The Flash‐ageing of Vat‐printed Viscose Rayon Fabrics

Abstract

The evolution of pad–steam processes for fixing printed vat dyes on cellulosic fabrics is traced. The behaviour of a wide range of vat dyes under laboratory “flash‐ageing” conditions with sodium dithionite (hydrosulphite)–caustic soda as the reducing system indicated the most satisfactory thickening agent, dyes, and padding and steaming conditions, and led to the design of a bulk‐scale steamer operating at about 7 yd./min. with a steaming time of about 20 sec. The percentage fixation of a selected range of dyes on viscose rayon and on cotton under flash‐ageing conditions compares favourably with that obtained by the “all‐in” sulphoxylate–carbonate printing process, where the steaming time is sixty times as long. Many vat dyes show improved brightness and/or colour value when steamed for short times, owing to the absence of leuco decomposition or over‐reduction. Such a flash‐ageing process is ideally suited to screen printers and to those machine printers of all‐vat styles who wish to augment their steaming capacity with little capital expenditure. A modified process is being worked out with thiourea dioxide as reducing agent in the printing paste, development being by padding with caustic soda before steaming. This is particularly suitable for designs of low coverage, or where it is essential to process other types of dyes alongside vat dyes. Preliminary experiments indicate that emulsion thickenings are unlikely to show much technological advantage over conventional thickenings in the flash‐ageing process.

Whither High‐temperature Steaming ?

“…Increasing the temperature by pressurising led to shorter steaming times and increased dye uptake. A further significant development was the flash ager for vat dyes [2] ; by applying the necessary chemicals in a separate stage, more powerful reducing agents could be used with a much shorter steaming time. For very good reasons, which need not be dealt with here, the process and equipment were satisfactorily adapted for reactive dyes, but some restriction in dye selection, mainly in relation to reactivity, had to be accepted.…”