FR2773167A1 - PROCESS FOR THE PREPARATION OF MIXED MINERAL FLOCKS BASED ON TIO2, COMPOSITION BASED ON TIO2 AND SIO2 AND ITS USE AS AN OPACIFYING AGENT IN PARTICULAR IN THE PAPER INDUSTRY - Google Patents

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

The subject of the present invention is a composition based on TiO2, useful for

  as an opacifying agent, especially in laminated papers and a

  preparation process for obtaining said composition.

  Laminated paper, commonly called decor paper, is the surface element with an opacifying and decorative function, used for the

  manufacture of laminated panels, intended for the furniture industry.

  A special feature of decor paper is that it has an extremely high rate of TiO2, up to 40% of the mass of the

dry leaf.

  For comparison, print-write type papers can

contain a maximum of 10%.

  In fact, this high content of TiO2 is explained by the level of opacity required for decorative paper. This paper undergoes a stratification process which tends to make it transparent. However, this is incompatible with its opacifying and decorative functions. It is therefore necessary to remedy it.

by adding an opacifying agent.

  Titanium dioxide is conventionally used for this application because it is the only white pigment that can provide the levels

  opacity required thanks to its high refractive index.

  WO 89/08739 however proposes to replace at a rate of 5 to 40% by weight, TiO2 by amorphous precipitated silica and to implement the corresponding mixtures, which are more advantageous in terms of

  economic, as charges in the paper industry.

  Conventionally, the sheets of paper are prepared from a mixture of cellulose fibers and mineral fillers, mainly TiO2 dispersed in water. This mixture is contained in a "headbox" which feeds a canvas where the sheet is formed by drainage and filtration. During this filtration, the cellulose fibers are retained on the canvas as well as part of the mineral filler, whether or not in interaction with the entangled fibers. We thus obtain "the fibrous mattress" which after drying

give the sheet of paper.

  In fact, it turns out that only a part of the initial amount of TiO2 is retained in the fibrous mat and further that this fraction is generally too agglomerated for the TiO2 to develop a

maximum opacity.

  To reduce this loss of TiO2 during the formation of the fibrous mat, papermakers generally introduce retention agents into their cellulose mixtures. These agents are conventionally cationic polymers which allow the fixation of TiO2 particles on the

  fibers by homo- and hetero-flocculation phenomena.

  However, in the case of the retention of opacifying fillers, such as titanium dioxide, the use of an electrically charged polymer results in a loss of efficiency in opacity due to too strong and too dense flocculation. Consequently, it appears that the simple act of retaining TiO2 in the fibrous mat is not sufficient in itself in terms of opacity yield. It would also be necessary to maintain the TiO2 in the fibrous mat, in a sufficiently dispersed form so that it can retain

  i5 its pigmentary properties and develop a good opacifying power.

  Advantageously, we could then obtain the same opacity while engaging less TiO2. The opacity yield of TiO2 would be significantly increased. To this end, international patent application WO 97/18268 proposes to carry out a surface treatment of the TiO2 particles. This treatment consists in covering them with a monolayer of inorganic particles such as silica and whose particle size remains smaller than that of the TiO2 particles. This particulate monolayer type coating

  allows spacing of the TiO2 particles from each other.

  The object of the present invention is precisely to propose a new composition based on TiO2 which meets all the requirements.

mentioned above.

  More particularly, it proposes a new opacifying agent system making it possible to improve both the retention of TiO2 during the formation of the fibrous mat, and to store it there in a structure of

  flocculation least penalizing possible for opacity.

  The inventors have thus demonstrated that a solution to the problem of agglomeration of the mineral filler consists in creating mixed mineral flocs by intercalating between the TiO2 particles of the particles, under

  form of aggregates or not, of a so-called inorganic spacer agent.

  The mixed mineral flocs obtained according to the invention are advantageous for several reasons: - they make it possible to keep the TiO2 particles sufficiently dispersed during the various stages of formation of the paper sheet so that the maximum of them can develop their character pigmentary and therefore participate in the opacity in the dry leaf, - they have an open structure, which is conducive to better retention, - they also prove to be sufficiently resistant to withstand the capillary forces which occur during draining and drying of the fibrous mat, as well as the shears that may be encountered during

the manufacture of the sheet.

  In fact, the internal cohesion of the mixed mineral flocs, resulting from the association with TiO2 of at least one inorganic spacer agent, is based on the solidity of ionic bonds established between TiO2 and the spacer agent. This cohesion stems directly from the process used to prepare said flocs

mixed minerals.

  More specifically, these flocs are obtained under operating conditions such as TiO2 and the inorganic spacer agent considered

  have opposite and significantly different surface charges.

  In particular TiO2 and the inorganic spacer agent considered must have sufficiently different isoelectric points for there to be a pH range in which these two mineral species have opposite charges. Under these conditions, the two mineral species manifest one towards the other an electrostatic attraction. The resulting attractive forces must be sufficient to lead on the one hand to a structural arrangement of the two compounds and on the other hand to stabilize them.

under this form.

