Note: Descriptions are shown in the official language in which they were submitted.
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COMPOSITIONS AND METHODS FOR PRODUCING
HIGH GLOSS RADIATION CURABLE COATINGS
BACKGROUND OF THF INV~NTION
Radiation curable coatings are highly desirable because
they can be cured without the need to evaporate substantial amounts
of solvent. Therefore, curing can be accomplished rapidly and with
less release of volatile organic compounds into the atmosphere.
Also, radiation curing can be carried out at relatively low
temperatures, thereby lending itself to use with temperature
sensitive substrates such as wood and plastic. The use of electron
beam radiation is particularly favored for curing thick coatings and
coatings that are heavily pigmented.
It is known to cure certain coating compositions to a
glossy finish using electron beam radiation to initiate the
crosslinking reactions that result in a cured film. The curing of
typical radiation curable coating compositions is inhibited by the
presence of oxygen, so it is conventional to provide a substantially
inert atmosphere (e.g., less than 200 parts per million oxygen) in
the electron beam exposure apparatus. It can be costly to maintain
the required degree of inertness accurately and constantly. Slight
variations in the oxygen concentration can lead to uneven coating
appearance with conventional methods and compositions. It would be
desirable to reduce the sensitivity of an electron beam curing system
to oxygen when producing glossy coatings.
It is known to add accelerator compounds (e.g., tertiary
amino compounds) to coating compositions to at least partially
overcome the inhibition of curing due to oxygen, thereby increasing
the amount of oxygen that can be permitted in the radiation curing
chamber. Although the accelerators provide processing advantages,
they generally have deleterious effects on the coating, such as
yellowing and/or surface roughness. The accelerators can sometimes
also have a negative effect on the ease of applying the coating
composition onto the substrate.
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Multi-step radiation curing processes, which may include a
combination of electron beam and ultraviolet radiation, have been
employed to produce low gloss coatings. In these prior art processes
oxygen (air) was intentionally present during the first radiation
curing step to initially inhibit polymerization~at surface portions
of the coating, and curing of the coating was completed in a
subsequent step in an inert atmosphere. Shrinkage of underlying
layers during the first step caused pigment particles to be driven
into the surface portions, whereby the surface contained a larger
0 amount of pigment than the body of the film which reduced the gloss
of the film without sacrificing film strength or rheology properties
of the coating composition. U.S. Patent Nos. 3,918,393 (Hahn) and
4,048,036 (Prucnal) illustrate this approach. Multi-step radiation
curing techniques have also been proposed for producing textured
finishes in U.S. Patent Nos. 4,421,784 (Troue), 3,840,448 (Osborne et
al.), and 4,411,931 (Duong) wherein surface wrinkling of the coating
is induced by the staged curing process. Those of skill in the art
would have considered two stage radiation cure processes
inappropriate for producing high gloss finishes.
SUMMARY OF T~. INV~NTION
By the present invention it has been found that electron
beam radiation can be employed to yield high gloss finishes from
certain coating compositions without requiring the fully inert (i.e.
less than 200 parts per million oxygen) required by prior art
processes. The process uses a two step curing process and selected
coating compositions. Achieving high gloss finishes from a two step
process is surprising in view of the fact that multi-step processes
have previously been used for producing low gloss finishes.
Furthermore, the substantial lessening of the requirement for a fully
inert atmosphere during the electron beam radiation step is highly
advantageous for the sake of reducing costs associated with
atmosphere control.
The first step of the process of the present invention
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involves irradiation with an electron beam, followed by a subsequent
step of exposure to ultraviolet radiation. The electron beam step
may take place in air or in a partially inert atmosphere including at
least 2 percent oxygen by volume, whereby the surface of the coating
remains wet or uncured. The ultraviolet radiation step takes place
in a substantially inert atmosphere (e.g., less than 1000 parts per
million oxygen) whereby curing of the coating is substantially
completed. Although an inert atmosphere is required in the
ultraviolet step, it may be noted that the conditions need not be
controlled as rigorously as in prior art electron beam processes.
