WO2002058812A2

WO2002058812A2 - Air filter with microencapsulated biocides

Info

Publication number: WO2002058812A2
Application number: PCT/US2001/047809
Authority: WO
Inventors: Stewart R. Kaiser; Joseph T. Terleski
Original assignee: R-Tec Technologies, Inc.
Priority date: 2000-12-18
Filing date: 2001-12-10
Publication date: 2002-08-01
Also published as: WO2002058812A3; AU2002248174A1

Abstract

An improved air filter (10) for removing and destroying harmful pathogens in the airstream of an HVAC system. The air filter (10) includes natural, synthetic or a combination of natural and synthetic fibers (12) woven to trap particulate matter. Interspersed in these fibers are microcapsules (16) containing a suitable biocide for disinfecting and/or deodorizing the exhaust air. The selected biocide is microencapsulated to provide a uniform diffusion rate of the biocide over the useful life of the filter (10). The method of preparing the microcapsules (16) for incorporation with the filter is disclosed using readily available broad spectrum disinfecting agents.

Description

INVENTOR: STEWART R. KAISER JOSEPH T. TERLESKI

ASSIGNEE: R-TEC TECHNOLOGIES, INC.

ATTY. REF.: R-TEC-1 PCT

AIR FILTER WITH MICROENCAPSULATED BIOCIDES

BACKGROUND OF THE INVENTION

This application claims priority of the provisional patent application 60/256,437 filed December 18, 2000. FIELD OF THE INVENTION [0001] The subject invention relates to filters for heating, ventilation and air conditioning systems, and more particularly to an air filter coated with microencapsulated biocides that substantially reduces the quantity of harmful and odor causing microorganisms, bacteria and virii in filtered air, as well as methods of using and making such an air filter. DESCRIPTION OF THE RELATED ART

[0002] Outdoor air contains many substances, ranging from eye-burning smog to sneeze-provoking pollen to headache-causing fumes to irritating cigarette smoke. When analyzed, samples of outdoor air may contain soot, smoke, ozone, silica, clay, decayed animal and vegetable matter, lint and plant fibers, metallic fragments, mold spores, bacteria, pollen, and a range of gaseous emissions from business and industrial processes. Unless properly filtered and cleaned, outdoor air drawn into a heating, ventilation and air conditioning (HVAC) system may introduce some of these same substances to the indoor air of homes and buildings. [0003] Air filters of various thicknesses, textures and materials have been used to filter airborne dirt and dust particles by straining or impinging the particles upon a filter medium as air passes through the filter. A replaceable filter uses polyester or fiberglass fibers as the filtration medium and is effective in removing particulate matter that is greater than 10μm (1 μm = 1 micron) in size. Another design to remove smaller particles is a high efficiency particulate air (HEPA) filter. A HEPA filter is capable of removing 99.97% of all particulate matter greater than 0.3μm in size. Although this is an improvement in air filtration technology, it still does not fully address the problem of harmful pathogens in the atmosphere.

[0004] While the above-mentioned air filters are effective in removing particulate matter, they are not very effective in the removal and destruction of bacteria, virii and other microorganisms. The removal of such impurities is particularly important when the filtered air is furnished to homes and buildings where people are present. These impurities can adhere to dust and particulate matter already trapped in the filter, multiply and grow on the surface of the dust, and may later become dislodged and pass through the filter and into the breathable indoor air. Thus, the air filter itself will act as if it were a medium of bacterial propagation exacerbating the problems of the indoor environment.

[0005] In response to this deficiency, a number of alternative methods have been proposed. One possible alternative is an atomization type filter where a disinfecting gas is injected into the air stream to destroy the microorganisms. An example of this type is disclosed in U.S. Patent No. 5,141 ,722. In that disclosure, the system uses a standard intake filter, but injects chlorine dioxide and ozone into the air downstream of the intake filter. Chlorine dioxide and ozone combine to form chlorine trioxide that sterilizes and deodorizes the air. This provides effective control of microorganisms present in the air. A catalytic layer at the exit point removes the ozone and chlorine gases from the air stream. Air exiting the system is therefore deodorized and disinfected. However, such a system has several drawbacks. Among these are that it is larger than most filters and cannot be readily adapted for residential use. Also, a system of this type will require frequent maintenance to replenish the chlorine dioxide supply and ensure that the amount of disinfecting gas being injected is adequate to control the microorganisms. Furthermore, the chlorine dioxide concentration must be carefully monitored since it is unstable in light and easily detonated if the concentration reaches 10% at atmospheric pressure. Lastly, the system will require changing the catalytic media on a periodic basis to prevent the introduction of ozone and chlorine gases, which are hazardous to human health, into normally occupied spaces. Such a system is costly to install, operate and maintain.

