EP1044242A1

EP1044242A1 - Preparation and use of mixed opacifiers based on titanium and silica oxides

Info

Publication number: EP1044242A1
Application number: EP98964523A
Authority: EP
Inventors: Patrice Le Cornec; Franck Fajardie; Michel Foulon
Original assignee: Millennium Inorganic Chemicals SA
Current assignee: Millennium Inorganic Chemicals SA
Priority date: 1997-12-30
Filing date: 1998-12-24
Publication date: 2000-10-18
Also published as: NO20003410D0; KR20010033733A; IL136900A0; FR2773167A1; CA2316281A1; WO1999035193A1; NO20003410L; JP2002500257A; AU1971099A; CN1284103A; BR9814542A

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 IFIANTING OPACS BASED ON TITANIUM AND SILICON OXIDES

The subject of the present invention is a composition based on TiO ₂ , useful as an opacifying agent in particular in laminated papers and a preparation process making it possible to obtain said composition.

5 Laminated paper, commonly called decorative 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 decorative paper is that it has an extremely high LO rate of TiO ₂ , which can reach up to 40% of the mass of the dry sheet.

For comparison, print-write type papers can contain a maximum of 10%.

In fact, this high content of TiO _{2 is} explained by the level is of opacity required for a 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 required levels of opacity due to its high refractive index.

Application WO 89/08739, however, proposes replacing TiO ₂ at a rate of 5 to 40% by weight with precipitated amorphous silica and using the corresponding mixtures, which are more advantageous from an economic point of view, as fillers. in the paper industry.

Conventionally, the sheets of paper are prepared from a mixture of cellulose fibers and mineral fillers, mainly TiO _2, dispersed in water. This mixture is contained in

30 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. This gives the "fibrous mat" which, after drying, gives the sheet of paper.

35 In fact, it turns out that only part of the initial quantity

TiO ₂ is retained in the fibrous mat and furthermore this fraction is generally too agglomerated for the TiO to be able to develop maximum opacity. To reduce this loss of TiO ₂ 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 TiO ₂ particles on the fibers by phenomena of homo- and hetero-flocculation.

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 TiO ₂ in the fibrous mat is not sufficient in itself in terms of opacity yield. It would also be necessary to maintain the TiO ₂ in the fibrous mat, in a sufficiently dispersed form so that it can retain its pigmentary properties and develop a good opacifying power. Advantageously, one could then obtain the same opacity while engaging less TiO _2. The opacity yield of TiO ₂ would be significantly increased.

To this end, international patent application WO 97/18268 proposes to carry out a surface treatment of the TiO ₂ particles. This treatment consists in covering them with a monolayer of inorganic particles such as silica and whose particle size remains less than

. that of TiO ₂ particles. This coating of the particulate monolayer type makes it possible to space apart the TiO ₂ particles from one another. ;.

The object of the present invention is precisely to propose a new composition based on TiO _{2 which} meets all of the requirements mentioned above.

More particularly, it proposes a new system of opacifying agent making it possible to improve both the retention of TiO ₂ , during the formation of the fibrous mat, and to store it there in a flocculation structure that is as penalizing as possible for the '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 interposing between the TiO ₂ particles particles, in the form of aggregates or not, of a agent known as inorganic spacer. The mixed mineral flocs obtained according to the invention are advantageous for several reasons:

- they make it possible to keep the TiO ₂ particles sufficiently dispersed during the various stages of formation of the paper sheet so that the maximum of them can develop their pigmentary character and therefore participate in the opacity in the dry sheet,

- they have an open structure, which is conducive to better retention, - they also prove to be sufficiently resistant to withstand the capillary forces which intervene during the draining and drying of the fibrous mat, as well as to the shears that the can be encountered during the manufacturing of the sheet.

In fact, the internal cohesion of the mixed mineral flocs, resulting from the association with TiO ₂ of at least one inorganic spacer agent, is based on the solidity of ionic bonds established between TiO ₂ and the spacer agent. This cohesion stems directly from the process used to prepare said mixed mineral flocs.

