MULTI-DIE MELT BLOWING SYSTEM FOR FORMING CO-MINGLED STRUCTURES AND METHOD THEREOF

Abstract

A melt blowing system for the manufacturing of pleatable/moldable nonwoven fabrics includes a collector positioned to receive a plurality of fibers, a first die in fluid communication with a first liquid polymer supply having a melt flow index of about 500 or lower, and a second die in fluid communication with a second liquid polymer supply having a melt flow index of about 500 or higher. The first die has a concentric air design, includes a plurality of spinneret nozzles facing the collector surface, and is configured to draw a first plurality of fibers. The second die includes a plurality of spinneret nozzles having smaller capillary diameter than those of the first die and is positioned to draw a second plurality of fibers such that the first and second pluralities of fibers form a co-mingled nonwoven web with varying fiber diameters on the collector surface.

Claims

1. A method of forming a nonwoven fabric of co-mingled meltblown fibers, the method comprising: drawing a first plurality of fibers from a first polymer having a melt flow index of 500 or lower, using a first die comprising a plurality of spinneret nozzles facing a collector surface having a surface positioned to receive a plurality of fibers, wherein the first die has a concentric air design comprising nozzles with a capillary diameter in the range of 500 microns to 850 microns and with individual concentric air jets surrounding each nozzle; and drawing a second plurality of fibers from a second polymer having a melt flow index of 500 or higher, using a second die comprising a plurality of spinneret nozzles having a capillary diameter in the range of 100 microns to 500 microns and facing the collector surface, and wherein the first and the second plurality of fibers are drawn in such a way and the second die is positioned in such a way, that the second plurality of fibers have fiber diameters of less than 10 microns and the first plurality of fibers and the second plurality of fibers form a co-mingled nonwoven web on the collector surface, wherein the co-mingled nonwoven web formed of fibers has varying fiber diameters and the first plurality of fibers has a larger average diameter than the second plurality of fibers.

2. The method of claim 1, wherein the second die has a concentric air design with individual concentric air jets surrounding each of the plurality of spinneret nozzles or which is a single-row-capillary type die design with impinging air streams from both sides of a die tip.

3. The method of claim 1, wherein the first polymer is of the same polymer species as the second polymer, and wherein the melt flow viscosity of the first polymer is different from the melt flow viscosity of the second polymer.

4. The method of claim 1, wherein the first polymer is from a different polymer species than the second polymer.

5. The method of claim 1, wherein the second polymer comprises a first polyolefin polymer and the first polymer comprises at least one of a polyamide, a polyester, elastomers, a second polyolefin, and combinations thereof.

6. The method of claim 1, wherein the second polymer comprises polypropylene and the first polymer comprises polybutylene terephthalate.

7. The method of claim 1, further comprising introducing a third material into the co-mingled web on the collector surface using an applicator, wherein the third material is selected from the group consisting of a particulate material, a fibrous material, a plurality of capsules, and combinations thereof.

8. The method of claim 1, further comprising introducing a third material into the co-mingled web on the collector using an applicator, wherein the third material is a carded web or a textile fabric comprising a plurality of fibers, a film-like material, or paper.

9. The method of claim 1, further comprising producing the co-mingled nonwoven web such that it is pleatable or moldable.

10. The method of claim 1, further comprising introducing a third material into the co-mingled web on the collector surface using an applicator.

11. A nonwoven fabric comprising co-mingled meltblown fibers of different diameters comprising a first plurality of fibers formed of a polymer and a second plurality of fibers formed of a polymer having a melt flow index of 500 or higher, the second plurality of fibers having fiber diameters of less than 10 microns and wherein the first plurality of fibers have a larger average diameter than the second plurality of fibers.

12. The nonwoven fabric of claim 11, wherein the second polymer comprises a first polyolefin polymer and the first polymer comprises at least one of a polyamide, a polyester, elastomers, a second polyolefin, and combinations thereof.

13. The nonwoven fabric of claim 11, wherein the second polymer comprises polypropylene and the first polymer comprises polybutylene terephthalate.

14. The nonwoven fabric of claim 11, wherein the nonwoven fabric is in the form of pleated or molded filtration media.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Having thus described the present disclosure in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

[0046] FIG. 1 is a schematic drawing illustrating the melt blowing stream of fibers being deposited onto a drum collector;

[0047] FIG. 2 is a schematic drawing of a system having dual melt blowing dies and a deposition slot for converging fiber streams and trapping materials deposited therein;

[0048] FIG. 3 is a schematic drawing of an embodiment of a filter formed from a single-layer pleatable nonwoven fabric described herein;

[0049] FIG. 4 is a schematic drawing including FIGS. 4A-4C illustrating meltblown fiber streams with varying degrees of convergence;

[0050] FIG. 5A is a schematic drawing illustrating a concentric-air die type technology having concentric air jets surrounding each nozzle;

[0051] FIG. 5B is a schematic drawing illustrating an impinging air die type technology having impinging air streams from both sides of a die tip;

[0052] FIG. 6 is a schematic drawing of a melt blowing system comprising two meltblown dies having different capillary diameters;

[0053] FIGS. 7A-7D are scanning electron microscope (SEM) images of meltblown fabrics made from a multi-die system with a high degree of converging;

[0054] FIGS. 8A-8D are SEM images of meltblown fabrics made from a multi-die system with a medium degree of converging;

[0055] FIGS. 9A-9D are SEM images of meltblown fabrics made from a multi-die system with a low degree of converging;

[0056] FIGS. 10A-10B are SEM images of meltblown fabrics made from a multi-die system with a high degree of converging;

[0057] FIGS. 11A-11B are SEM images of meltblown fabrics made from a multi-die system with a low degree of converging;

[0058] FIG. 12 is a graph comparing filtration performance of samples produced with a dual die apparatus to filtration performance of samples produced with a single die system;

[0059] FIGS. 13A-13D are SEM images of meltblown fabrics made from a multi-die system with a high degree of converging;

[0060] FIGS. 14A-14D are SEM images of meltblown fabrics made from a multi-die system with a low degree of converging;

[0061] FIG. 15 is a graph comparing filtration performance of polypropylene samples produced with a dual die apparatus to filtration performance of polypropylene/PBT samples produced with a dual die apparatus; and

[0062] FIGS. 16A and 16B are photographs of meltblown fabrics made from a multi-die system that have been pleated.

