JPS5928334B2 - Method for producing tetrafluoroethylene polymer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a tetrafluoroethylene polymer, and more specifically, to a method for producing a tetrafluoroethylene polymer, and more specifically, a process for controlling dimensional changes during firing of a molded product obtained by paste extrusion molding of a fine powder of the obtained polymer. The present invention relates to a method for producing a tetrafluoroethylene polymer that can be controlled or suppressed to a preferable value.

The above-mentioned fine powder is generally produced by polymerizing tetrafluoroethylene in an aqueous medium containing a polymerization initiator and a dispersant, solidifying an aqueous dispersion of a tetrafluoroethylene polymer using a weak shear stress, and then drying it. This can be obtained by doing so.

This tetrafluoroethylene polymer fine powder is mixed with a hydrocarbon-based lubricant, turned into a paste-like mass, and processed into so-called paste extrusion molding to make wire coatings, tapes, films, tubes, pipes, etc. . Among these extrudates, electric wire coatings, tubes, pipes, etc. are usually heated and fired to become final products. During this firing, the extrudate undergoes significant dimensional changes, typically shrinking in the direction of extrusion and expanding transversely to extrusion.

For example, if the extrudate is a tube, firing will cause it to shrink in its length direction and increase its inner and outer diameters. If this dimensional change is too large, the extrudate will lose its surface smoothness. Furthermore, if the wire is coated on the wire at the same time as extrusion, the adhesion to the wire will be incomplete after firing. For this reason, it is generally desirable that dimensional changes be small. The magnitude of the above dimensional change depends on the conditions of paste extrusion, but on the other hand it directly depends on the type of fine powder, that is, under what manufacturing conditions the fine powder was made. It is known to be a thing. Based on this fact, a method for producing a tetrafluoroethylene polymer fine powder that exhibits little dimensional change during firing of an extruded product has been developed. For example, in JP-A No. 52-5890, at least one of hexafluoropropene, ω-hydroperfluoroolefin, and trifluorochloroethylene is used as a copolymerizable modifier, and less than half of the amount is used in the nucleation stage of polymerization. By introducing the remaining modifier after 70% of the total amount of tetrafluoroethylene has been polymerized, the shrinkage rate in the extrusion direction due to firing of fine powder paste extrudates (hereinafter simply referred to as "extrudates") is reduced. It is stated that. However, since the method of JP-A-52-5890 mentioned above uses a copolymerizable modifier, it cannot be applied to the production of a homopolymer of tetrafluoroethylene. It has therefore been found that attempts to control the shrinkage rate by adjusting the amount of modifier used inevitably have the disadvantage of affecting other physical properties.

Furthermore, when using such a known method, in the actual manufacturing process, the polymerization conditions between batches and the materials used, such as monomers as raw materials, polymerization initiators, emulsifiers, various additives, aqueous media, etc. The degree of dimensional change during extrudate firing may vary due to extremely small fluctuations in the purity, etc. of It is extremely difficult to prevent or suppress variations in the degree of dimensional change. In such a case, even if a fine powder with a small degree of dimensional change during firing of the extrudate is obtained, the dimensions of the fired product will not be constant for each batch, making it impossible to control the quality of the fired product. Of course, the degree of dimensional change can be adjusted to some extent by adjusting the firing conditions. However, adjustment of such dimensional changes is limited and not sufficient, and the work is generally complicated. An object of the present invention is to provide a method for producing a tetrafluoroethylene polymer that solves all of the above problems. That is, in the present invention, when polymerizing tetrafluoroethylene in an aqueous medium containing a water-soluble persulfate and a dispersant, the dicarboxylic acid represented by the general formula is added by about 0.005 to 0.2 weight to the aqueous medium. ? The present invention relates to a method for producing a tetrafluoroethylene polymer, characterized in that a tetrafluoroethylene polymer is present.

In the present invention, it is essential to add the above-mentioned dicarboxylic acid to the polymerization system, and the degree of dimensional change during firing of the extrudate of the obtained tetrafluoroethylene polymer fine powder is reduced as much as that of the conventional method. be able to.

