FR2786510A1 - UNCOATED PAPER HAVING PSEUDO-FILIGRANE AND MANUFACTURING METHOD - Google Patents

Abstract

The invention concerns an uncoated paper comprising at least a mark resembling a watermark, characterised in that one or several specific zones of the paper has a reduced thickness with respect to the rest of the paper, the mass per unit area in said zone(s) of the paper being identical to that of the rest of the paper. The invention also concerns a method for making uncoated paper comprising at least a mark resembling a watermark characterised in that it consists of producing said mark after the paper-drying step, by the following steps which consist in: a) applying a wetting solution on at least one face of the uncoated paper; then b) applying pressure and heat in said zone(s) so as to evaporate said solution and densify the non-coated paper in said zone(s) with respect to the rest of the paper.

Description

UNCOATED PAPER COMPRISING A PSEUDO-FILIGRANE AND METHOD
MANUFACTURING
The present invention relates to an uncoated paper comprising at least one pseudofiligrane consisting of a mark giving the said paper a visual effect and a texture resembling that of a watermark.

 The present invention also relates to a method of manufacturing an uncoated paper according to the invention.

 Watermarked papers are generally used in the field of security papers such as means of payment such as banknotes or checks, check letters and official documents such as passports, identity cards, stamped papers, deeds notarized, or entry tickets to cultural or sporting events because the presence of the watermark limits the possibilities of reproduction by photocopying and counterfeiting, and offers a means of recognition and / or authentication of said paper. Watermark papers are also used in the field of personalized corporate papers, using the company's logo, name or brand as a watermark. It may turn out that these watermarked papers are requested in very low tonnages and / or in short manufacturing times.

 It is known to produce watermarks in different ways depending on whether real watermarks or pseudo watermarks are produced.

 Until now, various methods have been proposed with a view to producing watermarked papers which can be classified into two categories.

1-The real watermarks are obtained during the manufacture of the sheet of paper, in the wet part of the paper machine, by round shapes comprising imprints or embossings in hollow and / or in relief or using rollers watermarks with embossing and / or embossing associated with a flat table (Fourdrinier machine). A pattern is then obtained which includes light areas, when looking at the sheet of paper by observation in transmitted light, if the prints are made in relief, or dark or shaded areas, if the prints are made in hollow. The light areas are due to the fact that the thickness of the sheet and the quantity of fibers (areal mass) are smaller in the areas corresponding to the imprints than in the rest of the paper sheet. Conversely, the dark areas are due to the fact that the thickness of the sheet and the quantity of fibers are greater in the areas corresponding to the imprints.

 These wet part watermarking methods imply costly means of implementation specific for each type of watermark such as the watermark rollers which are produced by etching the watermark motif and therefore do not offer the necessary flexibility from an economic point of view and also desired by applicants for a custom watermark to make watermark papers in small quantities.

 2-lt is known to produce pseudo-watermarks by penetration or printing in specific areas of the paper with a composition which acts either by permanently transparentizing the fibrous mat of the sheet of paper, or by varnishing the surface. These methods significantly alter the surface properties of the paper thus treated, in particular the printability.

 W097 / 17493 has described papers coated with pseudo-watermarks resulting from a variation in the layer weight applied in determined zones which induces a variation in thickness and opacity in the said zones where the layer weight is reduced or increased.

 An object of the present invention is to provide an uncoated paper comprising pseudo-watermarks consisting of marks which locally modify the physical characteristics of the paper such as its mass volume and therefore certain properties such as its thickness and possibly its opacity.

 Another object of the present invention is to provide an uncoated paper having pseudo-watermarks consisting of marks which create a shade contrast with the rest of the sheet of paper.

 Another object of the present invention is to provide an uncoated paper comprising pseudo-watermarks produced without recourse to the application of a varnish on its surface so that the composition of the paper in the areas constituting the pseudo-watermark is not not significantly changed.

 Another object of the present invention is to provide uncoated paper with pseudo-watermarks whose usage properties, in particular the printability properties in the areas of the surface of the paper corresponding to said marks, are not significantly altered compared to other areas not marked by a so-called paper watermark.

 Another object of the present invention is to provide an uncoated paper having pseudo-watermarks which can be produced in variable and low quantities under more economical conditions than by the processes of the prior art and which can be obtained in widths and quantities. independent of the characteristics of the paper machine, in particular by a process whose implementation of the machines does not modify the operation of manufacturing the paper proper.

