EP2766199A1

EP2766199A1 - Method for preparing a masterbatch of natural rubber and magnesium-doped silica

Info

Publication number: EP2766199A1
Application number: EP12769677.1A
Authority: EP
Inventors: Benoît DE GAUDEMARIS; Géraldine LAFFARGUE; Julien Berriot
Original assignee: Michelin Recherche et Technique SA Switzerland; Compagnie Generale des Etablissements Michelin SCA; Michelin Recherche et Technique SA France
Current assignee: Compagnie Generale des Etablissements Michelin SCA
Priority date: 2011-10-11
Filing date: 2012-10-10
Publication date: 2014-08-20
Also published as: US20140256845A1; US9505890B2; FR2981080A1; WO2013053737A1; FR2981080B1

Abstract

The invention relates to a method for preparing a masterbatch of natural rubber and silica, comprising the following successive steps of: doping the silica with magnesium, preparing at least one dispersion of the thus-doped silica in water, bringing into contact and mixing a natural rubber latex and the aqueous dispersion of doped silica in order to obtain a coagulum, recovering the coagulum, and drying the recovered coagulum in order to obtain the masterbatch.

Description

PROCESS FOR THE PREPARATION OF A MASTER MIXTURE OF NATURAL RUBBER AND SILICON DOPED MAGNESIUM

The invention relates to the preparation of a masterbatch of natural rubber and silica comprising at least one silica (modified) and a natural rubber latex.

The term "masterbatch" (commonly referred to by its English name as "masterbatch") means an elastomer-based composite in which a filler and possibly other additives have been introduced. The present invention relates in particular to the use of such a masterbatch for the manufacture of inorganic filler-reinforced diene rubber compositions for the manufacture of tires or semi-finished products for tires, in particular treads of these tires. In order to obtain the optimum reinforcing properties conferred by a load in a tire tread and thus a high wear resistance, it is known that it is generally appropriate for this filler to be present in the elastomeric matrix under final form that is both finely divided possible and distributed in the most homogeneous way possible. However, such conditions can be achieved only to the extent that this charge has a very good ability, on the one hand to incorporate into the matrix during mixing with the elastomer and to deagglomerate, on the other hand to to disperse homogeneously in this matrix.

In a known manner, carbon black has such aptitudes, which is not generally the case for inorganic fillers, in particular silicas. Indeed, for reasons of mutual affinities, these inorganic filler particles have an unfortunate tendency in the elastomeric matrix to agglomerate with each other. These interactions have the detrimental consequence of limiting the dispersion of the charge and therefore the reinforcing properties to a level substantially lower than that which it would be theoretically possible to achieve if all the bonds (inorganic filler / elastomer) that can be created during the mixing operation, were actually obtained; these interactions tend on the other hand to increase the consistency in the green state of the rubber compositions and thus to make their implementation ("processability") more difficult than in the presence of carbon black.

Since fuel savings and the need to protect the environment have become a priority, it has been necessary to produce tires with reduced rolling resistance without penalizing their wear resistance.

This has been made possible in particular by the use, in the treads of these tires, of new load-reinforced rubber compositions. inorganic, in particular highly dispersible type specific silicas, capable of competing with a reinforcing point of view with a conventional pneumatic grade carbon black, while offering these compositions a lower hysteresis, synonymous with a lower rolling resistance for tires with them, as well as improved grip on wet, snowy or icy surfaces.

Treads loaded with such highly dispersible silicas (denoted "HD" or "HDS" for "highly dispersible" or "highly dispersible silica"), usable in tires with low rolling resistance sometimes called "Green Tires" for energy savings offered to the user ("Green Tire concept"), have been extensively described. Reference is made in particular to patent applications EP 501 227, EP 692 492, EP 692 493, EP 735 088, EP 767 206, EP 786 493, EP 881 252, WO99 / 02590, WO99 / 02601, WO99 / 02602, WO99 / 06480, WO00 / 05300, WO00 / 05301. These documents of the prior art teach the use of HD type silicas having a BET surface area of between 100 and 250 m ² / g. In practice, a high surface area silica HD referring in the field of "Green Tires" is in particular the silica "Zeosil 1165 MP" (BET surface equal to about 160 m ² / g) marketed by Rhodia. The use of this silica "Zeosil 1165 MP" makes it possible to obtain good compromises in terms of tire performance, in particular satisfactory wear resistance and rolling resistance.

The advantage of using a high surface area silica resides mainly in the possibility of increasing the number of bonds of the silica with the elastomer and therefore of increasing the level of reinforcement thereof. Therefore, it appears advantageous to use, in tire tread rubber compositions, silicas with a high specific surface area, possibly greater than that conventionally used, of the order of 160 m ² / g, in particular to improve the wear resistance of these treads. Nevertheless, the dispersibility of the charge and the increase in its specific surface area are considered as antinomic features. Indeed, a large specific surface area implies an increase in the interactions between charge objects, and therefore a poor dispersion thereof in the elastomeric matrix and a difficult implementation. Another type of solution has been envisaged which consists, in order to improve the dispersibility of the filler in the elastomeric matrix, of mixing the elastomer and the "liquid" phase filler. To do this, an elastomer in the form of latex has been used which is in the form of elastomer particles dispersed in water, and an aqueous dispersion of the filler, that is to say a dispersed silica. in water, commonly called "slurry". However, the contacting of the elastomer latex and the slurry does not allow coagulation in the liquid medium, coagulation which should allow to obtain a solid which after drying, results in obtaining the masterbatch of elastomer and silica desired.

Indeed, silica aggregates are typically hydrophilic in nature and have affinity to water, so silica aggregates have more affinity with water than with the elastomer particles themselves.