  Consequently, the first subject of the present invention is a process for preparing a composition based on TiO2, useful as an opacifying agent, characterized in that it comprises the steps according to which: - a dispersion is mixed with an aqueous dispersion of TiO2 aqueous of at least one inorganic spacer, the mixing of the two dispersions being carried out with stirring and at a pH between the respective isoelectric points of said TiO2 and spacer and chosen such that said TiO2 and spacer have surface charges opposite and sufficiently different to lead, under the effect of electrostatic forces, to their arrangement in mixed mineral flocs in which the TiO2 particles are globally spaced from one another by particles and / or aggregates of the spacing agent; - if necessary, the pH is regulated at the value fixed in step 1, - the resulting aqueous dispersion of mixed mineral flocs is matured at a temperature sufficient to strengthen the solidity of the bonds established between the TiO2 particles and the particles and / or aggregates of the spacing agent, said composition is recovered in the form of an aqueous dispersion of mixed mineral flocs and - optionally said composition is formulated in a dry form. FIG. 1 gives a schematic representation of the structure of mixed mineral flocs obtained according to the invention. It is confirmed by the

  transmission electron micrograph shown in Figure 2.

  For the purposes of the invention, the term floc is intended to denote mixed agglomerates of two mineral species of TiO2 type and inorganic spacer agent such as SiO2 for example. These agglomerates result from the association between aggregates of said spacer agent and TiO2 particles. A spacer is made up of particles or aggregates of

  particles which are interposed between the TiO2 particles.

  As regards the isoelectric point, it corresponds to the pH for which the particle of the mineral species considered has a generally zero surface charge. For a pH higher than this value, the charge is generally negative and for a lower pH, the charge is generally positive.

  The TiO2 used according to the invention is preferably a rutile TiO2.

  More preferably, it is a rutile TiO2 of pigmentary size. It can if necessary be coated with a mineral surface treatment. Preferably, this surface treatment comprises at least one compound chosen from alumina, silica, zirconia, phosphate, cerium oxide,

  zinc oxide, titanium oxide and mixtures thereof.

  The amount of oxide (s) can be of the order of 1 to 20% by weight or less or preferably of the order of 3 to 10% by weight or less, per

  io relative to the total weight of the pigment.

  By way of illustration of these titanium dioxides, mention may be made in particular of the two rutile pigments Rhoditan RL18 and RL62, marketed by Rhône-Poulenc. These two pigments are differentiated by the composition of their surface treatment and the Zeta potentials which

I5 result.

  RL 18 has a silica-alumina surface treatment

  (SiO2 / AI203) and a negative Zeta potential at pH 6, it is called "anionic TiO2".

  However, RL62 has a phosphate-alumina surface treatment (P205 / A1203) with a positive Zeta potential at pH 6, it is called "cationic TiO2". The choice of pH 6 is close to the start-up pH

industrial work.

  In the context of the present invention, the choice of TiO2, anionic or cationic, of course conditions the choice of the spacer agent

associated with it.

  In each case, an inorganic spacing agent is chosen having an isoelectric point sufficiently different from that of the TiO2 considered so that the electrostatic attractions can manifest between

  the two compounds which are necessary for their arrangement.

  The aqueous dispersion of TiO2, used according to the invention comprises approximately 5 to 80% by weight of TiO2 and preferably approximately 5 to%. In this regard, the limiting point is the viscosity of the suspension which must

  stay at a reasonable value to be easy to handle.

  According to a preferred embodiment of the invention, the TiO2 selected is a TiO2

  cationic pigment rutile and in particular Rhoditan RL62.

  As regards the inorganic spacing agents considered according to the invention, they must not interfere with the others

  reagents conventionally used in the paper industry.

  Preferably, they do not significantly absorb visible light. In general, the size of their particles is smaller than that of the TiO2 particles. However, these particles are preferably arranged in the form of aggregates whose size is then greater than those of the TiO2 particles. Preferably these aggregates have a size included

io between about 0.5 and 2pm.

  By way of illustration of the inorganic spacing agents which can be used according to the invention, mention may in particular be made of oxides of silicon, titanium, zirconium, zinc, magnesium, aluminum, yttrium, antimony, cerium and tin; barium and calcium sulfates; zinc sulfide; zinc, calcium, magnesium, lead and mixed metal carbonates; aluminum, calcium, magnesium, zinc, cerium and mixed metal phosphates; titanates of magnesium, calcium, aluminum and mixed metals; magnesium and calcium fluorides; zinc, zirconium, calcium, barium, magnesium, mixed alkaline earth and silicate minerals; alkali and alkaline earth aluminosilicates; calcium, zinc, magnesium, aluminum and mixed metal oxalates; zinc, calcium, magnesium and alkaline earth aluminates; Aluminum hydroxide and mixtures thereof. Of course, the choice of this spacing agent is made so that it has a sufficient isoelectric point difference

with the TiO2 form selected.

  As inorganic spacing agents which are particularly suitable for the present invention, mention may be made of inorganic oxides which are preferably chosen from oxides of silicon, zirconium,

  aluminum, antimony, cerium and tin and their mixtures.

  In the particular case where the cationic rutile pigment TiO2 is retained, this inorganic spacer agent is preferably a silica, a

  Alumina, a silicoaluminate or a mixture thereof.

  Regarding the relationship between TiO2 and the spacer, it

  is of course variable depending on the nature of the spacing agent selected.

  In general, the lower limit of this ratio is constituted by the minimum quantity of inorganic spacer agent which is necessary to observe a positive effect at the level of the opacifying yield and its upper limit by the maximum quantity of spacer agent beyond which effects undesirable would appear on the paper incorporating the composition obtained according to the claimed process. These undesirable effects can notably result in a fragility of the paper in particular in terms of

  resistance whether dry or wet.

  This spacing agent can generally be used in an amount of about 1 to 40% relative to the weight of TiO2, preferably in an amount of about 5 to 15% by weight and more preferably in an amount of about 10%.

in weight.

