EP0215044B1 - Method for preparing a fibrous sheet by using paper manufacturing techniques - Google Patents

Abstract

New method for the preparation of a fibrous sheet by using paper manufacturing techniques in order to improve the retention and particularly the opacity, said method being characterized in that 1) an aqueous suspension of fibrous is prepared; 2) an aqueous suspension of mineral filler and binder is prepared wherein the filler/binder mixture is preflocculated by means of a cationic flocculent; 3) at the head circuits of the paper machine and before the head box of the latter, the aqueous suspension of the preflocculated filler-binder mixture is introduced into the aqueous suspension of fibers so that (i) the average diameter of the flocks of the filler-binder mixture added into said fiber suspension is comprised between 0.01 and 0.3mm, and (ii) the contact duration of said filler-binder mixture with the fibers in the resultant suspension is comprised between 10 and 60 seconds; 4) before the heat box, a cationic flocculent is introduced into the aqueous suspension of the mixture of fibers-filler-binder obtained in the stage (3), so that the contact duration of said floculent with the mixture of fibers-filler-binder is shorter than 45 seconds; 5) the resulting aqueous suspension is introduced into the head box and forms a sheet on a paper machine which is pressed and dried.

Description

The present invention relates to a new process for preparing a fibrous sheet by the papermaking route in order to improve, in general, the retention and, in particular, that of opacifying agents, by limiting the manufacturing costs and by preserving or improving the mechanical properties compared to previously known techniques. It also relates, as a new industrial product, to the fibrous sheet obtained by this process.

This new process is particularly useful in the manufacture of paper and cardboard, especially in the field of printing-writing, packaging, coatings and special papers. It is particularly advantageous for the retension of mineral fillers such as opacifying agents in order to improve opacity, in particular opacity-contrast and opacity in oil.

We know that the Applicant has already recommended in FR-A-2 492 425 a process for the preparation of a fibrous sheet by the papermaking route, according to which the mixture of the mineral filler and the organic binder is pre-flocculated before being incorporated, level of the paper machine head circuits, in the aqueous suspension of fibers previously treated with an organic anionic polymer (which strengthens the anionic power of the fibrous suspension), various conventional additives in stationery then being introduced before the headbox in the resulting aqueous suspension comprising the fibers, the mineral filler and the binder. This process makes it possible to increase retention compared to conventional or previously known papermaking techniques.

• -optimisation du diamètre moyen des flocs charge-liant,
• -limitation des durées de contact, avant la caisse de tête, des flocs charge-liant avec les fibres, d'une part, et du floculant avec le mélange fibres-charge-liant, d'autre part.

According to the invention, a new process is proposed which leads to even better results compared to the process described in the aforementioned document FR-A-2 492 425, in particular as regards retention. According to the invention, the improvement in retention results from the implementation of specific operating methods:

• -optimization of the average diameter of the load-binder flocs,
• -limiting the contact times, before the headbox, of the load-binder flocs with the fibers, on the one hand, and of the flocculant with the fiber-filler-binder mixture, on the other hand.

According to another aspect of the invention, the new process which is recommended is particularly advantageous for improving the retention of opacifying means.

It is known that titanium oxide, in particular of rutile or anatase variety, is an opacity and whitening pigment commonly used in paints, inks, plastics, paper and cardboard, cosmetics and ceramics, in particular due to its high covering power conferred by its high refractive index and its absorption by UV rays

The increasingly excessive cost of TiO ₂ (around 10 to 11 FF / kg at present) has led users to consider more economical alternatives. In the paper industry, attempts to replace titanium oxide with a mixture of TiO ₂ and another more economical mineral filler chosen in particular from talc, kaolin, alumina hydrate and CaCO ₃ unsatisfactory in terms of both opacity and retention.

According to French patent application FR-A-2 575 170 by TALCS DE LUZENAC (filed on December 26, 1984 under No. 84 19957 and published on June 27, 1986, which constitutes an intermediate document which cannot be relied on to the novelty of the invention ) is recommended a new opacity pigment particularly interesting with regard to opacity-contrast and especially opacity in oil. This opacity pigment consists of a mixture of 10 to 70% by weight of TiO ₂ and 30 to 90% by weight of an adjuvant chosen from lamellar silicates, and is obtained by co-grinding of Ti0 ₂ and said lamellar adjuvant in order to generate an adsorption of titanium oxide on the fragmented particles of the adjuvant. Among the lamellar silicates which are suitable according to this patent application, mention is made in particular of talc (hydrated magnesium silicate), chlorite (hydrated magnesium and aluminum silicate), kaolin, mica, phlogopite and their mixtures.

