WO1984002936A1

WO1984002936A1 - Preparation of a packaging paper

Info

Publication number: WO1984002936A1
Application number: PCT/FR1984/000022
Authority: WO
Inventors: Daniel Gomez
Original assignee: Gascogne Papeteries
Priority date: 1983-01-31
Filing date: 1984-01-31
Publication date: 1984-08-02
Also published as: FI843838A0; DE3462162D1; FI79580B; FR2540152A1; EP0123567A1; FI843838L; JPS60500504A; EP0123567B1; FI79580C; ATE25118T1; FR2540152B1

Abstract

New process for preparing a fiber sheet by using paper techniques, with a pH comprised between 6.2 and 9.5, from fibers, an organic polymer binder, an organic cationic flocculant and an agent reducing the adherence of the sheet to the wet presses during the formation of the sheet. The sheet obtained according to said method, of which the pH of the aqueous extract is higher than 7, and which exhibits improved mechanical properties particularly as to the tensile strength, the bursting resistance, the impact resistance, the rupture energy and the machinability, is useful in the packaging field and particularly for paper bags.

Description

Preparation of wrapping paper

The present invention relates to a new process for preparing a fibrous sheet by papermaking techniques from fibers, a binding agent, a flocculating agent and a particular additive, said sheet, which has properties improved mechanical properties, being useful in the field of packaging and in particular that of so-called small, medium and large capacity paper bags. The invention also relates to the sheet of paper obtained by this process as a new industrial product.

We know that papermakers and packaging kraft processors have to face competition from many packaging means such as plastic bags, and so-called bulk or semi-bulk delivery packaging. As a result, papermakers and processors have been led to develop more technical products of lower weight compared to traditional materials (reduction in the grammage of paper or reduction, for example, in the number of folds in the bags) perfectly meeting the qualitative requirements of end users, that is to say automatic binding bagging conditions in a wide variety of industrial sectors such as, according to the language of the skilled person: a) construction materials: cement, plaster, hydraulic lime , coatings, floor coverings, insulating materials, sealants; b) mineral products: raw clays, kaolins, ceramics, refractory products based on clay, asbestos, lavas, marbles, crushed flint, barytes, chalk and gypsum blanks, ochres, talcs , siliceous and silica products, bentonites, activated carbon, sand, salt; c) human food: pastry and baking flour, sugars, milk powder, pasta, potatoes, fruits and vegetables, dehydrated food; d) animal feed: compound feed, meat and fish meal, molasses, oil cake, wheat, alfalfa flour, pulps, mineral concentrates, pet food, milk powder; e) chemical products: aluminum, sodium, sulfur and combinations, detergents, detergent soaps, polymers from petrochemicals or carbochemicals, insecticides, pesticides, synthetic resins, cellulose or starch derivatives, dyes, pigments; f) fertilizers: nitrogen fertilizers, phosphates, lime nitrate, potash, compound fertilizers, Thomas or potassium slag; and, g) miscellaneous products: seeds, sawdust, bitumens, charcoal, etc.

Depending on the different types of use, the paper bag, either with valve or with an open mouth, must meet a certain number of qualitative requirements: its shape must correspond to a certain volume of product; it must be adapted to the requirements of the transformer (machinability and printability), modern bagging techniques and rapid and automatic mechanization; it must withstand bagging constraints and shocks during various handling and overpressures; it must protect the content from physicochemical attacks and organoleptic degradations; it must also be suitable for palletizing and stacking with the minimum risk of slipperiness. For many years, kraft producers have had to define the important properties of paper, on the one hand, so that it can machine properly in the processor, on the other hand, so that the bag meets the above requirements . These studies show that the energy at break of the paper, mainly linked to an elongation at break and more particularly in the machine direction, is a fundamental property for the final resistance of the bag.

We know that for this purpose a certain number of technical solutions have been recommended. We know in particular the solution called "refining of cellulose pulp at high concentration", see in particular WC WEST, Tappi 47 (6), 313 (1964), French patent N ° 1 453 806 and its additions N ° 91 978 and 91 979, which starting with a cellulose content of 20 to 35% generally makes it possible to improve the tear strength and elongation at break by 5 to 10%, but which has the major disadvantage of consuming large quantities of energy compared to conventional refining techniques.

The so-called "airborne kraft" solution is also known, which has the advantage of limiting print runs during the manufacture of paper and thus improving the energy at break, but which has the following drawbacks: significant investments, costs very high energy, irregularities in the cross direction, the machine equipped according to this solution can not produce with good profitability more traditional paper products without particular requirement for the energy at break.

We also know the so-called "crepe kraft" solution, see in particular French patent No. 1261 100, which leads to elongations at break of at least 7 to 8% in the machine direction, but which is too flexible and low machinability for both the processor and the user filling the bag, especially due to the significant development of automation of bagging lines.

It is known that due to the serious difficulties encountered with crepe papers, another extensibility solution has been proposed which is notably known under the name of the "CLUPAK" process (trademark of the company CLUPAK INC.) For the preparation of stretch krafts or semi-extensible, see in particular American patent No 2 624 245, French patents 1 439 100, 1 442 574, and 1 550049, and which comprises compacting the sheet of paper still wet between the rubber press strip and a drying roller, the rubber band, by retracting, applying compression forces in the machine direction and causing local compaction hence a certain extensibility of the sheet, the desired percentage of elongation being mainly obtained by playing on the difference speed before and after the compacting device. This solution, in the current context, is the one that offers the best compromise between quality and price. However, the measures to lighten the bags that it can allow remain limited. Indeed, the levels of elongation at break of the semi-extensible krafts are generally of the order of 5 to 10% in the machine direction, and in particular of 5 to 7%, because beyond this threshold appear the drawbacks major flexibility therefore of machinability already mentioned for the crepe papers. Under these conditions, this solution therefore also has its limits for the energy at break. It is also very important to underline that the mechanical action of extensibility, favorable on the elongation properties of the paper, nevertheless has a very negative effect on the tensile strength of the paper, of the order of 15 to 30%. .

Furthermore, the so-called semi-extensible, extensible or creped krafts according to the last two aforementioned solutions, as also the so-called standard krafts, for making small, medium or large capacity bags, are obtained by using a fibrous suspension mainly composed of unbleached or bleached softwood fibers, with or without a small percentage of hardwood fibers depending on the uses, combined with cellulose waterproofing agents, commonly called "bonding agents", to reduce sensitivity to leaf water. These bonding agents are generally prepared from rosin constituted by different resin acids (in particular abietic, neoabietic and palustriic acids). These are either rosin glues most often saponified with soda, sodium carbonate, potash, ammonia or organic bases, or reinforced rosin emulsions, that is to say composed of modified rosin emulsified in the presence of a dispersing agent, of the resinate type or of a proteinaceous material. However, it turns out that these adhesives must always be used in an acid medium, that is to say at a pH between 4 and 6, and most often between 4 and 5. This acidity is commonly obtained by the use of sulfate aluminum alone or in combination with sulfuric acid. Aluminum sulphate acts as a flocculating and fixing agent for the bonding resin particles on the fibers.

