Abstract:
The present invention is directed toward a filter media for a high efficiency particulate air (“HEPA”) filter including a multiplicity of adjacent electrostatically charged piles of nonwoven fabric. The invention further includes a method of making a HEPA filter.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed toward a filter media for a high efficiency particulate air (“HEPA”) filter comprising a multiplicity of adjacent electrostatically charged piles of nonwoven fabric. The invention further comprises a method of making a HEPA filter. 
     2. Description of the Prior Art 
     HEPA filters have been used to filter radioactive particles out of air streams. HEPA filters are employed in a variety of environments, including hospital operating theaters, electronic manufacturing clean rooms, and nuclear power plants. HEPA filters are employed to remove submicron size particles from the air. The term “HEPA filter,” as used herein, refers to a filter that is capable of filtering out at least 99.97% of 0.3 micron size particles, as evidenced by a DOP test. 
     Fine glass fiber paper has traditionally been used in HEPA filters as well as in ultra low penetration air (“ULPA”) filters. The term “ULPA filter,” as used herein, refers to a filter that is capable of filtering out 99.99% of 0.3 micron size particles, as evidenced by a DOP test. 
     The use of fine glass fiber paper in HEPA filters and in ULPA filters has many disadvantages, including brittleness during processing, heavy weight, high pressure drop, and skin irritation. An additional disadvantage of fine glass fiber paper in this application is that it cannot be electrostatically treated or charged. It has been shown that synthetic charged fibers are well suited as filters in heating, ventilating, and air conditioning (“HVAC”) systems. 
     The present invention provides an improved filter media which is capable of meeting the filtration standards of HEPA filters and ULPA filters, while maintaining low pressure drops and low weight. Lower pressure drops result in energy savings. Lower weight results in lower transportation costs. Energy savings and lower transportation costs are advantages of the present invention. 
     An additional advantage of the present invention is that the material costs are less than 10% the material costs associated with fine glass fiber paper. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a particulate air filter media comprising a first pile of charged nonwoven fabric having a weight in the range of 5-40 grams per square meter and one or more fibers having a diameter in the range of 1.5-2.0 microns. The term “charged,” as used herein, refers to electrostatically charging. 
     The invention also comprises a second pile of nonwoven fabric having a weight in the range of 5-40 grams per square meter and one or more fibers having a diameter in the range of 1.5-2.0 microns, adjacent to said first pile. The invention further comprises a third pile of nonwoven fabric having a weight in the range of 5-40 grams per square meter and one or more fibers having a diameter in the range of 1.5-2.0 microns, adjacent to said second pile. 
     The present invention is also directed toward a method of making a particulate air filter. This method comprises charging at least three adjacent piles of nonwoven fabric, each of said piles having a weight in the range of 5-40 grams per square meter, and each of said piles comprising one or more fibers having a diameter in the range of 1.5-2.0 microns. This method further comprises collating a stiffening layer with said piles. The method embodiment of this invention further comprises bonding a stiffening layer of fabric to one of said piles to form a stiffened filter media having a top side and a bottom side. The stiffened filter media, comprising the piles of nonwoven fabric and the stiffening layer, is then scored. After scoring, the stiffened filter media is folded into a multiplicity of adjacent layers. The method of the present invention further comprises inserting a separator between adjacent layers. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a first embodiment of the filter media of the present invention. 
     FIG. 2 is a side view of a second embodiment of the filter media of the present invention. 
     FIG. 3 is a side view of a third embodiment of the filter media of the present invention. 
     FIG. 4 is a side view of a fourth embodiment of the filter media of the present invention. 
     FIG. 5 a  is a side view of a first embodiment of the filter of the present invention. 
     FIG. 5 b  is a top cross sectional view of the embodiment shown in FIG. 5 a.    
     FIG. 6 is a block diagram of a first method embodiment of the present invention. 
     FIG. 7 is a side view of the folding and separating steps of the first method embodiment of the present invention. 
     FIG. 8 a  is a side view of the gluing step of the second method embodiment of the present invention. 
     FIG. 8 b  is a side view of the folding step of the second method embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, the particulate air filter media embodiment of the present invention comprises a first pile of charged nonwoven fabric  10  having a weight in the range of 5-40 grams per square meter and one or more fibers having a diameter in the range of 1.5-2.0 microns. The electrostatic charging of the nonwoven fabric in the first layer may be accomplished by any method known in the nonwoven fabric arts, including but not limited to, those methods disclosed in U.S. Pat. Nos. 5,401,446 to Tsai, et al., U.S. Pat. No. 4,215,682 to Kubik, et al.; U.S. Pat. No. 4,904,174 to Moosmayer, et al., or U.S. Pat. No. 5,122,048 to Deeds. The disclosures of all of these patents is incorporated herein by reference. In a preferred embodiment, the charging is accomplished using the method and/or apparatus disclosed in U.S. Pat. No. 5,401,446. 
