Post-treatment of nonwoven webs

Abstract

A method for post-treating a precursor nonwoven web including consolidating the web laterally and thereby reducing the maximum pore size measure of the web. The resultant nonwoven web is also disclosed as is utilization of the product web as a filter.

What is claimed is: 
     
       1. A .Iadd.planar .Iaddend.nonwoven web made from a .Iadd.planar .Iaddend.precursor nonwoven web comprising randomly deposited nonelastomeric thermoplastic fibers and being uniaxially drawn and heat set, wherein the uniaxial heating and drawing are carried out continuously by passing the precursor web into an oven .Iadd.at a temperature in the range of 250° and 425° F. and between the softening temperature and the melting point of the thermoplastic fibers, thereby heat setting it .Iaddend.at a first linear velocity and withdrawing the precursor web from the oven at a second linear velocity and wherein the .[.ratio of the.]. second velocity .[.to.]. .Iadd.exceeds .Iaddend.the first velocity .[.ranges from about 1.1:1 to 2:1.]., whereby a majority of the fibers are .[.consolidated and.]. aligned generally in the direction of draw and a minority of segments of fibers are disposed crosswise or transversely to the direction of draw, whereby the web is .Iadd.consolidated in the cross-direction, longitudinally elongated and heat set with respect to the precursor and .Iaddend.characterized by a maximum pore size of less than 80% .Iadd.of .Iaddend.that of the precursor web and has an elasticity in the cross direction of at least 70% recovery from a 50% elongation in the cross direction. 
     
     
       2. The nonwoven web of claim 1 wherein the .[.web is composed of nonelastomeric thermoplastic meltblown.]. fibers .Iadd.are meltblown, .Iaddend.randomly bonded and .[.having.]. .Iadd.have .Iaddend.an average diameter of 0.5 to 8 microns. 
     
     
       3. The nonwoven web of claim 2 wherein the web has a mean flow pore size of between 3 to 40 microns. 
     
     
       4. The nonwoven web of claim 1 wherein the fibers are at least partially coated with a surfactant for increasing the water wettability of the fabric. 
     
     
       5. A laminate comprising: (a) the web of claim 1.[.;.]. and   (b) a polymeric substrate bonded to said web.   
     
     
       6. The .[.nonwoven web.]. .Iadd.laminate .Iaddend.of claim 5 wherein the substrate is an elastomeric web. 
     
     
       7. The .[.nonwoven web.]. .Iadd.laminate .Iaddend.of claim 5 wherein the substrate is a film. 
     
     
       8. The .[.nonwoven web.]. .Iadd.laminate .Iaddend.of claim 5 wherein the substrate is a textile that limits the cross direction extensibility of the laminate to less than 100% strain. 
     
     
       9. A .Iadd.planar .Iaddend.nonwoven web made from a .Iadd.planar precursor .Iaddend.nonwoven web, said precursor comprising a lamination of individual nonwoven webs having nonelastomeric .Iadd.thermoplastic .Iaddend.fibers and being uniaxially drawn and heat set, wherein the uniaxial heating and drawing are carried out continuously by passing the precursor web into a oven .Iadd.at a temperature in the range of 250° and 425° F. and between the softening temperature and the melting point of the thermoplastic fibers, thereby heat setting it .Iaddend.at a first linear velocity and withdrawing the precursor web from the oven at a second linear velocity and wherein the .[.ratio of the.]. second velocity .[.to.]. .Iadd.exceeds .Iaddend.the first velocity .[.ranges from about 1.1:1 to 2:1.]., whereby a majority of the fibers are .[.consolidated and .]. aligned generally in the direction of draw and a minority of segments of fibers are disposed crosswise or transversely to the direction of draw, whereby the web is .Iadd.consolidated in the cross-direction, longitudinally elongated and heat set with respect to the precursor and .Iaddend.characterized by a maximum pore size of less than 80% that of the precursor web and has an elasticity in the cross direction defined by at least 70% recovery from a 50% elongation in the cross direction. 
     
