Patent Publication Number: US-6986427-B2

Title: Three-dimensional non-woven media

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority from U.S. Provisional Patent Application Ser. No. 60/343,827, filed Oct. 23, 2001, for “Three-Dimensional Non-Woven Media,” by Thomas M. Aune, Clinton V. Kopp, Michael J. Madsen, Philip M. Rolchigo, and Travis G. Stifter. Reference is made to U.S. patent application Ser. No. 09/296,070, now U.S. Pat. No. 6,358,417, filed on Apr. 21, 1999, for “Non-Woven Depth Filter Element”, U.S. Provisional Patent Application Ser. No. 10/278,322, filed Oct. 23, 2002, for “Three-Dimensional Non-Woven Filter”, and U.S. patent application Ser. No. 10/279,043, filed Oct. 23, 2002, for “Process For Making Three-Dimensional Non-Woven Media”. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of melt-blown media, more specifically media that has reduced density while maintaining structural strength. Such media can provide beneficial application in many uses where desirable material properties include low density and high void volume while maintaining a relatively rigid structure, especially under pressure. Uses for such media include filtration media for various applications such as particle filtration, coalescing of oils and leukocyte filtration. Other uses envisioned include insulation, impact absorbing protective and conformable material, and wicking media for evaporators. 
     Numerous apparatuses and processes for forming melt blown media comprised of a plurality of substantially continuous filaments currently exist in the prior art. In this art, fiber forming devices or fiberizers such as those described in U.S. Pat. No. 3,825,379 issued to Lohkamp et al. and U.S. Pat. No. 3,825,380 issued to Harding et al. are used to spray filaments of synthetic resinous material toward a collection device. During this process, jets of air or other gases act on the filaments to attenuate such filaments to a comparatively fine diameter and convey the same to the collection device. Fibers continue to build up on the collection device until a mass of fibers of the desired size and morphology is achieved. 
     Several specific processes have evolved from this general concept. One of these processes is described in U.S. Pat. No. 3,849,241 issued to Buntin et al. It discloses a process die or fiberizer consisting of a die head containing separate passages for the filament material and the attenuating air. During operation, molten resinous material is forced through one or more small holes or nozzles in the die head toward a collection device and is attenuated by air streams positioned on the sides of the material outlet holes. The collection method utilized with this process includes a rotating drum to form a continuous mat. Another of these processes is described in U.S. Pat. No. 4,021,281, issued to Pall. It describes the continuous formation of a melt blown media web onto a rotating drum, being deposited in the form of a tubular web that can be slit into flat media. Another process is exemplified by U.S. Pat. No. 4,240,864 issued to Lin et al. This patent discloses a process die or nozzle block which delivers a plurality of filaments toward a rotating collection device. Associated with the filaments are attenuating air streams which function to attenuate the filaments as they travel toward the collection device. Lin et al. also disclose a press roll for varying the pressure applied to the accumulating fibers on the rotating mandrel so as to provide a filter of varying fiber density. Like the processes of Buntin et al. and Pall, the diameter of the individual filaments in the Lin et al. process is constant throughout the entirety of the media. However, contrary to Buntin et al. and Pall, in the Lin et al. process, the resultant media are continuously urged off the rotating mandrel via the noncylindrical press roll to produce a coreless depth filter element. 
     Another specific process is represented by U.S. Pat. Nos. 4,594,202 and 4,726,901, both issued to Pall et al. Similar to the processes described above, the Pall process includes a fiberizer or fiberizer die having a plurality of individual nozzles through which the molten filament resin is forced toward a collection mandrel. Also similar to the other processes described above, this process discloses the use of air or gas streams for the purpose of attenuating the filaments as they travel toward the collection mandrel. This process differs from the processes described above, however, in that it discloses a means for varying the fiber diameter throughout the radial dimension of the filter element, while maintaining a substantially constant voids volume for each level of fiber diameter variance. Pall et al. accomplish this by sequentially altering certain parameters which affect the fiber diameter during collection of the filaments on the rotating mandrel. 
     Although each of the above specific processes is generally acceptable for certain applications, each also has certain limitations. For example, one limitation of the Pall et al. (U.S. Pat. Nos. 4,594,202 and 4,726,901) process is that it is a non-continuous or semi-continuous process. In other words, a filter element of finite length is formed by building up a mat of attenuated filaments on a rotating mandrel. When the collected filament material reaches a desired thickness, the filter structure is removed and the process is commenced again for the next filter element. 
     Although the Pall et al. patents (U.S. Pat. Nos. 4,594,202 and 4,726,901) contemplate a depth filter element comprised of filaments with varying diameters, there are several limitations which exist. First, the process of Pall et al. is not a continuous process, but must be repeated for each filter manufactured. Second, although some filter elements of Pall et al. have filaments of varying diameters, the process of making such elements has limitations. Specifically, the filament diameter is varied by sequentially changing one of several operating conditions of the filament producing mechanism. Whenever such a change is introduced, however, the system takes time to respond to such changes before again reaching equilibrium. The time frame for response is proportional to the degree of change. Because these changes are introduced during the manufacture of each individual filter element, a less stable and more variable process results. Further, the changeover from a filament of one diameter to that of another occurs gradually as a time related transition, rather than abruptly such as where the structure is comprised of two or more discrete filaments. 
     An important attribute of the media structure is the percent void volume which is the ratio of the air volume in the structure to the total media structure volume. The percent voids volume in the melt-blown media should be as high as possible in order to achieve a number of desirable characteristics in filtration applications, such as high dirt holding capacity and lower initial pressure drop. Generally, achieving a high void volume results in lowering the density of the media mass. It is also desirable to lower the density of a media mass, because a lower density media requires less material usage, allowing for lower material costs, higher throughput, and faster production. 
     Another advantage of media with high void volume is that they are amenable to insertion of a significant percentage volume of active particles or fibers without inducing an unacceptable increase in pressure drop in filtration applications. For example, activated carbon particles may be dispersed in the media as they are formed. Moreover, masses with high void volumes and lower densities also generally provide advantages for other applications such as thermal insulation, evaporative wicking and impact absorption material. 
     However, in prior art melt blown media, there is an upper limit beyond which further increasing the percent voids volume becomes undesirable. Attempts to produce low density, high void volume, media structures using the prior art teachings result in reduced fiber-to-fiber bonding and typically insufficient structural strength. As the voids volume is increased in prior art structures, the fibrous media used in a depth filter are more readily compressed by the pressure drop generated by the fluid passing through it. This is particularly troublesome when the fluid is viscous. If the percent void volume is too high, the filter medium will begin to collapse at too low a differential pressure. As it collapses, the pores become smaller and the differential pressure increases, causing still more compression. The resulting rapid increase in pressure drop thus reduces the media&#39;s useful life and dirt holding capacity rather than—as might otherwise be expected with the increased void volume media—extending it. Use of a very low density (high voids volume) can also make the filter very soft and thereby more readily damaged in normal handling and more likely to compress and collapse in use. 
     A drawback of the prior art products is that the low density filters often are made using fine fibers and therefore have a fine micron rating, which is inherent to the finer fiber matrix. It would be desirable to use fine fibers to achieve low density, while maintaining the capacity to produce media with a larger pore structure. For a filtration application, this would mean a coarser micron rating, thereby allowing for filtration of a wider range of particles without premature clogging of the filter. This would require that the fine fiber network is somehow fixed in a more open structure, thereby avoiding the natural packing tendency of the fine fibers that inherently create a finer pore structure. 
     Although prior art methods exist for manufacturing melt blown media, each of the methods, as well as the products constructed from such methods, have limitations of compressive strength at low media densities. Accordingly, there is a need in the art for an improved, cost efficient melt blown media. A need also exists for a continuous method and apparatus for producing such media. 
     BRIEF SUMMARY OF THE INVENTION 
     A non-woven melt-blown filament medium includes a mass of essentially continuous melt-blown polymer filaments and an essentially continuous traversing melt-blown polymer filament extending through the mass. The mass has a depth dimension, a longitudinal dimension, and a latitudinal dimension. The mass includes a plurality of layers, each of the plurality of layers being generally oriented in the longitudinal and latitudinal dimensions. The traversing filament is generally oriented in the depth dimension and extends through one or more layers of the mass. In one embodiment, the mass is cylindrical in shape and the layers comprise concentric zones. In one embodiment, a core zone of the mass has filaments having a diameter; an intermediate zone of the mass has filaments having larger diameters than the filaments of the core zone; and an outer zone of the mass has filaments having larger diameters than the filaments of the intermediate zone. In one embodiment, the traversing fiber is a bonding fiber bonding one or more layers of the mass together. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram generally illustrating a system for continuously producing a non-woven depth filter element. 
         FIG. 2  is a schematic diagram illustrating the system configuration for continuously producing a depth filter element of the present invention. 
         FIG. 2A  is an enlarged view of the collection device of the apparatus of  FIG. 2 . 
         FIG. 3  illustrates an elevation view of a depth filter element of the present invention viewed from line  3 — 3  of  FIG. 2A . 
         FIG. 4  is a schematic diagram generally illustrating a second embodiment of the system for continuously producing a non-woven depth filter element. 
         FIG. 5  is a schematic diagram illustrating the system configuration for the embodiment of  FIG. 4 . 
