US2893754A - Special papers - Google Patents

Description

July 7 1959 G. A. RICHTERv ETAL l 2,893,754

' SPECIAL PAPERS v Filed Feb. 29, 1944 Ela Kon 7x .$1276 onze? s gmc/MMS United States Patent 2,893,754 'SPECIALPAPERS .George A, Richter, Rochester, NX., and Arthur -B..Lamb, Cambridge, Mass., assignors to the ,United States of America as-represente'd by the Secretary of the Navy Application-February, 1944, lSerial N 055245476 .5- Claims. -(Cl. 281-38) The present 4invention relates to the manufactureof rsheet material and more particularly to aspecial paper "suitable for 'the vpreparation of lreadily destructible `documents.

v"Bro'adly speaking, the object .of fthe present invention is to -provide aV sheetmaterial' suitable forY the preparation =of documents :that may berendered unintelligible or iin- A"decipherable, merely'by contacting vthe document with '.water.

fAfrnoreparticular-f object is to :provide a paper of such :a naturefthat any-'indicia or-characters thatvhave been Written,;printed `or otherwise Vvinscribed thereon become fillegibleWhen theidocument isl immersed in any aqueous :medium, including-sea water.

A further :object fis tol-.devise a book' assemblyrso .defsigned-.that waterawillV rapidly reach all-parts ofeverypage whenrthe assemblyisimmersed in water, Y`and bringabout 'the t obliteration-lofallcharacters, patterns kor l indicia that may `.be markedon the i-ndividuaLpages.

Still-another objectis'the-provision` of1a new Yand imvproved 2process.` of fabricating water-,destructible docu- .ments such as cipherY key. lists,- code books, codestrips and .'.the like. v v

.Other .objects ..and advantageswill be apparentas the .invention lis hereinafter more particularly described in connection withfthe two figures of .thej drawing l wherein:

.Eig f1lisY aperspective viewfofabound volume', and Fig.f2.is a similar -viewof an .unbound assembly, both in accordance withthepresentinvention.

A's fa l'result f an Vexte'riled investigation f it -has been ouridthat'the'foregoing objects maybe accomplished by 'utilizing Va ,special `type if' sheet material provided with 2,893,754 Patented VJuly '7, 1959 fabricated lby vany of the conventional methods, for example by treating a waterfleafpaper in the usual manner 'with relatively strong sulfuric acid for a short period of time, followed by thorough 4removal of :the acid `by'a water-washing operation.

For purposes of ythe present invention, ahigh'degree .of parchmentation -is desirable in order to'insure high- -case -of lpapers of the'usual bond zandledger-types. It

should beunderstood, however, that other sheet materials having similar properties may b'e used, includingglassine papers, vulcanizedpapers and regenerated cellulosesuch .-as that now marketed under Vthe .name cellophane However, 'for most purposes, parchmentpapers provide the most satisfactory and most V-readilyprocurable/base ffortheimproved fdestructible.paperof thevpresent invention.

A largenumber ofmaterials .are available thatcan be usedfto form:the colloidal, oil-impervious, water-dispersible lmwith Whichthe base sheetiscoated. One of the -most satisfactory of these lfilm-forming materials thus far .discovered `comprises water-,soluble -hydroxyethyl .cellulose. Another material whichservessubstantially aswell lis low viscosity, water-.soluble 'polyvinyl alcohol. Still .other types of oil-impervious, Water-dispersible colloidal lcoating materials may be employed where desired, particularly iflhey are. applied to the base with a greater covetage than isfoun'd necessary with the preferred material j(s'luble hydroxye'thyl cellulose). Thus one may. employ Ygelatinous, water-soluble starches; Vwater-soluble cellulose 'esters (;e ;g., partially hydrolyzed cellulose acetate); sodi- 'uinborophosphate resins and thelike.

fBecause of the colloidal vnature of the gel produced .when coatings ofthe foregoingtype are wetted, any in- *dicia Athat have been printed, inscribed or otherwise `marked'thereupon Withjah oili-bas'e'ink, are not only dis- -placed laterally when 'the i document is. submerged in -water, 4butalso are-individually disintegrated. 'Indeed "the rate of disintegration effectedby' the viscous'colloid a? vspecial `coating Cas I'he'reina'fter more particularly .de-

scribed.