By implementing the process of the invention, the retention is improved and savings are made on the manufacturing costs while preserving or improving the mechanical properties of the fibrous sheets compared to the previously known papermaking techniques, when Ti0 _{2 is used.} as the only opacifying filler, on the one hand, and with the said more economical opacity pigment, the subject of the aforementioned application FR-A-2 575 170, on the other hand.

It has been observed in particular (see for this purpose the results of comparative tests given below) using the abovementioned opacity pigment, that (i) the process of FR-A-2 492 425 gives manufacturing sheets having properties (in particular retention and opacity) much superior to those obtained according to conventional papermaking techniques, and (ii) the method according to the invention improves said properties compared to said conventional techniques and to FR-A-2 492 425.

Furthermore, when Ti0 ₂ is replaced by the opacity pigment according to the above-mentioned French application, the quantity of mineral filler entering the paper must be increased (for example, 4 parts by weight of Ti0 ₂ are replaced by 7 parts by weight d '' a co-ground mixture of 80% by weight of lamellar silicate and 20% by weight of Ti0 ₂ ) to obtain an equivalent opacity. Now, it is known that when the content of the mineral filler is increased, the mechanical properties of the resulting fibrous sheet are reduced.

According to the invention, the disadvantage of the increase in the content of mineral filler is overcome by retaining or even improving the mechanical properties such as in particular the resistance to rupture and bursting, in particular when a good opacity is sought. .

• 1°) on prépare une suspension aqueuse de fibres;
• 2°) on prépare une suspension aqueuse de charge minérale et de liant dans laquelle le mélange charge-liant est préfloculé au moyen d'un floculant cationique;
• 3°) au niveau des circuits de tête de la machine à papier et avant la caisse de tête de celle-ci, on introduit la suspension aqueuse du mélange charge-liant préfloculé dans la suspension aqueuse des fibres, de telle façon qui (i) le diamètre moyen des flocs du mélange charge-liant que l'on introduit dans ladite suspension des fibres soit compris entre 0,01 et 0,3 mm, et (ii) la durée de contact dudit mélange charge-liant avec les fibres dans la suspension résultante soit compris entre 10 et 60 seconds;
• 4°) avant la caisse de tête, on introduit un floculant cationique dans la suspension aqueuse du mélange fibres-charge-liant obtenue au stade 3°), de telle façon que la durée de contact dudit floculant avec ledit mélange fibres-charge-liant soit inférieure à 45 secondes;
• 5°) on introduit la suspension aqueuse résultante dans la caisse de tête et forme une feuille sur machine à papier que l'on presse et sèche.

According to the invention, a method of preparing a fibrous sheet is recommended by the papermaking route, in which.

• 1) an aqueous suspension of fibers is prepared;
• 2) an aqueous suspension of mineral filler and binder is prepared in which the filler-binder mixture is pre-flocculated by means of a cationic flocculant;
• 3) at the head circuits of the paper machine and before the head box thereof, the aqueous suspension of the charge-binder mixture pre-flocculated is introduced into the aqueous suspension of the fibers, in such a way that (i) the mean diameter of the flocs of the filler-binder mixture that is introduced into said suspension of fibers is between 0.01 and 0.3 mm, and (ii) the duration of contact of said filler-binder mixture with the fibers in the resulting suspension is between 10 and 60 seconds;
• 4) before the headbox, a cationic flocculant is introduced into the aqueous suspension of the fiber-filler-binder mixture obtained in step 3), so that the duration of contact of said flocculant with said fiber-filler-binder mixture is less than 45 seconds;
• 5) the resulting aqueous suspension is introduced into the headbox and forms a sheet on a paper machine which is pressed and dried.

By "fibrous sheet" here is meant a composite material comprising fibers, a mineral filler, a binding agent and at least one flocculating means. This composite material, which is prepared by the papermaking process, can also contain one or more conventional additives in stationery. With regard to this definition, the term "fibrous sheet" includes paper and board when the predominant fibers are cellulosic fibers, on the one hand, and nonwovens or synthetic papers when the fibers used are essentially non-cellulosic fibers. , on the other hand.

The composite material obtained according to the abovementioned process is useful as a print-write support, coating support, packaging support or for obtaining special supports (in particular photographic supports, supports for carbonless copies and supports for stratification).

