Filtering material and device and method of its manufacture

Abstract

A device and method for producing a porous fiber structure. One or more points of high surface curvature is produced in a liquefied polymer, such as a polymer solution or a polymer melt. The points of high surface curvature may be produced by forcing the liquefied polymer through narrow nozzles, or by wetting sharp protrusions with the liquefied polymer. The liquefied polymer is charged to a high negative electrical potential relative to a grounded moving belt. Thin jets of liquefied polymer emerge from the points of high surface curvature to impinge as fibers on the moving belt, thereby forming an unwoven fiber structure of relatively uniform porosity. A powdered aerosol is charged to a high positive electrical potential relative to the moving belt. As the belt moves past the aerosol, the aerosol particles are attracted to fill interstices in the fiber structure, thereby creating a composite filtering material.

What is claimed is:  
     
         1 . A device for transforming a liquefied polymer into a fiber structure, comprising: 
 (a) a precipitation electrode;    (b) a first mechanism for charging the liquefied polymer to a first electrical potential relative to said precipitation electrode; and    (c) a second mechanism for forming a surface on said liquefied polymer of sufficiently high curvature to cause at least one jet of the liquefied polymer to be drawn by said first electrical potential to said precipitation electrode;    wherein said first and second mechanisms are designed such that when a plurality of fibers are precipitated on said precipitation electrode, a high efficiency particulate air unwoven fiber structure, capable of filtering out at least 99.97% of 0.3 μm particulates in air flowing at 5 cm/sec is obtainable.    
     
     
         2 . The device of  claim 1 , wherein said first mechanism for charging the liquefied polymer to a first electrical potential relative to said precipitation electrode includes in combination: 
 (i) a source of high voltage; and    (ii) a charge control agent mixed with the liquefied polymer.    
     
     
         3 . The device of  claim 2 , wherein said first mechanism for charging the liquefied polymer to a first electrical potential relative to said precipitation electrode further includes: 
 (iii) a source of ionized air being in contact with said liquefied polymer.    
     
     
         4 . The device of  claim 1 , wherein said second mechanism is effected by at least one rotating wheel having a rim formed with a plurality of protrusions.  
     
     
         5 . The device of  claim 4 , wherein each of said protrusions is formed with a liquefied polymer collecting cavity.  
     
     
         6 . The device of  claim 4 , wherein each of said at least one wheel is tilted with respect to said precipitation electrode.  
     
     
         7 . The device of  claim 4 , wherein each of said at least one wheel includes a dielectric core.  
     
     
         8 . The device of  claim 1 , wherein said second mechanism is effected by a gas bubbles generating mechanism.  
     
     
         9 . The device of  claim 1 , wherein said second mechanism is effected by a rotating strap formed with a plurality of protrusions.  
     
     
         10 . The device of  claim 1 , wherein said precipitation electrode is operative to move past said mechanism for forming said surface of high curvature.  
     
     
         11 . The device of  claim 10 , wherein said precipitation electrode includes a belt.  
     
     
         12 . The device of  claim 1 , wherein said mechanism for forming said surface of high curvature includes at least one nozzle.  
     
     
         13 . The device of  claim 1 , wherein said mechanism for forming said surface of high curvature includes at least one protrusion made of a material which is wetted by the liquefied polymer, said at least one protrusion including a tip whereon said surface of high curvature is formed.  
     
     
         14 . The device of  claim 13 , wherein said at least one protrusion is disposed on a rim of a wheel with said tip pointing radially outward from said wheel.  
     
     
         15 . The device of  claim 13 , further comprising: 
 (d) a bath for holding the liquefied polymer;    wherein said at least one protrusion is operative to reciprocate within said bath, said jets of the liquefied polymer being formed at a closest approach of said at least one protrusion to said precipitation electrode.    
     
     
         16 . The device of  claim 1 , further comprising: 
 (d) an additional electrode, intermediate between said precipitation electrode and said mechanism for forming said surface of high curvature.    
     
     
         17 . The device of  claim 16 , wherein said additional electrode includes a plate having an aperture, opposite said mechanism for forming said surface of high curvature, where through said at least one jet of the liquefied polymer emerge towards said precipitation electrode.  
     
     
         18 . The device of  claim 1 , further comprising: 
 (d) an aerosol generator operative to supply an aerosol to said precipitation electrode at a second electrical potential difference from said precipitation electrode opposite in sign to said first electrical potential difference.    
     
     
         19 . The device of  claim 18 , wherein said aerosol generator includes: 
 (i) a pressure chamber; and    (ii) a partition between said pressure chamber and said precipitation electrode;    said pressure chamber and said partition cooperating to fluidize a filler powder which is drawn by said second electrical potential difference to said precipitation electrode.    
     
     
         20 . The device of  claim 18 , wherein said aerosol generator includes a slot sprayer.  
     
     
         21 . A method for forming a polymer into a high efficiency particulate air unwoven fiber structure capable of filtering out 99.97% of 0.3 μm particulates in air flowing at 5 cm/sec, comprising the steps of: 
 (a) liquefying the polymer, thereby producing a liquefied polymer;  
 (b) supplementing the liquefied polymer with a charge control agent;  
 (c) providing a precipitation electrode;  
 (d) charging said liquefied polymer to a first electrical potential relative to said precipitation electrode; and  
 (e) forming a surface on said liquefied polymer of sufficiently high curvature to cause at least one jet of said liquefied polymer to be drawn to said precipitation electrode by said first electrical potential difference, thereby forming the unwoven fiber structure capable of filtering out 99.97% of 0.3 μm particulates in air flowing at 5 cm/sec on said precipitation electrode.  
 
