JPS595037A - Manufacture of porous tetrafluoroethylene resin - Google Patents

Abstract

PURPOSE:To provide a uniform fine pore, by stretching a sintered nonporous tetrafluoroethylene resin by three times and below at the temperature under a secondary transition point, then stretching it by two times and more at the temperature above the secondary transition point and below a melting point. CONSTITUTION:After a sintered nonporous tetrafluoroethylene resin is stretched by three times and below at the temperature under a secondary transition point near 130 deg.C, it is stretched by two times and more at the temperature above the secondary transition point and below a melting point and the porous tetrafluoroethylene resin having a uniform and fine pore is obtained. A stretch velocity can be set at will according to a desired pore ratio and pore diameter and if it is in the range in which a uniform temperature and deformation stress are given to the starting material of the tetrafluoroethylene resin to be stretched, the larger the value of the stretch velocity is, the larger the deformation stress is made, so it is suitable for making the resulted pore diameter uniform and fine.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a novel method for producing porous tetrafluoroethylene resin having a uniform micropore size suitable for use in filters, electrolytic membranes, battery membranes, gas separation membranes, etc. It is something. Porous tetrafluoroethylene resin takes advantage of the excellent heat resistance, chemical resistance, electrical insulation, and water repellency of tetrafluoroethylene resin (hereinafter abbreviated as PTFE), and is used in various filters, diaphragms, and other waterproof and breathable materials. It is used for materials, wire covering materials, sealing materials, etc. Several methods are already known for producing the material, but the most commercially attractive method is to make it porous through a stretching operation. Basically, PTFE powder and liquid lubricant disclosed in Japanese Patent Publication No. 42-1'1560 are formed into a sheet or tube shape using a paste method, stretched in an unfired state, and then fired to make them porous. This is the way to do it.

This method has the advantage that it can be applied to a wide range of product forms and that it is possible to obtain porous bodies with high porosity. The object of the present invention is to provide a method for producing PTFE which is impossible to obtain.

The present invention is characterized in that the fired non-porous PTFE is stretched in two stages depending on the stretching temperature range to make it porous. This relates to a manufacturing method in which stretching is carried out by a factor of 3 times or less at a temperature below the secondary transition temperature, and then by a factor of at least 2 times or more at a temperature above the secondary transition temperature and below the melting point. It is possible to obtain porous PTFE having the following properties. Non-porous PTF whose particles are fused and integrated by firing
By performing the first stretching at low temperature using E as a starting material, many extremely fine racks are formed over the entire surface due to its large tensile stress, and in the subsequent second stretching operation at high temperature, the stress is relatively low. Therefore, it is assumed that the mechanism is that cracks grow together with micro cracks as nuclei. Such a manufacturing method was completely unknown in the past, and the present invention will be explained in more detail below in order of f.

(1) Production of non-porous PTFE Unfired PTFE powder is shaped into a desired product shape, then fired and cooled. Molding may be carried out by compression molding or ram extrusion, but more preferably, fine powder and liquid lubricant are mixed and this is further granulated into secondary particles of about 20 mesh. Liquid lubricant is PTFE
For example, solvent naphtha, white oil, etc., which can wet the surface and which can be removed by evaporation, extraction, etc. after molding, can be used. In addition, its blending amount is usually P
It is used in an amount of 17 to 35% by weight based on 100 TFE. This I'TFE fine powder is extruded or/
Then, it is formed into a film or tube shape by rolling. The liquid lubricant is then removed by evaporation or extraction. The green PTFE molded article obtained here is a porous body having a porosity depending on the amount of the liquid lubricant mixed, since the removed portion of the liquid lubricant remains as voids. When this is fired as it is, for example, in a heating furnace, the molten resins fuse together and become non-porous. In this case, since it tends to undergo some heat shrinkage in the direction of extrusion or rolling, it is preferable to sinter it so that it does not shrink. Therefore, in the normal firing method, although the length direction shrinks somewhat, the cross section is
By firing the product with almost the same dimensions as when molded and keeping it in the longitudinal direction so as not to shrink, it is possible to obtain a more complete fired product with thinner films or tubes that are non-porous. You can also do it. This compressive force can be applied to a film, for example, by passing it between heated rolls, and to a tube, by drawing it out with a gouer that is thinner than the outer diameter of the tube and a floating die that is thicker than the inner diameter of the tube.

