The traditional dyeing and printing process not only consumes a lot of energy and generates high carbon emissions, but also produces pollutants such as wastewater, waste gas, and waste residue. With the proposal of China's "dual carbon" goals, the dyeing and printing industry is transforming towards low-carbon, green, and environmentally friendly under the dual impetus of policy guidance and technological innovation, and has achieved breakthrough progress.

Pre-treatment modification technology

The principle of pre-treatment modification technology is to modify natural cellulose fibers such as cotton. First, the fibers are made to carry positive charges through chemical reactions or physical adsorption. Then, during the dyeing process, the negatively charged reactive dye molecules are adsorbed and combined with the fibers, significantly increasing the dye uptake rate. There is no need to use salt or alkali to increase the fixation rate, thus achieving low-salt and low-alkali (or salt-free and alkali-free) dyeing. Common modifiers are mainly cationic modifiers, including epoxy, nitrogen heterocycles, and quaternary ammonium salts, as well as modifiers extracted from natural biomass. The advantage of this technology is that it can increase the dye uptake rate on fabrics, reduce the use of salt and alkali, and also reduce the generation of dyeing wastewater, and can achieve water recycling. It has been industrialized in many places such as Xinjiang, Shandong, and Zhejiang. However, its drawback is that it can only be applied to cellulose fibers such as cotton and cannot be used for polyester, nylon, and other materials.

Supercritical carbon dioxide dyeing technology

The principle of supercritical carbon dioxide dyeing technology is to use supercritical carbon dioxide as the dyeing medium. By taking advantage of its solubility and high diffusivity, the dye can reach the fiber pores, achieving uniform and rapid dyeing. After the dyeing process, the carbon dioxide can be fully separated from the dye and recycled, without the need for subsequent washing and drying processes. Due to the poor solubility of polar dyes (such as reactive dyes) in carbon dioxide fluid, there are certain difficulties in dyeing natural fibers. Therefore, supercritical carbon dioxide technology is currently mainly applied to the dyeing of synthetic fiber fabrics such as polyester. The advantage of this technology is that it uses very little water, has high solvent and dye recycling rates, and is less likely to cause environmental pollution. However, its disadvantages include high requirements for equipment, processes, and operations, as well as the need for high-temperature and high-pressure environments, which not only consume a lot of energy but also pose certain safety risks. Therefore, large-scale promotion still requires some time.

Other non-aqueous medium dyeing technologies

The principle of non-aqueous medium dyeing technology is to utilize the hydrophilicity of reactive, acid, and other dyes to distribute them between the fiber and the non-aqueous medium, thereby improving the adsorption efficiency of the fiber for the dye. In the non-aqueous medium dyeing system, only a small amount of water is retained, which is used to swell the fibers and carry a water film, as well as to dissolve and penetrate the dye, promoting the reaction between the dye and the fiber. The non-aqueous medium cannot dissolve the dye or mix with water and is only used for material transmission and heat conduction. The dye is distributed in the non-aqueous medium in the form of high-concentration dye emulsion droplets or suspended particles. As the medium moves, the dye continuously contacts the water film on the fiber surface. Due to the low surface tension of the non-aqueous medium, the dye can uniformly contact the fiber surface, ultimately achieving a high dye uptake rate, and the dye fixation rate is much higher than that of traditional water bath dyeing. Common non-aqueous media include siloxanes, ethers, alkanes (isooctane, paraffin, white oil, vegetable oil, etc.), sulfones, and sulfoxides. The advantage of this technology is that it uses very little water and salt and does not generate dyeing wastewater. However, its drawback is that some non-aqueous media are toxic and have bioaccumulation properties, and their discharge can cause new environmental pollution.

Digital inkjet printing technology

The principle of digital inkjet printing technology is similar to that of inkjet printers. The pattern is input into the computer in digital form, edited through the computer system, and then the special dye is directly sprayed onto the fabric through nozzles to form the desired pattern. Compared with traditional printing methods that require complex processes such as plate-making and color paste preparation, digital inkjet printing is on-demand, environmentally friendly, and saves 20% to 30% of dyeing and finishing materials, water, and steam compared to traditional printing techniques. The advantages of this technology lie in its rich color range, high precision, short production cycle, and flexible batch sizes, which align with the current flexible production model of the textile and garment industry that emphasizes multiple varieties, small batches, high quality, and quick delivery. However, its disadvantages include high equipment investment costs and relatively high costs for high-quality ink and other consumables. Compared to other green dyeing and printing technologies, its water and energy-saving effects are average.

Other green dyeing and printing technologies

In research and exploration both at home and abroad, there are many other green dyeing and printing technologies, such as liquid ammonia dyeing, small-bath ratio batch dyeing, low-water continuous pad dyeing of polyester fabrics, cold pad-batch dyeing, low-salt and low-alkali reactive dyeing, and fully automatic complete set dyeing of yarns. These technologies have reduced the consumption of water, chemicals, and energy to varying degrees, and have decreased wastewater discharge, meeting the requirements of sustainable development in the textile industry. However, these technologies also share some common problems, such as high equipment investment, complex process control, and high requirements for dye and fabric compatibility. In the future, with further technological development and cost reduction, more green dyeing and printing technologies are expected to be more widely applied in the textile industry.