NO163806B - METHOD AND APPARATUS FOR PREPARING AN ELECTRIC FILTER MEDIUM. - Google Patents

Description

The present invention relates to a continuous

method and apparatus for producing an electret fuUter

me di um from dielectric material with an open and porous structure.

A discontinuous high-temperature method of manufacture

of an electret filter medium is described in US-A-4 308 223. In this method, a filter web with polypropylene fibers with a relatively high basis weight, more precisely 410 g/m<2>, and a thickness of 2 mm is charged by means of two effects, namely orientation of dipoles and implantation of charges in the fibers.

For optimal orientation of the dipoles, the charging is carried out by

high temperature (120°C) and consequently requires a relatively long time,

a total of 15 minutes.' The charge is carried out by inserting the mat between two electrodes, one of which is equipped with a large number of corona points which are connected to a high direct voltage.

The other electrode is grounded and covered with a dielectric

foil in order to prevent the ions generated by the corona from flowing to earth. For the incorporation of negative CO3-

ions, a negative corona voltage is used. In order to improve the evenness of the charge of the filter medium, the corona points are placed close to each other on the upper electrode. The coronas that originate in the points,

counteract each other and can work over a limited distance (a few millimetres). The points are placed at a small distance above or in the filter material to be charged. High corona voltages cannot be used, because these cause sparking

formation from the points to the grounded electrode. The risk of spark formation becomes noticeably greater when the points are pressed into the filter material. The sparks can then cause a short circuit that creates holes in both the foil and the filter material. The holes in the foil can even become so large that the foil becomes unusable.

The holes in the filter material allow the passage of dust particles to an unacceptable degree.

Because both the points and the upper electrode are located on a

high voltage level, the device may be unsafe. An outer

more serious problem is that the filter material is fixed with screws on the lower electrode, so that the part of the filter

the material remains uncharged and must be discarded.

furthermore, the charge is somewhat variable, as can be seen from the penetration data in table II of US-A-4 308 223. The variation from 1.3 to 7 mg of penetration is probably due to the uneven charging of the filter material, where some parts of the filter material are not charged optimally.

From US-A-4 375 718 there is known a continuous room-temperature process for the production of an electrostatically charged filter medium. In this method, a web of non-conductive thermoplastic fibers is brought into contact on each side with a more conductive web to form a unified, composite web. The composite web is charged by means of corona charging elements of opposite polarity on opposite sides of the composite web.

From EP-A-0 050 185 there is known a method for producing an electret filter medium from a dielectric material with an open or porous structure. The apparatus known from this publication for producing electret medium from a path of dielectric material comprises a corona device with a space between the corona electrodes, a substantially closed dielectric foil and means for guiding the dielectric foil together with the dielectric material through the space between the corona electrodes.

The method according to the invention is characterized by the further feature that the thickness of the path of dielectric material is reduced and the charging is carried out on the path with reduced thickness.

The introduction of the charge is carried out using a corona such as e.g. produced, with thin tungsten wires. Although either a positive or a negative corona can be used, the charging is suitably carried out with two coronas, one positive on one side and one negative on the other side of the combination of dielectric material and dielectric foil. With this device, one corona will provide the charge introduction, while the other acts as a counter electrode. This arrangement has the advantage that the risk of catastrophic flashover through the dielectric foil and/or the material to be charged is small, because instead of a metal counter electrode, one of the coronas, i.e. a plasma of air ions, serves as the counter electrode. The dielectric foil can be moved very quickly and without friction over the mentioned counter electrode of the plasma type, which is of great importance in a continuous charging process.

Because an excessively heavy and/or excessively thick filter web cannot be charged sufficiently high to achieve the desired low penetration of fine dust particles, the electret filter web can be built up as a stack of a number of layers that are charged separately. The division of teams can then be lifted, e.g. by pinning or welding.

The superior charging of the dielectric material is achieved by the thickness of the dielectric material being reduced during charging. It is possible to permanently reduce the thickness of the dielectric material before charging by compressing it so that it does not return to its original shape. This may be the case when producing shaped objects from an open dielectric material. The thickness of the material can also be reduced by stretching in the longitudinal and/or transverse direction. This applies in particular to materials that can be deformed elastically, e.g. foam. Temporary stretching during loading has the further advantage that the surface weight of the material is reduced.

A reduction in thickness can be achieved by pressing an open gas or a net into the dielectric material, e.g. in that rollers over which the gas runs are arranged in such a way that they press against the track. Moreover, it is possible to exert a pneumatic overpressure on a substantially closed foil which lies above the dielectric path, and thus achieve compression.

A thickness reduction is preferably achieved by placing the dielectric material in an essentially gas-tight space, one of the limitations of the space being flexible and arranged perpendicular to the thickness of the dielectric material, and providing a partial vacuum in the closed space. The gas-tight space may comprise an envelope of substantially closed dielectric foil immediately adjacent to the open dielectric material.

Surprisingly and unexpectedly, it has been found that the result of the aforementioned charging under partial vacuum, as shown by filter penetration data, is better than what is obtained by charging under other compression methods. Moreover, the vacuum method allows the convenient achievement of a high reduction in thickness (by a factor of 5 or more).

When charging a dielectric filter material with a curved surface, as e.g. used in breathing masks, the dielectric material will first be preformed, after which it can be charged according to the invention, e.g. in a partial vacuum. The forming is normally carried out under pressure and at high temperature. A pre-charged filter web can in this way lose part of its charge, and such a loss can be avoided by the charging being carried out after the forming operation.

It is clear that the charging of the dielectric material is accompanied by a simultaneous charging of the adjacent, substantially closed separation foil. When the same separating foil is to be used for successive charging of a number of pieces of dielectric material, it seems that discharging the foil after the charging operation improves the charging of the subsequent pieces.

