Reducing ionic impurities content in polycarbonate resins

Abstract: An aromatic polycarbonate composition having less than 300 parts per billion each of ionic impurities, particularly sulfate and chloride ions is obtained by devolatilizing the ionic impurities from a polycarbonate composition employing an aqueous medium of about 1% by weight based on the weight of the polycarbonate composition. Preferably the devolatilizing is carried out in an extruder during compounding the polycarbonate composition and preferably under a vacuum. Also, the extruded strands of polycarbonate resin are quenched in a water bath having a sulfate and chloride ion content of less than 300 ppb each.

Description: The present invention is directed to a thermoplastic aromatic polycarbonate composition having reduced ionic impurities, while maintaining good processibility properties. The product is a higher quality polycarbonate resin for producing higher quality molded articles such as computer hard drive platter carriers or pre-chip silicon wafer carriers for the computer industry. More specifically, this invention is directed to an improved process and to an improved product from such process as herein described. BACKGROUND OF THE INVENTION Polycarbonate resin may often contain certain impurities which in turn affect their property performance in the final molded article. For example, Sulfate and Chloride ions if in sufficient quantities will affect color and processability of polycarbonate resins for producing pre-chip silicon wafer carriers, or computer hard drive platter carriers. The Sulfate ions can react with residual ammonia on the surface of the silicon wafer to form ammonium sulfate which forms a white residue on the surface of a silicon pre-chip wafer. The wafer then requires cleaning before being processed into a computer chip. Additional heat phase exposure such as injection molding. extrusion or compounding thereof may also induce discoloration of the polycarbonate resin. Even with phosphite stabilizers, yellowing of the polycarbonate and hydrolysis of the phosphite can occur particularly at processing temperatures. It is believed that the phosphites being susceptible toward hydrolysis at elevated or processing temperatures form in-situ acid species which can then react with the polycarbonate possibly increasing chain scission and giving rise to side-reactions which can eventually generate color in the molded article. This can also occur during extrusion; compounding or molding of the polycarbonate resin. EP-A-0300485 discloses a process for producing a low-particle polycarbonate resin molding material suitable for use as molding material for optical materials which comprises directly supplying a polycarbonate resin wet powder having a water content of from 0.5 to 10.0% by weight of an organic solvent content of from 0.01 to 5.0% by weight, said wet powder, when dried, having a weight average particle diameter of from 5 to 60 mesh, to a vented extruder and extruding the wet powder while venting to obtain pellets containing not more than 20 ppm of a residual halogenated hydrocarbon. The molding material has a greatly decreased solvent content while being freed from a particle content increase. Also, in order to achieve certain properties or improved properties, certain additives are employed with the polycarbonate resin during extrusion, compounding or injection molding of the polycarbonate composition. The addition of such additives to achieve better properties should not affect processability. Additionally, it is known to stabilize polycarbonate resin against discoloration by employing phosphites and/or epoxies as stabilizing additives. These are broadly disclosed in such U.S. Patents as 4,381,358, 4,358,563 and 3,673,146. However, if certain known impurities can be removed without the use of additives to neutralize the impurities, the removal of the impurities would greatly enhance its properties without affecting processability as may occur with additives. Therefore, it is an object of this invention to provide a process for reducing ionic impurities in a polycarbonate resin. Another object of this invention is to reduce ionic impurities in a polycarbonate resin during melt blending of the polycarbonate composition. Yet another object of the invention is to reduce Sulfate and Chloride ions in a polycarbonate resin. Still another object of this invention is to produce a polycarbonate resin having, reduced ionic impurities. SUMMARY OF THE INVENTION This invention is directed to polycarbonate composition having reduced ionic impurities and to a process for producing reduced ionic impurities in polycarbonate