CA1079565A - Dot-etchable photopolymerizable elements and image reproduction process - Google Patents

Abstract

ABSTRACT
Photopolymerizable elements are described comprising a sheet support, a photopolymerizable layer having a thick-ness of, at most, 0.0006 inch (0.015 mm) and an optical density of at least 3.0 in the actinic region. Resist images made from these elements may be etched by a process of chemically undercutting the image areas and then spraying or rubbing them.
The elements are useful as contact speed lithographic films and for other graphic arts applications.

Description

79S~5 This invention relates to new photopolym~rizable - elements which can be used for preparing novel, dot-etchable - image-containing masks that are useful for copy preparation and related purposes~ Masks are used in imagewise exposing other photosensitive. elements with actinic radiation.
In the photomechanical trades, a mask containing an image that is opaque to actinic radiation is used in preparing a printing plate of some kind; whether'letter-press or lithographic,. the method of production is much .' the same. A metal (,or in some cases/ plastic) plate is . coated with a photosensitive resist-forming material, and is exposed to actinic light through the mask. After ex-~ posuxe, the plate is ~reated .~ith a solvent that removes : the unexposed materi~l, leaving a resist image which pro-15 tec'ts the~resist-co.vered' areas of the metal plate from ' th.e'acid used in etch'i'ng the plate (i.n the. case of letter-; press~ or from th~ Various hydrophilic coatings used in ., l$th~graphy~
For this purpose, the image in the mask must be of the highest possible contrast~ e.g~, completely opaque ' black in the image areas, and completely transparent .l (free from ~og) in the unexposed areas. For such masks, a film known as "litho~' ~ilm (or various proprietary names containing~it~ is used. Such ~ilms are usually s~l~er halide emulsions which produce extremely sharp images o~ high denslty and contrast; they are used in the ' graphlc arts t'or maklng line and screen images~ and in :~, æome phototypesetting systems~ The characteristic curve .' . ~ .
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o~ a lith ~ilm has a steeply rlslng stral~ht-llne portion~
and a very short toe .
When a litho film is exposed through a hal~tone screen and developed, it contains an image comprised of dots. The dots correspond to the areas o~` the ~llm under the transparent areas Or the halftone screen and are comprised o~ exposed and developed material. In silver halide litho films~ these dots may be r~duced in size by "dot-etching". Dot-etching is reducing the siæe of`, or - lO "etching" the halftone dots, thereby changing the tone ; values o~` the image. In silver halide ~ilms, this is - done chemically by treating the Iilms with a silver "solvent". This method is used in lithography when tone values or color strength must be changed during the photographic steps rather than on the printing plate.
Its importance is further illustrated in the processes for correcting color work.
When process color work is a part of the da~ly business of a lithographic shop, a certain amount o~
correction is o~ten necessar~ to accurately reproduce the colors in the original. A typical procedure requires the production of a set of three or ~our color separation I negatives, and from these negatives, a set Or halftone j positives ls made. Both o~` these operations are performed by a color cameraman~ and at a later stage~ the halftone positives are given to the dot etch artist for correction.
The dot etcher chemically treats the hal~tone positive dots to reduce their size in selected areas9 At some point~ the dot etcher is sat~s~ied that the work he has done wlll produce the necessary color correction :. '' . .. .

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~n the reproduction. He then turns the corrected posit~ve over to someone to make a contact halftone negative from ~t for platemaking~ This step is often performed by the con-tact printer. At times, i~ a deep-etch plate is required, -s the contact printer will first make a contact hal~tonenegati~e, and then, from this negative, make a contact hal~`tone posi~ive. The reaso~ for this seeming duplica-tion of effort is to get an image with a dot su~ficiently optically dense for the platemak~ng exposure, This t~pe of dot is known as a "hard" dot. The dot etching process reduces the optical density of silver halide dots and there~ore necessitates the extra step to ob~ain har~
dots~
An improved mask suitable for use as a litho ~ilm is needed which wil~ produce hard dots and whîch does not involve the expense and red light handling requirements o~ silver halide ~ilms. Any such mask~
; however, needs to have the characteristic o~ dot-etch-abillty so as to permit tQne correction.
; 20 Dot-etching of photopolymerizable elements is believed to be new~ Due to the differences in materials and mechanisms, the production of dot-etchable photopoly-merizable elements, which are particularly suited for use as litho films, has apparently not even been attempted.
; 25 In accordance with this invention improved photo-polymerizable elements are provided which comprise a support bearing a photopoly~erizable layer which is the outermost layer of the elemen~ or is contiguous to a removable cover sheet or an overcoat layer which is at least partially ~-soluble in or permeable to a solvent ~or the photopolymerizable ,;
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layer, characterized in that the photopolymerizable layer has an optical density in the actinic region of at least 3.0, and is no more than 0.0006 inch (0.015 mm) thick.
The photopolymerizable layer will ordinarily have a cover sheet or overcoat layer which is transparent to actinic radiation to protect the element against oxygen inhibition as well as physical damage. The invention there-fore provides a photopolymerizable element comprising in order, (1~ a support, (2) a photopolymerizable layer having a thickness of no more than 0.0006 inch (O.OlS mm) and an optical density of at least 3.0 in the actinic region, and (3) a removable cover sheet or overcoat layer which pre-ferably is transparent to actinic radiation. By the term "overcoat layer" as used throughout is meant a coating in dry form over the photopolymerizable layer which is a least partially soluble in or permea~le to a solvent for the photo-polymerizable layer.
The invention also provides a process of image reproduction havin-3 the steps of (1) imagewise exposing to actinic radiation a photopolymerizable layer on a support, the layer containing actinic radiation absorbiny material,

(2) removing the unexposed areas of the layer (e.g., by washing out from the side of the layer which was exposed to actinic radiation), leaving an image on the support com-prised of the exposed areas of the layer, and (3) reducing 1 the size of exposed image areas by removing the edges of j tlle areas on the si~e of the layer which was exposed to actinic radiation.
- The photopolymerizable elements of the invention can replace the use of expensive silver halide .naterials , ~.

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~or making photographic mask~ has b~en ~ound that the invention~ as de~ined, provides a photopolymeri~able element with a characteristic curve suitable for lithographic appli~
cations and which produce~ images which are tone correctable by dot etching without signi~lcant loss of image density.
The elements of the invention are also characterized by rapid processability, suitability~ ~or bright light handllng, and p~oduction of hal~tone dot ima~es that are ~harper and easier to reduce in size than those of silver hallde litho ~ilms with greatly reduced operator handling. These and other aspect~ o~
the improYement in t~e art provided by the invention are described hereina~ter.
IN THE DRA~INGS
Figure 1 i9 a schematlc (not to scal~) cross-sectional view of an exposed and developed element o~ the invention with exposed areas, whose edges have been undercut by development, remaining on the sheet support.
Figure 2 is a vertical v~ew of exposed and developed elem~nts showing the pattern of hal~tone dots composed o~
. 20 exposed~ opaque photopolymer~ Figure 2A represents 60~ dots, : whose corners in~ersect thereby de~ining holes~ Figure 2B
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: repre~ents 10~ dots. - :
Figure 3 is a schematic cross-~ectional view o~ an ; ~:
expo~ed and de~eloped element o~ the invention with exposed 25 areas whose und~rcut edge~ have been re~ovedO :~
Figure 4 is a rertical view of exposed and developed :~
elements o~ Figure 2 with ~ndercut edges remo~ed showing the re~uced size of the hal~tone dots o~ the exposed photopolymer image. Figure 4A represents the do~ image o~ Figure 2A a~ter dot etching. Figur~ 4B represents the dots o~ Figure 2B a~er dot etching~

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The present invention is based on the discovery of' the process of "etch~ng" exposed areas (espec~ally~
halftone dots) Or photopolymerizable material and that photo-polymerizable elements usable in this process can be obtained using a photopolymeri~able layer which is very thin, i.e., not mo~e than 0.~0006 inch (~.~015 mm) thick! and contains sufficient actinic radiation absorbing materials such that the optical density of said layer ln (i,e.~throughout) the act~nlc region is at least ~.O. Imagewise exposure to actinic radia-tion, e.g., in the region 35~-4~O nm., produces hardened areas in the photopolymerizable layer correspoi~ding to the transparent areas of the stencil or process transparency through which the element is exposedO De~elop~en~ VL ~hc elements af~er exposure is pre~erably by washou~ of un-exposed areas down to the base with a solvent ~or theunexposed areas. Continued action o~ the solvent dur~ng development undercuts (i~e.~ removes material beneath the edges of) the exposed, hardened areas; since the high optical density o~' the layer causes a gradient in degree o~ polymerization or hardening through the thickness o~
the layer, with the greates~ degree o~ polymerization or hardening at the surface of the layer. The remaining image areas consist o~ a hardened upper skin which rests on a so~`ter undervolume having a lesser degree of poly~er~
ization or hardening~ The sheet support ot the elements supports the exposed areas of the developed mask and in com-I bination with the removable cover sheet or overcoat layer ; pro~ides protection against oxygen inhibition on exposure.
Photopolymerizable materials are particularly suitable for the layer s~nce they can be washed out with .
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7~ 5 solvent in unexposed areas and undercut easily. Preferred - elements of this invention therefore comprise a transparent support, a transparent cover sheet o.r overcoat layer, and a single photopolymerizable layer, not more than 0 . 0006 inch ~o.nl5 mm) thick, between the support and cover sheet or over-coat containing;
a~ An ethylenically unsaturated compound capable OI` ~orming a high polymer by free radical initiated~ chain propagating~ addition poly~erizat~on, 10-30~ by weight, bo an organic poly~eric binder~ 10-60~o by weightg `~ c. a ~rec radical generating addit~on polymeri-zation initiator system activatable by actinic radqation, 001-20~ by weight, and, d. an actinic radiation absorbing material pxesent in such concentration as to impart an optical density to the photopolymerizQble layer o~ at least 3.0 oYer at least the spectral range OI' ~50~400 nm.
Particularly prererred are such elements having a thickness Or not more than 0.0004 inch ~0.010 mm~) having an opt}cal density in the photopolymerizable layer of at least 4Ø
In a partlcularly preferred element o~ this invention~ the polymeric binders are selected so that the unexposed photopolymerizable compositlon is soluble in predominantly aqueous solutions, e.g.~ dilute aqueous alkaline solutïons, but upon exposure to actinic radia- ~ -tion becomes relatively insoluble therein. ~ypiCAlly, ~ .
polymers wh~ch satisf~ these criterla are carboxylated, :

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56~i e.g~,, vlngl addition polymers containing f'ree carboxylic -- acid groups. However, a wide variety OI` binders may be - used, as disclosed in prior art photopolymerizable compo-sitions~ where aqueous development is not required.
The elements Or the in~ention are versatile -in that they can be used as litho masks and as "photo-masks"~ A mask suitable ~or contact speed lithographic woxk is readily providèd b~ the above elements by~ for example, imagewise expos~ng the ele~ent with a ha~tone ~0 screen through the cover sheet or overcoat layer, followed by peeling off the cover sheet or dissolving the overcoat layer partially or completely, and then developing by washing out ~he unexposed areas to leave a suitable dot-etchahle mask on ; the support film. Of course, in this application, dyes or - 15 pigments, e.g., colloidal carbon, would be adde~ to the photo-; polymerizable layer so that the image would be opaque in both the ultraviolet and visible regions of the spectrum.
Alternatively, a suitable "photomask" for use in exposing photoresists and tne like can be provided by a ~0 slightly different process. In this instance, for example, after removing the cover sheet or completely or partially removing the overcoat layer, the photopolymerizable layer on a temporary support can be laminated to a dimensionally stable and rigid support, for example, optical glass. Then, after lamination and exposure, the temporary support is re-moved and solvent development of the photopolymeri~able layer yields the photomask on the rigid support. In this application, the layer may contain dyes or pigments absorb-ins throughout at least the 350-400 nm. range. Thus, the photomask can be substantially transparent in a m~jor portion ~' .
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~ o795G5 of ~he visible region of the spectrum, but be substantially opaque in the actinic region (i.e., the region of the spectrum used to initiate photopo1ymerization).
Tne elements of the invention may be used in the ways described above deperlding on the process of development. Development can be s~opped when holes have been cleaned out down to the base and before there is substantial undercutting. The elements thus developed may be used as a photomask for circuit line images, for example, where no etching is desired. If the coating thickness is too great, undercutting may cause uncontrolled or excessive loss of portions of the ~mage. For use as a litho mask, the element is exposed with a half-tone screen and developed. When it is desired to correct 15 tone values in the image, development is carried out further `
to undercut the halftone dots in the image and permit size reduction of the dots. Where development is carried out -' to the extènt causing undercutting, the layer must be very thin so that dots in highlight areas are not com~
pletely removed oy the etching process. If the layer is :, , too thick, small dots which comprise a photohardened cap ~ .
preferably not more than one-third the thickness of the ~ ;
layer resting on a tall column of soluble, unexposed material, are exceedingly frag;le and subject to breaking .. . .
o~f.
The photopolymerizable layer must have an optical ,density great enough ~o produce with a very thin layer an - -!opaque image useful as a mask. Where dot etching is to be .
practiced, high optical density is also required to produce a yradient in degree of polymerization.

