WO1999035085A1

WO1999035085A1 - Optimally dispersible precipitated silica particulates, process for their preparation and their use

Info

Publication number: WO1999035085A1
Application number: PCT/IL1998/000634
Authority: WO
Inventors: Alexander Josef
Original assignee: Rotem Amfert Negev Ltd
Current assignee: Rotem Amfert Negev Ltd
Priority date: 1998-01-05
Filing date: 1998-12-31
Publication date: 1999-07-15
Anticipated expiration: 2000-07-05
Also published as: AU1780299A; WO1999035086A1; TW412504B; IL122851A0; AU1780199A

Abstract

An optimally dispersible precipitated silica particulate, characterized in that it possesses a Δ-value, corresponding to the peak value of the Differential Instrusion located at the Median Pore Diameter, as measured by Hg porosimeter, such that: Δ (ml/g.Å) x 103≥0.141 x CTAB (m2/g) - b, wherein the Median Pore Diameter is comprised between 100 and 400 Å, the CTAB is between 90 and 400 and the intercept b is 10 or less. Also provided are process for preparing said silica and uses thereof.

Description

OPTIMALLY DISPERSIBLE PRECIPITATED SILICA PARTICULATES. PROCESS FOR THEIR PREPARATION AND

THEIR USE

Field of the Invention

The present invention relates to a process for preparing optimal precipitated silica particulates from rocks such as porcelanite, diatomite and amorphic quartz, characteristically in the form of granules, powders or spherical beads, for use as a reinforcing filler material for elastomeric and rubber matrices, and to a novel precipitated silica.

Background of the Invention

Precipitated silica particulates have been used as filling material in mixtures for tires (as described in EP-A 0520862 and EP-A 0157703), and in rubber material (S. Wolff, v. 7, page 674, (1988), Kautschuk und Gummikunst).

Dispersible precipitated silica particulates - beads, powders and granulates - have been described as having characteristic particle sizes BET and CTAB specific surface areas (EP-A 0520862, corresponding to US 5,403,570).

In general, it is known in this specific art that if a filler is to provide optimum reinforcing properties, it must be as homogeneously distributed as possible (U.S. 5,403,570).

In the particular case where the filler is introduced initially in a granular

state, the granules have the capacity for incorporation into the matrix when

mixed with the elastomer, and for disinte ration in the form of a powder, and

the powder can in turn be dispersed homogeneously within the elastomer matrix (U.S. 5,587,416). For the reinforcement of elastomers, the art

describes particles having characteristic BET and CTAB specific surface

areas for total pore volume and pore size distribution. The specific surface

area is described by the BGT Brunauer-Emmet-Teller method (J. Amer.

Chem. Soc, Vol. 60, p. 309, 1938). The CTAB specific surface area is the

external surface area, determined in accordance with French NFT Standard

45007 (November 1987).

The prior art has addressed the question of the desired physical properties of

the precipitated silica particles and granules. However, the prior art has

failed to identify the physical parameter of the product which imparts to the

silica outstanding properties. As a result, the selection rules adopted by the

prior art are non-optimal on the one hand, and leave out silica which possesses outstanding properties, as being undesirable.

For instance, the inventors have found that silicas provided by US 5,403,570

(and its related U.S. Patent Nos. 5,547,502 and 5,587,416 - collectively

referred to as "the '570 patent", for the sake of brevity) do not teach optimal

silicas, nor do their describe an optimal selection rule for the silica in

question. Furthermore, and quite importantly, excellent silicas provided by

the present invention are not suggested not taught by the prior art. Another patent dealing with this subject is EP 647 591. This patent, in

practice, applies the very same criterion as the '570 patent. Superficially,

however, the '570 patent and EP 647 591 apply different numerical limits to

the silica in question. This, however, derives from the different measurement

method, as will be explained in detail hereinafter. Both the '570 patent and

EP 647 591 use the criterion of Equation 1:

V, (175 - 275 A)

= X% (Eq. 1) F, (< 400 )

Wherein in the '570 patent X>50, 60 or 65, in different cases.

The '570 patent uses the measurement method for the pores volume in which

the pore diameter is calculated by the Washburn equation employing an

angle of contact equal to 130° and a surface tension equal to 484 dynes/cm.

This is also the measurement method adhered to throughout the description

of the present invention. EP 647 591, on the other hand, carries out its

measurements according to the German DIN 66133 standard, in which the

contact angle is 140° (rather than 130°), and the surface tension is 480

dynes/cm (rather than 484 dynes/cm). When the results of the '570 patent are translated to the measurement method employed in EP 647 591, and the

results are compared, it is seen that the applicable range of X for EP 647 591,

in Equation 1, is 19 - 22%.

Both the '570 patent and EP 647 591 do not teach optimal silicas, however,

since they apply an unsuitable criterion and, as will be apparent to the

skilled person from the description to follow, excellent silica exists with

narrow pore size distribution, outside the ranges provided for by both the

'570 patent and by EP 647 591, as seen in Example 9 and in Example 10

hereof, respectively.

