Reactive Dyes for Printing on Cellulose

Reactive Dyes on Polyamide. In principle, the reactive dyes used for wool are also suitable for PA. Covalent binding of dyes to the terminal amino groups produces dyeings that have excellent wet- and rubfastness. The lightfastness corresponds to that of analogous acid dyes. The depths of color attainable are limited by the number of end groups and are higher for PA 6 than for PA 66. 

In the dyeing process, the dyeing bath is made weakly acidic (pH 4.5-5). The process is started at 20-45 °C, followed by heating at a rate of l°C/min and dyeing at near boiling temperature. Aftertreatment (soaping) is performed with a nonionic surfactant and sodium bicarbonate or ammonia at 95 °C. 

Reactive dyes are well suited to dye blends of cellulose and PA fibers. Clear shades with very good fastness are obtained. Like with vat dyes, the depth of shade of reactive dyes depends relatively strongly on the type of PA and structural differences. Dyeing is carried out in a three-step process with appropriately selected products. First, the reactive dyes in a weakly acidic liquor are allowed to absorb on the PA component. Salt is then added to improve the yield on the cellulose component. Finally, the liquor is made alkaline for reaction with the cellulose fiber. Dyes (e.g., with MTC anchor) that dye PA from a neutral liquor in the presence of salt are applied in a two-step process, as in the case of cellulose. In the reversal of this dyeing process, the cellulose component is dyed first at alkaline pH, followed by neutralization with acid, and the PA component is then covered at elevated temperature. 

For reactive dyes on cellulose-PAC fiber blends see Section 4.14.8. 

Recent developments show how vinyl sulfone reactive dyes can be applied on cotton by transfer printing [43], 

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