WO1986005530A1

WO1986005530A1 - Method for preparing a fibrous sheet by using paper manufacturing techniques

Info

Publication number: WO1986005530A1
Application number: PCT/FR1986/000073
Authority: WO
Inventors: Daniel Gomez
Original assignee: Papeteries De Gascogne
Priority date: 1985-03-18
Filing date: 1986-03-07
Publication date: 1986-09-25
Also published as: FR2578870A1; FI81157C; DE3671767D1; ES553296A0; EP0215044A1; ES8801009A1; FR2578870B1; FI864653A; FI81157B; FI864653A0; EP0215044B1; JPS62502696A

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

Process for the preparation of a fibrous sheet by the papermaking route.

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 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, in particular in the field of printing-writing, packaging, coatings and special papers. It is particularly advantageous for the retention 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 process, according to which the mixture of the mineral filler and the organic binder is pre-flocculated before being incorporated, into the 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), a flocculating agent then being introduced before the headbox into the suspension resulting aqueous 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.

According to the invention, a new method is proposed which leads to even better results compared to the method 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,

- limitation of the contact times, before the Lête box, 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 method 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 (of the order of 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 have been unsatisfactory in terms of both opacity and retention.

According to French patent application N ° 84 19957 of December 26, 1984 (TALCS DE LUZENAC), a new opacity pigment is recommended which is particularly advantageous with regard to opacity-contrast and especially opacity with 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 TiO 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 costs of manufacturing while preserving or improving the mechanical properties of the fibrous sheets compared to previously known papermaking techniques, when TiO _{2 is used} as the only opacifying filler, on the one hand, and with said more economical opacity pigment, object of the aforementioned French patent application No. 84 19957, on the other hand.

In particular, it has been observed (see for this purpose the results of comparative tests given below) using the abovementioned opacity pigment, that (i) the process of FR-A-2492425 gives fibrous sheets having properties ( in particular retention and opacity) much higher than those obtained according to conventional papermaking techniques, and (ii) the process according to the invention improves said properties compared to said conventional techniques and to FR-A-2 492 425. Furthermore when we replace TiO ₂ with 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 TiO ₂ are replaced by 7 parts by weight of a co-ground mixture of 80% by weight of lamellar silicate and 20% by weight of TiO ₂ ) to obtain equivalent opacity. However, 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 breakage and to bursting, in particular when a good opacity is sought. .

The process for preparing a fibrous sheet by paper techniques, which is recommended according to the invention to improve retention, in which the pre-flocculated mixture is introduced into the head circuits of the paper machine of a flocculating agent of the mineral filler and of the organic binder in the aqueous suspension of fibers, then a flocculating agent is introduced before the headbox into the resulting aqueous suspension, said process being characterized in that 1) an aqueous suspension of fibers is prepared; 2) an aqueous suspension of mineral filler and binder is prepared in which the charging mixture is pre-flocculated by means of a cationic flocculant; 3) at the head circuits of the paper machine and before the head box of the latter, the aqueous suspension of the pre-flocculated charge mixture is introduced into the aqueous suspension of the fibers, so that (i) the diameter means of the flocs of the filler-binder mixture which are 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 be 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" is meant here a composite material comprising fibers, an inorganic 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 laminated 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, use is made of noble wood fibers, 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, 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 the fibers of stage 1 ° 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 ) polymetacrylamides, polyacrylates and polymetacrylates sodium, 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 stronger 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 sulphate, colloidal silica, barium sulphate, titanium dioxide, white satin (hydrated calcium sulphoaluminate), magnesium hydroxide, or mixtures thereof. Also suitable is the opacity pigment described in the above-mentioned French patent application No. 84 19957.

This opacity pigment is obtained by co-grinding TiO ₂ and lamellar silicate so that the fragmented particles of TiO ₂ 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.

Co-grinding can be carried out dry by introducing TiO ₂ and the lamellar silicate into a gas stream adapted to entrain the particles of the two products at 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 mill of the "JET-O-MIZER" type (manufactured by the Company Fluid-Energy) or "COX" (manufactured by the Company Cox-Brothers and C °). The co-grinding can also be carried out by wet channel 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 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 TiO 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 TiO ₂ having an average particle size of 60 to 100 microns and of lamellar silicate having a medium 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 TiO ₂ and from 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 TiO ₂ (d ₅₀ = 80 μm) and 80% by weight of a talcchlorite weight mixture (1: 1) (d ₅₀ = 10 μm). The aforementioned French application recommends in particular for the opacity-contrast a pigment consisting of 30 to 50% by weight of TiO 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 TiO 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 print-write, 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 tape 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 native or modified by chemical, enzymatic or thermal means, dextrins, polyvinyl alcohols, casein, animal glue, vegetable proteins, cellulosic esters such as / carboxymethylcellulose, alginates, dispersions of synthetic polymers such as carboxylated or non-carboxylated styrenebutadiene latexes, acrylic latexes, s, 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, there may be mentioned in particular polyethyleneimine, polyamide-amine, especially crosslinked polyalkylamine, especially modified polyacrylamide, quaternary ammonium compounds such as in particular chloride / hydroxypropyltrimethylammonium and cationic starches.

