Patent Publication Number: US-2019168143-A1

Title: Filter media with a multi-layer structure

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 14/984,406, filed Dec. 30, 2015, which is a continuation of U.S. application Ser. No. 14/169,994, filed Jan. 31, 2014, which is a continuation of U.S. application Ser. No. 12/768,318 (now U.S. Pat. No. 8,679,218), filed Apr. 27, 2010, which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF INVENTION 
     The disclosure relates generally to filtration and, more particularly, to filter media, related systems, components and methods. 
     BACKGROUND OF INVENTION 
     Filter elements can be used to remove contamination in a variety of applications. Such elements can include a filter media which may be formed of a web of fibers. The fiber web provides a porous structure that permits fluid (e.g., gas, liquid) to flow through the media. Contaminant particles (e.g., dust particles, soot particles) contained within the fluid may be trapped on the fiber web. Depending on the application, the filter media may be designed to have different performance characteristics. 
     Dust holding capacity is a measure of the amount of dust captured on the filter media under certain conditions. Dust capture efficiency is a measure of the concentration of dust that passes through the filter as compared to the concentration of dust prior to passing through the filter. In some applications, it is desirable for the filter media to have a high dust holding capacity and/or a high dust capture efficiency. 
     SUMMARY OF INVENTION 
     Aspects presented herein relate to filter media. 
     In one aspect, a filter media is provided. The filter media includes a first layer comprising a first phase including a first plurality of fibers and a second plurality of fibers different from the first plurality of fibers; and a second phase including a third plurality of fibers and a fourth plurality of fibers different from the third plurality of fibers, the third plurality of fibers being the same as or different from the first or second plurality of fibers and the fourth plurality of fibers being the same as or different from the first or second plurality of fibers, wherein the air permeability of the first plurality of fibers is greater than the air permeability of the second plurality of fibers and the air permeability of the third plurality of fibers is greater than the air permeability of the fourth plurality of fibers. The filter media also includes a second layer attached to the first layer, wherein the second layer is a nanofiber layer. 
     In another aspect, a filter media is provided. The filter media includes a first layer including a plurality of cellulose fibers; and a second layer attached to the first layer, wherein the second layer is a nanofiber layer, and wherein the filter media has a dust holding capacity of at least 200 g/m 2  and an initial efficiency of at least 80%. 
     In a further aspect, a method of manufacturing a filter media is provided. The method includes forming a first layer comprising forming a first phase including a first plurality of fibers and a second plurality of fibers different from the first plurality of fibers; and forming a second phase including a third plurality of fibers and a fourth plurality of fibers different from the third plurality of fibers, the third plurality of fibers being the same as or different from the first or second plurality of fibers and the fourth plurality of fibers being the same as or different from the first or second plurality of fibers, wherein the air permeability of the first plurality of fibers is greater than the air permeability of the second plurality of fibers and the air permeability of the third plurality of fibers is greater than the air permeability of the fourth plurality of fibers. The method also includes forming a nanofiber layer; and adhering the first layer and the nanofiber layer together to form a fiber composite. 
     Other aspects, embodiments, features will become apparent from the following description. Each reference incorporated herein by reference is incorporated in its entirety. In cases of conflict or inconsistency between an incorporated reference and the present specification, the present specification will control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. In the drawings: 
         FIG. 1  depicts an embodiment of a filter media having a first layer and a second layer; 
         FIG. 2  illustrates an embodiment of a filter media having a first layer and a second layer where the first layer includes multiple phases; 
         FIG. 3  illustrates an embodiment of a filter media having a first layer, a second layer and an intermediate layer; 
         FIG. 4  illustrates an embodiment of a filter media having a first layer that includes multiple phases, an intermediate layer and a second layer; 
         FIG. 5  is a schematic view of a system configured to be used in the manufacture of filter media; 
         FIG. 6  depicts an embodiment of a corrugated filter media having a first layer, a second layer and an intermediate layer; 
         FIG. 7  illustrates a graph of performance characteristics of the examples provided; 
         FIG. 8  illustrates another graph of performance characteristics of the examples provided; and 
         FIG. 9  depicts a graph of further performance characteristics of the examples provided. 
     
    
    
