Filter medium

Abstract

The present invention relates to a filter medium, a method for the manufacture thereof and the use of the filter medium according to the invention. The filter medium according to the invention comprises at least two textile nonwoven layers which are joined to one another by needling. The needled composite is then subjected to an after treatment so that the holes present from the needling are reduced by at least 50%.

Claims

1. A filter medium comprising: a prefabricated first layer, wherein the prefabricated first layer comprises a first spunbonded nonwoven layer comprising synthetic polymer fibres; wherein the prefabricated first layer has a weight per unit area of 50 to 500 g/m.sup.2; wherein the synthetic polymer fibres of the prefabricated first layer have a linear density in the range of 0.7 to ?6 dtex; wherein the synthetic polymer fibres of the prefabricated first layer comprise a polyester; wherein the prefabricated first layer has no chemical binders; wherein the prefabricated first layer has an air permeability of 500-2000 l/m.sup.2 sec; and the prefabricated first layer comprises a linen embossing; a prefabricated second layer which is applied to at least one side of the prefabricated first layer, wherein the prefabricated second layer comprises a second spunbonded nonwoven layer of synthetic polymer fibres; wherein the prefabricated second layer has a weight per unit area of 50-500 g/m.sup.2; wherein the synthetic polymer fibres of the prefabricated second layer have a linear density in the range of 3.0 to ?15 dtex; wherein the synthetic polymer fibres of the prefabricated second layer have a diameter that is at least 10% greater than the synthetic polymer fibres of the prefabricated first layer; wherein the prefabricated second layer has no chemical binders; wherein the air permeability of the prefabricated second layer is greater than the air permeability of the prefabricated first layer; wherein one of the prefabricated first layer and the prefabricated second layer is consolidated using mechanical consolidation but not thermal consolidation and the other of the prefabricated first layer and the prefabricated second layer is thermally consolidated; wherein fibres of one or both of the prefabricated first layer and the prefabricated second layer comprise an antimicrobial finish; wherein the prefabricated first layer and the prefabricated second layer are coupled together by using mechanical needling of 20 to 100 punches per cm.sup.2 using punches that produce holes having diameters in the prefabricated first and second layers; and wherein at least some of the holes formed on an outer side of the prefabricated first layer that faces away from the prefabricated second layer by the mechanical needling are fully closed.

2. The filter medium according to claim 1, wherein the prefabricated first layer and the prefabricated second layer do not contain split fibres.

3. The filter medium according to claim 1, wherein the filter medium consists only of the prefabricated first layer and the prefabricated second layer.

4. The filter medium according to claim 1, wherein the prefabricated first and second layers consist of synthetic polymer fibres.

5. The filter medium according to claim 4, wherein the prefabricated first and second layers consist of bicomponent fibres.

6. The filter medium according to claim 1, wherein the prefabricated first and second layers comprise a respective first and second plurality of layers.

7. The filter medium according to claim 1, wherein the prefabricated first and second layers comprise thermoplastics.

8. The filter medium according to claim 1, comprising additives that reduce static or additives that enable electrostatic charging.

9. The filter medium according to claim 1, wherein the prefabricated first and second layers have no staple fibres and/or melt-blown fibres made of synthetic polymer materials.

10. The filter medium according to claim 1, wherein at least one of the prefabricated first and second layers comprises at least one of glass or mineral fibres.

11. The filter medium according to claim 1, comprising a housing.

12. The filter medium according to claim 1, wherein: at least one surface of the filter medium comprises no fibres protruding from the at least one surface.

13. The filter medium according to claim 1, wherein: the air permeability of the prefabricated second layer is between 550 and 2200 l/m.sup.2 s.

14. The filter medium according to claim 1, wherein: the polyester comprises polyethylene terephthalate.

15. The filter medium according to claim 1, wherein: the synthetic polymer fibres of one or both of the prefabricated first layer and the prefabricated second layer comprise bicomponent fibres having a first component of polyethylene terephthalate and a second component of modified polyethylene terephthalate.

