FR3012149A1 - COMPOSITE PARTICLES, METHOD OF PREPARATION AND USE IN THE MANUFACTURE OF PAPER - Google Patents

Abstract

Process for the preparation of composite particles, comprising the following steps: copolymerization of at least one vinyl monomer comprising a carboxylic acid function in the presence of at least one hemicellulosic polysaccharide and / or depolymerized starch; - addition of at least one type of mineral filler; precipitation, in the presence of said inorganic filler, of the copolymer thus formed; obtaining composite particles consisting of the mineral filler coated with said copolymer.

Description

The invention relates to the preparation of composite particles by coating mineral fillers by means of a copolymer based on hemicellulosic polysaccharide. The field of use of the present invention particularly relates to the paper industry. STATE OF THE PRIOR ART The papermaking processes use a paste suspended in an aqueous medium and its deposit on a fabric. Once the paste is deposited on the fabric, the water is removed, in particular by draining, so as to form a sheet of paper which is then dried.

In order to modify the properties of the paper sheet, fillers can be introduced into the dough, generally prior to the formation of the sheet. For example, the incorporation of mineral fillers may in particular make it possible to improve the optical properties of the paper.

Recurring problems related to mineral fillers concern the alteration of the mechanical properties of the paper sheet when it contains a large amount of mineral fillers. On the other hand, their retention within the sheet of paper is limited. In this regard, a retention agent, typically a polymeric compound, may be used. It may be compounds belonging to the family of polyacrylamides for example. Other methods have been implemented to retain the inorganic fillers in the paper sheet, and more particularly the coating of the fillers with organic polymers. For example, Shen has described the coating of mineral particles with water. starch or polyacrylamide (Shen J., Song Z., Qian X., Liu W., "Modification of papermaking grade fillers: A brief review." Bioresources, 2009, 4, 1190-1209).

The coating of calcium carbonate particles with starch can be achieved by forming a gel after cooking (Zhao Y., Hu Z., A. Ragauskas, Deng Y. precipitated calcium carbonate spin. "Tappi J., 2005 4, 3-7.).

The method of coating starch-containing kaolin particles may include pre-modifying the starch with fatty-chain acids (Cao S., Song D., Deng., Y., Ragauskas A. "Preparation of starch-fatty acid modified clay and its application in packaging papers. »Ind. Eng Chem Res, 2011, 50, 5628-5633.) The document WO 2006/297571 describes the coating of mineral fillers with a polysaccharide (cellulose or chitin) insoluble in water but soluble in ionic liquid. Even though some of the processes of the prior art are relatively satisfactory in terms of retaining mineral fillers and maintaining the mechanical properties of the paper sheet, there is nevertheless a need to develop alternatives. The Applicant has thus developed a method easy to implement for coating inorganic particles with an organic copolymer. The composite particles thus obtained are better retained in the paper sheet compared to the untreated particles. In addition, and in general, they can notably improve the optical and mechanical properties of paper.

SUMMARY OF THE INVENTION The Applicant has developed a process for preparing composite particles of the core / shell type, the core of which is made of inorganic material and the shell of organic polymer. In view of the properties of the shell, the retention rate of these particles in a sheet of paper is improved compared to uncoated particles. In addition, these particles make it possible to improve the mechanical properties of the paper sheet relative to the untreated particles.

The particles according to the invention are obtained by gelling a polysaccharide by copolymerization with a monomer in the presence of mineral fillers. More specifically, the present invention relates to a process for preparing composite particles, comprising the following steps: copolymerization of at least one vinyl monomer comprising a carboxylic acid function in the presence of at least one hemicellulosic polysaccharide; - addition of at least one type of mineral filler; precipitation, in the presence of said inorganic filler, of the copolymer thus formed; obtaining composite particles consisting of the mineral filler coated with said copolymer. According to a particular embodiment, the copolymerization of at least one vinyl monomer comprising a carboxylic acid function is carried out in the presence of at least one hemicellulosic polysaccharide and / or depolymerized starch. The particles thus obtained are of core / shell type, the core being a mineral filler, and the shell is a copolymer obtained by polymerization of at least one vinyl monomer comprising a carboxylic acid function in the presence of at least one hemicellulosic polysaccharide. This process therefore makes it possible to coat inorganic fillers with an organic copolymer.

