Acid dyes are water-soluble anionic dyes that bond with fibres carrying positive sites in an acidic bath. They are used mainly on wool, silk, nylon and leather.
Outside textiles, some acid dyes colour paper, inks, wood stains, anodised aluminium and household products. The grade for those uses is often different from the textile grade.
How they work
Wool, silk and nylon contain amino groups. In an acid bath these become positively charged and attract the negatively charged dye. The bond is ionic, helped by other weaker forces, so wet fastness depends a lot on the size of the dye molecule.
The pH of the bath controls how fast the dye goes on. Too low a pH can make the dye rush onto the fibre and dye unevenly. Mills use levelling agents and a controlled temperature rise to manage this, so ask the supplier for its recommended dyeing method and test it in your own lab.
Main groups
- Levelling acid dyes are small molecules that dye evenly but have modest wash fastness.
- Milling acid dyes are larger, dye in a weaker acid bath and give better wet fastness.
- Metal-complex dyes contain chromium or cobalt bound in the molecule, giving good light and wet fastness on wool and nylon.
What to watch when buying
Metal-complex dyes contain metal by design, so your customer's restricted substance list matters. Many lists accept bound metal in the dye but limit free metal and hexavalent chromium in the finished product. Read what heavy metal limits apply to dyes and pigments before you choose a range.
For leather, ask for results on leather, not on wool. For nylon, ask about barre coverage, since some acid dyes show yarn variations more than others.
Acid dyes also come under the banned-amine rules for textiles and leather, so ask for a recent test report from an accredited laboratory.
What to put in your order
Acid dyes are often sold by Colour Index name, but strength and shade still differ between makers. Agree a reference sample and test each lot against it, as you would for any dye. Read how to check batch-to-batch consistency in dyes.
- State the fibre and process, such as nylon carpet, wool yarn or chrome-tanned leather, so the supplier offers the right group.
- State strength against your standard, with an allowed range.
- State the shade tolerance as a colour difference value measured under agreed light sources.
- State the fastness grades you need for washing, light, perspiration and rubbing.
- State any restricted substance list the dye must meet, such as a brand list or the ZDHC MRSL.
- State the pack size, moisture-proof liner and labelling you expect.
Next step
We find acid dye makers in Gujarat, check their test reports and compare samples against your standard before you order. To begin, book a free sourcing review.