JPH0510961B2 - - Google Patents

Description

Claim 1: In manufacturing a filter medium for an electric filter from a dielectric material having an open structure or a porous structure, the web of said dielectric material is combined with a dielectric foil in a substantially closed form, and at least one of said webs of dielectric material is provided. feeding the web of dielectric material into a corona discharge device in a state adjacent to two major surfaces; reducing the thickness of the web of dielectric material; charging the web of dielectric material with reduced thickness by the corona discharge; A method for producing a filter medium for an electret filter, which comprises performing a treatment.

2. A method according to claim 1, characterized in that the thickness of the dielectric material is reduced by compression.

3. Claims characterized in that said compression is effected by applying high pressure by means of pneumatic forces to a substantially closed-shaped inductive foil present on said web of dielectric material. The method described in Section 2.

4. The compression is performed by placing an open gauze or mesh on the dielectric material and pressing the open gauze or mesh against the dielectric material. The method described in Scope No. 2.

5. The method according to claim 2, wherein the thickness of the dielectric material is temporarily reduced during the charging process.

6. A method according to claim 2, characterized in that, before carrying out the charging treatment, an operation is performed to permanently reduce the thickness of the dielectric material.

7. The method according to claim 1, wherein the dielectric material is temporarily stretched during the charging process.

8. A method according to claim 7, characterized in that the dielectric material is temporarily stretched in its longitudinal direction.

9. The method according to claim 7, characterized in that the dielectric material is temporarily stretched in its width direction.

10. The method of claim 7, wherein the dielectric material is temporarily stretched in two directions.

11 A dielectric material is disposed within a substantially airtight enclosure, at least one major interface of which is flexible and extends along a major surface of the dielectric; 2. The method of claim 1, further comprising reducing the thickness of said dielectric material by reducing the pressure within said substantially hermetic enclosure.

12. A method as claimed in claim 1, characterized in that the substantially closed dielectric foil is discharged at times between the successive charging operations.

13. A method according to claim 1, characterized in that the substantially closed dielectric foil is charged to a polarity opposite to that during the charging process of the web of dielectric material.

14 The web of dielectric material is subjected to a pre-charging treatment, the pre-charging treatment being characterized in that the web of dielectric material is charged to a polarity opposite to the polarity during the final charging operation. The method according to claim 1.

15. A method according to claim 1, characterized in that, before the final charging treatment, the web of dielectric material is subjected to an AC-corona.

16. The method according to claim 1, comprising the step of stacking a plurality of charged webs of the dielectric material to form a composite filter medium for an electret filter. .

17. The method according to claim 16, characterized in that a plurality of charged webs are integrated by stitching or welding.

18. A method according to claim 1, characterized in that the web of dielectric material is subjected to a preforming operation to permanently reduce the thickness of the web.

19. The method of claim 1, wherein said dielectric material comprises a web of dielectric fibers.

20. The method of claim 19, wherein the web of dielectric fibers is a mixture of certain fibers and fine fibers.

21. Claim 19, characterized in that said web of dielectric material consists of a laminate of a plurality of individual webs formed from dielectric fibers and dielectric fine fibers. Method described.

22 Apparatus for manufacturing a filter medium for an electret filter from a web of dielectric material having an open structure or a porous structure, comprising a corona device having spaces between a plurality of corona electrodes and a dielectric material having a substantially closed form. means for passing said dielectric foil with said dielectric material through said space between said foil and said corona electrode; and means for reducing the thickness of said web of dielectric material. An apparatus for manufacturing a filter medium for an electric filter, characterized in that:

23. Device according to claim 22, characterized in that said substantially closed dielectric foil is in the form of an endless belt guided by rotating rollers.

24. said substantially closed dielectric foil is in the form of a substantially hermetic enclosure, at least one interface of said enclosure being flexible and said dielectric foil being in the form of a substantially airtight enclosure; Claim 2, characterized in that it has a shape extending along the main surface of the material and has means for partially evacuating said airtight enclosure.
The device according to item 2.

