CA1197408A - Lithographic substrate and its process of manufacture - 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 a lithographically suit-able substrate on an aluminum surface can be provided by the firing of at least one monobasic phosphate solution on substrates having an aluminum surface.

Description

LITIIOG PIIIC SUBSTRATE AND ITS PROCESS OF MANUFACTURE

BACKGR0UND OF T~IE IN~ENTION
Lithographic printing plates have been widely used for many years.
One of -the basic concepts utilized in that technology is the establishment of differential wettability between areas on the planographic printing surface.
This is most usually effected by providing a substrate having a hydrophilic or water wettable surface and coating that surface with an imageable and develop-able (usually light or radiation sensitive) grease sensitive (oleophilic) and hydrophobic layer. After imaging and developing of this layer, the printing ~o plate variously exposes hydrophilic and hydrophobic surfaces. When the plate is then wet with water and then coated with grease or oil-based printing inks, only the hydrophobic areas o:E the image will hold the ink and provide a dark image when pressed against a receiving surface such as paper.
This lithographic process has been able to provide planographic print-ing plates which 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 single imaging and developing procedure must be performed, and ~he printing press need not be shut down during operation in order to change 2n plates.
The investigation of means for lengthening the running time of plano-graphic printing plates has been the focus of many studies and research. It is also necessary in the provision of more durable plates to nlaintain or improve such other desirable or essential characteristics of printing plates such as imaging speed, ease of development, non-polluting chemistry, and shelf life.
Most of the research in providing longer running and ~igher durability plano-graphic printing plates has centered on the imageable and hydrophobic coa~ing layer on X ~7k 3160~ CANlA ~-~t7~

the hydrophiLic surface of the substrate. As it is usually the breakdown o~ this layer which causes failure o~ the printing plate, the loyic oE that directiorl of research is ap~arant.
S In general, the subs~rate provided for the imayeable layer is an alurninum sheet which has been prepared Eor coating by a variety of cleanin~, rnechanical, and chemical treatments. For example, an aluminum surface is usually cleaned to remove oils and other contaminants ~his cleaning is then followed by a rnechanical or electro-chemical graining process, and finally an anodi~ ny ~rocess.
For example~ U.S~ Patent No. 3,963,59~ discloses the electrochemical etching of an alurninum substrate ~ith an aqueous solution of hydrochloric acid and gluconic acid. This provides a uni~or~nly coarse surface texture to the aluminu~ substrate. The process of ~hat ~a~ent ~rlnits the use oE lower ~ur~nt densiti~s that were ~oun to be necessary when using only hydrochloric acid in the bath. rrhe treatment also complexed dissolved aluminum ancl other impurities which form in the etching bathO Thix extended the life of the bath. Desmutting treatlnents were also shown in combination with tlle electrochemical etching.
British Paten~ No. 1,439,127 discloses an anodi~.ing treatment ~or aiuminum substrates in which the ~nodization is performed in an aqueous sulfuric acid bath ~t a tem~erature in excess oE 70C and with an anodi~ing current density of at least 50 amps/sq. t. This treat-ment redu~es the length of time in which the anodizing step is per~orln~d. Other etching solution comi~ositions and electrical paralneters are dis~losed in U~SO Pa-tents Nos. 4,072,5~9 and ~,052,275 and French Patent No.

2,322,015.
U.S. Patent ~lo. 3,030,210 and French Patent l~o.
~,025,S50 discloses lithographic plate substrates prepared by immersion in phosphate-con-taining solutions, rinsing, 7~3 . -3-and drying. I`he phosphate-containing layer produced by such a metho~ is com-paratively thin7 soft and hydrated and does not have the intrinsic wear-resisting properties conferred by firing a ceramic coating a-t elevated tempera-tures.
After electrochemical or mechanical etching o:f the aluminum surEace, an anodically oxidizing treatment is usually pe-rformed to render the aluminum surface both corrosion resistant and wear resistant. This is shown in British Patent 17~39,127 discussed above and in U.S. Patent No. 4,131,518. In this la~ter patent, aluminum -Eoil in the form of a continuous web, particularly when the aluminum carries a polymeric coating on one side thereo~, is anodizin~ so that energy requirements are reduced and anodizing speeds are increased.
U.S. Patent No. 3,181,461 discloses an anodizing process in which the sulfuric acid anodi~ing step is followed by treatment with an aqueous solution of sodium silicate. This treatment seals the pores of the anodic oxide and pro-vides a hydrophilic, ink~repelling surface ]ayer.
The practice of treating lithographic substrates of aluminum in order to increase surface areas and enable anodic coatings is well described in the art such as U.S. Patents ~os. 3,935,080; 3,9~9,591; 3,9807539; and 3,9887217.
Even though anodizing has bacome ~he most common Tneans of providing an aluminum substrate for durable planographic printing plates7 and even though some reduction of energy requirements has been made7 the process still requires large amounts of electrical energy in its operation and also generates efflu-ents that must be care*ully disposed of to avoid environmental pollution.
A novel method for providing textured surfaces on lithographic print-ing plate substrates of aluminum was disclosed in U.S. Patent No. 3,210,18~. A
layer of boehmite (aluminum oxide monohydratè) was produced on the aluminum sub-strate by bathin~ the aluminum in hot water or steam in the presence of a weak J~

