The general process of dyeing polyester with disperse dyes is: the dye is dispersed in the dye bath, and then dissolved in the dye liquor as single molecules. The single molecules of the dye are adsorbed on the surface of the fiber, and finally diffuse into the interior of the fiber at high temperature and continue to dissolve. , adsorption, and diffusion until an equilibrium state is reached.
For the detailed process, please refer to the picture below:
1. The dye is dispersed in the dye liquor; 2. The properties of the dye Single molecule dispersed and dissolved state; 3. Adsorption of dye on fiber surface; 4. Diffusion of dye molecules into fiber; 5. Equilibrium state of dyeing; 6. Recrystallization state of dye.
It can be seen from the above description that there are three main factors affecting the dyeing process: polyester, disperse dyes, and dyeing equipment. The following discusses from these three aspects why polyester is dyed with disperse dyes at 130°C.
Various synthetic fibers have different physical and chemical properties, so the requirements for dyes are also different. Generally speaking, it is required that the polarity, molecular structure size and shape of the dye must be compatible with the polarity and physical structure of the fiber. The basic component of polyester is polyethylene terephthalate, which is a relatively linear polymer. There are no side chains or side groups on the molecular chain, only less polar ester groups. Except for the end groups, There are few hygroscopic centers, so hygroscopicity is poor.
Polyester is a thermoplastic fiber, and its microstructure presents an amorphous zone and a crystalline zone, of which the amorphous zone accounts for 40%. The amorphous zone has a tight structure, and the micro gaps between fiber molecules are very small, while the amorphous zone The area is the main hydrogen bonding site of disperse dyes, so the resistance for disperse dyes to diffuse into polyester fibers at room temperature or low temperature is very high, and it is almost impossible to enter the interior of the fiber. The Tg (glass transition temperature) of crystalline and oriented polyester increases as the crystallinity of the fiber increases. When the polyester is heated above Tg, the molecular chain segments inside the amorphous region move, and the micro-gaps between fiber molecules increase and expand. When a certain temperature is reached, the instantaneous gap formed by the movement of fiber molecular chain segments increases, and dye molecules can enter the interior of the fiber, and the dyeing rate and dyeing rate will be significantly improved. Due to the tight structure of polyester and its high Tg, it requires a higher temperature to dye.
The dyeing process is a type of dye that is dyed in a dispersed state in the dye solution with the help of a dispersant, so it is called disperse dye. The molecular structure of disperse dyes does not contain water-soluble groups, such as sulfonic acid groups, carboxylic acid groups, etc., so they are difficult to dissolve in water and difficult to ionize in water. They are non-ionic dyes; however, there are some polar groups in the molecules, such as hydroxyl groups. , amino group, azo group, etc. Due to the existence of these polar groups, the dye can be dissolved in water in a trace single molecule state at room temperature. Its solubility is equivalent to 0.01% of direct dyes. As the temperature increases, the general solubility will also decrease. increase.
For example, Disperse Red 19 has the following structural formula. Its solubility in water at room temperature is 0.4 mg/L, and its solubility at 80°C is 18 mg/L. The addition of the dispersant forms an electric double layer on the surface of the dye particles (one or several dye molecules), which stabilizes the dispersion; when the critical micelle concentration is exceeded, the dye will be dissolved in the micelles to solubilize it. Generally, as the temperature increases, the solubilization effect increases and the solubility of the dye increases; the dispersant also plays a role in stabilizing the crystal form of the dye, mainly to prevent the conversion of the crystal form and the growth of the crystal form. In both cases, there is no It is beneficial to improve the solubility of dye. However, the trace solubility of the disperse dye itself, the solubilization effect of the dispersant, and the effect of temperature are not likely to make the solubility of the disperse dye too large, otherwise it will not be conducive to dyeing polyester.
The dyeing temperature of high-temperature and high-pressure dyeing is generally around 130°C. At this time, the dye uptake rate of disperse dyes is relatively high, and the general dye uptake rate is 90%. Above, the dyeing rate is appropriate, which is more suitable in terms of dyeing quality and dyeing cost. However, if the dyeing temperature is too low or appropriately lowered, the dye uptake rate will be low, resulting in waste of dye and an increase in wastewater treatment costs; and if the dyeing temperature is too high, the dye uptake rate will no longer increase and energy consumption will increase. At this time, the The pressure resistance requirements of the equipment have increased, so increasing the temperature is not worth the gain.
So the choice of disperse dyes for polyester dyeing is determined by the physical and chemical properties of polyester fibers and the characteristics of disperse dyes. The main reason for choosing 130°C for dyeing is the physical and chemical properties of polyester fibers. Secondly, it is related to the characteristics of disperse dyes. It is related to solubility and has little to do with dyeing equipment. </p