Examples of commercial silicas, and their properties, are shown in Table I below:

Table I

* The Pore Ratio is measured according to the '570 patent.

It is therefore an object of this invention to provide optimally dispersible precipitated silica particulates, which overcome the problems of the prior art. It is another purpose of this invention to provide a process for making such optimally dispersible silica particulates, which makes it possible to produce optimal silicas in a repeatable manner.

Other purposes and advantages of this invention will become apparent as the description proceeds.

Summary of the Invention

In one aspect, the invention is directed to an optimally dispersible precipitated silica particulate, which possesses a Δ-value, corresponding to the peak value of the Differential Intrusion located at the Median Pore Diameter, as measured by Hg porosimeter, such that:

(Eq.2)

A(mll g - A) x l0³ ≥ 0.141 x CTAB(m² 1 g) - b

wherein the Median Pore Diameter is comprised between 100 and 400 A, the value of CTAB is between 90 and 400 m²/g, and preferably between 90 and 210 m²/g, and the intercept b is 10 or less, and is preferably between 7.5 and 9.79, and more preferably between 8 and 9, and is most preferably 8.64. A preferred range of CTAB for many uses is between 120 and 180 m²/g. Hereinafter, for the sake of brevity, the above equation will be given omitting the units of Δ and CTAB, as follows:

Δx l0³ > 0Λ4l x CTAB - b According to a preferred embodiment of the invention, the optimally dispersible precipitated silica particulate has a Δ-value of at least 0.01 ml/g-A.

According to another preferred embodiment of the invention, the sihca particulate has a CTAB surface area comprised between 134 - 170 m²/g. The sihca particulate can be, e.g., a powder, but is preferably in granulated form.

In another aspect, the invention is directed to a method for producing a reinforced elastomer/rubber matrix, comprising the steps of:

a) providing non-cross-linked components mixture of the matrix; b) selecting a precipitated silica particulate having a Δ-value and CTAB-value such that

Δ x lO³ > 0.141 x CTAB - b wherein b is 10 or less, and is preferably between 7.5 and 9.79, and more preferably between 8 and 9, and is most preferably 8.64. c) homogeneously dispersing said precipitated silica particulate in said component mixture; and d) vulcanizing said mixture.

Thus, the invention is also directed to a reinforced elastomer/rubber matrix comprising a precipitated sihca particulate having a Δ-value and CTAB-value such that

Δx l0³ > 0.141 x C 45 -b wherein b is 10 or less, and is preferably between 7.5 and 9.79, and more preferably between 8 and 9, and is most preferably 8.64. In a further aspect the invention is directed to a process for the preparation of an optimally dispersible precipitated silica particulate having a Δ-value and CTAB-value such that

Δx lO³ > 0.141 x CTAB - b wherein b is 10 or less, and is preferably between 7.5 and 9.79, and more preferably between 8 and 9, and is most preferably 8.64, which process comprises the steps of:

a) Obtaining a sodium silicate solution from the treatment of sihca-containing minerals with NaOH in hot solution (120 - 150°C), said solution containing 15% - 17% of silica, filtering said solution to remove solids and diluting said solution with water, to obtain a sihca content of 3 - 10%;

b) Precipitating sihca by acidifying said solution with an acid combination selected from H2SO /NaHSO₄ and H2CO3/NaHCO3, wherein the H₂SO /NaHSO concentration ratio is comprised between 1 and 3, the concentration of free H2SO being in the range 4% - 10%, or the H2CO3/NaHCO3 concentration ratio is comprised between 0.64 and 1.92, the concentration of CO2 being in the range 1.5% - 5.0%, and wherein the addition time of the acidifying mixture to the silicate solution is comprised between 60 minutes and 110 minutes;

c) Carrying out a reinforcement stage by the addition of sodium silicate comprising 5% to 25% of total Siθ2 at a fixed pH, wherein the pH is maintained by adding required amounts of H2SO ; d) Lowering the pH by the addition of sulfuric acid, to pH 4-5;

e) Filtering the precipitated sihca; and

f) Mixing said precipitated silica with dry sihca to form a mixture having a moisture content not higher than 75%, preferably between 60 to 73%, granulating said mixture and subsequently drying the granules obtained in a fluidized bed.

When deahng with the process of the invention, all percentages given are by weight, unless specifically otherwise stated.

Its a also been found, and this is another object of the invention, that a most convenient silica-containing mineral to be used for the purposes of the invention is porcelanite.

It has further been found, and this is another object of the invention, that it is advantageous to employ as the raw material porcelanite containing organic material, and that such raw material leads to an improved product, in terms of lower metal contaminants and better surface properties, as compared with raw material which does not contain organic matter. Typically, but non-limitatively, contents of about 80 ppm of organic matter are sufficient to lead to such improved properties.