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 be used 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 paper machine's head circuits; - 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 essentially carried out in a maximum of one minute. 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 preflocculated 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 the 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 below "flocculant II", will be chosen from organic and mineral agents for ionic destabilization. Flocculant II can, 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 flocculant II can be identical to flocculant I.

When the flocculant II 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 (SO ₄ ) _{3 content} . To overcome this risk, two solutions are recommended: do not put the recycled white water in 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 field of printing-writing, the ionic demand will be in particular of the order of 10 milliequivalents per gram.

In addition to the fibers, the mineral filler, the organic binder, the anionic polymer and the cationic flocculants I 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 sizing 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 an acid medium,

- anti-foaming agents,

- optical brighteners,

- coloring or shading agents, - wet strength agents such as urea-formaldehyde, melamine-formaldehyde, glyoxal, crosslinked cationic polyalkyleneamines, melamine-formaldehyde condensation products 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 leaté-arate) and viscosity regulating agents (in particular there 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 of implementation of this method is given below.

Stage 1 ° a) the cellulose fibers in aqueous suspension coming from defibration in a pulper (non-integrated factory) or directly from the pulp manufacturing workshop (integrated factory) are stored at a concentration of 40-400 g / l under agitation in a vat room; 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, per using standard conical or double disc refiners, or in particular between 250 and 350 g / l with special refiners for high concentration refining, in particular in the case of the production of packaging supports, in order to obtain high tear resistance . At this level, organic fibers can be introduced, if necessary and / or mineral which 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 / l. This mineral filler can 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 ₅₀ = 0.5 -1.5 μm) obtained from 80% by weight of talc-chlorite (1: 1) by weight (d ₅₀ = 10 μm ) and 20% by weight of TiO ₂ (d ₅₀ = 80 μm). 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 an enzymatic degradation, of dextrins or of esters of starches, is put in the form of an aqueous preparation at a concentration ranging 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 I 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 I which 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 I 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 charging mixture thus pre-flocculated is collected continuously at the lower end of the mixer. 3rd stage

The charge-binder mixer pre-flocculated in aqueous suspension at 100-200 g / l is introduced continuously at the level of the head circuits into 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 duration of contact 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 retention. final on the fabric of the paper machine, the duration of contact of the flocculant II with the flocculated fiber-filler-binder mixture being less than 45 seconds and preferably between 8 and

30 seconds.

Remarks

The other aforementioned additives such as bonding agents, optical brighteners, etc. can be incorporated either after the refining of 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 process of the invention are referenced as being "Ex" products, the fibrous sheets according to the prior art are referenced as being "A" (classic technique) and "B" (technical) products. of FR-A-2 492 425). EXAMPLE 1

Preparation of a printing-writing medium A fibrous sheet is prepared according to the best method given above from cellulosic fibers constituted by a mixture of bleached bisulfite resinous fibers ... 40% by weight bleached kraft hardwood fibers ....... 60% by weight refining ................................ 35 ° SR and pigment opacity according to Example 1 of French Application No. 84 19957 obtained by co-grinding a mixture of

TiO ₂ (anatase), d ₅₀ = 80 microns ......... 20% by weight

Talc-chlorite (1: 1) by weight, d ₅₀

= 10 microns ............................. 80% by weight Average grain size of the oobroyat .......... d ₅₀ = 0.8 micron the other operating conditions being as follows

Average diameter (d _m ) of the flocs of the filler-binder mixture introduced in stage 3 ° ............................... 0 , 03-0.15 mm

Contact time (t ₁ ) or charge-binder mixtures pre-flocculated with the fibers .................................. .40 seconds

Contact time (t ₂ ) of the flocculant II with the fiber-charge mixture .................................. ..25 seconds

A fibrous sheet useful as a printing-writing medium having a basis weight of 64 g / m ^{2 is formed} .

COMPARATIVE TEST I The sheet obtained according to Example 1 was compared with control sheets obtained according to a conventional technique (A ₁ ) and according to the teaching of FR-A-2 492 425 (B ₁ ) having the same grammage (64 g / m ² ), from the same cellulosic fibers (35 °

SR) of the same variety of TiO ₂ (d ₅₀ = 0.8 micron after grinding of titanium dioxide of d ₅₀ = 80 micron) for A1, and of the same opacity pigment for B1, obtaining the fibrous sheet B1 further comprising the following terms:

Average diameter of the flocs of the filler-binder mixture introduced at the 3 ° stage ....... 0.35-0.50 mm Contact time t ₁ ................ ...... 80 seconds

Contact time t ₂ ...................... 60 seconds

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

The results of Table I show that Ex 1 obtained according to the method 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 is prepared in 64 g / m ² from the cellulosic fibers and the opacity pigment according to example 1 above with the same values for d _m , t ₁ . and respectively t ₂ .

