Textile Recycling Technologies, Colouring and Finishing Methods

4.1 Dyeing and Finishing Processes
The dyeing process involves the use of colourants to apply colour to yarn or fabrics. Colourants 
are substances that may include dyes or pigments. Dyestuffs are classified into natural and 
synthetic dyes and are most widely used in textile dyeing operations.
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Natural dyes are derived 
from plants and animals, whereas synthetic dyes are typically prepared from resources such as 
petroleum by-products and earth minteral.
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Natural dyes tend to be limited by the number of 
colours available, fastness properties, yield, and nonreproducibility,
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 therefore, synthetic dyes, 
which overcome these issues are more commonly used.
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Traditional dyeing methods can be divided into two types of methods: Exhaust dyeing (or batch 
dyeing) and Pad-steam dyeing (continuous dyeing). The exhaust dyeing method is most common 
and imparts colour to the textile material in a dye bath. As a batch process, it is suited for lot sizes 
from 10-1000 kg. The process requires the control of parameters including dyeing temperature, 
dyeing liquor, chemical additives, electrolytes, water or solvent, and mechanical agitation of the 
dye bath.
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 The pad-steam process is a continuous process in which the textile fabric is padded 
through dyeing solution, followed by steaming. During the process, dye molecules on the fibre 

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surface migrate into the swollen fibre structure and are subsequently fixed.
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Colour fastness 
properties from dyeing fabrics by this method tend to be low, along with higher effluent 
discharge.
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The interaction and fixation between dye and fibre impacts the discharge of dyes in effluent, 
colourfastness during consumer use, and the lesser characterized, dye removal from recycling 
processes at the material end-of-life stage. Sustainability of the dyeing process in industry is 
assessed by the ability to discharge low amounts of unfixed dyes (effluent) as drainage, with less 
discharge when the aggregation and chemical bonding present between the fibre and dye molecule 
is strong.
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 Table 14 summarizes the different dye classes on textiles.

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