US2003268A - Method of etching planographic plates, composition therefor, and resulting product - Google Patents

Description

Patented May 2a, 1935 2,003,268

UNITED STATES PATENT OFFICE PLATES, COMPOSITION THEREFOR, AND RESULTING PRODUCT William B. Wescott, Dover, Mara, assignmto l 'aph Company, Wilmington, DeL, a corporation of Delaware No Drawing. Application September 1:, less,

Serial No. 689,285

36 Claims. (Cl. ll-41.5)

This invention relates toa method of combined image has been delineated on the so-prepared etching and dampening metal planographic surface, the non-printing or non-image areas printing plates, to compositions effective for practhereof are invariably desensitized or "etched tieing Said m thod and to the product: and it prior to printing in order to remove the basic salt 5 comprises a method of treating the non-printing and deposit in its stead a fllm of an acidic salt or 5 areas of the printing surface of a plate with an an adsorbed layer 0i an acidic anic 0011016, aqueous solution comprising a passivator for theboth; and the etched plate is maintained in a metal, chosen from the designated class of passidamp condition throughout the printing opera- 4 vating agents effective in solution with an acid t by means of Water 810118 more commonly 1 phosphate of the alkali metal group, and such by means of a solution o cu erotic and a week in acid phosphate as is capable of rendering the acid. metal surface preferentially water-receptive and N w sta d the mflintenenee 0f the 11011- chemically inert to acidic inks and further capap int a a of the p at in a d p condible of displacing ink scums from said surface tion dur the i k d pri t operat on.

after said treatment, and sometimes but not alsom ink ay n ve e ess beco mec anica y 15 ways a peptizing agent for such products of -adherent to the plate in these areas, and etches reaction of the metal and the components of said Of Widely Variant 00111130511511!!! have been 5118- solution as may occur dissociate fromthe surface gested and used to efiect the deteehmeht 01 this of the plate, said peptizing agent being preferably extraneous 1111b During the Printing Operation,

but not necessarily hygroscopic and being capable the hen-printing areas 01 the P ate require not 20 of depressing the diffusion rate in said solution at only to he p p, but y need also to e the late surface and also preferably but not maintained in p e a y w receptive or necessarily capable of itself rendering the surface hydrophilie Co d and ee from extraneous of the treated plate 'hydrophilic, whereby said areas are rendered and may be maintained pas- Impwvements in method and in means for 5 -sive in and with respect to said solution, inert to filling the last three v e t oned requireacidic inks, hydrophilic and sometimes but not ments of pla e p c p int n p ates a e o always hygroscopic and also whereby crystalline ated a p of t e p se t i v nexcrescences on said areas and crystalline pre- Wh n the n ing areas are photog phi l cipitates in said solution are avoided; and it furdetermined by a l ghta d ed a b image, 30 ther comprises compositions, effective for practict is e desideletllm 0f the art that the we i ing said method, buifered'substantially at the of t a e substance under the nf ue ce of isoelectric point of light-hardened albumin the dampening a s he p at u whereby the swelling of said albumin, as the subwhereby th l f a d n -r t nt v apa ty of stance of a printing image, by said composition t e image under printing C t s y -1 35 is substantially held to a minimum; and it furp x a the ximum obta a ther comprises a planographic printing plate h present invention relates in P to having the non-image areas thereof substantially provements in method and in means of achieving passive when wet with a solution comprising an the ve-me ned d s e e 40 acid phosphate: all as more fully hereinafter de- The etches which have been heretofore sug- 40 scribed and claimed. gested or used for treating the non-printing areas The art of planographic printing from metal of planoeraph p nt p a af r th ima plates grew out of that of lithography and it is has been delineated thereon have all, so far as based upon the same fundamental principles as I am aware, failed to provide in one or more is that older art. These principles relate to the particulars the cooperative functional character- 45 immiscibility of oil and water, to the persistency istics of the ideal etch. of a greasy image on a surface chemically reac- It is a purpose of this invention to provide a tive with an acidic component of the image-formmethod of planographic etching which has none ing material, and to the greaseor ink-repellency of the deficiencies of prior methods and which 00 of a damp hydrophilic surface. achieves advantages not hitherto obtainable."

