Background and Field of the Invention The invention is directed to a filter element for filtering fluids, particularly for filtering air streaming into the passenger cabin of a vehicle, comprising an electret filter layer consisting of a non-woven material, and a reticular support structure for holding said electret filter laye .

When using filter material layers consisting of non-woven materials, normally the electret filter layer is required to be held by a reticular support structure or the like, which support structure is permeable to the fluid flow to be filtered. Such reticular support structures, consisting, e.g., of plastic, are also necessitated in non-woven electret filters if the filter element is given a wave or zig-zag shape to enlarge the filter element surface area exposed to the flow. Usually, such filter material layers are mechanically connected to the reticular support structure. The thus formed filter element is placed in the flow to be filtered in such a manner that - when seen in the flow direction - the electret filter layer is placed before the reticular support structure so that the electret filter layer is pressed against the reticular support structure by the fluid flow. This causes a decrease of the volume of the electret filter available for filtering, which in turn leads to a reduction of the storage or filtering capacity of the filter element. In a wave- or zig-zag-shaped filter element, the electret filter forms the surface of two faces forming a V-shaped filter element region. Since the electret filter layer comprises a non-woven material, individual fiber ends protrude into said V- shaped filter region and cause microturbulences in the flow; for this reason, the degree of the folding of the

filter element is limited as the individual filter element portions or folding portions of the filter element are typically oriented at a comparatively large angle to each other. A zig-zag-shaped filter element known from German Utility Model 89 01 798.6 comprises a particle filter ply and an absorber filter ply arranged behind the particle filter ply when seen in the flow direction. The particle filter ply has two electret filter layers with an air-permeable grid interposed between the two plies. A thin, sheet-like carrier body with attached absorber particles is behind the downstream second electret filter layer. Each of the particle filter ply and the adsorber filter ply is folded in a zig-zag manner, with the individual folds of each ply engaging each other so that the carrier body of the adsorber filter ply is in face-to-face abutment with the second electret filter layer of the particle filter ply. Because of the thin, air-permeable carrier body of the adsorber filter ply, the "compound" structure of the particle filter ply and the adsorber filter ply can be given a pleating. Thus, the carrier body of the adsorber filter ply is inherently stable limiting the volume of the second electret filter layer in the flow direction. The first electret filter layer is pressed against the grid support structure of the particle filter ply by the oncoming flow of air, and the second electret filter layer is pressed against the thin carrier body of the adsorber filter ply. This leads to condensed areas in the two electret filter layers and thus to a limitation of the take-up capacity of the particle filter ply.

Fiber ends protrude from the surface of the electret filter layers. Since the first electret fiber layer in flow direction is exposed to the oncoming air flow and forms the surface of the V-shaped recesses of the zig-zag-shaped filter element, the protruding fiber

ends cause air turbulences. These very small air turbulences impair the flow characteristics of the filter element, which has a negative effect on the filtering of air. Due to these air turbulences, the angles formed by the two side faces of a V-shamed recess of the zig-zag-shaped filter element must not be too small, so that the side faces do not adversely affect the air flow to be filtered.

It is the object of the invention to provide a filter element with improved filtering characteristics. For solving the above object, the invention provides that, in a filter element of the initially mentioned type, the reticular support structure is arranged before the electret filter layer when viewed in the flow direction of the gas and/or the liquid, with the non-woven material of the electret filter layer free to expand on the filter side facing away from the reticular support structure.

According to the invention, the reticular support structure for the non-woven material of the electret filter layer is the first component or layer of the filter material when seen in the flow direction. Therefore, the oncoming gas or the oncoming liquid will first contact the reticular support structure and, after passing therethrough, will then contact the electret filter layer. The non-woven material of the electret filter layer is connected to the reticular support structure by gluing, welding or in a mechanical manner and accordingly is held on the reticular support structure. Another property of the filter element of the invention is that the electret filter layer, while fluid (gas or liquid) is streaming therethrough, can freely expand to the side facing away from the reticular support structure, and become thicker. By this expansion, the volume of the electret filter layer becomes larger, which in turn increases the storage capacity of the filter element and thus considerably

lengthens the service life of the device. This is because, due to the enlarged distance of the individual fibers forming the electret filter layer, more space is available around the fibers for the binding of particles through the electrostatic charge of the fibers. Thus, in the invention, the electret filter layer has no layer, ply or other component arranged therebehind which might limit the expansion of the non- woven material. In the filter element of the invention, the first ply in the flow direction is not formed by the electret filter layer, but by the reticular support structure. The air-permeable reticular support structure provides that the number of fiber ends protruding from the surface of the filter element exposed to the fluid flow, is reduced as the web of the reticular support structure keeps down a large number of the protruding fiber ends of the non-woven material of the electret filter layer. Only the openings of the reticular support structure have fiber ends projecting therethrough; however, these fiber ends project beyond the surface of the reticular support structure only by a fraction of their length. Altogether, less fiber lengths protrude into the V-shaped recesses of the filter element if the latter has a zig-zag shape. This again offers the advantage that the filter element can be more densely folded, i.e., the individual filter element portions or the folding portions of the filter element can extend at a more acute angle With respect to each other. At an identical overall size of the filter element, it is thus possible to enlarge the surface exposed to the fluid flow if the filter element is folded in a tighter manner. Generally, the air resistance characteristics of the filter element are improved because the same air resistance is reached merely by increasing the filtering area, i.e., the face of the electret filter layer, said face being exposed

