FILTER MEDIUM MADE OF SYNTHETIC POLYMER

Description

[0001] The invention relates to a filter medium made of synthetic polymer with a first layer of nonwoven fabric, which is produced by the melt-spinning process.

[0002] Nonwoven filter media of this type are used in filters, e.g., in room air filters and air conditioning systems, but especially in air filters for vehicle interiors or engines.

[0003] The nonwovens are usually produced after a primary forming process using a melt-spinning process such as a spun-bond process or a melt-blown process, as described, for example, in DE 41 23 122 A1.

[0004] The intake air of internal combustion engines, e.g., in motor vehicles or off-highway applications, is typically filtered to protect the engine's combustion chamber from mechanical damage caused by particles drawn in from the ambient air. An important criterion in the design of the filter elements is to ensure a long filter service life while maintaining high separation efficiency for the intake particles.

[0005] On the other hand, motor vehicles have a precisely calculated energy distribution system. Only a limited amount of energy is available for heating, ventilation, and air conditioning. Due to increasingly strict emissions regulations, these energy quantities must be reduced ever further, particularly in electric vehicles, where mechanical energy should be used only for propulsion if possible. The costs of vehicle components must also remain within a narrow range. On the other hand, vehicle buyers are placing ever greater demands on comfort and safety. From this perspective, particulate filters with the lowest possible drop or differential pressure are particularly important, as the fan motor only needs to generate a low pressure, thus keeping energy consumption to a minimum.Furthermore, due to the lower power required, it also operates more quietly, which reduces noise and thus significantly increases driving comfort.

[0006] The demand for filter systems with low differential pressure competes with the required separation efficiency and the required service life, i.e. the time expressed in mileage that a filter can remain in the vehicle until it needs to be replaced.

[0007] For example, pollen filters that only filter pollen from the incoming air are not sufficient for vehicle interiors. The allergens to which the immune system of allergy sufferers reacts are proteins whose diameter is only a fraction of the pollen diameter. They are in the size range around 0.1 micrometers, i.e. in the range that poses the greatest problems for particle filters, the so-called MPPS (Most Penetrating Particle Size). Accordingly, the separation efficiency in this size range should be at least 50%, measured using an aerosol whose particles have roughly the same density, e.g. sodium chloride. At the same time, such filters should achieve a service life of at least 30,000 km when installed in vehicles.

[0008] In common filters, such as ring filters or frame filters, the filter material is pleated in a zigzag pattern. To prevent adjacent filter surfaces of a pleat from collapsing, thereby reducing the surface area exposed to airflow, spacers are commonly used. Such spacers are typically achieved by embossing the filter material. However, embossing is only possible satisfactorily with paper filters. Filters made of PP, PET, or polyester, on the other hand, are very difficult or impossible to emboss.

[0009] While maintaining an acceptable pressure drop, the targeted use of additional electrostatic separation mechanisms offers the opportunity to increase separation efficiency in the MPPS range. Electret filters incorporate the additional component of electrostatics, meaning that particles are separated not only by mechanical separation mechanisms such as interception, inertia or impaction, diffusion, and gravity, but also by Coulomb, dipole, and image charge forces. This can lead to significant improvements in filtration performance.

[0010] As described in US Pat. No. 6,524,360 B2, electret filter media are typically manufactured by extruding a polymer. Electrostatic charging of the filter medium is achieved via a corona discharge or a triboelectric process. A corona discharge method for electrostatically charging an electret filter medium is known, for example, from US Pat. No. 4,588,537.

[0011] Furthermore, it is known to improve the electrostatic effect of the electret filter medium by adding additives, e.g., fluorine compounds, as described in US 6,808,551 B2.

[0012] However, common electret filters have low charge stability, which limits the duration of the electrostatic effect during the filtration phase. Furthermore, common electret filters lose their electrostatic effect when exposed to moisture.

