How does polyester dyeing change when heated?
Polyester has good thermoplastic properties and produces different changes at different temperatures. The thermal changes of polyester are between the thermal changes of amorphous and crystalline polymer compounds. Above the glass transition temperature, only certain molecular chain segments with small inter-molecular chain interactions in the amorphous region can move. Molecular chain segments with large inter-molecular chain interactions are still difficult to move. Of course, molecular chains in the crystalline region cannot move. , so the fiber only appears to be relatively flexible, but not necessarily as elastic as the highly elastic state.
When it continues to be heated to 230-240°C, the softening point of polyester is reached. The movement of the molecular chains in the amorphous region of polyester intensifies, and the forces of intermolecular interaction are broken up. At this time, it is similar to a viscous flow state, but the chain segments in the crystallization zone have not been dismantled, so the fiber only softens, not melts. However, the use value of the fiber has been lost at this time, so it is not allowed to exceed during printing and dyeing processing. this temperature.
Transfer printing of polyester is achieved by utilizing the movement of heated molecular chains in the amorphous area. However, the temperature must be strictly controlled. If it exceeds the allowed range, the fabric will become rough and hard to the touch. When polyester is exposed to high temperatures of 258-263°C, the molecular chain segments in the polyester crystallization zone also begin to move, and the fiber melts. This temperature is polyester melting point.
Principle of heat-setting polyester
When polyester is heat-set, due to the different heat received, the fibers The microstructural changes are also different. When the setting temperature is 170-180°C, the absorption capacity of polyester for disperse dyes in exhaust dyeing is very low. When it exceeds 180°C, the absorption rate of polyester for dyes is proportional to the increase in temperature, which is related to the crystallization properties of polyester. This is related to the fact that at 170-180°C, polyester crystallizes quickly. Since more crystals are formed, less dye is absorbed. As the temperature increases, the degree of orientation of macromolecules may decrease, so the absorption of dye increases.
Polyester will undergo hydrolysis under the action of high-temperature steam for a long time, and the strength and dyeing performance of the fiber will decrease. However, it can withstand 140% in water and dye baths using water as the medium. ℃ high temperature.
Why are polyester disperse dyes dyed at 130°C?
Why 130 degrees?
The general process of dyeing polyester with disperse dyes is as follows: the dye is dispersed in the dye bath, and then dissolved as single molecules in the dye liquor. The single molecules of the dye are adsorbed on the surface of the fiber, and finally in a high temperature state Diffuses downward into the fiber, continuously dissolving, adsorbing, and diffusing 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 single molecule dispersed and dissolved state of the dye; 3. The adsorption of the dye on the fiber surface; 4. The diffusion of dye molecules into the fiber; 5. The equilibrium state of dyeing; 6. The recrystallization state of the 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.
1. 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.
2. Polyester is a thermoplastic fiber. 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 inter-fiber molecules are The microgaps are very small, and the amorphous region is the main hydrogen bonding site of disperse dyes. Therefore, the resistance of disperse dyes to diffusing into polyester fibers at room temperature or low temperature is very high, and it is almost impossible to enter the fiber interior. 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.
3. 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 a 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, and its solubility is equivalent to that of direct dyes.�0.01%. As the temperature increases, the solubility will generally 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 preventing 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 higher, 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 reduced, the dye uptake rate will be low, resulting in a waste of dye and an increase in wastewater treatment costs; while 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, and the choice of 130 The main reason for dyeing at ℃ is determined by the physical and chemical properties of polyester fiber. Secondly, it is related to the solubility of disperse dyes, but has little to do with dyeing equipment. </p
