Dasteck ChemicalsYantai Dasteck Chemicals Co., Ltd.
Direct answer
The purpose of polyester reduction clearing is to remove unfixed disperse dye and lift fastness, and replacing hydrosulfite with thiourea dioxide is the highest-success conversion in textile processing. Starting recipe: 0.5 to 1.0 g/L thiourea dioxide + 2.0 to 4.0 ml/L caustic soda (38 Be) + wetting agent, 70 to 95 °C for 20 to 30 minutes.
1. Why reduction clearing
Disperse dyes are applied to polyester at high temperature, and unfixed dye remains on the fibre surface after dyeing. That surface dye directly reduces rub and wash fastness; reduction clearing converts it into water-soluble products that rinse away.
2. Why thiourea dioxide suits the job
| Advantage | Detail |
|---|---|
| Controlled release | Dye already on the fibre is less likely to be over-reduced; shade drift is lower than with hydrosulfite |
| Odourless | No sulphur dioxide in the workshop |
| Lower dose | About one half to one third of the hydrosulfite weight |
| Safer logistics | Sealed, cool and dry storage is enough; no temperature- and humidity-controlled warehouse |
3. Recommended recipes
| Equipment | Thiourea dioxide | Caustic soda (38 Be) | Wetting agent / detergent | Liquor ratio | Temperature | Time |
|---|---|---|---|---|---|---|
| Cross winder or overflow | 0.5 to 1.0 g/L | 2.0 to 4.0 ml/L | 1.0 to 2.0 ml/L | 10:1 to 15:1 | 70 to 95 °C | 20 to 30 min |
| Continuous washing line | 2.0 to 3.0 g/L | 5.0 to 7.0 ml/L | 0.3 to 0.5 ml/L | - | 80 to 95 °C | 15 to 80 s |
4. Operating points
1. Alkali first, product second: in weakly acidic conditions thiourea dioxide is oxidised to formamidine sulfonic acid and loses its effect
2. Not below 70 °C: release is slow below that, and carrying over a low-temperature recipe leads to a false conclusion that the product does not work
3. Do not cut the alkali with the dose: halving the dose does not mean halving the caustic soda
4. Prepare and use immediately: cold water for the stock solution, alkali only shortly before use
5. Minimise air ingress: an exposed surface and vigorous agitation keep consuming the reducing agent
5. Quality assessment
Compare wet and dry rub fastness, wash fastness, shade shift as delta E, and brightness. Judge by fastness ratings rather than by eye. For deep shades, keep a small hydrosulfite addition in the blend.
6. Common questions
Sources
Recipes are the recommended values in our technical data sheet and must be confirmed by bench trial.