     
       10. A longitudinally extending .Iadd.planar .Iaddend.nonwoven web made from a precursor .Iadd.planar .Iaddend.nonwoven web having nonelastomeric, thermoplastic fibers characterized in that the majority of the fibers .[.have been consolidated in a direction transverse the length of the web and are.]. .Iadd.are .Iaddend.disposed generally in the longitudinal direction of the web, .Iadd.thereby consolidating the web in a direction transverse to the length of the web is longitudinally elongated and heat set with respect to the precursor, .Iaddend.wherein the consolidation is carried out by passing the passing the precursor web into an oven .Iadd.at a temperature in the range of 250° and 425° F. and between the softening temperature and the melting point of the thermoplastic fibers, thereby heat setting it, the passing being performed .Iaddend.at a first linear velocity and withdrawing the precursor web from the oven at a second linear velocity and wherein the .[.ratio of the.]. second velocity .[.to.]. .Iadd.exceeds .Iaddend.the first velocity .[.ranges from about 1.1:1 to 2:1.]., whereby the maximum pore size of the web represents less than 70% of the maximum pore size of the precursor web and, a minority of the fibers in the web .[.extent.]. .Iadd.extend .Iaddend.transverse the longitudinal direction of the web and a portion of which are bonded to the longitudinal fibers, .[.disposed.]. said web further being characterized as having a level of elasticity defined by at least 70% recovery from a 50% elongation in a direction crosswise of the longitudinal direction of the web. 
     
     
       11. A filter comprising meltblown web having nonelastomeric thermoplastic fibers having an average diameter of 0.5 to 8 microns, the majority of which are generally aligned in one direction, wherein the fibers are aligned by passing a .Iadd.planar .Iaddend.precursor web into an oven .Iadd.at a temperature in the range of 250° and 425° F. and between the softening temperature and the melting point of the thermoplastic fibers, thereby heat setting it, the passing being performed .Iaddend.at a first linear velocity and withdrawing the .[.precursor.]. web from the oven at a second linear velocity and wherein the .[.ratio of the.]. second velocity .[.to.]. .Iadd.exceeds .Iaddend.the first velocity .[.ranges from about 1.1:1 to 2:1.]., said web .Iadd.being planar and made from a planar precursor nonwoven web of randomly deposited nonelastomeric thermoplastic fibers, consolidated in the cross direction, longitudinally elongated and heat set with respect to the precursor, thereby reducing the average pore size and the pore size distribution in the web, and .Iaddend.having a mean flow pore size of 2 to 50 microns, a pore size distribution spread of 4 to 100 microns, and an elastic recovery of at least 40% from a 100% elongation in a direction normal to the direction of the aligned fibers. 
     
     
       12. The filter of claim 11 wherein the filter having a basis weight of less than 1.5 oz/sq. yd. is capable of capturing at least 50% of particles larger than or equal to 1 micron measured at an aerosol velocity of 10 centimeters/sec. 
     
     
       13. A face mask comprising the filter of claim 11. 
     
     
       14. The filter as defined in claim 11 wherein the fibers contain an electrostatic charge. 
     
     
       15. The filter as defined in claim 11 wherein the web has an electrostatic charge applied thereto. 
     
     
       16. A method for post treating a .Iadd.planar .Iaddend.nonwoven web which comprises: (a) selecting a .Iadd.planar .Iaddend.precursor nonwoven web made of randomly collected nonelastomeric thermoplastic fibers having a crystallinity of at least 30%, said web having a maximum processing draw ratio at break of less than 4 at a temperature at least 10° F. less than the melting point of the precursor thermoplastic and at a strain rate of at least 2500%/min, and a maximum pore size measure of from 4 to 250 microns based on ASTM F316-86;   (b) heating the precursor web at a temperature between the softening temperature and melting temperature of the thermoplastic,   (c) drawing .Iadd.and heat-setting .Iaddend.the heated web under tension in a substantially longitudinal direction to cause the web to .Iadd.be longitudinally elongated and heat set with respect to its precursor and .Iaddend.consolidate laterally .Iadd.without disrupting the planar integrity of the web .Iaddend.and thereby reduce the maximum pore size measure of the web by at least 20%, wherein the heating and drawing steps are carried out continuously by passing the precursor web into an oven .Iadd.at a temperature in the range of 250° and 425° F. .Iaddend.at a first linear velocity and withdrawing the precursor web from the oven at a second linear velocity.[., the ratio of.]. .Iadd.and wherein .Iaddend.the second velocity .[.to.]. .Iadd.exceeds the .Iaddend.first velocity .[.ranges from about 1.1:1 to 2:1.].; and   (d) cooling the web or permitting the web to cool.Iadd., whereby there is obtained the nonwoven web which is consolidated laterally, elastic in the cross direction, longitudinally elongated and heat set. .Iaddend.   
     
