EP1356159A2

EP1356159A2 - Method for producing a security document

Info

Publication number: EP1356159A2
Application number: EP01993724A
Authority: EP
Inventors: Thierry Dumery; Christian Vieu; Jean-Michel Grimal
Original assignee: Banque de France
Current assignee: Banque de France
Priority date: 2000-11-10
Filing date: 2001-11-09
Publication date: 2003-10-29
Also published as: WO2002038863A3; MD20030114A; AU2002218352A1; FR2816642A1; OA12412A; MA25922A1; FR2816642B1; WO2002038863A2; BG107790A

Abstract

The invention concerns a method for producing a security document having an opaque fibrous texture, so as to constitute on the surface of the document at least a highly transparent zone. The invention is characterised in that the method comprises a local treatment of the fibrous texture of the document in said zone, so as to enhance its transparency, and the method comprises the exposure of the locally treated zone to the vibration of a sonotrode at an ultrasonic frequency.

Description

The present invention relates generally to securing objects liable to be counterfeited.

More specifically, the invention relates to a method of manufacturing a security document having a generally opaque fibrous weft, the method comprising the constitution on the surface of the document of at least one zone of high transparency.

Numerous methods are known for integrating security elements into objects liable to be counterfeited. In the particular case of security documents (banknotes, checks, tickets, etc.), an important concern is to avoid that counterfeiters can make faithful copies of the document using reprographic means such as a photocopier.

And one way to avoid such counterfeiting of security documents would be to integrate a high transparency window into the document.

Indeed, due to the presence of such a window in a document, a copy of the document produced using a photocopier will present an opaque area at the location of the window, the opacity of which will be that of the support. used to make the copy. Thus, such a provision would make it possible to combat the counterfeiting of documents.

On the other hand, such a window would also constitute a distinctive sign easily recognizable by the general public.

Special documents are already known which include a transparent window. These known documents are produced by printing a transparent polymer support, the transparent window corresponding to an area on which no printing is carried out.

However, the example mentioned above is limited to the production of documents produced on a plastic support; and it would be interesting to obtain the same type of result with documents with fibrous weft (paper banknotes, or others). Furthermore, document US Pat. No. 6,036,230 discloses a security paper comprising a transparent window.

However, the production of this paper requires the implementation of a complex and costly process, during which a window must be cut out in the frame of the ticket, this gaping window then being covered with a transparent film.

And the fact of thus superimposing a transparent film on a gaping window also has the disadvantage of leading to non-homogeneous mechanical characteristics in the ticket (tensile strength, shear strength, folding and crumpling behavior ...) .

The object of the present invention is to make it possible to produce objects such as security documents with a fibrous frame, in particular made of paper, having a window of high transparency, so as to constitute a sign of security recognizable by the general public and prohibiting the reproduction of the object by a photocopier, these objects being freed from the drawbacks mentioned above.

In order to achieve this object, the invention provides a method of manufacturing a security document having an opaque fibrous screen, with a view to constituting on the surface of the document at least a zone of high transparency, characterized in that the method comprises a local treatment of the fibrous frame of the document in said zone, so as to increase its transparency, and the method comprises exposing the locally treated zone to the vibration of a sonotrode according to an ultrasonic frequency.

Preferred, but non-limiting aspects of the process according to the invention are the following:

The fibrous weft of the document is a cellulose weft,

The local treatment includes the impregnation of the opaque screen of the document with a varnish whose optical refractive index is substantially equal to the optical refractive index of the fibrous screen of the document, The varnish contains a solid filler, the optical refractive index of which is substantially equal to the optical refractive index of the fibrous web of the document,

The filler is selected from talc, kaolin or lime carbonate,

• the impregnation is carried out by screen printing, flexography or rotogravure,

The varnish has a component which can be polymerized under UV and the method comprises a step of exposing the locally treated area to UV radiation in order to polymerize the varnish after impregnation,

• the varnish is a hot-melt product and the process comprises a step of cooling the area treated locally after impregnation, • the process includes a step of preheating the area treated locally before impregnation,

The process comprises a step of cooling the area treated locally after impregnation, with a view to evaporating the aqueous part of the varnish, the sonotrode vibrates at a frequency of the order of 30 to 40 kHz,

The amplitude of the displacement of the sonotrode is of the order of 8 to 16 microns,

The document comprises a plurality of treated areas having a high transparency, said areas form a repeating pattern,

The method also includes the integration of additional security elements into the high transparency zone,

• said security elements may include a thread, fibers, or special pigments, • the document is a bank note on which a plurality of zones of high transparency are produced, said zones having the form of blocks having the form of aligned parallelograms. Other aspects, aims and advantages of the invention will appear better on reading the following description of preferred embodiments of the invention.