When the mineral filler consists essentially of or contains an opacifying means, the composite material according to the invention is particularly advantageous in particular in (i) the field of supports intended for impregnation with phenolic resins and / or melamines for the production of limited or laminated panels, in order to avoid the material becoming transparent, (ii) the field of packaging supports (food packaging in particular) which must remain opaque in contact with grease or after complexing with waxes, resins and / or polymers which tend to affect the opacity of the final product.

All the fibers are suitable for the preparation of the fibrous sheet according to the invention, in particular natural organic fibers (cellulosic fibers) or synthetic fibers (polyamide, polyester, polyalkylene, polyacrylate fibers) and mineral fibers (glass fibers, ceramic, acicular gypsum, carbon and rock wool) and their mixtures. Preferably, noble wood fibers are used, namely unbleached softwood and hardwood fibers, semi-bleached or bleached, optionally combined with recovered fibers, for example from old paper and textiles. It is also possible to combine cellulosic fibers with fibers of high synthetic polymers such as polyamide, polyester, polyethylene, polypropylene fibers or with mineral fibers such as glass, ceramic and calcium sulphate fibers. and carbon, or to cellulose regeneration fibers, or to their mixtures.

Advantageously, it is recommended to add an anionic means to the aqueous suspension of stage 1 ° fibers in order to make said fibers substantial by strengthening their anionic power. This means, which contributes to improving the fiber-filler connections and consequently the internal cohesion of the fibrous sheet, is introduced into said aqueous suspension of the fibers before the incorporation in stage 3 ° of the flocculated particles of the filler-binder mixture. Preferably, this anionic means will be chosen from polymers of the poyacrylic derivative type such as polyacrylamides (in particular modified polyacrylamides having a high average molecular weight - of the order of 5 × 10 ⁶ to 10 ⁷ ) sodium polymetacrylamides, polyacrylates and polymethacrylates , potassium and ammonium.

The amount of anionic means depends on the anionicity of the paste used, which is linked to the manufacturing process (kraft or bisulfite paste) but also to the conditions for washing the paste before use. A kraft pulp from an integrated factory has a much more anionic character than a dried and stored pulp before being sent to the paper machine. In practice, a dose of 0.02% to 0.5% by weight of anionic means relative to the weight of the fibrous sheet will be used, and preferably 0.05% to 0.2% of said means relative to the weight of the fibrous sheet.

The mineral fillers according to the invention are essentially non-binding mineral fillers. Particularly suitable are the usual mineral fillers in the stationery and paint industries such as, for example, talc, kaolin, natural calcium carbonate, precipitated or originating from the regeneration operations of black liquors extracted from the cooking of kraft pastes and more. particularly after the caustification operation, magnesium carbonate, alumina hydrates, calcium sulfate, colloidal silica, barium sulfate, titanium dioxide, satin white (hydrated calcium sulfoaluminate), magnesium hydroxide, or mixtures thereof. Also suitable is the opacity pigment described in the French patent application FR-A-2 575 170 mentioned above.

This opacity pigment is obtained by co-grinding of Ti0 ₂ and of the lamellar silicate in such a way that the fragmented Ti0 ₂ particles are adsorbed on the new activated surfaces resulting from the fragmentation of the lamellar silicate. The aforementioned French patent application demonstrates how and why this pigment is distinguished, both in terms of structure and in terms of final opacity, from the physical mixtures of the two ingredients previously envisaged.

The co-grinding can be carried out dry by introducing Ti0 ₂ and the lamellar silicate into a gas stream adapted to entrain the particles of the two products at a supersonic speed and to project them against each other in order to obtain the fragmentation and the aforementioned adsorption, in particular by means of a "JET-O-MIZER" type shredder (manufactured by the Fluid-Energy company) or "COX" (manufactured by the Cox-Brothers and C ° company). Co-grinding can also be carried out by the wet method by preparing a liquid suspension containing the starting products in divided form and by agitating this suspension in the presence of solid balls so as to break up the particles of the two products between the balls in order to obtain the aforementioned fragmentation and adsorption, in particular by means of a grinder of the "BABCOCK BALL MILL" type.

Said opacity pigment is a mixture consisting of 10 to 70% by weight of Ti0 ₂ and 90 to 30% by weight of lamellar silicate (talc, chlorite, kaolin, mica, phlogopite and their mixtures); it advantageously has an average particle size (as defined in said French application) of between 0.5 and 1.5 microns and is obtained by co-grinding of Ti0 ₂ having an average particle size of 60 to 100 microns and of lamellar silicate having an average particle size greater than 2.5 microns and preferably between 8 and 12 microns.

