted monomer is contained in the obtained polymer. Therefore, a drying volatilization treatment has been carried out as a post-treatment subsequent to the polymerization by a drying machine to remove a remaining volatile component such as the monomer therefrom. However, in the case that the removal of the remaining volatile component is made by the use of the drying machine; the residence time is prolonged, which is not economical. In addition, a large amount of the unreacted monomer and the like easily remains in the product, and in this case, molding failure occurs, and the transparency of films or the like deteriorates, so that such molded articles are not desirable any more as wrapping materials for foods. Therefore, it has been desired to develop a method for economically and efficiently removing a remaining volatile component such as an unreacted monomer from the SPS prepared by the non-solvent polymerization method or the slurry polymerization method. EP-A-0 380 968 deals with the problem of the insufficient molding with syndiotactic styrene-based polymers and describes that the problem can be overcome by providing a purified styrene-based polymer having a melt viscosity of between 10 3 and 10 6 measured at 300 °C and a shearing rate of 10/sec. Molding with these polymers is performed in the presence of an antioxidant. Disclosure of the Invention The present invention has been developed under such circumstances. That is to say, it is an object of the present invention to provide a process for stably and efficiently preparing, by a simple means, SPS pellets in which yellowing can effectively be inhibited and which can supply a molded article having a high quality. The present inventors have intensively researched with the intention of achieving the above-mentioned object. As a result, it has been found that if a styrenic polymer powder having a syndiotactic configuration is volatilized and extruded in the presence of an antioxidant by the use of an extruder to prepare pellets, SPS pellets in which yellowing is effectively inhibited can stably and efficiently be obtained. The present invention has been completed on the basis of the found knowledge. That is to say, the present invention is directed to a process for preparing a styrenic resin which comprises the step of extruding a styrenic polymer powder having a syndiotactic configuration in the presence of an antioxidant by the use of an extruder, such as disclosed in the attached claims 1 and 4. Best Mode for Carrying out the Invention The present invention is concerned with a process for preparing SPS pellets in which yellowing is inhibited. A styrenic polymer which is applicable in the preparation processes regarding the present invention has a syndiotactic configuration. Here, the syndiotactic configuration of the styrenic polymer means that its stereochemical structure has a syndiotactic structure, i.e., a steric structure in which phenyl groups and substituted phenyl groups which are side chains are located alternately in opposite directions on a main chain comprising carbon-carbon bonds. Its tacticity can be quantitatively determined by a nuclear magnetic resonance method using an isotopic carbon (a 13 C-NMR method). The tacticity which can be determined by the 13 C-NMR method can be called as follows in accordance with the number of existing plural continuous constitutional units. For example, in the case that the continuous units are two, the tacticity is called a diad; in the case that the continuous units are three, it is called a triad; and in the case that the continuous units are five, it is called a pentad. "The styrenic polymer having the syndiotactic configuration" referred to in the present invention means the styrenic polymer in which the content of the syndiotacticity is usually 75% or more, preferably 85% or more in terms of a racemic diad, or it is usually 30% or more, preferably 50% or more in terms of a racemic pentad. Examples of "the styrenic polymer" include polymers obtained by polymerizing aromatic vinyl compounds as monomers, for example, styrene; alkylstyrenes such as p-methylstyrene, o-methylstyrene, m-methylstyreno, 2,4-dimethylstyrene, 2,5-dimethylstyrene, 3,4-dimethylstyrene, 3,5-dimethylstyrene and p-t-butylstyrene; halogenated styrenes such as p-chlorostyrene, o-chlorostyrene, m-chlorostyrene, p-bromostyrene, o-bromostyrene, m-bromostyrene, p-fluorostyrene, o-fluorostyrene, m-fluorostyrene