Various solutions have been proposed to allow nevertheless to obtain this coagulation and a good dispersion of the charge in the elastomeric matrix in "liquid" phase by the combined use of an agent for increasing the affinity between the elastomer and silica, such as a coupling agent, and an agent for making the setting in mass, so the coagulation, called coagulation agent.

For example, US Pat. No. 5,763,388 proposes the incorporation of silica into the rubber latex by treating the silica with a coupling agent, mixing the silica thus treated in the presence of conventional coagulation agents.

The patent EP1321488 also proposes to put in contact an aqueous dispersion with negatively charged silica and a diene elastomer latex, with an emulsion containing a polysulfide coupling agent, in the presence of a coagulation agent such as a polyamine .

The Applicants have surprisingly discovered a method for obtaining a silica-elastomer masterbatch prepared in the "liquid" phase without using a coagulation agent or a coupling agent. Such a method allows, moreover, not only to achieve a very good rate of return (greater than 80% by mass) by respecting the feed rate previously introduced and with good dispersion of the filler in the elastomeric matrix.

Of course, such a method seems all the more interesting if it is implemented with highly dispersible silicas as presented above.

The method for preparing a masterbatch of natural rubber and silica, according to the invention, comprises the following successive stages:

- doping the silica with magnesium,

to prepare at least one dispersion of the silica thus doped in water,

contacting and mixing a natural rubber latex and the aqueous dispersion of doped silica to obtain a coagulum,

- recover the coagulum,

- Dry the recovered coagulum to obtain the masterbatch. According to one embodiment of the method, the coagulum recovery step is performed by a filtering operation.

According to another embodiment, the coagulum recovery step is performed by a centrifugation operation.

Preferably, the natural rubber latex is a concentrated latex of natural rubber. According to an alternative embodiment of the invention, the silica is a precipitated silica. Advantageously, one of the following conditions is satisfied:

(i) the formulation pH is between 3 and 6 and the magnesium doping rate of the silica is greater than 0.2% by weight,

(ii) the formulation pH is greater than or equal to 8.5 and the magnesium doping rate of the silica is greater than or equal to 0.4% by weight.

The invention also relates to a masterbatch of natural rubber and silica prepared according to the method which comprises the following successive stages:

- doping the silica with magnesium,

to prepare at least one dispersion of the silica thus doped in water,

- recover the coagulum,

- Dry the recovered coagulum to obtain the masterbatch.

The subject of the invention is also a rubber composition based on at least one masterbatch of natural rubber and silica prepared according to the method according to the invention, as well as a finished or semi-finished article, a tire tread a tire or semi-finished product comprising at least one such rubber composition.

The terms "dope" the silica with magnesium, the fact of modifying the surface of the silica so as to integrate the magnesium in the mesh of the outer layers of the silica and / or on the surface of this silica. By extension, will be called silica "doped" magnesium, a silica having magnesium in the mesh of its peripheral layers and / or on its surface. I. - MEASUREMENTS AND TESTS

1-1) Measurement of magnesium doping

This method is used to determine the surface magnesium of doped silicas by atomic emission spectrometry (ICP-AES). These silicas are prepared by doping with a commercial silica.

Silica is not digested, this method does not allow to measure the magnesium present in the heart of the silica. a) Principle

Magnesium is solubilized by hot sulfuric acid and then assayed by inductively coupled plasma atomic emission spectrometry (ICP-AES). The surface magnesium contents are calculated by subtracting the magnesium contents of the starting commercial silica. The calibration range used is 0 to 20 mg / l magnesium, two measurements are made per sample. (b) Apparatus

- Precision scale 0.1 mg,

- Funnels,

- Class A volumetric flasks of 100 ml,

- Class A volumetric flasks of 250 ml,

- 10 ml test tube or 10 ml acid dispenser,

- 50 ml test tube

- calibrated micropipette with variable volume 0.1-1 ml (ex: Eppendorf),

- calibrated micropipette with variable volume 0.5-5 ml (ex: Eppendorf),

- Cellulose acetate syringe filters with pore diameter 0.45μιη,

- Pill box for 30 ml passer,

- ICP spectrometer (ex: Jobin Yvon model Activa M).

- 250 ml wide neck Erlenmeyer flasks.

- Sand bath. c) Reagents

- Ultrapure water

- concentrated nitric acid (eg RP NORMAPUR REF 20.422.297)

d = 1.41

% HN0 ₃ = 65 - concentrated sulfuric acid (ex: RP MERCK EF 1.00731.1000)

d = 1.84

% H ₂ S0 ₄ = 95-97

- Standard solution of magnesium at 1 g / l (eg MERCK REF HC 812641) d) Procedure d) - 1 - Preparation of a sulfuric acid solution at 5% by volume

In a flask of class A IL, 200 ml of demineralized water are introduced using a test tube. 50 ml of concentrated sulfuric acid (3.4) are then introduced using a test tube. The whole is homogenized, then the cooling of the solution is awaited. The gauge line is filled with deionized water. d) -2- Preparation of silicas in open system on sand bath

The measurements will be done in duplicate. It is preferable to make a blank procedure during each series of measurements (preparation in the same conditions but without sample). The raw silicas before doping will also be analyzed.

In an Erlenmeyer flask, 250 mg of silica are weighed.

- Pour 20 ml of 5% sulfuric acid (§5.1).

- It is heated on a sand bath and brought completely dry.

- Let cool.

The walls of the Erlenmeyer flask are rinsed with a little water and then 12.5 ml of 65% concentrated nitric acid are added.

- We bring to a boil.

- It is allowed to cool and then transferred quantitatively into a volumetric flask of 250 ml.

- Complete with the gauge line with demineralized water.

- The solution is filtered using a syringe filter (0.45 μιη)

- The analysis is done by ICP-AES. d) -3- Preparation of the calibration range:

Preparation of the calibration range of magnesium.