Brief Description of the Drawings

- Fig. 1 is a graph showing the cumulative intrusion (A) and extrusion (B), given by ml of mercury/gram of sample, of a specific sample of sihca, as a function of the diameter of the pores (Angstrom); - Fig. 2 is a graph showing the Differential Intrusion (ml/g-A), derived from the values of Fig. 1, as a function of the pore diameter (A);

- Fig. 3 is a comparison of various Pore Ratio values obtained using samples according to the present invention and the criterion of the '570 patent;

- Fig. 4 is a graph of the Δ-value as a function of CTAB;

- Fig. 5 is a comparison of values obtained with three different silicas; and

- Fig. 6 is a schematic diagram of the drying process according to a preferred embodiment of the invention.

Exp er imental

The experimental methods described herein are conventional methods employed in the industry. One exception, is the Dispersability Test, to which reference is made herein, which has been developed by the apphcant. The test described below directly correlates with the ability of sihca particles to disperse in rubber: this test reflects the ability of the sihca granules of the present invention to undergo disintegration and to provide finely divided particles. This property, which is of a significant importance for various applications, in particular, when the silica is intended for rubber reinforcing applications, is measured by the D50 parameter, indicating the mean diameter of the particles obtained from the granules.

This value, hereinafter termed "D50", is obtained as follows. The sihca is charged into an ultrasonic bath. The bath employed in the following examples was integral with a MasterSizer Micro device (ex Malvern Instruments Ltd.), for the analysis of particle size distribution. The ultrasonic transducer operated at 40 kHz and 75 W. The samples are passed through a 18 Mesh (-1 mm) screen. About 0.2 gr of the sample are then dispersed into 600 ml demineralized water at room temperature. The dispersion is stirred with a mechanical stirrer at 2070 r.p.m., and the ultrasonic bath is operated for 5 minutes. At the end of this 5 minute period, the particle size distribution and the D50 parameter are determined. The above procedure of measurment was carried out at two time points: a few hours after the granules are formed, and seven days after the granules are formed. There were found to be no further signficant changes in the D50 parameter after the latter time point.

Representative D50 values of silicas obtained according to the invention are in the range 7.0 — 18.0 μm, preferably between 7 to 14 μm and more preferably between 11 to 14 μm. However, these values can vary and the invention is by no means hmited to any specific D50 value.

Detailed Description of Preferred Embodiments

The method of the invention can be illustrated by the following specific

numerical example. Fig. 1 is a graph showing the cumulative intrusion (A)

and extrusion (B), given by ml of mercury/gram of sample of a specific sample

of sihca, as a function of the diameter of the pores (Angstrom). The

measuring conditions were as described above, viz., the contact angle was

130°, and the mercury surface tension was 484.0 dyne/cm.

The differential of the graph of Fig. 1, over the pore diameter, is the value of

interest, viz., the Differential Intrusion (ml/g-A), shown in Fig. 2 as a function of the pore diameter (A). It has been surprisingly found, and this is

an object of the present invention, that this parameter provides for silica with

the desired properties for the stated purposes, as long as its value is at least

as provided for in Equation 2. As stated, all values referred to herein with

reference to surface properties, are measured according to the method of the

'570 patent, unless otherwise specifically indicated.

The sihca having the optimal values, viz. a Differential Intrusion of at least

0.01 ml/g-A, does not correlate with the V2/V1 values of the '570 patent, or of

EP 647 591- Fig. 3 is a comparison of various values obtained using the

criterion of the '570 patent, or EP 647 591, using samples according to the present invention. In this comparison, the values described in EP 647 591

have not been normalized to correspond to the measurement method used in

the present invention and in the '570 patent, but rather have been included

in the comparison as described the patent.

In Fig. 3, the ordinate refers to the value "X" of equation 1, ranging from 0 to

80%. On the abscissa, the lower scale represents values of the surface area

(m²/g), measured by the CTAB method (as described, e.g., in the '570 patent),

and the upper scale is the Medium Pore Diameter (A), as measured by the

method of the '570 patent. The line drawn over the sohd square points is that

which corresponds to the values of the '570 patent, and the line drawn over

the solid diamond points corresponds to the values of the EP 647 591 (as said, not normalized to the common system of the '570 patent and of the present

invention).

In Fig. 4 the ordinate gives values for Δ, which is the peak value obtained

from the graph of the Differential Intrusion vs. Diameter (viz., 0.0113 in Fig.

2). This value is shown for convenience multiphed by 10³ in the figure. The

sohd triangles are the values of Δ according to the present invention, some of

which appear in the examples given below.

Furthermore, Δ values have been calculated for the commercial product

ZEOSIL 1165 MP (ex Rhone-Poulenc Chemie) (point "M") in the figure, and

for the commercial product ULTRASIL 3370 (ex Degussa, Germany) (point "N') in the figure.

