KR20150079112A - Stretch machine for porous PTFE sheet and stretching process using the same - Google Patents

Abstract

The present invention relates to a porous PTFE separation membrane stretching apparatus and a porous PTFE membrane stretching method using the same. More particularly, the present invention relates to a porous PTFE separation membrane stretching apparatus which has improved a uniformity of thickness, and to a porous PTFE membrane stretching method using same.

Description

TECHNICAL FIELD [0001] The present invention relates to a porous PTFE membrane stretching apparatus and a porous PTFE membrane stretching apparatus using the porous PTFE membrane stretching apparatus.

The present invention relates to a porous PTFE separation membrane stretching apparatus and a porous PTFE separation membrane stretching method using the same, and more particularly, to a porous PTFE separation membrane stretching apparatus with improved thickness uniformity and a porous PTFE separation membrane stretching method using the same.

Polymeric polytetrafluoroethylene (hereinafter referred to as "PTFE") has characteristics such as high chemical resistance and low dielectric constant, and has a high melting point and excellent heat resistance. Therefore, And is also widely used for mechanical use such as a member used for a no-lubricating sliding portion by using a characteristic having a small coefficient of friction or a small surface tension.

However, despite the excellent physical and chemical properties as described above, PTFE does not dissolve in most solvents except for special solvents, and has a melt viscosity of 380 캜 to 10 ¹⁰ to 10 ¹¹ Pa s (10 ¹¹ to 10 ¹² P ), There is a problem that it is difficult to produce a PTFE molded article by a general method such as extrusion molding or injection molding, which is generally used for molding a thermoplastic resin having a melt viscosity of 10 ² to 10 ³ Pa · s.

Therefore, a method called a sintering method has been conventionally used as a method for producing a PTFE compact. The sintered compacting method is a method in which preformed PTFE fine powder particles are used as a starting material at a room temperature (in this case, a molding auxiliary agent may be added if necessary), and the formed preform is heated at a melting point of PTFE ) To obtain a PTFE molded body.

The detailed process of the sintering molding method is appropriately selected according to the shape of the molded body to be obtained. For example, a cylindrical PTFE molded body (for example, a PTFE block) is preformed and fired to form a series of steps To produce a PTFE film.

A conventional porous PTFE film production method includes a first step of preliminarily molding a paste form obtained by mixing a PTFE fine powder and a liquid lubricant, a second step of removing the liquid lubricant, a step of putting the extrudate into an extruder, A third step of forming the PTFE sheet by insertion, a fourth step of stretching the molded PTFE sheet, and a fifth step of winding up the stretched sheet.

The PTFE sheet formed above is stretched at a temperature higher than the PTFE melting point. First, the sheet is stretched in the longitudinal direction by the speed difference between the rollers, and then transversely stretched in the tenter unit orthogonal to the above direction.

On the other hand, in Patent Document 10-2009-0103700 (published on Oct. 01, 2009), both side edges of the polymer film are gripped by an openable and closable waveguide moving in the running direction of the polymer film, A stretching method of stretching a polymer film at a predetermined magnification.

However, in the conventional stretching method using the tenter, there is a problem in that it is difficult to produce a sheet having a uniform thickness because the side surface of the stretched sheet is stretched earlier than the center portion.

Accordingly, the present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a porous PTFE separation membrane elongation apparatus improved in thickness uniformity by controlling the point at which elongation of a PTFE sheet starts with a simple structure, Method.

The present invention includes the steps of pre-heating the sheet to a temperature T _1, and pre-heating the central portion of the sheet to the temperature T _2; Heating and stretching the preheated sheet; And sintering the stretched sheet to produce a porous PTFE separation membrane, wherein T ₂ is (1.1 to 1.4) × T ₁ and T ₁ is 25 to 160 ° C.

According to a preferred embodiment of the present invention, the width D ₂ of the central portion of the sheet may be 0.25 to 0.5 times the sheet width D ₁ .

According to another preferred embodiment of the present invention, the center preheating of the sheet may be performed using an apparatus including at least one of the group consisting of a ceramic heater, a far-infrared heater, and a hot air heater.

According to another preferred embodiment of the present invention, the preheating may include at least one of the group consisting of a ceramic heater, a far-infrared heater, and a hot air heater.

According to another preferred embodiment of the present invention, the stretching can be performed at a temperature of 180 to 250 ° C by 15 to 25 times.

According to another preferred embodiment of the present invention, the sintering may be performed at 327 to 400 ° C.

