Methods of making low-shrink polypropylene fibers

Abstract

Improved polypropylene fibers exhibiting greatly reduced heat- and moisture-shrink problems and including certain compounds that quickly and effectively provide rigidity to the target polypropylene fiber after heat-setting are disclosed herein. In such a manner, the “rigidifying” compounds provide nucleation sites for polypropylene crystal growth. After drawing the nucleated composition into fiber form, the fiber is then exposed to sufficient heat to grow the crystalline network, thus holding the fiber in a desired position. The preferred “rigidifying” compounds include dibenzylidene sorbitol based compounds, as well as less preferred compounds, such as sodium benzoate, certain sodium and lithium phosphate salts (such as sodium 2,2′-methylene-bis-(4,6-di-tert-butylphenyl)phosphate, otherwise known as NA-11).

What we claim is:  
     
         1 . A method of producing a polypropylene fiber, said method comprising the sequential steps of 
 a) providing a polypropylene composition in pellet or liquid form comprising at least 10 ppm of a nucleating agent;    b) melting and mixing said polypropylene composition of step “a” to form a substantially homogeneous molten plastic formulation;    c) extruding said plastic formulation to form a fiber structure,    d) mechanically drawing said fibers; and    e) exposing said drawn fiber of step “d” to a temperature of at least 110° C.; 
 wherein the temperature at which step “d” is performed is below that at which step “e” is performed.  
   
     
     
         2 . A method of producing a polypropylene fiber as in  claim 1 , wherein the temperature in the drawing step “d” is less than 100° C.  
     
     
         3 . A method of producing polypropylene fiber as in  claim 2 , wherein the fiber is exposed to a high temperature in step “e” for at most 1 second.  
     
     
         4 . A method of producing polypropylene fiber as in  claim 1 , wherein the fiber exhibits a shrinkage at 150° C. in hot air of at most 7%.  
     
     
         5 . The method of producing polypropylene fiber as in  claim 1 , wherein the fiber exhibits a shrinkage at 100° C. in boiling water of at most 3%.  
     
     
         6 . The method of producing polypropylene fiber as in  claim 1 , wherein said nucleating agent of step “a” is present in an amount of at least 100 ppm.  
     
     
         7 . The method of producing polypropylene fiber as in  claim 6 , wherein said nucleating agent of step “a” is present in an amount of at least 1250 ppm.  
     
     
         8 . The method of  claim 1  wherein said nucleating agent of step “a” is selected from the group consisting of p-MDBS, 3,4-DMDBS, 2,4,5-TMDBS, Liquid 3,4-DMDBS, DBS, sodium benzoate, NA-11, NA-21, and any mixtures thereof.  
     
     
         9 . The method of  claim 8  wherein said nucleating agent is p-MDBS.