MXPA00006278A

MXPA00006278A - Preparation and use of mixed opacifiers based on titanium and silica oxides

Info

Publication number: MXPA00006278A
Application number: MXPA/A/2000/006278A
Authority: MX
Inventors: Cornec Patrice Le; Franck Fajardie; Michel Foulon
Original assignee: Rhodia Chimie
Priority date: 1997-12-30
Filing date: 2000-06-23
Publication date: 2001-07-03

Abstract

The invention concerns a method for preparing a composition based on TiO2 useful as opacifier which consists in mixing with an aqueous TiO2 dispersion an aqueous dispersion of at least an inorganic spacer, in conditions such that the two mineral species combine into mixed mineral flocs wherein the TiO2 particles are globally spaced from one another by the spacer particles and/or aggregates. The invention also concerns a composition based on TiO2 and SiO2 characterised in that the TiO2 and SiO2 particles are combined therein in the form of mixed mineral flocs based on TiO2 and SiO2 wherein the TiO2 particles are globally isolated from one another by said silica aggregates and its use as opacifier, in particular in the paper industry.

Description

PREPARATION AND USE OF MIXED OPACITY AGENTS BASED ON TITANIUM AND SILICA OXIDES The object of the present invention is a compound with a TiO2 base for use as an opacity agent, specifically in laminated papers, and a preparation process that allows the obtaining the composition. Laminated paper, commonly known as decorative paper, is the surface element that has both an opacity function and an adornment function that is used in the manufacture of laminated panels designed for use in the furniture industry. A particularity of the decoration paper is that it has an extremely high percentage of Ti02, possibly up to 40% of the mass of the dry leaf. By comparison, papers for printing and writing can contain an absolute maximum of 10%. The level of opacity that is required for a paper whose purpose is decoration in fact explains this high content of Ti02. The paper is subjected to a bonding process with resin that makes it transparent. Since this is incompatible with its opacity and decoration purposes, this should be corrected by the addition of an opacity agent. Titanium dioxide is traditionally used for this application because it is the only white pigment that, due to its high refractive index, can offer IOJS required levels of opacity. The application WO 89/08739, however, suggests the replacement of Ti02 at a rate of 5 to 40% by weight for precipitated amorphous silica and the use of the corresponding mixtures as fillers in the paper industry because they are more economical. The paper sheets are conventionally prepared from a mixture of cellulose fibers and mineral fillers containing mainly Ti02 dispersed in water. This mixture is contained in a "front box" that feeds a canvas where the sheet is formed by drainage and filtration. During filtration, the cellulose fibers and a part of the mineral charge are conserved in the canvas, whether or not there are interactions with the entangled fibers. In this way a "fiber mat" is obtained which, after drying, results in a sheet of paper. It has been found, in fact, that only a part of the initial amount of Ti02 is conserved in the fiber mat and that this fraction is generally too agglomerated for the Ti02 to develop its maximum opacity. To reduce this loss of Ti02 during the formation of the fiber mat, papermakers generally introduce retention agents into their cellulose blends. These agents are conventionally cationic polymers that allow the fixation of the Ti02 particles on the fibers through phenomena through homoflocculation and heteroflocculation. However, in the case of retention of opacity charges such as for example titanium dioxide, the use of an electrically charged polymer results in a loss of opacity efficiency due to excessively extensive and excessively dense flocculation. As a result, it seems that the simple retention of Ti02 in the fiber mat is not sufficient in itself and in terms of the opacity rate. It would also be necessary to keep the TiO2 in the fiber mat in a sufficiently dispersed form in such a way that it will retain its pigmentation properties and develop a good opacity power. In a profitable manner, it would then be possible to obtain the same opacity with lower Ti02. In this way, the opacity rate of Ti02 would be significantly increased. For this purpose the international patent application WO 97/18268 proposes a method for the treatment of the surface of particular Ti02. The treatment consists in coating it with a single layer of mineral particles such as silica, whose grain size is smaller than the grain size of the Ti02 particles. This unique particle layer coating makes it possible to space the Ti02 particles between them. The object of the present invention is specifically to propose a new compound with a Ti02 base that meets the aforementioned requirements. More specifically, proposes a new opacity agent system that makes it possible to improve the retention of Ti02 during the formation of the fiber mat and maintain it in a flocculation structure that is as negative as possible for opacity. The inventors have also shown that a solution to the problem of agglomeration of the mineral charge consists in the creation of mixed mineral aggregates by the insertion of particles of an agent known as an inorganic spacer, whether in the form of aggregates or not, between the particles of Ti02. The mixed mineral agglomerates obtained in accordance with the present invention are advantageous for several reasons: they allow the Ti02 particles to remain sufficiently dispersed during the various steps of the formation of a sheet of paper in such a way that a maximum of them can develop their characteristics of pigment and contribute consequently to the opacity of the dry leaf, - they have an open structure, favorable to a better retention, - they have also proven to be sufficiently resistant to resist the capillary forces that develop during the drainage and drying of the fiber mat, as well as the shear stress that can occur during manufacture , of a sheet. In fact, the internal coherence of the mixed mineral agglomerates resulting from the association of at least one inorganic spacer with Ti02 is based on the strength of the ionic bonds established between Ti02 and the spacer. This coherence results directly from the process adopted to prepare the mixed mineral aggregates. More precisely, these aggregates are obtained under operating conditions such that the TiOr and the inorganic spacer in question have opposite and significantly different surface charges. Specifically, the Ti02 and the inorganic spacer in question must have isoelectric points sufficiently different to be in a pH range in which the two types of minerals have opposite charges. Under these conditions, the two types of minerals have an electrostatic attraction between them. The resultant forces of attraction must be sufficient to provide a structural arrangement of the two compounds on the one hand, and to stabilize them in this way on the other hand. Accordingly, the first object of the present invention is a process for the preparation of a compound having a TiO2 base which can be used as an opacifying agent which includes the steps according to which: an aqueous dispersion of at least one inorganic spacer an aqueous dispersion of Ti02 is mixed, and the mixture of the two dispersions carried out under agitation and at a pH between the respective isoelectric points of the Ti02 and the spacer is selected such that the Ti02 and the spacer have sufficiently opposite surface charges different to result, under the effect of electrostatic forces, in their arrangement in mixed mineral aggregates wherein the Ti02 particles are globally spaced from each other by the particles and / or aggregates of the spacer; - as necessary, the pH is adjusted to the value established in step 1, with the procedure being characterized in that a single word also includes the steps in accordance with which: - the resulting aqueous dispersion of mixed mineral aggregates is cured at a sufficient temperature to strengthen the strength of the bonds established between the Ti02 particles and the particles and / or aggregates of the spacer, the compound is recovered in the form of an aqueous dispersion of mixed mineral aggregates, and the compound can be formulated possibly in dry form. Figure 1 schematically represents the structure of mixed mineral aggregates obtained in accordance with the invention. It is confirmed by transmission electron micrograph, as illustrated in Figure 2. As defined in the invention the term "aggregate" means the mixed agglomerates of two types of minerals of the type Ti02, and an inorganic spacer Si02, for example. These agglomerates result from the association between the aggregates of the spacer and the Ti02 particles. A spacer is composed of particles or aggregates of particles that are inserted between the Ti02 particles. As in the case of the isoelectric point, it is the pH at which the particle of the type of mineral in question has a net surface charge of zero. In the case of a pH higher than this value, the net charge is negative; and in the case of a lower pH, the net charge is positive. The Ti02 employed according to the present invention is preferably a rutile Ti02. More preferably, it is a rutile TIO2 of pigment size. If required, it can be coated with a mineral surface treatment. This surface treatment preferably contains at least one compound selected from alumina, silica, zirconia phosphate, cerium oxide, zinc oxide, titanium oxide, and mixtures thereof.

