Reactive dyes are vastly used in dyeing and printing of cotton fibre. These dyes have a distinctive reactive nature due to active groups which form covalent bonds with -OH groups of cotton through substitution and/or addition mechanism. Among many methods used for dyeing cotton with reactive dyes, the Cold Pad Batch (CPB) method is relatively more environment friendly due to high dye fixation and non requirement of thermal energy. The dyed fabric production rate is low due to requirement of at least twelve hours batching time for dye fixation. The proposed CPB method for dyeing cotton involves ultrasonic energy resulting into a one third decrease in batching time. The dyeing of cotton fibre was carried out with CI reactive red 195 and CI reactive black 5 by conventional and ultrasonic (US) method. The study showed that the use of ultrasonic energy not only shortens the batching time but the alkalis concentrations can considerably be reduced. In this case, the colour strength (K/S) and dye fixation (%F) also enhances without any adverse effect on colour fastness of the dyed fabric. The appearance of dyed fibre surface using scanning electron microscope (SEM) showed relative straightening of fibre convolutions and significant swelling of the fibre upon ultrasonic application. The total colour difference values ΔE (CMC) for the proposed method, were found within close proximity to the conventionally dyed sample.

The eco-sustainable natural dyes and their application are increasing due to the control of pollution, biological protection and easy application. In this study, Marigold natural dye was extracted from the flowers of the indigenous crop and applied onto cotton fabric by ultrasound-assisted dyeing method. In the first stage, the extraction of natural dye was optimized. Afterward, mercerized cotton fabric was dyed with the extracted Marigold dye. The dyed fabrics produced yellow shades onto cotton substrate. Higher colored yields of the dyed fabrics up to the value of 25 and acceptable colorfastness were achieved at optimized conditions. However, dyeing by pre-mordanting method, washing fastness was marginally improved. Comparing conventional and ultrasound dyeing methods, savings in amount of water (around 16%), thermal energy (for lesser dyeing temperature) and salt concentration (around 14%) required for ultrasound dyeing were notable. Also, representative wastewaters showed around 30% reduction in the total dissolved solids contents. Additionally, tensile strength, stiffness and surface morphology of the dyed fabrics were also assessed and found acceptable.