AU2014208991B2 - Fibrous materials in sheet form having improved mechanical strength properties, and related method, use, and aqueous compositions - Google Patents

Abstract

The present invention relates to a fibrous material in sheet form corresponding to a sheet of paper or cardboard, optionally in the form of a manufactured item, characterized in that it is treated by impregnating same, deep within the body of said fibrous material, with an aqueous composition including: at least one alkaline metal silicate, and preferably a silicate of Na, K or Li or of a mixture of said alkaline metals; at least one inorganic filler, characterised in that the weight ratio of the inorganic filler to the alkaline silicate in the solids of the composition is 0.25 to 4, preferably 0.3 to 3, and more preferably 0.5 to 1.5. The invention also relates to uses thereof, to methods for treating a fibrous material, and to aqueous compositions that are particularly advantageous for performing such a treatment.

Description

FIBROUS MATERIALS IN SHEET FORM HAVING IMPROVED MECHANICAL STRENGTH PROPERTIES, AND RELATED METHOD, USE AND AQUEOUS COMPOSITIONS.

The present invention relates to the technical field of fibrous materials in sheet form, notably papers and cardboards, and more specifically papers used for making corrugated cardboards. More specifically, the invention proposes the use of aqueous compositions in an industrial method for manufacturing continuously fibrous materials, allowing the manufacturing of fibrous materials in sheet form which are recyclable and repulpable, having improved mechanical strength and more particularly improved compressive strength, a method for producing such fibrous materials in sheet form and particular aqueous compositions. A fibrous material is produced via a wet route, by draining through one or several webs, one or several jets of diluted pulp. The various jets form as many folds which may be assembled in one step or in separate steps.

The wet paper or cardboard sheet is wrung between two rollers for extracting a major portion of the water and for consolidating it before being dried in contact with interiorly heated cylinders with pressurized steam.

An aqueous composition may be sprayed or deposited between the folds or at the surface of the sheet in order to impregnate the surface or all or part of the thickness thereof. This composition has the purpose of binding the folds, reinforcing the mechanical strength of the material or imparting to it barrier or functional properties. A corrugated cardboard is formed by one or most often several sheets of fluted paper glued on one or most often several sheets of plane paper. The paper sheets used may stem from recycled papers (recycled test liner or flute) or virgin papers (kraft liner, semi-chemical fluting). The different sheets are generally glued together under the action of heat, with an adhesive, most often a starchy adhesive.

The corrugated card boards used for the manufacturing of packages intended to be used under strong hygrometry conditions (tropical conditions, refrigerated food packagings...) generally consist of covering papers based on kraft liner, and of flutes produced on the basis of virgin fibers (semi-chemical pulps). The corrugated cardboards are increasingly produced with recycled papers.

The main application of corrugated cardboards is the formation of packages which are often stored and/or used under humid conditions, i.e. in a steam-laden atmosphere. For example, this is the case of packages for fresh products, like fruit, vegetables, fish or further frozen products. Now, the compressive resistance of these packages is strongly degraded under strong hygrometry conditions.

In order to improve the mechanical strength, notably in the wet condition (steam-laden atmosphere), of packages or more generally fibrous materials in sheet form, many treatments have been proposed, either directly in the pulp used for making paper at the pulper or in upstream circuits of the paper-making machine, or by surface treatment. Starches are currently used for reinforcing the mechanical characteristics of papers, most often by treating the starch in a size press. The amounts of starch which may be incorporated into the paper are limited by the too high viscosity when the starch concentration is increased, and consequently it is very difficult to obtain desirable final strength characteristics, otherwise by increasing the basis weight of the papers used, which poses economical problems. Further, because of the hydrophilicity of starch, the mechanical characteristics, and in particular the compressive strength, then strongly drop in a wet medium (steam-laden atmosphere).

Many other surface treatments of the paper by means of chemical additives have also been proposed. A certain number of commercially available agents for insolubilizing starch have been used. Formaldehyde was used in the past, but is no longer used because of restrictive regulatory texts limiting the acceptable amount of formaldehyde vapor to very low levels. Glyoxal is also of a limited use for similar reasons. Metal salts have also been used as insolubilization agents, the most used being zirconium carbonate (AZC).

Surface treatments with synthetic latices and/or waxes have also been proposed. Patents US 3,308,006, US 4,117,199, US 5,658,971, JP 311 856 and US 5,750,237 describe the use of resins and/or waxes and paraffins for improving the strength in a humid condition of papers. Patent US 6,066,379, as for it, describes a surface treatment, forming a water barrier, consisting of a synthetic polymer, of a wax, which may be a paraffin or a polyethylene wax, or of a pigment.

Patent US 6,794,016 also describes a surface treatment of paper, forming a water barrier, consisting of a layer applied on at least one face of a corrugated cardboard, this layer including a synthetic resin in an emulsion and a mixture of pigments. This mixture of pigments contains from 5 to 40% by mass of a pigment having an average grain size from 5 to 15 microns and from 60 to 95% by mass of a pigment having an average grain size of less than 3 microns. The pigments may be inorganic, such as calcium carbonate for example as a precipitate, silica or kaolin; or organic like fine powders of acrylic resin, of benzoguanamine resin or of starch particles.

The solutions based on paraffins or microcrystalline waxes, which are the most widespread, pose recyclability problems which today have become redhibitory. Copolymers based on maleic anhydride (SMA) are also used, but are too expensive for applications in industrial packaging.

The other proposed solution is to manufacture corrugated cardboards by means of paper essentially consisting of virgin fibers (kraft liner and semi-chemical fluting) with high basis weights, which also poses economical problems, in many fields.

Moreover, soluble silicates have found a certain number of applications in the paper industry for producing cartons or other packaging materials. These applications relate to the use of soluble silicates, as adhesives in the manufacturing of paper mandrels or spiraled cardboard barrels.

Soluble silicates have also been used, as a material for impregnating kraft liner papers intended for the manufacturing of corrugated cardboard packages. These techniques generally give satisfaction in most cases, except when these materials treated with soluble silicates are subject to strong humidity, or come into contact with water. When this occurs, the silicates which are hygroscopic salts rehydrate and rapidly lose their adhesive properties and their stiffness. A strong humidity level or the presence of water also generates a migration of the silicates in the bulk of the paper, thereby lowering the mechanical characteristics.

On the other hand, coating methods with soluble silicates are more expensive than the starches currently used for reinforcing mechanical characteristics of papers, which to this day has limited their use. Consequently, in the paper industry, soluble silicates are essentially used for de-inking of old papers and for their adhesive properties in the manufacturing of packaging materials such as cardboard mandrels and barrels.

The following documents however describe the use of alkaline silicates for improving mechanical characteristics of paper: - Patent application WO 89/10448 describes a method which may be exclusively applied at the scale of the laboratory for improving the strength in a humid condition of substrates and papers, in particular by means of a surface treatment with a composition comprising an alkaline silicate, an agent for insolubilizing the silicate and a wax or a paraffin. After drying the composition coated on the paper, the obtained silicate surface film is brittle and contains micro-cracks which alter the water barrier properties of the layer coated on the surface of the paper. The role of the waxes and paraffins used is to fill in these micro-cracks, so as to preserve the continuity of the coated layer and to retain the water barrier properties. A synergistic effect between the silicate and the waxes or paraffins used is mentioned. The insolubilizing agent may be an oxide or a carbonate of a divalent or polyvalent metal, such as zinc, calcium, beryllium, copper, tin, boron or aluminum. In the case of zinc oxide, the intention is to use an amount of zinc oxide from 0.1 to 10% by dry weight based on the dry weight of silicate, by achieving heating so as to obtain a relatively long drying period from 30 to 60 minutes. - Patent application US 2007/0208125 describes a composition for surface treatment of paper which remains recyclable and repulpable after coating.

