DE20023273U1 - Composite filter for e.g. vacuum cleaner bag, comprises several non-prebonded tiers, each comprising filtration material, with tiers bonded together to form unitary stratified structure having boundary surfaces - Google Patents

Abstract

The composite filter comprises several non-prebonded tiers, each comprising at least one filtration material and are distinct from the adjacent tiers. The tiers are bonded together to form a unitary stratified structure having a boundary surface adapted to receive particulates entrained in air and another boundary surface adapted to discharge filtered air. An independent claim is also included for the production of the composite filter.

Description

VERSION TO BE ADDRESSED Multilayer filter and method for producing the same

Field of the invention

This invention relates to porous filter media for removing solid particles entrained in a moving gas stream. More particularly, it relates to a filter composition comprising a plurality of non-prebonded filter material layers arranged in preselected juxtaposition and bonded together to form a unitary layer structure useful for filtering particles from air.

background

Recently, the technology for filtering particles from gases has become highly advanced both in conventional applications such as consumer-oriented vacuuming of dirt and dust and in very demanding industrial applications such as removal of specific particle size fractions of a wide variety of contaminants from inert to biochemically sensitive. It is now well recognized that the contaminant particles in a gas stream can have a wide variety of sizes, geometric shapes, e.g. elongated and spherical, and chemical and physical compositions, e.g. odorless and odor-emitting particles.

Consequently, filtration technology has evolved to provide filter media that is customized to optimally filter specific portions of contaminating particles. This technology has also developed techniques for maximizing various performance characteristics of filters, such as maintaining a low pressure drop across the filter and increasing filter life, thereby extending the time between filter element replacements.

The traditional approach to achieve these goals has been to provide a multi-layer filter media consisting of separate, individually designed layers.

ten, each of which was designed to achieve primarily one and sometimes several specific filter functions. For example, a very open porous and thin scrim is often used to protect underlying filter layers from abrasion by fast moving, large and hard particles; a porous and bulky layer is typically used to capture or retain substantial amounts of mainly large particles, and a layer of low porosity and of ultrafine diameter filaments is usually designed to remove the smallest particles to increase filtration efficiency. From the wide selection available, separate filter layers are selected and combined in a preselected arrangement, then arranged as a group to form a multilayer and therefore multifunctional filter. The one or more adjacent layers may be bonded to one another, or the layers may be unbonded. Optionally, the individual layers may be sandwiched between covers, typically of paper, for structural integrity and ease of handling.

A disadvantage of the aforementioned multi-layer system for constructing multifunctional filters is that there is repeated machining of the filter media, which can be excessive. That is, the filter material in a given layer is first processed to form the single layer, then it is machined to assemble that layer into the multi-layer filter. Each step results in further compaction and coverage, even slight, of the final filter product. This tends to increase the pressure drop through the filter and reduce the dust holding capacity, thereby limiting the service life. It is desirable to have a multifunctional multi-component filter that can be manufactured with a minimum of filter media compaction and coverage.

Summary of the invention

Accordingly, the present invention now provides a multilayer filter according to claim 1.

Short description of the characters

Fig. 1 is a schematic representation of an inline process for manufacturing a multilayer filter according to a preferred embodiment of the present invention.

Fig. 2 is a schematic diagram showing a cross-section of an embodiment of the novel filter composition having a unitary layer structure of two layers.

Fig. 3 is a schematic diagram showing a cross-section of another embodiment of the novel filter composition having a unitary three-layer layer structure.

Fig. 4 is a schematic diagram showing a cross-section of another embodiment of the novel filter composition having a unitary four-layer layer structure.

Fig. 5 is a schematic diagram showing a cross-section of another embodiment of the novel filter composition having a unitary layer structure of five layers.

Fig. 6 is a schematic diagram showing a cross-section of another embodiment of the novel two-layer filter composition of Fig. 2 in combination with an adjacent filter layer.

Fig. 7 is a schematic diagram showing a cross-section of another embodiment of the novel three-layer filter composition of Fig. 3 in combination with an adjacent filter layer.

Fig. 8 is a schematic diagram showing a cross-section of another embodiment of the novel four-layer filter composition of Fig. 4 in combination with an adjacent filter layer.

Figure 9 is a schematic diagram showing a cross-section of another embodiment of the novel five-day filter composition of Figure 5 in combination with an adjacent filter layer.

Figure 10 is a schematic cross-sectional view showing the two-layer filter composition of Figure 6 bonded to an adjacent filter layer with an adhesive or ultrasonically bonded layer.

Figure 11 is a schematic cross-sectional view showing the three-day filter composition of Figure 7 bonded to an adjacent filter layer with an adhesive or ultrasonically bonded layer.

Figure 12 is a schematic cross-sectional view showing the four-layer filter composition of Figure 8 bonded to an adjacent filter layer with an adhesive or ultrasonically bonded layer.

Figure 13 is a schematic cross-sectional view showing the five-day filter composition of Figure 9 bonded to an adjacent filter layer with an adhesive or ultrasonically bonded layer.

Detailed description

The invention essentially provides a new filter composition comprising a plurality of stacked layers of filtration material bonded together to form a unitary layer structure. The composition of filtration material in any given layer is preselected to perform a desired filtering function. For example, fine (i.e., small diameter) and closely packed fibers may be selected to capture very small dust particles, such as those of about 5 microns or smaller. Electrostatically charged fibers may also be used to stop the passage of these and even smaller particles. Likewise, bulky, highly porous media designed to have a large dust holding capacity may be used to capture medium and large sized dirt particles.

The novel filter composition has the distinguishing characteristic that at least one and preferably all of the layers are not prebonded prior to and during stacking. The term "prebonded" as used herein means that a filter media composition, such as thermally bondable fusible fibers or adhesively bondable fibers, is treated in such a manner that the bonding mechanism is activated to thereby form a separate, free-standing, continuous and typically self-supporting web of said filter composition. Such a prebonded web can be mechanically processed by processes such as winding onto a roll, unwinding from a roll, cutting and the like. Thus, in one aspect of this invention, bonding of the at least one and preferably all of the layers to form the unitary structure is not begun until after stacking of all of the layers of a particular desired multi-layer filter structure has been completed. The resulting structure is a single body composed of different types of filtration material that appear as different layers when looking through the filter composition in cross section, as further explained in the description and drawings below.

As mentioned, the unitary structure is formed by forming a stack of layers of selected filtration materials. Since the layers are not prebonded, the components of each layer, i.e., fibers, granules, etc., are generally loosely laid down on the underlying layer by mechanical or air-laying techniques. Within a layer, the composition of filter material is largely uniform and there is a "fuzzy" interface between the layers, such as interface 36A in Fig. 2 and similarly shown as a dashed line in Figs. 3-13. Since it is desirable to provide a unitary structure, it is a feature of this invention that adjacent layers in a stack have different compositions. Nevertheless, a composition of a layer may be repeated in the stack, with at least one layer of a different composition being between layers of the same composition.

