> Pigment vs Dyes....
Dye
is a substance that is applied in order to impart colour with
some degree of permanence. Dyes are used principally to colour
textile fibres. Since textile dyes are usually applied from
aqueous solution, they are required to have a degree of solubility
in water or, as is the case with vat dyes, to be capable of
conversion to a water-soluble form. Major application of dyes
would be : Acid, Azoic, Basic, Direct, Disperse, Mordant, Reactive,
Sulfur and Vat.
The
word pigment comes from the Latin " pigmentum" meaning
coloured material. Pigments are generally distinguished from
dyes as colouring materials on the basis of their solubility
characteristics. Pigments are used mainly in the colouration
of paints, printing inks and plastics, although they are used
to a certain extent in a much wider range of applications including
textiles, ceramics, paper, and cosmetics. In contrast to dyes,
pigments are highly insoluble colouring materials, which are
incorporated into an applications medium by dispersion, and
they remain as discrete solid particles held mechanically within
a polymeric matrix. Pigments are thus required to resist dissolving
in solvents, which they may contact in application to minimize
problems such as 'bleeding' and migration. In addition to solvent
resistance, pigments are required to be fast to light, weathering,
heat and chemicals such as acids and alkalis to a degree dependent
on the demands of particular application.
> Organic vs Inorganic
Pigment....
Natural inorganic pigments, derived mainly
from mineral sources, have been used as colorants since pre-historic
times and a few, notably iron oxides, remain of some significance
today. The origins of the synthetic inorganic pigment industry
may be traced to the introduction of Prussian blue in the early
18th century, pre-dating the synthetic organic colorant industry
by some 150 years. In organic pigments tent to be the oxides,
sulfides, hydroxides, silicates, sulfates and carbonates of
metals. The colour of a pigment is due to its interactions with
light by scattering and absorption.
The synthetic organic pigment industry
emerged towards the end of the 19th century out of the established
synthetic textile dyestuffs industry. Many of the earliest organic
pigment were known as 'lakes'. These products were prepared
from established water soluble dyes by precipitation on to an
insoluble inorganic substrate. A further significant early development
in organic pigments was the introduction of a range of azo pigments.
One of the most critical events in the development of the organic
pigment industry was the discovery, in 1928, of copper phtalocyanine
blue. This was the first pigment to offer the outstanding intensity
and brightness of colour typical of organic pigments, combined
with an excellence range of fastness properties, comparable
with many inorganic pigments. Organic pigments generally provider
higher intensity and brightness of colour than inorganic pigments.
However, organic pigments are unable to provide the degree of
opacity offered by most inorganic pigments because of the lower
refractive index associated with organic crystals. As a rule
of thumb, following table can be used as a guide to explain
the difference between organic and inorganic pigments :
|
|
|
|
Derivation
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Colour
|
Tinctoral Strength
|
Opacity
|
Light Fastness
|
Solubility
|
Physiological properties
|
Chemical stability
|
Cost
|
|
Minerals
|
Often dull
|
Low
|
Opaque
|
Very Good
|
Insoluble in solvents
|
May have problematic
|
Often sensitive
|
Moderate
|
|
Synthesized from oil
|
Bright
|
High
|
Transparent
|
Poor to Very Good
|
May have some solubility
|
Usually safe
|
Usually good
|
Can be very expensive
|
|
> Types of Organic
Pigment....
> Forms of Organic
Pigment...
Organic pigments are typically synthesized
in an aqueous medium and isolated by filtration and washing
using a conventional plate and frame filter press. At this point
the pigment is physically in a PRESSCAKE form. This presscake
is merely water-wet pigment with a minimal amount of residual
inorganic present in the product. Typically our presscake products
are washed with natural water to an absolute conductivity of
400 micro mhos. In presscake form each pigment particle is a
discrete particle of a size ideal for optimum color strength
development. A pigment presscake may be dried and pulverized
as the next link in the manufacturing chain to give DRY TONER.
Presscake results in pigment with a particle
size and a physical form that is ideal producing a water based
dispersion. A dispersion of fluidized product is prepared by
thoroughly homogenizing the pigment in an aqueous system to
form a stable pigmented dispersion. Surface active agents may
be required to stabilize this dispersion dependant upon the
level of pigment and the type of system.
Reguler solids presscake represents the
product as discharged from the filter press and as such the
"pigment solids" or "dry content" will have
the same solids content all the time. In certain formulations
the low solids / high water content of reguler presscakes makes
it difficult to produce an ink or coatings formulation of correct
pigment / binder ratio. To solve these difficulties we make
HIGH SOLID PRESSCAKE.
Presscake maybe flushed into an oleoresinous
vehicle to give a FLUSHED COLOR designed to serve the specific
needs of the offset and letterpress ink manufacturer.
The term POLYETHYLENE FLUSH indicates a
pigmented polyethylene concentrate or perdispersion that is
prepared by flushing pigment as presscake from the aqueous phase
into a low molecular weight polyethylene carrier in a sigma
blade mixer or flusher. The advantage of this process is that
by starting with the wet, unagglomerated pigment presscake a
much finer dispersion results than from a dispersion made with
dry color.
> Flow Diagram....
