JPH11240917A - Tetrafluoroethylene copolymer and its use - Google Patents

Abstract

(57) [Problem] To provide a tetrafluoroethylene-based copolymer having excellent extrusion processability and useful for producing a uniform porous body by stretching. A tetrafluoroethylene and CH ₂ = CH-
It is manufactured by emulsion polymerization with a fluorinated comonomer represented by R ^f (R ^f is a perfluoroalkyl group), for example, (perfluorobutyl) ethylene, and the polymerization unit based on the fluorinated comonomer is 0.005 to 0.05 mol. % Included
A copolymer having a standard specific gravity of less than 2.155.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fine powder of a tetrafluoroethylene copolymer (hereinafter, referred to as PTFE) and a porous molded product thereof.

[0002]

2. Description of the Related Art Fine powder of PTFE is produced by coagulating polymer fine particles obtained by a so-called emulsion polymerization method in which polymerization is carried out using an emulsifier in an aqueous medium.
Copolymerizing tetrafluoroethylene (hereinafter referred to as TFE) with a relatively small amount of a copolymerizable copolymer to form PTF
Modifying E is known in the art. Further, it is known that modification of PTFE is effective in improving the processability at the time of paste extrusion by adding a suitable auxiliary to fine powder.

[0003] Japanese Patent Publication No. 56-262242, Japanese Patent Publication No. 56-2
6243 includes chlorotrifluoroethylene (hereinafter referred to as C).
In the production of modified PTFE using a comonomer such as TFE) and TFE, a method has been proposed in which the proportion of polymerized units based on a comonomer such as CTFE in the polymer formed substantially at the end of polymerization is increased.

Japanese Patent Publication No. 59-34724 discloses a modified PTF.
A method has been proposed in which the proportion of polymerized units based on various comonomers of E is increased in the early stage of polymerization.
E has a standard specific gravity of 2.2 or more, and has a low molecular weight, which is not sufficient for stretching.

[0005] Japanese Patent Publication No. 56-262242 proposes a method in which comonomer is added all the time during polymerization and the amount added in the initial stage is increased. In this case, it is assumed that the proportion of the polymerized unit based on the comonomer in the PTFE is large and insufficient for stretching.

In Japanese Patent Publication No. 3-66926, R ^f −
A method for modifying PTFE using CH = CH ₂ (R ^f is a perfluoroalkyl group having 1 to 10 carbon atoms) as a comonomer has been proposed. Here, a method is proposed in which the comonomer is continuously added to the middle of the polymerization so as to increase the degree of modification at an early stage.

The above modification is mainly performed for the purpose of improving the paste extrusion processability of fine powder, for example, reducing the extrusion pressure, and the content of the polymerized unit based on the comonomer is 0.5% by weight or less. However, it is contained in a relatively large amount of substantially 0.1% by weight or more. For this reason, it has substantially no melt moldability, but is accompanied by a considerable decrease in crystallinity. Also, modified PTFE
However, there is a problem that heat resistance is lowered due to the introduced comonomer structure. Furthermore, molecular orientation is unlikely to occur due to the comonomer structure, which causes breakage at the time of stretching, and cannot be practically used for producing a stretched porous body.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide PTFE which is excellent in extrusion processability, can be uniformly stretched, and can obtain a high-strength porous body.

[0009]

Means for Solving the Problems The inventor of the present invention has established a uniform and high-strength porous material by stretching, by limiting the amount of polymerized units based on a comonomer copolymerized with TFE to such an extent that the processability is not affected. It has been found that a modified PTFE suitable for the production of a densified body can be obtained.

That is, the present invention relates to TFE and a compound represented by the general formula CH
₂ = CH-R ^f (where R ^f is a perfluoroalkyl group having 1 to 10 carbon atoms) is a polymer with a fluorinated comonomer, wherein the content of polymerized units based on the fluorinated comonomer is 0 PTFE characterized by being 0.005 to 0.05 mol%. In addition, the present invention provides the PTFE, wherein the standard specific gravity of the PTFE is smaller than 2.155. Also, after extruding the powder of PTFE from the paste,
Provided is a polymer porous body which is stretched at a temperature of 250 ° C. or higher.

