EP3623528B1 - Method for manufacturing a sheet material, corresponding machine, sheet material and safety document - Google Patents

Description

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing a material consisting of fibers and shaped into a strip.

It also relates to a machine for manufacturing such a material. Other aspects of the invention relate to a sheet material, that is to say cut from the strip, as well as to a security and / or valuable document which includes it.

TECHNOLOGICAL BACKGROUND OF THE INVENTION

In the field of security documents and more particularly banknotes, the support usually used is made of paper. It has many advantages, in particular that of being able to integrate a certain number of security elements such as, for example, a watermark or a security thread.

Another advantage is, among other things, that it can be easily printed in intaglio, that is to say with reliefs easily detectable by touch. These elements are easily recognizable by the general public and provide a strong means of authentication.

In order to improve the physical resistance of the support, some national banks use a transparent polymer which is subsequently opacified in white and printed. In addition to being mechanically more resistant, it makes it possible to achieve effective security of the “transparent window” type making it possible to see through said support, from one or the other of its opposite faces.

This construction of a ticket, or rather of a “transparent” portion of a ticket, opened up many possibilities in terms of structure and made it possible to provide a new element that was easily recognizable by the general public.

There are of course techniques known as "transparentization" of paper, which make it possible to create less opaque zones in the mass of papermaking fibers. However, it is impossible to get an area totally transparent, unless you physically create an opening in the paper of the "window" type.

Banknotes with this type of perforation in "windows" are more and more in vogue today. Of course, the resulting windows are sealed with transparent security films to, on the one hand, offer physical resistance to handling in circulation and, on the other hand, serve as a "platform" (i.e. support), so as to accommodate security elements that can be authenticated from one and / or the other of the faces.

Thus, in order to seal the windows, it is necessary to seal a film, for example plastic, on the area of the paper immediately adjacent to the window. The corresponding processing methods are known to those skilled in the art under the term "lamination".

This lamination process differs from the widely used hot transfer process. Thus, in particular, in the lamination process, the carrier film of the security elements is directly deposited on the support, as opposed to the hot transfer process according to which the carrier film is rewound after the thin film containing the security elements has been transferred to the medium.

However, this lamination process suffers from several drawbacks. Thus, the film has mechanical properties, water absorption and contraction / expansion different from that of the paper support, so that the film, especially when it is deposited in a strip over the entire height of a ticket , tends to cause a "curl" effect on the cut sheet paper (which means that the paper tends to curl locally). In English, we speak of a phenomenon of " curl " .

These problems are all the more important when the film is deposited on reel-reel machines before cutting the paper to obtain individual sheets. The film may have been stretched when it was deposited on the support and tends to regain its initial shape at the time of cutting, thus causing the unwanted deformation of the paper.

A solution to this problem was provided by the marked application of a plastic film on the back of the paper having the laminated film on the front, said plastic film applied on the back being called, in terms of the trade, "back-foil" and allowing to counterbalance the effect of "curl" mentioned above. This is described in particular in the document EP 2077190 .

This technique responds to the problem of paper deformation but creates a problem of excess thickness on the paper reels at the level of the laminated film and the "counter-foil", due to the cumulative thickness at this location of the laminated film, of the paper. and the "counter-foil".

We describe in the document EP 2860040 a paper formed from two layers of paper assembled together. In each layer is formed a bowl (that is to say a recess which opens onto only one of the two faces of the layer), the two bowls being located opposite one another.

Thus, depressed areas are locally created in which the thickness of the paper is reduced and the presence of the cups is used to accommodate the aforementioned “foils” and “counter-foils” therein. Thus, the thickness of the final product is drastically reduced in this zone.

Several techniques for forming these cuvettes are mentioned, namely the preparation of a specific canvas for the main layer and the secondary layer, thus creating strips of paper of lower thicknesses, but also localized calendering, and localized shrinkage. of material in the secondary layer.

It emerges from this document that the localized reduction in the thickness of each layer requires additional specific means and that, during the assembly of the two layers, it is necessary to position them correctly with respect to each other, at the risk of having cuvettes which are not exactly in register.

An additional state of the art in this area consists of the following documents:
WO 2007/042718 describes a method of manufacturing a sheet material having at least one window. This window extends transversely to the direction of movement of the sheet material on the manufacturing machine ("running direction"). When the material is formed from two layers of paper joined together, the window is formed through the assembly of the two layers by spraying a pressurized fluid. This fluid extends in the form of a brush, the width of which corresponds to the longitudinal extent of the window to be formed.