[0013] DE 97 04 366 T2 discloses microfibers having an average diameter in the range of 0.5 to 30 µm (micrometers), preferably 0.5 to 10 µm (micrometers), and most preferably 1 to 5 µm (micrometers). Fibers within the broad range, but preferably within the narrow range, have a suitable diameter to remove particles in the size range of 0.1 to 20 µm (micrometers), which are the particles of the size removed by the filter nets. Nonwoven filter fabrics made from the microfibers have a weight between 0.0169 and 0.34 kg / m (0.5 and 10 oz / square yard), preferably between 0.024 and 0.10 kg / m (0.7 and 3 oz / square yard). The thickness of the fabric should be between 0.127 and 0.762 mm (5 to 30 mils). Considerable variations are possible within these limits.It is within the skill of the art to vary the architectural dimensions and features within these limits and to improve and ultimately optimize the filtration efficiency / flux density for the desired purpose. The microfiber filter fabrics can be used as part of a filter construction and can be laminated with a layer of another (generally melt-bonded) fabric, or they can take the form of a three-layer construction in which the melt-spun, particularly melt-blown, microfiber fabric forms the core, while the other melt-bonded fabrics form the outer layers. Melt-bonded nonwoven fabrics and their fibers, which are inherently suitable for such layered structures, can be manufactured by any suitable process, for example, by melt-spinning the polymer, as described in U.S. Patent Nos. 3,821,062 (Henderson), 3,563,838 (Edward), and 3,338,992 (Kinney).The various spunbonded fabric fibers of the layered structure should have a diameter of at least 20 µm (micrometers), which is significantly larger than the preferred microfiber diameter of 1 to 5 µm (micrometers) used for the inner fabric. The spunbonded layer provides mechanical strength and integrity, while the inner layer provides the desired microporosity and filtration properties.

[0014] WO 2011 / 002878 A2 discloses a spunbonded nonwoven fabric having a strength of less than 8.0% to about 4.0% and a ratio of effective fiber diameter to actual fiber diameter of at least 1.40, wherein the nonwoven fabric is substantially free of crimped fibers, gapped fibers and bicomponent fibers.

[0015] The object of the invention is to provide a filter medium with a long service life and high separation efficiency.

[0016] This object is achieved by a filter medium according to claim 1 and a filter according to claim 10.

[0017] The filter medium according to the invention has the advantage that the differential pressure, i.e. the pressure loss at a defined flow velocity, is low. Furthermore, the filter medium according to the invention has a high static charge capacity, whereby the electrostatic effect is particularly pronounced. The high electrostatic effect results in higher efficiency with lower pressure loss. While a filter medium with nanofibers achieves a separation efficiency of approximately 80% at a differential pressure of 110 Pa (Pascal), the filter medium according to the invention achieves a separation efficiency of 80% at a differential pressure of just 60 Pa. Furthermore, the filter medium according to the invention is rechargeable, i.e. the electrostatic effect can be regenerated if the static charge is lost due to a long period of inactivity or, for example, exposure to moisture.

[0018] A nonwoven fabric, as defined by the invention, is a nonwoven fabric in which fiber strands are laid on top of one another during the primary forming process and bonded together to form a nonwoven fabric. If necessary, a further step involves consolidation through calendering, thermobonding, hot air welding, and / or ultrasonic welding.

[0019] A polymer within the meaning of the invention is a pure polymer or a polymer mixture. In particular, it has a characteristic distribution of molecular chain length and / or a characteristic molecular structure.

[0020] Air permeability in the sense of the invention is the volume per square meter through which a fiber fleece flows at 200 Pa flow pressure per second.

[0021] Mass coverage or area weight within the meaning of the invention is the area-related mass and is determined in particular according to DIN EN 29073-1.

[0022] Applicable in the sense of the invention means that a layer or a filter medium can be electrostatically charged over a longer period of time, in particular at least for several days or months, and in particular that electrical charge can be stored on the layer or the filter medium.

[0023] The average pore size within the meaning of the invention is determined in accordance with the bubble point test according to the standards ASTM D6767, ASTM F316-0 and / or ISO 2942, ISO 4003, in particular using the Topas PSM 165 measuring device.

[0024] The separation efficiency in the sense of the invention is determined with NaCl particles, in particular with a size of 0.3µm to 0.5µm and according to DIN 71460-1, at an inflow velocity of 0.14m / s.

[0025] A fiber bundle within the meaning of the invention consists of several fiber strands.

[0026] Intertwined in the sense of the invention means that the fibers of two nonwoven layers are mixed and swirled together in such a way that the two nonwoven layers are firmly connected to one another.

[0027] Preferred embodiments are claimed in the subclaims.

[0028] In a preferred embodiment, the softness, elasticity, compressibility, best strength with puncture body, thickness, mass coverage, abrasion and / or tensile strength after a tensile test of the first layer of nonwoven fabric are such that at least one surface of the first layer of nonwoven fabric has a fluffy feel.