The preferred opacity pigment for the opacity of the fibrous sheets according to the invention will be a mixture as described above consisting of 10 to 50% by weight of Ti0 ₂ and 90 to 50% by weight of lamellar silicate, in particular a pigment having an average particle size d ₅₀ = 0.5 to 1.5 μm and obtained by co-grinding of a mixture of 20% by weight of Ti0 ₂ (d _5o = 80 μm) and of 80% by weight of a mixture by weight talc-chlorite (1: 1) (d ₅₀ = 10 pm). The aforementioned French application recommends in particular for the opacity-contrast a pigment consisting of 30 to 50% by weight of Ti0 ₂ and from 70 to 50% by weight of lamellar silicate, and for the opacity in oil a pigment consisting from 10 to 30% by weight of Ti0 ₂ and from 90 to 70% by weight of lamellar silicate.

The mineral filler-fiber weight ratio is not critical, it can be in particular between 0.01 and 6, depending on the desired applications.

For example, in printing-writing, the amount of mineral filler in the fibrous sheet may vary from 5 to 40% by weight, and in particular from 10 to 30% by weight relative to the weight of said fibrous sheet. For various coatings intended for the building, the loading rate may be greater than 50% by weight relative to the weight of the fibrous sheet. For packaging applications of the small, medium or large capacity bag type or for kraft envelopes or address band supports for example, the amount of mineral filler may vary between 2 and 15% by weight relative to the weight of the fibrous sheet. .

The organic binder which can be used in the process according to the invention is any organic binder, natural or synthetic, usually used in stationery. It ensures the connection of the constituents of the material together and makes it possible to improve the physical properties of the sheet material. Among the binders which are suitable according to the invention, mention may in particular be made of starches which are native or chemically, enzymatically or thermally modified, dextrins, polyvinyl alcohols, casein, animal glue, vegetable proteins, cellulose esters such as carboxymethylcellulose, alginates, dispersions of synthetic polymers such as carboxylated or non-carboxylated styrene-butadiene latexes, acrylic latexes, styrene-acrylic latexes, vinyl acetate latexes, neoprene latexes, acrylonitrile latexes, vinyl chloride latexes and mixtures thereof.

The amount of binder depends on the end use envisaged for the sheet material; it can in particular vary between 1 and 40 parts by weight, and preferably between 1 and 25 parts by weight, relative to 100 parts by weight of fibers and mineral filler.

The pre-flocculation of the mineral filler-binder mixture of stage 2 ° is carried out by means of a flocculant which aims to ionically destabilize said filler-binder mixture before mixing with the fibers. This product, hereinafter designated "flocculant I", will advantageously be chosen from cationic organic flocculants rather than from mineral cationic flocculants such as aluminum sulphate and aluminum polychlorides.

Among the organic cationic flocculants which are suitable in the process of the invention, mention may in particular be made of polyethyleneimine, polyamideamine, especially crosslinked polyalkylamine, especially modified polyacrylamide, quaternary ammonium compounds such as in particular hydroxypropyltrimethylammonium chloride and starches cationic.

• -la concentration de la suspension aqueuse de charge et de liant;
• -le temps de contact floculant-charge-liant, qui est lié à la configuration des circuits de tête de la machine à papier;
• -l'agitation, et
• -le pouvoir cationique du floculant.

The cationic organic flocculant which occurs in stage 2 ° is incorporated, in the form of an aqueous solution or suspension, preferably continuously, in the aqueous suspension containing the mineral filler and the organic binder, in an amount generally between 0.006 and 5 parts in by weight, and preferably between 0.01 and 2 parts by weight per 100 parts by weight of the mixture of mineral filler and binder. The exact amount to use depends on four factors:

• the concentration of the aqueous suspension of filler and binder;
• the flocculant-filler-binder contact time, which is linked to the configuration of the head circuits of the paper machine;
• - agitation, and
• the cationic power of the flocculant.

However, as a general rule, this quantity is adjusted so that the preflocculation of the filler-binder mixture is carried out essentially in one minute at most.

According to a characteristic of the invention, it is important that, during the implementation of the 3 ° stage, the average diameter of the flocs of the filler-binder mixture is between 0.01 and 0.3 mm, and preferably between 0.03 and 0.15 mm, when said pre-flocculated mixture is introduced into the aqueous suspension of fibers. If the average diameter of said flocs is less than 0.01 mm, the fabric losses may increase, and if the average diameter of said flocs is more than 0.3 mm, there is a serious risk that the fibrous sheet formed will not be uniform or the air is not good.

