improvements in preventing heat - and moisture - shrink problems in specific polypropylene tape fibers are provided . such fibers are basically manufactured through the initial production of polypropylene films or tubes which are then slit into very thin , though flat tape fibers thereafter . such fibers require the presence of certain compounds that quickly and effectively provide rigidity to the target polypropylene tape 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 . upon slitting of 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 sodium benzoate , certain sodium and lithium phosphate salts 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 .