Regenerated Cellulose Fibers

Keywords

Cellulose source • Wheat straw • Rice straw • bacterial cellulose • Acid tunic • Bamboo • Rayon

Apart from using conventional sources of cellulose such as wood, nonconventional resources such as sugarcane bagasse, bamboo, and bacteria have also been used to produce regenerated cellulose fibers. Viscose fibers with a linear density of 17 tex and length between 1.7 and 38 mm obtained from eucalyptus wood were modified with alkali solutions to improve the adhesive and surface properties [13Roj]. After treating with various concentrations of alkali, about 10-12 % decrease in % crystallinity was observed. Increase in diameter of fibers and decrease in contact angle after alkali treatment were also seen [13Roj].

Fabrics made from bamboo viscose were characterized for their structure and properties compared to traditional viscose rayon and cotton. Table 21.1 shows some of the comparative properties of the three types of fibers [12Mis]. Bamboo viscose had very similar properties compared to traditional viscose rayon. Compared to natural cellulose, viscose fibers and fabrics had considerably higher elongation. In addition, bamboo viscose is reported to have natural antibacterial property. Con­siderably lower numbers of bacterial colonies were observed on bamboo viscose compared to cotton and viscose as seen from Table 21.1.

Bacterial cellulose with a considerably high DP of 2700 was dissolved using NMMO and extruded into fibers on a laboratory scale. Fibers obtained had rela­tively low tenacity of 0.06-0.2 g/den and elongation of 3-8 % probably due to the poor drawing and orientation of the fibers during this process [11Gao]. Similar to using bacteria, bagasse obtained as the by-product after processing sugarcane was made into pulp and regenerated into fibers using NMMO/water as the solvent system. Pulp obtained was dissolved in NMMO/water at 120 °C for 2.5 h, and the cellulose solution was dry jet-wet spun into coagulation bath containing water, methanol, isopropanol, ethanol, or water/NMMO mixture (10 %) at 20 °C

Table 21.1 Comparison of the properties of regenerated cellulose fibers using bamboo as a source with the traditional viscose rayon and to fibers regenerated from 100 % cotton (from [12Mis]) Property Bamboo viscose Viscose rayon 100 % cotton Fabric weight [g/m2] 198 173 190 Wetting time [s] 1.36 1.71 5.02 Fiber length [mm] 38.2 37.9 28.4 Fiber fineness [tex] 12.1 11.9 12.3 Tenacity [g/den] 2.6 2.6 3.1 Strain [%] 23.4 24.5 7.3 E. coli colonies 142 245 280 S. aureus colonies 168 230 275

Table 21.2 Properties of regenerated cellulose fibers obtained from sugarcane bagasse using NMMO as the solvent and different coagulation baths [10Jal] Coagulant Draw ratio Fiber diameter [pm] Tenacity [g/den] Elongation [%] Modulus [g/den] Crystallinity [%] Water/ NMMO 17.5 38 3.4 15 ± 2 103 56 Ethanol 17 40 3.0 4.3 ± 1 155 60 Methanol 12.75 43 2.6 10 ± 1.5 97 58 Isopropanol 10.5 48 2.7 11 ± 1.8 130 56 Water 8.75 52 2.0 9.3 ± 1.6 80 48

[10Jal]. Fibers were hot drawn and also treated in an oven to orient the fibers. Some of the physical and tensile properties of the fibers obtained using various coagula­tion baths are reported in Table 21.2. Considerable variations in properties, espe­cially elongation, were seen with changing draw ratio and coagulation baths. Removal of NMMO from the fibers resulted in higher crystallinity and better orientation leading to higher strength and modulus. Stress-strain curves of fibers obtained from the different coagulation baths are shown in Fig. 21.1.

Wheat straw was steam exploded and grafted with various monomers to facili­tate dissolution and regeneration into fibers. Straw was steam exploded at 225 °C for 3 min and later treated with 2 % soda solution at 80 °C for 2 h and then bleached with 0.3 % sodium chlorite at 70 °C for 2 h. Acrylonitrile and methyl methacrylate were also grafted onto wheat straw. The treated or grafted straw was then added into dimethylacetamide at 150 °C for 30 min and later treated with lithium chloride solution (7 %) at 120 °C overnight until the samples were completely dissolved. Fibers were produced using both the dry and wet spinning system. Fibers obtained from grafted straw had considerably lower tensile properties especially at high draw ratios due to the difficulties in orienting the grafted polymers during extrusion and spinning [98Foc]. Table 21.3 provides some properties of fibers obtained from wheat straw and modified wheat straw.