This structure differs from that of conventional multi-layer filtration media, which are formed by laminating a plurality of individual filter media layers, each of which has been pre-bonded to form a self-supporting web prior to forming the multi-layered structure. The term "layer" here means a tape that

made of non-prebonded filter material as one layer of a uniform layer structure. In contrast, a "layer" means a separate, prebonded, self-supporting web of filter material.

The unitary layer structure of the new filter composition provides a number of significant advantages over conventional filter media. In one aspect, the unitary layer structure can be made more bulky to provide greater dust holding capacity than a layered structure of individual prebonded layers having compositions corresponding to the layers of the unitary structure, respectively. This is because each part of the conventional filter media is compressed at least twice: once when the individual layer is formed by bonding and a second time when the individual layers are layered to form the filter.

In another aspect, the new structure may have less adhesive than conventional filters. This comes from the ability to dry lay successive layers on the stack without using adhesive at the interfaces. Although adhesives, such as latex adhesives, are typically used sparingly, each additional application, however small, adds to the coverage. Thus, in general, the more adhesive used, the higher the pressure drop across the filter and the faster a filter can be expected to become clogged with dust and dirt particles.

In a further aspect, the new structure preferably comprises one or more layers, each of which is very loose. That is, they have such a high porosity or have such a small amount of solid filter material that the layer is not self-supporting. In other words, the composition of the layer lacks sufficient structural integrity of its own to form a separate, free-standing web that could be used in a conventional multi-layered layered structure. Therefore, the new filter composition provides the ability to comprise one or more individually loose but highly functional layers in a uniform composition. Due to this uniform structure, the composition has suitable strength, stiffness and other properties to be useful as a filter. For example, a very highly porous and therefore individually loose layer in a stack of the new filter

composition to provide high dust holding capacity even though a standalone layer of the same composition could not be made for use in a conventional layered structure. Similarly, a very thin, structurally weak layer of ultrafine fibers may be included in a stack to produce high quality fine dust filtration even though the same composition may be too weak to form a layer by itself.

The looseness of a layer can be tested as follows. If the substance of the layer cannot be laid on a carrier, then wound onto a roll and unwound from the roll, the substance is said to be loose within the meaning of this invention.

The filter compositions of this invention can be used in vacuum cleaner bags and more generally in vacuum filters. By "vacuum filter" is meant a filter structure designed to operate by passing a gas, preferably air, which usually entrains dry, solid particles, through the structure. In this application, the convention has been adopted to draw the side faces and layers of the structure with respect to the direction of air flow. That is, for example, the filter inlet side is "upstream" and the filter outlet side or discharge side is "downstream". Sometimes the terms "before" and "behind" are used herein to refer to the relative positions of structural elements as upstream and downstream, respectively. Of course, there will be a pressure gradient across the filter during filtration, sometimes referred to as "pressure drop". Vacuum cleaners commonly use bag-shaped filters. Typically, the upstream side of a vacuum cleaner bag filter is the inside and the downstream side is the outside.

In addition to vacuum cleaner bags, the new filter composition can be used in applications such as heating, ventilation and air conditioning (HVAC) systems, vehicle cabin air filters, high efficiency (so-called "HEPA") and clean room filters, emission control bag house filters, respiratory filters, surgical face masks and the like. Optionally, the filter composition can be used in such applications with an additional carbon filter or particle-containing layer in series with the new filter composition, e.g. to absorb odors or toxic contaminants.

Furthermore, certain applications such as HEPA and clean room filters may utilize additional layers in series with the new composition, such as a low porosity polytetrafluoroethylene (PTFE) membrane disposed on an edge surface of a suitable unitary layer structure as a composite filter.

In this text, the notations "x" and "^�Lgr;" are used to denote multiplication and exponential operations, respectively.

The DIN 44956-2 test is used to determine the increase in pressure drop of five different examples of vacuum cleaner bag designs after a dust load of fine dust at the following levels: 0, 0.5, 1.0, 1.5, 2.0 and 2.5 grams.

Air permeability after fine dust loading test: The dust loading part of DIN 44956-2 is carried out with 0.5 gram increments from 0 to 2.5 g/(m ^A 2 x s) for seven bags of each sample. However, the pressure drop values were not recorded again. The maximum sustained air permeability values were then determined for the bags exhibiting the specified dust loading levels.

Filter material compositions referred to in this patent application are described in more detail below:

Standard vacuum cleaner filter paper

This material, sometimes referred to as "standard paper", has traditionally been used as a single layer where it provides dust filtration and containment as well as the strength and abrasion resistance required for a vacuum cleaner bag. This material is also strong enough to allow easy manufacture on standard bag making equipment. This paper is composed predominantly of unbleached wood pulp with 6-7% of a synthetic fibre such as polyethylene terephthalate (PET)-type polyester and is manufactured by the wet-laid process. The standard paper typically has a basis weight of 30-80 g/m ^A 2 and often around 50 g/m ^A 2. The PET fibres typically have a fineness of 1.7 dtex and lengths of 6-10 mm. This pa-

pier has an air permeability in the range of about 200-500 l/(m ^A 2 &khgr; s) and an average pore size of about 30 mm. However, the performance, determined according to the DIN 44956-2 test, is only about 86%. Another characteristic is that the pores quickly become clogged with dust and the dust storage capacity is further limited by the very thin paper thickness of only about 0.20 mm.

Spunbond nonwoven

A nonwoven made of spunbond polymer fibers can be used as a second filtration layer in the structure. The fibers can be made of any spunbondable polymer, such as polyamides, polyesters or polyolefins. The basis weight of the spunbond nonwoven should be about 10-100 g/m ^A 2 and preferably about 30-40 g/m ^A 2 . The spunbond nonwoven should have an air permeability of about 500-10,000 l/(m ^A 2 × s) and preferably about 2,000-6,000 l/(m ^A 2 × s), measured according to DIN 53887. The spunbond can also be electrostatically charged.

Scrim or carrier fleece

Scrim generally refers to a very open, porous paper or nonwoven with a light basis weight. The basis weight of the scrim is typically about 10-30 g/m ^A 2 and often about 13-17 g/m ^A 2. The scrim, sometimes referred to as a support fleece, usually has an air permeability of about 500-10,000 l/(m ^A 2 x s). It is used primarily to protect other layers or plies from abrasion. The scrim can filter even the largest particles. The scrim, like any other layer of the filter composition, can be electrostatically charged, provided that the material has suitable dielectric properties.

Wet-laid paper with high dust capacity

High dust capacity wet laid paper, often referred to here as "wet laid capacity paper", is bulkier, thicker and more permeable than standard vacuum cleaner bag filter paper. It performs several functions. These include resisting impact loading, filtering large dirt particles, filtering a significant proportion of small dust particles, storing or retaining

large amounts of particles while allowing air to flow through easily, thereby providing a low pressure drop at high particle loads, which increases the service life of the vacuum cleaner bag.