Metal planographic plates are usually, though One ofthe objects of this invention is'to pronot necessarily, provided with a roughened or videanetching composition which concomitantly grained surface rendered in general chemically will render the non-printing areas (a) chemically reactive with acidic inks by a film of a neutral inert to acidic printing inks, and simultaneously or basic salt deposited thereon; and, after the (b) substantially inert to further reaction with 66 the etch, i. e., passive, (c) preferentially waterreceptive or hydrophilic and (d) moist. Another object is to maintain the non-printing areas moist by intermittent supply thereto of the etch throughout the printing operation whereby abrasions of or other damage to the etch-reacted surface of said areas are immediately repaired by said etch and said areas are kept in an ink-repellent and scum-free condition. A further object is to provide an etching composition which will maintain the said areas free from ink-retentive microscopic crystalline masses. Another object is to provide an etch having a component which is both capable of depressing the diffusion rate at the plate surface or reducing the rate of reaction with the metal and also capable of rendering the film of etch retained by the non-printing areas of the plate hygroscopic whereby these areas tend to remain moist and substantially inert to both the etch and the reactive elements of the atmosphere, such as oxygen and carbon dioxide. A still further object is to peptize such products of the reaction between the etch and the metal as may be formed in the etch dissociate from the surface of the plate while said products are still of molecular or, at most, colloidal dimensions; whereby is avoided the formation of crystalline precipitates capable of destructive attrition of an albumin image and abrasion of the reacted surface of the non-printing areas. A further purpose is to substantially maintain the integrity of the grain of the non-printing areas in, and notwithstanding, continuous contact with an etch which is effective to displace any extraneous ink which may have become mechanically attached to the plate in said areas, whereby the function of the grain is unimpaired and such displaced ink may,

be removed by the form ink-rolls. Still another .object is to provide a combined etch and dampening means which is bufiered substantially at or close to the isoelectric point of the light-hardened albumin substance of a printing image, whereby the swelling of said image substance under the influence of said etch and dampening means is maintained at a minimum and the active life and ink-retentive capacity thereof is maintained at a maximum. A further object is to provide an etch which by appropriate variations in concentration may serve as a powerful inkor scumremoving agent, as an effective though innocuous fountain solution, and by appropriate increase in viscosity as the dispersed phase of a dual-function emulsoid ink. And yet another purpose is to provide a planographic printing plate, the nonprinting areas of which are both hydrophilic and substantially passive in contact with an etching solution of the type hereinafter described. Other objects and advantages will appear from the following disclosure.

I have discovered as part of this invention that, when a zinc or aluminum printing plate is reacted with a metal saltchosen from the hereinafter designated classin solution with an acid phosphate of the alkali metal group, the metal surface undergoes a rapid modification and then appears to remain substantially unchanged, although contact with the reactive solution be maintained for a period of many hours. The said modification may not usually be detected by either macroscopic or microscopic visual inspection, although in the case of highly polished surface interference colors aresometimes observable and are thought to be due to internal reflection of light in an extremely tenuous surface film. The fact of surface modification may be readily demonstrated however by showing the ease with which acidic. inks are displaced from the modified surface by the salt solution used to effect the modification, or by other salt solution capable of detaching ink from desensitized surfaces.

Furthermore the modified surface will be found to be relatively more hydrophilic or preferentially water-wettable than previously. Even though the surface modification is of,extreme thinness, the presence therein of constituents of the treating solution may be demonstrated. Thus, if zinc be treated with nickel nitrate and ammonium acid phosphate for one or two seconds only and then washed thoroughly, nickel may be readily demonstrated to be upon the surface. The presence of the phosphate is usually not so readily demonstrable, but if the treatment be prolonged for several minutes (3 to 5 depending on the salt concentrationand other factors to be hereinafter considered) the presence of phosphate may then be demonstrated, as well as that of the nickel. The surface modification in this case is thought, but not conclusively determined to be, an insoluble complex of the zinc-nickel ammonium phosphate variety.

The method and compositions of this invention are in general as effective for the treatment of aluminum as they are for zinc; but inasmuch as zinc is the more commonly used metal and preferred because of its relative softness notwithstanding the fact that it is somewhat more difficult to control chemically for planographic purposes, it will be more extensively considered hereinafter than will its substitutable equivalent, aluminum.

After several minutes treatment, the abovedescribed reaction appears to stop and, although there is evidence that it does not entirely stop but is rather enormously reduced in rate, for practical purposes it is considered to have been substantially arrested and the metal surface rendered passive. Thus, in marked contrast to prior etching solutions, the compositions of this invention cause no appreciable erosion of or crystallization upon thetreated metal, as indicated for instance by the weight which is substantially constant even when treatment by immersion is prolonged for many hours.

The chemistry of the reactions of the compositions of this invention is not clearly understood, and since the improvements afforded by the methods of this invention are in some instances quantitatively relative, it is thought instructive to draw a comparison between that prior etch which casually seems most nearly to parallel those of this invention and one of the latter.

If, under strictly comparable conditions of salt concentration, metal surface condition, time, and temperature, two series of thin foils of strip zinc, each strip of which weighing about 0.05 gram per sq. cm. of total surface, be chosen, and be sensitized with a conventional nitric acid-alum counter-etch; and if one of these series of foils be immersed in that prior etch consisting of ammonium nitrate, ammonium acid phosphate, and water, and the other series, identical with the first both as to mass, surface area, and condition of each strip, be immersed in an etch of this invention consisting of nickel nitrate, ammonium acid phosphate, and water; and if both be allowed to remain so immersed and undisturbed for a period of twenty-four hours, the strips of the former will be found to have become encrusted with minute crystal masses and on the average to have acquired a mean weight in- 2,008,268 crement of the order of 2.90% while the strips.

of the lattertreated according to the method of this invention-will be found to have retained a bright metallic appearance and to have only ac uired a mean weight increment of the order of 0.23%.

