HK1067098B - Asymmetric porous polytetrafluoroethylene membrane for a filter - Google Patents
Asymmetric porous polytetrafluoroethylene membrane for a filter Download PDFInfo
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- HK1067098B HK1067098B HK05100603.4A HK05100603A HK1067098B HK 1067098 B HK1067098 B HK 1067098B HK 05100603 A HK05100603 A HK 05100603A HK 1067098 B HK1067098 B HK 1067098B
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Description
Technical Field
The present invention relates to an asymmetric porous polytetrafluoroethylene membrane for a filter. The present invention also relates to a filter material comprising an asymmetric porous polytetrafluoroethylene membrane for a filter and a reinforcing material.
Background
Porous polytetrafluoroethylene films (hereinafter referred to as porous PTFE films) have good chemical resistance and high tensile strength, and therefore are widely used not only in the fields of chemicals, foods, semiconductors, and the like for sealing, gaskets, and the like of production equipment, piping, and the like thereof, but also in filters for gas and liquid filtration, air/water permeable films for clothing, medical films, and the like.
In general, a mixture of PTFE fine powder and an extrusion aid such as naphtha, that is, PTFE paste, is extruded and rolled. Subsequently, the extrusion aid is removed from the rolled product, and then the product is stretched in a uniaxial or biaxial direction. In order to maintain the shape of the stretched porous PTFE film, a method of producing such a porous PTFE film by performing so-called heat setting at a temperature between 35 ℃ and the melting point of PTFE has been disclosed (for example, see U.S. Pat. No. 3953566, U.S. Pat. No. 3962153, U.S. Pat. No. 4096227, and U.S. Pat. No. 4187390).
There are also many documents that use a porous PTFE membrane as a filter, but there is a problem that the air permeability and the collection efficiency are insufficient (for example, refer to U.S. Pat. No. 5234739, U.S. Pat. No. 5395429, and U.S. Pat. No. 5409515).
Disclosure of Invention
The object of the present invention is to provide an asymmetric porous PTFE film-forming material for a filter, which has the characteristics of a conventionally known porous PTFE molded article, such as small change over time, water permeation resistance, gas permeation resistance, sealing characteristics, and electrical characteristics, and which has improved collection efficiency, improved air permeability, and improved pressure loss.
The conventionally known porous PTFE membrane has continuous bubbles, and the pore diameter distribution formed is uniform on the surface and inside of the membrane, and the pore diameter is uniformly formed in the entire membrane (the porosity is substantially constant in the membrane), that is, it is a symmetric porous membrane.
As a result of intensive studies, it has been found that when one surface of a membrane is a dense PTFE surface layer and the other surface is an asymmetric porous PTFE membrane comprising an open-cell porous layer having a lower density, the water resistance, air permeability and water vapor permeability of the porous PTFE membrane can be improved.
That is, the present invention relates to an asymmetric porous polytetrafluoroethylene membrane for a filter, which comprises a highly dense surface layer and a continuous-cell porous layer, wherein:
(1) the contact angle of the water on the surface of the surface layer is 120-140 degrees;
(2) the light diffusion reflectance is 91 to 94%.
The asymmetric porous polytetrafluoroethylene film is preferably produced by biaxial stretching.
The asymmetric porous polytetrafluoroethylene membrane for a filter preferably has a film thickness of 5 to 100 μm.
The filter material comprising the asymmetric porous polytetrafluoroethylene membrane for a filter preferably contains a reinforcing material comprising a synthetic resin or an inorganic fiber.
Reinforcing materials, preferably polyethylene, polypropylene, polyester, polyamide or glass fibres.
The present invention also relates to a method for producing an asymmetric porous PTFE membrane for a breather filter, characterized in that one surface of a stretched symmetric porous PTFE membrane is cooled to 0 ℃ or lower, and the other surface is subjected to a heat treatment at 260 to 380 ℃.