According to another preferred embodiment of the present invention, the porous PTFE separation membrane may have a coefficient of variation of 1 to 10% calculated by the following relational expression (1).

[Relation 1]

Coefficient of variation (%) = (standard deviation of thickness / average thickness) x 100

The present invention also relates to an inflow roll; A take-up roll for winding up the sheet fed from the feed roll; A tenter installed between the inlet roll and the winding roll to transport, stretch and sinter in the width direction; A preheater for preheating the sheet conveyed by the tenter to a temperature T ₁ ; And a heater for heating the sheet preheated by the preheating heater,

And an additional heating means installed at a central portion of the preheater for preheating a central portion of the sheet to a temperature T ₂ higher than T ₁ .

According to a preferred embodiment of the present invention, the additional heating means may include at least one of a ceramic heater, a far-infrared heater, and a hot air heater.

According to another preferred embodiment of the present invention, the additional heating means is a hot air heater, and the hot air supplied by the hot air heater may have an angle of 30 to 60 degrees with the seat.

According to another preferred embodiment of the present invention, the width D ₂ of the central portion of the sheet may be 0.25 to 0.5 times the sheet width D ₁ .

According to another preferred embodiment of the present invention, the T ₂ may be (1.1 to 1.4) × T ₁ and the T ₁ may be 25 to 160 ° C.

According to another preferred embodiment of the present invention, the additional heating means may be installed at the rear half of the preheating heater.

According to another preferred embodiment of the present invention, the additional heating means may be 0.15 to 0.2 times the length of the preheating heater.

According to another preferred embodiment of the present invention, the porous PTFE separation membrane elongating apparatus may be a porous air filter manufacturing apparatus.

Hereinafter, terms of the present invention will be described.

The term "sheet" used in the present invention includes both sheet form, film form, and thin film form.

The porous PTFE separation membrane stretching apparatus and the stretching method using the same of the present invention can improve uniformity of thickness by a simple structure. In addition, due to the uniformity of the thickness, the deviation of the differential pressure of the porous PTFE membrane can be reduced.

1 is a cross-sectional view illustrating a porous PTFE separation membrane stretching apparatus according to a preferred embodiment of the present invention.
FIG. 2 is a structural view showing a porous PTFE separation membrane stretching apparatus according to a preferred embodiment of the present invention and an enlarged view of a part for installing additional heating means.
3 is a flowchart illustrating a method of stretching a porous PTFE separation membrane according to an embodiment of the present invention.

Hereinafter, the present invention will be described in detail.

As described above, in the conventional stretching method in which the longitudinal stretching is performed in the longitudinal direction by the speed difference between the rollers and laterally in the tenter unit orthogonal to the above direction, the side of the stretched sheet is stretched earlier than the center portion of the stretched sheet, It is difficult to produce a thick sheet.

The purpose of this invention is to pre-heat the sheet at a temperature T _1, the central portion of the sheet is the step of pre-heated to a temperature T _2; Stretching the preheated sheet; And a step of sintering the stretched sheet to produce a porous PTFE separation membrane, wherein T ₂ is (1.1 to 1.4) × T ₁ and T ₁ is 25 to 160 ° C. Thereby solving the above-mentioned problem. Accordingly, there is an effect of providing a porous PTFE separation membrane stretching apparatus improved in thickness uniformity by a simple structure and a stretching method using the same.

FIG. 1 is a sectional view showing a porous PTFE separation membrane stretching apparatus according to a preferred embodiment of the present invention, FIG. 2 is a structural view showing a porous PTFE separation membrane stretching apparatus according to a preferred embodiment of the present invention, The porous PTFE separation membrane stretching apparatus of the present invention will be described with reference to FIG.

First, the inflow roll 210 will be described.

The inflow roll is an apparatus for providing the sheet 230 to be stretched, and is not particularly limited as long as it is usually used in a porous PTFE separation membrane stretching apparatus.

Next, the winding roll 220 will be described.

The winding roll is a device for winding up a drawn sheet introduced from an inlet roll, and is not particularly limited as long as it is usually used in a porous PTFE separation membrane stretching apparatus.

Next, the tenter 290 will be described.

The tenter is disposed between the inflow roll 210 and the take-up roll 220 and is a device for transporting and widthwise stretching and sintering the sheet 230. The tenter is generally limited to those specifically used in the porous PTFE separation membrane stretching apparatus. I never do that. For example, a clip tenter or a pin tenter can be used.

