JP2012144719A - Stretch material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a material capable of forming a polytetrafluoroethylene porous film which is easily biaxially stretched, can be stretched uniformly even at a high draw ratio, and exhibits only a low pressure loss.SOLUTION: The stretch material contains a polytetrafluoroethylene (A) having a standard specific gravity (SSG) of 2.130 to 2.230 and fibrillation property, and a low-molecular weight polytetrafluoroethylene (B) which has not been irradiated with electron beams or radiation, and has a melt viscosity at 380°C of 1×10to 7×10Pa s, and does not fibrillate.

Description

The present invention relates to a stretched material for producing a polytetrafluoroethylene porous membrane.

A polytetrafluoroethylene porous membrane is a porous body having innumerable fine pores inside using polytetrafluoroethylene (hereinafter referred to as “PTFE”) having excellent heat resistance and chemical resistance. It is used for

For example, Patent Document 1 relates to a polytetrafluoroethylene (PTFE) porous material used for a microfiltration filter and the like, and is composed of a mixture of PTFE having an average molecular weight of 2 million or more and PTFE having an average molecular weight of 1 million or less. PTFE porous bodies are described.

By the way, in Patent Document 2, in a material for forming a tape, a filament, a film, a rod, a tube, or the like, for the purpose of providing thermal stability, a stretched structure in which nodes are connected by fibrils is drawn. Porous polytetrafluoroethylene material, wherein the material comprises a mixture of two different polymers, one polymer is a polytetrafluoroethylene homopolymer and the other polymer is modified Is described.

Patent Document 3 describes porous polytetrafluoroethylene obtained by decomposing polytetrafluoroethylene radiochemically, mixing the decomposed polytetrafluoroethylene with a high molecular weight emulsion polymer, and extruding the mixture. Has been.

Patent Document 4 discloses a porous expanded PTFE article including an expanded PTFE resin including a first fine powder PTFE resin and a second fine powder PTFE resin, wherein the first PTFE resin is formed from the second PTFE resin. The second PTFE resin has the property of forming a thicker node than the node formed from the first PTFE resin, and the expanded PTFE article has a plurality of properties. A porous expanded PTFE article is described that includes nodes and fibrils and has a thickness of about 100 μm or greater.

JP-A-3-17136 Japanese National Patent Publication No. 10-505378 JP-A-7-53755 JP 2010-018800 A

As a material for producing a PTFE porous membrane, a material capable of forming a PTFE porous membrane that can be uniformly stretched and has low pressure loss is required. Although it is known that two types of PTFE are blended as in Patent Documents 1 to 4, with conventional materials, uniform stretching is performed, and a PTFE porous membrane with low pressure loss is manufactured. It was not possible to achieve both. For example, in the mixture described in Patent Document 2, biaxial stretching was difficult at a high stretch ratio even if the pressure loss was reduced. Moreover, the mixture described in Patent Document 3 was difficult to be biaxially stretched and easily broken during stretching.

An object of the present invention is to provide a material that can easily form a polytetrafluoroethylene porous membrane that is easy to biaxially stretch, can be uniformly stretched even at a high stretch ratio, and has low pressure loss.

As a result of intensive studies by the present inventors, it includes a high molecular weight polytetrafluoroethylene having a fibrillation property and a low molecular weight polytetrafluoroethylene, and is produced without being decomposed particularly by irradiation with an electron beam or radiation, and It has been found that the above-mentioned problems can be solved by a stretched material containing a low molecular weight polytetrafluoroethylene having a melt viscosity of 1 × 10 ^{2 to} 7 × 10 ⁵ Pa · s at 380 ° C. and exhibiting the property of not fibrillating. It was done.

That is, the present invention has a standard specific gravity (SSG) of 2.130 to 2.230, is not irradiated with an electron beam or radiation with polytetrafluoroethylene (A) having fibrillation property, and at 380 ° C. It has a melt viscosity of 1 × 10 ^{2 to} 7 × 10 ⁵ Pa · s, and includes a low molecular weight polytetrafluoroethylene (B) that does not fibrillate.

In the stretched material of the present invention, the mass ratio (A) / (B) of the polytetrafluoroethylene (A) to the low molecular weight polytetrafluoroethylene (B) is preferably 99 to 50/1 to 50.

The low molecular weight polytetrafluoroethylene (B) has a heat of fusion of 322 to 322 in PTFE (B) having no history of heating to a temperature of 300 ° C. or higher and a differential scanning calorimeter obtained at a heating rate of 10 ° C./min. It is preferable to have a peak top at 333 ° C.

The stretched material of the present invention is preferably obtained by coagulating an aqueous dispersion containing polytetrafluoroethylene (A) and low molecular weight polytetrafluoroethylene (B).

Polytetrafluoroethylene (A) is 333 to 347 ° C. in a heat of fusion curve obtained with a differential scanning calorimeter at a heating rate of 10 ° C./min for PTFE (A) that has not been heated to a temperature of 300 ° C. or higher. It is preferable to have a peak top.

The stretched material of the present invention has a heat of fusion curve obtained from a differential scanning calorimeter at a heating rate of 10 ° C./min for PTFE (A) that has not been heated to a temperature of 300 ° C. or higher. It is preferable to have a peak top at ˜347 ° C.

The present invention is also a polytetrafluoroethylene porous membrane formed by stretching the above-mentioned stretching material.

The present invention is also the use of the stretched material to produce a polytetrafluoroethylene porous membrane.

The present invention is also a method for producing a polytetrafluoroethylene porous membrane comprising a step of stretching the stretched material.

