EP3172379B1 - Security sheet resistant to crumpling and hard creasing - Google Patents

Description

The present invention relates to a wrinkle and crease resistant security sheet, its manufacturing method and a security document comprising the sheet. For the purposes of the invention "resistant to wrinkling and marked crease" means that the sheet hardly undergoes the impact of a wrinkle and crease marked thanks to a controlled stiffening of the cellulosic substrate.

Currently, many security documents, such as banknotes or identity documents, include paper supports or cellulosic substrates.

A disadvantage of the used paper supports is that they resist bad wrinkling and wrinkles. Thus, crumpled or folded areas have marked folds, irreversible, poor resistance to soiling, so that these wrinkled areas are weakened and often give rise to tears.

This is particularly disadvantageous in the case of documents which, during their handling, are frequently folded or wrinkled, such as for example banknotes, the presence of marked folds weakening and reducing their life, and making their automated processing difficult. , for example during verifications of authenticity or wear on a sorting machine.

Document is known US 2004/0023008 , security sheets, especially banknotes, resistant to the effects of circulation, using polyurethanes and a resin poly (aminoamide-epichlorohydrin), as a moisture resistance agent. However, this document does not mention aggregates of polymer (s) or cationic flocculation agent. On the contrary, the polyurethane is crosslinked. In addition, the process proposed in this document relates to surface treatment.

In addition, there is a need to improve the resistance of the sheets produced by the method described herein to the effects of the circulation and in particular, their resistance to wrinkling, marked crease and soiling.

The mass treatment of paper is one of the solutions that can be considered to improve the resistance to wrinkling.

Indeed, as detailed below, the use of flocculation, before the formation of the sheet, makes it possible to form a global fiber-flocculating agent-polymer network via the formation of bonds made possible by the presence of the flocculation agent. In other words, the flocculation of the polymer takes place directly on the fibers thanks to the presence of the flocculation agent. There is therefore no prior formation of polymer flocculates before contacting the fibers.

Thus, the document WO 2008/152299 discloses safety sheets comprising an anionic polymer having a glass transition temperature above -40 ° C and a cationic flocculation agent having good wrinkling resistance.

However, it has been found that the flocculation of anionic polymeric dispersions as proposed in WO 2008/152299 cause a decrease in certain mechanical properties such as tensile strength, especially the breaking lengths, dry and wet, compared to standard paper.

The document WO 2014/083527 discloses safety sheets comprising a polyurethane having an elongation at break greater than 600%, and a cationic flocculating agent, having good wrinkling resistance as well as good mechanical properties such as tensile strength, including breaking lengths dry and wet, resistance to double-fold or tearing.

However, the improvement of the mechanical properties of these safety sheets, in particular those described in the applications WO 2008/152299 and WO 2014/083527 , causes a decrease in rigidity. In addition, these sheets are not entirely satisfactory in terms of resistance to soiling.

Certain methods are moreover known for increasing the rigidity of the security sheets. For example, EP 1 466 755 and EP 1 466 756 describe safety sheets whose rigidity is increased on the corners with specific watermarks. However, these processes are not economically very attractive and more importantly, they do not allow to treat the entire cellulosic substrate.

There is therefore a need to provide a security sheet which exhibits an improvement in its rigidity on the totality of the cellulosic substrate and its resistance to soiling while retaining its mechanical properties such as tensile strength, especially the breaking lengths, dry and wet and double-fold resistance, and good resistance to wrinkling.

In fact, adding stiffness, for example at least 20%, to the safety sheets would further reduce the mechanical impact of the traffic. (wrinkles, creases ...) on the paper. As for the reduction of the physicochemical impact of the circulation such as soiling, wrinkling or folding marked on the paper, it would reduce the rejection by automatic sorting machines. In fact, the wrinkling, the marked bending and the soiling of a banknote paper lead to a decrease in its rigidity as well as a large number of rejects in automatic sorting machines. Indeed, studies show that the main reason for rejecting banknotes at the beginning of circulation, especially up to the first thirty percent of the life of a bank note, before the stains and tears, is the presence marked wrinkles that disturb the sensors and can therefore be interpreted as tears or perforations, in front of dirt and tears.

Unexpectedly, the inventors have found that such an objective could be achieved provided that an anionic dispersion of at least one polymer having a glass transition temperature greater than 23 ° C. is combined with an effective amount of an agent. cationic flocculation during the manufacture of a security sheet.

By " anionic dispersion " is meant an aqueous suspension of particles having an anionic surface charge. Said surface charge is either intrinsically present in the polymer or is provided by a surfactant compound (or surfactant) used in the formation of said particles. Thus, the polymer according to the present invention may be anionic or not. Preferably, said surface charge is provided by a surfactant compound (or surfactant) used in the formation of said particles. The term " dispersion " is here used generically and can therefore also refer to an emulsion.

• des fibres,
• des agrégats de polymère(s) présentant une température de transition vitreuse supérieure à 0 °C, et
• une quantité efficace d'un agent de floculation cationique, dit agent principal.

Thus, the present invention discloses a wrinkle-resistant security sheet comprising at least:

• fibers,
• aggregates of polymer (s) having a glass transition temperature above 0 ° C, and
• an effective amount of a cationic flocculation agent, said main agent.

• des fibres,
• des agrégats de polymère(s) présentant une température de transition vitreuse supérieure à 23 °C, et
• une quantité efficace d'un agent de floculation cationique, dit agent principal.

In particular, the present invention relates to a wrinkle-resistant security sheet comprising at least:

• fibers,
• aggregates of polymer (s) having a glass transition temperature greater than 23 ° C, and
• an effective amount of a cationic flocculation agent, said main agent.

"Polymer aggregates (s)" denotes the flocs formed by bringing into contact with the anionic polymer dispersion (s) with the fibers and the flocculation agent. Flocculation of the anionic dispersion of polymer (s) thus occurs as soon as the anionic dispersion of polymer (s) is mixed with the fibers and the flocculation agent. The aggregates are thus preserved within this mixture which is then subjected to the successive common steps of the papermaking process, namely dewatering, pressing and drying. Under the effect of said routine steps of the papermaking process and especially by the disappearance of water, said polymer aggregates (s) are thus dispersed within the network formed by the fibers and the flocculation agent.

• des fibres,
• des agrégats de polymère(s) présentant une température de transition vitreuse supérieure à 5 °C, et
• une quantité efficace d'un agent de floculation cationique, dit agent principal, lesdits agrégats de polymère(s) étant répartis au sein du réseau formé par les fibres et l'agent de floculation.

Thus, the present invention discloses a wrinkle-resistant security sheet comprising at least:

• fibers,
• aggregates of polymer (s) having a glass transition temperature above 5 ° C, and
• an effective amount of a cationic flocculating agent, said main agent, said polymer aggregates (s) being distributed within the network formed by the fibers and the flocculation agent.

