Can Thiourea Dioxide Really Replace Sodium Hydrosulfite?

Direct Answer

Direct Answer It can replace sodium hydrosulfite, but not on a weight-for-weight basis and not in every process without adjustment. What is actually being replaced is the reducing duty that hydrosulfite performs in one specific step, so dosage, temperature, alkali charge and order of addition all have to be re-established.

Where the replacement is well established

• Reductive final or intermediate cleaning of disperse dyeings and prints on polyester and polyester/cotton
• Stripping and rework of reactive and disperse dyeings
• Cleaning of dyeing machines, pipework and filters
• Dyeing cotton and cotton blends with sulphur dyes
• Vat dyeing by the pad-steam process, where it can replace sodium dithionite but may cause hue errors and require the dye formulation to be corrected
• Bleaching of polyamide
• Reductive stripping of wool and silk

Where care is needed

• Stripping black and grey shades: thiourea dioxide is weaker than hydrosulfite on carbon-black and some deep-shade combinations, and the trade commonly blends them at roughly 1 : 10 (thiourea dioxide : hydrosulfite)
• Processes that require a long-lived leuco vat: the leuco form is stable for a shorter time than with hydrosulfite, so long continuous dyeing needs separate evaluation
• Product lines where cost is extremely tight: compare chemical cost per tonne of fabric, not price per kilogram

Four parameters that must change together with the dosage

• Dosage: according to our technical data sheet, stripping uses about one third of the sodium dithionite weight
• Alkali: do not cut it in step with the reducing agent; usually hold or slightly increase it
• Temperature: thiourea dioxide develops its full reducing effect only above 70 C; reduction clearing normally runs at 70 to 95 C, and stripping and machine cleaning at the boil
• Order of addition: alkali first, then the reducing agent, with the alkali added only shortly before use

Bottom line

It can replace hydrosulfite, but what you are replacing is a set of process parameters, not a sack of chemicals.

Sources

1. 我司产品技术数据表《THIOUREA DIOXIDE Technical Data Sheet》（TDO-TDS，签发日期 2024-05-08）