The stretched material of the present invention is a material that can be easily stretched biaxially by the above structure, can be stretched uniformly even at a high stretch ratio, and can form a PTFE porous membrane with low pressure loss. Since the PTFE porous membrane of the present invention is produced by stretching the stretched material, the membrane appearance is good and the pressure loss is low.

FIG. 1 is a schematic cross-sectional view showing an outline of a roll stretching apparatus used in Examples. FIG. 2 is a schematic cross-sectional view showing the tenter stretching apparatus used in the examples.

The PTFE stretched material of the present invention is a mixture of polytetrafluoroethylene [PTFE] (A) and low molecular weight polytetrafluoroethylene [low molecular weight PTFE] (B).

In the stretched PTFE material of the present invention, the mass ratio of PTFE (A) to low molecular weight PTFE (B) is preferably (A) / (B) of 99 to 50/1 to 50. More preferably, it is 95-50 / 5-50, More preferably, it is 90-70 / 10-30. If the proportion of the low molecular weight PTFE (B) is too large, the stretchability may be inferior, and if it is too small, the pressure loss of the PTFE porous membrane obtained from the PTFE stretched material of the present invention may increase.

PTFE (A) has fibrillation properties. The presence or absence of fibrillation can be determined by “paste extrusion” which is a typical method for forming “high molecular weight PTFE powder” which is a powder made from a TFE polymer. Usually, paste extrusion is possible because high molecular weight PTFE has fibrillation properties. In the case where an unfired molded product obtained by paste extrusion does not have substantial strength or elongation, for example, when the elongation breaks when pulled at 0%, it can be regarded as having no fibrillation property. PTFE (A) has non-melt processability.

Since the stretched material of the present invention contains PTFE (A) having the above-described fibrillation property, a biaxial stretching is easy, a PTFE porous membrane that can be stretched uniformly even at a high stretching ratio and has a low pressure loss is formed. it can.

PTFE (A) has a standard specific gravity (SSG) of 2.130 to 2.230. The SSG is preferably 2.130 to 2.190, more preferably 2.140 to 2.170. If the SSG of PTFE (A) is too high, the stretchability of the stretched material may be inferior. If the SSG is too low, the rollability may deteriorate and the homogeneity of the porous film may deteriorate, There is a possibility that the pressure loss of the stretched film is increased. SSG is a value measured according to ASTM D 4895.

PTFE (A) has a peak top at 333 to 347 ° C. in a heat of fusion curve obtained with a differential scanning calorimeter at a heating rate of 10 ° C./min for PTFE (A) that has not been heated to a temperature of 300 ° C. or higher. It preferably has a (DSC melting point). If the DSC melting point in the melting heat curve is too high, the pressure loss may increase. More preferably, it has a peak top at 333 to 345 ° C, and still more preferably has a peak top at 340 to 345 ° C.
If the DSC melting point is too low, the stretchability of the stretched material may be inferior. If the melting point is too high, the rollability may be deteriorated and the uniformity of the porous film may be deteriorated. There is a risk that the pressure loss will increase.

More specifically, for example, the differential scanning calorimetry [DSC] described above uses RDC220 (manufactured by SII NanoTechnology Co., Ltd.) temperature-calibrated in advance using indium and lead as standard samples, and about PTFE powder. 3 mg is put into an aluminum pan (crimp container), and a temperature range of 250 to 380 ° C. is raised at 10 ° C./min under an air stream of 200 ml / min. The standard sample is calibrated with heat using indium, lead, and tin, and the empty aluminum pan is sealed and used as the measurement reference. The obtained heat of fusion curve uses Muse standard analysis software (made by SII Nano Technology) as the DSC melting point at the temperature showing the peak top of the heat of fusion.

PTFE (A) may be modified polytetrafluoroethylene (hereinafter also referred to as “modified PTFE”) or homopolytetrafluoroethylene (hereinafter also referred to as “homo PTFE”). However, it is preferable that it is a homo PTFE from a viewpoint of stretchability and breaking strength. The polymerization method for obtaining PTFE (A) may be emulsion polymerization or suspension polymerization, but can form a PTFE porous membrane that can be stretched more uniformly and has low pressure loss. Emulsion polymerization is preferred.

The modified PTFE is modified PTFE composed of tetrafluoroethylene [TFE] and a monomer other than TFE (hereinafter also referred to as “modified monomer”). It is preferable that the modified PTFE is uniformly modified.

The modified PTFE is composed of a TFE unit based on TFE and a modified monomer unit based on a modified monomer. In the modified PTFE, the modified monomer unit is preferably 0.005 to 0.500% by weight of the total monomer units. More preferably, it is 0.02 to 0.30% by weight. In the present specification, the modified monomer unit means a part derived from the modified monomer and part of the molecular structure of the modified PTFE, and the total monomer unit means all the single monomers in the molecular structure of the modified PTFE. It means a part derived from the body.

The modified PTFE preferably has a cylindrical extrusion pressure at a reduction ratio 1600 of 70 MPa or more. More preferably, the cylindrical extrusion pressure at the reduction ratio 1600 is 80 MPa or more. The upper limit of the extrusion pressure is not particularly limited, and cannot be extruded by an extruder, and may be high enough to exceed the limit of measurement, for example, 110 MPa. If the extrusion pressure is too small, the stretchability may be inferior. By setting the cylindrical extrusion pressure at the reduction ratio 1600 to 70 MPa or more, a material capable of forming a PTFE porous film having excellent stretchability can be obtained. Further, a molded article such as a PTFE porous membrane obtained from the stretched material of the present invention can be made excellent in homogeneity. The modified PTFE may have a cylindrical extrusion pressure at a reduction ratio 1600 of less than 70 MPa.