The reagents presented in the following table correspond to the concentrations previously specified in paragraph c). E0 El E2 E3 E4

0 5 10 15 mg / l magnesium

95 94.5 94 93.5 93 ml of ultrapure water

5 5 5 5 5 ml of nitric acid using a dispenser

0 0.5 1.0 1.5 2.0 ml of magnesium standard solution 1000 mg / 1 using a micropipette

100 100 100 100 100 total volume (in ml)

These stallions are kept for 4 months.

Preparation of a validation control at 10 mg / 1:

The verification control is prepared in each series of measurements in the same manner as the above standards by introducing 1.0 ml of 1000 mg / l magnesium standard solution of a different batch. It validates the calibration. The verification cookie does not keep after use. d) -4- ICP-AES Assay:

Sequence of analysis:

1- Calibration

2- Validation indicator at 10 mg / l (magnesium)

3- Samples + White procedure

4- Verification standard E4 (at 20 mg / l magnesium)

Calibration validation: for a magnesium range 0 - 20mg / l

Validation indicator (theoretical value: 10 mg / 1)

Tolerances: 9.8 mg / 1 <[Magnesium] <10.2 mg / 1 Validation of the analysis sequence (this validation demonstrates that there has been no drift): for magnesium range 0 - 20 mg / 1

Verification standard, E4 (theoretical value: 20mg / l)

Tolerances: 19.6 mg / l <[Magnesium] <20.4 mg / 1 Activa ICP-AES parameters:

Plasma and nebulization settings:

Spray chamber: cyclonic type (Scott chamber) - Pump speed: 20 rpm

Plasma gas flow: 12 1 / min

Sheathing gas flow rate: 0.2 1 / min

Auxiliary gas flow: 0 1 / min

- Nebulization flow *: 0.87 ml / min

Nebulization pressure *: 2.97 bar

Rinsing time: 20 s

Transfer time: 30 s

Stabilization time: 20 s

- Nebulizer: concentric type (Meinhard)

Generator power: 1,100 W

Detection parameters:

e) Results

The surface magnesium content of the sample (in mass%):

Mg mg% = area _if _this doped ii - ii% Mg _s gross _ice; example: Mgieo MP = 0% by mass The uncertainty of measurement was determined on the ICP-AES spectrometer: Jobin Yvon Activa M at the rate of three measurements per day during 6 days. The uncertainty given is three standard deviations.

For a 0.5% Mg doped silica, the uncertainty is ± 0.06% by mass, which corresponds to a relative uncertainty of 0.12%.

1-2) pH measurement

The pH is measured according to the following method deriving from the ISO 787/9 standard (pH of a suspension at 5% in water)

- H meter Mettler Toledo MP225

- Electrodes with automatic temperature compensation:

- Inlab®Reach Pro electrode (for synthesis and pH of slurries)

- Inlab®Solids Pro electrode (for pH formulations)

- Heidolph MR3003 Magnetic Heated Stirrer

Small equipment

- Glass beaker lOOmL (diameter: 4.7cm * height: 7cm) for pH of an aqueous dispersion of silica

- Glass beaker 250mL (diameter: 6.5cm * height: 9.3cm) for formulation pH

- Magnetic bars adapted to the size of the beakers

- Double glass envelope reactor of 5L

Operating mode

Procedure for measuring the pH of the aqueous dispersion or formulation:

1 / Calibration of the electrode with buffer solutions at pH 4.01, 7.01 and 10.01

2 / Aqueous dispersion (or formulation) stirred by magnetic stirring at 500tr / min 3 / Soaking the electrode in the beaker and reading the pH

Procedure for measuring pH during doping syntheses:

Reaction medium stirred with mechanical stirring (about 650 rpm)

3 / Soaking the electrode in the reactor and reading the pH

1-3) ATG Charge Rate Measurement

The purpose of this procedure is to quantify the categories of constituents of the rubber mixes. There are 3 temperature ranges that each correspond to a category of constituents:

- between 250 and 550 ° C, corresponding to organic materials: elastomers, oils, vulcanizing agents ...

- between 550 and 750 ° C, corresponding to intermediate losses (charcoal material), - above 750 ° C, corresponding to ashes and mineral products: ZnO, possibly silica ...

It applies to both raw and cooked mixes. a) - Apparatus

- Thermogravimetric analysis set on a METTLER TOLEDO brand analyzer: TGA 851 or TGA DSC1 model

- Balance at 1/100 mg scale brand and model

- Alumina crucibles 70μ1 (without lid) Mettler Toledo ref 00024123

- Miscellaneous laboratory equipment: pliers, scissors .... b) - Principle

The weight losses of a mixing sample subjected to a rise in temperature are monitored. The latter is done in 2 steps:

1 / Heating 25 to 550 ° C under an inert atmosphere (N ₂ ) to evaporate the volatile matter and carry out the pyrolysis of organic material. Volatility resulting products causes a corresponding weight loss in a ^1st step (300 ° C) and then the volatiles organics originally present in the mixture.

21 Continuation of heating up to 750 ° C under oxidizing atmosphere (Air or 0 ₂ ) to achieve the combustion of black (and / or charcoal). The resulting volatility of the products results in a weight loss corresponding to the initial amount of black (and / or charcoal material).

The products that remain after these treatments constitute ashes. It is generally inorganic materials such as ZnO, silica ... c) - Measures c) -l- Preparation of samples

The quantity of product analyzed must be weighed to 0.0 lmg and between 20 and 30 mg.

