COMPARATIVE CHARACTERISATION OF MAN-MADE REGENERATED CELLULOSE FIBRES

COMPARATIVE CHARACTERISATION OF MAN-MADE REGENERATED CELLULOSE FIBRES Thomas Röder 1, Johann Moosbauer 1, Gerhard Kliba 1, Sandra Schlader 2, Gerhard Zuckerstätter 2, and Herbert Sixta 3 1 Lenzing AG, Department FE, A-4860-Lenzing, Austria 2 Kompetenzzentrum Holz GmbH (Wood K plus), A-4021 Linz, Austria 3 Helsinki University of Technology, 02150 Espoo, Finland Phone: (+43) 07672-701-82; Fax: (+43) 07672-918-82; E-mail: t.roeder@lenzing.com Presented during the 8 th International Symposium Alternative Cellulose Manufacturing, Forming, Properties 03-04 th September 08 Rudolstadt/Germany Man made cellulose fibres show a large bandwidth of properties depending on their manufacturing process. Despite the differences in structure and morphology, molecular weight and its distribution, fibre properties and cross section, crystallinity and orientation, the chemical substrate is always cellulose in the modification II. The great variability of their properties is the basis for the manifold use of these fibres all over the world. Commercial man made cellulose fibres of different types (viscose, cuam and high modulus rayon, tire cord, lyocell) are compared against experimental Introduction regenerated cellulose fibres (e.g. fortisan, fibre B bocell, lyocell). Numerous characterisation methods (e.g.: REM, Raman/WAXS, SEC, NMR, fibre properties) are used to highlight similarities, differences, and characteristics of cellulose fibres. Structure property relationships will be presented and discussed. Keywords: man made cellulose fibres, fibre properties, viscose, lyocell, modal, fortisan, bocell, cuam, celsol In contrast to cotton the use of cellulose pulp made from wood to produce textiles needs a forming procedure based on cellulose dissolution followed by a forming (and a regeneration) step. The history of man-made cellulose fibres started in 1846 with the discovery of cellulose nitrate by Karl-Friedrich Schönbein. In 1892 the first factory was built in Besançon (France) by Comte Hilaire de Chardonnet producing Chardonnet silk. Cuprammonium rayon was discovered in 1857 by Eduard Schweizer, the first factory was built in 1899. Viscose (cellulose xanthate) was discovered in 1891 by Cross, Bevan, and Beadle. Nowadays a great variety of man-made cellulose fibres is known. But only a few are produced in industrial scale: viscose (CV, staple and filament), modal (CMD), lyocell (CLY), cupro (CUP), acetate (AC), and triacetate (CTA). The fibres are divided into two categories: cellulose fibres (e.g. CV, CMD, CLY, CUP) and cellulose derivative fibres (e.g. AC, CTA). We will focus on the cellulose fibres only. 98

Cellulose fibres can be produced via cellulose derivatisation followed by regeneration (involving the destruction of the derivative, e.g. viscose) or via dissolution in a non-derivatising solvent (e.g. NMMO, Cuoxam, ionic liquids) and consequent regeneration. Fibre samples In this study various commercial fibres are compared with a great variety of experimental fibres. Commercial fibres are viscose fibres (viscose, tire cord viscose, polynosic fibres), modal fibres, cupo fibres (Asahi Kasai), and lyocell fibres (Tencel ). Fibres based on cellulose derivatives are Fortisan fibres: A saponified cellulose acetate prepared by dry spinning a solution of cellulose acetate in acetone (Celanese Corp., sample kindly provided by Prof. H. Chanzy, Grenoble, France). Fibres are highly oriented. The yarn is no longer produced. Fibre B (Bocell): Anisotropic solutions of cellulose in superphosphoric acid spun via air-gap into acetone and saponification in water/soda. Fibres have high tenacities and are highly oriented (developed by H. Boerstoel (Akzo Nobel), sample kindly provided by Dr. S. Eichhorn, Manchester, UK). Viscose fibres: Cellulose xathogenate dissolved in sodium hydroxide and spun via wet spinning process into sulphuric acid/ sodium sulphate/ zinc sulphate. Modal fibres are produced by a modified viscose process. Celsol fibres: Cellulose is dissolved in sodium hydroxide and regenerated in water (Institute of Chemical Fibres, Lodz, Poland). Fibres based on direct dissolved cellulose are Cupro fibres: Cellulose dissolved in tetraaminecopper dihydroxide (complexation) and regenerated in water (Asahi Kasai). Lyocell fibres: Cellulose is directly dissolved in NMMO/water (Tencel ) or ionic liquids and spun through an air gap and precipitated in water. Cellulose solvents, resulting fibre types and their characterisation The cross section is a structural parameter of the fibre which is accessible via microscopy. Fibre diameters around 10 microns allow the use of light microscopy as well as scanning electron microscopy. Independent of the used solvent system for cellulose similar cross sections can be made (Figure 1) assuming the use of similar spinnerets. However, the same solvent system but a modified spinning bath leads to different cross sections (Figure 2). For viscose the acid concentration (same dope, same machine, all the other parameters are the same) influence the regeneration of the cellulose xanthate. A lower acid content leads to a softer regeneration. However, high acid content results in a massive loss of degree of polymerization (DP). The commercially available viscose type fibres CV, CV tire cord, CMD, and polynosic (Figure 3) differ in DP of cellulose (source), spinning process, and after treatment. Their fibre properties vary over a wide range. Nevertheless, further diversification needs the use of other solvent systems. Higher tenacity, e.g., is available by using the lyocell process. Crystallinity increases, too. The solvent for the commercial Lyocell process is NMMNO (Tencel ). Similar to Lyocell, BISFA defines a cellulose fibre obtained by a spinning process from an organic solvent mixed with water."solventspinning" means dissolution and spinning without derivatization. Therefore, every spinning process using ionic liquids to dissolve and to spin cellulose is a Lyocell process. 99