The widthwise stretching is carried out in a stretching section (B) in which the preheated sheet is heated and stretched in the width direction, and is not particularly limited as long as it is a condition usually used for stretching the porous PTFE separating membrane, but preferably 15 to 25 times can do.

The sintering is carried out in a sintering section (C) in which the preheating, heating and stretched sheets are cooled and stabilized, and is not particularly limited as long as it is a temperature higher than the melting point of ordinary PTFE, that is, a temperature of 327 캜 or higher.

Next, the preheater 240 will be described.

The preheating heater is installed in a preheating section A for preheating the sheet 230 conveyed by the tenter to the temperature T _1. The preheating heater is not particularly limited as long as it is usually used in the porous PTFE separator stretching apparatus, A ceramic heater, a far-infrared heater, and a hot air heater.

The T ₁ is not particularly limited as long as it is preheated before the porous PTFE separator is stretched, but it may preferably be 50 to 80% of the stretching temperature.

If preheated to less than 50% of the stretching temperature, the fibril differentiation at the node of the porous PTFE separation membrane may be small at the time of stretching in the biaxial direction, and the pore size may become uneven.

Next, the heater 250 will be described.

The heater is installed in the stretching section B for heating the sheet preheated by the preheating heater. The heater is not particularly limited as long as it is usually used in the porous PTFE separation membrane stretching apparatus. Preferably, the heater is a ceramic heater, a far- ≪ RTI ID = 0.0 > and / or < / RTI >

The heating is not particularly limited so long as it is a temperature condition of 327 DEG C or lower which is lower than a normal PTFE melting point.

Next, the additional heating means 270 will be described.

The additional heating means is installed at the center of the preheating heater (see the enlarged structure of FIG. 2), and preheats the central portion of the sheet to a temperature T ₂ higher than T ₁ , and is typically used in a porous PTFE separator stretching device But it may preferably include at least one kind selected from the group consisting of a ceramic heater, a far infrared ray heater and a hot air heater, and more preferably, it can be a hot air heater.

The additional heating means 270 discharges hot air through an air knife to heat a local portion of the sheet, and the air knife is positioned between the far infrared heaters.

The height of the hot air outlet of the additional heating means 270 is not particularly limited as long as hot air is not disturbed from the discharge, but it is preferably at the same height as the far infrared heater which is not disturbed by discharge of hot air, It is preferable to be positioned closer to the seat than the far infrared ray heater.

Further, in order to locally heat the width (D ₂ ) range of the central portion, the height of the hot air discharge port of the additional heating means 270 may be 5 to 12 cm higher than the sheet, more preferably 9 to 11 cm Lt; / RTI >

Further, in order to locally heat the width (D ₂ ) range of the central portion of the present invention, the discharge gap of the air knife of the additional heating means 270 can be adjusted, and the discharge gap is set to a condition that hot air can be discharged at a constant interval But is preferably from 0.05 to 1.5 mm, and more preferably from 0.5 to 1.0 mm.

The width (D ₂ ) of the central portion of the sheet is not particularly limited as long as it is smaller than the total sheet width (D ₁ ), but is preferably 0.25 × D ₁ D ₂ 0.5 D ₁ , More preferably 0.3 D ₁ ? D ₂ ? 0.4 D ₁ Lt; / RTI >

If D ₂ is less than 0.25 x D ₁ , The sheet may be shaken due to the wind force of the discharged hot air, and the sheet may be deformed and sagged in the preheating section. If D ₂ is less than 0.5 x D ₁ , The sheet temperature immediately before the inflow section B may not be locally increased but may be increased by heating the central part to a higher temperature.

The width D ₁ of the sheet means the distance between the clip of the tenter 290 and the clip, and the actual sheet width may be about 30 to 40 mm, including the portion where the tenter clip fixes the sheet.

According to an embodiment of the present invention, the width D ₁ of the sheet can be 50-150 mm, more preferably 70-100 mm.

The temperature T ₂ is not particularly limited as long as it is higher than T ₁ , but it may preferably be (1.1 to 1.4) × T ₁ .

If T ₂ is higher than 1.1 × T ₁ , it may be difficult to produce a sheet having a uniform thickness, which is an advantage of being obtained by preheating the central portion of the sheet at a temperature higher than the total preheating temperature of the sheet. ₂ is higher than 1.4 × T ₁ , the central portion of the porous PTFE separation membrane may be intensively stretched after entering the stretching section (B), which may cause a problem of uniform pore formation.