It is then placed in a 70 μΐ alumina crucible (without lid). C) -2- Definition of the "method" (temperature program)

- The following segments are successively defined:

. ^1st segment: dynamics from 25 to 550 ° C at 50 ° C / min, under nitrogen (40 ml / min)

. ^2nd segment: dynamics of 550 to 750 ° C at 10 ° C / min, in air (or 02) (40 ml / min)

- Activate the field "subtract curve to blank". Any measurement is automatically corrected for a blank curve. The latter is made under the same conditions as the measurement, with empty crucible. It is stored and used for all the following measurements (no new blank test required before each measurement). c) -3- Launch of the measure

We first make sure, by consulting the control window of the oven, that the flow rates of nitrogen and air are properly adjusted (40μ1 / ηιίη). Otherwise adjust them using the settings on the "gas box".

- Blank curve

The blank curve is made following the procedure described in the TGA User's Manual.

- Measure

The measurement is carried out following the procedure described in the TGA operating manual c) -4- Exploitation of the curve

Following the instructions in the TGA user manual

- we select and open the curve to exploit

- is delimited on this curve the bearing ^1, corresponding to the volatile matter, respectively between 25 and about 250 ° C.

the weight loss corresponding to the volatile matter content is calculated (in%)

- is delimited on this curve the 2 ^nd level, corresponding to the organic material, respectively, between the temperature of the bearing ^1, about 250 ° C and 550 ° C.

the weight loss corresponding to the organic matter content is calculated (in%)

- is delimited on this curve the 3 ^rd level, corresponding to the losses between

respectively 550 and 750 ° C

- we calculate the weight loss corresponding to these losses (in%)

- the residue or ash content is calculated in% c) -5- Presence of volatile compounds

For some mixtures containing volatile compounds that can evaporate at room temperature, there is a risk of loss of material between the sample preparation and the effective departure of the measurement.

These losses are not taken into account by the device.

To take into account these losses and to have the actual composition of the mixture, one can proceed as follows: The steps c) -l to c) -3 previously described are carried out with the following two instructions:

- when preparing the sample: record the weight of the empty crucible (PO) and the weight of the PI sample

- when launching the measurement: enter the "crucible weight" field by PO and the "sample weight" field by PI

For operation (step c) -4), the TGA takes into account, in order to determine the losses, the mass of the sample P2 which it calculates at the effective start of the measurement from the weight of the crucible, which is essential for the calculation of the residue; P2 is calculated by the TGA taking into account the mass P3 (Crucible + sample) at time T0 - PO.

The calculation of the levels of the various constituents and of the residue is made with respect to the sample weight PI defined during the preparation and not with respect to P2.

The volatile matter rate then calculated by the apparatus is erroneous since a part of MV, volatile matter (PI - P2) evaporated during the wait between the preparation and the actual start of the measurement.

VMs must be manually recalculated:

- in mass MV mg = (PI - P2) mg + losses ^1st level mg

- in Tx MV% = MV mg / PI * 100 or 100 - residue 1 ^st %%. c) -6- Charge rate in pcmo

This rate is expressed in pcmo, percentage of organic matter and obtained by calculation, when interpreting the measurement of ATG with the following formula:

Tx load (pcmo) = [(D) / (B + C)] * 100

In which B represents the percentage of organic matter (range 250-550 ° C), C the percentage of intermediate losses (between 550 and 750 ° C) and D the percentage of residue (above 750 ° C).

1-4) Measurement of coagulation yield

The coagulation yield corresponds to the ratio of the recovered dry mass (from which the mass of volatile matter as defined in the ATG measurement protocol has been removed in the preceding paragraphs) to the initial target mass. multiplied by one hundred. DETAILED DESCRIPTION OF THE INVENTION

The method for preparing a masterbatch of natural rubber and silica according to the invention comprises the following successive steps:

- doping the silica with magnesium,

to prepare a dispersion of the silica thus doped in water,

contacting and mixing a natural rubber latex and the doped silica dispersion,

recovering and drying the master mixture thus obtained.

II- 1) Preparation of the aqueous dispersion of silica

In the first step of the method, magnesium silica is doped.

This "doping" step of the silica can advantageously be carried out according to the protocol detailed in the patent application WO 02/051750.

The doping level obtained corresponds to the percentage by weight of magnesium per hundred parts by weight of silica.

For the invention may be used any silica (S1O2) known to those skilled in the art, especially any precipitated or fumed silica having a BET surface and a CTAB specific surface both less than 450 m ² / g, preferably 30 to 400 m ² / g.

In particular, it is possible to use highly specific silicas (known as "HDS"), for example the silicas Ultrasil 7000 and Ultrasil 7005 from the company Degussa, the silicas Zeosil 1165MP, 1135MP and 1115MP from the company Rhodia, the silica Hi-Sil EZ150G from PPG, the Zeopol 8715, 8745 and 8755 silicas from Huber, the high surface area silicas as described in WO 03/16837.

The doped silica obtained is then dispersed in water, preferably so as to obtain a dispersion whose viscosity is sufficient to be easily "manipulable". For example, an aqueous dispersion of silica doped with a silica content in water of 4% by weight can be produced.

Advantageously, the dispersion is sonifée to allow to obtain a stability of the aggregates in water, which improves the aqueous dispersion of silica doped in the master batch then produced.

This sonification can notably be carried out using a Vibracell generator manufactured by SONICS and Materials Inc. of 1500 Watts with a piezoelectric converter with PZT crystal (reference 75010), a booster for the probe and a 19mm diameter titanium alloy probe. (for a height of 127mm). It may be useful to add to this aqueous dispersion of doped silica, an acidifying agent such as strong acids or weak acids, to allow the pH of the aqueous dispersion of doped silica to be modified in order to obtain at the time of setting contact of the two dispersions described in the following, the pH of the targeted formulation.