The cylindrical extrusion pressure at the reduction ratio 1600 is a value measured according to ASTM D 4895. As a specific measuring method, 50 g of PTFE and 10.25 g of hydrocarbon oil (trade name Isopar G (registered trademark), manufactured by Exxon) as an extrusion aid were mixed in a glass bottle for 3 minutes, and room temperature (25 ± 2 ) For 1 hour, and then the above on an extrusion die with a cylinder (inner diameter 25.4 mm) (with an aperture angle of 30 ° and an orifice (orifice diameter: 0.65 mm, orifice length: 2 mm) at the lower end) The mixture is filled, a 1.2 MPa load is applied to the piston inserted in the cylinder and held for 1 minute. Thereafter, the mixture is immediately extruded from the orifice at a ram speed of 20 mm / min at room temperature to obtain a rod-like material. In the latter half of the extrusion, a value obtained by dividing the pressure at the portion where the pressure is in an equilibrium state by the cylinder cross-sectional area can be used as the extrusion pressure.

The modified PTFE preferably has a cylindrical extrusion pressure at a reduction ratio of 100 (RR100) of 5 MPa or more, more preferably 8 MPa or more, and preferably 15 MPa or less.

The cylindrical extrusion pressure at the reduction ratio 100 is a value obtained by the following method. 100 g of PTFE left at room temperature for 2 hours or more and 21.7 g of hydrocarbon oil (trade name: Isopar H (registered trademark), manufactured by Exxon) as an extrusion aid are placed in a glass bottle with a capacity of 900 cc and mixed for 3 minutes. After being left in a thermostatic bath at 25 ° C. for 2 hours, through an orifice (diameter 2.5 cm, land length 1.1 cm, introduction angle 30 °) under the conditions of reduction ratio 100, extrusion speed 51 cm / min, 25 ° C., Paste extrusion is performed to obtain a bead (extruded product). In this paste extrusion, the value obtained by dividing the load when the extrusion load is in an equilibrium state by the area of the cylinder used is defined as the column extrusion pressure at the reduction ratio 100.

The modified PTFE preferably has an average primary particle size of 0.05 to 0.5 μm.
The average primary particle size is determined by measuring the transmittance of the projection light having a wavelength of 550 nm with respect to the unit length of the aqueous dispersion whose polymer concentration is adjusted to 0.22% by mass, and the unidirectional diameter in the transmission electron micrograph. A calibration curve with the average primary particle diameter can be prepared, and the transmittance can be measured for the aqueous dispersion to be measured, and determined based on the calibration curve.

The modifying monomer is not particularly limited as long as it can be copolymerized with TFE. For example, perfluoroolefin such as hexafluoropropylene [HFP]; chlorofluoroolefin such as chlorotrifluoroethylene [CTFE]; Examples thereof include hydrogen-containing fluoroolefins such as trifluoroethylene and vinylidene fluoride [VDF]; perfluorovinyl ether; perfluoroalkylethylene and ethylene. Moreover, the modified | denatured monomer to be used may be 1 type, and multiple types may be sufficient as it.

It does not specifically limit as said perfluoro vinyl ether, For example, following General formula (1)
_{CF 2 = CF-ORf (1} )
(Wherein Rf represents a perfluoro organic group), and the like. In the present specification, the “perfluoro organic group” means an organic group in which all hydrogen atoms bonded to carbon atoms are substituted with fluorine atoms. The perfluoro organic group may have ether oxygen.

Examples of the perfluorovinyl ether include perfluoro (alkyl vinyl ether) [PAVE] in which Rf is a perfluoroalkyl group having 1 to 10 carbon atoms in the general formula (1). The perfluoroalkyl group preferably has 1 to 5 carbon atoms.

Examples of the perfluoroalkyl group in the PAVE include a perfluoromethyl group, a perfluoroethyl group, a perfluoropropyl group, a perfluorobutyl group, a perfluoropentyl group, and a perfluorohexyl group. The group is preferably a perfluoropropyl group. That is, the PAVE is preferably perfluoropropyl vinyl ether [PPVE].

As the perfluorovinyl ether, in the general formula (1), Rf is a perfluoro (alkoxyalkyl) group having 4 to 9 carbon atoms, and Rf is represented by the following formula:

(Wherein m represents an integer of 0 or 1 to 4), and Rf is a group represented by the following formula:

(In the formula, n represents an integer of 1 to 4).

The perfluoroalkylethylene (PFAE) is not particularly limited, and examples thereof include perfluorobutylethylene (PFBE) and perfluorohexylethylene.

The modified monomer in the modified PTFE is preferably at least one selected from the group consisting of HFP, CTFE, VDF, PAVE, PFAE, and ethylene. PAVE is more preferable, and PPVE is still more preferable.

The homo-PTFE is substantially composed of only TFE units. For example, it is preferable that the homo-PTFE is obtained without using a modified monomer.

The homo PTFE preferably has an average primary particle size of 0.05 to 0.5 μm. An average primary particle diameter can be measured by the same method as modified PTFE.

The homo-PTFE preferably has a cylindrical extrusion pressure of 10 to 35 MPa at a reduction ratio 100. More preferably, the cylindrical extrusion pressure at the reduction ratio 100 is 10 to 20 MPa. If the cylindrical extrusion pressure at the reduction ratio 100 is too large, the pressure loss of the porous membrane may be increased, and if it is too small, the stretchability may be deteriorated. The cylindrical extrusion pressure at the reduction ratio 100 can be measured in the same manner as in modified PTFE.