The sheet material 'utilized in accordance with the present invention is characterized-by'thelfact that, when uncoated surfaces thereof are pressed together and the assembly submerged in an aqueous medium, water will rapidly travel between'fthel opposed surfaces, reaching all areas thereof in a relatively short period of time, and zthence:rapidlyrpassV throughzthe-=sheet to 'the-opposite sur- :face thereof. Sheebmaterial fzthetforegoing typeis fcoatedfwith oil-impervious, .water-dispersible 'colloidal `:hint-forming material. The: resultingsheets are@ then uti- '.lized for the preparation L of S documents that vmay be rendered completelyf'indecipherable 'by sllbmer'siony in -any aqueous Alm'edi.ur'n,l including? sea water.

"Numerous `'sheet materials are available today that possess- 'the characteristics fcleseribed above. In general, it has beenffoun'd that Vsuth'`v sheet materials are further 'charvact'e'rize'd by vaill'iigh degree f -later'al expansion -when @vet-ted, Land""t-his"latter propertyfprovides -'a convenient "method f selecting aefsuitablef :sheet material "for the Yconstruction of" documents in accordance `with the present 5 invention. "Of the-available typesV of sheet material' hav- -isfmuch more rapidand morecomplete'than would be yeffected'by a non-colloidallm. For the foregoing realf-s'ons, itis especially desirable to employ colloidal Afilm- -"formingfmaterials ofthe character described.

ln order to give the coated paper-a surface'characteriFor lreasons explained hereinafter, where the ultimate lobject istolproduce,abook-like assembly, the' .lhigh-'wetexpansionsheet-material is preferably coated with'the water-dispersible lm, on only one side-of the paper.` y'In any event, the coating operation `may be satisfactorily accomplished by'means of any of the conventional applicater `rdevices such las those commonly used'in thecoated Ipaper. industry. vThedepth of the Water-insoluble coating may be controlled by-'regulating`theconcentration of the coating mixture, the temperature,"the viscosity, the

settingV of the' brush applicator, or anyother .suitable manner. The spreadingcan be accomplished'by means of scraper blades Aor other conventional devices.l

'After'V the sheet material has' been coatedin 'the'maniier described above it may then be dried. However, immediately following the coating operation, and before the evaporation of substantial proportion of the water, it is desirable to effect a quick-set of the gel lin order to minimize penetration of the colloid into the sheet during the final drying operation. Such quick setting may be accomplished by a variety of means, eg., by the use of infra-red rays,'or by impinging a blast of hot air against the liquid film before substantial drying is brought about,

or by quickly reducing the temperature of the colloid, thereby partially congealing the film. The sheet is then dried, preferably calendered and cut to suitable size.

The sheets prepared in the manner described above may be utilized in various ways for the production of waterdestructible documents. Thus, for example, where the ultimate document is to consist of a single leaf or a number of loose leaves, the sheets may be coated either on one surface or on both surfaces with the water-dispersible film. The information is then printed, inscribed or otherwise marked upon the outer surface of the film, using an oil-base ink. Because of the oil-impervious nature of the coating, the indicia or characters utilized for the transmission of the information will be supported upon the outer or exposed surface of the water-dispersible colloidal film. Accordingly, when the document is wetted with an aqueous medium, the film is converted to a gelatinous, slimy, viscous liquid, the nature of which is such that the indicia are not only displaced laterally over the surface of the base sheet but they are also individually disintegrated, thereby rendering the document completely and permanently indecipherable.