After being fired, the molded product is cooled, and the degree of crystallinity of the molded product changes depending on the cooling rate.

The degree of crystallinity is somewhat related to the molecular weight of the resin used, but generally, when rapidly cooled, the degree of crystallinity is 50 to 6o96. When this is slowly cooled, the slower the cooling rate, the higher the degree of crystallinity. In a more preferred implementation of the invention,
It is necessary to have a high crystallinity, and non-porous PTFE with a crystallinity of 6596 or higher even after firing will yield porous PTFE with more uniform and finer pores in the next drawing process. It turned out that. Another way to increase the crystallinity is to heat the fired PTFE molded product again at 880'C, where thermal decomposition and deterioration begin at a temperature above the melting point.
It may be heated to the following temperature and then slowly cooled.

In any case, it is thought that the cooling rate near the melting point will lead to more uniform and fine pore growth and fewer fractures as a result of the crystallinity.

(2) Stretching of fired non-porous PTFE" The non-porous PTFE obtained in step 1 (1) is first stretched by 8 times or less at a temperature below the secondary transition point near 130°C. conduct. This low temperature stretching operation requires extremely high stress to deform the PTFE and causes extremely fine racks to form at the amorphous-crystalline interface. Therefore, the lower the stretching temperature, the finer and more uniform the generated cracks, and the stretching is preferably carried out at 60°C or lower. The low-temperature stretching is performed at a stretching ratio of up to 3 times, but it is preferable to perform it at a stretching ratio of 1.2 times to 2 times in order to reduce the frequency of breakage.

The reason for this is that when the stretching ratio is increased, the number of cracks that occur does not increase, but only the size of the cracks increases, which ultimately makes them more likely to break. Therefore, porous PTFE obtained only by the first stretching at a low temperature has an extremely low porosity, but if the stretching ratio is further increased forcibly, only the pore diameters can vary widely.

- [and will grow together, resulting in uniform pores penetrating both surfaces of the film.

The high temperature stretching is performed at a stretching ratio of at least 2 times or more.

If the stretching ratio is lower than this, the growth of micropores will be insufficient.
Generally, the higher the stretching ratio, the higher the porosity. The setting of the specific stretching ratio is considered together with the stretching ratio in low-temperature stretching. Generally, it is preferable to carry out low-temperature stretching at a low stretching ratio and high-temperature stretching at a high stretching ratio.More preferable conditions include l:2 or higher, such as a ratio of the stretching ratio at low temperature to the stretching ratio at high temperature of 2 times and 4 times. It is best to use the value .

The stretching speed can be set arbitrarily depending on the desired porosity and pore diameter, but as long as a uniform temperature and deformation stress can be applied to the starting material PTFE to be stretched, the higher the stretching speed, the more the deformation stress can be reduced. It is preferable to increase the size of the pores, make the resulting pore diameter uniform, and make the pore diameter microscopic.

On the other hand, it is not possible to obtain porous PTFE having desired uniform fine pores only by high-temperature stretching. That is, when stretching is carried out at high temperatures, so-called necking occurs, and the porosity does not increase at all, but only the cross section of the molded product becomes stiff. In conclusion, by combining gentle low-temperature stretching with high stress and high-temperature stretching with good sliding properties between crystals and amorphous, uniform and fine Example 1゜PTFE Fine Powder (manufactured by Daikin Industries, Ltd., 9-product) can be produced. Name F104.) 27 parts by weight of a liquid lubricant (manufactured by Shell Oil Company, naphtha A5) is mixed with 100 parts by weight,
After preforming the mixture, it was extruded into a round bar shape and rolled into a film with a thickness of 0.1 mm.

Next, the film was heated to 160 to 200°C to volatilize and remove the naphtha, and then placed in a firing furnace at 355 to 870°C.

Passed through a slow cooling furnace at ~340'C, thickness Q, l mx,
Non-porous PTFE with a crystallinity of 68% was obtained. Using this film as a starting material, first, at a temperature of 20°C, Table 1
As shown in Figure 2, stretching was carried out in the length direction from 1.2 times or more to 3 times. When stretched at a ratio higher than 3 times, the film often broke and was unstable. Next, the low-temperature stretched film was stretched twice or more in the same direction in the temperature range of 180°C to 280°C as shown in Table-1. In this comparative example, the pores formed were larger than 0.1 μm, or only a porous 10-based film with a low porosity was obtained.