Figs. 3 and 4 illustrate the actual problem of selecting appropriate sihca. As

it can be seen, when operating according to the invention, there is a hnear

correlation between the Δ values and the Median Pore Diameter (area). This

permits, when operating according to the present invention, to provide silicas

which fit into the desired ranges, in a repeatable and smooth manner. These

figures also explains why the results obtainable when attempting to use the

'570 patent and EP 647 591 to obtain silicas of the invention are erratic and

non-operative: when operating in the most important range for green tire

uses, viz., in the range of surface area between 132-160 m²/g, and most

notably, between 140 - 145 m²/g, the behavior of the criterion of the prior art is very sharp and subject to extreme variations. It should be noted that, at

the time the '570 patent was apphed for it was common behef that the

optimal median pore diameter was 226A (viz., close to the arithmetic average

of 175A and 275A). This corresponds to a surface area of 156 m²/g (according

to the '570 patent). However, it has been found since then that a surface area

of 156 m²/g is in practice less suitable for the purpose of tire manufacturing,

because of the high viscosity of the resulting mixture.

As seen in Fig. 4, the points "M" and "N' are not optimal pore distributions, since they are in the neighborhood of Δ=0.006, well below the preferred lowest value of Δ=0.01. As a direct result of the above, the D50 of M and N silicas is substantially higher than that of the sihca of the invention (See Table VI below).

The art also makes reference to other parameters which are desirable of the silica particulates. Such parameters must of course also be in the applicable range in the silica of the present invention. Thus, for instance, for reasons of processabihty, the BET specific surface area is preferably in the range 110 - 180 m²/g, since BET values higher than 180 m²/g lead to too high viscosities, and BET values lower than 110 m²/g do not provide the desired reinforcement to the tyre. It is further important to ensure that the silica does not contain an appreciable amount of micropores (smaller than 20 A), since this impairs its ability to blend in the rubber. This property is checked by determining the BET/CTAB ratio, which preferably varies in the range of 0.9- 1.2. Other important properties, such as the content of contaminants, e.g., metal traces which react with other additives, the composition of the silica etc., which are needed on the basis of the specifications provided for each specific use, must of course be obeyed also by the silica of the invention, if the latter is to be suitable for the specific use. The various requirements and properties referred to above are well known to skilled persons operating in this field and, therefore, are not discussed herein in detail, for the sake of brevity.

Fig. 5 (A-D) is a comparison of the Δ-values obtained with three different silicas. The results are shown side by side in Fig. 5A, wherein the left graph relates to a product obtained according to the present invention, and (ZEOSIL 1165 MP) and (VN-3) are the commercial products used herein for comparison purposes. The graphs of Fig. 5A are also shown in enlarged form, in Figs. 5B - 5D, from which it can be seen that the product according to the present invention has a Δ-value of about 0.015, ZEOSIL has a Δ-value of about 0.0072, and VN-3 has a Δ-value of 0.0065.

The process according to the invention is of paramount importance in obtaining the optimal dispersible silica of the invention. As already stated, silica obeying the optimal criterion of the invention cannot be obtained at will by prior art processes. The preferred porcelanite used in the process of the invention is that found in Israel, in the Zin area, or an equivalent porcelanite. Unless specifically otherwise stated, all percentages herein are by weight. The process comprises the following stages:

A. Preparation of Sodium Silicate Solution

In this stage, a sodium silicate solution obtained from the treatment of porcelanite with NaOH in hot solution (120 - 150°C) is further processed. The preparation of the silicate solution is known in the art. However, in contrast to the prior art processes, which employ solutions containing 26% silica, according to the process of the invention solutions containing 15% - 17% silica are prepared. In this stage, said silicate solution is first filtered to remove sohds and washed by press filter, and is further diluted with water, to a silica content of 3 - 10%.

B. Silica Precipitation

In this stage, silica is precipitated by the acidification of the solution. Prior art processes employ sulfuric acid for this purpose. The present invention comprises two important novel features in this stage: the first is the use of a combination of H2SO₄/NaHSO₄,, or H2CO3 NaHCO3. The second, and more important feature, is the ratio between said two components and the rate of addition of the acidifying mixture to the silica solution, as described above.

C. Reinforcement

The reinforcement stage is a stage known in the art. It requires the addition of very small amounts of sodium silicate, while maintaining a fixed pH by adding required amounts of H2SO .

D. Acidification

In this stage, the pH is lowered by the addition of sulfuric acid, to pH 4-5. The addition of the acid is carried out with moderate mixing.

E. Filtration

At this stage the precipitated silica is filtered, typically under pressure or vacuum. F. Granulation and Drying

The filtered silica is dried according to the prior art in any suitable way. Prior art processes employ rotating dryers, or spray dryers. The process according to the invention can be carried out using conventional drying methods. However, according to a preferred embodiment of the invention, the wet silica obtained from Stage E, typically in the form of a wet cake having a moisture content of about 75 to 87 weight %, is mixed with dry silica, to form a mixture having a moisture content not higher than 75 weight %, and is subjected to granulation. Subsequently, the granules obtained are dried in a fluidized bed.