When, as is contemplated in the method of this invention, a suitable colloid or viscous material, chosen from the following category'of such materials, listed in the order of their decreasing preference: glycerin, gum tragacanth, ethylene glycol, gum arable, is added to an etch of this invention, there is produced a difference in the effect of the product which is of distinct advantage over that obtaining when any of these materials are added to usual etches. Thus an increase in the viscosity of the etch as a whole, such as may be produced by glycerin, or an increase produced locally, such as occurs closely adjacent to the metal surface because of inclusion of gum tragacanth which not only is adsorbed on the metal surface but also tends to be fiocculated near the surface, functions to depress the diffusion rate or to reduce the rate of the. reaction. This reduction in rate of the reaction is no doubt produced with prior etches containing gum arabic or glycerin as well as with those of this invention; but beyond that common reduction in rate there is a markedly different and, as stated, advantageous effect characterized by the absence of turbidity and crystalline precipitates in the etching solutions of this invention when in contact with metals under conditions which, in the absence of such added materials, would allow the production of turbidity and a highly undesirable crystalline precipitate in the etching solution dissociate from the metal surface. This effect is thought to be due' to the peptization of such products of reaction as may be formed dissociate from the surface while these products are still of molecular or, at most, col-' loidal dimensions.

Under the conditions of test above-described. the substitution of glycerin for some of the water of my nickel nitrate-ammonium acid phosphate etch reduces the mean weight increment from the noted 0.23% to a value of about the next lower order and also that of the experimental error and hence not definitely ascertainable. A similar substitution of glycerin for some of the water of the prior ammonium nitrate-ammonium acid phosphate etches produced under like conditions no significant change in the mean Weight increment; whichwas, as it was also in the absence of glycerin, noted to be of the order of 2.90%.

When the foils were not counter-etched but were treated in an oxidized condition, the weight increment effected by the said'prior art etch was again not appreciably influenced by the presence of glycerin in the etch; but not so the etch of this invention which, acting upon this oxidized zinc in the absence of glycerin but under conditions otherwise as above described, produced a mean weight increment of circa 1.98% and considerable turbidity and crystalline precipitate. When however, under like conditions, glycerin was substituted for some of the water of the said etch of this invention, the treated zinc foils were found to have acquired a mean weight increment of the order of 0.50% and substantially no turbidity nor any crystalline precipitate could be macroscopically observed in the etch. The turbidityjs occasioned by the suspension in the etch of fine crystalline bodies thought but not conclusively.

demonstrated to be of the same general composition'as the passivating layer on the metal.

It has been shown heretofore that the nitrate radical is reduced by hydrogen at the metal surface and therefore serves to depolarize the hydrogen film and permit uniform action of the acid. The metal nitrates are not of themselves alone capableof passivating zinc-they are destructive in fact.

It has also been stated in the literature of the art that any nitrate will desensitize zinc. But I have found no nitrate which will alone produce a desensitization of a metal useful in planographic printing remotely comparable to that which I have discovered to be effected by certain metal nitrates or chlorates in combination with certain primary phosphates of the alkali metal group whereby the, metal is not only-desensitized'to acidic inks but its surface is also rendered substantially inert to the salt solution which produced it. Nevertheless the salts of the metals which I have found useful in cooperation with acid phosphates for the purposes of this invention lower the desensitization capacity of the acid phosphates alone and preferably therefore should not be used in amounts substantially greater than is requisite to render and maintain the reacted surface substantially inert to further reaction with the cooperating phosphate and to prevent the undesirable crystallization on the surface, as heretof ore experienced, when reacted with the acid phosphate alone or in combination with the heretofore commonly used ammonium nitrate and or ammoniumfiuoride.

It has been discovered as part of this invention that the following salts in combination with the listed primary salts of the alkali metal group are effective to produce a substantially inert surface condition which is herein designated as a passive or passivated surface. The category of salts which will hereinafter be referredto as passivators or passivating agents and which are useful for the purposes of this invention comprises: ferric nitrate, nickel chlorate, nickel nitrate, cobalt chlorate, and cobalt nitrate.

The category of salts of the alkali metal group, which have a common marked ability to displace acidic inks from metal surfaces previously reacted therewith, and which in combination with "the above catalogued passivating agents are useful for the purposes of. this invention. comprises the acid phosphates of sodium, potassium, and ammonium.