The presence of aluminum ions is also necessary in the prior techniques for precipitating and fixing on the fibers the particles of residual resin contained in the pulp after kraft cooking of the softwoods. Otherwise, these deposits or pitch very significantly disturb the manufacturing conditions: fouling of the canvas, felts, excessive fluffing with wet presses with frequent sheet breaks, on the machine. The addition of alumina sulfate which facilitates the production conditions is detrimental to the overall mechanical characteristics of the paper under the operating conditions according to the invention.

To fix the ideas well it is recalled that in general the formation of the sheet on paper machine is favored by a high rate of refining of the fibers (it is necessary to have a degree SR higher than 35, and preferably a degree SR greater than or equal to 50). On the other hand, when one uses weakly refined cellulosic fibers (SR degree less than or equal to 35), one encounters difficulties: (i) the weakly refined fibers include tackifying substances which tend to deposit on the pressing rolls, this circumstance resulting in intensive linting and consequently significant risks of breakage, and (ii) if, in an alkaline medium, the high rate of refining contributes to good adhesion or internal cohesion of the sheet, on the other hand a low rate of refining is unfavorable for internal membership.

These difficulties explain why in the techniques known as "bonding in a neutral medium" (at a pH between 6.5 and 8.5) described in DE-A-3 000 367, FR-A-1 005 346 and FR-A- 1 218 904 very refined fibers (SR degree of 50) are used to avoid breakage and promote internal cohesion.

We also know from DE-A-2 809 422 a process for preparing wrapping paper according to which 0.01 to 3% by weight, relative to the weight, is incorporated before or at the head box final dry paper, of a composition comprising 2 to 15% by weight of a non-stick agent and 98 to 85% by weight of a synthetic rubber or a synthetic resin. Now it turns out that for the reasons given above, this process is carried out using highly refined fibers (SR degree of 68-70). According to the invention, a total technique is implemented different. To achieve energy savings, weakly refined fibers are used and, to compensate for the drawbacks associated with the use of these fibers, it is recommended to add a certain number of ingredients according to specific methods. This technique, which is particularly advantageous for so-called integrated stationeries which continuously manufacture pulp and paper, allows the preparation of kraft paper for the production of packaging bags.

According to the invention, a new technical solution is proposed which overcomes the aforementioned drawbacks and offers the advantage of leading to fibrous sheets having improved mechanical properties such as in particular impact resistance, energy at break, and the machinability, on the one hand, and savings in energy and raw materials, such as fibers and glue to laminate the fibrous sheets when making bags, on the other hand.

As an indication, according to the invention, the saving in cellulose, linked to the reduction in the number of sheets and / or in the grammage of the paper making up the bag, can be between 5 and 35%, and in particular between 10 and 15%; for adhesives, this saving can be of the order of 10 to 20%.

The process for preparing a fibrous sheet according to the invention by papermaking techniques from fibers, binder and flocculant, useful in particular in the field of packaging and in particular in that of paper bags, is characterized in an aqueous suspension is prepared having a pH of between 6.2 and 9.5 and containing 100 parts by dry weight of fibers, 0.01 to 1 part by dry weight of organic cationic flocculant, 0.2 to 5 parts by dry weight of organic polymeric binder and 0.01 to 1 part by dry weight of agent which reduces linting and adhesion on the wet presses of the sheet during its formation, introduces this suspension into the headbox, and forms a sheet on a paper machine which is pressed and dried.

It is important that in the head circuits and the headbox the pH is substantially neutral or alkaline. PH to be used will be between 6.2 and 9.5, advantageously between 6.7 and 8.5, and preferably greater than 7 and less than or equal to 8.5.

The quantity of agent which reduces linting and the adhesion of the sheet to wet presses will advantageously be between 0.01 to 1 part by dry weight per 100 parts by dry weight of fibers.

All cellulosic fibers are suitable, such as chemical, semi-chemical, mechanical, mechanical-chemical, softwood, hardwood, annual plant fibers, old paper recovery fibers and their mixtures. It is preferable to use unbleached, semi-bleached or bleached cellulosic fibers obtained by the chemical sulphate process called the kraft process. These fibers will be particularly chosen from kraft softwood fibers alone or in combination with kraft hardwood fibers. For certain applications, it will be possible to combine kraft softwood fibers, mixed where appropriate with kraft hardwood fibers, fibers originating from the recovery of old paper or else with fibers obtained by manufacturing processes different from the kraft process. It is also possible to mix the cellulosic fibers with synthetic organic fibers (polyamides, polyesters, polyalkylenes such as polyethylene or polypropylene) and / or mineral fibers (glass, calcium sulphate, rock wool). The preferred fibers will advantageously be chosen from the group consisting of

A) weakly refined cellulosic fibers, having a degree

SR between 16 and 35, and belonging to the set of unbleached kraft softwood fibers, bleached kraft softwood fibers and their mixtures; B) the associations of weakly refined cellulosic fibers, having a degree SR of between 16 and 35, comprising on the one hand, 60 to 90% by weight of fibers belonging to the set of unbleached kraft softwood fibers, softwood fibers bleached kraft and their blends, and on the other hand, 40 to 10% by weight of unbleached kraft hardwood fibers, bleached kraft hardwood fibers and and their blends;

C) fiber associations comprising, on the one hand, 40 to 90% by weight of weakly refined cellulosic fibers, having an SR degree between 16 and 35 and belonging to the set of unbleached kraft softwood fibers, bleached kraft softwood and their mixtures, and on the other hand, 60 to 10% by weight of recovered waste paper;

D) fiber associations comprising, on the one hand, 30 to 90% by weight of weakly refined cellulosic fibers, having a degree SR of between 16 and 35, and belonging to the set of unbleached kraft softwood fibers, fibers bleached kraft softwoods and their mixtures, and on the other hand, 70 to 10% by weight of fibers from the manufacture of cellulose pulps different from those of the kraft process (in particular fibers obtained by the bisulfite, mechanical-chemical or mechanical processes softwood or hardwood); and

E) recovery fibers from old paper.

Furthermore, if non-cellulosic fibers are used in combination with weakly refined cellulosic fibers, the weight ratio of non-cellulosic fibers to cellulosic fibers will be less than or equal to 0.1.

It is important that the cellulosic fibers are refined with care (that is to say with refining powers per tonne produced about 20 to 50% lower than those of conventional processes; i.e. for example 125 to 350 kWh -de preferably 125 to 250 kWh according to the invention instead of 250 to 450 kWh according to conventional techniques per tonne of paper produced for an SR degree between 16 and 35).