The dry silica employed in the process according to the present invention is preferably, and conveniently, recycled silica obtained from fractions of dried sihca having diameters larger or smaller than the required diameter of the finished product. A typical output from the dryer, before screening, comprises particles having diameters in the range 0.1 - 5.0 mm, while a typical desirable finished product should contain particles having diameters in the range 0.5 - 3.0 mm.

A preferred embodiment of the process according to the present invention is schematically shown in Fig. 6. Wet silica precipitate, WS, obtained from the precipitation process described above in the form of a wet cake, is fed into a mixing and granulating apparatus, indicated as Mx/Gr, together with two forms of dry sihca, indicated as FDS and CDS, consisting of recycled dry sihca from the fine fraction and coarse fraction, respectively, as will be further explained below. The combined silica is mixed and granulated in said mixing and granulating apparatus, the output of which is granules, F, entering the fluidized bed. The moisture content of the wet sihca, WS, is generally abotit 75 to 87 weight % moisture. The ratio between the amount of the wet sihca, WS, and the joined amounts of the dry sihca, FDS+CDS, is dictated by the desired content of moisture of the mixture fed into Mx/Gr, which must not be higher than 75 weight %, and preferably between 60 to 73 weight %. The exact amounts of the wet and dry sihca may be easily calculated accordingly, upon determining their initial moisture content by methods known in the art.

Preferably, when a change-can mixer manufactured by KENWOOD is employed as the mixing- granulation appartus, the mixing speed varies between 200 to 350 rpm, the mixing time being between 5 to 20 min. The exact conditions for each type of mixer may be adjusted by a skilled person.

In the fluidized bed the granules are preferably dried at a temperature between 75 to 100° C, and preferably between 80 to 95° C , the output being dried granules having water content in the range between 4 to 6 wt. %. These granules are screened, to obtain the fraction of the desired diameter range, P, from screen S2. Larger diameter fraction material, from screen Si, is optionally ground in a mill "M", and is then mixed with the smaller fraction from screen S3, providing, respectively, the joined amounts of CDS and FDS mentioned above, in the mixing- granulating apparatus Mx Gr.

The above and other characteristics and advantages of the invention will be further understood from the following illustrative, non-hmitative examples.

Example 1

A solution of sodium silicate was prepared from porcelanite. 6371 g of solution, containing 17.8% Siθ2 (module 3.0) and 105 ppm organic compound, and 12,031 g water were placed into a 25 liter reactor provided with a mixer and double-jacketed heater. The mixture was heated to 82°C, and agitation was maintained. 8,301 g of solution containing H2SO (5.53%) and NaHSO₄ (2.17%) were added until a pH value of 8.8 was attained in the reactor medium after 85 minutes. The temperature was then increased to 95°C, and 1124 g of sodium silicate solution was added to the silica sediment in two equal parts, with an interval of 30 minutes.

The simultaneous addition of sulfuric acid (6.5%) was carried out, while constantly maintaining a pH of 7.5. Adding additional H₂SO to the reaction mixture adjusted the pH to 4.0.

A suspension of precipitated silica was thus obtained, which was then filtered under vacuum. The silica cake was washed twice with 1.5 hters of water. 8,420 g of a silica pulp was obtained (85% moisture). Dry sihca was combined with the pulp silica, to obtain a mixture of 28% solids by weight (1,646 g). The mixing and granulation were carried out in a change-can mixer (KENWOOD). The product was then dried to 5% moisture in a fluidized bed with dry air (90°C). The dried granules were passed through a screen of mesh, and the diameter of the granules obtained was in the range 0.5-3.0 mm.

Example 2

This example illustrates the effect of natural porcelanite organic matters on the quality of the final product. In this example, the amount of organic matter present in the porcelanite is negligible, as compared to the very small - but still effective - content of 105 ppm in Example 1. A solution of sodium sihcate was prepared from the porcelanite of Example 1, which was previously calcined. 6,523.7 g of solution, containing 18.62% Siθ2, traces of about 2 ppm organic compound, and 12,010.4 g water, were placed into a 25 liter reactor provided with a mixer and double-jacketed heater. The mixture was heated to 82°C. 8300 g of solution containing H2SO (5.53%) and NaHSO (2.17%) were added, until a pH 8.8 was attained in the reactor medium after 85 minutes. The temperature was then increased to 95°C, and 1150 g sodium silicate solution were added to the sihca sediment in two equal parts with an interval of 30 minutes.