The homo-PTFE preferably has a stress relaxation time of 150 seconds or longer. More preferably, it is 300 seconds or more. The stress relaxation time is determined by the following method.
The bead (extruded product) prepared by measuring the paste extrusion pressure at the reduction ratio 100 is cut to an appropriate length, each end is fixed so that the gap between the clamps is 38 mm, and heated to 300 ° C. in an air circulating furnace. Then, the stretched body a2 is formed by stretching the clamp at a stretching rate of 1000% / second until the total stretching becomes 2400%. Furthermore, the stretched body a2 (total length: 25 cm) is fixed to the fixture in a state of being pulled tightly, and the time required from the time when it is left in an oven at a temperature of 390 ° C. until it breaks is determined as the stress relaxation time.
The stretched body a2 in the fixture is inserted into the oven through the (covered) slot on the side of the oven so that the temperature does not drop during placement of the stretched body a2 in the oven and is therefore No time is required for recovery as disclosed in US Pat. No. 4,576,869.

The homo PTFE preferably has a breaking strength of 5 to 50N. More preferably, it is 10-30N. The said breaking strength is calculated | required with the following method.
100 g PTFE fine powder left at room temperature for 2 hours or more and 21.7 g hydrocarbon oil (trade name: Isopar H (registered trademark) manufactured by Exxon), which is an extrusion aid, are placed in a 900 cc glass bottle. After mixing for 2 minutes and leaving in a thermostatic bath at 25 ° C. for 2 hours, the orifice (diameter 2.5 cm, land length 1.1 cm, introduction angle 30 ° under the conditions of reduction ratio 100, extrusion rate 51 cm / min, 25 ° C. ) Through which paste is extruded to obtain a bead (extruded product). This bead is cut to an appropriate length, each end is fixed so that the clamp interval is 51 mm, heated to 300 ° C. in an air circulating furnace, and then the clamp is stretched at a stretching rate of 100% / About the stretched body a1 created by stretching in seconds, the strength at break when pulled at a rate of 300 mm / min at room temperature using a tensile tester (trade name: AGS-500D, manufactured by Shimadzu Corporation) The value obtained by measurement is taken as the breaking strength.

Low molecular weight PTFE (B) is not irradiated with an electron beam or radiation, has a melt viscosity of 1 × 10 ^{2 to} 7 × 10 ⁵ Pa · s at 380 ° C., and does not fibrillate.

The low molecular weight PTFE (B) has a melt viscosity of 1 × 10 ^{2 to} 7 × 10 ⁵ Pa · s at 380 ° C. If the melt viscosity is too high, the pressure loss of the porous membrane may increase.If the melt viscosity is too low, the high-temperature volatile components increase, and cracking gas is likely to be generated in the process of producing the porous membrane. There is a possibility that the porous membrane may be colored.
The melt viscosity is in accordance with ASTM D 1238, and a 2 g sample that has been heated for 5 minutes at a measurement temperature (380 ° C.) in advance using a flow tester (manufactured by Shimadzu Corporation) and a 2φ-8L die is 0.7 MPa. It can be measured while maintaining the above temperature with a load of.

Low molecular weight PTFE (B) is not irradiated with an electron beam or radiation. When low molecular weight PTFE obtained by decomposing PTFE by irradiation with an electron beam or radiation is used, biaxial stretching is difficult and breaks easily during stretching. The low molecular weight PTFE (B) is preferably obtained directly by polymerization of TFE. Such low molecular weight PTFE is known as low molecular weight PTFE after polymerization.

The low molecular weight PTFE (B) preferably has a fluorine ion concentration of 1 ppm or less. Low molecular weight PTFE decomposed by irradiation with an electron beam or radiation increases the fluorine ion concentration to about 50 ppm, but low molecular weight PTFE (B) not irradiated with an electron beam or radiation has a fluorine ion concentration of 1 ppm or less. Low value. Therefore, it can be determined from the fluorine ion concentration of the low molecular weight PTFE whether or not the low molecular weight PTFE is irradiated with an electron beam or radiation.

The fluorine ion concentration can be measured, for example, by the following method.
10 g of sample powder is put in a container made of polyethylene that has been washed with deionized water in advance and dried, and further 10 ml of methanol / deionized water (mixing ratio 1: 1 (volume ratio)), buffer solution (per 10 l of deionized water, 10 ml of acetic acid (500 ml), sodium chloride (500 g), trisodium citrate dihydrate (5 g), sodium hydroxide (320 g) added) are added. Shake the container and leave at 25 ° C. for 18 hours. Using an appropriately calibrated fluorine ion meter (Orion Expandable Ion Analyzer EA940), the fluorine ion concentration of the extract is measured, and the fluorine ion concentration of the sample powder is determined from the ratio of the sample powder to the extract solution.

The low molecular weight PTFE (B) preferably has a number average molecular weight of 600,000 or less. If the number average molecular weight is too large, the pressure loss of the porous membrane may increase. If the number average molecular weight is too small, the high-temperature volatile components increase, and a decomposition gas is likely to be generated in the process of producing the porous film, and the porous film may be colored.

Low molecular weight PTFE (B) is not fibrillated, and a continuous extrudate (extruded strand) cannot be obtained by paste extrusion. If the low molecular weight PTFE has fibrillation properties, the pressure loss of the porous membrane may increase. The presence or absence of fibrillation can be determined by the method described above.

The low molecular weight PTFE (B) is preferably a low molecular weight PTFE having fluidity in the melting region for reasons of thermal stability.