Of the possible combinations of the passivators and acid phosphates hereinabove catalogued, in general nickel nitrate and cobalt chlorate are equally satisfactory with ammonium acid phosphate and glycerin. There are however certain factors of cost, plate surface condition and others relating to the mode and particulars of use which may dictate other choices. The principal factors to be considered. in choosing the particular combination best suited for the attainment of the desired result include the type of counter-etching or sensitizing treatment to which the plate has been previously subjected. Some counter-etches in common use so modify the plate surface as to render it wholly incompatible with some of my etches and not with others. The etches of this invention without exception give the best results when used on zinc or aluminum upon which has been formed by the counter-etching operation an extremely thin film of basic aluminum sulphate, For the continuous or protracted treatment of zinc surfaces which have been counter-etched with any of the usual acids, as phosphoric, acetic, hydrochloric, etc., I obtain the best'resuits by means of an etch containing glycerin and any one of the passivating agents excepting only cobalt nitrate while if the surface is oxidized, nickel chlorate must be excepted as well for best results. However, cobalt nitrate or nickel chlorate in cooperation with an acid phosphate give, as stated, excellent results on zinc or aluminum so counter-etched as to carry a thin film of basic aluminum sulphate. The inclusion of glycerin in my etching compositions is in general desirable though not always necessary. Considering only its functional relation to the other components of an etch of this invention I prefer to include glycerin in all etches to be used as fountain solutions or as the dispersed phase of emulsoid inks; that is, whenever the plate is to-be maintained wet by the etch for protracted periods as is required in printing large editions. When, however, an etch comprising cobalt nitrate or chlorate and ammonium acid phosphateis chosen for continuous use on a basic aluminum-sulphate coated plate, the choice as to the inclusion or omission of glycerin may be predicated entirely on whether or not a hygroscopic film of moisture on the plate surface is desired.

When the maximum contrast between image and plate background is desired, as when printing light colors, I prefer an etch containing glycerin and either nickel chlorate for use on zinc or cobalt chlorate for use on aluminum.

In general there is advantage in using an etch comprising the same reagents both for the preliminary etching of the plate and for the dampening thereof during the printing operation, and I prefer therefore to use but one combination of reagents on a given plate. I may however omit the glycerin from the etching solution when the plate is first etched and also use the etch minus glycerin in the event that a heavy scum or smear of ink is to be removed from the non-printing areas. The glycerin, as stated, tends to restrain the reaction presumably by depressing the diffusion rate at the plate surface and also to diminish slightly the rate at which the etch can displace ink from a desensitized surface. There is a slight advantage in point of time to be obtained in omitting the glycerin when giving a plate a preliminary etching or when removing scum. When glycerin has been so omitted for such purpose however, I prefer in general to dampen the plate with an etch or fountain solution containing glycerin, gum tragacanth, or both before the form rolls are contacted with the plate and when nickel chlorate is a component of the etch, glycerin must be included if the etch is to be used for a protracted period on plates other than those so counter-etched as to leave a film of basic aluminum sulphate and regardless of the nature of the image. If the image is determined by a light-hardened albumin or other colloid, as in direct photoplanography, it is essential that the integrity of the image substance be maintained against attraction of large editions are to be printed. In such case I find it highly desirable to include glycerin in all the etches of this invention when used as fountain solutions. Ethylene glycol may be substituted for glycerin in etches containing nickel nitrate but not so satisfactorily in etches containing, cobalt. I prefer to use glycerin. The advantage derived from the inclusion of glycerin is somewhat less in the case of cobalt chlorate, but inasmuch as I generally prefer to have the film of moisture on the plate slightly hygroscopic and therefore slow drying, I recommend the inclusion of glycerin with cobalt chlorate as well as with the other passivating agents above catalogued.

The following formula will serve to illustrate the preferred proportions of my etches when they are to be-used for the preliminary treatment of the plate or for removing scum as an incidental operation.

Ammonium acid phosphate half molar solution 10 parts by volume Cobalt chlorate half molar solution 2 parts by volume Glycerin C. P 3 parts by volume As above stated the glycerin may be omitted; and when omitted, I usually prefer to replace it with distilled water. Distilled water is strongly recommended for all the compositions of this invention. If ferric nitrate is chosen as the passivator, it may be substituted in one-third molar solution instead of the half 'molar solution as for the other metallic salts in the above and following formula. It is necessary to add to the ferric nitrate solution before admixture with the acid phosphate solution a sufiiciency of ammonium citrate to prevent precipitation, as is well known.

When under ordinary conditions an etch of this invention is used as a fountain solution for application to the plate between successive printings of a large edition, I prefer to add water to the above formulated etch as indicated below.

Ammonium acid phosphate half molar solution 10 parts by volume Cobalt chlorate half molar solution 2 parts by volume Glycerin C. P 3 parts by volume Distilled water 25 parts by volume Where glycerin is included to functionally assist in the passivation and prevent undesirable crystallization in the etch dissociate from the surface, I prefer to maintain the passivator-toglycerin ratio at about that above given though more glycerin does no harm so long as the solution is limpid enough to form a tenuous film over the non-printing areas and not so viscous as to cause streaks over the printed image.

