Physical Damage Formation on Automotive Finishes Due to Acidic Reagent Exposure

EPA/600/A-92/240
PHYSICAL DAMAGE FORMATION ON AUTOMOTIVE
FINISHES DUE TO ACIDIC REAGENT EXPOSURE
Douglas White, Raymond Fornes
Richard Gilbert, J. Alexander Speer
North Carolina State University
Box 8202
Raleigh, North Carolina 27695-8202
John Spence
United State Environmental Protection Agency
Atmospheric Research and Exposure Assessment Laboratory
Research Triangle Park, North Carolina 27711
ABSTRACT
Several types of automotive finishes with clear coatings were exposed to drops of acidic
reagents at 54 C. Surface damage was examined using visual observations, reflection opti-
cal microscopy, SEM, EDS, and profilometry. Reflection microscopy was the most useful
technique for observing surface damage. Scanning electron microscopy provided sulfur
mappings through the use of an EDS attachment.
A chamber dew with pH level of 3.4 created in a smog chamber designed to simulate real
environmental conditions was highly detrimental to the finishes with damage concentrated
in a ring with a diameter less than the original drop size. The form of this damage suggests
a free energy minimization process favoring a concentration of the damaging reagent at
the edge of the evaporating drop where stable nuclei are thought to form. Continued heat-
ing of the samples after the drop evaporation resulted in damage that increased with time,
with most of the visual damage located underneath material deposited from the evaporated
drop.
INTRODUCTION
Environmental damage to automobile paints has been observed recently. This damage
is generally in the form of circular, elliptical, or irregular spots that cannot be removed by
washing1. The major automotive manufacturers in the U. S. and other countries have some
concern regarding the effects of acidic pollutants on automotive paints2. It is suggested
that newer automotive paint formulations which contain unpigmented surface clear coats
are highly susceptible to acidic pollutant caused damage2.
Our studies have attempted to reproduce this type of damage through exposing several
types of automotive paints to a variety of acidic reagents with the objective of obtaining a

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better understanding of the paint degradation process. We also were interested in evaluating
techniques to assess the paint's surface damage. Such information would be helpful in
formulating paints that are more resistive to acid precipitation.
J. White and W. Rothschild linked acidic deposition/acidic pollutants to automotive
paint damage3. They observed spotting and in some cases pitting in locations on automobile
paint surfaces when vehicles were exposed in the field in Israel. They were able to reproduce
the color changes of the field exposed paints by spotting the specimens with either nitric
or sulfuric acid.
The natural environment is very complex and it is difficult to isolate the effects of
acidic pollutants from other variables for field exposed paints. It is also difficult to deter-
mine which techniques and parameters are optimum for determining the damage levels of
paints after exposure. Here we extend the spotting type tests done by White and Roth-
schild and others by using acidic reagents that closely resemble environmentally produced
precipitation. In an environmental research chamber at North Carolina State University,
it is possible to prepare complex mixtures of acid dew of varying composition, which are
representative of dews formed in real atmospheres. The composition of these dews can be
easily controlled making them useful for the damage analysis of paints. Several types of
automotive paints with clear coatings were exposed to these chamber dews and a variety
of other acidic solutions. In the study reported here, emphasis was on characterizing the
physical surface damage and proposing a possible model for the damage process.
EXPERIMENTAL METHODS
Paint Samples Studied
Several types of paint samples on sheet metal substrates were supplied by an automo-
tive paint manufacturer. All samples were 10 cm by 15 cm with a total thickness (substrate
plus paint) of about 0.84 mm. The paints were the same except for differing clear coat
compositions. Based on information provided by the company, the most likely composi-
tions were determined. All the paints had a black pigmented base coat which consisted of
an hydroxyl-containing acrylic resin crosslinked with melamine formaldehyde.
Exposure Procedures
The paint samples were first washed with deionized water and then buffed lightly with a
chamois before heating in order to remove dust from the surfaces of the sample. Then three
30 (ill drops of a pH 3.4 chamber dew produced in a smog chamber designed to reproduce
real environmental conditions were pippetted onto the surface of the plate equal distances
apart. Unless otherwise indicated, the plates were then placed into an oven and heated at
54 C for 50 minutes. After exposure, the plates were removed from the oven, allowed to
cool, and then rinsed with deionized water and hand buffed immediately to remove surface
deposits.
RESULTS AND DISCUSSION
The exposure method used in this study simulates the outdoor damage process of