According to another characteristic of the invention, it is important that the pre-flocculated filler-binder mixture is introduced at the 3 ° stage as close as possible to the headbox, so that the duration of contact of the filler-binder mixture with the fibers is between 10 and 60 seconds, and preferably between 10 and 45 seconds. If said contact time is greater than 60 seconds, there will be excessively large fiber-filler-binder flocs and consequently the fibrous sheets may not be uniform.

Finally, it is important that at stage 4 ° the cationic flocculant, which is incorporated into the fiber-filler-binder mixture resulting from stage 3 °, is introduced into the head circuits very close to the head box so that the duration of contact of said flocculant with the fiber-filler-binder mixture is less than 45 seconds and preferably between 8 and 30 seconds.

The cationic flocculant of stage 4 °, which is designated hereinafter "flocculant 11", will be chosen from organic and mineral ionic destabilizing agents. The flocculant II may, like flocculant I, be a cationic organic substance, or unlike flocculant I, a cationic mineral substance such as for example aluminum sulphate and poly aluminum chloride. Of course the flocculant It can be identical to the flocculant I.

When the flocculant It is a cationic mineral substance such as for example aluminum sulphate, there is a risk that the white water, which is collected in the wet part of the paper machine, in particular under the cloth, and that is recycled, increase the size of the load-binder flocs, because of their Al ₂ (S0 ₄ ) _{3 content} . To overcome this risk, two solutions are recommended: do not bring the recycled white water into contact with the aqueous suspension of the filler-binder mixture from stage 2 ° until the start of stage 3 °, or provide a shearing device to reduce the diameter by means of the flocs of said filler-binder mixture just before its introduction into the aqueous suspension of fibers at stage 3 °.

According to another characteristic of the invention, the ionic demand of the fibers in the suspension containing said fibers, the mineral filler and the binder obtained in stage 3 ° is less than or equal to 20 milliequivalents per gram of dry matter. For a fibrous sheet intended for the packaging field, in particular for bags, the ionic demand will be in particular from 1 to 4 milliequivalents per gram, and for a fibrous sheet intended for the printing-writing field, the ionic demand will be in particular of the order of 10 milliequivalents per gram.

• -les agents de collage utilisés habituellement en papeterie pour réduire la sensibilité à l'eau de la feuille, tels que les colophanes modifiées, les émulsions de paraffine, les alkylcétènes dimères,
• -les agents de régulation du pH, par exemple le sulfate d'aluminium (qui peut intervenir comme floculant II, comme indiqué ci-dessus), ou l'acide sulfurique pour régler le pH entre 4,5 et 6 pour un collage en milieu acide,
• -les agents anti-mousse,
• -les azurants optiques,
• -les agents de coloration ou de nuançage,
• -les agents de résistance à l'état humide tels que l'urée-formol, la mélamine-formol, le glyoxal, les polyalkylène-amines cationiques réticulées, les produits de condensation de mélamine-formaldéhyde et d'acide amino-caproïque,
• -les agents fongicides et/ou bactéricides ainsi que des additifs auxiliaires classiques des bains de couchage d'impression-écriture tels que agents dispersants (notamment l'hexamétaphosphate et le pyrophosphate du sodium), les agents lubrifiants (notamment les dérivés d'acide gras, par exemple lestéarate de sodium ou de calcium) et les agents régulateurs de viscosité (notamment la gélatine, l'éthylènediamine et l'urée).

In addition to the fibers, the mineral filler, the organic binder, the anionic polymer and the cationic flocculants 1 and II, it is possible to use in the process for the preparation of a fibrous sheet according to the invention various conventional adjuvants in stationery such as:

• -the bonding agents usually used in stationery to reduce the water sensitivity of the sheet, such as modified rosins, paraffin emulsions, dimeric alkyl ketenes,
• - pH regulating agents, for example aluminum sulphate (which can act as a flocculant II, as indicated above), or sulfuric acid to adjust the pH between 4.5 and 6 for bonding in the medium acid,
• - anti-foaming agents,
• -optical brighteners,
• - coloring or shading agents,
• wet strength agents such as urea formaldehyde, melamine formaldehyde, glyoxal, crosslinked cationic polyalkylene amines, condensation products of melamine formaldehyde and amino caproic acid,
• -fungicidal and / or bactericidal agents as well as conventional auxiliary additives in printing-writing coating baths such as dispersing agents (in particular hexametaphosphate and sodium pyrophosphate), lubricating agents (in particular fatty acid derivatives , for example sodium or calcium estearate) and viscosity regulating agents (in particular gelatin, ethylenediamine and urea).

