THE SORPTION BEHAVIOUR OF CELLULOSE FIBRES - Lenzing

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Introduction Conventional regenerated cellulose fibres are produced generally by the indirect viscose process (viscose fibres), while high-tenacity modal fibres are produced using a modification of the basic procedure. The fibre production processes are based on the derivatisation of cellulose using carbon disulfide [[4],[5]]. New lyocell fibres are produced by a more environmentally friendly procedure, by regenerating cellulose into fibre form out of a solution of cellulose in Nmethylmorpholine-N-oxide, without the formation of derivatives [[6],[7],[8],[9],[10]]. The natural, as well as the conventional and new regenerated cellulose fibres, consist more or less of a chemically clean cellulose. The natural fibres are always cleaned in order to remove the noncellulose compounds (as pectinic products, wax, proteins…). Different production processes of conventional viscose, modal, and new lyocell fibres cause differences in the structure and properties of the fibres in spite of having the same chemical composition. Cellulose fibres, natural as well as regenerated, have a crystalline/amorphous microfibrillar structure. Elementary fibrils consist of a succession of crystallites and intermediate less-ordered amorphous regions. Lateral tie molecules connect laterally adjacent amorphous regions [[11],[12]]. The differences between natural and particular types of regenerated cellulose fibres are, above all, in the size of crystallites and amorphous regions, amorphous and crystalline orientation, size and shape of the voids and the number of interfibrillar lateral tie molecules. In order to characterise cellulose accessibility, interaction with water is often employed, which is able to destroy weaker hydrogen bonds but cannot penetrate into the regions of high order [[13],[14]]). The adsorption properties of fibres can be obtained also on the basis of various methods for determining the dye, iodine, surfactant adsorption [[13],[15]]. Currently some additional methods especially sensitive to surface properties (determination of the electrokinetic properties of fibres) were applied in order to characterize the reactivity of the fibre surfaces [[16],[17]]. Analyses of the structure characteristics of new lyocell fibres and comparison with conventional viscose and modal fibres were performed. The 29 degree of polymerisation DP and the supermolecular structure (crystallinity index CrI, crystallite dimension Λ*, molecular orientation f∆n, void structure Vp, Sp, d) of the regenerated cellulose fibres were investigated [[2],[3]]. Differences in molecular and supramolecular structure of natural and different types of regenerated cellulose fibres cause different adsorption and electrokinetical properties of fibres. In addition the structural parameters (amorphous regions and void system) which significantly influence the adsorption properties of fibres (moisture adsorption, water retention, swelling of fibres in aqueous medium) were determined. The accessibility of free adsorption places in the amorphous part of cellulose fibres was investigated by the determination of electrokinetic properties, which were analysed by zeta potential (ζ) measurements. Experimental Materials. Three types of the regenerated cellulose fibres (viscose, modal, lyocell) and one of the natural cellulose fibres (100% cotton fabric; purified and NaOH mercerised), produced by <strong>Lenzing</strong> AG Austria, were investigated. In previous research work the analysis of structural parameters (molecular mass, degree of polymerisation, crystallinity index, orientation factor, void structure) and mechanical properties of investigated regenerated cellulose fibres was performed [[2],[3]]. In Table 1 and Table 2 the specifications and some of the most important 7fibre structure characteristics of investigated cellulose fibres were presented. The following surface and structure modification processes for the cotton cellulose fibres were applied: a) - Purifying treatment (the cellulose structure remains unchanged - cellulose I); symbolized as – Cotton 2%NaOH - Boiling - Removal of non-cellulose compounds - interfibrillar swelling: (20 g/l NaOH; pH = 11.5; t = 90 min; T = 95°C). b) - Treatment causing a structural modifications (the cellulose structure is changed to cellulose II); symbolized as – Cotton 24%NaOH - Mercerisation - interfibrillar and intrafibrillar swelling: (24% NaOH, pH = 13, t = 60s, T = 15°C) After each treatment the fibres were washed with distilled water until a conductivity of less than 3 ms/m was reached. The processed material was airdried. 29
Table 1. Specifications and structure characteristics of regenerated cellulose fibres [1]], [2]. Fibre type Viscose Modal Lyocell Symbol CV CMD CLY Linear density Tt [dtex] DIN 53 812 1.88 ±0.15 1.78 ±0.23 1.82 ±0.30 Fibre length l [mm] DIN 53 808 39.9 ±0.51 40.1 ±0.33 39.4 ±0.44 Fibre diameter d [µm] DIN 53 811 14.3 ±1.39 14.2 ±1.10 12.8 ±1.00 Density ρ [g/cm 3 ] DIN 53 479 1.5045 1.5141 1.5205 Degree of polymerisation DP DIN 54 270 235 ±5.13 507 ±3.61 642 ±4.58 Molecular mass M DIN 54 270 38005 ±881 82097 ±538 104021 ±730 Crystallinity index CrI * 0.25 0.37 0.44 Orientation factor f ∆n ** 0.58 ±0.07 0.69 ±0.08 0.71 ±0.04 Void volume Vp [cm 3 /g] *** 0.68 0.49 0.62 30 * Determined by X-ray wide angle diffraction (2θ = 5° - 45°) ** From birefringence measurements of the fibres *** Determined by size exclusion chromatography [[18],[19],[20],[21]] Table 2: The specifications of investigated natural cellulose fibres. Fibre type Cotton Symbol Co Degree of polymerisation DPη Metal content [ppm/0.5 g fibres] 4700±34.3 Mg 249 Fe 39.4 Mn 1.26 Zn 20.2 Methods. Moisture sorption of fibres was determined according to standard DIN 54 351. Cellulose fibres were exposed to standard atmosphere 20 ±2°C, 65% ±2% RH for 24 hours (DIN 53 802). Moisture sorption was calculated as a mass % of absolute dry material (T = 105°C, t = 4 hours). Water retention power of cellulose fibres was determined according to standard DIN 53 814. This method is based on a determination of the quantity of water, which the fibres can absorb and retain under strictly controlled conditions. This property is expressed as a ratio between the mass of water retained in the fibre after soaking (2 hours) and centrifuging (20 min), and the mass of absolute dry sample (T = 105°C, t = 4 hours). Swelling of the cellulose fibres in the aqueous medium was determined on the basis of fibre diameter measurements. Applied equipment: fibre diameter measurements were carried out with the Axiotech 25 HD microscope using the image analysis system and supporting Kontron KS 300 software. Electrokinetic properties of fibres were analysed by zeta potential ζ measurements, which were determined by streaming potential measurements as a function of the pH. It has been shown that the streaming potential measurement is the most appropriate electrokinetic technique for studying electrokinetic properties i.e. the zeta potential (ζ) of fibres systems [[16],[17]]. The ζ was calculated from the streaming potential (Us) data using the Smoluchowski equation. The channel′s geometry was taken into account according to Fairbrother and Mastin [[22]]. This measurements were always performed in a fibre cell using 0.001 n KCl as electrolyte solution. The pH of the electrolyte solution was always varied in an identical way. It was first adjusted to pH 10 using 0.1 n NaOH and then decreased gradually with 0.1 n HCl. The here mentioned zeta potential values were always those obtained at the constant part of the zeta potential - pH function in the alkaline region at pH = 9. Applied equipment: Electrokinetic Analyzer EKA, A. Paar KG.
Page 1: Abstract 28 THE SORPTION BEHAVIOUR
Page 5 and 6: 14.1%, lyocell 14.6%, viscose 15.1%

THE SORPTION BEHAVIOUR OF CELLULOSE FIBRES - Lenzing

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