Note: Descriptions are shown in the official language in which they were submitted.
7'~
1 - 20731-92~
The present invention re:lates to a radlation-polymerizable
composition. More particularly, this invention relates to a radiation-
polymerizable composition which may be employed to produce an improved
photographic element such as a lithographic printing plate. The
basic difference in the various approaches in the Eormulation of
lithographic printing plates from radiation polymerizable compositions
has been in the search for a system which is prepared Erom relatively
inexpensive ingredients, does not require prolonged imaging cycles (has
a high quantum efEiciency), is able to undergo an increased number of
press runs and results in the creation of high resolution images within
the composition which can be manifested without prolonged and elaborate
development.
Most such lithographic printing plates comprise a metal
substrate which is coated with a light sensitive diazonium compound in
admixture with suitable binding resins, photoinitiators, photopolymerizable
compositions, colorants, stabilizers, exposure indicators, surfactants
and the like.
Although ~he art is replete with photosensitive compositions
which may be used ~or lithographic printing plates, the priar art
composition's serviceability is restricted by their limited stability
and number of press runs.
This invention relates to a photopolymerizable composition
for use in forming photographic elements such as lithographic printing
plates, comprising in admixture
a) a polymeric binder;
b) a photoinitiator;
c) a dlazonium salt; and
d) a polymerizable mixture of
i) an acrylic monomer having from 3 to 6 unsaturated
0
groups; and
ii) an oligomer which is prepared by reacting one molar
2 2073~ 24
equivalent of a substantlally linear polymeric
compound havin~ an active hydrogen group at each
end thereof with at least two molar equivalents
: of a diisocyanate compound so as to form a
prepolymer having an isocyanate group at each
end thereof; and subsequently reacting said
prepolymer with at least two equivalents of an
ethylenically unsaturated compound having an
active hydrogen group to provide said prepolymer
with unsaturation at each end thereof.
~-~ This invention relates to a radiation~polymerizable
composition.
The polymeric binder useful in the practice of this
invention is preferably characterized as being a polymer which
does not react with
,~
'
7~2J~
itself or oLher compo-mds whell exposed to actinic radiation. The
preferred binder resin should be substantially organic solvent soluble
and is preferably one which is not substcmtially alkali, acid or water
soluble. The binder must have a molecular weight sufficient to provide
a tack-free surface when in admixture with the photopolymerizable
mixture and to provide a tough cohesive matrix which when used to produce
a lithographic printing plate is capable of providing numerous quality
impressions. The molecular weight must be low enough, though, to permit
solubility in formulation as well as during development. The
molecular weight of the binder should be greater than about 20,000,
preferably greater than about 30,~0 and, most preferably, greater than
about 40,000.
Examples of binders suitable for this invention include epoxy
resins, polyvinyl acetate, polyvinyl formals, polyvinyl acetals,
~ b~t rR~
L~ polyvinyl ~ , polyesters, polyamides, polye-thers, polyurethanes
and polyacrylic resins which are either homo- or copolymers of
acrylates (or methacrylates) and/or acrylic acid (or methacrylic acid)
and copolymers or terpolymers of any of the monomers ~orming the above
polymers. In the preferred embodiment, the polymeric binder is a
terpolymer o~ polyvinyl acetate, polyvinyl alcohol and polyvinyl formal
available as Formvar 12/85 commercially from Monsanto of St. Louis,
Missouri.
Preferably, the polymeric binder is present in the composition at
a percent solids level of from about 20% to about 75~ by weight.
A more preferred range is from about 30% to about 65% by weight and,
most pre~erably, the polymeric bincler is present at a percent solids level
of from about 35% ~o about 50% by weight.
Suitable pho~oinitiators whlch may be used in this invention are
preferably those free-radical pho~oinitiators having an absorption maximum
range from about 320 to about ~00nm. Examples include the acetophenones,
benzophenones, triazines, benzoins, benzoin ethers, xanthones, thioxanth-
ones, acridines and benzoquinones. More preferred of these are the
A~ ~r~
7'~
triazines havint~ the Eormula
,CC13
Ar - CH = CH _ ~ C~
N-C
R R2 \CC13
wherein Ar is ~
R3
` ~4
and Rl, ~2' R3 and R4 are, independently, hydrogen, chlorine,
bromine, alkoxy or alkyl. The most preferred photoinitiator is
2-stilbenyl-4,6-di(trichloromethyl) triazine.
