EPA-600/2-91-035 July 1991

RADIATION-CURABLE COATINGS

Prepared by:

Stephen A. Walata 111 C.R. Newman

Alliance Technologies Corporation 100 Europa Drive, Suite 150

Chapel Hill, NC 27514

EPA Contract NO. 68-D9-0173 Work Assignment OR02 and 1/130

Project Officer

Charles H. Datvln Air and Energy Engineering Research Laboratory

U.S. Envlronmental Protection Agency Research Trlangle Park, NC 2771 1

Prepared for:

Control Technology Center U.S. Environmental Protection Agency

Research Triangle Park, NC 2771 1

CONTROL TECHNOLOGY CENTER

SPONSORED BY:

Emission Standards Division Office of Air Quality Planning and Standards

U.S. Environmental Protection Agency Research Triangle Park, NC 27711

Air and Energy Engineering Research Laboratory Office of Research and Development

U.S. Environmental Protection Agency Research Triangle Park, NC 27711

Center for Environmental Research Information Office of Research and Development

U.S. Environmental Protection Agency Cincinnati, OH 45268 -

PREFACE

EPAs Control Technology Center (CTC) was established by EPAs Office of Research and

Development (ORD) and Office of Air Quality Planning and Standards (OAQPS) to provide technical

assistance to State and local air pollution control agencies. Three levels of assistance are available

through the CTC. First, a CTC HOTLINE has been established to provide telephone assistance on

matters relating to air pollution control technology. Second, more in-depth engineering assistance can

be provided when appropriate. Third, the CTC can provide technical guidance through publication of

technical guidance documents, development of personal computer software, and presentation of

workshops on control technology matters.

The technical guidance projects, such as this one, focus on topics of national or regional interest

that are identified through contact with State and local agencies. In this case, the CTC assisted the

Bay Area Air Quality Management District in San Francisco, CA, in identifying the scope and nature

of radiation-curable coatings and inks being used in industry. This document discusses advantages

and disadvantages of radiation-curables as well as potential future use.

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TABLE OF CONTENTS

Page

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv ListofTables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V

1.0 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2.0 BACKGROUND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.2 Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.3 Curing and Drying Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.4 Literature Sources . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.0 RADIATION-CURABLE SYSTEMS IN INDUSTRY . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1 Current Usage in Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2 Advantages of Radiation-Curable Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.3 Limitations of Radiation-Curable Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.3.1 Oxygen Inhibition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.3.2 Cure Penetration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.3.3 Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.4 Potential Applications for Radiation-Curable Systems . . . . . . . . . . . . . . . . . . . 15

4.0 TOXICITY DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4.1 General Toxicological Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4.2 Permissible Exposure Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.3 Safety and Radiation-Curable Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5.0 EMISSIONS DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 5.1 Theoretical Emissions Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 5.2 Airborne Hazard of Radiation-Curable Systems . . . . . . . . . . . . . . . . . . . . . . . 24 5.3 V O C Reduction Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

6.0 COST COMPARISON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.1 Costs Associated with Curing Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.2 Costs Associated with Coating and Ink Material . . . . . . . . . . . . . . . . . . . . . . . 27 6.3 Costs Associated with Controlling VOC Emissions . . . . . . . . . . . . . . . . . . . . . 30 6.4 Other Cost Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 6.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

7.0 CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

8.0 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

iii

LIST OF FIGURES

Number Page

2-1 UV-Curable System . Continuous Web Configuration . . . . . . . . . . . . . . . . . . . . . . . . 7

2-2 UV-Curable System . Sheet-Fed Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2-3 EB-Curable System . Continuous Web Configuration . . . . . . . . . . . . . . . . . . . . . . . . 8

2-4 EB-Curable System . Sheet-Fed Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

6-1 Cost of Coating a 1,000.Squar e.Foot Area with Various Types of Fiat Line Finishing Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

6-2 Pounds of VOCs Emitted While Coating a 1,000.Squar e.Foot Area with Various Types of Flat Line Finishing Equipment . . . . . . . . . . . . . . . . . . . . . . . 33

Number

LIST OF TABLES

Page

2-1 Comparison of the Main Characteristics of Radiation-Curable Systems . . . . . . . . . . . 5

2-2 Literature Sources for Radiation-Curable Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3-1 Current Applications of UV-Curable Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3-2 A Select Sample of Companies Using Radiation-Curable Systems . . . . . . . . . . . . . . 11

3-3 Viscosity Ranges for Various Printing Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4-1 Permissible Exposure Limits of Radiation-Curable Material . . . . . . . . . . . . . . . . . . . . . 19

4-2 Permissible Exposure Limits of Organic Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

5-1 VOC Emissions Reduction Resulting from Conversion to Radiation-Curable Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

6-1 Capital Cost Elements and Factors for Select Add-on Control Systems . . . . . . . . . . . 31

6-2 Typical Items Included in Annual Costs of Add-on Control Systems . . . . . . . . . . . . . 32

i v

EXECUTIVE 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 literature was conducted to assess the state of the technology and emissions from radiation-

curable processes. The data collected in the literature survey were used to evaluate the engineering

and economic concerns associated with radiation-curable systems and to identify technical problems

requiring future resolution.

Radiation-curable coatings and inks may be considered higher solids formulations than

conventional coatings and, consequently from an air pollution viewpoint, are considered to be a well

suited substitute for solvent-based thermal-curable systems. The radiation source for these systems

is either an ultraviolet (UV) light or an accelerated electron beam (EB). A radiation-cured surface

coating or printing process emits reduced levels of VOC-emissions in comparison to solvent-based,

thermal-curable processes due to the solventless nature of mosf radiation-curable systems. Radiation-

curable systems also require less energy to achieve a cured film and operate at lower temperatures

than thermal-curable systems. This allows radiationcurable systems to be used on temperature

sensitive substrates and can yield a 75 to 90 percent savings in energy costs. The curing equipment

for radiation-curable compounds typically requires 50 to 75 percent less floor space than thermal-curing

ovens. Higher production rates than thermalcurable systems are potentially achievable due to the

rapid curing rates of radiation-curable systems. Four types of polymerizable systems commonly used

for radiation-curable applications include unsaturated polyester resins, multifunctional acrylates, thiol-

polyene systems and cationic polymerized epoxides. The photoinitiators required for UV-curable

systems form the initiating radical by either photocleavage, hydrogen abstraction or cationic

photoinitiation.

Radiation-curable systems have several inherent limitations which may limit immediate

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 radicals.

The presence of air in the radiation-curable process can result in coatings with tacky surfaces.

Practical solutions to oxygen interference will differ between UV- and EBcuring 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 syrface of the coating. This generates a higher proportion of free

radicals at the surface in relation to the rest of the coating. With proper formulation, UV-curing can

be conducted in ambient conditions. The fairly uniform distribution of energy for EBcurable 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 formulations have a higher viscosity than thermal-curable systems.

The high viscosity of radiation-curable systems may restrict potential application methods. The

V

viscosity problem can be minimized by the addition of a reactive diluent or additional 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. Acrylate materials used in

radiation-curable systems are considered skin and eye irritants. The seventy of the irritation depends

on the material being used.

Cost per volume of coating formulation traditionally has been an economic factor used to

determine whether radiation-curable systems are an acceptable alternative to conventional thermal-

curable systems. The cost differential on this unit basis between radiation-curable and thermal-curable

systems is relatively large in favor of thermalcurable systems. The higher unit cost of radiation-curable

systems results in the user’s perception that radiation-curable systems are not an economically

attractive alternative printing or coating process. However, when the costs of the two systems are

compared on the basis of cost per unit area of substrate coverage, a better indication of the true cost

of the coating or printing process is given. Limited available data on cost per area coated indicate the

cost differential to be in favor of radiationcurable systems. ~

Available data indicate that some monomer emissions could be present in the exhaust for

processes using radiation-curable systems. Additional investigation to determine the quantity and

composition of emissions resulting from the use of radiation-curable coatings and inks would define and

determine the extent of the problem, if any. All other potential problem areas might be addressed by

good engineering and operating practices of the installed system.

Vi

¶

1 .O INTRODUCTION

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. Traditionally,

surface coating operations (e.g., furniture finishes, paper varnishes, protective varnishes on aluminum

cans) have used coating formulations consisting of a functional part, a solid polymer, dissolved in an

organic solvent. The formulation may be further diluted if required with more solvent (e.g., methyl ethyl

ketone, ethyl acetate, toluene, alcohols) to reduce the viscosity in order to facilitate application to a

substrate. After the coating has been applied, the solvent is removed by evaporation. Traditional

thermal-curing methods use a heat source to speed solvent evaporation and trigger the polymerization

reaction. The solvent evaporates into the air currents and is carried away from the substrate, leaving

a dry film. The solvent-laden air is then exhausted through a stack or a vent with or without being

subject to some vapor removal control system (Lankford,-1983). The polymer network formed by the

coating may require a thermally sensitive catalyst to initiate the polymerization reaction.

Radiation-curable coatings may be considered higher solids formulations used in various surface

coating industries as an alternative to energy- and solvent-intensive coatings and inks. Radiation-

curable coatings and inks are composed of organic material (i.e., polyesters, acrylates, epoxies) and,

unlike thermal coatings, use ultraviolet (UV) or electron-beam (EB) energy to initiate the reaction to

form a polymer network. Since the formulations contain little or no organic solvents, use of radiation-

curable systems may allow surface coating operations to comply with current VOC emissions

restrictions. Although radiation-curable systems have been used in commercial operations for 20 years,

little information is available to allow local air pollution control agencies to assess the technology's

effectiveness as a process material change to reduce VOC emissions.

The Bay Area Air Quality Management District in San Francisco, CA, requested the assistance

of EPAs Control Technology Center (CTC) in compiling information on radiationcurable coatings for

the purpose of evaluating this technology as a process change to achieve VOC reduction. CTC is

responsible for supporting State and l o c a l air pollution control agencies in the implementation of their

programs. Work has involved: (1) a suwey of the literature to assess the state of the technology and

emissions from radiation-curable coating processes; (2) an evaluation of engineering and economic

concerns associated with radiation-curable systems; and (3) an identification of technical problems

preventing universal adoption of this coating and printing technology.

1

2.0 BACKGROUND

2.1 HISTORY

The use of radiation-curable coatings dates back at least 4,000 years. The ancient Egyptians

used a type of UV-curable coating in the preparation of mummies which cured when exposed to

sunlight (AFWSME, 1986). An asphatbased oil coating that polymerized upon exposure to solar

radiation was also used by the ancient Egyptians as a sealant for ships (Decker, 1987).

