Saminathan Ratnapandian

Conclusions

1. Introduction

This chapter summarises the conclusions from the investigations into understanding the compatibility between natural dyes and continuous colouration processes. Based on the information found, further work could be undertaken to increase the level of knowledge regarding the performance and application of selected natural dyes using present-day colouration technology.

1. Padding of mordant dyes

It has been found that mordant dyes of natural origin are compatible with the padding process for dyeing of cotton and wool fabrics. An in-depth evaluation was conducted of pre-mordanting, post-mordanting and simultaneous mordanting techniques. Of the three techniques, post-mordanting produced the most level dyeing as well as the darkest shades in the present study. The combination of mordant and dye is unique in terms of the shade produced, processing sequence and dye-to-mordant ratio.

It was confirmed that, metal from the mordant chelate with the dye and textile material forming a covalent bonded complex. Quantitative assessment of the actual mass of metal retained from the mordant may be used to optimise padding recipes. Each dye- mordant combination has an individual ideal ratio and requires a unique process sequence. The same dye can yield different shades in combination with different metals. It was established that, the metal influences the bonds of the complex to such an extent that similar complexes are formed from different dyes. It should be noted that, the quantity of metal retained by the dyed fabrics was within the standard limits recommended by international organizations such as Oekotex.

When compared with current available literature, it was found that, for the dyes and mordants investigated, a low dye-to-mordant ratio was sufficient to produce deep and level shades at the examined concentration of dye. This fact combined with the low material-to-liquor ratio, as compared to the requirements for exhaust dyeing, make padding the process of choice for large scale production.

2. Improving padding of mordant dyes

Atmospheric pressure plasma pretreatment improves the wettability and dye uptake of wool. At treatment duration of 7 seconds, plasma generated from pure helium was equally effective as that from a mixture of helium and nitrogen in increasing the depth of shade obtained. When the treatment duration was increased to 14 seconds, the presence of nitrogen decreased the effect of plasma. Longer treatment in helium plasma results in darkest shades although a definite trend is yet to be established. Selective exposure to plasma can be employed to create tone-on-tone patterns on wool in a single step by padding. Apart from increasing the evenness of shade, the effect of plasma pretreatment was not significant on cotton.

The present work, of adding chitosan to the pad liquor and increasing the depth of shade on cotton, is the first published report on the simultaneous application of chitosan and natural dye by padding. The amount of chitosan that produces the desired result has to be carefully determined because excess chitosan reduces the shade obtained. As an added benefit, the use of chitosan imparts an antimicrobial character to the dyed fabric. While chitosan has medical applications in absorbable sutures and wound dressing materials, its use in general medical textiles has not been fully explored. It is hoped that the present work will stimulate research into application of fabrics dyed in the presence of chitosan in hospital bed linen.

The shades obtained from both improvement techniques bring out the influence of mordant in the bonding characteristics of the dye-mordant-textile complex. The increase in depth of shade varies according to the mordant employed.

3. Indigo colouration

This thesis is the first to report on the complete replacement of the chemicals used to reduce indigo, the most important among all the available natural dyes. The research is unique in that, cotton was dyed, using indigo reduced by the alternate chemicals, by employing a modified commercial vat dye padding process.

Natural indigo is commonly used for producing traditional fabrics. Although a variety of application methods are used by the craft industry, reduction of indigo is achieved by

the traditional fermentation method across the globe. The textile manufacturing sector employs sodium dithionite as reducing agent in the presence of sodium hydroxide for indigo colouration. In this research an alternate reducing agent, thiourea dioxide, has been evaluated for reducing indigo in the presence of either sodium hydroxide or sodium carbonate.

Cotton fabric can be dyed by both natural and synthetic indigo, reduced by either of the above alternate chemical combinations, employing a modified commercial vat-dye padding process of pad (dye) → dry → pad (reducing chemicals) → steam. The use of sodium carbonate as the alkali while reducing indigo results in a near equivalent shade to that produced in the presence of sodium hydroxide. The drawback is that a longer duration of steaming is necessary.

Similar to dyeing, printing with indigo is a traditional art with its own variety of processes. The common factor is the use of alkali-stable thickeners to ensure pattern integrity. As may be expected, the characteristics of print-paste thickener influences the shade obtained. In the present investigation, the thickener did not permit sodium carbonate to be as effective as sodium hydroxide and the result was a lighter shade produced in the presence of the former. In comparison to dyeing, the duration of steaming to obtain the darkest shade was also affected.

The difference in crystallinity between natural and synthetic indigo affects the depth of shade obtained. The less crystalline natural indigo produces a darker shade during dyeing. However, in printing, the lower crystallinity increases the interaction with the thickener hindering migration of the leuco-indigo to the fabric and thereby produces lighter shades.

In all cases irreversible reduction (over-reduction) of indigo results in light shades and has to be avoided. This generally occurs by steaming for longer than necessary duration.

The overall outcome is that natural dyes are compatible with continuous dyeing processes. The synergistic effect of a renewable dye source and the efficiency of present day colouration processes would make commercial colouration using natural

dyes sustainable. The above conclusions indicate areas for further research of which some are listed below.

2.