what is Vat dye？why we choose vat dye?

Vat dyes, also known as Shihlin dyes, are derived from English: Indanthrene (indanthrene). Vat dyes are also known as dyes that never fade because of their high fastness to sunlight.Basic knowledge of vat dyes

1.Definition:
Vat dyes are insoluble in water and contain two or more conjugated carbonyl groups (> C = O) in the molecule. They are first reduced to soluble leuco sodium with affinity for cellulose fibers under alkaline conditions when dyeing The fiber is dyed on the salt, and then oxidized to the original insoluble dye and fixed on the fabric.

2.Dyeing characteristics:

Good fastness to dyeing, fastness to sun> 6; fastness to soaping> 4
Bright color, complete color spectrum (lack of red, blue, green, purple, brown, olive is better)Like Vat green 3,Vat green 8,Vat violet 1,Vat blue 1
High cost (expensive intermediates and complex synthesis) Like Vat red 41,Vat orange 11
Poor leveling, easy to produce white core
Some dyes are lightly brittle (yellow, orange, red) and have poor rubbing fastness,Like Vat yellow 1, Vat yellow 2,Vat red 29
It is mainly used for dyeing cotton or polyester / cotton blended fabrics. It can also be used for vinylon dyeing, not for protein fiber

3.Structural Features of Vat Dyes
① There are multiple carbonyl groups.
② No water-soluble group
③ Most of them have good flatness

Generally, it can be divided into the following types:
Indigo system
Anthraquinones Anthraquinones Anthraquinones Anthraquinones
Anthraquinone oxazole. Anthraquinone oxazole. Anthraquinone oxazole
Condensed ring

Vat dye principle

1. Dyeing principle

Soluble leucosodium salt
Affinity to fiber
Oxidation is restored to the original insoluble dye, which is fixed on the fiber.

2. Dyeing steps

Dye reduction and dissolution
Upstaining
Oxidation of stained leucosomes
Soap

(I) Reduction and dissolution of vat dyes. Reduction and dissolution of reduction dyes

(1) Reduction method

Latent body potential: Under alkaline conditions, the dye is reduced to a leuco body, and under certain conditions, the leuco body is oxidized with an oxidant (red blood salt) and titrated to the amount of oxidant consumed by oxidation.

① Dry tank reduction: first reduce the dye and then add it to the dye bath. Reduction under high concentration of caustic soda and insurance powder.
② Full bath reduction: The dye is directly reduced in the alkaline dye bath.

(2) Extremely influential factors of reduction rate

① Dye structure itself (including electron-withdrawing group is easy to reduce)
② High temperature and high reduction rate
③ The higher the concentration of reducing agent, the higher the rate
④ The particles of the dye are large (that is, the size of the surface) and difficult to reduce.

(3) Commonly used reducing agents

① Insurance powder: sodium dithionite, low sulfite Na2S2O4
② Thiourea dioxide

a. Good stability b. Strong reduction ability
c. Poor solubility d. Easy to reduce excessively (less use)

(4) abnormal recovery phenomenon

① Crystal precipitation of leuco
A. Low reduction temperature b. Low solubility of leuco
C. Improper addition of salt d. Placed for too long
    The concentration of caustic soda and insurance powder is too high
② Excessive reduction (causes the dye affinity to decrease, it is difficult to oxidize after dyeing)
③Dehalogenation: high temperature
④ Phthalamido hydrolysis: high temperature and alkali concentration

(2) Up-dyeing of leucochromat of reducing dyes
When the vat dye leucochromite is impregnated, the initial dyeing rate is very high, but it is only adsorbed on the surface of the fiber in a state of circular dyeing, and then slowly diffuses into the interior, exhibiting slow dyeing characteristics and poor migration.

Vat dye leucochroma dyeing properties: poor leveling and transfection, with "white core" phenomenon.

(1) Performance of leuco

① The structure is complicated, the electrolyte is increased, and the temperature is lowered, which may cause aggregation.
② The normal amount of alkali is conducive to depolymerization, and the excess is easy to aggregate.

