METHOD FOR PREPARING UNSINTERED POLYTETRAFLUOROETHYLENE FILM AND POROUS FILM THEREOF

Abstract

A method for preparing an unsintered PTFE film capable of being continuously formed and with uniform density distribution and high density. The method for preparing the unsintered PTFE film includes filling a mixture obtained by adding a forming aid to PTFE fine powder in an extrusion forming die, extruding the filled mixture from the extrusion forming die to produce an extrusion forming body, rolling the extrusion forming body with a roll to produce a forming aid-removed film without the forming aid, and pinching the forming aid-removed film into a pinch roll made of a rubber roll formed by coating rubber on a metal shaft core at room temperature and compressing the forming aid-removed film so that thickness of the forming aid-removed film is reduced and density thereof is above 2.0 g/cm.sup.3.

Claims

1. A method for preparing an unsintered polytetrafluoroethylene film, wherein: making a mixture obtained by adding a forming aid to polytetrafluoroethylene fine powder obtained by emulsion polymerization of tetrafluorethylene, filling the mixture in an extrusion forming die, and extruding the filled mixture from the extrusion forming die to produce an extrusion forming body, calendering the extrusion forming body into a film shape with a roll to produce a forming aid-removed film without the forming aid, using two rubber rolls composed of a pair of rollers with one made of metal and the other made of rubber with D type hardness more than 80 as measured by JIS K 6253 coated on a metal shaft core, and pinching the forming aid-removed film with the pressure roll at room temperature and compressing the forming aid-removed film with a linear pressure of 50 kg/cm-200 kg/cm so that thickness of the forming aid-removed film is reduced at a reduction rate more than 24.6% and the density is 2.0 g/cm3.

2. The method for preparing the unsintered polytetrafluoroethylene film according to claim 1, wherein the forming aid-removed film is composed of more than two layers of forming aid-removed films.

3. The method for preparing the unsintered polytetrafluoroethylene film according to claim 2, wherein the more than two layers of forming aid-removed films contain a forming aid-removed film with different elongation from other layers.

4. A method for preparing a polytetrafluoroethylene porous film, wherein: stretching the unsintered polytetrafluoroethylene film according to claim 1 in length direction thereof and/or width direction thereof, and heating the stretched unsintered polytetrafluoroethylene film to above a melting point of unsintered polytetrafluoroethylene.

5. A method for preparing a polytetrafluoroethylene porous film, wherein: performing chemical etching treatment on one or both sides of the unsintered polytetrafluoroethylene film according to claim 1, and stretching the unsintered polytetrafluoroethylene film undergoing the chemical etching treatment in length direction thereof and/or width direction thereof. heating the stretched unsintered polytetrafluoroethylene film to above a melting point of unsintered polytetrafluoroethylene.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0038] FIG. 1 is an appearance view of a sealing tape.

[0039] FIG. 2 is an enlarged surface view of the sealing tape.

[0040] FIG. 3 is an enlarged view of an internal structure of the sealing tape.

[0041] FIG. 4 is a photograph showing transparency of the unsintered high-density PTFE film of Embodiment 1.

[0042] FIG. 5 is a surface photograph of the unsintered high-density PTFE film of Embodiment 1 taken with a scanning electron microscope.

[0043] FIG. 6 is a cross section photograph of the unsintered high-density PTFE film of Embodiment 1 taken with a scanning electron microscope.

[0044] FIG. 7 is a photograph of the compressed forming aid-removed film of Embodiment 8.

[0045] FIG. 8 is a photograph of the compressed forming aid-removed film of comparative example 4.

[0046] FIG. 9 is a photograph of the compressed forming aid-removed film of comparative example 5.

[0047] FIG. 10 is a photograph of the compressed forming aid-removed film of comparative example 6.

[0048] FIG. 11 is a photograph of the compressed forming aid-removed film of comparative example 6 in a calendered state.

[0049] FIG. 12 is a photograph of the compressed forming aid-removed film of comparative example 7.

[0050] FIG. 13 is a surface photograph of the sintered film of Embodiment 11 taken with a scanning electron microscope.

[0051] FIG. 14 is a cross section photograph of the sintered film of Embodiment 11 taken with a scanning electron microscope.

[0052] FIG. 15 is a surface photograph of the porous film of Embodiment 11 taken with a scanning electron microscope.

[0053] FIG. 16 is a surface photograph of the sintered film of Embodiment 12 taken with a scanning electron microscope.

[0054] FIG. 17 is a surface photograph of the porous film of Embodiment 12 taken with a scanning electron microscope.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0055] [Preparation Method]

[0056] The unsintered high-density PTFE film and the PTFE porous film of the embodiments of the present invention are prepared by the following preparation method.

