Composite filter and method of making the same Number:7,094,270 from the United States Patent and Trademark Office (PTO) owispatent The invention refers to a composite filter for filtering a stream of ambient air including at least one non-prebonded upstream tier and one non-prebonded downstream tier, wherein the ratio of absolute pore volume of upstream tier to downstream tier RAPV>2, and the absolute projected fiber coverage of upstream tier and of downstream tier APFC>95%. Further, the invention refers to a method of making such a composite filter including the steps of (a) laying down a filtration material onto a support to form the upstream non-prebonded tier, (b) depositing onto the upstream tier the downstream non-prebonded tier, and (c) bonding the tiers to form a composite filter having a unitary stratified structure.<H Composite, making, Composite, filte, 7094270.html, Patent, pto, 7094270

Title: Composite filter and method of making the same

Abstract: The invention refers to a composite filter for filtering a stream of ambient air including at least one non-prebonded upstream tier and one non-prebonded downstream tier, wherein the ratio of absolute pore volume of upstream tier to downstream tier RAPV>2, and the absolute projected fiber coverage of upstream tier and of downstream tier APFC>95%. Further, the invention refers to a method of making such a composite filter including the steps of (a) laying down a filtration material onto a support to form the upstream non-prebonded tier, (b) depositing onto the upstream tier the downstream non-prebonded tier, and (c) bonding the tiers to form a composite filter having a unitary stratified structure.

Patent Number: 7,094,270 Issued on 08/22/2006 to Schultink, et al.

Inventors: Schultink; Jan (Overpelt, BE), Schultink; Bas (Overpelt, BE)
Assignee: Airflo Europe N.V. (Overpelt, BE)

Appl. No.: 10/469,415

Filed: March 1, 2002

PCT Filed: March 01, 2002

PCT No.: PCT/EP02/02251

371(c)(1),(2),(4) Date: December 22, 2003

PCT Pub. No.: WO02/070107

PCT Pub. Date: September 12, 2002

Foreign Application Priority Data


Mar 02, 2001 [EP]			01105152

Current U.S. Class: 55/486 ; 15/347; 15/352; 428/340; 428/903; 442/389; 55/382; 55/485; 55/487; 55/528; 55/DIG.2; 95/287; 95/57; 96/66; 96/69

Current International Class: B03C 3/011 (20060101)

Field of Search: 95/57,78,287 15/347,352 55/485-487,DIG.2,382,528 96/15,69,66 428/340,903 442/389

References Cited [Referenced By]

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Foreign Patent Documents


195 44 790	May., 1997	DE
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WO 97/30772	Aug., 1997	WO
WO 98/11282	Mar., 1998	WO
WO 01/03802	Jan., 2001	WO

Other References

American Allergy Supply-3M Filtrete Vacuum Filter Bags Internet Web Publication http://www.neosoft.com/users/s/sreifler/3mbagidx.htm, Copyright 1994-1997. cited by other .
Jirsak, O. et al; Nonwoven Textiles; Carolina Academic Press; Durham, North Carolina; 1999; pp. 26-29. cited by other .
Ward, G.; "Micro Denier Nonwoven Process and Fabrics;" Nonwovens World; Summer 1998; pp. 37-40. cited by other.

Primary Examiner: Pham; Minh-Chau T.
Attorney, Agent or Firm: Brinks Hofer Gilson & Lione

Claims

The invention claimed is:

1. A composite filter for filtering a stream of ambient air comprising at least one non-prebonded upstream tier and one non-prebonded downstream tier, wherein the ratio of absolute pore volume of upstream tier to downstream tier RAPV>2, and the absolute projected fiber coverage of upstream tier and of downstream tier APFC>95%.

2. The composite filter of claim 1, wherein the ratio of average pore diameter of upstream to downstream tier RPD is 4<RPD<10.

3. The composite filter of claim 2, wherein the average pore diameter of the upstream tier PDU>60 .mu.m.

4. The composite filter of claim 1, wherein the upstream tier comprises a relative pore volume RPVU>94%, an apparent density ADU<0.05 g/cm.sup.3, and a thickness D in the range of 0.5 mm<D<2.5 mm.

5. The composite filter of claim 4, wherein the downstream tier comprises a relative pore volume RPVD being smaller than RPVU, an apparent density ADD in the range of 0.07 g/cm.sup.3<ADD<0.14 g/cm.sup.3, and a thickness D in the range of 0.1 mm<D<0.4 mm.

6. The composite filter of claim 1, wherein the upstream tier comprises fibers having a length in the range of 0.1 mm to 3.0 mm.

7. The composite filter of claim 6, wherein the orientation of the fibers in flow direction in the upstream tier is higher than in the downstream tier.