         FIG. 6  illustrates an elevation view of a second embodiment of a depth filter element of the present invention viewed from line  6 — 6  of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention of melt blown media with improved structural strength can be used in various formats for melt blown media including continuously formed cartridge filters, continuously formed webs, structural composite webs, and pre-impregnated fiber reinforcing mats. The melt blown media of the present invention comprise a mass of essentially continuous polymer filaments. The media have a length or longitudinal dimension, a width or latitudinal dimension, and a depth dimension. The primary filaments of the melt blown media are generally oriented in the length (x or longitudinal) and width (y or latitudinal or circumferential in the case of a cylindrical mass) dimensions. An important feature of the invention is that the media also comprise essentially continuous polymer filaments extending in the depth (z) dimension. The concurrent formation of primary fibers in the x and y dimensions and separate bonding fibers in the z dimension allows for design and engineering of specific zones of media for specific application needs. The invention also includes a method of continuously producing the melt blown media. 
     One important embodiment of the melt blown media comprises a cylindrical mass of essentially continuous polymer filaments. The cylindrical mass has a longitudinal or x dimension, a circumferential or y dimension and a radial or z dimension. The primary filaments of the cylindrical mass are generally oriented in the longitudinal and circumferential or x and y dimensions. The filament mass also comprises essentially continuous polymer filaments extending throughout the cylindrical mass in the radial or z dimension. These melt blown media are particularly useful for producing a filament mass to use in constructing a depth filter element. In a tubular filter, for example, the media of the present invention allow for the formation of a self supporting interior core zone that concurrently provides a zone of critical filtration. By placing a higher percentage of the bonding filaments in the core zone and those zones next to the core, the filter can be engineered to have both higher crush strengths and lower density than if the same amount of bonding filaments were evenly distributed throughout the media. The invention also includes a method of continuously producing the filament mass. 
     One useful demonstrated application for the invention is for particle filtration and more particularly for use in a depth filter cartridge comprising a filter element constructed of a plurality of substantially continuous filaments which are collected to form a generally tubular depth filter cartridge. The present invention also relates to a method and system for making such a filter cartridge. 
     To obtain the unique, combined benefit of a low density and rigidly fixed media structure requires implementation of two or more concurrently formed melt blown media. A very fine matrix of primary fibers with reduced fiber to fiber bonding is used to form a structure with low density. A second source of filaments is concurrently and intentionally placed in the z dimension onto the primary media as they are forming to provide improved fiber to fiber bonding as well as interlocking the mechanical structure. These z filaments thereby form a more rigid porous structure which has significantly greater mechanical strength. The primary media are typically formed in essentially two-dimensional layers with the fibers oriented in the x and y axes and with only incidental bonding between layers. It has been discovered that it is beneficial to place the bonding z filaments in the forming layers of primary media fibers and across two or more of the formed primary media layers, with these bonding z filaments essentially oriented in the z-axis with respect to the primary media. This permits concurrent, continuous production of very fine melt-blown media that are relatively rigid and wherein the fibers are structurally locked in place. 
     It has also been found beneficial to insert the bonding z filaments across the primary media as they are forming, so the bonding z filaments extend across one or more zones of the primary media. It has also been found beneficial for the bonding z filaments to extend across all the layers of the primary media, and thereby to traverse from one major surface of the finished primary media to the other major surface. In a described embodiment of this invention, the z filament is used as a bonding filament to produce low density primary media that have improved resistance to compression. It is envisioned that the insertion of the bonding z filament across one or more layers of the primary media as they are forming could be used to produce media with other significant benefits. For example, the z polymer could have significantly different physical or chemical characteristics which could result in a significant improvement in the composite media produced. The ability to engineer the placement, composition and physical attributes of these z filaments is very useful and offers the opportunities to explore media structures not possible in the current art. 
     Another aspect of the present invention is the application of a thin layer of bonding fibers at one or both surfaces of the forming media to provide a more finished porous surface. The bonding fibers adhere to the primary media fibers at the surface and thereby eliminate loose fibers at the media surface. Another significant benefit discovered is that the bonding fibers adhere to the primary surface fibers and conform to the texture of the surface. The bonding fibers then shrink as they cool, which intensifies the resulting surface roughness. The resulting finished surface was surprisingly found to have about twice the surface area of an unfinished primary media surface. This increased surface area provides a number of benefits, especially useful for particle filtration applications. Doubling the surface area of the shell can allow the shell to have a lower porosity while not causing an excessive pressure drop. Also, as the filter is used, a cake of particles can collect on the shell surface and also cause increased pressure drop. The high surface area permits extended operation before such pressure drop increases are incurred. Also, in a cartridge filter embodiment, the formation of a relatively hard shell avoids the necessity to encapsulate the filter in a support cage after the filter cartridge is produced. 
     A preferred embodiment of the present invention is an improved non-woven filament mass for use in constructing a depth filter element as well as a system and a method for continuously making such a mass. However, it should be understood that other embodiments are also contemplated. For example, while a cylindrical product is described in the preferred embodiment, the teachings of the invention may be adapted for flat, sheet, or planar products. Such a flat product may be produced, for example, by manufacturing the medium on a large drum and then cutting the resulting cylindrical medium along its length to obtain a sheet of material. 
       FIG. 1  generally illustrates one embodiment of a system which is used to continuously manufacture filament mass of indefinite length. The mass can then be cut into a plurality of individual filament elements of desired length. A similar system is disclosed in U.S. Pat. No. 5,340,479 by Szczepanski, et al., which is fully incorporated herein by reference. The illustrated embodiment of system  10  includes motor driven screw type extruder  12  which is supplied with thermoplastic polymeric material from a source (not shown). The particular thermoplastic polymeric material may be any one of a variety of synthetic resinous materials which can produce the filaments used in manufacturing the depth filter element of the present invention. Although the class of polymeric materials known as polypropylenes is preferred, polyesters, Nylon, polyurethanes and other materials may be used as well. 
     Within extruder  12 , the polymeric material is heated to a molten state, at which time it is metered and conveyed into heated delivery line  14 . The material is maintained or further heated in line  14  and is ultimately directed to a filament forming means, which in one embodiment is in the form of two filament delivery systems  16  and  18 . Each of the delivery systems  16  and  18  produces one or more substantially continuous polymeric filaments and directs the same along a predetermined path toward a collection means as will be described in greater detail below. 
     Filament delivery system  16  includes a motor driven gear type positive displacement metering pump  20  which receives molten polymeric material from heated delivery line  14  and pumps it to heater block  22 . The speed of motor  24  which drives metering pump  20 , and thus the rate at which the material is metered through pump  20  is electronically controlled by an appropriate controller  26 . 
     Heater block  22 , which is independently heated via heating means (not shown) is provided with internal passages which lead to a plurality of nozzles  27 ,  28 , and  29 . The heating means, and thus the temperature of the polymeric material within heater block  22 , is controlled by temperature control  30 . Each nozzle  27 ,  28 , and  29  includes an orifice, the size of which may be selected as desired to assist in achieving a desired filament size or diameter. The molten material fed to each nozzle  27 ,  28 , and  29  exits the respective orifice in a stream. 
     Associated with each nozzle  27 ,  28 , and  29  are attenuating mechanisms  31 ,  32 , and  33 , which comprise a plurality of gas or air jets. Gas flowing out of the attenuating mechanisms  31 ,  32 , and  33  function to attenuate the stream of molten material exiting from nozzles  27 ,  28 , and  29  to form polymeric filaments in a manner known in the art. Attenuating mechanisms  31 ,  32 , and  33  accordingly may be of any design known in the art including that described in U.S. Pat. No. 4,173,443 by Lin, the disclosure of which is incorporated herein by reference. 
     Attenuating mechanism  31  is associated with an optional gas heater  34  and gas supply source  36 . Gas supply source  36  provides gas via conduit  38  and appropriate valves and regulators to heater  34 . The temperature of heater  34  is elevated or lowered to the desired temperature via temperature control  40 . The gas is then fed from heater  34  through conduit  42  to attenuating mechanism  31 . Attenuating mechanisms  31 ,  32 , and  33  may be provided with gas from a common supply source such as described with reference to  FIG. 1  or alternatively, separately controlled gas sources may be employed for each attenuating mechanism  31 ,  32 , and  33 . 
     Filament delivery system  18  is substantially similar to that of system  16  described above, except that filament delivery system  18  preferably includes a means of delivering the filaments in such a manner as to actively intermingle with filaments produced by one or more of the nozzles used in system  16 . Filament delivery system  18  may include one or more polymer extrusion nozzles. One embodiment uses one nozzle  44  which includes a sweep mechanism for attenuator  54  (shown later with respect to  FIG. 2 ). Specifically, system  18  includes heater block  46 , independently driven positive displacement metering pump  48  and motor  50 . Heater block  46  is provided with nozzle  44  and temperature control  52 . System  18  is also provided with attenuating mechanism  54  associated with nozzle  44 . Pressurized gas is passed to attenuating mechanism  54  from gas supply source  56  via conduit  58 . As with delivery system  16 , each of the attenuators in system  18  can be associated with optional gas heaters, not shown. The provision of separate filament delivery systems  16  and  18  enables separate control and production of polymeric filaments produced by each system  16  and  18 . 
     Delivery systems  16  and  18  produce streams of discrete, essentially continuous polymer filaments which are distributed in flared patterns  66 ,  68 ,  70 , and  72  and directed from nozzles  27 ,  28 ,  29 , and  44  and attenuating mechanisms  31 ,  32 ,  33 , and  54 , respectively, toward filament collection device  74 . There is preferably some overlap in adjacent filament patterns  66 ,  68 , and  70  so that the filaments of each pattern connect with the filaments of the respective adjacent patterns, resulting in an integrated tubular filament mass. Filament collection device  74  includes central, rotatable collection device  76  such as a mandrel or drum, which extends from drive motor  78 . Press roll member  80 , which rotates about axle shaft  81 , is disposed adjacent to mandrel  76  and spaced therefrom. 
