JPS636581B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is based on polytetrafluoroethylene (hereinafter referred to as polytetrafluoroethylene).
PTFE). PTFE is an industrially significant material with excellent properties such as heat resistance, chemical resistance, mechanical properties, and electrical insulation, and its porous sheets can be used as filters for highly corrosive substances, diaphragms for batteries, It is used in diaphragms for separating isotopes such as uranium hexafluoride. Furthermore, in recent years, it has attracted attention for its combination of breathability and waterproofness, and it is rapidly becoming popular as a material for ski wear, windbreakers, tents, raincoats, diaper covers, etc. Conventionally, porous PTFE sheets have been produced by forming a mixture of PTFE powder and a liquid lubricant such as naphtha into a sheet, and then stretching the mixture at a temperature below the melting point of PTFE to make it porous. However, this unfired porous sheet has low mechanical strength and can stretch, break, or tear under the action of a slight external force, which often causes practical problems. To solve the above problem, after stretching, the unfired sheet is usually kept in its stretched state and PTFE is
Mechanical strength is improved by heating and firing to a temperature higher than the melting point of . Although it is possible to obtain a porous body with high mechanical strength using this method, it has the drawbacks of increasing the number of steps, complicating the manufacturing equipment, and consuming a huge amount of thermal energy during manufacturing. There was an urgent need to do so. The inventors of the present invention have conducted repeated research to solve the problems of the prior art, and first proposed a method (Japanese Patent Laid-Open No. 156067/1983) in which a PTFE sheet is stretched while being fired. According to this method, the PTFE sheet can be fired and stretched at the same time, and compared to the conventional method,
It has the advantage that a fired PTFE porous sheet can be obtained in a short process and that the amount of thermal energy consumed is low. The present invention further improves this simultaneous firing and stretching method,
This improves manufacturing efficiency and achieves further savings in thermal energy consumption. That is, the method for producing a fired porous PTFE sheet according to the present invention is to form a mixture of PTFE powder and an appropriate amount of liquid lubricant into a sheet by roll rolling, and then coat a predetermined number of sheets with silicone resin. They are overlapped with a thin layer interposed therebetween, and then the sheet multilayer is arranged so that both ends of the sheet multilayer in the direction in which it is to be stretched are outside the heating zone, and the portion of the sheet multilayer that is disposed inside the heating zone is heated to a temperature higher than the melting point of PTFE. It is characterized by stretching the sheet in at least one axial direction while heating and baking the sheet, and then peeling the sheets from each other. In the present invention, a homogeneous mixture of PTFE powder and liquid lubricant is first formed into a sheet by roll rolling. Compression or extrusion may be additionally performed during sheet forming. The liquid lubricant used in the sheet forming process is one that can wet the surface of PTFE and can be removed by evaporation, extraction, etc. after sheet forming. Specific examples include liquid paraffin, naphtha, and white lubricant. Hydrocarbon oil such as oil, aromatic hydrocarbons such as toluene and xylene, alcohols, ketones, esters, silicone oil, fluorochlorocarbon oil, and polymers such as polyisobutylene and polyisoprene dissolved in these solvents. Examples include solutions, mixtures of two or more of these, water or aqueous solutions containing surfactants, and the like. In the sheet forming process of the present invention,
The amount of liquid lubricant mixed with PTFE powder varies depending on the presence or absence of other additives, but usually 100% of PTFE powder is used.
