EP2523905A1

EP2523905A1 - Precipitated silica

Info

Publication number: EP2523905A1
Application number: EP10800734A
Authority: EP
Inventors: Rémi Valero
Original assignee: Bluestar Silicones France SAS
Current assignee: Elkem Silicones France SAS
Priority date: 2009-12-23
Filing date: 2010-12-20
Publication date: 2012-11-21
Also published as: JP2013515661A; WO2011076716A1; US20120329937A1; KR20120114319A; CN102753479A

Abstract

The present invention relates to a method for preparing precipitated silica, in particular in powder form. The invention also relates to the resulting precipitated silicas and to the use thereof, in particular for the reinforcement of silicone elastomer or silicone paste matrices.

Description

Precipitated silica

The present invention relates to a process for the preparation of precipitated silica, in particular in the form of a powder. The invention also relates to the precipitation silicas thus obtained, as well as their use, in particular for the reinforcement of silicone elastomer matrices or based on silicone pastes.

In the compositions of the silicone elastomer type or of the silicone pastes, for many years, as reinforcing fillers, combustion silicas have been used, that is to say silicas obtained by a process consisting in reacting at high temperature tetrachlorosilane compounds with hydrogen and oxygen. However, by their method of production, the combustion silicas are generally expensive. As a result, in applications for reinforcing silicone matrices, it was quickly sought to replace at least partially these high-cost silicas with so-called "precipitation" silicas obtained by precipitating a silica in an aqueous medium starting from a precursor such as a silicate, under appropriate pH conditions. Indeed, these silicas are of lower cost and may have the required dispersibility characteristics within a silicone-based matrix.

Since many years, it is therefore sought to replace at least partially these combustion silicas by precipitated precipitation silicas. There are various processes for the preparation of precipitation silicas, complex processes in which the temperature, the concentrations of reactants and the pH value are adjusted during the preparation, as described in French Patent 1,352,354.

But, in the most general case, precipitation silicas have a high affinity for water. It therefore turns out that the precipitated silicas do not often have good reinforcing properties of the silicone matrices because their compatibility with the silicone matrices in which they are incorporated is not always satisfactory. Thus, patent FR 2 61 1 196 describes a heat treatment at high temperature (minimum 700 ° C.) which is used to prepare precipitated silicas with low water uptake, but these treatments remain energy-intensive and complex to implement. because of the high temperatures used.

More recently, patent EP 1 860 066 describes a process for producing precipitated silica which is of particular interest for the reinforcement of silicone matrices, comprising a step of heat treatment at high temperature (300-800 ° C.) in a fluidized bed. Nevertheless, this step is expensive in energy and requires substantial industrial investments.

It has also been sought to improve the qualities of reinforcing agent for precipitated silicas for silicone applications by rendering the silicas hydrophobic by an appropriate surface treatment (use of, for example, silane or silazane). Hydrophilic silicas made hydrophobic by such a treatment and usable for silicone applications are described, for example, in French Patent 2,356,596. However, these treatments also make the processes relatively expensive.

Thus, an essential objective of the present invention is to provide a process for the preparation of precipitated silica which is simple to implement, does not require significant additional industrial investment or high energy expenditure compared to known methods and which allows the production of precipitated silica. obtaining dispersible precipitated silicas which can be used as fillers, in particular reinforcing fillers, in matrices containing silicones and to which they can confer good mechanical properties.

More specifically, the invention also aims to provide dispersible precipitation silicas that can be used as fillers, in particular reinforcing fillers, in matrices based on silicones and to which they can confer good mechanical properties.

Another object of the invention is to provide a precursor organopolysiloxane composition of silicone elastomers comprising this dispersible precipitated silica.

Another object of the present invention is to obtain a silicone elastomer comprising this dispersible precipitated silica. A final objective of the invention is the use of the precipitated silica thus obtained in tires, dentifrices, cosmetic compositions, compositions foodstuffs, pharmaceutical compositions, silicone compositions and elastomers.