• des fibres,
• des agrégats de polymère(s) présentant une température de transition vitreuse supérieure à 23 °C, et
• une quantité efficace d'un agent de floculation cationique, dit agent principal, lesdits agrégats de polymère(s) étant répartis au sein du réseau formé par les fibres et l'agent de floculation.

More particularly, the present invention discloses a wrinkle-resistant security sheet comprising at least:

• fibers,
• aggregates of polymer (s) having a glass transition temperature greater than 23 ° C, and
• an effective amount of a cationic flocculating agent, said main agent, said polymer aggregates (s) being distributed within the network formed by the fibers and the flocculation agent.

According to one embodiment of the invention, said sheet further comprises an auxiliary cationic flocculating agent, said secondary agent, in an amount of between 0.1% and 0.5% by dry weight relative to the total dry weight of fibers and polymer (s).

This embodiment is particularly advantageous when the proportion of polymer (s) by dry weight relative to the total weight of the fibers in dry is high, in particular when it exceeds 20% by dry weight relative to the total weight of the fibers, because the presence of the secondary cationic flocculating agent makes it possible to perfect the flocculation of the polymer (s).

• des fibres,
• des agrégats de polymère(s) présentant une température de transition vitreuse supérieure à 23 °C, dans une proportion comprise entre 1 % et 40 % en poids sec, par rapport au poids sec total en fibres et polymère(s), et
• un agent de floculation cationique, dans une proportion comprise entre 0,5 % et 5 % en poids sec, par rapport au poids sec total en fibres et polymère(s).

More particularly, the present invention relates to a security sheet
crease-resistant and marked-pleat material comprising at least:

• fibers,
• aggregates of polymer (s) having a glass transition temperature greater than 23 ° C, in a proportion of between 1% and 40% by dry weight, relative to the total dry weight of fibers and polymer (s), and
• a cationic flocculating agent, in a proportion of between 0.5% and 5% by dry weight, relative to the total dry weight of fibers and polymer (s).

The Applicant has found, as illustrated in the examples below, that the presence of aggregates of polymer (s) having a glass transition temperature greater than 23 ° C and flocculation agent (s) in the composition of the sheet according to the invention significantly improved its rigidity on the entire cellulosic substrate and the resistance to soiling while maintaining good resistance to traction, especially the breaking lengths, dry and wet, wet state and double-fold, and good resistance to creasing and tearing of said sheet.

In our experiments, we found that only sheets with aggregates of polymer (s) having a glass transition temperature greater than 23 ° C had good rigidity and resistance to soiling while retaining excellent resistance to wrinkling, dry and wet pulls and double-fold.

In order to highlight the interest of flocculation, as used in the context of the present invention, a description of this phenomenon is presented below, without it linking the inventors to this theory.

The phenomenon of flocculation is chemically reflected by the formation in aqueous medium of electrostatic bonds between a chemical compound having ionized groups, namely in the context of the invention the anionic dispersion of polymer (s) having a glass transition temperature greater than 23 ° C, and an agent of flocculation having ionized groups of charge opposite to that of the chemical compound. The flocculation agent forms weak atomic bonds (of the order of 5 kJ / mol) with the chemical compound according to a chemical interaction process taking place under mild conditions (for example, at room temperature) and obligatorily in medium solvated (eg water).

• adsorption de l'agent de floculation sur les charges anioniques des fibres,
• adsorption d'une partie des particules anioniques formant la dispersion anionique de polymère(s) sur les charges cationiques de l'agent de floculation, puis
• agrégation des particules par coalescence due à la diminution des charges anioniques de surface des particules et l'apparition de liaisons de Van der Waals lors des étapes courantes du procédé papetier, à savoir l'égouttage, le pressage et le séchage.

In the context of the present invention, namely using an anionic dispersion of polymer (s) and a cationic flocculation agent, the flocculation takes place in three steps:

• adsorption of the flocculation agent on the anionic charges of the fibers,
• adsorption of a portion of the anionic particles forming the anionic dispersion of polymer (s) on the cationic charges of the flocculation agent, then
• aggregation of particles by coalescence due to the decrease of the anionic surface charges of the particles and the appearance of van der Waals bonds during the common steps of the paper process, namely dewatering, pressing and drying.

According to a particular embodiment, the anionic dispersion of polymer (s) is introduced last, after having introduced the fibers and the flocculation agent.

Thus, the flocculation results in an inhomogeneous distribution of aggregates of polymers (s) intertwined in the network formed by the fibers and the flocculation agent. The flocculation system is used for a mass treatment (by volume) of the sheet and is thus carried out in wet part (period during which the fibers are dispersed in water, then forming a fibrous suspension), before the formation of leaf.

Thus, the aggregates of particles are distributed in the interstices left vacant between the fibers during the formation of the sheet. These interstices are no longer accessible after draining, pressing and drying of the sheet as filled by said aggregates. According to a particular embodiment, said aggregates preferably have hydrophobic and / or oleophobic properties, so that the penetration of the soil is less and the sheet thus has a better resistance to soiling.

Before the formation of the sheet according to the invention, there is, in addition to the bonds between the anionic particles of the polymer and the cationic charges of the flocculation agent, formation of bonds between the anionic charges of the cellulose and the cationic charges. flocculation agent. It thus forms a global network (fibers - agent flocculation - polymer) which is preserved during the formation of the paper sheet, which explains the improvement of the rigidity on the totality of the cellulosic substrate.

Moreover, as mentioned above, the resistance to soiling can also be improved by the chemical nature of the polymer aggregates (s), in particular when they have a hydrophobic and / or oleophobic nature.

According to a variant of the present invention, when the fibers used have a negative surface charge, these can be previously treated to promote the fixing of the cationic flocculation agent. We can then say that we add a preliminary step of "anionization" of the fibers. Such fibers may be cotton fibers. As an indication, the cellulosic fibers of wood do not generally have this low negative surface charge. However, they may also be subject to this prior treatment, where appropriate.

According to this variant, this preliminary step may consist in the addition of cellulose derivatives such as carboxymethylcellulose (CMC). Such polymers capable of increasing the negative surface charge of fibers may in any case be chosen from anionic polymers which are soluble in water and have an affinity with cellulose.

• adsorption sur les fibres d'un polymère anionique, ayant une affinité avec les fibres, tel que la CMC,
• adsorption de l'agent de floculation sur les charges anioniques des fibres,
• adsorption d'une partie des particules anioniques formant la dispersion anionique de polymère(s) sur les charges cationiques de l'agent de floculation, puis
• agrégation des particules par coalescence due à la diminution des charges anioniques de surface des particules et l'apparition de liaisons de Van der Waals lors des étapes courantes du procédé papetier, à savoir l'égouttage, le pressage et le séchage.