The flocculating agent will be chosen from cationic organic flocculants such as resins of the cationic synthetic organic products type such as polyamide-epichlorohydrin resins, polyamide-polyamine-epichlorohydrin resins, polyamine resins such as polyethyleneimine, polyethylene- modified imine, polypropylene-polyamide resins, glyoxal, quaternary ammonium derivatives such as chlorohydroxypropyltrimethyl-ammonium, modified polyacrylamides. The polyethylene-imine, polyamide-amine and crosslinked polyalkylamine resins are used in aqueous solution and at variable concentrations generally between 5 and 30% w / v. The amount of flocculating agent to be used will advantageously be between 0.05 to 1 part by dry weight per 100 parts by dry weight of fibers.

Suitable binders are those commonly used in the paper industry such as native straight or branched chain starches, or chemically, enzymatically or thermally modified starches, dextrins, polyvinyl alcohols, casein, animal glue, vegetable proteins, cellulose esters such as carboxymethyl cellulose, alginates, dispersions of synthetic polymers on the market presented in the form of aqueous dispersions of 300 to 600 g / l, containing:

- either 87 to 90 parts by weight of ethyl acrylate unit, 1 to 8 parts by weight of acrylonitrile unit, 1 to 6 parts by weight of N - methylolacrylamide unit and 1 to 6 parts by weight of acrylic acid unit, - either 60 to 75 parts by weight of ethyl acrylate unit, 5 to 15 parts by weight of acrylonitrile unit, 10 to 20 parts by weight of butyl acrylate unit, 1 to 6 parts by weight of N-methylolacrylamide unit,

either 60 to 65 parts by weight of butadiene unit, 35 to 40 parts by weight of acrylonitrile unit and 1 to 7 parts by weight of methacrylic acid unit, - either 38 to 50 parts by weight of styrene unit, 47 to 59 parts by weight of butadiene unit and 1 to 6 parts by weight of methylacrylamide unit,

- ie 53 to 65 parts by weight of styrene unit, 32 to 44 parts by weight of butadiene unit and 1 to 6 parts by weight of methylacrylamide unit. Native starches with a linear or branched chain are preferably chosen (the linear fraction or amylose having glucose units linked together by alpha 1 - 4 bonds, the degree of polymerization possibly being between 100 and 3000; the branched fraction or amylopectin having alpha 1 - 4 and alpha 1-6 bonds and a degree of polymerization between 200 and 2000), chemically modified and cold soluble starches and aqueous dispersions of synthetic polymer, such as latex, styrene- carboxylated or non-carboxylated butadiene and acrylic latexes.

The amount of binder to be used will advantageously be between 0.2 to 5 parts by dry weight per 100 parts by weight of fibers.

The agent which reduces linting and adhesion on wet presses is chosen in particular from the group consisting of (i) amino fatty acids, (ii) amino fatty acids condensed with at least one polyfunctional compound (such as in particular l epichlorohydrin, polyamines and mixtures or precondensates of the epichlorohydrin-diamine or alkylene diamine oxide where the alkylene oxide is ethylene or propylene oxide), (iii) fabric softeners and (iv) their mixtures.

In addition to the fibers, the binder, the flocculant and the agent reducing the adhesion of the sheet to wet presses, it is possible to incorporate, if necessary, various conventional additives in stationery such as those listed below.

1. A bonding agent in neutral medium also called water repellant to reduce the water sensitivity of the sheet. The water repellents are solutions of dimeric alkyl ketene at weight / volume concentrations between 5 and 12%, mixtures of ammonium salt of a copolymer of styrene and maleic anhydride (50:50) and a copolymer of acrylonitrile and acrylic acid in solution or dispersion at 20 - 60% (weight / volume), the ammonium salts of a copolymer of diisobutylene, maleic anhydride and maleic acid, in solution or dispersion at 20 - 60% (weight / volume), the ammonium salts of a copolymer of styrene, acrylic acid and maleic acid, in solution or dispersion at 20 - 60% (weight / volume), emulsions paraffin - wax at 30 - 50%. The amounts of water repellents are obviously variable depending on the nature of the products used and the quality objectives targeted, but, as a general rule, this amount varies from 0.05 to 2 parts by dry weight per 100 parts by weight of fibers.

2. Optionally a coloring and tinting agent.

3. Optionally an anti-foaming agent.

4. Possibly fungicides or bactericides.

5. Possibly one or more special additives to give the material certain specific properties such as:

- greasiness.

_ plasticity (with a plasticizing agent)

- resistance to insect attack

- anti-slip - anti-grip

6. Possibly a conventional mineral filler such as talc, kaolin, CaO or CaCO ₃ .

In practical terms, in order to obtain on the one hand the improved mechanical properties, and on the other hand to achieve savings in energy and raw materials, it is recommended to proceed as follows, at a pH of between 6.2 and 9, 5 and advantageously between 6.7 and 8.5: a) an aqueous suspension is prepared containing 20 to 100 g / l of fibers chosen from the group consisting of the fibers AE above. b) 0.05 to 1 part by dry weight of an organic cationic flocculant per 100 parts by weight of fibers is introduced into the aqueous suspension thus obtained, the resulting aqueous medium having a dry matter content of 20 to 80 g / l c) 0.2 to 5 parts by dry weight of organic polymeric binder per 100 parts by weight of fibers are introduced into the resulting aqueous medium, the aqueous medium thus obtained having a dry matter content of 15 to 50 g / l; d) 0.01 to 1 part by dry weight of lint and adhesion reducing agent is introduced into the resulting aqueous medium on the presses wet of the sheet during its formation, per 100 parts by weight of fibers, the resulting aqueous medium having a dry matter content of 2 to 30 g / 1; e) a sheet is formed which is pressed in the wet part by means of a pressing device having a press coating of hardness greater than or equal to 60 ° Shore, and dry.

If necessary, a mineral filler may be incorporated into the aqueous suspension of stage a) which contains the fibers, the mineral filler / fibers weight ratio being less than or equal to 0.1.

The neutralizing agent can be introduced either after stage c) and before stage d), or again after stage b) and before stage c). The fibrous sheet obtained according to the method of the invention can advantageously be subjected to a mechanical extensibility treatment in the machine direction, intervening when it is still wet.

This extensibility treatment can be carried out by (i) creping or (ii) by compacting between an elastic band and a heating roller.

The contact times are as follows:

- for the flocculant - at stage b) - from 15 seconds to 30 minutes and in particular from 15 seconds to 10 minutes, - for the binder - at stage c) from 15 seconds to 30 minutes and in particular from 15 seconds to 10 minutes,

- for the agent that reduces linting and adhesion on wet presses - in stage c) - from 15 seconds to 20 minutes and in particular from 15 seconds to 10 minutes, and - for the bonding agent from 15 seconds to 20 minutes and in particular from 15 seconds to 10 minutes.

When operating continuously, the contact times are generally between 15 seconds and 2 minutes. The best mode for implementing the method according to the invention is as follows:

1.- The fibers in aqueous suspension, coming either from defibration in a pulper (non-integrated factory), or directly from the pulp factory (integrated factory), are stored with stirring in a vat.