     DETAILED DESCRIPTION 
     Filter media are described. The filter media may include multiple layers, as described further below. In some embodiments, the filter media includes a nanofiber layer adhered to another layer. In some embodiments, the layer to which the nanofiber layer is adhered is formed of multiple fiber types (e.g., fibers that give rise to structures having different air permeabilities and/or pressure drops). The filter media may be designed to have advantageous properties including, in some cases, a high dust particle capture efficiency and/or a high dust holding capacity. 
     For example,  FIGS. 1 and 2  depict illustrative embodiments of a filter media  10  having a first layer  20  and a second layer  30 . In some embodiments, first layer  20  includes a number of different kinds of fibers and second layer  30  is a nanofiber layer. In some embodiments, layer  20  is upstream of layer  30  when fluid flows through the media. In other embodiments, layer  20  is downstream of layer  30  when fluid flows through the media. In some cases, first layer  20  may include a single phase, such as an open phase or a tight phase, described further below. In other cases, as illustrated in FIG.  2 , first layer  20  may be a multi-phase layer having a first phase  22  (e.g., open phase) and a second phase  24  (e.g., tight phase). In some embodiments, first phase  22  may be a tight phase and second phase  24  may be an open phase. It can be appreciated that any number of phases may be incorporated in first layer  20 . 
     It should be understood that the term “filter media” is used herein in singular or plural form. When referring to a single filter media, the term “filter media” may be used interchangeably with the term “filter medium.” 
     Further,  FIGS. 3 and 4  depict illustrative embodiments of a filter media  10  having an intermediate layer  40  disposed between first layer  20  and second layer  30 . In some embodiments, as shown in  FIG. 3 , intermediate layer  40  may be an adhesive, described further below. Depicted in  FIG. 4 , in other embodiments, intermediate layer  40  may be included in an arrangement where the intermediate layer  40  is disposed between a first layer  20  having a first phase  22  and a second phase  24 , and a second layer  30 . 
     I. Single-Phase or Multi-Phase Layer Structure 
     As discussed, filter media  10  having a layer  20  with multiple fiber types may include one or more phases that give rise to different air permeabilities and/or pressure drop characteristics within those phases. In some embodiments, such as that shown in  FIG. 2 , a filter media  10  may have a first phase  22  that has a larger air permeability and/or a smaller pressure drop than that of a second phase  24 . In some cases, a phase having a larger air permeability and/or a smaller pressure drop may be referred to as an open phase, whereas a phase having a smaller air permeability and/or a larger pressure drop may be referred to as a tight phase. 
     In an example, a filter media  10  may include an open phase  22  that is provided substantially over a tight phase  24 . For instances where open and tight phases are formed in a filter media, the position of open and tight phases may depend on the particular embodiment. It is possible for an open phase to be disposed over a tight phase and vice versa. Accordingly, an open phase may be positioned upstream relative to a tight phase and vice versa. However, it should be understood that a first layer  20  of a filter media  10  can have any number of open and/or tight phases. Indeed, a first layer  20  may include two or more open and/or tight phases. As described herein, a layer in a filter media may include one or more phases (e.g., open or tight). In embodiments in which a layer includes more than one phase, each phase may have different fiber types and/or different characteristics, as described further below. In some embodiments, phases may be laminated together or formed simultaneously on a papermaking machine, also as described below. 
     In some embodiments, filter media having a first layer with multiple phases (e.g., open and tight phases) may exhibit improved dust holding capacity and/or dust capture efficiency as compared to a filter media that has a first layer with a single phase structure. Though, alternatively, in some instances, a single phase layer may exhibit improved dust holding capacity and/or dust capture efficiency over multi-phase layers. 
     It should be understood that a first layer  20  can be utilized independently as a filter media. However, certain embodiments presented herein incorporate a first layer  20  into filter media  10  having other components. 
     Suitable multi-phase layers have been described in commonly-owned U.S. patent application Ser. No. 12/488,033, entitled “Multi-Phase Filter Medium,” filed on Jun. 19, 2009 which is incorporated herein by reference in its entirety. 
     A. Open Phase 
     An open phase in a filter media may include a first plurality of fibers and a second plurality of fibers, although in some embodiments, only a first plurality of fibers is used to form the open phase. 
     In some embodiments, the first plurality of fibers have an air permeability larger than that of the second plurality of fibers. For example, the first plurality of fibers can have an air permeability from about 50 cubic feet per minute (“CFM”) to about 350 CFM (e.g., from about 100 CFM to about 200 CFM), and the second plurality of fibers can have an air permeability from about 5 CFM to about 50 CFM (e.g., from about 8 CFM to about 37 CFM or from about 15 CFM to about 25 CFM). As used herein, air permeability of fibers is determined by measuring the air permeability of a hand sheet produced exclusively of such fibers and having a basis weight of 100 g/m 2  according to ISO 9237. 
     In some embodiments, the first plurality of fibers have a pressure drop smaller than that of the second plurality of fibers. For example, the first plurality of fibers can have a pressure drop from about 5 Pascals (“Pa”) to about 300 Pa (e.g., from about 10 Pa to about 250 Pa or from about 20 Pa to about 100 Pa), and the second plurality of fibers can have a pressure drop from about 300 Pa to about 1,000 Pa (e.g., from about 350 Pa to about 500 Pa). As used herein, pressure drop of fibers is determined by using a gas having a face velocity of 40 cm/s on a hand sheet produced exclusively of such fibers and having a basis weight of 100 g/m 2  according to ASTM F778-88. 
     In some embodiments, the first plurality of fibers have an average fiber length larger than that of the second plurality of fibers. For example, the first plurality of fibers can have an average fiber length from about 1.5 mm to about 6 mm (e.g., from about 2.5 mm to about 4.5 mm), and the second plurality of fibers can have an average fiber length from about 0.5 mm to about 2 mm (e.g., from about 0.7 mm to about 1.5 mm). 
     In general, the materials that can be used to form the first and second pluralities of fibers can vary as desired. In some embodiments, the first plurality of fibers are made from softwood fibers, cotton fibers, glass fibers, polyester fibers, polyvinyl alcohol binder fibers, and/or rayon fibers. Exemplary softwood fibers include fibers obtained from mercerized southern pine (“mercerized southern pine fibers or HPZ fibers”), northern bleached softwood kraft (e.g., fibers obtained from Robur Flash (“Robur Flash fibers”)), southern bleached softwood kraft (e.g., fibers obtained from Brunswick pine (“Brunswick pine fibers”)), or chemically treated mechanical pulps (“CTMP fibers”). For example, HPZ fibers can be obtained from Buckeye Technologies, Inc., Memphis, Tenn.; Robur Flash fibers can be obtained from Rottneros A B, Stockholm, Sweden; and Brunswick pine fibers can be obtained from Georgia-Pacific, Atlanta, Ga. 
     In some embodiments, the second plurality of fibers are made from hardwood fibers, polyethylene fibers, or polypropylene fibers. Exemplary hardwood fibers include fibers obtained from Eucalyptus (“Eucalyptus fibers”). Eucalyptus fibers are commercially available from, e.g., (1) Suzano Group, Suzano, Brazil (“Suzano fibers”), (2) Group Portucel Soporcel, Cacia, Portugal (“Cacia fibers”), (3) Tembec, Inc., Temiscaming, QC, Canada (“Tarascon fibers”), (4) Kartonimex Intercell, Duesseldorf, Germany, (“Acacia fibers”), (5) Mead-Westvaco, Stamford, Conn. (“Westvaco fibers”), and (6) Georgia-Pacific, Atlanta, Ga. (“Leaf River fibers”). In general, softwood fibers have a relatively large air permeability, small pressure drop, and large average fiber length as compared to hardwood fibers. 
     In some embodiments, an open phase can include a mixture of softwood fibers and hardwood fibers. In certain embodiments, the open phase can include only one type of fibers (e.g., softwood fibers) uniformly distributed within the open phase. 
     In some embodiments, an open phase can include a mixture of fibers with different characteristics (e.g., different air permeabilities and/or pressure drops). Fibers with different characteristics can be made from one material (e.g., by using different process conditions) or different materials. 
     In some embodiments, the first plurality of fibers can be formed from a material identical to the material used to form the second plurality of fibers. In such embodiments, the first and second pluralities of fibers can be prepared by using different preparation methods, or different conditions in the same preparation method, such that they have different characteristics (e.g., different air permeabilities or pressure drops). In certain embodiments, the first and second pluralities of fibers can be formed of the same material and also have the same characteristics. In some embodiments, the first plurality of fibers can be formed from a material different from the material used to form the second plurality of fibers. 
     Generally, the weight ratio of the first and second plurality of fibers can vary depending on the desired properties of the filter media. For instance, an open phase in a filter media may include fibers with a larger air permeability and/or a smaller pressure drop in an amount larger than the amount of fibers with a smaller air permeability and/or a larger pressure drop. In some embodiments, a weight ratio of the first and second pluralities of fibers described above can range from about 50:50 to about 97:3 (e.g., from about 50:50 to about 70:30 or from about 60:40 to about 70:30). As used herein, the weight of the first or second plurality of fibers refers to the initial weight of each group of fibers in a composition (e.g., a pulp) used to prepare the open phase. In certain embodiments, an open phase in a filter media can include fibers with a larger air permeability and/or a smaller pressure drop in an amount equal to or smaller than the amount of fibers with a smaller air permeability and/or a larger pressure drop. 
     In some embodiments, an open phase can include one or more pluralities of fibers in addition to the first and second pluralities of fibers. Each of the additional pluralities of fibers can have characteristics (e.g., air permeability and/or pressure drop) different from the first or second plurality of fibers, and/or can be prepared from a material different from those used to prepare the first or second plurality of fibers. In some embodiments, an additional plurality of fibers can be prepared from a material identical to a material used to form one of the first and second pluralities of fibers, but still have characteristics different from those of the first or second plurality of fibers. 
     B. Tight Phase 
     A tight phase in a filter media may include a third plurality of fibers and a fourth plurality of fibers. 
     In some embodiments, the third plurality of fibers can have the same characteristics (e.g., air permeability and/or pressure drop) and/or be made from the same type of material as the first or second plurality of fibers described above, and/or the fourth plurality of fibers can have the same characteristics and/or be made from the same type of material as the first or second plurality of fibers described above. 
