MXPA98008470A - Method for concentrating aqueous fluoropolymer dispersion - Google Patents

Abstract

A method for concentrating an aqueous dispersion of a fluoropolymer including one which is capable of easily forming fibers, such as PTFE, with a low energy consumption at a low cost in a short time merely by using a small amount of a surfactant selected from among various kinds thereof. An aqueous fluoropolymer dispersion containing 2.0 to 8.0%by weight, based on the solid content of the fluoropolymer, of a surfactant is fed to a microfiltration film having a pore size of 0.01 to 1&mgr;m through feeding means substantially free from shearing force to remove the aqueous medium containing the surfactant from the aqueous dispersion.

Description

METHOD FOR CONCENTRATING AQUEOUS DISPERSION OF POLYMER CONTAINING FLUOR TECHNICAL FIELD The present invention relates to a method for concentrating the aqueous dispersion of fluorine-containing polymer such as polytetra-loro-pyrethylene (PEFE) by micro-il-rac membrane.

TECHNICAL BACKGROUND An emulsion of PTFE emulsion is prepared by polymerizing emulsifying a monomer tetraluoroelen in the presence of an aqueous polymerization initiator and an emulsifying agent containing phosphorus (for example, US Patent 2,559,752). . Fluorine-containing polymers "in addition to PTFE" are also prepared almost by the same procedure "and a solid content thereof is usually from to 45% by weight. In the case of industrial uses such as coatings and binders for batteries, an aqueous dispersion having a high concentration (for example, almost 60% by weight) is demanded. However, it is difficult to obtain a high concentration simply by adding an additional amount of fluorine-containing polymer, and such a high concentration is usually achieved by a concentration method.

For example »the Patent of E.U.A. No. 2,478,228 describes a method for concentrating the aqueous dispersion of fluorine-containing polymer using an anionic surfactant. However, this method has not been practically adopted because there is a problem because by making the surfactant insoluble by adding a large amount of electrolyte, a part of PTFE particles causes avoidable irreversible coagulation. Also a method for using nonionic surfactant described in US Patent 3 037,953 can give an aqueous dispersion having a relatively high concentration without causing coagulation of PTFE particles and has been marketed. However the shortcomings of this method are that because a large amount of specific surfactant having a limited scale of cloud point (commonly 20 ° to 80 ° C) is used and after the concentration the agent surfactant (the hydrophilic portion) that has a deviated distribution of molecular weight should be seen as a floating liquid »a high cost in material is necessary in raw materials» and because a large consumption of heat energy and long-lasting process steps they are required »the effectiveness of the procedure is not good. In addition in a concentration method by evaporation detailed in 1 to Patent of E.U.A. 3 »31S» 201 »because the resulting concentrated dispersion contains an aqueous polymerization initiator and an emulsifying agent containing fluorine at high concentration which is used in a polymerization reaction, there is a defect in that the viscosity of the Aqueous dispersion is significantly changed with its temperature. Further »in the case of an aqueous dispersion having low dispersion stability such as an aqueous dispersion of PTFE» the primary particles readily coagulate during the concentration step to break the emulsion state. In an electrical decanting method described in GB 642,025 »because the resulting coagulated particles adhere to an electrode to obstruct the passage of electric current, the concentration effect is very low with a large amount of electrical power consumption. Therefore »that method is not suitable for practical use. On the contrary, a membrane separation method using a tracer membrane (UF membrane) having a pore size of up to 0.5 times the minimum particle size of fluorine-containing polymer particles (JP-B) is proposed. -2-34971 »US 436» 92S) This method of membrane separation using UF membrane has been used for filtration of high molecular weight materials »and also in the concentration of an aqueous dispersion of fluorine-containing polymer, has advantages as low energy consumption and low cost »use of various surfactants and a reduced amount of surfactant However, in the membrane separation method using UF membrane there are problems that must be solved such as: Removable molecular weight of UF membrane is as small as almost 1, 000 to hundreds of thousands (no more than 0.01 μm in pore size), the desired filtration performance can not be obtained unless the filtration pressure is adjusted to almost 1 MPa. Furthermore, the filtration rate is low and when the liquid flow rate increases, the UF membrane is clogged with the fluorine-containing polymer particles, and 2, because an aqueous dispersion must be forcedly fed to the UF membrane and a fluorine-containing polymer »in particular the PRFE is f? bilated by the necessary mechanical shear force» in the case of using a pump that has mechanically movable mechanism, the polymer is fibrillated by a shearing force generated in the moving parts or senate parts of the polymer. the pump, which causes the problem that the passage of the dispersion and the UF membrane are clogged with the resulting f-ibrilated product. According to the technique detailed in JP-B-2-34971 »the problem mentioned in number 2 is intended to be solved by using a pump which keeps the fluorine-containing polymer particles out of mechanical parts that cause friction eg a pump peristaltically or preferably a centrifugal pump. However, even with the use of a centrifugal pump which is considered preferable in the aforementioned patent publication, a shear force is generated in rotating parts and the technique does not give a substantial solution. An object of the present invention is to provide a method for concentration which makes improvements on the problems mentioned above in number 1 such as high filtration pressure, low filtration rate and long filtration time and can solve the problem with the fibrillation of the No. 2 mentioned above while maintaining the advantages of the membrane separation method UF »that type of surfactants is not limited and an amount of the surfactant can be decreased, improving and further exceeding its low energy consumption and its low cost.

