SYNOPSISThe tensile properties of polypropylene fibers, produced in a short-spin line, are correlated with the parameters of the three processing stages (spinning, drawing, and annealing), and with the molecular weight distribution. In general, tensile stiffness and strength increase with increasing molecular orientation, while the elongation at break decreases. The degree of orientation is determined by the deformation ratios and temperatures of the first two stages. Tensile modulus and strength also increase with increasing annealing stage shrinkage ratio. All the tensile properties, including the elongation at break, increase with increasing average molecular weight. The mechanisms of crystallization and deformation are related to the molecular weight distribution in different ways. Hence, the tensile modulus is highest for broad distributions when the draw ratio is low, and for narrow distributions when the draw ratio is high. The tensile strength increases and the elongation at break decreases as the width of the molecular weight distribution decreases, for all combinations of processing parameters. The distribution of tensile strength, for fibers with high draw ratios, broadens as the molecular weight distribution narrows. The total draw ratio of fibers, as experienced during processing and testing, and the true stress at break, are discussed in terms of deformation rates and relaxation times.

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SYNOPSISPolypropylene fibers produced in a compact-spinning process were studied by differential scanning calorimetry (DSC). With unrestrained fibers, the onset of melting increases with decreasing draw ratio, increasing MJM,, decreasing extrusion temperature, increasing annealing ratio, and increasing draw-down ratio. These trends are discussed in terms of restraints and reorganization. The onset of melting is shifted to lower temperatures as the heating rate increases for all combinations of material and processing parameters, indicating suppressed reorganization. At low draw ratios, the height and width of the endotherm are 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 MJM,,, increasing draw ratio, decreasing draw-down ratio, and decreasing heating rate, but its position mainly depends on the heating rate. This indicates that the secondary peak may be due to the melting of structures that have been reorganized during the heating scan. As the draw ratio increases, the melting regime broadens, especially towards lower temperatures, and several maxima emerge on the DSC curve. Reorganization and shrinkage during heating may explain these observations. 0 1995 John Wiley & Sons, Inc. I NTRODUCT! ONA DSC heating scan provides a fingerprint of the sample, containing information about the processing history and material parameters. Using statistical methods, a set of DSC curves can be "calibrated" to yield structural information otherwise only directly available by techniques such as wide-angle xray scattering (WAXS) . Furthermore, the melting dynamics observed by DSC provides supplementary information about structural restraints and molecular mobility. DSC analyses also have more direct applications, e.g., in assessing the bondability of fibers to be used in thermobonded nonwoven fabrics.DSC thermograms obtained for oriented polymers are generally not well understood. Endotherms with several maxima are often observed for oriented polymers. Different interpretations can be found in the literature. One of the debated issues is to what extent * To whom correspondence should be addressed.Journal of Applied Polymer Science, Vol. 57, 1075-1084 (1995) 0 1995 John Wiley & Sons, Inc.CCC 0021-8995/95/091075-10 structural reorganization during heating influence the thermograms. The position and the shape of the endotherm depend on the processing and material parameters of the polymer sample, the sample preparation technique, and the heating rate. There are variations in published data for seemingly similar samples, but DSC curves are reproducible for a given instrument and sample preparation technique.This article deals with polypropylene (PP) fibers produced in a compact-spinning process. The structure development in this process and the tensile properties of the fibers have been discussed Effects of processing parameters and molecular weight distribution (MWD) on the DSC thermograms wi...

SYNOPSISTypical polypropylene fibers for use in light nonwoven fabrics were produced in a fullscale compact-spinning line. Molecular weight distribution (MWD), extrusion temperature, draw-down ratio, and draw ratio were varied. The fibers were thermally bonded (welded) into nonwoven fabrics, at different bonding temperatures, using a pilot calender line. The tensile properties of the fabrics are influenced by the MWD and the processing conditions of the fibers, and the effects of these fiber parameters increase with increasing bonding temperature. The fabric strength increases with increasing MJM,, decreasing draw ratio, and increasing extrusion temperature, while in all these cases the fiber strength generally follows the opposite trend. Furthermore, the fabric strength, as well as the fiber strength, have a maximum as a function of draw-down ratio. The tensile properties of the fabrics seem to be governed by the bonding properties of the constituent fibers, not the fiber strength per se. Bond characteristics are discussed in terms of skin-core structures. Some details of the macroscopic fracture mechanisms of fabrics were revealed by scanning electron microscopy and the shape of load-elongation curves. 0 1995 John Wiley & Sons, Inc.

SYNOPSISThe cooling of the spinning stage in a commercial compact-spinning line has been studied. A rectangular fiber bundle is extruded from the spinneret and cooled by air entering from one side. The speed of the cooling air is considerably reduced through the fiber bundle. There are practical lower and upper limits for the cooling air entrance speed, corresponding to filament breakage a t the leeward and windward sides, respectively. These limits are functions of the material and processing parameters. Due to the nonuniform cooling, fibers sampled at the windward side generally have higher molecular orientation, lower amorphous fraction, higher density, and higher tensile modulus and strength. For most combinations of spinning and material parameters, the structure is either bimodally oriented a-crystalline or uniaxially oriented mesomorphic a t all spinneret positions. Fibers with different structure types show opposite windward/leeward side trends with regard to local order and melting behavior. The structure may be mesomorphic a t the leeward side and a-crystalline at the windward side, if the average spin-line stress is close to a critical value for orientationinduced crystallization, and the air speed difference across the spinneret is large. The cooling air speed affects the spin-line stress. Hence, the fiber-to-fiber variations due to nonuniform cooling are discussed in terms of the molecular orientation in the melt and the effective time available for arranging molecules into ordered structures. 0 1995 John Wiley & Sons, Inc.