KR20200043759A - Porous fluorine resin film and method for preparing the same - Google Patents

Abstract

Provided are a fluorine-based resin porous membrane showing excellent thickness uniformity and improved shrinkage rate, and a manufacturing method thereof. The thickness fluctuation rate of the fluorine-based resin porous membrane is 7% or less.

Description

Fluorine-based resin porous membrane and manufacturing method therefor {POROUS FLUORINE RESIN FILM AND METHOD FOR PREPARING THE SAME}

The present invention relates to a porous fluorine-based resin membrane having excellent thickness uniformity and reduced shrinkage, and a method for manufacturing the porous membrane.

Porous membranes used in various fields are required to have high filtration efficiency and gas and liquid permeability together. Accordingly, it is known to uniformly control the pore diameter distribution inside the porous membrane to increase the amount of fluid passing through the pores under a specific pressure.

The porous membrane of the fluorine-based resin may have characteristics such as high heat resistance, chemical stability, weatherability, non-flammability, strength, non-adhesiveness, and low friction coefficient, resulting from the fluorine-based resin itself, and in addition, when made of a porous sieve , Flexibility, liquid permeability, particle collection efficiency, and low dielectric constant.

In particular, porous membranes using polytetrafluoroethylene (PTFE) among these fluorine-based resins have high stability to various compounds, and in particular, semiconductor related fields, liquid crystal related fields, and food and medical related fields, gas and liquid It is often used as a fine filtration filter (membrane filter) for mixtures of types.

The PTFE membrane is made of a preform using a paste composed of a mixture of PTFE powder and a lubricant, and the preform is shaped into a sheet by a rolling or extrusion process, followed by heat treatment to remove the lubricant, and then transverse (TD). Alternatively, it is manufactured by uniaxially stretching in the longitudinal direction (MD), or biaxially stretching in the TD direction after stretching in the MD direction.

However, in the case of manufacturing the PTFE membrane by the above-described method, in processes such as extrusion, drying, and stretching, a phenomenon in which the pore shape or properties cannot be maintained due to a high temperature and high pressure environment may occur, and particularly, the surface is defective. Bubbles may occur, and accordingly, the manufactured PTFE porous membrane may not have sufficient strength and filtration performance. In addition, the PTFE membrane is to adjust the porosity in the membrane by stretching and sintering process, it is easy to secure the porosity of the separation membrane during stretching, but strength and pressure resistance in the transverse direction may be reduced, and contraction in the longitudinal direction is easy. There are problems that arise.

In addition, the PTFE porous membrane made of MD / TD stretching has high porosity and poor dimensional stability because it has a reversible node and fibril structure.

Thus, as a method of improving the high temperature shrinkage of the PTFE porous membrane, a method of performing heat fixation at a high temperature of Tm or more of PTFE has been proposed. In this case, the strength of the film increases while the residual stress decreases due to high temperature. However, since the fibril structure collapses due to the high temperature of Tm or more, the pore size is greatly changed, so there is a limit to lowering the shrinkage rate.

In addition, after the manufacture of the PTFE porous membrane, a process such as thickness inspection, defect inspection, and slitting is usually performed. At this time, there is a problem in that the shrinkage in the longitudinal direction increases as tension is applied in the longitudinal direction.

The present invention is to provide a fluorine-based resin porous membrane having excellent thickness uniformity and reduced shrinkage, and a method for manufacturing the porous membrane.

In order to solve the above problems, according to an embodiment of the present invention, the thickness variation rate calculated according to the following Equation 1 is 7% or less, and the longitudinal direction calculated according to the following Equation 2 after 30 minutes at 120 ° C. A fluorine-based resin porous membrane having a shrinkage of 5% or less is provided.

[Equation 1]

[Equation 2]

In addition, according to another embodiment of the invention, the step of stretching the porous fluorine-based resin base film at a temperature of 250 ° C to 340 ° C by 1 to 12 times (MD); The longitudinally stretched film is relaxed at a temperature above the longitudinal stretching temperature and at a temperature of 340 ° C. or less, with a relaxation rate of 10 to 30% relative to the longitudinal length of the longitudinally stretched film, to heat the longitudinal direction. Shrinking; And stretching the shrinked film 5 to 15 times transverse direction (TD) at a temperature of 150 to 320 ° C., and heat setting at a temperature of 340 to 390 ° C. for 5 seconds to 60 minutes. Provide a manufacturing method.