The method according to the invention, having regard to the above-mentioned contact durations, comprises a large number of continuous operations.

The best mode for implementing this method is given below.

Stage 1 °

a) the cellulosic fibers in aqueous suspension coming from defibering in a pulper (non-integrated factory) or directly from the pulp manufacturing workshop (integrated factory) are stored at a concentration of 0-400 g / I with stirring in a copper;

b) the cellulosic fibers are conventionally refined to a Schoepper-Riegler degree of between 15 and 65 depending on the applications, at a variable concentration of between 20 and 350 g / l, in particular between 20 and 60 g / l, using standard conical or double disc refiners, or in particular between 250 and 350 g / I with special refiners for high concentration refining, in particular in the case of the production of packaging supports, in order to obtain a high tear resistance . At this level we can introduce, if necessary, the organic and / or mineral fibers that we want to associate with cellulosic fibers;

c) the anionic means required as indicated above to make the fibers substantial are added with stirring, at a dose of between 0.02 and 0.5% and preferably 0.05 to 0.2% by weight relative to the weight of the fibrous sheet.

Stage 2 °

a) The mineral filler is dispersed in an aqueous medium at a concentration of between 150 and 600 g / l, preferably at a concentration of 300-400 g / I. This mineral filler may consist entirely of an opacifying agent or of a mixture of several fillers including in particular an opacifying agent. Preferably, the opacifying agent is here a co-ground mixture (d _n = _0, 5 to 1.5 microns) obtained from 80% by weight of talc-chlorite (1: 1) by weight (of _n = 10 pm ) and 20% by weight of TiO ₂ (d ₅₀ = 80 pm).

b) The organic binder ready to use if it is a latex or after baking if it is native starches, etherified, oxidized or subjected to enzymatic degradation, dextrins or esters of starches, is put into the form of an aqueous preparation at a concentration of between 20 and 300 g / l, preferably between 20 and 200 g / l. The preferred binder is native starch.

c) mixing is carried out with stirring, preferably continuously, of the aqueous suspension of the mineral filler and of the aqueous preparation of the binder. This operation is advantageously carried out in a dynamic conical mixer with a propeller which ensures perfect homogeneity of the filler-binder mixture. The flocculant is incorporated into said filler-binder mixture after having been diluted with water from 10 to 100 times (dilution greater than 10 times and less than or equal to 100 times). The amount of flocculant 1 that is introduced is between 0.006 and 5 parts by weight, and preferably between 0.01 and 2 parts by weight per 100 parts by dry weight of the filler-binder mixture.

In practice, the charge suspension is introduced continuously, on the one hand, and the aqueous preparation of the binder, on the other hand, each in the vicinity of the top of a dynamic conical mixer with a propeller; the flocculant 1 is continuously introduced in the vicinity of the middle of the height of said mixer which comprises, near its lower end, a dilution water inlet, in order to carry out the required dilutions. The charge-binder mixture thus pre-flocculated is collected continuously at the lower end of the mixer.

Stage 3 °

The charge-binder mixer pre-flocculated in aqueous suspension at 100-200 g / I is introduced continuously at the level of the head circuits in the aqueous suspension of the fibers prepared in stage 1 °, the mean flock diameter of said charge-binder mixture being included between 0.01 and 0.3 mm, and preferably between 0.03 and 0.15 mm, the introduction of said pre-flocculated mixture being carried out as close as possible to the headbox so that the contact time of said mixture load-binder with the fibers is less than 60 seconds and advantageously between 10 and 45 seconds.

Stage 4 °

Upstream of the headbox, a small amount of flocculant II is incorporated into the mixture resulting from stage 3 ° to strengthen the bonding of the flocs and improve the final retention on the fabric of the paper machine, the contact time of flocculant Il with the flocculated fibers-filler-binder mixture being less than 45 seconds and preferably between 8 and 30 seconds.

Remarks

The other additives mentioned above such as bonding agents, optical brighteners, etc. can be incorporated either after refining the cellulosic fibers in stage 1 °, or after the introduction of the charge-binder mixture pre-flocculated in stage 3 °.

