United States
Environmental Protection
Agency
Atmospheric Research and J*''
Exposure Assessment Laboratory -^ ,
Research Triangle Park NC 27711 »,,
Research and Development
EPA/600/S3-89/032 Sept. 1989
&ER& Project Summary
Paint Coatings: Controlled
Field and Chamber Experiments
Edward O. Edney
To determine the impact of pollu-
tion levels on the weathering rates of
coatings, laboratory chamber experi-
ments and controlled field exposures
at North Carolina and Ohio sites were
conducted in such a manner to sepa-
rate the contributions due to dry dep-
osition, wet deposition, precipitation
pH, etc. The results of these studies
confirm that acidic gases such as
SO2 and HNO3, as well as acids with-
in rain, promote the dissolution of al-
kaline components including CaCO3,
ZnO, and Al flake from paint films. It
is unclear from these studies wheth-
er the removal of these components
reduces the service life or protective
properties of the paint film. Other re-
searchers within the Coatings Effects
Program are conducting subsequent
analyses to determine micro-damage
of these paints. The uptake of acidic
gases to painted surfaces is a com-
plex process that depends on several
factors. The deposition rate of SO2 to
a wet, painted surface may be con-
trolled by the level of oxidants such
as H2O2.
This Project Summary was devel-
oped by EPA's Atmospheric Research
and Exposure Assessment Laboratory,
Research Triangle Park, NC, 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
Because of the high volume of paint
used in the United States, any decrease
in service life by pollution effects could
result in significant economic loss. Deter-
mining the impact of pollution levels on
weathering rates requires the develop-
ment of models that relate the amount of
damage to environmental factors. The
construction of such models requires un-
derstanding: (1) the wet and dry de-
position of pollutants to painted surfaces,
(2) interactions between deposited com-
pounds and interactions of these species
with components of the paint film and the
substrate, and (3) the relationships be-
tween reactions on a molecular scale and
macroscopic damage. To address some
of these issues we have developed lab-
oratory and field systems capable of ex-
posing materials to complex air mixtures
but in a manner such that the separate
contributions due to dry deposition, wet
deposition, precipitation pH, etc., can be
evaluated.
Project Objectives
1. To conduct controlled laboratory ex-
periments to determine the compo-
nents of a typical urban smog mixture
that deposit to painted surfaces in the
absence and presence of surface
moisture.
2. To conduct controlled field studies to
determine the separate effects of dry
deposition, wet deposition, and pre-
cipitation pH on the damage rates of
paint under ambient conditions.
Technical Approach
Laboratory Studies: Laboratory studies
were conducted using an exposure sys-
tem that consists of two parallel exposure
chambers and a smog chamber that is
operated as a continuously stirred tank
reactor and serves as a reservoir for the
exposure chambers. The flow rate
through the 11.3-m3 smog chamber is
100 Lpm, whereas turbulent conditions
are generated in the exposure chambers
by circulating air through the 26-L cham-
bers at a flow rate of 2500 Lpm. Each
exposure chamber has a chiller back
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plate that can be chilled below the air
dew-point to generate dew that, if pro-
duced in a sufficient quantity, can be
collected and analyzed. The air dew-point
is regulated using a computer-controlled
steam system. Hydrocarbon concen-
trations are measured using gas cnro-
matography; and NOX, NO, 03, and S02
levels are determined using commercial
instruments. Gas-phase and particulate
N03-, as well as SO4= concentrations,
are measured by ion chromatography.
The dew samples are analyzed using the
technique described below for runoff
analysis.
Controlled Field Studies: To separate
the effects of dry and wet deposition on
weathering of paints, covering/spray sys-
tems were designed and installed in Re-
search Triangle Park, NC, a relatively un-
polluted site, and in Steubenville, OH, a
site with high levels of S02. The auto-
mated systems are used to expose ma-
terials under the following conditions: (1)
dry deposition only, (2) dry plus ambient
wet deposition, and (3) dry deposition
plus deionized water (D1). Each system
consists of movable and stationary racks,
a shelter, moisture-sensing information,
and a D1-spray system.
At the onset of precipitation, the mova-
ble racks automatically are brought under
the shelter to avoid ambient precipitation.
The racks remain inside as long as the
moisture sensor is wet. Once the sensor
dries, the racks are returned to the ambi-
ent-exposure condition. The operation of
the D1-spray system at the Ohio site
differs from that at the North Carolina
site. The North Carolina system is com-
puter-controlled and the test panels,
located under the sheltered spray sys-
tem, automatically are sprayed for a fix
period of time (~50 s for each 0.13 cm of
ambient precipitation that accumulates in
the exposed tipping bucket). At the Ohio
site, the panels are sprayed after the
completion of the precipitation event.
At each site, runoff collectors are lo-
cated at some of the test-panel positions.
Runoff samples are collected on an event
basis and undergo detailed chemical
analyses. Each runoff sample is analyzed
for Na + , NH4 + , K*, HCOO", Cf, NO3',
HS03-/S03 = , and SO4= by ion
chromatography and for Zn and Ca by
atomic absorption spectroscopy. In
addition, the volume and pH of each
sample are determined.