The low molecular weight PTFE (B) has a peak top at 322 to 333 ° C. in a heat of fusion curve obtained with a differential scanning calorimeter at a heating rate of 10 ° C./min for modified PTFE having no history of heating to a temperature of 300 ° C. or higher. It preferably has a (DSC melting point). If the DSC melting point in the melting heat curve is too high, the pressure loss may increase. More preferably, it has a peak top at 325 to 332 ° C.
If the DSC melting point is too high, the effect of reducing pressure loss may be reduced. If the melting point is too low, the high-temperature volatile components increase, and a decomposition gas is likely to be generated in the process of producing the porous film, and the porous film may be colored.

The low molecular weight PTFE (B) can be produced by emulsion polymerization or suspension polymerization. Moreover, it can also manufacture by the method which combined emulsion polymerization and suspension polymerization. It may be produced by performing emulsion polymerization in the early stage of polymerization and suspension polymerization in the late stage of polymerization.

Low molecular weight PTFE (B) may be modified PTFE or homo-PTFE. The modified monomers constituting the modified PTFE may be those already exemplified.

Although the shape of the extending | stretching material of this invention is not specifically limited, For example, a powder is mentioned.

The stretched material of the present invention preferably has a standard specific gravity (SSG) of 2.130 to 2.190, more preferably 2.140 to 2.170.

The stretched material of the present invention preferably has a cylindrical extrusion pressure at a reduction ratio of 100 to 10 to 20 MPa. The measuring method of the cylindrical extrusion pressure in RR100 is the same as the measuring method of the cylindrical extrusion pressure in the reduction ratio 100 of the modified PTFE described above.

The stretched material of the present invention preferably has a breaking strength of 5 to 25N. More preferably, it is 5-15N. When the breaking strength is in the appropriate range, a PTFE porous membrane that can be stretched uniformly and has a low pressure loss can be formed.

The stretched material of the present invention preferably has a stress relaxation time of 100 to 600 seconds. The stress relaxation time can be measured by the same method as the stress relaxation time in homo-PTFE.

The stretched material of the present invention has a heat of fusion curve obtained from a differential scanning calorimeter at a heating rate of 10 ° C./min for PTFE (A) that has not been heated to a temperature of 300 ° C. or higher. It is preferable to have a peak top at ˜347 ° C. More preferably, it has a peak top at 325 to 332 ° C and 333 to 345 ° C.

The stretched material of the present invention may contain known additives and the like. For example, when the stretched material of the present invention is used as a material for producing a PTFE porous membrane, it is also preferable to include carbon materials such as carbon nanotubes and carbon black, pigments, photocatalysts, activated carbon, antibacterial agents, adsorbents, deodorants and the like. .

The stretched material of the present invention can be produced by various methods. For example, when the stretched material is a mixed powder, a PTFE (A) powder and a low molecular weight PTFE (B) powder are generally mixed. And a method of obtaining a co-coagulated powder by co-coagulating an aqueous dispersion containing PTFE (A) and low molecular weight PTFE (B). If it is such a method, even if it is any manufacturing method, a suitable extending | stretching material can be obtained.
Among them, since the pressure loss of the obtained PTFE porous membrane can be reduced, the stretched material of the present invention is an aqueous dispersion containing polytetrafluoroethylene (A) and low molecular weight polytetrafluoroethylene (B). It is preferable that it is obtained by coagulating.

From the viewpoint of more uniformly dispersing PTFE (A) and low molecular weight PTFE (B), the aqueous dispersion containing PTFE (A) and low molecular weight PTFE (B) is coagulated, that is, PTFE (A) and low A method of co-coagulating the molecular weight PTFE (B) is preferred.

Examples of the co-coagulation method include (i) a method of coagulating after mixing an aqueous dispersion of PTFE (A) and an aqueous dispersion of low molecular weight PTFE (B), and (ii) PTFE (A ) And then coagulating after adding the low molecular weight PTFE (B) aqueous dispersion, and (iii) adding the low molecular weight PTFE (B) powder to the PTFE (A) aqueous dispersion and coagulating. The method of analyzing is mentioned.
As the co-coagulation method, the method (i) is preferable because it is easy to uniformly disperse.

The co-coagulation is preferably carried out by adding an acid such as nitric acid, hydrochloric acid, or sulfuric acid; and metal salt such as magnesium chloride, calcium chloride, sodium chloride, aluminum sulfate, magnesium sulfate, or barium sulfate.

By molding the stretched material of the present invention, a PTFE porous membrane can be obtained.
A PTFE porous membrane formed by stretching the stretched material is also one aspect of the present invention. Since the PTFE porous membrane of the present invention is made of the above stretched material, the membrane appearance is excellent and the pressure loss is low. Moreover, the uniformity of the film is excellent.

The film thickness of the PTFE porous membrane is preferably 5 μm or more. More preferably, it is 10 micrometers or more, More preferably, it is 20 micrometers or more. If the film thickness is too thin, the mechanical strength may decrease. Moreover, although the upper limit of a film thickness is not specifically limited, For example, it is 100 micrometers.

The method for producing the PTFE porous membrane is not particularly limited, and a conventionally known method can be used. For example, a liquid lubricant such as solvent naphtha or white oil is added to the PTFE mixture, and paste extrusion is performed in a rod shape. Thereafter, the rod-like paste extrudate is rolled to obtain a PTFE green body (PTFE green tape). And can be produced by stretching the PTFE green tape.

