Elsevier

Dyes and Pigments

Volume 64, Issue 3, March 2005, Pages 231-241
Dyes and Pigments

An adsorption and kinetic study of lac dyeing on silk

https://doi.org/10.1016/j.dyepig.2004.06.009Get rights and content

Abstract

Adsorption and kinetic studies of lac dyeing of silk were investigated using, as optimal conditions, a pH of 3.0, a material to liquor ratio (MLR) of 1:100, an initial dye concentration of 450 mg/L and 60 min contact time. The progress of dye adsorption was monitored at λmax 487 nm by visible spectroscopy. The experimental data fitted well to the Langmuir and Freundlich isotherms with a high correlation coefficient (R2). The pseudo second-order kinetic model was indicated with the activation energy of 47.5 kJ/mol. It is suggested that the overall rate of lac dye adsorption is likely to be controlled by the chemical process. The values of the enthalpy (ΔH#) and entropy of activation (ΔS#) were 44.7 kJ/mol and −175.7 J/mol K, respectively. The free energy of activation (ΔG#) at 30 °C was 97.9 kJ/mol. The activation parameters with and without erythrolaccin in the lac dye were similar, consistent with erythrolaccin only having a slight effect on the lac dyeing of silk. The free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) terms for the lac dyeing were also determined, and the negative values of ΔG° and ΔH° obtained indicated that the lac dye adsorption process is a spontaneous and an exothermic one.

Introduction

Lac dye is a natural reddish dyestuff extracted from stick lac which is a secretion of the insect Coccus laccae (Laccifer lacca Kerr). The insect C. laccae is often found in South and Southeast Asia, especially in Thailand and India. In Thailand, the lac insect grows most commonly on the Rain tree, Samanea saman (Jacq.) Merr. (Pithecolobium saman, Mimosaceae) [1]. Lac dye, which is the soluble part of stick lac, is composed mainly of two major anthraquinone-based components: laccaic acids A and B [2], [3], [4], [5], [6], [7], [8]; the minor components, laccaic acids C, D and E, have also been isolated [9], [10] (Fig. 1). All have an anthraquinone moiety with dicarboxylic acid groups, except for laccaic acid D which is a monocarboxylic acid. The quantities of these laccaic acid components depend on the locality and the season. Erythrolaccin (Fig. 1) is another component of the lac dye [2], but this can be removed by ethanol extraction.

Lac dye is used extensively as a natural food additive [7], [11], and in cosmetics [7], as well as a colourant for silk and cotton dyeing [1]. In the north and the northeast of Thailand, it is used as a natural red dyestuff for cotton and silk dyeing but the fastness properties and reproducibility to give consistency in production are still problems to be solved. As part of the approach to tackle these problems, fundamental physical studies on the dyeing process are important.

The thermodynamics of adsorption of laccaic acids on silk has been studied [12], but without pH control and the results indicated that the adsorption isotherm of silk dyeing with laccaic acids, from which the erythrolaccin had been removed, was of the Langmuir type. Also values for the heat and entropy of dyeing were reported [12]. However, there have not been any complementary studies on the kinetics of the dyeing process and such studies are now reported in this paper. The adsorption rates were measured and determined quantitatively in relation to dye solution pH values, contact time, initial dye concentration, and material to liquor ratio (MLR). The Langmuir and Freundlich equations were used to fit the equilibria.

Section snippets

Silk yarn

The silk yarn used was purchased from villagers living in Nakhon Ratchasima, Thailand. To remove the sericin gum, the silk yarn (1 kg) was added to boiling water (5 L) to which soap flakes had been added (ca 100 g), sodium silicate (10 g), sodium carbonate (50 g) and 40% hydrogen peroxide (100 mL). The mixture was then boiled for 2 h. The silk was then removed, washed with water, squeezed to remove excess liquor and air dried. Finally, it was treated with 1 M HCl (ca 3 L) at room temperature for 30 min

Optimal conditions for lac dye onto silk

In order to investigate the adsorption of lac dye onto silk, the experiment parameters including pH, material to liquor ratio (MLR), contact time, initial dye concentration and temperature were determined to find the optimal conditions for adsorption.

Conclusions

The adsorption isotherm and kinetics of lac dyeing were studied. The following results were obtained:

  • 1.

    Adsorption data were modeled by using the Freundlich and Langmuir adsorption isotherms. A high correlation coefficient (R2 > 0.99) with both isotherms was obtained.

  • 2.

    Initially, the rate of adsorption of lac dye onto silk yarn was very fast. This was then followed by a slower rate, and gradually approached a plateau.

  • 3.

    Before equilibrium was reached, an increase in temperature lead to an increase in the

Acknowledgements

We gratefully acknowledge support from the Ministry of University Affairs, Thai Government for Montra Chairat. We also thank Suranaree University of Technology, Walailak University, and the University of Wollongong for support.

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