EXTRUDED THERMOPLASTIC FOAMS USEFUL IN FILTRATION APPLICATIONS

Abstract: A filter element made to fit within a filter element housing, the filter element comprising an extruded open-cell thermoplastic foam having a stucture substantially of cell walls and cell struts, an overall open-cell content of 50 percent or more, an average cell size of 1.5 millimeters or less, and a permeability coefficient of 6 x 10<-13> m<2> or more, and, optionally, retaining means for the foam, and a method of filtration comprising passing a liquid or gas through the filter element.

Description: EXTRUDED THERMOPLASTIC FOAMS USEFUL IN FILTRATION APPLICATIONS The present invention relates to a method of filtration of a gas or liquid by continuously passing either or both of them through extruded, open cell thermoplastic foams which exhibit exceptional permeability. The prior art relates various open cell foams which can be employed in filtration applications. They include both thermoset and thermoplastic foams. Disclosed thermoset foams include those described in U.S. Patent Nos. 3,873,281 ; 4,212,737; 4,303,533; and 4,099,943. Disclosed extruded thermoplastic foams include those described in U.S. Patent Nos. 4,384,032 and 4,427,548 and Great Britain 1 ,100,727. Disclosed thermoplastic emulsion polymer foams include those described in "Polymeric Foam Materials on Filtration Media," Filtration and Separation, Z. Blumgara, March 1995, pages 245-251 . Open cell foams have not been widely employed commercially in filtration applications due to deficiencies in performance compared to other materials, most notably nonwoven fibrous materials. The nature of these performance deficiencies vary according to the structure of the foam. Open cell thermoset foams are used commercially in certain filtration applications but have significant performance limitations. Conventional open cell thermoset foams have reticulated cellular structures (primarily of cell struts without cell walls) and can be manufactured in average cell size ranges above 120 micrometers. A foam with cell struts but without cell walls is illustrated at Handbook of Polymeric Foams and Foam Technology, by Klempner and Frisch, Hanser Publishers © (1991 ), page 23, Figure 2a. Commercial foams of this type exhibit relatively high arrestance (70-90 percent) but low efficiency (<20 percent) for particle sizes of 0.3-0.5 micrometers. These foams efficiently filter large particles but do not efficiently filter small particles. These foams are usually recommended for use as air filters for large particles or rocks in lawn or outdoor equipment or as pre- filters. There are commercially available microcellular urethane foams made via leaching of soluble salts. Pore sizes are advertised as being as low as 27 microns, but porosity is only up to 80 percent. The limited porosity results in limited permeability and possibly lower life capacity. Open cell emulsion polymer foams have not found significant commercial use in filtration applications. Low density open cell emulsion polymer foams typically have reticulated (primarily cell struts without cell walls) cellular structures and can be manufactured to very small cell sizes of 10 micrometers or less. The open nature of the reticulated cellular structure limits potential filtration efficiency and capacity since small particles can easily pass through it. They are also usually mechanically fragile and subject to flexure and/or compression when a pressure drop is applied across it. Open cell extruded polymer foams have not found significant commercial use in filtration applications. Open cell extruded thermoplastic foams typically have a relatively rigid cellular structure defined substantially by both cell struts and cell walls. Prior art extruded foams disclosed as useful in filtration applications have exhibited higher pressure drops than porous, non-woven fibrous materials of equivalent efficiency due to their relatively low permeability. Prior art foams, even those of relatively high open cell content, that is 90-100 percent open cell, typically have relatively small pores within their cell walls and limited pore incidence level throughout. Thus, they exhibit relatively low permeability. The low permeability translates into high pressure drop for a given foam thickness. It would be desirable to have an extruded, open-cell thermoplastic foam which has a relatively rigid cellular structure defined substantially by both cell walls and cell struts and which exhibits relatively high permeability for liquids and gases. It would also be desirable to have a foam which exhibits low pressure drop for a given desired efficiency, capacity, and/or foam thickness. It would also be desirable to have a foam which exhibits improved efficiency in filtering small particles of 0.3-0.5 micrometers. This invention provides a filter element made to fit within a filter element housing, the filter element comprising an extruded open-cell thermoplastic foam having a structure substantially of cell walls and cell struts, an overall open-cell content of 50 percent or more, an average cell size of 1.5 millimeters or less, and a permeability coefficient of 6χ10 '13 m 2 or more, and, optionally, retaining means for the foam. This