It is used in an amount of about 5 to 50 parts by weight, preferably 10 to 30 parts by weight. Furthermore, in the present invention, when mixing the liquid lubricant with the PTFE powder, various additives are added, such as pigments for coloring, improving strength against compression, improving wear resistance, and preventing low-temperature flow. Carbon black, graphite, silica powder, asbestos powder, glass powder, metal powder, metal oxide powder, metal sulfide powder, etc. can also be mixed. Unfired PTFE obtained by the above molding process
Although the sheet contains a liquid lubricant, in the present invention, after forming the sheet, the liquid lubricant is removed by a heating method, an extraction method, or a combination thereof. The liquid lubricant is thus removed from the unfired
Next, predetermined PTFE sheets are stacked together with a thin layer of silicone resin interposed therebetween. The sheets can be stacked together via a thin layer of silicone resin in various ways. For example, the surface of each sheet is soaked in a silicone resin liquid,
A method can be adopted in which the silicone resin is applied by spray coating or roll coating, and the sheets are polymerized with the coated surfaces facing each other, or a method in which a silicone resin liquid is applied to the surface of one sheet and an uncoated sheet is polymerized on the coated surface. . In any case,
A thin layer of silicone resin is interposed between the contact surfaces of the sheets to be overlapped. The silicone resin liquid used is an emulsion, solution, oil, grease, varnish, etc. in a state that can be applied to the surface of the PTFE sheet. Commercially available products include Shin-Etsu Chemical Co., Ltd. under the trade names KM763, KM764E,
KM765, KM722, KM740, KM751, KM410,
Examples include KF410, KS700, KS701, KS751, KS785, TSM630 (trade name manufactured by Toshiba Silicone Company), and SH7024 (trade name), SH7036, SH7050 (manufactured by Toray Silicone Company). In addition, when using a silicone resin emulsion, solution, or varnish, it is preferable that the silicone resin concentration is about 0.2 to 30% by weight in terms of ease of application work. In addition, when these silicone resin liquids are used, the solvent is removed by drying after application, and the silicone resin is fixed and formed in a thin layer on the surface of the PTFE sheet. Drying temperature varies depending on the type of solvent, but is usually around 50-300℃
That's about it. In the present invention, the silicone resin thin layer interposed between the superposed PTFE sheets is intended to prevent the sheets from fusing together during firing and stretching of the multilayered sheet, which is performed by heating to a temperature higher than the melting point of PTFE. and its thickness is approximately 0.1 to 10 μm
It is preferable that If the thickness of the silicone resin thin layer is within the above range, polymerization is possible.
Of course, it is possible to prevent fusion of PTFE sheets, and although the reason is not clear, the silicone resin flows into the micropores formed in the sheet during stretching and closes the micropores, resulting in a porous structure. There is almost no inconvenience such as reducing the air permeability of the sheet, or the silicone resin remaining on the surface of the peeled porous sheet becomes an obstacle when the porous sheet is bonded to cloth or the like. In the present invention, the PTFE sheets are stacked on top of each other with a silicone resin thin layer interposed therebetween as described above, and then arranged so that both ends of the sheet stack in the direction in which it is to be stretched are outside the heating zone, The part of the sheet layered material placed inside the heating zone is
As well as being heated and fired above the melting point of PTFE,
Each sheet is stretched in at least one axial direction with the two ends as starting points, and countless micropores are formed in each sheet. In the firing/stretching process of the present invention, arranging both ends of the sheet multilayer material in the direction in which it is to be stretched is outside the heating zone is most effective when the multilayer material is hot stretched at a high temperature higher than the melting point of PTFE. By keeping the temperature at both ends of the layered material, where a large force acts, below the melting point of PTFE, preferably below the softening point, to prevent softening or melting of the both ends, the mechanical strength of the both ends can be maintained, and hot stretching can be achieved. This is to prevent the multilayered material from breaking during the process. At this time, the portion of the multilayered sheet placed in the heating zone is heated to a temperature higher than the melting point of PTFE, but in order to perform the firing uniformly and in a short time, and to prevent deterioration due to heating, the heating is performed at approximately 340 to 420°C. It is preferable to do so. In addition, the stretching speed is usually about 100%/
It is preferably 10%/second or less. In this step, as described above, the portion of the layered sheet placed within the heating zone is heated and baked and stretched in at least one axis. The purpose of stretching is to form countless micropores in each sheet that makes up the multilayered material, and the stretching rate is determined according to the purpose.