In the modern era, scientific interest in developing radiation-curable systems began only in the

1940’s. At that time, the first patent was granted for an unsaturated polyester styrene printing ink that

polymerized under UV exposure (Decker, 1987). One of the first attempts at applying radiation-curable

systems to a manufacturing system was made in the late 1960’s. This high-energy electron beam

curing system developed by the Ford Motor Company, however, was abandoned due to complexities

of the process and associated high costs (Lawson and- Kaminsky, 1982). Successful commercial

application of radiation-curable systems did not evolve until th3 early 1970’s. The publicized driving

forces behind the development of commercially viable systems were the energy crisis of the early

1970’s and the growing environmental concerns about VOC emissions resulting from conventional

thermal-curable systems (Kubisen, et al., 1984; Lankford, 1983). However, the primary motivations for

the use of radiation-curable systems are improved product performance and productivity (Lawson and

Kaminsky, 1982).

The early applications of radiation-curable systems were limited to flat line processes mainly in

the lumber and printing industries (AFP/SME, 1986). This was primarily because radiation-curable

processes of that time could only be cured when in the line-of-sight of the radiative source. Starting

in 1974, UV-curable inks and overvarnishes were used for decorating aluminum beverage cans

(LeFevre, 1980). Improvements in plant engineering, such as rotating conveyers and adjustments to

the curing equipment, have allowed the three-dimensional application of radiationcurable coatings and

inks (Lawson and Kaminsky, 1982). In addition, advances in the field of polymer science have

provided a wide variety of materials for formulations which can exhibit the characteristics required by

an end-user in specific industrial applications (Decker, 1987).

2.2 CHEMISTRY

The basic chemistry of radiationcurable systems involves two components which make up the

polymer network of a coating or ink formulation. One component of a formulation is reactive resins,

which form the backbone of the coating. Usually referred to as oligomers, these compounds typically

have a relatively high molecular weight and are highly viscous. A second component in a formulation

is the reactive diluents. Reactive diluents are low molecular weight materials used to reduce the

viscosity of a formulation (Danneman, 1988). These reactive diluents become part of the polymer

2

network and enhance or add to the physical properties of the coating or ink. The formulation may also

contain other components such as pigments, flow modifiers, and fillers.

The polymer network is formed by either bridging existing polymer chains (e.g., vulcanization of

rubbers) or by promoting the polymerization of compounds that contain at least two compatible reactive

functionalities in their molecules. The latter characteristic results in a chain reaction which develops

rapidly when radiation (UV or EB) is used to produce the initiating species and can ultimately lead to

a network exhibiting a very high crosslink concentration (Decker, 1987). In EB-curable formulations,

the radicals required for crosslinking the oligomers and/or monomers are generated through electron

bombardment (Lankford, 1983). UV-radiation, on the other hand, does not have enough energy to

produce initiating species in sufficient quantity and thus requires a photoinitiator to initiate the chain

reaction. Photoinitiators are compounds which absorb incident light and produce initiating species with

high efficiency.

The various photoinitiators in use are classified into three major categories depending on the

mechanism involved in their photolysis. The most efficient category of photoinitiators forms radicals

by photocleavage, a process by which the photo intitiator molecule is fragmentated by use of

electromagnetic energy. UV radiative energy dissociates a bond in the photoinitiator, resulting in the

formation of two radical molecules. Included in this category are aromatic carbonyl compounds,

benzoin derivatives, benzilketals, acetophenone derivatives, and hydroxyalkylphenones, where the

benzoyl radical is the major initiating species. Other initiators of the photocleavage type include

acylophosphine oxides and the substituted a-amino ketones (Decker, 1987).

The next category of photoinitiators generates radicals by hydrogen abstraction. Aromatic

ketones, when promoted to an excited state by UV-radiation, abstract a hydrogen atom from a

hydrogen-donor molecule to the oxygen in the carbonyl group, which generates a ketyl radical and the

donor radical. The hydrogen-donor molecules most often used in this type of initiation are tertiary

amines. In this case, the hydrogen-donor radical initiates the polymerization of the formulation. The

ketyl radicals are consumed mainly through a radical coupling process. Compounds included in this

type of photoinitiation are benzophenone and thiozanthone.

The last category of photoinitiators are cationic photoinitiators. Aryldiazonium salts undergo a

fast fragmentation under UV-radiation with formation of free Lewis acids which are efficient initiators

for the polymerization of epoxy monomers (Decker, 1987).

Four different types of polymerizable systems are commonly used for radiation-curable

applications. Early radiationcurable systems were based on unsaturated polyester resins. The

reactive diluent used in these type of systems is usually styrene monomer. The resulting cured

material is very hard and solvent-resistant. The curing rate of this system, however, is relatively slow,

and problems with air pollution may arise from the volatility of the styrene diluent. Due to their low

cost, polyester resins are used mostly in industries where large tonnages are involved (Decker, 1987).

3

Multifunctional acrylates are the most widely used compound class in radiation-curable systems

due to their high reactivity, moderate cost, and low volatility (Bean, 1984). The variety of multifunctional

acrylates allows this type of compound class to be used as both an oligomer and a reactive diluent.

Multifunctionaldy of a compound means that it has more than one potential reaction site for the

polymerization process (AFP/SME, 1986). A large variety of prepolymer material is available to the

end-user, allowing for the creation of polymer networks with tailor-made properties. Because of the

high viscosity of the oligomers, reactive diluents must be added to the formulation. These are usually

low volatility mono- or multifunctional acrylates that provide good application properties and increase

the cure speed (Decker, 1987). The diluent material reacts with the oligomers to become part of the

coating. However, depending on the vapor pressure of the material, a small fraction may be emitted

into the surroundings before the curing process is completed.

Thiol-polyene systems use the photochemically-induced addition of a thiol to an olefinic double

bond as the network-forming reaction, provided multifunctional materials are involved. Using hydrogen

abstraction photoinitiators, thiol-polyene systems are not very sensitive to oxygen inhibition because

radicals formed by oxygen scavenging can still abstract hydrogen from a thiol group and continue the

chain reaction. A broad range of properties can be obtained for the cured material by varying the

functionaldy and the backbone structure of both the polyene and polythiol.

The last type of polymerizable system discussed is the use of cationic polymerization of

epoxides. These systems involve the cationic ring opening polymerization of epoxy resins, when

catalyzed by a Lewis acid or protonic acid. Such systems are less sensitive to oxygen inhibition than

unsaturated polyester or multiiunctional acrylate systems but have a relatively slow cure rate. Claims

have been made that systems using cationic polymerization of epoxides cause less skin irritation than

acrylate systems (Vrancken, 1980; Decker, 1987). The main characteristics of the various radiation-

curable systems are compared in Table 2-1.

2.3 CURING AND DRYING EQUIPMENT

When a coating or ink has been applied to a substrate, the "wet" coating or ink must be cured

before proceeding in the manufacturing process. Conventional thermal-curable systems use a dryer

tunnel. Frequently, high velocity heated air is used to drive off solvents from the coating, leaving a dry

film (Lankford, 1983). In the case of solution and reactive coatings, the temperature initiates the

polymerization reaction. Radiationcurable systems, on the other hand, cure at room temperature using

radiative energy in the form of accelerated electron energy (EB) or electromagnetic energy (UV-light)

to cause the formation of the polymer network (AFP/SME, 1986).

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TABLE 2-1. COMPARISON OF THE MAIN CHARACTERISTICS OF RADIATION-CURABLE SYSTEMS

OXYGEN EB uv SYSTEM REACTIVITY INHIBITION DURABILITY CURABLE CURABLE

~ ~~

Polyester/styrene Low High Poor No Yes

Acrylates High High Moderate Yes Yes

Thiol-polyene High Low High Yes Yes

Cationic Epoxies Medium Low Fair No Yes

Sources: Decker, C.. ‘UV-Curing Chemistry: Past, Present, and Future,’ J. ol C08thp Tech., 59(751):97-108, 1987. Vrandcen. A, ’Market Trends for Irradiation Curable Coatings and Printing Inks in Eumpe,” fmm the pmceedings of Radiation Curing V; A L& to the 80’s. ksoaation of Finishing Processes of M E , Sept. 23-25, Boston, MA. 1980.

UV-curable systems typically use medium-pressure meicury vapor lamps to generate the UV

radiation energy for curing. These lamps provide electromagnetic energy with a wavelength of 200 to

400 nanometers (Frecska, 1987; Garratt, 1984). UV-radiation in this range provides the requisite

energy to the photoinitiators to form the radicals which begin the polymerization process. A UV-curable

system also has reflectors to enhance the focus and distribution of UV-light onto the substrate.

Shutters may be a part of the UV-curing system to protect heat-sensitive substrates which may be

under the lamp when the conveyor stops. These shutters are normally linked with a press or belt

switch and close automatically when the coating or printing line stops (Knight, 1979). UV-curing

systems require shielding to minimize exposure to UV-radiation by the curing process personnel. Since

UV-light is not visible to the eye, workers may not be aware of its presence or related dangers. The

skin and eyes are particularly vulnerable to injury caused by overexposure to UV-radiation. Common

injuries caused by overexposure are skin erythema (i.e., sunburn), heat stress, eye conjunctivitis, and

lesions on the cornea known as photokeratitis. Chronic overexposure can result in skin cancer,

cataracts, retinal damage, keratoconjunctivitis, genetic alteration, and asthenopia (Kovachik, 1987b).

UV-curing systems also may require a ventilation system for the irradiation area to remove ozone

generated when short wavelength UV photons are absorbed by oxygen. Ozone generation by the UV-

curing system can be minimized with the use of quartz filters fitted to the lamp which filter out the short

wavelength photons (Kovachik, 1987b):

Most EB-curable systems include a power supply, vacuum system, and an electron beam gun,

which is used to bombard the coated substrate with accelerated electrons (Lankford, 1983). As the

electrons lose their energy in the coated material, the energy from the electron beam interacts with the

unsaturated prepolymer material, forming the radical required for the polymerization process to begin.

The amount of energy lost by the electrons is directly related to the curing completeness of the coating.

5

Personnel can receive ionizing radiation exposure from both electrons and x-rays. Protection from

ionizing radiation from electrons can be achieved by preventing direct exposure to the flux of electrons

from the electron gun. Protection from x-rays is normally achieved by lining the unit with material of low atomic number followed by material having high atomic number. Current federal guidelines for

occupational exposure limit the dose-equivalent to the whole body to 3 rems per year, and an

accumulated occupational dose-equivalent to the whole body not exceeding 5(N-18) rem, where N

equals the individual age in years.

