JP5204266B2 - Polytetrafluoroethylene porous membrane manufacturing method and bag filter filter medium manufacturing method - Google Patents

Description

  The present invention relates to a method for producing a polytetrafluoroethylene (PTFE) porous membrane and a method for producing a filter material for a bag filter.

  Bag filter type dust collectors are used, for example, to remove soot and dust from exhaust gas discharged from waste incinerators and industrial waste incinerators, and as a means to remove harmful substances such as dioxin contained in exhaust gas together with soot and dust. Is also emphasized.

  In the bag filter type dust collector, the dust in the gas flow is deposited as a dust cake layer on the bag filter and removed from the gas flow. The dust cake layer is removed from the bag filter by regular backwashing. Backwashing is performed by applying to the bag filter a pulse pressure that is applied in the direction opposite to the gas flow permeation direction.

  PTFE porous membranes excellent in heat resistance and chemical resistance are used in various filters and have characteristics suitable for bag filters. When used as a filter medium, the PTFE porous membrane is often reinforced by being joined to a breathable support material such as felt or nonwoven fabric.

The PTFE porous membrane is generally produced by further stretching a porous PTFE sheet obtained by extruding or stretching a PTFE paste. Patent Documents 1 and 2 disclose a method of stretching a PTFE sheet to form a PTFE porous membrane for use in a bag filter. In general, a PTFE sheet is first stretched in the sheet flow direction (MD direction; longitudinal direction for long sheets), which is the direction of extrusion or stretching, and then is orthogonal to the sheet flow direction (TD direction; long length). The sheet is stretched in the width direction). According to Patent Document 1, the draw ratio in the MD direction is preferably 5 to 30 times, particularly 10 to 20 times, and the draw ratio in the TD direction is preferably 10 to 50 times.
JP 2004-223334 A Japanese Patent Laid-Open No. 2006-81984

  In order to prevent the bag filter pressure loss from rising by sufficiently removing the dust cake layer, it is necessary to apply a large pulse pressure during backwashing. For this reason, when a filter medium in which a PTFE porous membrane and a breathable support material are joined is used as a bag filter, the PTFE porous membrane is broken or peeled off from the breathable support material due to repeatedly applied pulse pressure. Sometimes. The destruction or peeling of the PTFE porous membrane reduces the dust collection efficiency of the bag filter.

  The degree of destruction and peeling of the PTFE porous membrane varies depending on the type of the air-permeable support material to which the PTFE porous membrane is bonded. Since the glass woven fabric has excellent heat resistance, the glass woven fabric itself is suitable for use in a bag filter. However, in a bag filter obtained by joining a PTFE porous membrane and a glass woven fabric, the dust collection efficiency is likely to decrease due to the destruction or breakage of the PTFE porous membrane. This is because relatively large irregularities exist on the surface of the glass woven fabric, so that the PTFE porous membrane and the glass woven fabric are likely to be insufficiently joined. If the pulse pressure during backwashing is reduced in order to prevent the PTFE porous membrane from being broken or peeled off, the dust cake layer will not be sufficiently detached, and the pressure loss of the bag filter will increase.

  For this reason, despite its excellent heat resistance, glass woven fabrics have been considered unsuitable as breathable support materials for bag filter media. Patent Document 1 mentions cotton, acrylic, polypropylene, and the like as a material for a woven fabric used as a breathable support material, but does not mention glass fiber (paragraph 0026). In Patent Document 2, a felt made of thermoplastic resin fiber is used as a breathable support material to be joined to the PTFE porous membrane (Claim 1).

  In view of the above circumstances, the present invention provides a new method for producing a filter material for a bag filter in which a PTFE porous membrane and a glass woven fabric are bonded together in order to utilize the excellent heat resistance of the glass woven fabric for the filter material for a bag filter. The purpose is to provide. Another object of the present invention is to provide a PTFE porous membrane suitable for use in the above production method.