[0057] A mixture is made by adding a forming aid to PTFE fine powder obtained by emulsion polymerization of TFE (generally referred to as a “preformed body” in the technical field of the invention).

[0058] The mixture is filled in an extrusion forming die, and the filled mixture is extruded from the extrusion forming die to produce a flaky or rod-shaped extrusion forming body.

[0059] The extrusion forming body is calendered by a pair of pressure rolls to make thin film.

[0060] The film is heated in a heating furnace to dry and remove the forming aid, so that a forming aid-removed film is produced.

[0061] The forming aid-removed film is pinched by the pinch roll for compression so as to reduce thickness thereof and produce a high-density unsintered PTFE film.

[0062] The pinch roll is composed of one metal roll and one rubber roll formed by coating hard rubber on a metal shaft core.

[0063] The high-density unsintered PTFE film is stretched in a length and/or width direction.

[0064] The stretched unsintered PTFE film is sintered in the heating furnace at a temperature above a melting point of the unsintered PTFE film to produce a PTFE porous film.

[0065] Properties of the film and the roll are measured according to the following points.

[0066] The thickness is measured by a dial thickness gauge (SM-112 produced by TECLOCK Corporation).

[0067] Weight is obtained by rounding off a measured value below 3 decimal places obtained by an electronic balance (ASPRO electronic balance ASP213 produced by AS ONE Corporation).

[0068] The density is calculated by cutting a sample into 5 cm squares and measuring weight and thickness thereof.

[0069] Transparency of the film is obtained by observing degree of perspective of an image on a slice overlapped under the film to determine whether the film is transparent, semi-transparent or opaque.

[0070] Change of the melting point is measured by a differential scanning calorimeter (DSC produced by Shimadzu Corporation) at a heating rate of 10° C./min.

[0071] The strength and elongation are obtained by measuring a test piece obtained by cutting the test piece into JIS K 6251 3 dumbbell by a test piece cutting knife with a precision universal testing machine (Autograph produced by Shimadzu Corporation) at a tensile speed of 50 mm/min.

[0072] Surface roughness is measured using a microfigure measuring instrument (SURFCORDER ET3000 produced by Kosaka Laboratory Ltd.) with a cutoff value of 0.8 mm, an evaluation length of 8.0 mm and a feed rate of 0.1 mm/sec.

[0073] For the density distribution in the length direction, the density is calculated by cutting the sample into the length of 1000 mm and the width of 25 mm and measuring the weight and thickness of 100 mm units, and the density distribution is confirmed by the difference from the average value.

[0074] The surface state is determined by observing the photographs taken with a scanning electron microscope.

[0075] The distance between clearances of the rolls is measured by adjusting position of a bearing of the roll and using a clearance gauge plate.

Embodiment 1

[0076] PTFE F-106 (produced by Daikin Industries, Ltd.) was used as PTFE fine powder.

[0077] Petroleum solvent Isopar H (produced by Exxon Mobil Corporation) was used as a forming aid.

[0078] A mixture (preformed body) of the PTFE fine powder and the forming aid was made by adding the forming aid amounting to 22 parts by weight to the PTFE fine powder amounting to 100 parts by weight.

[0079] The mixture was filled in an extrusion forming die, and the filled mixture was extruded from the extrusion forming die to produce a flaky extrusion forming body.

[0080] The extrusion forming body was rolled by a pressure roll to make thin film.

[0081] The film was heated in a heating furnace to dry and remove the forming aid, so that a 155 mm wide and 295 μm thick forming aid-removed film was produced.

[0082] The forming aid-removed film was compressed by a pinch roll at room temperature of 26° C., so that about 50 m high-density unsintered PTFE film was produced. The thickness 295 μm before compression was reduced to 210 μm (at a reduction rate of 28.8%).

[0083] The pinch roll was composed of a pair of rolls, one was a rubber roll formed by coating the surface of a metal roll with hard rubber with a hardness of 88 as measured by D type hardness tester, and the other was a metal roll made of chrome plating on the surface of a metal roll, and outer diameter of both rolls was 200 mm and the length thereof was 400 mm.

[0084] In addition, a rotating linear speed of the pinch roll during compression was 2 m/min, and total load at a contact line of the pinch roll was 6 t (a linear pressure was 150 kg/cm).

[0085] As shown in FIG. 4, the unsintered high-density PTFE film prepared by the above preparation method became transparent. The opaque portion of the left and right ends of the film in FIG. 4 was an uncompressed portion, and the middle portion except the left and right ends was a compressed portion, and the compressed portion was the portion that became semi-transparent.

[0086] The density of the unsintered high-density PTFE film prepared by the above preparation method was 2.18±0.01 in the length direction and approximately uniform.

