AbstractThe effects of the conditions fiber formation on structural parameters and tensile properties of melt‐spun, highly isotactic polypropylene filaments were investigated to determine the feasibility of producing supertenacity polypropylene filaments. Meltspun polypropylene monofilaments were prepared from five highly isotactic polymers (94 or more by extraction), under different conditions of extrusion, quenching, drawing, and annealing. The molecular weight, molecular weight distribution, crystalline structure, crystallinity, crystallite orientation, and total molecular orientation (as expressed by total draw ratio) of the experimental filaments were determined, and the relationships between these structural parameters and the tensile properties of the filaments are discussed in the paper. Several of the experimental polypropylene filaments produced had tenacities greater than 12 g./den. and one of them had a tenacity of 13 g./den. Polypropylene filaments having tenacities greater than 13 g./den. appear feasible. To produce the super‐tenacity polypropylene filaments, a polymer with a weight‐average molecular weight of about 470,000 was melt spun into undrawn filaments having the paracrystalline structure, and the filaments were drawn to a maximum and annealed. The tenacity of the polypropylene filaments increased with increasing fiber molecular weight. A narrow molecular weight distribution was beneficial, but was not necessary for obtaining high tenacities. High draw after spinning was important in producing high tenacity. The maximum amount that a fiber could be drawn depended upon its molecular weight, the amount of stretch introduced during spinning, and the drawing time and temperature (filament temperature). Filaments having the paracrystalline structure, which was obtained with proper extrusion and quenching conditions, could be drawn more easily than filaments having the crystalline structure, and hence, under equivalent conditions, the former could be drawn more. The more the filaments were stretched during spinning, the lower were their tenacities. The relationship between crystallite orientation and the tensile properties of drawn and annealed filaments stretched during spinning was different from the relationship for drawn and annealed filaments that were not stretched during spinning. The highly drawn filaments contained voids, but they could be removed by proper annealing conditions, which also increased the tenacity of the fila
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