Note: Descriptions are shown in the official language in which they were submitted.
D-6037 12~8985
PHOTOSENSITIVE ELASTOMERIC POLYMER
COMPOSITlON FOR FLEXOGRAPHIC PRINTING PLATE
DESCRIPTIOM OF THE INVEMTION
Field of the Inven~ion
"
This invention rela~es ~o high molecular weight
butadiene-acrylonitrile copolymers ~NBR~ con~aining
carboxyl groups blended with medium molecular weight
butadiene-acrylonitrile copolymers also containing
carboxyl groups to produce photosensitive elastomeric
articles which are designed to be used primarily as
flexographic printing plates.
Back~_ound of the Invention
British Patent No. 1,358,062 discloses photosensi-
, tive compositions consisting of a nitrile:rubber with a
photopolymeriæable tri- or tetra-unsaturated ester
derived rom acrylic or methacrylic acid and in addition,
~0
a polymerization initiator activatable by act~nic radia-
tion.
: UOS. Patent No. 3,825,430 discloses a photosensltlve
compositio~ containing a continuous phase of a light
sensitive~organic material and a discontinuous phase of a
;~ ~ finely divided, elastomeric o~ganic compound uniformly
~ distributed through the continuous~phase, i.e. a carboxyl
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containing, high molecular weight butadiene-acrylonitrile
copolymer.
U.S. Patent No. 4,177,074 discloses a photosensitive
composition containing a high molecular weight butadiene-
acrylo~itrile copolymer wh~ch contains carboxyl groups alow molecular weight butadiene polymer which may or may
not contain carboxyl groups and an ethy:Lenical~y unsatur
ated monomer, and a free-radical generating system. This
composition is said to be useful for flexographic print-
ing plates.
U.S. Patent No. 4 9 272,608 discloses photosensitive
elastomeric compositions comprising a high molecular
weight carboxylated butadiene-acrylonitrile copolymer in
which at least 25% of the carboxyl groups are neutralized
lS with a metal cation of Groups IIA or IIB,of the periodic
table and an ethylenically unsaturated monomer and a free
radical generating system.
The art is continually seeking photosenæitive
elastomeric compositions for use in flexographic printing
plates which have more rapid cure rates, i.e., require
less exposure time without the attendant disadvantages
normally associated with said improved sensitivity, e.g.,
shoulder build-up.
a~e~D3~5~L~laL~ E~laY
The present invention is a ~lexible photopolymer
~ printing plate composi~ion comprising a blend of:
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~A) a high molecular weight butadiene-acrylonitrile
copolymer having a number average molecular
weight (~n) of about 30,000 to 125,000, an
acrylonitrile content o 10 to 5~Z by weight, a
c~rboxyl content of 2 to 15~ by weight and a
Mooney viscosity (ML-4 at 100C) of 45 or
higher;
(B) a medium molecular weight butadiene-acryloni
trile copolymer having a number av~rage mole-
13 cular weight of about 15,000 to 40,000, an
acrylonitrile content of lO to 40~ by weight, a
carboxyl content of 1-15% by weight and a
Mooney viscosity (ML-4 at 100C) of from 15 to
~ less than 45;
(C~ a photopolymerizable, ethylenic,ally unsaturated
cross-linking agent compatible with polymers `
~: ~A~ and (B); and
(D) an addition polymerization initiator activat-
able by actinic radiation.
This invention provides photosensitive materials for
~ : pr~oducing flexographic printing plates with the advan-
:: tages o~ inereased productio~ r~tes,:reduced costs, and
labor savings over conventLonal platemaking procedures. :~
: The~blends~of the high medium molecular weight~
25: carboxylated copolymers al8o have:improved ink compati- ~
billty over prior art blends, due to;the absence o~ ;
iquid copolymer.s. Not onl~ do~these composltions also
: : show improved sensitometric properties tfaster cure ra~e)
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. requiring less exposure to actinic radiation to reproduce
finely detailed images but serendipitously exhibit very
little shoulder build-up.
DETAILED DESCRIPTION OF THE INVENTION
The compositions of this invention comprise a high
molecular weight butadiene-acrylonitrile copolymer (A)
having a number average molecular weight of from 30,000
to 125,0009 preferably from 45,000 to 110,000. The
acrylonitrile content of this copolymer is from about 10
to 50% preferably from about 20 to 40% by weight, a
carboxyl content of from about 2 to 15~ preferably about
3 to 10%, most preferably from about 4 to 8% by weight.