All of these objects, among others, are achieved by the present invention which relates to a process for preparing a precipitated silica X having improved dispersibility and reinforcing properties comprising the steps of: a) reacting at least one of a silicate with at least one acidifying agent, so as to obtain a suspension A of precipitated silica,

b) filtering and washing said suspension A precipitated silica, so as to obtain a filter cake B,

c) the filter cake B is dried to obtain a precipitated silica powder C, and d) the precipitated silica C is crushed and dried, these two operations are carried out simultaneously in a mechanical mill Z and at a temperature between 50 and 190 ° C, preferably between 60 and 150 ° C and even more preferably between 65 and 130 ° C and recovering precipitated silica X.

By mechanical mill is meant an apparatus in which takes place the reduction of the particles by mechanical means (for example a jaw, hammer or knife mill). This term does not include fluid jet mills such as air jet mills where the particles are entrained by an air jet in an enclosure designed so that the particles undergo a large number of shocks.

One of the advantages of mechanical mills is that the temperature at which milling takes place only slightly affects the size of the particles obtained, unlike fluid jet mills where the temperature of the fluid affects its flow rate and consequently the mill performance.

To achieve this objective, the inventors have had the merit of surprisingly and surprisingly revealing that a precipitation silica exhibiting dispersibility characteristics, a density and a moisture content particularly suitable for their use for the reinforcement of Silicone-based matrices can be obtained according to a silica precipitation process, provided that the grinding and drying step is simultaneously carried out in a mechanical grinder. Steps a), b) and c) of the process according to the invention are well described in the art.

Anterior and known to those skilled in the art. In general, the preparation of silica precipitated is carried out by precipitation reaction of a silicate, such as an alkali metal silicate (sodium silicate for example) with an acidifying agent (sulfuric acid for example). The mode of precipitation of the silica (step a)) can be arbitrary: in particular, by addition of acidifying agent to a silicate base stock or by simultaneous total or partial addition of acidifying agent and silicate on a stock of water or silicate. At the end of these operations, a silica slurry is obtained which is then separated (liquid-solid separation). This separation generally consists of a filtration which can be done according to any suitable method, for example press filter or belt filter or rotary vacuum filter, this filtration leading to the obtaining of a "filter cake" This cake of filtration obtained is subjected to one or more washing operation, generally with water, so as to reduce its salt content (step b)). Optionally, it can also be subjected to a disintegration operation prior to the drying step. Drying of the filter cake (step c) is preferably by spray drying. For this purpose it is possible to use any suitable type of atomizer, in particular turbines, nozzles, liquid pressure or two-fluid atomizers. In general, the precipitated silica thus separated, filtered, optionally washed and dried can be subjected to a subsequent grinding so as to obtain the desired particle size. Different types of mill can be used such as air jet mills or mechanical grinders.

In silica manufacturing processes, drying is preferably carried out prior to grinding. Indeed, during grinding the density of the precipitated silica will greatly decrease and consequently the powder volumes to be dried and transported increase considerably. In addition, the handling of small particle size powders imposes very important hygiene, safety and environmental constraints. It is therefore in the interest of any industrial to carry out the grinding step as late as possible in the process for producing a precipitated silica.

According to a preferred embodiment, a means for simultaneously grinding and drying is to regulate in step d) the temperature in the mechanical mill Z by means of an air supply heated to a temperature of between 50.degree. and 190 ° C, preferably between 60 and 150 ° C and even more preferably between 65 and 130 ° C. The temperature can be regulated also by means of a supply of a heated inert fluid (for example nitrogen or argon), but this variant leads to a higher operating cost. According to an even more preferred embodiment, the grinding in step d) is carried out by means of a mechanical grinder Z by attrition and more particularly by means of a mechanical grinder Z by attrition in a grinding chamber provided with a rotor and a stator.

