Dyeing technology, machinery, and sustainability

After completion of the dye-fiber reaction step, the fabric contains three forms of the dye covalently bonded dye, absorbed but unreacted dye, and hydrolyzed dye. The unreacted and hydrolyzed dyes are unfixed. This causes poor wash-fastness and requires removal of all the unfixed dye in a wash-off step. Residual alkali and absorbed salts must also be removed. The wash-off process requires large quantities of water (Shukla, 2007). Approximately 75% of the water used in reactive dyeing occurs during washing off (Knudsen and Wenzel, 1996). 

50 years have resulted in major improvements to sustainability, lower water and energy use, lower dye and salt use, and hence reduced disposal to the environment. Such dyeing machines are highly recommended as the best available technology for exhaust dyeing (European Commission, 2(X)3). 

From a sustainability perspective, padding-based dyeing methods for cotton fabrics are the most preferred coloration pathway (Schramm and Jantschgi, 1999). Relative to exhaust dyeing, pad-based dyeing methods offer (Leube, 2003) the following  

The lowest possible hquor-to-fiber ratio in exhaust dyeing is 3 1 using ultra-low liquor-ratio dyeing machines. Pad dyeing reduces this to between 1 1 and 0.5 1. Such low liquor ratios further increase dye take-up. As a result further reductions in quantities of dye and water are discharged as effluent at the end of the process. However, these methods are normally avoided on knitted fabrics and small lots due to certain limitations (Smith, 2003) such as dimensional instability and higher pollution load (European Commission, 2003). 

A range of fixation technologies have been developed to take advantage of pad dyeing. Some have been evolutionary, whereas others have been developed specifically for a particular dye-fiber product. A summary of the key features of the various fixation technologies and their sustainability issues is given in the following sections. 

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