Page 38 - Curriculum Visions Dynamic Book

(Above and below) Synthetic dyes used with synthetic fibres tend to be best in outdoor conditions, retaining their strength of colour much longer than natural materials.
To overcome this problem, the dyes have to be transferred using a nonwater-based solvent such as benzyl alcohol.
There are no pores in synthetic fibres, so the dye has to be deposited on the surface. One way is to dip the yarn in a hot liquid, so the fibres relax and let in some liquid and its dye, then cool the fabric so that the fibres close again.
Azo dyes are widely used with synthetic fibres. They are entirely synthetic and are based on coal and oil. The first azo dye was called aniline, and towards the end of the 19th century it revolutionized the dyeing process. The dye can actually be made while the fabric is being dyed. The dye does not necessarily have to be made up
beforehand. Dyeing in this way is a two- stage process. First, one solution, called the coupler, is used. Then the fabric is dipped in a solution of an azo salt. The coupler and azo salt react on the surface of the fabric to produce a colour. This is called ingrain dyeing.
Although the techniques are slightly different, in the majority of cases the fibres are dyed after they have been made, just as would happen with natural fibres. This means that a large bulk of neutral colour polymer can be produced.
Some synthetics simply will not take up any kind of dye. In this case colour
is added to the polymer while it is being made and before it is extruded. Acrylic fibres and polypropylene fibres are of this kind. In these cases the colouring material is usually a pigment, that is,
a solid material with colour. Adding pigments during fibre-making also helps resist change due to light, something that is very desirable if the fabric is to be used permanently outdoors.
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