EP2118366B1 - Offset-printable coated white paper having a high fluorescence intensity and method for producing same - Google Patents

Abstract

White coated sheet material comprises a base sheet and a printable surface layer comprising a white pigment, a binder, an optical brightener and a support for the optical brightener. The optical brightener is present in an amount of at least 1% by weight of pigment and the support is present in an amount of more than 2% by weight of pigment. Producing coated sheet material as above by applying an aqueous pigment composition by curtain coating.

Description

The present invention relates to an offset printable coated white sheet, at least the surface layer of which has a high fluorescence power by virtue of its composition. It also relates to its manufacturing process.

In the field of papers as well as in the case of plastic sheets treated by applying a pigmented layer to obtain offset printability, fluorescent whitening agents, also known as optical brighteners, have long been used, which act by absorbing natural light in the interior of the room. ultraviolet, as well as in the visible near below 420 nm, then re-emit light fluorescence at around 440 nm so in the blue or blue-violet part of the visible spectrum. They increase the whiteness of these leaves by moving strongly their shade from yellow to blue.

The use of these fluorescent whitening agents, unsaturated organic molecules containing two to six sulphonated groups, has been abundantly described, in particular with regard to the need, at the level of the sheet, for a good fixation of these molecules. in their trans isomeric form which is only active.

Among the existing raw materials, the most efficient to ensure this fixation is by far the cellulose because of the interactions between the electrons of the hydroxyl groups and the unsaturated structure of the fluorescent whitening agent. In fact, it is easy to obtain visually very white uncoated papers by the addition of a sufficient amount of a suitable fluorescent whitening agent, said papers having a high fluorescence power of at least 55, or even more. from 60 to 70 as measured according to International Standard ISO 11475: 2004.

In the field of offset printability, it is known that for a good rendering of printing, in particular an ink-surface interaction as homogeneous as possible and a shorter ink drying time, it is necessary to apply on the surface of the base sheet a layer composed of at least one inorganic or organic pigment and at least one natural or synthetic binder. According to the composition and the method of application, the weight of this layer after drying is at least 5 g / m ² in dry weight per side to see an improvement in printability, preferably 10 g / m ² per face and if we want a high level of offset printability, many more, possibly applied in several stages.

The problem with such layers is that the fluorescent whitening agents are poorly fixed by the components of the layers and that it is difficult to increase the fluorescence without causing the counter-whitening effect called "greening" which corresponds to an evolution of the shade of the leaf lying from blue to green. This evolution can be quantified by the variation Δa * of the tristimulus coordinate a * of the CIELAB space measured under the conditions of the ISO 11475: 2004 standard.

Another problem is that the layers based on their weight per square meter act more or less as a UV filter of the incident light vis-à-vis the fluorescent whitening agent present in the base sheet.

To remedy these two problems several solutions, described below, have been proposed in the prior art and are implemented alone or in combination.

A first solution, in the case of a paper sheet, is to confer on the fibrous base a high fluorescence power which is then more or less attenuated according to the composition and the layer thickness deposited thereon. In practice, this route uses large amounts of fluorescent whitening agent and limits the deposition layer to about 8 g / m ² dry weight per face if it is desired to maintain a high fluorescence intensity.

Another solution is to introduce into the layer composition chemical substances which are a carrier for the fluorescent whitening agent, that is to say which play a role of fixing the agent analogous to that of cellulose. Among these carrier substances, there may be mentioned starch, polyvinyl alcohol (PVA), carboxymethylcellulose (CMC), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), etc ... which are macromolecules rich in groups. hydrophilic. In practice, the amount of whitening agent is limited, the amount of 0.2 to less than 1% by dry weight of fluorescent whitening agent and 0.3 to 2% by weight is usually used relative to the total dry weight of pigments. dry of substance support. Indeed, if we want to increase the amount of fluorescent whitening agent, we observe a very critical greening from and above a value of 1 of the Δa * We can correct the shade by adding dyes, but this decreases brightness, so the perception of whiteness. If we want to increase the amount of support substance beyond 2%, to push back this greening limit, then we observe another problem related to the quality of offset printing (especially with inks drying in the air free) due to a non-uniform ink-surface interaction, which gives an irregular, cloudy appearance to printing, called "mottling defect" or "mottling".

