Aug 5 l952 w. G. ALExEWlcz ET AL 2,606,130
STRIPPING TISSUE Filed July 16, 1948 25m-*ner* jexemz Ez azurwna Flai-.F17
fabel-d .21 ma @am mdmwsc Patented ug. 5, 1952 UNITED sTAT Es PATENT orifice A,
' STRIPPING TISSUE bankfoaiif.,
i j "ri L, Appuatipnmly 16, ina-serial Nogsalislvf 'Y iciaim. (Cim-'16) Y y. ,i This invention concerns generally an improved strippingtissue .of the type employed in certain of the photographic arts. Our improved stripping tissue, in common with many previously known products, comprises a temporary supporting sheet, ordinarily of paper; a nlm of unhardened'gelatin; and an intermediate layer between and temporarily uniting the supporting sheet and the gelatin film. kThe intermediate layer has the dual function of forming a firm and effective bond between the gelatin and the paper temporary support while the stripping tissue is dry; and of effectively releasing the soft gelatin layer after the stripping tissue'has been immersed for a short time in water (or in a weak aqueous solution) and the exposed gelatinsurface has beenpressed into firmjcontact with a suitable new support. The' presentapplication is a continuation in part of our copending application, Serial No. 634,278, iiled December l1, 1945. l L j Previous stripping tissues of a. type which might be used with soft gelatin layers depend forstripping action upon `application of a solu-l tion which is a solvent for the material of the stripping layer. That has the obvious disadvantage of raising the possibilityof contaminating the gelatin layer either with`the solvent itself o-r with the dissolved Vmaterial of the stripping layer, and thus interferingwith the subsequent processing. Such contamination may in some instances be avoidable in theory by suicient washing of the gelatin layer. after stripping, but in practice there is the danger that enough of the material of the stripping layer, may lremain to afiectlater processing either chemically or by affecting such physical properties'of the soft gelatin as the degree of its swelling and the rate of diffusion of' variousi substances through it. Any such effect is rendered more serious by the probability vthat `different areas of the gelatin will be affected dilferently, yielding a mottled or otherwise non-uniform final product. u
An important advantage of the present invention is the complete elimination of any such contamination of the gelatin layer. That is accomplished by providingv a stripping layer which is insoluble in water and in solutions ordinarily encountered in the art, and which does not depend for its stripping action upon-being wholly or. partially dissolved. Hence, during transfer of the gelatin layer to a new support the entire stripping layer remains'firmlyattachedA to the temporary paper support, and is removed with `vit. Whether pure water-is' used as 'the stripping bath,
or whether 'that bath ls a dilute alkaline'v or acid solution, no `part of'th'e strippinglayer Ais dissolved.v Hence the softlgelatin layercomes out of the transfer onits new support without possibility of contamination by ldissolved material of the stripping layer. t l f The invention is particularly effective'in connection with layers `of pigmented unhardened gelatin of the' type used for m'aking'photographic prints by the carbro. and-'the carbon processes, both of which involve'foirnationof` anjim'age of selectively hardened gelatin'within the layer of initially soft gelatin;v Such gelatin layers lare considerably thicker than the gelatin layers ordinarily used for other photographic' processes. A stripping layerLwhi'ch'is' suitable vfor 'use with a relatively thin for hard Slayer of gelatin is not necessarily practicable for u'seiwith the'relatively thick and soft 'carbon'andjcva'rbro gelatin layers, for which the presentjinvention 'is particularly well adapted.
A complete understanding of our invention and of its further objects and advantages will be had from the following detailed description of a preferred manner` of lcarryin'g it into eifectand of certain typical'illustrative processes in which it finds useful application. The "accompanying drawing forms a part 'Ofthatdescription and is a schematics'ection'.taken transversely through a stripping tissue jin accordanoe 'with' the invention.
We have found that a stripping tissue with the desirable properties describedabove can be .made in the following way. ,The paper'su'p'port lll is coatedwitha solution of vinyl acetate resin, a polymerization product of vinyl ace-tate V(such as type AIYA'l1 supplied'commercially `:by Carbide and CarbonChemicals,Corp), Adissolved in a suitable volatile solvent,;which' may befor example, a mixture of approximately %*isopropyl alcohol, 5% acetone and"2Q% `water-;fAfterrthe resulting strippinglayer ll has-dried, the layer l2 of unhardened gelatin is coated on top of it by the usual coating4 procedure.
