Improvements in preventing heat- and moisture-shrink problems in specific
polypropylene tape fibers are provided. Such tape fibers are basically
manufactured through the initial production of polypropylene films or
tubes which are then slit into very thin, though flat (and having very
high cross sectional aspect ratios) tape fibers thereafter. These
inventive tape fibers (and thus the initial films and/or tubes) require
the presence of relatively high amounts of certain compounds that quickly
and effectively provide rigidity to the target polypropylene tape fiber.
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 allowing such an oriented
polymer 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. Subsequent to
slitting the initial film and/or tube, 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 [2.2.1]heptane-bicyclodicarboxylic acid, otherwise
known as HPN-68, 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 inventive tape fibers, as
well as fabric articles made therefrom, are also encompassed within this
invention.