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automobile paints due to exposure to precipitation followed by heating from the sun. Tem-
peratures during outdoor exposure can commonly exceed 90 C1. In these experiments,
exposures at 54 C for 50 minutes were used in order to observe damage levels under mod-
erate exposure conditions. A 6pot type test was used to simulate the damage process that
occurs after rain or dew exposure to automobile paints. Beading of water on the paint sur-
face occurs during the drying process after rain or dew exposure which forms drops on the
paint surface. The exposure of the paints to reagent drops in the spot test simulates this
type of exposure and this type of test also permits comparisons with adjacent unexposed
areas of the paint.
Reflectance Microscopy Observations
Ring shaped blistering damage was observed on all the paint surfaces. Figure 1 is of
a paint surface with a clear coat having an hydroxyl containing acrylic resin crosslinked
with diisocyanates and melamine formaldehyde. The clear coat appears to be separating
into layers, and fracturing of the ring can be observed. This is a result of the dew being
absorbed into the paint coating followed by the dew reacting with the paint surface. As
the dew reacts with the clear coat, the elasticity of the paint coating decreases resulting
in splitting of the surface. The ring shaped form of the damage suggests a free energy
minimization process favoring a concentration of the damaging reagent at the edge of the
evaporating drop. This process will be discussed in a separate section of this paper.
Scanning Electron Microscopy Observations
Scanning Electron Microscopy permitted the obtaining of qualitative sulfur concentra-
tions on the surface of the damaged areas through X-ray compositional sulfur mapping.
Damage to a paint with a clear coat composed of an acrylic resin containing hydroxyl
groups, modified with a polyester resin and crosslinked with melamine formaldehyde ob-
served using SEM is shown in Figure 2. To the left is a sulfur mapping of the damaged area.
The lighter a particular part of the map, the higher the concentration of sulfur (probably
in the form of SO*-) in that particular part of the damage area. On the right is an image
of the damaged area. The part of the image indicated by arrows is not a damaged area, but
is a scratch identification mark. Deposition of sulfur 1s generally located in a central area.
These samples were rinsed and buffed after exposure, so either the sulfur had chemically
combined with the paint, or was in some other form resistant to buffing.
Profilometry Observations
Profilometry was also used to assess damage levels. This technique can give quantitative
comparison of damage amounts between samples since it measures the actual size (width,
height, or depth) of the damaged areas. Unfortunately, due to the unevenness of the paint
coatings, the background noise was high which limited the precision and usefulness of this
technique for studying the samples. Figure 3 shows a profile of damage formed on a paint

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Figure 1 Chamber Dew #7 Damage - Paint #2
50 Minute Exposure at 54 C
Figure 2 Chamber Dew #6 Damage - Paint #7
50 Minute Exposure at 54 C
(EDS Sulfur map on left; Photomicrograph on right)
rrows indicate scratch marks for identification of damage location).

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with a clear coat having an acrylic resin containing hydroxyl groups and crosslinked with
diisocyanates. A crater shaped blister was formed with wall heights up to 0.6 micrometers.

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Figure 3 Profile of Chamber Dew #7 Damage - Paint #4
50 Minute Exposure at 54 C
Results from inspection of the profilometer analysis and photomicrographs of the ex-
posed paints indicate that the damage areas are not necessarily etches as would be expected
from acid attack. What seems to be occurring is instead a separation of the clear coat into
layers. The acidic dews at elevated temperatures generally form an uplifted crater on the
paints. The center of the crater is approximately the same height as the rest of the paint.
However, the sides of the crater are higher than the rest of the paint.
Chamber Dew Evaporation Process on Paint Surface
Visual observations of the evaporation process of the chamber dews on the paint surfaces
show that no change of the clear coat surfaces of the paints tested occurs until a critical
drop size is reached. At this critical size, deposition of material from the drop begins
and continues until the drop has completely evaporated. After this evaporation, an area of
deposition can be observed which is much smaller than the initial drop 6ize with the highest
levels of deposits located in a circular ring. After washing and buffing, a circular ring of
damage is observed which is located at the same places on the surface where deposits were
formed.
Deposition of material from the reagent drop occurs only after the drop reaches a criti-
cal size or concentration. Once this concentration is reached, precipitation occurs, resulting
in material being deposited on the paint surface with the highest levels occurring at the
edge of the drop. This location appears to be the most favorable for the creation of nu-
cleating centers of the depositing material to form. Favorable locations for nucleation in
solutions occur at the surface of solutions (the drop surface in our case), or at the walls of
the vessel containing the solution (the drop-paint interface in our case)"'. These locations
£