More specifically, these flocs are obtained under operating conditions such as TiO ₂ and the inorganic spacer agent considered have opposite and significantly different surface charges. In particular, the TiO ₂ 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 in this form.

Consequently, the first object of the present invention is a process for preparing a composition based on TiO ₂ , useful as an opacifying agent, characterized in that it comprises the steps according to which:

- An aqueous dispersion of at least one inorganic spacer agent is mixed with an aqueous dispersion of TiO ₂ , the mixture of the two dispersions being carried out with stirring and at a pH between the respective isoelectric points of said TiO ₂ and spacing agent and chosen in such a way that said TiO ₂ and spacing agent have opposite surface charges and sufficiently different to conduct, under the effect of electrostatic forces, in their arrangement in mixed mineral flocs in which the TiO ₂ particles are globally 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 flocs is matured at a temperature sufficient to strengthen the solidity of the bonds established between the TiO ₂ 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 microscopy image presented in Figure 2.

For the purposes of the invention, the term floc is intended to denote mixed agglomerates of two mineral species of the TiO ₂ type and inorganic spacer agent such as SiO ₂ for example. These agglomerates result from the association between aggregates of said spacer agent and TiO ₂ particles. A spacer is made up of particles or aggregates of particles which are interposed between the TiO ₂ 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 TiO ₂ used according to the invention is preferably a rutile TiO ₂ . More preferably, it is a rutile TiO ₂ 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 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, 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 *, sold by Rhône-Poulenc. These two pigments are differentiated by the composition of their surface treatment and the resulting Zeta potentials.

RL 18 has a silica-alumina surface treatment (SiO ₂ / AI ₂ O ₃ ) and a negative Zeta potential at pH 6, it is called "anionic TiO ₂ ".

On the other hand, RL62 has a phosphate-alumina (P ₂ Os AI ₂ O ₃ ) surface treatment with a positive Zeta potential at pH 6, it is called "cationic TiO ₂ ". The choice of pH 6 is close to the industrial implementation pH.

In the context of the present invention, the choice of TiO ₂ , anionic or cationic, of course conditions the choice of the inorganic spacer agent associated with it.

In each case, an inorganic spacing agent is chosen having an isoelectric point sufficiently different from that of the TiO ₂ considered so that the electrostatic attractions between the two compounds which are necessary for their arrangement can manifest themselves.

The aqueous dispersion of TiO ₂₎ used according to the invention comprises approximately 5 to 80% by weight of TiO ₂ and preferably approximately 5 to 40%. In this regard, the limiting point is the viscosity of the suspension which must remain at a reasonable value to be easily manipulated.

According to a preferred embodiment of the invention, the TiO ₂ selected is a cationic pigmented rutile TiO ₂ and in particular Rhoditan RL62. As regards the inorganic spacing agents considered according to the invention, they must not interfere with the other 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 TiO ₂ particles _. However, these particles are preferably arranged in the form of aggregates whose size is then greater than those of the TiO ₂ particles. Preferably these aggregates have a size of between approximately 0.5 and 2 μm.

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; silicates of 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 their mixtures.

Of course, the choice of this spacing agent is made so that it has a sufficient isoelectric point difference with the TiO ₂ form used.

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 TiO ₂ is retained, this inorganic spacer agent is preferably a silica, an alumina, a silicoaluminate or one of their mixtures.

As regards the ratio between TiO ₂ and the spacing agent, it is of course variable depending on the nature of the spacing agent retained. 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 in particular result in a fragility of the paper, in particular in terms of resistance, whether in the dry state or in the wet state.

This spacing agent can generally be used in an amount of approximately 1 to 40% relative to the weight of TiO ₂ , preferably in an amount of approximately 5 to 15% by weight and more preferably in an amount of approximately 10% by weight. 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 TiO ₂ 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 made after the precipitation agent precipitation step.

