TheN-Methylmorpholine-N-Oxide (NMMO) Process of Producing Regenerated Fibers
The NMMO process is considered to be the most environmentally friendly method of producing regenerated cellulose fibers on a commercial scale. Regenerated cellulose fibers generally called “lyocell” (Lenzing) and also available in trade names such as “New Cell” (Akzo Nobel) and “Tencel” (Courtaulds) are regenerated cellulose fibers that are commercially available and are claimed to have considerable advantages over the traditional regenerated cellulose fibers produced through the viscose or cuprammonium process. Schematics of the steps involved in the dissolution, production, and regeneration of the fibers are shown in Figs. 18.1 and 18.2. It has been well documented that the properties of the fibers produced using the NMMO process can be varied to a large extent by controlling the spinning parameters such as type of solvent, extrusion speed, air gap distance, coagulation conditions, etc. [00Dre, 01Fin]. Similarly, post-fiber treatments such as solvent exchange during precipitation from methanol to water or posttreatment with hot water and aqueous NaOH changes the crystallinity, fibrillar structure, and therefore fiber properties [01Fin]. Changes in the tensile properties and fibrillation of the fibers with varying air gap distance and conditions in the air gap are given in Table 18.1. As seen in the table, elongation and fibrillation index are affected by the spinning conditions to a greater extent than the tenacity or modulus because of the changes in the orientation and crystallinity of the fibers. Similar changes in fiber properties were observed when the concentration of cellulose or % water in the solution was changed as seen in Table 18.2. Lower concentration of cellulose will allow the fibers to relax leading to lower tensile properties but less fibrillation [96Mor1]. Morphologically, fibers obtained through the NMMO process have a circular cross section compared to the irregular cross section seen in conventional viscose-type fibers.