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organic base. Printing plates using this textured su~strate wer~ shown to pro-vide increased numbers of copies under comparable condit:ions as compared to printing plates using mechanically roughened aluminum substrates.
German Patent (Ofenlegungsschrift) 24 3~ 098 discusses -firing a com-position of aluminum phosphate and silicon carbide particles onto a metal sur-face of the purpose of increasi.ng i-ts wear resistance. However, the invention of German Patent 2~ 34 098 is not suitable for use on lithographic plates and is not performed on aluminum surfaces.
U.S. Patent Nos. 3,871,881 and 3,975,1~7 disclose another method of enhancing the physical properties o aluminum surfaces. Various types of part-iculate material are bonded to the surface of the aluminum by an in situ formed binder of aluminum hydroxyoxide. The enhanced aluminum article is suggested for use as a substrate for printing plates.
It is disclosed in the description of the present invention that a novel process for treating lithographic quality aluminum foil can provide a durable, long-running substrate for lithographic plate constructions. This pro-cess provides a novel substrate which can be resistant to the chemical action of printing plate developers and press solvents. The novel substrate also en-ables the bonding of many photoreactive imaging layers to the treated aluminum substrate without the need for primers or other adhesion promoting agents.
SU~ARY OF T~IE INVENTION
A process is disclosed for firing a solution of at least one mono-basic phosphate, preferably monoaluminum phosphate, on a alumi.num substrate or aluminized surface of a substrate. This process produces an aluminum sheet bearing a layer o glass or polymorphic forms of aluminum ~i7~