Where the ultimate document is to consist of several leaves more or less tightly compressed together (as for example in the case of a bound volume), it is preferable to employ a method f assembly which will insure that water will reach all areas of every page when the document is immersed in an aqueous medium. To this end, the assemblies described hereinafter are particularly de- 4 consists of a bare sheet 11' (prepared either from any suitable paper stock or from parchment) that has been coated with a similar water-dispersible, oil-impervious colloidal film, the information being imprinted with an oil-base ink upon this film. The bleeder element consists of two opposed leaves 8 of high wet expansion paper coated only on the surfaces 12 which are unopposed to each other, the coating likewise consisting of a water-dispersible, oil-impervious colloidal film. It will thus be seen that the coated surfaces of the bleeder element are in face-to-face relation with the printed and coated side of the indicia-bearing pages. Even when the resulting assembly or any assembly prepared from a multiple of such elements, is compressed orl folded together in booklike form and either bound or enclosed in a perforated envelope or other water pervious container, water will rapidly travel between the opposed uncoated surfaces 13 of the bleeder element, thence pass through the pervious base sheet of the bleeder element and produce sirable inasmuch as they take full advantage of the fact that water will travel laterally between opposed uncoated surfaces of the special base sheet, much more rapidly than between opposed surfaces of ordinary paper stock, or between opposed coated surfaces of the special base sheet. For this reason, the preferred assembly for documents consisting of several leaves more or less compressed together, is constructed from a plurality of sheets of highwet-expansion paper, each of which has been coated only on one surface with a water-dispersible, oil-impervious colloidal film. The singly-coated sheets are then assembled in accordance with either of the procedures described hereinafter.

According to the rst method of assembly (Fig. l), the indicia 3 for the transmission of the information are written, printed or otherwise inscribed upon the outer surface of the water-dispersible film, using an oil-base The resulting leaves 4 and 5 are then assembled into book form in such a manner that the coated surface 6 of each leaf is in face-to-face relation with the coated surface of the adjacent leaf. When the resulting volume is immersed in ari aqueous medium, the water rapidly passes between the opposed uncoated surfaces 7 of the individual leaves and thence penetrates through the sheet, converting the coating on the opposite surface to a viscous, gelatinous liquid that brings about the disintegration of the indicia imprinted on the coated surface.

An alternative method of constructing a document is of particular utility in preparing unbound documents wherein one or more sheets are placed in an envelope, folder or other container. The method, however, is also applicable to the preparation of a bound volume. According to this alternative method (diagrammatically shown in Fig. 2), coated special sheets 8 of the character described above, are employed as bleeder elemen which are placed against the indicia-bearing sides of another sheet 9. In this case, the indicia-bearing page 10 a slimy, viscous liquid not only from the water-dispersible iilm on the reverse face of the bleeder element but also from the corresponding film on the contacting surface of the indicia-bearing page. Any printed matter disposed on the film covering the indicia-bearing page will become disintegrated in the resulting viscous, geiatinous liquid.

Although any of the conventional oil-base inks may be employed in preparing the documents, it has been found that certain inks having a tendency to harden on aging, produce printed letters which take on the character of a film. Although such film-like characters are lifted from the sheet surface when the coating becomes jellied, they may sometimes retain their full structure even though they may be greatly displaced. When this occurs the result may be characterized as alphabet soup, It is, therefore, preferable to employ inks which exhibit much less tendency for the retention of full letter structure and in such instances, there appears to be a strong tendency for the printed characters to break into small sections or portions and, therefore, to become more completely disintegrated.

In order to assure still more complete illegibility when the document is immersed in water, segmented lettering may be employed in preparing'the document. Such lettering may be produced, for example, by the use of photographic screens or by other devices capable of producing letters consisting of a multiplicity of small dots. Segmented characters disperse much more readily and bring about a complete scrambling of the information.

In order to describe the nature of the present invention still more clearly, a specic example will hereini after be described in considerable detail. It should be clearly understood, however, that this is done solely by way of illustration and is not to be construed as a limitation upon the spirit or scope of the appended claims.

