Abstract:

Significance Reactive dyes dominate the dyeing market for cellulosic fibers owing to their wide color gamut, brilliant shades, broad applicability, and favorable color fastness. However, in conventional dyeing processes, reactive dyes are prone to hydrolysis, resulting in the discharge of substantial amounts of unfixed dyes into wastewater, which leads to considerable resource waste and environmental pollution. Low wet pick-up dyeing technology, because of its potential to reduce the input of water and chemical agents at the source, has become one of the key research directions for promoting the green and low-carbon transformation of the textile dyeing industry.

Progress In order to improve the utilization efficiency of reactive dyes, a series of low wet pick-up dyeing technologies have been developed. According to the underlying technical principles and liquor-application modes, such systems can be broadly classified into two main categories, i.e.,the expression-type and topical-type. The expression-type approaches include vacuum dewatering wet steaming, low-moisture pad-bake steaming, and high-pressure air dewatering, and the topical-type techniques include foam dyeing, transfer-application-based dyeing, spray dyeing, and inkjet-based coloration. Among these, foam dyeing has completed the transition from laboratory research to industrial application and is currently in the stage of industrial promotion. Low wet pick-up technologies, such as spray dyeing and inkjet-based coloration, which enable precise control of dye-liquor application, have developed rapidly,and are regarded as important technologies for the green transformation of the textile dyeing and printing industry. It can be learnt from the review that the expression-type low wet pick-up dyeing benefits from an initially high wet pick-up, which facilitates adequate liquor penetration into the cellulosic fiber's assembly and subsequent diffusion of dye molecules into the cellulosic fiber's interior, thereby providing the basis for level dyeing. However, this route presents two major dilemmas. First, although the dewatering stage effectively controls the final wet pick-up, it does not reduce the overall water consumption of the process. Second, the energy consumed during dewatering counteracts the energy savings achieved during fixation. Owing to its inherent high-to-low wet-pick-up profile, the reduction in fixation energy is often offset by the substantial energy demand of the dewatering unit, making net energy savings across the whole process difficult to realize. By contrast, metered-application-type low wet pick-up dyeing is characterized by the precise application of dye liquor at the initial stage. However, unlike conventional pad dyeing, it lacks the full-bath impregnation and nip pressure that promote uniform liquor penetration, thereby limiting subsequent dye transport into the cellulosic fiber's interior. This directly leads to two challenges: insufficient penetration and non-uniform surface distribution. In addition, much of the existing research remains limited to comparisons of macroscopic dyeing performance, such as K/S values, whereas understanding of the underlying transport and distribution mechanisms remains insufficient, which has become a key constraint on the transition from laboratory research to industrial application.

Conclusion and Prospect This paper systematically scrutinized two principal technical routes for low wet pick-up reactive dyeing, namely, the expression-type and the metered application-type route. Future research should focus on synergistic process-equipment innovation, strengthening theoretical research and evaluation systems, and advancing technology integration. The development of low-energy dewatering units, penetration-enhanced metered-application systems, standardized evaluation methods, and digitally controlled adaptive application technologies will be essential for further progress

Key words: reactive dye, low wet pick-up dyeing, cleaner dyeing, water-saving technology, wet pick-up, green and low-carbon, energy conservation and emission reduction