2. What is supercritical fluid?
Any gas that is above its
critical temperature is able to retain
the free mobility of gaseous state but if
pressure is increased its density will
tend to increase towards liquid. Such
highly compressed gases are
supercritical fluids and that is the
reason they are able to combine
properties of both liquid and gas.
3. Contd…..
The 3 main stages of matter at ordinary
temperatures and pressures are
gas, liquid and solid. The molecules in a
solid are so close together that the
forces between them hold them in a
given shape. When more energy is
added, these forces are overcome and
the substance becomes a liquid. Add
more and it becomes a gas. When both
the temperature and pressure get high
enough, liquid and gas phases become
indistinguishable and the phase is called
a supercritical fluid.
4. CO2 QUALIFICATIONS
Carbon dioxide is also considered the best
supercritical fluid for the dyeing process. It is naturally
occurring, chemically inert, physiologically
compatible, relatively inexpensive and readily available.
Other attributes of carbon dioxide are:
It is an inexhaustible resource.
Its use does not release volatile organic compounds
(VOCs).
It is biodegradable as a nutrient for plants.
There are no disposal issues. It can be recovered and
reused from the dyeing process.
It is non-flammable and non-corrosive.
It is non-toxic and low cost.
The critical point of the carbon dioxide is well within the
manageable range (31C and 73 bar).
5. Principles
Supercritical fluids are highly
compressed gases that have
properties of both a liquid and gas and
this offers advantages for textile
processing . Supercritical CO2 may act
as both a solvent and a solute.
Supercritical fluids have higher
diffusion coefficient and lower
viscosities than liquids, as well as the
absence of surface tension, allowing
better penetration into materials.
6. Dyeing process
Roll of fabric is inserted into the cylindrical
dyeing chamber on a retractable carriage. In
dyeing, CO2 is heated to 120°C and
pressurized to 250 bar.
CO2 penetrates synthetic fibres, thereby
acting as swelling agent and enhances
diffusion of dye into the fibres. In other
words, glass transition temperature of fibre is
lowered by the penetration of CO2 molecules
into polymer. This accelerates the process for
polyester by a factor of two. Finally, the CO2
is able to transport the necessary heat from a
heat exchanger to the fibre.
7. Contd…
CO2 loaded with dyestuff penetrates deep
into the pore and capillary structure of fibres.
This deep penetration provides effective
coloration of these materials which are
intrinsically hydrophobic. The process of
dyeing and the act of removing excess dye
can be carried out in the same plant.
During the dyeing, the CO2 is circulated
through a heat exchanger, through a vessel
where the dye is delivered to the textile. After
the dyeing cycle the CO2 is gasified, so that
the dye precipitates and the clean CO2 can
be recycled by pumping it back to the dyeing
vessel.
8. Auxiliaries free process
Current disperse dyes contains 40%
detergents and salts to enable the
solubilisation of hydrophobic dyes in the
water. When applying carbon dioxide, none of
these additives are required and pure
dyestuff can be used.
Another advantage, specifically for
polyester, is that under supercritical
conditions the CO2 molecules penetrate and
swell the polymer. This plasticises the fibres
and increases diffusion coefficient of dyes
inside the polyester by one order of
magnitude, relative to aqueous dyeing.
9. Contd…
In the case of cotton dyeing, fixation of
99-100% is achieved by CO2.
Developed reactive dyes can also be
used on synthetic polymers by CO2
dyeing process, enabling dye houses
to dye bends such as cotton-polyester
with a single dyestuff in a single run-
reducing process time by factor five
and achieving tremendous savings on
energy and water
10. Comparison b/w conventional
dyeing and supercritical dyeing
Conventional dyeing Supercritical dyeing in CO2
Energy requirements higher Energy requirements are only around
20%
Large quantities of waste water with
residual dye chemicals, etc.
No question of waste water, dye stays
as powder. Dispersing, levelling
agents not needed.
Dyeing, washing, drying times per
batch is generally longer.
Much shorter
Traditional Systems Newer machines
11. Advantages
Contaminated waste water streams are eliminated.
No need of dispersants for making disperse dyes water
soluble.
Lower viscosities enabling the circulation of dye
solutions easier.
CO2 causes the polymer fiber to swell slightly that gives
a faster diffusion within the polymer.
Faster penetration of voids between fibers because of
no surface tension and the miscibility of CO2 with air
under pressure.
Higher diffusiveness in the fluid resulting in faster mass
transfer in the fluid.
Supercritical CO2 is ecologically harmless, non-toxic
and nonexplosive.
No drying process involved so no drying devices
required.