Drawings
FIG. 1 is a schematic view showing an example of a heat treatment apparatus.
FIG. 2 is an SEM image (magnification:. times.3000) of a cross section of an asymmetric porous PTFE membrane after heat treatment (340 ℃ C., 10s) was applied to one surface of the membrane. The dense layer (heated side) is the upper white portion.
Detailed Description
The stretched porous PTFE film used in the present invention can be produced basically by the following known 6 steps.
(1) Paste extrusion step of PTFE Fine powder
A paste-like mixture of PTFE fine powder produced by emulsion polymerization and an extrusion aid such as naphtha was extruded by an extruder to obtain cylindrical, prismatic, or plate-like extrudates.
The PTFE fine powder is obtained by precipitating an aqueous dispersion of a polymer produced by an emulsion polymerization method, separating the polymer, and drying the polymer. The polymer composition is a Tetrafluoroethylene (TFE) alone or a copolymer of TFE and a small amount (usually 0.5 wt% or less) of a perfluoroalkyl vinyl ether or hexafluoropropylene (modified PTFE).
In this step, the PTFE is pressed as strongly as possible, which is preferably at a point where it can be smoothly subjected to the next stretching step. The suppression of orientation can be achieved by appropriately selecting the reduction ratio of paste extrusion (preferably in the range of 300: 1 or less, usually 20: 1 to 150: 1), the PTFE/extrusion aid ratio (usually 77/23 to 80/20), the die angle of the extruder (usually about 60 °), and the like.
In addition, as the extrusion aid, mineral oil having high lubricity, such as naphtha, is generally used.
(2) Rolling procedure for paste extrudates
The paste extrudate produced in (1) is rolled by a calender roll or the like in the extrusion direction or in the direction perpendicular to the extrusion direction to be formed into a sheet shape.
(3) Extrusion aid removal step
The rolled product produced in (2) is subjected to extraction with a solvent such as trichloroethane or trichloroethylene by heating or to remove the extrusion aid.
The heating temperature may be appropriately selected depending on the extrusion aid, but is preferably 200 to 300 ℃. It is particularly preferable to heat at about 250 ℃. If the temperature exceeds 300 ℃ and particularly exceeds the melting point of PTFE, that is, 327 ℃, the material tends to be sintered.
(4) Drawing step
The rolled material prepared in (3) without extrusion aid was subjected to stretching. In the case of the stretching method, stretching may be carried out in a uniaxial direction or a biaxial direction, but in order to make the pore size distribution narrower and to obtain a desirable porosity in terms of ventilation, biaxial stretching is preferably carried out. In the case of biaxial stretching, the stretching may be carried out sequentially or simultaneously. The pre-heating to about 300 ℃ can be carried out before stretching.
The stretch ratio should be carefully selected because it affects the tensile strength of the film and the like. The draw ratio is preferably 300 to 2000%, more preferably 400 to 1500%. If the stretch ratio deviates from this range, the target pore diameter and porosity tend not to be obtained.
(5) Heat setting Process
The stretched product obtained in (4) is preferably heat-set by heating the product at a temperature slightly higher than the melting point (about 327 ℃) of PTFE and lower than the decomposition temperature, i.e., 340 to 380 ℃ for a relatively short time (5 to 15 seconds). If the temperature is less than 340 ℃, the heat setting tends to be insufficient; when the temperature exceeds 380 ℃ and the setting time becomes short, the time control tends to be difficult.
(6) Production of asymmetric porous PTFE film
In the present invention, an asymmetric porous PTFE film is produced by cooling one surface of the stretched symmetric porous PTFE film produced in this manner, heating the other surface, and then cooling the film. One of the apparatus for manufacturing and the method thereof is shown in fig. 1, for example, but not limited thereto.
The production method of the present invention will be specifically described below with reference to fig. 1.
