Multi-layer filter medium

Abstract

The invention relates to a multi-layer filter medium (10) for insertion in a filter element, comprising: i) an electrically conductive first conductor layer (12), ii) a counter layer (14), wherein the counter layer (14) comprises: ii.a) an activated carbon layer (16), comprising activated carbon in a proportion by mass of 70 % or more, based on the mass of the activated carbon layer (16), and iii) a multi-layer intermediate layer (18) disposed between the first conductor layer (12) and the counter layer (14), wherein the intermediate layer (18) comprises: iii.a) a particle filter layer (20), comprising a first textile sheet-like structure, wherein the first textile sheet-like structure consists at least partially of an electret material, iii.b) a protective layer (22) disposed between the activated carbon layer (16) and the particle filter layer (20) and comprising a second textile sheet-like structure.

Description

[0001] Multi-layer filter medium

[0002] Description

[0003] The invention relates to a multi-layer filter medium for use in a filter element, in particular for use in an electrostatic precipitator with filter functionality, and to a corresponding filter element. Modern filtration technology today offers numerous efficient alternatives and further developments to the traditional methods for separating unwanted components from fluid streams. These methods are generally based on essentially mechanical separation, usually on a partially permeable filter element, which may, for example, be made of a porous material.

[0004] One of these developments, which is used particularly for gas purification, is so-called electrostatic precipitation, whereby the devices used are also known as electrostatic precipitators. In these electrostatic precipitators, the particles to be removed are first ionized by an electrode and then deposited on an oppositely charged collecting electrode. Such electrostatic precipitators are used primarily in industrial plants.

[0005] However, other requirements are placed on the performance characteristics of cabin air filters, particularly for use in vehicles, whereby, for example, the reliable separation of pollen and / or particulate matter and / or odors and / or exhaust gases is also desired.

[0006] To meet these requirements for cabin air filters, the concept of electrostatic separation, which in many cases is not strictly speaking filtration, was combined with mechanical separation processes to obtain high-performance filter elements. By combining conventional filter materials with an ionization device, the separation efficiency of filters can be significantly increased in many cases. For the purpose of clear identification, the corresponding filter elements are referred to as electrostatic precipitators with filter functionality within the scope of the present invention. However, it has been observed that the separation efficiency of such filter elements can decrease over the service life of the filter element, which is often attributed to a reduction in the electrostatic charge in the filter material over time.

[0007] To solve this problem, filter elements have been proposed that feature two electrically conductive layers separated from each other by an insulating material in the layered filter medium. These layers can be used to polarize the filter material in the filter element by applying a voltage through the creation of an electric field. In combination with an ionization device, this polarization enables efficient separation, which can advantageously usually be maintained over the entire service life of the filter element. Examples of corresponding filter elements or filter media, as well as further background information on the basic technology, are disclosed, for example, in WO 2007 / 135232 A1 or EP 3 448 540 B1.

[0008] Despite the potentially advantageous properties, the production and use of corresponding polarizable filter elements for electrostatic precipitators proves to be very challenging in practice. To ensure proper function, contact between the two layers in the filter medium acting as electrodes must be prevented. This is often made difficult by the fact that the layered filter media for use in electrostatic precipitators with filter functionality usually have to be folded to create a so-called pleated filter medium.

[0009] In addition to ensuring good processability, there is ongoing interest in the field of technology in optimizing the performance of the filter media used, particularly with regard to particle separation efficiency and the absorption of potentially harmful gaseous contaminants. Although various basic concepts for the construction of suitable filter media for such electrostatic precipitators with filter functionality are known in principle from the state of the art, there is great interest in the field of technology in further optimizing the designs and material selection in this very young field of technology - at least compared to conventional filtration.

[0010] The primary object of the present invention was to eliminate or at least mitigate the disadvantages of the prior art.

[0011] In particular, it was an object of the present invention to provide a high-performance filter medium which is particularly efficient when used in an electrostatic precipitator with filter functionality and with which particularly good separation performance can be achieved, particularly in combination with an ionization device, in particular with regard to the degree of separation of dust particles as well as of potentially harmful substances, wherein the advantageous degree of separation should desirably be guaranteed for a wide range of particle sizes.

[0012] It was a further object of the present invention that the filter media to be specified and the filter elements based thereon should show an advantageous differential pressure and a high air flow despite an excellent filter performance.

[0013] Furthermore, it was an object of the present invention that the filter media to be specified should be particularly mechanically resilient and should not exhibit undesirable delamination during production, particularly during folding, but also during subsequent operation, e.g., in a vehicle, and should be particularly resistant to unwanted short circuits. In this respect, a further object of the present invention was that the filter media to be specified and corresponding filter elements should exhibit an extended replacement interval compared to the prior art.

[0014] In addition, it was a supplementary task that the filter media to be specified should be as light and thin as possible while achieving high separation efficiency in order to achieve space and weight savings.

[0015] The inventors of the present invention have found that the objects described above can be achieved in a particularly advantageous manner if a multi-layer intermediate layer is used between two layers acting as electrodes for the polarization, one of which comprises an activated carbon layer particularly rich in activated carbon, which intermediate layer comprises a textile fabric made of a specific material, namely an electret material, as a particle filter layer, if a protective layer made of a textile fabric is arranged between the particle filter layer and the activated carbon layer, as defined in the claims.

[0016] The combination of the specifically selected features surprisingly results in a particularly high-performance filter medium which combines excellent separation efficiency with advantageous differential pressures and air passages at a low overall weight, while demonstrating high mechanical strength and good processability.

[0017] The above-mentioned objects are accordingly achieved by the subject matter of the invention as defined in the claims. Preferred embodiments of the invention emerge from the subclaims and the following statements.

[0018] Such embodiments, which are designated as preferred below, are combined in particularly preferred embodiments with features of other embodiments designated as preferred. Combinations of two or more of the embodiments designated as particularly preferred below are thus very particularly preferred. Likewise preferred are embodiments in which a feature of one embodiment designated as preferred to any extent is combined with one or more further features of other embodiments designated as preferred to any extent. Features of preferred filter elements result from the features of preferred filter media.

