Patent Publication Number: US-8986432-B2

Title: Meltblown filter medium, related applications and uses

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 12/266,892, filed Nov. 7, 2008 and entitled “Meltblown Filter Medium”, which claims priority under 35 U.S.C. §119(e)(1) to U.S. Patent Application No. 60/986,642, filed Nov. 9, 2007 and entitled “Meltblown Filter Medium”, both of which are hereby incorporated by reference in their entireties. 
    
    
     FIELD 
     The disclosure generally relates to filter media, as well as related assemblies, systems and methods. 
     BACKGROUND 
     Filter media are used in a variety of systems. The media are typically used to remove undesirable materials (e.g., particles) from a liquid or gas by passing the liquid or gas through the media. 
     SUMMARY 
     The disclosure generally relates to filter media, as well as related assemblies, systems and methods. 
     In one aspect, filter elements are provided. In one embodiment, the filter element comprises an inlet, an outlet, and a filter medium positioned between the inlet and the outlet. The filter medium includes a first, substrate layer facing the inlet, the substrate layer having an air permeability of at least 70 CFM (e.g., at least 100 CFM, at least 125 CFM, at least 150 CFM). The filter medium may also include a second layer comprising a meltblown material positioned downstream of the first layer. 
     In another embodiment, a filter element comprises an inlet, an outlet, and a filter medium positioned between the inlet and the outlet. The filter medium includes a first, uppermost layer comprising a substrate and a second layer comprising a meltblown material positioned downstream of the first layer. The filter medium has a dust holding capacity of at least 70 g/m 2 . 
     In another embodiment, the filter element comprises an inlet, an outlet, and a filter medium positioned between the inlet and the outlet. The filter medium includes a first, substrate layer facing the inlet, the substrate layer having an air permeability of at least 70 CFM (e.g., at least 100 CFM, at least 125 CFM, at least 150 CFM). The filter medium may also include a second layer comprising a plurality of synthetic fibers having an average diameter of less than 1.5 microns. In some embodiments, the synthetic fibers may be meltblown fibers or electrospun fibers. 
     In another aspect, methods of filtering a fluid are provided. In one embodiment, a method comprises passing a fluid (e.g., a gas or a liquid) across a filter medium. The filter medium includes a first, uppermost layer comprising a substrate having an air permeability of at least 70 CFM (e.g., at least 100 CFM, at least 125 CFM, at least 150 CFM). The a second layer includes a meltblown material positioned downstream of the first layer. 
     In another embodiment, a method of filtering a fluid includes passing a fluid across a filter medium. The filter medium includes a first, uppermost layer comprising a substrate and a second layer comprising a meltblown material positioned downstream of the first layer. The filter medium has a dust holding capacity of at least 70 g/m 2 . 
     In the filter media described above, the first, substrate layer may have an air permeability of between 100 CFM and 300 CFM (e.g., between 160 CFM and 200 CFM). The first, substrate layer may have a basis weight of at least 150 g/m 2 . The thickness of the substrate may be at least 400 microns. In certain embodiments, the first, substrate layer comprises an organic polymer. 
     The second layer may comprise meltblown fibers having an average diameter of at most 1.5 microns (e.g., an average diameter of at most 0.8 micron or at most 0.6 micron). The thickness of the second layer may be from 30 microns to 100 microns thick. 
     Optionally, the filter medium includes a third layer comprising a scrim or an adhesive material. Additionally, the filter medium may be substantially free of an oil coating. 
     The filter medium may have a dust holding capacity of at least 150 g/m 2  (e.g., at least 200 g/m 2 ). Furthermore, the filter medium may have an air permeability of from 100 to 200 CFM. The filter medium may have an initial dust capture efficiency of at least 90% for 10 micron particles, and/or an initial dust capture efficiency of at least 95% for 20 micron particles. 
     The filter element may be an automotive filter (e.g., an automotive air filter). 
     In one aspect, the disclosure features an article that includes first, second and third layers. The second layer includes a meltblown material. The third layer includes an adhesive and is between the first and second layers. 
     In another aspect, the disclosure features an article that includes a first layer, a second layer and a third layer. Optionally, the third layer can be a scrim. The second layer includes a meltblown material. The scrim is between the first and second layers, or the second layer is between the first layer and the scrim. 
     In a further aspect, the disclosure features an article that includes first, second and third layers. The second layer includes a meltblown material, and the third layer is between the first and second layers. The article is a filter medium. 
     In an additional aspect, the disclosure features an assembly that includes a housing and a filter medium supported by the housing. The filter medium can be, for example, any of the articles described in the preceding three paragraphs. 
     In yet another aspect, the disclosure features a filter medium having an initial dust capture efficiency of at least 90%, and a dust holding capacity of at least 50 g/m 2 . 
     In still another aspect, the disclosure features a filter medium having a periodic dust capture efficiency of at least 90%, and a dust holding capacity of at least 50 g/m 2 . 
     In another aspect, the disclosure features a filter medium having an initial cleanability test time of at least four hours. 
     In a further aspect, the disclosure features a filter medium having a soot particle capture efficiency of at least 80%. 
     In an additional aspect, the disclosure features a filter medium having a NaCl particle filtration efficiency of at least 30% and a NaCl particle capture test time of at least 40 minutes. 
     In yet a further aspect, the disclosure features a filter medium having a liquid filtration efficiency of at least 45%. 
     In still another aspect, the disclosure features a method that includes forming any of the articles and/or filter media described in the preceding paragraphs of summary. 
     In another aspect, the disclosure features a method that includes adhering a meltblown material to an article comprising a substrate to provide a filter medium. 
     In an additional aspect, the disclosure features a method that includes supporting a meltblown material with a scrim to provide a first article, and bonding the first article and a substrate together to provide a filter medium. 
     In a further aspect, the disclosure features a filter medium that includes first and second layers. The second layer may be different from the first layer. The second layer includes a first meltblown material. The first layer can be, for example, a meltblown material or an electrospun material. In some embodiments, the first and second layers are supported by another layer. 
     In another aspect, the disclosure features a filter medium that includes first, second and third layers. The second layer includes a plurality of fibers, and the second layer has a thickness of at least five microns. The third layer includes a scrim or an adhesive material. 
     In yet another aspect, the disclosure features any of the articles and/or filter media described in the preceding paragraphs of the summary, with an aged cleanability test time that is at least 70% of the initial cleanability test time. 
     In still another aspect, the disclosure features a filter medium having aged cleanability test time that is at least 70% of the initial cleanability test time. 
     In an additional aspect, the disclosure features an article or filter medium as described in any of the preceding paragraphs of the summary, with a liquid filtration retention efficiency of at least 60%. 
     In a further aspect, the disclosure features a filter medium having a liquid filtration retention efficiency of at least 60%. 
     In an additional aspect, the disclosure features an article that includes a substrate and a meltblown material bonded to the substrate. 
     In another aspect, the disclosure features a meltblown layer having a plurality of crests and valleys, a distance between adjacent valleys being at least 400 microns. 
     In yet another aspect, the disclosure features an article that includes a first layer and a second layer bonded to the first layer, the second layer comprising fibers having an average diameter of at most 1.5 microns. 
     In one aspect, the disclosure feature an article that includes a first layer having first and second sides; a second layer comprising a meltblown material; and a material between the first side of the first layer and the second layer. The article has a corrugation channel width of at least 150 mils, a corrugation depth of at least 8 mils on the first side of the first layer, and a corrugation depth of at least 8 mils on the second side of the first layer. 
     In another aspect, the disclosure features an article that includes a first layer having first and second sides; a second layer comprising fibers; and a material between the first side of the first layer and the second layer. At least 5% of the fibers in the second layer extend a distance of at least 0.3 micron in a direction substantially perpendicular to a surface of the second layer. The article has a corrugation channel width of at least 150 mils, a corrugation depth of at least 8 mils on the first side of the first layer, and a corrugation depth of at least 8 mils on the second side of the first layer. 
     In a further aspect, the disclosure features an article that includes a first layer having first and second sides; a second layer comprising fibers having a fiber diameter geometric standard deviation of greater than 1.3; and a material between the first side of the first layer and the second layer. The article has a corrugation channel width of at least 150 mils, a corrugation depth of at least 8 mils on the first side of the first layer, and a corrugation depth of at least 8 mils on the second side of the first layer. 
     In an additional aspect, the disclosure features a first layer having first and second sides; a second layer comprising a meltblown material; and a material between the first side of the first layer and the second layer. The article is a corrugated article having a retained corrugation of at least 25%. 
     In one aspect, the disclosure features an article that includes a first layer having first and second sides; a second layer comprising fibers, at least 5% of the fibers in the second layer extending a distance of at least 0.3 micron in a direction substantially perpendicular to a surface of the second layer; and a material between the first side of the first layer and the second layer. The article is a corrugated article having a retained corrugation of at least 25%. 
     In another aspect, the disclosure features an article that includes a first layer having first and second sides; a second layer comprising fibers having a fiber diameter geometric standard deviation of greater than 1.3; and a material between the first side of the first layer and the second layer. The article is a corrugated article having a retained corrugation of at least 25%. 
     In a further aspect, the disclosure features a article that includes a first layer; a second layer comprising a meltblown material; and an adhesive material between the first and second layers. The adhesive material is present in at least 70% of the area between the first and second layers. 
     In an additional aspect, the disclosure features an article that includes a first layer; a second layer comprising fibers; and an adhesive material between the first and second layers. The adhesive material is present in at least 70% of the area between the first and second layers, and at least 5% of the fibers in the second layer extend a distance of at least 0.3 micron in a direction substantially perpendicular to a surface of the second layer. 
     In one aspect, the disclosure features an article that includes a first layer; a second layer comprising fibers; and an adhesive material between the first and second layers. The adhesive material is present in at least 70% of the area between the first and second layers, and the fibers in the second layer have a fiber diameter geometric standard deviation of greater than 1.3. 
     In another aspect, the disclosure features an article that includes a first layer; a second layer comprising a meltblown material; and an adhesive material between the first and second layers. The mean peel strength between the first and second layers is at least 0.5 ounce per inch. 
     In a further aspect, the disclosure features an article that includes a first layer; a second layer comprising fibers, at least 5% of the fibers in the second layer extending a distance of at least 0.3 micron in a direction substantially perpendicular to a surface of the second layer; and an adhesive material between the first and second layers. The mean peel strength between the first and second layers is at least 0.5 ounce per inch. 
     In an additional aspect, the disclosure features an article that includes a first layer; a second layer comprising fibers having a fiber diameter geometric standard deviation of greater than 1.3; and an adhesive material between the first and second layers. The mean peel strength between the first and second layers is at least 0.5 ounce per inch. 
     In one aspect, the disclosure features an article that includes a first layer; a second layer comprising a meltblown material; and an adhesive material between the first and second layers. The adhesive material has an open time of at least 15 seconds. 
     In another aspect, the disclosure features an article that includes a first layer; a second layer comprising fibers; and an adhesive material between the first and second layers. The adhesive material has an open time of at least 15 seconds, and at least 5% of the fibers in the second layer extend a distance of at least 0.3 micron in a direction substantially perpendicular to a surface of the second layer. 
     In a further aspect, the disclosure features an article that includes a first layer; a second layer comprising fibers; and an adhesive material between the first and second layers. The adhesive material has an open time of at least 15 seconds, and the fibers in the second layer have a geometric standard deviation of greater than 1.3. 
     In an additional aspect, the disclosure features an article that includes a first layer, and a second layer comprising a meltblown material. The article has a Beta decay of at most 20% at a particle size of four microns. 
     In yet another aspect, the disclosure features a method that includes using a pressure of from 20 pounds per linear inch to 40 pounds per linear inch to adhere a meltblown material to an article comprising a substrate to provide a filter medium. 