The present invention is also the use of the PTFE mixture of the present invention to produce a PTFE porous membrane. Moreover, this invention is also a manufacturing method of the polytetrafluoroethylene porous membrane characterized by including the process of extending | stretching the mixture of this invention.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples.

In addition, each data in an Example is obtained with the following measuring method.

Standard specific gravity (SSG)
Measured according to ASTM D 4895.

Polymer (solid content) concentration of aqueous dispersion Based on the heating residue (Zg) of the aqueous dispersion (Xg) heated at 150 ° C. for 3 hours, the formula: P (solid content concentration) = Z / X × 100 (% ).

Average primary particle size The average was determined by measuring the transmittance of projection light having a wavelength of 550 nm with respect to the unit length of an aqueous dispersion whose polymer concentration was adjusted to 0.22% by mass and the directional direction diameter in a transmission electron micrograph. A calibration curve with the primary particle diameter can be prepared, and the transmittance can be measured for the aqueous dispersion to be measured and determined based on the calibration curve.

RR1600 paste extrusion pressure Measured according to ASTM D 4895.
50 g of PTFE fine powder and 10.25 g of hydrocarbon oil (trade name Isopar G (registered trademark), manufactured by Exxon), which is an extrusion aid, are mixed in a glass bottle for 3 minutes, and at room temperature (25 ± 2 ° C.) for 1 hour. Mature. Next, the above mixture is filled in an extrusion die with a cylinder (inner diameter 25.4 mm) (with an aperture angle of 30 ° and an orifice (orifice diameter: 0.65 mm, orifice length: 2 mm) at the lower end) and inserted into the cylinder. A load of 1.2 MPa is applied to the piston and held for 1 minute. Thereafter, the mixture is immediately extruded from the orifice at room temperature at a ram speed of 20 mm / min to obtain a rod-like material. In the latter half of the extrusion, a value obtained by dividing the pressure at the portion where the pressure is in an equilibrium state by the cylinder cross-sectional area is defined as the extrusion pressure.

Measurement item of low molecular weight PTFE Melt viscosity In accordance with ASTM D 1238, using a flow tester (manufactured by Shimadzu Corporation) and a 2φ-8L die, 2 g of a sample heated in advance at a measurement temperature (380 ° C.) for 5 minutes. Measurement was carried out while maintaining the above temperature with a load of 0.7 MPa.

Apparent density It measured based on JISK6891.

Using an average particle diameter laser diffraction type particle size distribution measuring device (manufactured by JEOL Ltd.), measuring the particle size distribution at a pressure of 0.1 MPa and a measurement time of 3 seconds without using a cascade, and 50% of the obtained particle size distribution integration Equal to the value corresponding to.

Evaluation of membrane appearance The PTFE sheet prepared by the method (1) below was stretched 5 times by 36 times by the method (2) below, and the resulting stretched sheet (PTFE porous membrane) was visually observed. And evaluated.

(1) Preparation of PTFE sheet 3 kg of PTFE powder and 780 g of extrusion aid (product name: Isopar M, manufactured by Exxon) were put into a 15 L plastic bottle, mixed at 100 rpm for 20 minutes, and placed in a furnace at 40 ° C. for 12 hours. Let stand and allow the extrusion aid to fully penetrate.

Next, the TFE polymer fine powder mixed with the above-mentioned extrusion aid is put into a 100φ mm preforming machine, and after reaching a pressure of 3 MPa, it is held for 10 minutes to obtain a preform. Subsequently, the preform is put into an extruder having an inner diameter of 100 mm, in which a die having an inner diameter of 16 mmφ is set at 50 ° C., and extruded. Furthermore, it rolls with the 400 mm diameter rolling roll heated at 60 degreeC, and makes a sheet | seat of thickness of 200 micrometers. The obtained sheet is heated to 180 ° C. to completely remove the extrusion aid and obtain a PTFE sheet.

(2) Stretching method Using the stretching apparatus provided with a plurality of rolls shown in FIG. 1, the TFE polymer sheet is unwound from the unrolled film unwinding roll 1 at a speed of 1.0 m / min, and the final winding speed is 5 m / min. For 5 minutes at a temperature of 250 ° C. (Uniaxial stretching)
The obtained 5-fold stretched sheet is stretched at a stretch ratio of 36 times in the width direction (biaxial stretching) using a device (tenter) shown in the left half of FIG. A PTFE porous membrane was obtained. At this time, the stretching temperature was 290 ° C., the heat setting temperature was 360 ° C., and the stretching speed was 500% / second.

The evaluation criteria are as follows.
○: Uniform Δ: Unevenness ×: Breaks during stretching

Using a PTFE porous membrane thickness gauge (1D-110MH type, manufactured by Mitutoyo Corporation), measure the total thickness by stacking 5 PTFE porous membranes stretched 5 × 36 times above. A value obtained by dividing the value by 5 was defined as one film thickness.

Pressure loss The PTFE porous membrane stretched 5 times in length x 36 times in width is set in a filter holder with a diameter of 100 mm, the inlet side is pressurized with a compressor, and the flow rate of air permeated with a flowmeter is 5.3 cm / sec. Adjusted. And the pressure loss at this time was measured with the manometer.

DSC melting point DSC melting point is a value measured using differential scanning calorimetry [DSC]. DSC uses RDC220 (manufactured by SII NanoTechnology Co., Ltd.) as a standard sample in advance and temperature calibrated using indium and lead. About 3 mg of tetrafluoroethylene [TFE] polymer powder is placed in an aluminum pan (crimp container). ), And a temperature range of 250 to 380 ° C. is raised at 10 ° C./min under an air stream of 200 ml / min. In addition, the amount of heat was calibrated using indium, lead, and tin as standard samples, and the above-described empty aluminum pan was used as a measurement reference. For the obtained heat of fusion curve, using Muse standard analysis software (manufactured by SII Nanotechnology), the temperature showing the peak top of the heat of fusion was taken as the DSC melting point.