[0006] Another method for disinfecting air is the ultraviolet light type filter. In this filter, air passing through is bathed in ultraviolet light of a specific wavelength designed to destroy microorganisms. Such a device is disclosed in U.S. Patent No. 5,523,057. In that disclosure, a filter chamber was described with a conventional intake filter to remove large particulates, a series of ultraviolet lamps producing a specific wavelength of ultraviolet light to destroy airborne bacteria and an activated charcoal filter to remove odors at the exit point. Despite it apparent simplicity, such a system requires more maintenance than a simple replaceable filter. Maintaining this system would require changing the conventional intake filter, the charcoal exit filter, and the ultraviolet lamps within the sealed chamber. It also requires an external power source for operating the ultraviolet lamps.

[0007] A third alternative is the use of an electrostatic precipitator type filter system. A system of this type uses electricity to charge the particulate matter in the air stream and an opposing grounded collector plate for collecting the charged particulates. Although this works to remove particulates from the atmosphere, including harmful pathogens, it does not provide a method for destroying these pathogens and they will accumulate and pose a threat to human health inside the electrostatic precipitator. An improvement on this were disclosed in U.S. Patent No. 5,993,738 that combined an electrostatic precipitator with a photocatalyst and ultraviolet light to destroy the pathogens accumulating on the collector plates. However, this is a large system requiring frequent maintenance to clean the plates and exposing personal to potential hazards within the system.

[0008] Another type of filter system has received much attention over the years.

This system combines the use of a conventional filter in conjunction with a form of germicidal or biocidal agent embedded or incorporated into the filter material to remove harmful pathogens from the environment. A recent example is U.S. Patent No.

5,840,245 that issued to Coombs et al. on November 24, 1998. That patent discloses the use of inorganic antimicrobial agents made up of specific salts of transition metals.

Also, the patent discloses a preferred method for application of these antimicrobial agents as being incorporated into the woven fibers of the filter media. In the alternative, a solution of the antimicrobial agent may be sprayed directly onto the filter media.

[0009] U.S. Patent No. 4,534,775, issued to Frazier on August 13, 1985, discloses an air treatment filter containing an antimicrobial agent that is incorporated into a liquid within the filter itself. That disclosure reveals a filter for removing odors and harmful gases from the air while the antimicrobial agent inhibits growth of microorganisms.

[0010] Of general interest is U.S. Patent No. 4,118,226 issued to Bourassa on

October 3, 1978. In that patent, a combination aromatic and disinfecting filter was disclosed for use in forced air type ventilation systems. [0011] Similarly to Bourassa, U.S. Patent Nos. 3,820,308, 3,116,969 and

3,017,239 disclose methods of applying germicides to air filters. All three disclosures teach mixing the selected germicide in a solvent and applying the resulting mixture to the air filter media to destroy airborne pathogens and prevent trapped pathogens from multiplying. [0012] In another area of commerce, it is known that n-alkyl (50% C14, 40% C12, 10% C16) dimethyl benzyl ammonium saccharinate, in small quantities of 1% or less, can be used as a disinfectant to sanitize, control and prevent mold and mildew along with their odors. This saccharinate composition can kill 99.9% of listed bacteria, fungi, virii (pathogens), rhino virus (common cold), herpes simplex virus type 1 & 2, adenovirus type 2, rotavirus, salmonella, staphlococcus aureus (staph), polio virus type 1 , klebsiella pneumolae (k. pneumoniae), E-coli, respiratory syncytial virus, hepatitis A, and campylobacter. This chemical composition is used as an active ingredient in disinfecting sprays for surfaces and the air. Due to rapid dissipation, it is only suitable for short term disinfecting as an aerosol mist.

[0013] Therefore, it is an objective of the subject invention to provide a sterilizing and deodorizing air filter which overcomes the deficiencies of the prior art. [0014] Accordingly, it is another objective of the subject invention to provide an air filter that will not only collect particulate matter from an air stream, but remove and destroy any microbial matter, such as bacteria, fungi and virii.