  As explained above, these two compounds are brought into contact in the form of corresponding aqueous dispersions, under operating conditions such as are produced by heterocoagulation of TiO2 with the particles and / or aggregates of particles of the inorganic spacer agent, mixed mineral flocs. The spacer can also be precipitated in situ. In this case the pH adjustment conducive to heterocoagulation

  will be performed after the spacing agent precipitation step.

  These operating conditions conducive to the manifestation of the phenomenon of heterocoagulation between the inorganic spacer agent and TiO2 are in particular the choice of a pH within a range defined by their respective isoelectric points. This pH should be chosen so that the two compounds have opposite and sufficiently different surface charges. For implementation reasons, it is desirable that the isoelectric points of the spacer agent and of the TiO2 be spaced at least

a pH unit.

  The mixed mineral flocs making up the expected composition therefore form with stirring of said dispersions, generally at room temperature and at a pH as defined above. It may be necessary, during the reaction, to adjust the pH for the

  maintain at a value conducive to the formation of said flocs.

  The attraction is immediate. However, it is preferable to maintain agitation for approximately 15 minutes so as to stabilize the system before

the ripening stage.

  According to a preferred variant of the invention, TiO2 is used in a cationic pigmented rutile form and preferably is RL62, and the agent

associated spacer is silica.

  More preferably, the silica used is a silica with a large specific surface, in particular between about 20 and 300 m 2 / g. It can be in the form of aggregates of sizes between approximately 0.5 and μm. The use of silica as a spacing agent in accordance with

  The present invention is advantageous for several reasons.

  First of all, it has an isoelectric point around 2, which is a sufficiently different value from that of the isoelectric point of the

  cationic form of TiO2 (6.5 to 7).

  Furthermore, silica has the advantage of not significantly adsorbing visible light, which is favorable in terms of

whiteness of the leaf.

  The pH for bringing the two corresponding dispersions into contact is between the isoelectric points of the spacer agent and of TiO2. Normally the upper bound is imposed by the isoelectric point of the TiO2 considered and the lower bound should be imposed by the isoelectric point of the spacing agent concerned. In this case, this pH should be between 2 and 6.5. However, in the particular case of RL62, it is necessary to avoid the dissolution of its surface treatment. To this end, the pH range will be limited between 4.5 and 6.5. More preferably the process

  according to the invention is implemented at a pH of the order of 5.5.

  In the particular case of the preparation of a composition comprising TiO2 in a cationic pigmented rutile form associated with aggregates of silica particles, the silica is used in an amount of at least

  1% by weight relative to the weight of TiO2.

  It is only from this level of silica that a significant gain is observed in terms of retention and opacity. This silica content can be increased up to approximately 20% by weight of TiO2. Beyond this value, we

  is confronted with the problem of brittleness of the paper mentioned above.

  Consequently, the silica is preferably used at a rate of approximately 5 to 15% by weight of the weight of TiO2 and more preferably at a rate of 10% by weight D5 The silica can be introduced either in the form of an aqueous dispersion of particles slurry type silica or be generated in situ by

  acidification of a solution of silicates.

  In the particular case where the silica is precipitated in situ in the dispersion of TiO2, the pH of the reaction medium is adjusted after the precipitation step to a value conducive to the manifestation of electrostatic forces between TiO2 and the silica thus generated. These forces

  are therefore necessary for their heterocoagulation.

  The second step required according to the claimed process, in fact corresponds to an operation of ripening the mixed mineral flocs

trained in the previous step.

  As mentioned above, the mixed mineral flocs obtained according to the claimed process are in particular intended to be used as an opacifying agent in the paper industry. This implies a whole

  succession of manipulations of said flocs.

  It is therefore necessary that these flocs are strong enough to resist shearing, if necessary the effect

  flocculant of polymeric derivatives such as PAE (polyamino-amide-

  epichlorohydrin), as well as the removal of water during formation and drying

of the leaf.

  It is therefore important that the TiO2 particles present in the composition obtained according to the invention are not only sufficiently dispersed to improve their opacity yield, but also better

  retained during the formation of the sheet.

  Consequently, the ripening operation carried out according to the claimed process proves to be particularly advantageous for reinforcing the chemical or even steric interactions established within the mixed mineral flocs. It is moreover likely that some of the ionic bonds will be converted into covalent bonds at the end of this step of

ripening.

  In the particular case of the preparation of a composition of mixed mineral flocs based on TiO2 and SiO2, this ripening step is carried out at a temperature above 40 C. Preferably, the temperature

  is between about 60 C and 100 C.

  As for the duration of the heating, it is at least 30 minutes and if necessary can be extended up to three hours. At the end of this heating step, the resulting composition is allowed to cool to

  room temperature and can be recovered as is.

  This composition can be used directly under

  this form as an opacifying agent.

  However, one can also consider formulating it in a dry form. To this end, it is possible to apply to the dispersion obtained

  according to the invention different conventional drying techniques.

  In particular it may be spray drying or thin layer drying. However, a simple drying will not lead to a product properly redispersed. Since the flocs agglomerate on drying, it is preferable to deagglomerate the product by an air jet grinding step (micronization). According to another variant of the claimed process, the mixed mineral flocs obtained at the end of the first or of the second step of the process can undergo a mineral surface treatment. This comprises at least one hydrated oxide as defined above. The latter can be precipitated within the reaction medium after bringing the

  dispersions of TiO2 and the spacer.

  The mineral surface treatment represents approximately 16% by weight or less or preferably of the order of 10% by weight or less, relative

  the total weight of the mixed mineral flocs thus treated.

  The present invention extends to compositions based on TiO2

  likely to be obtained according to the claimed process.