BRIEF DESCRIPTION OF THE INVENTION Typically, the present invention relates to a method for concentrating an aqueous dispersion of fluorine-containing polymer particles by feeding the aqueous dispersion of fluorine-containing polymer particles and containing a surfactant to a micro-lining membrane having a size from O.O.sub.l to 1 .mu.m, preferably 0.05 to 0.5 .mu.m, with aqueous dispersion feed means which do not substantially generate the shear and remove an aqueous medium containing the surfactant from the aqueous dispersion mentioned above by of the microfiltration membrane. In the present invention, the pore size means a minimum particle size of the polymer particle that does not pass through the membrane. The pore size of the microfiltration membrane is preferably selected on the scale of 0.2 to 1.5 times the average particle size of the polymer particles. It is preferable that the aqueous dispersion of the fluorine-containing polymer particles be flowed in parallel with a surface of the microfiltration membrane by the so-called flow filtration tangential method.

(TFF). With respect to the aqueous dispersion feed means, it is preferable to generate a force to feed the aqueous dispersion by applying a static pressure thereto and it is particularly preferable to generate the force for feeding by placing the aqueous dispersion in an air-tight sealed container and Pressurizing the dispersion with clean compressed air or inert gas. In the case where the fluorine-containing polymer is PTFE »The pore size of the micro-filtration membrane is preferably 0.1 to 0.3 μm. In addition, the present invention relates to a method for concentrating an aqueous dispersion of fluorine-containing polymer particles containing a surfactant agent consisting of. in a first closed container and a second closed container communicating with one another through a microfiltration membrane »alternately repeat the following steps until a desired concentration of the aqueous dispersion is obtained; a step for feeding the aqueous dispersion of fluorine-containing polymer particles containing the surfactant from the first sealed container to the second closed container with aqueous dispersion feed means that do not generate substantially shear and a step to feed the aqueous dispersion from the second container closed to the first closed container with aqueous dispersion feed means.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic flow diagram for explaining the concentration method of the present invention. Figure 2 is a flow diagram of a practical concentration system for carrying out the concentration method of the present invention. Figure 3 is a flow chart of a practical concentration system for carrying out the comparative example of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION The characteristics of the concentration method of the present invention are: that the membrane separation is preferably carried out by the TFF method with a microfiltration membrane having a pore size of 0.2 to 1.5 times »preferably 0.6 to 1.0 times the size average particle of polymer particles; and as the means for feeding the aforementioned aqueous dispersion of fluorine-containing polymer particles, means are used which do not substantially generate a shear stress. For example, a means is adopted for placing the aqueous dispersion in a closed container and then applying pressure. to the dispersion with compressed inert gas. A basic concept of one embodiment of a system that performs the aforementioned concentration method of the present invention it is explained in accordance with the schematic flow diagram of FIG. 1. In FIG. 1, the numbers 1 and 2 represent the first and second closed containers respectively to store the aqueous dispersion of the fluorine-containing polymer before the concentration. And the containers are communicated with each other through a pipe 5 and a filter apparatus 4 having a micro-filtration membrane 3. Both upper parts of the first closed container and upper part 2a of the second closed container 2 are connected to a air pump G. On the microfuge apparatus 4 is provided a pipe 7 for carrying out a separate aqueous medium B containing a surfactant agent. The separated aqueous medium B is placed in the container 8. In addition on the path of the pipe 5 a pipe 10 is provided for removing the concentrated aqueous dispersion of the fluorine-containing polymer through a valve 9. The concentrated aqueous dispersion is preserved in a container 11. In the concentration operation »first an aqueous dispersion A to be concentrated is introduced into the first closed container 1 (or it can be introduced into the second closed container 2), and clean compressed air is supplied inside the upper part of the first closed container 1 with the air pump S. At that time »the upper part 2a of the second closed container 2 is left open to the air through a reducing valve ( not shown) and the valve 9 for removing the concentrated aqueous dispersion is closed. The aqueous dispersion A to be concentrated in the first closed vessel 1 is fed to the microfiltration membrane 3 in the micro filtering apparatus 4 with the compressed air "and there the aqueous medium B containing the surfactant is separated and passes through the the pipe 7 to be conserved in the container 8. The concentrated aqueous dispersion which has passed through the microfuge apparatus 4 »when concentrated to a desired concentration by a filtration step» is carried within the container 11 through the pipe IO opening the valve 9. In the case where the concentration of the concentrated aqueous dispersion at the outlet of the micro filtering apparatus 4 is not "desired" the valve 9 is left closed and the aqueous dispersion is fed to the second container closed 2. When almost all the aqueous dispersion A to be concentrated in the first closed container 1 was transferred to the second closed container 2 »the compressed air filled in the first closed container 1 is released, and the interior of the first closed container l is restored to atmospheric pressure and at the same time, compressed air is supplied to the upper part 2a of the closed second container 2 with the air pump 6. Thus the interior from the second closed vessel 2 is pressurized with the compressed air to return the aqueous dispersion A to the micro filtering apparatus 4 to carry out the concentration by filtration. These operations are repeated until the desired concentration is obtained and finally the valve 9 is opened to remove the concentrated dispersion of fluorine-containing polymer within the container 11 through the pipe 10. Preferred operating conditions for the filtration method n of the present invention are as follows. (1) Temperature from 20 ° to 40 ° C »normally from 20 ° to 25 ° C. The filtration can be carried out at room temperature, and coloring does not occur by heating or decreasing the dispersion stability of the aqueous dispersion. Also the energy consumption can be decreased. (2) Method for feeding the aqueous dispersion to the microfiltration membrane. As a method for the aqueous dispersion to the microfiltration membrane, there is a neutral filtration method to feed the aqueous dispersion vertically to the membrane and a tangential flow filtration method (TFF) to feed the aqueous dispersion