Hereinafter, a fluorine-based resin porous membrane according to a specific embodiment of the present invention, a method for manufacturing the same, and a filter using the same will be described in more detail.

The term "fluorine-based resin porous membrane" used in the present invention is manufactured using a fluorine-based resin such as polytetrafluoroethylene (PTFE), and refers to a membrane containing pores. In particular, the present invention removes foreign substances and the like. It includes the meaning of the filtration membrane used to.

In the present invention, in the production of a porous membrane for a fluorine-based resin, a porous fluorine-based resin-based film stretched in the longitudinal direction is shrunk at a high temperature by using a reversible structure between nodes and fibrils in the porous fluorine-based resin film stretched at a temperature below the melting point After the (relaxation), by performing transverse stretching, thickness uniformity and heat shrinkage were improved.

Specifically, the fluorine-based resin porous membrane according to one embodiment of the invention, the thickness variation rate calculated according to the following equation (1) is 7% or less,

After 30 minutes at 120 ° C., the longitudinal shrinkage calculated according to Equation 2 below is 5% or less:

[Equation 1]

[Equation 2]

In the present invention, the thickness fluctuation rate of the fluorine-based resin porous membrane is measured by dividing the thickness of the porous membrane by dividing it by 50 points at the same interval, and then obtaining the average and standard deviation of the measured values, and standard deviation of the average as in Equation 1 above. It was determined from the ratio.

As described below, the fluorine-based resin porous membrane can control the thickness fluctuation rate in the transverse direction by stretching in the longitudinal direction and then stretching in the transverse direction before stretching in the longitudinal direction. This is due to the special properties of the porous membrane of the fluorine-based resin, and the fluorine-based resin has a high crystallinity of 98% or more, and because the interaction between polymer chains is very low, fibrillation is very easy during stretching. Do. In addition, the fibril thus formed has a reversible characteristic that contracts again when heated and returns to the node. Accordingly, after performing MD stretching that is higher than the actual stretching ratio, shrinking causes the orientation of the resin to change and the stress received by the sheet in the stretching process is reduced. In the present invention, by controlling this action, the fluctuation rate of the thickness of the porous membrane of the fluorine-based resin is controlled. The properties, namely thickness uniformity, were improved.

The thickness variation rate means thickness uniformity, and the lower the better. In the present invention, the thickness variation rate can be lowered by controlling the shrinkage ratio during the relaxation process, and as a result, the thermal shrinkage in the MD direction can be greatly reduced.

On the other hand, the fluorine-based resin porous membrane according to an embodiment of the present invention, a fine structure composed of a plurality of fibrils and a plurality of nodules connected to each other by the fibrils form a porous porous structure.

The pores included in the structure can be adjusted by controlling the draw ratio and conditions during the manufacturing process. Accordingly, in the present invention, the average diameter (Mean Pore Size) of the pores included in the fluorine-based resin porous membrane through optimization of manufacturing conditions is 0.02 μm to 0.3 μm, and the maximum diameter of the pores (Max Pore Size) is 0.03 to 10.0 μm. Can be By including pores satisfying the above range, it is possible to exhibit excellent filtration efficiency without deteriorating the permeability. When considering the remarkable effect of improving the control of pore size, more specifically, the average diameter of the pores contained in the fluorine-based resin porous membrane is 0.1 to 0.3 μm, and the maximum diameter of the pores may be 0.3 to 0.5 μm, , More specifically, the average diameter is 0.2 to 0.3 µm, and the maximum diameter of the pores is 0.3 µm or more and less than 0.4 µm, and the ratio of the average diameter of the pores and the size of the maximum diameter (the ratio of the maximum pore diameter / average pore diameter) is Micropores of uniform size may be formed in 1 or more and less than 2.

In addition, the fluorine-based resin porous membrane has a porosity of 70 to 90%, more specifically a porosity of 75 to 88%. As described above, as the average pore size is small, the permeability can be remarkably improved as the porosity is increased.

Meanwhile, in the present invention, porosity of the porous membrane can be determined according to the following Equation 3 after obtaining the density from the volume and weight of the porous membrane:

[Equation 3]

In this case, in Equation 3, the true density is 2.2 g / cm ³ of the fluorine-based resin.

In addition, the fluorine-based resin porous membrane has a thickness of 20 to 100 μm, more specifically 25 to 40 μm, and even more specifically 30 to 40 μm. By satisfying the thickness range in addition to the above-described pore conditions, it is possible to balance the filtration efficiency, permeability, and dimensional stability.

