Application of natural dyes by padding technique on textiles
Creators: name: Ratnapandian, Saminathan
Publisher: name: RMIT University
Year: 2024

description: This thesis presents the findings from investigations into adapting the padding process for dyeing cotton and wool using natural dyes. The aim was to apply traditional dyes employing current and emerging technologies of textile colouration. This synergistic amalgamation led to a cleaner production process with low environmental impact.<br><br>Mordant 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 (padding sequence, mordanting method and mordant concentration). Post-mordanting with copper (II) sulfate or iron (II) sulphate yielded the darkest shades for both dyes. Ideal process sequence for copper (II) sulfate was pad (dye) → dry → steam followed by pad (mordant) → steam → dry while for iron (II) sulfate it was pad (dye) → steam → dry followed by pad (mordant) → steam → dry for both dyes. Similarly, use of the first mordant yielded a beige shade while a yellow-grey shade was obtained with the second. Optimum mordant concentration for 10 g/l dye was 15 g/l copper (II) sulfate or 5 g/l iron (II) sulphate for both dyes. These were lower than those recommended for exhaust dyeing (5% OWM) and padding (60 g/l) reported in the literature.<br><br>Atomic absorption spectroscopy revealed higher amount of metal in the dyed fabric compared to identically mordanted fabric, confirming the formation of a dye-metal-textile complex. FTIR spectra were distinctly different for the two dyes investigated. However, the spectra for dyed samples, coloured using the above dyes in combination with the same mordant, exhibited minimal differences. This indicated that metal plays a major role in defining the bonds created during dye-metal-textile complex formation causing a similarity in shade.<br><br>Darker shades were obtained by 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. Exposure to either plasma improved the wettability of wool. Wool treated in pure helium plasma for 14 seconds exhibited a 30% increase in the depth of shade. A tone-on-tone pattern was created in a single padding operation by selective plasma pretreatment. Incorporation of 0.05% chitosan in the pad liquor resulted in a 20% darker shade while imparting antimicrobial properties to the dyed fabric.<br><br>Indigo was applied on cotton by the vat dyeing process of pad (dye) → dry → pad (reducing chemicals) → steam. In this process, sodium dithionite and sodium hydroxide, the common reducing agent and alkali in exhaust dyeing of indigo, were replaced by thiourea dioxide and sodium carbonate respectively. Although equivalent shades were obtained, the use of sodium carbonate required comparatively longer steaming time. Natural indigo produced a darker shade when reduced using the alternative chemicals perhaps due to its higher reactivity resulting from its lower crystallinity as compared to synthetic indigo. However, printing with synthetic indigo resulted in darker shades. The discrepancy between printing and dyeing may be attributed to interference by the print paste thickener on diffusion of reduced indigo into the fabric.