MXPA96005925A - New process for the preparation of precipitated silice, new precipitated silicks that contain zinc and its use to reinforce elastome - Google Patents

Abstract

A new method for preparing precipitated silica having good dispersibility and highly satisfactory reinforcing properties is disclosed. New precipitated silicas are also revealed in the form of powder, granules or substantially spherical beads. These silicas are characterized by having a specific area of CTAB surface of 90-250m2 / g, a specific area of BET surface of 90-250m2 / g, an absorption of DOP oil of less than 300 m1 / 100 g and a zinc content of 1- 5% by weight, and in that, the number N of acid-stearic molecules consumed per nm2 of the silica surface, when the stearic acid is reacted with said silica in xylene for 2 hours at 120 ° C, is less than 1. In addition, it is revealed the use of said silicas as reinforcement fillers for elastomers, in particular to improve the rheological properties of the same.

Description

New process for the preparation of precipitated silica, new precipitated silicas containing zinc and their use to reinforce elastomers The present invention relates to a new process for the preparation of precipitated silica, to precipitate silicas that are particularly in powder form, from practically spherical or granular beds, and for their application as a reinforcing filler for elastomers. It is known that precipitated silica has been used for a long time as a white reinforcing filler in elastomers. However, like any filling, it is appropriate that it is capable, on the one hand, of being handled and, above all, on the other hand, it is capable of being easily incorporated into the mixtures. It is generally known that, in order to obtain the optimum reinforcing properties conferred by a filler, it is appropriate that the latter be present in the elastomer matrix in the final form which is as finely divided as possible and distributed as homogeneous as possible. . However, these conditions can only be achieved so far, on the one hand, while the filling has a very good ability to be incorporated into the matrix during mixing with the elastomer (capacity to incorporate the filling) and to disintegrate or deagglomerate in a of a very fine powder (disintegration of the filler) and, on the other hand, while the powder resulting from the aforementioned disintegration process itself can, in turn, disperse perfectly and homogeneously in the elastomer (dispersion of the powder). Moreover, for the reasons of mutual affinity, the silica particles have an unfortunate tendency, in the elastomer matrix, to agglomerate with each other. This silica / silica interaction has the negative consequence of limiting the reinforcement properties to the level that is practically lower than that which theoretically could be expected, if all the silica / elastomer interactions, capable of being created during the mixing operation, were obtained really (this theoretical number of the silica / elastomer interaction is, as is well known, directly proportional to the external surface of the silica used). In addition, in the raw state, the silica / silica interaction tends to increase the rigidity and consistency of the mixture, making it more difficult to process. The problem arises from the availability of fillers that, while being able to be of a relatively large size, have a very good dispersibility in the elastomers.

The object of the present invention is to overcome the aforementioned disadvantage and solve the aforementioned problem. More specifically, the aim is, in particular, to propose a new process for the preparation of precipitated silica which, advantageously, have a very good dispersibility (and disintegration capacity) and very satisfactory reinforcing properties, in particular, when used as a filler of reinforcement for elastomers, imparts excellent rheological properties to the latter while providing them with good mechanical properties. The invention also relates to precipitated silicas, preferably, which are in the form of powder, of practically spherical beds or, optionally, of granules, and which, while of a relatively large size, have satisfactory reinforcing properties and, in a preferred way, a very good dispersibility (and disintegration capacity). Finally, it is related to the use of precipitated silicas as reinforcement of elastomer fillings. In the description that follows, the BET surface area is determined according to the Brunauer-Emmet-Teller method described in the Jounal of the American Chemical Society Vol. 60, page 309, of February 1938 and corresponds to the norm NFT 45007 (November 1987). The CTAB surface area is the outer surface determined in accordance with NFT 45007 (November 1987) (5.12). DOP oil uptake is determined in accordance with NFT 30-022 (March 1053) when using dioctyl phthalate. The density of the package (OD) is measured according to standard NFT 030100. The pH is measured according to ISO 787/9 (pH of a suspension at a concentration of 5% in water). Finally, it is specified that the given pore volumes are measured by means of a mercury porosimetry, the diameter of the pores is calculated from the Washburg ratio with the theta contact angle equal to 130 ° and a surface of gamma voltage equal to 484 dyne / cm (9300 Micromeritics porosimeter). The dispersibility and the disintegration capacity of the silica according to the invention can be quantified by means of the specific disintegration test. The disintegration test is carried out according to the following procedure: the cohesion of the agglomerators is assessed or determined by a measurement of the particle size (using laser dispersion), developed in a suspension of silica previously disintegrated by an ultrasonic treatment; the capacity of disintegration of silica is measured later (breaking of the objects from 0.1 to a few tenths of microns). The ultrasonic disintegration is carried out with the help of a Vibracell Bioblock sonic transducer (600 W) equipped with a 19 mm diameter specimen. The measurement of the particle size is carried out by means of a laser blaster in a Sympatec particle size analyzer. Measure 2 grams of silica outside in a specimen tube (height: 6 cm and diameter: 4 cm) and make up to 50 grams by adding demineralized water; and an aqueous suspension containing 4% silica is produced, which is homogenized for 2 minutes by magnetic stirring. The ultrasonic disintegration is then carried out as follows: with a specimen immersed at a depth of 4 cm, the powder is adjusted in such a way that a deflection is obtained in the needle of the energy dial indicating 20% (corresponding to a energy of 120 watts / cm2 dissipated by the end of the specimen). The disintegration is carried out for 420 seconds. The measurement of the particle size is carried out after a known volume (expressed in me) of the homogenized suspension has