This composition preferably used for coating corrugated paper is compatible with the use of starchy adhesives currently used for producing corrugated cardboard. The composition described in this document comprises from 10% to 70%, and preferentially from 40 to 60%, by mass of sodium silicate; from 10 to 60%, and preferentially from 30 to 50%, by mass of a synthetic resin as an emulsion; from 10% to 50% of paraffins as an emulsion in water; and from 5 to 25% by mass of water. This composition applied as a coating, preferentially coated on a single face of the corrugated paper, for economical reasons, does not include any insolubilization agents. The masses of deposited compositions are comprised between 3 and 25 g/m2. - Patent US 5,358,554 describes a composition for treating mats of cellulose fibers, such as papers and cardboards, for improving their mechanical strength, hydrophobicity and recyclability, combining a paraffin wax and an alkaline metal silicate solution. This composition is applied as a surface treatment on the paper or cardboard and neither includes any mineral filler, nor agent for insolubilizing silicate. The silicate is present in the composition, in an amount of about 10 to 50% by weight, based on the total level of dry materials, and preferably in an amount of about 25 to 40%, and preferentially from 30 to 35%. - Patent GB 1,423,253 describes a method for preparing paper, flutings or cardboards, into which are introduced at the pulper, before making paper, an alkaline metal silicate and a thixotropic agent and/or a synthetic resin as an emulsion, as well as an acid and/or an acid salt. The method is particularly suitable for making paper pulp comprising a mixture of hemicellulose and of fibers from recycled paper. The addition of silicate is shown as giving the possibility of increasing the mechanical characteristics, while reducing the amount of hemicellulose thereof of higher cost than that of recycled papers. In this document, there is no unique composition, since the different additives are added to the pulp. In order to improve the compressive strength in the dry condition of recycled papers, adding sodium silicate to starch solutions has moreover been proposed, as an agent for gluing recycled papers, as described in the article of Pengje Peng, Xiaofan Zhou & Jinxia MA "Water glass compound starch used as surface sizing agent to improve the strength of linerboard" - Bioresources 6 (4) 4158-4167. However, the treatment with sodium silicate being more expensive than with starch, this route did not seem to have any viable economical interest.

Moreover, in order to reduce the price costs for making papers, introducing less expensive mineral fillers than paper fibers, into packaging papers intended for producing corrugated cardboard, was contemplated on the other hand.

The introduction of mineral fillers into the paper leads to a reduction in the mechanical characteristics and in particular to a reduction in the compressive strength. At a strong concentration, the mineral fillers cause powdering problems on a paper-making machine and on transformation machines. Indeed it is generally recognized that the incorporation of large amounts of mineral fillers into the paper reduces the binding forces between fibers and between fibers and mineral fillers, thereby drastically reducing the mechanical strength of the material. In particular, the article "A new analysis of filler effects on paper strength" (Journal of Pulp and Paper Science Vol. 28 No. 8 August 2002) L. Li, A. Collis and R. Pelton provides the following conclusions: - the mineral fillers reduce the mechanical characteristics of the paper, and certain fillers are more harmful than others; - for a given filler, the fillers having smaller grain size are the most harmful; - the distribution of the fillers is seldom uniform in the thickness of the paper; - the grain size distribution of the fillers in the paper is not identical with the distribution of the fillers of the dispersion of fillers used, because of flocculation problems.

In the article "Developing a new paradigm for linerboard fillers" (TAPPI JOURNAL March 2008), Yulin Zhao, Dongho Kim, David White et at. specify that increasing the amount of mineral fillers reduces the mechanical characteristics of the paper. In the article "The structure and strength of floes of precipitated calcium carbonate induced by various polymers used in papermaking" (14th Fundamental Research Symposium, Oxford September 2009), Roger Gaudreault, Nicolas Di Cesare, Theo G.M van den Ven and David A. Weitz recall and confirm that mineral fillers reduce the mechanical characteristics of the paper by reducing the fiber-fiber binding surfaces.

Several approaches have been explored for increasing the amounts of mineral fillers, unsuccessfully. However, pre-flocculation of the mineral fillers on the fibers of the paper has been tried out, without giving tangible results. Chemically treating the mineral fillers has also been proposed so as to improve the mineral fillers—binder and fibers-mineral fillers bonds so as to avoid weakened areas which alter the mechanical properties of the paper.

Other solutions have been proposed: - Kuboshima K. uses acrylic acid or vinyl acetate for improving the chemical bonds between the polymers and the mineral fillers. "On functional fillers for papermaking" (High Performance Paper Soc.(21) 31-38 (1982); - patent CA 2,037,525 describes a method for improving fibers-mineral fillers bonds by means of epichlorhydrin and polyamino-amide or polyamine; - patent application WO 00/59965 describes sulfonated polymers which improve the bonds between the fibers and the mineral fillers, in particular when they are used with starch; - the use of alkoxysilanes for treating mineral fillers is also known.

All these treatments are however expensive and cannot be contemplated in the manufacturing of recycled papers for use as packagings.

Mention may also be made of documents US 4,376,674 and US 1,676,727 which nevertheless come under a quite different goal, which is to provide fibrous materials and treatment compositions for obtaining resistance to water in its liquid form. Document US 4,376,674 describes a method for making particle panels uninflammable by treating the surface of fibrous panels with an aqueous dispersion of sodium silicates and of calcium carbonate, for which the surface is hard, uninflammable and resistant to liquid water. The invention described in this document relates to a method for making insulating panels of wood fibers having a fire- and water-resistant surface coating. For this, a surface coating is provided; the applied treatment is very thick (243 to 585 g/m2), the reactions conducted at high temperatures (150-200°C) at low flow rates for long drying periods: 45 minutes, in order to obtain the desired characteristics. In column 3, lines 33 to 36, it is specified that too large penetration of the dispersion and insufficient surface retention of the components of the dispersion, are contrary to the goals of patent US 4,376,674. Document US 1,676,727, as for it, describes water-resistant laminated products consisting of several layers joined together during a discontinuous process with long reaction times of an alkaline solution of sodium silicate and of calcium carbonate for 2 to 3 days. This dispersion of calcium carbonate and of sodium silicate is then used as an adhesive, and remains at the surface of the various layers to be bound together, as illustrated in the single figure.

The analysis of all these technologies and of the proposed prior solutions confirms that the problem of treatment of papers or cardboards, or more generally of fibrous materials in sheet form, with view to improving their mechanical characteristics is not solved satisfactorily, under acceptable economical conditions and this in particular in the case of papers consisting of 100% recycled fibers intended for making corrugated cardboards for packages used in a humid environment. Indeed, for these applications, the technical requirements are high in terms of compressive strength, first of all in the dry condition, and for certain applications under strong hygrometry. Further, the treated papers should be able to be assembled with conventional starchy adhesives, be repulpable and recyclable. The treatments with waxes and paraffins of the prior art for example pose redhibitory problems of recycling.

Further, the acceptable economical conditions for the relevant markets are very limited, which excludes a certain number of too expensive technologies, such as treatments outside a paper-making machine, the use of synthetic copolymers, of waxes and paraffins at a high concentration, of mineral fillers surface-treated beforehand.

In this context, disclosed herein is a method and compositions for treating fibrous materials in sheet form, and in particular of paper or cardboards, and fibrous materials of the paper or cardboard type, notably for use as a package, which are acceptable economically and which fit the following criteria: - mechanical strength and more particularly satisfactory compressive strength in the dry condition and, even for at least certain compositions, in humid atmospheres, - capability of adhering with starchy adhesives under standard industrial conditions, - repulpability, - recyclability, - and, at least for certain alternative embodiments, suitability for food contact (dry and non-fatty contact).

The invention proposes compositions for treating a fibrous material in sheet form, in order to improve the mechanical strength thereof in the dry condition, and for some of them, also in the humid condition, preferably by impregnation in a size press and the method for continuously manufacturing the associated fibrous material in sheet form. Within the scope of the invention, by strength in the humid condition, is conventionally meant, the strength under conditions of strong hygrometry, i.e. in a steam-laden atmosphere. The compositions according to the invention are adapted so as to be directly applied onto a sheet of fibrous material already formed in its final shape or during its formation, during a continuous manufacturing method.

The present invention relates to aqueous compositions for treating a fibrous material in sheet form comprising: - at least one alkaline metal silicate, and preferably a silicate of Na, K or Li or of a mixture of these alkaline metals, - at least one mineral filler, characterized in that the mass ratio between the mineral filler and the alkaline silicate in the dry extract of the composition is from 0.25 to 4, and preferably from 0.3 to 3 and preferentially from 0.5 to 1.5.

The present invention relates to aqueous compositions for treating a fibrous material in sheet form comprising: - at least one alkaline metal silicate, and preferably a silicate of Na, K or Li or of a mixture of these alkaline metals, - at least one mineral filler capable of releasing multivalent metal ions which may be substituted for the alkaline ions of the silicate and thus forming precipitates of water-insoluble silicates and, preferably selected from zinc oxide, zinc carbonate, barium carbonate, barium sulfate, calcium sulfate, beryllium carbonate, strontium carbonate and calcium carbonate, the calcium carbonate being preferred for obtaining an improvement in the mechanical properties in the humid condition.