Example In the following procedure, a parchment paper is selected as a preferred wet expansion paper. Such parchment paper is readily available in the form of rolls from various parchment paper producers, including the Paterson Parchment Paper Company, the Kalamazoo Vegetable Parchment Company and other sources.

The coating mixture described below is prepared and then applied continuously to one side of the sheet as it travels progressively past the 4applicator device. The rate of sheet travel will, of course, depend upon the apparatus employed and upon the capacity of the drying chamber into which the sheet is delivered following the application of the coating. Ordinarily, however, linear speeds of 20 to 100 feet per minute are practical.

The coating composition preferably consists of approximately a 5 to 10% solution containing the watersoluble, film-forming material, fillers, plasticizers, etc.

, x The following coating solution -has been A.found 'very satisfactory:

The water-soluble hydroxyethyl `cellulose may be obtained .from Carbide and -Carbon Chemicals Corporation, and as received usually consists-of an aqueous `solution containing solids. 'In preparing the coating solution, water is added to the hydroxycthyl cellulose solution as received, in an amount suficient to bring the concentration down to 7.5% solids. Thereafter the starch and Cellite, both dispersed in water, are added to this solution. 'Ihe resulting mixture is passed through a colloid mill to break up any agglomerates. Finally, the lglycerin is added to the homogenized solution. Violent agitation is to be avoided in order to minimize the foaming tendency. All steps in the preparation of the solution may be carried out at yroom temperature.

It is to be noted that because of the nature of the parchment and also because of the relatively high viscosity of the coating mixture, there is comparatively little penetration of the coating ingredients into the sheet, and every effort is preferably made to hold the coating mixture on the surface. The coating may be applied to the paper by means of a cloth application roll and smoothed by a series of vibrating brushes. The amount of coating is controlled by the pressure of the application belt on the paper. Control determination of the dry weight added to the paper may be made at the coating end. A weight increase of 3.5 lbs. per 1000 sq. ft. has been found quite satisfactory.

It is also desirable to direct continuous jets of steam against the uncoated side at the edge of the paper. This prevents the edge of the sheet from curling under, as happens when parchment is wetted on one side only.

The paper leaving the coating end of the applicator is preferably ygiven a quick-set by means of infra red irradiation and blasts of hot air. Thereafter the sheet is dried continuously by hot air, care being taken to avoid over-drying, inasmuch as over-dried parchment is brittle and hard to handle during calendering. The dryer may consist of a large diameter heated roll upon which the uncoated Side of the paper rests. If preferred, however, the drying operation may be carried out by means of hot air and by the use of a so-called festoon dryer. In either case, provision is made to have sufficient drying capacity to take care of the evaporation of the water. The sheet emerges from the drying chamber and is preferably reeled.

Where it is desirable to smooth the sheet surface or to improve the printability of the coated side, the paper is calendered. It has been found, however, that a high gloss 0n the face of the sheet produces a paper which sticks to the rubber blanket customarily employed in a multilith printing operation. Accordingly, the paper is preferably calendered with high pressure on the back of the sheet, thereby maintaining a semi-mat finish on the face. The coating and backing of the product may be carried out in any convenient manner. The roll of paper is out into large sheets, and each sheet may be inspected and sorted for defects. The -good sheets may then ybe square-cut into the -required sizes without -any further sorting. The culled sheets are also cut to smaller size and these are further inspected and sorted.

With the hydroxyethyl cellulose coating mixture, it has been found that a coverage of from about 2.5 to about 3.5 lbs. of coating solids per 1000 sq. ft. of paper produces a satisfactory product. The concentration, however, may be adjusted to meet -any specified con- 6 A ditions. The ratio of hydroxyethyl cellulose, filler `and' glycerin may be adjusted,` andindeed may desirably be vfrequently changed vinorder to take care of slight differences which may occur in the base paperitself. The filler maybe starch, solid'or powdered .celluose, talc or any equivalent material.