The pulp is refined to a conventional paper concentration in a combined chain of disc refiners - conical refiners, at a refining power 20 to 50% lower than that commonly used in a known manufacturing process of unbleached or bleached kraft for the manufacture of bags. The desired degree of refining of the dough, expressed in Schoepper-Riegler degrees, is between 16 and 35 and in particular between 17 and 25. At this stage, the fibrous suspension will have a pH between 6.2 and 9.5 and a concentration from 20 to 100 g / 1.

2.- The flocculant is incorporated batchwise or continuously into the fibrous suspension. At this stage, the resulting suspension has a concentration of between 20 and 80 g / l and in particular between 20 and 60 g / l. This flocculating agent is used in aqueous solution at a dose of between 0.05 and 1 part by dry weight, preferably between 0.05 and 0.8 part by dry weight per 100 parts by weight of fibers. The exact amount to use depends on:

- the concentration of the synthetic organic product in aqueous solution and its cationic power;

- the amount of organic binder; and

- the final properties required for the sheet material and in particular its wet strength characteristics. 3.- The organic binder, preferably continuous, is incorporated continuously into the resulting suspension containing the fibers and the flocculant if it is a synthetic polymer presented in the form of an aqueous dispersion of 300 to 600 g / 1 concentration or after cooking if it is especially native or modified starches. The latter are generally sold in the form of aqueous compositions of 20 to 200 g / l.

At this stage, the resulting suspension has a concentration of between 15 and 50 g / l and usually between 15 and 30 g / l. The binder will be used in an amount between 0.2 and 5 parts by dry weight and in particular between 0.2 and 3 parts by dry weight per 100 parts by weight of fibers. 4.- The other auxiliary additives for obtaining the final properties of the sheet material, such as the bonding agent in particular, can be added either to the storage tank for the refined paste, or continuously in the head. 5.- The agent reducing adhesion on wet presses, previously diluted from 1 to 10 times, is introduced continuously (by metering pump) into the head circuits of the paper machine, the resulting suspension having a concentration included between 2 and 30 g / 1 and in particular between 2 and 20 g / 1.

The amount of agent reducing adhesion on wet presses can be between 0.01 and 1 part by dry weight per 100 parts by weight of fibers and in particular between 0.01 and 0.5 part by dry weight per 100 parts by weight of fibers. 6.- The resulting suspension which is at a pH between 6.2 and 9.5 is sent to the table of the paper machine for the sheet setting of the material respecting certain conditions for wet pressing, partial printing wet and in the dryer. It is preferable to choose wet press sections with suitable coatings and a minimum hardness of 60 ° Shore.

The dry and wet prints will be as low as possible to maximize the elasticity and energy at break of the sheet.

The fibrous sheet obtained according to the invention has improved mechanical properties and is characterized in that the pH of its aqueous extract according to French standard NF Q03-005 (according to which one measures, after extraction of a 2g sample for 1 h with 100 ml of distilled or deionized water having a conductivity less than or equal to 0.1 mS / m, the pH of the extract at a temperature of 20 - 25 ° C) is higher than 7.

The fibrous sheet according to the invention can be manufactured on a conventional table-top paper machine, with one or more jets, horizontal, vertical or inclined for the production of unbleached or bleached krafts intended for the packaging industry and in particular for manufacturers of small, medium and large capacity bags. These paper machines are generally equipped with drying cylinders.

The material which is the subject of the invention can also be used on a machine equipped with a tunnel for airborne drying of the sheet, in order to further promote the mechanical properties of the material, but, as already mentioned previously, these conditions manufacturing will be much less interesting economically, given the high energy consumption required for airborne drying and this compared to traditional drying technologies on cylinders.

According to the invention, the fibrous sheet can be obtained on a paper machine with conventional dryers and, preferably, associated with a known mechanical process for the improvement of elongation in the machine direction and of the energy at break such as creping or semi-stretch or stretch kraft manufacturing systems of the aforementioned "CLUPAK" type. In fact, the process which is the subject of the invention has the major advantage of eliminating the drawbacks mentioned above of extensibility techniques in the machine direction of a paper, while benefiting from the known advantages of these systems. This association therefore makes it possible to obtain an unbleached or bleached sheet material, endowed with mechanical characteristics and machinability or processing properties clearly higher than those of known materials intended for the packaging industry and in particular for manufacturers. of small, medium and large capacity bags.

The sheet according to the invention can be used alone or in combination with other substances such as cellulose or its derivatives, plastic, aluminum. It can be coated, by conventional means, with products in an aqueous medium, solvent or hot-melt based on organic, vegetable, animal, synthetic binder, combined or not with usual additives in the processing industry such as fillers. mineral and / or organic, plasticizers, dyes, antioxidants, lubricants, special resins, etc.

The sheet according to the invention can also be, as required, tarred, coated with foam, laminated, extruded, smoothed, calendered, grained, rubbed, glazed, glossy.

The material according to the invention can also lend itself to surfacing and / or coating operations on a paper machine or outside of a paper machine, with conventional stationery means, such as Size-Press, Champion doctor blade, bill-blade, the trailing blade, the air knife, to impart or improve certain specific properties of the material, such as:

- greasiness

- water repellency

- resistance to insect attack - rot-proof properties

- fire resistance

- anti-slip

- increased resistance to bending

- barrier properties vis-à-vis certain chemicals or solvents.

The material can also lend itself to the usual methods of printing one or more colors, such as flexography, offset or rotogravure.

In the more specific context of making bags, the sheet according to the invention has good aptitude for folding, creasing, gluing, sewing and punching operations. It can also receive different surfactants, such as colloidal silica derivatives in aqueous emulsion, to increase the anti-slip properties of the final bag and promote palletization. The reduction in the number of folds of a small, medium or large capacity bag, linked to the advantageous mechanical properties of the sheet according to the invention, is also very favorable to the rapid deaeration of the bags after bagging, therefore to the palletizing and loading before shipment.

Other advantages and characteristics will be better understood on reading which will follow from nonlimiting examples given by way of illustration.

For comparison, kraft packaging was prepared in a first series (examples A-1 to A-8) according to known and conventional methods, with or without mechanical extensibility devices (of the "CLUPAK" type of creping). The materials obtained were tested (the results relating to the mechanical properties are recorded in Table I) and used for the manufacture of small, medium and large capacity bags. - Example A-1

Unbleached kraft softwood fibers are refined in a combined chain of disc refiners - conical refiners. The refining energy is 300 to 350 kWh per tonne of paper produced. The level of fattening of the cellulose, controlled by the Shoepper Riegler degree, is between 16 and 30. A conventional adhesive based on saponified rosin is incorporated into the fibrous suspension, with a concentration of between 20 and 60 g / l. precipitated and fixed on the cellulose with alumina sulfate, At this stage, the suspension, which has a dry matter concentration of between 2 and 20 g / 1 and a pH of between 4.5 and 5, is sent on a conventional paper machine with a flat table, to a horizontal jet (wet pressing, drying cylinders) for the production of krafts for large capacity bags. This machine was not equipped with a sheet extensibility system (of the "CLUPAK" type or crepe blade). The sheet is produced in 70 g / m2.