And for dampening purposes in general I find it satisfactory to keep the glycerin content between 7.5% as in the above formula and about 15% as a maximum. The water in the above formula may be increased to fifty or one hundred parts if so desired, as when only small editions of the order of one thousand copies are to be printed; but I do not in general recommend that the diluting water be added in amounts greater than fifty parts particularly if the etch is to be used in connection with an albumin image. At a dilution of 20 to 30 parts the etch has an acidity corresponding to pH 4.2 and ample buffer capacity provided by the acid phosphate to remain at this hydrogen ion concentration while in contact with the plate under operating conditions.

In compounding the etching compositions of this invention I recommend as highly desirable though not necessary that chemicals of the C. P. grade be used and that, whatever the grade of chemicals used my be, the mixtures be allowed to stand until any precipitates formed (usually by reason of traces of impurities) may be removed by filtration. If heat has been used to accelerate the solution of the acid phosphate, it is necessary that the solution be cooled before admixture with the nitrate or chlorate solution to avoid precipitation.

It has been discovered as part of this invention that the cleaning, scum-removing, or inkdisplacing capacity ,of the salts of the alkali metal group effective for the purpose is not due to the alkali component of "these salts nor is it to any appreciable extent dependent upon such acidity as they may possess; but is due substantially if not entirely to the acid radical component thereof..

Thus ink, rolled up on a zinc plate, treated with an etch of this invention containing ammonium acid phosphate, and therefore presumed to have a surface film comprising zinc ammonium phosphate, is as readily displaced therefrom by tertiary (basic) ammonium phosphate as it is by the acid (primary) ammonium phosphate or by a solution of phosphoric acid of a somewhat higher acidity than is tolerated by an albumin image. The tertiary phosphates, or dilute phosphoric acid, or other scum-removing agents as the molybdates and arsenates generally of the alkali metal group may be used for momentary treatment of a badly scummed plate though I do not recommend such treatment particularly if the image comprises a base of a light-hardened colloid.

The precise mechanism of ink displacement from a desensitized surface by the compositions of this invention is not understood, although it is believed that it depends upon the direct aocessibility of the surface underlying the ink to the composition and upon the nature of the surface particularly in respect of its relatively higher wettability by that composition than by the ink. To test the relative effectiveness of the etching compositions of this invention, it is deemed important that the ink should be rolled up on a .dry surface in precisely the same manner for each test to the end that the thickness and particularly that the porosity of the ink film be substantially reproduced.

In general the rate of ink displacement by etching compositions comprising any of the acid phosphates herein catalogued, is greater (everything else being equal) from dry surfaces which have been previously reacted with an etch comprising that particular phosphate than from a dry surface which had been similarly reacted with an etch comprising any other salt of the alkali metal group provided however that the etches contained no organic colloid adsorbable by the metal. The water wettability of organic colloids, such as gum tragacanth and gum arabic which are adsorbable from aqueous solutions by metal surfaces, is when so adsorbed in general somewhat higher than that of the insoluble salts of the metal which may be formed on its surface. All of the etching compositions of this invention will readily displace ink from a clean,

dry metal surface previously contacted with a solution of gum tragacanth or gum arabic. This is true even' though the contact be very short or the washing to remove any gum not firmly adsorbed by the metal be very thorough.

The etches of this invention are not only useful for general etching and dampening purposes but are particularly serviceable for use on plates carrying a light-hardened albumen image when,

in addition to the other stated advantages, these etches induce a minimum swelling of the albumin image substance by reason of the fact that they are effectively buflfered at about the isoelectric point of the image substance which has been determined to lie between pH 3.8 and pH 4.2 and has been reported as being circa pH 4. The swelling of the albumin image substance may for practical purposes be considered as constant over the range of hydrogen-ion concentration lying between pH 3.6 and pH 4.4. It is recognized that the herein above-mentioned and commonly used ammonium nitrate-ammonium phosphate etch is, when the acid phosphate is used, buffered at pH 4.2 or substantially at the desired value; but such an etch will not otherwise fulfill the requirements of the method of this invention in several respects. It does not render the metal surface inert or passive, and hence it is unsuited for the long-continued contact with the plate required of a dampening means or socalled fountain solution. Moreover it tends to form as stated a crystalline deposit on the metal surface and a particularly exuberant and objectionable crystalline growth on surfaces previously counter-etched or sensitized as zinc in a bath of dilute nitric acid and alum, aluminum in a bath of dilute sulphuric acid and copper chloride, or other counter-etch which deposits basic aluminum sulphate on the metal as is common practice and as is the preferred treatment of plates to be etched and dampened by the compositions of this invention. So far as I am aware no heretofore suggested etch, capable of rendering and maintaining the non-printing areas of a metal plate chemically insensitive to acidic inks and of subsequently displacing extraneous ink or scum therefrom, is also capable of maintaining these areas in a substantially inert state throughout long contact with and. in respect of that etch the while said etch retained a substantially constant acidity.

