Low-shrink polypropylene fibers

Abstract

This invention relates to improvements in preventing heat- and moisture-shrink problems in specific polypropylene fibers. Such fibers require the presence of certain compounds that quickly and effectively provide rigidity to the target polypropylene fiber after heat-setting. Generally, these compounds include any structure that nucleates polymer crystals within the target polypropylene after exposure to sufficient heat to melt the initial pelletized polymer and upon allowing such a melt to cool. The compounds must nucleate polymer crystals at a higher temperature than the target polypropylene without the nucleating agent during cooling. In such a manner, the “rigidifying” nucleator 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). Specific methods of manufacture of such fibers, as well as fabric articles made therefrom, are also encompassed within this invention.

What we claim is:  
     
         1 . A polypropylene fiber possessing at most 5,000 denier per filament and exhibiting a heat-shrinkage in at least 150° C. hot air of at most 11%, wherein said fiber further comprises at least one noncoloring nucleating agent.  
     
     
         2 . The polypropylene fiber of  claim 1 , wherein said fiber possesses a denier per filament of at most 1,000, wherein said fiber exhibits a heat shrinkage in at least 150° C. hot air of at most 9%, and wherein said at least one nucleating agent is present in an amount of at least 100 ppm.  
     
     
         3 . The polypropylene fiber of  claim 2 , wherein said fiber possesses a denier per filament of at most 500, wherein said fiber exhibits a heat shrinkage in at least 150° C. hot air of at most 8%, and wherein said at least one nucleating agent is present in an amount of at least 1250 ppm.  
     
     
         4 . The polypropylene fiber of  claim 3 , wherein said fiber possesses a denier per filament of at most 100 and wherein said fiber exhibits a heat shrinkage in at least 150° C. hot air of at most 8%.  
     
     
         5 . The fabric article of  claim 1  wherein said noncoloring nucleating agent 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.  
     
     
         6 . The polypropylene fiber of  claim 5  wherein said noncoloring nucleating agent is p-MDBS.  
     
     
         7 . A yarn comprising at least one fiber as defined in  claim 1 .  
     
     
         8 . A yarn comprising at least one fiber as defined in  claim 2 .  
     
     
         9 . A yarn comprising at least one fiber as defined in  claim 3 .  
     
     
         10 . A yarn comprising at least one fiber as defined in  claim 4 .  
     
     
         11 . A yarn comprising at least one fiber as defined in  claim 5 .  
     
     
         12 . A yarn comprising at least one fiber as defined in  claim 6.