Method for the manufacture of an electret fibrous filter

Abstract

A method for the manufacture of an electrically charged fibrous filter from a highly molecular non-polar fiber material wherein a .[.web.]. .Iadd.film .Iaddend.of the fiber material is continuously fed and stretched. At least one side of the stretched .[.web.]. .Iadd.film .Iaddend.is homopolarly charged by a plurality of corona charging elements. The charged .[.web.]. .Iadd.film .Iaddend.material is then fibrillated, collected and processed into a filter.

I claim: 
     
       1. A method for the manufacture of an electrically charged fibrous filter from a high molecular non-polar material, comprising the steps of: continuously feeding a film of high molecular isotactic polypropylene material at a rate of 12.2 meters per minute;   stretching said film in two stages, said first stage of stretching stretches said film at a ratio of 1:6 at a temperature of approximately 110° C., said second stage of stretching stretches said film at a ratio of 1:1.5 at a temperature of substantially 130° C.;   homopolarly charging at least one side of the stretched film with a plurality of corona charging elements, said plurality of corona discharge elements being connected to minus 10 KV, said step of charging including the use of a metal plate connected to an opposite polarity voltage source and a grid whereby said charging corona elements are between said metal plate and said film, said metal plate being connected to a voltage of plus 3 KV, said grid being connected to a voltage of minus 2.3 KV, said corona charging elements being spaced substantially 5 mm from the means for supporting said film during said second stage of stretching;   fibrillating the charged film into fiber material; .Iadd.and .Iaddend.   collecting the fiber material .[.; and processing the collected fiber material into a filter.]. .Iadd.to form a filter.Iaddend..   
     
     
       2. A method for the manufacture of an electrically charged fibrous filter from a high molecular non-polar material, comprising the steps of: continuously feeding a film of high molecular isotactic polypropylene material at a rate of 12.2 meters per minute;   stretching said film in two stages, said first stage of stretching stretches said film at a ratio of 1:6 at a temperature of approximately 110° C., said second stage of stretching stretches said film at a ratio of 1:1.5 at a temperature of substantially 130° C.;   homopolarly charging at least one side of the stretched film with a plurality of corona charging elements, said plurality of corona discharge elements being connected to minus 3.2 KV, said step of charging including the use of a metal plate connected to an opposite polarity voltage source whereby said charging corona elements are between said metal plate and said film and said metal plate being connected to a voltage of plus 3 KV, said corona charging elements being spaced substantially 5 mm from the means for supporting said film during said second stage of stretching;   fibrillating the charged film into fiber material; .Iadd.and .Iaddend.   collecting the fiber material .[.; and processing the collected fiber material into a filter.]. .Iadd.to form a filter.Iaddend..   
     
     
       3. A method as in claim 2 wherein said step of homopolarly charging said film takes place simultaneously with said second stage of stretching. 
     
     
       4. A method as in claim 2 wherein said step of homopolarly charging takes place simultaneously with the second stage of stretching at the region of highest heat application on said moving film. 
     
     
       5. A method as in claim 2 wherein said film is bi-laterally charged by a plurality of coronoa elements on each side of said film. 
     
     
       6. A method as in claim 2 wherein said step of homopolarly charging includes the step of using a grid connected to the same polarity voltage source as said plurality of charging corona elements such that said grid is between said charging corona elements and said film. 
     
     
       7. A method as in claim 5 wherein said step of stretching is accomplished in two stages and said step of homopolarly charging said film takes place simultaneously with said second stage of stretching. 
     
     
       8. A method as in claim 7 wherein said step of homopolarly charging takes place simultaneously with the second stage of stretching at the region of highest heat application on said moving film. 
     