25. Claim 24, characterized in that said substantially airtight enclosure is a pack made of foil within which said dielectric material is sealed. Equipment described in Section.

26. Said substantially gas-tight enclosure includes a perforated rotatable drum, providing a suction space within said drum, and said substantially closed dielectric foil comprises an endless belt. 25. Device according to claim 24, characterized in that it is of the form.

Description The present invention has an open structure or a porous structure.
In a method for continuously manufacturing electret filter media from a structured dielectric material, a web of dielectric material is formed into a substantially closed (i.e. void-free) dielectric foil.
and feeding the foil into a corona discharge device with said foil adjacent to at least one major surface of said web to reduce the thickness of said web of dielectric material so that said web of dielectric material has a reduced thickness. The present invention relates to a method for manufacturing a filter medium for a filter, characterized in that a web of dielectric material is charged with a corona discharge means. The invention also relates to an apparatus for carrying out the method.

A process for producing filter media for electret filters, which is somewhat similar to the above process but is carried out batchwise at high temperatures, is described in US Pat. No. 4,308,223.

In this known method, a filter web (ie a filter medium in the form of a web) made of polypropylene fibers with a thickness of 2 mm and a relatively heavy basis weight of 410 g/m ² is subjected to an electrostatic treatment. This charging process has two effects, one of which is the orientation of the dipoles and the other is the implantation of charge into the fiber. The charging process is carried out at high temperature (120℃) to obtain the optimal dipole orientation.
This process therefore takes a very long time, ie 15 minutes. This charging process is carried out as follows: the fiber mat is inserted between two electrodes, one of the electrodes is provided with a number of corona points, and these corona points are connected to a high voltage AC power source. . The other electrode is grounded,
It is also covered with dielectric foil, the purpose of which is to prevent ions generated by the corona from flowing to the ground plate. A negative corona voltage is used to release negative _CO3 ions. In order to further improve the homogeneity of the charging process of the filter medium, the corona points are located close to each other on the upper electrode. The coronas originating from these points are opposite each other and give a corona effect over a limited distance (approximately 2-3 millimeters). These corona points are placed at a location slightly above or within the filter material to be electrostatically charged (i.e., the filter medium for the filter, or the raw material for its manufacture). High corona voltage cannot be used. This is because such high voltages cause sparks to fly from the above locations to the ground electrode. This risk of sparking is even greater if the corona spots are pushed towards the filter material. This spark can cause a short circuit that can puncture both the foil and filter material.
Additionally, the width of the holes in the foil may become so large that the foil becomes unusable. Holes in the filter material are undesirable because they provide passageways for dust particles to pass through in an unacceptable manner.

Since a high voltage is applied to the corona point and the upper electrode, the above member arrangement is not preferred from a safety standpoint. Furthermore, since the filter material is placed on the lower electrode by screw engagement, a portion of the filter material remains unchanged (uncharged state).
It also has the disadvantage that the parts remain intact and must be discarded.

Furthermore, as is clear from the data on the particle penetration of the filter media in Table 1 in US Pat. No. 4,308,223, the effect of the above-mentioned charging treatment is not always constant and varies somewhat each time. The data dispersion of this permeation amount ranges from 1.3 mg to 7 mg.

(Particle permeation amount and transmittance will be explained later). This is probably because the charging process of the filter material was performed non-uniformly, and some of the filter material did not reach the optimal charging state.

US Pat. No. 4,375,718 discloses a method for continuously producing an electrostatically charged filter medium at room temperature. in this way,
A composite web is formed by contacting each side of non-conductive thermoplastic fibers with a somewhat conductive web. The composite web is then charged by a corona discharge means, and this charging treatment consists of applying charges of opposite polarity to each opposing surface of the composite web.