These operating conditions conducive to the manifestation of the phenomenon of heterocoagulation between the inorganic spacer agent and TiO ₂ 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 TiO ₂ be spaced by at least one 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. If necessary, it may be necessary to adjust the pH during the reaction in order to maintain it at a value suitable for the formation of said flocs.

The attraction is immediate. However, it is preferable to keep stirring for about 15 minutes so as to stabilize the system before the ripening stage. According to a preferred variant of the invention, TiO ₂ is used in a cationic pigmented rutile form and preferably is RL62, and the associated spacer agent is silica.

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

The use of silica as a spacer agent in accordance with the present invention is advantageous for several reasons.

First of all, it has an isoelectric point around 2, that is to say a sufficiently different value from that of the isoelectric point of the cationic form of TiO ₂ (6.5 to 7).

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

The pH for bringing the two corresponding dispersions into contact is between the isoelectric points of the spacer agent and of TiO ₂ . Normally the upper limit is imposed by the isoelectric point of the TiO ₂ considered and the lower limit 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 carried out at a pH of the order of 5.5.

In the particular case of the preparation of a composition comprising TiO ₂ 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 TiO ₂ .

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 TiO ₂ . Beyond this value, we are faced with the problem of brittleness of the paper mentioned above. Consequently, silica is preferably used at a rate of approximately 5 to 15% by weight of the weight of TiO ₂ and more preferably at a rate of 10% by weight.

The silica can be introduced either in the form of an aqueous dispersion of silica particles of the slurry type or can 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 TiO ₂ , the pH of the reaction medium is adjusted after the precipitation stage to a value conducive to the manifestation of electrostatic forces between TiO ₂ 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 formed 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 series of manipulations of said flocs.

It is therefore necessary that these flocs are strong enough to resist shearing, if necessary the flocculating effect of polymeric derivatives such as PAE (polyamino-amide-epichlorohydrin), as well as the removal of water during the sheet formation and drying.

It is therefore important that the TiO ₂ 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 are converted into covalent bonds at the end of this ripening step.

In the particular case of the preparation of a composition of mixed mineral flocs based on TiO ₂ and SiO ₂ , this ripening step is performed 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 left to cool to room temperature and can be recovered as it is.

This composition can be used directly in this form as an opacifying agent.

However, one can also consider formulating it in a dry form. To this end, it turns out to be possible to apply to the dispersion obtained according to the invention various 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. As 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 from the reaction medium after bringing into contact the dispersions of TiO ₂ and the spacer agent.

The mineral surface treatment represents approximately 16% by weight or less or preferably of the order of 10% by weight or less, relative to the total weight of the mixed mineral flocs thus treated.

The present invention extends to compositions based on TiO _{2 which} can be obtained according to the claimed process.

It also relates to a composition based on TiO ₂ and

SiO ₂ characterized in that the particles of TiO ₂ and SiO ₂ are arranged therein in the form of mixed mineral flocs in which the particles of TiO ₂ are generally spaced from one another by aggregates of said silica. These mixed mineral flocs of TiO and SiO ₂ are stabilized thanks to electrostatic forces established between the particles of TiO ₂ and the aggregates of SiO ₂ . Furthermore, this stability of the mineral flocs is reinforced by the fact that they undergo a ripening as described above. This ripening operation in particular contributes to creating covalent bonds between TiO ₂ and SiO ₂ 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 TiO ₂ particles. This distribution is discontinuous. Figures 1 and 2 give an idea of the structure of said flocs.

The TiO ₂ is preferably a rutile TiO ₂ 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, relative to the total weight of the pigment. The TiO ₂ is preferably a rutile TiO ₂ cationic pigment.

Preferably, the TiO ₂ is RL62.

More preferably, the silica used is a silica with a large specific surface, in particular between around 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. It can also be a silica generated in situ by acidification of a silicate solution.

