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

EPA/600/S3-88/044
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