**3.1.3 Cellulose-aqueous alkali hydroxide interactions**

The process of caustic treatment was devised in 1844 by John Mercer and he was later granted a patent(Mercer, 1851) for work he had done on vegetable fabrics and fibrous material (cotton, flax) and other textures with treatment of caustic soda, potassium hydroxide, dilute sulfuric acid or zinc chloride. The process did not become popular, however, until H. A. Lowe improved it into its modern form in 1890; by holding the cotton during treatment to prevent it from shrinking, Lowe found that the fibre gained a lustrous appearance and good dyeability.(Beaudet, 1999) Afterwards, treatments with aqueous sodium hydroxide solutions found numerous applications as the primary step in many industrial practices leading to acetyl, ether or ester derivatives of cellulose.

One of the most important steps in cellulosic fabric processing is the treatment in aqueous solutions of alkali hydroxides, particularly aqueous sodium hydroxide solution. In general, mercerization and alkalization, respectively of cellulose is a conventional swelling treatment of cellulosics substrates with aqueous alkali solutions, usually carried out in concentrations between 6.8 to 7.6 mol dm-3 NaOH, and is a prerequisite to the preparation of many cellulose derivatives, the spinning of viscose ramie and lyocell, or the manufacturing of cellulose-based sponges*.*(Porro et al., 2007) In its interaction with cellulose, aqueous sodium hydroxide above a certain concentration is able to penetrate the cellulose crystalline lattice to yield a series of more or less well-defined crystalline complexes holding a number of sodium ions and water molecules within their crystalline lattice. From analysis of these complexes by X-ray diffraction it has been found that cellulose and alkali solutions interact strongly with the consequence of swelling of cellulose fibres. This interaction at low alkali concentration with cellulosic fibres causes an increase in their cross-sectional area with simultaneous shrinkage in length; the extent of the dimensional changes caused by the swelling depends on the alkali treatment concentration and temperature.(Krässig, 1993a) This effect is most probably influenced by the degree of swelling observed previously for cotton(Warwicker, 1969), which herein at its maximum (between 3.00 and 3.75 mol dm-3 for continuously treated lyocell fabrics) decreases with increasing temperature of the treatment liquor.(Široký et al., 2010) Treatment with aqueous sodium hydroxide solution improves the fabric's mechanical and chemical properties such as dimensional stability, fibrillation tendency, tensile strength, dyeability, reactivity, lustre and smoothness.(Široký et al., 2009) Therefore, the strong swelling tendency of lyocell in alkali solutions substantially influences substrate properties such as the pore structure, crystallinity, fibrillation tendency, and surface characteristics in fibres, and yarn crimp and stiffness in fabrics.(Široký et al., 2010) In addition, during wet processing, lyocell fabrics are sensitive to the generation of permanent crease marks and alkali pre-treatments are recommended to counteract this (Široký et al., 2009).

Recently, Goswami *et al.*(Goswami et al., 2009) observed that sodium hydroxide treatment causes the density, orientation and crystallinity of lyocell fibres to decrease with increasing sodium hydroxide concentration, and that the greatest change in fibre properties occurs between 3.0 and 5.0 mol dm-3 NaOH. This was attributed to the onset of formation of sodium (Na)-cellulose II at 3.0 mol dm-3 NaOH; a fully formed Na-cellulose II structure was observed above 6.8 mol dm-3 NaOH. In addition, alkalization without tension can increase the moisture regain at a given relative humidity to 1.5 times its previous value;

The process of caustic treatment was devised in 1844 by John Mercer and he was later granted a patent(Mercer, 1851) for work he had done on vegetable fabrics and fibrous material (cotton, flax) and other textures with treatment of caustic soda, potassium hydroxide, dilute sulfuric acid or zinc chloride. The process did not become popular, however, until H. A. Lowe improved it into its modern form in 1890; by holding the cotton during treatment to prevent it from shrinking, Lowe found that the fibre gained a lustrous appearance and good dyeability.(Beaudet, 1999) Afterwards, treatments with aqueous sodium hydroxide solutions found numerous applications as the primary step in many