Examples for these are given in Figures 1 and 4. The shape and the overall properties of these fibres are comparable to Tencel fibres. The use of special pulp sources (e.g. cotton linters pulp) and special spinning conditions allows the manufacturing of fibres with higher tenacity and higher modulus. However, the fibre properties are in the same range as for NMMO fibres, which could be improved by the named changes in pulp source and spinning conditions, too. NMMO and ionic liquids are the most recent cellulose solvents used and lie in the focus of actual research. Other solvents for cellulose like hydrates of inorganic salts, e.g., zinc chloride and lithium perchlorate; lithium chloride and dimethylacetamide (DMAc); and metal complex solvents, e.g., copper ethylendiamine (Cu-en), are known and some are used for analytical purpose (LiCl-DMAc for SEC; Cu-en for intrinsic viscosity measurement), but they are not used to make fibres. Another interesting approach is the CELSOL concept: Cellulose is dissolved in sodium hydroxide. Known from the viscose process, where cellulose reacts with sodium hydroxide to sodium cellulose, sodium hydroxide is able to dissolve hemicellulose and shorter cellulose molecules. The idea is to use sodium hydroxide directly without cellulose xanthate as for viscose to produce man made fibres. The disadvantage is in the limited solubility of longer cellulose chains. Therefore, presumably only fibres lower than viscose quality will be possible. During World War 2 a process for manufacturing of cellulose silk for parachutes was developed: Fortisan (Figure 4). These fibres have a high crystallinity, high orientation, and high modulus. Therefore they are very lean and thin. Fortisan was produced via a dry spinning process. Figure 1. SEM figures of regenerated cellulose fibres with round shape. Figure 2. Influence of acid content in spinbath for viscose types of fibres. 100

Figure 3. SEM figures of different viscose types of fibres. Figure 4. SEM figures of regenerated cellulose fibres with different cross sections. Table 1. Fibre data. titre Tenacity cond. Elongation cond. Tenacity wet Elongation wet BISFA modulus modulus cond. Commercial/ experimental [dtex] [cn/dtex] [%] [cn/dtex] [%] [cn/tex/5%] [cn/tex/%] fibre Tencel 1,3 40,2 13,0 37,5 18,4 10,8 8,8 com. polynosic 1,8 38,2 9,7 26,0 11,0 12,1 n.a. com. cupro 2,5 22,3 24,3 17,6 51,3 5,0 9,9 com. EMIM-Cl 1,7 43,0 9,6 35,9 11,6 14,0 8,1 exp. BMIM-Cl 1,5 50,1 9,3 39,4 10,4 17,8 n.a. exp. CV 1,4 23,9,1 12,5 22,0 2,4 5,3 com. CV tire cord 1,9 52,3 15,1 38,4 22,9 3,1 11,1 com. CMD 1,3 33,1 13,5 18,4 14,1 5,2 6,3 com. fibre B 1,6 89,4 6,8 75,8 7,9 39,0 23,5 exp. EMIM-Ac 1,8 44,7 10,4 38,1 11,9 13,2 10,0 exp. Fortisan 0,7 23,9 3,2 27,7 5,1 27,0 n. a. exp. Celsol 3,32 16,96 7,8 7,2 11,2 4 n. a. exp. Up to now the highest crystallinity, orientation, and tenacity is known from the so-called fibre B. This experimental fibre is produced from liquid crystalline solution of cellulose in superphosphoric acid and spun into acetone followed by saponification. This has similarities to the old Fortisan process where cellulose acetate is spun into acetone, too. In contrast to viscose type fibres (wet spinning) all these fibres are produced via air-gap in a dry-wet (Lyocell) or in a dry spinning process (followed by saponification: Fortisan, fibre B). Characteristic fibre data are given in Table 1. 101

