NL8300439A - Continuous electret filter medium manufacture - uses a substantially closed supporting dielectric foil - Google Patents

Abstract

An electret filter medium is manufactured from a dielectric material with an open or porous structure by (a) continuously feeding a web of the dielectric material with a substantially closed dielectric foil adjacent to at leawt one major face, into a corona discharge device; (b) reducing the web thickness; (c) charging the web within the corona discharge.

Description

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Method and device for the production of an electretfinter medium.

The invention relates to a method for manufacturing an electret filter medium of dielectric material with an open structure, which material is charged with at least one side adjacent a substantially closed dielectric film. Such a method is known from US patent 4,308,223.

According to this known method, a flash mat of polypropylene fibers with a relatively high surface weight, namely 410 g / m 2 and a thickness of 2 mm, is charged. Charging is achieved by two effects, namely by aligning dipoles and by implanting charges * in the fibers. For an optimal orientation of the dipoles, the charging is carried out at a high temperature (120 ° C) and therefore takes quite a long time, a total of 15 minutes. Charging is performed by placing the mat between two electrodes, one of which has a large number of corona points. These corona points are connected with a high DC voltage and must be placed at a short distance above or in the mat. The other electrode is grounded and a dielectric film is placed on this electrode to prevent the ions generated by the corona from flowing to earth. A negative corona voltage is used to inject stable negative COg ions.

The known charging method has a number of drawbacks. Charging must be carried out at a high temperature, takes a long time and is done in batches, so not continuously.

To increase the uniformity of the charging of the filter medium, the corona points should be placed close together on the top electrode. As a result, however, the corona's emanating from the points start to work against each other, so that they only work to a limited distance (a few mm's). In connection with this, the tips must always be placed at a short distance above or in the filter material to be charged, but the charging uniformity still remains defective. Furthermore, high corona voltages cannot be used, otherwise sparks will jump from the points to the grounded electrode. The probability of this increases considerably when the tips are pressed into the filter material. The sparks cause a short circuit, creating holes in both the film and the filter material. The holes in the 40 film can even become so large that the film is no longer usable 8300439 2 *. »Is. The holes in the filter material represent a fatal damage, which inadmissibly permits the particles to be filtered.

Because both the tips and the top electrode are under high voltage, the arrangement is not safe. A further drawback is that the filter is screwed onto the bottom electrode, so that part of the filter material is not charged and must be disposed of as waste.

Furthermore, the charge is not very reproducible as can be seen from the penetration results listed in the table of the above-mentioned U.S. Patent which range from 1.3 to 7 mg. This is most probably related to the fact that the known charging of the filter material is not uniform, but shows a certain point geometry, whereby some parts of the filter material are not optimally charged.

The charged filter mats were tested according to NIOSH regulations with a quartz dust aerosol. The average penetration of the charged filters was 3.2 mg. According to the NIOSH requirement, 1.5 mg is maximum permissible, which requirement is not met by the filters charged according to the known method, despite the fact that the pressure drop is quite high; 12.5 mm H2O at an air velocity of 1Ό cm / s.

The object of the invention is to provide a method of the type mentioned in the opening paragraph, wherein the above-mentioned drawbacks are avoided.

According to the invention, this object is achieved in that the dielectric material is charged with a thickness reduced by compression.

This reduction in the thickness of the dielectric material can also be achieved in that the material is compressed prior to charging 30 such that it no longer bounces back and has undergone a permanent reduction in thickness.

Furthermore, the thickness can be reduced by stretching the material in the length and / or width direction. This is especially true if the material is elastically deformable, such as foam. The temporary stretching during charging provides the additional advantage that the surface weight of the material is also reduced.

It has been found that by reducing the thickness, the charge injection and thus the charge is increased considerably and can be effected quickly. This makes the method according to the invention particularly suitable to be carried out as a continuous process. Because the charge 8300439 * 4 fc 3 injection is so strong, it is no longer necessary to aim for alignment of permanent dipoles, so that dielectric materials without polar groups can be used. These have a higher insulation resistance than polar dielectric materials, which greatly improves the long-term stability of the injected charge. In addition, the method according to the invention can be carried out at room temperature.

In addition, it has been found that by compression even a heavy and thick layer of dielectric material (for example for polypropylene heavier than 450 g / cm 2 and thicker than 9 mm measured according to ASTM standard D461) can still be charged efficiently. As a result, a filter mat can generally be charged as a single layer, so that loading in separate layers and then stacking and merging into one mat is unnecessary to achieve the desired low penetration for particulate matter. This means that finished filter media, i.e. in the final stage of their manufacture, a permanent electret charge can be imparted. The loading of finished filter material instead of fiber material has the advantage that it is no longer necessary to machine charged fibers. Because of their charge, such fibers have a tendency to adhere to metal parts, which makes their processing into filter material, for example on a carding and needle-making machine, more difficult.

The charge injection is preferably effected by means of a corona which is generated, for example, with thin tungsten wires. Although a corona, whether positive or negative, can be used, it is preferable to charge with two corona wires, one positive on one side and a negative on the other side of the combination of dielectric material and dielectric film. In this arrangement, one corona provides the charge injection, while the other functions as a counter electrode. In effect, the injected field is doubled. In addition to the advantage of doubling the injecting field, said arrangement has the further advantage that the risk of sparking through the dielectric film and / or the material to be charged is small, because instead of a metal counter electrode, one of the coronas, ie a plasma of air ions, is used as a counter electrode. The dielectric film can be moved frictionlessly and rapidly over this plasma-shaped counter electrode, which is of great importance in a continuous charging process. It is easiest to perform the corona charge in air. However, instead of 40 air, another gas can also be used.

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Preferably, the thickness reduction is achieved in that the dielectric material is enclosed in a substantially gas-tight space, at least one of the boundaries of which run perpendicular to the thickness direction of the dielectric material and the pressure in the space is reduced.

Surprisingly and unexpectedly, it has been found that the result of this charge under reduced pressure, measured by penetration, is better than that of mechanical compression charge with the same thickness reduction. Moreover, a large thickness reduction by a factor of 5 or more can easily be achieved by vacuum.

It is clear that during the charging of the dielectric material, the adjacent mainly closed foil or separating foil is also charged. When the same separation foil is used for successively charging several layers of dielectric material, it appears that by discharging this foil between successive charging operations, the charge of the following layers is increased.

Surprisingly, a further increase in charging has been achieved in that prior to using the separating foil for charging the dielectric material, this foil is charged with a polarity opposite to that used for charging the material.

It has further been found that the dielectric material is charged more bipolar if it is subjected in advance to a charge with a polarity opposite to that used during the final charge.

A similar improvement in the bipolarity of the dielectric material can be achieved by subjecting the dielectric material to an alternating voltage corona prior to final charging. It has further been found that filter material charged in a substantially gastight space at lower pressure or by means of overpressure is completely bipolar.

Due to the improved bipolarity of the dielectric material, the polarity of charged particles to be filtered is not important, moreover highly inhomogeneous fields are created in the filter, resulting in a better capture effect of uncharged particles to be filtered.

For good filtering effect, the dielectric material consists of fibers which are preferably as fine as possible. Furthermore, it is advantageous to include a powder material with special properties in the fiber material, including, for example, powder material for absorbing undesired gases.

When a filter mat with a curved surface, such as, for example, in mouth masks, has to be charged, the dielectric material is first preformed, after which the charging according to the invention, in particular under vacuum, can be applied. Usually the design takes place under pressure and high temperature. A loaded filter mat can lose part of its load here; this is avoided if charging is performed after shaping.

The invention also relates to a device for the serial production of a large number of electret filter mats of dielectric material with an open structure, comprising a corona device, in the corona of which a substantially closed dielectric film is substantially perpendicular to the corona field strength. extends. This device is characterized in that a supply device 15 for supplying a blown film and the dielectric material to be charged is provided, which connects to the supply device a vacuum packaging device, which packs the film and the dielectric material into evacuated packages and that the corona device has a positive and negative corona between which the evacuated packages 20 are transported.

Another embodiment according to the invention is characterized in that the electrode is a rotatable drum, the cylindrical circumferential wall of which has through holes and the film is guided at a distance from the cylindrical outer surface of the drum to provide a passage space for the dielectric between them. material, the film being drawn to the drum by suction of the passage space through the holes in the drum wall opening into it.

Preferably, the film is an endless belt passed over rotatable rollers.

Yet another embodiment according to the invention is characterized in that a stationary body is provided in the interior of the drum opposite the holes in the drum wall which lead out into the passage space for the dielectric material and which is provided with two axes in the axis direction of the drum. sealing surfaces which abut the inner side of the cylindrical drum wall near the outer holes leading into the passage space, and that the surface of the body extending between the sealing surfaces, which faces the holes leading into the passage space, is spaced from the drum- 40 wall is to define a closed extraction area, which is connected to an extraction 8300439 4 * * 6 suction line.