 Another object of the present invention is to provide an uncoated paper having a pseudo-watermark in which said watermark is produced after the last manufacturing operation, that is to say at the outlet of the dryer, possibly on the finished paper, c is to say off-line manufacturing.

To do this, the present invention provides a method of manufacturing an uncoated paper having at least one mark resembling a watermark, characterized in that said mark is produced after the step of drying the paper, or in particular off line. making paper, performing the steps in:
a) applying a rewetting solution to at least one face of the
uncoated paper, in one or more specific area (s) then,
b) applying pressure and heat in the said zone or zones of the
uncoated paper rewetted so as to evaporate said solution and
densify the uncoated paper in said area (s) relative to the
rest of the coated paper.

According to the present invention by uncoated paper is meant that the paper is not coated with a pigmented layer comprising at least fine mineral pigments in particular kaolin and / or calcium carbonate, and at least one binder or adhesive, in particular starch or latex, as well as, optionally, any additives commonly used by those skilled in the art, the function of which is to improve the rheological properties of the sauce and to confer particular properties on the layer.

On the other hand, the paper can be impregnated or surface treated with a non-pigmented composition such as a sizing composition, in particular to improve its printability, and / or its mechanical resistance and / or its resistance to soiling and / or its bactericidal properties.

According to the present invention by rewetted paper is meant that, in the said zone or zones, the rewetting solution has penetrated inside the paper and is not yet evaporated.

According to the present invention, by exerting pressure on the whole sheet of uncoated paper or only on said areas and by increasing the temperature of the paper, said rewetting solution is evaporated and the paper is densified in the areas where said solution was initially applied. This results in an increase in the density of the paper in said zones relative to the rest of the sheet of paper and more precisely a reduction in thickness with a surface mass identical to that of the rest of the paper. Said zones can then present an opacity contrast (reduction of opacity) and / or a color contrast, in particular of color nuance and brightness with the rest of the sheet of paper.

 The opacity of the paper is partly linked to the presence of air in the fibrous and / or pigmented interstices of the paper. During the rewetting step, the solution replaces the air in said interstices. Then during step (b), the solution is evaporated and the fibers and / or pigments being densified, the air interstices generally occupy a reduced volume and in particular are found in a smaller quantity than initially, which generates deopacification.

 It should be noted that, in an original way, in the process according to the invention, the densification of the paper comes from a reduction in thickness of the areas treated relative to the rest of the paper, the surface mass remaining constant relative to the rest of the paper. On the contrary, in a true watermark the surface mass is less in the light areas because the fibers deposited are in a smaller quantity compared to the rest of the sheet.

 In the pseudo-watermark described in WO 97/17493, the surface mass is not constant because the weight of the layer is reduced in certain areas.

 Rewetting the paper makes it easier to rearrange the fibers and / or pigments during calendering. B can be additives in the rewetting solution which facilitate the rearrangement of the fibers and / or pigments and / or which promote the greater or lesser penetration of said solution into the paper.

 The method of the present invention can also create a change in the color of the paper in the said zone or zones when said re-wetting solution comprises a coloring agent, in particular a tinting coloring agent. However in the case of a multijet paper, in particular bijet, the color contrast in the said zone or zones can also result from the modification of the opacity in one of the surface jets when there is a difference in shade initially between said surface jet and the jet linked to it.

 The method according to the present invention is applicable to all uncoated, white or colored papers, without limitation of grammage, possibly laminated or multijet.

 In a preferred embodiment in step (b), said pressure and said heat are applied by calendering the paper.

 The rollers of the radiator grille can be heated, but in any event the friction produced by the radiator grille generates heat.

 The calendering operation is carried out according to the present invention with calenders known to those skilled in the art.

 In general, the calenders can be of the supercalender, soft calender (also called elastic) or smooth type. They consist of several rollers, the number and nature of the material constituting them vary depending on the purpose and the paper to be treated. The goal is to smooth the surfaces of the papers, by more or less strong compression of the sheet between the rollers and by the more or less large extent of the contact area between the rollers and the sheet, in order to give them a certain smoothed as well as a certain softness to the touch and a good aptitude for writing and printing. The ridges and reliefs are flattened more or less strongly at the time of passage between the rollers and the sheet is densified. The purpose of calendering can also be to make the surfaces shiny or satin.

The calender rollers are made of hardened cast iron or steel, some of which may be covered with a fabric, cardboard or plastic for example to make elastic rollers.

