Will Thiourea Dioxide Affect the Color Shade?

Direct Answer

Direct Answer Less than with hydrosulfite, but shading does occur, and the extent depends on the dye system and the dose.

Why it shifts less than hydrosulfite

Hydrosulfite is a strong reducing agent with uncontrolled release, so it readily over-reduces disperse or reactive dye already on the fibre, which shows up as a paler, yellower or bluer shade. Thiourea dioxide releases in a controlled way, so the window in which reduction is just sufficient is wider and the shade lands closer to target.

When shading still occurs

• Excessive dose, most visible on deep shades
• Excessive temperature or dwell time
• Alkali charge beyond what the process needs
• Dyes that are themselves reduction-sensitive, such as certain anthraquinone disperse dyes and some reactive blues

How to judge it properly

In the bench trial measure both colour difference as delta E and the change in hue angle. Visual judgement alone is not enough. As a rule of thumb, delta E within 0.5 is acceptable, and above 1.0 the recipe needs adjusting.

A practical order of correction

If shading appears after the switch, first reduce the dose, then lower the temperature or shorten the time, and only then consider adjusting alkali or changing dye. In most cases the dose or the temperature is simply too high rather than the dye being unsuitable.

Bottom line

Shade stability is better than with hydrosulfite, but any one of dose, temperature or alkali running out of range will shift the shade. Judge with delta E, not by eye.