DETAILED DESCRIPTION

[0063] The present inventions will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. As used in this specification and the claims, the singular forms a, an, and the include plural referents unless the context clearly dictates otherwise.

[0064] Embodiments of the present invention provide a multiple array of melt blowing dies for producing co-mingled fibrous webs. As used herein, the term co-mingled is used to refer to a fibrous web comprising two or more pluralities of fibers at least partially mixed. The degree to which the two or more pluralities of fibers are co-mingled (i.e., mixed/blended) can be varied as described in more detail below. In some embodiments, other materials can be introduced into the co-mingled fibrous web by depositing the material onto the converging fiber streams.

[0065] As used herein, the term fiber is defined as a basic element of textiles which has a high aspect ratio of, for example, at least about 100 times. In addition, filaments/continuous filaments are continuous fibers of extremely long lengths that possess a very high aspect ratio. Staple fibers are cut lengths from continuous filaments. The term multicomponent fibers refers to fibers that comprise two or more polymers that are different by physical or chemical nature including bicomponent fibers. The term nonwoven as used herein in reference to fibrous materials, webs, mats, batts, or sheets refers to fibrous structures in which fibers are aligned in an undefined or random orientation.

[0066] The fibers according to the present invention can vary, and include fibers having any type of cross-section, including, but not limited to, circular, rectangular, square, oval, triangular, and multi-lobal. In certain embodiments, the fibers can have one or more void spaces, wherein the void spaces can have, for example, circular, rectangular, square, oval, triangular, or multi-lobal cross-sections. The fibers may be selected from single-component (i.e., uniform in composition throughout the fiber) or multicomponent fiber types including, but not limited to, fibers having a sheath/core structure and fibers having an islands-in-the-sea structure, as well as fibers having a side-by-side, segmented pie, segmented cross, segmented ribbon, or tipped multi-lobal cross-sections.

[0067] The means of producing a nonwoven web can vary. In general, nonwoven webs are typically produced in three stages: web formation, bonding, and finishing treatments. Web formation can be accomplished by any means known in the art. For example, webs may be formed by a drylaid process, a spunlaid process, or a wetlaid process. In various embodiments of the present invention, the nonwoven web is made by a melt blowing process.

[0068] Melt blowing technology is a melt-spun process that can be used to produce microfibers by injecting a molten polymer stream into high velocity gas jets. As illustrated in FIGS. 1, 5A and 5B, for example, a high-velocity gas jet impinges upon the polymer as it emerges from the spinneret 4. An extruder 1 can feed a polymer to a first die 3 and through spinneret 4. Air can come in the air intake 5 and into the air manifold 2. High pressure air is then used to draw the polymer into a fiber which can be collected on collector 6. The drag force caused by the air attenuates the fiber rapidly, and reduces its diameter as much as a hundred times from the nozzle diameter. Melt blown webs are typically reported to have fibers in the range of 0.1-10 m, high surface area per unit weight, high insulation value, self-bonding, and high barrier properties yet breathability.

[0069] The fiber formation process in melt blowing can be critically dependent on the aerodynamics of the process. For example, the drag force due to the high-speed air is the main cause of fiber attenuation. Primary air systems include high-speed air jets that impinge upon the molten polymer streams once they exit the die. Secondary cross flow air streams can be employed to provide cold or ambient air to quench the extruded filaments. See, e.g., U.S. Pat. No. 5,080,569 to Gubernick et al., which is incorporated by reference herein. See also the secondary air quenching system for a spun-bonding die system disclosed in U.S. Pat. No. 5,098,636 to Balk, herein incorporated by reference. Accelerating the air below the die face by recessing the die tip above the die face in order to increase the air velocity can thereby increase the drag force and fiber attenuation. See, e.g., U.S. Pat. Pub. No. 2003/0173701 to Arseneau et al. It has been shown that for such inset dies, the maximum turbulence intensity occurs right at the die face, where the constriction is at its smallest and where a molten polymer fiber might start to vibrate and stick to the die tip. The use of a cold secondary air stream followed by an air constrictor can be used to increase the fiber attenuation after fibers solidified. See, e.g., WO 2006/037371, which is herein incorporated by reference. However, such rapid cooling of the fibers near the die face can result in a larger fiber diameter, and the cross flow direction can make the fibers sticks or accumulate on the edges of the air constrictors.

[0070] Melt blowing is generally capable of providing fibers with relatively small diameters. Diameter and other properties of meltblown fibers can be tailored by modifying various process parameters (e.g., die design (discussed in more detail below), die capillary size, polymer throughput, air characteristics, collector placement, and web handling). Die capillary size refers to the diameter of the holes in a die through which a polymer is fed during a melt blowing process. A larger die capillary diameter can contribute to forming fibers having larger diameters. Polymer throughput can be measured in grams of polymer per hole per minute (ghm). A larger throughput can contribute to producing coarser (i.e., larger diameter) fibers. Attenuating the air pressure affects fiber size, as higher pressures typically yield finer fibers (e.g., up to about 5 microns, such as about 1-5 microns) and lower pressures yield coarser fibers (e.g., up to about 30 microns, such as about 10-30 microns).

[0071] In certain embodiments of the present invention, the nonwoven web comprises meltblown fibers having average diameters in the range of about 1 to about 10, e.g., about 2 to about 5 microns. In some embodiments, the mean flow pore size of a meltblown nonwoven web can be about 20 microns or less, about 10 microns or less, about 8 microns or less, about 5 microns or less, about 2 microns or less, or about 1 micron or less. The meltblown fibers typically comprise single component fibers.

[0072] As disclosed herein, a multiple array of melt blowing dies can be used for producing co-mingled fibrous webs. See, for example, the system illustrated in FIG. 2. As shown in FIG. 1, for example, the system can comprise a first extruder to feed a first polymer to a first die and a second extruder to feed a second polymer to a second die. The system can further comprise a collector positioned to receive fibers from the first and second dies. The first and second die can be positioned such that a co-mingled web is formed on the collector. Optionally, the system can comprise an applicator configured to optionally introduce a third material into the co-mingled web.

[0073] Embodiments of the present invention provide a system using two or more die types and/or configurations where multiple fiber streams converge, wherein each die represents different technology such that different types and/or sizes of fibers can be co-mingled to form a unique hybrid nonwoven structure. Conventional meltblown die technologies can be roughly classified into two categories: (1) single-row-capillary or impinging air type die design, which is also well-known as Exxon design; (2) multiple-row-concentric-air type design, which is also known as Biax/Schwarz design.