been introduced into the analyzer cell of the particle size. The value of the average diameter f50 that is obtained is proportionally smaller than the greater capacity of disintegration of the silica. The average (10 x dispersion volume introduced (in me)) / optical density of the suspension detected by the particle size analyzer is also determined (this optical density is of the order of 20). This average is an indicator of the proportion of fines, this is the content of particles smaller than O.lμm, which are not detected by the particle analyzer. This average, called the ultrasonic disintegration factor (FD), is proportionally greater than the higher capacity of silica disintegration. One of the objects of the invention is a process for the preparation of precipitated silica of the type included in the reaction of a silica with an acidifying agent, by means of which the suspension of precipitated silica is obtained, followed by a separation and the drying of this suspension, in which the precipitation is carried out in the following manner: (i) the initial base raw material comprising a silicate is formed, the silicate concentration (expressed as SiO2) in the initial base raw material is less than 100 g / 1 and the electrolytic concentration in the initial base raw material is lower than 17 g / 1, (ii) the acidifying agent is added to the base raw material until the pH value of the reaction mixture is obtained. , at least, about 7. (iii) the acidifying agent and the silicate are simultaneously added to the reaction mixture, and in which a suspension having solid contents of no more than 24% by weight is dried, characteristic because the process includes one of the following two operations (a) or (b): (a) at least one zinc compound and then a basic agent are added to the reaction mixture after The phase (iii), and when the separation comprises a filtration and the cake disintegration that originates from the filtration, the disintegration is preferably carried out in the presence of at least one aluminum compound. (b) a silicate and at least one aluminum compound A are added simultaneously to the reaction mixture after step (iii) and, when the separation comprises a filtration and a cake disintegration, which originates from filtration, disintegration is preferably carried out in the presence of at least one aluminum B compound. Therefore, it has been found that the introduction of aluminum - this being in accordance with a particular method - combined with a low silicate concentration (expressed as Si02) and electrolytes in the raw material base and in the appropriate solids content of the suspension that is dried constitutes an important condition for imparting good properties to the product that is obtained, especially the very satisfactory reinforcing properties (in particular in relation to the rheology of the elastomers) and preferably a remarkable dispersibility. It should be noted, in general, that the process concerned is a process for the synthesis of precipitated silica, which means an acidifying agent with a silicate under very special conditions. The selection of the acidifying agent and the silicate is made in the manner that is well known per se. It can be remembered that the acidifying agent which is generally employed is a strong inorganic acid such as sulfuric acid, nitric acid or hydrochloric acid, or an organic acid such as acetic acid, formic acid or carbonic acid. The acidifying agent may be diluted or concentrated; its normality can be between 0.4 and 36 N, for example between 0.6 and 1.5 N. In particular, in the case where the acidifying agent is sulfuric acid, its concentration can be between 40 and 180 g / 1, for example between 60 and 130 g / 1. In addition, it is possible to employ as silicate any common silicate form such as metasilicates, disilicates and advantageously alkyl metal silicate, especially sodium or potassium silicate. The silicate can have a concentration, expressed as silica, of between 40 and 330 g / 1, for example between 60 and 300 g / 1, in particular between 60 and 250 g / 1. In general, sulfuric acid is used as the acidifying agent, and sodium silicate is used as the silicate. In the case where the sodium silicate is used, the latter generally has an average weight of Si02 / Na20 of between 2 and 4, for example between 3.0 and 3.7. As regards, more particularly to the preparation process of the invention, the precipitation is done in a specific manner according to the following steps. First of all, the base raw material is formed, which includes some silicate and an electrolyte (stage (i)). The amount of silicate present in the initial base raw material advantageously represents only a part of the total amount of silicate that is introduced into the reaction. The term "electrolyte" is understood herein in its accepted normal meaning, which means that it denotes any ionic or molecular substance which, when in solution, decomposes or disassociates to form ions or charged particles. An electrolyte which can be mentioned as a salt of the alkyl group and the earthy alkali metal salts, especially the metal salt of the initial silicate and the acidifying agent, for example sodium sulphate in the case of the reaction of a silicate of sodium with sulfuric acid.

According to a characteristic of the process of the preparation of the present invention the electrolyte concentration in the initial base raw material is (greater than Og / l and) less than 17 g / 1, preferably lower than 14 g / 1. According to another feature of the preparation process of the invention, the silicate concentration in the initial base raw material is (greater than 0 g / 1 y) less than 100 g of SiO 2 per liter. This concentration is preferably less than 90 g / 1, especially less than 85 g / 1. In some cases it may be less than 80 g / 1. The second step consists in adding the acidifying agent to the base raw material of the composition described above in step (ii)). This addition, which causes the corresponding decrease in the pH of the reaction mixture, is carried out until the pH value is reached is at least about 7, generally between 7 and 8. Once the At the desired pH value, a simultaneous addition of acidifying agent is carried out (step (iii)). This simultaneous addition is preferably carried out so that the pH vapor remains the same (within +/- 0.1) that is achieved at the end of step (ii). According to the essential characteristics of the preparation process according to the invention, the latter includes one of two operations, (a) or (b) that were mentioned above, that is: (a) at least one compound of zinc and then a basic agent, after step (iii) to the reaction mixture and, when using the separation in the process comprises the filtration and the disintegration of the cake that originates from the filtration, the disintegration it is preferable that it be carried out in the presence of