     
       17. The .[.process as.]. .Iadd.method of .Iaddend.claim 16 wherein the drawing step is sufficient to provide the web with a means flow pore size measure at least 20% smaller than the mean flow pore size of the precursor web. 
     
     
       18. The .[.process.]. .Iadd.method .Iaddend.of claim 16 wherein the drawing step is sufficient to provide the web a packing density at least 20% greater than that of the precursor web. 
     
     
       19. The method of claim 16 wherein the cooling step is carried out prior to release of the drawing tension and cools the web to a temperature below the softening temperature of the thermoplastic. 
     
     
       20. The method of claim 16 wherein the nonwoven precursor web is a meltblown web having an average fiber diameter of 0.5 to 8 microns and having an elongation at break of less than .Badd..[.30%.]..Baddend. .Iadd.40% .Iaddend.based on ASTM D5035-90. 
     
     
       21. The method of claim 20 wherein the precursor nonwoven web has a breaking elongation less than .Badd..[.40%.Baddend. .Iadd.30% .Iaddend.based on ASTM D 5035-90. 
     
     
       22. The method of claim 16 where in the thermoplastic is a polyolefin selected from the group consisting of polypropylene, polyethylene, and copolymers thereof, and the heating step is carried out at a temperature of between 190 to 350 degrees Fahrenheit. 
     
     
       23. The method of claim 22 wherein the meltblown precursor web has randomly distribute throughout fiber-fiber bonding. 
     
     
       24. The method of claim 16 wherein the thermoplastic is selected from polyesters, polyamides, cellulose triacetate, cellulose diacetate, .[.poly-4-methylpenetene-1.]. .Iadd.poly-4-methylpentene-1.Iaddend., polyphenylene sulfide, liquid crystal polymers and fluoropolymers. 
     
     
       25. The method of claim 16 wherein the precursor nonwoven web is a spun-bond web having fibers of from 7 to 50 micron average diameter and having spaced apart bonds distributed through the web. 
     
     
       26. The method of claim 16 and further comprising the step of passing the draw web over a bar or roller having a surface to impart transverse compression forces inwardly of the width of the web. 
     
     
       27. The method of claim 16 wherein the .[.first.]. velocity .Iadd.of passing the precursor web into the oven .Iaddend.is controlled by passing the precursor web through the nip of first counter rotating rollers prior to the heating step, and the .[.second.]. velocity .Iadd.of withdrawing the web from the oven .Iaddend.is controlled by passing the consolidated web through the nip of second counter rotating rollers after the heating step, and wherein the consolidated web is permitted to cool to a temperature below the softening point of the thermoplastic prior to passing through the nip of the second counter rotating roller. 
     
     
       28. The method of claim 16 wherein the precursor nonwoven web is made of a laminate of at least two distinct nonwoven webs, each web made of randomly collected nonelastomeric thermoplastic fibers and each web has distributed throughout fiber-fiber bonding. 
     
     
       29. The method of claim 28 wherein the precursor web is a composite comprising a spunbound layer/a meltblown layer/and a spunbound layer, wherein the layers are thermally bonded together at spaced apart locations. 
     