In a mechanical grinder by attrition, the grinding of the particles takes place between the rotor and the stator. The fragmentation of the particles depends on the probability of impact between the grinding bodies and the particles. Thus, the same particle can be crushed several times while others will never be crushed. The product obtained therefore has a wide particle size distribution. In order to obtain a better calibrated product, to increase the energy efficiency and to avoid over-grinding, the mechanical grinders can be equipped with particle classification systems by flight.

According to a preferred variant of the invention, in step d) the mechanical mill Z is equipped with an integrated particle classification system for recovering precipitated silica particles X.

And according to another preferred variant of the invention in step d) the mechanical mill Z is connected to an independent particle classification system for recovering precipitated silica particles X.

While not wishing to be bound in any way to a particular theory, it seems possible to be argued that simultaneously grinding and drying allows the small particles resulting from grinding to be dried immediately, which will be less likely to reagglomerate later and will exhibit better dispersibility.

According to another embodiment of the invention, step d) takes place at atmospheric pressure.

Precipitated silicas are commonly characterized according to the methods and measurements detailed below:

The BET specific surface area which is measured according to the BRUNAUER-EMMET-TELLER method described in the Journal of the American Chemical Society, vol. 60, page 309 (February 1938). The specific surface area called CTAB, determined according to standard NFT 45007 (November 1987).

The pH measured according to ISO 787/9 (pH of a suspension at 5% in water). The moisture content (or residual water content) determined by the weight loss measured after heat treatment at 105 ° C for 2 hours (in% weight) The tap density or packed fill density (DRT) is determined according to standard NF T 30-042.

In addition, the precipitation silicas generally contain, at least in trace amounts, a salt resulting from the action of the acidifying agents used on the silicates used. Thus, when the process of the invention specifically employs an alkali silicate as the precursor of silica and sulfuric acid as an acidifying agent, the precipitation silicas contain an alkali sulfate. Generally, the content of alkali sulphate in the silicas thus obtained is relatively low, most often such that the mass of sulphate ions present generally represents at most 1% by weight relative to the total mass of the dry material. The control of the sulfate content of the precipitated silica is important for some applications. By way of example, sulfate levels in precipitated silica greater than 0.7% by weight lead to a coloration (yellowing) of the elastomers containing this silica. It is also known also that a high sulphate rate promotes a significant water recovery, hence the interest of keeping this sulphate rate as low as possible.

According to a preferred embodiment of the invention in step c) the precipitated silica C has the following characteristics:

a BET surface area of between 50 and 300 m ² / g,

a CTAB surface area of between 50 and 300 m ² / g,

the BET-CTAB value <50 m ² / g,

a moisture content of between 4 and 10% by weight,

a pH of between 4 and 8,

a sulphate content S0 ₄ ^" <1, 2% by weight, and

- a density typed> 100 g / l

According to an even more preferred embodiment of the invention in step c) the precipitated silica C has the following characteristics:

a BET surface area of between 130 and 250 m ² / g, a CTAB surface of between 130 and 250 m ² / g,

the BET-CTAB value <30 m ² / g,

a moisture content of between 4 and 9% by weight,

a pH between 4.5 and 7.5,

a sulfates content S04 ^" <0.7% by weight, and

a density typed> 150 g / l

And, according to an even more preferred embodiment of the invention in step c), the precipitated silica C has the following characteristics:

a BET surface area of between 155 and 185 m ² / g,

a CTAB surface area of between 155 and 185 m ² / g,

the BET-CTAB value <15 m ² / g,

a moisture content of between 4 and 8% by weight,

a pH of between 5 and 6.5,

a sulphate content S04 ^" <0.5% by weight, and

- a density typed> 200 g / 1

Preferably, the precipitated silica C is a water-dispersible silica on the type of silica Z160 ^® marketed by Rhodia, silica Ultrasil ^® marketed by Degussa or silica DRX190 ^® marketed by PPG.

It is important to note that some of the characteristics of the precipitated silica C are not modified during step d) of the process where the grinding-drying is carried out simultaneously. This is the case, for example, with CTAB, BET, pH and sulphate level. On the other hand, step d) of the process according to the invention enables us to improve the properties of the precipitated silica C by decreasing its moisture content, by decreasing its particle size and by decreasing its typed density.