     Embodiments may exhibit one or more of the following advantages. 
     Embodiments may provide one or more of the following advantages. In certain embodiments, the filter medium can be relatively durable, relatively good at capturing fine particles, relatively good at holding material (e.g., dust), exhibit relatively good cleanability, relatively good soot capture, and/or relatively good liquid filtration. In some embodiments, the filter medium can simultaneously exhibit advantages that are typically not simultaneously provided by at least some known filter media. As an example, in certain embodiments, the filter medium can be effective at capturing fine particles while also being relatively durable. As another example, in some embodiments, the filter medium efficiently captures particles while also having a good ability to hold materials (e.g., dust). As a further example, in certain embodiments, the filter medium can exhibit increased particle capture efficiency while maintaining or increasing particle capture capacity. In certain embodiments, the filter medium, as well as related filter systems, can be prepared in a relatively quick, inexpensive and/or simple manner. In some embodiments, the processes disclosed herein can be used to provide a corrugated article (e.g., corrugated filter medium) that has good corrugation properties. As an example, a corrugated article can include a substrate (that is corrugated as provided) having an adhesive and additional layer disposed thereon, whereon the corrugation properties (e.g., corrugation depth) of the final article is not substantially different from the corrugation depth of the substrate as provided prior to having the adhesive and additional layers disposed thereon. As an another example, a corrugated article can include a substrate (that is corrugated as provided) having an adhesive and additional layer disposed thereon, whereon the corrugation properties (e.g., corrugation depth) of the side of the article having the adhesive and additional layer are not substantially different from the corrugation depth of the other side of the article. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The disclosure may be better understood based on the description below as well as the figures, in which: 
         FIG. 1  is a cross-sectional view of a filter medium; 
         FIG. 2  is a cross-sectional view of a pleated filter medium; 
         FIG. 3  is partial cut-away perspective view of a filter assembly including a filter medium; 
         FIG. 4  is a cross-sectional view of a corrugated filter medium; 
         FIG. 5  is schematic view of a system configured to be used in the manufacture of a filter medium; 
         FIG. 6  is a cross-sectional view of a meltblown layer; and 
         FIG. 7  is a schematic view of a system configured to be in the manufacture of a filter medium. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure generally relates to filter media, as well as related assemblies, systems and methods.  FIG. 1  is a cross-sectional view of an exemplary filter medium  10  that includes a substrate  12 , an intermediate layer  14  and a meltblown layer  16 .  FIG. 2  depicts a typical pleated configuration of filter medium  10 .  FIG. 3  shows a cut-away perspective of an exemplary filter assembly  100  including a filter housing  101 , a filter cartridge  102 , an inner screen  108  and an outer screen  103 . Filter medium  10  is disposed in filter cartridge  102 . During use, a gas enters assembly  100  via an opening  104  and then passes through inner screen  108 , filter medium  10  and outer screen  103 . The gas then exits filter assembly  100  via opening  106 . 
     I. Filter Medium 
     A. Substrate 
     Substrate  12  is generally used to provide mechanical integrity to filter medium  10 . 
     Substrate  12  can be formed from one or more layers of material. Examples of materials include glasses, celluloses, synthetic materials, ceramics, polymers (e.g., organic polymers), cotton, hemp, carbon and metals. In some embodiments, a substrate is comprised of an organic polymer. Typically, substrate  12  includes fibers of one or more materials. Exemplary classes of fibers include natural fibers, synthetic fibers, organic fibers (e.g., organic polymer fibers) and inorganic fibers. Combinations of fibers and/or materials can be used. In certain embodiments, substrate  12  may include one or more layers that do not contain fibers. Examples of non-fibrous materials that may be used in substrate  12  include open cell foam structures. Open cell foam structures can, for example, be made of polymers, such as polyolefins and polystyrenes. In certain embodiments, the substrate is substantially free of an oil coating. 
     Substrate  12  may be made using any suitable method. In some embodiments, substrate  12  is made by a method that includes web forming (e.g., wet laid, dry laid, direct laid), carding, spun bonding, melt blowing and film fibrillation. The particular configuration of the substrate can depend on the intended application of the filter media, and the particular configuration can be varied to achieve the desired structural properties, including stiffness, strength, pleatability, temperature resistance. As an example, when filter medium  10  is designed for use in heavy duty air filtration systems, gas turbine filtration systems, automotive air filtration systems, and/or pulse cleaning applications, substrate  12  may be a wet-laid paper, such as cellulose or a synthetic/cellulose blend. As another example, when filter medium  10  is designed for use in HVAC filtration systems, liquid filtration systems, HEPA filtration systems, and/or battery separators, substrate  12  may be a wet-laid paper (e.g., made from cellulose, glass and/or synthetic fibers), carded nonwovens, spunbonds, meltblowns, or air laid (e.g., synthetic or cellulose). 
     In general, substrate  12  may have any desired thickness. Typically, substrate  12  is at least 200 microns (e.g., 300 microns, 400 microns, 500 microns, 600 microns) thick, and/or at most 1500 microns (e.g., 1400 microns, 1300 microns, 1200 microns, 1100 microns, 1000 microns) thick. In some embodiments, substrate  12  has a thickness of from 200 microns to 1500 microns (e.g., 200 microns to 1000 microns, 400 microns to 1000 microns). As referred to herein, the thickness of substrate  12  is determined according to TAPPI T411. 
     The basis weight of substrate  12  is usually selected so that substrate  12  provides a desired amount of mechanical integrity to filter medium  10 . In certain embodiments, substrate  12  has a basis weight of at least 25 g/m 2  (e.g., at least 50 g/m 2 , 75 g/m 2 , 100 g/m 2 , 125 g/m 2 , 150 g/m 2 , 200 g/m 2 ), and/or at most 500 g/m 2  (e.g., at most 400 g/m 2 , 300 g/m 2 , 200 g/m 2 , 150 g/m 2 ). For example, in some embodiments, substrate  12  has a basis weight of from 25 g/m 2  to 200 g/m 2  or from 25 g/m 2  to 400 g/m 2  (e.g., from 50 g/m 2  to 200 g/m 2 , from 75 g/m 2  to 150 g/m 2 ). As referred to herein, basis weight is determined according to ASTM D-846. 
     Substrate  12  can be designed to have any desired air permeability. In some embodiments, substrate  12  has an air permeability of at least three cubic feet per minute (CFM) (e.g., at least 10 CFM, 25 CFM, 50 CFM, 75 CFM, 100 CFM, 140 CFM, 160 CFM, 180 CFM), and/or at most 400 CFM (e.g., at most 300 CFM, 200 CFM, 150 CFM, 100 CFM). As an example, in certain embodiments, substrate  12  has an air permeability of from two CFM to 400 CFM (e.g., from 10 CFM to 300 CFM, from 25 CFM to 200 CFM, from 100 CFM to 300 CFM, from 160 CFM to 200 CFM). As used herein, air permeability is determined at a pressure of 0.5 inch water column according to ASTM F778-88. Substrate  12  can also be designed to have any desired filtration efficiency. In certain embodiments, substrate  12  has a NaCl particle filtration efficiency (measured with flow rate of 32 liters per minute) of less than 10% (e.g., less than 8%, less than 5%) (see discussion below regarding the test for NaCl particle filtration efficiency). 
     While shown in  FIG. 1  as being continuous, in some embodiments, substrate  12  can be discontinuous. For example, substrate  12  could be formed of filaments (yarns) which themselves could be continuous or discontinuous. Additionally or alternatively, substrate  12  could be in the form of a material with holes in it (e.g., in the form of a mesh). Additionally or alternatively, substrate  12  could be in the form of patches (e.g., dots) of material. 
     B. Intermediate Layer 
     1. Adhesive 
     In some embodiments, layer  14  is formed of an adhesive (e.g., a hot melt adhesive, a pressure sensitive adhesive, a thermoplastic adhesive, a thermoset adhesive) that is adhered to layers  12  and  16 . 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, layer  14  is formed of a web of fibers less than 4 micron in diameter with a geometric standard deviation of 1.4. 
     The thickness of adhesive layer  14  can generally be selected as desired. In some embodiments, the thickness of adhesive layer  14  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 adhesive layer  14  can be from five microns to 100 microns (e.g., from five microns to 75 microns, from five microns to 50 microns, from 30 microns to 100 microns) as determined by scanning electron microscopy. 
     In general, the basis weight of adhesive layer  14  can be selected as desired. In some embodiments, adhesive layer  14  has a basis weight of 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, adhesive layer  14  can have a basis weight of from 0.5 g/m 2  to 10 g/m 2  (e.g., from 1 g/m 2  to 8 g/m 2 , from 2 g/m 2  to 5 g/m 2 ). 
     While shown in  FIG. 1  as being continuous, in some embodiments, adhesive  14  can be discontinuous. For example, adhesive  14  could be in the form a material with holes in it (e.g., in the form of a mesh). Additionally or alternatively, adhesive  14  could be in the form of patches (e.g., dots) of material. Typically, the amount of adhesive  14  between layers  12  and  16  is sufficient to provide appropriate adhesion between layers  12  and  16  when considering the intended use of article  10 . For example, in some embodiments, adhesive  14  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 layers  12  and  14 . 
     In general, adhesive  14  is selected to such that the mean peel strength between layers  12  and  16  is at least 0.5 ounce per inch of width (e.g., at least one ounce per inch of width, at least 1.5 ounces per inch of width). In some embodiments, adhesive  14  is selected so that the mean peel strength between layers  12  and  16  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 inches by seven inches coupons, 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 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, adhesive  14  is selected to have an appropriate open time for the manufacturing process below. For example, the open time of adhesive  14  should be sufficient so that it does not become non-adhesive between the time it is applied to one layer (e.g., layer  12  or layer  16 ) and the time adhesive  14  contacts the other layer (e.g., layer  16  or layer  12 ). In some embodiments, adhesive  14  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, layer  14  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, layer  14  serves as a carrier layer (scrim) for meltblown layer  16  (see process discussion below). In such embodiments, scrim  14  is typically bonded together with layers  12  and  16  (e.g., laminated together). 
     Scrim  14  can be, for example, formed of a polymer. Examples of polymers include polyesters, polyamides and polyolefins. Optionally, scrim  14  is formed of a spunbond nonwoven material or a carded nonwoven material. In some embodiments, scrim  14  is formed of a spunbond polypropylene. 
     Generally, the thickness of scrim  14  can be selected as desired. In certain embodiments, scrim  14  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 scrim  14  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 scrim  14  is determined according to TAPPI T411. 
     In general, the basis weight of scrim  14  can be selected as desired. In some embodiments, scrim layer  14  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, scrim layer  14  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 ). 
     While shown in  FIG. 1  as being continuous, in some embodiments, scrim  14  can be discontinuous. For example, scrim  14  could be in the form a material with holes in it (e.g., in the form of a mesh). Additionally or alternatively, scrim  14  could be in the form of patches (e.g., dots) of material. 
     C. Meltblown Layer 
     Layer  16  is formed via meltblown process, as discussed below. In general, layer  16  is 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, at most 0.8 micron, at most 0.6 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, layer  16  is formed of fibers having an average diameter of from 0.2 micron to 1.5 microns (e.g., from 0.3 microns to 1.4 microns, from 0.4 micron to 1.3 microns). As another example, in certain embodiments, layer  16  is formed of fibers having an average diameter of from 0.2 micron to 0.5 micron (e.g., from 0.3 micron to 0.5 micron, from 0.4 micron to 0.5 micron, from 0.2 micron to 0.4 micron, from 0.2 micron to 0.3 micron, from 0.3 micron to 0.4 micron). In other embodiments, layer  16  is formed of fibers having an average diameter of from 0.2 micron to 0.8 micron (e.g., from 0.3 micron to 0.8 micron, from 0.4 micron to 0.8 micron, from 0.2 micron to 0.8 micron, from 0.2 micron to 0.6 micron, from 0.3 micron to 0.6 micron). In general, 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 micron in a direction substantially perpendicular to a surface of the second layer as indicated by the arrow in  FIG. 1 . 