Moreover, it has been found that the rate of this reduction has a unique property in that it is centrally determined by the amount of dicarboxylic acid added under certain polymerization conditions. That is, as the amount of dicarboxylic acid added increases, the degree of dimensional change during firing of the extrudate decreases in a certain relationship. Therefore, as shown in Figure 1, by experimentally determining the relationship between the amount of dicarboxylic acid added and the degree of dimensional change under certain polymerization conditions, the dimensional change can be freely controlled according to the purpose. . In this way, even if variations in dimensional changes occur between polymerization batches for some reason, such variations can be reduced by appropriately increasing or decreasing the amount of dicarboxylic acid added based on the above relationship. Furthermore, it is a great advantage of the present invention that the addition of dicarboxylic acid hardly affects the physical properties and processability of the obtained polymer, other than the shrinkage rate of the extrudate. For example, the molecular weight of the polymer obtained by adding dicarboxylic acid does not decrease,
Therefore, there is no risk that the mechanical strength of the fired product will be impaired.As described above, the present invention is directed to the firing of fine powder paste extrudates of polymers obtained by adding the above-mentioned dicarboxylic acid to the tetrafluoroethylene polymerization system. This makes it possible to reduce the dimensional change during the process and to control the dimensional change by increasing/decreasing the amount of dicarboxylic acid added, and can be implemented industrially with great advantage.

Next, a general method for carrying out the polymerization of the present invention will be explained.

First, deionized and deoxidized water is placed in a polymerization tank equipped with a temperature control mechanism, and after adding dicarboxylic acid and a dispersant to adjust the temperature, tetrafluoroethylene is pressurized to a certain pressure, and a water-soluble persulfate is added. Polymerization is initiated by adding and stirring. It is also desirable to add the dicarboxylic acid to the polymerization system before the start of polymerization. However, it may be added after the initiation of polymerization, as long as it is in the nucleation stage, which is the very early stage of polymerization. Polymerization temperatures range from about 10 to 100°C. Moreover, the polymerization pressure is about 3 to 50 kg/Crl. Approximately G is suitable. The pressure in the polymerization tank is maintained by the vapor pressure of tetrafluoroethylene. As the polymerization progresses, tetrafluoroethylene is consumed and the pressure decreases, so
Tetrafluoroethylene is supplied accordingly to maintain the initially set pressure. This supply may be carried out continuously. Alternatively, the internal pressure of the polymerization tank may be maintained by, for example, injecting tetrafluoroethylene every time the pressure decreases by about 1k9/d. In the case of the latter supply method, the amount of polymer produced can be determined from the amount of tetrafluoroethane injected at one time (pressure difference) and the number of times of injecting at a constant polymerization temperature. Thus, the polymerization reaction is terminated after the tetrafluoroethylene polymer in the tank reaches a dispersion concentration of about 25 to 40% by weight. If the concentration is further increased, coagulation of the produced polymer will occur and the uniformity of the polymer properties will be lost, which is not preferable.
Polymerization can usually be terminated by exhausting the tetrafluoroethylene in the tank and cooling the polymerization tank.
Examples of the dicarboxylic acid of general formula [1] used in the present invention include malonic acid, succinic acid, glutaric acid, adipic acid, and pimelic acid.

These dicarboxylic acids are usually added at a concentration of about 0.00 to an aqueous medium.
5-0.2 weight? , especially in the range of about 0.01 to 0.2% by weight, so that the degree of dimensional change during firing of the resulting extrudate can be controlled within the desired range. As mentioned above, the larger the amount added, the smaller the degree of dimensional change during firing of the extrudate. However, if an excessively large amount of dicarboxylic acid is added, the baked product may be colored or the polymerization rate may be reduced. Therefore, it is generally preferable to keep the amount of dicarboxylic acid at about 0.1% by weight or less based on the aqueous medium. Furthermore, when the number of carbon atoms in the alkylene group R of the dicarboxylic acid is 4 or more, coloration tends to occur even when the amount added is small.
Therefore, it is more preferable to add the dicarboxylic acid whose alkylene group R has 1 to 3 carbon atoms in an amount of about 0.01 to 0.1% by weight to the aqueous medium. Dispersants used in the present invention include conventionally known dispersants, such as Japanese Patent Publication No. 46-14
Various surfactants described in No. 466 can be used.