The wet-laid capacity paper usually comprises a fiber mixture of wood pulp fibers and synthetic fibers. It typically comprises up to about 70% wood pulp and correspondingly more synthetic fibers, such as PET, than the standard paper described above. It has a greater thickness than the standard paper of about 0.32 mm with a typical basis weight of 50 g/m ^A 2. The pore size is much larger, the average pore size can be larger than 160 mm. This means that the paper can hold much more dust in its pores before it becomes clogged. The basis weight of the wet-laid capacity paper is typically about 30-150 g/m ^A 2 and preferably about 50-80 g/m ^A 2.

The wet-laid capacity paper has a fine dust particle filtration performance of about 66-67%, determined according to DIN 44956-2. It is essential that the wet-laid capacity paper has an air permeability that is higher than that of the standard filter paper. The lower limit of the permeability should therefore preferably be at least about 500 l/(m ^A 2 × s), more preferably at least 1000 l/(m ^A 2 × s) and most preferably at least about 2000 l/(m ^A 2 × s). The upper limit of the permeability is defined to ensure that the paper filters and retains a majority of the dust particles larger than about 10 mm. Accordingly, the secondary or second high-performance filter medium arranged downstream can filter out and capture fine particles for a much longer period before a significant increase in pressure drop across or through the filter occurs. Accordingly, the air permeability of the wet-laid capacity paper should preferably be at most 8000 l/(m ^A 2 × s), more preferably at most about 5000 l/(m ^A 2 × s), and most preferably at most 4000 l/(m ^A 2 × s). It can be seen that the wet-laid capacity paper is exceptionally well designed as a multi-purpose filtration layer to be placed upstream of the secondary high-efficiency filtration layer.

Dry laid paper with high dust capacity

Dry-laid paper with high dust capacity, sometimes referred to here as "dry-laid capacity paper", was not used as a filter in vacuum cleaner bags. Dry-laid paper is not formed from a water suspension or slurry, but is produced using an air-laying technique and preferably a fluff pulp process. Hydrogen bonding, which plays a major role in mutual attraction of molecular chains, is not effective in the absence of water. Thus, for the same basis weight, dry-laid capacity paper is usually much thicker than standard paper and wet-laid capacity paper. For example, at the typical weight of 70 g/m ^A 2 the thickness is 0.90 mm.

The dry laid capacity paper structures can be bonded primarily by two methods. The first method is latex bonding, in which the latex binder is applied from water-based dispersions. Impregnation techniques such as spraying or dipping and squeezing (padder roll application), in both cases followed by a drying and heat curing process, can be used. The latex binder can also be applied in discrete patterns such as dots, diamonds, crosshatches or wavy lines by means of gravure rollers, followed by drying and curing.

The second method is thermal bonding, e.g., by using binder fibers. Binder fibers, sometimes referred to herein as "thermally bondable fusible fibers," are defined in the Nonwoven Fabric Handbook (1992 edition) as "fibers having a lower softening point than other fibers in the web. Under the application of heat and pressure, they act as a binder." These thermally bondable fusible fibers generally melt completely at locations where sufficient heat and pressure are applied to the web, causing the matrix fibers to adhere to one another at their crossover points. Examples include co-polyester polymers, which adhere to a wide range of fibrous materials when heated.

In a preferred embodiment, thermal bonding can be achieved by adding at least 20%, preferably up to 50%, of a bicomponent ("B/C") polymer fiber to the dry-laid web. Examples of B/C fibers

comprise fibers having a core of polypropylene ("PP") and a sheath of a more heat sensitive polyethylene ("PE"). The term "heat sensitive" means that the thermoplastic fibers become soft and sticky or heat fusible at a temperature 3-5 ^° C below the melting point. The sheath polymer should preferably have a melting point in the range of about 90-160 ^° C and the core polymer should have a higher melting point, preferably at least about 5 ^° C higher than that of the sheath polymer. For example, PE melts at 121 ^° C and PP melts at 161-163 ^° C. This aids in bonding the dry-laid web when it is passed between the nip of a thermal calender or in a through-air oven by obtaining thermally bonded fibers with less heat and pressure to produce a less compact, more open and breathable structure. In a further preferred embodiment, the core of the core/sheath of the B/C fiber is arranged eccentrically with respect to the sheath. The closer the core is arranged to one side of the fiber, the more likely the B/C fiber will curl during the thermal bonding step, thereby increasing the volume of the dry-laid capacity paper. This will of course increase its dust holding capacity. Therefore, in a further preferred embodiment, the core and sheath are arranged side by side in the B/C fiber and the bonding is achieved by means of a through-flow oven. A thermal calender, which would compress the web more than in through-flow bonding, is less preferred in this case. Other polymer combinations used in core/sheath or side-by-side B/C fibers include PP with low melting co-polyester polymers and polyester with nylon 6. The dry-laid high capacity layer can also be formed substantially entirely from bicomponent fibers. Other variations of bicomponent fibers may be used in addition to "sheath/core", such as the "side-by-side", "islands in the sea" and "orange" embodiments disclosed in "Nonwoven Textiles", Jirsak, �O., and Wadsworth, LC ₁ Carolina Academic Press, Durham, North Carolina, 1999, pp. 26-29, the entire disclosure of which is incorporated herein by reference.

In general, the average pore size of dry laid capacity paper is between the pore size of standard paper and wet laid capacity paper. The filtration efficiency, as determined by the DIN 44956-2 test, is about 80%. Dry laid capacity paper should have approximately the same basis weight and the same

Permeability as the wet-laid capacity paper described above, ie in a range of about 500-8000 l/(m ^A 2 &khgr; s), preferably about 1000-5000 l/(m ^A 2 &khgr; s) and most preferably about 2000-4000 l/(m ^A 2 &khgr; s). It has an excellent dust holding capacity and has the advantage of being much more uniform in weight and thickness than the wet-laid papers.

Several preferred embodiments of dry-laid capacity paper are contemplated. One is a latex-bonded fluff pulp fiber composition. That is, the fibers comprising this paper consist essentially of fluff pulp. The term "fluff pulp" means a nonwoven component of the filter of this invention which is produced by mechanically shredding pulp rolls, i.e., fibrous material made of wood or cotton, then aerodynamically transporting the pulp to web forming components of air-laying or dry forming machines. A Wiley mill can be used to shred the pulp. So-called Dan-Web or M and J machines are useful for dry forming. A fluff pulp component and the dry-laid layers of fluff pulp are isotropic and are thus characterized by disordered fiber orientations in the directions of all three orthogonal dimensions. That is, they have a large proportion of fibers oriented away from the plane of the nonwoven and particularly perpendicular to the plane, as compared to three-dimensional anisotropic nonwovens. Fluff pulp fibers used in this invention are preferably about 0.5-5 mm long. The fibers are held together by a latex binder. The binder can be applied as either a powder or an emulsion. The binder is typically present in the dry laid capacity paper in the range of about 10-30 wt% and preferably about 20-30 wt% binder solids based on the weight of the fibers.