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are favorable because the interfacial free energy of a nucleating center is lowered in these
locations. A nucleus located at the edge of the drop exists at both favorable locations
simultaneously, resulting in a further decrease in the free energy of formation compared
with either location alone or the interior of the solution.
CONCLUSIONS
The results from these investigations indicate that acid deposition causes considerable
damage to automotive coatings with clear coat surfaces. The chamber dews formed uplifted
craters on the paints. The center of the crater is approximately the same height as the rest
of the paint, but the walls are higher. The visual damage on the paint surface appeared to
occur as a result of the interaction at elevated temperature between the deposited material
from the evaporated drop and the clear coat surface with the damage levels on the paint
surface increasing as the time of heating increased. The ring shaped damage produced by
the dews appeared to be a result of a nucleation process which favored the deposition of
the damage producing material at the edge of the evaporating drop.
Reflection microscopy, scanning electron microscopy, and profilometry were found to
be useful techniques to evaluate the damage on the paint surfaces. Reflection microscopy
and profilometry were helpful in determining the physical structure of the damage to the
paint surface. Scanning electron microscopy was most useful for obtaining sulfur mapping
of the paint surfaces through the use of an EDS attachment to the SEM.
REFERENCES
1. 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): 1638-1648(1990).
2. T. C. Simpson and P. J. Moran, The John Hopkins University, Baltimore, MD, personal
communication, 1990.
3. J. White and W. Rothschild, "Defects in the Finish of Motor Cars," Metal Finishing
85(5): 15-18 (1987).
4. D. Elwell and H. J. Schell, Crystal Growth from High-Temperature Solutions, Academic
Press, London, 1975, p.100.
DISCLAIMER
This paper has been reviewed in accordance with the U. S. Environmental Protec-
tion Agency's peer and administrative policy for publication. Mention of trade names of
commercial products does not constitute endorsement or recommendation for use.

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TECHNICAL REPORT DATA
1, REPORT NO.
EPA/600/A-92/240
2.
3,
V
4. TITLE AND SUBTITLE
PHYSICAL DAMAGE FORMATION ON AUTOMOTIVE FINISHES DUE
TO ACIDIC REAGENT EXPOSURE
5.REPORT DATE
6.PERFORMING ORGANIZATION CODE
7. AUTHOR(SJ
White, D., Fornes, R., Gilbert, R., Speer, A., and
Spence, J.
8.PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
North Carolina State University
Raleigh, NC 27695
U. S. Environmental Protection Agency
Research Triangle Park, NC 27711
10.PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
CR-814121
12. SPONSORING AGENCY NAME AND ADDRESS
Atmospheric Research and Assess. Laboratory
Office of Research and Development
U. S. Environmental Protection Agency
Research Triangle Park, NC 27711
13.TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/09
15. SUPPLEMENTARY BOTES
j 16. ABSTRACT
^Several types of automotive finishes with clear coatings were exposed to drops of
acidic reagents at 54° C. Surface damage was examined using visual observations,
reflection optical microscopy, SEM, EDS, and profilometry. Reflection microscopy
was the most useful technique for observing surface damage. Scanning electron
microscopy provided sulfur mappings through the use of an EDS attachment.
A chamber dew with pH level of 3.4 created in a smog chamber designed to simulate
real environmental conditions was highly detrimental to the finishes with damage
concentrated in a ring with a diameter less than the original drop size. The form
of this damage suggests a free energy minimization process favoring a concentration
of the damaging reagent at the edge of the evaporating drop where stable nuclei are
thought to form. Continued heating of the samples after the drop evaporation
resulted in damage that increased with time, with most of the visual damage located
underneath material deposited from the evaporated drop.
17.	KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
b.IDENTIFIERS/ OPEN ENDED TERMS
c.COSATI



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