Nonwoven elastic fibrous webs and methods for making them

Abstract

A coherent nonwoven fibrous web comprises directly formed elastic fibers that have a molecular orientation sufficient to provide a birefringence number of at least 1×10 −5 and preferably at least 1×10 −2 . The web can be made by a method that comprises a) extruding filaments of elastic-fiber-forming material; b) directing the filaments through a processing chamber in which gaseous currents apply a longitudinal stress to the filaments that attenuates and draws the filaments; c) maintaining the filaments at their orienting temperature while the filaments are under attenuating and drawing stress for a sufficient time for molecules within the filaments to become oriented along the length of the filaments; d) cooling the filaments to their orientation-locking temperature while the filaments are under attenuating and drawing stress and further cooling the filaments to a solidified fiber form; and e) collecting the solidified fibers as a fibrous nonwoven web. In a preferred aspect, the method includes the further step of annealing the collected fibers by exposing them to a temperature that is above their shrinking temperature but less than their relaxation temperature, and preferably bonding the fibers after (or before) the annealing step. Dimensionally stable webs comprising elastic oriented fibers are obtained.

1 . A coherent nonwoven dimensionally stable fibrous web comprising directly formed elastic fibers that have a molecular orientation sufficient to provide a birefringence number of at least 1×10 −5 .  
   
   
       2 . A web of  claim 1  in which said directly formed elastic fibers have a molecular orientation sufficient to provide a birefringence number of at least 1×10 −2 .  
   
   
       3 . A web of  claim 1  in which the elastic fibers comprise ethylene-based polymer and the fibers exhibit a birefringence number of at least 1×10 −2 .  
   
   
       4 . A web of  claim 1  in which the elastic fibers comprise propylene-based polymer and the fibers exhibit a birefringence number of at least 1×10 −2 .  
   
   
       5 . A web of  claim 1  in which the elastic fibers comprise urethane-based polymer and the fibers exhibit a birefringence number of at least 1×10 −2 .  
   
   
       6 . A web of  claim 1  in which the elastic fibers comprise a styrenic block copolymer.  
   
   
       7 . A web of  claim 1  in which the elastic fibers comprise aliphatic polyester or aliphatic polyamide.  
   
   
       8 . A web of  claim 1  that has been annealed by heating the elastic fibers above their shrinking temperature while retaining said molecular orientation sufficient to provide a birefringence number of at least 1×10 −5 .  
   
   
       9 . A web of  claim 1  that exhibits a shrinkage in width of no more than 10% when heated to 70° C.  
   
   
       10 . A web of  claim 1  in which the elastic fibers exhibit an elongation to break of at least 200%.  
   
   
       11 . A web of  claim 10  in which the elastic fibers have sufficient elastic recovery that when released from tension stretching them to twice their original length they will retract to no more than one-and-one-fourth their original length.  
   
   
       12 . A web of  claim 11  in which the elastic fibers exhibit a birefringence number of at least 1×10 −2 .  
   
   
       13 . A web of  claim 1  in which the elastic fibers have been thermally bonded.  
   
   
       14 . A web of  claim 13  in which the bonds comprise autogenous bonds.  
   
   
       15 . A web of  claim 1  in which the fibers of the web have been hydroentangled to mechanically bond the fibers.  
   
   
       16 . A web of  claim 1  in which the elastic fibers exhibit strain-induced crystallization.  
   
   
       17 . A coherent nonwoven dimensionally stable fibrous web comprising directly formed elastomeric fibers that have a molecular orientation sufficient to provide a birefringence number of at least 1×10 −2 .  
   
   
       18 . A web of  claim 17  in which the elastomeric fibers comprise an ethylene-based or propylene-based polymer.  
   
   
       19 . A web of  claim 17  in which the elastomeric fibers have been annealed and subsequently bonded while retaining said molecular orientation sufficient to provide a birefringence number of at least 1×10 −2 .  
   
   
       20 . A web of  claim 17  in which the elastomeric fibers exhibit strain-induced crystallization.  
   
   
       21 . A fiber-forming method comprising a) extruding filaments of elastic-fiber-forming material; b) directing the filaments through a processing chamber in which a longitudinal stress is applied to the filaments that attenuates and draws the filaments; c) maintaining the filaments at their orienting temperature while the filaments are under attenuating and drawing stress for a sufficient time for molecules within the filaments to become oriented along the length of the filaments; d) cooling the filaments to their orientation-locking temperature while the filaments are under attenuating and drawing stress and further cooling the filaments to solidified elastic fibers; and e) collecting the solidified elastic fibers as a fibrous nonwoven web.  
   
   
       22 . A method of  claim 21  in which the filaments enter the processing chamber at a temperature higher than the glass transition temperature or melting point of the filaments.  
   
   
       23 . A method of  claim 21  in which the largest longitudinal stress is applied to the filaments after they leave the processing chamber.  
   
   
       24 . A method of  claim 22  in which the largest longitudinal stress is applied to the filaments after they leave the processing chamber.  
   
   
       25 . A method of  claim 21  in which the filaments pass through the processing chamber at a rate of at least 2800 meters/minute.  
   
   
       26 . A method of  claim 22  in which the filaments pass through the processing chamber at a rate of at least 2800 meters/minute.  
   
   
       27 . A method of  claim 24  in which the filaments pass through the processing chamber at a rate of at least 2800 meters/minute.  
   
   
       28 . A method of  claim 21  in which the filaments pass through the processing chamber at a rate of at least 4000 meters/minute.  
   
   
       29 . A method of  claim 21  including the further step of annealing the collected fibers by exposing them to a temperature that is above their shrinkage temperature but less than the relaxation temperature of the fibers.  
   
   
       30 . A method of  claim 21  including the further step of thermally bonding the fibers after they have been annealed.  
   
   
       31 . A method of  claim 21  in which the collected fibers of the nonwoven web exhibit an elongation to break of at least 200 percent.  
   
   
       32 . A method of  claim 21  in which the filaments comprise an ethylene-based polymer or a propylene-based polymer.  
   
   
       33 . A method of  claim 21  in which the filaments comprise a urethane-based polymer.  
   
   
       34 . A method of  claim 21  in which the filaments comprise a styrenic block copolymer.  
   
   
       35 . A method of  claim 21  in which the filaments comprise an aliphatic polyester or an aliphatic polyamide.  
   
   
       36 . A method for preparing a fibrous web comprising 1) preparing extruded filaments from an elastic-fiber-forming liquid, 2) processing and attenuating the extruded filaments to solid collectible fibers having molecular orientation, 3) collecting the fibers as a nonwoven web, 4) annealing the collected fibers by exposing them to a temperature that is above their shrinking temperature but less than their relaxation temperature to make the web dimensionally stable while retaining sufficient molecular orientation that the fibers exhibit a birefringence of at least 1×10 −5 , and 5) bonding the web to give it increased coherency.  
   
   
       37 . A method of  claim 36  in which step (4) is performed after step (5).  
   
   
       38 . A method of  claim 37  in which bonding of the web in step (5) comprises hydroentangling the web.  
   
   
       39 . A method of  claim 36  including the further step of thermally bonding the fibers after they have been annealed.  
   
   
       40 . A method of  claim 36  in which the filaments are processed and attenuated by passing the filaments through a processing chamber at a rate of at least 2800 meters/minute.