Curing Methods for Coatings - American Chemical Society

35 Curing Methods for Coatings VINCENT D.McGINNISS1and GERALD W. GRUBER2 Columbus Laboratories, Battelle, Columbus, OH 43201 PPG Industries, Allison Park, PA 15101 1

Downloaded by PURDUE UNIV on January 15, 2015 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch035

2

IR Drying Radio Frequency and Microwave Radiation Light Energy Radiation Light Source Photoinitiators Free Radical Initiated Radiation Curable Coating Compositions Acid Intermediate Initiated Radiation Curable Coating Compositions Electron Curing Glow Discharge Polymerization

In conventional organic solvent based coating systems, a preformed functional polymer component is dissolved or dispersed [nonaqueous dispersion (NAD) technologies] in an organic solvent (30-80% solids), and a cross-linking oligomer and various flow agents, catalysts, pigments, etc., are added to make up a complete coating formulation. The coating formulation is applied to a substrate by conventional methods, for example, spray, r o l l coating, flow coating, etc., and subsequently cured in gas or infrared thermal oven equipment. Curing of conventional solvent-based coatings involves both solvent removal and thermal initiation of chemical reactions between the preformed functional polymers and cross-linking oligomers that are involved in developing final film properties through three-dimensional network formation of cross-link sites. Similar analysis can be established for waterborne polymer solutions or dispersions (emulsions) in that the solvent (water) must be removed followed by polymer-polymer or polymer-cross-linking oligomer interaction for formation of network structures ( 1 - 4 ) (Figure 1). The development of high performance coating systems requires the use of special energy-related chemical reaction mechanisms in order to effect the formation of complex network structures. These network formation reactions take place between high molecular weight 0097-6156/ 85/ 0285-0839506.00/0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


C o a t i n g System

Figure 1.

Thermal e n e r g y i n p u t f o r s o l v e n t removal

Conventional t h e r m a l l y processes.

formu-

A d d i t i o n o f pigments, w e t t i n g agents, f l o w a g e n t s , l u b r i c a n t s and o t h e r additives

A p p l i c a t i o n of the coating l a t i o n to a substrate

C o a t i n g System

y

Thermal e n e r g y input for cure i n i t i a t i o n and r e a c t i o n p r o c e s s e s

•—^

Excess heat r e moval from t h e substrate

cured coating systems and

Preformed f u n c t i o n a l p o l y mers d i s s o l v e d o r d i s p e r s e d i n w a t e r o r water and c o solvent mixtures

Water-based

Addition of c r o s s l i nking oligomers ( m a t e r i a l s w h i c h have a h i g h c o n c e n t r a t i o n of r e a c t i v e f u n c t i o n a l i t y f o r t h e i r m o l e c u l a r s i z e and w e i g h t )

Preformed f u n c t i o n a l p o l y mers d i s s o l v e d o r d i s p e r s e d in solvents

Solvent-based


F i n a l coated substrate product

η m

73

m

•v Ο

"0

>

oo

ê


35.