  It also relates to a composition based on TiO2 and SiO2, characterized in that the particles of TiO2 and SiO2 are arranged therein in the form of mixed mineral flocs in which the particles of TiO2 are generally spaced from one another by aggregates of said silica. These mixed mineral flocs of TiO2 and SiO2 are stabilized thanks to electrostatic forces established between the particles of TiO2 and the aggregates of SiO2. Furthermore, this stability of the mineral flocs is reinforced by the fact that they undergo a ripening as described above. This ripening operation contributes in particular to creating covalent bonds

  between TiO2 and SiO2 within the flocs.

  In the case of mixed mineral flocs according to the invention, there is no uniform distribution of the aggregates of the inorganic spacer agent around the TiO2 particles. This distribution is discontinuous. Figures 1 and

  2 give an idea of the structure of said flocs.

  TiO2 is preferably a rutile TiO2 of pigment size.

  If necessary, it can be coated with a mineral surface treatment. This surface treatment can be chosen from phosphate, alumina, silica, zirconia, cerium oxide, zinc oxide, titanium oxide and

their mixtures.

  The amount of oxide (s) can be of the order of 1 to 20% by weight or less or preferably of the order of 3 to 10% by weight or less, per

relative to the total weight of the pigment.

  The TiO2 is preferably a cationic pigmented rutile TiO2.

  Preferably, the TiO2 is RL62.

  More preferably, the silica used is a silica with a large specific surface, in particular between about 20 and 300 m 2 / g. It is in the form of aggregates of sizes between approximately 0.5 and 10 μm. The silica is preferably a precipitation silica. He can also

  it is a silica generated in situ by acidification of a solution of silicates.

  The silica is preferably present in an amount of approximately 1 to 20% by weight of the weight of TiO2 and more preferably in an amount of approximately 5 to% by weight and more preferably 10%. Where appropriate, these mineral flocs based on TiO2 and SiO2 can be coated with at least one mineral surface treatment as defined above. The amount of mineral surface treatment can be of the order of 16% by weight or less or preferably of the order of 10% by weight or

  less, compared to the total weight of the mixed mineral flocs.

  The compositions as defined above or obtained according to the invention prove to be advantageous for the preparation of paper including laminated paper and very particularly advantageous in terms of retention at the level

  cellulose fibers and opacity yield of the TiO2 used.

  The conventional processes for preparing laminated paper or decorative paper generally use, in addition to cellulose fibers of anionic nature, and the opacifying agent, a polymeric agent of character

  cationic acting as a reinforcing agent in the wet state and as a retention agent.

  In the case of the use of a composition based on TiO2 / SiO2 mineral flocs as opacifying agent, it is observed that the chemical retention by electrostatic attraction is advantageously reinforced.

  compared to TiO2 in individual cationic form.

  This increase in terms of retention can be explained by the

next way.

  In the absence of cationic polymer, the cationic TiO2 is attracted by the anionic cellulose fibers which is favorable for the retention of TiO2. On the other hand, in the presence of the polymer, the fiber-TiO2 interactions change and the retention of TiO2 decreases. This phenomenon is interpreted by a cationization of the cellulose fibers, resulting from their covering by the

cationic polymer.

  Conversely, in a composition based on TiO2 / SiO2 mineral flocs, there is a mixture of cationic TiO2 and anionic charges, SiO2, whose zeta potential is generally negative. Mixed mineral flocs therefore behave like anionic charges. Under these conditions one can imagine that the mixed flocs could enter into attractive interaction with the cellulose fibers positivated by the cationic polymer, via the negatively charged silica aggregates which they contain. this results in a

gain in terms of retention.

  The compositions based on mixed mineral flocs claimed and obtained according to the invention are particularly advantageous as an agent

  opaque and especially in the paper industry.

  The gain in opacity measured on the sheets prepared using a composition based on mixed mineral flocs according to the invention, clearly results from the combination of two phenomena: the increase in the quantity of TiO2 retained on the sheet, resulting from better retention at the time of the formation of the fibrous mattress, and the improvement of the opacity yield resulting

  _the best dispersion of the titanium particles contained in said flocs.

  Furthermore, it was noted that these compositions favored the

  whiteness of the paper incorporating them.

  In addition to this application in the paper industry, the compositions claimed and obtained according to the invention are also advantageous to

  as an opacifying agent in the paint and plastics industries. The examples and figures given below are presented for

  illustrative and not limiting of the present invention.

  Figures: Figure 1: schematic representation of spaced TiO2 pigments

by aggregates of SiO2.

  Figure 2: electron micrograph in transmission of

  mixed mineral flocs based on TiO2 and SiO2.

  Figure 3: evolution of charge retention for different mixed minerals as a function of the stirring speed imposed on the mixture

  "cellulose / PAE / load" before formation of the fibrous mat.

MATERIALS AND METHODS

  The products used are commercial products: - The titanium dioxide used in the examples is rutile titanium dioxide marketed under the name of Rhoditan RL62 by the company RHONE-POULENC. This pigment consists of rutile TiO2 coated by a phosphate alumina surface treatment (P205 / AI203). At pH 6, its zeta potential is positive. His

  isoelectric point is located around 6.5-7.

  - Cellulose fibers: dry leaves of a 70/30 mixture of short fibers / long fibers previously refined to 35 SR, supplied

by the company ARJO WIGGINS.

  j 5 - Silica is a precipitation silica with a large specific surface area between 20 and 300 m2.g- 'and having agglomerates of sizes between 0.5 and 10 µm. Its isoelectric point is located at

around 2.

