Elastomeric matrix reinforced with precipitated silicas

Details Elastomeric matrix reinforced with precipitated silicas Elastomeric matrix reinforced with precipitated silicas

1999-11-29

2001-04-10

Mulcahy, Peter D. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C524S269000

Reexamination Certificate

active

06214912

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to new precipitated silicas which are in particular in the form of powder, of substantially spherical beads or of granules, to a process for their preparation and to their use as reinforcing filler for elastomers.
BACKGROUND OF THE INVENTION
It is known that precipitated silica has been employed for a long time as a white reinforcing filler in elastomers, in particular in tires.
However, like any reinforcing filler it should be capable of, on the one hand, being handled and, on the other hand and above all, being easily incorporated into mixtures.
It is known, in general, that, to obtain the optimum reinforcing properties imparted by a filler, the latter must be present in the elastomeric matrix in a final form which is at the same time divided as finely as possible and distributed as homogeneously as possible. Now, such conditions can be achieved only insofar as, on the one hand, the filler has a very good capacity for being incorporated into the matrix when mixed with the elastomer (incorporability of the filler) and for being broken up or disintegrated into the form of a very fine powder (disintegration of the filler) and insofar as, on the other hand, the powder resulting from the abovementioned disintegration process can, in its turn, be completely and homogeneously dispersed in the elastomer (dispersion of the powder).
In addition, for mutual affinity reasons, silica particles have an unfortunate tendency to agglomerate together in the elastomer matrix. A detrimental consequence of these silica/silica interactions is that they limit the reinforcing properties to a level which is appreciably lower than that which it would be theoretically possible to reach if all the silica/elastomer interactions capable of being created during the mixing operation were actually obtained (this theoretical number of silica/elastomer interaction being, as is well known, directly proportional to the external surface of the silica employed).
Furthermore, such silica/silica interactions tend, in the raw state, to increase the stiffness and the consistency of the mixtures, thus making them more difficult to process.
The problem which arises is to have available fillers which, while capable of being of a relatively large size, have a very good dispersibility in the elastomers.
SUMMARY AND OBJECTS OF THE INVENTION
The objective of the present invention is to circumvent the abovementioned disadvantages and to solve the abovementioned problem.
More precisely, the invention relates to new precipitated silicas which are generally in the form of powder, of substantially spherical beads or, optionally, of granules, and which, while being capable of being relatively large in size, have a dispersibility (and disintegrability) and reinforcing properties which are very satisfactory.
It also has the objective of providing a process for the preparation of these precipitated silicas.
Finally, it relates to the use of the said precipicated silicas as a reinforcing filler for elastomers, to which they impart very good mechanical properties.
The main subject of the invention consists of new precipitated silicas which have a dispersibility (and disintegrability) and reinforcing properties which are very satisfactory, the said silicas preferably being of a relatively large size.
DETAILED DESCRIPTION OF THE INVENTION
In the description which follows, the .BET specific surface is determined according to the Brunauer-Emmet-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 external 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) using diocoyl phthalate.
The packing density in the temped state (TPD) is measured according to NFT standard 030100.
Finally, it is specified that the pore volumes given are measured by mercury porosimetry, the pore diameters being calculated by 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 disintegrability of the silicas according to the invention can be quantified by means of a specific test for disintegration.
The disintegration test is carried out according to the following method:
the cohesion of the agglomerates is assessed by a particle size measurement (using laser scattering) performed on a suspension of silica disintegrated ultrasonically beforehand; the disintegrability of the silica is thus measured (rupture of the objects from 0.1 to a few tens of microns). The ultrasonic disintegration is performed with the aid of a Vibracell Bioblock (600 W) sound generator equipped with a probe of 19 mm diameter. The particle size measurement is performed by laser scattering on a Sympatec particle size analyzer.
2 grams of silica are weighed into a pill container (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. Ultrasonic disintegration is then carried out as follows: with the probe immersed over a length of 4 cm, the output power is adjusted so as to obtain a needle displacement on the power scale indicating 20% (which corresponds to an energy dissipated by the end of the probe of 120 watts/cm
2
). 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 analyzer.
The value of the median diameter ø
50
which is obtained is proportionally lower the higher the disintegrability exhibited by the silica. The ratio (10×volume of suspension introduced (in ml))/optical density of the suspension detected by the particle size analyser is also determined (this optical density is of the order of 20). This ratio gives an indication of the fines content, 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 disintegrability exhibited by the silica.
According to a first embodiment of the invention, a new precipitated silica is now proposed, characterized in that it has:
a CTAB specific surface (S
CTAB
) of between 100 and 140 m
2
/g, preferably between 100 and 135 m
2
/g,
a median diameter (ø
50
), after ultrasonic disintegration, smaller than 2.8 &mgr;m, preferably smaller than 2.7 &mgr;m, for example smaller than 2.5 &mgr;m.
The silica according to the first embodiment of the invention generally exhibits an ultrasonic disintegration factor (F
D
) higher than 3 ml, preferably higher than 4 ml, for example higher than 4.5 ml.
A second embodiment of the invention consists of a new precipitated silica characterized in that it has:
a CTAB specific surface (S
CTAB
) of between 100 and 140 m
2
/g, preferably between 100 and 135 m
2
/g,
a pore distribution such that the pore volume consisting of the pores whose diameter is between 175 and 275 Å represents less than 55% of the pore volume consisting of the pores of diameters smaller than or equal to 400 Å,
a median diameter (ø
50
), after ultrasonic disintegration, smaller than 4.5 &mgr;m, preferably smaller than 4 &mgr;m, for example smaller than 3.8 &mgr;m.
One of the characteristics of the silica according to the second embodiment of the invention consists therefore also of the distribution, or spread, of the pore volume, and more particularly of the distribution of the pore volume which is produced by the pores of diameters smaller than or equal to 400 Å. This latter volume corresponds to the usable pore volume of the fillers which are employed for reinforcing elastomers. Analysis of the programs shows that th

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