Although the formation of the window takes place while the material is still wet, it is indicated in this document that the pressure of the fluid is between 50 and 4000 bar, for example between 1000 and 2000 bar. We readily understands that this range of values is high and requires a significant amount of energy.

FROM 3,431,577 relates to the formation of watermarks in the thickness of a paper during manufacture, which makes use of partial removal of material using jets of water, air or laser radiation.

US 2001/0004007 is interested in cutting jagged edges from sheet material during manufacture.

It is the same for the document US 2003/01196531 in which water jet cutting is used. This action is performed after the sheet material has passed between drying rollers.

• US 4 943 093 décrit un matériau papier à deux couches, dont une couche comporte des fenêtres individuelles notamment visibles aux figures 2, 3a, 3b, 3c, 5a, 5b et 5c ont soit des bords droits, soit deux bords convergents dans la direction de la deuxième couche de papier. Plus précisément, sur les figures 2 à 3a seuls les bords orientés dans le "sens travers" (direction perpendiculaire au "sens marche") présentent des bords convergents ;
• WO 2014/199296 décrit également un matériau à deux couches dont l'une comporte une fenêtre à "bords droits", c'est à dire non convergents ;
• US 2008/036197 décrit un procédé de fabrication de papier dans lequel des fenêtres sont découpées après que la masse papetière ait séché ;
• WO 2009/022072 enseigne une technique de fabrication de papier avec formation d'une zone d'épaisseur nulle par projection d'un jet de fluide sous pression, sans autre précision.

Otherwise :

• US 4,943,093 describes a two-layer paper material, one layer of which has individual windows, particularly visible to figures 2, 3a, 3b, 3c , 5a, 5b and 5c have either straight edges or two edges converging in the direction of the second layer of paper. More precisely, on figures 2 to 3a only the edges oriented in the "cross direction" (direction perpendicular to the "direction of travel") have converging edges;
• WO 2014/199296 also describes a material with two layers, one of which has a window with "straight edges", that is to say not converging;
• US 2008/036197 describes a papermaking process in which windows are cut out after the papermaking mass has dried;
• WO 2009/022072 teaches a papermaking technique with formation of a zone of zero thickness by spraying a jet of pressurized fluid, without further details.

Starting from this prior art, it is clear that there remains a need for a technique for manufacturing a material consisting of fibers and shaped as a strip, in which at least one depression is formed which is as simple as possible to realize, very energy efficient and which requires, for its installation, only a minimal modification of the existing papermaking machines, with a minimum impact on the visual quality of the border areas of this depression.

SUMMARY OF THE INVENTION

Thus, according to a first aspect, the present invention relates to a method of manufacturing a material consisting of fibers and shaped as strip, by wet papermaking, this material being multilayer and comprising a first layer of fibers and at least a second layer of fibers assembled together, a process during which each layer of fibers is formed individually by draining on a forming fabric in a forming section, then conveyed in a strip to an assembly section in which said layers are assembled to form the multilayer material, and then to a dewatering (more commonly called pressing) and drying section,
characterized in that the removal, by means of a pressurized fluid, between said forming and assembly sections, of at least one longitudinal strip of fibrous material in only one of said layers so as to form therein a continuous longitudinal window, that use is made, for each longitudinal strip, of two pressurized fluid jets, which are positioned so as to delimit the opposite edges of said longitudinal window, and that the 'said jets are oriented converging, namely that they are oriented towards the center of said longitudinal strip to be removed, so as to form with the plane in which said strip is contained an angle x ° less than 90 °, in particular between 45 ° and 90 °, preferably between 45 ° and 85 ° and more preferably between 50 ° and 75 °.

The opposite edges of said longitudinal window are the edges oriented in the "running direction", i.e. parallel to the direction of movement of the material from the forming section to the dewatering and drying section.

Thus, use is made, within the machine, of an area where there is a space available for placing a source of pressurized fluid therein. In addition, the moisture content of the strip being very high in this area, a relatively low pressure of fluid is required to clear the window, resulting in significant energy gains. Finally, the particular orientation of the jets results, on the finished product, in an excellent visual quality of the border zones for removing the tape.

