Study On The Cotton Modification And Its Application In The Dyeing With Natural Dyes

Natural dyes, derived from nature, are biodegradable, and compatibile with eco-environment. They are harmless to human health and consistent with the demands for health and environmental protection. However, most natural dyes show the problems of low dye-uptake and poor colorfastness on cotton. In order to solve those problems, cationic modifications of cotton were studied and applied in the dyeing with natural dyes, leading to a significant improvement in the dyeing performance. In this paper, the reactive modifier H and non-reactive modifier BE-2 were used to modify cotton fabrics. The modifying process and dyeing process of natural dyes were optimized, and the dye-uptake increased remarkably. The stability of natural dyestuffs was studied by UV-Visible spectroscopy, and the thermal analysis and Zeta potential analysis. The physical and mechanical properties of cotton before and after modification such as whiteness, strength, moisture content and moisture regain were investigated. Adsorption isothermals and dyeing rate curves were made to study the thermodynamics and kinetics properties of natural dyes on the modified cotton.The optimized modification process for the reactive modifier H was as follows: Firstly, 4% (o.w.f) modifier H and 0.5-1 % (o.w.f) peregal O were mixed together at room temperature with bath ratio 1:30 and the cotton fabrics were dipped in this solution. The above solution was heated up to 70℃and kept for 30 min. Then, 0.5-1 g/L NaOH was added into the solution to react with the cotton fabrics and modifier H for 20 min. Finally, the modified fabrics were washed with cold water. In addition, the optimized modification process for the non-reactive modifier BE-2 was as follows: cotton fabrics was dipped in 4 % (o.w.f) BE-2 solution at 70℃for 10-15 min with bath ratio 1: 30.The cotton fabrics modified by the above two processes exhibit positive Zeta potentials and slightly decreased thermal stability. However, the physical and mechanical properties show little changes. The dyed samples after modification have high color yield, good color fastness, levelness, as well as excellent UV-protection properties. In order to improve the theoretical guidance to process control and process optimization, the absorption kinetics and thermodynamics of the natural dyes on modified cotton were studied. Pseudo first- and second-order kinetic models were used to examine the adsorption kinetic data and some dyeing kinetic parameters were calculated. Furthermore, three theoretic adsorption models, Langmuir, Freundlich and Nernst isotherm, were used to simulate the thermodynamic experimental data and some dyeing thermodynamic parameters were also calculated. The results indicated that the adsorption kinetics of the three natural dyes on modified cotton fabrics was follow the pseudo second-order kinetic model, which means the adsorption of the natural dyes on modified cotton was chemical adsorption including multi-molecular layer adsorption. Batch isotherm studies showed that the adsorptions of these three natural dyes on modified cotton were described by the Langmuir and Freundlich isotherms. The adsorptions were not only specific site adsorption, but also non-specific site absorption.