Why does reactive dark black printing change color?

1. The mechanism of discoloration

The reason for the color change of reactive dark black printing should start from the reactive group of the dye. According to the different reactive group, reactive dyes can be divided into s-triazine type, halogenated pyrimidine type, vinyl sulfone type, double reactive group type, etc. The black dyes used for printing are mostly homogeneous chlorotriazine type (K type), vinyl sulfone type (KN type), double reactive group type (such as KD type, KP type, M type, B type).

K-type black has low reactivity, strong alkali resistance and high stability, but the fixation rate is not high. Its printing has many floating colors, large dosage, and it is not easy to print very deep blacks. The improved P-type black dye has a greater increase in depth. Its monochrome structure P-type black is a very ideal printing depth. black. At the same time, it should be noted that there are many types of P-type blacks on the market, which are not a single matrix structure. They are composed of several P-type dyes, and their colorability, alkali resistance, and stability are all poor. . KN-type black has higher reactivity than K-type, and it is easy to print dark colors, but its alkali resistance is poor, and it is easily hydrolyzed under alkaline conditions. It is mostly used in reactive black anti-printing printing. The application of dual-active base black is more extensive. It is mostly a K-type plus a KN-type structure. Its reactivity is higher than that of K-type, and it is easier to print dark colors, but its alkali resistance is still not satisfactory.

2. The specific causes and solutions of the discoloration

As we all know, dark black color change, especially light color, reddish, etc., have seriously affected the quality of printed products, and sometimes it will cause the entire batch of products to drop, which is very harmful.

2.1 Pre-treatment of printed fabric

The effect of pre-treatment on the dark black color change is not very obvious, and the hair effect requirements are better, which is conducive to dark black coloring. One thing to note is that the cloth surface should be washed clean and the alkali content should not be too high, especially for some mercerized printing blanks. If the alkali removal is insufficient, the fiber will contain caustic soda, which will easily cause the reactive dyes to be hydrolyzed during the treatment process. The printing color becomes lighter and discolored, especially the dark black with poor alkali resistance.

2.2 Handling of tracing

Printing is generally composed of multiple colors, divided into deep, medium, and light. Generally, the pattern structure has overlapping colors of light colors on dark colors and multicolor processing from light colors to dark colors. In the overlapping and multi-color parts, because the darkest black is the darkest and the lighter colors are repeatedly overprinted, the alkali agent of different colors is accumulated locally, which causes the local alkali agent content to be too high and causes the black dye to hydrolyze, which causes the black color to change.

2.3 Reasonable formulation of printing process

Reactive printing relies on alkali agents to fix the color. It does not mean that the more alkali agents, the better, the stronger the alkalinity, the better. K-type and P-type s-triazine dyes with low reactivity, good stability and excellent alkali resistance, the amount of alkali agent can be higher, mainly NaHC03, and sometimes Na2C03 can be added. For dyes with high reactivity, high fixing rate and poor alkali resistance, only weak fixing agents such as NaHC03 can be used for fixing, and the dosage should be strictly controlled to reach the scope of process insurance. The use of alkalis depends on the color depth in addition to the variety of dyes.

2.4 The impact of steaming and washing

If production management is not in place, the impact is very obvious.

First, the printing should be steamed in time after the machine is off, and washed in time after steaming. Because as long as the alkali agent is not fully washed and removed from the cloth surface, due to the moisture absorption of the cloth, the color paste printed on the cloth surface always has a certain amount of water, which promotes the dye to slow down under the action of the alkali agent. It will be hydrolyzed slowly, and if it accumulates more, the color will change.

Second, steaming to ensure a certain humidity is very beneficial for color fixation, but if the humidity is too high, the dark part of the overprint is too much urea, it will cause excessive accumulation of water, which will aggravate the hydrolysis. Therefore, the active steaming must ensure a certain humidity without excessive, and each factory will formulate different humidity control parameters according to the specific production conditions.

Third, the washing must be clean. After washing and drying, there should be no alkali agent on the cloth surface. pH=4.5-7.5 is appropriate. If it is not washed, although the alkali content of the cloth surface is already very small, in the later processing, storage, sales, and consumption, due to the long-term micro hydrolysis of a small amount of alkali agent on the cloth surface, the black color will change and become widespread. Red. In many printing and dyeing factories, the color of the cloth surface is positive when it leaves the factory, and there is no phenomenon such as color change or redness, but after a few months, sometimes the dark black color of the cloth surface changes to redness, which is more or less the same as the cloth surface. Alkali is not completely related.