In general, any water-soluble (or partially soluble) hemicellulose polysaccharide having free OHs can be so modified. However, and advantageously, this method particularly relates to polysaccharides of low molar masses which can not form gels at high concentration or at high temperature.

Thus, the hemicellulosic polysaccharide has a number average degree of polymerization advantageously less than or equal to 200, more advantageously still between 100 and 200.

The hemicellulose polysaccharide is advantageously derived from wood or annual plants.

It can especially be a compound selected from the group comprising xylan, glucuronoxylan, arabinoxylan, glucomannan, galactomannan, galactoglucomannan, xyloglucan, and mixtures thereof, however, and according to a particular embodiment it may also be depolymerized starch. According to a preferred embodiment, the hemicellulosic polysaccharide is xylan.

It can especially be xylan from birch wood or beech for example. According to a particular embodiment, the hemicellulosic polysaccharide may be modified before the grafting step.

It may especially be a bleaching step hemicellulose (hydrogen peroxide for example) and / or the introduction of carboxylic acid functions on the main chain of hemicellulosic polysaccharide. The introduction of carboxylic acid functions can be carried out by succinylation, by reacting succinic anhydride with the hemicellulosic polysaccharide. Without issuing any theory, the Applicant considers that this modification improves the solubility, and therefore the reactivity of the hemicellulosic polysaccharide in an aqueous medium. More specifically, it allows the expansion of the polysaccharide, xylan for example having a tendency to form aggregates in water. The Applicant has thus observed that the specific surface of a succinylated xylan / PAA type copolymer is much greater than a xylan / PAA (PAA = acrylic acid poly) copolymer. The aggregation of xylan is due to the interactions between the hydroxyl chains (formation of hydrogen bonds). The presence of negatively charged groups allows a repulsion between the chains and a better solubilization. Thus, and advantageously, 0.1 to 1 substitutions are made per unit sugar of the hemicellulose polysaccharide, more preferably 0.3 to 0.4 substitutions. According to a preferred embodiment, the hemicellulose polysaccharide is treated xylan. with succinic anhydride, prior to the copolymerization. The amount of succinic anhydride is advantageously of the order of 2 molar equivalents of succinic anhydride per xylose unit. It is more advantageously equal to 2 molar equivalents. In the process step relating to the copolymerization, the main chain of the hemicellulosic polysaccharide is grafted with the vinyl monomer and / or its polymer (polymer resulting from the polymerization of the vinyl monomer or monomers comprising a carboxylic acid function). This grafting corresponds to the formation of branches on the main chain of the hemicellulosic polysaccharide. The grafting thus makes it possible to increase the viscosity of the hemicellulosic polysaccharide, but also its ability to form a gel.

Once the grafting has been carried out, the (copolymerization) medium can be neutralized, for example by the addition of a reducing agent. The neutralization may consist in eliminating, if appropriate, the traces of the initiator having initiated the copolymerization.

In general, the reducing agent is added in solution until disappearance of the coloration of the initiator. The reduction of the initiator (passage CeIV to CeIII for example) allows above all a discoloration of the medium. It facilitates its removal by washing with water, although traces may remain.

The mineral fillers are then added and coated by precipitation of the copolymer. The precipitation may especially be obtained by adding a non-solvent liquid of the copolymer, such as acetone. The inorganic filler may especially be chosen from the group comprising calcium carbonate, kaolin, titanium dioxide, and talc. As already indicated, the vinyl monomer comprises a carboxylic acid function. By carboxylic acid function is also meant the salts of this carboxylic acid function. By salt is meant salts of alkaline, alkaline earth or ammonium (quaternary in particular). The formation of the salts is advantageously carried out after polymerization and before precipitation, in particular by adding a base such as sodium hydroxide for example.

In general, the vinyl monomer used comprises a carboxylic acid function but, according to a particular embodiment, it may comprise several.