According to a further elaboration, a stationary sealing surface is provided, which closes the holes which do not open into the passage space for the dielectric material, the substantially closed space of the drum being connected to an extraction pipe.

The invention will be explained in more detail below with reference to the drawings. In the drawings shows:

Fig. 1 an embodiment of a device for charging dielectric material according to the invention; FIG. 2 a device for continuously carrying out the method according to the invention;

Fig. 3 another embodiment for continuously performing a method according to the invention; and

Fig. 4 graphs illustrating the achieved penetrations in percent as a function of overpressure and underpressure respectively, the charging being carried out in four separate layers.

The invention will be described hereinafter with reference to dielectric fiber electret filters, however it is clear that other open structure dielectric materials can be charged with the same advantages according to the invention, such as for example dielectric foam, porous membranes, sintered powder , etc., while the material can also be used for purposes other than filtration.

Furthermore, the fibers can be fine and / or coarse and take any shape, such as, for example, round, lobed, rectangular, hollow, etc. Furthermore, staple fibers, non-woven, spun or atomized fibers, etc. can be charged, or a mixture of various fibers. The dielectric filter material can also consist of several layers, for example two layers, one layer of coarse and the other of fine fiber. If desired, different dielectric materials can be used for the layers.

The current trend when designing filters is to increasingly capture not only coarse dust, but also fine dust. This is important for both air treatment systems and personal protection, since particles with a size of less than 1 µm are the most dangerous. These are inhalable and often contain heavy metals. Furthermore, particulate matter must be kept out of dust-free rooms ("clean rooms") in which micro-electronic components are manufactured, and in intensive care units of hospitals, etc.

40 Furthermore, many production processes produce very fine dust and also • 83 0 0 4 3 9 <«7 atmospheric dust contains many submicron particles, which must be removed to protect health.

Fine dust can only be captured well if the fibers are fine. However, an electrostatic charge on the fibers also makes a great contribution to the capture of particles. The electret filters according to, for example, Dutch Patent Specification 160,303 are based on this.

It has been found in the past that the permanent electrostatic charging of a dielectric material with an open or porous structure, in particular fibers, presents difficulties with regard to breakdown. To prevent this breakdown, a closed dielectric film is first strongly stretched according to Dutch patent 160,303, after which it is charged and fibrillated.

Fibers provided with an electrostatic charge need not be particularly fine for some applications. The fibers in the electret filters according to the said patent are relatively coarse because they are based on fissile fibers (8 x 40 µm). Despite the fact that these fibers have been given a high charge, their charge is mainly made ineffective by very fine dust, but in the long run. As a result, they cannot be used long enough in some applications (especially in air handling systems).

In this context, charging a finished microfibre fiber mat opens entirely new perspectives. Several good methods are known for making microfibre fibers (see, for example, German Offenlegungsschrifts 2,328,015, 2,032,072 and 2,620,399; U.S. Patent 4,230,650 and German Offenlegungschrift 2,940,170).

Fig. 1 shows a preferred embodiment of a device with which the method according to the invention can be carried out.

Two corona plasmas 1 and 2 are used in this device, one positive and one negative. Between the two corona plasmas 1 and 2 there is a substantially closed dielectric foil 3 with the filter mat 4 to be charged thereon. The dielectric foil 3 has the function here of separating the positive and negative ions from each other and thus blocking their mutual neutralization. That is why this film is also called separating or blocking film. In this arrangement, the positive corona causes ion injection into the filter mat and the negative corona charges the blocking foil on the bottom 40 strongly negative, resulting in positive ions with an 8300439

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8 extra large field are injected. In fact, the injecting field is doubled. By using a corona, i.e. a plasma of air ions, as a counter electrode instead of a metal electrode, the chance of sparking through the separating foil is small. Furthermore, the combination of filter mat and foil can be transported frictionless and at high speeds (if desired at more than 50 m per minute) between the corona wires. Due to the reduced risk of spark breakdown and the frictionless transport, a very thin (for example 2 µm thick) dielectric film can be used, which enhances the charging of the fiber mat 10 because only little voltage loss occurs over the film. The use of thin films is particularly advantageous in the charging of thin non-woven membranes which will be discussed later.

