United States
Environmental Protection
Agency
Atmospheric Research and
Exposure Assessment Laboratory
Research Triangle Park NC 2771 1
Research and Development
EPA/600/S3-88/044 Feb. 1989
&EPA Project Summary
Analytical Techniques for
Measuring the Effects of Acid
Deposition on Coatings on Wood
C. M. Balik, R. E. Fornes, and R. D. Gilbert
Preliminary experiments have been
carried out to characterize the
potential deleterious effects of acidic
deposition on three representative
paints: an oil alkyd paint and two
acrylic latex formulations. The base
polymer latex common to both latex
paints was also studied individually.
Free films of paint have been
exposed to relatively high levels of
gaseous SO2 and ultraviolet light,
and have been immersed in aqueous
SC>2 at pH 2.0. Several analytical
techniques have been used to
assess the resulting chemical and
physical changes in the paint films,
including sorption and diffusion
measurements, attenuated total
reflectance infrared spectroscopy,
dynamic mechanical analysis, sol-
gel analysis, contact angle meas-
urements, differential scanning
calorimetry, and electron spin
resonance. All techniques show
promise for characterizing the early
stages of damage to paint films
caused by acidic deposition. The
major effects noted in this study
include leaching of acid-soluble
extender components upon immers-
ion in aqueous SOz, and enhanced
degradation of the base polymer
upon exposure to gaseous SOa and
ultraviolet light
This Project Summary was devel-
oped by EPA's Atmospheric Research
and Exposure Assessment Laboratory,
Research Triangle Park, NC, to
announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title (see Project Report
ordering information at back).
Introduction
The objectives of this project are: (1) to
study the effects of acid deposition
products in combination with near-uv
radiation of the micro- and macro-
structures of acrylic latex paint coatings,
(2) to assess microdamages experienced
by acrylic copolymers upon exposure to
acid deposition; (3) to determine
mechanistic paths that lead to polymer
structural changes and microdamage;
and, (4) to relate molecular structural
changes to macrodamages that affect the
service life of exterior coatings used on
wood substrates.
The following analytical techniques will
be employed: Intrinsic viscosity; Sol/Gel
Analyses; Gel Permeation Chroma-
tography; Surface Contact Angles; Dif-
ferential Scanning Calorimetry (DSC);
Dynamic Mechanical Analysis (DMA);
Stress-Strain Analysis; Fatigue Tests;
Infrared Spectroscopy; Electron Spin
Resonance Spectroscopy; ESCA and
SIMS; SEM - Fracture Surfaces studies.
Changes will be followed in molecular
weight, molecular weight distribution,
glass transition temperature, changes in
bulk properties, the amount of polymer
chain scission, crosslinking and oxidation
upon exposure of the paint coatings to
ultra-violet radiation, S02, NOX and
mixtures of the SC>2 and NOX with air
and/or water.
The resulting data will permit deter-
mination of the mechanistic paths that
result in the degradation of the base
polymer in the paint coating and allow
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the prediction of useful lifetimes of
coatings exposed to acid rain pollutants.
Experimental Procedures
Films of the base and compounded
latexes were prepared by the drawdown
procedure, using standard draw-down
bars, on release paper. They were dried
at R.T. for 4 hr, stored in a desiccator
oven drierite for 48 hr and then heated at
40°C for 48 hr under vacuum. The films
prepared in this manner absorbed
negligible amounts of water when stored
at R.T. and 64% R.H.
A sample of each film before and after
exposure to various environments was
obtained and placed in tetrahydrofuran
(THF) for 3 days. The soluble portion
was separated from the insoluble portion
by filtration, the insoluble portion dried
and weighed to determine the gel
content. The amount of sol was
determined by evaporation of the THF.
Intrinsic viscosities were determined at
25°C using modified Cannon-Fenske
viscometers.
Thermal analyses were performed with
either a Perkin-Elmer DSC-2 or -7,
each equipped with a data station.
Samples 1 cm by 6 cm were cut from
each film using care to obtain uniform
widths. The thicknesses were measured
with a micrometer. Dynamic mechanical
analyses (DMA) tests were made with an
Autovibron DDV-II-C at a frequency of
11 hz and a scanning rate of 2.5°C/min.
The films were exposed to uv light
(254 or 350nm) in a Rayonnet U.V.
reactor. The samples were mounted in
quartz tubes equipped with inlet tubes for
SOa, NOX, air or various combinations.
Contact angles were measured with an
NRL goniometer using distilled water.
The solubility/diffusivity data for gases
in polymers is obtained using conven-
tional weight-gain techniques. A
sensitive electrobalance system en-
closed in a vacuum chamber was
constructed for this purpose. Provisions
are made for maintaining constant
diffusant pressure and temperature.