- Example A-2

According to the methods of Example A-1, a kraft sheet is made having a grammage of the order of 90 g / m2. - Example A-3

According to the methods of Example A-1, a kraft sheet is produced in 100 g / m2.

- Example A-4

The fibrous suspension of Example A-1 contains a conventional urea-formaldehyde resin from the paper mill, at a dose of 0.6 parts by dry weight per 100 parts by weight of cellulose, in order to reinforce the mechanical properties in the state wet.

All the other operating methods are those described in Example A-1. The sheet is produced in 70 g / m2. - Example A-5

The fibrous suspension of Example A-1 contains a urea-formaldehyde resin and a cationic starch incorporated into the mass by conventional stationery techniques and at a dose of 0.6 parts by dry weight and 0.3 parts by weight, respectively. dry weight per 100 parts by weight of cellulose. These additives are used to enhance the dry and wet properties of the sheet. All the other conditions respect those defined for example A-1; the sheet is produced in 70 g / m2.

- Example A-6 The preparation conditions are those of Example A-1 but the sheet of 9D g / m2 is compacted according to the "CLUPAK" technique. The target elongation at break is 4 to 6%.

The sheet is produced in 90 g / m2. - Example A-7

The operating methods are those of Example A-1, but the 90 g / m2 sheet is compacted according to the "CLUPAK" technique. The target elongation at break is 8 - 10%. The sheet is produced in 90 g / m2. - Example A-8

The sheet obtained in Example A-1, in 70 g / m2, is subjected to a polyethylene extrusion (the amount of low density polyethylene provided is approximately 15 g / m2) to improve the moisture barrier properties of the paper.

Also for comparison, a second series (examples A-9 to A-14) was prepared of materials existing on the market and commonly used for making 50 or 25 kg capacity bags in very varied industrial sectors (materials of construction, human or animal food, fertilizers, etc ...), These sheet materials were tested (the results relating to the mechanical properties are recorded in Table II) and used to make bags.

- Example A-9 Extensible unbleached Kraft of 75 g / m2, obtained by a conventional "CLUPAK" technique, the fibers of this paper are only unbleached kraft softwood fibers and this paper contains a resin for reinforcing the resistance to wet state based on urea-formaldehyde. - Example A-10

Stretch kraft prepared according to example A-9 in 90 g / m2

- Example A-11

Unbleached semi-extensible 100 g / m2 kraft, obtained by a classic CLUPAK technique, the fibers of this paper are only unbleached kraft softwood fibers.

- Example A-12

Ecru kraft crepe of 95 g / m2, obtained by a classic papermaking technique. The fibers in this paper are unbleached softwood fibers only.

- Example A-13 Standard bleached Kraft of 75 g / m2. This wrapping paper is made, from bleached softwood fibers, on a conventional flat table paper machine, with a jet, horizontal not equipped with a sheet extensibility system. - Example A-14

A sheet is prepared from old paper recovery fibers on a conventional machine with a flat table, with a jet, horizontal and equipped with a "CLUPAK" extensibility system, for a target grammage of 90 g / m2.

All the products of Examples A-1 to A-14 are obtained from a fibrous suspension in an acid medium, that is to say at a pH between 4 and 6 and, more often, between 4 and 5.

In a third series, fibrous sheets were prepared according to the invention which were tested (the results relating to the mechanical properties are recorded in Table III) and used in the manufacture of paper bags.

- Example 1

Unbleached kraft softwood fibers are refined in a combined chain of disc refiners - conical refiners, of the type of that of example A-1. The refining energy is from 160 kWh to 180 kWh / per ton of paper produced. The fattening level of the cellulose, controlled by the Schoepper-Riegler degree, will be between 16 and 30. At this stage, the fibrous suspension will have a pH between 6, 7 and 9.5. The following ingredients are incorporated into the aqueous suspension which contains 20 to 60 g / l of fiber:

- 0.2 part by dry weight of flocculant (polyamide-polyamine-epichlorohydrin resin in aqueous solution) per 100 parts by weight of fiber; - 1.5 parts by dry weight of binder per 100 parts by weight of fibers, the binder consisting of an aqueous dispersion of a polymer containing 90 parts by weight of ethyl acrylate pattern, 1 to 8 parts by weight of acrylonitrile unit, 1 to 6 parts by weight of N-methylolacrylamide unit and 1 to 6 parts by weight of acrylic acid unit;

- 0.15 part by dry weight of dimeric alkyl ketene per 100 parts by weight of fibers; and

- 0.05 part by dry weight of agent reducing the lint and the adhesion of the sheet on wet presses per 100 parts by weight of fibers, said agent being a derivative of amino stearic acid., (Stearic acid mined and modified with oxyethylenediamine then solubilized with a acetic acid).

At this stage, the resulting aqueous suspension has a pH of between 6.7 and 9.5.

The sheet setting is carried out on a flat table machine with a jet, horizontal and with conventional dryer cylinders without system of sheet extensibility and respecting the conditions defined according to the preferred mode of implementation.

The target grammage of the sheet is 90 g / m2.

- Example 2 A sheet is prepared according to the methods of the example

1, the organic binder being a native starch, the degree of polymerization of which is between 100 and 3000. This binder is used at the rate of 2 parts by dry weight per 100 parts by weight of fibers.

The target grammage of the sheet is 70 g / m2. - Example 3

We proceed according to the operating methods of the example

2, the target grammage of the sheet being 85 g / m2.

- Example 4

The procedure is as in Example 2, the target grammage of the sheet being 90 g / m2.

- Example 5

The procedure is as in Example 2, the target grammage of the sheet being 100 g / m2.

- Example 6 We proceed according to the operating methods of the example

2, the target grammage of the sheet being 120 g / m2.

- Example 7

The procedure according to Example 1 is carried out using a mixture of 80 parts by weight of unbleached kraft softwood fibers and 20 parts by weight of unbleached kraft hardwood fibers (eucalyptus), instead of 100 parts by weight unbleached kraft softwood fibers, the use of short hardwood fibers may be useful in certain applications to improve leaf formation. the target grammage is 90 g / m2. - Example 8 A sheet is prepared according to the methods of the example

1, from a fibrous suspension consisting of a mixture of 80 parts by weight of unbleached kraft softwood fibers and 20 parts by weight of waste paper fibers mainly coming from recycled packaging kraft, the use of old paper fibers may be useful in certain applications, in order to reduce material costs.

The target grammage is 70 g / m2.

- Example 9

A sheet is prepared according to Example 1 but with a fibrous suspension of bleached kraft softwood fibers. The flocculating agent is a modified polyamine-polyethyleneimine resin, used at a rate of 0.4 parts by dry weight per 100 parts by weight of fibers.