The amount of oxide (s) may be from 1 to 20% by weight or less, or preferably of the order of 3 to 10% by weight or less, relative to the total weight of pigment. As examples of these titanium dioxides, there can be specifically mentioned the two rutile pigments Rhoditan RL18 and R162, manufactured by Rhéne-Poulenc. These two pigments are differentiated by the composition of their surface treatment and the resulting Zeta potentials.

RL18 has a silica / alumina surface treatment (Si02 / Al203) and a negative Zeta potential at a pH of 6, and is known as "anionic Ti02". RL62 has a phosphate / alumina surface treatment (P205 / A1203) with a positive Zeta potential at a pH of 6, and is known as "cationic Ti02". The choice of pH = 6 approaches the pH for industrial use. In the present invention the selection of anionic or cationic TiO2 is evidently determined by the choice of the associated inorganic spacing agent. In each case, an inorganic spacing agent with an isoelectric point sufficiently different from the Ti02 in question is chosen in such a way that the electrostatic attractions between the two compounds, necessary for its arrangement, can occur. The aqueous dispersion of Ti02 used according to the invention contains approximately 5 to 80% by weight of TiO2, preferably approximately 5 to 40% by weight. Regarding this aspect, the limiting factor is the viscosity of the suspension that must remain at a reasonable value to be easily handled. According to a preferred embodiment of the invention, the Ti02 adopted is a Ti02 of cationic pigment rutile, especially Rhoditan RL62. As in the case of the inorganic spacing agents considered in accordance with the invention, they should not interfere with the other reagents traditionally employed in the paper industry. Preferably, they should not absorb visible light significantly. In general, its particle size is smaller than the size of Ti02 particles. However, these particles are preferably used in the form of aggregates with a size which is therefore greater than the size of the Ti02 particles. The aggregates have a size preferably of between about 0.5 and 2 μm. As examples of inorganic spacing agents which can be used according to the invention, the following can be specifically mentioned: silica, titanium, zirconium, zinc, magnesium, aluminum, yttrium, antimony, cerium and tin oxides; barium and calcium sulfates; zinc sulfide; zinc, calcium, magnesium, lead and mixed metal carbonates; phosphates of aluminum, calcium, magnesium, zinc, cerium and mixed metals; titanates of magnesium, calcium, aluminum and mixed metals; magnesium and calcium fluorides; silicates of zinc, zirconium, calcium, barium, magnesium, mixed alkaline earths, and silicate minerals; alkaline aluminosilicates and alkaline earths; oxalates of calcium, zinc, magnesium, aluminum and mixed metals; aluminates of zinc, calcium, magnesium and alkaline earths; aluminum hydroxide, and mixtures thereof. The spacing agent is obviously selected such that it has an isoelectric point sufficiently different from the adopted Ti02 form. As inorganic spacing agents particularly suitable for the present invention, there may be mentioned the following inorganic oxides, which are preferably chosen from silicon, zirconium oxides. Aluminum, antimony, cerium and tin, and mixtures thereof. In the specific case where the cationic pigment rutile Ti02 is adopted, the preferred inorganic spacing agent is a silica, an alumina, a silicoaluminate, a mixture thereof. The relationship between Ti02 and the spacing agent is obviously variable, depending on the type of spacing agent adopted. The lower limit of this ratio is formed in general by the minimum amount of the inorganic spacing agent necessary to observe a positive effect in terms of opacity level, and its upper limit by the maximum amount of spacing agent beyond which undesirable effects would appear at the level of the paper that incorporates the compound obtained in accordance with the claimed process. These undesirable effects can be translated specifically into paper fragility, particularly in terms of strength either in the dry state or in the wet state. The spacing agent may generally be employed at a rate of about 1 to 40% by weight relative to the weight of TiO2, preferably at a rate of about 5 to 15% by weight, and preferably at a rate greater than about 10. % in weigh. As explained above, the two compounds are joined in the form of corresponding aqueous dispersions under operating conditions developed by the heterocoagulation of Ti02 with particles and / or aggregates of particles from the inorganic spacing agent, the mixed mineral aggregates. The spacing agent can also be precipitated in situ. In this case, the pH that favors heterocoagulation will be adjusted after the precipitation step of the spacing agent. These favorable operating conditions for the appearance of heterocoagulation between the inorganic spacing agent and Ti02 are remarkably the choice of a pH in a range defined by their respective isoelectric points. It is advisable to select a pH in such a way that the two compounds have sufficiently different surface charges. For operational reasons, it is desirable that the isoelectric points of the spacing agent and Ti02 be spaced apart by at least one pH unit. The mixed mineral aggregates that make up the expected compound are therefore formed under agitation of the dispersions, generally at room temperature and at a pH according to that defined above. If necessary, the pH can be adjusted during the reaction in order to maintain a favorable value for the formation of the aggregates. The attraction is immediate. However, it is preferable to continue stirring for about 15 minutes in order to stabilize the system before the curing step. In accordance with the preferred variant of the invention, Ti02 is used in the form of cationic pigment rutile and is preferably RL62® and the associated spacer is silica. With even greater preference, the silica used is a silica with a large specific surface area between 20 and 300 m2 / g. It can take the form of aggregates of approximately 0.5 to 10_ μ in size. The use of silica as a spacer people in accordance with the present invention is helpful for several reasons. First it has an isoelectric point in the vicinity of 2, which is a value sufficiently different from the value of the isoelectric point of the cationic form of Ti02 (from 6.5 to 7). In addition, silica has the advantage of not significantly absorbing visible light, which is favorable for leaf bleaching. The pH for the exposure of the two corresponding dispersions between them is between the isoelectric points of the spacing agent and the Ti02. The