Agitating – Rubber or heavy plastic working – With specified vent means
Reexamination Certificate
1999-03-29
2001-11-06
Cooley, Charles E. (Department: 1723)
Agitating
Rubber or heavy plastic working
With specified vent means
C366S089000
Reexamination Certificate
active
06312148
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention concerns the mixing of rubber. In particular, it relates to the continuous preparation of rubber compounds containing silica as reinforcing agent.
U.S. Pat. No. 5,626,420 discloses a continuous mixing apparatus for preparing a rubber compound. This apparatus has an elongated mixing chamber containing at least one rotor in rotation in a stator, an upstream end and a downstream end, between which the material progressively flows, one or more inlets through which said mixing chamber receives the constituents of the rubber compound and an outlet at the downstream end through which the rubber compound is discharged. The continuous mixing apparatus has the advantage, notably, of making it possible to obtain rubber compounds in which the reinforcing agent—e.g., carbon black—is remarkably well dispersed.
Patent EP 0,501,227 showed that rubber compounds containing silica as reinforcing agent in very large proportions could have remarkable properties for use as a tire tread, on condition, notably, that the silica should have an excellent dispersion in the elastomeric matrix.
It was therefore sought to use a continuous mixer, such as presented in U.S. Pat. No. 5,626,420, in order to employ rubber compounds containing silica as reinforcing agent.
It has been discovered, contrary to expectations, that the use of such a mixer did not make it possible, in all cases, to obtain satisfactory rubber compounds, notably, when those compounds contain silica as reinforcing agent. In particular, instabilities of flow of material into the mixer were sometimes observed. Furthermore, the properties of those compounds proved to be different from those obtained with the known “batch” mixing methods: For example, greater hardness and reduced calcining time (time necessary, at a given temperature, for the vulcanization reaction to start in a sample of the rubber compounds after mixing).
SUMMARY OF THE INVENTION
The invention concerns a continuous mixing method and apparatus making it possible to attain better results, particularly when rubber compounds containing silica are prepared.
In what follows, when the mixing chamber in a given zone is totally occupied by the constituents to be mixed, it is said by convention that the filling rate of that zone is equal to 1. On the other hand, when the volume of the zone of the mixing chamber considered is not totally occupied by the constituents to be mixed, it is then said that the filling rate is less than 1.
The continuous mixing process, according to the invention, for preparing a rubber compound in an elongated mixing chamber defined by the volume between the radially inner surface of a stator and the radially outer surface of at least one rotor in rotation in said stator, comprises:
continuously introducing in the mixing chamber, at one or more points of introduction, different constituents, among which there is at least one base elastomer and at least one reinforcing filler, said constituents being such that their mixing is capable of producing inside the mixing chamber the appearance of at least one product in gaseous phase,
channeling the different constituents from their points of introduction to an outlet of the mixing chamber, and
circulating, in at least part of the mixing chamber, a stream of gas intended to carry along said gaseous phase out of said mixing chamber.
The circulation of that gas is intended to make possible the extraction from the mixing chamber of the products in gaseous phase likely to appear in the chamber on mixing. This method makes it possible to obtain, notably, for rubber compounds containing silica as reinforcing agent, satisfactory properties. The zone in which the stream of gas is circulated, the “circulation zone,” is, in particular, a zone where the filling rate of said constituents is less than one. That stream of gas is preferably circulated in a direction opposite the direction of flow of materials in the course of mixing.
The circulation of said gas also makes it possible to better control the thermal development of the rubber compound.
The invention also concerns a continuous mixing apparatus for preparing a rubber compound which has an elongated mixing chamber containing:
at least one rotor in rotation in a stator;
an upstream end and a downstream end, between which the material progressively flows;
one or more material inlets through which said mixing chamber receives said constituents of the rubber compound;
an outlet situated at the downstream end, through which the rubber compound is expelled; and
means for circulating a gas in at least a portion of said chamber called circulation zone.
Said mixing chamber is characterized in that, in the circulation zone, the stator and/or rotor contain a groove.
The stator and/or rotor preferably have at least one continuous thread in the circulation zone, and the groove adjoins the back face of that continuous thread. The groove can have a U or inclined V shape. That groove has the advantage of regulating the circulation of gas, as well as the stability of the flow of material in the mixing chamber.
The means for circulating the gas in the circulation zone preferably circulate that gas against the direction of flow of the material in the mixing chamber. That counterflow circulation increases the speed of the gas relative to the material and promotes good extraction of the products in gaseous phase.
The circulation zone is advantageously situated below the inlet or inlets of the constituents in the mixing chamber.
This makes it possible to disturb the incorporation of those constituents in the elastomeric matrix of the compound as little as possible.
To obtain the tightness of the circulation zone and promote a stable circulation of the gas above and below that zone, the shape of the rotor and/or stator surface is such that one encounters, above and below, a zone whose filling rate is equal to 1.
This has the advantage of strictly limiting the circulation of gas to that circulation zone without disturbing mixing up the line.
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Deal Michel
Van Meyel Christian
Baker & Botts L.L.P.
Cooley Charles E.
Sedepro Societe Anonyme
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