[0019] The invention relates in particular to a multi-layer filter medium for use in a filter element, in particular for use in an electrostatic precipitator with filter functionality, comprising: i) an electrically conductive first conductor layer, ii) a counterlayer, wherein the counterlayer comprises: ii.a) an activated carbon layer, comprising activated carbon in a mass fraction of 70% or more, based on the mass of the activated carbon layer, and iii) a multi-layer intermediate layer arranged between the first conductor layer and the counterlayer, wherein the intermediate layer comprises: iii.a) a particle filter layer, comprising a first textile fabric, wherein the first textile fabric consists at least partially of an electret material, iii.b) a protective layer arranged between the activated carbon layer and the particle filter layer, comprising a second textile fabric.

[0020] The multilayer filter media according to the invention are intended in particular for use in an electrostatic precipitator with filter functionality and demonstrate excellent separation performance in these. At least in principle, however, it is also conceivable to use a corresponding multilayer filter medium as the filter medium of another filter element that does not require electrostatic separation, although this is less preferred. The multilayer filter medium according to the invention initially comprises a first electrically conductive layer, which for this purpose is referred to as the conductor layer. This conductor layer consists of an electrically conductive material or is equipped with a corresponding electrically conductive material, resulting in a macroscopic electrical conductivity of the conductor layer.

[0021] The first conductive layer forms the first part of the layers that enable the polarization of the multilayer filter medium. The second part of the corresponding layer arrangement is the so-called counter layer, which is an electrically conductive layer or includes an electrically conductive layer.

[0022] The counterlayer comprises an activated carbon layer, which is described in more detail below. It is conceivable that the activated carbon layer itself functions as a second electrically conductive layer, in which case the activated carbon layer itself can be electrically contacted during later use in the filter element to achieve the desired polarization effect. Such a design is particularly suitable if the activated carbon layer is designed with good macroscopic electrical conductivity, for example, through particularly high activated carbon contents and / or the use of an electrically conductive binder. This can be achieved by selecting a suitable binder, for example, despite the phase or grain boundaries occurring in the material.

[0023] The counterlayer can consist of the activated carbon layer or comprise additional layers. At least in principle, it is possible for the activated carbon layer of the counterlayer to form the outermost layer of the multi-layer filter medium. This is particularly advantageous if the activated carbon layer has a high mechanical strength through the selection of a suitable adhesive or through the use of a foam structure.

[0024] However, with the generally preferred use of particulate activated carbon and relatively low binder contents, it is preferable to apply a cover layer to the side of the activated carbon layer facing away from the intermediate layer to ensure advantageous mechanical stability, even with direct electrical contact with the activated carbon layer. This cover layer covers the activated carbon layer and protects it, for example, from mechanical stress but also from the dissolution of activated carbon. In this case, a multi-layer filter medium according to the invention is preferred, wherein the counter layer additionally comprises: ii.b) a cover layer comprising a fifth textile fabric, wherein the activated carbon layer is arranged between the intermediate layer and the cover layer.

[0025] If the activated carbon layer is used for polarization, the cover layer can advantageously be designed as an insulating cover layer. Particularly preferred embodiments of the cover layer result from the features of preferred protective layers, as disclosed below.

[0026] However, both with regard to durability, in particular mechanical resistance, and the achievable filter performance, in particular with regard to the most efficient and reliable polarizability possible, it has proven particularly advantageous to equip the activated carbon layer in the counter layer with a dedicated additional conductive layer which, in particularly preferred embodiments, corresponds in terms of design to the electrically conductive first conductor layer.

[0027] For the vast majority of applications, a multi-layer filter medium according to the invention is preferred, wherein the counter layer additionally comprises: ii.c) an electrically conductive second conductor layer, wherein the activated carbon layer is arranged between the intermediate layer and the second conductor layer.

[0028] In practice, the person skilled in the art readily distinguishes between an electrically conductive material and an electrically insulating material based on his or her expert and clear technical understanding of these terms. In this respect, a material is considered to be electrically conductive within the scope of the present invention in particular if its specific electrical resistance at 20 °C is 100,000 Ω mm 2 / m or less, preferably at 1000 Q mm 2 / m or less, particularly preferably at 100 Q mm 2 / m or less. Conversely, a material is considered to be an electrically insulating material within the scope of the present invention if its specific electrical resistance at 20 °C is 10 8 Q mm 2 / m or more, preferably at 10 10 Q mm 2 / m or more, particularly preferably at 10 12 Q mm 2 / m or more. Accordingly, metals and alloys, but also carbon blacks, in particular conductive carbon black, and plastics made conductive by additives, are electrically conductive materials, whereas typical, unmodified plastics such as PET or PE are also electrically insulating materials, as are most glasses and ceramics. The person skilled in the art will understand that the electrical conductivity of conductor layers, for example the first conductor layer, can in many cases, for example when using composite materials, result from the presence of conductive material paths that run in an otherwise largely insulating material. For example, a dielectric fleece whose fibers are provided with a thin layer of conductive carbon black can be an electrically conductive layer, even though the macroscopic specific resistance of the composite material is relatively high due to the dielectric carrier material.

[0029] The first conductor layer and the counterlayer, in particular a second conductor layer possibly arranged in the counterlayer, serve to apply a voltage between them, which enables polarization of the multilayer filter medium. According to the inventors, a particularly promising option for designing the conductor layers is to provide the underlying layers of the intermediate layer or the activated carbon layer with a graphene layer acting as a conductor layer, which provides the necessary electrical conductivity. Information on the technological background and suitable methods for applying corresponding graphene layers are disclosed, for example, in WO 2021 / 239663 A1. The use of graphene layers can advantageously produce particularly light and thin filter media, which are particularly suitable for weight-optimized high-performance applications.Since, in the inventors' estimation, corresponding graphene layers can only make an insignificant contribution to the overall mechanical stability of corresponding filter media, it is, in the inventors' estimation, the use of the protective layer provided for in the invention that enables the efficient use of such graphene layers in the first place. However, due to the small contribution to overall mechanical stability, the inventors propose that advantageously only one conductor layer should be designed as a graphene layer. According to the inventors' findings, such a graphene layer can be applied particularly efficiently to the particle filter layer of the intermediate layer, which is preferred over application to the activated carbon layer.For certain weight-optimized applications, a multilayer filter medium according to the invention is preferred, wherein the first conductor layer and / or the second conductor layer, preferably the first conductor layer, particularly preferably the first conductor layer contacting the particle filter layer, comprises a graphene layer, preferably consists of a graphene layer. Particularly preferred is a multilayer filter medium according to the invention, wherein the graphene layer is produced or can be produced by coating a carrier layer, preferably the particle filter layer, with an aqueous dispersion comprising: x) graphene, preferably graphene nanoplatelets, in a combined total weight in the range from 1 to 65 g / L, y) binder, in particular polymeric binder, in a combined total weight in the range from 10 to 500 g / L, and z) wetting agent, in a combined total weight in the range from 1 to 50 g / L.