at least one aluminum compound, or (b) a silicate and at least one zinc compound are added, simultaneously, after step (iii), to the mixture of The reaction and, when the separation used in the process comprises a filtration and a cake disintegration that originates from the filtration, the disintegration is preferably carried out in the presence of at least one aluminum compound. In the first alternative form of the preparation process according to the invention (which means when it includes operation (a)), the following successive steps are carried out, advantageously, after the operation is carried out. precipitation according to steps (i), (ii) and (iii) described above: (iv) at least one zinc compound is added to the reaction mixture (i.e. to the suspension of the reaction or pulp) obtained), (v) a basic agent is added to the reaction mixture, preferably, until the pH value of the reaction mixture is obtained between 7.4 and 10, in particular between 7.8 and 9, ( vi) the acidifying agent is added to the reaction mixture, preferably until the pH value of the reaction mixture of 7 is obtained, in particular between 7 and up to 8.5, in particular between 7 and 8. After the simultaneous addition of step (iii), it may be advantageous to carry out the maturation of the reaction mixture, it is possible that this maturation lasts, for example, from 1 to 60 minutes, in particular from 3 to 30 minutes. In this first alternative form it is desired, between step (iii) and step (iv), and especially before optional maturation, to add an additional amount of acidifying agent to the reaction mixture. This addition is generally carried out until the pH value of the reaction mixture is obtained between 3 to 6.5, in particular between 4 and 6. The acidification agent which is used during the addition is generally identical to that used during the addition. steps (ii), (iii) and (iv) of the first alternative form of the preparation process according to the invention. A maturation of the reaction mixture is usually carried out between step (v) and (vi), for example, for 2 to 60 minutes, in particular for 5 to 45 minutes. Similarly, the maturation of the reaction mixture, in most cases, takes place after step (vi), for example for 2 to 60 minutes, in particular for 5 to 30 minutes. The basic agent employed during step (iv) may be a solution of aqueous ammonium or, preferably, a solution of sodium hydroxide (or soda). In a second alternative form of the preparation process according to the invention (ie when the latter includes operation (b)), step (iv) is carried out after steps (i), (ii) and ( iii) that were described above, which consist of adding a silicate and at least one zinc compound simultaneously to the reaction mixture. After the simultaneous addition of step (iv), it may be advantageous to carry out the maturation of the reaction mixture, it being possible for this to last for a duration of 2 to 60 minutes, in particular for 5 to 60 minutes. up to 30 minutes. In this second alternative form, after step (iv), and especially after optional ripening, an additional amount of acidifying agent is added to the reaction mixture. This addition is generally made until a pH value of the reaction mixture of at least 7 is obtained, in particular between 7 and 8.5, for example between 7 and 8. The acidification agent employed during this addition is generally identical which is employed during steps (ii) and (iii) of the second alternative form of the process of the preparation according to the invention. The maturation of the reaction mixture is usually carried out after the addition of the acidification agent, for example for 1 to 60 minutes, in particular for 3 to 30 minutes.

The zinc compound used in the process of the preparation according to the invention is generally an orchronic and inorganic zinc salt. By means of the examples of an organic salt, mention will be made herein, in particular, of the salts of carboxylic acid or of polycarboxylic acids, such as the salts of acetic, citric, tartaric or oxalic acid. Examples of an inorganic salt include, especially, halides and oxyhalides (such as chlorides and oxychlorides), nitrates, phosphates, sulfates and oxysulfates. In practice, the zinc compound can be used in the form of a solution, generally aqueous. In zinc sulfate it is preferably used as a zinc compound. The temperature of the reaction mixture is generally between 70 and 98 degrees centigrade. According to the alternative form of the invention, the reaction is carried out at a constant temperature between 75 to 96 degrees centigrade. According to another (preferred) alternative form of the invention the temperature at the end of the reaction is greater than the temperature at the beginning of the reaction, the temperature at the beginning of the reaction is preferably maintained between 70 and 96 degrees centigrade and then the temperature increases for a few minutes, preferably to a value between 80 and 98 degrees Celsius, whose value is maintained until the end of the reaction; operations (a) or (b) are thus, usually carried out at a constant temperature value. At the end of the steps just described, a silica paste is obtained which is then separated (liquid-solid separation). In the first alternative form of the process for the preparation according to the invention (i.e. when the latter includes operation (a)), generally this separation comprises a filtration (followed by a washing if necessary) and a disintegration, the disintegration it is preferably carried out in the presence of at least one aluminum compound and, preferably, in the presence of an acidification agent as described above (in the latter case the aluminum compound and the acidifying agent are advantageously added in a simultaneous). The disintegration operation, which can be carried out, for example, by passing the filter cake through a colide or bed type mesh, makes it possible, in particular, to decrease the viscosity of the suspension to subsequently dry. In the second alternative form of the preparation process according to the invention (ie when the latter includes operation (b)), the separation also comprises, in general, a filtration (followed by a wash if needed) and a disintegration , the disintegration is preferably carried out in the presence of at least one aluminum compound and, in general, in the presence of an acidification agent as described above (in the latter case the aluminum compound and the acidifying agent are advantageously added simultaneously).

The aluminum compound generally consists of an alkali metal, especially potassium, or, preferably, sodium aluminate. The amount of the zinc compound used in the preparation process according to the invention is preferably such that the precipitated silica preparation contains between 1 and 5%, in particular between 1.5% and 4%, for example between 1.5 and 2.5% by weight of zinc.

