A dye can generally be described as a coloured
substance that has an affinity to the substrate to which it is being
applied. The dye is usually used as an aqueous solution and may
require a
mordant to improve the fastness of the dye on the fibre. (In
contrast, a
pigment generally has no affinity for the substrate, and is
insoluble)Archaeological evidence shows that, particularly
in
India and the Middle East, dyeing
has been carried out for over 5000 years. The dyes were
obtained from either animal, vegetable or mineral origin with no or very
little processing. By far the greatest source of dyes has been from the
plant kingdom, notably roots, berries, bark, leaves and wood, but only a few
have ever been used on a commercial scale. Natural dyes
Animal Origin
Tyrian purple Vat dye
Kermes Mordant dye
Cochineal Mordant dye
Vegetable Origin
Safflower Direct (substantive) dye
Turmeric Direct (substantive) dye
Indigo Vat dye
Woad Vat dye
Alizarin (Madder) Mordant dye
Dyer's Broom Mordant dye
Logwood Mordant dye
Brazilwood Mordant dye
Quercitron bark Mordant dye
Weld Mordant dye
Old Fustic Mordant dye
Cudbear
Classification
The first man made organic dye,
mauveine, was discovered by
William Henry Perkin in 1856. Many
thousands of dyes have since been prepared
and because of vastly improved properties
imparted upon the dyed materials quickly
replaced the traditional natural dyes. Dyes
are now classified according to how they are
used in the dyeing process.
Acid dye
- Water soluble anionic dyes that are
applied to fibres such as
silk,
wool,
nylon and
modified acrylic fibres from neutral
to acid dyebaths. Attachment to the fibre
is attributed, at least partly, to salt
formation between anionic groups in the
dyes and cationic groups in the fibre.
Acid dyes are not substantive to
cellulosic fibres.
Basic dye
- Water soluble cationic dyes that are
mainly applied to acrylic fibres but find
some use for wool, and silk. Usually
acetic acid is added to the dyebath to
help the take up of the dye onto the
fibre. Basic dyes are also used in the
coloration of
paper.
Direct (Substantive) dye
- Dyeing is normally carried out in a
neutral or slightly alkaline dyebath, at
or near the boil, with the addition of
either sodium chloride (NaCl)
or sodium sulphate (Na2SO4).
Direct dyes are used on cotton, paper,
leather, wool, silk and nylon. They
are also used as
pH indicators and as
biological stains.
Mordant dye
- As the name suggests these dyes
require a mordant. This improves the
fastness of the dye on the fibre such as
water, light and perspiration fastness.
The choice of mordant is very important as
different mordants can change the final
colour significantly. Most natural dyes
are mordant dyes and there is therefore a
large literature base describing dyeing
techniques. The most important mordant
dyes are the synthetic mordant dyes
(chrome dyes) used for wool, these
comprise some 30% of dyes used for wool
and are especially useful for black and
navy shades. The mordant used is potassium
dichromate applied as an after-treatment.
Vat dye
- These dyes are essentially insoluble
in water and incapable of dyeing fibres
directly. However, reduction in alkaline
liquor produces the water soluble alkali
metal salt of the dye. In this leuco form
these dyes have an affinity for the
textile fibre. Subsequent oxidation
reforms the original insoluble dye.
Reactive dye
- First appeared commercially in 1956,
after their invention in 1954 by Rattee
and Stephens at the ICI Dyestuffs Division
site in Blackley, Manchester, UK. They are
used to dye cellulosic fibres. The dyes
contain a reactive group, either a
haloheterocycle or an activated double
bond, that, when applied to a fibre in a
weakly alkaline dyebath, forms a chemical
bond with an hydroxyl group on the
cellulosic fibre. Reactive dyeing is now
the most important method for the
coloration of cellulsic fibres. Reactive
dyes can also be used to dye wool and
nylon, in the latter case they are applied
under weakly acidic conditions.
Disperse dye
- Originally developed for the dyeing of
cellulose acetate. They are
substantially water insoluble. The dyes
are finely ground in the presence of a
dispersing agent then sold as a paste or
spray dried and sold as a powder. They can
also be used to dye nylon, triacetate,
polyester and acrylic fibres. In some
cases a dyeing temperature of 130 deg C is
required and a pressurised dyebath is
used. The very fine particle size gives a
large surface area that aids dissolution
to allow uptake by the fibre. The dyeing
rate can be significantly influenced by
the choice of dispersing agent used during
the grinding.
Azoic dye
- A dyeing technique in which an
insoluble azo dye is produced directly
onto or within the fibre. This is achieved
by treating a fibre with a diazo component
and a coupling component. With suitable
adjustment of dyebath conditions the two
components react to produce the required
insoluble azo dye. This technique of
dyeing is unique in that the final colour
is controlled by the choice of the diazo
and coupling components.
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One other class which describes the role
dyes have rather than their mode of use is
food dyes.
- This is a special class of dyes of
very high purity. They include direct,
mordant and vat dyes. Their use is
strictly controlled by legislation. Many
are azo dyes but
anthraquinone and triphenylmethane
compounds are used for colours such as
green and blue. Some naturally occurring
dyes are also used.
A number of other classes have also been
established and these include:
Oxidation bases mainly hair and fur
Sulfur dyes textile fibres
Leather dyes leather
Fluorescent brighteners textile fibres and paper
Solvent dyes wood staining,
producing coloured lacquers,
producing solvent inks,
colouring oils, waxes and fats.
Fabric Definition
Fiber
Definition
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