Production Example 1 (Production of PTFE (A))
The following experiment was conducted according to the method described in Example 4 of JP-B-58-39443.
A stainless steel (SUS316) autoclave with stainless steel (SUS316) anchor-type stirring blades and temperature control jacket, 6 liters in volume, deionized water 2980 ml, paraffin wax 120 g and ammonium perfluorooctanoate 3.0 g Then, while heating to 70 ° C., the inside of the system was replaced with nitrogen gas three times and TFE gas twice to remove oxygen. Thereafter, the internal pressure is adjusted to 0.85 MPa with TFE gas, the mixture is stirred at 250 rpm, and the internal temperature is maintained at 70 ° C.
Next, an aqueous solution in which 18 mg of ammonium persulfate is dissolved in 20 ml of deionized water is press-fitted with TFE, the autoclave internal pressure is 0.8 MPa, the reaction temperature is 70 ° C., and the stirring speed is maintained at 250 rpm. TFE is continuously supplied so that the internal pressure of the autoclave is always kept at 0.90 ± 0.05 MPa.
When the consumption of TFE monomer reached 378 g, an aqueous solution in which 12 mg of hydroquinone was dissolved in 20 ml of deionized water was injected with TFE, and the reaction was continued.
When the amount of TFE monomer consumption reached 900 g, stirring and monomer supply were stopped, and the gas in the autoclave was immediately released to normal pressure, the reaction was terminated, and an aqueous dispersion of homo-PTFE was obtained. The obtained aqueous dispersion had a polymer concentration of 23.0% by weight and an average primary particle size of 0.33 μm.
Next, a stainless steel (SUS316) stirring blade, a baffle plate, and a temperature control jacket are provided, and the polymer concentration is 14% by weight by filtering the paraffin into a 6 liter stainless steel (SUS316) coagulation tank. 3 L of an aqueous PTFE dispersion diluted with deionized water was added.
After the temperature of the contents is adjusted to 20 ° C., stirring is started (450 rpm). At this time, 1 ml of nitric acid is charged into the coagulation tank as a coagulant. Stirring is stopped when the polymer powder separates from water. The obtained wet powder was newly washed with 3 L of deionized water. After this water washing operation was repeated twice, the TFE polymer powder was obtained by drying with a hot air circulating dryer at 160 ° C. for 18 hours (SSG 2.160, DSC melting point 343.7 ° C.).

Production Example 2 (Production of PTFE (B))
The following experiment was conducted according to the method described in Comparative Example 1 of International Publication No. 2009/020187.
In the same autoclave as in Production Example 1, 3300 ml of deionized water and 5.0 g of ammonium perfluorooctanoate were charged, and the system was purged with nitrogen gas three times and TFE gas twice to remove oxygen, and then chained. As a transfer agent, 70 mg of propane was injected with TFE, and the internal pressure was adjusted to 0.10 MPa with TFE. The temperature inside the tank was raised under stirring at 500 rpm, and when the pressure in the tank reached 55 ° C., TFE was injected again to make the internal pressure 0.75 MPa.
Next, an aqueous solution in which 850 mg of ammonium persulfate was dissolved in 20 ml of deionized water was press-fitted with TFE, and the internal pressure of the autoclave was adjusted to 0.80 MPa. TFE is continuously supplied so that the internal pressure of the autoclave is always kept at 0.80 ± 0.05 MPa. During the reaction, the temperature inside the tank was controlled to 55 ± 1 ° C. and the stirring speed was controlled to 500 rpm.
When the amount of TFE monomer consumption reached 850 g, stirring and monomer supply were stopped, and the gas in the autoclave was immediately released to normal pressure, the reaction was terminated, and an aqueous dispersion of low molecular weight PTFE was obtained. The resulting aqueous dispersion had a polymer concentration of 20.4% by weight and an average primary particle size of 0.18 μm.
Into the same coagulation tank as in Production Example 1, 3 L of an aqueous dispersion of low molecular weight PTFE diluted with deionized water to a polymer concentration of 14% by weight was prepared, and the temperature was adjusted so that the temperature of the contents was 25 ° C. Start stirring (450 rpm). At this time, 3.5 ml of nitric acid is charged into the coagulation tank as a coagulant. If the polymer powder separated from water, it was neutralized, and 10 g of a 24 wt% aqueous sodium hydroxide solution was added and stirring was continued for 5 minutes. The obtained wet powder was filtered off, washed with water and dried in the same manner as in Production Example 1 to obtain a low molecular weight PTFE powder. (Melt viscosity 1.7 × 10 ⁴ Pa · s, DSC melting point: 329.0 ° C., average particle size: 4.5 μm, apparent density: 0.36 g / ml).

Example 1
2850 g of the TFE polymer powder obtained in Production Example 1 and 150 g of the low molecular weight PTFE powder obtained in Production Example 2 are charged into a 15 L plastic bottle, mixed for 5 minutes with a tumbler mixer, and the TFE polymer and low molecular weight PTFE are mixed. (DSC melting point: 329.0 ° C. and 343.7 ° C.). Various measurements and evaluations were performed on the obtained mixture.

