EPA/600/A-92/237
A STUDY OF THE EFFECTS OF ACIDIC POIXUTATSTTS
ON AUTOMOTIVE FINISHES
Naraporn Rungsimuntaku!, Douglas White, Raymond Fornes,
Richard Gilbert, Chunshan Zhang
Physical and Mathematical Sciences Research
North Carolina State University
P.O. Box 8209, Raleigh, N. C. 27695
John Spence
Atmospheric Research and Assessment Laboratory
United States Environmental Protection Agency
Research Triangle Park, N. C. 27711
ABSTRACT
Automotive finishes of various compositions on metal substrates were exposed vertically in a
smog chamber to UV and acidic atmospheres that were generated from combinations of SC>2, NO,
propylene, water and air. Dews of different compositions were generated and collected for spot
testing. Spot tests were performed by placing drops (IOOjiL) of dews on the surfaces of the paints
and heating in an air-circulating oven at 90 °C for 24 hours. Visual observation, reflection optical
microscopy, profilometry, SEM and EDS were used to examine surface damage. Various degrees
of damage occurred depending upon the dew composition and surface properties. In general, the
damage areas were in the form of rings with diameters smaller than the original drop. After rinsing
and buffing, the damage was still visible. Microscopy and SEM revealed that the rings consisted of
numerous small areas of damage and that swelling, pitting blistering and cracking had occurred.
EDS showed aluminum and sulfur at the damage surface, while the surrounding area did not Since
the base coat contained A1 flakes, this suggested that the acidic dew had penetrated through the top
coat into the base coat
INTRODUCTION
Environmentally-related damage to automotive finishes (paint coatings on metal substrates)
has been observed. Weather and acid resistance of materials have concerned coatings and
automotive manufacturers and government agencies. Paints exposed to acidic pollutant atmospheres
are typically subjected to UV light, oxygen/ozone, temperature and humidity. Understanding the
damage characteristics and the influencing factors and mechanisms of photodegradation of materials
in an acidic environment will provide highly useful information and guidelines, possibly leading to
the development of the acid resistant materials.
Paint damage can occur at the macroscopic level, such as darkening or fading of the
pigments, decrease in gloss, chalking12 pitting, blistering, peeling and cracking2*3- It can also
occur at the microscopic level, such as increased crosslinking, adsorption or film component
dissolution2.
McEwen et al.1 studied accelerated weathering of automotive paint using xenon-arc and
quartz filtered ultraviolet light and compared the results with materials exposed to Florida sunlight.
They used gloss meter analysis and attenuated total reflectance infrared spectrometry (ATR-IR). The
ATR-IR gave useful information about the degradation and indicated that some surface oxidation
occurred. However, the paint film had to be peeled away from the metal substrate in order to be
characterized using the ATR-IR.
Simson and Moran2 discussed the effects of UV radiation and oxygen/ozone on the
degradation of the polymer films. They stated that UV excitation of sufficient energy caused
excitation of oxidants resulting in free radical formation. Free radical reaction with polymer films
could induce crosslinking or chain scission of the polymer. If the backbone of a polymer chain were