Another subject of the invention concerns the precipitated silica X obtainable by the method of the invention and which has the following characteristics:

an average particle size D _v 50 <20 μηι,

a moisture content <5% by weight, and

- a density typed <100 g / l

The size of the precipitated silica particles is measured with a laser granulometer (Malvern 2000 apparatus). The distribution values are expressed in cumulative volume. Thus 10% of the particles in volume have a size smaller than the value indicated by the _Dv 10, 50% of the particles in volume have a size smaller than the value indicated by the Dv50 and 90% of the particles in volume have a smaller size than the value indicated by D _v 90.

According to a preferred embodiment of the invention, the precipitated silica X obtainable by the process of the invention has the following characteristics:

an average particle size D _v 50 <14 μηι,

a moisture content <4% by weight, and

- a density typed <80 g / 1

According to an even more preferred embodiment of the invention, the precipitated silica X that can be obtained by the process of the invention has the following characteristics:

an average particle size D _v 50 <14 μηι,

a moisture content <3% by weight, and

a typed density <80 g / 1 Preferably, the precipitated silica X according to the invention has a D _v 10 <12 μηι and even more preferentially D _v 10 <8 μηι.

Preferably, the precipitated silica X according to the invention has a D _v 90 <25 μηι and even more preferentially D _v 90 <22 μηι.

The present invention also relates to a precursor organopolysiloxane composition of silicone elastomers comprising precipitated silica X according to the invention or as obtained by the process according to the invention. As an example of organopolysiloxane compositions comprising precipitated silica X, we can mention the compositions making it possible to obtain hot vulcanized (EVC) and cold (EVF) elastomers, the compositions making it possible to obtain LSRs ("Liquid Silicone Rubber"). ), and RTV ("Room Temperature Vulcanisizing" monocomponents (such as mastics and cold glues) and two-component. In general, the organopolysiloxane compositions for obtaining the EVCs according to the invention comprise (in parts by weight):

a) 100 parts of at least one diorganopolysiloxane gum (1) having a viscosity greater than 1 million mPa.s at 25 ° C,

b) 0.1 to 7 parts of an organic peroxide (2),

c) 5 to 150 parts of a precipitated silica (3) according to the present invention, and d) from 0 to 15 parts of at least one diorganopolysiloxane oil (4) with a viscosity of at most 5000 mPa.s at 25 ° C. The diorganopolysiloxane gum (1) with a viscosity greater than 1 million mPa.s at

25 ° C, may for example be a chain of siloxyl units of formula R ₂ SiO _2/2 , blocked at each end of its chain by a siloxyl unit of formula R ₃ SiOi _{/ 2} and / or a radical of formula OR '; in these formulas the R symbols, which are identical or different, represent methyl, ethyl, n-propyl, phenyl, vinyl or 3,3,3-trifluoropropyl radicals, at least 60% of these radicals being methyl and at most 3% being vinyl, the symbol R 'represents a hydrogen atom, an alkyl radical having 1 to 4 carbon atoms, a betamethoxy-ethyl radical.

The diorganopolysiloxane oil (4) having a viscosity of at most 5,000 mPa.s at 25 ° C. can be formed of a chain of siloxyl units of formula R " ₂ SiO _2/2 blocked at each end of its chain by a radical of formula OR 'in these formulas the symbols R ", identical or different, represent methyl, phenyl or vinyl radicals, at least 40% of these radicals being methyl and the symbol R' has the meaning given above.