Example 2
The same coagulation tank as in Production Example 1 was charged with 2.85 L of the aqueous TFE polymer dispersion obtained in Production Example 1 and 0.15 L of the low molecular weight PTFE aqueous dispersion obtained in Production Example 2. . The aqueous dispersion obtained by removing the polymerization additive such as paraffin from the aqueous dispersion obtained after the aqueous dispersion polymerization of TFE by filtration was diluted with deionized water to a solid content concentration of 14% by weight. Is.
After the temperature is adjusted so that the temperature of the contents becomes 20 ° C., stirring is started (300 rpm). At this time, 10 ml of nitric acid as a coagulant is charged into the coagulation tank. The obtained wet powder was filtered, washed with water and dried in the same manner as in Production Example 1 to obtain a mixed powder of TFE polymer and low molecular weight PTFE (DSC melting points: 329.0 ° C. and 343.7 ° C.).
Various measurements and evaluations were performed on the obtained mixed powder in the same manner as in Example 1.

Example 3
The amount of the aqueous dispersion of the TFE polymer obtained in Preparation Example 1 was changed to 2.1 L, and the amount of the aqueous dispersion of the low molecular weight PTFE obtained in Preparation Example 2 was changed to 0.9 L. Co-coagulation was carried out in the same manner as in Example 2 to obtain a mixed powder of TFE polymer and low molecular weight PTFE (DSC melting points: 329.0 ° C. and 343.7 ° C.).
Various measurements and evaluations were performed on the obtained mixed powder in the same manner as in Example 1. In Example 3, the amount of the extrusion aid used for producing the PTFE sheet was changed to 750 g.

Comparative Example 1
The TFE polymer powder obtained in Production Example 1 was subjected to various measurements and evaluations as in Example 1.

Production Example 3
The TFE polymer powder obtained in Production Example 1 was irradiated with an electron beam of 180 kGray and further pulverized using an atomizer to obtain a low molecular weight PTFE powder (melt viscosity 1.9 × 10 ⁴ Pa · s). DSC melting point: 329.9 ° C., average particle size: 4.1 μm, apparent density: 0.39 g / ml).

Comparative Example 2
Except for changing the low molecular weight PTFE powder used as a raw material to the powder obtained in Preparation Example 3, mixing was performed in the same manner as in Example 1 to obtain a mixed powder of TFE polymer and low molecular weight PTFE (DSC melting point: 329.9 ° C and 343.7 ° C).
Various measurements and evaluations were performed on the obtained mixed powder in the same manner as in Example 1.

The results of each example and each comparative example are shown in Table 1.

As is clear from the results shown in Table 1, in Examples 1 to 3, a low pressure loss and an excellent film appearance can be achieved. In Comparative Example 1, the film appearance is excellent, but the pressure loss is high. In Comparative Example 2, the sample breaks during the stretching in the width direction (biaxial stretching), and a porous film cannot be obtained.

The stretched material of the present invention is suitable as a material for producing a PTFE porous membrane.

1: Unwinding film unwinding roll 2, 18: Winding roll 3, 4, 5, 8, 9, 10, 11, 12: Roll 6, 7: Heat roll 13: Unwinding roll 14 for longitudinally oriented film : Preheating zone 15: Stretching zone 16: Heat fixing zone 17: Laminate roll

Stretching material of the present invention is the heat of fusion curve by shows difference scanning calorimetry obtained at heating rate 10 ° C. / min, preferably has a peak top in the three hundred twenty-two to three hundred thirty-three ° C. and 333-347 ° C.. More preferably, it has a peak top at 325 to 332 ° C and 333 to 345 ° C.

Claims (9)

Polytetrafluoroethylene (A) having a standard specific gravity (SSG) of 2.130 to 2.230 and having fibrillation properties;
Low molecular weight polytetrafluoroethylene (B) which is not irradiated with an electron beam or radiation and has a melt viscosity at 380 ° C. of 1 × 10 ^{2 to} 7 × 10 ⁵ Pa · s and which is not fibrillated;
A stretched material comprising: The stretched material according to claim 1, wherein the mass ratio (A) / (B) of the polytetrafluoroethylene (A) to the low molecular weight polytetrafluoroethylene (B) is 99 to 50/1 to 50. The low molecular weight polytetrafluoroethylene (B) has a heat of fusion of 322 to 322 in PTFE (B) having no history of heating to a temperature of 300 ° C. or higher and a differential scanning calorimeter obtained at a heating rate of 10 ° C./min. The stretched material according to claim 1 or 2, which has a peak top at 333 ° C. The stretched material according to claim 1, 2 or 3, obtained by coagulating an aqueous dispersion containing polytetrafluoroethylene (A) and low molecular weight polytetrafluoroethylene (B). Polytetrafluoroethylene (A) is 333 to 347 ° C. in a heat of fusion curve obtained with a differential scanning calorimeter at a heating rate of 10 ° C./min for PTFE (A) that has not been heated to a temperature of 300 ° C. or higher. The stretched material according to claim 1, wherein the stretched material has a peak top. PTFE (A) having no history of heating to a temperature of 300 ° C. or higher has a peak top at 322 to 333 ° C. and 333 to 347 ° C. in a heat of fusion curve obtained at a heating rate of 10 ° C./min with a differential scanning calorimeter. The stretched material according to claim 1, 2, 3, 4 or 5. A polytetrafluoroethylene porous membrane obtained by stretching the stretched material according to claim 1, 2, 3, 4, 5 or 6. Use of the stretched material according to claim 1, 2, 3, 4, 5 or 6 for producing a polytetrafluoroethylene porous membrane. A method for producing a porous polytetrafluoroethylene film comprising a step of stretching the stretched material according to claim 1, 2, 3, 4, 5 or 6.

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