[0015] It is another objective of the subject invention to provide an air filter coated with a biocide where the germ killing properties of the biocide are released over a period of time equal to the useful life of the air filter medium. [0016] A third objective of the subject invention is to maintain an air filter free of harmful pathogens so that the filter does not become a breeding ground for various harmful pathogens and a source for releasing these pathogens into the surrounding environs.

[0017] A further objective of the subject invention is to provide a biocidal coating for an air filter wherein the biocide is suspended in a viscous solvent and the viscous solvent traps pathogens by impingement. [0018] Another objective of the subject invention is to provide an air filter coated with a biocide that is effective in killing pathogens when laden with particulate matter. [0019] A final objective of the subject invention is provide an air filter coated with a biocide that does not reduce the air flow characteristics of the filter. SUMMARY OF THE INVENTION

[0020] The above stated objects are met by a new and improved air filter coated with a microencapsulated biocidal agent. The subject air filter comprises a filter medium made from tightly woven materials, such as fiberglass or polyester, preferably with openings in size of one micrometer (1μm) or less. This tightly woven filter medium will function to remove dust, dirt or any large particulate matter from an air stream. The filter medium will be coated with a microencapsulated biocidal agent to kill germs, bacteria, virii, fungi, mold and such, that pass through the air filter, adhere to the other particulate matter, or adhere to the filter medium itself. The biocidal agent is microencapsulated in a porous shell that allows the biocide's pathogen killing effect to be uniformly released over time. Additionally, the biocidal agent is suspended in a viscous solvent within the porous shell. Diffusion of the biocidal containing solvent maintains a uniform effectiveness of the biocide and also traps pathogens by viscous impingement. Preferably, the biocide will be released over a period of time equal to the useful life of the filter medium. The new and improved air filter can be employed in a typical residential or commercial HVAC system to drastically reduce bacteria, fungi, virii, dirt, dust and odors in homes, schools, hospitals and commercial buildings. In fact, the subject air filter can be adapted for any HVAC system, including those used on planes, buses, ships and other locales where filtered air is desirable. Additionally, the new and improved filter can be adapted for use in military applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a perspective view of the air filter with microencapsulated biocides of the subject invention.

[0022] FIG. 2 is an enlarged view of a microencapsulated biocide in accordance with the subject invention.

[0023] FIG. 3 is a cross-sectional view of a filter medium with a plurality of microencapsulated biocides applied to the front and back surface. [0024] FIG. 4 is a cross-sectional view of the air filter of FIG. 1 employing two layers of filter medium. DESCRIPTION OF THE INVENTION

[0025] Referring to the FIGS., a new and improved air filter 10 is provided to remove and destroy all microbial matter, as well as removing dust, dirt and particulate matter from the airflow through an HVAC system. The air filter 10 comprises a filter medium 12 and a frame 14. The filter medium 12 of the air filter 10 is coated with microcapsules 16. The filter medium 12 will normally remove particulate matter, including dirt, dust, leaves and insects. Inside the microcapsules 16, is the core material 18 containing a biocide suspended in a viscous solvent. Over time, the core material 18 diffuses through the microcapsule's 16 shell and coats the filter medium 12. This has the added benefit of trapping pathogens by viscous impingement. Therefore, the microcapsules 16 will kill any airborne microbial matter (i.e. bacteria, fungi, virii or any other microorganism), any microbial matter that has attached itself to any retained particulate matter or to the filter medium itself. Thus, contaminated air passing through the air filter 10 will leave the filter, and be essentially free of any particulates and harmful microbial matter. [0026] The filter medium 12 of the air filter 10 can be any known medium in the art. One such air filter medium is the dry, fibrous type. Dry, fibrous media are made of glass fibers, cellulose fibers, or synthetic fibers. The media can be formed into blankets of varying thicknesses, fiber sizes, and densities or into fiber mats of random fiber size and density, which are then supported by a wire or cardboard frame 14. In addition, dry fibrous filters may contain an electric charge. These filters contain fibers that are positively-charged, negatively-charged or both. They have the added benefit of capturing charged particles from the air stream. The subject invention does not interfere with the electrostatic properties of such a filter and is suitable for use on these filters as well.