[0030] Although the use of thin graphene layers is a promising alternative, particularly for weight-optimized high-performance applications, the inventors consider it advantageous in the vast majority of cases if the conductor layers are formed by textile fabrics. It is theoretically possible for the textile fabrics to consist of an electrically conductive material, for example, by comprising fibers made of a conductive plastic. However, in the inventors' opinion, with a view to cost-efficient production and streamlining the required raw material base, it is particularly preferable to ensure the electrical conductivity of the corresponding textile fabrics by finishing them with a conductive material, for example, by coating or impregnation.Accordingly, a multi-layer filter medium according to the invention is preferred, wherein the first conductor layer comprises a third textile fabric, preferably a third textile fabric coated or impregnated at least in sections with an electrically conductive first material, and / or wherein the second conductor layer comprises a fourth textile fabric, preferably a fourth textile fabric coated or impregnated at least in sections with an electrically conductive second material.

[0031] The term "textile fabric" is clear to the skilled person and refers to two-dimensional textile products, which can be woven, warp-knitted, needle-punched, or knitted, for example. According to the skilled person's understanding, non-textile, fiber-containing fabrics, such as paper and cardboard, in which the fibers are primarily held together by binding agents, are not considered textile fabrics.

[0032] Even though a wide range of textile fabrics could in principle be considered for the textile fabrics of the conductor layers, the inventors have been able to achieve the best performance characteristics with so-called "non-woven" fabrics, and in particular with spunbonded fabrics. A corresponding textile fabric made of a non-conductive material can advantageously be achieved by equipping the textile fabric with a conductive material, whereby, in the opinion of the inventors, a variety of possible methods are suitable in principle. Therefore, a multi-layer filter medium according to the invention is preferred, wherein the third textile fabric and / or the fourth textile fabric, preferably the third textile fabric and the fourth textile fabric, is a nonwoven fabric, preferably a spunbonded fabric.A multi-layer filter medium according to the invention is preferred, wherein the third textile fabric and / or the fourth textile fabric, preferably the third textile fabric and the fourth textile fabric, consists of a plastic to a mass fraction of 80% or more, preferably 90% or more, particularly preferably 95% or more, very particularly preferably substantially 100%, wherein the plastic is preferably selected from the group consisting of polyacrylonitriles, polyolefins, polyesters and mixtures of these plastics, particularly preferably polyacrylonitriles, polypropylene, polyethylene terephthalate and mixtures of these plastics, especially preferably polyethylene terephthalate, based on the mass of the textile fabric.Additionally or alternatively, a multi-layer filter medium according to the invention is preferred, wherein the first conductor layer and / or the second conductor layer, preferably the first conductor layer and the second conductor layer, are produced or can be produced by equipping the respective textile fabric, in particular the nonwoven fabric, with the respective electrically conductive material using a process selected from the group consisting of impregnation, dip coating, in particular using a padding system, spray coating, application coating, in particular by means of a dispersion in the kiss-coat process, and print coating.Particularly preferred is a multi-layer filter medium according to the invention, wherein the electrically conductive first material and / or the electrically conductive second material, preferably the electrically conductive first material and the electrically conductive second material, is selected from the group consisting of metals, conductive plastics and carbon, preferably carbon, in particular carbon black or graphene, wherein the electrically conductive material is particularly preferably identical for the first conductor layer and the second conductor layer.Additionally or alternatively, a multi-layer filter medium according to the invention is also particularly preferred, wherein the first conductor layer and / or the second conductor layer, preferably the first conductor layer and the second conductor layer, as a result of the coating of the respective textile fabric, is electrically conductive over at least 80% or more of the area, preferably over 90% or more, particularly preferably over 95% or more, very particularly preferably over 98% or more, particularly preferably substantially over the entire area.

[0033] Even though the conductor layers can contribute to the filtration performance of the filter media, particularly when designed as textile fabrics, the inventors consider it advantageous if the conductor layers are designed with sufficient open pores so that they primarily restrict the air flow through the filter medium as little as possible. With regard to the weight of the conductor layers, at least when using textile fabrics, there is a trade-off between the influence on the overall weight of the filter medium and a beneficial stabilizing effect of the conductor layers on the filter medium, thus improving the mechanical properties. In this respect, the inventors have succeeded in identifying suitable basis weights for the textile fabrics that advantageously resolve this trade-off.Against this background, a multi-layer filter medium according to the invention is preferred, wherein the first conductor layer and / or the second conductor layer, preferably the first conductor layer and the second conductor layer, have an air passage in the range of 500 to 30,000 L / (m) at 200 Pa. 2 s), preferably in the range of 2000 to 20000 L / (m 2 s). Additionally or alternatively, a multi-layer filter medium according to the invention is preferred, wherein the third textile fabric and / or the fourth textile fabric, preferably the third textile fabric and the fourth textile fabric, has a basis weight in the range of 5 to 200 g / m 2 , preferably in the range of 10 to 150 g / m 2 , particularly preferably in the range of 15 to 50 g / m 2 , has.

[0034] Another important component of the counter layer is the activated carbon layer. Activated carbon is known to experts in the field of filtration technology as an important material and is commercially available from numerous manufacturers.

