CA1144412A - Element including a layer containing aromatic o-dialdehyde dye former and a radiation responsive image-forming composition and a superimposed polymer layer - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An imaging element which relies upon an aromatic dialdehyde to produce a dye provides improved maximum neutral densities when certain polymers are superimposed over the element. Such polymers seal the element to reduce loss of the dialdehyde during development. An imaging method is also disclosed.

Description

IMAGING ELEMENTS
(1) Field o~ the Invention This invention rela1;es to an imaglng element which contains an aromatic dialdehyde as a dye-rorming component of a radiation-responsive image-rorming compositlon, and a method of using such an element. More specifically, an element is provided which includes a layer of a polymer that seals the dialdehyde into the element as a means Or 1ncreaslng the maximum neutral densities available from the lmaglng 10 element-

(2) Background of the Invention Imaging elements have been devised which rely upon the photodestruction of o-phthalaldehyde whlch~ where not destroyed, forms a dye when suitably developed. Examples 15 are disclosed in US Patent No 3,102,811 whereln poly(vinyl-pyrrolidone) and poly(vinyl alcohol) are listed as exemplary binders for an o-phthalaldehyde image-~orming composition. o-Phthalaldehyde ls also used as a dye-formlng material in imaging elements which rely upon the reduction o~ cobalt(III) 20complexes, as descrlbed in Research Disclosure, Vol 158, June, 1977, Publication No 15874, published by Industrial Opportunities Ltd, Hampshire, United Kingdom.
Such imaging elements are susceptible to loss Or phthalaldehyde during element formation, due to extreme 25volatillty Or the compound. Such losses can decrease dras-tically tne amount Or dye density available durlng devel-opment. One solution to this problem ls to use as a binder ror the phthalaldehyde a materlal which is adapted to retain the phthalaldehyde in the element during manufacturing.
30Particularly userul binders which provlde superior levels of retention are described in commonly owned Canadian Application Serial No. 323,823, by Fletcher et al ~iled on March 20, 1979, entitled "Imaging Elements and Compositlons Featuring Aro-matlc Dialdehyde-Retaining Binders".
Although the binders described in the a~oresald application greatly increase the avallable dye denslty in elements using phthalaldehyde as the dye-forming material, some ph~halaldehyde can still be lost by vol~till2atlon ~1~4~1Z

during image processing. Losses partlcularly can occur when the exposed element is heated for image development. Accord-ingly, further retention of phthalaldehyde ls desirable.
Various polymers have been used ln the past as overcoats for a variety of imaging elements and for a variety Or purposes. Common among such polymers are poly-(vinyl pyrrolidone) and poly(vinyl alcohol). However, these without more do not provide a significant lncrease in image densities when applied to a phthalaldehyde-lmaging element.

In accord with the present inventlon, there is advantageously featured an imaging element having the capability of producing increased maximum neutral densltles when exposed and developed, as compared with a simllar 15 imaging element lacking the hereinafter described invention.
Such an advantageous result is based upon the discovery that certain layers superimposed over and sealing the imaging element increase the available maximum neutral density. More specifically, there is provided ln an imaging 20 element including a support bearing at least one layer comprising a radiation-responsive image-forming composltion capable of lmagewise-converting an aromatic dlaldehyde into a dye, the composition including the dialdehyde, a compatible polymer superimposed over such layer and capable, when coated at a pH of 3.0 ln an amount of about 21.5 mg/dm2, dried, exposed for 0.5 second to a 400-watt medium-pressure mercury arc lamp and developed by heating for 5 seconds at 130 C, of produclng wlth the first-mentioned layer a maximum neutral density that ls at least 10% greater than is produced under ldentlcal condi-tlons by the same element, but wlthout the superimposed polymer. "Compatlble" ls used here to mean havlng other physlcal properties approparlate to an lmage element, e.g., sufflclent adheslon to the light-sensitlve underlayer, transparency to activatlng radiation~ and freedom from cracking.