The silica is preferably present in an amount of about 1 to 20% by weight of the weight of TiO ₂ and more preferably in an amount of about 5 to 15% by weight and more preferably 10%.

Where appropriate, these mineral flocs based on TiO ₂ and SiO ₂ 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, relative 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 of the cellulose fibers and of opacity yield of TiO ₂ used. The conventional processes for preparing laminated paper or decor paper generally use, in addition to cellulose fibers of anionic nature, and the opacifying agent, a polymeric agent of cationic nature playing the role of reinforcing agent in the state moist and retention agent.

In the case of the use of a composition based on TiO ₂ / SiO ₂ mineral flocs as opacifying agent, it is observed that the chemical retention by electrostatic attraction is advantageously reinforced compared to TiO ₂ in individual cationic form.

This amplification in terms of retention can be explained as follows.

In the absence of cationic polymer, the cationic TiO ₂ is attracted by the anionic cellulose fibers which is favorable for the retention of TiO ₂ . On the other hand, in the presence of the polymer, the fιbre-TiO ₂ interactions change and the retention of TiO ₂ decreases. This phenomenon is interpreted by a canonization of the cellulose fibers, resulting from their covering by the cationic polymer.

Conversely, in a composition based on TiO ₂ / SiO ₂ mineral flocs, there is a mixture of cationic charges TiO ₂ , and anionic charges, SiO ₂ , 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 opacifying agent and in particular 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 TiO ₂ retained on the sheet. , resulting from better retention at the time of the formation of the fibrous mat, and the improvement of the opacity yield resulting from the better dispersion of the titanium particles contained in said flocs. Furthermore, it has been noted that these compositions favor 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 as an opacifying agent in the paints and plastics industries.

The examples and figures appearing below are presented by way of illustration and without limitation of the present invention.

Figures:

Figure 1: schematic representation of TiO ₂ pigments spaced apart by SiO ₂ aggregates.

Figure 2: electron microscopy image in transmission of mixed mineral flocs based on TiO ₂ and SiO ₂ .

Figure 3: evolution of charge retention for different mixed minerals as a function of the stirring speed imposed on the "cellulose / PAE / charge" mixture 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 sold under the name of Rhoditan RL62 by the company RHONE-POULENC. This pigment consists of rutile TiO ₂ coated by a phosphate alumina surface treatment (P ₂ Os AI ₂ O ₃ ). At pH 6, its zeta potential is positive. Its isoelectric point is located around 6.5-7.

Cellulose fibers: dry leaves of a 70/30 mixture of short fibers / long fibers previously refined at 35 ° SR, supplied by the company ARJO WIGGINS.

Silica is a precipitation silica with a large specific surface area between 20 and 300 m ² . g ^"1 and having agglomerates of sizes between 0.5 and 10 μm. Its isoelectric point is around 2.

- PAE resin (polyaminoamide epichlorohydrin), R4947 * from the company CECA

A. "Retention Form" Test

Apparatus:

- Dispermat * and Pendraulik * rapid dispersers

- Mixing tanks

- "Retention form", TECHPAP company

Procedure:

- Preparation of the fiber / TiO ₂ 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 TiO ₂ slurry or of product suspension according to the invention is added, so as to introduce 15 g dry of TiO ₂ . The extract of the TiO ₂ slurry or of the suspension according to the invention will therefore be taken into consideration. This addition is carried out in a mixing tank. It is followed by a dilution to 4 liters with deionized water.

- Preparation of the test portion

A 500 ml test portion 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 inverted 2 times to mix well. This test portion is then introduced into the retention form to obtain a sheet.

- Measurement of retention

The formation of the sheet is triggered after a stirring time of 30 s at a speed of 1300 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 the nearest 10 g.

The retention rate is given by: P2 / P1.

P1 = weight of fillers (TiO ₂ + SiO ₂ ) in an initial sample of 500 ml P2 = weight of ash after calcination of the prepared sheet.

