PROCESS FOR PRODUCING LOW-DUST TRANSPARENT THERMOPLASTIC RESIN SOLUTION

Abstract: A transparent thermoplastic resin, e.g. polystyrene, is dissolved in a water-immiscible solvent, preferably of lower solubility than water, such as an aliphatic or aromatic hydro­ carbon, the solution preferably having a viscosity of 10 to 3000 cps and concentration of 10 to 25 wt %; pure water of a low dust content may be mixed in amount of 1 to 200 wt %. The solution is centrifuged at 3000 G or more and allowed to stand to separate into an aqueous phase and a resin solution. The dust particles are mainly left on the inter­ face on the aqueous side so that the resin solution is recove­ red containing less than 10,000 particles of dust of a size of at least 0.5 µm per gram of solid resin. The centrifuging can be repeated if necessary. Con­ ventional microfiltration can also be used.

Description: This invention relates to a novel process for producing a highly pure transparent thermoplastic resin solution from which dust has been substantially removed. More specifically, it relates to a process for producing a low-dust transparent thermoplastic resin solution, by which one can easily reduce the amount of dusts having a size of at least 0.5 µm to 10,000/g or less of solid resin, and under most preferred operating conditions, can reduce it to substantially zero, which is difficult to achieve by means of conventional microfilters. Heretofore, removal of fine dust, from transparent thermoplastic resin has been carried out, for example, by use of microfiltered reaction materials including monomers, catalysts, solvents, etc., or microfiltration of a prepared resin solution or a solution obtained by redissolving the produced resin in a solvent. These methods require use of a very precise filter set to remove the fine dusts by filtration. When the resin solution is to be filtered, the pressure drop in filtration becomes higher as its viscosity becomes higher. This leads to the defect that the amount of the resin solution to be treated decreases, and the filtration efficiency is poor. Filters are naturally blocked up by dusts, and in order to perform efficient purification by filtration, the filters must be frequently exchanged, and moreover, the amount of the resin solution to be filtered is very large. A method improved in both operation and cost has therefore been sought heretofore. In the purification of a transparent thermoplastic resin solution containing a substantially water-immiscible solvent, a centrifugal separating method is used in the prior art to separate the resin solution from water used for washing. Usually, the purpose of this centrifugal separation is to separate from the resin solution phase the washing water which is used to remove the unreacted monomers, catalyst and other impurities in the resin solution. This method can cause to some extent removal and separation of the aqueous phase, but its effect of decreasing the amount of dust is substantially small because the interface between the two phases is usually handled as the resin solution phase. Moreover, the centrifugal force, the separation treatment time, etc. for removal of free water are naturally insufficient for the purpose of dust removal. FR-A-2174670 discloses a process for removing water-soluble impurities from a solution of polyalkylene polyol, wherein the polyol is dissolvedin a water-insoluble solvent, water is mixed with the resulting mixture, and centrifuging is carried out to separate the water; preferably at 1500 to 16000 G. The present inventors have found in the course of thoroughly analyzing and examining the process of manufacturing polycarbonate resins with the aim of reduction of dust content that the distribution of dust particles during purification of a resin solution by water washing decreases in an interface between an aqueous phase and a resin solution phase, the resin solution phase, and the aqueous phase, in the order stated. Accordingly this invention provides a process for producing a low-dust transparent thermoplastic resin solution by centrifuging a solution of a transparent thermoplastic resin containing free water in an amount of at least 0.5% by weight of the resin solution so as to separate it into resin solution phase, interface and an aqueous phase, and separating the resin solution phase, characterised in that the centrifuging is carried out at a force of at least 3000 · 9.80665 m/s² (3000 G), and that the resin solution phase is separated from the aqueous phase and the interface. In a preferred embodiment, the transparent thermoplastic resin is selected from polystyrene, a styrene/acrylate copolymer, styrene/maleic anhydride copolymer, styrene/maleimide copolymer, styrene/ maleic anhydride/maleimide copolymer, acrylic resins, acrylate/polycarbonate block copolymers, styrene/polycarbonate block copolymer, styrene/maleic anhydride/polycarbonate copolymer, styrene/maleimide/polycarbonate copolymer, styrere/maleic anhydride/maleimide/polycarbonate copolymer, polycarbonate, and polyphenylene oxide, and a solution of the resin having a viscosity of 10 to 3000 · 10 _3 Pa.s (10 to 3,000 cp), preferably 50 to 300 · 10 _3 Pa.s (50 to 300 cp), is continuously centrifuged at a centrifugal force of at least 3,000 (wherein G is the acceleration of gravity which is 9.80665 m/s²), especially preferably at least 5,000 G. The treated solution is transferred to a standing vessel where it is