~4) Descri~tion o~ the Preferred Embodiments Although this invention is hereinafter described in connection with phthalaldehyde as the preferred aromatic dialdehyde, the invention is not li.mited thereto. Rather, it can be used advantageously with any volatile aromatic dialdehyde capable of reacting to form a dye, e.g., other aromatic dialdehydes which are dye-forming materials, for example, 4-hydroxy-, 4-benzyloyloxy-~ 4-methacryloyloxy-, 4-t-butyl- and 4-bromo-1,2-dicarboxaldehyde; 5,6,7,8-tetrahydro-10 5,5,8,8-tetramethylnaphthylene-2,3-dicarboxaldehyde; and 2,3-naphthalenedicarboxaldehyde.
Preferably, phthalaldehyde is only one component of a radiation-responsive image-forming composition con-taining a material for imagewise-generating a product reac-15 table with the phthalaldehyde to form the dye. Although thepreferred embodiments hereinafter disclosed feature mate-rials for generating amines as the reaction product, the invention is not limited to such embodiments. Any compo-sition capable of imagewise-converting phthalaldehyde to a 20 dye can be incorporated into the imaging element of this invention.
ortho-Phthalaldehyde, herein abbreviated as phthalaldehyde~ or PA, is a convenient dye-forming material capable of selective reaction with amines to form a black 25 dye. By "amines" we refer to ammonia and prirnary amines.
The dye reaction sequence, in the case of NH3, is believed to be as follo~ls:
H\ ~OH

(1) NH3 + ~ ~-CHO -~~~~~~ ~/ ~ I ~C, OH 1~ -q----H20 ~0~ H20 H l~

~4412 A convenient form of the lnvention features a layer of an image-rorming composition comprising phthal-aldehyde and a binder, which layer imagewise generates and responds to the presence of amines to form the oligomer dye B noted above. In accordance with one aspect of the inven-tion, it has been discovered that, through the selection of certain polymers applied over the image-forming composition layer, improved maximum neutral density values can be ob-tained for dye B. As used herein, "maximum neutral density"
10 refers to the densities available from an imaging composi-tion or element at a level of exposure to activating radi ation which produces at least three 0.15 log E steps when using a step tablet. Of course, a density value will be exactly equivalent to the so-called D or shoulder densi 15 ties depicted on a conventional density-log exposure curve plotted for the element in question, only if the exposure level is in fact a maximum exposure level for the compos-ition in question. However, it has been found that the selection of an exposure level to produce at least three 200.15 log E steps does, in fact, also produce a density which is generally comparable with the DmaX or shoulder density, as is well-known. Actual density comparisons are made at the same level of exposure in all cases, that level being selected to produce at least the three 0.15 log E steps in 25all cases-The preferred embodiments feature such super-imposed polymers as overcoats, due to the manufacturing convenience resulting therefrom. However, other methods of superposition can be used to achieve the same improvement in 30maximum neutral densities.
Thus, overcoats of a wide variety of polymers have permitted an enhancement of the densities achieved from phthalaldehyde-containing imaging chemistries. Specifically, it has been found that, using compatible polymers of this 35invention, the maximum neutral density values of the image-forming composition are significantly greater than when using no overcoat, or when using poly(vinylpyrrolidone), an overcoat polymer of other imaging elements. As used herein, "significantly greater" means by an amount which is statisti-cally significant, that is, is more than experimen~al error, determined to be about 7~O. Further, it has been found that there are a number of compatible polymers which give improve-ments of about 40% or more. The superior images form pos-sibly becallse of the superior retention of phthalaldehyde when developing an exposed phthalaldehyde-containing imaging element.
Overcoat polymers having the above-noted property lO of producing increased maximum neutral density values include gelatin, gelatin grafted with recurring units of acrylonitrile and bisacrylamidoacetic acid, and polymers or copolymers having recurring units Or the formula:

R' CHz-CH ~ CHz-C~y O-C C=O
NH2 ~H
~ m R4 N~l~C~G

tC ~CH2'z ;~ CHz-CR4-)z 25 O=C C=O
D D"
1 2 1 ll l R -~nC-CHz~C-R3 R2_cH

30 wherein R is alkylene containing from 1-3 carbon atoms, such as methylene, propylene and the like;
R is alkyl containing from 1-3 carbon atoms, for ample~ methyl, ethyl, propyl, isopropyl and the like;
Rl and R4 are the same or different and each is hydro-gen or methyl;
G is =O or hydrogen;
m and n are the same or different and each is l or O;