The Mooney viscosity (ML-4 at lQ0C) is 45 or higher,
preferably 45 to 75.
The blends of this invention further contain a
medium molecular weight carboxylated butadiene-acryloni-
trile copolym r (B) having a number average molecular
weight of from 15,G00 to 40,000 preferably of fro= 20,000
to 30,000; an acrylonitrile content of from about 10 to
40%, preferably from about 20 to 35% by weight; a car-
boxyl content of from about 1 to 15% preferabl~ abvut 4
to l0% by welght (B); and a Mooney viscosity (MI-4 at
100C) of lS to: less than 45; preferably lS to 40.
The ~A) / ~B) weight ratio may range from 80/20 to
30/70,~ u~ually from 70/30 to 40/60, most preferably :
65/35-45/55, provided that~ the Mooney vis~cosity of such
blend falls within a range of rom about 20-55,
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preferably from about 25-50 and mast preferably from
about 30-50 (ML-4 at 100C).
Furthermore, the photosensitive composition~ of this
inven~ion comprise ethylenically unsaturated crosslinking
agents ~C) including unsaturated esters of polyols,
particularly such esters with alpha-methylene carboxylic
acids, e.g. ethylene glycol di~crylate, diethylene glycol
diacrylate, glycerol diacryl~te 9 glycerol triacrylate,
mannitol polyacrylate, sorbitol polyacrylate, ethylene
glycol dimethacrylate, 1,3-propanediol dimethacrylate,
1,3,4-butanetriol trimethacrylate, 1,4-butanediol
diacrylate or dimethacrylate, 1,4-cyclohexanediol dla-
crylate, 1,4-benzenediol dimethacrylate, isosorbidene
diacrylate, pentaerythritol di-, tri- and tetrametha-
crylate~ dipentaerythritol polyaerylate, dipentaery-
thritol polyacrylate, pentaerythritol di, tri- and
tetra-acrylates, 1,3-propanediol diacrylate, 1l6-
hexanediol diacrylate or dimethacrylate 9 1,5-pentanediol
dimethacrylate, trimethylolpropane triacrylate, the
bisacrylat~s and methacrylate~ of polyethylene glycols of
molecular weight 200 to 4000, unsaturated amides,
particularly those of the alpha-methylene carboxylic
- acid~ and especially those o~ alpha, omega-diamines and
: oxygen-int~rrupted omega-diamine~ such as methylene
bisacryl~mide, methylene bismethacrylamide, ethylene
bismethacrylamide, 1,6-mexamethylene bisacrylamide,
diethylene triamine, trismethacrylamide,~
: 1,2~di(gamma-methacrylamidopropoxy)ethane,
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beta methacrylaminoethyl methyacrylate, N-(bet~-hydroxy-
ethyl)-2-(methacrylamido~ ethyl acrylate and N,N-
bi~(beta-methacryloxethyl~acrylamide; vinyl esters such
as divinyl succinate, divinyl adipate, di~inyl phthaLate,
divinyl terepthalate, divinyl benzene-1,3-disulfonate,
divinyl butane-1,4-disulfonate and unsaturated aldehydes,
such as sorbaldehyde (hexadien~l). Preferred addition
polymerizable crosslinking agents are the esters and
amides of alpha-methylene carboxylic acids and sub-
stituted carboxylic acids with polyols and polyamineswherein the molecular chain between the hydroxyls and
amino groups is solely carbon or oxygen-interrupted
carbon,
The amount of unsaturated crosslinking agent added
should be about 2 ~o 40 par~s by weight preferably about
5 to 25 parts by weigh~ based on 100 parts of the elasto-
mer content. The exact amount will vary for best results
depending on the partîcular composition used.
The most preferred unsaturated crosslinking agents
are 19 4-hexandiol diacrylate ? 1,4-hexanediol dime~hyl-
acrylate, trimethylolpropane triacrylate and penta-
erythritol triacrylate. The preferred compositions are
- those in which the high molecular weight copoly~ers are
compatible ~ith the unsaturated compounds to form clear,
25 ~on-light scattering compositions in layers less than
.250 inch tca. 6.4 mm) thick.