     It should be understood that while meltblown layer  16  is shown positioned upstream of substrate  12  in  FIG. 1 , in other embodiments meltblown layer may be positioned downstream of substrate  12 . For example, a filter medium including a substrate and a meltblown layer may be configured within a filter element such that the substrate faces an inlet of a filter element, and the meltblown layer is positioned downstream of the substrate. In such embodiments, a fluid flowing through the filter element passes across the substrate before passing across the meltblown layer. Other configurations are also possible. 
     Typically, the meltblown material is formed of one or more polymers. Exemplary polymers include polyolefins (e.g., polypropylenes), polyesters (e.g., polybutylene terephthalate, polybutylene naphthalate), polyamides (e.g., nylons), polycarbonates, polyphenylene sulfides, polystyrenes, polyurethanes (e.g., thermoplastic polyurethanes). Optionally, the polymer(s) may contain fluorine atoms. Examples of such polymers include PVDF and PTFE. In certain embodiments, the meltblown material is substantially free of an oil coating. Layer  16  can generally have any desired thickness. In some embodiments, layer  16  is at least 5 microns (e.g. at least 10 microns, at least 20 microns, at least 40 microns, at least 60 microns) thick, and/or at most 250 microns (e.g., 200 microns, 150 microns, 100 microns, 750 microns) thick. For example, layer  16  can be from five microns to 250 microns (e.g., from 10 microns to 200 microns, from 20 microns to 150 microns, from 50 microns to 100 microns) thick. The thickness of layer  16  is determined using scanning electron microscopy. Without wishing to be bound by theory, it is believed that, using the processes described herein, it is possible to obtain a layer of meltblown fibers that is thicker than is typical and/or economically feasible for a layer of electrospun fibers, and/or that is thinner and/or economically feasible than is typical (e.g., due to considerations of, for example, mechanical integrity) for a layer of meltblown fibers. 
     The basis weight of layer  16  can typically be selected as desired. In some embodiments, the basis weight of layer  16  is at least one 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, layer  16  has a basis weight of from one g/m 2  to 100 g/m 2  (e.g., from 1 g/m 2  to 10 g/m 2 , from 1 g/m 2  to 20 g/m 2 , from 10 g/m 2  to 90 g/m 2 , from 25 g/m 2  to 75 g/m 2 ). Without wishing to be bound by theory, it is believed that, using the processes described herein, it is possible to obtain a layer of meltblown fibers that has a basis weight greater than is typical and/or economically feasible for a layer of electrospun fibers, and/or that has a basis weight that is less than is typical (e.g., due to considerations of, for example, mechanical integrity and/or instantaneous lamination) for a layer of meltblown fibers. 
     The air permeability of layer  16  can also be varied as desired. In some embodiments, layer  16  has an air permeability of at most 500 CFM (e.g., at most 400 CFM, at most 300 CFM, at most 250 CFM, at most 200 CFM, at most 150 CFM, at most 100 CFM, at most 75 CFM, at most 50 CFM), and or at least 20 CFM (e.g., at least 50 CFM, at least 75 CFM, at least 100 CFM, at least 150 CFM, at least 200 CFM). For example, in some embodiments, the air permeability of layer  16  can be from 20 CFM to 500 CFM (e.g., from 30 CFM to 100 CFM, from 50 CFM to 250 CFM, from 100 CFM to 200 CFM). Typically, the air permeability of layer  16  (Perm) is determined by the equation (1/Perm)=(1/Perm 1 )+(1/Perm 2 ), where Perm is the air permeability of filter medium  10  (including layers  12 ,  14  and  16 ), Perm 1  is the air permeability of meltblown layer  16 , and Perm 2  is the air permeability of substrate layer  12 . For example, the air permeability of filter medium  10  where meltblown layer  16  has an air permeability of 300 CFM, and substrate  12  has an air permeability of 70 CFM would be 56.8 CFM because 1/56.8=1/300+1/70. 
     While shown in  FIG. 1  as being continuous, in some embodiments, layer  16  can be discontinuous. For example, layer  16  could be in the form a material with holes in it (e.g., in the form of a mesh). Additionally or alternatively, layer  16  could be in the form of patches (e.g., dots) of material. 
     D. Filter Medium Properties 
     In general, the thickness of filter medium  10  may be selected as desired. The thickness of filter medium  10  is the distance from the outer surface of layer  12  to the outer surface of layer  16 . In embodiments in which layer  14  is an adhesive, filter medium  10  can have a thickness of the article that is at least 200 microns (e.g., at least 300 microns, at least 400 microns, at least 500 microns, at least 600 microns), and/or a thickness of most 1500 microns (e.g., at most 1400 microns, at most 1300 microns, at most 1200 microns, at most 1100 microns, at most 1000 microns, at most 750 microns). For example, in such embodiments, filter medium  10  has a thickness of from 200 microns to 1500 microns (e.g., from 300 microns to 1000 microns, from 400 microns to 750 microns). In embodiments in which layer  14  is a scrim, filter medium  10  can have a thickness of the article is at least 200 microns (e.g., at least 300 microns, at least 400 microns, at least 500 microns, at least 600 microns), and/or a thickness of most 2500 microns (e.g., at most 2000 microns, at most 1500 microns). For example, in such embodiments, filter medium  10  has a thickness of from 200 microns to 2500 microns (e.g., from 300 microns to 2000 microns, from 400 microns to 1500 microns). 
     Generally, filter medium  10  can have any desired basis weight. In embodiments in which layer  14  is an adhesive, filter medium  10  can have a basis weight of at most 500 g/m 2  (e.g. at most 400 g/m 2 , at most 300 g/m 2 , at most 250 g/m 2 , at most 200 g/m 2 ), and or at least 30 g/m 2  (e.g., at least 75 g/m 2 , at least 100 g/m 2 , at least 150 g/m 2 ). In embodiments in which layer  14  is a scrim, filter medium  10  can have a basis weight of at most 600 g/m 2  (e.g. at most 500 g/m 2 , at most 400 g/m 2 , at most 300 g/m 2 ), and or at least 50 g/m 2  (e.g., at least 100 g/m 2 , at least 150 g/m 2 , at least 200 g/m 2 ). 
     The air permeability of filter medium  10  can usually be selected as desired. In some embodiments, the air permeability of filter medium  10  is at most 300 CFM (e.g., at most 200 CFM, at most 180 CFM, at most 150 CFM, at most 100 CFM), and/or at least one CFM (e.g., at least 10 CFM, at least 25 CFM, at least 50 CFM, at least 70 CFM, at least 100 CFM, at least 125 CFM, at least 150 CFM). For example, in some embodiments, filter medium  10  can have an air permeability of from one CFM to 300 CFM (e.g., from 10 CFM to 200 CFM, from 25 CFM to 100 CFM, from 100 CFM to 200 CFM, from 140 CFM to 190 CFM). 
     In some embodiments, filter medium  10  can exhibit a good ability to capture dust. For example, in some embodiments, filter medium  10  has an initial dust capture efficiency of at least 80% (e.g., at least about 85%, at least about 90%) (see discussion below for description of initial dust capture efficiency test). In certain embodiments, filter medium  10  has a periodic dust capture efficiency that is at least about 90% (e.g., at least about 95%, at least about 97%) (see discussion below for description of periodic dust capture efficiency test). In some embodiments, filter medium  10  has an initial dust capture efficiency of at least 80% (e.g., at least about 85%, at least about 90%) and a periodic dust capture efficiency that is at least about 90% (e.g., at least about 95%, at least about 97%.) 
     In certain embodiments, filter medium  10  can have good dust holding properties. For example, in certain embodiments, filter medium  10  can have a dust holding capacity of at least 50 g/m 2  (e.g., at least 60 g/m 2 , at least 70 g/m 2 , at least 80 g/m 2 , at least 100 g/m 2 , at least 125 g/m 2 , at least 150 g/m 2 , at least 175 g/m 2 , at least 200 g/m 2 , at least 225 g/m 2 , at least 250 g/m 2 ) (see discussion below for description of dust holding capacity test). The dust holding capacity of the filter medium may depend on the mechanism of dust capture. For instance, a filter medium designed for depth filtration may have a higher dust holding capacity than a filter medium designed for surface filtration. In some embodiments, the filter medium is designed for depth filtration, for example, when a substrate layer is positioned upstream of a meltblown layer as described herein. 
     In certain embodiments, a filter medium described herein has a dust holding capacities described above while being substantially free of an oil that coats all or a portion of the filter medium. For example, certain depth filters known in the art include one or more layers (e.g., a fibrous layer) treated with an oil to increase the filter&#39;s ability to capture dust. While oil may be used to enhance dust capture ability in some embodiments described herein, other embodiments do not include such oil-treated layer(s) for certain applications, such as certain automotive air filtration applications. 
     In some embodiments, filter medium  10  has both good dust capture and good dust holding properties. As an example, in some embodiments, filter medium  10  has an initial dust capture efficiency of at least 80% (e.g., at least about 85%, at least about 90%) and a dust holding capacity of at least 50 g/m 2  (e.g., at least 60 g/m 2 , at least 70 g/m 2 ). As another example, in some embodiments, filter medium  10  has a periodic dust capture efficiency of at least 90% (e.g., at least about 95%, at least about 97%) and a dust holding capacity of at least 50 g/m 2  (e.g., at least 60 g/m 2 , at least 70 g/m 2 ). Without wishing to be bound by theory, it is believed that, simultaneously providing good dust capture and good dust holding properties can result, in some embodiments at least, from the processes described herein, by which the article can include a layer of meltblown fibers that has a relatively small average fiber diameter (e.g., 0.8 micron or less) and is less dense and is thicker than electrospun coatings. 
     In certain embodiments, filter medium  10  has good fine particle capture properties. As an example, in some embodiments, filter medium  10  has a NaCl particle capture test time of at least 40 minutes (e.g., at least 50 minutes, at least 60 minutes, at least two hours (see discussion below for description of NaCl particle capture test). 
     In some embodiments, dust can be relatively easily removed from filter medium  10 . For example, in some embodiments, filter medium  10  has an initial cleanability test time of at least four hours (e.g., at least five hours, at least six hours) (see discussion below for description of initial cleanability test). In certain embodiments, filter medium  10  has an aged cleanability test time of at 70% (e.g., at least 80%, at least 90%) of the initial cleanability test time (see discussion below for description of aged cleanability test). 
     In some embodiments, filter medium  10  can exhibit good NaCl particle filtration efficiency and good NaCl particle capture. For example, in some embodiments, filter medium  10  can have a NaCl particle filtration efficiency of at least 30% (e.g., at least 40%, at least 50%) and a NaCl particle capture test time of at least 40 minutes (e.g., at least 50 minutes, at least 60 minutes) (see discussion below for description of NaCl particle capture efficiency test and NaCl particle capture test). Without wishing to be bound by theory, it is believed that, simultaneously providing good NaCl particle filtration efficiency and good NaCl particle capture can result, in some embodiments at least, from the processes described herein, by which the article can include a layer of meltblown fibers that has a relatively small average fiber diameter (e.g., 0.8 micron or less) and is less dense and is thicker than electrospun coatings. 