8. The composite filter of claim 1, wherein the upstream tier comprises a dust retention DR with respect to dust particles with a diameter corresponding to the average pore diameter of the downstream tier of DR>99%.

9. The composite filter of claim 1, wherein the upstream tier comprises dry-laid, thermally bondable fusing, bicomponent or monocomponent polymer fibers and the downstream tier comprises meltblown fibers.

10. The composite filter of claim 9, wherein the upstream tier 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 natural fibers, staple fibers or a mixture thereof, and a blend of at least about 10 wt % monocomponent polymer thermally bondable fusing fibers with a complementary amount of fluff pulp fibers, staple fibers or a mixture thereof.

11. The composite filter of claim 10, wherein the bicomponent polymer fibers have a sheath of one polymer and a core of a different polymer having a melting point higher than the one polymer.

12. The composite filter of claim 11, wherein the core is polypropylene and the sheath is polyethylene.

13. The composite filter of claim 12, wherein the core is disposed eccentric relative to the sheath.

14. The composite filter of claim 9, wherein the upstream tier further comprises fibers selected from at least one of uncharged split film fibers, charged split film fibers and mixed electrostatic fibers.

15. A vacuum cleaner bag comprising a composite filter in accordance with claim 1.

16. A method of making a composite filter in accordance with claim 1, comprising the steps of (a) laying down a filtration material onto a support to form the upstream non-prebonded tier, (b) depositing onto the upstream tier the downstream non-prebonded tier, and (c) bonding the tiers to form a composite filter having a unitary stratified structure.

17. A composite filter produced by the method of claim 16.

18. A vacuum cleaner bag comprising a composite filter produced by the method of claim 16.

19. The composite filter of claim 2, wherein the average pore diameter of the upstream tier PDU is >80 .mu.m and >200 .mu.m.

20. The composite filter of claim 1, wherein the upstream tier comprises a relative pore volume RPVU>96%.

Description

This application is based on, and claims priority to International Application No. PCT/EP02/02251, filed Mar. 1, 2002 and European Application No. 011 05 152.1, filed Mar. 2, 2001.

FIELD OF THE INVENTION

The invention relates, to a composite filter for removing solid particles entrained in a stream of ambient air. More specifically, it relates to a composite filter comprising at least one non-prebonded upstream tier and one non-prebonded downstream tier useful for filtering particulates from ambient air.

The term "pre-bonded" means herein that a composition of filter medium, such as thermally bondable fusing fibers or adhesively bindable fibers, is treated in a manner effective to activate the binding mechanism thereby forming a separate, free-standing, cohesive, and typically self-supporting web of that filter composition. Such a pre-bonded web can be mechanically manipulated by such processes as winding on a roll, unwinding from a roll, cutting and the like.

The term "tier" herein means a band formed from non-prebonded filter material into a stratum of unitary stratified structure. In contrast, a "layer" means a separately, prebonded, self-supporting web of filter material.

BACKGROUND AND PRIOR ART

In recent times, the technology for filtering particulates from gases has become quite sophisticated in both commonplace applications such as consumer oriented vacuum cleaning of dirt and dust as well as very demanding industrial applications such as removal from gases of specific particle size fractions of wide varieties of contaminants including from inert to biochemically sensitive, among others. It is now well appreciated that the contaminating particulates 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., odor-free and odoremitting particles.

Consequently, filtration technology has evolved to provide filter media which are adapted to optimally filter specific fractions of the contaminating particulates. Also, this technology has developed techniques for optimizing various performance characteristics of filters such as maintaining low pressure drop across the filter and increasing the filter service life so as to extend the length of time between filter element replacements.

The traditional approach to achieving these objectives has been to provide a multilayer filter medium composed of separate, individually designed layers which are each intended to accomplish 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 substantial amounts of chiefly large particles, and an ultrafine diameter filament, low porosity layer is usually prescribed for removing the smallest particles to increase filtration efficiency. From the many choices available, separate filter layers are selected and combined in a preselected sequence then assembled as a group to form a multilayer, and therefore multifunctional filter. The one or more adjacent layers can be bonded to each other or the layers can be unbonded. Optionally, the individual layers can be sandwiched between covers, typically of paper, for structural integrity and ease of handling.

A drawback of the aforementioned multilayer system of constructing multifunctional filters is that there is repetitive processing of the filter media which can be excessive. That is, the filter material in a given layer is first processed to form the individual layer, then it is processed to assemble that layer in the multilayer filter. Each step adds to the compaction and cover, if ever slight, of the final filter product. This tends to raise the pressure drop through the filter and reduce dust holding capacity, thereby limiting service life.