16. A filter medium comprising: a prefabricated first layer, wherein: the prefabricated first layer comprises a first spunbonded nonwoven layer comprising synthetic polymer fibres; the prefabricated first layer has a weight per unit area of 50 to 500 g/m.sup.2; the synthetic polymer fibres of the prefabricated first layer have a linear density in the range of 0.7 to ?6 dtex; the prefabricated first layer has no chemical binders; the prefabricated first layer has an air permeability of 500-2000 l/m.sup.2 sec; and the prefabricated first layer comprises a linen embossing; a prefabricated second layer which is applied to at least one side of the prefabricated first layer, wherein: the prefabricated second layer comprises a second spunbonded nonwoven layer of synthetic polymer fibres; the prefabricated second layer has a weight per unit area of 50-500 g/m.sup.2; the synthetic polymer fibres of the prefabricated second layer have a linear density in the range of 3.0 to ?15 dtex; the synthetic polymer fibres of the prefabricated second layer have a diameter that is at least 10% greater than the synthetic polymer fibres of the prefabricated first layer; the prefabricated second layer has no chemical binders; the air permeability of the prefabricated second layer is greater than the air permeability of the prefabricated first layer; one of the prefabricated first layer and the prefabricated second layer is consolidated using mechanical consolidation but not thermal consolidation and the other of the prefabricated first layer and the prefabricated second layer is thermally consolidated; the prefabricated first layer and the prefabricated second layer are coupled together by using mechanical needling of 20 to 100 punches per cm.sup.2 using punches that produce holes having diameters in the prefabricated first and second layers; and at least some of the holes formed on an outer side of the prefabricated first layer that faces away from the prefabricated second layer by the mechanical needling are fully closed.

17. A filter medium comprising: a prefabricated first layer, wherein: the prefabricated first layer comprises a first spunbonded nonwoven layer comprising synthetic polymer fibres; the prefabricated first layer has a weight per unit area of 50 to 500 g/m.sup.2; the synthetic polymer fibres of the prefabricated first layer have a linear density in the range of 0.7 to ?6 dtex; the prefabricated first layer has no chemical binders; and the prefabricated first layer has an air permeability of 500-2000 l/m.sup.2 sec; a prefabricated second layer which is applied to at least one side of the prefabricated first layer, wherein: the prefabricated second layer comprises a second spunbonded nonwoven layer of synthetic polymer fibres; the prefabricated second layer has a weight per unit area of 50-500 g/m.sup.2; the synthetic polymer fibres of the prefabricated second layer have a linear density in the range of 3.0 to ?15 dtex; the synthetic polymer fibres of the prefabricated second layer have a diameter that is at least 10% greater than the synthetic polymer fibres of the prefabricated first layer; the prefabricated second layer has no chemical binders; the air permeability of the prefabricated second layer is greater than the air permeability of the prefabricated first layer; one of the prefabricated first layer and the prefabricated second layer is consolidated using mechanical consolidation but not thermal consolidation and the other of the prefabricated first layer and the prefabricated second layer is thermally consolidated; fibres of one or both of the prefabricated first layer and the prefabricated second layer comprise an antimicrobial finish; the prefabricated first layer and the prefabricated second layer are coupled together by using mechanical needling of 20 to 100 punches per cm.sup.2 using punches that produce holes having diameters in the prefabricated first and second layers; and at least some of the holes formed on an outer side of the prefabricated first layer that faces away from the prefabricated second layer by the mechanical needling are fully closed.

Description

DETAILED DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a filter medium in which the textile nonwoven layer (layer 1) and the further textile nonwoven layer (layer 2) were needled to one another. The punch holes are clearly identifiable and significantly frayed.

(2) FIG. 2 shows a filter medium in which the textile nonwoven layer (layer 1) and the further textile nonwoven layer (layer 2) have been needled to one another and subjected to the aftertreatment according to the invention. The number of identifiable punch holes is reduced significantly and the surface has almost no protruding fibres or fibre ends. The size of the punch holes still present is significantly reduced.

