Process for making syndiotactic polystyrene containing pellets

Abstract: Disclosed is a method for producing a molding material, which comprises melt-extruding a styrenic polymer essentially having a syndiotactic structure or a composition containing the polymer to give strands, then rapidly cooling the resulting strands to thereby make the surface temperature of the strands not higher than the melting point of the styrenic polymer or the composition containing the polymer, and thereafter gradually cooling them to thereby make the surfacel temperature of the strands not higher than a temperature higher by 20°C than their glass transition temperature, and finally pelletizing the strands into pellets. The pellets produced have a high degree of crystallinity and can be well molded into good moldings.

Description: FIELD OF THE INVENTION The present invention relates to a method for producing a molding material, and, more precisely, to a stable method for producing a molding material with a high degree of crystallinity, which comprises a styrenic polymer essentially having a syndiotactic structure. BACKGROUND OF THE INVENTION A styrenic polymer essentially having a syndiotactic structure and a composition containing the polymer, which have been developed recently, are widely known as engineering plastics with good heat resistance and good chemical resistance. The polymer will be hereinunder referred to as SPS. SPS has various applications in various molding methods. For example, it is molded into films, sheets and the like through extrusion molding. Extrusion molding of SPS pellets having a low degree of crystallization is problematic in that the SPS pellets as heated at the glass transition temperature of SPS or higher are softened and aggregated in the feed zone near the inlet of the extrusion-molding machine and that the softened pellets adhere to the screws in the extrusion-molding machine and even to the inner wall of the barrel of the machine. As a result, it is often difficult to constantly feed the SPS pellets to the extrusion-molding machine, or as the case may be, the screws are often stopped. One measure to solve these problems is disclosed in Japanese Patent Application Laid-Open No. 02-194044, in which they say that the SPS pellets to be fed to an extruder shall have a specifically-controlled melt viscosity and a specifically-controlled degree of crystallinity. A method for producing such SPS pellets is disclosed in JP-A 02-194044 and US-A-5,292,587 which comprises melt-extruding SPS, and then cooling the resulting SPS strands in air or cooling them in water followed by re-heating the resulting SPS pellets. The document US -A- 5,292,587 describes in a working example how polystyrene of syndiotactic configuration is extruded at 300°C, how the extruded strands are quenched with air at room temperature and how the cooled strand was then pelletized by using a pelletizer. The resulting pellet had a Ctystallinity of 35%. Another example describes the cooling of the strands with water of 20°C. However, the process comprising the step of cooling the SPS strands in water followed by the step of re-heating the resulting SPS pellets, which is to increase the degree of crystallinity of SPS, is complicated. On the other hand, the process comprising the step of melt-extruding SPS followed by the step of cooling the resulting SPS strands in air is simple, but is problematic in that the SPS strands are often cut in some unsuitable cooling conditions, and that the degree of crystallinity of the SPS pellets obtained often differs or could not be satisfactorily increased, depending on the cooling conditions employed. Thus, in this process, it is difficult to stably produce SPS pellets having a high degree of crystallinity. Accordingly, it is greatly desired to develop a method for stably and efficiently producing SPS pellets with a high degree of crystallinity that are suitable for molding, especially for extrusion molding. SUMMARY OF THE INVENTION The present invention has been made in consideration of the current situation as above, and its object is to provide a method for stably and efficiently producing, in a simplified manner not requiring any complicated operations, SPS pellets with a high degree of crystallinity that are suitable for molding, especially for extrusion molding. This object is met by the method of claim 1. In one preferred embodiment of the method, the strands are rapidly cooled to thereby make the surface temperature of the strands fall within a range between a temperature higher by 80°C than the glass transition temperature of the styrenic polymer and a temperature lower by 30°C than the same glass transition temperature. In still another preferred embodiment of the method, the rapidly-cooled strands are gradually cooled to thereby make the surface temperature of the strands fall within a range between a temperature of the glass transition temperature of the styrenic polymer and room temperature. In the method of the invention, the strands first rapidly cooled and then gradually cooled may be again rapidly cooled to thereby make their surface temperature not higher than a temperature lower by 20°C than their glass transition temperature. PREFERRED MODES OF CARRYING OUT THE INVENTION Now, the invention is described in detail hereinunder. 