     In some embodiments, the third plurality of fibers can have one or more characteristics (e.g., air permeability and/or pressure drop), and/or be made from a type of material, different from those of the first or second plurality of fibers, and/or the fourth plurality of fibers can have one or more characteristics, and/or be made from a type of material, different from those of the first or second plurality of fibers. 
     In some embodiments, a tight phase can include a mixture of softwood fibers and hardwood fibers. In certain embodiments, the tight phase can include only one type of fibers (e.g., hardwood fibers) uniformly distributed within the tight phase. 
     In some embodiments, a tight phase can include a mixture of fibers with different characteristics (e.g., different air permeabilities and/or pressure drops). Fibers with different characteristics can be made from one material (e.g., by using different process conditions) or different materials. 
     In some embodiments, the third plurality of fibers are made from softwood fibers, cotton fibers, glass fibers, polyester fibers, polyvinyl alcohol binder fibers, and/or rayon fibers. Exemplary softwood fibers include mercerized southern pine fibers or HPZ fibers, northern bleached softwood kraft fibers (e.g., Robur Flash fibers), southern bleached softwood kraft fibers (e.g., Brunswick pine fibers) or CTMP fibers. 
     In some embodiments, the fourth plurality of fibers are made from hardwood fibers, polyethylene fibers, or polypropylene fibers. Exemplary hardwood fibers include Eucalyptus fibers (e.g., Suzano fibers, Cacia fibers, Tarascon fibers, Acacia fibers, Westvaco fibers, Leaf River fibers). 
     In some embodiments, the third plurality of fibers can be formed from a material identical to the material used to form the fourth plurality of fibers. In such embodiments, the third and fourth pluralities of fibers can be prepared by using different preparation methods, or different conditions in the same preparation method, such that they have different characteristics (e.g., different air permeability or pressure drop). In certain embodiments, the third and fourth pluralities of fibers can be formed of the same material and also have the same characteristics. In some embodiments, the third plurality of fibers can be formed from a material different from the material used to form the fourth plurality of fibers. 
     Generally, the weight ratio of the third and fourth plurality of fibers can vary depending on the desired properties of the filter media or its intended uses. A tight phase in a filter media may include fibers with a higher air permeability and/or a smaller pressure drop in an amount smaller than the amount of fibers with a smaller air permeability and/or a larger pressure drop. For example, a weight ratio of the third and fourth pluralities of fibers described above can range from about 3:97 to about 50:50 (e.g., from about 25:75 to about 50:50 or from about 70:30 to about 50:50). As used herein, the weight of the third or fourth plurality of fibers refers to the initial weight of each group of fibers in a composition (e.g., a pulp) used to prepare the tight phase. In some embodiments, a tight phase in a filter media can include fibers with a higher air permeability and/or a smaller pressure drop in an amount larger than or equal to the amount of fibers with a smaller air permeability and/or a larger pressure drop. 
     In some embodiments, a tight phase can include one or more pluralities of fibers in addition to the third and fourth pluralities of fibers. Each of the additional pluralities of fibers can have characteristics (e.g., air permeability or pressure drop) different from the third or fourth plurality of fibers, and/or can be prepared from a material different from those used to prepare the third or fourth plurality of fibers. In some embodiments, an additional plurality of fibers can be prepared from a material identical to a material used to form one of the third and fourth pluralities of fibers, but still have characteristics different from those of the third or fourth plurality of fibers. 
     In general, the weight ratio of the open and tight phases in a filter media can vary as desired. For example, a basis weight ratio of the open and tight phases can range from about 10:90 to about 90:10 (e.g., from about 30:70 to about 70:30, from about 40:60 to about 60:40, or from about 30:70 to about 90:10). In certain embodiments, the basis weight ratio of the open and tight phases is about 30:70 or greater (e.g., about 40:60 or greater, about 50:50 or greater, or about 60:40 or greater). As used herein, basis weight of an open or tight phase refers to the weight of the phase over a unit area (e.g., per square meter). For example, the basis weight of an open or tight phase can have a unit of grams per square meter. 
     C. Additional Features of a Single-Phase or Multi-Phase Layer 
     As discussed above, filter media  10  may include a first layer  20  that is single-phase or multi-phase, having an open phase, a tight phase or both. 
     The first layer  20  of a filter media  10  may include any desired percentage of softwood fibers. For example, the first layer  20  may include a weight percentage of softwood fibers of at least about 50%, at least about 60% or at least about 80% of the weight composition of the first layer. 
     Similarly, the first layer  20  of a filter media  10  may include any desired percentage of hardwood fibers. For example, the first layer  20  may include a weight percentage of hardwood fibers of less than about 50%, less than about 30%, less than about 20%, less than about 10% or 0% of the weight composition of the first layer. 
     Further, in some embodiments, additional fibers may be included in the first layer  20  of a filter media  10 . For instance, additional fibers may include a weight percentage of synthetic fibers (e.g., polyester, polypropylene) in the first layer of 0%, greater than 5%, greater than 10%, greater than 20% or greater than 30% of the weight composition of the first layer. 
     In some embodiments, synthetic fibers included in the first layer  20  of a filter media  10  may have a denier of between about 0.5 and about 6.0, between about 0.5 and about 1.5, between about 1.5 and about 3.0, or between about 3.0 and about 6.0. In some embodiments, synthetic fibers may have a denier of at least about 1.5 or at least about 3.0. 
     Additional fibers of any particular denier may be included in the first layer  20  of a filter media  10  in any desired amount. In some embodiments, the first layer  20  may include a weight percentage of synthetic fibers in the first layer having a denier of at least 1.5 of 0%, greater than 5%, greater than 10%, greater than 20%, or greater than 30% of the weight composition of the first layer. In other embodiments, the first layer  20  may include a weight percentage of synthetic fibers in the first layer having a denier of at least 3.0 of 0%, greater than 10%, greater than 20%, or greater than 30% of the weight composition of the first layer. 
     In general, the thickness of the first layer  20  of a filter media  10  may vary as desired. For example, the first layer  20  can have a thickness from 100 microns to 2,000 microns (e.g., from 200 microns to 1,600 microns or from 400 microns to 1,200 microns). The thickness may be determined according to the standard TAPPI T411. 
     The first layer  20  of a filter media  10  may have any suitable basis weight. For example, the first layer  20  can have a basis weight of at least about 10 g/m 2  and/or at most about 500 g/m 2  (e.g. between about 25 g/m 2  and about 230 g/m 2  or between about 80 g/m 2  and about 150 g/m 2 ). As used herein, basis weight of the first layer  20  refers to the weight of the media over a unit area (e.g., per square meter). A typical unit for the basis weight is g/m 2 . 
     The air permeability of the first layer  20  of a filter media  10  can usually be selected as desired. For example, the air permeability of the first layer  20  can range between about 1 CFM and about 300 CFM (e.g., between about 1 CFM and about 250 CFM, between about 2 CFM and about 250 CFM, between about 2 CFM and about 300 CFM, between about 30 CFM and about 120 CFM, or between about 100 CFM and about 200 CFM). As used herein, air permeability of a filter media is determined according to ISO 9237. 
     The average pore size of the first layer  20  of a filter media  10  can vary as desired. For example, the first layer  20  can have an average pore size from at least about 3 microns and/or at most about 1,000 microns (e.g., between about 30 microns and 400 microns). In some embodiments, the average pore size of the first layer  20  having a single phase (e.g., open phase) may range between about 50 microns and about 200 microns or between about 75 microns and about 150 microns. In other embodiments, the average pore size of an open phase portion of the first layer  20  may range between about 100 microns and about 400 microns or between about 150 microns and about 200 microns. In further embodiments where the first layer  20  has more than one phase, the average pore size of a tight phase portion of a filter media may range between about 30 microns and about 200 microns or between about 50 microns and about 125 microns. As used herein, the average pore size refers to the mean flow pore size measured by using a Coulter Porometer as described in ASTM F316-03. 
     In general, the properties of filter media may be assessed using a Palas flat sheet test. Such testing is based on the following parameters: A 100 cm 2  surface area of a filter media is challenged with a fine dust particles (0.1-80 μm) at a concentration of 200 mg/m 3  with a face velocity of 20 cm/s for one minute. The dust capture efficiency is measured using a Palas MFP2000 fractional efficiency photodetector. The dust capture efficiency is [(1-[C/C0])*100%], where C is the dust particle concentration after passage through the filter and C0 is the particle concentration before passage through the filter. The dust capture efficiency is measured after one minute and is referred to herein as the initial dust capture efficiency. The dust holding capacity is measured when the pressure drop across the media reaches 1,800 Pa, and is the difference in the weight of the filter media before the exposure to the fine dust and the weight of the filter media after the exposure to the fine dust. 
     The first layer  20  of a filter media  10  can exhibit an advantageous ability to capture dust. For example, the first layer  20  can have an initial dust capture efficiency of at least about 25% (e.g., at least about 40%, at least about 60) and/or at most about 70% measured according to the Palas flat sheet test, as described above. 
     The first layer  20  of a filter media  10  can also have good dust holding properties. For example, the first layer  20  can have a dust holding capacity (DHC) of at least about 30 g/m 2  (e.g., at least about 100 g/m 2 , at least about 200 g/m 2 , or at least about 300 g/m 2 ) and/or at most about 400 g/m 2  (e.g., at most about 350 g/m 2 , at most about 300 g/m 2 , at most about 250 g/m 2  or at most about 200 g/m 2 ) according to a Palas flat sheet test. As used herein, DHC is the weight of dust captured divided by the test area of the Palas test sheet. As another example, the first layer  20  can have a specific dust holding capacity of at least about 0.001 g/g (e.g., at least about 0.004 g/g) and/or at most about 1.0 g/g (e.g., at most about 0.9 g/g). As used herein, specific dust holding capacity can be calculated by dividing the weight of dust captured over a unit weight (e.g., per gram) of the Palas test sheet. Alternatively, specific dust holding capacity can be calculated by dividing the weight of dust captured over a unit thickness (e.g., per millimeter) of the Palas test sheet. In such alternate case, the specific dust holding capacity shall be expressed in units of g/mm. 
     In some embodiments, the first layer  20  of a filter media  10  has both advantageous dust capture and dust holding properties combined. As an example, the first layer  20  can have an initial dust capture efficiency of at least about 25% (e.g., at least about 40%, at least about 60%) and a dust holding capacity of at least about 30 g/m 2  (e.g., at least about 100 g/m 2  at least about 200 g/m 2 , or at least about 300 g/m2). 