in parallel with the membrane. In the present invention "although both methods can be used" the TFF is preferred from the point that the pore clogging of the membrane hardly occurs and the feed of the aqueous dispersion can be carried out at relatively low pressure. (3) Filtration pressure The filtration pressure is represented by an average pressure of a membrane pressure tap (hereinafter also referred to as "Pin") and a pressure output of the membrane (hereinafter also referred to as "Pin"). referred to as "Pout"). The average filtration pressure is, in the case of tangential flow filtration (TFF), from 0.01 MPa to 0.3 MPa, preferably from 0.1 MPa to 0.2 MPa. When the average filtration pressure is too high »the coagulation of the particles tends to occur easily» and when it is too low »the efficiency of the filtration tends to be markedly diminished. The filtration pressure is an important element to determine not only the concentration velocity, but also the feed rate of the aqueous dispersion to the filtration membrane. Normally in the method that uses the circulation pump, because the performance of the pump is limited, the filtration pressure is restricted by the circulation flow rate of the aqueous dispersion (in other words, when the circulation flow velocity is increased »the filtration pressure must be set at a low pressure). In the present system "because the feed flow rate of the aqueous dispersion is set by the differential pressure of the closed containers, the filtration pressure is not affected by the feed flow rate of the aqueous dispersion and can be established freely. (4) Means for feeding the aqueous dispersion. Usable means which do not substantially give a shear stress to the aqueous dispersion, for example, means for applying a pressure to the dispersion with clean air or inert gas. Means for feeding the aqueous dispersion having movable parts such as pumps can not be used because the movable parts generate frictional force to produce more or less shear force. Examples of gas erte are nitrogen gas »etc. From the point of view of cost »clean air is preferable.In the case where the pressure is applied with the compressed air, a pressure to be applied can be determined in such a way that the filtration pressure becomes an average pressure of the inlet pressure and the outlet pressure mentioned above, of the membrane. (5) Flow rate of the aqueous dispersion to the microfiltration membrane. The flow rate of the aqueous dispersion varies depending on the type and pore size of the microfiltration membrane and the method of feeding the aqueous dispersion. It is usually preferred to establish a linear velocity at O.5 to 7 m / sec »more preferably at 3 m / sec in the tangential flow filtration (TFF). The flow rate of the aqueous dispersion is established by a differential pressure of the feed container of the aqueous dispersion and the recipient of the receiving side. (6) Pore size In the micro-membrane used in the present invention, it is advantageous to make the pore size smaller from the point where it is possible to filter the dispersion containing particles having a smaller particle size. . However, if the pore size is too low, it is disadvantageous from the point that the filtration pressure becomes higher and the filtration rate is decreased. On the other hand, when the size is larger, it is advantageous from the point that the pressure is low and the filtration time is short. However, when the size of per is too large, there is a problem that fluorine-containing polymer particles having a small particle size pass through the membrane. Therefore in principle the size of per is 0.2 to 1.5 times, preferably from O.S to 1.0 times the average particle size of the fluorine-containing polymer particles. The aqueous dispersion to be concentrated by the method of the present invention is an aqueous dispersion containing fluorine-containing polymer particles. The fluorine-containing polymer can be one that forms a stable aqueous dispersion by the addition of a surfactant. Examples of fluorine-containing polymer are fresh fluorine-containing such as PTFE (including a modified PTFE containing O.OOl at 1.0% by weight of other monomers such as hexafluoropropene, chlorotrifluoroethylene, perfluoroalkyl ether, tri-1-uoroeti). firewood »perfluoroalkal le oo ether per luoroal cox inil co» Low molecular weight PTFE, tetraf1 uoroeti le o-ether perfluoroalk 1 vinyl ico copolymer (PFA) »tetra luoroeti leno-hexafluoropropyl wood copolymer (FEP)» pol ivi no 1 edeno fluoride CPVdF), ethylene-chlorotri luoroethylene copolymer (ECTFE) and polychlorotri luoroethene (PCTFE); fluorine-containing elastomers such as vi-fluoride elastomer or 1-hexafluoropropyl elastomer »vinyl ideide-tetrafluoroethylene-hexafluoroethane elastomer elastomer» vinylidene fluoride chlorotrifluoroethylene elastomer, tetra-chloro-lime elastomer elastomer, tetrafluoroethylene elastomer 1 in-propylene »elastomer hexafluoropropy len-eti le, elastomer fluoroalkyl vinyl ico-olefin elastomer, and g luorofosfazona; and the similar ones. Among them, the concentration of PTFE (including a modified PTFE) was particularly difficult because PTFE is easy to filter. As mentioned above, these fluorine-containing polymers are obtained as an emulsion polymerization product having an average particle size of about 0.01 μm to 0.5 μm. Example of the aqueous medium for the aqueous dispersion is water which may contain various water-soluble organic solvents such as ethylene glycol and toluene. The aqueous dispersion of fluorine-containing polymer to be concentrated according to the method of the present invention contains a surfactant in an amount of 2.0 to 8.0% by weight »preferably 4.0 to 7.0% by weight based on the polymer co or mentioned above . This amount of the surfactant is a markedly decreased amount because in the aforementioned method of darkening point a large amount of surfactant (almost 10 to 13% by weight) must be used. The type of surfactant is not particularly limited. As long as the dispersion stability is obtained »any anionic» cationic »nonionic and ampholytic surfactant can be used, and various types of the aqueous dispersion of fluorine-containing polymer can be prepared. For example, in the case of a nonionic surfactant "as mentioned above" because the method of the present invention is not affected by an obscuration point of the surfactant. A selected compound within a broad molecular weight scale can be used. When a compound having a low molecular weight is used "in the coating or impregnation process" it is possible to adopt a relatively low drying temperature and thermally decompose the surfactant for a very short time at a high temperature and it is also possible to avoid that a coated article is colored due to the decomposition products of the surfactant. On the other hand »when a surfactant having a slightly high molecular weight and a high darkening point even at high temperatures in summer» the storage stability of products can be improved. When an anionic surfactant is used it is possible to provide an aqueous dispersion of PTFE used as a coating material for the Mash method (method for obtaining a paste containing water by coagulating an aqueous dispersion of PTFE with a coagulating agent) which is used to produce a bearing without oil.