     During operation, the essentially continuous polymer filaments of streams  66 ,  68 , and  70  are directed in a flared pattern toward rotating mandrel  76  and collected thereon in a manner known in the art. While mandrel  76  is shown, it is contemplated that other collection devices may also be used, such as large diameter drums. Simultaneously, reciprocating or oscillating stream  72  deposits an essentially continuous filament or fiber stream which spans the distance between a far edge  82  of stream  66  and a far edge  84  of stream  70  and traverses the layers of filaments laid down by streams  66 ,  68 , and  70 . Rotating press roller  80  engages the filaments which have accumulated on rotating mandrel  76 . As sufficient filaments are built up on mandrel  76 , press roller  80  forces non-woven filament mass or fiber structure  86  off the axial end of mandrel  76  in the direction of arrow  88  to produce a continuous filament mass  86  of indefinite length. Filament mass  86  has a radial dimension, a longitudinal dimension, and a circumferential dimension. The entire filament collection device  74  may be similar to that described in U.S. Pat. No. 4,240,864 by Lin, the disclosure of which is incorporated herein by reference. 
     For a more complete understanding of the present invention, reference is made to  FIG. 2 , which is a schematic diagram illustrating the apparatus of  FIG. 1  configured for continuously producing a depth filter element of the present invention. As shown in  FIG. 2 , four filament producing devices are employed, each of which comprises a nozzle and an attenuating mechanism, such as nozzles  27 ,  28 ,  29 , and  44  and attenuating mechanisms  31 ,  32 ,  33 , and  54 . Nozzles  27 ,  28 , and  29  are longitudinally aligned along common axis  90 , which is preferably about 0–15 degrees offset from parallel to mandrel  76 . In a preferred embodiment, nozzles  27 ,  28 , and  29  are positioned about 4 inches apart. Each nozzle  27 ,  28 , and  29  includes an orifice which defines an axis  92 ,  94 , and  96 , respectively, that is preferably perpendicular to axis  90  and about 0–15 degrees offset from perpendicular to mandrel  76 . Axes  93 ,  94 , and  96  generally correspond to the flow axis of molten polymer exiting the respective nozzle orifice. In one preferred embodiment, nozzles  27 ,  28 , and  29  are located approximately 35–40 inches from mandrel  76 , which preferably spins at a rate of about 400 RPM. This orientation results in flared filament patterns  66 ,  68 , and  70  being directed toward mandrel  76 . 
     Filament patterns  66 ,  68 , and  70  are comprised of polymer filaments having diameters of between less than about 1 micron to about 100 microns. In a preferred embodiment, filament pattern  66  comprises filaments of the smallest diameter; filament pattern  68  comprises filaments of intermediate diameter; and filament pattern  70  comprises filaments of the largest diameter. As a non-limiting example, polymer filaments of filament patterns  66 ,  68 , and  70  were produced in a depth filter by extruding polypropylene heated to a temperature of between about 325° C. and about 400° C. through a nozzle having an orifice size of about 0.016 inch at a rate of about 11 pounds per hour and passing an ambient gas at a temperature of about 25° C. at a rate of about 13 standard cubic feet per minute over the molten polymer stream exiting the nozzle orifice. It will be appreciated that a person skilled in the art can readily determine other suitable parameter combinations. It can be appreciated that the operating parameters may be varied between filament patterns  66 ,  68 , and  70  to produce zones of varying densities and fiber sizes. 
     Opposite filament patterns  66 ,  68 , and  70 , nozzle  44  and attenuating mechanism  54  produce filament pattern  72 . As better seen in  FIG. 2A , filament pattern  72  comprises pattern  72 A which moves in a reciprocating, transverse pattern, preferably covering the distance between the primary pattern edges  82  and  84 . Alternatively, filament pattern  72  covers less than the distance between edges  82  and  84 . Filament pattern  72  preferably originates from one or more nozzles  44  located in a position above or below press roll  80  so that pattern  72  travels from nozzle  44  to mandrel  76  and lands on the forming filament mass  86  without spraying directly onto press roll  80 . 
     Attenuating mechanism  54  preferably includes servo driven sweep mechanism  98  (see  FIG. 2 ) which allows attenuating mechanism  54  to sweep through an angle so that the filament pattern  72 A (see  FIG. 2A ) traverses back and forth among fiber patterns  66 ,  68 , and  70 , along a longitudinal dimension of filament mass  86 . As pattern  72 A traverses fiber patterns  66 ,  68 , and  70 , it deposits essentially continuous polymer filaments across the overall laydown pattern which extends between the primary pattern edges  82  and  84 . In formed filament mass  86 , the fibers of filament pattern  66  deposited along edge  82  will form a first major surface  97  (shown in  FIG. 3 ), and the fibers of filament pattern  70  deposited along edge  84  will form a second major surface  99  (shown in  FIG. 3 ). In another embodiment, nozzle  44  may be oscillated back and forth to sweep bonding filament pattern  72 . 
     In a preferred embodiment shown in  FIG. 2 , sweep mechanism  98  comprises a servo drive motor with a cam and follower mechanism. Other suitable devices, such as AC/DC driven mechanical cranks and push rod mechanisms, for example, are also acceptable. In a preferred embodiment, sweep mechanism  98  runs at about 950 oscillations per minute. As depicted, attenuating mechanism  54  of nozzle  44  is oriented to produce gas streams which result in flared filament pattern  72  being directed toward mandrel  76 . 
     In one preferred embodiment, nozzle  44  is located approximately 18–22 inches from mandrel  76 . Because nozzle  44  is positioned much closer to mandrel  76  than nozzles  27 ,  28 , and  29 , the fibers of filament pattern  72  have less time to cool before contacting filament mass  86  and are therefore hotter and more adhesive than fibers of filament patterns  66 ,  68 , and  70 . Preferably, the fiber of filament pattern  72  is still relatively liquid when it contacts the fibers of filament patterns  66 ,  68 , and  70 . Because a skin or shell has not completely formed on the fiber of filament pattern  72 , it instantaneously adheres to the fibers of filament patterns  66 ,  68 , and  70  upon contact. However, some attenuation or cooling of the fiber of filament pattern  72  is required to avoid melting of the fibers of filament patterns  66 ,  68 , and  70 . 
     In an alternative embodiment, rather than locating nozzle  44  closer to mandrel  76  than nozzles  27 ,  28 , and  29 , attenuating mechanism  54  may use less air or warmer air than attenuating mechanisms  31 ,  32 , and  33 . This arrangement will also result in fibers of filament pattern  72  being hotter and more adhesive than fibers of filament patterns  66 ,  68 , and  70 . Other process alternatives known in the art may be used to deliver fibers of filament pattern  72 . For example, it is also envisioned that fibers of pattern  72  could be colder than those of filament patters  66 ,  68 , and  70  so as to lend mechanical advantage rather than thermal bonding, as taught in an embodiment above. 
     Filament pattern  72  is comprised of polymer filaments having diameters of between less than about 1 micron to about 100 microns. As a non-limiting example, polymer filaments of filament pattern  72  are produced in the depth filter of the instant invention by passing polypropylene heated to a temperature of between about 325° C. and about 400° C. through a nozzle having an orifice size of about 0.016 inch at a rate of about 8 pounds per hour and passing at an ambient gas at a temperature of about 25° C. at a rate of about 7 standard cubic feet per minute over the molten polymer stream exiting the nozzle orifice. It will be appreciated that a person skilled in the art can readily determine other suitable parameter combinations. 
     As more completely shown in  FIG. 2A , which is an enlarged view of the collection device of  FIG. 2 , an accumulated mass of filaments  86  is produced on mandrel  76 . Filament pattern  72  comprises reciprocating cone-shaped filament pattern  72 A, which sweeps between pattern edges  82  and  84  to produce an overall wider cone-shaped pattern  72 . In one embodiment, press roller  80  is oriented at an angle relative to mandrel  76  with nip  100  in contact with mandrel  76 . As a non-limiting example, outer surface  102  of press roller  80  is angularly displaced by about 3° relative to mandrel  76 . In one embodiment, nip  100  contacts mandrel  76  close to edge  82  of filament pattern  66 . Because of the angular placement of press roller  80 , compression of filaments in collective filament mass  86  varies along the length of press roller  80 . This results in a filament mass having a varying density gradient in the radial dimension, with the filament density of filament pattern  66  being generally greater than that of the filament mass comprised of filament patterns  68  and  70 . 
     Fibers from filament patterns  66 ,  68 , and  70  form a generally two-dimensional mat or layer of material that is continuously formed on mandrel  76  to build up filament mass  86  composed of many layers of fibers. These fibers can be described as being laid down in an X-Y plane, or in the longitudinal and circumferential or latitudinal dimensions. As the fibers are built up, layer upon layer, they produce a radial or depth dimension. The sweeping motion of filament pattern  72 A, combined with the rotation of mandrel  76  causes the fibers coming from nozzle  44  to integrate into mass  86  as a “z” direction fiber, extending radially through the zones produced by filament patterns  66 ,  68 , and  70 . 
       FIG. 3  illustrates an elevation view of filament mass  86  viewed from line  3 — 3  of  FIG. 2A . Filament mass  86  comprises first major surface  97 , second major surface  99 , and concentric filtration zones  104 ,  106 , and  108 , with additional filament mass strength in the radial direction provided by filament  110 . Filament  110  serves as a fiber structure strengthening element. Filament  110  extends throughout filament mass  86  and extends in the radial, longitudinal, and circumferential dimensions. 
     Generally, filament zone  104  is produced by filament pattern  66 ; filament zone  106  is produced by filament pattern  68 ; filament zone  108  is produced by filament pattern  70 ; and filament  110  is produced by filament pattern  72 . Filtration zones  104 ,  106 , and  108  preferably possess different physical characteristics. For example, filtration zone  104  may comprise relatively smaller diameter filaments; filtration zone  106  may comprise intermediate diameter filaments; and filtration zone  108  may comprise larger diameter filaments. Filtration zones  104 ,  106 , and  108  preferably have filaments having diameters ranging in size from less than about 1 micron to about 100 microns. Filaments  110  and  172  may have diameters which are equal to, greater than, or less than an average diameter of the filaments of filtration zones  104 ,  106  and  108 . In some embodiments, filtration zone  104  may have a relatively high density of filaments; filtration zone  106  may have an intermediate density of filaments, and filtration zone  108  may have a lower density of filaments. In another embodiment, filtration zones  104 ,  106  and  108  may have other variations in density. 
     In one embodiment, there is generally an absence of fiber-to-fiber bonding within each of the masses  104 ,  106 , and  108 . The primary bonding within filament mass  86  is accomplished by the bonding between “z” direction fiber  110  and the filaments of zones  104 ,  106 , and  108 . Selected zones of the media can be made very rigid to provide a filtering layer which also carries the resultant mechanical loads, thereby eliminating the need for separate structural elements in a given filter device. 
