Saminathan Ratnapandian

List of tables

Table 1.1 Typical shades of natural dyes 9

Table 1.2 Continuous dyeing vs. batch dyeing 15
Table 2.1 Fabric specifications 29
Table 2.2 Chemicals used and their suppliers 32
Table 3.1 Pad liquor (dye) recipe 47
Table 3.2 Atomic mass of elements present in mordants 50
Table 3.3 Calculation to determine maximum mass of metal applied on 0.2 g of fabric
. 50
Table 3.4 FTIR band assignment 54
Table 3.5 Typical K/S values at 440 nm for dyed cotton fabric 58
Table 3.6 K/S values at 440 nm for cotton fabric padded with different mordant concentrations 59
Table 3.7 Metal retained (mg) measured using AAS 62
Table 3.8 Comparison of dyeing process consumables (10 g fabric sample) 63
Table 3.9 Fastness testing results for pad-dyed cotton 63
Table 4.1 Plasma treatment vs. traditional wet processing 68
Table 4.2 Mordant concentration and sequence for padding with 10 g/l dye 72
Table 4.3 Wicking test results 73
Table 4.4 Fastness properties of wool dyed after plasma treatment 78
Table 5.1 Padding liquor recipe incorporating chitosan 83
Table 5.2 Fastness ratings of cotton fabric dyed with Acacia dye and 0.05% chitosan.89 Table 5.3 Antimicrobial testing results 91
Table 6.1 Indigo exhaust dyeing recipe 96
Table 6.2 Pad liquor recipe 98
Table 6.3 Constitution of reducing chemical pad baths 98
Table 6.4 Print paste recipe for indigo 100
Table 6.5 Comparative colour strength that demonstrates effect of steaming time 104
Table 6.6 Fastness testing results (dyeing) 108
Table 6.7 Fastness testing results (printing) 111

Abstract

This thesis presents the findings from investigations into adapting the pad-dyeing process for dyeing cotton and wool using natural dyes. The aim was to apply traditional dyes employing current and emerging technologies of textile colouration. The scope of this research was to add to the knowledge regarding continuous dyeing methods for natural dyes with an ultimate goal of large-scale sustainable colouration.

The growing niche market for sustainable products is prompting the reintroduction of natural dyes on a commercial scale. However, the dominance of synthetic dyes for the past 150 years has stifled in-depth studies on industrial-scale use of natural dyes.

Hobbyists and craftspeople, a minority, who continue to use natural dyes for textile colouration, adhere primarily to the conventional home-scale exhaust dyeing method. Such methods are not readily compatible with commercial application methods. Hence, this work was undertaken to explore the continuous dyeing method of padding for the application of natural dyes.

Two prominent classes of natural dyes namely mordant dyes (represented by dyes derived from Acacia catechu and Acacia nilotica) and vat dyes (Indigofera tinctoria) were evaluated.

The work on mordant dyes focused on determining optimal process parameters such as padding sequence, mordanting method and mordant concentration. Copper (II) sulfate and iron (II) sulfate, two widely used mordants, were employed in the research. Among the three possible mordanting methods, post-mordanting was found to yield the darkest shades for both mordants. A process sequence of pad (dye) → dry → steam followed by pad (mordant) → steam → dry resulted in the deepest shades when copper (II) sulfate was the mordant, while for iron (II) sulfate it was pad (dye) → steam → dry followed by pad (mordant) → steam → dry. A distinct shade of beige was obtained on mordanting with copper (II) sulfate while iron (II) sulfate yielded a yellow-grey shade. Ideal mordant concentration required for a 10 g/l dye concentration was either 15 g/l copper (II) sulfate or 5 g/l iron (II) sulfate. These concentrations were lower than those recommended for both exhaust dyeing (5% on the weight of material) and padding (60

g/l) as reported in the literature. The results reported above were similar for both mordant dyes (Acacia catechu and Acacia nilotica).