Other advantages and characteristics of the invention will be better understood on reading the following description of examples of preparation and comparative tests. All of these elements are not limiting but are given by way of illustration for a particular purpose: improvement of retention and opacity.

For convenience, the fibrous sheets obtained according to the method of the invention are referenced as being "Ex" products, the fibrous sheets according to the prior art are referenced as being products "A" (conventional technique) and "B" (technique of FR-A-2 492 425).

Example 1 Preparing a print-write medium

et du pigment d'opacité selon l'exemple 1 de la demande FR-A-2 575 170 précitée obtenu par cobroyage d'un mélange de

les autres conditions opératoires étant les suivantes

A fibrous sheet is prepared according to the best mode given above from cellulosic fibers constituted by a mixture of

and the opacity pigment according to Example 1 of the aforementioned FR-A-2,575,170 obtained by co-grinding a mixture of

the other operating conditions being as follows

A fibrous sheet useful as a writing printing medium having a grammage of 64 g / _m ^{2 is formed} _.

Comparative Trial l

The sheet obtained according to Example 1 was compared with control sheets obtained according to a conventional technique (A1) and according to the teaching of FR-A-2 492 425 (B2) having the same grammage (64 g / m ² ) , from the same cellulose fibers (35 ° SR) of the same variety of Ti0 ₂ (d _so = 0.8 micron after grinding of titanium dioxide of d ₅₀ = ₈₀ micron) for A1, and the same opacity pigment for B1, obtaining the fibrous sheet B1 further comprising the following methods:

The quantities of the constituents, the mechanical properties and the material costs of the said sheets were recorded in Table 1 below.

The results of Table I show that Ex 1 obtained according to the process of the invention has mechanical properties (average rupture length, average burst index and above all retention) significantly improved compared to A1 and B1, for an identical opacity.

Example 2 Preparation of a fibrous sheet for printing-writing

A fibrous sheet in 64 g / m ² is prepared from the cellulose fibers and the opacity pigment according to example 1 above with the same values for d _m , t ₁ and respectively t ₂ .

Comparative essay It

The fibrous sheet of Example 2 was compared with a fibrous sheet (A2) obtained under substantially similar conditions with regard to the choice of cellulosic fibers, binder and flocculant I, the opacity pigment being replaced by Ti0 ₂ as in comparative test 1 above.

The amounts of the constituents, the mechanical properties and the material costs of the said sheets have been recorded in Table II below.

The results given in said table II confirm those recorded in table I. It can be seen in particular that Ex 1 and Ex 2 allow savings of 7 to 10% and are very favorable, for identical opacity, to improving retention and mechanical properties. The economic balance can be increased by preparing Ex 1 and Ex 2 sheets in 60 g / m ² instead of 64 g / m ² having regard to the very good mechanical properties of the products according to the invention.

Example 3 Preparation of a fibrous sheet for packaging

du pigment d'opacité selon l'exemple 1 ci-dessus avec des valeurs d_m, t₁ et t₂ identiques à celles dudit exemple 1.A fibrous sheet for packaging in 70 g / m ² is prepared from a mixture of cellulosic fibers comprising

opacity pigment according to example 1 above with values d _m , t ₁ and t ₂ identical to those of said example 1.

Comparative Trial III

The product obtained according to Example 3 was compared with a conventional fibrous sheet (A3) in 70 g / m ² in which the opacity pigment was replaced by TiO ₂ , the cellulosic fibers and the average particle size of the Ti0 ₂ of the A3 paper being respectively identical to the fibers of Example 3 and to the average particle size of the opacity pigment. The compositions of Ex 3 and A3, as well as the results which were obtained are reported in Table III.

Example 4 and comparative test IV Preparation of a basic support for carbonless

le pigment d'opacité est identique à celui de l'exemple 1 ci-dessus ainsi que les valeurs d_m, t₁ et t₂. Le support ainsi obtenu a un grammage de 40 g/m².A basic support for carbonless carbon is prepared according to the invention (Ex 4) from a mixture of cellulosic fibers comprising:

the opacity pigment is identical to that of Example 1 above as well as the values d _m , t ₁ and t ₂ . The support thus obtained has a grammage of 40 g / m ² .

The comparison product (A4) is obtained from the same fibers, replacing the opacity pigment with TiO ₂ (d ₅₀ = 0.8 micron). The corresponding results are reported in Table IV below.

