EP0428447B1 - Security paper comprising a new security thread reacting to falsification - Google Patents

Abstract

A printable security sheet. It comprises at least one security member arranged at least partially between the faces of the sheet, the said member being coloured in transmitted light, characterised in that the security member consists of a substrate at least partially coated with a layer consisting of at least one binder which is degradable in basic medium and at least one base-sensitive dye, the binder being a means for protecting the dye in a nonalkaline medium. Application as banknote paper.

Description

The invention relates to security paper, in particular banknote paper.

Papers are already known which include strips or threads consisting of a transparent film provided with a continuous metallic layer, for example aluminum deposited under vacuum on polyester. Such threads, when introduced into paper which is then printed, are barely seen by the public in reflected light and become much more apparent in transmitted light. The purpose of these wires is to prevent a counterfeiter, from a common paper, from imitating an authentic document, by printing on this paper a dark line on the surface. In this case of counterfeiting, there is no difference between the visibility of the line in reflection and in transparency.

EP-A-279 880 describes a transparent wire comprising metallic micro-prints obtained for example by demetallizing areas not protected by varnish. The prints are barely visible by reflection and more visible by transparency using a magnifying device.

In application EP-A-319 157, a paper has been described comprising a thread provided with negative macro-impressions, which can be read by transparency with the naked eye.

The purpose of these security threads is to avoid counterfeiting, that is to say printing on non-authentic paper.

However, a new problem appears, namely the falsification of documents, in particular banknotes, by washing the original printing and reprinting the false document on the authentic paper.

In fact, in some countries, all banknotes denominations have the same format and are printed on the same paper. It therefore seems advantageous for a counterfeiter to erase the impression of a low cut and to reprint a high cut. Such print washes are possible by using solutions attacking the printing ink, for example in the case of banknotes, alkaline solutions and / or very polar solvents.

The invention aims to provide a support comprising a security element betraying falsification.

It is known to introduce into the paper during its manufacture, active chemical compounds intended to react to falsification. These compounds are intended to react to ad hoc forgery, for example modification of a sum or signature on a check. Such papers have drawbacks, however. Indeed, it is possible to subject them to the prolonged action of a liquid, water or organic solvent, to extract the active compound therefrom.

In the case of falsification of banknotes, it is necessary for the counterfeiter to completely erase the duplex printing before reprinting. To obtain a reprintable white surface, the counterfeiter must subject the paper to the prolonged action of solutions attacking the printing ink. So if the paper contained only known compounds which prevent adulteration, these may elute and not fulfill their function.

EP-A-330 733 describes a security paper comprising a security element disposed at least partially between the faces of the paper, said element having characters visible in transmitted light and consisting of a support coated at least partially a layer consisting of at least one binder and at least one dye.

However, such a security element does not have the function of becoming colorless in light transmitted by the action of solutions attacking the colored printing.

Now, we now wish to obtain a printable security sheet comprising a security wire visible in transmitted light and having a certain color, this wire being such that by the action of solutions attacking the printing inks, it becomes colorless and therefore invisible. Thus, if the counterfeiter subjects the printed paper to such an action of solutions, for example alkaline, the wire would become colorless and the counterfeiter could not reproduce the coloring effect by transmitted light by passing a stroke of a colored pencil, because it would then obtain a colored line which would be seen in reflected light.

The problem to be solved in order to obtain a colored wire which becomes colorless under the action of alkaline solutions is that this wire must be introduced into a sheet obtained by the wet method, the introduction taking place during the manufacture of the sheet. The dyes placed on the wire should therefore not pollute the aqueous suspension of fibers from which the sheet is made.

Another problem to be solved is that the color of the wire must disappear under the action of alkaline solutions more quickly than the surface prints of the paper.

Indeed, if the color of the wire disappeared more slowly than the surface prints, the counterfeiter - after having erased the surface prints - would have a white paper comprising a colored security thread and he could reprint the surface of the paper to obtain a ticket not comprising no evidence of tampering.

One might think of printing a security thread with an ink used for the surface printing of paper since this ink is sensitive to alkaline solutions. But then the thread would not discolor faster than the surface impression. Indeed, the wire being at the heart of the paper, the paper fibers form a barrier to the alkaline solution. In addition, in general during falsification, the counterfeiter, to accelerate the discoloration of the ink on the surface of the paper, performs a rubbing and / or abrasion action on the surface of the paper. Such an action is not possible on the wire and therefore it would discolor less quickly than the surface.

The invention therefore relates to a printable security paper comprising at least one security element disposed at least partially between the faces of the sheet, said element being colored in transmitted light, the sheet being such that the security element consists of a support coated at least partially with a layer consisting of at least one binder degradable in basic medium and at least one dye sensitive to the bases, so that the coloring of the element, including the part arranged between the faces of the paper , disappears under the action of alkaline compositions more quickly than a surface impression of the paper subjected to the same treatment, the binder being a means of protecting the dye in a non-alkaline medium and the binder allowing, in the presence of an alkaline aqueous solution, diffusion of the dye out of the security element or its destruction.