MCGINNISS AND GRUBER

Curing Methods for Coatings

841

polymer chains containing reactive functional groups or between functional polymer chains and cross-linking oligomers that have a very high concentration of functional groups per molecular weight and molecular size dimensions (figure 2). Examples of t y p i c a l functional polymer systems, cross-linking reactions, and mechanisms commonly used in the coating industry are listed in Table I. The cross-linking or curing reactions associated with many of the polymer systems l i s t e d in Table I usually require the use of elevated thermal energies to effect formation of three-dimensional network structures and high-performance coating properties in a time frame useful enough to be commercially acceptable for a wide variety of i n d u s t r i a l applications. A major step in reducing paint or coating drying times was the i n s t a l l a t i o n of i n d u s t r i a l drying ovens. Present-day convection ovens are suitable for continuous or batch operations and can accommodate a wide variety of object sizes and shapes. However, diminishing natural gas supplies and environmental considerations are lending impetus to the development of alternative methods and a l t e r n a t i v e energy sources for curing coatings. Some of these alternatives to present-day convection oven drying of paints are listed in Table II (9). In understanding how the various energy sources and curing methods work, i t is helpful to r e c a l l that energies, on the order of a few electron volts, are required to break a chemical bond or raise a molecule to an e l e c t r o n i c a l l y excited state. Thus, i t i s clear that infrared, microwave, and radio frequency radiation produces chemistry similar to what one would expect from heating a coating in a convection oven. Ultraviolet and accelerated electron chemistry can be characterized by bond breakage and subsequent polymerization resulting from electronic excitation or ionization processes. IR Drying IR is produced by any warm object. Its energy distribution depends upon the nature and temperature of the heated object. Dominant IR wavelength (emitted by a tungsten filament) as a function of temperature were 1.45, 1.15, 0.965, and 0.905 ym at 2000, 2500, 3000, and 3200 °C, respectively (10). The intensity of the radiation is a function of the fourth power of the temperature of the heated object. This means that a radiant heat source operating at several hundred degrees w i l l have a high rate of heat transfer toward a substrate at temperatures up to a few hundred degrees, providing the substrate is not highly reflective or transparent toward the emitted infrared. In practice, a filament is heated by gas or e l e c t r i c i t y and the radiant energy i s directed toward the substrate by means of reflectors. Since u t i l i z a t i o n of IR depends upon absorption, i t becomes essentially a line of sight process. Of course, a highly conductive substrate (e.g., metal) w i l l distribute the thermal energy out of the line of sight. Conversely, an insulating substrate (e.g., wood) w i l l do l i t t l e to distribute the heat, and hot spots can develop i f exposure is nonuniform. While most organic coatings absorb somewhere in the IR, there are advantages to its selectivity. Leather is largely transparent i n the region from 1 to 1.5 ym. This makes i t p o s s i b l e to s e l e c t i v e l y heat glue on the inside of a shoe and cause i t to dry


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A P P L I E D POLYMER SCIENCE

development of threedimensional networks or cured polymer structures

Figure 2.

Idealized polymer curing (cross-linking) reactions.



ι

Addition of hydroxyl groups to the isocyanate to form urethane linkages Unblocking or removal of phenol to form a free isocynate followed by addition of hydroxyl groups to form urethane linkages

Isocyanates and hydroxyl groups Phenol-blocked isocyanates and hydroxyl groups

Polyfunctional isocyanates and hydroxylcontaining polymers

Block polyisocyanates and hydroxyl polymers

Polyurethanes (two-component mixtures)

Polyurethanes (single-component system)

s-

ο

Addition followed by ring opening

Oxirane, primary, and secondary amines, carboxy groups

Polyfunctional amines or polyfunctional carboxyl materials

Epoxy (twocomponent mixtures)

!S-

I'

Transesterification or transetherification reactions between polymers and cross-linking oligomers (condensation)

C GO

73

Ό Ο

ζ

>

2

η

Carboxyl and hydroxyl groups—methylol and ethers of methylol groups

Reference

Urea formaldehyde and melamine resins

Air oxidation, peroxide formation, and coupling between polymer chains (free radical)

Cross-Linking Reaction (Mechanisms)

Conjugated double bonds and other unsaturation sites

Functional Groups

2

Polyesters, acrylics, epoxy (single-com ponent systems)

Air-drying oils or polyester resins (single component)

Cross-Linking Oligomers

Typical Polymer Cross-Linking Oligomer Curing Reactions Used in the Coatings Industry

Polymer Systems

Table I.



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A P P L I E D P O L Y M E R SCIENCE

Table II.

Alternative Energy Sources to Gas Fuel Convection Ovens for Cure of Coatings

Energy Source

Energy (eV)

Mechanism

Infrared

lo-i

Thermal

Microwave

10"3

Thermal

Radio frequency

10~6

Thermal

Visible

1

Electronic excitation

Ultraviolet

5

Electronic excitation

Electron beam

105

Ionization excitation

Gamma-ray

108

Ionization excitation


35.