The simultaneous addition of sulfuric acid (6.5%) was carried out, while constantly maintaining a pH of 7.5. Adding additional H2SO₄ to the reaction mixture adjusted the pH to 4.0. A suspension of precipitated sihca was thus obtained which was then filtered under vacuum. The sihca cake was washed twice with 1.5 liters of water. 7088 gram of a pulp of precipitated silica was thus obtained (82.3% moisture). Dry silica was combined with the pulp silica, to obtain a mixture of 28% solids by weight (1,090 g). The mixing and granulation were carried out in a change-can mixer (KENWOOD). The precipitated silica then produced was in the form of granules with a diameter less then 3 mm. The said product was dried to 5% moisture in a fluidized bed with dry air (90°C).

The granules were passed through a mesh screen, and the diameter of the granules recovered and analyzed in Table II below was 0.5-2.0 mm.

Table II below details the contents of the various contaminants in the silica granules of Examples 1 and 2, as well as the physical properties of the products obtained. It can be seen that the content of Fe, Ca, Mg, Cu and Zn in the product of Example 1 is much lower, while the surface area (both CTAB and BET) and the Δ-value for the product of Example 1, are substantially higher. In order to obtain suitable Δ-value using this starting material, the CTAB should be improved (increased) by suitable process steps.

Table II

Example 3

Sihca granules were prepared using a procedure similar to the of Example 1. This sample was compared with commercial samples of silicas which are suitable for use in the anufacturing of tires. Table III lists the properties of the Δ-value (diff. intrusion in units of ml g-A, which was obtained from the maximum of the curve of differential intrusion versus pore diameter, and D50 (the results of the distribution of silica agglomeration particles after ultrasonic bath activity for 5 minutes).

Table III

(l)Ultrasonic bath test.

B: measured seven days after the granules are formed

As is seen from the table, the product of Example 1 obtains much better results, both in the ultrasonic bath test, and in the Δ-value.

Example 4

A solution of sodium silicate was prepared from porcelanite. The 6371 g solution, containing 17.74% Siθ2, (3.02 module) and 12,617 g water were placed into a 25 liter reactor provided with a mixer and double -jacketed heater. The mixture was heated to 82°C, and agitation was maintained. With a peristaltic pump, 8,301 g solution of a couple of acids, H2SO 5.95% and NaHSO , 2.33% (ratio weight H₂SO /NaHSO₄ 2.55) were added, until a pH value of 8.6 was attained in the reactor medium after 85 minutes. A sihca sediment was obtained.

The temperature was then increased to 95°C, and 1125 g of sodium silicate was added in two equal parts, with an interval of 30 minutes. The simultaneous addition of sulfuric acid (7.0%) was carried out, while maintaining a constant pH of 7.5. Adding additional H2SO (7%) to the reaction mixture adjusted the pH to 4.0.

The suspension of precipitated silica was filtered under vacuum, and the silica cake was washed twice with 1.5 hters of water. 8,772 g of a sihca pulp was obtained (85.6% moisture). Dry sihca was combined with the pulp silica, to obtain a mixture of 28% sohds by weight (1,780 g). The mixing and granulation were carried out in a change-can mixer (KENWOOD). The product was then dried to 5% moisture in a fluidized bed with dry air (90°C). The powder was passed through a mesh screen, and the diameter of the granules obtained was 0.5-2.0 mm.

Example 5 (Comparative)

The same sample as that of Example 4 was prepared, but the drying step was changed. The sihca cake was fluidized with an A.P.V. Gaulin Type: ISMR-BIBA homogenizer, to obtain a fluid suspension of silica, which was then dried in a Niro Atomizer - Mobile Minor spray-drier with a gas temperature of 320°C. A sihca powder was obtained. The properties of the products of Examples 4 and 5 are compared in Table IV below. Example 5 is not within the scope of the present invention and is included for the purpose of comparison.

Table IV drying method Example 4 Example 5

C.T.A.B. 150 150 moisture % 4.6 4.8

Δ ml/g-A 0.012 0.0095

A: measured a few hours after the granules are formed 3: measured seven days after the granules are formed Example 6

Two commercial silicas (KS408 ex Akzo, Holland and R-P (ex Rhone-Poulenc Chemie, France, according to the aforementioned '570 patent) were tested in rubber, and compared with the product of Example 1. The results are summarized in Table V, from which it can be seen that the product of Example 1 obtained an outstandingly better performance.

Table V

#: Tyre performance parmeters are calculated according to the above equations, wherein G * is the complex modulus and G" is the loss modulus (Futamara, Tire Science & Technology 18 2 (1990); Futamara, Rubber Chemistry and Technology 64, 57 (1990)) Example 7 The dispersabihty of the product of Example 1 was compared with that of a commercial product, Zeosil 1165 (ex Rhone-Poulenc Chemie), in the dispersion test. From the results shown in Table VI it can be seen that the D- 50 obtained with the silica of the invention, having a Δ-value of 0.0115, is much better (lower) than that obtained with the commercial product, having a Δ-value of 0.0065.