The present invention relates to a method for manufacturing a filter medium for an electric filter or an electrostatic filter that exhibits superior filtration performance (in other words, particle collection performance) to filter media for filters manufactured according to the prior art. It is something. It has now been found that the use of a supporting dielectric foil in a substantially closed form is advantageous since this substantially prevents electrical damage or sparking during the charging process. Served.

The method of the invention is highly suitable for continuous operation.

Since the charge injection is so strong that permanent dipole alignment operations are unnecessary, the method can be started with dielectric materials without polar groups. This has a much higher insulation resistance than dielectric materials with polar groups, and therefore the long-term stability of the ejected charge is considerably improved. Furthermore, the method of the invention can be carried out at room temperature.

Charge injection is preferably performed by a corona generated by a thin tungsten wire or the like. Although positive or negative coronas can be used, the charging process is conveniently carried out with two types of corona, i.e. one side of the dielectric material and dielectric foil combination is positively charged. It is best to charge the other side with a negative charge and charge the opposite side with a negative charge. In this embodiment, one corona performs the charge injection and the other corona acts as a counter electrode. The advantage of this embodiment is that there is very little risk of catastrophic damage to the dielectric foil and/or the dielectric material to be charged. This is because instead of a metal counter-electrode, one of the coronas, ie a plasma of air ions, acts as a counter-electrode. The dielectric foil can move quickly over this plasma-type counter electrode without friction. This is very advantageous in continuous charging operations.

An extremely heavy and/or extremely thick filter web cannot be sufficiently charged, so that a charged filter web with low transmission of fine dust particles cannot be obtained. It is therefore advantageous to separately charge a plurality of layers and to stack them together to form an electric filter web. It should be noted that the multilayer structure described above can be eliminated by stitching the layers together with a needle or by welding them together (heat bonding), ie, an integrated structure can be formed.

By reducing the thickness of the dielectric material during the charging process, the effect of the charging process on the dielectric material is greatly improved. It is also possible to permanently reduce the thickness of the dielectric material by compressing it prior to charging. In this case, this thickness is no longer restored. The permanent compression described above can be carried out when forming molded articles from open structure dielectric materials. It is also possible to reduce the thickness by stretching the dielectric material in its longitudinal and/or transverse direction. In the case of dielectric materials that can be elastically deformed, e.g. in the form of a foam,
The stretching method described above is particularly advantageous. Temporary stretching only during the charging process has the additional effect of reducing the area weight of the material.

The thickness of the dielectric material can be reduced by placing a gauze or mesh having an open structure on the dielectric material and moving the gauze with a roller to compress the web. The dielectric material can also be compressed by placing a substantially closed foil on top of the web of dielectric material and applying high air pressure to the foil.

The dielectric material is placed in a substantially gas-tight space, at least one of the interfaces of the space being flexible and at right angles to the thickness direction of the dielectric material. By reducing the pressure in this closed space (creating a partial vacuum state),
Preferably, the thickness of the dielectric material is reduced.
Said hermetic space is an envelope comprised of a substantially closed dielectric foil lying adjacent to said dielectric material having an open structure.
It may be surrounded by

Surprisingly, it has been found that the results of the charging process under partial vacuum described above are even better than those of charging processes under compression using other compression methods. [can be determined using particle transmittance data (see below)]. moreover,
This "vacuum method" is very convenient because it allows for a fairly large reduction in thickness (for example, down to 1/5 or less of the original thickness).

Dielectric filter materials with curved surfaces are used in respirators, etc., but when performing charging treatment on dielectric filter materials with such curved surfaces, the dielectric material is first preshaped, and then According to the invention, this can be subjected to a charging treatment, for example under partial vacuum conditions. This preforming can generally be carried out at high temperatures and under pressure. Pre-charged filter webs may lose some amount of charge during the forming operation. This loss of charge can be avoided by carrying out a charging process after the molding operation has been carried out.

