CA1101260A - Presensitized positive-proofing sheet containing a poly-complex oxygenated anion-diazonium salt and a colour layer - Google Patents

Abstract

ABSTRACT

A presensitized positive acting sheet capable of providing both overlay and single sheet color pre-press proofing. The overlay system utilizes a sheet structure comprising a clear colorless transparent backing having a coating thereon of an appropriate pigmented polyvinyl acetal resin and a light sensitive layer thereover comprising a poly-complex oxygenated anion-diazonium salt. In the single sheet proofing system, the aforementioned structure also contains a barrier layer, and typically a pressure sensitive adhesive layer.

Description

PRESENSITIZED POSITIVE-PROOFING SYSTEM

The present lnvention relates to the photo-mechanical production of images, and has particular ~
application in the prooflng of color separation positives -preparatory to multi-color lithographic reproduction.
In printing pictorial matter, the half-tone process is typically used, wherein the actual printing image is composed of a multitude of minute dots per square inch of a slngle color ink. The density variation which one observes when viewing a pictorial is created by a ' controlled variation in the size of the dots relative to the unprinted areas between the dots. In black'and white pictorial matter the dots are printed in black ink only. ` ' To print a full color reproduction, however, the three basic process colorsg yellow, magenta and cyan are needed~
In some instances a black printer is also used~ i.e., in ' -~
under color removal methods. Each color ink is printed utilizing a separate prlnting plate, on which can be found a pattern of dots which represents the array of ~ -color, for example magenta, ~ound on the original.
In order to prepare these plates, the original color picture is "separated", either photographically with filters, masks, contact screens, etc., or electronically, iOe. with color scanners, into a set of three or four ~ half-tone negatives or positives. Each of these represents - 25 one of the colors~ and contains an array of dots repre-sentative of the quantity and dispersion of that color in the originalO

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Before actual color printing is undertaken, the color separation must typically be proofed in some manner.
One method of undertaklng this proofing is to prepare a "press proof" ln which actual printing plates are prepared and a full color pictorial is printed therefrom. This is, of course, economically unattractlve. Alternatively, varlous pre-press proofing systems are available such that actual plates need not be prepared.
One commercially available pre-press proofing system involves an overlay system wherein four separate and separable sheets of colored images, each contained on a clear substrate, which slngularly represent the yellow, magènta, cyan, and, if necessary, black portions of the original, are overlayed ln superimposed relatlonshlp. In this manner a full color proof of the original can be visually lnspected. Thls system is of relatively low cost and provides the ability to quickly illustrate the color progressives utilizing the same proof. Such an overlay system, utilizing a positive-actlng photosensitive system, \;is disclosed in U.SO Patent 3,211,553. While this type of color proo~ing system has en~oyed considerable acceptance~
the overlay of such a multiplicity of sheets contains inherent drawbacks. For this reason, a single sheet proofing system has been disclosed wherein the three or four colored images are actually làminated in intimate contact one upon another in perfect register. This provides a significant reduction of haze and light scatter over that of the overlay system, thereby resulting in a close approximation of actual press results. One such system, which has en~oyed considerable acceptance, utilizes :
a negati.ve-acting photo.sensiti~e syste~ and ls dlsclosed in U.S. Patent 3,671,236 ~ .
Both the overlay and single sheet color proofing :
systems have utility., and it is often desirable to utilize both systems for proofing. For example, the overlay system may be utilized as an ln-house pre-press proofing check of the quality of the film separation, whereas the slngle sheet proof may typically be used as a flnal customer prooflng ~ :
check It would therefore be advantageous to have a complete color-matched system wherein the overlay proof, the single sheet proof, and the printing ink use matchlng .
colors and densit.les so that. errors are not introduced ~ :
during color correction, i.e. the steps that are taken to make the color proof match the original. Additionally, ~ ~
prooflng sys.tems should be cons~stent as to "colorl' in all ~ ~:
its aspects together with fidelity of reproduction, so that the craftsman can have con~idence in his judgments as to the type and degree of correctlon needed. ;~
Present positive-acting photosensitive systems ~.
have found app~ication ln the overlay systems, as afore- ~-mentloned, and in the preparatlon of actual printing .~`
platesO At.tempts at single sheet positive proofing :;
systems have heretofore been confined primarily to the .
utilizatio~ of tacky photopolymer-colored toner systemsO ~.
~:~ 2~ In these instances, the color is typically applied by ~: hand, by the customer, in his own shopO Because of this, such a system has many drawbacksO For example, it has been ascertained that the amount o~ colored toner accepted by such a system will vary with ambient con-ditions encountered in each individual shop, which provides :~