As the fluorinated comonomer, (perfluoroethyl) ethylene, (perfluorobutyl) ethylene or (perfluorooctyl) ethylene is obtained.
It is preferable from the viewpoint of the stretchability of the FE and the uniformity and mechanical strength of the stretched product, and (perfluorobutyl) ethylene is particularly preferable. These fluorinated comonomers are adding ethylene in the presence of a peroxide to iodine compound represented by IR ^f, it is readily prepared by subsequent de HI with a basic compound such as KOH.

In the present invention, the content of the polymerized unit based on the fluorinated comonomer needs to be strictly controlled from the viewpoint of stretchability. The content is 0.005 in the modified PTFE.
０．０５0.05 mol%. If it exceeds 0.05 mol%, the crystallinity of the polymer is slightly reduced, and the paste extrusion pressure is reduced, but the stretchability is significantly reduced. Also,
If the content is less than 0.005 mol%, it is difficult to substantially obtain the effect of modification such as improvement in physical properties of the stretched product. In particular, 0.01 to
It is preferably 0.04 mol%.

The PTFE of the present invention has a modified PTFE content (0.17% by weight) described in Examples of Japanese Patent Publication No. 3-66926 described above in which the content of polymerized units based on fluorinated comonomers is 0.17% by weight.
Although substantially less than the above (corresponding to 0.069 mol% or more), it is distinguished from the conventional modified PTFE because the paste extrusion processability is not different from that of the TFE homopolymer.

Further, it is preferable that the molecular weight of PTFE is sufficiently high from the viewpoint of stretchability. Generally, the standard specific gravity of a polymer having a correlation with the molecular weight is used as a measure of the molecular weight. That is, the higher the molecular weight, the smaller the standard specific gravity. In the case of a copolymer, the molecular weight based on the standard specific gravity is strictly different from the homopolymer in principle, but in the present invention, the standard specific gravity is adopted as a measure of the molecular weight for convenience because the amount of fluorinated comonomer added is small. I have.

The standard specific gravity of the PTFE of the present invention is 2.155.
Preferably, it is smaller, ie of high molecular weight.
If the molecular weight is too high, the crystallinity will be extremely reduced and the heat resistance will be reduced. Therefore, the standard specific gravity of PTFE is 2.130.
It is preferable that it is above.

The PTFE of the present invention is produced by emulsion polymerization commonly used for producing PTFE. This polymerization method is disclosed in U.S. Pat. Nos. 3,142,665 and 3,391,099.
It is described in the issue.

After water, a normal free-radical polymerization initiator, a paraffin wax for suppressing the formation of aggregates, and an emulsifier are charged into an autoclave, TFE is injected while stirring. After this, the autoclave is gently stirred and the polymerization is carried out at a suitable temperature and pressure. The emulsified dispersion obtained upon completion of the polymerization can be used as it is, but for molding use, generally, fine powder obtained by aggregating polymer fine particles by a known method is generally used.

In the production of the PTFE of the present invention, it is preferable to add the required amount of fluorinated comonomer all at once in the beginning, without continuous or intermediate addition. By the initial batch addition, the ratio of the polymerized unit based on the fluorinated comonomer in the core of the polymer fine particles generated in the initial stage of polymerization is high,
As the polymerization proceeds, the ratio gradually decreases, and a homopolymer substantially containing no fluorinated comonomer is formed in the shell portion of the polymer fine particles at the end of the polymerization. Such a structure is preferable from the viewpoint of stretchability and physical properties of a stretched product.

In the method of continuously adding the fluorinated comonomer or the method of adding the fluorinated comonomer until a certain period of polymerization, a relatively high proportion of the polymerized unit based on the fluorinated comonomer occupies the whole particles, which is advantageous for paste extrusion. Even with this, it is difficult to obtain fine powder that is advantageous for stretching.