In addition, in the fluorine-based resin porous membrane, the fluorine-based resin can be used without limitation as long as it is usually used for a fluorine-based resin membrane. Specific examples include polytetrafluoroethylene (PTFE) and tetrafluoroethylene-perfluoroalkyl vinyl Ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), ethylene-tetrafluoroethylene copolymer resin (ETFE), tetrafluoroethylene-chlorotrifluoroethylene copolymer (TFE / CTFE) Or ethylene-chlorotrifluoroethylene resin (ECTFE), and any one or a mixture of two or more of them may be used. Of these, PTFE can be used in view of improving the chemical resistance, heat resistance, weather resistance and non-flammability of the porous membrane.

The fluorine-based resin porous membrane having the above-described structure comprises: stretching the porous fluorine-based resin base film at a temperature of 250 ° C. to 340 ° C. in a 1 to 12-fold longitudinal direction (MD) (step 1); Relaxing the longitudinally stretched film at a temperature range of not less than the longitudinal stretching temperature and 340 ° C. or less, with a relaxation rate of 10 to 30% relative to the longitudinal length of the longitudinally stretched film to undergo longitudinal thermal contraction. (Step 2); And stretching the contracted film 5 to 15 times transversely at 150 to 320 ° C., and heat setting at a temperature of 340 to 390 ° C. for 5 seconds to 60 minutes (step 3); have. Accordingly, according to another embodiment of the present invention, a method for manufacturing the above-described fluorine-based porous membrane is provided.

Hereinafter, the present invention will be described in detail for each step.

Step 1 is a step of longitudinally (MD) stretching the porous fluorine-based resin base film.

The porous fluorine-based resin-based film can be used without particular limitation, as long as it is usually used for preparing a porous membrane of a fluorine-based resin.

As an example, the porous fluororesin base film may include extruding and rolling a fluorine-based resin-containing composition comprising a fluorine-based resin and a lubricant to prepare a fluorine-based resin base film; And removing the lubricant by heat-treating the fluorine-based resin base film. Accordingly, the method for manufacturing a porous fluorine-based resin membrane according to an embodiment of the present invention may further include a manufacturing step of the porous fluorine-based resin-based film.

Specifically, the fluorine-based resin-containing composition may be prepared by mixing a fluorine-based resin with a lubricant.

At this time, as the fluorine-based resin, resins as described above may be used. In addition, when a PTFE resin is used as the fluorine-based resin, the PTFE resin may be prepared by a conventional method such as emulsion polymerization, and may be used in powder form.

In addition, the lubricant serves to facilitate extrusion while wetting the surface of the fluorine-based resin powder, and can be used without particular limitations as long as it can be removed by a method such as evaporative extraction by heat after molding into a sheet form. As specific examples, hydrocarbon oils such as liquid paraffin, naphtha, white oil, toluene, and xylene, various alcohols, ketones, and esters may be used.

When the porous membrane is prepared by stretching the fluorine-based resin-based film, fine fibrils are formed from the fluorine-based resin particles under high temperature and high pressure conditions, and fine pores can be formed by the nodule structures connected to each other by the fibrils. In order to form a bond between the Brill and the bond between the fluorine-based resin particles tightly and to produce a porous membrane having small pores, it is necessary to minimize the use of lubricant. However, when the content of the lubricant compared to the resin is too small, the surface film is clogged and a smooth surface is formed because the load on the surface of the preform is increased in the process of manufacturing a preform, rolling, extrusion, etc. Anger may occur. When the surface filming phenomenon occurs, since the pores disappear on the surface, the lubricant cannot be discharged outside in the drying process, and the lubricant that has not been discharged outside is vaporized by high heat in a process such as stretching, and then inside the membrane This may cause peeling or excite the inside of the sample, causing defects. Accordingly, in the present invention, the lubricant may be used in an amount of 10 to 30 parts by weight, more specifically 15 to 30 parts by weight based on 100 parts by weight of the fluorine-based resin.

The mixing of the fluorine-based resin and the lubricant may be performed according to a conventional method, and after mixing, a process of aging for a certain period of time for uniform mixing of each component in the mixture may be selectively performed. The aging can be carried out by specifically maintaining at a temperature of 30 to 50 ℃ for 12 to 24 hours.

In addition, after the mixing and optionally aging process, prior to performing the extrusion process, a process of forming a preform by applying pressure to the mixture, that is, the fluorine-based resin-containing composition may be selectively performed. The preform forming process may be specifically performed by applying a pressure of 1 to 5 MPa to the mixture or the aged mixture.

