COLOUR
Colour
is the by-product of the spectrum of light, as it is reflected or absorbed, as
received by the human eye and processed by the human brain.
Primary
colours
The three defining colours of a colour wheel, from
which all other colours are built. Primary colours
are sets of colors that can be combined to make a useful range of colours. For
human applications, three primary colors are usually used, since human color
vision is trichromatic. The primary colors normally are red, green, and blue. It is primarily used in art and art education,
particularly painting
Primary colours
Secondary colours
Secondary colors are
made by mixing two primary colors. Each
secondary color is made from the two primary colors closest to it on the color
wheel. Examples include the following: green,
orange, and violet (purple).
Green (●) + red (●) =
yellow (●)
Red (●) + blue (●) =
magenta (●)
Blue (●) + green (●) =
cyan (●)
Dyes
A dye is a colored
substance that has an affinity to the substrate to which it is being applied.
The dye is generally applied in an aqueous solution, and requires a mordant to
improve the fastness of the dye on the fiber.
Both dyes and pigments
appear to be colored because they absorb some wavelengths of light more than
others. In contrast with a dye, a pigment generally is insoluble, and has no
affinity for the substrate. Some dyes can be precipitated with an inert salt to
produce a lake pigment, and based on the salt used they could be aluminum lake,
calcium lake or barium lake pigments. The dyes were obtained from animal,
vegetable or mineral origin, with none to 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.
Classes
of dyes
a) Naturals
dyes
b) Chemicals
dyes
Natural
dyes
Before, all dyes were derived directly from natural
sources such as the leaves, flowers, berries, stems or roots of plants, from
insects and shellfish, and even a number of minerals.The colouring of the
fibres results from the chemical reaction of the functional groups within the
fibres and those within the dye stuff.The difficulty with most natural dyes is
that they lack colour fastness.However the treatment of the fabrics with
certain natural acids or oxides (metallic salts) improves their colour fastness.These
substances are called mordants;
they react both with the dyestuff and the fibre to form an insoluble compound
thus fixing the colour within and on the fibre.
Natural dyes may be grouped according to their
chemical structures
A) Carotenoids
(give yellow and orange colours): carotenes (carrots, red pepper oranges and
tomatoes) and Xanthophylls (found in marigolds)
B) Flavonoids: flavones, flavonols (have yellowish
colours, sensitive to ph., becomes much deeper in colour in high ph.)
C) Anthracenes: anthraquinones (e.g. madder) and
naphthoquinones (e.g. walnuts)
Chemical dyes
The first human-made organic dye, mauveine, was
discovered serendipitously by William Henry Perkin in 1856, the result of a
failed attempt at the total synthesis of quinine. Many thousands of synthetic
dyes have since been prepared.
Synthetic dyes quickly replaced the traditional
natural dyes. They cost less, they offered a vast range of new colours, and
they imparted better properties to the dyed materials. Dyes are now classified
according to how they are used in the dyeing
Classes of chemical dyes
Acid
dyes: are
water-soluble anionic dyes that are applied to fibers such as silk, wool, nylon
and modified acrylic fibers using neutral to acid dye baths. 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. Most synthetic food colours fall in this category.
Basic
dyes: are 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 dye bath to help the uptake
of the dye onto the fiber. Basic dyes are also used in the coloration of paper.
Direct or substantive dyeing is normally carried out
in a neutral or slightly alkaline dye bath, at or near boiling point, with the
addition of either sodium chloride (NaCl) or sodium sulfate (Na2SO4) or sodium
carbonate (Na2CO3). Direct dyes are used on cotton, paper, leather, wool, silk
and nylon. They are also used as pH indicators and as biological stains.
Mordant
dyes: require a
mordant, which improves the fastness of the dye against water, light and
perspiration. 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, or 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, potassium dichromate,
is applied as an after-treatment. It is important to note that many mordants,
particularly those in the heavy metal category, can be hazardous to health and
extreme care must be taken in using them.
Vat
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,
which, in this leuco form, has an affinity for the textile fibre. Subsequent
oxidation reforms the original insoluble dye. The colour of denim is due to
indigo, the original vat dye.
Reactive
dyes: utilize a
chromophore attached to a substituent that is capable of directly reacting with
the fibre substrate. The covalent bonds that attach reactive dye to natural
fibers make them among the most permanent of dyes. "Cold" reactive
dyes, such as Procion MX, Cibacron F, and Drimarene K, are very easy to use
because the dye can be applied at room temperature. Reactive dyes are by far
the best choice for dyeing cotton and other cellulose fibers at home or in the
art studio.