[0087] Moreover, the unsintered high-density PTFE film was photographed by a scanning electron microscope. FIG. 5 shows a surface photograph, and FIG. 6 shows a cross section photograph. It can be confirmed from these photographs that clearances existing in the film were broken and the PTFE particles were once filled without clearances and became close to the most dense filling state.

[0088] [Changes of the Film Before Compression by the Pinch Roll and after Compression by the Pinch Roll]

[0089] In the above Embodiment 1, changes in length, width, thickness, density, transparency, peak melting point, temperature, tensile strength, elongation, and surface roughness were determined or calculated for the forming aid-removed film before compression by the pinch roll and for the unsintered high-density PTFE film after compression by the pinch roll.

[0090] Table 1 shows the results.

TABLE-US-00001 TABLE 1 Embodiment 1 Film not compressed by (Film compressed by the pinch roll the pinch roll) Length (mm) 50000 50000 Width (mm) 155 155 Thickness (μm) 295 210 Rubber hardness of the rubber roll of D88 the pinch roll Linear pressure of the pinch roll during 150 compression (kg/cm) Density (g/cm3) 1.55 Density distribution: 2.18 ± 0.01 Transparency Opaque Semi-transparent Peak melting point temperature (° C.) 344 344 Tensile strength (MPa) and elongation Tension 450 at Tension 500 at (%) strength 6 strength 15 Surface roughness Rubber surface 0.03~0.05 0.95 (μm) side Ra (arithmetic Metal surface 0.03~0.05 0.03 mean roughness) side

[0091] Comparison between the film before compression by the pinch roll and the film after compression by the pinch roll showed that the length and the width did not change and only the thickness decreased, as a result, the density increased, and the film changed from opaque state to transparent state.

[0092] The reason could be considered that removing the forming aid from the forming aid-removed film before compression by the pinch roll made the portions occupied by the forming aid become voids. Therefore, when the forming aid-removed film was irradiated by light, the light diffused due to the voids in the forming aid-removed film, and the light could not pass through, so the film became opaque.

[0093] However, it could be considered that when the forming aid-removed film was compressed by the pinch roll, the voids existing in the forming aid-removed film is broken after compression, resulting in volume reduction and density increase, thus the forming aid-removed film became the unsintered high-density PTFE film. Moreover, the voids in the forming aid-removed film were broken. As a result, when the unsintered high-density PTFE film obtained by compressing the forming aid-removed film was irradiated by light, diffuse reflection caused by the voids in the film reduced and the light was transmitted, so the film became semi-transparent.

[0094] The change of melting point was measured, and the peak melting point temperature was 344° C., without any change.

[0095] In addition, when the forming aid-removed film before compression by the pinch roll was stretched, the film broke at an elongation of 450% at a tensile strength of 6 MPa. In contrast, when the unsintered high-density PTFE film after compression by the pinch roll was stretched, the film broke at an elongation of 500% at a tensile strength of 15 MPa. Therefore, the result was that the film after compression by the pinch roll had lower elongation relative to strength and higher tensile strength.

[0096] [Change of Pinch Roll]

Embodiment 2

[0097] A mixture (preformed body) was made by adding the forming aid changed from 22 parts by weight in Embodiment 1 to 20 parts by weight to the PTFE fine powder amounting to 100 parts by weight. The thickness of the resulting forming aid-removed film was 272 μm (density increased to 1.62 g/cm3 with the thickness change). In addition, a forming aid removal film of Embodiment 2 was made in the same way as in embodiment 1.

[0098] The forming aid-removed film in the Embodiment 2 was compressed by a pinch roll at a linear pressure of 50 kg/cm.

[0099] As a result, the thickness decreased from 272 μm to 195 μm (at a reduction rate of 28.3%), and the density increased to 2.17 g/cm3. After compression, the film had neither corrugated undulation nor breakage, and became semi-transparent.

Embodiment 3

[0100] The forming aid-removed film in the Embodiment 2 was compressed by a pinch roll at a linear pressure of 100 kg/cm.

[0101] As a result, the thickness decreased from 272 μm to 187 μm (at a reduction rate of 31.3%), and the density increased to 2.24 g/cm3. After compression, the film had neither corrugated undulation nor breakage, and became semi-transparent.

Embodiment 4

[0102] The forming aid-removed film in the Embodiment 2 was compressed by a pinch roll at a linear pressure of 150 kg/cm.

[0103] As a result, the thickness decreased from 272 μm to 185 μm (at a reduction rate of 32.0%), and the density increased to 2.25 g/cm3. The film had neither corrugated undulation nor breakage, and became semi-transparent.

Embodiment 5

[0104] The forming aid-removed film in the Embodiment 2 was compressed by a pinch roll at a linear pressure of 175 kg/cm.