Of course, for the sake of simplification, the rest of the on-line process is not described here, but the strip undergoes other processing steps (sizing or "sizing" in English, additional drying, calendering, etc.) before to be wound on itself in a coil.

• ledit fluide sous pression est de l'eau ou de l'air ;
• la pression de fluide est inférieure à 10 bar, de préférence comprise entre 5 et 9 bar ;
• la teneur en eau desdites couches de fibres, lorsque l'on procède audit enlèvement, est supérieure à 70% ;
• ladite section de formation comprend des moyens d'égouttage à travers une toile d'une forme ronde baignant partiellement dans une suspension aqueuse de fibres ou d'une table plate alimentée par une suspension aqueuse de fibres, ou par injection et égouttage de matière fibreuse sur une toile de cylindre rotatif ;
• on oriente les deux jets de fluide sous pression dans le sens de déplacement dudit matériau en bande, de ladite section de formation vers ladite section d'assemblage, préférentiellement selon un angle y° inférieur à 90°, préférentiellement compris entre 45° et 90° ;
• on fait usage de jets ayant de zones d'impact ponctuelles ou quasi-ponctuelles ;
• on procède à l'enlèvement dudit ruban longitudinal par le dessous de ladite couche ;
• on insuffle de l'air sur ladite couche, par le dessus de celle-ci, au moins dans une zone en repérage de celle où lesdits fluides sous pression opèrent, ceci afin de faciliter l'enlèvement du ruban longitudinal de matière fibreuse.

According to other non-limiting and advantageous characteristics of this process:

• said pressurized fluid is water or air;
• the fluid pressure is less than 10 bar, preferably between 5 and 9 bar;
• the water content of said layers of fibers, when said removal is carried out, is greater than 70%;
• said forming section comprises means for draining through a cloth of a round shape partially bathed in an aqueous suspension of fibers or from a flat table fed with an aqueous suspension of fibers, or by injection and draining of fibrous material on a rotating cylinder fabric;
• the two jets of pressurized fluid are oriented in the direction of movement of said strip material, from said forming section towards said assembly section, preferably at an angle y ° less than 90 °, preferably between 45 ° and 90 ° ;
• use is made of jets having point or quasi-point impact zones;
• said longitudinal tape is removed from below said layer;
• air is blown on said layer, from above it, at least in an area marking that where said pressurized fluids operate, in order to facilitate the removal of the longitudinal strip of fibrous material.

Further, the present invention provides a papermaking machine, sheet material, and security and / or valuable document.

This papermaking machine, which has a forming section with at least two stations for forming a wet paper web in which are individually formed at least two layers of fibers, an assembly section in wherein said layers are assembled to form the multilayer material, as well as a pressing and drying section, is characterized by the fact that it comprises, between said forming section and assembly section, removal means, to using a pressurized fluid, at least one longitudinal strip of fibrous material in only one of said layers so as to form a continuous longitudinal window therein.

This single sheet paper material, which comprises a first layer of paper and at least a second layer of paper assembled together, said first layer being a surface layer, is characterized in that said first layer of paper comprises at least one longitudinal window, the opposite edges of which converge in the direction opposite to said second layer, while said second layer has no window facing that of said first layer and is of constant thickness.

The opposite edges of said longitudinal window are the edges oriented in the "running direction".

By the expression “converging in the direction opposite to said second layer” is meant that the lines formed by said edges converge towards a point located “felt side”, that is to say located, with respect to the first layer of paper, on the side opposite to said second layer, as shown in figure 3 .

The security and / or value document is remarkable in that it consists, at least in part, of a material according to the preceding characteristic.

BRIEF DESCRIPTION OF THE DRAWINGS

• la figure 1 est une vue schématique et partielle d'une machine de fabrication de papier conforme à la présente invention ;
• la figure 2 est une coupe transversale d'une couche de fibres formée au sein de la machine de la figure 1 et de moyens qui permettent d'enlever dans cette couche un ruban longitudinal de fibres ;
• la figure 3 est une vue partielle en coupe transversale de la couche de la figure 2, ce schéma étant destiné à montrer comment s'opère l'enlèvement de matière du ruban longitudinal ;
• la figure 4 est une vue partielle en coupe transversale de la couche de la figure 3 et de moyens de soufflage d'air ;
• la figure 5 est une vue en coupe transversale de la couche des figures précédentes après réalisation d'une fenêtre ;
• enfin, la figure 6 est une vue en coupe transversale partielle de deux couches de fibres assemblées l'une à l'autre.

