Pigments:
Pigment is a substance in particulate form which is
insoluble in water but which can be mechanically dispersed in this medium to
modify its color and light scattering properties. They are organic or inorganic
colouring materials.
Pigments have no direct affinity to textile fibers. They are
fixed on textile materials (fabric/yarn) with the help of a binding agent in
form of a this invisible coating.
In pigments auxochrome groups are absent. A number of
organic pigments are prepared from dyes by combining the dyes with a dye
precipitant.
Pigment are abundant in nature. The cause of color is not
only the light absorption of molecules but also the purely physical effect of
optical diffraction at grating systems. Pigments are applied at thickened
pigment coating from oil-in-water or water-in-oil emulsion systems.
Properties of pigments:
i.
It is insoluble in liquids but can be
mechanically dispersed in the medium.
ii.
Pigments have no direct affinity towards fiber,
so binding is used as a binding agent in case of pigment colouration which
sticks the pigment particle on the fiber surface.
iii.
Pigment are applicable to all kinds of fibers.
iv.
No aftertreatment is required after coloration
process, only drying and curing are required.
v.
Only rubbing fastness of pigment colouration is
poor, but water wash and light fastness are very good.
vi.
A wide range of bright shade can be produced by
pigments.
vii.
In pigments auxochrome groups are absent.
Differences between dyes and pigments:
The difference between dye and pigment is not a clear one.
Most organic pigments are closely related to dyes with respect to their
chemical structure. There are dyes which become pigments after application. Vat
dye is a dye when used in dyeing, when vat dyes are oxidized and made insoluble
in fabric(after vatting) that is called pigment form of vat dyes. Vat dyes act
like pigment when used in printing. Now some differences are mentioned below:
1 ) The basic difference between dye and pigment is that,
dye has three parts-
i.
Chromophore, which is responsible for color.
ii.
Auxochrome, which is responsible for dye
properties
iii.
Chromogen, which holds the above two together.
But pigment has two part:-
i.
Chromophore, i.e. color bearing part.
ii.
Chromogen.
2 ) Pigments are insoluble in the materials that they are
used to color, whereas most dyes are soluble in them.
3 ) pigments are water insoluble, but dyes are more or less
water soluble.
4 ) Pigments are used as colorants in the physical for in
which they are manufactured. But dyes are first dissolved in water to form a
dyebath, then they diffuse as single molecules from these solutions into
textile fibers to colour them.
5 ) Pigments have no direct affinity towards textile fibers
but most dyes have that.
6 ) Pigments are metallic compounds may be organic or
inorganic. But dyes are mostly organic compounds, few are inorganic.
Classification of pigments:-
There are mainly two types of pigments, namely,
1. Organic
pigments and
2. Inorganic
pigments
Organic pigments are further classified as below:-
I.
Dyes converted into pigments.
It has two types,
1) Dyes made insoluble by precipitating on
substrates.
2) Vat dyes converted into pigments
II.
ISO indolinone pigments
III.
Azo pigments:
It has two types
1) Metal free{Water insoluble azo compounds
(group1)}
2)Metal containing
there are two types: chelates(group-3) and
Water soluble azo compounds(group-2)
–low soluble, make heavy metal
salts(group-2a)
–high solubility precipitate heavy metal
salts or basic substrates(group-2b)
IV.
Quinacridone pigments
V.
Phthalocyanine pigments
Inorganic pigments are classified by two types
I.
White pigments and
II.
Coloured pigments
There are three types of coloured pigments:
1. Ultramarine
2. Cadmium
pigments and
3. Iron
oxide pigments.
Binder:
Binders are film forming polymers which play important roles
in achieving optimum properties. Pigments have no affinity towards textile
materials. So actually pigment particles are sticked on the surface of fabric.
Binders are those adhesive type coating forming polymeric materials which
sticks pigment particles on fiber/fabric surface. Binders are white, milk-like
liquid. If forms a very thin invisible film on fabric surface during curing.
Under this film pigment particles are remain sticked. So in pigment dyeing and
printing binder is a very important and essential ingredient.
Binders with a particle size range of 0.05 to 2 microns are
usually available. These contain about 6% by weight of reactive groups for
crosslinking of the binder to textile. The reaction involved is as below:
Binder-CH2-OR + HO-textile = Binder-O-textile +
HOR
[where Ris H or CH3 ]
Crosslinking increases the rubbing washing and drycleaning
fastness properties, but it affects the handle of fabric.
Types of binder:
Binder can be classified according to many aspects, Some are
mentioned below:
1. According
to origin: According to origin there are two types of binder,
I.
Natural binder- glue, gelatin etc.
II.
Synthetic binder- TROL, SLN, IG, Acramin binders
etc.
2. According
to chemical groups: According to chemical group present in binder there are two
types of binder, those are:-
I.
IG Binders: Emulsion copolymer of vinyl acetate
and butyl acrylate with a modified urea formaldehyde.
II.
Ag Binders: They are made by emulsion copolymerization
of olifinic unsaturated monomers in an aqueous medium. These monomers include:-
a) Acrylic
acid ester(butyl or ethyl acrylate)
b) Styrene
c) Acrylonitrile
d) Vinyl
chloride
e) Asymmetric
dichloroethane
f) Vinyl
acetate
g) Diolifine
such as butadiene
Required properties of a binder:
The binder must have certain qualities. For example:
i.
It should not get coagulated due to shear
forces.
ii.
The binder film must be clear/transparent.
iii.
The binder film must be of even thickness,
smooth, neither too hard nor too soft.
iv.
It should be elastic in nature and should have
good adhesion to the substrate without being tacky.
v.
It should have good resistance to chemical and
mechanical stress.
vi.
It should be easily removable from equipments.