invention provides a method of filtration comprising passing a liquid or gas through a filter element made to fit within a filter element housing, the filter element comprising an extruded open-cell thermoplastic foam having a structure substantially of cell walls and cell struts, an overall open-cell content of 50 percent or more, an average cell size of 1.5 millimeters or less, and a permeability of 6x10 '13 m 2 or more, and, optionally, retaining means for the foam. Also provided by this invention is a method of filtration comprising passing a liquid or gas through an extruded, open-cell thermoplastic foam having a structure substantially of cell walls and cell struts, an overall open-cell content of 50 percent or more, an average cell size of 1 .5 millimeters or less, and a permeability of 6x10 "13 m 2 or more. Figure 1 shows a foam filter useful in the present invention. The filter has a cylindrical shape. Figure 2 shows another embodiment of a foam filter useful in the present invention. The filter is in the form of a pleated foam sheet. Figure 3 shows another embodiment of a foam filter for liquids or gases useful in the present invention. The filter is partially perforated. This invention provides a filter element made to fit within a filter element housing. Filter element housings are well known in the art in various applications involving the filtration of fluids such as gases and liquids. The filter element of this invention may be substially the foam itself, or the filter element may employ retaining means to hold the foam. The retaining means may be any material know in the art including plastics, metals, wood, paper, etc. The filter element and the method of filtration of the present invention employ certain extruded, open-cell thermoplastic foams which exhibit outstanding filtration performance for both liquids and gases. The foams exhibit a greater permeability than would be expected for foams having a substantially cell strut/cell wail structure. The enhanced permeability enables such foams to be efficacious in conventional commercial, residential, and industrial filtration applications. The extruded open cell foams exhibit permeability coefficients for air of 6 x 10 '13 m 2 or more, or 1.44 x 10 "12 m 2 or more, desirably 3 x 10 '12 m 2 or more, more desirably 4.28 x 10 "12 m 2 or more, and preferably 8 x 10 "12 m 2 or more, more preferalby 1 .6 x 10 "11 m 2 or more, and still more preferalbly 1.6 x 10 '11 m 2 or more. Permeability is determined according to the following formula: AP where A = area (square meters (m 2 )) Q = volumetric flow rate of air (cubic meters/second (m 3 /sec)) P = differential pressure (Pascals (Pa)) L = thickness of the foam (meters (m)) μ = kinematic viscosity of air (Newton - seconds/square meter (Ns/ m 2 )) K Θ = permeability coefficient (square meters (m 2 )) The formula is described in Handbook of Filter Media by Derek B. Purchas, 1 sl Edition, Elsevier Science Ltd. © 1996, pages 488-89. In the description herein involving numerical values for permeability, it should be understood that occasionally the trerms "permeability" and "permeability coefficient" are used interchangeably. Although not bound by any particular theory, extruded foams useful in the present invention are believed to derive their enhanced permeability from their unique foam structure. The foams have an open cellular structure having substantially both cell walls and cell struts wherein the cellular structure defines any or a combination of the following: enhanced (greater) incidence of pores within cell walls, enhanced (larger) pores sizes within the cell walls, enhanced (greater) proportion of cell walls generally vertical and horizontal to the extrusion direction having pores therein, and a minor proportion of cell walls missing or substantially missing. Prior art extruded filtration foams lack these structural features to the extent required to achieve the permeabilities disclosed herein for the foams employed in the present method. The enhanced permeability affords improved design flexibility with respect to efficiency, capacity, pressure drop, and foam thickness. Extruded foams useful in the present invention afford improved performance advantages over reticulated prior art foams. The substantially cell wall/cell strut cellular structure of the foam of the present method provides a relatively tortuous pore pathway for the liquid or gas to traverse and provides substantial pockets, crevices, and surface area into which particles can lodge themselves. The cellular structure affords improved design flexibility with respect to efficiency, capacity, pressure drop, and foam thickness compared to reticulated foams. The foam has an open ceil content of 50 percent or more, preferably 70 percent or more, more preferably 90 percent or more, and most preferably 95 percent or more according to ASTM D2856-A. Permeability can increase as open cell content increases. The present invention is efficacious at a wide range of foam cell sizes. The foam desirably has an average cell size of 1.5 millimeters or less, more desirably, an average cell size of 1 millimeter or less, preferably, an average cell size of 0.1 millimeter or less, and more preferably 0.01 to 1.0 millimeters (10 to 1000 micrometers) according to ASTM D3576-77. One useful foam embodiment has an average cell size of 0.2 to 0.7 millimeters (200 to 700 micrometers) according to ASTM D3576-77. Another useful foam embodiment has an average cell size of 0.01 to 0.07 millimeters (10 to 70 micrometers) according to ASTM D3576-77. The foam can have a mean flow pore diameter (m.f.p.d.) of from 0.1 to 50 micrometers. Desired levels of m.f.p.d. will vary depending on the sizes of microparticles to be filtered, the medium to be filtered (liquid or gas or both), desired efficiency levels, and desired pressure drop levels. Useful m.p.f.d. ranges include 0.1 micrometers or more, 1 micrometer or more, 5 micrometers or more, 10 micrometers or more, and 15 micrometers or more. Average cell size and m.p.f.d differ in that average cell size relates to average cell dimension in the foam and m.p.f.d relates to the mean pore size at which one half of total air flow occurs through pores greater in size than the mean pore size and half through pores less in size than the mean pore size. The m.f.p.d. may be determined using an automated porometer, such as the Perm Porometer 200 PSI by PMI (Porous Materials, inc.), with a perfluoro compound C5-18 fluid such as Fluoroinert FC-40 (Sigma Chemical Corp.). Preferred ranges for alternative embodiments include m.f.p.d. ranges of from 3 to 8 micrometers, from 8 to 20 micrometers and from 20 to 50 micrometers. Foams which exhibit a wide range of efficiencies are possible in the present invention. A particular efficiency level or range may depend upon the desired commercial application. Typically, efficiencies will range from 40 to substantially 100 percent and any point in between for air streams. Efficiency can be determined by one of two methods single-pass challenge tests. In ASTM 2986-71 a monodisperse aerosol of 0.3 micron DOP particles is generated via an aerosol generator. The aerosol is drawn through the specimen at 10 fpm (5 m/sec). The penetration is measured through the aid of a particle counter on each side of the filter. The penetration P is defined as: P = (Particle cone, downstream / Particle Cone, upstream) Efficiency, E, defined as one minus the penetration, or E = 1 - P A second method involves the generation of a polydisperse aerosol concentration and measuring the particle concentration via a size-discriminating detector. Penetration and efficiency are measured in the same manner described above. The test method used for the data in this case utilized a polydisperse aerosol of NaCI and KCI solutions. The particles were generated via an AT300 generator from TSI Incorporated. An HIAC/Royco 5109 particle counter was used to determine the efficiency. Efficiencies were measured over a range of face velocities. The face velocity is the volumetric flow rate divided by the cross-sectional are of the filter media. Values reported are performed at a phase velocity of 10 feet per minute. Desirable efficienies for the foams for use in this invention include 40 percent or more for particle sizes of 0.3 micrometers, 90 percent or more for particle sizes of 0.3 micrometers, 99 percent or more for particle sizes of 0.3 micrometers. A preferred efficieny for high efficiency filters is 99.991 percent or more for particle sizes of 0.3 micrometers. A preferred range of efficiencies for one embodiment is from 40 to 70 percent or more for particle sizes of from 0.3 to 0.5 micrometers. The foam has a density of preferably from 16 to 250 kilograms per cubic meter (kg/m 3 ) and more preferably from 25 to 100 kg/m 3 according to ASTM D- 1622-88. One useful embodiment of the invention employs an extruded foam having a structure substantially of cell walls and cell struts, an overall open-cell content of 50 percent or more (preferably 90 percent or more), and an average cell size of 0.1 millimeters (100 micrometers) or less. Foams of such cell sizes are useful because they permit gases and liquids to be filtered for particles of sizes too small for known extruded foam filters. Foaming with small cell sizes induces formation of thinner cell walls, which can have pores more easily formed therein with respect to both incidence level and pore size. Use of such a foam as a filter is heretofore unknown in the art. Another useful embodiment the invention employs an extruded foam have a structure substantially of cell walls and cell struts, an overall open-cell content of 50 percent or more, an average cell size of 0.1 millimeters (100 micrometers) or more, and a mean flow pore diameter of 10 micrometers or more. Foams of such cell sizes are useful because they permit gases and liquids to be filtered for particles of size 0.3 to 0.5 micrometers at moderate efficiency levels for air (40 to 70 percent) with a permeability coefficient of 1 .6 x 10 "11 m 2 or more for air. Use of such a foam is heretofore unknown in the filtration art. Additional foam embodiments useful in the present method are set forth below in Table 1. As is apparent, the foams may have ranges of desirable physical property and performance parameters. The foams of #1 generally correspond in performance to those of HEPA non-woven fiber filters. The foams of #2 generally correspond in performance to those of