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In photopolymerizable compositions the molecular weight Or at least one componen~ of the compo-sition is increased by exposure to actinic radiation3 causing a rhan~e in the rheological and thermal properties oY the exposed areas and xenderin~ the exposed ~reas relatively less soluble in solvents than the unexposed areas, thereby producing a solvent developable ~age~
Pho~opolymerizable compositions ~or use ln the in~ention usually comprise a photoinitiator, an ackinic radiation absorber to opaci~ the element in the actinic re~ion, a polymeri~able monomer, and a polymeric binder, as descrIbed ~urther below.
(a) The Photoinitiator :, The photopoly~erizable composition contains an organic, ~ree-radical generating syste~ activatable by actinic radia~ion which initlates polymerization of the .
ethylenically unsaturated compound and does not subsequently terminate the reaction. The free-radical generating system ` should have at least one component that has an active radia- ;
~0 tion absorption band with a molar extinction coefficient of at least about 50 within the range of about 300-500 nm.
"Active radiation absorption band" means a band of radiation which is active to produce the free radicals necessary to initiate the polymerization. The free-radical generating system can comprise one or more compounds which directly furnish free radicals when activated by radiation. It can also comprise a plurality of compounds, one of which yields free radicals ater havlng been caused to do so by a ~ sensitizer which is activated by the radiation.
Photoinitiators which can be utilized ' ' ' ' .

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in the practice of this invention include aromatic ketones such as benzophenone, Michler's-ketone (4,~t_ bis-(dimethylamino)benzophenone)~ 4,4'-bis(diethyl~mlno) benzophenone, 4;methoxy~4'-dimethylaminobenzophenone~ :
2-ethylanthraquinone, phenanthraquinone, and other aromatic ketones, benzoin, benzoin ethers such as benzoin methyl ether, benzoin ethyl ether an~ benzoin phenyl ether, methylbenzoin, ethylbenzoin and other benzoins, and 2,4,5~triar~1im~daæole dimers SUCh as 2~o-chlorophenyl)-4,5-diphenylimidazole dimer, 2-(o-chlorophenyl)-4,5-(m~methoxyphenyl)imidazole di~er, 2~(o-fluorophenyl)-4, . 5-diphenylimidazole dimer, 2-(o-metho~yphe~nyl)-4,5-diphe~-limidazo}e dimer, 2~ ethoxyphenyl)-~,5 d~phenyl-imidazole dimer, 2,4-di~-methoxyphenyl)_5-phenylimidazole dimer3 2-(2,4 dimethoxyphenyl)-4,5-dlphenylimidazole ~mer, : 2-(~-methylmercaptophenyl)-4,5 diphenylimidazole dlmer, and the llke disclosed in U.S. Patent 3,479,1853 in . British Patent 1,~47,569~ published November 9, 196~, ;
:~' and U~S~ Patent 3,78~557.
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. 20 Particularly usef'ul inltiators are the 2~435 . .
triaryli~idazole dimers (also known as hexaarylbiimidazoles).
These are used with a rree~radical producing electron donor agent~ such as 2-mercaptobenzoxazole, leuco crystal violet or tris(4-diethylamino-2-methylphenyl)-methane~ Such -i 25 sensitizers as Michler's ketone may be addedO Various .. ....
energy transl~er dyes~ such as Rose Bengal and Eosin Y, can also be used~ ~dditional examples oX suitable initiators are disclosed in U.S. Patent 2,760,~63p Other .. use~'~I systems employ a kriarylimidazole dimer and a ~ree- :
radical producin~ eleGtron donor agent~ with or without , ~ ,.

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the use o~ a sensLtizing compound 3S descr~be~ in U.S.
Patent 3~479,185. Another use~'ul group o~ initiators is those ~ixtures described in U.S. Patent 3,427,161.
The ~ree-radical generating system is employed in a concentration suf`~icient to initiate polymerization~
w~ich is u~ually about 0.1~20~ by wei~ht based on total com~osition, and pxe~'erably about 2 to 12~ by weight.
(b) A tinic Radiation Absorber Compound~, in addition to the photoinitiator system, e~fective in absorbing actinic radiation~ which for most ~hotopolymerizable compositions includes at least the range of 350-400 nm,can be present in the layer in relatively high con-centrations to provide a very opaque mask and cause a poly merization gradient required for image undercutting. Such a ~radient is provided in such compositions by an optical density o~ at least 3.0 in the layer over at least the spectral r~nge of 35~-4~0 nm. inclu~ive. Mixtures of actinic radiation absorb-ing photoinitiator systems3 dye~, and pigments are frequently used to provide the absorption ovex the visible and actinic range providing an optical density of at least 3.0 throughout it or at least over the range of 300-500 nm.
Examples o~ ultraviolet d~es, ultraviolet absorbers and other dyes which can be used in thls lnvention are listed below:
-. 25 2~2'-dihydro.Yy-4-methoxy-benzophenone 4-dodecyloxy-2-hydroxybenzophenone ` 2,4-dihydroxybenzophenone hydroxyphenylbenzotriazole 2(2l-hydroxy-5'-methox~phenyl)benæotxiazole resorcinol-monobenzoate .: 13 -- - ~ . . . . - . ..... . ... .

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2-hydroxy-4-m~tho~Yyb2nzophenone ~
?~2'-d~hydroxy 4, 4'~dimPthoxy~benzophenone ~:
2,2'~4,4~-tetrahydroxybenz~phenone 2-hydroxy-4-methoxy-benzophenone-5-sulfonic acid (al~o ~odlum ~alt Of ~bôve) ;, e~hyl-2-cyano~3,3-diphenylacrylate 2 ethylhe~yl-2~cyano~3,3-diphen~l~çrylate ~u~ol~ Orange GRL Color Index ~25 ~Sol~ent Orange) GS " ll ~5624 lû " " R " " #20 Plasto'~ Orange M ll ll ~21 RS " " #22 " ll :
-~ra~ol~ Fa~t Orange 2RN Itl~ #33 11 "
Oil Orange " " ~12055 (Solverlt Yellow #14) Sudan Orange RA " " #12055 (Solvent Yellow ~14) Luxol~ Yellow G Color Index ~5 (Sol~nt Y~llow) T " " #4!7 Plast~6~ Yell~w GP~ " " #39 " " :
" " MGS i' " ,~40 " "
20011 Yell~w 3~ 29 . -. ... .
D 1~ 1~ It 11 ~2 Sudan Yellow " " ~30 Lu~col~ Fa~t Blue AR Color Index No. 37 (Solvent Blue) Lllxol~ Fa~t Black L "" " 17 (Solvent Bla~k) Primro~e Yellow "" A~77603 (Pigment) Chrome ~el~ow L~ght ~~77603 Chro~e ~ell~w Medium Dit ~ 7600 Dlspersed Manganese di~xide Toluidlne Yell~w GW Color Index ~ 1~80 (Pi~ment3 Molybdate Ora~ge ~ #77605 , : ' ., '' '' ' .' , ,.

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Dalamar Yellow Color Indcx ~1171~1 (Pi~ment) Green Gold ~ 12775 Graphtol Yellow " " Plgment Yellow ~61 Graphtol Orange " " " Orange t~l3 ~ Colloidal carbon is a particularly pre~erred pigment.
To obtain an optical density Ln a thin layer of - at least 3.0 throughout the 350 400 nm. region frequently requlres a high percentage o~ dyes and/or pi~ments, ~sually between 15-4~o~ by weight, o~ the photopolymerizable layer. When the element of ~his inven~ion is to serve as a mask for only a limited number Qf exposures to ultraviolet radiation a high concentratio~ of photoinitiator alone (0~
dyes and/or pigments) is sufficient to provide the required optical density. Preferably, however, to provide a mask suitable for many exposures high concentration~ of ultra- ;
violet stable dyes and/or pigments are present.
~c) The Monomer The instant inven~ion is no~ limited ~o the use of any particular polymexizable monomer, it being requiied only that the ~onomer be ethylenically unsaturated and capable of addition polymerization. A large number of use~ul monomers is available, generally characterized by one or more terminal ethylenic groups~ Among the suitable monomers may be mentioned various vinyl and vinylidene monomers~ eOg., vinyl carboxylates~ alpha-alkyl acrylates, alpha-substituted acrylic acids a~d esters thereo~, vinyl esters~ vinyl hydrocarbons~ acrylic and alpha-subsituted ac~ylic acid esters o~ the polymethylene glycols and ether alcohols~ all as diæclosed in U.S~ P~tents 2,760,863 .:

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and 2,791,504; the various compounds disclosed in U.S.
Patent 2,927,022 and especially those ha~ing a p~urality of addition polymerizable ethylenic linkages, particularly ~hen present as terminal llnkages~ ~nd more-~specially those wherein at least one and preIerably mos~ o~ such : linkages are con~ug~ted with a doubly bon~ed carbon~
including carbon dou~ly bonded to carbon or to such hetero-atoms as nitro~en, oxygen and sulfur; esters OI' penta-aerythritol compounds o~ the kind dlsclosed in U.S, Patent 3,261,686; and compounds ol the l~ind described in U.S. Patent 3~3809~31; eOg., the react~on product o~
trimethylolpropane, ethylene oxide, and acrylic and metha- :
crylic acids.
The concentration o~ the monomer or monomers employed is normally about 7 5-35% by weight based on the total solids o~ the composition, and pre~erably between 15-25%.
(d) m e Binder . The binder used ~s an organic polymeric mat~rial that is pre~erably solid at 50C., and it is necessary that the binder be compatible with the polymerizable monomer and the polymerization ~nitiator system. Tt may ~requently be desirable, but it is not required, that the binder be thexmoplastic. The binder.may be o~ the same general type as the polymerizable monomer being used and ~-.
may be soluble therein and plas~icized therebyO ~:
A wide variety or suitable binders, both thermo~ . .
plastic and nonthermoplastic, is disclosed in U.S. Patent . ~ -

3,060~023, e.g., cellulose ethers or esters; polyall~ylene ethers; condensation polymers o~ glycols with dib~s~c ~ .
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5~i5 ac~ds; polymers and copolymers of v~nyl esters; acry~ic acids and esters; polyvinyl alcohol; cellulose; phenolic resins; and the llke. Other ~inders~ includlng a number of vinylidene polymers, are disclosed ~n UOS. Patents 2,760,863 and Z,791,504. Still other use~ul bin~ers are the N-methoxymethyl polyhexamethylene adipamide mixtures o~ British Pa~ent 826,272, the polyester, polyacetal or mixed polyesteracetal mixtures of U.S. Patent 2,892~716;
the f'usible poly~inyl alcohol derivatives o~ U.S, Patent 2,902~36~; ~he ~usible blends or selected organic soluble~
base soluble cellulose derivatives o~ U.S. Patent 2,902,365;
the f~usible blends o~' selected organic-soluble, base~
solu~le cellulose derivatives o~ U.S. Patent ~,927,022;
the polyvinyl acetals having extralinear vinylidene groups of U. S. Patent 2,902,710; the linear polyamides containing extralinear N-acrylyloxymethyl groups of U. S. Patent 2,972,540;
' and the 1,3-butadienes of U. S. Patent 3,024,180.
The binder or binder mixture usually comprises ' between lO-60%, by weight, o~ the photopolymerizable layer.
Particularly pre*erred as binders are acidic, .' polymeric, organic compounds since the photopolymerl~able composition resulting is developable in solely aqueous alkaline solvent devoid o~' organic solvents. This is : :
advantageous since organic sol~ents are costly~ may be hazardous with respect to toxiclty and/or ~'lammability, may become scarce due to petrochemical shortages, and ! may pollute the air and water. -One class o~ ilm-~'orming binders which is .
- soluble in aqueous alkaline medla and is useful ~n the composltions o~' the present in~ention is vinyl addition .' .

~L~79565 polymers contalning ~ree carboxylic acid groups, which are prepared from 30 to 94 mole percent of one or more alkyl acrylates and 70 to 6 mole percent of one or more alpha-beta-ethylenically unsaturated carboxylic acids, and ~ore pre~erably prepared from 61 to 94 mole percent of two ælkyl acrylates and 39 to 6 mole percent of an alpha-beta-ethylenically unsatllrated carboxylic acid.
Suitable alkyl acrylates for use in prepar~ng these polymexic bin~ers include methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate~ methyl methacrylate, ethyl methacrylate~ bu~yl methacrylate, etc. Suitable alpha-beta-ethylenicall~ ~unsaturated carboxylic acids incl~de acrylic acid, methacrylic acid and the lile.
Binders o~ ~his type, including their preparation, are described in German Application, oS 2,320,849~ published 11/8/73~
The advantages of using acidlc binders can also be ob~ained by selec~ing a pre~ormed, compatible macro-molecular polymeric binding agent which ~s a copolymer : :
of (1) a styrene-type of a vinyl monomer and (2) an unsaturated carboxyl-containing monomer, as described . :
in detail in British Patent 1,361,298~
Another preferred photopolymerizable compos~tion is obta~ned b~ using a pref~rmed, compatible macromolecular polymeric binding agent mixture, the camponents thereo~
taken trom two selected classes. The use of the mixtures . as described in Belgian Patent 828,237, eliminates the need - :for organic solvent~ in developing. These are mixtures of two types of binders. ~he first type is preferably selected from 3~ a copo~ymer or vinyl acetate and crotonic ~c~; a ter- .