Another component (D) of the photosensitive composi-
tion of this inv~ntion is an addition polymPrization
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and/or crosslinking initiator acti~atable by actinic
radiation and thermally inactive at or below 185C. Such
initiators include aroma~ic ketones such as benzophenone
and substituted benzophenones, the substituted or unsub-
stituted polynuclear quinone,s which are compounds having
two intracyclic carbonyl groups attached to intracyclic
carbon atoms in a conjugated gix-membered carbocyclic
ring, there being at least one aromatic carbocyclic ring
fused to the ring containing the carbonyl groupis. Such
suitable initiators include 9,10-anthraquinone, l-chloro-
anthraquinone, 2-chloroanthraquinone, 2-methylanthraqui-
none, 2-tert-butylanthraquinone, octamethylanthraquinone,
1,4-naphthoquinone, 9,10-phenanthrenequinone, 1,2-benzan-
thraquinone, 2,3-benzanthraquinone, 2-methyl-1,4-naptho-
quinone, 2,3-dichloronaphthoquinone 9 1,4-dimethylanthra-
quinone, 2,3-dimethylanthraquinone, 2-phenylanthraqui-
none, 2,3-diphenylanthraquinone, sodium salt of anthra-
quinone, alpha-sulfonic acid, 3-chloro-2-methylanthraqui-
none, retenequinone, 7,8,9,10-tetrahydronaphthac~nequi-
none, and 1,2,3,4-te~rahydrobenz-alpha-anthracene-7,12-
dione. Other photoinitiators which are useful for the
purpose of this invention are described in U.S. Patent
- No. 2~760,863 and include vicinal ketaldonyl compounds
such A diacetyl benzil, etc., alpha-ketaldonyl alcohols,
such ais benzoin, pivaloin, etc. acyloi~ ethers, e.g.
benzoin methyl and ethyl ethers, etc. 9 alpha-hydrocarbon
substituted aromatic acyloins, including alpha-methylben-
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zoin, alpha-allylbenzoin and alpha-phenylbenzoin and 2, 2
dialkoxy-2-phenylacetophenone.
The amount of pho~oinitiator added should be from
about 0.01 to 10 parts by weight based on 100 parts of
the total elastomer, the preferred range being from about
0.2 to 4.
The photosensitive compo~ition can be prepared by
mixing the components, i.e. (A) the high molecular weight
butadiene-acrylonitrile carboxyl containing copolymer,
(B) the medium molecular weight butadiene-acrylonitrile
carboxyl containing copolymer, ~C) the compatible ethy-
lenically unsaturated crosslinking agent, and (D) the
free-radical generating system to a preferably homo-
geneous blend either by solvent blending in suitable
solvents such as chlorinated hydrocarbons, e.g.,
trichloroethylene, trichloroethane and chlorotoluene;
ketones, e.g., methyl ethyl ketone, diethyl ketone, and
methyl isobutyl ketone; aromatic hydrocarbons, e.g~,
toluene and tetrahydrofuran or by open milling or by
internall~y mixing for a time long enough to blend the
ingredients thoroughly, preferably ho~ogeneously.
~ The non-light scattering composition~ are then
- formed into sheets in any de~red manner of thicknesses
from o.ooas to .250 inch ~0.0013-0.64 cm). Such method~
are, for example~ Bolvent-caBting~ hot pressing,
calendering or extrusion at room tempersture or at
elevated temperature. The sheet~may be supported during
: forming by a suitable supporting substrate or the sheet
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may be applied to a supporting substrate after forming
The supporting substra~e can be any natural or syn~he~ic
support which exis~s in a flexible or rigid film or sheet
form. Commonly used supports for use by this composition
in flexographic printing include metal sheets such as
steel or tin coated steel and plastic films such as
polyester or polyamide. The preferred supporting sub~
strates are dimensionally stable and resistant to the
washout solutions.