non-woven fiber filters typically used in applications requiring 98 percent ASHRAE filters. The foams of #3 generally correspond in performance to those of non-woven fiber filters typically used in applications requiring 60-70 percent ASHRAE filters. Table 1 Additional Foam Embodiments -M.F.P.D. = Mean Flow Pore Diameter -m 2 = meters squared -efficiency is determined according to ASTM D2986-71 for air at 10 feet/minutes (0.05 meters/second) phase velocity Photomicrographs of the cellular structure of foams useful in the present invention include those shown in Figures 1 -5 of U.S. Serial No. 09/096,029, filed June 1 1 , 1998. The foam may take any physical configuration known in the art such as sheet, plank, pleat, tube, rod, or cylinder. Desirable sheet foams include those less than 0.375 inch in thickness in cross-section. Desirable plank foams include those having in cross-section thickness of 0.375 inch (0.95 cm) or more. Useful sheet foams can be made by skiving or slicing extruded plank foams by knife or hot (heated) wire into two or more plies or sheets. Desirably, the skin layers of the foam are sliced, skived, or planed off or otherwise removed to better expose the open cell structure of the foam. The present method is useful in a variety of conventional residential, industrial, and agricultural liquid/gas filter applications. Liquid filter applications include water filters, industrial and agricultural waste/effluent and/or recycle filters, chemical filters, oil filters, and backflushing filters. Suitable gas filtration applications include air intakes in furnaces and air conditioners, industrial air circulation and blower systems, residential air cleaners, dust collection and removal systems, fume and vapor emission control systems, compressed air systems, and respiratory masks. A useful industrial application is a sheet foam formed into a flexible bag for use as a bag house filter. The foam is also useful as a pre-filter upstream of other filter materials. Figure 1 shows a cylindrical filter 10 for gases or liquids. Cylinder 10 defines a channel 12 extending from one end of filter 10 to the other. As shown, a liquid or gas to be filtered may enter cylinder 10 under pressure through an orifice 14. A liquid or gas may also enter cylinder 10 under pressure through orifice 16, or, alternately, orifice 16 may be blocked off in favor of orifice 14. Upon entering channel 12, the liquid or gas is forced radially through a cylindrical-shaped foam media 18 to effect filtration. Cylindrical filters such as the one shown in Figure 1 are typically employed commercially in cylindrical cartridges for residential, industrial, and agricultural applications. One embodiment of such a cylindrical filter has an outside diameter (foam media diameter) of approximately 3 inches (7.62 centimeters) and an inside diameter (channel diameter) of approximately 1 inch (2.54 centimeters). Figure 2 shows a pleated filter 20 having a sheet foam media 22 which is folded continuously along its length to form pleats 24 therein. Gas or liquid is forced through filter 20 under pressure to effect filtration. Another embodiment of a pleated filter is a filter having pleats within pleats (not shown). In other words, the filter would have relatively larger primary pleats and relatively smaller secondary pleats within the primary pleats. Use of a pleated configuration allows the gas or liquid to be exposed to a greater surface and volume of foam media. Pleated filters are commonly employed in home and industrial furnace air intakes. Figure 3 shows a filter 30 having a foam media 32 which has a plurality or multiplicity of perforations 34 extending partially through it from a face or surface 36 opposite to the gas or liquid stream. The presence of such perforations is to enable pressure drop across the foam media to be reduced yet provide much of the advantage of the efficiency and capacity afforded by a thicker foam. Alternately, a foam (not shown) may have perforations extending partially through it from a face or surface which is directional to the gas or liquid stream. In that instance, the perforations effectively increase the surface area to which the gas or liquid stream is exposed, which can increase filtration efficiency and capacity. Filtration may take place through the extruded foam in the extrusion direction or transverse (vertical or horizontal) direction or in a combination of the three directions. If desired, the foam can be manufactured and/or fabricated to have a gradient through it wherein the m.f.p.d. of the cellular structure of the foam changes from one surface of the filter to the opposing surface. For instance, the foam can have a gradient wherein the m.f.p.d. changes from relatively high to relatively low or vice versa. The liquid or gas can be passed from the region of high m.f.p.d. to the region of low m.f.p.d. or vice versa depending upon the desired function and performance of the filter. Typically, liquid or gas will be passed from a region of high m.f.p.d. to a region of low m.f.p.d. so that larger particles will be filtered out first and be less likely to clog small pores; smaller particles are then subsequently filtered in the region of low m.f.p.d. A preferred method of