JPH0820611A - Method for concentrating aqueous emulsion of fluoropolymer - Google Patents

Abstract

PURPOSE:To concentrate an aqueous emulsion of a fluoropolymer with a small amount of a surfactant without aggregating polymer particles by transferring water from the aqueous emulsion of a fluoropolymer separated by ion exchange membranes to aqueous solutions of electrolytes. CONSTITUTION:An aqueous emulsion of a fluoropolymer exists in one zone of a device which has at least two zones for holding materials mutually separated by ion exchange membranes and an aqueous solution of an electrolyte exists in other zone. Water in the aqueous emulsion of the fluoropolymer is transferred through the ion exchange membranes to the zone of the solution of the electrolyte to concentrate the aqueous emulsion of the fluoropolymer. One of the ion exchange membranes is a cation exchange membrane and the electrolyte is an acid or a salt. The other ion exchange membrane is an anion exchange membrane and the electrolyte is a base or its salt. The acid (salt) electrolyte separated by the cationic exchange membrane exists in one side and the base (salt) electrolyte separated by the anionic exchange membrane, exists in the other side so as to sandwich the aqueous emulsion of the fluoropolymer in-between to provide the objective concentrating method. The exchange membrane is a filmy membrane or a tubular membrane.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for concentrating a fluoropolymer aqueous emulsion.

[0002]

2. Description of the Related Art Polytetrafluoroethylene (hereinafter PT
Aqueous emulsions called FE) are described in US Pat.
Emulsion polymerization method as disclosed in No. 9,752, that is, by press-fitting and polymerizing tetrafluoroethylene into an aqueous medium containing a water-soluble polymerization initiator and an anionic surfactant having a fluoroalkyl group as a hydrophobic group, It is produced by a method of producing colloidal particles of PTFE in a medium. The concentration of the as-polymerized PTFE aqueous emulsion obtained by such a method is generally 15 to 45% by weight.
Aqueous emulsions of fluoropolymers other than PTFE are also prepared in much the same way. When such an emulsion is used for coating metal substrates, impregnating glass fibers, etc., it is required that the emulsion has a higher concentration (for example, about 60% by weight). On the other hand, such an aqueous emulsion as it is polymerized (hereinafter referred to as "stock solution") itself has relatively poor stability and is accompanied by boiling when evaporating and concentrating the water in the stock solution, and further with pumping and metering when transferring the stock solution. The mechanical action causes the colloidal particles to aggregate with each other to form secondary particles, which causes emulsion breakage. For this reason, even when used for the above-mentioned purposes, in most cases, even in the process of concentration or transfer, an emulsion is carried out with a nonionic surfactant such as P-alkylphenyl polyethylene glycol ether (alkyl group has 8 to 10 carbon atoms). We are trying to stabilize.

As the currently known concentration method, in the anionic surfactant method disclosed in US Pat. No. 2,478,229, a low concentration emulsion having a PTFE concentration of 2 to 8% by weight, for example, dodecyl is used. Anionic surfactant such as sodium benzenesulfonate is added to the emulsion in an amount of about 5% based on the weight of PTFE to stabilize the emulsion, and then a large amount of ammonium carbonate (for example, 1.7 times based on the weight of PTFE) is added to the surface active agent. Is temporarily insolubilized. As a result, most of the PTFE particles undergo reversible aggregation and settle. On the other hand, the upper layer becomes a supernatant. After removing the supernatant, the lower layer is cooled to crystallize ammonium carbonate to solubilize the surfactant again, release the PTFE particles from the agglomerated state, crystallize ammonium carbonate and a small amount of irreversibly solidified PTFE particles. Is a method for obtaining a concentrated liquid. In the method using an anionic surfactant as seen in this method, when a large amount of electrolyte (for example, ammonium carbonate) is added and dissolved in order to insolubilize the anionic surfactant, a part of the PTFE particles is irreversible. It is currently rarely used because it causes coagulation, which results in a loss of fluoropolymer.

In the nonionic surfactant method disclosed in US Pat. No. 3,037,953, a nonionic surfactant such as P-nonylphenol polyethylene glycol ether is added to an emulsion having a PTFE concentration of 30 to 50% by weight. Stabilize the emulsion by adding 6 to 12% by weight relative to the weight of PTFE, and then add a small amount of electrolyte and heat to 50 to 80 ° C at which the nonionic surfactant reaches the cloud point, and the PTFE particles will settle in the upper layer. The supernatant can be formed. The supernatant is removed and the liquid temperature is returned to room temperature to obtain a concentrated PTFE emulsion.
The method utilizing the cloud point of the nonionic surfactant as seen in this method has a drawback in that the amount of the surfactant used is more than necessary for stabilizing the emulsion, but the PTFE particles are coagulated. Is the only method currently used to concentrate PTFE aqueous emulsions industrially because it is less likely to occur.