     In some embodiments, at least one of the first, second, third, and fourth pluralities of fibers described above are made from an organic polymeric material (e.g., softwood fibers, cotton fibers, hardwood fibers, or synthetic organic polymers such as polyester or rayon). In certain embodiments, more than one (e.g., two, three, or all) of the first, second, third and fourth pluralities of fibers are made from an organic polymeric material. In some embodiments, at least one of the first, second, third, and fourth pluralities of fibers described above are made from cellulose. 
     In some embodiments, the first layer  20  of a filter media  10  can further include a binder. In general, including a binder in the first layer  20  can significantly increase its strength (e.g., tensile strength measured according to ISO 1924-2 or Mullen Burst strength measured according to DIN 53113). The binder can include a polymeric material, such as polyvinyl acetate, an epoxy, a polyester, a polyvinyl alcohol, an acrylic (e.g., a styrene acrylic), or a phenolic resin. In some embodiments, the binder can be at least about 2% and/or at most about 50% (e.g., at most about 35%, at most about 25%, at most about 15%, or at most about 5%) of the weight composition of the first layer. In some embodiments, the binder can be present in the first layer between about 10% and about 50%, or between about 15% and about 30% of the weight composition of the first layer. In general, the binder can be present in the first layer with or without the presence of cross-linking agents (e.g., melamine, hexamine, or an epoxy hardener) or other additives (e.g., silicones, fluorocarbons, or catalysts such as ammonium chloride). 
     In some cases, a filter media having a multi-phase structure includes an interface between a first phase and a second phase. In some embodiments, when the multi-phase structure is prepared in a continuous wet laid process (e.g., forming first and second phases in a continuous liquid-based coating process), the interface can take the form of a transition phase which includes at least a portion of each of the first, second, third, and fourth pluralities of fibers intermingled with each other. Without wishing to be bound by theory, it is believed that an interface prepared by such a process can be substantially non-linear due to the interaction between the fibers in the first and second phases. In addition, as no adhesive is typically used in a wet laid process, the interface is typically substantially free of any adhesive. First and second phases which include an interface that is substantially free of an adhesive may be joined by, for example, physical interactions between the fibers in each of the phases, or by other suitable methods that do not involve the use of an adhesive to join the phases. In some cases, first and second phases which include an interface that is substantially free of an adhesive are not joined by lamination. 
     In some embodiments, and as discussed above, a first layer of a filter media can include one or more phases in addition to a first phase and a second phase. An additional phase can be the same as or different from a first phase or a second phase. For example, a multi-phase layer having three phases may have any combination of open and/or tight phases. 
     D. Methods of Manufacturing a Multi-Phase Layer 
     A multi-phase layer for incorporation in a filter media can be made by any suitable methods. 
     In some embodiments, a multi-phase layer can be prepared by a wet laid process as follows: First, a first dispersion (e.g., a pulp) containing first and second pluralities of fibers in a solvent (e.g., an aqueous solvent such as water) can be applied onto a wire conveyor in a papermaking machine (e.g., a fourdrinier or a rotoformer) to form a first phase supported by the wire conveyor. A second dispersion (e.g., another pulp) containing third and fourth pluralities of fibers in a solvent (e.g., an aqueous solvent such as water) is then applied onto the first phase. Vacuum is continuously applied to the first and second dispersions of fibers during the above process to remove the solvent from the fibers, thereby resulting in an article containing the first phase and a second phase. In some embodiments, the second phase is brought into contact with the first phase as the vacuum is applied at nearly the same time or soon thereafter. 
     The article thus formed is then dried and, if necessary, further processed (e.g., calendared) by using known methods to form a multi-phase layer. In some embodiments, first and second phases in a multi-phase layer do not have macroscopic phase separation as shown in a conventional multi-phase layer (e.g., where one sub-layer is laminated onto another sub-layer), but instead contain an interface in which microscopic phase transition occurs depending on the fibers used or the forming process (e.g., how much vacuum is applied). 
     In some embodiments, a polymeric material can be impregnated into a multi-phase layer either during or after the multi-phase layer is being manufactured on a papermaking machine. For example, during the manufacturing process described above, after the article containing first and second phases is formed and dried, a polymeric material in a water based emulsion or an organic solvent based solution can be adhered to an application roll and then applied to the article under a controlled pressure by using a size press or gravure saturator. The amount of the polymeric material impregnated into the multi-phase layer typically depends on the viscosity, solids content, and absorption rate of the multi-phase layer. As another example, after the multi-phase layer is formed, it can be impregnated with a polymeric material by using a reverse roll applicator following the just-mentioned method and/or by using a dip and squeeze method (e.g., by dipping a dried multi-phase layer into a polymer emulsion or solution and then squeezing out the excess polymer by using a nip). A polymeric material can also be applied to the multi-phase layer by other methods known in the art, such as spraying or foaming. 
     II. Filter Media with a Nanofiber Layer 
     As described above, second layer  30  of filter media  10  may be a nanofiber layer  30 . For example, the nanofiber layer  30  may be adhered to the first layer  20 . In aspects described herein, the first layer  20  may be a single-phase or a multi-phase layer. In some embodiments, a nanofiber layer may be formed from a meltblown process. Suitable meltblown layers have been described in U.S. Patent Publication No. US2009/0120048 which is incorporated herein by reference in its entirety and is based on U.S. patent application Ser. No. 12/266,892, entitled “Meltblown Filter Medium,” filed on Nov. 7, 2008. 
     In other embodiments, the nanofiber layer may be formed via other suitable processes such as meltspun, melt electrospinning and/or liquid electrospinning processes. 
     A. Nanofiber Layer 
     In some embodiments, the nanofiber layer  30  may be formed of fibers having an average diameter of at most 1.5 microns (e.g., from 0.1 microns to 1.5 microns, from 0.2 microns to 1.5 microns, from 0.3 microns to 1.5 microns, from 0.3 microns to 1.4 microns, from 0.4 microns to 1.3 microns, from 0.1 microns to 0.5 microns, from 0.1 micron to 0.2 microns, from 0.1 microns to 0.3 microns, from 0.5 microns to 1.5 microns, from 0.5 microns to 1.0 micron, from 0.2 microns to 0.8 microns, from 0.2 microns to 0.5 microns, from 0.3 microns to 0.5 microns, from 0.4 microns to 0.5 microns, from 0.4 microns to 0.6 microns, from 0.4 microns to 0.8 microns, from 0.6 microns to 0.9 microns, from 0.2 microns to 0.4 microns, from 0.2 microns to 0.3 microns, from 0.3 microns to 0.4 microns). 
     In some embodiments, a nanofiber layer manufactured from a meltblown process may be formed of fibers having an average diameter of at most 1.5 microns (e.g., at most 1.4 microns, at most 1.3 microns, at most 1.2 microns, at most 1.1 microns, at most one micron), and/or at least 0.2 micron (e.g., at least 0.3 micron, at least 0.4 micron, at least 0.5 micron), as measured using scanning electron microscopy. As an example, in some embodiments, the meltblown layer is formed of fibers having an average diameter of from 0.2 micron to 1.5 microns (e.g., from 0.3 microns to 1.5 microns, from 0.3 microns to 1.4 microns, from 0.4 micron to 1.3 microns). As another example, in certain embodiments, the meltblown layer is formed of fibers having an average diameter of from 0.2 microns to 1.5 microns (e.g., from 0.5 microns to 1.5 microns, from 0.5 microns to 1.0 micron, from 0.2 microns to 0.8 microns, from 0.2 microns to 0.5 microns, from 0.3 microns to 0.5 microns, from 0.4 microns to 0.5 microns, from 0.4 microns to 0.6 microns, from 0.4 microns to 0.8 microns, from 0.2 microns to 0.4 microns, from 0.2 microns to 0.3 microns, from 0.3 microns to 0.4 microns). In some cases, at least 5% (e.g., at least 10%, at least 25%, at least 50%, at least 60%, at least 75%) of the fibers in the meltblown material extend a distance of at least 0.3 microns in a direction substantially perpendicular to a surface of a second layer. 
     The meltblown layer may be formed of fibers having an average length. For example, in some embodiments, the meltblown layer may be formed of fibers having an average length from about 0.1 inches to about 1,000 inches or between about 1 inch and about 100 inches. 
     The fibers in the meltblown layer may have an average aspect ratio. For example, in some embodiments, fibers in a meltblown layer may have an average aspect ratio between about 5 and about 1,000,000 or between about 10 and about 100,000. 
     The meltblown material may be formed of one or more polymers (e.g., thermoplastic polymers). Exemplary polymers include polyolefins (e.g., polypropylenes), polyesters (e.g., polybutylene terephthalate, polybutylene naphthalate), polyamides (e.g., nylons), polycarbonates, polyphenylene sulfides, polystyrenes, polybutylene terephthalate, and polyurethanes (e.g., thermoplastic polyurethanes). Optionally, the polymer(s) may contain fluorine atoms. Examples of such polymers include PVDF and PTFE. 
     The meltblown layer can generally have any suitable thickness. In some embodiments, the meltblown layer is at least 5 microns (e.g. at least 10 microns, at least 20 microns, at least 30 microns) thick, and/or at most 500 microns (e.g., at most 400 microns, at most 200 microns, at most 150 microns) thick. For example, the meltblown layer can be from 5 microns to 500 microns (e.g., from 5 microns to 250 microns, from 10 microns to 200 microns, from 20 microns to 150 microns, from 30 microns to 500 microns, from 50 microns to 100 microns) thick. The thickness of the meltblown layer is determined using scanning electron microscopy in cross-section view. 
     The basis weight of the meltblown layer can typically be selected as desired. In some embodiments, the basis weight of the meltblown layer is at least 1 g/m 2  (e.g., at least 10 g/m 2 , at least 25 g/m 2 ), and/or at most 100 g/m 2  (at most 90 g/m 2 , at most 75 g/m 2 ). For example, in certain embodiments, the meltblown layer has a basis weight of from 1 g/m 2  to 100 g/m 2  (e.g., from 10 g/m 2  to 90 g/m 2 , from 25 g/m 2  to 75 g/m 2 , from 2 g/m 2  to 20 g/m 2 , from 4 g/m 2  to 12 g/m 2 ). 
     The average pore size of the meltblown layer can vary as desired. For example, the meltblown layer can have an average pore size that may range between about 5 microns and about 50 microns, between about 10 microns and about 30 microns or between about 10 microns and about 20 microns. As used herein, the average pore size refers to the mean flow pore size measured by using a Coulter Porometer as described in ASTM F316-03. 
     The air permeability of the meltblown layer can also be varied as desired. In some embodiments, the meltblown layer has an air permeability of at most 500 CFM (e.g., at most 250 CFM, at most 200 CFM), and or at least 20 CFM (e.g., at least 50 CFM, at least 100 CFM). For example, in some embodiments, the air permability of the meltblown layer can be from 20 CFM to 500 CFM (e.g., from 50 CFM to 250 CFM, from 100 CFM to 200 CFM). In some embodiments, the air permeability of a meltblown layer may be appropriately tailored to meet composite properties based on the permeability of the first layer (e.g., single-phase or multi-phase). 
     B. Intermediate Layer
         1. Adhesive       