making a foam having a m.f.p.d. gradient through it is to extrude a plank foam of 10 millimeters thickness or more in cross-section. Thick extruded foams will typically exhibit a cellular structure which varies in physical properties such as density, cell size, and pore size depending upon position in the cross-section. Density typically increases from the middle region to a skin region. Cell size and pore size typically decrease from the middle region to a skin region. An extruded foam can be cut or sliced at a given region of the foam to yield a foam portion of a desired gradient cellular structure. If desired, filtration efficiency and capacity can be enhanced by rendering the foams electrostatic by any means know in the art. Electrostatic agents can be incorporated into the polymer resin during manufacture of the foam or be deposited on the internal surfaces of the foam. The internal surfaces of the foam may also be exposed to corona discharge or plasma treatment. The foam may also be exposed to an electrostatic field when filtration is occurring. Regardless of the method of treatment, the entire foam may be treated or merely one or more surfaces or regions thereof. Extruded thermoplastic foams are generally prepared by heating a thermoplastic material to form a piasticized or melt polymer material, incorporating therein a blowing agent to form a foamable gel, and extruding the gel through a die to form the foam product. Prior to mixing with the blowing agent, the polymer material is heated to a temperature at or above its glass transition temperature or melting point. The blowing agent may be incorporated or mixed into the melt polymer material by any means known in the art such as with an extruder, mixer, blender, or the like. The blowing agent is mixed with the melt polymer material at an elevated pressure sufficient to prevent substantial expansion of the melt polymer material and to generally disperse the blowing agent homogeneously therein. Optionally nucleating agent may be blended in the polymer melt or dry blended with the polymer material prior to plasticizing or melting. The foamable gel is typically cooled to a lower temperature to optimize or attain desired physical characteristics of the foam. The gel may be cooled in the extruder or other mixing device or in separate coolers. The gel is then extruded or conveyed through a die of desired shape to a zone of reduced or lower pressure to form the foam. The zone of lower pressure is at a pressure lower than that in which the foamable gel is maintained prior to extrusion through the die. The lower pressure may be superatmospheric or subatmospheric (vacuum), but is preferably at an atmospheric level. As the extrudate exits and expands from the die, the foam is optionally elongated by mechanical means to assist in pore formation and open cell formation. Elongation is discussed below. To assist in extruding open-cell thermoplastic foams, it may be advantageous to employ a polymer different than the predominant polymer employed in the thermoplastic material. Employing a minor amount of a polymer different than the predominate polymer enhances open cell content development. For example, in making a polystyrene foam, minor amounts of polyethylene or ethylene/vinyl acetate copolymer may be employed. In making a polyethylene foam, minor amounts of polystyrene may be employed. Useful teachings to preferred different polymers are seen in U.S. Serial No. 08/880,954. Extruded open- cell thermoplastic foams may be made according to the general extrusion teachings herein and optionally may be exposed to the additional processing step of elongating the extrudate as it exits and expands from the extrusion die to form the foam. Elongation can increase the relative proportion of cell walls having pores therein and/or increase the average size of existing pores. Extensive teachings to elongation are seen in U.S. Serial No. 60/049181 , filed June 1 1 , 1997, and U. S. Serial No. 09/096,029, filed June 1 1 , 1998. The foam may be formed of any thermoplastic which can be formed or blown into an open cell foam of sufficient permeability. Useful thermoplastics may include natural and synthetic organic polymers. Suitable plastics may include poiyolefins, polyvinylchloride, alkenyl aromatic polymers, cellulosic polymers, polycarbonates, starch-based polymers, polyetherimides, polyamides, polyesters, polyvinylidene chlorides, polymethylmethacrylates, copolymer/polymer blends, and rubber modified polymers. Suitable alkenyl aromatic polymers include polystyrene and copolymers of styrene and other copolymerizable monomers. Some very desirable embodiments are those in which the thermoplastic material compromises in polymerized form 50 percent or more by weight alkenyl aromatic monomeric units, or the thermoplastic material compromises in polymerized form 50 percent or more by weight ethylenic monomeric units, or the thermoplastic material compromises in polymerized form 50 percent or more by weight propyleneic monomeric units, or the thermoplastic material compromises in polymerized form 50 percent or more by weight ester monomeric units. If desired, the foam can be blown from a thermoplastic material which is partially or substantially