left to stand to separate it into a resin solution phase, an interface, and an aqueous phase. As a result, a low dust transparent thermoplastic resin solution is produced which has 10,000 dust particles/gram or less of the transparent thermoplastic resin, in a most preferred embodiment substantially zero/g of solid resin. One centipoise (cp) = 10 _3 Pa.s. The transparent thermoplastic resin solution to be treated in accordance with this invention is a solution of a transparent thermoplastic resin obtained by a solution polymerization method using a substantially water-immiscibie solvent, or a solution of a produced solid resin in a substantially water-immiscible solvent. Examples of the solution are solutions of various transparent thermoplastic resins having a viscosity of 10 to 3000 · 10 _3 Pa.s (10 to 3,000 cp), preferably 50 to 300 · 10 _3 Pa.s (50 to 300 cp) and a resin concentration of 1 to 50% by weight, preferably 10 to 25%. The resins have a viscosity average molecularweight (Mv) of 1 × 10³ to 200 × 10³. Specific examples of the transparent thermoplastic resins are polystyrene, styrene/acrylate copolymers, styrene/maleic anhydride copolymer, styrene/maleimide copolymer, styrene/maleic anhydride/maleimide copolymer, acrylic resins, acrylate/polycarbonate block copolymers, styrene/polycarbonate block copolymer, styrene/maleic anhydride/polycarbonate copolymer, styrene/maleimide/polycarbonate copolymer, styrene/maleic anhydride/maleimide/polycarbonate copolymer, polycarbonate, and polyphenylene oxide. Of these, the resins other than acrylic resins and polyphenylene oxide are preferred. The solvent may be any substantially water-immiscible solvent which dissolves these transparent thermoplastic resins and separates from the aqueous phase. Preferred solvents have lower solubilities in water, and a large difference in specific gravity from water. Aliphatic hydrocarbons or aromatic hydrocarbons are used. They may, for example, be chlorinated aliphatic or aromatic hydrocarbons such as methylene chloride, 1,1-dichloroethane, chlorobenzene and chlorotoluene. For the resins such as polystyrene, styrene/methyl methacrylate copolymer, styrene/maleic anhydride copolymer, styrene/maleimide copolymer, styrene/ maleic anhydride/maleimide copolymer and polyphenylene oxide, aromatic hydrocarbons such as benzene, toluene or xylene are preferred. For the resins such as acrylic resins, acrylate/polycarbonate block copolymer, polystyrene/ polycarbonate block copolymer, styrene/maleic anhydride/ polycarbonate copolymer, styrene/maleic anhydride/maleimide/ polycarbonate copolymer and polycarbonate, aliphatic hydrocarbons such as methylene chloride are preferred and halogenated aliphatic hydrocarbons are particularly preferred. The "pure water" as referred to in this invention is used to remove fine dust from the resin solution, and naturally, it preferably has the lowest possible dust content. Pure water having a low dust content as a result of microfiltration, etc. is naturally preferred. Pure water obtained by conventional ion exchange which usually has a dust content of 500 to 1,000/ml less may be used in an early stage of a separation operation carried out in a multiplicity of phase-separating steps or when thorough removal of the dust is not necessary. As required, the pure water is mixed with the transparent thermoplastic resin solution to form a transparent thermoplastic resin solution containing free water, and the solution is subjected to the centrifugal separation in accordance with this invention. The amount of the free water (separable water) whose presence in the solution is effective to decrease the dust is usually at least 0.5% by weight, preferably at least 1% by weight, from the standpoint of phase separability. There is no particular upper limit, but for economic reasons, its amount should be small, usually 200% by weight or less, especially preferably in the range of 5 to 35% by weight. The transparent thermoplastic resin solution containing free water is centrifuged to separate it into a resin solution phase and an aqueous phase, and the resin solution phase is separated from the interface and the aqueous phase to prepare a low dust transparent thermoplastic resin solution. Centrifugation is carried out to separate the resin solution phase and the aqueous phase more strictly than in the prior art. Investigations of the present inventors (see Examples 1 and 2 given hereinbelow) have led to the presumption that when a solution of the transparent thermoplastic resin solution containing a large amount of free water is subjected to phase separation, it looks as if it was separated into a resin solution phase and an aqueous phase, but fine particles of water are dispersed in the resin solution phase and also the resin solution is dispersed in fine particle form in the aqueous phase, and that large amounts of dust exist in the dispersed water portion of the resin solution. The centrifugal separation in accordance with this invention is carried out for the purpose of removing the free water dispersed in fine particulate form efficiently from the resin solution and to reduce the content of the dusts. The resin solution phase and the aqueous phase separated by centrifugation are maintained in the separate state even during conveyance by ordinary pipes or other means substantially free from a stirring operation unless they are mixed by, for example, stirring. By ordinary