TFE can be added at once and polymerized by adding it all at once, but TFE is continuously added in order to increase the ratio of polymerized units based on fluorinated comonomers in the core of polymer fine particles formed at the beginning of polymerization. It is preferable to carry out divisional addition during the polymerization and polymerization.

In the present invention, one or more fluorinated comonomers copolymerizable with TFE can be used, and other copolymerizable monomers can be used in combination. The structure of the monomer used in this case is not particularly limited as long as it is a polymerizable compound copolymerizable with TFE, but a perfluoro polymerizable compound is preferable from the viewpoint of heat resistance of the obtained modified PTFE.

The size of the polymer particles in the emulsified dispersion is
It can be controlled by a known method. For example, US Pat.
As described in JP-A No. 099, a desired particle size can be obtained by controlling the addition of an emulsifier. Also, JP-A-60-765
As shown in FIG. 16, it can be controlled by varying the TFE pressure during the polymerization.

As the emulsifier, a salt of perfluoroalkanecarboxylic acid or a salt of perfluoroalkanesulfonic acid which does not participate in chain transfer can be used. Particularly, ammonium perfluoroalkanecarboxylate having 7 to 9 carbon atoms is preferably used.

As the initiator, a peroxide such as disuccinic peroxide or diglutaric peroxide or a persulfate such as ammonium persulfate or potassium persulfate is used alone or in combination. In addition, it is used as a redox system in common with a reducing agent such as sodium sulfite. Further, during the polymerization, the radical concentration during the polymerization can be adjusted by adding a radical scavenger such as hydroquinone or catechol, or by adding a peroxide decomposing agent such as ammonium sulfite.

The polymerization is usually carried out at a temperature of 50 to 120 ° C. and a pressure of 6 to 40 kg / cm ² . Usually, when the concentration of the polymer reaches 20 to 40% by weight, unreacted monomers are released to the outside of the system, stirring is stopped, and the polymerization is terminated. Then, the polymer fine particles are aggregated.

Aggregation can be performed by a known method. That is, the polymer is diluted with water so as to have a concentration of 10 to 20% by weight, and then vigorously stirred to cause aggregation. Depending on the case, the pH may be adjusted, or a coagulation aid such as an electrolyte or a water-soluble organic solvent may be added. Then, by performing appropriate stirring, the agglomerated polymer is separated from water, granulated and sized, and then dried to obtain a polymer in the form of powder particles, that is, a fine powder.

Drying is usually carried out in a state where the wet powder obtained by agglomeration does not flow much, preferably still, and is subjected to vacuum, high frequency, hot air or the like. Fine powder has the property of easily fibrillating even with a small shearing force and losing the state of the crystal structure after completion of the original polymerization. Particularly in stretching applications, contact or friction between the powders at particularly high temperatures is not preferred in order to prevent a reduction in workability. Drying is 10
To 250 ° C., particularly preferably at 100 to 250 ° C.

[0028] Ordinary modified PTFE has a high content of polymerized units based on a fluorinated comonomer and cannot be stretched and is broken. However, the expanded porous PTFE of the present invention can be produced by a general method. That is, 5 to fine powder
After adding and mixing 20% by weight of a lubricant, the mixture is fully aged in a closed container.

The lubricants used are, for example, petroleum solvents such as solvent naphtha and white oil, hydrocarbon oils such as toluenes, ketones and esters, silicone oils, fluorine oils and fluorine-containing compounds.
Any material can be used as long as it can wet the fine powder and can be easily evaporated and removed from the extruded product after the extrusion.

The fine powder to which the lubricant is added is
After preforming at a pressure of about 50 kg / cm ² , the paste is extruded. In some cases, the extruded material is formed into a sheet by calendaring or the like.

In the extrusion, it is important to promote the orientation of the fine powder particles, and the reduction ratio R / R of the extruder (the ratio of the area of the barrel to the area of the extrusion die).
Is preferable. R / R = 50-800, preferably 80-200. At this time, the pressure applied to the paste is referred to as an extrusion pressure, which is usually 100 to 1000 kg / cm ² . The lower the extrusion pressure, the better the extrusion processability. However, if the extrusion pressure is too low, the orientation of the particles is not sufficient, and the stretchability decreases. R / R is set from these points.