As will be readily understood, charging a dielectric material involves the simultaneous charging of a substantially closed separating foil used in close proximity thereto. If the same isolating foil is used many times when charging multiple dielectric materials in sequence, these dielectric materials can be removed by discharging the foil after each charging process. Therefore, the charging effect of the electrostatic material can be considerably improved.

Surprisingly, the isolating foil before being used in the charging of said dielectric material is charged, and the charging of said foil is then carried out to a polarity opposite to that during the charging of said dielectric material. It has been found that the charging effect of the dielectric material can be further improved by carrying out this treatment.

Furthermore, if a preliminary operation is performed in which the dielectric material is previously exposed to a charge having a polarity opposite to the polarity during the charging treatment of the dielectric material, the dielectric material It was also found that the ions were more likely to charge bipolarly.

Furthermore, it has also been found that when the dielectric material is subjected to an AC corona before being subjected to the final charging treatment, the bipolar charging effect of the dielectric material is further improved. Furthermore, filter materials that are charged under low pressure or high pressure in a substantially gas-tight space appear to be fully bipolar charged materials.

The polarity of the web of dielectric material is determined by the “Monroe
It can be measured by reading the surface potential of the web using a non-contact measuring device (probe) such as "Isoprobe Electrostatic Voltmeter".In a bipolar charged web, charges with different signs exist at the same concentration. Since the polarity of each charge almost completely cancels each other out, the instrument's pointer in this case will point to a substantially zero value.

Since this dielectric filter material has good bipolar charging properties, the polarity of the charged particles to be collected by the filter is not important; moreover, the strong heterogeneous electric field formed in the filter can also collect uncharged particles. Can be collected effectively.

In order to increase the efficiency of the filter, the dielectric filter material preferably consists of very fine fibers.

The invention also relates to an apparatus for continuously producing webs for electric filters. One embodiment of this device includes a corona device into which a substantially closed dielectric foil extends in a direction perpendicular to the corona electric field. exist. The device is characterized in that said dielectric foil forms part of an endless belt, said endless belt is guided by rotating rollers, and said corona device generates at least one positive corona and one negative corona. However, this corona acts on the portion of the foil in the belt, and within the area of action of this corona the dielectric material resting on the belt travels, i.e. the dielectric material travels between these coronas. It is structured so that the process progresses.

Another specific example of the device of the present invention will be described. In this specific example, a grounded metal electrode is placed facing the corona. This electrode consists of a rotating metal roller or belt. The inductive foil is placed on the periphery of this roller or on one side of the belt (ie the side facing the corona). The dielectric material travels over the dielectric foil and passes through the corona. The dielectric foil can also be adhered to the surface of the roller or belt as a covering material.

As will be readily understood, the dielectric material can also be run between the dielectric foil and a metal roller or belt. In this case, it is also possible to press the foil towards the roller or belt, thereby compressing the dielectric material.

The invention also relates to an apparatus for continuously producing a large number of electric filter webs from dielectric material having an open structure. The production apparatus has a corona device in which a substantially closed-shaped dielectric foil extends primarily in a direction perpendicular to the corona electric field. The feature of this manufacturing equipment is that it incorporates a feeding device for the foil and the dielectric material to be charged, and next to this feeding device is a vacuum pack forming device. said foil and dielectric material are processed into a vacuum pack (evacuated pack), a corona device generates pressure and a negative corona;
The structure is such that the vacuum pack body advances through this corona.

Another specific example of the present invention will be described.
A feature of this manufacturing device is that the electrode is in the form of a rotatable drum, the cylindrical circumferential wall of which is provided with holes, and the blocking foil is guided into an area at a distance from the cylindrical outer surface of the drum. and advancing the dielectric material into and through the space between the foil and the outer surface and opening towards the delivery space. The foil is drawn toward the drum by evacuating the feeding space through the holes in the drum.

This foil is preferably in the form of an endless belt guided by rotatable rollers.