[0105] As a result, the thickness decreased from 272 μm to 185 μm (at a reduction rate of 32.0%), and the density increased to 2.25 g/cm3. After compression, the film had neither corrugated undulation nor breakage, and became semi-transparent.

Comparative Example 1

[0106] The forming aid-removed film in the Embodiment 2 was compressed by a pinch roll at a linear pressure of 12.5 kg/cm.

[0107] As a result, the thickness decreased from 272 μm to 225 μm (at a reduction rate of 17.3%), and the density increased to 1.91 g/cm3. After compression, the film had neither corrugated undulation nor breakage, but the film kept opaque instead of becoming semi-transparent.

Embodiment 6

[0108] The pinch roll made of a rubber roll formed by coating the surface of Embodiment 1 with hard rubber with a hardness of 88 as measured by a D type hardness tester was changed to a pinch roll made of a rubber roll formed by coating the surface with hard rubber with a hardness of 83 as measured by a D type hardness tester to compress the forming aid-removed film in the Embodiment 2 at a linear pressure of 100 kg/cm.

[0109] As a result, the thickness decreased from 272 μm to 205 μm (at a reduction rate of 24.6%), and the density increased to 2.07 g/cm3. After compression, the film had neither corrugated undulation nor breakage, and became semi-transparent.

Embodiment 7

[0110] The pressure roll made of a rubber roll formed by coating the surface of Embodiment 1 with hard rubber with a hardness of 83 as measured by a D type hardness tester was changed to a pinch roll made of a rubber roll formed by coating the surface with hard rubber with a hardness of 83 as measured by a D type hardness tester to compress the forming aid-removed film in the Embodiment 2 at a linear pressure of 175 kg/cm.

[0111] As a result, the thickness decreased from 272 μm to 195 μm (at a reduction rate of 28.3%), and the density increased to 2.15 g/cm3. After compression, the film had neither corrugated undulation nor breakage, and became semi-transparent.

Embodiment 8

[0112] Two 272 μm thick forming aid-removed films in the Embodiment 2 were overlapped. The overlapped forming aid-removed films with a thickness of 544 μm were compressed by a pinch roll made of a rubber roll formed by coating the surface the same as Embodiment 1 with the hard rubber with a hardness of 88 as measured by a D type hardness tester at a linear pressure of 150 kg/cm.

[0113] As a result, the thickness decreased from 544 μm to 290 μm (at a reduction rate of 46.7%), and the density increased to 2.24 g/cm3. The compressed film is as shown in FIG. 7. The two overlapped forming aid-removed films were integrated without corrugated undulation and breakage, and became semi-transparent. In FIG. 7, the left half is the uncompressed portion, and the right half is the compressed portion.

Comparative Example 2

[0114] The forming aid-removed film in the Embodiment 2 was compressed by a pinch roll made of a rubber roll formed by coating the surface with soft rubber with a hardness of 100 as measured by a A type hardness tester at a linear pressure of 100 kg/cm.

[0115] As a result, the thickness decreased from 272 μm to 246 μm (at a reduction rate of 9.6%), and the density increased to 1.79 g/cm3. After compression, the film had neither corrugated undulation nor breakage, but the film kept opaque instead of becoming semi-transparent.

Comparative Example 3

[0116] The forming aid-removed film in the Embodiment 2 was compressed by a pinch roll made of a rubber roll formed by coating the surface with soft rubber with a hardness of 100 as measured by a A type hardness tester at a linear pressure of 175 kg/cm.

[0117] As a result, the thickness decreased from 272 μm to 235 μm (at a reduction rate of 13.6%), and the density increased to 1.85 g/cm3. After compression, the film had neither corrugated undulation nor breakage, but the film became the state that the semi-transparent portion was mixed with the opaque portion.

Comparative Example 4

[0118] The forming aid-removed film in the Embodiment 2 was compressed by a pinch roll composed of a pair of chromeplated mirror metal rolls without a clearance between the two rolls instead of the rubber roll at a linear pressure of 25 kg/cm.

[0119] As a result, the thickness decreased from 272 μm to 195 μm (at a reduction rate of 28.3%), and the density increased to 2.15 g/cm3. The compressed film, a shown in FIG. 8, had no breakage, but had corrugated undulation, two ends thereof remained opaque in the width direction, and the central portion became semi-transparent.

Comparative Example 5

[0120] The forming aid-removed film in the Embodiment 2 was compressed by a pinch roll composed of a pair of chromeplated mirror metal rolls without a clearance between the two rolls instead of the rubber roll at a linear pressure of 100 kg/cm.