As a preamble to this description, it should be recalled that the invention relates mainly to the manufacture of security documents whose fibrous web is similar to that of paper.

The cellulose fibers, constituting the main part of the paper, have a natural transparency.

And after cellulose, the second volume component of a sheet of paper is the air between these cellulose fibers. As the optical refractive indices of air and cellulose are different, the light passing through a sheet of paper is scattered, which results in an opacity of the sheet of paper for an observer.

It is recalled that the transparency is defined by the complement to 100 of the opacity, a sheet having an opacity of a value N between 0 and 100 then having a transparency of (100-N); high transparency is equivalent to reduced opacity. According to a first main embodiment of the invention, at least one zone of reduced opacity is produced on a security document with a fibrous weft of paper, by reducing the volume of air present. in the frame of this particular area.

This local reduction in air volume can in a first variant be obtained by calendering.

During calendering, the fibrous weft of the area to be treated is compressed, so that part of the air interposed between the cellulose fibers is expelled, the porosity of the paper decreasing in this particular area. It is also possible to reduce the amount of air present in the weft of the ticket by prolonging the refining of the paper pulp used to form the sheet of the ticket.

However, this technique can only be applied to treat the entire surface of a sheet, and does not make it possible to treat a particular area of the ticket. However, it is possible to combine this technique with calendering, in order to prepare a paper pulp making it possible to produce ticket sheets having over their entire surface a reduced opacity per se, calendering a particular area of each ticket then allowing to achieve a particularly high transparency window.

It is also possible, in a second preferred variant of this first embodiment of the invention, to locally compress a particular zone of a sheet between an anvil which can be fixed, and a sonotrode vibrating at an ultrasonic frequency, according to a direction substantially perpendicular to the surface of the sheet.

The paper can run in an uncut continuous strip between the anvil and the sonotrode.

The frequency of displacement of the sonotrode can be of the order of 30 to 40 KHz, and the amplitude of this displacement can be of the order of 8 to 16 μm.

The zone of the sheet interposed between the sonotrode and the anvil is strongly compressed, which again leads to a reduction in the volume of air in the frame of this zone, and consequently to a reduction in the opacity of the bill in said area. And the shapes of the anvil and the sonotrode, as well as the speed and the direction of travel of the paper, will be adapted to obtain on the tickets areas of desired geometry.

In this variant, the exposure of the areas to be treated to the vibration of the sonotrode causes vitrification of the area, the transparency of which is increased. According to a second main embodiment of the invention, instead of locally compressing the ticket so as to expel the air from the fibrous fabric from a given area, part of this air is replaced by a solid product or liquid having a refractive index substantially identical to that of the fibers which constitute the weft of the paper.

These fibers are generally cellulose fibers; It is on the basis of this element that various variants of this second embodiment will be described below.

According to this second main embodiment, an impregnation of the area to be treated is thus carried out with an impregnation varnish having desired characteristics of temperature, viscosity, penetration time in the fibrous weft of the ticket, replacement rate of air by the impregnating varnish, and possibly drying, hardening or polymerization time of the impregnating varnish.

The varnish may comprise at least one solid filler whose optical refractive index is substantially equal to that of cellulose, said filler being intended to replace at least part of the air initially included in the frame of the ticket, and to remain in the treated area so as to increase its transparency.

We can thus choose fillers such as talc, kaolin or carbonate of lime for example.

Several possibilities are possible for carrying out the impregnation of the paper with the impregnation varnish. It will be preferred according to the invention to use a technique for printing the impregnation varnish, preferably a printing technique by screen printing, by flexography or by rotogravure, because such a technique makes it possible to produce patterns of desired geometry.

Such patterns can constitute a single zone on each ticket, or even a plurality of zones of reduced opacity which can constitute a repeating pattern. As regards the physical characteristics of the impregnation varnish, it is possible in a first variant to impregnate the zone or zones of the paper to be treated with a varnish of high viscosity, liquid or pasty. Such a varnish is produced from an oily or greasy base, and does not change state once integrated into the weft of the ticket paper.

This first variant is possible, but it can significantly modify the areas to be treated, and induce a feeling of greasy to the touch. According to a second variant which constitutes a preferred variant, the impregnation varnish is in the form of a liquid whose physical characteristics will change after impregnation of the paper.