The cationic and ampholytic surfactants offer the same effects that it is possible to lower a processing temperature of the aqueous dispersion of fluorine-containing polymer and to prevent a coated article from coloring due to the decomposition products of the surfactant. Examples of each surfactant are as follows. (Anionic surfactant) Higher fatty acid salts such as mixed fatty acid soda soap »oleic acid potash soap» castor oil potash soap, tallow fatty acid soda soap semi-hardener »And semi-hardening beef tallow fatty acid potash soap; salts of alkyl sulfates such as sodium lauryl sulfate, sodium higher alcohol sulfate, lauryl sulfate triethanolamine, and ammonium lauryl sulfate; alkylbenzenesulfonate salts such as sodium dodecyl benzene sulfonate; alkyl naphthalene sulfonate salts such as sodium naphthalene sodium hydrophonate; dialkyl sulfosucinate salts such as sodium dialkyl sulfosucinate; Alkali metal salts such as sodium alkyldisulfonate; salts of alkyl phosphates such as diethanolamine alkyl phosphate and potassium alkylsulfate; condensation products naphthalenesulonate and formaldehyde such as sodium salts of G-na talensul onate and formaldehyde condensation product; condensation product of aromatic sulfonate and formaldehyde such as sodium salt of aromatic sulfonate and formaldehyde condensation product; and salts of polyethylene glycol 1 and alkylater sulfates such as polyoxyoxyethylene lauryl ether sulphate, sodium polyoxyethylene alkali ether sulphate and polyoxyethylene glycol sodium sulfate.

((Non-ionic surfactant) The polyoxyethylene ethenoalkyl ethers (the ethylene oxide portion can be partially replaced by the propylene oxide portion) such as polyoxyethylene lauryl ether, polyoxyethylene cetyl ether, polyoxyethylene stearyl ether, ether polyoxyethylene oleic acid and polyoxyethylene higher alcohol ether; polyoxyethylene alkyl ethenyl ethers (a part of the ethylene oxide portion may be block portion with propihenoxide portion) such as phenylethyl ether of polyethylene oxide and nonyl enyl ether polyoxyethylene; oxy-oxypropylene wood block polymer; sorbitan fatty acid esters such as sorbitan laurate »sorbitan palmitate» sorbitan stearate and sorbitan oleate; sorbitan polyoxyethylene fatty acid esters such as lauryl ester »palmityl ester» ester stearyl and polyoxyetholenorbitan oleyl ester »mcnogl céridos de fatty acids such as stearic acid and oleic acid; polyethylene glycol fatty acid esters such as lauryl ester, stearyl ester and polyethylene glycol oleyl ester; and polyoxyethylene sheets such as polyoxyethylene sheet and polyoxyethylene sheet; and derivatives thereof.

(Cationic surfactant) Alkylamines salts such as cocoamine acetate, stearylamine acetate, cocoamine hydrochloride, tinari hydrochloride laminate and stear oleate; and quaternary ammonium salts such as lauryl chloride, 1-trimethylammonium chloride, stearyl trimethylammonium chloride, disteary chloride Idimeti ammonium chloride and alkyl 1-benzimimethylammonium chloride.