Preferably, the mineral filler used within the scope of the invention, has an average grain size D50 comprised in the range from 20 nm to 20 microns, preferably comprised in the range from 100 nm to 10 microns.

Advantageously, in the preferred aqueous compositions within the scope of the invention, the mass of alkaline silicate(s) and of mineral filler(s) represents from 20 to 100% of the dry extract, preferably from 50 to 100% of the dry extract, and preferentially from 70 to 100% of the dry extract. In certain embodiments of the invention, the aqueous compositions may contain a mass of alkaline silicate(s) and of mineral filler(s) representing from 90 to 100% of the dry extract, preferably from 95 to 100%.

The compositions according to the invention may comprise an agent for insolubilizing silicate, other than the present mineral filler(s). This further allows an improvement in the mechanical strength properties in the humid condition. The agent for insolubilizing the silicate other than a mineral filler may be selected from organic acids, mineral acids, salts of mineral or organic acids, organic or mineral products releasing protons, esters, organic carbonates and multivalent metal salts. In particular, in the dry extract of the compositions according to the invention, the mass ratio between the agent for insolubilizing the silicate other than a mineral filler and the silicate is from 0.01 to 0.1, and preferably from 0.03 to 0.05.

Within the scope of the invention, the alkaline metal silicate is preferably selected from silicates of formula (M20)xSi02 wherein M is Na, K or Li or a mixture of these alkaline metals, and x is the molar ratio between S1O2 and M2O and advantageously belongs to the range from 0.5 to 4. Preferably, the molar ratio x of the alkaline silicate is greater than 2.5 and is preferably greater than 3.

The compositions according to the invention may further comprise a plasticizer. The plasticizer is for example selected from glycerol, sucrose, polyethylene glycols or preferably copolymers as an emulsion such as emulsions of carboxylated styrene butadiene or not, of acrylonitrile butadiene styrene, or acrylic styrene. In particular, in the dry extract of the compositions according to the invention, the mass ratio between the plasticizer and the silicate is from 0.01 to 0.06, and preferably from 0.02 to 0.04.

Advantageously, the compositions according to the invention neither contain any wax or paraffin.

In the compositions according to the invention, the dry extract may represent from 10 to 75% of the total mass of the composition, and preferably from 20 to 50%.

The invention also relates to the use of a composition according to the invention in the manufacturing of a fibrous material in sheet form (in particular, a paper or cardboard), by impregnation in depth into the thickness of said fibrous material in sheet form with said composition, for reinforcing the mechanical strength properties in the dry condition and optionally in the humid condition of the obtained fibrous material. However it should be emphasized that the fibrous materials obtained within the scope of the invention are not intended to be resistant to water in its liquid form as such. In the case when an improvement in the mechanical strength properties in the humid condition is desired, these are properties in humid atmospheric environments, often with significant water absorptions, such as may be found in cold chambers of agro-food circuits.

Within the scope of the invention, the impregnation of the fibrous material with the aqueous composition, is achieved in depth in the thickness of the material. A core impregnation is therefore achieved, preferably extending over the whole thickness of the material. A distribution of the different components of the composition is thereby obtained: selected mineral filler(s) and alkaline silicate(s)... in the whole thickness of the fibrous material. The mineral filler(s) and the alkaline silicate(s), which are present, are thus homogenously distributed in the thickness of the fibrous material and are not only found present at the surface but also in the core of the latter. The invention also relates to a method for treating a fibrous material in sheet form, carried out continuously on at least one of the faces of the fibrous material in sheet form with an aqueous composition according to the invention. In particular, the treatment is carried out on each of the faces of the fibrous material in sheet form and in its bulk. The treatment preferably is integrated to a continuous manufacturing method for a fibrous material in sheet form and is applied on the latter when running, in a finished condition or during design. The treatment preferably is carried out by impregnation, preferentially in a size press. According to the invention, in a totally unexpected way, unlike the traditional methods with a size press, proposed in the prior art, a core treatment with a composition based on a mineral filler does not lead to a weakening of the mechanical strength properties of the fibrous network, but on the contrary reinforces them.

The fibrous material most often within the scope of the invention consists of virgin or recycled cellulose fibers.

The invention also relates to fibrous materials in sheet form treated with one of the aqueous compositions according to the invention, and notably treated for obtaining a deposited dry mass of composition belonging to the range from 3 g/m2 to 35 g/m2, and notably belonging to the range from 8 g/m2 to 25 g/m2, as well as to the fibrous materials in sheet form obtained according to a method defined within the scope of the invention. Such fibrous materials may notably be found as a paper or cardboard sheet, optionally as a manufactured article.

The invention also relates to a corrugated cardboard at least partly consisting of a fibrous material as defined in the invention.

Disclosed herein are corrugated cardboards at least partly consisting of a paper as defined within the scope of the invention.

The disclosure therefore provides an improvement in the mechanical strength of fibrous materials in sheet form, of the paper or cardboard type. Such materials selected from among paper or cardboard sheets are treated in their thickness, by impregnation of the core, with an aqueous composition comprising a mixture, in adequate proportions, of an alkaline metal silicate and of a mineral filler, giving the possibility of improving the compressive strength. Such a treatment may be applied on a size press and is compatible with rapid industrial methods, with short drying periods and producing high throughputs.

Within the scope of the invention, the mineral filler present in a strong concentration, allows reduction in the cost of the fibrous materials obtained with such a composition on the one hand, notably as compared with prior treatments with starch compositions and when the mineral filler is reactive towards the silicate, in order to reinforce the cohesion of the silicates and to insolubilize them in an aqueous solution on the other hand. Indeed, silicates are very sensitive to water, and preferably require the use of insolubilization agents which, within the scope of the invention, are at least partly formed with the mineral filler incorporated into the composition.

Each of the constituents which may be present in the aqueous phase applied by impregnation, and therefore in the fibrous material obtained after treatment with such a composition, will now be described in detail.

Alkaline metal silicates

Alkaline silicates play the role of a binder for the fibers making up the paper and condition the obtaining of good properties in terms of mechanical strength.

Alkaline metal silicates soluble in water, notably of formula (M20)xSi02 wherein M is Na, K, Li or a mixture of these alkaline metals, may be made by melting mixtures in variable proportions of sodium carbonate, potassium carbonate or lithium carbonate and of pure sand. This melting is conventionally carried out in an oven at a temperature of about 1,400°C. After cooling of the casting, a glassy silicate is obtained as a limpid glass. Liquid silicates are obtained by dissolving glassy silicates in water in an autoclave. Such silicates are commercially available, notably as an aqueous solution.

Depending on the molar ratio x, the alkaline metal silicates of formula (M20)xSi02 wherein M is Na, K, Li or a mixture of these alkaline metals, have different properties. Preferably, an alkaline silicate having a molar ratio generally varying from 0.5 to 4 will be used. The alkaline silicates are most often available as an aqueous solution having a dry extract of 25 to 65% by mass. Alkaline silicates having lower molar ratios generally have a higher wetting power, but are more difficult to dry and slow down the production rates; those which have a higher molar ratio have a higher binding power and are easier to dry.

Within the scope of the invention, it was found advantageous to use alkaline metal silicates having a molar ratio of more than 2.5 and preferably of more than 3. Indeed it is this type of silicate which gives the possibility of having the best binding power which allows better cohesion of the fibrous blanket, and which is the most easy to dry, thereby allowing gains in productivity and for applications in a humid medium, better resistance in the humid condition.

All the alkaline metal silicates may be used in the composition of the invention, but sodium silicate being the most common and the most economical, this is the one which is preferred.

Mineral fillers A mineral filler is characterized by quasi-insolubility or very low solubility in water, notably less than 0.1 g/l in water at 20°C. The use of mineral fillers has the purpose of lowering the costs. The currently used mineral fillers in the paper industry are notably: - Alumina trihydrate - Kaolin - CaCC>3 - Mica - Talcum - Montmorillonite - Bentonite - Attapulgite

Among these mineral fillers or pigments, a distinction is made between those which react with silicates, by releasing in an aqueous medium a multivalent metal ion which will be exchanged with the ion of the alkaline metal of the silicate in order to form an insoluble precipitate in water, so-called mineral fillers "reactive" towards silicates (such as sodium silicate), or designated as agents for insolubilization of the silicates which are, notably:

- ZnO - ZnC03 BaC03 BaS04 CaSC>4 BeC03 - SrC03 - CaCC>3, notably finely milled CaCC>3 (GCC) or precipitated CaCC>3 (PCC).