As concrete examples of siloxyl units of formulas R ₂ SiO _2/2 and R ₃ SiO _1/2 and radicals of formula OR ', may be mentioned those of formulas:

(CH ₃ ) ₂ SiO _2/2 , CH ₃ (CH ₂ = CH) SiO _2/2 , CH ₃ (C ₆ H ₅ ) SiO _2/2 , (C ₆ H ₅ ) ₂ SiO _2/2 , CH ₃ (C ₂ H ₅ ) SiO _2/2 , (CH ₃ CH ₂ CH ₂ ) CH ₃ SiO _2/2 , CH ₃ (nC ₃ H ₇ ) SiO _2/2 , (CH ₃ ) (C ₆ H ₅ ) ( CH ₂ = CH) Si _1/2 ,

-OH, -OCH ₃ , -OC ₂ H ₅ , -O-nC ₃ H ₇ , -O-isoC ₃ H ₇ , -O-nC ₄ H _g , -OCH ₂ CH ₂ OCH ₃ .

The diorganopolysiloxane oil (4) may be present in an amount of from 0 to 15 parts, preferably from 0.3 to 12 parts per 100 parts of gum (1). This oil or these oils are linear polymers of relatively low viscosity, at most 5,000 mPa.s at 25 ° C, preferably at most 4,000 mPa.s at 25 ° C, of which the diorganopolysiloxane chain is formed essentially of the units of the aforementioned formula R " ₂ Si0 _2/2 , this chain is blocked at each end by a radical of the aforementioned formula OR '. At least 40% of the radicals R "are methyl radicals, preferably at least 45%. The meaning of the symbols R" and R 'has been explained previously. Preferably, are used:

dimethylpolysiloxane oils blocked at each end of their chain by hydroxyl, methoxyl or betamethoxyethoxyl radicals and whose viscosity is between 10 and 200 mPa.s at 25 ° C .;

methylphenylpolysiloxane oils, consisting of CH ₃ (C ₆ H ₅ ) SiO _{2/2 units} , blocked at each end of their chain by hydroxyl and / or methoxyl radicals, and whose viscosity is between 40 and 2000 mPa. s at 25 ° C.

The organic peroxides (2) are used in a proportion of 0.1 to 7 parts, preferably 0.2 to 5 parts, per 100 parts of the gums (1). They are well known to those skilled in the art and more particularly include benzoyl peroxide, 2,4-dichloro-benzoyl peroxide, dicumyl peroxide, 2,5-dimethyl (tert-butylperoxy) -2,5-hexane, sodium perbenzoate and the like. t-butyl, peroxy t-butyl and isopropyl carbonate, di-t-butyl peroxide, bis (t-butylperoxy) -1,1 trimethyl-3,3,5 cyclohexane. These various peroxides decompose at sometimes different temperatures and speeds. They are chosen and their quantity is adapted according to the desired conditions.

Another subject of the present invention is a silicone elastomer comprising precipitated silica X according to the invention or as obtained by the process according to the invention.

A final subject of the invention relates to the use of precipitated silica X according to the invention or as obtained by the process according to the invention in tires, dentifrices, cosmetic compositions, food compositions, pharmaceutical compositions silicone compositions and elastomers.

Indeed, in addition to their applications as fillers in silicone matrices, the precipitated silicas of the present invention can also be advantageously used as reinforcing filler in matrices based on organic polymers, and in particular in matrices based on one or more elastomers, natural or synthetic, and in particular in matrices based on rubber, and more particularly based on natural or synthetic rubbers, SBR type or butyl rubber in particular. In fact, the silicas obtained according to the process of the invention have good quality of dispersibility and reinforcement within polymer and elastomer matrices, where they make it possible in particular to increase the resistance to abrasion, which can prove to be interesting in the context of the constitution of tires.

The precipitated silicas of the present invention may also be advantageously used as thickening agents in organic or aqueous media, preferably in aqueous media, and especially in toothpastes.

Furthermore, the silicas obtained according to the invention may be useful in many other fields of customary use of precipitated silicas, for example for the manufacture of paints or papers. They are particularly useful as a carrier in food or cosmetic compositions.

The silicas obtained according to the process of the present invention are, moreover, silicas which are particularly well suited in the field of galenics. Thus, the silicas of the present invention are particularly suitable as fillers, carriers and / or excipients within pharmaceutical compositions.

The object and the advantages of the present invention will appear even more clearly in view of the various implementation examples described below in a non-limiting manner.

