Process for making fine spunbond filaments

Abstract

A method is provided for preparing webs of spunbond fibers having reduced diameter. The spunbond fibers can be single component fibers or multiple component fibers having a symmetric cross-section, or combinations thereof. The fibers are re-heated and drawn in a secondary drawing step after quenching to provide fibers having at least a 5% reduction in average fiber diameter compared to fibers that have not been re-heated and drawn in a secondary drawing step.

1 . A method for preparing a spunbond nonwoven fabric, comprising the steps of: 
 a. melt spinning a plurality of continuous polymeric filaments from a spinneret, wherein the continuous filaments are selected from the group consisting of single component filaments and multiple component filaments having a symmetric cross-section and comprising at least a first polymeric component and at least a second polymeric component;    b. drawing the filaments in a first drawing step;    c. quenching the drawn filaments;    d. passing the quenched filaments through a pneumatic draw jet,    e. supplying the draw jet with a gaseous stream, the gaseous stream applying a tension to the filaments as the filaments and the gaseous stream pass through and exit the draw jet;    f. heating the filaments while under the tension applied by the gaseous stream to a temperature sufficient to draw the filaments in a second drawing step, thereby reducing the average filament diameter by at least 5 percent compared to the average filament diameter that is achieved when the filaments are not heated and drawn in a second drawing step in an otherwise identical process; and    g. collecting the filaments on a collecting surface to form a nonwoven web.    
   
   
       2 . The method of  claim 1 , wherein the heating step comprises blowing hot air on the filaments as they exit the draw jet.  
   
   
       3 . The method of  claim 1 , wherein the heating step comprises exposing the filaments to radiant heat as they exit the draw jet.  
   
   
       4 . The method of  claim 1 , wherein the multiple component filaments comprise bicomponent filaments that have a concentric sheath-core cross-section.  
   
   
       5 . The method of  claim 4 , wherein the sheath comprises a polymer selected from the group consisting of polyethylenes and polyester copolymers and the core comprises a polymer selected from the group consisting of polypropylenes, polyesters, and polyamides.  
   
   
       6 . The method of  claim 5 , wherein the sheath comprises a polymer selected from the group consisting of polyethylenes and polyester copolymers and the core comprises a polyester.  
   
   
       7 . The method of  claim 6 , wherein the core comprises poly(ethylene terephthalate).  
   
   
       8 . The method of  claim 7 , wherein the sheath comprises linear low density polyethylene.  
   
   
       9 . The method of  claim 5 , wherein the sheath comprises a polyester copolymer selected from the group consisting of poly(ethylene terephthalate) modified with di-methyl isophthalic acid, and poly(ethylene terephthalate) modified with 1,4-cyclohexanedimethanol.  
   
   
       10 . The method of either of claims  1  or  6 , wherein the filament diameter is reduced by at least 10 percent compared to the average filament diameter that is achieved when the filaments are not heated and drawn in a second drawing step.  
   
   
       11 . The method of either of claims  1  or  6 , wherein an average filament diameter of less than 14.5 micrometers is achieved after the second drawing step compared to the average filament diameter that is achieved when the filaments are not heated and not drawn in a second drawing step in an otherwise identical process.  
   
   
       12 . The method of  claim 11 , wherein an average filament diameter of less than 10 micrometers is achieved after the second drawing step.  
   
   
       13 . The method of  claim 1 , wherein the continuous filaments comprise single component filaments.  
   
   
       14 . The method of  claim 13 , wherein the single component filaments comprise polyester filaments.  
   
   
       15 . The method of  claim 14 , wherein the polyester comprises poly(ethylene terephthalate)  
   
   
       16 . The method of  claim 1 , wherein the multiple component filaments comprise bicomponent filaments that have a segmented pie cross-section having an even number of alternating segments, wherein adjacent segments comprise different polymers.  
   
   
       17 . The method of  claim 16 , wherein adjacent polymer segment combinations are selected from the group consisting of polypropylene/polystyrene, polypropylene/poly(ethylene terephthalate), polypropylene/polyamide, polyethylene/polystyrene, polyethylene/poly(ethylene terephthalate), polyethylene/polyamide polystyrene/polyamide and poly(ethylene terephthalate)/polyamide.  
   
   
       18 . The method of  claim 17 , wherein the adjacent polymer segment combination is polypropylene/poly(ethylene terephthalate)  
   
   
       19 . The method of  claim 17 , wherein the adjacent polymer segment combination is polyethylene/poly(ethylene terephthalate).  
   
   
       20 . An apparatus for forming a nonwoven web of polymeric continuous filaments, comprising: 
 a. a spinneret for spinning continuous polymeric filaments;    b. a quenching means positioned below the spinneret;    c. a pneumatic draw jet having a filament inlet and a filament exit positioned below the quenching means, through which the polymeric filaments are passed together with a gaseous stream that is fed to the pneumatic draw jet;    d. a means for heating the polymeric filaments to a temperature sufficient to draw the filaments after the filaments exit the draw jet, the heating means located no more than 20 cm below the exit of the draw jet; and    e. a collecting surface for collecting the filaments to form a nonwoven web.    
   
   
       21 . The apparatus of  claim 20 , wherein the heating means comprises a hot air jet.  
   
   
       22 . The apparatus of  claim 20 , wherein the heating means comprises a radiant heat source.  
   
   
       23 . The apparatus of  claim 22 , wherein the radiant heat source comprises an infrared heat source.  
   
   
       24 . The apparatus of either of claims  21  or  22 , wherein the heating means is located no more than 0.5 cm from the exit of the draw jet.