Regenerated Cellulose Fibers

Keywords

Cellulose • Green solvent • Toxicity • Fibrillation • High temperature

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.

© Springer-Verlag Berlin Heidelberg 2015

N. Reddy, Y. Yang, Innovative Biofibers from Renewable Resources, DOI 10.1007/978-3-662-45136-6_18

spinning pump with

filter and spinneret

spinning bath

Fig. 18.1 The NMMO process for producing regenerated cellulose fibers (reproduced with permission from Elsevier)

water

Fiber

Fig. 18.2 Depiction of the steps in producing regenerated cellulose fibers from wood using the lyocell process

The draw ratios of the fibers were also found to significantly affect the properties of the regenerated fibers including those produced using NMMO as the solvent [96Mor2, 96Mor3]. As seen in Table 18.3, increasing the draw ratio substantially increased (more than three times) the strength and modulus and decreased the

elongation. This was mainly due to the better orientation of the fibrils in the fibers as indicated by the increasing orientation factor.

Fibers obtained from the traditional xanthate and the new NMMO process (lyocell fibers) show considerable variations in tensile, mechanical, and perfor­mance properties [04Car]. The lateral order index (LOI) (1,420/893 cm-1) and total crystallinity index (TCI) (1,375/2,900 cm-1) calculated using FTIR spectrums showed that lyocell fibers had higher % crystallinity than viscose fibers which was also confirmed using X-ray diffraction studies as given in Table 18.4. Lyocell fibers were also more thermally stable which was related to the higher crystallinity

and better crystal orientation in the fibers [04Car]. Similar results were also reported by Xu et al. who compared the structure and thermal properties of Tencel (cellulose produced using the NMMO system) and bamboo viscose and conven­tional viscose fibers. Tencel was found to have higher % crystallinity (69 %) and thermal stability than the viscose fibers [07Xu]. In a theoretical study using viscoelastic models, it was determined that regenerated cellulose obtained using the lyocell process had higher tensile properties in the dry and wet conditions attributable to the higher molecular weights and crystallinity [13Zha]. Microwave heating has been used to decrease the dissolution time and energy consumption during NMMO process of fiber production [09Dog].