JP4812826B2 - Polytetrafluoroethylene resin fine powder or its extrusion - Google Patents

Description

  The present invention relates to an ethylene tetrafluoride resin fine used for medical materials such as gas separation membranes, liquid separation membranes, gas-liquid diaphragms for the purpose of degassing liquids or dissolving gases in liquids, and artificial blood vessels. The present invention relates to powder or an extruded product thereof.

  A porous body made of tetrafluoroethylene resin (hereinafter abbreviated as PTFE) (referred to as a fluororesin porous body) has excellent material characteristics such as chemical resistance and heat resistance, and is uniform and fine. It has a porous structure and has been conventionally used in many applications such as separation membranes, artificial blood vessels, catheters, and incubators. It is also used as a degassing of aqueous liquids utilizing the water repellency, or conversely as a gas-dissolving diaphragm for aqueous liquids.

  This fluororesin porous body is usually prepared by mixing a liquid lubricant (auxiliary) with PTFE fine powder as a raw material, pressing it into a cylindrical shape by pre-molding, and extruding the paste into a predetermined shape. The product is manufactured by a method of stretching the product in the major axis direction to make it porous, and then firing it. In addition to the stretching method, there are methods such as extraction of a pore-forming agent after mixing with a pore-forming agent and foaming with a foaming agent, but porosity such as porosity, filter performance, or strength that represents the degree of porosity. From this aspect, the previous stretching method is excellent.

  The fluororesin porous body is required to have characteristics such as an average pore diameter, a transmittance, a permeation flow rate, and strength at desired levels according to various uses. Of these characteristics, the filter performance is mainly determined by the permeation flow rate of the gas or liquid that permeates the porous body and the limit value of the size of the particles to be separated by filtration, that is, the pore diameter. These two characteristics are often substituted by a porosity, which is the proportion of air in the porous body, and a bubble point, which is the permeation start pressure of air in alcohol. The bubble point is a method using a capillary phenomenon, and the smaller the hole diameter, the larger the capillary phenomenon. Therefore, the higher the bubble point, the smaller the hole.

Usually, if the pore size and bubble point, which are the separation ability of filtration, are the same, it is desirable that the porosity indicating the filtration processing efficiency is high. However, as a method for producing a fluororesin porous body, if a method of making holes by stretching is adopted, the porosity increases as the stretching rate increases, but at the same time the pore diameter increases, Filtration performance decreases. Thus, there is a correlation between the porosity and the bubble point. Normally, if the porosity is determined, the bubble point is also determined.
Therefore, those having a small pore diameter and high filtration performance have a small permeation flow rate, and in order to increase the permeation flow rate, an attempt to increase the porosity increases the pore diameter, resulting in a decrease in filtration performance.
Therefore, for example, if it is intended to be used as a filter having a small pore diameter so as to have a higher filtration performance associated with higher integration of semiconductors, only a filter having a very small filtration processing capacity can be obtained, and practical application is difficult.

Thus, the PTFE stretched porous body is basically a relationship between the pore diameter and the porosity, although the pore diameter and porosity can be changed to some extent by changing the speed and temperature conditions during stretching due to problems in the production method. The degree of freedom is small. Of course, in addition to stretching, the pore generation mechanism of the porous body is influenced by the degree of connection between PTFE particles due to the pressure during extrusion and the degree of firing due to the heating condition during firing, as will be described later. However, there is a limit value in the former as described later, and the latter has high filter performance if the firing is weak, but the strength and durability are impaired. is there.
Therefore, a PTFE porous body made of PTFE fine powder as a raw material has not been able to be realized with better filtration performance and higher processing efficiency.

  An object of the present invention is to provide a fluororesin porous body that improves the properties of PTFE fine powder in stretching, has good filtration performance, and has high processing efficiency.

  As a result of intensive research on this problem, the present inventor has applied radiation to the PTFE fine powder at the raw material powder stage or the extruded product at the extrusion stage before the stretching process in the fluororesin porous body manufacturing process. , The bubble point and the porosity have a correlation, and when trying to increase the porosity, the bubble point becomes smaller and the inherent disadvantage of the PTFE fine powder is modified. A fluororesin porous body having good performance and high processing efficiency was obtained.

  According to the present invention, it is possible to control the stretchability of PTFE fine powder, and it is possible to obtain a fluororesin porous body having higher filtration performance with the same permeability by selecting an appropriate radiation dose. It becomes possible. Further, the performance can be known in advance by analyzing the melting peak with a differential scanning calorimeter, and industrially well-controlled production becomes possible. Although there is no restriction | limiting in particular in the use of these invention goods, It can use suitably for a separation membrane use etc.