" The photoinitiator is preferably present in the com-
position at a percent solids level of about 1.5% to about 8.~
by weight, more preferably about 2.0 to about 6.0% by welght and
is most preferably present at a percent solids level of from
about 3.0~ to about 4.~% by weight.
The diazonium salt which is useful in the practice of
this invention may be any suitable light-sensitive diazonium
polymeric or monomeric compound wh:ich are well known to the
skilled artisan although the polymeric diazonium compounds are
preferred. Suitable diazonium compounds include those condensed
with ~ormaldehyde such as disclosed in United States Patents
2,063,631 and 2,667,415, the polycondensation products such as
disclosed in United States Patents 3,849,392 and 3,867,147, and
the high speed diazos such as disclosed in United States Patent
4,436,804.
Preferably~the diazonium salt comprises the 1:1 poly-
condensation product of 3-methoxy-diphenyl amine-4-diazonium
sulEate and 4,4'-bis-methoxYmethyl-diphenyl ether, precipitated
as the mesikylene sulfonate, such as is taught in United States
.:
3,849,392.
4 -
t ~
3 ~L~ 7~
The diazonium ~alt is pre~er~bly present in the
composition of the su~ject invention at a percent solids
level oE from about 3% to about
- 4a -
.,, -s ~
7~
2~% by weight. More preEerably it i5 present at about 5% to about 18% by
~eigllt and most preferably the diazonium salt Ls present at a percent
solids level of from about 10% to about 15% by weight.
The photopolymerizable mixture of the subject invention is
comprised of, in admixture, an acrylic monomer which has Erom 3 to 6
unsaturated groups and a urethane oligomer preferably a diacrylated
polyurethane, which is hereinafter described.
The monomer is an ethylenically unsaturated compound having
from three to six unsaturated groups and being capable of reacting
with the urethane oligomer upon exposure to imaging radiation. Preferably
the unsaturated groups are acrylate or methacrylate esters. The
preferred monomer is either a solid or liquid having a viscosity of
greater than about 7~0 cps (mPa.5~ at 25C, preferably greater than
about 2000 cps at 25DC. Most preferably, the monomer has a viscosity
of greater than about 4000 cps at 25C.
Examples of compounds which are suitable for use as the monomer
of this invention include trimethylolpropane triacrylate and the
ethoxylated or propoxylated homologs thereof, trimethylol propane
t7~
.
-- 6 --
tri-methacrylate and the ethoxylated or propoxylated homolog~
thereof, pentaerythritol triacrylate, pentaerythritol
tr-methacrylate, dipentaerythritol monohydroxY pentdacrylate,
dipentaerythritol monohydroxy pentamethacrylate,
dipentaerythritol hexaacrylate, dipentaerythritol
hexamethacrylate, pentaerythritol tetraacrylate and
pentaerythritol tetramethacrylate. Preferably, the monomer is
dipentaerythritol monohydroxy pentaacrylate, although a
combination of suitable monomers is also advantageous.
The monomer is present at a percent solids level which is
preferably in the range of from about 1~ to about 35% by weight.
More preferably, the monomer is present at a percent solids level
of from about 15% to about 30% by weight and most preferably from
about 15~ to about 25~ by weight.