Porosity, pore diameter, stretching direction of PTFE porous sheet,
It is determined depending on the number of stretching axes, etc., but usually about 15~
It is approximately 1500%, and considering the effect of forming micropores by stretching, the uniformity of the diameter of the micropores, etc., it is preferably approximately 20 to 1200%. In the present invention, when uniaxial stretching is performed in the firing/stretching process, both ends in the non-stretching direction are placed outside the heating zone in the same way as both ends in the stretching direction, or the ends in the non-stretching direction are placed within the heating zone. It is preferable to fix both ends with chucks, clips, etc., and to regulate the dimensions so that the distance between the ends does not change, since this facilitates the formation of micropores during stretching. After the sheet multilayer product has undergone the above firing and stretching process, the sheets are peeled off from each other. Peeling the sheets together is easy because a thin layer of silicone resin is interposed between the sheets.
A plurality of fired porous PTFE sheets are obtained. The porosity and the pore diameter of the porous sheet obtained by the method of the present invention are determined by the stretching ratio,
Although it varies depending on the stretching direction, the number of stretching axes, heating and firing temperature, etc., the porosity is usually about 35 to 95%, and the diameter of the micropores is about 0.01 to 50 μm. In order to improve the dimensional stability of the fired porous PTFE sheet, in the present invention, the stretched state of the porous sheet is maintained before or after peeling, in other words, the length in the stretching direction is dimensionally regulated. Heat treatment can be performed. By performing this heat treatment, the stretched state of the porous sheet can be fixed, and a sheet with excellent dimensional stability when used at high temperatures can be obtained. If the above-mentioned restrictions are not carried out during this heat treatment, the micropores will be significantly reduced or even disappear, which is not preferable. Incidentally, if the length of the porous sheet in a direction other than the stretching direction is further regulated during the heat treatment, reduction and disappearance of micropores can be more effectively prevented. Measures for controlling the dimensions of the porous sheet during the heat treatment mentioned above include, for example, grasping both ends of the porous sheet in the stretching direction with chucks, clips, etc., and heating while maintaining the gap, or using a feed roll that rotates at approximately constant speed. Examples include a method of heating between a winding roll and a winding roll. The heat treatment temperature shall be higher than the operating temperature of the porous sheet. Although the basic aspects of the present invention are as described above, the present invention also includes the following aspects (a) to (c). (a) An unfired PTFE sheet obtained by roll rolling is pre-stretched at a temperature below the melting point of PTFE,
The pre-stretched sheets or the pre-stretched sheet and the non-pre-stretched sheet are stacked together with a silicone resin thin layer interposed therebetween, and then stretched in the pre-stretching direction and/or in a direction other than the stretching direction in the firing/stretching process. While baking,
A mode in which the sheets are then peeled off from each other. (b) The sheets are stacked together with a thin layer of silicone resin interposed therebetween, and the sheet stack is pre-stretched at a temperature below the melting point of PTFE, and then in the firing/stretching step, the sheets are stretched in the pre-stretching direction and/or in a direction other than the stretching direction. A mode in which the sheets are fired while being stretched in the same direction, and then the sheets are peeled off from each other. (c) A mode in which the sheet multilayer material is fired and stretched, then stretched at a temperature below the melting point of PTFE in the stretching direction in the firing and stretching step and/or in a direction other than the stretching direction, and then the sheets are peeled from each other. Also included. In the embodiments (a) and (b) above, when stretching in the firing/stretching process is performed in a direction other than the preliminary stretching direction, the length in the preliminary stretching direction is dimensionally regulated during the firing/stretching process, and (c ) In the case where the stretching after the firing/stretching step is performed in a direction other than the stretching direction in the firing/stretching step, the firing/stretching