[0035] A key feature of the multilayer filter media according to the invention is that the activated carbon layer is formed from a high mass fraction of activated carbon, so that, for example, the proportion of fibers or other carrier materials in the activated carbon layer should be kept as low as possible. Based on the inventors' experiments, this achieves a particularly good separation effect or cleaning performance while simultaneously minimizing thickness or weight, with mechanical stability being enabled synergistically by the protective layer when using corresponding activated carbon-rich layers. The inventors have therefore found that particularly high-performance filter media can be obtained with the purest possible activated carbon layers, which preferably consist essentially exclusively of activated carbon apart from the binder.Accordingly, a multi-layer filter medium according to the invention is preferred, wherein the activated carbon layer comprises activated carbon in a mass fraction of 80% or more, preferably 90% or more, particularly preferably 95% or more, based on the mass of the activated carbon layer. Additionally or alternatively, a multi-layer filter medium according to the invention is also preferred, wherein the activated carbon layer comprises a mass fraction of less than 1%, preferably less than 0.1%, particularly preferably less than 0.01% of fibrous materials that are not binders. Particularly preferred is a multi-layer filter medium according to the invention, wherein the activated carbon is fixed in the activated carbon layer by a binder, wherein the activated carbon layer comprises activated carbon and binder in a combined mass fraction of 90% or more, preferably 98% or more, particularly preferably 98% or more, especially preferably 99% or more, based on the mass of the activated carbon layer.

[0036] The inventors have succeeded in identifying particularly preferred ranges for the selection of activated carbon and for the dimensioning of the activated carbon layer, which have resulted in excellent filter performance in their own experiments. A multi-layer filter medium according to the invention is preferred, wherein the activated carbon is particulate activated carbon, wherein the activated carbon preferably has an average diameter in the range of 0.1 to 4 mm, preferably in the range of 0.2 to 2 mm. Additionally or alternatively, a multi-layer filter medium according to the invention is preferred, wherein the activated carbon has a specific surface area of ​​500 m 2 / g or more, preferably 1000 m 2 / g or more, particularly preferably 2000 m 2 / g or more. Additionally or alternatively, a multi-layer filter medium according to the invention is also preferred, wherein the activated carbon layer has a basis weight in the range of 50 to 1000 g / m 2, preferably in the range of 100 to 600 g / m 2 , has.

[0037] Between the first conductor layer and the counterlayer there is arranged a multi-layer, i.e. at least two-layer, composite which is referred to as an intermediate layer in the context of the present invention. Those skilled in the art will understand that the multi-layer filter medium, in addition to the first conductor layer, the counterlayer and the intermediate layer, can also comprise further layers which contact the conductor layer or the counterlayer on the side facing away from the intermediate layer and which cannot be assigned to the intermediate layer, even if the inventors believe this is only advantageous in a few cases. In the context of the present invention, however, for the purpose of a simple definition, all layers arranged between the first conductor layer and the counterlayer are assigned to the intermediate layer, the structure of which is described in more detail below.

[0038] In the course of developing the present invention, it has become apparent that corresponding activated carbon-rich activated carbon layers, although advantageous in terms of filter effectiveness, can be associated with significant processing problems, particularly when pleating the corresponding multi-layer filter media. In particular, during pleating, it has frequently been observed that portions of the activated carbon can push through from the activated carbon layer into the adjacent layers, which, in filter media structures not according to the invention, can lead to undesirable short circuits or adversely reduced polarization voltages. For this reason, the inventors consider the use of a protective layer, as further disclosed below, to be essential, whereby particularly high activated carbon contents of the activated carbon layer can be advantageously realized in filter media according to the invention without unduly adversely affecting processability.

[0039] As explained above, the protective layer is of particular importance in filter media according to the invention. This protective layer is arranged as part of the intermediate layer between the activated carbon layer and the particle layer further characterized below and serves the particular purpose of preventing the activated carbon from pushing through into the particle layer. Furthermore, the protective layer, particularly due to its central arrangement in the multi-layer filter medium, allows the realization of filter media according to the invention with excellent mechanical properties, since the protective layer acts as a central carrier, which is particularly advantageous when using one or more graphene layers as conductor layers. According to the invention, the protective layer comprises a second textile fabric.In contrast to the textile fabrics used in preferred embodiments of the particle layer, as further disclosed below, the inventors' experiments for the protective layer, which inherently has different requirements, achieved particularly good results with spunbonded nonwovens. Accordingly, a multi-layer filter medium according to the invention is also preferred, wherein the second textile fabric is a nonwoven, preferably a spunbonded nonwoven.

[0040] A particularly advantageous influence of the protective layer on the polarizability of multi-layer filter media according to the invention was observed in those embodiments in which the protective layer is designed to be as electrically insulating as possible. In a synergistic manner, this has the advantage that typical, commercially readily available insulating plastics can be used and, in contrast to the conductor layers, no additional processing steps are necessary for the provision of an electrically conductive coating. Accordingly, a multi-layer filter medium according to the invention is preferred, wherein the specific electrical resistance of the material of the second textile fabric at 20°C is 10 8 Q mm 2 / m or more, preferably at 10 10 Q mm 2 / m or more, particularly preferably at 10 12 Q mm 2 / m or more. Additionally or alternatively, a multi-layer filter medium according to the invention is preferred, wherein the second textile sheet structure consists of a plastic to a mass fraction of 80% or more, preferably 90% or more, particularly preferably 95% or more, very particularly preferably substantially 100%, wherein the plastic is preferably selected from the group consisting of polyesters, polyamides, polyolefins and mixtures or combinations of these plastics, particularly preferably polyethylene terephthalate, polycaprolactam, polypropylene and mixtures or combinations of these plastics, in particular polyethylene terephthalate, based on the mass of the textile sheet structure.

[0041] With regard to the design of the protective layer, there is also a conflict of objectives between the desired high air permeability at low weight on the one hand and an advantageous mechanical stabilizing effect on the other. Due to the desired stabilizing effect of the protective layer and the desired protective effect against the activated carbon being pushed through from the activated carbon layer, the inventors propose that it is preferable to use at least somewhat higher surface weights than for the nonwovens possibly used in the conductor layers. A multi-layer filter medium according to the invention is preferred, wherein the protective layer has an air permeability in the range of 500 to 30,000 L / (m 2 s), preferably in the range of 2000 to 20000 L / (m 2 s). Additionally or alternatively, a multi-layer filter medium according to the invention is preferred, wherein the protective layer has a basis weight in the range of 15 to 400 g / m 2, preferably in the range of 50 to 200 g / m 2 , has.