Those skilled in the art will then proceed to several contacts to adjust the pH of the aqueous dispersion to obtain the desired formulation pH. Indeed, the skilled person knows that it is not possible to determine a priori based on the volumes poured and the pH of each of the dispersions what will be the pH of the formulation, because of the many variables related to the nature of the elastomer latex which influences the pH changes.

II-2) Natural rubber latex

As said before, the elastomer latex is a particular form of elastomer which is in the form of elastomer particles dispersed in water.

Natural rubber exists in various forms, as detailed in Chapter 3, Latex concentrates: properties and composition, K.F. Gaseley, A.D.T. Gordon and TD Pendle in "Natural Rubber Science and Technology," AD Roberts, Oxford University Press - 1988. In particular, several forms of natural rubber latex are marketed: so-called "latex field" natural rubber latexes. , the so-called "natural concentrated rubber rubber latex", the epoxy latex ("ENR"), the deproteinized latex or the prevulcanized latex. Field natural rubber latex is a latex in which ammonia has been added to prevent premature coagulation and the concentrated natural rubber latex is a field latex which has been treated to a wash followed by a new concentration. The different categories of concentrated natural rubber latex are listed in particular according to ASTM D 1076-06.

The latex can be used directly or be previously diluted in water to facilitate its implementation.

II-3) Contacting the two dispersions The two dispersions are brought into contact. To allow the proper mixing of these solutions, they are for example poured into a beaker with magnetic stirring. Any type of apparatus allowing an "effective" mixing of two products in the liquid phase can also be used, so it will be possible to use a static mixer such as static mixers marketed by Noritake Co., Limited, TAH in the US, KOFLO with USA, or TOKUSHU KIKA KOGYO Co., Ltd. or a blender performing high shear such as mixers marketed by TOKUSHU KIKA KOGYO Co., Ltd., or by the company PUC in Germany, by CAVITRON in Germany or by SILVERSON in the United Kingdom.

It is clear that the more efficient the mixing step, the better the dispersion.

During this phase of mixing the two dispersions, a coagulum of elastomer and silica is formed either as a single solid element in the solution, or in the form of several separate solid elements. As soon as the contacting of the two dispersions has taken place, the pH, here named formulation pH, of this new dispersion is measured according to the protocol described previously in the tests.

Surprisingly, it has been found that in order to effectively obtain a coagulum, with a coagulation yield of greater than or equal to 80%, and which corresponds to obtaining a master mixture that complies with the initial ratio by weight of filler with respect to elastomer, thus a difference of 20% compared to the ratio calculated initially is considered acceptable, it was necessary to be in one of the following cases:

(i) the formulation pH is 3 and 6 and the magnesium doping rate of the silica is greater than 0.2% by weight;

And it is still preferable to be in the case where the condition (i) is the following: (i) the formulation pH is between 3 and 6 and the magnesium doping rate of the silica is between 0.25 and 0.25; and 0.9% by weight.

Even more preferably, the formulation pH is greater than or equal to 8.5 and the magnesium doping level of the silica is greater than or equal to 0.4% by weight. More particularly, the formulation pH is less than 10.

The volumes of the two dispersions to put in contact and in particular the silica dispersion volume depends on the target silica level for the masterbatch to be produced. So the volume will be adapted accordingly. Advantageously, the target silica content for the masterbatch is between 20 and 150 phr (parts by weight per hundred parts of elastomer), preferably between 30 and 100 phr and more preferably between 30 and 90 phr, more preferably between 30 and 90 phr. and 70 pce. II-4) Recovery of the formed solid.

The solids recovered are filtered or centrifuged. Indeed, the filtering operation that can be performed using a filtration screen, may be unsuitable when the coagulum is in the form of many small and solid elements. In such a case, an additional centrifugation operation is preferably carried out.

At the end of this filtering or centrifugation step, the coagulum obtained is dried, for example in an oven. At the end of this operation, the ATG loading rate and the coagulation yield are measured.

II-5) Rubber composition Advantageously, the masterbatches thus produced can be used in rubber compositions, in particular for tires.

Advantageously, the level of magnesium present in the masterbatch can be limited by limiting the doping rate of the silica to 4.5% by weight. The tire rubber compositions based on the masterbatches according to the invention also comprise, in a known manner, a coupling agent and a vulcanization system.

It will be recalled here that "coupling agent" is understood to mean, in known manner, an agent capable of establishing a sufficient bond, of a chemical and / or physical nature, between the inorganic filler and the diene elastomer; such a coupling agent, at least bifunctional, has for example as simplified general formula "Y-Z-X", in which:

Y represents a functional group ("Y" function) which is capable of binding physically and / or chemically to the inorganic filler, such a bond being able to be established, for example, between a silicon atom of the coupling agent and the surface hydroxyl (OH) groups of the inorganic filler (for example surface silanols in the case of silica);

X represents a functional group ("X" function) capable of binding physically and / or chemically to the diene elastomer, for example via a sulfur atom;

Z represents a divalent group making it possible to connect Y and X.

Coupling agents, in particular silica / diene elastomer, have been described in a very large number of documents, the best known being bifunctional organosilanes. carriers of alkoxyl functions (i.e., by definition, "alkoxysilanes") as "Y" functions and, as "X" functions, functions capable of reacting with the diene elastomer such that for example polysulfide functions. Among the known alkoxysilane-polysulfide compounds, mention should be made particularly of bis 3-triethoxysilylpropyl tetrasulfide (abbreviated "TESPT"), of formula [(C ₂ H ₅ O) ₃ Si (CH ₂ ) 3 S 2] 2, sold especially by the company Degussa under the name "Si69" (or "X50S" when supported at 50% by weight on carbon black), in the form of a commercial mixture of polysulfides S _x with a mean value for x which is close to 4.