       FIG. 3 . illustrates, for one embodiment, approximately the orientation of “z” fiber  110 , as it is laid down during one revolution of mandrel  76  (shown in  FIG. 2A ). In this embodiment, the relation between the rate of movement of the servo driven sweep of “z” fiber  110  and the rate of rotation of mandrel  76  are such that the “z” fibers  110  are placed in a continuous manner from the core or bottom zone  104  to the shell or top zone  108  and back to the core zone  104  of mass  86  during approximately 120 degrees or less of rotation during the forming of mass  86 . The path of “z” fiber  110  in one rotation of mandrel  76  can be described as follows. When filament pattern  72 A is near pattern edge  82 , “z” fiber  110  is laid onto filament mass  86  near the core of core zone  104 . As filament pattern  72 A sweeps toward pattern edge  84 , “z” fiber  110  is laid across zones  104 ,  106 , and  108  until it reaches the outside of shell zone  108 . Mandrel  76  spins while filament pattern  72 A sweeps so that “z” fiber  110  also travels in a circumferential direction around filter mass  86 . Thus, “z” fiber  110  runs radially, longitudinally, and circumferentially throughout filter mass  86 . In the case where mass  86  is planar rather than cylindrical, “z” fiber  110  may be described as extending in the length, width, and thickness dimensions of mass  86 . 
     Filter mass  86  is built up only after many revolutions of mandrel  76 , and thus filter mass  86  includes a web of “z” fibers  110  which act to hold together fibers from zones  104 ,  106 , and  108  in all three dimensions, thereby lending strength to filament mass  86  and providing tensile support. Because the fibers of mass  86  are held in place in all three directions, bending moments of the fine fibers are minimized, thereby minimizing dirt release and channeling at increased pressure drops. Such undesirable dirt release and channeling would otherwise be expected when using such fine fibers in a low density media. 
     In one embodiment, the fibers of zones  104 ,  106 , and  108  comprise about 75–95 percent of the fibers of filter mass  86 , and “z” fibers  110  comprise about 5–25 percent of the fibers of filter mass  86 ; more preferably, the fibers of zones  104 ,  106 , and  108  comprise about 80–90 percent of the fibers of filter mass  86 , and “z” fibers  110  comprise about 10–20 percent of the fibers of filter mass  86 ; most preferably, the fibers of zones  104 ,  106 , and  108  comprise about 85 percent of the fibers of filter mass  86 , and “z” fibers  110  comprise about 15 percent of the fibers of filter mass  86 . In a preferred embodiment, sweep mechanism  98  is adjustable to control the amount of “z” fiber  110  deposited in each zone  104 ,  106 , and  108 . In one embodiment, a higher percentage of “z” fiber  110  is deposited in core zone  104  than in zones  106  and  108 . This may be accomplished by slowing the sweep of mechanism  98  in core zone  104 . For example, “z” fiber  110  may make up about 25% of the total fibers in core zone  104  and about 3% in shell zone  108 . This configuration provides added strength to the core region of filter mass  86 , which is required to maintain the filter&#39;s crush resistance as it is used. 
     The fibers of zones  104 ,  106 , and  108  may be comprised of different materials, may be of different sizes, or may otherwise have differing properties. For example, the diameters of the fibers in each zone may get progressively larger from core zone  104  to shell zone  108 . Each zone may also possess a different density from each adjacent zone. For example, the density of the zones may decrease progressively from core zone  104  to shell zone  108 . Other alternatives will be evident to one skilled in the art. 
     The unique construction of filament mass  86  allows for a high void volume without sacrificing strength by fixing the fibers into an open, yet supported structure. Thus, the filament mass  86  of the present invention displays significantly greater mechanical strength to weight ratios than media of the prior art. Filament mass  86  may be formed to any thickness desired. In one embodiment, filament mass  86  has an inside diameter of about 1.15 inch and an outside diameter of about 2.5 inches. In one embodiment, filament mass  86  has a mass of about 95 grams or less per ten inch section and a crush strength of at least about 40 psi. A high void volume results in a filament mass  86  with greater dirt holding capacity, longer element life, and lower pressure drop. Moreover, it allows filament mass  86  to be produced faster and with less material, compared with conventional filters. In a preferred embodiment, a ten inch section of filament mass  86  can be produced in about 15 seconds and has a retention rating of 90% at 20 microns. 
       FIG. 4  is a schematic diagram generally illustrating a second embodiment of the system for continuously producing a non-woven depth filter element.  FIG. 4  is similar to  FIG. 1  but further includes filament delivery system  114 , nozzle  116 , attenuating mechanism  118 , flare pattern  120 , and shell-forming filament delivery system  122 . Additional nozzle  116 , attenuating mechanism  118 , and flare pattern  120  are similar to the nozzles  27 ,  28  and  29 ; attenuating mechanisms  31 ,  32 , and  33 ; and flare patterns  66 ,  68  and  70  described above. While four such nozzles, attenuating mechanisms, and flare patterns are shown for filament delivery system  16 , it is contemplated that more or fewer may be used. In one embodiment, nozzles  27 ,  28 ,  29  and  116  are positioned about 35 inches to about 40 inches from mandrel  76 . 
     Filament delivery system  114  is substantially similar to that of system  16  described above, except that filament delivery system  114  preferably includes a means of delivering the filaments in such a manner that they intermingle with filaments produced by one or more of the nozzles used in system  16 . Filament delivery system  114  may include one or more polymer extrusion nozzles. One embodiment uses one nozzle  124  with attenuator  126 , positioned at an acute angle relative to mandrel  76  to deliver a filament pattern or stream  128  which contacts filament mass  127  in an elliptical pattern which intermingles with filament patterns  66 ,  68 ,  70  and  120  and those of filament delivery system  18 . 
     Specifically, system  114  includes heater block  130 , independently driven positive displacement metering pump  132  and motor  134 . Heater block  130  is provided with nozzle  124  and temperature control  136 . System  114  is also provided with attenuating mechanism  126  associated with nozzle  124 . Pressurized gas is passed to attenuating mechanism  126  from gas supply source  138  via conduit  140 . As with delivery system  16 , attenuators  126  can be associated with an optional gas heaters, not shown. The provision of separate filament delivery systems  18  and  114  enables separate control and production of polymeric filaments produced by each system  18  and  114 , although each of the filament delivery systems  18  and  114  produces filaments which traverse filament mass  127  in a radial, or z, dimension. In one embodiment, the source of material for filament delivery system  114  is extruder  12  via delivery line  14 ; in another embodiment, the material source for system  114  is separate to provide alternate materials to those used in filament delivery systems  16 ,  18  and  122 . 
     Delivery system  114  produces a stream of a discrete, essentially continuous polymer filament which is distributed in flared pattern  128  and directed from nozzle  124  and attenuating mechanism  126  toward filament collection device  74 . During operation, the filament of stream  128  is directed in a flared pattern toward rotating mandrel  76 . In one embodiment, filament pattern  128  spans the distance between a far edge  82  of stream  66  and a far edge  142  of stream  120 . In an alternative embodiment, filament pattern  128  does not span the distance between far edges  82  and  142 , but does cover a significant portion of the forming layers of filament mass  127 , e.g., the distance covered by filament pattern  128  is greater than the distance covered by each primary filament stream  66 ,  68 ,  70  and  120  individually. Preferably the distance covered by filament pattern  128  is greater than the distance covered by two or more adjacent primary filament streams  66 ,  68 ,  70  and  120 . In one embodiment, nozzle  124  is placed about 10–13 inches from mandrel  76 . In one embodiment, nozzle  124  is placed at an acute angle of about 10° to about 20° relative to mandrel  76 , and more preferably about 15° relative to mandrel  76 . 
     Shell-forming filament delivery system  122  is substantially similar to system  16  described above, except that shell-forming filament delivery system  122  is preferably configured and positioned to produce a relatively smooth outer shell zone  112  (see  FIG. 6 ) on the exterior cylindrical surface of filament mass  127 . Shell-forming filament delivery system  122  preferably uses a different location, polymer throughput rate, and air attenuation setting relative to filament delivery system  16 . Compared to system  16 , nozzle  144  is preferably placed closer to mandrel  76  and uses a lower polymer throughput rate; additionally, attenuating mechanism  146  uses less air attenuation. Similar to system  16 , shell-forming filament delivery system  122  includes heater block  148 , metering pump  150 , motor  152 , temperature control  154 , gas supply source  156 , and conduit  158 . 
     As a non-limiting example, polymer filaments of filament pattern  162  was produced in a depth filter by extruding polypropylene heated to a temperature of between about 270° C. and about 325° C. through nozzle  144  having an orifice size of about 0.016 inch at a rate of about 1 pound per hour and passing an ambient gas at a temperature of about 25° C. at a rate of about 1.5 standard cubic feet per minute over the molten polymer stream exiting the nozzle orifice. In one embodiment, nozzle  144  is placed about 3–6 inches from mandrel  76 . It will be appreciated that a person skilled in the art can readily determine other suitable parameter combinations. 
     Nozzle  144  is preferably placed so that the filament produced thereby is deposited on the outer zone  170  formed by filament pattern  120  (as shown in  FIG. 6 ). This configuration produces a very shallow zone or shell  112  with significant fiber-to-fiber bonding, including some bonding between the fibers of shell  112  and the fibers of outer zone  170 . The fiber-to-fiber bonding of shell  112  essentially eliminates the presence of loose fibers on the surface  99  of the finished filament mass  127  and significantly increases the surface area of the resulting surface  99 . 
       FIG. 5  is a schematic diagram illustrating the system configuration for the embodiment of  FIG. 4 . As shown in one embodiment in  FIG. 5 , filament delivery system  16  includes four filament producing devices, each of which comprises a nozzle and an attenuating mechanism, such as nozzles  27 ,  28 ,  29  and  116  and attenuating mechanisms  31 ,  32 ,  33  and  118 . Nozzles  27 ,  28 ,  29  and  116  are longitudinally aligned along common axis  90 , which is preferably about 0–15 degrees offset from parallel to mandrel  76 . In a preferred embodiment, nozzles  27 ,  28 ,  29  and  116  are positioned about 4 inches apart. Each nozzle  27 ,  28 ,  29  and  116  includes an orifice which defines an axis  92 ,  94 ,  96  and  160 , respectively, that is preferably perpendicular to axis  90  and about 0–15 degrees offset from perpendicular to mandrel  76 . Axes  93 ,  94 ,  96  and  160  generally correspond to the flow axis of molten polymer exiting the respective nozzle orifice. In one preferred embodiment, nozzles  27 ,  28 ,  29  and  116  are located approximately 40 inches from mandrel  76 , which preferably spins at a rate of about 400 RPM. This orientation results in flared filament patterns  66 ,  68 ,  70  and  120  being directed toward mandrel  76 . 
     Filament patterns  66 ,  68 ,  70  and  120  are comprised of polymer filaments having diameters of between less than about 1 micron to about 100 microns. In a preferred embodiment, filament pattern  66  comprises filaments of the smallest diameter; filament pattern  68  comprises filaments of intermediate diameter; filament pattern  70  comprises filaments of larger diameter; and filament pattern  120  comprises filaments of the largest diameter. As a non-limiting example, polymer filaments of filament patterns  66 ,  68 ,  70  and  120  were produced in a depth filter by extruding polypropylene heated to a temperature of between about 325° C. and about 400° C. through a nozzle having an orifice size of about 0.016 inch at a rate of about 11 pounds per hour and passing an ambient gas at a temperature of about 25° C. at a rate of about 13 standard cubic feet per minute over the molten polymer stream exiting the nozzle orifice. It will be appreciated that a person skilled in the art can readily determine other suitable parameter combinations. It can be appreciated that the operating parameters may be varied between filament patterns  66 ,  68 ,  70  and  120  to produce zones of varying densities and fiber sizes. 
