United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-94/043 May 1994
EPA Project Summary
Evaluation of Supercritical
Carbon Dioxide Technology to
Reduce Solvent in Spray Coating
Applications
Kenneth J. Heater, Alice B. Parsons, and Robert F. Olfenbuttel
Product quality, waste reduction,
and economic issues were evaluated
for a spray paint application technol-
ogy using supercritical carbon diox-
ide to replace some of the solvent in
conventional solvent-borne coatings
formulations. Product quality was
evaluated by comparing product fin-
ishes for a coating applied by conven-
tional spray with that of a similar
coating applied by supercritical carbon
dioxide (CO2) technology. Waste reduc-
tion and economics were documented
from company records and interviews
with key company personnel. The tech-
nology was found to have good poten-
tial to reduce waste without affecting
product quality.
This Project Summary was developed
by the EPA's Risk Reduction Engineer-
ing Laboratory, Cincinnati, OH, to an-
nounce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
This program was conducted by Battelle
for the Pollution Prevention Research
Branch (PPRB) of the U.S. Environmental
Protection Agency with the cooperation of
Union Carbide Corporation,* Nordson Cor-
poration, and Pennsylvania House Furni-
ture Company. The PPRB is evaluating
and demonstrating new technologies for
' Mention of tradenames or commercial products does
not constitute endorsement or recommendation for
use.
pollution prevention through the Pollution
Prevention Clean Technology Demonstra-
tion (CTD) Program.
The report summarized here reviews
the use of supercritical carbon dioxide
(CO2) technology for paint spray applica-
tion. Specifically, it describes how this tech-
nology is used by Pennsylvania House,
where supercritical CO2 coating technol-
ogy (UNICARB™) has been used for more
than a year to apply a nitrocellulose lac-
quer finish to oak and cherry furniture on
a chair-finishing line. At current Pennsyl-
vania House production rates, more than
250 furniture units per day are coated
with nitrocellulose lacquer by this process.
During the subject technology evalua-
tion, three aspects of this technology were
examined:
• Product Quality: To show that coating
applied by this spray technology
meets company standards for a quality
finish
• Pollution Prevention Potential: To
demonstrate that use of this spray
application technology for solvent
replacement in coatings reduces
volatile organic compounds (VOCs)
released during finishing operations
• Economic Ramifications: To document
the cost to install and operate this
pollution prevention technology on an
existing spray coating finish line.
In the supercritical CO2 spray process,
the solvent-like properties of supercritical
CO2 are exploited to replace a portion of
the solvent in the conventional solvent-
borne coating formulation. The addition of
supercritical CO2 acts as a diluent solvent
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oSO Printed on Recycled Paper
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to thin the viscous coating just before ap-
plication, so that the coating can be
atomized and applied with a modified spray
gun. Typically, most of the volatile, fast-
drying solvents and some of the medium-
drying solvents are eliminated; enough
medium- and slow-evaporating solvents
are retained to obtain proper leveling and
film coalescence. The remaining solvent
blend is adjusted to optimize performance
without changing the resin chemistry or
pigment-loading levels. The actual solvent-
content reduction that can be achieved is
affected by the type of coating, the de-
sired film thickness, the desired proper-
ties of the applied coating, and the
environment in which the coating is being
applied. Thermosetting, thermoplastic, air-
dry, and two-component formulations, in
clear, pfgmented, and metallic coating sys-
tems, have been developed for use with
the UNICARB™ process.
Special equipment is needed to intro-
duce the CO2 into the reduced solvent
formulations and to heat and pressurize
the resultant mixture before spraying. Typi-
cally 10% to 50% by weight CO,, may be
introduced. Usually, the coating is heated
to 40° to 70°C with spray pressures of
1200 to 1600 psi. UNICARB™ coatings
are applied with spray guns similar to those
used for airless applications. However, the
spray-gun nozzle design was slightly modi-
fied to optimize the spray pattern because
the decompression of supercritical CO,
results in finer atomization of the sprayed
coating and smaller particles than is com-
mon with use of airless spray equipment,
Pennsylvania House uses supercritical
CO, spray technology on the chair-finish-
ing line to apply nitrocellulose lacquer fin-
ishes while reducing VOC emissions'from
their finishing operation. To bring this
technology to production-line use, Penn-
sylvania House worked closely with
manufacturers and suppliers to optimize
the basic UNICARB™ process, equip-
ment, and coatings formulations. In the
conventional finishing process, two coats
of nitrocellulose lacquer (21-23% solids)
are applied manually with airless spray
equipment. The supercritical CO2 finishing
process uses only one coat of nitrocellu-
lose lacquer to achieve the desired film
build and finish quality. The nitrocellulose
lacquer formulation optimized for the su-
percritical CO2 spray system has approxi-
mately a 41% solids content.