(Ampholytic surfactant) Alkis 1 etains such as lauri 1 etaine, cocobetaine and steari 1 betaine; amine oxide such as lauryl-dimethylamine oxide; and imidazole im'obetaine such as imidazole i ni olaur Icarboximeti 1 idroxieti 1 betaine. With respect to the microfiltration membrane used in the microfilled apparatus, a known or commercially available micro-iltration membrane can be used as such as long as it retains the primary particles of the fluorine-containing polymer without passing them. Restricted from the membrane material are »for example» synthetic resins such as nylon »polypropylene» polyether sulfone »resin containing fluorine and polyester; resins are synthetic such as cellulose acetate; ceramics such as fiberglass »porous glass and high purity aluminum; and the similar ones. Among them a membrane made of ceramics is preferred to prevent the membrane from becoming clogged because the resulting accumulated particles of fluorine-containing polymer adhered to the membrane can be controlled at high pressure and it is also more advantageous from the standpoint of mechanical strength and it lasts i 1 idad. The filtration membrane can be in any form such as a film »(flat membrane,) tubular form (in the form of a hose) and tubularly shaped. The tubular shape (in the form of a hose) and the multitubular shape are advantageous because the area of membrane available to pass the liquid can be made wider. A module is constructed to install several of these units (elements) in a nozzle »and one or a plurality of the modules are used. It is particularly efficient to connect a plurality of modules in parallel or in series with respect to the circulation system. Among the fluorine-containing polymers »PTFE is easily fibrillated when a shear force is applied. Thus PTFE filtration was very difficult and the tube concentration to be carried out by conventional methods such as the darkening point method. In consideration of such characteristics of PTFE, the foregoing mentioned JP-B-2-34971 proposes that "a pump that can maintain PTFE in the state of not being in contact with parts generating friction force" is used. However in fact such a pump does not exist »and when a dispersion is passed through moving parts such as mechanically rotating parts or sealing parts» the cutting force inevitably appears in the dispersion and fibrous PTFE particles are produced. When an aqueous dispersion containing such fibrous PTFE particles is passed through a microfiltration membrane having a relatively large pore size, the fibrous PTFE particles accumulate on the membrane to clog the membrane. Therefore »in the present invention. in the particular case where the fluorine-containing polymer is PTFE (including modified PTFE), it is desirable to use the aqueous dispersion feed means which are not provided with a movable part and a sealable part and which therefore do not substantially apply a cutting force to PTFE. The aforementioned means are preferably used as the aqueous dispersion feed means which do not substantially apply a cutting force to the dispersion. Then a more preferred mode of apparatus for achieving the concentration method of the present invention is explained according to the schematic flow chart of Figure 2. The closed container mentioned below is an air-tight enclosed container that is not provided. with nothing except pipes and flanges that are not open in normal state. A microfuge apparatus 31 containing a microfiltration membrane (s) 30 made of ceramic is located between a closed first container 20 and a second closed container 21 and is connected to the containers 20 and 21 with pipes to the dispersion feeders 51, 52 and 53. The containers can store an aqueous dispersion of fluorine-containing polymer to be concentrated and have the same volumetric size. As illustrated, both sides 32 and 33 of the icrofiltration ceramic membrane 30 are connected to the dispersion feed pipes 51 and 52, respectively, so as not to cause leakage of the dispersion. Both sides 32 and 33 are communicated with one another through one or more holes that pass through (not shown) in the form of straight tubes provided inside the ceramic membrane 3? of micro iltration. The holes that cross are communicated with the outside of the ceramic membrane 30 of microfiltration through micropores and also to the outlet 34 of the micro-traction apparatus 31. The outlet 34 and the closed type container 40 for backwash are communicated with each other through a supply pipe of. In addition, the backwash container 40 is connected to an open container 22 through a dispersion feed pipe 55. On the pipe 55 there is provided a selenoid type on and off valve 41 for the dispersion which is open when it is electrically switched off A dispersion feed pipe 56 provided with a valve 42 that is normally closed is connected to a concentrated dispersion receiver container 23 from a union of the pipes 52 and 53. The dispersion feed pipe 56 may be provided on the waste side. the pipe 51. The valve 42 is opened after completing the concentration. Then the elements to apply an air pressure are explained. As illustrated »the compressed air to be supplied from which the contaminants were removed to a necessary degree is supplied from a source of compressed air 43 to the first closed vessel 20 and to the second closed vessel 21 through a pressure regulating valve 44 to apply pressure, a pressure regulating valve 45 for residual pressure and a solenoid valve 46 for changing direction. A backwash air that is branched from the compressed air source 43 is supplied to the backwash container 40 through a pressure regulating valve 47 and an on / off switch valve 48 that is closed when the power is turned off. . Then the operations of the concentration system are explained immediately. In the initial state, an aqueous dispersion is not in all the containers 20, 21, 22, 23 and 40 and the tracer apparatus 31. A virgin dispersion to be concentrated is fed into a storage container and the container It is sealed. In the following explanation »the virgin dispersion is placed in the first closed container 20 and the container is sealed, but alternatively the virgin dispersion can be put in the second closed container 21. In that state, all the pressure regulation valves have been established at given pressures. For example, it is desirable that the pressure regulating valves 44, 45 and 47 are set at almost 0.17 MPa, almost 0.13 MPa and almost O.4 MPa, respectively. When the direction change solenoid valve 46 is pressed as to supply air within the closed container 20, a pressure inside this container becomes greater than that of the second closed container 21. Therefore the dispersion in the first closed container 20 it moves to the second closed container 21. The air in the second closed container 21 is released into the atmosphere through the pressure regulating valve 45 to adjust the internal pressure. While the dispersion moves from the first closed container 20 to the second closed container 21, a pressure greater than the atmospheric pressure is applied to the interior of the microfiltration membrane 30 'and thus only a filtrate containing a surfactant that passed through the micropores it flows into the backwash container 40. When the container 40 is filled with the filtrate, the filtrate flows into the open container 22 through the filtrate feed pipe 55. An amount of the dispersion flowing into the second closed container 21 decreases by an amount that has passed through. of the micropores, so the aqueous dispersion is concentrated. When the residual amount of the dispersion in the first closed vessel 20 reaches a given value, it is detected with a level meter and a flotation sensor. When the given quantity is detected »the direction change solenoid valve 46 is switched to the opposite direction. Because the pressure in the second closed vessel 21 becomes higher, the aqueous die flow flows into the first closed vessel 20 from the second closed vessel 21. Also in this case »in the same way as explained above» the flow within of the backwash container 40 occurs. When the residual amount of the dispersion in the second closed vessel 21 reaches a given value, the direction change solenoid valve 46 is switched to the opposite direction. It is desirable that the commutation be repeated until a quantity of the liquid containing a surfactant stored in the open container 22 reaches a given value. When the desired concentration was carried out (say when the amount of liquid in the container 22 reaches a given value), the valve 42 is opened to remove the concentrated aqueous dispersion of fluorine-containing polymer within the container 23. It is explained below the method of backwash of the membrane of m? ltration. In the normal state where the backwash operation is not performed the solenoid valve 41 for switching on and off is opened and the solenoid valve 48 for switching on and off is in the closed position. In order to carry out the backwash »these valves are operated almost at the same time, let's say the solenoid valve 41 of change is closed and the solenoid valve 48 of change is open. Because the pressure regulating valve 47 for backwash is set at a higher pressure than the other pressure regulating valves, an air flows into the closed backwash container 40. and with this air pressure, the liquid containing a surfactant in the closed container 40 flows back through the prícroporas of the microfiltration membrane 30, then washing the inside of the membrane 30. It is desirable that these selenoid valves are actuated only for almost one second necessary for the backwash and that independently of the switching of the solenoid valve 46 previously mentioned changeover »the actuation of the valves for reversal is carried out at a given interval of 10 seconds. at 10 times the time period taken for the backwash.

