Application of supercritical carbon dioxide in staining

Abstract: the staining technology of supercritical carbon dioxide without water has completely changed the traditional method of textile staining and achieved cleaner production without environmental pollution. It is characterized by short dyeing process, uniform coloring, good color fastness, low production cost, remarkable economic and environmental benefits. Suitable for textile printing and dyeing enterprises promotion role.

1 introduction

Dyeing and finishing is a highly polluting and water-consuming industry, and water is a very precious resource. In the entire dyeing and finishing process, except for burning wool and mechanical finishing. All in water, that is, wet processing. According to calculation, printing and dyeing processing of fabrics and the discharge of waste water weight at 1:15 0-120 estimates by the national printing and dyeing wastewater by the year of 1 billion tons, waste water treatment is expensive, 35% of industrial waste water emissions, because its waste water to join the various dyeing agent, additives and precipitant, so it is difficult to effective recycling and environmental protection of dyeing and finishing industry cost is very high. With the improvement of national environmental protection standards and the improvement of workers' health protection, environmental protection has become one of the urgent problems to be solved in the dyeing and finishing industry. The dyeing and finishing that I passed has always been the "bottleneck" of textiles. No matter compared with the world's advanced level, or compared with the research and development of China's fabrics and clothing, it is in a backward state. Through the development of environmental protection technology, it can also promote the improvement of technology and improve the research and development level of the whole industry.

Since water is a source of pollution, people have been trying to get textile dyes out of water: in organic solvents or supercritical fluids. Everyone wants to find a substitute for water that will reduce pollution while maintaining or even improving staining. East China university and the national dyeing and finishing engineering technology research center use supercritical carbon dioxide as the dyeing medium, the dyeing process does not use water completely, does not cause pollution. Now they have developed the first experimental equipment for supercritical staining; As global water resources become increasingly exhausted and environmental pollution become increasingly serious, the traditional printing and dyeing process using as the medium has been severely challenged. A new process of nonpolluting supercritical CO2 dyeing without water as medium appears.

2. Supercritical carbon dioxide staining principle

Carbon dioxide is a colorless, odorless, and non-combustible gas, and its weight is 1.5 times that of air. The molecules are linear with two oxygen atoms on both sides of carbon, and they are distributed symmetrically. Therefore, they are not polar. Although they have a large molecular weight and water, they have a low boiling point and are gases at room temperature. Its critical temperature is 31.10c, and it is easy to liquefy under pressure. Because the molecules are non-polar, liquid carbon dioxide has a higher solubility to polar materials, and carbon dioxide has a higher solubility to non-polar or hydrophobic fibers. If carbon dioxide is put into a closed system, when the temperature and pressure exceed the critical temperature (31.10c) and the critical pressure (7.39mpa), the critical fluid state is exceeded. Carbon dioxide then has many unique properties. No matter how you heat it up above the critical temperature, it won't turn into a gas. Likewise, at the critical pressure, it won't turn into a liquid or a solid. Because it is different from gas, liquid and solid, the liquid state is called supercritical fluid. In supercritical fluid CO2 as well as gas, can be evenly distributed throughout the container, by controlling the pressure, can achieve the same size and the liquid density (0.3-1 g/cm3) above), it has the very strong osmosis of objects,'s ability to dissolve substances is much bigger than gas, and even stronger than those of liquids, therefore, many of the properties of supercritical CO2 fluid with gas and liquid, more different from solid, this is its unique properties. The density of supercritical CO2 fluid is hundreds of times that of gas, close to that of liquid, but its viscosity has a gas equivalent. Its diffusion coefficient is about 1% of gas, but it is hundreds of times larger than liquid. This is the reason why substances dissolved in supercritical CO2 fluid are easy to diffuse and have strong permeability. The process makes use of the principle that the solubility of the fuel in supercritical carbon dioxide increases with the density of the fluid, which decreases the density of the fluid and the amount of the fuel in the solution when the temperature is raised, and promotes the diffusion of the fuel on the fiber.