     
       30. The method of claim 16 wherein the precursor web has a width of 6 to 160 inches and a thickness of 2 to 100 mils and wherein the drawn web has a width of less than 80% of the precursor web and thickness of 2 to 10 mils and wherein the thickness ratio of the drawn web and precursor web ranges from 1:1 to 1.5:1. .Iadd.31. The method of claim 16 wherein the planar integrity of the web is maintained during drawing by developing sufficient stresses in the longitudinal direction to reorient the fibers and by developing sufficient compressive stresses to laterally consolidate the fibers, thereby reducing the average pore size and pore size distribution of the web. .Iaddend. .Iadd.32. The nonwoven web of claim 1 wherein the crystallinity of the fibers is high enough to provide a room temperature breaking elongation of less than 40%. .Iaddend. .Iadd.33. The nonwoven web of claim 9 wherein the crystallinity of the fibers is high enough to provide a room temperature breaking elongation of less than 40%. .Iaddend. .Iadd.34. The nonwoven web of claim 1 wherein the pore size distribution has been reduced by at least 20% as compared to that of the precursor nonwoven web. .Iaddend. .Iadd.35. The nonwoven web of claim 9 wherein the pore size distribution has been reduced by at least 20% as compared to that of the precursor nonwoven web. .Iaddend. .Iadd.36. The process of claim 16 wherein the pore size distribution is reduced by at 
     
     
        least 20% as compared to that of the precursor web. .Iaddend. .Iadd.37. The nonwoven web of claim 1 wherein the reduction of the maximum pore size is by at least 20% and the pore size distribution is reduced by at lest 20%. .Iaddend. .Iadd.38. The nonwoven web of claim 1 wherein the crystallinity of the fibers is in the range of 30 to 70%. .Iaddend. .Iadd.39. The nonwoven web of claim 1 wherein the porosity of the precursor web is in the range of 50 to 95%. .Iaddend. .Iadd.40. The nonwoven web of claim 1 wherein the fibers are randomly fiber-to-fiber bonded. .Iaddend. .Iadd.41. The nonwoven web of claim 40 wherein the area of the web bonding ranges from 5 to about 25%. .Iaddend. .Iadd.42. The nonwoven web of claim 1 wherein the fibers are meltblown fibers. .Iaddend. .Iadd.43. The nonwoven web of claim 1 wherein the fibers are spunbound fibers. .Iaddend. .Iadd.44. The nonwoven web of claim 1 wherein the composite web is a spunbound/meltblown/spunbound web. .Iaddend. .Iadd.45. The nonwoven web of claim 40 wherein the meltblown fibers have an average 
     
     
        diameter of 0.5 to 8 microns. .Iaddend. .Iadd.46.  The nonwoven web of claim 1 wherein the drawing of the thermoplastic fibers is performed while in the rubbery state and the temperature is in the range of 250° to 
     
     
        350° F. .Iaddend. .Iadd.47.  The nonwoven web of claim 9 wherein the drawing of the thermoplastic fibers is performed while in the rubbery state and the temperature is in the range of 250° to 350° F. .Iaddend. .Iadd.48. The nonwoven web of claim 10 wherein the passing is performed while in the rubbery state and the temperature is in the range 
     
     
        of 250° to 350° F. .Iaddend. .Iadd.49.  The method of claim 16 wherein the drawing of the web is performed while the fibers of the web are in the rubbery state and the temperature is in the range of 250° to 350° F. .Iaddend. .Iadd.50. The nonwoven web of claim 1 wherein the upper temperature in the range is within 15° F. below the melting point of the thermoplastic fibers. .Iaddend. .Iadd.51. The nonwoven web of claim 9 wherein the upper temperature in the range is within 15° F. below the melting point of the thermoplastic fibers. .Iaddend. .Iadd.52. The nonwoven web of claim 10 wherein the upper temperature in the range is within 15° F. below the melting point of the thermoplastic fibers. .Iaddend. .Iadd.53. The method of claim 16 wherein the upper temperature in the range is within 15° F. below 
     