In another preferred embodiment, the dry-laid capacity paper comprises a thermally bonded blend of fluff pulp fibers and at least one of split film fibers or bicomponent polymer fibers. The blend of fluff pulp fibers more preferably comprises fluff pulp fibers and bicomponent polymer fibers.

Split film fibers

Split film fibers are essentially flat, rectangular fibers that are electrostatically charged before or after being incorporated into the composite structure of the invention. The thickness of the split film fibers can be between 2-100 micrometers, the width can be between 5 and 500 micrometers, and the length can be between 0.5 and 15 mm. However, the preferred dimensions of the split film fibers are a thickness of about 5 to 20 micrometers, a width of about 15 to 60 micrometers, and a length of about 0.5 to 8 mm.

The split film fibers of the invention are preferably made of a polyolefin such as polypropylene. However, any polymer suitable for making fibers can be used for the split film fibers of the composite structure of the invention. Examples of suitable polymers include, but are not limited to, polyolefins such as homopolymers and copolymers of polyethylene, polyterephthalates such as poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT), poly(cyclohexyl dimethylene terephthalate) (PCT), polycarbonate and polychlorotrifluoroethylene (PCTFE). Other suitable polymers include nylon, polyamides, polystyrenes, poly-4-methylpentene-1, polymethyl methacrylates, polyurethanes, silicones, polyphenylene sulfides. The split film fibers can also comprise a mixture of homopolymers or copolymers. In the present application, the invention is exemplified using split film fibers made of polypropylene.

The use of PP polymers with different molecular weights and morphologies in layered film structures has been demonstrated to produce films with an appropriate balance of mechanical properties and brittleness needed to produce split film fibers. These PP split film fibers can also subsequently be given the appropriate amount of crimp. All dimensions of the split film fibers can of course be changed during the production of the fibers.

A method for producing the split fibers is disclosed in US Patent No. 4,178,157. Polypropylene is melted and extruded into a film, which is then blown into a large tube (balloon) into which ambient air is introduced or

which allows penetration, according to conventional bubble stretching techniques. Charging the balloon with air serves to quench the film and biaxially orient the molecular structure of the PP molecular chains, resulting in increased strength. The balloon then collapses and the film is stretched between two or more pairs of rollers in which the film is held in the nip of two contacting rollers, with different pressures applied between the two contacting rollers. This results in additional stretching in the machine direction, which is achieved by operating the second set of rollers at a higher surface speed than the first set. The result is even greater molecular orientation of the film in the machine direction, which subsequently becomes the long dimension of the split film fibers.

The film may be electrostatically charged before or after it has been cooled. Although various electrostatic charging techniques may be used to charge the film, two methods have been found to be most suitable. The first method involves passing the film approximately midway through a gap of about 1.5 to 3 inches between two DC corona electrodes. Corona bars with metallic wire emitter pins may be used in which one corona electrode has a positive DC voltage potential of about 20 to 30 kV and the opposite electrode has a negative DC voltage of about 20 to 30 kV.

The second preferred method uses the electrostatic charging techniques described in U.S. Patent No. 5,401,456 (Wadsworth and Tsai, 1995), referred to as Tantret Technique I and Technique II, which are further described herein. Technique II, in which the film is suspended from insulated rollers as the film is passed around the inner periphery of two negatively charged metal cups with a positive corona wire in each cup, has been found to impart the greatest voltage potentials to the films. In general, Technique II can impart positive 1000 to 3000 volts or more to one side of the films and similar magnitudes with negative signs to the other side of the charged film.

Technique I, in which films touch a metal roller with a direct current voltage of -1 to -10 kV and a wire with a direct current voltage of +20 to + 40 kV approximately

Placing the film coil 1 to 2 inches above the negatively biased roll, with each side of the film sequentially subjected to this roll-wire charging configuration, results in lower voltage potential as measured on the surfaces of the films. Technique I typically achieves voltages of 300 to 1500 volts on the film surface with generally equal but opposite polarities on each side. However, it has been found that the higher surface potentials achieved with Technique II do not result in better measurable filtration performance of the webs of split film fibers. Therefore, and because it is easier to insert the film and feed it through the Technique I apparatus, this method is primarily used to charge the films prior to the splitting process.

The cooled and stretched films can be electrostatically charged hot or cold. The film is then simultaneously stretched and slit into narrow widths, typically down to about 50 microns. The slit, flat filaments are then collected with a tow which is crimped at a controlled number of crimps per centimeter and then cut into the desired staple length.

In a particularly preferred embodiment, the dry-laid high dust capacity paper comprises a mixture of fluff pulp fibers, bicomponent polymer fibers and electrostatically charged split film fibers. Preferably, the fluff pulp fibers are present at about 5-85 wt.%, more preferably about 10-70 wt.% and most preferably about 40 wt.%, the bicomponent fibers are present at about 10-60 wt.%, more preferably about 10-30 wt.% and most preferably about 40 wt.%. This dry-laid high dust capacity paper can be thermally bonded, preferably at high temperatures of 90-160 ^° C, more preferably at a temperature less than 110 ^° C and most preferably at about 90 ^° C.

Mixed electrostatic fibers

Other preferred embodiments of the dry laid capacity paper include a thermally bonded paper with 100% "mixed electrostatic fibers", a mixture of 20-80% mixed electrostatic fibers and 20-80% B/C fibers, and a mixture of 20-80% mixed electrostatic fibers, 10-70% fluff pulp and 10-70% B/C fibers. "Mixed electrostatic fibers" filters are made by mixing fibers with very different triboelectric properties and rubbing them against each other or against metal parts of machinery such as wires or carding cylinders during carding. This makes one type of fiber more positively or negatively charged with respect to the other types of fibers and improves the Coulomb attraction for dust particles. The manufacture of filters with these types of mixed electrostatic fibers is taught in U.S. Patent No. 5,470,485 and European Patent Application No. EP 02 246 811 A2.

In US Patent 5,470,485, the filter material consists of a mixture of (I) polyolefin fibers and (II) polyacrylonitrile fibers. The fibers (I) are bicomponent PP/PE fibers of the core/sheath or side-by-side type. The fibers II are "halogen-free." The (I) fibers also have some "halogen-substituted polyolefins," while the acrylonitrile fibers have no halogen. The patent mentions that the fibers must be thoroughly washed with nonionic detergent, alkali or solvent, and then rinsed well before being mixed together so that they are free of lubricants or antistatic agents. Although the patent teaches that the nonwoven fabric produced should be needle punched, these fibers may also be cut to 5-20 mm lengths and blended with thermal bicomponent bonding fibers of similar length, and also with the possible addition of fluff pulp, so that dry-laid thermally bonded paper can be used in this invention.