WO 01/03802 discloses a composite filter comprising at least one non-prebonded upstream tier and one non-prebonded downstream tier. However, as will be shown in detail below (FIG. 2), in this composite filter a relatively high pressure drop across the composite filter occurs. Further, also the service life time of this filter is low.

In view of this, the objective problem underlying the invention is to provide a composite filter in which the pressure drop across the filter is maintained low and which has a long service life time.

SUMMARY OF THE INVENTION

This objective problem is solved by a composite filter for filtering a stream of ambient air comprising at least one non-prebonded, upstream tier and one non-prebonded downstream tier, wherein the ratio of absolute pore volume of upstream tier to downstream tier RAPV>2, and the absolute projected fiber coverage of upstream tier and of downstream tier APFC>95%.

Due to the parameters of this composite filter, the pressure drop across the filter medium is kept low and the service life time of the filter is increased.

Further, this invention enables to provide a composite filter made up of at least two stacked tiers of filtration material bonded together to form a unitary stratified structure. The composition of filtration material in any given tier is preselected to perform a desired filtering function. For example, fine, (i.e., small diameter) and densely packed fibers can be selected to capture very small dust particles such as those of about 5 micrometers and smaller. Additionally, electrostatically charged fibers can also be used to stop passage of these and even smaller particles. Similarly, bulky, highly porous media designed to have large dust holding capacity can be utilized to trap medium to large size dirt particles.

Since the composite filter of the invention comprises non-pre-bonded tiers, the bonding of at least one and preferably all of the tiers to form the unitary structure is begun only after the stacking of all the tiers of a particular desired composite filter structure has been completed. The resulting structure is a single body composed of different types of filtration material which appear as distinct strata.

In view of this, the stratified structure is formed by building up a stack of tiers of selected filtration materials. Because the tiers are non-prebonded, the components of each tier, that is, fibers, granules, etc., generally are laid loosely by mechanical or air-laying processes onto the layer lying below. Within each tier the composition of filter material is largely uniform and there is a "fuzzy" interface between the tiers.

Preferably, the composite filter of the above mentioned kind comprises a ratio of average pore diameter of upstream to downstream tier RPD in the range of 4<RPD<10.

Due to this ratio, the dust holding capacity of the upstream tier is greatly increased, such that the upstream tier acts as a pre filter for the downstream tier without increasing the pressure drop across the composite filter.

Additionally but not exclusively, such composite filter may comprise an average pore diameter of the upstream tier PDU, with PDU>60 .mu.m, preferably in the range 80 .mu.m<PDU<200 .mu.m.

All above discussed composite filters may comprise upstream tiers with a relative pore volume RPVU>94%, preferably RPVU>96%, an apparent density ADU<0.05 g/cm.sup.3, and a thickness D in the range of 0.5 mm<D<2.5 mm. Choosing these parameters results in an upstream tier with the required RAPV and APFC.

Further, this composite filters may also comprise downstream tiers with a relative pore volume RPVD being smaller than RPVU, an apparent density ADD in the range of 0.07 g/cm.sup.3<ADD<0.14 g/cm.sup.3, and a thickness D in the range of 0.1 mm<D<0.4 mm. Choosing these parameters results in an downstream tier with the required RAPV and APFC.

Further, the upstream tier of any of the above discussed composite filter may preferably comprise fibers having a length in the range of 0.1 mm to 3.0 mm.

Due to such a structure, the upstream tier can be made bulkier to provide greater dust holding capacity.

Preferably, the above discussed composite filters may comprise an upstream tier having a dust retention DR with respect to dust particles with a diameter corresponding to the average pore diameter of the downstream tier of DR>99%.

This feature avoids an clogging of the downstream tier, and therefore, further maintains a low pressure drop across the filter and further increases the service life time of the composite filter.

Additionally but not exclusively, this effect can be increased in a composite filter in which the orientation of the fibers in flow direction in the upstream tier is higher than in the downstream tier. Such structure further improves maintaining the pressure drop across the filter.

The composite filters, as discussed above, may comprise an upstream tier of dry-laid, thermally bondable fusing, bicomponent or monocomponent polymer fibers and a downstream tier of meltblown fibers. In this aspect a single tier is constituted of a single type of filter medium, for example, 100% bicomponent polymer fibers, melt blown, staple fibers, or spunbond filaments.

Alternatively, the composite filter may comprise an upstream tier having 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 natural fibers, such as fluff pulp fibers or kokon fibers, staple fibers or a mixture thereof, and a blend of at least about 10 wt % monocomponent polymer thermally bondable fusing fibers with a complementary amount of fluff pulp fibers, staple fibers or a mixture thereof.

In this aspect, a single tier is constituted by a blend of media, such as an air-laid, usually uniform blend of bicomponent polymer fibers and fluff pulp (FP) fibers.