1. Styrenic polymer essentially having a syndiotactic structure: (1) Styrenic polymer essentially having a syndiotactic structure: The syndiotactic structure as referred to herein indicates the stereo-structure of the styrenic polymer, in which the side chains of phenyl groups or substituted phenyl groups are alternately positioned in the opposite sides relative to the main chain composed of carbon-carbon bonds, and its tacticity is determined according to its nuclear magnetic resonance with an isotopic carbon ( 13 C-NMR). The tacticity to be determined according to 13 C-NMR indicates the proportion of a plurality of continuous constitutional units in a polymer. For example, a polymer comprising two continuous constitutional units is referred to as a diad; that comprising three continuous constitutional units is referred to as a triad; and that comprising five continuous constitutional units is referred to as a pentad. The styrenic polymer essentially having a syndiotactic structure as referred to herein generally has a racemic diad syndiotacticity of 75 % or higher, preferably 85 % or higher, or has a racemic pentad syndiotacticity of 30 % or higher, preferably 50 % or higher, including, for example, polystyrene, poly(alkylstyrenes), poly (halogenostyrenes), polypoly(halogenoalkylstyrenes), ), poly(alkoxystyrenes), poly(vinylbenzoates), hydrogenated derivatives of those polymers, and copolymers consisting essentially of those polymers. The poly(alkylstyrenes) include, for example, poly(methylstyrene), poly(ethylstyrene), poly(isopropylstyrene), poly(tert-butylstyrene), poly(phenylstyrene), poly(vinylnaphthalene), poly(vinylstyrene), etc. The poly(halogenostyrenes) include, for example, poly(chlorostyrene), poly(bromostyrene), poly(fluorostyrene), etc. The poly (halogenbalkylstyrenes) include, for example, poly(chloromethylstyrene), etc. The poly(alkoxystyrenes) include, for example, poly(methoxystyrene), poly(ethoxystyrene), etc. Of those styrenic polymers, especially preferred are polystyrene, poly(p-methylstyrene), poly(m-methylstyrene), poly(p-tert-butylstyrene), poly(p-chlorostyrene), poly(m-chlorostyrene), poly(p-fluorostyrene), hydrogenated polystyrene, and copolymers comprising those constitutional units. The molecular weight of the styrenic polymer for use in the invention is not specifically defined, but its weight-average molecular weight is preferably not smaller than 10,000, more preferably from 50,000 to 1, 000, 000. The molecular weight distribution of the polymer is not also specifically defined, and the polymer may have any undefined molecular weight distribution. The method of producing SPS for use in the invention is not specifically defined. For example, styrenic monomers (including their mixtures) corresponding to the intended styrenic polymers may be polymerized or copolymerized optionally along with any other comonomers, in the presence or absence of inert hydrocarbon solvents, using various known catalysts. (2) Other components except SPS: In the composition containing SPS of the invention, the amount of SPS may be generally not smaller than 10 % by weight, preferably not smaller than 30 % by weight, more preferably not smaller than 50 % % by weight, even more preferably not smaller than 70 % % by weight. The other components constituting the composition are, for example, as follows: (a) Inorganic fillers: Various inorganic fillers are employable herein, including, for example, fibrous, granular and powder ones. The fibrous inorganic fillers include, for example, glass fibers, carbon fibers and whiskers. They may be in any form of fabrics, mats, chopped strands, short fibers and the like, and may also be whiskers themselves. The chopped strands preferably have a length of from 0.05 to 50 mm and a fiber diameter of from 5 to 20 µm. The granular or powdery inorganic fillers include, for example, talc, carbon black, graphite, titanium dioxide, silica, mica, calcium carbonate, calcium sulfate, barium carbonate, magnesium carbonate, magnesium sulfate, barium sulfate, oxysulfates, tin oxide, alumina, kaolin, silicon carbide, metal powder, glass powder, glass flakes, glass beads, etc. Of those inorganic fillers, especially preferred are glass fillers such as glass powder, glass flakes, glass beads, glass filaments, glass fibers, glass rovings and glass mats. The inorganic fillers are preferably surface-treated with coupling agents or the like to thereby improve their adhesiveness to resins. The coupling agents may be any conventional ones, including, for