Acid & Direct Dyes

Acid dyes are typically used to dye natural protein (wool and silk), synthetic polyamide (nylon) and to a small extent acrylics and blends of these fibres. They are so called because they are applied to these fibres from dyebath in acidic or neutral conditions.

An acid dye is a dye that is typically applied to a textile at low pH. They are mainly used to dye wool, not cotton fabrics. Some acid dyes are used as food colorants, and some can also be used to stain organelles in the medical field. Acid dyes are anionic, soluble in water and are essentially applied from acidic bath. Overall wash fastness is poor although light fastness is quite good. Acid dyes are generally divided into three classes according to their fastness requirements, migration ability, and dyeing Ph. Acid dyes affix to fibres by hydrogen bonding, Van der Waals forces and ionic bonding. While some acid dyes work in water, many choose to activate dyes in acid dye-baths instead. In textiles, acid dyes are effective on protein fibres, i.e. animal hair fibres like wool, alpaca, and mohair. They are also effective on silk. They are effective in dyeing the synthetic fibre nylon, but of minimum interest in dyeing any other synthetic fibers. Their are three Classes of Acid Dyes.

• LEVELING ACID DYES-Levelling, or equalising acid dyes have good levelling properties and are applied from a bath containing sulphuric acid to achieve exhaustion. They exhibit Low Wet Fastness hence they are not suitable for use as Apparel Fabric and they require acidic dye bath together with levelling Agents.
• MILLING DYES-They exhibit wet fastness, which is useful for dyeing wool materials. Milling acid dyes are sometimes called 'Neutral acid dyes' as they do not require an acidic dye bath. They are commonly applied using Acetic acid.
• METAL COMPLEX ACID DYES-These dyes are composed of acid dye molecules complexed with a metal ion, giving them low mobility and high wet fastness. They are commonly used on nylon and synthetic polyamide fibres. Metal complex acid dyes are economical, however, they produce relatively dull shades.

Acid dyes can also be used as food colouring, helping to increase the attractiveness of certain foods, and thus becoming more appealing to customers. Some examples include erythrosine, tartrazine, sunset yellow and allura red, to name a few, many of which are azo dyes. These dyes can be used in frosting, cookies, bread, condiments or drinks. In order to prevent health hazards, a dye must be approved for consumption before it can be marked as edible. Some separation methods that can be used to identify unapproved dyes include the solid phase extraction process, the overpressured thin layer chromatography process, and the use of reversed-phase plates.

Download Brochure