-------
disrupted, significant degradation could result. They also discussed the effect of the presence of
moisture and stated that it could result in some swelling of the polymer film and lead to increased
permeation of pollutants.
Campbell et al. used six techniques to assess air-pollution damage to three kinds of paint on
steel: alkyd industrial maintenance paints, coil coating finishes and automotive refinishes (titanium
in nitrocellulose/acrylic).4 The techniques used were: erosion rate, ATR-IR, gloss and sheen,
surface roughness, tensile strength and scanning electron microscopy (SEM). They concluded that
the combination of the gravimetric erosion rate, ATR-IR and SEM analysis gave the most useful
information in assessing the damage. SEM micrographs showed that the automotive refinish
exposed for seven months at Chicago, Illinois and Leeds, North Dakota were unaffected, but that
the paints exposed to 1 ppm of SO2 had slighdy greater surface roughness. The erosion rate
appeared to be linearly related to (SO2 or O3) concentration.
White and Rothschild3 found dark spots of several millemeters in diameter on cars exposed
in Israel. Some spots were in the form of shallow pits in the paint films. They reproduced the
appearance of the spots by leaving drops of strong nitric and sulfuric acid on the paint for some
time.
Wolff et al.5 studied the damage to automotive finishes exposed outdoors in Florida
(subjected to heat, UV radiation, wind, rain, dew and air pollution) for five weeks. The damage
occurred as circular, elliptical or irregular spots that appeared as deposits or precipitates. They
summarized that wetting events such as rain and dew were a prerequisite for damage to occur. The
damage was enhanced without exposure to additional wetting events when the deposits remained on
the test panels for several days while exposed to daytime heating, sunshine and cooling. Their
electron dispersive spectroscopy (EDS) data showed that the precipitate associated with the ringlet
shaped spots was composed mainly of calcium sulfate (CaSC^). When the surface was washed,
most of the CaSC>4 was removed but the surface remained scarred.
Simson and Moran reviewed Edney's work (EPA report 1988) on an oil-based maintenance
paint, which they exposed in a smog chamber containing mixtures of C3H^/NOx/SO2, under wet
and dry conditions for 21 h.2 The pollutant concentrations used were: 722ppb of SO2, 230 ppb of
O3 , 180 ppb of NO* , 380 ppb of HCHO and 7ppb of HNO3 . The results indicated that this paint
was fairly inert at this exposure condition.
The extent of damage to polymer films caused by acidic pollutants depends on many factors
such as the type and concentration of pollutants, the exposure temperature and time and the presence
of UV radiation, ozone and humidity. Jellinek et al. reported that chain scission took place in
poly(methyl methacrylate) when the polymer was exposed to UV/O2 and UV/O2/SO2.6 Sankar et
al.7 reported the results of elemental analysis, x-ray photoelectron spectroscopy (XPS) and 13C
magic angle spinning nuclear magnetic resonance (13C MAS NMR) studies of a UV/SO2/H2O
exposed acrylate copolymer and a terpolymer of n-butyl acrylate, vinyl acetate and vinyl chloride
films. There was clear evidence of the incorporation of sulfur into the polymers and there was
significant loss of the acetate group in the terpolymer. The results suggested a synergistic interaction
between SO2 and UV leading to rapid degradation.
EXPERIMENTAL
We developed our spot test to simulate the outdoor damage process of automobile paint
exposed to precipitation followed by heating from the sun. In this study we exposed automotive
finishes in a smog chamber for one week, followed by dew exposure in the form of a spot test at
90 °C for 24 h. (Note that the outdoor temperature of the automobile body can surpass 90 °C6.)
The spot test was performed by placing 100 |iL drops of the dews onto the surface of the paints,
followed by heating the samples in an air circulating oven at 90 °C for 24 h. The samples were
analyzed using reflection light microscopy, profilometry, SEM and EDS.
Samples
2

-------
The paint samples' dimensions were 10 cm x 15 cm x 0.058 cm. They were prepared in a
manner that is consistent with the method of the preparation of automobile paint coatings. Two
kinds of paints, white and metallic grey, were studied. The white sample consisted of a layer of
titanium dioxide in an acrylate blend topcoat on a metal substrate. The metallic grey sample
consisted of layers of an acrylate blend topcoat and aluminum flakes on an acrylate blend base coat
on a metal substrate. The samples were rinsed with deionized water and lightly buffed with a
chamois before each exposure to remove dust
Chamber Exposure
Samples were positioned vertically and exposed to UV light and pollutants in a smog
chamber at North Carolina State University. Pollutants such as SO2, NO and propylene were
introduced into the chamber and mixed with dry clean air and deionized water. The chamber was
surrounded by banks of UV light which simulated the UV component of the sunlight spectrum. The
chamber was operated as a continuously stirred reactor in which NOx, ozone and various acids were
formed Each sample holder was placed on a chiller plate so that sample could be chilled below the
dew point temperature to generate a wet surface, i.e., dew.
The samples were exposed at approximate average pollutant concentrations of 300 ppb of
NO*, 460 ppb of C3H6,90 ppb of SO2 and 120 ppb of O3 for one week, under the cycle of 5 h wet
-2 h dry - 5 h wet - 12 h dry surface conditions. The sample temperatures under dry deposition and
wet deposition were about 30 °C and 5 °C, respectively. The samples were removed from the
chamber, observed for surface change and stored in a desiccator. For the spot tests, dews were
generated on teflon films placed on the chiller plates and the run-off s were collected twice a day and
kept in amber glass botdes under refrigeration. The pH and composition of the dew were measured
by using a Fisher pH meter and a Dionex Ion Chromatograph model 2010i, equipped with an
IonPac column AS-10.
Dew Exposure
Three drops of filtered chamber dews (100 p.1 each) were pipetted onto the surface of the
samples. The samples were kept at room temperature for 1 h and then placed in an air-circulating
oven at 90 °C for 24 h. Finally, the samples were rinsed with deionized water and lightly buffed
with a chamois before surface analyses were performed.
Surface Analysis of the Painted Samples
A reflection light microscope (Zeiss model AXIOMAT) was used to examine the damaged
surface areas. An Alpha Step 200 profilometer manufactured by Tencor Instruments Co., with a
stylus in the shape of a 60° cone rounded at the end to a spherical tip of 12.5 |im radius was used to
measure the depth or height of the damage by dragging it across the surface. An SEM, (JEOL 840)
equipped with an EDS (Kevex model 8000) was used to examine the details of the damage and the
elemental composition of the surface.
RESULTS AND DISCUSSION
From visual observations, there was no significant change of the surface appearance on
either of the chamber exposed samples. The samples were then subjected to spot testing using two
chamber dews with pH 3.35 and 3.50, respectively. The composition of the dews are shown in
Table 1.
Table 1	dH and composition of the chamher dews.	
Chamber.	pH	Ion Concentration (mg/L)
DeW#	F- CI" S04= NO3- CH3COO- HCOO
91-3
91-6
3.35 1.21 0.54 20.51 10.14 20.49
3.50 1.59 0.99 14.91 7.99 23.07
1.83
32.81