Thereafter, the lubricant contained in the extruded product is removed by evaporation, and the extruded product is stretched 2 to 50 times at a temperature of 250 ° C. or more to obtain a preferable porous material. 25
If the temperature is lower than 0 ° C., only a porous body having a small stretching ratio is easily obtained, and if the temperature is 350 ° C. or higher, the PTFE melts, so that it is difficult to obtain a porous body.

Preferably, the porous body is stretched uniformly. The term “uniform” means that the entire molded body is uniformly stretched by stretching, and specifically, that the weight distribution and the pore size distribution in the stretching direction of the porous body are uniform.

The stretching ratio is not particularly limited. However, when the stretching ratio is 2 or less, the porous structure is not substantially obtained, and when the stretching ratio is 50 or more, the porous structure does not become stable. It is preferable to carry out at about 50 times. The stretching speed is not particularly limited, but is usually 50 to
Performed at 1000% / sec.

The emulsified dispersion of PTFE of the present invention can be used as a coating material, and can be used for coating rolls, cooking utensils, impregnating glass cloth, and the like. In particular, it can be suitably used for a stretched product. The stretched porous PTFE body of the present invention is particularly excellent in stretch uniformity and strength. This porous body is particularly suitable for industrial products requiring durability, for example, bag filters,
Useful for packing, gaskets, and other coating applications.

[0036]

The present invention will now be described with reference to Examples (Examples 1-3, 6-
8) and Comparative Examples (Examples 4 and 5), but the present invention is not limited thereto.

[Example 1] 100 equipped with a baffle plate and a stirrer
A 1 liter stainless steel autoclave was charged with 35 g of ammonium perfluorooctanoate, 78 g of paraffin wax, and 63.4 liters of deionized water. After the autoclave was replaced with nitrogen and further replaced with TFE, the temperature was raised to 72 ° C. while stirring. 19 kg of TFE
/ Cm ^2, and 5.0 g of (perfluorobutyl) ethylene (hereinafter referred to as PFBE) and 5.4 g of disuccinic peroxide dissolved in 3 liters of water were injected. The internal pressure dropped to about 18.5 kg / cm ^{2 in} about 3 minutes.

The polymerization was allowed to proceed while adding TFE so as to keep the internal pressure of the autoclave at 19 kg / cm ² . T
When the amount of the FE added reached 720 g, 65 g of ammonium perfluorooctanoate dissolved in 3 liters of water was injected. The reaction was terminated when the amount of TFE added reached 25 kg, and TFE in the autoclave was released to the atmosphere. The resulting emulsified dispersion was cooled to remove the supernatant paraffin wax. The concentration of polymer particles in the emulsified dispersion was about 28.1% by weight, and the average particle size of the polymer particles was 0.210 μm.

This emulsified dispersion was diluted with ion-exchanged water to a polymer fine particle concentration of 10% by weight and stirred vigorously until it solidified. After coagulation, the mixture was further stirred for 5 minutes, and the coagulated polymer was dried at 200 ° C. The standard specific gravity of the obtained polymer was 2.149. Further, the content of polymerized units based on PFBE in the polymer determined by an infrared spectrometer is based on the charged P
The amount of all FBE reacted was 0.02 mol%. Table 1 shows the test results obtained by performing a stretching test shown below using the obtained polymer.

The properties of the obtained polymer were measured by the following methods. (1) Standard specific gravity: The standard specific gravity of the powdery polymer is ASTM.
It was determined by the amount of water displaced by a standard molding test sample according to the D1457-69 method. This standard molding test sample was prepared as follows. First, a powdery polymer12.
0g into a 2.86cm diameter mold and 352kg /
Hold for 2 minutes under a pressure of cm ² and preform. The preform is then heated in an oven from 300 ° C. to 380 ° C. at 2 ° C./min, held at 380 ° C. for 30 minutes, then
After cooling to 294 ° C at a rate of ° C / min, the sample was taken out of the oven, kept at 23 ° C for 3 hours or more, and used as a test sample.