Other characteristics and advantages of the invention will appear on reading the following description of a preferred embodiment of the invention. This description is made with reference to the accompanying drawings in which:

• the figure 1 is a schematic and partial view of a papermaking machine according to the present invention;
• the figure 2 is a cross section of a layer of fibers formed within the machine of the figure 1 and means which make it possible to remove from this layer a longitudinal ribbon of fibers;
• the figure 3 is a partial cross-sectional view of the layer of the figure 2 , this diagram being intended to show how the removal of material from the longitudinal strip takes place;
• the figure 4 is a partial cross-sectional view of the layer of the figure 3 and air blowing means;
• the figure 5 is a cross-sectional view of the layer of the preceding figures after production of a window;
• finally, the figure 6 is a partial cross-sectional view of two layers of fibers joined together.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, the present invention relates to a method of manufacturing a material consisting of fibers and shaped into a strip.

By the expression “strip-shaped” is meant that the material is manufactured continuously so as to constitute a single and unique strip of a few hundred to a few thousand meters in length. Of course, in the end, this strip will be cut into a series of individual sheets.

This process is implemented in a machine for manufacturing such a material, which will be referred to below as a "paper machine".

In general, the figures which accompany this text are intended for illustrative purposes only. This means that the dimensions including thicknesses, clearances and angles are exaggerated for the sake of simplicity and readability and do not accurately reflect reality.

This paper machine M is represented partially and symbolically at the figure 1 . Only the sections of this machine are visible which are of interest in relation to the subject of the invention. This is in particular a training section 1 and an assembly section 2.

However, such a machine advantageously comprises a set of presses, a dryer set and a press-size assembly not shown here. In the end, the material produced in this machine is intended to be rolled up on itself.

In a manner known per se, the training section 1 comprises two training stations 10 and 11 which are substantially identical. Thus, the station 10 comprises a tank 100 containing a suspension of fibers S which can be cellulose fibers, more particularly cotton, synthetic fibers, reconstituted natural fibers with or without additional chemical grafting or any mixture of at least two of these different fibers.

In this tank is partially immersed a rotating cylinder 101 in fabric which defines, when it is driven by a rotational movement in the direction counterclockwise to the arrow f, a peripheral surface in contact with which a layer 4 of fibers is continuously formed.

Similarly, the second training station 11 comprises a tank 110 in which is contained a suspension S identical or different in composition from the first mentioned above and in which a cylinder 111 similar to cylinder 101 and with the same direction of rotation f is bathed. . This second station makes it possible to form a second layer 5 of fibers. These fibers are of the same nature or of a different nature from that of layer 4.

Of course, the speed of rotation of cylinders 101 and 111 is one of the parameters which conditions the thickness of layers 4 and 5.

In an embodiment not shown, the aforementioned two cylinders are immersed in one and the same tank.

With respect to the direction of advance of layers 4 and 5 thus created, referred to as machine direction or running direction S _M , station 10 is located upstream with respect to station 11.

When the layers 4 and 5 are formed, they are pressed against and guided in the machine direction against a felt 3 which is designed to have good retention of the fine fibers and fillers contained in the layer of fibers but which remains sufficiently permeable to pass air and water.

Voluntarily, at the figure 1 , the felt 3 is shown slightly removed from the layer 4, but only in order to clearly distinguish it from this layer.

It is for example a woven fabric of polyester and polyamide having a permeability of the order of 350 CFM ( Cubic Feet per square feets of sample per Minute ). Such a fabric is referred to as an SSB ( Sheet Support Binder ) fabric which qualifies a multilayer fabric whose binding threads are part of the structure, which makes it possible to bind the paper side to the machine side while serving as a support for the strip. constitution course.

Within this relatively traditional architecture of a paper machine M, there exists between the two stations 10 and 11 an interval I which is, in accordance with the invention, taken advantage of to proceed at this level, that is to say between the two stations. sections of formation 1 and assembly 2, on removal with the aid of a pressurized fluid, of at least one longitudinal strip R of fibrous material in the single layer 4 formed within the station 10. Of course The term “longitudinal strip” is understood to mean a strip which extends in the direction of the strip being formed, in this case in the machine direction S _M.