The purpose of the present invention is to overcome the drawbacks of the prior art (defined by the documents WO-A1-01 / 07714 , DE-A1-100 55,592 , DE-A1-104 44 131 , EP-A1-1 416 088 , WO-A2-2006 / 035234 and EP-A1-1 577 438 ) and these alternatives to obtain coated sheets appearing very white and offset-printable, in particular by offset inks drying in the open air, without noticeable defect of mottle.

The applicant remedies by providing paper sheets but also plastic sheets, said sheets being coated and having a high fluorescence power by incorporating fluorescent whitening agent and carrier substance in relatively high amounts compared to those used in the prior art. The person skilled in the art was not inclined to use such quantities for the reasons set out above.

• au moins des pigments de couchage blancs et au moins au liant de couchage,
• au moins un agent blanchissant fluorescent en une quantité totale supérieure ou égale à 1% en poids sec par rapport au poids sec desdits pigments,
• au moins une substance support dudit agent blanchissant en une quantité totale en poids sec supérieure à 2% par rapport au poids sec desdits pigments.

The invention thus provides a white coated sheet having a fluorescence power greater than 55, measured according to International Standard ISO 11475: 2004, by difference between the value of CIE whiteness under illuminant D65 and the value of this same whiteness CIE after interposition of a filter eliminating wavelengths shorter than 420 nm, and being printable by offset printing without mounded defects, said coated sheet comprising at least one base sheet and a printable white surface layer whose pigmented composition comprises:

• at least white coating pigments and at least the coating binder,
• at least one fluorescent whitening agent in a total amount greater than or equal to 1% by dry weight relative to the dry weight of said pigments,
• at least one support substance of said whitening agent in a total amount by dry weight greater than 2% relative to the dry weight of said pigments.

It is meant by the fact that the coated sheet according to the invention is "offset-printable without mounded defects" that it does not present on its surface when it is printed by offset, in particular by in-line drying inks, noticeable mottling defect. This mocking defect can be evaluated in particular by the test described in the examples presented below which gives a mock index.

Said coated sheet according to the invention has a high fluorescence power and therefore appears very white and moreover its surface is printable in offset, in particular by offset inks drying in the open air, without noticeable mottle defect.

More particularly, the Applicant has demonstrated that said layer of The surface should be as uniform as possible, even if the surface of the base sheet and / or sub-layers are very irregular. This regularity of the deposition of the layer makes it possible to have a layer of composition that is homogeneous on the surface after drying and to avoid the flaw of molding during printing. Indeed, it has been found that during the drying of the layer deposited in an aqueous medium, the support substance which is in fact water-soluble, migrates irregularly with water depending on the irregularities of the deposition of the layer, these irregularities are notably related to structural irregularities of the base on which it is applied and / or a certain uneven penetration of the layer, resulting in offset printing (especially with inks drying in the open air) a non-uniform ink-surface interaction , which gives an irregular appearance to the printing, which can explain the lack of mottle observed in the prior art when the support substances are used in greater quantity.

According to a particular case of the invention, said coated sheet may comprise at least one sublayer comprising pigments and at least one binder disposed under said surface layer.

More particularly according to the invention the weight of said surface layer is at least 5 g / m ² dry, per side, preferably at least 10 g / m ² , more particularly between 15 and 30 g / m ² .

More particularly according to the invention, only this most surface-deposited layer comprises a large quantity of fluorescent whitening agent and of support substance, the base sheet and / or the sub-layers possibly having intrinsically low fluorescence power, even zero in order to reduce the cost of these fluorescent whitening agents and carrier substances whose price is high.