Among these, particularly preferred are fluorine-based surfactants, such as alkali metal salts such as perfluorooctanoic acid and omega hydroperfluorononanoic acid, and ammonium salts. The amount of such dispersant used is usually in the range of about 0.001 to 5% by weight based on the aqueous medium. A wide variety of known water-soluble persulfates can be used as the polymerization initiator, such as potassium persulfate and ammonium persulfate.

The amount of these water-soluble persulfates used is 0 for the aqueous medium.
．． 0001 to 0.1 weight 70. In addition, when polymerizing at a low temperature, a reducing agent can be added to the water-soluble persulfate to form a redox system. In the polymerization method of the present invention, a dispersion stabilizer may also be present in order to improve the dispersion stability of the resulting polymer.

The dispersion stabilizer is preferably selected from saturated hydrocarbons that are liquid at the above polymerization temperature and hardly cause chain transfer. The polymerization of the present invention is usually homopolymerization of tetrafluoroethylene.

However, in the present invention, various copolymerizable modifiers known to be used in the production of tetrafluoroethylene polymers can also be used in combination, depending on the intended use of the resulting polymer fine powder. For example, if the method using a copolymerizable modifier described in JP-A No. 52-5890 is carried out in the presence of the dicarboxylic acid used in the present invention, when the resulting polymer extrudate is fired, The degree of dimensional change can be further reduced. Specifically, tetrafluoroethylene and a copolymerizable modifier are polymerized in an aqueous medium containing a water-soluble persulfate and a dispersant to form a modified tetrafluoroethylene containing 0.002 to 2% by weight of the modifier. When producing a fluoroethylene polymer, (1) hexafluoropropene, general formula CF2=CF (CF2CF2) as the copolymerizable modifier. H(n
is an integer of 1 to 5) and at least one of ω-hydrobubbleooloolefins and trifluorochloroethylene
(2) 0 to 30% of the total amount of tetrafluoroethylene to be polymerized; During polymerization, less than half of the modifier to be copolymerized is introduced into the polymerization system, and the inner shell portion of the resulting polymer particles contains 0.001 weight 70 or more in proportion to the final amount of polymer produced. (3) After at least 70% of the total amount of tetrafluoroethylene to be polymerized, the remainder of the modifier to be copolymerized is introduced into the polymerization system. 50 to 99.95 weight of modifier to be copolymerized with? is carried out in the presence of the dicarboxylic acid of the present invention. Also, JP-A-51-3
By carrying out the polymerization method using a copolymerizable modifier described in No. 6291 or Japanese Patent Publication No. 48-17863 in the presence of a dicarboxylic acid, extrusion molding can be performed at low extrusion pressure even at a high extrusion drawing ratio, Moreover, the degree of dimensional change during firing of the extrudate can be reduced. That is, tetrafluoroethylene and chlorotrifluoroethylene as a modifier are polymerized in an aqueous medium containing a water-soluble persulfate and a dispersant to form a modified tetrafluoroethylene containing 0.001 to 2 mol% of chlorotrifluoroethylene. When producing a fluoroethylene polymer, (1) at least 50% of the chlorotrifluoroethylene to be introduced into the polymer is present in the shell portion of the particles corresponding to 30% of the total weight of the final product; and (2) the outermost particulate shells representing 10% of the total weight of the final product always contain at least 0.
A method in which chlorotrifluoroethylene is introduced into the polymerization system so that it is present at a concentration of 0.01 mol % (JP 5136291) or in an aqueous medium essentially consisting of water, a water-soluble persulfate and a dispersant. When polymerizing tetrafluoroethylene in the aqueous medium, the last 3070% of the tetrafluoroethylene to be polymerized is added to the aqueous medium.
The total polymerization reaction rate is at least 570 times lower than when an equivalent amount of tetrafluoroethylene is polymerized without the presence of a modifier in the same reaction medium continuously saturated with tetrafluoroethylene at the same temperature and pressure during the polymerization. A method of polymerization containing a modifier consisting of at least one of perfluoroalkyltrifluoroethylene having 3 to 10 carbon atoms and oxyperfluoroalkyltrifluoroethylene having 3 to 10 carbon atoms, which has the effect of keeping the
(Japanese Patent Publication No. 48-17863) is carried out in the presence of the dicarboxylic acid of the present invention. If the known polymerization method is carried out as described above in the presence of the dicarboxylic acid of the present invention, the degree of dimensional change during firing of the extrudate according to the present invention can be reduced without detracting from all the advantages of the known method. The effect of reduction is achieved and, in turn, this effect and the specific effects of each of the above-mentioned methods are fully exploited. This fact is also one of the advantages of the present invention. The aqueous dispersion of the tetrafluoroethylene polymer thus obtained is diluted appropriately by a conventional method, coagulated, and dried to obtain a tetrafluoroethylene fine powder.