[0027] To achieve the microbial killing effect of the filter 10, a biocide is to be applied to the filter medium 12. For the purpose of this application, a biocide is understood to be any chemical agent capable of destroying living organisms and virii. Any of the following terms could be classified as a biocide: germicide, disinfectant, antibacterial solution, anti-mold solution, antifungal solution and anti-spore solution. Typical chemical compositions which would act as a biocide for the purpose of this invention are alkyl dimethyl benzyl ammonium sacharinate, Quaternary ammonium chlorides, Triclosan, and phenylphenol. Once the biocide to be used is determined, the biocide is microencapsulated so it will be released slowly over time. [0028] Generally, a core material 18 is microencapsulated when natural and/or man-made (synthetic) materials are used to initiate a polymerization reaction that envelops the core material 18 and forms a shell 20 around the entire core material 18, which is usually roughly spherical in shape. In time-released microcapsules, the shell wall 20 is porous so that over time the core material 18 is released by mass diffusion. The porous nature of time-released microcapsules is controlled during polymerization and is varied according to the desired amount and time the core material 18 is to be released from the shell 20.

[0029] The core material 18 should be a viscous solvent with properties such that its time release profile can be controlled and be easily engineered to produce the desired release profile i.e.; amount released over a desired time period. It should also coat the fibers of the filter media and have the properties of trapping particles by viscous impingement. The viscous solvent can be selected from any of a number of naturally occurring oils from vegetables, nuts or other plants. It may also be a synthetic product, or a combination of natural and synthetic materials. [0030] In microencapsulating the biocide in accordance with the subject invention, the material used as a solvent for the afore-mentioned biocide (solute) can be any one of the following: almond oil, soybean oil, peanut oil, olive oil, linseed oil, corn oil, vegetable oil, any and all cooking oils, lanolin, glycerine, triethanolomine, ethyl alcohol, tallow, fatty acids (i.e. oleic, linoleic, palmitic, myristic, stearic, and arachidic acids), water, and organic solvents. The process of polymerization will be controlled so that the resulting microcapsules 16 will have a time-release profile that matches the useful life of the filter medium 12. Therefore, the microcapsules 16 ensure complete and substantially uniform diffusion of the core material 18 over the useful life of the filter medium 12. [0031] Referring to FIGS. 3 and 4, the dry, free-flowing microcapsules 16 can be applied to the filter medium 12 in many ways depending on the type of filter medium used. When using the dry, fibrous type medium, the filter medium is coated with an adhesive base. This adhesive base may be a naturally occurring substance such as gum or latex, a synthetic material such as a polymeric compound, or a combination of natural and synthetic adhesives. To coat the air filter, the microcapsules are then spread, sprayed, or otherwise applied in a uniform manner on the coated filter medium. Drying time of the adhesive may be accelerated by passing heated air across or through the filter. Additionally, a radiant heat source, such as a heat lamp, may be used as an alternative method for shortening the drying time. In the case of a viscous impingement type medium, no additional adhesive material is to be applied since the viscous substance will retain the microcapsules. Preferably the intake side of the filter is coated with the biocide. As an option, the exhaust side may also be located for increased protection. [0032] In one preferred embodiment, the invention will be a water based slurry with a microcapsule concentration of 25% to 35% by weight. The slurry will also contain an adhesive that constitutes between 1% and 5% by weight of the total slurry. This adhesive can be a natural product such gum arabic or a synthetic polymeric adhesive. This slurry is easily atomized and sprayed onto a filter medium. Additionally, the slurry may be brushed, rolled or otherwise similarly applied to the filter medium 12. It is easy to adapt this technique for use on a wide range of filter media such as, but not limited to, pure synthetics, such as polyester, natural media, such as a cotton weave, or a combination of natural and synthetic filter media.

[0033] Incorporated into the core material 18 is an antimicrobial agent with a concentration between 0.5% to 5% by weight depending upon the particular antimicrobial agent selected. The most desirable property of the chosen antimicrobial agent is the ability to destroy a wide variety of pathogens and microorganisms that pose a health hazard to humans. Industry has provided a number of compounds that may be used as an antimicrobial agent. One such compound is n-alkyl (50% C14, 40% C12, 10%C16) dimethyl benzyl ammonium saccharinate and is particularly effective as a broad spectrum biocide. In addition, other quaternary ammonium salts may be used. Other effective biocidal agents include Triclosan, o-Benzyl p- chlorophenol, 2-phenylphenol or N-Alkyl N-Ethyl Morpholinium Sulphates, any of which may be used as the antimicrobial agent.