Surprisingly, a further improvement in the charge is achieved if the separation foil, before it is used for charging the dielectric material, is charged with a polarity opposite to that used for charging the material.

It has also been found that the dielectric material is charged more bipolarly if it is previously exposed to a charge with a polarity opposite to that used during charging.

A similar improvement in the bipolarity of the dielectric material can be achieved if, before receiving its final charge, the material is exposed to an alternating current corona. Furthermore, it seems that filter material charged in a mainly gas-tight space under negative or positive pressure becomes completely bipolar.

The polarity of the dielectric material can be determined by reading the surface potential of the web using a non-contacting probe, e.g. an electrostatic voltmeter of the Monroe Isoprobe type. Bipolar tracks will give a reading of essentially zero due to the virtually complete compensation of charges of each polarity in the material at equal concentrations of charges of opposite sign.

Due to the improved bipolarity of the dielectric filter material, the polarity of the charged particles to be filtered is not important, and the highly inhomogeneous fields produced in the filter provide improved capture of uncharged particles.

To achieve a high filter efficiency, the dielectric filter material should preferably consist of very fine fibres.

The apparatus according to the invention is of the type mentioned above in connection with the mention of EP-A-0050185, and is characterized by means for reducing the thickness of the path of dielectric material, so that the charging is carried out on the path with reduced thickness. Preferred features of the apparatus are the subject of claims 23-26.

The invention will be explained in more detail below with reference to the drawing. - Fig. 1 shows an embodiment of an apparatus for charging the dielectric material according to the invention.

- Fig. 2 shows an apparatus which allows the method

according to the invention to be carried out continuously.

- Fig. 3 shows a further development of the embodiment of

fig. 2.

- Fig. 4 shows another embodiment of continuous

execution of the method according to the invention.

- Fig. 5 shows an apparatus for continuous

implementation of the method according to the invention.

- Fig. 6 shows another embodiment of continuous

implementation of the method according to the invention.

- Fig. 7 is a diagram showing filter penetration for electret filter mats according to the invention as a function of the number of layers that are charged separately.

- Fig. 8 is a diagram showing achieved penetrations i

percent as a function of overpressure or underpressure, as the charging is carried out with four separate layers.

The invention will be described below by reference

for electret filters made from dielectric fiber material.

However, it is clear that other dielectric materials with

open structure, e.g. dielectric foam, porous membranes, sintered powder, etc., can also be charged according to the present invention with the same advantages. It should also be clear that the charged material can be used for purposes other than filtration.

Furthermore, the fibers can be fine and/or coarse and assume which one

any form, e.g. round, leaf-shaped, rectangular, hollow, etc.

Furthermore, it is also possible to charge staple fibres, non-woven,

spun, melt-blown, solution-blown and sprayed fibers or a mixture of several of these fibers. The dielectric filter material can comprise different layers, e.g. a coarse layer of fibers and a layer of fine fibers. Optionally, several different dielectric materials can be used for the layers.

The current trend in the construction of filters

is concentrated on capturing not only coarse but also fine dust more and more efficiently. This is of importance both in terms of

in connection with air conditioning systems and for personal protection, because particles with dimensions smaller than one micrometer are the most dangerous. These particles can be inhaled and often contain heavy metals. Furthermore, fine dust should be excluded from dust-free rooms ("clean rooms") where micro-electronic components are manufactured, and from intensive care units in hospitals, etc. Furthermore, very fine dust is produced in many manufacturing processes, and atmospheric dust also contains many small particles that can be harmful to health.

In ordinary fiber filters, the fine dust is captured effectively only if the fibers are very fine. The ability to capture fine particles is greatly improved by applying an electrostatic charge to the fibers. This forms the basis for the electret filters according to e.g. NL patent 160 303 or US patent RE 30 7 82.

In the past, permanent electrostatic charging of a dielectric material with an open or porous structure, especially fibers, has presented problems arising from unwanted dielectric overshoot. According to NL-C-160 303 or

US-E-30 782 this overshoot is avoided by using a closed dielectric foil which is first stretched in the longitudinal direction, then charged and then fibrillated into fibers.

For a plurality of applications, fibers carrying an electrostatic charge need not be fine. The fibers in the electret filters described in NL-C-160 303 or US-E-30 782 are relatively coarse because they are formed from split fibers (10 x 40 µm). Although these fibers carry a high charge, they are adversely affected by very fine dust. For this reason, they are not particularly useful for certain long-term applications, e.g. in air conditioning systems.

The present invention opens up new perspectives, as it makes it possible to charge existing fiber webs made of microfine fibers. Several good methods are known for the production of microfine fibers and mixtures of microfine

fibers and staple fibers. See e.g. DE-A-2 328 015, 2 032 072 and 2

620 399, US-A-4 230 650 and DE-A-2 940 170. See also

US-A-3,016,559 and 4,118,531 and Van A. Wente, "Superfine Thermoplastic Fibers", Industrial and Engineering Chemistrv.

Vol. 49, page 1342 ff. (1956).

In fig. 1 shows an embodiment of an apparatus with which the method according to the invention can be carried out.

In this device, two corona plasmas 1 and 2 are used, one positive

and a negative. Between the two corona plasmas 1 and 2, an essentially closed dielectric foil 3 is placed there, which carries the filter mat 4 to be charged. The dielectric foil 3

acts as a barrier that separates the positive and negative ions and thereby blocks their mutual neutralization.

