Why does polyester need alkaline bath dyeing?

The dyeing conditions of disperse dyes are usually to dye polyester under high temperature (130℃±5℃) and acid (pH=4~5). There are two main reasons:

Polyester has poor stability under alkaline conditions, and alkaline hydrolysis may occur on the surface of the fiber, resulting in a rough surface of the fiber, a decrease in gram weight, and even a decrease in strength.

Disperse dyes have the defect of poor alkali resistance and stability, that is, alkaline hydrolysis is prone to occur in high temperature alkaline baths (especially disperse dyes containing ester groups, cyano groups, amide groups, etc. in the molecule), resulting in serious decolorization, discoloration, and decolorization. chromatic aberration problem.

However, when polyester is dyed at high temperature (130℃±5℃) and acid (pH=4~5), there is a problem of polyester oligomers. Especially for polyester yarns or polyester fabrics after alkali reduction, in high temperature dyeing baths, the semi-condensation polymers (also known as oligomers or oligomers) with low degree of polymerization (n=2, 3, 4) contained in polyester. , the content is about 1%~2%), it will migrate from the fiber when the polyester swells and the structure becomes relaxed. During the process of dyeing and cooling, these oligomers temporarily dissolved in the high temperature dye bath will precipitate due to the decrease in solubility. These precipitated off-white powders are easy to adhere to the dyed fabrics, making the cloth surface gray and dull, as if covered with a layer of white fog. Also, due to the extremely low solubility of oligomers, effective removal is difficult. What's more, these oligomers in the dye bath will also combine with dye aggregates, fiber debris, etc. to form a sticky tar-like substance. Once adhered to the fabric, it will cause irreparable dyeing defects.

 Advantages of alkaline bath dyeing

1. The oligomers in the dye bath are easy to remove

Under alkaline conditions, polyester oligomer is in a better dissolved state, and it is not easy to precipitate during the cooling process, which can effectively improve or eliminate the harm of oligomer to dyeing quality.

2. Shorten the cleaning process, improve production efficiency, and save neutralizing auxiliaries

The PH value of polyester after alkali reduction or pretreatment is required to be lower, as long as the PH value is less than 9, it can be dyed. Before solid dyeing, the cleaning process can be omitted, the auxiliaries used in neutralization can be saved, and the production efficiency can be improved; the one-bath method of degreasing dyeing can not only enhance the degreasing effect, but also shorten the process flow

3. Alkaline bath dyeing has a certain cleaning effect on the agglomerates such as dyes and oil stains adhering to the fabric.

4. Alkaline bath can improve the smoothness of certain fabrics in the tank and reduce fabric scratches

Key Problems to be Solved in Alkaline Bath Dyeing

1. It is necessary to select the appropriate disperse dyes correctly. This series of disperse dyes is stable in color and light in the range of PH value = 8-9, and does not hydrolyze, and the strength is not affected.

2. The pH value needs to be stably controlled between 8 and 9; that is, a pH value stabilizer is required, which has a large buffering capacity for pH value and can effectively overcome the acid brought in by the outside world (fabric, dyes, auxiliaries, water quality). The effect of alkaline substances on the pH value of the dyebath and maintain the stability of the pH value of the dyebath