MCGINNISS A N D GRUBER


845

fast enough to be an inline process (10). This is perhaps the most unexploited attribute of IR. The emergence of infrared lasers has only recently provided highly monochromatic sources of IR energy with high efficiency, particularly in energy regions where thermal emitters are i n e f f i c i e n t . For example, CO2 lasers emit at 10.6 ym and have powers of several kilowatts (11). Actually, this suggestion serves to introduce the subjects of microwave and radio frequency radiation. RF radiation is being used to cure adhesives on laminates for furniture and for drying coatings, while microwave has been used to vulcanize rubber, convert plastics, and dry inks.


Radio Frequency and Microwave Radiation Radio frequency radiation and microwave radiation cause coatings to dry or cure by thermal activation. The most important mechanism of activation involves rotation of polar molecules so as to align their dipoles in an electric f i e l d . The rate at which electrical energy can be dissipated in a d i e l e c t r i c material i s proportional to the frequency of the energy and to the square of the e l e c t r i c f i e l d strength. The relationship is expressed in the equation (12) Ρ = Kf(Ef)(tan Ô)(E2) where Ρ = rate of energy dissipation (watts), f = f i e l d frequency (hertz), E = substrate dielectric constant, tan δ = loss tangent or power factor, and Ε = f i e l d strength (volts per centimeter). The d i e l e c t r i c constant and loss tangent are the substrate variables. The loss tangent is apparently related to variables such as the molecular weight, viscosity, and conductivity of the material to be dried. Some examples w i l l serve to illustrate the difficulty in predicting the rate at which a material w i l l be heated. Water absorbs microwave energy about 103 times better than ice. This i s apparently due to the c r y s t a l structure of ice i n h i b i t i n g rotation of the water molecules. D i s t i l l e d water i s a polar insulator, and while i t absorbs microwave rather well, a 0.5 molal sodium c h l o r i d e - d i s t i l l e d water s o l u t i o n absorbs 50% more efficiently. Carbon is nonpolar, but its conductivity is such that i t i s a respectable microwave absorber. In practice, researchers have elucidated many of the principles involved in microwave and RF energy use and one can formulate an ink or coating with these p r i n c i p l e s in mind, but the ultimate test i s the rate at which the formulated product interacts with microwave energies. Microwave generation i s somewhat expensive, but at least one economic analysis has suggested that RF radiation compares most favorably with IR i n that increased energy efficiency leads to substantially lower energy costs (13). F

Light Energy Radiation The interaction of l i g h t energies ( v i s i b l e and u l t r a v i o l e t ) on photosensitive liquid materials leading to their hardening or curing has been known since ancient times but only until recently has this process found commercial success in major industries such as



846


photoresist manufacture, p r i n t i n g , wood f i n i s h i n g , d e n t a l composites, metal decorating, vinyl floor coatings, and adhesives. The concept of light energy radiation curing or photocuring of coatings can be divided into f i v e basic segments: (1) A stable l i g h t source i s needed—one capable of producing u l t r a v i o l e t wavelengths of light or visible light (near- and far-UV, 200-400 nm; v i s i b l e , 400-750 nm) with sufficient power or intensity to be commercially feasible. (2) A photoinitiator or photoactive catalyst capable of absorbing ultraviolet radiation, at appropriate wavelengths of energy emitted from the light source, is needed. (3) Active free radical intermediates or strong acid species must be produced through the a c t i o n of l i g h t absorption by the photochemically active photoinitiator. The free radicals initiate polymerization of unsaturated monomers, oligomers, and polymers while the photochemically released strong acid compounds initiate ring opening polymerization reactions of epoxy resins and other monomers, oligomers, and polymers sensitive to cation-induced polymerization reaction mechanisms. (4) Unsaturated (high b o i l i n g acrylic, methacrylic) monomers, oligomers, cross-linkers, and low molecular weight polymers make up the f l u i d , low viscosity, free radical light-induced curable coating system while low molecular weight epoxy monomers and polymers make up the cationic l i g h t induced curable coating systems. (5) After free radical or cationic initiation of the reactive liquid coating, the monomers, oligomers, and polymers propagate into a f u l l y cured, cross-linked s o l i d coating or film (14). Light Source. Early UV processing of coatings involved the use of low-pressure mercury lamps for curing styrenated polyesters. Because these lamps emit only 1 W or less of ultraviolet per inch of arc length, they soon gave way to the so-called medium-pressure mercury arcs. In general, these lamps operate at 200 W/in. input power with UV output being in the region of 30-50 W (15). This type of lamp is offered by several manufacturers, and i t has become the industry standard. These arcs have a lifetime of a few thousand hours, and about 50% of the input power emerges as infrared. But excessive substrate heating can be a problem. The arc must operate at greater than 600 °C, but at the same time the arc electrodes must be cooled to below 300 °C. Clearly temperature control around the arc is a serious problem. It has been stated that about 1000 CFM of air is required to cool three 40-in. lamps (16). Temperature control around a mercury arc i s necessary, and an obvious way to do this is to cool with air. Because oxygen inhibits free radical polymerizations, air cure does not afford the superior properties required for many end uses. Much progress has been made in this area, and indeed most of the present-day UV curing l i n e s operate in air atmosphere; however, new developments in UV hardware are now providing alternatives. One alternative system utilizes water cooling of electrodes and lamp reflector housings so that minimal amounts of nitrogen can be used as an inert blanket in order to exclude oxygen and produce a very high quality coating f i n i s h . Other systems have made use of special infrared absorbing f i l t e r s or combinations of inerted low-pressure lamps and medium-pressure lamps that operate in a i r . The low-pressure lamps require only nominal cooling, which allows for modest nitrogen use. In such a



