EP0072481B1 - Tamperproof paper - Google Patents

Description

With a number of special papers, such as. B. those for money transactions, securities, identity papers and documents, there is great interest in protecting them from counterfeiting. In particular, such papers are to be protected from the fact that manuscripts such. B. have been done with ink, removed by erasing.

A larger number of possibilities have been described in the patent literature as to how erasing with acids, alkalis, oxidizing agents, reducing agents and organic solvents can be made more difficult by the fact that colored substances are formed during this treatment. Most of the proposed substances react to alkali, acid or chlorine (see, for example, US-A-4 037 007, US-A-3 437 555 or FR-A-2 365-656).

A common counterfeiting method uses both a strong oxidizing agent and a reducing agent, such as the potassium permanganate / sodium bisulfite system - preferably in the acidic range. Potassium permanganate quickly and completely discolors ink; the resulting manganese dioxide is then made colorless with bisulfite. Eraser water on this basis is available in the form of ready-to-use solutions in small bottles (e.g. “lnk Eradicator Les Siamoises X 2”. I. Herbin S.A., Paris).

Protection against this counterfeiting method is particularly difficult since a substance is required which is colorless and becomes clearly colored with permanganate, but cannot be decolorized again with bisulfite.

It has now been found that this counterfeiting method also has no chance of remaining undetected if a paper is used which uses 2- (4'-amino-3'-sulfophenyl) -6-methyl-benzothiazole-7-sulfonic acid ( I) or their salts.

Suitable salts are, in particular, the alkali and alkaline earth salts, preferably the Na, K, Li, Mg, Ca or ammonium salts.

2- (4'-Amino-3'-sutfophenyt) -6-methyt-benzthiazo) -7-sulfonic acid is known.

The papers according to the invention are produced in such a way that the compound of the formula (I) is applied during or after paper manufacture by adding it to the pulp or by applying it to the paper surface or by an appropriate surface treatment.

This compound is generally used in the form of the aqueous solution of its readily soluble salts, in particular alkali salts.

Although these solutions are pale yellow in a concentrated form, they become practically colorless when diluted below approx. If, according to a preferred production variant, the diluted solutions are applied to white paper alone or in combination with a binder, the appearance of the paper practically does not change.

Z. B. solutions of starch or carboxymethyl cellulose or anionic plastic dispersions, for. B. based on polymers of butadiene and styrene or acrylic acid ester, and polyvinyl alcohol solutions.

The paper is normally sized in the pulp, e.g. B. with resin glue. However, you can also omit the paper pulp sizing completely or partially and combine the anti-counterfeiting reagent with an anionic surface sizing agent, possibly also using starch at the same time. This allows the solution of the reagent to penetrate deeper into the paper and, at the same time as the equipment, the sizing required for writing on the paper is achieved. Commercially available products are suitable as surface sizing agents, e.g. B. aqueous solutions or dispersions based on copolymers of maleic anhydride or their ammonium salts.

The paper treated with compound (1) becomes distinctly yellow when spotted with a dilute potassium permanganate solution and also remains yellow when the dark brown manganese oxides formed on the paper are decolorized with a dilute solution of sodium bisulfite.

To achieve a clearly visible gel color, depending on the absorption capacity of the paper, a surface treatment with a 0.01 to 0.5% solution of the anti-counterfeiting reagent is necessary.

In principle, the reagent can also be added to the paper pulp (i.e. the paper fiber suspension). However, it can be expected that some of the product will go into wastewater during paper production. However, the yield in paper can be improved by adding aluminum sulfate or cationic retention or fixing agents (e.g. based on polyacrylamide, polyamidamine or dicyandiamide formaldehyde). In general, 0.1-5% reagent (based on the paper pulp) is required for this production variant.

From US-A-2075538 it was known to produce security paper using the sodium or ammonium salt of dehydrothiopara-toluidine monosulfonic acid. However, such papers show an undesirable discoloration under the long influence of light.

Claims (3)

1. Forgery-proof paper which exhibits a distinct colouration when treated with an oxidising and, if appropriate, reducing agent, characterised in that it contains, as the reagent, 2-(4'-amino-3'-sulphophenyl)-6-methyl-benzothiazole-7-sulphonic acid or its salts.

2. Forgery-proof paper according to Claim 1, characterised in that it contains the reagent in the form of the alkali metal salts.

3. Process for the preparation of the forgery-proof papers according to one of Claims 1-2, characterised in that the reagent are applied during or after the manufacture of the paper, by addition to the paper pulp or by application on to the surface of the paper or by an appropriate surface treatment.