Zero-water discharge and rapid natural dyeing of wool by plasma-assisted spray-dyeing

The application of plasma on textiles is a highly beneficial technique to improve dyeability and reduce the consumption of water, energy, and time. This is advantageous for the industries to demonstrate high productivity and quality of the product but at a lower cost. Plasma treatment usually is applied as a pre-treatment before the conventional dyeing process, where the generation of dye wastewater remains a challenge. Plasma post-treatment can make a huge difference in reducing the dye wastewater; however, this has received little attention and more specifically the effectiveness of plasma on natural dye fixation is unknown. Natural dyes from waste, such as onion skin (Allium cepa L.) are greatly abundant and a low-cost green option for sustainable dyeing of textiles. In this study, a clean method of dyeing wool fabric with onion skin is proposed using a plasma-assisted spray-dyeing technique with no wastewater discharge. Four different plasma parameters were used varying the plasma operating power (40 W and 64 W) and treatment duration (30 s and 60 s). Mordanting and dyeing were done by a successive spraying-drying method in multiple rounds to reach the identical shade of a parallelly conducted conventional dyeing process. The removal of the cuticle layer of wool by plasma treatment was perceived from the morphology of the treated fabric, possibly contributing to the wettability. A higher plasma operating power and longer treatment time were beneficial and affected the overall dyeability. The chemical structure of wool showed a higher hydroxyl peak intensity after dyeing and a lower peak intensity of C–H groups after plasma post-treatment. The colour fastness properties (washing and crocking) improved after plasma post-treatment and confirmed the possible attachment of unfixed dyes with fabric. Overall, the time and energy consumed by the plasma-assisted spray-dyeing technique was nearly one-third of conventional dyeing, with no wastewater generation and led to a better colour strength (20.4 compared to 15.8) and similar colour fastness. Results showed the great potential for this newly proposed method as a sustainable textile dyeing alternative.