isoelectric point of the Ti02 in question normally imposes the upper limit, and the isoelectric point of the spacing agent employed should impose the lower limit. In the current case, this pH should be between 2 and 6.5. However, in the particular case of R162, it is necessary to avoid the dissolution of its surface treatment. To achieve this, the pH range should be limited to a value between 4.5 and 6.5. With even greater preference, the process according to the present invention is implemented at a pH of about 5.5. In the specific case of the preparation of a compound containing Ti02 in the form of cationic pigment rutile bound to silica particle aggregates, silica is used at a rate of at least 1% by weight relative to the weight of TiO2. It is only with this amount of silica that a significant gain in opacity retention begins to occur. This amount of silica can be increased to about 20% by weight of Ti02. Above this level, the problem of the fragility of the paper described above is presented. Accordingly, the silica is preferably used at a rate of about 5 to 15% by weight of the weight of TiO2, preferably even greater than about 10% by weight. The silica can be introduced either in the form of an aqueous dispersion of the silica particle paste type or it can be generated in situ by acidification of a silicate solution. In the specific case where the silica is precipitated in situ in the Ti02 dispersion, after the precipitation step, the reaction medium is adjusted to a value favorable to the manifestation of electrostatic forces between the Ti02 and the silica generated in this way . These forces are necessary for its heterocoagulation. The second step required in accordance with the claimed procedure is in fact an operation to cure the mixed mineral agglomerates formed in the previous step. As mentioned above, the mixed mineral agglomerates obtained in accordance with the claimed process are specifically designed to be used as an opacity agent in the papermaking industry. This includes a whole sequence of handling of the agglomerates. Therefore, it is necessary that these agglomerates are sufficiently strong to withstand shear stress, and if the case arises, the flocculation effect of polymer derivatives such as PAE (polyaminoamide-epichlorohydrin) and the removal of water during formation. and the drying of the leaf. It is therefore important that the Ti02 particles present in the compound obtained according to the invention are not only sufficiently dispersed to improve their opacity without, but that they are also better preserved during the formation of the sheet. As a consequence, the curing operation carried out in accordance with the claimed procedure is especially advantageous for reinforcing the chemical, even spherical actions established within the mixed mineral agglomerates. However, it is likely that some of the ionic bonds are converted into covalent bonds at the end of this curing step. In the specific case of preparing a compound of mixed mineral agglomerates having a base of Ti02 and Si02, this step of curing is carried out at a temperature higher than 40 ° C. Preferably, the temperature is between about 60 ° C and 100 ° C. The heating time lasts at least 30 minutes, and if necessary, it can be extended up to 3 hours. At the end of the heating step, the resulting compound is allowed to cool to room temperature and can be recovered in this state. The compound can be used directly in this form as an opacity agent. However, it is also conceivable to formulate it in dry form. For this purpose, it has been found possible to apply conventional drying techniques to the dispersion obtained according to the invention. In particular, it may be a spray drying or thin film drying. However, drying will not always result in a properly redispersed product. Since the agglomerates are grouped during drying, it is preferable to spray the product using a grinding step with air jets (micronization). According to another variant of the claimed process, the mixed mineral agglomerates obtained at the conclusion of the first step or second step of the process may be subjected to a mineral surface treatment. This includes at least one hydrous oxide in accordance with that defined above. The latter can be precipitated in a reaction medium after exposure of Ti02 dispersions and spacing agent between them. The mineral surface treatment is about 16% by weight or less, or preferably about 10% by weight or less, of the total weight of the mixed mineral agglomerates treated in this way. The present invention encompasses compounds based on Ti02 that can be obtained in accordance with the claimed procedure. Another object of the present invention is a compound having a base of Ti02 and Si02 characterized in that the Ti02 and Si02 particles are arranged there in the form of mixed mineral agglomerates where the Ti02 particles are globally spaced from each other by the aggregates of the silica . These mixed agglomerates of Ti02 and Si02 are stabilized through the electrostatic forces established between the Ti02 particles and the Si02 aggregates. further, the stability of the mineral agglomerates is reinforced by the fact that they are subjected to the curing described above. This curing operation contributes particularly to the creation of covalent bonds between Ti02 and SiO2 within the agglomerates. In the case of the mixed mineral agglomerates according to the invention, there is no uniform distribution of the aggregates of the inorganic spacing agent around the Ti02 particles. This distribution is discontinuous. Figures 1 and 2 provide a representation of the structure of the agglomerates. The preferred Ti02 is a rutile Ti02 of pigment size. If necessary, it can be coated with a mineral surface treatment. This surface treatment may be chosen from phosphates, alumina, silica, zirconia, cerium oxide, zinc oxide, titanium oxide, and mixtures thereof. The amount of oxide (s) may be from about 1 to 20% by weight or less, or preferably from about 3 to 10% by weight or less relative to the total weight of the pigment. The preferred Ti02 is a Ti02 of cationic pigment rutile. The preferred Ti02 is RL62. The silica used is more preferably a silica having a large specific surface, specifically between about 20 and 300 m2 / g. It is present in the form of aggregates whose sizes are approximately between 0.5 and 10 μm. The preferred silica is precipitation silica. It can also be a silica generated in situ by the acidification of a silicate solution. Silica is preferably present at a rate of about 1 to 20% by weight of the weight of TiO2, and preferably at a rate of about 5 to 15% by weight, and preferably even higher, 10%.