The present invention
A PTFE porous membrane production method in which a PTFE sheet is stretched in two directions perpendicular to each other to form a porous membrane,
Extruding or rolling a paste prepared by mixing PTFE powder and a liquid lubricant to form a PTFE sheet,
The sheet is stretched at a temperature lower than the melting point of PTFE at a stretching ratio of 5 to 20 times in the width direction of the sheet, which is a direction perpendicular to the flow direction of the sheet, which is the direction in which the paste is extruded or rolled. Stretched,
Removing the liquid lubricant from the sheet stretched in the width direction;
The sheet from which the liquid lubricant has been removed has a draw ratio of 2 to 15 times in the length direction of the sheet, which is the flow direction of the sheet, at a temperature lower than the melting point of PTFE, and the draw ratio in the width direction. Stretching so that the product of the lengthwise draw ratio is 40-75,
By firing the sheet stretched in the length direction at a temperature equal to or higher than the melting point of PTFE,
For each of the width direction and the length direction, a PTFE porous membrane having a tensile strength of 0.25 N or more and an elongation of 100% or more is obtained.
A method for producing a PTFE porous membrane is provided.

  Here, the tensile strength is defined by a pair of grips in which a porous membrane sample cut so that the length in the direction orthogonal to the measurement direction is 25 mm is arranged so that the distance between them is 50 mm. The test is held so that the measuring direction coincides with the measurement direction, and a test is performed by pulling the sample along the measurement direction at a speed of 100 mm / min with a pair of grips until the sample breaks. Determined by the stress applied to the tool.

The elongation percentage is calculated by {(L ₁ −50) / 50} × 100 (%), where L ₁ (mm) is the distance between the pair of grips when the sample breaks in the above test. It is determined by the value.

The present invention also provides:
A method for producing a filter material for a bag filter, comprising joining a PTFE porous membrane and a glass woven fabric to obtain a filter material,
Performing the above method to produce a PTFE porous membrane,
The PTFE porous membrane and the glass woven fabric are joined to obtain a filter medium.
Manufacturing method of filter material for bag filter,
I will provide a.

According to the present invention, there is provided a filter material for a bag filter in which a PTFE porous membrane and a glass woven fabric are joined, and the glass woven material is held in a state in which the filter medium is held so that the ventilation surface is a circle having an area of 98.5 cm ^2. To provide a filter material for a bag filter in which cracks are not observed on the vent surface even when a test is repeated 20000 times by applying a pseudo backwash operation in which 0.1 MPa of pulsed air is applied to the vent surface from the cloth side for 0.1 second. Can do.

  In the present invention, a PTFE porous membrane having a high degree of elongation and a high tensile strength in the in-plane direction while having an air permeability required for use in a filter material for a bag filter is used. Since this PTFE porous membrane has a high elongation rate, it follows the shape change of the glass woven fabric well and is not easily peeled off, and since it has a high tensile strength, it is difficult to break even if it is stretched.

  Moreover, the PTFE porous material according to the present invention is set to have a high elongation and tensile strength in both the MD direction and the TD direction. Therefore, the PTFE porous membrane according to the present invention eliminates the direction in which the elongation rate and the tensile strength are extremely small, and the PTFE porous membrane in the bag filter is unlikely to break or peel off. ADVANTAGE OF THE INVENTION According to this invention, the PTFE porous membrane and glass woven fabric join, and the filter medium for bag filters by which peeling and damage of the PTFE porous membrane at the time of backwashing were suppressed can be manufactured.

FIG. 1 is a diagram for explaining a method for measuring tensile strength and elongation. FIG. 2 is a diagram for explaining a method of a backwash test.

  Hereinafter, first, a method for producing a PTFE porous membrane according to the present invention will be described.

  The PTFE porous membrane is produced by stretching a PTFE sheet to make it porous. The PTFE sheet can be produced from a PTFE paste prepared by mixing PTFE powder and a liquid lubricant.

  In preparing the PTFE paste, the amount of the liquid lubricant is preferably 5 to 50 parts by mass with respect to 100 parts by mass of the PTFE powder. As the PTFE powder, unfired PTFE fine powder (for example, Polyflon F104 manufactured by Daikin Industries, Ltd.) may be used. As the liquid lubricant, hydrocarbon oils such as liquid paraffin and naphtha may be used.