     As shown in FIG. 1, the invention also comprises a second pile of nonwoven fabric  12  having a weight in the range of 5-40 grams per square meter and one or more fibers having a diameter in the range of 1.5-2.0 microns, adjacent to said first pile. In a preferred embodiment, the fabric of a second pile is charged. 
     The invention further comprises a third pile  14  of nonwoven fabric having a weight in the range of 5-40 grams per square meter and one or more fibers having a diameter in the range of 1.5-2.0 microns, adjacent to said second pile. In a preferred embodiment, the fabric of the third pile is charged. 
     In a preferred embodiment, the nonwoven fabric of each pile is a meltblown fabric. In another preferred embodiment, the meltblown fabric comprises polypropelene. 
     In a preferred embodiment, the filter media further comprises a fourth pile  13  adjacent to the third pile, as shown in FIG.  2 . In this embodiment, the weight of each pile is less than or equal to 35 grams per square meter. 
     In a preferred embodiment, the filter media further comprises a fifth pile  15  adjacent to the fourth pile, as shown in FIG.  3 . In this embodiment, the weight of each pile is less than or equal to 25 grams per square meter. 
     In a preferred embodiment, the filter media further comprises a sixth pile  17  adjacent to the fifth pile, as shown in FIG.  4 . In this embodiment, the weight of each pile is less than or equal to 20 grams per square meter. 
     In a preferred embodiment, the invention further comprises a stiffening layer  16 , as shown in FIG. 5 a.  In this embodiment, stiffening layer  16  is bonded to at least one of said piles such that the bonding area is in the range of 0.1% to 30% of the surface area of the bonded pile, as shown in FIG. 5 b.  In a preferred embodiment, the bonding is ultrasonic bonding and said bonding area is greater than or equal to 1% of the surface area of the bonded pile. The bonding area is represented by the circular regions  29  in FIG. 5 b.  In a preferred embodiment, the stiffening layer comprises a spunbond fabric. 
     In another embodiment, the invention is directed toward a particulate air filter comprising at least three adjacent piles  10 ,  12 ,  14  of nonwoven fabric. Each of these three piles has a weight in the range of 5-40 grams per square meter and comprises one or more fibers having a diameter in the range of 1.5-2.0 microns. These piles further comprise a first outer surface  19  and a second outer surface  20 . In a preferred embodiment, the nonwoven fabric of each adjacent pile comprises meltblown polypropelene. 
     A preferred embodiment of the particulate air filter comprises a first stiffening layer  16  bonded to said first outer surface such that the bonding area is in the range of 0.1-30% of the surface area of the first outer surface. In a preferred embodiment, the invention further comprises a second stiffening layer  22  bonded to the second outer surface such that the bonding area is in the range of 0.1-30% of the surface area of the second outer surface, as shown in FIG.  5 . In a preferred embodiment, the bonding area of the first stiffening layer to the first outer surface and the second stiffening layer to the second outer surface is in the range of 1%-30% of the surface area of the first and second outer surfaces, respectively. 
     The present invention is also directed toward a method of making a particulate air filter. In a first embodiment, this method comprises charging at least three adjacent piles of nonwoven fabric wherein each of the piles has a weight in the range of 5-40 grams per square meter and each of the piles comprises one or more fibers having a diameter in the range of 1.5-2.0 microns, as shown in Block  30  of FIG.  6 . This method further comprises collating a stiffening layer of fabric with the piles, as shown in Block  32  of FIG.  6 . 
     This method further comprises bonding the stiffening layer to one of the piles to form a stiffened filter media having a top side and a bottom side, as shown in Block  34  of FIG.  6 . In a preferred embodiment, the bonding is ultrasonic bonding. 
     This method further comprises scoring the stiffened filter media as shown in Block  32  of FIG.  6 . This method further comprises folding the stiffened filter media into a multiplicity of adjacent layers  52  as shown in Block  38  of FIG.  6  and in FIG.  7 . This method further comprises inserting a separator  50  between two adjacent layers, as shown in Block  40  of FIG.  6  and in FIG.  7 . In a preferred embodiment, the separator is made from aluminum. 
     In a second method embodiment of the present invention directed toward making a particulate air filter, the first through fourth steps of the method are the same as those depicted in Blocks  30 ,  32 ,  34  and  36  of FIG.  6 . 
     This method further comprises applying glue to a side of the stiffened filter media as shown in FIG. 8 a,  folding the filter media into a series of V shaped pleats, as shown in FIG. 8 b,  and cutting the pleated filter media into a multiplicity of pieces having a predetermined size. In a preferred embodiment, the stiffened filter media is dried after gluing and prior to folding. In a preferred embodiment the cutting is performed with a cutting blade  54  positioned above the pleated filter media. 
     The pleated filter media shown in FIG. 8 b  may be used in cylindrical filters or cubical filters. For cubical filters, glue is applied to both sides of the stiffened filter media. 
     The foregoing disclosure and description of the invention are illustrative and explanatory. Various changes in the size, shape, and materials, as well as in the details of the illustrative construction may be made without departing from the spirit of the invention.