The reactive/ disperse dye combination for polyester/ cotton fabric is very popular. In this process usually polyester is dyed first by using a range of disperse dye.
Why PET is dyed first?
If cotton is dyed first, the dyeing temperature of polyester at 120–130 °C can change the shade of the dyeing cotton as cotton dyes are less stable at a high temp. However, disperse dye leaves some stains on the cotton which are later removed by Reduction Clearing with an Alkaline Solution of Hydroz. This stained cotton has poor light and wash-fastness and it will fast transfer colour to other synthetic materials if washed together.
Reduction Clearing
This process involves treatment with
alkaline hydros (2 g/l NaOH, 2 g/l Na2S2O4 . 2H2O) and
a surfactant ( 1 g/l)
for 20 min
at 70 °C.
The reduction clearing temperature is well below the glass transition temperature of the polyester. This reduces and eliminates the azo pigment on the fibre surface and reactive dyes on cotton surfaces
Dyeing Methods
There are 4 major types of process used for the batch dyeing of cotton/ polyester materials using a combination of reactive and disperse dyes. There are also a number of variants designed to take advantage of specific dye products.
Most procedures are quite lengthy, the major problem being the long times needed for the complete removal of the hydrolzsed reactive dyes from the cotton.
The most conservative method uses four separate steps:
Dye the polyester with the disperse dyes at 130 °C;
Reduction clear to remove any disperse dyes staining the cotton;
Apply the reactive dyes to the cotton;
Rinse and scour to remove hydrolyzed reactive dyes from the cotton.
The overall process can take as long as 10–12 h. The required time is less if the reactive dyes are added to the cooled exhausted disperse dye bath without draining, re-filling and reduction clearing. Some clearing of loosely held disperse dye particles from the fabric occurs during scouring to remove unfixed reactive dyes from the cotton.
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