     Filament pattern  72  comprises pattern  72 A which moves in a reciprocating, transverse pattern, preferably covering the distance between the primary pattern edges  82  and  142 . Alternatively, filament pattern  72  covers less than the distance between edges  82  and  142 . Attenuating mechanism  54  preferably includes servo driven sweep mechanism  98  which allows attenuating mechanism  54  to sweep through an angle so that the filament pattern  72  traverses back and forth among fiber patterns  66 ,  68 ,  70  and  120 , along a longitudinal dimension of filament mass  127 . As pattern  72 A traverses fiber patterns  66 ,  68 ,  70  and  120 , it deposits essentially continuous polymer filaments across the overall laydown pattern which extends between the primary pattern edges  82  and  142 . In formed filament mass  127 , the fibers of filament pattern  66  deposited along edge  82  will form a first major surface  97  (shown in  FIG. 6 ), and the fibers of filament pattern  70  deposited along edge  84  will form a second major surface  99  (shown in  FIG. 6 ). In another embodiment, nozzle  44  may be oscillated back and forth to sweep bonding filament pattern  72 . 
     Fibers from filament patterns  66 ,  68 ,  70  and  120  form a generally two-dimensional mat or layer of material that is continuously formed on mandrel  76  to build up filament mass  127  composed of many layers of fibers. These fibers can be described as being laid down in an X-Y plane, or in the longitudinal and circumferential or latitudinal dimensions. As the fibers are built up, layer upon layer, they produce a radial or depth dimension. The sweeping motion of filament pattern  72 A, combined with the rotation of mandrel  76  causes the fibers coming from nozzle  44  to integrate into mass  127  as a “z” direction fiber, extending radially through the zones produced by filament patterns  66 ,  68 ,  70  and  120 . 
     In the embodiment shown in  FIG. 5 , filament pattern  128  is preferably produced simultaneously by nozzle  124  and attenuating mechanism  126 , located about 13 inches from mandrel  76 . In one embodiment, nozzle  124  and attenuating mechanism  126  are preferably static or stationary, in that filament pattern  128  does not oscillate or reciprocate like filament pattern  72 A. In an alternative embodiment, pattern  128  is oscillated or reciprocated. The filament from pattern  128  preferably mixes with the filament from pattern  72  across filament patterns  66 ,  68 ,  70  and  120 . This is accomplished in one embodiment by introducing filament pattern or stream  128  at an acute angle relative to mandrel  76 , resulting in a highly elliptical cross section of filament pattern  128  contacting the rotating, forming filament mass  127 . 
     As shown in  FIG. 5 , sweeping filament stream  72 A intercepts filament stream  128 , helping to secure the filaments of stream  128  to the forming filament mass  127 . Further, nozzle  144  and attenuating mechanism  146  preferably direct shell-forming filament pattern  162  onto a portion of filament mass  127  which has substantially reached its finished circumference. 
       FIG. 6  illustrates an elevation view of a second embodiment of a depth filter element of the present invention viewed from line  6 — 6  of  FIG. 5 . Filament mass  127  includes first major surface  97 , second major surface  99 , and concentric filtration zones  164 ,  166 ,  168  and  170 , with additional filament mass strength in the radial direction provided by filaments  110  and  172 . Filaments  110  and  172  serve as a strengthening element for fiber structure  127 . Filaments  110  and  172  extend throughout filament mass  127  and extend in the radial, longitudinal, and circumferential dimensions. 
     Generally, filament zone  164  is produced by filament pattern  66 ; filament zone  166  is produced by filament pattern  68 ; filament zone  168  is produced by filament pattern  70 ; filament zone  170  is produced by filament pattern  120 ; filament  110  is produced by filament pattern  72 ; and filament  172  is produced by filament pattern  128 . Filtration zones  164 ,  166 ,  168  and  170  preferably possess different physical characteristics. For example, filtration zone  164  may comprise relatively smaller diameter filaments; filtration zones  166  and  168  may comprise intermediate diameter filaments; and filtration zone  170  may comprise larger diameter filaments. Filtration zones  164 ,  166 ,  168  and  170  preferably have filaments having diameters ranging in size from less than about 1 micron to about 100 microns. In another embodiment, for example, filtration zone  164  may have a relatively high density of filaments; filtration zones  166  and  168  may have an intermediate density of filaments, and filtration zone  170  may have a lower density of filaments. 
     In one embodiment, there is generally an absence of fiber-to-fiber bonding within each of the masses  164 ,  166 ,  168  and  170  produced by filament patterns  66 ,  68 ,  60  and  120 , respectively. The primary bonding within filament mass  127  is accomplished by the bonding between “z” direction fibers  110  and  172  and the filaments of zones  164 ,  166 ,  168  and  170 . Selected zones of the media can be made very rigid to provide a filtering layer which also carries the resultant mechanical loads, thereby eliminating the need for separate structural elements in a given filter device. 
     Fibers  110  are produced as described with reference to  FIG. 3  above. Fibers  172  are formed as follows: when the filament stream of filament pattern  128  is near pattern edge  82 , “z” fiber  172  is laid onto filament mass  127  near the area of surface  97 . As the filament stream of filament pattern  128  flares toward pattern edge  142 , “z” fiber  172  is laid across zones  164 ,  166 ,  168  and  170  until it reaches the outside of outer zone  170 . Mandrel  76  spins while filament pattern  128  sprays so that “z” fiber  172  also travels in a circumferential direction around filter mass  127 . Thus, “z” fiber  172  runs radially, longitudinally, and circumferentially throughout filter mass  127 . In the case where mass  127  is planar rather than cylindrical, gluing fiber  172  may be described as extending in the length, width, and thickness dimensions of mass  127 . 
     In a preferred embodiment, filament pattern  128  is positioned so that the elliptical cross sectional area that contacts fiber mass  127  transverses one or more zones  164 ,  166 ,  168  and  170 ; however, filament pattern  128  need not transverse all zones  164 ,  166 ,  168  and  170 . The elliptical cross section of filament pattern  128  results in a longitudinal component of orientation. The forming fiber mass  127  upon which filament stream  128  is laid has a conical shape resulting in a radial component of orientation. Mandrel  26  spins, providing filament  172  with a circumferential component of orientation around filter mass  127 . Thus, “z” fiber  172  runs radially, longitudinally and circumferentially throughout filter mass  127 . While one nozzle  124  is shown to produce filaments  172 , it is contemplated that a different number of nozzles with other positions and configurations may also be used. 
     In one embodiment, the fibers of zones  164 ,  166 ,  168  and  170  comprise about 75–95 percent of the fibers of filter mass  127 , and “z” fibers  110  and  172  comprise about 5–25 percent of the fibers of filter mass  127 ; more preferably, the fibers of zones  164 ,  166 ,  168  and  170  comprise about 80–90 percent of the fibers of filter mass  127 , and “z” fibers  110  and  172  comprise about 10–20 percent of the fibers of filter mass  127 ; most preferably, the fibers of zones  164 ,  166 ,  168  and  170  comprise about 85 percent of the fibers of filter mass  127 , and “z” fibers  110  and  172  comprise about 15 percent of the fibers of filter mass  127 . 
     A new and unexpected property of the media of the present invention is that a strong integral filtration core may be produced without significantly increasing the density of the media. This is accomplished by depositing bonding fibers  110  and  172  onto the primary filtration fibers of zones  164 ,  166 ,  168  and  170  during the melt blowing process. The additional heat energy of bonding fibers  110  and  172  allow the highly amorphous polypropylene primary filtration fibers to significantly increase in crystallinity, which, in turn, strengthens the media. 
     The fibers of zones  164 ,  166 ,  168  and  170  may be comprised of different materials, may be of different sizes, or may otherwise have differing properties. For example, the diameters of the fibers in each zone may get progressively larger from core zone  164  to outer zone  170 . Each zone may also possess a different density from each adjacent zone. For example, the density of the zones may decrease progressively from core zone  164  to outer zone  170 . Moreover, in one embodiment, one or both of “z” fibers  110  and  172  have different material properties than the primary fibers of zones  162 ,  166 ,  168  and  170 . For example, fibers  110  and/or  172  may be catalysts for reactions or absorbent or adsorbent materials for toxins, viruses, proteins, organics, or heavy metals. In a preferred embodiment, the diameters of structural strengthening fibers or filaments  110  and  172  are comparable to the diameters of the primary filtration fibers in zones  164 ,  166 ,  168  and  170  so that the fibers  110  and  172  contribute not only to the strength of filament mass  127 , but also to its filtration capabilities. Other alternatives will be evident to one skilled in the art. 
     Depth filter elements formed in the manner described herein have demonstrated excellent particle filtration and fluid throughput capabilities. For example, the depth filter of the present invention has been demonstrated to have about twice the life and dirt holding capacity compared to similarly rated filters (e.g., 90% effective at removing 20 micron particles). Furthermore, the depth filter element of the present invention allows fluid throughput with a minimal drop in fluid pressure across the filter. 
     Filter performance depends on a combination of a number of factors, including the following: the size of contaminants that the filter can remove (efficiency), the amount of contaminants the filter can hold before plugging (dirt holding capacity), and the reliability of the filter function throughout its life or under variable operating conditions. 
     For any given filter, the dirt holding capacity (DHC) and filter efficiency are generally inversely related. The mass of a particle varies with the cube of its radius; therefore, lower efficiency filters that trap only larger particles and let the smaller particles pass can gain more weight before plugging. 
     To one skilled in the art, it is evident that DHC and filter cartridge weight are also generally inversely related. Reducing the weight of a fixed-volume filter cartridge is accomplished by taking material out of the cartridge, which in turn leaves more space (void volume) in which the trapped contaminants may accumulate. It is also apparent that taking material out of the cartridge, with all other variables held constant, makes the cartridge weaker (lower filter crush strength). 
     Filter crush strength is a typical measurement used to gauge the durability of a filter cartridge. If a filter is too soft, it will not function reliably throughout its service life or under variable operating conditions. To one skilled in the art of melt blowing, it is apparent that filter crush strength at a fixed filter weight can be manipulated by changing the fiber diameter as well as other process parameters; generally, larger fibers produce higher crush strength. Changing the filter construction to larger fibers generally increases the pore size to some degree, resulting in lower retention efficiency. 
     In order to take these major filter performance and construction variables into consideration for the purpose of filter comparison, the Madsen performance ratio (M) has been developed.
 