  The PTFE paste is formed into a PTFE sheet by rolling or extrusion. The thickness of the PTFE sheet is preferably in the range of 0.1 to 0.7 mm.

  The PTFE sheet is stretched to become a PTFE porous membrane. The PTFE porous membrane may be stretched by biaxial stretching in the MD direction (longitudinal direction) and the TD direction (width direction), but may be uniaxial stretching only in the TD direction. The stretching of the PTFE sheet may be performed by a roll method, a tenter method, or the like, as conventionally performed.

  In the present invention, in order to make the elongation and the tensile strength equal to or higher than the predetermined values in each of the MD direction and the TD direction, the stretching ratio, temperature, stretching order (when biaxial stretching), lubricant removal and stretching are performed in stretching. It is necessary to appropriately control the stretching conditions such as the context.

  Conventionally, the required characteristics of a PTFE porous membrane used as a filter medium have been air permeability and average pore diameter that define pressure loss and collection efficiency. If the values indicating these characteristics are in an appropriate range, it can be used without any problem as a filter medium to which a large stress is not applied like a clean room air filter. However, in order to obtain the PTFE porous membrane according to the present invention, it is necessary to control the stretching conditions of the PTFE sheet by paying attention to the elongation rate and the tensile strength. Further, in order to maintain the strength when used by being bonded to a glass woven fabric, it is also required that the direction in which the elongation rate or the tensile strength is extremely small is eliminated. However, in the conventional stretching, neither aiming at achieving both the elongation rate and the tensile strength nor variation due to the direction of these characteristics has been sufficiently considered.

The PTFE porous membrane preferably has an air flow rate measured by the Frazier method in the range of 1 to 10 cm ³ / cm ² / sec. Conventionally, a PTFE porous membrane having such a degree of air permeability has been generally produced by removing a lubricant from a PTFE sheet and then biaxially stretching sequentially in the MD direction and the TD direction. When a stretching ratio that is normally applied in this biaxial stretching (for example, a magnification of 10 to 20 times in the MD direction, which is preferable in Patent Document 1) is applied, the resulting PTFE porous membrane is stretched in the MD direction. The rate is low (see results in Tables 1 and 2 for sheets FH). This is thought to be because the fibrils become too long because the draw ratio in the MD direction is high. When biaxial stretching as described above is applied, in order to ensure the elongation in the MD direction, the stretching ratio in the MD direction is made lower than that in the past, and the stretching ratio in the TD direction is preferable for air permeability. It may be set so as to compensate for the low stretch ratio in the MD direction so as to be in the range.

  The PTFE porous membrane can be obtained not only by the biaxial stretching as described above but also by a method of stretching only in the TD direction. As confirmed in the examples, a PTFE porous membrane having desirable characteristics can be obtained by stretching an unfired PTFE sheet containing a liquid lubricant according to the following conditions.

  a) The liquid lubricant is removed and stretched in the MD direction at a magnification of 2 to 8 times, and then stretched in the TD direction at a magnification of 15 to 50 times. Both the MD direction and the TD direction are stretched at a temperature lower than the melting point of PTFE. The product of the draw ratio in the MD direction and the draw ratio in the TD direction is 100 to 125.

  b) The film is stretched 2 to 5 times in the MD direction to remove the liquid lubricant, and then stretched 15 to 25 times in the TD direction. Both the MD direction and the TD direction are stretched at a temperature lower than the melting point of PTFE. The product of the draw ratio in the MD direction and the draw ratio in the TD direction is 50 to 75.

  When the liquid lubricant is removed after stretching in the MD direction as in the above b), it becomes a raw fabric with less fibrils as compared with the case (a)) in which the liquid lubricant is removed after stretching in the MD direction.

  c) The liquid lubricant is removed, the film is stretched 20 to 70 times in the TD direction, and then stretched in the MD direction at a magnification of 5 times or less, or stretching in the MD direction is not performed. Both the MD direction and the TD direction are stretched at a temperature lower than the melting point of PTFE. The product of the stretching ratio in the MD direction and the stretching ratio in the TD direction is 50 to 100 (the stretching ratio when the stretching is not performed is calculated as 1).