It is thus possible to use a liquid varnish, printed on the areas to be treated, which solidifies after printing in one of the following ways:

• by photochemical reaction. This constitutes a particularly privileged mode of implementation of the invention. The varnish then comprises a polymerizable phase which is activated, preferably by exposing the varnished areas to ultraviolet radiation. Such ultraviolet polymerization causes solidification of the varnish, and thus makes it possible to produce areas having a high transparency. It is thus possible to obtain areas of greater or lesser transparency, depending on the amount of charge present in the paper - the important thing is to constitute areas whose transparency is sufficient to be recognizable to the naked eye by an observer. , and to produce an area opaque to photocopying in order to prevent duplication of tickets by photocopying,

• by cooling. In this case, the impregnation varnish also optionally comprises a filler having substantially the same refractive index as cellulose. The varnish is in this case in the form of a hot-melt product such as a wax, and the areas of the banknotes to be treated are preferably preheated before applying the impregnation varnish, • by evaporation. It is also possible to evaporate, optionally with additional heating, the aqueous phase of the impregnation varnish. The aqueous phase may or may not include a solvent. Note however that this solution does not constitute a preferred variant of the invention, because during evaporation part of the aqueous phase is replaced by air, so that the decrease in opacity of the treated areas is less important than in the case of UV drying varnishes for example.

It is thus possible according to the invention to treat certain areas of security documents with a fibrous weft, so as to obtain for these areas a homogeneous refractive index substantially equal to that of the material of the fibers (cellulose in particular).

And as we said, we can implement the invention to constitute on a ticket several zones of high transparency, these zones being able to constitute a repeating pattern having all desired geometrical characteristics.

In one embodiment of the invention, the zones of high transparency of the ticket are in the form of blocks which can have the form of aligned parallelograms. And these pavers will preferably be produced by printing by screen printing, flexography or rotogravure with an impregnation varnish which can polymerize under exposure to ultraviolet radiation.

Finally, it should be noted that the processing of particular areas with a view to increasing their transparency can be combined with other means of securing documents. It will thus be possible to include in said zones other safety signs, such as threads, fibers or pigments with specific properties (optical, magnetic, electrical, etc.).

Claims

1. Method for manufacturing a security document having an opaque fibrous frame, with a view to constituting on the surface of the document at least a zone of high transparency, characterized in that the method comprises a local treatment of the fibrous frame of the document in said zone, so as to increase its transparency, and the method comprises exposing the locally treated zone to the vibration of a sonotrode according to an ultrasonic frequency.

2. Method according to claim 1, characterized in that the fibrous weft of the document is a cellulose weft.

3. Method according to one of the preceding claims, characterized in that the local treatment comprises impregnating the opaque screen of the document with a varnish whose optical refractive index is substantially equal to the optical refractive index of the document fibrous frame.

4. Method according to the preceding claim, characterized in that the varnish contains a solid filler whose optical refractive index is substantially equal to the optical refractive index of the fibrous frame of the document.

5. Method according to the preceding claim, characterized in that the filler is selected from talc, kaolin or carbonate of lime.

6. Method according to one of claims 3 to 5, characterized in that the impregnation is carried out by screen printing, flexography or rotogravure.

7. Method according to one of claims 3 to 6, characterized in that the varnish comprises a component polymerizable under UV and the method comprises a step of exposure of the locally treated area to UV radiation to polymerize the varnish after impregnation .

8. Method according to one of claims 3 to 6, characterized in that the varnish is a hot-melt product and the method comprises a step of cooling the area treated locally after impregnation.

9. Method according to the preceding claim, characterized in that the method comprises a step of preheating the locally treated area before impregnation.

10. Method according to one of claims 3 to 6, characterized in that the method comprises a step of cooling the area treated locally after impregnation, in order to evaporate the aqueous part of the varnish.

11. Method according to one of the preceding claims, characterized in that the sonotrode vibrates at a frequency of the order of 30 to 40 KHz.

12. Method according to one of the preceding claims, characterized in that the amplitude of the displacement of the sonotrode is of the order of 8 to 16 microns.

13. Method according to one of the preceding claims, characterized in that the document comprises a plurality of treated areas having high transparency.

14. Method according to the preceding claim, characterized in that said zones form a repeating pattern.

15. Method according to one of the preceding claims, characterized in that the method also comprises the integration into the high transparency zone of additional security elements.

16. Method according to the preceding claim, characterized in that said security elements may comprise a thread, fibers, or special pigments.

17. Method according to one of the preceding claims, characterized in that the document is a banknote on which a plurality of zones of high transparency are produced, said zones being in the form of blocks having the form of aligned parallelograms.