[0029] Tests have shown that a filter medium with a first layer of fleece with a fluffy feel significantly increases the electrostatic capacity of the filter medium. This improves the separation efficiency due to the electrostatic effect of the filter medium.

[0030] In a further preferred embodiment, the separation efficiency of the first layer of nonwoven fabric before application of the charge is substantially 5% to 50%, preferably 10% to 30%, more preferably 15% to 20% and after application of the static charge is substantially 50% to 95%, more preferably 60% to 90%, even more preferably 70 to 80% and most preferably 75%.

[0031] In a further preferred embodiment, the first nonwoven layer has a mass coverage of about 35 to 60 g / m 2< , preferably about 40 to 55 g / m 2< , particularly preferably about 45 to 50 g / m 2< and most preferably about 47.5 g / m 2<.

[0032] In a further preferred embodiment, the first layer of nonwoven fabric has a thickness of approximately 0.4 mm to 0.7 mm, preferably approximately 0.5 mm to 0.6 mm and most preferably approximately 0.55 mm.

[0033] These ranges of mass density and thickness have proven to be particularly advantageous with regard to the service life and differential pressure of the filter medium.

[0034] In a further preferred embodiment, the first layer of nonwoven fabric has an air permeability of approximately 800 l / m 2< s to 1300 l / m 2< s, preferably approximately 900 l / m 2< s to 1200 l / m 2< s and most preferably approximately 1000 l / m 2< s.

[0035] In a further preferred embodiment, the first layer comprises nonwoven fabric with an average pore size of more than 10 µm to 60 µm, preferably from 20 µm to 50 µm, more preferably from 30 to 40 µm and most preferably from 35 µm.

[0036] In a further preferred embodiment, the first nonwoven layer is produced at a polymer processing temperature of approximately 230°C to 280°C, preferably approximately 240°C to 270°C and most preferably approximately 250°C to 260°C.

[0037] This processing temperature has proven particularly advantageous for polymers with the claimed melt flow index.

[0038] In a further preferred embodiment, 50% to 100% and preferably 80% to 90% of the fibers of the first layer are arranged in fiber bundles of at least two fibers.

[0039] The fiber bundles increase the stiffness of the first layer of nonwoven fabric, improving its pleatability. Furthermore, the fiber bundles also ensure greater internal stability of the first layer of nonwoven fabric, preventing it from collapsing even at high air velocities. This ensures the nonwoven's open porosity and thus a low differential pressure.

[0040] In a further preferred embodiment, the first layer has a volume density between 0.01 g / cm 3 and 0.12 g per cm 3 , preferably between 0.03 g / cm 3 and 0.1 g / cm 3 and most preferably 0.07 g per cm 3 .

[0041] The low density enhances the haptic effect of the fluffiness of the fleece of the first layer, which further increases the electrostatic capacity and keeps the differential pressure at a low level.

[0042] In a further preferred embodiment, the fibers of the first layer predominantly have a fiber thickness of 3 µm to 10 µm and preferably of 4 µm to 5 µm.

[0043] Due to the comparatively high fiber thickness, e.g., compared to a filter medium with nanofibers, the filter medium according to the invention is relatively easy to manufacture. The spray nozzles in a spinning beam for the larger fiber thickness are significantly less susceptible, and other disruptive factors are also less influential in a production process for fibers with a larger fiber thickness.

[0044] According to the invention, the filter medium has a second layer of nonwoven fabric for stabilization, in particular for producing pleatability.

[0045] Due to the softness and low density of the first layer, it may be necessary for certain applications to add a second layer to the filter medium. The second layer need not have any or at least only minimal filtering properties. Its primary purpose is to stabilize the first layer of the filter medium.

[0046] This is also expressed by the fact that the second layer in a further preferred embodiment has a higher volume density (in particular) preferably from 0.12 to 0.2 g / cm 3< and even more preferably from 0.15 to 0.18 g / cm 3<.

[0047] In a further advantageous embodiment, fibers of the first layer of nonwoven fabric and fibers of the second layer of nonwoven fabric are interlaced in a boundary layer.

[0048] Due to the interlacing of the fibers, which is preferably achieved during the primary forming process, the first and second layers adhere to each other without any additional aids.

[0049] According to the invention, the second layer has thicker fibers than the first layer, in particular with a fiber thickness of 7 µm to 25 µm, preferably 10 µm to 20 µm and particularly preferably 15 µm to 18 µm.

