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.