Indeed, despite the low fluorescence power of the base and / or any sub-layers, the Applicant has unexpectedly found that the sheets according to the invention have a high fluorescence power, and that they do not exhibit offset printing of noticeable mottle defect.

Preferably, the total amount of fluorescent whitening agent in said surface layer is greater than 1 and less than or equal to 4% by dry weight relative to the total dry weight of said coating pigments, in particular between 1.5 and 3. %.

Preferably, the total amount of support substance in said surface layer is less than or equal to 10% by dry weight relative to the total dry weight of said coating pigments, in particular between 4 and 8%.

According to a particular case of the invention, the total amount of fluorescent whitening agent in said surface layer is equal to 1% by dry weight relative to the total dry weight of said coating pigments, and the total amount of carrier substance in said surface layer is greater than 5% by dry weight relative to the total dry weight of said coating pigments.

Preferably said coated sheet according to the invention has a fluorescence power greater than 60, this power being measured according to the international standard ISO 11475: 2004 as previously described.

According to a particular case of the invention, said base sheet, optionally coated with one or more sub-layers, has a fluorescence power of between 0 and 20.

According to a particular case of the invention, the coating pigments of said layer are chosen from calcium carbonates, kaolins, talcs, titanium dioxide and plastic pigments. For example, the plastic pigments are hollow microspheres of a (styrene-acrylic) copolymer having an average size of between 1 and 0.1 μm.

More particularly, the surface layer comprises at least one coating binder chosen from acrylic polymers, styrene-butadiene polymers and possibly other monomers commonly used in coating, as well as other commonly used additives such as dyes. shading. The coating binders are employed as a stabilized aqueous dispersion (latex).

According to the invention, the fluorescent whitening agent is more particularly chosen alone or as a mixture among the various derivatives of stilbene disulfonic acid having altogether 2, 4 or 6 sulphonic groups, especially as marketed. Possibly these may be commercial preparations already containing a very small amount of a carrier substance.

According to the invention, the support substance is chosen alone or in a mixture from polyvinyl alcohols (PVA), carboxymethylcellulose (CMC), polyvinylpyrrolidone (PVP) and N-vinylformamide-based polymers. The PVAs are preferably chosen from those having a high degree of hydrolysis, especially greater than 98%.

The base sheet may be any fibrous sheet, particularly cellulosic and / or synthetic fiber based sheet, such as paper including cardboard.

The base sheet can also be a sheet or a plastic film, for example a said synthetic paper based on a polyolefin extruded film of POLYART ^® brand, or extruded polypropylene sheet manufactured and sold by the company PRIPLAK ^®.

The base sheet may also be a complex of a fibrous sheet, especially a paper and a film or plastic sheet or a paper coated with an extruded plastic layer.

The invention also relates to the method of manufacturing said coated sheet.

The Applicant has also found that among the various means of depositing an aqueous-pigmented composition on a substrate, an outline method such as the curtain coating method contributes to the objects of the invention, in particular in the case of a substrate. substrate with a not very regular surface. Indeed, this method makes it possible to obtain a surface layer deposited with a very regular thickness and thus avoids, during drying, the non-homogeneous migration of the water-soluble support substance, and to avoid the mottle defect during offset printing.

The invention therefore also relates to a method of manufacturing said offset coated white sheet with the characteristics as described above according to the invention, comprising a step of forming said surface layer by depositing the pigmented composition defining it, and carried out in an aqueous medium, by a curtain coating on said base sheet, the latter being optionally coated with underlayer (s).

More particularly, this coating method applies to a fibrous base sheet, especially a paper. Curtain coating can be done online during paper making or offline.

According to a particular case of the invention, the process is such that the pigmented composition of said surface layer is deposited simultaneously with that of a pigmented sub-layer described above by means of a multilayer curtain coating head.

However, according to a particular case of the manufacturing process, the said underlayer (s) may be applied (s) by means of a gluing press for example on the paper machine on which the base of the paper is produced or offline for a plastic base sheet. The gluing press may be of the conventional type or it may be a modified model, for example a "Speedsizer" as marketed by Voith. Other coating methods may be used provided that they are suitable for the application of the desired layer weights, several sub-layers can be applied by different methods. A first underlayer may be coated in a size press and a second pigmented sub-layer may be applied by blade coating, for example.