If the resulting polymer is sized by adding a known granulation step during or after the coagulation, a powder product with excellent handling properties can be obtained. Further, an inorganic or organic filler may be uniformly mixed during this coagulation. Examples and comparative examples are listed below to further clarify the present invention.

However, the molecular weight, paste extrusion molding, baking and dimensional changes were measured as follows. First, the molecular weight (M
n number average molecular weight), first measure the specific gravity (S.G.) of the polymer, and then calculate the S.G. G. This is the value obtained from the value of by the following formula.

The specific gravity (S.G.) of the polymer is determined by the following method.

That is, sample powder 5f was heated in an atmosphere whose temperature was controlled to 23 to 25°C
1 in a circular mold with a diameter of 32 mm.
Compress it with a pressure of 9/~, take it out from the mold and press 38'C
Place in an air oven and bake for 30 minutes until the temperature reaches 7'C/Hr. After cooling down to 30°C at a cooling rate of 30°C, it was taken out of the furnace and allowed to cool at room temperature. S. G. is the value determined as the ratio of the weight of the thus molded sample in air to the weight of the same volume of water at 23°C. Next, paste extrusion and other tests are performed as follows.

Fine Powder I 509 was mixed well with a hydrocarbon lubricant mainly composed of isoparaffin (trade name "Isopa I E", manufactured by Etsuso Standard Oil Co., Ltd., boiling point approximately 125°C) 109, and a die was attached to the lower end with an inner diameter of 25. .Fill into a 4mm cylinder.

The die has an aperture angle of 30. It has an orifice with an inner diameter of 2.54 mm and a land length of 7 mm at the lower end. The mixture is then extruded from the top through the orifice at a ram speed of 20 mm/min and the pressure during extrusion is recorded. The extrusion pressure is about 15? Cut it to a length of , and mark a scale of 10 cTrL on the surface in the length direction. Next, tie the ends of the sample with wire so that it can be hung inside the electric furnace.

The sample was quickly placed in a furnace heated to 37°C, removed after 4 minutes, and left to cool to room temperature. The degree of dimensional change is evaluated by reading the shortened scale width as described above and calculating the shrinkage rate from this.

Example 1 Deionized and deoxygenated water 0.7511 paraffin wax (melting point 56309,
Ammonium perfluorooctanoate as a dispersant 0.
3751 and 75 mg of succinic acid were charged, and the air in the autoclave was replaced with nitrogen gas several times and then with tetrafluoroethylene several times.

During this time, adjust the temperature to 80°C. The internal pressure is 8.0 kg/? with tetrafluoroethylene. G, 20 mg of ammonium persulfate was added, and the mixture was heated at 600 rpm. p. Stirring is started at m to initiate polymerization. Tetrafluoroethylene is continuously fed so that the internal pressure is maintained at 8.0 kg/(1-J mo V1G). The polymerization is allowed to proceed to a sufficiently high yield before a large amount of coagulated polymer is generated. During this time, the polymerization temperature is kept constant. 8
``Adjust to maintain C. The polytetrafluoroethylene aqueous dispersion obtained had a concentration of 3,501 parts per 1 part water, the number average length diameter of the particles was 0.223 μm, and the molecular weight was 2,730,000 as determined by optical particle size measurement using centrifugal sedimentation.

