JPH11216760A - Polytetrafluoroethylene sheet and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To decrease local nonuniformity of strength characteristics in a sheet face by improving extrusion molding process of a polytetrafluoroethylene sheet. SOLUTION: A paste contg. polytetrafluoroethylene and a lubricant is drawn in the orifice passing direction by passing the paste through a first orifice 4. In addition, while local difference in flow of the paste is suppressed by draw caused by projection 7 provided in the path, this paste is fed into a second orifice 6 and in addition, it is drawn into a sheet-like shape by the second orifice 6 to obtain an unsintered polytetrafluoroethylene sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polytetrafluoroethylene (hereinafter, referred to as "PTFE") sheet and a method for producing the same. More specifically, the present invention relates to a PTFE sheet useful as a porous film, a seal tape, and the like. The present invention relates to the manufacturing method.

[0002]

2. Description of the Related Art PTFE sheets are widely used as base materials for porous membranes, seal tapes, and adhesive tapes. In each of such applications, it is desired that the PTFE sheet has low anisotropy in strength. Tokiko Sho 42
No. -14178 discloses a method for producing a PTFE sheet with reduced anisotropy. This method uses P
A preform obtained by mixing TFE fine powder and a lubricant is first drawn through a first orifice into a rod shape, and then drawn through a second orifice into a ribbon (sheet) shape. is there. The preform is first stretched in the longitudinal direction by the first orifice to form a rod. Due to this stretching, a shear stress acts on the PTFE fine powder in the longitudinal direction, and the small fibers are oriented in the rod along the major axis thereof. Subsequently, this rod is
The sheet is stretched in the lateral direction by the second orifice to form a sheet. By this drawing, small fibers oriented in the direction orthogonal to the small fibers are generated, and as a result, a PTFE sheet in which the small fibers are oriented in the biaxial direction is produced. If the orientation of the fibrils is appropriately adjusted, the anisotropy of the strength of the PTFE sheet can be substantially eliminated.

[0003]

Problems to be Solved by the Invention Japanese Patent Publication No. 42-1417
According to the production method described in Japanese Patent Publication No. 8 (1999) -1994, the fibrils are oriented biaxially, the anisotropy of the strength is reduced at one point in the sheet surface, and the difference in the elongation rate of the sheet is different. An unsintered PTFE sheet with reduced anisotropy can be obtained.
However, the PTFE sheet formed by the above method is
Actually, both ends in the sheet width direction (the direction orthogonal to the extrusion direction) have a wakame-like appearance, and there is a problem that the strength and elongation characteristics are significantly different between the center in the width direction and the both ends. . Such non-uniformity of local properties in the unsintered PTFE sheet has resulted in non-uniform strength of the above-mentioned products using the unsintered PTFE sheet and a decrease in the production yield of the sheet. .

The present invention has been made to solve the above-mentioned conventional problems.
An object of the present invention is to provide a PTFE sheet in which local unevenness in strength in a sheet surface is reduced by improving a step of extruding a PTFE sheet, and a method for producing the same.

[0005]

Means for Solving the Problems In order to achieve the above object, the present inventors have made intensive studies and found that the non-uniformity of the appearance and strength characteristics in the sheet width direction of the PTFE sheet manufactured by the above method is as follows. First orifice and second
It has been found that the flow of the paste containing the PTFE powder and the lubricant between the orifice and the orifice is different between a portion corresponding to the central portion in the sheet width direction and a portion corresponding to the both ends. Was done.

In order to reduce such a difference in the flow of the paste in the sheet width direction, the method of manufacturing a PTFE sheet of the present invention is characterized in that the preform containing PTFE and a lubricant is passed through a first orifice. A first stretching step of stretching the preformed body in a direction in which the shaped body passes through the first orifice, and passing the stretched preformed body through a second orifice to form the preformed body. A second stretching step of stretching the sheet in a direction perpendicular to the direction to form a sheet, wherein the first stretching step and the second stretching step, A step of passing the preform through a path having a cross section that is smaller in an end portion than a central portion and that can deform a cross-sectional shape of the formed body that has passed through the first orifice. And wherein the Mukoto.