     In some embodiments, the intermediate layer  40  includes an adhesive (e.g., a hot melt adhesive, a pressure sensitive adhesive, a thermoplastic adhesive, a thermoset adhesive) that is adhered to the first layer and the nanofiber (e.g., meltblown) layer. Generally, the adhesive is a polymer. Examples of polymers include ethylene vinyl acetate copolymers, polyolefins (e.g., polyethylenes, polypropylenes, amorphous polyolefin), polyamides (e.g., nylons), epoxies, cyanoacrylates, polyurethanes (e.g., moisture cured polyurethanes) and polyesters. In some embodiments, the adhesive is an ethylene vinyl acetate copolymer. Examples of commercially available materials include amorphous polyolefin adhesives available from Bostik (Wauwatosa, Wis.) under tradenames HM 4379, M2751 and H3199, and from Heartland (Germantown, Wis.) under tradename H312. Examples of commercially available materials also include copolyesters available from Bostik (Wauwatosa, Wis.) under tradenames HM4199, HM4156 and Vitel 4361B. Examples of commercially available materials further include polyamides available from Bostik (Wauwatosa, Wis.) under tradenames HM 4289LV and HM4229. 
     In some embodiments, the intermediate layer is formed of a web of fibers less than 4 micron in diameter with a geometric standard deviation of 1.4. 
     The thickness of an adhesive layer can generally be selected as desired. In some embodiments, the thickness of an adhesive layer is at least five microns (e.g., at least 10 microns, at least 25 microns), and/or at most 100 microns (e.g., at most 75 microns, at most 50 microns). For example, the thickness of an adhesive layer can be from five microns to 100 microns (e.g., from five microns to 75 microns, from five microns to 50 microns) as determined by scanning electron microscopy. 
     In general, the basis weight of an adhesive layer can be selected as desired. In some embodiments, an adhesive layer has a basis weight of at most 20 g/m 2  (at most 10 g/m 2 , at most 8 g/m 2 , at most 5 g/m 2 ), and/or at least 0.5 g/m 2  (e.g., at least 1 g/m 2 , at least 2 g/m 2 ). For example, in some embodiments, an adhesive layer can have a basis weight of from 1 g/m 2  to 20 g/m 2  (e.g., 3 g/m 2  to 10 g/m 2 , 0.5 g/m 2  to 10 g/m 2 , from 1 g/m 2  to 8 g/m 2 , from 2 g/m 2  to 5 g/m 2 ). 
     Although in many cases, an adhesive layer is continuous, in some embodiments, an adhesive can be discontinuous. For example, an adhesive may be in the form a material with holes in it (e.g., in the form of a mesh). Additionally or alternatively, an adhesive could be in the form of patches (e.g., dots) of material. Typically, an amount of adhesive between a first layer and a nanofiber (e.g., meltblown) layer is sufficient to provide appropriate adhesion between the layers when considering the intended use of the filter media. For example, in some embodiments, an adhesive is present in at least 70% (e.g., at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 99%, 100%) of the area between a first layer and a nanofiber (e.g., meltblown) layer. 
     In general, an adhesive is selected so that the mean peel strength between a first layer and a nanofiber (e.g., meltblown) layer is at least 0.5 ounces per inch of width (e.g., at least one ounce per inch of width, or at least 1.5 ounces per inch of width). In some embodiments, an adhesive is selected so that the mean peel strength between a first layer and a nanofiber (e.g., meltblown) layer is at most four ounces per inch of width. As used herein, the “mean peel strength” of a first layer/adhesive/second layer configuration is determined as follows. The test is a modified version of ASTM D903, using a Thwing-Albert Intellect II tensile tester. Samples are cut to two inch by seven inch strips, and the peeling is done in the machine direction. TUFFLEX (TF4150 85447) tape from Intertape Inc. (Montreal, Quebec, Canada) is applied to the length of the coated surface of the specimen to firmly bond to the top layer to be able to separate the laminate. A half inch prepeel is used to start the delamination. The tester cross head and top air grip moves at a speed of 12 inches per minute from the stationary bottom air grip. The test is complete when the cross-head and the top air grip moves four inches from the initial position. The maximum peel strength and minimum peel strength are recorded as a function of the load measured by the load cell. The mean peel strength is calculated from the loads measured by the load cell during the entirety of the test. All peel strengths are divided in half to report a peel strength per inch width by dividing by two. 
     Generally, an adhesive may be selected to have an appropriate open time for the manufacturing process below. For example, the open time of an adhesive should be sufficient so that it does not become non-adhesive between the time it is applied to one layer (e.g., a first layer or a meltblown layer) and the time the adhesive contacts the other layer (e.g., a first layer or a meltblown layer). In some embodiments, an adhesive has an open time of at least 15 seconds (e.g., at least 20 seconds, at least 30 seconds at least 40 seconds). In certain embodiments, an adhesive layer has an open time of at most 60 seconds. As used herein, the “open time” of an adhesive is determined according to ASTM D4497 using a 1/16 inch wide bead of adhesive. 
     2. Scrim 
     In some embodiments, a scrim is included in the filter media. In some cases, a scrim is located above a nanofiber layer  30 . In some embodiments, a filter media includes a first layer  20 , an intermediate layer  40 , a nanofiber layer  30  and a scrim disposed over the nanofiber layer  30 . 
     A scrim can be, for example, formed of a polymer. Examples of polymers include polyesters, polyamides and polyolefins. Optionally, a scrim is formed of a spunbond nonwoven material or a carded nonwoven material. In some embodiments, a scrim is formed of a spunbond polypropylene. 
     Generally, the thickness of a scrim can be selected as desired. In certain embodiments, a scrim is at least 50 microns (e.g., at least 100 microns, at least 200 microns) thick, and/or at most 1000 microns (e.g., 900 microns, 750 microns) thick. For example, the thickness of a scrim can be from 50 microns to 1000 microns (e.g. from 100 microns to 900 microns, from 250 microns to 750 microns) thick. As referred to herein, the thickness of a scrim is determined according to TAPPI T411. 
     In general, the basis weight of a scrim can be selected as desired. In some embodiments, a scrim has a basis weight of at most 100 g/m 2  (at most 90 g/m 2 , at most 75 g/m 2 ), and/or at least five g/m 2  (e.g., at least 10 g/m 2 , at least 20 g/m 2 ). For example, in some embodiments, a scrim can have a basis weight of from five g/m 2  to 100 g/m 2  (e.g., from five g/m 2  to 90 g/m 2 , from five g/m 2  to 75 g/m 2 ). 
     III. Filter Media with Single-Phase or Multi-Phase and Nanofiber Layers 
     A. Performance Characteristics 
     Filter media having a first layer (e.g., single phase or multi-phase layer) and a nanofiber layer adhered thereon may exhibit advantageous properties including increased dust holding characteristics such as capture efficiency and dust holding capacity. 
     According to the Palas flat sheet test, in some embodiments, the dust holding capacity of the filter media having a nanofiber layer adhered to a first layer (e.g., single-phase or multi-phase) may be greater than about 10 g/m 2 , greater than about 20 g/m 2 , greater than about 50 g/m 2 , greater than about 200 g/m 2 , greater than about 300 g/m 2 , greater than about 350 g/m 2 , greater than about 400 g/m 2 , or greater than about 450 g/m 2 . In some embodiments, filter media subjected to the Palas flat sheet test may exhibit a dust holding capacity that is at most about 500 g/m 2 . 
     It should be understood that the filter media described herein may exhibit any of the above-noted dust holding capacities in combination with the following characteristics. 
     The initial dust particle capture efficiency may be characterized using the Palas flat sheet test described above. In some embodiments, the initial dust particle capture efficiency of filter media having a nanofiber layer adhered to a first layer (e.g., single-phase or multi-phase) may be greater than about 25%, greater than about 40%, greater than about 70%, greater than about 80%, greater than about 90% or greater than about 99%. In some embodiments, the initial dust particle capture efficiency of such filter media may be at most 99.5%. 
     The initial pressure drop may also be characterized using the Palas flat sheet test. In some embodiments, the initial pressure drop of filter media having a nanofiber layer adhered to a first layer may be greater than about 10 Pa, greater than about 15 Pa, greater than about 20 Pa or greater than about 30 Pa. In some embodiments, the initial pressure drop of such filter media may be at most about 300 Pa or at most about 500 Pa. 
     According to the Palas flat sheet test, a certain period of time will elapse until the pressure drop reaches 1,800 Pa. In some embodiments, the period of time for filter media having a first layer and a nanofiber layer may be at least about 60 minutes, at least about 70 minutes, at least about 80 minutes (e.g., about 83 minutes). 
     Filter media having a nanofiber layer adhered on to a first layer may give rise to certain values of air permeability as well as mechanical performance. For example, the air permeability of such filter media using the above described permeability test may be greater than about 1 CFM, greater than about 2 CFM, greater than about 10 CFM, greater than about 30 CFM, greater than about 50 CFM, greater than about 75 CFM or greater than about 100 CFM. In some embodiments, the filter media may exhibit air permeability of at most about 120 CFM, at most about 200 CFM or at most about 250 CFM. 
     In some embodiments, a filter media having a first layer and a nanofiber layer may exhibit mechanical strength based on a Mullen Burst test. Mullen Burst test values for such a filter media may be at most about 80 kPa, at most about 90 kPa or at most about 100 kPa according to DIN 53113. In some embodiments, Mullen Burst test values according to the above standard for filter media described may be greater than about 10 kPa, greater than about 15 kPa or greater than about 20 kPa. 
     B. Structural Characteristics 
     Various structural properties may be measured of filter media with a nanofiber layer adhered on to a first layer (e.g., single phase or multi-phase layer). 
     Filter media with a first layer and a nanofiber layer may have any suitable basis weight. For example, filter media having a nanofiber layer adhered on to a first layer can have a basis weight ranging between about 10 g/m 2  and about 500 g/m 2 , between about 25 g/m 2  and about 230 g/m 2 , between about 30 g/m 2  and about 250 g/m 2 , between about 50 g/m 2  and about 200 g/m 2 , between about 80 g/m 2  and about 150 g/m 2 , or between about 90 g/m 2  and about 160 g/m 2 . 
     The thickness of a filter media having a first layer and a nanofiber layer may vary as desired. For example, filter media with a nanofiber layer adhered on to a first layer can have a thickness, according to TAPPI T411, ranging between about 100 microns and 2 mm, between about 200 microns and 1.6 mm, between about 300 microns and about 2.0 mm, between about 400 microns and 1.2 mm, between about 500 microns and about 1.7 mm or between about 700 microns and about 1.5 mm. 
     The average pore size of a filter media having a first layer and a nanofiber layer may vary as desired. For example, filter media with a nanofiber layer adhered on to a first layer can have an average pore size ranging between about 5 microns and about 50 microns, between about 10 microns and about 30 microns or between about 10 microns and about 20 microns. 
     C. Methods of Manufacturing 
