Dyeing behaviour of lyocell fabric: effect of NaOH pre-treatment

Abstract

To understand the effect of alkali pretreatment on the dyeing of lyocell fabrics, samples are pre-treated with 0.0-7.0 mol dm -3 NaOH using a pad-batch process and then dyed with different types of reactive dyes. Exhaustion, fixation, and visual colour strength (K/S values) are measured. It is observed that sodium hydroxide pre-treatment significantly improves the colour yield, exhaustion, and fixation for all dyes used. Highest K/S values are obtained when the fabrics are pre-treated with 2.0-2.5 mol dm -3 NaOH. Cross-sectional analysis shows that below this optimum concentration the core fibres in the yarn are not dyed; at optimum concentration all fibres in yarn cross-section are homogeneously dyed. Cross-sectional analysis shows that as the pre-treatment concentration of NaOH increases above 2.5 mol dm -3 , the fibres change progressively from a circular to angular cross-section, forming a solid unit. The decrease in K/S above the treatment concentration of 2.5 mol dm -3 , though the %E and %F remains almost constant, is attributed to the distribution of dye over a larger surface area of the outer fibres in the yarn cross section, forcing the K/S at k max to decrease.

Sorption of dyes on cellulose II: effect of alkali treatment of fibre and dye structure

“…However, from an industrial perspective the findings will enable dyehouses to achieve a higher depth of shade on lyocell by pre-treatment of the substrate; the optimum concentration to achieve highest sorption is 2.0 mol dm -3 NaOH. This work compliments our previous research (Goswami et al 2009b) where it was also observed that highest visual colour yield is achieved when lyocell was pre-treated with 2.0 mol dm -3 NaOH.…”

“…The work concluded that formation of Na-cellulose II causes plasticization of the lyocell fibre as both inter-and intra-molecular hydrogen bonds are broken by these higher sodium hydroxide concentrations. We have also observed previously using reactive dyes that NaOH pre-treatment significantly improves the colour yield, exhaustion, and fixation of lyocell fabric (Goswami et al 2009b), and that an understanding of reactive dyeing of lyocell is particularly important in relation to fibrillation properties of lyocell (Goswami et al 2007), which can be affected by NaOH treatment. The purpose of the work reported herein is to gain a deeper understanding of the sorption system involved when cellulose II fibres (lyocell) are pre-treated with aqueous sodium hydroxide solution, and how this affects sorption of dye molecules of varying molecular size.…”

“…This explains the observations herein that sorption at 2.0 mol dm -3 NaOH is more thermodynamically favourable than at 0.0 mol dm -3 NaOH; however, this does not explain why sorption at 4.0 mol dm -3 NaOH is less favourable than at 2.0 mol dm -3 NaOH, despite the further reduction in x c that would be expected. This can be further explained in terms of the micro and macro state of the sorbent: on the micro-level, with increasing alkali concentration x c decreases and more amorphous regions in the polymer are accessible for sorption and diffusion; however, on the macro-level, as we have shown in other work (Goswami et al 2009b), as the pre-treatment concentration of NaOH increases, lyocell fibres change progressively from a circular to angular cross-section, forming a solid unit, which provides a barrier to diffusion (at higher concentrations) and accessibility to the core of the substrate is limited by pressure the individual fibres exert on each other-optimum alkali concentration to achieve maximum sorption using 2% omf RR120 was shown to be 2.5-3.0 mol dm -3 NaOH. From previous work and the work herein, we can conclude that pre-treatment of lyocell with 2.0 mol dm -3 NaOH provides the substrate that achieves the most thermodynamically favourable system for sorption of Table 1 Summary of isotherm constants and correlation for Freundlich and Langmuir isotherms for RO84 (M r 1,813.4 g mol -1 ) NaOH (mol dm -3 ) Freundlich Langmuir…”

Physical Modification of Lyocell® and Modal® Fabrics and its Effect on Fabric Dyeability

“…It has been reported that swelling of lyocell and modal fabrics causes the expansion of void spaces within the semi-crystalline morphology, thus forming a water fiber two phase structure (7,17,18) . The expansion of internal structure leads to a very high internal wetted surface area, meaning that a high proportion of the polymer hydroxyl groups become accessible to the swelling medium.…”

“…Although results regarding the dyeability of lyocell and modal fabrics with reactive and direct dyes have been published (5)(6)(7) , a detailed study of the effect of pre-swelling in metal salts on their dyeability with reactive and direct dyes has not appeared.…”