TEXTILE NONWOVEN LAYER (LAYER 1)

(3) Nonwovens of synthetic polymer fibres are used as textile nonwoven layer (layer 1), where the textile nonwoven layer can be formed from various synthetic polymer fibres. Preferably used are so-called bicomponent fibres (BiCo fibres). Furthermore, the textile nonwoven layer (layer 1) can also be constructed as multilayer per se. In this case, the individual layers can differ in regard to the selected various synthetic polymer fibres and/or have different fibre diameters.

(4) The nonwovens comprise staple fibre nonwovens, here in particular wet nonwovens as well as spun-bonded nonwovens or dry-laid nonwovens which are consolidated by means of thermal and/or mechanical consolidation but have no chemical binders.

(5) The nonwovens preferably comprise spunbonded nonwovens of endless synthetic fibres. Spunbonded nonwovens, i.e. so-called spunbonds, are produced by a random deposition of freshly melt-spun filaments. The filaments are endless synthetic fibres of melt-spinnable polymer materials, in particular based on thermoplastics

(6) Suitable polymer materials are, for example, thermoplastics, preferably polyamides such as, for example, polyhexamethylene diadipamide, polycaprolactam, aromatic or partially aromatic polyamides (aramids), aliphatic polyamides such as, for example, Nylon, partially aromatic or fully aromatic polyesters, polycarbonate (PC), polyphenylene sulfide (PPS), polyphenylene oxide (PPO), polystyrene (PS), polyvinylcarbazole (PVK), polyacetal (POM), polyarylether, polyarylsulfone, polyethersulfone, polymers having ether and keto groups such as, for example, polyetherketone (PEK) and poly-etherether ketone (PEEK), polyolefins such as, for example, polyethylene or polypropylene or polybenzimidazole. Particularly preferred are polyesters, polyolefins such as, for example, polyethylene or polypropylene or aromatic or partially aromatic polyamides (aramids), aliphatic polyamides such as, for example, Nylon.

(7) The spunbonded nonwovens preferably comprise or consist of melt-spinnable polyesters. In principle, all known types suitable for fibre production can be considered as polyester material. Such polyesters predominantly consist of building blocks which are derived from aromatic dicarboxylic acids and from aliphatic diols. Common aromatic dicarboxylic acid building blocks are the divalent radicals of benzene dicarboxylic acids, in particular of terephthalic acid and isophthalic acid; common diols have 2 to 4 C atoms, with ethylene glycol being particularly suitable. Spunbonded nonwovens consisting of at least 85 mol. % polyethylene terephthalate are particularly advantageous. The remaining 15 mol. % is then made up of dicarboxylic acid units and glycol units which act as so-called modifying agents and which enable the person skilled in the art to specifically influence the physical and chemical properties of the filaments produced. Examples for such dicarboxylic acid units are radicals of isophthalic acid or of aliphatic dicarboxylic acid such as, for example, glutaric acid, adipic acid, sebacic acid; examples of diol radicals having a modifying action are those of longer-chain diols, e.g. of propane diol or butane diol, of di- or triethylene glycol or, if present in small quantity, of polyglycol having a molecular weight of about 500 to 2000.

(8) Particularly preferred are polyesters containing at least 95 mol % polyethylene terephthalate (PET), particularly those comprising unmodified PET.

(9) The polyesters contained in the spun-bonded nonwovens preferably have a molecular weight corresponding to an intrinsic viscosity (IV) measured in a solution of 1 g polymer in 100 ml dichloroacetic acid at 25? C., of 0.6 to 1.4.