example, silane-type coupling agents, titanium-containing coupling agents and the like. One or more of those inorganic fillers are usable herein either singly or as combined, if desired. (3) To shorten the time for crystallization of SPS, a nucleating agent may be added to the styrenic polymer The nucleating agent may be any conventional one, including, for example, metal carboxylates such as aluminium di(p-t-butylbenzoate), metal phosphates such as sodium methylene-bis(2,4-di-t-butylphenol) acid phosphate, talc, phthalocyanine derivatives, etc. One or more of those nucleating agents are usable herein either singly or as combined, if desired. (4) To SPS of the invention, various additives may be added without interfering with the object of the invention. The additives include, for example, antioxidants, nucleating agents, plasticizers, mold lubricants, flame retardants, flame retardant promoters, antistatic agents, etc. (5) Where various additives are added to SPS of the invention, the method of mixing them is not specifically defined. The order of adding the components to a mixing device, the type of mixing systems to be employed, and other conditions for the method may be suitably determined. If desired, the resulting mixture may be melted and kneaded, and the melt is within the scope of the invention. The method of melting and kneading the mixture is not specifically defined, and any conventional melt-kneading methods are employable herein. 2. Method for producing molding material: In the invention, the strands as obtained through melt extrusion of SPS are rapidly cooled to thereby make the surface temperature of the strands not higher than the melting point of SPS in the further step the strands are gradually cooled to thereby make the strands completely not higher than a temperature higher by 20°C than the glass transition temperature of SPS, and thereafter they are pelletized. 1 ○ Melt extrusion of SPS: In the invention, SPS is heated generally at a temperature not lower than the melting point of SPS, but preferably at a temperature falling between the melting point of SPS and a temperature higher by 100°C than the melting point, and the resulting SPS melt is extruded into strands. The melt extruder to be employed herein is not specifically defined, and may be any ordinary, single-screw, but preferably double-screw, vented or vent-free extruder that is generally used for melt extrusion of resins. 2 ○ Rapid cooling of strands: In conventional methods, SPS strands as extruded out through the die of an extruder are generally cooled in water. In this rapid cooling step, the SPS strands are entirely cooled to a temperature not higher than the glass transition temperature of SPS. In this, therefore, the SPS strands are solidified to have a low degree of crystallization. Accordingly, the SPS pellets as produced by pelletizing those solidified strands shall naturally have a low degree of crystallization. Apart from this, SPS strands may be gradually cooled in air, whereupon the solidified SPS strands shall surely have a high degree of crystallization. However, the gradual cooling of SPS strands in air to make their temperature not higher than their glass transition temperature takes a lot of time. In addition, where the thus-cooled SPS strands are continuously pelletized using a pelletizer, the cooling time depends on the take-up speed of the pelletizer used and on the distance between the extruder and the pelletizer. Therefore, in this case where the cooling time is long, the take-up speed of the pelletizer must be lowered, or the distance between the extruder and the pelletizer must be prolonged. In order to increase the take-up speed of the pelletizer, the cooling zone must be relatively prolonged, thereby inevitably resulting in that the distance between the extruder and the pelletizer is prolonged. Accordingly, where the SPS melt strands are cooled and pelletized in those conditions, they will dangle without being stretched in the space between the extruder and the pelletizer, and if the worst comes to the worst, they will be cut so that they could not be continuously pelletized. The present invention is free from those problems inevitably occurring in the conventional methods. Specifically, in the method of the invention, the extruded melt strands are rapidly cooled in water to thereby make their surface temperature not higher than their melting point. In this rapid cooling step, the strands are so cooled that their surface temperature is not higher than the melting point of SPS while their inner temperature is not lower than the melting point of SPS. The thus-cooled strands have a high surface strength and therefore do neither dangle nor cut while they are drawn from the extruder. After stretching the strands in air, the SPS strands are gradually coolled, whereby they are wholly crystallized due to their inner heat to have a high degree of crystallization. Thus, in