More particularly, the total dry weight of all the layers of a sheet according to the invention is greater than 10 g / m ² in sec per side and preferably 15 g / m ² , or even 30 g / m ² per side or more. .

The coated sheet according to the invention may comprise said surface layer on each of its faces.

The invention will be better understood with the aid of the non-limiting or comparative examples and the tests presented below.

THE TESTS:

The fluorescence power is measured according to ISO 11475: 2004 by difference between the values of the CIE whiteness under illuminant D65 and the value of this quantity measured after the interposition of a filter eliminating wavelengths below 420 nm.

As an indication, the measurements of the diffuse reflectance factor in blue R457 according to ISO 2469: 1994 and ISO 2470: 1999 under illuminant D65 as well as the whitening power at 457 nm are indicated by difference with the value of diffuse reflectance. R457 after interposition of a filter eliminating wavelengths below 420 nm.

The greening Δa * is calculated by the difference between the value of the trichromatic coordinate a * in the presence of the filter at 420 mm for the coated sheet without fluorescent whitening brightener and that under the same conditions of measurement for the coated sheet with a given percentage of fluorescent whitening agent.

The homogeneity of the printing, achieved by offset printing on a ROLAND 200 4-color machine with NOVAFIT 918 SUPREME BIO free-air offset inks from FLINT GROUP GERMANY GmbH, is evaluated by the melt index which is determined by image analysis on a KHEOPS device marketed by the company TECHPAP, the index being on a scale of 1 (perfect) to 9 (very bad), with considered unacceptable for this use from note 7.

EXAMPLE 1

Example 1, in connection with Table 1, comprises Comparative Examples 1a to 1c and Example 1d according to the invention.

• un mélange de pigments carbonate de calcium (95 parts) et de talc (5 parts)
• un liant copolymère (styrène-butadiène), à raison de 7 % en poids sec par rapport au poids sec des pigments
• un agent blanchissant fluorescent A, qui est un dérivé stilbènique disulfoné commercialisé par 3Vsigma sous la référence d'OPTIBLANC NL, en quantité en poids sec par rapport au poids total des pigments tel qu'indiqué dans le Tableau 1,
• une substance support de l'agent blanchissant, qui est un alcool polyvinylique basse viscosité à haut degré d'hydrolyse du type 4 - 98, en quantité en poids sec par rapport au poids total des pigments tel qu'indiqué dans le Tableau 1.

On a fibrous base (paper) already pre-surfaced and precoated, the total layer weight of 30 g / m ² dry per side, with pigmented compositions of calcium carbonate, styrene-butadiene starch and copolymer binders and having a fluorescence power of 39, is coated again, at a rate of 17 g / m ² dry per side, a surface pigmented composition made in an aqueous medium and comprising essentially:

• a mixture of calcium carbonate (95 parts) and talc (5 parts) pigments
• a copolymeric binder (styrene-butadiene), at a rate of 7% by dry weight relative to the dry weight of the pigments
• a fluorescent whitening agent A, which is a disulphonated stilbene derivative marketed by 3Vsigma under the reference of OPTIBLANC NL, in an amount by dry weight relative to the total weight of the pigments as indicated in Table 1,
• a carrier substance for the whitening agent, which is a polyvinyl alcohol of low viscosity with a high degree of hydrolysis of type 4 - 98, in an amount by dry weight relative to the total weight of the pigments as indicated in Table 1.

The surface compositions of Comparative Tests 1a-1c are coated by a drag-plate method (steel) and the aqueous compositions are adjusted to the same solids (66%) and viscosity.

The pigmented surface composition of the test 1d according to the invention is deposited by curtain coating.

Matt coated papers with a total coating weight of 47 g / m ² dry per side are then obtained, as used for sheet-fed offset printing applications (sheet-by-sheet printing with air-drying inks). high quality.