[0121] As a result, the thickness decreased from 272 μm to 140 μm (at a reduction rate of 48.5%), and the density increased to 2.33 g/cm3. The compressed film, as shown in FIG. 9, had corrugated undulating and breakage, and became semi-transparent as a whole.

Comparative Example 6

[0122] The forming aid-removed film in the Embodiment 2 was compressed by a pinch roll composed of a pair of chromeplated mirror metal rolls with a clearance of 0.06 mm between the two rolls instead of the rubber roll at a linear pressure of 100 kg/cm.

[0123] As a result, the thickness decreased from 272 μm to 185 μm (at a reduction rate of 32.0%), and the density increased to 2.25 g/cm3. The compressed film, a shown in FIG. 10, had no breakage, but became largely undulating, and kept a slightly opaque state as a whole. In addition, when the film before compression was compressed by drawing lines in the width direction at 2 cm intervals in the length direction, as shown in FIG. 11, which proved that the film was slightly stretched in the length direction.

Comparative Example 7

[0124] The forming aid-removed film in the Embodiment 2 was compressed by a pinch roll composed of a pair of chromeplated mirror metal rolls with a clearance of 0.15 mm between the two rolls instead of the rubber roll at a linear pressure of 100 kg/cm.

[0125] As a result, the thickness decreased from 272 μm to 205 μm (at a reduction rate of 24.6%), and the density increased to 2.05 g/cm3. The compressed film, a shown in FIG. 12, had no breakage, but had corrugated undulation, two ends thereof remained opaque in the width direction, a portion thereof was cut into a quadrilateral for the sake of comparison, and the film lacked transparency as a whole.

Comparative Example 8

[0126] The forming aid-removed film in the Embodiment 2 was compressed by a pinch roll composed of a pair of chromeplated mirror metal rolls with a clearance of 0.20 mm between the two rolls instead of the rubber roll at a linear pressure of 100 kg/cm.

[0127] As a result, the thickness decreased from 272 μm to 210 μm (at a reduction rate of 22.8%), and the density increased to 2.02 g/cm3. The compressed film had neither breakage nor corrugated undulation, but two ends thereof remained opaque in the width direction, and film lacked transparency as a whole.

Comparative Example 9

[0128] The forming aid-removed film in the Embodiment 2 was compressed by a pinch roll composed of a pair of chromeplated mirror metal rolls with a clearance of 0.25 mm between the two rolls instead of the rubber roll at a linear pressure of 100 kg/cm.

[0129] As a result, the thickness decreased from 272 μm to 240 μm (at a reduction rate of 11.8%), and the density increased to 1.80 g/cm3. After compression, the film had neither corrugated undulation nor breakage, but kept opaque.

[0130] Moreover, when measuring the changes in the clearance distance of the metal roll in the above Comparative Examples 6-9, the clearance distance of the metal roll was determined by adjusting position of the bearing portion of the metal roll. In addition, when the film was passed through the clearance of the metal roll, the film was compressed by applying a linear pressure of 100 kg/cm in such a way that the metal roll would not float and the clearance distance would increase.

[0131] Table 2 shows the results of the above Embodiments 2-7 and Comparative Examples 1-9.

TABLE-US-00002 TABLE 2 Before com- pression by the pinch Embodiments Comparative examples roll 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 9 One “Yes” “Yes” “Yes” “Yes” “Yes” “Yes” “Yes” “Yes” “Yes” “Yes” “No” “No” “No” “No” “No” “No” rubber roll and one metal roll Two “No” “No” “No” “No” “No” “No” “No” “No” “No” “No” “Yes” TES “Yes” “Yes” “Yes” “Yes” metal rolls Rubber D88 D88 D88 D88 D83 D83 D88 D88 A100 A100 hardness of rubber roll (hardness tester) Clearance 0 0 0 0 0 0 0 0 0 0 0 0 0.06 0.15 0.20 0.25 between rolls (mm) Linear 0 50 100 150 175 100 175 150 12.5 100 175 25 100 100 100 100 100 pressure (kg/cm) Thickness 272 195 187 185 185 205 195 225 246 235 195 140 185 205 210 240 (μm) 544 290 Reduction 28.3 31.3 32.0 32.0 24.6 28.3 46.7 17.3 9.6 13.6 28.3 48.5 32.0 24.6 22.8 11.8 rate of thickness (%) Weight 1.11 1.06 1.05 1.05 1.05 1.06 1.05 1.08 1.10 1.10 1.05 0.76 1.04 1.05 1.06 1.08 per 5 cm 2.22 1.63 square (g) Reduction 0 4.6 5.2 5.6 5.6 4.3 5.2 27 2.9 0.7 1.2 5.2 31.0 6.1 5.2 4.3 2.5 rate of weight (%) Density 1.62 2.17 2.24 2.25 2.25 2.07 2.15 2.24 1.91 1.79 1.85 2.15 2.33 2.25 2.05 2.02 1.80 (g/cm3) Trans- x ∘ ∘ ∘ ∘ ∘ ∘ ∘ x x Δ Δ ∘ Δ Δ Δ x parency Weight ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ x x x x ∘ ∘ reduction (FIG. (FIG. (FIG. (FIG. rate 7) 8) 9 11) (%) and FIG. 10) Evaluation ∘ ∘ ∘ ∘ ∘ ∘ ∘ x x x x x x x x x Transparency: ∘ The film is semi-transparent Δ The film is either semi-transparent mixed with opaque, or opaque at both ends and semi-transparent at the center. x The film is opaque. State of the film: ∘ The film has neither corrugated undulation nor breakage. x The film has corrugated undulation or breakage. Evaluation: ∘ The film is semi-transparent, without corrugated undulation and breakage. x The film is opaque, or mixed with opaque portion, or the film has corrugated undulation or breakage.