The oligomeric component of the photopolymerizable mixture of the
composition of the subject invention is a photocurable oligomer
or polymer which is prepared by reacting one molar equivalent of
a substantially linear polymeric compound having an active
hydrogen group at each end thereof with a least two molar
equivalents of a diisocyanate compound so as to form a prepolymer
having an isocyanate group at each end thereof; and subsequently
reacting said prepolymer with at least two equivalents of an
ethylenically unsaturated compound having an active hydrogen
group to provide said prepolymer with unsaturation at each end
thereof. The preferred oligomer is one havin~ a polyester
backbone prepared from an aliphatic dicarboxylic acid and an
aliphatic diol. The preferred dicarboxylic acid is linear and
has from about 2 to ~ carbon atoms. The polyester is prepared in
such a way that the compound is symmetrical and
hydroxy-terminated. A procedure for doing so would be known to
the skilled artisan, for example as shown in Sandler and Karo,
'7~3
-- 7 --
Pol~mer Synthesis, Vol. 2, Academic Press 1977, pp 1~167. The
pol~ester polyol is in turn reacted with preEerably an aliphatic,
more preferably a cyclic aliphatic, diisocyana~.e having from
about 2 to 13 carbon atoms. The diisocyanate is reacted with the
polyester polyol so that one of the two isocyanate groups is
reacted with the terminal hydroxy group on the end of the
polyester backbone. The remaining isocyanate group is
subsequently reacted with a hydroxy-containing acrylate or
methacrylate. For example, 1,6-hexane diol is reacted with
adipic acid in a mole ratio of greater than 1:1
to form a polyester and then reacted with
dicyclohexyl-methane-4,4'-bis diisocyanate in a 2:1 mole ratio
(diisocyanate/polyester). The product is reacted with 2-hydroxy
ethyl acrylate in a 2:1 mole ratio (acrylate/diisocyanate-
polyester product) to for,m an oligomer useful in this invention.
The oligomer may be characterized as follows:
U-D-R-D-U
wherein:
R prior to the reaction to form the oligomer is an
essentially linear polymeric compound having two end groups
with active hydrogen functionality,
D prior to the reaction to form the oligomer is a
diisocyanate compound, and
U prior to the reaction to form the oligomer is a compound
having ethylenic unsaturation and a group with an active
hydrogen.
Examples of compounds which can be used as the ~ group are
polyesters obtained by reacting a dicarboxylic acid with a diol
in such a fashion that the rnole ratio of diol to dicarboxylic
acid is greater than 1:1 so as to have a symmetrical hydroxyl
terminated polymer; polyethers obtained by reacting a diol with
an alkylene oxide
747~
and epo~ies obtained by reacting a
symmetrical diglycidyl compound with a diol in such a fashion
that the mole ratio of diol to diglycidyl compound is greater
than 1:1 so as to have a symmetrical hydroxyl terminated polymer.
More specifically, R groups which are polyesters are prepared
from dicarboxylic acids such as oxalic, malonic, succinic,
glutaric, adipic, pimelic, suberic, azelaic and sebacic acids,
and from diols such as ethylene glycol, diethylene glycol,
neopentyl glycol, propylene glycol, dipropylene glycol,
1,3-butane diol, 1,9-butane diol, 1,6-hexanediol and
2-ethyl-1,6-hexane diol.
. .
R groups which are polye~hers are prepared from diols such as
ethylene glycol, diethylene glycol, propylene glycol, dipropylene
glycol, neopentyl glycol, 1,3-butane diol, 1,4-butane diol,
1,6-hexane diol and 2-ethyl-1,6-hexane diol, and alkylene oxides
such as ethylene oxide or propylene oxide, or tetrahydrofuran.
R groups which are epoxies are prepared from diols such as
ethylene glycol, diethylene glycol, propylene glycol, dipropylene
glycol, neopentyl glycol, 1,3-butane diol, l,~-butane diol,
1,6-hexane diol and 2-ethyl-1,6-hexane diol, and diglycidyls such
as diglycidyl iscphthalate, diglycidyl terephthalate, diglycidy
phthalate and bisphenol-A diglycidyl ether.
Examples oE compounds which can be used as the D group are
ethylene diisocyanate, propylene diisocyanate, tetramethylene
diisocyanate, dicyclohexyl-methane-4,~'-diisocyanate,
hexamethylene diisocyanate, l-methyl-2,3-diisocyanato-
cyclohexane, l-methyl-2,6-diisocyanatocyclohexane, lysine
7~ 3
_ 9 _
di~socyanate, 4,~'-ethylene-bis-(cyclohexyl isocyanate) and
isaphorolle diisocyanate.