The coated papers of the tests 1a to 1d are tested in offset printing on the Roland 4-color machine and their roughness index is evaluated as described above.

Comparative test la illustrates a standard coated paper for offset printing, both in terms of its coating formulation and its coating process: It is found that the quality of printing is good but the fluorescence power is insufficient.

Comparative test 1b shows the negative effect of the percentage increase fluorescent whitening agent which leads after greening, the opposite effect of that sought.

Comparative test 1c shows that under standard coating conditions it is not possible to increase the percentage of carrier substance without unacceptably degrading the printability (mottle defect).

Test 1d illustrates the invention, a coated sheet appears very white because of a high fluorescence power and for which there is a good quality of offset printing without noticeable mottle defect. Table 1 Trial 1a comparative 1b comparative 1c comparative 1d % substance support 1 1 6 6 % fluorescent whitening agent A 0.4 1.5 0.4 1.5 Sleeping type Blade Blade Blade Curtain Fluorescence power of the reclining leaf 47 41 52 62 Puffiness index 4 3.5 8 3

It should be noted that the coated paper 1d according to the invention has a CIE whiteness of 136, a diffuse reflectance factor in the R457 blue of 107% and a bleaching power of 22%.

EXAMPLE 2

Example 2, in conjunction with Table 2, includes Comparative Examples 2a and 2b and Examples 2c and 2d with surface compositions in accordance with those defined for the invention.

Two aqueous pigment compositions for mat coating are prepared consisting of the same mixture of calcium carbonate pigments and a copolymer binder (styrene-butadiene) at a rate of 11% by dry weight relative to the dry weight of the pigments.

One of the surface compositions, which is not in accordance with the invention, also comprises standard percentages according to the prior art (0.4%) of the carrier substance of Example 1 and of a fluorescent whitening agent B which is a tetrasulphonated stilbene derivative marketed by CLARIANT under the reference LEUCOPHOR LCPE.

The other composition includes the same products but with quantities according to the invention.

These two compositions are each coated at a rate of 15 g / m ² dry per side on two fibrous bases (paper) of 120 g / m ² weak (16) or strong (69) fluorescence power.

Table 2 shows that, unlike the surface compositions according to the invention (tests 2c and 2d), it is not possible with a standard coating composition of the prior art (tests 2a and 2b) to reconcile a deposit of a layer compatible with the printability requirement, that is to say a layer comprising a low level of support substance so as not to have a mottle defect, and a high fluorescence power of the coated sheet (greater than 55 ), even taking as a basis a paper having a very high fluorescence power (test 2b). Table 2 Trial 2a 2b 2c 2d Fluorescence power of the fibrous base 16 69 16 69 % substance support 0.4 6 % fluorescent whitening agent B 0.7 1.3 Fluorescence power of the reclining leaf 45 51 59 63

EXAMPLE 3

Example 3, in conjunction with Table 3, includes Examples 3a and 3b with surface compositions in accordance with those defined for the invention.

These examples illustrate that even from a base with a virtually zero fluorescence power, the pigmented compositions according to the invention allow the production of coated sheets with very high fluorescence power.

Table 3 reports the results obtained by coating a fibrous base with the same pigmented composition as in Example 2, this time in the presence of 6% (by dry weight relative to the dry pigments) of the carrier substance and 2, 5% (dry weight relative to the dry pigments) of fluorescent whitening agent B. Table 3 Trial 3a 3b Fluorescence power of the fibrous base 2 Layer weight (g / m ² dry per side) 15 30 Fluorescence power of the reclining leaf 62 69

The coated papers 3a and 3b according to the invention respectively have a whitening power of 22% and 25%.

EXAMPLE 4

Example 4 comprises Comparative Examples 4a to 4d and Examples 4 to 4q with surface compositions in accordance with those defined for the invention.

These examples show that, contrary to the usual conditions, the surface compositions conforming to those defined for the invention do not exhibit critical greening.