One of the most important steps in cellulosic fabric processing is the treatment in aqueous solutions of alkali hydroxides, particularly aqueous sodium hydroxide solution. In general, mercerization and alkalization, respectively of cellulose is a conventional swelling treatment of cellulosics substrates with aqueous alkali solutions, usually carried out in concentrations between 6.8 to 7.6 mol dm-3 NaOH, and is a prerequisite to the preparation of many cellulose derivatives, the spinning of viscose ramie and lyocell, or the manufacturing of cellulose-based sponges*.*(Porro et al., 2007) In its interaction with cellulose, aqueous sodium hydroxide above a certain concentration is able to penetrate the cellulose crystalline lattice to yield a series of more or less well-defined crystalline complexes holding a number of sodium ions and water molecules within their crystalline lattice. From analysis of these complexes by X-ray diffraction it has been found that cellulose and alkali solutions interact strongly with the consequence of swelling of cellulose fibres. This interaction at low alkali concentration with cellulosic fibres causes an increase in their cross-sectional area with simultaneous shrinkage in length; the extent of the dimensional changes caused by the swelling depends on the alkali treatment concentration and temperature.(Krässig, 1993a) This effect is most probably influenced by the degree of swelling observed previously for cotton(Warwicker, 1969), which herein at its maximum (between 3.00 and 3.75 mol dm-3 for continuously treated lyocell fabrics) decreases with increasing temperature of the treatment liquor.(Široký et al., 2010) Treatment with aqueous sodium hydroxide solution improves the fabric's mechanical and chemical properties such as dimensional stability, fibrillation tendency, tensile strength, dyeability, reactivity, lustre and smoothness.(Široký et al., 2009) Therefore, the strong swelling tendency of lyocell in alkali solutions substantially influences substrate properties such as the pore structure, crystallinity, fibrillation tendency, and surface characteristics in fibres, and yarn crimp and stiffness in fabrics.(Široký et al., 2010) In addition, during wet processing, lyocell fabrics are sensitive to the generation of permanent crease marks and alkali pre-treatments are recommended to counteract this

Recently, Goswami *et al.*(Goswami et al., 2009) observed that sodium hydroxide treatment causes the density, orientation and crystallinity of lyocell fibres to decrease with increasing sodium hydroxide concentration, and that the greatest change in fibre properties occurs between 3.0 and 5.0 mol dm-3 NaOH. This was attributed to the onset of formation of sodium (Na)-cellulose II at 3.0 mol dm-3 NaOH; a fully formed Na-cellulose II structure was observed above 6.8 mol dm-3 NaOH. In addition, alkalization without tension can increase the moisture regain at a given relative humidity to 1.5 times its previous value;

industrial practices leading to acetyl, ether or ester derivatives of cellulose.

**3.1.3 Cellulose-aqueous alkali hydroxide interactions** 

(Široký et al., 2009).

mercerization under tension does not cause such a large increase,(Morton & Hearle, 1993) and generally causes an increase in fibrillar (crystalline) orientation.(Krässig, 1993a) Moreover, cellulose is susceptible to limited degrees of dissolution in alkali solutions, leading to weight loss in substrates. For instance, lyocell fibre mass reduces by 2-10% with increasing NaOH concentration up to 10 mol dm-3.(Jaturapiree, 2007; Zhang et al., 2005)

The ability of cellulose to adsorb alkali hydroxide ions was originally considered to be a chemical process. In 1907, Vieweg observed two ranges of caustic soda concentrations at which the uptake of sodium hydroxide reached a plateau by applying so-called "change-intiter" method;(Krässig, 1993a) the principle of this method was to measure the drop in alkali concentration of a given caustic soda solution by a known weight of immersed cellulose, and, at the time, had been use for a decade as a convincing observation for the occurrence of a defined chemical reaction following stoichiometric rules. Nevertheless, Vieweg's findings (and those of other research groups) were subjected to criticism by Leighton (1916) who introduced the centrifugation method; he mainly criticized the assumption made by Vieweg that no water is taken up by solid phase and he showed that "his method" allowed the separation of the uptake of water and the uptake of sodium hydroxide. Moreover, Leighton's method revealed that there is initially a preferential water uptake, which has a maximum at medium concentrations, whereas the uptake of sodium hydroxide steadily increases over the whole concentration range, except at the lowest concentrations. The balance of the two separate uptakes explains the plateau in Vieweg's curve of apparent sodium hydroxide uptake.

From these observations, it was concluded that the uptake of alkali hydroxides by cellulose is an adsorption phenomena based on the Donnan equilibrium theory,(Donnan, 1924; Neale, 1929) rather than a chemical process. It is assumed that cellulose, behaving as a very weak monobasic acid, forms a sodium salt to an extent increasing with the concentration of the alkali. Excess alkali diffuses into the mechanically separable cellulose phase. It results in an unequal distribution of ions what causes an osmosis or movement of water which distends the cellulose until the osmotic pressure is balanced by the forces arising from the cohesion of the gel and therefore, the cellulose swells. In addition, Neale's theoretical osmotic curves showed that as the temperature falls the maximum osmotic pressure rises, and the maximum occurred at a lower concentration of alkali. One of the difficulties with the absorption theory was based on the fact that uptake by native cellulose substrates is not smooth and reversible as might be expected for an adsorption process.(Krässig, 1993a) Another issue with the earlier investigations and results interpretation is that the effect of the morphology, fine structure, and accessibility of the cellulosic fibre substrate was, in most cases, ignored and it was assumed that the fibres were equally accessible throughout.