biodegradable. Useful polymers include cellulosic polymers, starch-based polymers, and blends of starch-based polymers and phenoxy ether polymers. A particularly useful thermoplastic foam comprises an alkenyl aromatic polymer material. Suitable alkenyl aromatic polymer materials include alkenyl aromatic homopoiymers and copolymers of alkenyl aromatic compounds and copolymerizable ethylenically unsaturated comonomers. The alkenyl aromatic polymer material may further include minor proportions of non-alkenyl aromatic polymers. The alkenyl aromatic polymer material may be comprised solely of one or more alkenyl aromatic homopoiymers, one or more alkenyl aromatic copolymers, a blend of one or more of each of alkenyl aromatic homopoiymers and copolymers, or blends of any of the foregoing with a non-alkenyl aromatic polymer. Regardless of composition, the alkenyl aromatic polymer material comprises greater than 50 and preferably greater than 70 weight percent alkenyl aromatic monomeric units. Most preferably, the alkenyl aromatic polymer material is comprised entirely of alkenyl aromatic monomeric units. Suitable alkenyl aromatic polymers include those derived from alkenyl aromatic compounds such as styrene, alphamethylstyrene, ethylstyrene, vinyl benzene, vinyl toluene, chlorostyrene, and bromostyrene. A preferred alkenyl aromatic polymer is polystyrene. Minor amounts of monoethylenically unsaturated compounds such as C2-6 a|k y! acids and esters, ionomeric derivatives, and C4.6 dienes may be copolymerized with alkenyl aromatic compounds. Examples of copolymerizable compounds include acrylic acid, methacrylic acid, ethacrylic acid, maleic acid, itaconic acid, acrylonitrile, maleic anhydride, methyl acrylate, ethyl acrylate, isobutyl acrylate, n-butyl acrylate, methyl methacrylate, vinyl acetate and butadiene. Useful foams can comprise substantially (that is greater than 95 percent by weight) and most preferably entirely of polystyrene. Useful foams can comprise polystyrene of 125,000 to 300,000 weight average molecular weight, 135,000 to 200,000 weight average molecular weight, 165,000 to 200,000 weight average molecular weight, and 135,000 to 165,000 weight average molecular weight according to size exclusion chromatography. Useful extruded thermoplastic foams include extruded microcellular alkenyl aromatic polymer foams of high open cell content and processes for making as disclosed in WO 96/34038. The disclosed foams have an average cell size of 70 micrometers or less and an open cell content of 70 percent or more. In the process disclosed in WO 96/34038, useful blowing agents include 1 ,1 - difluoroethane (HFC-152a), 1 ,1 ,1 -trifluoroethane (HFC-143a), 1 ,1 ,1 ,2- tetrafluoroethane (HFC-134a), chlorodifluoromethane (HCFC-22), carbon dioxide (CO2), and difluoromethane (HFC-32). Preferred blowing agents are HFC-152a, HFC-134a, and carbon dioxide. The above blowing agents will comprise 50 mole percent or more and preferably 70 percent or more of the total number of moles of blowing agent. The balance may be made up of other blowing agents. The amount of blowing agent employed is from 0.06 to 0.17 gram-moles per 100 grams of polymer, preferably from 0.08 to 0.14 gram-moles per 100 grams of polymer, and most preferably from 0.08-0.12 gram-moles per 100 grams of polymer. The use of a relatively small amount of blowing agent allows formation of a foam with a high open ceil content. Preferred foaming temperatures will vary from 1 18°C to 160°C. Most preferred foaming temperatures will vary from 1 18°C to 130°C, especially for larger size foam production lines. Smaller lines will run hotter. The amount of nucleating agent employed may range from 0.01 to 5 parts by weight per hundred parts by weight of a polymer resin. The preferred range is from 0.1 to 3 parts by weight. Another extruded alkenyl aromatic foam of larger average cell size and processes for making are seen in WO 96/00258. Open-cell content is 30 percent or more according to ASTM D2856-87. The disclosed foams have a density of 1.5 pcf to 6.0 pcf (24 kg/m 3 to 96 kg/m 3 ) and preferably a density of 1 .8 pcf to 3.5 pcf (32 kg/m 3 to 48 kg/m 3 ) according to ASTM D-1622-88. The present foam has an average cell size of from 0.08 millimeters (mm) to 1 .2 mm and preferably from 0.10 mm to 0.9 mm according to ASTM D3576-77. In the process for making the foam in WO 96/00258, the foaming temperature, which is relatively higher than that for making closed-cell foams (less than 10 percent open-cell according to ASTM D2856-87), may vary from 1 18°C to 145°C. Foaming temperature will vary according to nucleating agent composition and concentration, blowing agent composition and concentration, polymer material characteristics, and extrusion die design. The foaming temperature for the present open-cell foam varies from 3°C to 15°C and preferably 10°C to 15°C higher than the highest foaming temperature for a corresponding closed-cell foam (less than 10 percent open-cell according to ASTM D2856-87) of substantially equivalent density and cell size made with a substantially equivalent composition (including polymer material, nucleating agent, additives, and blowing agent) in a substantially equivalent process. A preferred foaming temperature