According to the manufacturing method of the present invention, the flow of the paste in the longitudinal direction at the end of the path can be restricted by passing through the path having the cross section.
The difference in the flow of the paste in the vertical direction between the portion corresponding to the central portion in the sheet width direction and the portion corresponding to both ends can be reduced. Thus, by suppressing the difference in paste flow in the sheet width direction, it is possible to manufacture a PTFE sheet whose appearance and strength characteristics are more uniform than before.

Further, the PTFE sheet of the present invention is a PTFE sheet manufactured by extrusion molding, and in a green state obtained by the extrusion molding, a variation rate of sheet strength depending on a measurement point is 10% or less. , Preferably 5% or less.

This PTFE sheet has little local variation in mechanical properties in the extruded state as it is. Therefore, for example, by further rolling or stretching, it can be used as a seal tape or the like whose properties are more uniform than before. Furthermore, it is also possible to sinter as needed and provide for each of the above applications. As described above, according to the present invention, it is possible to provide PTFE sheet-applied products having little local variation in strength characteristics. In other words, according to the present invention, it is possible to improve the production yield of the product.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. The manufacturing method of the present invention can be carried out, for example, by using the die 1 shown in FIGS.

As shown in FIG. 1, which is a cross-sectional view of the die 1, the die 1 basically includes a cylindrical compression chamber 3 and a ram 2 slidable in the compression chamber 3 up and down. And an orifice portion extending downward from the compression portion. In the orifice portion, a first orifice 4 and a second orifice 6 are arranged continuously. As shown in FIG. 3 which is an II section in FIG. 1 and FIG. 5 which is also a III-III section in FIG.
The first orifice 4 has a rectangular sectional shape, and the second orifice 6 has a slit-like sectional shape.

By passing through the first orifice 4, the preform made of the paste containing the PTFE fine powder and the lubricant supplied from the compression chamber 3 is stretched in the extrusion direction (vertical direction in FIG. 1). It becomes a rod.
Inside the rod, small fibers are formed in the longitudinal direction of the rod due to shear stress acting on the preform when passing through the first orifice. The first orifice 4
May be any shape as long as the small fibers can be oriented in the extrusion direction, and is not limited to the shape shown in FIG.

The rod-shaped paste that has passed through the first orifice 4 is supplied to a path 5 that continues to a second orifice 6. As can be seen with reference to FIGS. 1 and 2 (die cross section from a direction perpendicular to the cross section of FIG. 1), the path 5 between the two orifices 4, 6 is basically a cross section of the two orifices 4, 6. It is configured by connecting the shapes with straight lines. However, as shown in FIG. 2, a projection 7 is formed in the path 5, and the path 5 is formed by the projection 7.
Is partially narrowed. As shown in FIG. 4 which is a cross-sectional view taken along the line II-II of FIG. 1, the narrowed narrowed portion has a wide portion corresponding to the center portion in the sheet width direction to be formed, and a portion corresponding to the both ends. Has a narrow cross-sectional shape. Therefore, the flow of the paste containing the PTFE fine powder and the lubricant passing through the constricted portion is restricted at both ends of the path cross section, and the speed at both ends is relatively slower than when there is no protrusion 7. . In this manner, the projections 7 act to alleviate local differences in paste flow, which tend to be relatively slow at the center and relatively fast at the ends, and to make the paste flow as a whole uniform. I do.

As shown in FIG. 4, it is preferable that the cross section of the aperture portion where the projection 7 is formed is gradually narrowed from the center to the end. The cross-sectional shape in which the width gradually decreases in this manner may be, for example, a rhombus or the like, but it is preferable that the cross-sectional shape is configured by a curve in which the degree of narrowing decreases as approaching the end from the center, for example, As shown in FIG. 4, it is preferable that the light emitting device is constituted by a curve that roughly draws a sine curve.

The path 5 in which the projection 7 is formed
May be constituted by a straight line, but FIG.
It is preferable that the stop portion is formed by a curved line as shown in FIG. 2, for example, as shown in FIG.

