How is thiourea dioxide used on wool and silk?

Direct answer

Wool and silk are protein fibres, sensitive to both alkali and high temperature. When using thiourea dioxide for reductive stripping, the key is mild alkali, moderate temperature, controlled time: far less alkali than for cotton, 60 to 80 °C, and 20 to 30 minutes.

Why it suits protein fibres

• Thiourea dioxide is a reducing agent rather than an alkali, so its chemical attack on the fibre is milder than a strong alkaline system
• Release is controlled, so there is no sudden local concentration spike as with hydrosulfite
• Unlike sodium formaldehyde sulfoxylate (Rongalite) systems, it releases no formaldehyde

Reference process

Control points

• Use less alkali rather than more: protein fibres hydrolyse readily in strong alkali and strength loss is irreversible
• Do not push the temperature: prolonged treatment above 85 °C clearly damages handle and strength
• Bench trial first: run blank, hydrosulfite control and thiourea dioxide arms on the same wool or silk sample and compare stripping efficiency, shade, tensile strength and handle
• Stripping depth is limited: deep shades need a blend or repeat treatments rather than one very high dose

Pass criteria

Alongside stripping efficiency and shade shift (delta E), measure retained tensile strength. For protein fibres we suggest keeping retention above 90%; below that, reduce temperature or shorten the time.

Sources

Process parameters are our recommended starting points and must be confirmed by bench trial for the specific fibre, dye and machine.