Thus, in the embodiment shown in the figures, preferably pressurized water is used and to do this, pairs of nozzles 7 are used which direct jets of fluid so as to have specific impact zones or quasi-punctual on the layer of fibers 4. This is shown in particular on figure 2 where the jets J thus make it possible to make very fine cutouts D within the layer 4.

In this interval I, the water content of the layers of fibers 4 and 5 is greater than 50% and advantageously greater than 70, or even 80%. Therefore, the fluid pressure that it is necessary to use to proceed with the removal of the tape R is relatively low and advantageously between 5 and 9 bars, due to the low mechanical strength of the layer of fibers at this stage. This is in no way comparable to the high pressures used in the state of the art, pressures which may be of the order of 1000 bar, or even higher.

As also visible at the figure 2 , the jets J are oriented convergently, namely that they are oriented towards the center of the longitudinal strip R to be removed, so as to form with the plane in which the layer 4 is contained an acute angle x ° between 45 and 90 ° and this in order to make a clean cut without bead on the edges of the layer 4 intended to be attached to the layer 5.

The fact of using jets J with point or quasi-point impact zones, which jets are inclined at the aforementioned angle x °, has many advantages. Among these, we particularly note the fact that a point jet requires less water pressure than a nozzle which would provide a jet in the form of a wide and flat brush.

In addition, as shown in the figure 3 , the relative inclination of the jets J will make that the fibers of layer 4 hit by the jets are released towards the center of the window or fall by gravity as shown by the arrows g of the figure 3 . This clearance towards the center of the window or this fall by gravity is facilitated by the shape conferred, in particular because of the relative inclination of the jets J, the cutouts D to the tape R.

Thus, a layer 4 is finally obtained, in which cuts have just been made, the longitudinal edges 40 of which are in theory of the shape indicated in figure 3 .

In order to complete the removal of the longitudinal tape R, it is possible to blow air on layer 4 from above it, and in a zone in registration with that where the jets J operate, in order to facilitate removing the ribbon R of fibrous material. This is represented at figures 1 and 4 in which the compressed air source is referenced 8, while the air jet projected against the layer 4 bears the reference P ( figure 4 ).

On the figure 1 , it is noted that the means 7 for spraying pressurized fluid and the means 8 for spraying pressurized air direct jets which are inclined in the longitudinal direction at angles y °, respectively z °.

The inclination according to the angle y ° makes it possible to make a cut in the layer of fibers in the direction of advance (machine direction S _M ) of the strip of paper, which makes it possible to make a clean cut. This angle can of course be adjusted according to the operating speed of the paper machine.

The angle z ° (oriented opposite to the angle y ° but without the absolute values of these angles being necessarily equal) and the distance of the air nozzles 8 from the fabric 3 can be adjusted to have a perfect detachment of the paper tape cut by the water jets. In addition, these air nozzles must be positioned according to a careful registration vis-à-vis the area in which the water nozzles act. If they are badly positioned, they will peel off the edges of the layer 4 which must remain on the canvas 3 to join the second layer 5, thus generating defects on the final assembly. The width of the pressurized air jet must be at most equal to the width of the strip R of cut paper. In an optimized setting, it can be narrower for more positioning flexibility.

In fact, if the angle x ° is properly adjusted, the edges of the cut-out paper tape R will tend to come off on their own. Thanks to the blowing in the center of the paper tape, it will come off the canvas 3.

These nozzles 8 are positioned so that the ribbon R of paper falls back into a vat of pulp and is then recycled.

As indicated above, the window F which is formed in the layer 4 has longitudinal edges 40 which are generally inclined. This is of particular importance in the assembly 6 of the layers which is finally formed. Thus, if the angle x ° of the air nozzles is too high, that is to say too close to 90 °, then the cut is too blunt and practically perpendicular to the edges of the layer 4 of fibers remaining on the carpet and then sits visible on the final paper. Indeed, the border delimiting the window F is then clear with a marked difference in thickness generating a visible black line. In addition, with such an angle too large, banks are formed at left and right of the cut tape R on layer 4, so that they are also visible on the final sheet.

On the contrary, with an angle that is too small, the nozzles generating the jets J are too close to the paper and the space involved is significant with less flexibility to move them.

The angle x ° is therefore in particular between 45 ° and 90 °, preferably between 45 ° and 85 ° and more preferably between 50 ° and 75 °.

