The influence of drawing, twisting, heat setting, and untwisting on the structure and mechanical properties of melt-spun high-density polyethylene fiber

Abstract

The variation of crystalline morphology and mechanical properties of polyethylene fibers was studied as they were sequentially melt spun, drawn, twisted, heat set, and untwisted. Twisting of as-melt spun fibers was also investigated. The morphology was characterized using wideangle x-ray diffraction, small-angle x-ray diffraction, and scanning electron microscopy techniques. Drawing results in high crystalline orientation, fibrillation, and large increases in modulus and tensile strength. Effects due to variation of spinning conditions were noted. Twisting either as-spun or drawn fibers decreased the axial orientation, modulus, tensile strength, and usually also the elongation to break. The changes in these properties increased with twist angle. Twisting also caused transformation of a small fraction of the sample to the monoclinic form of polyethylene. Heat setting caused healing of voids generated during drawing and increased the perfection and periodicity of the stacking of lamellar crystals along the fiber axis. Heat setting also caused the monoclinic polyethylene to transform back to the orthorhombic form, and it increased the modulus and tensile strength. Untwisting returned the orientation in the fiber to essentially that which it would have if it had not been twisted, but untwisting also resulted in the formation of kink bands.