CA1268985A - Photosensitive elastomeric polymer composition for flexographic printing plate - Google Patents

Abstract

ABSTRACT
The present invention relates to a flexible photo-polymer printing plate composition comprising a homo-genous blend of:
(A) a high molecular weight butadiene-acrylonitrile copolymer having a number average molecular weight of about 30,000 to 125,000, an acrylonitrile content of 10 to 50% by weight, a carboxyl content of 2 to 15% by weight, and a Mooney viscosity (ML-4 at 100°C) of 45 or higher; and (B) a medium molecular weight butadiene-acryloni-trile copolymer having a number average molecular weight of about 15,000 to 40,000, an acrylonitrile content of 10 to 40% by weight, a carboxyl content of 1-15%, and a Mooney viscosity (ML-4 at 100°C) of from 15 to less than 45.

Description

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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2 6~9 ~ 5 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.

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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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91~35 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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s 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 .: , ' .

~ ~ 6~ ~ ~5 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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~l2~ 5 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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~26~ 35 g 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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~;26~5 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.

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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, ~L2~ 9~35 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.

6~ ~8 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 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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~ 2 ~ 5 - 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. -' : 25 ~ : :

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Claims (9)

What is claimed is:

1. A photosensitive elastomeric composition which comprises:
(A) a butadiene-acrylonitrile copolymer having a number average molecular weight of about 30,000 to 125,000, an acrylonitrile content of 10 to 50% by weight, a carboxyl content of 2 to 15%
by weight, and a Mooney viscosity (ML-4 at 100°C) of 45 or higher;
(B) a butadiene-acrylonitrile copolymer having a number average molecular weight of about 15,000 to 40,000, an acrylonitrile content of 10 to 40% by weight, a carboxyl content of 1-15% by weight and a Mooney viscosity (ML-4 at 100°C) of from 15 to less than 45;
(C) from about 2 to 40 parts by weight based on 100 parts of the elastomer content, of a photopoly-merizable, ethylenically unsaturated crosslink-ing agent compatible with copolymers (A) and (B); and (D) from about 0.01 to about 10 parts by weight based on 100 parts of the elastomer content of an addition polymerization initiator activat-able by actinic radiation, wherein the weight ratio of copolymer (A) to copolymer (B) from 80:20 to 30:70 and the Mooney viscosity (ML-4 at 100°C) of the composition is from about 20 to 55.

2. The elastomeric compositions of claim 1 wherein:
copolymer (A) has a number average molecular weight of about 45,000 to 110,000 and a Mooney viscosity (ML-4 at 100°C) from about 45 to 75, and copolymer (B) has a number average molecular weight of about 20,000 to 30,000 and a Mooney viscosity (ML-4 at 100°C) from about 15 to 40.

3. The elastomeric composition of claim 2 wherein:
copolymer (A) has an acrylonitrile content of from 20 to 40% by weight; and copolymer (B) has an acrylonitrile content of from 20 to 35% by weight.

4. The elastomeric composition of claim 2 wherein:
copolymer (A) has a carboxyl content of from 3 to 10% by weight; and copolymer (B) has a carboxyl content of from 4 to 10% by weight.

5. The elastomeric composition of claim 1 wherein the Mooney viscosity (ML-4 at 100°C) of said composition is from about 25 to 50.

6. A photosensitive elastomeric composition which comprises:
A) A butadiene-acrylonitrile copolymer having a number average molecular weight of from about 45,000 to 110,000, an acrylonitrile content of from about 20 to 40%
by weight, a carboxyl content of from about 4 to 8% by weight, and a Mooney viscosity (ML-4 at 100°C) of 45 to 75;
B) a butadiene-acrylonitrile copolymer (B) having a number average molecular weight of from about 20,000 to 30,000, an acrylonitrile content of from about 20 to 35 by weight, a carboxyl content of from about 4 to 10% by weight and a Mooney viscosity (ML-4 at 100°C) of from 15 40;
C) from about 5 to 25 parts by weight based on 100 parts of the elastomer content of a photopolymerizable, ethylenically unsaturated crosslinking agent compatible with copolymers (A) and (B); and D) from about 0.2 to 4 parts by weight based on 100 parts of the elastomer content of an addition poly-merization initiator activatable by actinic radiation;
wherein the weight ratio of copolymer (A) to copolymer (B) is from 65:35 to 45:55 and the Mooney viscosity (ML-4 at 100°C) of the composition is from about 30 to 50.

7. A photosensitive flexographic plate which comprises a substrate having thereon a layer of the photosensitive composition of claim 1.

8. A photosensitive flexographic plate which comprises a substrate having thereof a layer of the photosensitive composition of claim 5.

9. A photosensitive flexographic plate which comprises a substrate haying thereon a layer of the photosensitive composition of claim 6.