The vinyl monomer may especially be chosen from the group comprising acrylic acid, methacrylic acid, ethylacrylic acid, itaconic acid, and the salts of these acids. Advantageously, it is acrylic acid and / or an acrylic acid salt. It may be itaconic acid and / or an itaconic acid salt. Acrylic acid is particularly suitable in view of its ability to form hydrogels. Thus, according to a preferred embodiment, the copolymer obtained is a polyacrylic acid xylan copolymer (or poly acrylic acid, PAA).

Without issuing any theory, the Applicant believes that the carboxylic acid functions from the polymerized vinyl monomer, increase the swelling capacity of the copolymer. They thus make it possible to form a network for coating the mineral filler. It is the copolymer that adsorbs on the mineral filler. The affinity between the copolymer and the cellulosic fibers is certainly due to the presence of hemicellulose. As already indicated, the composite particles according to the invention are of the core / shell type. They comprise a core of inorganic material and a copolymer shell based on at least one hemicellulosic polysaccharide and at least one vinyl monomer comprising carboxylic acid functions. According to an advantageous embodiment, the core / shell composite particles according to the invention comprise: 1 to 50% by weight of copolymer based on hemicellulosic polysaccharide, more advantageously 15 to 30% by weight; 99 to 50% by weight of mineral filler, more preferably 85 to 70% by weight. The copolymerization of at least one vinylic monomer and hemicellulosic polysaccharide can be carried out according to conventional techniques that are within the knowledge of a person skilled in the art. It is a radical polymerization in homogeneous solution. It is advantageously carried out in aqueous solution. As already indicated, it can implement an initiator such as, preferably, cerium and ammonium nitrate (CAN). According to another embodiment, it may be potassium persulfate.

The present invention also relates to composite particles obtainable by the process described above, but also their use in a papermaking process. The core / shell composite particles are advantageously insoluble in water.

Indeed, after formation, the composite particles are advantageously washed with water and centrifuged (in general, 3 to 4 cycles of washing / centrifugation). All water-soluble entities are removed with the wash water. Characterization and determination of the grafting rate of the composite particles are carried out after washing. The copolymer forms the shell of the composite particles, it is not resolubilized under the conditions of use of the composite particles. In general, the size of the inorganic filler particles (TiO2 for example) alone is between 50 and 250 nanometers. By size, we mean the largest dimension of the particles, that is to say its diameter when it is spherical. The shell of the composite particles, that is to say the copolymer layer, has a thickness which may be of the order of 15 nanometers. The thickness of the shell generally has a thickness that can be between 10 and 20 nanometers.

In addition, when the copolymer content is high, a network comprising a plurality of mineral filler particles can be formed. The dimensions of this network are then greater than those of the composite particle.

In general, the papermaking process using the composite particles according to the invention is carried out on a paper machine, and comprises the following steps: suspending the pulp in an aqueous suspension; addition of the composite particles in the aqueous suspension of pulp; forming a sheet of paper on the web of the paper machine; pressing of the sheet; drying the leaf.

The paper pulp is obtained by resuspending dry pulp sheets, waste paper, or directly from a pulping plant, according to conventional papermaking processes forming part of the human knowledge of the pulp. job. It includes in particular cellulosic fibers.

Those skilled in the art will be able to adjust the amount of composite particles according to the invention to be added to the dough during the manufacture of paper. This amount may possibly vary depending on the optional addition of a retention agent. It is generally between 0.1 and 2 tons per ton of dry pulp, more preferably between 0.5 and 1 ton per ton of dry pulp. As already indicated, and contrary to the processes of the prior art, the present invention makes it possible in particular to improve the retention of mineral fillers without requiring the addition of a retention agent. Coating the particles with the hemicellulosic polysaccharide / vinyl monomer copolymer makes it possible to obtain a compromise between the optical properties and the mechanical properties of the paper sheet. Indeed, while the mineral fillers of the prior art significantly degrade the mechanical properties of the paper sheet, the charges according to the invention make it possible to stabilize them.