In applying stripping layer Il it is important to control its thickness closely. 4This is done by properly regulating the'rate of coating and the viscosity of the solution, which inturn'depends primarily upon the concentration of resin Vin the solutionl and uponljthe temperature.l The Nproper thickness is such `that after the strippingllayer has dried the 'surfaceof' the paper is` just covered essentially comfple'tz'ely.v If paper bersl protrude beyond thestripping'layerlto too' great an extent, `they will so fincrease ythe Lbond y with .l the gelatin layeras to-prevent easy stripping. l If; on
resin layer can be caused to have a thickness such as has been described, and the layer will then form a suitable intermediate layer for a stripping tissue.
The amount of resin required per unit? area y' to form a satisfactory stripping layer .depends largely upon the depth of penetration of the solution into the paper before evaporation of the solvent. Conditions which leadto fast drying,
such as lo'w humidity, for example, usually require a coating solution of lower resin concentra 4 The new support may be of any material to which the swelled gelatin will adhere firmly, paper or a plastic sheet having a grained surface being preferred. Since the adhesion between the gelatin and the new support is, greater than that between the gelatin and the intermediate stripping layer, when the paper temporary support is stripped away the entire intermediate layer is Vremoved with it, leaving the coherent gelatin layer on the new support.
An important illustrative application of the Lnew type of` stripping tissue is in making photographic prints, by the carbro process when a final picture of the highest quality is desired. For that .purpose the gelatin layer of the stripping tissue contansa suitable pigment dispersed in it.
, That pigmentmay, for example. be carbon black tion. other factors remaining the same.
When paper of the type of photographic raw paper is used, ordinary coating methods are .generally found to give a layer `ofthe required thickness if the concentration 'of vinyl :acetate resin in the coating solution, (using,V for example, the preferred solvent composition defined above) has some definite value between about f3 and about 8% of the solution by weight, depending upon the exact conditions used. That assumes conditions of temperature, humidity.' coating' speed and the like withinsthe normally encountered ranges. ,When any of those factors are abnormal, or when a special type of paper is used, it may be desirable to use a resin concentration which is not included in the abovey stated'range. For example, with a paper less porous than usual a resin concentration as low as 2%v vmay be advisable. In practice itv is most convenient tohold other factors substantially jconstant'and to vary the resin concentration ofy the coating solution until a suitable coating.` as' defined above,y is obtained. In general, with photographic raw paper of the type normally used, a suitableii'nished stripping layer in accordance with the invention is found to have between 0.8,'and 1.8 grams of resin per square foot of paper surface, depending primarily upon the depth of'penetration of the resin into the paper. 'I'he extreme lightness of such a coating is apparent from the equivalent thickness of the resin layer (assuming a homo.- geneous layer for purposes of illustration), The range of 0.8 to 1.8 grams per square foot corresponds to an equivalent thickness of only about 0.0003 to 0.000,65 inch. I
The gelatin layer` I2.' which is coated in the usual manner and which directly overliesy the stripping layer Vjust described, may vary considerably according to the type of process for which the stripping tissue is intended.- However, the gelatin layer is unhardened and should have al thickness of at least 0.0015 inch. y
In the stripping process,the stripping tissue is first soaked briefly in water orin aweak aqueous solution until the exposed gelatin surface becomes tacky. An advantage of stripping tissue in accordance with the present invention is the fact that the stripping bath in which the gelatin is rendered tacky is not limited to any particular pH characteristic, but may beslightly alkaline, neutral or slightly acid. Particularly for use with the carbro process, a slightly acid stripping bath is preferred. When the gelatin has become tacky but is not yetfullyL s we11ed,,its exposed surface is brought into contact with the face `off the new support to which it is to be transferred and firm adhesion isestablished bypressure.
if a black and white print is to be made, or may be any of a wide variety of colored pigments. In either instance a pigmented stripping tissue made in accordance with the invention has marked `advantages over the, conventional nonstrippingV carbon tissue's/p'reviously. available. In the carbro process. as ordinarily carried out, a positive bromide print is. first made and processed inthe usual manner. n carbon tissue. comprising' ordinarily a pigmented gelatin layer coated on a paper'suppo't without any intermediate strippinglayer, is then sensitized by soaking in a suitable well-known aqueous solution, and the outer faceof ,the pigmented gelatin layer .isbrought into contacts-with the bromide emulsion, intimate contactv being established by pressure. AChemical actionfo'rms. a latent` image of hardened pigmentedgelatin extending inward from the gelatin-bromide interface. The pigmented tissue is then separated from the bromide and is wet and squeegeed face down upon a developing support, which is' typically a sheet of plastic. The image is .developedf in hot water. which first melts the unhardened tissue gelatin, releasing the papervbacking which can then be stripped off, and then dissolves all Vthe. remaining soft gelatin, leaving the image of hardened pigmentedgelatin adhering to4 the surface of the developing support. After drying, the gelatin image can be re-soaked and transferred to any desired permanent support, such as paper.