The foil is therefore designated as a separation or block

ering foil. The use of a corona, i.e. a plasma of air ions, instead of a metal electrode as a counter electrode reduces the risk of sparking through the separation foil. In addition, the combination of filter web and foil can be transported between the corona fields at high speed. The reduction in the risk of sparking and the frictionless transport allows the use of a very thin dielectric foil (e.g. 2 micrometers thick).

This increases the charge of the fiber mat because the voltage loss over

a thin foil is low. The use of the thin foils is particularly useful when loading thin fiber pile.

The corona plasmas 1 and 2 are provided by thin tungsten wires 5 and 6

which are respectively placed above and below the combination of

the filter mat 4 and the foil 3. The wires 5 and 6 are connected with

respectively a positive and a negative voltage of e.g. 7 kV

supplied from voltage sources 7 or 8. When the threads are placed perpendicular to the longitudinal direction of the filter mat, a uniform charge is deposited. Compared to known charging techniques with corona points, the corona wires have the advantage that they are less prone to damage, are more difficult to break and are less exposed

for spark erosion. In addition, threads provide a better charging

geometry. The corona device is equipped with two grounded plates 9

and 10 which strengthens the ionization of the air, so that many more ions are available for injection. In addition, the grounded plates 9 and 10 make the arrangement electrically safe.

In the apparatus of fig. 1, a wide mesh dielectric gas (not shown) can be pressed mechanically against the dielectric material to be charged, and the separation foil 3. For that purpose, the foil 3 is stretched over a frame (not shown) which is mounted in a stationary position. In a similar way, the gas is stretched over a frame (not shown) and pressed in the direction of the foil.

The charging of the filter material is carried out continuously, as long as the filter material is inside the corona device, it is continuously compressed by means of the gas which is rotated by means of a roller system (not shown).

The corona charging that is used is fast (can be carried out in one second or less) and can be carried out in a large temperature range, but preferably at room temperature.

The charges that are implanted in the dielectric material by means of the coronas are strongly bound to structural defects in the material; in other words, they are caught in so-called "traps"

(retention centers). As will be understood, storage of charge in the traps will normally be achieved at room temperature. Only if the material also contains shallow traps in addition to deep traps, is it preferable to carry out the charging at a higher temperature, because high temperatures favor the filling of the deep traps, while the shallow ones remain empty.

The charge injection is so strong that one no longer needs to aim for an orientation of permanent dipoles in the filtering material, so that one can start from dielectric materials without polar groups. Due to the higher insulation resistance in non-polar materials compared to polar materials, the stability of the introduced charges is much better in the long run.

Many dielectric materials have been found to be suitable, including

polymers such as polypropylene, linear LD polyethylene, polymethylpentene, polytetrafluoroethylene, polytrifluorochloroethylene, polystyrene, poly-

carbonate, polyester and others.

Excellent charging can be achieved by the dielectric material

during loading is compressed in the direction of its thickness. This compaction is possible because a filter contains a lot of air, while the fill or packing density (volume of fibers/total volume)

often only make up a few percent. The filters can often be compressed to a thickness of one fifth or less of their original thickness.

The compression can be achieved in different ways. Firstly

the filter mat can be compressed mechanically on the blocking film,

e.g. using a wide-masked gas. Furthermore, the filter mat can be loaded between two foils instead of one. In this case, the filter mat can be compressed by exposing the foils to a pneumatic overpressure. Optionally, the filter mat can be permanently compressed, e.g. in a high-pressure press, possibly at a high temperature, or by being fed between hot press rollers (calan-

dring). Compression can also be achieved by shaping the filter material into an object, e.g. a breathing mask.

Particularly good results are achieved when the compression of

the filter mat or filter fleece is achieved by pressure reduction. IN

in this case, the fiber carpet is enclosed in a mainly gas-tight space, if at least one limiting surface (a main surface)

perpendicular to the thickness direction of the dielectric material is flexible. When the pressure in the said room is reduced, the fiber carpet is compressed. The limitations of the mentioned room can consist of e.g. an upper and a lower foil. One of these foils

can, at least locally, have a low porosity, which makes it possible to create a certain negative pressure in the space between the two foils.

It seems that such a low porosity does not affect the blocking effect of the separating foil. Alternatively, air can

is sucked away at the edges of the filter mat, and in any case the separation foil does not need to be porous.

The limits of the space can be formed by so-called blowing foil (hose) which completely encloses the fleece, and which is also able to serve as a separation foil during the loading process. The air is sucked away at the open end of the blowing foil. If necessary, the blown film can later serve as packaging to protect against moisture and dust.

Surprisingly, it has been found that charging at a reduced pressure for the same thickness reduction gives a better result, i.e. a lower particle penetration, than what is the case with other forms of compression.

The great advantage of the invention, especially when compression is used, can be read in tables A, B, C, and D. Testing has been carried out in accordance with British Standard 400, which is a test of the penetration of a standard dispersion of sodium chloride particles produced by atomizing an aqueous solution of NaCl. The aerosol concentration is measured in a hydrogen flame. The air flow has a speed of 20 cm/s.

The quantity Q is a property specification for filter media defined mathematically by the expression

q = - ln(% P/ 100) {1)

A P

where %P is percentage penetration, AP is the pressure drop in Pascal and ln stands for the natural logarithm. This property specification or quality number always has a positive sign and increases with reduced penetration. Conversely, the value of Q will be reduced as the pressure drop increases.

In the event that the filter path is made thicker by adding material, the penetration of relatively fine particles has been found to be expressed approximately by the mathematical expression:

%P = 100e"k"w (2)

where W is the base weight, k is a constant and e is the base of the natural logarithm. Base weight is defined as weight per area unit of the track, e.g. in grams per square meters. (It is assumed

and degree of compression as the original material, and that interface effects and depth filling effects can be neglected).