Ex-mple of compounds which may be used as the U group include
hyaroxyethyl acrylate, hydroxypropyl methacrylate, hydroxypropyl
acrylate, hydroxyethyl methacrylate, 1,3-butane diol acrylate,
1,3-butane diol methacrylate, 2,4-butane diol acrylate,
1,4-butane diol methacrylate, neopentyl glycol acrylate,
neopentyl glycol methacrylate, pentaerythritol triacrylate,
pen~aerythritol trimethacrylate and the mono acrylate and
methacrylate of polyethylene glycol, polypropylene glycol and
polycopolymers of ethylene glycol and propylene glycol.
Useful oligomers in the practice of this invention include those
oligomers preferably having a molecular weight of from about 1500
to about 4000, more preferably from about 2000 to about 3500 and,
most preferably, the oligomer has a molecular weight of about
,
3a00. It is desirable that the oligomer be in solid form or
semi-solid form, i.e. having a viscosity of greater than about
~8~,0~0 cps at 25C. The composition preferably contains the
oligomer at a percent solids level of from about 10~ to about 35
by weight. More preferably the oligomer is present in the
radiation polymerizable composition of this invention in an
amount of from about 15~ to about 30~ by weight and it is most
preferably present at a percent solids level of from about 15% to
about 25~ by weight.
One of the 5igniEicant aspects of this invention is that the
unique combination of photoinitiator, diazonium salt and
photopolymerizable mixture eliminates the need for use of an
oxygen barrier layer or the necessity for processing in a
nitrogen environment, although the exact mechanism for
this in unclear. Among the advantages derived is elimination of
i7~'71~
-- 10 --
1, the inconvenience of applying a second coating, 2) the concern
o~ r thè refractive index and solubility of the oxygen barrier
la er, 3) the concern over blinding on a printing press due to
residue from the oxygen barrier layer, and ~) the concern over
potential image gain due to the oxygen barrier layer.
Other components which may be included in the radiation-
polymerizable composition of this invention include acid
stabilizers, exposure indicators, plasticizers, photoactivators
and colorants.
Suitable acid stabilizers useful within the context of this
invention include phosphoric, citric, benzoic, m-nitro benzoict
p(p-anilino phenylazo) benzene sulfonic acid, 4,4'-dinitro-2,2'-
stilbene disulfonic, ita~onic, tartaric and p-toluene sulfonic
acid, and mixtures thereof. Preferably, the acid stabilizer is
phosphoric acid. When used, the acid stabilizer is prefera~ly
present in the radiation-polymerizable composition in the amount
of from about 0.3% to about 2.0~, and most preferably from about
~.75% to about 1.5~, although the skilled artisan may use more or
less as desired.
Exposure indicators (or photoimagers) which may be useful in
conjunction with the present invention include
4-phenylazodiphenylamine, eosin, azobenzene, CalcozineFuchsine
dyes and Crystal Violet and Methylene Blue dyes Preferably, the
exposure indicator is 9-phenylazodiphenylamine. The exposure
indicator, when one is used, is preferably present in the
composition in an arnount ~ from about 0.001~ to about C.0035~ by
weight. A more preferred range is from about 0.002~ to about
~. C3~b and, most preferably, the~ exposure indicator is present in
an amount of from about 0.005~ to about ~.20~, althou~h the
skilled artisan may use more or less)as desired.
7~
Th~ photoactivator which may be included in the composition of
this invention should be an amine-containing photoactivator which
combines synergistically with the free-radical photoinitiator in
order to extend the e~fective half-life of the photoinitiator,
which is normally in the approximate ranye of about 10 9 to 10 15
seconds. Suitable photoactivators include 2-(n-butoxy) ethyl-4-
dimethylamino benzoate, 2-(dimethylamino) amino benzoate and
acrylated amines. Preferably the photoactivator is
ethyl-4-dimethylamino benzoate. The photoactivator is preferably
present in the composition of this invention in an amount of from
about 1.~% to about 4.0% by weight, although the skilled artisan
may use more or less as desired.
A plasticizer may also bé included in the composition of this
invention to prevent coating brittleness and to keep the
composition pliable if desired. Suitable plasticizers include
dibutylphthalate, triarylphosphate and, preferably,
dioctylphthalate. The plasticizer is preferably present in the
composition of this invention in an amount of from about 0.5% to
about 1.25~ by weight, although the s~illed artisan may use more
or less as desired.
