ACID DYE

The acid dyes are large dyes containing one or more sulfonic or carboxylic acid salt functional groups. These dyes are dyed onto fibers from acid solution, since positive charge development within the fiber in acid solutions acts as a driving force for dye diffusion and migration into the fiber. Only fibers which develop a positive charge in the presence of acid, such as wool, silk, and other protein fibers, nylon, and certain modified synthetics, are readily dyed by acid dyes, Acid dyes on fibers are reasonably colorfast to light and laundering, but mordanting (more complete insolubilization of the dye through reaction with a metal salt) will improve the overall fastness properties of the dye. The color of the dye may be affected somewhat by mordanting; however, pre-metallized acid dyes are a special class of acid dyes which have been reacted wi th a mordant prior to dyeing and which have sufficient solubility to be dyed under conditions normally used for acid dyes.

One method  of  classifying  acid  dyes  involves  dividing  them  into three

groups  according  to  their application   and  wetfastness.   Leveling   acid  dyes

dye  evenly  to a  moderate  degree  of  fastness.   For  wool  and protein  fibers

they  are small  molecules  requiring   highly  acidic  dyebaths  for good  exhaustion.    For  nylon,  leveling   dyes  are  somewhat   larger   molecules   and  are

applied  from  weakly  acidic   dyebaths.   Milling   dyes  have  better  washfast-

ness,  but  generally  give  duller   shades  and  lack  good  leveling  character-

istics.   They   are  larger  than  leveling  dyes  and  are  applied  from  dilute

acetic  acid  solutions.   Acid milling   or super  milling  dyes are  applied  from

neutral   solutions   and  have  poor   leveling  characteristics    due  to   their

larger  molecular   size.   They  generally   are applied   to wool  and  have  high

wetfastness   and lightfastness.