If required, these mineral agglomerates based on Ti02 and Si02 can be coated with at least one mineral surface treatment in accordance with what is defined above. The amount of mineral surface treatment may be about 16% by weight or less, preferably about 10% by weight, relative to the total weight of the mixed mineral agglomerates. The compounds defined above or obtained according to the invention have proved to be interesting for the preparation of paper including rolled paper, and especially useful in terms of retention of Ti02 at the level of the cellulose fibers and the opacity rate of the Ti02 used. The conventional process for the preparation of laminated paper or decorative paper generally employs, in addition to anionic cellulose fibers and the opacifying agent, a polymeric agent which is cationic by nature as a "reinforcing agent in the wet state and a retention agent. In the case in which a compound based on mineral agglomerates Ti02 / Si02 are used as an opacity agent, it is observed that the chemical retention by electrostatic attraction is refounded in a useful way in comparison with Ti02 in individual cationic form. This retention amplification can be explained as follows: In the absence of a cationic polymer, the cationic Ti02 is attracted by the anionic cellulose fibers which is favorable for the retention of Ti02. In contrast, in the presence of the polymer, Ti02-fiber interactions change and Ti02 retention decreases. This phenomenon results in the cationization of the cellulose fibers, which results from their coating by the cationic polymer. Conversely, in a compound whose base is mineral agglomerates Ti02 / Si02, there is a mixture of charges of cationic Ti02 and anionic Si02 whose net zeta potential is negative. The mixed mineral agglomerates therefore behave as anionic charges. Under these conditions it can be considered that the mixed agglomerates can enter into an active interaction with the cellulose fibers that became positive due to the cationic polymer, through the negatively charged silica agglomerates they contain. This results in an increase in retention. Compounds based on mixed mineral agglomerates claimed and obtained in accordance with the invention are of particular interest as an opacifying agent, particularly in the papermaking industry. The increased opacity measured on sheets prepared using a compound based on mixed mineral agglomerates in accordance with the present invention visibly results from the accumulation of two phenomena: an increase in the amount of Ti02 retained in the paper sheet resulting in a better retention at the time of the formation of the fiber sphere, and an improved opaque capacity that results in the best dispersion of the titanium particles contained in the agglomerates. In addition, it was observed that these compounds increase the white character of the paper in which they are incorporated. In addition to this application in the papermaking industry, the compounds claimed and obtained in accordance with the present invention are also useful when used as opacifiers in the paint and plastics industries. The examples and figures presented below are given by way of illustration and do not limit the scope of the present invention. Figures Figure 1: schematic representation of Ti02 pigments spaced by Si02 aggregates. Figure 2: transmission electron micrograph of mixed mineral agglomerates based on Ti02 and Si02. Figure 3: change in charge retention for different mixed minerals as a function of the agitation speed imposed on the "cellulose / PAE / filler" mixture before the fiber sphere is formed. MATERIALS AND METHODS The products used they are commercial products: - The titanium dioxide used in the examples is rutile titanium dioxide sold under the trade name of Rhoditan RL62 by the Rhône-Poulenc company. This pigment is formed from rutile Ti02 coated by a surface treatment of aluminum phosphate (P2O5 / AI2O3). It has a positive zeta potential at pH 6. Its isoelectric point is between approximately 6.5-7. - Cellulose fibers: dry leaves with a 70/30 mixture of short / long fibers previously refined at 32 ° SR, supplied by the company Arjo Wiggins. - Silica is a precipitation silica - with a high specific surface comprised between 20 and 300 m2g_1 that has agglomerates of a size comprised between 0.5 and 10 μM. Its isoelectric point is approximately 2. - PAE resin (polyaminoamide apochlorohydrin) R4947® from the company CECA. A. "Sheet retention" test "_I _ 1 Equipment: - Dispermat® and Pendraulik® rapid dispersers - Mixing tanks -" Test sheet retention "machine, Techpap company Method of operation: - Preparation of dispersion of fibers / Ti02 The amount of paste or substance Ti02 of the product according to the present invention necessary to introduce 15 g of dry Ti02 is added to 15 g of fibers redispersed in 500 ml of deionized water for 10 minutes in the Dispermat at 3000 rpm. This takes into consideration the suspension or paste extract Ti02 according to the invention.This addition is carried out in a mixing tank.This is followed by dilution with deionized water to 4 liters. A test sample of 500 ml of the well-homogenized mixture is decanted into a test tube.With a micropipette, the desired amount of PAE resin is added (commercial solution diluted 10 times). The test tube is inverted twice to carry out a complete mixture. This test sample is then fed into a test sheet holding machine to obtain a sheet. Measurement of the retention The formation of the leaf is caused after agitation during 30 seconds at a speed of 1300 revolutions per minute followed by a resting time of one second. The obtained leaf is recovered in the canvas in the form of "mass", dried in the dryer, and then calcined at a temperature of 800 ° C. The ashes obtained are then weighed to approximately 10"4 g.

