JPS6144656B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a novel method for producing a porous tetrafluoroethylene resin molded product having a uniform micropore diameter suitable for use as filters, electrolytic membranes, battery membranes, gas separation membranes, and the like. Porous tetrafluoroethylene resin is called tetrafluoroethylene resin (hereinafter referred to as
(abbreviated as PTFE) has excellent heat resistance, chemical resistance,
Utilizing electrical insulation and water repellency, various filters,
In addition to diaphragms, it is used in waterproof and breathable 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, Tokuko Showa 42-13560
This is a method in which PTFE powder and a liquid lubricant are formed into a sheet or tube shape using a paste method, which is then stretched in an unfired state, and then fired to make it porous. 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, but on the other hand, it produces porous bodies with extremely fine pore diameters, for example, pore diameters finer than 0.1μ. The drawback was that it was impossible. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a method for producing porous PTFE molded products such as films, tubes, rods, and filaments having uniform and extremely fine pore diameters. The present invention is characterized in that the fired non-porous PTFE molded product is stretched in two stages depending on the stretching temperature range to make it porous.
The molded product is heated at a temperature below the second-order transition point near 130°C.
This relates to a manufacturing method in which a porous PTFE molding having uniform and extremely fine pores is carried out by stretching at least twice as much, and then stretching at least twice as much at a temperature above the secondary transition temperature and below the melting point. can get things. By performing the first stretching at low temperature using a non-porous PTFE molded product in which particles are fused and integrated by firing, many extremely fine clutches are formed on the entire surface due to the large tensile stress. In the second stretching operation performed at a high temperature, the mechanism is presumed to be that the cracks grow together with the fine cracks as nuclei due to relatively low stress. Such a manufacturing method was completely unknown in the past, and the present invention will be explained in more detail in the order of the steps below. (1) Manufacture of non-porous PTFE molded product After molding unfired PTFE powder into the desired product shape, it is fired and cooled. The molding may be performed by compression molding or ram extrusion, but it is more preferable to mold by paste extrusion from a mixture of fine powder and liquid lubricant.
The fine powder mentioned here is a fine powder produced by coagulating dispersion obtained by emulsion polymerization, and its primary particles have a diameter of 0.2 to 0.4 μ.
This is further granulated into secondary particles of about 20 mesh. The liquid lubricant that can wet the PTFE surface and can be removed by evaporation, extraction, etc. after molding, such as solvent naphtha or white oil, can be used. Also, the amount added is usually
It is used in a range of 17 to 35% by weight based on PTFE100. The mixture in which the liquid lubricant is uniformly and sufficiently dispersed and permeated into the PTFE fine powder is formed into a film-like or tube-like shape by extrusion and/or rolling. The liquid lubricant is then removed by evaporation or extraction. The unfired PTFE molded product obtained here is a porous body having a porosity depending on the amount of the liquid lubricant mixed, since the removed 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 without compressing it. Therefore, with a normal firing method, it is possible to obtain a fired product having approximately the same cross-sectional dimensions as when molded, although it shrinks somewhat in the length direction. On the other hand, by applying compressive force to the thickness or radial direction of the molded product before or during firing, and firing while maintaining the lengthwise direction so as not to shrink, thinner films or non-porous materials such as tubes can be created. It is also possible to obtain a fired product with more complete oxidation. This compressive force can be applied to a film, for example, by passing it between heated rolls, and to a tube, by drawing it with a die 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 in general, when rapidly cooled, the degree of crystallinity is 50 to 60%. When this is slowly cooled, the slower the cooling rate, the higher the degree of crystallinity. In a more preferred implementation of the present invention, it is necessary that the degree of crystallinity is high, and even after firing, a non-porous PTFE molded product with a degree of crystallinity of 65% or more is uniformly and Porous PTFE with fine pores
It was found that molded products can be given. Another way to increase the crystallinity is to heat the fired PTFE molded product so that it begins to decompose and deteriorate again at a temperature above its melting point.
It may be heated to a temperature of 380°C or less and then slowly cooled. In any case, the cooling rate near the melting point has a large effect on the degree of crystallinity. By performing an operation to increase the degree of crystallinity, new microcrystals are generated in the previously amorphous portion, which disperses the deformation stress during the stretching operation.
This is thought to lead to more uniform and finer pore growth and fewer fractures. (2) Stretching of the fired non-porous PTFE molded product First, the non-porous PTFE molded product obtained in the step (1) above is stretched at a temperature below the second order transition point near 130°C by a factor of 3 times or less. Perform stretching. This low temperature stretching operation
Extremely high stress is required to deform PTFE, and extremely fine cracks are generated at the interface between amorphous and crystalline materials. Therefore, the lower the stretching temperature, the finer and more uniform the cracks produced, and the stretching is preferably carried out at 60°C or lower. In addition, this low-temperature stretching is performed at a stretching ratio of up to 3 times.
It is preferable to do this in the range 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, a porous PTFE molded product obtained by only the first stretching at a low temperature has an extremely low porosity, but if the stretching ratio is further increased forcibly, only a product with large variations in pore diameter can be obtained. Next, a second stretching is performed at a temperature above the secondary transition point and below the melting point. By carrying out the stretching at this high temperature, the ultrafine cracks generated during the low-temperature stretching grow mutually as nuclei, and uniform pores penetrating both surfaces of the film are generated. 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 is insufficient, and in general, the higher the stretching ratio is, the higher the porosity is. 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 are that the ratio of the stretching ratio at low temperature to that at high temperature is 1:2 or more, such as 2 times and 4 times. It is better to do it by value. The stretching speed can be set arbitrarily depending on the desired porosity and pore size, but as long as it can give uniform temperature and deformation stress to the starting material PTFE to be stretched,
The larger the stretching speed, the larger the deformation stress.
As a result, it is preferable to make the diameter of the pores emitted uniform and to make the diameter of the pores fine. On the other hand, it is not possible to obtain a porous PTFE molded product having desired uniform fine pores only by high-temperature stretching. That is, when stretching is carried out at high temperatures, so-called netting occurs, and the porosity hardly increases and only the cross-sectional area of the molded product decreases. Therefore, even if the material is stretched and deformed to near the breaking point, only a porous PTFE molded product with extremely low porosity will be obtained. In conclusion, porous PTFE molded products with uniform and fine pores can only be obtained by combining low-temperature stretching, where high stress acts, and high-temperature stretching, which has good sliding properties between crystals and amorphous. The present invention will be explained below with reference to examples, but the scope of the present invention is not limited thereto. Example 1 PTFE fine powder (manufactured by Daikin Industries, Ltd.,
Mix 27 parts by weight of a liquid lubricant (manufactured by Shell Oil Co., Ltd., Naphtha No. 5) to 100 parts by weight of product name F104,
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 is heated to 160-200℃ to volatilize and remove the naphtha, and then heated in a firing furnace at 355-370℃.
Passed through a slow cooling furnace at 340℃, thickness 0.1mm, crystallinity
A 68% non-porous PTFE film was obtained. Using this film as a starting material, it was first stretched at a temperature of 20° C. from 1.2 times or more to 3 times in the length direction as shown in Table 1. When stretched at a magnification higher than 3 times, the film often broke and was unstable. Next, the low-temperature stretched film was stretched more than twice in the same direction in a temperature range of 180°C to 280°C to obtain a porous PTFE film.
The above stretching conditions and properties of the obtained film are shown in the table below.
Shown in 1. Furthermore, as comparative examples, Table 1 also shows the results when only low-temperature stretching was performed and when only high-temperature stretching was performed. In this comparative example, the generated pores
Porous films with porosity larger than 0.1μ or with low porosity have been 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 molded product 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 5000 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.