(10) In a further preferred embodiment of the invention, the nonwoven, in particular the spunbonded nonwoven, is a melt-binder-consolidated nonwoven, in particular based on bicomponent fibres, i.e. the consolidation is accomplished by means of a thermoplastic binder which is preferably present in fibre form or as a fibre component. The melt-binder-consolidated nonwoven therefore comprises carrier and hot-melt adhesive fibres and/or bicomponent fibres having carrier and binder components. The carrier and hot-melt adhesive fibres or components can be derived from any thermoplastic fibre-forming polymers and carrier fibres can furthermore also be derived from non-melting fibre-forming polymers. Such melt-binder consolidated spunbonded nonwovens are described, for example, in principle in EP-A-0,446,822 and EP-A-0,590,629.

(11) Examples for polymers from which the carrier fibres or carrier fibre components can be derived are polyacrylonitrile, polyolefins such as polyethylene or polypropylene, substantially aliphatic polyamides such as Nylon 6.6, substantially aromatic polyamides (aramids) such as poly-(p-phenylene terephthalate) or copolymers containing a fraction of aromatic m-diamine units for improving the solubility or poly-(m-phenylene isophthalate), substantially aromatic polyesters such as poly-(p-hydroxybenzoate) or preferably substantially aliphatic polyesters such as polyethylene terephthalate.

(12) The proportion of the two fibre types to one another can be selected within wide limits where it should be noted that the fraction of the hot-melt adhesive fibres is selected to be sufficiently high that due to adhesive bonding of the carrier fibres to the hot-melt adhesive fibres, the nonwoven acquires a sufficient strength for the desired application but on the other hand the required air permeability is ensured. The fraction of the hot-melt adhesive coming from the hot-melt adhesive fibres in the nonwoven is usually less than 50 wt. % (relative to the weight of the nonwoven).

(13) In particular, modified polyesters having a melting point reduced by 10 to 50? C., preferably by 30 to 50? C. with respect to the nonwoven raw material come into consideration as hot-melt adhesives. Examples of such hot-melt adhesives are polypropylene, polybutylene terephthalate or polyethylene terephthalate modified by condensation of longer-chain diols and/or of isophthalic acid or aliphatic dicarboxylic acids.

(14) The hot-melt adhesives are preferably introduced into the nonwovens in fibre form or in the form of so-called bicomponent fibres, wherein the previously designated materials for the carrier fibres form the mechanical strength and the previously designated materials for the hot-melt adhesive fibres form the second component of the bicomponent fibres which is used for the consolidation.

(15) Preferably carrier and hot-melt adhesive fibres are constructed from one polymer class. By this it should be understood that all the fibres used are selected from one substance class so that these can easily be recycled after use of the nonwoven. If the carrier fibres, for example, consist of polyesters, the hot-melt adhesive fibres will also be selected from polyesters or from a mixture of polyesters, e.g. as bicomponent fibres with PET in the core and a lower-melting polyethylene terephthalate copolymer as cladding; furthermore however bicomponent fibres constructed from different polymers are also possible. Examples for this are bicomponent fibres of polyester and polyamide (core/cladding).

(16) The single fibre titre of the carrier and the hot-melt adhesive fibres can be selected within the said limits.

(17) The fibres making up the nonwovens can have an almost round cross-section or also other shapes such as dumbbell-shaped, kidney-shaped, triangular or tri- or multilobal cross-sections. Hollow fibres and bi- or multicomponent fibres can also be used. Furthermore the hot-melt adhesive fibres or hot-melt adhesive component can also be used in the form of bi- or multicomponent fibres.

(18) The fibres forming the nonwoven can be modified by usual additives, for example, by antistatics such as soot or additives which enable an electrostatic charging. Furthermore, the fibres can have an antimicrobial finish.

(19) The synthetic polymer fibres forming the nonwoven preferably comprise no staple fibres and/or so-called melt-blown fibres of synthetic polymer materials.

(20) In addition to the said synthetic polymer fibres, additional glass fibres can also be present so that a mixture of glass and/or mineral fibres and synthetic polymer fibres is present as nonwoven-forming fibres.