The retention rate is given as follows: P2 / P1 Pl = load weight (TÍO2 + SÍO2) in the initial 500 ml removed. P2 = weight of ashes after calcination of the prepared leaf. B. Opacity test ___ Opacity rate tests were carried out using test sheets manufactured for the purpose of understanding the spatial distribution of titanium dioxide in the dry leaf. The test sheets were manufactured in accordance with the method of operation described in the paragraph below. The optical properties of the impregnated and pressed test sheets were also measured in accordance with the method described below. 1. Manufacture of the test sheets i) Formulation of paper pulp Cellulose; 15g (representing 100 parts) Opacifying compound: 100 parts (expressed in Ti02), or 15 g PAE: 0.8% dry relative to cellulose ii) Preparation of the pulp: removal of fibers _ After humidification in water, Cellulose is cut into small squares manually. The small cellulose squares are gradually added to 500 ml of water with stirring in the Dispermat vessel at 1000 revolutions per minute. After the addition of the cellulose, the speed is raised to 3000 rpm and the stirring is continued for 10 minutes. iii) Mixture of opacifying agent / test compound Cellulose without fiber is diluted to 1 liter. Then it is placed with stirring in a mixer with a blade. The opacifying compound is added as a powder or as a suspension, then the mixture is stirred for 5 minutes. Finally, the entire mixture is diluted to 4 liters in order to manufacture sheets of 80 g / m2 of substance. iv) Fabrication of the test sheets _ 500 ml of a well homogenized suspension is placed in the test tube. Add one ml of PAE (commercial solution diluted 10 times to obtain an acceptable sample volume). The test tube is inverted several times to mix well. The content of the test tube is emptied into the container and filled with 6 liters of distilled water to extract the test sheet. For 10 seconds, mixing is carried out by bubble formation followed by a rest period of 10 seconds, and then the sheet is manufactured by vacuum extrusion. The sheet is then recovered on a felt substrate, then dried in vacuum for 7 minutes. Afterwards, the blade is weighted accurately and the volume removed is adjusted with the object of obtaining the desired weight of substance (rule of three). If a sheet has the desired weight and has no manufacturing defect, it is selected for the remaining operations, ie chemical and optical tests. 2. Measurement of the amount of ash _ The amount of Ti02 present in the sheet of 80g / m2 is measured by calcination of a third part of the test sheet at a temperature of 80 ° C for one hour. The percentage of Ti02 present in the sheet is calculated as follows: Itl after calcination - I - in vacuum Amount of ash (%) = Calcination ufantes - If-in vacuum The quantity of ash corresponds to the amount of mineral charges present in the sheet. This determination is made using the method NF 03-047 (taken from French Paper, Carton, and Pulp Standards: test methods, volume A, fourth edition, 1985). 3. Measurement of the opacity of the impregnated and pressed sheet. i) Preparation of melamine formaldehyde resin (Incam 3240 ECSC resin) 400 g of water were heated at 60 ° C. When the temperature is reached, 245 g of previously weighed resin are gradually emptied into a direct current. Once all is dissolved, stirring is allowed to continue for 30 minutes at a temperature of 60 ° C. After cooling, filtration is carried through the 50 μM tarpaulin. ii) Impregnation-pressing Paper strips of 7 cm by 10 cm are cut. The strips are then impregnated by capillary action by placing them in the resin for 1 minute. Center two glass rods and dry for 2 minutes in a dryer at a temperature of 120 ° C. The strips are impregnated a second time by immersion in resin for one minute. Then they are squeezed between a steel bar and a glass bar. They are dried for 3 minutes in a dryer at a temperature of 120 ° C. These sheets are fixed on a substrate formed from the bottom to the top by 2 white barriers and 3 kraft barriers, and the rest of the sheet is in direct contact with the kraft barriers. The obtained laminates are pressed for 8 minutes at a temperature of 150 ° C under a pressure of 100 bar. iii) Measurement of optical properties The opacity of the laminates is measured by evaluating the contrast ratio for each of the papers to be tested between the area on the kraft substrate and the area on the white substrate, using the "opacity" function of the Datacolor Elrepho 2000 spectrocolorimeter. EXAMPLE 1 Preparation of a mixed mineral agglomerate compound according to the invention in the form of an aqueous suspension RL62 is used in the form of an aqueous suspension, titrating 40 g / l. The mineral agglomerates are generated by heterocoagulation of the Ti02 particles with the addition of silica. The heterocoagulation process consists of the addition of a part of silica at a regulated pH at the bottom of a stirred tank containing a Ti02 solution. The pH of the heterocoagulation can be between 4.5 and 6.5, but preferably at a pH of 5.5. The pH is regulated by the simultaneous addition of a solution of HCl to the pulp. This operation is carried out at room temperature. The final suspension contains 10% silica by mass in comparison with the pigment content of Ti02, and the overall dry extract (Ti02 + Si02) is approximately 11%.