EXAMPLES

Table 1 below describes the commercial silicas used to obtain the silicas according to the invention.

Table 1 - Characteristics of Commercial Silicas

D _v volume distribution of the particles for all the examples

The uncrushed commercial silicas 1, 2 and 3 are then:

· Is simultaneously milled and dried according to the invention in an ACM mill from Hosokawa supplied with hot air at 70 ° C. and equipped with a particle classification system making it possible to recover the silicas according to the invention (silicas S1, S2 and S3).

• are conventionally milled in standard grinders (grinding only) to provide us with comparative examples C1, C2 and C3.

The results are shown in the following Table 2. Table 2 - Characteristics of Crushed (Comparative) and Crushed and Dried Silica Simultaneously (Invention)

Simultaneous milling-drying of the various commercial precipitated silicas makes it possible to obtain a quality of precipitated silica which can be repeated even starting from silicas of different quality. The average particle size D _v 50 is between 9 and 12.5 microns and the moisture content is less than 5% by weight.

The milled and simultaneously dried silicas, obtained by the process according to the invention (silicas S1 to S3), as well as those milled conventionally, that is to say without simultaneous drying (comparative silicas C1 to C3), were used as reinforcing fillers in two heat-vulcanizable silicone compositions.

Silicone composition A (all parts are by weight)

In a Z-arm kneader kneader, the compounds listed in the following Table 3 are introduced. Mix for 30 minutes. Then, the temperature of the kneader is raised to 150 ° C in one hour, then maintained for one hour at Ι δΟ'Ό. Then, the heating is stopped and mixing is continued for one hour. The mixer is still maintained under a slight nitrogen sweep. Table 3: Formulation of silicone composition A (parts by weight)

(a) Vi corresponds to vinyl for all of the examples The composition thus obtained is introduced into a two-roll mill and 1.25 parts of 2,4-dichloro-benzoyl peroxide diluted to 50% by weight in a silicone oil. added as a catalyst. A fraction of the homogeneous mass obtained on the kneader is used to measure the mechanical properties of the silicone elastomer resulting from the hot vulcanization of the polyorganosiloxane composition. To do this, the homogeneous mass fraction taken is then vulcanized in press for 8 minutes at 1 ~ 5 ° C operating in a suitable mold to obtain plates 2 mm thick. Thus plates are obtained in the non-annealed state (NR). These plates are then annealed or aged for 4 hours at 200 ° C. Standardized samples are then taken from all these plates and the following properties are measured:

· Shore A hardness (DSA) according to AFNOR standard NFT 46-004

• Resistance to fracture (R / R) in MPa according to AFNOR standard NFT 46-002

• Elongation at break (A / R) in% according to AFNOR standard NFT 46-002

• Elastic modulus (ME) at 100% MPa according to ASTM D412

• Tear resistance (RD) in kN / m according to ASTM D624-73

· Compression set (% C) in accordance with ASTM D395-03 Method B (25%, 177Ό, 22 hours)

The following Table 4 shows the mechanical properties of the silicone elastomers obtained by using the milled silicas and simultaneously dried by the process according to the invention (Examples A1 to A3 with the silicas S1 to S3) and the conventionally milled silicas, that is to say ie without simultaneous drying (Comparative Examples AC1 to AC3 with C1 to C3 silicas). Table 4: Mechanical properties of elastomers derived from silicone compositions A

These results show that the elastomers formulated with the silicas crushed and dried simultaneously according to the method of the invention (Examples A1, A2 and A3) have a breaking strength (RR) and an elongation at break (AR) much greater than that of silicas crushed conventionally, that is to say without simultaneous drying (Comparative Examples AC1, AC2 and AC3).