-------
The samples with dew drops placed on their surfaces were placed into an oven and heated at
90 °C. The behavior of the drops was essentially the same as reported by White8. From the
damage size and shape, it appears that at a critical concentration the acidic chamber dew attacks the
painted surfaces.
Previously, the damage was found to be more pronounced when the exposure temperature
was increased from 54 to 90 °C, and when the exposure time was increased from 1 to 24 h8. This
also suggests that the extent of the attack directly depends on the exposure temperature and that the
attack continues with the exposure time.
Various degrees of damage occur depending upon the dew composition and the surface
properties. For the white paint (#7), reflection light microscopy reveals that the damage is in the
form of a ring with some damage in the middle. Some surface cracking also occurs (Figure la).
The corresponding surface profile of this damage (Figure lb) shows that the damage is a blister with
a height of 0.8 mm. (Note that the 4.5 |im value in the figure represents the height of the deposits or
reaction products.) The white paint (#7B), which was exposed to pollutants in the smog chamber
for one week prior to the spot test, has a larger damage area and a slighdy higher blister (0.9 mm)
than #7, as shown in Figures 2a and 2b, respectively. When drops of acidic dew containing various
acids such as H2SO4, HNC^ HCOOH, CH3COOH, were dried, deposits were left on the paint
surface. It appears that they penetrated the top coat and caused swelling and cracking of the paint
The chamber exposed (#1B) and unexposed metallic grey paint samples (#1) were subjected
to spot tests using chamber dews #91-3 and #91-6. The height and depth of the damage area are
reported in Table 2. The damage on the chamber exposed samples is greater than on the unexposed
sample. The chamber dew #91-3, pH 3.35, causes more damage than the chamber dew #91-6, pH
3.50.
Table 2 Maximum height (H) and depth (D) of the damage on the paint samples
exposed to chamber dews at 90 °C for 24 h, measured by profllometry scans
Chamber pH "PaTnt #T- ~ "PaTnt"#2~	~Paint~#3	Paint #4 ~
Dew#	H D	H	PHD HP
#91-3 3.35 0.90, -0.1 2.60 -- 0.86, -0.90 0.90 -0.80
#91-6.	2JQ	CL22	11	L52	-	NA	NA	0,60 -0,94
Microscopy and SEM reveal that the ring shaped damages on paint #1 and #1B consist of
many small areas of damage and that swelling, pitting, blistering and cracking occurred. The EPS
spectrum of the damage surface on paint #1B shows aluminum and sulfur (Figure 3a), while the
surrounding area does not (Figure 3b). This suggests that the chamber dew has attacked the top coat
and penetrated through cracks into the base coat, exposing the aluminum in the base coat. It also
suggests that the sulfuric acid in the dew may have reacted with the acrylate polymer.
SUMMARY
This preliminary study of the effects of acidic pollutants on acid resistant automotive finishes
showed that one week of exposure to moderately high concentration of pollutants in a smog chamber
does not visually damage the paints. However, the spot test at 90 °C for 24 h using a chamber dew
containing various organic and inorganic acids causes blistering and cracking of the top coat. EDS
data provide evidence that the acidic dew can penetrate through the top coat into the base coat.
Further studies are underway in an attempt to understand the mechanism of the attack.
REFERENCES
1.	D. J. McEwen, M.H. Verma and R.O. Turner, "Accelerated Weathering of Automotive Paints
Measured by Gloss and Infrared Spectroscopy", J. Coat. Tech.. 52(755): 123 (1987).
2.	T.C. Simson and P.J. Moran, TTie Johns Hopkins University, Baltimore, MD, personal
4

