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...
Utility Patent
1997-02-28
2001-01-02
Griffin, Steven P. (Department: 1754)
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...
C423S335000
Utility Patent
active
06169135
ABSTRACT:
The present invention concerns a novel process for the preparation of precipitated silica, in particular precipitated in the form of a powder, substantially spherical spherules, or granules, and their application as a reinforcing filler for elastomers.
FIELD OF THE INVENTION
Precipitated silica has long been used as a white reinforcing filler for elastomers, in particular for tires.
However, like all reinforcing fillers, it must capable of ready manipulation in, and above all incorporation into the mixtures.
BACKGROUND OF THE INVENTION
It is generally known that, in order to produce optimal reinforcing properties from a filler, the latter must be present in the elastomer matrix in a final form which is both as finely divided as possible and distributed as homogeneously as possible. These conditions can only be satisfied if the filler can be extremely easily incorporated into the matrix during mixing with the elastomer (filler incorporability) and can disaggregate or deagglomerate to a very fine powder (filler disaggregation), and if the powder produced by the disaggregation process described can itself disperse perfectly and homogeneously in the elastomer (powder dispersion).
Further, the silica particles have an annoying tendency to agglomerate among themselves in the elastomer matrix because of mutual attraction. These silica/silica interactions limit the reinforcing properties to a level which is far lower than that which could theoretically be achieved if all the silica/elastomer interactions which could be formed during the mixing operation were formed (this theoretical number of silica/elastomer interactions is known to be directly proportional to the external surface area of the silica used).
These silica/silica interactions also tend to increase the rigidity and consistency of the mixtures in the uncured state, making their use more difficult.
It is difficult to find fillers which, while having a relatively large size, readily disperse in elastomers.
SUMMARY OF THE INVENTION
The present invention aims to overcome the drawbacks described above and resolve the difficulty mentioned above.
More precisely, it provides a novel process for the preparation of precipitated silica which advantageously has improved dispersion (and deagglomeration) abilities and/or reinforcing properties, in particular when used as a filler for elastomers, to provide the latter with an excellent compromise between their different mechanical properties.
The invention also concerns precipitated silicas, preferably in the form of a powder, substantially spherical spherules or, optionally, granules and which, while relatively large in size, have highly satisfactory dispersion (and deagglomeration) abilities. They also have improved reinforcing properties.
Finally, the invention concerns the use of said precipitated silicas as reinforcing fillers for elastomers.
One of the objects of the invention is thus to provide a process for the preparation of a precipitated silica comprising reacting an alkali metal M silicate with an acidifying agent to produce a suspension of a precipitated silica, then separating and drying said suspension, characterised in that precipitation is carried out as follows:
(i) forming an initial seed comprising a portion of the total amount of the alkali metal M silicate used i the reaction, the concentration of silicate expressed as SiO
2
in said seed being less than 20 g/l,
(ii) adding acidifying agent to said initial seed until at least 5% of the quantity of M
2
O present in said initial seed is neutralised,
(iii) simultaneously adding acidifying agent and the remaining quantity of alkali metal M silicate to the reaction medium such that the ratio of the quantity of silicate added (expressed as SiO
2
)/quantity of silicate present in the initial seed (expressed as SiO
2
), the consolidation ratio, is greater than 4 and at most 100.
It has thus been discovered that a very low concentration of silicate expressed in SiO
2
in the initial seed and an appropriate consolidation ratio during the simultaneous addition step constitute important conditions for ensuring that the products obtained have excellent properties.
It should be noted that, in general, the process concerned is a process for the synthesis of precipitated silica, ie., an acidifying agent is reacted with an alkali metal M silicate.
The choice of acidifying agent and silicate is made in known fashion. The acidifying agent is usually a strong mineral acid such as sulphuric acid, nitric acid or hydrochloric acid, or an organic acid such as acetic acid, formic acid or a carboxylic acid.
The silicate can be in any known form such as a metasilicate, disilicate or, advantageously, an alkali metal M silicate where M is sodium or potassium.
In general, the acidifying agent used is sulphuric acid, and the silicate used is sodium silicate.
When using sodium silicate, this generally has a SiO
2
/Na
2
O molar ratio of between 2 and 4, more particularly between 3.0 and 3.7.
Specifically, precipitation is carried out using the following steps of the process of the invention.
Firstly, a seed containing silicate is formed. The quantity of silica present in the initial seed advantageously only represents a portion of the total quantity of silicate used in the reaction.
According to an essential feature of the preparation process of the invention, the concentration of silicate in the initial seed is least than 20 g of SiO
2
per liter.
This concentration can be at most 11 g/l, optionally at most 8 g/l.
Particularly when the separation carried out at the end of the process of the invention comprises filtration using a filter press, this concentration is preferably at least 8 g/l, in particular between 10 and 15 g/l, for example between 11 and 15 g/l; the subsequent drying step in the process of the invention is then advantageously carried out by atomisation using a spray diffuser.
The conditions imposed on the silicate concentration in the initial seed partially determine the characteristics of the silicas obtained.
The initial seed may contain an electrolyte. Nevertheless, it is preferable that no electrolyte is used during the preparation process of the invention; in particular, the initial seed preferably contains no electrolyte.
The term “electrolyte” has its normal meaning here, ie., it signifies any ionic or molecular substance which, when in solution, decomposes or dissociates to form ions or charged particles. A salt from the group formed by alkali metal and alkaline-earth metal salts can be cited as an electrolyte, in particular the salt of the starting metal silicate and the acidifying agent, for example sodium sulphate in the case of the reaction of sodium silicate with sulphuric acid.
The second step consists in adding the acidifying agent to the seed with the composition described above.
Thus, in the second step, the acidifying agent is added to said initial seed until at least 5%, preferably at least 50%, of the quantity of M
2
O present in said initial seed is neutralised.
Preferably, the acidifying agent is added to said initial seed in said second step until 50% to 99% of the quantity of M
2
O present in said initial seed is neutralised.
The acidifying agent can be dilute or concentrated; the concentration can be between 0.4 and 36N, for example between 0.6 and 1.5N.
In particular, when the acidifying agent is sulphuric acid, its concentration is preferably between 40 and 180 g/l, for example between 60 and 130 g/l.
When the desired concentration of neutralised M
2
O has been reached, simultaneous addition (step (iii)) of the acidifying agent and a quantity of alkali metal M silicate is commenced so that the consolidation ratio, i.e., the ratio of the quantity of silicate added (expressed as SiO
2
)/quantity of silicate present in the initial seed (expressed as SiO
2
) is greater than 4 and at most 100.
In one embodiment of the invention, simultaneous addition of the acidifying agent and quantity of alkali metal M silicate is carried out such that the consolidation ratio is more particularly between 12 and 100, prefera
Chevallier Yvonick
Prat Evelyne
Burns Doane Swecker & Mathis L.L.P.
Griffin Steven P.
Hendrickson Stuart L.
Rhone Poulenc Chimie
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