Bicomponent Fibers and Methods for Making Them

Abstract

The present invention relates to bicomponent polymer fibers, and to processes for forming those fibers. Bicomponent polymer fibers are described, having a core comprising a core polymer and a sheath comprising a sheath polymer, wherein the sheath polymer is a polyolefin having an Mw less than about 65,000 g/mol. The core polymer has an Mw at least about 20,000 g/mol greater than the Mw of the sheath polymer. Processes for forming bicomponent fibers are also described, comprising (i) forming a molten blend of a core polymer and a sheath polymer; (ii) extruding the molten polymer blend using an extrusion die having a length to diameter ratio greater than or equal to about 10 and under shear conditions sufficient to drive the sheath polymer to the die wall; and (iii) forming meltblown fibers having a core comprising the core polymer and a sheath comprising the sheath polymer.

1 . A bicomponent polymer fiber having a core comprising a core polymer and a sheath comprising a sheath polymer, wherein the sheath polymer is a polyolefin having a weight average molecular weight (Mw) less than about 65,000 g/mole and wherein the Mw of the core polymer is at least about 20,000 g/mole greater than the Mw of the sheath polymer. 
     
     
         2 . The polymer fiber of  claim 1 , wherein the core polymer is selected from a propylene-based polymer, an ethylene-based polymer, a propylene-ethylene block copolymer, a styrenic block copolymer, an acrylate, or a combination of the foregoing. 
     
     
         3 . The polymer fiber of  claim 1 , wherein the core polymer is a propylene-based polymer comprising from about 5 to about 30 wt % ethylene and/or a C 4 -C 12  alpha olefin and having a triad tacticity greater than about 90% and a heat of fusion less than about 75 J/g. 
     
     
         4 . The polymer fiber of  claim 1 , wherein the propylene-based polymer comprises from about 8 to about 20 wt % ethylene. 
     
     
         5 . The polymer fiber of  claim 1 , wherein the propylene-based polymer comprises from about 12 to about 18 wt % ethylene. 
     
     
         6 . The polymer fiber of  claim 1 , wherein the Mw of the core polymer is at least about 75,000 g/mole greater than the Mw of the sheath polymer. 
     
     
         7 . The polymer fiber of  claim 1 , wherein the Mw of the core polymer is at least about 100,000 g/mole greater than the Mw of the sheath polymer. 
     
     
         8 . The polymer fiber of  claim 1 , wherein the Mw of the sheath polymer is less than about 50,000 g/mole and the Mw of the core polymer is greater than about 100,000 g/mole. 
     
     
         9 . The polymer fiber of  claim 1 , wherein the sheath polymer is selected from a polypropylene wax, a polyethylene wax, and combinations thereof. 
     
     
         10 . The polymer fiber of  claim 1 , wherein the sheath polymer comprises a backbone of propylene or ethylene with maleic anhydride grafted to the backbone. 
     
     
         11 . The polymer fiber of  claim 1 , wherein the sheath polymer is a polyethylene or polypropylene wax comprising at least one functionalized end group providing a polar character to the polymer. 
     
     
         12 . A nonwoven fabric comprising polymer fibers according to  claim 1 . 
     
     
         13 . A process for forming bicomponent polymer fibers comprising:
 a. forming a molten blend of a core polymer and a sheath polymer;   b. extruding the molten polymer blend using an extruder having a die with a length to diameter ratio greater than or equal to about 10 and under shear conditions sufficient to drive the sheath polymer to the die wall; and   c. forming meltblown fibers having a core comprising the core polymer and a sheath comprising the sheath polymer;   
       wherein the sheath polymer is a polyolefin having a weight average molecular weight (Mw) less than about 65,000 g/mole and wherein the Mw of the core polymer is at least about 20,000 g/mole greater than the Mw of the sheath polymer. 
     
     
         14 . The process of  claim 13 , wherein the core polymer is selected from a propylene-based polymer, an ethylene-based polymer, a propylene-ethylene block copolymer, a styrenic block copolymer, an acrylate, or a combination of the foregoing. 
     
     
         15 . The process of  claim 13 , wherein the core polymer is a propylene-based polymer comprising from about 5 to about 30 wt % ethylene and/or a C 4 -C 12  alpha olefin and having a triad tacticity greater than about 90% and a heat of fusion less than about 75 J/g. 
     
     
         16 . The process of  claim 13 , wherein the propylene based polymer comprises from about 8 to about 20 wt % ethylene. 
     
     
         17 . The process of  claim 13 , wherein the Mw of the core polymer is at least about 75,000 g/mole greater than the Mw of the sheath polymer. 
     
     
         18 . The process of  claim 13 , wherein the Mw of the core polymer is at least about 100,000 g/mole greater than the Mw of the sheath polymer. 
     
     
         19 . The process of  claim 13 , wherein the Mw of the sheath polymer is less than about 50,000 g/mole and the Mw of the core polymer is greater than about 100,000 g/mole. 
     
     
         20 . The process of  claim 13 , wherein the sheath polymer is selected from a polypropylene wax, a polyethylene wax, and combinations thereof. 
     
     
         21 . The process of  claim 13 , wherein the sheath polymer comprises a backbone of propylene or ethylene with maleic anhydride grafted to the backbone. 
     
     
         22 . The process of  claim 13 , wherein the sheath polymer is a polyethylene or polypropylene wax comprising at least one functionalized end group providing a polar character to the polymer. 
     
     
         23 . A nonwoven fabric comprising polymer fibers made according to  claim 13 . 
     
     
         24 . An article comprising a nonwoven fabric according to  claim 23 . 
     
     
         25 . The fiber according  claim 1 , further comprising at least one of carbon black, clay, talc, calcium carbonate, mica, silica, silicate, hindered phenol, hindered amine, phosphate, sodium benzoate, oleamide, erucamide, calcium stearate, hydrotalcite, and calcium oxide.