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2.2 Colour change materials
2.2.1 Description
This chapter deals with textile structures treated with dyestuff that change colour for either functional
or purely aesthetic reasons, in response to some external stimulus. These external stimuli might be for
instance acids, alkalis, sunlight, water, mechanical loading and electrical power. The colour changing
effect is based on physical and chemical changes in dyestuff molecules and can be either temporary
or permanent. In literature these dyestuffs are referred to as smart dyestuffs. Although smart dyestuffs
are not yet widely used due to poor resistance to visible light, heat, other external influences and
costs, they have the potential to be used to produce smart textile materials. In the following, different
types of smart dyestuffs are presented and in which way and extend they are applied on textiles.
In general, forms of colour changing material are produced in an encapsulated form as
microencapsulation helps to protect these sensitive chemicals from the environment [138].
2.2.2 Technology
Thermochromism
Microencapsulated thermochromic systems pioneered by the ink industry constitute the active element
that received fibre and fabric finish embodiments. The systems comprise a colourless electrondonating
chromatic organic compound, an electron acceptor, and a reaction medium generally made
up of one or two components. Above the medium’s melting point, one of its two components solvates
the electron donor and the system is colourless. As temperature drops to the medium’s solidification
point, solvation ceases, and the electron donor turns to the electron acceptor, which produces a
coloured addition compound. These processes are all reversible. The chemical structures of the
electron donor and acceptors determine the colours. The reaction medium components, on the other
end, control the temperatures at which the colour appears and disappears, as well as the sharpness of
these transitions and the colour intensities. Incorporation of ordinary dyestuffs or pigments increases
the number of colour combination options [139].
Thermochromic dyes
Thermochromic liquid crystals are commonly used to visually display temperature changes. The
crystals are capable of displaying different colours at different temperatures, and with this ability have
found a number of niche utilities. Fabrics treated with thermochromic dye change from being a
colourful to a colourless product at the time of activation and, if applied with other pigments then
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