Example 5 and Comparative Test V Preparation of a support for stratification

le pigment d'opacité est identique à celui de l'exemple 1 ci-dessus, ainsi que les valeurs d_m, t₁ et t₂. On obtient un support pour stratification de 90 g/m².A support for lamination (Ex 5) according to the invention is prepared from a mixture of cellulosic fibers comprising:

the opacity pigment is identical to that of Example 1 above, as well as the values d _m , t ₁ and t ₂ . A support for stratification of 90 g / m ² is obtained.

The comparison product (A5) is obtained from the same fibers, replacing the opacity pigment with Ti0 ₂ (d _so = 0.8 micron). The corresponding results are recorded in Table V below. It is noted that the saving on the cost of the composition exceeds 25%.

• (i) le pigment d'opacité selon la demande de brevet français précitée N° 84 19957 (TALCS DE LUZENAC) obtenu par cobroyage est particulièrement intéressant sur le plan industriel, les résultats obtenus ici confirmant ceux donnés dans ladite demande,
• (ii) le procédé selon FR-A-2 492 425 améliore la rétention et par suite l'opacité des feuilles fibreuses quand on utilise ledit pigment d'opacité, et
• (iii) le procédé selon la présente invention est très favorable pour l'opacité, dès lors que pour une opacité identique, il améliore la rétention et les propriétés mécaniques et conduit à des économies de matières premières importantes par rapport à l'art antérieur.

The study of Tables 1 to V allows us to conclude that

• (i) the opacity pigment according to the aforementioned French patent application No. 84 19957 (TALCS DE LUZENAC) obtained by co-grinding is particularly advantageous from an industrial point of view, the results obtained here confirming those given in said application,
• (ii) the method according to FR-A-2 492 425 improves the retention and consequently the opacity of the fibrous sheets when said opacity pigment is used, and
• (iii) the method according to the present invention is very favorable for opacity, since for an identical opacity, it improves retention and mechanical properties and leads to significant savings in raw materials compared to the prior art.

Claims (9)

1. A method for the preparation of a fibrous sheet by a papermaking technique, wherein:

1°) an aqueous suspension of fibers is prepared;

2°) an aqueous suspension of mineral filler and binder is prepared, in which the filler/binder mixture is preflocculated by means of a cationic flocculant;

3°) at the head circuits of the paper machine and upstream of its headbox, the aqueous suspension of the preflocculated filler/binder mixture is introduced into the aqueous suspension of fibers in such a way that (i) the mean diameter of the flocs of the filler/binder mixture introduced into the said suspension of fibers is between 0.01 and 0.3 mm, and (ii) the contact time between the said filler/binder mixture and the fibers in the resulting suspension is between 10 and 60 seconds;

4°) upstream of the headbox, a cationic flocculant is introduced into the aqueous suspension of fiber/ filler/binder mixture obtained in stage 3°), in such a way that the contact time between the said flocculant and the said fiber/filler/binder mixture is shorter than 45 seconds; and

5°) the resulting aqueous suspension is introduced into the headbox and a sheet is formed on the paper machine, pressed and dried.

2. A method according to claim 1, wherein the mineral filler contains an opacifier.

3. A method according to claim 2, wherein the mineral filler contains an opacifier selected from pigments consisting of a mixture of 10 to 70% by weight of Ti0₂ and 90 to 30% by weight of lamellar silicate and obtained by co-grinding Ti0₂ with the lamellar silicate so as to cause fragmented particles of Ti0₂ to be adsorbed by the activated surfaces of the fragmented particles of lamellar silicate.

4. A method according to claim 1 or 3, wherein the fibers of stage 1°) are made substantive by the addition of an anionic agent.

5. A method according to claim 4, wherein the anionic agent is an anionic organic polymer used in a proportion of 0.02 to 0.5% by weight and preferably 0.05 to 0.2% by weight, relative to the weight of the fibrous sheet.

6. A method according to claim 1 or 3, wherein the mean diameter of the flocs of the filler/binder mixture introduced in stage 3°) into the aqueous suspension of fibers is between 0.03 and 0.15 mm.

7. A method according to claim 1 or 3, wherein the contact time between the preflocculated filler/binder mixture introduced in stage 3°) and the fibers is between 10 and 45 seconds.

8. A method according to claim 1 or 3, wherein the contact time between the cationic flocculant introduced in stage 4°) and the fiber/filler/binder mixture is between 8 and 30 seconds.

9. A method according to claim 1 or 3, wherein the ionic requirement of the fibers is less than or equal to 20 milliequivalents per gram of solids.