35.

McGlNNISS AND GRUBER


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system, the low-pressure source dries the surface of a coating. This provides an oxygen barrier so that the final stage can achieve the bulk cure in air. A high-intensity, electrodeless ultraviolet source has recently been introduced. This UV source i s distinguished from other commercially available UV lamps by the fact that no electrodes are inside the lamp. Electrical energy is supplied in the form of radio frequency power that i s coupled into an evacuated quartz tube containing mercury and other additives. As compared to conventional medium-pressure mercury arcs with electrodes, the electrodeless system has several attributes that make i t unique (17). These are summarized as follows: (1) instant on-off; (2) modular lamp design; (3) increased lamp lifetime; (4) variable spectrum; (5) excellent energy conversion. One of the disadvantages of electrode lamps i s that they take about 3 min to warm up from a cold start and about 5 rain to restart after being turned off. Electrodeless lamps need no more than 15 s to come to f u l l power from a cold start or 60 s from a hot start. This hot start time can be reduced to instant starting (less than 1 s) i f required. Presumably this would a l l e v i a t e the need for shutters on commercial curing lines. A unique characteristic of the electrodeless system i s the modular lamp construction. The basic lamp is 10 in. in length, and because there are no electrodes, i t emits from the entire length. Thus, a 40-in. web can be cured by butting four 10-in. modules next to each other. The elimination of glass-metal seals results in a simpler, less stressed bulb. As a r e s u l t , the electrodeless lamp potential lifetimes should be superior to the electrode type. In fact, a mechanism for failure has not yet been identified. Because there are no electrodes in the radio frequency discharge lamp, less care need be taken to avoid f i l l materials that interact unfavorably with them. Additives can be used in small or large quantities to vary the spectral output distribution. The same basic lamp system and power supply can be used with different f i l l s to provide a wide range of spectral characteristics, including sources with enriched high- or low-energy photon outputs. There is also a wider range of mercury pressures easily available with electrodeless lamps, so that various ratios of line to continuum radiation can be obtained. It would seem that these v a r i a b l e s would be of considerable value in promoting the cure of titanium dioxide pigmented coatings. Because the electrodeless lamp can be cooled uniformly, i t can be made to run cooler than the electrode type, but at the same time i t can be driven at higher power. This allows for lamp input powers up to 600 W/in. as compared to the conventional 200 W/in. of electrode lamps, without any serious degradation of lamp lifetime. Because the plasma inside has a higher electron temperature, the spectrum is shifted further toward ultraviolet. With the same f i l l , the same lines and continuum appear as with an electrode lamp but the output in the UV i s much greater. The actual UV output from a 320 W/in. electrodeless lamp i s 117 W/in. This represents a 36% energy efficiency as compared to about 20% for 200 W/in. electrode arcs. Of course, i t must be added that considerable energy losses occur in converting ac to radio frequency.