Radiation-curable system equipment is generally operated as either a continuous web or sheet-

fed configuration. Figures 2-1 to 2-4 provide the outlines of the basic configurations used in radiation-

curable systems with roll coating application.

2.4 LITERATURE SOURCES

Table 2-2 lists literature sources that were reviewed in this project. This list represents the

sources available during the literature search and should not be considered an all inclusive list of

information sources on radiation-curable systems.

TABLE 2-2. LITERATURE SOURCES FOR RADIATION-CURABLE SYSTEMS

Radiation Curing (periodical)

Journal of Radiation Curing (periodical)

Journal of Coatings Technology (periodical)

Modern Paint and Coatings (periodical)

Furniture Design and Manufacturing (periodical)

Paint and Resin (periodical)

Conference Proceedings and Literature published by the Association for Finishing Processes of the Society of Manufacturing Engineers

6

Roll

Coating Reservoir

Unwind Roll Station

Rewind Roll Station

Figure 2-1. UV-curable system - continuous web configuration.

Feed Conveyor

0 Feed

n Stacking

Figure 2-2. UV-curable system - sheet-fed configuration.

7

Electron Chamber /- Beam Gun

lnotina

Roll

I

Beam Collector

Unwind Roll Rewind Roll Station Station

Figure 2-3. EB-curable system - continuous web configuration.

Coating Reservoir 7

Feed Conveyers Stacking

Figure 2-4. EB-curable system - sheet-fed configuration.

a

3.0 RADIATION-CURABLE SYSTEMS IN INDUSTRY

3.1 CURRENT USAGE IN INDUSTRY

Radiation-curable systems became a generally viable commercial process in the early 1970’s.

Among the early successful radiation-curing processes were printing operations and wood processing

operations (Bean, 1984; Garratt, 1984; Currier, 1989; AFP/SME, 1986). Since their introduction into

manufacturing, radiation-curable systems have been used in many applications in the surface coating

and printing industries. Table 3-1 provides a general overview of the main applications of radiation-

curable systems. One of the primary uses of radiation-curable systems is in resilient flooring, where

no-wax floor coatings are radiatively cured onto vinyl tiles (Pincus and Sickinger, 1988; Pincus, 1983;

Prane, 1980). The graphic arts industry is another large user of UVcurable inks and varnishes,

accounting for almost 50 percent of the radiationcurable market (Decker, 1987; Morris, 1984). A

relatively new, yet quickly growing, industry using radiation-curable systems is the microelectronics

industry (Frecska, 1987).

After the initial success of UV systems in wood processing, no-wax resilient floors, and printing

inks, experts predicted radiation-curable technology would soon capture at least 20 percent of the

overall surface coating and ink market. However, the facilities using radiation-curable systems have

been limited to 2 to 3 percent of the overall surface coating and ink market (Schessler, 1984).

Intense competition among equipment manufacturers and material suppliers has led to dollar and

product volumes being regarded as proprietary information, which makes it difficutt to assess market

size (Pincus, 1983). However, hundreds of companies, ranging from large corporations to small job-

shops, use radiation-curable systems in manufacturing processes. Table 3-2 provides a small sample

of companies which are currently using radiation-curable technologies.

3.2 ADVANTAGES OF RADIATION-CURABLE SYSTEMS

Radiation-curable systems have advantages over conventional thermalcurable systems in

energy efficiency, operating environment (i.e., working condiiions), facility usage, and productivity. In

general, the majority of current radiation-curable systems contain none of the organic solvents found

in conventional coatings and inks. In fact, the film forming components in a radiationcurable system

may be considered to be essentially 100 percent reactive, which means all of the material is converted

into the polymer network, and nothing evaporates before the coating or ink is considered dry (Lankford,

1983). The exception to this is some photoinitiators for UVcurable systems, which may not become

part of the formed polymer system of the film and are available for migration, extraction, and/or

absorption (Bean, 1984). Consequently, no additional energy is required to volatilize an organic

solvent. This results in EB and UV systems using only about 20 percent of the energy consumed by

thermal systems (Clark, 1990). The absence of heat energy, in contrast to the thermal systems, means

9

TABLE 3-1. CURRENT APPLICATIONS OF UV-CURABLE SYSTEMS

Process Estlmated Usage in 1985

(million pounds)

Surface Treatment Graphic Arts' Wood Finishes Metal Coatings Plastic Coatings Paper Varnishes Resilient Flooring

Electronics Printed Circuits (negative photoresists) Sealants (encapsulation)- Protective Coatings (optical fibers) Patterning (video disks, compact disks)

Pigmented Resins UV-Curable Inks Print Plates Dental Material

Adhesives Laminates Sealants Bonding Pressure Sensitive

13 10 N/A 15 16

1 42

11 14 N/A

N/A N/A N/A N/A

'Gnphic Arts usage is retleded in Metal and Plastic Coatings. Paper Varnishes. UV-Curable Inks and Print Plates. b a a nflect industry-wide usage. NIA - Data not available 1 pound - ,454 kilogrpm

Dedcer, C.. 'UV-Curing Chemistry: Past, Present. and Future,' J. of Coati- Tdmobgy, 59(751)97-106,1987. Prane, J.W., 'Ultraviolet Curable 'halings and Inks - Markets and Pmjediom,' fmm the pmceedings of R d W n Curing V: A Lodc lo the BO!! Association of Finishing Pmcesws of WE. Sept. 23-25. Boston. MA, 1980.

that the radiation-curable systems operate at or near room temperature. Because of the low operating

temperature, radiation-curable systems can be used on temperature-sensitive substrates (e.g.,

polyethylene, polypropylene, polyzgstyrene, wood, low quality paper). The reduced energy

requirements result in a 75 to 90 percent savings on energy costs over thermal-curable systems

(AFPISME, 1986; Lawson and Kaminsky, 1982).

10

TABLE 3-2. A SELECT SAMPLE OF COMPANIES USING RADIATION-CURABLE SYSTEMS

Company Usage

Coors Container Company

Loewenstein, Inc.

The Stanley Works

Corning Glass

Solder Woodworking

Universal Wood

Westvaco

Metalized Products

American Telephone & Telegraph

Doubleday

Armstrong World Color

General Motors

James River C o p

Hallmark

Metal Coating

Furniture

Metal Coating

Fiber Optics

Furniture

Furniture

Paper Coating

Thermal Blankets

Fiber Optics

Paper Coating

Resilient Flooring

Sealants and Adhesives

Graphic Arts

Graphic Arts

Sources: Proceedings of Radation Curing V: A Look to the 80s, Association of Finishing Processes of SME, 1980. Proceedings of Radcure ‘84 Conference, Association of Finishing Processes of SME, 1984.

The second advantage of radiation curing systems over thermal equipment is the different facility

space requirements. The basic requirement of the curing equipment, regardless of type, is to provide

a sufficient residence time for the requisite energy to cure the coating or ink. In thermally-cured

systems, the coated material is subject to heat transfer rates. Time is required to heat the substrate

and coating to the proper temperature to vaporize the solvent and cause chemical crosslinking.

Depending on the volatility of the solvent, the initial temperature of the coating and the speed of the

production line, the length required for the dryer tunnel to evaporate and exhaust the solvent and cause

potential curing reactions to be completed can vary between 15 and 230 feet (4.6 and 70.2 meters),

(Lankford, 1983; Clark, 1990). Radiation-curable systems, on the other hand, require a short time

period for the radiative energy to generate the initiating species. A chain reaction then takes place to

cure the coating. Consequently, UV- or EB-curing equipment is more compact than the thermal

11

counterpart and requires less space. For example Coors Container Corporation replaced a 230-foot

thermal oven used to cure paint on beer cans with a six-foot UV curing system (Clark, 1990). Such

an example may be extreme, but, on average, radiation curing systems require 50 to 75 percent less

space than thermal-curing systems (Arnold, 1984).

Another advantage of radiation-curable systems is the potential for higher production rates than

thermal-curable systems due to their rapid curing rates (Morris, 1984; Lawson and Kaminsky, 1982).

The more rapid curing rate of radiation-curable systems allows for faster production throughputs, either

from faster line speeds, elimination of an oven pass, or more rapid handling in subsequent operations.

The highly crosslinked nature of radiation-curable systems provides advantageous characteristics

for the resulting coating film. Characteristics, such as solvent and heat resistance, provide the coated

substrate with better stain resistance and resistance to any possible heating operation than most

conventional coatings (AFPISME, 1986). High scratch resistance and improved resistance to natural

weathering over conventional coatings can be achieved with systems that have high crosslink density

characteristics (Morris, 1984; Decker, 1987).

The fact that radiation-curable coatings and inks do not cure until exposed to the proper radiation

source provides a distinct advantage overthermal-curable systems. This allows for easier maintenance

of the application and curing equipment, since the coating or ink will not cure in the production

equipment during normal operations. The absence of heat in the process allows for a more rapid and

efficient maintenance of the curing equipment. Since radiation curing equipment systems are “instant

on” systems, there is no need for lengthy cool-down and warm-up periods as required by thermal

ovens (LeFevre, 1980).

3.3 LIMITATIONS OF RADIATION-CURABLE SYSTEMS

Although radiation-curable systems have advantages over conventional coatings and inks, they

also have some inherent limitations, which may restrict potential applications to commercial

environments. The more severe limitations are examined in the following pages.

3.3.1 Oxygen Inhibition

Some coatings lose some of their reactivity when exposed to air. Such inhibition has a retarding

effect on the curing process due to molecular oxygen’s affinity for the free radical, forming a peroxide

(Decker, 1987). Such reactions deplete the number of free radicals. They are therefore no longer

available to react with unsaturated bonds to continue the polymerization chain reaction. Further, such

copolymerization of oxygen into the polymer network presents potential reactive sites for subsequent

photodegradation. Relatively small amounts of oxygen can result in a coating with a tacky surface after

curing because of reduced chain length or molecular weight (Vrancken, 1980). UVcurable systems

12

a

typically are not as sensitive to oxygen inhibition as EB-curable systems, because a high concentration

of photoinitiated radicals is found at the surface. A high concentration of radicals at the surface

interface is required to offset the competition from molecular oxygen (Vrancken, 1980). If oxygen

inhibition becomes a problem for a UVcurable system, increasing the light intensity or the addition of

oxygen barriers, such as nitrogen blanketing, can minimize the undesired effects (Decker, 1987).