is at 33°C or more higher than the glass transition temperature (according to ASTM D-3418) of the alkenyl aromatic polymer material. A most preferred foaming temperature is from 135°C to 140°C. The amount of blowing agent incorporated into the polymer melt material to make a foam-forming gel is from 0.2 to 5.0 gram-moles per kilogram of polymer, preferably from 0.5 to 3.0 gram-moles per kilogram of polymer, and most preferably from 0.7 to 2.0 gram-moles per kilogram of polymer. A nucleating agent such as those described above may be employed. To make foams of the physical properties described in WO 96/00258 which have the pore size and pore incidence level to be effective in the present invention, it may be necessary to incorporate different polymers into the alkenyl aromatic polymer material such as polyolefins of melting temperatures of 70°C or less, ethylene/styrene interpolymers, and styrene/butadiene copolymers or other rubbery homopoiymers or copolymers. Useful extruded, open cell thermoplastic foams include those made of styrene/ethylene interpolymers and blends of such interpolymers with alkenyl aromatic polymers and ethylene polymers described in U.S. Patent No. 5,460,818, WO 96/14233, and U.S. Serial No. 60/078091 , filed March 16, 1998. Such interpolymers are particularly useful in making foams having an average cell size of greater than 100 micrometers. In making extruded foams, other additives may be incorporated such as inorganic fillers, pigments, antioxidants, acid scavengers, ultraviolet absorbers, flame retardants, processing aids, and extrusion aids. The following are examples of the present invention, and are not to be construed as limiting. Unless otherwise indicated, all percentages, parts, or proportions are by weight. EXAMPLES Various extruded thermoplastic foams useful in the present invention were prepared and tested for filtration capability. The foams were prepared in two apparatuses each comprising an extruder, a mixer, a cooler, and an extrusion die in series. Polymer and additives were melted and mixed in the extruder and conveyed to the mixer in the form of a polymer melt. Blowing agent was incorporated in the melt in the mixer to form a polymer gel. The gel was conveyed through the cooler to cool it to a desirable foaming temperature. The gel was then conveyed through an orifice in the extrusion die into atmospheric pressure to expand to form the foam. Forming plates were positioned at the exit of the die to restrict expansion in the vertical direction and facilitate expansion in the horizontal and extrusion directions. A polystyrene resin of 135,000 weight average molecular weight according to size exclusion chromatography was employed. The other polymer employed was an ethylene/styrene interpolymer (ESI) of 69 percent styrene monomeric content by weight, less than 5 percent styrene homopolymer (polystyrene), and the remainder ethylene monomeric content by weight. Additives included talc as a nucleating agent, ethylene/vinyl acetate copolymer (EVA) to aid in open cell formation, and calcium stearate (CaSt) as an extruder lubricant. Polymers, additives, and blowing agents employed as well as certain process conditions are set forth in Table 2. Foam and filtration properties are set forth in Table 3. The density of the foam was measured according to ASTM D- 1622-88. ASTM D2856-A was used to determine the open cell content of the foam and cell size was determined according to ASTMD3576-77. The mean flow pore diameter (m.f.p.d.) was determined by using an Automated Perm Porometer 200 PSI from Porous Materials, Inc. (PMI) with Fluoroinert FC-40 compound. The test procedure and equipment are discussed in PMI literature as well as papers presented at INDA-TEC 1996 and FILTRATION 97 (page 12.0). In this test, the mean flow pore diameter is interpreted as the pore size at which half of the total air flow through a sample occurs through pores larger than the mean and half of the air flow occurs through pores smaller than the mean. The test procedure and equipment used to determine the efficiency and pressure drop across the foam filter are described in Filtration and Separation, March 1998 pages 1 18-122. The open cell foams used as filters in this test were 24" by 24 " and ranged in thickness froml .52 to 0.16" in thickness. Sampling time for the tests was 60 seconds. A polydisperse aerosol of KCI was used as the test dust. Efficiencies and pressure drops were measured at media face velocities of 10, 20 30 and 40 feet per minute. Table 2 Table 3 -kPa = Kilopascals -m2 = Meters squared -kg/m3 = Kilograms per Cubic Meter -Example 2a was taken from the core of the foam of Example 2 of Table 2 and Example 2b from closer to the skin of that foam -efficiency was based on 0.3 - 0.5 micron particles at a filter face velocity of 10 or 20 feet per minute (see table) (.05 meters/second) While embodiments of the foam and the methods of the present invention have been shown with regard to specific details, it will be appreciated that depending upon the manufacturing process and the manufacturer's desires, the present invention may be modified by various changes while still being fairly within the scope of the novel teachings and principles herein set forth.

