The pretreatment of pure cotton or polyester-cotton fabric is mainly to remove impurities on the cotton fiber, so that the fabric has good printing and dyeing processing performance. The conventional desizing, scouring and bleaching processes are set up for different impurities. Which of the commonly used additives in printing and dyeing, sodium hexametaphosphate or soda ash, has better softening effect?

When dyeing artificial cotton with direct dyes, soft water is an important condition to ensure quality, especially for some direct copper salt dyes. When dyeing cotton fabrics, soda ash soft water is often used, but when dyeing viscose fabrics, the desired effect may not be achieved.

First, let's take a look at the comparison of sodium hexametaphosphate and soda ash:

The pH value of soda ash is higher. Due to the low degree of polymerization of viscose fiber, such as the dyeing temperature is set at the boiling point, it is easy to cause the breakage and hydrolysis of the fiber molecular chain, which affects the strength and causes the fabric to feel rough. In addition, it can cause the reduction of aldehyde groups and glucose products in the alkaline dyeing bath, which will damage some direct dyes that are not resistant to reduction and cause peeling.

Sodium hexametaphosphate is easily soluble in water, and the pH value of 10g/L aqueous solution is 5.8-7.3 close to neutral. It forms complexes with metal ions such as calcium and magnesium to "block" the metal, thereby achieving the purpose of softening water. Therefore, it does not have the above shortcomings of soda ash, and it is an ideal water softener.

Soda ash is an electrolyte, which will cause the increase of sodium ions in the dye bath and produce a dye-promoting effect. For rayon, the relationship is even greater. Some dyes with greater dyeing affinity and very sensitive to electrolytes, especially direct copper salt dyes. If soda ash is used to soften water, serious surface staining will often occur, which will affect product quality.

Sodium hexametaphosphate is an inorganic electrolyte polyphosphate with good dispersion effect.

The adsorption of sodium hexametaphosphate dispersant on the surface of the particles can not only significantly increase the absolute value of the surface potential of the particles, thereby generating a strong electrostatic repulsion effect of the electric double layer, but also the sodium hexametaphosphate can also enhance the wetting of the surface of the particles by water. degree. The adsorption on the particle surface also enhances the wettability of the surface and increases the strength and thickness of the solvated film, thereby further enhancing the mutual repulsion of the particles.

Heavy metal compounds, such as CuSO₄, organic copper compounds, tin compounds, zinc salts, etc., are sometimes added to the sizing slurry in textile mills as inhibitors. These compounds are not conducive to enzymatic desizing. Sodium hexametaphosphate complexing agent can be added for treatment during desizing. The chemical fiber desizing process uses a chelating agent to make the slurry easier to puff, separate from the fiber, and disperse into a colloid for removal.

Sodium hexametaphosphate has the strong dispersing power of a chelating agent, which can prevent impurities and slurry from being deposited on the fiber again. Furthermore, adding sodium hexametaphosphate as a chelating agent in the cleaning solution after desizing can make the base cloth whiter, desizing more complete, and make the equipment cleaner.