Dyes used
for dyeing denim are:
- Indigo
- Vat
- Direct
- Sulphur
- Reactive
- Pigment Colors
Among all
these dyes indigo dye is the most popular and widely used consisting of almost
67% of all the dyes used for denim production.
Indigo for Denim
The indigo color
is the principal source of the almost magical appeal of denim. The dye imparts
a brilliant blue hue to fabric. In fact, there is no other dye that creates
such an intense blue color with so few carbon atoms in its molecule Indigo is
unique in its ability to impart surface color due to partial penetration in cotton
fibers. When cotton yarn dyed with indigo is untwisted, the inner layers remain
uncolored. The dye also fades and abrades continually. This characteristic of
indigo lets denim fabric have its final worn look with different types of
washing and finishing applications. It enables denim fabric to respond to
finishing applications that give a real life to the fabric.
Despite
many other blue dyestuffs, indigo has kept its popularity for denim dyeing. This
no doubt is achieved by the fact that indigo has several properties that have
not yet been achieved by another single dyestuff. The unique feature of indigo
dyed denim is the possibility of achieving wash down effects on repeated
washing without losing the freshness of the color. Another important feature of
indigo is that unlike many other dyes, indigo dyed denim does not pose health
hazards. In fact, indigo is so safe for living things that it has long been
used to color polyester medical sutures. Indigo is also used as food color and
as a medical indicator applied intravenously.
Technical
features of indigo include possibility of dyeing at room temperature, favored
shades from black to navy to sky blue that are not too brilliant or not too
dull, reduced indigo solution is not sensitive to water hardness and this
allows the dyeing of greige or minimum pretreated cotton, does not strongly
bond to the fiber, repeated washings slowly remove the dye without losing
freshness and most importantly the competitive price. In fact, many synthetic
blue dyes have superior qualities compared to indigo, and this would have
ruined synthetic indigo as well if denim had not begun its invasion to Western
culture after the Second World War.
Other Dyes Used for Denim
Since denim
is made of 100% cotton, therefore in addition to indigo it can be dyed with dye
classes such as direct, vat (other than indigo), reactive, sulphur dyes and pigment
colors. In the last decade, denim was mainly dyed with indigo (67%), sulphur
black (26%) and other sulphur colors (6%). The changing fashion trends have also
led to vat, reactive and direct dyestuffs, as well as pigments being used to color
denim.
Direct dyes
are currently being applied in garment dyeing of jeans in light colors for the
higher fashion market. Direct dyes can also be used for tinting denim fabrics
by adding to finishing formulation. Reactive dyes are often used in garment
dyeing. There have been attempts to apply reactive dyes on indigo machinery,
but control of dyeing is nearly impossible because of the large dye boxes on
most indigo equipment, much of the dye being hydrolyzed before it can react
with cotton. Therefore, reactive dyes should be applied in a small box, then
dried, chemical padded and steamed. The only practical alternative for most
denim companies is to apply reactive dyes during sizing with special
procedures. There are few vat dyes that can be mixed with indigo for casting;
however, pure vat colors are best applied on denim with specially designed rope
ranges with an additional set of drying cylinders, steamer and small pad boxes
for dye pad, dry, chemical pad, steam methods. However, vat dyes produce shades
with high fastness properties and therefore it is difficult to get the classic
denim look.
Sulphur
dyes are of low cost and can be applied on indigo machinery, fabric dyeing machinery,
jiggers, pad-batch and garment dyeing machinery with properly designed methods.
Sulphur dyes have an appearance that is more natural than reactives or directs,
having a softer appearance and allowing versatile wash down effects in
laundering. Therefore, among the different dye classes other than indigo,
sulphur dyes are more popular and are used for bottoming or topping or over
dyeing of finished indigo dyed garments to produce a variety of shades with
fancy looks. Pigments are relatively easy to apply, and since they are a
surface coloration, can produce a distressed look, but they present color
fastness problems in darker shades and have a harsher feel than dyed garments.