The supercalender, which has a large number of rollers (of the order of 12 or more), is often located outside the machine due to maintenance, changing of the rollers and sheet breaks. The soft calender can be located on the paper machine, the number of rollers is lower and some are elastic. The boom is located on a machine and has a few rollers, often made of steel, and is not covered.

The linear pressure exerted between the rollers on the paper is of the order of 0.5 to 5000 kN / cm. The temperature of the rollers when heated can be from 50 to 300 C.

 According to the present invention, for a given piece of equipment, the different calendering parameters such as the temperature, the hardness of the rollers, the coating, if any, of the rollers, the extent of the contact area between the rollers and the sheet as well as the pressure are chosen according to the nature of the desired final contrast. These various parameters are adapted as a function of the calendering speed, which is itself controlled by the speed of application of the rewetting solution. Preferably, a calender comprising 2 to 6 rollers is used and, where appropriate, with a plastic coating (for its impermeability to the rewetting solution).

 According to the present invention, the mark may correspond to the said zone or zones and constitute a given pattern appearing in positive contrast with respect to the rest of the sheet of non-rewetted paper. Conversely, the given pattern may appear as a negative contrast in so far as the mark corresponds to said remainder of the sheet of untreated paper, that is to say not rewetted initially. In particular, the said zone or zones may define a visual pattern under observation in transmitted light resulting from a reduction in opacity in the said zone or zones. The said zone or zones can also define a visual pattern by observation in reflected light resulting from a difference in color of the said zone or zones.

 Preferably, said mark corresponds to the said area or areas treated.

 Preferably, in order to meet the volume objectives of the tailor-made markets, step (a) is carried out on off-line production paper, that is to say on finished paper.

 In step a), the rewetting solution according to the invention can be applied using a rotogravure device comprising a rotogravure cylinder whose imprints or hollow cells have a shape such that they allow 'applying said rewetting solution according to a pattern corresponding to said mark in said zone or zones.

 Preferably, the re-wetting solution is applied using a device of the type of those used in the ink jet printing processes, in particular the ink jet printing processes on a reel, it being understood that the 'said ink is replaced by said aqueous rewetting solution. Ink-jet printing devices, in particular digital ink-jet printing devices, make it possible to create marks according to various patterns which can be modified quickly and at low cost from one manufacture to another.

 Said rewetting solution can advantageously comprise a wetting agent so as to improve or accelerate the penetration of the solution into the paper. Said wetting agent can be advantageous, in particular depending on the characteristics of the paper, in particular its porosity and the possible or not presence therein of a wetting agent and also according to the amount of re-wetting solution applied. As an illustration, a quantity of 2 to 20 g / m2 of rewetting solution can be applied.

 Advantageously, said rewetting solution is a solution of a hydrophilic polar solvent.

 As wetting agent, it is possible in particular to use ethanol or 2-pyrrolidone. Preferably, the rewetting solution is an aqueous solution, the use of a non-aqueous solvent inducing additional costs and risks of pollution.

 According to a particular embodiment, a hydroalcoholic solution can be used as rewetting solution.

 More particularly, an aqueous solution containing from 1 to 10% by volume of ethanol, in particular 2%, in water, or an aqueous solution containing from 1 to 10% by weight of 2-pyrrolidone, in particular 2%, can be used. in water.

 In some cases, it may be possible to use water without a wetting agent as an aqueous rewetting solution, in particular in the case of a very porous paper and / or which contains wetting agents and / or particular pigments having a great capacity for absorbing water in its composition, like certain silicas.

 Optionally, the re-wetting solution can also contain additives such as dyes, in particular tinting dyes, fluorescent whitening agents (optical brighteners) or, conversely, fluorescence inhibiting agents as well as any additives known to those skilled in the art. art to allow the recognition or authentication of so-called security papers. In particular, the re-wetting solution may comprise an antifalsification agent or colorless authentication agent capable of being revealed by reaction with a determined co-reactant or under specific conditions.

 The present invention also relates to an uncoated paper comprising at least one mark resembling a watermark, characterized in that one or more determined zone (s) of the paper have a reduction in thickness compared to the rest of the sheet. of paper, the surface mass in the said zone or zones of the paper being identical to that of the rest of the paper. In particular, the grammages are identical in the said zone or zones and in the rest of the paper, it being understood that the variation in weight which may result from the presence of an additive not evaporated from the rewetting solution is not significant.