These mineral fillers are quasi-insoluble or very sparingly soluble in water. They will therefore react very slowly with sodium, potassium or lithium silicates present in the composition which are initially insoluble in water, in order to form a precipitate insoluble in water. As the reaction is very slow since it is controlled by the release of multivalent metal ions, no turbidity occurs which may end up with gelling or bulk setting of the composition for at least 24 hours without stirring at 20°C. Most often gelling or bulk setting of the composition occurs between 24 and 72 hours when the composition is left without stirring at 20°C. In the case of the use of a mineral filler of the Zn, Ca, ..., type, the sodium, lithium or potassium of the silicate is substituted with the polyvalent metal ion released by the mineral filler, thereby forming a then insoluble silicate complex. In every case, because of the very small solubility of the mineral filler in an aqueous medium, the kinetics of the reaction are very slow, which makes the use of the composition compatible in a method for industrial treatment of a fibrous material in sheet form.

These mineral fillers are therefore described as reactive with alkaline metal silicates, notably in water and at 20°C, with reaction rates which depend on the temperature, pH, concentration and grain size conditions.

The composition according to the invention includes at least one such mineral filler and in particular a so-called reactive mineral filler, in a relative amount based on the silicates of more than 25% by mass and preferably greater than or equal to 50%. The presence of such an amount of mineral filler allows reduction in the cost of the composition on the one hand, the amount of silicate then being less. On the other hand, the presence of these mineral fillers does not affect the mechanical properties in the dry condition imparted by the presence of silicates and furthermore, the use of so-called reactive mineral fillers allows improvement in the mechanical compressive strength in the humid condition, by insolubilizing the silicates. This is in particular the case when ZnO, zinc carbonate or preferably calcium carbonate is used. The mineral filler(s) present in the composition, such as CaC03, play(s) a dual role of reinforcing cohesion and of insolubilizing the present silicates. Advantageously, within the scope of the invention, the mineral fillers present such as ZnO, Ζη(Χ>3, CaC03, have not been subject to any preliminary coating, for example with alkoxysilanes as this was the case in certain techniques of the prior art.

In a quite unexpected way, it was discovered that the introduction of such amounts of mineral fillers, like milled calcium carbonate with a small grain size, or precipitated calcium carbonate, in association with alkaline silicates did not alter the mechanical characteristics of the papers and allowed retention of the beneficial effect provided by the alkaline metal silicate, considering the mechanical properties of the obtained paper. As such effect was by no means predictable, notably considering the work of Yuhin Zho, Dongho Kim et al. ("Developing a new paradigm for linerboard fillers" TAPPI Journal March 2008 suprei) who noticed that the addition of nonsurface-treated mineral fillers, in covering papers intended for corrugated cardboard packages, in order to lower the price cost cause a significant loss in mechanical characteristics.

The reactive mineral fillers dispersed in water, like ZnO, ZnCC>3, or CaC03 are not very soluble in water and slowly react with silicates. The compositions according to the invention are consequently very stable with a viscosity which does not change very much over time, the gelling reaction being very slow in solution. From among these reactive mineral fillers, CaCC>3 is the preferred one because of its harmlessness, of its great availability and of its low cost and of its optimal insolubilization capabilities at room temperature.

As an example, the composition may comprise, as a mineral filler, calcium carbonate, kaolin or a mixture of both of these mineral fillers.

It was also noticed that by selecting a reactive mineral filler with fine and regular grain size, it was further possible to improve the mechanical characteristics of the papers treated with a composition according to the invention even under strong hygrometry. Indeed, even with a small grain size, the mineral fillers used are not very soluble and gradually release cations over time, which gives the possibility of avoiding gelling of the composition for quite a long time and leads to homogeneity of the insolubilization of the silicates and therefore to more homogeneous papers and therefore with more satisfactory properties.

Also, advantageously, the mineral filler(s), such as notably ZnO, or calcium carbonate, present in the composition according to the invention has (have) a controlled grain size belonging to the range from 20 nm to 20 microns. It was notably found to be advantageous to use finely ground calcium carbonate (GCC) with an average grain size D50 belonging to the range from 0.4 to 10 microns or precipitated calcium carbonate (PCC) with a grain size belonging to the range from 20 to 500 nm. The average grain size D50 may be measured by laser diffraction according to the ISO 13320:2009 standard, notably with a Beckman Coulter LS 13320 apparatus. The average grain size D50 is the value in microns or nm for which 50% by number of the distribution of the diameters of the particles is below this value and 50% by number of the distribution is above this value.

The deflocculation effect, well known for silicates, moreover gives the possibility within the scope of the invention of obtaining very good dispersion of the mineral fillers in the bulk of the treated paper.

The composition according to the invention may also contain mineral fillers or pigments, said to be inert towards silicates, like: - Alumina trihydrate - Kaolin - Mica - Talcum - Montmorillonite - Bentonite - Attapulgite - Quartz, - T1O2, and/or - Fe203.

Advantageously, the silicate (and in particular sodium silicate)/calcium carbonate pair or the silicate (and in particular sodium silicate)/kaolin pair will be used in the compositions according to the invention.

The insolubilizina agents other than a mineral filler

The composition according to the invention may include one or several agents insolubilizing silicates, other than the so-called reactive mineral fillers towards silicates. These insolubilizing agents notably in an aqueous solution react with the silicate so as to form an insoluble precipitate in water and cause precipitation of the silicate according to faster reaction kinetics than mineral fillers: the reaction kinetics are such, that turbidity of the solution would be observed as soon as the first minutes or hours after the mixing of the ingredients if the latter were introduced in amounts as significant as the mineral fillers.

In a known way, alkaline silicates may be made insoluble in water with two types of reaction: a) by a gelling reaction - polymerization by lowering the pH of the silicate solution below a pH of 10.7.

The simplest way for lowering the pH is to use one or several organic or mineral acids, but the reactions are very fast. A large number of other products may also be contemplated, in order to be able to better control the gelling time. These are notably organic or mineral products which release protons, such as salts of organic or mineral acids, esters (reacting by hydrolysis in order to form the corresponding carboxylic acid), certain organic carbonates ....

As examples of insolubilizing agents other than a mineral filler (also designated in the continuation of the description as additional agent for insolubilizing silicate) may be used within the scope of the invention, mention may be made of:

- HCI - H2SO4 - HN03 - triacetin - diacetin - monoacetin - sodium bicarbonate - citric acid - formic acid - acetic acid - propionic acid - NaH2P04 - NH4H2PO4 - C02 - ethylene carbonate - propylene carbonate

When the solutions of silicates are acidified, orthosilicic Si(OH)4 is released, which ends up by polymerizing in order to form a precipitate. b) by precipitation by means of multivalent metal salts in an aqueous solution.

The soluble silicates almost instantaneously react with multivalent metal cations in order to form the corresponding insoluble metal silicate. The metal ions which react with the silicates include Ca2+, Mg2+, Zn2+, Cu2+, Fe3+, Al3+....

As non-limiting examples, mention may be made of the following multivalent metal salts: - CaCI2 - MgS04 - AI2(S04)3 CaS04

The reactions between the alkaline metal silicates and the multivalent metal salts are generally very fast.

Also, the problem posed during their use in treatment compositions on a papermaking machine is to control the gelling time of the compositions when the insolubilizing agents are introduced. The gelling kinetics depends on many parameters: > the nature of the insolubilizing agents, > the molar ratio of the alkaline silicate (notably SiO^Na^ in the case of sodium silicate), given that when the molar ratio increases, the degree of polymerization increases, > the concentration of the various constituents, > the shear rates of the coating process, > the temperature, > the pH.

The insolubilizing agents like CaCI2 or salts of mineral acids in general, cause immediate reaction with the alkaline metal silicates. Such insolubilizing agents if they were used alone could only be applied on paper in a two-step method with introduction of the insolubilizing agents before or after application of the silicates. With such a two-step method, when the silicates would be applied by means of a size press, the method would lead to non-homogeneous papers in the bulk because of the heterogeneity of the insolubilization, which is unsuitable.

This is why in the scope of the invention, the insolubilizing agent for the additional silicate selected from among mineral or organic acids, salts of mineral or organic acids, organic or mineral substances releasing protons, esters, organic carbonates and multivalent metal salts is optional and optionally acts as an addition to mineral fillers.