American Chemical Society Library 1155 Science; 16th St., N.W. In Applied Polymer Tess, R., et al.; Washington, D.C. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

848



Many other types of l i g h t sources can also be used for photopolymerization reactions, for example, low-pressure mercury arcs, flash lamps, fluorescent lamps, tungsten halide sources, and even l a s e r s . A complete review of l i g h t sources used i n photopolymerization reactions can be found in Reference 18. Photoinitiators. Many theories of photoninitiated polymerization reactions with different l i g h t - s e n s i t i v e catalysts have been reviewed in References 19-21. There are, however, two general classes of photoinitiators: (1) those that undergo direct photofragmentation upon exposure to UV or visible light irradiation and produce active free radical or cationic intermediates; (2) those that undergo hydrogen abstraction electron transfer reactions f o l l o w e d by rearrangement i n t o a free r a d i c a l or c a t i o n i c intermediate. It i s also important to select photosensitizers and photoini tiators with absorption bands that overlap the emission spectra of the various commercial UV and visible light sources (only the light absorbed by a molecule evokes a photochemical reaction). Some of the most common photoinitiator systems in use today are as follows:

0 9-alkyl

C-CR]^ R2

ο

hv 365 nm

derivative of acetophenone where R^=H, alkyl, aryl, alkoxy and R2=H, alkyl, aryl

O-alkyl

SxC- +

•CRi R2

free radical intermediates

Ar Ar-S X I Ar

hv 365 nm

Ar S* 2

+ Ar* + X"

radical cations, radicals and lewis acid intermddiates

where JT - BF , PF , AsF , SbF , 4

6

6

6

free radical intermediates

R . N-CHR R In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

35.


MCGINNISS A N D GRUBER

849

Other p h o t o i n i t i a t o r s t r u c t u r e s (free r a d i c a l and acid intermediate generators) have been reviewed in References 22 and 23.

Free Radical Initiated Radiation Curable Coating Compositions. Conventional thermally cured coating systems are generally based on the following polymer backbone chemical structures: (1) Epoxy

0,

A


c—c-c-o^oH-^>- o-c-c—c (2) Urethane

0

0

-j O-C-NH-R χ -NH-C-0-R2J^ (3) Polyester

0

0

+ 0-C-R -C-0-R f 1

2

(4) Acrylic C-0 0

1

R

Present-day thermal curing coatings systems utilize these types of polymer structures as well as f i l l e r s and pigments dissolved or dispersed i n an organic s o l v e n t for coatable a p p l i c a t i o n v i s c o s i t i e s . These solvents are then thermally removed and the coating i s cross-linked into a three-dimensional network by an energy-rich chemistry requiring a high degree of thermal energy to convert the polymers into useful commercial acceptable properties. Radiation curable polymer systems are based on the same chemical structural design as the conventional polymer systems, but certain modifications are made in order to accommodate reactive unsaturation sites necessary for a radiation-induced free radical curing mechanism. Examples of these modifications of conventional polymer structures to form radiation curable polymers are as follows: (1) Unsaturated (Acrylic/Methacrylic) Epoxy 0 OH CH =CH-C-0—! 2

0H 0

Ç3 H

/

0^/@>_j-< grK>—0-C-C=CH v

2


850 (2)

A P P L I E D POLYMER SCIENCE

Unsaturated Urethane 0

0

0

CH2=CH-G-0-CH -CH -0—C-NH-R -NHC-0-R —CH=CH 2

(3)

2

1

2

2

Unsaturated Polyester 0

0

«

n

I

-0-C-C»C-C-0-R J m Downloaded by PURDUE UNIV on January 15, 2015 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch035