The length of the stretching process in the stretching direction is regulated. If the above-mentioned size restrictions are not applied, the stretched state in the direction in which the size should be restricted cannot be maintained, which is not preferable. According to these embodiments (a), (b), and (c), a porous sheet with a high porosity or a relatively large micropore diameter can be obtained. Accordingly, in the present invention, unfired PTFE sheets are stacked on top of each other with a thin layer of silicone resin interposed therebetween, and both ends of the stacked sheet in the direction in which it is to be stretched are placed outside the heating zone. PTFE porous sheets can be fired and stretched at the same time without thermal adhesion, and porous PTFE sheets can be efficiently produced, and thermal energy consumption can be saved. Hereinafter, the present invention will be explained in more detail by way of Examples with reference to the drawings, but these are not intended to limit the present invention. In addition, "parts" in the examples means "parts by weight." Example 1 100 parts of PTFE powder (product name: Teflon 6J, manufactured by Mitsui Fluorochemical Co., Ltd.) to 100 parts of liquid lubricant Naphtha No. 1 20
The mixture was uniformly mixed and pre-compressed at a pressure of 20 kg/cm ² , then extruded into a round bar shape, and then passed between a pair of metal rolling rolls to a thickness of 110 μm. , to obtain a long sheet with a width of 115 mm. Heat the sheet at 120°C for 2 minutes to remove the liquid lubricant. Next, a water-based emulsion of silicone resin (manufactured by Shin-Etsu Chemical Co., Ltd., trade name KM-765) was diluted to a concentration of 5% by weight and applied to one side of the unfired PTFE sheet by roll coating, and the thickness of the coating was thickened using a doctor knife. By adjusting the thickness and heating at a temperature of 150° C. for 1 minute, water is removed and a thin layer of silicone resin with a thickness of 0.2 μm is fixedly formed on one side of the sheet. After that, a thin layer of silicone resin was formed.
Two PTFE sheets are stacked one on top of the other so that the thin layers face each other, and wound into a roll around a tubular core. Thereafter, as shown in FIG. 1, the sheet multilayer material 1 wound on a tubular core is set on the feeding side of the stretching device, and one end of the sheet in the longitudinal direction is guided to the winding roll 2, and the heating zone 3 is heated. Keep the temperature at 360℃,
The rotation speed of the pair of pinch rolls 5 provided on the exit side of the heating zone 3 is set higher than the rotation speed of the pair of pinch rolls 4 provided on the entrance side of the heating zone 3, and the sheet multilayer material 1 is heated in the heating zone. While firing the placement part, it is stretched by 30% in the length direction at a stretching rate of 2%/sec. In this case, pinch rolls 4 and 5 have a temperature of 20
C. cold air was blown to prevent the base point 6.7 from softening or melting during stretching of the portion of the sheet multilayer material 1 disposed in the heating zone. Note that 8 and 9 are guide rolls, and 10 is a cooling roll. After firing and stretching, the sheets are peeled off to a thickness of 100μ.
m, width 103mm, porosity 33%, average pore diameter 0.05μ
Two fired PTFE porous sheets (sample 1) of m were obtained. In addition, the porosity is ASTM-D1457-
56T, the true specific gravity (P ₀ ) was measured using n-butanol, and the apparent specific gravity (P ₁ ) was measured using water, and calculated using the following formula. Porosity (%)=P ₀ −P ₁ /P ₀ ×100 In addition, the pore diameters of the micropores were each measured by the BET method. Furthermore, the adhesiveness and air permeability of this porous sheet were measured. The data obtained are shown in Table 1 below. "Adhesion" is a PTFE porous sheet with a thin silicone resin layer formed on the nylon/cotton mixed woven fabric (warp nylon thread 70 denier, weft thread 80 denier double thread) with a polyamide-based hot-melt adhesive. 35%, one cycle is 9 minutes of washing, 10 minutes of rinsing, and 6 minutes of dehydration, depending on the number of cycles until peeling occurs between the porous sheet and the nylon/cotton fabric. evaluated. The porous sheet and the nylon/cotton mixed woven fabric were bonded together by heating and pressing for 30 seconds at a temperature of 140° C. and a pressure of 3 kg/cm ² using a hot melt adhesive manufactured by Nippon Rilsan Co., Ltd. under the trade name Platamide H105P. In addition, the washing machine manufactured by Hitachi, trade name Aozora PF2350 was used. Moreover, "air permeability" was measured by JIS-P-8117, Gurley densometer type A. On the other hand, except for setting the thickness of the silicone resin thin layer or the stretching conditions as shown in Table 1, all operations were carried out in the same manner as in the case of Sample 1, and Samples 2 to 2 were
A porous sheet of No. 6 was obtained. For reference, data for a porous sheet (sample 7) obtained by firing and stretching one PTFE sheet used to obtain these porous sheets under the same conditions as sample 1 is also shown. Example 2 A porous sheet having a length 11.4 times the length before stretching (stretching ratio 1040%) was produced by performing all operations in the same manner as Sample 2 except that firing and stretching were repeated five times in the longitudinal direction of the sheet. (Sample 8) was obtained. Example 3 All cases are Sample 4 except that a thin silicone resin layer is formed on one PTFE sheet, the thin layer is not formed on the other PTFE sheet, and both PTFE sheets are overlapped with a thin silicone resin layer interposed therebetween. A porous sheet (sample 9) was obtained in the same manner as above.