After cooling the aqueous dispersion to room temperature, it is coagulated and stirred, and 120 is further dried at C for 14 hours to obtain a fine powder. The extrusion pressure when extruding this into paste was 116 kg.
/ml. When the extruded product was fired, a shrinkage rate in the extrusion direction was 27%. Comparative Example 1 Polymerization was carried out under the same conditions as in Example 1 except that succinic acid was not added.

Table 1 shows the results of the polymer yield, number average length and diameter, molecular weight, extrusion pressure, shrinkage rate in the extrusion direction, and colored state of the fired product in the same manner as in Example 1. Example 2 Polymerization was carried out in the same manner as in Example 1 except that the amount of succinic acid added was increased to 300 mg.

Example 3 Polymerization was carried out in the same manner as in Example 1 except that the amount of succinic acid added was increased to 790 mg.

Example 4 Polymerization was carried out in the same manner as in Example 1, except that malonic acid 1329 was used in place of succinic acid.

Example 5 Polymerization was carried out in the same manner as in Example 1, except that 84 mg of glutaric acid was used in place of succinic acid.

Example 6 Polymerization was carried out in the same manner as in Example 1, except that 95 mg of adipic acid was used in place of succinic acid.

Example 7 Polymerization was carried out in the same manner as in Example 1, except that the polymerization temperature was 85°C, and the dispersants 1.09 mg of ammonium perfluorooctanoate and 7 mg of ammonium persulfate were used.

Comparative Example 2 Polymerization was carried out in the same manner as in Example 7 except that succinic acid was not added.

Example 8 Polymerization was carried out in the same manner as in Example 1 except that 20 cc (1 atm) of hexafluoropropene gas was introduced into the autoclave immediately before the start of polymerization.

Comparative Example 3 Polymerization was carried out in the same manner as in Example 8 except that succinic acid was not added.

The yield, number average length and diameter, molecular weight, extrusion pressure, shrinkage rate in the extrusion direction, and colored state of the fired products of the polymers obtained in Examples 2 to 8 and Comparative Examples 2 and 3 above were measured in the same manner as in Example 1. The results are shown in Table 1. Incidentally, the variation in the measurement results of the shrinkage percentage in Table 1 is within ±0.5% in all cases. As is clear from the results in Table 1, when the dicarboxylic acid of the present invention is not used (Comparative Examples 1 and 2), the shrinkage rate is considerably large, whereas when dicarboxylic acid is used (
In Examples 1 to 7), the shrinkage rate is considerably small.

Further, as is clear from Examples 1 to 3, as the amount of dicarboxylic acid added increases, the shrinkage rate decreases.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the results of measuring the shrinkage rate of the extrusion nozzle when the amount of succinic acid added was changed under the same polymerization conditions as in Example 1.

Claims (1)

[Claims] 1. When tetrafluoroethylene is polymerized in an aqueous medium containing a water-soluble persulfate and a dispersant, the general formula HOOC
About 0.005% of the dicarboxylic acid represented by RCOOH [I] [in the formula, R represents an alkylene group having 1 to 5 carbon atoms] to the above aqueous medium.
A method for producing a tetrafluoroethylene polymer, characterized in that it is present in an amount of ~0.2% by weight. 2. The manufacturing method according to claim 1, wherein at least one of hexafluoropropene, ω-hydroperfluoroolefin, and trifluorochloroethylene is used as a copolymerizable modifier. 3. The manufacturing method according to claim 1, wherein chlorotrifluoroethylene is used as a copolymerizable modifier. 4. The method according to claim 1, wherein at least one of perfluoroalkyltrifluoroethylene having 3 to 10 carbon atoms and oxyperfluoroalkyltrifluoroethylene having 3 to 10 carbon atoms is used as a copolymerizable modifier. Production method. 5. The manufacturing method according to any one of claims 1 to 4, wherein R of the dicarboxylic acid is an alkylene group having 1 to 3 carbon atoms. 6 The amount of dicarboxylic acid added is 0.01 to aqueous medium
The manufacturing method according to any one of claims 1 to 5, wherein the content is 0.1% by weight.