[0074] As illustrated in FIG. 5B, for example, a conventional meltblown technology (i.e., single-row-capillary or impinging-air type die design) has a single row of spinning capillaries with impinging air streams from both sides of the die tip to draw the fibers. The safe operation pressure of this process is less than about 100 bar, for example. The concentric air-type meltblown die technology features multiple rows of spinning nozzles with individual concentric air jets to attenuate the fibers. It also tolerates high melt pressures at the spinneret and therefore can utilize higher viscosity polymers with a wide operation window. See, e.g., R. Zhao, Melt Blowing Polyoxymethylene Copolymer, International Nonwoven Journal, Summer 2005, pp. 19-21 (2005), herein incorporated by reference. As such, the different die types can return different sized fibers and utilize different polymers.

[0075] Currently available polymers of interest have melt flow rates mostly lower than 100 compared to common meltblown grade polypropylene resins that have melt flow rates higher than 100 and as much as 2400, for example. In other words, one can expect high melt pressure during melt blowing of these polymers. A concentric-air design die type can be used with polymers with lower melt flow rates. For example, a concentric-air design die type can be used to draw polymers with a melt flow of about 1600 to about 30 melt flow or less. A concentric-air design die type can comprise capillaries each having a diameter that ranges from about 100 to about 850 microns.

[0076] In various embodiments of the present invention, an array of dies are arranged in a co-mingled system, wherein the system comprises a first die and a second die. The first die can be a concentric-air type die design and the second die can be a concentric-air type die design or a single-row-drilled-hole type die design. Use of the first die in the form of a concentric air-type die is advantageous to form larger diameter fibers on a co-mingled structure, particularly where the larger diameter fibers (e.g., polybutylene terephthalate fibers) are constructed of a polymer having a melt flow index of 500 or lower. The second die is used to form smaller diameter fibers (e.g., polypropylene fibers), which can be for example, adapted primarily for purposes of providing good filtration performance in the co-mingled structure. Advantageously, the melt flow index of the smaller diameter fibers can be about 500 or higher.

[0077] The dies can be arranged to allow a wide range of angles between each die and the collector from about 10 to about 90 degrees. The distance between the two dies and the collector surface and the angle between the dies can also be adjusted to allow various degrees of co-mingling. For example, the overall die to collector distance can range from about 100 mm to about 500 mm.

[0078] In some embodiments of a co-mingled meltblown system, the system can comprise a first die and a second die, wherein the first die can have a first capillary size and the second die can have a second capillary size. In certain embodiments, the first die comprises capillaries in the range of about 500 microns to about 850 microns in order to produce larger diameter fibers. In certain embodiments, the second die comprises capillaries in the range of about 100 microns to about 500 microns in order to produce finer fibers.

[0079] In some embodiments of a co-mingled meltblown system, the system can comprise a concentric air-type die where capillaries have different sizes either in rows or in an alternate pattern. The concentric air-type die is advantageously configured to produce larger diameter fibers to add strength to a co-mingled meltblown structure, such as a concentric air-type die with capillaries sized about 500 to about 600 microns. The system can further comprise a second concentric air-type die or a single-row-capillary type die with a capillary size that produces finer fibers (e.g., capillaries sized about 100 to about 300 microns). In a multi-rowed concentric air type die design, it is also possible to vary the capillary size between rows of nozzles such that different size fibers are provided by the same single die.

[0080] In some embodiments, the first die can comprise a first nozzle size and a first nozzle geometry. The second die can comprise a second nozzle size and a second nozzle geometry. In certain embodiments, the first nozzle size and the first nozzle geometry are different from the second nozzle size and the second nozzle geometry.

[0081] The manner in which the two meltblown polymer streams converge can determine the level of co-mingling (e.g., a fully co-mingled structure or a structure composed of two layers with only some of the fibers being co-mingled). As illustrated in FIG. 4A, for example, at about 10 degrees to about 30 degrees, the webs can be layered with some of their respective fibers co-mingled. At higher angles, the degree of co-mingling increases, as demonstrated in FIGS. 4B and 4C, for example.

[0082] In various embodiments of the melt blowing system described herein, the two or more dies can be arranged to vary the degree of co-mingling between the fibers, as illustrated in FIG. 6, for example. The multi-die melt blowing system 60 can comprise a first die 62 and a second die 64. The first die can be designed to produce finer fibers and the second die can be designed to produce coarser fibers, for example. Dies having different sized capillaries can be used depending on the polymer used and the desired type of meltblown fibers to be produced (e.g., fine fibers or coarse fibers). Varying the alignments of the two dies can control the degree of converging between the two fiber types. In a first alignment, die 62 can be arranged such that a low converging stream A is provided from die 62. With the low degree of convergence setting, fibers from the second melt blowing die 64 are collected on the forming surface (e.g., the drum collector 64) first and then fibers from the first die 62 are laid over the fibers from the second melt blowing die 64. A convergence angle can be defined as the angle between the two streams coming from the two melt blowing dies. The convergence angle used for this low convergence setting can be about 15 to about 25, for example. In a second alignment, melt blowing die 62 can be arranged such that a medium converging stream B is provided from die 62. The medium converging stream B partially intermingles with stream D coming from the second melt blowing die 64 just before or approximately at the location where the streams met the drum collector 66. The convergence angle used for this medium convergence setting can be about 35 to about 45, for example. In a third alignment, melt blowing die 62 can be arranged such that a high converging stream C is provided from die 62. The high converging stream C intermingles with stream D coming from the second melt blowing die 64 in the air before the two streams hit the surface of the drum collector 66 such that a high amount of co-mingling occurs between the two streams. The convergence angle used for this high convergence setting can be about 50 to about 60, for example.

[0083] In various embodiments of the present invention, at least one thermoplastic polymer and/or a blend thereof can be used to fabricate the co-mingled nonwoven structures described herein. Fibers used in forming nonwoven co-mingled webs can include, for example, one or more thermoplastic polymers selected from the group consisting of: polyesters, co-polyesters, polylactic acid, polyamides, polyolefins, polyacrylates, thermoplastic liquid crystalline polymers, elastomers such as PBAX, ELASTOLLAN, KRATON and HYTREL, and combinations thereof. In some embodiments a single layer meltblown structure can be fabricated from fibers comprising polyester, co-polyester, polypropylene, polyethylene, or polyamide type polymeric materials, or combinations thereof. In various embodiments, at least about 50%, or about 60%, or about 70%, or about 80%, by weight of the nonwoven web comprises a polyamide, polypropylene, polybutylene terephthalate, polyvinylidene fluoride (PVDF), perfluoroalkoxy alkane (PFA), ELASTOLLAN, KRATON, HYTREL, or a combination thereof. Other fiber grade polymers can also be used to form the nonwoven webs described herein. The polymer compositions used to form the fibers of the co-mingled structures of the invention can optionally include other components not adversely affecting the desired properties thereof. Examples include, without limitation, antioxidants, stabilizers, particulates, pigments, and the like. These and other additives can be used in conventional amounts.