[0042] Additionally or alternatively, a multi-layer filter medium according to the invention is also particularly preferred, wherein the protective layer has a Gurley flexural rigidity, determined according to ASTM D6125-97, in the range from 0.4 to 40 mN, preferably in the range from 0.5 to 20 mN, particularly preferably in the range from 0.6 to 10 mN.

[0043] As a particularly advantageous embodiment of filter media intended for later use in room air filters, the protective layer can be treated with active ingredients that exhibit an anti-allergenic or biocidal effect in order to enhance the cleaning and protective effect of the corresponding filter media. In this case, a multi-layer filter medium according to the invention is preferred, wherein the protective layer comprises one or more active ingredients selected from the group consisting of anti-allergenic and biocidal active ingredients.

[0044] In addition to the protective layer, which enables the use of corresponding activated carbon-rich layers in the counterlayer, the intermediate layer also comprises a particle filter layer, which, according to expert understanding, has the particular task of mechanically removing particles present in the filtered fluid, but also plays an important role in the desired polarization of the filter medium. For this purpose, the particle filter layer comprises a first textile fabric, which is preferably designed as a nonwoven. In contrast to the textile fabrics of the conductor layers and the protective layer, the inventors' experiments have shown that the use of spunbonded nonwovens is less preferred than that of other nonwovens.A multi-layer filter medium according to the invention is preferred, wherein the first textile fabric is a nonwoven fabric, wherein the first textile fabric is preferably not a spunbonded fabric, and / or wherein the first textile fabric is preferably selected from the group consisting of needle-punched nonwovens and meltblown nonwovens. Additionally or alternatively, a multi-layer filter medium according to the invention is preferred, wherein the particle filter layer has a basis weight in the range of 10 to 200 g / m. 2 , preferably in the range of 15 to 150 g / m 2 , particularly preferably in the range of 20 to 110 g / m 2 , has.

[0045] It is essential for the present invention that not just any material is used for the first textile sheet of the particle filter layer. Rather, the inventors have recognized that in order to obtain particularly high-performance filter media, which are optimized in particular for use in electrostatic precipitators with filter functionality, textile sheet materials must be used which comprise an electret material, wherein it is preferred if the first textile sheet material consists as largely as possible of the electret material. Accordingly, a multi-layer filter medium according to the invention is preferred, wherein the first textile sheet material consists of the electret material to a mass fraction of 70% or more, preferably 80% or more, particularly preferably 90% or more, very particularly preferably 95% or more, particularly preferably essentially 100%, based on the mass of the first textile sheet material.

[0046] However, in the inventors' experiments, good results were also achieved with a composite material in which a textile fabric, which as described above consists as largely as possible of an electret material and is dimensioned with regard to the basis weight as described above, for example with about 50 g / m 2 , as a first partial particle filter layer, is combined with a second partial particle filter layer made of a non-electret material as a carrier and needled thereto, wherein the carrier is expediently designed with a lower basis weight, for example with approximately 15 g / m 2. The resulting textile fabric comprises the electret material in absolute quantities that are comparable, for example, to the use of a pure electret fleece, but relatively comprises a smaller proportion of electret material. Against this background, the person skilled in the art understands that lower relative proportions of electret material can also be used if, for example, multi-component particle filter layers are used, provided that a somewhat higher total surface weight is then selected. Accordingly, additionally or alternatively, a multi-layer filter medium according to the invention is preferred, wherein the first textile fabric consists of the electret material to a mass fraction of 50% or more, preferably 60% or more, particularly preferably 70% or more, based on the mass of the first textile fabric, and wherein the particle filter layer has a surface weight in the range of 15 to 250 g / m 2, preferably in the range of 20 to 200 g / m 2 , particularly preferably in the range of 25 to 150 g / m 2 , has.

[0047] Electret materials are generally known to those skilled in the art of filtration technology and are commercially available from various manufacturers. Electret materials are materials with low electrical conductivity into which a largely permanently stored electrical charge is introduced or an essentially permanent alignment of the electrical dipoles in the material is created through the manufacturing process chosen and / or subsequent processing. Thus, electret materials exhibit an electric field, which is why electret materials are sometimes compared to permanent magnets. In the field of filtration technology, the use of electret materials from so-called electret filters is known. However, these rely precisely on the inherent electrical charge of the electret materials and do not implement additional electrical polarization, as occurs in an electrostatic precipitator with filter functionality.Due to the lower design effort, such electret filters are particularly suitable for simple applications in which no means for ionizing the gas and polarizing the filter media can or should be provided.

[0048] In this context, it should be noted that the occasionally encountered term "electret" for electrostatic precipitators is also misleading, since, unlike the filter media of the present invention, the electrostatic precipitators known from the prior art do not use electret materials, whereas classic electret filters dispense with the ionization of the gas and the polarization of the filter media, which are the basis of the electrostatic precipitator concept. In this respect, it was surprising to the inventors that such advantageous results could be achieved by combining elements from two very different approaches or fields of application, namely electrostatic precipitators and electret filters.This is especially true since the use of an electret material when using active polarization in the electrostatic precipitator may at first glance appear redundant, since the electrostatic charging of the electret materials in the electrostatic precipitator used in electret filters is intended to be generated by polarization anyway, so that the advantageous effects found were not to be expected. A multilayer filter medium according to the invention is preferred, wherein the electret material is a fibrous plastic material, wherein the plastic material is preferably selected from the group consisting of polyolefins, in particular polytetrafluoroethylene, polytetrafluoroethylene-propylene, polypropylene and polyethylene, polyesters, in particular polyethylene terephthalate, polycarbonates, polyamides, polyvinylidene fluoride, polyacrylonitrile, and polylactides.Additionally or alternatively, a multi-layer filter medium according to the invention is preferred, wherein the specific electrical resistance of the electret material at 20 °C is 10. 8 Q mm 2 / m or more, preferably at 10 10 Q mm 2 / m or more, particularly preferably at 10 12 Q mm 2 / m or more.

