Colorants are classified as either pigments or dyes. Pigments are practically insoluble in the medium in which they are incorporated. Dyes dissolve while being applied and in the process lose their crystal or particulate structure. It is thus by physical characteristics rather than by chemical composition.

Dyestuffs are used in the surface coating and printing industries, where a very high level of transparency is required, for the production of colored metal foils and films, flamboyant finishes in the paint industry and for wood stains, among others.

Due to advances in resin technology, dyes are now also being used in specific automotive finishes because of their color and transparency properties. The dyestuffs for those applications are soluble in various solvents.

Dyestuffs are also used for the mass coloration of those thermoplastics that are in a glass transition state at normal service temperatures. The dyestuffs selected for the coloration of polymers are soluble within those substrates.

The dyestuffs used fall into three main categories:

• Basic dyestuffs
• Fat soluble dyes
• Metal complex dyestuffs

Basic Dyestuffs
These are cationic dyes with good solubility in polar solvents such as alcohols, glycols and water. Basic dyestuffs are used by the printing ink industry with laking agents such as tannic acid. They are used for the clean bright shades. However, their light-fastness is poor which limits much of their usefulness.

Fat Soluble Dyestuffs
These include non-ionic, metal free azo and anthraquinone dyestuffs which have good solubility in less polar solvents such as aromatic and aliphatic hydrocarbons.

Azo dyestuffs are mainly used for aliphatic/aromatic solvent based woodstains and in the coloration of styrene polymers.

Anthraquinone dyes have a much wider usage in the coloration of plastics and fibers due to their much wider range of resistance properties.

Metal Complex Dyestuffs
These are mainly anionic chromium and cobalt complexes of azo dyestuffs. The cation is either a sodium ion or a substituted ammonium ion. Substituted soluble phthalocyanines also fall into this category. These types of dyes are normally soluble in alcohols, glycolethers, ketones and esters.

Dyestuffs for wood coating
Normally a wood stain is a transparent or semi-transparent solution of colorants

designed to decorate and protect the wood. The solution will penetrate into the wood without hiding it grain enhancement is often desired.

The main requirements of wood colorants are transparency, high saturation/brilliance, good bleed and chemical resistance, good solvent compatibility and light-fastness.

A number of factors influence staining results, such as,

• Substrate: wood type/color, cut of the wood, with/without pre-treatment
• Stain: concentration, application method (spraying, brushing, wiping …)
• Clear coat: over-lacquer resistance, color of clear coat
  
  

Dyestuffs exhibit great brilliance offering bright shades and excellent transparency. Transparency is a prime requirement in the wood coating industry allowing the grain of the wood to be seen.  

Ciba offers 3 dyestuffs suitable for wood coatings.

• Ciba ORASOL -Solid solvent dyes  
• Ciba IRGASPERSE-U2 - Liquid acid dyes  
• Ciba IRGASPERSE-W - Liquid reactive dyes