[0084] In various embodiments, the first polymer fed to the first die is of the same species as the second polymer fed to the second die. In certain embodiments, the first polymer is different from the second polymer. In some embodiments, a first polymer component having a first melt viscosity can be fed through a first die and a second polymer component having a second melt viscosity can be fed through a second die. In certain embodiments, the first melt viscosity can be different from the second melt viscosity such that the first melt viscosity is either higher or lower than the second melt viscosity. In certain embodiments, the first melt viscosity can be the same as (i.e., equal to) the second melt viscosity. The melt viscosities, expressed as melt flow index, can range from about 30 to about 2400, for example. MFI/MFR can be calculated using ISO 1133:2005 and expressed in grams per 10 minutes. Higher melt flows can be possible by adding, for example, an additive such as peroxide to unzip the polymer and reduce its molecular weight. Accordingly, melt flows as high as 7000 can be possible.

[0085] In a preferred embodiment, for example, the polymer fed to the second die (e.g., used to form the finer fibers) can be a first polyolefin and the polymer fed to the first die (e.g., used to form the coarser fibers) can comprise at least one of a polyamide, a polyester, elastomers, a second polyolefin, and combinations thereof. The melt viscosity of the polymer used for the coarser fibers can be higher than the melt viscosity of the polymer used to form the finer fibers. In a preferred embodiment, a liquid polymer supply having a melt flow index of about 500 or lower (e.g., about 400 or lower or about 300 or lower such as about 10 to about 400) can be fed to the first die to produce coarser fibers and a second liquid polymer supply having a melt flow index of about 500 or higher (e.g., about 600 or higher or about 700 or higher such as about 500 to about 2400) can be fed to the second die to produce finer fibers. In certain embodiments, the first die can be a concentric air-type die configured to produce coarser fibers from polymers having a melt flow index of about 500 or lower.

[0086] In some embodiments, a third material can be deposited into an individual fiber stream and/or the co-mingled fiber stream such that the third material would be inter-locked into the structure. If the two nozzles are converging, the process allows for the deposition of fibers (e.g., nano and/or micro fibers (i.e., fibers with a diameter in the range of several m down to below 100 nm), continuous fibers, multilobal fibers, a plurality of cut fibers, sub-micron fibers such as those produced by electrospinning or other means, a plurality of staple fibers, and the like), particles (including nanoparticles), a web of fibers (e.g., a carded web or another textile fabric), absorbents, other materials like films or paper, and combinations thereof into the converging streams, for example. The third material can then be co-mingled with the fibers formed from the first die and the second die. For example, an electrospun fiber can be deposited onto the converging stream to form a co-mingled structure composed of fine and coarse meltblown fibers (e.g., from the concentric air-type die and single-row-capillary type die, respectively) as well as electrospun fibers in the middle of the converging stream. Since the fibers are still tacky and not completely solidified, the deposited materials form a bond with the fibers.

[0087] Various embodiments of the present invention describe a method of forming a pleatable/moldable single-layer nonwoven fabric. Embodiments of the method can comprise positioning a first die and a second die to form a co-mingled web, wherein the first dies is a single-row-drilled-hole type die design wherein impinging air streams from both sides of a die tip are configured to draw a first fiber comprising a first polymer having a first melt viscosity, and wherein the second die is a concentric air design such that it features nozzles with individual concentric air jets, and wherein the second die is configured to draw a second fiber comprising a second polymer having a second melt viscosity. The position of each die relative to each other and the collector can control the degree of co-mingling of the extruded fibers as explained above.

[0088] The method can further comprise positioning a collector to receive the first fiber and the second fiber. Optionally, the method can comprise positioning an applicator configured to optionally introduce a third material into the co-mingled web. The die collector distance is adjustable to allow the degree of consolidation of the webs to be set to a desired amount. For example, the distance can be adjusted from about 10 cm to about 50 cm after the two or more layers of webs are co-mingled.

[0089] In a preferred embodiment, a method of forming a nonwoven fabric of co-mingled meltblown fibers can comprise introducing a first liquid polymer and a second liquid polymer into the melt blowing systems described herein, drawing a first plurality of fibers from the first die and drawing a second plurality of fibers from the second die. The method can further comprise collecting the first plurality of fibers and the second plurality of fibers on the surface of the collector to form a co-mingled meltblown fabric. The co-mingled meltblown fabric can comprise a first plurality of fibers formed of a polymer having a melt flow index of about 500 or lower and a second plurality of fibers formed of a polymer having a melt flow index of about 500 or higher, for example. Both the first plurality of fibers and the second plurality of fibers can have fiber diameters of less than about 10 microns, but the first plurality of fibers can have a larger average diameter (e.g., about 5 to about 10 microns) than the second plurality of fibers (e.g., about 1 to about 5 microns). Fiber diameters can be determined by visual inspection of the fibers using SEM images. Fiber diameters noted herein refer to individual fibers rather than agglomerated sections of multiple fibers within the fabric where greater diameters may be observed.

[0090] Advantageously, the present invention provides a method and system for forming co-mingled meltblown fabrics that can be formed from polymers having different melt flow index and different average fiber diameter, with the coarser fiber component (e.g., a polyester such as PBT) providing increased strength to the co-mingled web and the finer fiber component (e.g., a polyolefin such as PP) providing good filtration performance. The use of a concentric-air design die to produce the coarser fiber component enables one to use polymers of higher melt flow index at good throughput rates. It has been surprisingly discovered that forming co-mingled meltblown fabrics according to the invention can provide filtration media exhibiting good filtration performance. In certain embodiments, the co-mingled meltblown fabric is pleatable/moldable without the use of additional scrims or other structures that can complicate manufacturing.

