Organic and inorganic pigments solvent dyes

Because of the requirements of insolubility in water, in organic solvents and in the medium that it is being used to colour, the application processes for using pigments are quite different from those for dyes. Coloration with pigments is essentially a process of dispersion of solid particles of the pigment in a semi-solid medium. 

The chemical nature of most organic pigments is closely similar to that of the synthetic dyes that have been discovered during the past 150 years. In fact, with the exception of the phthalocyanines, almost every chemical class of pigments has been developed first for dyestuff use. There are some signs that this may not continue to be the case, with the development of some new organic pigments for specialised uses. 

Finally, passing mention must be made of the two most important organic pigments in our world, both natural products. These are chlorophyll and haemoglobin, which are absolutely vital in the strict meaning of the word, but only chlorophyll has found a commercial use as a colorant in food preparation. 

The methods used for making pigments from these and other natural dyes were more like recipes than scientific procedures and were probably derived from the work of alchemists and herbalists. The former spent their lives trying to prepare gold by dissolving all kinds of cheap substances in acids and then re-precipitating them (hence their discovery of many precipitants), while the latter sought to extract compounds of medicinal value from plants and some of their extracts must have included natural dyes. 

Among the precipitants employed were tannic acid, tartar emetic, rosin soaps, fatty acid (stearic, oleic) soaps, sulphonated oils (Turkey red oil), earth lakes (mixed natural silicates), phosphates, casein and arsenious acid. The fastness properties of these pigments 

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Chapter 2 Organic and inorganic pigments solvent dyes... 

Adhesives Organic and inorganic bases Coolants Cleaners colorants (pigments and dyes) Fragrances Inks Paints Plasticisers (e.g., in plastics) Polymers (rubbers and plastics) Sealants Solvents Stabilisers Synthetic fibres Varnishes Biocides Detergents Fertilisers Fuels Foodstuff additives and flavourings to foodstuffs Medicinal products (for humans and animals) Plant protection products Radioactive substances... 

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