GB2209304A - Printing and writing medium - Google Patents

Abstract

A writing or printing medium, e.g. paper or coated plastics is characterized in that it contains at least one metallic salt, preferably a hydrated cobalt salt of oxidation degree 2, which has different colours in the moist and dry states, the metallic salt being incorporated into or onto the medium in a dry weight quantity relative to the medium such that the colour of the medium at room temperature is substantially the colour of the medium without the metallic salt, and that the colour of the medium is reversibly changed when the temperature of the medium is raised. Such a medium can be thermally authenticated and is reactive to chemical falsification.

Description

PRINTING AND WRITING MEDIUM The present invention relates to a printing or writing medium, particularly of paper, which is reactive to any attempt at falsification using chemical means and can be authenticated by heat.

It relates in particular to the field of security papers for use in making handwritten payment documents and official documents such as cheques, banknotes, Savings Bank bonds, account passbooks, commercial accounts books, securities, stamped paper, notarial deeds, travel authorisations and, generally, to all security documents and articles comparable therewith, such as credit cards, etc.

For all such security papers it is necessary to guard against any attempt at falsification of the writing or stamp carried on these papers. The falsification may be mechanical (erasing or scraping) or chemical: with the aid of chemical agents which make it possible to eliminate cleanly the coloured inks at present in use for writing by hand or printing by inking rubber stamps.

Security papers are known which contain, in addition to the watermark and other physical security elements, such as wires, fibres, etc., certain reagents contributing a sensitivity to chemical agents which may be used for falsifying writing by chemically decolouring the inks.

For example, papers have already been proposed which contain chemical substances reactive to chemical forging agents such as acids, alkalis, chlorinated bleaching agents such as chlorine water and Javel water, ink erasing pencils (FR-A-2 365 656, FR-A-2 399 505, FR-A-2 402 739, and FR-A-2 406 027), and oxidizing/reducing agents used in erasers of the "CORRECTOR" R type (EP-A-174 885).

However, the majority of commercially available security papers do not have a simple and reversible authentication process which can be combined with a protection against falsification.

The use of thermochromism for the authentication of security papers has been described in DE-A-1 228 972 and AT-A-362 658. However, these processes require little utilized products which are costly and difficult to use in the papermaking industry. GB-A-l 565 243 describes authentication by thermochromism with the aid of thermochromic and thermomagnetic substances. Moreover, FR-A-2 597 897 describes a security paper which can be authenticated by thermochromism and which contains a violet lactone and an organic acid, but such a paper has a tendency to colour with time.

Furthermore, the protection of paper against forgeries must retain the non-fluorescent nature of the paper.

Indeed, fluorescence limits the security of these papers, as non-fluorescence is a very frequently used recognition sign, particularly with banks and the public at large. It is known that papers, particularly for writing and printing, are practically always fluorescent due to aids for improving the whiteness of the paper.

Given that supplies of non-fluorescent writing and printing paper are very closely supervised, forgers, unless they manage to obtain non-fluorescent paper, will, in all likelihood, use fluorescent paper. A non-fluorescent paper therefore affords a most effective safeguard against forgery and, when working on such a paper after having managed to make it non-fluorescent, it is obviously essential that the security agents incorporated into the paper should not themselves introduce the fluorescence characteristic which one had managed to avoid.

Among all the falsification agents, sodium bisulphite is an efficient reducing agent for deletion. It is used in deleting products of CORRECTOR R type, and can be very effective for shading out or bringing about the disappearance of blots caused by falsification of so-called "non-falsifiable" papers.

In "Verpackungs-Rundschau 11/1985 pp.1250-1253" a process is described for ascertaining the degree of humidity of air. Blotting paper is used, which is thus extremely absorbent, on which an anhydrous salt of blue cobalt chloride is deposited. As the paper absorbs the moisture of the air it turns pink. This paper must not be in contact with the perspiration of the hands to prevent unwanted moistening.

Furthermore, US-A-2 005 105 describes a paper which is reactive to bleaching or oxidizing agents, said paper being treated with an ink containing cobalt ferrocyanide.

In US-A-l 804 978 there is described a paper which is reactive to ink deletors, being treated with a solution comprising water, alcohol, iodine, cobalt nitrate and sodium thiosulphate.

DE-A-42 260 describes a paper reactive to chlorine-based deletors, said paper being treated with an ink containing a sulphated manganese oxide or cobalt chloride.