The paste whose flow velocity has been locally adjusted as described above is supplied to the second orifice 6 and passes through the orifice 6 to form an unsintered PTFE sheet. This PTFE sheet is usually formed as a continuous sheet whose extrusion direction is the sheet longitudinal direction. In the unsintered PTFE sheet, as a result of adjusting the flow of the paste between the orifices 4 and 6, the uniformity of the appearance and the strength characteristics in the sheet width direction is improved as compared with the related art. Further, the uniformity of the properties in the sheet longitudinal direction can be ensured by adjusting the raw materials, making the extrusion conditions uniform and stabilizing.

The PT using the dice as described above
In the method of forming the FE sheet, conventionally used raw materials are used without any particular limitation. For example, PTFE
It is preferable to use fine powder having a particle size of about 0.1 to 0.5 μm. Lubricants are various organic solvents that have been used to adjust the PTFE to fibrillation,
For example, liquid paraffin, petroleum solvents, hydrocarbon oils, aromatic hydrocarbons, alcohols, ketones, silicone oils, fluorocarbon oils, and mixtures thereof can be used. The amount of the lubricant is preferably in the range of 5 to 50 parts by weight, particularly 20 to 30 parts by weight based on 100 parts by weight of PTFE. In addition, raw materials include, as necessary, carbon black, graphite, silica powder, asbestos powder, glass powder, glass fiber, silicates, carbonates, metal powder,
A filler such as metal oxide powder, ammonium chloride, sodium chloride, plastic or rubber powder or solution may be mixed. The mixture of each raw material is pre-compressed and shaped so as to be inserted into a die to be used.

Further, the operating conditions in the conventional extrusion molding method using a die can be applied to the above method. For example, it is preferable in consideration of the orientation of the fibrils that the longitudinal stretching of the preform by the first orifice is about 10 to 70% greater than the transverse stretching by the second orifice.

The unsintered PTFE sheet obtained by the above-described method may be subjected to removal of a lubricant, sintering, and porosity formation by stretching, if necessary, similarly to the conventional unsintered PTFE sheet. It is selectively applied as appropriate. For example, sintering is 35
The heating is preferably performed by heating at 0 to 500 ° C (heating time is within several minutes). Even after such treatment, the improved properties of the unsintered PTFE sheet result in a P
The strength characteristics of the TFE sheet become more uniform.

[0020]

The present invention will be described more specifically with reference to the following examples. (Example) PTFE fine powder (CD made by Asahi ICI)
-123) 25 parts by weight of liquid paraffin as a lubricant was mixed with 100 parts by weight, and this mixture was preformed into a cylindrical shape. This preform is extruded using a die similar to that shown in FIGS.
An E sheet was obtained. The appearance of the obtained PTFE sheet was smooth over the whole. Further, the width direction of the sheet (the direction perpendicular to the extrusion direction) is equally divided into six, and five measurement points are determined so as to be equally spaced from each other, and these measurement points (points A to E in order from one). When the tensile strength and the elongation at break were measured, the results shown in Tables 1 and 2 were obtained. Both characteristics were measured at each point in the MD direction (extrusion direction; sheet longitudinal direction) and TD direction (direction orthogonal to the extrusion direction; sheet width direction).

(Comparative Example) The same as the above embodiment except that there is no projection 7 in FIG. 2 and a conventional die 11 having a cross section as shown in FIG. 6 when viewed from the same direction as FIG. An unsintered PTFE sheet was prepared in this manner, and the characteristics were evaluated in the same manner as in the above example. The results are shown in Tables 1 and 2. This PTFE sheet was wavy at both ends in the sheet width direction, and had a lack of flatness.

As shown in Table 1, the variation rate (variation coefficient) of the tensile strength in the sheet width direction of the unsintered PTFE sheet according to the present invention is 5% in both the MD and TD directions.
It was as follows. Also, as shown in Table 2, the rate of change of the elongation at break in the sheet width direction of the sheet is the MD direction,
It was 5% or less in both the TD direction. Such a low fluctuation rate cannot be realized by the conventional manufacturing method. In addition, a plurality of points were selected in the sheet longitudinal direction of the unsintered PTFE sheet obtained in the above example, and similar characteristics were evaluated. However, as in the sheet width direction, the variation rate was low. It was confirmed that the rate of change in strength of the sheet due to in-plane measurement points was 5% or less.