• des pigments de carbonate de calcium,
• un liant copolymère (styrène-butadiène) à raison de 8 % en poids sec par rapport au poids des pigments,
• un agent blanchissant fluorescent en quantité en poids sec par rapport au poids total des pigments tel qu'indiqué dans le Tableau 4,
• une substance support en quantité en poids sec par rapport au poids total des pigments tel qu'indiqué dans le Tableau 4.

On a fibrous base (paper) previously surfaced and precoated, for a total layer weight of 15 g / m ² dry per side, with pigmented compositions of calcium carbonate and a mixture of binders starch and copolymer (styrene-butadiene) ), said precoated base having a fluorescence power of 43, is coated at a rate of 12 g / m ² per side, a surface pigmented composition produced in an aqueous medium essentially comprising:

• calcium carbonate pigments,
• a copolymer binder (styrene-butadiene) at a rate of 8% by dry weight relative to the weight of the pigments,
• a fluorescent whitening agent in an amount by dry weight relative to the total weight of the pigments as indicated in Table 4,
• a support substance in an amount by dry weight relative to the total weight of the pigments as indicated in Table 4.

The support substance employed is that mentioned in the preceding examples, the fluorescent whitening agent may be the agent A (disulphonated stilbene derivative) or B (tetrasulphonated stilbene derivative) already mentioned in the preceding examples, or another agent C which is a stilbene derivative. hexasulfonated marketed by ROBAMA under the reference of RESISTOL SLK (see Table 4).

Semi-matte coated sheets of paper are obtained.

Only tests 4e to 4q with pigmented surface compositions in accordance with those of the invention, compared with comparative tests 4a to 4d, have both a high fluorescence power and a greening Δa * less than 1.

According to the design mode of the coated sheet, the best quality / cost compromise will be chosen industrially to achieve the desired fluorescence power with a greening Δa * of less than 1, preferably less than 0.6. Table 4 Trial 4a 4b 4c 4d 4th 4f 4g 4h 4i 4d 4k 4l 4m 4n 4o 4p 4q % substance support 0.8 3 6 9 % fluorescent whitening agent A 0.4 1.5 2.5 4 1.5 1 1.5 2.5 4 1.5 2.5 4 % fluorescent whitening agent B 1 1.5 2.5 4 % fluorescent whitening agent C 1.5 Fluorescence power of the reclining leaf 47 40 36 31 59 63 64 63 59 58 63 66 68 65 66 67 63 Δa * of the test 0.24 0.70 1.21 1.72 0.59 0.38 0.52 0.64 0.94 0.26 0.34 0.39 0.47 0.24 0.42 0.57 0.79

EXAMPLE 5

This example 5, in connection with Table 5, includes Examples 5a to 5c which illustrate the use of mixtures of carrier substances to produce pigmented compositions in accordance with those of the invention.

The coating conditions of Example 4 are repeated with 2% (by dry weight relative to the dry weight of the pigments) of the fluorescent whitening agent A (stilbene disulfonated derivative), 0.8% (by dry weight per relative to the weight of the dry pigments) of the PVA carrier substance described in the previous examples and 5% (by dry weight relative to the dry weight of the pigments) of one of the following carrier substances:
S1 = CMC marketed by HERCULES under the reference BLANOSE 7L1C1.
S2 = polymer based on N-vinylformamide marketed by BASF under the reference LUPAMIN 4500.
S3 = PVP marketed by BASF under the reference LUMITEN PPR 8450. Table 5 Trial 5a 5b 5c Substance support S1 S2 S3 Fluorescence power of the reclining leaf 59 60 62 Whiteness CIE of the reclining leaf 141 142 144

It should be noted that the coated papers 5a, 5b and 5c according to the invention respectively have a measurement of the reflectance factor R457 of 104% (for 5a), 104.5% (for 5b) and 105% (for 5c). ), and a whitening power respectively of 21% (for 5a), 21% (for 5b) and 22% (for 5c).