FR-A-1 509 715 describes a paper containing manganese ferrocyanide and a water-insoluble metallic salt, the paper being intended to react to bisulphate by forming a coloured ferrocyanide which imparts a colour to the paper. The metallic salt may be a transition metal salt.

Thus, according to these publications, transition metal salts are always used in combination with other salts to obtain either a reaction to bleaching or oxidizing agents, or a reaction to bisulphite. The specialist has therefore been led to associate metallic salts with other salts.

According to the invention, there is provided a medium for writing or printing, intended to be thermally authenticated, characterized in that at least one metallic salt which is coloured in the moist state and has a different colour in the dry state is incorporated in or on the medium, the dry weight quantity of the salt relative to the medium being such that the colouration of the medium at ambient temperature is substantially the colouration of the medium without the metallic salt, and that the colouration of the medium is reversibly changed when the temperature of the medium is raised.

It has been found, after intensive and lengthy tests, that by means of the invention a writing and printing medium can be obtained which has non-fluorescent security, can be reversibly thermally authenticated, is not appreciably coloured in its normal1 state, is compatible with other known reagents for sensitization to acids, bases, oxidizers, deleting pencils, and organic solvents, and is reactive to bisulphite.

Preferably, the metallic salt or salts incorporated in the medium in accordance with the invention are able to change from a crystalline state in one colour to a crystalline state of another colour, and are preferably cobalt salts of oxidation degree 2. Preferably the cobalt salt is a hydrated cobalt salt, preferably selected from the group comprising cobalt sulphate, cobalt chloride, cobalt acetate, cobalt nitrate, and cobalt acetyl-acetonate.

The writing or printing medium is preferably a cellulosefibre-based paper, or a plastic support coated with a layer enabling it to be printed or written on, and generally contains from 5 to 7% water after it has been through the drier. Given that the dry weight quantity of cobalt salt incorporated in the medium is low relative to the medium, preferably between 0.1 and 4%, the cobalt salt remains in the hydrated state, even after the sheet of paper or other medium has been through the drier. In the case of cobalt chloride, this is then pink but the amount of it is sufficiently low not to impart any colouration to the paper which remains white.

Indeed, cobalt salts of oxidation degree 2 are generally coloured, but their introduction in relatively low quantities into paper does not affect the original colouring of the paper under normal conditions of dryness, and does not contribute fluorescence. At around 1000C such paper assumes a blue or violet colouring, which quickly fades away as soon as the paper is returned to ambient temperature. This colouration phenomenon, which is brought about by the presence of a cobalt salt of oxidation degree 2 in the paper, is visible only at high temperatures and therefore has no effect under normal daily conditions in which security papers are used. Thus, this colouration at the time of the dehydration of the paper may be used as an easy, non-destructive thermal authentication system for a security paper of this type.

The paper may be authenticated by localized heating, for example by means of heating jaws, blowing hot air, or a heating pen.

If desired, it is possible to add a drying salt, such as sodium chloride, into the paper, which reduces the amount of water available and thereby lowers the temperature threshold of the colouration change.

Moreover, it has been observed, surprisingly, that the addition of cobalt salt to the paper results in a reaction (therefore providing protection) with several falsifying agents, such as hypochlorite, alkalis and sodium bisulphite. This reaction is obtained by introducing the cobalt salt on its own. Thus, the invention enables a thermally authenticatable and chemically non-falsifiable writing or printing medium to be obtained by introducing one compound only into the medium.

The medium in accordance with the invention may have any fibrous composition, purely cellulose-based or partly synthetic, from synthetic fibres or coated synthetic films, referred to briefly hereinunder as "paper", and may incorporate any of the additives standard in the stationery trade, such as mineral charges, various resistance agents, binders, resins, shading pigments, sizing products (neutral, acid or basic), alumina sulphate for acid sizing or pH adjustment, etc.

Since cobalt salts are generally very soluble, their introduction into the medium in accordance with the invention may be made by impregnation with the aid of sizing or coating equipment.

The reaction to heat of cobalt salts of oxidation degree 2 is perceptible to the naked eye at from 500 ppm of the salt in the medium, but, for very visible and fast reactivity, concentrations of between 0.1% and 4% cobalt of oxidation degree 2 are preferred.

Papers in accordance with the invention may also contain within their mass, in the dispersed pigmentary state, water-insoluble but organo-soluble pigments, with a view to preserving written entries or marks made on such papers in the event of attempts at forging using organic solvents. Moreover, these papers may be watermarked or may contain various devices intended to ensure recognition.

Thus, the invention enables manufacture of a white, nonfluorescent paper which is reversibly thermally authenticatable and which, in response to an attempt at chemical falsification, develops colourations detectable with the naked eye on the paper.