Here, methods for measuring each physical property will be explained. The degree of crystallinity was determined by measuring the specific gravity of the PTFE in acetone and from the relationship between the specific gravity and the degree of crystallinity. The relationship between specific gravity and crystallinity is determined by X-ray diffraction and infrared absorption spectroscopy.

The porosity of the porous PTFE was calculated by measuring the specific gravity by an underwater floating method and comparing it with the specific gravity of the starting material.

The size of the generated pores can be measured using a mercury porosimeter, but it can also be evaluated using an electron micrograph taken at a magnification of 5,000 times or more. A simple method for determining whether a hole is a through hole is to drop a solvent capable of wetting PTFE onto the molded product, and if the solvent permeates and the molded product becomes transparent, it is a through hole. I understand.

did not exist.

×2) There is large variation in hole size.

Example 2 An unfired PTFE film with a thickness of 0.1 mm produced in the same manner as in Example 1 is pasted onto an endless belt as shown in FIG. 1 while applying a compressive force.

Here, to explain the firing process according to FIG. 1, the unfired PTF E film 9 is sent out from the supply roll (2)
It is crimped onto the endless belt 8'' by the crimping roll 2. Firing was performed by heat transfer from the heating roll 8. Using this, low-temperature stretching and high-temperature stretching were successively carried out under the conditions shown in Table 2 to obtain a film with the porous properties shown in Table 2.

Using the same mixture as in Example 1, after preforming, the outer diameter was 4 B.
, a tube with an inner diameter of 3 mytt was extruded and passed through a drying furnace, a firing furnace, and a slow cooling furnace to obtain a crystallinity of 741% with the above dimensions.
A non-porous PTFE tube was obtained. Using this Chee 13-use, low-temperature stretching and high-temperature stretching were successively carried out under the conditions shown in Table 3 to obtain tubes with porous properties shown in Table 3.

After obtaining an unsintered PTFE film with a thickness of 0.1111M by the same method as in Example 1, it was passed through a heating furnace and immediately cooled at room temperature after firing (referred to as sample A).
When slowly cooling at a cooling rate of hr (sample B), an additional 1
When slowly cooling at a cooling rate of 5°C/hr (sample C)
Nonporous films were manufactured using three different cooling rates.

When the crystallinity of each sample was measured, sample A was 55%, sample B was 65%, and sample C was 71%. Next, each sample was sequentially subjected to low-temperature stretching and high-temperature stretching under the conditions shown in Table 4.
A film with porous properties shown in Table 4 was obtained.

×1) Breakage easily occurs during stretching.

[Brief explanation of drawings]

FIG. 1 schematically illustrates an apparatus suitable for applying compression in the thickness direction and firing to make the material non-porous. 1 is a supply roll, 2 is a pressure roll, 3 is a heating roll, 4
・ is a cooling roll, 5 is a take-up roll, 6 is a take-up roll, 7
'C Annealing furnace, 8C Endless belt, 97j1 Figure 189-

Claims (6)

[Claims] (1) Calculated non-porous tetrafluoroethylene resin II) °C
Stretching is performed by 8 times or less at a temperature below the nearby secondary transition point (hereinafter referred to as low temperature), and then stretching is performed at least 2 times or more at a temperature above the secondary transition temperature and below the melting point (hereinafter referred to as high temperature). A method for producing porous tetrafluoroethylene resin, characterized by: (2) The manufacturing method according to claim 1, characterized in that the low-temperature stretching is carried out at a temperature of 60° C. or lower. (3) The manufacturing method according to claim 1, wherein the stretching at low temperature is performed in a range of 1.2 to 2 times. (4) The ratio of the draw ratio at low temperature to the draw ratio at high temperature is 1:
The manufacturing method according to claim 1, wherein the value is 2 or more. (5) The manufacturing method according to claim 1, characterized in that the nonporous tetrafluoroethylene resin to be stretched is heat-treated to have a crystallinity of 65% or more = 1-. (6) Claims characterized in that the non-porous tetrafluoroethylene resin to be stretched is molded by a paste method using a liquid lubricant and then fired. Manufacturing method of paragraph 1.