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FLUORINE-BASED RESIN POROUS MEMBRANE AND METHOD FOR PREPARING THE SAME

20210039053 ยท 2021-02-11

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Abstract

The present disclosure provides a fluorine-based resin porous membrane exhibiting high mechanical strength and low heat shrinkage rate while having a fine pore size, and a method for preparing the same.

Claims

1. A fluorine-based resin porous membrane comprising a plurality of fluorine-based resin fibrils, wherein a ratio of an average thickness of fibrils in a surface region to an average thickness of fibrils in an inner region of the porous fluorine-based resin membrane is 1.8 to 3, and wherein the surface region is a region within 10% of the total thickness of the membrane from the surface of the porous fluorine-based resin membrane, and the inner region is a region excluding the surface region.

2. The porous fluorine-based resin membrane of claim 1, wherein the average thickness of the fibrils in the surface region of the membrane is 50 to 140 nm, and the average thickness of fibrils in the inner region of the membrane is 40 to 70 nm.

3. The porous fluorine-based resin membrane of claim 1, wherein the inner region corresponds to a region of greater than 10% and less than 90% of the total thickness of the membrane from a surface of the porous fluorine-based resin membrane.

4. The porous fluorine-based resin membrane of claim 1, wherein an average pore diameter of pores presented in the porous fluorine-based resin membrane is 0.1 to 0.25 m, and the maximum pore diameter is 0.3 to 0.45 m.

5. The porous fluorine-based resin membrane of claim 1, wherein the porous fluorine-based resin membrane has a porosity of 70 to 90%.

6. The porous fluorine-based resin membrane of claim 1, wherein the porous fluorine-based resin membrane has a thickness of 20 to 100 m.

7. The porous fluorine-based resin membrane of claim 1, wherein the porous fluorine-based resin membrane has a heat shrinkage rate of 10% or less, the heat shrinkage being as calculated according to Equation 2 using a length value in the transverse direction changed after heat treatment at 120 C. for 30 minutes, and a length value in the transverse direction before the heat treatment, and wherein a tensile strength in the machine direction is 60 to 100 MPa and a tensile strength in the transverse direction is 70 to 120 MPa, the tensil strength in the machine and transverse directions being as measured according to ASTM D 882. Heat .Math. .Math. shrinkage .Math. .Math. rate .Math. .Math. ( % ) = 100 [ ( Length .Math. .Math. in .Math. .Math. transverse .Math. .Math. direction before .Math. .Math. heat .Math. .Math. treatment - Length in .Math. .Math. transverse .Math. .Math. direction .Math. .Math. after heat .Math. .Math. treatment ) Length .Math. .Math. in .Math. .Math. transverse .Math. .Math. direction before .Math. .Math. heat .Math. .Math. treatment . [ Equation .Math. .Math. 2 ]

8. The porous fluorine-based resin membrane of claim 1, wherein the fluorine-based resin is at least one selected from polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkylvinylether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, ethylene-tetrafluoroethylene copolymer resin, tetrafluoroethylene-chlorotrifluoroethylene copolymer and ethylene-chlorotrifluoroethylene resin.

9. The porous fluorine-based resin membrane of claim 1, wherein the fluorine-based resin comprises polytetrafluoroethylene having a standard specific gravity of 2.14 to 2.22 as measured using JIS K6892.

10. A method for preparing a fluorine-based resin porous membrane comprising the steps of: extruding a fluorine-based resin composition prepared by mixing a fluorine-based resin and a lubricant into a sheet to prepare a fluorine-based resin sheet; stretching the fluorine-based resin sheet in a ratio of from 1 to 12 times in the machine direction at a temperature of 200 C. to 340 C.; stretching the sheet stretched in the machine direction, in a ratio of from 5 to 25 times in the transverse direction at 200 C. to 320 C.; and then heat setting the sheet stretched in the machine direction and the traverse direction at a temperature of 370 C. to 390 C. for 5 seconds to 60 minutes.

11. The method for preparing a fluorine-based resin porous membrane of claim 10, wherein the fluorine-based resin sheet is prepared by applying pressure to the fluorine-based resin composition to form a preformed body; extruding the preformed body in a form of a sheet using a die, followed by rolling; and then conducting a heat-treating the sheet at 120 C. to 200 C., and wherein the fluorine-based resin has a standard specific gravity of 2.14 to 2.22 as measured using JIS K6892.

12. A filter comprising the porous fluorine-based resin membrane according to claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0072] FIG. 1a is a photograph of fibrils on the surface of the porous fluorine-based resin membrane in Example 1 observed with a scanning electron microscope, and FIG. 1b is a photograph of fibrils inside the porous membrane observed with a scanning electron microscope.