Note that the air permeability and adhesiveness data in this case are those of a porous sheet obtained from a sheet on which a thin layer of silicone resin is formed. Example 4 A thin layer of silicone resin with a thickness of 5 μm was formed on the same unfired PTFE sheet used in Example 1, and then pre-stretched at a temperature of 120° C. to a stretching ratio of 100% in the longitudinal direction. This pre-stretched product has a thickness of 100 μm, a porosity of 66%, and an average pore diameter of 0.1 μm.
Twelve m sheets are stacked together with a thin layer of silicone resin interposed therebetween. Thereafter, this multi-layered sheet was stretched in the longitudinal direction at a stretching rate of 200% at a temperature of 380°C and a stretching rate of 0.2%/sec, and the sheets were peeled off from each other to obtain a porous sheet (sample 10). Ta. Example 5 The same sheet multilayer material 1 used in Example 1 was
As shown in the figure, the material is passed through a pinch roll 4 and introduced into a heating zone 3 in which the width of the heating region increases from the inlet side to the outlet side. Heating zone 3
The temperature is maintained at 350℃. Then, both ends of the sheet stack 1 in the width direction are grasped by the chucks of the tenter-type stretching machine 11 installed outside both ends of the heating zone 3, and the portion of the molded product 1 disposed inside the heating zone is stretched in the width direction. At the same time, the film is stretched by 200% and stretched in the length direction by the speed difference between the pinch rolls 4 and 5. Note that the stretching speed was 0.5%/sec in the length direction and 1.5%/sec in the width direction. After firing and stretching, the sheets were peeled off from each other to obtain a porous sheet (Sample 11). Example 6 All samples were the same as Sample 5 except that after firing and stretching, the longitudinal direction of the sheet layered product was restricted and the sheet was stretched in the width direction at a temperature of 130°C and a stretching rate of 3.6%/sec (stretching rate of 400%).
A porous sheet (sample 12) was obtained in the same manner as above.

[Table] As is clear from the above examples and reference examples,
According to the method of the present invention, the silicone resin does not block the micropores during firing and stretching.
It is also seen that there is no decrease in adhesiveness due to residual silicone resin.

[Brief explanation of the drawing]

FIG. 1 is a side view schematically showing a porous sheet manufacturing apparatus used in the present invention, and FIG. 2 is a plan view showing the main parts of another apparatus. 1...Sheet multilayer material, 3...Heating zone, 4,
5...Pinch roll.

Claims (1)

[Claims] 1 A mixture of polytetrafluoroethylene powder and an appropriate amount of liquid lubricant is formed into a sheet by roll rolling, a predetermined number of these sheets are stacked together with a thin layer of silicone resin interposed therebetween, and then the sheet stack is The sheet is arranged so that both ends in the direction in which it is to be stretched are outside the heating zone, and at least one axis of the multilayer sheet is heated and fired at a temperature higher than the melting point of polytetrafluoroethylene. 1. A method for producing a fired porous polytetrafluoroethylene sheet, which comprises stretching the porous sheet in a direction and then peeling the sheets together.