In fabricating a'book assembly containing secret vinformation, the .photolithographic printing process v.is particularly satisfactory, especiallywhere itis desired to sproduce individual letters or characters consisting ofaimultiplicity of small dots. `As previously indicated, it is also preferable to employ annif-base ink whi'chhas little tendencyltoharden by oxidation.

In assembling the leaves into a book form, the coated and printed surfaces 6 of successive 'leaves may be placed face to face as shown in Fig. 1. It may then be bound in any suitable manner, the page arrangement specified rendering it unnecessary to resort to special binding procedure. When the resulting book is immersed in sea water, the water rapidly passes between the contiguous uncoated faces 7 of the individual parchment sheets and thence penetrates through the sheet, dissolving and/or gelling the coating on the opposite surface thereof. Even when the sheets are tightly packed together in the form of books or pamphlets, because of the colloidal nature of the resulting film, the obliteration of the indicia on the sheets is remarkably rapid and the information contained on the page becomes unintelligible.

A satisfactory production control test may be carried out to determine the destructibility of the document.4 Thus, for example, a small 2" square book of 50 pagesl may be bound between two perforated metal plates and the assembly placed in a tray under an applied pressure of 20 grams per sq. cm. A 3% salts solution is then poured into the tray covering the booklet to a depth of 6". At the end of 11/2 hours to 5 hours the book is removed and inspected. The printing on a satisfactory sample will be thoroughly disintegrated at the end of that time.

In view of the foregoing detailed description it will be apparent to those skilled in the art that many modifications may be made without departing from the spirit of the invention. All such modifications are to be understood as included within the scope of the invention as defined in the appended claims.

We claim:

1. An assembly of the character described, including at least one indicia-bearing page, a bleeder element, said indicia-bearing page positioned in contact with said bleeder element, said indicia-bearing page comprising sheet material having a water-dispersible, oil-impervious colloidal film thereon adapted to support said indicia on the outer surface of said film; said bleeder element comprising at least two opposed leaves of sheet material having a highwet-expansion, coated only on the surfaces which are unopposed to each other, with a -water-dispersible, oilimpervious colloidal film adapted to contact the corresponding film on an adjacent indicia-bearing page.

2. An assembly of the character described composed of at least one pair of leaves of sheet material having consecutive indicia-bearing pages, a bleeder element, said indicia-bearing pages separated from each other by said bleeder element disposed therebetween, said indicia-bearing pages having a water-dispersible, oil-impervious colloidal film thereon adapted to support said indicia on the outer surface of said lm; said bleeder element comprising at least two leaves of sheet material having a high expansion when wetted, coated only on the surfaces which are unopposed to each other with a water-dispersible, oilimpervious colloidal film and disposed to contact the corresponding film on an adjacent indicia-bearing page.

3. A document of the character described, including a. base sheet of cellulosic material having a high-wet-expansion; a ywater-destructible oil-impervious, colloidal coating on at least one surface of said sheet, and inked indicia,`

comprising segmented printed characters, upon the outer Vsurface of said coating.

ble, oil-impervious, colloidal film, inked indicia, compris- Il ing segmented characters, being disposed on the outer surfaces of said water-destructible lm.

5. A book assembly comprising `a plurality of leaves of cellulosic sheet material of high Wet-expansion bound together, said leaves bein-g coated on one surface only j with a water-dispersible oil-impervious colloidal film and arranged so that the coated surface of each leaf is in faceto-face relation to the coated surface of an adjacent leaf,

and inked disintegratable indicia disposed on Vthe outer' surface of said water-destructible lm.

References Cited in the le of this patent e UNITED STATES PATENTS 171,871"v Schloble Ian, 4, 1876 2,225,520 Eisenberg Dec. 17, 1940 i l2,235,798 COllingS et al Mar. 18, 1941 2,240,999 Moreton May 6, 1941 2,285,490l Broderick .Tune 9, 1942 2,300,787 Ingliss Nov. 3, 1942 2,322,887 Schwartz June 29, 1943 Wysong Sept. 26, 1944

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