Table VI

A: measured a few hours after the granules are formed B: measured seven days after the granules are formed

Example 8

A solution of sodium sihcate was prepared from porcelanite. The 6,951 g solution, containing 16.4% Siθ2, (3.1 module) and 10,948 g water were placed into a 25 hter reactor provided with a mixer and double-jacketed heater. The mixture was heated to 84°C, and agitation was maintained. With a peristaltic pump, 8,829 g solution of a couple of acids, H2SO 5.0% and NaHSO , 2.5% were added, until a pH value of 8.4 was attained in the reactor medium after 85 minutes. A sihca sediment was obtained.

The temperature was then increased to 95°C, and 1,226 g of sodium silicate were added in two equal parts, with an interval of 30 minutes. The simultaneous addition of sulfuric acid (6.1%) was carried out, while maintaining a constant pH of 7.5. Adding additional H2SO₄ (6%) to the reaction mixture adjusted the pH to 4.0.

The suspension of precipitated silica was filtered under vacuum, and the silica cake was washed twice with 1.5 liters of water. 8,676 g of a sihca pulp was obtained (84.0% moisture). Dry silica was combined with the pulp sihca, to obtain a mixture of 28% sohds by weight (1,795 g). The mixing and granulation were carried out in a change-can mixer (KENWOOD). The product was then dried to 5% moisture in a fluidized bed with dry air (90°C). The dried granules were passed through a mesh screen, and the diameter of the granules obtained was 0.5-3.0 mm.

The results are shown in Table VII below.

Table VII

Example 9

A solution of sodium silicate was prepared from porcelanite. The 6,561 g solution, containing 17% Siθ2, (3.0 module) and 11,401 g water were placed into a 25 hter reactor provided with a mixer and double-jacketed heater. The mixture was heated to 84°C, and agitation was maintained. With a peristaltic pump, 8,983 g solution of a couple of acids, H2SO 5.9% and NaHSO , 2% were added, until a pH value of 8.6 was attained in the reactor medium after 85 minutes. A silica sediment was obtained. The temperature was then increased to 95°C, and 1,158 g of sodium sihcate were added in two equal parts, with an interval of 30 minutes. The simultaneous addition of sulfuric acid (6.0%) was carried out, while maintaining a constant pH of 7.5. Adding additional H2SO (6%) to the reaction mixture adjusted the pH to 4.0.

The suspension of precipitated silica was filtered under vacuum, and the silica cake was washed twice with 1.5 hters of water. 8,648 g of a sihca pulp was obtained (85.6% moisture). Dry sihca was combined with the pulp silica, to obtain a mixture of 28% sohds by weight (1,755 g). The mixing and granulation were carried out in a change-can mixer (KENWOOD). The product was then dried to 5% moisture in a fluidized bed with dry air (90°C). The dried granules were passed through a mesh screen, and the diameter of the granules obtained was 0.5-3.0 mm.

The results are shown in Table VIII below.

Table VIII

As can be easily verified, the silica of example 9 satisfies the criterion according to the present invention, i.e.,

Δ x lO³ > 0.141 x CTAB - b wherein b is less than 10, and does not satisfy the criterion according to the prior art (the '570 patent), according to which the pore ratio, V2/V1 should be higher than 50%. On the other hand, the commercially available sihca satisfies the criterion according to the prior art, but does not satisfy the criterion according to the present invention. It is apparent from the above comparison that the sihca of the present invention exhibits superior dispersion properties, as reflected by the D50 parameter.

Example 10

A solution of sodium silicate was prepared from porcelanite. The 6,951 g solution, containing 16.4% Siθ2, (3.1 module) and 11,451 g water were placed into a 25 hter reactor provided with a mixer and double-jacketed heater. The mixture was heated to 82°C, and agitation was maintained. With a peristaltic pump, 8,301 g solution of a couple of acids, H2SO₄ 6.4% and NaHSO , 2.2% were added, until a pH value of 8.6 was attained in the reactor medium after 85 minutes. A sihca sediment was obtained.

The temperature was then increased to 95°C, and 1,226 g of sodium silicate were added in two equal parts, with an interval of 30 minutes. The simultaneous addition of sulfuric acid (6.5%) was carried out, while maintaining a constant pH of 7.5. Adding additional H2SO (6.5%) to the reaction mixture adjusted the pH to 4.0.

The suspension of precipitated silica was filtered under vacuum, and the sihca cake was washed twice with 1.5 hters of water. 8,842 g of a silica pulp was obtained (85.6% moisture). Dry silica was combined with the pulp silica, to obtain a mixture of 28% sohds by weight (1,795 g). The mixing and granulation were carried out in a change-can mixer (KENWOOD). The product was then dried to 5% moisture in a fluidized bed with dry air (90°C). The dried granules were passed through a mesh screen, and the diameter of the granules obtained was 0.5-3.0 mm.