phosphate or mixtures oE aluminum phosphates on at least one surface thereof.
This coated layer has been found to provide an excellent surface Eor adhesion of organic materials. In particular, -the a].uminum phosphate layer provides excell-ent adhesion for diazonium resins and photopolymeric compositions used in the printing art and particularly in the planographic printing artr According to one aspect of the present invention there is provided a process for coa-ting an aluminum or aluminized substra-te wherein a layer compris-ing at least one monobasic phosphate is applied on at least one aluminum surface of the aluminum or aluminized substrate and the monobasic phosphate is then fired at a temperature of at least 230C to form a ceramic coating having a thickness oE between 0.2 and 15 micrometers on said aluminum wherein said at least one sur-face of said aluminum or aluminized surface has a microscopic, lithographically useful texture in the aluminum and wherein said ceramic coating is etched aEter firing.
According to another aspect of the present invention there is provided an article comprising an aluminum or aluminized substrate bearing on at least one alumi.num or aluminized surface thereof a fired ceramic coating comprising a poly-meric form of aluminum phosphate or mixtures of aluminum phospha-tes, said coating being substantially free of particulate material, wherein a pho-tosensitive layer is coated on said ceramic layer.
DETAILED DESCRIPTION OF THE INVENTION
The process of forming aluminum phosphate coatings on substrates accor-ding to -the present invention provides a number of improvements over prior ar-t processes for producing substrates for pho-toimaging elemen-ts, particularly in -the continuous manufacture of substrates. No-t only does the coa-ted substra-te of the present invention have equivalent or improved properties as compared -to materials of the prior art, b~t also provides significant economic advantages in i-ts manu-facture. Apparatus used in the process consists of fewer separate items of equip-ment, thus requiring a lower capital investment than conven-tional forms of con-tinuous substrate formation. The significant equipment eliminated includes the anodizing facili-ty which is itself costly to operate because of high energy re-quirements and the need for safe effluen-t disposal. Such equipment is desirably eliminated from a substrate manufacturing line because of associa-ted electrochem-ical corrosion problems of other equipment on the same produc-tion line.
A coating of a monobasie phosphate solution is applied to a clean a]um-inum or aluminized surface. Suitable monobasic phosphates include -monoaluminum phosphate and alkaline earth acid phosphates such as monomagnesium phosphate, monocalcium phosphate, and mix-tures thereof. This coating is fired at a tempera-ture of at least 450F (230C), preferably at least 500 or 550F (260 or 290C), to produee a ceramic coating of a glass or polymorphie form of aluminum phosphate or mixture of aluminum phosphates. The eeramie surface may be etched to provide desired -5a-7~0~
~5--texture to the surace. This e-tciling is ~nost conveniently perforlned for substrates to be used ln liti~o~rapl~ic plates at the same ti~ne in which silica~in(3 of th~ substrate is bein~ eefected~ ThiS can be accomplished by using known alkaline silicate solutions which will etch and deposit a silicate coating at the salne timeO Where no si:Licatiny is re~uired or where ~he subsequently applied light sensitive composition would not be compatible Wit}l a silicate surface, the etch Inay be performed in al]caline phosphate or alu~ninate solu~ions, for example. The aluminuln or alu,ninized substrate shoul~ initially have a texturized surface so tllat etching of the ceramic coating will restore the relief provided by the underlying ~exture under ~he cera,nic coating and provide additional lnicroscopic texture.
This texture from bo~h sources, which is a rnicrQscOpiC
texturing visible by light scattering or under magniEication, provides a physical structure to ~hich su~equ~ tly applied ligll~ s,ensitive coatiny com~ositions may adhere. ~ny o~ the available known proce~ses Eor ~rovi~1inc~ texture to ~he substrate Inay be used, but i~ is preferred to use rnechanical graining such as slurry brush yraining.
The post-firing etch Inay remove whatever amount of the dehydrated ceramic coatin~ is necessary to ~rovide ~he character required in the texture oE the substrate.
As little as five percent and as much as sixty percen~ b~
weight or more of the ceramic coating may have to be relnoved, but generally ~etween fi~teen and Eifty perce~nt of the coating is removed, and preerably between twenty-f ive and Eorty percent is rernoved. The length of tlme of tlle etch is regulate~ by the temperature ancl p~ of t~le etchinc3 ~nvironment. Higher temperatures and higher p~l levels provide ~aster etches. The pH may be controlled ~y the addition of alkaline llydroxides such as sodium or potassium hydroxide. ~eplenishing solutions may be added ~urin(~ tile ~ontinuous processing operation ~o replacc? dny naterial~ such as the alkali component, which i5 depletecl _~ -7 clurillg the etch. The combined etch and silicatin.~ solu-tions are generally optimized to emphasi~e the silicatiny treatment since the silica~e etch has a wider ~erformance latitude than phosphate or aluminate etchin~ solutions.
The silica~es used for the combined etching and silicating baths are preferably at the high silica content ent3 of the sommercially available materials. Such materials as asi ~ 1", or "S-35~ of the Philadelphia Quartz Co~, or mixtures of "S-35~ wi-th a fine silica sol (e.y., 'INalcoag #1115" of l~alco Chemical Co.) are particularly useEul when diluted ~ith water to give solutions having approxirnately one percent silica on a dry weight basisO
The texturized substrates produced by the etchiny of the ceramic coated aluminum or aluminized substrate may then be coated with a light sensitive composition. A~ oligomeric diazoni~n resin and/or an or~anic negative acting pho~osensitive composition may be desirably applied to the texture~ surface4 The surface p~ovided on the alumin~m substrate ~0 is highly water receptive and has been shown to be at least as hydrophilic as anodized aluminum. The sur~ce provides excellent adhesion for polymeric and oligomeric compositions. The sur~ace has been found to provide excellent adhesion Eor positive acting photosensi tive ~5 compositions such as those containing diazo oxides and dia~o sulfides.
The -thickness of tlle ceramic coating can readily be varied as desired, for example, between 0.2 and 15 mi.crolneters. Pre~erably, for use as a sub~trate for ~lano-graphic printing plates, the coating layer is between 0~3 al~d 10 microme~ers and more preEerabl~ is between 0.5 and 5 Inicrolneters.
The firing temperatures used in the practice of the present invention must be higher than 450 or 500F
(230 or 260C) and preferably are at least 5$0~'F (285~)~
Temperatures higher than 700F (370C) do not o~fer any significant advantages and tend to raise the energy ~ ~rA~