According to an embodiment of the present invention, the additional heating means may be installed at the rear half of the preheating heater. As a result, it is possible to obtain the effect of producing a sheet having a uniform thickness by solving the problem that it is difficult to produce a sheet having a uniform thickness by stretching the side of the sheet earlier than the central portion at the time of stretching.

The length (L ₂ ) of the additional heating means is not particularly limited as long as it is shorter than the length of the preheating heater (L ₁ ), but may preferably be 0.15 × L ₁ ≦ L ₂ ≦ 0.2 × L ₁ .

If L ₂ is smaller than 0.15 x L ₁ , it is not easy to reach the target temperature in the center of the seat, and if the temperature of the discharged hot air is increased to overcome the problem, there is a problem that the temperature of the entire preheating sheet may be affected If L ₂ is larger than 0.2 × L ₁ , there may occur a problem that the temperature of the sheet is increased as a whole due to a large amount of hot air.

According to another embodiment of the present invention, the additional heating means is an air knife, and the hot air supplied by the air knife may have an angle of 30 to 60 degrees with the sheet, Lt; / RTI >

If the angle formed by the hot air supplied from the air knife and the sheet is less than 30 DEG, the center portion of the preheated sheet is cooled before being introduced into the stretching section (B), so that the center portion of the sheet can be preheated It is difficult to produce a sheet having a uniform thickness. If the angle formed by the hot air supplied from the air knife and the sheet exceeds 60 째, the sheet is deformed or hot air flows into the stretching section (B) A problem may occur that affects the stretching temperature.

A barrier 280 may be provided between the preheating section and the elongating section to prevent the above-described problem and the heat of the elongating section from reaching the preheating section.

The barrier ribs 280 may be disposed between the preheating zone, the stretching zone, the stretching zone and the sintering zone.

The porous PTFE separation membrane elongating device as described above is not particularly limited as long as it is used in a process of producing a porous PTFE separation membrane through stretching, but it can be preferably used in a porous air filter manufacturing process.

3 is a flowchart illustrating a method of stretching a porous PTFE separation membrane according to an embodiment of the present invention. Referring to FIGS. 1 to 3, a porous PTFE separation membrane stretching method of the present invention will be described as follows.

First, the sheet 230 is preheated to a temperature T ₁ , and the central portion of the sheet is preheated to a temperature T ₂ (S 1, A: preheating period). This can be done through the preheater 240 and the additional heating means 270 of FIGS.

The sheet 100 is not particularly limited as long as it is a sheet that is usually obtained by purchase or manufacture and is required to be stretched, but may preferably be a uniaxially (longitudinally) stretched sheet, Or a stretched porous PTFE sheet.

Further, the above-mentioned T ₁ is not particularly limited as long as it is preheated before the stretching of the porous PTFE separator, but it may be preferably 25 to 150 ° C, more preferably 130 to 150 ° C.

If preheated to less than 25 캜, the fibril differentiation at the node of the porous PTFE separator may be small during stretching, and the pore size may become uneven.

In addition, the preheating is not particularly limited as long as it is carried out using an apparatus used for drawing a porous PTFE separation membrane. Preferably, the preheating may include at least one of a ceramic heater, a far-infrared heater and a hot air heater .

Further, the above-mentioned T ₂ is not particularly limited as long as it is higher than T ₁ , but it may preferably be (1.1 to 1.4) × T ₁ , more preferably (1.15 to 1.3) × T ₁ .

If T ₂ is lower than 1.1 × T ₁ , it may be difficult to produce a sheet having a uniform thickness, which is an advantage that it can be obtained by preheating the central portion of the sheet at a temperature higher than the total preheating temperature of the sheet. ₂ is higher than 1.4 × T ₁ , the central portion of the porous PTFE separation membrane may be intensively stretched after entering the stretching section (B), which may cause a problem of uniform pore formation.

The width (D ₂ ) of the center portion 270 of the sheet is not particularly limited as long as it is smaller than the total sheet width (D ₁ ), but is preferably 0.25 × D ₁ D ₂ 0.5 D ₁ , More preferably 0.3 D ₁ ? D ₂ ? 0.4 D ₁ Lt; / RTI >

If D ₂ is less than 0.25 x D ₁ It is possible to cause a problem that the sheet is deformed and sagged in the preheating section, and when D ₂ is less than 0.5 x D ₁ , There may arise a problem that the sheet temperature immediately before the inflow of the stretching section is increased not entirely but locally.

Further, the preheating of the central portion of the sheet can be carried out through the additional heating means 270 shown in FIGS. 1 and 2, and is not particularly limited as long as it is carried out by using an apparatus usually used for drawing a porous PTFE separation membrane. May be performed using an apparatus including at least one of a ceramic heater, a far-infrared heater, and a hot air heater, and more preferably, it may be a hot air heater.