Subsequently, a process for producing a fluorine-based resin-based film by performing extrusion and rolling on a mixture or a preform obtained through the above-described process is performed. At this time, the thickness of the fluorine-based resin base film prepared after extrusion may be 1 to 3 mm, and the thickness of the fluorine-based resin base film produced after rolling may be 100 to 300 μm.

The extrusion and rolling process may be performed according to a conventional method, except that the thickness of the fluorine-based resin-based film to be produced has the above range. Specifically, the extrusion process may be performed under a temperature of 25 to 50 ° C and a pressure of 1 to 40 MPa, and the rolling process may be performed under a temperature of 30 to 100 ° C and a pressure of 10 to 30 MPa by a method such as calendering. . In addition, the rolling process may be performed once in consideration of the thickness of the above-described fluorine-based resin base film, or may be performed in multiple steps of two or more times.

Next, a process of removing the lubricant by performing heat treatment on the produced fluorine-based resin base film is performed.

The temperature at the time of the heat treatment is not particularly limited as long as the temperature at which the lubricant can be removed, specifically, 120 to 200 ° C, and more specifically 150 to 180 ° C, may be performed for a period during which the lubricant can be completely removed.

Next, using the fluorine-based resin-based film prepared by the above method or manufactured according to a conventional method, the stretching process in step 1 for preparing a fluorine-based resin porous membrane according to an embodiment of the present invention is performed.

The stretching process may be performed between rolls rotating at different speeds, or may be performed using a tenter in an oven.

Specifically, the stretching process may be performed by stretching the fluorine-based resin base film in the longitudinal direction at a stretching ratio of 1 to 12 times, or 1 to 9 times, or 1 to 3 times. The stretching ratio during the longitudinal stretching process affects the structure of the porous membrane of the fluorine-based resin to be produced, in particular, the pore size, and can exhibit excellent shrinkage by having an appropriate pore size when performing the longitudinal stretching process under the above conditions. . If longitudinal stretching is not performed, or if the stretching ratio is less than 1 time, the size of the pores becomes too large, and if the stretching ratio exceeds 12 times, the length of the fibril becomes too long, and as a result, the shrinkage may increase. There is

In addition, the stretching process may be carried out at a temperature of 250 ℃ to 340 ℃, more specifically 280 ℃ to 330 ℃. It is advantageous to form a porous structure when the stretching process is performed in the above temperature range. In the stretching process, if the temperature is less than 250 ° C, there is a fear of stretching uniformity, and when it exceeds 340 ° C, melting of the fluorine-based resin occurs, the structural properties of the sheet change, and accordingly, the physical properties change, thereby changing the TD under the same conditions. There is a risk of breakage during stretching.

Next, step 2 for producing a fluorine-based resin porous membrane according to an embodiment of the invention, the film stretched in the longitudinal direction in step 1, the longitudinal stretching temperature is higher than 340 ℃ temperature range, in the longitudinal direction It is a relaxation step of heat shrinking with a relaxation rate of 10 to 30% relative to the longitudinal length of the stretched film.

The larger the longitudinal stretching ratio, the smaller the pore size of the prepared porous membrane. When controlling the pore size of the membrane by using these characteristics, there is a fear of shrinkage reduction due to an increase in the stretching ratio. At this time, the shrinkage rate can be improved through relaxation after MD stretching under the above conditions. In addition, the orientation characteristic of the fluorine-based resin porous membrane that has undergone relaxation after MD stretching may be changed, and the uniformity of thickness may be improved after TD stretching due to small stress during stretching.

Specifically, the relaxation rate after stretching is performed at about 10 to 30% of the longitudinal length of the film stretched in the longitudinal direction. If the relaxation rate after stretching is less than 10%, the effect of reducing the thickness fluctuation rate by performing the relaxation process and the longitudinal direction The shrinkage reduction effect is negligible. On the other hand, if the relaxation rate after stretching exceeds 30%, there is a fear that the rate of change in thickness, such as wrinkles formed in the film, may increase. More specifically, the relaxation process may be performed with a relaxation rate of 20 to 30%.