  The fluororesin porous body of the present invention can be produced, for example, by the following technique. First, a mixed paste of PTFE fine powder and a lubricant is extruded into a sheet shape or a tube shape in an extrusion process, rolled as necessary, and then stretched. In this step, fibers are formed in the stretching direction so that yarns are pulled between crack-like holes formed such that the resin powders pressure-bonded by extrusion are torn apart by stretching. Thereafter or simultaneously, the fluororesin porous body can be obtained by heating to at least the melting point of PTFE of 327 ° C. or higher and sintering.

  In the present invention, the PTFE fine powder is irradiated with radiation such as heavy ion rays, alpha rays, beta rays, that is, electron rays, gamma rays, X rays, ultraviolet rays, etc. at any stage before the stretching step in the above process. This line type is not very suitable because it can cause uneven effects on PTFE molecules with large particle beams such as heavy ions, and X-rays and ultraviolet rays have little energy to be applied to PTFE molecules. And gamma rays are desirable, and gamma rays with high permeability are most desirable from the viewpoint that uniform irradiation treatment to powder and extrusion molding is possible.

The irradiation dose before stretching in the present invention needs to be a very small amount compared with the irradiation dose of a general polymer treatment. Specifically, it is 10 Gy or more and less than 1000 Gy , and it is desirable that it is in the range of 100 Gy to 500 Gy from the viewpoint of controlling the stretchability described later.

  The function of the present invention will be described below. The secondary particles of several hundred μm of the PTFE fine powder are formed by collecting primary particles having a size of about 0.2 μm, and in the primary particles, a band-like crystal is present in a folded state. The connection between the primary particles formed by particles rubbing by the flow in the extrusion process connects both particles, and when the particles are stretched during stretching, the folded crystal band in the primary particles unwinds and forms a fiber It becomes a structure. That is, the porous structure formed by stretching is determined by two factors, that is, the amount of connection formation and the degree of squeezing of the crystal band from the folded structure.

  Since porosity correlates with stretch ratio, the size of holes at the same porosity is inversely correlated with how many holes are generated per unit length of the extruded product before stretching, that is, with the density of holes generated. . That is, if there are many holes generated by stretching, many small holes are available, and if there are few holes, each hole will be large. In general, the higher the extrusion pressure, the more shear stress is applied to the particles, the greater the amount of connections formed, and the higher the hole density.

  In the prior art, as a method of increasing the extrusion pressure, a method of reducing the amount of oil as an extrusion aid has been used. However, as a result of investigations by the inventors, the number of connections is 2 per particle, that is, the state where adjacent particles are in a state where hands are connected to each other is the limit, and even if the extrusion pressure is increased further, the number of connections may not increase. know.

  As previously mentioned, the porous structure due to stretching is determined by the amount of connection formation and the extent of the banding of crystals from the folded structure. As a result of intensive studies on the method of controlling the porous structure other than the limited connection formation amount, the present inventors have found that the length of fibers that can be generated from PTFE fine powder particles can be changed by irradiation. did. That is, when the PTFE fine powder is irradiated with radiation, the folded structure of the crystal band in the primary particles is partially broken, and the amount of thread to be pulled out becomes shorter.

  This phenomenon can be shown by the relationship between the irradiation amount and the elongation in the tensile test of the extruded product after irradiation. The elongation of the extruded product depends on the radiation dose, and the phenomenon of elongation begins to be detected from a dose of about 10 Gy, and hardly becomes stretched by irradiation of about 1 kGy or more. That is, the fiber forming ability is lost and the film cannot be substantially stretched. This decrease in elongation is not different between irradiation of the raw material powder before extrusion and irradiation after extrusion. The tensile strength correlates with the extrusion pressure but does not correlate with the irradiation amount. In other words, irradiation does not affect the connection formation during extrusion, but exclusively inhibits the thread from the particles, that is, the stretching of the folded structure of the crystal band.

  The present inventors have also found that these phenomena can be captured by observing the endothermic curve of PTFE with a differential scanning calorimeter. The PTFE fine powder produced from emulsion polymerization has an endothermic peak at 347 ° C., which is 20 ° C. higher than the 327 ° C. possessed by molding powders made by other polymerization methods and once heated PTFE. It was found that irradiation changed the melting point peak at 347 ° C. to around 335 ° C.

PTFE fine powder is known to have a peak at 347 ° C. and a shoulder near 335 ° C. When the PTFE fine powder is irradiated with radiation, an endothermic peak begins to appear in the shoulder near 335 ° C from about 10 Gy, and when the dose is increased further, the peak at 347 ° C decreases and conversely the peak at 335 ° C. Is getting bigger.
And with irradiation of about 1 kGy or more, the peak at 347 ° C. can no longer be detected. (See Figure 2)
As shown in FIG. 2 Chart A, an extruded product made of PTFE in a state where the peak at 347 ° C. cannot be detected is, when subjected to a tensile test, fractured with almost no elongation, and is actually impossible to stretch. Become.