Next, the preheated sheet is heated and stretched (S2, Fig. 2B).

The heating as can be performed through the heater of Figure 1, if the typically conditions used in the stretching process of the porous PTFE membrane is not particularly limited, preferably be heated to T ₃ temperature in the seat bottom and an upper pre-heating have. T ₃ is not particularly limited as long as it is a temperature condition of 327 ° C or lower which is lower than a normal PTFE melting point.

In addition, the heating is not particularly limited as long as the heating is carried out using a device usually used in stretching the porous PTFE separation membrane. For example, a ceramic heater, a far-infrared heater, or a hot air heater can be used.

The above stretching is not particularly limited as long as it is carried out using a device used for tenter-like stretching of the porous PTFE separation membrane.

In addition, the stretching is not particularly limited as long as it is an apparatus usually used for stretching a porous PTFE separation membrane. For example, a clip tenter or a pin tenter can be used.

The stretching is not particularly limited as long as it is an elongation usually used for stretching the porous PTFE separation membrane, but it is preferably stretched at an elongation of 1000 to 2500%.

On the other hand, the stretched porous PTFE separation membrane as described above may have improved sheet thickness uniformity than the porous PTFE separation membrane drawn by the conventional stretching method. The porous PTFE separation membrane of the present invention has a coefficient of variation of 1 To 15%.

[Relation 1]

Coefficient of variation (%) = (standard deviation of thickness / average thickness) x 100

Specifically, in Experimental Example 1, the standard deviation of the thickness of the porous PTFE separation membranes of Examples 1 and 2 stretched by the stretching method of the present invention was remarkably larger than that of the porous PTFE separation membrane of Comparative Example 2 drawn by the conventional stretching method .

Next, the stretched sheet is sintered to produce a porous PTFE separator (S3, Figs. 1 and 2C).

The sintering is carried out in a sintering section (C) for cooling and stabilizing the stretched sheet. The sintering is not particularly limited as long as it is a temperature above the melting point of ordinary PTFE, that is, a temperature of at least 327 ° C.

Hereinafter, the structure and effect of the present invention will be described in more detail with reference to examples and comparative examples. However, this embodiment is only an example for explaining the present invention in more detail, and the scope of the present invention is not limited to these embodiments.

[ Example ]

Example  One.

A PTFE paste was prepared by mixing PTFE polymer (DF-204, Shangdong dongyue) and liquid paraffin (Exopon Mobil product, trade name: Isopar-H) as a liquid lubricant at a weight ratio of 8: 2. The PTFE paste was compressed at 20 占 폚 under a pressure of 2 MPa to form a preform. The preform was introduced into the ram extruder, and the condition inside the ram extruder was extrusion molded into a rod shape having an outer diameter of 15 mm by performing an extrusion process at a pressure of 9 MPa (90 kg / cm 2) and at 60 캜. Thereafter, the extruded rod was passed through a rolling device (calender roll) to form a sheet having a thickness of 500 mu m and a width of 150 mm. The formed PTFE sheet was heated at 220 DEG C for 5 minutes to remove liquid paraffin. The sheet thus prepared was stretched 20 times in the uniaxial direction at 300 DEG C to prepare a 200 mu m thick and 120 mm wide first stretched sheet.

The temperature of the hot air discharged from the air knife was set to 180 DEG C, and the hot air discharged from the air knife was heated to 150 DEG C, The width (D ₂ ) of the contacting sheet was 0.5 times the sheet width (D ₁ ). Also, the angle between the hot air discharged from the air knife and the sheet was 60 ˚, and the length of the air knife was 0.5 times the preheating section length. At this time, the height of the air knife discharge port was 10 cm and the discharge gap was 1 mm.

Thereafter, the stretched section was heated with a far infrared heater and stretched 20 times in the biaxial direction (width direction) by heating at 200 ° C to prepare a porous PTFE separator.

Comparative Example  One.

A porous PTFE separation membrane was prepared in the same manner as in Example 1 except that an air knife as an additional heating means was not used.

Comparative Example  2.

A porous PTFE separator was produced in the same manner as in Example 1, except that the temperature of the air discharged from the air knife was 225 ° C.

Experimental Example  One.

