Method for preparing porous membrane of fluorine-based resin

Abstract

The present invention provides a method for preparing a porous membrane of a fluorine-based resin having an improved shrinkage while maintaining excellent filtration efficiency and air permeability.

Claims

1. A method for preparing a porous membrane of a fluorine-based resin, comprising: stretching a fluorine-based resin film in a machine direction to prepare a porous fluorine-based resin film; thermal-treating the stretched porous fluorine-based resin film at a temperature equal to or more than 330° C. and less than 340° C. for 5 to 12 seconds; and stretching the thermal-treated porous fluorine-based resin film in a transverse direction; and then sintering the stretched thermal-treated porous fluorine-based resin film at a temperature equal to or more than a melting point of the fluorine-based resin film, wherein the sintering is performed at a temperature of 360 to 380° C. for 9 to 100 seconds, and the porous membrane of a fluorine-based resin has a transverse direction shrinkage of 30% or less at 100° C. or less and a mean pore size of 100 to 2000 nm.

2. The method of claim 1, wherein the stretching in the machine direction is performed at a stretching ratio of 2 to 20 times at a temperature of 100 to 320° C.

3. The method of claim 1, wherein the stretching in the transverse direction is performed at a stretching ratio of 2 to 50 times at a temperature of 100 to 400° C.

4. The method of claim 1, wherein the fluorine-based resin includes at least one selected from polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, ethylene-tetrafluoroethylene copolymer resin, tetrafluoroethylene-chlorotrifluoroethylene copolymer, and ethylene-chlorotrifluoroethylene resin.

5. The method of claim 1, wherein the fluorine-based resin includes polytetrafluoroethylene.

6. The method of claim 1, wherein the fluorine-based resin film is prepared by extruding and rolling a fluorine-based resin-containing composition containing the fluorine-based resin and a lubricant; and removing the lubricant.

7. The method of claim 6, wherein the lubricant includes at least one selected from hydrocarbon oils, alcohols, ketones and esters.

8. The method of claim 6, wherein the lubricant includes at least one selected from liquid paraffin, naphtha, white oil, toluene, and xylene.

9. The method of claim 6, wherein the fluorine-based resin-containing composition includes 10 to 30 parts by weight of the lubricant, based on 100 parts by weight of the fluorine-based resin.

10. The method of claim 6, wherein the removing of the lubricant is performed by thermal-treating the fluorine-based resin film at 120 to 200° C.

11. The method of claim 1, wherein the porous membrane of the fluorine-based resin further has a transverse direction shrinkage equal to or less than 50% at 120 to 200° C.

12. A porous membrane of a fluorine-based resin prepared by the method of claim 1.

13. The method of claim 1, wherein the porous membrane of the fluorine-based resin has a thickness of 5 to 100 μm.

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

15. The method of claim 1, wherein the porous membrane of the fluorine-based resin has a dimensional stability of 10% or less.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows results of evaluating thermal shrinkage of the porous membranes of a fluorine-based resin manufactured in Examples 1 and 2, and Comparative Example 1.

DETAILED DESCRIPTION OF EMBODIMENTS

(2) Hereinafter, preferred examples will be presented in order to assist in the understanding of the present invention. However, the following examples are provided only in order to easily understand the present invention, and a content of the present invention is not limited thereto.

Example 1

(3) 22 parts by weight of Isopar H™ (manufactured by ExxonMobil) as a lubricant was mixed with 100 parts by weight of PTFE resin F106C™ (manufactured by Dakin) to prepare a fluorine-based resin-containing composition, and then the composition was aged at room temperature for 24 hours. Then, a pressure of 4 MPa was applied to the composition to prepare a preform block, and the preform block was extruded into a sheet form using a paste extruding device, and then rolled to have a thickness of 500 μm using a rolling roll to manufacture a PTFE film.

(4) The manufactured PTFE film was thermal-treated in a heating oven at 200° C. through a roll-to-roll process to completely remove the lubricant, stretched 6 times in the machine direction using roll speed difference at 300° C., and then thermal-treated at 335° C. for 9 seconds. The PTFE film thermal-treated and stretched in the machine direction was stretched 15 times in the transverse direction using a tenter at 300° C., and sintered using the tenter at 380° C. for 13 seconds to manufacture a porous PTFE membrane (thickness: 25 μm).