     A nanofiber layer may be manufactured and adhered on to a single-phase or multi-phase layer in any appropriate manner. In some embodiments, a nanofiber layer may be positioned downstream with respect to a single-phase or multi-phase layer or vice versa. 
     As discussed above, for some embodiments, a nanofiber layer may be produced from a meltblown process. For example, meltblown processes and manufacturing methods described in U.S. Patent Publication No. 2009/0120048, entitled “Meltblown Filter Medium,” may be used.  FIG. 5  illustrates an embodiment of a system  200  that can be used to form a filter medium having an adhesive layer. System  200  includes a first roll-to-roll system  210  and a second roll-to-roll system  220 . 
     System  210  includes rollers  212   a ,  212   b ,  212   c  and  212   d  that move a continuous belt  214  as the rollers rotate. System  212  also includes an extruder  216 . As rollers  212   a - 212   d  are rotating, the polymer(s) (e.g., optionally with one or more additives) are vacuum drawn into the extruder  216 , and the polymer is heated (e.g., slowly heated) from the beginning of the extruder to the end, allowing the polymer(s) to flow more easily. The heated polymer(s) is(are) fed into a melt pump which controls the throughput (lb/hr) of the polymer(s). The polymer(s) then goes through a die tip with a series of holes. In some embodiments, the throughput of polymer per hole may have a relatively strong effect on fiber diameter. Heated, high velocity air impinges the polymer on either side of the die tip as the polymer comes out of the die tip. 
     The size of the holes and number of holes per inch for a die can generally be selected as desired. In some embodiments, the die can have 35 holes per inch with 0.0125″ holes. In certain embodiments, the die can have 70 holes per inch with 0.007″ holes. Other dies can optionally be used. 
     Meltblown material that is to be formed into a nanofiber layer may be softened (e.g., melted) by an increased temperature. As an example, in some embodiments, the material is heated to a temperature of at least 350° F. (e.g., at least 375° F., at least 400° F.), and or at most 600° F. (e.g., at most 550° F., at most 500° F.). For example, the material can be heated to a temperature of from 350° F. to 600° F. (e.g., from 375° F. to 550° F., from 400° F. to 500° F.). 
     In one aspect, process air is heated on either side of a die tip where fibers are formed. This heated air (typically the same temperature as the die tip) impinges the fibers and helps attenuate the fibers to the final fiber size. In some embodiments, as the air volume increases, the fiber diameter can decrease. The process air volume can be selected as appropriate. In some embodiments, the process air volume is at least 2500 pounds/hour-meter (e.g., at least 2750 pounds/hour-meter, at least 3000 pounds/hour-meter), and/or at most 4000 pounds/hour-meter (e.g., at most 3750 pounds/hour-meter, at most 3500 pounds/hour-meter). For example, the process air volume can be from 2500 pounds/hour-meter to 4000 pounds/hour-meter (e.g., from 2750 pounds/hour-meter to 3750 pounds/hour-meter, from 3000 pounds/hour-meter to 3500 pounds/hour-meter). 
     The vacuum created by vacuum  218  can be selected as appropriate. In some embodiments, the vacuum is at least 10 inches of water (e.g., at least 12 inches of water, at least 14 inches of water), and/or at most 26 inches of water (e.g., at most 23 inches of water, at most 20 inches of water). For example, the vacuum can be from 10 inches of water to 26 inches of water (e.g., from 12 inches of water to 23 inches of water, from 14 inches of water to 20 inches of water). 
     System  220  includes rollers  222   a ,  222   b ,  222   c  and  222   d  that move a first layer (e.g., single-phase or multi-phase) as the rollers rotate. Between rollers  222   a  and  222   b , system  220  includes a station  226  that applies an adhesive to the first layer. In a region adjacent rollers  222   b  and  212   a , the adhesive contacts the meltblown layer, and the meltblown layer is removed from belt  214  while adhered to the adhesive to form a composite filter media. In some cases, the first layer/adhesive/meltblown layer composite then passes through a charging unit  228 . 
     It can be appreciated that a nanofiber layer formed from a process other than a meltblown process can be positioned on a belt  214  that is moved by rollers  212   a - 212   d . In such a case when a meltblown process is not used to form a nanofiber layer, an extruder  216  is not necessary. Accordingly, once adhesive is applied to the first layer by station  226 , in a region adjacent rollers  222   b  and  212   a , the adhesive contacts the nanofiber layer, and the nanofiber layer is removed from belt  214  while adhered to the adhesive to form a composite filter media. Similar to that described above, a first layer/adhesive/nanofiber layer composite may then pass through a charging unit  228 . 
     Station  226  can generally be selected as desired. In some embodiments (e.g., when it is desirable to have a relatively high coverage of adhesive), station  226  can be a metered adhesive system. The metered adhesive system can be configured to apply a relatively highly dispersed and uniform amount of adhesive. In certain embodiments, station  226  is an adhesion applicator system having 12 nozzles arranged per inch that provides dispersed adhesion lanes with a two millimeter gap between center points of the lanes and with each nozzle having a 0.06 inch diameter orifice. 
     The belt  214  used to carry the nanofiber layer can be made of any material that allows the formation of a nanofiber layer on the belt, and also allows for the removal of the nanofiber layer from the belt when the nanofiber layer contacts the adhesive layer. Examples of materials from which a belt can be made include polymers (e.g., polyesters, polyamides), metals and/or alloys (e.g., stainless steel, aluminum). 
     The speed at which a belt  214  moves can be selected as desired to form a nanofiber layer. In some embodiments, the belt  214  may move at a speed of least 10 ft/min (e.g., at least 20 ft/min, at least 30 ft/min), and/or at most 300 ft/min (e.g., at most 200 ft/min, at most 100 ft/min). For example, the belt  214  can move at a speed of from 10 ft/min to 300 ft/min (e.g., from 20 ft/min to 200 ft/min, from 30 ft/min to 100 ft/min). 
     In general, any belt configuration can be used. For example, in some embodiments, the belt has an open structure, such as a mesh structure. In some cases, an open structure may result in a nanofiber material having a complementary structure to that of the belt from the nanofiber material being under the force of the blown air. 
     As discussed above for other processes to form a nanofiber layer from meltblown material, the temperature may be selected to properly soften (e.g., melt) the material that is to be formed into the nanofiber layer. Similarly, process air volume, vacuum strength, and the speed at which the belt moves can be selected as appropriate. 
     In certain embodiments where an adhesive layer is used, the manufacturing method may involve applying the adhesive layer to a first layer, and the adhesive layer is subsequently joined to the nanofiber layer formed on the collector belt as both layers are passed through a nip roll. Thus, in the filter media, the first layer and the nanofiber layer are both adhered to the adhesive layer. As discussed above, the first layer may be a single-phase or a multi-phase (e.g., dual-phase) layer. 
     In some embodiments, manufacture of a filter media with an adhesive layer involves a continuous (e.g., roll-to-roll) process. The process can, for example, involve the use of multiple roll-to-roll systems. As an example, one roll-to-roll system can be used to form a nanofiber layer, and another roll-to-roll system can be used to adhere an adhesive layer to the first layer. In such a system, the roll-to-roll systems can be configured so that, in a continuous fashion, the adhesive layer contacts the nanofiber layer as these two layers become adhered to each other. 
     A roll-to-roll system may include rollers that move the first layer as the rollers rotate. The system may include a station between rollers that applies an adhesive to the first layer. In a region adjacent to rollers, the adhesive contacts the nanofiber layer, and the nanofiber layer is removed from a belt and is adhered to the adhesive as it passes through a nip roll or similar device. 
     The first layer may be fed through the adhesive station by a pulling force generated via a nip formed at rollers  212   a  and  222   b . By contacting the first layer adjacent a roller  222   b  with a nanofiber layer adjacent roller  212   a , the speed of belt  214  and the first layer are synchronized (e.g., so that the first layer moves at approximately the same speed as the belt). The pressure between rollers  212   a  and  222   b  is generally selected as desired for the intended use of the filter media. For example, in embodiments in which the filter media is corrugated, the pressure between rollers  212   a  and  222   b  may be selected to achieve good corrugation depth and conformity. In some embodiments, the pressure between rollers  212   a  and  222   b  is from 20 pounds per linear inch to 40 pounds per linear inch (e.g., from 25 pounds per linear inch to 35 pounds per linear inch, from 28 pounds per linear inch to 32 pounds per linear inch, from 29 pounds per linear inch to 31 pounds per linear inch, 30 pounds per linear inch). 
     When applied to the first layer, the temperature of the adhesive can be selected so that it has an appropriate level of tack when it comes into contact with the nanofiber layer. In embodiments in which the adhesive is a hot melt adhesive, this can involve heating the adhesive prior to its application to the first layer. For example, prior to being applied to the first layer, the adhesive can be heated to a temperature of at least 350° F. (e.g., at least 370° F., at least 380° F.), and or at most 450° F. (e.g., at most 430° F., at most 420° F.). For example, the material can be heated to a temperature of from 350° F. to 450° F. (e.g., from 370° F. to 430° F., from 380° F. to 420° F.). 
     In some embodiments, once formed, a first layer/adhesive/nanofiber layer composite passes through a charging unit. The charging unit is used to charge the composite (in general, particularly the nanofiber layer) which may result in a filter media having enhanced fine particle capture properties. 
     In general, any of a variety of techniques can be used to charge the first layer/adhesive/nanofiber layer composite to form an electret web. Examples include AC and/or DC corona discharge and friction-based charging techniques. In some embodiments, the composite is subjected to a discharge of at least 1 kV/cm (e.g., at least 5 kV/cm, at least 10 kV/cm), and/or at most 30 kV/cm (e.g., at most 25 kV/cm, at most 20 kV/cm). For example, in certain embodiments, the composite can be subjected to a discharge of from 1 kV/cm to 30 kV/cm (e.g., from 5 kV/cm to 25 kV/cm, from 10 kV/cm to 20 kV/cm). 
     In some embodiments, similar to that described above for the adhesive, manufacture of a filter media with a scrim layer may involve a continuous (e.g., roll-to-roll) process. The process can, for example, involve the use of multiple roll-to-roll systems. As an example, one roll-to-roll system can be used to form a nanofiber layer on to a scrim, and another roll-to-roll system can be used to carry the first layer. 
     In such a system, the roll-to-roll systems can be configured so that, in a continuous fashion, the nanofiber layer/scrim composite contacts the first layer to form a three layer composite (or four layer including an adhesive), and the three layers are subsequently bonded together. 
