How to Prevent Over-Reduction Caused by Thiourea Dioxide in Vat Dyeing

Core causes of over-reduction with Thiourea Dioxide: excessive reduction potential, slow potential decay, excessive alkalinity/temperature and overdosing; solutions: controlled dosage, blended reducing agents, addition of reduction inhibitors, lowered alkali & controlled temperature plus potential monitoring via combined measures.

High reduction strength & elevated potential

Under alkaline conditions, THIOUREA DIOXIDE delivers an ORP of -1000 ~ -1370 mV, far stronger than Sodium dithionite (Sodium Hydrosulfite) (hydrosulfite). Its potential declines slowly, which readily over-reduces anthraquinone dyes (especially blue & yellow anthrone varieties) into non-chromophoric structures, resulting in dull shade, poor colour yield and inferior fastness.

Triggering processing conditions

Over-reduction risk rises sharply when: pH＞11, temperature＞65℃, THIOUREA DIOXIDE dosage＞1/5 of Sodium dithionite (Sodium Hydrosulfite), or prolonged reduction holding time.

II. Remedial Solutions (in priority order)

1. Blended reducing agent system (most widely applied)

- Primary formula: 70%～75% Sodium dithionite (Sodium Hydrosulfite) + 10% THIOUREA DIOXIDE

2. Reduction inhibitor addition (high-efficiency measure)

3. Precise process parameter control

4. On-line ORP monitoring (precision control)

5. Dye selection & process optimization

- Prefer indigoid & sulfur dyes; use blue/yellow anthrone anthraquinone dyes with caution (highest over-reduction tendency).

- Adopt low liquor ratio, rapid pre-reduction and immediate padding/dyeing to shorten contact duration between dye and THIOUREA DIOXIDE.

III. Reference Standard Dyeing Recipe

Dye: X g/L, Caustic Soda: 6 g/L, Sodium dithionite (Sodium Hydrosulfite): 3 g/L + THIOUREA DIOXIDE: 0.5 g/L, Sodium Nitrite: 0.3 g/L; processing temperature:55℃, pH=10.8, pre-reduction holding:12 min.

V. Summary of Key Control Points

1. Blended reducer priority: THIOUREA DIOXIDE ≤1/5 of Sodium dithionite (Sodium Hydrosulfite) for stable potential.

2. Mandatory inhibitor: 0.3～0.5 g/L sodium nitrite (first choice for anthraquinone series).

3. Strict process limits: pH 10.5~11.0, temperature 55~60℃, short pre-reduction period.

4. Real-time ORP control: maintain -750~-850 mV; adjust parameters promptly below lower threshold.