     
       9. A method as in claim 6 wherein said film is bi-laterally charged by a plurality of corona elements on each side of said film. .Iadd. 10. A method for the manufacture of an electrically charged fibrous filter from a highly molecular non-polar material, comprising the steps of: continuously feeding a film of said material;   heating said film .Iaddend.   stretching said film along the longitudinal axis thereof as defined by the path of movement of said film;   homopolarly electrically charging at least one side of said film;   fibrillating the charged film into fiber material; and   collecting the fiber material to form a filter. .Iadd. 11. A method as in claim 10 wherein said film is stretched substantially nine times the original length thereof. .Iaddend..Iadd. 12. A method as in claim 10 wherein said step of stretching includes the step of heating said film wherein the temperature is slightly below the melting temperature of said film. .Iaddend..Iadd. 13. A method as in claim 10 wherein said step of stretching includes the step of heating said film and the temperature is dependent on the speed at which the film is continously fed, said temperature increasing with increaing speed of film feeding. .Iaddend..Iadd. 14. The method as in claim 10 wherein said step of stretching includes the step of stretching said film in two stages wherein both stages of stretching include the application of heat to said film and wherein the temperature in both stages is slightly lower than the melting temperature of said film. .Iaddend. .Iadd. 15. A method as in claim 10 wherein said step of stretching includes the step of stretching said film in two stages wherein both stages of stretching include the application of heat to said film and wherein the temperature of the second stage of stretching is greater than the temperature of the first stage of stretching. .Iaddend..Iadd. 16. A method for the manufacture of an electrically charged fibrous filter from a highly molecular non-polar material, comprising the steps of:   continuously feeding a film of said material;   heating said film   stretching said film;   homopolarly electrically charging at least one side of said film;   fibrillating the charged film into fiber material substantially in the longitudinal direction thereof as defined by the path of feeding of the film; and   collecting the fiber material to form a filter. .Iaddend..Iadd. 17. A method as in claim 16 wherein said step of fibrillating employs a needle roller having a higher peripheral velocity than the speed of feeding of said film. .Iaddend..Iadd. 18. A method as in claim 16 wherein said step of fibrillating includes spreading the film fibers, which spreading is enhanced by the electrostatic charges injected during said step of charging. .Iaddend. .Iadd. 19. A method for the manufacture of an electrically charged fibrous filter from a highly molecular non-polar material, comprising the steps of:   continuously feeding a film of said material;   heating said film;   stretching said film;   homopolarly electrically charging at least one side of said film;   fibrillating the charged film into fiber material; and   collecting the fiber material on a roller and simultaneously taking one or more layers lying on top of the other together from said roller to form a filter. .Iaddend..Iadd. 20. A method for the manufacture of an electrically charged fibrous filter from a highly molecular non-polar material, comprising the steps of:   continuously feeding a film of said material;   heating said film;   stretching said film;   homopolarly electrically charging at least one surface of said film using a corona device spaced from the surface of said film whereby the electric charge is sprayed onto the surface of said film;   fibrillating the charged material into fiber material; and   collecting the fiber material to form a filter. .Iaddend..Iadd. 21. The method as in claim 20 wherein said step of charging coincides with said step of stretching. .Iaddend. .Iadd. 22. A method as in claim 21 wherein said step of charging includes the steps of inserting a metal grid between said corona device and the surface of said film, and supporting said film on a grounded support member. .Iaddend..Iadd. 23. A method as in claim 22 wherein said step of charging includes the step of applying an alternating voltage to the corona wires of said corona device. .Iaddend..Iadd. 24. A method as in claim 22 wherein said step of charging increases the spraying intensity of the charge onto the surface of the film by positioning a metal plate a greater distance from the film surface than said corona device. .Iaddend..Iadd. 25. A method as in claim 20 wherein said step of homopolarly charging at least one side of said film includes charging both surfaces of said film by respective corona devices spaced from the respective surfaces of said film. .Iaddend..Iadd. 26. A method as in claim 25 wherein the step of charging the film surface includes the step of applying equal but opposite charging to the respective opposite surfaces of said film. .Iaddend..Iadd. 27. A method as in claim 20 wherein said step of charging includes ageing the film thermally to obtain high charge persistence and also to increase the thermal stability of the charge of said film. .Iaddend. .Iadd. 28. A method as in claim 20 wherein said step of charging includes a first step of charging one surface of the film with one charge polarity coincidentally with said step of stretching by a corona device spaced from said film surface, and a second subsequent step of charging