[0091] In various embodiments, the meltblown system is capable of forming a pleatable/moldable single-layer nonwoven fabric comprising co-mingled meltblown fibers of different diameters. In a preferred embodiment, a nonwoven fabric comprising co-mingled meltblown fibers of different diameters can be formed from the methods and systems described herein. The nonwoven fabric can comprise first plurality of fibers formed of a polymer having a melt flow index of about 500 or lower and a second plurality of fibers formed of a polymer having a melt flow index of about 500 or higher. Both the first plurality of fibers and the second plurality of fibers can have fiber diameters of less than about 10 microns, but the first plurality of fibers can have a larger average diameter than the second plurality of fibers.

[0092] The relative amount of the coarse and fine fiber components can vary, although it is advantageous for the coarse fiber component to be the predominate fiber component in the co-mingled meltblown fabric from a weight perspective. For example, the course fiber component can be present in an amount of at least about 50 weight percent, such as at least about 55 or at least about 60 weight percent. In certain embodiments, the coarse fiber component is present in an amount of at least about 70 weight percent or at least about 80 weight percent, based on the total weight of the fabric.

[0093] Basis weight (W) and thickness (t) of the meltblown webs can be measured. For example, Standard Test Methods for Mass Per Unit Area (Weight) of Fabric (ASTM D3776) can be used to measure the basis weight of the fabrics described herein. In various embodiments of the present invention, co-mingled meltblown fabrics can have a basis weight of about 40 to about 150, about 40 to about 90, about 40 to about 50, or about 40 to about 45 grams per square meter (gsm). In various embodiments of the present invention, co-mingled meltblown fabrics can have a thickness of about 300-1200 m, about 400-800 m, or about 400-600 m.

[0094] Solidity of the fabrics can also be calculated. Solidity () can be calculated using the following equation:

[00001]$=\frac{W}{{}_{f}t}$

where .sub.f is fiber density.
In various embodiments of the present invention, co-mingled meltblown fabrics can have a solidity of about 4 to about 20%, about 5 to about 15%, or about 6 to about 12%.

[0095] In addition, filtration properties of the fabrics can be measured. Filtration performance can be evaluated with TSI 3160 filter tester at face velocity of 5.3 cm/sec, for example. 0.3 micron particles can be used to measure filtration efficiency. Filtration efficiency (E), pressure drop (p) and quality factor (Q.F.) can be recorded. Quality factor is defined as:

[00002]$Q.F.=-\frac{\ln .Math..Math.P}{.Math.p}$

where P is penetration (P=1E/100).

[0096] In various embodiments of the present invention, co-mingled meltblown fabrics can have a filtration efficiency of about 20 to about 50 percent at a pressure drop of about 5 to about 20 Pa. Co-mingled meltblown fabrics can have a filtration efficiency of about 25 to about 55 percent at a pressure drop of about 20 to about 40 Pa, for example. Co-mingled meltblown fabrics can have a filtration efficiency of about 35 to about 55 percent at a pressure drop of about 40 to about 60 Pa, for example. In various embodiments of the present invention, co-mingled meltblown fabrics can have a quality factor of about 0.01 to about 0.05 Pa.sup.1, about 0.01 to about 0.025 Pa.sup.1, or about 0.01 to about 0.015 Pa.sup.1. In certain embodiments, the quality factor can be expressed as at least about 0.010 Pa.sup.1, or at least about 0.014 Pa.sup.1, or at least about 0.020 Pa.sup.1.

[0097] In some embodiments, a filtration media can be formed from the pleatable/moldable single-layer nonwoven fabric. See for example, the filter 30 with pleats 35 illustrated in FIG. 3. See also FIG. 16 illustrating a pleated co-mingled nonwoven fabric of the present invention.

EXPERIMENTAL

[0098] The present invention is more fully illustrated by the following examples, which are set forth to illustrate the present invention and are not to be construed as limiting thereof.

Example 1

[0099] Co-mingled fine and coarse fibers can be formed by having two or more dies that have capillaries that are different in size. Alternatively, two or more similar or dissimilar dies can be used but the capillary throughput is adjusted such that fine and coarse fibers are formed. In a preferred embodiment, two different die types are usedone based on a concentric air design and the other based on single-row-drilled-hole type die design.

Example 2

[0100] Co-mingled meltblown and electrospun webs can be formed by having two or more meltblown dies and a stream of electrospun fibers that are injecting the electrospun fiber webs into the meltblown fiber streams. The drum collector acts as the ground for the Electrospinning system. The advantage of such a system is that it can form a larger fiber layer (for pleating/molding) and also for pre-filtering using a concentric air-type die and a polymer such as PET, PBT and the like, and a layer of electrospun fibers that are deposited into the middle of the of the two converging meltblown fiber streams. As such, a third layer of fibers can be inserted between and/or at least partially co-mingled with layers of fine fiber.

Example 3

[0101] Co-mingled cut fibers and meltblown fibers can be formed by having two or more meltblowing dies and depositing cut fibers in the form of loose fibers and/or staple fiber webs by depositing fibers and/or the web into the converging stream of meltblown fibers. The fibers are deposited by using a similar technique to depositing particles in powder coating units where they are dosed and added using a rotating brush or collector.

Example 4

[0102] Co-mingled particles, pulp and the like and meltblown fibers can be formed by having two or more meltblowing dies and depositing powders, particles, and/or pulp into the converging stream of meltblown fibers. The particles can be activated carbon, Metal Organic Frame Works (MOFs), ZIFs, ceramic, metal oxides, and the like.

Example 5

[0103] Co-mingled fine and coarse fibers can be formed by having two or more similar die types that have capillaries that are different in size. In a preferred embodiment, two similar die types are usedone die based on a concentric air design with capillaries in the range of 500 to 850 microns and a second die also based on a concentric air design with capillaries in the range of about 100 to about 500 microns.

Example 6

[0104] Co-mingled fine and coarse fibers can be formed by having two or more similar die types that have capillaries that are different in size. In a preferred embodiment, two similar die types are usedone die based on a single-row-impinging-air design with a single row of capillaries in the range of 500 to 850 microns and a second die also based on a single-row-capillary type design with capillaries in the range of about 100 to about 500 microns.

Example 7

[0105] Co-mingled fine and coarse fibers can be formed by having two or more similar die types that have capillaries that are different in size. In a preferred embodiment, two similar die types are usedone die based on a single-row-impinging-air design with a single row of capillaries with 20 to 30 capillaries per inch with diameters in the range of 500 to 850 microns and a second die also based on a single-row-capillary type design with 30 to 50 capillaries per inch and with diameters in the range of about 100 to about 500 microns.

