**4.4 Woven lyocell structure effect on sodium hydroxide release (wash-off)**

While in previous sections the discussion was oriented to the extended study of lyocell plain woven fabrics with effect of alkali concentration, treatment temperature and applied tension based on the previous work (Široký et al., 2010, 2011b; Široký et al., 2009), in this chapter, the influence of fabric structure (plain-, twill-, or sateen-woven fabrics) on NaOH release from lyocell after pad-batch pre-treatment (Široký et al., 2011a) will be explored by conductivity measurements in the system of deionized water-NaOH impregnated assemblies-the washoff bath. Weaving of the fabric is distinguished according to the manner the yarns or threads, longitudal-warp and lateral-weft, are interlaced to form fabric or cloth. Three basic weaves are used the most, plain-, twill- or sateen-weaving. Differences in fabric weaving or construction provide also differences in terms of accessibility, substrate surface and compactness, fabric diffusion, swelling, and fabric bulk density and porosity.

Different fabric construction of the same fibre and yarn, herein weaving, plays a crucial role in liquid-fabric interactions. For example, wet pick-up (*WPU*) differs, which in turn effects the swelling, bulk density, or porosity of such a porous substrate as it shown in Fig. 12. Due to mathematical complexicity, only the changes on the macro-level were further investigated by measuring the conductivity in the wash bath and applying Crank's approximation, the Crank's equation for the flow through a membrane(Crank, 1975):

Alkali Treatments of Woven Lyocell Fabrics 199

(2011a)), which alters the dimensions of the fibre, and this, in turn, causes changes in both physical properties (size, shape, stiffness, and permeability of yarns and fabrics, sorption/desorption characteristics)(Morton & Hearle, 1993; Široká et al., 2008) and mechanical properties (tensile modulus, breaking stress).(Kongdee et al., 2004) Moreover, the "skin-core'' effect of lyocell fibres exists, wherein a fibre structure consists of different regions, which may lead to a diffusional boundary layer and hence, cause significant

Fig. 13. Trend lines based on calculated liquid transport coefficients (*K*) at different NaOH

An alkali transport coefficient (*K*) was established to quantify alkali release, which represents the liquid-side mass transfer of alkali release after a pad-batch process into the wash-bath.(Široký et al., 2011a) Three regions of NaOH-release behavior were observed (Fig. 13) and two competitive phenomena related to swelling, an increase of the substrate surface, and the substrate compactness, have been recognized. Firstly (up to 2.25 mol dm-3), the influence of fabric-alkali interactions and non-uniform access of NaOH determines alkali release, and also the absolute K value is mainly determined by the bulk densities (plain 0.4998, twill 0.4622 and sateen 0.4096 g cm-3) and porosities (plain 0.671, twill 0.696 and sateen 0.731) and thus, it increases continuously as the material is still not fully swollen. The maximum swelling of cellulose fibers occurs between 2.25-3.00 mol dm-3 NaOH; here, maximum K is observed as well as dependency on fabric construction is still determining factor. Above 3.75 mol dm-3 NaOH, the treated substrates are highly swollen sheets and the K does not show dependency on fabric constructions, thus, the fabric construction is not relevant at this range anymore for the alkali-diffusion and the approximation of plain sheet diffusion corresponds to the real situation. As shown in this study, alkalization and release during wash-off are governed by fabric structure and alkali concentration. This finding is of particular relevance for an optimized processing of fabrics from regenerated cellulose fibers.

changes in concentration gradient within the substrate.

treatment concentrations.

$$\frac{\mathbf{M\_{\tilde{t}}}}{\mathbf{M\_{\infty}}} = 1 - \frac{8}{\mathbf{n}^2} \exp\left(\frac{-\mathbf{n}^2 \mathbf{D} \mathbf{t}}{\mathbf{l}^2}\right) \tag{1}$$

where, *Mt* is conductivity at a given time (mS cm-1), *M∞* is conductivity at wash-off equilibrium or the corresponding conductivity during infinite time (mS cm-1), *D* is the diffusion coefficient (m2 s), *l* is half of the sheet (layer) thickness, because diffusion takes place from both its sides (m), and *t* is time (s). Herein, the factor (*-π2 D/l2*) was used as "alkali transport coefficient" *K* and was determined from the slope of ln(1 – *Mτ/M∞*) = *f* (*t*).

Fig. 12. Changes in wet pick-up and porosity of loom-state fabric (A-untreated) and swollen fabric after dropping NaOH solution of different concentration: B-0.7, C-3.0, or D-5.0 mol dm-3 on fabric of different construction (plain-, twill-, or sateen-woven fabric). An optical microscope (Krüss, Germany) combined with a digital camera (Canon Power Shot S40) was used for taking microphotographs. A sample of 1 x 1 cm size was placed on a glass slide and NaOH solution was dropped on the substrate. After 10 s, the picture was taken.