EXAMPLES The concentration method of the present invention is; explained then on the basis of examples and comparative examples »but the present invention is not limited to those examples.

EXAMPLE 1 Tetrafluoroethylene monomer was stirred in a water-soluble polymerization initiator and an aqueous dispersion of a fluoro-containing emulsifying agent to be polymerized under pressure, thus giving an aqueous dispersion of PTFE. This aqueous dispersion of the emulsified polymer contained almost 30% by weight of PTFE (average particle size: nearly 0"28 μm, standard specific gravity: 2189) based on liquid weight. Almost 7% by weight of alkyl oxyethylene ether of polyoxyethylene (trade name: Triton X-100 available from Union Carbide CO. Ltd.) were added to the aqueous dispersion based on a weight of polymer solid content and pH it was adjusted to be 9.5 with aqueous ammonia. Standard specific gravity (SSG) was measured by water displacement method (specific gravity measurement method) using a sample formed according to ASTM D4B95-89. Almost 20 liters (24 Kg) of the aqueous dispersion were concentrated with the concentration system shown in Figure 2. As the micro-illation membrane -30. a micro-layer membrane module made of ceramic and having a pore size of 0.2 μm (pore size / average particle size = to 0.71) (trade name: Ceraflo MSDWO402O available from Mikuni Kikai Co., Ltd.) was used so that the tangential flow filtration was carried out. The aqueous dispersion was placed in a closed container 20 and transferred to another container with the compressed air in such a way that the average filtration pressure becomes almost 0.15 MPa (Pin = 0.17 MPa, Pout = 0.13 MPa). In this case, the aqueous dispersion of PTFE was filtered through the microfiltration membrane 30 and almost 2.2% by weight of the filtrate containing a surfactant was obtained. After transferring to another container »the pressurization of the empty container 20 was stopped and the internal pressure was released. Then a pressure inside the container 21 containing the aqueous dispersion of PTFE was increased with the compressed air and the aqueous dispersion was fluid to the container 20 through the micro-iltration membrane 30. This operation was repeated until the calculated PTFE concentration with a quantity of the filtrate reaches almost 60% by weight »and thus a concentrated dispersion was obtained. The concentrated dispersion obtained did not contain coagulated and fibrous products. The microfiltration membrane 30 was not clogged since the backwash was carried out for 0.5 seconds at a 1 minute interval by using the filtrate and compressed air at almost 0..4 MPa. An average flow rate of the aqueous dispersion of PTFE to the microfiltration membrane was almost IO liters / inuto (linear velocity: 1.1 m / sec), and the period of time taken for the concentration was almost 5.5 hours. The average particle size of the polymer particles of the present invention was obtained by measuring particle sizes of 100 particles using a scanning electron microscope available from Hitachi, Limited and then taking an average of them.

EXAMPLE 2 The microfiltration was carried out with almost 20 liters (24 kg) of an aqueous PTFE dispersion in the same way as in Example 1 except that the two micro-layered membrane modules made of ceramic were connected in parallel to give a dispersion. concentrated having a PTFE concentration of almost 60% by weight. Coagulated and fibrillated products were not seen in the concentrated dispersion. An average flow rate of the aqueous dispersion of PTFE to the microfiltration membrane was almost 20 liters / minute (linear speed: 1.1 m / sec) »and the period taken for the concentration was almost 2 hours and 40 minutes.