Mass changes (as small as a few
micrograms) are monitored as a function
of time, and continuously displayed on a
chart recorder. A second electrobalance
system that can be used in corrosive
environments, to be built in the second
year, will greatly speed up the data
collection process. An Analect FX-6260
FTIR spectrometer equipped with an
MCT detector and a flat-plate ATR
sampling accessory was used to obtain
the infrared spectra. A ZnSe paral-
lelogram crystal with an angle of
incidence of 45° was used. All ATR
spectra were collected at 4cm-1
resolution, and 128 scans were accum-
ulated for each sample. The paint films
for FTIR Analysis were cast on a clean
glass plate surface, dried overnight in air,
and transferred to a desiccator containing
anhydrous CaSC>4 for a minimum of
three days. Film thickness was measured
using a sensitive micrometer having a
precision of ± 1.2 nm. The thickness of
the films used in this study was 127 urn
(about 5 mils).
A coating device for putting a thin,
uniform film of paint on a cylindrical ATR
crystal was constructed for FTIR analysis
of diffusion kinetics and in-situ analysis
of chemical reactions. The associated
gas delivery system for these experi-
ments has also been completed. The
system will be tested, and should be
operational by July 1988.
Experimental Results
Preliminary Mechanical/
Structural Effects of SO?
The effect of uv light (350 nm) in the
absence and presence of SOa on the
intrinsic viscosity, gel content, contact
angle, Tg, and dynamic modulus of the
contained polymer in the base latex is
shown in Table 1. All the films were from
a single casting.
Preliminary Solubility and
Diffusivity Data for SOa
Sorption isotherms at 28°C for SC>2 in
the Latex without CaCOs (LO), Latex with
CaCC>3 (LC) and polyer base samples
have been obtained. The solubility of
SOa in the samples varies linearly with
pressure up to 650 torr (Henry's law
behavior) for the two paint samples and
the polymer base. The paint sample
containing CaC03 superimposes on the
base polymer isotherm, while the sample
without CaCC>3 has a slightly higher
slope. This difference could be due to an
inconsistency in the reported amount of
polymer in this paint; a value of 35% was
used to normalize the data for both paint
samples to grams polymer. Reproduci-
bility tests are in progress to check this.
Diffusion coefficients (D) were obtained
for S02 In each sample at each
experimental pressure. D was found to
increase with increasing pressure. This
trend has been observed before for SOa
In a polyimide.This is attributed to
plasticization of the polymer by the
penetrant. The increase in D is most
pronounced for the paint sample without
CaC03. |
The total flux of SO? through these
samples is proportional to the
permeability, P, which is equal to the
product of the diffusivity D and the
solubility ko (Henry's law constant). The
pressure-dependent diffusivity trans-
lates to a pressure-dependent perme-
ability. The permeabilities of the polymer
base and the CaCOs-containing paint
are again very comparable, and are in the
neighborhood of 10'7 cc(STP) cm2/cm3
polymer-torr-sec. The permeabilities of
the CaCC>3-free paint are an order of
magnitude higher.
Simple Immersion Tests in
Aqueous SO2
Free films of LC and LO latex paints,
as well as the polymer base, were
exposed to aqueous SOa (pH = 2.0) for
periods ranging from 1 minture to 14
days. The polymer base showed no
weight loss after 14 days. However,
significant weight loss occurred upon
exposure to sulfurous acid for the two
latex paint samples. For the latex paint
without the CaCOa extender, only 7.2%
of the sample weight is lost after 14 days
of exposure. For the latex paint with
CaCOs extender, the weight loss levels
off after 4 hours; the maximum weight 4
loss being 27% after 14 days. The weight
loss of the LC samples in deionized
distilled water (pH = 5.4, due to the
presence of HgCOs) is on'y 8.5% after 14
days. This clearly indicates that the
presence of SOa In water accelerated the
rate at which materials are leached out of
the films. The initial linearity of the weight
loss vs. t°-5 plot (curve A, Figure 1)
suggests a Fickian diffusion mechanism
for removal of material from the sample.
The diffusion coefficient is 1.84 x 10'9
cm2/sec.
The ATR spectra of the polymer binder
and the two latex paints are shown in
Figure 2. Due to the multicomponent
nature of the paints and the presence of
infrared-absorbing inorganics, assign-
ment of all the bands is difficult,
especially in the region between 400
cnrr1 and 1600 cm'1 where over-
lapping of bands is evident. The polymer
binder is a terpolymer of vinyl acetate,
vinyl chloride, and butyl acrylate, but the
relative composition is not known.
