Gases Chemistry Worksheet - Chapter 13, An Introduction To Chemistry Page 32
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49514
Chapter 13
Gases
A Greener Way to Spray Paint
S
T
PECIAL
OPIC 13.1
is 31 °C. Above
United States industries use an estimated 1.5 billion
The critical temperature of CO
2
liters of paints and other coatings per year, and much
this temperature, carbon dioxide can be compressed
of this is applied by spraying. Each liter sprayed from
to a very high pressure and relatively high density
a canister releases an average of 550 grams of volatile
supercritical fluid. Like a liquid, the supercritical carbon
organic compounds (VOCs). Some of these VOCs are
dioxide will mix with or dissolve the blend of coating
hazardous air pollutants.
and low volatility solvent to form a product that is thin
The mixture that comes out of the spray can has two
enough to be sprayed easily, in very small droplets.
kinds of components: (1) the solids being deposited
The supercritical CO
has a very high volatility, so it
2
on the surface as a coating and (2) a solvent blend that
evaporates from the droplets almost immediately after
allows the solids to be sprayed and to spread evenly. The
they leave the spray nozzle, leaving a mixture that is
solvent blend must dissolve the coatings into a mixture
thick enough not to run or sag when it hits the surface.
The mixture is sprayed at temperatures of about 50 °C
that has a thin enough consistency to be easily sprayed.
But a mixture that is thin enough to be easily sprayed
and a pressure of 100 atm (about 100 times normal
will be too runny to remain in place when deposited
room pressure).
on a surface. Therefore, the solvent blend contains
Because carbon dioxide is much less toxic than the
additional components that are so volatile they will
VOCs it replaces and because it is nonflammable and
evaporate from the spray droplets between the time the
relatively inert, it is much safer to use in the workplace.
spray leaves the spray nozzle and the time the spray hits
It is also far less expensive. Moreover, because the
the surface. Still other, slower-evaporating components
CO
can be obtained from the production of other
2
do not evaporate until after the spray hits the surface.
chemicals, the new process does not lead to an increase
They remain in the coating mixture long enough to
in carbon dioxide in the atmosphere. In fact, since the
cause it to spread out evenly on the surface. Since the
VOCs that it replaces can form CO
in the atmosphere,
2
more volatile solvents have escaped, the mixture that hits
the supercritical fluid spray process actually leads to a
the surface is thick enough not to run or sag.
decrease in atmospheric CO
levels compared to what
2
The Clean Air Act has set strict limitations on the
they would be with the traditional spray process.
emission of certain VOCs, so new safer solvents are
Source: M. D. Donohue, J. L. Geiger, A. A. Kiamos, and K. A.
needed to replace them. One new spray system has
Nielsen, “Reduction of VOC Emissions During Spray Painting.”
In Green Chemistry; ed. P. T. Anastas and T. C. Williams (American
been developed that yields a high-quality coating while
Chemical Society: Washington DC, 1996), pp. 153-167.
emitting as much as 80% fewer VOCs of all types and
none of the VOCs that are considered hazardous air
pollutants. This system is called the supercritical fluid
spray process. The solvent mixture for this process still
contains some of the slow evaporating solvents that
allow the coating to spread evenly but replaces the fast
evaporating solvents with high pressure CO
.
2
Some gases can be converted to liquids at room
temperature by being compressed into a smaller volume,
but for each gas, there is a temperature above which
the particles are moving too fast to allow a liquid to
form no matter how much the gas is compressed. The
temperature above which a gas cannot be liquefied is
called the critical temperature. The very high pressures
that are possible for a gas above its critical temperature
allow the formation of gas systems in which the density
By taking advantage of the properties of gases
approaches the densities of liquids. At this high pressure
at high temperatures and pressures, scientists
and density, the substance takes on characteristics of
have invented a new, environmentally friendly
both gases and liquids and is called a supercritical fluid.
spray-painting.
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