(Μm) of the porous PTFE separation membrane prepared in Example 1 and Comparative Examples 1 and 2, an average thickness (탆) of the porous PTFE separation membrane as an average of the thickness, a standard deviation (탆) of the thickness of the porous PTFE separation membrane, The standard deviation (%) of the thickness of the porous PTFE separation membrane and the elongation percentage (%) elongated using a tenter-type stretching apparatus are shown in Table 1 below.

Specifically, the thickness of the porous PTFE membrane was measured at 10 points in the biaxial direction, and the average of the thickness values at the 10 points was used as the average thickness of the porous PTFE membrane. Respectively.

[Relation 1]

Coefficient of variation (CV,%) = (standard deviation of thickness / average thickness) x 100

As can be seen in Table 1, the coefficient of variation of the porous PTFE separator of Example 1 prepared by preheating the middle portion of the sheet of the present invention was higher than that of Comparative Example 1 prepared without additional preheating of the central portion of the sheet, Which is significantly lower than that of Example 2.

Therefore, it can be confirmed that the porous PTFE separation membrane stretching apparatus of the present invention and the stretching method using the same are suitable for manufacturing a porous PTFE separation membrane having a uniform thickness than a conventional stretching apparatus and a stretching method using the same, without additional preheating of the center portion of the sheet.

As can be seen from Examples, Comparative Examples and Experimental Examples, it is possible to provide a porous PTFE separation membrane stretching apparatus capable of producing a porous PTFE separation membrane of uniform thickness with a simple structure and a stretching method using the same, There is an effect of providing a porous PTFE separation membrane which reduces variation in differential pressure of the porous PTFE separation membrane due to uniformity.

Claims (15)

Preheating the sheet to a temperature T ₁ and heating the middle of the sheet to a temperature T ₂ ;
Heating and stretching the preheated sheet; And
And sintering the stretched sheet to produce a porous PTFE separator,
Wherein the T ₂ is (1.1 to 1.4) × T ₁ and the T ₁ is 25 to 160 ° C. The porous PTFE separation membrane elongation method according to claim 1, wherein the width (D ₂ ) of the central portion of the sheet is 0.25 to 0.5 times the sheet width (D ₁ ). The porous PTFE separation membrane according to claim 1, wherein the central portion of the sheet is preheated by using an apparatus including at least one of a ceramic heater, a far-infrared heater and a hot air heater. The method of claim 1, wherein the preheating comprises at least one of a ceramic heater, a far-infrared heater, and a hot air heater. The method according to claim 1, wherein the stretching is conducted at a temperature of 180 to 250 ° C by 15 to 25 times.  The porous PTFE separation membrane elongation method according to claim 1, wherein the sheet is a sheet stretched in the longitudinal direction of the sheet. The porous PTFE separation membrane according to any one of claims 1 to 6, wherein the porous PTFE separation membrane has a coefficient of variation of 1 to 10% calculated by the following relational expression (1).
[Relation 1]
Coefficient of variation (%) = (standard deviation of thickness / average thickness) x 100 Inflow roll;
A take-up roll for winding up the sheet fed from the feed roll;
A tenter installed between the inlet roll and the winding roll to transport, stretch and sinter in the width direction;
A preheater for preheating the sheet conveyed by the tenter to a temperature T ₁ ;
And a heater for heating the sheet preheated by the preheating heater,
And additional heating means provided at a central portion of the preheater for preheating a central portion of the sheet to a temperature T ₂ higher than T ₁ . The porous PTFE separation membrane stretching apparatus according to claim 8, wherein T ₂ is (1.1 to 1.4) × T ₁ and T ₁ is 25 to 160 ° C. The porous PTFE separation membrane stretching apparatus according to claim 8, wherein the additional heating means includes at least one of a ceramic heater, a far-infrared heater, and a hot air heater. The porous PTFE separation membrane elongation apparatus according to claim 10, wherein the additional heating means is a hot air heater, and the hot air supplied by the hot air heater has an angle of 30 to 60 degrees with the sheet. The porous PTFE separation membrane stretching apparatus according to claim 8, wherein a width (D ₂ ) of a central portion of the sheet is 0.25 to 0.5 times a sheet width (D ₁ ). 9. The porous PTFE separation membrane stretching apparatus according to claim 8, wherein the additional heating means is installed at the rear half of the preheating heater. 14. The porous PTFE separation membrane stretching apparatus according to claim 13, wherein the additional heating means is 0.15 to 0.2 times the length of the preheating heater. The porous PTFE separation membrane elongation apparatus according to any one of claims 8 to 14, wherein the porous PTFE separation membrane elongation apparatus is a porous air filter production apparatus.

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