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~795 Ei~i -polymer o~` ethyl acrylate, methyl methacrylate, and acrylic acid; and cellulose acetate succlnateO The second type is preferably selected from tolu~ne sul~onamlde fo~maldehyde;
a copolymer oi methyl methacrylate and methacrylic acidj a terpolymer Or methyl methacrylate, ethyl acrylate~ and hydrogen maleate; a terpolymer o~ vinyl chlorlde, vinyl acetate~ and maleic acid; a copoly~er o~ styrene and maleic anhydride; and a terpolymer o~ ~ethyl met~acrylate, ethyl acrylate, and methacrylic ac~d.
Photopolymerizable elements can be prepared by coating the photopolymerizable compositions from solvents on-to supports in accordance with usual coating practices~ Suit-able su~port materials include rilms composed or high polymers, which are cast as ~ilms ~rom molten polymer, such as polyamides, e.g., polyhexamethylene sebacamide, polyhexamethylene adipamide, polyole~ins, e.g., poly-propylene; polyeskers, e.g~, polyethylene terephthalate, polyethylene terephthalata/isophthalate; vinyl polymers9 e.g., vinyl acetals, vinylidene chloride/v~nyl chlor~de copolymers, polystyrene, polyacrylonitrile; and cellu-: losicæ, e.g~a cellulose acetate, cellulose acetate/
; butyrate, cellophaneO A part~cularly pref`erred SUpp3 rt ~-material is polyethylene terephthala~e Iilm o~ the kind ; described in Alles ek al., U.S. Patent 2,627,088, and Alles, UOS. Patent 2,779368l~, with or without the sur~ace coating described in the former patentO The support may ha~e a resin "sub" or other layer thereon which may or .: may not be soluble and which for purposes o~ this inven-tion is considered part o~ the support~ However, the 30 total thickness of the photopolymerizable layer and any , ~ .
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5~

soluble su~ or underlayer should not exceed o.ooo6 lnch ~ (0~015 mm,)~ B~ "soluble" is meant solubility in a so~vent in-which the photopolymerizable layer is developable.
Preferred ~oluble sub layers have a thickness not exceeding about 0.0002 inch (0.005 mmO). I~ere the particular appli-- cation does not xequire that the base support be trans-parent, the photopolymerizable composition may usel`ully be coated on an opaque support~ such as paper, especially water_proo~` photographic paper, thin metal sheets, especially aluminum and copper sheet5, cardboard and the l~keO The optimum coating th~ckness ~or a particular purpose t~Jlll ~ depend on such ~actors as the use to which the coat~ng - Will be put~ th~ par~icular l~ght-sensitlve comps:)sition employed, and the nature of other compounds which may be present ''33n ~he coating. Pxeferred dry coating thick~esses c~
be from about 0.0001 to o.ooo6 inch (000025 to Q.015 mm~) -Even a~ter evaporation o~ the solvent, ~any of the photopolymerizable coatings made from the various com-ponents outlined above are somewhat soft, sticky, or tacky.
To facilitate storage and handling and to protect against oxygen inhibition during exposure~ there is applied a removable cover sheet or overcoat layer, which may be either a previously cast ilm or an additional coating. The coversheet is a nonphbtosensitive sheet that is completely remova~le from the entire photopolymerizable layer in one operation, i.e., by stripping~ A convenient and suitable material for a cover sheet that can be mechanically stripped ~ off is any of the several commercially available varieties 3 0~ polyethylene terephthalate or polypropylene film. Al-ternatively, polymeric materials, e.g., polyvinyl alcohol, ~, .

5~5 gelatin, etc., may be coated in solution over the photo-polymerizable stratum to leave, after removal of solvent, a dry overcoatl~yer, which depending on its composition,is at least partially soluble in or permeable to the solvent for the photopolymerizable layerO Some overcoat layers are described in U. SO Patent 3,458,311.
In practicing an embodiment of the inv~ntion~
an element containing an image-yielding photopolymerizable stratum is made by coating a layer of a photopolymerizable composition disclosed herein on a suitable film support.
After drying the photopolymerizable stratum, there is lami-nated to the surface thereof a removable cover sheet or coated thereon an overcoat layer. The photopolymerizable composition is prefera~ly coated to give a dry coating thickness of about 0.0002 inch (50 mg/dm2) ~0.0051 mm.). A
suitabl~ support film may be chosen from a wide variety of films composed of high polymers, e.g., polyamides, poly- -olefins,polyesters, vinyl polymers, and cellulose esters and may have a thickness of from 0.00025 inch tO.006 mm.) to o.oo8 inch (00203 mm~) ox more~ ~Y exposure is to be through the support and before removin~ the Support film, it must~ of course, transmit a substantial ~raction o~`
the actinic radiation lncident Upon ik. I~ the Support ~ilm iS remo~ed prior to e~posure~ no such restrictions apply. I~ the support is to remaln on the layer, as in the case o~ a litho mask, it must be transparent~ A
particularly suitable film is a transparent pol~Jethylene terephthalate film havi~g a thickness O~ about 00004 inch (00102 mm~)O Suitable removable cover sheets may be chosen from the same group of high polymcr rilms des-~.
:, :

~721~6~

cribed above and ~an have the same wide range oi thick-neSses. ~ere exposure is to be through the cover sheet or layer, the same should be transparent to actinic radiation. A cover sheet of 0.0005 inch thick ~0~013 mm.) polyethylene terephthalate is especially suitable. Support and cover films as described above provide good protection to the photopolymerizable layer. The overcoat layers have also been described above. The support and/or the cover film may have other layers thereon as desired, e.g., a silver halide emulsion layer.
Preferred photopolymerizable layers have a thickness 0~ less than o .ooo6 lnch, e~gO~ 0.000~ inch ~0.01 mm.).
Thicknesses o~ 0.0002 inch (0.005i n~n.) to ~.0003 inch (0.~077 mm.) are particularl~ preIerred. The optical density o~ the layer, ~Jhich is the result o~ the light absorbance of the radiation absorber, the photoinitiating ; system, and all other materiale in the layer, is at least 3.0 and pre~erably at least 4~0. A layer ha~in~ an optical density o~ 3~0 in the actinic region absorbs 99.9% of the incident actinic radiation~ ~rhile a layer with an optical density Or 4.0 absorbs 99.99~o.
In carrylng out the process o~ this invention, a photopolymerizable element is pxepared as already described.
The element is then exposed to actinic radiation, preferably through the transparent cover sheet or overcoat layer. The exposure may be by means of a light source which is rich in i ultraviolet radiation through a halftone imag~ transparency, e.g., process negative or positive (an image-bearing transparency consist~ng solely o~ substantially opa~ue and substantially tra~sparent areas where the opa~ue areas are substantially ' ' ..
: - ~2 -:.

356~

OL~ the same optical density)O 'l'he transparency may also have a line image such as en~ineerin~ drawlngs. The image or transparency may or may not be in contact with the surface of the element. i.eO, contact exposure or pro ~ection exposure. For exposures through transparent ~ilm supports the time required will range ~rom a f`ew seconcls to several minutes depending on the intensity Or the exposing radiation and the inherent photographic speed of the composition. After exposure, the cover ~heet or sheet support through which the element was exposed is removed~ and the exposed layer with its exposecl, harclened areas and its unexposed or underexposed, unhardened are~s is developed by removal o~ the latter from elementO The removal o~ the unexposed areas may be accomplished by treating ~he si~e OL' ~he element which was exposed with ~he deveLoper solution, thereby washing out the unexposed areas. Alternatively, if an overcoat layer is present in-stead of a cover sheet, the developer for the unexposed photopolymerizable layer will remove the overcoat layer com-pletely above the unexposed areas and at least partiallyabove the exposed areas. Since free-radical generating initiators activatable by actinic radiation generally exhibit their maximum sensitivity in the ultraviolet range, ~he radicltion source shouId usually Lurnish an e~ec_ tlve amount o~` this rcldiation. Bo~h point and broad radiation sources are eI`iective~ Such sources inc~ucle carbon arcs, ~enon arcs, mercury vapor arcs~ ~luorescent . .
lamps with ultraviolet radiation emittln~ phosphors~
~ 30 argon glow lampsa electronic f`lash units and photo~raphlc ; .:

~, . , ~ . . , , , , . ., , - , .:

11[17~5 flood lamps. 0~` these, the tungsten lOOOW quartz iodide lamp is most suitable. 'l'he sunlamp mercury-vapor arcs are customarily used as a distance o~ 30~61 cmO ~rom the photopolymerizable layer. The point sources are generally used at a distance o~ 50-125 cm. from the element. It is no~ed, however, that in certain circumstances it may be ad~antageous to expose with ~isible ll~ht, using a photoînitiator sensitive in the v~s~ble re~ion o~ the spectrum. ~n such cases, the radiatLon source should f'urnlsh an e~`~ective amount o~' ~isible radiatlon. Many o~ the radiation sources listed above furnish the required amount of' visible light.
Alte exposure, th~ elemen~ is developed, e.g., by washlng out with a suitable solvent the unexposed, unhardened, soluble portions o~ the photopolymerizable layer on the side of the layer that was exposed. This may be carried out by impingement oi spray jets, with agitated immersion, brushing or scrubbing, to leave the desired ;
colored, harde~ed, insoluble image~ The result is a resist image which has sur~icient opacity in the actinic region to be used as an intermediate element, "rl~ster,' or mask ~`or ~'urther exposing photosensitive compositlon~.
The resist image may be used as a mask directly, or it may be sub~ected to "etching," which is the chemical undercutting and subsequent removal o~ the edges of the image areas. I~en the ima~e aleas are in khe ~orla o~ dots, ~ this process is re~erred to as "dot etchlng".
-l The developer solvent ~s a ~luid in which khe unexposed areas are suf~iciently soluble to be removed from the support and in which the exposed areas are ' relatively insoluble. The developer sol~ent selected ; 24 -'~
. . - . . . . .
~. . . . . .

7~3565 thexe~ore depends upon the pho~opolymerizable composition.
Thus, the solvent used in coating the composition can be us~ed. Wea~ly ac-ldic photopol~meriza~le composit~ons ~y be con~eniently de~-eloped with solvent mixtures as ~isclosed in u. s. Patent 3,475,171~ The more pre~erred aqueously de~elopable photopolymerizable compositions are developed with aqueous bases, ice " aqueous solutions o~ water-soluble bases in concentra~lons generally in the range from 0~017~ to lO~o by ~eight.
Suitable bases I~or the development include the alkali metal hydroxides~ e~g~ lithlum, sod~um and potassium hydroxide; the base-reacting alkcali metal salts o~ weak acids~ e~g" lithium, sodium, and potassium carbonates and bicarbonates~ ammonium hydroxide and tetra-substituted ammonium hydroxides, e.g., tetramethyl-~' tetraethyl-, trimethylbenzyl-, and trimethylphenyl-ammonium hydroxides, su}fonium hydro~ides, e.g., tri methyl-, diethylmeth~l-, dimethylbenzyl-, sulfonium hydroxides, and the basic soluble salts thereol, e~g., the carbona~es, bicarbon~tes and sul-~'ides; alkaLi metaL ~' phosphates and pyrophosphates, e.gO~ sodium and potassium triphosphates and sodium and potassium pyrophosphat~s;
tetra-subskituted (pre~'erably l~rholly alkyl) phos~h~n~unl, arsonium, and stibonium hydroxide, e.~ etramethyl~
phosphonium hydroxide~
The same solvents woul~ also be use~ul in the dot etch process, wherein the parti~lly h~rdened dot is under- ~' cut and then reduced in size by mechanical abrasionO
The elements o~' the inventlon provide a cost saving alternat~e to silver halide masks. A black, machine processable~ contact-speed, pho~opolymer litho ~.. . . .

~;

.3S65 film has been prepared according to the invention with properties equivalent or superior to competitive silver halide films. For example, a 30-second, contact, tungsten lamp exposure gives image reproduction equal to that of a silver halide litho film. The above exposure is suffi-cient to reproduce a 150 line/inch (59.05 line/cm.), 2-98%
dot tonal range. Transmission density of background areas is comparable to silver halide (0.05 using blue filter) and good developer latitude, dot sharpening, and dot etching ~ ' have been demonstrated for the preferred element; this element comprises a 0.0005 inch (0.0125 mm.) thick cover sheet of polyethylene terephthalate, a 0.0003 inch (0.0076 mm.) thick photopolymerizable layer with an optical density of 4.0 in 350-400 nm. region, conta~ning collo~dal carbon, coated on a 0.004 inch (0.102 mm.) thick polyethylene tereththalate film substrate, which contains a 0.00002 inch (0.0005 mm.) thick soluble subbing. Further, this element can be developed in a 1QW cost, automatic processor which consists of an immersion developmen~ compartment and a water ! spray wash section. At the end of the process the film is dried. The whole process requires less than one minute. Finally, soft dot reproduction with the photo-polymer film is either equal to or better than that o~
silver halide.
A related product may be prepared which is .
, particularly useful ln vlsual registration. This product i~ is a co1ored, vis;ble light-transparent, ultraviol2t-:~, opaque, photopolymerizable element which does not require a dark room, can accurately reproduce 2-98% dots, 150 `~ lines/inch screen ~59.05 line/cm.), is dot etchable, and has good exposure and development latitude.
The photopolymerizable elements of the invention . .
.. .. . . .
- 2 6 - ~

~ .