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for difficult control of color denslty and hue. This in turn undermines the reliabllity of any ~udgment as to the type and degree o~ color correction necessary in the color .
separatlon positlves. Photosensltive colored sheets designed for color proofing can:be more uniformly and closely controlled at t.he manufacturer's source than in the customers' shop, and as such would lead to a more consistent process.
While lt would.be extremely desirable to obtain a positlve-acting imaging system capable of finding application not only in the area of llthographic printing plates, but also in providing a color-matched set of overlay and single sheet pre-press proofing systems, to my knowledge such has not heretofore been avallable.
In accordance with the invention there is provided positi.ve overlay and single sheet color proofing systems for the pre-press proofing of colored pictorial matter, In the overlay system, appropriately colored ~~ 20 presensitized sheets are used, each comprislng a clear colorless transparent backin~ sheet having on one surface thereof a thin layer of appropriately pigmented polyvinyl acetal resin~ and overlylng the resinous layer a light-sensitive layer comprising a poly-complex oxygenated anion-diazonium salt capable Or decomposing in light struck areas.such that it is readily removable in such areas with a solvent developerO Upon exposure of each colored sheet t~o the ap~ropriate color separation positive ;;
and image developmentg the sheets can be placed in regis-tration to provide a color proof of the original o For the slngle sheet prooflng system, a pre-sensitized sheet comprises a carrier sheet having a smookh release surface, a coating thereover of the aforementloned diazonium salt, a plgmented polyvinyl acetal resin layer over the diazonium salt layer, a colorless barrier layer over the pigmented layer, the barrier layer being insoluble in the sol~ent :developer for the diazonlum salt, preferably a pressure sensiti.ve adhesive layer over the barrier layer, and a removable protective fllm overlying the adhesive layer, In use, the pro~ective film is stripped Yrom the pressure sensitive surface and the sheet is laminated to ~-~
a suitable substrateO The carrier sheet is then removed and the light sensitive system exposed through the appropriate separation posltive and developed in an image-wlse manner, A second presensltlzed sheet is laminated over the first constructlon and similarly processed, The balance of sheets necessary to provide a full color proo~ ~
are simllarly laminated and processed to thereby provide ; ~ :-: i a single substra~e: having a full color pre-press proo~
~ r 2Q of an original thereon, The aforementioned U,S. Patent 3,211,553 utilizes hetero- or iso polycomplex oxygenated anion salts of ~ :
diazonium com~ounds as the positive-acting light-sensitive system therein, The composltlon disclosed relies upon the adherence of the unexposed diazonium salt to the ba~e material, iOe, metal or polyester, and in some instances to an overlying layer of a resin/pigment dispersion for image retentionO In the light-exposed areas~ the decomp-osition of the diazonium salt yields products in that layer which are attackable and removable by an alcohol-water developer. Any overlying resin/pigment layer is correspondingly removed therewith during development.
Furthermore, upon appllcat~on to a polyester backing~ a separate resin is typically coated onto t.he backing to increase adheslon o~ the diazonium salt to the base material, Upon exposure and development, this resin remains in the exposed areas and may thereby result in a tendency toward unclean background areas as pigment particles and colored dlazonlum decompositlon products may adhere sllghtly to the resin~
Also, ln order to remove the unexposed residual colored diazonlum-complex salt from the image areas, so ~.
as to insure attainment of the correct color hue, the dlazonium salt typlcally must be raised up through the overlying color layer without damaging same, in a process called bleaching or fixingO Furthermore, with the llght-sensitlve diazonlum salt underlying the colored layer, exposure of colors which are highly ultraviolet absorbing, such as black or yellow, has to be made through a trans- :
parent base sheet or backingO If exposure is undertaken through the color layer, extremely long exposures to light would be necessaryO With a single sheet proofing system, it is often desirable to place the proo~ on a relatively opaque or diffuse substrate such as paper or metalj whereupon exposure would have to be undertaken ~:
through the overlying colored layer, resulting in the ~: -aforementioned long exposure timèsO
It has surprisingly been found that very high quality images can be obtained when the complex dia~onium salt is applled as an overlayer upon a :~