Another specific example of the present invention will be described.
A feature of this embodiment is that a stationary body is arranged in an area inside the drum opposite to the inside of said hole in the drum wall which is open towards said feeding space for feeding said electrically conductive material. However, this stationary body has two sealing surfaces in the axial direction of the drum, and these sealing surfaces are one of the holes inside the drum (the holes that open toward the feeding space). a surface of the stationary body that is in close contact with the outermost hole of the body, exists between these sealing surfaces, and faces said hole (i.e., a hole that is once open towards the feeding space); There is a space between the drum and the wall of the drum, and a suction conduit is connected to this space so that the space can be used as a closed space for suction.

The attached drawings will be explained.

FIG. 1 is a schematic illustration of a specific example of an apparatus for charging a dielectric material according to the present invention.

FIG. 2 is a schematic illustration of an example of an apparatus that can be used when carrying out the method of the invention continuously.

FIG. 3 is a schematic illustration of a modified example of the apparatus shown in FIG.

FIG. 4 is a schematic illustration of another embodiment of an apparatus that can be used in continuously carrying out the method of the invention.

FIG. 5 is a schematic illustration of yet another embodiment of an apparatus that can be used when carrying out the method of the invention continuously.

FIG. 6 shows another embodiment for carrying out the method of the invention continuously.

FIG. 7 shows the relationship between the particle transmittance (so-called in-filter transmittance of particles) in the mat for an electric filter according to the present invention and the number of layers (separately charged layers) constituting the mat. It is a graph.

Figure 8 shows the pressure (high pressure to low pressure) used when charging the four separate layers;
It is a graph showing the relationship between the layer and the particle permeability.

Next, a filter medium for an electric filter made from a dielectric fiber material according to the present invention will be described.

However, dielectric materials with other open structures (e.g., dielectric foams, porous semipermeable membranes, sintered powders, etc.) may also be electrostatically treated in accordance with the present invention. You can achieve the same effect. Of course, it is also possible to use the charged material for purposes other than filtration.

The fibers may be fine fibers (fine fibers) and/or coarse fibers (coarse fibers), and may have any shape such as round, filamentous, square, hollow, etc. It's fine. In addition, staple fibers (short fibers), nonwoven fibers, spun fibers,
Melt-blown spun fibers, solvent-blown spun fibers, and spray-spun fibers can also be used, and mixed fibers containing multiple types of these fibers can also be used. Furthermore, the dielectric filter material may consist of various layers, for example, one layer of coarse fibers and one layer of fine fibers. If desired, it is also possible to use several dielectric materials as raw materials for these layers.

Current trends in filter design are focused on creating filters that can capture not only coarse dust but also fine dust. This is important in both air conditioning equipment and equipment for protecting the human body (pollution prevention).

This is because particles with a diameter of 1 micrometer or less are the most dangerous. Such particles are easily inhaled by humans and often contain heavy metals. Furthermore, fine dust must also be removed from dust-free rooms (clean rooms), which are rooms where small electronic components are made, and from treatment rooms in hospitals. Additionally, many manufacturing plants produce very fine dust;
Dust in the atmosphere also contains many fine particles with a diameter of 1 micron or less, and these are often harmful to health.

In conventional fibrous filters, fine dust can only be effectively captured if the fibers are very fine. By subjecting such fibers to electrostatic charging treatment, the fine particle collection efficiency can be greatly improved. Electret filters made on this principle are known, for example, from Dutch Patent No. 160303 and US Patent (Reissue) No.
It is described in the specification of No. 30782.

Conventional processing methods for permanently electrostatically charging dielectric materials (especially fibers) with open or porous structures have the important drawback that undesired dielectric breakdown may occur. It was hot. Dutch Patent No. 160303 and US Patent (Reissue)
No. 30782 discloses the use of a closed dielectric foil that is first longitudinally stretched, then electrically charged and then fibrillated. It is disclosed that the above-mentioned destruction can be avoided by subjecting the fibers to fibers.