By means of the method of the invention, a paper material is generated which comprises a first perforated layer 4 as well as a second layer 5 of constant thickness. Usually, the window produced is not detectable with the naked eye but by vision systems, generally installed at the end of the paper machine. The area where the window is present is therefore controlled online and, if any drifts appear, the adjustment parameters of the water and air nozzles can be adjusted during production.

Purely as an indication, one can remove according to the present invention, a tape 20 millimeters wide and a thickness of 12 micrometers with the following parameters: Setting Value Angle x ° 60 ° Angle y ° 65 ° Angle z ° 90 ° Pressure of the jets (J) of pressurized fluid (water) 7 bars Blown air pressure 2 bars Fiber layer (4) 15 g / m ² Fiber Layer (5) 70 g / m ²

Nozzles 7 and 8 can be easily adjusted if they are mounted on a graduated rail.

According to the invention, it is therefore possible to remove a ribbon of fibers at a stage where the layer of fibers has very little mechanical strength and this without impacting the visual quality of the border zones for removing the ribbon.

Claims (12)

1. A method for manufacturing a material (6) consisting of fibers and shaped in a strip, by wet papermaking process, this material (6) being multilayered and including a first layer of fibers (4) and at least a second layer of fibers (5) assembled together, method during which each layer of fibers (4, 5) is formed individually by draining on a forming fabric (3) in a forming section (1), then conveyed in a strip towards an assembling section (2) in which said layers (4, 5) are assembled to form the multilayer material (6), and then towards a pressing and drying section,
   characterized in that at least one longitudinal ribbon (R) of fibrous material is removed, using a pressurized fluid, between said forming (1) and assembling (2) sections, from only one (4) of said layers (4, 5) so as to form therein a continuous longitudinal window (F), that two jets (J) of pressurized fluid which are positioned so as to delimit the opposite edges (40) of said longitudinal window (F) are used for each longitudinal ribbon, and that said jets (J) are oriented in a convergent manner, namely that they are oriented towards the center of said longitudinal ribbon (R) to be withdrawn, so as to form with the plane in which said strip is contained an angle x° less than 90°, in particular comprised between 45° and 90°, preferably comprised between 45° and 85° and more preferably comprised between 50° and 75°.
2. The method according to claim 1, characterized in that said pressurized fluid is water or air.
3. The method according to any of claims 1 or 2, characterized in that the fluid pressure is less than 10 bars, preferably comprised between 5 and 9 bars.
4. The method according to claims 1 to 3, characterized in that the water content of said layers of fibers (4, 5), when said removal is carried out, is higher than 70%.
5. The method according to any of claims 1 to 4, characterized in that said forming section (1) comprises means for draining through a fabric of a rounded shape partially immersed in an aqueous suspension (S) of fibers or of a flat table fed by an aqueous suspension of fibers or by injection and draining of fibrous material on a rotating cylinder fabric.
6. The method according to any of claims 1 to 5, characterized in that said two pressurized jets (J) are oriented in the direction of displacement of said strip material, from said forming section (1) to said assembling section (2), preferably at an angle y° less than 90°, preferably comprised between 45° and 90°.
7. The method according to any of claims 1 to 6, characterized in that jets (J) having point or quasi-point impact areas are used.
8. The method according to any of claims 1 to 7, characterized in that said longitudinal ribbon is removed from below said layer (4).
9. The method according to claim 8, characterized in that air is blown over said layer (4), from above the latter, at least in an area in identification of the area where said pressurized fluids operate, in order to facilitate the removal of the longitudinal ribbon of fibrous material.
10. A paper manufacturing machine (M) which includes a forming section (1) with at least two forming stations (10, 11) of a wet paper strip within which are individually formed at least two layers of fibers (4, 5), an assembling section (2) in which said layers are assembled to form a multilayer material (6), as well as a draining and drying section, characterized in that it includes, between said forming section (1) and assembling section (2), means for removing (7), using a pressurized fluid, at least one longitudinal ribbon (R) of fibrous material in only one (4) of said layers so as to form therein a continuous longitudinal window (F).
11. An individual-sheet paper material which includes a first layer of paper (4) and at least a second layer of paper (5) assembled together, said first layer (4) being a surface layer, characterized in that said first layer of paper (4) includes at least one longitudinal window (F) whose opposite edges (40) converge in the direction opposite to said second layer, while said second layer (5) has no window facing that of said first layer (4) and has a constant thickness.
12. A Security and/or valuable document made, at least partly, of a material according to claim 11.

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