It is customary to etch planographic plates for very short times (i. e., from 30 seconds to one or two minutes) in a. desensitizing solution, to wash them thoroughly and particularly so if the plates carry an albumin image and then to dampen them during the early stages of the printing operation with water alone and later with an aqueous solution of gum arabic and at most a very dilute acid. This customary procedure is based on experience and is an acknowledgement that water alone will not keep the plate clean for long and that acid added to the fountain or dampening solution to overcome scumming or thickening of the image, which is a localized scumming, is detrimental to the life of the plate. The problem sought to be solved by the method of this invention is most recently and clearly stated on page 94, Research Bulletin No. 6 of the Lithographic Foundation as follows: If water alone will not keep the plate clean, and the acid solutions ordinarily used either sharpen or blind the work and damage the grain, there should exist an intermediate condition of the fountain solution which will maintain the plate most nearly in its original condition. Such a fountain solution has not been developed in practice because of lack of knowledge regarding the effect of acid on the plate, and also because of the lack of a satisfactory means for controlling-acidity. If an ideal fountain. solution could be developed, it should be possible for the pressman to start the plate and run it through the edition without change, provided of course no troubles arise due to incorrect adjustment of the solution was not at fault and could look for the trouble among the other factors. He would thereby avoid the extreme changes in acidity that are often made in present practice and which are detrimental to all types of lithographic plates."

' The method of combined etching and dampening oi. this invention not only meets the requirements of the problem as above quoted, but still other advantages accrue as well.

Thus the inclusion of glycerine not only functions as an active component of the etch but renders the film of etch on the plate hygroscopic and hence persistently moist as aforesaid, and thereby avoids, except for relatively long periods and for storage, the heretofore experienced necessity of gumming up the plate to protect it from drying out and oxidation, which is of advantage.

Moreover the passivity imparted to the plate by the etches of this invention is not appreciably affected by such electrolytic potentials as may exist when electrolytic contact is established by the etch between the plate and the commonly used brass core of the dampening roll, which is an advantage. I have found for instance that a clean zinc plate which had for twenty-four hours constituted one electrode of a closed circuit zincbrass cell, the electrolyte of which entirely covered the electrodes and consisted of a high concentration (approximately 0.35 molar) etch comprising nickel nitrate and ammonium acid phosphate, had undergone no microscopically discernible change either as to sharpness of grain or as to other visually detectable surface modification. That a change in the nature of the surface had taken place however was demonstrated by the fact that when the plate was rolled up with an acidic ink after thorough washing and drying, the ink was readily displaced by the etching solution which had been used as electrolyte.

The rate of corrosion or loss of weight of zinc in a usual etch rises rapidly as the acidity thereof rises or as the pH value falls below circa pH 5.5; but, as has been stated, the optimum acidity of an etch for use in the presence of an albumin image lies between pH 3.8 and pH 4.2 from which it is clear that the requirements of minimum swelling of the albumin image substance necessary to long service life and avoidance of corrosion of the metal also essential to long service life cannot both be obtained by adjustment of the acidity alone whatever the buffer capacity of the etching solution may be. The etches of this invention are effectively buffered at pH 4.2 and therefore induce substantially the minimum swelling of the albumin image substance. Nevertheless, and notwithstanding the fact that the acidity in terms of pH is such as would in any prior etch known to me induce a detrimentally high rate of dissolution of the zinc, the etches of this invention passivate the metal and therefore permit no detrimental dissolution of the metal even when reacted therewith for such protracted periods as are required to print editions running into the tens of thousands.

The combined etching and dampening compositions of this invention moreover provide means of not only maintaining the integrity of the surface grain by the substantial elimination of corrosion and of assuring a substantial minimum swelling of the albumin image substance over long periods as stated, but they also provide for the continued presence on the plate surface of an effective cleaning agent, whereby such scum as might otherwise occur because of faulty machine adjustment or otherwise is either prevented or so displaced as to be readily and rapidly removed by the form ink-rolls.

Moreover the compositions of this invention may be used with impunity at such high salt concentrations as to be substantially immune to such chemically and detrimentally reactive substances as may be derived under ordinary conditions from either paper or ink.

Chlorides and sulphates as well as free alkali in solution with the etch tend to "poison the passivating action of the etches of this invention or activate the dissolution of the metal and hence care should be exercised to avoid their inclusion as impurities in said etches. I prefer therefore to use distilled water as stated and chlorideand sulphate-free salts to make up my etch solutions. When these solutions are made up to the preferred concentrations, as have been hereinabove described, their tolerance for activating impurities is well above the probable contamination, under ordinary working conditions, from paper or ink. Because of the activating impurities in commercial grades of gumarabic, I do not recommend its use as a component of my passivating etches but prefer to use gu'm tragacanth of the better grades.