The separation used in the process of the preparation according to the invention usually includes a filtration which is carried out by means of a desirable method, for example by means of a belt filter, a rotary vacuum filter or, preferably, a filter press. The suspension of the precipitated precipitated silica (filter cake) is then dried. According to one of the characteristics of the process of the preparation according to the invention, this suspension must have, immediately before drying, a solid content of not more than 24% by weight, preferably not more than 22% by weight. This drying can be done according to any method known per se. Drying is done, preferably by spraying. Any type of drying suitable for this purpose can be used, especially a turbine, a nozzle, liquid pressure or spray of two fluids. According to one embodiment of the invention, the suspension that is dried has a solids content greater than % by weight, preferably greater than 17% by weight and, for example, greater than 20% by weight. Drying is preferably carried out by means of a nozzle sprayer. The precipitated silica obtainable according to the embodiment of the invention and preferably when using a filter press is advantageously in the form of practically spherical beds, preferably of an average size of at least 80 μm. It should be noted that the dried material, for example silica in the powder form can also be added to the filter cake after filtration, in a subsequent stage of the process. At the end of drying, a grinding step can be carried out on the recovered product, especially on the product obtained by drying the suspension having a solids content higher than 15% by weight. The silica precipitate obtained in this way is generally in the form of a powder, preferably with an average size of at least 15 μm, in particular between 15 and 60 μm, for example between 20 and 45 μm. The milled products with the desired particle size can be separated from any non-conforming product by means, for example, of a vibration screen having appropriate maya sizes, and conforming products that are recovered in this way can be returned to Grind. Similarly, according to another embodiment of the invention, the drying suspension has a solids content of at least 15% by weight. Drying is then usually carried out by means of a turbine sprinkler. The precipitated silica obtained in this way, according to this embodiment of the invention and preferably when using the vacuum filter by rotation is generally in powder form, preferably with an average size of at least 15 μm, in particular between 30 and 150 μm, for example between 45 and 120μm Finally, the product that has dried (especially of a suspension having a solids content of at least 15% by weight) or ground can, according to another embodiment of the invention, be subjected to an agglomeration step. By agglomeration is meant any process that allows finely dividing the objects to be joined in order to put them in the form of large objects that are mechanically stronger. These methods are, especially direct compression, wet path granulation (ie the use of bonds such as water, silica paste, etc.), extrusion and, preferably, dry compaction. When this last technique is used, it can be found advantageous, before starting the compaction, the air is removed from the pulverulent products (an operation that is also known as predensification or degassing), in order to remove the air included in them and to ensure greater uniformity in the compaction. The precipitated silica obtainable according to the embodiment of the invention, advantageously, is in the form of grains, preferably in a size of at least 1 mm, in particular between 1 and 10 mm. At the end of the stage of agglomeration products can be classified into a desired size, for example, by means of the screen, and then packaged for future use. The powders, as well as the beds, of precipitated silica is obtained by means of the process according to the invention, furthermore it offers the advantage, among others, of providing access to granules such as those mentioned above, in a simple, efficient and economic, especially by operations in a conventional manner, such as, for example, granulation or compaction, without the latter resulting in degradation capable of covering, or even inhibiting, the good intrinsic properties associated with these powders or beds, as may be the case in the prior art. when conventional powders are used. Other objects of the invention consist of new precipitated silicas which, advantageously, have a very good dispersibility (and disintegration capacity) and very satisfactory reinforcing properties, in particular that, when used as a reinforcing filler for the elastomers, They impart excellent rheological properties to the latter while providing them with good mechanical properties. Therefore, the precipitated silica of which he now proposes, according to the invention, is characterized in that it has: a CTAB surface area of between 90 and 250 m2 / g, preferably between 120 and 230 m2 / g, a specific BET surface between 90 and 250 m2 / g, preferably between 120 and 240 m2 / g, an oil uptake of less than 300 ml / lOOg, preferably between 200 and 295 ml / lOOg, a zinc content of between 1 and 5% by weight , preferably between 1.5 and 4% by weight, and in that the number N of the stearic acid molecules that are consumed per nm2 of the silica surface, when the stearic acid is reacted with the silica in silene for 2 hours, a temperature of 120 degrees centigrade is at least 1, preferably at least 1.2, in particular at least 1.5. The silica according to the invention preferably has a zinc content of between 1.5 and 4% by weight; this content can be especially between 1.5 and 2.5% by weight. One of the essential characteristics according to the invention is its consumption, in a medium model (silene), of a rubber vulcanization ingredient (stearic acid). The Applicant Company has found that the precipitated silicas have a particular number N, in combination with other characteristics mentioned in the present description, which makes it possible especially to grant excellent rheological properties to the elastomers while providing excellent mechanical properties. To determine these characteristics (number N (, the stearic acid is reacted in the presence of silica in silene, for 2 hours at a temperature of 120 degrees centigrade.) The amount of stearic acid in the silene after the reaction is determined by the spectrometry of infrared, the amount of stearic acid that silica consumes can then be deduced and therefore the number N of stearic acid molecules that is consumed per nm2 of silica surface.The operating method used to determine these characteristics is described more Then 60.2 g (which is 70 ml) of silene are added to a round-bottomed flask containing 3.17 g of stearic acid.The flask is stopped and then stirred magnetically for a few minutes. The flask is then placed in an oil bath at a temperature of 120 degrees Celsius under reflux (a condenser is fitted to it). The flask is stirred magnetically for 105 minutes. Agitation is stopped and the flask is left in an oil bath for another 15 minutes. The total duration of the reaction, at a temperature of 120 degrees centigrade, is 2 hours. The condenser is removed and the flask is removed from the oil bath. The contents of the flask are filtered in a microfiltration system (Milipore unit with Durapore membrane filters made of polyvinyldene fluoride (pore size: 0.45μm)). 