The NaOH/Urea Systems of Producing Regenerated Cellulose Fibers
An extension of the alkali system of dissolving cellulose and the most recent development in the production of regenerated cellulose fibers has been the dissolution of cellulose using NaOH/urea or NaOH/thiourea systems [04Rua, 06Che]. In one such approach, cotton linter pulp (DP ~ 550) of 4–5 wt% was dissolved using NaOH (9.5 %) and thiourea (4.5 %) solution that was precooled to −8 to −10 °C [01Zha, 10Zha]. After dissolution, the solution was filtered, degassed, and extruded through a spinneret into a coagulation bath. Various chemicals (mainly acids or salts) were added into the coagulation bath, and it was found that aqueous solutions of sulfuric acid, hydrochloric acids, acetic acid, or ammonium salts were best for fiber formation. Fibers were produced using a laboratory wet spinning system at a pressure of 0.15 MPa and with a spinneret diameter of 0.12 mm [04Cai, 06Che]. Morphologically, the fibers obtained had a circular cross section contrary to traditionally produced viscose fibers that have a distinguishing irregular cross section. Unlike the conventional viscose process where complete dissolution of cellulose occurs, the new solvent system is considered to be a physical sol–gel process that helps to retain the circular shape of the fibers [04Rua]. Some of the properties of the fibers obtained using the NaOH/urea systems are compared to the traditional viscose fibers obtained from the NMMO system in Table 17.1.