[0049] Electret materials can be further characterized by the process used to manufacture them. Various methods are used in the prior art, which are based in particular on injecting free charge carriers into the non-conductive base material or on achieving the most permanent orientation possible of the dipoles in this non-conductive base material, for example by producing them within a strong electric field. For example, it is possible to expose the textile fabrics to a plasma, in particular a low-temperature plasma such as a corona discharge, or to treat them with reactive oxygen species (ROS). In principle, exposure can take place after the textile fabric has been manufactured or even during production. The use of additional auxiliary materials, such as water or additives, is conceivable.An example of a multi-layer filter medium according to the invention is accordingly one in which the electret material is produced or can be produced by injecting free charge carriers into an insulating base material, for example by means of plasma discharge, particle irradiation, or orientation of dipoles in an insulating base material, for example by shaping the molten material in a strong electric field. Furthermore, production can also be carried out by means of so-called “hydrocharging,” in which an additive is added to the plastic, for example polypropylene, which, for example, has a high melamine content in its structural formula, in order to subsequently align the charge carriers in the plastic during fiber production by spraying with water. In this respect, a multi-layer filter medium according to the invention is preferred, wherein the electret material is produced or can be produced by a production process comprising one or more of the following steps:

[0050] - electrostatic charging of a starting material, preferably a starting film, and splitting the electrostatically charged starting material, and / or

[0051] - Forming the electret material from a melt by solidification in an electric field, and / or

[0052] - Charge separation using the triboelectric effect, and / or

[0053] - Charging by hydrocharging.

[0054] Particularly preferred is a multi-layer filter medium according to the invention, wherein the first textile fabric is a triboelectric nonwoven.

[0055] The inventors of the present invention have endeavored in their own experiments to identify those electret materials which are particularly suitable for use in filter media according to the invention. In doing so, the inventors have recognized that the suitability for use in filter media according to the invention can be advantageously expressed via the change in the separation efficiency or in the retention, which is measured between the starting material before and after a deliberate discharge of the electret material, which can be achieved by storage in isopropanol, since these variables can serve as a measure of the extent of the electret property. The particle layer is particularly well suited for use in filter media according to the invention if the quotient of the original separation efficiency divided by the separation efficiency after a corresponding discharge is 1.2 or more.if the quotient of the retention in the discharged state divided by the retention in the initial state is 1.2 or more. A multi-layer filter medium according to the invention is therefore preferred, wherein the particle filter layer: a) has a separation quotient A = AOutput / AEntiaden of 1.4 or more, and / or b) a retention quotient R = REntiaden / ROutput of 1.4 or more, wherein AOutput is the initial separation efficiency of the particle filter layer, wherein AEntiaden is the separation efficiency of the particle filter layer after discharge in isopropanol according to DIN EN 779:2012-10, wherein ROutput = 1 - AOutput and Rentiaden = 1 - AEntiaden, wherein the separation efficiency is determined according to DIN EN 779:2012-10 with a di-ethyl hexyl sebacate (DEHS) test aerosol for a particle size of 0.4 pm at a flow velocity related to the filter medium of 0.2 m / s.

[0056] The usefulness of distinguishing between the two different quotients, which are defined once by the separation efficiency and once by the retention, arises from the fact that the reductions in the separation efficiency in high-efficiency electret media and less high-efficiency electret media caused by discharge in isopropanol lead to a different relative change, whereby the use of the separation efficiency and the retention makes it possible to specify suitable particle filter layers for both high-efficiency and less high-efficiency media.A multi-layer filter medium according to the invention is preferred, wherein the particle filter layer has a deposition quotient A of 1.5 or more, preferably 2 or more, particularly preferably 4 or more, very particularly preferably 10 or more, particularly preferably 20 or more, and / or wherein the particle filter layer has a retention quotient R of 1.5 or more, preferably 2 or more, particularly preferably 4 or more, very particularly preferably 10 or more, particularly preferably 20 or more.

[0057] For high-efficiency particle layers, a multi-layer filter medium according to the invention is preferred, wherein the particle filter layer has an initial separation efficiency AOutput of 80% or more and a retention quotient R of 1.4 or more. For less high-efficiency particle layers, a multi-layer filter medium according to the invention is preferred, wherein the particle filter layer has an initial separation efficiency AOutput of less than 80% and a separation quotient A of 1.4 or more.

[0058] As in many areas of filtration technology, it is fundamentally desirable to achieve a high cleaning and separation efficiency with the subsequent filter elements. In the case of the present filter media, which are intended to be used in electrostatic precipitators, however, the special feature applies that for the above-mentioned definition of the separation efficiency, i.e. determined according to DIN EN 779:2012-10 with a diethylhexyl sebacate (DEHS) test aerosol for a particle size of 0.4 pm, a high system performance of the filter element can in practice only be achieved through the interaction of the ionizer with the polarized filter medium in the corresponding filter element. Accordingly, the filter media according to the invention can advantageously be designed such that they have a comparatively low separation efficiency for non-ionized test fluids, which is advantageously combined with advantageous air passages.However, in the opinion of the inventors, the relevant initial separation efficiency should not be chosen too low. In this respect, a multi-layer filter medium according to the invention is preferred, wherein the particle filter layer has an initial separation efficiency AOutput of 30% or more, preferably of 35% or more, and / or wherein the particle filter layer has an initial separation efficiency AOutput in the range of 30 to 70%, preferably in the range of 35 to 60%. A multi-layer filter medium according to the invention is preferred, wherein the particle filter layer has an air flow rate in the range of 200 to 8000 L / (m) at 200 Pa. 2 s), preferably in the range of 500 to 4000 L / (m 2 s).

[0059] Particularly for particle filter layers with relatively low separation efficiencies, particularly suitable electret materials can advantageously be characterized more easily via the absolute difference between the separation efficiencies. A multi-layer filter medium according to the invention is preferred, wherein the particle filter layer has an initial separation efficiency AOutput of 70% or less, preferably 60% or less, and wherein the particle filter layer has an absolute difference between AOutput (in %) and AEntiads (in %) of 7% or more, preferably 9% or more.