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herelnafter defined resi~/pigment colored layerO There ls llttle evldence of undercutting, iOeO migration of the solubilizlng photo.-produced decompos~tlon products into une~posed areas taking place, even in the fine half-tone image areas, which can typically cause image ~ossO
Whlle not wlshing to be bound by theory, it ls believed that ultraviolet. exposure of the relatlvely ;~
insoluble hetero- or lsopoly-complex oxygenated anion-diazonium salt layer, which overlies the colored res~n layer, results in decomposition products which increase :
the solubillty of the diazonium salt layer and probably the underlying resin layer, thereby allowing removal :.
of both by a sui.table developer. The solubillty of the unexposed area remains unchanged and ls therefore .
insoluble in the developer~ The heteropoly anion~ e.gO, phosphotungstate, resulting ~rom photodecomposition, ls a heavy, large anlon and probably does not migrate . :
outwardly very readily It also may be forming some sort ;.
of association product with the underlying pigmented :
resin~
The positioning of t:he diazonlum salt layer above the colored layer allows for lmaging o~ opaque layers on opaque backgrounds with llttle or no lncrease in exposure times necessary It also allows ~or easier and more complete removal of the colored diazonium salt from the sur~ace of the colored resln layer Further-more, since there is no concern for bonding of the diazonium salt to the base support~ cleaner backgrounds resultO In factg by use of a thin developer-soluble 0 ''~
barrier between the diazonium salt and the colored layer, the diazonium salt itself can be removed during development, thereby cornpletely eliminating the necessity for bleaching/fixing.
The hetero- or iso-polycomplex oxygenated anion-diazonium salts, which are suitable in my invention, ;
are disclosed, and their method of preparation is described, in aforementioned U.S. Patent 3,211,553.
Generally, compounds capable of providing a poly-complex oxygenated anion are formed from the weak acids of certain amphoteric metals. These weak acids are characterized by the ease with which they condense to form anions containing several molecules of the acid anhydride. They may be ~ `
condensed to form an anion containing only one type of acid ~-anhydride, in which case they are known as isopoly acids or more specifically compounds having an iso-polycomplex oxygenated anion. They may further be condensed with other acids in which case they form heteropoly acids, i.e. compounds having a hetero-polycomplex oxygenated anion? an example of which is phosphotungstic acid.
These heteropoly and isopoly compounds providing the '; ' polycomplex oxygenated anions are ordinarily of high solubility in water. They~ form a highly water-insoluble ;~
reaction product with various diazo materials~ the reaction taking place through the light-sensitive diazo ~`
group to thereby form a complex diazonium salt. Not-withstanding the formation of the stable water-insoluble reaction product, the diazo group retains its light-sensitive character. Therefore, upon exposure of 6~

the diazoni.um salt to actinic light, the dlazo group is destroyed and th.e poly-complex-dlazo reaction product is broken down Apparently, the poly-complex oxygenated anion component reverts to, or sub~tantlally tO1 its :~
initial solub.le character.
~xemplary diazonium salts o~ poly-complex oxygenated anlons useful in my invention include:
the sal~ of p-dlazo diethylaniline ' zinc chloride double salt and sodium 12-molybdophosphate; .' :~
the salt of 4-diazo-4'-methoxy diphenyl amine chloride ancl sodium 12-molybdophosphate, the salt of 4~tolyl me~capto-2,5 diethoxy benzene diazonium chlorlde zinc chloride double salt and sodium 9-molybdophosphate;
the salt of p-diazo diphenyl amine sulfate and sodium 12-molybdophosphate;
the salt of 4~phenyl acetamido 2,5 diethoxy benzene diazonium chloride zinc chloride double salt and sodium 12-tungstophosphate;
the salt of 2,4 dinltro diazo benzene diazonium chloride ' zinc chloride double salt and sodium 12-tungstophosphate;
the salt of 4-nitro diazo benzene diazonium chloride zinc chloride double salt and sodium 12-tungstophosphate;
the salt of p-diazo diphenyl amine sulfate and sodium 12-tungstophosphate;
the salt of 4'-methoxy'diphenyl amine 4-diazonlum chloride and sodium 12-tungstophosphate;