Moreover, the said relaxation process is performed in the temperature range of 340 degreeC or less and more than the temperature in a longitudinal stretch process. When the temperature in the relaxation process is lower than the temperature in the longitudinal stretching process, it is difficult to run because relaxation does not occur, and there is a fear that the thickness fluctuation rate and the heat shrinkage in the MD direction may increase significantly. In addition, when the temperature during the relaxation process exceeds 340 ° C, physical properties of the porous film are completely changed during the process of melting and recrystallization of the porous film, so physical property control is not easy, and as a result, there is a possibility that the thickness fluctuation rate and shrinkage rate may increase. . Considering the excellent effect of improving the thickness variation rate and shrinkage rate according to the temperature control during the relaxation process, it can be carried out more specifically at 280 to 320 ℃.

In addition, the time during the heat shrinking process may be appropriately determined under the conditions satisfying the above-described temperature conditions and shrinkage rates.

Next, step 3 for preparing a fluorine-based resin porous membrane according to an embodiment of the present invention is a step of stretching and heat-setting the fluorine-based resin-based film contracted in step 2 in the transverse direction.

Specifically, the stretching process in step 3 may be performed by stretching the fluorine-based resin base film in the transverse direction at a stretching ratio of 5 to 15 times, more specifically 10 to 15 times. The transverse stretching process affects the porosity and the basic physical properties of the porous membrane of the fluorine-based resin to be produced. If the stretching ratio during the transverse stretching process is less than 5 times, the porosity is low and the air permeability and flow rate tend to decrease. In addition, when the stretching ratio exceeds 15 times, porosity is excessively increased and shrinkage is high, making it difficult to secure dimensional stability.

In addition, the stretching process may be performed at a temperature of 150 to 320 ° C, more specifically 200 to 320 ° C. Under the above conditions, the shrinkage resistance of the porous membrane, particularly the shrinkage resistance in the transverse direction, can be improved by increasing the porosity while reducing the average pore size when performing the transverse stretching process.

Subsequently, a heat fixing process is performed on the transversely stretched fluorine-based resin base film.

The heat fixing process is to prevent heat shrinkage of the final manufactured fluorine-based film porous membrane, at a temperature of 340 to 390 ° C. for 5 seconds to 60 minutes, and more specifically 5 to 20 seconds at a temperature of 350 to 390 ° C. Can be performed during. The pore size in the fluorine-based resin porous membrane finally produced by the heat setting treatment under the above conditions can be made uniform. However, if the temperature is less than 340 ° C or performed within 5 seconds during heat fixation, the effect of preventing heat shrinkage is not sufficient, and if it is performed above 390 ° C or exceeding 60 minutes, fracture may occur due to problems such as melting of fibrils Can occur.

When the porous membrane of the fluorine-based resin is produced as described above, by performing at a low stretching ratio during transverse stretching, the porous membrane of the produced fluorine-based resin can realize physical properties / structural characteristics as described above.

In addition, in the conventional micro-thickness porous membrane, the shape or the diameter of pores distributed therein may change due to the applied pressure during filtration, and the filtration characteristics may be significantly deteriorated due to rupture of the membrane itself, etc. The fluorine-based resin porous membrane manufactured according to the method has excellent mechanical properties, and also has a characteristic in which its shape, shape, and shape of internal pores, etc. are not significantly changed during the manufacturing process and the filtration operation process.

Accordingly, according to another embodiment of the present invention, there is provided a fluorine-based resin porous membrane having a significantly reduced thickness fluctuation rate by the above-described manufacturing method and exhibiting an improved heat shrinkage.

Specifically, as described above, the porous membrane of the fluorine-based resin has a longitudinal shrinkage of 5% or less, and more specifically 1 to 5, calculated according to Equation 2 using the dimensional values changed after 30 minutes at 120 ° C. %, More specifically 2 to 5%. As such, having a low longitudinal shrinkage, handling properties are improved, and assembling properties may be improved when an application using a fluorine-based resin porous membrane is applied. For example, when applied as a processing filter, shrinkage due to heat generated in the assembly process is prevented, thereby improving assembly yield.

In addition, the fluorine-based resin porous membrane has a transverse heat shrinkage of 3% or less, more specifically 1 to 3%, more specifically, calculated according to the following Equation 4 using the changed dimensional value after 30 minutes at 120 ° C. Specifically, it is excellent in shape stability at a high temperature of 2 to 3%. Accordingly, when the product of the porous membrane of the fluorine-based resin is applied, the shape stability can be maintained even in conditions of contact with high temperature sulfuric acid.