11~4~12 D, D' and D" are the same or different and each ls -NH- or -0-;
Z is the atoms necessary to complete one or more saturated or unsaturated heterocyclic rlngs containing from 4-9 ring atoms, such as l-imidazole, 2-pyridine, 4-pyridlne, 2-pyrrole, 2-pyrazole and the llke; and x, y, z and zl are weight percents Or the recurring units such that:
50 - x - 90;
10 - y - 50;
O - z - 1 0;
o < zl < 10 The aforenoted gelatin grafts can have recurring units with the structure:
1' (II) -~CH2-CH ~ CHz-CH~z"
CN C=0 L~
CH-COzH
20 GEL ~-. ~ ~....................... D' C=O
~CHz-CH~:
2~ wherein:
GEL ls gelatin;
D and D' are as defined above; and 50 - x" - 90;
10 - y" - 50;
~- 30 0 < z,, c 10.
Noninterfering recurring units other than those mentioned can be included in the copolymers useful in the invention.
The gelatin overcoats, or those having recurring units of formula (I) with pendant active methylene O O
.. ..
(~CCH2C~) or primary hydroxyl groups, can be further improved for handling by crosslinking. In the case of gelatin, such ~7--crosslinking improves the toughness and water resistance of the overcoat. Useful crosslinking agents include formal-dehyde and a 5 weight percent aqueous solution of hexa-methoxymethyl melamine.
Preparation of the polymers of formula (I) pro-ceeds via conventional addition polymerization techniques such as by using redox initiator systems, such as persul-fate-bisulfite or hydrogen peroxide, or organic soluble free-radical-generating initiating systems such as 2,2'-azobis(2-methylpropionitrile). Similarly, the graft poly-mers of structure (II) are available via conventional techniques.
The following example is included by way of illustration:
Preparation of poly(acrylamide-co-N-vinyl-2-pyrrolidone-1, co-2-acetoacetoxyethyl methacrylate) (50:45:5)*
. . . _ _ . . _ To a 5-liter round-bottom flask, fitted with a stirrer, reflux condenser and nitrogen inlet, were added 3240 ml of dlstilled water, 360 g Or denatured ethanol, 200 2 g (2.81 mole) of acrylamide, 180 g (1.62 mole) of vinyl-pyrrolidone and 20 g (0.81 mole) of 2-acetoacetoxyethyl methacrylate. The contents were purged with nitrogen for 20 min and then the flask was immersed in a 60C water bath.
Nitrogen bubbling and stlrring were continued ror an addi-2- tional 10 min and then 4.0 g (0.024 mole) of 2,2'-azobls(2-methylpropionitrile) dissolved in 60 ml acetone were added.
The solution was stirred under nitrogen for an additional 5 hr at 60 C.
The resultant viscous polymer solution, when

3~ diluted to 5.1% solids with distilled water, had a bulk viscosity of 40 centipoise at room temperature. After - dialysis, the polymer had an inherent viscosity, as measured in 1 N NaCl at 0.25 g/dl, of 1.27 at 25 C.
It is not completely understood why these poly-3~ mers provide improved maximum neutral density values.
*
As used herein, unless otherwise stated, all percentages ofrecurring units are weight ratios of monomers as starting materials.