  When the film is stretched in the MD direction after being stretched in the TD direction as in the above c), the width shrinkage is likely to occur in the TD direction as compared with the case where the film is stretched in the MD direction and then stretched in the TD direction (a)).

  d) The film is stretched 5 to 20 times in the TD direction to remove the lubricant, and then stretched 2 to 15 times in the MD direction. Both the MD direction and the TD direction are stretched at a temperature lower than the melting point of PTFE. The product of the draw ratio in the MD direction and the draw ratio in the TD direction is 40 to 75.

  When the liquid lubricant is removed after stretching in the TD direction as in d) above, more fibrils are generated in the MD direction than when the lubricant is removed and then stretched in the MD direction (c)). In addition, the elongation in the MD direction is reduced.

  In the above a) to d), the stretching temperature is set to a temperature lower than the melting point of PTFE. However, if the stretching temperature is too low, the film may break. The stretching temperature should be set appropriately considering the balance with the stretching ratio and the like.

  In any of the above-mentioned a) to d), it is preferable to perform firing at a temperature equal to or higher than the melting point of PTFE, for example, 327 to 470 ° C. If the firing temperature is too high, the porous PTFE membrane may be altered. The firing time is, for example, 0.2 second to 1 minute. This firing improves the tensile strength of the porous PTFE membrane, stabilizes the dimensions, reduces the pressure loss, and facilitates handling.

  The thickness of the PTFE porous membrane is preferably 5 to 60 μm, particularly preferably 10 to 50 μm.

  The PTFE porous membrane obtained as described above is bonded to a glass woven fabric and becomes a filter material for a bag filter. The PTFE porous membrane and the glass woven fabric may be joined using a hot-melt adhesive or the like, but preferably by thermal lamination. This is because thermal lamination is not only advantageous in terms of manufacturing cost but also keeps the pressure loss of the filter medium low and does not impair heat resistance.

  Thermal lamination can be performed by laminating a PTFE porous membrane and a glass woven fabric and pressing them by passing between a pair of heated rolls. The temperature of the heat laminate is preferably a temperature equal to or higher than the melting point of PTFE.

The glass woven fabric used for bonding with the PTFE porous membrane includes a double woven fabric, a twill woven fabric, a satin woven fabric, a triple woven fabric, etc., but a double woven fabric or a twill woven fabric is preferable from the viewpoint of heat resistance. . The basis weight is preferably 500 to 800 g / m ² . A glass woven fabric impregnated with PTFE may be used. In this case, 5 to 30% by mass of PTFE may be impregnated with respect to the glass woven fabric.

  Hereinafter, the present invention will be described in more detail with specific examples.

A PTFE porous membrane was formed as follows. 19 parts by mass of naphtha as a liquid lubricant was uniformly mixed with 100 parts by mass of unsintered PTFE fine powder (polyflon F104 manufactured by Daikin Industries, Ltd.) to prepare a PTFE paste. Next, this PTFE paste was preformed at a pressure of 20 kg / cm ² , extruded into a round bar shape, and then rolled with a pair of metal rolls to obtain a PTFE sheet having a thickness of 0.2 mm.

  Subsequently, the sheet was stretched under the conditions shown in Table 1. Stretching in the MD direction (extrusion direction) was performed by a roll method, and stretching in the TD direction (direction orthogonal to the extrusion direction) was performed by a tenter method. The lubricant was removed by holding it in a furnace set at 150 ° C. for 1 minute. The stretched porous PTFE membrane was fired by being held in a furnace set at 400 ° C. for 5 seconds.

  The obtained PTFE porous membrane was measured for tensile strength (membrane strength), elongation, film thickness, and air permeability.

  The strength and elongation were measured using a tensile tester (Tensilon manufactured by Shimadzu Corporation). The tensile strength and the elongation rate in the MD direction (longitudinal direction) and the TD direction (width direction) were individually measured by preparing different samples.