Example 2

(5) 22 parts by weight of Isopar H™ (manufactured by ExxonMobil) as a lubricant was mixed with 100 parts by weight of PTFE resin F106C™ (manufactured by Dakin) to prepare a fluorine-based resin-containing composition, and then the composition was aged at room temperature for 24 hours. Then, a pressure of 4 MPa was applied to the composition to prepare a preform block, and the preform block was extruded into a sheet form using a paste extruding device, and then rolled to have a thickness of 500 μm using a rolling roll to manufacture a PTFE film. The manufactured PTFE film was thermal-treated in a heating oven at 200° C. through a roll-to-roll process to completely remove the lubricant, stretched 3 times in the machine direction using roll speed difference at 300° C., and then thermal-treated at 335° C. for 9 seconds. The PTFE film thermal-treated and stretched in the machine direction was stretched 20 times in the transverse direction using a tenter at 300° C., and sintered using the tenter at 380° C. for 13 seconds to manufacture a porous PTFE membrane (thickness: 30 μm).

Comparative Example 1

(6) A porous PTFE membrane was manufactured in the same manner as in Example 1, except that the stretched PTFE film was not subjected to a thermal-treatment process after being stretched in the machine direction in Example 1.

(7) Specifically, 22 parts by weight of Isopar H™ (manufactured by ExxonMobil) as a lubricant was mixed with 100 parts by weight of PTFE resin F106C™ (manufactured by Dakin) to prepare a fluorine-based resin-containing composition, and then the composition was aged at room temperature for 24 hours. Then, a pressure of 4 MPa was applied to the composition to prepare a preform block, and the preform block was extruded into a sheet form using a paste extruding device, and then rolled to have a thickness of 500 μm using a rolling roll to manufacture a PTFE film.

(8) The manufactured PTFE film was thermal-treated process in a heating oven of at 200° C. through a roll-to-roll process to completely remove the lubricant, and stretched 6 times in the machine direction using roll speed difference at 300° C. The resulting stretched PTFE film in the machine direction was stretched 15 times in the transverse direction using a tenter at 300° C., and sintered using the tenter at 380° C. for 13 seconds to manufacture a porous PTFE membrane (thickness: 25 μm).

Comparative Example 2

(9) 22 parts by weight of Isopar H™ (manufactured by ExxonMobil) as a lubricant was mixed with 100 parts by weight of PTFE resin F106C™ (manufactured by Dakin) to prepare a fluorine-based resin-containing composition, and then the composition was aged at room temperature for 24 hours. Then, a pressure of 4 MPa was applied to the composition to prepare a preform block, and the preform block was extruded into a sheet form using a paste extruding device, and then rolled to have a thickness of 500 μm using a rolling roll to manufacture a PTFE film.

(10) The manufactured PTFE film was thermal-treated in a heating oven at 200° C. through a roll-to-roll process to completely remove the lubricant, stretched 6 times in the machine direction using roll speed difference at 300° C., and then thermal-treated at 340° C. for 9 seconds. The PTFE film thermal-treated and stretched in the machine direction was stretched in the transverse direction using the tenter at 300° C. However, the strength of the PTFE film was excessively increased due to high temperature at the time of thermal-treatment, such that the PTFE film was not stretched during the stretching in the transverse direction but was fractured.

Comparative Example 3

(11) 22 parts by weight of Isopar H™ (manufactured by ExxonMobil) as a lubricant was mixed with 100 parts by weight of PTFE resin F106C™ (manufactured by Dakin) to prepare a fluorine-based resin-containing composition, and then the composition was aged at room temperature for 24 hours. Then, a pressure of 4 MPa was applied to the composition to prepare a preform block, and the preform block was extruded into a sheet form using a paste extruding device, and then rolled to have a thickness of 500 μm using a rolling roll to manufacture a PTFE film.