and o-methyl-p-fluorostyrene; alkenylstyrenes such as divinylbenzene, p-isopropenylstyrene, 4-vinylbiphenyl, 3-vinylbiphenyl and 2-vinylbiphenyl; alkoxystyrenes such as methoxystyrene and ethoxystyrene; and aminostyrenes such as dimethylaminostyrene and diphenylaminostyrene, copolymers obtained by copolymerizing two or more kinds of these aromatic vinyl compounds as comonomers, mixtures of these polymers and copolymers, and these substances to which various kinds of additives such as pigments are added. Furthermore, no particular restriction is put on the molecular weight of this styrenic polymer, but its weight-average molecular weight is preferably 10000 to 3000000, more preferably 50000 to 1500000. In addition, the width of a molecular weight distribution is not limited, either, and styrenic polymers having various molecular weight distributions are applicable. The melting point of the styrenic polymer having the syndiotactic configuration is in the range of 160 to 310°C, and hence, the heat resistance of this kind of styrenic polymer is much better, as compared with a conventional styrenic polymer having an atactic configuration. No particular restriction is put on the preparation method of the styrenic polymer having the syndiotactic configuration, but this styrenic polymer can be prepared, for example, by polymerizing one or copolymerizing two or more of styrenic monomers (which correspond to the above-mentioned styrenic polymers) in the presence of a known catalyst comprising (a) a transition metal compound, (b) a coordinated complex compound comprising a cation and an anion in which a plurality of groups are bonded to a metal, or an aluminoxane, and if necessary, (c) an alkyl group-containing metallic compound. In the present invention, the styrenic polymer having the syndiotactic configuration is extruded by an extruder while volatilized, thereby forming pellets. The operation of volatilization·extrusion is required to be carried out by the extruder in the presence of In the process of the present invention, the operation of the volatilization·extrusion is carried out in the presence of the antioxidant to obtain SPS pellets which are scarcely yellowed. In general, the value of YI can be controlled to 30 or less, preferably 20 or less. In this case, the amount of the antioxidant is usually in the range of 0.001 to 5% by weight, preferably 0.01 to 2% by weight, more preferably 0.1 to 0.4% by weight based on the weight of the styrenic polymer powder having the syndiotactic configuration. This antioxidant may directly be fed to the extruder, or may be added to the styrenic polymer powder. Alternatively, the antioxidant may be fed to the molten and thus plasticized styrenic polymer powder. Furthermore, when the extrusion is carried out by the use of the extruder having a plurality of vents, the feed of the antioxidant may be done after the volatilization has been made through the optional vent on the upstream side of the final vent. In the case that the extrusion is done by the use of a plurality of the extruders connected in series, the feed of the antioxidant may be done after the volatilization has been made through the optional vent on the upstream side of the final vent of the extruder positioned on the most downstream side. The antioxidant which is in the state of a powder may be added without any treatment, or it may be dissolved in an aromatic hydrocarbon solvent and then added. The antioxidant which can be used in the present invention can be selected from the group consisting of phenolic antioxidants and phosphoric antioxidants. The employment of the plural antioxidants is also acceptable. No particular restriction is put on the phenolic antioxidants, and typical examples of these phenolic antioxidants include 2,6-di-t-butyl-4-methylphenol, 2,6-di-t-butyl-4-ethylphenol, n-octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate, pentaerythrityl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate], 2-t-butyl-6-(3-t-butyl-2-hydroxy-5-methylbenzyl)-4-methylphenyl acrylate, 2-[1-(2-hydroxy-3,5-di-t-pentylphenyl)ethyl]-4,6-di-t-pentylphenyl acrylate, triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl) propionate], 1,6-hexanediol-bis-[3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate], 3,9-bis-[1,1-di-methyl-2-[β-(3-t-butyl-4-hydroxy-5-mathylphenyl)-propionyloxy] ethyl]-2,4,8,10-tetraoxaspiro[5,5)undecane, 1,3,5-trimethyl-2,4,6-tris (3,5-di-t-butyl-4-hydroxybenzyl)-benzene, tris(3,5-di-t-butyl-4-hydroxybenzyl) isocyanurate, tris(4-t-butyl-2,6-di-methyl-3-hydroxybenzyl) isocyanurate and 2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine. Above all, pentaerythrityl-tetra-kis[3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate] and 2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine are preferable. No particular restriction is put on the phosphoric antioxidants, and typical examples of these phosphoric antioxidants include tetrakis(2,4-di-t-butylphenyl)-4,4'-biphenylene phosphonite, tris(2,4-di-t-butylphenyl) phosphite, 2,2'-methylenebis(4,6-di-t-butylphenyl)octyl phosphite, bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite, bis(2,6-di-t-butyl-4-methylphenyl)pentaerythritol diphosphite, di(nonylphenyl)pentaerythritol diphosphite, phenyl-bisphenol A-pentaerythritol diphosphite, distearylpentaerythritol diphosphite, dioctylpentaerythritol diphosphite, dilaurylpentaerythritol diphosphite, diphenylpenta-erythritol diphosphite, dicyclohexylpentaerythritol diphosphite, bis (2,4,5-tri-t-butylphenyl)pentaerythritol diphosphite, 2,2'-ethylidenebis(4,6-di-t-butylphenyl) fluorophosphite, trisindecyl phosphite, trisdodecyl phosphite, phenylisooctyl phosphite, phenylundecyl phosphite, phenyldodecyl phosphite, diphenylisooctyl phosphite, diphenylisodecyl phosphite, diphenyldodecyl phosphite, triphenyl phosphite, tris(monononylphenyl) phosphite and tris(dinonylphenyl) phosphite. Above all, bis(2,6-di-t-butyl-4-methylphenyl)-pentaerythritol diphosphite is preferable. In the process of the present invention, as the extruder which can be used for this volatilization·extrusion, a vent type extruder having one vent or plural vents can be used, but this type extruder may optionally serially be combined with one or more of a single screw extruder, an intermeshing type one-directional rotation twin-screw extruder, an intermeshing type extruder, a none-intermeshing type opposite-directional rotation twin-screw extruder and the like. If necessary, a gear pump may be installed on the downstream side of the extruder. The internal pressure of each vent is in the range of 0 to atmospheric pressure, preferably 0 to 26.6 kpa ( 200 Torr), more preferably 0 to 6.66 kpa (50 Torr). If the internal pressure of the vent is too high, it is difficult to accomplish the sufficient volatilization. The extrusion is preferably carried out at a molding temperature in the range of from the melting point of the polymer to 400°C at an extrusion rate meeting the equation Q/(D×H×V)=1.5×10 4 to 6.0×10 5 (wherein Q is an extrusion rate (kg/hr); D is a screw diameter (m); H is a screw groove depth (m); and V is a screw peripheral speed (m/sec)). In the case that a plurality of the serially connected extruders is used, the above-mentioned extrusion rate means an extrusion rate at the outlet of the most downstream extruder. The screw peripheral speed of the extruder depends upon the diameter of the screw, the amount of the polymer to be treated and the like, but it is preferably in the range of 0.1 to 3.5 m/sec in order to efficiently promote surface renewal at the vent. If the peripheral speed is in excess of 3.5 m/sec, heat is abnormally generated, and the required power cost also increases inconveniently. In the present invention, the resin temperature at the time of the extrusion is advantageously in the range of from the melting point of the applicable styrenic polymer to 400°C. If the resin temperature is more than 400°C, the styrenic polymer tends to decompose. Accordingly, the resin temperature is preferably in the range of from the melting point of the polymer to 370°C. In this case, the cylinder temperature of the extruder is preferably set in the range of from room temperature to 400°C, more preferably from a glass transition temperature to 370°C, most preferably from the glass transition temperature to 320°C. In the present invention, in order to sufficiently inhibit the yellowing, an inert gas such as nitrogen, argon, helium or carbon dioxide or an inert liquid can be poured into the extruder. Next, the present invention will be described in more detail with reference to examples. Example 1 To a styrenic polymer powder of a syndiotactic configuration having a viscosity-average molecular weight of 250000 and an average particle diameter of 200 µm and containing 29% by weight of styrene as a remaining monomer on the basis of