The organic binder is a polymer dispersion containing 60 to 75 parts by weight of ethyl acrylate unit, 5 to 15 parts by weight of acrylonitrile unit, 10 to 20 parts by weight of butyl acrylate unit, 1 to 6 parts by weight of N-methylolacrylamide motif. This binder is introduced into the resulting suspension in an amount of 1.5 parts by dry weight per 100 parts by weight of fibers.

The other adjuvants and operating methods are those described in Example 1. The target grammage is 70 g / m2.

- Example 10

A sheet of paper is prepared according to the methods of Example 2 but the sheet is laid out on a machine with a flat table, with a jet, horizontal, with conventional drying cylinders and equipped with a system of extensibility of the "CLUPAK" type paper. The target elongation at break is 5 to 6%.

The sheet is produced in 90 g / m2.

- Example 11 We proceed according to the operating methods of Example 2 but the material is sheeted on a flat table machine, with a horizontal jet, with conventional dryer cylinders and equipped with a "CLUPAK" type paper extensibility system. The target machine direction elongation at break is 6 to 7%.

The sheet is produced in 90 g / m2.

- Example 12

The material according to the invention is prepared according to the methods of Example 11, but the elongation at break in the machine direction concerned is 9-10%.

- Example 13

A fibrous sheet is prepared according to the methods of Example 1, but the target grammage of the sheet is 60 g / m2.

- Example 14 We proceed according to the operating methods of the example

1 but there is incorporated into the fibrous suspension, after the refining operation and before the introduction of the flocculating agent, an anti-grease agent of the ammonium orthophosphate bis [2-Nethyl- (perfluorooctane-1-sulfonamido-ethyl) type ] at a rate of 0.3 parts by dry weight per 100 parts by weight of fibers, to give the sheet resistance to the penetration of fatty substances.

The target grammage of the sheet is 60 g / m2.

- Example 15 A sheet is prepared according to the methods of the example

9 then it is subjected to a treatment on one side on a paper machine to improve its surface properties. The surfacing bath, of the type commonly used in printing-writing, has the following composition: - kaolin: 80 parts by dry weight;

- calcium carbonate: 20 parts by dry weight;

(these mineral fillers being previously dispersed in an alkaline medium (pH 8 - 8.5) and at a concentration of between 500 and 650 g / 1);

- oxidized starch: 10 parts by dry weight (used after cooking at 90 ° C);

- styrene butadiene latex (in aqueous dispersion): 20 parts by dry weight; - lubricant based on calcium stearate; 1 part by dry weight.

The amount of dry matter deposited on one side is of the order of 10 to 12 g / m2.

The sheet thus obtained is slightly smoothed (40 kg / cm) at the end of the paper machine. The final grammage of the targeted sheet is 75 g / m2.

- Example 16

A sheet is prepared according to the operating methods of Example 10 from recovery fibers from old paper instead of unbleached kraft softwood fibers, for a target grammage of 90 g / m2.

- Example 16a

A sheet is prepared according to the operating methods given in Example 2 from a fibrous composition comprising 70 parts by weight of unbleached kraft softwood fibers and 30 parts by weight of old paper fibers containing long fibers, short fibers and a mineral filler (talc), all the other ingredients being those referred to in Example 2. The sheet is produced on a paper machine under a compacting device, for a targeted grammage of 70 g / m2. the ash rate of this sheet is between 5 and 8% by weight relative to the weight of said dried sheet (this ash rate corresponds to a mineral filler-fiber weight ratio of less than 0.1). - Example 17 to 19

A sheet is prepared according to the operating methods of Example 10 from fibers B, C and respectively D instead of unbleached kraft softwood fibers, for a target grammage of 90 g / m2.

Without departing from the scope of the invention, by following the operating methods described above, in particular in Example 10, it is possible to obtain lower grammages, of the order of 40 to 60 g / m2, or higher, of the order of 100 to 200 g / m2, or even more than 200 g / m2.

Furthermore, the anti-flaking and anti-adherent agent used in Examples 1-16, 16 bis and 17-19 can be replaced by an amino fatty acid derivative (where the fatty acid group comprises from 12 to 24 carbon atoms), by example a lubricant such as ammonium stearate, stearoyl - and lauroyl - aminopropyleneamines (oxyethylenated, if necessary, with 2 to 22 and in particular 7 molecules of ethylene oxide for 1 molecule of amine) and aminopropyleneamino acids - and amino - stearic, - oleic and - lauric, if necessary oxyethylenated with 2 to 22 molecules of ethylene oxide.

The results of the comparative tests given in Tables I, II and III with the previous products (A-1 to A-14), and with the products according to the invention (Ex 1 to Ex 16), were obtained after conditioning at 65% RH and 20 ° C and under the following conditions:

- Air permeability

The air volume in cm3 passing through 1 cm2 of paper in 1 second under a pressure difference of 1 millibar (NF Q 03-001) is the absolute porosity. AFNOR porosity is equal to the product of absolute porosity by the grammage of the paper expressed in g / m2.

- The breaking load

The breaking load (determined according to NF Q 03-004) is expressed as the breaking length according to the relationship

or

RT = average tensile strength expressed in newtons for a 15 mm test piece, L = Average breaking length expressed in m, G = Weight of the sheet expressed in g / m2.

- Burst resistance

The bursting strength is expressed in kilopascals and is determined according to standards NF Q 03-053 and NF Q 03-054.

- Tear resistance The tear resistance is expressed in millinewtons and is determined according to standard NF Q 03-011.

- Dynamic perforation

The test piece is subjected to the shock of an indenter (standard NF Q 03-034) attached to a pendulum whose drop height corresponds to the known potential energy.

The work absorbed by the perforation of the material to be tested is expressed in decijoule (dJ).

- Kodak rigidity

The specimen embedded at one end is caused by adjusting its free length to vibrate in resonance at a known frequency. The stiffness is calculated from the mass su m2 and the free resonance length, according to the equation:

where R ≈ Kodak stiffness expressed in 10 ^-3 mN,

L = Length in mm,

M = grammage in g / m2

The findings which result from the study and comparison of the properties of the products in Tables I, II and III have been given below.

COMMENTS ON TABLE I

- The comparison of Example A-1 with Examples A-4 and A-5 shows that the incorporation into the mass of a urea-formaldehyde resin with or without cationic starch in an acid medium plays mainly on the improvement of the properties in the wet state (+ 60 to 70%), on the other hand, the tensile strengths, the bursting strength and the energies at break only increase by 5 to 7%, 3 to 7% respectively and 7 to 15%. Under these conditions, the tear strength is affected by 20% on average. The sheet obtained is more fragile when folded.

