Regenerated Cellulose Fibers

Keywords

Cellulose dissolution • Low temperature • Urea-sodium hydroxide • Fiber cross section • Sol-gel process

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.

The degree of polymerization (DP) of the fibers obtained from the NaOH/urea system was similar to that of the DP of the cotton linters used for fiber production indicating that there was no significant degradation of cellulose during the dissolu­tion using the new system. Tensile properties of the fibers were similar to that of the traditional viscose and cuprammonium fibers but lower than that of the fibers obtained using the NMMO system mainly because of the better drawing of the fibers during the commercial-scale NMMO production process. Recently, the

Table 17.1 Comparison of the properties of regenerated cellulose fibers obtained through the NaOH/urea system compared to the traditional viscose process and the NMMO process

industrial-scale production of regenerated cellulose fibers using the NaOH/urea system has been reported [10Li]. Fibers with strength (2.0 ± 0.2 g/den) and elonga­tion of 19 %, similar to that of commercial rayon, were obtained. Figure 17.1 shows the digital images of the dissolution and fiber production process, and Fig. 17.2 shows the images of the actual fibers and cross section of the fibers obtained [10Li]. Although fibers with properties similar to those commercially available were produced using the NaOH/urea system, there is an upper limit on the degree of polymerization and molecular weights of cellulose that can be dissolved using this

system [08Wan]. For instance, a 6 % NaOH/4 % urea aqueous solution could only completely dissolve cotton linters with Mw of up to 6.7 x 104 g/mol and cellulose II with Mw up to 11.2 x 104 g/mol [08Wan]. To overcome this limitation, enzymatic pretreatment was used to promote the dissolution of cellulose with high molecular weight. Increase in cellulose solubility from 30 to 65 % and shortening of dissolu­tion time were observed after the enzymatic treatment [08Wan]. Unlike the NaOH/ urea treatment which breaks inter — and intra-cellulose bonds, enzyme treatment attacks and cuts the cellulose crystal and increases the accessibility to NaOH and urea solutions and therefore allows the use of high molecular weight cellulose [08Wan].

Properties of the fibers obtained from the NaOH/urea system were heavily dependent on the conditions used for cellulose dissolution. As seen in Fig. 17.3, increasing the concentration of NaOH continually increased the solubility of cellulose up to an NaOH concentration of 8 %. Similarly, increasing temperature above —10 °C considerably decreases dissolution as seen in Fig. 17.4. A phase transition between gel formation and solution form occurs with the change of the temperature and concentration as seen in Fig. 17.5.

Instead of using pure cellulose, attempts were also made to use modified cellulose for dissolution and production of fibers using the NaOH/urea method of

dissolution. In one such attempt, hydroxyethylated cellulose (HEC) with low levels of substitution was dissolved in alkali solutions and extruded into fibers [13Wan]. HEC was added into 8 % NaOH/8 % urea and 6.5 % thiourea solution and cooled to —10 °C. The solution was stirred at 0 °C for 2 h to dissolve the cellulose. Dissolved cellulose was extruded into a coagulation bath consisting of 12 % sulfuric acid and 10 % sodium sulfate to precipitate the fibers which were later washed and dried [13Wan]. Structure and properties of the regenerated cellulose fibers obtained from HEC were considerably different compared to those obtained using unmodified cellulose through the traditional processes as seen in Table 17.2. The original cellulose seen in Fig. 17.6a had the cellulose I crystal structure, whereas the etherified cellulose (HEC) in Fig. 17.6b and the fibers obtained from HEC (Fig. 17.6c) had the typical cellulose II structure. The comparison of the properties between the HEC fibers and regenerated fibers obtained using the lyocell

process shows that etherification resulted in considerable decrease in % crystallinity and the fibers were also much weaker compared to the regenerated cellulose fibers obtained using the lyocell process [13Wan].

References

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