  - PAE resin (polyaminoamide epichlorohydrin), R4947 from the company CECA A. Test of the "retention form" io Apparatus: - Rapid dispersers DispermatC and PendraulikO - Mixing tanks - "Retention form", company TECHPAP Procedure: - Preparation of the fiber / TiO2 dispersion To 15 g of fibers redispersed in 500 ml of deionized water for 10 min at Dispermat at 3000 rpm, the necessary amount of TiO2 slurry or product suspension according to the invention is added, so as to introduce g dry of TiO2. The extract of the TiO2 slurry or of the suspension according to the invention will therefore be taken into consideration. This addition is made in a tank

  mixture. It is followed by a dilution to 4 liters with deionized water.

  - Preparation of the test sample A 500 ml test sample of the well-homogenized mixture is drawn off in a test tube. We add to the micropipette, the amount of PAE resin desired (commercial solution diluted 10 times). The test tube is turned over 2 times to mix well. This test portion is then introduced

  in the retention form to obtain a sheet.

  - Measurement of retention The formation of the sheet is triggered after an agitation time of 30 s at a speed of 1,300 rpm followed by a rest time of 1 s. The sheet obtained is recovered on the canvas in the form of a "paton", dried in an oven and then calcined at 800 C. The ash obtained is then weighed

to within 104 g.

  The retention rate is given by: P2 / P1.

  P1 = load weight (TiO2 + SiO2) in an initial 500 ml sample

  P2 = weight of ash after calcination of the prepared sheet.

  B. Opacity yield test The opacity yield tests were carried out using manufactured formulas in order to determine the spatial distribution of the dioxide

titanium in the dry sheet.

  The formulas were produced in accordance with the mode

  procedure described in the paragraph below.

  The optical properties of the impregnated and pressed form have

  also were measured according to the method described below.

  1. Manufacture of the formulas i) Formulation of the cellulose paper pulp: 15 g (which represents 100 parts) Opacifying composition: 100 parts (expressed as TiO2) or 15 g PAE: 0.8% dry compared to cellulose ii ) Preparation of the dough: defibraqe The cellulose is torn by hand into small squares after having moistened it with water. The small cellulose squares are gradually added to 500 ml of water with stirring in the Dispermat bowl at 1000 rpm. After adding o cellulose, the speed is increased to 3000 rpm and left stirring for min. iii) Mixture of opacifying composition-fibers The defibrated cellulose is diluted to 1 liter. Then, it is stirred in a mixer with paddle. The opacifying composition is added in the form of a powder or a suspension and then the mixture is stirred for 5 min. Finally, the whole is diluted to 4 liters in order to manufacture

grammage at 80 g / m2.

  iv) Manufacture of formulas 500 ml of well-homogenized suspension are removed in a test tube. We add the PAE (commercial solution diluted 10 times to have an acceptable intake volume), or 1 mi. The test tube is turned over several times to mix well. The contents of the test tube are poured into the bowl of the form filler filled with 6 liters of distilled water. It is mixed by bubbling for s, it is left to stand for 10 s, then the form is produced by drawing under vacuum. The form is then collected on a cardboard support, then

  placed in a vacuum dryer for 7 min.

  We then weigh the form accurately and correct the volume

  sampled to reach the desired grammage (rule of three).

  If a sheet has the desired grammage and has no manufacturing defect, it is selected for the rest of the operations, that is to say,

  chemical and optical characterizations.

  2. Measurement of the ash rate The quantity of TiO2 present in the sheet of 80 g / m2 is measured

  by calcining a third of the form at 800 C for one hour.

  The percentage of TiO2 present in the sheet is thus calculated: my next calcination - m vacuum ash rate (%) = m before calcination - my vacuum The ash rate measures the quantity of mineral fillers

  present in the sheet. This determination is made according to method NF 03-

  047 (Collection of French Standards Paper, Cardboard, and Pulp: method

  of essays, volume A, 4th edition, 1985).

  3. Measurement of the opacity of the impregnated and pressed sheet i) Preparation of the melamine formaldehyde resin (Inilam 3240 resin from the company CECA) 400 g of water is heated to 60 C. When this temperature is reached, poured in rain gradually with magnetic stirring the 245 g of resin previously weighed. Once everything is dissolved, the mixture is left stirring at 60 ° C. for 30 minutes. After cooling, it is filtered through a 50 μm fabric. ii) Impregnation - pressing Strips of paper 7 cm by 10 cm are cut. The strips are then impregnated by capillarity by placing them for 1 minute on the 1o resin. Express between two glass rods and dry for 2 minutes

in an oven at 120 C.

  The strips are impregnated a second time by immersion in the

  resin for 1 min. We express between a steel rod and a glass rod.

  It is dried for 3 min in the oven at 120 C.

  These sheets are fixed on a support made up from the bottom of 2 white barriers and 3 kraft barriers, the form being in direct contact with

kraft barriers.

  The laminates obtained are pressed for 8 min at 150 ° C. under a

pressure of 100 bars.

  iii) Measuring the optical properties The opacity measurements on the laminates are made by evaluating the contrast ratio, for each of the papers to be tested, between the area on a kraft background and the area on a white background, using the "opacity of the

  Elrepho 2000 spectrocolorimeter from DATACOLOR.

EXAMPLE 1

  Preparation of a composition of mixed mineral jibs according to the invention in the form of an aqueous suspension RL 62 is used in the form of an aqueous suspension

grading 40 g / I.

  The mineral flocs are produced by heterocoagulation of

  TiO2 particles with silica aggregates.

  The heterocoagulation process involves adding to regulated pH

  the silica slurry in a stirred base stock containing the TiO2 suspension.