  The measurement method will be described below with reference to FIG. Sample 1 was cut in advance so that the length W in the direction orthogonal to the measurement direction was 25 mm. The sample 1 was held by a pair of chucks 2 and 2 arranged so that the distance D between them was 50 mm so that the measurement direction (MD direction or TD direction) coincided with the direction defining the distance D. As shown in FIG. 1, the chucks 2 and 2 hold the sample 1 in its full width (25 mm). From the state shown in FIG. 1, the sample 1 was pulled by the pair of chucks 2 and 2 along the measurement direction at a speed of 100 mm / min until the sample 1 was broken. The stress applied to the pair of chucks 2 and 2 when the sample broke was measured, and this stress was taken as the tensile strength.

Further, the elongation percentage is {(L ₁ −50) / 50} × 100 (%), where L ₁ (mm) is the distance between the chucks 2 and 2 when the sample 1 is broken in the above test. It was determined by the calculated value.

  The film thickness was measured using a Mitsutoyo thickness gauge. The air permeability of the PTFE porous membrane and the filter material for bag filter described later was measured according to JIS L 1096 using a Frazier tester (manufactured by Toyo Seiki).

  The measurement results are summarized in Table 2.

Each PTFE porous membrane obtained from the above was overlapped with a glass woven fabric (“T790QB” manufactured by Unitika Glass Fiber Co., Ltd., basis weight 800 g / m ² ) and passed between a pair of hot rolls heated to 370 ° C. The filter material for bag filters was produced by joining the porous membrane and the glass woven fabric.

  The air permeability, the collection efficiency after the washing test, and the presence or absence of cracks after the test were examined in the produced filter material for bag filter.

  In the washing test, six samples cut into B5 size (182 mm × 257 mm) from the filter material for bag filter to be measured are put into a washing machine (manufactured by Sanyo Electric) together with water, and the washing mode is set after washing for 20 minutes. The cycle was set to 9 minutes and the cycle was repeated 4 times. This is a test for evaluating the damage of the filter medium due to the collision with the inner wall of the washing machine or the collision between the filter mediums.

  The presence or absence of damage and the collection efficiency were measured for each filter medium after the washing test. The presence or absence of cracks was judged by observing the state of the PTFE porous membrane in each sample by visual observation. When cracks were confirmed even in one of the six samples, “cracked” was set.

The collection efficiency was measured as follows. First, the periphery of the sample is supported by a holder so that the effective ventilation area is 100 cm ² , and pressure is applied between the upstream side and the downstream side of the sample so that the flow velocity of air passing through the sample is 5.3 cm / second. Gave a difference. In this state, JIS11 (Kanto loam) dust is supplied to the upstream side so that the dust concentration is 2 g / m ^3, and the particle counter (for the dust whose volume average particle diameter is in the range of 2 to 5 μm is targeted. The dust concentration in the space on the upstream side and downstream side of the sample was measured using KC-03 manufactured by Rion Co., Ltd.

The collection efficiency in each sample was determined by the following formula, and the average value of the six samples was taken as the collection efficiency for the filter medium.
Collection efficiency (%) = {1− (downstream dust concentration / upstream dust concentration)} × 100

As shown in Table 3, the bug was made using a PTFE porous membrane having a tensile strength and elongation in the MD direction and TD direction of 0.25 N (0.25 N / 25 mm) or more and 60% or more, respectively. All of the filter media (sheets A to E and sheets 1 to 5) were free from cracks in the washing test, and the collection efficiency after the test was good. Further, these filter media had an air flow rate measured by the Frazier method of 1 to 6.2 cm ³ / cm ² / sec, more specifically 1.2 to 6.2 cm ³ / cm ² / sec.

Further, using the apparatus shown in FIG. 2, a pulsed air application test simulating backwashing was performed. This test was conducted by sandwiching a filter medium formed by joining the PTFE porous membrane 11 and the glass woven fabric 12 between a pair of cylindrical bodies 21 and 21 made of acrylic resin having a circular cross section. The cylindrical bodies 21 and 21 have a circular space with an area of 98.5 cm ^{2 at} the bottom, a circular shape with an area of 26 cm ² at the top, and a height of 40 cm. Ring-shaped rubber packings 22 and 22 are interposed between the filter medium and the cylindrical body for airtightness. In the filter medium sandwiched between the cylindrical bodies 21 and 21, a circular portion having an area of 98.5 cm ² is exposed to the internal space of the cylindrical body to form a ventilation surface. The operation of applying the pulse air 23 for 0.1 second is performed 20,000 times on the circular filter medium so that a pressure of 0.3 MPa is applied to the surface where the glass woven fabric 12 is exposed. The speed of air passing through the filter medium was 0.18 L / min. Then, the presence or absence of crack generation was confirmed visually. The results are also shown in Table 3.