[0050] Due to the thicker fibers, the second layer has greater stability.

[0051] In a further advantageous embodiment, at least one polymer, in particular a polyester, selected from the group consisting of polyethylene (PE), polyethylene terephthalate (PET), polycarbonate (PC), polyamide (PA), polybutylene terephthalate (PBT), polypropylene (PP) and / or polyvinyl chloride (PVC) is suitable for the synthetic first and / or second polymer.

[0052] In a further advantageous embodiment, 50% to 100%, preferably 80% to 100% and particularly preferably 80% to 90% of the fibers of the first layer and / or second layer are arranged in fiber bundles of at least two fibers.

[0053] According to the invention, the first layer comprises fiber bundles made of fibers of various synthetic polymers. In a further advantageous embodiment, the second layer comprises fiber bundles made of fibers of various synthetic polymers.

[0054] By forming bundles of fibers of different synthetic polymers, the electrostatic capacity of the material can be further increased.

[0055] In a further advantageous embodiment, the individual fibers and / or the fiber bundles of the first layer and / or second layer are arranged predominantly in parallel.

[0056] In a further advantageous embodiment, the first layer and / or the second layer is hydrophobic.

[0057] In a further advantageous embodiment, the first layer and / or second layer is designed in such a way that the filter medium has a differential pressure of less than 70 Pa, preferably less than 60 Pa.

[0058] According to the invention, several of the embodiments of the invention described above can also be combined with one another as desired.

[0059] The above and other advantages, features, and possible applications of the present invention will become apparent from the following descriptions of the preferred embodiments with reference to the drawings. These are shown as follows: Figure 1 is a partially schematic representation of a cross-section through a filter medium according to one embodiment; Figure 2a is a scanning electron microscope image and Figure 2b is a partially schematic representation of a cross-section through a filter medium according to a second embodiment of the invention; Figure 3a is a scanning electron microscope image and Figure 3b is a partially schematic representation of a pleated filter medium according to the second embodiment of the invention; Figure 4a is a scanning electron microscope image and Figure 4b is a partially schematic representation of an electron microscope image of a first layer of nonwoven fabric; Figure 5is a schematic representation of an electron microscope image of a first layer of nonwoven fabric; Figure 6a is a scanning electron microscope image and Figure 6b is a partially schematic representation of an electron microscope image of a first layer of nonwoven fabric; Figure 7a is a scanning electron microscope image and Figure 7b is a partially schematic representation of an electron microscope image of a cross-section through a filter medium according to the second embodiment of the invention; Figure 8 is a schematic representation of an electron microscope image of a second layer of nonwoven fabric according to the present invention.

[0060] Referring to Figure 1 A filter medium 1 is explained in more detail.

[0061] The filter medium 1 according to the invention comprises a first nonwoven layer 2, which is produced by a melt-spinning process. The first nonwoven layer 2 preferably consists of a polymer with a melt flow index of 20-200, preferably 40-100, and even more preferably 60. The melt flow index is determined according to DIN EN ISO 1133.

[0062] Preferably, the following typical test conditions are used to determine the MFI index of some plastics: Test load / kg Test temperature / °C 0,325 1,2 2,16 3,8 5 10 21,6 125 EVA 150 EVA 190 PE PE PE EVA POM PP WPC 200 PS 220 ABS SAN ASA 230 PP PMMA PP PMMI PVDF 235 PA -12 PA -12 PA -11 PA -11 PA -11 250 PBT 260 PBT PMP PMMI 275 PA 280 PPE / PS PET PPE / PS PPE / PS PPE / PS 300 PC PPE / PS PA -GF PPE / PS PPE / PS 315 PPS 330 PC PA6T 340 PC PEI 343 PSU 360 PES PPSU PSU 400 PES PPSU PEEK

[0063] The melt flow index is preferably determined via the melt mass flow rate with the unit g / 10min.

[0064] A first surface 3 of the nonwoven preferably has a fluffy or soft surface. This means that the surface 3 has a high degree of softness. This "softness" can be characterized by the softness, elasticity, compressibility, the best strength with a puncture body according to DIN ISO 12625-9, the thickness according to DIN ISO 12625-3, the mass coverage according to DIN ISO 12625-6:2005, the abrasion, and / or the tensile strength according to a tensile test according to DIN 12625-4. The fluffy feel can preferably also be determined by a test person feeling the surface structure of the first surface 3. The second surface 4 of the first layer 2 also preferably has a fluffy feel. However, the second surface 4 is particularly preferably rather smooth, since in a melt-spinning process for producing the filter medium 1, the first layer 2 is usually deposited on the second surface 4, whereby it takes on the shape of the substrate.