That is, in order to successfully control the stretching in realizing the present invention, both the 347 ° C. peak indicating that the elongation is increased and the 335 ° C. peak indicating that the elongation is reduced are applied to the PTFE irradiated with radiation. There will be a need to have a peak.
It should be noted that at least in the above-mentioned range of irradiation, the amount of heat necessary for crystal melting, which is an index of the amount of crystal, that is, the latent heat of fusion, is not different from the case of non-irradiation. It has been suggested that this is not due to PTFE crystal destruction.

  As a conventional technique for this kind of change in melting point peak, for example, there is a heat treatment of PTFE fine powder disclosed in Japanese Patent Publication No. 58-145735 and Japanese Patent No. 2533229. The former is a method in which the fine powder has a melting point shoulder of 338 ° C. and a single peak at 347 ° C., and the latter is characterized by a single peak at 335 ° C. The biggest difference between these methods and the present invention is that the method of the present invention does not involve a change in latent heat of fusion as described in the previous section. That is, in the conventional method in which PTFE fine powder is heat-treated, the crystal is melted to some extent by heating, and the melting point is changed, so that the change in the amount of crystals is unavoidable. In the method of the present invention, there is almost no change in the latent heat of fusion, that is, the amount of crystals, even if irradiation is performed until a single peak of 335 ° C. is obtained, which is similar to the fusion heat peak obtained by the method disclosed in Japanese Patent No. 2533229. It is considered that the heating mechanism is different from these heating methods.

  An application example of the technology of the present invention will be specifically described in the case of reducing the pore diameter without changing the porosity. Ordinary extruded products vary depending on conditions, but generally can be stretched by 1000% or more. When it is desired to reduce the pore diameter, the stretching ratio is lowered. However, for example, in the case of a uniaxially stretched tube having a pore diameter of 0.1 μm, the stretching ratio must be suppressed to 100 to 150%. Unirradiated can cope with stretching of 1000% or more, so that the holes generated in the initial stage become reasonably large to some extent and do not lead to the generation of the next hole. That is, the density of the holes does not increase so much.

  On the other hand, in the case of an irradiated product in which the amount that can be stretched is limited, even if the stretch rate is low, the fibers in the perforated portion do not extend any more and other perforated portions can be generated, resulting in the opening. It is possible to increase the pore density and obtain a material having a smaller pore diameter at the same stretch rate, that is, porosity.

  The technique of the present invention is not limited to the tube in the above example, but can be applied to a sheet-like porous body or a rod-like porous body having a higher degree of freedom of molding than biaxial stretching or the like. In addition to this example, for example, an electron beam with a low accelerating voltage that only affects the surface layer is used instead of highly transmissive radiation such as gamma rays, and then the surface is irradiated and stretched. By doing so, it becomes possible to create a porous body in which the pore diameters of the irradiated part and the non-irradiated part inside or on the back side are different. It is also possible to obtain a porous body having a non-uniform porous structure not in the thickness direction but in the plane direction by irradiating the extruded molded body with a mask such as a metal net, pattern, or pictograph. It can be used for marking on a porous PTFE body that does not use a dye. Since the technique of the present invention reduces only the elongation of PTFE and does not affect the strength, various applications can be considered.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated concretely, this invention is not limited only to these Examples.

As the PTFE fine powder, tetrafluoroethylene resin fine powder CD-123 manufactured by Asahi IC Fluoropolymers Co., Ltd. was used. The powder was irradiated with gamma rays having Co60 as a radiation source at doses of 100, 300, 500, 1000 and 1500 Gy. Each powder was mixed with 22 parts by weight of solvent naphtha and extruded into a tube shape having an inner diameter of 1.5 mm and an outer diameter of 2.7 mm. Further, naphtha was removed in a 60 ° C. drying furnace to obtain a dry extruded product. These were designated as Examples 1 to 3 and Reference Examples 4 and 5 . A dry extrudate obtained by similarly obtaining unirradiated powder was used as Comparative Example 1.

Further, Reference Examples 1 and 2 were obtained by irradiating the unexposed dry extruded product of Comparative Example 1 above with gamma rays using Co60 as a radiation source at a dose of 300 Gy and 500 Gy. For the above Examples 1 to 3, Reference Examples 1, 2, 4, 5 and Comparative Example 1, a tensile test was performed at a tensile temperature of 200 ° C., a tensile speed of 100 mm / min, and a chuck distance of 2.5 cm. The results are shown in Table 1.