Thus, according to this variant, the flocculation takes place in four stages:

• adsorption on the fibers of an anionic polymer, having an affinity with the fibers, such as CMC,
• adsorption of the flocculation agent on the anionic charges of the fibers,
• adsorption of a portion of the anionic particles forming the anionic dispersion of polymer (s) on the cationic charges of the flocculation agent, then
• aggregation of particles by coalescence due to the decrease of the anionic surface charges of the particles and the appearance of van der Waals bonds during the common steps of the paper process, namely dewatering, pressing and drying.

In order to clearly distinguish this phenomenon of flocculation from the crosslinking, which occurs especially during a coating process, the Applicant has found it useful to describe hereinafter the crosslinking phenomenon.

Unlike the phenomenon of flocculation, the crosslinking phenomenon is chemically translated by the formation of covalent bonds between two entities: a chemical compound and a crosslinking agent.

The crosslinking agent, composed of reactive groups, forms strong atomic bonds (of the order of 500 kJ / mol) with certain groups of the chemical compound according to a chemical reaction process taking place under specific energy conditions (temperature, pressure, radiation. ..). The bonds formed are homogeneously distributed. They are permanent and resistant, which gives the compound / crosslinking mixture rigidity and homogeneous dimensional stability, properties which are the opposite of those sought for a security sheet contemplated by the invention which must be flexible and resistant.

Flocculation has the advantage of providing stiffness over the entire cellulosic substrate, in particular by virtue of the good distribution of the aggregates of polymer (s) in the substrate. It should also be noted that large amounts of polymer (s) are not necessary to achieve this improvement of the rigidity property.

For comparison, the surfacing by the same polymer, has the disadvantage of leading to a brittle layer structure, which resists poorly to circulation, including wrinkling.

In addition, such a mass treatment makes it possible to envisage an additional surface treatment, in particular a protective treatment, for example of the antifouling type and / or printing treatment, on the security sheet obtained according to the process of the invention. . By contrast, the surface treatments do not make it so easy to carry out additional surface treatment for obvious reasons of surface affinities between them and modification of the properties provided by the first surface treatment, which must first be taken into account. account.

• des fibres,
• une dispersion anionique d'au moins un polymère présentant une température de transition vitreuse supérieure à 0 °C, de préférence supérieure à 5 °C, et
• une quantité efficace d'un agent de floculation cationique, dit agent principal, puis à égoutter, presser et sécher ladite feuille.

According to another of its aspects, the present invention describes a manufacturing method of forming said wet sheet from an aqueous suspension comprising at least:

• fibers,
• an anionic dispersion of at least one polymer having a glass transition temperature greater than 0 ° C, preferably greater than 5 ° C, and
• an effective amount of a cationic flocculating agent, said main agent, and then to drain, squeeze and dry said sheet.

• des fibres,
• une dispersion anionique d'au moins un polymère présentant une température de transition vitreuse supérieure à 23 °C, et
• une quantité efficace d'un agent de floculation cationique, dit agent principal, puis à égoutter, presser et sécher ladite feuille.

Thus, the present invention relates to a manufacturing method of forming said wet sheet from an aqueous suspension comprising at least:

• fibers,
• an anionic dispersion of at least one polymer having a glass transition temperature above 23 ° C, and
• an effective amount of a cationic flocculating agent, said main agent, and then to drain, squeeze and dry said sheet.

According to yet another of its aspects, the present invention relates to a wrinkle and crease-resistant security sheet comprising fibers, obtained by mass treatment of said fibers by a flocculation system, said flocculation system comprising an anionic dispersion of at least one polymer having a glass transition temperature above 23 ° C and a cationic flocculating agent.

According to yet another of its aspects, the invention relates to a wrinkle-resistant security sheet comprising fibers, said fibers comprising en masse aggregates of particles formed following the flocculation of an anionic dispersion of at least one polymer having a glass transition temperature above 23 ° C, and a cationic flocculation agent.

fibers

As stated above, the security sheet according to the invention comprises fibers.

In the present application, the expression " total fiber weight " should be understood as meaning " total dry fiber weight " unless otherwise indicated.

According to one embodiment of the invention, the fibers used in the composition of the sheet comprise natural fibers.

Among the natural fibers, mention may be made of cellulosic fibers, such as wood fibers, for example of eucalyptus, of softwood or their mixture, of cotton, bamboo, viscose, straw, abaca, asperto , hemp, jute, flax, sisal or their mixtures.

The fibers may be bleached, semi-bleached or unbleached.

Preferably, the fibers in the composition of the sheet comprise cellulosic fibers, particularly cotton fibers.

According to one variant, the fibers comprise fibers previously anionized with polymers capable of increasing the negative surface charge of the fibers such as anionic polymers which are soluble in water and have an affinity with cellulose. According to this variant, such anionic polymers may be chosen from cellulose derivatives such as carboxymethylcellulose (CMC).

In particular, said cellulosic fibers are present in a proportion greater than 60% by dry weight relative to the total dry weight of fibers and polymer (s), in particular in a proportion of greater than 70%.

According to a particular embodiment of the invention, said cellulosic fibers represent at least 70% by dry weight of the total amount of fibers.

In particular, said cellulosic fibers are cotton fibers and represent at least 70% by dry weight of the total amount of fibers.

Preferably, according to another embodiment of the invention, the fibers used in the composition of the sheet may comprise synthetic fibers.

This embodiment is particularly advantageous because it makes it possible to further improve the tear-resistance properties of the sheet according to the invention.

Indeed, in the course of its research, the Applicant has found that, surprisingly, the use of synthetic fibers, generally used to strengthen the paper, has a synergistic effect with the use according to the invention of a polymer presenting a specific Tg. Indeed, the Applicant has measured that the sheets containing synthetic fibers, while being rigid and retaining resistance to creasing and high soiling, also had a particularly high tear resistance. The tear resistance of the sheets according to this particular embodiment of the invention is greater than the tear resistance of the sheets according to the invention without synthetic fibers and the tear resistance of sheets comprising synthetic fibers. but no aggregates of polymer (s).

According to a preferred embodiment of the invention, the synthetic fibers are in a proportion of between 5% and 30% by dry weight relative to the total dry weight of fibers and polymer (s), and better still between 10% and 15% by weight. %.

According to a particular embodiment of the invention, the sheet comprises cotton fibers in a proportion of at least 70% by dry weight relative to the total weight of the fibers and synthetic fibers in a proportion of between 10% and 30% by weight. % by dry weight relative to the total weight of the fibers, the sum total of the cotton fibers and synthetic fibers being equal to 100.