Althou~h understanding is not essential to the practice of the invention, it is believed that, in part, the overcoats of this inYention are superior materials for the retention of phthalaldehyde~ a volatile molecule. However, there is 5 not an exact correspondence between best retention o~ phthal-aldehyde and best maximum neutral denslty values~
The molecular weight of the polymer selected for the overcoa~ does not appear to be critical to the rormation of improved maximum neutral density values. Furthermore, the molecular weights are subject to wide variation even within a given class of polymers, depending on the prepa-ration conditlons, as is well-known. For example, useful terpolymers of acrylamide of the type described above can have molecular weights within and beyond the range evidenced 1- by inherent viscosities from about 0.1 to about ~.0, measur-ed as a 0.25 weight percent solutlon in dimethyl~ormamide.
A preferred range of inherent vlscosities is from about 0.5 to about 2Ø
~he image-~orming composition preferably comprises, 2~ as noted, phthalaldehyde and a binder. The binder selected for the image-forming composition is not believed to be critical, inasmuch as even binders which are relatively pervious to phthalaldehyde can be used ln such an image-forming composition if the overcoat of the lnvention ls also 2~ used. However, the best results are achleved when uslng as the image-forming composition binder one of those disclosed in the arorementioned Canadian Application S.N. 323,823, by Fletcher et al. Particularly preferred examples Or such polymers include homopolymers and copolymers such as 30 polyacrylonitriles, e.g., poly(methacrylonitrlle~, and polysul~onamides such as poly[N-(4-methacryloyloxyphen-ylmethanesulfonamide], poly(ethylene-co-1,4-cyclohexyl-enedimethylene-l-methyl-2~4-benzenedisulfonamide), poly-(ethylene~co-1,4-cyclohexylenedlmethylene-1-chloro-2~4-35 benzenedisulronamide), poly~ethylene-co-1,4-cyclohexyl-enedimethylene-1,2-dichloro-3,~-benzenedisulfonamide), poly(ethylene-co-1,4-cyclohexylenedimethylene-1-chloro-3,5-benzenedisul~onamlde) J poly(ethylene-co 1,3--xylylene-. .

11~441Z
g l-methyl-2,4-benzenedlsulfonamide), poly(l,4-cyclohex-ylenedimethylene-l-methyl-2,4-benzenedisulfonamide), pol~(l,3-xylylene-1-methyl-2,4-benzenedisulfonamide) and poly(ethylene-co-hexamethylene-l-methyl-2,4-benzenedi-sulfonamide. Of these, poly(ethylene-co-1,4-cyclohex-ylenedimethylene-l-methyl-2,4-benzenedisulfonamide) (50:50) is highly preferred. Preparation of the poly-(acrylonitriles) proceeds via conventional processes. The above-mentioned polysulfonamides can be prepared either as 10 condensation polymers, wherein an -NHSO2- group is ln the backbone of the polymer, or as addition polymers, wherein an -NRlSO2- group is a pendant moiety, Rl being H or methyl. The former is made by a direct solution poly-condensation reaction, preferably using aromatic disul-- 1 fonyl chlorides and diamines in the presence of an acid scavenger. The latter is preferably a polymerization of vinyl monomers containing a sulfonamide pendant moiety.
The image-forming composition also preferably includes an amine-generating material responsive to acti 2C vating radiation. The amines when formed react with the phthalaldehyde to form a dye. Any amine-generating material can be used. Preferred materials for generating the amines are the cobalt(III) complexes with or without a destabi-lizer, as disclosed in the aforesaid Fletcher application.
2- Examples of the complexes lnclude those ln the followlng Table 1. Ihe suffix (U) deslgnates those whlch are ther-mally unstable above about 100 C and whlch therefore do not require a destabilizer.

Table 1 . _ s~a~ ~
hexa-ammine cobalt(III) benzilate hexa-ammine cobalt(III) thiocyanate hexa-ammine cobalt(III) ~rifluoroacetate hexa-ammine cobalt(III) hexafluorophosphate hexa-ammine cobalt(III) trifluoromethane sulfon-ate chloropenta-ammine cobalt(III) perchlorate bromopenta-ammine cobalt(III) perchlorate a~uopenta-ammine cobalt(III) perchlorate bis(methylamine) tetra-ammine cobalt(III) hexa-fluorophosphate aquopenta(methylamine) cobalt(III) nitrate (U) chloropenta(ethylamine) cobalt(III) perfluoro-butyrate(U) trinitrotris-ammine cobalt(III) trinitrotris(methylamine) cobalt(III) (U) ~-superoxodeca-ammine dicobalt(III) perchlorate (U) penta-ammine carbonato cobalt(III) perchlorate tris(glycinato) cobalt(III) A highly preferred form o~ the material capable of generating amines is a composition comprising a cobalt(III) 20 complex that is thermally stable at temperatures slightly above 100 C containing releasable amine ligands and a destabilizer which serves to initiate release of amines from the complex in response to activating radiation. Such a destabilizer compound can be a compound responsive to heat,~
25 of which the following are examples: organometallics such as ferrocene, l,l-dimethylferrocene and tricarbonyls such as N,N-dimethylaniline chromium tricarbonyl; and organic materials such as 4-phenylcatechol, sulfonamidophenols and naphthols, pyrazolidones, ureas such as thiourea, aminimides 30 in polymeric or simple compound form, triazoles, barbitu-rates and the like.
Alternatively, the destabilizers can be pho-toactivators which respond to exposure to light to form a reducing agent for the cobalt(III) complex, whereby 35 cobalt(II) and free amines are formed. Such photoac-tivators can be spectral sensitizers such as are described in Research Disclosure, Vol 130, Publication No 13023.