The coating compositions of the present invention are
characterized by: a resin binder curable by radiation exposure in
the presence of at least one photoinitiator compound, a relatively
non-volatile photoinitiator compound, and a flow control agent such
as a siloxane.
DETA~T~T~n DESCRIPTION
The binder or vehicle in the coating composition of the
present invention comprises at least one resin (monomer, oligomer, or
polymer) which is curable by exposure to radiation in the presence of
one or more of the photoinitiators disclosed herein. Binder may
constitute 10 to 99, preferably 50 to 99, percent by weight of the
total coating composition. Many such resins are known in the art and
may be used in the present invention. The resins suitable for use in
the present invention are characterized by inhibition of curing by
the presence of oxygen (such as in air). Oxygen inhibition permits
maintaining an at least partially uncured surface layer during the
initial curing step with the electron beam. A particular category of
useful radiation curable compounds are characterized by a plurality
of acrylyloxy groups and the ability to free radically addition
polymerize upon being initiated by a photoinitiator or ionizing
radiation. Unless otherwise indicated either directly or by context,
acrylic unsaturation is used in its broad sense to mean the
unsaturation provided by unsubstituted acrylyl groups or
W O 94/22596 PCTAJS94/02939
2~s6293
a-substituted acrylyl groups such as methacrylyl, ethacrylyl and
a-chloroacrylyl. Examples of these compounds are the diacrylates and
dimethacrylates of ethylene glycol, 1,3-propanediol, propylene
glycol, 2,3-butanediol, 1,4-butanediol, 2-ethylbutane-1,4-diol,
1,S-pentanediol, 1,6-hexanediol, 1,7-heptanedioll 1;8-octanediol~
1/9-nonanediol/ 1/10-decanediol, 2,10-decanediol,
1,4-cyclohexanediol, 1,4-dimethylolcyclohexane,
2,2-diethylpropane-1,3-diol, 2,2-dimethylpropane-1,3-diol,
3-methylpentane-1,4-diol, 2,2-diethylbutane-1,3-diol, 4,5-nonanediol,
o diethylene glycol, triethylene glycol, propylene glycol, neopentyl
glycol, 5,5-dimethyl-3,7-dioxanonane-1,9-diol,
2,2-dimethyl-3-hydroxypropyl 2,2-dimethyl-3-hydroxypropionate; the
triacrylates and diacrylates of glycerol, 1,1,1-trimethylolpropane
and trimethylolethane; and the tetraacrylates, triacrylates, and
diacrylates of pentaerythritol and erythritol. The acrylyloxy groups
in each of the molecules are usually the same, but they may be
different as exemplified by the compound
2/2-dimethyl-1-acrylyloxy-3-methacrylyloxypropane.
Further examples of satisfactory polyacrylyloxy compounds
that may be included in the radiation curable resin include
polyacrylyloxy functional polyesters/ polamides/ polyacrylates,
polyethers, polycarbonates or polyurethanes as well as polyacrylyloxy
functional compounds of mixed functionality such as polyacrylyloxy
functional poly(ester-urethanes), poly(ester-amides) and
poly(ether-urethanes). Mixtures of compounds having a plurality of
acrylyloxy groups may be used, if desired.
The amount of polymerizable compound having a plurality of
acrylyloxy groups present in the coating composition is subject to
wide variation. The compound is ordinarily present in an amount in
the range of from about 10 to 99 percent by weight based on the
weight of the binder of the coating composition. An amount in the
range of from about 20 to 97 percent is typical. From about 30 to 95
percent by weight of the binder is preferred.
Monomers having monoacrylic functionality which crosslinks
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with the compound having polyacrylyloxy functionality may optionally
be present in the coating composition. Examples of monoacrylic
functional monomers which may be used are methyl acrylate, ethyl
acrylate, propyl acrylate, butyl acrylate, hexyl acrylate, hexyl
ethyl acrylate, hexyl butyl acrylate, 2-ethyl hydroxy acrylate, octyl
acrylate, hydroxy ethyl acrylate, hydroxy butyl acrylate,
caprolactone-hydroxyl alkyl acrylate reaction products, and 2-ethyl
hydroxy acrylate. The preferred monoacrylic functional monomers are
liquid compounds miscible with the polyacrylyloxy compound. A
0 benefit from the use of one or more monoacrylic functional monomers
is that the monoacrylic functional monomer may act as a reactive
solvent for the polyacrylyloxy functional compound, thereby providing
coating compositions having a satisfactory low viscosity while using
relatively small amounts or no volatile, nonreactive solvent.