Impurities in the metal may cause local activation or breakdown of the passivated surface and therefore I recommend, particularly in respect of zinc, the use of the purest metal obtainable. Zinc, as usually supplied to the lithographic trade is suitable for use in conjunction with the etches of this invention.

It should be pointed out here that there is another condition which promotes activation and which should be assiduously avoided if the best results from the use of the herein described etches are to be obtained, and that is a triple metal-air-etch boundary. A disadvantageous activation at the triple boundary with an at-- tendant local corrosion of the metal is observable whether the metal be dry or wet with water as it is after development of a photographic image thereon. I therefore recommend that in .the first instance'the treatment of the plate with the etches of this invention be carried out as expeditiously as possible to the end that the entire surface of the plate be wetted with the etch in the minimum time. And for the same reason I recommend that, when the press is stopped for more than a fewminutes, the dampening roll be removed from contact with the plate, particularly if the etch contains no hygroscopic inclusion as ethylene glycol, or preferably glycerin, to thereby prevent local activation at the triple boundary which would otherwise occur at the junction of the dampening-roll contacted surface, the adjacent drying surface, and the atmosphere.

When it is desired to emulsify the dampening means in the printing ink and thereby avoid, as do the so-called Dry Lithographic processes, the

conveyance of the dampening etch to the plate by a separate operation, the etches of this invention' may be so utilized with all the attendant advantages hereinabove recited. For such use it is wise protected from dustand particularly dust of an abrasive character-a filmiform protective coating is recommended as in usual practice although, as stated, I do not advocate the use of gum arabic for this purpose but prefer instead to use gum tragacanth of the best grade. After an interval in printing from a plate of this invention, the plate should for best results be thoroughly dampened by contacting the dampening roll for several revolutions of the plate or form cylinder before the form ink-rolls are again brought into contact therewith. However, I have found that, even if the form ink-rolls are contacted with the plate while the latter is dry or apparently so and if the plate is rolled up with ink over its entire surface, its non-printing areas will free themselves from ink under the influence of the etches of this invention. While I do not recommend the inking of the plates of this invention while in the dry state, the fact that the nonprinting areas will clean themselves is of advantage; and the rapidity with which this cleaning is accomplished is very striking, particularly when glycerin has been present in the dampening 7 solution previously used to treat the plate and when the non-printing areas thereof, though apparently dry, have retained but the merest trace of adsorbed moisture.

By the terms planographic printing plates, metal plates, printing plates, etc. as herein used are included suitably prepared sheets, surfaces or coatings of aluminum or zinc, adapted for use asplanographic printing surfaces by usual image .forming means andby the etches and dampening compositions of this invention as specifically designated in the context.

By the expression acidic ink as herein used is meant any usual lithographic ink comprising a linseed or other oil or varnish vehicle which is ordinarily acidic by nature and is intended to include any ink which is not alkaline.

I claim: 1. The method of treating the non-printing areas of the surface -of a planographic printing plate with an aqueous solution-comprising a passivator for the metal chosen from the designated class of passivator agents and an acid phosphate.

2. The method of treating the non-printing areas of the surface of a planographic printing plate with an aqueous solution comprising a passivator for the metal chosen from the designated class of passivator agents and an acid phosphate of the alkali metal group.

3. The method of treating the non-printing areas of the surface of a planographic printing plate with an aqueous solution comprising a passivator for the metal chosen from the designated class of passivator agents, an acid phosphate and a peptizing agent.

4. The'method of treating the non-printing areas of the surface of a planographic printing plate with anaqueous solution comprising a passivator for the metal chosen from the designated class of passivator agents, an acid phosphate and a hygroscopic peptizing agent.

15. The method of treating the non-printing areas of the surface of a planographic printing plate with an aqueous solution comprising a passivator for the metal chosen from the designated class of passivator agents, an acid phosphate and a hygroscopic peptizing agent capable of depressing the diffusion rate in said solu tion at the surface of the plate.

6. The method of treating the non-printing areas of the surface of a planographic printing plate with an aqueous solution comprising a passivator for the metal chosen from the designated class of passivator agents, an acid phosphate and a peptizing colloid capable of itself rendering the .surface of the treated plate hydrophilic.

7. The method of treating the non-printing areas of the surface of a planographic printing plate with an aqueous solution comprising a passivator for the metal, chosen from the designated class of passivator agents, an acid phosphate and a hydrophilic peptizing colloid, adsorbable by said areas of the surface of the plate.

8. The method of treating the non-printing areas of the surface of a planographic printing plate with an aqueous solution comprising a passivator for the metal, chosen from the designated class of passivator agents, an acid phosphate and an agent for peptizing such reaction products as are formed dissociate from the surface of the plate, while said products are. still of colloidal dimensions.