, .

Preferred photoactivators are photoreductants such as metal carbonyls, e~g., benzene chromium tric~r-bonyl; p-ketosulfide, e.g., 2-(4 tolylthio)chromanone;
disulfidesi diazoanthrones~ dlazophen~nthrones; aromatic azides; carbaæides; diazosulfonates; ~-ketosulfides;
diketones; carboxylic acid a2ides; organic benzilates;
dipyridinium salts; diazonaphthones; phenazines; and particularly quinone photoreductants.
The quinones which are particularly useful as photoreductants include ortho- and ~ -benzoquinones and ortho- and para-naphthoquinones, phenanthrenequinones and anthraquinones. The quinones may be unsubstituted or lncorporate any substituent or combination of substituents which do not lnterfere with the conversion of the quincne to the corresponding reducing agent. A variety of such substltuents are known in the art and include, but are not limited to, primary, secondary and tertiary alkyl, alkenyl and alkynyl, aryl, alkoxy~ aryloxy, alkoxyalkyl, acyl-oxyalkyl, aryloxyalkyl, aroyloxyalkyl, arylo~yalkoxy, alkylcarbonyl, carboxy, primary and secondary amino~
aminoalkyl, amidoalkyl, anilino, piperidinog pyrrolidino, morpholino, nitro, halide and other similar substituents.
Such aryl substituents are preferably phenyl substltuents and such alkyl, alkenyl and alkynyl substituents, whether present as sole substituents or present ln combination with other atoms 9 typically incorporate about 20 or fewer (preferably 6 or fewer) carbon atoms.
3 A highly preferred class of photoreductants is that of internal hydrogen source quinones~ that is, qui-nones incorporating labile hydrogen atoms. These quinones are more easily photoreduced than quinones which do not incorporate lablle hydrogen atoms.
Particularly preferred internal hydrogen source quinones are 5,~-dihydro-1,4-naphthoquinones having at least one hydrogen atom in eash of the 5- and 8-ring positions9 or those which have a hydrogen atom bonded to a carbon atom to which is also bonded the oxygen atom of an _ oxy substltuent or a nltrogen atom of an amlne substltuent wlth the further provlsion that the carbon-to-hydrogen bond ls the third or fourth bond removed from at least one quinone carbonyl double bond. As employed ln the dls-5 cussion of photoreductants herein, the term "amine sub-stltuent" ls lnclusive of amide and imine substltuents.
Further details and a list of useful quinone photoreductants of the type descrlbed above are set forth in Research Dlsclosure, Vol 126, Oct, 1974, Publication No 10 12617. Still others which can be used include 2-lsopropoxy-3-chloro-1,4-naphthoquinone and 2-isopropoxy-1,4-anthraquinone.
The quinone photoreductants rely upon a light exposure between about 300 nm and about 700 nm to form the reduclng agent which reduces the cobalt(III) complex. It 1~ ls noted that heatlng is not needed after the llght expo-sure to cause the redox reaction to take place. However, an additional thermal exposure can be used as part of the exposure to drlve the reaction to a more timely comple-tion. Furthermore, the heat is deslrable to form the dye 2C B.
An imaglng element prepared in accordance wlth the invention preferably comprises the amine-generating material, phthalaldehyde and the binder all mlxed together, ln a slngle layer on the support, overcoated 2~ wlth a polymer of the type described. Alternatively, however, the material generating the amines in response to the radlation exposure can be associated with a separate phthalaldehyde layer. In this case, such a radiatlon-exposure layer comprlsing a cobalt(III) complex~ and a 3 destabilizer, without phthalaldehyde, can be simply applied, as by coating over the phthalaldehyde-contalning layer to form an integral element. To avoid yet another overlayer, the binder for the cobalt(III) complex layer can be the overcoat of the invention as described above. However, for 3~ the best density values, it ls preferred that the overcoat of the invention be applied over the cobalt co~plex layer.

41~

Still another, and the currently prererred, embodiment is an element prepared by superlmposlng, such as by coating a second layer over the rirst overcoat, a polymer which can be different rrom the above-described polymers used in the ~lrst overcoat. Such a technlgue allows, e.g., the use Or a more rçadily hardenable second overcoat whlch would not adhere well to the image-~ormlng composltion if coated directly. For example, poly(acryl-amide-co-N-vinyl-2-pyrrolidone-co-2-hydroxyethyl acrylate 10 (45:45:10) can be applied over gelatln as the rlrst over-coat. Alternatlvely, water-soluble cellulose acetate, crosslinked using the above-described melamlne, can be coated over crossllnked gelatln. At least one Or the overcoat layers should comprlse one Or the polymers de-15 scrlbed above as produclng an lncreased maxlmum neutraldenslty.
As yet another alternative, an amplirler can be lncluded, such as phthalaldehyde, the lntermedlate product A of reaction (1) servlng as a reducing agent for remain-20 lng cobalt(III) complexes. Or the amplifier can be acompound whlch will chelate wlth cobalt(II) to form a reduclng agent ror remalnlng cobalt(III) complexes. Such chelatlng compounds contaln conJugated bonding systems.
Typical ampllfiers Or thls class, and necessary restrlc-25 tlons concernlng pKa values Or the anlons whlch can beused in the cobalt(III) complex in such clrcumstances, are described in US Patent No 4,075,019 issued Feb 21, 1978, and ln ~esearch Dlsclosure, Vol 135, July, 1975, Pub-llcatlon No 13505.

In some lnstances, even thermally stable cobalt(III) complexes can be used wlthout a destablllzer.
Examples include compositions and elements containing the complex and a tridentate-chelate-forming ampllrier, 35 exposed to a pattern of lncldent electron radiatlon as described ln Research Dlsclosure, Vol 146, Publicatlon No 14614, June, 1976.

In commonly owned Canadian Application Serial No.
299,193, filed on M~rch 17, 197~, by Adin, entitled "Inhi-bition of Image Formation Utilizing Cobalt(III) Com-plexes", there is disclosed the use Or photolytlcally activated compositions which inhiblt the reductio~ of cobalt(III) complexes, whereby a positive-working element can be achieved. To the extent that such photoinhibitors are generally compatible with the binders of this lnven~
tion, they can also be included in the compositlons and~or 10 elements herein described.
Other layers not particularly effective in en-hancing the maximum neutral density, but added for other purposes, can be disposed between the one or more overcoats described herein, and the one or more layers comprising the 15 image-forming composition, without interfering with the function of the overcoat of this invention.
Manufacturing Techniques To form an imaging element, the image-formln~
composition is preferably coated onto a support, particu-20 larly ir the coatin~ is not self-supporting. Any conventional photographic support can be used ln the practice of this inventionO Typical supports include transparent supports such as ~ilm supports and glass supports, as well as opa~ue supports such as metal and 25 photographic paper supports. The support can be elther rigid or flexib:le. The most common photographic supports ror most applications are paper, including those with matte finishes, and transparent film supports such as poly(ethylene terephthalate) film. Suitable exemplary 3 supports are disclosed in Product Licensin~ Index, Vol 92, Dec, 1971, Publication No 9232, at p 108, and Research Disclosure, Vol 134, June, 1975~ Publicatlon No 13455.
The support can incorporate one or more subbing layers for the purpose of altering its surface properties so as to 35 enhance the adheslon of the radiation-responsi~e composition to the support.
Supports such as poly(ethylene terephthalate) are particularly preferred because they tend to be rela-z tively impervious at most processing temperatures to thevolatile aromatic dialdehydes. As a result, phthalal-dehyde is not lost through the support during the deYel-opmental heating of the exposed element. However, even supports which are not resistant to such a loss can be used, provided they are given a protective coating of one of the polymers described above for the overcoat of the element. In such a case, the result is an image-forming composition sandwiched between two protective layers, each 10 of which comprises a polymer which results ln increased maximum neutral densities. Also, if the support is omitted, the overcoat is preferably applied to both sides to create such a sandwich.
The aforedescribed image-forming composition, and 15 thereafter the overcoat are successively coated out of a suitable solvent onto the support. Preferably, the coating solvent is a nonaqueous solvent, such as acetone, a mixture of acetone and 2-methoxyethanol, or dimethylformamide, to permit the use of other components such as photoactivators 20 which are soluble in nonaqueous solvents.
The proportions of the nonbinder reactants com-prising the image-forming composition to be coated can vary widely, depending upon which materials are being used.
Where cobalt(III) complex is present, the molar amoùnts for 25 such compositions can be expressed per mole o~ complex.
Thus, if destabilizer materials are incorporated in addition to cobalt(III) complex, they can vary widely from about 0.004 mole per mole of complex, such as ferrocene, to about 5 moles per mole for succinimide. For example, 5,5-diphenyl-30 hydantoin can be present in an amount of between about 0.1mole and about 2 moles per mole of the complex. With respect to the phthalaldehyde, it can be present in an amount from about 1 to about 15 moles per mole of cobalt(III) complex.
A convenient range of coating coverage of 35 phthalaldehyde îs between about 2.5 and about 25 mg/dm2.
~onveniently, the overcoat is applied at a coverage of between about 3 and about 100 mg/dm2. The total combined thicknesses of dual overcoat, if used, can be within the range noted above for a single overcoat. Preferably~ such dual coverage, when using crosslinked gela~in, is about 20 mg/d~2 with the gelatin being about 5 mg/d~2~
Typically 9 the solutlons are coated by such means as whirler coating, brushlng, doctor-blade coat1ng, hopper coatin~ and the like. Therearter, ~he solvent ls evapora~ed. Other exemplary coating procedures are set forth in Product Licensing Index, Vol 92, Dec, 1971~
Publication ~o 9232, at p 109. Addenda such as coating aids and plasticlzers can be inrorporated into the coating 10 composition. A particularly use~ul addendum ts the over-coat is one Or the conventional mattln~ agents.
Examples The ~ollowing examples further illustrate the invention.
15 Examples 1=3:
To demonstrate the manner 1n which varlous overcoat polymers affect the maximum neutral denslty available rrom a pre~erred imaging element, the rollowing machine coatin~ W2S prepared for each of the examples on a 20 subbed poly(ethylene terephthalate) support:

poly~ethylene-co-1,4-cyclohexylene- 75.5 mg/dm2 dimethylene-l-methyl-2,4-benzene-disul~onamide 2 phthalaldehyde 25.1 mg/dm2 25 hexa-ammine cobalt~III) trlfluor- 12.5 mg/dm acetate 2-~sopropoxy 3-chloro-1~4-naphtho- o.36 mgfdm2 quinone The overcoats~ listed in ~able 2, were prepared 30 as aqueous solutions, ad~usted to pH 3.0 and applled to give a dry coverage of 21.6 mg/dm2. Each coating was then dried in the following order: 48 sec at about 38 C3 2 mln at about 60 C, 2 mln at about 70 C, 2 min at about 80 C and 2 min at about 27 C.
A~ter 10 days of lab keeping at approximately 24 C and 65~ RH, samples o~ each coatlng were exposed ~or 0.5 sec in a copier obtained from IBM under the trade-mark IBM Micromas~er Diazo Copierz ~odel II~, to -16a a 0.15 log E step tablet and processed ~or 5 sec, support side to heated sur~ace, on a 130 C hot block. The maxi-mum neutral density was measured and recorded.

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~I E ~ o s a~ ~ s ~18-T~lese examples demonstrated that poly(N-vinyl-2-pyrrolidone Or an ave. mole wt. of 350~000 gave a result which was not sta~istically slgnificant compared with the use of no overcoat at all; that is, the difference was less than the 7% experimental error. On the other hand, the overcoats of the invention gave more than 10% lmprove-ment. Thererore, the remaining Examples 4-16 used as the comparative control PVP K-90, which is assumed to be equlva-lent to no overcoat at all.
Examples 4-16:
The procedure of Examples 1-3 was repeated, using a different batch Or overcoat polymers identiried ln Table 3. For comparative purposes, two other controls, a lower molecular-weight poly(N-vinyl-2-pyrrolidone) and poly(vinyl 1~ alcohol) were also tested. The results Or Table 3 were measured as ror Examples 1-3, and ~urther ~ncluded speed results as the number Or 0.15 log E steps which were fully developed to a density Or greater than 1Ø

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The slightly higher values obtained ln tests of these Examples compared with similar overcoats tested in Examples 1-3 are explainable due to the different batches of chemicals and different batches of polymers whlch were used.
Comparative Exam~les 1 and 2:
Comparative Example 1:
Example 13 was repeated except that the over-coat was poly(acrylamide-co-N-vinyl-2-pyrrolldone) (25:75). The maximum neutral density produced was 2.76 at 1~ three 0.15 log E steps, a result that was not signifi-cantly better than Control 2 of Table 2.
Comparative Example 2:
Example 1 was repeated, except that the overcoat was sodium cellulose sulfate. The maxlmum neutral density 1' produced was 2.29 at three 0.15 log E steps.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope 2~ of the invention.

Claims (7)

What is claimed is:

1. In an imaging element including a support and a radiation-responsive image-forming composition which upon exposure converts an aromatic ortho-dialdehyde into a dye, said composition including said dialdehyde and comprising at least one layer on said support, a compatible polymer superimposed over said one layer, said polymer being capable, when said com-position coated at a pH of 3.0 in an amount of about 21.5 mg/dm2, dried, exposed for 0.5 second to a 400-watt medium pressure mercury arc lamp and developed by heating for 5 seconds at 130°C, of producing with said one layer a maximum neutral density that is at least 10 percent greater than is produced under identical conditions by the same element but without said superimposed polymer.

2. An element as defined in Claim 1, wherein said polymer has recurring units of the formula:

(I) wherein:
R2 is alkylene containing 1 to 3 carbon atoms;
R3 is alkyl containing from 1 to 3 carbon atoms;

R1 and R4 are the same or different and each is hydrogen or methyl;
G is =O or hydrogen;
m and n are the same or different and each is 1 or 0;
D, D' and D" are the same or different and each is -NH- or -O-;
Z is the atoms necessary to complete one or more saturated or unsaturated heterocyclic rings containing from 4 to 9 ring atoms; and x, y, z and z1 are weight percents of the recurring units such that:
50 ? x ? 90, 10 ? y ? 50, 0 ? z ? 10, 0 ? z1 ? 10.

3. An element as defined in Claim 2 wherein R1 is hydrogen; G is =O, Z, together with the C and N atoms, forms a 2-pyrrolidone ring; R3 and R4 are methyl; D and D' are -O-; R2 is ethylene; m is 0; n is 1; x is 50; y is 45; z is 5; and z1 is 0.

4. An element as defined in Claim 1 wherein said polymer is gelatin.

5. An element as defined in Claim 1 wherein said dialdehyde is -phthalaldehyde.

6. In an imaging element including a support and a radiation-responsive image-forming composition capable of imagewise-converting -phthalaldehyde into a dye, said composition including said phthal-aldehyde and comprising at least one layer on said support, a compatible polymer superimposed over said layer, said polymer having recurring units with the formula:

(I) wherein:
R2 is alkylene containing 1 to 3 carbon atoms;
R3 is alkyl containing from 1 to 3 carbon atoms;
R1 and R4 are the same or different and each is hydrogen or methyl;
G is =O or hydrogen;
m and n are the same or different and each is 1 or 0;
D, D' and D" are the same or different and each is -NH- or -0-;
Z is the atoms necessary to complete one or more saturated or unsaturated heterocyclic rings containing from 4 to 9 ring atoms; and x, y, z and z are weight percents of the recurring units such that:
50 ? x ? 90, 10 ? y ? 50, 0 ? Z ? 10, 0 ? z1 ? 10.

7. An element as defined in Claim 6 wherein R1 is H; G is =O; Z, together with the C and N atoms, forms a 2-pyrrolidone ring; R3 and R4 are methyl; D and D' are -O-;

R2 is ethylene; m is 0; n is 1; x is 50; y is 45; z is 5;
and z1 is 0.