As these are photo-activated systems, it is neces-
sary to protect the above mentioned sheets from surface
contamination by dirt and dust during storage before
being exposed and washed into a finished relief imaged
element. This is accomplished by lamination or applica-
tion of a flexible protective eover sheet to the side of
the element opposite that of the substrate. As these
elements can be tacky, it is also desirable to apply a
r lease film to the surface of the element before appli-
cation of the protective cover sheet. This release film
may be about .0002 inch (0.0051 cm) thick flexible
polymer~c film. Typical polymeric materials are polya-
mide copolymers or vinyl acetate copolymers. Similarly
- familiar relPase agents may al~o be used for this pur-
pose, e.g., silicone, fluorooarbons and waxes as iong as
they are transparent and do no~ interfere with exposure
to actinic radiation. These release age~ts or films
promote good intimate contact between the surace of the
element opposite to~he substrate and ~he image bearing
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negative or transparency applied to the element. Thisintimate contact is essential for accurate reproduction
of the image on the e7ement.
Relief prin~ing plates are produced from the compo-
sition of t~is invention from the above described sheets.
The construction usually comprises a polyester substrate
from .001 to .020 inch (0.025~ 51 mm) and preferably
0.004-0.006 inch (0.010-O.Q15 cm) 9 adhered to a sheet of
the composition of this in~ention from .005 to .250 inch
(0.25-0.64 cm) thick coated with a ca. 0.0002 inch (0.005
mm) thick polyamide copolymer release film on the side
opposite of the substrate. This polyamide film may be
covered and adhered to a second polyester ~ilm (ca, 0.005
inch; 0.013 cm) which is a protective cover sheet. This
construction is then exposed to actinie radiation through
the substrate, if necessary depending on thickness, to
crosslink via addition polymeriæation the non-relief
backing area o the plate. This exposure will control
the depth o~ relief produced in the plate (thin plates do
not require this step). The plate is then turned over,
and the protective cover sheet removed. Phot~graphic
négatives or transparencies or other image bearing media,
- wi~h ~ransparent and opa~ue areas used to produce the
image, are laid in iotimate eontact with the polyamide
release coated æurface of the sheet of the composition of
the ~n~ention. :Good contact be~ween the nega~ive and
release surface can be provided by a vacuum system that
presses the negatiYe against the sheet surface. This
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sandwich is then exposed to actinic radiation, e,g.,
ultraviolet radiation at a wavelength between 2500 A and
O O
5000 A, with preferred being 3600 A. On exposure, the
transparent areas of the negative permit addition poly-
merization ~r crosslinking to take place. E~posure is ofsufficient duration to crosslink the image to the sub-
strate or the previously cros~linked sheet backing.
Crosslinking will occur only in the exposed areas of the
sheet (transparent areas of the negative) with no signif-
icant crosslin~ing occurring in the non-image, unexposed
areas of the sheet (opaque areas of the negative).
Actinic radiation may be provided from any source
such as, carbon arcs and high pressure mercury lamps.
Preferred sources are very high output so-called black-
light fluorescent types due to their hi~h ratio ofultraviolet ~o infrared output. Exposure times wil~
vary, depending on the output of the lamps, distance from
the lamp, the relief depth desired and the thickness of
the plate. ~xposure times may vary from a few seconds to
several mi~utes.
On c~mpletion of the exposure; the negative is
removed9 and the exposed sheet i~ ready for development
- of the relief image by treating with a semi-aqueous basic
solution or solvent wash. These wash systems remove the
unexposed, uncrosglinked composition from the e~posed
sheet~and do not adversely affect the exposed, cross-
linked composition that forms the raised relief image.
The semi-aqueous basic wash method is preferred.
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Solvent wash may be accomplished as in the prior art
with organic solvents such as 2-butanone, benzene,
toluene, xylene, trichlorethane, trichlorethylene,
tetrachlorethylene, methylchloroform, and solvent mix-
tures such as tetrachlorethylene with n-butanol.
Suitable semi-aqueous washout mixtures include
sodium hydroxide/isopropyl alcohol/water, sodium car-
bonate/2-butoxyethanol/water, sodium borate/2-butoxy-
ethanol/water, sodium silicate/-2-butoxyethanollgly-
lo cerol/water, sodium carbonate/2-~2-butoxyethoxy~
ethanol/water, sodium hydroxide/2-(2-butoxyethoxy)
ethanol/water and sodium hydroxi.de/1,2-ethanediol/water.
Normal use of ~ese mixtures is at elevated temperature.