     In certain embodiments, filter medium  10  can have good liquid filtration properties. For example, in certain embodiments, filter medium  10  has a liquid filtration efficiency of at least 45% (e.g., at least 50%, at least 60%) at a given particle size (see discussion below for description of liquid filtration efficiency test). As another example, in some embodiments, filter medium  10  has a liquid filtration retention efficiency of at least 60% (e.g., at least 65%, at least 70%) at a given particle size and time (see discussion below for description of liquid filtration efficiency test). 
     In certain embodiments, article  10  can have a Beta decay of at most 20% (e.g., at most 15%, at most 10%, at most 5%) at a particle size of four microns. In some embodiments, article  10  has a Beta decay of at least 1% at a particle size of four microns. As used herein, the “Beta decay at a particle size of four microns” of an article is determined according to the ISO 16889:1999 test procedure. 
     In some embodiments, an article can be corrugated. Optionally, a corrugated article can also be pleated. 
       FIG. 4  shows an article  30  having a substrate  12 , an adhesive  14  and a meltblown layer  16 . Article  30  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, article  30  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, article  30  has a corrugation depth on a side  12 A of substrate  12  that is depicted by a distance “d 1 ”, which is the distance from a peak of layer  16  to a valley of layer  16  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, article  30  has a corrugation depth on a side  12 B of substrate  12  that is depicted by a distance “d 2 ”, which is the distance from a peak of side  12 B of substrate  12  to a valley of side  12 B of substrate  12  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, article  30  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 article  30  is determined by dividing the corrugation depth “d 1 ” by the distance from a peak of side  12 A substrate  12  to a valley of side  12 A of substrate  12  (measured before layer  14  is applied to side  12 A of substrate  12 ) 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 layer  12  is formed on a separate web from layer  16 , 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 some embodiments, for instance, when used for certain depth filtration applications such as automotive air applications, a filter medium described herein has a basis weight of from 60 lbs/ream to 90 lbs/ream (e.g., about 70 lbs/ream). A thickness of the filter medium may be from 400 microns and 700 microns (e.g., about 500 microns). The filter medium may have a permeability of from 150 CFM to 190 CFM (e.g., about 145 CFM). The filter medium may have an MD Gurley stiffness of from 4,000 mg to 5,000 mg (e.g., about 4,500 mg). The as is Mullen may be from 10 psi to 50 psi (e.g., about 40 psi). The as is wet Mullen may be from 15 psi to 55 psi (e.g., about 45 psi). The cured wet Mullen may be from 15 psi to 55 psi (e.g., about 45 psi). 
     In some such instances, the filter medium may have an initial dust capture efficiency for 10 micron particles of at least 90% (e.g., at least 92%, at least 94%) and/or an initial dust capture efficiency for 20 micron particles of at least 95% (e.g., at least 96%, at least 98%). The dust holding capacity of the filter medium may be at least 200 g/m 2  (e.g., at least 225 g/m 2 . When the filter medium is tested using the Palas test, the time to reach 1,800 Pa may be at least 52 minutes (e.g., at least 70 minutes, at least 90 minutes, at least 100 minutes). The filter medium may be corrugated or flat. 
     The filter medium may include one or more layers comprising a meltblown material. In some instances, the meltblown material includes PBT or any other suitable fibers. The fibers may have an average fiber diameter of from 0.3 microns to 0.6 microns (e.g., about 0.4 microns). The basis weight of the meltblown layer may be from 0.2 to 8 g/m 2  (e.g., about 2.5 g/m 2 ). 
     The filter medium may optionally include an adhesive layer having a basis weight of from 2 g/m 2  to 4 g/m 2  (e.g., about 3 2 g/m 2 ). Any suitable adhesive may be used. In some cases, an adhesive that is stable to at least 200 degrees F. (e.g., at least 220 degrees F.) is desirable. 
     Such filter media may further include a substrate. The substrate may be formed of any suitable material, such as an organic polymer. In some cases, the substrate comprises cellulosic fibers. The substrate may optionally include a resin such as a precured solvent phenolic resin. The substrate may have an FR Rating of F1 or higher (e.g., K1). In certain embodiments, the substrate has a smolder rating of S1 or higher. The basis weight of the substrate may be from 60 to 80 lbs/ream (e.g., about 65 lb/ream). The thickness of the substrate may be from 480 microns to 600 (e.g., about 530 microns). The substrate may have a permeability of from 160 CFM to 200 CFM (e.g., about 185 CFM). The substrate may have an MD Gurley stiffness of from 4,000 mg to 5,000 mg (e.g., about 4,500 mg). The as is Mullen of the substrate may be from 10 psi to 50 psi (e.g., about 40 psi). The as is wet Mullen of the substrate may be from 15 psi to 55 psi (e.g., about 45 psi). The substrate may include a cured wet Mullen of from 30 psi to 80 psi (e.g., about 65 psi). The substrate may further include a wet Mullen ratio of from 60% to 80% (e.g., about 70%). When integrated into a filter element, the substrate may be positioned upstream with respect to the meltblown layer, e.g., such that the substrate faces an inlet of the filter element. 
     II. Filter Assemblies and Systems 
     Filter assembly  100  can be any of a variety of filter assemblies. Examples of filter assemblies include gas turbine filter assemblies, heavy duty air filter assemblies, automotive air filter assemblies, HVAC air filter assemblies, HEPA filter assemblies, vacuum bag filter assemblies, fuel filter assemblies, and oil filter assemblies. Such filter assemblies 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, the filter medium can optionally be prepared by coating a paper with the meltblown material. In certain embodiments, the filter medium can be prepared using a wet laid or dry laid product (e.g., cellulose, polymer, glass). The filter medium can optionally be pleated into any of a variety of configurations (e.g., panel, cylindrical). 
     The orientation of filter medium  10  relative to gas flow through a filter assembly/filter system can generally be selected as desired. In some embodiments, meltblown layer  16  is upstream of substrate  12  in the direction of gas flow through the filter assembly/system. In certain embodiments, meltblown layer  16  is downstream of substrate  12  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, meltblown layer  16  can be upstream of substrate  12  in the direction of gas flow through the filter assembly/system. As another example, in some embodiments in which improved depth filtration is desired, meltblown layer  16  can be downstream of substrate  12  in the direction of gas flow through the filter assembly/system. 
     III. Methods of Manufacturing Filter Medium 
     1. Adhesive 
     In general, in embodiments in which adhesive layer  14  is used, the manufacturing method involves applying layer  14  to substrate  12 , and subsequently applying meltblown layer  16  to adhesive  14 , so that, within filter medium  10 , substrate  12  and meltblown layer  16  are both adhered to adhesive layer  14 . 
     In some embodiments, manufacture of filter medium  10  with adhesive layer  14  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 meltblown layer  16 , and another roll-to-roll system can be used to adhere layer  14  to substrate  12 . In such a system, the roll-to-roll systems can be configured so that, in a continuous fashion, adhesive layer  14  contacts meltblown layer  16  and these two layers become adhered to each other. 
       FIG. 5  shows an embodiment of a system  200  that can be used to form filter medium  10  having adhesive layer  14 . 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 extruder  216 , and the polymer is heated (generally slowly) 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. It is believed that, in some embodiments, the throughput of polymer per hole can have a relatively a strong affect 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. It is believed that this air can attenuate the fiber to the final fiber size. It is believed that, in some embodiments, as the process air throughput increases, the fiber diameter can decrease, and/or that, as the process air temperature increases, the fiber diameter can decrease. In the area where fiber attenuation occurs, quench air is present, which forms an area where fiber formation occurs at the same temperature year round. The distance from the die tip to the collector allows us to control the density of the material (e.g., as the collector distance is increased, the fiber velocity is decreased and the fiber temperature is reduced so packing of the fibers is less dense, resulting in a more lofty web). As the distance is increased, the velocity of the fiber is generally decreased, making a loftier filter media. The collector suction is also controlled, which also impacts the loft of the material. It is believed that, in some embodiments, as the belt speed is increased, the web basis weight of the filter medium can decrease, and/or that, as the polymer throughput increases, the basis weight of the filter medium can increase. 
     The size of the holes and number of holes per inch for the 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. 
     System  220  includes rollers  222   a ,  222   b ,  222   c  and  222   d  that move substrate  12  as the rollers rotate. Between rollers  222   a  and  222   b , system  220  includes a station  226  that applies an adhesive to substrate  12 . In a region adjacent rollers  222   b  and  212   a , the adhesive contacts meltblown layer  16 , and meltblown layer  16  is removed from belt  214  and adhered to the adhesive. The substrate/adhesive/meltblown layer composite then passes through a charging unit  228 . Charging unit  228  is used to charge the composite (in general, particularly the meltblown layer). It is believed that this can result in a filter medium having enhanced fine particle capture properties. It is believed that the charging process can embed charges in the meltblown material. 
     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 a Nordson Precision Metered Gear Adhesion applicator system with Signature nozzles, which can have 12 nozzles per inch that provide 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. 
     In general, the temperature is selected to properly soften (e.g., melt) the material that is to be formed into layer  16 . 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., 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 general, the process air is the heated air on either side of the die tip where the 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. It is believed that, 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). 
     Belt  214  generally can be made of any material that allows the formation of layer  16  on belt  214 , and also allows the removal of layer  16  from belt  214  when layer  16  contacts adhesive layer  14 . Examples of materials from which belt  214  can be made include polymers (e.g., polyesters, polyamides), metals and/or alloys (e.g., stainless steel, aluminum). 
     The speed at which belt  214  moves can be selected as desired to form layer  16 . In some embodiments, the belt  214  moves 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, 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, when applied to substrate  12 , the temperature of the adhesive can be selected so that it has an appropriate level of tack when it comes into contact with layer  16 . In embodiments in which the adhesive is a hot melt adhesive, this can involve heating the adhesive prior to its application to substrate  12 . For example, prior to being applied to substrate  12 , 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., 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.). 
     Substrate  12  is typically fed through the adhesive station by the pulling force generated via a nip formed at rollers  212   a  and  222   b . By contacting substrate  12  adjacent roller  222   b  (e.g., a rubber roller, such as a 70 Shore A EPDM rubber roller) with meltblown material  16  adjacent roller  212   a  (e.g., a stainless steel roller, such as a stainless steel roller that is crowned by 0.025 inch), the speed of belt  214  and substrate  12  are synchronized (e.g., so that substrate  12  moves at approximately the same speed as belt  214 ). The pressure between rollers  212   a  and  222   b  is generally selected as desired for the intended use of article  10 . For example, in embodiments in which article  10  is corrugated, the pressure between rollers  212   a  and  222   b  is typically selected to achieve good corrugation depth and conformity for article  10 . 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). 
     In general, any of a variety of techniques can be used to charge the substrate/adhesive/meltblown 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). Exemplary processes are disclosed, for example, in U.S. Pat. No. 5,401,446, which, to the extent it is not inconsistent with the present disclosure, is incorporated herein by reference. 
     In general, any belt configuration can be used. For example, in some embodiments, the belt has an open structure, such as a mesh structure. Without wishing to be bound by theory, it is believed that such an open structure results in the meltblown material having a complementary structure to that of the belt because the meltblown material is under the force of the blown air.  FIG. 6  shows a cross section of the meltblown material  60  having a series of crests  64  and valleys  62  resulting from the complementary shape of the belt. Without wishing to be bound by theory, it is believed that this structure may be present in the meltblown material in the filter medium, and that, during collection of dust or other particles, the dust may build up in the valleys, allowing for good dust removal during pulsing. In some embodiments, the distance d between adjacent valleys  62  is at least 400 microns (e.g., at least 500 microns, at least 700 microns), and/or at most 2000 microns (e.g., at most 1500 microns, at most 1200 microns). In some embodiments, the distance d between adjacent valleys  62  is from 400 microns to 2000 microns (e.g., from 500 microns to 1500 microns, from 700 microns to 1200 microns). In some embodiments, the distance h from a crest  64  to a valley  62  is at least 50 microns (e.g., at least 100 microns, at least 300 microns), and/or at most 2000 microns (e.g., at most 1500 microns, at most 1000 microns). In some embodiments, the distance h from a crest  64  to a valley  62  is from 50 microns to 2000 microns (e.g., from 200 microns to 1500 microns, from 300 microns to 1000 microns). 