~ 5~ 5 completely displace the use of silver ;n litho films thereby providing great cost and hanclling advantages.
In addition, the elements have improved resistance to scratching and abrasive markings which ruin masks of the silver type. Advantageously, dyes and colorants added to -~he layer can be controlled to attain just the optical density needed for further copy preparation. When it is necessary for the technician to use more than one mask to make a copy, the transparent image type is clearly v;sible under safelights used during exposure, resulting in perfect registry. An important commercial advantage is that the use of applicant's elements and processes requires less handling of the photopolymerizable elements in making masks and that they are dot-etchable.
The etching process may be carried out by continuing development until substantial undercutting of exposed areas has occurred and then reducing the size of the exposed areas by removing material from the ; edges of their upper surfaces (i.e.~ the surfaces that were exposed to actînic radiation).
In making masks for exposing litho plates, an exposed and developed mask is usually made and then ; .
examined by a color proofer who determines whether tone correction is required. This can be done by actually making a litho plate (a "press proof"), by measuring ; -the halftone dots with a densitometer and comparing the colors in the original to color chips keyed to dot size, by a special color proofing film (as described in U.S.
3,649,268), or by any o~her suitable method of color proofing. If it is determined that tone correction is desired~ the mask can be dot-etched by returning it to the .. ," , .

~'9~s developer apparatus which is equipped with spray nozzles or other means to provide the mechanical action for dot etch;ng. The amount of dot etching will be proportional to the amount of time in the developer apparatus, so that the process of tone correction by dot etching can be quite easily controlled. Instead of returning ~he mask used for proofing to the developer, a new mask can be prepared and simply developed for a longer period of time, so as to yield a dot-etched mask of proper tonal balance. Wi~h either alternative, the process usually involves a size reduction in the dots of at least 5% between the mask used for proofing and the subsequently dot etched mask.
More commonly, a size reduction of 10% or more is carried out with the dot-etch process. As illustrated in the examples, a dot size reduction of 75% or more ma~y be achieved with elements of the invention.
.
If it is determined that only a portion of the image on the mask needs tonal correction, developer solvent may be applied to that area alone, followed by rubbing - 20 with a wet pad or spraying, to dot-etch that area selectively.
When an element of the invention is exposed wi-th -~;
a halftone screen and then developed, the image thereon is comprised of normally square halftone dots of polymeric material~ It will be noted that square dots of a size of 50% or greater touch at the corners and thereby de-fine , .
clear "holes" in the mask. Dot etching of such large dots therefore may be viewed as hole enlargement; however, hole enlargement occurs through reduc~ion in size of the exposed, poly~eric dots. In all cases do~ etching results in less opaque polymeric exposed area on the mask ~nd there-.

~L~'795~

fore a reduction in the overall density of the image due to larger spaces between the opaque polymeric exposed areas. The drawings illustrate the effect or dot etch-ing on large and small size dots. F;gure 1 represents a dot image that has been undercut but not subjected to physical abrasion to reduce dot size. Figure 2A shows 60% dots before dot etching, and 2B shows 10% dots before dot etching. Figure 3 represents a dot image tha~ has been dot etched. The upper surfaces of the dots have been broken off, thereby reducing dot size. Figure 4A
shows the dot image of Figure 2A after dot etching. The dots have been reduced in size by physical abrasion, thereby enlarging the holes between them. Figure 4B
; shows the dots of Figure 2B after dot e-tching which has reduced their size.
The invention accordingly provides a process; ~
comprising the steps of imagewise exposin~ the element,~ ~;
removing the unexposed areas of the photopolymerizable layer, :" , and reducing the size of exposed image areas by removing the edges of said areas. The latter two steps can be carried out sequentially in a single pass through a de~eloper apparatus suitably equipped with spray, brush-ing, or other means to apply mechanical action to the ~;
dot surface. Alternatively7 these steps may be carried out by hand, as by immersion ;n developer and rubbiny. -The first step of the process, imagewise exposing, especially through a hal~tone screen, thus produces - -; a dot etchable mask from an element of the invention.
The mask contains a tone correctable image comprised of- :
opaque polymer;c dots which are reducible in size by mechanical action (e~g., rubbing, brushing, or spraying~
.1 . .

- ' . .
., . . . . . :

~ 7 9 S~ ~

on thP imate bearing surface of the mask. Such a mask beIore and a~er size reduction by mech~nical action iS
represented by Figures 1 and 3, respectivelyO

AS explained above, the major utillty Oï dot etching is ln lithographic plate making, wherein a mask of the invention is one of a p~urality or color separation masks and reducing the s~ze of the exposed areas (dots) iS
carried out to an extent whereby a composite color image produced With the masks ~on a lithographic plate o~ on a proofing ~ilm) has the same tonal balance as the original color image. The elements o~ the inven~ion are capable -~
o~ functioning in thiS process, thereby providing a new and improved rep~acement or silver halide l~tho masks~
The elements of this invention are also useful as color-coded photomasks. Such masks can be used to expose photosensitive printing plates which will print yellow, magenta, cyan process colors, and usually also black, thus forming a full-color reproduction of the original. The mask~ themselves are colored in these shades and thus pro-vide a simple, rapid method for proofing.
- Color-coded photomasks meet the optical density !;
and thickness requirements set forth above, i.e.~ have an optical density throughout the actinic region of at least 3.0 and a thickness no greatex than 0.015 mm. The high optical density is achieved by adding relatively high concen-trations of ultraviolet absorbers to the photopolymerizable layer. Each layer is also colored in a visible region of the spectrum with transparen~ dyes or pigments corresponding `, with inks to be later used; generally yellow, magenta, cyan and black.

~ . . .
~ - 30 -.
~'' - , - . ~ .

.

In usel a printer desiring to prepare full-color prints of a process transparency, would expose the yellow photomask of this invention to a halftone image of ~he yellow component of the process transparency, the magenta photomask to the magenta component, the cyan photomask to the -cyan component, and, usually, a black photomask to the gray-to-black component. A~ter exposure and development, the imaged masks would be assembled, one on top of the other in register, to yield a full color proof of the original, e.g., when viewed with transmitted light. If the color reproduction was satisfactory, each imaged photomask would bé used for exposing a positive-working photosensitive printing plate, which when processed and inked with an nk corresponding in color to the color of the photomask, would yield prints of that color. Multiple printing from plates exposed through each of the yellow, magenta, cyan, and black ; ~
imaged masks, and inked accordingly, would yield a faith- ;
fully reproduced full-color print of thé original process transparency.
At other times, when the color-coded imaged photo-masks ha~e been assembled for proofing, it may be observed that the full color proof is, for example, too yellow, or too red, etc., either overall or in certain regions. In this instance, the imaged photomask corresponding to the excess color would be corrected by dot etching, either ` overall or locally. After dot etching, the color-coded imaged photomasks would be reassembled and inspected again. ~urther ~` corrections may be necessary, and they would be made until the assembled color~coded masks gave the full-color re-- 30 production desired. T.~hen satisfactory, each mask would then be used to expose a photosensitive printing plate.
; .
- - 31 ~

~)7~65 The color-coded photomasks o- this inventionl when assembled in register, provide a very convenient proof-ing system. The above description applies only to positive--working photosensitive printing plates. ~hen negative-working plates are to be exposed, an extra process step is involved.
Each color-coded mask is used to prepare a contact litho-graphic negative film, which is then used for modulating -~
exposure of the printing plate.
The following examples illustrate embodiments of the invention in which a photopolymerizable composltion, compris-ing a polymerizable monomer and a polymeric binder is used for the photopolymerizable lay~r. Such compositions have been ~ound to work p~rtlcularly well in the invention~possibly due to migration o~ the monomer through the blnder toward the ; radiation source, resulting in a high gradient of polymerl~atlon through the thickness of the l~yer and thereby promotion o~ ~;
dot-etchability. Such composition~ are there~ore particuarly pre~erred.
The developed images of the examples o~ khe inventlon had high density in the image areas and little or no density in the nonimage areas. The lack of density in the nonimage ~reas (background) provides an important advantage over diazo-type masks, which have substantial background densiky, This adYantage is facilitated by the use of a soluble sub layer.
Elements of the invent-ion can be provided which are capable o~ having an optical density to radiation in the 350-400 nm.

, :
' 30 ~. .
- .. . - :
: - : . . , . . : . .
- - :, . ... - . ~ . . .

~ID7`~jS

re~ion in the nonimage areas after development not exceedin~
0.3 and preferably being virtually nil. Such elements con-taining a dispersed pigment, e.g.~ carbon black~ as the r~diation ab~orber are particularly preferred since they ~ j , can provide elements with very good photospeed.
In carrying out the process of ~his inven~lon, exposures sufflcient to polymerize the surface, but not more than about one-third the photopolymerizable layer,are preferr~d with the time of exposure not exceedlng about 3 minutes.

The invention will be further illustrated by the follow.ing examples, wherein parts and percentag~s are by weight unless otherwise noted~

A coating composition was prepared from solvent solutions of the following ingredients:

Amount Component ~ Solids;~ -(a) Methylene chloride solvent433.7 (b) Methanol solvent 47,3 (c) Copolymer Or methylmetha~rylate ~9~7 40.17 (90 mole ~0) and methacrylic acid (lO Mole %), mol. wt.
range about 30,000-50,000 (d) Polymethyl~ethacrylate, high2.6 2.ll mol. wt" 0.9 inherent viscosity (e~ 2~2'-Bis~2-chlorophenyl)-4~4'~ 8.2 6.60 5,5~-tetraphenyl biimidazole (f) 4~4~-Bis(dimethylamino)benzop~enone 3.2 2~58 ~Michler~s ketone2 (g) Mixed ester of tr~ethylene glycol- 1~8 1.42 dicaproate and d~cacrylate~
refr~ct~ve index lol~60 at 25C.
~........
.

~ - 33 -...

.., , . , ., ... :

~7~565 Amount ~0 Components ~gramslSolids~
(h) Trime~hylol propane triacrylate 33~3 26.97 (i) 2-Mercaptobenzothiazole 1.2 0.96 5 (j~ Grasol Fast Orange 2RN, C.I. Sol- 23.7 ls.ls vent Orange 33 (a mixture o~ Sol-vent Red 30 9 0 . I. 27291 and di-cyclohexylamine salts of a bisazo-disulfo acid dye).
(*) in the coating composition After thorough mixing, a portion of the photo-;poly~erizable composition thus formed was coated an the sub layer of a U.007 inch thick (0.17~ mm.) polyethylene terephthalate clear film support, prepared as described in Example IV of assignee's Alles U.S. Patent 2~779,684, containing an insoluble resin sub layer on one side only.
After hot air dry;ng, the dry photopolymerizable layer was abou~ 0.0003 inch (0.0076 mm.) thick and had an optical density of 4 over the spectral ranye 300-SOO nm., as measured on a Cary recording spectrophotometer. The resulting product was laminated with a 0.0005 inch thick (0.0127 mm.) clear, biaxially oriented and heat-set polyethylene terephthalate film removable cover sheet to yield a composite element. This element was then image-wise exposed for 15 seconds, through the cover sheet slde, by a fluorescent, black light source rich ln ultraviolet radiation, in a standard (nuArc~) vacuum frame with a silver image ~ransparency bearing an opaque image simu-lating a printed circuit. The radiation source contained 9 closely-packed, 24-watt, high output bulbs (General ~ -Electric Corp., F24T 12-BL-HO); this source was placed - 30 3 inches (7.62 cm.) from the photopolymerizable element.
., :

. , . . ~ .

.

9~

After exposure the cover sheet was stripped o~`r an~ the ~~ -layer developed, by washing out the unexposed3 unpolymerized portions of the coating, using a predominantly aqueous solution Or a solvent f`or the unexposed material o~ the layer o~ the following composi~ion:
Grams Sodium carborlate monohydrate 35 Diethylene glycol monobutyl ether 22605 Distilled water 3~90 A~ter developlng with the above solution, rins~r~ with water~ and air drying, the resulting film contained an - orange, polymeric, ultraviolet absorbing image ~,rhich was transpaxent to visible light ~ut had an optical density of 4 over the spectral range 300 500 nmO This i~ged element, an exact replica o~ ~he silver image trans-parency original~ was useful as a target, ste~cil, or mask, for preparir~ printed circuits using the process and materials described in U. S. Patent 3,469,982.
A~other composite element made as previously described was given a 30 second exposure to the compact black light source above through a 150 line hal~tone screen hard dot target through the cover sheet side.
After removing the cover sheet, the unexposed areas were washed away by ir~mersing the element ~n the developer described abo~e at 73-7~F. (23C.) ~or 90 seconds, .:
immersing ~he element in water to stop development and then rubbing the element with a developer-laden pad to dot . etch and elimlnate residual une~posed m~ter-i.al. Transmission , . . .
: density o~ background areas is comparable to si:lver halide ~ilms (0005 using a blue filter)0 The process . ~
:' -35 - .
:. .
~' .

