Look around you, at your clothes, the walls, the floor. Chances are that you see before you a riot of colour. Humans have been fascinated by colour for thousands of years and use colours to warn, to seduce and primarily to decorate. So, what about the chemistry behind the decoration? What makes one molecule coloured, and another not? Why do some clothes fade in the wash?
The chemical basis of colours is the reason many people choose to do chemistry. The basis of this project is the chemistry behind fabric dyes- what are they? What are the origins of dyes? How are they made? What affects the way they attach to different fibres?
The search for highly coloured, colour fast dyes has fuelled major industry from ancient times right up to the present, from the Roman dye factories at Tyre, to modern chemical companies such as ICI. Nowadays, the chemist with a knowledge of organic chemistry is at the forefront of new dye development, altering the structures of known dyes, and inventing new ones.
Essentially, dyes have an affinity for the substrate (fibre) they are applied to. Dyes are also either soluble, or dispersible in a solvent (the particles of a dispersed dyes are essentially aggregates of a few molecules)1.
Pigments have no affinity for the substrate they are applied to, and tend to be present as an insoluble suspension in a drying oil or other resinous vehicle. Pigment particles also tend to be of the order of 1μm in diameter1.
However, the boundary between pigments and dyes is not sharp. The method of application of some dyes requires them to form insoluble compounds within the fabric. See Protein Textile Dyes for a greater discussion of this effect.
When we say something is coloured, what do we mean? We are implying that it either reflects or absorbs electro-magnetic radiation in a narrow region- the visible spectrum. This is the only region in the entire electro-magnetic spectrum which our eyes can detect. Its effective range is 380-720nm. The ultra-violet region starts below 360nm, and the infra-red region starts above 780nm1.
The hue refers to the major wavelength or wavelengths reflected from the material. Different wavelength indicate different hues. Approximately 150 hues can be detected in the visible spectrum1.
The brightness of any substance depends on the amount of reflected light1.
The strength of a coloured surface is inversely proportional to the amount of white light the surface reflects. This is because white light dilutes the wavelengths which give the surface its hue1.
This is the ability of the dye to resist fading. The causes of fading are UV radiation from sunlight, and washing. See Colour Fastness for more.
These will be mentioned in passing as well, since they play an equal part in the dying process. An understanding of the functional groups on different fibres can help give an indication of the type of dye that should be used, as well as suggest ways of developing new dyes.