[0060] At least theoretically, it is conceivable that the filter medium according to the invention, in addition to the layers defined above, comprises further layers as part of the intermediate layer. In the opinion of the inventors, however, in view of the outstanding performance that can already be achieved through the use of the layers defined above and from the point of view of a desired weight optimization, it is particularly preferable to dispense with the use of further layers. Accordingly, a multi-layer filter medium according to the invention is preferred for particularly demanding applications, wherein the multi-layer intermediate layer comprises one or more further layers, for example layers of fine fibers with a fiber diameter of 5 pm or less, preferably 1 pm or less, for example produced by meltblown or electrospinning processes, preferably electrospinning processes, or microporous membranes.In most cases, however, a multi-layer filter medium according to the invention is preferred, wherein the multi-layer intermediate layer does not comprise any further layers arranged between the first conductor layer and the counterlayer. Additionally or alternatively, a multi-layer filter medium according to the invention is preferred, wherein the multi-layer filter medium consists of the first conductor layer, the counterlayer and the intermediate layer, preferably of the first conductor layer, the second conductor layer, the activated carbon layer and the intermediate layer, to a mass fraction of 95% or more, preferably of 98% or more, particularly preferably of 99% or more, and / or wherein the intermediate layer consists of the particle filter layer and the protective layer to a mass fraction of 95% or more, preferably of 98% or more, particularly preferably of 99% or more.

[0061] Based on the fact that it is preferable to avoid using further layers wherever possible, it follows that the layers defined above contact one another as directly as possible in particularly preferred embodiments. Accordingly, a multi-layer filter medium according to the invention is preferred, wherein the activated carbon layer contacts the protective layer, preferably the protective layer and the second conductor layer, preferably over the entire surface. Additionally or alternatively, a multi-layer filter medium according to the invention is also preferred, wherein the protective layer contacts the activated carbon layer, preferably the activated carbon layer and the particle filter layer, preferably over the entire surface. Again, additionally or alternatively, a multi-layer filter medium according to the invention is preferred, wherein the particle filter layer contacts the first conductor layer, preferably the first conductor layer and the protective layer, preferably over the entire surface.

[0062] Also preferred is a multi-layer filter medium according to the invention, wherein the layers of the multi-layer filter medium are at least partially, preferably predominantly, particularly preferably substantially all, bonded to one another, preferably by means of an adhesive.

[0063] The inventors have succeeded in identifying preferred dimensions for the filter medium according to the invention, which yield a filter medium that is excellently suited for use in interior filters, particularly vehicle interior filters. A multi-layer filter medium according to the invention is preferred, wherein the multi-layer filter medium has a total surface area of ​​1000 mm on one side. 2 or more, preferably 40000 mm 2 or more, particularly preferably 90000 mm 2or more. Additionally or alternatively, a multi-layer filter medium according to the invention is also preferred, wherein the multi-layer filter medium has an average thickness of 10 mm or less, preferably 8 mm or less, particularly preferably 6 mm or less, very particularly preferably 4 mm or less, at an applied pressure of 2.5 kPa. Additionally or alternatively, a multi-layer filter medium according to the invention is also preferred, wherein the multi-layer filter medium has a total basis weight of 2000 g / m 2 or less, preferably 1500 g / m 2 or less, particularly preferably 1000 g / m 2 or less. Additionally or alternatively, a multi-layer filter medium according to the invention is also preferred, wherein the multi-layer

[0064] Filter medium a total air flow at 200 Pa of 200 L / (m 2 s) or more, preferably 400 L / (m 2 s) or more, particularly preferably 600 L / (m 2s) or more. Additionally or alternatively, a multi-layer filter medium according to the invention is also preferred, wherein the multi-layer

[0065] Filter medium has a Gurley flexural rigidity, determined according to ASTM D6125-97, of 3 mN or more, preferably of 100 mN or more, particularly preferably of 200 mN or more.

[0066] The invention also relates to a filter element comprising a multi-layer filter medium according to the invention.

[0067] A filter element according to the invention is preferred, wherein the filter element is an electrostatic precipitator with filter functionality. Additionally or alternatively, a filter element according to the invention is preferred, wherein the filter element comprises a device for ionizing dust particles. Additionally or alternatively, a filter element according to the invention is thus also preferred, wherein the filter element is configured to cause or promote the separation of electrically charged particles, in particular ionized particles, upon application of a voltage between the first conductor layer and the second conductor layer as a result of polarization of the filter medium.

[0068] Particularly preferred is a filter element according to the invention, wherein the filter element is an air filter, preferably an interior air filter, particularly preferably an interior air filter for vehicles. Additionally or alternatively, preferred is also a filter element according to the invention, wherein the filter element contains the multilayer filter medium as a pleated filter medium.

[0069] The invention and preferred embodiments of the invention are explained and described in more detail below with reference to the accompanying figure. The figure shows: Fig. 1 shows a schematic cross-sectional view through a multi-layer filter medium according to the invention in a preferred embodiment.

[0070] Fig. 1 shows a schematic cross-sectional view through a multi-layer filter medium 10 according to the invention, which comprises a multi-layer intermediate layer 18 arranged between a first conductive layer 12 and a counter-layer 14 and bonded to them via an adhesive. In addition to the activated carbon layer 16, the counter-layer 14 comprises a second conductive layer 24. Both the first conductive layer 12 and the second conductive layer 24 are formed from a PET spunbonded fabric that has been made conductive by coating with conductive carbon black and whose basis weight is approximately 30 g / m 2 amounts.

[0071] In the example shown, the activated carbon layer 16 comprises particulate activated carbon with an average diameter of approximately 0.25 to 0.6 mm and a specific surface area of ​​approximately 2000 m 2 / g, wherein the activated carbon layer 16 has a combined basis weight of about 215 g / m 2 with about 15 g / m 2 attributable to the binding agent.

[0072] Arranged above the activated carbon layer 16 is the protective layer 22 of the intermediate layer 18, which in the example shown consists of a non-conductive polyester spunbonded nonwoven. Between the protective layer 22 and the first conductive layer 12 is also arranged the particle layer 20, which in the example shown in Fig. 1 is designed as a needle-punched nonwoven made entirely of an electret material, which is a triboelectric nonwoven.