Oxygen inhibition is more of a problem in EB-curing systems. Unlike UV-curable systems, radical

formation in EB-curable systems is not higher at the surface. This means that an insufficient quantity

of free radicals is present at the surface to surmount the effect of molecular oxygen. To solve the

oxygen inhibition problem, EB-curing is typically carried out within an inert cure zone using a nitrogen

gas blanket as the oxygen barrier (Vrancken, 1980).

3.3.2 Cure Penetration

One of the major limiting factors of UV-curable systems is the thickness of the film which can

be cured, due to both the light’s inability to penetrate the organic matter which comprises the

prepolymer material and the depletion of light energy due to absorption by surface photoinitiators

(Decker, 1987). Studies have shown that as the percentage of photoinitiator in an UV-curable

formulation increases, the quantity of light energy penetrating the coating decreases (Lewarchik and

Hurwitz, 1984). Most of the applications of UV-curable systems, therefore, have been restricted to thin

films and coatings. Another limitation of UV-curing involves the curing of pigmented coatings. The

pigment in a formulation blocks the light and restricts the spectral region available for efficient utilization

by photoinitiators (Garratt, 1984). Most UV-curable systems operate in the wavelength range of 200

to 400 nanometers. Successful applications of clear UV-curable systems have been achieved with film

thicknesses of 130 to 250 pm (0.005 to 0.01 inches) (Lewarchik and Hurwitz, 1984). Pigmented UV-

curable systems, however, have been cured with film thicknesses up to only 50 pm (0.002 inches)

(Bean, 1984).

Cure penetration can be improved by reducing the concentration of the photoinitiator in the

coating formulation, which improves the depth of cure penetration, but lengthens the cure rate. High

intensity light, such as pulsed sources or lasers, may improve the depth of penetration of the light in

some applications. Development of photoinitiators which become transparent to UV light upon

photolysis would also improve cure penetration (Decker, 1987). Pigments with good hiding power have

also been developed for printing inks a 9 can be cured at film thicknesses of 10 to 15 pm (0.0004 to

0.0006 inches) at practical line speeds (Garratt, 1984). Increasing the number of lamps is another

means of improving the curing process for pigmented coatings. However, this compromises the space-

saving and energy cost advantages to some extent (Schessler, 1984).

13

3.3.3 Viscosity

The high viscosity characteristic of radiation-curable coatings or inks is not well suited for

traditional coating equipment. Over two-thirds of traditional, thermally-cured finishes are applied by a

spray process which requires a relatively low viscosity (Lawson and Kaminsky, 1982). The application

of inks in various printing processes also requires a specific viscosity as shown in Table 3-3.

Typically, before a reactive resin can be applied to a substrate in an industrial process, the

viscosity must be reduced to meet the requirements of the application equipment. Several viscosity

reduction methods have been devised, including a method involving the addition of low-viscosity

reactive diluents. Instead of evaporating during curing like solvent diluents in conventional systems,

most reactive diluent materials react with the growing polymer chain to become part of the finished film

(Danneman, 1988). Thus, only a fraction of these reactive diluents are emitted to the atmosphere.

Use of reactive diluents, however, does have potential limitations. One such limitation is that these

reactive diluents may result in unwanted physical changes (e.g., brittleness, yellowing) in the resulting

film (Lawson and Kaminsky, 1982). The reactive diluents may also possibly cause problems with the

cure rate of the system because some low-viscosity reactive diluents have poor curing rates (Bean,

1984). Emissions problems may also arise, since the reactive diluents with the lowest viscosities are

likely to be the most volatile, hence a greater fraction evaporates before reaction (AFPBME, 1986).

Another method of reducing the viscosity of a radiation-curable coating or ink is the application

of heat to the formulation. Studies have shown that a temperature increase of 15°C would frequently

reduce the viscosity of a formulation by half (Lawson and Kaminsky, 1982; Bean, 1984). As with

adding reactive diluent, heating the formulation to reduce the viscosity also has limitations. For highly

viscous formulations, heating may not reduce the viscosity to a degree necessary for the application

equipment. In addition, applying too much heat may cause the formulation to decompose and to emit

acrid smoke and irritating or toxic fumes (Sax and Lewis, 1989). Furthermore, heating increases the

evaporation rate and may resutl in greater VOC emissions.

Reducing the viscosity of radiation-curable coatings or inks can also be accomplished by the

addition of an organic solvent to the formulation. This method has been gaining wide acceptance in

many application processes, especially in the furniture and wood processing industries (Riedell, 1989;

Currier, 1989). This method incorporates a quantity of conventional solvent into the formulation in order

to adjust rheological properties (i.e., viscosity) to permit the use of spray equipment (Garratt, 1984).

The solvents used are usually fast evaporating (i.e., ketones and alcohols) and provide the proper

viscosity for good atomization and rapid flow over on the substrate when using spraying equipment

(Lawson and Kaminsky, 1982). Before the formulation can be exposed to the radiation source, flash

time is required to remove the solvent (FDM, 1989b). Sprayable UV-curable systems used in the

furniture industry generally contain 47 percent by volume (40 percent by weight)

14

TABLE 3-3. VISCOSITY RANGES FOR VARIOUS PRINTING PROCESSES

Printing Process Viscosity at 25°C (Centipoise)

Letterpress Lithography News Ink Flexography Gravure Screen Printing

1,000 - 50,000 10,000 - 80,000

200 - 1,000 50 - 500 30 - 200

1,000 - 50,000

Source: Radiation Curing: An Introduction to Coatings, Varnishes, Adhesives and Inks, Second Edition, The Association for Finishing Processes of the Society of Manufacturing Engineers, 1986.

conventional volatile organic solvents. A conventional thermal-curable coating for similar use contains

85 percent by volume (79 percent by weight) organic solvent (Currier, 1989). In addition, there are a

fewspray applications of radiation-curable systems which require high organic solvent contents in the

range of 70 to 80 percent by volume. When these organic solvent-containing, radiation-curable

systems are used, removal of the organic solvents from the coating must be conducted in a manner

similar to thermal-curable compounds. Additional safety factors must be included since most

UV-curable units are not equipped for handling solvent laden atmospheres (Lawson and Kaminsky,

1982). An alternative may be found in water-based UV-curable acrylate resins being developed in

Europe for low-viscosity, low VOC-emitting inks and top coats (Ashcroft, 1987). Similarly, there are

waterborne radiation-curable nitrocellulose systems are available in the United States.

The use of organic solvents to reduce the viscosity of radiation-curable coatings and inks

compromises some of the advantages inherent to the systems. For example, increased space would

be required and a potential for increased energy demand for the coating process to evaporate the

organic solvent prior to initiating the polymerization process. Additionaliy, an undesired effect of using

organic solvents as a viscosity reducing agent is the need to control VOC emissions. Likely sources

of VOC emissions resulting from organic solvent-containing, radiation-curable systems would be the

spray booths and drying areas and may require the facility to install an emissions controls system to

meet local regulations.

3.4 POTENTIAL APPLICATIONS FOR RADIATION-CURABLE SYSTEMS

Radiation-curable systems have the potential to be used in nearly every application which

employs conventional thermal-curable coatings and inks. In most cases, the application equipment

used for the thermal-curable systems can be used with some modifications for the radiation-curable

systems. The more promising uses of radiation curable systems involve application techniques which

apply materials having relatively high viscosity. Among such application techniques are roll coating,

offset lithographic printing and screen printing. Roll coating processes are used in the wood industry

for applying fillers and top coatings and in the paper, plastics, and metal industries for applying clear

protective top coats (Jones, 1980). Roll coating applications can operate in either a continuous web

or sheet-fed configuration. Offset lithographic processes are being used with radiation-curable inks in

the metal decorating industry (e.g., two-piece beverage cans) (Schessler, 1984). The use of UV-

curable inks in conjunction with screen printing processes has been used in the electronics industry.

Spray and dip-coating applications of radiation-curable systems are possible with the addition

of a volatile organic solvent to control the viscosity. Spray application of UV-curable top coats with a

low VOC content 3 to 5 pounds per gallon (360 to 600 grams per liter) is currently being used in the

wood furniture industry (FDM, 1989a).

16

4.0 TOXICITY DATA

4.1 GENERAL TOXICOLOGICAL DATA

Unstable chemistry of formulations and dermatitis problems from formulations used in graphic

arts left early radiation-curable systems with a reputation for high toxicity (Pincus and Sickinger, 1988).

This perception continues to the present time to some degree and may be the result of workers not

fully understanding or respecting the materials being used (AFPISME, 1986). More recently developed

radiation-curable systems have shown a continuing shift towards lower toxicity levels. Today, the

toxicity of a radiation-curable system depends greatly on the formulation technique. As a general tule,

manufacturers try to exclude from use those ingredients which are suspected carcinogens, tumorogens,

and mutagens (Lawson and Kaminsky, 1982). This, however, is not always possible. Some functional

acrylic-based polymers or copolymers may exhibit toxic properties (Currier, 1989). As a whole, most

acrylic unsaturated compounds are not considered toxic (Vrancken, 1980). In general, reactive

diluents and some oligomers are skin and eye irritants (AFP/SME, 1986). Draize scores, a measure

of skin and eye irritation, range between 1.5 and 6.2 for monomers used in radiation-curable systems

(Bean, 1984; Kubisen et al., 1984). A draize score of 8.0 constitutes a severe skin irritant. In addition

to being skin irritants, some radiation-curable materials have been shown to be oncogenic or

carcinogenic. A skin study with male mice indicated that 2-ethylhexyl acrylate and neopentyl glycol

diacrylate are oncogenic (DePass, 1982). Other skin painting tests have shown tetraethylene glycol

diacrylate, triethylene glycol diacrylate, and 2-ethylhexylglycoldiacrylate to be very weak skin

carcinogens (AFP/SME, 1986). However, due to the low volatility of radiation-curable systems,

inhalation risks to workers can be minimized (Vrancken, 1980; AFP/SME, 1986).

The solvents used in conventional thermal-curable systems have their own set of risks. The high

volatility of these solvents requires worker protection from inhalation of the compounds. Methyl ethyl

ketone, a commonly used solvent, causes irritation of the eyes and nose and is considered an

experimental teratogen (Sax and Lewis, 1989). Nearly all of the ketones used as solvents for paints,

varnishes, and lacquers are considered eye and nose irritants (Sittig, 1985). Methanol, an industrial

solvent for inks, can cause mild dermatitis upon contact. Systemic effects of chronic exposure to

methanol include optic nerve damage (Siig, 1985). Glycol ethers, a class of solvents used for inks,

paints, lacquers, and varnishes are suspected carcinogens (Schessler, 1984). The cleaning chemicals

for radiation-curable systems should be similar to those used in thermal-curable systems and should

present no increased risks.