     2. Scrim 
     In general, in embodiments in which scrim layer  14  is used, the manufacturing method involves applying meltblown layer  16  to scrim  14 , subsequently applying substrate  12  to scrim  14 , and then bonding the three layers together. 
     In some embodiments, manufacture of filter medium  10  with scrim layer  14  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 meltblown layer  16  on scrim  14 , and another roll-to-roll system can be used to carry substrate  12 . In such a system, the roll-to-roll systems can be configured so that, in a continuous fashion, the meltblown layer/scrim composite contacts substrate  12  to form a three layer composite, and the three layers are subsequently bonded together. 
       FIG. 7  shows an embodiment of a system  300  that can be used to form meltblown layer  16  on scrim  14 . System  300  includes rollers  302   a ,  302   b ,  302   c  and  302   d  that move a continuous belt  304  as the rollers rotate. Scrim  14  is applied to belt  304 . System  302  also includes an extruder  306 . As rollers  302   a - 302   d  are rotating, extruder  306  is heated, and the material from which layer  16  is to be formed (e.g., a polymer in pellet form) is introduced into heated extruder  306 . The material is softened (e.g., melted) and forced through a die  307  in the form of filaments. The filaments are moved toward scrim  14  under the influence of a vacuum  308  on the opposite side of belt  304  relative to die  307 . The effect of the vacuum is to stretch the filaments and force them against the surface of scrim  14  to provide meltblown layer  16  disposed on scrim  14 . 
     The process conditions used for the process described in  FIG. 7  can generally be selected as desired to form layer  16 . In general, the temperature is selected to properly soften (e.g., melt) the material that is to be formed into layer  16 . 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., 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 general, the process air is the heated air on either side of the die tip where the 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. It is believed that, in some embodiments, increasing the air volume can result in a reduced fiber diameter. 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  308  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). 
     The speed at which belt  304  moves can be selected as desired to form layer  16 . In some embodiments, the belt  304  moves 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, belt  304  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). 
     The scrim/meltblown layer composite is removed from belt  304 , and substrate  12  is disposed on scrim  14 . Typically, this involves bringing scrim  14  onto a belt (e.g., belt  214 ) and then blowing the meltblown fibers directly onto scrim  14 . Scrim  14  can have adhesive applied before the meltblown is blown on or the force and the heat of the meltblown fibers can be used to adhere the two layers together. The relevant process conditions are generally the same as above. 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, meltblown layer  16 , scrim  14  and substrate  12  can be joined by applying ultrasonic energy between an aluminum vibrating horn (½″ contact width, from Branson Ultrasonics, Danbury, Conn.) and an engraved contact roll. In certain embodiment, the method involves using a horn pulsing at 20 kHZ applying 20 to 30 psi of contact pressure at an amplitude of 20 to 35 microns at a feed rate of 25 to 45 ft/min bonds the composite together at points comprising of less than 10% (e.g., less than 8%, less than 5%, less than 3%) of the total area as determined by the engraving on the contact roll. 
     The following examples are exemplary and not intended as limiting. 
     IV. Examples 
     A. Test Protocols 
     1. NaCl Particle Filtration Efficiency Test 
     A 100 cm 2  surface area of the filter medium was tested with NaCl (sodium chloride) particles having a 0.26 micron mass mean diameter with a geometric standard deviation less than 1.83, a concentration of 15 to 20 mg/cm 3 , and a face velocity of 5.3 cm/s by a TS18130 CertiTest™ automated filter testing unit from TSI, Inc. equipped with a sodium chloride generator. The instrument measured a pressure drop across the filter media and the resultant penetration value on an instantaneous basis at a flow rate less than or equal to 115 liters per minute (lpm). Instantaneous readings were defined as 1 pressure drop/penetration measurement. This test is described in ASTM D2 986-91. The NaCl particle filtration efficiency is [100−(C/C 0 )]*100%, where C was the particle concentration after passage through the filter and C 0  was the particle concentration before passage through the filter. 
     2. Initial Dust Capture Efficiency, Periodic Dust Capture Efficiency and Dust Holding Capacity 
     A 100 cm 2  surface area of the filter medium was challenged with a fine dust (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 was measured using a Palas MFP2000 fractional efficiency photodetector. The dust capture efficiency was [(1−[C/C0])*100%], where C was the dust particle concentration after passage through the filter and C0 was the particle concentration before passage through the filter. The dust capture efficiency was measured after one minute and is referred to herein as the initial dust capture efficiency. The dust capture efficiency was also measured periodically after one minute and is referred to herein as the periodic dust capture efficiency. The dust holding capacity is measured when the pressure reaches 1800 Pa, and is the difference in the weight of the filter medium before the exposure to the fine dust and the weight of the filter medium after the exposure to the fine dust. 
     3. Initial Cleanability Test and Aged Cleanability Test 
     An AC fine dust at 16 g/hr was sent through the filter medium at a face velocity of 5 cm/s, and then subject to 150 millisecond pulse at 4 bar to remove particles from the medium when the medium reaches a pressure of 10 mBar. This process (exposure to the AC fine dust under the stated conditions until reaching a pressure of 10 mBar) is repeated a total of 30 times, and the initial cleanability time is the amount of time it takes to complete the 30 cycles. The medium is then aged by continuous exposure to the AC dust (12 g/hr) for 10,000 Cycles and pulsed 14 times per minute. After this ageing process, the filter medium is again exposed to the AC fine dust under the conditions noted above 30 times, and the aged cleanability time is the amount of time it takes to complete these 30 cycles. This test is performed on a Palas MMTC-2000 Cleanability test stand by VDI-3926 type 2 Procedure with a test area of 177 cm 2 . 
     4. NaCl Particle Capture Test 
     A surface area of 100 cm 2  was exposed to an aerosol of 0.4 to 0.5 .mu.M NaCl particles at 2% concentration at a face velocity of 8.3 cm/s with a total flow volume of 45 liters/min. The NaCl particle capture test time is the amount of time it takes to reach a pressure of 1800 Pa. 
     5. Liquid Filtration Efficiency Test and Liquid Filtration Retention Efficiency 
     Using a FTI Multipass Filter Test Stand (Fluid Technologies Inc., Stillwater, Okla.), an A2 fine dust is fed at a rate of 0.3 liters per minute into Mobil MIL-H- 5 606 fuel for a total flow rate of 1.7 liters per minute to contact the filter medium per ISO 16889 until a terminal pressure of 174 KPa above the baseline filter pressure drop is obtained. Particle counts (particles per milliliter) are taken at the particle size selected (in this case 4, 5, 7, 10, 15, 20, 25 and 30 microns) upstream and downstream of the media are taken at ten points equally divided over the time of the test. The average of upstream and downstream particle counts are taken at each selected particle size. From the average particle count upstream (injected −C 0 ) and the average particle count downstream (passed thru-C) the liquid filtration efficiency test value for each particle size selected is determined by the relationship [(100−[C/C 0 ])*100%]. The liquid filtration retention efficiency as a function of time and particle size can also be measured by comparing the upstream and downstream particle counts (and determining efficiency [(100−[C/C0])*100%]) at the sequential ten points in the test. 
     B. Examples 
     1. Sample A 
     Sample A was prepared by forming a 7 g/m 2  (gsm) meltblown web from 0.8 micron polypropylene fibers (Exxon PP3546 G, ExxonMobil Chemical Company, Houston, Tex.) produced at a polymer heated to 475° F. at rate of 36 lbs of polymer/hr from a 35 holes per inch die blown by process air heated to 475° F. at a flow rate of 3900 lbs/hr while quenching with 55° F. air of 390 lbs/hr. The meltblown material was collected and instantaneously bonded onto a 10 gsm spunbond polypropylene nonwoven scrim (Celestra from Fiberweb Corporation, Nashville, Tenn.) moving on a collector belt moving at 55 ft/min with a vacuum pressure of approximately 18 inches of water through a 7 inch wide slot. The meltblown was adhered to the spunbond giving a composite structure with caliper of 0.0055″, a basis weight of 18 gsm, air permeability of 91 cfm at 0.5″ water column. The resulting filter media had a pressure drop of 1.5 mm H2O @ 10.5 FPM face velocity, as determined with a TSI 8130 filtration tester. The NaCl particle filtration efficiency was 82.2%. 
     The meltblown nanofiber/scrim combination was adhered to a support layer formed from a cellulose fiber containing 17% vinyl acetate resin and 83% cellulosic fiber wet laid nonwoven with a basis weight of 139 gsm and an air permeability of 80 cfm at 0.5″ water. The meltblown nanofiber/spunbond was ultrasonically point bonded (3% bond area) to a cellulose support with the cellulose support positioned on the downstream side of the meltblown nanofiber scrim with the scrim positioned upstream of the meltblown nanofiber. 
     The filter media had a basis weight of 156 gsm, a caliper of 0.030″ and an air permeability of 38 cfm at 0.5″ water column. The NaCl particle filtration efficiency was 87.5%. This is an improvement over the uncoated cellulose substrate (approximately 11%). The spunbond scrim has essentially no ability to capture fine particles. 
     2. Sample B 
     Sample B was prepared by forming a 1 gsm meltblown web from 0.25 micron polypropylene fibers (Exxon PP3546 G, ExxonMobil Chemical Company, Houston, Tex.) produced at a polymer heated to 425° F. at rate of 2 lbs of polymer/hr from a 70 holes per inch die blown by process air heated to 450° F. at a flow rate of 3250 lbs/hr while quenching with 55° F. air at 350 lbs/hr. The meltblown material was collected and instantaneously bonded onto a 10 gsm spunbond polypropylene scrim (Celestra from Fiberweb Corporation, Nashville, Tenn.) moving on a collector belt moving at 30 ft/min with a vacuum pressure of approximately 20 inches of water through a 7 inch wide slot. The meltblown was adhered to the spunbond giving a composite structure with caliper of 0.0034″, a basis weight of 11 gsm, air permeability of 328 cfm at 0.5″ water column. The resulting filter media had a pressure drop of 0.4 mm H2O @ 10.5 FPM face velocity, as determined with a TSI 8130 filtration tester. The NaCl particle filtration efficiency was 47%. 
     The meltblown nanofiber/scrim combination was adhered to a support layer formed from a cellulose fiber containing 17% vinyl acetate resin and 83% cellulosic fiber wet laid nonwoven with a basis weight of 139 gsm and an air permeability of 80 cfm at 0.5″ water. The meltblown nanofiber/spunbond was ultrasonically bonded to a cellulose support with the cellulose support positioned on the downstream side of the meltblown nanofiber scrim with the scrim positioned upstream of the meltblown nanofiber. 
     The filter media had a basis weight of 156 gsm, a caliper of 0.032″ and an air permeability of 53 cfm at 0.5″ water column. The NaCl particle filtration efficiency was 53%. This is an improvement over the uncoated cellulose substrate (approximately 11%). The spunbond scrim has essentially no ability to capture fine particles. 