     
        the melting point of the thermoplastic. .Iaddend. .Iadd.54.  The nonwoven web of claim 1 wherein the ratio of the difference in breaking elongation in the cross direction between the nonwoven consolidated web and the precursor to the cross direction breaking elongation of the precursor is at least 1.87. .Iaddend. .Iadd.55. The nonwoven web of claim 9 wherein the ratio of the difference in breaking elongation in the cross direction between the nonwoven consolidated web and the precursor to the cross direction breaking elongation of the precursor is at least 1.87. .Iaddend. .Iadd.56. The nonwoven web of claim 10 wherein the ratio of the difference in breaking elongation in the cross direction between the nonwoven consolidated web and the precursor to the cross direction breaking elongation of the precursor is at least 1.87. .Iaddend. .Iadd.57. The method of claim 16 wherein the ratio of the difference in breeaking elongation in the cross direction between the nonwoven consolidated web and the precursor to the cross direction breaking elongation of the 
     
     
        precursor is at least 1.87. .Iaddend. .Iadd.58.  As a product, a segment of the nonwoven consolidated web of claim 1 which has two pairs of sides and a ratio of a/c of at least 1.3, wherein "a" is the width of the web entering the oven and "c" is its width exiting the oven. .Iaddend. .Iadd.59. As a product, a segment of the nonwoven consolidated web of claim 9 which has two pairs of sides and a ratio of a/c of at least 1.3, wherein "a" is the width of the web entering the oven and "c" is its width exiting the oven. .Iaddend. .Iadd.60. As a product, a segment of the nonwoven consolidated web of claim 10 which has two pairs of sides and a ratio of a/c of at least 1.3, wherein "a" is the width of the web entering the oven and "c" is its width exiting the oven. .Iaddend. .Iadd.61. As a product, a segment of the nonwoven consolidated web of claim 16 which has two pairs of sides and a ratio of a/c of at least 1.3, wherein "a" is the width of the web entering the oven and "c" is its width exiting the oven. 
     
     
        .Iaddend. .Iadd.62.  The nonwoven web of claim 1 which is longitudinally elongated by at least 5% and heat set with respect to the length of its precursor. .Iaddend. .Iadd.63. As a product, a segment of the nonwoven consolidated web of claim 1 which has two pairs of sides and is longitudinally elongated by at least 5% and heat set with respect to its precursor. .Iaddend. .Iadd.64. The nonwoven web of claim 1 which is longitudinally elongated from by at least 10% and heat set with respect to 
     
     
        the length of its precursor. .Iaddend. .Iadd.65.  The nonwoven web of claim 16 which is longitudinally elongated from by at least 10% and heat set with respect to the length of its precursor. .Iaddend. .Iadd.66. The nonwoven web of claim 1, the nonwoven precursor of which has not been necked by drawing at ambient temperature. .Iaddend. .Iadd.67. The nonwoven web of claim 9, the nonwoven precursor of which has not been necked by drawing at ambient temperature. .Iaddend. .Iadd.68. The nonwoven web of claim 10, the nonwoven precursor of which has not been necked by drawing at ambient temperature. .Iaddend. .Iadd.69. The filter of claim 11, the nonwoven precursor web of which has not been necked by drawing at ambient temperature. .Iaddend. .Iadd.70. The method of claim 16 wherein the precursor nonwoven web has not been necked by drawing at ambient temperature. .Iaddend. .Iadd.71. The nonwoven web of claim 1, the nonwoven precursor web of which has a room temperature elongation (strain) at break between 2 and 40 percent, based on test method ASTMD 1117-77. .Iaddend. 
     
     
        .Iadd.72.  The nonwoven web of claim 9, the nonwoven precursor web of which has a room temperature elongation (strain) at break between 2 and 40 percent, based on test method ASTMD 1117-77. .Iaddend. .Iadd.73. The nonwoven web of claim 10, the nonwoven precursor web of which has a room temperature elongation (strain) at break between 2 and 40 percent, based on test method ASTMD 1117-77. .Iaddend. .Iadd.74. The filter of claim 11, the nonwoven precursor web of which has a room temperature elongation (strain) at break between 2 and 40 percent, based on test method ASTMD 1117-77. .Iaddend. .Iadd.75. The method of claim 16 wherein the nonwoven precursor nonwoven web has a room temperature elongation (strain) at break between 2 and 40 percent, based on test method ASTMD 1117-77. .Iaddend.