- The comparison of Example A-2 with Examples A-6 and A-7 shows, as mentioned above, that the technique of extensibility of the sheet of the "CLUPAK" type allows an improvement in the energy at break of 50% for an elongation in the running direction of 5 - 6% and 70% for an elongation in the running direction of 8 - 9%, but the tensile strength can drop by 25 - 30%. also that excessive compaction of the sheet (elongation in the machine direction greater than 8%) has an unfavorable influence on the machinability properties of the finished material COMMENTS CONCERNING TABLE II

- The properties measured on different materials, used mainly for making small, medium and large capacity bags, further highlight (example A-9, A-10 and A-11) the advantage of the "CLUPAK" system for energy at break but the tensile strength levels, measured on these papers, remain relatively low (break length - average running direction and cross direction - from 5000 to 5500 m).

As soon as this technology, very conventional on paper machines which produce unbleached or bleached krafts for bags, is combined with a treatment in the cellulosic mass of the urea - or melamine-formaldehyde type in an acid medium, to improve the properties at wet state of the sheet, the tear resistance is degraded by 10 - 15% while the energy at break and the length of break remain substantially equivalent (comparison of examples A-6 and A-7 with example A -10). - Example A-12 highlights the favorable action of creping on energy at break and tear but this technique is not at all effective for tensile strength and stiffness. COMMENTS ON TABLE III

The results recorded in Table III demonstrate the surprising and unexpected nature of the mechanical properties exhibited by the products obtained according to the invention compared with the previous embodiments. - If, first, we compare the mechanical properties of the sheets obtained in 70, 90 and, respectively, 100 g / m2 according to the method of the invention (examples 2, 4 and, respectively, 5) on a non-paper machine equipped with an extensibility system, with those of the neighboring sheets produced according to a prior conventional technique (products A-1, A-2 and, respectively, A-3) also on paper machine not equipped with a system of extensibility, the improvements made by the invention and given in Table IV below are observed.

- If, secondly, the mechanical properties of the sheets obtained according to the process of the invention on a paper machine not equipped with an extensibility system are compared with those of standard sheets prepared in an acid medium according to a prior art commonly used by manufacturers of kraft packaging on a paper machine equipped with an extensibility device, the improvements made by the invention and given in Table V below are observed.

- If, thirdly, we compare the mechanical properties of the sheets obtained according to the process of the invention on a paper machine with extensibility device, with those of standard sheets prepared according to an earlier technique commonly used by manufacturers of krafts d packaging on a paper machine also equipped with an extensibility device, we observe, in particular for sheets of 90 g / m2 as indicated in Table VI, the significant improvements conferred by the invention.

The application examples which follow illustrate in particular the savings in materials made with the products of the invention. Examples of applications

These examples were chosen in a packaging sector where the invention has very important applications: paper bags.

The bags are classified into 3 main categories.

EXAMPLE 20 - Small capacity bags

1 kg flour bag

- The standard bag is generally made from bleached I kraft of 75 g / m2 (according to example A-13).

- The new bag will consist of 1 bleached kraft obtained according to the method of the invention in 70 g / m2 (according to Example 9).

Cellulose savings: 7 - 8%. EXAMPLE 21 - Medium capacity bags

5 kg S.O.S. bag for animal feed

- The standard bag is generally made from:. of bleached kraft I of 75 g / m2 (according to example A-13) and. 70 g / m2 ecru kraft I (according to example A-1). - The new bag is made up of:. 1 bleached kraft of 70 g / m ² (according to example 9) and. 1 unbleached kraft of 60 g / m ² (according to example 13). obtained by the process which is the subject of the invention. Cellulose savings> 10 - 11%.

The area of large capacity bags which is very important has been the subject of a more in-depth comparative study.

EXAMPLE 22 - 50 kg cement bag with valve

. The standard bag consists of 2 ecru semi-extensible krafts in 100 g / m ² , of the type of that of example A - 11.. The new bag is made from 2 unbleached krafts obtained according to example 4. Cellulose saving: 10 to 12% The light bags made with the fibrous sheet according to the invention are very efficient on a fast bagging machine, for example a 4 turbine nozzles (25 to 40 bags per minute). The breakage rate recorded during bagging and during various handling operations: falls on conveyor belts, palletization, loading, etc., is less than 3 per 1000 bags while the usual rate is 5 to 8 for 1,000 bags.

The new bags are more rigid, which facilitates the opening of the valve, therefore the introduction of the bag into the spout of the bagger. For this application, it would also be possible to envisage making the bag with 2 sheets of the type of examples 10, 11 or 12 but in 80 g / m ² .

EXAMPLE 23 - 40 kg plaster bag with valve

. The standard bag is generally made up of 3 stretch krafts of 75 g / m ² of the type of example A - 19.

. The new bag is made, either from 3 ecru krafts of

70 g / m2 for example 2, (cellulose saving: 8%); either from 2 unbleached krafts of 90 g / m2 obtained according to the example

1 (cellulose saving: 14 to 15%). On a turbine bagger, with 4 nozzles and a bagging rate of 1,300 to

1,500 bags per hour. The breakage rates during bagging and handling are respectively as follows:

EXAMPLE 24 50 kg flour sack with open mouth

. The standard bag is generally made up of 3 stretch krafts of 75 g / m ² obtained according to A-9.

. The new bag is made either from 3 unbleached krafts otenus according to Example 2 of 70g / m2 (cellulose saving: 8%), or from 2 unbleached krafts obtained according to Example 10,11 or 12 (economy cellulose: 14 - 15%). On a bagging machine with 6 chutes, with 3 lateral vibrators for tamping (bagging rate of 600 to 800 bags / hour for example), with automatic shaping and sewing, the results, obtained with the new lightened bags, are particularly efficient compared to standard bags obtained with stretch krafts. It should be noted that handling is generally, in large mills, extremely severe.

A bag made up of 3 sheets obtained according to the conditions of Example 12 but with a grammage of 60 g / m ² may be suitable.

EXAMPLE 25 - 50 kg fertilizer bag with open mouth

. The standard bag is generally obtained with 3 ecru krafts of 70 g / m2 (example A-1) and unbleached kraft of 70 g / m2 coated with polyethylene (example A-8).

. The new bag is made with 2 ecru kraft of 70 g / m2 according to example 2, associated with an ecru kraft of 70 g / m2 coated with polyethylene (type example A-8).

Cellulose saving: 25%

EXAMPLE 26 - Large capacity trash bag

. The classic bag is, depending on the uses, either made from ecru I-kraft 100 g / m2 (according to example A-11), or from 2 standard krafts of 70 g / m2 (according to example A-1). . With the fibrous sheets according to the invention, the following combinations are carried out:

.. The 1 sheet 100 g / m2 bag is replaced by a new 90 g / m2 I bag (according to example 8); .. The 2-sheet 70 g / m2 bag is replaced either by a new 1-sheet 120 g / m2 bag (according to example 6) or by a 2-sheet 60 g / m2 bag (according to example 13). Cellulose savings are between 10 and 15%. EXAMPLE 27 - 25 kg open-milk powder packaging bag

. The classic bag is generally obtained from bleached 75 g / m2 I kraft, according to example A-13, and from 2 ecru krafts of 70 g / m2 of the type of that of example A-1, and a polyethylene sheath. . The new bag is obtained from a 75 g / m2 sheet, according to Example 15, from a 70 g / m2 sheet, according to Example 2, and from a polyethylene sheath. The cellulose economy is more than 30%. EXAMPLE 28 10 kg bag of dog food The contents of dog food bags generally have a high fat content (of the order of 10 to 12% by weight). For this use requiring the use of greaseproof unbleached kraft I, the composition of the bag is as follows: - 1 exterior bleached kraft, according to Example 15, combined with 1 internal unbleached kraft of 60 g / m2, according to the example 14.