 In one embodiment, the one or more zones have a reduction in opacity relative to the rest of the paper.

 According to an alternative embodiment, the said zone or zones have a color, in particular a shade and / or a brightness, different from that of the rest of the paper.

 In the papers according to the object of the present invention, the paper may have a reduction in thickness and opacity in the said zone or zones relative to the rest of the paper.

 According to the present invention, the paper can present, in the said zone or zones, agents chosen from coloring agents, fluorescent agents, fluorescence inhibiting agents, agents allowing recognition or authentication. These agents may have been deposited by incorporating the re-wetting solution containing them and may have remained after evaporation of the solution. Anti-forgery or authentication agents are well known to those skilled in the art in the field of fiduciary and security papers.

 According to the invention, the paper can comprise two or more fibrous jets. According to a variant, only one of the surface jets has a reduction in thickness and / or a difference in shade and / or brightness in the said zone or zones relative to the rest of the multi-jet paper.

 According to the invention, the paper can comprise two or more sheets of laminated paper. According to a variant, the lamination adhesive is colored. In one embodiment, only one sheet of the laminated paper has a reduction in thickness and / or a difference in shade and / or a difference in brightness in the said zone or zones relative to the rest of the laminated paper.

 Other characteristics and advantages of the present invention will become apparent in the light of the following exemplary embodiments.

GENERAL CONDITIONS FOR PRODUCING THE PSEUDO-FILIGRANE OF
EXAMPLES 1 to 11:
In simulation of digital inkjet printing systems on a reel, a Hewlett Packard Deskjet 560 C desktop printer is used fitted with a cartridge reference 51626A previously emptied of its ink, cleaned then filled with a rewetting solution consisting distilled water and 2% volume of ethanol, in order to rewet the papers below. In these examples, the marks are positive, unless otherwise specified, and the quantity of rewetting solution provided, for a rewetted solid color with a resolution of 600 × 300 dots per inch, and the so-called coarse-graining conditions, of known quality mail and intensity control said normal, is of the order of 12 g / m2.

The papers, immediately rewetted, are passed through a laboratory calender with two rollers heated to 50 ° C. under a linear pressure of 3.0 kN / cm.

EXAMPLE 1: OPACITY CONTRAST
On a sheet of uncoated white paper of 80 g / m2 A4 format, marketed under the brand RGO of the company GUERIMAND SA, a pattern is represented by rewetting and calendering according to the conditions given above. hand a closed umbrella, using MICROSOFT POWER POINT software (E). We obtain a paper with marks according to the pattern similar to a translucent shaded watermark. The variable thickness reduction in the treated area can be up to 40% compared to the rest of the paper.

EXAMPLE 2: OPACITY CONTRAST
On a sheet of uncoated white paper of 80 g / m2 in A4 format, marketed under the brand RG (D of the company GUERIMAND SA), a re-marbling and calendering is represented, according to the conditions given above. using MICROSOFT POWER POINT (E) software. We obtain a paper with marks according to the pattern similar to a translucent shaded watermark. The reduction in variable thickness in the treated area can be up to 40% compared to the rest paper.

EXAMPLE 3: OPACITY CONTRAST AND COLOR GAP (SHADE AND BRIGHTNESS)
On a sheet of uncoated yellow paper of 80 g / m2 A4 format, marketed under the brand POPSET (3 from the company ARJO WIGGINS SA), the pattern of the re-wetting and calendering is represented according to the conditions given above. character holding a closed umbrella in his hand, using MICROSOFT POWER POINT @ software, producing a paper with marks according to the pattern similar to a translucent shaded watermark.

The color difference AE * (according to the colorimetric coordinates of the CIELAB 1976 system) is 5.97. The variable thickness reduction in the treated area can be up to 40% compared to the rest of the paper.

EXAMPLE 4:
On a sample of uncoated white paper of 115 g / m2 satin, therefore already calendered, the motif is reproduced in two different places, according to an identity photograph of a person, according to the general conditions mentioned above, however for one of the reproductions (a), we made by rewetting the dark parts of the photograph, and for the other reproduction (b), we made by rewetting the light parts of the photograph, thus appearing in negative. After calendering and drying, the paper thus obtained is a security paper personalized by the double pseudo-watermark of the identity photograph which we recognize.
By direct vision, in reflected light, in the form of shade contrast, we observe (a). By observation in transmitted light, we observe (b) in the form of opacity contrast (rewetted areas become less opaque and therefore brighter), being understood that, conversely, (a) in transmitted light and (b) in direct vision in reflected light are each perceived as an imprint of the negative of identity photography.