The ultrafiltration method disclosed in US Pat. No. 4,369,266 is based on the stabilization of a PTFE aqueous emulsion by the use of a surfactant, and then the water in the emulsion is removed by an ultrafiltration membrane. It is a method of concentrating by doing. The method using an ultrafiltration membrane as seen in this method forces the water in the PTFE aqueous emulsion to be expelled by pressure through the ultrafiltration membrane.
There is a drawback that the ultrafiltration membrane is easily blocked by the PTFE particles. A common drawback of these methods is that the removed water cannot be directly discarded as it contains a large amount of surfactant in the removed water. Further, it is economically disadvantageous in itself to dispose of a large amount of surfactant.

Although the evaporative concentration method is the most commonly used method for concentrating a dilute solution, in the case of a poor stability such as a PTFE aqueous emulsion, PT is used.
There is a drawback that emulsion breakage is likely to occur due to aggregation of FE particles to form secondary particles.

[0007]

SUMMARY OF THE INVENTION A first object of the present invention is to concentrate a fluoropolymer aqueous emulsion without fear of agglomeration of fluoropolymer particles.
The purpose of this is to concentrate the fluoropolymer aqueous emulsion by using a small amount of surfactant as compared with the above-mentioned existing concentration method, and the third purpose is to make the surfactant active in the water removed to concentrate the fluoropolymer aqueous emulsion. It does not contain a drug.

[0008]

SUMMARY OF THE INVENTION A method of concentrating a fluoropolymer aqueous emulsion according to the present invention is a fluoropolymer aqueous emulsion in one zone of an apparatus having at least two zones separated from each other by an ion exchange membrane, the other zone. An aqueous solution in which an electrolyte is dissolved is allowed to exist, and water in the fluoropolymer aqueous emulsion is moved to the area of the electrolyte solution through the ion exchange membrane.

The basic structure of an apparatus used for carrying out the present invention will be described with reference to the accompanying drawings. FIG. 1 is a plan view seen from above, and FIG. 2 is a sectional view taken along line AA of FIG. In one area of the device having at least two zones 2 (A-chamber) and 3 (B-chamber) separated from each other by an ion exchange membrane 1, for example A-chamber with fluoropolymer aqueous emulsion, the other zone, for example B-chamber. An aqueous solution in which an electrolyte is dissolved is allowed to exist. Moisture in the fluoropolymer aqueous emulsion is transferred through the ion exchange membrane to the area of the electrolyte solution (B chamber), and the fluoropolymer aqueous emulsion in the A chamber is concentrated.

In the present invention, the fluoropolymer means a polymer of tetrafluoroethylene, chlorotrifluoroethylene or vinylidene fluoride, or a copolymer thereof. For example, polytetrafluoroethylene, tetrafluoroethylene / hexafluoropropylene copolymer,
Examples thereof include tetrafluoroethylene / fluoroalkyl vinyl ether copolymer, tetrafluoroethylene / ethylene copolymer, polychlorotrifluoroethylene, poly (vinylidene fluoride) and vinylidene fluoride / hexafluoropropylene copolymer.

In the present invention, the aqueous emulsion refers to an emulsion containing 1 to 75% by weight of fluoropolymer colloidal particles having an average particle diameter of 0.1 to 0.3 μm in water. The average particle size of the colloidal particles can be measured by the centrifugal sedimentation method. In the present invention, a value measured by a centrifugal sedimentation type particle size distribution measuring device (SA-CP4L manufactured by Shimadzu Corporation) is shown.

The emulsion stabilizer of the fluoropolymer aqueous emulsion may be either a nonionic surfactant or an anionic surfactant. In the case of an anionic surfactant, it should be used in combination with a cation exchange membrane. This is advantageous because it is possible to prevent the movement of the surfactant into the electrolyte solution more reliably than the nonionic surfactant.

Examples of the anionic surfactant include alkylsulfonic acid, alkylsulfuric acid, alkylarylsulfonic acid, alkylarylsulfuric acid, salts of higher fatty acid, phosphoric acid alkyl ester or phosphoric acid alkylaryl ester, oxyalkylated sulfonic acid or a salt thereof. Salts, as well as esters or salts of sulfosuccinic acid are mentioned.

Examples of nonionic surfactants include those described in US Pat. No. 3,925,292. Examples include: Table by the general formula, R-Ph-O- (CH 2 CH 2 O) n H [ alkyl group wherein R is carbon atoms of 4 to 20, Ph is a phenyl radical, n is a number from 4 to 20] Alkylphenol polyethylene glycol ether (provided that part of the ethylene oxide units may be replaced with propylene oxide units). Formula, R-O- [the number of wherein n is 4-20, R is an alkyl group having 4 to 20 carbon _{atoms] (CH 2 CH 2 O)} n H aliphatic expressed by Alcohol polyoxyethylene glycol ether (however, a part of the ethylene oxide units may be replaced with propylene oxide units). And the general formula, [Wherein x is 2 to 20, y is 10 to 40, and z is 2 to 2]
It is a number of 0], the ethylene oxide-propylene oxide-block copolymer represented by. It is also possible to use nonionic condensation products having amines, especially aliphatic amines, or of ethylene oxide, or of mixtures of ethylene oxide and propylene oxide. It is also possible to use mixtures of the compounds mentioned.

