US 2333979 A
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Patented Nov. 9, 1943 IUNITED STATES PATENT OFFICE SAFETY PAPER Y Rexford H. Bradt, Battle Creek, Mich, assignor to Fox River Paper Corporation, Appleton, Wis., a corporation of Wisconsin N Drawing.
The invention relates to an improved process for the manufacture of safety paper usable for documents, bonds, checks, notes, and the like, and to the safety paper produced by said process.
One purpose is the provision of a safety paper in which any erasure becomes immediately apparent by the revelation of an underlying layer of a different color than the surface layer of the paper.
Another purpose is the provision of a paper, which may be absorbent or not, but which has a relatively absorbent coating of a different color whereby the ink which is applied to the coating penetrates entirely through the coating and also to or into the base paper, whereby a removal of the coating is not sufiicient to remove the ink.
Another purpose is the provision of a safety paper having a coating sensitive to ink removers whereby a staining of the coating results when an ink remover is applied thereto.
Another purpose is the provision of a relatively absorptive coating impregnated with or including a chemical or chemicals responsive to an ink eradicator whereby, when an ink eradicator is applied, the coating takes up a large amount of the eradicator, thus assuring the production of a deep stain.
Other purposes will appear from time to time in the course of the specification and claim.
The method of formation of the paper is as follows: An ordinary base paper, such as a bond or ledger paper, is employed, and this base paper may, if desired, be somewhat ink absorptive. Ink which is applied to the coating and passes through the coating will penetrate to or into the base paper. A relatively absorbent coating of a different color than the base paper is applied to the paper.
A wide variety of coatings may be employed, but it is important that whatever coating is used be relatively ink absorbent, preferably sufficiently absorbent so that the amount of ink normally used, and the amount of pressure normally applied in writing or typing on the paper, will be sufficient to cause some of the ink to pass, under normal circumstances, entirely through the coating and to or into the base paper, whereby a mere erasure of the coating will not be enough to eliminate the ink. A colored coating may be used, or a colored paper, or coating and paper of different colors.
Any such coating may include a relatively porous filler, and any suitable binder, so that the coating, when it dries, will be ink absorptive, and
Application August 15;, 1941, Serial No. 407,011
1 Claim. (c1. 1 17 -1) will adhere firmly to the base paper, and not dust off. 7
Whereas a diiferential coloring of the absorbent coating and the absorbent base makes the erasure of the ink impossible without detection, because of the change in color produced by the removal of the coating, it is also found important to protect the coating against ink eradicators. Any suitable chemical or chemicals which react by a color change to the presence of or the application of ink eradicators may, therefore, be applied to or included in the coating. While a wide variety of such chemicals may be employed, the employment of benzidine is taken as an example, or a chemical or chemicals of the benzidine group. In employing benzidine in connection with the coating, it is found desirable to cut the benzidine with a suitable acid and to make it into a colloidal paste, for example in a ball mill. As an example of such an acid, phosphoric acid, in a chemical equivalent proportion, gives a paste that is free from any tendency to body or jell the coating mixture. The benzidine combines chemically with the phosphoric acid.
As a specific example of the coating composition, the following may be employed:
The ratio of filler, especially the calcium carbonate, to the casein controls the absorptiveness of the coating. The casein is the binder that holds the coating to the surface of the sheet. Enough casein must be used to keep the coating from dusting ofi. The above given ratio of filler and casein produces a coating which will not dust off, but provides a surface which can readily be written upon by pen and ink, but is absorbent enough to take up an appreciable quantity of the ink.
The benzidine phosphate responds to the usual ink removers which contain a chlorine oxidizing agent. The calcium carbonate in the coating reacts with any acid which may be used alone or in conjunction with the ink remover. It will be'understood, of course, that, whereas a, practical set of proportions is illustrated, substantial variations from the proportions above given may be employed without departing from the present invention.
The coating, including the suitable chemical, such as the benzidine phosphate paste, may be applied to the base paper in any suitable manner. The base paper may for example be passed through a bath or body of the coating in liquid form; the coating may thereafter be doctored in any suitable manner, and the coated paper dried.
The end product is a safety paper which has the following elements and characteristics:
(a) An ordinary base paper, which, if desired, may be somewhat ink absorptive.
(b) A coating which adheres firmlyto *the surface of the base paper sheet.
(c) A differentiation in color between the coating and the base paper. H V (d) A coating preferably sufiiciently ink ab-v sorptive so that the ink penetrates entirely through the coating and to or into the.ba-se paper.
(6 A coating including a chemical or chemicals, such as benzidine phosphate, which stains 'ing.
when an ink eradicator is applied, the depth of the stain being intensified by the liquid absorptive characteristic of the coating. 7 g V It will he realized that various changes may be made in the details or steps-of the ethod of manufacturing the paper, in the pro rtions used, and in the characteristics of the paper itself. It is possible, for example, to apply benzidine phosphate, or some other suitable chemical to the base paper itself, in addition to or in place of the benzidine phosphate in the coat- It will be realized, therefore, that the disclosure herein may be taken as in a broad sense illustrative, instead of a limitation to the precise characteristics of the paper and the precise sequence of .steps herein described.
What is claimed is:
A safety paper including a base adapted to receive and retain ink, a coating of a color differentiated from that of the and suificiently ink absorptive to permit ink supplied to the sur-, face of said coating to penetratereadily to the surface of the base, said coating firmly adhering to the base and not normally separable therefrom andhaving a mineral filler including clay and calcium carbonate and a binder. and a color changing agent of the benzidine group chemically neutralized with phosphoric acid.
REXFORD H. BRADT.