According to a preferred embodiment of the invention, said synthetic fibers are chosen from polyester, polyamide, rayon and viscose fibers, preferably they are polyamide fibers and / or polyester fibers. It may be, for example, polyamide 6-6 fibers or polyester fibers marketed by Kuraray under the trade name EP133®.

Polymer with a Tg greater than 23 ° C

A security sheet according to the invention further comprises at least polymer aggregates (s) having a glass transition temperature (Tg) greater than 23 ° C.

According to a preferred embodiment of the invention, the polymer (s) used in the context of the present invention have a glass transition temperature greater than 35 ° C, and more preferably greater than 40 ° C.

These polymers are described as "rigid" in the context of the present invention.

The term " glass transition temperature " means the temperature below which the polymer is rigid. As the temperature increases, the polymer passes through a transition state that allows free macromolecular chains amorphous domains to slide relative to each other and the polymer softens.

The measurement of the glass transition temperature can be carried out by differential scanning calorimetry (" Differential Scanning Calorimetry " or " DSC ").

According to one particular embodiment of the invention, the polymer (s) may be present in a proportion of between 1% and 40% by dry weight, in in particular between 2% and 25% by dry weight, and preferably between 2% and 15% by dry weight, relative to the total dry weight of fibers and polymer (s).

In particular, the polymer (s) having a glass transition temperature greater than 23 ° C are chosen from polyacrylics, polyacrylates, polyacrylamides, polystyrenes, polyurethanes, polyvinyls, polyethylenes, and mixtures thereof.

According to a preferred embodiment, the polymer (s) having a glass transition temperature greater than 23 ° C. are chosen from acrylic (or polyacrylic) polymers, that is to say homopolymers or copolymers comprising at least less an acrylic monomer, namely acrylic homopolymers or acrylic copolymers.

• les vinyl-acryliques, par exemple le produit Orgal VA-HP commercialisé par la société Organik Kimya (Tg = +41 °C),
• les styrène-acryliques, par exemple le produit Acronal DS2416 commercialisé par la société BASF (Tg = +38 °C), et
• les polyuréthane-acryliques, par exemple le produit Joncryl U6336 commercialisé par la société BASF (Tg = +40 °C).

As acrylic copolymers, may be mentioned in particular:

• vinyl acrylics, for example the Orgal VA-HP product marketed by the company Organik Kimya (Tg = +41 ° C.),
• styrene-acrylics, for example the product Acronal DS2416 sold by the company BASF (Tg = + 38 ° C.), and
• polyurethane-acrylics, for example the Joncryl U6336 product marketed by BASF (Tg = + 40 ° C.).

Preferably, the polymer (s) having a glass transition temperature greater than 23 ° C. are chosen from acrylic homopolymers.

• auprès de la société Tanatex Chemicals, sous la dénomination Edolan AH (Tg = +36 °C),
• auprès de la société Organik Kimya, sous la dénomination Orgal NA 302 (Tg = +26 °C),
• auprès de la société Icap Sira, sous la dénomination Acrilem 7105 (Tg = +50 °C), et
• auprès de la société BASF, sous la dénomination Acronal DS 2416 (Tg = +38 °C).

Such polymers are available in anionic dispersion, for example:

• from Tanatex Chemicals, under the name Edolan AH (Tg = + 36 ° C.),
• from the company Organik Kimya, under the name Orgal NA 302 (Tg = + 26 ° C),
• from Icap Sira under the name Acrilem 7105 (Tg = + 50 ° C), and
• from BASF under the name Acronal DS 2416 (Tg = + 38 ° C.).

The polymer (s) may be non-crosslinkable, crosslinkable with the aid of an external crosslinking agent or else self-crosslinkable.

According to a particular embodiment, the polymer (s) are self-crosslinkable. Thus, as is apparent hereinafter in connection with the description of the process, the drying step may lead to crosslinking of the polymers, in a step subsequent to the implementation of the process according to the invention.

According to the invention, the anionic dispersion of the polymer (s) is monophasic or multiphasic. A multiphasic dispersion corresponds in particular to core-shell particles comprising two types of polymers of which at least the most abundant (the core) is a rigid polymer considered according to the invention. The other polymer (s) may have a function other than the addition of rigidity, it (s) may (in particular) be used as crosslinking agent and / or binder.

Flocculation agent

A security sheet according to the invention further comprises at least an effective amount of a cationic flocculation agent, said main agent.

For the purposes of the present invention, the term " effective amount " means the amount of cationic flocculating agent advantageously required to allow the anionic dispersion of the polymer to flocculate. This amount of cationic flocculating agent is to be considered with regard to the associated polymer (s) and may therefore vary from one polymer to another. From his general knowledge, the skilled person is able to adjust the amount of cationic flocculating agent necessary to allow flocculation of the polymer (s).

Preferably, the cationic flocculating agent, called the main agent, is present in an amount of between 0.5% and 5% by dry weight, in particular between 1% and 3% by dry weight, relative to the total dry weight. fiber and polymer (s).

The cationic flocculating agent, said main agent, present in the security sheet according to the invention is chosen from water-soluble cationic polymers, in particular from polyacrylamides, polyethyleneimines, polyvinylamines and their mixtures.

According to a preferred embodiment of the invention, the cationic flocculating agent, said main agent, is a cationic resin. In particular, this resin is a polyamide-amine-epichloridin resin, called PAAE resin.

According to another embodiment of the invention, the cationic flocculation agent, said main agent, is selected from polyacrylamides, polyethyleneimines, polyvinylamines and mixtures thereof.

According to a particular embodiment of the invention, the security sheet comprises, in addition to the cationic flocculating agent, said main agent, described above, a secondary cationic flocculating agent, said secondary agent.

Such a secondary cationic flocculating agent is present in an amount of between 0.1% and 0.5% by dry weight relative to the total dry weight of fibers and polymer (s).

Preferably, said cationic flocculation agent, said secondary agent, is selected from polyacrylamides, polyethylenimines, polyvinylamines and mixtures thereof.

Security element

According to one embodiment of the invention, the security sheet comprises at least one security element.

In particular, said security element is chosen from optically variable devices (OVD), in particular interference-effect elements, in particular iridescent elements, holograms, liquid crystals and interferential multilayer structures. These security elements may be affixed to security threads, foils, patches, boards, pigments or luminescent fibers and / or magnetic and / or metal, and their combinations. As another element of security, one can also mention the watermarks.

In addition, the sheet according to the invention may comprise an RFID device (radio frequency identification device).

According to another embodiment of the invention, the security sheet comprises at least one at least partially fiber-free zone, also known as a " window " zone .

According to another embodiment, the security sheet according to the invention comprises an embedded security thread or strip (e), in particular totally or partially, in said sheet, and preferably appearing in at least one window.