This additional insolubilizing agent is advantageously present in a small amount. Preferably, the mass percentage of the additional insolubilizing agent based on the silicate in the composition is comprised in the range from 0.01 to 0.1 and preferably in the range from 0.03 to 0.05, so as to control the gelling time. Indeed, the use of reactive mineral fillers such as zinc oxide, zinc carbonate, or calcium carbonate, as a main agent for insolubilizing the silicate, very substantially allows an increase in the gelling times.

The plasticizers

The papers impregnated with silicate(s) gain rigidity and may be brittle depending on the concentration used of silicate(s). In order to increase the flexibility of the paper obtained after treatment with the composition according to the invention, the latter will preferably contain at least one plasticizer.

Plasticizers like glycerol, sucrose, polyethylene glycols may be used. Nevertheless, as these products are very soluble in water, it was found to be advantageous to use copolymers as an emulsion such as: - styrene-butadienes either carboxylated or not, - acrylonitrile-styrene-butadiene, - acrylic styrenes.

Preferably, in the compositions according to the invention, between the plasticizer and the silicate in the dry extract of the composition, the proportion is from 0.01 to 0.06 and preferably from 0.02 to 0.04.

The waxes and paraffins migrate in the silicates and may be difficult to use as plasticizers, since this would pose adhesive bonding problems on the corrugator during the manufacturing of corrugated cardboard. Also, preferably, the composition according to the invention will neither contain any wax, or paraffin.

Without this being a limitation, it may be advantageous to add to an aqueous composition according to the invention: • one or several native natural or transformed binders such as starch (in particular, corn starch, wheat, potato, manioc starch ...), carboxymethyl cellulose, hydroxyethyl cellulose, guar or carob gums, soya or casein and/or • one or several synthetic binders such as more or less hydrolyzed polyvinyl acetates, synthetic latices like styrene-butadiene copolymers, carboxylated styrene-butadiene copolymers, styrene-acrylic copolymers, or styrene-butadiene-acrylic copolymers and/or • one or several water-repellent agents for paper such as silanes, siloxanes, for obtaining fibrous materials resistant to water (liquid) and/or • one or several dispersants such as sodium polyacrylate and/or • one or several biocidal agents and/or • one or several antifoam agents and/or • one or several coloring agents or pigments and/or • one or several antistatic agents and /or • these different additives being conventionally used in the paper-making industry.

The composition according to the invention is prepared, by incorporating the different constituents into water. Preferably, the silicate is introduced into the composition before the mineral filler(s). Advantageously, the composition is subject to stirring by any suitable notably mechanical device so as to obtain a homogeneous mixture.

Within the scope of the invention, it was noticed that unlike what is observed with compositions of starch where the dry extract of the compositions is limited to about 10%, the viscosity of the compositions made with alkaline silicates and mineral fillers remains low even with much higher dry extracts. In particular, the compositions according to the invention may have a dry extract from 10 to 60% and/or a Brookfield viscosity from 10 to 100 mPa.s, notably measured at 50°C with stirring at 100 rpm. The Brookfield viscosity may for example be determined according to the ISO 1652 standard.

With such a viscosity and/or dry extract, it is possible to substantially increase the composition masses deposited on the fibrous supports during impregnation treatments and to gradually increase the mechanical properties, and more particularly the compressive strength.

It was demonstrated, within the scope of the invention, that the combined use of alkaline silicates and of mineral fillers, in particular of reactive mineral fillers, optionally combined with one or several additional insolubilizing agents, as a product for treating paper, as a replacement for the starch treatments presently used, has the following advantages: - improvement in the dry compressive strength by increasing the deposited weights, - in certain cases, improvement in the wet compressive strength, in particular in the case of the use of reactive mineral filler(s) towards silicates, and in particular calcium carbonate, - an economic advantage, - reduction in the consumption of energy.

Further, the compositions according to the invention are compatible for the treatment of papers intended for manufacturing packaging cardboard for food use notably. Indeed, even if certain insolubilizing agents may have a sanitary risk (borax, metaborates, glyoxal, ...), others like citric acid, sodium bicarbonate, NH2PO4... are perfectly harmless at the concentrations of use and will notably be preferred for food applications.

Alkaline silicates and mineral fillers, like CaCC>3, used within the scope of the invention are substances classified as non-hazardous in the sense of European regulations (Regulation CLP 1272-2008); CaCC>3 is exempted from REACH (Article 2) registration. Alkaline silicates and CaCC>3 are substances which may enter the making of materials suitable for dry and non-fatty food contact. They are classified in the GRAS (Generally Recognized As Safe) category by the FDA (Food and Drug Administration).

According to another aspect, the invention relates to the use of an aqueous composition defined within the scope of the invention for producing papers having improved mechanical strength.

Method for applying the composition

The composition according to the invention may be applied on various fibrous supports in sheet form, of the paper or cardboard type, at different stages of their manufacturing process and according to various techniques.

In the wet portion of a paper or cardboard production line, for example in several folds, the composition may be applied between two folds or mixed with the pulp making up one or several jets.

In the dryer section or outside the paper-making machine, the composition may be applied at the surface on one or both faces of the papers and cardboards.

The composition, which is mainly, but not exclusively, intended for treating paper may be directly applied on a paper-making machine, after the wet portion of the machine with any method known in the paper-making industry such as application on the size press, spray coating, with etched rollers or not, by Champion bars, by Mayer bars, by an air gap or by other systems known to the person skilled in the art. The treatment in a size press is preferred since it allows better distribution of the fillers in the bulk of the paper.

The compositions according to the invention, and in particular those including silicates and carbonates, further have lifetimes in liquid form of several days, and a low viscosity at a strong concentration, which makes their use very easy in a treatment method, and allows application of these compositions with a traditional size press.

The composition according to the invention may be applied during a step of a method for making a fibrous material in sheet form, which may notably be a sheet of paper, or on the finished material, i.e. directly after its making. The fibrous material in sheet form is then running and application of the composition is achieved and is followed by a drying step, which may either correspond or not to the last drying step. Rapid running rates are compatible with the application of an aqueous composition comprising an alkaline metal silicate/mineral filler mixture contemplated within the scope of the invention. Most often, another preliminary drying step is present before applying the composition. In particular, the composition will be applied on a sheet of paper having a humidity level from 5 to 14%, preferably from 8 to 10%.

An application through the faces, notably in a size-press, allows homogeneous impregnation of the described compositions in the whole thickness of the fibrous material in sheet form, and in particular of the sheet of paper.

During the application of the composition, the following parameters either alone or as a combination, will preferably be used: - running rate of the sheet from 100 to 1,500 m/min, - temperature of the applied composition from 40 to 70°C, - temperature of the sheet at the instant of application from 80 to 105°C, - after application of the composition, drying at a temperature of less than or equal to 140°C for a period of less than one minute, and preferably belonging to the range from 100 to 120°C.

The fibrous support may be formed with virgin fibres, fibres exclusively stemming from recycled papers and/or cardboards or from a mixture of such fibres. The fibrous support may also contain fibres from annual plants, hemicelluloses, galactomannans, MCC etc.

Although this is not the preferred application within the scope of the invention, the sheet may comprise a small or even no proportion of recycled fibres.

The papers treated with the compositions according to the invention show good capability for repulping and recycling.

The following composition examples are given as an example, and do not limit the scope of the invention described in the claims.

Example 1:

The following concentrated composition was prepared by cold mixing the following substances or preparations by means of a mixer rotating at 1500 rpm.

Water 464 liters

Sodium silicate 340-3840 (Silmaco) 456 kg

Dry extract = 38%

Density =1.38 Molar ratio x=3.4 pH (1%)= 11

Self-crosslinkable carboxylated styrene-butadiene latex VL 10703 (Synthomer) 10 liters

Dry extract = 50%

Tg = 58°C pH = 8.5 ISO 976 Standard

Viscosity: (Brookfield LVF 60 rpm) ISO 1652 standard 200 mPa.s Insolubilizing agent: CaC03 70 kg

The CaC03 used is HYDROCARB 90 (OMYA)

Average diameter D50 0.7 microns, dry extract 75% and density = 1.89 i.e.: 30% by dry weight based on the weight of dry silicate

Total 1,000 kg

Dry extract of the composition = 23% pH = 11 ISO 976 standard

Viscosity of the composition: 30 mPa.s ISO 1652 standard

This composition is very stable, no gelling of the bath is observed for more than 24 hours, and there is no variation of viscosity during this period.

This composition was directly applied on a paper-making machine by means of a traditional size press on a cover paper of 136g/m2 based on 100% of recycled fibers.

