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ABSTRACT:The structures of polypropylene fibers, spun according to a factorial experimental design, have been studied with the aid of wide angle X-ray diffraction and birefringence measurements. From statistical analysis of the results, the fibers have been characterized in terms of their crystallographic order and the overall orientation of their constituent polymer chains. These properties have been quantitatively assessed as responses to seven specially selected process control parameters in the extrusion equipment used to process the fibers. For both crystallographic order and overall orientation, the metering pump speed (MPS) at which the fibers are extruded and the speed (WS) at which the extruded fibers are wound exert significant effects. Moreover, the interaction, WS ϫ MPS, between these two control parameters also significantly influences orientation. For crystallographic order, two further significant parameters are the melt flow index (MFI) of the grade of polypropylene used and the temperature (ST) at which the polymer melt passes through the spinneret. The roles of these two factors in the development of crystallites within the fibers are discussed. No interaction effects, however, appear to be significant for crystallographic order. Models that specify the direction of change of the significant parameters for increasing or reducing both responses are given.

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As-spun polypropylene (PP) fibres, obtained under a one-step statistical experimental design using orthogonal arrays and involving all the common spinning parameters were studied with wide angle X-ray scattering (WAXS). The WAXS results revealed that the micro-structure of PP fibres can be altered progressively from less-ordered para-crystalline states to highly-ordered crystalline states with an α-monoclinic lattice. The degree of structural order has been evaluated quantitatively in terms of (W½)−1, the reciprocal of the half-height width of the first WAXS reflection peak. On the basis of such quantitative evaluation, which is shown to be consistent with the conclusions derived from “traditional” qualitative assessment, comprehensive quantitative analysis of the effects of processing parameters and their interactions on the degree of structural order is accomplished, using systematic but simple statistical methods. Parameters significantly affecting structure, including types of raw materials, winding speed, spinning temperature, application speed of spin finish and metering pump speed are identified, and correlation between structure and parameter setting is established in a concise statistical model. The model indicates clearly the combination of parameter levels for increasing or reducing the degree of structural order over the whole range from essentially amorphous to highly crystalline forms. Comparisons with the “conventional” approach of “one factor at a time” demonstrated the validity of the new approach and its significant advantages. The latter includes the economy in experiment scale, efficiency in result analysis and a comprehensive overview of the process being studied, which covers effects arising from synergistic interactions as well as from individual parameters.

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