In a similar way, the pressure drop can be calculated from the expression:

where k' is a constant and W is the base weight.

A combination of equations (1), (2) and (3) gives

This shows that Q is an index that is independent of the direct effect of the basis weight on the filter performance. Thus, Q can be used to compare the filtering performance of paths with different basis weights. (See William C. Hinds, Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, John Wiley and Sons, New York, Chapter 9). If Q is different between two paths, it may be because either k or k' or both are different, k may be different because the fibers are differently charged. This is evident from the first two lines in table A, which show the beneficial effect of charging with reduced surface weight and thickness. In a similar way, k' can be increased if the path is permanently compressed. This is evident from the increased pressure drops that can be read from lines 5 and 6 in table A.

Table A relates to a non-carded filter web of polypropylene split fibres. These were relatively coarse and had a rectangular cross-section (dimensions 9 x 45 µm). The filter material was charged in two ways, i.e. without compression with one separating foil and in vacuum with compression between two separating foils. The charging was carried out on an apparatus similar to that shown

fig. 1, in approx. 1 second at 25 C with corona voltages of + 7 kV and with one or two micrometer thick blocking foils of MYLAR, the two foils forming an essentially gas-tight enclosure for charging in partial vacuum (only one foil is shown in

fig. 1).

The table shows that free charging in four layers instead of one reduces salt penetration from 62 to 30%. A comparable result was obtained when the entire mat was charged at once in a partial vacuum of 30 kPa. A much better result was achieved when the filter material was charged in a partial vacuum of 30 kPa in four layers, and the salt penetration was then reduced to just 5%.

In the last two examples, the filter material was compressed permanently in a press under a high pressure of 11.8 MPa at 25°C, which prevents the material from returning to its original height after charging. These examples show the surprising advantage of charging in four layers in a vacuum.

The filter material in Table B was formed from polypropylene fibers and loaded in one layer, or it was a stack of stacked layers. The charging was carried out on an apparatus similar to that shown in fig. 1, in a partial vacuum of 30 kPa at room temperature during approx. 1 second and with corona voltages of + 7 kv as well as with two separating foils of two micrometer thick MYLAR. The latter foils formed an essentially gas-tight space (only one foil is shown in Fig. 1).

The very fine fibers are meltblown polypropylene microfibers produced by the method described by Van A. Wente, "Superfine Thermoplastic Fibers", Industrial and Engineering Chemistrv, Vol. 48, pages 1342 et seq. (1956). The fibers are a mixture of sizes ranging from less than one micrometer to several micrometers in diameter.

The fine fibers constitute a mixture of meltblown polypropylene fibers as described above and polypropylene staple fibers with a diameter of approximately 25 micrometers added in a proportion of approximately 30 percent by weight using the method described in US-A-4,118,531.

The breath ash filling is a very fine fiber similar to the above-mentioned very fine fibers taken from a 3M Company 8710 disposable dust and fog respirator.

Table B shows that loading and compression in partial vacuum allows efficient loading of filter materials with a high basis weight and large thickness. This is particularly expressed through the results for the filter material which consists of a mixture of fine fibers and coarse fibers. For a basis weight of 318 g/m<2> and a filter thickness of 5.8 mm, e.g. the salt penetration decreases from 25% to 0.5% after charging in partial vacuum. Even material with a basis weight of 450 g/m<2> and a thickness of 8.7 mm can still be charged.

The test results given in Table B for very fine fibers show that four layers with a total basis weight of 133 g/m<2> and a total thickness of 1.8 mm can be loaded simultaneously, whereby the salt penetration decreases to 0.7% after charging. The laminated filter material in face masks can also be charged very efficiently in a stack of four layers. The salt penetration then falls from 9% to 1.2%.

Although the use of compression during charging (or permanent compression prior to charging) greatly improves filter efficiency (defined as a percentage of 100 minus the percentage penetration), it is still profitable to build up a filter path from layers charged separately during compression and later assembled into one lane. This is evident from tables C and D.

Table C relates to the filter properties of polypropylene split fiber filters of 190 g/m<2>, compressed during charging and/or charged in four layers. The charging was carried out at 25°C in an apparatus similar to that shown in fig. 1, during approx. 1 second and with corona voltages of approx. +7 kV. The compression was achieved respectively by compression with an open gas lying above the filter, by vacuum suction and under the effect of an overpressure. In the tests to which the fourth and fifth lines of the table relate, the filter web was enclosed between two blocking foils which were welded together. The space between the two foils was evacuated during the test according to line four down to a pressure shown in the table, while the numbers in the fifth line relate to the case where the foils were compressed at an overpressure of 30 cm Hg

(40 kPa) produced in a chamber which enclosed the entire apparatus and had means for the outlet of air from the otherwise gas-tight space which was limited by the blocking foils. Finally, a filter was compacted permanently by compression in a high-pressure press at room temperature under a pressure of 11.8 MPa. In the latter case, the filter web did not regain its original shape, so the pressure drop was higher, i.e. 71 Pa instead of 20 Pa. The results of free loading without compaction show that the salt penetration decreased from 60 to 30% when dividing the fiber web into

four layers. Compressing twice reduces this to 20%. Vacuum and compression by overpressure are more effective and lead to a reduction in penetration to 6% and 14% respectively Table D calculates the loading results for three types of polypropylene filter material, more specifically split fibres, fine fibers and very fine fibres. The fine and very fine fibers are the same as mentioned in connection with table B. The charge lasted for approx. 1 second and was carried out at 25°C with corona voltages of + 7 kV and with a blocking film of 2 micrometer thick MYLAR, again on the apparatus shown in fig. 1, except that the sealed bags of blocking foil were used for the vacuum charging. Before the split fiber web was loaded, it was split into four layers, which were reassembled into a single fiber web after

be charged separately. The table again shows that the NaCl penetration for the split fiber webs is lowest when vacuum charging is used. The latter method reduces the penetration from 90 to 5%. This may be due to the high compression of the filter web, whose thickness decreases from 1.5 to 0.2 mm. The X-ray beam charge is used to obtain samples that are known to be completely uncharged.