` --8-requirements of the process. The firing should be performed for a long enough period of time at these temperatures -to insure substantially complete dehydra-tion of the dried coating. This may take place in as little as -fifteen seconds dwell time at the described ~emperatures depending upon the thickness of the coating and the temperature and other parameters of the firing process. These temperatures refer to the sur-face temperature of the coating as measured by con-tacting that surface with the bare junction o-f a thermocouple. The surface tem-perature may vary quite markedly from the control temperature of various ovens and so this type of measurement is desirable.
Particulate matter such as magnesium oxide, silica, alumina, chromia, and ferric oxide may be added to the monobasic phosphata solution with conse-quent benefits. The addition of some of these ma-terials, such as magnesium oxide and alumina in particular, provide increased resistance to attack from basic developers and provide additional qualities to the graininess of the coat-ing layer. The improvement is the subject matter of assignee's copending Canadian Patent Application S.N. 411458, filed September 15, 1982 in the name of L. A. Brey and D. ~. Cadwell~ Flocculation inhibitors such as gluconic acid may also be added to the slurry, but alkaline dispersants such as alkali phos-phates are not preferred even though they do not destroy the function of the presen-t invention.
The process can be readily performed in a continuous manner and has been Eound to provide satisfactory results when performed in this fashion on a web.
Lithographically useful composition may~ of course, be coated on the coated surface. Such compositions would comprise 1) oligomeric diazonium resins, 2) positive acting diazo oxides or esters, 3) photopolymerizable organic compositions (particularly such as ethylenically unsaturated materials in the presence of free radical photoinitiators)~ 4) oligomeric diazonium X

resin undercoats wih photopolyinerizable oryanic co~posi-tion overcoats, and 5) any otiler various well known litho~3raphically useful photosensitive co~positions.
These and other aspects of the present invention will become apparent from the following examples.

Exam~
A precleaned, ungrained aluminum foil was coated with a solution of 25 weight percent monoal~ninum pho.sphate in water and dried above 100C to a coating thickness of about 3 ~nicro,neter.s. The surface tem~erature of the coating ~as raised to 550F (260C~ in ninety seconds in an oven and removed after thirty seconds at thdt temperature. A positive actiny photosensitive composition as described in Example 3 of U.S. Patent 4,247,616 was coa~ed on~o the treated surface after rinsin~ and drying. The composition adhered well to the ~ub.~trate and develo~ed of f cleanly after expoSur~-.

~J~ _ 2 A precleaned~ ungrained aluminum foil was coatecl ~ith a composition comprisin~, b~ weight, 12% alumina (nominally 0.5 micrometers diameter), 15% monoaluMinum phosphate, 0.75~ mag~esium oxide ~particle size less than ~5 200 mesh), and 72025~ water. The coating was dried to a thickness o~ about 3 Tnicro,neters.
The coated fil-ln was placed in an oven and the surEace temperature of the coating was raise-~ to 550F
(260~C) in thirt~ seconds. Dwell time in the oven was one and one half minutes. The coated film was cooled, rinsed, and dried, then rolled u~.
The foil was subse~uently unrolled and coated with the ~ositive acting photosensitive com~osition of the previous example. The photosensitive layer adhered well to the substrate and developed off c-leanly with no undesirable undercuttin~ of the half tone image.

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Example 3 The procedure of Example 2 was repeate-~ except that 1% zillc oxide was used in place of the magnesium oxide and corre~pondingly less water was used. The coated aluminu,n was found to be somewhat less resistant to developer chemicals than the sheek of Example 2, but still provided excellent adherence to the photosensitive layer and provided a ~aseful printi~g plake surface.

Example 4 The procedure of Example 2 was repeated, using the ~ame coating cornposition, but with firing èf~ected at 600F (310C). No differences were observed between the mechanical or chemical pro~erties o~ the materidls. Both were presumed to be fully dehydrated.