Since it is also desirable to provide a stratified structure, adjacent tiers in a stack may have different compositions. Nonetheless, a composition of one tier can be repeated in a stack although at least one tier of different composition should be present between the tiers of same composition.

This structure of the composite filter differs from that of conventional multilayer filtration media which are formed by laminating a plurality of individual filter medium layers that have each been pre-bonded to form a self-supporting web prior to formation of the multilayer laminate.

Such unitary stratified structure provides a number of significant advantages over conventional filter media. In one aspect, the unitary stratified structure can be made bulkier to provide greater dust holding capacity than a laminate of individually, pre-bonded layers having compositions corresponding respectively to the tiers of the unitary structure. This is because each portion of the conventional filter medium is compressed at least twice: once when the individual layer is formed by bonding, and a second time when the individual layers are laminated to form the filter.

Preferably, the bicomponent polymer fibers of this structure may have a sheath of one polymer and a core of a different polymer having a melting point higher than the one polymer. The core may comprise polypropylene and the sheath may comprise polyethylene.

Additionally, the core may be disposed eccentric relative to the sheath. In such a structure, the fibers will crimp with the result that the bulkiness of the tier is further increased.

Preferably and alternatively, the above discussed composite filter may comprise an upstream tier further having fibers selected from at least one of uncharged split film fibers, charged split film fibers and mixed electrostatic fibers.

Accordingly, the present invention now provides a composite filter comprising at least two nonprebonded tiers each tier independently comprising at least one filtration material and being distinct from the adjacent tier, in which the tiers are bonded together to form a unitary stratified structure having a first boundary surface adapted to receive particulates entrained in air and a second boundary surface adapted to discharge filtered air, this composite filter showing a reduced pressure drop and a prolonged service life time.

All above discussed composite filters may be embodied in a vacuum cleaner bags, and more generally in vacuum filters. By "vacuum filter" is meant a filter structure intended to operate by passing a gas, preferably air, which entrains usually dry solid particles, through the structure. The convention has been adopted in this application to refer to the sides, tiers and layers of the structure in relation to the direction of air flow. That is, the filter inlet side is "upstream" and the filter discharge side is "downstream" for example. Occasionally herein the terms "in front of" and "behind" have been used to denote relative positions of structure elements as being upstream and downstream respectively. Of course, there will be a pressure gradient, sometimes referred to as "pressure drop", across the filter during filtration. Vacuum cleaners typically use bag shaped filters. Normally, the upstream side of a vacuum bag filter is the inside and the downstream side is outside.

In addition to vacuum cleaner bags, the composite filter of the invention can be utilized 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, respirators, surgical face masks and the like. Optionally, the composite filter can be used in such applications with an additional carbon fiber or particle-containing layer in series with the composite filter of the invention, for example to absorb odors or toxic contaminants. Moreover, certain applications, such as HEPA and clean room filters can employ additional layers in series with the composite filter of the invention, such as low porosity polytetrafluorethylene (PTFE) membrane laminated to a boundary surface of an appropriate unitary stratified structure, composite filter.

The present invention also provides a method of making a composite filter of the above kind, comprising the steps of (a) laying down a filtration material onto a support to form the upstream non-prebonded tier, (b) depositing onto the upstream tier the downstream non-prebonded tier, and (c) bonding the tiers to form a composite filter having a unitary stratified structure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram showing in cross section an embodiment of the composite filter in accordance with the invention having a unitary stratified structure of two tiers.

FIG. 2 is a diagram showing the pressure drop of the composite filter in FIG. 1 and a composite prior art filter.

FIG. 3 is a schematic diagram showing in cross section another embodiment of the composite filter in accordance with the invention having a unitary stratified structure of three tiers.

FIG. 4 is a schematic diagram showing in cross section another embodiment of the composite filter in accordance with the invention having a unitary stratified structure of four tiers.

FIG. 5 is a schematic diagram showing in cross section another embodiment of the composite filter in accordance with the invention having a unitary stratified structure of five tiers.

FIG. 6 is a schematic diagram showing in cross section another embodiment of the two-tiered composite filter of FIG. 1 in combination with a filter layer adjacent thereto.

FIG. 7 is a schematic diagram showing in cross section another embodiment of the three-tiered composite filter of FIG. 3 in combination with a filter layer adjacent thereto.

FIG. 8 is a schematic diagram showing in cross section another embodiment of the four-tiered composite filter of FIG. 4 in combination with a filter layer adjacent thereto.

FIG. 9 is a schematic diagram showing in cross section another embodiment of the five-tiered composite filter of FIG. 5 in combination with a filter layer adjacent thereto.

FIG. 10 is a schematic cross section diagram showing the two-tiered composite filter of FIG. 6 bonded to an adjacent filter layer with an adhesive or ultrasonically bonded layer.