10 g of filtrate are obtained which is then diluted with 10 g of silene; a solution S is obtained. In parallel, the normal solution of stearic acid in silene was prepared (having a stearic acid content less than 2 mass%) and the IR spectrum (from 400 to 4000 cm ":) of each of The characteristic peak of stearic acid stands at 1710cm ~: The intensity of this peak associated with the stearic acid content of the solution makes it possible to draw a straight line of the stearic acid content of the solution as an absorption function GO to 1710 cm "1; The equation of the calibration of the straight line is obtained by a linear regression. Similarly, the IR spectrum of the S solution is produced. The characteristic peak value of the stearic acid referred to in the equation of the straight calibration line makes it possible to determine the content of stearic acid present in the solution S; taking into account the mass of silene that is added during the dilution, the stearic acid content of the reaction filtration is obtained. The content, and therefore the amount of stearic acid that the silica consumes during the reaction is deducted from the initial stearic acid content and from the stearic acid content after the reaction (the latter is the stearic acid content of the filtration ). The number N of the stearic acid molecules that is consumed per nm2 of the silica surface is then determined. The zinc present in the silica precipitate according to the invention is preferred that it is not in crystalline form but, better in an amorphous form (this can be determined by X-ray diffraction). According to an alternative (preferred) form of the invention, the silica precipitate has: a CTAB surface area of between 90 and 185 m2 / g, in particular between 120 and 185 m2 / g, for example between 140 and 180 m2 / g, the average diameter (050) after the disintegration with ultrasound, is smaller than 4μm, preferably smaller than 3μm, the ultrasonic disintegration factor (FD) is greater than 6ml, preferably greater than 10ml. The mean diameter (050), after the ultrasonic disintegration, of the silica precipitate according to this alternative form of the invention can be smaller than 2.8 μm, for example smaller than 2.5 μm. The ultrasonic disintegration factor (FD) of the precipitated silica according to the alternative form of the invention may be greater than 11 ml, for example greater than 14 ml. The BET surface of the precipitated silica according to the alternative form of the invention is generally between 90 and 195 m2 / g, in particular between 120 and 195 m2 / g, for example between 150 and 190 m2 / g. According to another alternative form of the invention the precipitated silica has: a CTAB surface area greater than 185 m2 / g and less than 220m2 / g, the average diameter (050) after the ultrasound disintegration is smaller than 7μm, preferably smaller than 5.5μm. The average diameter (050) after the ultrasonic disintegration of the precipitated silica according to this alternative form of the invention may be smaller than 4μm. The ultrasonic disintegration factor (FD) of the precipitated silica according to the alternative form of the invention can be greater than 6ml. The specific surface of precipitated silica according to the alternative form of the invention is generally between 185 and 230 m2 / g. According to the particular embodiment of the invention, one of the characteristics of silica also lies in the distribution, or distribution, of the volume of the pores and especially in the distribution of the pore volume and especially in the distribution of the pore volume that is produced by pores with a diameter less than or equal to 400 Á. This last volume corresponds to the volume of the useful pore of the fillings that are used in the reinforcement of the elastomers. The analysis of the pores shows that the silica according to a particular method of the invention, then have a pore distribution such as the pore volume, which consists of the pores whose diameter is between 175 and 275 A, represents at least 50. %, for example at least 60%, of the pore volume consisting of pores with a diameter less than or equal to 400 Á.

According to a very preferred alternative embodiment of the invention, the silica has a specific surface area average of BET / CTAB between 1.0 and 1.2, that is, it is preferred that it has a very low microporosity. The pH of the silica according to the invention is, generally between 8.0 and 9.0, for example between 8.3 and 8.9. The silica according to the invention can be in the form of powders, of practically spherical beds or, optionally, in the form of granules, and is characterized in particular by the fact that, while they are relatively large in size, they have an excellent dispersibility and disintegration capacity and very satisfactory reinforcing properties. Therefore, they have a desipersability and disintegration capacity which are advantageously superior to that of the silicas of the prior art, which are identical or very closely related in the specific surface and in an identical or very closely related in size. The silica powders according to the invention preferably have a size of at least 15 μm; the latter is, for example, between 15 and 60 μm (especially between 20 and 45 μm) or between 30 and 150 μm (especially between 45 and 120 μm). Preferably, they have a DOP oil uptake of between 240 and 290 ml / lOOg. The packing density (PD) of the powders is generally at least 0.17 and, for example, between 0.2 and 0.3. The powders generally have a total pore volume of at least 2.5 cm3 / g and, more particularly, between 3 and 5 cm3 / g. This makes it possible, in particular, to obtain a very good compromise between the process and the mechanical properties in the vulcanizing step. They also constitute the preferred precursors for the synthesis of granulates as will be described below. The practically spherical beds according to the invention preferably have an average size of at least 80μm. According to certain alternative forms of the invention, this bed size is at least 100μm, for example at least 150μm, is generally at least 300μm and preferably lies between 100 and 270μm. This average size is determined according to the norm NF X 11507 (December 1970) by the drying screen and the determination of the diameter corresponding to a cumulative size over 50% Preferably, it has a DOP oil uptake of between 240 and 290 ml / lOOg. The packing density (PD) of the beds (or beads) is generally at least 0.17 and, for example, between 0.2 and 0.34%. Usually, they have a total pore volume of at least 2.5 cm3 / g and, more particularly, between 3 and 5 cm3 / g. As indicated above, the silica in the form of substantially spherical beds which are advantageously solid, homogeneous and low in powder and has a good purity capacity, has an excellent disintegrability and