EXAMPLE 3 The microfiltration was carried out with almost 20 liters (24 kg) of an aqueous dispersion of PTFE in the same way as example l except that the average filtration pressure was adjusted to almost 0.05 MPa (Pin = Q.OB MPa » Pout = 0.02 MPa) and the average flow velocity of the aqueous dispersion of _ PTFE to the micro-ionic membrane was adjusted to almost 20 liters / minute (linear velocity: 2.2 m / sec) to give a concentrated dispersion having a PTFE concentration of almost 60% by weight No coagulated or fibrous products were seen in the concentrated die dispersion The period taken for the concentration was almost 5 hours.

EXAMPLE 4 The microfiltration was carried out with almost 20 liters (24 kg) of an aqueous dispersion of PTFE in the same manner as in Example 1 except that the average filtration pressure was adjusted to almost O.IO MPa (Pin = O. 12 MPa, Pout = 0"OB MPa) and the average flow velocity of the aqueous dispersion of PTFE to the microfiltration membrane was adjusted to almost 15 1 itroe / minute (linear velocity: 1.7 m / sec) to give a dispersion concentrate having a PTFE concentration of almost 60% by weight. IMo clotted and fibrillated products were seen in the concentrated dispersion. The period taken for the concentration was almost 5 hours.

EXAMPLE 5 The microfiltration was carried out with almost 20 liters (24 kg) of an aqueous dispersion of PTFE in the same manner as in Example 1 except that the average filtration pressure was adjusted to almost 0.20 MPa (Pin = O.21 MPa »Pout = 0.19 MPa) and the average flow velocity of the aqueous dispersion of PTFE to the microfiltration membrane was adjusted to almost 5 liters / minute (linear velocity O.5 m / sec) to give a concentrated dispersion having a concentration of PTFE of almost SO% by weight. Coagulated and fibrillated products were not seen in the concentrated dispersion. The period taken for the concentration was almost 7 hours.

COMPARATIVE EXAMPLE 1 The concentration of 600 g of an aqueous dispersion of PTFE was carried out in the same manner as in Example 1 except that instead of 1 to a microfiltration membrane 30 'an ultrafiltration membrane (nominal molecular weight separable: 300,000, PTMK0MS10 made from polyether sulfone and available from Nippon Millipore Co., Ltd.) was used (pore size / average particle size = 2.5 x 10-z). The average filtration pressure was adjusted to almost O.5 MPa (Pin = 0.6 MPa, Pout = 0.4 MPa) and the average flow velocity of the aqueous dispersion of PTFE to the ultrafiltration membrane was adjusted to almost 0.3 ltr / minute . The operation was carried out for almost 1 hour. Over time, the filtration membrane was clogged due to accumulated PTFE particles and filtration became impossible. Therefore, the concentration was stopped. In the aqueous dispersion of PTFE (virgin dispersion), the coagulated products were recognized. A concentrated dispersion was obtained in an amount of only about 40 grams "and concentration has hardly been achieved.

COMPARATIVE EXAMPLE 2 The concentration was carried out with the ultra filtering membrane in the same way as in Comparative Example 1 except that the average filtration pressure was decreased to almost 0.15 MPa. As a result »almost no concentrated dispersion was obtained, and almost no concentration has been achieved.

The following comparative example 3 was carried out using a concentration system shown in Figure 3. In the concentration system shown in Figure 3 a virgin dispersion tank 60 for storing an aqueous dispersion of fluorine-containing polymer particles to be concentrated is communicated with a membrane module 63 through a circulation pump 61 »a pressure gauge 62 for measuring the inlet pressure of the membrane and a dispersion feeder pipe 70. The membrane module is connected to the pipes of the membrane. dispersion feed 70 and 71 so as not to cause fugae of the dispersion as the microfiltration ceramic membrane 30 in FIG. 2. In addition, the membrane module 63 is connected to the virgin dispersion tank 60 through a pressure gauge 64 to measure the membrane outlet pressure. a valve 65 and a dispersion feed pipe 71. In addition, a dispersion feed pipe 72 provided with a valve 66 that is normally closed is connected to a vessel 67 to receive a concentrated dispersion obtained by filtration with the membrane module 63. In the concentrating operation, first an aqueous dispersion A to be concentrated is placed in the virgin dispersion tank SO and the circulation pump 61 is operated to feed the aqueous dispersion A to be concentrated to the membrane module 63. The aqueous dispersion was withdrawn out of the container 67 by opening the valve 66. A portion of the dispersion that had not been concentrated in the Membrane module 63 is returned to tank 60 through the dispersion feed pipe 71.

COMPARATIVE EXAMPLE 3 In the concentrating system shown in figure 3. the concentration was carried out with almost 20 liters (24 kg) of the aqueous dispersion of PTFE that was prepared in example 1 using the microfiltration ceramic membrane module. (pore size: 0.2 μm) (pore size / average particle size = 0.71) of Example 1 as the membrane module 63 and a centrifugal pump (available from Mikuni Kikai Co., Ltd.) was used as the pump circulation 61. The circulation of the aqueous dispersion was carried out with the average filtration pressure of ca. 0.12 MPa (Pin = 0.15 MPa »Pout = 0.09 MPa) and the flow velocity of the aqueous dispersion PTFE circulation of almost 20 li ros / minute (linear speed: 2.2 m / sec). The PTFE began to be fibr side due to a shear force generated in moving parts and sealing parts of the pump »and the obstruction of the filtration membrane occurred due to the fibrillated PTFE. Thus the operation became impossible. In the concentrated dispersion obtained, fibrillated PTFE was observed.