Paper in accordance with the present invention may be printed by any one of the known printing methods, and it is usable for handwriting when it is desired to secure permanence of the entries made, and the discovery of any future attempts at falsifying these handwritten entries.

It is particularly suitable for the production of handwritten non-fluorescent payment documents, such as cheques, cash vouchers, savings bank books, official deeds, banknotes, etc.

Although most security papers used are white, it is also possible to make a security paper in accordance with the invention which is not white, but coloured.

Several examples of the medium in accordance with the invention will now be described by way of further, non-limiting, illustration.

EXAMPLE 1 A priting-writing paper has an aqueous coating solution deposited on its surface by a standard stationery technique (sizing press), the coating solution comprising, per litre of liquid: - 100 g starch - 15 g hexahydrated cobalt chloride.

A white paper is obtained which has an orange reaction to sodium bisulphite or to the subsequent addition of a solution of sodium bisulphite and permanganate.

0 The paper, if heated locally to about 100 C, assumes a blue tinge which disappears after a few minutes when heating has stopped.

EXAMPLE 2 A paper medium identica!l with that of Example 1 has an aqueous coating solution deposited on its surface by a standard stationery technique (sizing press), the aqueous coating solution containing, per litre of liquid: - 100 g starch - 15 g hydrated cobalt sulphate.

Results similar to those of Example 1 are obtained.

EXAMPLE 3 A paper medium is treated the same way as in Example 1 except that the coating solution contains 18 g of hydrated cobalt nitrate instead of cobalt chloride. Similar results are obtained.

EXAMPLE 4 A paper medium is treated the same way as in Example 1 except that the coating solution contains 15 g tetrahydrated cobalt acetate instead of cobalt chloride.

Similar results are obtained.

EXAMPLE 5 A paper medium is treated the same way as in Example 1 except that the coating solution contains 15 g cobalt acetyl acetonate instead of cobalt chloride. Similar results are obtained.

EXAMPLE 6 Banknote paper has a qoating solution deposited on its surface by a standard stationery technique (sizing press), the coating solution containing, per litre of liquid: - 100 g polyvinyl alcohol - 30 g hexahydrated cobalt chloride - 10 g sodium chloride.

A white paper is obtained which assumes a blue pigmentation beginning at 800C. It is therefore easily authenticated by localized heating of the paper. The reaction is reversible in a few minutes after removal of the heat. Moreover, the paper has a strong orange reaction to bisulphite.

EXAMPLE 7 A high density polyethylene plastic medium is coated, using a standard paper technique (doctor blade, air brush, etc.), with an aqueous coating slurry containing, per litre of liquid: - 400 g dispersed charges (kaolin, talcum, carbonate) - 200 g latex - 30 g hydrated cobalt chloride.

The coated plastic is white, but on the application of heat a very clear blue pigmentation becomes visible.

Claims (10)

1. A medium for writing or printing, intended to be thermally authenticated, characterized in that at least one metallic salt which is coloured in the moist state and has a different colour in the dry state is incorporated in or on the medium, the dry weight quantity of the salt relative to the medium being such that the colouration of the medium at ambient temperature is substantially the colouration of the medium without the metallic salt, and that the colouration of the medium is reversibly changed when the temperature of the medium is raised.

2. A medium according to claim 1, in which the metallic salt is able to change from a crystalline state of one colour to a crystalline state of another colour, and is preferably a cobalt salt of oxidation degree 2.

3. A medium according to claim 2, in which the cobalt salt is a hydrated salt, preferably selected from the group comprising cobalt sulphate, cobalt chloride, cobalt acetate, cobalt nitrate, and cobalt acetyl-acetonate.

4. A medium according to any one of claims 1 to 3, in which the dry weight quantity of the metallic salt relative to the medium is between 0.1 and 4%.

5. A medium according to any one of the preceding claims, containing a ,siccative salt which lowers the temperature to which the medium is raised to alter its colouration.

6. A medium according to claim 5, in which the siccative salt is sodium chloride.

7. A medium according to any one of the preceding claims, which is reactive to bisulphite so that it cannot be falsified by sodium bisulphite.

8. A medium according to any one of the preceding claims, in which the metallic salt is deposited onto the surface of the medium by a standard paper-making technique from an aqueous coating solution containing said metallic salt.

9. A medium according to claim 1, substantially as herein described in any one of Examples 1 to 7.

10. A security document or article comprising a medium according to any one of the preceding claims.