2

(4) Unsaturated Acrylic u

\J m

C-0 OH 0 i I II 0—CH -CH-CH -0-C-CH=CH 2

2

2

The monomer in radiation-curable coatings is the analog of the solvent in a conventional paint. Although i t performs l i k e a solvent by being a medium for a l l of the other ingredients and by providing the necessary liquid physical properties and rheology, i t differs in that i t enters into the copolymerization and is not lost on cure. Most radiation-curable monomers contain single unsaturation sites and are high-boiling acrylic esters, although in the wood area some coatings use styrene as the monomer. Usually, where styrene is used, most or a l l of the polymer-polyester unsaturation is fumarate rather than acrylic. Cross-linking oligomers in conventional thermosetting coatings formulations are u s u a l l y melamine r e s i n s ( a c i d , h y d r o x y l transetherification cross-linking reactions), amine/amide hardeners (oxirane r i n g opening r e a c t i o n s ) , and blocked isocyanate prepolymers. Oligomers and cross-linking materials in radiation curing systems are similar to single v i n y l functional monomers except they contain d i - , t r i - , or multifunctional unsaturation sites. These multifunctional components cause polymer propagation reactions to proceed into three-dimensional network structures of a cured film.


35. MCGINNISS A N D G R U B E R

Curing Methods for Coatings hv

PI

PIfree radical intermediate

photoinitiator PI-

851

multifunctional unsaturated

+

monomers and polymers


three-dimensional network formation

Formulation of radiation curable coatings requires a balance in composition among three variables: single unsaturated functional monomers, cross-linking agents (multiunsaturated oligomers), and unsaturated polymers. For example, as the composition of a coating changes from monomer- to polymer-rich mixtures at a constant crosslinking oligomer concentration, the coating viscosity increases, the rate of cure for the coating may decrease, and the f i n a l f i l m properties would be expected to have good adhesion and better extensibility. If the coating composition changes from a monomerrich to cross-linking oligomer rich mixture at constant polymer concentration, then the rate of cure for the coating w i l l be increased but the f i n a l cured f i l m may be b r i t t l e and have very l i t t l e adhesion to certain substrates (24). The physical properties of the cured f i l m depend upon the i n i t i a l l i q u i d formulation ingredients, their chemical structure, unsaturated reactivity, and individual component concentrations. Acid Intermediate Initiated Radiation Curable Coating Compositions. In this process, photogenerated acid intermediates can cause ring opening reactions of various oxirane monomers and prepolymers that can then further polymerize i n t o three-dimensional network structures. hv

RH

•> Ar S + Ar* + R» + HX 2

Coating compositions for this process can contain cyclic ethers, vinyl ether monomers, organosilicone monomers, and a wide variety of mono-, d i - , and polyepoxy f u n c t i o n a l m a t e r i a l s . Further modifications to this process include the addition of free radical p o l y m e r i z a b l e monomers i n combination with the c a t i o n i c polymerizable monomers so that both curing processes may take place at the same time. The photoinitiator systems that generate acid catalyst intermediates also generate free radical intermediates or can be combined with other photosensitizer materials such that both


852


ring opening and radical v i n y l addition reactions can occur. A major advantage to this hybrid system (free radical vinyl addition and acid-catalyzed ring opening photochemical reactions) i s that very fast tack-free cured surfaces can be achieved through the vinyl polymerization reaction while the slower curing ring opening reaction causes the development of excellent adhesive bonding capabilities of the cured film to the substrate.