the first cooling step of the invention, the melt strands are rapidly cooled to make their surface temperature not higher than their melting point. In this, preferably, the melt strands are rapidly cooled to thereby make their surface temperature fall within a range between a temperature not higher than their melting point and a temperature not lower than a temperature lower by 30°C than their glass transition temperature, more preferably, they are rapidly cooled to thereby make their surface temperature fall-within a range between a temperature higher by 80°C than their glass transition temperature and a temperature lower by 10°C than their glass transition temperature, even more preferably, within a range between a temperature higher by 50°C than their glass transition temperature and their glass transition temperature. In this rapid cooling step, the strands are cooled at a mean cooling rate of not lower than 25°C/sec, preferably falling between 50 and 300°C/sec or so, more preferably falling between 100 and 250°C/sec, on the basis of the surface temperature of the strands. If the cooling rate is lower than 25°C/sec, the strands will be cooled not only on their surface but also even in their inside, resulting in that they could no more be crystallized even though they are further gradually cooled in the next step. On the other hand, the mean cooling rate of higher than 300°C/sec is unfavorable, since the surface temperature of the strands being cooled at such a high cooling rate is difficult to control and since some special cooling equipment is needed. The mean cooling rate as referred to herein is obtained by dividing the difference between the temperature at which the cooling is started and the temperature at which the cooling is finished, by the cooling time. The means of rapidly cooling the melt strands to be employed herein is specifically defined to be performed in water. The preferred cooling means varies, depending on the type of SPS constituting the strands to be cooled. The method of the invention uses hot or lukewarm water having a temperature falling between 40 and 90°C desirably between 50 and 70°C or so. If the temperature of water used for the cooling is lower than 40°C, it is difficult to control the surface temperature of the strands being cooled. If so, therefore, the strands will be cooled not only on their surface but also even in their inside, resulting in that they could no more be crystallized even though they are further gradually cooled in the next step. On the other hand, if the temperature of water used for the cooling is higher than 90°C, the surface temperature of the strands being cooled would be lowered insufficiently, often resulting in that the resulting strands will dangle or cut in the next gentle cooling step. 3 ○ Gentle cooling of strands: The strands are gradually cooled in a gentle cooling step, in which the strands are entirely cooled and SPS constituting those strands is crystallized. The strands are further cooled in the gentle cooling step, thereby making their temperature not higher than a temperature higher by 20°C than the glass transition temperature of SPS, preferably making their temperature fall between a temperature higher by 10°C than the glass transition temperature of SPS and room temperature, more preferably between the glass transition temperature of SPS and 50°C. If the surface temperature of the strands thus gradually cooled. in this step is higher by 20°C than the glass transition temperature of SPS, the strands could not be pelletized well in the next pelletizing step. As has been mentioned hereinabove, the inner temperature of the strands being cooled is higher than the surface temperature thereof so that the crystallization of SPS constituting the strands is promoted by the inner heat of the strands. Therefore, the surface temperature of the strands that have been gradually cooled in this gentle cooling step may often be higher than that of the strands that have just been rapidly cooled in the rapidly cooling step. In this gentle cooling step, the strands may be gradually cooled at a mean cooling rate falling between -10 and 20°C/sec, preferably between -5. and 15°C/sec, more preferably between 1 and 10°C/sec, even more preferably between 2 and 6°C/sec, on the basis of the surface temperature of the strands being cooled. If the mean cooling rate is higher than 20°C/sec, the strands will be entirely cooled to their glass transition temperature before they are fully crystallized. If so, the degree of crystallinity of the thus-cooled strands is not satisfactorily increased. On the other hand, even if the mean cooling rate is lower than -10°C/sec, the degree of crystallinity of the cooled strands is no more increased over the intended value. The mean cooling rate as referred to herein is obtained in the same manner as above. In the gentle cooling step of the invention, the strands are gradually cooled for at least 2 seconds or so, preferably for 3 to 30 seconds or so, more preferably for 4 to 10 seconds or so. If the gentle cooling time is shorter than 2 seconds, SPS having a sufficiently increased degree of crystallinity could not be obtained. The gentle cooling time may be controlled by suitably varying the gentle cooling distance, or that is, the distance between the rapid cooling device and the pelletizer within which the strands are moved prior to being pelletized, and the take-up speed of the pelletizer. 