Enactment of the Toxic Substance Control Act (TSCA) in 1977 gave EPA broad authority to

regulate new and existing chemicals. Under this Act, EPA is charged with assessing the risks posed

by toxic chemicals to human health and ecosystems (Plamondon and Keener, 1984). Most of the

materials used in radiation-curable systems are under the jurisdiction of this Act. The development of

17

new compounds for radiation-curable systems is regulated under Section 5 of TSCA, which requires

manufacturers or importers of a chemical not included in the TSCA inventory to submit a

Premanufacturing Notice (PMN) for a new chemical substance (NCS) 90 days prior to its manufacture

(Svoboda and Schwebke, 1987). During the 90-day period, EPA must decide whether or not the NCS

presents an unreasonable risk. If EPA takes no action within 90 days after the PMN submission, the

NCS is presumed not to pose an unreasonable risk, and the submitter is free to manufacture the

substance (Plamondon and Keener, 1984). Over the past few years, the safety of acrylate material

has been questioned and, thus, few if any new acrylate materials have cleared the PMN regulatory

process (Kovachik, 1987a). EPA has increasingly invoked Section 5(e) of TSCA for radiation-curable

materials which allows EPA to ban or place limits on the NCS that may pose the unreasonable risks

(Plamondon and Keener, 1984). As a result, the industry feels that TSCA regulations have inhibited

innovations (Nelson, 1987). Industry groups (Basic Acrylic Monomer Manufacturers Association and

Specialty Acrylates and Methacrylates Program) have been working with EPA to resolve the regulatory

differences, but some regulatory problems still remain (Clark, B90).

The recently passed Clean Air Act Amendments (CAAA) of 1990 may also have an impact on

radiation-curable systems. Under Title Ill of the CAAA, EPA is required to develop emissions standards

for ail stationary source categories which have emissions of 189 listed hazardous air pollutants.

Several of the compounds used in radiation curable systems are included on this list (e.g.,

acetophenone, benzophenone, styrene). The list also includes several compounds used to reduce the

viscosity of radiation-curable formulations, such as methanol, methyl ethyl ketone, toluene, and xylene.

The emission standards developed by EPA for new and existing sources of hazardous air pollutants

will require the maximum degree of reduction in emissions after considering the cost of achieving the

emission reduction, any nonair quality health and environmental impacts, and energy requirements.

Emissions standards for all 189 compounds are to be promulgated by the year 2000.

4.2 PERMISSIBLE EXPOSURE LIMITS

The Occupational Safety and Health Administration (OSHA), the National Institute for

Occupational Safety and Health (NIOSH), and the American Conference of Governmental Industrial

Hygienists (ACGIH) have developed exposure limits for workers for a variety of chemicals used in

industrial settings. Some of the components of radiation-curable systems have permissible exposure

limits (PEL) determined by these agencies. Table 4-1 provides PEL values for radiation-curable

materials. This list should not be considered all inclusive given the large quantity of material used in

radiation-curable processes. Table 4-2 provides PEL values for potential organic solvents used in

conventional thermal-curing processes.

18

TABLE 4-1. PERMISSIBLE EXPOSURE LIMITS OF RADIATION-CURABLE MATERIAL

Compound &hour time weighted average limit

Acetophenone Benzophenone Caprolactone acrylate 2,2-Dimethyltrimethylene acrylate 1 ,CDivinyl benzene Epoxy acrylate Ethoxyethoxyethylacrylate Ethoxyethyl acrylate 2-Ethylhexyl acrylate 2-Ethylhexyl methacrylate Glycerolpropoxytriacrylate Hexanediol diacrylate Hydroxypropyl acrylate Methylcarbamoyloxyethyl acrylate Pentaerythritol triacrylate Phenyl glycidyl ether Styrene Trimethylolpropane triacrylate Trirnethylolpropane tri(3-mercaptopropionate) Tripropyleneglycoldiacrylate Vinyl cyclohexene dioxide n-Vinyl pyrrolidone

No Standard Available No Standard Available No Standard Available No Standard Available

No Standard Available No Standard Available No Standard Available

No Standard Available No Standard Available

'- No Standard Available 0.5 ppm'

No Standard Available No Standard Available

10 PPm 100 ppm

No Standard Available No Standard Available No Standard Available

10 PPm No Standard Available

10 PPm

50 PPm

4.3 SAFETY AND RADIATION-CURABLE SYSTEMS

The compounds used in radiation-curable systems can be handled safely if the personnel

involved fully understand the nature of the hazards. One principal source of information on the

chemicals used in a formulation is the material safety data sheet (MSDS). Required by the

Occupational Safety and Health Administration (OSHA) for chemicals used in the workplace, MSDS's

contain product hazard and safety information aimed at helping workers and customers use products

safely (Kovachik, 1987b). As a general rule, radiation-curable systems can be handled safely if

19

measures are taken to eliminate dermal contact with the compounds. The protective clothing used

should be compatible with the compound being handled. Proper eye protection should always be used

when handling radiation-curable materials in liquid form. Good hygiene practices should be followed

including use of barrier cream before starting work, never using a solvent to wash, and proper

housekeeping of the work area (Scott, 1987).

20

TABLE 4-2. PERMISSIBLE EXPOSURE LIMITS OF ORGANIC SOLVENTS

Compound &hour time weighted average limit (ppm)

n-Butoxyethanol Butyl acetate n-Butyl glycidyl ester Butyl lactate p-tert-Butyl toluene Carbon tetrachloride Diisopropyl ether N,N-Dimethylacetamide Dipropylene glycol methyl ether Dipropyl ketone 2-Ethoxyethanol 2-Ethoxyethyl acetate Ethyl acetate Ethyl butyl ketone Ethyl ether sec-Hexyl acetate lsophorone Isopropanol Isopropyl acetate Mesityl oxide Methyl acetate Methylal Methyl alcohol Methyl n-amyl ketone Methyl n-butyl ketone Methylcyclohexane Methyl ethyl ketone 5-Methyl-3-heptane Methyl isoamyl ketone n-Propyl alcohol Propylene glycol monomethyl ether 1 ,I ,2,2-Tetrachloroethane Tetrahydrofuran Toluene Xylenes

50 150 50 5- 10 10 500 10 100 50 200 100 400 50 400 50 25 400 250 25 200 1,000 200 100 100 500 200 25 50' 200 100' 5 200 200 100

'Note: No Federal standard, value recommended by ACGIH. Source: Sittig, M., Handbookof Toxicand Hazardous Chemicals andCafcinogens, 2nd edition, Noyes Publications, Park

Ridge, NJ. 1985.

21

5.0 EMISSIONS DATA

There is a general lack of published data about the volatile organic compound (VOC) emissions

from radiation-curable systems. One study reported the volatile component emitted by a traditional

100 percent reactive radiation-curable system comprised about 2 percent of the formulation (Lawson

and Kaminsky, 1982). This volatile component consisted largely of monomers and dimers. Another

study evaluating a UVcurable coating on metal showed approximately 1 percent loss after UV-curing

and a total 8.5 percent loss after a subsequent baking (AFP/SME, 1986). The study, however, did not

identify the composition of the volatile components. One study, which included sampling with a gas

chromatograph in the neighborhood of a printing press using an UV-curable varnish, detected an

ambient concentration of less than 3 ppm of monomer (Vrancken, 1980). This would be expected,

since a majority of the compounds associated with radiation-curable systems have low vapor pressures

(AFPSME, 1986). Low vapor pressures are an indication that a compound will not readily volatilize

at ambient temperatures.

Another obstacle in evaluating emissions from radiation-curable systems is the lack of data

regarding the type and quantity of chemicals used by the industry. For the most part, the formulations

used in radiation-curable systems are considered proprietary and are only described in general terms

or by the trade name in the literature. The formulations provided in the literature did not include the

extent to which each is used by the industry. There are, however, several general statements which

can be made about emissions resulting from the application of radiation-curable systems.

5.1 THEORETICAL EMISSIONS RATE

The flux (emissions rate per unit area) of a compound into the air is defined by Fick's law and

can be expressed as (Shewood et al., 1975):

where N, = Flux [(g mole/(sec)(cm2)], DAB = Diffusion coefficient (cm2/sec), P = Pressure (atm), R = Ideal gas constant ((cm3(atm)/(g mole)(K)], T = Temperature (K),

Ye,, YE, = Mole fraction of compound in air, and yo = Distance in direction of diffusion (cm).

This equation assumes that a thin layer of stagnant air is in contact with the coating and that the air

does not dafuse into the coating. Assuming the environmental conditions (i.e., temperature and

22

pressure) do not change, the driving forces of the flux are the diffusion coefficient and the mole fraction

difference from the coating surface to the edge of the stagnant layer. This equation shows that there

is a proportional relationship between the flux and diffusion coefficient and the natural logarithm of the

difference in the mole fractions. The diffusion coefficient of a compound can be calculated by the

theoretical equation derived by Chapman and Enskog (Lyman et al., 1982):

DAB = 1.858 x l o 3 T1 .5(M,)0.5 [ 1 where

M, (MA + MB)/MAMB MA = Molecular weight of air; 28.97 g/g mole, MB = Molecular weight of the compound of interest (g/g mole), CAB = Collision diameter, and L? = Collision integral.

The collision diameter and collision integral are functions of ‘iemperature and are based on the

Lennard-Jones potential (Welty et al., 1976). A solution to this equation has been proposed by Fuller,

Schettler and Gibbings and is based on the temperature, molecular weights and molar volumes (Lyman

et al., 1982). This solution is expressed as:

1 0-3 ~ 1 . 7 5 ~ , 0 . 5

DAB P ( V p + V p ) ’

where VA = Molar volume of air; 20.1 cm3/g mole, and V, = Molar volume of the compound of interest (cm3/g mole).

(5-3)

This relationship shows that the heavier compounds will have a smaller diffusion coefficient and thus,

be less likely to volatilize from the surface.

To simplify the Equation 5-1, the mole fraction at the edge of the stagnant layer is assumed to

approach zero. The mole fraction at the interface of the coating and air can be estimated as (Lyman,

et al., 1982):

YB, = H/RT(&MW~/XX,MW,) (5-4)

where H = Henry’s law constant (atm m3/g mole), and X = Weight fraction of component in coating.