standing, they can be easily separated from each other. To increase the operating efficiency of the centrifuge, therefore, it is preferred to transfer the centrifuced solution to a standing vessel, leave it to stand therein to separate it into phases, and to separate the interface and the aqueous phase from the resin solution phase. The centrifuge used may be those ordinarily used. From the standpoint of the treating efficiency, they are preferably continously operable at the centrifugal force of 3,000 G or more and particularly at 5,000 G or more. One centrifuging operation and one phase-separating operation will suffice if a sufficient centrifugal force can be used. When the centrifugal force is insufficient or it is desired to increase the treating efficiency, it is also preferred to repeat the separating operations two or more times to decrease the dusts successively. The standing vessel for leaving the centrifuged solution to stand may be any ordinary vessel used for this purpose. Although the present invention reduces the amount of dusts by centrifugation, it is possible, if desired, to also use filtration on a conventional microfilter. This can be practised industrially having regard to economy and operability. The following Examples illustrate the present invention more specifically. Unless otherwise indicate, all percents, parts, ratios and the like are by weight. EXAMPLE 1 A methylene chloride solution of a polycarbonate resin obtained by interfacial polymerization from 2,2-bis(4-hydroxyphenyl)propane (hereinafter referred to as "BPA") as a main raw material was extracted with alkali. The organic phase was neutralized with phosphoric acid, and then washed with water once. The resulting solution had a content of dust particles having a size of at least 0.5 µm (hereinafter referred to as "a fine particule number") of 30,000/g of solid resin, a viscosity of 180 · 10 _3 Pa.s (180 cp), a polycarbonate (Mv = 20,000) concentration of 17%, a pH of 3.5 and a free water content of 0.6%. The solution was treated by a centrifuge with a residence time of 10 minutes under the conditions shown in Table 1, and then the amount of dusts having a size of at least 0.5 µm was examined at the resin solution phase, the interface (since in this example, the amount of free water was small, the aqueous phase was also dealt with as the interface), and the entire well surface. The results obtained are shown in Table 1 below. The amount of the resin was 200 g. The number of fine particles was measured by using a fine particle counter made by HIAC-ROYCO Co. The number of fine particles on the interface was measured by using measuring samples taken from the resin solution phase and the aqueous phase. The results showed that almost all fine particles were present on the interface on the aqueous phase side. In the tables, the number of fine particles (*1) was in accordance with the following standards. Starting resin solution: number/g of polycarbonate resin interface: number/ml of interfacial solution Entire wall surface: the number of fine particles adhered to the entire wall surface Aqueous phase: number/ml of aqueous phase. The balance of the number of fine particles is not 100%, but such a degree of variations in balance is tolerable in view of the present state of art. EXAMPLE 2 Example 1 was repeated except that a solution obtained by mixing the same polycarbonate resin solution as in Example 1 with 50%, based on the resin solution, of ultrapure (10 dust/ml) was used as the starting solution. The results obtained are shown in Table 2 below. It is understood from Table 2 that if washing and centrifugation are carried out using pure water, a sufficient centrifugal force and a sufficient time can give a resin solution containing substantially zero fine particle number/g of solid resin. With increasing centrifugal force, the number of dust particles tends to decrease both in the resin solution phase and the aqueous phase and the dusts tend to gather concentratingly in the interface. From this tendency and the measurement of the number of fine particles in the interface in Example 1, it is presumed that most of the dusts (fine particles) adhere in great amounts to the free water in the resin solution or in the free resin solution in the aqueous phase, and with increasing centrifugal force, these dusts move to the interface and thus result in an increased number of dusts in the interface. EXAMPLE 3 Example 1 was repeated except that a solution obtained by mixing the same polycarbonate resin solution as used in Example 1 with 100%, based on the resin solution, of ultrapure water (10 dusts/ml) was used, and the centrifugation time was varied. The results obtained are shown in Table 3 below. EXAMPLE 4 Each of an acrilic resin (general-purpose brand name PMMA (VH) made by Mitsubishi Rayon Co., Ltd.; hereinafter referred to as "PMMA"), polystyrene resin (brand name Diarex® HH 102 made by Mitsubishi-Monsanto Co., Ltd.; hereinafter referred to as "PS"), styrene/methyl methacrylate copolymer (brand name "Estyrne® MS-600" made by Shin Nittetsu Chemical Co., Ltd.; to be referred to as "MS"), styrene/maleic anhydride copolymer (brand nane "Dylark® 232" made by Sekisui Chemiclal Industry Co., Ltd.; hereinafter referred to is "S-MA") and styrene/polycarbonate copolymer having styrene grafted at the terminals (styrene content 20%; hereinafter referred to as "S-PC") was dissolved in methylene chloride to