disperse ability. Additionally, it has good reinforcing properties. Silica also constitutes a preferred precursor for the synthesis of powders and granules according to the invention. Silica in the form of substantially spherical beds constitutes a highly advantageous alternative form of the invention. The dimensions of the granules according to the invention are preferably at least 1 mm, in particular between 1 and 10 mm, together with the axis of their longest dimension (length). Preferably, they have an oil uptake of between 200 and 260 ml / 100 g. The granules can be of different forms. The forms that can be especially mentioned by means of the examples are the spherical, cylindrical, parallelepipedal, tablet, flake, tablet and extruded circular or polilobar section. The packing density (PD) of the granules is generally at least 0.27 and can vary up to 0.37. Generally, they have a total pore volume of at least 1 cm3 / g and, more particularly, between 1.5 and 2 cm3 / g. The silicas according to the invention, especially in the form of powder or of practically spherical or granulated beds are preferably prepared according to one of the appropriate alternative forms of the preparation process according to the invention and the above description. The silicas according to the invention or prepared by the process according to the invention find a particularly advantageous application in the reinforcement of natural or synthetic elastomers. They impart excellent rheological properties to elastomers while providing good mechanical properties and, in general, good resistance to abrasion. Additionally these elastomers are preferably less exposed to heat. Therefore, the invention also relates to the use of these silicas to improve the rheological properties (which are illustrated, for example, by the Mooney consistency and the minimum torque) of the elastomers. The following examples illustrate the invention, however, do not limit its scope. EXAMPLE 1 (comparative) It is p >repairs a silica (reference Al) according to Example 12 of European Patent Application EP-A-0520862 (registered under number 92401677.7). The characteristics of the silica that is obtained in the form of practically spherical beds are the following: CTAB surface area 160m2 / g BET specific surface 170m2 / g DOP oil capture 276ml / 100g - zinc weight content < 0.005% pore volume VI represented by the pore of d < 400Á 0.90cm3 / g pore volume V2 represented by the pore of 175Á < d < 275Á 0.55cm3 / g average V2 / V1 61% pH 6.5 - average particle size 260μm The number N of stearic acid molecules consumed per nm2 of silica surface, when reacted with silica Al in sileno for 2 hours , at a temperature of 120 degrees Celsius (according to the procedure of the operation in the description) is equal to 0.5. The silica Al is subjected to a disintegration test as defined above in the description. After disintegration with ultrasound it has an average diameter of (050) of 4.3 μm and an ultrasonic disintegration factor (FD) of 6.5 ml. EXAMPLE 2 The following is introduced in a stainless steel reactor provided with a stirring system that uses a propeller with heat using a cover: 624 liters of water 11.2 Kg of Na2SO4 310 liters of aqueous sodium silicate having an average weight Si02 / NaO equal to 3.45 and a relative density, at a temperature of 20 degrees Celsius of 1,230. The concentration of silica, expressed as SiO2, in the initial base raw material is 79 g / 1. The mixture is then heated to a temperature of 80 degrees centigrade while being kept in agitation. Dilute sulfuric acid with a relative density, at 20 degrees centigrade, of 1.050, at an average flow of 7.0 1 / min, is introduced until the pH value of 8.0 (measured at its temperature) is obtained in the reaction mixture. . The temperature of the reaction is 80 degrees centigrade for the first 30 minutes; then the temperature rises from 80 to 94 degrees centigrade in about 15 minutes and then it is held at 94 degrees centigrade until the completion of the reaction. The aqueous sodium silicate of the type described above, a flow average of 2.4 1 / ml, and the sulfuric acid, also of the type described above, at an average flow rate which is controlled to maintain the pH, during the introduction of the period, at a constant value equal to 8.0 ± 0.1 are then introduced together into the reaction mixture for 30 minutes. After this simultaneous addition an aqueous solution containing 85 g / 1 of zinc sulfate is added thereto for 12 minutes at an average flow of 9.3 1 / min. At the end of this addition, an aqueous solution containing 180 g / 1 of sodium hydroxide is introduced into the reaction mixture until a pH is obtained in the same reaction mixture 8.0. The introduction of sodium hydroxide is then stopped and the reaction mixture is kept stirring for 10 minutes. Sulfuric acid of the type described above is then introduced until the pH of the reaction mixture equals 7.1. The introduction of acid is stopped and the maturing of the reaction mixture is carried out for 5 minutes at a temperature of 94 degrees centigrade. The total period of the reaction is 128 minutes. A paste or suspension of the silica precipitate is obtained, which is subsequently filtered and washed by means of a filter press. Then the paste obtained is fluidized by mechanical and chemical action (simultaneous addition of sulfuric acid and a quantity of sodium aluminate corresponding to an average weight of A1 / S02 of 0.20%). After the disintegration operation the resulting paste, with a pH equal to 6.6 and a loss in ignition equal to 78.0% (and therefore a solid content of 22.0% by weight), is sprayed by means of a nozzle spray . The characteristics of the silica Pl obtained in the form of practically spherical beds (according to the invention) are the following: CTAB surface area 151m2 / g surface area BET 158 m2 / g oil uptake DOP 262ml / lOOg content zinc weight 1.80 % - pore volume VI, represented by the pores of d <; 400 Á 0.92 cm3 / g pore volume V2 represented by pores 175 Á < d < 275 Á0.47 cm3 / g average V2 / V1 51% - pH 8.5 average particle size 260 μm average particle size 260 μm The number N of stearic acid molecules that is consumed per nm2 of silica surface, when reacted the stearic acid with the silica Pl in sileno for 2 hours at a temperature of 120 degrees Celsius (according to the procedure of the operation mentioned in the description), is equal to 1.2. The silica Pl that is subjected to the disintegration test as defined above in the description. After disintegration with ultrasound, an average diameter (050) of 2.2 μm and an ultrasound disintegration factor (FD) of 14.1 ml are obtained. EXAMPLE 3 This example illustrates the use and behavior of the silica according to the invention and of the silicas that are not according to the invention in the industrial rubber formulation. The following formulation is used (the parts are expressed by weight): - rubber SBR (1) 50 rubber KBR01 (2) 25 natural rubber SMR5L 25 ZnO (3) active 1.82 stearic acid 0.35 6DPD (4) 1.45 CBS (5 ) 1.1 - DPG (6) 1.4 Sulfide (7) 0.9 Silane X50S (8) 8.13 (1) solution of butadiene styrene copolymer Buna type VSL 1955 S25 (2) Polybutadiene (3) zinc oxide grade rubber (4) N- ( 1,3-dimethylbutyl). N'-phenyl-p-phenylenediamine (5) N-cyclohexyl-2-benzothiazyl sulfenamide (6) Diphenylguanidine (7) vulcanization agent (8) silica / rubber coupling agent (product sold by Degussa) The formulations are prepared as follows: The following is introduced in an internal mixer (of the Banbury type), in this order and in the mixing time and temperature shown below in parentheses: SBR, KBR01 and natural rubber (to ) (60 degrees SBR, KBR01 and natural rubber (to) (60 degrees Celsius) X50S and 2/3 silica (to + 1 min) (80 degrees Celsius) s) - ZnO, stearic acid, 6PPD and 1/3 silica (to + 2 min) (100 degrees Celsius) The mixer discharge (mix drops) is carried out when the chamber temperature reaches 165 degrees centigrade (ie at approximately to + 5 min 15 s). The mixture is introduced in a roller grinder, the rollers is maintained at a temperature of 30 degrees Celsius to be calendered. The CBS, the DPG and the sulfur are introduced in the grinder. After the homogenization and passing through three fine meshes, the final mixture is calendered in the form of sheets of 2.5 to 3 mm thick. The results of the tests are as follows: 1- Rheological properties The measurements are carried out in the raw state formulations. The results are reported in Table I below. The device used to drive the measurements is shown TABLE I (1) Mooney Viscometer MV 2000E (Mooney wide (1 + 4) measures at 100 degrees Celsius) (2) Remitter Monsanto 100 S The formulations obtained from the silicas according to the invention result in lower values. These express the high processing capacity of the mixtures prepared from the silicas according to the invention, in particular in relation to the extrusion and calendering operations that are carried out, frequently during the manufacture of the elastomer compositions (energy expenditure smaller to process the mixture, greater injection release during the composition, less swelling during extrusion, less reduction to calendering, etc.) 2 .. Mechanical properties The measurements of the formulations carried out in the vulcanization formulations. The vulcanization is carried out by heating the formulations at a temperature of 150 degrees centigrade for 40 minutes. The following standards are used: (i) stress test (tensile strength, elongation at break): NFT 46-002 or ISO 37-1977 (ii) test of abrasion resistance DIN 53-516 The results obtained are listed In the table II below. TABLE II (1) The measured value is the loss of abrasion: the lower it is, the better the abrasion resistance. This last result demonstrates the good reinforcing effect of the silicas according to the invention. Therefore, while they result in clearly better rheological properties, the silicas according to the present invention provide the mechanical properties that are at least appreciably equivalent or even better than those obtained with the silicas of the prior art. On the other hand, the high reinforcing power of the silicas according to the invention is confirmed by the high values obtained for the tension force and the elongation at the break. Moreover, it can be seen that the silica according to the invention has a satisfactory behavior with respect to the abrasion resistance. 3. Dynamic properties The measurements are carried out in the vulcanization formulations. The vulcanization is obtained by heating the formulations to a temperature of 150 degrees centigrade for 40 minutes. The results (illustrate the tendency to overheat) are reported in Table III below (the less the value, the lower the tendency to overheat). The device used to conduct the measurements is shown. TABLE III (1) Instron% viscoelasticimeter The tendency for overheating is obtained from the silicas according to the invention is actually lower.

Claims (36)

1. CLAIMS 1.- The process for the preparation of precipitated silica of the type that includes the reaction of a silicate with an acidification agent, whereby a suspension of precipitated silica is obtained, then the separation and drying of this suspension, in the which precipitation is carried out in the following manner: (i) the initial base raw material comprising a silicate is formed, the silicate concentration (expressed as SiO2) in the initial base raw material is less than 100 g / 1 and the electrolytic concentration in the initial base raw material is less than 17 g / 1, (ii) the acidifying agent is added to the base raw material until the pH value of the reaction mixture of at least about 7. (iii) the acidifying agent and the silicate are simultaneously added to the reaction mixture, and in which a suspension having solid contents of no more than 24% by weight is dried, characteristic because the process includes one of the following two operations (a) or (b): (a) at least one zinc compound is added and a basic agent is added to the reaction mixture after step (iii), when separation comprises filtration and cake disintegration originated from filtration, disintegration is carried out in the presence of at least one aluminum compound B, (b) a silicate is added and, less, a zinc compound simultaneously to the reaction mixture after step (iii) and, when the separation comprises a filtration and cake disintegration, which originates from the filtration, the disintegration is preferably carried out in presence of at least one aluminum compound.
2. 2. The process according to claim 1, which includes the reaction of a silicate with an acidification agent, whereby a suspension of precipitated silica is obtained, then the separation and drying of these suspensions, in which: the precipitation is carried out in the following manner: (i) the initial base material comprising a silicate is formed, the silicate concentration (expressed as SiO2) in the initial base raw material is less than 100 g / 1 and the electrolytic concentration in the initial base raw material is less than 17 g / 1, (ii) the acidifying agent is added to the base raw material until the pH value of the reaction mixture of at least about 7 g / l is obtained. (iii) the acidifying agent and the silicate are simultaneously added to the reaction mixture, then the following successive steps are carried out: (iv) at least one zinc compound is added to the reaction mixture (which means the suspension of the reaction or paste obtained), (v) a basic agent is added to the reaction mixture, preferably, until the pH value of the reaction mixture is obtained between 7.4 and 10, in particular between 7.8 and 9, (vi) the acidifying agent is added to the reaction mixture, preferably until the pH value of the reaction mixture of at least 7 is obtained, in particular between 7 and 8.5, the separation comprises a filtration and the cake disintegration that originates from the filtration, the disintegration is carried out in the presence of at least one aluminum compound, a suspension is dried having a solid content of at least 24% by weight.
3. 3. The process according to claim 2, characterized in that, between step (iii) and step (iv), acidification agent is added to the reaction mixture, preferably until a value of pH of the reaction mixture between 3 and 6.5.