The filter paths of fine and very fine fibers were not divided. The penetrations obtained for these webs (of 146 g/m<2> and 33 g/m<2>) were respectively 2% and 5% after vacuum charging. Their free charging, i.e. without compression, also gave quite good results, i.e. salt penetrations of 18% and 15% respectively.

With the method according to the invention, it is even possible to charge existing filters permanently to obtain electret filters. An example of this is given in Table E.

Table E reproduces the test results for a very effective bag filter available on the market. The microfine fiber material was produced by Carl Freudenberg according to the method in GB-A-1 346 231. According to the invention, the filters were subjected to post-charging using an apparatus as, for example, shown in fig. 1 together with sealed, evacuated cover foils.

The bag filter consisted of a carrier web that carried the filter layer itself, which consisted of microfine polycarbonate fibers, which in turn was protected with a cover web. The table shows that aftercharging reduces penetration from 22% to 4.5%.

Because charging is performed with two corona plasmas and not with rigid charging electrodes, even preformed, non-planar filter materials can be charged. Such materials are used e.g. in breathing masks. The shaping into a cup shape is often carried out under pressure and at high temperature. Charged filter material can thereby lose some of its charge. This is avoided by the charging being carried out after forming.

The charging is preferably carried out in a partial vacuum, as the pre-formed filter material is enclosed in a thin blown foil tube where the pressure is reduced. The coronas are placed above and below the filter material. For optimal charging, the shape should be adapted to the shape of the curved mesh. Optionally, the corona on the concave side of the face mask can be replaced with a thin metal foil. Alternatively, the separation foil can be metallized on the concave side. These foils have the advantage that they adapt to the curved surfaces of the mask, especially when vacuum is used.

Another possibility is to position a corona on the hollow side, as an air current is circulated in this cavity in such a way that the ions are blown towards the separation foil.

A breathing mask generally consists of three layers: a protective covering web, the actual filter web formed by micro-fine fibers and a carrier web. The filtering effect of the covering web and the supporting pile is small, because these consist of relatively coarse fibres. The cover layer is primarily intended to give the mask a certain rigidity. It is preferred not to charge the relatively thick and heavy cover layer, because such a charge would otherwise damage the charge of the filter layer itself (which is thin and light). Table F contains an example of a complete breathing mask that has been left in vacuum. In this case, a disposable dust and mist respirator (type 3M Company 8710) was used. Again, the apparatus and charging conditions are the same as those used in previous examples.

It is clear that recharge reduces salt intrusion by a factor of 1.4. A much greater yield is achieved when, instead of loading the completely finished filter, you only load the polypropylene fiber fleece together with the carrier web. This yield is due to the fact that the heavy cover pile consists of coarse fibers and thus hardly helps to retain fine dust, while it requires a large part of the charging voltage.

Table G, which reproduces the results of simultaneous vacuum charging of a stack of three stacked layers of fine fiber material as before, shows that even when vacuum charging is used, it will still pay to build up a filter mat from thin layers that are charged separately .

Also in these examples, an apparatus as indicated in fig. 1 with sealed, evacuated foils and corona which worked in the same way as in the previous examples. Charging was carried out at room temperature.

Table G shows that the penetration in the middle layer is less satisfactory (greater penetration) than in the outer layers. The lower charge of the middle layer will have less effect on the overall performance when the stack has a smaller thickness, in other words when a thinner web is charged. The thin webs are stacked to form a single filter web after they are charged separately, after which the connection between the layers is improved by e.g. pin stapling or welding.

In light of what has been said above, the fiber web should be as thin and/or as light as possible and/or the compression should be as great as possible to achieve the greatest possible charge. However, little additional benefit is obtained by exceeding a certain number of layers or by using a lower than a certain underpressure or a greater than a certain overpressure. This is shown by the graphs in fig. 7 and 8.

The graphs in fig. 7 represents test results for a polypropylene split fiber filter web with a basis weight of 185 g/m<2> under different loading conditions. The ordinate gives penetration data in percent, measured in a NaCl sample at 20 cm/s. The abscissa gives the number of layers with which the filter is loaded. Graphs a, b, c and d relate respectively to free charging, charging with compression twice, charging with a partial vacuum of 30 kPa and charging of a filter permanently pre-compressed at a pressure of 11.8 MPa at room temperature.

Fig. 8 shows the test results for a similar filter mat as discussed in connection with fig. 7. The same NaCl test was used. Again, the penetration in percentage is indicated along the ordinate, while the abscissa indicates overpressure or underpressure i

kPa. Graph a concerns charging in four layers under overpressure, while graph b gives the results for charging in four layers in a partial vacuum. In both cases, the filter medium was charged between two blocking foils of 2 micrometer thick MYLAR at 25°C and with corona voltages of + 7 kV and in an apparatus similar to that shown in fig. 1.

The separation foil should preferably be in intimate contact with the material to be loaded, especially when this is a thin web. The intimate touch is promoted by the use of a thin separating film that can be easily bent, because such a film conforms easily to the web's contours, not least because it is helped by the electrostatic attraction between the fiber web and the film.