Structural parameters and fibre properties Fibre properties can be varied within a wide range by using the same dope and the same spinning process. The level of stretching determines the tenacity and elongation level. A high degree of stretching results in a relatively tearresistant and low-strain fibre. The product of tenacity and elongation is called working capacity. This working capacity is mainly determined by the process and the solvent used. The molecular weight of the cellulose should be in an optimum range for the used solvent. The length of cellulose chains should be high enough to be able to form fibres. Cellulose chains should, however, not be too long, since this would increase the viscosity of the dope too much. For every manufacturing process there exists a different optimum range. Structural parameters like orientation are mainly influenced by the stretching during fibre making. The liquid crystalline structure of the dope for fibre B is one reason for the high crystallinity of the fibres. The structure of the cellulose inside the dope, the width of cellulose molecular weight distribution, and the velocity of regeneration process mainly determine the crystallinity of the fibres. WAXS (wide angle X-ray scattering) is the standard method for the determination of crystallinity of man-made cellulose fibres (Hermans and Weidinger 1949) [1]. The information concerning crystallinity of cellulose II can be obtained by other methods as well, e.g. Fourier Transform (FT) Raman, FT-infrared (IR), or solid state 13 C NMR (nuclear magnetic resonance) spectroscopy. Raman and IR based methods normally use a calibration with WAXS data [2] [3]. Therefore these methods can only be as good as the reference method itself. One of the factors determining the properties of cellulose fibres is the Tenacity cond [cn/tex] 50 40 CLY, same dope, different draft 0,10 0,15 0, 0,25 0, 0,35 f 2 C(WAXS) *X IR Figure 5. CLY fibre manufacturing with the same dope at various draft levels correlation of tenacity and structural parameters orientation and crystallinity. Figure 6. WAXS versus FT-Raman crystallinity [2]. alignment of the fibrillar elements relative to the fibre axis. A relationship between the degree of orientation and the tensile strength of viscose fibres was put forth by Krässig and Kitchen [4]: Strength or tenacity in the conditioned state FFc ~ f 2 c and strength or tenacity in the wet state. FFn ~ f 2,5 tot The classical method to quantify orientation of polymer fibres is wide angle X-ray scattering (WAXS). The WAXS results were compared with Raman results to obtain a Raman-based method to determine the degree of orientation of cellulose II (Figure 7 and 8). 102

Figure 7. Calibration of crystalline fibre orientation - WAXS versus Raman. Figure 8. Calibration of total fibre orientation WAXS versus Raman. The higher resolution of Raman spectroscopy can be used as a tool for determining the local fibres orientation. Another common method to measure the fibre orientation is the birefringence. A microscope equipped with polarisers and a compensator is used. The birefringence n is obtained by dividing the measured retardation of polarised light by the respective fibre thickness. By relating n to the maximum birefringence of cellulose n max, the orientation factor f (Hermans orientation factor) can be calculated according to f = n/ n max. Factor f = 1 means perfect orientation parallel to the fibre direction, f = 0 for random orientation, and -1 for perfect transverse orientation. This method is very useful for fibres with a round cross section, e.g. lyocell, and no crimp and distortion. Unfortunately, most of commercial fibres have no round cross section, are crimped, or both. This results in large differences of orientation values. According to Krässig and Kitchen [4] a linear correlation between the degree of orientation and the fibre tenacity in conditioned as well as in wet state was found. Especially a series of CLY made from the same dope spun under different drafts showed a very good correlation 103