4. 4. The process according to claim 1, which includes the reaction of a silicate with an acidification agent, whereby a suspension of silica is obtained, then the separation and drying of this suspension, in which it is carried The precipitation is carried out in the following manner: (i) the initial base raw material comprising a silicate and an electrolyte is formed, the silicate concentration (expressed in SiO2) in the initial base raw material is less than 100 g / 1 and the electrolytic concentration in the initial base raw material is less than 17 g / 1, (ii) the acidifying agent is added to the base raw material until the pH value of the reaction mixture is obtained from at least about 7. (iii) the acidifying agent and the silicate are simultaneously added to the reaction mixture, (iv) they are added a silicate and at least one zinc compound simultaneously to the reaction mixture, and in which the suspension having a solid content of at least 24% is dried.
5. 5. The process according to claim 4, characterized in that, after step (iv), the acidification agent is added to the reaction mixture.
6. 6. The process according to any of claims 4 and 5, characterized in that the separation comprises a filtration and disintegration of the cake that originates from the filtration, the disintegration is carried out in the presence of at least one compound of aluminum.
7. The process according to one of claims 1 to 6, characterized in that the amounts of zinc compounds that are used in such a way that the prepared precipitated silica contain between 1 and 5% by weight of zinc .
8. The process according to one of claims 1 to 7, characterized in that the zinc compound is an inorganic or organic aluminum salt, the organic salt is preferably selected from the salts of carboxylic or polycarboxylic acids and the inorganic salt is preferably selected from halides, oxyhalides, nitrates, phosphates, sulfates and oxysulfates.
9. 9. The process according to one of claims 1 to 8, characterized in that the zinc compound is a zinc sulfate.
10. 10. The process according to one of claims 1 to 9, characterized in that the aluminum compound is an alkali metal aluminate.
11. 11. The process according to one of claims 1 to 10, characterized in that the aluminum compound is a sodium aluminate.
12. 12. The process according to one of claims 1 to 11, characterized in that the preparation includes a filtration carried out by means of a filter press.
13. 13. The process according to one of claims 1 to 12, characterized in that the drying is carried out by spraying.
14. 14. The process according to claim 13, characterized in that the suspension having a solid content greater than 15% by weight, preferably greater than 17% by weight, is dried.
15. 15. The process according to one of claims 12 to 14, characterized in that the drying is carried out by means of a multiple nozzle dryer.
16. 16. The process according to one of claims 12 to 15, characterized in that the dried product is subsequently milled.
17. 17. The process according to claim 16, characterized in that the ground product is subsequently agglomerated.
18. 18. The process according to claim 13, characterized in that the suspension having a solids content of more than 15% by weight is dried.
19. 19. The process according to claim 18, characterized in that the dry product subsequently agglomerates.
20. 20. The precipitated silica capable of being obtained by the process according to one of claims 1 to 19.
21. 21. The precipitated silica which is characterized in that it has: a CTAB surface area of between 90 and 250 m2 / g, a specific BET surface area between 90 and 250 m2 / g, a DOP oil uptake of less than 300 ml / lOOg, a zinc content of between 1 and 5% by weight, and in which the number N of stearic acid molecules is consumed per nm2 of silica surface, when the stearic acid has to be reacted with the silica in silene for 2 hours at a temperature of 120 degrees centigrade, is at least 1.
22. 22. The silica according to claim 21, which It is characterized because zinc does not have a crystalline form.
23. 23. The silica according to any of claims 21 and 22, characterized in that it is at least 1.2, preferably at least 1.5.
24. The silica according to one of claims 21 to 23, characterized in that it has a zinc content between 1.5 and 4% by weight.
25. 25. The silica according to one of claims 21 to 24, characterized in that it has: a CTAB surface area of between 90 and 185 m2 / g, the average diameter (050) after the ultrasound disintegration, is smaller than 4μm, preferably smaller than 3μm, the ultrasonic disintegration factor (FD) is greater than ßi-il, preferably greater than 10ml.
26. The silica according to one of claims 21 to 24, characterized in that it has: a CTAB surface area greater than 90 and 185 m2 / g, the average diameter (050) after the ultrasound disintegration, it is smaller than 7μm, preferably smaller than 5.5μm.
27. 27. The silica according to one of claims 21 to 26, characterized in that it has a DOP oil uptake of between 200 and 295 ml / lOOg.
28. The silica according to one of claims 21 to 27, characterized in that it has a pore distribution, such that the volume of the pores consists of pores whose diameter is between 175 and 275 A, represents at least 50% of the pore volume consisting of pores with a diameter less than or equal to 400 Á.
29. 29. The silica according to one of claims 21 to 28, characterized in that it is in the form of substantially spherical beds with an average size of at least 80 μm.
30. 30. The silica according to claim 29, characterized in that the average size is at least 100 μm, in particular at least 150 μm.
31. 31. The silica according to one of claims 21 to 28, characterized in that it is in powder form with an average size of at least 15 μm.
32. 32. The silica according to one of claims 29 to 31, characterized in that it has a DOP oil uptake of between 240 and 290 ml / lOOg.
33. 33. The silica according to one of claims 21 to 28, characterized in that it is in the form of granules of at least 1 mm in size.
34. 34. The silica according to claim 33, characterized in that it has a DOP oil uptake of between 200 to 260 ml / lOOg.
35. 35. The use, as reinforcing fillers for the elastomers, of a silica obtained by the process according to one of claims 1 to 19 or of a silica according to one of the claims from 20 to 33
36. 36. The use of silica obtained by means of the process according to one of claims 1 to 19, or of a silica according to one of the claims of 20 to 1 to 34, with the objective to improve the rheological properties of elastomers.