The preferred solution is sodium hydroxide/2-(2-butoxy-
ethoxy) ethanol/water, i.e., (0.5% sodi~m hydroxide inwater)/2-(2-butoxyethoxy)ethanol (5jl ratio by volume) at
50-70C.
As can be seen from the above, the flexible
photopolymer printing plates of the present invention can
be readily processed in basic aqueous, or semi-aqueous
medium solutions as we~l as in halogenated hydrocarbon
solutlons or blends of halogenated hydrocarbons and
alcohols. The plates exhLbit excellent solvent resis-
tance to a wide variety of printing inks, including
water, alcohol, and hydrocarbon based inks. The
compositions also exhibit excellent abrasion resistance
and better clarity than certain other photosensitive
printing plates.
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The above composition has preferred utility in
raised image printing processes such as le~terpress,
le~terset and flexographic printing, however, the com-
positions also have utility in other applications where
image formation is utilized such as photoresists, plano-
graphic plates, "silk screen" printing and stencils.
The following examples are illustrati~e of the
presen~ invention and are not intended as a limitation
upon the scope ther~of.
EXAMPLE 1
A polymer ~olution was prepared by mixing the
following ingredients (all in parts by weight).
CNBR-ltl) 60
15 CNBR-2(2) 40
TMRTA(3) 10
Photoinltiator(4)
Toluene 190
Methyl ethyl ketone47
by first blending the solvents and the antioxidant, and
while heating the mixture to about 50C, adding the
remaining materials. Agitation was maintained untiI
- solids were analyzed to be 32% (by weight).
~5 REMARKS
(1~ Carboxylated NBR; Mooney VisC08ity (ML-4 at 100C~ -
55, acrylonitrile (ACN) content: 3~X, carboxyl
content: 9Z, ~n ca. 75~000.
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(2) Carboxylated NBR; Mooney viscosity (ML-4 at 100C) =
35, ACN = 27%, carboxyl: 3~4, ~n ca~ 30,0000
(3) Trime~hylolpropane trimethacrylate (TMPTA)
(4) 2,2-dimethoxy-2-phenylacetophenone SDMPA)
The homogenous solution was then ca9t on a support
sheet of polyester film (0.005 inch, 0..127 mm) via a
doctor blade, in multiple pas~es to build the photosensi-
tive composition to a dry film thickness of .075 inch
~0.19~ cm3. The doctor blade was set .006 inch ~0.015
cm) over the polyester substrate or support film at the
start of the casting and adjusted to .006 inch over the
cast material as the thickness was built to .075 inch.
Each additional pass was air dried for S minutes before
the next pass was applied. When the desired .075 inch
(0.19 cm) thickness was reached, the plate was then oven
driPd at 55-65 for 6 hours to remove any remaining
solvent.
~ The final composition had a Mooney viscosity (ML-4
: : at 100C) o 47.
The plate was then sprayed with a dry release, high
molecular weight wax coating on ~he side opposite the
polyester support sheet. The p~ate was then exposed on
the back, through the polyester support, for 30 seconds
with ultraviolet lamps (Sylvania blacklight very high
~: : 25 output lamps, TR-48T12 - BIL-VH0-180). The:lamps were
: spaced apart on 2-inch (5.08 cm) centers. The plate wa~
placed 2 inches (5.08 cm~ from th~ lamps. After back
exposure, the plate was placed phato~en~itive c~mposition
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up (polyester backing down) under the lamps. A By~hrome
(trademark) negative having three identical percentage-
calibrated screen tints was placed in int;mate contact
with the release coated surface using the vacuum frame of
the exposure unit. The plate was exposed to the ultra-
violet light ~hrough three negatives for 8, 10 and 15
minutes, respectively. After-exposure, the negatives
were rPmoved, and the plate was washed or 15 minutes in
a rotary brush processor using a mixture of 75Z tetra-
chloroethylene and 25% n-butanol. After washing, the
pla~e was dried in a forced air oven at 65C for l hour.
The resultant printing plate had exceptional
clarity, excellent image sharpness at an average relief
image of 0.030 inch (0.076 cm) reproducing the 5 to 90%
tones in 211 three nega~ives from 65 lin~ to 150 line
half tone. Plate hardness was 52 tShore A~, and the
plate did not crack when flexed 180 upon itself; sur-
prisingly, the rate of should build-up was much slower
than ordinarily expected, even over ~he rather wide range
of exposure times.