The corona plasmas 1 and 2 are generated by thin tungsten wires 5 and 5, respectively. 6, which are above resp. foil 3 are placed under the combination of filter mat 4 and 15. The wires 5, 6 are connected by means of the voltage sources 7 and 6, respectively. 8 on a positive resp. negative voltage of, for example, 7 kV or more. By placing the threads perpendicular to the longitudinal direction of the filter mat, a uniform charge is achieved. Compared to the known charging method with coro-20 points, the corona wires have the advantage that they are less fragile, less dirty and less wear due to spark erosion. In addition, they provide a better charging geometry. Furthermore, the corona device is also provided with two earthed plates 9, 10, which reinforce the ionization of the air, so that extra many ions are available for injection. Furthermore, the grounded plates 9, 10 make the arrangement safer.

The device of Fig. 1, with only the components discussed so far, has been used to charge a filter mat of dielectric material with an open structure, in particular a nonwoven fabric. It has been found that this fiber web can only be charged properly if the surface weight standardized on the fiber density and the thickness thereof do not exceed certain values, i.e. 0.22 m resp. 6 mm. It is therefore necessary to start with thin layers of filter material with a low surface weight. Such layers are generally not sufficient to achieve the desired low penetration for microfibre dust, such that the filter mat must be built up of a number of thin layers which must be charged separately. In addition, in order to prevent "fluttering" of these layers, they still have to be joined, for example by needling or welding.

It has been found that when the filter mat is compressed in the 40 thickness direction during charging, it can be charged with a surface weight normalized to 8300439% * 9 fiber density and an original thickness greater than the limits just mentioned. After all, a filter contains a lot of air, because the degree of filling or packing, i.e. volume of fibers / total volume, is often only a few percent.

5 Usually, the filters can be compressed to a thickness that is five times or more times smaller than the original. The compression of the filter mat can be achieved in various ways.

In the device of Fig. 1, a wide-mesh dielectric mesh 11 is mechanically pressed onto the separating foil 3. Foil 3 is hereby stretched, for example, in a frame (not shown), which is fixedly arranged. The mesh 11 is also stretched in a frame (not shown) which is pressed in the direction of the film.

Preferably, the charging of the filter material is carried out continuously, the filter material, insofar as it is located in the coro-15 device, being continuously compressed by the mesh which is guided by means of a roller system (not shown).

Furthermore, the filter mat can be charged between two films instead of one. In this case, the filter mat can be compressed by applying an overpressure to the films. Optionally, the filter mat can also be permanently compressed, for instance by pressing the mat before charging in a high-pressure press, possibly at a high temperature, or by passing it between hot press rolls (calender).

Although, as described above, a corona is used on either side of the combination of filter mat and blocking foil, substantially the same results are obtained with one corona and a metal counter electrode. A single, in particular a positive corona, has the advantage of producing less ozone than a two-sided one.

It is to be expected that when a single coro-30 is used, a higher voltage must be connected to the corona wires.

Due to the high compressibility of most filters, it has been found that the size and doubling of the corona voltage by using two corona wires are of less importance, so that one corona can suffice if desired and the foil in combination with a ground metal electrode can be used.

The corona charging used is fast and can be carried out at various temperatures, but preferably at room temperature.

The charges implanted into the dielectric material by the corona wires are energetically bound to structural defects in the material, in other words they are captured in so-called 8300439 • <·.

10. The "traps" (capture centers). As mentioned, the charge storage in the "traps" can generally be accomplished at room temperature. Only if the material contains deep as well as shallow "traps", it is recommended to charge at higher temperature, because then preferably the deep traps are filled and the shallow ones remain empty.

The charge injection is so strong that it is no longer necessary to aim for alignment of permanent dipoles in the filter mat, so that dielectric materials without polar groups can be used. Due to the high insulation resistance of these non-polar materials, compared to polar materials, the long-term stability of the charge injection is much more favorable.

Various dielectric materials have been found to be suitable, including, for example, the polymers polypropylene, linear low pressure polyethylene, polymethylpentene, polytetrafluoroethylene, polytrifluoro-chloroethylene, polystyrene, polycarbonate, polyester, and others.

Particularly good results are obtained when the compression of the filter mat or the nonwoven is brought about by pressure reduction. The nonwoven fabric is hereby enclosed in a substantially gas-tight space, of which at least one of the boundaries running perpendicular to the thickness direction of the dielectric material is flexible. Because the pressure in the room is lowered, the fiber web is compressed. The boundaries of the said space are formed, for example, by an upper and a lower foil. One of the 25 films, at least locally, has a low porosity in order to be able to realize a certain underpressure within the two films. It has been found that a low porosity has no consequences on the blocking action of the separating film. However, the air can also be sucked away at the edges of the filter mat and in this case the separating film need not be porous.