(21) Instead of glass fibres, it is also possible to use mineral fibres based on alumosilicate, ceramic, dolomite fibres or fibres of vulcanites such as, for example, basalt diabase, melaphyre. Diabase (green stone) and melaphyre (so-called paleobasalts) can also be used.

(22) Among the glass fibres, the glass fibres used are not subject to any substantial restriction in regard to glass type so that in principle all glass types such as E glass, S glass, R glass and C glass can be used. For economic reasons E glass or C glass is preferred. Biosoluble glasses are particularly preferred.

(23) The glass fibres can be formed from filaments, i.e. infinitely long fibres or from staple fibres, the latter being preferred. The average length of the staple fibres is between 3 and 100 mm, preferably 6 to 18 mm. The staple fibres can also have different lengths.

(24) The diameter of the glass fibres lies between 0.5-15 ?m, preferably 8 to 15 ?m.

(25) The fraction of glass fibres in the textile nonwoven layer (layer 1) is up to max. 50 wt. %, preferably up to max. 30 wt. %, particularly preferably up to max. 10 wt. %.

(26) The weight per unit area of the textile nonwoven layer (layer 1) is between 50 and 500 g/m.sup.2, preferably 80 and 300 g/m.sup.2, in particular 100 and 250 g/m.sup.2.

(27) As already mentioned, the nonwovens comprise those which are consolidated by means of thermal and/or mechanical consolidation but which have no chemical binders. This consolidation is preferably accomplished by means of calendering with the result that the air permeability of 500 to 2000 l/m.sup.2 sec is set.

(28) In a preferred embodiment of the invention, the textile nonwoven layer (layer 1), preferably the spunbonded nonwoven after consolidation by means of a calender has a smooth or linen embossing.

Further Textile Nonwoven Layer (Layer 2)

(29) Nonwovens of synthetic polymer fibres are used or produced on the textile nonwoven layer (layer 1) as further textile nonwoven layer (layer 2).

(30) The further textile nonwoven layer (layer 2) preferably has a gradient with regard to the fibre diameter which decreases in the direction of the textile nonwoven layer (layer 1). This gradient is produced by various synthetic polymer fibres having different fibre diameters.

(31) The further textile nonwoven layer (layer 2) preferably comprises staple fibre nonwovens, here in particular wet nonwovens as well as spunbonded nonwovens or dry-laid nonwovens, but preferably spunbonded nonwovens of endless synthetic fibres. Spunbonded nonwovens, i.e. so-called spunbonds are produced by random deposition of freshly melt-spun filaments. The filaments are endless synthetic fibres of melt-spinnable polymer materials.

(32) Due to the manufacture, e.g. in the case of spunbonded nonwovens, the previously designated gradient can also be produced by a multilayer structure per se.

(33) In this case, the individual layers differ in regard to the selected fibre diameters and possibly also by use of various synthetic polymer fibres.

(34) The further textile nonwoven layer (layer 2) has no chemical binders.

(35) Suitable polymer materials for the further textile nonwoven layer (layer 2), in particular for spunbonded nonwovens are, for example, thermoplastics, preferably polyamides such as, for example, polyhexamethylene diadipamide, polycaprolactam, aromatic or partially aromatic polyamides (aramids), aliphatic polyamides such as, for example, Nylon, partially aromatic or fully aromatic polyesters, polycarbonate (PC), polyphenylene sulfide (PPS), polyphenylene oxide (PPO), polystyrene (PS), polyvinylcarbazole (PVK), polyacetal (POM), polyarylether, polyarylsulfone, polyethersulfone, polymers having ether and keto groups such as, for example, polyetherketone (PEK) and poly-etherether ketone (PEEK), polyolefins such as, for example, polyethylene or polypropylene or polybenzimidazole. Particularly preferred are polyesters, polyolefins such as, for example, polyethylene or polypropylene or aromatic or partially aromatic polyamides (aramids), aliphatic polyamides such as, for example, Nylon.