  The heterocoagulation pH can be chosen between 4.5 and 6.5, but it is preferable to work at pH = 5.5. The pH is regulated by simultaneously adding an HCl solution to the silica slurry. This operation takes place at room temperature. The final suspension contains 10% by mass of silica relative to the content of pigment TiO2 and the overall dry extract (TiO2 + SiO2) is approximately

D 11%.

  After contact for 15 minutes at a regulated pH of 5.5, the suspension, still stirred, is brought to a temperature between C and the boiling temperature for 1 to 3 h and then cooled to room temperature. o0 All the samples prepared according to this protocol, were tested by preparing formulas (round sheet). For all tests the volume of "fiber + load + PAE" mixture taken from the mixing tank was adjusted

  so as to obtain sheets of the same grammage: 80 g / m2.

  - Part of the form is calcined to determine the amount of oxide present in the dry sheet (SiO2 + TiO2). Knowing the amount of silica added relative to TiO2, we deduce the% of TiO2 present in the dry sheet. The formette training protocol and the principle for calculating the TiO2 rate are detailed in the previous chapter,

Materials and methods.

  It was verified by assaying TiO2 and SiO2 by X-ray fluorescence in the sheets obtained that there was no preferential retention of one or the other of the mineral species. The SiO2 / TiO2 ratio is retained throughout

  along the process of forming the sheet.

  - The other part of the form is impregnated with resin and pressed so as to obtain a laminated paper whose opacity and whiteness are then measured. Impregnation and opacity measurement protocols are

also described above.

  Some samples were also tested in the retention form test to assess the shear resistance of the flocs. This test consists in subjecting the "fiber + filler + PAE" mixture to lively and shearing agitation for a certain time immediately before forming the formette. 19) The contribution of each of the two phenomena (retention and spacer effect) to the total gain in opacity measured for each of the tests is

  detailed in Tables 1 and 2.

  Table 2 below also reports the results obtained with a control composition 1 (T1). This is prepared in

  simply mixing silica and TiO2.

TABLE 1

  Tests Opacity Ash rate ___ _ (% TiO2) N SiO2 Curing Measures Aoa. Aret AespMesures Acend

* 1 0% None 91.1 Ref. Ref. Ref. 35.6 Ref.

  2 1% 3 h at boiling point 92.0 +0.9 +0.6 +0.3 37.8 +2.2 3 5% 3 h at boiling point 92.9 9 +1.8 +1.1 +0, 7 39.9 +4.3 4 10% 3 h at boiling point 92.8 +1.7 +1.0 +0.7 39.5 +3.9 10% 1 hr at boiling point 93 +2.0 +1 , 1 + 0.9 39.7 + 4.1

TABLE 2

  Opacity Rate of test taking n SiO2 Curing (%) resin ash

_ (% 1-_ (%)

  6 0% none 90.5 36.5 106 7 10% 1 h at boiling point 91.8 41.5 105 T1 10% simple mixing 89.5 39.5 130 Acend = gain in ash rate resulting from better retention of TiO2 during leaf formation Aopa = Aret + Aelp = total opacity gain Arét = Acend * slope = opacity gain resulting from the increase in the TiO2 level retained in the leaf (better retention) Aesp = Aopa - Aret = gain d opacity resulting from the better dispersion of TiO2 retained in the sheet thanks to the spacer effect of the aggregates

silica.

  The results of tests 1 to 5 clearly show that the use of mixed mineral flocs not only improves first pass retention (higher ash rate) but also improves the opacity yield of the TiO2 retained since in all cases the gain in opacity (dopa) is greater than the gain in opacity normally expected by

  an increase in the ash rate (Aret).

  On the other hand, the comparison of the results of control test 1, T1 with those of tests 6 and 7 clearly reveals that a simple mixture, that is to say without taking any particular precaution in terms of pH and

  ripening, does not lead to an improvement in terms of opacity.

  The significant increase in resin uptake is indicative of a

porous and non-homogeneous structure.

  With regard to the influence of the silica content, we note that the tests carried out with 5 and 10% of silica are clearly more efficient

  than the test carried out with 1% silica.

  Consequently, the previous results show that compared to a conventional formulation (TiO2 without silica), the use of a mineral mixture containing 5 and 10% of silica should make it possible to improve the opacity from 0.6 to 0, 9 point at equivalent rate of ash. This gain corresponds to the yield

  opacity measured for the most efficient tests.

  It is also conceivable with compositions according to the invention, to use less TiO2 while retaining the same level of opacity as a conventional formulation without silica since the mixed minerals improve the ash rate and the opacity yield. The potential gain in TiO2 can be estimated by evaluating the gain in ash rate corresponding to an opacity gain equal to the opacity yield. This value related to the ash rate of the tests without silica corresponds to the percentage of TiO2 which can

  be saved while remaining at the same level of opacity as the control test.

  Under these conditions, the use of a mineral mixture containing 10% silica

  should save at least 7-10% of TiO2 while retaining the same level of opacity as a conventional formulation without silica.

EXAMPLE 2:

  Using the “retention form” test, the ability to retain the product obtained according to the invention was compared with those

of "classic" titanium oxides.

  Product A: product according to the invention SiO2 = 10% Product B: product according to the invention, SiO2 = 15% Product C: TiO2 Rhoditan RL18

Product D: TiO2 Rhoditan RL62.

  The results of the "retention form" test are given to

table 3.

  At a PAE rate of 0.8%, the usual rate in application, the products according to the invention lead to retention rates higher than that of RL62 and equivalent to that of RL18. This is due to their anionic character but also to their particular structure conferred by the synthesis process

subject of the invention.