The monoacrylic functional monomer, or mixtures of
monoacrylic functional monomers, may be employed over a broad range,
although none is required. The amount of such monomer when used
should be sufficient to provide a liquid, flowable,
interpolymerizable mixture. When used, the monomer will ordinarily
be present in the coating composition in the range of from about 0 to
about 80 percent by weight of the binder of the coating composition.
Typically, the monoacrylic functional monomer will be present in the
range of from about 0 to about 30 percent by weight of the binder.
Other monovalent functional monomers may be employed as known in the
radiation curing art, including N-vinyl-2-pyrolidone, vinyl
neodecanoate, and other ethylenic unsaturated monomers known to be
suitable for radiation curable coatings.
The present invention involves a coating composition
containing a photoinitiator. Photoinitiators absorb radiation and
thereby obtain energy to form free radicals that initiate
polymerization of the binder resin. The photoinitiator in the
present invention is one which forms free radicals upon exposure to
actinic radiation, viz., ultraviolet light. In order to produce the
high gloss finishes of the present invention, it has been found
W O 94/22596 21 S 6 2 9 3 PCT/US94/02939
desirable to select photoinitiators that are non-volatile, which is
generally related to the molecular weight of the photolnitiator or
its ability to copolymerize with other constituents of the
composition. Therefore, for the purpose of the present invention,
the non-volatile photoinitiator is generally characterized by a
molecular weight of at least 260. Preferred examples had molecular
weights greater than 300, with the best results achieved with
molecular weights greater than 1000. Alternatively, the non-volatile
photoinitiator may be characterized as having a copolymerizable
functionality such as an acrylate group, even though its molecular
weight may be slightly lower than 260. An example of a polymerizable
photoinitiator is 4-(2-acryloxyloxyethoxy)
phenyl-(2-hydroxyl-2-propyl) ketone.
One suitable class of photoinitiators from which the
photoinitiators of the present invention may be selected are the
acylphosphine oxides. These photoinitiators cleave when exposed to
ultraviolet radiation, and their residues in the cured film
advantageously do not impart unwanted coloration to the film.
Acylphosphine oxide photoinitiators are disclosed in U.S. Patent Nos.
3,668,093 and 4,447,520 and may be characterized by the formula:
O O
Il 11
R--P--C--R "
R'
where R and R ' may be linear or branched 1 to 6 carbon alkyl,
cyclohexyl, cyclopentyl, aryl, halogen-, alkyl-, or
alkoxy-substituted aryl, or 5- or 6-membered S- or N-heterocyclic
groups; R ' may additionally be 1 to 6 carbon alkoxy, aryloxy, or
arylalkoxy, or forms a ring with R; R" is linear or branched 2 to 18
carbon alkyl, a 3 to 12 carbon cycloaliphatic group, an alkyl- or
(thio)alkoxy-substituted phenyl or naphthyl group, or a 5- or
6-membered S- or N-heterocyclic group which can contain other
functional groups, or an -X-CO-P(=O)R-R' group (where X is a
WO 94l22~96 2 ~ 5 ~ 2 ~ 3 PCTrUS94/02939
-- 7
phenylene or 2 to 6 carbon (cyclo)aliphatic divalent group. R, R',
or R" may include unsaturation. A particular example of an
acylphosphine oxide is 2,4,6-trimethyl benzoyl diphenyl phosphine
oxide, which is sold under the name "Lucirin TPO" by BASF
Corporation. In selecting photoinitiators, one of skill in the art
would consider it expedient to select compounds that are soluble and
stabile in the particular composition.
