The Influence of Particle Size on the Dyeing Properties of Vat Dyes

An attempt has been made to assess the importance of the particle size of commercial vat dyes as a factor influencing level dyeing, when using the padding technique in dyeing viscose rayon cakes. A number of dyes have been subjected to the Hampson filtration test and classified according to particle size. A similar classification has been based on filtration of dye from the padding liquor during the actual dyeing process. A fair degree of correlation between the two methods has been established. Some factors affecting the filtration properties of the vat dyes have been investigated. The classification of dyes according to particle size has been compared with actual dyeing results. It has been found that in the case of dyes with a high rate of strike and poor levelling properties some advantage is obtained by virtue of the more, level pigmentation which results from the use of dyes of small particle size. In general, however, it is concluded that particle size is not a major factor affecting the dyeing of viscose rayon cakes with vat dyes, and that a more important part is played by the migration of the leuoo vat dye. This migration is promoted by dyeing at elevated temperatures, and with many dyes is influenced by the concentration of Dispersol VL.