The boundaries of the space can also be formed by a so-called bubble film that completely encloses the fleece and can also serve as a separating film during the charging process. The air is drawn away on the open side of the bladder film. This blown foil may continue to serve as a packaging optionally later, for reasons of protection against moisture and keeping the charged material clean with respect to ambient dust.

Surprisingly, it has been found that with the same reduction in thickness by compression, charging at reduced pressure or vacuum gives a better result, i.e. lower penetration.

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The big advantage of. the invention in particular when compressed under reduced pressure is shown in Table A.

Table A

filter material total total total penetration measurement 5 number of cloth thickness NaCl; 20cm / sec.

layers weight mm g / m2 ___ pressure penetration Z drop before op- after 10_____Pa load__load very fine fibers 1 34 0.45 43.4 70 6.7 2 61 0.9 72 1.8 3 99 1.35 105 1 .5 4 133 1.8 136 0.7 15 fine and coarse fibers mixed 1 146 2.9 76 50 2.3 _2 318 5.8 149 25 0.5 __3 450 8.7 258 16 0.3 mouth mask filling 20 ( fine fibers + carrier loss) 4 428 3 447 9 1.2

The filter material consisted of polypropylene fibers and was charged stacked on one or more layers. The charge was carried out at room temperature for one second, with corona voltages of +7 kV, two 2 µm thick Mylar separation foils and in a vacuum of 30 kPa.

Table A shows that by compression and charging in vacuum filter material with a high cloth weight and a large thickness can still be charged efficiently. This is especially evident from the results of the filter material, in which fine and coarse fibers are mixed. For example, for a fabric weight of 318 g / m2 and a filter thickness of 5.8 mm, the salt penetration after charging in vacuum drops from 25 to 0.5%. Even material with a cloth weight of 450 g / m2 and a thickness of 8.7 mm can still be charged.

The measurement results for very fine fibers listed in Table A show that four layers with a total fabric weight of 133 g / m2 and a total thickness of 1.8 mm can be charged simultaneously, with the salt penetration falling to 0 after vacuum charging. 7%. Laminated 40 filter material from mouth masks can also be stacked on top of each other in four layers, 8300439 12 can still be charged reasonably efficiently. The salt penetration of this drops from 9 to 1.2%.

According to the invention it is even possible to permanently charge finished filters into electret filters. An example of this is given at -5 in Table B.

Table B

penetration measurement NaCl; 20cm / sec.

10 filter material fabric weight after storage / total total pressure penetration drop% ___Pa_ 15 highly efficient bag filter cover fleece 82) none 164 22 microfine polycarbonate fibers 72) 203 20 carrier loss 49) microfine polycarbonate fibers + carrier loss { 121 vacuum 164 4.5 25 Table B shows the measurement results of a high-efficiency bag filter, which are commercially available. According to the invention, this filter is subject to post-charging according to the invention.

The bag filter consisted of a carrier loss with the actual filter layer of microfine polycarbonate fibers on top. These fibers are at 30. side facing away from the carrier loss with a covering fleece. The table shows that the penetration by post-charging according to the invention falls from 22 to 4.5%.

Since the charging is done with two corona plasmas, not with rigid charging electrodes, preformed non-flat filter material can also be charged. Such a material is used, for example, in mouth masks. Usually the molding to "cup" takes place under pressure and at high temperature. Charged filter material can lose part of its charge, this is avoided if the charge is carried out after molding.

Preferably this is done again with a vacuum, whereby the preformed 8300439 13 filter material is enclosed in a thin bubble film, in which the pressure is reduced. The corona wires are placed above and below the filter material. For optimal charging, they should be adapted to the curved shape of the mask. If necessary, the corona on the concave side of a mouth mask can be replaced with a thin metal foil. Preferably, the separating foil is metallized on the concave side instead. Especially when the film is thin, it follows the curved surface of the mask perfectly. Particularly after vacuum suction, the film is very close to the mask face.

Another possibility is to place a corona on the concave side, in which the air is circulated, in such a way that the ions on the concave side are blown against the separating foil.

In general, a mouth mask consists of three layers: a covering layer, the actual filter layer of microfine fibers and a carrier fleece. The filtering effect of the cover layer and the support fleece is low, because they consist of fairly coarse fibers. The cover layer is primarily intended to give the mask a certain strength. Preferably, therefore, the rather thick and heavy cover layer is not charged, otherwise the real filter layer (which is thin and light) is charged much less efficiently. Table C shows an example of post-charge in vacuum of a complete mouth mask filter.