Despite these complications, differences
between the spectra of samples with and
without CaCOs extender can be
observed, as seen by comparing spectra
A and B in Figure 2. The two peaks at ^
1416 cm'1 and 875 cm'1 are not found
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040 -
0.32 -
0.24 -
"0
•
0.16 -
008 -
40
60 80
tos(mmf5
100
120
140
Figure 1. Weight loss of latex paint film with exposure time fa) Films with CaCO3 at pH
= 2.0, (b) films without CaCOa at pH = 2.0: (c) films with CaCOa in deionized
distilled water.
1728
1416
4000 3600 3200 2800 2400 2000 1600 1200 800 400
Wave Number
Figure 2. FTIR-A TR spectra of fa) latex with CaCOs, (b) latex without CaCOi (c) base polymer
latex.
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in the spectrum of the LO sample, and
are identified as the stretching and
wagging vibrations of the C0$2~ group
of 003, respectively. The broad band
between 850 and 450 cnr1 is absent in
the spectrum of the polymer base and
therefore has been attributed to the
pigment (TiC>2) or the china clay
(aluminum silicate) added to the paints.
The vibrational frequencies of these
additives are located in this region. The
band at 1728 cnrr1, due to the C = 0
stretching mode of the polymer, is
common to all three spectra. This peak is
well separated from others and is
suitable for use as a reference for
spectral subtraction.
In Figure 3, ATR spectra of the LC
samples exposed to sulfurous acid for 1,
6, and 30 minutes are shown. The in-
tensities of the bands at 1416 cnv1 and
875 cnr1 decrease markedly with
increasing exposure time, and are
essentially gone after 30 minutes of
exposure, indicating a loss of CaCO^.
The broad absorbance centered around
700 cnv1 becomes sharper with
increasing exposure. This result is also
found for the spectra of exposed LO
samples. In contrast, the spectra of the
polymer base show essentially no
change after 14 days of exposure.
Subtraction of the spectrum of the
polymer base exposed for 14 days from
the spectrum of an unexposed sample
resulted in a flat line across the entirJ
spectral range.
Determination of D from the
FTIP-ATR Data
Difference spectra between the
unexposed LC and LO samples revealed
the two CaCOa peaks at 875 and 1416
cm~1. The integrated intensities of these
two peaks were followed as a function ol
exposure time to aqueous SOa- The
penetration depth in an ATR experimenl
is only a few pm, therefore the CaCOa
will be removed from this thin surface
layer in a much shorter time than is
required to remove all of the CaCO$ from
10
-e
8
•Q
2000 7700 1400 1100
Wave Number
800
450
Figure 3.
FTIR-ATR spectra of latex paint containing CaCO3 exposed to aqueous SOz fpH
= 2.0) for la) 0 mm; (b) 1 min.; (c) 6 mm., (d) 30 mm
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the bulk film. The penetration depths are
2.3 and 1.4 urn for the 875 and 1416
cnr1 peaks respectively. The very rapid
loss of CaCOa from the surface layer is
evident in Figures 4-(a) and (b).
The diffusion coefficient cannot be
obtained from Figures 4-(a) and (b) in a
straightforward manner. Exposure times
below 1 minute could not be
reproducibly controlled, thus the (linear)
short-time regions of the curves could
not be obtained. Secondly, there is
considerable uncertainty in the assign-
ment of the effective film thickness ;,
given that the amplitude of the
evanescent wave in an ATR experiment
decays exponentially with depth into the
sample, and that the concentration profile
of the diffusant remaining in the surface
layer are nonlinear.
The results of this analysis for the data
in Figure 4 are D = 1.77 x 10'9
cm2/sec for the 1416 cm'1 peak, and
2.25 x 10-9 cm2/sec for the 875 cm-1
peak. Both are in very good agreement
with the value of 1.84 x 10-9 cm2/sec
obtained from the bulk weight loss data.
The solid lines in Figure 4 were obtained
by fixing these values for D and back-
calculating the expected values of It/loo.
Discussion
Mechanism of UV/SO2 Effects
The data in Table 1 demonstrate that
exposure to uv causes both chain
12
t°s(min)°*
(b)
Figure 4. Data points: removal of CaC03 from LC paint films exposed to sulfurous acid
as measured by the integrated intensity ratio of la) the CaCOa band at 875 cm'\-
(b) the CaCOs band at 1416 cm'\ Solid lines: calculated variation of the integrated
intensity ratio with f°'5.
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Table 1. Effect of U.V. and/or SOg on Base Latex Properties
Sample
Reference No.
1-75-BL
1-77-BL1
1-78-BL2
1-79-BL3
1-80-BL4
1-81-BL5
1-81-BL6
Exposure Time (Hr) SC>2
UV (350nm) (ml/min)
0
48
96
48
96
0
0
0
0
0
2
2
2 (48 hr)
2 (96 hr)
In]
(dl/gm)
1.00
0.82
0.50
0.17
0.11
1.00
1.00
Gel Content
(%)
22
32
60
66
68
22
22
Contact
Angle (6)
78
71
65
52
*
63
67
Tg E1X1010 dynes/cm2
(°C) (at -30"C)
22
22
24
35
33
34
34
—
83
97
100
80
35
25
24
24
2.50
2.50
2.16
2.72
2.40
1.90
1.90
1.24
1.27
4.54
2.29
1.49
1.64
2.44
1.25
2.07
"Water drop spread immediately
scission and crosslinking of the polymer.
The intrinsic viscosity decreases and the
gel content increases with uv exposure
time. The higher molecular weight
species will be preferentially converted
to gel (crosslinked polymer) increasing
the fraction of low molecular weight
species in the soluble portion and
resulting in a decrease in intrinsic
viscosity. However, the decrease in
contact angle indicates an increase in the
carbonyl group content which must result
from chain scission. The small increase
in Tg confirms crosslinking has occurred.
Exposure to SC>2 alone has relatively
little effect. However, the combination of
uv and SOa results in rapid polymer
microstructural changes. The intrinsic
viscosity decreases by nearly an order of
magnitude, the gel content triples and
the Tg increases markedly. For example,
the Tg increases from approximately
22 °C for the unexposed base latex to
about 75-80 °C for samples exposed for
48 hrs to both uv and SC>2 The polarity
of the surface also significantly
increases. This indicates that uv and SC>2
in combination cause rapid microstruc-
tural polymer changes, namely chain
scission and crosslinking.
Mechanism of CaCOa Removal
The bulk weight loss data and FTIR
spectra clearly suggest that the CaCOa
extender added to the latex paint is
removed when the film is exposed to
solutions containing acidic ions.
Exposure to deionized distilled water, in
which the acidic species come from
HaCOa, rsults in loss of less than 10%
of the sample weight in 14 days.
However, upon exposure to pH = 2.0
sulfurous acid, essentially all of the
CaCOj is leached out in about 4 hours.
The approximate composition (in weight
%) of the dry LC latex samples is 37%
polymer base, 21% CaC03, 35% TiOa,
and 7% china clay. The maximum weight
loss is 27%, which is nearly equal to the
sum of the weights of CaCOa and china
clay. The changes in the infrared spectra
also support the loss of these two
components. Although the sharpening of
the broad band between 450 and 850
cm~1 could be attributed to loss of TiOg
rather than china clay, the extremely low
solubility of TiC>2 in acids, relative to
china clay makes this rather unlikely. No
detectable evidence for sulfite or bisulfite
ion was found in the FTIR spectra of
exposed films.
The mechanism of removal of CaCOa
from the latex film must involve three
steps: (1) diffusion of the components of
the SOa aqueous solution into the film,
(2) reaction and/or dissolution of CaCOa,
and (3) diffusion of CaCO^ (or its ions)
out of the water-swollen film. Given the
Fickian kinetics for the overall removal of
material from the film, the rate limiting
step must be diffusion controlled.
Therefore, if any of the above chemical
reactions do occur, they must be very
rapid in comparison. It is not possible to
distinguish between steps (1) and (3)
with the present data, although one of
these processes should be much fastei
than the other, otherwise, Fickian kineticj
would not be observed. Thus, the
diffusion coefficient measured in these
experiments represents either step (1) oi
step (3) in the mechanism proposec
above, but not a combination of the two
It would seem that diffusion of CaCOa (oj
its ions) out of a water-swollen filnr
could occur much faster than the initia
diffusion of water, SOa, and ions into £
dry, unswollen film.
Summary
During the first year of this project
preliminary studies were conducted tc
determine the best techniques to identify
chemical and physical effects or
polymeric coatings used on wood due tc
wet and dry acid deposition. The
following techniques were employed
Fourier transform IR, gravimetric
measurements to obtain diffusivities anc
solubilities, dynamic mechanical analysis
(DMA), sol-gel analyses, intrinsic
viscosities (IV), differential scanninc
calorimetry (DSC), contact angle anc
electron spin resonance (ESR). Emphasis
during the initial phase of ou
investigations has been on free films o
latex paint. Some preliminary work hai
begun on alkyd paint films and coating;
on wood substrates.
Preliminary exposure studies show tha
the techniques employed are well suitei
to follow changes in the microstructure c
latex films as a function of exposure ti
acid deposition. A
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C. M. Batik, R. E. Fornes, and R. D. Gilbert are with North Caromia oia.c
University, Raleigh, NC 27695.
J. W. Spence is the EPA Project Officer (see below).
The complete report, entitled "Analytical Techniques for Measuring the Effects of
Acid Deposition on Coatings on Wood," (Order No. PB 89-127 518/AS; Cost:
$15.95, 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 Project Officer 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
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