COMPARATIVE EXAMPLE 4 In the concentration system shown in Figure 3, the concentration was carried out with almost 400 g of an aqueous dispersion of FEP (average particle size: almost 0.15 μ) using the trapping membrane (nominal molecular weight separable: 300,000) (pore size / average particle size = 4.7 x 10-2 of comparative example 1 co or membrane module S3 and a tubular pump (available from IMippon Millipore Co., Ltd.) was used as the circulation pump 61. The average filtration pressure was adjusted to almost 0, .5 MPa (Pin = 0.6 MPa »Pout = 0.4 MPa) and the average flow velocity of the circulating aqueous dispersion of FEP was adjusted to caei 0.27 1 i troe / minute The operation was carried out for almost an hour, with a lapse of time the filtration membrane was blocked due to an accumulated resin and the filtration became impossible, therefore the concentration was stopped. observed coagulated particles of FEP "and almost no concentration could be achieved. The concentrations of fluorine-containing polymer and surfactant in the concentrated and filtered dispersions (filtered aqueous medium containing the tectosin agent) obtained in Examples 1 to 5 and Comparative Examples 1 to 4 respectively were determined. The results are shown in table 1.

TABLE 1 Concentrated Dispersion Filtration Concentration Concentration Concentration of polymeric agent concentration Fluorine-containing agent polymer particle coagulated surfactant or surfactant-containing fluoride (by weight) (by weight) fibrillated (by weight) (by weight) PTFE Before concentration 30.1 8.2 None - - Ex. 1 63.2 4.0 None No of 0.01 2.2 Ex. 2 61.2 4.1 None Not of 0.01 2.1 Ex. 3 61.2 4.0 None Not of 0.01 2.2 Ex. 4 61.8 4.0 None No those of 0.01 2.1? or.

Ex. 5 60.8 4.0 None Not those of 0.01 2.3 Ex. Conp.1 The concentration was iposible Ex. Conp.2 The concentration was iiposible Ex. Coip.3 53.0 3.9 Discovered Not those of 0.01 2.2 FEP Before the concentration 30.1 6.0 None - - Ex. Coip.4 The concentration was iiposible According to the concentration method of the present invention, energy consumption and cost can be reduced. Various types of surfactants can be used and the amount of surfactant can be reduced. In addition, the concentration period can be shortened "and the method can be applied to a fluorine-containing polymer such as PTFE which is easily fibrillated.

Claims (5)

NOVELTY OF THE INVENTION CLAIMS

1. - A method for concentrating an aqueous dispersion of fluorine-containing polymer particles »consisting of feeding the aqueous dispersion of fluorine-containing particles containing a surfactant to a microfiltration membrane having a pore size of 0.01 to 1 μm with media of aqueous dispersion feed which do not generate substantial shear stress and »remove an aqueous medium containing the surfactant from the aqueous dispersion.

2. The method according to claim 1 further characterized in that the pore size of the membrane of m traction is 0.2 to 1.5 times the average particle size of the fluorine-containing polymer particles.

3. The method according to re-indication 1 or 2 further characterized in that said aqueous dispersion of fluorine-containing polymer particles is fluid in parallel with a surface of the micro-ltration membrane.

4. The method of conformance with any of the claims 1 to 3 »further characterized in that the pore size of the micro-membrane diaphragm membrane is 0.05 to 0.5 μm.

5. The method according to any of the re indications 1 to 4 further characterized in that said aqueous dispersion feed means provide a force to feed the aqueous dispersion by applying a static pressure thereto. & The method according to any of claims 1 to 5 »further characterized in that the said aqueous dispersion feed means are means for giving a force to feed the aqueous dispersion by placing the dispersion in an air-tight sealed container and then pressurizing with clean air or inert gas. 7. The method according to any of claims 1, 2 »3» 5 and 6 »further characterized in that said aqueous dispersion of fluorine-containing polymer particles is an aqueous dispersion of pol i tetraf1? Oroet 1 not and the size The pore size of the membrane of the membrane is 0.1 to 0.3 μm. B. The method according to any of claims 1. 2 »4, 5» 6 and 7 »further characterized in that said aqueous dispersion of fluorine-containing polymer particles is fluid in parallel with a surface of the micro-lining membrane at a linear speed of 0.5 to 7 m / sec. 9. A method for concentrating an aqueous dispersion of fluorine-containing polymer particles containing a surfactant comprising »a first closed container and a second closed container communicating with each other through a membrane of m Fig. 3, illustrating alternately the following steps until the desired concentration of the aqueous dispersion is obtained; a step for feeding the aqueous dispersion of fluorine-containing polymer particles containing surfactant from the first closed container to the second closed container with aqueous dispersion feed medium that does not generate substantially sharp cutting and a step for feeding the aqueous dispersion from the second container closed to the first container closed with the aqueous dispersion feeder means.