It will be noted that it is possible to envisage the introduction of the coupling agent in the preparation of the masterbatch in order to directly obtain a masterbatch of elastomer and silica also comprising a coupling agent. The coupling agent can thus be added before or during the bringing into contact of the aqueous dispersion of doped silica and the natural rubber latex

These rubber compositions in accordance with the invention may also comprise all or part of the usual additives normally used in elastomer compositions intended for the manufacture of tires, in particular treads, for example plasticizers or lubricating oils. extension, whether these are aromatic or non-aromatic, pigments, protective agents such as anti-ozone waxes, chemical antiozonants, anti-oxidants, anti-fatigue agents, reinforcing resins, acceptors ( for example phenolic novolac resin) or methylene donors (for example HMT or H3M) as described for example in the application WO 02/10269, a crosslinking system based on either sulfur or sulfur donors and / or peroxide and / or bismaleimides, vulcanization accelerators. Preferably, these compositions comprise, as preferential non-aromatic or very weakly aromatic plasticizing agent, at least one compound chosen from the group consisting of naphthenic, paraffinic, MES, TDAE, ester (especially trioleate) oils. glycerol, the hydrocarbon plasticizing resins having a high Tg preferably greater than 30 ° C, and mixtures of such compounds.

These compositions may also contain, in addition to the coupling agents, coupling activators, covering agents (comprising, for example, the only Y function) of the reinforcing inorganic filler or, more generally, processing aid agents capable of in a known manner, by improving the dispersion of the inorganic filler in the rubber matrix and by lowering the the viscosity of the compositions, to improve their ability to implement in the green state, these agents being for example hydrolysable silanes such as alkylalkoxysilanes (especially alkyltriethoxysilanes), polyols, polyethers (for example polyethylene glycols), primary, secondary or tertiary amines (for example trialkanol amines), hydroxylated or hydrolysable POSs, for example α, ω-dihydroxy-polyorganosiloxanes (in particular α, ω-dihydroxy-polydimethylsiloxanes), fatty acids such as, for example stearic acid.

The additives described above, oil, antioxidant, coating agent, could also be incorporated into the masterbatch before the formation of the coagulum.

These compositions may in particular also contain, in addition to the masterbatch, one or more other diene elastomers. It is recalled that, by elastomer or "diene" rubber, must be understood in known manner an elastomer derived at least in part (ie, a homopolymer or a copolymer) from monomers dienes (monomers bearing two carbon-carbon double bonds, conjugated or otherwise ).

II-6). Manufacture of rubber compositions The rubber compositions of the invention are manufactured in appropriate mixers, using two successive preparation phases according to a general procedure well known to those skilled in the art: a first phase of work or thermomechanical mixing (sometimes referred to as a "non-productive" phase) at a high temperature, up to a maximum temperature of between 130 ° C and 200 ° C, preferably between 145 ° C and 185 ° C, followed by a second phase of work mechanical (sometimes called a "productive" phase) at a lower temperature, typically less than 120 ° C, for example between 60 ° C and 100 ° C, finishing phase during which is incorporated the crosslinking system or vulcanization. According to a preferred embodiment of the invention, all the basic constituents of the compositions of the invention, with the exception of the vulcanization system, namely the masterbatch, the coupling agent (if it is not is not already present in the masterbatch) and the carbon black, if any, are intimately incorporated by mixing with the masterbatch during the so-called non-productive first phase, i.e. at least one of the various basic constituents is introduced into the mixer and mechanically kneaded in one or more steps until the maximum temperature of between 130 ° C. and 200 ° C. is reached, preferably between 145 ° C and 185 ° C. By way of example, the first (non-productive) phase is carried out in a single thermomechanical step during which all the necessary constituents, the possible coating agents, are introduced into a suitable mixer such as a conventional internal mixer. or other complementary additives and other additives, with the exception of the vulcanization system. The total mixing time in this non-productive phase is preferably between 1 and 15 minutes. After cooling the mixture thus obtained during the first non-productive phase, the vulcanization system is then incorporated at low temperature, generally in an external mixer such as a roller mixer; the whole is then mixed (productive phase) for a few minutes, for example between 2 and 15 min.

When a coating agent is used, its incorporation can be carried out entirely during the non-productive phase, together with the inorganic filler, or in full during the productive phase, together with the vulcanization system, or still split over the two successive phases.

It should be noted that it is possible to introduce all or part of the coating agent in a supported form (the setting is carried out beforehand) on a solid compatible with the chemical structures corresponding to this compound. For example, during the fractionation between the two successive phases above, it may be advantageous to introduce the second portion of the coating agent on the external mixer, after being placed on a support in order to facilitate its incorporation and its dispersion. .

The vulcanization system itself is preferably based on sulfur and a primary vulcanization accelerator, in particular a sulfenamide type accelerator. To this vulcanization system may be added, incorporated during the first non-productive phase and / or during the productive phase, various known secondary accelerators or vulcanization activators, such as for example zinc oxide, fatty acids such as stearic acid, guanidine derivatives (in particular diphenylguanidine), etc. Sulfur is used at a preferential rate of between 0.5 and 12 phr, in particular between 1 and 10 phr. The primary vulcanization accelerator is used at a preferred level of between 0.5 and 10 phr, more preferably between 0.5 and 5.0 phr. The final composition thus obtained is then calendered, for example in the form of a sheet or a plate, in particular for a characterization in the laboratory, or else extruded in the form of a rubber profile that can be used, for example, as a tread. tire for passenger vehicle. III EXAMPLES OF CARRYING OUT THE INVENTION

III.1 Preparation of magnesium-doped silica

Procedure for the Mg doping synthesis performed in the laboratory (doping at 0.5% w targeted)

Synthesis of Mg doping on a silica marketed under the name Zéosil ZI 165MP by the Rhodia company at 0.5% w referred to (theoretical mass of doped product produced:

166.87g) - for 0.5% w Mg target, introduced 6% w Mg because the doping yield is about 8.33% w.