In Examples 1 to 3 and Reference Examples 1 , 2 , 4 , and 5 , a decrease in elongation at break was observed according to the irradiation amount of gamma rays compared to Comparative Example 1. With respect to the yield point strength, the extrusion resistance of the powder is lowered by irradiation and the fluidity is increased, so that the extrusion pressure is lowered when extruded in the same shape with the same number of auxiliary components. The yield point strength depends on the amount of connection formed between the powder particles, and therefore depends on the shear stress that forms the connection, that is, the extrusion pressure. On the contrary, in Examples 6 and 7 irradiated after extrusion, there is not much reduction.

Further, with respect to Examples 1 to 3, Reference Examples 1, 2, 4, 5 and Comparative Example 1, the melting point peak was observed with a differential scanning calorimeter. In Comparative Example 1, there was one peak at 347 ° C., but in Example 1, it was observed that the peak at 335 ° C. gradually increased as the dose increased. The heights of the peak at 335 ° C. and the peak at 347 ° C. were reversed between Examples 1 and 2. Absorption around 347 ° C. remained only slightly in Reference Example 4 and almost disappeared in Reference Example 5 .

Dry extrudates were obtained in the same manner as in Examples 1 to 3 and Reference Examples 4 and 5 except that the number of auxiliaries was 16.5 parts and the gamma ray irradiation amount was 500 Gy. This was stretched in a furnace with a furnace length of 1 m and a furnace temperature of 550 ° C. at a draw rate of 75% and a winding wire speed of 5.5 m / min, and then in the same heating furnace to a furnace temperature of 600 ° C. and a winding wire speed of 5 m / min. And fired. This stretched porous tube was taken as Example 6.

  Reference Example 3 was obtained in the same manner as in Example 6 except that the dried extruded product of Reference Example 1 was used and the stretch ratio was 100%. Moreover, it was set as the comparative example 2 like Example 6 except having used the dried extrusion product obtained by carrying out similarly to the comparative example 1 except having set it as 21 parts of adjuvant. Further, Comparative Example 3 was obtained in the same manner as Reference Example 1 except that the dried extruded product of Comparative Example 1 was used.

  The production conditions of Example 6, Reference Example 3 and Comparative Examples 2 and 3 and the porous characteristics thereof are shown in Table 2 and FIG.

  In Example 6 and Comparative Example 2, the number of auxiliaries was adjusted in order to match the extrusion pressure, and Reference Example 3 and Comparative Example 3 used the same dry extruded product. Referring to FIG. 1, it can be seen that the example has a characteristic that the pore diameter is small with substantially the same porosity as each corresponding comparative example.

  This is presumably because the dried extrudates used in the examples have stretchability limited by radiation irradiation and are being stretched near the limit at which they can be stretched. Referring to Table 1, the unirradiated product can be stretched by nearly 800%. In the 75-100% stretching as in Comparative Examples 2 and 3, not all the holes were generated. On the other hand, in the examples, stretching of 75 to 100% was close to the limit of the stretchable range, and innumerable small holes were formed.

  In addition, in Example 6, Reference Example 3 and Comparative Examples 2 and 3 described above, as a result of analysis by a differential scanning calorimeter, the melting point peak was in the range of 327 to 329 ° C., and the latent heat of fusion was 25 J / g or less. It was considered that the difference between the examples and the comparative examples was not due to the difference in the degree of baking.

Production conditions and strength characteristics of Examples 1 to 3, Reference Examples 1, 2, 4, 5 and Comparative Example 1

  Production conditions and porous properties of Example 6, Reference Example 3 and Comparative Examples 2 and 3

表中、ＢＰはバブルポイント。ＡＳＴＭ−Ｆ−３１６−８０の方法により、イソプロピルアルコールを用いて測定した。

In the table, BP is a bubble point. Measurement was performed using isopropyl alcohol by the method of ASTM-F-316-80.

It is a graph which shows the relationship between the bubble point and porosity of Example 6, Reference Example 3 and Comparative Examples 2 and 3 of the present invention. It is a figure which shows the change of melting | fusing point peak by radiation irradiation in the differential scanning calorimeter chart of PTFE fine powder.

Claims (2)

  There is tetrafluoroethylene resin fine powder irradiated with radiation of 10 Gy or more and less than 1000 Gy, and the heat absorption curve in the differential scanning calorimetry analysis shows the 347 ° C. absorption peak inherent in tetrafluoroethylene resin fine powder and radiation irradiation. A tetrafluoroethylene resin fine powder characterized by having both an absorption peak at 335 ° C. that appears by Extrusion product using tetrafluoroethylene resin fine powder irradiated with radiation of 10 Gy or more and less than 1000 Gy, wherein the heat absorption curve in differential scanning calorimetry analysis of the tetrafluoroethylene resin fine powder is tetrafluoride An extruded product characterized by having both a 347 ° C. absorption peak inherent in ethylene resin fine powder and a 335 ° C. absorption peak that appears upon irradiation.

Images