The kinetics of the system is a complex system with many parameters/components involved. Briefly, cellulose accessibility is dependent on the available inner surface, supramolecular order (range of degrees of order), fibrillar architecture, and pore structure. Interactions of cellulose with water which consequently destroys weak hydrogen-bonds, but cannot penetrate into regions of high order, in contrast to aqueous solutions of sodium hydroxide. Additionally, a significant role associated with pores, capillaries, and the network of voids, which do not have uniform size and shape need to be considered, and also NaOH treatment causes changes in pore structure (shape and size).(Bredereck & Hermanutz, 2005) Liquid absorption leads to swelling of the substrate (see Široký et al.

<sup>M</sup> <sup>2</sup> <sup>8</sup> <sup>π</sup> D t <sup>t</sup> 1 exp <sup>M</sup> 2 2 <sup>π</sup> <sup>l</sup> 

where, *Mt* is conductivity at a given time (mS cm-1), *M∞* is conductivity at wash-off equilibrium or the corresponding conductivity during infinite time (mS cm-1), *D* is the diffusion coefficient (m2 s), *l* is half of the sheet (layer) thickness, because diffusion takes place from both its sides (m), and *t* is time (s). Herein, the factor (*-π2 D/l2*) was used as "alkali

Fig. 12. Changes in wet pick-up and porosity of loom-state fabric (A-untreated) and swollen fabric after dropping NaOH solution of different concentration: B-0.7, C-3.0, or D-5.0 mol dm-3 on fabric of different construction (plain-, twill-, or sateen-woven fabric). An optical microscope (Krüss, Germany) combined with a digital camera (Canon Power Shot S40) was used for taking microphotographs. A sample of 1 x 1 cm size was placed on a glass slide and

The kinetics of the system is a complex system with many parameters/components involved. Briefly, cellulose accessibility is dependent on the available inner surface, supramolecular order (range of degrees of order), fibrillar architecture, and pore structure. Interactions of cellulose with water which consequently destroys weak hydrogen-bonds, but cannot penetrate into regions of high order, in contrast to aqueous solutions of sodium hydroxide. Additionally, a significant role associated with pores, capillaries, and the network of voids, which do not have uniform size and shape need to be considered, and also NaOH treatment causes changes in pore structure (shape and size).(Bredereck & Hermanutz, 2005) Liquid absorption leads to swelling of the substrate (see Široký et al.

NaOH solution was dropped on the substrate. After 10 s, the picture was taken.

transport coefficient" *K* and was determined from the slope of ln(1 – *Mτ/M∞*) = *f* (*t*).

(1)

(2011a)), which alters the dimensions of the fibre, and this, in turn, causes changes in both physical properties (size, shape, stiffness, and permeability of yarns and fabrics, sorption/desorption characteristics)(Morton & Hearle, 1993; Široká et al., 2008) and mechanical properties (tensile modulus, breaking stress).(Kongdee et al., 2004) Moreover, the "skin-core'' effect of lyocell fibres exists, wherein a fibre structure consists of different regions, which may lead to a diffusional boundary layer and hence, cause significant changes in concentration gradient within the substrate.

Fig. 13. Trend lines based on calculated liquid transport coefficients (*K*) at different NaOH treatment concentrations.

An alkali transport coefficient (*K*) was established to quantify alkali release, which represents the liquid-side mass transfer of alkali release after a pad-batch process into the wash-bath.(Široký et al., 2011a) Three regions of NaOH-release behavior were observed (Fig. 13) and two competitive phenomena related to swelling, an increase of the substrate surface, and the substrate compactness, have been recognized. Firstly (up to 2.25 mol dm-3), the influence of fabric-alkali interactions and non-uniform access of NaOH determines alkali release, and also the absolute K value is mainly determined by the bulk densities (plain 0.4998, twill 0.4622 and sateen 0.4096 g cm-3) and porosities (plain 0.671, twill 0.696 and sateen 0.731) and thus, it increases continuously as the material is still not fully swollen. The maximum swelling of cellulose fibers occurs between 2.25-3.00 mol dm-3 NaOH; here, maximum K is observed as well as dependency on fabric construction is still determining factor. Above 3.75 mol dm-3 NaOH, the treated substrates are highly swollen sheets and the K does not show dependency on fabric constructions, thus, the fabric construction is not relevant at this range anymore for the alkali-diffusion and the approximation of plain sheet diffusion corresponds to the real situation. As shown in this study, alkalization and release during wash-off are governed by fabric structure and alkali concentration. This finding is of particular relevance for an optimized processing of fabrics from regenerated cellulose fibers.

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