Disperse
dyes: were
originally developed for the dyeing of cellulose acetate, and is water
insoluble. The dyes are finely ground in the presence of a dispersing agent and
sold as a paste, or spray-dried and sold as a powder. Their main use is to dye
polyester but they can also be used to dye nylon, cellulose triacetate, and acrylic
fibres. In some cases, a dyeing temperature of 130 °C (266 °F) is required, and
a pressurised dye bath 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.
Sulfur
dyes: are two part
"developed" dyes used to dye cotton with dark colors. The initial
bath imparts a yellow or pale chartreuse colour;this is aftertreated with a
sulfur compound in place to produce the dark black we are familiar with in
socks for instance. Sulfur Black 1 is the largest selling dye by volume.
Dyeing
Principles
Ø In a dye house an infinite variety of fibres and
fabric structure must be dyed on shade in a minimum amount of time at a minimum
cost.
Ø The colourfastness expectations and colouration
desires of consumers must be met.
Ø A dyer must know how to dye a wide array of fabrics
composed of natural and manufactured fibres and various blends.
Ø Fibre manufacturers continuously produce modifications
of their fibres, and each modification alters the way the fibre behaves when it
is dyed.
Ø Dye variant fibres: e.g. deep dyeing nylon fibres,
basic dyeable nylon or polyester.
Ø Dyeing equipment is continuously being developed to
accommodate new fabrics and dyes therefore the dyer must continuously keep
abreast of changes in the field in order to remain competitive.
Ø Textiles are usually dyed by being immersed into a
prepared aqueous dye bath. Usually the dye bath contains dyes that differ in
the wavelength of light absorbed and reflected.
Ø When the yarn or fabric to be dyed contains different
fibres, the dyebath may contain different classes of dye so that each type of
fibre can absorb dye.
Ø The aqueous dye bath usually contains salts, acids or
alkalis and other auxillary chemicals that cause the dye to have a greater
affinity for the fibres than for the solution.
Ø They assist the migration of the dye from the solution
to the fibre surfaces and aid the diffusion of the dye into the fibre.
Ø Either the substrate or dye solution must be agitated
to promote uniform dyeing. The dyeing time varies from a few minutes to several
hours. The temperature of the dye bath also varies depending on the dye class.
Dyeing
is a chemical process involving the principles of:
Migration: first the dye must migrate from the solution to the
fibre surface and be adsorbed (the adhesion of a thin layer of molecules of the
dye to the surface of the substrate) on the surface of the fibre.
Diffusion: then it must diffuse from the surface toward the
centre of the fibre
Retention: finally the fixation of the dye by covalent or
hydrogen or other forces must occur in order for the fibres to retain the dye
METHODS OF DYEING:
1)
Bale Dyeing: This is a low cost method to dye
cotton cloth. The material is sent without scouring or singeing, through a cold
water bath where the sized warp has affinity for the dye. Imitation chambray
and comparable fabrics are often dyed this way.
2)
Batik Dyeing: This is one of the
oldest forms known to man. It originated in Java. Portions of the fabric are
coated with wax so that only un-waxed areas will take on the dye matter. The
operation may be repeated several times and several colours may used for the
bizarre effects. Motifs show a mlange, mottled or streaked effect, imitated in
machine printing.
3)
Beam Dyeing: In this method the warp is dyed
prior to weaving. It is wound onto a perforated beam and the dye is forced
through the perforations thereby saturating the yarn with colour.
4)
Burl or speck Dyeing: This is done mostly on
woollens or worsteds, coloured specks and blemishes are covered by the use of
special coloured links which come in many colours and shades. It is a hand
operation.
5)
Chain Dyeing: This is used when
yarns and cloth are low in tensile strength. Several cuts or pieces of cloth
are tacked end-to-end and run through in a continuous chain in the dye colour.
This method affords high production.
6)
Cross Dyeing: This is a very popular
method in which varied colour effects are obtained in the one dye bath for a
cloth which contains fibres with varying affinities for the dye used. For
example, a blue dyestuff might give nylon 6 a dark blue shade, nylon 6, 6 a
light blue shade, and have no affinity for polyester area unscathed or white.
7)
Jig Dyeing: This is done in a jig, kier, vat,
beck or vessel in an open formation of the goods. The fabric goes from one
roller to another through a deep dye bath until the desired shade is achieved.