Mineral loads

According to one embodiment of the invention, the security sheet comprises mineral fillers in an amount of 1% to 10% by dry weight relative to the total dry weight of the composition of the sheet.

In particular, said mineral fillers are present in a proportion of between 1% and 5% by dry weight relative to the total dry weight of the composition of the sheet.

These fillers are chosen, for example, from calcium carbonate, kaolin, titanium dioxide or mixtures thereof.

Other fillers and adjuvants

The security sheet according to the invention may further comprise fillers and additives commonly used in the paper industry.

These are in particular synthetic fillers, retention agents and / or wet strength agents.

According to the particular embodiment described above relating to the use of a fibrous composition having a small negative surface charge, particularly in the case of a fibrous composition comprising cotton fibers, the security sheet according to The invention further comprises carboxymethylcellulose (CMC).

Additional treatment

According to another embodiment of the invention, the security sheet may further comprise an additional treatment, in particular an impregnation, an external surfacing layer, a coating or a coating. It can be an external treatment.

These surfacing layers, coated on at least one side of a sheet, are well known to those skilled in the art and make it possible, for example for a layer based on a polyvinyl alcohol, to improve the properties of double strength. -plugging and pulling the leaf.

According to another example, the security sheet according to the invention may comprise a surfacing layer intended to reinforce its durability properties, such as, for example, a layer whose composition is described in the application EP 1 319 104 and which comprises a transparent or translucent elastomeric binder, such as a polyurethane, and a colloidal silica.

Thus, according to a particular embodiment, the security sheet according to the invention further comprises an outer surfacing layer, preferably based on a polyvinyl alcohol, an acrylic polymer or a transparent or translucent elastomeric binder. , such as a polyurethane, and optionally a colloidal silica.

Manufacturing process

The invention also relates to a method of manufacturing the security sheet described above.

• des fibres,
• une dispersion anionique d'au moins un polymère présentant une température de transition vitreuse supérieure à 23 °C, et
• une quantité efficace d'un agent de floculation cationique, dit agent principal, puis à égoutter, presser et sécher ladite feuille.

According to the invention, the manufacturing method consists of forming said wet sheet from an aqueous suspension comprising at least:

• fibers,
• an anionic dispersion of at least one polymer having a glass transition temperature above 23 ° C, and
• an effective amount of a cationic flocculating agent, said main agent, and then to drain, squeeze and dry said sheet.

According to the invention, said fibers, said anionic dispersion of at least one polymer having a glass transition temperature greater than 23 ° C and said cationic flocculating agent, are advantageously mixed in bulk.

In particular, said polymer aggregates (s) are formed by contacting the anionic dispersion of polymer (s) with the fibers and the flocculation agent.

1. (i) de mise en contact d'une dispersion anionique d'au moins un polymère présentant une température de transition vitreuse supérieure à 23 °C, avec des fibres et une quantité efficace d'un agent de floculation cationique pour former des agrégats de polymère(s) répartis sur lesdites fibres, puis
2. (ii) d'égouttage, de pressage et de séchage.

In other words, the present invention relates to a method for manufacturing a crease-resistant and marked-ply security sheet comprising at least the steps of:

1. (i) contacting an anionic dispersion of at least one polymer having a glass transition temperature above 23 ° C with fibers and an effective amount of a cationic flocculating agent to form polymer aggregates (s) distributed on said fibers, then
2. (ii) dewatering, pressing and drying.

According to one embodiment of the invention, said aqueous suspension further comprises an auxiliary cationic flocculating agent, said secondary agent, in an amount of between 0.1% and 0.5% by dry weight relative to the total dry weight fiber and polymer (s).

The process of the invention makes it possible, by using polymer (s) having a specific Tg and flocculation agent (s), to flocculate the polymer (s) on the fibers and to obtain a very rigid safety sheet with particularly high resistance to creasing and soiling.

According to the invention, the anionic dispersion of the polymer (s) having a glass transition temperature greater than 23 ° C and the cationic flocculating agent, said main agent, are mixed in mass in the fibrous suspension before dewatering .

This process is clearly distinguishable from a coating process.

Thus, according to a particular embodiment of the invention, said aqueous suspension is obtained from a mixture of fibers and said cationic flocculation agent, said main agent, to which said anionic dispersion of polymer (s) and optionally said cationic flocculation agent, said secondary agent, before proceeding with the formation of said sheet.

This embodiment has the advantage of bringing the polymer (s) and the flocculation agent into contact in order to facilitate flocculation.

This embodiment also has the advantage that it can be applied to aqueous suspensions of " standard " fibers used for the manufacture of safety sheets because they include wet strength agents which can also be used as main flocculation in the context of the present invention.

According to a particular case of the process, adding said anionic dispersion of polymer (s) before said flocculating agent said secondary agent.

According to one embodiment of the invention, crosslinking can take place after flocculation. Thus, the polymers, once flocculated, can crosslink under the effect of the high temperature dedicated to drying.

According to one embodiment of the invention, the method of manufacturing the security sheet comprises a step in which at least one face of said sheet, after dewatering of said suspension is coated with a surfacing layer.

This surfacing layer may allow, for example, to improve the marked bending and / or tensile strength properties or even the durability properties of said sheet, as described above.

The invention also relates to a security document comprising the security sheet object of the invention, or as obtained by the method of the invention, described above.

In particular, the invention relates to a means of payment, such as a bank note, a payment card, a check or a restaurant ticket, an identity document, such as an identity card, a visa, a passport or a driving license, a card, including access, a lottery ticket, a ticket or a ticket to cultural or sports events, a loyalty card, a service card, a ticket a membership card, a playing or collectible card, a voucher or a voucher.

This is preferably a bank note. One of the specificities of banknotes is indeed to be subjected to wrinkles and marked folds, and to have to maintain its rigidity and good resistance to wrinkles and creases marked despite the effects of traffic to minimize rejects in automatic machines and automatic sorting machines. A banknote generally has a thickness of between 70 μm and 170 μm, for example 100 μm.

Characterization of the security sheet 1. Weight measurement

The grammage measurements can be performed according to ISO 536.

2. Measurement of the thickness

Thickness measurements can be made according to ISO 534.

3. Resistance to soiling

In order to evaluate the resistance to soiling of the security sheet, it is possible to evaluate IS dirt indices.

Preparation of compositions

Three compositions are prepared for making the measurements.