After 15 minutes of contact at a regulated pH of 5.5, the suspension, still under stirring, is brought to a temperature between 60 ° C and boiling point for 1 to 3 hours, and then cooled to room temperature. All samples prepared according to this protocol were tested by preparing test sheets (round sheets). For all tests, the volume of the mixture "fiber + charge + PAE" removed in the mixing tank was adjusted to obtain sheets with the same weight of substance: 80 g / m2. - A section of the test sheet is calcined to determine the amount of oxide present in the dry leaf (Si02 + Ti02). Knowing the amount of silica added in relation to Ti02, the percentage of Ti02 present in the dry leaf is then calculated. The protocol for the formation of the test sheet and the principle for calculating the amount of Ti02 are presented in detail in the previous section, "Materials and Methods". By quantifying Ti02 and Si02 with X-ray fluorescence in the obtained leaves, it was verified that there is no preferred retention of one or the other type of minerals. The Si02 / Ti02 ratio was maintained throughout the leaf formation process. - The other section of the test sheet is impregnated with resin and pressed to obtain a laminated paper for which the opacity and whiteness are measured afterwards. The protocols for impregnating and measuring the opacity are also described above. Some samples were also tested in the test sheet retention test to evaluate the strength of agglomerates to shear stress. This test consists of subjecting the mixture "fiber + load + PAE" to a rapid agitation and applying a shear stress during a certain time immediately before the formation of the test sheet.