(2) Particle size of polymer fine particles: laser diffraction type particle size measuring device (LPA-3000 / 3 manufactured by Otsuka Electronics Co., Ltd.)
Using 100), measurement was performed three times at 25 ° C. at a concentration of about 0.1% by weight of the polymer fine particles at 25 ° C., and the average value was defined as the particle diameter of the polymer fine particles.

(3) Preparation of stretch processability evaluation sample: 50 g of a polymer and 11.8 g of an extrusion lubricant (Supersol FP, manufactured by Idemitsu Petrochemical Co., Ltd.), which is a hydrocarbon oil, were mixed at 25 ° C.
And mature for 1 hour or more. Next, the cylinder (internal diameter 9.95m
m) with an extrusion die (diameter 1 mm at a drawing angle of 30 ° C)
Is filled with the above mixture, and the
Apply a load of g to the piston inserted in the cylinder and hold for 10 minutes. Thereafter, an extruded rod is obtained at a ram speed of 100 mm / min. In the second half of the extrusion, put the extrudate in the part where the pressure is in an equilibrium state into the oven,
At 180 ° C., the lubricant is evaporated off. About 50mm
Into a sample for evaluation of stretching.

(4) Stretching workability test: The above sample was stretched 10 times in length using a tensile tester equipped with a thermostat at a distance between chucks of 10 mm, a temperature of 250 ° C. and a tensile speed of 1000% / sec. The stretchability under these conditions was evaluated from the viewpoint of appearance and uniformity. The appearance was evaluated on the basis of A: smooth, B: slightly uneven, C: fairly uneven, D: cut during stretching. (5) Strength Evaluation of Stretched Rod: The strength of the five stretched rods obtained in (3) and (4) was measured, and the average value was shown as the strength (kg) per rod.

(6) Evaluation of uniformity of porous body: center of sample before stretching (10 mm between chucks) (5 mm from chuck)
, And the distance L (mm) from the position of the center (50 mm from the chuck) of the sample (100 mm) after stretching to the marking was measured, and the value according to the following equation was used as an index of uniformity (%). The larger this value is, the higher the uniformity is.

[0045]

## EQU1 ## Uniformity (%) = ((100 / 2-L) / (10
0/2)) × 100

Example 2 A polymer was obtained in the same manner as in Example 1 except that 2.5 g of PFBE was used. The concentration of polymer particles in the emulsified dispersion is about 27.6% by weight, the average particle size of the polymer particles is 0.233 μm, and the standard specific gravity of the polymer is 2.15.
It was 0. P in the polymer determined in the same manner as in Example 1.
The content of polymerized units based on FBE was 0.01 mol%. Table 1 shows the results of the stretching test performed in the same manner as in Example 1.

Example 3 A polymer was obtained in the same manner as in Example 1 except that 12 g of PFBE was used. The concentration of the polymer fine particles in the emulsified dispersion is about 28.3% by weight, the average particle size of the polymer fine particles is 0.208 μm, and the standard specific gravity of the polymer is 2.150.
Met. PF in polymer determined by the same method as in Example 1.
The content of polymerized units based on BE was 0.048 mol%. Table 1 shows the results of the stretching test performed in the same manner as in Example 1.

Example 4 A polymer was obtained in the same manner as in Example 1 except that 43 g of PFBE was used. The concentration of the polymer particles in the emulsified dispersion was about 28.0% by weight, the average particle diameter of the polymer particles was 0.204 μm, and the standard specific gravity of the polymer was 2.141.
Met. PF in polymer determined by the same method as in Example 1.
The content of polymerized units based on BE was 0.17 mol%. Table 1 shows the results of the stretching test performed in the same manner as in Example 1.