FIG. 11 is a schematic cross section diagram showing the three-tiered composite filter of FIG. 7 bonded to an adjacent filter layer with an adhesive or ultrasonically bonded layer.

FIG. 12 is a schematic cross section diagram showing the four-tiered composite filter of FIG. 8 bonded to an adjacent filter layer with an adhesive or ultrasonically bonded layer.

FIG. 13 is a schematic cross section diagram showing the five-tiered composite filter of FIG. 9 bonded to an adjacent filter layer with an adhesive or ultrasonically bonded layer.

FIG. 14 is a schematic diagram of an inline process for producing a composite filter according to a preferred embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the following, and before discussing explicitly discussing the preferred embodiments of the invention, different filter material compositions which suitably used in the present invention are described in greater detail:

Regarding the discussion below, DIN 44956-2 test has been employed to determine the increase in pressure drop of five different examples of vacuum cleaner bag constructions after dust loading with 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 the DIN 44956-2 is performed at 0.5 gram increments from 0 to 2.5 g/(m.sup.2.times.s) on seven bags of each sample. However, the pressure drop values are not recorded again. The maximum sustainable air permeability values are then determined on the bags, which had the specified levels of dust loading.

Standard Vacuum Cleaner Filter Bag Material:

This material, sometimes referred to as "standard paper" has traditionally been used as a single ply in which it provides dust filtration and containment, as well as the strength and abrasion resistance required of a vacuum cleaner bag. This material is also rigid enough to enable easy fabrication on standard bag manufacturing equipment. The paper is predominantly composed of unbleached wood pulp with 6 7% of a synthetic fiber such as poly[ethylene terephthalate] (PET) type polyester, and is produced by the wet laying process. The standard paper typically has a basis weight of about 30 80 g/m.sup.2 and commonly about 50 g/m.sup.2. The PET fibers typically have a fineness of 1.7 dtex and lengths of 6 10 mm. This paper has air permeability in the range of about 200 500 L/(m.sup.2.times.s) and an average pore size of about 30 mm. However, the efficiency as determined from the DIN 44956-2 Test is only about 86%. Another characteristic is that the pores are quickly clogged with dust and the dust holding capacity is further limited-by the very thin paper thickness of only about 0.20 mm.

Spunbond Nonwoven:

A nonwoven of spunbond polymer fibers can be deployed as a filtration tier in the structure. The fibers can be of any spunbond-capable polymer such as polyamides, polyesters or polyolefins. Basis weight of the spunbond nonwoven should be about 10 100 g/m.sup.2 and preferably about 30 40 g/m.sup.2. The spunbond nonwoven should have an air permeability of about 500 10,000 L/(m.sup.2.times.s), and preferably about 2,000 6,000 L/(m.sup.2.times.s) as measured by DIN 53887. The spunbond can also be electrostatically charged.

Scrim or Supporting Fleece:

Scrim refers to a generally light basis weight, very open porous paper or nonwoven web. Basis weight of the scrim is typically about 10 30 g/m.sup.2, and frequently about 13 17 g/m.sup.2. The scrim, sometimes referred to as a supporting fleece usually has air permeability of about 500 10,000 L/(m.sup.2.times.s). It is primarily employed to protect other tiers or layers from abrasion. The scrim can also filter the very largest particles. The scrim, as well as any tier of the filter composite, can be electrostatically charged provided the material has suitable dielectric properties.

Wet-Laid High Dust Capacity Material:

Wet-laid High Dust Capacity material, frequently referred to herein as "wet-laid capacity paper" is bulkier, thicker and more permeable than the standard vacuum cleaner bag filter paper. It performs multiple functions. These include resisting shock loading, filtering of large dirt particles, filtering a significant portion of small dust particles, holding large amounts of particles while allowing air to flow through easily, thereby providing a low pressure drop at high particle loading which extends the life of the filter.

The wet-laid capacity paper usually comprises a fiber mixture of wood pulp fibers and synthetic fibers. It typically contains up to about 70% wood pulp and correspondingly more synthetic fiber, such as PET, than the standard paper described above. It has a greater thickness than the standard paper of about 0.32 mm at a typical basis weight of 50 g/m.sup.2. Pore size also is much greater, in that the average pore size can be greater than 160 mm. Thus, the paper is able to hold much more dust in its pores before clogging up. Basis weight of the wet-laid capacity paper typically is about 30 150 g/m.sup.2 and preferably about 50 80 g/m.sup.2.