EP 02 246 811 describes the triboelectric effect when two different types of fibers are rubbed against each other. It teaches to use similar types of fibers as US Patent 5,470,485, except that the -CN groups of the polyacrylonitrile fibers are substituted by halogen (preferably fluorine or chlorine). After a sufficient amount of substitutions of -CN by Cl groups, the

Fibers may be referred to as "modacrylic" if the copolymer comprises 35-85 wt.% acrylonitrile units. EP 0 246 811 teaches that the ratio of polyolefin to substituted acrylonitrile (preferably modacrylic) may range from 30:70 to 80:20 by surface area, and more preferably from 40:60 to 70:30. Similarly, US Patent 5,470,485 teaches that the ratio of polyolefin to polyacrylonitrile fibers is in the range of 30:70 to 80:20 with respect to a surface area of the filter material. Thus, these ranges of ratios of polyolefin to acrylic or modacrylic fibers may be used in the above proportions in the dry-laid, thermally bonded capacity paper.

Meltblown fleece

A synthetic polymer fiber meltblown web can optionally be used as a layer between a multipurpose layer and a high efficiency filtration layer. The meltblown web layer increases the overall filtration efficiency by trapping particles that have passed through the multipurpose filtration layer. The meltblown web layer can also optionally be electrostatically charged to assist in filtering fine dust particles. The inclusion of a meltblown web layer results in an increase in pressure drop for a given dust loading compared to compositions that do not include a meltblown web layer.

The meltblown nonwoven preferably has a basis weight of about 10-50 g/m ^A 2 and an air permeability of about 100-1500 l/(m ^A 2 &khgr; s).

High-volume meltblown nonwoven

Another discovery from recent research to develop improved vacuum cleaner bags was the development of a high bulk MB web or ply that could be used upstream of the filtration grade MB web as a pre-filter instead of a wet laid capacity paper or dry laid capacity paper. The high bulk MB pre-filter can be manufactured in a meltblown process that uses a cooled quench air or cooling air at a temperature of about 10 ⁰ C. In contrast, conventional MB usually uses room air at an ambient temperature of 33-45 ⁰ C. Also, the collection bag

· t ·

The distance between the MB nozzle exit to the nonwoven take-up belt is increased to 400-600 mm in the high bulk MB process. For normal MB production, the distance is normally 200 mm. In addition, high bulk MB nonwoven is produced using a lower temperature stretch air temperature of approximately 215-235 ⁰ C instead of the normal stretch air temperature of 280-290 ⁰ C and a lower MB melt temperature of 200-225 ⁰ C compared to 260-280 ⁰ C for filtration grade MB production. The colder quench air, lower stretch air temperature, lower melt temperature and larger collection distance cools the MB filaments more. The removal of heat results in less stretching of the filaments and thus larger fiber diameters than would be found in typical filtration grade MB nonwovens. The cooler filaments are less likely to thermally melt together when deposited on the collector. This would give the high-bulk meltblown nonwoven a more open surface. Even with a basis weight of 120 g/m ^A 2 , the air permeability of the high-bulk meltblown nonwoven is 806 l/(m ^A 2 × s). In contrast, a much lighter (e.g. 22 g/m ^A 2) filtration grade MB-PP nonwoven has a maximum air permeability of only 450 l/(m ^A 2 × s). The filtration performance of the high-bulk MB nonwoven, as determined by the DIN 44956-2 test, was 98%. When the two were placed together with the high-bulk MB nonwoven on the inside of the bag, the air permeability was still 295 l/(m ^A 2 × s) and the filtration efficiency of the pair was 99.8%. The high-bulk meltblown nonwoven can be uncharged or optionally electrostatically charged, provided that the nonwoven is made of a material with suitable dielectric properties.

The high bulk MB nonwoven of this invention should be distinguished from the "filtration grade MB" also used in the multilayer vacuum filter structure of this disclosure. The filtration grade MB nonwoven is a conventional meltblown nonwoven generally characterized by a lower basis weight, typically about 22 g/m ^A 2 , and a small pore size. Additional typical characteristics of filtration grade MB nonwoven made of propylene are shown in Table I. A preferred high bulk MB nonwoven made of polypropylene optimally comprises about 5-20 wt.% ethylene vinyl acetate. A filtration grade MB nonwoven generally has a high dust removal efficiency, i.e., greater than about 99%.

Table 1

Preferred More preferred Most preferred

Filtration grade MB PP

Weight g/m ^A 2 Thickness, mm

Air permeability, l/(m ^A 2 &khgr; s) Tensile strength, MD, N Tensile strength, CD, N Fiber diameter, µm&igr;&tgr;&igr;

5-100 10-50 25 0.10-2 0.10-1 0.26 100-5000 100-2000 450 0.5-15 1.0-10 3.7 0.5-15 1.0-10 3.2 1-15 1-5 2-3

High-volume MB PP

Weight, g/m ^A 2 Thickness, mm

Air permeability, l/(m ^A 2 &khgr; s) Tensile strength, MD, N Tensile strength, CD, N Fiber diameter, pm

30-180 60-120 80 0.3-3 0.5-2 1.4 300-8000 600-3000 2000 1.0-30 2-20 10 1.0-30 2-20 9.2 5-20 10-15 10-12

High bulk MB nonwoven is similar in filtering performance to the dry-laid and wet-laid capacity papers mentioned above. Therefore, high bulk MB nonwoven is well suited to removing large amounts of large dust particles and retaining large amounts of dust. Accordingly, high bulk MB nonwoven sheet is suitable to be placed upstream and as a pre-filter of the filtration grade MB sheet in a vacuum filter structure of this invention.

(Modular) spunblown nonwoven

A new type of melt blow technology as described in G. Ward, "Nonwovens World", Summer 1998, pp. 37-40 is available to produce a (modular) spunblown nonwoven suitable to be used as a coarse filter layer in the present invention. Alternatively, the spunblown nonwoven can be used as

Filtration grade meltblown nonwoven layer as desired in the new structure may be used. Specifications of the (modular) spunblown nonwoven are shown in Table II.

The process for producing the (modular) spunblown nonwoven is generally a meltblown process with a coarser modular nozzle and using colder stretching air. These conditions produce a coarse meltblown nonwoven with higher strength and air permeability at a comparable basis weight to conventional meltblown nonwovens.

