How should polyester/cotton blends and elastane fabrics be reduction cleared?

Direct answer

Reduction clearing of polyester/cotton blends and elastane fabrics is not about whether there is enough reducing power; it is about selectivity. The bath has to remove loose disperse dye from the polyester without attacking the reactive dye on the cotton or destroying the elasticity of the elastane. Thiourea dioxide is milder than hydrosulfite, but temperature, alkali and time still have to be established for each fabric.

1. Two opposing constraints in blends

• The cotton side: alkali and temperature act on the reactive dye as well, and too much alkali or too long a time shifts the shade and lowers fastness
• The polyester side: loose disperse dye needs a real reduction step to be cleared

Because the two pull in opposite directions, start conservative and work down:

2. The extra risk with elastane

Elastane is a polyurethane fibre, and hot strong alkali combined with a strong reducer attacks it, showing up as lost elastic recovery, a stiffer hand, yellowing or even broken filaments. Key controls:

• Hold the temperature at 60 to 70 C rather than the high temperatures used for pure polyester
• Use the lower end of the alkali range and keep pH at 9.5 to 10.5
• Keep the time short, typically 10 to 15 min
• Thiourea dioxide is milder than hydrosulfite, but still verify elastic recovery on a bench sample

3. Three sets of results to check every time

• Shade and depth, assessed separately on the polyester and the cotton
• Wash and rub fastness, the purpose of the operation
• Strength and elastic recovery, the key properties for fabrics containing elastane

4. Operating points

1. Alkali first, then the reducer: thiourea dioxide needs alkali present to release, but the alkali should go in shortly before use

2. Dissolve in cold water: hot water accelerates decomposition

3. Never in the same bath as an oxidant: hydrogen peroxide and thiourea dioxide simply consume each other

4. Rinse well afterwards: residual reducer must not carry into later processes

Sources

Starting values are the recommended ranges from our technical data sheet and published literature. Blend ratio, elastane content and machinery vary widely, so confirm on a bench sample before running production.