If the material to be loaded, e.g. is a web of flat split fibres, the web is preferably stretched in the plane of the separation foil to improve the mutual contact and thereby optimize the charging of the fibres. The use of thin separation foils for charging thin fiber webs is advantageous because they produce less voltage loss than thick foils. The separation film can consist of a thin film of a number of insulating polymers, e.g. polyethylene terephthalate (PET), polypropylene (PP), polyethylene (PE) or polytetrafluoroethylene (PTFE). PET of 2 micrometer thickness and polypropylene of 10-50 micrometer thickness give good results.

Furthermore, it seemed that the thinner the web to be charged, the separating foil should be thinner.

Weathering tests with polypropylene split fiber filter material loaded in a vacuum have shown that penetration increased from 3.2 to 5% when the filter material was exposed to a humid atmosphere with a relative humidity of 100% at an elevated temperature (45°C) for 34 days . The stability thus appears to be very good.

When the same blocking foil is used in successive charging operations, it has been found that the foil is strongly charged with a polarity corresponding to the polarity of the charge carriers to be implanted. Discharging the foil between successive charging operations, preferably by means of an alternating current corona, gives an improvement in the charge of approx. 10%. Apparently, the charging of the foil where it comes into contact with the fibers will counteract the charging of the fibers. Surprisingly, it has been found that opposite polarization of the foil further increases the charge on the fibers. By opposite polarization is meant that the foil before the charging of the web acquires a polarity which is opposite to the polarity of the final charge carriers to be implanted.

Table H shows the effect of said discharge and opposite polarization on the blocking or B foil.

The results in Table H were obtained at a charge temperature of 25°C, a feed rate of 10 m/min, corona voltages of + 7 kV and a corona length of 20 cm in an apparatus of the type shown in fig. 2 or (for discharged or oppositely charged foil) in fig. 3.

A quick look at Table H seems to show that charging the B foil with an opposite polarity does not provide appreciable improvement over discharging the B foil. However, it should be noted that the filter material in the example relating to the charging of B-

the foil, had a lower basis weight, namely 170 g/m<2>. On an equal weight basis, the improvement in penetration with opposite charging will be higher (see also the differences in pressure loss). This is demonstrated by the value of Q.

It is known that a positive corona contains only very few negative ions (IO<-3> times the number of positive ions) and vice versa (see R. S. Sigmond in "Electrical Breakdown of Gases", page 361, Wiley, New York, 1978, edited by J.M.. Meek and J.D.

Cragg). Fibers that are freely charged on a blocking foil with a positive corona can therefore be expected to carry a unipolar positive charge. However, surprisingly, the fibers are not charged unipolarly, but almost bipolarly, probably due to a random charging process with polarity opposite to the charge-

the polarity. This reverse charge probably takes place when the filter material is no longer exposed to corona-

the charge and removed from the blocking foil.

The achieved bipolarity is beneficial because charged particles are then captured effectively regardless of the sign of the charge. Moreover, bipolarity also promotes the trapping of uncharged particles, because it provides highly inhomogeneous electrostatic fields.

However, it has been found that fibers charged freely or with mechanical compression using positive or negative corona actually carry an excess of positive and negative charges, respectively. The bipolarity seems to be more balanced when the fibers are charged in a closed space under positive or negative pressure.

The bipolarity of the fibers during free charging can be improved by first exposing the dielectric material to a corona with a polarity opposite to the corona polarity used during the final charge. The aforementioned free loading is hereinafter referred to as pre-filling. Table J shows that the prefill improves the penetration results.

Although an AC corona provides both positive and negative ions, it has surprisingly been found that precharging using an AC corona instead of a DC corona gives approximately the same result.

The results according to table J were obtained by loading polypropylene split fibers in four layers at 25°C for 1 second at corona voltages of approx. + 7 kV and with a 2 micrometer thick MYLAR blocking foil. Prefilling was achieved by charging on an apparatus of the type shown in fig. 1, but without blocking foil. The actual charging was then performed with a blocking foil.

So far, the invention has been described as a discontinuous process, but the described methods are preferably carried out continuously.

Figure 2 shows an embodiment of an apparatus for continuous execution of the method according to the invention. In the aforementioned apparatus, the separating foil is formed by an endless belt 11 of a substantially closed dielectric foil which runs over rollers 12-15. These rollers can rotate freely in a frame (not shown). The upper race 16 of the endless belt 11 runs through a double corona device which on each side of the race 16 has respectively the positive corona plasma 17 and the negative corona plasma 18. The coronas 17 and 18 are provided by means of the tungsten wires 19 and 20 which is connected to a high-voltage source (not shown) of + 7 kV and - 7 kV respectively. On the side facing away from the coronas, there are arranged grounded plates 21 and 22. The dielectric material with an open structure, in particular the fiber web 23, is transported between the two coronas in contact with the run 16 of the endless belt 11. The tungsten wires 19 and 20 is arranged perpendicular to the feeding direction in order to obtain a uniform charge of the fiber web 23.

The apparatus shown in fig. 3, and which is intended for continuous charging of the fiber path 23, is similar to the apparatus shown on

fig. 2 with respect to the charging operation. Accordingly, corresponding parts have the same reference number. In the apparatus of fig. 3, a further corona device is used where the barrel 25 is discharged with an AC corona provided by tungsten wires 26 or 27, or charged with a polarity opposite to that provided by the upper corona device. In the further lower corona device is

where as for arranged earthed plates 28 and 29.

The endless belt 11 in the devices in figures 2 and 3 can be formed by a grounded metal belt whose outer surface is covered with a dielectric material. In this arrangement, the corona wires 20 and 26 with grounded plates 22 and 28 are omitted. Preferably, the tire adheres to the surface of the belt. Optionally, the endless belt 11 can consist of dielectric foil metallized on the inside.