Microdenier spunbond nonwoven

A spunbond ("SB") nonwoven, sometimes referred to herein as a microdenier spunbond, can also be used in this invention in the same manner as the aforementioned coarse filter layer or filtration grade meltblown nonwoven layer. Specifications of microdenier spunbond are listed in Table II. A microdenier spunbond is particularly characterized by filaments having a diameter of less than 12 pm, which corresponds to 0.10 denier for polypropylene. For comparison, conventional SB nonwovens for disposable items typically have filament diameters averaging 20 pm. A microdenier spunbond can be purchased from Reifenhauser GmbH (Reicofil III), Koby Steel, Ltd., (Kobe-Kodoshi Spunbond Technology) and Ason Engineering, Inc. (Ason Spunbond Technology).

Table II

Preferred More preferred Most preferred

[Modular) Spunblown 10-150 10-50 28 Weight g/m ^A 2 0.20-2 0.20-1.5 0.79 Thickness, mm 200-4000 300-3000 1200 Air permeability, l/(m ^A 2 &khgr; s) 10-60 15-40 43 Tensile strength, MD, N 10-50 12-30 32 Tensile strength, CD, N 0.6-20 2-10 2-4 Fiber diameter, pm • · · &idr; · * •

Microdenier spunbond PP (Ason, Kobe-Kodoshi. Reicofil III)

Weight, g/m ^A 2 10-50 20-30 17

Thickness, mm 0.10-0.6 0.15-0.5 0.25

Air permeability, l/(m ^A 2 &khgr; s) 1000-10,000 200-6000 2500

Tensile strength, MD, N 10-100 20-80 50

Tensile strength, CD, N 10-80 10-60 40

Fiber diameter, μηη 4-18 6-12 10

A preferred process for making an embodiment of the novel filter composition having a unitary layer structure of MB and FP compositions is shown in Figure 1. The process shown provides a product laminated to a scrim, paper or nonwoven to facilitate handling, holding or packaging. It is also possible to provide an unlaminated filter composition by replacing the scrim, paper or nonwoven with a carrier tape to carry the non-prebonded layers through the process. The unitary filter composition ultimately consists of at least two layers, each layer may contain more than one type of fiber or other material, and generally consists of three to five layers that are thermally or latex bonded. Electrostatic charging of the filter composition is preferably carried out in-line by the "cold" Tantret electrostatic charging process, although MB fibers can be "hot" charged in-line as they exit the MB nozzle. Also, the split film fibers that have been electrostatically charged during their manufacture can be introduced through the FP applicators. In addition, "mixed electrostatic fibers" that have opposite polarities after being rubbed against each other due to different triboelectric properties can be incorporated into the composition through the FP applicators.

Referring now to Fig. 1, an optional unwinding device 1 is arranged at the starting point of the line to allow the introduction of an optional carrier layer 2, which may be a scrim, paper or nonwoven. Components 1, 2, 4 and 5 are optional in that the inventive unitary filter composition is attached to a scrim, paper or nonwoven only for ease of handling,

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tens or packaging. A conveyor belt 3 extends the entire length of the line; however, it may also be divided into shorter sections, with one conveyor belt section introducing the assembly of plies into the next sections as required in the process. Also at the starting point of the line is an optional adhesive applicator 4 to dispense an adhesive 5 in the form of glue or hot melt adhesive. This adhesive applicator station may be used to laminate a carrier layer to the unitary ply structure of the new composition in-line. It should be noted, however, that the applicator 4 is not intended for pre-bonding plies within the unitary structure.

Next, as shown in Figure 1, there are at least one and preferably two FP applicator units 6 and 8. The primary function of the FP applicator units at the beginning of the line is to produce and deposit dry laid layers 7 and 9 on the optional adhesive layer 5 or on the conveyor belt 3 when the optional carrier layer 2 and adhesive 5 are not used. The dry laid layers 7 and 9 have different compositions and properties to meet the requirements of the final product. In all respects, the role of the layers 7 and 9 is primarily to support and protect the MB or related filter media layers 12 and 14. In the illustrated embodiment, the FP layers 7 and 9 are composed primarily of "pulp" and bicomponent (B/C) fibers. Various types of B/C fibers may be used as described above. For example, a preferred type has a core of a higher melting point fiber such as PP and a sheath of a lower melting point fiber such as PE. Other preferred compositions of pulp and B/C core/sheath PP/PE are 50% pulp/50% B/C fibers in layer 7 and 25% pulp/75% B/C fibers in layer 9. If no latex binder is applied in section 23, at least 20% B/C fibers or other types of thermal binder fibers should be used. On the other hand, if latex binder is subsequently applied in sections 23 and 27, then 100% pulp fibers can be applied by the FP applicator heads 6 and 8. It is also possible to apply 100% B/C fibers from the FP applicator 6 or applicator 8 or from both applicator heads 6 and 8.

In additional embodiments, instead of 100% B/C fibers, regular monocomponent staple fibers made of PP, PET, polyamide and other fibers can be used for up to 80%

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of the B/C or thermal bonding fibers which may be applied by any of the FP applicator heads 6, 8, 15, 18 and 20. Many types of thermally bondable fibers which melt completely, also known as "melt fibers", may also be used in place of the B/C fibers, except in dry-laid ply components where 100% B/C fibers would be used.

Fig. 1 further illustrates an optional compactor 10 that reduces the thickness of the web and increases the fiber-to-fiber adhesion of the FP layers 7 and 9. It should be noted that the extensive pre-bonding typically used to create the layers individually is not the goal of this optional compaction step used in this inventive in-line process. The compactor 10 may be a calender, which may or may not be heated. The MB or related filter media 12 and 14 may be deposited onto the FP layers 7 and 9 through one or more MB nozzles 11 and 13. The primary function of the MB component is to serve as a high performance filter, i.e., to remove small percentages of small size particles (less than about 5 microns). The specifications of filtration grade MB media and related types of ultrafine fiber diameter filter media are given in Table I.

The process may include at least one or more MB nozzles 11 and/or one or more related fine denier fiber applicators 13 (ultrafine fiber diameters), designated as X. For example, if two identical MB units are used, units 11 and 13 are the same. Other variations that fall within the breadth of this invention include having the first unit as a (modular) spunblown or microdenier spunbond (SB) system first to form a filter gradient from coarser to finer high performance filters. Another contemplated variation is to use one or more (modular) spunblown or microdenier SB systems in series. Another variation is to use a microdenier SB followed by a spunblown system first.

The next device component shown in Fig. 1 is another FP applicator 15 which deposits a FP web onto the layer 14 (or onto the layer 12 if a second MB layer 13 is not included). Then the non-prebonded assembly moves from

layers with layer 16 as the uppermost through another optional compactor 17. Next, the intermediate product is conveyed under one or more additional FP units 18 and 20. The FP applicator heads 15 and 18 insert the dry laid capacity layer into the structure. The FP applicator 20 is primarily designed to produce a very open (i.e., bulky) FP primarily for dust holding capacity rather than as a filter. The very open FP layer 21 is preferably made of 100% bicomponent B/C fiber or blends of B/C with ratios of B/C to "pulp" characterized as higher than those normally used to produce coarse prefilter FP webs. One or both of the FP layers 16 and 19 may also comprise split film fibers and "blended electrostatic fibers." If B/C fibers or other types of thermal bonding fibers are not used in the FP layers 16 and 19, then latex binder should be applied at units 23 and 27 to bond the layers. If B/C fibers or other types of thermal bonding fibers are included in either of the FP applicator heads 15 and 18, then latex binder can still be applied at units 23 and 27.