In order to gain a better understanding of the functioning of mordant in the dyeing process, the dyes, mordants and dyed fabrics were analysed using Atomic Absorption Spectroscopy (AAS) and Fourier Transform Infra-Red (FTIR) spectroscopy. AAS revealed the absorption of a significantly higher amount of metal by the dyed fabric as compared to a mordanted fabric. This confirmed the formation of a dye-metal-textile complex during dyeing. The FTIR spectra were distinctly different for the dyes derived from Acacia catechu and Acacia nilotica. However, when the dyed samples, coloured using the above dyes in combination with the same mordant, were analysed, the differences between the spectra diminished. This indicated that metal plays a major role in defining the bonds created during dye-metal-textile complex formation. This may be the reason for the similarity in shades observed for the dyes evaluated.

Improvement to the optimised padding process was achieved by employing atmospheric pressure plasma pretreatment of the textile, or including chitosan in the pad liquor. Pure helium and a 95/5 helium/nitrogen mixture were evaluated as the plasma gas. Irrespective of the gas used or the duration of treatment, exposure to plasma improved the wettability of wool fabric. Wool treated in pure helium plasma for 14 seconds exhibited an increase of 30% in the depth of shade as compared to an untreated fabric, both pad-dyed with 10 g/l dye solution. A tone-on-tone pattern was created in a single padding operation by selective pretreatment of the fabric.

Incorporation of 0.05% chitosan in the pad liquor for cotton resulted in a 20% darker shade and imparted antimicrobial properties to the dyed fabric. However addition of an excess quantity of chitosan inhibited dye uptake by the fabric.

Indigo was applied, at a concentration of 16 g/l, on cotton by the vat dyeing process of pad (dye) → dry → pad (reducing chemicals) → steam. Sodium dithionite and sodium hydroxide, the common reducing agent and alkali employed in the exhaust dyeing of indigo, were replaced by thiourea dioxide (TUD) and sodium carbonate respectively. The advantage of TUD is its thermal stability in comparison to sodium dithionite. This made the dyeing process easy to control and avoided the effluent problems associated

with sodium dithionite. Sodium carbonate, being a milder alkali than sodium hydroxide, poses reduced handling and disposal issues.

Experiments on colouration with indigo were conducted in two stages. First, the alternate reducing agent (TUD) was used in combination with sodium hydroxide to reduce the indigo. In the next stage, both reducing agent and alkali were replaced. The shade obtained in both cases was equivalent to that obtained by exhaust dyeing of a 1.5% shade of indigo. When TUD was used in the presence of sodium hydroxide, a steaming time of 90 seconds was sufficient to reduce the natural indigo and produce the darkest shade. However, when the alkali was also substituted, a prolonged steaming of up to 6 minutes was necessary. This increase may be due to the higher reaction energy requirements for sodium carbonate as compared to sodium hydroxide. A comparison between samples dyed using natural and synthetic indigo by the alternative chemicals indicated that the former produced a darker shade. This may be due to the higher reactivity of the less crystalline natural indigo.

Similar experiments were conducted on ‘table strike-off’ trials using a K-Bar hand coater that simulated screen printing. Cotton fabric printed with an indigo-containing paste was dried. The dried fabric was padded in a reducing bath containing either TUD and sodium hydroxide or TUD and sodium carbonate and then steamed for durations equal to that followed for dyeing. The results differed in that an equivalent shade was not obtained, neither between natural and synthetic indigo nor between the two alkalis. Overall, the darkest shade was produced by synthetic indigo reduced in the presence of sodium hydroxide. Natural indigo yielded a shade 20% lighter than synthetic indigo.

The discrepancy in results between printing and dyeing may be attributed to the interference by the print paste thickener on the diffusion of the reduced indigo into the fabric.

The outcome of the research is a clear indication that natural dyes can be used in conjunction with current continuous dyeing practices. The reaction between natural dye, mordant and textile is unique and optimum combinations have to be determined in each case. The role of the mordant is critical in regards to the final shade and, as seen in this investigation; similar shades may be obtained from dyes derived from different plants. Judicial use of plasma treatment and chitosan can produce darker shades for a

given amount of dye, leading to a cleaner production process. Any consequent benefit opens additional markets for the dyed materials. Colouration with indigo by the vat dyeing method and the use of alternate reduction chemicals increases process control while lowering the impact on the environment.

Publications