Results and Discussion
Product Quality Evaluation
The specific objective of the product
quality evaluation was to determine
whether nitrocellulose lacquer applied by
the UNICARB™ process provided a wood
finish of equal or better quality than does
the conventional nitrocellulose formulation
and spray technique previously used by
Pennsylvania House. At Pennsylvania
House, the appearance and quality of the
final finish are judged through visual ex-
amination by inspectors on the coating
line. Special attention is given to gloss,
smoothness, and lack of surface defects
such as blisters or pinholes.
Three sets of chair-back splats were
finished on the Pennsylvania House chair
line. One set of samples was finished
using the one-coat UNICARB™ process,
and the other two sets were finished us-
ing one and two coats (respectively) of
the old nitrocellulose formulation and the
airless spray equipment still in place on
the chair-finishing line. Liquid samples of
the conventional nitrocellulose lacquer and
of the reformulated nitrocellulose lacquer
were collected for laboratory analysis.
Product quality was evaluated through
independent evaluations performed by
Pennsylvania House staff members and
coatings experts in the Battelle Polymer
Center along with a group of other Battelle
staff members making up a panel consid-
ered representative of the consumer mar-
ket. Nine chair-back splats (three sample
sets) were prepared by Pennsylvania
House staff on the chair-finishing line dur-
ing a site visit by Battelle staff. All panels
were finished by the same production
methods that typically are used on the
chair line at Pennsylvania House.
The product quality evaluation demon-
strated that a coating applied by the
supercritical CO, spray process yielded a
product with a finish quality equal to or
better than the finish quality obtained by
conventional methods. Splats finished by
the supercritical CO2 process and by the
conventional process were rated as ac-
ceptable by all three groups. The splats
sprayed with one coil coat by the conven-
tional process were not acceptable. These
results are supported by Pennsylvania
House records for consumer acceptance.
Based on the number of furniture units
requiring rework because of finish defects,
it appears that production efficiency has
improved since the UNICARB™ process
was implemented.
Pollution Prevention Potential
The pollution prevention potential of this
technology is based on reducing emission
of organic solvents without adding to other
wastestreams. In this evaluation, the pol-
lution prevention potential of the
UNICARB™ process was determined by
carefully considering all wastestreams. The
nitrocellulose lacquer finishing process
used on the chair line can contribute to
pollution in two ways: VOC emissions from
the coating formulation, and spray-booth
wastes, including solvent-laden filters and
nitrocellulose dust. In conventional spray
coatings, a blend of fast-evaporating sol-
vents, medium-evaporating solvents, and
slow-evaporating solvents is used. In the
supercritical CO2 spray process, most of
the fast- and medium-drying solvents are
replaced by supercritical CO2 and the slow-
drying solvents are adjusted slightly for
better film formation. Although reducing
VOC emissions is important, it is equally
important to demonstrate that the super-
critical CO2 process does not add pollut-
ants to other wastestreams.
Pennsylvania House records indicate
that it takes approximately 16 oz of the
conventional formulation to apply the two
coats needed to achieve the desired qual-
ity in the finished product. The UNICARB™
process required about 7 oz of the re-
duced solvent formulation per furniture unit
to achieve the same quality. There are
two reasons that a smaller volume of coat-
ing is required when the supercritical CO2
process is used: (1) the higher solids-
content of the UNICARB™ formulation
means that more resin is transferred to
the substrate per volume of formulation
sprayed and (2) the increased viscosity of
the film deposited by the process inhibits
film buildup by soaking into the wood sub-
strate.
The Material Safety Data Sheets (MSDS)
data for each of the formulations indicate
that the UNICARB™ coating is formulated
using 16.7% less solvents (on an absolute
basis) than the conventional formulation.
Only 9.67% of the UNICARB™ formulation
is comprised of Hazardous Air Pollutants
(HAP) materials, compared to 30.46% for
the conventional formulation. On a per-
gallon-of-coating-sprayed basis, this would
result in a relative decrease in VOC emis-
sions of 22.81%, with a 68.25% decrease
in HAPs using the UNICARB™ formula-
tion. VOC contents are reported as 4.7 Ib/
gal for the UNICARB™ formulation and
5.9 Ib/gal for the conventional system.
Assuming an average yearly production
of 50,000 units and the use of 7 oz for the
one-coat UNICARB™ process and 16 oz
for the two-coat conventional formulation,
the UNICARB™ formulation needs 2734.4
gal and the conventional formulation needs
6495.5 gal to finish the units. Using the
VOC content reported on the MSDSs, this
corresponds to an annual reduction in VOC
emissions of 67.5% when the newer pro-
cess is used. Even if the VOC content of
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the formulations were the same, an an-
nual reduction in VOC emissions of 57.9*&
would be achieved because of the de-
creased amount of formulation needed per
unit with the UNICARB™ process to
achieve the same quality of finish.