The first composition, called artificial sweat composition A, is prepared from the weight proportions indicated in Table 1 below: <i><u> Table 1 </ u></i> compounds Weight (g) Distilled water 500 NaCl 5 Lactic acid 5 Urea 0.5 Histidine hydrochloride 0.5 Water Qs 1000

The second composition, called powdery color composition B, is prepared from the weight proportions indicated in Table 2 below: <i><u> Table 2 </ u></i> compounds Weight (g) Yellow iron oxide marketed by Lanxess under the name Bayferrox® 420 0.833 Brown iron oxide marketed by Lanxess under the name Bayferrox® 610 0.833 Carbon black sold by the company BASF under the name Black Microlite CK 0.083 Vermiculite 2D marketed by EFISOL 248.25

The compounds of Table 2 are mixed using a Turbula mixer for 30 minutes in a plastic bottle with six large 20 mm diameter ceramic beads.

The third composition, called fat composition C, is prepared from the weight proportions indicated in Table 3 below: <i><u> Table 3 </ u></i> compounds Weight (g) Lanolin marketed by Lanolines de la Tossée under the name Lanoline Lanor TEC 200 Ethoxylated tridecyl alcohol marketed by the company Nopco under the name Disponil® TD 0785 20

The mixture of the two compounds of Table 3 is homogenized until the lanolin melts. 28 g of water are then added with stirring, followed by 16 g of ethoxylated ether of behenic alcohol (sold by the company Nopco under the name Eumulgin® BA 10). The whole is mixed by heating until a homogeneous preparation.

The mixture is then allowed to cool to room temperature and then 24 g of water are then added by mixing for 15 minutes. 190 g of water are then added, incorporating it very slowly at the beginning.

An emulsion containing 42% lanolin is obtained.

Water-based stain resistance test

Each sample is crumpled beforehand 4 times with an IGT creasing device.

The samples (75 mm x 140 mm) are mounted on wooden blocks (60 mm x 35 mm x 35 mm), around which they are held with three exposed faces.

90 ml of composition A, 10 g of composition B, 20 g of composition C and 25 ceramic beads with a maximum diameter of 20 mm are placed in a plastic jar of 2 L. The ceramic beads may be alumina beads, they must not weigh less than 300 g. They are packaged before use by placing them in the Turbula mixer with an abrasive cream and a little water for 1 hour and 30 minutes.

The mixture is agitated then the four wooden blocks, dry, on which the samples were placed are introduced. The jar is closed and stirred with a Turbula mixer at 67 rpm for 20 minutes.

The samples are then rinsed with running water and dried using two blotters. They are then put in the oven at 60 ° C for 3 minutes.

The samples are then calendered one by one with one passage per side, at 15 bar pressure, at minimum speed.

dans laquelle :

• B₁ est la blancheur mesurée sur les échantillons après le test,
• B₀ est la blancheur avant le test,
• J₁ est le degré de jaunissement après le test, et
• J₀ est le degré de jaunissement avant le test.

The IS soil indexes are then evaluated, thanks to the formula: $$I_S=\frac{\left(B_1-B_0\right)+\left(J_1-J_0\right)}{2}$$

in which :

• B ₁ is the whiteness measured on the samples after the test,
• B ₀ is the whiteness before the test,
• J ₁ is the degree of yellowing after the test, and
• J ₀ is the degree of yellowing before the test.

The whiteness is an average value of three measurements according to the NF ISO 2470-2 6167 standard and the degree of yellowing is an average value of three measurements, which are well known to those skilled in the art (measurements carried out with the Datacolor Elrepho 3000 apparatus of the company Lorentzen & Wettre).

4. Measurement of stiffness

In order to evaluate the flexural strength (in Newton), and in particular the rigidity, measurements can be made according to the ISO 2493-1 standard, in particular with a Taber stiffener (Model 150-B) or with a Franck rigidimeter ( Model 58565). The index of flexural strength (Nm ⁶ / Kg ³ ) is determined by taking into account the basis weight of the paper.

According to this method, the sheets according to the invention advantageously have a stiffness gain of at least 15%, or even at least 20%, with respect to the rigidity of a sheet comprising 100% cotton fibers.

5. Double-fold resistance (Schopper folding)

The double-fold resistance measurements can be carried out according to the ISO 5626 standard.

6. Tear resistance

Tear resistance measurements can be made according to ISO 1974. The tear index is obtained by dividing the tear strength by the basis weight.

7. Tensile strength in the dry state

In order to evaluate the tensile strength, especially the breaking length, in the dry state, it is possible to measure the length at which the sheets break under the effect of their own weight according to the ISO 1924-2 standard.

8. Tensile strength in the wet state

In order to evaluate the tensile strength, especially the breaking length, in the wet state, it is possible to measure the length at which the sheets break under the effect of their own weight according to standard NF Q03-056.

9. Porosity and wrinkle resistance Crease test

In order to evaluate the crease resistance of the security sheet, BENDTSEN porosity measurements can be made before and after creasing.

Indeed, the wrinkling operation causes, because of the folds formed, more or less pronounced alteration of the surface of the paper, leading to an increase in its porosity and therefore its fragility. Comparing the value of the porosity of the paper before and after creasing can therefore be evaluated the crease resistance of the latter. The less the increase in porosity between the initial sheet and the crumpled sheet, the less the paper is wrinkle resistant. The goal is therefore to obtain values of porosity after crumpling the lowest possible.

The wrinkle test therefore makes it possible to determine the wrinkle resistance of papers such as bank note papers and wrapping papers.

The apparatus used corresponds to that described by the National Bureau Of Standards ( Carson, FT Shaw, MB Wearing quality of experimental currency type papers, J. Research NBS 36, 256-257 (1946) RP 1701 ).

1. a) un dispositif pour rouler l'éprouvette de papier en un cylindre. Ce dispositif est constitué d'un manchon fendu à l'intérieur duquel est placée une fourche mobile à deux dents.
2. b) un tube dont l'une des extrémités est pourvue d'un couvercle mobile.
3. c) un guide cylindre glissant à l'intérieur du tube.
4. d) un guide cylindrique permettant de maintenir à l'intérieur et en position verticale le piston dont la base inférieure repose à l'extrémité d'un levier. Le guide cylindrique est conçu de telle manière que le tube peut coulisser entre ce guide et le piston.
5. e) un levier monté sur un pivot.
6. f) un poids à l'extrémité du bras long du levier, opposée à celle du bras court qui supporte le piston.

The apparatus comprises:

1. a) a device for rolling the paper test specimen into a cylinder. This device consists of a split sleeve inside which is placed a movable fork with two teeth.
2. (b) a tube with one end provided with a movable lid.
3. c) a sliding cylinder guide inside the tube.
4. d) a cylindrical guide for maintaining inside and in vertical position the piston whose lower base rests at the end of a lever. The cylindrical guide is designed such that the tube can slide between this guide and the piston.
5. e) a lever mounted on a pivot.
6. f) a weight at the end of the long arm of the lever, opposite to that of the short arm which supports the piston.