[0132] In Embodiments 2-8, the rubber roll coated with hard rubber with a hardness of 83 or 88 as measured by a D type hardness tester was used to compress in the range of 50-175 kg/cm linear pressure, and good results were obtained in terms of the state and transparency of the film for the unsintered PTFE film with a high density more than 2.0 g/cm3.

[0133] In Comparative Example 1, a rubber roll coated with hard rubber with a hardness of 88 as measured by a D type hardness tester was used. However, because the linear pressure was 12.5 kg/cm (weight reduction rate: 17.3%), the density only increased to 1.91 g/cm3. As a result, the film did not change to semi-transparent.

[0134] In Comparative Examples 2 and 3, the rubber roll coated with soft rubber with a hardness of 100 as measured by a A type hardness tester was used, and the density was measured at the linear pressure of 100 kg/cm (thickness reduction rate: 9.6%) and 175 kg/cm (thickness reduction rate: 13.6%) respectively, and the resulting density was 1.79 g/cm3 and 1.85 g/cm3 respectively. As there was no high density, the film did not change to semi-transparent.

[0135] In Comparative Examples 4 and 5 in which only metal rollers were used and no gap was provided between the rollers, even if linear pressure was reduced to 25 kg/cm (a thickness reduction rate of 28.3%), a film had undulatory undulations and bad transparency. When the linear pressure was increased to 100 kg/cm (the thickness reduction rate of 48.5%), although the film became translucent, the film was broken.

[0136] Therefore, like Comparative Examples 6-9, measurement was performed by providing a gap between the metal rollers while maintaining the line pressure at 100 kg/cm. Until distance between the gaps reached 0.15 mm, the state of the film was abnormal. When the distance exceeded 0.20 mm, the state of the film was slightly improved. However, because density was not increased, the transparency is not good.

[0137] According to the forgoing content, the result of various changes of a pinch roller was that one side of the pinch roller used a rubber roller coated with hard rubber of hardness of 80 or more with a D-type durometer. When the line pressure was in the range of 50-175 kg/cm, the film was compressed to the thickness reduction rate of 24% or more. When the film was densified to density of 2.0 g/cm3 or more, a uniform, semi-transparent, non-uniform unsintered PTFE film without undulatory undulation and break on the surface thereof could be obtained.

[0138] As a PTFE fine powder, in addition to replacing polytetrafluoroethylene F-106 (produced by DAIKIN INDUSTRIES, Ltd.) with polytetrafluoroethylene F-302 (produced by DAIKIN INDUSTRIES, Ltd.), a forming aid-removed film was changed from 155 mm in width and 295 μm in thickness to 150 mm in width and 220 μm in thickness. Besides the forming aid-removed film was cut to the width of 25 mm, the forming aid-removed film was produced in the same manner as in Embodiment 1.

[0139] In the same manner as in Embodiment 1, the forming aid-removed film cut to the width of 25 mm was compressed with the pinch roller to produce a densified unsintered PTFE film with the thickness of 137 μm and density of 2.12 g/cm3.

[0140] Then, a laminated film obtained by laminating 8 sheets of the unsintered PTFE film compressed by the pinch roller was produced. The laminated film was produced by winding the unsintered PTFE film on a metal or ceramic core body in a way that the unsintered PTFE film was wound into 8 layers, as described in Patent Document 2, for example. The laminated film wound on the core body was covered with glass cloth, and sintered in a heating furnace at 365° C. for 2 h. The sintered laminated film had thickness of 1.2 mm. After that, the sintered laminated film was punched into a dumbbell shape specified in JIS K 6251 No. 3 using a test piece punching knife. The tensile strength and the elongation rate of a test piece were measured using a precision universal testing machine (an automatically generated chart produced by Shimadzu Corporation) at a tensile speed of 50 mm/min. The tensile strength is 25 MPa and the elongation rate is 350%.