Table IX

* Measured by the method of EP 647 591.

A: measured a few hours after the granules are formed

B: measured seven days after the granules are formed

As can be easily verified, the sihca of example 10 satisfies the criterion according to the present invention, i.e.,

Δ x lO³ > 0.141 x CTAB - b wherein b is less than 10, and does not satisfy the criterion according to the prior art (established by the method of measurement defined in EP 647591), according to which the pore ratio, V2/V1 , should be between 19 to 46%. On the other hand, the commercially available sihca satisfies the criterion according to said prior art, but does not satisfy the criterion according to the present invention. It is apparent from the above comparison that the sihca of the present invention exhibits superior dispersion properties, as reflected by the D50 parameter.

While embodiments of the invention have been described by way of illustration, it will be understood that the invention can be carried out by persons skilled in the art with mary modifications, variations and adaptations, without departing from its spirit or exceeding the scope of the claims.

Claims

1. An optimally dispersible precipitated sihca particulate, characterized in that it possesses a Δ-value, corresponding to the peak value of the Differential Intrusion located at the Median Pore Diameter, as measured by Hg porosimeter, such that:

Δ (ml/g-A) x 10³ > 0.141 x CTAB (m²/g) - b

wherein the Median Pore Diameter is comprised between 100 and 400 A, the CTAB is between 90 and 400 m²/g and the intercept b is 10 or less.

2. A silica particulate according to claim 1, wherein b is between 7.5 and 9.79.

3. A sihca particulate according to claim 2, wherein b is between 8 and 9.

4. A silica particulate according to claim 3, wherein b is 8.64.

5. A sihca particulate according to claim 1, having a CTAB of between 90 and 210 m /g.

6. A silica particulate according to claim 5, having a CTAB surface area comprised between 134 and 170 m²/g.

7. An optimally dispersible precipitated sihca particulate according to claim 6, having a Δ-value of at least 0.01 ml/g-A.

8. A sihca particulate according to any one of claims 1 to 7, which is a powder.

9. A sihca particulate according to any one of claims 1 to 7, which is in granulated form.

10. A method for producing a reinforced elastomer/rubber matrix, comprising the steps of: a) providing non-cross-hnked components mixture of the matrix; b) selecting a precipitated sihca particulate having a Δ-value and CTAB-value such that Δ (ml/g-A) x 10³ > 0.141 x CTAB(m²/g) - b, wherein b is 10 or less; c) homogeneously dispersing said precipitated sihca particulate in said component mixture; and d) vulcanizing said mixture.

11. A reinforced elastomer/rubber matrix comprising a precipitated silica particulate having a Δ-value and a CTAB value such that

Δ (ml/g-A) x 10³ > 0.141 x CTAB(m²/g) - b, wherein b is 10 or less.

12. A reinforced elastomer/rubber matrix, whenever prepared by the process of claim 10.

13. A process for the preparation of an optimally dispersible precipitated silica particulate having a Δ-value and a CTAB value such that

Δ (ml/g-A) x 10³ > 0.141 x CTAB (m²/g) - b, wherein b is 10 or less, comprising the steps of: a) Obtaining a sodium silicate solution from the treatment of sihca-containing minerals with NaOH in hot solution (120 - 150°C), said solution containing 15% - 17% of sihca, filtering said solution to remove sohds and diluting said solution with water, to obtain a silica content of 3 - 10%;

b) Precipitating silica by acidifying said solution with an acid combination selected from H₂SO /NaHSO and H₂CO3/NaHCO₃, wherein the H2SO /NaHSO concentration ratio is comprised between 1 and 3, the concentration of free H2SO being in the range 4% - 10%, or the H2CO3 NaHCO3 concentration ratio is comprised between 0.64 and 1.92, the concentration of CO2 being in the range 1.5% - 5.0%, and wherein the addition time of the acidifying mixture to the silica solution is comprised between 60 minutes and 110 minutes;

c) Carrying out a reinforcement stage by the addition of sodium silicate comprising 5% to 25% of total Siθ2 at a fixed pH, wherein the pH is maintained by adding required amounts of H2SO₄;

d) Lowering the pH by the addition of sulfuric acid, to pH 4-5;

e) Filtering the precipitated sihca; and

f) Mixing said precipitated sihca with dry sihca to form a mixture having a moisture content not higher than 75%, preferably between 60 to 73%, granulating said mixture and subsequently drying the granules obtained in a fluidized bed.

14. A process according to claim 13, wherein the sihca-containing minerals is porcelanite.

15. An optimally dispersible precipitated silica particulate, whenever prepared by the process of claim 13 or 14.

16. A process for making silica particulates, comprising precipitating silica particles from a sihca solution obtained by the acidification of porcelanite containing organic material.

17. A process according to claim 16, wherein the content of organic matter in the porcelanite is at least about 80 ppm.

18. Sihca particulates, substantially as described and illustrated.