The main purpose of pre-dyeing treatment for natural fibers is to remove their symbionts. For synthetic fibers such as polyester, artificial “impurities” are removed, including oil added during spinning or weaving, as well as contaminated oil, dust, pigments, etc. Among them, the most noteworthy is oil.
Pre-dyeing treatment
The oil contains lubricants, emulsifiers, antistatic agents, etc. . Applying oil is necessary for spinning and weaving of synthetic fibers such as polyester. However, the oil must be washed away during dyeing and finishing. If the oil is not washed before dyeing and dyed with the oil, the oil will form a “dye-resisting film” on the surface of the polyester, preventing the dye from evenly diffusing and penetrating into the fiber.
Therefore, it is easy to cause uneven coloring and dyeing defects such as color flowers and spots. Moreover, it will also increase the floating color and affect the color fastness. If the pre-dyeing treatment process is not in place and the oil on the fiber is unevenly removed, cloud-like flowers may occur; if during the dyeing process, the dye dispersion stability in the dye liquor is poor and agglomeration occurs, these oils will interact with the fibers. The aggregates of dyes combine and adhere to the dye to produce stains. It must be noted that it must be cleaned after pre-treatment, otherwise it will easily cause flower stains.
Dye Selection
Disperse dyes do not contain sulfonic acid groups (-SO3Na) or carboxylic acid groups Hydrophilic groups such as (COONa) only contain some polar groups such as hydroxyl groups, amino groups, ethanol groups, azo groups, and substituted amino groups. Therefore, they have poor hydrophilicity and are almost insoluble in water. The dye particles can only be coated with anionic dispersants and then dispersed in water.
However, under the dual factors of high temperature above 100℃ and rapid flow of dye liquor, the “coating energy” between dispersant molecules and dye particles will decrease. If the coated dye particles are freed, they will reaggregate into larger dye particles. This phenomenon is the “high temperature cohesion” of disperse dyes. Disperse dyes with different structures have different thermal cohesion phenomena.
It can be roughly divided into three types:
One is that thermal condensation is not easy to occur. These dyes do not agglomerate due to heating, cooling or collision with each other. Dyeing with such dyes generally does not cause uneven dyeing or stains.
The second reason is that thermal agglomeration is easy to occur, but as the dye molecules diffuse into the fiber, the dye aggregates will depolymerize again. As long as the dye is evenly adsorbed on the fiber surface, it will generally not cause Color flowers and stains.
The third reason is that thermal condensation is easy to occur, and it is difficult to depolymerize during the dyeing process. When dyeing with this type of dye, especially when the dispersant or leveling agent used has poor high-temperature dispersion capabilities, it is easy to cause uneven color due to thermal condensation of the dye. Even tar compounds are produced due to the combination of dye aggregates with oligomers, oils, fiber scraps and other impurities in the dye liquor. The high-temperature and high-pressure dyeing method has strong filterability for the dye liquor. The dispersion uniformity of the dye liquor is very demanding.
Therefore, once the dye is improperly selected, the dye liquor will aggregate during the heating or cooling process. The outer and inner layers of the barrel shaft will produce a diminishing color difference. If the roll tension and density are uneven. Each part of the cylinder axis will also produce flowers with different shades of color.
Countermeasures:
Choose disperse dyes with small thermal cohesion properties. Disperse dyes with different structures have different thermal cohesion properties. Use dyes with low thermal cohesion to prevent color flowers and stains. Varieties with low high-temperature cohesion include: dispersed golden SE-3R, dispersed blue E-4R (B-56#), dispersed red 5E-G5 (R-153#), dispersed bright yellow SE-4GL, dispersed purple HFRL (V -26#), disperse brilliant blue R5E (B-l83#), etc. Varieties with a high tendency to condense at high temperatures include: dispersed ruby S-5BL (S-2GFL) (R-167#), dispersed gray S-BL, dispersed red KB-SE, dispersed emerald blue S-GL (B-60#) , dispersed orange G-SF (O-73#), etc.
Heating rate
High temperature and high pressure dyeing is usually divided into four steps:
1. The dyeing material is first run in a buffer bath composed of high-temperature dispersant and acetic acid-sodium acetate to soak it evenly and eliminate the air in the dyeing material, and at the same time start to heat up.