(12) The manufactured PTFE film was thermal-treated in a heating oven at 200° C. through a roll-to-roll process to completely remove the lubricant, stretched 6 times in the machine direction using roll speed difference at 300° C., and then thermal-treated at 300° C. for 9 seconds. The PTFE film thermal-treated and stretched in the machine direction was stretched 15 times in the transverse direction using a tenter at 300° C., and sintered at 380° C. for 13 seconds using the tenter to manufacture a porous PTFE membrane (thickness: 30 μm).

Comparative Example 4

(13) A porous PTFE membrane was manufactured in the same manner as in Example 1, except that the porous fluorine-based resin film was subjected to a thermal-treatment process for 4 seconds after being stretched in the machine direction.

Comparative Example 5

(14) The same manner was performed as in Example 1, except that the porous fluorine-based resin film was subjected to a thermal-treatment process at 335° C., which is a temperature equal to or more than the melting point of the PTFE, for 10 minutes, after being stretched in the machine direction. Thereafter, the thermal-treated PTFE film was subjected to the stretching process in the transverse direction, but fracture occurred during the stretching process, such that it was impossible to manufacture a porous PTFE film.

Experimental Example 1

(15) Thermal shrinkage and a change in shrinkage of the porous membranes of the fluorine-based resin manufactured in Examples 1 and 2, and Comparative Example 1 depending on temperature were measured. The results are shown in FIG. 1.

(16) Specifically, it was measured the dimensions changed when the porous membranes of the fluorine-based resin manufactured in Examples 1 and 2, and Comparative Example 1 were cut at 5 cm in the machine direction and at 5 cm in the transverse direction, placed in ovens each maintaining each temperature of 30° C., 40° C., 50° C., 60° C., 70° C., 100° C., 120° C., 150° C. and 200° C. and then left in a free standing state for 30 minutes. The TD thermal shrinkage was calculated according to the following Equation 1 using the measured results.

(17) $\begin{array}{cc}\mathrm{TD}\mathrm{thermal}\mathrm{shrinkage}\left(\%\right)=100\%\times \frac{\begin{array}{c}(\mathrm{TD}\mathrm{length}\mathrm{before}\mathrm{thermal}\mathrm{treatment}-\\ \mathrm{TD}\mathrm{length}\mathrm{after}\mathrm{thermal}\mathrm{treatment})\end{array}}{\mathrm{TD}\mathrm{length}\mathrm{before}\mathrm{thermal}\mathrm{treatment}}& \left[\mathrm{Equation}1\right]\end{array}$

(18) wherein the transverse length before thermal-treatment is 5 cm.

(19) As a result of the experiments, the porous PTFE membrane of Comparative Example 1 in which the thermal-treatment was not performed between the stretching process in the machine direction and the stretching process in the transverse direction had the transverse direction shrinkage of greater than 50% at 100° C. or less. However, the porous PTFE membranes of Examples 1 and 2 had the transverse direction shrinkage of 30% or less, and specifically 25% or less at 100° C. or less. In addition, the transverse direction shrinkage of the porous PTFE membrane was greatly increased at 100° C. or more. However, the porous PTFE membranes of Examples 1 and 2 still exhibited a low transverse direction shrinkage, and specifically, the transverse direction shrinkage of 50% or less at 120 to 200° C., as compared with the porous PTFE membrane of Comparative Example 1. From this, it may be appreciated that the thermal shrinkage of the porous PTFE membrane can be greatly improved at the time of manufacturing thereof according to the present invention.

(20) In addition, a change in the shrinkage (ΔShrinkage) was calculated according to the following Equation 3:
ΔShrinkage=(Shrinkage of Comparative Example 1).sub.T1−(Shrinkage of Example 1 or 2).sub.T1  [Equation 3]

(21) As a result, as shown in FIG. 1, the porous PTFE membranes of Examples 1 and 2 exhibited a large difference in the shrinkage from Comparative Example 1, and the shrinkage was increased at a relatively constant slope up to 100° C. Slopes of the shrinkage for the porous PTFE membranes of Examples 1 and 2, and Comparative Example 1 were increased in the range of 100 to 120° C., but the porous PTFE membranes of Examples 1 and 2 exhibited the greater slope of the shrinkage than that of Comparative Example 1.

