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Advantages of pigments over dyes
Jun 23, 2018

 Advantages of pigments over dyes

Previously it had been indicated that water-soluble colours were technically inferior to water-insoluble
(pigments) colours. The reasons for this are given below. Migration
Drying is an integral part of the coating process and, as a consequence, water will leave the film coat continuously as the coat is formed. If the colour is in the form of insoluble particles, then no migration takes place. However, a water-soluble colour tends to follow the escaping water molecules to the tablet surface and produce a mottled finish to the coating.
Opacity Pigments are much more opaque than dyes, hence they offer a much greater measure of protection against light than dye-coloured film coats.
Colour stability Edible colours for medicinal products have an established use by virtue of their low order of toxicity. Some of their technical attributes, for example colour stability, can represent somewhat of a compromise. In general the inorganic pigments, e.g. iron
• Sunset Yellow
• Tartrazine
• Erythrosine.
• Titanium dioxide
• Iron oxide yellow, red and black
• Talc.
• Riboflavine
• Carmine
• Anthocyanins.
oxides, have an excellent stability while the synthetic organic dyes are much less satisfactory in this respect. The lake forms of many of the synthetic organic dyes, however, provide a degree of improvement in this respect.
Pigments decrease the permeability of films to water vapour and oxygen thereby offering the possibilities of increased shelf-life.
Coating solids
Pigments contribute to the total solids of a coating suspension without significantly contributing to the viscosity of the system. Thus faster processing times by virtue of more rapid drying is possible. This
is particularly significant with aqueous-based processes. Anti-tack activity
Tack is a concept that is widely used to describe the forces involved in the separation of two parallel surfaces separated by a thin film of liquid. Such considerations are important during the coating process as excess tack can cause troublesome adhesion of tablets to each other or to the coating vessel. Since the early days of film coating it has been appreciated that solid inclusions, including pigments, in the formula have a part to play in combating the effects of tack. Chopra & Tawashi (1985) have quantified
the action of titanium dioxide, talc and indigo carmine lake on the tackiness of coating polymer solutions. They have shown that, at high polymer concentrations, increasing the pigment concentration and decreasing the pigment particle size, reduced the effect of tack, whereas at low polymer
concentration only talc was effective in reducing tack. Alternative methods of tack evaluation have been utilized by other workers such as Massoud & Bauer (1989) and Wan & Lai (1992).

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