Material:

- 1 5L double jacketed reactor, equipped with a Teflon hexagon stirring plate - 1 Heidolf stirring motor, model RZR2101

- 1 Huber thermostatic bath, model CC245

- 2 Masterflex peristaltic pumps (model 7523-25 for 10 to 600rpm, or model 7523-37 for 1 to 100rpm), equipped with an easy-load pump head, model 7518-60

- 1 pH electrode Mettler Toledo Inlab®Reach Pro calibrated, with temperature compensation + 1 pH meter Mettler Toledo MP225

- 1 magnetic stirrer + 1 magnetic bar

- Tygon pipes, glassware

- Ultra-Turrax® T25B rotor-stator homogenizer equipped with an N-type S25N-18G packing generator shaft and Vibracell ultrasonic generator manufactured by SONICS and 1500 Watt Materials Inc. with a PZT crystal piezoelectric converter (reference 75010), a booster for the probe and a titanium alloy probe of diameter 19mm (for a height of 127mm)

The reagents are presented in the following table:

speeds

addition

Reagents Details Quantities in the

middle

157.43g dry equivalents

160MP MV: 6.48% w

that is 168.33g weighed

Demineralized water 3.935L

15mL / min Mg (S04), 7H20 purity salt: 99% 96,90g in 182g water

11-15mL / min 5M NaOH a few mL

15mL / min H2S04 0.82M a few mL Operating mode

The contents of 2 beakers [2 * (83.97 g of 160 MP in 485.17 g of water)] were sheared using the abovementioned homogenizer for 5 min at 17500 rpm and then sonicating at 100% of the maximum power. a 1500W probe, for 8min.

The slurry thus sheared is introduced into the reactor and 2964.67 ml of demineralized water are added to obtain an initial silica concentration of 40 g / l, ie 3.8% w.

The medium is stirred at 650 rpm and heated to 60 ° C. (control of the temperature of the medium using a temperature probe integrated into the electrode).

The Mg (SO ₄ ) 7H ₂ O salt is added at 15 ml / min and the pH of the medium is adjusted to 7.5 by simultaneous addition of sodium hydroxide.

The reaction medium is left stirring and heating for 30 minutes (pH regulation at 7.5), then the pH is lowered to 4.5 by addition of H ₂ SO ₄ .

The mixture is left stirring and temperature again for 10 minutes with pH stabilization at 4.5, then the mixture is filtered off and the charge cake thus produced is washed with 10L of demineralized water, using a RC30VxR type wringer marketed by Rousselet Centrifugation SA company

The cake obtained is resuspended in demineralized water at a concentration of about 10% w.

A measurement of volatile matter is made on the suspension contained in the suspension bottle (for use in the manufacture of masterbatches) to know the exact mass concentration of the suspension. Material:

- Halogen Moisture Analyzer (= thermobalance) mettler toledo, model HR73, connected to a computer

- disposable magnesium cup (consumable adapted to the device). Operating mode:

After filling in the information on the sample, the measurement conditions (160 ° C without temperature ramp, duration of 30 minutes) and the taring of the magnesium cup, about 2.5 g of sample are introduced into the cup and start the measurement. The protocol to be followed for different doping levels is identical with the exception of the amount of magnesium precursor salt to be introduced as summarized in Table 1 which follows in which appear the amounts of magnesium salt used according to the doping levels referred.

Table 1

It should be noted that those skilled in the art will be able to adjust the amounts of Mg (SO ₄ ) salt reactant in order to obtain the desired doping level, by extrapolating from these values.

III.2 Preparation of masterbatches

The magnesium-doped silicas previously obtained are dispersed in water so as to obtain a concentration of 4% by weight of silica in water.

Each dispersion of silica sonified and then left stirring for 10 minutes (possible adjustment of the pH in the last minute), is brought into contact with a concentrated natural rubber latex maintained with magnetic stirring, the aqueous dispersion of silica being poured very rapidly in this latex.

The volume of the aqueous dispersion of doped silica is adjusted in relation to the volume of the latex as a function of the concentration of the silica and the concentration of the latex, so that, when the two dispersions are brought into contact (silica and elastomer latex), ) the desired charge rate.

For the examples, an amount of silica of 50 parts by weight per hundred parts of elastomer was chosen, which corresponds here to 50 phr (in effect the masterbatches described here only comprise silica and the diene elastomer). As soon as the whole of the doped silica aqueous dispersion has been poured, the pH measuring electrode is introduced into the mixture in order to measure the formulation pH.

As explained above, to obtain the desired pH, it is necessary to adjust the pH of the aqueous dispersion of doped silica after shearing thereof but before it comes into contact with the latex. It is therefore necessary to carry out an experiment to adjust the pH of the aqueous dispersion of doped silica. The mixture is maintained for a few minutes with magnetic stirring before recovery of the coagulum formed.

In order to have for the different tests identical conditions of the operating mode, the coagulum formed or the solids formed (commonly called "crumbs") are centrifuged, including in cases where the visual appearance of the coagulum made it possible to envisage a filtering operation.

The centrifugation is carried out after transfer into a 250mL nalgene flask using a Sigma 4K15 scoop centrifuge at 8000 rpm for 10 minutes. The coagulum thus recovered is dried under a fume hood at room temperature for 24 hours and then under vacuum in an oven for 24 hours at 65 ° C. under 300mbar in order to remove the last traces of water.

The ATG loading rate and the coagulation yield are then measured.

III-3 Example 1

This example is intended to demonstrate the proper functioning of the method according to the invention, in particular vis-à-vis the formulation pH measured at a given doping rate of the silica.