to the fluid flow. A larger surface area entails a larger volume of the electret materials and thus a larger storage capacity.

In a preferred embodiment of the invention, there is provided a cover layer for keeping the non-woven material of the electret filter layer together, being arranged on the side of the electret filter layer facing away from the reticular support structure and being attached, particularly by gluing, to the surface of the electret filter layer. This cover layer does not restrict the expansion of the non-woven material; instead, when the filter element has fluid passing therethrough, the cover layer will move away from the reticular support structure to the extent to which the non-woven material is expanded. The cover layer merely fulfills the function of keeping the non-woven material together to prevent individual fibers from becoming detached from the electret filter layer or torn out of the filter while fluid is passing through the filter element.

Preferably, the cover layer consists of a strengthened non-woven material. The strengthening of the non-woven material can be performed by the usual treating and processing techniques known to that purpose. Particularly, the cover layer consists of (staple) fibers being multiply thermally bonded and randomly arranged. The cover layer has greater strength (against tearing) than the electret filter layer, whose non-woven material, even if strengthened, e.g., by needle tacking, are connected in a relatively loose manner as compared to the cover layer. This strengthened non-woven material has no inherent stability, i.e., the zig-zag-shaped folds of the filter element are maintained exclusively by the reticular support structure.

The reticular support structure, the non-woven material of the electret filter layer and, if provided.

the strengthened non-woven material of the cover layer preferably consist of polypropylene. This is advantageous especially with regard to recycling since all of the components consist of the same material. The filter element of the invention is inserted into the to-be-filtered fluid flow in such a manner that the reticular support structure is the first layer of the filter element to have the fluid passing therethrough and to come into contact with the fluid. The reticular support structure is then followed in the flow direction by the electret filter layer and - if provided - the cover layer. For maintaining the fluid flow, there is provided a suitable means, e.g., a blower or the like, for blowing the air through the filter element. The term "means" has a very wide meaning in the context of the invention; it is meant to comprises all devices and circumstances providing for a (fluid) flow passing through the filter element. Ultimately, such a means can also be the vehicle itself because the vehicle, provided with the filter element of the invention, generates an airflow while being driven, with the airflow streaming through the filter elemen .

Brief Description of the Drawings

An embodiment of the invention will be explained here-under in greater detail with reference to the drawings.

Fig. 1 is an exploded view of the layered structure of a filter element arranged in zig-zag folds.

Fig. 2 is a view of the filter element of Fig. 1 in the assembled condition, with the filter frame schematically indicated in the lower portion of the Figure.

Fig. 3 is a sectional view through a bending area of the zig-zag-shaped filter element with no gas passing therethrough.

Fig. 4 is a cross-sectional view through a bending area of the filter element while gas is passing therethrough.

Detailed Description of the Invention

According to Fig. 1, the filter element 10 comprises a reticular support structure 12 of plastic material arranged in zig-zag folds, a layer 14 of non- woven material serving as a filtering layer, and a cover layer 16 of strengthened non-woven material. When seen in the flow direction 18, these three layers are arranged one behind the other in the above order. The non-woven material layer 14 is comparatively thick and rather loose; the fibers of layer 14, preferably made from polypropylene, are connected to each other in a relatively loose manner. The cover layer 16, in contrast, is comparatively thin; the mutual connection of the fibers of cover layer 16 - which again are preferably made from polypropylene - is tight; in so far, this is a strengthened layer of fiber material which, however, has no inherent stability of shape. The zig-zag shape of filter element 10 is defined exclusively by the reticular support structure 12 with its zig-zag folds.