     3. Sample C 
     Sample C was prepared by forming a 2 gsm meltblown web from 0.32 micron polypropylene fibers (Exxon PP3546 G, ExxonMobil Chemical Company, Houston, Tex.) produced at a polymer heated to 425° F. at rate of 12 lbs of polymer/hr from a 70 holes per inch die blown by process air heated to 450° F. at a flow rate of 3250 lbs/hr while quenching with 55° F. air at 350 lbs/hr. The meltblown material was collected and instantaneously bonded onto a 10 gsm spunbond polypropylene scrim (Celestra from Fiberweb Corporation, Nashville, Tenn.) moving on a collector belt moving at 75 ft/min with a vacuum pressure of approximately 20 inches of water through a 7 inch wide slot. The meltblown was adhered to the spunbond giving a composite structure with caliper of 0.0052″, a basis weight of 12 gsm, air permeability of 335 cfm at 0.5″ water column. The resulting filter media had a pressure drop of 0.3 mm H2O @ 10.5 FPM face velocity, as determined with a TSI 8130 filtration tester. The NaCl particle filtration efficiency was 36%. 
     The meltblown nanofiber/scrim combination was adhered to a support layer formed from a cellulose fiber containing 17% vinyl acetate resin and 83% cellulosic fiber wet laid nonwoven with a basis weight of 139 gsm and an air permeability of 80 cfm at 0.5″ water. 
     The meltblown nanofiber/spunbond was ultrasonically bonded to a cellulose support with the cellulose support positioned on the downstream side of the meltblown nanofiber scrim with the scrim positioned upstream of the meltblown nanofiber. 
     The filter media had a basis weight of 156 gsm, a caliper of 0.031″ and an air permeability of 56 cfm at 0.5″ water column. The NaCl particle filtration efficiency was 49%. This is an improvement over the uncoated cellulose substrate (approximately 11%). 
     4. Sample D 
     Sample D was prepared by forming a 5 gsm meltblown web from 0.7 micron polypropylene fibers (Exxon PP3546 G, ExxonMobil Chemical Company, Houston, Tex.) produced from a polymer heated to 425° F. at rate of 20 lbs of polymer/hr from a 70 holes per inch die blown by process air heated to 450° F. at a flow rate of 325° lbs/hr while quenching with 55° F. air at 490 lbs/hr. The meltblown material was collected and instantaneously bonded onto a 10 gsm spunbond polypropylene nonwoven scrim (Celestra from Fiberweb Corporation, Nashville, Tenn.) moving on a collector belt moving at 50 ft/min with a vacuum pressure of approximately 20 inches of water through a 7 inch wide slot. The meltblown was adhered to the spunbond giving a composite structure with caliper of 0.004″, a basis weight of 15 gsm, air permeability of 111 cfm at 0.5″ water column. 
     The meltblown nanofiber/scrim combination was adhered to a support layer formed from a cellulose fiber containing 17% vinyl acetate resin, 15% polyester fibers and 68% cellulosic fiber wet laid nonwoven with a basis weight of 122 gsm and an air permeability of 94 cfm at 0.5″ water. 
     The meltblown nanofiber/spunbond was adhered to the cellulose support by application of a hot melt glue (Bostik HM 4379 Amorphous Polyolefin (APO)) spray at an areal weight of 4 g/m2 to the cellulose support and then immediately bonding the glue applied layer to the meltblown/nanofiber cellulose by contact pressure between two rubber rolls. The article was made into a filter element having the scrim layer facing the inlet and the meltblown nanofiber in the middle and the cellulose support facing the downstream side. 
     The filter media had a basis weight of 136 gsm, a caliper of 0.031″ and an air permeability of 51 cfm at 0.5″ water column. The NaCl particle filtration efficiency was 68%. This is an improvement over the uncoated cellulose substrate (approximately 11%). 
     5. Sample E 
     Sample E was prepared by forming a 5 gsm meltblown web from 0.5 micron polypropylene fibers (Exxon PP3546 G, ExxonMobil Chemical Company, Houston, Tex.) produced from a polymer heated to 425° F. at rate of 20 lbs of polymer/hr from a 70 holes per inch die blown by process air heated to 450° F. at a flow rate of 3250 lbs/hr while quenching with 55° F. air at 490 lbs/hr. The meltblown material was collected on the bare collector belt moving at 45 ft/min with a vacuum pressure of approximately 20 inches of water through a 7 inch wide slot. The free standing nanofiber meltblown had a caliper of less than 0.001″, a basis weight of 5 gsm, air permeability of 100 cfm at 0.5″ water column. 
     The meltblown nanofiber was adhered to a support layer formed from a cellulose fiber containing 17% vinyl acetate resin 15% polyester fibers and 68% cellulosic fiber wet laid nonwoven with a basis weight of 122 gsm and an air permeability of 94 cfm at 0.5″ water. 
     The meltblown nanofiber/spunbond was adhered to the cellulose support by application of a hot melt glue (Bostik HM 4379 APO) spray at an areal weight of 4 g/m2 to the cellulose support and then immediately bonding the glue applied layer to the meltblown/nanofiber cellulose by contact pressure between a belt used to collect the meltblown fibers and a rubber roller. The article was made into a filter element having the meltblown nanofiber facing the inlet and the cellulose support facing the downstream side. 
     The filter media had a basis weight of 133 gsm, a caliper of 0.029″ and an air permeability of 50 cfm at 0.5″ water column. The NaCl particle filtration efficiency was 63%. This is an improvement over the uncoated cellulose substrate (approximately 1%). The mean peel strength of nanofiber layer from the base substrate was 0.5 ounce per inch width. 
     6. Sample F 
     Sample F was prepared by forming a 5 gsm meltblown web from 0.7 micron polypropylene fibers (Exxon PP3546 G, ExxonMobil Chemical Company, Houston, Tex.) produced from a polymer heated to 425 F at rate of 20 lbs of polymer/hr from a 70 holes per inch die blown by process air heated to 435 F at a flow rate of 3900 lbs/hr while quenching with 55° F. air at 520 lbs/hr. The meltblown material was collected on the bare collector belt moving at 60 ft/min with a vacuum pressure of approximately 20 inches of water through a 7 inch wide slot. The free standing nanofiber meltblown had a caliper of less than 0.001″, a basis weight of 5 gsm, air permeability of 172 cfm at 0.5″ water column. 
     The meltblown nanofiber was adhered to a support layer formed from a cellulose fiber containing 17% vinyl acetate resin and 83% cellulosic fiber wet laid nonwoven with a basis weight of 125 gsm and an air permeability of 32 cfm at 0.5″ water. 
     The meltblown nanofiber was adhered to the cellulose support by application of a hot melt glue (Bostik HM 4379 APO) spray at an areal weight of 4 g/m2 to the cellulose support and then immediately bonding the glue applied layer to the meltblown/nanofiber cellulose by contact pressure between a belt used to collect the meltblown fibers and a rubber roller. The article was made into a filter element having the meltblown nanofiber facing the inlet and the cellulose support facing the downstream side. 
     The filter media had a basis weight of 134 gsm, a caliper of 0.027″ and an air permeability of 27 cfm at 0.5″ water column. The NaCl particle filtration efficiency was 50%. This is an improvement over the uncoated cellulose substrate (approximately 20%). 
     7. Sample G 
     Sample G was prepared by forming a 3 gsm meltblown web from 0.7 micron polypropylene fibers (Exxon PP3546 G, ExxonMobil Chemical Company, Houston, Tex.) produced from a polymer heated to 425° F. at rate of 20 lbs of polymer/hr from a 70 holes per inch die blown by process air heated to 435° F. at a flow rate of 4250 lbs/hr while quenching with 55° F. air at 520 lbs/hr. The meltblown material was collected on the bare collector belt moving at 100 ft/min with a vacuum pressure of approximately 17 inches of water through a 7 inch wide slot. The free standing nanofiber meltblown had a caliper of less than 0.001″, a basis weight of 3 gsm, air permeability of 300 cfm at 0.5″ water column. 
     The meltblown nanofiber was adhered to a support layer formed from cellulose fiber containing 17% vinyl acetate resin and 83% cellulosic fiber wet laid nonwoven with a basis weight of 125 gsm and an air permeability of 32 cfm at 0.5″ water. 
     The meltblown nanofiber/spunbond was adhered to the cellulose support by application of a hot melt glue (Bostik HM 4379 APO) spray at an areal weight of 4 g/m2 to the cellulose support and then immediately bonding the glue applied layer to the meltblown/nanofiber cellulose by contact pressure between a belt used to collect the meltblown fibers and a rubber roller. The resulting article was made into a filter having the meltblown nanofiber facing the inlet and the cellulose support facing the downstream side. 
     The filter media had a basis weight of 129 gsm, a caliper of 0.025″ and an air permeability of 29 cfm at 0.5″ water column. The NaCl particle filtration efficiency was 37%. This is an improvement over the uncoated cellulose substrate (approximately 20%). 
     8. Sample H 
     Sample H was prepared by forming a 3 gsm meltblown web from 0.7 micron polypropylene fibers (Exxon PP3546 G, ExxonMobil Chemical Company, Houston, Tex.) produced from a polymer heated to 425° F. at rate of 20 lbs of polymer/hr from a 70 holes per inch die blown by process air heated to 440° F. at a flow rate of 4360 lbs/hr while quenching with 55° F. air at 490 lbs/hr. The meltblown material was collected on the bare collector belt moving at 100 ft/min with a vacuum pressure of approximately 17 inches of water through a 7 inch wide slot. The free standing nanofiber meltblown had a caliper of less than 0.001″, a basis weight of 3 gsm, air permeability of 307 cfm at 0.5″ water column. 
     The meltblown nanofiber was adhered to a support layer formed from cellulose fiber containing 17% vinyl acetate resin and 83% cellulosic fiber wet laid nonwoven with a basis weight of 139 gsm and an air permeability of 89 cfm at 0.5″ water. 
     The meltblown nanofiber was adhered to the cellulose support by application of a hot melt glue (Bostik HM 4379 APO) spray at an areal weight of 2 g/m2 to the cellulose support and then immediately bonding the glue applied layer to the meltblown/nanofiber cellulose by contact pressure between a belt used to collect the meltblown fibers and a rubber roller. The resulting article was made into a filter having the cellulose support facing the inlet and the meltblown nanofiber layer facing the downstream side. 
     The filter media had a basis weight of 143 gsm, a caliper of 0.029″ and an air permeability of 69 cfm at 0.5″ water column. The NaCl particle filtration efficiency was 30%. This is an improvement over the uncoated stiff backer support substrate (approximately 9%). 
     9. Sample I 
     Sample I was prepared by forming a 11 gsm meltblown web from 0.7 micron polypropylene fibers (Exxon PP3546 G, ExxonMobil Chemical Company, Houston, Tex.) produced from a polymer heated to 425° F. at rate of 30 lbs of polymer/hr from a 70 holes per inch die blown by process air heated to 4400 F. at a flow rate of 4360 lbs/hr while quenching with 55° F. air at 490 lbs/hr. The meltblown material was collected on the bare collector belt moving at 37 ft/min with a vacuum pressure of approximately 20 inches of water through a 7 inch wide slot. The free standing nanofiber meltblown had a caliper of less than 0.003″, a basis weight of 11 gsm, air permeability of 66 cfm at 0.5″ water column. 
     The meltblown nanofiber was adhered to a stiff backer carded non-woven support layer formed from polymeric fibers with a basis weight of 107 gsm and an air permeability of 435 cfm at 0.5″ water. 
     The meltblown nanofiber was adhered to the stiff backer carded non-woven support by application of a hot melt glue (Bostik HM 4379 APO) spray at an areal weight of 4 g/m2 to the cellulose support and then immediately bonding the glue applied layer to the meltblown/nanofiber by contact pressure between a belt used to collect the meltblown fibers and a rubber roller. The resulting article was made into a filter having the stiff backer carded nonwoven support facing the inlet and the meltblown nanofiber layer facing the downstream side. 