The wet-laid capacity paper has a fine dust particle filtration efficiency of about 66 67% as determined by the DIN 44956-2. Importantly, the wet-laid capacity paper has air permeability higher than the standard filter paper. The permeability lower limit thus preferably should be at least about 500 L/(m.sup.2.times.s), more preferably at least about 1,000 L/(m.sup.2.times.s) and most preferably at least about 2,000 L/(m.sup.2.times.s). The upper limit of permeability is defined to assure that the paper filters and holds a major fraction of the dust particles larger than about 10 mm. Consequently, a secondary high efficiency filter medium positioned downstream is able to filter out and contain fine particles much longer before showing indication of a substantial pressure drop increase across the filter. Accordingly, the air permeability of the wet-laid capacity paper preferably should be at most about 8,000 L/(m.sup.2.times.s), more preferably at most about 5,000 L/(m.sup.2.times.s), and most preferably at most about 4,000 L/(m.sup.2.times.s). It is thus seen that the wet-laid capacity paper is especially well designed as a multipurpose filtration tier to be positioned upstream of the secondary high efficiency filtration tier.

Dry-Laid High Dust Capacity Material:

Dry-laid high dust capacity material, sometimes referred to herein as "dry-laid capacity paper", had not been used as a filter in vacuum cleaner bags. Dry-laid paper is not formed from a water slurry, but is produced with air-laying technology and preferably by a "fluff pulp" process. Hydrogen-bonding which plays a large roll in attracting the molecular chains together does not operate in the absence of water. Thus, at the same basis weight, dry-laid capacity paper, is usually much thicker than standard paper and the wet-laid capacity paper. For a typical weight of 70 g/m.sup.2, the thickness is 0.90 mm, for example.

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

The second method is thermal bonding, for example by utilizing binder fibers. Binder fibers sometimes referred to herein as "thermally bondable fusing fibers" are defined by the Nonwoven Fabric Handbook, (1992 edition) as "Fibers with lower softening points than other fibers in the web. Upon the application of heat and pressure, these act as an adhesive." These thermally bondable fusing fibers generally completely melt at locations where sufficient heat and pressure are applied for the web, thereby adhering the matrix fibers together at their cross-over points. Examples include co-polyester polymers which when heated adhere a wide range of fibrous materials.

In a preferred embodiment thermal bonding can be accomplished by adding from at least 20% preferably up to 50% of a bicomponent ("B/C") polymer fiber to the dry-laid web. Examples of B/C fibers include fibers with a core of polypropylene ("PP") and a sheath of more heat sensitive polyethylene ("PE"). The term "heat sensitive" means that thermoplastic fibers soften and become sticky or heat fusible at a temperature of 3 5 degrees C. below the melting point. The sheath polymer preferably should have a melting point in the range of about 90 160 degrees C. and the core polymer should have a higher melting point, preferably by at least about 5 degrees C. higher than that of the sheath polymer. For example, PE melts at 121 degrees C. and PP melts at 161 163 degrees C. This aids in bonding the dry-laid web when it passes between the nip of a thermal calendar or into a through-air oven by achieving thermally bonded fibers with less beat and pressure to produce a less compacted, more open and breathable structure. In a more preferred embodiment the core of the core/sheath of the B/C fiber is located eccentric of the sheath. The more that the core is located towards one side of the fiber the more likely that the B/C fiber will crimp during the thermal bonding step, and thereby increase the bulk of the dry-laid capacity. This will, of course, improve its dust holding capacity. Thus, in a still further preferred embodiment the core and sheath are located side-by-side in the B/C fiber, and bonding is achieved with a through-air oven. A thermal calendar, which would compress the web more than through-air bonding and is less preferred in this case. Other polymer combinations that may be used in core/sheath or side-by-side B/C fibers include PP with co-polyester, low melting polymers, and polyester with nylon 6. The dry-laid high capacity tier can also be constituted essentially completely by bicomponent fibers. Other variations of bicomponent fibers in addition to "sheath/core", can be used, such as "side-by-side", "islands in the sea" and "orange" embodiments disclosed in Nonwoven Textiles, Jirsak, O., and Wadsworth, L. C., Carolina Academic Press, Durham, N.C., 1999, pp. 26 29.

Generally, the average pore size of dry-laid capacity is intermediate between the pore size of the standard paper and wet-laid capacity paper The filtration efficiency as determined by the DIN 44956-2 Test is approximately 80%. Dry-laid capacity paper should have about the same basis weight and the same permeability as the wet-laid capacity paper described above, i.e., in the range of about 500 8,000 L/(m.sup.2.times.s), preferably about 1,000 5,000 L/(m.sup.2.times.s) and most preferably about 2,000 4,000 L/(m.sup.2.times.s). It has 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 the paper consist essentially of fluff pulp. The term "fluff pulp" means a nonwoven component of the filter of this invention which is prepared by mechanically grinding rolls of pulp, i.e., fibrous cellulose material 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 grind the pulp. So-called Dan Web or M and J machines are used for dry forming. A fluff pulp component and the dry-laid tiers of fluff pulp are isotropic and are thus characterized by random fiber orientation in the direction of all three orthogonal dimensions. That is, they have a large portion of fibers oriented away from the plane of the nonwoven web, and particularly perpendicular to the plane, as compared to three-dimensionally anisotropic nonwoven webs. Fibers of fluff pulp utilized in this invention preferably are from about 0.5 5 mm long. The fibers are held together by a latex binder. The binder can be applied either as powder or emulsion.

