US2016517A - Method of preparing printing surfaces - Google Patents

Description

Patented Oct. 8, 1935 UNITED STATES METHOD OF PREPARING PRINTING SURFACES George S. Rowell, Cleveland, Ohio, assignor to Multigraph Company, Wilmington, Del., a corporation of Delaware No Drawing. Application September 2, 1932, Serial No. 631,588

4 Claims. (01. 41-415) This invention relates to improvements in printing surfaces and method of preparing metal planographic printing plates for re-use, whereby the regraining of such plates is avoided, and it comprises the co-action of a grease solvent capable of dissolving and removing a greasy image from the surface of a metal printing plate and an aqueous solution of salts, preferably containing glycerin, capable of rendering the surface of said plate readily wettable by oil-immiscible fluids, as water or glycerin, and particularly that surface of said plate from which said greasy image has been removed by said grease solvent, whereby the entire surface of the plate is rendered readily wettable by oil-immiscible fluids and grease repellent; and it further comprises the steps of rinsing the so-treated plate and to remove the said solution of saltsand of glycerin, of drying, and of the application of an image in a greasy material sufliciently acid or alkaline to so react with the said surface as to render it grease-retentive and after such reaction and removal of the reacting image-forming material to leave the so-reacted image area in a condition more readily receptive of grease than of oil-immiscible fluids, all as more fully hereinafter set forth and claimed.

Planographic printing depends upon the maintenance of the non-printing areas of a metal printing plate free from-ink by means of a thin film of an oil-immiscible fluid present thereon. Such afilm cannot be obtained on a surface not strictly free from grease, and ink-repellent films of the tenuousness required for cleancut work are not obtainable even on clean metal surfaces unless the surface has been modified or etched, as by reaction with certain salts, such as ammonium phosphate, ammonium nitrate, so as to render it readily wettable by oil-immiscible fluids.

To prepare a planographic printing plate for re-use it is necessary to remove the greasy printing image and all residual traces of grease, render the surface grease-receptive, heretofore accomplished by regraining the entire surface, put on the new image, and condition the non-printing areas for ready receptivity of the ink-repelling fluid.

Prior practice, so far as I know, always com- I prised the step of gumming up" the plate before the image was dissolved; that is, the plate was plate for re-use because of the well-known fact that a thin film of gum arabic, generally believed to be monomolecular, is so strongly adsorbed on the surface-as not to be susceptible of removal by washing.

' tend on evaporation to leave behind a layer of grease, often so thin as to be quite invisible, but nevertheless such as to make it impossible to maintain that ink-repelling condition of the nonprinting areas which is essential to clean work. 15 Because of the aforesaid difficulty and the excessive cost of the large amount of solvent required and further because the non-printing areas are left in a hydrophilic etched or desensitized condition, it has been the custom to recondition 20 used planographic plates by subjecting the plates, after first removing the bulk of the greasy image with solvents therefor, to the attrition of "a regraining process. Regraining is objectionable in at least two major respects, namely, (a) that it of 25 necessity involves such abrasive wear as to permit but few re-uses; and (b) that the graining operation, particularly on small plates, is expensive and tedious.

It is an object of this invention to avoid the 30 objectionable features of the reconditioning processesheretofore practiced, whereby the cost of reconditioning is reduced to a lower order and the effective life of the plate is greatly increased. It

is another object of this invention, not only to 35 avoid the regraining of the plate but also to provide a method whereby all traces of the greasy image may be removed and the entire surface of a plate simultaneously rendered readily wettab e by oil-immiscible fluids. And it is a further obje t of this invention to form an image on a so-co ditioned plate by means of greasy materials which will self-sensitize the image areas ofthe plate and upon subsequent removal of said imageforming material, leave the said area in a sensitized condition, that is, more readily receptive of grease than of oil-immiscible fluids. Other objects and advantages will becomeapparent from its more detailed description.

It is well-known that, while clean metal surfaces such as those of zinc and aluminum are not readily wettable by water or polyhydric alcohols, such as glycerin or glycol, they may be converted to surfaces having a. marked a'filnity for such fluids by allowing the metal to react with any of a large number of inorganic salts, among which ammonium nitrate and ammonium phosphate are preferred. Heretofore it has been customary to apply the image to the sensitized or counter-etched surface of the metal plate in any well-known manner and thereafter to desensitize or modify the water wettability of the remaining non-image areas of the plate by later appropriate etching treatment.

I havediscovered as part of this invention that I can completely remove a greasy, or oily ink, image from the surface of a used printing plate by dissolving the same in a suitable solvent, as benzene, carbon tetrachloride or the like; in the presence of a solution of salts reactive with the metal of the plate to etch the same and render it readily wettable by oil-immiscible fluids.

I have further discovered that after the image has been removed and the entire surface de-' sensitized by the process of this invention, the surface may be sensitized" or deprived of its hydrophilic character by swabbing thereover a solution of ammonia or other suitably'weak alkali.

I have also further discovered that a greasy image may be formed on and made to adhere to a metal surface even though previously etched or desensitized to grease by means of greasy crayon or oily inks rendered sufliciently acid, as by oleic acid, or alkaline as by alkaline soap to self-sensitize the previously desensitized surface.

