Regenerated Cellulose Fibers

Keywords

Cellulose • Dissolution • Room temperature • Ionic interaction • Solubility • Production properties

Room temperature ionic liquids are considered green solvents and typically have low vapor pressure and good dissolution power and are easily recyclable [10Mak]. Ionic liquids used to dissolve cellulose should have low melting points, should not decompose cellulose, and should be stable and easily recoverable and relatively inexpensive. Considerable research has been done on dissolving cellulose using ionic liquids. Some of the ionic liquids that have been used to dissolve cellulose with concentrations of 10 % or above are listed in Table 19.1 [10Mak]. Dissolution of cellulose by ionic liquids is mainly related to the combined properties of the cations and anions and the basicity of the hydrogen bonds. Smaller cations were found to be more efficient in dissolving cellulose, and cations containing hydroxyl end groups had lower solubility [05Zha]. This is because the hydroxyl groups in the cations react with acetate of chloride anions and compete with cellulose to form hydrogen bonds. Ionic liquids with high hydrogen bond basicity were also found to have better solubility for cellulose. Ionic liquids are considered to be non-derivatizing solvents for cellulose, and therefore changes to the structure of cellulose are not expected. However, depolymerization of cellulose has been observed at high temperature when ionic liquids such as 1-allyl-3- methylimidazolium chloride [AMIM][Cl] were used [05Zha], whereas [BMIM] [Cl] did not depolymerize cellulose.

Although there are no reports on commercial-scale production, several labora­tory and pilot-scale studies have been conducted on producing regenerated cellu­lose fibers using ionic liquids [08Kos]. Eucalyptus pre-hydrolysis sulfate pulp (DP 569) and cotton linter pulp (DP 454) were dissolved between 90 and 130 °C under pressure (700-705 mbar) in several ionic liquids and spun into fibers. Solubility of cellulose in the solvents ranged from 10 to 17 %. Some of the

Table 19.1 List of some of the ionic liquids that have been used to dissolve cellulose in concentrations above 10 % Ionic liquid % Dissolved Condition References [EMIM][Cl] 15.8 85 °C [08Kos] [EMIM][OAcl] 13.5 85 °C [08Kos] [AMIM][Cl] 14.5 80 °C [05Zha] [BMIM][Cl] 10-18 85-100 °C [02Swa] [3MBPy][Cl] 39 105 °C, 1 h [00Hei, 05Hei] N, N,N-triethyl-3,6,9-trioxadecylammonium formate 10 110 °C [09Zha] [HDBU][Cl] 100 100­150 °C [08DAn]

Table 19.2 Properties of cellulose fibers produced using ionic liquids [08Kos] Ionic liquid BMIMCl EMIMCl BMIMAc BMIMAc EMIMAc NMMO- MH Cellulose concentration 13.6 15.8 13.2 18.9 19.6 13.5 Spinning temperature [°C] 116 99 90 98 99 94 Fiber fineness [dtex] 1.46 1.84 1.67 1.64 1.76 1.49 Fiber tenacity [g/den] 6.0 6.0 5.0 5.5 5.2 4.9 Elongation [%] 13.1 12.9 15.5 12.9 11.2 16.7 Modulus [g/den] 77 102 80 81 77 106 Wet modulus [g/den] 35 35 21 31 8 22

properties of the fibers produced from cellulose dissolved in various ionic solvents are given in Table 19.2 [08Kos].

Fibers produced from ionic solvents have similar tensile strength but lower elongation and modulus compared to the properties of the fibers produced by the NMMO system. It was also seen that cellulose dissolved using the chloride anion produced fibers that had higher tenacity but lower elongation than those produced using acetate anions.

Two direct solvents 1-ethyl-3-methyl imidazolium acetate [EMIM][OAc] and 1-ethyl-3-methyl imidazolium diethyl phosphate ([EMIM][DEP]) which have melt­ing points below room temperature and have good solubility and recoverability were used to produce fibers from cotton linters (DP 759) and eucalyptus sulfite pulp (DP 592). The pulp was dissolved in the solvent at 85 °C for 2 h and later wet and dry spun into fibers. Both dry and wet spinning produced fibers with properties similar to that of the fibers obtained using the NMMO process except that the fibers