Binder is usually 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 weight of fibers.

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" and bicomponent polymer fibers. More preferably, the blend of fluff pulp fibers comprises fluff pulp fibers and bicomponent polymer fibers.

Split Film Fibers:

Split film fibers are essentially flat, rectangular fibers which may be electrostatically charged before or after being incorporated into the composite structure of the invention. The thickness of the split film fibers may range from 2 100 micrometers, the width may range from 5 micrometers to 500 micrometers, and the length may range from 0.5 to 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 which is suitable for making fibers may be used for the split film fibers of the composite structures of the invention. Examples of suitable polymers include, but are not limited to, polyolefins like 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 nylons, polyamides, polystyrenes, poly-4-methylpentene-1, polymethylmethacrylates, polyurethanes, silicones, polyphenylene sulfides. The split film fibers may also comprise a mixture of homopolymers or copolymers. In the present application, the invention is exemplified with split film fibers made of polypropylene.

The use of PP polymers with various molecular weights and morphologies in laminate film structures has been shown to produce films with a proper balance of mechanical properties and brittleness required to produce split film fibers. These PP split film fibers may also be subsequently given the desired level of crimp. All dimensions of the split film fibers may, of course, be varied during manufacture of the fibers.

One method for production of the split fibers, is disclosed in U.S. Pat. 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 allowed to enter, in accordance with conventional blow stretching technology. Inflating the balloon with air serves to quench the film and to bi-axially orient the molecular structure of the PP molecular chains, resulting in greater strength. The balloon is then collapsed 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 the application of varying amounts of pressure between the two contacting rollers. This results in an additional stretch in the machine direction which is accomplished by driving the second set of rollers at a faster surface speed than the first set. The result is an even greater molecular orientation to the film in the machine direction which will subsequently become the long dimension of the split film fibers.

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

The second, preferred, method utilizes the electrostatic charging technologies described in U.S. Pat. No. 5,401,446 (Wadsworth and Tsai, 1995), which is referred to as Tantret(tm) Technique I and Technique II, which are further described herein. It has been found that Technique II, in which the film is suspended on insulated rollers as the film passes around the inside circumference of two negatively charged metal shells with a positive corona wire of each shell, imparts the highest voltage potentials to the films. Generally, with Technique II, positive 1,000 to 3,000 volts or more may be imparted to on one side of the films with similar magnitudes of negative volts on the other side of the charged film. Technique I, wherein films contact a metal roller with a DC voltage of -1- to -10 kV and a wire having a DC voltage of +20 to +40 kV is placed from about 1 to 2 inches above the negatively biased roller with each side of the film being exposed in succession to this roller/wire charging configuration, results in lower voltage potentials as measured on the surfaces of the films. With Technique I, voltages of 300 to 1,500 volts on the film surface with generally equal but opposite polarities on each side are typically obtained. The higher surface potentials obtained by Technique II, however, have not been found to result in better measurable filtration efficiencies of the webs made from the split film fibers. Therefore, and because it is easier to lace up and pass the film through the Technique I device, this method is now predominately used to charge the films prior to the splitting process.

The cooled and stretched film may be hot or cold electrostatically charged. The film is then simultaneously stretched and split to narrow widths, typically up to about 50 micrometers. The split, flat filaments are then gathered into a tow which is crimped in a controlled numbers 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 blend of all of fluff pulp fibers, bicomponent polymer fibers, and electrostatically charged split film fibers. Preferably, the fluff pulp fibers will be present at about 5 85 wt %, more preferably about 10 70 wt %, and most preferably about 40 wt %, the bicomponent fibers at about 10 60 wt %, more preferably about 10 30 wt % and most preferably about 20 wt %, and the electrostatically charged split film fibers at about 20 80 wt %, and more preferably about 40 wt %. This dry-laid high dust capacity may be thermally bonded, preferably at a high temperature of 90 160 degrees C., more preferably, at a temperature lower than 110 degrees C. and most preferably at about 90 degrees C.