In the apparatus of fig. 4, a fiber web 30 is charged by a continuous process by means of a one-sided corona device. The fiber web 30 is guided over a rotatable metal roller 31 which is grounded. The separation foil is located on the circumference of the roller 31 and preferably adheres as a cover to the surface of the roller. A corona device with corona wires 32 is arranged above the part of the fiber web 30 which lies against the foil or cover. The corona wires 32 are connected to a voltage source (not shown) of e.g. +7 kV and provides a positive corona plasma 33 for charging the fiber path 30. On the side of the corona wires 32 facing away from the corona, there is arranged a grounded metal plate 34. It is clear that the fiber path can also be charged with a negative corona, and to for this purpose, a voltage source of e.g.

-7 kV is connected to the corona wires 32.

Although not shown, the blocking film on the roller will be continuously discharged or reversed, e.g. with a corona device.

The separation foil can also be placed between the fiber webs 30 and the corona plasma 33, the foil in this case being guided as an endless belt formed of a substantially closed dielectric material, over a series of rollers in a manner not shown. Preferably, the fiber web 30 is pressed with the foil belt against the metal roller, e.g. using rollers not shown. The resulting compression of the fiber web noticeably improves charging.

The same placement of the foil and the pressure it exerts in the direction of the fiber web can also be used in the devices in Figures 2 and 3 when the endless belt 11 consists of a grounded metal belt.

Furthermore, the compression of the fiber web can be achieved by stretching a belt of gas material over the web in a manner not shown. To that end, an endless belt of gaseous material is passed over rotating rollers. The run of the belt that engages with the track is pressed against it by pressing the rollers on both sides of the run in the direction towards the track.

Fig. 5 is a diagram of an embodiment of an apparatus suitable for continuous charging of packages of filter material. Filter material 112 which is wound up on a spool 113 is unwound from the spool and supplied to a vacuum packing device 114 via a supply device (not shown). A blown foil tube 115 which is wound on a spool 116 is unwound from this and supplied to the packaging device 114 via a supply device which is also not shown. In the packaging device 114, the supplied filter material 112 and the blown foil 115 are vacuum packed. The band 117 of packets of filter material produced in this way is fed through a corona device comprising corona wires 118 and 119 and grounded metal plates 120 and 121. The voltages indicated by + and - on the corona wires 118 and 119 are produced by voltage sources not shown on respectively +7 kV and - 7 kV. The strip of packages 117 can be wound up on a reel for storage or transport to the consumer. During storage or transport, the filter material is protected by its packaging against moisture and dust. Packages can of course be cut off from the belt 117 after they have left the corona device, and stored or transported separately. After removing the blown film from the packages, the filter material is immediately available for use in filters.

In the apparatus of fig. 5, the filter material is enclosed between two foils and is charged in a two-sided corona device. However, the filter material can also be charged between a blocking foil and a grounded metal counter electrode with either a positive or

negative corona. The results are given in table K.

Table K gives the penetration results for split fiber filters charged in vacuum (in four layers) in two ways. The charging was carried out with corona voltages of + 7 kV at 25°C for 1 second and with a 2 micrometer thick MYLAR foil. Table K simultaneously compares the two corona charging methods, i.e. charging between two foils with two coronas and charging between a blocking foil and a grounded metal counter electrode with a corona. A quick review shows that the second charge is less effective because the measured salt penetration is higher. However, the surface weight of the track in this case is 25% lower. If one corrects using equation (2), the salt intrusions will be quite similar.

In the apparatus of fig. 6, the fiber web 122 is charged by a continuous process by means of a one-sided corona device and with a blocking foil. The fiber web 122 is guided over a rotatable and earthed metal roller 123. A corona device with corona wires 124 is mounted over the part of the fiber web 122 that abuts the roller 123. The corona wires 124, which are connected to a voltage source of e.g. +7 kV (not shown), provides a positive corona plasma 125 for charging the fiber path 122.

A grounded metal plate 126 is placed on the side of the corona wires 124 opposite the corona plasma. It is clear that the fiber path 122 can also be charged with a negative corona, and for this purpose a negative voltage source of e.g. -7 kV connected to the corona wires 124.

The separation foil 127 lies between the fiber path 122 and the corona plasma 125, the foil being fed over rollers 128 and 129 as an endless belt of essentially closed dielectric foil. Preferably, the run of the foil belt 129 which borders the fiber web 122 is pressed against this web, e.g. in that the rollers 128 and 129 are forced downwards. The fiber web is thereby compressed, which improves charging. This can be further improved by discharging the separating foil 127 or by continuous connection to a pole with the opposite polarity by means of a device 130. This device can contain either an AC corona device or a corona device that gives the belt an opposite polarity the one for the corona threads 124.

Even better results are achieved when charging is carried out in a partial vacuum. For this purpose, the roller 123 is provided with holes 131. The interior of the roller 123 contains a stationary body 132 which is provided with labyrinth seals 133 and 134 near the inner surface of the roller 123. A suction chamber 135 is defined by the inner wall of the roller 123 and a recess 136 in the surface of the body 13 2.

The suction chamber 135 and the labyrinth seals 133 and 134 extend in the direction of the axis of the roller 123. The pressure in the suction chamber 135 is reduced by means of a vacuum pump via a suction line (both not shown). Through the holes 131 in the roller 123, the suction chamber 135 will be connected to a space 137 for feeding through the fiber web 122. The negative pressure created in the feed-through space is almost equal to that in the suction chamber 135. The reduced pressure in the feed-through space 137 forces the foil 127 against the roller 123, whereby the fiber web is compressed and charging takes place in a partial vacuum.

Furthermore, rollers 138 and 139 are arranged to guide the fiber web 122 in the desired direction.