Comparative Example 10

[0141] Instead of compressing a forming aid-removed film cut to a width of 25 mm in Embodiment 9 with a pinch roller, a laminated film made of 8 laminated films was produced in the same manner as in Embodiment 9, and the laminated film was sintered in a heating furnace at 365° C. for 2 h. The sintered laminated film had thickness of 1.1 mm. The sintered laminated film was punched into a dumbbell shape in the same manner as in Embodiment 9, and the tensile strength and the elongation rate of the laminated film were measured at a tensile speed of 50 mm/min. The tensile strength was 7 MPa, and the elongation rate was 300%.

[0142] Table 3 shows the results of Embodiment 9 and Comparative Example 10.

TABLE-US-00003 TABLE 3 Embodiment 9 Comparative example 10 Film compressed by Film not compressed by the pinch roller the pinch roller Thickness (μm) 137 220 Line pressure (g/cm) 150 Density (g/cm3) 2.12 1.40 Transparency Translucent Opaque Melting point peak Temperature (° C.) 335 335 Thickness after sintering in layers 1.2 1.1 (mm) Tensile strength (MPa) and Elongation of 350 Elongation of 300 elongation rate (%) at strength of 25 at strength of 7

[0143] After the film of Embodiment 9 compressed by the pinch roller was superimposed by 8 sheets and then sintered, the tensile strength of the film was relatively high. However, after the film of Comparative Example 10 which was not compressed by the pinch roller was superimposed by 8 sheets and sintered, the tensile strength of the film was relatively lower.

[0144] This also showed that after one side of the pinch roller used a rubber roller coated with hard rubber of hardness of 88 with a D-type durometer, and the film was compressed with linear pressure of 150 kg/cm to be densified to density of more than 2.12 g/cm3 and sintered, the tensile strength and the elongation rate of the film became high.

Embodiment 10

[0145] As chemical etching, the forming aid-removed film produced with 155 mm in width and 295 μm in thickness in Embodiment 1 was compressed by a pinch roller, and the densified unsintered PTFE film was treated with surface defluorination. The results of the surface defluorination treatment showed that if a treatment agent was uniformly coated on the surface of the film, the chemical etching treatment of the film surface can be performed.

[0146] In the surface defluorination treatment, TETRA-ETCH B (manufactured by JUNKOSHA Inc) surface treatment agent was used as a fluororesin surface treatment agent in which a defluorination agent was dispersed in a solvent.

[0147] The surface defluorination treatment was performed by applying the fluororesin surface treatment agent to a densified unsintered PTFE film. After the surface defluorination treatment, first, the surface of the PTFE film changes to brown, and as the treatment progresses, the depth of color of the surface color increased and changed to dark brown.

[0148] When observing the surface of the PTFE film after the surface defluorination treatment, there are no gaps between PTFE particles of a compressed, densified unsintered PTFE. Therefore, a fluororesin surface treatment agent was only uniformly applied to the surface of the PTFE film after the surface defluorination treatment.

Comparative Example 11

[0149] Regarding a forming aid-removed film of Embodiment 10, the same surface defluorination treatment as in Embodiment 10 was performed without compression with a pinch roller. Observing the surface, a fluororesin surface treatment agent penetrated into the gaps between the PTFE particles, and the fluororesin surface treatment agent was applied unevenly and sparsely.

[0150] Table 4 showed results of Embodiment 10 and Comparative Example 11.

TABLE-US-00004 TABLE 4 Embodiment 10 Comparative example 11 Film compressed by Film not compressed by the pinch roller the pinch roller Width(mm) 155 155 Thickness (μm) 210 295 Density (g/cm3) Density distribution 1.55 2.18 ± 0.01 Transparency Translucent Opaque The state of the Because there were no gaps The treatment agent penetrated surface after between the PTFE particles, the into the gaps between the PTFE surface treatment agent was only particles and was applied defluorination uniformly applied to the unevenly and sparsely. treatment surface of the film.

[0151] Based on the forgoing content, it was shown that the unsintered PTFE film compressed and densified by a pinch roller of Embodiment 10 was uniformly coated with the fluororesin surface treatment agent on the film surface. Therefore, the film surface can be chemically etched. The unsintered PTFE film of Embodiment 11 that was not compressed by the pinch roller was not suitable for chemical etching treatment of the film surface because the film surface was unevenly coated with the fluororesin surface treatment agent.