There are many other photoinitiators which may be used in
the coating compositions of the present invention. Another class of
o compounds useful as photoinitiators in the present invention are
acetophenone derivatives meeting the definition of non-volatile set
forth above. Many acetophenone derivatives are known as
photoinitiators, a large number of which are disclosed in U.S. Patent
No. 4,229,274 (Carlblom). AcetophPn~n~ derivative photoinitiators
may be generally characterized by the formula:
O Rl
Il I
0C--C--R2
R3
where Rl, R2, and R3 may, for example, include independently
hydrogen, alkyl usually having from 1 to 6 carbon atoms (preferably 1
to 4 carbon atoms), alkoxy, cycloalkyl, or substituted or
unsubstituted phenyl groups, and 0 is a phenyl group.
,l,.~,
W O 94/22596 PCTrUS94/02939
A particularly useful family of acetophenone derivatives
has the following structure:
o R4
ll I
0C -C -OH
R5
lo where R4 and Rs are alkyl groups having from 1 to 4 carbon atoms, and
0 is a phenyl group.
The amount of photoinitiator present in the coating
composition is preferably at least 0.01 weight percent based on total
solids content of the coating composition. Although larger amounts
could be used, it is usually uneconomical to use more than 5 percent
of the photoinitiator. Typically, the photoinitiator may be present
in an amount of at least 0.1 percent, most often in the range of from
0.5 percent to 2 percent. Mixtures of more than one photoinitiator
compound may be used.
The coating compositions of the present invention include
a surfactant of the type that serves as a flow control agent or
leveling agent. A large number of products are commercially
available for this purpose, and many of them are siloxane or
fluorocarbon compounds. The preferred class of flow control agents
are siloxane compounds, examples of which are "BYK -310, a polyester
modified dimethylpolysiloxane ~ from Byk Chemie, Wallingford,
Connecticut, and "Versaflow* 102," a modified methyl siloxane from
Shamrock Technologies, Inc., Newark, New Jersey. Examples of
fluorocarbon flow control agents are the "Zonyl ~ surfactants from
DuPont and the "Fluororad " surfactants, particularly "FC430," from
3M. The flow control agents for use in the present invention are
characterized by the property of assisting leveling of the coatings
of the present invention after application onto a substrate, without
substantial interference with the application process itself. It is
also desirable for the flow control agent to have relatively low
volatility. The flow control agent may be present in the composition
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in amounts ranging from 0.01 to 1.0 percent by weight based on total
solids content of the composition.
Pigments may be included in the coating composition.
Examples of opacifying pigments include titanium dioxide (rutile or
anatase), zinc oxide, zirconium oxide, zinc sulfide and lithopone.
Examples of coloring pigments include iron oxides, cadmium sulfide,
carbon black, phthalocyanine blue, phthalocyanine green, indanthrone
blue, ultramarine blue, chromium oxide, burnt umber, benzidine
yellow, toluidine red, aluminum powder and aluminum flakes. Examples
0 of extender pigments include silica, barytes, calcium carbonate,
barium sulfate, talc, aluminum silicates, sodium aluminum silicates,
potassium aluminum silicates and magnesium silicate. A single
pigment may be used or mixtures of pigments may be employed. When
the pigment is ultraviolet light absorbing, it should be used in
amounts which do not preclude curing of the interior of the coating.
The maximum amount is therefore related to the thickness of the
coating to be cured. Thin coatings may tolerate more ultraviolet
light absorbing pigment than thick coatings. When the pigment does
not significantly absorb ultraviolet light, there is usually greater
latitude in the amounts which may be employed. When pigment is used,
it is generally present in an amount in the range of from about 0.1
to about 70 percent by weight of the coating composition. Often it
is present in an amount in the range of from about 0.5 to about 50
percent. Usually it is present in an amount in the range of from
about 1 to about 35 percent by weight of the coating composition.
Dyes and tints may optionally be included in the coating composition
as replacements for all or some of the pigment content.
Other optional ingredients are resinous pigment
dispersants, viscosity control agents (e.g., cellulose acetate
butyrate or resinous acrylics), plasticizers, or grinding vehicles
such as non-reactive acrylics. There are many resinous additives
which are commercially available which may be used for these
purposes. These additives are used in the manner and in amounts
known to the art, such as 0 to 20 weight percent of the total
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21s6293 - 10 -
composition.