~l~lZ60 the salt of 4-N-morphollno-2g5 di-N-butoxy-benzene diazonium chloride ' z~nc chloride double salt and sodium 12-tungstophosphate;
the salt of 4-N-morpholino benzene diazonium chloride zinc chloride double salt and sodium 12-tungsto-phosphate; -the salt of p-diazodiethyl aniline zinc chloride double salt and sodium 12-tungstophosphate;
the salt of 4-methyl dlazobenzene and sodium 12-tunstophosphate; and the salt of 4-N-benzamido 2,5 diethoxy ben3ene diazonium chloride zinc chloride double salt and sodium 12-tungstophosphate.
~arious resins can be included with the diazonium salt to provide particular characteristlcs to the light-sensitive layer, l.e. to improve the bonding of the light~
sensitive layer to the underlying pigmented resin layer, etcD Such resins must be soluble in the application solvent from which the dia30nium complex is coated and resistant to acids and alcohol-water developer solutions.
Exemplary resins include Saran F-310, trade~ for a vinylidene chloride/acrylonitrile copolymer commercially ~ h~
available from the Dow Chemical Corp,, and VAGH, trade~e for a vinyl chloride-acetate resln commercially avallable -from the Unlon Carbide CorpO Typically up to about 50 percent by weight of such resins can be included in the light-sensitive layer.
The colored resinous underlayer ls comprised -~
of a polyvinyl acetal resin having a color pigment dispersed therein. Acetals are formed by a well-known --10-- ~

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reactlon between aldehydes and alcoholsO Stoichlo-metrically~ the addltion of one molecule of an alcohol to one molecule of an aldehyde produces a hemiacetal~
Because of the inherent instabillty of hemlacetals, they are rarely lsolated, and instead are further reacted with another alcohol molecule to provlde a stable acetal, iOe. a resin contalning _ CH2 - CH ~ CH2 CH
l _ -- C---- ... -.
'., /\
: H R

wherein R is hydrogen or a lower alkyl group, i.e.
havlng less than about 4 carbon atoms.
15 It is of course known that by control of reactlon conditions and concentration of alcohols and aldehydes, the proportion of hydroxyl groups, acetal groups, and acetate groups in the resultant resin can be variedO To function within the context of my invention, it is believed the acetal functionality, iDeO the percentage of acetal groups with~n the resin must be at least about 5~ percent by weight, the balance being primarily hYdroxyl and acetate groupsO
Exemplary polyvinyl acetals include polyvinyl formal resins, commercially designated Formvar~by the Monæa~to Chemical CoO, wherein R in the formula above is hydrogen, and polyvinyl butyral, commercially designated Butvar by the Monsanto Chemical CoO, where~n ~ f~ ~J~.~ J

2~0 R in the formula above ls CH3CH2CH2-o While not wishing to be bound by theory, it is believed ~hat the heteropoly anlon, eOgO phosphotungstate, being rela~lvely heavy, may migrate downward into the poly~inyl acetal resin layer, where an associatlon product may be formed or a breaking down of the acetal group may be evidenced, in either event affecting the solubility characteristics of the resin such that same can be removed in light exposed areas.
The polyvinyl acetal layer contains the approp-riake colored pigment therein, typically from about 30 to about 50 percent, and preferably from about 40 to about 45 percent, by weight to maintain adequate film strength, scratch resistance, and developabillty of the layerO Decreasing pigment concentrations require higher coating weights to attain the appropriate color density of the layer, while lncreaslng concentrations increase the difficulty of image development~
~he colored layer can be applied to a suitable :~
support in conventional fashion, typically by coatlng techniques. The coating solution can be conveniently prepared by dispersing the desired plgment in a suitable solvent, addlng the polyvinyl acetal resin and ball mill~ng or roller milllng the mixture to thoroughly dlsperse the solld pigment in the resin mlxtureO The mixture can then be diluted wlth solvent to the coating application concentratlon, from about 1 to about 10 percent solids being sultable depending on the coating method utllizedO