[0065] The volume density of the first layer 2 is preferably between 0.01 and 0.12 g / cm 3 ; particularly preferably between 0.03 and 0.10 g / cm 3 , and most preferably 0.07 g / cm 3 . This is a rather low density for a filter medium 1 of the type mentioned.

[0066] The fiber thickness of the fibers of the first layer is preferably 3 µm to 10 µm and particularly preferably 4 µm to 5 µm. Due to the relatively low fiber thickness of the first layer 2, a sufficiently large surface area of ​​the fibers is achieved to ensure an electrostatic capacity of the first layer 2. Preferably, the separation efficiency of the first layer of nonwoven fabric 2 before application of the charge is substantially 5% to 50%, preferably 10% to 30%, more preferably 15% to 20%, and after application of the static charge, it is substantially 50% to 95%, more preferably 60% to 90%, even more preferably 70 to 80%, and most preferably 75%. Further preferably, the first layer of nonwoven 2 has a mass coverage (basis weight) of about 35 g / m 2< to 60 g / m 2< , preferably of about 40 g / m 2< to 55 g / m 2< , particularly preferably of about 45 g / m 2< to 50 g / m 2< and most preferably of about 47.5 g / m 2<.Further preferably, the first layer of nonwoven fabric 2 has a thickness of approximately 0.4 mm to 0.7 mm, preferably approximately 0.5 mm to 0.6 mm, and most preferably approximately 0.55 mm. Further preferably, the first layer of nonwoven fabric 2 has an air permeability of approximately 800 l / m 2< s to 1300 l / m 2< s, preferably approximately 900 l / m 2< s to 1200 l / m 2< s, and most preferably approximately 1000 l / m 2< s. Further preferably, the first layer of nonwoven fabric 2 has an average pore size of more than 10 µm to 60 µm, preferably 20 µm to 50 µm, more preferably 30 µm to 40 µm, and most preferably 35 µm. Further preferably, the first layer of nonwoven fabric 2 is produced with a processing temperature of the polymer of approximately 230°C to 280°C, preferably approximately 240°C to 270°C and most preferably approximately 250°C to 260°C.

[0067] The respective limit values ​​only represent an approximate range and values ​​adjacent to the specified values ​​are also possible for an implementation.

[0068] In particular, an electrostatic charge is preferably applied to the first layer of nonwoven fabric 2 using a corona discharge or a triboelectric process step.

[0069] With reference to the Figures 2a and 2bthe structure of the first layer 2 is shown schematically, enlarged under the electron microscope. The individual fibers 6 are therefore preferably arranged for the most part in fiber bundles 7. The distances between the fibers 6 or the fiber bundles 7 from one another are relatively large, so that large spaces arise between the fibers. A large part of the fiber bundles 7 consists of two to four fibers, although more fibers can also make up a fiber bundle. Preferably, 80% to 100%, particularly preferably 80% to 90% of the fibers are arranged in fiber bundles. In one embodiment, all fibers 6 consist of one polymer. In a further embodiment, fibers made of different polymers are present. In a further preferred embodiment, fiber bundles 7 are present which consist of fibers 6 of different polymers and / or polymer mixtures.Tests have shown that polypropylene, which has an MFI according to the values ​​mentioned above, is particularly well suited for the first layer.

[0070] The fiber bundles 7 are created in the primary forming process of the melt spinning process by closely arranging the spray nozzles.

[0071] With reference to the Figures 3a and 3b A first layer 2 is shown again enlarged under an electron microscope. The magnification scale is shown in the Figures 3a and 3b higher than in the Figures 2a and 2b . Again, the individual fibers 6 as well as the fiber bundles 7 consisting of at least two fibers and preferably of two to four fibers are clearly visible.

[0072] Figure 4a shows a scanning electron microscope image and Figure 2bA schematic representation of a scanning electron microscope view of the first layer 2 with even higher magnification. Here, too, the fiber bundles and fibers are clearly visible.