[0152] [Extension]

Embodiment 11

[0153] A densified unsintered PTFE film compressed by a pinch roller produced in Embodiment 1 was extended 5 times in a longitudinal direction between heated rollers, and a part of the extended film was extracted. Both ends in a rolling direction were fixed to a frame of 250 mm, but a width direction was not fixed. The film was heated and sintered in a heating furnace at a heat treatment temperature of 365° C. for 2 h to produce a porous film. FIG. 13 was a photograph obtained by photographing the surface of the porous film using a scanning electron microscope, and FIG. 14 was a photograph obtained by photographing a cross section of the porous film using the scanning electron microscope. As shown in these photographs, a blackspot with finely divided nodes and no unevenness in thickness and density was produced and was a PTFE porous film with a very uniform structure.

[0154] In addition, after extending 5 times in a longitudinal direction as described above, sintering was not performed, but the film was extended 25 times in a width direction. Sintering was performed at a heat treatment temperature of 390° C. to produce the porous film. FIG. 15 is a photograph obtained by photographing the surface of the porous film with a scanning electron microscope. As shown in the photograph, a blackspot without uneven thickness and uneven density was produced and was the porous PTFE film with a very uniform structure.

Embodiment 12

[0155] A densified unsintered PTFE film compressed by a pinch roller produced in Embodiment 1 was extended 9 times in a longitudinal direction between heated rollers, and a part of the extended film was extracted. Both ends in the rolling direction were fixed to a frame of 250 mm, but a width direction was not fixed. The film was heated and sintered in a heating furnace at a heat treatment temperature of 365° C. for 2 h to produce a porous film. FIG. 16 is a photograph obtained by photographing the surface of the porous film with a scanning electron microscope. As shown in the photograph, a blackspot with finely divided nodes and no uneven thickness and density was produced and was a PTFE porous film with a very uniform structure.

[0156] In addition, after extending 9 times in a longitudinal direction as described above, sintering was not performed, but the film was extended 25 times in a width direction. Sintering was performed at a heat treatment temperature of 390° C. to produce the porous film. FIG. 17 is a photograph obtained by photographing the surface of the porous film with a scanning electron microscope. As shown in the photograph, compared with Embodiment 11, a blackspot without unevenness in thickness and density was produced and was a more uniform PTFE porous film with a dense structure and even fibers.

[0157] Table 5 below showed comparative data of Embodiments 11 and 12.

TABLE-US-00005 TABLE 5 Embodiment 11 Embodiment 12 A densified PTFE film compressed by a pinch roller The film was extended 5 The film was extended 9 times in a longitudinal times in the longitudinal direction, and sintered at direction, and sintered at Before heat treatment temperature heat treatment temperature extending of 365° C. (a porous film) of 365° C. (the porous film) (film of Before After Before After Embodiment 1) sintering sintering sintering sintering Thickness (μm) 210 170 180 160 180 Width(mm) 155 130 115 125 110 Density (g/m3) Density 0.65 0.66 0.56 0.66 distribution 2.18 ± 0.01 Porosity(%) 69 69 Air permeability (Pa) — — Average mobility 0.73 0.74 pore diameter (μm) The film before sintering The film before sintering was extended 24 times in was extended 24 times in a width direction and the width direction and sintered at heat treatment sintered at heat treatment temperature of 390° C. temperature of 390° C. (the porous film) (the porous film) Thickness (μm) 12 7 Width(mm) 1800 1800 Density (g/m3) 0.052 0.034 Porosity(%) 97 98 Air permeability (Pa) 450 490 Average mobility 0.33 0.27 pore diameter (μm) Porosity = 100 − (density ÷ specific gravity of a sintered product 2.10) × 100 Air permeability: pressure when a speed of wind passing through the film is 5.5 cm/second.

[0158] A bubble point method; measurement was performed using a reagent GALWICK (manufactured by Porous Materials Co., Ltd.).

[0159] The table showed that the densified unsintered PTFE film produced in Embodiment 1 compressed by a pinch roller was used, and an unsintered PTFE porous film produced by being extended in a length direction and a width direction in Embodiments 11 and 12 and being sintered had a porosity of 97% in Embodiment 11 and a porosity of 98% in Embodiment 12, both of the porosities were relatively high. It also showed that the bore diameter of the film in Embodiment 11 was 0.33 μm, and the bore diameter of the film in Embodiment 12 is 0.27 μm. Both of bore diameters were relatively small and had a dense structure. It also showed that the air permeability in Embodiment 11 was 450 Pa, and the air permeability in Embodiment 12 was 490 Pa. Both of the air permeability were good.

[0160] The embodiments of the present invention are described above, but the embodiments are presented as examples and are not intended to limit the scope of the invention. This embodiment can be implemented in other various forms, and various omissions, substitutions, and changes can be made without departing from the spirit of the invention. This embodiment and its variants are included in the range and theme of the present invention, and similarly, are included in the present invention described in claims and its equal range.