The symmetric porous PTFE film that has been heat-set and cooled in step (5) is sent out by the symmetric porous PTFE film sending-out roller 4, and passes through between the heating device 2 and the brine cooling tank 1. Here, the surface temperature of the PTFE film is measured by the temperature sensor 6 and read by the temperature reading unit 7. Next, the data on the temperature is sent to the heating device control unit 8, and the temperature of the hot air discharged from the heating device 2 through the hot air outlet 3 is controlled in accordance with the data. In the brine cooling tank 1, a cooling liquid is circulated and maintained at a constant temperature. The PTFE film passing through the gap is wound by an asymmetric porous PTFE film winding roll 5.
In this case, the heat treatment temperature by the heating device 2 is preferably 260 to 380 ℃, and more preferably 340 to 360 ℃. When the heat treatment temperature is less than 260 ℃, the dense layer tends not to be formed sufficiently; if the temperature exceeds 380 ℃, the control of the production of the asymmetric PTFE film becomes difficult, and the film tends to be densified as a whole.
The cooling treatment temperature by the brine tank 1 on the other side is preferably 0 ℃ or lower, more preferably-10 ℃ or lower. If the cooling treatment temperature exceeds 0 ℃, control of asymmetric film production becomes difficult, and the film tends to be densified as a whole and to have reduced air permeability.
The heating and cooling treatment time is preferably 5 to 15 seconds, more preferably 6 to 10 seconds.
Under the above conditions, one surface of the symmetric porous PTFE membrane after heat setting was cooled to form a continuous-cell porous layer, and the other surface was again subjected to heat treatment to modify the membrane surface, thereby producing an asymmetric porous PTFE membrane having a highly dense surface layer.
Here, the term "densification" refers to a layer in which only one surface is modified by heat treatment to further densify a porous structure, and which exhibits properties different from those of an original symmetric porous PTFE film, such as a contact angle with water and a diffuse reflectance with light; the continuous bubble property is a layer having a porous structure substantially the same as that of the film before heat treatment.
Further, when 0.1 to 0.2mL of an aqueous solution having an n-propanol content of 60% is dropped on the surface of the film, the aqueous solution directly permeates into the film on the non-heat-treated porous layer surface, and the white porous layer surface becomes transparent; on the other hand, the surface layer after heat treatment and densification was not easily penetrated with the aqueous solution, and the dropped surface film remained white.
The contact angle with water on the surface layer of the asymmetric porous PTFE membrane of the present invention is 120 to 140 °, preferably 125 to 135 °. If the contact angle is less than 120 degrees, the densification of the heat-treated surface is insufficient, and the collection efficiency tends to be lowered; when the degree exceeds 140 degrees, the densification tends to be excessive and the air permeability tends to be lowered.
The asymmetric porous PTFE membrane surface layer of the present invention has a considerably higher water repellency than the symmetric porous PTFE membrane, as shown by the contact angle (110 to 118 °) to water.
Here, the contact angle to water can be obtained by the following equation.
Contact angle 2tan-1(h/r)
Where h is the height of the spherical water droplet and r is the radius of the spherical water droplet.
The surface layer of the asymmetric porous PTFE membrane for a filter of the present invention has a light diffusion reflectance of 91 to 94%. The light diffusion reflectance is an index indicating the modified layer, and less than 91% indicates insufficient densification; if it exceeds 94%, the densification is excessive. This indicates that the light diffusion reflectance was higher than that of the symmetric porous PTFE film (90 to 91%).
In terms of the porous structure, the conventional symmetric porous PTFE membrane was formed into a uniform porous structure as a whole from the SEM image; in the asymmetric porous PTFE membrane of the present invention, the surface layer is formed as a dense layer, and the porous layer has the same porous structure as that of the conventional symmetric porous PTFE membrane. The porosity of the entire membrane is preferably 30 to 95%, more preferably 50 to 90%. If the porosity is less than 30%, the pressure loss tends to increase; if the concentration exceeds 95%, the collection efficiency tends to be low.
