Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – By gas forming or expanding
Reexamination Certificate
2001-05-17
2004-04-06
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
By gas forming or expanding
C152S158000, C264S054000
Reexamination Certificate
active
06716380
ABSTRACT:
BACKGROUND OF INVENTION
The present invention relates to a process for expanding at least one cross-linkable or cross-linked blank which is intended to constitute, in the cross-linked, expanded state, all or part of an elastomeric safety support of cellular structure having closed cells, said support being intended to be mounted on a wheel rim within a tire. The invention also relates to a process for cross-linking and expanding said or each blank, a device for implementing said expansion or cross-linking/expansion process, and finally a section of cross-linked, expanded support and such a support which are obtained by this process.
The safety supports according to the invention can be used for equipping tires, for example, of machinery or vehicles of the two-wheeler, automobile or heavy-vehicle type.
The use of elastomeric safety supports having closed cells is well-known for competition tires which are intended to travel on bumpy courses of the “cross-country rally” type.
These cellular supports, which are usually of substantially toric form, are supposed to permit travel following a significant drop in pressure over a distance which depends in particular on the more or less severe conditions characterizing this travel, for example, following perforation of the tire.
Such supports are generally obtained by extruding a cross-linkable, expandable rubber composition which has been subjected to thermomechanical working in order to obtain a blank, then by cross-linking and expanding the blank, the origin of the expansion being the thermal decomposition of a blowing agent which is initially present in the rubber composition.
More precisely, in a first step of thermomechanical working, the rubber composition, which comprises in particular a diene elastomer such as butyl rubber (copolymer of isoprene and isobutylene), a reinforcing filler such as carbon black, a blowing agent to permit later obtaining of the expanded cellular structure, and a cross-linking system, is kneaded.
In a third step, which is carried out in an oven, the blank thus obtained is preheated, to a temperature usually of between 70° C. and 100° C.
In a fourth step, performed in a mold, at least partial cross-linking of the preheated blank is effected, at a temperature usually of between 130° and 150° C.
In a fifth step, which is performed in an oven, the demolded blank is subjected to expansion, at atmospheric pressure and at a temperature usually of between 130° C. and 150° C.
Thus a cross-linked, expanded support is obtained.
French patent specification FR-A-2 095 535 describes a process for foaming and hardening an elastomeric filler material, such as polyisoprene, dimethyl-methylvinyl polysiloxane or polybutadiene, within a mounted assembly formed of a previously-vulcanised tire which is mounted on a wheel rim. This filler material is intended to equip tires intended for vehicles of the heavy-vehicle type, and the main object desired is to minimize the surface oxidation within the body of the vulcanised tire.
This object is achieved in that document by implementing a process consisting of heating in an oven the mounted assembly by means of saturated steam at a temperature of approximately 149° C., then either removing the mounted assembly from the oven or carrying out therein a plurality of heating cycles followed by removal of the steam within the oven, in order to permit blowing of the filler material.
It will be noted that this mounted assembly is intended to form an enclosure of the mold type for the filler material which it contains, owing to the fact that the foam being blown presses on the wall of this mounted assembly.
It will also be noted that the foam which presses against the wall of the mounted assembly is the seat of local chemical reactions at the location of this wall (rim or tire), which induces heterogeneity of structure of the foam which is finally formed.
The conventional cross-linking/expansion processes which use a mold for cross-linking have one major drawback, which is linked to the density characteristics of the cross-linked, expanded supports which are obtained. In fact, the cross-linked, expanded supports which are obtained by these processes using a mold are generally characterized by an intermediate layer having a significantly higher density (usually in a ratio of 1.5) than that of the core or center of the support. There is shown diagrammatically in
FIG. 5
appended to the present description a view in cross-section through such a support having, radially towards the inside of its outer surface (at the location of which there is a surface layer or skin A of the support), this intermediate layer B and said core C.
Now, experience shows that this density gradient is at the origin of increased propagation towards the core of the support of the deformations which are imposed on the latter when travelling at zero pressure, which results in likewise increased internal heating of the support, which may cause the destruction thereof within a relatively short period of travel.
SUMMARY OF THE INVENTION
The object of the present invention is to propose a process for expanding at least one partially cross-linked blank which is intended to form, in the cross-linked, expanded state, all or part of an elastomeric safety support of cellular structure having closed cells, said or each support being intended to be mounted on a wheel rim within a tire, which makes it possible to overcome the aforementioned disadvantage in relation to the use of a mold for the cross-linking.
To this end, an expansion process according to the invention is such that it consists of curing said or each blank in a bath of fluid at excess pressure which is contained in an enclosure, the temperature of said bath having a maximum value of between 110° C. and 210° C., and the absolute pressure of said bath having at least a value equal to or greater than 5 bar and a final pressure-relief value substantially equal to 1 bar, for expanding said or each blank such that the increase in volume thereof is unlimited with respect to said enclosure.
According to one example of embodiment of the invention, this expansion process consists in varying, discretely or continuously, said absolute pressure of said fluid between a maximum value less than or equal to 26 bar and a value less than said maximum value.
According to the invention, this expansion process consists in using a liquid fluid, such as water, or a gaseous fluid, such as hot air, steam or nitrogen, for curing said or each blank.
According to an optional characteristic of the invention, this expansion process consists of cooling said bath to a temperature less than or equal to 100° C. and to an absolute pressure of between 1 and 26 bar, following the curing of said or each blank.
According to another characteristic of the invention, this expansion process consists of curing a plurality of blanks of linear and/or curved forms in said bath.
It will be noted that the supports or parts of supports thus obtained may have forms and dimensions which are variable according to the desired applications.
The subject of the present invention is also a process for cross-linking and expanding at least one cross-linkable, expandable blank which is intended to form, in the cross-linked, expanded state, all or part of an elastomeric safety support of cellular structure having closed cells, said support being intended to be mounted on a wheel rim within a tire.
This cross-linking and expansion process according to the invention also makes it possible to overcome the aforementioned drawback in relation to cross-linked, expanded supports, which drawback results from the use of a mold for the cross-linking, and it is such that:
said cross-linking consists of curing said or each expandable or expanded blank in a bath of fluid at excess pressure which is contained in an enclosure, such that said or each blank interacts with said bath independently of said enclosure, the temperature of said bath having a maximum value of between 110° C. and 210° C., and the absolute pressure of said bath having at
Durif Pierre
Menard Gilbert
Morel Noel
Peyron Georges
Baker & Botts L.L.P.
Kuhns Allan R.
Michelin & Recherche et Technique S.A.
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