[0073] Comparative experiments:

[0074] In the following, the invention and preferred embodiments of the invention are further explained and described with reference to comparative experiments.

[0075] Three multilayer filter media were manufactured by combining the layers listed in Table 1, using the materials listed in Table 2. Table 1 - Structure of the multilayer filter media

[0076] Table 2 - Materials

[0077] Key characteristics of the filter media produced in this way are summarized in Table 3, with the maximum polarization voltage being given as the average value of 8 produced filter media.

[0078] Table 3 - Characteristics of the filter media or assembled filters

[0079] The filter characteristics show that the filter elements E1 and E2 according to the invention were designed to be comparable to the non-inventive reference medium V1 in terms of application-relevant properties such as air permeability, thickness, and weight, allowing a good comparison of the effective separation performance. In this respect, the overall characteristics achieved are characteristic of an advantageously light and thin filter medium.

[0080] The separation efficiency of the filter media produced in this way was determined according to DIN EN 779:2012-10 using a diethylhexyl sebacate (DEHS) test aerosol at a face velocity of 0.2 m / s relative to the filter medium. The separation efficiency was determined both in the new state (AOutput) and after treatment with isopropanol according to DIN EN 779:2012-10 (Aentiaden). The data obtained are summarized in Table 4, with the separation efficiencies given for different particle sizes and as the overall separation efficiency.

[0081] Table 4 - Separation efficiency of the filter media

[0082] The separation efficiencies shown in Table 4 clearly demonstrate that the filter media according to the invention, even when new and outside of an electrostatic precipitator, achieve significantly higher separation efficiencies than the comparison filter medium V1. The fact that the relative advantages of E1 and E2 compared to V1 are significantly reduced after discharge demonstrates that this positive effect is due to the use of the electret material. The separation efficiency thus demonstrates a significant improvement over filter materials that also comprise an advantageously activated carbon-rich activated carbon layer, but do not rely on the use of an electret material in the particle layer.

[0083] The filter media were then installed in an electrostatic precipitator, demonstrating excellent processability. The separation efficiency was again determined according to DIN EN 779:2012-10 using a diethylhexyl sebacate (DEHS) test aerosol at a flow velocity of 0.2 m / s relative to the filter media. This involved polarization of the filter media and ionization of the test aerosol.

[0084] A uniform ionizer was used for all tests. The maximum possible polarization voltage was applied in each case. The measured separation efficiencies are again presented for the different particle sizes and as the overall separation efficiency.

[0085] Table 5 - Separation efficiency of the filter elements

[0086] The separation efficiencies shown in Table 5 clearly demonstrate that the filter media according to the invention achieve excellent separation efficiencies in electrostatic precipitators, even for a wide range of different particle sizes. The values ​​compiled in Table 5 demonstrate that particularly advantageous results can be achieved with filter media according to the invention in electrostatic precipitators with comparable characteristics.

[0087] Reference symbol

[0088] 10 multi-layer filter medium

[0089] 12 first conductor layer

[0090] 14 Counter layer 16 Activated carbon layer

[0091] 18 Intermediate layer

[0092] 20 particle filter layer

[0093] 22 Protective layer

[0094] 24 Second conductor layer

Claims

Claims 1. Multi-layer filter medium (10) for use in a filter element, comprising: i) an electrically conductive first conductor layer (12), ii) a counter layer (14), wherein the counter layer (14) comprises: ii.a) an activated carbon layer (16) comprising activated carbon in a Mass fraction of 70% or more, based on the mass of the activated carbon layer (16), and iii) a multi-layer intermediate layer (18) arranged between the first conductor layer (12) and the counter layer (14), wherein the intermediate layer (18) comprises: iii.a) a particle filter layer (20) comprising a first textile A sheet-like structure, wherein the first textile sheet-like structure consists at least partially of an electret material, iii.b) a protective layer (22) arranged between the activated carbon layer (16) and the particle filter layer (20), comprising a second textile sheet-like structure.

2. Multi-layer filter medium (10) according to claim 1, wherein the counter layer (14) additionally comprises: ii.c) an electrically conductive second conductor layer (24), wherein the activated carbon layer (16) is arranged between the intermediate layer (18) and the second conductor layer (24).

3. Multi-layer filter medium (10) according to one of claims 1 or 2, wherein the first conductor layer (12) and / or the second conductor layer (24) comprises a graphene layer.

4. Multi-layer filter medium (10) according to one of claims 1 to 3, wherein the first textile fabric is a nonwoven fabric.

5. Multi-layer filter medium (10) according to one of claims 1 to 4, wherein the first textile fabric consists of the electret material to a mass fraction of 70% or more.

6. Multi-layer filter medium (10) according to one of claims 1 to 5, wherein the particle filter layer (20): a) has a separation quotient A = AOutput / AEntiaden of 1.4 or more, and / or b) a retention quotient R = REntiaden / ROutput of 1.4 or more, wherein AOutput is the initial separation efficiency of the particle filter layer (20), wherein AEntiaden is the separation efficiency of the particle filter layer (20) after discharge in isopropanol according to DIN EN 779:2012-10, wherein ROutput = 1 - AOutput and REntiaden = 1 - AEntiaden, wherein the separation efficiency according to DIN EN 779:2012-10 with a diethylhexyl sebacate (DEHS) test aerosol for a particle size of 0.4 pm at a pressure applied to the filter medium (10) relative flow velocity of 0.2 m / s.

7. Multi-layer filter medium (10) according to one of claims 1 to 6, wherein the first textile fabric is a triboelectric nonwoven.

8. Multi-layer filter medium (10) according to one of claims 1 to 7, wherein the second textile fabric is a nonwoven fabric.

9. Multi-layer filter medium (10) according to one of claims 1 to 8, wherein the specific electrical resistance of the material of the second textile fabric at 20 °C is 10 8 Q mm 2 / m or more.

10. Multi-layer filter medium (10) according to one of claims 1 to 9, wherein the protective layer (22) has a Gurley flexural rigidity, determined according to ASTM D6125-97, in the range of 0.4 to 40 mN.

11. Filter element comprising a multi-layer filter medium (10) according to one of claims 1 to 10.

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