Morphology and properties of melt-spun polypropylene

The purpose of this investigation was the experimental study of structural and morphological development during the melt spinning of an isotactic polypropylene and the correlation of developed structure and morphology with observed mechanical properties.

Experimental data was obtained from spinning apparatus consisting of an extruder-spin head mounted on a column so that data could be obtained from the moving spinline at specified distances from the spinneret. Measurements made included birefringence and surface temperature as well as x-ray diffraction.

Fibers were spun at three melt temperatures, and at varying take-up velocity. Spinning conditions were chosen so that the effect of stress and the effect of cooling rate could be treated separately.

Quantitative conclusions based on the experimental data have been made. At low measured spinline stress polypropylene fibers possess spherulitic morphology. As spinline stress is increased a "row structure" morphology is observed with chain or c-axis orientation becoming parallel to the fiber axis. This morphological change correlates with a change in fiber properties from "cold drawable" to "hard elastic."

The effect of bimodal chain or c-axis orientation on calculated Hermann-Stein orientation functions is shown.

Crystallization kinetics were observed to change with changes in both engineering stress and cooling rate. These changes could be discussed in terms of "constant cooling transformation" curves.