EXAMPLE 5:
An uncoated 170 g / m2 bijet paper is produced on a laboratory form. One of the jets is white in color and has a grammage of 90 gsm and the other jet is of green shade and also has a grammage of 80 g / m2
After drying the bijet paper, on the face of the white jet, the pattern of the character holding a closed umbrella in his hand, using the POWER POINT @ software, is represented by rewetting and calendering according to the general conditions mentioned above. MICROSOFT. We obtain a paper with marks according to the pattern appearing in transmitted light similar to a translucent and nuanced shaded watermark. In direct vision in reflected light, the pattern is observed because of the green shade of the jet below the pattern. The white spray being quite thick, the rewetting solution did not penetrate the green spray.

EXAMPLE 6
An uncoated laminated paper of 210 g / m2 is produced by laminating two uncoated white papers having a grammage of 90 g / m2 using an adhesive based on poly (vinyl acetate) and comprising 3%. by weight of a shading dye, VIOLET BB 200%.

 On one of the faces, the reworking and calendering according to the general conditions given above, represents the motif of the character holding in his hand a closed umbrella, using the POWER POINT O software from MICROSOFT. We obtain a paper with marks according to the pattern appearing in transmitted light similar to a translucent and nuanced shaded watermark.

In direct vision in reflected light, the pattern is observed due to the nuance of the lamination glue. The rewetting solution did not penetrate all of the paper due to the presence of glue.

EXAMPLES 7 and 8: COLOR CONTRAST
Example 7:
To the rewetting solution described above, water-ethanol, 0.01% by weight of a powder of a shade dye, VIOLET BB 200%, is added.

 On a sheet of white uncoated paper of 80 g / m2 A4 format, marketed under the brand RUZ of the company GUERIMAND, a rewetting and calendering is represented according to the conditions given above, a pattern which is a character holding on to the hand a closed umbrella, using POWER POINT (E) software from MICROSOFT. We obtain a paper with marks according to the pattern similar to a translucent watermark visible in transmitted light. The pattern is also visible in reflected light thanks to its purple shade.

Example 8:
To the rewetting solution described above water-ethanol, only 0.005% by weight of a powder of a shading dye, VIOLET BB 200%, is added,
The pseudofiligrane is produced as in Example 7. A paper is obtained having marks according to the pattern similar to a translucent watermark but it has better visibility in transmitted light, since the amount of dye is less than that of the example 7.

We also observe the pattern in reflected light thanks to its purple shade.

EXAMPLE 9: COLORED INFALSIFICATION REACTION
To the re-wetting solution described in the general water-ethanol conditions, 4% by weight of an infalsification dye, the dye called yellow DHTD, which passes from colorless to yellow-brown under the action of hypochlorite, is added. sodium.

 On a sheet of white uncoated paper of 80 g / m2 in A4 format, marketed under the brand RUZ of the company GUERIMAND SA, a word and a drawing are represented by rewetting and calendering, under the conditions given above. using POWER POINT @ software from MICROSOFT. One obtains a paper having non-colored marks according to the pattern similar to a translucent watermark visible in transmitted light. If you apply sodium hypochlorite (bleach) c

EXAMPLE 10: OPACITY AND FLUORESCENCE CONTRAST
To the water-ethanol rewetting solution described in the general conditions, 2% of a whitening fluorescent agent sold under the brand Blancophor O is added.
BSU PN.

 On a sample of uncoated vellum paper of 80 g / m2, made from cotton fibers and containing no whitening fluorescent agent, the character's motif is represented by rewetting and calendering according to the general conditions mentioned above. holding in hand a term umbrella. One obtains a paper having non-colored marks according to the pattern similar to a translucent watermark visible in light transmitted by contrast of opacity and which, in direct vision in reflected light, is more or less revealed by contrast of shade and fluorescence depending on whether incident light, natural or artificial, contains more or less ultraviolet radiation.

EXAMPLE 11:
The rewetting solution consists of distilled water and 2% by weight of 2pyrrolidone (98% purity).

After calendering at 50 ° C. a paper similar to that of Example 3, under a linear pressure of 1.0 kN / cm, a paper is obtained with a pseudo-watermark in accordance with the invention, by shade contrast and opacity.