Example 5 A polymer was obtained in the same manner as in Example 1 except that PFBE was not used. The concentration of the polymer particles in the emulsified dispersion was about 28.0% by weight, the average particle size of the polymer particles was 0.258 μm, and the standard specific gravity of the polymer was 2.151. Table 1 shows the results of the stretching test performed in the same manner as in Example 1.

In each of Examples 1 to 3 and Example 5, a stretched porous body was obtained. In Example 4 having a high PFBE content, the paste extrusion pressure was low and the extrusion processability was excellent.
The porous body was broken during the stretching process and could not be obtained. Further, in Examples 1 to 3, a porous body having excellent strength as compared with Example 5 was obtained.

[Example 6] 5.0 g of (perfluoroethyl) ethylene (hereinafter referred to as PFEE) instead of PFBE
A polymer was obtained in the same manner as in Example 1 except that The concentration of the polymer particles in the emulsified dispersion was about 27.9% by weight, the average particle size of the polymer particles was 0.225 μm, and the standard specific gravity of the polymer was 2.150. The content of polymerized units based on PFEE in the polymer, determined in the same manner as in Example 1, was 0.02 mol%. Table 1 shows the results of the stretching test performed in the same manner as in Example 1. In Example 6, a porous body having excellent uniformity and strength as compared with Example 5 was obtained.

Example 7 A polymer was obtained in the same manner as in Example 1 except that 5.0 g of (perfluoctyl) ethylene (hereinafter, referred to as PFOE) was used instead of PFBE. The concentration of the polymer particles in the emulsified dispersion was about 28.1% by weight, the average particle size of the polymer particles was 0.211 μm, and the standard specific gravity of the polymer was 2.149. PF determined by the same method as in Example 1
The content of polymerized units based on OE was 0.02 mol%. Table 1 shows the results of the stretching test performed in the same manner as in Example 1. In Example 7, a porous body having better uniformity and strength than that of Example 5 was obtained.

Example 8 A polymer was obtained in the same manner as in Example 1 except that 1.0 g of PFBE was used. The polymer particle concentration of the emulsified dispersion was about 27.8% by weight, the average particle diameter of the polymer particles was 0.236 μm, and the standard specific gravity of the polymer was 2.151. Determined in the same manner as in Example 1,
The content of polymerized units based on PFBE in the polymer is 0.00
It was 4 mol%. Table 1 shows the results of the stretching test performed in the same manner as in Example 1.

[0054]

[Table 1]

[0055]

By co-polymerizing a small amount of a comonomer having a perfluoroalkyl group with TFE, a polytetrafluoroethylene polymer particularly useful for stretching can be obtained. The stretched porous article made of this polymer exhibits properties excellent in uniformity and strength.

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Claims (6)

[Claims] (1) tetrafluoroethylene and a compound of the general formula CH ₂ =
A polymer with a fluorinated comonomer represented by CH- ^Rf (where ^Rf is a perfluoroalkyl group having 1 to 10 carbon atoms), wherein the content of the polymerized unit based on the fluorinated comonomer is 0.005; A tetrafluoroethylene-based copolymer, characterized in that the amount is from 0.05 to 0.05 mol%. 2. The tetrafluoroethylene copolymer according to claim 1, wherein the fluorinated comonomer is (perfluorobutyl) ethylene. 3. A tetrafluoroethylene copolymer in which at least a core portion of polymer fine particles produced by emulsion polymerization of tetrafluoroethylene and a fluorinated comonomer is the polymer according to claim 1 or 2. 4. The tetrafluoroethylene copolymer according to claim 1, wherein the copolymer has a standard specific gravity of less than 2.155. 5. A polymer having a structure in which the content of polymerized units based on fluorinated comonomers in the core portion of the polymer fine particles produced by emulsion polymerization is larger than the content of polymerized units based on fluorinated comonomers in the shell portion. A powder comprising the tetrafluoroethylene-based copolymer according to claim 1, obtained from fine particles. 6. A powder comprising the tetrafluoroethylene-based copolymer according to claim 5, which is extruded into a paste, and then extruded.
A polymer porous body, which is stretched at a temperature of not less than ° C.