The invention and the advantages thereof will appear more clearly from the following figures and examples given to illustrate the invention and not in a limiting manner. DESCRIPTION OF THE FIGURES FIG. 1 illustrates the steps of a particular embodiment of the process for preparing composite particles according to the invention. FIG. 2 corresponds to the graph of the percentage of charges retained in a sheet of paper as a function of the variation of the burst index, for charges according to the invention and charges according to the prior art. FIG. 3 corresponds to the graph of the percentage of charges retained in a sheet of paper as a function of the variation of the tensile index, for charges according to the invention and charges according to the prior art. FIG. 4 corresponds to the graph of the percentage of charges retained in a sheet of paper as a function of whiteness, for charges according to the invention and charges according to the prior art.

FIG. 5 corresponds to the graph of the percentage of charges retained in a sheet of paper as a function of opacity, for charges according to the invention and charges according to the prior art. FIG. 6 illustrates the rate of charges retained in a sheet of paper, for charges according to the invention and charges according to the prior art. EXAMPLES OF CARRYING OUT THE INVENTION Example 1 Synthesis of TiO 2 / PAA / xylan Composite Particles (FIG. 1) The reaction is carried out under an inert atmosphere in a 100 ml three-necked flask equipped with a refrigerant. Xylan obtained from birch wood (1 g, 7.58 mmol, from Aldrich), and previously bleached with hydrogen peroxide and esterified with succinic anhydride (0.3 substitution per xylose unit), is solubilized in 50 ml of demineralised water. The xylan solution thus obtained is stirred for 10 minutes at 40 ° C. under a stream of nitrogen.

A cerium ammonium nitrate initiator (0.54 g, 1 mmol) is then added, followed by the addition of 1 g of acrylic acid. After 3 hours of reaction (40 ° C.), the solution is neutralized by addition of a solution of sodium hydroxide (1N). A few drops of a saturated solution of sodium hydrosulphite are also added, until disappearance of the yellow color of the initiator, in order to reduce the excess of cerium nitrate.

TiO 2 is then introduced into the medium with stirring, and the copolymer (acrylic xylanpolyacid) is precipitated by addition of acetone in order to coat the inorganic filler.

The composite is recovered by centrifugation, before being thoroughly washed with water. It is thus subjected to washing / centrifugation cycles.

TiO 2 / PAA / xylan composites with two levels of inorganic / organic: 85/15% and 70/30% (w / w) were prepared (PAA = polyacrylic acid). Characterization of the composite thus obtained: The morphology of TiO 2 / PAA / xylan particles was studied by transmission electron microscopy. For example, for the composite (85/15%), there is the presence of a polymer layer of about 15 nanometers which coats the titanium dioxide charges. Example 2: Manufacture of paper in the presence of mineral fillers (FIGS. 2-6) Sheets were formed from a bleached Kraft pulp made from hardwood (Birch mixture, Beech, Eucalyptus in equal proportion), refined at 34 ° C. SR. This paste is disintegrated using a Lhomargy® laboratory disintegrator (10000 turns, standard 1505263) and then diluted to 3 g / L.

The protocol used for the preparation of the sheets on a "FRET" retention tablet is as follows: 710 ml of 3 g / L paste are introduced into the FRET bowl (Techpap, Grenoble, France) and stirred mechanically for 1 min. at 1000 rpm. In this case, unlike the conventional procedure, and in order to evaluate the self-retention of the composite charge, no retention agent was used.

At the end of the stirring cycle, the suspension is left standing for 2 seconds, then it is drained under a vacuum of 400 mbar and the retained fibers are distributed on the fabric to form a sheet.

This sheet is recovered on a blotter, pressed and then dried under vacuum for 7 min. It is then stored under standard conditions of temperature and humidity (23 ° C, 50% RH). The sheets obtained have a diameter of 19 cm and a basis weight of 75 g / m.sup.2. For the tablets containing composite fillers, these are added to the dough diluted at different rates in the form of slurry, before introduction into the bowl. FRET.

This technique is commonly used in the paper industry to replicate the steps of paper making on a larger scale on an industrial machine. FIGS. 2 and 3 show that the use of modified mineral fillers according to the invention (TiO 2 coated with the xylan-PAA copolymer) makes it possible to limit the degradation of the mechanical properties of the paper relative to untreated TiO 2 particles. On the other hand, the strength of the paper increases further as the copolymer content in the composite filler increases by 15 to 30%.

