Porous fluorine resin film and preparation method thereof

Abstract

A porous fluorine resin film in which a fibril structure is stabilized through impregnation coating of a water-repellent and oil-repellent polymer having a high oil repellency grade, and free shrinkage also proceeds before impregnation and application of the water-repellent and oil-repellent polymer, thereby improving dimensional stability, and a method for preparing the porous fluorine resin film.

Claims

1. A porous fluorine resin film comprising: a porous fluorine resin layer having pores formed therein; and a coating layer formed on at least one surface of the porous fluorine resin layer and an outer surface of the pores, wherein, the coating layer has water repellency and oil repellency and includes a (co)polymer of a perfluoroalkyl acrylate having 1 to 10 carbon atoms, an alkyl acrylate having 1 to 10 carbon atoms, vinyl chloride and a crosslinkable monomer, and wherein, the porous fluorine resin film has a change rate of air permeability of 1% of less before and after bending thereof within a curvature radius of at least 2 mm as calculated by Equation 1:
Change rate of air permeability before and after bending (%)=[(Pa−Pb)/Pa]×100  [Equation 1] wherein, in the above Equation 1, Pa is an air permeability value of the porous fluorine resin film measured before bending thereof, Pb is an air permeability value of the porous fluorine resin film measured after bending thereof, wherein bending is applied by winding the porous fluorine resin film around a stainless steel cylinder having a diameter (R) of 4 mm to 12 mm to which a tension of 0.5 kgf is applied, holding the wound film for 30 seconds, removing the tension and spreading the film, and wherein, the air permeability value is measured by a Gurley method according to JIS P 8117.

2. The porous fluorine resin film of claim 1, wherein an omnidirectional bending shrinkage due to the bending of the film is 1% or less as calculated by Equation 2:
Bending shrinkage (%)=5×exp[−0.8×curvature radius (mm)]  [Equation 2]

3. The porous fluorine resin film of claim 2, wherein each of the omnidirectional Bending shrinkage in the machine direction (MD) and omnidirectional Bending shrinkage in the transverse direction (TD) is 0% to 1% within a range of a curvature radius of 2 mm to 6 mm.

4. The porous fluorine resin film of claim 2, wherein the omnidirectional shrinkage due to the bending is a ratio of the omnidirectional Bending shrinkage in the MD direction to the omnidirectional Bending shrinkage in the TD direction.

5. The porous fluorine resin film of claim 1, wherein the crosslinkable monomer is a monomer having a hydroxyl group, a carboxyl group, an epoxy group, and an isocyanate group, urethane, amine, amide, or urea.

6. The porous fluorine resin film of claim 1, wherein the coating layer is formed on more than one surface of the porous fluorine resin layer and any one surface of the porous fluorine resin film and another surface forming the remaining part of the film have an oil repellency grade (AATCC-118) of 6 or higher, respectively.

7. The porous fluorine resin film of claim 6, wherein the oil repellency grade of any one surface and the oil repellency grade of another surface forming the remaining part have symmetry with each other.

8. The porous fluorine resin film of claim 1, wherein it has a porosity of 40 to 90%, a maximum pore size of 300 nm to 4000 nm, and a density of 0.10 to 1.30 g/cm.sup.3.

9. The porous fluorine resin film of claim 1, wherein the porous fluorine resin layer is provided through free shrinkage.

10. The porous fluorine resin film of claim 1, wherein the fluorine resin layer includes one or more fluorine-based compounds selected from the group consisting of polytetrafluoroethylene (PTFE), a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), a tetrafluoroethylene-hexafluoropropylene copolymer (FEP), an ethylene-tetrafluoroethylene copolymer resin (ETFE), a tetrafluoroethylene-chlorotrifluoroethylene copolymer (TFE/CTFE), and an ethylene-chlorotrifluoroethylene resin (ECTFE).

11. A method for preparing the porous fluorine resin film of claim 1, comprising the steps of: uniaxial stretching of a porous fluorine resin layer; heat-setting of the uniaxially stretched porous fluorine resin layer; free shrinking of the heat-set porous fluorine resin layer at a temperature of at least 150° C. or higher; and impregnating of the free shrunk porous fluorine resin layer into a water-repellent and oil-repellent agent-containing solution diluted to have a solid content of 2 to 10% by weight to produce the porous fluorine resin film, wherein the water-repellent and oil-repellent agent includes a (co)polymer of a perfluoroalkyl acrylate having 1 to 10 carbon atoms an alkyl acrylate having 1 to 10 carbon atoms vinyl chloride and a crosslinkable monomer.