resins. This invention is also directed to a aromatic polycarbonate composition having reduced ionic impurities. The polycarbonate composition may be either injection molded, extruded into sheet or film, profile extruded, co-extruded or extruded blow molded. The process of this invention comprises the devolatilization of the impurities downstream in a melt blending process such as an extruder during compounding of the aromatic polycarbonate resin formulation by employing an aqueous medium. A small amount of the aqueous medium, preferably water, may be added to the formulation during melt blending in an extruder and then removing the ionic impurities by devolatilization, generally, under a vacuum downstream in an extruder. While removal of the Sulfate ions is the preferred ion removal, it has been found that other ions are also removed such as Chloride ions. The Sulfate and Chloride ions are each removed down to less that 300 parts per billion (hereinafter ppb). In a melt blending process, the formulation is extruded through a die into strands which are then pelletized. The strands, before pelletizing are passed through an aqueous medium cooling or quenching bath. Since water has a fairly high ionic concentration, namely Sulfate and Chloride ions, the polycarbonate resin strands become re-contaminated with these ions. Therefore, the process of this invention further requires using an aqueous cooling bath through which the extruded strands are passed having a low ion concentration, particularly a low Sulfate and Chloride ion content. Therefore, the water bath should be analyzed for at least Sulfate and Chloride ion concentration which should be less than about 300 ppb each or at least employing deionized water. The process of compounding or melt blending the aromatic polycarbonate resin employed herein is well known to those skilled in the art of compounding or melt blending an aromatic polycarbonate formulation and are disclosed in numerous articles and patents for preparing polycarbonate molding formulations. Preferably, the composition is first compounded or melt blended with the additive materials, generally, in an extruder. The compounded formulation is extruded into strands which are generally quenched in an aqueous bath pelletized, dried and processed under heat and pressure into the finished article. The finished article may be injection molded, profile extruded, sheet or film extruded, co-extruded or extruded blow molded into hollow shapes such as single or multi-layer plastic objects such as bottles, computer hard drive platter carriers or silicon pre-chip wafer carriers for the computer industry. A small amount of an aqueous medium is added to the compounding formulation to enhance the removal of the ionic impurities as well as employing an aqueous quenching bath having a low ion concentration. The amount of aqueous medium, preferably water, is that amount sufficient to reduce ionic impurities, particularly Sulfate and Chloride ions to less than 300 ppb. The amount of aqueous medium added is 0.25 to 2.0 weight % based on the weight of the polycarbonate formulation, and preferably 0.75 to 1.5 weight %. 1.0% has been found to be optimum. The cooling or quenching aqueous bath should have a low ion concentration, preferably wherein the Sulfate and Chloride ion concentration is each less than 300 ppb, and in particular each being no greater than 100 ppb and more particularly each being less than 50 ppb. The aqueous medium may be added with the ingredients in the feed hopper of the extruder or may be added downstream to the melt. Obviously, the aqueous medium should be added prior to devolatilization of the aqueous medium and removal of ionic impurities. The aqueous medium may be added as one shot or may be added in several increments such as part in the feed hopper and the balance downstream in the extruder or in increments down from the feed hopper. The aromatic polycarbonate resin employed herein may be any of the known aromatic polycarbonates or co-polymers or terpolymers thereof, or blends of polycarbonates with other polymers, co-polymers or terpolymers thereof. The aromatic polycarbonate employed in the practice of this invention may be prepared by reacting a dihydric phenol with a carbonate precursor in the presence of an acid acceptor and generally a molecular weight regulator. Any dihydric phenol may be employed in preparing the polycarbonate resin disclosed herein. Preferably, they are mononuclear or polynuclear aromatic compounds containing as functional groups two hydroxyl radicals, each of which is attached directly to a carbon atom of an aromatic nucleus. Examples of some of the dihydric