(36) The spunbonded nonwovens preferably comprise or consist of melt-spinnable polyesters. In principle, all known types suitable for fibre production can be considered as polyester material. Such polyesters predominantly consist of building blocks which are derived from aromatic dicarboxylic acids and from aliphatic diols. Common aromatic dicarboxylic acid building blocks are the divalent radicals of benzene dicarboxylic acids, in particular of terephthalic acid and isophthalic acid; common diols have 2 to 4 C atoms, with ethylene glycol being particularly suitable. Spunbonded nonwovens consisting of at least 85 mol % polyethylene terephthalate are particularly advantageous. The remaining 15 mol. % is then made up of dicarboxylic acid units and glycol units which act as so-called modifying agents and which enable the person skilled in the art to specifically influence the physical and chemical properties of the filaments produced. Examples for such dicarboxylic acid units are radicals of isophthalic acid or of aliphatic dicarboxylic acid such as, for example, glutaric acid, adipic acid, sebacic acid; examples of diol radicals having a modifying action are those of longer-chain diols, e.g. of propane diol or butane diol, of di- or triethylene glycol or, if present in small quantity, of polyglycol having a molecular weight of about 500 to 2000.

(37) Particularly preferred are polyesters containing at least 95 mol % polyethylene terephthalate (PET), particularly those comprising unmodified PET.

(38) The polyesters contained in the spun-bonded nonwovens preferably have a molecular weight corresponding to an intrinsic viscosity (IV), measured in a solution of 1 g polymer in 100 ml dichloroacetic acid at 25? C., of 0.6 to 1.4.

(39) In a further preferred embodiment of the invention, the nonwoven, in particular the spunbonded nonwoven, is a melt-binder-consolidated nonwoven, in particular based on bicomponent fibres, i.e. the consolidation is accomplished by means of a thermoplastic binder which is preferably present in fibre form or as a fibre component. The melt-binder-consolidated nonwoven therefore comprises carrier and hot-melt adhesive fibres and/or bicomponent fibres having carrier and binder components. The carrier and hot-melt adhesive fibres or components can be derived from any thermoplastic fibre-forming polymers and carrier fibres can furthermore also be derived from non-melting fibre-forming polymers. Such melt-binder consolidated spunbonded nonwovens are described, for example, in principle in EP-A-0,446,822 and EP-A-0,590,629.

(40) Examples for polymers from which the carrier fibres or carrier fibre components can be derived are polyacrylonitrile, polyolefins such as polyethylene or polypropylene, substantially aliphatic polyamides such as Nylon 6.6, substantially aromatic polyamides (aramids) such as poly-(p-phenylene terephthalate) or copolymers containing a fraction of aromatic m-diamine units for improving the solubility or poly-(m-phenylene isophthalate), substantially aromatic polyesters such as poly-(p-hydroxybenzoate) or preferably substantially aliphatic polyesters such as polyethylene terephthalate.

(41) The proportion of the two fibre types to one another can be selected within wide limits where it should be noted that the fraction of the hot-melt adhesive fibres is selected to be sufficiently high that due to adhesive bonding of the carrier fibres to the hot-melt adhesive fibres, the nonwoven acquires a sufficient strength for the desired application but on the other hand the required air permeability is ensured. The fraction of the hot-melt adhesive coming from the hot-melt adhesive fibres in the nonwoven is usually less than 50 wt. % (relative to the weight of the nonwoven).

(42) In particular modified polyesters having a melting point reduced by 10 to 50? C., preferably by 30 to 50? C. with respect to the nonwoven raw material come into consideration as hot-melt adhesives. Examples of such hot-melt adhesives are polypropylene, polybutylene terephthalate or polyethylene terephthalate modified by condensation of longer-chain diols and/or of isophthalic acid or aliphatic dicarboxylic acids.