Tests El, E2, E3 and E4 were carried out according to the method detailed in the preceding paragraph with:

a concentrated "High Ammonia" natural rubber latex from the BEETEX supplier with a solids content measured at 61.68%, and which has been diluted by 2,

a magnesium-doped silica with a doping level of 0.5% by weight,

a quantity of silica when the two dispersions of 50 phr are brought into contact.

The only difference between these four tests consists, during the previously detailed operating method, in modifying the pH of the doped silica aqueous dispersion in order to modify the formulation pH.

Thus the El, E2, E3 and E4 tests are distinguished from each other by their formulation pH when the dispersions (aqueous dispersion of silica and elastomer latex) are brought into contact as follows:

for El, the formulation pH was 2.2,

for E2, the formulation pH was 3.5,

- for E3, the formulation pH was 7.2 for E4, the formulation pH was 9.5 (it will be noted that to obtain this formulation pH, it was not necessary to add an acid to the aqueous dispersion of silica. yield and charge rate) for these four tests are presented in Table 2 which follows:

Table 2

It is found that tests E1 and E3 (whose formulation pH is 2.2 and 7.2) did not allow the coagulation of the elastomer with the doped silica. Silica and latex were demixed during the centrifugation recovery step, so no coagulum was obtained.

For tests E2 and E4, both acceptable silica levels (between 40 pcmo and 60 pcmo) and a yield greater than 80% are obtained.

It clearly appears that it is important to be in a given pH range to meet the desired criteria in terms of the rate of charge observed and the yield obtained.

III-4 Example 2

This example is intended to demonstrate the proper functioning of the method according to the invention, in particular vis-à-vis the formulation pH measured at a given doping level different from Example 1.

Tests E '1, E'2 and E'3 were carried out according to the method detailed in the preceding paragraph with:

a concentrated latex of natural rubber identical to that of Example 1

a magnesium-doped silica with a doping level of 0.25% by weight,

a quantity of silica when the two dispersions of 50 phr are brought into contact. The only difference between these three tests consists, during the previously detailed operating method, in modifying the pH of the doped silica aqueous dispersion in order to modify the formulation pH.

Thus the tests ΕΊ, E'2 and E'3 are distinguished from each other by their formulation pH when the dispersions (aqueous dispersion of silica and elastomer latex) are brought into contact as follows:

for E '1, the formulation pH was 4,

for E'2, the formulation pH was 7.2

for E '3, the formulation pH was 9.2 (it will be noted that in order to obtain this formulation pH, it was not necessary to add an acid to the aqueous dispersion of silica).

The results obtained for these three tests are presented in Table 3 which follows.

Table 3

It can be seen that only the E 'I test made it possible to obtain a masterbatch with a yield greater than 80% and a silica content of 20%> with respect to the target 50%. Tests E'2 and E'3 did not allow coagulation of the elastomer with the doped silica. Silica and latex were demixed during the centrifugation recovery step, so no coagulum was obtained.

Claims

1) Method for preparing a masterbatch of natural rubber and silica comprising the following successive steps:

- doping the silica with magnesium,

to prepare at least one dispersion of the silica thus doped in water,

contacting and mixing the aqueous dispersion of doped silica and at least one natural rubber latex to obtain a coagulum,

- recover the coagulum,

- Dry the recovered coagulum to obtain the masterbatch.

2) Method according to claim 1, wherein the coagulum recovery step is performed by a filtering operation. 3) Method according to claim 1, wherein the step of recovering the coagulum is performed by a centrifugation operation.

4) Method according to any one of claims 1 to 3, wherein the natural rubber latex is a concentrated latex of natural rubber.

5) Method according to any one of claims 1 to 4, wherein the silica is a precipitated silica.

6) Method according to any one of claims 4 or 5, wherein one of the following conditions is satisfied:

(i) the formulation pH is between 3 and 6 and the magnesium doping rate of the silica is greater than 0.2% by weight;

7) The method of claim 6, wherein the condition (i) is as follows:

(i) the formulation pH is between 3 and 6 and the magnesium doping rate of the silica is between 0.25 and 0.9% by weight. The method of claim 6, wherein the formulation pH is greater than or equal to 8.5 and the magnesium doping level of the silica is greater than or equal to 0.4% by weight.

9) Method according to any one of claims 6 to 8, wherein the formulation pH is less than 10. 10) Method according to any one of claims 1 to 9, wherein the amount of silica in contacting two dispersions is between 20 phr and 150 phr, parts per hundred parts by weight of elastomer.

11) Method according to any one of claims 1 to 10, wherein the amount of silica in contacting the two dispersions is between 30 phr and 100 phr. 12) Method according to any one of claims 1 to 11, wherein the amount of silica in contacting the two dispersions is between 30 phr and 90 phr.

13) Method according to any one of the preceding claims, wherein an aqueous dispersion of coupling agent is added before or during the contacting of the magnesium-doped aqueous silica dispersion and the natural rubber latex.

14) Masterbatch of natural rubber and silica prepared according to any one of claims 1 to 13.

15) Masterbatch according to claim 14, wherein the silica content is between 20 phr and 150 phr.

16) Masterbatch according to any one of claims 14 or 15, wherein the silica content is between 30 phr and 100 phr.

17) Masterbatch according to any one of claims 14 to 15, wherein the silica content is between 30 phr and 90 phr, preferably between 30 and 70 phr. 18) A rubber composition based on at least one masterbatch of natural rubber and silica prepared according to any one of claims 1 to 13.

19) A finished or semi-finished article comprising a composition according to claim 18. 20) A tire tread comprising a composition according to claim 18.

21) Pneumatic or semi-finished product comprising at least one rubber composition according to claim 18.