 [Equation 4]

In addition, in the fluorine-based resin porous membrane, the ratio of the longitudinal shrinkage to the transverse shrinkage (= longitudinal shrinkage / transverse shrinkage) is 0.8 to 3, more specifically, under conditions that satisfy the aforementioned longitudinal and transverse shrinkage. May be 1 to 2.5. As it has a ratio of the longitudinal shrinkage ratio to the transverse shrinkage ratio in the above range, it may exhibit better shape stability.

Accordingly, the fluorine-based resin porous membrane can be widely used as a filter medium for corrosive gases and liquids, a permeable membrane for filter electrolysis for a processor, and a battery separator, and can also be used to precisely filter various gases and liquids used in the semiconductor industry. You can.

According to another embodiment of the invention, a filter comprising the above-described fluorine-based resin porous membrane, and a filter device are provided.

The filter may further include a filter element such as a non-woven fabric, a fabric, a mesh, or a screen, in addition to the above-described fluorine-based resin porous membrane, and may have various shapes such as a flat plate, a pleated shape, a spiral shape or a hollow cylinder shape.

The fluorine-based resin porous membrane according to the present invention can exhibit improved heat shrinkage by having excellent thickness uniformity. In addition, the pores contained in the membrane are small, but have high porosity, and thus excellent dimensional stability. Accordingly, it can be particularly useful as a filter for a process requiring excellent dimensional stability with high mechanical strength and low shrinkage.

Hereinafter, preferred embodiments are provided to help understanding of the present invention. However, the following examples are only provided to more easily understand the present invention, and the contents of the present invention are not limited thereby.

Example  One

A fluorine-based resin-containing composition was prepared by mixing 22 parts by weight of a lubricant (Isopar H ™, manufactured by Exxon) with respect to 100 parts by weight of PTFE resin 650J ™ (manufactured by MCF), and then aged at 38 ° C. for 24 hours. Subsequently, a preform block was prepared by applying a pressure of 4 MPa, and extruded into a sheet shape of 1 mm thickness using paste extrusion equipment, and then rolled to a thickness of 300 μm through calendering to prepare a PTFE base film. The prepared PTFE base film is roll-to-roll in a heating oven at 200 ℃. The lubricant was completely removed by heat treatment.

After the heat-treated PTFE base film is stretched three times in the longitudinal direction (MD) using a difference in roll speed at 300 ° C, the roll speed is slowed so that the MD direction shrinkage can occur 10% at 310 ° C. Was carried out.

Thereafter, the film was stretched 10 times in the transverse direction (TD) using a difference in roll speed at 280 ° C, and the film was heat-set at 370 ° C for 9 seconds using a heating roll to prepare a PTFE porous membrane.

Example 2

After the MD stretching in Example 1, a PTFE porous membrane was prepared in the same manner as in Example 1, except that relaxation was performed by slowing down the roll speed so that MD direction shrinkage may occur at 310 ° C at 20%.

Example 3

After the MD stretching in Example 1, a PTFE porous membrane was prepared in the same manner as in Example 1, except that relaxation was performed by slowing down the roll speed so that MD direction shrinkage may occur at 310 ° C in 30%.

Comparative Example 1

After the MD stretching in Example 1, a PTFE porous membrane was prepared in the same manner as in Example 1, except that a TD stretching process was performed without a relaxation process.

Comparative Example 2

A fluorine-based resin porous membrane was prepared in the same manner as in Example 2, except that the MD draw ratio was 5 times and the relaxation process was performed at a relaxation rate of 40%.

However, wrinkles occurred due to unstable running in the manufacturing equipment during the manufacturing process, and as a result, normal sample preparation was difficult.

Comparative Example 3

A fluorine-based resin porous membrane was prepared in the same manner as in Example 2, except that the relaxation process was performed at a temperature of 350 ° C.

However, during the manufacturing process, the width shrinkage was severe and wrinkles were generated.

Comparative Example 4

A fluorine-based resin porous membrane was prepared in the same manner as in Example 2, except that the relaxation process was performed at a temperature of 280 ° C.

However, during the manufacturing process, the longitudinal shrinkage did not occur sufficiently, and thus, the running was unstable, and the production of a normal porous membrane was difficult.

Test Example 1

The PTFE porous membranes prepared in Examples and Comparative Examples were evaluated by the following method, and the results are shown in Table 1 and FIGS. 3 to 5 below.

1) Thickness (µm) and Thickness Variation (%): After measuring the thickness of the porous membrane by dividing it by 50 points, the average and standard deviation of the measured values were obtained, and the thickness variation was calculated according to Equation 1 below.