A comprehensive study of parameters, such as *sodium hydroxide concentration* and *temperature*, which influence the formation of various sodium-celluloses was conducted by Sobue et al. (1939) using an X-ray technique. They proposed a phase diagram of ramie cellulose and its various Na-cellulose crystalline complexes. They also revealed a new highly swollen and poorly crystalline allomorph, namely *Na-cellulose Q*, occurring at NaOH concentrations between 7% to 10% (1.75 to 2.50 mol dm-3 NaOH) and temperatures between –10 and 1°C. More recently, it was found that if an adequate pre-treatment of cellulose was applied, within the *Q phase*, total dissolution of cellulose can occur;(Kamide et al., 1984) solutions of 5% (w/v) cellulose could be obtained at a sodium hydroxide concentration of

Alkali Treatments of Woven Lyocell Fabrics 193

effect of chemicals to be swollen and hence, the accessibility of fabric decreases significantly. This effect can be seen in wash-off or alkali release from alkali treated lyocell fabrics which is discussed and shown in recent publication(Široký et al., 2011a). On the level of fabric, the fabric construction has a crucial effect as the swelling and wet pick-up differs for various

fabric constructions (*e.g.* plain-, twill- or sateen-woven lyocell fabrics).

Fig. 10. Picture of apparatus used in continuous alkali pre-treatment process.

previous work.(Široký et al., 2009)

**4.1 Influence of physical properties** 

The continuing process of alkali pre-treatment was done with a washer simulating real alkali process which is given in Fig. 10. It was divided into four stages (A, B, C, D), each with two sub-compartments (1 and 2) that could be heated independently; the four stages were alkali treatment (A2), stabilisation treatment (B2), washing (C1 and C2), neutralisation (D1), and final washing (D2). The fabric was passed through the apparatus over a series of rollers including tension compensators (T) and pressurised squeeze rollers (P). The fabric after passing through the last compartment (D) was wound on a take-up roller (R). In treatment stage, the NaOH concentration, applied tension and temperature varied. Due to changes in the concentration in the treatment stage (caused by process), the "effective" concentration of alkali was established at 0.0, 2.53, 3.33, 4.48, 4.65 and 7.15 mol dm-3 for treatment bath and 0.0, 0.73, 1.08, 1.18, 1.48 and 2.15 mol dm-3 for stabilization bath. Residence time was adjusted at 75±2 s and 71±3 s in treatment bath and stabilization bath respectively. Tension in treatment compartments was applied either at 49 N m-1 or 147 N m-1, tension in stabilisation compartment was applied at 147 N m-1, and tension in washing and neutralization compartments was applied at 49 N m-1. During the alkali treatment stage the temperature of the solution was set at 25 °C or 40 °C, and in the stabilization stage the temperature was 60°C. The speed of passage of fabric through the system was set at 2 m min-1. Entire process and its all adjustments during the process are described in detail in our

Alkali treatment has a substantial influence on dimensional and mechanical properties as well as on morphological, molecular and supramolecular properties of cellulose II fibres (*e.g.* lyocell type) causing changes in their structure and performance. These physical changes were observed and examined by dimensional change, flexural rigidity (in dry and wet

9% (2.25 mol dm-3 NaOH) and temperature of 4°C and below. It is generally accepted that the immersion of ramie or cotton fibres into 12-16% (w/v) NaOH (3.0 to 4.0 mol dm-3 NaOH) yields *Na-cellulose I*, whereas the use of more concentrated alkali, typically 20-30% (w/v) NaOH (5.0 to 7.5 mol dm-3 NaOH), gives another allomorph: *Na-cellulose II*. The conversion of *Na-cellulose I* into *Na-cellulose III* is obtained by vacuum drying *Na-cellulose I*, whereas washing *Na-cellulose I* and *Na-cellulose II* until neutrality, yields the sodium-free *Nacellulose IV*, which in its turn will give *cellulose II* upon drying. As Porro *et al.*(2007) stated, the analysis of the X-ray data alone may not be sufficient to give a clear picture of the interaction of the Na+ ions with the cellulose molecules. Therefore, Porro suggested a thorough analysis of 13C solid-state NMR data of the various Na-cellulose complexes due to the low spectral resolution of X-ray which did not lead to a clear assignment of the resonances belonging to the various complexes. These authors proposed a phase diagram, based on the occurrence of *Na-cellulose I* and *Na-cellulose II* together with the *Q region* where cellulose is essentially soluble. It defines six regions, two of them being border zones where two phases are present at the same time.
