Masters Theses

Date of Award

3-1982

Degree Type

Thesis

Degree Name

Master of Science

Major

Polymer Engineering

Major Professor

J. E. Spruiell

Committee Members

J. L. White, J. F. Fellers

Abstract

Fibrillation is a phenomenon which arises in the processing of synthetic fibers. Fibrillation is related to a transformation from a spherulitic or cylindritic structure characteristic of the morphology of melt spun filaments to a fibrillar morphology characteristic of drawn filaments. The fibers break up into small "Fibrils" during the plastic deformation of drawing. A similar effect can occur in the processing of tapes and films and is generally detrimental to the physical and mechanical properties. Fibrillation is apparently due, at least in part, to the growth of longitudinal microvoids during deformation and transformation into the final fiber structure. The degree of fibrillation depends on many variables such as draw ratio, draw temperature, the initial structure of the spun filaments, and the molecular characteristics of the polymer. This research was undertaken in order to experimentally determine the effects of such variables on the level of fibrillation.

Various fibrillated polypropylene fibers were prepared at different spinning and drawing conditions. Hercules Profax(R) (6323,6423) and Diamond Shamrock(R) (5831-106-7) having a range of melt flow indices (6.6-25) were selected for experimental study. Each polymer was spun at two spinning conditions representing low and high molecular orientation in the, spun filaments. These filaments were then drawn 3.5X and 6X at 25°C, 60°C, 90°C, and 150°C.

The structure of the spun and drawn filaments were characterized using scanning electron microscopy, wide angle x-ray diffraction, small angle x-ray scattering, birefringence and density measurements. Based on preliminary qualitative measurements, the small angle x-ray scattering from selected samples was measured quantitatively using the 10 meter Small Angle X-ray Scattering facility at the National Center for Small Angle Scattering at Oak Ridge, Tennessee (NCSASR). The volume fraction of microvoids was computed from the invariant of the scattered intensity, and the size, shape, and number density studied by subjecting the scattered intensity to Guinier Analysis.

The tensile mechanical properties of both as-spun and drawn filaments were measured. A series of compression molded polypropylene sheets (6423,6323, 5831-106-7) were also drawn using an Instron tensile testing machine. Two drawing processes were examined: cold drawing at 25°C include draw ratios of 4X and 6X; hot drawing at 150°C include draw ratio of 4X and 6X. The structure and mechanical properties of the drawn sheets were also characterized.

Wide Angle X-ray Scattering (WAXS) patterns of drawn samples showed that cold drawing transformed the monoclinic form of as-melt spun filament into a more oriented but disordered structure, while hot drawing produced a highly oriented well-formed monoclinic structure. Samples drawn at 150°C showed somewhat lower level of "birefringence and c-axis crystalline orientation function (fc).

The volume fraction of microvoids computed from the Small Angle X-ray Scattering (SAXS) technique was found to correlate well with microvoid fractions estimated from a combination of crystallinity (DSC technique) and density measurements. These showed that the volume fraction of microvoids increased with; 1) increase in draw ratio, 2) decrease in draw temperature, 3) increased orientation in the as-spun filaments, and 4) increased molecular weight of the polypropylene. "Volume fractions of microvoids varied from about 0.4 x 10-3 (0.04%) to about 0.28 x 10-1 (2.8%).

The Guinier analysis showed that the average void size had dimensions of 250-400 Å parallel to the fiber axis and o of order 150-200 Å perpendicular to the fiber axis. Based on results of void fraction, void dimensions, void number density and Scanning Electron Microscopy (SEM) photomicrographs it was found that a) fibrillation increased with increasing draw ratio, b) drawing at 25°C is accompanied by very extensive fibrillation, while the amount of fibrillation decreases as drawing temperature increases, c) the high melt index samples showed less fibrillation than those with low melt index, and d) the filament with low take-up velocity (30 mm/min) during spinning fibrillate less comparatively than those with higher take-up.

The drawn fibers showed better mechanical properties than melt spun fibers. Under the experimental conditions investigated, the mechanical properties such as; tensile strength, tangent modulus and elongation to break of the melt spun and drawn fibers all correlated with the birefringence. It was also found that mechanical properties are mainly a function of the orientation and are not very much affected by changes in fibrillation per se.

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