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Table C

penetration measurement NaCl; 20cm / sec.

5 filter material fabric weight after recharge / m total total pressure penetration drop% ___Pa__ 10 mouth mask cover fleece 202) microfine polypropylene fiber 60) 302 none 127 23 15 carrier loss 40) cover fleece + fibers + carrier loss 302 vacuum 127 17 charge 20 fibers + backing fleece 100 vacuum- 127 2 charge

We see that the salt penetration by the post-charge decreases by a factor of 1.4. A much greater profit is achieved if not only the complete filter, but only a polypropylene fiber fleece is charged with the carrier fleece. This is because the heavy, coarse fiber covering fleece hardly contributes to the capture of the fine dust, while at the same time it does much of the charging voltage for 30 is claimed.

The use of thin separating films for charging thin nonwovens is advantageous because of a smaller voltage loss compared to thick films. Especially for vacuum charging, where the filter material to be charged is enclosed between two films, it is important that the films are thin.

As the material for the separation film, thin films of any insulating polymers such as polyethylene terephthalate (PET), polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE), and others can serve. With 40 µm thick PET and 10-50 µm thick PP, good 40 results have been obtained.

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Furthermore, it has been found that the thinner the membranes to be charged, the thinner the separating film must be.

Experiments with vacuum-charged polypropylene fissile fiber filter material showed that the penetration increased from 3.2 to 5% 5 after the filter material was exposed to a humid atmosphere with a relative atmosphere at elevated temperature (at 45 ° C) for 34 days. humidity of 100%. The stability thus appears to be extremely favorable.

When the same blocking films were used in successive charging operations, this film was found to be highly charged with a polarity corresponding to the polarity of the charge carriers to be implanted. By discharging the film between each charging operation, preferably by means of an alternating voltage corona, an improvement in charging of about 10% was found. It appears that the charge of the film, at the location of the adjacent fibers, counteracts the charging of the fibers. It has unexpectedly been found that the fibers can be charged higher by counter-polarizing the film. By opposites is meant that the film for charging the web has a charge polarity which is opposite to that of the charge carriers ultimately to be implanted.

The influence of the charging or discharging treatment of the blocking or B film which is mentioned hereinafter is shown in Table D.

Table D

25 filter material B-film filter pressure drop penetration,% weight Pa NaCl, 20 cm / s __g / m2_ charged splitting- not discharging 195 26 5.8 30 non-woven fiber discharging 200 25 4.8 charged (opposite pola 170 15 - 3 5__rity) __

The results in Table E were obtained under the following conditions: a charging temperature of 25 ° C, a throughput speed of 10 m / min, a corona voltage of plus / minus 7 kV and a corona length of 40 cm.

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In a cursory consideration of the penetration values, charging the opposite polarity B film appears to yield only a slight improvement over the B film discharge. In the case of the opposite charge of the B-foil 5, however, the filter material has a lower weight, namely 170 g / m 2. Had the same filter weight been assumed in both cases, the gain in penetration with opposite charge would be much more pronounced.

As is known, in a positive corona only very few negative ions occur (factor 10 ^ less) and in a negative corona very few positive ions, see RS Sigmond in "Electrical breakdown of gases", page 361, Wiley, New York, 1978 edited by JM Meek and J.D. Gragg. Therefore, one would expect fibers charged freely on a positive corona blocking foil to carry a unipolar positive charge. Surprisingly, however, it appears that the fibers are not charged unipolar but substantially bipolar. This could indicate a charging process with a polarity opposite to the charging polarity performed. This reverse charge could occur as soon as the filter material is no longer exposed to the corona charge and is removed from the blocking foil.

The bipolarity achieved is favorable because charged particles are then effectively captured regardless of the sign of their charge. Moreover, a bipolarity is also favorable for the capture of uncharged particles, because a strongly homogeneous electrostatic field is generated by the bipolar charged fibers.

However, it has been found that the fibers charged free or by mechanical compression using a positive or negative corona nevertheless carry a net surplus of positive and negative charges, respectively. The bipolarity appears to be much more complete when charged in a closed space under overpressure or underpressure.

The bipolarity of the fibers can be improved upon free charging by first subjecting the dielectric material, in particular the fiber material, to a corona having a polarity opposite to the corona polarity used for the final charging. This pre-charge is hereinafter also referred to as pre-charge. Table E shows that this does indeed improve penetration; the salt penetration drops from 30 to 24%.