[0073] FIG. 2a is a photograph of fibrils on the surface of the porous fluorine-based resin membrane in Example 2 observed with a scanning electron microscope, and FIG. 2b is a photograph of fibrils inside the porous membrane observed with a scanning electron microscope.

[0074] FIG. 3 is a result of evaluating the change in the heat shrinkage rate according to the change in the thickness ratio of fibrils on the surface and in the inside of the porous fluorine-based resin membrane in Experimental Example 2.

[0075] FIG. 4 is a result of evaluating the change in the tensile strength in the transverse direction (TD) according to the change in the thickness ratio of fibrils on the surface and in the inside of the porous fluorine-based resin membrane in Experimental Example 2.

[0076] FIG. 5 is a result of evaluating the change in the tensile strength in the machine direction (MD) according to the change in the thickness ratio of fibrils on the surface and in the inside of the porous fluorine-based resin membrane in Experimental Example 2.

[0077] Hereinafter, the present disclosure will be described in further detail with reference to the following preferred Examples so as to facilitate better understanding of the present disclosure. However, it should be understood that the following Examples are given by way of illustration of the present disclosure only, and are not intended to limit the scope of the present disclosure.

EXAMPLE 1

[0078] 100 parts by weight of PTFE resin (650J, manufactured by MCF, SSG (measured by JIS K6892): 2.163) was mixed with 22 parts by weight of a lubricant (Isopar H, manufactured by Exxon) to prepare a fluorine-based resin-containing composition, and then aged at 50 C. for 24 hours. Then, a preform block was prepared by applying a pressure of 2 MPa, and extruded into a sheet with a thickness of 1 mm using a extrusion equipment for a paste equipped with a die, and then rolled to a thickness of 300 m through a calender to prepare a PTFE film. The prepared PTFE film was heat-treated by a Roll to Roll process in a heating oven at 200 C. to completely remove the lubricant.

[0079] The heat-treated PTFE film was stretched in a ratio of 3 times in the machine direction (MD) by using a difference in roll speed at 300 C., and stretched in a ratio of 10 times in the transverse direction (TD) using a TD tenter, and then the stretched film was heat-set at 380 C. for 9 seconds using a heating roll to prepare a PTFE porous membrane.

EXAMPLE 2, AND COMPARATIVE EXAMPLES 1 TO 3

[0080] A PTFE porous membrane was prepared in the same manner as in Example 1, except that the conditions described in Table 1 were carried out.

COMPARATIVE EXAMPLES 4 AND 5

[0081] A PTFE porous membrane was prepared in the same manner as in Example 1, except that it was extruded in the form of a rod during extrusion in Example 1, and the heat setting process was performed under the conditions described in Table 1.

COMPARATIVE EXAMPLE 6

[0082] For comparison of effects when the ratio of a thickness of fibrils on the surface of the porous fluorine-based resin membrane and a thickness of fibrils present in the inner region of the membrane was greater than 3, the heat fixing step was performed at a high temperature of 400 C. to prepare a fluorine-based resin porous membrane.

[0083] In detail, as disclosed in Table 1 below, an attempt was made to produce the PTFE porous membrane to produce in the same manner as in Example 1, except that the heat setting step was performed at 400 C. for 9 seconds. However, after the thickness unevenness was deepened, a rupture was occurred, making it impossible to produce a film.

TABLE-US-00001 TABLE 1 Example Comparative Example 1 2 1 2 3 4 5 6 Extrusion Extruded into Extruded into Extruded into Extruded into condition a sheet a sheet a rod a sheet Heat setting 380 375 360 350 340 340 380 400 (H/S) temperature( C.) Heat setting time 9 9 15 20 25 25 9 9 (residence time) (sec)

EXPERIMENTAL EXAMPLE 1

[0084] The surface and the inside of the PTFE porous membranes produced in Examples 1 and 2 were observed by scanning electron microscope (SEM), respectively, and the results are shown in FIGS. 1a to 2b.

[0085] From the observation results, it was confirmed that in the case of the PTFE porous membranes of Examples 1 and 2 prepared by the production method according to the present disclosure, the thickness of fibrils on the surface thereof is larger as compared with that of the fibrils in the inside.

EXPERIMENTAL EXAMPLE 2

[0086] The PTFE porous membranes produced in Examples and Comparative Examples were evaluated by the following method, and the results are shown in Table 2 and FIGS. 3 to 5.

[0087] 1) Thickness (m): The thickness of the PTFE porous membranes prepared in Examples and Comparative Examples was measured using a Mitsutoyo 7327 thickness gage.

[0088] 2) Porosity: The weight, thickness, and area of the PTFE porous membrane were measured, respectively, and the porosity was measured according to Equation 1 below. At this time, the thickness of the PTFE porous membrane was measured using Mitsutoyo dial thickness gauge.