B. Opacity performance test

The opacity yield tests were carried out using manufactured formulas in order to know the spatial distribution of titanium dioxide in the dry sheet.

The formulas were manufactured in accordance with the procedure described in the paragraph below.

The optical properties of the impregnated and pressed form were also measured according to the method described below.

1. Manufacture of formettes

i) Formulation of the pulp

Cellulose: 15 g (which represents 100 parts)

Opacifying composition: 100 parts (expressed as TiO ₂ ) or 15 g

PAE: 0.8% dry compared to cellulose

in Preparation of the dough: defibraαe

Cellulose is torn by hand into small squares after moistening 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 the cellulose, the speed is increased to 3000 rpm and left to stir

10 mins. iii) Mixing 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 make grammage formulas at 80 g / m ² . iv) Manufacture of formettes

500 ml of well homogenized suspension are taken in a test tube. PAE is added (commercial solution diluted 10 times to have an acceptable intake volume), ie 1 ml. 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 10 s, it is left to stand for 10 s, then the form is made 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 with precision and correct the volume taken 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 subsequent operations, that is to say, chemical and optical characterizations.

2. Measurement of the ash rate

The amount of TiO ₂ present in the sheet of 80 g / m ^{2 is} measured by calcining a third of the form at 800 ° C for one hour.

The percentage of TiO ₂ present in the sheet is thus calculated:

ITI after calcination "You have empty ash rate (%) =

Tlavant calcination "Tlè vide

The ash rate measures the quantity of mineral fillers present in the sheet. This determination is made according to the method NF 03- 047 (Collection of French Standards Paper, cardboard and paste: test method, volume A, 4 ^th ^"16th 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 are heated to 60 ° C. When this temperature is reached, the rain is gradually poured with magnetic stirring

245 g of resin previously weighed. Once everything is dissolved, the mixture is left stirring at 60 ° C. for 30 min. After cooling, it is filtered through a 50 μm cloth.

ii) Impregnation - pressing

Strips of paper 7 cm by 10 cm are cut. The strips are then impregnated by capillary action by placing them for 1 minute on the resin. It is expressed between two glass rods and dried 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 the kraft barriers.

The laminates obtained are pressed for 8 min at 150 ° C. under a pressure of 100 bars.

iii) Measuring 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 company DATACOLOR.

EXAMPLE 1

Preparation of a composition of mixed mineral flocs 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 / l.

The mineral flocs are produced by heterocoagulation of the TiO ₂ particles with the silica aggregates.

The heterocoagulation process consists in adding the silica slurry to a regulated pH in a stirred base stock containing the suspension of TiO ₂ . The heterocoagulation pH can be chosen between 4.5 and 6.5, but it is better 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 TiO ₂ and the overall dry extract (TiO ₂ + SiO ₂ ) is approximately 11%.

After contact for 15 minutes at a regulated pH of 5.5, the suspension, still stirred, is brought to a temperature between 60 ° C. and the boiling temperature for 1 to 3 h and then cooled to room temperature. All the samples prepared according to this protocol, were tested by preparing formulas (round sheet). For all the tests, the volume of "fiber + filler + PAE" mixture taken from the mixing tank was adjusted so as to obtain sheets of the same grammage: 80 g / m ² .

- Part of the form is calcined to determine the amount of oxide present in the dry sheet (SiO ₂ + TiO ₂ ). Knowing the amount of silica added relative to TiO ₂ , we deduce the% of TiO ₂ present in the dry sheet. The formette training protocol and the principle for calculating the rate of TiO ₂ are detailed in the previous chapter, Materials and methods.

It was verified by assaying TiO ₂ and SiO ₂ by X-ray fluorescence in the leaves obtained that there was no preferential retention of one or the other of the mineral species. The SiO / TiO ₂ ratio is maintained throughout the sheet formation process.

- 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. The 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. 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 by simply mixing the silica and the TiO ₂ .