phenols which can be employed in the practice of this invention are bisphenols such as 1,1-bis (4-hydroxyphenyl) methane, 2,2 - bis(4-hydroxyphenyl) propane, 4,4 - bis(4 - hydroxyphenyl) heptane, dihydric phenol ethers such as bis(4 - hydroxyphenyl ether, bis(3,5 - dichloro- 4- hydroxyphenyl) ether, dihydroxy diphenyls such as p,p' - dihydroxydiphenyl, 3,3' - dichloro - 4,4' -dihydroxy-diphenyl, dihydroxyaryl sulfones such as bis(4-hydroxyphenyl) sulfone, bis(3,5 - dimethyl - 4 - hydroxyphenyl) sulfone, bis (3 - methyl - 5 - ethyl - 4 - hydroxyphenyl) sulfone, dihydroxy benzenes, resorcinol, hydroquinone; halo- and alkyl-substituted dihydroxy benzenes such as 1,4 - dihydroxy - 2 - chlorobenzene, 1,4-dihydroxy - 2,3 - dichlorobenzene, 1,4 - dihydroxy - 2-methylbenzene, and dihydroxy diphenyl sulfoxides such as bis(4-hydroxyphenyl) sulfoxide, bis(3,5-dibromo-4-hydroxyphenyl) sulfoxide. The carbonate precursor employed in the practice of this invention can be either carbonyl halide or a haloformate. The carbonyl halides which can be employed herein are carbonyl bromide, carbonyl chloride, carbonylfluoride, or mixtures thereof. The haloformates suitable for use herein include bishaloformates of dihydric phenols (bischloroformates of hydroquinone) or glycols (bishaloformates of ethylene glycol, neopentyl glycol, polyethylene glycol, etc). While other carbonate precursors will occur to those skilled in the art, carbonyl chloride, also known as phosgene, is preferred. The reaction disclosed above is preferably known as an interfacial process or reaction between the dihydric compound and a carbonyl chloride such as phosgene. Another process for preparing the aromatic polycarbonate of this invention is the transesterification process which involves the transesterification of an aromatic dihydroxy compound and a diester carbonate. This process is known as the interfacial melt process. In the practice of this invention, the process of producing the aromatic polycarbonate is not critical. The critical feature of this invention is preparing the aromatic polycarbonate resin formulation by devolatilization of the aqueous medium containing the ionic impurities as described above. As used herein, aromatic polycarbonate shall mean and include any of the aromatic polycarbonates and combinations thereof as set forth previously. The polycarbonate composition of the invention may also include such additives as UV stabilizers, thermal stabilizers, release agents, fillers and reinforcing fillers such as glass fibers (short or long glass fibers), carbon fibers, talc, silica and other known additives employed in polycarbonate compositions. DETAILED DESCRIPTON OF THE EXAMPLES OF THIS INVENTION This invention can be further described by means of the following Examples; it being understood, however, that this invention shall in no way be restricted by these Examples. In the Examples where amounts are in terms of percent, they are percent by weight. Example 1 Four samples of about 1000 grams each of a polycarbonate resin were prepared as follows: Sample A was an aromatic polycarbonate powder having an intrinsic viscosity of about 0.50 deciliters per gram (dl/g) as measured at 20°C in methylene chloride. The polycarbonate powder was not melt extruded in an extruder. Sample B was prepared using the polycarbonate powder of Sample A by melt blending in an extruder a formulation of the polycarbonate powder of Sample A and standard additives of mold release, thermal stabilizers and colorants. This sample was melt blended in a vented extruder at about 330°C and at an extrusion pressure of 8274 kPa (1200 psi.) The extruded strands of Sample B were quenched in a water bath employing regular city water. Sample C was also an aromatic polycarbonate powder but having an intrinsic viscosity of 0.45 dl/g as determined under the conditions as employed with Sample A. The Sample C was also not melt extruded in an extruder. Sample D was prepared employing the polycarbonate powder of Sample C and was melt blended in an extruder under the same extruder conditions as employed with Sample B. The formulation was essentially the same as the formulation of Sample B except that 1.0 weight % of water was added to the formulation based on the weight of the formulation. Downstream in the extruder the formulation was devolatilized under a vacuum of greater than 68 kPa (20 inches of mercury) through the vent in the extruder. The water bath employed for quenching the extruded polycarbonate strands was deionized water having a Sulfate content of 12 ppb and a Chloride content of 12 ppb. Each formulation contained the same weight % of standard additives. Each