Colorants useful herein include dyes such as Rhodamine,
Calcozinej Victoria Blue and methyl violet, and such pigments as
the anthraquinone and phthalocyanine types. Generally, the
colorant is present in the form of a pigment dispersion which may
comprise a mi~ture of one or more pigments and~or one or more
dyes dispersed in a suitable solvent or mixture of solvents.
When a colorant is used, it is preferably present in the
composition of this invention in~an amount of from about 1.5~ to
about 4.0~ by weight, more preferably from about 1.75~ to about
7~
.
- 12 -
3.~% and most preferably from about 2.C~ to about 2.75~, although
the s~illed drtisan may use more or less as desired.
.
In order to form a coating composition for the production of
lithographic printing plates, the composition of this invention
may be dispersed in admixture in a solvent or mixture of solvents
to facilitate application of the composition to the substrate.
Suitable solvents for this purpose include tetrahydrofuran,
propylene glycol monomethyl ether, butyrolactone, methyl
cellosolve, ethylene glycol ethers, alcohols such as ethyl
alcohol and n-propanol, and ketones such as methyl ethyl ketone,
or mixtures thereof. Preferably, the solvent comprises a mixture
of tetrahydrofuran, propylene glycol monomethyl ether and
butyrolactone. In general, the solvent system is evaporated from
the coating composition o,nce it is applied to an appropriate
substrate, however, some insignificant amount of solvent may
remain as residue.
Substrates useful for coating with the composition of this
invention to form a lithographic printing plate include sheets of
transparent films such as polyester, aluminum and its alloys and
other metals, silicon and similar materials which are well known
in the art. Preferably, the substrate comprises aluminum. ~hè
substrate may first be pretreated by standard graining and/or
etching and/or anodizing techniques as are well known in the art,
and also may or may not have been treated with a composition such
as polyvinyl phosphonic acid, sodium silicate or the li~e
suitable for use a~ a hydrophilizing agent.
In the production of photographic elements such as lithographic
printing plates, an aluminum substrate is first preferably
grained by art recognized methods such as by means of a wire
brush, a slurry of particulates or by chemical or electrochemical
- 13 -
mear.s, for example in an electrolyte solution comprising
hy ochloric acid. The grained plate is preferably th~n an~ized
for example in sulfuric or phosphoric acid in a manner well known
in the art. The grained and anodized surface is preferably then
rendered hydrophilic by treatment with polyvinyl phosphonic acid
by means which are also known to the skilled artisan. The thusly
prepared plate is then coated with the composition of the present
invention, preferably at a coating weight of from about ~.6g/m2
to about 2.5g/m2, more preferably from about 0.8g/m2 to about
2.0g/m2 and most preferably from about 1.2g/m2 to about 1.5g/m2,
although these coating weights are not critical to the ?ractice
of this invention, and dried.
Preferably the thusly prepared lithographic printing plate is
exposed to actinic radiation through a negative test flat so as
to yield a solid 6 on a 21 step Stouffer exposure wedge after
development. The exposed plate is then developed with a suitable
developer composition such as an organic solvent based developer,
preferably one comprising 2-propoxyethanol, a nonionic surfactant
and an ino~ganic salt such as is disclosed in U.S. Patents 4,3~8,340 and
4,381,340. In conventional use, the developed plate is finished
with a subtractive finisher such as a hydrophilic polymer.
Examples include cold water-soluble dextrin and/or polyvinyl
pyrrolidone, a nonionic surfactant, a humectant, an inorganic
salt and water, as taught by U.S. Patent 4,213,887.
E;or the purpose of improving the press performance of 2 plate
prepared as described above, it is kno~n that baking of the
exposed and developed plate can result in an increase in the
number of quality impressions over that otherwise obtainable. To
properly bake the plate, it is first treated with a solution
designed to prevent loss of hydrophilicity of the background
during baking. ~n example of an effective solution is disclosed
7'~7~
in U.S. Patent ~,355,096. The thusly prepared plate is thcn heat
trea.ted by baking at a temperature of from about 18aC up to the
annealing temperature of the s~lbstrate, most preferably about
24~C. The effective baking time is inversely proportional to
the temperature and averages in the range of from about 2 to
about 15 minutes. At 24~C the time is about 7 minutes.