TABLE 1

O

TABLE 2

Δcen = gain in ash rate resulting from better retention of

TiO ₂ during the formation of the sheet Δopa = Δret + Δesp = total opacity gain

Δrét = Δcend ^* slope = gain of opacity resulting from the increase in the TiO _{2 level} retained in the sheet (better retention) Δesp = Δopa - Δrét = gain of opacity resulting from the better dispersion of the

TiO ₂ retained in the sheet thanks to the spacer effect of the silica aggregates.

The results of tests 1 to 5 clearly show that the use of mixed mineral flocs not only improves the first pass retention (higher ash rate) but also improves the opacity yield of the TiO ₂ retained since in all cases the gain in opacity (Δ _o p _a ) is greater than the gain in opacity normally expected by an increase in the ash rate (Δr _é t) -

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 an indication of a porous and non-homogeneous structure.

With regard to the influence of the silica content, it is noted that the tests carried out with 5 and 10% of silica are clearly more efficient than the test carried out with 1% of silica.

Consequently, the previous results show that compared to a conventional formulation (TiO ₂ 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 opacity yield measured for the most efficient tests.

It is also possible to envisage, with compositions according to the invention, to use less TiO ₂ 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 TiO ₂ can be estimated by evaluating the gain in ash rate corresponding to an opacity gain equal to the opacity yield. This value compared to the rate of ash from the silica-free tests corresponds to the percentage of TiO ₂ 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% of silica should make it possible to save at least 7 to 10% of TiO ₂ while retaining the same level of opacity as a conventional formulation without silica.

EXAMPLE 2:

Using the "retention form" test, the retention ability of the product obtained according to the invention was compared with that of "conventional" titanium oxides.

Product A: product according to the invention SiO ₂ = 10%

Product B: product according to the invention, SiO ₂ = 15%

Product C: TiO ₂ Rhoditan RL18 15 Product D: TiO ₂ Rhoditan RL62.

The results of the “retention form” test are given in Table 3.

At a PAE rate of 0.8%, the usual rate in application, the 0 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 imparted by the synthesis process which is the subject of the invention.

At 0% PAE, the self-retentive nature of the product is demonstrated. Although more anionic than RL18, the products according to the invention have a clearly more marked self-retentive character. This is indeed the 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 30 (naturally anionic ) and the charges. TABLE 3

EXAMPLE 3

Determining the influence of the ripening stage

a) Effect in terms of retention:

The shear strengths of certain mineral flock compositions identified in Example 1 were tested by the "retention form" test.

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 formation of the fibrous mat.

It is noted that the retention of the 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 shearing to maintain good first pass retention. The results also show that: • the 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 stirring speed. 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 good quality sheet of paper from an RL62 SiO ₂ mixture with 10% SiO ₂ , which has not undergone a ripening. As soon as cellulose and PAE are added to the mixing tank, there is agglomeration and the formation of "balls".

The ripening stage is therefore a stage necessary for the formation of effective mixed mineral flocs.

EXAMPLE 4:

Effect on the whiteness of the laminated paper sheets of the mixed mineral flock compositions 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 collated in Table 4 below.

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

TABLE 4

In general, it is observed that the use of the compositions according to the invention improves the whiteness of the laminated sheet and this all the more so as the silica content increases. For 5 and 10% of silica we measure a gain of around 0.2 point in L ^* and especially 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 retained TiO ₂ , they therefore also lead to an improvement in the whiteness of the laminated sheet.

EXAMPLE 5

Preparation of a composition of mixed mineral flocs in powder form.

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 the other is subjected to 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 to 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 TiO ₂ RL62 are introduced. In the case of the products according to the invention, 15 g of the TiO ₂ + SiO ₂ assembly are introduced.

The rest of the operating mode is that described in the "Opacity yield" test.

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 lamination, 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 rate of TiO ₂ in the whole sheet. quite comparable.