[0073] Referring to Figure 5 A filter medium according to one embodiment of the invention is explained in more detail. The filter medium 1 now comprises a first layer 2 and a second layer 5. The configurations of the second embodiment can be combined with those of the first embodiment as far as technically possible.

[0074] Preferably, the fibers of the second nonwoven layer are interlaced in a boundary layer. This interlacing occurs during the master mold manufacturing process. The individual layers are sprayed from different spinnerets onto a substrate moving transversely to them. If the jets are sprayed directly one after the other or even into each other, a boundary layer is created in which fibers from both layers are present in different concentrations and in which the fibers are intermingled and swirled.

[0075] The first layer 2 and the second layer 5 preferably each consist of a synthetic polymer, of at least one polymer from the group polyethylene (PE), polyethylene terephthalate (PET), polycarbonate (PC), polyamide (PA), polybutylene terephthalate (PBT), polypropylene (PP), polyester and / or polyvinyl chloride (PVC).

[0076] The fibers may consist of the same synthetic polymer or of different polymers or of mixtures of different polymers.

[0077] The second layer 5 of the filter medium 1 preferably has a volume density of 0.12 to 0.20 g / cm 3 ; more preferably of 0.15 to 0.18 g / cm 3 , and thus preferably a higher density than the first layer 2. The fibers 8 of the second layer 5 are also preferably thicker than the fibers 6 of the first layer 2 and preferably have a fiber thickness of 7 µm to 25 µm, more preferably of 10 µm to 20 µm, and particularly preferably of 15 µm to 18 µm.

[0078] The respective limit values ​​only represent an approximate range and values ​​adjacent to the specified values ​​are also possible for an implementation.

[0079] The second layer 5 preferably has no electrostatic charge. However, it is particularly preferred to apply an electrostatic charge to the second layer 5. This preferably occurs in the same step of the manufacturing process as the charging of the first layer 2, particularly preferably by corona discharge or a triboelectric process.

[0080] The first layer 2 and / or the second layer 5 of the filter medium 1 are hydrophobic.

[0081] In particular, the filter medium 1 is preferably washable, whereby it can be regenerated.

[0082] With reference to the Figures 6a and 6ba scanning electron microscope image and a schematic of a scanning electron microscope image of the filter medium 1 according to the second embodiment of the invention are shown. The first layer 2 and the second layer 5 are clearly visible. The fibers of the individual layers are predominantly arranged in the same direction perpendicular to the surface of the filter medium 1. This alignment of the fibers 6 of the first layer 2 and the fibers 8 of the second layer 5 results in particular from the speed of a conveyor belt or substrate on which the polymer(s) is / are deposited during the primary forming process of the filter medium 1. The second layer 5 also has fiber bundles 9 formed from several fibers 8 of the second layer 5. Both the fibers 6 of the first layer 2 and the fibers 8 of the second layer 2 are preferably loosely combined and even more preferably connected to one another to form fiber bundles 7, 9. The connection is preferably already created during the primary forming process.It is also possible for individual fibers 6, 8 of a fiber bundle 7, 9 to be partially connected to one another, partially only loosely connected, and partially or partially separated from one another. The joining of the fibers 6, 8 to form fiber bundles 7, 9 can also occur in a later process step, in particular by heating.

[0083] In an intermediate region 10 between the first layer 2 and the second layer 5, fibers of the first layer 2 are preferably entangled and / or connected to fibers of the second layer 5, so that the first layer 2 and the second layer 5 adhere to one another.

[0084] Regarding the Figures 7a and 7bA schematic enlargement of the second layer 5 and a diagram of the same magnification under a scanning electron microscope are shown. The fiber bundles 9 consisting of two or more fibers of the second layer 5, 8 are clearly visible. Furthermore, compared to the first layer 2, more nodes 11 are visible, at which individual fiber bundles 9 meet and are preferably connected to one another. Preferably, 80% to 100%, particularly preferably 80% to 90% of the fibers of the first layer 2 and / or the second layer 5 consist of fiber bundles consisting of at least two fibers. The second layer 5 serves in particular to stabilize the filter medium 1. This is achieved by the thicker fibers 8.

[0085] In particular, the stabilization of the filter medium 1 ensures that the filter medium 1, as in Figure 8 shown, is pleatable. The second layer 5 supports the first layer 2.

[0086] Preferably, the second layer 5 has no or only a slight filtering effect, whereas the first layer 2 is preferably primarily responsible for the filtering effect of the filter medium 1. In another preferred embodiment, however, both layers 2, 5 serve as filter materials, wherein the layers 2, 5 are preferably designed such that different particle sizes can be filtered.