Examples 8-22

[0106] Examples of co-mingled meltblown structures were produced with a multi-die melt blowing system having two concentric air-type die types, as illustrated in FIG. 6, for example. The multi-die melt blowing system 60 comprises two meltblown dies where one melt blowing die 62 has capillaries with a diameter of 228 m to form fine fibers and a second die 64 has capillaries with a diameter of 508 m to form coarse fibers. Dies having different sized capillaries can be used depending on the polymer used and the desired type of meltblown fibers to be produced (e.g., fine fibers or coarse fibers).

[0107] Varying the alignments of the two dies controls the degree of converging, as illustrated in FIG. 6, for example. A convergence angle can be defined as the angle between the two streams coming from the two melt blowing dies. The convergence angle used for this low convergence setting can be about 15 to about 25. The convergence angle used for this medium convergence setting can be about 35 to about 45The convergence angle used for this high convergence setting can be about 50 to about 60.

[0108] Examples 8-22 utilize polypropylene resin METOCENE MF650 W with melt flow rate of 500 supplied by Lyondellbaseell. The same polymer was used for both dies. Air flow and throughput were varied to change fiber diameters. The degree of convergence was also varied. Detailed production conditions of examples 8-22 are provided in Table 1 below. The fiber ratio is reported as the mass/weight ratio of the two plurality of fibers forming the co-mingled web.

TABLE-US-00001 TABLE 1 Production Conditions of Examples 8-22 Die 1: Die 2: Capillary Diameter = Capillary Diameter = 0.009 (228 m) 0.0020 (508 m) Die 1 to Die 2 to Fiber collector collector Ratio Basis Example Converging Throughput Air distance Throughput Air distance Die 1: weight No. Degree (ghm) (psi) (cm) (ghm) (psi) (cm) Die 2 (gsm) 8 High 0.035 8 38 0.102 8 30 41:59 45 9 High 0.035 11 38 0.102 8 30 41:59 44 10 High 0.035 11 38 0.102 11 30 41:59 45 11 High 0.035 8 38 0.138 8 30 34:66 44 12 High 0.035 8 38 0.138 8 30 34:66 88 13 Medium 0.035 8 30 0.102 8 30 41:59 43 14 Medium 0.035 11 30 0.102 8 30 41:59 42 15 Medium 0.035 11 30 0.102 11 30 41:59 44 16 Medium 0.035 8 30 0.138 8 30 34:66 43 17 Medium 0.035 8 30 0.138 8 30 34:66 72 18 Low 0.035 8 23 0.102 8 30 41:59 50 19 Low 0.035 8 23 0.138 8 30 34:66 47 20 Low 0.035 11 23 0.138 8 30 34:66 40 21 Low 0.035 11 23 0.138 11 30 34:66 42 22 Low 0.035 11 23 0.138 11 30 34:66 97

[0109] Filtration performance was also evaluated with a TSI 3160 filter tester at face velocity of 5.3 cm/sec. 0.3 micron particles were used to measure filtration efficiency. Filtration efficiency (E), pressure drop (p) and quality factor (Q.F.) were recorded. Properties of the fabrics produced in examples 8-22 are provided in Table 2 below.

TABLE-US-00002 TABLE 2 Fabric Properties of Examples 8-22 Filtration Testing Results Pressure Quality Example Thickness Solidity Drop Efficiency Factor ID (m) (%) (Pa) (%) (Pa.sup.1) 8 521 9.5 35.1 41.2 0.015 9 731 6.9 34.5 44.3 0.017 10 507 9.3 36.2 43.0 0.016 11 349 13.7 24.9 28.5 0.013 12 711 13.7 47.2 47.8 0.014 13 390 12.1 22.0 28.1 0.015 14 352 13.1 24.6 30.5 0.015 15 367 13.4 26.0 30.2 0.014 16 688 7.1 11.5 20.8 0.020 17 481 15.7 33.7 33.2 0.012 18 516 10.8 29.7 35.0 0.014 19 855 6.0 14.9 24.1 0.019 20 660 6.3 16.5 26.9 0.019 21 626 7.4 15.3 22.8 0.017 22 596 18.0 44.1 41.0 0.012

[0110] SEM images of examples with a high degree of converging are illustrated in FIGS. 7A-7D, for example. Each fabric sample has two sides, a small capillary side and a large capillary side. FIG. 7A illustrates the small capillary side of fabric example number 10 produced according to the parameters listed above. FIG. 7B illustrates the large capillary side of fabric example number 10 produced according to the parameters listed above. FIG. 7C illustrates the small capillary side of fabric example number 11 produced according to the parameters listed above. FIG. 7D illustrates the large capillary side of fabric example number 11 produced according to the parameters listed above. As illustrated in the SEM images, the two sides of a meltblown sample with a high degree of convergence are similar in appearance and difficult to distinguish.

[0111] SEM images of examples with a medium degree of converging are illustrated in FIGS. 8A-8D, for example. As stated above, each fabric sample has two sides, a small capillary side and a large capillary side. FIG. 8A illustrates the small capillary side of fabric Example 15 produced according to the parameters listed above. FIG. 8B illustrates the large capillary side of fabric Example 15 produced according to the parameters listed above. FIG. 8C illustrates the small capillary side of fabric Example 16 produced according to the parameters listed above. FIG. 8D illustrates the large capillary side of fabric Example 16 produced according to the parameters listed above. As illustrated in the SEM images, the two sides of a meltblown sample with a medium degree of convergence are similar in appearance, but the two sides are more distinguishable than the two sides of a high convergence sample.

[0112] SEM images of examples with a low degree of converging are illustrated in FIGS. 9A-9D, for example. Each fabric sample has two sides, a small capillary side and a large capillary side. FIG. 9A illustrates the small capillary side of fabric Example 20 produced according to the parameters listed above. FIG. 9B illustrates the large capillary side of fabric Example 20 produced according to the parameters listed above. FIG. 9C illustrates the small capillary side of fabric Example 21 produced according to the parameters listed above. FIG. 9D illustrates the large capillary side of fabric Example 21 produced according to the parameters listed above. As illustrated in the SEM images, the two sides of a meltblown sample with a low degree of convergence are fairly distinct in appearance. The two sides are more distinguishable than the two sides of a high or a medium convergence sample.