83 0 0 4 3 9 5 17

Table E

pre-filling filter weight pressure drop penetration, X

__g / m ^ _Pa_NaCl, 20 cm / s none 142 18 30 with ions of opposite polarity__140_19_24_ 10 Although an alternating voltage corona generates both positive and negative ions, it has been surprisingly revealed that the filling by means of an alternating voltage corona instead of a DC voltage corona the result was approximately the same.

The loading conditions for achieving the results of Table E were: loading of polypropylene fissile fibers in four layers at 25 ° C and for 1 sec. with corona voltages of 7 kV and a 2 µm thick Mylar blocking foil.

When preparing the electret filter, the fiber dielectric material is charged according to one of the methods of the invention, the dielectric material can advantageously be provided with a powder material or materials with special properties. For example, a powder material for absorbing gases can be used.

The graphs of Figure 4 show measurement results of a filter mat 25 with a weight of 185 g / m 2 of polypropylene splitting fibers for two charging conditions. Penetration is plotted in percent along the ordinate, which was measured with a NaCl test at 20 cm / sec. The overpressure and the underpressure are plotted in kPa along the abscissa, with which the filter is charged. Graph a applies to four-layer charging under positive pressure, while graph b shows the result of four-layer charging under negative pressure. In both cases, the filter medium between two blocking foils was charged at 25 ° C in 1 sec with +7 kV. The graphs show that little profit is made below a certain underpressure or above a certain overpressure, because the penetration then decreases little more.

Figure 2 schematically shows an embodiment of a device suitable for continuously charging packages of filter material. The filter material 12 to be supplied which is wound on a roll 13 is unwound from it and supplied via a feeder 40 (not shown) to a vacuum packaging device 14. An 8300439 18 blown film 15 which is wound on a roll 16 is unwound therefrom and fed to the packaging device 14 via a feed device (also not shown). In this packaging device 14, the supplied filter material 12 and the supplied blown film 15 are vacuum packed. The web 17 of packets of filter material thus manufactured is passed through a corona device, which consists of the corona wires 18, 19 and the earthed metal plates 17, 18. The voltages indicated with + and - on the corona wires 18 and 19 are generated. by means of non-disconnected voltage sources of, for example, -10 + and -7 kV, respectively. The package web 17 can be wound on a roll for storage or further transport to the user. During storage or transport, the filter material is protected against moisture or dust by its packaging. Of course, after leaving the corona device, the packages can be cut from the web 17 and stored or transported as separate packages. After removing the bubble film from the packages, the filter material is immediately suitable for use in a filter.

In the device according to Figure 2, the filter material is enclosed between two films and charged in a double-sided corona device 20. However, the filter material may also be charged between a blocking foil and a single corona ground metal counter electrode, either positive or negative. This is shown in Table F.

Table F

charging method filter weight pressure drop penetration in% 25 of filter mat / g / m ^ _Pa_NaCl; 20cm / sec.

between 2 blocking foils and with 2 corona wires 178 22 6 between 1 blocking 30 foil and metal counter electrode, and with 1 corona 135 20 12

Table F shows penetration results of split fiber filters charged in two ways in vacuum (in four layers). Charging was performed with corona voltages of +7 kV, at 25 ° C, for one second and with a 2 µm thick PET film. In Table F, a comparison is made between the two charging modes, namely charging between two foils and with two corona wires respectively charging between one blocking foil and a grounded metal TE electrode and with one corona wire. The last charge appears to be less efficient at first sight, because the measured salt penetration is higher · However, the fabric weight in this case is 1.3 times lower. If corrected for this, the salt penetrations are almost the same.

In the device shown in figure 3, a nonwoven fabric 22 is charged by means of a single-sided corona device and one blocking foil according to a continuous process. This nonwoven fabric 22 is guided over a rotatable metal drum 23 which is grounded. Above the portion of the fiber web 22 which abuts against the drum 23 is a corona device with corona wires 24. Connected to a voltage source of +7 kV, for example, not shown, the corona wires 24 generate a positive corona plasma 25 for charging the nonwoven fabric 22. A grounded metal plate 26 is present on the side of the corona wires 24 opposite the coronaplasm. It is clear that the nonwoven fabric 22 can also be charged with a negative corona, for which purpose a negative voltage source of, for example, -7 kV is connected to the corona wires 24.