In this application, the cellulose saving is at least 10%.

In these examples of applications, we have only mentioned the savings in cellulose which are rapidly quantifiable, but it is obvious that a bag made from 2 sheets instead of 3 sheets will consume less glue, will be easier to manufacture (reduction in production costs - less waste and handling). Transport costs will also be lower.

Claims

1. A method of preparing a fibrous sheet by paper techniques from fibers, binder and flocculant, useful in particular in the field of packaging and in particular in that of paper bags, said method, which allows '' improving the mechanical properties such as in particular breaking strength, burst strength, impact resistance, breaking energy and machinability, being characterized in that an aqueous suspension is prepared having a pH of between 6.2 and 9.5 and containing 100 parts by dry weight of fibers, 0.05 to 1 part by dry weight of organic cationic flocculant, 0.2 to 5 parts by dry weight of polymeric organic binder and 0.01 to 1 part by weight dry agent reducing lint and adhesion on the wet presses of the sheet during its formation, introduced this suspension into the headbox, and formed a sheet on a paper machine which was pressed and dried.

2. Method according to claim 1, characterized in that in the head circuits and the head box of the paper machine the aqueous suspension has a pH between 6.7 and 8.5.

3. Method for preparing a sheet of paper useful in the field of packaging and in particular that of paper bags, from cellulose fibers, a flocculant, a binder, a lint reducing agent and the adhesion of the sheet to wet presses during the formation thereof, said method which is intended to improve the mechanical properties such as breaking strength, burst strength, impact resistance, energy to rupture and machinability, and which allows savings of energy and raw materials, being characterized in that, at a pH always between 6.2 and 9.5 and advantageously between 6.7 and 8.5, 1) an aqueous suspension is prepared containing 20 to 100 g / 1 of fibers chosen from the group consisting of:

A) The slightly refined cellulosic fibers, having a SR degree between 16 and 35, and belonging to the set of unbleached kraft softwood fibers, bleached kraft softwood fibers and their mixtures;

B) the associations of weakly refined cellulosic fibers, having a SR degree of between 16 and 35, comprising, on the one hand, 60 to 90% by weight of fibers belonging to the set of unbleached kraft softwood fibers, bleached kraft softwoods and their blends, and, on the other hand, 40 to 10% by weight of fibers belonging to the whole of unbleached kraft hardwood fibers, bleached kraft hardwood fibers and their blends;

C) fiber associations comprising, on the one hand, 40 to 90% by weight of weakly refined cellulosic fibers, having a degree SR of between 16 and 35 and belonging to the set of unbleached kraft softwood fibers, fibers of bleached kraft softwood and their mixtures, and, on the other hand, 60 to 10% by weight of recovered waste paper;

D) fiber associations comprising, on the one hand, 30 to 90% by weight of weakly refined cellulosic fibers, having an SR degree between 16 and 35, and belonging to the set of unbleached kraft softwood fibers, fibers bleached kraft softwood and mixtures thereof, and, on the other hand, 70 to 10% by weight of fibers from the manufacture of cellulose pulp different from that of the kraft process

(in particular fibers obtained by bisulfite, mechanical-chemical, mechanical, softwood or hardwood processes); and

E) recycled fibers from old paper; 2) 0.05 is introduced into the aqueous suspension thus obtained. 1 part by dry weight of an organic cationic flocculant per 100 parts by weight of fibers, the resulting aqueous medium having a dry matter content of 20 to 80 g / 1; 3) 0.2 to 5 parts by dry weight of organic polymeric binder per 100 parts by weight of fibers are introduced into the resulting aqueous medium, the aqueous medium thus obtained having a dry matter content of 15 to 50 g / l; 4) 0.01 to 1 part by dry weight of agent for reducing linting and adhesion to the wet presses of the sheet are introduced into the resulting aqueous medium during its formation, per 100 parts by weight of fibers, the resulting aqueous medium having a dry matter content of 2 to 30 g / 1; 5) a sheet is formed which is pressed in the wet part by means of a pressing device having a press coating of hardness greater than or equal to 60 ° Shore, and dry.

4. Method according to claim 3, characterized in that one introduces after stage 3 °) 0.05 to 2 parts by dry weight of water-repellent agent per 100 parts by weight of fibers.

5. Method according to claim 3, characterized in that one introduces before stage 3 °) 0.05 to 2 parts by dry weight of water-repellent agent per 100 parts by weight of fibers.

6. Method according to any one of claims 1 and 3, characterized in that, after formation of the sheet, it is subjected thereto still wet to an extensibility treatment in the machine direction.

7. Method according to claim 6, characterized in that the extensibility treatment is carried out by compaction between an elastic band and a heating roller.

8. Method according to claim 6, characterized in that the extensibility treatment is carried out by creping.

9. Method according to claim 3, characterized in that successively introduced into an aqueous suspension of 100 parts by weight of fibers chosen from the group consisting of unbleached kraft softwood fibers of degree SR between 16 and 30, waste paper recovery fibers and their mixtures, - 0.3 parts by dry weight of cationic organic flocculant ,

- 2 parts by dry weight of starch,

- 0.15 part by dry weight of dimeric alkyl ketene as a water-repellent agent,

- 0.05 part by weight of agent reducing linting and the adhesion of the sheet to wet presses during its formation.

10. Method according to claim 1, characterized in that non-cellulosic fibers are associated with cellulosic fibers, the weight ratio of non-cellulosic fibers - cellulosic fibers being less than or equal to 0.1.

11. Method according to claim 3, characterized in that a mineral filler is introduced into the aqueous suspension of the fibers of stage 1 °), the weight ratio of mineral filler to fibers being less than or equal to 0.1.

12. Method according to claim 1, characterized in that the agent reducing linting and adhesion on wet presses is chosen from the group consisting of (i) amino fatty acids, (ii) condensed amino fatty acids with at least one polyfunctional compound, (iii) fabric softeners and (iv) mixtures thereof.

13. Sheet of paper useful in the field of packaging and in particular the manufacture of paper bags, characterized in that it is obtained according to the method of any one of claims 1 to 12.

14. Sheet of paper according to claim 13, characterized in that its aqueous extract, obtained by extraction of a 2g sample for 1 hour with 100 ml of distilled or deionized water having a conductivity less than or equal to 0.1 mS / m, has a pH, measured at 20-25 ° C, greater than 7.