[0087] The resulting filter medium 1 has a separation efficiency of preferably approximately 5% to 20% without electrostatic charge and a separation efficiency of preferably approximately 60% to 90% with electrical charge. This represents a surprisingly high difference in separation efficiency for both embodiments with and without electrostatic charge and is based in particular on the structure of the first layer 2. Both embodiments shown are preferably electrostatically regenerable, i.e., an electrostatic charge can be reapplied to the filter medium 1 after the electrostatic charge has been lost. In contrast, the separation efficiency of the second layer 5 without the first layer 2 would only be approximately 40%.

[0088] The separation efficiency is determined using common measurement methods. Preferably, the separation efficiency is determined using sodium chloride particles of 0.4 µm at an inlet velocity of V = 0.14 m / s.

[0089] Referring to Figure 8 A pleated filter medium 1 is shown schematically. Due to the stabilization provided by the second, stiffer layer 5, the filter medium 1 can be easily folded, and the first surface 3 preferably serves as the flow-directed filter surface.

[0090] The effect according to the invention can also be achieved at the edges of the respective ranges. Furthermore, all parameter values ​​mentioned are average values, which may vary significantly at individual points within the respective layers. List of reference symbols:

[0091] filter medium 1 First layer of fleece 2 First surface 3 Second surface 4 Second layer of fleece 5 Fiber (first layer) 6 Fiber bundle (first layer) 7 Fiber (second layer) 8 Fiber bundle (second layer) 9 Intermediate area 10 Junction 11

Claims

1. Filter medium (1) made of a first synthetic polymer, comprising a first layer of fleece (2), which is produced by means of a melt-spinning process, wherein an electrostatic charge is applicable to the first layer of fleece (2), and a second layer of fleece (5) for stabilization made from the first and / or a second polymer, in particular for producing the pleatability, wherein the first synthetic polymer has a melt flow index of 20 to 200, preferably of 40 to 100 and more preferably of 60, determined according to DIN EN ISO 1133, wherein the second layer (5) comprises thicker fibres (8) than the first layer (2) and wherein the first layer (2) comprises fibre bundles of fibres of different synthetic polymers.

2. Filter medium (1) according to one of the preceding claims, wherein the first layer of fleece (2) has a mass coverage of about 35 to 60 g / m2, preferably of about 40 g / m2 to 55 g / m2, particularly preferably of about 45 to 50 g / m2 and most preferably of about 47.5 g / m2.

3. Filter medium (1) according to one of the preceding claims, wherein the first layer of fleece (2) has an air permeability of about 800 l / m2s to 1300 l / m2s, preferably of about 900 l / m2s to 1200 l / m2s and most preferably of about 1000 l / m2s.

4. Filter medium (1) according to one of the preceding claims, wherein the first layer has a volume density between 0.01 g / cm3 and 0.12 g / cm3, preferably between 0.03 g / cm3 and 0.10 g / cm3 and most preferably of 0.07 g / cm3.

5. Filter medium (1) according to one of the preceding claims, wherein the fibres (6) of the first layer (2) have a fibre thickness of 3 µm to 10 µm and preferably of 4 µm to 5 µm.

6. Filter medium (1) according to claim 1, wherein fibres of the first layer of fleece (2) and fibres (6, 8) of the second layer of fleece (5) are entangled with one another in a boundary layer.

7. Filter medium (1) according to one of claims 1 to 6, wherein the second layer of fleece (5) has a higher volume density than the first layer (2), in particular from 0.12 g / cm3 to 0.20 g / cm3, more preferably 0.15 g / cm3 to 0.18 g / cm3.

8. Filter medium (1) according to one of claims 1 to 7, wherein the second layer (5) comprises fibres (8) with a fibre thickness of 7 µm to 25 µm, preferably 10 µm to 20 µm and particularly preferably 15 µm to 18 µm.

9. Filter medium (1) according to one of the preceding claims, wherein the synthetic first and / or second polymer consists of at least one polymer from the group polyethylene (PE), polyethylene terephthalate (PET), polycarbonate (PC), polyamide (PA), polybutylene terephthalate (PBT), polypropylene (PP), polyester and / or polyvinyl chloride (PVC).

10. Filter with a filter medium according to one of the preceding claims.