  At 0% PAE, the self-retentive nature of the product is highlighted. Although more anionic than RL18, the products according to the invention have a clearly more marked self-retentive character. This is indeed proof of a particular geometric structure made up of loose and sparse flocs, and shows that, in this case, the retention is far from being solely the result of interactions of an electrostatic nature between the fibers.

  (naturally anionic) and charges.

TABLE 3:

  Ir PAE rate (% dry / fiber) Ref. product 0 0.2 0.4 0.8 1.2 Retention rate (%)

A 38 64 74 72 73

B 24 46 55 69 73

C 2.5 36 44 79 74

D 49 57 47 40 42

EXAMPLE 3

  Determination of the influence of the ripening stage t0 a) Effect in terms of retention: The shear resistances of certain compositions of mineral flocs identified in example 1 were tested by the "formette" test

retention ".

  FIG. 3 shows the evolution of the charge retention for different compositions as a function of the stirring speed imposed on the

  "cellulose / PAE / filler" mixture before the fibrous mat is formed.

  Note that the retention of mixed mineral flocs decreases when the stirring speed increases. However, they remain significantly higher than those obtained from the filler without silica. We can therefore conclude that the mixed mineral flocs are sufficiently resistant to

  shears to maintain good first pass retention.

  The results also show that: a reduction in the maturing time from 3 h to 1 h has little effect on the resistance of the flocs, * the system containing 10% silica provides better retention than the system containing 5% silica which whatever the speed

  of agitation. This result confirms that it is preferable to use 10% silica.

  b) Effect in terms of opacity In fact, it turns out that it is not possible to obtain a sheet of good quality paper from an RL62 SiO2 mixture containing 10% SiO2,

  have not undergone a ripening stage.

  As soon as cellulose and PAE are added to the mixing tank,

  is confronted with an agglomeration and the formation of "balls".

  The ripening stage is therefore a stage necessary for the

  formation of effective mixed mineral flocs.

io EXAMPLE 4:

  Effect on the whiteness of the laminated paper sheets of the mixed mineral coatings according to the invention The whiteness of the laminated sheets (the measurement is carried out on the white background area) was measured for each test. The results are

  grouped in Table 4 below.

  The whiteness measurements were carried out according to the CIE l * a * b * scale, on an ELREPHO 2000 spectrocolorimeter from the company.

DATACOLOR.

TABLE 4

  __ Whiteness tests No SiO2 Ripening L * AL * b * Ab *

1 0% None 93.8 Ref. 5.0 Ref.

  2 1% 3 h at boiling point 93.9 +0, 1 4.7 -0.3 3 5% 3 h at boiling point 94.0 +0.2 4.5 - 0.5 4 10% 3 h at boiling point 94 , 3 +0.5 _ 4.4 -0.6 10% 1 h at boiling point 94.1 +0.3 4.4 -0.6 1 ',, -J In general, we observe that the use compositions according to the invention improves the whiteness of the laminated sheet and this all the more as the silica content increases. For 5 and 10% of silica, a gain of about 0.2 point in L * is measured and above all a reduction in b * from 0.4 to 0.6 point. Such a decrease in b * gives a pronounced blue undertone to the

  laminated sheet and reinforces the impression of whiteness.

  In addition to an improvement in the retention and opacity yield of the TiO2 retained, they therefore also lead to an improvement in

  io the whiteness of the laminated sheet.

EXAMPLE 5

  Preparation of a composition of mixed mineral jibs

in the form of Powder.

  In this example, the heterocoagulation is carried out according to the method described in Example 1. After the ripening step (1 h at 90 ° C.), the product is dried in a thin layer (15 h in an oven at 150 ° C.). The product obtained is divided into two parts: one is used as it is while it is subjected

  on the other, air jet grinding (micronization).

  These two products are subjected to the opacity yield test. They are used on cellulose fibers after slurrying to a dry extract of 40%. They are compared with a control product: titanium oxide RHODITAN RL62 put in slurry at 40%. In this example, the formulation used is as follows: Cellulose fiber: 100 parts (15 g) Opacifying pigment: 100 parts (15 g) PAE resin: 0.8% dry / fibers

  In the case of the control product, 15 g of TiO2 RL62 are introduced.

  In the case of the products according to the invention, 15 g of the whole are introduced.

TiO2 + SiO2.

  The rest of the procedure is that described in the test

"Opacity yield".

  The results are given in Table 5. These results clearly show that a product produced according to the invention which is subjected to a single drying (test 2) is not better than a standard product (test 1). On the other hand, this same product, after a micronization step (test 3) releases all of its potential for improving opacity yield. It in fact leads to a sheet of paper which, after laminating, gives off an opacity 2.4 points higher than that of the laminated paper produced from the reference product RHODITAN RL62, and this for

  a completely comparable level of TiO2 in the sheet.