Deposited mass = 14 g/m2 dry material

Final paper mass 150 g/m2

Final humidity of the paper: 8%

The speed of the paper-making machine was 500 m/min, the drying period of the composition of about 20 seconds and the temperature of the paper at the outlet of the drier was 105°C.

Alkaline silicates, which retain less water than starches traditionally used, gave the possibility of obtaining a gain in energy of about 10%.

The mechanical characteristics of the paper were measured, and in particular the compressive strength SCT, according to the ISO 9895 standard.

These measurements were conducted after conditioning the specimens at 23°C and with 50% of relative humidity, and after passing in the oven at 25°C and 85% of relative humidity for 24 hours.

The quality of the treatment is evaluated according to the initial measurements of compressive strength for applications in a dry medium and by the ratio of the measurements conducted at 50% of relative humidity and at 85% of relative humidity for applications in a humid medium.

This ratio is expressed as a percentage relatively to the resilience ratio.

These measurements were compared with a paper support of identical quality treated in a gluing press with native cornstarch used alone. A loss of the mechanical characteristics is seen under conditions of strong humidity, less than in the case of the composition containing calcium carbonate. Example 1 which includes a reactive mineral filler towards the silicate which behaves like an agent insolubilizing silicates, has good characteristics in the dry condition and in the humid condition (resilience level of 62%).

Figs. 1 and 2 are microphotographs corresponding to overall views in a Transmission Electron Microscope (TEM) in a composition mode of the transverse section of the paper, with a magnification of x200 and xlOOO respectively. These micrographs show that fillers of very small size are again found in the whole thickness of the paper and in all the areas.

Examples 2 to 3 - Influence of the nature of the insolubilizina aaent

Identical compositions were prepared by replacing the calcium carbonate of example 1 with finely milled zinc oxide ZnO in example 2 according to the invention, with citric acid in comparative example 1, and with kaolin with a grain size of less than 2 microns in example 3 according to the invention, with the same proportion of insolubilizing agents as in example 1 (30% by dry weight based on the dry weight of the silicate), except for the comparative example 1 which only includes 5% of citric acid by weight based on the silicate.

The compositions with zinc oxide (example 2) and kaolin (example 3) are also very stable and no gelling of the silicate is observed for 24 hours. The mechanical characteristics in the dry condition are good in every case. On the other hand, the resilience levels (ratio of the compressive strength in both atmospheres with 85% RH and 50% RH) are better with calcium carbonate and zinc oxide (62 and 60% respectively) than with kaolin. But in every case, the mechanical characteristics in the humid condition are greater than those obtained with starch, but are not optimum in the case of kaolin notably.

The papers treated with the composition with the citric acid of comparative example 1 retain their properties in the humid condition (resilience level of 60%), but the composition used begins to gel after 30 minutes, with a viscosity which rapidly increases, which makes the method very difficult to handle on a paper-making machine. The use of insolubilization agents for sodium silicates such as weak acids or organic esters releasing an acid, such as diacetin or triacetin for example, leads to compositions which rapidly gel, with the gelling times which vary between 10 mins and one hour depending on the concentrations used. In order to be able to use these insolubilizing agents on a paper-making machine under good industrial running conditions, it is necessary to only use these products in low concentrations in order to retain stable viscosity. Unfortunately, under these conditions, their power for insolubilizing the silicates is limited and not sufficient.

These compositions were applied in a size press on a paper-making machine under the same conditions as in example 1, on a paper with the same basis weight.

The treatment with these compositions was compared in terms of compressive strength with a paper of the same basis weight traditionally treated with native corn starch.

The whole of the results are shown in TABLE 1: TABLE 1

These different tests show that alkaline silicates are more hygroscopic than the currently used starches, and preferably require the use of an insolubilizing agent.

The most efficient insolubilizing agents are generally the faster agents and a short gelling time would imply a two-step method since, in the case of fast gelling, the viscosity of the composition used in a size press changes rapidly over time and the method becomes uncontrollable and incompatible with industrial utilization. A two-step treatment for the introduction of the insolubilizing agents before or after application of the silicates leads to heterogeneous insolubilization with loss of the characteristics in the humid condition. The gain in strength in the dry condition is independent of the nature of the insolubilizing agent since the latter only has the function of making the silicate structure insensitive to water.

Only CaC03, ZnO and citric acid have an insolubilization effect on the silicate. With a mass ratio of only 5% of insolubilizing agent based on the silicate, citric acid has an insolubilizing effect, but with a gelling time of about 30 minutes, the method is difficult to handle on a paper-making machine.

Example 4:

The following composition was prepared by mixing the following substances or preparations by means of a mixer rotating at 1,500 rpm.

Water 227 liters

Partly hydrolyzed wheat starch, prepared at a concentration of 17% 294 liters i.e.: 20% by dry weight based on the dry extract of the composition Sodium silicate 340-3840 (Silmaco) 410 kg

Dry extract = 38%

Density = 1.38 Molar ratio x = 3.4 pH (1%) = 11 i.e.: 60% by dry weight based on the dry extract of the composition Insolubilizing agent: CaCC>3 70 kg

The CaC03 used is HYDROCARB 90 (OMYA)

Average diameter D50 0.7 microns, dry extract 75% and density = 1.89 i.e.: 20% by dry weight based on the dry extract of the composition

Total

Dry extract of the composition = 25% pH = 11 ISO 976 standard

Viscosity of the composition: 50 mPa.s ISO 1652 standard

This composition was directly applied on a paper-making machine by means of a traditional size press on fluted paper of 134 g/m2 based on 100% of recycled fibers.

Deposited mass = 16 g/m2 of dry material

Final mass of the paper 150 g/m2

Final humidity of the paper: 8%

The rate of the paper-making machine was 800 m/min, the drying time of the composition of about 10 seconds and the temperature of the paper at the outlet of the drier was 105°C.

The mechanical characteristics of the paper were measured, and in particular the compressive strength SCT, according to the ISO 9895 standard.

These measurements were conducted after conditioning specimens at 23°C and with 50% relative humidity.

These measurements were compared with a paper support of identical quality treated in a size press with native wheat starch used alone.

An improvement in the mechanical characteristics of about 20% in a dry medium, is seen, while simultaneously noticing energy gains of about 15%.

The whole of the results are shown in TABLE 2: TABLE 2

Example 5 - Influence of the grain size of the mineral fillers

Paper treatment compositions according to the invention were prepared with finely milled reactive mineral fillers of very small grain size. These mineral fillers also play the role of an insolubilizing agent for silicates. Different forms of CaCC>3 in an aqueous dispersion (slurry) were studied, and more particularly milled CaCC>3 (GCC) Hydrocarb 90 from Omya with a mean grain size of 0.7 micron, milled CaCC>3 (GCC) Setacarb 85 OG from Omya with an average grain size D50 of 0.4 micron, and precipitated CaC03 of Solvay SOCAL 31 with an average grain size D50 comprised between 50 and 100 nm. A dry CaCOs/sodium silicate ratio of 1/1 was used.

It was noticed that the reactivity depended on the grain size: the finer the grain size and the more the mineral filler is reactive. However Hydrocarb 90, with a grain size less small than the other CaC03, remains the cheapest, therefore this is the one which was studied subsequently.

Examples 6 to 9 - Influence of the CaCOW silicate ratio

The compositions were prepared under the same conditions as in Example 1. TABLE 3 shows the different studied compositions. TABLE 3

These examples correspond to the following weight ratios (dry silicate over dry calcium carbonate). - Comparative Example 3 100% silicate - Comparative Example 4 90% silicate -10% CaC03 (CaC03/silicate mass ratio of 0.11) - Example 6 75% silicate - 25% CaC03 (CaC03/silicate mass ratio of 0.33) - Example 7 50% silicate - 50% CaC03 (CaC03/silicate mass ratio of 1) - Example 8 25% silicate - 75% CaC03 (CaC03/silicate mass ratio of 3).

These compositions were applied on a paper-making machine by means of a size press on a paper consisting of 100% of recycled fibers with a final basis weight of 120 g/m2, and the thereby made papers were compared with a 100% recycled paper of the same basis weight and of the same composition treated with starch.

The whole of the results are shown in TABLE 4: TABLE 4

In Comparative Example 5, a paper of 120g/m2 was treated with a deposited weight of starch of 6 g/m2, and it is industrially difficult to obtain a greater deposited weight, because of the too large viscosity of the starchy solutions at concentrations of more than 12% of dry extract. A starchy solution having a viscosity of more than 60 mPa.s is actually very difficult to use in a size press because of the instability induced from the coating process causing impregnation irregularities and production incidents.