.

produced an in~ge on the f~lm support corn~rised o~`
~-- polymeric dots reduced in slze ~rom the size corresponaing to the hal~tone screen valuesc ~lternatively, ths element could have been developed in an apparatus ha~ing scrubbing members~
'~'he use o~ a weak ~eveloper ~mproves develop~r latitude3 iOeo~ the amount by which khe time o~' develop~
men~ can be varied withou-t signi~icant e~ect 5 Immersion in water quickly stops the action o~ the developer and minimizes the requiremen~ f`or a wide developer latitudeO
A~'ter immersion in ~rater the element can be rub~e~ with a wa~er-wet pad to remove material which, although not dissolved~ remains loose o~ t~le element.
Another composite element made as previously described was dot-etched by the Pollo~ing procedure. The etch~ng solution used was similar to developer with the addition of a sur~actant~ octyl phenoxy polyethoxy et'nanol (Triton~ X-l~O), and a thickener (Cellulose ~um). T~e l,est Sample was first immersed in etch-ng s~lution for lO seconds and taken out and allowed to stand (on horizonta] surface) for a variable a~nount o~ time, a~ter which it was i~mlersed in water and then rubbed with ~ developer laden pad. Total etch time vs, dot equivalency i~ given ~elo~
' ~ ;

.'~ '~ ' , _ 36 -~7~5~;5 ETC~I TI~ - DOT EQUI~ALENCY

Seconds O g2 ~4 ~0 63 Microdensitometer Measu~ements Measurements were ta~cen in 2 etched al~eas o~
the tonal range. The ~irst was a 90~ test area reduced ...
; to a 60~ dot equi~alency~ This area is chaxacteri~ed by mostl~ solid background with cir~cular clear holes dispersed ln a regular pat~ern. Etching produced laxger holes which remain cixcular. The other area, which is characteristic of the low tonal range up to 50~p dot ' equivalency~ is comprised of a regular pattern OI' dots.
`~ Results o~ microdensitometer scans are given below:
Original Dot ~quivalency 90 ~5 Element hole dot : Etched:Dot E~ui~alency 60 10 . Etch Time ~sec.) 100 60 ..
~. ~ Density loss in element 6~ 20~ ..
-1 This illustrates that the value o~ the tones can be drasti-cally altered ~lthout seriously a~'~'ecting top density~

',: ' '.'.

30 ~ . .
::

~ 37 ~
: ' ':

.

79~i~;5 150 (59.05) Line Halftones Reproduction ~ Dot equivalencies o~ the developed test sample - be~'ore etching in the 5-93~o region o~ the tonal range are shown below:
-5 . Original SllverPhotopolymer Halide Hard DotReproduction Tar~et ~ Dot* ~ Dot . ~3 ~1 *Since the photopolymer s~stem is negative working, these numbers represent the anti- ~-cipated values to be obtained ~rom this 2Q orlginal.
~hese data show that the extremes o~' ~he tone range (5 to 93~) were reproduced simultaneously.
Exposure Latitude :
The rollowing data compare exposure latitude of - 25 photopolymer test samples prepared and developed as above~
without etching vs. a silver halide contact litho masl~.
A test target was exposed at various exposure times and four target areas were measured to determine dot equivalency, 3~
-_ 38 _ . .

~ 5~ 5 Exposure Photopolymer Mask (seconds) ~ 29 9 34 1~8- 94 ~ 9 39 54 97 .

Silver Halide ~sk Dot Size ~Top density Too Low at This Exposure) ; 10 7 33 53~ 93 7 34 . 54 B 9 ~0 7 37 59 96 1~ 80 7 4 63 97 ` 160 9 ~ 72 99 , The~most critica~ area in both cases is on the , 50% dot size range where a 12~ vaxiation is noted lrom 10 40 seconds ~A ~ B)~ indicating equivalent exposure latitude ~or the photopolymer mask and the silver halide maskO It is there~ore demonstrated that the elements o~`
the in~ention, which contain no sllver halide~ have equivalent sensitometr~ Lor contact lltho mask applications to silver halide masks o~' the prior art.
~ EXA~E 2 :
.
:, This example shol~rs various applications oi' the elements o~' the invention as masks~ A coaking composi~
tion identical to that describe~ in ~he precedin~ example was prepared bu~ containing7 in addition, 4205 gO o~` Baslc `
Blue 7 (C.I. 42595). Using the elemenk preparatlon ' , : :
~3~ _ , ~.

3L~79~5 procedure~ exposure device, and development process detailed in ~xample 1, a photographic, silver hal~de negative test image was used as ~arget during exposureO
rl~e resultant black, polymeric, positive-i~age, having --a thickness of 0~0003 inch (o.0076 mm. ) and absorbing ; with optical density greatex than 3~0 in the range 300-500 nm., was used as a mask in exposing the ~ollowing light sensiti~e materials:
1G A photopolymer printing plate described in U. S. Patent 3,458,3Il was exposed through this mask f~or 45 seconds at 3~ inches (91.4 cm.) under a carbon axc. The line and hal~tone paGtern was de~eloped wîth the developer composition dlsclosed -Ln the abo~e re~erence and a suitable ~rinting plate was obtained.
2. A 'Itacky'l photopolymer system as described in U.S Patent3,649,268 was prepared and coated to a dry thick-ness o~ 00004 inch (0~01 mm.) on resin subbed polyethylene ;~ terephthalate and overcoated by laminating a 0.0075 inch thick (0,019 mm,) polyethylene terephthalake sheet on top.
Exposure was carried out wlth a xenon arc for 15 seconds at 18 inches (45.7 cm.). The image~ which is I`ormed by the hardening Or the tacky polymer in the area which was exposed, was developed a~er removal oL the polyethylene terephthalate cover sheet by dusting in the con~entional manner with a ~ry5 colored ~ye which adhered to those tacky portions. This material is described in Heiart~ U.S. Patent 3~307~943O ~he resultant clyed image as trans~erred to coated paper stock by lamination at 110C. and an excellent positive copy Or the photor~sk was thus securedO

. ' .

, ~t7~ 3~ 5 3. A moderately slow, ortho-sensitive silver halide film was exposed for 10 seconds with the photo-polymer mask using a General Electric 100-watt, 20-volt standard incandescent lamp at a distance of 53 inches (135 cm.) through a 0.6 neutral density filter and a No. 47 Wratten filter. The image was developed in a standard multiprocess type developer (metol-hydroquinone) followed by fixing in a conventional photographic fixer, washed and dried as known to those skilled in the art. A
very high resolution black and white image was obtained.

A black photopolymerizable layer, similar to Example l, was prepared containing the following ingredients:
Component (of Example l) Percent (by weight) Binder (c) 38.20 Photoinitiator (e) 5.92 Photoinitiator (f) 2.34 -~
Plasticizer (g) 1.30 Monomer (h) 24.50 ~;
Chain transfer agent (i) 0.87 Orange dye (j) 13.10 Luxol Fast Black L 13.8 (C o I ~ Solvent Black 17) The above materials were dissolved in methylene chloride/
. . ~ .
2-ethoxyethanol (7:3 ratio by volume) to yield a coating solution containing 20% solids. This solution was coated on a polymeric film support at a coating weight of 95-100 mg/dm2 to yield a dried, photopolymerizable layer with a :..
thickness of 0.0004 inch (0.01 mm.) and an optical density ;~ 30 greater than 3 0.

The 0.004 inch (0.101 mm~) thick polyethylene .1 , .

~:, '. . .

~ 795~5 terephthalake ~ilm support, in this Example, had a 0.0002 inch (0.0051 mm.) thick soluble substratum which was a modification of that disclosed in Rawlins, U~S. Patent - 3,443,950. This modification conslsted essentially of -;~
adding two acidic terpolymers in a weight ratio of approximately 1.2:8.01:1, (Rawlins sub:~irst polymer:second polymer) wherein the rirst polymer is ~'ormed I'rom ethyl acrylate ~56~), methyl methacrylate (37~0) and acrylic acid (7%)~ and the second polymer is f;ormed ~rom 665 methyl methacrylate, 29~ ethyl acrylate~ and 55' meth~
acryllc acid, to the substratum described in th~ above cited patent.
Af'ter completing element preparation, by laminating the cover sheet of' Example 1 f~ollowing the process of Example 1, the element was exposed through hard and soft dot targets. Exposure was carried out f'or 30 seconds with a 2500 watt, pulsed xenon nuArc~
(Model FT26M-2) radiation source at a distance o~' ~3.2 cm. Then, after removing the cover sheet, the black ~mage was developed~ The developer of Example 1 was ~ound to perform well with a 30 second im~ersion at ~0F. (27C.)~ An automatlc processor is used which haæ a development and a wash stop stage~ A~`ter being submerged in a developer section~ the film is trans-ported through a water spray which not only removesdeveloped matrix but dilutos residual developer and stops its action; the element is then dried. I~mersion step takes 30 seconds and second stage takes an equal amount o~' timeO Good hard and sort dot reproduction ' 30 wa~ obtained. The mask lmage could be dot-etched if' desired~
' .

- ~.

~ 7~5~:;5 A use~ul, black, solely aqueous alkaline develop-able, photopolymer~zable composition was prepared contain ing the following ingredients:
Component ~
~a) Solvent (a) o~ Example 1 720 ~ .
(b) Solvent ~b) o~ Example 1 80 (c) Terpolymer ~ormed from 56~ ethyl acrylate~ 15~2 37~ methyl methacrylate and 7~ acrylic acid, MolO Wt~ ca 260~000~ acid number 76-85.
(d) 1:1 copolymer o~ styrene and maleic anhy- 25.1 dride, partially esterified with isoprop~l : alcohol, mol. wt. ca. 1700, acid number ca. 270 :
(e) Photoinitiator (e) o~ Example 1 5.3 (~) Photoinitiator ~f) o~ Example 1 2.1 (g) Chain transfer agent (i) of` Example 1 o.8 (h) Triethylene glycol dimethacrylate 12.0 . ~ ~
(i) Dye (j~ ol` Example 1 20.0 :
(~) Blue dye of Example 2 (C.I. 42595) 4~0 (k~ Nonionic fluorocarbon sur~actant FC_430, 0.3 Minn. Mining and Mfg~ Corp., 10~ solution in C~2cl2 This coating solution was coated by the use of a so-called "doctori' kni~e3 to y~eld a dry 0 000~ inch (o.oo76 mm~) thick layer having an optical density greater than 3~0 in the 300-500 mm. region, onto the substrate and subbing layer described in Example 3e ~lement preparation was comp~eted by laminationa at 75C.3 as described in Example 1. The finished prod~ct was given a 1 minute exposure, through a hal~-tone target~ using an xenon lamp ~nuArc~
"Flip-Top" P1ate Maker Model ~T26M-2). Development (15 seconds) with a 4~ aqueous sodium carbonate solution at ~ ~ .

.

5~5 80F~ (27C.) gave an exact posltive copy of the original which was useful for preparlng other copies.

:
- m e following composition was made ~or use . ~S~
with a neat organie sol~ent as developer:

(a) Methylene chloride 1000 ~b) 2-Ethoxyethanol 100 (c) Poly(methyl methacrylate) 70 mol. wt. ca. 30,000 (d) Poly~methyl methacrylate~ 40 molO wt. ca. 100,000 (e) Photoinitiator (e) of Example 1 15~
(f) ~lotoinitia~or (f) of Example 1 6.3 ~g) Triethylene glycol diacetate 17 (h) Monomer (H) of Example 1 70 (i) Dye (j) of Example 1 37.2 (~) Tris(~-diethylam~no-o-tolyl) methane 3~1 -(k) 4~4',4"-Mekhylidynetris (N~N-dlmethyl-o.63 ` 20 anillne)~ Leuco Cr~stal Vio~et ; The above composition was coated, usin~ a o.ool~ inch ~;
~0.101 mmO) doctor knife, to give a dried layer 0.0005 inch thick (0.013 mm.) having an optical density greater than 3.0 on the substrate described in Example 1~ The co~er sheet described in Example 1 was laminated (at 100C.~ over the photosensitive layer to f`orm the ele~ent~ The element was exposed, through a transparency, Ior 2 mlnutes using the black light device o~`-Example 1~ After delamination, a : black image was obtained by de~eloping in a spra~ o~ methyl . .
chlororoxm (for 9 seconds), followed by a water rinse and - drying, ~he resulting f~lm was sultable for use as a mask.