The contribution of each of the two phenomena (retention effect and spacing) to the total increase in opacity measured for each of the tests is presented in detail in tables 1 and 2. Table 2 (below) also shows the results obtained with a control compound 1 (TI). It was prepared by simple mixing of silica and Ti02. "TABLE 1 Test Op, Acid No. Si02 Cured Measurements? Opa? Ret? Spac 1 -0% none 91.1 Ref. Ref. Ref. 2 1% 3 hrs. To 92.0 + 0.9 +0.6 +0.3 boiling temp 3 5% 3 hrs at 92.9 +1.8 +1.1 +0.7 boiling temp 4 10% 3 hrs at 92.8 +1.7 +1.0 +0.7 boiling temp 5. 10% 1 hr. to 93 +2.0 +1.1 +0.9 boiling temperature Amount of ash (% Ti02) or Measurements? ash 1 -35.6 Ref. 2 37.8 +2.2 3 39.9 +4.3 4 39.5 +3.9 5. 39.7 +4.1 TABLE 2 No. of S Sii02 Cured Opacity Amount of Test Absorption (%) ash (%) resin (%) 6 0% None 90.5 36.5 106 77 1100 %% 1 hr. a 91.8 41.5 105 temp. boiling TI 10% mixture 89.5 39.5 130 Simple? ash = increase in the amount of ash resulting from the best retention of Ti02 during sheet formation. ? opa =? ret +? spac = increase in total opacity? ret =? ash * slope = the increase in opacity resulting from the increase in the amount of Ti02 retained in the sheet (better retention). ? spac =? opa-? ret = increase of the opacity resulting from the best dispersion of the Ti02 retained in the sheet during the spacing effect of the silica aggregates. The results of tests 1 to 5 clearly illustrate that the use of mixed mineral agglomerates makes it possible not only to improve the retention of first pass (higher amount of ash), but also makes possible and improve the opacity of retained Ti02 since in In each case, the increase in opacity (? opa) is greater than the increase in opacity normally expected with an increase in the amount of ash (? ret). In contrast, a comparison of the results of the control test 1 (TI) with the results of the tests 6 and 7 clearly indicates that a simple mixture, that is, without particular attention to the level of pH and curing conditions, does not result in an improvement of the opacity. The significant increase in resin absorption indicates a porous and inhomogeneous structure. As for the influence of the amount of silica, it is observed that the tests carried out with 5 and 10% silica were clearly more productive than the test carried out with 1 & amp;; of silica. Therefore, the previous results show that compared to a conventional formula (Ti02 without silica), the use of a mixed mineral containing 5 and 10% silica should make it possible to increase the opacity from 0.6 to 0.9 points with a equivalent of ash. This increase corresponds to the opacity rate measured for the best performance test. With these compounds according to the invention, it is also conceivable to use less Ti02 while at the same time maintaining the same level of opacity as in the case of a conventional silica-free formula since the mixed minerals will improve the amount of ash and the rate of ash. opacity. The potential gain in Ti02 can be estimated by evaluating the increase in the amount of ash that corresponds to an increase in opacity equal to the production of opacity. This value reported in the amount of ash in the silica-free tests corresponds to the percentage of Ti02 that can be saved while maintaining the same level of opacity as in the control test. Under these conditions, using a mixed mineral containing 10% silica would allow a saving of at least 7 to 10% of Ti02 and would allow to maintain the same level of opacity as a conventional formula free of silicone. EXAMPLE 2 Using the "sheet retention" test, the retention capacity of the product obtained according to the invention was compared with the "conventional" titanium oxide product holding capacity. Product A: produced in accordance with - the invention, Si02 _ = 10% Product B: produced according to the invention, Si02 = 15% Product C: Ti02, Rhoditan RO 8 Product D: Ti02, Rhoditan RL62 The results of the test " sheet retention "appear in table 3. At a PAE rate of 0.8%, a standard rate in this application, products in accordance with the present invention result in retention rates in excess of the RL62 rates and equivalent to the rates of RL18. This is due to its anionic properties as well as to the particular structure, contributed by the synthesis process that is the object of the invention. In a 0% PAE, the self-locking nature of the product becomes evident. While more anionic than the RL18, the products according to the present invention have a much more remarkable self-retention character. It is an evident proof of a particular geometric structure formed of loose agglomerates that are not very dense and in this case it is demonstrated that the retention is far from being solely the result of electrostatic interactions between the fibers (naturally ionic) and the charges. TABLE 3 Reference Amount of PAE (dry% / fiber) of product 0.2 0.4 0.8 1.2 Retention rate (%) A 38 64 74 72 73 B 24 46 55 69 73 2. 5 [sic] 36 44 79 74 D 49 57 47 40 42 EXAMPLE 3 Determination of curing influence a) Effects in terms of retention: The shear strength of certain mineral agglomerate compounds identified in example 1 was tested using the "retention sheet test". Figure 3 shows the change in charge retention for different compounds as a function of the stirring speed imposed on the "cellulose / PAE / filler" mixture before the formation of the fiber mat. It is evident that the retention of the mixed mineral agglomerate decreases as the agitation speed is raised. However, it remains clearly higher than that obtained with a load free of silica. We can therefore conclude that the mixed mineral agglomerates are sufficiently resistant to shear to maintain good first-pass retention. The results obtained also show that: • the reduction of the curing time from 3 hours to 1 Jaora has little influence on the strength of the agglomerates. • the system containing 10% silica offers better retention than the system containing 5% silica, regardless of the rate of agitation. This result confirms that it is preferable to use 10% silica. b) Effects in terms of opacity It has been found, in fact, that it is not possible to obtain a good quality sheet of paper with a mixture of RL62 Si02 with 10% SiO2 that has not been subjected to the curing step. As soon as cellulose and PAE are added to the mixing tank, the formation of agglomeration and "balls". The curing step is therefore an essential step for the formation of effective mixed mineral agglomerates. EXAMPLE 4 Effect of mixed mineral agglomerates according to the invention on the whiteness of the laminated paper sheets _ The whiteness of the laminated sheets (measured in the white background area) was measured in each test. The results appear in table 4, below. The whiteness measurements were carried out in accordance with the CIÉ scale 1 * a * b * using a Datacolor Elrepho 2000® colorimeter spectrum. Table 4 Tests Bleached No. Si02 Cured L *? L * b *? B * 1 0% none 93.8 Ref. 5 . 0 Ref. 2 1% 3 hrs. to 93.9 + 0. 1 4. 7 - 0 3 temp. boiling 3 5% 3 hrs. to 94. 0 + 0 2 4. fifty . 5 temp. boiling 4 10% 3 hrs. to 94.3 +0.5 4.4 -0.6 temp. boiling 5 10% 1 hr. to 94.1 +0.3 4.4 -0.6 temp. Boiling It can generally be seen that the use of compounds according to the present invention improves the whiteness of the laminated sheet, even more with an increase in the amount of silica. With 5 and 10% silica, a gain of approximately 0.2 points in L * and a decrease b * of 0.4 to 0.6 points is measured. This decrease in b * offers a marked blue undertone to the laminated sheet which reinforces the whiteness impression. In addition to improving the retention and opacity rate of the retained Ti02, they also result in an improvement in the whiteness of the laminated sheet. EXAMPLE 5 Preparation of mixed mineral agglomerate compound in powder form In this example heterocoagulation is carried out according to the example described in example 1. After the curing step (1 hour at 90 ° C), the product is dried in a thin layer (15 hours in a dryer, at 105 ° C). The product obtained is divided into two parts. One part is used in this state, while the other part is ground with air jets (micronization). Both products are subjected to an opacity test. Cellulose fibers are used after placement in a paste with 40% dry extract. They are then compared with a control product: Rhoditan RL62, titanium oxide, placed in 40% paste. The formula used in this example: Cellulose fiber: 100 parts (15g) Opacity pigment: 100 parts (15g) PAE resin: 0.8% dry / fibers For the control product, 15g of RL62 Ti02 was introduced. For products according to the invention, 15 g of the combination Ti02 + Si02 were introduced. The rest of the method of operation is equal to the method described in the "opacity rate test". The results appear in table 5. These results clearly demonstrate that a product made according to the present invention subjected to a single drying (test 2) is not superior to a standard product (Test 1). On the other hand, after a micronization step (test 3), all the opacity improvement potential for this same product is obtained. In fact, it results in a sheet of paper, which after lamination, provides an opacity that these 2.4 points greater than the opacity of the laminated paper developed from the product d reference Rhoditan RL62, and with a totally comparable amount of Ti02 in the sheet . TABLE 5 No. of Si02 Dried Curaclo «= n micronizí Drying test 1 0% - - - 2 10% 1 Hr at 90 ° C 15 hr at 150 ° C no 3 10% 1 hr at 90 ° C 15 hr at 150 ° C yes No. of Opacity Quantity of Test Quantity (%: 1 ash (%) Ti02 (5) 1 89 .: L 38.2 38.2 2 89. .3 41.6 37.8 3 91. .5 41.6 37.8