2. Beat the dye thoroughly with a mixer to make a dye dispersion, and add it at 50 to 60°C.
3. Raise the temperature to 130°C at a rate of 1 to 2°C/min, and keep dyeing for 30 to 60 minutes.
4. Cool down at a rate of 1~2°C/min, wash with water, and perform reduction cleaning if necessary, but the heating rate should not be too fast. Otherwise, it is easy to cause the dye to be unevenly colored and cause flowers to bloom. Especially when dyeing medium to light colors and whitening (with the addition of bluing agents such as dispersed purple HFRL or dispersed blue 2BLN), color flower dyeing defects are most likely to occur.
This is because the dyeing rate of disperse dyes is directly proportional to the dyeing temperature. As the dyeing temperature increases, the expansion speed of polyester and the coloring speed of dye will be significantly accelerated.
According to experience, the speed of heating rate is related to the levelness of the dye used. If the levelness is good, it can be done faster; if it is poor, it can be done slower. ②Related to the dyeing depth. Dyeing can be faster when dyeing dark colors, and slower when dyeing light colors. ③Related to the heat resistance of polyester. The glass transition temperature of polyester is 67~8l℃. At 85℃, the microstructure of polyester is in a glassy state and absorbs color very slowly, so the temperature can be increased faster. At 85-110℃, polyester swells rapidly and its color absorption ability is significantly enhanced, so the temperature rise should be slower. 110-130℃, because there is less dye in the dye liquor, the temperature can be raised faster. ④Circulation status with dye liquor� off. If the pressure of the dye liquor is high and the penetrating power is strong, the dye liquor can come into close contact with the fiber quickly. Then the heating can be faster; otherwise, it should be slower.
Countermeasures:
The heating rate must be correctly controlled. The actual heating rate must be reasonably set based on the levelness of the dye used, the depth of dyeing, the circulation state of the dye liquor, and the different dyeing stages. The usual heating rate is 1 to 2°C/min. If it is faster, it can be faster. , when you shouldn’t be fast, you must be slow, and one size does not fit all.
When the dyeing temperature is below 85°C, the dye rarely dyes and can be heated directly. 85℃-110℃ is the temperature area where the dyeing speed increases the fastest, and the temperature rise speed needs to be strictly controlled below 1.5/min. It can be slightly faster within the range of 115℃-130℃, but the temperature rise rate needs to be controlled at about 2℃/min. The holding time here is for medium-depth colors, light colors can be appropriately shortened, and dark colors can be appropriately extended. The heat preservation time should be appropriate. If it is too short, it will cause ring dyeing. If it is too long, it will easily produce lint. Generally it should be controlled within 30-60 minutes.
After dyeing, the temperature should be cooled down quickly, because oligomers will be produced during the slow cooling process, and the residual thread surface will be tar-like and difficult to remove.
Heating time
Since disperse dyes rely on hydrogen bonds and van der Waals forces to dye polyester, It has good dye migration properties under high temperature conditions. Therefore, appropriately extending the heat preservation time (60 minutes for dark colors, 40 minutes for medium colors, and 30 minutes for light colors) can correct the uneven color absorption produced during the heating stage through dye migration and achieve uniform color. This has a noticeable effect on dyeing darker shades.
Experience in printing and dyeing: Prevention and treatment of color circles when dyeing low-count cheese yarn. The reasons for this phenomenon are: the dye particles are too large or not ground; the water hardness is too high and agglomerates with the dye; and the additives are improperly selected.
Prevention methods:
The grinding of dyes should be strengthened; chelating dispersants should be added to the dye bath; dyeing auxiliaries should be preferred, For example, Ningbo Huake Additive’s color point and color circle preventer. Low-count yarns are opaque when dyed and have “white knots”. This is because low-count yarns contain more impurities and the impurities are not completely removed in pre-treatment, resulting in part of the yarn being dyed opaquely. Or due to the uneven wool effect after scouring, some yarns absorb less color and appear “white knots”. When dyeing the package yarn, the air is not exhausted, and the air is brought into the package yarn during the circulation of the dye liquor, resulting in bubbles and white spots. It is also possible that the water cannot be washed thoroughly after scouring, and the yarn has alkali spots and calcium spots, causing “white knots” during dyeing. The pre-treatment process should be strengthened, and attention should be paid to exhausting the air when dyeing. </p