The intermediate top layer product 21 then moves through a further densifier 22 and thereafter through a section of the production line in which the previously loose, unbonded layers are subjected to one or more bonding process steps which act cumulatively to form the unitary layer structure of the multi-layer filter. Preferably, all of the filter components which will be incorporated into the unitary layer structure are incorporated together in the intermediate product at this stage prior to bonding of the layers.

With further reference to Fig. 1, it can be seen that the bonding steps begin in the illustrated embodiment with a latex binder 24 applied by the applicator 23. The latex may be sprayed from a liquid dispersion or emulsion, applied by a roller or gravure applicator, or sprayed as a dry powder onto the substrate and then thermally melted or bonded thereto. The latex also serves as a sealant by eliminating dust that may emanate from the outer surfaces of the FP layer. After adding latex binder at 23, the intermediate product moves through a heating unit 25 which dries and cures the latex binder to bond the composition. The heating unit may be a heated calender, or a

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Infrared, microwave or convection oven. A combination of these may also be used. A through-air oven is preferred. If B/C fibers or other types of thermally bondable melt fibers are present in the intermediate product, then ovens 25 and 29 may serve to thermally melt these fibers to continue the bonding and formation of the unitary structure.

After oven 25, the intermediate product is cooled by system 26 and then a second latex binder is applied at 27. As shown, the travel path and spray unit 27 are arranged to apply the latex binder to the opposite side of the first application. The intermediate product comprising the second latex binder 28 is then passed through a second through-flow oven 29 and through another cooling section 30. Next, the fully bonded multilayer film is charged into a uniform layer structure in cold electrostatic charging station 31, preferably a Tantret-J system. Finally, the multilayer film 32 is slit to a desired width or multiple widths on the slitting device 33 and rolled up by the winch 34. Although electrostatic charging is illustrated as occurring towards the end of the process, it is also intended that charging may be carried out at a stage prior to the application of latex binder, provided that the binder and subsequent processing steps do not significantly strip the charge from the intermediate product.

Representative products according to the present invention are shown schematically in Figures 2-13, which are described in detail below. In the figures, the air device is indicated by arrow A. In Figure 2, a two-layer unitary filter composition 36 is shown. The inner (dirty air side) layer 37 is a dry-laid FP capacity layer having the general weight of 10-150 g/m ^A 2 , typical weight range of 20-80 g/m ^A 2 and with a preferred weight of 50 g/m ^A 2 . The FP layer 37 has various blends of pulp fibers, bicomponent film layer (B/C) fibers, split film fibers and "mixed electrostatic fibers". Split film fibers and "mixed electrostatic fibers" are not used in all variations of Layer 37, but at least 10% and at least 20% B/C fibers or other types of thermally bondable fusion fibers should be used to achieve adequate thermal bonding. In general,

at least 10% and preferably at least 20% pulp fibers are used to improve coverage and filtration performance. The outer layer 38 is a high performance MB component weighing 5-100 g/m ^A 2 . The independently assembled layers 37 and 38 meet at the interface 36A. This interface is different from that in a laminate of two prebonded layers in a multilayer composition. Due to the fact that forming a prebonded layer is not required to produce the unitary structure 36, at least one of the layers 37 and 38 may be sufficiently fluffy that it cannot be formed as a freestanding web for inclusion as a layer in a conventional multilayer composition.

Fig. 3 shows a unitary filter composition 39 of three layers. The inner layer 40 is a coarse dry-laid component of 100% B/C fibers. It serves primarily as a pre-filter and protects the downstream filter material. The broadest weight range is 10-100 g/m ^A 2 with a typical weight range of 20-80 g/m ^A 2 and a preferred weight of 50 g/m ^A 2. The middle layer 41 is a dry laid FP capacity component with the broadest weight range of 10-150 g/m ^A 2, typical weight range of 30-80 g/m ^A 2 and a preferred weight of 50 g/m ^A 2. The layer 41 typically has at least 10% and preferably at least 20% B/C fibers, 10% and preferably at least 20% FP and may also comprise split film fibers and "mixed electrostatic fibers". At least 10% and preferably at least 20% B/C fibers or other types of thermally bondable fusion fibers should be used to achieve proper thermal bonding. Generally, at least 4% and preferably at least 20% pulp fibers are used to improve coverage and filtration performance. The layer may be free of B/C fibers or other types of thermally bondable melt fibers when latex binders are used. The outer layer 42 is made of high filtration performance MB media or other materials with ultrafine fiber diameters such as modular spunblown or microdenier spunblown.

Fig. 4 is a graphical representation of a unitary multilayer filter 43 made of four layers of material. The inner layer 44 is composed of dry laid FP or 100% B/C fibers. The widest weight range is 10-100 g/m ^A 2 , which is typical.

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The typical weight is 20-80 g/m ^A 2 and the target weight is 50 g/m ^A 2. The second layer 45 is a dry laid FP capacity layer with the widest weight range of 10-150 g/m ^A 2, typical weight range of 30-80 g/m ^A 2 and a preferred weight of 50 g/m ^A 2. The layer 45 comprises at least 10% and preferably at least 20% B/C fibers, 10% and at least 20% pulp fibers and may comprise various amounts of charged or uncharged split film fibers. It may comprise various amounts of "mixed electrostatic fibers". At least 10% and preferably at least 20% B/C fibers or other types of thermally bondable melt fibers should be used to achieve proper thermal bonding. Generally, at least 10% and preferably at least 20% pulp fibers are used to increase coverage and filtration performance. The layer may be free of B/C fibers or other types of thermally bondable fusible fibers if latex binder is used. The third layer 46 comprises MB filter media and a widest weight range of 5-100 g/m ^A 2 , a typical weight of 10-50 g/m ^A 2 and a preferred weight of 25 g/m ^A 2 . The outer layer 47 is a dry laid FP of air-laid pulp and B/C fibers.

Fig. 5 is a graphical representation of a unitary multilayer filter 48 comprising five layers of material. The inner layer 49 is composed of dry laid FP or 100% B/C fibers. The broadest weight range is 10-100 g/m ^A 2 , the typical weight is 20-80 g/m ^A 2 and the target weight is 50 g/m ^A 2 . The second layer 50 is a dry laid FP capacity component with the broadest weight range of 10-150 g/m ^A 2 , typical weight range of 30-80 g/m ^A 2 and a preferred weight of 50 g/m ^A 2 . The component 50 comprises at least 10% and preferably at least 20% B/C fibers, at least 10% and preferably at least 20% pulp fibers and may comprise varying amounts of charged and uncharged split film fibers. At least 10% and preferably at least 20% B/C fibers or other types of thermally bondable fusible fibers should be used to achieve proper thermal bonding. Generally, at least 10% and preferably at least 20% pulp fibers are used to increase coverage and filtration performance. The layer may be free of B/C fibers or other types of thermally bondable fusible fibers if latex binder is used. Component 41 includes carbon granules or carbon fibers to absorb odors and remove pollutants and toxic gases from the air.

