Liquid carbon dioxide is already used by some dry-cleaning systems in
place of tetrachloroethene (commonly referred to as "perc"),
which is a very volatile organic compound and suspected carcinogen.
Even mild exposure can have unpleasant effects, and dry cleaners must
dispose of spent material as hazardous waste. Other solvents, including
chlorinated fluorocarbons (CFCs), are used for degreasing machine parts
and other industrial cleaning.
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|A fully automated
process using supercritical carbon dioxide has been developed for
cleaning photoresist and etching residues from semiconductor wafers.
Manufacturers normally use sulphuric acid, hydrogen peroxide and
organic solvents, and may typically generate 18 million litres of
wastewater per day.
The cleaning is done using supercritical carbon dioxide with a small
amount of propylene carbonate, an environmentally benign material.
This acts a bit like a detergent, aiding the dissolving of the residues.
A final "rinse" with pure scCO2 ensures the wafers are
spotlessly clean and dry, and both the carbon dioxide and propylene
carbonate can be re-used.
Machine Component Degreasing
often requires very thorough cleaning of metal components. This
has traditionally been done using organic solvents, but the treatment
of used solvent is an additional expense. Very thorough cleaning
can be achieved using scCO2, and equipment is now available for
In many applications materials are required in
the form of a very fine powder. This can be particularly important in
the manufacture of pharmaceutical products. The normal approach is to
re-crystallise the material from a solvent and then grind it (known
A more even result can be produced by dissolving the material in scCO2,
and then releasing the pressure rapidly from a nozzle. Very fine powders
can be made in this way.
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Much research has gone into finding alternatives to
the many volatile organic solvents in use in products such as paints.
One of the most common solvents, however, is water. It is used in very
large quantities by some industries, but cannot be discharged unless
it is free from contamination. There are several areas where scCO2 may
be introduced as an alternative.
||It is not possible to "capture"
the solvent vapour released when spray-painting large objects like
ships and aircraft. A system using scCO2 as a solvent
is now available, and has been of particular interest for military
use. It greatly reduces the quantity of VOCs released into
|Yarn sizing for woven
||High-speed fabric weaving requires
smooth yarn, and so it is usually coated (or "sized")
to achieve this, and the size is removed afterwards. The process
uses a lot of water and heat energy, but research has shown that
the size could be applied using scCO2 instead of water,
and heat is no longer needed for drying the yarn. Find
||"Dry" cleaning and industrial
de-greasing can be carried out with scCO2 and small quantities
of environmentally friendly additives. The cleaning of integrated
circuit wafers for computer use is one area where using scCO2
can reduce the amount of contaminated wastewater needing treatment.
Find out more
||The production of a fine powder with
even-sized particles is not as easy as it sounds. It is, however,
often vital for the effective absorption of drugs into the body.
The rapid de-pressurisation of scCO2 containing the dissolved
material is one way of achieving this. Follow
this link for more information
|Dyes and preservatives
||Effective penetration of dyes and
preservatives into materials is often slow and incomplete using
liquid solvents. If scCO2 is used instead, it will penetrate
as effectively as a gas, but carry the quantities of dye or preservative
normally associated with a liquid solvent, giving a better result.
Yarn used in weaving is "sized" before
weaving. The process, known as "slashing", involves coating
the yarn with a material such as starch or poly(ethanol) (polyvinyl
alcohol, PVA) to glue down loose fibres and allow it to slide more easily
during weaving. The size is washed off afterwards.
Both sizing and washing require very large quantities
of water, which must be treated to remove excess size before it can
be re-used or discharged. A water-based size also needs time, space
and energy to dry before the yarn can be used, and consequently the
machinery occupies a lot of space.
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A method has been developed using supercritical
carbon dioxide (scCO2) as a solvent for the size. This has the potential
· Reduce the energy used in the slashing process, and
the space occupied
· Produce a more even layer of size, which can therefore
· Drastically reduce the amount of water used for sizing
Key to this development is the ability to coat yarn in a rapid, continuous
process at the high pressures necessary. This has been achieved on a
laboratory scale using a specially constructed "venturi" tube,
shaped to create the pressure drop needed to precipitate the size. A
series of baffles reduce pressure towards the ends of the tube - this
is to prevent size and carbon dioxide being squirted out the ends!
The system is not yet used commercially, but several
companies have shown interest.