To completely remove a greasy image from a zinc plate for instance and simultaneously protect the non-image areas from grease and etch the printing areas, as these become exposed, I may flood the same with an excess of an etch, comprising a solution of salts, such as ammonium nitrate and ammonium phosphate, and preferably, though not always, also comprising glycerin, which will of course not wet the plate wherever there is a trace of grease, and then swab the surface with benzene to dissolve the greasy image; or I may swab the surface simultaneously with two tuffs of cotton or other suitable material, one charged with benzene and the other with the etch.

In either case I find that such treatment fully protects the non-image areas from grease and yields a surface readily wettable by oil-immiscible fluids, as water or glycerin, throughout its entire area.

As a theory useful in visualizing what apparently takes place, but not relied upon as a statement of fact, the following is suggested. It would appear that, as the solvent dissolves the grease and exposes grease sensitive metal, the immiscible -etch solution of salts displaces the solvent,

reaches and reacts with the so-exposed surface and the oil-immiscible solution, which .wets and underlies the then grease-free surface-thereafter protects it from contamination by the grease held in the solvent or deposited by the evaporation thereof.

After the surface has been thoroughly degreased and reacted with the aqueous solution of salts and thoroughly wet therewith it may be rinsed and dried and is then in condition for the reception of an image formed by self-sensitizing" greasy materials, as above stated, or it may be sensitized throughout its entire area by treatment with a weak alkali, as ammonium hydroxide,

and used in the ordinary way as a freshly grained plate would be.

While not essential to the removal of the greasy image by the method of this invention, the inclusion of glycerin or glycol in the solution of salts is desirable, particularly when the greasy image is so extensive, heavy bodied, or old as to require drastic rubbing for its solution and removal. A 5 surface when wet with an oil-immiscible solution of salts containing glycerin or glycol is decidedly more resistant togrease under mechanical pressure than when merely wet with water, and this sometimes is of advantage, particularly as when 1 swabbing must be carried out vigorously in order to dissolve and dislodge the greasy image. When no glycerin or glycol has been used, merely drying of the plate is sufllcient to prepare it for sensitizing" or the reception of a new self-sensitizing 15 image, although rinsing is in general desirable. When glycerin or glycol is used it is always desirable to remove the same by washing in order that sensitizing with an alkali may be readily carried out and to permit a greasy self-sensitiz- :0 ing image to fully contact the plate. While in general the procedure is undesirable, a greasy image may, if circumstances require, be applied directly on the plate even when the latter is still water-wet, by means of an acid or alkaline greasy 35' self-sensitizing" crayon or ink if these are applied with sufilcient pressure to cut through and displace the overlying moist film.

It is obvious that by the process of my invention a grained plate may be re-used, without re- 30 graining, as often as desired while and so long as the actual grain of the plate persists and that a plate treated by the method of this invention, and particularly when the non-image areas are left in a hydrophilic condition or readily wettable 35 by oil-immiscible fluids, may be advantageously used with emulsoid inks, containing dispersed therein oil-immiscible fluids, as glycerin, glycol or the like, adapted to deliver the pigmented oily vehicle thereof to the image and the contained 40 oil-immiscible fluid to the non-image area.

Among the advantages accruing from the use of the process of this invention, the following may be mentioned: 1. The cost of regraining small-plates, which is much higher than the relative cost of graining large metal sheets, from which the small plates may subsequently be cut, is eliminated. 2. The loss of time incurred by the regraining of used plates is avoided. 3. The stock of plates required by the user may be greatly reduced because of the aforementioned diminution in the time necessary to recondition the plates for further .use. 4. Because the non-printing areas of the reconditioned plate are in condition to be readily water-wet, they therefore are available for immediate use with the self-sensitizing greasy image-forming materials without the necessity of the usual intervening etching operations. 5. The possibility of damage to the printing image by etching the plate after ap- 0 plication of the image,-as has heretofore been necessary, is avoided,

I claim:

- 1. Method of preparing a planographic printing surface having grease thereon, consisting in 5 the following steps, removing the grease from the printing surface with a solvent for the grease in the presence of an aqueous etching solution spread over the entire surface and thereafter forming an image on the treated surface with a greasy material reactive with such surface.

2. Method of preparing for re-use a pianographic printing surface having a greasy image thereon consisting in the following steps, removing the greasy image from the surface with a solvent for the greasy substance of the image in the presence of an aqueous etching solution containing ammonium phosphate and forming an image thereon with a greasy material reactive with the treated surface.

3. Method of preparing for re-use a pianographic printing surface having a greasy image thereon consisting in the following steps, removing the greasy image from the surface with a solvent for the greasy substance of the image in the presence of an aqueous etching solution containing ammonium phosphate, rinsing to remove a portion of said solution and thereafter raming an image thereon with a greasy material reactive with said treated surface.

4. Method of preparing for re-use a plane'- graphic printing surface having a greasy image thereon consisting in the following steps, removing the greasy image from the surface with a solvent ior the greasy substance of the images in the presence of an aqueous etching solution contaming ammonium phosphate, rinsing to remove a portion of said solution, drying and thereafter 10 forming an image thereon with a greasy material reactive with said treated surface.

GEORGE S. ROWELL.