A respective gain of about 5% and 15% of the burst and tensile indices is observed for the fillers comprising 30% by weight of xylan-PAA copolymer, relative to the absence of mineral fillers.

Figure 4 shows that the sheets containing composite fillers according to the invention have lower whiteness properties than a sheet comprising untreated titanium dioxide. This is due to the initial color of xylan. A whitening treatment adapted to wood hemicelluloses makes it possible to improve the degree of whiteness. Finally, the sheets of paper comprising the composite fillers according to the invention have a good opacity comparable to that of a sheet of paper containing untreated TiO 2 (91 vs. 93%).

Retention of composite fillers: In general, the mineral fillers are not easily retained in the dough. It is therefore common to introduce into the dough polyelectrolytes, a cationic polyacrylamide for example, to improve their retention.

The composite fillers studied were introduced without addition of retention agent. The percentage of the charges retained was determined by incineration of the sheets at 600 ° C.

Figure 6 shows that the percentage of charge retention increases with the percentage of organic copolymer. In other words, the TiO2 charge retention is less than that of the TiO 2 / PAA-xylane (85/15) charges, which is itself less important than the TiO 2 / PAA-xylane (70/30) charges. . Coating TiO 2 with a layer of xylan-PAA copolymer improves its retention by about 30 points.

In summary, the mineral fillers (composite particles) according to the invention make it possible simultaneously to improve the mechanical properties and the optical properties of the paper without influencing the whiteness. On the other hand, the retention of the charges on the elements of the suspension is much greater than that of the reference TiO 2. As already indicated, this process for preparing composite particles is not specific to TiO 2. It can be applied to other mineral fillers, for example calcium carbonate.25

Claims (2)

REVENDICATIONS1. A process for the preparation of composite particles, comprising the steps of: copolymerizing at least one vinyl monomer comprising a carboxylic acid function in the presence of at least one hemicellulosic polysaccharide and / or depolymerized starch; addition of at least one type of mineral filler; precipitating, in the presence of said inorganic filler, the copolymer thus formed; obtaining composite particles consisting of the mineral filler coated with said copolymer. Process for the preparation of composite particles according to Claim 1, characterized in that the hemicellulosic polysaccharide is chosen from the group comprising xylan, glucuronoxylan, arabinoxylan, glucomannan, galactomannan, galactoglucomannan and xyloglucan, and mixtures thereof. Process for the preparation of composite particles according to claim 1 or 2, characterized in that the hemicellulosic polysaccharide is treated with succinic anhydride prior to the copolymerization step. Process for the preparation of composite particles according to Claim 3, characterized in that the hemicellulosic polysaccharide is xylan. Process for the preparation of composite particles according to one of Claims 1 to 4, characterized in that the mineral filler is chosen from the group comprising calcium carbonate, kaolin, titanium dioxide and talc. Process for the preparation of composite particles according to one of Claims 1 to 5, characterized in that the vinyl monomer is acrylic acid, itaconic acid or a salt of these acids. 10 2. Process of preparing composite particles according to one of Claims 1 to 6, characterized in that the composite particles comprise: 1 to 50% by weight of copolymer with hemicellulose polysaccharide base, advantageously from 15 to 30%; - 99 to 50% by weight of mineral filler, preferably 85 to 70%. 8. Process for preparing composite particles according to one of claims 1 to 7, characterized in that the copolymerization is carried out in aqueous solution. 9. Composite particles obtainable by the method according to one of claims 1 to 8, characterized in that they comprise a core of inorganic material and a copolymer shell based on at least one hemicellulose polysaccharide and at least one vinyl monomer comprising a carboxylic acid function. 10. Use of the composite particles of claim 9 for the manufacture of paper. 11. A process for producing paper on a paper machine, comprising the following steps: - aqueous suspension of pulp; - addition of composite particles according to claim 9 in the aqueous suspension of pulp; - forming a sheet of paper on the canvas of the paper machine; - pressing the sheet; - drying of the sheet.