Unlike most organic compounds, dyes have colours because they have the ability to absorb light in the visible spectrum, possess at least one chromophore or the colour-bearing group, with a conjugated system or a structure with alternating single and double bonds, and shows resonance of electrons. These four features must be all present in the molecular structure, otherwise the colour is lost. Apart from the colour-bearing group, most colour dyes also contain the colour helpers group, or auxochromes. They don’t have direct relation with the colour itself but they can change the colour of a colourant. They are mostly used to influence dye solubility.
When it comes to solubility, there are two kinds or organic colourants: dyes and pigments. The main difference is that colour dyes are soluble in water or organic solvent while pigments are not in both media. Dyes are used to colour substrates while pigments are useful to colour polymeric substrate in a different mechanism. Pigments provides surface-only colouration unless they are mixed with the polymer before moulded article formation. The largest family of organic colour dyes include: the acid dyes for protein substrates such as wool, silk and nylon; disperse dyes for hydrophobic substrates such as acetate and polyester; and direct and reactive dyes for cellulosic substrates such as paper, linen, cotton and rayon.
The affinity between the dye and substrate is often affected by the effectiveness of a dyeing or printing process. It is therefore important that colour dyes are designed with a certain substrate in mind. They must have greater affinity for such substrate than the medium and a high level of permanence under end-use conditions. Apart from dye-substrate affinity, another crucial consideration in dye design is the safety of the products, both to the end-user and the environment. As such, synthetic dyes can’t be commercialized unless they pose insignificant risks. Raw materials used in manufacturing synthetic dyes must not contain compounds known to cause health risks such as the aromatic amines, which are either cancer agents or mutagens.
A.S. Paterson Company provides a wide range of colour dyes soluble in water, alcohol and other solvents. Some of them can be dissolved in melted plastic giving a transparent colour effect.