12. The method for preparing the porous fluorine resin film of claim 11, wherein the free shrinking step includes a step of performing the free shrinking of the heat-set porous fluorine resin layer at 150 to 250° C. for 3 to 30 minutes.

13. The method for preparing the porous fluorine resin film of claim 11, further comprising a step of allowing the porous fluorine resin layer to stand room temperature for 3 to 30 minutes, after the free shrinking step.

14. The method for preparing the porous fluorine resin film of claim 11, wherein the heat-setting step is carried out for the uniaxially stretched porous fluorine resin layer at a temperature equal to or higher than the melting point of a fluorine resin for 3 to 30 seconds.

15. The method for preparing the porous fluorine resin film of claim 11, wherein the water-repellent and oil-repellent agent-containing solution contains one or more solvents selected from the group consisting of water, alkanes having 4 to 16 carbon atoms, alcohols, carboxylic acids, ketones, ethers, and fluorine alkanes having 5 to 12 carbon atoms.

16. The method for preparing the porous fluorine resin film of claim 11, wherein the porous fluorine resin layer in the step of uniaxial stretching is prepared by a method comprising the steps of: preparing a preform using a composition containing a fluorine resin and a lubricant; and extruding the preform, and drying and uniaxially stretching the extruded preform in the MD direction.

17. An automotive vent filter comprising the porous fluorine resin film of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a simplified illustration of a process for preparing the porous fluorine resin films of Comparative Examples 1 to 3 and Examples 1 to 3.

(2) FIG. 2 shows an electron microscope photograph (SEM) of a PTFE filtration film used for producing the porous fluorine resin films of Examples 1 to 3 of the present invention ((a), 1K magnification, (b): 5K magnification)

(3) FIG. 3 shows the radius of curvature after bending the porous fluorine resin film of Comparative Examples 1 to 3 and Examples 1 to 3.

(4) FIG. 4 shows a comparison of changes in shrinkage ratio of the porous fluorine resin films of Comparative Examples 1 to 3 and Examples 1 to 3 according to the average radius of curvature.

Detailed Description of the Embodiments

(5) The invention will be described in more detail by way of examples provided below.

(6) However, the following examples are illustrative of the present invention, and the scope of the present invention is not limited to or by the examples.

Examples 1 to 3: Preparation of Water-Repellent and Oil-Repellent Coated PTFE Porous Film

(7) A uniaxially stretched porous fluorine resin layer (uniaxially stretched PTFE filtration film) was produced through Preform—Extrusion—Calendering & Drying—MD Stretching—Heat Setting processes.

(8) That is, 22 parts by weight of a liquid lubricant (trade name: “Isopar H”, Exxon Co.) was mixed with 100 parts by weight of polytetrafluoroethylene powder (CD145E, AGC) to prepare a single layer preform.

(9) Then, the single layer preform was extruded at a temperature of 50° C. at a rate of 50 mm/min to prepare a sheet having a thickness of about 300 μm.

(10) The sheet thus prepared was heated at a temperature of about 200° C. to completely dry and remove the liquid lubricant.

(11) After the drying step, the preform was uniaxially stretched under the conditions shown in Table 1 below.

(12) The cloth means a uniaxially stretched porous fluorine resin layer. The bending shrinkage of the cloth was the maximum MD 8%/TD 0% level.

(13) In addition, the oil repellency grade of the cloth was grade 4 level.

(14) TABLE-US-00001 TABLE 1 Drying .Math. curing condition Initial cloth Thickness, μm 207 Porosity, % 67 Pore size mean/max, nm 315/664 Gurley value, s ~50 Water pressure resistance, MPa 0.15

(15) Next, as shown in FIG. 1, the uniaxially stretched porous fluorine resin layer subjected to heat setting was freely shrunk at 150° C. for 30 minutes and left at room temperature for 10 minutes.

(16) At this time, the heat setting was carried out for 10 seconds at 350° C., which is equal to or higher than the melting point of the polytetrafluoroethylene powder.

(17) A water-repellent and oil-repellent agent (AG-E 550D, AGC from Asahi Glass Co., Ltd.) including a (co)polymer containing a repeating unit derived from a perfluoroalkyl acrylate having 1 to 10 carbon atoms was dissolved using a solvent (ethanol) so that the solid matter content was 4.2% by weight.

(18) Next, the porous fluorine resin layer subjected to the free shrinkage was impregnated with the water-repellent and oil-repellent agent-containing solution for 30 seconds, and the water-repellent and oil-repellent agent was entirely coated onto the inner and outer surfaces of the porous fluorine resin layer to prepare a porous fluorine resin film of a patch type (water-repellent and oil-repellent coated PTFE porous film).