, . . , '.
~ . , ~' .

The following formulation, related to Example 5, .demonstrates a dif~'erent initiator system and preparing a photomask on glass~
ngredient ~ ~ Solids Methanol 160 Solvent ~a) o~ Example 5 6928 Binder (c) of Example 5 35106 22.60 Binder (d~ o~ Example 5 233.3 14.99 Copolymer o~ Methyl methacxylate and 58.7 3077 gamma methacryloxypropyltrime~hoxy silane~ see UOS.P. 3,758,306 ' Photoinitiator (Is) o~` Example 5 2.42 0'.0156 ~enzophenone 11~2 0.72 Plasticizer (g) of Example 5 98.5 6.33 Monomer (h) of Example 5 ~10 26.35 : Dye (1) of' Example 5 389 25~0 This solution conta'ined abou~ 13.7~ solids and was doctor knif'e coated on 0.001 inch thick ~0.025 ~m.) clear, biaxially orien~ed and heat.set polyethylene . terepht~alate f`ilm. Arter drying, the ~ilm was laminated with 0.001 inch thick (00025 mm.) polyethylene to protect the resulting W -absorbing layer which had a dxied thickness of about .0003 inch (oO0076 mm~).
, 25 This W -absorbing layer having an optical density ''`' greater than 3.0 in the 300-500 nm~ region ~JaS nexk trans~
.l f`erred to a 3" x 4" (7.6 x 10.2 cmO) pxo~ector sllde cover ~. :
~, glass, which had been thoroughly cleaned by scrubbing with : ;:
' solventsO Transf'er was accomplished by removing the poly~
. I ~ , . . .
~ 30 ethylene cover shee~.and laminating the uv layer to the '';

~; _ 45 , 3S6~

glass plate through heated rollers at 100-120Co ~t approx-imately 183 cm~/~in, The photopolymerizable layer was th~n given a 10 - second exposure at 40.6 cm. with a 1~00 watt-quartz iodide lamp through a negative containing a standard ~est pattern comprised o~ simulated electronic printed circuitry. The polyethylene terephthalate sheet was removed by strlpping and the resulting image developed by spraying 10 seconds with methyl chlorororm, rollowed by a water spray, and the resulting orange image dried by blowing with com- -; pressed air. The solvent used herein dissolved the unhardened areas o~ the photoresist, leaving a suitable, positive mask on a rigid glass support useful ~or imaging photoresists with light sensitivity o~` between 300-500 nm.
EXAMPL~ 7 An orange photopolymerizable composition, similar . to Example ~, but containing an oxygen impermeable cover : layer, was prepared containing the ~ollowing ingredients: :
Component of Exa~ Solids -Solvent (a) 363.0 Solvent (b) Bindex (c) 15~8 17.96 Binder (d) 26.2 29.69 Photoinitiator (e) 505 6.21 .
Photoini~iator (r) 2.2 2.46 I Chain transIer agent (g) o.83 o.g4 ; Monomex (h) 12.6 14.30 Dye (i) 25.0 28.36 ~ 30 Surfactant (k) 0 o7 o ~ o8 ~ -~ ::
.

,.. .

9~6~

The above coating solution was coated on the substrate described in Example 3, to give a dried coating weiyht of 80-90 mg./dm.2 with a thickness of about 3.0003 inch (0.0076 mrn.). An~oxygen barrier layer, which had previously been coated on the cover sheet described in Example 1, was then laminated oYer the above photosensitiYe layer. The barrier composition, dry coating weight 6-10 mg./dm.2, was a modification of an aqueous-alcoholic solution of poly-vinyl alcohol (medium viscosity, 99,~ saponified) contain-ing 2% of a polyoxyethylene surfactant as dîsclosed in U.S. Patent 3,458,311, Example 1. The modîfication com-prises adding to the polyvinyl alcohol, in a ratio of 5 1~ a copolymer of vinylpyrrolidone and vinyl acetate ~89/11); the viscosity of a 15~ ethanol solution of this copolymer ranges between 38-44 centistokes at 75F. (24C~
In addi~ion, to insure good wettability, the barrier layer contains a small amount of a surfactant of the following st~ucture:
' CH2- CH3 ,0 -' ~C8F17 - S02 - N - CH2 - C - 0~) K~
` 20 After removing the cover sheet, the removable oxygen-barrier layer remains on the photosensitive layer.
The element ;s then exposed, through a process transparency, ~`
by the black light device of Example 1 for 30 seconds. An orange image of the original suitable for use as a photo-mask, was obtained on developing for 80 seconds at 68-70F.
(20Co~ ~ by immersion in a developer composition containing 2.33% sodium bicarbonate and 0.~7% sodium carbonate in distilled water, followed by washing with water, scrubbing .
.
: :.

,, ' . .

~ S 6 S

with a water~laden pad~ then drying~ The developer remoYes the oxygen-barrier layer and then the unexposed areas o~`
the photopolymerizable layerO The following data show that as development is continued ~or longer periQds of time, etch . .
ing of ~he line i~age occurs:
DEVELOP~NT vs~ LINE WIDTH
Xmmerslon Time Line Width .

124 ~underdev.) Origlnal line target = 124 microns clear space.

:
This example demonstrates the necessity o~ hlgh optical densities in order to achieve dot etchabillty.
Four photopolymerizable compositions were prepared as des- :
- cribed in Example 7. Composition A is identical w:ith Example 7; Compositions B~D vary as ~ollows:
Composition Variations ~grams) , ; Component A B C D
~ . . .
CH Cl - 363 340~8 318 279.3 CH30H 41 37.9 35.3 31 Michler's Ketone 2~2 1.65 1.1 0.55 Dye 25.0 2000 15.0 6025 Optical Density 4 - 3 2 300-500 ~m A~`ter exposure, as in ~xample 7, to a target containing 125 ~icron lines, the decrease in line widths with increasing de~elopment was measured, using the '~ .

_ 48 _ :

- ~7~565 de~eloper and process of Example 7. ~he following data -- were obtained.
Development Time Line W~dths (~1) ~se~ A B C D
_ - 5 40 118 120 1~7 120 112 114 ~12 120 109 ~ 117 120 10 The results indicate that composition A ~E~o 7), with an O.D. OI' 4 over 300-500 nmO, is dot (line~ etchable in a regular pattern~ Composition B (O~D. o~ 3)~ is also dot etchable. but less readll~ so. Composition C~ (O.D. o~ 2) gave somewhat erratic results, but ls obviously substan-tiall~ not dot etchable. Compositîon D, (O.D7 o~ 1) is clearly stabilized, presumably polymerized, down to the base~ and thus not etchable (undercut) at all.
EXAMPLE g Part A
.
m is example illustrates a photopolymerizable~
aqueously developable, contact-speed~ lithographic system containing a pigment. The follo~ring coating disperslon, 10~ so}ids in methylene chlorîde~ was prepared:
(by weight) ~a) Binder (c) o~ Example 4 16~4 (b) Binder (d) o~ Example 4 30~.3 (c) Photoinitiator ~e) o~` Example 4 5.8 '!1 (d) Photoinitiator (f) of Example 4 2~3 ~i~ 30 (e~ Surfactant ~k)~or Example ~ O.o9 (f) Monomex (h) of Exampl~ 1 27~2 ;' ~ 49 ~ -.

;5 ~ (by weight) of solids (g) Plasticizer ~g) of Example l l.7 (h) Colloidal carbon 16.4 In preparing the above coating formulation, components (b) through (g) are dissolved directly in the solvent.
~ Components (a) and (h) are mixed separately in CH2Cl2, ; ` sand milled to obtain a small particle size, then the other components are added to this dispersion. Many carbon blacks can be used; ~urnace black, particle size ca. 75 nm. was used in this example. The well mixed ooating composition was coated on the substrate of Example 3 to yield a layer with a dry thickness of 0.0002 inch (0.005 mm.) having an optical density of greater than 3.0 in the 300-500 nm. reglon, and laminated with the cover sheet of Example l. The element was ~hen ;~:
exposed, through a halftone target, by a BRH lamp (lO00 watt, tungsten-iodide), for 30 seconds, the lamp was at a distance of l meter from the photopolymerizable layer.
After removal of the coYer sheet, a black image was - obtained by developing the film for 45 seconds at room temperature with an aqueous developer oontaining 2% each of Na2C03 and NaHC03, followed by a water rinse and drying.
Further, the positive halftone dots obtained were dot etched by swabbing the film in the same developer for 30 seconds.
Thus, this film provldes a suitable lithographic-type element for preparing further copies.
Part B
Elements, similar to that described in Part Ag 30 were used to illustrate dot etching as performed in the -~
printing trade. A positive image master (mask~ was made ~ ~ ,~.. ....
. .- . ' .

~L~7~S6~i .
w~th the elements ~'or each o~' the f~our standard colors (magenta, cyan, yellow and blac~) normally used to prepare plates rOr printing a color picture~ These elements were exposed using the device in Example 9 and d~veloped in an ~ -automatic processor described in Example 3 with the developer solution described in ~xamples 10-12 ~o provide ~our dot-etchable masks. A ~ull color proo~' on color proofing rilm (as described in U~. Patent 3,649,2~8) was made ~xom these mas~s to simulate a press print. On analyzing the proof' it was observed that there was too much yellow in all parts Or the picture and tha~ one area ; appeared "too redl'. To correct these problems the yellow positive image should be "~lat etched" (all the dots in the picture reduced a small percentage in size) and the ~agenta positive image shou~d be "locally etched" (the dots in the too-red area reduced a substantlal percentage in size)~
To accompl~sh this~ copies were made o~' the ~ellow and ma.genta masters using the same exposure and development techniques (the positive image masters were cupied to ne~ative image intermediates which were then copled to positive image duplicates o~' the ori~inals).
The resulting yellow duplicate was then rerun through the processor with twice the normal development timeO
~ 25 This served to dot etch all the dots on the imageO
;, Examples of' the amounk o~ reduction in the size (coverage) of various dots (150 line screen) are:

.
. .

, ~ , . : , ~ ~ ~9 ~ 6 ~

Dot size ln Dot size in Or~. Mask Etched Mask _ 98.3% 92.1%
55.4 35 11 2.3 Measurements o~ the change in sur~`ace area o~ the dots under magnification were compared to the change in integrated - density for a gi~en area. These measurements con~'irmed that the change in integrated dens~ty was totally a result of the change in dot area and not a Loss o~ density within the dot.
The magenta positive image duplicate was then locally etchedO The area to be etched was visually ascertained, a~d the rest ol the image was protected by painting with a "staging solution'i containing:
CC12FCClF2 240 g.
. . ~ .
poly(n-butyl methacrylate) 15 g-Inherent viscosity = 0~53 in a solution containing 0~25 g.
polyrner in 50 ml~ CEC13 ~easured at 20~C. using a No~ 50 Cannon~Fenske viscometer.
Tris~ diethylamino-o-tolyl)methane 0.5 g.
C.I. Solvent Blue 36 0.3 g.
When the stage had dried, the designated area was etched by imrnersing the whole ~iIm in a solution o~ 655~ Butyl Carbitol~, 17~5~' ethylene ~lycol and 17.5,C~ water, by volume, for 10-20 y seconds and then rinsed with an 1mpact w~ter spray. The dot s$ze ~ras measured and ~he process ~ras repeated until ~he proper sized dot was achieved. In this instanc~, the reduc- -tion was ~rom l~o~ dots to 10~ dots and again there was no loss in density within the dot when tes~ed by comparing . , .
., .