 M   ratio =( DHC× crush strength)/(μm@90%×filter weight)
         DHC is in grams   crush strength is in pounds per square inch (psi)   μm@90% refers to the particle size (μm) at which the filter produces 90% efficiency   filter weight (unused filter) is in grams       

     Higher ratio values indicate better utilization of the material in the filter, meaning that the filter has a better balance of strength, dirt holding capacity, and removal efficiency than filter with lower ratio values. 
     Although the description of the preferred embodiments and methods have been quite specific, it is contemplated that various modifications could be made without deviating from the spirit of the present invention. Accordingly, it is intended that the scope of the present invention be dictated by the appended claims rather than by the description of the illustrated embodiments. For example, it is contemplated that the teachings of the present invention may be adapted for flat or sheet type filters and products of other configurations. Additionally, the invention may also be practiced using “z” fibers  172  without “z” fibers  110 , or vice versa. One advantage of the use of both “z” filament delivery systems  18  and  114  is that a system of multiple sources offers an operator a greater degree of control. Additionally, while one filament delivery system of each type  16 ,  18 ,  114  and  122  is shown, it is contemplated that multiple systems of one or more types may also be used. 
     Moreover, it is contemplated that the roles of the filaments from the various delivery systems may be interchanged. For example, in one embodiment, the primary filtration filaments are produced by system  16  and bonding or structural strengthening filaments are produced by systems  18  and  114 . In another embodiment, the primary filtration filaments are produced by one or both of systems  18  and  114 , and bonding or structural strengthening filaments are produced by system  16 . The operating parameters and conditions can be manipulated by one of ordinary skill to obtain the desired combination of filaments in a mass. 
     EXAMPLE 1 
     Comparing a filter of the present invention with a standard filter, the following results were found for filters for a 10 micron particle size (A.C. fine test dust): 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                   
                 crush strength 
                 weight of 10″ 
                 life 
                 dirt holding 
               