     
     
         22 . The method of  claim 21 , wherein charging said liquefied polymer to said first electrical potential relative to said precipitation electrode is followed by recharging said liquefied polymer to a second electrical potential relative to said precipitation electrode, said second electrical potential is similar in magnitude, yet opposite in sign with respect to said first electrical potential.  
     
     
         23 . The method of  claim 21 , wherein said liquefying is effected by dissolving the polymer in a solvent, thereby creating a polymer solution.  
     
     
         24 . The method of  claim 23 , further comprising the step of: 
 (f) providing vapors of said solvent proximate to said surface of high curvature.    
     
     
         25 . The method of  claim 21 , wherein said charge control agent is selected from the group consisting of biscationic amides, phenol and uryl sulfide derivatives, metal complex compounds, triphenylmethanes, dimethylmidazole and ethoxytrimethylsians.  
     
     
         26 . The method of  claim 21 , wherein said forming of said surface of high curvature is effected by causing said liquefied polymer to emerge from a nozzle, said surface of high curvature being a meniscus of said liquefied polymer.  
     
     
         27 . The method of  claim 21 , wherein said forming of said surface of high curvature is effected by wetting a protrusion having a tip with said liquefied polymer, said surface of high curvature being a surface of said liquefied polymer adjacent to said tip.  
     
     
         28 . The method of  claim 21 , further comprising the step of: 
 (f) moving said precipitation electrode so that the unwoven fiber structure is formed on said precipitation electrode as a sheet.    
     
     
         29 . The method of  claim 21 , further comprising the step of: 
 (f) vibrating said surface of high curvature.    
     
     
         30 . The method of  claim 29 , wherein said vibrating is effected at a frequency between about 5000 Hz and about 30,000 Hz.  
     
     
         31 . The method of  claim 21 , further comprising the steps of: 
 (f) charging a filler powder to a second electrical potential relative to said collection surface, said second electrical potential being opposite in sign to said first electrical potential, thereby creating a charged filler powder; and    (g) exposing the unwoven fiber structure on said precipitation electrode to said charged powder, thereby attracting said charged filler powder to the unwoven fiber structure.    
     
     
         32 . The method of  claim 31 , wherein said liquefied polymer is charged negatively relative to said precipitation electrode and wherein said charged powder is charged positively relative to said precipitation electrode.  
     
     
         33 . The method of  claim 21 , further comprising the step of: 
 (f) supplementing the liquefied polymer with an additive selected from the group consisting of a viscosity reducing additive, a conductivity regulating additive and a fiber surface tension regulating additive.    
     
     
         34 . The method of  claim 32 , wherein said viscosity reducing additive is polyoxyalkylein, said conductivity regulating additive is an amine salt and said fiber surface tension regulating additive is a surfactant.  
     
     
         35 . A high efficiency particulate air filter comprising unwoven fibers of a polymer, the filter being capable of filtering out at least 99.97% of 0.3 μm particulates in air flowing at 5 cm/sec, having a pressure drop of about 0.75 mm H 2 O to about 13 mm H 2 O, and having a dust load to filter weight per area ratio of about 1 to about 1.8.  
     
     
         36 . The high efficiency particulate air filter of  claim 35 , wherein the filter is substantially electrically neutral.  
     
     
         37 . The high efficiency particulate air filter of  claim 35 , wherein said fibers have a diameter of about 0.1 μm to about 10 μm  
     
     
         38 . A high efficiency particulate air filter comprising unwoven fibers of a polymer, the filter being capable of filtering out at least 99.97% of 0.3 μm particulates in air flowing at 5 cm/sec, having a pressure drop of about 0.75 mm H 2 O to about 13 mm H 2 O, the filter is substantially electrically neutral.  
     
     
         39 . The high efficiency particulate air filter of  claim 38 , wherein the filter has a dust load to filter weight per area ratio of about 1 to about 1.8.  
     
     
         40 . The high efficiency particulate air filter of  claim 38 , wherein said fibers have a diameter of about 0.1 μm to about 10 μm  
     
     
         41 . A high efficiency particulate air filter comprising unwoven fibers of a polymer having a diameter of about 0.1 μm to about 10 μm, the filter being capable of filtering out at least 99.97% of 0.3 μm particulates in air flowing at 5 cm/sec, having a pressure drop of about 0.75 mm H 2 O to about 13 mm H 2 O.  
     
     
         42 . The high efficiency particulate air filter of  claim 41 , wherein the filter has a dust load to filter weight per area ratio of about 1 to about 1.8.  
     
     
         43 . The high efficiency particulate air filter of  claim 41 , wherein the filter is substantially electrically neutral  
     
     
         44 . A high efficiency particulate air filter comprising unwoven fibers of a polymer, the filter being capable of filtering out at least 99.97% of 0.3 μm particulates in air flowing at 5 cm/sec and having a pressure drop of about 0.75 mm H 2 O to about 13 mm H 2 O, wherein at least about 90% of said fibers having a diameter in a range of X and 2X, where X is in a range of about 0.1 μm and about 10 μm.  
     
     
         45 . A high efficiency particulate air filter comprising unwoven fibers of a polymer, the filter being capable of filtering out at least 99.97% of 0.3 μm particulates in air flowing at 5 cm/sec and having a pressure drop of about 0.75 mm H 2 O to about 13 mm H 2 O, the filter featuring pores formed among said fibers, wherein at least about 90% of said pores having a diameter in a range of Y and 2Y, where Y is in a range of about 0.2 μm and about 10 μm.