~ 52 ~

~7956S

, integrated densit~T and dot siz~. The ~ilm was dried by - hot air and the stage was removed by care~'ully wiping with solvent (CC12FCClF23 and then drled.
Another ~ull color proo~' was ~ade~frorn the master cyan and black masks and the etched yello~J and rnagen~a masks, which conrirmed that the required color shi~t had been accomplishedO Thus~ these masks ma~ be employed for exposing photopolymerizable printing plates ~rhich~ in turn, would give a I'ull color press print ~0 with the correct color balance~
~A.~ - 2 The ~ollowing examples are similar to Example 9, but illustrate the use of multiple monomers and alternative photoinitiators. m e ~ollol~ing photopolymerizable coating compositions, about 15,~ solids in CH2C12, ~Jere prepared:
. Amount (g) Component Ex~ 10 ~x~ 11 Ex~ 12 , ~ . . .
(a) Binder (c~ of Example 4 20.0 10~8 10.8 (b) Binder (d) of Example 4 38.7 19 19 .
(c) Photoinitiator (e) ol~ ample 4 11.4 (d) Photoinitiator (f) of Example ~ 2~8 __ _ (e) Surfactant (k) o~ E~amp}e 40.5 1.0 1.0 (f) Monomer (h) o~` Example 1 2.3 2.0 2~0 (g) Colloidal carbon 1~.4 7.2 7.2 (h) ~nomer (h) o~' Example 4 22.1 9.8 9.8 (i) Phenanthrenequinone : __ 1.5 --(;) Benzoin methyl ether ~ 1.5 As in Example 9, the carbon and binder (a) were . . .
rinely dispersed b~ mil1in~, and the other components added to this disperslon. rl~en~ the well-mixed coating compositions ,'' . . . . .
-- 5 3 ~
'.
.
-- . .. . . . . . . . . ...
: - . ~

s~6s were coated separatel~ on the substrate o~` Example 3~ and lam~nated with the cover sheet of Example 1. The dried -coating thickness of Example 10 was ca~ o.ool~ mm., and o~
Examples 11-12 caO o~oo76 mm. The Example 10 i`ilm ~ad a visible opkical density of 300, and 4.0 in the region 300-500 mm~; the ~ilms o~` Examples 11-12 had optical densities ~ 4Do in both the visible and W regions of the spectrum.
The Example 10 ~'ilm was exposed i~or 20 seconds as in Example 9, but with the radiation source 152a5 cm.
~rom the sampleO m e ~ilms o~ Examples 11-12 were exposed f~or 10 minutes to a xenon lamp using tl~ device described in Exarnple ~O Then, upon removal o~ the co~er sheet, the -; ~ilm of ~xample 10 was developed in the automatic processor ~ Example 3, 5 seconds, 27C., using as developer a solu-tion o~ ~aHC03 (1~o6 g.) and Na2C03-~20 (27-4 g~) in IJ~ter (1 liter), ~'ollo~ed by a water spray rinse and drying.
The resulting ~ilm image ~as an accurate reproduction OI' the hal~'tone target; the "hard'1 dots exhibited sharp edges, and wexe dot etchableO
The ~'ilms ol Examples 11-12 t~ere developed in a tray (15 seconds, 24C~) in an aqueous solution containing Na2C03 (2~) and NaHC03 (l~o~ . A~ter a water rinse and dryin~, sections o~' the imaged ~'ilms were selectively dot etched producing a dot size reduction o~ approximately 5 10~
using an aqueous solution containing Na2C03 (0.450) and NaHC03 (O.~,f ) EX~IELES 13 , The ~ollowing two examples~ also similar to Example 9, illustrate alternative binders useful in the . . .. .:
~ 54 -': ' . .

r~~

invention. Coat;ng compositions containing the following components, in CH2C12/2-ethoxy-ethanol, were prepared:
Amount (% of Solids) Component Ex. 13 Ex.~14 5 (a) Binder (c) of Example 4 46.7 (b) Photoinitiator (e) of Example 4 7.5 7.5 (c) Photoinitiator (f) of Example 4 2.1 2.5 (d) Surfactant (k) of Example 4 0.1 0.1 (e) Monomer (h) of Example 4 20.0 2~.0 (f) Colloidal carbon 18.0 17.6 (g) Copolymer of methyl me~hacrylate 5.2 --(90%) and methacrylic acid (10%), mol. wt. ca. 50,000 15 (h) High molecular weight tetrapolymer -- 46.5 from methyl methasrylate (30%), butyl acrylate (25%), acrylonitrile (30~) and methacrylic acid (15%) -~ Following the milling, mixing, coating and lamination pro-cedures and using the exposure device O~ Example 9, the two films were exposed for 15 and 10 seconds, respectively.
The film of Example 13 had a thickness of 0.00025 inch (0.0063 mm.) and O.D. ~ 3, and that of Example 14 0.0002 inch (.0051 mm.) and O.D. ~ 3. Follow;ng development as described in Example 10 ~pH = 10.3, 29~C., 10-15 seconds), the dried film hal~ tone images were dot etched. The etch-ing solution was identical to the developer, but was diluted with 3 parts of water. The etching procedure involved protecting portions of the film with tape, then gently swabbing the unprotected area with a cotton pad soaked in etching solution (room temperature~, followed by a water rinse and drying. Of course, this etching procedure could be repea~ed until the desired effect is -.~
'' , ' . ,~
.,~,' ., .
- 55 - ~

s~s achieved~ As the :L`ollolring data indicate, both f'llms are dot etchable with no loss in top d~nsity (i.e~ the density of the dots~ e drop in the overall (~isible) density of the elem~nts indicates-dot-etchin~ (i.ei~reduction in size o~ the dots of the image).
p~ical Densit~ (O~D. Units) O~erall Density~
~ _ 50~ Dots Ex~ Be~'ore A~texBefore .A~ter No~ Etchin~ Etchl~Etchin~ tchin~
}3 3.5 3~5 0~27 0,21 14 3.0 3.0 0.30 0~18 The ~ollo~ing two examples, yielding use~ul orange photomasks similar to Example 4, illustrate the use o~ alte~native dyes. The i~ollowing coating composi tions were prepared:

Component OI` Example 4Ex. 15 Ex, 16 Solvent (a) 81 81 .
Solvent (b) 9 9 ; Binder (c) 1.58 1~58 Binder (d) 2~62 2~62 Photoinitiator (e) o~55 o~55 . 25 Photoinitiator (~) 0.22 0.22 Chaln transfer agent (g) oOo8 o~o8 Monomer (h) 1.26 1.26 Sur~`actant ~k) 0.07 0~07 Dye A* 0~9 0 9 Dye B~
Dye C* 1.75 l~'f5 Dye I:* ~- 005 ' : `
56 ~

.
- - , .: ~

~ ~ 7 9 5 ~ ~

* Dyes A-D are all solvent soluble and axe dLcyclohexylamine salts Or acid dyes. Dye A is Solvent Red 30, C~I. 27291, Dye C is the salt OI' Tartrazine~ C~I. 19140. Dye ~ is - - the salt o~ C.I~ 19135. Dye B is the salt of the acid - -~- dye ~ormed on diazo'Gizing and coupling 2-amino-5-chloro-benzenesulI`onic acid to 3-methyl~ 2,5-dichloro~ sulto-phenyl)-pyrazolone~
m e above coating solutions ~rere coated~ laminated and exposed as in Example ~. Dry coatin~ ~Jeights were 118 125 mg,/dm.2, ca. 0.0004 inch (0~010 mm.) thlck; both were exposed 3 minutes~ Development, ~ollowin~ removR1 o~ the cover sheet~ emplo~ed an aqueous solution contain-ing 2~ sodiu~ bicarbonate and o.675~ sodium carbonate, 1or 2 minutes, at 21C. A~ter rinsing ~Jith water and drying, the resulting orange image had an O.D. ~ 4 in the region 300-~0~ nm~
EX~PLE 17 Carbon-pigmented photopolymerizable layers are oxygen-sensitive and must be protected from oxygen o~ the air during exposure~ This is rrequently accomplished by ;~ use o~ a thin removable ~`ilm, which is laminated ~o the sur~ace of the photopolymer layer. It can also be accom-plished by overcoatin~ the photopolymer layer wlth a solution o~ an oxygen baxrier polymer, which adheres to the photopolymer layer (a~ter evapora~on o-~ its solvent), gives good oxygen pro~ection, and is removable in the developer solvent. An example o~ such an overcoat solu-- ~ion is given belo~
~ ';

,.

.
.:

, . ~ - : , . , :: :: ~ , - -s Water 12~0 g.
Polyvinyl alcohol (98-99~ saponi~led, 200 g.
lo~ viscoslty) Copolymer of vinyl pyrrolidone/vinyl 24~4 g, 5 acetate (60/40, medium mol. wt~) Ethyl CELLOSOLVE* 24.6 g.
Pol~oxyethylene sur~ac~nt o~ the formula C8Hl7_ ~ _ 0(CH2C~2)9~l 2.3 g. . .

Denatured alcohol 17.3 For use in coating a composition similar to Example 9, Part A, a ~olution containlng 3~5~ solld~ was prepared as follows:
Water 683 g.
Overcoat solutlon above 210 g.
15 Surfactant above (10~ aqueous solution) 504 ml.
Blue Pigment (Inmont Blue 3G) 1.8 g, Thi~ ~olution was coated on ~lements similar to that of Example 9, Part A but without a cover ~ilm~ using an extrusion die; the coated element was dried at 200F.
(93.3C.). In three coating~, drie~ coating weights of 9.2, 15.0, and 2401 mg./dm.2 were obtain~d. These three films were then compare~ to the control, a compo-sition similar to Example 9~ Part A ~with a protective cover film). The two overcoated elements with lower 25 coatlng weights exhibited lower photospeed than the ~
control; fu~thermore, these two were rapidly a~fected - :
by the atmosphere (th~ overcoat lost its gloss and i~
cracks appeared~. The element with the highest overcoat weight, however, exhib~ted good stability and with both photospe~d and dot range equiYalent to the control.
, * denotes tr~de mark ~ .

56~i 500 g. of polyvinyl alcohol (98~98.8~ saponified, low viscosity) were added to 5000 g. of distilled water and heated at 85C for 2 hours. lOD g. of the solution was mixed with 262 g. distilled water, 18 g. polyoxyethylene sur-factant (10~ aqueous solutionj described in Example 17, 10 g.
ethyl cellosolve, and 10 g. ethyl alcohol. To 100 g. of the above mixture was added 2.7 g. of a 30% colloidal silica dispersion containing particle sizes in the xange of 1~ to lS millimicrons and 30 g. of distilled water. This resulted in an overcoat ormulation in grams as follows:
polyvinyl alcohol 2.25 distilled water 122.50 polyoxyethylene surfact- 0.45 ant ethyl cellosolve 2.50 ethyl alcohol 2.50 colloidal silica 2.70 A photopolymer composition as described in Ex-.
ample 10 was used to prepare a photopolymerizable element ` 20 also described in Example 10.
The cover sheet was removed. Using a 2-mil (0.05 mm) doctor knife, the above overcoat solution was coated directly on the photopolymerizable surface and allowed ~o dry. The coatin~ weight of the overcoat was l~.Q mg~dm2.
The overcoated element was exposed and developed as described in Example 10. It was found that the over-coated element and an element having a cover sheet as in ~xample 10 had substantially the same photospeed Iwithin one~ ~ step).
V

, :
~ 59 ~
~ .
-~ : , .... .

7~S~S

The following tests were conducted to de~ermine - retention of the overcoat:
A. GRAVIMETRIC
A sample of the overcoated film was given an overall imaging exposure (10 seconds). It was found that only 9.0 mg/dm of overcoat could be dissolved off this sample (as opposed to 10.0 mg/dm2 for unexposed film.) This in-dicates that 10~ of the overcoat remains in the exposed areas.
B. X-RA~ FLUORESCENCE
The above gravimetric data are in good agreement with x-ray fluorescence analysis, which indicatés that ex~
posed and processed image areas retain 12% of the silica initially present in the raw-~tock.
; C. SCANNING ELECTRON MICROSCOPY (SEM) Additional evidence for overcoat material remain ing on exposed and processed film is given by scanning - electron microscopy. ~M studies of ilms overcoated with polyvinyl alcohol based overcoats that contain~d 1-4% of

4 micron silica particles show that significant numbers of these 2n particles remain on exposed and processed image surfaces.
EXA~PLE 19 Into a 3-gallon (11.36 liter) emulsion can is placed 4500 g. of distilled water and 100 g. of decationized gelatin, and the mixture is allowed $o soak for 15 minutes ~5 at room temperature followed by heating to 125 to 130F
(52 to 5S C~ for 30 minutes. The mixture is cooled to 95 to 100F (35 to 39C). The following ingredients are , added-' :
- 60 ~
. .
' 56~

50g. ethanol 50g. ethyl cellosolve - .
50 ml. of a 10% aqueous solution of the $urfactant described in Example 17 75 g. methylmethacrylate (66%)/ethyl acrylate (29%);
acrylic acid (5~) (30% solids in water), 600 ml. mucochloric acid, 2% aqueous solution The above solution is coated on the sur~ace of .~ photopolymerizable layer of a photopolymerizable elemcnt described in Example 10 to a dry coating weight of 10 mg/dm2.
The gelatin layer is dried at 230F (110C). :
Two samples of the coating, one square decimeter, are exposed, front and kack sides for 1 minute each as described in Example 10, and the weights of the samples are . 15 recorded with the following results: . .
Sample No. Weight (g?
1 . 1.4347 : :~
2 1.4393 Samples~l and 2 are processed as described in Example 10.
Ater drying, the two samples are reweighed with the fol-,, lowing results, Sample No. 1 2 . .
. initial weight 1.4347 1.4393 weight after 1.4339 1.4386 :
processing weight Loss 0.0008 0.0007 . -` 3n -' . ; ~' 1~1179S6S

The average wei~ht loss is 0.75 mg/dm2. This example illustrates that approximately 92.5~ by-weight hardened gelatin remains on the photopolymeriz~d surface of. a photo-pol~merizable element, The unexposed unpolymerized photo- -polymerizable layer and its gelatin overcoat are completelyremove~ during processing.