               
                 product 
                 (psi) 
                 cartridge (g) 
                 (minutes) 
                 capacity (g) 
               
               
                   
               
             
            
               
                 invention 
                 93 
                 133 
                 60 
                 60 
               
               
                 standard 
                 125  
                 205 
                 29 
                 33 
               
               
                 control 
                 42 
                 143 
               
               
                   
               
            
           
         
       
     
     In all three examples, the standard product is made by any of the prior art methods discussed in the background of the invention. The control product is made by the same method as the invention, but without the z filaments  110  and  172 . This is accomplished by turning off the material pumps to nozzles  44  and  124 . Additionally, the control filter cartridge was allowed more time to form in order to compensate for the decreased input of material and allow it to reach a comparable weight compared to the invention product using z filaments  110  and  172 . Other operating conditions for formation of the “invention” and the “control” cylindrical cartridges are described below. 
     In this example, the filter of the present invention was much lighter than the standard filter, lasted about twice as long, and had about twice the DHC of the standard filter. However, it had a lower crush strength. Applying the formula above, M ratio-invention =4.2 and M ratio-standard =2.0. Thus, the filter of the present invention performs better than the standard product. The control product was tested only for crush strength. At a weight comparable to the invention product, the control product exhibited less than half the crush strength of the invention product. 
     In this example, the filter of the present invention was produced using primary fiber filament patterns  66 ,  68 ,  70  and  120  created by extruding polypropylene heated to between about 360° C. and about 400° C. through nozzles  27 ,  28 ,  29  and  116  having an orifice size of about 0.016 inch at a rate of about 9.5 pounds per hour. Filament streams  66  and  68  were heated to about 400° C., and filament streams  70  and  120  were heated to about 360° C. Attenuating mechanisms  31 ,  32 ,  33  and  118  passed ambient gas at a temperature of about 25° C. and had flow rates between about 10.5 to about 15 cubic feet per minute over the molten polymer streams exiting from nozzles  27 ,  28 ,  29  and  116 . The flow rate of attenuating mechanism  31  was at about 15 cubic feet per minute over nozzle  27  and the flow rates of attenuating mechanisms  32 ,  33  and  118  progressively decreased to a flow rate of about 10.5 cubic feet per minute at attenuating mechanism  118  over nozzle  116 . Nozzles  27 ,  28 ,  29  and  116  were positioned at a distance of between about 35 and about 37 inches from mandrel  76 . 
     “Z” fiber filament pattern  128  was produced by extruding polypropylene heated to about 370° C. through nozzle  124  having an orifice size of about 0.016 inch at a rate of about 5.5 pounds per hour. Attenuating mechanism  126  passed ambient gas at a temperature of about 25° C. and had a flow rate of about 9 cubic feet per minute over the polymer stream exiting nozzle  124 . Nozzle  124  was positioned at a distance about 13 inches from mandrel  76 . 
     “Z” fiber filament pattern  72 A was produced by extruding polypropylene heated to about 370° C. through nozzle  44  having an orifice size of about 0.016 inch at a rate of about 5.5 pounds per hour. Attenuating mechanism  54  passed ambient gas at a temperature of about 250° C. and had a flow rate of about 7 cubic feet per minute over the polymer stream exiting nozzle  44 . Nozzle  44  was positioned at a distance about 21 inches from mandrel  76 . 
     Shell forming fiber filament pattern  162  was produced by extruding polypropylene heated to about 280° C. through nozzle  144  having an orifice size of about 0.016 inch at a rate of about 1.0 pound per hour. Attenuating mechanism  146  passed ambient gas at a temperature of about 25° C. and had a flow rate of about 1.25 cubic feet per minute over the polymer stream exiting nozzle  144 . Nozzle  144  was positioned at a distance about 3.5 inches from mandrel  76 . 
     EXAMPLE 2 
     Comparing a filter of the present invention with a standard filter, the following results were found for filters for a 20 micron particle size (A.C. coarse test dust): 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                   
                 crush strength 
                 weight of 10″ 
                 life 
                 dirt holding 
               