9. The method of treating the non-printing areas' of the surface of a planographic printing plate with an aqueous solution comprising a passivator for the metal chosen from the designated class of passivator agents, an acid phosphate and a peptizing agent whereby said treated surface is rendered passive said solution.

10. Method of treating the non-printing areas of the surface of a planographic plate carrying a filmiform deposit of basic aluminum sulphate, with an aqueous solution comprising a passivator for the metal, and an acid phosphate, whereby said treated surface is rendered passive with respect to said solution.

in and with respect to -11. Method of treating the non-printing areas of the surface of a planographic plate carrying a filmiform deposit of basic aluminum sulphate, with an aqueous solution comprising a passivator for the metal, and an acid phosphate, whereby said treated surface is rendered passive with respect to said solution and hydrophilic.

12. Method of treating the non-printing areas of the surface of a planographic printing plate carrying a light-hardened colloid image, with an aqueous solution comprising a passivator for the metal and an acid phosphate, characterized by being buffered substantially at the isoelectric point of the light-hardened colloid image substance, whereby the metal is passivated and the swelling of said image substance is maintained at a minimum.

13. Method of treating the non-printing areas of the surface of a planographic printing plate with anaqueous solution comprising a passivator for the metal and an acid phosphate, characterized by being buffered substantially at pH 3.6 to 4.4.

14. A combined etching and dampening composition for metal planographic printing plates, comprising a passivator chosen from the designated class of passivating agents, an acid phosphate and water.

15. A combined etching and dampening composition for metal planographic printing plates, comprising a passivator chosen from the designated class of passivating agents, an acid phosphate of the alkali metal group and water.

16. A combined etching and dampening composition Ior metal plancgraphic printing plates, comprising a passivator chosen from the designated class of passivating agents, an acid phosphate, a 'peptizing agent and water.

17. A combined etching and. dampening composition for metal planographic printing plates, comprising a passivator chosen from the designated class of passivating agents, an acid phosphate, a hygroscopic peptizing agent and water.

18. A combined etching and dampening composition for metal planographic printing plates, comprising a passivator chosen from the designated class of passivating agents, an acid phosphate, a peptizing agent and a hydrophilic organic colloid agent adsorbable by the metal.

19. A combined etching and dampening composition for metal planographic printing plates,

comprising a passivator chosen from the designated class of passivating agents, an acid phosphate, a peptizing agent, a hydrophilic agent adsorbable by the metal and water.

20. A combined etching and dampening composition i'or metal planographic printing plates, comprising a passivator chosen from the designated class of passivating agents, an acid phosphate characterized by a hydrogen ion' of pH 3.6 to 4.4 and water.

21. A combined etching and dampening composition for metal planographic printing plates, comprising a passivator chosen from the designated class of passivating agents, an acid phosphate characterized by a high buffer capacity at pH 4.2 and water.

22. A combined etching and dampening composition for metal planographic printing plates comprising nickel nitrate, acid phosphate, and water.

23. A combined etching and dampening composition for metal planographic printing plates comprising cobalt chlorate, acid phosphate, and water.

24. A combined etching and dampening composition for metal planographic printing plates comprising cobalt nitrate, acid phosphate, and water.

25. A combined etching and dampening composition for metal planographic printing plates comprising nickel nitrate, acid phosphate, water, and glycerin.

26. A combined etching and dampening composition for metal planographic printing plates comprising cobalt chlorate, acid phosphate,

water, and glycerin.

27. A combined etching and dampening composition for metal planographic printing plates comprising cobalt nitrate, acid phosphate, water, and glycerin. 28. A combined etching and dampening composition for metal planographic printing plates comprising nickel nitrate, acid phosphate, water, and gum tragacanth.

29. A combined etching and dampening composition ifor metal planographic printing plates comprising cobalt chlorate, acid phosphate, water, and gum tragacanth.

30. A combined etching and dampening composition for metal planographic printing plates comprising cobalt nitrate, acid phosphate, water,

and gum tragacanth.

31. A combined etching and dampening composition for metal planographic printing plates comprising cobalt chlorate, acid phosphate, water, glycerin, and gum tragacanth.

32. A metal planographic printing plate having the non-printing areas thereof substantially passive with respect to a solution comprising a passivator for the metal and an acid phosphate.

33. A metal planographic printing plate having the non-printing areas thereof substantially passive with respect to a solution comprising a passivator for the metal, an acid phosphate, and a hydrophilic agent.

34'. A metal planographic printing plate having the non-printing areas thereof substantially passive and containing a metal constituent of the passivator and a phosphate.

35. The method of passivating zinc by treating it with a metal salt chosen from the designated class of passivating agents in solution with an acid phosphate of the alkali metal group.

36. The method of passivating zinc by treating it with nickel nitrate in solution with an acid phosphate of the alkali metal group.

- WILLIAM B. WESCO'I'I.