its wet state, Irganox 1010 made by Ciba-Geigy was added in an amount of 0.2% by weight based on the weight of the polymer powder, and they were then mixed. Next, the resulting mixture was volatilized and extruded under conditions of a cylinder temperature of 290°C, a molding temperature of 300 to 320°C, a first vent internal pressure of 26.6 kpa (200 Torr), a second vent internal pressure of 2.7 kpa (20 Torr), a screw peripheral speed of 0.5 m/sec (co-rotation) and an extrusion rate of 28 kg/hr, i.e., Q/(D×H×V)=1.32×10 5 by the use of a twin-screw-extruder made by The Japan Steel Works, Ltd. [TEX44-XCT (L/D = 38.5, D = 47 mm, H = 9.0 mm and vent number = 2)]. The resulting polymer was collected in the form of pellets, and its YI value was 18. Incidentally, Irganox 1010 is pentaerythrityltetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate]. Example 2 The operation of volatilization·extrusion was carried out by the same procedure as in Example 1 except that, instead of the addition of Irganox 1010 made by Ciba-Geigy to the styrenic polymer powder, a 10% by weight ethylbenzene solution of Irganox 1010 was fed in an amount of 2.0% by weight based on the weight of the powder after the plasticization and melting of a styrenic polymer powder in an extruder. The YI value of obtained pellets was 17. Example 3 The operation of volatilization·extrusion was carried out by the same procedure as in Example 1 except that Irganox 1010 made by Ciba-Geigy was replaced with PEP36 made by Adeca-Argas Co., Ltd. The YI value of obtained pellets was 20. Incidentally, PEP36 is bis (2,6-di-t-butyl-4-methylphenyl)pentaerythritol diphosphite. Comparative Example 1 The operation of volatilization·extrusion was carried out by the same procedure as in Example 1 except that Irganox 1010 made by Ciba-Geigy was not added. The YI value of obtained pellets was 41. Possibility of Industrial Utilization According to the present invention, a styrenic polymer powder having a syndiotactic configuration can be volatilized and extruded by an extruder to stably and efficiently prepare pellets in which yellowing is inhibited, or pellets containing a less amount of a remaining volatile component such as an unreacted monomer. Therefore, a styrenic resin prepared by the present invention can suitably be utilized as a material for various industrial products, and hence, its industrial significance is extremely large.

Claims: A process of volatilisation extrusion for preparing a styrenic resin, which comprises the step of extruding a styrenic polymer powder having a syndiotactic configuration and containing 0.1 to 50 % by weight of a remaining volatile component, in the presence of an antioxidant, by the use of an extruder to remove the volatile components, wherein the antioxidant is fed to the extruder, followed by the extrusion. The process for preparing a styrenic resin according to Claim 1 wherein the antioxidant is at least one selected from the group consisting of phenolic antioxidants and phosphoric antioxidants. The process for preparing a styrenic resin according to Claim 1 or 2 wherein the extrusion is carried out in the presence of the antioxidant in an amount of 0.001 to 5% by weight based on the weight of the styrenic polymer powder. A process of volatilization extrusion for preparing a styrenic resin, comprising the step of extruding a styrenic polymer powder having a syndiotactic configuration and containing 0.1 to 50% by weight of a remaining volatile component, in the presence of an antioxidant wherein the styrenic polymer powder to which the antioxidant is added is fed to the extruder, followed by the extrusion. The process for preparing a styrenic resin according to claim 4, wherein the antioxidant is at least one selected from the group consisting of a phenolic antioxidant and a phosphoric antioxidant. The process for preparing a styrenic resin according to claim 4, wherein said antioxidant is present in an amount of 0.001 to 5 % by weight, based on the weight of the styrenic polymer powder.

Inventors: FUKADA KAZUYOSHI, ONODERA TSUTOMU, SAEKI TAKASHI, FUKADA, KAZUYOSHI, ONODERA, TSUTOMU, SAEKI, TAKASHI, Fukada, Kazuyoshi, c/o Idemitsu Petrochem. Co, Ltd, Onodera, Tsutomu, c/o Idemitsu Petrochem. Co., Ltd, Saeki, Takashi, c/o Idemitsu Petrochem. Co., Ltd.
Applicants: IDEMITSU PETROCHEMICAL CO, IDEMITSU PETROCHEMICAL CO., LTD.
Application year: 1996
Main classification: C08F6
IPC codes: C08F6