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.
