Dyes and Pigments

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Dyes and Pigments
Pure transition metals and their alloys have many uses but their compounds are also extremely useful. Compounds of
metals other than the transition metals tend to be colourless when crystalline or in solution. However, transition
metal compounds are generally coloured. If a transition metal compound is soluble (dissolves) it can be used as a dye.
If it is insoluble it can be used as a pigment.
Dyes
Dyes are transitional metal compounds that are soluble (dissolve for example in
water) and result in solutions. They will form chemical bonds to a fabric placed in the
solution. Some dyes require another chemical to be added to them for them to bond to
the fabric ­ this compound is called a mordant.
Dyeing has been carried out for over 5000 years. The dyes were obtained from animal,
vegetable or mineral origin, with no or very little processing. The greatest source of dyes
has been from the plant kingdom, notably roots, berries, bark, leaves and wood. Most
dyes used nowadays are artificial dyes, the sources of which are coal and oil. These are
non-renewable resources and are also in demand as fuels. The manufacturing processes
involved in making the dyes use a lot of corrosive chemicals and even carcinogenic chemicals; you have to be
very careful on how you dispose of them. The effluent (liquid waste or sewage discharge into a river or the sea)
from dyeing works was once a major problem resulting in polluted rivers and, even now, it is a very costly part of the
process.
Since coal and oil are becoming more expensive it will be necessary to find another source of the chemicals from
which dyes can be produced. In the future it would be better to rely on renewable biomass (sugar, straw etc) as a
source of raw materials.
An example of a dye is lead acetate which is a white solid that is used to dye cotton and to make enamels and
varnishes. However, it is poisonous.
Pigments
Pigments are transitional metal compounds that are insoluble and result in suspensions. The powdered pigment,
eg titanium dioxide (a white pigment) is mixed with a liquid to form a suspension. The suspension could then be used
as a paint.
Emulsion paints have the pigment dispersed in tiny droplets of water with a
polymer such as PVC or acrylic resin. The water droplet evaporates leaving the
pigment particles on the surface bound by the polymer. Evaporation is not a chemical
reaction.
Gloss paints use oil droplets instead of water. The solvent evaporates as the paint
dries and other chemicals harden as they react with oxygen in the air (an oxidation
reaction). The paint surface becomes harder as it `dries'.
An example of a pigment is red iron oxide which is a red pigment that is often used in paints and cosmetics.
Issues
Some transition metals are toxic (e.g. lead) so can be harmful to humans and possible the environment. Dyes and
pigments that have these metals in must be disposed of carefully so that they cannot cause harm. For example certain
paints shouldn't be washed away in the sink as this is where a lot of people get there drinking water from.
Some of the chemicals are carcinogenic (cancer causing) so must be handled carefully during manufacturing
processes and by the users. The processes used to make the dyes and pigments use some very corrosive
chemicals, you have to dispose of these chemicals carefully to protect the environment.

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