     The filter media had a basis weight of 113 gsm, a caliper of 0.024″ and an air permeability of 57 cfm at 0.5″ water column. The NaCl particle filtration efficiency was 88%. This is an improvement over the uncoated cellulose substrate (approximately 20%). 
     10. Sample J 
     Sample J was prepared by forming a 24 gsm meltblown web from 0.5 micron PBT fibers (Ticona Celanex 2008) produced from a polymer heated to 530° F. at rate of 20 lbs of polymer/hr from a 35 holes per inch die blown by process air heated to 550° F. at a flow rate of 2600 lbs/hr. The meltblown material was collected on the bare collector belt moving at 30 ft/min with a vacuum pressure of approximately 20 inches of water through a 7 inch wide slot. The free standing nanofiber meltblown had a caliper of 0.008″, a basis weight of 24 gsm, air permeability of 79 cfm at 0.5″ water column. 
     The meltblown nanofiber was adhered to a support layer formed from a cellulose fiber containing 17% vinyl acetate resin 85% cellulosic fiber wet laid nonwoven with a basis weight of 165 gsm and an air permeability of 12 cfm at 0.5″ water. 
     The 4 layers of the meltblown nanofiber/spunbond was ultrasonically bonded to a cellulose support with the cellulose support positioned on the upstream side and the meltblown nanofiber scrim on the downstream side. 
     The filter media had a basis weight of 287 gsm, a caliper of 0.045″ and an air permeability of 7 cfm at 0.5″ water column. 
     11. Sample K 
     Sample K was prepared by forming a 10 gsm meltblown web from 2 micron polypropylene fibers (Exxon PP3546 G, ExxonMobil Chemical Company, Houston, Tex.) produced at a polymer heated to 500° F. at rate of 240 lbs of polymer/hr from a 35 holes per inch die blown by process air heated to 500° F. at a flow rate of 3250 lbs/hr while quenching with 55° F. air at 350 lbs/hr. The meltblown material was collected and instantaneously bonded onto a 10 gsm spunbond polypropylene scrim (Celestra from Fiberweb Corporation, Nashville, Tenn.) moving on a collector belt moving at 250 ft/min with a vacuum pressure of approximately 20 inches of water through a 7 inch wide slot. To this meltblown/scrim composite, a nanofiber layer was added by forming a 4 gsm meltblown web from 0.5 micron polypropylene fibers (Exxon PP3546 G, ExxonMobil Chemical Company, Houston, Tex.) produced at a polymer heated to 425° F. at rate of 20 lbs of polymer/hr from a 35 holes per inch die blown by process air heated to 450° F. at a flow rate of 3250 lbs/hr while quenching with 55° F. air at 350 lbs/hr. 
     The resulting three layer composite had a nanofiber meltblown on the top surface, a conventional meltblown structure underneath and a scrim on the bottom surface. The resulting composite had a basis weight of 25 gsm, a caliper of 0.012″ and an air permeability of 84 cfm at 0.5″ water column. The NaCl particle filtration efficiency was 88%. 
     12. Sample L 
     Sample L was prepared by forming a 5 gsm meltblown web from 0.4 micron polypropylene fibers (Exxon PP3546 G, ExxonMobil Chemical Company, Houston, Tex.) produced from a polymer heated to 450° F. at rate of 20 lbs of polymer/hr from a 35 holes per inch die blown by process air heated to 450° F. at a flow rate of 4360 lbs/hr while quenching with 55° F. air at 490 lbs/hr. The meltblown material was collected on the bare collector belt moving at 45 ft/min with a vacuum pressure of approximately 20 inches of water through a 7 inch wide slot. The free standing nanofiber meltblown had a caliper of less than 0.001″, a basis weight of 5 gsm, air permeability of 150 cfm at 0.5″ water column. 
     The meltblown nanofiber was adhered to a corrugated support layer formed from a cellulose fiber containing 20% vinyl acetate resin 80% cellulosic fiber wet laid nonwoven with a basis weight of 114 gsm and an air permeability of 16 cfm at 0.5″ water. The corrugated support layer had a corrugation channel width of 0.170″. The corrugation depth of the support layer was 0.022″ on the felt side to be coated, the opposite (Wire) side had a corrugation depth of 0.022″ as measured with the IAS Laser Corrugation Gauge. 
     The meltblown nanofiber was adhered to the cellulose support by application of a hot melt glue (Bostik M2751 adhesive) heated to 400° F. and sprayed at 410° F. at an areal weight of 6 g/m 2  to the cellulose support and then immediately bonding the glue applied layer to the meltblown/nanofiber cellulose by contact pressure between a stainless steel belt used to collect the meltblown fibers and a rubber roller at a nip pressure of 30 pounds per linear inch (PLI). The resulting article was made into a filter having the meltblown nanofiber facing the inlet and the cellulose support facing the downstream side. 
     The filter media had a basis weight of 125 gsm, a caliper of 0.026″ and an air permeability of 14 cfm at 0.5″ water column. The NaCl particle filtration efficiency was 62%. This is an improvement over the uncoated cellulose substrate (approximately 26%). This composite had a corrugation depth of 0.012″ on the meltblown nanofiber coated side and 0.016″ on the reversed, uncoated (wire) side. The mean peel strength of the nanofiber layer to the base substrate was 2.4 ounces per inch width. 
     13. Sample M 
     Sample M was prepared by forming a 5 gsm meltblown web from 0.4 micron polypropylene fibers (Exxon PP3546 G, ExxonMobil Chemical Company, Houston, Tex.) produced from a polymer heated to 450° F. at rate of 20 lbs of polymer/hr from a 35 holes per inch die blown by process air heated to 450° F. at a flow rate of 4360 lbs/hr while quenching with 55° F. air at 490 lbs/hr. The meltblown material was collected on the bare collector belt moving at 45 ft/min with a vacuum pressure of approximately 20 inches of water through a 7 inch wide slot. The free standing nanofiber meltblown had a caliper of less than 0.001″, a basis weight of 5 gsm, air permeability of 150 cfm at 0.5″ water column. 
     The meltblown nanofiber was adhered to a corrugated support layer formed from a cellulose fiber containing 20% vinyl acetate resin 80% cellulosic fiber wet laid nonwoven with a basis weight of 114 gsm and an air permeability of 16 cfm at 0.5″ water. The corrugated support layer had a corrugation channel width of 0.22″. The corrugation depth of the support layer was 0.022″ on the felt side to be coated, the opposite (Wire) side had a corrugation depth of 0.022″ as measured with the IAS Laser Corrugation Gauge. 
     The meltblown nanofiber was adhered to the cellulose support by application of a hot melt glue (Bostik M2751 adhesive) heated to 400° F. and sprayed at 410° F. at an areal weight of 6 g/m2 to the cellulose support and then immediately bonding the glue applied layer to the meltblown/nanofiber cellulose by contact pressure between a stainless steel belt used to collect the meltblown fibers and a rubber roller at a nip pressure of 30 pounds per linear inch (PLI). The article was made into a filter element having the meltblown nanofiber facing the inlet and the cellulose support facing the downstream side. 
     The filter media had a basis weight of 125 gsm, a caliper of 0.029″ and an air permeability of 14 cfm at 0.5″ water column. The NaCl particle filtration efficiency was 63%. This is an improvement over the uncoated cellulose substrate (approximately 26%). This composite had a corrugation depth of 0.016″ on the meltblown nanofiber coated side and 0.018″ on the reversed, uncoated (wire) side. The mean peel strength of the nanofiber layer to the base substrate was 2.0 ounces per inch width. 
     14. Sample N 
     Sample N was prepared by forming a 5 gsm meltblown web from 0.4 micron polypropylene fibers (Exxon PP3546 G, ExxonMobil Chemical Company, Houston, Tex.) produced from a polymer heated to 450° F. at rate of 20 lbs of polymer/hr from a 35 holes per inch die blown by process air heated to 450° F. at a flow rate of 4360 lbs/hr while quenching with 55° F. air at 490 lbs/hr. The meltblown material was collected on the bare collector belt moving at 45 ft/min with a vacuum pressure of approximately 20 inches of water through a 7 inch wide slot. The free standing nanofiber meltblown had a caliper of less than 0.001″, a basis weight of 5 gsm, air permeability of 150 cfm at 0.5″ water column. 
     The meltblown nanofiber was adhered to a corrugated support layer formed from a cellulose fiber containing 20% vinyl acetate resin 80% cellulosic fiber wet laid nonwoven with a basis weight of 122 gsm and an air permeability of 28 cfm at 0.5″ water. The corrugated support layer had a corrugation channel width of 0.170″. The corrugation depth of the support layer was 0.013″ on the felt side to be coated, the opposite (Wire) side had a corrugation depth of 0.013″ as measured with the IAS Laser Corrugation Gauge. 
     The meltblown nanofiber was adhered to the cellulose support by application of a hot melt glue (Bostik M2751 adhesive) heated to 400° F. and sprayed at 410° F. at an areal weight of 6 g/m2 to the cellulose support and then immediately bonding the glue applied layer to the meltblown/nanofiber cellulose by contact pressure between a stainless steel belt used to collect the meltblown fibers and a rubber roller at a nip pressure of 30 pounds per linear inch (PLI). The resulting article was made into a filter having the meltblown nanofiber facing the inlet and the cellulose support facing the downstream side. 
     The filter media had a basis weight of 134 gsm, a caliper of 0.021″ and an air permeability of 24 cfm at 0.5″ water column. The NaCl particle filtration efficiency was 62%. This is an improvement over the uncoated cellulose substrate (approximately 20%). This composite had a corrugation depth of 0.08″ on the meltblown nanofiber coated side and 0.011″ on the reversed, uncoated (wire) side. The mean peel strength of the nanofiber layer to the base substrate was 2 ounces per inch width 
     15. Sample O 
     Sample O was prepared by forming a 25 gsm meltblown web from 0.6 micron PBT fibers (Ticona JKX) produced from a polymer heated to 550° F. at rate of 80 lbs of polymer/hr from a 35 holes per inch die blown by process air heated to 575° F. at a flow rate of 2500 lbs/hr. The meltblown material was collected on the bare collector belt moving at 40 ft/min with a vacuum pressure of approximately 20 inches of water through a 7 inch wide slot. 
     The meltblown nanofiber was adhered to a support layer formed from a cellulose fiber containing 20% phenolic resin and 80% cellulosic fiber wet laid nonwoven with a basis weight of 200 gsm and an air permeability of 2 cfm at 0.5″ water. Overall thickness was 0.029″ and corrugation depth was 0.013″. 
     The meltblown nanofiber was adhered to the cellulose support by application of a hot melt glue (Bostik Vitel 4361B adhesive) heated to 450° F. and sprayed at 450° F. at an areal weight of 8 g/m2 to the cellulose support and then immediately bonding the glue applied layer to the meltblown/nanofiber cellulose by contact pressure between a stainless steel belt used to collect the meltblown fibers and a rubber roller at a nip pressure of 35 pounds per linear inch (PLI). The resulting structure was made into a filter media having the meltblown nanofiber facing the inlet and the cellulose support facing the downstream side. 
     The filter media had a basis weight of 233 gsm, a overall caliper of 0.024″ and an air permeability of 1.9 cfm at 0.5″ water column. This composite had a corrugation depth of 0.06″ on the meltblown nanofiber coated side and 0.010″ on the reversed, uncoated (wire) side. The mean peel strength of the nanofiber layer to the base substrate was 3.5 ounces per inch width. 