Silicone composition B (all parts are by weight)

In a Z-arm kneader mixer, the compounds listed in the following Table 5 are introduced. Mix for 30 minutes. Then, the temperature of the kneader is raised to 150 ° C in one hour, then maintained for one hour at Ι δΟ'Ό. Then, the heating is stopped and mixing is continued for one hour. The mixer is still maintained under a slight nitrogen sweep. Table 5: Formulation of silicone composition B (parts by weight)

The composition thus obtained is introduced into a two-roll kneader and 0.6 parts of 2,5-dimethyl-2,5bis (tert-butylperoxy) -2,5-hexane diluted to 75% by weight in a silicone oil is added as a catalyst. A fraction of the homogeneous mass obtained on the kneader is used to measure the mechanical properties of the silicone elastomer resulting from the hot vulcanization of the polyorganosiloxane composition. To do this, the homogeneous mass fraction taken is then vulcanized in press for 10 minutes at 170 ° by operating in a suitable mold to obtain plates of 2 mm thick. Thus plates are obtained in the non-annealed state (NR). These plates are then annealed or aged for 4 hours at 200 ° C. Standardized samples are then taken from all these plates and the same properties are measured as for the silicone composition A.

Table 6 below shows the mechanical properties of the silicone elastomers obtained by using the milled silicas and dried simultaneously by the process according to the invention (Examples B1 to B3 with the silicas S1 to S3) and those milled conventionally, that is to say say without simultaneous drying (Comparative Examples BC1 to BC3 with C1 to C3 silicas). TABLE 6 Elastomeric Mechanical Properties Resulting from the Silicone B Compositions

These results show that the elastomers formulated with the milled and dried silicas according to the invention (Examples B1, B2 and B3) exhibit a breaking strength (RR), an elongation at break (AR) and a compression set (DRC) much higher than that of the crushed silicas by the suppliers (Comparative Examples BC1, BC2 and BC3).

Claims

1 - Process for preparing a precipitated silica X having a dispersibility and improved reinforcing properties comprising the following steps: a) reacting at least one silicate with at least one acidifying agent, so as to obtain a suspension A precipitated silica,

2 - Process according to claim 1 wherein in step d) the temperature in the mechanical mill Z is controlled by means of an air supply heated to a temperature between 50 and 190 ° C, preferably between 60 and 1 50 ° C and even more preferably between 65 and 130 ° C.

3 - Process according to claim 1 wherein step d) is carried out by means of a mechanical grinder Z by attrition.

4 - Process according to claim 1 wherein step d) is carried out by means of a mechanical grinder Z by attrition in a grinding chamber provided with a rotor and a stator.

5 - Process according to any one of the preceding claims wherein in step d) the mechanical mill Z is equipped with an integrated particle classification system for recovering precipitated silica particles X. 6 -Procédé according to any one of claims 1 to 4 wherein in step d) the mechanical mill Z is connected to an independent particle classification system for recovering precipitated silica particles X.

7 - Process according to any one of the preceding claims wherein step d) takes place at atmospheric pressure.

8 - Process according to any one of the preceding claims wherein in step c) the precipitated silica C has the following characteristics:

a BET surface area of between 50 and 300 m ² / g,

a CTAB surface area of between 50 and 300 m ² / g,

the BET-CTAB value <50 m ² / g,

a moisture content of between 4 and 10% by weight,

a pH of between 4 and 8,

a sulphate content S0 ₄ ^" <1.5% by weight, and

a typed density> 100 g / l 9 - precipitated silica X obtainable by the process as described according to any one of claims 1 to 8 and having the following characteristics:

an average particle size D _v 50 <20 micrometers,

a moisture content <5% by weight, and

- a density tapped <100 g / l.

10 - Composition of organopolysiloxane precursor of silicone elastomers comprising the precipitated silica X as described in claim 9 or as obtained according to the process described according to any one of claims 1 to 8.

11 - silicone elastomer comprising the precipitated silica X as defined in claim 9 or as obtained according to the process described according to any one of claims 1 to 8. 12 - Use of precipitated silica X as defined in claim 9 or as obtained according to the process described according to any one of claims 1 to 8 in tires, dentifrices, cosmetic compositions, food compositions, pharmaceutical compositions, silicone compositions and elastomers.