4 ○ Pelletization of strands: The strands thus fully crystallized in the previous gentle cooling step are directly, or after having been again rapidly cooled, pelletized into pellets in any known manner, for example, using an ordinary pelletizer. In the case of having been again rapidly cooled before being pelletized, the strands have been preferably cooled to make their surface temperature at least not higher than a temperature lower by 20°C than their glass transition temperature. Optionally, the strands may be again rapidly cooled for the following reasons: In general, when the temperature of the strands is lowered to be not higher than their glass transition temperature, the strands are usually no more crystallized. Therefore, even though the strands that have been lowered to a temperature not higher than their glass transition temperature are further gradually cooled, the degree of crystallization of the resulting pellets would no more differ so significantly, and there is no necessity for the additional gentle cooling of the strands. Contrary to this, however, if the strands that have been rapidly cooled and then gradually cooled are again rapidly cooled, the necessary gentle cooling distance can be shortened. Accordingly, this additional rapid cooling that may be effected optionally is irrelevant to the first rapid cooling that is indispensable in the invention, and the means of the additional rapid cooling is not specifically defined. The cooling rate of the additional rapid cooling may be either higher or lower than that of the indispensable rapid cooling. Preferably, the additional rapid cooling may be effected in water. According to the method of the invention described in detail hereinabove, stably obtained are SPS pellets having a high degree of crystallization of not lower than than 30 %. Even when heated at their glass transition temperature or higher in a molding machine, especially in an extrusion-molding machine from which they are melt-extruded to be molded, those SPS pellets are not softened but are directly heated up to their melting point or higher and are thus melted. Accordingly, the SPS pellets are free from the problems that they are aggregated in the feed zone near the inlet of the extrusion-molding machine and they adhere to the screws in the extrusion-molding machine or even to the inner wall of the barrel of the machine. Thus, the molding material comprising those SPS pellets is good. Now, the method for producing the molding material of the invention is described in more detail with reference to the following Examples, which, however, are not intended to restrict the scope of the invention. Production Example 1: Production of SPS 1.0 liter of pure styrene and 1 mmols of triethyl aluminium were put into a 2-liter reactor, and heated at 75°C, to which was added 16.5 ml of a pre-mixing catalyst (comprised of 90 µmols of pentamethylcyclopentadienyl titanium trimethoxide, 90 µmols of dimethylanilinium tetrakis(pentafluorophenyl) borate, 29.1 mmols of toluene, and 1.8 mmols of triisobutyl aluminium), and the, monomer was polymerized therein at 75°C for 4.8 hours. After the reaction, the product was repeatedly washed with methanol, and dried to obtain 380 g of a polymer. The above production process was repeated to obtain more than 20 kg of the polymer. The weight-average molecular weight of the polymer was measured through gel permeation chromatography at 130°C, using a solvent of 1,2,4-trichlorobenzene, to be 272,000. From its melting point and 13 C-NMR data, the polymer was identified as SPS. Example 1: The SPS powder as obtained in Production Example 1 was melt-extruded through an extruder TEX30 (manufactured by Nippon Steel Works, Co. - this rotates at 200 rpm). For the extrusion, the amount of the SPS powder charged was 20 kg/hr, the take-up rate of the SPS strands formed was 1 m/sec, and the die temperature was 300°C. The strands as extruded out through the die were dipped in a water bath at 60°C for 1 second, and thereafter stretched in air at 25°C for 5 seconds. The surface temperature of the strands just drawn up from the water bath was 115°C, and that of the strands after having been stretched in air for 5 seconds was 95°C. From those data, it is known that the mean cooling rate of the surface of the strands being cooled in the water bath was 185°C/sec. The mean cooling rate of the surface of the strands being