TABLE 5

  Test n SiO2 Curing Drying Micronization Opacity (%) Rate of Rate of TiO2 ash oven (% o) (%)

1 0% - - 89.1 38.2 38.2

  2 10% 1 hr at 90 C 15 hr at 150 C no 89.3 41.6 37.8 3 10% 1 hr at 90 C 15 hr at 150 C yes 91.5 41.6 37.8 o0

Claims (32)

  1. Method for preparing a composition based on TiO2 useful as an opacifying agent, characterized in that it comprises the steps according to which: - an aqueous dispersion of TiO2, an aqueous dispersion of at least one inorganic spacer agent is mixed , the mixing of the two dispersions being carried out with stirring and at a pH between the respective isoelectric points I0 of said TiO2 and spacing agent and chosen in such a way that said TiO2 and spacing agent have opposite surface charges and sufficiently different to lead, under the effect of electrostatic forces, when arranged in mixed mineral jibs in which the TiO2 particles are generally spaced from one another by particles and / or aggregates of the spacer agent; - if necessary, the pH is regulated at the value fixed in step 1, - the resulting aqueous dispersion of mixed mineral frocs is matured at a temperature sufficient to strengthen the solidity of the bonds established between the TiO2 particles and the particles and / or aggregates of the spacing agent, - said composition is recovered in the form of an aqueous dispersion of mixed mineral flocs and - optionally said composition is formulated in a dry form. 2. Method according to claim 1 characterized in that the   titanium dioxide used is a rutile TiO2.   3. Method according to claim 1 or 2 characterized in that the   titanium dioxide used is a rutile TiO2 of pigment size.   4. Method according to claim 1, 2 or 3 characterized in that   TiO2 is coated with a mineral surface treatment.   5. Method according to claim 4 characterized in that the surface treatment comprises at least one compound chosen from. alumina, silica, zirconia, phosphate, cerium oxide, zinc oxide, oxide titanium and their mixtures.   6. Method according to one of claims 1 to 5 characterized in that   that the aqueous dispersion of TiO2 comprises approximately 5 to 80% by weight of TiO2. 7. Method according to claim 6 characterized in that the   aqueous dispersion of TiO2 comprises approximately 5 to 40% by weight of TiO2.   8. Method according to one of claims 1 to 7 characterized in that   that the inorganic spacer agent is chosen from oxides of silicon, titanium, zirconium, zinc, magnesium, aluminum, yttrium, antimony, cerium and tin; barium and calcium sulfates; zinc sulfide; zinc, calcium, magnesium, lead and mixed metal carbonates; aluminum, calcium, magnesium, zinc, cerium and mixed metal phosphates; titanates of magnesium, calcium, aluminum and mixed metals; magnesium and calcium fluorides; zinc, zirconium, calcium, barium, magnesium, mixed alkaline earth silicates and silicate minerals; alkali and alkaline earth aluminosilicates; calcium, zinc, magnesium, aluminum and mixed metal oxalates; zinc, calcium, magnesium and alkaline earth aluminates; Aluminum hydroxide and mixtures thereof.   9. Method according to one of claims 1 to 8 characterized in that   that the inorganic spacing agent is chosen from silicon oxides,   zirconium, aluminum, antimony, cerium, tin and their mixtures.   10. Method according to one of claims 1 to 9 characterized in   what the inorganic spacer is used at about 1 to 40% by weight relative to the weight of TiO2.   11. Method according to one of claims 1 to 10 characterized in   what the inorganic spacer is used at about 5 to 15% by weight relative to the weight of TiO2.   12. Method according to one of claims 1 to 11 characterized in   that TiO2 is a cationic pigmented rutile TiO2.   13. Method according to claim 12 characterized in that the inorganic spacer agent is a silica, an alumina, a silicoaluminate or 0o one of their mixtures.   14. Method according to one of claims 1 to 13 characterized in   that the inorganic spacer is a silica and that TiO2 is a TiO2 cationic pigment rutile.   15. The method of claim 14 characterized in that the   silica has a specific surface of between approximately 20 and 300 m2 / g.   16. The method of claim 14 or 15 characterized in that the silica is in the form of aggregates of size between about 0.5 and 10 pm.   17. Method according to one of claims 14 to 16 characterized in   that silica is generated in situ by acidification of a solution of silicates.   18. The method of claim 17 characterized in that the pH is adjusted after precipitation in situ of the silica to a value conducive to the manifestation of electrostatic forces between the TiO2 and the silica thus generated.   19. Method according to one of claims 14 to 17 characterized in   that the two aqueous dispersions are brought together at a pH of around 5.5.   20. Method according to one of claims 14 to 19 characterized in   that silica is used in an amount of about 5 to 15% by weight relative to the TiO2 weight.   21. Method according to one of claims 14 to 20 characterized in   that the ripening step is carried out at a temperature between   about 60 C and 100 C for at least 30 minutes.   22. Method according to one of claims 1 to 21 characterized in   what the mixed mineral flocs obtained after the first or second   stage undergo mineral surface treatment.   23. The method of claim 22 characterized in that the mineral surface treatment lo represents about 16% by weight or less by   relative to the total weight of the mixed mineral flocs treated.   24. Composition based on TiO2 capable of being obtained according to   the method defined according to one of claims 1 to 23.   25. Composition based on TiO2 and SiO2 characterized in that the particles of TiO2 and SiO2 are arranged therein in the form of mixed mineral jibs based on TiO2 and SiO2 in which the particles of TiO2 are generally spaced apart from one another others by aggregates of said silica.   26. Composition according to claim 25 characterized in that   the silica is present in an amount of approximately 5 to 15% by weight relative to the TiO2.   27. Composition according to claim 25 or 26 characterized in   that TiO2 is a cationic pigmented rutile TiO2.   28. Composition according to one of claims 25 to 27   characterized in that the silica has a specific surface of between approximately 20 and 300 m2 / g and / or is in the form of size aggregates   between about 0.5 and 10 pm.   29. Composition according to one of claims 25 to 28   characterized in that the mixed mineral flocs based on TiO2 and SiO2   are coated with a mineral surface treatment.   30. Composition 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 flocs.   31. Use of a composition obtained according to one of   claims 1 to 23 or of a composition defined according to one of   Claims 24 to 30 as an opacifying agent.   32. Use according to claim 31 in the paper industries, o0 plastics and paints.