Single Fibre Swelling Behavior for Natural and Man-Made Cellulose Fibres Under Industrial Steeping Conditions

13 Swelling behavior of cotton, dissolving wood pulp (DWP), viscose staple fibre (VsF), and Tencel staple 14 fibre (TsF) in varying sodium hydroxide (NaOH) were investigated by means of optical microscopy and 15 were characterized by molecular mass distribution, X-ray diffractometer, and dynamic vapor 16 sorption. Effect of temperature (20-45 °C) and duration (0-120 min) was studied. The results reveal 17 that the swelling ratio of fibre in alkali solution depends on fibre accessibility and NaOH 18 concentration. Among all the materials, VsF exhibited the highest swelling ratio and lowest swelling 19 ratio has been observed for cotton fibre. The results suggest that the swelling is limited by the 20 presence of plant cell wall structures in cotton and DWP, rather from fringed-fibrillar, semi-crystalline 21 sub-structures, which result from the inherent tendency of cellulose molecules to form such 22 structures during the biosynthesis of plant cell walls as well as during the formation of regenerated 23 cellulosic textile fibre in wet-spinning.


Introduction 54
Cellulose-based plant fibres are highly interesting sources of polymers for producing 55 bio-based products. Native or derivatized cellulose has been used as a renewable which locally prevent swelling of S wall. Finally, also S1 layer bursts and gets 99 fragmented and dissolved in the form of aggregates. The same mechanisms apply to 100 cotton fibres. However, cotton is considered more difficult to swell than wood pulp, 101 which could be explained by the many sub-layers in its cell wall structure, with 102 different direction of orientation (Klemm et al. 2005). 103 Various factors must be taken into account to understand fibre swelling and dissolution 104 behavior: their chemical composition, properties of primary and secondary layers in 105 natural fibres or properties of a skin in some man-made fibres, porosity, and cellulose 106 crystallinity and molecular weight (Abu- Rous et al. 2006). For instance, viscose fibre 107 has folded perimeter and a skin-core structure, due to immediate solidification of 108 outermost fibre surface, under which there is a properly oriented skin layer. The core 109 is poorly oriented, due to, i.e. the non-Newtonian flow of viscose dope through 110 spinneret orifices. On the other hand, Lyocell fibre has a more uniform fibre structure 111 because of slower solvent exchange between the fibre and the coagulation media.

112
To the best of our knowledge, there is no report comparing the swelling behavior of 113 natural, chemical pulp, and man-made fibres, under the same conditions. In man-made

Dynamic vapor sorption (DVS) 162
A DVS intrinsic apparatus (Surface Measurement system, London, UK)) with a 163 measuring accuracy of 0.1 µg was used for water vapor sorption analysis. Around 10 164 mg of pre-dried grinded fibres were placed in the sample pan and preconditioned at a 165 relative humidity (RH) of 0% at 25 °C until the equilibrium was reached using nitrogen 166 (flow rate 100 cm 3 s -1 ). The adsorption cycle was performed in 0% RH steps to a 167 maximum of 90% RH, and vice versa for the desorption cycle. In both cases, the RH 168 was kept constant until a defined equilibrium condition was reached, for example, 169 when sample mass change was below 0.001% per minute over 10 min. Then RH was 170 increased to 90% and sample weight evolution and time to reach equilibrium (ts) were MMCFs demonstrate the Cell-II allomorph, as expected. The CRI is around 32.2% and 220 39% for VsF and TsF, respectively (Table 1) Figure 6 and 7.

338
VsF and TsF fibre swell (Figure 6 and 7) more in 12 wt% NaOH solution, which is 339 similar to DWP swelling those fibres swelling increase as the concentration is reduced.

340
According to literature, pulp swelling to a maximum at around 11 wt% NaOH 341 concentration (Wilkes, 2011). VsF and TsF swelling radially in a homogeneous way. The microfibrillar structure of a man-made cellulosic fibre resembles strongly the 382 original fibrillar structure of DWP cell wall layers. To make a spinning/processable 383 dope, we know that cellulosic spinning dopes are not molecularly dispersed systems.

Time (min) (b)
As the purpose of this study was to gain better understanding of the sources of poorly 389 swollen and poorly dissolved residues in viscose spinning dope by comparing the 390 swelling of cotton, DWP, VSF and TSF, the experimental conditions used in this study 391 don't represent the real swelling behaviour during the steeping process with large scale.

392
During the steeping stage of the viscose process, the raw material concentration is 393 higher than suppresses the penetration of the liquor into the raw material due to the 394 reduced solid to liquor ratio. On the other hand, the formation of the raw material also 395 influences the swelling behaviour. In case of DWP, pulp sheet may be used instead of 396 powders for steeping. The extreme compact structure and low surface area of the pulp 397 sheet seriously lower the swelling rate of the cellulose. As for the raw material with 398 long fibre format, the surface area of the material has also to be reduced by means of 399 grinding in order to improve the penetration of the liquor into the materials. Grinding