While the non-woven material layer 14 is connected to the reticular support structure 12, e.g., by bonding, the cover layer 16 is connected to the side of non-woven material layer 14 facing away from the reticular support structure 12. In the assembled condition, the filter element 10 is arranged as shown in Fig. 2. Filter element 10 comprises substantially linear filter element portions or folding portions 20 which are interrupted by bending areas 22. Respectively, two adjacent folding portions separated

from each other by a bending area 22, extend at an acute angle to each other so that V-shaped protrusions and recesses are formed. The surface provided in this manner is the surface of filter element 10 exposed to the oncoming air from the direction of arrow 18. On both of the longitudinal sides of filter element 10, there are arranged tensioning bands 26 connected to the longitudinal lateral edges of filter element 10, (i.e., at least to the reticular support structure 12 and, if desired, additionally to the nonwoven material layer 14) . Because of its zig-zag shape, filter element 10 can be stretched and compressed, respectively, in the direction of the twin arrow 28 in Fig.. 2. Stretching of the filter element 10 is limited by the tensioning bands 26 which are put under stress when expanded; in the stretched condition, the tensioning bands 26 provide for stability of shape of filter element 10 so that the zig-zag shape of filter element 10 is kept up. The tensioning bands 26 are attached to the side edges of filter element 10 over the complete length thereof by bonding or ultrasonic welding. The thus-assembled filter element 10 is inserted in a filter frame 30 as Fig. 2 indicates in a rough manner only.

Fig. 3 is a sectional view of the filter element 10 in a bending area 22 joined by folding portions 20 on both sides. Some of fiber ends of the non-woven material layer 14 protrude through the air-permeable reticular support structure 12 preferably made from polypropylene. This number of fibers projecting out of the filter element surface exposed to the oncoming flow of the medium is considerably less than if the non- woven material layer 14 were directly exposed to the flow. Accordingly, the folded portions 20 can be oriented at considerably more-acute angles to each other, while precluding that the fiber ends protruding into recess 24 have any noteworthy effect on turbulences in the flow. Thus, the volume of the non-

woven material available for filtering as well as the surface of the filter element are enlarged.

As can be seen in Fig. 4, the non-woven material layer 14 is widened while gas is flowing through the filter element. By the expansion of the non-woven material layer 14 in a direction leading away from reticular support structure 12, a larger volume of the non-woven layer can be used for filtering; thereby, the take-up capacity of the filter element is improved. However, the above increase of volume of the non-woven material layer 14 is possible only if the non-woven material is allowed to expand in the flow direction. The cover layer 16 will hardly, if at all, impair this process; instead, cover layer 16 will be "moved along" due to the enlargement of the volume of the non-woven material layer 14, the result being that the distance of cover layer 16 from reticular support structure 12 becomes larger when seen in flow direction 18.

Example

It is especially advantageous if the filter element as described above and shown in the Figures is arranged in the manner described hereunder and consists of the parts and materials indicated hereunder. As a material for the webs of the reticular support structure 12, there are selected fibers having a diameter of about 0.45 mm. The openings of the reticular support structure 58 are diamond-shaped and have a size of about 3.6 x 4.1 mm. The thickness (height) of the reticular support structure is about 0.85 mm. The fibers consist of polypropylene or other polymers. The non-woven material of the electret filter layer 14 consist of split fibers 10 by 40 microns in diameter. The 2 basis weight of the non- woven material is about 85 g/m ² . The fiber material comprises needle tacked fibers randomly arranged. The thus treated fiber material layer has a thickness of

2.0 mm. The fibers comprise polypropylene with electret characteristics provided by the known methods in the art. Connection between the electret filter layer 14 and the reticular support structure 12 is performed by gluing with an adhesive compatible with polypropylene. As materials for the electret filter layer, there can be used the products distributed under the designation 3M Filtrete™ or 3M SBMF by the Minnesota Mining and Manufacturing Company. Alternatives to the above-mentioned non-woven material would consist in a non-woven material obtained in a melt-blown process, or in circular fibers of 1 - 5 micron diameter.

The cover layer 16 comprises a non-woven spun- bonded material produced in a known manner from fibers being multiply thermally bonded and randomly arranged. The basis weight of this non-woven spun-bounded material is about 10 - 30 grams/ ² . Connection between the cover layer 16 and the electret filter layer 14 is effected by known adhesives. As materials for the fibers of cover layer 16, polyester, polyamide and preferably polypropylene are suitable. The thickness of the cover layer is preferably 0.22 mm.

The tensioning bands 26 comprise the same material as the cover layer and are produced in the same manner as these.

With such reticular support structures, electret filter layers and cover layers, there can be made filter elements, arranged in zig-zag folds, which, with an overall size of 100 x 300 mm, have a folding portion length of 25 mm and a folding, i.e., a distance from one bending line to another, of about 0.8 - 1.5 cm, preferably 1.0 cm. Thus, a V-shaped recess or projection area of the filter element is preferably 1.0 cm wide at the wider portion thereof facing away from the tip, with the length of the legs or the depth (height) being about 25 mm.

A filter element of the above type can receive or let through 300 m ³ /h of air to be filtered. The pressure drop will be in the range of 120 Pa. The novel construction allows reduction of this pressure drop by up to about 10 - 20 % with simultaneous enlargement of the filter surface and the filter material quantity. The resulting main advantage is to be seen in the lengthened life span of the filter element of the invention.