Although the above mostly relates to split fibres, the method according to the invention also allows charging of other fiber structures, e.g. meltblown fibers and others.

When electret split fibers are used in dust filters, it is desirable to shrink the fibers to increase their dust-absorbing capacity. The process described in NL-A-76 14376, in which a "closed" foil, e.g., a blown foil, is charged on both sides, provides highly bipolar charged fibers. However, the attraction of oppositely charged faces of the fibers causes an undesirable attraction force which opposes the shrinkage of, for example, bicomponent split fibers. The fibers produced according to the invention are weakly unipolarly charged and will therefore mutually repel each other, so that shrinkage is not prevented. The fibers are therefore preferably charged after shrinkage.

Fiber webs obtained by fibrillation of foils, especially blown foils, can be charged in a number of ways. E.g. two flat foils can be obtained from the blown foil hose. After fibrillation, one foil can be bombarded with positive charges and the other with negative charges, after which they are assembled into a so-called macrobipolar filter path. Alternatively, one of the foils may remain unfibrillated to serve as a separation foil for the other fibrillated foil.

Claims (26)

1. Method for producing an electret filter medium from a dielectric material with an open or porous structure, where a web (23; 30; 112; 122) of the dielectric material with a substantially closed dielectric foil (11; 31a; 115; 127) lying until at least a major surface of the web is continuously fed into a corona discharge device (19, 20; 32; 108, 119, 120) and the web (23; 30; 112; 122) of dielectric material is charged by corona discharge (17, 18; 33; 118, 119, 120, 121; 124), characterized in that the thickness of the web (23; 30; 112; 122) of dielectric material is reduced and the charging is carried out on the web with reduced thickness. 2. Method as stated in claim 1, characterized in that the thickness of the web (23; 30; 112; 122) of dielectric material is reduced by compression. 3. Method as stated in claim 2, characterized in that the compression is achieved by exerting a pneumatic overpressure against the essentially closed dielectric foil (11; 31a; 115; 127) which covers the path (23; 30; 112; 122) of dielectric material . 4. Method as stated in claim 2, characterized in that the compression is achieved by an open gas or a net being placed on the dielectric material and pressed against it. 5. Method as stated in claim 2, characterized in that the thickness of the dielectric material is temporarily reduced during charging. 6. Method as stated in claim 2, characterized in that the thickness of the dielectric material is permanently reduced before charging. 7. Method as stated in claim 1, characterized in that the dielectric material is temporarily stretched during charging. 8. Method as stated in claim 7, characterized in that the dielectric material is temporarily stretched in its longitudinal direction. 9. Method as stated in claim 7, characterized in that the dielectric material is temporarily stretched in its width direction. 10. Method as stated in claim 7, characterized in that the dielectric material is temporarily stretched in two directions. 11. Method as stated in claim 1, characterized in that the thickness of the dielectric material is reduced by placing the material in an essentially gas-tight cubicle, with at least one main limitation of the cubicle being flexible and extending along a main surface of the dielectric material, and the pressure in the mainly gas-tight compartment is reduced. 12. Method as stated in claim 1, characterized in that the essentially closed dielectric foil is discharged between successive charging operations. 13. Method as stated in claim 1, characterized in that the essentially closed dielectric foil is charged with a polarity opposite to the polarity used for charging the path of dielectric 14. Method as stated in claim 1, characterized in that the path of dielectric material is subjected to a pre-charge with a polarity that is opposite to that used in the final charging of the path of dielectric material. 15. Procedure as stated in claim 1, characterized in that the path of dielectric material is subjected to an AC corona before final charging. 16. Procedure as stated in claim 1, characterized in that a number of charged paths of the dielectric material are stacked to form a composite electret filter medium. 17. Procedure as stated in claim 16, characterized in that the charged paths are united by pinning or welding. 18. Procedure as stated in claim 1, characterized in that the thickness of the web of dielectric material is permanently reduced by deformation of the web. 19. Procedure as stated in claim 1, characterized in that the dielectric material comprises a path of dielectric fibers. 20. Procedure as stated in claim 19, characterized in that the path of dielectric fibers comprises a mixture of coarse fibers and fine fibers. 21. Procedure as specified in claim 19, characterized in that the path of dielectric material comprises a stack of several individual paths which are formed of coarse dielectric fibers and fine dielectric fibers. 22. Apparatus for producing electret filter media from a web (23; 30; 112; 122) of dielectric material with an open or porous structure, comprising a corona device with a space between the corona electrodes (19, 20; 32; 118, 119, 120, 121; 124), a substantially closed dielectric foil (11; 31a; 115; 127) and means (12-16; 31; 123) for guiding the dielectric foil together with the dielectric material (23; 30; 112; 122) through the space between the corona electrodes (19, 20; 32; 118, 119, 120, 121; 124), characterized by means for reducing the thickness of the path (23; 30; 112; 122) of dielectric material, so that the charging is carried out on the track with reduced thickness. 23. Apparatus as specified in claim 22, characterized in that the substantially closed dielectric foil (11; 115; 127) is an endless belt which is guided over rotatable rollers. 24. Apparatus as specified in claim 22, characterized in that the mainly closed dielectric foil comprises a mainly gas-tight compartment, at least one main limitation of the compartment being flexible and extending along a main surface of the dielectric material, and there are means for partially evacuating the gas-tight compartment. 25. Apparatus as specified in claim 24, characterized in that the mainly gas-tight compartment comprises a package formed by a foil to which the dielectric material is welded. 26. Apparatus as specified in claim 24, characterized in that the mainly gas-tight enclosure comprises a rotatable roller (123) with small holes and a suction chamber (13 5) and the mainly closed dielectric foil in the form of an endless belt.