     In an embodiment, a system may include rollers that move a continuous belt and as the rollers rotate, a scrim is applied to the belt. Material from which a nanofiber layer is to be formed (e.g., a polymer in pellet form) may be introduced through a heated extruder while rollers rotate. The material is softened (e.g., melted) and forced through a die in the form of filaments. The filaments are moved toward the scrim under the influence of a vacuum, for example, on the opposite side of the belt relative to the die. In some embodiments, the vacuum stretches the filaments and forces them against the surface of the scrim to provide a nanofiber layer disposed on the scrim. 
     The scrim/nanofiber layer composite is removed from the belt, and the first layer is disposed on the scrim, typically by adhesive disposed (e.g., sprayed) on to the first layer. In some cases, where nanofibers are formed from a meltblown process, this involves bringing the scrim onto a belt and then blowing meltblown fibers directly onto the scrim. The scrim can have an adhesive applied before the meltblown material is blown on and/or the force and the heat of the meltblown fibers can be used to adhere the two layers together. The three layers are then bonded together. During this process, the three layers can optionally be laminated together. In some embodiments, the layers are ultrasonically bonded together (e.g., ultrasonically point bonded together). In some embodiments, a nanofiber layer, a scrim and the first layer can be joined by applying ultrasonic energy between an aluminum vibrating horn and an engraved contact roll. 
     D. Corrugation 
     In some embodiments, filter media can be corrugated. Optionally, corrugated filter media can also be pleated. Any suitable method may be used to corrugate and/or pleat filter media described. 
       FIG. 6  shows a filter media  10  including a first layer  20 , an intermediate layer  40  (e.g., adhesive) and a nanofiber layer  30 . In some embodiments, first layer  20  includes a single phase or multiple phases. Filter media  10  has a repeat corrugation pattern with a corrugation channel width depicted by a distance “c”, which is the distance from one peak to its nearest neighboring peak in the repeat corrugation pattern. In general, filter media  10  can have any desired corrugation channel width. In some embodiments, corrugation channel width “c” is at least 150 mils (e.g., at least 160 mils, from 167 mils to 173 mils, at least 225 mils, at least 250 mils, from 247 mils to 253 mils, from 150 mils to 335 mils). 
     In some embodiments, filter media  10  has a corrugation depth that is depicted by a distance “d 1 ”, which is the distance from a peak of layer  30  to a valley of layer  30  in the repeat corrugation pattern. In some embodiments, corrugation depth “d 1 ” is at least 8 mils (e.g., at least 10 mils, at least 12 mils, at least 14 mils, at least 16 mils), and/or at most 25 mils (e.g., at most 20 mils). 
     In certain embodiments, filter media  10  has a corrugation depth that is depicted by a distance “d 2 ”, which is the distance from a peak of first layer  20  to a valley of first layer  20  in the repeat corrugation pattern. In some embodiments, corrugation depth “d 2 ” is at least 8 mils (e.g., at least 10 mils, at least 12 mils, at least 14 mils, at least 16 mils), and/or at most 25 mils (e.g., at most 20 mils). 
     In some embodiments, filter media  10  has a retained corrugation of at least 25% (e.g., at least 30%, at least 40%, at least 50%, at least 60%, at least 70%). As referred to herein, the “retained corrugation” of filter media  10  is determined by dividing the corrugation depth “d 1 ” by the distance from a peak of a side of first layer  20  to a valley of a side of first layer  20  (measured before layer  40  is applied to first layer  20 ) in the repeat corrugation pattern, and multiplying this value by 100%. Without wishing to be bound by theory, it is believed that the retained corrugation may result from the processes disclosed herein in which first layer  20  is formed on a separate wire from layer  30 , and these layers are subsequently adhered to each other. In some instances, selecting appropriate pressure can enhance the retained corrugation, if the pressure selected is high enough to achieve desired adhesion while being low enough to achieve advantageous retained corrugation properties. 
     In embodiments where filter media  10  is corrugated and a roll-to-roll system is employed, as described above, to adhere a first layer  20  and a nanofiber layer  30  (e.g., formed by a meltblown process) to each other, the pressure between the rollers may be selected to achieve advantageous corrugation depth and conformity for filter media  10 . In some embodiments, the pressure between the rollers may range between about 20 pounds per linear inch and about 40 pounds per linear inch (e.g., between about 25 pounds per linear inch and about 35 pounds per linear inch, between about 28 pounds per linear inch and about 32 pounds per linear inch, between about 29 pounds per linear inch and about 31 pounds per linear inch, and about 30 pounds per linear inch). 
     IV. Filter Assemblies and Systems 
     Filter assemblies for filtration applications can include any of a variety of filter media and/or filter elements. The filter elements can include the above-described filter media. Examples of filter elements include gas turbine filter elements, dust collector elements, heavy duty air filter elements, automotive air filter elements, air filter elements for large displacement gasoline engines (e.g., SUVs, pickup trucks, trucks), HVAC air filter elements, HEPA filter elements, vacuum bag filter elements, fuel filter elements, and oil filter elements (e.g., lube oil filter elements or heavy duty lube oil filter elements). 
     Filter elements can be incorporated into corresponding filter systems (gas turbine filter systems, heavy duty air filter systems, automotive air filter systems, HVAC air filter systems, HEPA filter systems, vacuum bag filter systems, fuel filter systems, and oil filter systems). Vacuum filter bag systems are commonly used in home vacuum cleaners. In such embodiments, a filter media can optionally be prepared by coating a paper with meltblown material. In certain embodiments, the filter media can be prepared using a wet laid or dry laid product (e.g., wood, polymer, glass). Dry laid products may be used, for example, in HVAC, HEPA, auto air and cabin air applications. The filter media can optionally be pleated into any of a variety of configurations (e.g., panel, cylindrical). 
     Filter elements can also be in any suitable form, such as radial filter elements, panel filter elements, or channel flow elements. A radial filter element can include pleated filter media that are constrained within two open wire meshes in a cylindrical shape. During use, fluids can flow from the outside through the pleated media to the inside of the radial element. 
     When a filter element is a heavy duty air filter element, each of the open and tight phases in a single filter media can, by way of example, include a mixture of softwood fibers (e.g., Robur Flash fibers) and hardwood fibers (e.g., Suzano fibers). The weight ratio of the open and tight phases can be about 30:70 or greater. The weight ratio of the softwood fibers and hardwood fibers in the open phase can be, for example, about 85:15, and the weight ratio of the softwood fibers and hardwood fibers in the tight phase can be, for example, about 49:51. 
     When a filter element is an automotive air filter element, each of the open and tight phases in a single filter media can, by way of example, include a mixture of softwood fibers and hardwood fibers (e.g., Suzano fibers). The weight ratio of the open and tight phases can be about 50:50. The weight ratio of the softwood fibers and hardwood fibers in the open phase can be about 93:7, and the weight ratio of the softwood fibers and hardwood fibers in the tight phase can be about 65:35. Each of the open and tight phases can be made from two types of different softwood fibers (e.g., Robur Flash fibers and HPZ fibers). The weight ratio of the two types of different softwood fibers in the open phase can be about 83:10 (e.g., about 83% mercerized southern pine fibers and about 10% of Robur Flash fibers). The weight ratio of the two types of different softwood fibers in the tight phase can be about 40:25 (e.g., about 40% HPZ fibers and about 25% of Robur Flash fibers). 
     When a filter element is a fuel filter element, each of the open and tight phases in a single filter media can include a mixture of softwood fibers and hardwood fibers. The weight ratio of the open and tight phases can be about 50:50. The weight ratio of the softwood fibers and hardwood fibers in the open phase can be about 60:40, and the weight ratio of the softwood fibers and hardwood fibers in the tight phase can be about 6:94. The open phase can be made from two types of different softwood fibers with a weight ratio of about 40:20 (e.g., about 40% HPZ fibers and about 20% of Robur Flash), and a type of hardwood fibers (e.g., about 40% of Suzano fibers). The tight phase can be made of three types of different hardwood fibers with a weight ratio of about 48:36:10 (e.g., about 48% Suzano fibers, about 36% of Tarascon fibers, and about 10% Acacia fibers) and a type of softwood fibers (e.g., about 6% HPZ fibers). 
     The orientation of a filter media having first and second phases relative to gas flow through a filter element/filter system can generally be selected as desired. In some embodiments, the second phase is located upstream of the first phase in the direction of gas flow through a filter element. In certain embodiments, the second phase is located downstream of the first phase in the direction of gas flow through a filter element. As an example, when the gas filter element is a gas turbine filter element or a heavy duty air filter element, the second phase can be located upstream of the first phase in the direction of gas flow through the filter element. As another example, when improved depth filtration is desired, the second phase can be located downstream of the first phase in the direction of gas flow through the filter element. 
     The orientation of a filter media having a nanofiber layer attached to a first layer relative to gas flow through a filter assembly/filter system can generally be selected as desired. In some embodiments, the nanofiber layer is located upstream of the first layer in the direction of gas flow through the filter assembly/system. In certain embodiments, the nanofiber layer is located downstream of the first layer in the direction of gas flow through the filter assembly/system. As an example, in some embodiments in which the gas filter system is a gas turbine filter system or a heavy duty air filter system, the nanofiber layer can be located upstream of the first layer in the direction of gas flow through the filter assembly/system. As another example, in some embodiments in which improved depth filtration is desired, the nanofiber layer can be located downstream of the first layer in the direction of gas flow through the filter assembly/system. 
     V. Examples 
     The following examples are illustrative only and not intended as limiting. A single-phase layer and two dual-phase layers were produced from 20-30% polyester, Robur Flash fibers and HPZ XS fibers using methods described above for forming phases. Fiber sheets used to manufacture single-phase and dual-phase layers were saturated with resin binder such that the resin binder comprised 25% by weight of the sheet. The overall make up of the single-phase and dual-phase layers are provided in Table 1 and structural features of the layers can be found in Table 2 shown below. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Single-Phase and Dual-Phase Layer Fiber Formulations 
               