For some applications, it is not necessary that the electrostatically charged fibers be fine fibers. The electret filter fibers described in Dutch Patent No. 160,303 and US Pat. No. 30,782 are relatively coarse fibers. This is because it is composed of split fibers (10 x 40 micrometers). Although this fiber can carry a lot of charge, it is susceptible to degradation by very fine dust. Therefore, applications that are used for a long period of time (for example, filters for air conditioning systems)
The above mentioned fibers are by no means very useful.

According to the present invention, a fibrous web (ready-made web) made from microfine fibers can be subjected to charging treatment.

The manufacturing methods of microfine fibers, mixed fibers thereof, and staple fibers are known, for example, as disclosed in German Patent Publications Nos. 2328015, 2032072, and
It is described in Publication No. 2620399, US Patent No. 4230650, German Published Patent Application No. 2940170, etc.
Furthermore, U.S. Patent No. 3,016,072 and
4118531, and Van A. Wente,
“Superfine Thermoplastic Fibers”, Industrial
and Engineering Chemistry, Vol.48, p.1342
(1956).

FIG. 1 is an illustration of a specific example of an apparatus that can be used to carry out the method of the invention.

In this device, two corona plasmas 1 and 2 are used as plus plasma and minus plasma, respectively. A dielectric foil 3 carrying a filter mat (ie, mat-shaped filter material) 4 to be charged is placed between the two plasmas 1 and 2. The dielectric foil 3 is of substantially closed shape (i.e. pores).
]. The dielectric foil 3 acts as a barrier to keep the cations and anions separated, in other words to prevent neutralization of these ions. This foil is therefore referred to as an isolation foil or blocking foil. By using a corona instead of a metal electrode as the counterelectrode, ie a plasma of air ions, the risk of damage due to sparks through the isolation foil can be avoided. Furthermore, the filter web and foil combination can be advanced through the corona at high speed. It is possible to use very thin dielectric foils (for example 2 micrometers thick), since there is no risk of damage due to sparks and they can be advanced without friction. In this case, the amount of charge on the fiber mat increases because there is less voltage loss across the thin foil.

The use of thin foils is particularly advantageous when charging thin fiber fleeces.

Corona plasmas 1 and 2 are generated by thin tungsten wires 5 and 6. Tungsten wires 5 and 6 are arranged above and below the filter mat 4 and foil 3 combination, respectively. A positive voltage and a negative voltage (the value of which is, for example, 7 KV) are applied to the tungsten wires 5 and 6 from high voltage power supplies 7 and 8, respectively. By arranging the tigsten wires in a direction perpendicular to the longitudinal direction of the filter mat, the mat can be uniformly charged. Compared to known charging methods using dotted coronas, the advantage of using a corona generating wire is that the wire is less likely to break, is less susceptible to contamination, and has a lower degree of spark erosion. Furthermore, this metal wire can be processed into geometric forms with good charging effects. The corona generator is equipped with two ground plates 9 and 10, which promote ionization of the air, thereby making more ions available for charge injection. Furthermore, the earth plates 9 and 10 also serve as a safety device for this electrical device.

In the apparatus shown in FIG. 1, it is also possible to press the open dielectric gauze against the dielectric material to be charged and the isolation foil 3 by means of mechanical pressing means. For this purpose, foil 3 is attached to a frame (not shown) mounted in place.
can be stretched, said gauze is also stretched on a frame (not shown) and this gauze can be pressed in the direction of the foil 3.

Preferably, the charging treatment of the filter material is carried out continuously. When this filter material is present in the corona discharge device, it is advantageous to ensure that it is always kept pressed by the gauze, which is rotated by a roller arrangement (not shown).

The corona charging process in this case is rapid (it will generally be completed within 1 second) and can be carried out at various temperatures, but is preferably carried out at room temperature.