Electron Curing Accelerated electrons were first used to cure coatings in the 1930s. It was not u n t i l the late 1960s, however, that any serious commercial interest in such a process was apparent. In the past several years electron processing has been the perennial bridesmaid but seldom the bride. Many reasons have been cited for the failure of electrocuring to be accepted in the marketplace. Probably the dominant reasons, though, are the relatively large investment needed for an electron processing l i n e and perhaps the thought that the ultimate equipment was not yet available. The past several years have seen the development of electron cure equipment that promises to reduce the investment cost, increase the efficiency, and simplify the system. Early devices were aptly described as producing electron beams, which were then scanned magnetically. For various reasons they operated at 300-500 keV. The large accelerating potential was undesirable for two reasons: (1) When an accelerated electron undergoes a c o l l i s i o n , the energy is released as a highly penetrating X-ray that must be shielded, and the amount of shielding needed i s related to the X-ray energy (25). A 300-keV beam i s around 200 keV at the workpiece due to losses at the accelerator window and in the air gap between the window and the substrate. For efficient energy use, the coating thickness should be on the order of 8-12 mils. The new generation of electron accelerators employs linear cathodes that need not be scanned and are specifically designed for coating processing. As a result, the energy at the workpiece is on the order of 100-125 keV and the unit density coating can be 4-8 mils and have optimum energy coupling. The linear cathode allows for reduced dose rates at the same t o t a l dose. This has clear mechanical advantages, while in principal the lower dose rate should have chemical advantages. The compact linear cathode devices reduce the shielding requirements such that the total electron cure package is compatible with many existing curing lines. The time has come for electron curing. The economics are acceptable for high-volume uses; the equipment has been designed for coatings, and the chemistry for curing pigmented materials i s superior to ultraviolet approaches. One other advantage of electron curing of coatings over light-induced curing reactions i s the elimination of photoactive catalyst systems from the coating formulation. The energetic electrons from the processor are adsorbed d i r e c t l y in the coating i t s e l f where they create the initiating free radicals uniformly in depth. Since electron energies of only lOOeV or less are required to break chemical bonds and to ionize or excite components of the coating system, the shower of scattered electrons produced in the coating leads to a uniform population of free radicals (excited


35. MCGINNISS AND GRUBER Curing Methods for Coatings

853

atoms or ions) throughout the coating, which then initiate the polymerization reaction. In the liquid-phase systems of interest here for coatings work, the polymerization process will propagate until the activity of the growing chain is terminated. These energetic electrons are capable of penetrating many different types of pigmented coatings and are capable of producing through cure down to the substrate-polymer coating interface. The coating materials used in electron curing processes are the same as those previously described for free radical initiated radiation curable coating compositions.


Glow Discharge Polymerization Glow discharge polymerization (GDP) refers to the formation of polymers by means of an electric discharge. The breakdown of molecules in gas discharges has been known for 100 years (26, 27). Glow discharge polymers can be produced quite readily. One needs only a reaction vessel containing monomer and an electrical source of sufficient power to cause the monomer to glow. The details of how GDP works are a matter for speculation (22), but the energy input is large enough to break any chemical bond and as a result even materials like benzene can be polymerized (29). Because glow discharge polymerization is accomplished in an evacuated system, it is not ideal for continuous processing, although methods for doing so have been commercialized (30). Literature Cited 1. Blank, W. J. J. Coat. Technol. 1982, 54(687), 26. 2. Barrett, Κ. E. J. "Dispersion Polymerization in Organic Media"; Wiley: New York, 1975. 3. "Treatise on Coatings—Formulations"; Myers, R. R.; Long, J. S., Eds.; Marcel Dekker: New York, 1975; Vol. 4, Part 1. 4. Schultz, À. R. "Encyclopedia of Polymer Science and Technology"; Interscience: New York, 1965; Vol. 4, p. 331. 5. Patton, T. C. "Alkyd Resin Technology"; Interscience: New York, 1962. 6. Widmer, G. "Encyclopedia of Polymer Science and Technology"; Interscience: New York, 1965; Vol. 2, p. 1. 7. Lee, H.; Neville, K. "Handbook of Epoxy Resins"; McGraw-Hill: New York, 1967. 8. "Treatise on Coatings--Film-Forming Compositions"; Myers, R. R.; Long, J. S., Eds.; Marcel Dekker: New York, 1967; Vol. 1, Part 1. 9. Moore, N. L. "Radiation Drying of Paints and Inks"; Watford College of Technology: Hertis, 1973. 10. Summer, W. "Ultraviolet and Infrared Engineering"; Sir Isaac Pitman and Sons: London, 1962. 11. Ladstadter, E.; Hanus, H. D. preprints of OCCA Conference, June, 1973. 12. Readdy, A. F. "Plastics Fabrication by Ultraviolet, Infrared Induction, Dielectric and Microwave Radiation Methods"; Plastics Technical Evaluation Center, Picatinny Arsenal: Dover, N.J.



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APPLIED POLYMER SCIENCE

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Curing Methods for Coatings - American Chemical Society

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