Table 19.3 Comparison of the properties of regenerated cellulose fibers produced using two ionic solvents in comparison to the NMO process of fiber production [EMIM][OAc] Wet spun Dry spun EMIM[DEP] NMMO-MH Dope concentration 6-10 6-10 10 10 Dissolution temperature [°C] 85 85 85 120 Dissolution time [hours] 1-2 1-2 1-2 4-5 DP 590 500 524 556 Crystallite length [nm] — 8.62 5.87 6.14 Fiber fineness [dtex] 1.6 4.1 4.9 4.7 Fiber tenacity [g/den-dry] 2.5 2.8 3.0 2.9 Fiber tenacity [g/den-wet] 2.2 2.4 2.7 2.6 Elongation [%-dry] 8.0 3.8 6.0 8.5 Elongation [%-wet] 9.8 5.2 6.6 9.7

Spinning speed	50 m/min	65 m/min	80 m/min
Linear density [dtex]	2.22	2.22	2.22
Tenacity [g/den]	4.0	4.4	4.8
Elongation [%]	6.6	6.5	6.2
Modulus [g/den]	72	80	82
% Crystallinity	67	72	73
Crystal orientation [%]	74.1	82.7	83

Table 19.4 Influence of spinning speed on properties of the fibers [12Jia]

obtained from [EMIM][OAc] by dry spinning had low elongations as seen from Table 19.3. Fibers obtained from the ionic liquids had good strength and elongation retention when wet [13Ing].

Cotton pulp (5 %) with a DP of 514 was dissolved using [BMIM][Cl] by heating at 90 °C, and the solution was extruded through a spinneret with an orifice diameter of 0.15 mm. Various spinning speeds and draw ratios were chosen to study their influence of fiber properties. Properties of the fibers obtained at three different spinning speeds are compared in Table 19.4. Fibers obtained at different spinning speeds had considerably lower strength and elongation mainly due to the poor drawing. Increasing spinning speeds increased the crystallinity and crystal orienta­tion [12Jia]. Cai et al. dissolved wood pulp (8 %) with a DP of 722 using [BMIM] [Cl] and obtained fibers with a tenacity of 3.3 g/den and an elongation of 8 % [10Cai]. Fibers produced in both these studies had circular cross section similar to that of lyocell fibers, and the cellulose had diffraction patterns typical of cellulose II, suggesting that cellulose was transformed during the fiber formation. [BMIM] [Cl] was also used to dissolve wood pulp and bagasse pulp and produce fibers on a laboratory scale. Properties of the fibers are given in Table 19.5 along with the fibers produced from various other cellulose sources. Morphologically, regenerated cellulose fibers produced using ionic liquids as solvents had a circular cross section similar to the fibers obtained using the NMMO or NaOH/urea systems as seen from Fig. 19.1.

Table 19.5 Properties of fibers produced from various cellulose sources using [BMIM][Cl] as solvent Source Fineness [tex] DP Tenacity [g/den] Elongation [%] Modulus [g/den] Crystallinity [%] Bagasse 234 800­ 1,200 2.3 ± 0.2 3.9 ± 0.5 — 71-74 Wood 270 1,800­ 2,000 2.1 ± 0.2 4.8 ± 0.7 — 65-67 Cotton pulp 22 514 42.1 6.2 90.6 73 Eucalyptus 14.6 569 53.4 13.1 77.0 — Reproduced from [08Kos, 11Jia, 12Jia]

Fig. 19.1 Longitudinal and cross-sectional images of regenerated cellulose fibers produced using (a) [BMIM][Cl] and (b) NMMO (lyocell process). Fibers have circular cross sections unlike the irregular cross sections produced by the conventional rayon and viscose processes [10Cai]. Reproduced with permission from Wiley

In addition to using traditional pulp, microcrystalline cellulose and bleached kraft pulp were converted into different degree of polymerization, dissolved using 1-ethyl-3-methyl imidazolium acetate [EMIMAc] at 70 °C for 12 h, and the relationships between DP and viscosity and mechanical properties were studied. A linear relationship was found between DP and tenacity in the dry and wet state.

Fiber tenacities ranged from 0.9 to 1.8 g/den when dry and between 0.1 and 0.5 g/ den when wet with the DP varying from 330 to 1,340. Elongation of the fibers varied between 5 and 12 % but did not show a strong correlation with DP [13Ols]. Kim and Jang used 1-ally-3-methylimidazolium chloride [AMIM]Cl to dissolve microcrystalline cellulose (DP 1740), filter paper (DP 2310) and cotton fabrics (DP 2730), and produced fibers with fineness between 10.7 and 12.0 tex. Fibers produced from cellulose with high DP resulted in fibers with higher strength and modulus but lower elongation [13Kim].

References

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