Substituting Equations 5-3 and 5-4 into Equation 5-1 shows that the highly volatile organic solvents

used in thermal-curing systems will have a large flux. Conversely, the high molecular weight and low

23

volatility compounds used in radiation-curable systems will have a small flux. Although some reactive

diluents may be highly volatile, they will have a relatively small flux, because they only make up a small

percentage of the formulation.

5.2 AIRBORNE HAZARD OF RADIATION-CURABLE SYSTEMS

An airborne hazard from radiationcurable systems results from the potential overspray from

spray applications techniques. A buildup of overspray inside the plant should be avoided since it will

probably not cure and will become a source of worker contamination and skin irritation (Lawson and

Kaminsky, 1982). The exposure to facility personnel can be minimized through the use of an enclosed

automatic spray system. Robotics can be utilized in the enclosed area when complex or unusual

shapes are being coated (Lewarchik and Hunrvitz, 1984). Use of electrostatic spray equipment is

another means of reducing the amount of overspray (Riedell, 1989). The overspray can be recovered

for reuse in the coating operation, or the exhaust from the spray booth can be treated with a water

wash or dry filters to remove overspray particles depending on the economics of the recovery process

or waste disposal.

5.3 VOC REDUCTION POTENTIAL

As a means of reducing VOC emissions from surface coating operations, radiation-curable

coating can be very effective. For facilities involved with flat coating process (e.g., flatwood production,

paper coating, metal decoration, fabric coating), a near 100 percent reduction in VOC emissions over

conventional coatings can be achieved by using radiation-curable coatings. These are processes for

which radiation-curable systems have traditionally been well suited (Schessler, 1984). Even when

organic solvents are used to modify the viscosity of a radiation-curable system, a substantial reduction

in VOC emissions is possible in some applications. However, organic solvents used to reduce coatings

to a sprayable viscosity can constitute up to 80 percent by volume of a sprayable UV-curable system.

This translates into a VOC content of approximately 720 kg/m3 (6.0 Ib/gal) of coating. This is nearly

the same degree as conventional nitrocellulose lacquer which typically has an organic solvent content

of 80 percent by weight or 780 kg/m3 (6.5 Ib/gal) of coating (Riedell, 1989).

The degree of VOC reduction to be achieved from converting to radiation-curable systems can

be estimated by comparing emissions from various formulations. The base case for this comparison

is one metric ton (1.1 short ton) of conventional nitrocellulose lacquer finish a material commonly used

in spray applications in the wood furniture industry with an organic solvent content of 85 percent by

volume (Currier, 1989). The resulting uncontrolled emissions from this material are approximately

790 kg (1738 Ib) of VOC. Emissions from three UV-curable formulations were compared with

emissions from this thermally-cured coating as shown in Table 5-1. Case 1 is a sprayable UV-curable

system diluted to a sprayable viscosity with reactive diluents. It is assumed that only 2 percent by

24

weight will evaporate from the formulation during the curing process. Case 2 represents the typical

solvent-containing UV-curable system used in the wood furniture industry. This sprayable formulation

has an organic solvent content of 47 percent by volume. Case 3 illustrates the worst case for solvent-

containing radiation-curable systems. This high-solvent radiation-curable coating has an organic

solvent content of 80 percent by volume.

For the purpose of the comparison, it is assumed that equal quantities of solid material are

contained in each of the coating formulation cases and that each coating will cover equal areas with

an equal film thickness. It should be noted that this situation may not conform with real world

conditions. However, this assumption can provide a crude approximation to show the potential VOC

emissions reduction for each radiation-curable system and illuminate a few interesting facts. As one

would expect, a conversion to the 100 percent reactive UV-curable formulation (Case 1) provides the

greatest VOC emissions reduction over the thermal-curable coating. Depending on the compounds

emitted, the conversion to a 100 percent reactive UV-curable formulation might eliminate the necessity

of VOC control devices. A conversion to the high-solvent UV-curable formulation (Case 3) affords only

a relatively small improvement in VOC emissions over the thermalcurable coating. This means the

high-solvent UV-curable formulation will require approximately the same size VOC control device as

the thermal-curable system to meet VOC emission standards. A conversion to the typical sprayable

UV-curable formulation (Case 2) would provide an 82 percent reduction in VOC emissions over the

thermal-curable system even though this formulation is nearly half organic solvent. The VOC device

required to handle the evaporating organic solvents would be smaller and less costly than that needed

for the thermalarable coating.

25

TABLE 5-1. VOC EMISSIONS REDUCTION RESULTING FROM CONVERSION TO RADIATION-CURABLE COATINGS

CondRion Estimated VOC Emissions Projected Percent Reduction' in Kilograms (Pounds) due to UV-Curable Conversion

Base Case2

Case i3

Case z4

Case 35

790 (1,738)

4 (36)

140 (308)

630 (1,386)

99

82

20

Thermal Coating Emissions - Radiation Coating Emissions 'Percent Reduction- ~ x 1 0 0

b n e metric ton of conventinal nitrocellulose lacquer lopcoat used in the furniture industry containing 85 percent by volume organic solvent.

'A sprayable UV-curable formulation diluted to a spray viscosity with reactive diluents and has an evaporation rate of two percent for the mixture.

'A sprayable UVwable formulation typically used in the furniture industry and diluted to a spray viscosity with conventional solvents to 47 percent VOC by volume.

'A sprayable UVGurable formulation diluted with conventional solvents 0 80 percent VOC by volume.

Thermal Coating Emissions

26

6.0 COST COMPARISON

Another obstacle to widespread use of radiation-curable systems has been the perception of

higher costs than for conventional thermalcurable systems. In some cases, a cursory cost review may

have led facility management to conclude that radiation-curable systems are not as cost-effective as

conventional thermal systems. The emergence of competing technologies such as water-borne, high

solids, and powder systems has seemingly reduced the economic effectiveness of converting to a

radiation-curable system. However, there are several intrinsic characteristics of radiation-curable

systems which may outweigh the perceived cost disadvantage.

6.1 COSTS ASSOCIATED WITH CURING EQUIPMENT

The cost of curing and application equipment form the basis of the total capital expenditure

required for the facility. The choice between thermal- or radiation-curable equipment will dictate, to a

certain extent, some faciliy design features including size, suppbrt structures, utility requirements and

auxiliary systems (e.g., ventilation systems). It has generally been the perception in the thermal-

curable coating industry that radiationcuring equipment is more expensive than conventional thermal

ovens. Industry literature, however, indicates that this is not always the case. The cost for a UV-curing

oven can range from $25,000 to $100,000 (Pincus, 1983). The cost of a conventional thermal-curing

system is generally twice the cost of UV-curing equipment (LeFevre, 1980). EB-curing equipment,

which costs approximately $500,000 per unit, is more expensive than UV- or thermal-curing equipment

(Newcomb, 1985).

Radiation-curing equipment, however, has a distinct advantage over thermal equipment. In

addition to lower actual equipment costs for UV systems, radiation-curing equipment, for both EB and

UV systems, requires 50 to 75 percent less space than comparable thermal-curing systems. This

translates into the need for a smaller faciliiy to house the UV or EB equipment. This can be viewed

as an advantage when both converting an existing thermal process to a radiation-curable system or

constructing new production capacity. Radiation-curing systems generally can be housed in existing

facilities, whereas increasing capacity using thermal systems may require construction of a complete

facility, including all auxiliary physical plant equipment. When the Coors Container Company converted

to a UVcurable ink, for example, the 230-foot long thermal-curing oven was replaced with a six-foot

long UV-curing oven (LeFevre, 1980).

6.2 COSTS ASSOCIATED WITH COATING AND INK MATERIAL

The cost of coating or ink material is a major component of the annual operating cost of any

surface coating or printing operation. Some literature indicates that radiation-curable material, on a unit

basis, normally costs more than thermal-curable material. This has resutted in many considering a

27

conversion to radiation-curable systems to conclude that these types of systems are not an

economically attractive substitute material.

The cost of a radiation-curable coating can be up to four times the cost of an equal quantity of

thermally-curable formulation (Currier, 1989). The cost of radiation-curable inks may be 25 to

75 percent greater than conventional inks. This higher material cost may have prevented their use in

newspaper, magazine, and book production and has limited the use of radiation-curable inks to

specialty applications (Schessler, 1984). On an overall economic basis, the Coors Container Company

experienced a 20 percent increase in ink costs after converting to a UV-curable system (LeFevre,

1980). The cost differential between radiationcurable and thermally-cured systems generally seems

to be related to the viscosity of the formulation. The lower the viscosity requirement of the application

equipment, the greater the cost differential between both types of systems. It should be noted,

however, that the major component of a thermally-cured coating or ink is a solvent that, once cured,

is no longer present in the film. On an equal material basis, radiation-curable systems frequently have

two to four times the coverage of conventional systems (FDM,'1989b). Thus, less radiation-curable

material is required to obtain the same results as a thermal-curable system. This ability to provide

greater coverage can negate the disadvantage caused by the higher material costs. A study for the

wood furniture industry used costs of $5.80 per unit volume of nitrocellulose lacquer topcoat and $22.00

per unit volume of UV-curable clear topcoat. This is a cost differential of nearly four times. However,

when comparing the cost of coating a 1,000-square-foot ( 91 square meters) area with an equally thick

film, the cost for the UV-curable system was only slightly more expensive than the thermal system

(Currier, 1989). For various application techniques, this cost differential was between 6 and 12 percent.

Figure 6-1 shows the difference in cost for the two coating systems for different application techniques.

Depending on the application technique, the coating cost for a UV-curable system in a flat line

installation varies from approximately $28 to $52 per square foot ($307 to $571 per square meter).

Coating costs for a comparable thermal system can vary from approximately $25 to $49 per square

foot ($274 to $538 per square meter) (Currier, 1989). Although data are currently available for only

the wood furniture industry, investigations into other industrial applications of radiation-curable systems

may result in the similar comparisons.

Another cost associated with the type of material used is the energy required to cure the coating

or ink. As stated earlier, the energy required by radiation-curable systems is used to initiate the

polymerization chain-reaction. In contrast, energy required by thermal-curable systems is used to

evaporate the organic solvent in the formulation as well as to cure the material. When Coors Container

Company converted to UV-curable inks, the energy savings achieved by the switch was 60 percent

(LeFevre, 1980). The energy savings of 60 to 80.percent per square foot of surface coated with

radiation-curable systems is significant. However, the total cost savings can fluctuate with changes in

energy costs.