Table 21.3 Properties of regenerated cellulose fibers obtained from steam exploded and grafted wheat straw [98Foc] Sample Solution concentration [%] Draw ratio Strength [g/den] Elongation [%] Modulus [g/den] Bleached whole straw 4 0.39 1.8 6.6 100 4 0.78 1.8 8.1 146 6.9 0.39 2.3 — 106 6.9 0.78 2.5 6.0 142 PAN grafted straw 7 1.02 0.8 4.4 65 7 1.26 0.8 2.5 63 7 1.52 0.8 3.1 52 PMMA grafted straw 7 1.02 0.9 3.9 69 7 1.26 1.0 4.1 72 7 1.52 0.8 4.5 60

Similar to wheat straw, rice straw has also been used to produce regenerated cellulose fibers. Rice straw chopped to about 10 cm in length was pretreated with 20 % NaOH to remove lignin and hemicellulose and then dissolved in NMMO at 100 °C. Fibers were extruded into a coagulation bath containing water and then drawn using take-up rollers. Untreated straw had a cellulose content of 44 %, 26 % hemicellulose, 15.8 % lignin, and 14 % ash compared to 60-94 % cellulose, 3-18 % hemicellulose, 1-5 % lignin, and 2-5.6 % ash after treatment depending on the extent of alkali used. Extruded fibers had diameters ranging from 10 to 25 ^m with circular cross section, and the typical fibrillated surface of regenerated cellulose fibers obtained by the NMMO process was observed. Fibers had a tenacity ranging from 1.9 to 3.1 g/den and modulus between 85 and 100 g/den similar to the fibers obtained from steam exploded and grafted wheat straw [01Lim].

Table 21.4 Comparison of the properties of regenerated cellulose fibers produced from ascidian tunic at different winding speeds using NMMO as the solvent [02Koo] Winding speed [m/min] Fineness [denier] Tenacity [g/d] Elongation [%] Modulus [g/d] Crystallinity index [%] Dry Wet Dry Wet Dry Wet 20 3.83 3.62 3.55 14.54 9.5 116 84 76.6 40 3.52 3.72 3.62 13.02 8.36 121 87 77.2 60 2.92 3.97 3.88 11.09 7.25 130 95 79.4 80 2.46 3.98 3.90 8.03 7.06 131 95 80.3 100 2.06 4.00 3.95 6.79 6.24 138 103 80.7

Fig. 21.2 Hollow regenerated cellulose fibers produced from sugarcane bagasse using NMMO solvent system. Hollow structures were obtained by changing the concentration of NMMO in the coagulation bath from 30, 40, and 50 % [11Yam]

Ascidic tunic, a by-product, was made into pulp and used to regenerate cellulose fibers. About 33 % of the tunic was obtained as pulp (DP 918) after bleaching and was dissolved (6 %) using NMMO/water (87/13) at 120 °C for 40 min. Cellulose solution was then extruded into a water bath at 30 °C. Dry and wet tensile properties of the fibers obtained and the crystallinity index are listed in Table 21.4. Properties of the fibers were found to be related to the concentration of the cellulose solution and the winding speed which governed the orientation of the cellulose crystals in the fibers. Increasing winding speed produced finer fibers with higher crystallinity and orientation leading to higher strength and modulus but lower elongation. Another distinguishing feature of the fibers was their high wet strength, more than 95 % of the dry strength due to the higher DP of the ascidic tunic pulp and higher crystallinity of the fibers [02Koo, 03Wan].

In another research, bagasse dissolved using NMMO system was extruded as fibers into a coagulation bath containing various concentrations of NMMO [11Yam]. Unique hollow fibers shown in Fig. 21.2 were obtained when the NMMO concentration in the coagulation bath was higher than 30 %. The size of the hollow center increased with increasing concentration of NMMO. When high solvent concentrations are present in the coagulation bath, the fibers do not precipi­tate completely but are rapidly coagulated in the washing step with water. During washing, the outer surface precipitates, but the inner core dissolves in water leading to the formation of the hollow center. Figure 21.2 shows the SEM picture of the hollow fibers obtained at three different concentrations of NMMO in the coagulation bath. Despite being hollow, the fibers obtained had tensile properties similar to that of the commercially produced lyocell fibers.

References

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[13Roj] Rojo, E., Alonso, M. V., Dominguez, J. C., Saz-Orozco, D., Oliet, M., Rodriquez, F.: J. Appl. Polym. Sci. 130, 2198 (2013)