Here, the porosity is obtained from the following equation based on the measured density.
Porosity (%) - (1-apparent density of PTFE/true density of PTFE) × 100
The apparent density (g/cc) of PTFE was defined as the weight (W)/volume (V) of the porous PTFE film, and the true density (g/cc) was defined as 2.15 (literature value).
The maximum pore diameter of the porous layer of the asymmetric porous PTFE membrane of the present invention is preferably 0.03 to 2 μm, more preferably 0.05 to 1 μm. When the maximum pore diameter is less than 0.03. mu.m, the pressure loss tends to increase; when the particle size exceeds 2 μm, the collecting efficiency tends to be lowered.
Here, the maximum pore diameter is determined by the following method.
First, it was found from SEM images (x 20,000) that the pore diameter, structure, and the like of the porous layer did not change before and after the heat treatment with respect to the symmetric porous PTFE membrane and the asymmetric porous PTFE membrane produced by heat-treating the symmetric porous PTFE membrane. It is one of the features of the present invention that only the surface layer is modified without changing the pore size, structure, and the like of the porous layer after the heat treatment.
Next, the maximum pore diameter of the symmetric porous PTFE membrane was measured by a Porosimeter (porometer), and this value was substituted with the maximum pore diameter of the asymmetric porous PTFE membrane.
In a sample chamber of a Porosimeter (porometer PMI-1500 manufactured by poros Materials inc., ltd.), a film sample was mounted, and measurement was started in an automatic mode. After the start of the measurement, a gas (nitrogen gas) was introduced on one side of the membrane in the sample chamber. The rate of introduction of the gas is automatically controlled.
During periods when the pressure of the introduced gas is low, the sample membrane becomes a barrier and the pressure in the chamber is continuously increased gradually. The barrier properties of the sample membrane are lost as the pressure increases, so that the gas starts to permeate and the pressure increase in the sample chamber is stopped, and the pressure is measured.
The pressure measurement was carried out by measuring the dried membrane and the membrane wetted with Porewick solution to obtain the respective pressures P1、P2。
Further, the Porewick solution is a trade name of a standard solution prepared by Porous Materials, Inc. and having a surface tension adjusted to 16 dyn/cm.
The maximum pore diameter is calculated by the following equation.
d＝C·(τ/ΔP)
Where d is the maximum pore diameter (μm), C is 0.415, τ is the surface tension of the wetting solution (dyn/cm), and Δ P is P2-P1(psi)。
The asymmetric porous PTFE membrane for a filter of the present invention has a membrane thickness of preferably 5 to 100 μm, more preferably 10 to 70 μm. When the film thickness is less than 5 μm, the collecting efficiency tends to be lowered; when the average particle diameter exceeds 100. mu.m, the air permeability tends to be lowered. The thickness of the surface layer is preferably 0.04 to 40%, more preferably 0.1 to 30% of the total thickness. If the thickness of the surface layer is less than 0.04% of the total thickness, the collecting efficiency tends to be lowered; if the content exceeds 40%, stress damage tends to increase.
The following describes a filter material using the asymmetric porous PTFE membrane for a filter of the present invention.
One or both surfaces of the asymmetric porous PTFE membrane for a filter of the present invention are preferably reinforced with a mesh, woven fabric, nonwoven fabric, or the like having high air permeability, and the function as a filter can be preferably maintained for a long period of time.
The reinforcing material can be combined by a method of partially bonding the reinforcing material with an adhesive, or by various methods such as needling and attaching the reinforcing material to the filter holder so as to be simply superposed on the reinforcing material.
As the reinforcing material, synthetic resin, which is a material having high strength and less active in chemical properties, or air-permeable woven fabric, nonwoven fabric, mesh screen, or the like of inorganic fibers can be used. Examples of the synthetic resin include polyethylene, polypropylene, polyester, and polyamide. Examples of the inorganic fibers include glass fibers and carbon fibers.