28

1 1 I 1 0 LD

0

29

6.3 COST ASSOCIATED WITH CONTROLLING VOC EMISSIONS

One of the most common means of reducing VOC emissions for specific operations to meet air

emissions standards are add-on control systems. A number of variables affects the investment cost

and annualized operating cost of add-on control systems and must be accounted for in the planning

process. Tables 6-1 and 6-2 show typical items to consider for add-on control systems. The purchase

cost of this type of equipment for surface coating operations can range from $1 00,000 to $3,000,000 depending on the volume of solvent-laden exhaust air and operating conditions of the facility and on

regulatory requirements (Lankford, 1983). The cost of purchasing and maintaining VOC control

equipment can significantly add to the operating costs of applying each pound of coating or ink to the

substrate. In the past, such increased operating costs have made smaller facilities using conventional

coatings and inks uncompetitive (Lankford, 1983). Radiation-curable systems have the potential for

greatly reducing the necessity for VOC control equipment. The available data suggest that only a small

quantity of material will volatilize from a radiation-curable system consisting of 100 percent reactive

material (Lawson and Kaminsky, 1982; Vrancken, 1980). These emissions should only be a small

fraction of the VOC emissions resulting from a thermal-curable system. However, this is not substantial

proof of the reduced VOC control requirements extended by radiation curable systems.

As discussed in Section 3.3.3, organic solvents are sometimes used to reduce the viscosity of

radiation-curable systems (Lawson and Kaminsky, 1982). This is a common practice in the wood

furniture industry, allowing the use of UV-curable topcoats in spray application techniques (Garratt,

1984). The organic solvents in these formulations generally comprise 47 percent of the formulation

by volume, although some solvent-containing radiation-curable formulations consist of up to 80 percent

by volume of organic solvents for some spray processes (Currier, 1989). Even with a substantial

percentage of the formulation consisting of organic solvent, some radiationcurable systems emit

substantially fewer VOCs than conventional coatings because of the superior coverage radiation

curable systems offer on a square-foot basis. A study of VOC emissions from the wood furniture

industry compared a conventional nitrocellulose lacquer topcoat with an UV-curable clear topcoat

containing 47 percent by volume organic solvents. VOC emissions from the coating of a 1,000-square-

foot area with a UV-curable system emitted only 20 percent of the VOC emitted by the conventional

coating (Currier, 1989). Figure 6-2 shows the difference in VOC emissions for these coating systems

for various application techniques. The lower emissions generated by such a UV-curable system would

result in less-intensive VOC control requirements because less air would be required to carry the VOC

from the process, resulting in lower overall operating and capital costs.

TABLE 6-1. CAPITAL COST ELEMENTS AND FACTORS FOR SELECT ADD-ON CONTROL SYSTEMS'

Cost Element Incinerators Adsorbers Absorbers Condensers

DIRECT COSTS Purchased Equipment Cos* 1 .oo 1 .oo 1 .oo 1 .oo

Other Direct Costs: Foundation and Support Erection and Handling Electrical Piping Insulation Painting

0.08 0.14 0.04 0.02 0.01 0.01

0.08 0.14 0.04 0.02 0.01 0.01

0.1 2 0.40 0.01 0.30 0.01 0.01

0.08 0.14 0.08 0.02 0.1 0 0.01

Total Direct Cost 1.30 1.30 1.85 1.43

INDIRECT COSTS Engineering and Supervision Construction and Field Expenses Construction Fee Start up Performance Tests

0.10 0.05 0.1 0 0.02 0.01

.0.10 0.05 0.1 0 0.02 0.01

0.1 0 0.1 0 0.1 0 0.01 0.01

0.10 0.05 0.05 0.02 0.01

Total Indirect Cost 0.28 0.28 0.32 0.28

CONTINGENCY 0.05 0.05 0.07 0.05

TOTAL" 1.63 1.63 2.24 1.76

Note: 'As fractions of total purchased equipment cost. They must be applied to the total purchased equipment cost qotal of purchased costs of major equipment and auxiliary equipment and others, which include instrumentation and controls at 10 percent, taxes and freight at 8 percent of the equipment purchase cost Tor retrofit applications, multiply the total by 1.25

SOurCe: U.S. Environmental Protection Agency. Handbook - Control Technologies for Hazardws Air Pollutants. EPA/625/6-86X)14. Air and Energy Engineering Research Laboratory. Research Triangle Park, NC. 1976.

Other means of reducing VOC emissions from surface coating or printing operations include the

substitution of coating or ink material. Among the possible materials are water-borne and high solids

coating and inks. However, these materials also have drawbacks limiting their use. Water-borne

systems require more energy to cure than the current thermal-curable systems. In addition, water-

borne systems cannot be used with several application techniques, such as screen printing, because

they become tacky and cause the application equipment to stick to the substrate. Higher solid systems

have a lower organic solvent content than current thermal-curable systems, but are more expensive

on a cost per gallon basis but nearly identical on a cost per square-foot coated basis. They do require

31

higher temperatures and slower line speeds to cure (Schessler, 1984). This would restrict their use

with more temperature-sensitive substrates.

TABLE 6-2. TYPICAL ITEMS INCLUDED IN ANNUAL COSTS OF ADD-ON CONTROL SYSTEMS

Direct Operating Costs

Utilities (where applicable)

Natural Gas Fuel Oil Water Steam Electricity Solvent

Operating Labor:

Operating Labor Operating Labor Supervision

Maintenance:

Labor Maintenance Material and Operating Supplies (e.g., lubricants, paper)

Replacement:

Parts Labor

indirect Operating Costs

Overhead (80 percent of operating, maintenance and replacement labor) Property Tax (One percent of Total Capital Cost) Insurance (One percent of Total Capital Cost) Administration (One percent of Total Capital Cost) Capital Recovery (Based on average interest rate)

Credits Salable product or energy recovered by control system

Source: US. Environmental Protection Agency. Handbook - Control Technologies for Hazardous Air Pollutants. EPA1625/6-86/014. Air and Energy Engineering Research Laboratory. Research Triangle Park, NC. 1976.

32

I "

1 "

I W

I- O

0 0 0 0 0 0 co 10 d- m cv 7 0

33

c m

6.4 OTHER COST ISSUES Converting to radiation-curable processes, as with any process conversion, results in equipment

and personnel training costs. The application equipment used at the facility may need modification

before beginning the application of radiation-curable systems. Such components as rollers, blankets,

plates, and doctor blades will require examination to ensure compatibility with radiation-curable

systems (Schessler, 1984). Special attention is required to ensure that personnel involved in

application and curing processes understand exactly the hazards associated with each compound being

used. Protective clothing worn by the facility personnel also will need to be compatible with the

compounds in use.

One additional cost issue may eventually restrict the use of radiation-curable inks. The

crosslinked nature of radiation-curable inks has shown to advantageous in improved chemical and

abrasion resistance. This crosslinking, however, becomes a disadvantage when trying to de-ink prints

and makes recycling paper difficult (Schessler, 1984). The widespread use of radiation-curable inks

may substantially increase in the cost of recycling paper, thereby reducing the economic benefits of

using recycled paper products. More information, however, is required to adequately address this

issue.

6.5 SUMMARY

In the past, radiation-curable systems have not been perceived as an economically attractive

replacement for conventional thermal-curable systems. This perception is based on the relatively large

difference in the cost per gallon of radiationcurable and thermal-curable materials. This perception

does not account for advantages in coverage and energy savings provided by radiation-curable

materials. When these factors are included in the cost analysis and examined on a square-foot basis,

radiation-curable systems are only slightly more costly than thermal systems. The additional operating

cost associated with the larger VOC control equipment required for conventional coatings may provide

a cost differential in favor of higher solids radiation-curable materials. For the same area of coverage,

pure radiation-curable formulations and solvent-containing higher solids radiation-curable formulations

will generate less VOC emissions than conventional thermal systems.

34

7.0 CONCLUSIONS

Radiation-curable coatings and inks have found uses within various surface coating and printing

processes. Although radiation-curables, like any other single technology, may not be universally

applicable to all industrial applications, they have the potential to replace thermalcurable systems in

a substantial number of industrial processes to cost-effectively reduce VOC emissions. Radiation-

curable systems are currently being used in metal-decorating, flatwood production, and paper coating

processes. Depending on the operation and formulation, radiation-curable coatings and inks have

shown the potential to provide at least an 80 percent reduction in VOC emissions over thermal-curable

coatings and inks. Other advantages of radiation-curable systems over thermal-curables include a

reduction in the energy required to cure the material, a reduction in the space requirements, and an

increase in productivity. The low-temperature nature of radiationcurable systems may allow for a wider

use on substrates considered unsuitable for thermal-curable systems. The polymer films formed by

the radiation-curable systems have also been found more dwable than films from thermal-curable

systems in terms of hardness, solvent resistance, and heat resistance.

Radiation-curable systems, however, have several disadvantages, which are currently restricting

wider use in surface coating and printing processes. The high viscosity of many radiation-curable

compounds has tended to limit their use to application techniques such as roll coating, curtain coating,

lithographic, and screen printing. The majority of conventional industrial finishes, however, are applied

through spray techniques. In nearly all cases, the viscosity of radiation-curable systems is unsuitable

for use in spray applications. The current practice in the wood furniture industry is to modify the

viscosity of a radiationcurable system by adding organic solvents (up to 40 percent by volume and

more). This can compromise the environmental benefits of potential VOC emissions reduction through

use of radiation-curable systems. However, data exist which indicate that solvent-containing higher

solids radiation-curable systems still provide a substantial reduction in VOC emissions over

conventional thermal-curable systems.

The early radiationcurable systems were found to be fairly toxic due to unstable formulations

and resulting dermatitis problems. Most of these problems have been addressed by the industry.

Formulations have since been developed that remain stable for lengthy periods of time. Problems

associated with dermatitis can be avoided through a full understanding of the hazards involved and a

minimization of exposure to radiation-curable material. The industry has made great strides in reducing

the toxicity of radiationcurable systems, but like most industrial chemicals, some hazard will remain.