For both the runoff and laboratory ex-
periments, it is convenient to express the
aqueous concentrations in terms of runoff
concentrations Rj and runoff rates F\, that
is,
R.=
R
.
1 t.
N
(2)
(3)
(4)
where i denotes the precipitation or dew
event; t, is the exposure time for the ith
event; V( is the aqueous volume col-
lected; A is the surface area; R is the ac-
cumulated runoff concentration through N
events; and F is the average runoff rate.
The paints investigated included latex
paints with and without CaC03 (latex and
iatex-c, respectively), an oil-based paint
with ZnO and CaC03 (oil-cz), and a
maintenance oil-based paint (oil). Each of
the white paints employed Ti02 as the
pigment. For the laboratory experiments,
latex, Iatex-c, and oil-cz were applied to
galvanized steel substrates, whereas for
the field studies, red cedar was used for
the substrate. A steel substrate was used
for the oil paint for both applications.
The laboratory experiments consisted
of exposing films in the absence and
presence of surface moisture to a series
of irradiated C3H6/NOX/SO2 mixtures in
air, where the only parameter varied was
the SO2 level. The objective of the ex-
periments was to determine the impact of
incremental changes in the S02 level on
the dissolution rates of alkaline paint
components. Exposure experiments were
conducted for S02 levels of 0, 9, 18, 25,
50, 82, 134, 193, 326, 396, 534, and 722
ppb. Approximate steady-state concen-
trations for other compounds were O3,
230 ppb; NOX, 180 ppb; HCHO, 380 ppb;
and HN03, 7 ppb. Each experiment con-
sisted of a 21-h exposure where thin films
of moisture were generated during the
last 4 h in one of the chambers. Another
set of panels remained dry throughout
the exposure. At the completion of each
experimental run, both the dry and dew
panels were rinsed with 10 mL of D1 and
the rinse was analyzed chemically.
The North Carolina paint field exposure
experiment began March 15, 1988, and
the results reported here cover the time
period through November 6, 1988. The
exposure results at the Ohio site cov<
the time period between July 25, 198
and October 25, 1988.
Results
The accumulated runoff concentratior
for selected compounds for the entire s<
quence of laboratory S02-exposure e;
periments are shown in Table 1. Resul
of the analysis of the runxoff from ga
vanized steel panels also are included
Table 1 for comparative purposes. Figui
1 shows the SO4 = runoff concentratic
for dew panels as a function of SO2 coi
centration. The D1-spray and ambiei
runoff rates for the North Carolina ar
Ohio field studies are presented in Table
2 and 3.
Discussion
The laboratory runoff results show tr
dew samples consisted of complex mi
tures of ions whose composition d<
pended upon both the reactivity of tf
material and the moisture condition. Tf
deposition of acid gases such as HNC
and SO2 led to the dissolution of CaCC
in Iatex-c, ZnO and CaCO3 in oil-cz, ar
to Zn corrosion productions on ga
vanized steel. The presence of Zn in tt
Iatex-c and latex samples suggests th
Zn from the substrate was leache
through the latex paints. The domine
cation in the oil paint runoff was H + , a r
suit consistent with the lack of alkalii
compounds in the paint. The effect
surface moisture on NO3" precurso
(e.g., HNO3) was dependent on the cor
position of the coating. The uptakes
NO3" precursors in complex air mixtun
to dry panels were larger for latex, late
c, and galvanized steel than for the d«
panels, whereas the situation w,
reversed for oil and oil-cz. The uptake
SO2 appears to be a function of surfa
reactivity: The largest deposition to
place on galvanized steel, while tl
lowest value occurred for oil.
Figure 1 suggests that the uptakes
SO2 to moisture-covered, relatively in<
surfaces such as oil, oil-cz, or latex we
not directly proportional to the S02 lev
The uptakes may have been control!
by the level of available oxidants such
H2O2, a situation similar to that cc
trolling the acidification of cloud wat
However, for more alkaline surfaces su
as Iatex-c and, in particular, galvaniz
steel, the absorption rate is proportioi
to the S02 level.
The North Carolina and Ohio runi
results are consistent with the laboratc
results in that the dry and wet deposit!
of acid gases dissolves CaCO3 in late)
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Table 1.
Sample
Latex
Latex
Latex-c
Latex-c
Oil
Oil
Oil-cz
Oil-cz
G-stee/c
G-steel
Accumulated Runoff Concentration for Laboratory Incremental S02 Experiments R(nmole/cmz)
Condition Vol-ma
Dew
Dry
Dew
Dry
Dew
Dry
Dew
Dry
Dew
Dry
1.89
0.00
2.06
0.00
1.57
0.00
4.26
0.00
5.99
0.00
H*
4.6
18.2
13.8
3.0
144.4
4.9
38.6
4.3
21.2
2.1
Ca
2.4
2.6
357.0
255.5
3.2
0.8
109.6
40.6
Zn HCOO'
74.9
31.6
59.6
30.8
37.2
9.4
171.2
50.0
1.7 2277.6
0.9
425.3
47.5
2.3
123.6
7.0
55.2
2.7
105.7
4.0
704.1
7.7
NO3' HSO3'
108.3
129.8
161.1
497.8
120.1
22.6
306.6
147.2
517.2
764.1
12.7
0.0
368.6
0.0
0.0
0.0
13.4
0.0
2235.6
0.0
SCV HCHO
32.5
4.8
86.8
7.3
96.8
4.4
100.7
5.2
692.0
46.1
53.7
ND*
533.6
ND
164.9
ND
249.7
ND
3368.1
ND
* Total volume of dew collected.
o No data.