formulation was analyzed by Ion Chromatography (IC) analysis for Sulfate and Chloride ion content. The results obtained were as follows in TABLE 1 below. The test method consists of dissolving about 3 - 5 grams of the polycarbonate sample in 25 ml. Of methylene chloride. The ions are then extracted with 15 to 20 ml. Of deionized water. A 5 ml. aliquot of extracted deionized water is injected on an Ion Chromatograph to determine total free ions in the sample. Sample Total Free Sulfate Ion Content Total Free Chloride Ion Content A 565 1945 B 450 550 C 525 1555 D 276 156 Example 2 This Example is set forth to show the amount of leachable Sulfate and Chloride ions that accumulate on the surface of the polycarbonate resin strands from plain city water as compared to deionized water after passing the strands of the polycarbonate resin through an aqueous quenching medium. The amount of leachable ions is primarily those ions on the surface of the polycarbonate strands picked up from the water bath. In this test procedure 25 ml. of deionized water is added to about 10 grams of polycarbonate sample The sample is then kept in an oven at 55°C for about 16-20 hours. A 5 ml. aliquot of extracted deionized water from the sample is injected on an Ion Chromatograph to determine leachable-free ions on the surface of the polycarbonate as picked up from the quenching water bath. Pellets from Sample D of Example 1 above and pellets from Sample E which was prepared in accordance with the procedure for preparing Sample D above except that the quenching water bath employed standard city water instead of deionized water. The results obtained were as follows: Leachable Chloride Ions Leachable Sulfate Ions Sample D Pellets 10 20 Sample E Pellets 50 100 As can be seen from the Examples, a polycarbonate composition employing water in the formulation during compounding, then devolatilizing the water which is probably in the form of water vapor or steam and then cooling the extruded strands in a water bath having a low ion concentration reduces the ionic impurities in the polycarbonate formulations particularly Sulfate and Chloride ions.

Claims: A process for preparing a polycarbonate composition having reduced ionic impurities, said process comprising melt blending a polycarbonate resin with sufficient aqueous medium to reduce the concentration of ionic impurities the majority of which is comprised of Sulfate and Chloride ions to less than about 300 parts per billion of each as determined by Ion Chromatography, devolatilizing the water with ionic impurities from the composition and then extruding the polycarbonate composition into an aqueous medium quenching bath which bath has a low concentration of ionic impurities wherein the sulfate and chloride ion content of the quenching bath is each no greater than 300 ppb. The process of Claim 1 wherein the melt blending and devolatilization is carried out in an extruder under conditions of temperature and pressure and which extruder has a vent and a die whereby the aqueous medium containing the ionic impurities exits the extruder through the vent in the extruder prior to the polycarbonate composition exiting the die of the extruder. The process of claim 1 or claim 2 wherein the devolatilization is performed under a vacuum. The process of any preceding claim wherein the amount of aqueous medium added is 0.25 to 2.0 weight % based on the weight of the polycarbonate composition. The process of claim 4 wherein the aqueous medium added is 0.75 to 1.5 weight %. The process of any preceding claim wherein the aqueous medium is water. The process of any preceding claim wherein the sulfate and chloride ion content of the aqueous medium quenching bath is each less than 100 ppb. The process of any preceding claim wherein the sulfate and chloride ion content of the aqueous medium quenching bath is each no greater than 100 ppb. The process of any preceding claim wherein the sulfate and chloride ion content of the aqueous medium quenching bath is each less than 50 ppb. The process of any preceding claim wherein the aqueous medium quenching bath is deionized water. The process of any preceding claim wherein the polycarbonate composition is extruded or molded into an article. The process of claim 11 wherein the article is a computer hard drive platter or a pre-chip silicon wafer carrier.

Inventors: BRADLEY DAVID A, GOHR ERIC THOMAS, HAMILTON DOUGLAS GEORGE, LINDWAY MARTIN J, SKABARDONIS JOHN G, BRADLEY, DAVID A., GOHR, ERIC THOMAS, HAMILTON, DOUGLAS GEORGE, LINDWAY, MARTIN J., SKABARDONIS, JOHN G.
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1998
Main classification: C08G64
IPC codes: C08G64, C08L69