Porosity (%)={1(Weight [g]/(Thickness [cm]Area [cm.sup.2]True Density [g/cm.sup.3]))}100 [Equation 1]

[0089] wherein, in Equation 1, the true density was represented by a true density (2.2 g/cm.sup.3) of the fluorine-based resin.

[0090] 3) Average pore diameter (m) and maximum pore diameter (m): The average pore size and the maximum pore size were measured using PMI Capillary Flow Porometer.

[0091] In detail, the PTFE porous membrane was attached to the above measuring device, and completely immersed in a surface tension test solution (GALWICK), and air or nitrogen was injected into the porous membrane in the vertical direction. When the pressure increases constantly and reaches a certain pressure, a drop of test solution filling the largest hole in the pores was broken up. The pressure at this time was defined as a bubble point. Then, if the pressure continued to increase, then all of the solution filling the non-broken remaining small pores was also broken up. At this time, the flow rate (Wet Curve) according to the pressure was recorded and the size of the pore was calculated. The dry porous membrane that was not wetted with the test solution had a constant increase in flow rate as the pressure increases (Dry Curve). At this time, the average pore size of the pores corresponding to the pressure at the intersection of the graph where the dry curve is , and the wet curve was defined as the average pore size.

[0092] 4) Fibril thickness (nm): an image of the surface of the PTFE porous membrane was photographed using a field emission scanning electron microscope (FE-SEM) instrument. In the case of the fibril thickness of the inner region, the surface layer of the membrane was peeled off by about 5 m using a tape, and then the inner layer was observed by FE-SEM.

[0093] Subsequently, the thickness (or diameter) of the fibrils on the surface of the porous membrane and the fibrils in the inside of the membrane was measured from the images photographed using the software connected to the instrument, and each average value and thickness ratio (average thickness ratio of fibrils in the surface region/the inner region) were calculated.

[0094] At this time, the surface region is within 10% of the total thickness of the membrane from the surface of the porous membrane, and the inner region was a region of more than 10% and less than 90% of the total thickness of the membrane from any one surface of the porous membrane.

[0095] 5) Heat shrinkage rate (120 C., 30 min) (%): After cutting the PTFE porous membrane into a size of 5 cm in the machine direction (MD) and 5 cm in the transverse direction (TD), and then the changed dimensions when left in the free standing state at 120 C. for 30 minutes. The heat shrinkage rate was calculated according to Equation 2 below.

[00002] Heat .Math. .Math. shrinkage .Math. .Math. rate .Math. .Math. ( % ) = 100 [ ( Length .Math. .Math. in .Math. .Math. transverse .Math. .Math. direction before .Math. .Math. heat .Math. .Math. treatment - Length in .Math. .Math. transverse .Math. .Math. direction .Math. .Math. after heat .Math. .Math. treatment ) Length .Math. .Math. in .Math. .Math. transverse .Math. .Math. direction before .Math. .Math. heat .Math. .Math. treatment ] [ Equation .Math. .Math. 2 ]

[0096] wherein, in Equation 2, the length in the transverse direction before heat treatment is 5 cm, and the length in the transverse direction after heat treatment is a length in the transverse direction changed after being maintained at 120 C. for 30 minutes.

[0097] 6) Tensile strength: Tensile strength in the TD and MD directions was measured according to the test method as defined in ASTM D 882.

TABLE-US-00002 TABLE 2 Example Comparative Example 1 2 1 2 3 4 5 Thickness (m) 35 35 34 33 32 28 30 Porosity(%) 80 80 79 78 75 75 76 Average pore 0.21 0.21 0.20 0.19 0.18 0.18 0.19 diameter(m) Maximum pore 0.42 0.42 0.40 0.38 0.36 0.34 0.38 diameter(m) Average Surface 131.5 134.8 78.0 71.5 52.6 48.1 60.2 thickness of Inside 46.5 67.4 45.2 55.0 46.2 44.1 45.8 fibrils(nm) Average thickness ratio 2.8 2.0 1.7 1.3 1.1 1.1 1.3 of the surface/inside fibrils Heat shrinkage rate 3 7 12 24 40 43 6 (120 C., 30 min) (%) TD tensile 100 72 58 44 25 18 75 strength(MPa) MD tensile 75 60 45 35 19 14 54 strength(MPa)

[0098] In the porous fluorine-based resin membrane of Examples 1 and 2 prepared by the production method according to the present disclosure, the ratio of thickness of fibrils in the surface and inner regions of the membrane is in the range of 1.8 to 3, which exhibits a significantly reduced heat shrinkage rate while exhibiting excellent mechanical strength properties, as compared with the porous membrane of Comparative Examples having a thickness ratio of less than 1.8. Meanwhile, when the ratio of thickness of fibrils in the surface and inner regions of the membrane exceeds 3, breakage occurred during the film production process and thus, the membrane could not be produced (see Comparative Example 6).