Another ingredient which is often included in coating
compositions of this type is a non-reactive, volatile organic
solvent. However, in preferred embodiments of the present invention,
no such non-reactive solvent need be included. In other embodiments
of the invention, solvent may be present, but in lesser amounts than
conventional. It is generally advantageous to minimize the amount of
organic solvent, but if reduction of viscosity is desired for a
particular application, the present invention does not preclude
o adding larger amounts of a non-reactive solvent or mixtures of
several solvents. Examples of suitable non-reactive organic solvents
are acetone, methyl ethyl ketone, methyl isobutyl ketone, methyl
alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl
alcohol, secutyl alcohol, isobutyl alcohol, tert-butyl alcohol, amyl
alcohol, hexyl alcohol, 2-ethylhexyl alcohol, cellosolve, ethyl
cellosolve, cellosolve acetate, 2-ethylhexyl acetate,
tetrahydrofuran, and aliphatic naphtha. When solvent of this type is
used it is ordinarily present in the coating composition in the range
of from about 0.1 to about 40 percent by weight of the vehicle of the
coating composition. From about O to about 15 percent is typical.
The preferred compositions are solvent-free.
The listing of optional ingredients discussed above is by
no means exhaustive. Other ingredients may be employed in their
customary amounts for their customary purposes so long as they do not
seriously interfere with good coatings practice or the obtaining of
cured coatings of high gloss.
The coating compositions of the invention are usually
prepared by simply admixing the various ingredients. The compounds
comprising the photocatalyst system may be premixed and then admixed
with the other ingredients of the coating composition or they may be
added separately. Although mixing is usually accomplished at room
temperature, elevated temperatures are sometimes used. The maximum
temperature which is usable depends upon the heat stability of the
ingredients. Temperatures above about 2000F ~930C) are only rarely
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WO 94/22596 PCTrUS94/02939
employed.
The radiation curable coating compositions of the
invention are generally used to form cured adherent coatings on
-' substrates. The substrate is coated with the coating composition
using substantially any technique known to the art. These include
spraying, curtain coating, dipping, roller application, printing,
brushing, drawing and extrusion. Wet, uncured coatings as applied to
a substrate have thicknesses of at least 1.0 mil (0.025 millimeter),
preferably at least 2.5 mils (0.06 millimeters), in order to achieve
0 the low gloss effect of the present invention. Theoretically there
is no upper limit for wet coating thickness, but in order to effect
first stage curing of lower strata at practical radiation power
levels, it is expedient to limit coating thickness to 5 to 8 mils
(0.13 to 0.2 millimeters). Cured coatings of the ultraviolet light
curable coating composition of the invention usually have thicknesses
in the range of from 1 to 5 mils (0.025 to 0.13 millimeter). More
often they have thicknesses in the range of from 2 to 4 mils (0.05 to
0.1 millimeter).
Substrates which may be coated with the compositions of
this invention may vary widely in their properties. Organic
substrates such as wood, fiberboard, particle board, composition
board, paper, cardboard and various polymers such as polyesters,
polyamides, cured phenolic resins, cured aminoplasts, acrylics,
polyurethanes and rubber may be used. Inorganic substrates are
exemplified by glass, quartz and ceramic materials. Many metallic
substrates may be coated. Exemplary metallic substrates are iron,
steel, stainless steel, copper, brass, bronze, aluminum, magnesium,
titanium, nickel, chromium, zinc and alloys.
The method of curing the coating composition of the
present invention involves a two step radiation exposure wherein the
applied coating layer is cured in a subsurface portion in a first
step by exposure to ionizing radiation (e.g., electron beam radiation
or laser) in the presence of oxygen whereby curing at the surface is
inhibited. In a subsequent step the curing is completed throughout
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the remainder of the coating thickness by means of ultraviolet
radiation in an at least partially inert atmosphere (i.e., less than
1000 ppm oxygen).
