Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
1999-07-23
2002-10-22
Nguyen, Ngoc-Yen (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C423S335000, C524S492000, C524S493000
Reexamination Certificate
active
06468493
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a 371 of PCT/FR98/00975 filed May 15, 1998.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a novel process for the preparation of precipitated silica, in particular precipitated silicas in the form of powder, substantially spherical beads or granules, and to their application in reinforcing elastomers.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98.
It is known that precipitated silica has long been used as a white reinforcing filler in elastomers.
However, as with any reinforcing filler, it is convenient for it to be manipulable, on the one hand, and especially for it to be readily incorporated in mixtures, on the other hand.
It is known, in general, that in order to obtain the optimum reinforcing properties imparted by a filler, this filler 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 only be achieved provided that, on the one hand, the filler has a very good capacity to be incorporated in the matrix during mixing with the elastomer (incorporability of the filler) and to become disintegrated or deagglomerated in the form of a very fine powder (disintegration of the filler) and provided that, on the other hand, the powder obtained from the above mentioned disintegration process can itself, in turn, be dispersed fully and homogeneously in the elastomer (dispersibility of the powder).
BRIEF SUMMARY OF THE INVENTION
Furthermore, for reasons of mutual affinity, silica particles have an annoying tendency, in the elastomer matrix, to agglomerate together. These silica/silica interactions have the harmful consequence of limiting the reinforcing properties to a level substantially lower than that which could theoretically be achieved if all the silica/elastomer interactions liable to be created during the mixing operation were effectively obtained (this theoretical number of silica/elastomer interactions being, as is well known, directly proportional to the external surface area of the silica used).
In addition, 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 use.
These drawbacks are encountered in particular in the case of silicas with a relatively high specific surface area, which, furthermore, generally do not have very good reinforcing properties.
The problem arises of providing fillers which, while having a relatively high specific surface area, show a satisfactory dispersibility in elastomers and good reinforcing properties.
DETAILED DESCRIPTION OF THE INVENTION
The aim of the present invention is to avoid the abovementioned drawbacks and to solve the problem mentioned above.
With this aim, the invention proposes a novel process for the preparation of precipitated silica, of the type comprising the reaction of a silicate with an acidifying agent, whereby a suspension of precipitated silica is obtained, followed by separation and drying of this suspension, characterized in that:
the precipitation is carried out in the following way:
(i) an initial stock solution is formed containing at least some of the total amount of the silicate used in the reaction and at least one electrolyte, the concentration of silicate (expressed as SiO
2
) in the said initial stock solution being between 50 and 60 g/l,
(ii) the acidifying agent is added to the said stock solution until a pH value of between 7 and 8.5 for the reaction medium is obtained,
(iii) the acidifying agent is added to the reaction medium along with, where appropriate, simultaneously, the remaining amount of the silicate,
the separation comprises a filtration and washing operation using a filter equipped with a means of compacting,
a suspension having a solids content of less than 17% by weight is dried by spraying.
It has thus been found that a certain, relatively low, concentration of silicate, expressed as SiO
2
in the initial stock solution, combined with the use of a filter equipped with a means of compacting, preferably at a low compacting pressure, and with a suitable solids content of the suspension to be dried constitute important conditions for giving the products obtained their good properties.
It should be noted, in general, that the process concerned is a process for the synthesis of precipitation silica, i.e. an acidifying agent is reacted, under specific conditions, with a silicate.
The choice of acidifying agent and of silicate is made in a manner which is well known per se.
It may be recalled that a strong inorganic acid, such as sulphuric acid, nitric acid or hydrochloric acid, or an organic acid, such as acetic acid, formic acid or carbonic acid, is generally used as acidifying agent.
The acidifying agent can be dilute or concentrated; its normality can be between 0.4 and 8 N, for example between 0.6 and 1.5 N.
In particular, when the acidifying agent is sulphuric acid, its concentration can be between 40 and 180 g/l, for example between 60 and 130 g/l.
It is moreover possible to use, as silicate, any common form of silicate, such as metasilicates, disilicates and advantageously an alkali metal silicate, in particular sodium or potassium silicate.
The silicate can have a concentration, expressed as silica, of between 40 and 330 g/l, for example between 60 and 300 g/l, in particular between 60 and 250 g/l.
In general, sulphuric acid is used as acidifying agent and sodium silicate is used as silicate.
When sodium silicate is used, it generally has an SiO
2
/Na
2
O weight ratio of between 2 and 4, for example between 3.0 and 3.7.
As more particularly regards the preparation process of the invention, the precipitation is carried out in a specific manner according to the following steps.
A stock solution which comprises silicate and an electrolyte is first formed (step (i)). The amount of silicate present in the initial stock solution advantageously represents only some of the total amount of silicate used in the reaction.
According to an essential characteristic of the preparation process of the invention, the concentration of silicate in the initial stock solution is between 50 and 60 g of SiO
2
per litre. Preferably, this concentration is between 55 and 60 g/l.
The initial stock solution comprises an electrolyte. The term electrolyte is understood here in its normal accepted meaning, i.e. it means any ionic or molecular substance which, when in solution, decomposes or dissociates to form ions or charged particles. As electrolyte, mention may be made of a salt from the group of salts of alkali metals and alkaline-earth metals, in particular the salt of the metal of the starting silicate and of the acidifying agent, for example sodium chloride in the case of the reaction of a sodium silicate with hydrochloric acid, or, preferably, sodium sulphate in case of the reaction of a sodium silicate with sulphuric acid.
If the electrolyte used is sodium sulphate, its concentration in the initial stock solution is preferably between 12 and 20 g/l, in particular between 15 and 20 g/l.
The second step consists in adding the acidifying agent to the stock solution of composition described above (step (ii)).
This addition, which entails a corresponding decrease in the pH of the reaction medium, is carried out until a pH of between 7 and 8.5 is reached; in particular between 7 and 8, for example between 7.5 and 8.
Once the desired pH has been reached, the third step (step (iii)) is then carried out.
In the (preferred) case of an initial stock solution comprising only some of the total amount of silicate used in the reaction, a simultaneous addition of acidifying agent and of the remaining amount of silicate is carried out in step (iii).
This simultaneous addition is preferably carried out such that the pH is constantly equal (to within ±0.2) to the value reached after step (ii).
In general, in a subsequent step, an additional amount of acidifyi
Chevallier Yvonick
Cochet Philippe
Fourre Patrick
Nguyen Ngoc-Yen
Rhodia Chimie
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