gradually cooled in air was 4°C/sec. Next, the strands were again dipped in the water bath, whereby they were further cooled to 70°C, and thereafter they are pelletized into pellets, using a pelletizer. The SPS pellets thus obtained had a degree of crystallization of 30 %. These pellets were sheeted into sheets, whereupon they were not aggregated in the hopper and also near the screws in the sheeting machine, and they were smoothly and stably fed into the sheeting zone to give good sheets. During the sheeting operation, the amount of the pellets being extruded out did not vary, and the pellets were continuously sheeted into good sheets of which the thickness was uniform in the machine direction. Example 2: In the same manner as in Example 1, except that the strands that had been gradually cooled were further cooled in air to 90°C in place of being cooled in water to 70°C, the strands were pelletized into pellets, using a pelletizer. The SPS pellets obtained herein had a degree of crystallinity of 35 %. Like those in Example 1, the pellets were also stably sheeted into good sheets. Example 3: The pellets as obtained in Example 1 were fed into a stirred hopper (manufactured by Kawata Co. - this has an inner volume of 70 liters) at a feeding rate of 10 kg/hr, into which was introduced hot nitrogen gas heated at 150°C at a flow rate of 2.4 Nm 3 /hr to heat the pellets therein. After having been heated for 20 minutes, the pellets were sampled out through the sampling valve, and their degree of crystallinity was measured to be 35 %. Like those in Example 1, the pellets obtained herein were also stably extruded out and sheeted into good sheets. Comparative Example 1: The SPS strands were dipped in a water bath at 30°C for 10 seconds and then directly pelletized into pellets, using a pelletizer. In this process, the surface temperature of the strands just having been drawn up from the water bath was 70°C. From those data, it is known that the mean cooling rate of the surface of the strands in the water bath was 23°C/sec. The SPS pellets thus produced had a degree of crystallinity of 10 %. Those pellets were sheeted using a sheeting machine in which, however, the pellets were partly aggregated around the screws and above them so that it was difficult to constantly feed the pellets. The sheets thus produced had streaks on their surface. As has been described in detail hereinabove, the method of the invention stably and efficiently produces SPS pellets having a high degree of crystallinity. The SPS pellets thus produced by the method of the invention are good molding materials for, for example, extrusion molding Where the SPS pellets are molded through an extrusion-molding machine, they are not aggregated in the hopper and even near the screws in the machine, and the machine is smoothly run to give good moldings.

Claims: A method for producing a molding material, which comprises melt-extruding a styrenic polymer essentially having a syndiotactic structure, to give strands, then rapidly cooling the resulting strands in water having a temperature falling between 40°C and 90°C at a mean cooling rate of not lower than 25°C/sec on the basis of the surface temperature of the strands to thereby make the surface temperature of the strands not higher than the melting point of the styrenic polymer, further stretching the rapidly cooled strands in air and thereafter gradually cooling the stretched strands at a mean cooling rate of not higher than 20°C/sec on the basis of the surface temperature of the strands for at least 2 seconds, to thereby make the surface temperature of the strands not higher than a temperature higher by 20°C than the glass transition temperature of the polymer, and finally pelletizing the strands into pellets. The method for producing a molding material as claimed in claim 1, in which the strands are rapidly cooled to thereby make the surface temperature of the strands fall within a range between a temperature higher by 80°C than the glass transition temperature of the styrenic polymer and a temperature lower by 30°C than the same glass transition temperature. The method for producing a molding material as claimed in claim 1 in which the rapidly-cooled strands are gradually cooled to thereby make the surface temperature of the strands fall within a range between a temperature of the glass transition temperature of the styrenic polymer and room temperature. The method for producing a molding material as claimed in claim 3 in which the gradually-cooled strands are again rapidly cooled to thereby make their surface temperature at least not higher than a temperature lower by 20°C than their glass transition temperature.

Inventors: HIROSE KENZI, ISHIBASHI HIROFUMI, HIROSE, KENZI, ISHIBASHI, HIROFUMI
Applicants: IDEMITSU PETROCHEMICAL CO, IDEMITSU PETROCHEMICAL CO., LTD.
Application year: 1997
Main classification: B29B9
IPC codes: B29B9, C08F12, C08J3, C08F6, B29B13