[Equation 1]

2) Porosity: The weight, thickness, and area of the PTFE porous membrane were measured, and porosity was measured according to Equation 3 below. At this time, the thickness of the PTFE porous membrane was measured using a dial thickness gauge manufactured by mitsutoyo.

[Equation 3]

At this time, in Equation 3, the true density was set to 2.2 g / cm ³ of the fluorine-based resin.

3) Average pore size (µm) and maximum pore size (µm) = Average pore size and maximum pore size were measured using PMI's Capillary Flow Porometer equipment.

In detail, after mounting the PTFE porous membrane on the measuring device, it was completely wetted in the surface tension test solution (GALWICK), and air or nitrogen was injected vertically into the porous membrane. When the pressure is constantly increased and a certain pressure is reached, a drop of the test solution filling the largest hole among the pores bursts out, and the pressure at this time is taken as the bubble point. Subsequently, if the pressure continued to increase, all the solutions that had not filled and the remaining small pores burst out as droplets. At this time, the size of the pores was calculated by recording the Flow Rate (Wet Curve) according to the pressure. For the porous membrane in the dry state that is not wetted with the test solution, the flow rate increases constantly as the pressure increases (Dry Curve), which corresponds to the pressure at the point where the graph where the Dry Curve becomes 1/2 and the Wet Curve cross. The pore is defined as the average pore size.

4) Heat shrinkage (120 ℃, 30min) (%): After cutting the PTFE porous membrane to 5cm in the longitudinal direction (MD) and 5cm in the transverse direction (TD), free standing for 30 minutes at 120 ℃ When it was placed in the measured dimensions, the longitudinal and transverse thermal shrinkages were calculated according to the following equations 2 and 4, respectively.

[Equation 2]

[Equation 4]

In Equations 2 and 4, before heat treatment, the transverse length and the longitudinal length of the fluorine-based resin porous membrane are 5 cm, respectively.

Example 1 Example 2 Example 3 Comparative Example 1 Comparative example
2 Comparative example
3 Comparative example
4 MD draw ratio (times) 3.3 3.8 4.3 3 5.0 3.8 3.8 MD stretching temperature (℃) 300 300 300 300 300 300 300 Relaxation (%) 10 20 30 0 40 20 20 Relaxation temperature (℃) 310 310 310 - 310 350 280 Relaxation heat shrink direction Bell Bell Bell - Bell Bell Bell Actual draw ratio (times) 3 3 3 3 3 3 3 Thickness (㎛) 35 33 32 35 28 37 36 Porosity (%) 77 78 78 78 78 80 77 Average pore diameter (㎛) 0.20 0.20 0.20 0.20 0.20 0.24 0.22 Maximum pore diameter (㎛) 0.39 0.38 0.38 0.40 0.39 0.45 0.38 Heat shrinkage MD
(120 ℃, 30min)
(%) 5 3 2 10 2 3 12 Heat shrinkage rate TD (120 ℃, 30min)
(%) 2 2 2 2 2 8 3 Thickness fluctuation rate
(%) 6.13 5.50 4.40 8.12 12.2 18.2 10.1

As a result of the experiment, the fluorine-based resin porous membranes of Examples 1 to 3 produced by the production method according to the present invention exhibited a heat shrinkage in the MD direction reduced to 5% or less with a low thickness variation rate of 7% or less.

On the other hand, in the porous membrane of Comparative Example 1, which did not perform the relaxation process after MD stretching, the thickness, porosity, average pore size, and maximum pore size showed the same level as in Examples 1 to 3, but the rate of thickness fluctuation was as large as 8.12%. It increased, and the heat shrinkage in the MD direction also increased significantly to 10%.

In addition, in the case of Comparative Example 2 in which the relaxation process was performed after MD stretching, but the relaxation process was performed at a relaxation rate of 40%, the sheet variation was significantly increased to 12.2% compared to Examples 1 to 3 by increasing sheet non-uniformity. . In addition, in the case of Comparative Example 2, the thickness of the porous membrane was smaller compared to Examples 1 to 3 due to the high MD draw ratio. This is because it has a structurally reversible characteristic by contraction after MD stretching, but since it is not 100%, the larger the stretching magnification, and the more the relaxation is made, the thickness becomes thinner.

