Fabrication of high-strength fibers from ultrahigh-molecular-weight polyethylene

Abstract

Production of high-strength fibers from flexible-chain polymers having mechanical properties close to theoretically possible is one of the most important problems of physical chemistry and technology of polymers. Essential progress in this field has been achieved with the advent of gel spinning. Further enhancement of the mechanical properties of flexible chain polymers is possible only by complex investigations and, on their basis, goal-oriented formation of an optimal gel structure at all stages of the technological process. In this paper the structural transitions in ultrahigh-molecular-weight polyethylene (UHMWPE) in the course of gel spinning of high strength fibers were studied by optical spectroscopy, electron microscopy, X-ray diffraction, and differential scanning calorimetry. Special attention was devoted to such important stages of the gel spinning process as the liquid‒gel transition and the transition of gel fibers into a highly oriented state. It was shown that the structure of thermoreversible UHMWPE gels can described in terms of a modified Keller model. According to this model, the nodes of the spatial gel network are lamellar microcrystallites 4-5 nm in thickness and 20 nm in lateral dimension. During orientation drawing of UHMWPE gel fibers, gradual straightening of molecular folds occurs, and the structure comprising crystallites on folded chains transforms to the structure composition fibrillar crystallites on straight chains. Investigation on a solvent-free technology of fabrication of high-strength film fibers from UHMWPE reactor powders (RPs) was initiated. The technology includes three stages: RP consolidation (compaction), fabrication of a monolithic film (monolithization), and orientation drawing of the monolithic film. A special cell for consolidation and monolithization is was developed. The effect of temperature and applied pressure on the consolidation and monolithization of the UHMWPE RP was studied. It was found that at ambient temperature and a pressure of 136 MPa the consolidation process is complete within 15 min. The optimal temperature range for the monolithization process is 135–140°С. First results of attempted orientation drawing of UHMWPE monolithic films are reported.