Low-shrink polypropylene fibers, fabrics made therefrom, and methods of making thereof

This invention relates to improvements in heat and moisture-shrink problems in polypropylene fibers. Such fibers require the presence of compounds that quickly and effectively provide rigidity to the fiber after heat-setting. These compounds include structures that nucleate polymer crystals within polypropylene after exposure to sufficient heat to melt the polymer and allowing the melt to cool. The compounds must nucleate polymer crystals at higher temperatures than the polypropylene without the nucleating agent during cooling. Thus, the "rigidifying" nucleator compounds provide nucleation sites for polypropylene crystal growth. After drawing the nucleated composition into fiber form (11), the fiber is exposed to sufficient heat to grow the crystalline network (26), 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).