In addition, after the MD stretching, a relaxation process was performed, and in the case of Comparative Example 3, which performed a relaxation process at a high temperature of 350 ° C., the thickness, porosity, average pore size, and maximum pore size showed the same level as Examples 1 to 3 However, as the sheet property became non-uniform as the relaxation process was performed at a temperature of Tm or more, the rate of change in thickness was most increased to 18.2%, and as a result, the heat shrinkage in the TD direction was also significantly increased. The reason for the increased thickness non-uniformity is that when the PTFE film is processed above Tm, amorphous locking (a phenomenon in which the resin melts and recrystallizes and structurally changes so that the fibril cannot easily escape from the node) occurs. Because it increases, and in the process of re-crystallization after the sheet melts due to contact with the roll surface in the roll-to-roll process, the contact with the surface is not perfectly uniform, and it is difficult to achieve uniform stretching in the TD stretching process afterwards. to be.

In addition, amorphous relaxation occurs when relaxation is performed at a temperature above Tm after MD stretching, and the tendency to increase the pore size as fibrillation becomes difficult at the node in the subsequent TD stretching process.

In addition, in the case of Comparative Example 4 in which the relaxation process was performed after MD stretching, but the relaxation process was performed at a temperature lower than the temperature during the MD stretching process, wrinkles occurred due to inadequate relaxation of the PTFE film and thickness unevenness occurred. , The rate of thickness variation increased. In addition, due to the relaxation at a low temperature, the residual stress is not effectively removed, and the MD shrinkage is increased.

From the above experimental results, it can be seen that in order to reduce the thickness variation rate and the thermal contraction rate in the MD direction, the contraction direction and conditions in the relaxation process after MD stretching should be optimized.

Claims (12)

The rate of change in thickness calculated according to Equation 1 below is 7% or less,
A fluorine-based resin porous membrane having a longitudinal shrinkage of 5% or less calculated according to the following Equation 2 after 30 minutes at 120 ° C .:
[Equation 1]

[Equation 2]

According to claim 1,
The fluorine-based resin porous membrane, wherein the longitudinal shrinkage is 1 to 5%.
According to claim 1,
The fluorine-based resin porous membrane is a fluorine-based resin porous membrane having a lateral heat shrinkage of 3% or less calculated according to the following Equation 4 by using the dimensional values changed after 30 minutes at 120 ° C .:
[Equation 4]

According to claim 3,
The transverse shrinkage is 1 to 3%, fluorine-based resin porous membrane.
According to claim 3,
The ratio of the longitudinal shrinkage to the transverse shrinkage (= longitudinal shrinkage / transverse shrinkage) is 0.8 to 3, fluorine-based resin porous membrane.
According to claim 1,
The fluorine-based resin porous membrane having an average pore diameter of 0.2 to 0.3 µm and a maximum pore diameter of 0.3 µm or more and less than 0.4 µm of pores included in the fluorine-based resin porous membrane.
According to claim 1,
The fluorine-based resin porous membrane has a porosity of 75 to 88%.
According to claim 1,
The fluorine-based resin porous membrane has a thickness of 30 to 40㎛, fluorine-based resin porous membrane.
According to claim 1,
The fluorine-based resin is polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, ethylene-tetrafluoroethylene copolymer resin, tetrafluoroethylene-chloro A fluorine-based resin porous membrane comprising any one or two or more selected from the group consisting of trifluoroethylene copolymers and ethylene-chlorotrifluoroethylene resins.
Stretching the porous fluorine-based resin base film at a temperature of 250 ° C to 340 ° C in a 1 to 12-fold longitudinal direction;
Relaxing the longitudinally stretched film at a temperature above the longitudinal stretching temperature and at a temperature of 340 ° C. or less with a relaxation rate of 10 to 30% relative to the longitudinal length of the longitudinally stretched film;
The method of manufacturing a porous membrane of claim 1, comprising; stretching the shrinked film at 150 to 320 ° C. for 5 to 15 times transverse direction, and heat setting at a temperature of 340 to 390 ° C. for 5 seconds to 60 minutes.
The method of claim 10,
In the above manufacturing method, before the longitudinal stretching of the porous fluorine-based resin base film, the composition containing a fluorine-based resin and a fluorine-based resin containing a lubricant is pressed to form a preform, and the preform is extruded and rolled to extrude and roll the fluorine-based resin base film. After manufacturing, further comprising the step of producing a porous fluorine-based resin base film by heat treatment, a method for producing a porous fluorine-based resin film.
A filter comprising the porous membrane of the fluorine-based resin according to any one of claims 1 to 9.