DYES & PIGMENTS
DYES
A dye is a type of small soluble colorant. Here, solubility means that the dye dissolves well in the binder or the
application fluid, which can be water. Also, dyes are more transparent, allowing it to glaze over the material.
Generally, dye molecules have elements which help to bind to the surface. These elements form either van der
Waal’s forces, hydrogen bonds or even ionic bonds based on the characteristics of the substrate.
The part of the dye molecule responsible for the production of the color is known as the
chromophore. Most dyes contain auxochromes, which are either basic or weakly acidic groups, increasing the
color of the dye. Some dyes used in the textile industry are acid dyes, basic dyes, direct dyes, azoic dyes, sulfur
dyes, etc.
PIGMENT
A pigment is an insoluble material ground into a fine powder used as a colorant. Here, as they are insoluble,
pigments are suspended in the binder. Therefore, the binder is responsible for holding pigments in place on the
material. Another two characteristic features of pigments are their opacity and low tinting strength. As they are
opaque, pigments can completely cover color on the material. Here, the tinting strength refers to how well two
colors mix with each other, fading their colors to each other.
Most of the pigments are inorganic metals such as titanium, cadmium, lead, etc. Usually, pigments
are mainly used to produce inks, paints, plastics, and other polymeric materials. Significantly, the lightfastness is
high in pigments, which refers to the resistance of a colorant to fading it is when exposed to light.
Similarities
Dye and pigment are two types of compounds which can give a color to a material.
Therefore, they are known as colorants.
Also, both can be either natural or synthetic.
They have various applications in the textile industry, printing, painting, candle making,
etc.
Differences
A dye refers to an intensely colored or fluorescent organic substance, which imparts color to a substrate by
selective absorption of light while a pigment refers to the colored, black, white or fluorescent particulate organic
or inorganic solids, usually insoluble in, and essentially physically and chemically unaffected by, the vehicle or
substrate in which they are incorporated. Thus, this is the main difference between dye and pigment.
Size
A dye particle is very small while a pigment is comparatively large
Transparency
Transparency is another difference between dye and pigment. Dyes are more transparent while pigments are
comparatively less transparent.
Solubility
Solubility is also a difference between dye and pigment. Dyes are soluble in water while pigments are not soluble in
water and many solvents.
Types of compound
Most dyes are organic while pigments can be either organic or inorganic.
Auxochrome groups
Auxochrome groups can be present in dyes while no auxochrome groups are present in pigments.
Availability
Also, one other difference between dye and pigment is that dyes are available in a large number while pigments are
less available
Affanity
Moreover, dyes have a direct affinity to the material while pigments have no direct affinity to the material.
Binding Agents
Dyes do not require binding agents while pigments require binding agents. Hence, this is another difference between
dye and pigment.
Change in the structure during Application process
The structure of dyes temporary alters during the application process while the structure of pigments does not alter
during the application process.
Diffsion
Furthermore, dyes diffuse in the fabric while pigments diffuse on the fabric.
Method of Imparting colors
Dyes impart colors by the selective absorption while pigments impart colors by scattering of light or by selective
absorption.
Light Fastness
Dyes have low lightfastness while pigments have high lightfastness.
Product Resistance
The product resistance of dyes is lower while the product resistance of pigments is higher.
Combustive Properties
Dyes are compatible with burning while pigments tend to clog the wick during burning.
Long Lasting Factor
Additionally, dyes do not last long while pigments last longer.
Types of Materials
Dyes are compatible with a wide range of materials while suitable materials for pigments are limited.
Conclusion
A dye is a type of colorant molecule used to give a color to a material. Generally, dyes are fine particles that are
mostly soluble. They are available in large number. On the other hand, pigments are comparatively large
compounds, which are insoluble. However, pigments last longer than dyes. Therefore, the main difference between
dye and pigment is their size and durability.
Manufacturing of DYES
AZO DYE
Dye manufacturing consists of five basic steps: diazotization, coupling, isolation-filtration, drying, grinding, and
standardization. You must watch for one factor that influences all these processes: purity of intermediates. If you
use impure or tainted intermediates, your result is contaminated dyes.
1) Diazotization
In the first step, we produce a diazonium salt to react with a coupling component. The process involves
nitrosation of primary aromatic amines. We do this by introducing a strong acid like HCL and Sodium Nitrite,
with ice to control the temperature. After the reaction, we test the product with starch iodide paper and
create a blue reaction. The more times we perform a diazo reaction, the different class of dye. These include
dye products like disazo, trisaxo, and polyazo.
2) Coupling
To complete the synthesis of an azo dye, the diazonium salt reacts as an electrophile with an electron-rich
coupling component, like a phenol or an aniline. This is done through an electrophilic aromatic substitution
mechanism. We must control this reaction by monitoring pH, volume, time, and temperature throughout the
coupling reaction.
An end point test will show if the reaction completed. You achieve this by testing a salted sample of the
coupling solution with a diazo or coupling. Normally, a slight excess of either the diazo or coupling exists.
Whichever component was in excess will react after performing this test. Once you complete the test and the
results look good, we can move to the next step.
3) Isolation and Filtration
In the isolation-filtration process, you adjust the coupling liquid for pH, salt content, volume, and
temperature. Then, filter the coupling. This process separates the liquid from the dyesalt crystals in
the coupling liquid. Sometimes the product does not isolate and the coupling goes directly into the
spray dryer, completing the drying phase.The isolation-filtration stage produces a paste the
consistency of thick mud. It contains at least 50% water, which we’ll remove by drying in the next
step.
4) Drying
A common way to dry the paste is to spread it onto trays and place it in a tray dryer. After spreading, the
trays go on a rack dryer and dry at a specific temperature. We want to avoid charring during this process, so
it’s critical to monitor this phase.Once dry, we grind it to a specific particle size (usually the consistency of
flour salt). If we used an alternative drying method, like spray drying, we can skip this step.
5) Testing the Dye
The last step is to test the dye to ensure it meets specifications for the end product. These include
concentration, shade, solubility, and often use-specific tests.Once completed, the dye is
standardized based on customers’ requests.