Examples 23-28

[0113] Examples of co-mingled meltblown structures were produced with a multi-die melt blowing system having two concentric air-type die types, as illustrated in FIG. 6 and described above.

[0114] Examples 23-28 comprise polypropylene resin METOCENE MF650 W with melt flow rate of 500 supplied by Lyondellbaseell. The same polymer was used for both dies. A low throughput of 0.023 ghm was used in a first die to comingle fine fibers into coarse fibers provided by a second die having a throughput of 0.102 ghm. The degree of convergence was varied, as described in more detail above. Detailed production conditions of Examples 23-28 are provided in Table 3 below.

TABLE-US-00003 TABLE3 Production Conditions of Examples 23-28 Die 1: Die 2: Capillary Diameter = Capillary Diameter = 0.009 (228 m) 0.0020 (508 m) Die 1 to Die 2 to Fiber collector collector Ratio Basis Example Converging Throughput Air distance Throughput Air distance Die 1: weight ID Degree (ghm) (psi) (cm) (ghm) (psi) (cm) Die 2 (gsm) 23 High 0.023 8 38 0.102 8 30 31:69 44 24 High 0.023 11 38 0.102 8 30 31:69 44 25 High 0.023 14 38 0.102 8 30 31:69 45 26 Low 0.023 8 26 0.102 8 30 31:69 47 27 Low 0.023 11 26 0.102 8 30 31:69 50 28 Low 0.023 14 26 0.102 8 30 31:69 48

[0115] Filtration performance was evaluated with TSI 3160 filter tester at face velocity of 5.3 cm/sec. 0.3 micron particles were used to measure filtration efficiency. Properties of the fabrics produced in examples 23-28 are provided in Table 4 below.

TABLE-US-00004 TABLE 4 Fabric Properties of Examples 23-28 Filtration Testing Results Pressure Quality Example Thickness Solidity Drop Efficiency Factor ID (m) (%) (Pa) (%) (Pa.sup.1) 23 430 11.4 33.3 35.8 0.0133 24 600 8.3 27.0 33.2 0.0150 25 897 5.7 19.3 28.7 0.0175 26 616 8.2 24.7 31.7 0.0154 27 501 11.1 26.0 30.8 0.0142 28 709 7.9 24.8 28.8 0.0137

[0116] SEM images of examples with a high degree of converging are illustrated in FIGS. 10A-10B, for example. Each fabric sample has two sides, a small capillary side and a large capillary side. FIG. 10A illustrates the small capillary side of fabric Example 25 produced according to the parameters listed above. FIG. 10B illustrates the large capillary side of fabric Example 25 produced according to the parameters listed above.

[0117] SEM images of examples with a low degree of converging are illustrated in FIGS. 11A-11B, for example. Each fabric sample has two sides, a small capillary side and a large capillary side. FIG. 11A illustrates the small capillary side of fabric Example 28 produced according to the parameters listed above. FIG. 11B illustrates the large capillary side of fabric Example 28 produced according to the parameters listed above. Similar to examples 8-22 above, the two sides of a meltblown sample with a low degree of convergence are fairly distinct in appearance. The two sides are more distinguishable than the two sides of a high convergence sample.

Comparative Examples 29-31

[0118] Comparative Examples 29-31 are produced with a single die REICOFIL melt blowing apparatus (single-row-impinging-air design). The die had capillaries with a diameter of 400 microns. Basis weight of these samples was about 40 gsm, similar to some of Examples 8-28. Examples 29 and 30 comprise polypropylene with a 500 melt flow rate. Specifically, METOCENE MF650 W supplied by Lyondellbaseell was used for Examples 29 and 30. Example 31 comprises polypropylene with a 1200 melt flow rate. Specifically, METOCENE MF650 X supplied by Lyondellbaseell was used for Example 31. Detailed production conditions of Examples 29-31 are provided in Table 5 below.

[0119] Basis weight and thickness of the webs were measured and filtration performance were evaluated with TSI 3160 filter tester at face velocity of 5.3 cm/sec. 0.3 micron particles were used to measure filtration efficiency. Properties of the fabrics produced in Examples 29-31 are also provided in Table 5 below.

TABLE-US-00005 TABLE 5 Production Conditions and Fabric Properties of Examples 29-31 Sample Production Process Condition Filtration Testing Results Through- Air Basis Thick- Filtration Quality Sample put DCD flow weight ness Solidity Pressure Efficiency Factor ID Polymer (ghm) (cm) (m.sup.3/hr) (gsm) (m) (%) drop (Pa) (%) (Pa.sup.1) 29 PP- 0.6 30.0 1100 37 343 12.0 19.5 21.3 0.0006 500MFR 30 PP- 0.9 22.5 1100 38 340 12.1 20.8 20.6 0.0007 500MFR 31 PP- 0.6 30.0 1100 40 337 13.0 50.0 45.7 0.0021 1200MFR

[0120] FIG. 12 is a graph comparing filtration performance of examples produced with a dual die apparatus (Examples 8-28) and Comparative Examples 29-31 produced with a single die system (i.e., no co-mingling of multiple fiber streams). As illustrated therein, examples produced with a dual die system showed higher filtration efficiency at the same approximate pressure drop. Additionally, the Quality Factor for Examples 8-28 are much higher than the Quality Factor of Comparative Examples 29-31. This indicates improved filtration performance for meltblown structures according to the invention with co-mingled fine and coarse fibers.

Examples 32-42

[0121] Examples of co-mingled meltblown structures were produced with a multi-die melt blowing system having two concentric air-type die types, as illustrated in FIG. 6 and described above. Examples 32-42 comprise polypropylene resin METOCENE MF650 W with melt flow rate of 500 supplied by Lyondellbaseell and polybutylene terephthalate (PBT) CRASTIN FGS600F40NC010, MFR 33 g/10 min supplied from DuPont. A coarse fiber stream is issued from Die 2 (e.g., die 64 in system 60 of FIG. 6) comprising the PBT. A fine fiber stream is issued from Die 1 (e.g., die 62 in system 60 of FIG. 6) comprising the polypropylene. Detailed production conditions of Examples 32-42 are provided in Table 6 below.

