A dye can generally be described as a colored 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 fiber. (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
Inorganic dyes
Iron Buff
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 fibers
such as
silk,
wool,
nylon and
modified acrylic fibers from neutral to acid dyebaths.
Attachment to the fiber is attributed, at least partly,
to salt formation between anionic groups in the dyes
and cationic groups in the fiber. Acid dyes are not
substantive to cellulosic fibers.
Basic dye
Water soluble cationic dyes that are mainly applied
to acrylic fibers 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 fiber. 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.
As the name suggests these dyes require a mordant. This improves
the fastness of the dye on the fiber such as water, light and perspiration
fastness. The choice of mordant is very important as different mordants
can change the final color 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
fibers 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 fiber. 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 fibers. The dyes contain
a reactive group, either a haloheterocycle or an activated double bond,
that, when applied to a fiber in a weakly alkaline dyebath, forms a
chemical bond with an hydroxyl group on the cellulosic fiber. Reactive
dyeing is now the most important method for the coloration of cellulsic
fibers. 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 fibers. 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 fiber. 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 fiber. This is achieved by treating a fiber 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
color is controlled by the choice of the diazo and coupling components.
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
colors such as green
and blue. Some naturally
occurring dyes are also
used.
A number of other classes
have also been established
and these include:
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