The crumpling force is adjusted by the position of the weight on the arm of the lever, so that the pressure on the piston is 10 kg / cm ² ± 0.1 kg / cm ² .

The various cylindrical parts: guide, tube, piston must be able to slide freely and in particular slide under their weight.

Tube and piston being in place in the guide, the piston must fall or rise as one raises or lowers the weight at the end of the lever.

Sampling and packaging

Sampling and conditioning of the specimens are done in accordance with standards NFQ 03-009 and NFQ 03-010. For a particular purpose, the specimens can be measured as such. Since the specimen is constantly handled, it is necessary, in order to avoid moisture exchange with the operator, that the operator wears gloves of a moisture-barrier material during the preparation of the specimens and the run the test.

Preparation of test pieces

67 mm test pieces are cut using a template. The running direction is marked on each specimen.

Operating mode

With the fork and the two slots of the sleeve aligned, introduce the test tube in the direction of travel up to its half, then roll it by rotation of the fork.

The tube, closed lid, is then slid in continuity with the sleeve and the rolled test piece is transferred there by a return movement of the fork.

The tube, held by the lid with one hand, is then placed in a vertical position on the piston. The crumpling is done by pressing on the cover until the end of the long arm of the lever is raised above its rest position. It is important that the pressure exerted is sufficient to lift the weight, but neither too strong nor too fast for the lever to come into abutment. One way to control the effort is to use both hands on each other to press the lid.

The lid is open and the test piece is crumpled in the shape of a small accordion, and out of the tube. It is put flat by carefully straightening the hand: acting too abruptly could produce notches on the side that would lead to tearing of the specimen.

The straightened specimen is presented again for rolling in front of the slot of the sleeve but rotated 90 ° with respect to the first introduction; the complete cycle is repeated.

This is done eight cycles, with rotation each time 90 °, and returning the specimen after the fourth time.

In particular, it is possible to wait a certain time between each cycle, for example thirty minutes, to allow the self-repairing material to restructure itself. This allows to get closer to the actual traffic conditions that the wrinkle resistance test is trying to reproduce. Indeed, during the circulation of the information media, the constraints are applied repeatedly but episodically.

Measure wrinkle resistance

The air permeability of each specimen is measured before and after wrinkling using a BENDTSEN porosimeter according to ISO 5636-3.

The measurement to be made under the same conditions before and after wrinkling, it is necessary in both cases to remove the stopper of the head of the porosimeter if the normal stroke is insufficient to be able to slide the wrinkled specimen.

After wrinkling, the porosity is measured as follows: each specimen is straightened until it is reasonably flat. This can easily be done by holding between thumb and forefinger, the test piece by two opposite sides and then stretching it in three or four places. This operation is repeated by the other two sides; to do this operation in all four times is usually sufficient to obtain a sufficiently flat specimen.

In order to form on the test specimen a circular surface whose flatness is such that surface leakage is negligible with respect to the porosity measurement, each specimen is introduced between the jaws of the clamping device of a brightness meter and is applied during two seconds enough pressure to mark the paper. Porosity is measured by ensuring that the BENDTSEN porosimeter head is centered on the surface squeezed to the brightnessometer.

Loyalty

The number of test pieces to be tested depends on the sample studied. The reproducibility of the test is such that a sample per sample sheet is sufficient.

Results

According to this method, the porosity of the sheets according to the invention after creasing can vary from 1 to 20 ml / min, in particular from 1 to 12 ml / min.

Throughout the description, including the claims, the phrase " having one " should be understood as being synonymous with " having at least one ", unless the opposite is specified.

Expressions " between ... and ...", " ranging from ... to ..." and " varying from ... to ...", must be understood inclusive, unless otherwise specified .

In the text, the expressions " greater than ..." and " less than ..." mean that the terminals are not included, unless otherwise indicated.

The invention is illustrated in more detail by the examples presented below, by way of illustration and without being limiting to the scope of the invention.

EXAMPLES

The sheets A to F, and 1 to 3, tested below, are prepared according to the standard paper-making process, that is to say, in particular by draining, pressing and then drying aqueous suspensions whose compositions in dry weight are given in the tables. 1 to 3 below.

The amount of polyamide-amine-epichloridrine resin (PAAE resin) is set at 2.5% by dry weight relative to the total dry weight of fibers and polymer (s).

No flocculation step is carried out in comparative examples A, D, E and F. The PAAE resin thus plays a role of wet strength agent in Comparative Examples A, D, E and F .

A flocculation step is carried out in Examples B, C and 1 to 3. Thus, in Examples B, C and 1 to 3, the PAAE resin has a role of cationic flocculation agent and also resistance to corrosion. wet state.

Three types of cellulosic fibers have been used. These are cotton fibers of different origins and / or different degrees of refining.

Said sheets then undergo, according to a common practice in the paper industry, an external surfacing, as described above, deposited at a rate of 5 g / m ² / face. Said surfacing is carried out by impregnation.

Thus, the sheets A to C are impregnated with a layer based on a polyvinyl alcohol (PVA). Sheets D and E and 1 to 3 are impregnated with a layer based on a polyvinyl alcohol (PVA). As for the sheet C, it is impregnated with a layer based on an acrylic polymer having a Tg of 38 ° C.

The sheets E, F and 3 have, before surfacing, a basis weight of 85 g / m ² .

The sheets A to F, and 1 to 3 thus obtained, respectively corresponding to Comparative Examples A to F and Examples 1 to 3 according to the invention, were subjected to the tests described above.

Results:

Table 1: 1 ^era series Examples Weight (g / m ² ) ( after surfacing ) Thickness (μm) ( after surfacing ) Rigidity Franck (N) Franck stiffness index (Nm ⁶ / kg ³ ) Comparative Example A: 96.5 114 0.0678 75.4 100% Cellulosic Fibers 1 + external PVA surfacing Comparative Example B: 102.3 117 0.0611 57.1 10% carboxylated styrene-butadiene copolymer dispersion (Tg = -25 ° C) + 90% Cellulosic fibers 1 + external PVA surfacing Comparative Example C: 103.9 120 0.0596 53.1 10% Polyurethane-polyester dispersion (Elongation at break = 600%) (Tg = -50 ° C) + 90% Cellulosic fibers 1 + external PVA surfacing

Comparative Example A relates to standard, untreated paper.

Comparative Example B relates to a sheet obtained according to the process as described in document WO 2008/152299 .

Comparative example C relates to a sheet obtained according to the process as described in the document WO 2014/083527 .