The asymmetric porous PTFE membrane for a filter of the present invention can significantly improve the efficiency of collecting fine particles in the air without pressure loss, and can improve the permeation rate of gas and liquid by 2 to 4 times and the tensile strength by 20 to 60% although the porosity is substantially the same as that of the symmetric porous PTFE membrane.
The present invention will be described in detail with reference to examples, but the present invention is not limited to these examples.
< pore diameter >
Measurement was performed by SEM (MODEL S570, manufactured by Hitachi Co., Ltd.).
< porosity >
The weight (W) and volume (V) of the porous PTFE membrane were measured and determined by the following equation.
Porosity (%) - (1-apparent density of PTFE/true density of PTFE) × 100
The apparent density (g/cc) of PTFE was W/V, and the true density (g/cc) was 2.15 (literature value).
< maximum pore diameter >
The measurement was carried out in an automatic mode using a Porosimeter (Porosimeter PMI-1500 manufactured by Porous Materials, Inc.).
The dried membrane and the membrane wetted with a Porewick liquid (available from Porous Materials, Inc.) were subjected to filtrationMeasuring the respective pressure P1、P2The measurement was carried out, and the maximum pore diameter was determined by the following equation.
d＝C·(τ/ΔP)
Where d is the maximum pore diameter (μm), C is 0.415, τ is the surface tension of the wetting solution (dyn/cm), and Δ P is P2-P1(psi)。
< contact Angle with Water >
The contact angle was determined by the following equation using a contact angle measuring instrument CA-D manufactured by Kyowa Kagaku K.K.
Contact angle 2tan-1(h/r)
Where h is the height of the spherical water droplet and r is the radius of the spherical water droplet.
< Heat of fusion of Crystal >
DSC-7 using a differential analyzer manufactured by Perkin-Elmer at 60cm3The temperature was measured at a temperature rising rate of 20 ℃ per minute in a nitrogen flow per minute. Higher heat of fusion indicates higher crystallinity of PTFE.
< light diffusion reflectance >
The measurement was carried out according to ASTM E308 (wavelength: 400 to 700nm) using a Mini Scan XE Plus (product of The Color Management Company).
< tensile Strength of film >
Measured according to ASTM D-1456.
< degree of elongation at break of film >
Measured according to ASTM D-1456.
< pressure loss >
In MODEL8130 manufactured by TSI, air flow rate is 35.9L/min, and pressure difference is 150mmH2And O, performing measurement.
< IPA flow Rate >
Measured according to ASTM F-317.
< Frazier (フレ - ジヤ one) airflow >
Measured according to ASTM D-726-58.
< collection efficiency >
In MODEL8130 manufactured by TSI corporation, a porous PTFE membrane was set, the air flow rate on the outlet side was adjusted to 35.9L/min by pressure adjustment, colloidal particles having a particle diameter of 0.3 μm were included, the number of permeated particles was measured by a particle measuring instrument, and the collection efficiency was calculated by the following equation.
Collection efficiency (%) - [1- (concentration of permeated particles on the downstream side)/(concentration of particles in the air on the upstream side) ] × 100
Examples 1 to 3
A paste-like mixture of 80 parts by weight of PTFE fine powder and 20 parts by weight of naphtha prepared by emulsion polymerization was extruded by an extruder at a reduction ratio of 80: 1 to give a rod-like extrudate having a diameter of 18 mm. The rod-like extrudate was rolled in the same direction as the extrusion direction by a calender roll having a diameter of 500mm to obtain a rolled sheet having a width of 260mm and a thickness of 0.2 mm. The plate was heated to 260 ℃ in an oven to remove the naphtha. Subsequently, the sheet was preheated to 300 ℃, and then simultaneously biaxially stretched at a stretch ratio of 500% in the rolling direction and at a stretch ratio of 300% in the direction perpendicular thereto. While maintaining this stretched state, heat setting was completed by heating at 340 ℃ for 15 seconds. After that, the symmetric porous PTFE film was cooled to room temperature, and the film had a thickness of 20 to 25 μm, a maximum pore diameter of 0.5 μm and a porosity of 90%.