Preferably, the security element is a wire, but it can also be in the form of confetti or boards, strips, etc. According to one embodiment of the invention, the support is transparent. It can be made of polyester. When it is mentioned in the description that the binder is degradable in basic medium, this means that it is degradable at pH greater than 8, preferably 10. However, it is not degradable at the pH of the aqueous suspension of fibers from which the sheet is formed.

The binder according to the invention can swell and become porous in the presence of an alkaline aqueous solution to allow diffusion of the dye out of the security element and / or its destruction. The binder can for example be chosen from homo or copolymers with a high acid number, for example copolymers of (meth) acrylic ester, copolymers of phthalic acid ester.

According to a characteristic of the invention, the dyes are soluble in an acid medium and have no affinity for cellulose. The dyes can for example, in basic medium, have a colorless and soluble structure. They can split in alkaline medium into precursors, at least one of which is soluble. They can also become colorless in basic medium by redox reactions within the binder. For example, the dye can be an organic amine salt, an eosin derivative, a triphenylmethane derivative. The dye can also be a combination of a dye precursor with a Lewis acid or a mixture of a triphenylmethane derivative with a reducing agent.

The following examples will help to understand how the invention can be put into practice.

Example 1

A polyester film 23 micrometers thick is printed with an ink of the following composition (parts by dry weight): - a binder 97 games copolymer of acrylic ester sold under the name MOWILITH by HOESCHT which is dissolved in 10% in a 50/50 mixture by volume of methyl ethyl ketone and ethyl acetate. - a dye 3 parts which is a solution of Malachite green sold by BAYER, at 2% in dimethylformamide.

The film thus printed is dried. It is cut in the form of a wire. An aqueous suspension of cellulose fibers, optionally synthetic fibers, optionally fillers, optionally bonding agents is formed. The suspension is placed on a flat table on a Fourdinier machine or on a round form. The wire is introduced simultaneously into the suspension, in a known manner. The wire can be introduced completely into the heart of the paper or can come out superficially in certain places. We obtain a paper with a thread visible to the naked eye, green in color. If the paper is washed with a 2% aqueous sodium hydroxide solution for about an hour and a half, the green color disappears.

Example 2

A 23 micrometer polyester film is printed with an ink having the following composition: - a binder according to Example 1 99 games - a dye 1 part which is a solution of red dye precursor 6 B sold by CIBA, and of 2-4 dihydroxy-benzophenone in methyl ethyl ketone.

The film is dried and cut into strips which are introduced into the paper. A paper is obtained comprising a thread visible to the naked eye, red in transparency. If the paper is washed with the same aqueous sodium hydroxide solution for about an hour and a half, the red color disappears.

Claims (11)

1. Printable security paper comprising at least one security element disposed at least partly between the faces of a sheet, said element being colored against transmitted light, characterized by the fact that the security element is constituted of a support coated at least partly with a layer composed of at least one binder which is degradable in a basic medium and at least one coloring agent which is sensitive to the bases, so that coloring of the element, including of the part situated between the faces of the paper, disappear under the action of alkaline compositions quicker than a superficial printing of the paper selected to the same treatment, the binder being a means of protecting the coloring agent in non-alkaline medium, and the binder enabling, in the presence of an alkaline aqueous solution, the diffusion of the coloring agent outside the security element or its destruction.
2. Paper according to claim 1, characterized by the fact that the binder is selected from the homo-or copolymers having a high acid index and mixtures thereof.
3. Paper according to claims 1 and 2, characterized by the fact that the binder is selected from the copolymers of (meth) acrylic ester, the copolymers of phtalic acid ester.
4. Paper according to claims 1 to 3, characterized by the fact that the coloring agents are soluble in basic medium and show no affinity towards cellulose.
5. Paper according to claims 1 to 3, characterized by the fact that the coloring agents have a structure which is colorless and soluble in basic medium.
6. Paper according to claims 1 to 3, characterized by the fact that the coloring agents split up in alkaline medium into precursors of which at least one is soluble.
7. Paper according to claims 1 to 3, characterized by the fact that the coloring agents become colorless in basic medium by oxydo-reduction reactions within the binder.
8. Paper according to claim 4, characterized by the fact that the coloring agent is an organic amine salt, an eosine derivative.
9. Paper according to claim 5, characterized by the fact that the coloring agent is a triphenylmethane derivative.
10. Paper according to claim 6, characterized by the fact that the coloring agent is a combination of a coloring agent precursor with a Lewis acid.
11. Paper according to claim 7, characterized by the fact that the coloring agent is a mixture of a triphenylmethane derivative with a reducing agent.