Interpretation of results:

As shown in Table 1 of the results, with respect to a sheet comprising 100% cellulosic fibers (Comparative Example A), the sheets of Examples B and C, also based on flocculation techniques but comprising polymers not in accordance with the invention, have a decrease in their rigidity. <b><u> Table 2: 2 </ u>^{<u> th </ u>}<u> series </ u></b> Examples Weight (g / m ² ) (after surfacing ) Thickness (μm) ( after surfacing ) Soiling index Rigidity Franck (N) Franck stiffness index (Nm ⁶ / kg ³ ) Comparative Example D: 90.4 102 18.7 0.0289 39.1 100% Cellulosic fibers 2 + external PVA surfacing Example 1 according to the invention 91.0 104 14.2 0.0353 46.8 10% Acrylic polymer dispersion (Tg = 50 ° C.) Acrilem 7105 (Icap Sira company) + 90% Cellulosic fibers 2 + external PVA surfacing Example 2 according to the invention 91.6 103 13.7 0.0353 46.0 10% Dispersion of Acrylic Polymer (Tg = 36 ° C.) Edolan AH (Tanatex Chemicals Company) + 90% Cellulosic fibers 2 + external PVA surfacing

Interpretation of results:

As shown in Table 2 of the results, the sheets of Examples 1 and 2 according to the invention have a clear rigidity gain compared to the sheet of Comparative Example D.

In addition, the sheets of Examples 1 and 2 show an improvement in the soiling index of 4 to 5 points.

Thus, with respect to a given fibrous composition, the tests have made it possible to demonstrate the improvement of the rigidity and the resistance to soiling by the use of polymer according to the invention.

Interpretation of results:

As shown in Table 3 of the results, the sheet of Example 3 according to the invention has a much higher rigidity than that of Comparative Example E, while exhibiting values of tensile strength, tear strength and double-fold comparable to those of Comparative Example E, which shows that the use of the polymers according to the invention does not degrade the properties of tensile strength, tear and double-fold.

Thus, with respect to a given fibrous composition, the tests have made it possible to demonstrate the improvement of the rigidity by the use of a polymer dispersion according to the invention.

Interpretation of results:

Comparison of Example F with Example 3 makes it possible to show that the surfacing of a rigid acrylic polymer does not improve the rigidity of the substrate, despite the large amount of polymer supplied (6 g / m ² against 1.5 g / m ² of PVA).

Claims (14)

1. Security sheet resistant to crumpling and hard creasing comprising at least:

   - fibres,

   - aggregates of polymer(s) having a glass transition temperature above 23°C, in a proportion of between 1% and 40% by dry weight, relative to the total dry weight of fibres and polymer(s), and

   - a cationic flocculating agent, in a proportion of between 0.5% and 5% by dry weight, relative to the total dry weight of fibres and polymer(s).
2. Security sheet according to Claim 1, characterized in that the polymer (s) has (have) a glass transition temperature above 35°C, preferably above 40°C.
3. Security sheet according to either of the preceding claims, characterized in that the proportion of polymer(s) is between 2% and 25% by dry weight, and preferably between 2% and 15% by dry weight, relative to the total dry weight of fibres and polymer(s).
4. Security sheet according to any one of the preceding claims, characterized in that the polymer or polymers are chosen from acrylic polymers, polyacrylates, polyacrylamides, polystyrenes, polyurethanes, polyvinyls, polyethylenes, and mixtures thereof, preferably chosen from acrylic polymers, and more preferentially chosen from acrylic homopolymers.
5. Security sheet according to any one of the preceding claims, characterized in that the proportion of cationic flocculating agent is between 1% and 3% by dry weight, relative to the total dry weight of fibres and polymer(s) .
6. Security sheet according to any one of the preceding claims, characterized in that the cationic flocculating agent is chosen from water-soluble cationic polymers, in particular from polyacrylamides, polyethyleneimines, polyvinylamines and mixtures thereof, preferably a cationic resin, and better still a polyamide-amine-epichlorohydrin resin.
7. Security sheet according to any one of the preceding claims, characterized in that said fibres comprise cellulosic fibres, in particular cotton fibres, and preferably cotton fibres previously anionized by polymers capable of increasing the negative surface charge of the fibres such as anionic polymers that are water-soluble and that have an affinity with cellulose, in particular chosen from cellulosic derivatives such as carboxymethyl cellulose (CMC), said cellulosic fibres preferably being present in a proportion of greater than 60% by dry weight relative to the total dry weight of fibres and polymer(s), in particular in a proportion of greater than 70%.
8. Security sheet according to any one of the preceding claims, characterized in that said fibres comprise synthetic fibres, in particular chosen from polyamide fibres and/or polyester fibres, said synthetic fibres preferably being present in an amount of between 5% and 30% relative to the total dry weight of fibres and polymer(s), and better still between 10% and 15%.
9. Security sheet according to any one of the preceding claims, characterized in that said sheet comprises an extra cationic flocculating agent, referred to as secondary cationic flocculating agent, chosen from polyacrylamides, polyethyleneimines, polyvinylamines and mixtures thereof, in an amount which may be between 0.1% and 0.5% by dry weight relative to the total dry weight of fibres and polymer(s).
10. Security sheet according to any one of the preceding claims, characterized in that said sheet comprises at least one security element, in particular chosen from watermarks or optically variable devices (OVDs), especially elements having an interference effect, in particular iridescent elements, holograms, liquid crystals and multilayer interference structures, said optically variable devices possibly being affixed to security threads, foils, patches, planchettes, luminescent and/or magnetic and/or metallic pigments or fibres and combinations thereof, and/or comprises an RFID device.
11. Security sheet according to any one of the preceding claims, characterized in that said sheet comprises an outer external surfacing layer, preferably based on a polyvinyl alcohol, on an acrylic polymer or on a transparent or translucent elastomer binder, such as a polyurethane, and a colloidal silica.
12. Process for manufacturing a security sheet as described in any one of Claims 1 to 11, consisting in forming said sheet via a wet route from an aqueous suspension comprising at least:

    - fibres,

    - an anionic dispersion of at least one polymer having a glass transition temperature above 23°C, and

    - an effective amount of a cationic flocculating agent, then draining, pressing and drying said sheet.
13. Process according to Claim 12, characterized in that said fibres, said anionic dispersion of at least one polymer having a glass transition temperature above 23°C and said cationic flocculating agent are mixed in bulk.
14. Security document, characterized in that it comprises a security sheet as described in any one of Claims 1 to 11 or obtained according to the process as defined in either of Claims 12 and 13, said security document preferably being a means of payment, such as a banknote, a payment card, a cheque or a restaurant voucher, an identity document, such as an identity card, a visa, a passport or a driving licence, a card, in particular an access card, a lottery ticket, a transport pass or else an entrance ticket to cultural or sporting events, a loyalty card, a benefit card, a season ticket, a playing or trading card, a gift token or a voucher, and more preferentially is a banknote.