These emulsion stabilizers are added to the concentrated fluoropolymer aqueous emulsion in an amount of 0.5 to 12% by weight, preferably 2 to 8% by weight, more preferably 2 to 8% by weight based on the weight of the solid content of the fluoropolymer in the emulsion. It is added at a concentration of 2-5% by weight.

As the ion exchange membrane, a cation or anion exchange membrane having a cross-linking structure such as sulfonated polystyrene-divinylbenzene resin, phenol sulfonic acid resin, quaternary ammonium resin, or the like, or a functional group such as sulfonic acid group or carboxylic acid group can be used. It may be a cation exchange membrane of a fluoropolymer having. The form of the ion exchange membrane may be either a film form or a tube form, and the tube form is particularly advantageous because a large membrane area for water permeation can be obtained. Ion exchange membranes generally cannot apply high pressure because they have lower mechanical strength than microfiltration membranes, ultrafiltration membranes and reverse osmosis membranes. However, the higher the pressure applied to fluoropolymer aqueous emulsion, the more water moves. The increase in speed is the same as when using the filtration membrane.

As the electrolyte, when concentration is carried out through a cation exchange membrane, acids such as mineral acids such as sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid and acetic acid, propionic acid, other organic acids or salts thereof, and anion exchange are used. When the concentration is carried out via a membrane, a base such as caustic soda, caustic potash, ammonium hydroxide or a salt thereof can be used.

As described above, when the fluoropolymer aqueous emulsion and the electrolyte solution are present through the ion exchange membrane, the force for diluting the electrolyte solution between the emulsion and the solution according to Van't Hoff's law, that is, Osmotic pressure is generated and water moves to the electrolyte solution side through the ion exchange membrane. On the other hand, when the electrolyte is an anion, for example, an acid such as sulfuric acid or hydrochloric acid, by using a cation exchange membrane, the electrolyte is a cation, for example, caustic soda,
In the case of a base such as ammonium hydroxide, the use of anion exchange membranes can block migration of the electrolyte into the fluoropolymer aqueous emulsion. This is effective in maintaining the difference in the concentration of the electrolyte between the emulsion and the solution, and is also effective in preventing the contamination of the emulsion with the electrolyte. However, depending on the type of ion exchange membrane, there are some that cannot sufficiently block the movement of electrolyte ions into the emulsion. In such a case, as shown in FIGS. 2) and the cathode 5 in the emulsion area (room A), it is possible to prevent migration of, for example, sulfate ions or chloride ions to the emulsion area. The potential difference between both electrodes is not limited, and it is sufficient that it can prevent the movement of electrolyte ions to the emulsion area, and also in the sense of preventing the danger of hydrogen and oxygen generated when electrolyzing water. Also, it is desirable that it is lower than the electrolysis voltage of water in view of its economical efficiency. V mentioned above
As predicted from an't Hoff's law, the migration rate of water increases as the electrolyte concentration increases. Therefore, it is desirable to keep the electrolyte concentration high. On the other hand, the temperature should be kept below the temperature at which water boils. This is because the fluoropolymer colloidal particles may become unstable and agglomerate at the temperature at which water boils. This is because the desirable temperature is 50 ° C. or lower, and the viscosity of the emulsion increases and the moving speed of water decreases from around this temperature.
In particular, the increase in viscosity is remarkable when a nonionic surfactant is used as an emulsion stabilizer of the emulsion.

In the process of the present invention, the fluoropolymer aqueous emulsion is concentrated, while the electrolyte solution is diluted, which becomes waste. Therefore, it is desirable to concentrate and reuse the diluted electrolyte solution. In such a case, a diluted electrolyte solution is placed in both chambers A and B shown in FIGS. 3 and 4, and an anion exchange membrane is used when the electrolyte is an acid, and a cation exchange membrane is used when the electrolyte is a base. use,
When a current is passed between the anode 4 and the cathode 5, when the electrolyte is an acid, the acid ions move from the B chamber to the A chamber, and the electrolyte solution on the A chamber side is concentrated and can be reused. On the other hand, the electrolyte solution on the B chamber side becomes a very dilute electrolyte solution due to the movement of the acid ions to the A chamber, and can be neutralized and discarded. It goes without saying that when the electrolyte is a base, the B chamber side is concentrated.