We have found'thatthe` skin of hard gelatin which gradually forms upon the exposed surface of a pigmented lgelatin layer Vduring'storage frequently leads to difiiculty'in obtaining completely clear highlights in a carbro print` .When a latent image vis producedV in thatfsame surface, as in the' ordinary carbro process.; the hard skin becomes essentially a partof that image. When such a latentimageis transferred toanother support for development, andthe-remaining soft portion of the gelatin is'dissolvedaway, eventhe lightest parts of the developed image will contain a certain minimum thickness Yof pigmented gelatin, corresponding to the Adepth to which the surface hardening had progressedbe'fore, the Apaper was used. 1" I This effect is avoidedrin our new process by use of the .special pigmented' gelatin stripping tissue describedabove, which makes it possible to alter theusualA procedurein the following illustrative manner. Before the pigmented gelatin layer is sensitized and contacted to the bromide print, it is transferred from itsoriginal paper support .to another, preferably relatively rigid. support. That transfer isV accomplished, as indicated above, by brey soaking the stripping tissue in water to swell ,thegelatim thenY pressing the exposed gelatin surface into firm contact with the new support, stripping off the temporary paper support and the stripping layer attached to it, and drying the gelatin layer on its new support. The gelatin on its new support is then sensitized and contacted to the bromide print, the remainder of the process being carried out substantially as usual.
It will be seen that a result of the improved procedure just described is that the bromide print comes in contact with that face of the pigmented gelatin which was initially covered and protected by the intermediate layer, and which is therefore still uniformly soft. When a latent image is formed in this freshly exposed surface, as by contact with a bromide print, the extreme high light areas of the image may be entirely free of hardened gelatin. During development those areas will therefore wash out completely, leaving clear unveiled high lights in the developed image. The original hardened skin is on the opposite side of the gelatin layer from the image, and is everywhere separated from the image by still soft gelatin (since the gelatin layer is always thick-er than the thickest parts of the image). When the soft gelatin is dissolved away during development, the hardened skin adheres to the con tacting support and is thus completely removed, leaving the uncontaminated image upon the assembly support. Such a disposal of the original surface layer of the pigmented gelatin cannot be obtained in the ordinary carbro process, nor by any carbro process using conventional non-stripping tissues.
Moreover, the advantages of obtaining a fresh gelatin surface in which to form the latent image cannot be fully exploited if the process of obtaining that fresh surface results in the possibility of contaminating the very thing desired. As has been explained, with a stripping layer in accordance with the invention, such contamination is avoided, and the described improved carbro process can therefore be counted on to give uniformly satisfactory results.
When color photographs are to be printed by the usual carbro process, in which the various color component images are separately formed and then assembled in registered superposition, the problem of obtaining completely clear highsurface of pigmented gelatin. A coordinated procedure for carrying out such a color printing process is described in the above identied copending patent application. Further useful application lights is increasingly difficult. The difliculty can be overcome in that operation also by using pigmented stripping tissues in accordance with the invention, and by forming each of the three or more color component images in a freshly exposed of the present invention in carbro printing are also described in the said copending application, including the production of carbro color prints with automatic instead of manual registration of the component color images.
y The above description of a typical embodiment of our invention is intended to be illustrative of the Various forms in which the invention can be carried out, and is not intended as a limitation upon the scope of the invention, which is defined in the following claim.
A stripping tissue for use in making color prints by the carbro or carbon process comprising a paper support, a stripping layer coated on one face only of the paper support, and a layer of pigmented unhard-ened gelatin at least 0.0015 inch thick. coated directly on the stripping layer, the gelatin layer being bodily separable from the stripping layer after the stripping tissue has been soaked in an aqueous liquid without changing the physical state of the stripping layer, the stripping layer comprising vinyl acetate resin in amount between about 0.8 and 1.8 grams of resin per square foot of paper, said stripping layer being insoluble in aqueous liquids and being of such thickness as to substantially completely cover and follow the surface contour of the paper support whereby substantially no paper fibres extend up through the stripping layer, so that gelatin layer will be bonded to the stripping layer enough to withstand Separation during storage and handling but not enough to prevent its separation when the gelatin layer of the wetted tissue is pressed into contact with a second support.
'WERNER G. ALEXEWICZ. LAWRENCE PLOTXN. RICHARD MC .1 AW.
REFERENCES C'llEi) The following references are of record in the file of this patent:
UNITED STATES PATENTS Number Name Date 1,627,935 Stinchfield May 10, 1927 1,933,824 Seel Nov. 7, 1933 2,168,051 Smith et al. Aug. l, 1939 2,237,017 Thinius Apr. l, 194i