The reason why leuco is easy to gather: its structure is strong in planarity and conjugate. Therefore, anthraquinones are easier to aggregate than indigo

(2) Dyeing method (hypochromatosis)

Method A: It is suitable for dyes with complex structure, high affinity, poor diffusivity, and easy aggregation. It is dyed at 60 ° C without catalysis by salt. Slower dyes should be added to reduce the initial dyeing rate. Alkali and insurance powder are higher.

Method B: It is suitable for dyes with simpler structure, lower affinity, and lower initial dyeing rate. It is dyed at 45-50 ℃, and the amount of alkali and insurance powder is low.

Method C: used for dyes with simple structure. Dyeing at 25-30 ℃, the amount of alkali and insurance powder can be reduced, and salt can be added to promote dyeing.

The choice of method:

Indigo: High temperature reduction and low temperature dyeing.If the method is particularly easy to gather.C is not easy to gather.Containing phthaloamino groups cannot be used at higher temperatures to prevent hydrolysis of phthaloamino groups by the B method..The large molecular structure has a large diffusion barrier, and it is dyed at about 60 ° C.Different dyeing methods are mainly determined according to the directness, solubility and stability of leucosomes.The initial dyeing speed of vat dye leucochrome is very fast, and the general dyeing temperature cannot exceed 70 ° C; because of the fast dyeing speed, false equilibrium phenomenon will occur;For tightly structured fabrics, the dye preferentially dyes the outer fiber, which makes it difficult to penetrate the fabric, causing the fabric to "white core", such as viscose bobbin dyeing

The reasons for the white core are:Lower temperature, affecting the diffusion rate,The concentration of electrolyte is high, which causes the dye to aggregate and affect the diffusion of the dye.And the initial dyeing speed is fast, so that the leuco is quickly adsorbed on the outer layer and causes a white core.The high initial dyeing rate of leucosome is generally considered to be caused by the presence of a large amount of electrolyte in the dye solution. Zh

(III) Oxidation of leucochromat of reducing dyes
The dyed leuco is oxidized to the original dye, which is fixed on the fiber and develops color.

Precautions:

Easily oxidize first (with alkali) and then oxidize by air, or add weak oxidant to oxidize.
Excessive oxidation results in grains, reduced rubbing fastness, and increased chromatic aberration.
Not easy to oxidize under acidic conditions.
Low affinity is first oxidized with oxidant and then rinsed.
High affinity for washing with water or air oxidation
If the blue fabric is washed first and then oxidized, the color will be more vivid.
Common oxidants: oxygen in water, oxygen in air; hydrogen peroxide and aqueous sodium perborate solution
Oxidation method: water-wash oxidation, water-wash wind-through oxidation, water washing and oxidant oxidation.

(IV) Soaping: Soaping, washing, washing, washing, washing, etc.

1. The role of soaping
① Remove the floating color on the fiber surface
② Change the physical state of the dye on the fiber, improve the fastness and color of the dye (bright, pure, stable).

2.Soaping time
The longer the time, the greater the particle growth, the formation of surface aggregation, and the decrease in rubbing fastness.
3.Soaping process

note:
When removing the floating color, first wash it thoroughly with water and then soap it, otherwise the oxide will form a gel and it will be more difficult to wash; the residual color will become floating color after oxidation.

4. Effect of soap cooking on dye structure

The leuco dyes are transformed into insoluble dyes, which have a small attraction between them and the fibers, and are in a highly dispersed state;
Under the action of heat and humidity during the soaping process, the dye molecules move, form aggregates, and even form microcrystals.
The orientation of the dye molecules from the original parallel state with the fiber chain tends to the perpendicular state with the fiber molecular chain, which causes a change in the absorption spectrum or color of the dye. 

In the process of soap cooking, the isomerization of dye molecules, such as cis and trans isomerism, may also cause the change of color light. Zh

Recommended Products:

Vat Blue VB

Vat Red 41

Vat Blue 4

Vat Blue 1

Vat Green 8

Vat Green 3

Vat Violet 1