Eco Friendly In Textile Wet
Processing
By
Aravin Prince Periyasamy
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Supercritical
Carbon Dioxide
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What is supercritical carbon
dioxide?
A supercritical fluid is a substance above its
critical temperature and critical pressure.
Under
these
conditions
the
distinction
between gases and liquids does not apply
and the substance can only be described as
a fluid.
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Supercritical Fluid of CO2
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Above 31.10C and 73 atm carbon
dioxide behaves as a supercritical fluid
and shows properties of both a liquid
and a gas
It fills the container, like a gas, and
dissolves substances like a liquid.
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Supercritical fluid of CO2
With
an
increase
in
temperature the meniscus
begins to diminish.
The liquid density falls due
to expansion and the gas
density rises as more of the
substance evaporates.
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Supercritical fluid of CO2
Once
the
critical
temperature and pressure
have been reached the two
distinct phases of liquid and
gas are no longer visible.
The meniscus can no longer
be seen. One homogenous
phase
called
the
"supercritical fluid" phase
occurs.
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Industrial extraction with
supercritical CO2
Decaffeinating coffee and tea
Extracting bitterness to make beer
Defatting cocoa powder
Extracting spices and aromatic plants
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Significant properties of
supercritical fluids
Diffusivity
gas > supercritical fluid > liquid
Viscosity
gas < supercritical fluid < liquid
Surface tension gas = supercritical fluid < liquid
Density
gas < supercritical fluid < or =
liquid
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Benefits of using supercritical CO2 for
extraction
It is having the penetration power of a gas
and the extraction power of a liquid.
It acts as a solvent to extract organic
compounds with a low molecular weight.
It leaves no residual trace on treated
material
It respects the environment.
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Supercritical CO2 as solvent
Purification of specialty chemicals and
useful natural products
An alternative solvent in coatings industry
(replacing 40 - 90% of volatile solvents)
Degreasing and dry cleaning applications
Increasingly applied as a solvents in
synthetic industrial processes
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Use of supercritical CO2 in dyeing processes
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Use of supercritical CO2 in dyeing processes
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Enzymes
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Electron-ray treatment
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Microwaves
Microwaves are electromagnetic waves whose
frequency ranges from 1000MHz to10,00,000
MHz.
Microwave dyeing takes into account only the
dielectric and the thermal properties.
The dielectric property refers to the intrinsic
electrical properties that affect the dyeing by
dipolar rotation of the dye and influences the
microwave field upon the dipoles.
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The aqueous solution of dye has two components
which are polar, in the high frequency microwave
field oscillating at 2450MHz. It influences the
vibrational energy in the water molecules and the
dye molecules.
The
heating
mechanism
is
through
ionic
conduction, which is a type of resistance heating.
Depending on the acceleration of the ions through
the dye solution, it results in collision of dye
molecules with the molecules of the fiber.
This helps and affects the penetration of the dye
and also the depth to which the penetration takes
place in the fabric. This makes microwave superior
to conventional dyeing techniques
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Ultrasonic Treatments
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Electrochemical dyeing
Vat and sulphur dyeing involves both a reducing and an
oxidising step, which are carried out with chemical oxidants
and reducing agents. The environmental concerns associated
with the use of these chemicals are many. An attractive
alternative technique is to reduce and oxidise the dye by
means of electrochemical methods.
With direct electrolysis the dye itself is reduced at the surface
of the cathode. In indirect electrolysis the reducing power of
the cathode is transferred to the solution by a soluble
reversible redox system (e.g. based on antraquinone chemistry
or iron complexes). With this reversible redox system the
reducing agent is continuously regenerated at the cathode,
which thus allows full recycling of the dye bath and the
reducing agent.
Direct cathodic reduction in an electrochemical cell is
applicable to sulphur dyes. Vat dyes are reduced by indirect
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electrolysis.