            
           
           
               
               
               
               
            
               
                   
                 Single-Phase 
                 Dual-Phase 
                 Dual-Phase 
               
               
                   
                 Layer 
                 Layer I 
                 Layer II 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Upstream Layer (%) 
                  100% 
                   
                   50% 
                   
                   50% 
                   
               
               
                 Upstream Layer 
                 125 
                   
                 62.5 
                   
                 62.5 
               
               
                 (g/m 2 ) 
               
               
                   
               
               
                   
                   
                 Mass 
                   
                 Mass 
                   
                 Mass 
               
               
                   
               
               
                 HPZ XS 19 
                 65.0% 
                 81 g 
                 70.0% 
                 44 g 
                 60.0% 
                 38 g 
               
               
                 Polyester 
                 20.0% 
                 25 g 
                 20.0% 
                 13 g 
                 20.0% 
                 13 g 
               
               
                 (6 mm, 3 denier) 10 
               
               
                 Polyester 
                  0.0% 
                  0 g 
                  0.0% 
                  0 g 
                 10.0% 
                  6 g 
               
               
                 6 mm, 1.5 denier) 
               
               
                 15 
               
               
                 Robur Flash 101 
                 15.0% 
                 19 g 
                 10.0% 
                  6 g 
                 10.0% 
                  6 g 
               
               
                 Upstream Layer 
                   
                 125 g  
                   
                 63 g 
                   
                 63 g 
               
               
                 Total 
               
               
                 Downstream 
                   
                   
                   50% 
                   
                   50% 
               
               
                 Layer (%) 
               
               
                 Downstream 
                   
                   
                 62.5 
                   
                 62.5 
               
               
                 Layer (g/m 2 ) 
               
               
                 HPZ XS 19 
                   
                   
                 60.0% 
                 38 g 
                 50.0% 
                 31 g 
               
               
                 Polyester 
                   
                   
                 20.0% 
                 13 g 
                 20.0% 
                 13 g 
               
               
                 (6 mm, 3 denier) 
               
               
                 10 
               
               
                 Polyester 
                   
                   
                  0.0% 
                  0 g 
                 10.0% 
                  6 g 
               
               
                 (6 mm, 1.5 denier) 
               
               
                 15 
               
               
                 Robur Flash 101 
                   
                   
                 20.0% 
                 13 g 
                 20.0% 
                 13 g 
               
               
                 Downstream 
                   
                   
                   
                 63 g 
                   
                 63 g 
               
               
                 Layer Total 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Structural Properties of Single-Phase and Dual-Phase Layer 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                 Caliper 
                   
                   
                   
               
               
                   
                 Basis 
                 Pressure 
                 Air permeability 
                 Pore size [μm] 
                 Burst Strength 
               
               
                   
                 weight 
                 1 N/cm 2   
                 At 200 Pa 
                 IPA 
                 Mullen 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Sample 
                 [g/m 2 ] 
                 [mm] 
                 [l/m 2 s] 
                 max. 
                 mean 
                 [kPa] 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Single-Phase 
                 1 
                 123.4 
                 0.82 
                 1512 
                 119.7 
                 112.3 
                 23 
               
               
                 Layer 
               
               
                   
                 2 
                 121.6 
                 0.82 
                 1568 
                 123.9 
                 113.8 
                 36 
               
               
                   
                 3 
                 123.6 
                 0.88 
                 1522 
                 122.8 
                 114.7 
                 30 
               
               
                   
                 4 
                 128.1 
                 0.89 
                 1498 
                 124.7 
                 116.4 
                 30.4 
               
               
                 Dual-Phase 
                 1 
                 123.1 
                 0.83 
                 1514 
                 122.1 
                 117.6 
                 27 
               
               
                 Layer 
               
               
                   
                 2 
                 121.2 
                 0.82 
                 1406 
                 123.3 
                 112.9 
                 23.4 
               
               
                   
                 3 
                 122.0 
                 0.84 
                 1464 
                 120.7 
                 114.5 
                 25 
               
               
                   
                 4 
                 125.4 
                 0.84 
                 1380 
                 &lt;124.7 
                 115.7 
                 27 
               
               
                 Dual-Phase 
                 1 
                 116.1 
                 0.83 
                 1606 
                 &gt;124.7 
                 121.3 
                 29 
               
               
                 Layer 
               
               
                   
                 2 
                 123.8 
                 0.85 
                 1514 
                 124.7 
                 116.4 
                 23.8 
               
               
                   
                 3 
                 122.6 
                 0.87 
                 1496 
                 124.7 
                 115.5 
                 27 
               
               
                   
                 4 
                 124.3 
                 0.91 
                 1494 
                 124.1 
                 117.5 
                 23.4 
               
               
                   
               
            
           
         
       
     
     A nanofiber layer (0.076 mm thick, Air Perm. 207 CFM) was adhered to each of the single-phase and dual-phase layers. The nanofiber layer was a layer of polybutylene terephthalate fiber produced using a meltblown process. The nanofiber layer was measured to have a basis weight of 8 g/m 2 , an air permeability of 207 CFM, a thickness prior to adhesion of 3 mils at 2.65 psi, and a mean flow pore size of 7 microns. The nanofiber layer was positioned downstream with respect to each of the single-phase and dual-phase layers. The nanofiber layer was subsequently adhered to each of the single-phase and dual-phase layers, using a roll-to-roll system as described earlier, with a copolyester adhesive. 
     Basis weight values of the nanofiber layer, the single-phase layer, and the dual-phase layer before and after adherence of the nanofiber layer were measured according to ASTM F778-88. Caliper values of the nanofiber layer, the single-phase layer, and the dual-phase layer before and after adherence of the nanofiber layer were measured under a pressure of 1 N/cm 2  according to TAPPI T411. Air permeability values of the nanofiber layer, the single-phase layer, and the dual-phase layer before and after adherence of the nanofiber layer were measured according to ISO 9237. Pore size values of the nanofiber layer, the single-phase layer, and the dual-phase layer before and after adherence of the nanofiber layer were measured by a method of correlating the pressure measured by a manometer from blowing air through a sample immersed in isopropyl alcohol with calibrated pore sizes. Mullen Burst strength values of the single-phase layer and the dual-phase layer before and after adherence of the nanofiber layer were measured according to DIN 53113. 
     The Palas flat sheet test was used to measure dust holding capacity, capture efficiency and pressure drop of the single-phase and dual-phase layer filter media before and after the nanofiber layer was added thereon. The test results can be found in  FIGS. 7, 8 and 9 .  FIG. 7  illustrates the change in dust particle capture efficiency of the media for the above described examples with time.  FIG. 8  depicts the change in pressure drop across the media of the above described examples with time.  FIG. 9  depicts the dust holding capacities of media for the above examples. 
     As illustrated in  FIG. 7 , the nanofiber layer was observed to provide an overall efficiency between 80% and 90% on the single-phase and dual-phase layers after adherence. The increased efficiency is considered to be largely affected by presence of the nanofiber layer. Prior to adherence of the nanofiber layer, the single-phase and dual-phase layer materials had efficiencies on the order of 45%. 
     Further, depicted in  FIG. 8 , filter media composites with a nanofiber layer adhered to a single-phase or dual-phase layer exhibited dust holding capacity performances that took longer than 100 minutes to reach a pressure drop of 1,800 Pa. For instance, filter media having a nanofiber layer adhered to a single-phase or dual-phase layer exhibited nearly double the run time of similar composites not having the nanofiber layer adhered thereon, indicating a significantly longer filter life upon addition of the nanofiber layer. 
       FIG. 9  depicts the dust holding capacity of the above described filter media. The single-phase layer exhibited a DHC of 280 g/m 2 . The single-phase layer with the nanofiber layer adhered thereon exhibited a DHC of 261 g/m 2 . Dual-phase layer I exhibited a DHC of 280 g/m 2 . Dual-phase layer I with the nanofiber layer adhered thereon exhibited a DHC of 267 g/m 2 . Dual-phase layer II exhibited a DHC of 312 g/m 2 . Dual-phase layer II with the nanofiber layer adhered thereon exhibited a DHC of 286 g/m 2 . 
     Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.