EXAMPLE 12:
On a soft, low-width grille, a SCITEX 6240 inkjet printer printhead was installed one meter from the calender rolls. Printing and calendering speed are synchronized. As a rewetting solution for the paper, a mixture of demineralized water and 5% by volume of ethanol is used. The grille consists of two rollers; one is made of steel and the counter roller has a plastic covering. These rollers are heated to 80 C and the linear pressure between the rollers is 2.5 kN / cm.

The reel is unwound from a sheet of white paper, a pattern is made with the rewetting solution using the printing head on the sheet, then the said sheet is passed between the two rollers of the calender. The running speed of the paper sheet is 20 m / min.

The dry paper sheet has a thickness of 60 µm and has been densified in the rewetting areas by a thickness reduction of 16 µm. A paper is obtained with a pseudo-watermark according to the invention, by reduction of thickness.

Claims (21)

 b) pressure and heat are applied to the said area (s) of rewetted uncoated paper so as to evaporate said solution and to densify the uncoated paper in said area (s) relative to the rest of the paper.  a) applying a rewetting solution to at least one face of the uncoated paper, in one or more determined zones, then  CLAIMS 1. Method for manufacturing uncoated paper, comprising at least one mark resembling a watermark, characterized in that said mark is produced after the step of drying the paper, by carrying out the steps in which: 2. Method according to claim 1, characterized in that in step a) said rewetting solution is applied using an inkjet printing device on paper, in particular a printing device digital inkjet, in which the ink is replaced by said rewetting solution. 3. Method according to claim 1 or 2, characterized in that in step b) said pressure and said heat are applied by calendering the uncoated paper. 4. Method according to one of claims 1 to 3, characterized in that said rewetting solution is an aqueous solution. 5. Method according to one of claims 1 to 4, characterized in that said rewetting solution comprises additives chosen from wetting agents, coloring agents in particular shading agents, fluorescent whitening agents, fluorescence inhibiting agents, anti-forgery or authentication agents. 6. Method according to claim 4, characterized in that said aqueous solution is a hydroalkolic solution. 7. Method according to one of claims 4 to 6, characterized in that said aqueous solution contains from 1 to 10% by volume of ethanol in water. 8. Method according to claim 4 or 5, characterized in that said aqueous solution contains from 1 to 10% by weight of 2-pyrrolidone in water. 9. Method according to one of claims 1 to 8, characterized in that the said zone or zones define a visual pattern under observation in transmitted light resulting from a reduction in opacity in the said zone or zones. 10. Method according to one of claims 1 to 9, characterized in that the said zone or zones define a visual pattern under observation in reflected light resulting from a difference in color in the said zone or zones.  11. Uncoated paper comprising at least one mark resembling a watermark obtainable by the method according to one of claims 1 to 10. 12. Uncoated paper comprising at least one mark resembling a watermark, characterized in that one or more determined zones of the uncoated paper have a reduction in thickness compared to the rest of the uncoated paper, the surface mass in the or said areas of the paper being identical to that of the rest of the paper. 13. Uncoated paper having at least one mark resembling a watermark according to one of claims 11 to 12, characterized in that the said area or areas have a reduction in opacity compared to the rest of the paper. 14. Uncoated paper according to one of claims 11 to 13, characterized in that the said zone or zones have a color, and in particular a shade and / or a brightness, different from that of the rest of the paper. 15. Uncoated paper according to one of claims 11 to 14, characterized in that it has a reduction in thickness and opacity in the said zone or zones relative to the rest of the paper. 16. Uncoated paper according to one of claims 11 to 15, characterized in that the said zone or zones comprise agents chosen from coloring agents, fluorescent agents, fluorescence inhibiting agents, antifalsification or authentication agents. 17. Uncoated paper according to one of claims 11 to 16, characterized in that it comprises two or more fibrous jets. 18. Uncoated paper according to the preceding claim, characterized in that only one of the surface jets has a reduction in thickness and / or a difference in shade and / or brightness in the said zone or zones with respect to the rest of the multijet paper. 19. Uncoated paper according to one of claims 11 to 18, characterized in that it comprises two or more sheets of laminated paper. 20. Uncoated paper according to the preceding claim, characterized in that the lamination adhesive is colored. 21. Paper according to claim 19 or 20 characterized in that only one of the sheets of the laminated paper has a reduction in thickness and / or a difference in shade and / or a difference in brightness in the said zone or zones relative to the rest of the laminated paper.