(19) In this case, Examples 1 to 3 were classified as follows. Example 1: obtained after bending a porous fluorine resin film subjected to a freely shrinking step with a radius of curvature of 2 mm. Example 2: obtained after bending a porous fluorine resin film subjected to a freely shrinking step with a radius of curvature of 4 mm. Example 3: obtained after bending a porous fluorine resin film subjected to a freely shrinking step with a radius of curvature of 6 mm.

Comparative Examples 1 to 3: Preparation of PTFE Porous Film

(20) A PTFE porous film coated with a water repellent and an oil repellent was prepared in the same manner as in Example 1, except that the freely shrinking step was not carried out.

(21) At this time, Comparative Examples 1 to 3 were classified as follows. Comparative Example 1: obtained after bending a porous fluorine resin film not subjected to a freely shrinking step with a radius of curvature of 2 mm. Comparative Example 2: obtained after bending a porous fluorine resin film not subjected to a freely shrinking step with a radius of curvature of 4 mm. Comparative Example 3: obtained after bending a porous fluorine resin film not subjected to a freely shrinking step with a radius of curvature of 6 mm.

Experimental Example

(22) Bending was imparted to the porous films of the examples and comparative examples under the following conditions, and then the shrinkage and the air permeability before and after bending were measured. The results are shown in Table 2 below.

(23) Specifically, as shown in FIG. 3, the bending was imparted to the porous film, and the air permeability within a radius of curvature of 2 mm or more was measured.

(24) The change rate of air permeability of each porous film was measured according to the following Equation 1.
Change rate of air permeability before and after bending (%)=[(Pa−Pb)/Pa]×100  [Equation 1]

(25) (In the above Equation 1,

(26) Pa is an air permeability value of a fluorine-based porous film measured before imparting bending,

(27) Pb is an air permeability value of a fluorine-based porous film measured after imparting bending to the fluorine-based porous film).

(28) Further, the omnidirectional shrinkage due to bending for the porous fluorine resin film was calculated according to the following Equation 2.
Bending shrinkage (%)=5×exp[−0.8×curvature radius (mm)]  [Equation 2]

(29) Method of Imparting Bending

(30) As shown in FIG. 3, one end of the porous fluorine resin film was fixed so as to be in contact with a stainless steel cylinder having a diameter (R) of 4 to 12 mm, a weight was attached by a clamp to the opposite end that was not fixed, a tension of 0.5 kgf was applied thereto, and the porous film was wound around the cylinder to the end.

(31) After maintaining the state for 30 seconds, the clamp was removed, the tension was removed, the porous film was again spread, and the air permeability was measured.

(32) In addition, the air permeability can be measured by a Gurley method according to a standard such as JIS P 8117.

(33) That is, a Gurley number (unit: s or s/100 ml), which is the time required to pass 100 ml of air, was evaluated as the air permeability.

(34) Gurley number was determined by using a conventional Gurley Type Densometer based on the JIS P8117 standard.

(35) FIG. 3 shows the radius of curvature after imparting bending to the porous fluorine resin film of Comparative Examples 1 to 3 and Examples 1 to 3.

(36) FIG. 4 shows a comparison of shrinkages according to the average radius of curvature of the porous fluorine resin films of Comparative Examples 1 to 3 and Examples 1 to 3.

(37) TABLE-US-00002 TABLE 2 Comparative Example Example Category 1 2 3 1 2 3 Curvature 2 4 6 2 4 6 radius, mm Size, mm 49.0 × 50.0 49.5 × 50.0 49.8 × 50.0 49.5 × 50.0 49.9 × 50.0 50.0 × 50.0 (MD × TD) Shrinkage, % 2/0 1/0 0.4/0 1/0 0.2/0 0/0 (MD/TD) Air 18.fwdarw.17 21.fwdarw.20 19.fwdarw.17 15.fwdarw.15 14.fwdarw.14 15.fwdarw.15 permeability before and after bending, Gurley, s/100 ml

(38) As can be seen in Table 2 and FIG. 4, in the porous fluorine resin film (PTFE porous filtration film) products prepared in Examples 1 to 3, the shrinkage due to bending was reduced by 50% or more as compared with Comparative Examples 1 to 3.

(39) In particular, in Examples 1 to 3, since the internal structure of the porous film was not changed after bending, the change in air permeability was small.

(40) On the other hand, in Comparative Examples 1 to 3, the change rate of air permeability of the porous film after bending was about 5 to 10%, and as the air permeability was changed, the physical properties were changed.

(41) In addition, in Comparative Examples 2 and 3, even if the shrinkage ratio was 1% or less, the change rate of air permeability was large, which was not preferable to apply to products.