8)
Piece Dyeing: The dyeing of fabrics
in the cut, bolt or piece form is called piece dyeing. It follows the weaving
of the goods and provides a single colour for the material, such as blue serge,
a green organdy.
9)
Random Dyeing: Colouring only certain
designated portions of the yarn.
PRINTING
TECHNIQUES
Textile printing
is the process of applying colour
to fabric
in definite patterns
or designs. In properly printed
fabrics the colour is bonded with the fiber,
so as to resist washing
and friction.
Textile printing is related to dyeing
but in dyeing properly the whole fabric is uniformly covered with one colour,
whereas in printing one or more colours are applied to it in certain parts
only, and in sharply defined patterns.
Roller
printing, cylinder printing, or machine printing
This
elegant and efficient process was patented and worked by Bell in 1785 only
fifteen years after his application of the engraved plate to textiles. Bell's
first patent was for a machine to print six colours at once, but, owing
probably to its incomplete development, this was not immediately successful,
although the principle of the method was shown to be practical by the printing
of one colour with perfectly satisfactory results. The difficulty was to keep
the six rollers, each carrying a portion of the pattern, in perfect register
with each other.
Advantages
of roller printing
·
high productivity,
10,000 to 12,000 yards being commonly printed in one day of ten hours by a single-colour
machine,
·
its capacity of being
applied to the reproduction of every style of design, ranging from the fine
delicate lines of copperplate engraving and the small repeats and limited
colours of the perrotine to the broadest effects of block printing and to
patterns varying in repeat from I to 80 in.
·
The wonderful exactitude with which each
portion of an elaborate multicolour pattern can be fitted into its proper place
without faulty joints at its points of repetition.
Stencil
printing
The
art of stenciling is not new. It has been applied to the decoration of textile
fabrics from time immemorial by the Japanese, and, of late years, has found
increasing employment in Europe for certain classes of decorative work on woven
goods for furnishing purposes.
The
pattern is cut out of a sheet of stout paper or thin metal with a sharp-pointed
knife, the uncut portions representing the part that is to be reserved or left
uncoloured. The sheet is now laid on the material to be decorated and colour is
brushed through its interstices.
It
is obvious that with suitable planning an all over pattern may be just as
easily produced by this process as by hand or machine printing, and that
moreover, if several plates are used, as many colours as plates may be
introduced into it.
The
peculiarity of stenciled patterns is that they have to be held together by ties,
that is to say, certain parts of them have to be left uncut, so as to connect
them with each other, and prevent them from falling apart in separate pieces.
For instance, a complete circle cannot be cut without its center dropping out,
and, consequently, its outline has to be interrupted at convenient points by
ties or uncut portions.
The necessity for ties exercises great
influence on the design, and in the hands of a designer of indifferent ability
they may be very unsightly. On the other hand, a capable man utilizes them to
supply the drawing, and when thus treated they form an integral part of the
pattern and enhance its artistic value whilst complying with the conditions and
the process.
Screen-printing
Screen printing
is by far the most used technology today. Two types exist: rotary screen
printing and flat (bed) screen printing. A blade (squeegee) squeezes the
printing paste through openings in the screen onto the fabric.
Digital
textile printing
Digital textile printing,
often referred to as direct to garment printing, DTG printing, and digital
garment printing is a process of printing on textiles and garments using
specialized or modified inkjet technology. Inkjet printing on fabric is also
possible with an inkjet printer by using fabric sheets with a removable paper
backing. Today major inkjet technology manufacturers can offer specialized
products designed for direct printing on textiles, not only for sampling but
also for bulk production. Since the early 1990s, inkjet technology and
specially developed water-based ink (known as dye-sublimation or disperse
direct ink) has offered the possibility of printing directly onto polyester
fabric. This is mainly related to visual communication in retail and brand
promotion (flags, banners and other point of sales applications). Printing onto
nylon and silk can be done by using an acid ink. Reactive ink is used for
cellulose based fibers, such as cotton and linen. Using inkjet technology in
digital textile printing allows for single pieces, mid-run production and even
long-run alternatives to screen printed fabric.
Transfer printing
This
is when the print is applied to paper and then transferred to the fabric by the
use of heat and pressure. It s less expensive than roller printing.
Batik
printing
This
is when wax is applied to the cloth in areas that are not to receive the
colour. After dyeing wax is boiled off, but lately it s done by machine.
Tie
dyeing
Puffs
of fabric are wrapped in wax thread or sewn and tightly gathered, then dipped
in dye chating intriguing sunburst effects and penetrates fabric unevenly.
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