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Since this embodiment of my lnvention is basically for use in overlay color proofing, colorless transparent sheet material should be utilized as the support for the pigmented resin layer and light sensitive overcoating, which preferably is dimensionally stable.
Exemplary materials include polyesters, e.gO polyethylene terephthalate. The choice of support, however, is not critical in terms of image formation, and if for example a printing plate is the desired end product, any con- ~ ~
ventional support is acceptable. r Having now described this embodiment of my invention in a general manner, the same will now be specifically lllustrated by the use of the following nonlimiting example, wherein all parts are by weight unless otherwlse specified.
EXAM~LE I
In one vessel, 1342 grams of paradiazo diphenylamine sulfate were dissolved in 64.5 liters of 60Co water, following which the solution was filtered to remove any undissolved material~ In a second vessel, 4473 grams of sodium 12-tungstophosphate was dissolved in 13-.3 liters of waterO
With constant stirring of the diazo solution, the second solution was added thereto. A precipitate formed, indicating formation of the insoluble comple~
diazonium saltO The reactlon mixture was stirred for one hour to allow the reaction to go to completion, following which the precipitate was recovered by filtration in a filter press.

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The filter cake was washed with water until constant wash water pH was obtained. The thus washed filter ca~e was dispersed in isopropyl alcohol until the preclpitate chan~ed from a yellowish color to an o~ange color, and refiltered. The recovered filter cake was dried in an oven at 32C for 2~ hours.
A millbase was prepared by dispersing and milling the following mixture on a ball mill:

Formvar 12/85 (tradename for a polyvinyl formal resin commercially available from Monsanto Chemical Co.) 3.2 parts ~ ;

Formvar 7/95E (tradename for a polyvinyl formal resin commercially available from Monsanto Chemical Co.) 1.6 parts Monastral Blue BT-297-D (tradename for a phthalocyanine Blue Pi~ment a~ailable from DuPont Co.) 3.2 parts 1,1,2 Trichloroethane Solvent 92.0 parts `~

The resultant millbase was then diluted by adding further solvent to yield a 3% solids solution.
This dispersion was then coated on a 2 mil transparent colorless polyethylene terephtalate polyester base and dried at 160F. to provide a dry coating weight of 500 milligrams per s~uare meter.
Fourteen parts of the diazonium salt were stirred in ~6 parts of acetone until a fine suspension resulted. To the suspension, 0.16 part of concentrated hydrochloric acid was added to effect solution of the diazonium salt, following which the solution was filtered to remove undissolved residue. To the solution, sufflcient Saran F-310 ttradename for a polyvinylidene chloride/acrylonitrile copolymer resin commercially available from the Dow Chemical Co.) was added to provide a 5:1 diazonium salt:resin weight ratio.
Sufficient methyl ethyl ketone was added to the solution to provide a 1:1 by volume acetone/methyl ethyl ketone ratio~
The light sensitive coating was then coated --over the colored layer under sub~ued light and low ;
humidity to a dry coating welght of 450 milligrams per square meter.
In similar fashion, magenta and yellow overlay proofing sheets were prepared, companions to the fore-going cyan colored sheet. To prepare the magenta sheet~
Watchung Red RT-698-D, tradename for a magenta pigment commercially available from the DuPont Co., was substltuted for the cyan pigment. Similarly for the yellow sheet, Benæidine Yellow YT~564-D, trade~e for a yellow pigment commercially available from the DuPont Co., was utilized.
The proofing sheets were exposed from the coated side through camera separation positives to actinic light utilizing a carbon arc exposure unit.
Following exposure, each sheet was imagewise developed with a solution consist~ng of 7~.2 parts water, 19.4 parts n-propyl alcohol, 1.3 parts Sipon WD
(tradename for a wetting agent commercially available from American Alcolac) and 0 ol part potassium borate.
Each sheet was rubbed with a soft pad to thereby effect removal of the light-decomposed diazonium and the L2~i0 underlying colored layerO The sheets were then cold water rinsed and driedO
Because the light sensitive diazonium compound is itself yellowish in color, the resultant sheets were not representative of the true colors of the pigment retained on the sheetsO To overcome this, each sheet~ ~:
was treated with a solution of the following components: ~

Water 42 parts ~- ;
Sodium Acetate 3O0 parts Ethyl alcohol 44 parts Tergitol 15-S-9 ~ :
(trade ~ for a Union Carbide surfactant) 8.0 parts Sipon WD 3.0 parts The sheets were immersed in the solution for . ., ., , ~ .
1 minute followed.by cold water rinsing and drying.
True color images corresponding~in detail to the half-tone images of the camera positives from the original color separation were obtainedO When the three :: ~-sheets were superimposed in registry on a white background 5 ~;
an accurate proof of the three color prints was obtainedO
To eliminate the necessity for fixing or bleaching the imaged sheet construction, a developer solvent-soluble layer can be lnterposed between the pigmented polyvinyl acetal layer and the light-sensitive layer Thls layer is effective to provide a barrier between the sensitizer layer and the colored layer, such that upon development, the diazo layer is effectively removed, thereby eliml~ating the necessi~y of fixlngO