(22) Although absolute values of the shrinkage in the porous PTFE membranes of Examples 1 and 2, and Comparative Example 1 were different, differences between the absolute values tended to increase as a temperature increases. A thermal-treated sample had better thermal stability at temperature of 100° C. or less.

Experimental Example 2

(23) The porous PTFE membranes manufactured in the Examples and the Comparative Examples were evaluated by methods below, and the results were shown in the following Table 1.

(24) 1) Mean pore size (nm) and bubble point (psi): The mean pore size and the bubble point were measured using a Capillary Flow Porometer instrument manufactured by PMI Co. Ltd.

(25) Specifically, the porous PTFE membrane was mounted on the measurement instrument, and then completely wetted in a surface tension test solution (GALWICK™), and air or nitrogen was vertically injected into the porous membrane. When a pressure increases constantly and then reaches a specific pressure, drops of a test solution filling the largest hole in the pores burst. The pressure at this time was referred to as a bubble point.

(26) Then, when a pressure increases continuously, the solution filling all of remaining small pores which have not burst, will also burst into drops. In this case, pore sizes were calculated by recording a flow rate (wet curve) according to pressure. In a porous membrane in a dry state in which it is not wetted in the test solution, a flow rate increases constantly as a pressure increases (dry curve). In this case, a pore corresponding to the pressure at the point where a graph in which the dry curve is ½ intersects with a wet curve is defined as a mean pore size.

(27) 2) Porosity: The weight, thickness, and area of the porous PTFE membrane were measured, respectively, and porosity was measured according to the following Equation 2. In this case, the thickness of the porous PTFE membrane was measured using a dial thickness gauge manufactured by Mitsutoyo Co. Ltd.
Porosity (%)={1−(weight [g]/(thickness [cm]×area [cm.sup.2]×true density [g/cm.sup.3]))}×100%  [Equation 2]

(28) wherein the true density was 2.2 g/cm.sup.3, which was the true density of the fluorine-based resin.

(29) 3) Dimensional stability: The dimensional stability was measured according to the following Equation 4 by using a measured value of dimension changed when the porous membrane of the fluorine-based resin was cut at 5 cm in the machine direction and at 5 cm in the transverse direction, and left in a free standing state under room temperature and atmospheric pressure (23±5° C., 1±0.2 atm) conditions for 67 hours. The smaller the numerical value, the better dimensional stability.

(30) $\begin{array}{cc}\mathrm{Dimensional}\mathrm{stability}\left(\%\right)=100\%\times \frac{\begin{array}{c}(\mathrm{Transverse}\mathrm{direction}\mathrm{length}\mathrm{before}\mathrm{experiment}-\\ \mathrm{Transverse}\mathrm{direction}\mathrm{length}\mathrm{after}67\mathrm{hours}\mathrm{under}\mathrm{room}\\ \mathrm{temperature}\mathrm{and}\mathrm{atmospheric}\mathrm{pressure}\mathrm{conditions})\end{array}}{\mathrm{Transverse}\mathrm{direction}\mathrm{length}\mathrm{before}\mathrm{experiment}}& \left[\mathrm{Equation}4\right]\end{array}$

(31) wherein the length in the transverse direction before experiment is 5 cm.

(32) TABLE-US-00001 TABLE 1 Comparative Comparative Comparative Comparative Comparative Example 1 Example 2 Example 1 Example 2* Example 3 Example 4 Example 5* Mean pore 200 190 190 Not 200 200 Not size (nm) measurable measurable Bubble point 18 22 22 Not 16 18 Not (psi) measurable measurable Porosity (%) 86 87 88 Not 88 86 Not measurable measurable Dimensional 2 4 17 Not 14 16 Not stability (%) measurable measurable *For Comparative Examples 2 and 5, the porous membrane was fractured during the stretching in the transverse direction, such that it was impossible to evaluate the mean pore size, bubble point, fibril length, porosity and dimensional stability.

(33) As a result of experiments, the porous membrane of the fluorine-based resin of Examples 1 and 2 having a low transverse direction shrinkage of 30% or less at 100° C. or less exhibited remarkably improved dimensional stability at room temperature, as compared with the porous membrane of the fluorine-based resin of Comparative Examples 1, 3 and 4 having equivalent levels at porosity and the pore size.