(Figure 5). The comparison of different man-made cellulose fibres used for analytical method development is given in Figures 9 and 10. Despite the CV tire cord the fibres showed more or less a linear correlation between tenacity and fibre orientation. Tenacity cond [cn/tex] 60 50 40 10 f C...degree of orientation of crystalline regions X IR...crystallinity determined by infrared spectroscopy CLY CMD Polynosic CV Cupro CV tire cord 0 0,0 0,1 0,2 0,3 f 2 *X C(WAXS) IR Figure 9. Correlation between tenacity (conditioned state) and structural parameters crystalline fibre orientation and crystallinity. Tenacity wet [cn/tex] 45 40 35 25 CLY 15 CMD Polyosic 10 CV Cupro 5 CV tire cord 0 0,00 0,05 0,10 0,15 f 2.5 *X tot(waxs) IR Figure 10. Correlation between tenacity (wet state) and structural parameters total fibre orientation and crystallinity. As mentioned above, the molecular weight distribution is another important parameter. State of the art is the determination of the molecular weight distribution by size exclusion chromatography (SEC). Essential is the choice of a suitable solvent system, e.g. LiCl-DMAc. Another possibility is the use of cellulose derivatives, e.g. cellulose nitrate. In this study SEC based on LiCl- DMAc was used (Figure 11). differential molecular weight differential molecular weight 1,5 fortisan fibre "B" bocell CV tire cord 1,2 0,9 0,6 0,3 0,0 1,5 1,2 0,9 0,6 0,3 0,0 3,5 4,0 4,5 5,0 5,5 6,0 log molar mass CV polynosic celsol 3,5 4,0 4,5 5,0 5,5 6,0 log molar mass Figure 11. Comparison of molecular weight distribution of man-made cellulose fibres. Lyocell fibres (Tencel and ionic liquid fibres) have a higher molecular weight than viscose fibres. The spectrum depends on the used pulp. While the cellulose is degraded in the ripening step in the viscose process, the degradation during the lyocell process is prevented by added stabilizers. Therefore, the molecular weight of lyocell fibres is mainly determined by the used pulp. Normally, the degradation during the lyocell process should be less than 10%. The elastic modulus (Young s modulus) is the linear slope of stress (tenacity) versus strain (elongation). It is a measure of the stiffness of a material. A higher elastic modulus means a higher resistance of the fibre against deformation. A comparison of typical stress-strain-curves is given in Figures 12 and 13. The highest value for the elastic modulus and tenacity as well was reached by fibre B (bocell). The lowest value was determined for viscose fibres. To get fibres in between these two values, the solvent and/or the 104

manufacturing process has to be chosen carefully. tenacity cond [cn/tex] Figure 12. Stress-strain curve of viscose type man made fibres. tenacity [cn/tex] 40 CV tire cord CMD CV 10 0 0 2 4 6 8 10 12 14 16 18 50 40 10 Tencel Fortisan Fibre B 0 0 5 10 15 elongation [%] Figure 13. Stress-strain curve of high modulus fibres compared with Tencel. Conclusions elongation cond [%] Man-made cellulose fibres are manifold and open a fascinating field of research and application. The further development of these fibres will surely direct to some surprising properties and facts. After more than 100 years of viscose fibres, even there the development is not completed. Acknowledgements The authors thank Dr. Hans-Peter Fink and Dr. Andreas Bohn (IAP Golm, Germany) for the WAXS measurements concerning crystallinity, Prof. Dr. Peter Zipper and Dr. Boril Cernev (University of Graz, Austria) for the WAXS measurements concerning orientation, and Anneliese Flachberger (Lenzing AG, Austria) for the textile measurements. We would like to thank Prof. Dr. Henry Chancy (Grenoble, France) for the kind donation of the Fortisan fibre, Dr. Frank Dürsen (Obernburg, Germany) for the viscose tire cord fibres, Dr. Birgit Kosan (TITK Rudolstadt, Germany) for the ionic liquid fibres, and Dr. Stephen J. Eichhorn (University of Manchester, UK) for the fibre B (bocell). References [1] P. H. Hermans and A. Weidinger. X- ray studies on the crystallinity of cellulose, J. Polymer Sci. 4 (1949) 135-144. [2] T. Röder, J. Moosbauer, M. Fasching, A. Bohn, H.-P. Fink, T. Baldinger, and H. Sixta. Crystallinity determination of native cellulose -comparison of analytical methods, Lenzinger Berichte 86 (06), 85-89. [3] T. Baldinger, J. Moosbauer, and H. Sixta. Supermolecular structure of cellulosic materials by Fourier Transform Infrared Spectroscopy (FT- IR) calibrated by WAXS and 13 C NMR, Lenzing Berichte, 79 (00) 15-17. [4] H. Krässig and W. Kitchen. Factors influencing tensile properties of cellulose fibres, J. Polymer Sci. 51 (1961) 123-172. 105