Supercritical CO2 is used in the
UNICARB™ process to decrease VOC
emissions. The reduced solvent
UNICARB™ formulation used by Penn^
sylvania House requires approximately
2.43 Ib of CO2 for every gallon of coating
concentrate sprayed with the UNICARB™
process. The amount of CO2 released an-
nually into the atmosphere can be de-
termined based on annual usage of the
UNICARB™ formulation. Using 2734.4 gal
for an annual production of 50,000 units
as a basis, the annual emission of CO2
from the finishing process is expected to
be 6645 Ibs (2.43 Ib/gal x 2734.4 gal).
Carbon dioxide is not being produced
through use of the UNICARB™ process.
It is simply being used as a substitute
solvent to thin and aid in the spray atorrii-
zation process. The CO2 used in this tech-
nology is supplied by various distributors
of CO2 which obtain CO2 as a by-prpdu'ct
of other chemical processes. Thus, ''CO.
used in processes such as the supercritical
CO2 method of applying coatings does not
actually contribute to the emission of ad-
ditional CO2 into the atmosphere.
Coating overspray at Pennsylvania
House is collected on dry filters that are
compressed and stored in 55-gal drums
for disposal by landfill. Waste products
include dry and solvent-laden filters and
nitrocellulose dust, both loose and trapped
in the filters. The solid waste was not
increased or decreased by implementing
the supercritical CO2 technology.
The pollution prevention analysis indi-
cates that VOC emissions are reduced
when the supercritical CO2 spray process
is used. The only new by-product of the
process introduced into the wastestream
is CO,, but market information indicates
the CO2 sold commercially is itself a by-
product of other production processes.
Economic Analysis
The objective of the economic analysis
was to determine the payback period for
the switch to the supercritical CO2 pro-
cess from the previously used conven-
tional system. The initial investment in
capital equipment and installations costs
were considered along with operating costs
(materials, waste disposal, labor, and utili-
ties). A return on investment (ROI) was
calculated, based on the costs associated
with capital expenditures, including equip-
ment and installation and the return gen-
erated through lower personnel, operating,
and materials costs.
The initial investment for the
UNICARB™ process was $58,000, of
which $46,000 was for equipment pur-
chase and $12,000 for installation of the
equipment. The operating costs were
based on production of 50,000 chairs per
year. The UNICARB™ process costs of
$45,546 included $35,848 for the coatings
formulation and $9,698 for the CO2 equip-
ment rental and concentrate. The conven-
tional formulation cost was $46,883. By
converting from a two-coat process to the
one-coat UNICARB™ process, Pennsyl-
vania House was able to decrease its
utility costs by $11,000 because there was
one less booth to operate and labor costs
by $46,000 because one less finisher and
one less sander were needed. No change
was assumed for line waste handling and
disposal costs or for finishing line mainte-
nance.
The economic evaluation demonstrated
a positive return on investment after the
first year, with a total payback period within
3 years if gas utility savings are included,
and 5 years if gas utilities are not in-
cluded.
This analysis reflects the economics of
the actual operation in use on the chair
line at the time of this evaluation. Imple-
menting the UNICARB™ finishing process
on the chair line at Pennsylvania House
resulted in substantial annual savings in
both utilities and labor. Cost savings could
be realized from a decrease in raw mate-
rials costs, but these savings are offset by
the leasing fees for the CO2 tank and
pump at Pennsylvania House. Additional
savings could be realized by decreasing
the size of the existing ovens to reflect the
change to a one-coat system.
Conclusions
This evaluation shows that supercritical
CO2 spray technology has potential as a
waste-reduction option in application of
solvent-borne coatings. The wood furni-
ture facility where this evaluation was con-
ducted maintained product quality with a
nitrocellulose lacquer finish and reduced
VOC emissions from the coating process.
No additional waste entered the
wastestream. A 100% ROI should be
achieved in 5 years after implementation.
This supercritical CO2 technology is not
limited to one coating type but could be
used to reduce the solvent level required
to spray apply a variety of solvent-borne
coatings.
The full report was submitted in fulfill-
ment of Contract No. 68-CO-0003, Work
Assignment 3-36, by Battelle Memorial In-
stitute under the sponsorship of the U.S.
Environmental Protection Agency. It cov-
ers work done from November 1992
through September 1993.
•frV.S. GOVERNMENT PRINTING OFFICE: MM - 550-067/8023*
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Kenneth J. Heater, Alice B. Parsons, and Robert F. Olfenbuttel are with
BatteUe, Columbus, OH 43201.
Paul Randall Is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of Supercritical Carbon Dioxide
Technology to Reduce Solvent in Spray Coating Applications," (Order No.
PB94-160629; Cost: $19.50, subject to change) will be available only
from:
National Technical Information Service
5285 Port Royal Road
Sprlngfield,VA22161
Telephone: 703-487-4650
The EPA Project Off leer can be.contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
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EPA
PERMIT No. G-35
EPA/600/SR-94/043
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