•♦ »

Component 52 is a high filtration performance MB media with a widest weight range of 5-100 g/m ^A 2 , typical weight of 10-50 g/m ^A 2 and a preferred weight of 22 g/m ^A 2 . Component 53 is a dry laid FP composed of air laid pulp and B/C fibres.

Fig. 6 illustrates a unitary filter assembly 54 of the same construction as shown in Fig. 2, composed of two layers 55, 56 bonded to an outer carrier layer 57 of paper, scrim or nonwoven having a weight between 10 and 100 g/m ^A 2 .

Fig. 7 illustrates a unitary filter assembly 58 of the same construction as shown in Fig. 3, composed of three plies 59, 60 and 61 bonded to an outer layer 62 of paper, scrim or nonwoven having a weight between 10 and 100 g/m ^A 2 .

Fig. 8 illustrates a unitary filter assembly 63 of the same construction as in Fig. 4, composed of four plies 64-67 bonded to an outer layer 68 of paper, scrim or nonwoven having a weight of 10-100 g/m ^A 2 .

Fig. 9 illustrates a unitary filter assembly 69 of the same construction as in Fig. 5, composed of five plies 71-75 bonded to an outer layer 76 of paper, scrim or nonwoven having a weight of 10-100 g/m ^A 2 .

Fig. 10 illustrates a layered structure of a unitary filter composition 77 of the same construction as shown in Fig. 2, composed of two plies 78, 79 bonded to an outer carrier layer 81 of paper, scrim or nonwoven having a weight between 10 and 100 g/m ^A 2 , except that the outer layer is bonded by means of an adhesive or bonding agent 80, the latter of which could be a latex binder or a hot melt adhesive.

Fig. 11 illustrates a layer structure of a unitary filter composition 82 of the same construction as shown in Fig. 3, composed of three layers 83-85

is bonded to an outer layer 87 of paper, scrim or nonwoven having a weight between 10 and 100 g/m ^A 2 , apart from the fact that the outer layer is bonded by means of an adhesive or an adhesive 86.

Fig. 12 illustrates a layered structure of a unitary filter composition 87A of the same construction as in Fig. 4, composed of four layers 88-91 bonded to an outer layer 93 of paper, scrim or nonwoven having a weight of 10-100 g/m ^A 2 , except that the outer layer is bonded by means of an adhesive or bonding agent 92.

Fig. 13 illustrates a layered structure of a unitary filter composition 94 of the same construction as in Fig. 5, composed of five layers 95-99 bonded to an outer layer 101 of paper, scrim or nonwoven having a weight of 10-100 g/m ^A 2 , except that the outer layer is bonded by means of an adhesive or bonding agent 100.

When interlayer bonding is indicated in the embodiments of Figs. 10-13, conventional interlayer bonding techniques such as ultrasonic bonding may be used instead of or in conjunction with the adhesive-to-adhesive bonding mentioned above.

Claims (16)

1. A multilayer filter comprising a plurality of non-prebonded layers, each layer independently comprising at least one filtration material and being different from adjacent layers, the plurality of layers bonded together to form a unitary layer structure having a first edge surface configured to receive air-entrained particles and a second edge surface configured to discharge filtered air, and wherein a first layer comprises dry-laid, thermally bondable bicomponent or monocomponent polymer melt fibers and wherein a second layer comprises meltblown fibers, and wherein the first layer is disposed closer to the first edge surface than the second layer. 2. A multi-layer filter according to claim 1, wherein at least one layer is non-self-supporting. 3. A multi-layer filter according to claim 2, wherein all non-self-supporting layers individually lack effective integrity to be laid down, rolled up and unwound as a single unit. 4. A multilayer filter according to claim 1, comprising 2 to 5 layers. 5. The multilayer filter of claim 4, wherein the layers are placed next to each other in order of decreasing porosity such that the layer with the highest porosity is adjacent the first edge surface and the layer with the lowest porosity is adjacent the second edge surface. 6. A multi-layer filter according to claim 5, wherein the layers each have a characteristic dust holding capacity and are placed next to one another in order of decreasing dust holding capacity such that the layer with the highest dust holding capacity is adjacent to the first edge surface and the layer with the lowest dust holding capacity is adjacent to the second edge surface. 7. The multi-layer filter of claim 1, wherein the first layer has a composition selected from the group consisting of 100 wt. % bicomponent polymer fibers, a blend of at least about 10 wt. % bicomponent polymer fibers with a complementary amount of fluff pulp fibers, staple fibers, or a blend of these, and a blend of at least about 10 wt. % thermally bondable monocomponent polymer melt fibers with a complementary amount of fluff pulp fibers, staple fibers, or a blend of these. 8. A multilayer filter according to claim 7, wherein the bicomponent polymer fibers have a sheath of one polymer and a core of another polymer having a higher melting point than that of the one polymer. 9. A multilayer filter according to claim 8, wherein the core is polypropylene and the sheath is polyethylene. 10. A multilayer filter according to claim 8, wherein the core is arranged eccentrically with respect to the shell. 11. The multilayer filter of claim 1, wherein the first layer further comprises fibers selected from at least uncharged split film fibers, charged split film fibers, or mixed electrostatic fibers. 12. The multilayer filter of claim 1, further comprising a prefilter layer disposed closer to the first edge surface than the first layer and consisting essentially of dry-laid bicomponent polymer fibers having a sheath of polymer and a core of another polymer having a higher melting point than that of the one polymer, and wherein the second layer comprises a nonwoven fabric selected from the group consisting of filtration grade meltblown nonwoven fabric, modular spunblown and microdenier spunbond. 13. The multilayer filter of claim 12, further comprising a support layer disposed closer to the second edge surface than the second layer and comprising a dry-laid blend of bicomponent polymer fibers and fluff pulp fibers. 14. The multi-layer filter of claim 13, further comprising an odor absorbing layer disposed between the first layer and the second layer and comprising an odor absorbing agent. 15. The multilayer filter of claim 14, wherein the odor absorbing layer comprises a dry laid blend of bicomponent polymer fibers with either carbon granules or carbon fibers. 16. A vacuum cleaner bag comprising a filter element for filtering dirty air having particle contamination and an air inlet means on the filter element for guiding the dirty air into the filter element, the filter element comprising a multi-layer filter according to any one of the preceding claims.