Eco-friendly dyeing of raw cotton fibres in an ethanol–water mixture without scouring and bleaching pretreatments

Abstract

Raw cotton fibres (RCFs) are generally scoured and bleached to remove impurities before the dyeing process, which consumes large amounts of chemicals and water. Therefore, the development of an eco-friendly dyeing technology for RCFs that avoids the need for scouring and bleaching pretreatments is highly important for both the economic and ecological reasons. Herein, we report on a green approach for industrial-scale dyeing involving salt-free coloration of RCFs using an eco-friendly ethanol (EtOH)–water (H₂O) mixture. The method involved initially allowing the RCFs to swell in the EtOH–H₂O mixture followed by dyeing in another EtOH–H₂O mixture with 95% of ethanol by volume. The results showed that the swelling solution could be reused for at least five times, and the investigated dyes exhibited superior exhaustion and total fixation levels, indicating the high rate of dye utilization and low levels of effluent discharge. The research elucidated in detail the swelling and dyeing mechanisms of RCFs in relation to the EtOH–H₂O mixture. Additionally, the industrial-scale dyeing of RCFs revealed the successful establishment and verification of the proposed coloration technology. Meanwhile, we designed and established an automatically controllable EtOH recovery system for the reuse of EtOH from the EtOH–H₂O mixture, which accelerates its industrialization. Compared with the conventional dyeing method for RCFs, this eco-friendly approach consumes fewer chemicals and energy. Therefore, the eco-friendly dyeing of RCFs in the EtOH–H₂O mixture without scouring and bleaching pretreatments is a promising approach for protecting the environment by reducing effluent discharge.