Subsequently, the temperature of the brine tank 1 for cooling was maintained at-10 ℃ by the heat treatment apparatus shown in FIG. 1, the temperatures of the hot air discharged from the heating apparatus 2 through the hot air outlet 3 were adjusted to 260 ℃, 300 ℃ and 340 ℃, respectively, and the film passage time in the hot air outlet region was adjusted to 7 seconds, and only one surface of the PTFE film was heat-treated to obtain an asymmetric porous PTFE film, and the evaluation results are shown in Table 1.
Comparative example 1
The evaluation results of the symmetric porous PTFE membrane produced in example 1 are shown in table 1.
TABLE 1
| Example 1 | Example 2 | Example 3 | Comparative example 1 | |
| Temperature of Heat treatment (. degree.C.) | 260 | 300 | 340 | - |
| Film thickness (mum) | 20 | 23 | 25 | 25 |
| Porosity (%) | 85.8 | 89.8 | 90.9 | 89.7 |
| Pore size (mum) | 0.09～0.17 | 0.08～0.15 | 0.09～0.19 | 0.10～0.19 |
| Contact angle with water (°) | 128 | 129 | 131 | 117 |
| Light reflectance (%) | 92.4 | 92.6 | 93.8 | 90.8 |
| Film tensile Strength (MPa) | 5.96 | 7.3 | 8.5 | 5.07 |
| Degree of elongation at break (%) | 109 | 134 | 107 | 166 |
| Frazier air flow (× 10)4ft3/min·ft2) | 14.7 | 30.1 | 33.6 | 8.7 |
| IPA flow rate (ml/mm. cm)2) | 5.1 | 11.8 | 10.8 | 2.6 |
| Pressure loss (mmH)2O) | 150.7 | 150.7 | 150.8 | 150.7 |
| Collection efficiency (%) | 99.8 | 99.9 | 99.8 | 71.6 |
Industrial applicability
The present invention relates to an asymmetric porous PTFE membrane for a filter. Also disclosed is a filter material comprising an asymmetric porous PTFE membrane for a filter and a reinforcing material.
Claims (6)
1. The application of an asymmetric porous polytetrafluoroethylene membrane in a ventilation filter, wherein the asymmetric porous polytetrafluoroethylene membrane is composed of a high-compactness surface layer and a continuous bubble porous layer; and is
(1) The contact angle of the water on the surface of the surface layer is 120-140 degrees;
(2) the light diffusion reflectance is 91 to 94%.
2. The use according to claim 1, wherein the asymmetric porous polytetrafluoroethylene film is made by biaxial stretching.
3. The use according to claim 1 or 2, wherein the asymmetric porous polytetrafluoroethylene film has a film thickness of 5 to 100 μm.
4. Use of the asymmetric porous polytetrafluoroethylene membrane according to claim 1 and a reinforcing material, which is a synthetic resin or an inorganic fiber, in an air filter.
5. Use according to claim 4, wherein the reinforcement material is polyethylene, polypropylene, polyester, polyamide or glass fibre.
6. A method for producing an asymmetric porous PTFE membrane for a breather filter, characterized in that one surface of a stretched symmetric porous PTFE membrane is cooled to 0 ℃ or lower, and the other surface is subjected to a heat treatment at 260 to 380 ℃.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2003099359A JP3795027B2 (en) | 2003-04-02 | 2003-04-02 | Asymmetric porous polytetrafluoroethylene membrane for filters |
| JP99359/2003 | 2003-04-02 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| HK1067098A1 HK1067098A1 (en) | 2005-04-01 |
| HK1067098B true HK1067098B (en) | 2007-07-06 |
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