               
                 product 
                 (psi) 
                 cartridge (g) 
                 (minutes) 
                 capacity (g) 
               
               
                   
               
             
            
               
                 invention 
                 83 
                 119 
                 85 
                 118 
               
               
                 standard 
                 100  
                 160 
                 46 
                  65 
               
               
                 control 
                 36 
                 129 
               
               
                   
               
            
           
         
       
     
     In this example, the filter of the present invention was lighter than the standard filter, lasted about twice as long, and had almost twice the DHC of the standard filter. However, it had a lower crush strength. Applying the formula above, M ratio-invention 
=4.1 and M ratio-standard =2.0. Thus, the filter of the present invention performs better than the standard product. The control product was tested only for crush strength. At a weight comparable to the invention product, the control product exhibited less than half the crush strength of the invention product. 
     In this example, the filter of the present invention was produced using primary fiber filament patterns  66 ,  68 ,  70  and  120  created by extruding polypropylene heated to about 370° C. through nozzles  27 ,  28 ,  29  and  116  having an orifice size of about 0.016 inch at a rate of between about 10 to about 11 pounds per hour. Nozzles  27  and  28  had a flow rate of about 10 pounds per hour and nozzles  29  and  116  had greater flow rates of approximately 11 pounds per hour. Attenuating mechanisms  31 ,  32 ,  33  and  118  passed ambient gas at a temperature of about 25° C. and had flow rates between about 10.5 to about 15 cubic feet per minute over the molten polymer streams exiting from nozzles  27 ,  28 ,  29  and  116 . The flow rate of attenuating mechanism  31  was at about 15 cubic feet per minute over nozzle  27  and the flow rates of attenuating mechanisms  32 ,  33  and  118  progressively decreased to a flow rate of about 10.5 cubic feet per minute at attenuating mechanism  118  over nozzle  116 . Nozzles  27 ,  28 ,  29  and  116  were positioned at a distance of between about 38 and about 40 inches from mandrel  76 . 
     “Z” fiber filament pattern  128  was produced by extruding polypropylene heated to about 370° C. through nozzle  124  having an orifice size of about 0.016 inch at a rate of about 6 pounds per hour. Attenuating mechanism  126  passed ambient gas at a temperature of about 25° C. and had a flow rate of about 12 cubic feet per minute over polymer stream exiting nozzle  124 . Nozzle  124  was positioned at a distance about 13 inches from mandrel  76 . 
     “Z” fiber filament pattern  72 A was produced by extruding polypropylene heated to about 370° C. through nozzle  44  having an orifice size of about 0.016 inch at a rate of about 6 pounds per hour. Attenuating mechanism  54  passed ambient gas at a temperature of about 25° C. and had a flow rate of about 11 cubic feet per minute over the polymer stream exiting nozzle  44 . Nozzle  44  was positioned at a distance about 22 inches from mandrel  76 . 
     Shell forming fiber filament pattern  162  was produced by extruding polypropylene heated to about 290° C. through nozzle  144  having an orifice size of about 0.016 inch at a rate of about 1.1 pound per hour. Attenuating mechanism  146  passed ambient gas at a temperature of about 25° C. and had a flow rate of about 1.75 cubic feet per minute over the polymer stream exiting nozzle  144 . Nozzle  144  was positioned at a distance about 3.5 inches from mandrel  76 . 
     EXAMPLE 3 
     Comparing a filter of the present invention with a standard filter, the following results were found for filters for a 30 micron particle size (A.C. coarse test dust): 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                   
                 crush strength 
                 weight of 10″ 
                 life 
                 dirt holding 
               
               
                 product 
                 (psi) 
                 cartridge (g) 
                 (minutes) 
                 capacity (g) 
               
               
                   
               
             
            
               
                 invention 
                 75 
                 113 
                 105 
                 120 
               
               
                 standard 
                 80 
                 152 
                  50 
                  73 
               
               
                 control 
                 43 
                 106 
               
               
                   
               
            
           
         
       
     
     In this example, the filter of the present invention was lighter than the standard filter, lasted about twice as long, had a much greater DHC, and had a comparable crush strength. Applying the formula above, M ratio-invention =2.7 and M ratio-standard 
=1.3. Thus, the filter of the present invention performs better than the standard product. The control product was tested only for crush strength. At a weight comparable to the invention product, the control product exhibited significantly lower crush strength compared to the invention product. 
     In this example, the filter of the present invention was produced using primary fiber filament patterns  66 ,  68 ,  70  and  120  created by extruding polypropylene heated to about 360° C. through nozzles  27 ,  28 ,  29  and  116  having an orifice size of about 0.016 inch at a rate of between about 10 to about 11 pounds per hour. Nozzles  27  and  28  had a flow rate of about 10 pounds per hour and nozzles  29  and  116  had greater flow rates of about 11 pounds per hour. Attenuating mechanisms  31 ,  32 ,  33  and  118  passed ambient gas at a temperature of about 25° C. and had flow rates between about 10.5 to about 15 cubic feet per minute over the molten polymer streams exiting from nozzles  27 ,  28 ,  29  and  116 . The flow rate of attenuating mechanism  31  was at about 15 cubic feet per minute over nozzle  27  and the flow rates of attenuating mechanisms  32 ,  33  and  118  progressively decreased to a flow rate of about 10.5 cubic feet per minute at attenuating mechanism  118  over nozzle  116 . Nozzles  27 ,  28 ,  29  and  116  were positioned at a distance of between about 38 and about 40 inches from mandrel  76 . 
     “Z” fiber filament pattern  128  was produced by extruding polypropylene heated to about 360° C. through nozzle  124  having an orifice size of about 0.016 inch at a rate of about 6 pounds per hour. Attenuating mechanism  126  passed ambient gas at a temperature of about 25° C. and had a flow rate of about 12 cubic feet per minute over the polymer stream exiting nozzle  124 . Nozzle  124  was positioned at a distance about 13 inches from mandrel  76 . 
     “Z” fiber filament pattern  72 A was produced by extruding polypropylene heated to about 360° C. through nozzle  44  having an orifice size of about 0.016 inch at a rate of about 6 pounds per hour. Attenuating mechanism  54  passed ambient gas at a temperature of about 25° C. and had a flow rate of about 11 cubic feet per minute over the polymer stream exiting nozzle  44 . Nozzle  44  was positioned at a distance about 22 inches from mandrel  76 . 
     Shell forming fiber filament pattern  162  was produced by extruding polypropylene heated to about 280° C. through nozzle  144  having an orifice size of about 0.016 inch at a rate of about 1.1 pound per hour. Attenuating mechanism  146  passed ambient gas at a temperature of about 25° C. and had a flow rate of about 1.75 cubic feet per minute over the polymer stream exiting nozzle  144 . Nozzle  144  was positioned at a distance about 3.5 inches from mandrel  76 .