Saminathan Ratnapandian

Plasma pretreatment for improving padding of natural dyes

1. Introduction

This chapter presents the investigations into one of the pathways of improving the dyeing process mentioned in Section 1.2.2.5. Atmospheric Pressure Plasma (APP) pretreatment has been applied to improve the pad-dyeing of natural dyes. The significance of APP is that it is a continuous process that does not require either water or chemicals [60, 143]. The following sections introduce plasma technology, describe the experimental procedures and discuss the results obtained.

2. Plasma technology

Plasma, also known as the fourth stage of matter, is defined as a collection of nearly equal numbers of positive and negative charges obtained by ionisation of a gas. This mixture of excited ions, molecules, electrons, neutrons, protons and free radicals (Figure 4.1) is highly reactive [144]. The plasma gas is ionised by addition of energy in the form of temperature or the application of a high electric field. Heat originated plasma, with a high volume of charged particles and gas densities, is described as ‘hot’ or thermal plasma and has temperatures above 700^○C. Non-thermal or ‘cold’ plasma, generated by the application of a high electric field, exists at near room temperature and is better suited for treating textile materials [144, 145].


Gas ^Plasma

Energy / Temperature	Molecule	Molecule (excited)
Ions	Free electron	Free radical

Figure 4.1 Schematic representation of plasma [144]

Shishoo [60] summarises the classification of cold plasma as low-pressure (vacuum) and atmospheric pressure, depending on the gas pressure at which the plasma is generated. The former, a mature technology developed in non-textile areas, operates at vacuum pressures between 10^-2 and 10^-3 millibars. The latter, operating at atmospheric pressures, is sub-divided into corona treatment, dielectric barrier discharge and glow discharge, based on shape and positioning of electrodes. The major disadvantages of
the first two types are non-uniformity of treatment, the requirement for precise substrate positioning and low inter-electrode spacing (~1 mm). Glow discharge imparts uniform treatment by generating the plasma between two parallel-plate electrodes in an inert atmosphere.

Treatment by either vacuum or atmospheric pressure plasma yields comparable results in terms of the functionality achieved [145, 146]. Although APP for textiles is a fairly recent development, it is economical and convenient for continuous production as it avoids working under a vacuum [60]. A typical characteristic of all plasma treatment is that it affects only the surface (<1000 angstroms, 1 angstrom = 1 x 10^-10m) and leaves the bulk properties unaffected.

Desirable functionalities such as altered moisture relations (absorbance or repellence), antimicrobial property, soil repellence, stain resistance, soft handle and improved dyeing are widely achieved on textiles by wet finishing processes. Such processes employ a variety of chemicals [11]. Plasma pretreatment followed by wet finishing has been employed to impart similar functional finishes to textile materials with lesser amount of chemicals and in some cases without the use of chemicals or water [147- 154]. The effect of plasma treatment may be altered by varying process parameters such as supply frequency, discharge power, treatment time, type and pressure of gas.

For example oxygen or helium plasma increases moisture absorbance while fluorocarbon increases water repellence. Several surface phenomena such as adsorption, desorption, etching, cleaning, surface activation and cross-linking occur singly or in combination on exposure to plasma [154-156]. A concise comparison between traditional finishing and plasma treatment is given in Table 4.1 [60].

Table 4.1 Plasma treatment vs. traditional wet processing [60]