The charge applied to the dielectric material by the corona discharge is energetically trapped in structural defects in the material, or in other words, trapped in a trap. As mentioned above, trapping charges within the traps can generally be carried out at room temperature. If the dielectric material has shallow traps as well as deep traps, the charging process is preferably carried out at a relatively high temperature. This is because high temperatures promote the trapping of charge in deep traps, leaving shallow traps empty.

Said charge injection is so strong that it is no longer necessary at all to take account of the orientation of the permanent dipoles in the filter material, and this charging process therefore You can start with a non-dielectric material. Such non-polar materials have higher insulation resistance than polar materials, so
The stability of the charge in the charged body is very good during long-term operation.

Several types of dielectric materials have been found suitable, including polypropylene, linear low density polyethylene, polymethylpentene, polytetrafluoroethylene, polytrifluorochloroethylene, polystyrene, polycarbonate. ,
Examples include polymers such as polyester.

By compressing the dielectric material in the direction of its layer thickness during the charging operation, charging is carried out very conveniently. This compression is possible. This is because filter materials contain large amounts of air, and for example, in fibrous filter materials, the fiber packing density (volume of fibers/total volume) is generally only a few percent. In general, filter materials can often be compressed to a thickness that is one-fifth or less of its original thickness.

The compression described above can be carried out in various ways. The first method is to mechanically compress the filter mat onto blocking foil, such as with open gauze. A second method is to use two foils instead of one and compress the filter mat between them. In this case, the filter mat can be compressed by applying high air pressure to these foils. Optional operations can also be carried out, for example by compressing the filter mat in a high-pressure press (this operation can be carried out at elevated temperatures if desired) or by pressing the filter mat against hot press rollers (calender). Permanent compression can be applied to the filter mat by compression.

It is also possible to perform compression during the operation of forming the filter material into a device such as a respirator.

Particularly good results are obtained when the filter mat or fiber fleece described above is compressed under reduced pressure. In this case, such a fiber fleece is placed in a substantially airtight enclosed space. The enclosed space of the lever is therefore flexible at one or more boundary surfaces (principal surfaces) in a direction perpendicular to the direction of the layer thickness of the dielectric material. By reducing the pressure within this space, the fiber fleece therein can be compressed. The bounding surface of this space may for example consist of an upper foil and a lower foil. One of these foils may be low-pressure in at least some areas, thereby keeping the space between the two foils under some degree of low pressure. can. This low density has no negative effect on the blocking activity of the isolation foil. Alternatively, it is also possible to remove the air (suction removal) from the edges of the filter mat, in which case the isolation foil need not be porous.

The boundaries of the space can be formed by so-called blow-molded foils (tubular foils). The fleece is completely encapsulated within this tubular foil. This tubular foil can serve as an isolation foil during the charging operation. Air is evacuated from the open end of the blow-molded tubular foil. If necessary, this tubular foil can also serve as a pack or wrapper to protect against moisture and dust at a later stage.

Surprisingly, when the degree of decrease in layer thickness is the same,
It has been found that charging under reduced pressure gives better results than other types of compression, ie lower particle permeability values.

Test results are shown in Tables A, B, C and D below showing the great effects of the present invention, especially the very great effects obtained when compression is also applied. This test was carried out according to the method described in British Standard BS-400. That is, a test was conducted regarding the filter media permeability of a standard dispersion of sodium chloride particles produced by spraying an aqueous solution of NaCl.

Aerosol concentrations were measured in a hydrogen flame. The airflow velocity was 20 cm/sec.

Q in the table is a value mathematically defined by the following formula: Q=-1 _o (%P/100)/△P (1). This value indicates In the above formula, P (%) represents the transmittance, ΔP (Pascal) represents the degree of pressure drop, and 1 _o is a symbol representing the natural logarithm. The value of Q is always a positive value, and as the value of transmittance decreases, the value of Q increases. On the other hand, as the degree of pressure drop increases, the value of Q becomes smaller.

It has been found that when the filter web is made thicker by the addition of material, the transmittance of relatively fine particles can be expressed with good approximation by the following formula:

%P=100 _e ^-k