Suitable electron beam radiatlon for use in the first
curing step may constitute a dose of 2 to 10 megarads, preferably 3
to 7 megarads, at 150 to 300 kiloelectron volts, preferably about 250
kiloelectron volts. Line speeds of 50 to 120 feet per minute (15 to
36 meters per minute) are suitable. The exposure in the first step
is chosen so as to substantially cure the portion of the coating
o closest to the substrate. A portion of the coating thickness nearest
to the surface will remain at least partially uncured due to oxygen
inhibition. The atmosphere in the vicinity of the coating during the
electron beam exposure should include at least 2 percent oxygen by
volume.
Any suitable source which emits ultraviolet light, viz.,
electromagnetic radiation having a wavelength in the range of from
about 180 to about 400 nanometers, may be used in the practice of the
second curing step. Suitable sources are mercury arcs, carbon arcs,
medium pressure mercury lamps, high pressure mercury lamps,
swirl-flow plasma arc, ultraviolet light-emitting diodes and
ultraviolet light emitting lasers. Particularly preferred are
ultraviolet light emitting lamps of the medium or high pressure
mercury vapor type. Such lamps usually have fused quartz envelopes
to withstand the heat and transmit the ultraviolet radiation and are
ordinarily in the form of long tubes having an electrode at either
end.
The time of exposure to ultraviolet light and the
intensity of the ultraviolet light to which the coating composition
is exposed may vary greatly. For practical commercial line speeds,
lamps rated at 200 watts per inch (7900 watts per meter) or greater
are preferred. Generally the exposure to ultraviolet light should
continue until either the film is thermoset throughout or at least
cured to the point where subsequent reactions cause the film to
become thermoset throughout. Exposure of the coating to ultraviolet
~5~
WO 94/22596 PCTrUS94/02939
light may be accomplished in the presence of an inert atmosphere,
viz., an atmosphere either containing no oxygen or only a
concentration of oxygen which insignificantly inhibits polymerization
of the coating surface (less than 1000 parts per million oxygen).
s Gases such as nitrogen, argon, carbon dioxide or mixtures thereof are
typically the major components of inert atmospheres, although other
unreactive gases may be used. Nitrogen is generally employed for
this purpose.
Coatings produced in accordance with the present invention
o exhibit gloss and distinctness of image comparable to that of
coatings produced with full inerting (less than 200 parts oxygen per
million). Gloss may conveniently be determined by the Standard
Method of Test for Specular Gloss, ASTM Designation D-523-67
(Reapproval 1971). Using a Gardner 60~ glossmeter, the gloss of
cured coatings of the present invention may exceed 70 percent
reflected light, preferably greater than 80 percent, and with
specific examples being in the range of 83 to 86 percent.
Another measure of the gloss of coatings is "distinctness
of image" (D.O.I.). The particular distinctness of image test used
to evaluate the present invention employs a technique specified by
General Motors Standard Engineering Test: Distinctness of Image
GM91013 Page Reference W-65.201. In this test, a light source (a
Model GB11-8 Glowbox* made by I2R Corporation) is used to project a
series of successively smaller images of the letter "C" onto the
2s coated surface from a fixed distance. The smaller the image that is
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WO 94/22596 ~ PCTrUS94/02939
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reflected, the higher the D.O.I. rating on a scale of O to 100. High
gloss coatings generally exhibit D.O.I. ratings of at least 80,
preferably at least 85. Coatings cured with full inerting during the
electron beam exposure step may have D.O.I. ratings in the range of
90 to 100.
EXAMPLE 1
This example demonstrates a specific embodiment of the
composition and method of the present invention.
Co~titu~nt Perc~nt by weight
Diacrylate resinl 92.5
Cellulose acetate butyrate resin 4.0
Lampblack pigment2 1.2
15 Dispersant3 1.2
Photoinitiator4 1.0
Flow control agent5 0.1
1Low viscosity polyester diacrylate "SR606A" from Sartomer
Corporation, Exton, Pennsylvania, U.S.A.
2Grade #6 amorphous, acidic carbon black from General Carbon
Company, Los Angles, California, U.S.A.
