United States
Environmental Protection
Agency
Atmospheric Research and
Exposure Assessment Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/S3-89/066 Aug. 1989
£EPA Project Summary
Effects of Acid Deposition on
Painted Wood Substrates
C. M. Balik, R. E. Fornes, R. D. Gilbert, and R. S. Williams
This report summarizes the
progress that has been made within
the Coatings Effect Research
Program that EPA conducts for Task
Group VII within the National Acid
Precipitation Assessment Program
(NAPAP). The major objective of this
phase of the research program is to
identify early failure processes
associated with acidic deposition on
painted wood substrate systems.
On the basis of the findings of this
phase of the study it is recom-
mended that research be continued
to develop dose-response
relationships for the effects of acid
deposition on coated wood substrate
systems.
This Project Summary was
developed by EPA's Atmospheric
Research and Exposure Assessment
Laboratory, Research Triangle Park,
NC, to announce 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
The Atmospheric Research and
Exposure Assessment Laboratory
(AREAL) of the U. S. Environmental
Protection Agency (EPA) is conducting a
program, Effects of Acid Deposition on
Materials, that is planned by Task Group
VII, Effects on Materials and Cultural
Resources. This task group is one of
several groups that are conducting
research within the National Acid
Precipitation Assessment Program
(NAPAP). Within Task Group VII, EPA
has the responsibility for studying effects
on materials of construction, primarily
metals and paints, whereas the National
Park Service and U.S. Geological Survey
are studying cultural stone materials. One
of the primary missions of Task Group VII
is to provide NAPAP and EPA with scien-
tific data on the cause and effect
relationships (damage function models)
of acid deposition on man-made and
naturally occurring materials. These
models will be used by NAPAP to
evaluate the economic loss associated
with acid deposition damage to materials.
Because of their economic importance
and exposure to acid deposition, exterior
coatings are one of the types of materials
that Task Group VII and EPA is interested
in studying.
Before beginning a study of exterior
coatings, EPA held a workshop to review
a planning document entitled " Research
Plan for Determining the Effects of Acid
Deposition on Exterior Coatings." The
plan was approved in January 1987. In
addition to this document, EPA had
prepared an internal report entitled
"Strategy and Documentation for
Conducting Research on the Effects of
Acid Deposition on Coating-Substrate
Systems: The Data Quality Objective
Process" which supplements the
research plan and provides guidance to
researchers who are conducting effects
research on coating -substrate systems.
These two documents provide the
cornerstone for the EPA coatings
research program.
This report reviews the progress that
has been made under Task 3 of the
research plan in which preliminary
exposure studies were conducted using
an array of analytical techniques to
identify early failure processes
associated with acidic deposition on
coated wood substrate systems. Task 3
as proposed in the plan is now
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completed. The report recommends Task
4 of the research plan which involves
conducting field and exposure chamber
studies from which dose-response
functions can be derived for predicting
the effects of acidic deposition on the film
failure processes be initiated.
Results
A summary of the damage to coated-
wood substrate systems that were
observed during the Task 3 short-term
accelerated exposures to acidic deposi-
tion are as follows:
A. Structural and Property Effects
• It has been shown that S02 in the
presence of UV radiation reacts
with films of the base acrylic
terpolymer of the latex paint, thus
accelerating the rate of degra-
dation. A number of competing
reactions cause major micro-
structural changes in the polymer.
These changes affect the paint
film properties as follows:
1. Dramatic increase in the
glass transition temperature
2. Large increase in the gel
fraction
3. Extensive polymer back-
bone chain scission
4. Loss of the acetate side
groups
5. Dehydrodehalogenation
6. Discoloration
7. Increase in the surface pola-
rity
B. Solubility and Permeability
Characteristics
• S02 has relatively high solubility
and permeability in acrylic latex
and alkyd paints. If these paints
are applied on substrates that can
react with and/or absorb S02 then
rapid diffusion of SO2 is observed.
• In the absence of UV, essentially
all absorbed S02 in latex films is
removed upon desorption while a
significant amount of SO2 (about
10-20% of the amount absorbed)
is retained in alkyd films.
C. Dissolution of Particles
• Both laboratory and field studies
confirm that dry deposition of acid
gases such as SO2 and HN03
dissolves alkaline compounds
(e.g., CaC03, ZnO) in paint films,
and that acids deposited during
precipitation causes additional
dissolution.
• Quantitative data for the rate of
removal of CaC03 from latex paint
films have been obtained as a
function of pH. As the pH
decreases, the rate of CaC03
removal increases dramatically.
CaC03 removal from a latex paint
film is accompanied by a collapse
of the polymer to fill voids created
by the dissolution of the CaC03
particles, resulting in a decrease
in the film thickness and an
increase in the solubility and
permeability of gases in the film.
• After three month exposure at the
North Carolina and Ohio field
sites, discoloration or yellowing of
CaC03 containing paints on wood
substrates is being observed.
• Leaching of alkaline particles also
causes dimensional changes in
the film. If the film is on a
substrate, these changes may
result in local stresses at the paint
substrate-interface.
D. Effects on Wood Substrates
• The weathering of wood prior to
painting decreases paint
adhesion. Sulfurous, sulfuric, and
nitric acids increase the rate of
weathering significantly.
• S02 and sulfurous acid readily
diffuse through the coatings and
are absorbed by wood. Recent
studies have shown that the sulfur
compounds accumulate primarily
at the paint wood interface.
• A significant amount of SO2 is
retained in the wood following
sorption-desorption. S02 is likely
reacting with the lignin of wood.
• Lignin model compounds that
were encased in paint films
degraded within several weeks of
exposure to either gaseous S02
or sulfurous acid.
Recommendations
The following recommendations are
made regarding future exposure studies:
• Conduct exposures of coatings
wood substrate systems ii
environmental chambers at Nortl
Carolina State University (Raleigh
NC), U.S. Environmental Protectioi
Agency (Research Triangle Park
NC), and Forest Product!
Laboratory (Madison, Wl) at atmos
pheric pollutant levels to develof
dose-response relationships
Generic coatings that represen
commercial coatings of knowr
ingredients should be used in these
exposure studies. Thest
investigations should be directed t<
the discoloration or "yellowing" o
coatings and the loss of film
substrate strength that may resul
in blistering and peeling of late)
coatings, formulated with anc
without CaCO3 that are applied t<
Southern pine and Western rec
cedar substrates.
• Continue short-term exposures o
base acrylic latex (supplied by <
major producer) and commercia
paints on wood substrates and free
films. These tests would serve as «
guide for establishing degradatior
mechanisms and for designing
chamber exposures unde
atmospheric pollutant levels.
• Design and conduct a fiel<
exposure study using covering
spray devices at the Researcf
Triangle Park, NC, ant
Steubenville, OH materials expo
sure sites to investigate the
formation of mildew associated witf
the loss of ZnO, the discoloration o
yellowing of CaCO3 containing
paints, and the loss of filrr
substrate strength (peeling, etc.)
This study would complement anc
support the chamber exposure
studies.
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C. M. Balik, R. E. Fornes, and R. D. Gilbert are with North Carolina State
University, Raleigh, NC 27695 and R. S. Williams is with Forest Products
Laboratory, Madison, Wl 53705.
J. W. Spence is the EPA Project Officer (see below)..
The complete report, entitled "Effects of Acid Deposition on Painted Wood
Substrates," (Order No. PB 89-220 0241 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 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
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Official Business
Penalty for Private Use $300
EPA/600/S3-89/066
000085833 PS
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