Claims: WHAT IS CLAIMED IS: 1 . A filter element made to fit within a filter element housing, the filter element comprising an extruded open-cell thermoplastic foam having a structure substantially of cell walls and cell struts, an overall open-cell content of about 50 percent or more, an average cell size of about 1 .5 millimeters or less, and a permeability coefficient of about 6x10 '13 m 2 or more, and, optionally, retaining means for the foam. 2. The filter element of Claim 1 , wherein the foam has a permeability coefficient of about 1 .44x10 "12 m 2 or more. 3. The filter element of Claim 2, wherein the foam has a permeability coefficient of about 3x10 "12 m 2 or more. 4. The filter element of Claim 3, wherein the foam has a permeability coefficient of about 4.28x10 "12 m 2 or more. 5. The filter element of Claim 4, wherein the foam has a permeability coefficient of about 8x10 "12 m 2 or more. 6. The filter element of Claim 5, wherein the foam has a permeability coefficient of about 1 .6x10 "11 m 2 or more. 7. The filter element of Claim 6, wherein the foam has a permeability coefficient of about 4.18x10 '11 m 2 or more. 8. The filter element of one of Claims 1 -7, wherein the foam has a mean flow pore diameter of about 0.1 micrometers to about 50 micrometers. 9. The filter element of one of Claims 1 -7, wherein the foam has a mean flow pore diameter of about 1 micrometers or more. 10. The filter element of Claim 9, wherein the foam has an mean flow pore diameter of about 5 micrometers or more. 1 1 . The filter element of Claim 10, wherein the foam has a mean flow pore diameter of about 10 micrometers or more. 12. The filter element of Claim 1 1 , the foam having a mean flow pore diameter of about 15 micrometers or more. 13. The filter element of one of Claims 1 -7, the foam having a mean flow pore diameter of from about 3 to about 8 micrometers. 14. The filter element of one of Claims 1 -7, the foam having a mean flow pore diameter of from about 8 to about 20 micrometers. 15. The filter element of one of Claims 1 -7, the foam having a mean flow pore diameter of from about 20 to about 50 micrometers. 16. The filter element of one of Claims 1 -15, wherein the thermoplastic material compromises in polymerized form 50 percent or more by weight alkenyl aromatic monomeric units. 17. The filter element of one of Claims 1 -15, wherein the thermoplastic material compromises in polymerized form 50 percent or more by weight ethylenic monomeric units. 18. The filter element of one of Claims 1 -15, wherein the thermoplastic material compromises in polymerized form 50 percent or more by weight propyleneic monomeric units. 19. The filter element of one of Claims 1 -15, wherein the thermoplastic material compromises in polymerized form 50 percent or more by weight ester monomeric units. 20. The filter element of one of Claims 1 -19, wherein the open-cell content is 70 percent or more. 21 . The filter element of Claim 20, wherein the open-ceil content is 90 percent or more. 22. The filter element of Claim 21 , wherein the open-cell content is 95 percent or more. 23. The filter element of one of Claims 1 -22, wherein the foam has an average cell size of 1 millimeter or less. 24. The filter element of Claim 23, wherein the foam has an average cell size of 0.1 millimeter or less. 25. The filter element of one of Claims 1 -22, wherein the foam has an average cell size of from 0.01 to 1.0 millimeter. 26. The filter element of Claim 25, wherein the foam has an average cell size of from 10 to 70 micrometers, or wherein the foam has an average cell size of from 200 to 700 micrometers. 27. The filter element of one of Claims 1 -22, wherein the foam has an average cell size of 100 micrometers or more. 28. The filter element of one of Claims 1 -27, wherein the foam has an efficiency of 40 percent or more for particle sizes of 0.3 micrometers. 29. The filter element of Claim 28, wherein the foam has an efficiency of 90 percent or more for particle sizes of 0.3 micrometers. 30. The filter element of Claim 29, wherein the foam has an efficiency of 99 percent or more for particle sizes of 0.3 micrometers. 31. The filter element of Claim 30, wherein the foam has an efficiency of 99.991 percent or more for particle sizes of 0.3 micrometers. 32. The filter element of one of Claims 1 -27, wherein the foam has an efficiency of from 40 to 70 percent or more for particle sizes of from 0.3 to 0.5 micrometers. 33. The filter element of one of Claims 1 -32, wherein the density of the foam is from 16 to 250 kg/cubic meter. 34. The filter element of Claim 33, wherein the density of the foam is from 25 to 100 kg/cubic meter. 35. A method of filtration comprising passing a liquid or gas through an extruded, open-cell thermoplastic foam having a structure substantially of cell walls and cell struts, an overall open-cell content of 50 percent or more, an average cell size of 1.5 millimeters or less, and a permeability of 6x10 '13 m 2 or more. 36. A method of filtration comprising passing a liquid or gas through a filter element made to fit within a filter element housing, the filter element comprising an extruded open-cell thermoplastic foam having a structure substantially of cell walls and cell struts, an overall open-cell content of 50 percent or more, an average cell size of 1 .5 millimeters or less, and a permeability of 6χ10 '13 m 2 or more, and, optionally, retaining means for the foam. 37. The method of one of Claims 35-36, wherein the liquid or gas is passed through the foam substantially in the extrusion direction. 38. The method of one of Claims 35-36, wherein the liquid or gas is passed through the foam substantially transverse to the extrusion direction.

Inventors: BLAND DAVID G, BLAND, DAVID, G.
Applicants: BLAND DAVID G, DOW CHEMICAL CO, BLAND, DAVID, G., THE DOW CHEMICAL COMPANY
Application year: 1999
Main classification: B01D39
IPC codes: B01D39, B01D46