a concentration of 15%. The number of fine particles in the solution was as shown in Table 4 below. One hundred grams of the solution was mixed with 100 g of ultrapure water (2.7 fine particles/ml) for 0.5 minute with stirring. The mixture was treated by a centrifuge at a centrifugal force of 4,700 G for 20 minutes, and then transferred to another vessel. The number of dusts having a size of at least 0.5 µm was examined in the resin solution phase, the interface and the aqueous phase. The results obtained are shown in Table 4 below. Since the PMMA solution had low phase-separability, the above measurement was made after it was additionally centrifuged at a centrifugal force of 9,000 G for 20 minutes. Furthermore, because of the low phase-separability of the PMMA solution, the measured numbers of fine particles in the interface and the aqueous phase might have been excessive. EXAMPLE 5 Each of the PS, MS and S-MA resin solutions used in Example 4 was mixed with 50 g (per 100 g of the resin solution) of ultrapure water with stirring for 5.0 minutes. The resulting emulsion was centrifuged in the same way as in Example 4 at a centrifugal force of 9,000 G and varying residence times in the centrifuge. The results obtained are shown in Table 5 below. It is seen that the same explanation as made with regard to the results given in Table 2 can be applied to the results given in Table 5. EXAMPLE 6 Each of 10% polyphenylene oxide and PS resin solutions was mixed with 100 ml (per 100 ml of the resin solution) of ultrapure water. The resulting mixture was centrifuged in the same way as in Example 4 at a centrifugal force of 4,700 G for 20 minutes. The results obtained are shown in Table 6 below. The reason that the dust number in the starting resin solution is very large is that polyphenylene oxide and polystyrene which are the conventional molding materials being not subjected to dust control were used, and the reason that the dust balance differs is that almost all of dusts were adhered on the wall of the centrifugal machine. The following advantages can be obtained in the preferred embodiments of the process of this invention for producing a low dust transparent thermoplastic resin solution as can be seen from the foregoing statement including working examples. (1) Since a conventional microfilter is substantially unnecessary for the purification of the resin solution, a complicated filtering operation is not required. (2) The conventional microfilter is necessary for the production of pure water. However, the operation of filtering water is easy, and the amount of water can be smaller than tie resin solution. (3) A resin solution containing 10,000 or less dust particles having a size of at least 0.5 µm/g of solid resin can be easily obtained. (4) A resin solution having substantially zero dust/g of solid resin can be obtained by the preferred embodiment of this invention. Such a resin solution cannot be easily obtained by using a microfilter. Since the operation for decreasing dusts is easy and economical, the process of this invention is not limited to special uses such as optical uses and can also be applied to ordinary transparent thermoplastic resin materials.

Claims: 1. A process for producing a low-dust transparent thermoplastic resin solution by centrifuging a solution of a transparent thermoplastic resin containing free water in an amount of at least 0.5% by weight of the resin solution so as to separate it into resin solution phase, interface and an aqueous phase, and separating the resin solution phase, characterised in that the centrifuging is carried out at a force of at least 3000 · 9.80665 m/s² (3000 G), and that the resin solution phase is separated from the aqueous phase and the interface. 2. A process as in Claim 1, wherein the transparent thermoplastic resin is selected from polystyrene, a styrene/acrylate copolymer, styrene/maleic anhydride copolymer, styrene/maleimide copolymer, styrene/maleic anhydride/maleimide copolymer, acrylic resins, acrylate/Polycarbonate block copolymers, styrene/polycarbonate block copolymer, styrene/maleic anhydride/polycarbonate copolymer, styrene/maleimide/polycarbonate copolymer, styrene/maleic anhydride/maleimide/polycarbonate copolymer, polycarbonates and polyphenylene oxide. 3. A process as in Claim 1 or 2, wherein the resin solution has a viscosity in the range of 50 to 300 · 10 _3 Pa.s (50 to 300 centipoises). 4. A process as in any of Claims 1 to 3, wherein the resin solution centrifuged continuously is conveyed to a standing vessel where the resin solution phase is separated from the aqueous phase and the interface. 5. A process as in any preceding claim, wherein the number of dust particles having a size of at least 0.5 µm in the resulting low-dust transparent thermoplastic resin solution is 10,000/gram or less of the transparent thermoplastic resin. 6. A process as claimed in any preceding claim, wherein the resin is dissolved in an aliphatic or aromatic hydrocarbon as solvent. 7. A process as claimed in any preceding claim, wherein the solution of a transparent thermoplastic resin is a solution from which unreacted monomers, catalyst and other water-soluble impurities have been substantially removed.

Inventors: MAEDA, KENJI, MEGUMI, TAKEAKI, OHTANI, YUKI, UMEMURA, TOSHIKAZU
Applicants: MITSUBISHI GAS CHEMICAL COMPANY, INC.
Application year: 1987
Main classification: C08J3
IPC codes: G02B1, C08G85, C08G65, C08F6