TABLE-US-00006 TABLE 6 Production Conditions of Examples 32-42 Die 1: Die 2: Capillary Diameter = Capillary Diameter = 0.009 (228 m) 0.0020 (508 m) Die 1 to Die 2 to Fiber collector collector Ratio Basis Example Converging Throughput Air distance Throughput Air distance Die 1: weight ID Degree (ghm) (psi) (cm) (ghm) (psi) (cm) Die 2 (gsm) 32 High 0.038 8 38 0.334 8 30 18:82 43 33 High 0.038 8 38 0.334 8 30 18:82 91 34 High 0.038 8 38 0.334 8 30 18:82 139 35 High 0.038 11 38 0.334 8 30 18:82 43 36 High 0.038 11 38 0.334 8 30 18:82 87 37 High 0.038 11 38 0.334 8 30 18:82 134 38 Low 0.038 8 26 0.334 8 30 18:82 44 39 Low 0.038 8 26 0.334 8 30 18:82 89 40 Low 0.038 8 26 0.334 8 30 18:82 138 41 Low 0.038 11 26 0.334 8 30 18:82 45 42 Low 0.038 11 26 0.334 8 30 18:82 88

[0122] Basis weight and thickness of the webs were measured and filtration performance were evaluated with TSI 3160 filter tester at face velocity of 5.3 cm/sec. 0.3 micron particles were used to measure filtration efficiency. Properties of the fabrics produced in Examples 32-42 are provided in Table 7 below.

TABLE-US-00007 TABLE 7 Fabric Properties of Examples 32-42 Filtration Testing Results Pressure Quality Example Thickness Solidity Drop Efficiency Factor ID (m) (%) (Pa) (%) (Pa.sup.1) 32 432 8.3 9.7 25.9 0.0308 33 550 13.8 22.2 33.8 0.0186 34 773 15.1 33.6 43.6 0.0170 35 361 10.1 10.7 23.5 0.0251 36 577 12.6 24.2 37.8 0.0196 37 863 13.0 30.9 45.2 0.0194 38 775 4.7 8.4 31.2 0.0445 39 902 8.3 17.6 40.8 0.0297 40 1027 11.3 32.0 51.2 0.0224 41 782 4.8 8.2 33.4 0.0495 42 884 8.3 18.4 47.0 0.0346

[0123] SEM images of examples with a high degree of converging are illustrated in FIGS. 13A-13D, for example. Each fabric sample has two sides, a small capillary side and a large capillary side. FIG. 13A illustrates the small capillary side of fabric Example 32 produced according to the parameters listed above. FIG. 13B illustrates the large capillary side of fabric Example 32 produced according to the parameters listed above. FIG. 13C illustrates the small capillary side of fabric Example 35 produced according to the parameters listed above. FIG. 13D illustrates the large capillary side of fabric Example 35 produced according to the parameters listed above.

[0124] SEM images of examples with a low degree of converging are illustrated in FIGS. 14A-14D, for example. Each fabric sample has two sides, a small capillary side and a large capillary side. FIG. 14A illustrates the small capillary side of fabric Example 38 produced according to the parameters listed above. FIG. 14B illustrates the large capillary side of fabric Example 38 produced according to the parameters listed above. FIG. 14C illustrates the small capillary side of fabric Example 41 produced according to the parameters listed above. FIG. 14D illustrates the large capillary side of fabric Example 41 produced according to the parameters listed above. Similar to previous examples described above, the two sides of a meltblown sample with a low degree of convergence are fairly distinct in appearance. The two sides are more distinguishable than the two sides of a high convergence sample.

[0125] FIG. 15 is a graph comparing filtration performance of polypropylene samples produced with a dual die apparatus to filtration performance of PP/PBT samples produced with a dual die apparatus in Examples 32-42 and the PP co-mingled samples of Examples 8-28. It is apparent that the co-mingled PP/PBT fabrics exhibited higher filtration efficiency at comparable pressure drops as compared to the PP-only fabrics.

[0126] For certain commercially available filter media, when discharged with IPA immersion according to EN 779 standard before testing to remove electrostatic charge introduced during filter manufacturing, filtration efficiency is about 2-15% at about 10-25 Pa pressure drop, and the filtration efficiency is about 5-25% at about 25-40 Pa pressure drop, and the filtration efficiency is about 20-30% at about 40-60 Pa pressure drop. Accordingly, it is apparent from FIG. 15 that the co-mingled samples produced with a dual die apparatus according to the present invention showed enhanced filtration results over certain commercially available filter media.

[0127] Example fabrics 33 and 39 were pleated with a digital CNC controlled blade pleating machine made by JCEM GmbH. As illustrated in FIGS. 16A and 16B, for example, these fabrics were successfully pleated and self-supporting.

Comparative Examples 43-44

[0128] Comparative Examples 43 and 44 are produced using only the second die 64 (concentric air design) of the multi-die system illustrated in FIG. 6. Examples 43 and 44 comprise polypropylene with 500 melt flow rate. Specifically, Metocence MF650 W supplied by Lyondellbaseell was used for Examples 43 and 44. Detailed production conditions of Examples 43 and 44 are provided in

[0129] Table 8 below.

TABLE-US-00008 TABLE 8 Production Conditions of Examples 43 and 44 Die 1: Die 2: Capillary Diameter = Capillary Diameter = 0.009 (228 m) 0.0020 (508 m) Die 1 to Die 2 to collector collector Basis Example Converging Throughput Air distance Throughput Air distance weight ID Degree (ghm) (psi) (cm) (ghm) (psi) (cm) (gsm) 43 N.A. 0 (not used) 0 0.102 11 30 42 44 N.A. 0 (not used) 0 0.138 11 30 45

[0130] Basis weight and thickness of the webs were measured and filtration performance were evaluated with a TSI 3160 filter tester at face velocity of 5.3 cm/sec. 0.3 micron particles were used to measure filtration efficiency. Properties of the fabrics produced in Examples 43 and 44 are provided in Table 9 below.

TABLE-US-00009 TABLE 9 Fabric Properties of Examples 43 and 44 Filtration Testing Results Pressure Quality Example Thickness Solidity Drop Efficiency Factor ID (m) (%) (Pa) (%) (Pa.sup.1) 43 331 14.1 16.6 18.7 0.0125 44 474 10.3 8.9 11.3 0.0135

[0131] Comparing the filtration testing results for these comparative examples to the data of FIG. 15, it is apparent that the single die comparative examples (i.e., no co-mingling of multiple fiber streams) do not provide the same filtration performance. For example, all of the co-mingled samples from the present examples exhibited a filtration efficiency of greater than 20% at a pressure drop of between 10 and 20 Pa.

[0132] Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing description. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.