New dyeing technique for enhancing colour strength and fastness properties of cotton fabric dyed with natural dyes

Dyeing behaviour of four different natural dyes extracted from the leaves collected in a sustainable manner from respective dye containing plants has been studied on cotton fabric. A novel and ecofriendly mordanting technique has been devised for creating aluminium-tannate complex on cotton matrix. The new mordanting technique not only involves application of a modified sequence of mordanting chemicals but also employs addition of specific salts in water used in the dyeing process. The dyeing of the mordanted fabric has been carried out under acidic and alkaline conditions at different temperatures. The modified dyeing technique shows an overall improvement of 140-300% in colour strength along with the excellent fastness properties, such as washing, rubbing, perspiration and light. The overall results indicate that all the four natural dyes can be effectively and economically used for dyeing on cotton fabric. The entire chain of collection, extraction and dyeing methodology is environmentally and ecologically compatible. In view of the polluting aspects of synthetic dyes and long term non-sustainable nature of these petrochemical derivatives, natural dyes are gaining importance. Besides, natural colorants derived from flora and fauna, being non-toxic and bio-degradable in nature, are ecologically safe. In this context, a search of plant kingdom in the western ghats of India for identifying sustainable sources of dye containing plants led to the identification of a series of leaf based sources of natural dyes. Majority of natural dyes need a chemical in the form of a metal salt to create an affinity between the fibre and the pigment. These chemicals are known as mordants. Beside metallic salts, natural products containing tannins like harda and pomegranate shell are also used widely as mordants. Some of the commonly used mordants are alum, potassium dichromate, ferrous sulphate, copper sulphate, stannous chloride and stannic chloride. Treatment of cotton with tannins, either natural or synthetic, introduces additional hydroxyl and carboxyl groups on the fibre matrix. A subsequent treatment of the tannin-treated cotton with metal salts introduces the respective cations in the fibre which leads to the formation of metal complex. This tannin-metal complex creates affinity on cotton for the natural dye. Literature available so far indicates that the mordanting techniques employed 1-5 play a significant role in the colour strength and fastness properties during the subsequent dyeing operations onto cotton. The present work is aimed at studying the dyeing behaviour of four different natural dyes onto cotton using a novel mordanting technique. A ready-to-dye bleached plain weave poplin cotton fabric having the specifications 26 picks/cm, 26 ends/cm and 123 g/m 2 was used. The fabric contained no additives and was used as such for mordanting. Four different dyes were extracted separately from Meyna laxiflora (Helu), Maba angustifolia (Kalindrin), Wendlandia thyrsiodea (Shevara), and Eupatorium repandum (Ranmodi) wild plant species. The dried leaf was powdered fine in a ball mill and directly used for extraction. The required quantity of leaf powder was agitated in water at room temperature and filtered. The clear coloured extract was used for dyeing. Aluminium potassium sulphate (alum) (Qualigens AR grade) and tannic acid (Qualigens AR grade) were used as mordants. Mordanting was carried out using following techniques: