CA1189377A - Lithographic substrate including an aluminized support, a ceramic layer and an organic photosensitive layer - Google Patents

Abstract

LITHOGRAPHIC SUBSTRATE AND ITS PROCESS OF MANUFACTURE

ABSTRACT

Lithographic printing substrates ordinarily require a mechanical or chemical graining of aluminum surfaces. Such substrates are difficult and expensive to make. It has been found that lithographically suit-able substrates can be prepared by the firing of monobasic phosphate solutions with or without dispersed particles therein. The presence of reactive metallic oxides enhances the properties of the surface.

Description

32, ~66 3~7~
LITHOGRAPHIC SUBSTRATE AND ITg PE`(OCESS OF MANUFACTURE

BACKGROVND OF T~IE XNVEN'rION
Lithographic printing plates have been widely used for many years. One of the basic concept~ utili~ed in that technology is the establishment of differential we~tabili~y between areas on the planographic printing surace. This is most usually e~ected by providing a substrate having a hydrophilic or water wettable surface and coating that ~urface with an imageable (usually visible li~ht or radiation sensitive) and developable grease compatible ~oleophilic) and hydrophobic layer, After imaging and developing of this layer9 the printin~ plate imagewise exposes hydrophilic and hydrophobic surfaces.
When the plate is first wet with water and then coated with grease or oil-based printing lnks, only the hydrophobic areas of the image will hold the ink and provide a trans-erred dark ima~e when pressed against a receiving surEace such as paper.
This lithographic process has been able to provide planographic printing plates whjcil are capable of producing as few as hundreds of copies or as many as several hundred thousand high quality copies. The desirability of being able to produce large numbers of copies from a single plate are quite apparent~ Only a sin~le imaging and developing procedure must be performed, and the printing press need not be shut down during operation in order to change plates.
The investigation o means for lengthening the running time of planographic printing pla~es has been the ocus of many studies and research. It is also necessary in the provision o~ more durable plates to maintain or improve such o~her desirable or essential characteristics o printing plates such as imagin~ speed r ease of development, non-polluting chemistry, and shelf life.

,, . ~

7~ - 2-Most o~ the research in providincJ longer running and higher dura~ility planograpllic printin~ plates has centered on the imageable and hydrophobic coating la~er on the hydrophilic surEace of the ~ubstrate. As it is usually the breakdown of this layer which causes failure of the printing plate, the logic o~ that direction oE
research is apparent~
In general, the substrate provided for the image~
able layer is an alumin~n sheat which has been prepared for coating by a variety of cleaning, mechanical, and chernical treatments. For example, an alumin~n surface is usually cleaned to remove oils and other contaminants. This cleaning is then followed by a mechanical or electro-chemical graining process, and finally an anodizing 15 process.
For example, U~S. Patent No. 3~963/594 discloses the electrochemical etching of an aluminum substrate with an aqueous solution of hydrochloric acid and glucGnic acidO
This provides a uni~ormly coarse surface texture to the aluninum su~strate~ The process of ~hat patent permits the use of lower current densities than were found to be necessary when using only hydrochloric acid in the bath. The treatment also complexad dissolved aluminum and other impurities which ~orm in the etching bath. This extended the liEe of the bath. Desmutting treatments were also shown in co~nbination with the electrochemical etching.
British Patent No. 1,439,12~ disclosas an anodi~ treatlnent ~or alwninuJIl substrates in which the 30 anodizatiGn i8 performed in an aqueous sulfuric aci~ bath at a telnperature in excess of 709C and with an anodizing curr~nt density of at least 50 amps/sq. Et. This treat-ment reduces the length of time in which the anodizing step is performed. Other etching solution compositions and electrical parameters are disclosed in U.S. Patents Nos. 4,072,589 and 4,052,275 and French Patent No.

2,322,015.

~fter electrochemical or mechanical etchiny of the alw~inurn surEace, an anodically oxidiziny treatment is usually performed to rencler the aluininum surface both corrosion resistant and wear resistant. This is shown in aritish Patent 1,439,127 discussed above and in U.S.
Patent NO. 4,131,518. In this latter patent, aluminum Eoil in the Eorm of a continuous web, particularly when the aluminuln carries a polymeric coating on one sicle thereof, is anodizecl so that energy requirements are reduced and anodizing speeds are increased.
U.S. Patent No. 3,181,4~1 discloses an anodizing process in which the sulfuric acid anodizing step is ~ollowed by trea~ment with an aqueous solution of sodium silicate. This treatment seals the pores of the anodic oxide surface and provides a hydrophilic, ink-repelling surface layer.
German Patent (Offenlegungsschr1ft) 24 34 098 discusses firing a composition of aluminu~ phosphate and silicon carbide particles onto a metal surface for the purpose of inrreasing its wear resistance~ ~owever, this invention of German Patent 24 34 098 is not suitable for use on lithographic plates and is not coated on aluminum surfaces.
The practice of treating lithographic substrates o aluminum in order to increase surface areas and enable ano~7ic coatings is well described in the art such as U.S.
Patents Nos. 3,935,080; 3,929,591; 3,980,539; and

3,98%,217.
Even though anoclizing has become the most common means of providing an alulninum substrate for durable plano-gra~hic printing plates, and even though some reduction of eneryy rec~uirements has been made, the process still re~uires large alnounts of electrical energy in its opera-tion and also generates effluents that must be care~ully disposed oE to avoid environmental pollution.
A novel Inethod for providing kexturecl surfaces on lithoyraphic printing plate substrates of aluminum was 3t7~
~I:LSClOSe(3 in ~.S. Pal-ent NO. 3,210,184. A 1aYer OE
boehrnite (alulnlnum oxide monohydrate) was produced on the alumillum substrate by bathing the aluminum in hot water or steam in the presence of a weak organic base. Printing plates using the textured substrate were shown to provide increased numbers of copies under comparable conditions as compared to printing plates using mechanically roughened aluminurn substrates.
U~S. Patent Nos. 3,871,881 and 3,975~197 dis-close another me~hod of enhancing the physical propertiesof aluminum surfaces, Various types of particulate material are bonded to the surface of the aluminum by an in_situ foLrned binder of aluminum hydroxyoxide, The enhanced aluminum article is suggested Eor use as a susbstrate ~or printing plates~
U~S. Patent No. 4,319,924 discloses an aqueous coating cornposition comprisingg in coating forming propor-tions; dissolved phosphate; dissolved dichromate;
dixsolved aluminu~; and ~ispersed solid particulate material, preferably an aluminum-containing material.
This composition is capable of being heat~cured at elevated temperatures into a water-insoluble material.
The composition ~urther comprises diethanolamine in an alnount suE~icient to reduce the temperature at which said composition can be cured into the water-insoluble material .
It is disclosed in the description of the present invention that a novel process for treating litho~raphic quality aluminum foil can provide a durable, long-running substrate Eor lithographic plate construc-tions. This process prov.ides a novel substrate which can be ~esistant to the chernical action of the printing plate ~evelopers and press solvents. The novel substrate also enables the bonding of many photoreactive imaging layers ~o the treated aluminum substrate without the need for primer.s or other adhesion promoting agents.

7~
SUMMARY OF THE INVENTION
A process is disclosed for firing a slurry of particles and a phosphate binder. The particles are preferably weakly basic or amphoteric non-metallic particles and the composition is fired on an aluminum or aluminized substrate.
This process produces an aluminum substrate or aluminized surface of a sheet bea-ring a textured layer of hard particles bonded together by a water resistant phase, or phases, of a dehydration products of monobasic phosphates such as those of aluminum and/or magnesium, alone or in combination with -the products of their reaction with some or all of the particles, on at least one surface of the sheet.
This textured layer has been found to provide an excellent surface for adhesion of organic ma-terials. In particular, the phosphates strongly bind and adhere the particles into a layer having excellent adhesion for diazonium resins and photo-polymeric compositions used in the printing art and particularly in the planogra-phic printing art. This adhesion is enhanced by the surface texture and improved chemical stability imparted by the particles.

DETAILED DESCRlPTION OF THE INVENTION
_ According to one aspect of the present invention there is provided a process for preparing a photosensitive subs-trate comprising the steps of A. Providing a ceramic layer on an aluminum substrate or aluminized surface of a substrate which comprises applying a slurry of at least one mono-basic phosphate and inorganic non-metallic particles on at least one surface of the aluminum or aluminized substrate and firing the slurry at a temperature of at least 230C to form a ceramic coating on said aluminum or aluminized surface;
B. Coating on said ceramic layer an organic photosensitive lithogra-phic layer.
According to another aspect of -the present invention there is provided a photosensitive article comprising:

~8~37~
A. an aluminum or aluminized subs-trate bearing on at least one alumi-num or aluminized surface thereof a ceramic layer comprising 1) non-metallic in-organic particles and 2) a water-resistan-t phase, or phases, of a dehydration product of at least one monobasic phospha-te; and B. a photosensitive lithographic coating over said ceramic layer.
The process of forming phosphate coatings on substra-tes according to -the present invention provides a number of improvements over prior art processes for producing substrates for photoimaging elemen-ts, particularly in -the con-tinu-ous manufacture of subs-trates. Not only does the coated substrate of the present invention have equivalent or improved properties as compared to materials of the prior art, but also it provides significant economic advantages in its manufact-ure. Apparatus used in the process consists of fewer separate items of equipment, thus requiring a lower capital investment than conventional forms of continuous substrate formation. The significant equipmen-t eliminated includes the anodizing facility which is itself costly to operate because of high energy requirements and the need for safe effluent -5a-~ G-disposal. Such ec~uip~ent is desirably eliminated from a substrate manufacturing line because oE a~sociated electro-chemical corrosion problems oE other equipment on the same production line. The coating layer produced according to the present invention does not require e-tching or other texturizing. Such othe-r texturizing may be optionally perEormed to provide additional characteristics to the surfaceO The elilnination oE such texturizing processes provides a Eurther increment of cost reduction. The process can be readily per~ormed in a continuous Eashion and has been found to provide results subs~antially iden-tical to those of a batch process.
A coating of a monobasic phosphate solution E~rming a slurry with metal oxide particles is applied to a clean aluminum or aluminiæed surface~ This coating is fired at a temperature of at least 450F (230C), preferably at leas~ 500 or 550~ (260 or 290C), to produce a textured ceramic coating o~ a phosphate glass~
This may comprise non-metallic and especially meta~ oxide particles embedded in a vitreous, water resistant phase or phases o~ a dehydration product oE monobasic phosphates such as those of aluminum and/or magnesium, alone or in combination with the products of their reaction with some or all oE the par-ticles. Other monobasic phosphates which are acceptable include those of zinc, calcium~ iron, and beryllium. When the metal oxide is alumina, the bondin~
phase between the phospha~e glass and the alumina is most likely aluminum orthophosphate. The firing should be ~er~ormed for a long enou~h tilne at these temperatures to insure substan~ially complete dehydration of the coating.
This Inay t~ke place in as li-ttle time as three seconds at the descrihed temperature depending upon the thickness of the coatiny and the temperature and other parameters oE
the Eiring process. The ceramic surface may be Eurther treated as by e~ching to provide particularly desired te~;~ures and properties to the surface, but the surEace resulting Eroln the firiny already is textured. This ~ 7~
oç~tional treating is not critical or essential to the present inver1tion anc~ is most generally performed in con-junction witl1 imaging systems (e.g~, lithographic printing L~lates~ wl1ich are optirnized hy a silicate treatment of the substrate or sorne other analogous layer. Etching, for example, can be accomplished s~y using known alkaline silica~e solutions which will deposit a silicate coating at the same time. Where no silicating is required or where the subsequen~ly applied ligh~ sensitiv~ composition would not be compatible with a silicate surface, the etch may be performed in alkaline phosphate or aluminate solu-tionsr Eor example. The aluminum or aluminized substrate may initially have a texturized surface so that etching of the cerarnic coating will expose the texture through the ceramic coating. This is unnecessary, however, in the practice o~ the present in~ention because of the natural texture produced in ~he process. This natural texture, which is a microscopic texturing visible by light scattering or under magniEication, provides a physical structure to which subsequently applied light sensitive coating compositions may adhere.
An optional post-firing etch may remove whatever amount of the dehydrated ceramic coating is necessary to pro~ide the character required in the texture of the substr~te. As little as five percent and as much as sixty percent by weight or more oE the ceramic coating may be relnoved. Irhe length oE -time oE the etch is regulated by the temperature and pF of the etching environment. Higher ternperatures and higher p~I le~els provide ~aster etches.
The pH may be controlled by the addition of alkaline hydroxides such as sodiurn hydroxide. Replenishing solu-tions Inay be adde~ durin~ the continuous proCeSSinCJ oper~-tion to replace any material, such as the alkali colnponent, which is deple-ted during the etch. The coms~ined etch and silicating solutions are generally optisnized to emphasize the silicating treatment, since the silicate etch has a wider performance latitude than '7~
phosphate or alulnlnate etching solutions. The silicates used Eor the combined etching and silicating ~aths are ~referably at the high silica content end of the com~ r-~ cially av~ilable materials. Such materials as "Kasil ~1"
or "S-35" of the Philadelphia Quartz Co~ or mixtures of "S-35" ~7ith a fine silica sol (e~g., Sol~ 1115 of Nalco Chemical Co~ are particularly useful when diluted with water to give solutions having approximately one percent silica on a dry weight basis.
The texturized substrates produced on the ceramic coated aluminum or aluminized substrate by the firing step may then be coated with a llght sensitive composition either directly or with an intermediate sublayer. An oligomeric diazonium resin andjor an organic negative or positive acting photosensitive composition may be desirably appli~d to the textured surface.
A broad range of photosensitive compositions having va~ious diEEerent fields of utility are known to those oE ordinary skill. The types of photosensitive coatings useful in the practice oE the present in~ention are those which would ordinarily be considered for use in the printing plate art, and speciically, lithographic and relief compositions. These compositions wGuld include both ~egative and positive acting lithographic composi-tions and negative acting relief compositions.
Neyative acting compositions ordinarily perEormby having a photopolymerizable composition which becornes less soluble and/or more highly polymeri~.ed when struck by actil~ic radiation in an imagewise pattern. This is ordinarily acco~plished by havin~ a mixture of materials including such various ingredients as binders, polymeriz-able Inaterials ~mono!ners, oligomers and polymers~, photo-initiator catalysts Eor the polymerizable materlals, sensitizers Eor the photocatalysts, and various other ingredients such as oleophilicity enhancers, pigments, surfactants, coating aids and other ingredients known in the art. Afte~ i~nagewise exposure to actinic radiation, ~ ~ r~

~ ~q¢~ 7 the unreacted, ~nore soluble are~s of the coating corn-position are ren~oved by washing with various solvents includin.3 water, aqueous alkaline solutions and organic developers. Amongst the most common materials used in the forrnation of negative acting lithographic printing compositions are ethylenically unsaturated monomers and photoinitiated free radical generators.
Organic and methacrylic polymerizable materials are tne most frequently utilized components, but all other ethylenically unsaturated materials and copol~merizable ingredients are useul and known in the artO See, for example, U.S. Patent Nos. 4~ 315,9~9, 4,228,232, 3,895,949, 3,887,450, and 3,~7~955.
Positive acting lithographic compositions ordinarily comprise a binder of thermoplastic or partially crosslinked compcsitions which con~ain a por~itive acting photosensitizer which, when struck by light, becomes more soluble in selected solvents than the non-light exposed sensi~izer. The most common photosensitizers used in the art are o-quinone diazide compounds and polymers. Various binders such as acrylic resins, phenol formaldehyde resins (particularly novolaks)l polyvinyl acetal resins, and cellulosic esters are also yPnerally known in the art Eor use ~ith this type of photosensitizerO See, for example~
U.S. Patent Nos. 4,1g3,797; 4,189,320; 4llÇ9,732 and

4,247,616.
Negative acting relief compositions work in similar fa~shion to negative acting lithographic systems, except that the layers are considerably thicker and that molds and embossing are often used to impress the surface structure when forming the relief image.
The cera~nic surf~c~ provided on the al ulninurn substrate is hi~hly water receptive and has been shown to be at least as hydrophilic as anodized aluminum. The sur~ace provides excellent adhesion Eor polymeric and oligomeric compositions. The surface has been found to provide parti~ùlarly excellen~ adhesion for positive 3,~ 7~7 -1o-acting photosensitive compositions such as those containing diazo oxides and diazo sulfides, and provides good resistance to the developiny solutions use-3 which are generally highly alkaline.
The thickness oE the ceramic coating can readily be varied as desired, Eor example, between 002 and 15 microTneters. Preferably, for use as a substrate for plano graphic printing plates~ the coatlng layer is between 0.3 an~ 10 micrometers and more preferably is between 0.5 and

5 micrometars.
The firiny temperatures used in the practice of the present invention must b~ hi~her than 450 or 500~F
(230 or 260C~ and preferably are at least 550~ (285C).
Telnperatures higher than 700F (370C) do nvt ofEer any significant advantages and raise the energy requirements of the process and may distort or weaken the substrate.
These temperatures refer to the surface temperature of the coatin~, measured by contacting the coating surEace with the bare junction of a thermocouple. It will be under-stood that many dif~erent types of ovens having a variety oE control characteristics may be used Eor achieving the required surface temperatures. The control temperature may in fact difEer substantially Erom the surface temperature measured in the manner described above~
The particulate matter added to the monobasic phosphate to Eorm a slurry may have a considerable range and constitute substantially any non-metallic inorganic particle. By non metallic, it is meant that the particle s~lould not have such a high proportion of free metal ~greater than 25% by weight on the surface~ as would interEere with lithographic colnpositions.
By the term 'particulate,l as used in the practice of the present invention, it is understood that particles within a broad size range are useful. Particles smaL1 enough to Eorm colloidal dispersions, e.g.~ as small as average si~es of 5 x 10-3 microme~ers and pre~erably no smaller ~han 1 x 10-3 microme~ers ara quite useful up ~o 3;1'7~ 1 :L
~5 microlneters. The preEerred size rancJe is between 10-2 and 45 micrometers, and the most preferred range is between lo-2 and 5 micrometers. If highly reactive particles are used, they will in effect dissolve away (in whole or in part) by reaction of material from the sur~ace of the par~icle. Thus, surprisingly large particles can be used which would no~ interfere with the physical properties oE the surEace. Agglomerated particles having an effective large size which can be broken down during processing may also be used.
The metal oxides, Eor example, may include alumina (in various phases such a~ alpha, betaJ theta, and gamrna alumina), chromia, titania~ zirconia, zinc oxide, stannous oxide, stannic oxide, beryllia, boriay silica, ' magnesium oxide, etc. Certai~ oxides and even certain different phases of the oxides or mixtures thereof perEorm better than others. For example, the use oE mixtures of alpha alumina and certain reactive transition aluminas such as the theta and gamma alumina provides a surface having improved properties over that obtained wi~h particles of a single alumina phase. The presence of the alpha phase in the slurry provides a ceramic coating with increased wear resistance and the presence of the other phases, or other metal oxide particles, tends to provide a more Einely textured surface than alpha alumina by itself.
By 'reactive' it is meant that the oxide reac~s with mono-basic phosphate during firing conditions.
Particulate materials which react with the acid or rnonobasic phosphate ~ive more alkali-resistant binder compositions. When such materials are added in excess of the amount that will react Eully with phosphate, they may additionally contribute to the surface texture oE the Eired coating. If they are in a Eorm that reacts slowly or not at all at room temperature, they may contribute to the consistency or viscosity oE the slurry, giving a Eormulation that may be particularly well-adapted Eor L~articular coating methods. Furthermore, some of these 7~;7 mate~rials detract ~rom or add to the hydrophilicity of the phosphate coating in a controlled way. In summary, then, there can be added to the slurry a blend of reactive~
fine, particulate ma erials that maximize resistance to alkaline attack, optimize slurry consistency for the slurry solids fraction and the paxticular coating process, and give the hydrophilicity and microtexture that are most compatible with the intended image coating system.
A reactive inorganic material most desirably present is a magnesium compound. Magnesium carbonate, rnagnesium oxide r magnesium hydroxide, and monomagnesium phospha~e have been used, and the characteristic, desirable, contribution to alkali resistance was obtained in each case. Zinc oxide, zinc carbonate or zinc phospha~e could be substituted for the magnesium compound~
though slightly poorer alkali resistance was obtained with the zinc compounds than with the magnesium compounds.
The arnount of magnesia, i.e. t magnesium oxide or magnesiuln hydroxide, generally a~ded is not enough to yield a fully tribasic ~dehydrated) phospha~e composition upon Eiring, so additions oE other materials supplying polyvalent cations will in general give Eurther alkali resistance~ Hydrated or transition aluminas, sols of rnany metal oxides, or blends of these materials, may be added, generally together with magnesia and generally in such proportions as to optirnize the coating as noted abo,ve.
The graded gamma~theta alurrlina designated "G~ 2500~ by its supplier, the Micro A~rasives Co~p. oE Westfieldt Mass~, the hydrated alumina "Hydral 71~; supplied by the ~luminum Co. of ~me~ica, the predominantly boehmite product, "Dispural'~ supplied by Condea Chemie G ~ H, 2000 Hambur~J
13, West Germany; the "Aluminum Oxide C" of Degussa Pigrnents Division, D-6000 FrankEurt 1, West Germany; and the alumina, chrornia, yttria, ~irconia, an~ ceria sols supplied by Nyacol Inc., Ashland, Mass., are examples o~
reactive materials use~ul in combination with the sug-~ested ,nagnesium compounds. This list is intended ~o be t/~ ~,D~-7t7 -1 3-sugyestive rather than exhaustive. Many other compounds are expectd to be useful as reactive materials in the ~orm of sols or ine powders: compounds such as titanium ~ioxide, calcium hydroxide, calcium oxide~ calcium fluoride, or calcium phosphate, beryllium oxide, ammonillm fluorotitanate, and tungstic oxide are suggested by the li-terature as useful constituents of phosphate glasses.
The resulting fired slurries tolerated moderate levels of alkali metal oxides, such that commercial or technical gra~es of materials are gener~lly acceptable;
however, compounds containing majo~ amounts of alkali metal cations are not recommended. When silica sol is added, compensatin~ extra metallic oxide should also be a~ded to optimize alkali resistance.
In addition to reactive inorganic particulate materials or some blend thereof~ particles of hard, wear-resistant materials may be added with good effect. In general these ar~ expected to react only superficially with the phosphate bonding material~ and not to contribute subs~antially to insolubilization. Graded alpha alumina o the type of "331200 Alundum'l~ supplied by ~he Norton Co. of Troy, N.Y., Gr "WSK l200" white alumina supplied by Tr~ibacher GMBH of Treibach, Austria, and various grades o~ "WCA" alumina supplied by Micro ~brasives Corp., are ~seful. It is necessary to burn off organic contaminants, as hy kiln Eiring, ~rom some of these ma~erials~ Graded tin oxide supplied by Transelco~ Inc. of Penn Yan, N.Y., was substituted for or blended with alumina in several propor-tions witllout loss of properties. Graded fine sands of other hard ~aterials such as quartz, amorphous silica, cerium oxide, zirconia, zircon, spinel, aluminum silicate (mullite), and even hydrophobic particles such as silicon carbide, among many others, would be expected to function sa~isfac~orily, though perhaps (as compared to alu~ina) ~o have less attractive co~t or availability as closely-sized powders. It is preEerred to use hydrophilic parti~le~ in the practice of the present invention, and especially metal o~ides.

These hard par~icles contribute wear resistance and coarse roughness to the fired coating. 'rhe amount and size to be added depends on development of optimum com-patibility with the desired image coating system, to the extent such compatibility depends on coarse roughness, measurable as "Arithmetic Average Roughness" using a profile measuring instrument. ~xamples o~ such instru ments are the Federal Products Corp~ (Providence, R.I~) ~ "Surfanalyzer"~ and the Bendix Co. (Ann Arbor~ Mich.) "ProEicorder"~
It is well known that diferent lithographic substrate texturizi~g processes and he variation of conditions within these processes provide diferent ranges oE ~rithmetic average roughness and difEerent combinations of arithmetic average roughness and specific surface area.
The optimum combina~ion oE ~hese characteristics depends upon the type o~ use to which the final lithographic plate is subjected and the particular photosensitive composition applied thereto. Important characteristics to be considered with reyard to selecting combinations of these charact~ri~tics with specific photosensitive compositions include the mechanical adhesion and release properties (e.g., developability) of the imaged coating. By adjusting the size~ type, fraction, and mix oE particles in the coating composition, the coating process o~ the present invention can provide substrates over most of the range of roughness and specific surEace areas produced hy all of the prior art processes. The present process is highly El~xible and can be readily al~ered to provide substrates which fit changed r~quirements.
It should be noted that one can practice the present invention by Eorming monoalumin~n phosphate or other acid phosphates in situ during the Eiring step.
This can be accomplished, or example, by coating a slurry oE phosphoric acid and a stoichiometric excess o~ reactive alumind onto the alulninum or aluminized surEace to be Eired. Aluminum phosphate is ~enerated durin~ the initial ~rh~e ~

firing period, an~ with a stoichiome~ric excess of alurnina present, reactive alumina particulate material will remain in the reactive composition during the continued firing.
This is sufficient to provide coatings according to the present invention, and the formation cf monoaluminum phos-phate or other aluminum phosphates or mixed phosphates of aluminum and other metals in situ is contemplated as being within the scope o~ practicing the present invention.
In addition to the ~oregoing in situ formation by reaction oE excess metal oxide or hydroxide with phosphoric acid, it is also possible to form monoaluminum phosphate in situ or mixed phosphates of alwninum and other metals in situ by reacting stoichiometrically ey~i~alent amounts of reactive alumina~ i.e. 9 aluminum oxide or aluminwm hydroxide, wi~h either pho~phoric acid or an alkaline earth acid phosphate such as monomagnesium phosphate solution. The interchang~ability of mono-alulninum phosphate with a reaction mixture containing alulnina and phosphoric acid or an alkaline earth acid phosphate is shown by ~he following equivalences:

3Ca(H2PO4)2 ~ A123 = 2~1(H2PO4)3 ~ 3Ca~
3MgtH2PO4)2 + ~12O3 = 2Al(H2PO4)3 ~ 3MgO

Al(OH)3 ~ 3H3PO~1 = A1(H2PO4)3 + 3H20 1~1203 ~r 61~3P04 = 2Al(H2po4)3 t 3~20 The foregoing reactions are not necessarily of practical si~niEicance.
3~ Further, alkaline earth acid phosphates may be created in solution by mixing an appropriate alkaline ~arth tribasic phosphate with phosphoric acid. For example, Ca3(PO4)2 ~ 4H3PO4 - 3Ca(H2pO4)2 Mg3(po4)2 -~ 41I3PO4 = 3M~(~3~PO~)2 Mg3(po4)2 + H3PO4 - 3MgHPO4 ~ lG
The ~ollowincJ reactions dernonstrate the processes which create orthophosphates during firing with phosphoric acid and/or a suitable acidic phosphate starting material:

2H3PO4 + A123 ~ 2AlPO~ + 3H20 Al(H2PO4~3 + A12O3 ~---, 3~1PO~ ~ 3H20 Al(H2PO~)3 + 3MgO -~ AlPO4 + Mg3(po4)2 + 3~2~

3,~g(~l2PO4~2 + 2A1203 - ~ 4AlPO4 -~ Mg3(po4)2 ~ ~2 GCa(QH)2 + 3Mg(H2Po4~2 ~ 2Ca3(PO4)2 ~ Mg3(po4)2 + 12~20 It has been Eound that addition oE calcium-containing inorganic compounds to the slurry reduces the necessity Eor close monitoring o the firing temperature, by providing a mixture that i5 efEectively dehydra~ed at somewhat lower temperatures or shorter times. The calcium addition is preferably in the ~orm of calcium hydroxide, calcium ca~bonate, calcîum oxide, calcium phosphate, or mixtures thereof. To achieve this useful result it is also desirable to add the magnesium-containin~ compound as a soluble phosphate and the aluminum-containing compound in an ultraEine and/or dissolved condition.
It has also been ound that addition of calcium-containing inorganic compounds to the slurry contributes to a coating colnposition that may be fired at a lower 3Q tempera~ure or in less time, while retaining lithographic -~uality at yreater coating thicknesses than previously L~ossil~le. The alulninum-con~aining compound may be a ~umed alumina, e.g. that produced by flame hydrolysis of anl)ydrous alulninulll chloride, having mean ultimate parkicle diameter less than 20 nanometers. When all the consti-tuents which are required to react to form an intimately dispersed, uniforrn phosphate phase are in the most rapidly ~ 17~

r:edutive ~orms currently available as chemical raw materials, better functional properties at the lower Eiring times and/or temperatures are obtained. It should be noted that too great a stoichiometric excess of reactive cation-supplying materials over anion supplying materials, i~e~ phosphate and silicatel tends to weaken the coating mechanically, giving less abrasion resistance of the type described by ~NSI ASTM tests C501-66 and D1044-76, or a test combining some of the features o each of tl~e two standard tes~s.
The slurries or solu~ions generally contain between 85 and 95% (by volume) of material that i5 vola~ile upon firing, the actual amount being that re~uired to yield a satisfactory coating weight and satisfactory coatability with the chosen coating tech~
nique. A mix~ure of so% aqueous monoaluminu~ phosphate solution with an equal weiyht of water, ~alls easily in this range, as do slurries which contain a greater number oE additives. The volatile material is generally water, although water-miscihle volatile liquids such as alcohols may be added in substantial partO The volume of volatile material stated above includes bo~h the volatile material added as solvent or diluent, and that generated by reac-tions such as pyrolysis of organics or acid-base metathesis~
The second largest constituent of the slurries or solutions, which constituent may or may not be present, is a Eorm of hard particles, such as the alpha aluminas men~ioned previously. These may comprise between zero and 55~ of the volume oE ~he non-volatile portion. The size, shape, volume and type oE hard partic]es is varled as needed to accolnmodate the intended specific use o the ceramic-coated materialO ~ coating containing more than 65% hard particles by volume will be too lean in binder material with the result that the hard particles will not 35 be securely bonded, and ~he voids between them may tend to trap ink in areas required to remain free of ink when the Co,ltill~ is used as a lithographic substrate. A fired ~ilm containillg 45-60% by volume hard p~rticles is dominated by relatively coarse features. This -~ype of surface is desirable for use with sorne image coatings. Fired films containing less than 50~ by volume hard particles may have their surfaces increasingly dominated by ultrafine featur~s, because the relative volume of hard particles declines, while the relative volume of ultrafine particles increases. The ultrafine features are more desirable Eor use with some image coatings~ Sedimentation and agglomera-tion of hard particles may be a source of coating problems.Where a formulation i~s required wherein the main charac-teristic is ease of coating with simple apparatus, the reduc~ion of the volume of, or ~he complete elimination oE, ~he coarser particles may be preferableO The hard particles considered for use in this invention react so slowly with phosphate solu~ions that they are presumed to undergo negligible reaction during firing The remaining consl-ituent of the slurry ~ormula-tion is a matrix or bonding constituent. In the simplest form, phosphorîc acid alone may be coated and fired; how-ever, useEul properties are enhanc~d by using acid phos-phates, such as monoaluminum phosphate, and/or reactive particles, such as alumina.
Greater chemical resistance than can be obtained with a coating derived from phosphoric acid employed by it~lf is essential or at least preferred. To obtain greater chesnical resistance, reactive particles which yield metal ions should he added to khe pho.sphate solution in quantities sufficient ~hat an orthophosphate or mixture 30 of orthophosphates is created upon firing.
Where the reactive particles are of a chemical species which yield calcium ions, a quantity ranging Erom about 0 to about 20% of the arnount ~hat would combine with the available phosphate ions to give calciu~n ortho~
35 phosphate is used~ Calcium additions in excess of 20~
result in coagulation of the slurry, and are li]cely to result in a decline in acid resistance oE the final article. Quantities in the range of about 4 to about 7%
give the rnaximum reduction oE firing temperature with no noticeable degradation of acid resistance or slurry viscosity. A solution o~ monocalcium phosphate, rather than particles which yield calcium ions~ may be used to provide the calcium content.
Where the reactive particles are of a chemical species which yield magnesium ions, a quantity ranging from about 0 to about 35~ oE the amount that would combine with the available phosphate to give magnesium orthophosphate, is used. Quantities in the range of about 10 to about 20% are preferred. Where magnesium-containing particles are added at a level greater than 35% to monvalulninum phosphate, the resulting coating will flake lS away from the aluminum substrate upon cooling down rom the ~iring temperature. ~nounts of magnesium at lower than the 10% level are found to provide a less than optirnum contribution ~o alkali resistance. Comlnercial monomagnesium phosphate solution, containing 5.3% MgO and 32.6~P205 by weight, may be used to provide magnesium ions since the amounts of magnesium and phosphate provided by this solu~ion, witn no other phosphate addition, give a magnesium ion addition oE 19.1~ of the amount required to yield magnesium orthophosphate.
~ Reactive particles yielding aluminum ions may be added in quantities ranging ~rom about 0 to about 200% of the amount which, in combination with all other metal and phosphate ions present or liberated during firing, would give ~oichiometric orthophosphate. The pre~erred range is 100-150~. For example, if the quantity of calcium-containing material in the phosphate solution slurry were 5% oE the a,nount that would give calcium orthophosphate, and if the quantit~ of Inagnesium-containing material in the phosphate solution slurry were 15% of the amount that would give magnesium orthophosphate, then the preEerre~
alnount o aluminwn ion-containing material to be added would be in the range oE 80-130% of the amount that would 3~
yiel~ aluminum orthophosph~te. I another reactive cation were present, sufficient additional aluminum ion would have to be provided in excess o~ the 100-150~ orthophos-phate level to form the other reactive cation's ortho-cornpound for the preferred compositions (e~g. f if theotner cation were silica, then sufficient additional aluminu,-n ion-containing material would have to be provided to form aluminum orthosilicate). Aluminum contents below 100~ result in coatinys lacking chemical resistance, and alurninum contents above 150%, in combination with a typical loading of hard particles, result in decreased wear resistance of the coating. The excess oE unr~acted fine particles have utility as a filler or flatting agent, imparting a useful rnicrotexture or chemical character to the surface of the Eired ~ilm. It is concei~able that the total fine aluminum reactive parkicle content could be raised into the 150-200g range, with the resul~ing improve-ment in anchorage ~or the overcoated material compensating ~or the loss of basic abrasion resistance.
Dispersants such as gluconic acid may also be added to the slurry. Alkaline dispersants such as sodium tripolyphosphates are not preferred even though they do not destroy the Eunction of the present invention.
Pre~reatment oE the coarsest particles as with colloidal silicar may make the resulting slurries more stable a~ainst sedimentation.
Lithographically useEul compositions may, of course, be coated on the cerarnic surEace. Such composi-~ions would comprise 1) oligomeric diazonium resins, 2) positive acting diazo oxides or esters, 3) photopoly-merizable organlc compositions (particularly such as ethylenically unsaturated ma~erials in the presence of ree radical photoinitiators~, 4) oligomeric diazonium resin undercoats with photopolymerizable organic composi-tion overcoats, and 5) any other various well known litho-~raphically useful photosensitive compositions.
Tllese and other aspects oE the present invention will beco~ne apparent ~roln the ~ollowing examples.

7t~ ~L_ Example 1 A precleaned aluminum oil was coa'ced with a solu~ion of 50 weight percent monoaluminum phosphate in water and dried below 100C to a coating thickness of about 3 micrometers. The surface temperature of the coating was raised to 550F (260C) in ninety seconds in an oven and removed after thirty seconds at khat tempera-ture. A positive acting photosensitive composition as described in Example 3 of U.S. Patent No. 4,247,616 was coated onto the treated surface a~ter rinsing and drying.
The composition adhered to the substrate and developed off cleanly after exposure.

Example_2 lS A precleaned alumin~n foil was coated with a composition comprising, by weight, 1~ transition alumina (nominally 0~ 5 micrometers diameter of gamma or theta -phase alumina), 15% monoaluminum phosphate~ 0.75%
magnesium oxide tparticle size less than 200 mesh~, and 72.25~ water. The coatiny was dried to a thickness of about 3 ~nicrometers.
The coated foil was placed in an oven and the surface tempera~ure of the coating was raised to 550F
(260C) in thirty seconds. Dwell time in the oven was one and one half minutes~ The coated ~oil was co~led, rinsed, and dried, then rolled up.
The foil was subsequently unrolled and coated with the positive acting photosen.sitive composition of the ~revious example~ The photosensitive layer adhered well to ~he substrate and developed o~f cleanly with no undesir-able undercuttin~ of the half tone image. The background areas were less readily attacked by alkaline developer than was the ~ubstrate oE Example 1. There was also better adhesion on press because of reduced undercutting and a Eurther increase in adhesion due ~o surface texture provided.

~ rhe procedure oE Example 2 was repeated except that 1% æinc oxide was used in place of the magnesium oxide and correspondingly less water was used. The coated aluminum was found to be somewha less resistant to developer chemicals than ~he sheet of Example 2, but still provided excellent adherenee to the photosensltive layer and provided a useful prin~ing plate surface.

Example ~
The procedure of Example 2 was repeated, using the same coating composition, but with iring effected at 600F (310C~. No differences wer~ obs2rved between the mechanical or chemical properties of the materials. Both were presumed to be ~ully dehydrated.

The procedure of Example 2 was repeated using a coating composition having 8 percent reactive (e.g., theta transition alumina~ alumina (nominally 0.5 microns diameter), 0.5 percent dead-burned magnesia (parkicle size less than 200 mesh~, and 4.0 percent alpha alumina (nominally 1.0 microns average diameter) ~hese were dispersed as a 30% raw solids slurry in 0~5 percent aqueous gluconic acid. A water and monoaluminum phosphate solution was used to provide the above weight percentages of metal oxides and 15 percent by weight of monoalu~inum phosphate and 66 percent water. The alpha alumina content conferred additional wear resistance and a higher arithmetic average roughnes~ to the fired coating~ These additional properties are advantageous in many image coating systems.

Example 6 The ~ired substrate of Example 2 was immersed eor 2-1/2 Ininutes in a solution containing 5% by weight Phila. Quartz Co. "Star" brand sodium silicate solution at 95C, then rinsed in a deionized water spray and dried.

~ 7 -23-.~t was coatecl with a negative o]..igomeric diazonium resin and an ~ ~nicron dried thickness coating of the negative ~hotopolymeric composition of Example 2 oE U~. Serial No.
103,712, filed December 14, lg79. The image develoepd cleanly upon conventional exposure and development. It gave Inany thousands of good inked impressions during an accelerated press wear test, at least equalling the wear performance of the same image material on conventional c3rained and anodized aluminum.
Example 7 A slurry was prepared consisting of l~o 1~ (wt~ ) Norton Co. "~381200 Alundum~' brand alpha alumina (prior to ad~ition it had been cleaned by firing overnight in air at 600C); 2.4% Alcoa "Hydral 710" alumina trihydrate; 0 75 Condea Chemie l'Dispural 10/2" alumina monohydrate, 0.36%
Martin Marietta "MagChem 10" magnesia, and 14.4% 50~
monoal~minum phosphate solution, plus some surface-active materials as ~ollows. The dry powders were milled ~ogether as a 30~ solids slurry~ First the alpha alumina was milled alone in a solution containing all the water o~
the 30% slurry and 2~ oE ~he weight of alumina as Nalco "Nalcoag 1115" colloidal silica, for one hour. Then enough 50~ yluconic acid was added to make the aqueous phase 0.5% acid, and the balance of the dry solids were ad~ed, and milling continued for 12 hours~ A weighed sample of the 30% slurry was added to a container with enough additional distilled water and 50% rnonoaluminum phospila~e solution to yield th~ above ~roportions, and the combination stirred with a propellor mixer and roll coated onto sheets o~ alulninum alloy cleaned by trisodium i~hosphate (3% solution at 160F) etching, rinsing, and desmutting in 50~ HNO3, r.insing, and drying. The coated sheets dried in roorn air, and were fired 2 min. in an oven 35 wl~ose circulating air temperature was ~iven by the oven colltroL thermolneter as 650F. (~rhis thermal exposure had E>reviou~ly been Eound to yield a sur~ace te~lnpera~ure oE

~ 3 ~ 2~-550F duriny the final 30 sec.) When cooled and coated with a positive-working image composition of -the diazo oxide-phenolic resin type well known in the art, the resulting lithoyrapllic plate was capable of state-of-the-art performance, with regard to image contrast, developer resistance, and wear life, as determined b~
accelerated wear tests.

Example 8 A slurry was prepared~ coated and fired similarly to that of Example 7 except that the proportions were grade 1200 alpha alumina, 17~8%; Alcoa l'~ydral 710", 3.2%; Condea "Dispural 10i2" 0O7%; MgO, 0.38%; the ~nilling tir~es were 30 min. for alpha alumina in silica 501, and ~
hours ~or all solids at 40% solids; the phosphate addition was 15.4% 50% monoaluminum phosphate solution. The resulting slurry was readily roll coatable on etched and desmutted alulninum, and also onto lithographic aluminum alloy precleaned by brushi~g with a motor-driven cylindrical "Scotch-brite" Type A Very Fine ~l~p brush, while being sprayed with 150F~ 3% solution of Oakite "#166" metal cleaner, then rinsing and air drying. The resulting lithoplates conformed to the standards established ~or Example 7.
Example 9 The slurry of Example B was roll coated onto Bethlehem Steel Co9 "Duraskin" sheet, which is steel sheet plated with a predominately alwninum, aluminum-zinc-silicon alloy, and rerolled to .010 in.
thickness. The "Duraskin"~sheet was precleaned using the same "Scotch-brite" brushing technique described in Ex-ample 8. The fired material functioned very well as the substrate for either neyative- or positive working, long-wear life, lithographic plates; no significant differencesbeiny observed between it and the lithoplates of Example 8 in its image contrast or resolution, or its wear characteristics.
~ t~,~

A slurry was prepared consisting of 19.05~
(w~/wt) of "Imsil ~-10~ amorphous silica, said by the Illinois Minerals Co. oE Cairo, Ill~, to have a mean particle diameter of 2.2 micrometers; 0.65% `'~luminum Oxide C", said by the Degussa Pigments Division of Frankfurt, Germany to have a mean particle diameter oE
0.02 rnicrometers; 32.9% deionized water; and 47O4% of the Mobil Chemical Co. 50~ monoaluminum phosphate solution~
The silica and colloidal alumina were Eirst mixed together in the water, then the phosphate solution was added. This slurry was coated onto the subs~r~e o Example 7 and fired as in the same Example It was then immersed 25 se~onds in an 80C solution containing 4.8 wt. % "Kasil No. 1" potassium silicate solution available from the Philadelphia Quartæ Co., rinsed in deionized water and air dried. The resulting sheet was primed with a negative-working diazo compound and overcoated with a neyative-working photopolymer of a type well-known in the art. The resulting printing plate was exposed to a tes~ negative and developed with 3M Subtractive Developer. It was found to have commercially acceptable photospeed, contrast, resolution, and press life.

Example_ll A slurry was prepared consistiny of 15.08~ (wt.) Norton Co. "~381200 ~lundum" brand alpha alumina (prior to addition it has been cleaned by firing overnight in air at 600C), 0~45~ (wt.) "Nalcoag 1115" 15% aqueous colloidal 30 silica suspension, 0.45~ (wt.) 50~ aqueous gluconic acid solution, 62.12% (wt.) deionized water, 2.72~ (wt.) "Aluminum Oxide C", manufactured by Deyussa Pigments Division of Frankfurt, Germany, 0.96~ (wt.) Condea Chemie "Dispural 10/2" alumina monohydrate, 4.13~ (wt.) 50 aq~eous monoaluminum phosphate solution from Mobil Chemiccll Co., 13.48% (wt.) coMmercial magnesium phosphate solution containing 3.3~ magnesium and 140 2% phosphorus t ~e~

from ~obil Chemical Co., and 0.61~ (wt.) reagent calcium hydroxide. This ~ormulation was coated onto lithographic aluioinum at a coatiny weight in the range oE 325 to 400 mg~square foot and fired at a temperature of approximately 550F for one minute. When cooled and coated with a posi-tive working composition of the diazo oxide-phenolic resin type well known in the art, the resulting lithographic plate was capable o state-of-the-art perEormance, with regard to resolution, ink/water balance, and wear life, as determined by accelerated wear tests.

Example 12 ___ A slurry was prepared consis~ing of 18.64% (wt.) Trc-ibacher "WSK" F1200 alumina (prior to addition it had been cleaned by calcinin~ at 600C) 0O56% (wt.) "Nalcoag lllS" 15~ aqueous colloidal silica suspension, 0.44% ~wt.) 50% aqueou~ gluconic acid solution, 58.64~ (wt.) deionized water, 4.21~ (wt.) "Aluminum Oxide C~', manufactured by Degussa Pigments Division of Frank~urt, Germany, 17.00~
(wt.) cornmercial magnesium phosphate solution containing 3.3~ magnesium and 14.2% phosphorus ~rom Mobil Chemical Co., and 0.51% (wt.) reagenk calcium hydroxide~ This formulation was coated onto lithographic aluminum at a coating weight in the range oE 325 to 400 mg/square foot and fired at a temperature of approximately 550F for one Ininute. When cooled and coated with a positive working composition of the diazo oxide-phenolic resin type well known in the art, the resulting litho~raphic plate was capable oE state~o~-the-art performance, with regard to resolution~ ink/water balance, and wear life, as deter-mined by accelerated wear tests.

Claims (28)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for preparing a photosensitive substrate comprising the steps of A. Providing a ceramic layer on an aluminum substrate or aluminized surface of a substrate which comprises applying a slurry of at least one mono-basic phosphate and inorganic non-metallic particles on at least one surface of the aluminum or aluminized substrate and firing the slurry at a temperature of at least 230°C to form a ceramic coating on said aluminum or aluminized surface;
B. Coating on said ceramic layer an organic photosensitive litho-graphic layer.

2. The process of claim 1 wherein said particles comprise metal oxide particles having average sizes of from 1 x 10-3 to 45 micrometers.

3. The process of claim 2 wherein said non-metallic particles comprise reactive alumina particles.

4. The process of claim 3 wherein said metal oxide particles further additionally comprise alpha alumina particles.

5. The process of claim 2 wherein said metal oxide particles comprise a reactive alumina and a metal oxide, the orthophosphate of which oxide is insol-uble in an aqueous solution having a pH of from 6 - 12.

6. The process of claim 2 wherein firing is at a temperature of at least 260°C to form a dehydrated grained ceramic coating.

7. The process of claim 5 wherein said ceramic coating comprises alumina particles bound into an amorphous phase by one or more of the polymorphic forms of aluminum orthophosphate.

8. The process of claim 6 wherein the slurry comprises about 5 to about 15 percent by volume of materials that are not volatile upon firing, at least 35 percent by volume of said nonvolatile materials being a matrix comprising phosphates and reactive metal oxides and/or reactive metal hydroxides.

9. The process of claims 4 or 5 wherein said ceramic coating has a thickness of between 0.2 and 15 micrometers.

10. The process of claim 8 wherein said slurry contains a metal oxide, the orthophosphate of which oxide is insoluble in an aqueous solution having a pH of from 6 - 12.

11. The process of claim 2 wherein said monobasic phosphate is formed in situ during firing by the reaction of phosphoric acid with a tribasic phosphate.

12. The process of claim 2 wherein said monobasic phosphate is formed in situ during firing by the reaction of phosphoric acid with alumina and/or aluminum hydroxide,

13. The process of claim 2 wherein said monobasic phosphate is formed in situ during firing by the reaction of alumina with a monobasic phosphate not consisting essentially of monoaluminum phosphate.

14. The process of claim 1 wherein a calcium-containing compound is included in the slurry.

15. The process of claim 14 wherein said calcium-containing compound is calcium hydroxide, calcium carbonate, calcium oxide, calcium phosphate, or mixtures thereof.

16. The process of claim 8 wherein said ceramic coating has a thickness of between 0.5 and 5 micrometers.

17. The process of claim 2 wherein only a portion of the metal oxide particles comprise magnesia.

18. A photosensitive article comprising:
A. an aluminum or aluminized substrate bearing on at least one aluminum or aluminized surface thereof a ceramic layer comprising 1) non-metallic inorganic particles and 2) a water-resistant phase, or phases, of a dehydration product of at least one monsobasic phosphate;
and B. An organic photosensitive lithographic coating over said ceramic layer.

19. The photosensitive article of claim 18 wherein said particles comprise metal oxide particles having an average size of from 1 x 10-3 to 45 micrometers.

20. The photosensitive article of claim 19 wherein said metal oxide particles comprise alumina.

21. The photosensitive article of claim 19 wherein said ceramic layer has a thickness between 0.2 and 15 micrometers said substrate is aluminum in the form of a film or sheet, and said lithographic coating comprises a polymeriable composition.

22. The photosensitive article of claim 19 wherein said ceramic layer has a thickness between 0.2 and 15 micrometers and said substrate is aluminum in the form of a film or sheet, and said lithographic coating comprises a positive acting o-quinone diazide in a polymeric binder.

23. The photosensitive article of claim 20 wherein said ceramic layer has a thickness between 0.5 and 5 micrometers.

24. The photosensitive article of claim 19 wherein said ceramic layer con-tains a reaction product of alumina with a monobasic phosphate and a reaction product of at least one metal oxide other than alumina with a monobasic phos-phate, the orthophosphate of said at least one metal oxide being insoluble in an aqueous solution having a pH of from 6 - 12.

25. The photosensitive article of claim 18 or 19 wherein said coating com-prises an oligomeric diazonium resin.

26. The photosensitive article of claim 19 wherein said particles are bound into an amorphous phase of aluminum phosphates formed by dehydration of monoaluminum phosphate.

27. The photosensitive article of claim 19 wherein said metal oxide part-icles comprises magnesia.

28. The photosensitive article of claim 18 wherein the ceramic layer further comprises the reaction product of said at least one monobasic phosphate with said particles.