[Table] Example 2 A 0.1 mm thick unfired PTFE film produced in the same manner as in Example 1 was pasted onto an endless belt as shown in FIG. 1 while applying compressive force. Here, the firing process will be explained according to Fig. 1.
An unfired PTFE film 9 is sent out from the supply roll 1 and is crimped onto the endless belt 8 by the crimping roll 2. The film fired by heat transfer from the heating roll 3 is heat-treated in a slow cooling furnace 7, then allowed to cool on a cooling roll 4, passed through a take-up roll 5, and then wound onto a take-up roll 6. The obtained film was a non-porous film with a thickness of 0.075 mm and a crystallinity of 70%. Using this, low-temperature stretching and high-temperature stretching were successively carried out under the conditions shown in Table 2 to obtain a film having the porous properties shown in Table 2.

[Table] Example 3 Using the same mixture as in Example 1, after preforming, a tube with an outer diameter of 4 mm and an inner diameter of 3 mm was extruded, dried in a drying oven,
A non-porous PTFE tube having the above dimensions and a crystallinity of 74% was obtained by passing through a firing furnace and a slow cooling furnace. Using this tube, low-temperature stretching and high-temperature stretching were successively performed under the conditions shown in Table 3 to obtain tubes with porous properties shown in Table 3.

[Table] Example 4 Unfired 0.1 mm thick by the same method as Example 1
After obtaining the PTFE film, it was passed through a heating furnace, and after firing, it was immediately cooled at room temperature (Sample A), and when it was slowly cooled at a cooling rate of 70℃/hr (Sample B), it was further cooled to 15℃/hr. Non-porous films were produced in which the film was slowly cooled at a cooling rate of hr (sample C) and at three different cooling rates. When measuring the crystallinity of each sample, sample A was 55
%, sample B was 65%, and sample C was 71%. Next, each sample was successively subjected to low-temperature stretching and high-temperature stretching under the conditions shown in Table 4 to obtain films with porous properties shown in Table 4.

【table】 [Brief explanation of the drawing]

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 winding roll, 7 is a lehr, 8 is an endless belt,
9 is PTFE film.

Claims (1)

[Claims] 1. A fired non-porous tetrafluoroethylene resin molded product having a second-order transition point near 130°C is stretched three times or less at a temperature below the second-order transition point (hereinafter referred to as low temperature). A method for producing a porous tetrafluoroethylene resin molded product, which comprises stretching at least twice or more at a temperature above the secondary transition temperature and below the melting point (hereinafter referred to as high temperature). 2. A method for producing a porous tetrafluoroethylene resin molded product according to claim 1, characterized in that the low-temperature stretching is carried out at a temperature of 60° C. or lower. 3. The method for producing a porous tetrafluoroethylene resin molded product according to claim 1, characterized in that the stretching at low temperature is carried out in a range of 1.2 to 2 times. 4. The method for producing a porous tetrafluoroethylene resin molded product according to claim 1, wherein the ratio of the draw ratio at low temperature to the draw ratio at high temperature is 1:2 or more. 5. The method for producing a porous tetrafluoroethylene resin molded product according to claim 1, wherein the crystallinity of the nonporous tetrafluoroethylene resin molded product is 65% or more.