-------
communication, 1990.
3.	J. White and W. Rothschild."Defects in the Finish of Motor Cars," Metal Finishing. 85(5), 15
(1987),.
4.	G.G. Campbell, G.G. Schurr and D.E. Slawikowski, "A Study to Evaluate Techniques of
Assessing Air Pollution Damage to Paints," EPA-R3-73-040, U.S.Environmental Protection
Agency, Research Triangle Park, 1972.
5.	G.T Wolff, W.R. Rodgers, D.C. Collins, M.H. Verma and C.A. Wong,"Spotting of
Automotive Finishes from the Interactions Between Dry Deposition of Crustal Material and Wet
Deposition of Sulfate," J.Air Waste Manage. Assoc.. 40(12), 1638 (1990).
6.	HJLG. Jellinek, F. Flajsman and FJ. Krymn,"Reaction of SO2 and NQ2 with Polymers,"
J.AppI- Polvm. Sci.. 13, 107 (1969).
7.	S.S. Sankar, D,Patil, R. Schadt, R.E. Fornes and R.D. Gilbert,"Environmental Effects on
Latex Paint Coatings. II: CP/MAS 13C-NMR and XPS Investigations of Structural Changes in the
Base Polymer," J. AppI. Polvm. Sci..41.1251 (1990).
8.	DJF. White,"Investigations of Techniques to Assess Physical Damage to Polymeric Automotive
Coatings on Metals by Acidic Deposition," M.S. Thesis, North Carolina State University, (1992).
DISCLAIMER
This paper has been reviewed in accordance with the U.S. Environmental Protection
Agency's peer and administrative policy for publication. Mention of trade names of commercial
products does not constitute endorsement or recommendation for use.
T			sou	tuuu	rxro—'—-vS	
Horizontal Distance (jim)
(b)
Figure 1. Paint#7, spot tested with chamber dew #91-3,90 °C, 24h.
a. Optical micrograph	b. Surface profile of the damage

-------
(a)
J


'•/**
-	-	
X

1	 1000 ' IMU	55	
Horizontal Distance (pm)
(b)
Figure 2. Paint #?B, one week chamber exposed and spot tested with chamber dew #91-3,
90 °C. 24 h.
a. Optical micrograph	b. Surface profile of the damage.
,,
«*»
i

it ?>• i
(a)
m,M.|
ili,w
S ©#• l**i*
I J 		
ii.fii M*
(b)
Figure 3. EDS spectra of Paint # IB, one week chamber exposed and spot test with chamber
dew #91-3, 90°C, 24 h
a,	EDS spectrum of the dew exposed area
b.	EDS spectrum of the unexposed area

-------
TECHNICAL REPORT DATA
1. REPORT *0,
EPA/600/A-92/237
2.
3'
4. TITLE AND SUBTITLE
A STUDY OF THE EFFECTS OF ACIDIC POLLUTANTS OK
AUTOMOTIVE FINISHES
5.REPORT DATE
6.PERFORMING ORGANIZATION CODE
7. AUTHORCSJ
Rungsimuntakul, N., White, D,, Fornes, R.,
Gilbert, R., Zhang, C., and Spence, J.
8.PERFORMING ORGANIZATION REPORT NO.
9. FERFORHING ORGANIZATION KAHE AND ADDRESS
North Carolina State University
Raleigh, NC 27695
U. S. Environmental Protection Agency/AREAL
Research Triangle Park, NC 27711
10.PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
CR-814121
12. SPONSORING AGENCY NAME AND ADDRESS
Atmospheric Research & Exposure Assess. Laboratory
Office of Research and Development
U. S. Environmental Protection Agency
Research Triangle Park, NC 27711
13.TYPE OF REPORT AND PERIOD COVERED
1*. SPONSORING AGENCY CODE
EPA/600/09
15. SUPPLEMENTARY NOTES
,1
'/v '
16. ABSTRACT
; Automotive finishes of various compositions- on metal substrates were exposed
vertically in a smog chamber to UV and acidic atmospheres. The pollutants were
generated from combinations of SO^, NO, propylene, water, and air. Dews of
different compositions were generate# and collected twice a day. Spot tests were
performed by placing drops of (100 juL) of.--dews on the surfaces of the paints and
heating in an air-circulating oven at 90*C for 24 hours. Visual observation,
reflection optical microscopy, profilometry, SEM, and EDS were used to examine
surface damage. Various degrees of damage occurred depending upon the dew
composition and surface properties. In general, the damage areas were in the form
of rings with diameters smaller than the original drop. After rinsing and buffing,
the damage was still visible. Microscopy and SEM revealed that the rings consisted
of numerous small areas of damage and that swelling, pitting, blistering, and
cracking had occurred. EDS showed aluminum and sulfur at the damage surface, while
the surrounding area did not. Since the base coat contained Al flakes, this
suggested that the acidic dew had penetrated through the top coat into the base
coat./^.-:.----
17. KEY WORDS AND DOCUMENT ANALYSIS
«. DESCRIPTORS
b.IDENTIFIERS/ OPEN ENDED TERMS
e.COSATI



18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
•
19. SECURITY CLASS (this Recort)
UNCLASSIFIED
21.HO. OF FAGES
%
•
20. SECURITY CLASS (This P*«e)
"UNCLASSIFIED
22. PRICE

-------