Saminathan Ratnapandian

Dye	Thar (A. catechu)
Caspian (A. nilotica)
Mordant	CuSO4.5H2O (15 g/l)		FeSO4.7H2O (5 g/l)		CuSO4.5H2O (15 g/l)		FeSO4.7H2O (5 g/l)
Washing test	3–4		3–4		3–4		3–4
Light fading	4–5		3–4		4–5		3–4
Rubbing	Dry	Wet	Dry	Wet	Dry	Wet	Dry	Wet
	5	5	5	5	5	5	5	5

7. Conclusions

Combining atmospheric pressure plasma treatment and padding enhances the process efficiency, quantified by the depth of shade obtained from a given dye concentration, of continuous application of natural dyes on wool. Plasma pretreatment provides a benign alternative to the existing wet-treatment methods for enhancing dye uptake on wool.
Compared to 7 s, 14 s exposure of wool fabric to plasma leads to an increase in the depth of shade obtained, highlighting the progressive surface modification of wool. The hydrophilic character of the natural dyes used is identified by their enhanced uptake by plasma-treated wool. In this case, copper (II) sulfate performs better than iron (II) sulfate as a mordant in terms of increasing the depth of shade. Partial blocking of plasma pretreatment that brings about differentiated dyeing may be used to produce one-step patterning effects aimed at niche markets. Apart from improving the levelness, plasma pretreatment did not significantly increase the depth of shade on cotton fabrics. The fastness properties of the dyed fabrics were unaffected by the pretreatment. The next chapter will be aimed at improving the padding of cotton with natural dyes.

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Chapter 5

Improving dyeing performance by using chitosan in padding of natural dyes

1. Introduction

There are many approaches for increasing colour yield during dyeing. These could be simple changes to process parameters (such as temperature and time), pretreatment of the substrate and addition of auxiliaries [11, 12, 50]. The process gains popularity when the increase in colour yield is brought about by using a byproduct or waste product [78, 79, 162, 163].

Chitin is a byproduct of the seafood industry because it is abundantly present in the inedible shells of crustaceans such as crabs, shrimps and lobsters. It is also the second most widely present natural polymer next to cellulose. Chitosan is derived by de- acetylation of chitin [164]. The interaction between chitosan and textile dyes has been utilised in a variety of applications [163-168]. However, application of chitosan simultaneously with dye, especially by padding, has not been reported. This chapter details the work done to improve the padding of natural dyes by incorporating chitosan in the dye bath. A brief discussion of chitosan and its typical characteristics is followed by a description of the experiments and comprehensive analyses of the results obtained.

Chitosan is a linear polysaccharide composed of randomly distributed β-(1-4)-linked D- glucosamine (de-acetylated unit) and N-acetyl-D-glucosamine (acetylated unit) [166]. As depicted in Figure 5.1, it is poly-cationic with three reactive groups, namely the amino (–NH₂) group at C-2 and the two hydroxyl (–OH) groups at C-6 and C-3 in each repeat unit. This poly-cationic character has been utilised in different areas, such as cosmetics, weight loss, health care, water treatment and textile dyeing [164, 168].

Figure 5.1 Schematic structure of chitosan [164]

1. Application of chitosan in dyeing

The characteristic of multiple reaction sites in chitosan has been utilised for the decolourisation of dyeing effluents by agglomeration [169, 170]. Alternately, the ability
to agglomerate dyes has been used to improve the dye-ability of cotton and wool by pretreatment with chitosan. The mechanism proposed was that chitosan forms a film and increases the number of reaction sites on the substrates for dyes [168, 171]. An improvement in dye uptake leading to darker shades has been reported for both synthetic and natural dyes [96, 97, 163, 172-174]. Rippon [175] pretreated cotton fabric with chitosan to lower the differences in depth of shade between immature and mature fibres. Similar effects of chitosan pretreatment on damaged and undamaged wool have been reported by Davidson and Xue [174]. It should be noted that these dyeing studies applied chitosan in a separate step and primarily used the exhaust method of dyeing.
Hence combining chitosan and dye in one-step pad-dyeing offers an unexplored research area.

2. Antimicrobial activity of chitosan

The use of chitosan has been known to impart a durable antimicrobial functionality to the textile substrate [96, 176, 177]. Such functionality is desirable because it avoids bad odours and occasional damage of textile caused by microbial growth. The antimicrobial activity of chitosan is generally attributed to interaction between the cationic chitosan and anionic cell surface of microbes. The interaction leads to extensive microbial cell surface alterations and damage, inhibiting metabolism and finally killing the microbe [176, 178, 179]. Chitosan acts as a biocide for some microbes and as biostatic for others [180]. The lowest amount of a substance required to inhibit microbial activity is termed the Minimum Inhibition Concentration (MIC). Chitosan has been reported to have an MIC of 0.05% [181]. Hence, including chitosan in the padding liquor of natural dyes is likely to impart antimicrobial features to the dyed fabric. Such an additional benefit would open specialty markets in medical textiles. Hence the antimicrobial effectiveness of the dyed fabric also needs to be evaluated.

2.