United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S2-91/035 Sept. 1991
Project Summary
Radiation-Curable Coatings
S.A. Walata, III, and C.R. Newman
This report evaluates radiation-cur-
able coatings as a technology for re-
ducing volatile organic compound (VOC)
emissions from surface coating opera-
tions. A survey of the literature was
conducted to assess the state of the
technology and emissions from radia-
tion-curable processes. The information
collected from the literature was used
to evaluate the engineering and eco-
nomic issues associated with radiation-
curable systems and to identify the
requirements for implementing the tech-
nology and any problems arising from
its use.
Topics discussed in the report in-
clude coating characteristics, potential
VOC reduction capability, potential
health problems associated with the use
of ultraviolet (UV) coatings, and the eco-
nomic impacts of conversion to UV coat-
ings. The report provides Information
to permit an informed Judgement on
when and how to apply radiation-cur-
able technologies for industrial applica-
tion.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research Laboratory, Research
Triangle Park, NC, to announce key find-
ings of the research project that Is fully
documented In a separate report of the
same title (see Project Report ordering
Information at back).
Summary
This report presents the results of an
evaluation of radiation-curable coatings as
a technology for reducing volatile organic
compound (VOC) emissions from surface
coating operations. A survey of the litera-
ture was conducted to assess the state of
the technology and emissions from radia-
tion-curable processes. The data collected
in the literature survey were used to evalu-
ate the engineering and economic con-
cerns associated with radiation-curable
systems and to identify technical prob-
lems.
Radiation-curable coatings and inks are
higher solids formulations than conven-
tional coatings and, consequently from an
air pollution viewpoint, are considered to
be well suited substitutes for solvent-based
thermal-curable systems. The radiation
source for these systems is either an ultra-
violet (UV) light or an accelerated electron
beam (EB). A radiation-cured surface coat-
ing or printing process emits reduced lev-
els of VOC emissions in comparison to
solvent-based, thermal-curable processes
due to the solventless nature of most ra-
diation-curable systems. Radiation-curable
systems also require less energy to achieve
a cured film and operate at lower tempera-
tures than thermal-curable systems. This
allows radiation-curable systems to be used
on temperature sensitive substrates where
they can save 75-90% in energy costs.
The curing equipment for radiation-curable
compounds typically require 50-75% less
floor space than thermal-curing ovens.
Higher production rates than thermal-cur-
able systems are potentially achievable
due to the rapid curing rates of radiation-
curable systems. Four types of polymeriz-
able systems commonly used for
radiation-curable applications are unsatur-
ated polyester resins, multifunctional acry-
Printed on Recycled Paper
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lates, thiol-polyene systems, and cationic
polymerized epoxides. The photoinrtiators
required for UV-curable systems form the
initiating radical by photocleavage, hydro-
gen abstraction, or cationic photoinitiation.
Radiation-curable systems have sev-
eral characteristics which may limit imme-
diate applications in some commercial
environments. Atmospheric oxygen has a
retarding effect on the curing rate of free
radical polymerization processes due to
molecular oxygen's affinity for free radi-
cals. The presence of air in the radiation-
curable process can result in coatings with
tacky surfaces. Practical solutions to oxy-
gen interference will differ between UV-
and EB-curing systems due to the inherent
differences in the way the two forms of
energy interact with the coating material.
UV energy tends to be strongly absorbed
at the surface of the coating. This gener-
ates a higher proportion of free radicals at
the surface in relation to the rest of the
coating. With proper formulation, UV-cur-
ing can be conducted in ambient condi-
tions. The fairly uniform distribution of en-
ergy for EB-curable systems requires the
cure zone to be kept free of oxygen by
blanketing with an inert gas. Due to the
higher solids content, radiation-curable for-
mulations have a higher viscosity than ther-
mal-curable systems. The high viscosity of
radiation-curable systems may restrict po-
tential application methods. The viscosity
problem can be minimized by adding a
reactive diluent or heat or by thinning with
an organic solvent. In addition, some early
radiation-curable systems were considered
toxic. Current materials, however, have
been classified as only slightly toxic. Aery-
late materials used in radiation-curable sys-
tems are considered skin and eye irritants.
The severity of the irritation depends on
the material being used.
Cost per volume of coating formulation
has been used traditionally to determine
whether radiation-curable systems are ac-
ceptable alternatives to conventional ther-
mal-curable systems. The cost differential
on this unit basis between radiation- and
thermal-curable systems is relatively large
in favor of thermal-curable systems. The
higher unit cost of radiation-curable sys-
tems results in the user's perception that
radiation-curable systems are not economi-
cally attractive alternative printing or coat-
ing processes. However, comparing the
costs of the two systems on the basis of
cost per unit area of substrate coverage
better indicates the true cost of the coating
or printing process. Limited available data
on cost per area coated indicate the cost
differential to be in favor of radiation-cur-
able systems.
Available data indicate that some mono-
mer emissions could be present in the
exhaust for processes using radiation-cur-
able systems. Additional investigation to
determine the quantity and composition of
emissions resulting from the use of radia-
tion-curable coatings and inks would de-
termine the extent of any problems. All
other potential problems might be ad-
dressed by good engineering and operat-
ing practices of the installed system.
S. Walata and C. Newman are with Alliance Technologies Corp., Chapel Hill, NC 27514.
Charles H. Darvln is the EPA Project Officer, (see below).
The complete report, entitled "Radiation-Curable Coatings' (Order No. PB91-219550/
AS; Cost: $15.00, subject to change) will be available only from:
National Technical Information Seivice
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA PERMIT NO. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-91/035
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