Vat dyes, also known as Shilin dyes, are derived from the English word: Indanthrene. Vat dyes are also known as dyes that will never fade due to their high fastness to sunlight.
Basic knowledge of vat dyes
1 Definition:
Vat dyes are insoluble in water. The molecules contain two or several conjugated carbonyl groups (>C=O). When dyeing, they are first dyed under alkaline conditions. It is reduced to a soluble leuco sodium salt with affinity for cellulose fibers to dye the fiber, and then oxidized to the original insoluble dye and fixed on the fabric.
2 Dyeing characteristics:
Good dye fastness to sunlight Fastness>6; soaping fastness>4
Bright color, complete chromatogram (lack of bright red, blue, green, purple, brown and olive are better)
The cost is relatively high (intermediates are expensive and synthesis is complicated)
Poor level dyeing and white core is easily produced
Some dyes are light brittle (yellow, orange, red), and indigo has poor rubbing fastness
Mainly used for cotton or polyester/cotton blended fabrics Dyeing, can also be used for vinylon dyeing, but cannot be used for dyeing protein fibers
3 Structural characteristics of vat dyes
①There are multiple carbonyl groups
②There is no water-soluble group
③Most of them have Very good planarity
Generally can be divided into:
Indigo series
Anthraquinone
Anthraquinoneoxazole
Fused ring
Vat dye dyeing principle
1. Dyeing principle
Soluble leucosodium salt
has affinity for fibers
The dye is oxidized and restored to its original insoluble state, which is fixed on the fiber.
2. Staining steps
Reduction and dissolution of dyes
Uptake of chromatoleucosomes
Oxidation of chromatoleucosomes
Soap boil
(一 ) Reduction and dissolution of vat dyes
(1) Restore method
leuco body potential: under alkaline conditions, use insurance powder to reduce the dye to leuco body. Under certain conditions, use oxidant (erythremia salt) to oxidize the leuco body, and titrate to the oxidation point. The amount of oxidant consumed.
① Dry vat reduction: Reduce the dye first and then add it to the dye bath. Reduction under high concentration caustic soda and insurance powder.
②Full bath reduction: The dye is directly reduced in an alkaline dye bath.
(2) Reduction rate and its influencing factors
①Dye structure Itself (containing electron-withdrawing groups and easy to reduce)
②High temperature, high reduction rate
③The higher the concentration of reducing agent, The higher the rate
④The larger the dye particles (i.e. the size of the specific surface) are, the less likely they are to be reduced.
(3) Commonly used reducing agents
① Insurance powder: Sodium dithionite, low sulfite Na2S2O4
② Thiourea dioxide
a. Good stability
b. Strong reducing ability
c. Poor solubility
d. Easy to over-restore (use less amount)
(4) Abnormal reduction phenomenon
① The leuco body crystallizes out
a. Low reduction temperature
b. Low solubility of the leuco body
c. Improper addition of salt
d. Leaving it for too long
e. The concentration of caustic soda and insurance powder is too high
②Excessive reduction (causing a decrease in dye affinity and difficulty in oxidation after dyeing)
③Dehalogenation: high temperature
④Phthalamine hydrolysis: high temperature and alkali concentration
(2) Vat dye leucobody Dyeing
When vat dye leuco body is dip-dyed, the initial dyeing rate is very high, but it is only adsorbed on the surface of the fiber in a ring dyeing state, and then slowly diffuses into the interior, showing a slow upward dyeing. Dyeing properties, poor dye migration.
Dyeing properties of vat dye leucobody: poor level dyeing and dyeing through, and “white core” phenomenon.
(1) Performance of leuco bodies
① The structure is complex, the increase in electrolyte and the decrease in temperature can easily cause aggregation.
② A normal amount of alkali is conducive to depolymerization, but an excessive amount is easy to aggregate.
The reason why leucosomes are easy to aggregate: their structure is highly planar and has many conjugates. Therefore, anthraquinones are more likely to aggregate than indigos
(2) Dyeing method (leucobody dyeing)
Method A: Suitable for dyes with complex structure, high affinity, poor diffusivity, and easy aggregation. Use 60℃ dyeing without salt catalysis. Add retarder to reduce the initial dyeing rate. The dosage of alkali and insurance powder is relatively high.
Method B: Suitable for dyes with a simple structure, low affinity, and low initial dyeing rate. It is dyed at 45-50°C and uses less alkali and insurance powder.
C method: used for dyes with simple structure. Use 25-30℃ for dyeing, reduce the dosage of alkali and insurance powder, and add salt to promote dyeing.
Selection of methods:
Indigo: high temperature reduction and low temperature dyeing
If it is easy to assemble, use method A
If it is not easy to assemble, use method C
If it contains phthalamine, higher temperature cannot be used. Method B is used to prevent the hydrolysis of phthalamine.
Those with large molecular structures have large diffusion barriers and will stain at around 60°C.
Different dyeing methods are mainly determined based on the directness, solubility and stability of the leucosome.
The initial dyeing speed of vat dye leucobody is very fast, and the dyeing temperature generally cannot exceed 70℃; because of the fast dyeing speed, false balance will occur;
For fabrics with a tight structure, dyes preferentially dye the outer fibers, and it is difficult to dye the fabric through, resulting in a “white core” phenomenon in the fabric, such as viscose cheese dyeing
The reasons for producing white core:
The temperature is low, which affects the diffusion rate.
The concentration of electrolyte is high, which makes the dye Aggregation affects dye diffusion.
And the initial dyeing speed is fast, so that the leuco body will be quickly absorbed into the outer layer to form a white core.
The high initial dyeing rate of leucosomes is generally believed to be mainly caused by the presence of a large amount of electrolytes in the dyeing solution.
(3) Oxidation of vat dye leucobody
The dyed leuco body is oxidized to the original dye, which is fixed on the fiber and develops color.
Notes:
If the product is easily oxidized, rinse it first (with alkali) and then ventilate it for oxidation, or add a weak oxidant for oxidation.
Excessive oxidation will produce crystal grains, reduce the rubbing fastness, and increase the color difference.
Not easily oxidized under acidic conditions.
Those with low affinity are first oxidized with oxidants and then rinsed.
High-affinity fabrics are washed with water or air oxidized
If blue fabrics are washed first and then oxidized, the color will be more vivid.
Commonly used oxidants: oxygen in water, oxygen in the air; hydrogen peroxide and sodium perborate aqueous solution
Oxidation method: water washing oxidation , washed with ventilation and oxidized, washed with oxidant and oxidized.
(4) Soaping
1. The role of soaping
①Remove floating color on the fiber surface
②Change the physical state of the dye on the fiber and improve the dye Fastness and color (bright, pure, stable).
2. Soaping time
The longer the soaping time, the greater the particle growth. Large, surface aggregation is formed and friction fastness is reduced.
3. Soaping process
Note:
When removing floating color, wash thoroughly with water first and then soap, otherwise The oxide forms a gel, which is more difficult to wash; the residual color becomes a floating color after oxidation.
4. Effect of soap cooking on dye structure
Dye leucocolor The bodies are transformed into insoluble dyes, which have less attraction between them and fibers and are in a highly dispersed state;
Under the action of heat and moisture during the soap cooking process, the dyes The molecules move, form aggregation, and even form microcrystals
The orientation of the dye molecules changes from the original parallel state with the fiber chain to the perpendicular state with the fiber molecular chain, causing the dye absorption spectrum or Color change.
During the soap cooking process, the isomerization of dye molecules, such as cis and trans isomerization, may also be the cause of the color change.
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