Claims

CLAIMS 1. A process for the preparation of nrr compound based on Ti02 for use as an opacity agent including the steps according to which: - an aqueous dispersion of at least one inorganic spacer is mixed in an aqueous dispersion of Ti02, and the mixture of the two dispersions is carried out under agitation and at a pH between the respective isoelectric points of the Ti02 and the spacer and selected in such a way that the Ti02 have opposite surface charges and sufficiently different to result, under the effect of electrostatic forces, in its arrangement in mixed mineral agglomerates wherein the Ti02 particles are globally spaced from each other by the particles and / or aggregates of the spacer; - as necessary adjust the pH to the value established in step 1; with said process characterized in that it also includes the steps in accordance with which: the resulting aqueous dispersion of mixed mineral agglomerates is cured at a temperature sufficient to reinforce the strength of the bonds established between the Ti02 particles and the particles and / or aggregates of the spacer; the compound is recovered in the form of an aqueous dispersion of mixed mineral agglomerates; and - the compound can be formulated possibly in dry form.
2. The process according to claim 1, characterized in that the titanium dioxide used is rutile Ti02.
3. The process according to claim 1 or according to claim 2, characterized in that the titanium dioxide used is a rutile TiO ^ of pigment size.
4. A process according to claim 1,2 or 3, characterized in that the Ti02 is coated with a mineral surface treatment.
5. A process according to claim 4, characterized in that the surface treatment contains at least one compound selected from alumina, silica, zirconium, phosphate, cerium oxide, zinc oxide, titanium oxide, and mixtures thereof.
6. A process according to any of claims 1 to 5, characterized in that the aqueous dispersion of Ti02 contains from about 5 to 80% by weight of Ti? 2.
7. A process according to claim 6, characterized in that the aqueous dispersion of Ti02 contains from about 5 to 40% by weight of Ti02.
8. A process according to any of claims 1 to 7, characterized in that the inorganic spacer is selected from oxides of silicon, titanium, zirconium, zinc, magnesium, aluminum, yttrium, antimony, cerium, and tin; barium and calcium sulfates; zinc sulfide; carbonates of calcium, magnesium, lead and mixed metals; phosphates of aluminum, calcium, magnesium, zinc, cerium, and mixed metals; titanates of magnesium, calcium, aluminum and mixed metals; magnesium and calcium fluorides; silicates of zinc, zirconium, calcium, barium, magnesium, mixed alkaline earths and silicate minerals; alkaline aluminosilicates and alkaline earths; oxalates of calcium, zinc, magnesium, aluminum and mixed metals; aluminates of zinc, calcium, magnesium and alkaline earths; aluminum hydroxide, and mixtures thereof.
9. A process according to any of claims 1 to 8, characterized in that the inorganic spacer is selected from oxides of silicon, zirconium, aluminum, antimony, cerium, and tin, and mixtures thereof.
10. A process according to one of claims 1 to 9, characterized in that the inorganic spacer is used at a rate of approximately 1 to 40% by weight relative to the weight of Ti02.
11. A process according to one of claims 1 to 10, characterized in that the inorganic spacer is used at a rate of about 5 to 15% by weight relative to the weight of Ti02.
12. A method according to one of claims 1 to 11, characterized in that the Ti02 is a Ti02 of cationic pigment rutile.
13. A process according to claim 12, characterized in that the inorganic spacer is a silica, an alumina, a silicoaluminate, or a mixture thereof.
14. A process according to one of claims 1 to 13, characterized in that the inorganic spacer is a silica and that Ti02 is a cationic pigment rutile Ti02.
15. A process according to claim 14, characterized in that the silica has a specific surface comprised between approximately 20 and 300 m2 / g.
16. A process according to claim 14 or 15, characterized in that the silica occurs in the form of aggregates of a size between about 0.5 and 10 μm.
17. A process according to one of claims 14 to 16, characterized in that the silica is generated in situ through the acidification of a silicate solution.
18. A process according to claim 17, characterized in that the pH is adjusted after the in situ precipitation of the silica at a favorable value for the manifestation of electrostatic forces between the Ti02 and the silica generated in this way.
19. A process according to any of claims 14 to 17, characterized in that the two aqueous dispersions are bonded at a pH of the order of 5.5.
20. A process according to one of claims 14 to 19, characterized in that the silica is used at a rate of about 5 to 15% by weight relative to the weight of TiO2.
21. A process according to one of claims 14 to 20, characterized in that the curing step is carried out at a temperature comprised between 60 ° C and 100 ° C for at least 30 minutes.
22. A procedure according to claim 1, characterized in that the mixed mineral agglomerates obtained from the first step and the second step are subjected to mineral surface treatment.
23. A process according to claim 22, characterized in that the mineral surface treatment represents approximately 16% by weight or less relative to the total weight of the treated mixed mineral agglomerates. A compound based on Ti02 which can be obtained by the process defined according to one of claims 1 to 23. A compound based on Ti02 and Si02, characterized in that the particles of Ti02 and Si02 are arranged here in the form of mixed mineral agglomerates. with a base of Ti02 and Si02 in which the Ti02 particles are globally spaced from each other by aggregates of said silica. A compound according to claim 25, characterized in that the silica occurs at a rate of about 5 to 15% by weight relative to TiO2. A compound according to claim 25 or 26, characterized in that Ti02 is a Ti02 of cationic pigment rutile. A compound according to one of claims 25 to 27, characterized in that the silica has a specific surface comprised between approximately 20 and 300 m2 / g and / or occurs in the form of aggregates of a size comprised between approximately 0.5 and 10 μm. A compound according to one of claims 25 to 28, characterized in that - the mixed mineral agglomerates based on TiQ2 and SiO2 are coated with a mineral surface treatment. A compound according to claim 29, characterized in that this mineral surface treatment represents approximately 16% by weight or less relative to the total weight of the mixed mineral agglomerates. The use of a compound obtained according to one of claims 1 to 23 or a compound defined according to one of claims 24 to 30 as an opacifying agent. The use according to claim 31 in the paper, plastics and paints industries.