Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
1998-08-24
2001-04-24
Marcheschi, Michael (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S483000, C423S335000, C423S339000, C524S492000, C524S493000
Reexamination Certificate
active
06221149
ABSTRACT:
The present invention relates to a new process for the preparation of precipitated silica, to precipitated silicas which are in particular in the form of powder, of substantially spherical beads or of granules, and to their application as a reinforcing filler for elastomers.
It is known that precipitated silica has been employed for a long time as a white reinforcing filler in elastomers.
However, like any reinforcing filler, it is appropriate that it should be capable of, on the one hand, being handled and above all, on the other hand, of being easily incorporated into the mixtures.
It is known in general that, to obtain the optimum reinforcing properties conferred by a filler, it is appropriate that the latter should be present in the elastomer matrix in a final form which is both as finely divided as possible and distributed as homogeneously as possible. However, such conditions can be achieved only insofar as, on the one hand, the filler has a very good ability to be incorporated into the matrix during mixing with the elastomer (incorporability of the filler) and to disintegrate or to deagglomerate into the form of a very fine powder (disintegration of the filler) and as, on the other hand, the powder resulting from the abovementioned disintegration process can itself, in its turn, be perfectly and homogeneously dispersed in the elastomer (dispersion of the powder).
Moreover, for reasons of mutual affinities, silica particles have an unfortunate tendency, in the elastomer matrix, to agglomerate with each other. These silica/silica interactions have a detrimental consequence of limiting the reinforcing properties to a level that is substantially lower than that which it would be theoretically possible to expect if all the silica/elastomer interactions capable of being created during the mixing operation were actually obtained (this theoretical number of silica/elastomer interactions being, as is well known, directly proportional to the external surface of the silica employed).
Furthermore, in the raw state, such silica/silica interactions tend to increase the stiffness and the consistency of the mixtures, thus making them more difficult to process.
The problem arises of having available fillers which, while being capable of being relatively large in size, have a very good dispersibility in elastomers.
The aim of the present invention is to overcome the abovementioned disadvantages and to solve the abovementioned problem.
More precisely, its aim is especially to propose a new process for the preparation of precipitated silica which, advantageously, has a very good dispersibility (and disintegratability) and very satisfactory reinforcing properties, in particular which, when employed as a reinforcing filler for elastomers, imparts excellent rheological properties to the latter while providing them with good mechanical properties.
The invention also relates to precipitated silicas which, preferably, are in the form of powder, of substantially spherical beads or, optionally, of granules, and which, while being of relatively large size, have a very good dispersibility (and disintegratability) and very satisfactory reinforcing properties.
It relates finally, to the use of the said precipitated silicas as reinforcing fillers for elastomers.
In the description which follows, the BET specific surface is determined according to the Brunauer-Emmett-Teller method described in the Journal of the American Chemical Society, Vol. 60, page 309, February 1938 and corresponding to NFT standard 45007 (November 1987).
The CTAB specific surface is the outer surface determined according to NFT standard 45007 (November 1987) (5.12).
The DOP oil uptake is determined according to NFT standard 30-022 (March 1953) by using dioctyl phthalate.
The packing density (PD) is measured according to NFT standard 030100.
The pH is measured according to ISO standard 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 mercury porosimetry, the pore diameters being calculated from the Washburn relationship with an angle of contact theta equal to 130° and a surface tension gamma equal to 484 dynes/cm (MICROMERITICS 9300 POROSIMETER®).
The dispersibility and the disintegratability of the silica according to the invention can be quantified by means of a specific disintegration test
The disintegration test is carried out according to the following procedure:
the cohesion of the agglomerates is assessed by a particle size measurement (using laser scattering), performed on a silica suspension previously disintegrated by ultrasonic treatment; the disintegratability of the silica is thus measured (rupture of objects from 0.1 to a few tens of microns). The disintegration under ultrasound is performed with the aid of a VIBRACELL BIOBLOCK (500 W)® sonic transducer equipped with a probe 19 mm in diameter. The particle size measurement is performed by laser scattering on a Sympatec particle size analyser.
2 grams of silica are measured out into a specimen tube (height 6 cm and diameter 4 cm) and are made up to 50 grams by adding demineralized water, an aqueous suspension containing 4% of silica is thus produced, which is homogenized for 2 minutes by magnetic stirring. The disintegration under ultrasound is next performed as follows: with the probe immersed to a depth of 4 cm, the power is adjusted so as to obtain a needle deflection on the power dial indicating 20% (which corresponds to an energy of 120 watts/cm
2
dissipated by the end of the probe). The disintegration is performed for 420 seconds. The particle size measurement is then carried out after a known volume (expressed in ml) of the homogenized suspension has been introduced into the cell of the particle size analyser.
The value of the median diameter Ø
50
which is obtained is proportionally smaller the higher the disintegratability of the silica. The ratio (10×volume of dispersion introduced (in ml))/optical density of the suspension detected by the particle size analyser (this optical density is of the order of 20) is also determined. This ratio is an indication of the proportion of fines, that is to say of the content of particles smaller than 0.1 &mgr;m, which are not detected by the particle size analyser. This ratio, called the ultrasonic disintegration factor (F
D
) is proportionally higher the higher the disintegratability of the silica.
One of the subjects of the invention is a process for the preparation of precipitated silica of the type including the reaction of a silicate with an acidifying agent, whereby a suspension of precipitated silica is obtained, followed by the separation and the drying of this suspension, in which the precipitation is carried out in the following manner
(i) an initial base stock comprising a silicate of alkali metal M and an electrolyte is formed, the silicate concentration (expressed as SiO
2
) in the said initial base stock being lower than 20 g/l,
(ii) the acidifying agent is added to the said base stock until at least 5% of the quantity of M
2
O present in the said base stock is neutralized,
(iii) acidifying agent and a silicate of alkali metal M are added simultaneously to the reaction mixture such that the ratio of the quantity of silicate added (expressed as SiO
2
) (the quantity of silicate present in the initial base stock (expressed as SiO
2
, called the degree of consolidation, is greater than 4 and at most 100,
characterized in that the said process includes one of the following two operations (a) or (b):
(a) at least one aluminium compound A and then a basic agent are added to the reaction mixture after stage (iii), the said separation comprising a filtration and a disintegration of the cake originating from this filtration, the said disintegration being performed in the presence of at least one aluminium compound B,
(b) a silicate and at least one aluminium compound A are added simultaneously to the reaction mixture after stage (iii) and, when the said separation comprises a filtration and a disintegration of the cake originating
Bomal Yves
Chevallier Yvonick
Cochet Philippe
Marcheschi Michael
Rhodia Chimie
Seugnet Jean-Louis
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