Example 5 An aluminum foil similar to that of the previous examples except roughened by a rotary brush Eed with an abrasive slurry such as pumice, to give a mechenically a~ra~ed, lithogra~hically use~ul surfa~e texture ~s is well-known in the art, was coated with a solution of 25 weight percent monoaluminwn phosphate in water, (~ried one Ininute in still air and 300F, a second minute at 300F in moving air, and ~ired for one ~ninute during which the surEace temperature rose to 550F. The coated film was then immersed 90 seconds in a solukion containing 4O8~ by weight of "Kasil #l" brand potassium sili~ate solution (Philadelphia Quartz Co.~, .03~ potassium hydroxide, and the ~alance water, at ~5Cq The silicate treated Eoil was rill~cd in a .~pr~y o~ deioni~.ed w~ter [o~ 30 seconds, dried, and coated with a 12 Inicron nega-tive acting photo-~olymeric colnposikion having acrylate monQmers and a photo~ensitizer. Upon ~eing mounted side by-side as a hal~ plate, the other half being a factory~inade plate oE
thc same photopolymer com~o.sition on standard an3dized aluminuln, and run ~or thousands of impressions, the aluminum phosphate coated plate gave at least as many good impressions beEore showing wear, as the :Eactory-made anodized plate.

Exalnple 6 A grained aluminum foil prepared, phosphate coated and fired as .in Example 5, and similarly etched except in a 5.25 weight percent solution of Philadelphia Quartz S-35 sodium silicate in water, coated with the same imageable layer and tested similarlyr also equalled the performance of the commercial anodized plate.

Example 7 A grained aluminum foil prepared, phosphate coated and ired as in Example 5, and similarly etched exce~t in a p~ 10.4 solution of sodium pyrophosphate at 70C, was coated with a positive-working photopolyllleric c~ln~osition, and tested silnilarly a~ainst a ~actor~made~
pla~e. It gave several times as many copies before showing wear as the Tartan 25 plate, which i.5 ~rush ~3rained similarly to the experimental plate but is not hard coated or anodi2ed.

A grained aluminum foil prepared, phosphate coated and fired as above, was etched as above except in a ~H 10.4 solution of "~alco 68 ~ sodiwn aluminate, mad~ by the Nalco Chelnical Co~ ancl te.sted simi.l.arly. It yav~
several tilnes as many copies before showing wear as t~le ac tory-made plate .

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Claims (15)

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

1. A process for coating an aluminum or aluminized substrate wherein a layer comprising at least one monobasic phosphate is applied on at least one alu-minum surface of the aluminum or aluminized substrate and the monobasic phosphate is then fired at a temperature of at least 230°C to form a ceramic coating having a thickness of between 0.2 and 15 micrometers on said aluminum wherein said at least one surface of said aluminum or aluminized surface has a microscopic, litho-graphically useful texture in the aluminum and wherein said ceramic coating is etched after firing.

2. The process of claim 1 wherein said monobasic phosphate is monoaluminum phosphate.

3. The process of claim 1 wherein firing is at a temperature of at least 260°C to form a dehydrated coating.

4. The process of claim 1 wherein firing is at a temperature of at least 280°C to form a dehydrated coating.

5. The process of claim 3 wherein said ceramic coating is a polymorphic form of aluminum phosphate or mixtures of aluminum phosphate.

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

7. An article comprising an aluminum or aluminized substrate bearing on at least one aluminum or aluminized surface thereof a fired ceramic coating compris-ing a polymeric form of aluminum phosphate or mixtures of aluminum phosphates, said coating being substantially free of particulate material, wherein a photo-sensitive layer is coated on said ceramic layer.

8. The article of claim 7 wherein said aluminum or aluminized substrate has a lithographically useful microscopic texture on said at least one surface bearing the ceramic coating.

9. The article of claim 8 wherein the ceramic coating has been etched in an alkaline solution, rinsed and dried.

10. The article of claim 7 wherein said coating is between 0.2 and 15 micrometers and said aluminum is in the form of a film or sheet.

11. The article of claim 7 wherein said coating is between 0.5 and 5 micro-meters.

12. The article of claim 7, 8 or 9 wherein a photosensitive layer compris-ing organic polymerizable composition is coated on said ceramic layer.

13. The article of claim 10 or 11 wherein a photosensitive layer comprising organic polymerizable composition is coated on said ceramic layer.

14. The article of claim 7, 8 or 9 wherein a photosensitive layer compris-ing an oligomeric diazonium resin is coated on said ceramic layer.

15. The article of claim 10 or 11 wherein a photosensitive layer comprising an oligomeric diazonium resin is coated on said ceramic layer.