The perceived disadvantage of radiationcurable systems in terms of cost may be the most

difficult to resolve. The main issue of cost involves the fact that radiation-curable materials cost more

on a unit basis than thermal-curable material. The capital expense of radiation-curing equipment has

also been cited as a reason for rejecting radiation-curable systems. This view ignores the fact that

35

radiation-curable systems can provide improved coverage on the substrate, substantial reduction in

energy costs and floor space, and reduction in the size or in some cases, the need for VOC control

equipment. In addition, some data show UV-curing equipment actually costs less than thermal-curing

equipment. Data in the literature suggest that, on a cost-per-areacoated basis, the expenditures are

essentially the same for both radiation-curable and thermal-curable systems. This conclusion is based,

however, on only one industrial use for radiationcurable systems. Additional investigations on a cost-

per-area-coated basis could resolve this issue of c o s t .

Two issues have not been fully resolved. The first is the impact that greater use of radiation-

curable inks will have on paper recycling. The difficulty in de-inking radiation-cured paper may

significantly increase the cost to recycle this paper. The second issue is the question of volatile

emissions from radiationcurable systems. There is a general lack of data presented in the literature

concerning reactive components which may evaporate from the coating or ink. Only limited data are

available for determining which radiation-curable systems are environmentally suitable.

36

8.0 REFERENCES

1 . Arnold, H.S., Sr., "Three-Dimensional UV Finishing in Practice," from the proceedingsof Radcure '84 Conference, Association of Finishing Processes of SME, September 10-13, Atlanta, GA, 1984.

2. Ashcroit, W.R., 'Water-Based UV Curable Acrylate Resins," Paint and Resin, 57(5):4-6, 1987.

3. Association for Finishing Processes of SME, Radiation Curing: An introduction to Coatings, Varnishes, Adhesives and Inks, Second Edition, 1986.

4. Bean, A.J., "The Technology of Using UV and EB Curable Inks and Coatings for Decorative and Protective Packaging," from the proceedings of Radcure '84 Conference, Association of Finishing Processes of SME, September 10-13, Atlanta, GA, 1984.

5. Clark, K., "High-Solids Coatings, Electron Beam Curing and Ultraviolet Curing," Proceedings of the Conference "Waste Reduction-Pollution Prevention: Progress and Prospects within North Carolina,"North Carolina Department of Natural Resources and Community Development, 1990.

6. . Currier, G., "Interest in UV-Curable Coatings on the Rise," furniture Design and Manufacturing, 61(6):54-58, 1989.

7. Danneman, J., "UV Process Provides Rapid Cure for Compliant Wood Finishes," Modern Paint and Coatings, 78(2):28-29, 1988.

8. Decker, C., "UV-Curing Chemistry: Past, Present, and Future," J. of Coatings Technology, 59(751):97-106, 1987.

9. DePass, L.R., "Carcinogenicity Testing of Photocurable Coatings," Radiation Curing, 9(3):18-23, 1982.

10. Frecska, T., "UV Curing: The Process and the Equipment," J. of Radiation Curing, 14(4):26-28, 1987.

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12. Furniture Design and Manufacturing, "UV Coatings Update," 61 (1 2):llO-118, 1989b.

13. Garratt, P.G., "The Use of Unsaturated Polyester Resins in UV Curable Paint Formulations for Use in the Furniture Industry," from the proceedings of Radcure '84 Conference, Association of Finishing Processes of SME, September 10-13, Atlanta, GA, 1984.

14. Jones, D.T., "Roll Coating Equipment - Application and Limitations," from the proceedings of Radiation Curing V: A Look to the 80's, Association of Finishing Processes of SME, Sept. 23-25, Boston, MA, 1980.

15. Knight, R.E., "UV-drying Equipment, Design and Installation," J. of Radiation Curing, 6(1):14-18, 1979.

16. Kovachik, C.R., "Radiation Curing: A Market Overview," Radiation Curing, 14(1):18-19, 1987a.

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Lankford, A.P., II, "History and Growth of Radiation Curing in the Converting Industry," Radiation Curing, 1 O(3) 120-27, 1983.

Lawson, K.R., and M.I. Kaminsky, "Radiation Curable Coatings for Three-Dimensional Shapes," Radiation Curing, 9(2):28-35, 1982.

LeFevre, P.H., "UV Cure of Two-Piece Can Decoration," from the proceedings of Radiation Curing V: A Look to the 80k, Association of Finishing Processes of SME, Sept. 23-25, Boston, MA, 1980.

Lewarchik, R.J., and D.A. Hurwitz, "New Developments in UV Curables for Automotive and Product Finishes," from the proceedings of Radwre '84 Conference, Association of Finishing Processes of SME, September 10-13, Atlanta, GA, 1984.

Lyman, W.J., W.F. Reehl, and D.H. Rosenblatt, Handbook of Chemical Property Estimation Methods - Environmental Behavior of Organic Compounds, McGraw-Hill, Inc., New York, NY, . 1982.

Morris, W.J., "Formulation of High Performance Urethane UV/EB Curable Systems," from the proceedings of Radwre '84 Conference, Association of Finishing Processes of SME, September 10-13, Atlanta, GA, 1984.

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Newcomb, W.T., "Graphic Arts Research and Radiation Curing," Radiation Curing, 12(1):2-6, 1985.

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Pincus, A.H., and G. Sickinger, "Two Curing Leaders Radiate Ideas About Their Industry," J. of Radiation Curing, 15(2):8-10, 1988.

Plamondon, J., and R.L. Keener, "Recent Developments in the Regulation of Industrial Chemicals under TSCA," from the proceedings of Radwre '84 Conference, Association of Finishing Processes of SME, September 10-13, Atlanta, GA, 1984.

Prane, J.W., "Ultraviolet Curable Coatings and Inks - Markets and Projections," from the proceedings of Radiation Curing V: A Look to the 803, Association of Finishing Processes of SME, Sept. 23-25, Boston, MA, 1980.

Riedell, A., "Back to Basics to Beat VOC Emission Problems," Furniture Design and Manufacturing, 61 (1):32-34, 1989.

Santa Ana, M.M., "Printing UV Curable Inks on Narrow Webs," from the proceedings of Radiation Curing V: A Look to the 803, Association of Finishing Processes of SME, Sept. 23-25, Boston, MA, 1980.

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Sax, N.I., and R.J. Lewis, Sr., Dangerous Pmpertiesof lndustrialMaterials, Seventh Edition, Van Nostrand Reinhold, New York, NY, 1989.

Schessler, P.G., "Printing Ink - A Changing Industry," Radiation Curing, 11(1):17-22, 1984.

Scott, LC., "Hazardous Raw Materials Used in the Paint and Ink Industries," Radiation Curing, 14( 2) :20-22, 1987.

Shewood, T.K., R.L. Pigford, and C.R. Wilke, Mass Transfer, McGraw-Hill, Inc., New York, NY, 1975.

Sittig, M., Handbook of Toxic and Hazardous Chemicals and Carcinogens, Second Edition, Noyes Publications, Park Ridge, NJ, 1985.

Svoboda, G.R., and G. Schwebke, "Is A 5(e) Consent Order Workable?" Radiation Curing, 14(2):24-25, 1987.

U.S. Environmental Protection Agency. Handbook - Control Technologies for Hazardous Air Pollutants. EPN625/6-86/014. Air and Energy Engineering Research Laboratory. Research Triangle Park, NC, 1976.

40. Vrancken, A., "Market Trends for Irradiation Curable Coatings and Printing Inks in Europe," from the proceedings of Radiation Curin0 V: A Look to the ~ O ' S , Association of Finishing Processes of SME, Sept. 23-25, Boston, MA, 1980.

41. WeRy, J.R., C.E. Wicks and R.E. Watson, Fundamentals of Momentum, Heat, and Mass Transfer, Second Edition, John Wiley and Sons, Inc., New York, NY, 1976.

39

(Plcasc read Iurtructiom on lhe reverse beforc- compIcIiq) 1 . REPORT NO. 2.

EPA-600 /2-91-035

Radiation- Curable Coatings

3. RECIPIENT'S ACCESSION NO.

4. T I T L E A N 0 S U B T I T L E 5 . REPORT DATE

July 1991 6. PERFORMING ORGANIZATION CODE

7. A U T H O R G ) 8. PERFORMING ORGANIZATION REPORT NO Stephen A. Walata, 111, and C. R. Newman

9. PERFORMING ORGANIZATION NAME AND ADDRESS

Alliance Technologies Corporation 100 Europa Drive, Suite 150 Chapel Hill, North Carolina 27514 68- D9-0173, Tasks 0 /702

and 1/130 12. SPONSORING AGENCY NAME AND ADDRESS 13. TYPE OF REPORT AND PERIOD COVER

EPA, Office of Research and Development Task Final; 6/90 - 5/91 A i r and Energy Engineering Research Laboratory 14.SP0NS0R1NG AGENCY

I Research Triangle Park, North Carolina 27711 E P A / 600 / 13 I I

15.SUPPLEMENTARY NOTES AEERL project officer is Charles H. Darvin, Mail Drop 61, 91! 541-7633.

16. AesTRAcTThe report gives results of an evaluation of radiation- curable coatings as technology for reducing volatile organic compound (VOC) emissions from surface coating operations. A survey of the literature was conducted to assess the state of the technology and emissions from radiation- curable processes. The information collected from the literature was used to evaluate the engineering and economic issues associated with radiation-curable systems and to idenitify the requirements for implementing the technology and any problems arising from its use. Topics dis cussed in the report include coating characteristics, potential VOC reduction capa- bility, potential health problems associated with the use of ultraviolet (UV) coating: and the economic impacts of conversion to U V coatings. The report provides infor- mation to permit an informed judgement on when and how to apply radiation-curabll technologies for industrial application. Radiation- curable coatings and inks are higher solids formulations than conventional coatings and, consequently from an ai: pollution viewpoint, are considered to be well suited substitutes for solvent-based thermal-curable systems. The radiation source for these systems is either an UV light or an accelerated electron beam (EB).

-

a. DESCRIPTORS

Pollution Inks Coatings Ultraviolet Radiation Radiation Electron Beams Curing Organic Compounds Volatility

18. D ISTRIBUTION STATEMENT

Release to Public

EPA Form 2220-1 (9-73) 40

) . IDENTIFIERS/OPEN ENDED TERMS ~ ~~

Pollution Control Stationary Sources Volatile Organic Com-

pounds (VOCs)

19. SECURITY CLASS (ThisReport).

Unclassified !O. SECURITY CLASS (Thispage)

- Unclassified

~.

. COSATI Field/Group

13B 14E 11C 20F 14G 20H 13 H 07C 20M

!1. NO. OF PAGES

46 !2. PRICE