3"Sartomer* 802" acrylic , I ?r with pigment dispersing
characteristics from Sartomer Corporation, Exton, Pennsylvania,
- U.S.A.
4"Esacure* KIPlOOF~ photoinitiator comprising 70~ oligo
{2-hydroxy-2-methyl-l-[4-(methylvinyl) phenyl] prop~non~} and -
30~2-hydroxy-2-methyl-1-phenyl propan-l-one, from Sartomer
Corporation, Exton, Pennsylvania, U.S.A. Molecular weight is
reported ta be 2000.
5~Versaflow* 102" modified methyl siloxane from Shamrock
Technologies, Inc., Newark, New Jersey.
A portion of the diacrylate together with the dispersant and the
lampblack pigment were ground in a pigment mill with ceramic media
The grind was then flushed out of the mill with an additional portion
of the diacrylate. The composition was then let down with the
r~ i nd~r of the diacrylate premixed with the cellulose acetate
butyrate, the photoinitiator, and the siloxane flow control agent.
The composition above was applied at a wet film thickness
of 2.5 mils (0.06 millimeters) with a curtain coater onto medium
density fiberboard that has been filled, sealed, and sanded smooth
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W O 94t22596 ~ fi ~ 3 PCTAJS94/02939
The coated substrate was cured by at a line speed of 100 feet per
minute ~30.5 meters per minute), first with an electron beam
manufactured by Energy Sciences, Inc., of Woburn, Massachusetts, USA,
set at 250 kilovolts terminal voltage and 23 milliamperes beam
s current, yielding 5 megarads of energy, in an atmosphere containing
lO~ oxygen by volume. In a second curing step the coated substrate
was subjected to ultraviolet exposure from four medium pressure
mercury vapor lamps of 200 watts per inch (80 watts per centimeter)
manufactured by Aetek International, Plainfield, Illinois, USA, at a
line speed of 80 feet per minute (24 meters per minute) in a fully
inerted atmosphere (nitrogen with less than 200 parts per million
oxygen). A highly glossy finish was produced as reported in Table 1.
EXAMPLE 2
This example employs the same two stage curing process
with electron beam and ultraviolet stages as used in Example 1, but
uses a conventional radiation curable coating composition.
Therefore, a polymeric surfactant was not included in the composition
of this example. Also, a lower molecular weight photoinitiator was
used ("Irgacure* 651" from Ciba-Geigy Corp., Hawthorne, New York,
having a molecular weight of 256). Substantially lower gloss was
produced in this example as can be seen in Table 1.
EXAMPLE 3
This example employs the same prior art coating
composition as in Example 2, but curing was carried out in a single
stage process using only the electron beam described in Example 1.
The electron beam stage was fully inerted. Even though curing was
carried out in a fully inerted atmosphere, the results as reported in
Table l were not as good as Example 1.
EXAMPLE 4
This example is the same as Example 4, with the exception
that no photoinitiator was included in the coating composition in
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view of the use of a single stage electron beam curing process. The
results as reported in Table 1 are the same as Example 3, indicating
that the presence of photoinitiator was not the cause of the inferior
gloss of Example 3 relative to the present invention as represented
by Example 1.
TABLE 1
Gloss D.O.I.
Example 1 (The invention) 85 9o
o Example 2 (Prior art composition)70 50
Example 3 (Single stage) 85 80
Example 4 (Single stage) 85 80
The examples set forth above demonstrate that the
particular composition of the present invention (Example 1), when
cured by a two stage process that does not employ full inerting in
the electron beam stage, can yield finishes having gloss subtantially
superior to that attained by a typical prior art compositions cured
by the same process (Example 2). That this result can be attained
without the need to fully inert the electron beam stage is highly
advantageous and surprising. Also surprising is finding that the
gloss attained by Example 1 (the invention) is better even than that
produced by a fully inerted electron beam (Examples 3 and 4).
The invention has been disclosed in connection with
specific embodiments in order to provide the best mode of the
invention, but it should be understood that other variations and
modifications as would be known to those of skill in the art can be
resorted to within the scope of the invention as defined by the
claims which follow.