[0020]

Examples 1 to 3 Experiments were conducted using the apparatus shown in FIG.
Room A (internal volume = height 10 cm x width 10 cm x length 5 cm)
And room B (internal volume = height 10 cm x width 10 cm x length 5 c
m) has an opening with an area of 88 cm ² , which is closed by a film of sulfonated fluoropolymer (trade name Nafion 117). Concentration 43.8 stabilized with sodium dioctyl sulfosuccinate in room A
% PTFE aqueous emulsion (500 ml) and room B sulfuric acid aqueous solution (800 ml) were placed in the chamber B for 4 hours. As shown in Table 1, the higher the concentration of sulfuric acid, the more water moved from chamber A to chamber B. On the other hand, a small amount of sulfuric acid is B
Moved from room to room A. Colloidal particles of PTFE and dioctylsulfosuccinic acid were not detected in chamber B.

[0021]

[Table 1]

[0022]

Examples 4 to 6 Experiments were conducted using the same apparatus as in Example 1. In chamber A, various concentrations of PT stabilized with nonylphenol polyethylene glycol ether (n = 9)
500 ml of an FE aqueous emulsion and 800 ml of a sulfuric acid aqueous solution having the same concentration were placed in the chamber B and left for 4 hours. Table 2 shows the results. The movement of water from room A to room B was hardly affected by the concentration of PTFE. In addition, transfer of nonylphenol polyethylene glycol ether from room A to room B was not observed.

[0023]

[Table 2]

[0024]

Embodiments 7 to 9 As shown in FIGS. 3 and 4, 10 cm × 10 cm at both ends of the chambers A and B of the apparatus of FIGS. 1 and 2.
The lead plate was attached, and the chamber A was used as the cathode and the chamber B was used as the anode. A
The chamber was filled with PTFE aqueous emulsion with a concentration of 44.0 wt% stabilized with sodium dioctyl sulfosuccinate, and the B chamber was filled with 800 ml of a sulfuric acid aqueous solution with a concentration of 26.4 wt%.
A potential difference of 1.5 V, 3.5 V, and 4.5 V was applied between both electrodes, and the electrodes were allowed to stand. The distance between both electrodes was 12 cm. The results are shown in Table 3. The greater the potential difference, the less migration of sulfuric acid from chamber B to chamber A. On the other hand, from room A to B
The transfer of water to the chamber was almost the same. Room A to B of PTFE colloidal particles and sodium dioctyl sulfosuccinate
No transfer to the room was allowed.

[0025]

[Table 3]

[0026]

EFFECT OF THE INVENTION The fluoropolymer aqueous emulsion can be concentrated without fear of aggregation of the fluoropolymer particles, and the amount of the surfactant used as a stabilizer may be smaller than that in the existing concentration method. Since the water removed for concentration does not contain a surfactant, wastewater treatment is easy.

[Brief description of drawings]

FIG. 1 is a plan view of an apparatus used in the present invention as seen from above.

FIG. 2 is a sectional view taken along line AA of the device shown in FIG.

FIG. 3 is a plan view of an apparatus provided with an electrode used in the present invention as seen from above.

FIG. 4 is a sectional view taken along line AA of the device shown in FIG.

[Explanation of symbols]

 1 Ion Exchange Membrane 2 One Area (Room A) 3 Other Area (Room B) 4 Electrode (Anode) 5 Electrode (Cathode)

Claims (6)

[Claims] 1. A fluoropolymer aqueous emulsion is present in one zone of an apparatus having at least two zones separated from each other by an ion exchange membrane, and an aqueous solution containing an electrolyte is present in the other zone, and water in the fluoropolymer aqueous emulsion is present. A method of concentrating a fluoropolymer aqueous emulsion, which comprises transferring the fluoropolymer through an ion exchange membrane to an area of electrolyte solution. 2. The ion exchange membrane is a cation exchange membrane,
The method for concentrating a fluoropolymer aqueous emulsion according to claim 1, wherein the electrolyte is an acid or a salt thereof. 3. The ion exchange membrane is an anion exchange membrane,
The method for concentrating a fluoropolymer aqueous emulsion according to claim 1, wherein the electrolyte is a base or a salt thereof. 4. A region sandwiching a fluoropolymer aqueous emulsion, one of which has an acid or a salt thereof separated by a cation exchange membrane as an electrolyte, and the other of which has a base or a salt separated by an anion exchange membrane as an electrolyte. The method for concentrating a fluoropolymer aqueous emulsion according to claim 1, which is an area to be used. 5. The method for concentrating a fluoropolymer aqueous emulsion according to claim 1, wherein the ion exchange membrane is a film-shaped membrane. 6. The method for concentrating a fluoropolymer aqueous emulsion according to claim 1, wherein the ion exchange membrane is a tubular membrane.