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The coating weight of such a layer should be merely sufficient to provide a barrier, i e. 50 to 100 milli-grams per square meterO
An exemplary material for such a barrier layer is methyl ether of cellulose. The material must ~ ~r be such that its coating solvent must not attack the polyvinyl acetal layer, and the material must be inert relative to the diazonium salt coating solvent i.e~ not attackable thereby. Such a layer has not been found to substantially hinder the development characteristics of the colored layer after exposure, iOe. apparently the heavy anion of the diazonium layer can effectively penetrate the barrier to produce the desired develop-ment characteristics in the colored layerO
Alcohol-water mlxtures are satisfactory developing solutions for the exposed diazonium salts and the polyvinyl acetal layers.
As aforementioned, a second embodimen~ o~ my invention involves the preparation of a single sheet system for pre-press proofing of color lithography. In this instance, the principles enunclated in UOS~ Patent

3,671,236, relating to a negative-acting system, are applicable O
To more aptly describe my single sheet system, reference is made to the drawings 9 wherein Figure l represents a broken~away edge view of a light-sensitive posit~ve-acting proofing sheetO Figure 2, likewise a broken-away edge view, lllustrates the structure of Figure 1 followlng lamination to a substrate, removal : . . ~ , , of the carrier sheet, and exposure through a camera separation positiveO Figure 3 shows the structure of Figure 2 after image development.
Referring to Figure 19 a carr~er sheet 10 is prGvided with a release sur~ace 12~ whlch may either be a smooth surface of the carrier itself, or a surface coating thereonO 0verlying surface 12 is a light-sen-sltive diazonium salt layer 14 and pigmented color layer 16 as hereinbefore de~inedO Overlying the color layer 16 is a continuous solvent-resistant resinous protective film or layer 18, the exposed surface of which has a thin adhesive layer 20, e g. pressure sensitive adhesive, applied theretoO The outer surface of the adhesive can typically be protected from contamination by dirt or grease by removable protective liner 22.
In use, protective liner 22 is first stripped from the adhesive surface and the remaining structure is laminated onto substrate 30 (~'igure 2). Thereafter, carrier sheet 10 is stripped from the structure, where-upon the structure is exposed in an imagewise fashion ;~
through the appropriate color separation positive corresponding to the color of coating 160 During image development, the light exposed portions of the diazonium salt layer 14a and the under-lying color layer 16a are removed~ leaving the color ;~
image in areas 16b. During processing, the protective layer 18 serves as a barrier to protect the substrate and adhesive from solutions utilized during processing.

The drawing ls for illustrative purposes only~
and it is not lntended that the various layers and components be depicted in their true dimensions or proportions. In actuality the layers are extremely thin.
Following the above-described photo-mechanical production o~ the first color image on the substrate, similar sheets containing the other colors necessary for the proo~ are successlvely applied and the images produced ;
over the structure illustrated in Flgure 3 to yield a three or four color proo~O
The release layer must provide release of the system from either the carrier sheet itself or the overlying dlazonium salt layer without disruption o~
the bond between the adhesive layer/substrate or the dlazonium salt layer/pigmented color layer. Typically, release is less efficient between two in situ ~ormed layers~ i~eO the release layer/diazonium salt layer~
and there~ore it is much more probable that release will ~' occur between the carrier sheet and the release layerO ~.
In such an instance, the release layer must be trans-parent to actinic light and furthermore must be soluble in the developer solution utlllzed. ~:
The release coating must also be insoluble in and resistant to the particular solvent utillzed to prepare the diazonium salt coating solutionO Exemplary release coatings meeting the foregoing criteria include polyvinyl alcohol, cellulose ether, and gelatinO

