Nascent Morphology, the Important Link between Chemistry/Catalysis and Rheology/Processing pp. 179-194
Authors: (Piet Lemstra)
Abstract: Solution(gel)-spun polyethylene fibers are nowadays produced commercially and make an inroad in specific application such as ballistic protection (helmets, bulletproof vests) replacing aramid fibers such as KevlarŪ. The base material for high-performance polyethylene (PE) fibers is ultra-high molecular weight PE (UHMW-PE) , an intractable polymer in view of its high molar mass, > 106 gram/mole ,which makes melt-processing impossible. The term solution(gel)-spinning is misleading because UHMW-PE is spun from a true solution and gels are formed upon quenching the as-spun filaments which consists of 90% or more solvent. Upon quenching, the UHMW-PE crystallizes and the as-spun filaments become gelly-like. The essential feature is that upon dissolution of UHMW-PE, the very long chain molecules become dis-entangled from each other and this dis-entangled state is preserved in the as-spun filaments promoting ultra-drawing, even after removal of the solvent before the drawing process. The major disadvantage of these solution(gel)-spinning processes is that a large amount of solvent is needed, 90% or more, which has to be recovered fully. Solvent-free routes are currently explored using dis-entangled UHMW-PE produced directly in the reactor. In principle, upon compacting UHMW-PE reactor powder, consisting of folded-chain crystals, below the melting point renders films which are ultra-drawable, similar to solution-spun/cast UHMW-PE fibers and films. These solvent-free routes could well take over the current solution(gel)- spinning processes. In this chapter the development of high-performance PE fibers is discussed in retrospect and with an outlook to the future.