Mixed Electrostatic Fibers:

Other preferred embodiments of the dry-laid capacity paper comprises a thermally bonded paper with 100% "mixed electrostatic fibers", a blend of 20 80% mixed electrostatic fibers and 20 80% B/C fibers, and a blend of 20 80% mixed electrostatic fibers, 110 70% fluff pulp and 10 70% B/C fibers. "Mixed electrostatic fiber" filters are made by blending fibers with widely different triboelectric properties and rubbing them against each other or against the metal parts of machines, such as wires on carding cylinders during carding. This makes one of the types of fibers more positively or negatively charged with respect to the other type of fiber, and enhances the coulombic attraction for dust particles. The production of filters with these types of mixed electrostatic fibers is taught in U.S. Pat. No. 5,470,485 and in European Patent Application EP 0 246 811.

In U.S. Pat. No. 5,470,485, the filter material consists of a blend 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 11 are "halogen free". The (I) fibers also have some "halogen-substituted polyolefins": whereas, the acrylonitrile fibers have no halogen. The patent notes that the fibers must be thoroughly washed with nonionic detergent, with alkali, or solvent and then well rinsed before being mixed together so that they do not have any lubricants or antistatic agents. Although the patent teaches that the fiber mat produced should be needle-punched, these fibers could also be cut to lengths of 5 20 mm and mixed with similar length bicomponent thermal binder fibers and also with the possible addition of fluff pulp so that dry-laid thermally bonded paper can be utilized in this invention.

EP 0 246 811 describes the triboelectric effect of rubbing two different types of fibers together. It teaches using similar types of fibers as U.S. Pat. No. 5,470,485, except that the --CN groups of the polyacrylonitrile fibers may be substituted by halogen (preferably fluorine or chlorine). After a sufficient amount of substitution of --CN by --Cl groups, the fiber may be referred to as a "modacrylic" if the copolymer comprises from 35 to 85% weight percent 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, U.S. Pat. No. 5,470,485 teaches that the ratio of polyolefin to polyacrylonitrile fibers is in the range of 30:70 to 80:20, relative to a surface of the filter material. Thus, these ranges of ratios of polyolefin to acrylic or modacrylic fibers may be used in the above stated proportions in the dry-laid thermally bonded capacity paper.

Meltblown Fleece:

A synthetic polymer fiber meltblown fleece can optionally be deployed as an tier between a multipurpose tier and a high efficiency filtration tier. The meltblown fleece tier increases overall filtration efficiency by capturing some particles passed by the multipurpose filtration tier. The meltblown fleece tier also optionally can be electrostatically charged to assist in filtering fine dust particles. Inclusion of a meltblown fleece tier involves an increase in pressure drop at given dust loading as compared to composites not having a meltblown fleece ter.

The meltblown fleece preferably has a basis weight of about 10 50 g/m.sup.2 and air permeability of about 100 1,500 L/(m.sup.2.times.s).

High Bulk Meltblown Nonwoven:

Another discovery from recent research to develop improved vacuum cleaner bags was the development of a high bulk MB web or tier which could be used upstream of a filtration grade MB fleece as a pre-filter in place of the wet-laid capacity paper or dry-laid capacity paper. The high bulk MB pre-filter can be made in a meltblowing process using chilled quench air with a temperature of about 10 degrees C. In contrast, conventional MB normally uses room air at an ambient temperature of 35 45 degrees C. Also the collecting distance from the MB die exit to the web take-up conveyer is increased to 400 600 mm in the high bulk MB process. The distance normally is about 200 mm for regular MB production. Additionally, high bulk MB nonwoven is made by using a lower temperature attenuation air temperature of about 215 235 degrees C. instead of the normal attenuation air temperature of 280 290 degrees C., and a lower MB melt temperature of about 200 225 degrees C. compared to 260 280 degrees C. for filtration grade MB production. The colder quench air, lower attenuation air temperature, lower melt temperature and the longer collecting distance cool down the MB filaments more. Removing beat results in less draw down of the filaments, and hence, in larger fiber diameters than would be found in typical filtration grade MB webs. The cooler filaments are much less likely to thermally fuse together when deposited onto the collector. Thus, the High Bulk Meltblown nonwoven would have more open area. Even with a basis weight of 120 g/m.sup.2, the air permeability of the High Bulk Meltblown nonwoven is 806 L/(m.sup.2.times.s). By contrast, a much-lighter (e.g., 22 g/m.sup.2) filtration grade MB PP web had a maximum air permeability of only 450 L/(m.sup.2.times.s). The filtration efficiency of the High Bulk MB nonwoven as determined by the DIN 44956-2 Test was 98%. When the two were put together with the High Bulk MB nonwoven on the inside of the bag, the air permeability was still 295 L/(m.sup.2.times.s), and the filtration efficiency of the pair was 99.8%. The high bulk meltblown nonwoven can be uncharged, or