Acid Dyes

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Acid Dyes

An Acid dyes are water soluble anionic dyes have chemically less ammonium salt and more sodium salt of sulphuric, carboxylic or phenol organic acid. They possess affinity for amphoteric fibres while lack affinity of direct dyes for cellulose fibres. While dyeing, ionic bonding with fibre cationic is found responsible for fixation of colored anions in dyed material. Acid dyes are added to dyeing baths to increase the number of protonated amino-groups in fibres. The chemistry of Acid dyes is complex to understand.

Types of Acid dyes

Acid dyes are classified in three groups based on dyeing level properties, fastness requirements and its cost. The classification may partially subset each other depending on type of fibre to be colored and the process used for dyeing.

Equalizing/Leveling Acid dyes have high level dyeing properties. The molecular structure is very small which makes migration before fixation. Typically, used combining with trichromatic shades. They offer poor wet fastness makes it unsuitable for apparel fabrics. They give single color strong effect that long last. Fabric dyed with acid dyes should not be washed in warm water. Hand wash or dry wash is more preferable.

Milling Acid Dyes are generally not combinable and used for self-shading purpose. They show low light fastness in pale shades, otherwise, they are medium to high wet fastness.

Metallic complex Acid dyes is the most recent innovation by chemical industry. They are produced combining transition metals with dye precursors. They give dull shades with highest wet fastness and light fastness properties. They are very economical.

Limitations
  • Are not substantive to cellulosic fibres
  • Seems dangerous to handle but nothing to worry then used with proper knowledge and process
  • Risk of injury or disease due to dyes by exposure or ingestion to dye dust
  • Show rapid decrease in usage as many cheap and safer alternatives are available in the market
Attributes
  • Soluble in water
  • Are typically applied in presence of sulphuric, acetic or formic acid
  • Are soluble in alcohol
  • Gives very live and varied color
  • Acid dyes are fast wash types
  • When treated with a reducing agent get generally decolorized
  • Typically used combined with basic dyes
  • Have good light fastness and moderate washing fastness depending on levelling characteristics
  • Is not costly option as it is developed fast
Applications

Acid dyes are mainly used in fibres and medical purposes. Acid dyes used for textile proves effective on protein fibres viz. animal hair fibres like mohair, alpaca and wool. They give good results when applied on fibres like silk, synthetic fibre nylon, modified acrylic fibres using neutral to acid dye baths. Acid dyes for medical purposes are used to stain basic tissue proteins while microscopic examination for research and diagnosis. Some acid dyes are also used as food