(43) The hot-melt adhesives are preferably introduced into the nonwovens in fibre form or in the form of so-called bicomponent fibres, wherein the previously designated materials for the carrier fibres form the mechanical strength and the previously designated materials for the hot-melt adhesive fibres form the second component of the bicomponent fibres which is used for the consolidation.

(44) Preferably carrier and hot-melt adhesive fibres or carrier fibre and hot-melt adhesive fibre components are constructed from one polymer class. By this it should be understood that all the fibres used are selected from one substance class so that these can easily be recycled after use of the nonwoven. If the carrier fibres, for example, consist of polyesters, the hot-melt adhesive fibres will also be selected from polyesters or from a mixture of polyesters, e.g. as bicomponent fibres with PET in the core and a lower-melting polyethylene terephthalate copolymer as cladding; furthermore however bicomponent fibres constructed from different polymers are also possible. Examples for this are bicomponent fibres of polyester and polyamide (core/cladding).

(45) The single fibre titre of the carrier and the hot-melt adhesive fibres can be selected within the said limits.

(46) The fibres making up the nonwovens can have an almost round cross-section or also other shapes such as dumbbell-shaped, kidney-shaped, triangular or tri- or multilobal cross-sections. Hollow fibres and bi- or multicomponent fibres can also be used.

(47) Furthermore the hot-melt adhesive fibres or hot-melt adhesive component can also be used in the form of bi- or multicomponent fibres.

(48) The fibres forming the nonwoven can be modified by usual additives, for example, by antistatics such as soot or additives which enable an electrostatic charging. Furthermore, the fibres can have an antimicrobial finish.

(49) The synthetic polymer fibres forming the nonwoven preferably comprise no staple fibres and/or so-called melt-blown fibres of synthetic polymer materials.

(50) In addition to the said synthetic polymer fibres, additional glass fibres can also be present so that a mixture of glass and/or mineral fibres and synthetic polymer fibres is present as nonwoven-forming fibres.

(51) Instead of glass fibres, it is also possible to use mineral fibres based on alumosilicate, ceramic, dolomite fibres or fibres of vulcanites such as, for example, basalt diabase, melaphyre. Diabase (green stone) and melaphyre (so-called paleobasalts) can also be used. Glass fibres are preferred however as a result of their economic availability.

(52) Among the glass fibres, the glass fibres used are not subject to any substantial restriction in regard to glass type so that in principle all glass types such as E glass, S glass, R glass and C glass can be used. For economic reasons E glass or C glass is preferred. Biosoluble glasses are particularly preferred.

(53) The glass fibres can be formed from filaments, i.e. infinitely long fibres or from staple fibres, the latter being preferred. The average length of the staple fibres is between 3 and 100 mm, preferably 6 to 18 mm. The staple fibres can also have different lengths.

(54) The diameter of the glass fibres lies between 0.5-15 ?m, preferably 8 to 15 ?m.

(55) The fraction of glass fibres in the textile nonwoven layer (layer 2) is up to max. 50 wt. %, preferably up to max. 30 wt. %, particularly preferably up to max. 10 wt. %.

(56) The weight per unit area of the textile nonwoven layer (layer 1) is between 50 and 500 g/m.sup.2, preferably 80 and 300 g/m.sup.2, in particular 100 and 250 g/m.sup.2.

(57) The further textile nonwoven layer (layer 2) has a higher air permeability than the air permeability of the textile nonwoven layer (layer 1), this is preferably at least 10% higher, particularly preferably at least 50%. The air permeability of the further textile nonwoven layer (layer 2) is therefore preferably between 550-2200 l/m.sup.2 sec.

(58) The diameters of the fibres of the further textile nonwoven layer (layer 20 are preferably greater than the diameter of the fibres of the textile nonwoven layer (layer 1), particularly preferably by at least 10%.