" Galvanized steel.
Table 2.
Sample
Latex
Latex
Latex-c
Latex-c
Oil
Oil
Oil-cz
Oil-cz
North Carolina Average Runoff Rates
Condition
Dl
Ambient
Dl
Ambient
Dl
Ambient
Dl
Ambient
H*
0.28
4.78
0.13
1.29
0.49
4.16
0.88
1.32
Na +
2.09
3.02
1.67
3.36
0.44
0.85
0.35
0.69
K +
1.14
1.78
0.74
2.11
0.27
0.93
0.87
0.34
for Selected Compounds R(nmole/cm2-day)
Ca
0.59
2.12
4.25
11.75
0.63
0.83
1.19
3.63
Zn
ND
NO
ND
ND
ND
ND
0.64
1.98
HCOO'
0.24
0.98
0.29
0.94
0.06
0.46
1.42
0.44
cr
0.35
7.29
4.36
10.17
1.01
1.68
1.23
2.70
N03'
1.22
573
1.80
6.91
0.79
2.22
1.04
4.19
SO« =
1.22
5.17
1.46
6.45
0.59
2.70
0.52
3.37
Table 3. Ohio Average Runoff Rates for Selected Compounds R(nmolelcm2-day)
Sample Condition H+ Na+ K + Ca Zn HCOO' Cl~
N03
Latex
Latex
Latex-c
Latex-c
Oil
Oil
Oil-cz
Oil-cz
Dl
Ambient
Dl
Ambient
Dl
Ambient
Dl
Ambient
0.10
5.40
0.08
1.91
0.25
3.50
0.14
1.59
3.43
0.78
4.23
0.85
5.11
0.60
2.42
0.87
0.73
0.90
0.85
1.03
0.68
0.69
0.33
0.76
5.18
15.15
6.94
24.14
3.36
13.69
3.24
1996
ND
ND
ND
ND
ND
ND
0.99
3.47
1.41
0.07
1.47
0.07
1.78
0.07
1.17
0.09
6.38
6.67
6.36
8.50
7.42
4.55
4.36
547
1.06
11.02
1.22
13.01
0.80
9.60
0.76
11.46
4.22
19.15
4.73
24.31
3.34
15.38
2.90
20.50
and oil-cz and Zn compounds in oil-cz
and galvanized steel. In general, the
amounts dissolved at the Ohio site were
larger than the corresponding values in
North Carolina, a result qualitatively con-
sistent with the higher pollution levels in
Ohio. The presence of large amounts of
Ca in the Ohio samples raises the
question as to whether neutralization of
acid gases by dry-deposited alkaline par-
ticles occurred in Ohio.
Conclusions
The following conclusions are based on
results reported here and those that have
been previously reported in the literature.
1. Laboratory experiments suggest an
field studies confirm that dry deposi-
tion of acid gases such as S02 and
HNO3 dissolve alkaline compounds in
paint films. Field studies also show
that wet-deposited acids in precipita-
tion produce additional dissolution.
Alkaline compounds susceptible to
acidic deposition include CaCO3,
ZnO, and Al compounds in aluminum
flake paints.
2. The uptake of acid gases to painted
surfaces is a complex process that
depends upon the moisture condition,
surface reactivity, permeability of the
coating, and reactions with other
compounds deposited on the surface.
Those painted surfaces that react
readily with deposited acids tend to
more readily absorb these at-
mospheric acids than the more nert
surfaces.
Laboratory and field studies suggest
that factors other than just the SO2
level are important in determining the
deposition rates of S02 to wet,
painted surfaces. The levels of oxi-
dants such as H202 could be a con-
trolling factor in the deposition
process.
The leaching of ZnO, a mildewcide,
is a form of damage, but it is not
clear that removal of extenders such
as CaC03 constitutes a process that
reduces the paint service life. The
most significant pollution-induced ef-
fect may be those processes where-
by gaseous pollutants are absorbed
into the film and alter polymer
oxidation processes, possibly chang-
ing the protective properties.
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G-Stee//5
20 -
10
Latex-c
100
200
300 400 500
SO2 Concentration (ppb)
Figure 1. SO4 = Runoff concentration as a function of SO2-
600
700
800
The EPA author, Edward O. Edney, is with the Atmospheric Research and
Exposure Assessment Laboratory Research Triangle Park, NC27711.
The complete report, entitled "Paint Coatings: Controlled Field and Chamber
Experiments,' (Order No. PB 89-189 8491'AS; Cost: $13.95, subject to change) will
be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA author can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-89/032
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