the other film surface with a charge of opposite polarity by a second corona device spaced from said other film surface. .Iaddend..Iadd. 29. A method as in claim 21 wherein said step of stretching includes the application of heat to said film and said step of charging includes positioning said at least one corona device in a region of the highest temperature applied during said step of stretching. .Iaddend..Iadd. 30. A method for the manufacture of an electrically charged fibrous filter from a highly molecular non-polar material, comprising the steps of:   continuously feeding a film of said material;   stretching said film in two stages along the longitudinal axis thereof as defined by the path of movement of said film, and including the step of applying heat to both said two stages;   homopolarly charging said film in said second stage;   fibrillating the charged film into fiber material substantially along said longitudinal axis; and   collecting the fiber material to form a filter. .Iaddend..Iadd. 31. A method as in claim 30 wherein said step of fibrillating employs a needle roller having a higher peripheral velocity than the speed of feeding said film. .Iaddend. .Iadd. 32. A method as in claim 30 wherein said fiber material is collected on a roller, said step of collecting including the step of taking one or more layers lying on top of one another together simultaneously from said roller. .Iaddend..Iadd. 33. A method as in claim 30 wherein the temperature in each of said two stages is dependent upon the speed at which said film is continuously fed, said temperature increasing with increasing speed of film feeding and extending to a temperature slightly lower than the melting temperature of said film. .Iaddend..Iadd. 34. A method as in claim 33 wherein said step of homopolarly charging said film includes the step of spraying electrical charge onto at least one film surface by corona effect and at least one grounded film support member. .Iaddend..Iadd. 35. A method as in claim 34 wherein said step of spraying includes placing at least one corona effect device in spaced relationship to said at least one film surface and positioning a metal grid between said at least one corona effect device and said at least one grounded film support member. .Iaddend..Iadd. 36. A method as in claim 35 wherein said step of spraying includes applying an alternating voltage to the corona wires of said corona effect device. .Iaddend..Iadd. 37. A method as in claim 35 wherein the step of spraying increases the spraying intensity of the charge by positioning a metal plate a greater distance from the film surface than said corona effect device. .Iaddend. .Iadd. 38. A method as in claim 34 wherein said step of spraying is implemented where the temperature of the film is the highest. .Iaddend..Iadd. 39. A method as in claim 38 wherein said film is stretched in stretching ratios of 1:4 and 1:1.5 in said first and second stage, respectively. .Iaddend..Iadd. 40. A method as in claim 34 wherein said step of spraying is applied at the initial portion of said second stage. .Iaddend..Iadd. 41. A method as in claim 34 wherein said film is homopolarly charged in two stages wherein the first charging stage corresponds to the second stretching stage and including spraying electrical charges onto said one surface in the first charging stage and spraying electrical charges onto the other film surface by a corona effect with a second film support member in the second stage of charging. .Iaddend..Iadd. 42. A method as in claim 41 wherein the electrical charges sprayed onto the respective film surfaces are of opposite polarity. .Iaddend..Iadd. 43. A method as in claim 41 wherein charges sprayed onto each of the film surfaces are of the same polarity and the potential applied to said second support member is the same as the potential for spraying electrical charges in the first charging stage and the magnitude of the potential for spraying in the second charging stage is greater than the potential of said second support member. .Iaddend. .Iadd. 44. A method for the manufacture of an electrically charged fibrous filter from a highly molecular non-polar material, comprising the steps of:   continuously feeding a film of said material;   stretching said film along the longitudinal axis thereof as defined by the path of movement of said film and including the step of applying heat to said film;   homopolarly charging the stretched film on both surfaces thereof by spraying one surface with a charge of one polarity and spraying the other surface of said film with an equal charge of the opposite polarity;   fibrillating the charged film into fiber material substantially along said longitudinal axis; and   collecting the fiber material to form a filter. .Iaddend..Iadd. 45. A method as in claim 41 wherein said step of homopolarly charging is coincidental with said step of stretching. .Iaddend..Iadd. 46. A method as in claim 44 wherein each film surface is sprayed by placing a respective corona effect device in spaced relationship to a respective film surface and positioning a metal grid between each of said corona effect devices and the respective film surface. .Iaddend..Iadd. 47. A method as in claim 46 wherein the step of spraying increases the spraying intensity of the charge by positioning a metal plate a greater distance from the film surface than said corona effect device. .Iaddend. .Iadd. 48. A method as in claim 46 wherein said step of spraying includes applying an alternating voltage to the corona wires of each of the cornoa effect devices. .Iaddend.