EXAMPLE 6

This example 6 in conjunction with Table 6 illustrates the invention using as a base sheet a plastic sheet.

The coating composition of Test 4o (9% of carrier substance PVA in dry weight relative to dry pigments and 1.5% in dry weight relative to to the dry pigments of the fluorescent whitening agent A (disulfonated stilbene derivative)) with a coating weight of 23 g / m ² dry per side deposited on a plastic sheet (based on an extruded polyolefin film coated with a pigmented precursor) of 150 g / m ² marketed by the company ARJOBEX Ltd under the trademark POLYART®, having a fluorescence power of 8.

Table 6 shows that a sheet with a high fluorescence power is obtained. Table 6 Trial 6 Fluorescence power of the reclining leaf 68 Δa * of the test 0.6

Claims (15)

1. A coated white sheet, comprising at least one base sheet and a printable and white surface coat having a pigmented composition which comprises:

   • at least some white coating pigments and at least one coating binder;

   • at least one fluorescence whitening agent; and

   • at least one support substance for said whitening agent,

   characterized in that the pigmented composition comprises a total amount of whitening agent of more than 1% dry weight with respect to the dry weight of said pigments, and a total amount of support substance of more than 2% dry weight with respect to the dry weight of said pigments, and in that the sheet has a fluorescence intensity of more than 55, measured in accordance with international standard ISO 11475: 2004, by the difference between the value for the CIE whiteness under D65 illuminant and the value for this same CIE whiteness after interposing a filter that eliminates wavelengths shorter than 420 nm, and is offset-printable with no mottling.
2. A coated sheet according to claim 1, characterized in that it comprises at least one sub-coat comprising pigments and at least one binder, disposed below said surface coat.
3. A coated sheet according to any one of the preceding claims, characterized in that the total quantity of dry weight of said fluorescent whitening agent in said surface coat is lower than or equal to 4% with respect to the dry weight of said coating pigments.
4. A coated sheet according to any one of the preceding claims, characterized in that the total quantity of dry weight of said support substance in said surface coat is 10% or less with respect to the dry weight of said coating pigments.
5. A coated sheet according to any one of the preceding claims, characterized in that the coating pigments of said surface coat are selected from calcium carbonates, kaolins, tales, titanium dioxide and plastic coating pigments.
6. A coated sheet according to any one of the preceding claims, characterized in that said fluorescent whitening agent of said surface coat is selected, alone or as a mixture, from stilbene disulphonic acid derivatives containing a total of 2, 4 or 6 sulphonic groups.
7. A coated sheet according to any one of the preceding claims, characterized in that said support substance is selected, alone or as a mixture, from polyvinyl alcohols, carboxymethylcellulose, polyvinylpyrrolidone and polymers based on N-vinylformamide.
8. A coated sheet according to the preceding claim, characterized in that the polyvinyl alcohols have a high degree of hydrolysis, in particular more than 98%.
9. A coated sheet according to one of the preceding claims, characterized in that the weight of said surface coat is at least 5 g/m² dry weight per face, preferably at least 10 g/m², more particularly comprised between 15 and 30 g/m².
10. A coated sheet according to one of the preceding claims, characterized in that said surface coat comprises a coating binder selected from acrylic polymers, polymers (styrene-butadiene) and mixtures thereof.
11. A coated sheet according to one of the preceding claims, characterized in that said fluorescence intensity of said coated sheet is more than 60.
12. A coated sheet according to one of the preceding claims, characterized in that said base sheet, if appropriate coated with sub-coat(s), has fluorescence intensity comprised between 0 and 20.
13. A process for producing the coated sheet defined in one of claims 1 to 12, characterized in that said surface coat is formed by depositing its said pigmented composition in an aqueous medium by curtain coating onto said base sheet, if appropriate coated with sub-coats.
14. A process according to the preceding claim, characterized in that the pigmented composition of said surface coat is deposited by curtain coating simultaneously with a pigmented sub-coat.
15. A process according to claim 13 or claim 14, characterized in that the base sheet is a paper.