3 characteristics of supercritical carbon dioxide staining

Supercritical carbon dioxide fluid has many special properties, which are different from water and organic solvents. As a dyeing medium, it is especially suitable for dyeing hydrophobic synthetic fibers with low polarity disperse dyes.

Disperse dyes are a kind of insoluble and water. In the water group, they should be non-ionic dyes which exist in suspension, and in the water group, they should be kept dispersed by a large number of dispersing agents. In addition to the dispersions of most of the fine grains into the suspension and in the water, there are few parts that are molecular soluble and some that exist in the micelles with dispersants and other auxiliaries and maintain dynamic equilibrium with each other. Dyes in the molecular state can be stained with fibers, which dissolve into water until the dye is finished. Due to the low solubility of the dye, the dyeing speed is greatly limited at low temperature. In addition, most dyes exist in suspension extraction, so the dispersion stability of dye solution is not high, which is prone to grain condensation, crystal transformation and grain growth. In serious cases, precipitation will occur again, causing difficulty or unevenness of dyeing. Although the presence of dispersant improves the dispersion stability of dye suspension, it not only increases the production cost, but also pollutes the water quality.

Due to the low viscosity of carbon dioxide, it has little interaction with dye molecules, and the dye spreads quickly in the supercritical carbon dioxide. Moreover, the diffusion boundary layer near the fiber surface in this fluid is very thin, so the dye can be quickly absorbed and attached to the fiber surface. Due to its strong plasticizing effect on fiber, the dyeing speed is fast. Good levelness and penetration.

The major breakthrough of supercritical CO2 dyeing and dyeing industry has solved the problem of pollution that is difficult to be solved for many years, with the following advantages :(1) no water use, no waste water pollution, and the dyeing and finishing process of environmental protection; (2) after the dyeing, the pressure is reduced, the CO2 is quickly vaporized, and the dyeing drying is not required, which not only shortens the technological process, but also saves the energy required for drying; (3) fast dyeing speed, good levelling and penetration performance, excellent reproducibility of dyeing; (4) CO2 itself is non-toxic, tasteless, non-combustible and reusable; (5) the fuel can be reused, and there is no need to add any dispersant, levelling agent, buffer and other auxiliary agents when dyeing, which not only reduces the production cost and improves the utilization rate of the dye, but also is conducive to environmental protection and reducing pollution. (6) applicable fiber varieties are relatively wide, and some synthetic fibers (such as polypropylene and aramid) that are difficult to be dyed can also be normally dyed. In addition, the system can also be used for wool degreasing, in order to replace perchloroethylene and benefit environmental protection. The department of chemistry of zhejiang university also used laboratory equipment to study the feasibility of this technology. This green process will bring a revolution to textile dyeing.

4. Problems and development directions of supercritical carbon dioxide staining

However, the application of supercritical CO2 fluid dyeing is still in the stage of small batch test, and there are still some problems to be solved. 2. The applicable dye varieties are not complete enough, and the dyeing process needs to be further studied and improved. Supercritical CO2 fluid dyeing is mainly used non-ionic type of solvent disperse dye, dyed fibers include polyester, polyamide, acetate, also studied for polypropylene fiber and aramid fiber dyeing, but with natural fibers such as cotton, wool and other hydrophilic fiber supercritical carbon dioxide dyeing, these fibers have to be modified, can use disperse dye dyeing, not only increased the technological process, and there are still problems in the process of fibrosis, need further in-depth study. One of the most promising first to push polyester dyeing. The application of supercritical CO2 fluid in textile industry can be used for pretreatment and finishing of fabrics in addition to dyeing. Its industrialization prospect: the high pressure system that can be used in industrial production has been developed and applied in yarn dyeing.

In conclusion, supercritical carbon dioxide dyeing is an environmentally friendly dyeing process with high technical content, which represents the development direction of the dyeing and finishing industry and is suitable for the popularization and application of textile printing and dyeing enterprises.