Table I

Table II

Table III

Table IV

[0034] Tables l-IV show the effectiveness of a preferred embodiment of the subject invention against certain pathogens. A coating containing 25% to 35% by weight of the microcapsules was uniformly applied to a filter, and an untreated filter was used for comparison. Both the treated and untreated filters were inoculated with the test organisms. At specific time intervals, the test organisms were eluted from the samples in known amounts of neutralizing solution. The number of bacteria present in this liquid was determined, and the percentage reduction was calculated. All three qualitative tests were performed by a certified laboratory using the American Association of Textile Chemists and Colorists Test Method 100-1999: Antibacterial Finishes on Textile Materials. Table I shows the percent reduction of S aureus bacteria as time elapses. It is shown that 99% of these bacteria are killed after 1 minute of exposure to the treated filter. In Table II, a 97.9% reduction of K pneumoniae was achieved after a 1 minute exposure to the treated filter. In Table III shows that after 20 minutes of exposure, there were virtually no surviving tuberculosis (mycobacterium terrae) organisms. In comparison, the untreated filter demonstrated no killing of these microorganism after 120 minutes of incubation. Finally, Table IV shows the effectiveness of the subject invention against Bacillus subtilis, which is a proxy for Bacillus anthracis, the bacterium that causes anthrax. After 30 second of exposure, the subject invention reduced the population of bacteria by 49%. Maximum reduction of 84% was achieved after 10 minutes of exposure.

Table V

[0035] Evidence of the subject invention's efficacy over time is shown in Table V. A test was performed in accordance with ASTM 6329-98 "Standard Guide for Developing Methodology for Evaluating the Ability of Indoor Materials to Support Microbial Growth Using Static Environmental Chambers." A 24"x24" treated air filter was dust loaded until a pressure drop of 1"H₂O across the filter was achieved. After the desired dust loading was achieved, the treated air filter was inoculated with the test organism, Aspergillus versicolor, using an ASHRAE 52.2 test rig. The treated air filter was placed in the 52.2 test rig and an air flow rate of 1000 ft³ per minute was established. The test filter was inoculated by nebulizing a suspension of the test organism into the 52.2 test rig for at least twenty minutes. Following inoculation, the filter was removed from the test rig, cut into pieces and replicate pieces placed into one of the static chambers maintained at 94% relative humidity.

[0036] Replicate pieces were removed from the chambers at intervals over a three month period. The initial level of organisms was determined on day 0. On each test day, sample pieces were placed in sterile receptacles containing buffered detergent, and shaken on a wrist-action shaker for 30 minutes, after which the sample/buffer suspension was diluted and plated on Sabourads Dextrose agar. These plates were incubated and colony forming units (CFU) were counted shortly after visible growth was first noted and again as moderate growth become visible. Each sample's CFUs were compared to day 0 and the results are reported in Table V. [0037] The samples from day 0 show a mean of 1 ,820 CFUs and represents the initial level of inoculation. After one month, none of the samples had any measurable CFUs. This is an expected result for this type of test and is not indicative of antimicrobial activity. (350 is the minimum number of CFUs detectable). After two months, 60% of the sample pieces showed no detectable growth while 7 out of 15 samples showed no detectable growth after three months. Due to imprecise application, the test filter was not uniformly treated with the biocidal agent. Therefore, some filter pieces showed no effectiveness at inhibiting growth of the test organism. Previous tests of filters had varied, including areas of little growth, but did not exhibit areas of no detectable growth as in the test of the subject invention. [0038] Preferably, the time release microcapsules should be sized between range of 50μm and 500μm depending on the desired release profile of the core material. Microencapsulation is a well known technique and a multitude of techniques can easily be found in the literature. Any microencapsulation method that provides for the time release of its core material with the desired release profile may be used. A steady and constant release of the core contents over 3 to 6 months is preferred. [0039] Depending on the specific application of the air filter, (i.e. residential, commercial, or high efficiency applications), the quantity and size of the microcapsules will vary considerably. They may be applied to both surfaces of the filter medium or to different layers of filter medium. Typically, a second non-coated layer 24 of filter medium will be attached to rear of the initially coated filter 12 to catch any excess dirt, residue or microcapsules which may escape from the first filter layer 12. The second filter layer may be from 1/32 of an inch to several inches from the initial layer and additional layers may be added as required by the intended usage.

[0040] After assembly, the treated filter will be placed in an airtight package with an internal pressure slightly greater that atmospheric pressure. This prevents the biocide from diffusing out of the microcapsules and ensures a stable shelf-life making the product easier to store and ship. Prior to installing a treated filter into an HVAC system, the airtight package is opened and the biocide will begin to diffuse thereby activating the treated filter.

