MELTING BEHAVIOR OF POLYPROPYLENE FIBERS STUDIED BY DIFFERENTIAL SCANNING CALORIMETRY

Polypropylene fibers produced in a compact-spinning process were studi ed by differential scanning calorimetry (DSC). With unrestrained fiber s, the onset of melting increases with decreasing draw ratio, increasi ng M(w)/M(n), decreasing extrusion temperature, increasing annealing r atio, and increasing draw-down ratio. These trends are discussed in te rms of restraints and reorganization. The onset of melting is shifted to lower temperatures as the heating rate increases for all combinatio ns of material and processing parameters, indicating suppressed reorga nization. At low draw ratios, the height and width of the endotherm ar e affected by the spinline stress, and a secondary peak or shoulder is observed on the high temperature side of the main peak. The magnitude of the secondary peak increases with decreasing M(w)/M(n), increasing draw ratio, decreasing draw-down ratio, and decreasing heating rate, but its position mainly depends on the heating rate. This indicates th at the secondary peak may be due to the melting of structures that hav e been reorganized during the heating scan. As the draw ratio increase s, the melting regime broadens, especially towards lower temperatures, and several maxima emerge on the DSC curve. Reorganization and shrink age during heating may explain these observations. (C) 1995 John Wiley & Sons, Inc.