COMPARATIVE TEST II

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 TiO ₂ as in comparative test I above.

The quantities of the constituents, the mechanical properties and the costs of the materials of said sheets have been recorded in table II below. the results given in said table II confirm those recorded in table I. It is observed in particular that Ex 1 and Ex 2 allow savings of 7 to 10% and are very favorable, for an identical opacity, to the improvement of 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

A fibrous sheet is prepared for packaging in

70 g / m ² from a mixture of cellulose fibers comprising bleached kraft softwood fibers ......... 50% by weight bleached kraft hardwood fibers ......... 50% by refining weight .................................. 35 ° SR of the opacity pigment according to example 1 ci above with values d _m , t ₁ and t ₂ identical to those of said example 1.

COMPARATIVE TEST 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 TiO _2. 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 have been obtained are shown in Table III

EXAMPLE 4 AND COMPARATIVE TEST IV

Preparation of a base support for carbonless A base support for carbonless is prepared according to the invention (Ex 4) from a mixture of cellulose fibers comprising: bleached kraft softwood fibers ......... 60% by weight bleached kraft hardwood fibers ......... 40% by weight refining ........................... ....... 50 ° SR 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

A support for stratification (Ex 5) according to the invention is prepared from a mixture of cellulosic fibers comprising: bleached bisulfite softwood fibers ........ 40% by weight bleached kraft hardwood fibers ... ......... 60% by weight refining .................................... .28 ° SR 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 TiO ₂ (d ₅₀ =

0.8 micron). The corresponding results are reported in Table V below. It is noted that the saving on the cost of the composition exceeds 25%.

The study of Tables I 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 interesting from an industrial point of view, the results obtained here confirming those given in said application, (ii) the process according to FR-A-2 492 425 improves the retention and therefore the opacity of the fibrous sheets when said opacity pigment is used, and (iii) the process according to the present invention is very favorable for opacity, therefore that for an identical opacity, it improves the retention and the mechanical properties and leads to significant savings in raw materials compared to the prior art.

Claims

1. A method of preparing a fibrous sheet by the papermaking route to improve retention, said method, in which the mixture of the mineral filler and the pre-flocculated organic binder is introduced into the head circuits of the paper machine. the aqueous suspension of fibers, then introduced before the headbox a flocculating agent into the resulting aqueous suspension, being characterized in that

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 level of the head circuits of the paper machine and before the head box of the latter, the aqueous suspension of the preflocculated charge-binder mixture is introduced into the aqueous suspension of the fibers, so that (i ) the average diameter of the flocs of the filler-binder mixture which 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.

2. Method according to claim 1, characterized in that the mineral filler contains an opacifying agent.

3. Method for preparing a fibrous sheet by the papermaking process to improve retention and opacity, said method, in which (i) is introduced at the level of the head circuits of the paper machine, the mixture of the mineral filler and of the organic binder pre-flocculated in the aqueous suspension of fibers, the mineral filler containing an opacifying agent chosen from pigments constituted by a mixture of 10 to 70% by weight of TiO ₂ and from 90 to 30% by weight of lamellar silicate and obtained by co-grinding of TiO ₂ and of lamellar silicate with a view to generating an adsorption of fragmented TiO ₂ particles on the activated surfaces of the fragmented particles of lamellar silicate, then (ii) introduced before the headbox a flocculating agent in the resulting aqueous suspension, being characterized in that

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 thereof, the aqueous suspension of the charge-binder mixture pre-flocculated is introduced into the aqueous suspension of the fibers, so that (i) the mean floc diameter of the charge-binder mixture that fibers are introduced into said suspension to be between 0.01 and

0.3 mm, and (ii) the contact time 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.

4. Method according to any one of claims 1 and 3, characterized in that the fibers of stage 1 ° are made substantive by the addition of an anionic means.

5. Method according to claim 4, characterized in that the anionic means is an anionic organic polymer used at a content 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. Method according to any one of claims 1 and 3, characterized in that the average diameter of the flocs of the filler-binder mixture which is introduced in stage 3 ° into the aqueous suspension of the fibers is between 0.03 and 0.15 mm.

7. Method according to any one of claims 1 and 3, characterized in that the contact time of the charge-binder mixture pre-flocculated and introduced in the 3 ° stage, with the fibers is between 10 and 45 seconds.

8. Method according to any one of claims 1 and 3, characterized in that the duration of contact of the cationic flocculant introduced in stage 4 ° with the fiber charge-binder mixture is between 8 and 30 seconds.

9. Method according to any one of claims 1 and 3, characterized in that the ionic demand of the fibers is less than or equal to 20 milliequivalents per gram of dry matter.

10. Fibrous sheet, characterized in that it is obtained according to the method of any one of claims 1 to 9.