The following examples are illustrative of the invention but it
is understood that the invention is not limited thereto. None of
the plates prepared in Examples 1-5 have an oxygen barrier
layer thereon nor were they processed in a nitrogen
environment.
' '
Example 1
An 8" x 25" section of lithographic grade 1100 aluminum alloy is
deqreased with an aqueous alkaline degreasing solution and
electrochemically grained using 9~0 coulombs of alternating
current in a ~edium of nitric acid and aluminum nitrate. The
grained plate is well rinsed and anodized in a sulfuric acid bath
wherein the aluminum is made anodic. Sufficient current and
voltage is used to produce an oxide layer of 2.8g/m2. The
anodized plate is well rinsed and hydrophilized by immersing the
plate into a solution of polyvinyl phosphonic acid. The plate is
well rinsed and dried. The thusly prepared plate is whirler
coated with a solution having the followin~ composition:
% w/w
A terpolymer of polyvinyl acet~te~ y~ y~ alcohol
and polyvinyl formal commercially available as
Formvar 12/85 4-59
Dipentaerythritol monohydroxy pe~ntaacrylate 2.01
Diacrylated urethane oligomer formed by reacting 1,6-
hexane diol with adipic acid in a mole ratio of greater
than 1:1 ' -o form a
polyester and then reacting with dicyclohexyl-methane-
i7'~
- 15 -
4, '-bis diisocyanate in a 2:1 mole ratio tdiisocyanate/
polyester). The product is then reacted with 2-hydroxy
et:-.yl acrylate in a 2:1 mole ratio (acrylate/
diiocyanate polyester product) 2.el
Polycondensation product of 3-methoxy~4-diazo-
diphenyl amine sulfate and 4,4~bis-methoxy methyl
dlphenyl ether isolated as the mesitylene sulfonate 1.22
2-(4-Styryl-phenyl)-4,6-di(trichloromethyl)triazine 3.20
Methyl cellosolve 87.02
The coated and dried plate is exposed to actinic radiation
through a negative exposure flat so as to yield a solid seven on
a 21 step Stouffer step wedge. The plate is developed using the
following composition
% w/w
2-propoxy ethanol 14.85
n-propanol 11.15
g 4 7H2O 8.00
NaH2PO4 (anhydrous) 1.50
Polyvinyl pyrrolidone 1.50
Trycol OP-407 ~ 75
polyethylene glycol 20~ 1.00
2 Balance
M~'l = 10, 000
**
nonyl phenol polyoxyethylene ether ~ 40 moles ethylene oxide)
as a 70% solution in H2O.
and finished with the following composition
~ w/w
Dextrin 5.52
sodiu,n octyl sulfate 1.61
****
Triton X-100 1.00
Givgaurd DXN a.05
3 4 2.37
H2O Balance
.
7~
- 16 -
~ ~*
hydrolyzed tapioca dextrin
****
isooctyl phenol polyoxyethylene ether ( ~.5 moles ethylene
oxide)
**~ **
1,4-dimethyl-6-acetoxy-dioxane
and run on a Solna sheet fed press ~sing abrasive ink,
over-packing, and a ~ahlgren dampening system until image
breakdown is achieved. Under these conditions the plate provides
595,000 acceptable impressions.
Example 2
A lithographic printing plate is prepared and processed as
described in Example 1 except that the diacrylated urethane
oligomer is omitted. Under these conditions the plate provides
only 315,000 acceptable impressions.
Example 3
A lithographic printing plate is prepared and processed as
described in Example 1 except that the dipentaerythritol
monohydroxy pentaacrylate is omitted. Under these conditions the
plate provides only 340,000 acceptable impressions.
Example 4
A lithographic printing plate is prepared and processed as
described in Example 1 except that the diazo composition is
omitted. Under these conditions the plate provides only 220,t5t50
acceptable impressions.
Example 5
A lithographic printing plate is preparea ana nrocessed as
described in Example 1 except that the 2-t4-styryl-phenyl)-4,6-di-
(trichloromethyl)triazine is omi~ted. Under these conditions theplate provides only 335,(50a acceptable impressions.
~374'7~
- 17 -
It can be readily observed that plates prepared according to this
invention (Example l) show substantially increased press runs.
.