[0041] Although the present invention has been described with reference to the foregoing specification and drawings, many modifications, changes, additions and deletions to the invention may be made within the scope and spirit thereof. For example, different methods of microencapsulating than those disclosed may be used as well as alternative antimicrobial agents in lieu of those mentioned previously. Also, different combinations of filter media may be used to increase the efficiency of the final product for its designed purpose. It is to be understood that the foregoing description of the invention is illustrative and not limiting the scope of the invention being defined by the following claims.

Claims

WHAT IS CLAIMED IS:

1. An air filter comprising: a frame; a filter medium of a porous material uniformly disposed within said frame and securely attached to said frame; and a coating formed of a plurality of microcapsules, each said microcapsule containing a biocide in a polymerized material for uniform diffusion over time of said biocide onto said filter medium, and an adhesive material for uniformly distributing and securely attaching said microcapsules to the filter medium.

2. The air filter of claim 1 , wherein each said microcapsule further includes a viscous solvent.

3. The air filter of claim 2, wherein said filter is used in a heating, ventilation and air-conditioning system.

4. The air filter of claim 3, wherein the filter medium is of the dry, fibrous type.

5. The air filter of claim 4, wherein said filter medium is woven to capture particles greater than or equal to 1 μm in size.

6. The air filter of claim 5, wherein the microcapsules are between 50μm and 500μm in size.

7. The air filter of claim 6, wherein said biocide is n-alkyl (50% C14,

40%C12, 10%C16) dimethyl benzyl ammonium saccharinate.

8. The air filter of claim 7, wherein each said microcapsule further includes a concentration of 0.5% to 5.0% by weight of said biocide.

9. The air filter of claim 8, wherein said filter is a high-efficiency particulate air (HEPA) filter.

10. The air filter of claim 9, wherein said HEPA filter comprises fibers woven to capture particles greater than or equal to 0.3μm in size.

11. A method for coating an air filter with a biocide, which comprises: microencapsulating a biocide and a viscous solvent in a polymerizing material to form microcapsules; forming slurry of an adhesive material, water and said microcapsules; applying said slurry uniformly to an air filter; and drying said slurry to bond the microcapsules biocides to the filter medium.

12. A method as defined in claim 11 , wherein the slurry-forming step comprises a concentration of 1 % to 5% by weight of the adhesive material.

13. A method as defined in claim 12, wherein the microencapsulating step comprises microcapsules between 50 and 500 μm in size.

14. A method as defined in claim 13, wherein said biocide is n-alkyl (50% C14, 40%C12, 10%C16) dimethyl benzyl ammonium saccharinate.

15. A method as defined in claim 14, wherein the slurry-forming step comprises a concentration of 25% to 35% by weight of the microcapsules.

16. A method as defined in claim 15, wherein the applying the slurry step comprises spraying the slurry onto the filter to uniformly coat said filter.

17. A method as defined in claim 15, wherein the applying the slurry step comprises brushing the slurry onto the filter to uniformly coat said filter.

18. A method as defined in claim 16, wherein the drying the slurry step comprises passing heated air across the filter to evaporate excess moisture.

19. A method as defined in claim 17, wherein the drying the slurry step comprises passing heated air across the filter to evaporate excess moisture.

20. A method as defined in claim 16, wherein the drying the slurry step comprises using a radiant heat source to evaporate excess moisture.

21. A method as defined in claim 17, wherein the drying the slurry step comprises using a radiant heat source to evaporate excess moisture.

22. A method as defined in claim 15, wherein the applying the slurry step comprises rolling the slurry onto the filter to uniformly coat said filter.

23. A method as defined in claim 22, wherein the drying the slurry step comprises passing heated air across the filter to evaporate excess moisture.

24. A method as defined in claim 22, wherein the drying the slurry step comprises using a radiant heat source to evaporate excess moisture.

25. An antimicrobial coating comprising: a plurality of microcapsules, each said microcapsule containing a biocide and a viscous solvent in a polymerized material for uniform diffusion over time of said biocide; and an adhesive material for uniformly distributing and securing attaching said microcapsules.

26. The coating of claim 25, wherein said microcapsules are between 50μm and 500μm in size.

27. The coating of claim 26, wherein said biocide is n-alkyl (50% C14, 40% C12, 10% C16) dimethyl benzyl ammonium saccharinate.

28. The coating of claim 27, wherein each said microcapsule further includes a concentration of 0.5% to 5.0% by weight of said biocide.