     16. Comparison Example 
     The comparison example 1 was prepared by forming a 5 gsm meltblown web from 0.7 micron polypropylene fibers (Exxon PP3546 G, ExxonMobil Chemical Company, Houston, Tex.) produced from a polymer heated to 425° F. at rate of 20 lbs of polymer/hr from a 35 holes per inch die blown by process air heated to 435° F. at a flow rate of 3900 lbs/hr while quenching with 55 F air at 520 lbs/hr. The meltblown material was collected on the bare collector belt moving at 60 ft/min with a vacuum pressure of approximately 20 inches of water through a 7 inch wide slot. The free standing nanofiber meltblown had a caliper of less than 0.001″, a basis weight of 5 gsm, air permeability of 172 cfm at 0.5″ water column. 
     The meltblown nanofiber was adhered to a corrugated support layer formed from a cellulose fiber containing 17% vinyl acetate resin and 83% cellulosic fiber wet laid nonwoven with a basis weight of 125 gsm and an air permeability of 32 cfm at 0.5″ water. The corrugated support layer had a corrugation channel width of 0.170″. The corrugation depth of the corrugated support layer was 0.015″ on the felt side to be coated, the opposite (Wire) side had a corrugation depth of 0.015″ as measured with the IAS Laser Corrugation Gauge. 
     The meltblown nanofiber was adhered to the cellulose support by application of a hot melt glue (Bostik 4379 PVA copolymer adhesive) spray at an areal weight of 3 g/m 2  to the cellulose support and then immediately bonding the glue applied layer to the meltblown/nanofiber cellulose by contact pressure between a belt used to collect the meltblown fibers and a rubber roller. The resulting article was made into a filter having the meltblown nanofiber facing the inlet and the cellulose support facing the downstream side. 
     The filter media had a basis weight of 134 gsm, a caliper of 0.027″ and an air permeability of 27 cfm at 0.5″ water column. The NaCl particle filtration efficiency was 50%. This is an improvement over the uncoated cellulose substrate (approximately 20%). This composite had a corrugation depth of less than 0.001″ on the meltblown nanofiber coated side and 0.015″ on the reversed, uncoated (wire) side. This media was used to construct an element with a pleat height of 1.13″, element height of 14.375″ and pleat count of 155 on a 3″ center tube (16.5 pleats per inch center tube ID). The dust holding capacity was reduced by 25% at a face velocity of 300 cfm per SAE J726 protocol in comparison to conventional media. Examining the filters and pleat packs it was found that the unbonded meltblown Nanofiber was blocking the filter inlets formed by the pleating knuckles adding air resistance. 
     This media was also used in a element with a pleat height of 0.88″, element height of 3.125″ and a pleat count of 200 around a 10″ tube (6.4 pleats per inch center tube ID). The element was tested per SAE J726 protocol using a face velocity of 65 cfm. With this less dense pleated construction (.about.6 pleats/in vs. 17 pleats/in), dust holding capacity was 11% higher than conventional media. 
     Examples L-O demonstrated that using the method disclosed herein results in a corrugated filter medium having superior corrugation properties, such as corrugation depth. 
     C. Discussion 
     The following discussion provides some general observations based on relevant data. The fiber diameters of 100 fibers at 1000× were measured using scanning electron microscopy (SEM). The fiber diameter was calculated (D, Log D, RMS D, D2/D) along with the geometric standard deviation to determine the distribution of fiber diameters. The average (log D) fiber diameters were used as a reference to characterization the different samples. The meltblown nanofibers were considerably finer than normal meltblown fibers, approaching that of electrospun, but with a significantly broader distribution (.about.2 GSTD meltblown nanofibers vs. &lt;1.3 for Electrospun nanofibers). 
     The meltblown nanofibers were considerably finer, but not as fine as the electrospun Nanofibers. 
     Looking at the cross-sectional areas, the meltblown nanofiber was shown to be quite different from the electrospun nanofiber. The electrospun nanofiber had a nanofiber layer thickness of one micron to four microns, whereas the meltblown nanofiber had a nanofiber layer thickness of 17 microns to 30 microns. 
     For industrial cleaning applications, application of meltblown nanofibers can allow the use of a more open base material (Samples B and E), which would lower restriction and extend useful filter life, while maintaining and slightly increasing efficiency of dust capture. Filter life of meltblown nanofiber coated media offers a substantial increase in operating time when compared to standard cellulose application grade (H&amp;V FA6176). 
     Samples E, F and G had greater dust holding capacity than the standard application base material (approximately 16% to 40% improvement over standard application grade material). It is notable that in the comparison of Samples F and G that Sample G had a lower applied weight of meltblown nanofiber and also less dust holding capacity, suggesting that the quantity of meltblown nanofibers plays a role in the total dust holding capacity of the composite. 
     The meltblown fibers seemed to create a better, more homogenous dust cake, and the dust cake itself was easier to remove with pulsing giving a unique form of surface and depth filtration. With an open, low density, meltblown structure with some depth (Samples B, D and E), dust was formed in open funnels that are easily removed. In comparison to electrospun nanofiber, which only has surface filtration characteristics, the cleanable dust capacity might be more limited. For standard application grade cellulose media, the pressure rise from the aging was prohibitively high and therefore can be interpreted as having practically no cleanability behavior after aging. Meltblown nanofiber coatings of Sample B, D and E have shown very good retention of cleanability behavior after ageing (more than 70% of initial). 
     It was clear that fine particle capture efficiency was greatly improved by the presence of the meltblown nanofibers, as was also true for the electrospun nanofibers over the standard application grade cellulose. The meltblown nanofiber had the unique property of added particle capacity which lessens the pressure rise across the filter with accumulation of fine particles and nearly doubles filter life in comparison to standard application grade cellulose. It is believed that electrospun nanofiber actually would decrease filter life due to collection of fine particles at the outermost surface when the nanofiber is applied upstream and at the cellulose/nanofiber interface when the nanofiber layer is positioned downstream, greatly increasing pressure drop. In the case of the meltblown nanofiber composite, the particles were collected also in the meltblown layer increasing fine particle capture capacity. 
     The same observations for the heavy duty air grades can be made as for the Auto Air grades. Differences in particle capture efficiency for the heavy duty air grades between the nanofiber coated grades to the standard application cellulose are less dramatic due its fine pore structure and lower permeability. However, this fine base pore structure becomes clogged with fine particles rapidly resulting in a rapid pressure rise, greatly limiting useful operating life. The application of meltblown nanofibers through improved particle collection greatly extends operating life by over 300%. 
     The capacity of the meltblown nanofiber grades greatly exceeded that of the standard application cellulose and the electrospun nanofiber coated. 
     It should be noted that a second standard application cellulose was compared to sample B due to the coating orientation of the cellulose base sheet being on the more open side (felt), whereas for auto air the coating would normally be on the wire side. Challenging the wire side (which has a finer pore structure) of any cellulose media with a dust stream will reduce capacity due to the presence of fine pores. Under these conditions, the meltblown nanofiber has a diminished effect on the fine particle capacity of the composite media. 
     According to the liquid filtration efficiency test, the nanofiber coated cellulose greatly improved filtration performance for cellulose media, whereas electrospun nanofiber gives only a temporary improvement in performance and relatively quickly loses its advantage due to degradation of the fine fiber structure. Particle capture efficiency at particle sizes of four microns and 10 microns are improved by the addition of meltblown nanofibers over conventional cellulose media. At a particle size of 25 microns, particle capture efficiency was not improved by the addition of meltblown nanofibers. The size particles that can be captured at 90% efficiency and 99% efficiency, respectively, is considerably finer for Sample J (5.5 microns and 8.1 microns, respectively) compared to cellulose (11.1 microns and 17.9 microns, respectively). The size particles that can be captured at 90% efficiency and 99% efficiency, respectively, is also considerably finer for Sample J (5.5 microns and 8.1 microns, respectively) compared to electrospun fiber (9.8 microns and 14.6 microns, respectively). 
     While certain embodiments have been described, other embodiments are possible. 
     As an example, while embodiments have been described in which a scrim is disposed between a substrate and a meltblown layer, in certain embodiments, the meltblown layer can be between the substrate and the scrim. 
     As another example, while embodiments have been described in which a filter medium includes three layers, a filter medium can optionally include more layers. In some embodiments, a filter medium may have more than one substrate, more than one intermediate layer (e.g., more than one adhesive, more than one scrim), and/or more than one meltblown layer. As an example, in some embodiments, a filter medium can include a meltblown layer with fibers having an average fiber diameter and a second meltblown layer with fibers having a different average fiber diameter. A filter medium can also include additional layers. 
     As a further example, while embodiments have been described in which a filter medium has one meltblown layer, a filter medium can optionally include more than one meltblown layer. In certain embodiments, a filter medium can include a meltblown layer disposed on a meltblown layer. 
     As an additional example, while certain methods have been described for making a filter medium, other methods may also be used. As an example, in some embodiments, the substrate can be formed of a bicomponent film (e.g., a relatively low melting point material and a relatively high melting point material), onto which the meltblown material is formed. Subsequently, the relatively low melting point material is heated to melt the material (e.g., via the heat from the meltblown material and/or via heating in an oven), followed by cooling (e.g., to room temperature) to provide a filter medium that includes the meltblown material directly bonded to the substrate. As another example, in some embodiments, the substrate can be formed of two layers with one layer being formed of the relatively low melting point material and the other layer being formed of the relatively high melting point material. In such embodiments, the meltblown material can be deposited on the relatively low melting point material. Subsequently, the relatively low melting point material is heated to melt the material (e.g., via the heat from the meltblown material and/or via heating in an oven), and cooling (e.g., to room temperature) provides a filter medium that includes the meltblown material directly bonded to the substrate. The materials (relatively low melting point material, relatively high melting point material) from which the substrate is formed can be any desired material with the appropriate melting characteristics. Typically, such materials are polymers. In some embodiments, the relatively low melting point material can be one of the adhesives described above (e.g., the substrate can be a film formed of a composite that includes the adhesive). Optionally, a scrim and/or additional other layers of material can be incorporated into the filter medium. In certain embodiments, the substrate can include one or more additional materials. 
     As a further example, while embodiments have been described in which a meltblown material is bonded with a substrate via chemical bonding using an adhesive or mechanical bonding using ultrasonic bonding or melting/cooling, in some embodiments, other types of mechanical bonding can be used. Examples include stitching, sewing, hydroentangling and needling. In some methods, such as needling and hydroentangling, the meltblown material can become intermingled with another layer (e.g., the substrate). 
     As yet another example, while embodiments have been described in which meltblown material is used, additionally or alternatively, other materials can be used. More generally, without limitation to the material used or the process of forming the fibers, a material with fibers having an average diameter 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, can be used in what is described above as the meltblown layer. In some embodiments, the material is formed using melt processing (e.g., a meltblown process, spun bond, extrusion and blown film extrusion). In some embodiments, the small average diameter material can be formed by other methods. As an example, the small average diameter material can be made by taking a fiber of a relatively large diameter and stretching it to form the small average diameter material. Other methods include the “islands in the sea” and “segmented pie” methods of forming fibers, such as described in U.S. Pat. Nos. 5,783,503; 5,935,883; and 6,858,057, which are hereby incorporated by reference only to the extent that they are consistent with the remainder of the disclosure herein. In some embodiments, the material is non-polymeric (e.g., a glass, a ceramic). For example, the material can be a wet laid glass. In some embodiments, the substrate can be formed of a wetlaid glass with a relatively large average diameter (e.g., at most two microns, at most three microns, three microns to four microns), and, rather than a meltblown layer, the filter can include a layer of wetlaid glass fiber with a relatively small average diameter (e.g., at most 1.5 microns), where the filter medium may or may not include an adhesive material. 
     As a further example, although embodiments have been described in which the filter medium is corrugated and/or pleated, more generally, the filter medium can be shaped in any of a variety of desirable fashions. Such shapes are generally know in the art. Examples of shapes include dimpled, fluted, embossed and glue bead separated, bag structure or tubular structure. 
     Other embodiments are in the claims.