The compositions of silicates and carbonates remain fluid even at a strong concentration (Brookfield viscosity ISO 1652 standard from 30 mPa.s to 25% of dry extract, in examples 6 to 8 for example).

The mechanical characteristics, and in particular the compressive strength, are proportional to the amount of binder (starch or silicate) impregnated in the paper with the size press.

With dry extracts of silicate twice more significant than in the case of examples 6 and 7, than the dry extracts of the starchy compositions of comparative example 5, it is possible to obtain deposited weights of silicates of 13 g/m2 after drying, whereas, in the case of starch, under the same conditions, it is difficult to obtain deposited weights of more than 6 g/m2 with a paper of 120 g/m2.

The compressive strength SCT then passes from 2.1 kN/m (comparative example 5 starch) to 2.7 kN/m (example 7 Silicate-CaC03 50%-50%), i.e. a compressive strength gain of 25% in the dry condition.

In comparative example 6, the starch gives the possibility of obtaining a dry compressive strength (50% RH) of 2.7 kN/m, with a paper of 150 g/m2. The starch substituted in an amount quasi equivalent with an alkaline silicate leads to a paper having the same compressive strength in the dry condition. Moreover, the resilience level of a paper coated with starch is at a level of 57%, while in comparative example 3, with the silicate alone it is possible to attain a resilience level of 54%. The gain in energy noticed for the drying of the papers exclusively treated with silicates does not compensate for the overcost due to the use of silicates, and the strength in a humid condition is not improved in this configuration. Within the scope of the invention, it was noticed that the addition of a mineral filler which allows reduction in the cost of the composition, did not cause, quite unexpectedly, the loss of compression strength of the papers treated with compositions containing alkaline metal silicates, even at significant concentrations of mineral fillers. Moreover, the use of mineral fillers in increasing amounts gave the possibility of observing the following facts: > by gradually increasing the level of reactive mineral fillers from 20 to 50% by dry weight, with reduction of the dry weight of the silicate, no loss of compressive strength properties is observed (Comparative Example 4 and Examples 6 and 7). It is only from a filler level of 75% (Example 8) by dry weight based on the dry weight of silicate + calcium carbonate that a lowering of the compressive strength is observed. The compressive strength is proportional to the weight of binder deposited in the paper (starch or silicate), the mineral fillers strictly having no binding power. Indeed, in the Comparative Example 4, by treating a 100% recycled paper of 120 g/m2 treated with a dry deposited weight of 13 g/m2 and a filler level of 10% based on the dry material silicate + calcium carbonate, the deposited binder weight is of 11.7 g/m2 of binder, while in Example 7 for a same deposited weight, and a filler level of 50% based on the dry material silicate + calcium carbonate, the deposited weight of binder is only 6.5 g/m2. Now, quite unexpectedly, the (short) compressive strength SCT (2.7 N/m) remained unchanged, in spite of a reduction of about 50% of binder in the paper. The silicate/CaCC>3 composition thus forms a high-performance mineral composite material which allows very substantial improvement in the compressive strength under advantageous economical conditions, because of the low cost of CaC03 as compared with silicate. > Resilience gradually increases and is stabilized from a 25% filler level by dry weight corresponding to a CaCC>3/silicate dry weight ratio of 0.33 (example 5). This demonstrates the insolubilization effect on silicate with the reactive mineral fillers, in spite of the very low solubility of these mineral fillers.

With a 50% mineral filler level by dry weight for 50% by dry weight of silicate, the price cost of these compositions is less than the traditionally used starch compositions. Now, the use of these compositions provides a gain in energy of about 10% during the drying of the paper, and better compressive strength in a humid medium.

The following conclusions may be provided: > for a deposited dry weight of a composition including 50% by dry weight of silicate and 50% by dry weight of calcium carbonate, as compared with a same deposition of a starch-based composition, compressive strength values in the dry condition are seen and comparable (Comparative Example 5 and Example 9 with 6 g/m2 of deposited weight on a paper of 120 g/m2). The strength in the humid condition of papers treated with compositions including 50% of reactive mineral fillers by dry weight based on the dry weight of silicate is however much greater than that of papers treated with starch (Example 9: resilience 62% - Comparative Example 5: resilience 58%). > for a deposited dry weight of a composition including 50% of silicate and 50% of calcium carbonate by dry weight, of about double the deposited dry weight with a starch-based composition, much greater compressive strength values are observed. In Example 7, a paper of 120 g/m2 was treated with a deposit of 13 g/m2 of a composition including 50% of CaCC>3 and 50% of silicate by dry weight; the measured compressive strength SCT is 2.7 kN/m and the resilience is 62%. These values were compared with those which are obtained for a paper of 150 g/m2 treated with a deposit of 6 g/m2 of a starch composition (Comparative Example 6) which gives a compressive strength SCT of 2.7 kN/m and a resilience of 57%.

Thus, it is possible to advantageously replace a traditional starch-treated paper of 150 g/m2 with a paper of 120 g/m2 treated with a composition including 50% of silicate and 50% of CaC03; both papers have a comparable compressive strength in the dry condition. The paper treated with the silicate composition however has better compressive strength in the humid condition (resilience of 62% for Example 7 and resilience of 57% for Comparative Example 6).

On the other hand, these tests demonstrate that the use of mineral fillers gives the possibility of reinforcing the cohesion of alkaline silicates. Indeed, it appears that there is no loss of the mechanical characteristics, in spite of the very high mineral filler levels of up to more than 50% of fillers.

Without intending to be bound by any scientific theory, it appears in the Examples above that the CaC03 used has a dual role of reinforcing the cohesion of the silicates, but also of insolubilizing alkaline silicates as demonstrated by the increase in the resilience which passes from 54% to 62% by addition of CaCC>3.

The known effect of deflocculation of the pigments of the silicates moreover gave the possibility of obtaining very good dispersion of the mineral fillers in the bulk of the paper, as observed with scanning electron microscopy of the transverse section of the paper.

Example 10: A composition was prepared by adding to the composition of Example 7, 3% of triacetin by dry weight based on the dry weight of the sodium silicate used, and this composition was compared with a paper consisting of virgin fibres (semi-chemical pulp) without any starch, or silicate (comparative example 7).

The gelling time was of about one hour.

The obtained results are shown in TABLE 5: TABLE 5

These results show that the additional use of suitable insolubilizing agents either in an acid form which lowers the pH below 10.7, or in the form of multivalent salts or of organic esters, again gives the possibility of improving the compressive strength in the humid condition. As demonstrated by example 10, which describes a composition identical with the composition of example 7, to which were added 3% of triacetin by dry weight based on the dry weight of silicate: the addition of triacetin gives the possibility of passing from a resilience from 62% to 64%.

This demonstrates that in spite of the very significant amounts of reactive mineral fillers used, insolubilization of the silicates is only partial and may still be improved by adding a second acid insolubilizing agent or an acid-releasing agent such as diacetin or triacetin.

Example 11 - Preparation of corrugated cardboards

The thereby treated papers with compositions according to the invention were transformed into corrugated cardboard on a corrugator under normal conditions, with traditional starchy adhesives without any loss of adherence between the various supports.

Claims (36)