  The filter media for bag filters (sheets A to E and sheets 1 to 5) that showed good results in the washing test were free from cracks even in the pseudo backwash test. Thus, the filter medium according to the present invention has excellent bending resistance that passes a test that a conventional filter medium using a glass woven fabric could not withstand.

  In the above test, the crack was typically observed as a scratch having a length of about 0.5 to 5 cm on the surface of the porous PTFE membrane.

  From the results of the washing test, it is confirmed that when cracks occur, the collection efficiency of the filter medium is less than 90%. In other words, the result that cracks do not occur is that the collection efficiency of the filter medium is maintained at 90% or more, preferably 95% or more even after the backwash test.

  According to the present invention, it is possible to provide a filter material for a bag filter that has a long life and is hardly peeled off due to stress applied during backwashing while using a glass woven fabric having high heat resistance.

1 Sample 2 Chuck 11 PTFE Porous Membrane 12 Glass Woven 21 Tubular Body for Backwash Test 22 Packing 23 Pulse Air

Claims (3)

A method for producing a polytetrafluoroethylene porous membrane, in which a polytetrafluoroethylene sheet is stretched in two directions perpendicular to each other to form a porous membrane,
Extruding or rolling a paste prepared by mixing polytetrafluoroethylene powder and a liquid lubricant to form a polytetrafluoroethylene sheet,
The sheet is stretched at a temperature below the melting point of polytetrafluoroethylene at a stretching ratio of 5 to 20 times in the width direction of the sheet, which is a direction perpendicular to the flow direction of the sheet, which is the direction in which the paste is extruded or rolled. Stretched so that
Removing the liquid lubricant from the sheet stretched in the width direction;
The sheet from which the liquid lubricant has been removed is stretched at a temperature lower than the melting point of polytetrafluoroethylene at a stretching ratio of 2 to 15 times in the length direction of the sheet, which is the flow direction of the sheet. Stretching so that the product of the draw ratio and the draw ratio in the length direction is 40 to 75,
By firing the sheet stretched in the length direction at a temperature equal to or higher than the melting point of polytetrafluoroethylene,
For each of the width direction and the length direction, a polytetrafluoroethylene porous membrane having a tensile strength of 0.25 N or more and an elongation of 100% or more is obtained.
A method for producing a polytetrafluoroethylene porous membrane.
Here, the tensile strength is determined by a pair of grips in which a porous membrane sample cut so that the length in the direction orthogonal to the measurement direction is 25 mm is arranged so that the distance between each other is 50 mm. The specified direction is held so as to coincide with the measurement direction, and a test is performed by pulling the sample along the measurement direction at a speed of 100 mm / min with the pair of grips until the sample breaks. Determined by the stress applied to the pair of grips when ruptured,
The elongation percentage is calculated by {(L ₁ −50) / 50} × 100 (%), where L ₁ (mm) is the distance between the pair of grips when the sample breaks in the test. Determined by the value to be determined. The manufacturing method of the polytetrafluoroethylene porous membrane of Claim 1 which has the air flow rate measured by the Frazier method of the said polytetrafluoroethylene porous membrane in the range of 7-10 cm < ³ > / cm < ² > / sec. A method for producing a filter material for a bag filter, comprising joining a polytetrafluoroethylene porous membrane and a glass woven fabric as a filter material,
Performing the method of claim 1 to produce a polytetrafluoroethylene porous membrane;
Bonding the polytetrafluoroethylene porous membrane and the glass woven fabric to obtain a filter medium,
A method for producing a filter medium for a bag filter.

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