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The diazonium salt layer and the pigmented polyvinyl acetal color layer preparation are as previ- ;
ously discussed relative to the multiple sheet overlay color proofing system.
The barrier layer overlying the color layer must be resistant to the developing solution and to the solvent used for the adhesive coating application.
This layer must also be continuous, colorless 7 and transparent to actinic light, to allow for visual ~ ~
inspection of the proof. Furthermore, the interrelation ~ ;
between the barrier layer and pigmented layer is ;
important, since there must be a sufficient bond formed `
between the two layers such that the twc are not pulled apart during removal of the carrier sheet, yet the bond must not be so great as to prevent removal of the ~ ;~
pigmented layer from the barrier layer in light exposed areas. It has been found that the desired interrelation is present where at least a degree of physical incom- ;
patibility exists between the resins comprising the color and barrier layers. Exemplary resins for the ; ;
barrier layer include the preferable polacrylates and methacrylates and to a lesser degree polyvinyl chloride-acetate copolymers.
Transparent and colorless pressure sensitive ~;~
adhesives and protective liners which are useful in the invention are the same as those disclosed in aforemen-tioned U.~. Patent 3,671,236.

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Having described this embodiment generally, it will now be specifically illustrated by the following non-limiting example EXAMPLE II
A 2-mil film of smooth, transparent~ colorless polyethylene terephthalate polyester was coated with a :
r~ solution constituted as follows:

Methocel A-15 (trade ~ 1c5 parts for a methyl ether of cellulose commercially available from Dow ~:
Chemical Corp.) Water 65 parts ~ .`
n-propyl alcohol 33 parts Upon extrusion coating and drying at 70Co, a dry coatlng we~ght of 1000 milligrams per square meter was obtained for the release layer. ~ .
A light sensitive diazonium salt solutlon is prepared as per Example I and is extrusion coated onto the release layer, under subdued light and low humidity conditions, to a dry coating weight of 400 milllgrams per square meterO
The diazonium salt layer was then dip coated under subdued light with the cyan pigmented-polyvinyl formal resln dispersion of Example I and drled to provide a dry coating weight of 500 milligrams per square meterO
The pigmented color layer was then extrusion coated under subdued light with a solution constituted as follows: :.

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,~bR~ :
Elvacite 2041 (trade~e 3O0 parts r for a polymethylmeth- -acrylate resin commercially available from the DuPont : :
CoO) Ethyl acetate 97O0 parts :
The coating was allowed to dry at room temperature for one minute and then oven dried at 160F. to provide a ~-~
dry coating weight of 1000 milligrams per square meterO :; ;:
A clear colorless pressure sensitive adhesive (e.gO, as disclosed in Ulrich PatO ReO 24,906, granted December 13, 1960), was coated onto the acrylate barrier layer at a dry coating weight of 2000 milligrams per square meter. Following drying, a protective liner ~`
of sllicone-coated paper, commerclally available from the H.P. Smith Co. and designated by the tradename SP-8029, was placed against the adheslve for protection .~rom dirt, etcO
In the ~oregoing illustration, a cyan color proofing sheet is describedO The companion magenta, yellow, and black ~if necessary) structures can be ~-similarly prepared employlng the same technlques and ;
constituents with the exception o~ the incorporatlon of :
the appropriate pigments therein, ~or example, Watchung Red RT 761-D, Benzidine Yellow YT 564-D3 and Cabot Regal 300 R carbon black The llght sensit~ve sheets can then be converted into standard sizes~ packed in sultable light-proor containers and shipped in commerceO The sheets can be stored in thelr sensitized condltion~

and then used months later as successfully as immediately following manufacture.
In using the sheets to produce a color proo~, any suitable receptor substrate can be usedO
Frequently, the sheet stock on which the particular printing job will be performed is used. This is particularly advantageous when the printing stock is something other than white paper, such as colored paper, card stock or paperboard, plastic film or metal foil.
Where the printing stock is to be white paper3 I prefer to assemble the proof on a bright white stock to provlde optimum viewing conditionsO A particularly sutiable backing, because o~ dimensional stability, whiteness, and moisture resistance is "3M" Brand ~'Transfer Base~'.
In preparing a single sheet color proo~
composite, the colors are processed individually and consecu~ively. A sheet o~ the color represented by the first positive to be proofed, preferably cyan, 5to minimiæe halation) is prepared for processing by removing the silicone liner and laminating the color sheet to the backing sheetO Pressure applied by hand with a rubber roller is sufficient to achieve laminationO
Following lamination, the carrier sheet of polyethylene terephthalate is stripped awayO The light sensitive layer now on the backing sheet is contact exposed through the corresponding color separation positive.
The light-imaged composite is then developed and bleached in an identical manner, and with identical solutions, to that employed for the multiple sheet overlay syst2m described aboveO An image is thereby deflned, fa1th-fully representing the reproductlon and full color range which would result if the complete platemaklng and printing operation, using appropriately matched ink, were carried through with that color separation positive.
A sheet of the second color to be proofed 3 preferably yellow~ is laminated to the cyan image-containing substrate. The corresponding color separation positive is then positioned in exact reglster with the cyan imageO This is commonly pro- -vlded for by a pre-registratlon of all the separation positives and the backing sheet by a system of register marks or punches. The light-sensitive layer is exposed and processed in the same manner as the first color. The remaining magenta ~and black if required) sheets are thereafter processed in turn, thereby faithfully reproducing the result which would occur in printing, were printing plates employed, prepared from the same color separation posltives

Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A presensitized positive-acting sheet material suitable for use in preparing color-proofing film, comprising a clear colorless transparent backing sheet having on one surface thereof a thin color layer comprised of a polyvinyl acetal resin having a pigment dispersed therein, and overlying said color layer a light-sensitive layer comprising an actinic light decomposable poly-complex oxygenated anion-diazonium salt, said light sensitive layer and said color layer being readily removable by an aqueous developing medium in actinic light exposed areas upon exposure of said sheet to actinic light.

2. The sheet material of claim 1 wherein said polyvinyl acetal resin is a polyvinyl formal resin.

3. A presensitized positive-acting color-proofing sheet comprising a carrier sheet having a smooth release surface, a light sensitive layer over-lying said release surface comprising an actinic light decomposable poly-complex oxygenated anion-diazonium salt, and overlying said light-sensitive layer a color layer comprising a polyvinyl acetal resin having a pigment dispersed therein, overlying said color layer a continuous, water-insoluble, transparent, colorless barrier layer, said barrier layer being insoluble in an aqueous developing medium for said diazonium salt, said light sensitive layer and said color layer being readily removable from said barrier layer by said developing medium in actinic light exposed areas upon light exposure of said sheet.

4. The presensitized sheet of claim 3 wherein a transparent colorless pressure-sensitive adhesive layer is bonded to the surface of said barrier layer away from said color layer.

5. The presensitized sheet of claim 4 wherein a release liner is bonded to said pressure sensitive adhesive layer.

6. The presensitized sheet of claim 3 wherein said polyvinyl acetal resin is a polyvinyl formal resin.

7. A method for obtaining a positive multi-colored color proofing sheet image on a single substrate comprising:
(a) bonding a first pre-sensitized sheet of claim 3 to a substrate, (b) removing said carrier sheet, (c) exposing said presensitized sheet through a color separation positive corresponding to said color layer to create a latent image, (d) developing said image with said solvent developing medium, whereby exposed diazonium salt and color layer associated therewith are removed.
(e) bonding a second presensitized sheet of claim 3 to the developed first presensitized sheet, (f) repeating steps (b) - (d), and (g) repeating steps (e), and (b) - (d) in that order for further presensitized sheets of claim 3, each of said color proofing sheets being of a different color, whereby there is provided said multi-colored proofing sheet on a single substrate.

8. A presensitized positive-acting color proofing sheet, comprising:
(a) a clear colorless transparent backing sheet having on one surface thereof a thin color layer comprised of a polyvinyl acetal resin having a pigment dispersed therein, and overlying said color layer a light-sensitive layer comprising an actinic light decomposable poly-complex oxygenated anion-diazonium salt, said light sensitive layer and said color layer being readily removable by an aqueous developing medium in actinic light exposed areas upon exposure of said sheet to actinic light; or (b) a carrier sheet-having a smooth release surface, a light sensitive layer overlying said release surface compris-ing an actinic light decomposable poly-complex oxygenated anion-diazonium salt, and overlying said light-sensitive layer a color layer comprising a polyvinyl acetal resin having a pigment dispersed therein, overlying said color layer a contin-uous, water-insoluble, transparent, colorless barrier layer, said barrier layer being insoluble in an aqueous developing medium for said diazonium salt, said light sensitive layer and said color layer being readily removable from said barrier layer by said developing medium in actinic light exposed areas upon light exposure of said sheet.