The separating film 27 is arranged between the nonwoven fabric "22" and the corona plasma 25, which film is guided as an endless belt of mainly closed dielectric material over rollers 28, 29. Preferably, the portion of the film tape abutting the fiber web 22 is pressed against it, for example, by pressing down the rollers 28, 29. This compresses the fiber web to improve charging. For further improvement, the separating foil 27 is continuously discharged or reverse-poled by means of the device 30. This device 30 can be formed by an alternating current corona device or a corona device, which imparts a polarity opposite to that of the corona wires 24 to the tape.

Even better results are achieved when the charging takes place in vacuum. For this purpose, the drum 23 is provided with openings 31. Inside the drum 23 there is a stationary body 32, which is provided with labyrinth seals 33, 34 which have a small distance from the inner surface of the drum 23. The extraction space 35 is delimited on the one hand by the inner wall of the drum 23 and on the other hand by a recess 36 in the surface of the body 32.

The suction space 35 and the labyrinth seals 33, 34 run in the direction of the axis of the drum 23. In order to reduce the pressure in the suction space 35, it is connected via a suction line (not shown) to a vacuum pump (not shown). The extraction space 35 communicates via the openings 31 in the drum 23 with 8300439 20 the passage space 37 of the nonwoven fabric 22, so that in the passage space there is practically the same underpressure as in the extraction space 35. Due to the reduced pressure in the passage space 37 the film 27 is pressed against the drum 23, whereby the nonwoven fabric is compressed and the charging takes place in vacuum.

Rollers 38, 39 are further provided to guide the nonwoven fabric 22 in the desired direction.

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Claims (15)

1. A method for manufacturing an electret filter medium from an open structure dielectric material, said material being charged with a substantially closed dielectric film abutting at least on one side, characterized in that the dielectric material is reduced in thickness by compression charged. Method according to claim 1, characterized in that the dielectric material is enclosed in a substantially gas-tight space, at least one of the boundaries of which extend perpendicular to the thickness direction of the dielectric material and the pressure in the space is lowered. Method according to claim 1 or 2, characterized in that the same substantially closed foil 15 is discharged between the charging operations before successive charging operations. Method according to claim 1 or 2, characterized in that for the charging operation of the dielectric material, the substantially closed film is charged with a polarity opposite to that used for charging the material. Method according to claim 1, 3 or 4, characterized in that the dielectric material is subjected to a pre-charge with a polarity opposite to that used during the final charge. 6. A method according to any one of claims 1-4, characterized in that prior to the final charging, the dielectric material is subjected to an alternating voltage corona. Method according to any of the preceding claims, characterized in that the dielectric material consists of fibers. 8. Method according to claim 7, characterized in that a powder material is incorporated in the fiber material. Method according to claim 7 or 8, characterized in that the dielectric material consists of coarse and fine fibers, whether or not built up in layers. 10. A method according to claim 7, 8 or 9, characterized in that the dielectric material is preformed into a filter body. An apparatus for manufacturing electret filter mats of open structure dielectric material according to any of the methods of claims 1-10, comprising a corona device, in the corona of which a substantially closed dielectric film extends substantially perpendicular to the corona field strength 8300439 characterized in that a blower film feeder and the dielectric material to be charged is provided, which connects to the feeder a vacuum packaging device which packages the foil and dielectric material into evacuated packages and which the corona device has a positive and negative corona between which the evacuated packets are transported. 12. Device for manufacturing an electret filter mat of an open structure dielectric material according to any of the methods of claims 1-9, comprising a corona device and a grounded metal electrode between which a substantially closed dielectric film is arranged, with the characterized in that the electrode is a rotatable drum, the cylindrical circumferential wall of which has through holes and the foil is guided at a distance from the cylindrical outer surface of the drum to form a passage space for the dielectric material therebetween, the film is drawn to the drum by suction of the passage space through the holes in the drum wall opening into it. Device according to claim 12, characterized in that the film is an endless belt guided over rotatable rollers. Device as claimed in claim 12 or 13, characterized in that a stationary body is provided in the interior of the drum opposite the holes in the drum wall opening into the passage space for the dielectric material, which is provided in the axis direction of the drum two sealing surfaces which abut the inner side of the cylindrical drum wall near the outer holes opening into the passage space, and that the plane of the body extending between the closing surfaces, which faces the holes opening in the passage space, is spaced apart of the drum wall to define a closed extraction space, which is connected to an extraction line. Device according to claim 12 or 13, characterized in that a stationary sealing surface is provided, closing the holes which do not open into the passage space for the dielectric material, and in that the substantially closed space in the drum is connected to an extraction pipe. ********* 8300439