Structure formation of regenerated cellulose materials from NMMO-solutions

Abstract

Cellulose as the most abundant regrowing organic material exhibits outstanding properties and useful applications, but also a tremendous challenge with regard to an economical and environmentally friendly chemical processing. In recent years the N-methylmorpholine-N-oxide (NMMO)-technology turned out to be a simple physical alternative to the yet dominating viscose-technology for producing regenerated cellulosic fibers, films, food casings, membranes, sponges, beads, and others without hazardous byproducts. With consideration of own results, the present paper reviews the state of the art knowledge on structure formation of fibers and films via the NMMO-route comprising the cellulose–NMMO–water phase system, the state of solution, the dry jet-wet shaping, the precipitation, and the drying stages. Dissolving pulp as the starting material can be dissolved easily without pretreatment in NMMO-monohydrate. The fairly (8–12%) concentrated solution of cellulose in NMMO-monohydrate is characterized by a marked elastic behavior similar to a polymer melt which can be quantified by rheological measurements of the storage and loss moduli. As found by light scattering experiments of diluted cellulose–NMMO solutions, there exist aggregates of molecules even in the diluted solution, with the number of molecules corresponding to solid state morphological units (crystallites, microfibrils). As shown by WAXS-RDF analysis of the concentrated solutions at elevated temperature, the typical short-range order of a pure NMMO–water system is only slightly disturbed by the cellulose molecules. Fiber formation occurs in a dry jet-wet spinning process, with several physical factors (e.g. nozzle and air-gap dimensions, draw-down ratio, take-up speed) and dope characteristics (cellulose DP and concentration, temperature, modifiers) influencing the shaping process and the final fibers properties. The precipitation process has been shown to be another stage capable to affect the structure and properties of the fibers as, e.g. by a two-step precipitation leading to a skin–core structure and improved fiber properties (reduced fibrillation). The NMMO method offers for the first time the possibility to produce blow-extruded tube-like films similar to the polyolefine blown film processing. The influencing parameters are discussed and the properties of the new blown cellulosic films are shown to be superior to cellophane. Finally, the structures and properties of the NMMO-type fibers and films have been investigated and differences between the new materials and the traditional viscose based fibers and films were shown and related to the different structure formation routes.