(Table 1)-----------------------------------Tensile strength (kg / cm ² ) Point A Point B Point C Point D Point E Average value Standard deviation Fluctuation rate (%) −−−−−−−−−−−−−−−−−−−−−−−−−−−−−− −−−−−−−− Example MD direction 26 25 27 25 25 25.6 0.89 3.5 TD direction 28 27 27 28 26 27.2 0.84 3.1 −−−−−−−−−−−−−−−−−−−−−− −−−−−−−−−−−−−−−−− Comparative example MD direction 26 23 19 22 25 23.0 2.74 11.9 TD direction 23 25 28 26 23 25.0 2.12 8.5 −−−−−−−−−−−−−−−− −−−−−−−−−−−−−−−−−−−−−−−−−−−

(Table 2) Elongation at break (%) Point A-------------------------------------- Point B Point C Point D Point E Average value Standard deviation Fluctuation rate (%) −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− −−− Example MD direction 400 410 380 400 400 398 10.9 2.7 TD direction 380 390 380 360 390 380 12.2 3.2 −−−−−−−−−−−−−−−−−−−−−−−−−− −−−−−−−−−−−− Comparative Example MD direction 400 450 550 480 400 456 62.6 13.7 TD direction 460 400 350 390 450 410 45.3 11.0 −−−−−−−−−−−−−−−−−− −−−−−−−−−−−−−−−−−−−−−−

[0025]

As described above in detail, according to the present invention, the preform containing PTFE and the lubricant is passed through the first orifice, so that the preform is converted into the preform. A first stretching step of stretching in a direction passing through one orifice, and passing the stretched preform through a second orifice to stretch the preform in a direction orthogonal to the direction. A method for manufacturing a PTFE sheet, comprising: a second stretching step of forming a sheet shape, wherein the preformed body is located at an edge rather than at a center between the first stretching step and the second stretching step. The method further comprises a step of passing through a path having a cross section capable of deforming a cross-sectional shape of the molded body having a narrow portion and having passed through the first orifice. It will be able to suppress the difference of the paste flow in the orifice in the direction corresponding to the direction, appearance and strength characteristics are uniform than the conventional PT
An FE sheet can be manufactured.

According to the present invention, there is provided a PTFE sheet produced by extrusion molding, and in a green state obtained by the extrusion molding, a variation rate of the sheet strength at a measurement point is 10% or less. To this extent, a PTFE sheet with uniformized strength properties is provided. Therefore, according to the present invention, it is possible to provide various PTFE sheet-applied products, such as seal tapes, having a more uniform strength than before.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a die that can be used in the manufacturing method of the present invention.

FIG. 2 is a cross-sectional view of the die shown in FIG. 1 from another direction.

FIG. 3 is a sectional view taken along line II of FIG. 1;

FIG. 4 is a sectional view taken along line II-II of FIG.

FIG. 5 is a sectional view taken along the line III-III of FIG. 1;

FIG. 6 is a cross-sectional view of a die used in a conventional manufacturing method, taken from the same direction as in FIG.

[Explanation of symbols]

 1, 11 Dice 2, 12 Ram 3, 13 Compression Chamber 4, 14 First Orifice 5 Path 6, 16 Second Orifice 7 Projection

Claims (3)

[Claims] 1. A polytetrafluoroethylene sheet produced by extrusion molding, wherein in an unsintered state obtained by the extrusion molding, a variation rate of sheet strength at a measurement point is 10% or less. Characterized polytetrafluoroethylene sheet. 2. The polytetrafluoroethylene sheet according to claim 1, wherein the variation rate is 5% or less. 3. A preform including polytetrafluoroethylene and a lubricant is passed through a first orifice to extend the preform in a direction in which the preform is passed through the first orifice. A first stretching step, and a second stretching step in which the stretched preform is passed through a second orifice to stretch the preform in a direction perpendicular to the direction to form a sheet. A method for producing a polytetrafluoroethylene sheet, comprising: between the first stretching step and the second stretching step, wherein the preformed body has an end portion narrower than a central portion, A method for producing a polytetrafluoroethylene sheet, further comprising the step of passing the molded product through a path having a cross section capable of deforming the cross sectional shape of the molded product that has passed through one orifice. .