EX~MPLE 2
-The composition and procedure of Example l was
repeated except that the exposed plate was washed in a
: 25 semi-aqueous mix~ure 0.5% NaOH in a l/4 ~by volume)
: 2-(2-butoxyethoxy)ethanol/water mixture. The resultant
printing plate exhibi~ed the ~ame excellent characteris-
tics as the solvent washed plate of E~ample l.
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EX~MPLE 3
Additional plate compositions are prepared using the
procedure of Example l, i.e., blends of high and medium
molecular weight (high and medium ML-4) carboxylated
nitrile rubbers, nam~ly:
Run No. - 3-1 3-2 3-3
CNBR-3 70 - -
CNBR-4 - 60
CNBR- 5 - - 3 0
CNBR- 6 - 4 0
CNBR-2 30 - 70
TMPTA 10 10 10
DMPA 2 2 2
ML-4 (100C)(1) 41 44 47
REMARKS:
CNBR- ~DE(2) ACN COOH Ml-4(3) Mn
-3 66.6 27. 3.~ 45 30,000
-4 67 32 1.0 50 30,000
-5 63.5 32 4.5 75
-6 6~ 29 5 34
(1) After evaporation of solvent
: 25 ~2) l~3-butadi:ene
~ ~3) Mooney viscosi y (ML-4 at 100C)
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After exposure and a semi-aqueou~ washing s~ep,
about the same photosensitive plate characteristics arè
observed as with the blends of Example 1.
. _XAMPLE 4
After following essentially the procedure outlined
in Example 1, the shoulder ang-les of several photopolymer ~5
compositions were evaluated, namely, Runs No. 4-1, 4-2
and 4-3, wherein No. 4-l is identical to the composit:ion
of Example 1. No. 4-2 represents a typical composition
f Canadian patent application S.N. 434,153 filed August
9, 1983 t and Nos. 4-3 and 4-4 are a composition believed
to be representative o U.S. Patent No. 4,177,074
tProskow).
Resolution of images is ~ery critical and determine ..
the quality of a photosensitive plate. The resolution~of
a plate ultimately depends on the materials from which
the plates are made. One way to measure such resolution .:
is by examining the angle the s~oulder makes with ~he
printing surface of the relie~ image. For best results,
an angle of about 60-70 or greater is desired, a 55-60
: range is acceptable, wherea~ an angle of less than 55 ~s
: - :less than acceptable by the standards of this invention
: and the industry. .:
: 25 Commercially a~ceptabte 10 second Durometer hardness
alues ~Shore A values determ~ned after 10 second plunger
` ~ contact with material) for flexographic pho.topolymer
; printing plates lie in the range of from about 18 to 55, :'
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- preferably from about 20-45. Shore A 10 second values
below 18 are considered ~o be too soft for necessary
image r~tention.
All three plates were treated identically except for
face and back exposure times. The treatment and results
are summarized below:
,--
Run No. 4-1 4-2 4-3 4-4
Back exposure, minu~es 0.33 0.33 4.0 0.33
10 Face exposure, minutes 4.0 4.0 6.0 4.0
Shoulder angle, degrees 60 45 50 55 (gu~my)
Durometer Hardness
(Shore A after 10 seconds) 22 25 29 10
Clearly9 the compositions of this invention can
provide improved image resolution over prior art com-
positio~s.
The high-low molecular weight photosensitive blends
such as exempiifled by the 4-3 and 4-4 composition no~
only are slow cure blends, i.e., require long exposure
times but invariably experience shoulder build-up
problems. One would expect that in a blend having a
faster cure rate at a given exposure, the shoulder
: buil~-up problems would be exacerbated. Surprisingly~ a8
can be seen from the:abo~e, fast cure blend~ of this
invention have significantly less shoulder build-up than
; - the high-lo~ blend~ of the prior art.
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Although the invention has been illustrated by thepreceding examples ~ it is not to be construed as being
limited to the materials employed therein, but rather,
the invention encompasses the generic area as herein-
before disclosed. Various modifications and embodimentscan be made without departing from the spirit and scope
thereof. -'
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