1. ) I Claims: > ► ) 1. A fibrous material in sheet form corresponding to a paper or cardboard J sheet, optionally as a manufactured article, characterized in that it is treated by a deep impregnation in the thickness of said fibrous material with an aqueous | composition comprising: \ - at least one alkaline metal silicate, and preferably a silicate of Na, K or Li or of a mixture of these alkaline metals, > 1 - at least one mineral filler, characterized in that the mass ratio between the mineral filler and the alkaline silicate in the dry extract of the composition is from 0.25 to 4, and preferably from 0.3 to 3 and preferentially from 0.5 to 1.5.
2. 2. The fibrous material in sheet form according to claim 1, characterized in that the mineral filler is capable of releasing multivalent metal ions which will be substituted for the alkaline ions of the silicate in order to form precipitates of silicates insoluble in water, and is preferably selected from among zinc oxide, zinc carbonate, barium carbonate, barium sulfate, calcium sulfate, beryllium carbonate, strontium carbonate and calcium carbonate, calcium carbonate being preferred.
3. 3. The fibrous material in sheet form according to claim 1, characterized in that the composition comprises as a mineral filler, calcium carbonate, kaolin or a mixture of both of these mineral fillers.
4. 4. The fibrous material in sheet form according to any one of the preceding claims, characterized in that the mineral filler has an average grain size D50 comprised in the range from 20 nm to 20 microns, preferably comprised in the range from 100 nm to 10 microns.
5. 5. The fibrous material in sheet form according to any one of the preceding claims, characterized in that the aqueous composition further comprises at least one ) I compound acting as co-binders, preferably selected from among starch, > > carboxymethyl cellulose, hydroxyethyl cellulose, guar gums, carob gums, soya, ) ^ casein, more or less hydrolyzed polyvinyl acetates, and synthetic latices like * styrene-butadiene, carboxylated styrene-butadiene, styrene-acrylic or acrylic styrene-butadiene copolymers.
6. ) 6. The fibrous material in sheet form according to any one of the preceding j claims, characterized in that the mass of alkaline silicate(s) and of mineral filler(s) < represents from 20 to 100%, and preferably from 50 to 100%, and preferentially > I from 70 to 100%, of the dry extract of the composition.
7. 7. The fibrous material in sheet form according to any one of the preceding claims, characterized in that they comprise an agent insolubilizing the silicate, other than a mineral filler.
8. 8. The fibrous material in sheet form according to claim 7, characterized in that the agent insolubilizing the silicate other than a mineral filler is selected from among organic or mineral acids, salts of mineral or organic acids, organic or mineral substances releasing protons, esters, organic carbonates and multivalent metal salts.
9. 9. The fibrous material in sheet form according to claim 7 or claim 8, characterized in that the mass ratio between the silicate-insolubilizing agent other than a mineral filler and the silicate in the dry extract of the composition is from 0.01 to 0.1, and preferably from 0.03 to 0.05.
10. 10. The fibrous material in sheet form according to any one of the preceding claims, characterized in that the alkaline metal silicate is selected from among silicates of formula (M20)xSi02 wherein M is Na, K or Li or a mixture of these alkaline metals, and x is the molar ratio between Si02 and M20 and advantageously belongs to the range from 0.5 to 4. 1
11. 11. The fibrous material in sheet form according to claim 10, characterized in ! that the molar ratio x of the alkaline silicate is greater than 2.5 and is preferably ) ( greater than 3.
12. 12. The fibrous material in sheet form according to any one of the preceding | claims, characterized in that they further comprise a plasticizer.
13. \ ] 13. The fibrous material in sheet form according to claim 12, characterized in that the plasticizer is selected from among glycerol, sucrose, polyethylene glycols, > or preferably copolymers as an emulsion such as styrene-butadiene emulsions either carboxylated or not, acrylonitrile-styrene-butadiene or acrylic styrene emulsions.
14. 14. The fibrous material in sheet form according to claim 12 or claim 13, characterized in that the mass ratio between the plasticizer and the silicate in the dry extract of the composition is from 0.01 to 0.06, and preferably from 0.02 to 0.04.
15. 15. The fibrous material in sheet form according to any one of the preceding claims, characterized in that the aqueous composition neither contains wax nor paraffin.
16. 16. The fibrous material in sheet form according to any one of the preceding claims, characterized in that the dry extract represents from 10 to 75%, and preferably from 20 to 50%, by mass of the total mass of the composition.
17. 17. The fibrous material in sheet form according to any one of the preceding claims, characterized in that it consists of virgin or recycled cellulose fibers.
18. 18. The fibrous material in sheet form according to any one of the preceding claims, characterized in that it is treated with the aqueous composition for obtaining a deposited dry mass of composition belonging to the range from 3 g/m2 to 35 g/m2, and notably to the range from 8 g/m2 to 25 g/m2. I
19. 19. A corrugated cardboard at least partly consisting of a fibrous material as ! defined in any one of the preceding claims.
20. ) ) 20. The use of an aqueous composition as defined in any one of claims 1 to 16 in the making of a fibrous material in sheet form corresponding to a paper or * cardboard sheet by deep impregnation in the thickness of said fibrous material in j sheet form with said composition, for reinforcing the mechanical strength properties j in the dry condition and optionally in the humid condition of the obtained fibrous < material. > i
21. 21. A method for treating a fibrous material in sheet form corresponding to a paper or cardboard sheet, characterized in that the treatment is carried out continuously on each of the faces of the fibrous material in sheet form and in its bulk, by deep impregnation in the thickness of said fibrous material in sheet form, with an aqueous composition as defined in any one of claims 1 to 16.
22. 22. The method according to claim 21, characterized in that the treatment is integrated to a method for continuously manufacturing a fibrous material in sheet form and is applied on the latter when running, in a finished condition or during manufacturing.
23. 23. The method according to claim 21 or claim 22, characterized in that the treatment is achieved by impregnation in a size press.
24. 24. The method according to any one of claims 21 to 23, characterized in that the treatments is achieved with the aqueous composition in order to obtain a deposited dry mass of composition belonging to the range from 3 g/m2 to 35 g/m2, and notably to the range from 8 g/m2 to 25 g/m2.
25. 25. An aqueous composition for treating a fibrous material in sheet form comprising: Η 1 - at least one alkaline metal silicate, and preferably a silicate of Na, K or Li > or on a mixture of these alkaline metals, ) } - at least one mineral filler, H with a mass ratio between the mineral filler and the alkaline silicate in the dry J extract of the composition being from 0.25 to 4, and preferably from 0.3 to 3 and > preferentially from 0.5 to 1.5, | < characterized in that they comprise a silicate-insolubilizing agent, other than a I mineral filler.
26. 26. The aqueous composition for treating a fibrous material in sheet form according to claim 25, characterized in that the mineral filler is capable of releasing multivalent metal ions which will be substituted for the alkaline ions of the silicate in order to form precipitates of silicates, insoluble in water, and preferably is selected from among zinc oxide, zinc carbonate, barium carbonate, barium sulfate, calcium sulfate, beryllium carbonate, strontium carbonate and calcium carbonate, calcium carbonate being preferred.
27. 27. The aqueous composition for treating a fibrous material in sheet form according to claim 25, characterized in that they comprise, as a mineral filler, calcium carbonate, kaolin or a mixture of both of these mineral fillers.
28. 28. The aqueous composition for treating a fibrous material in sheet form according to any one of claims 25 to 27, characterized in that the mineral filler has an average grain size D50 comprised in the range from 20 nm to 20 microns, preferably comprised in the range from 100 nm to 10 microns.
29. 29. The aqueous composition for treating a fibrous material in sheet form according to any one of claims 25 to 28, characterized in that they further comprise at least a one compound acting as co-binders, preferably selected from among starch, carboxymethyl cellulose, hydroxyethyl cellulose, guar gums, carob gums, soya, casein, more or less hydrolyzed polyvinyl acetates, and synthetic latices like styrene-butadiene, carboxylated styrene-butadiene, styrene-acrylic or acrylic styrene-butadiene.
30. 30. The aqueous composition for treating a fibrous material in sheet form according to any one of claims 25 to 29, characterized in that the mass of alkaline silicate(s) and of mineral filler(s) represents from 20 to 100%, and preferably from 50 to 100%, and preferentially from 70 to 100%, of the dry extract of the composition.
31. 31. The aqueous composition for treating a fibrous material in sheet form according to any one of claims 25 to 30, characterized in that the silicate— insolubilizing agent other than a mineral filler is selected from among organic or mineral acids, salts of mineral or organic acids, organic or mineral substances releasing protons, esters, organic carbonates and multivalent metal salts.
32. 32. The aqueous composition for treating a fibrous material in sheet form according to any one of claims 25 to 31, characterized in that the mass ratio between the silicate-insolubilizing agent other than a mineral filler and the silicate in the dry extract of the composition is from 0.01 to 0.1, and preferably from 0.03 to 0.05.
33. 33. The aqueous composition for treating a fibrous material in sheet form according to any one of claims 25 to 32, characterized in that the alkaline metal silicate is selected from among silicates of formula (M20)xSi02 wherein M is Na, K or Li or a mixture of these alkaline metals, and x is the molar ratio between S1O2 and M2O and advantageously belongs the range from 0.5 to 4.
34. 34. The aqueous composition for treating a fibrous material in sheet form according to claim 33, characterized in that the molar ratio x of the alkaline silicate is greater than 2.5 and preferably greater than 3.
35. 35. The aqueous composition for treating a fibrous material in sheet form according to any one of claims 25 to 34, characterized in that they neither contain wax nor paraffin.
36. 36. The aqueous composition for treating a fibrous material in sheet form according to any one of claims 25 to 35, characterized in that the dry extract represents from 10 to 75 %, and preferably from 20 to 50%, by mass of the total mass of the composition.