00

Claims

1. Process for the preparation of a composition based on TiO ₂ useful as an opacifying agent, characterized in that it comprises the steps according to which:

- An aqueous dispersion of TiO _2ι is mixed with an aqueous dispersion of at least one inorganic _spacing agent, the mixing of the two dispersions being carried out with stirring and at a pH between the respective isoelectric points of said TiO ₂ and spacing agent and chosen from such that said TiO ₂ and spacing agent have opposite surface charges which are sufficiently different to lead, under the effect of electrostatic forces, to their arrangement in mixed mineral flocs in which the particles of TiO ₂ are generally spaced apart from one another others 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 TiO ₂ particles and the particles and / or aggregates of the spacer 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 TiO ₂ .

3. Method according to claim 1 or 2 characterized in that the titanium dioxide used is a rutile TiO ₂ of pigment size.

4. Method according to claim 1, 2 or 3 characterized in that the TiO ₂ is coated with an inorganic 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, titanium oxide and their mixtures.

6. Method according to one of claims 1 to 5 characterized in that the aqueous dispersion of TiO ₂ comprises approximately 5 to 80% by weight of TiO ₂ .

7. Method according to claim 6 characterized in that the aqueous dispersion of TiO ₂ comprises approximately 5 to 40% by weight of TiO ₂ .

8. Method according to one of claims 1 to 7 characterized in that the inorganic spacer is chosen from oxides of silicon, titanium, zirconium, zinc, magnesium, aluminum, yttrium, d 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; silicates of 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 their mixtures.

9. Method according to one of claims 1 to 8 characterized in that the inorganic spacer agent is chosen from oxides of silicon, zirconium, aluminum, antimony, cerium, tin and their mixtures.

10. Method according to one of claims 1 to 9 characterized in that the inorganic spacer agent is used in an amount of about 1 to 40% by weight relative to the weight of TiO ₂ .

11. Method according to one of claims 1 to 10 characterized in that the inorganic spacer agent is used in an amount of about 5 to 15% by weight relative to the weight of TiO ₂ .

12. Method according to one of claims 1 to 11 characterized in that the TiO ₂ is a cationic pigmented rutile TiO ₂ .

13. Method according to claim 12 characterized in that the inorganic spacer agent is a silica, an alumina, a silicoaluminate or one of their mixtures.

14. Method according to one of claims 1 to 13 characterized in that the inorganic spacing agent is a silica and that the TiO ₂ is a rutile cationic pigmented TiO ₂ .

15. The method of claim 14 characterized in that the silica has a specific surface between about 20 and 300 m ² / 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 microns.

17. Method according to one of claims 14 to 16 characterized in that the silica is generated in situ by acidification of a silicate solution.

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 TiO ₂ and the silica thus generated.

19. Method according to one of claims 14 to 17 characterized in that the two aqueous dispersions are brought into contact at a pH of the order of 5.5.

20. Method according to one of claims 14 to 19 characterized in that the silica is used in an amount of approximately 5 to 15% by weight relative to the weight of TiO ₂ .

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 that the mixed mineral flocs obtained at the end of the first or second step undergo a mineral surface treatment.

23. The method of claim 22 characterized in that the mineral surface treatment represents approximately 16% by weight or less relative to the total weight of the mixed mineral flocs treated.

24. Composition based on TiO ₂ capable of being obtained according to the process defined according to one of claims 1 to 23.

25. Composition based on TiO ₂ and SiO ₂ characterized in that the particles of TiO ₂ and SiO ₂ are arranged therein in the form of mixed mineral flocs based on TiO ₂ and SiO ₂ in which the particles of TiO ₂ are generally spaced from each other 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 TiO ₂ .

27. Composition according to claim 25 or 26 characterized in that the TiO ₂ is a cationic pigmented rutile TiO ₂ .

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 m ² / g and / or is in the form of aggregates of size between approximately 0, 5 and 10 μm.

29. Composition according to one of claims 25 to 28 characterized in that the mixed mineral flocs based on TiO ₂ and SiO ₂ are coated with an inorganic 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 plastics and paint industries.