This example illustrates preparing color-coded photomasks of this invention useful both as an alternative proofing system and for preparing printing plates for full color printing.
Three coating solutions were prepared, each ~on-taining the fol~owing ingredients:
Component of Ex. 4 Amount g _.
Solvent (a) 81 Solvent (b) 4 Photoinitiator (e) 1.0 Photoinitiator (f) 0.1 Binder (c) 4.1 Binder (d) 6.6 Mono~er (h)` 3.2 Surfactant (k) 0.04 Ultraviolet ~sorber 1.0 (2,2'-dihydroxy-4-methoxybenzo-phenone) 25 ~o each individual solution was also added one of the fol-lowing dyes: _ ~ Amount g I
Grasol~ Fast Brill.Red RL 1.0 (CI Solvent Red 86j Irgacet~ Yellow 2RL 1.0 ~CI Solvent Yellow 91) Irgacet~ ErilI~ Blue 2GLN 1.0 (C~ So~vent Blue 48) .

, - 6~ -.. '' ' :.

6~ii Each of the above three solutions was then coated, via a "doctor" knife, onto the substrate arld ~ubbing la~er deæcribed in Example 3. Ai:~ter hot air dry~ng9 the dried coating weights were approxima~ely 150 mg/dm2, ~he dried coating thicknesses approxlmately 0.0004 inch (0.01 mm).
Each photopolymeri~able layer was then overcoated with a thin oxygen barrler polymer, sub~tantially as described in Example 18. ~rhe re~ulti~g three elements of the inventlon all ab60rbed strongly throughout the 300-400 nm 6pectrhl 10 range; the opkical ~en~ities var~ed rrom 3.0-3.2. The maximu~ absorption in the vlslble portion c~f the spec~rum occurred at 675 and 630 nm ;e`or the cyax~ ~ilm, at 560 and 450 nm ~or the magenta ~ , and at 470 nm ~or the yellow - film.
Each colored element was then exposed as de~cribe~
in Example 4, but with a 90-seco~d exposure to the hal~tone color separation negative correspondiI~g to its color~ i.e., magenta, yellow and cyan~ Devels)pmeIIt (15 seconds) wlth an aqueous solutlon (p~I 10.~) ;o~ po~a6sium carbonate (105% ) and 20 potassium bicarbonate (1.5~ at 29C, followe~ by rinslng in warm water (38C), and drying~9 gave a ~aith~ul image of the hal~tone target~, one in m~genta, one in yellow, and one in cyan.
q~he cyan film was dot etched by soaking the imaged film in the developer described above (30 se¢onds, 29C) IOl- ;
lowed by a ~pray ~ith warm ~ater at 38C. A~ter drying, measurement showed a dot size reduction of a 50% dot to a 45% dot, wlthout an;sr loss o~ dens~ty ~ithin a dot~ In ~ similar manner, the yellow and mager~ta films c~n be do~
30 etched.

3 ~-f !~

Claims (53)

1. In a photopolymerizable element comprising a support bearing a photopolymerizable layer which is the outermost layer of the element or is contiguous to a removable cover sheet or an overcoat layer which is at least partially soluble in or permeable to a solvent for the photopolymerizable layer, the improvement comprising that the photopolymerizable layer has an optical density in the actinic region of at least 3.0, and is no more than 0.015 mm thick.

2. An element according to Claim 1 wherein the layer is no more than 0.010 mm thick and has an optical density or at least 4.0 in the 350 to 400 nm region.

3. An element according to Claim 1 wherein the photopolymerizable layer is contiguous to a re-movable cover sheet or overcoat layer.

4. An element according to Claim 1 wherein the support comprises a sheet bearing a nonphotosensitive sub layer between the sheet and the photopolymerizable layer, the sub layer being soluble in a developer solvent for the photopolymerizable layer, the combined thickness of the sub layer and photopolymerizable layer being no more than 0.015 mm.

5. An element according to Claim 3 wherein the support is a polymeric film.

6. An element according to Claim 5 wherein the photopolymerizable layer has a thickness of no more than 0.010 mm.

7. An element according to Claim 6 wherein the polymeric film support has a subbing layer between the polymeric film and the photopolymerizable layer.

8. An element according to Claim 1 wherein the photopolymerizable layer is comprised of a photoiitiator, an actinic radiation absorber, a polymerizable monomer, and polymeric binder.

9. An element according to Claim 7 wherein the sub layer is soluble in a developer solvent for the photopolymer-izable layer, and the combined thickness of the sub layer and the photopolymerizable layer is no more than 0. 015 mm.

10. An element according to Claim 9 wherein the photopolymerizable layer has an optical density of at least 4.0 throughout the 350 to 400 nm region,

11. An improved photopolymerizable element comprising a support bearing a photopolymerizable layer comprised of a photoinitiator comprising an organic free-radical generating system activatable by actinic rediation present in an amount of 0.1 to 20% by weight of the photopolymerizable layer, a polymer-izable monomer present in an amount of 7.5 to 35% by weight of the photopolymerizable layer, and a polymeric binder present in an amount of 10 to 60% by weight of the photopolymerizable layer, the photopolymerizable layer being the outermost layer of the element or being contiguous to a removable cover sheet or an overcoat layer which is at least partially soluble in or permeable to a solvent for the photopolymerizable layer, the improvement comprising that the photopolymerizable layer has an optical density in the actinic region of at least 3.0 and is no more than 0.015 mm thick.

12. An element according to Claim 11 wherein the photopolymerizable layer is developable in predominantly aqueous developer solvents.

13. An element according to Claim 1 wherein the overcoat layer is transparent to actinic radiation.

14. An element according to Claim 1 wherein the polymeric film support is transparent to actinic radiation.

15. An element according to Claim 14 wherein the polymeric binder comprises at least one vinyl addition polymer containing free carboxylic acid groups.

16. An element according to Claim 1 which is capable of having an optical density in the 350-400 nm region in the nonimage areas after development not exceeding 0.3.

17. An element according to Claim 1 containing a dispersed pigment in the photopolymerizable layer.

18. An element according to Claim 11 containing a dispersed pigment and being capable of having an optical density is the 350-400 nm region in the nonimage areas after development not exceeding 0.3,

19. An element according to Claim 18 wherein the dispersed pigment is carbon black.

20. A dot-etchable mask prepared by imagewise exposing to actinic radiation a photopolymerizable element which comprises a transparent support bearing an addition photopolymerizable layer which is the outermost layer of the element or is contig-uous to a removable cover sheet or an overcoat layer which is at least partially soluble in or permeable to a solvent for the photopolymerizable layer, said photopolymerizable layer being no more than 0.015 mm thick, and removing unexposed areas of the layer, said mask containing a tone correctable image comprised of polymeric dots having a hardened upper skin which rests on a softer undervolume having a lesser degree of polymerization or hardening, said dots having an optical density greater than 3.0 in the 350-400 nm region and being reducible in size by undercutting the polymeric dots with a solvent for the softer undervolume and removing hardened polymer from the edges of the hardened upper skin by mechanical action on the image bearing surface of the mask.

21. A dot-etchable mask according to Claim 20 wherein the dots have a thickness of no more than 0.015 mm.

22. A dot-etchable mask according to Claim 21 wherein the mechanical action is rubbing, brushing, or spraying.

23. A dot-etchable mask according to Claim 20 wherein the polymeric dots are reducible in size by solvent and mechan-ical action.

24. A process of image reproduction which comprise (1) imagewise exposing to actinic radiation a photopolymerizable element which comprises a support bearing a photopolymerizable layer having a maximum thickness of 0.015 mm and an optical den-sity in the actinic region of at least 3.0, said photopolymer-izable layer being the outermost layer of the element or being continuous to a removable cover sheet or an overcoat layer which is at least partially soluble in or permeable to a solvent for the photopolymerizable layer, the photopolymerizable layer con-taining actinic radiation absorbing material, (2) removing the unexposed areas of the layer, and (3) reducing the size of exposed image areas by removing the edges of the areas on the side of the layer which was exposed to actinic radiation, the steps of removing the unexposed areas of the layer and reducing the size of exposed image areas being carried out by treating the side of the layer which was exposed to actinic radiation with a developer solvent and subjecting the same side of the layer to mechanical action.

25. A process according to Claim 24 wherein the photopolymerizable element has a removable cover sheet or over-coat layer which is removed prior to removing the unexposed areas of the photopolymerizable layer.

26. A process according to Claim 24 wherein the photopolymerizable element has an overcoat layer which is com-pletely or partially removed during the removal of the unexposed areas of the photopolymerizable layer.

27. A process according to Claim 26 wherein the developer solvent is a predominantly aqueous solution.

28. A process according to Claim 27 wherein the exposed image areas are comprised of halftone dots and the sur-face of at least some of the dots in the exposed image areas on the side of the photopolymerizable layer which was exposed to actinic radiation is reduced in size by at least 5 percent.

29. A process according to Claim 28 wherein the support comprises a polymeric film having a nonphotosensitive sub layer between the polymeric film and the photopolymerizable layer, the sub layer being soluble in the developer solvent and the combined thickness of the sub layer and the photopoly-merizable layer being no more than 0.015 mm.

30. A process according to Claim 29 wherein the mechanical action is rubbing, brushing, or spraying.

31. A process according to Claim 24 wherein the element produced by the exposure and development is one of a plurality of color separation masks and the reduction in size of the exposed image areas is carried out to an extent that a composite color image produced with the several masks has the same tonal balance as the original color image.

32. A process according to Claim 24 wherein the imagewise exposing comprises exposing through a continuous tone original and a vignetted halftone screen.

33. A process of image reproduction which comprises exposing imagewise to actinic radiation a photopolymerizable element comprising a support bearing a photopolymerizable layer having a maximum thickness of 0.015 mm and an optical density in the actinic region of at least 3.0, said photopolymerizable layer being the outermost layer of the element or being contig-uous to a removable cover sheet or an overcoat layer which is at least partially soluble in or permeable to a solvent for the photopolymerizable layer, the exposure being sufficient to poly-merize the surface but to a depth not more than one-third the thickness of the photopolymerizable layer, and then developing the layer with a solvent for unexposed areas of the layer to form an image.

34. A process according to Claim 33 wherein the image formed comprises a plurality of photopolymerized caps resting on underlying columns, the columns being soluble in a solvent in which the caps are insoluble.

35. A process according to Claim 34 wherein the image areas are reduced in size by treating the side of the layer exposed to actinic radiation with said solvent and subjecting the same side de of the layer to mechanical action.

36. A color proofing system which comprises, in combinations at least two dot-etchable masks according to Claim 20, in register with each other, characterized in that each mask has been exposed through a different color separation negative and has been colored in a spectral region corresponding to the color separation negative.

37. A color proofing system according to Claim 36 characterized in that the dot-etchable masks and color separa-tion negatives correspond, respectively, to yellow, magenta and cyan.

38. A color proofing system according to Claim 37 characterized in that a dot-etchable mask corresponding to the grey-black separation negative is present in the combination of masks.

39. An element according to Claim 11 wherein the photoinitiator also includes an actinic radiation absorber present in the amount of up to 40% by weight of the photopoly-merizable layer.

40. An element according to Claim 39 wherein the actinic radiation absorber compound is a dye and/or pigment in an amount of 15 to 40% by weight of the photopolymerizable layer.

41. A process according to Claim 31 wherein each of said color separation masks has been exposed through a different color separation negative and has been colored in a spectral region corresponding to the color separation negative.

42. A process according to Claim 41 wherein the masks and color separation negatives correspond, respectively, to yellow, magenta and cyan.

43. A process according to Claim 42 wherein the reduction of size of exposed image areas is accomplished with the masks superimposed in register.

44. A dot-etchable mask according to Claim 20 wherein exposure to actinic radiation is sufficient to polymerize the surface but not more than about one third the photopolymerizable layer thickness.

45. A dot-etchable mask according to Claim 20 wherein the polymeric dots are reducible in size by at least 5 percent.

46. A dot-etchable mask comprising a transparent support bearing an addition photopolymerized tone correctable image having image areas which consist of hardened upper skins which rest on softer undervolumes having a lesser degree of polymerization or hardening, said image areas being no more than 0.015 mm thick and having an optical density greater than 3.0 in the 350-400 nm region and being reducible in size by undercutting the image areas with a solvent for the softer undervolume and removing hardened polymer from the edges of the hardened upper skin by mechanical action on the image bearing surface of the mask.

47. A dot-etchable mask according to Claim 46 wherein the image areas have a thickness of no more than 0.010 mm and an optical density of at least 4Ø

48. A dot-etchable mask according to Claim 46 wherein the thickness of the upper skin is not more than about one-third the thickness of said image areas.

49. A dot-etchable mask according to Claim 46 wherein the image areas comprise a plurality of polymeric dots having photopolymerized caps resting on underlying columns the columns being soluble in a solvent in which the caps are insoluble.

50. A dot-etchable mask according to Claim 46 wherein the mechanical action is rubbing, brushing, or spraying,

51. A dot-etchable mask according to Claim 46 wherein the image areas are reducible in size by a solvent and mechanical action.

52. A dot-etchable mask according to Claim 49 wherein the polymeric dots are reducible in size by at least 5 percent.

53. A dot-etchable mask according to Claim 20 wherein the tone correctable image comprises a plurality of polymeric dots having photopolymerized caps resting on underlying columns, the columns being soluble in a solvent in which the caps are insoluble.