(59) As already stated, the textile nonwoven layer (layer 1) and the further textile nonwoven layer (layer 2) are joined to one another by means of mechanical needling. As a result of the preceding method, holes are obtained at the punch points of the needles on the outer side of the textile nonwoven layer (layer 1) facing away from the further textile nonwoven layer (layer 2). These holes are, as already set out, appreciably reduced (number and size) or eliminated by means of the action of a heated surface, e.g. calender, on the layer side 1.

(60) The filter medium according to the invention is used in air/gas and liquid filtration, in particular in the automobile sector, in air-conditioning systems, interior filters, pollen filters, clean room filters, domestic filters and as oil filters and hydraulic filters. Preferably the filter medium is used for engine air intake filters which require a very good degree of separation.

(61) The filter medium according to the invention has a degree of separation of more than 99%, preferably more than 99.3%, in particular more than 99.5%, particularly preferably min. 99.8%.

(62) The subject matter of the present invention is therefore also filters, filter modules or cartridges which contain the filter medium according to the invention. Here the filters, optionally in pleated form are installed in housings or other enclosures. Corresponding configurations can be deduced, for example, from U.S. Pat. No. 5,883,501.

(63) General Measurement Methods:

(64) Separation Efficiency:

(65) The separation efficiency is tested by means of a filter test rig from Palas (Model MFP 2000) using ISO fine test dust (ISO 12103-1). The measurement is made at a flow rate of 0.33 m/s up to a final differential pressure of 2000 Pa.

(66) Dust Capacity:

(67) The dust capacity was tested by means of a filter test rig from Palas (Model MFP 2000) using ISO fine test dust (ISO 12103-1). The measurement is made at a flow rate of 0.33 m/s up to a final differential pressure of 2000 Pa.

(68) Air Permeability:

(69) The air permeability is determined in accordance with DIN EN ISO 9237.

(70) Weight Per Unit Area:

(71) The weight per unit area is determined in accordance with DIN EN ISO 29073-1.

(72) Determination of Nonwoven Thickness:

(73) The thickness is determined in accordance with DIN EN ISO 9073-2.

(74) Measurement of Fibre Diameter:

(75) The fibre diameter is determined in accordance with DIN EN ISO 1973 (as of 1995).

(76) The present invention is explained by means of the following examples without however being restricted to these.

EXAMPLE

(77) A textile spunbonded nonwoven layer (layer 1) based on bicomponent fibres (PET/mod. PET) having a titre of 1.7 dtex and a weight per unit area of 150 g/m.sup.2 and a further textile spunbonded nonwoven layer (layer 2) based on polyethylene terephthalate fibres (PET) having a titre gradient of 9.9 dtex and 5.6 dtex and having a weight per unit area of 160 g/m.sup.2 are supplied and needled to one another. The needle density is 41 punches/cm.sup.2. The needled composite is then supplied to a calender having two rollers, the calender gap is 1.4 mm. After the calender treatment the final composite has a thickness of 2.5 mm.

(78) The surface temperature of roller 1 of the calender, i.e. the roller which is in contact with the outer side of the textile nonwoven layer (layer 1) facing away from the further textile nonwoven layer (layer 2) is 210? C., the surface temperature of the opposite roller 2 is 70? C.

(79) The filter medium according to the invention is then tested with a filter test rig from Palas (Model MFP 2000) using ISO fine test dust (ISO 12103-1).

(80) The filter medium produced according to the invention was tested at a flow rate of 0.33 m/s up to a final differential pressure of 2000 Pa.

(81) The filter medium produced according to the invention showed an average mass-related separation efficiency of 99.8%, the specific dust capacity is 830 g/m.sup.2.

(82) The filter medium produced according to the invention is compared with a filter medium having an identical structure which however was produced without the thermal treatment by a calender according to the invention. The specific dust capacity was 980 g/m.sup.2 but the separation efficiency is only 99% as compared with 99.8% for the product according to the invention. The product not according to the invention therefore has a permeability a factor of 5 higher (permeability 1% vs. 0.2%).

(83) The filter medium produced according to the invention shows a significantly improved separation efficiency with only moderately reduced specific dust capacity.