CH508829A - Graft polymers from hydrocarbyl elastomer silicone - Google Patents

Abstract

Grafting is pref. carried out in the presence of a peroxide, such as dicumyl peroxide. (I) is pref. an ethylene-propylene copolymer, an acrylonitrile-butadiene copolymer, polybutadiene, polyisoprene, or an ethylene-propylene terpolymer. (II) is pref. a polysiloxane. The elastomer pref. contains approx. equal amounts of (I) and (II). Vulcanisation can be carried out at elevated temp., this pref. being followed by irradiation with gamma rays in a close of 15-30 megarad. The elastomer material is intended for use as a sealing material at gas pressures, which can vary rapidly between high pressures, at which considerable gas absorption in the material occurs, and low pressures, at which considerable gas desorption occurs. The elastomer is esp. useful as a packing for gas-cooled nuclear reactors.

Description

  

  
 



  Device comprising an enclosure containing gases
 The present invention relates to a device comprising an enclosure containing gases undergoing rapid and significant decompression at the time of dismantling and a seal ensuring the tightness of said enclosure.



   The production of seals intended to work normally under a high gas pressure and to withstand variations in pressure, such as cycling, without tearing and without excessive or residual swelling after decompression poses numerous problems.



   The materials currently used (acrylonitril or ethylene-propylene butandiene copolymers, butyl) have a too slow desorption rate, especially butyl. Other materials, the desorption rate of which would be satisfactory, have mechanical properties which make them unacceptable.



   This process is found in particular for the production of seals intended for use in nuclear reactors cooled by a gas to insulate the interior of a pressure resistance enclosure, occupied for example by carbon dioxide under a pressure. pressure of 60 bars, and the outside; the current commercial elastomers, and in particular the acrylonitrile butadiene copolymers, during their decompression, exhibit significant swelling, which can go up to 1000 / o, and a return time to their initial level of several hours, incompatible with the operating requirements of the reactor.

  As for the gaskets made up of mixtures based on silicone elastomer, they return fairly quickly to the initial dimensions but they are unusable because their mechanical properties are insufficient and result in detachment and tears within the material constituting the gasket.



   The object of the present invention is to remedy these drawbacks.



   The device according to the invention is characterized in that the material constituting the said seal comprises in the gully a silicone elastomer and a hydrocarbon elastomer, said elastomers being grafted and crosslinked.



   The hydrocarbon elastomer can be chosen from the group consisting of an ethylene-propylene copolymer, an acrylonitrile butadiene, polybutadiene, polyisoprene and a terpolymer based on ethylene and propylene.



   The silicone elastomer can be a polysiloxane, the silicone and hydrocarbon elastomers are chemically grafted in the presence of an accelerator also allowing the conjugate crosslinking of said elastomers. It can be a peroxide, for example dicumyl peroxide.



   However, it would also be possible to envisage grafting by irradiation.



   A method of manufacturing the seal forming part of the present device is as follows:
 First, an intimate mixture of the silicone and hydrocarbon elastomers is produced, after which an accelerator intended to cause chemical grafting of said elastomers and various fillers is added to the mixture. The choice between the different hydrocarbon elastomers that can be used is made according to the conditions of use of the seal; in particular if the atmosphere in which the seal will have to work contains oil vapors, it will be preferable to use a butadiene acrylonitrile copolymer.

  The mixing of the elastomers can be carried out by stirring in a mixer; The hydrocarbon elastomer is generally in the form of a gelatinous mass to which the silicone elastomer is added
 To this mixture must be added a setting accelerator, the role of which is essential for causing the grafting and the conjugated crosslinking of the hydrocarbon and silicone elastomers. Among the accelerators capable of fulfilling these roles, use may be made of appropriate peroxides, such as, for example, dicumyl peroxide, optionally added with calcium carbanate as diluent.



   Dicumyl peroxide, added in powder form, is also thoroughly mixed with the main constituents.



   Various fillers can still be added to the mixture. They can in particular include; carbon black, intended primarily to improve the hardness of the end mixture; sulphide in small proportion, or sulfur-donor accelerators, as well as zinc oxide and suitable anti-oxygenates chosen from those which do not intervene or which only affect the peroxides.

  By way of example, the following composition may be cited:
 Parts
 Weight composition
 Terpolymer based
 ethylene and propylene 100
 Carbon black 40
 Sulfur 1.5
 Zinc oxide 5
 Nickel dibutyldithiocarbonate 0.25
 Zinc mercaptobensimidazolate 0.25
 Pentamethylenethiluram disulfide 1,5
 Polysiloxane 100
 Dicumyl peroxide 1,2
 Benzothiazyl disulfide 0.5
 Of course the relative proportions of silicone and hydrocarbon elastomer which, in the aforementioned composition, are in the ratio 100/100, can be used in other proportions and tend towards a predominance of silicone elastomer or towards a predominance of elastomer. hydrocarbon, depending on the desired results.



   Once the intimate mixture of all the constituents has been made, the material should preferably be used within a few days, to avoid the onset of vulcanization in place which occurs even in the absence of heating. The material is shaped and placed in a mold and then heated to cause a pre-vulcanization: this operation is generally carried out for the hydrocarbon elastomers mentioned above at a temperature between 150 and 160 "C (that is to say at a temperature which is that generally used for the pre-vulcanization of silicone elastomers) for a period which obviously depends on the size of the joint The pre-vulcanization is stopped as soon as the temperature of 150 to 1600 ° C. is reached in the core.



   The joints are then demolded and then undergo a final polymerization treatment. This can constitute:
   (- either by vulcanization in an oven, at a temperature of the same order as the pre-vulcanization for a period which may be of the order of 36 hours,
 (B) - either by irradiation with gamma rays at a dose between 15 and 30 megarads,
   ((,) - either - and this solution appears to be the best - by vulcanization in an oven followed by irradiation under the conditions defined above.



   By applying the above process and using the mixture whose composition was given above, joints were produced having the following mechanical characteristics: Mechanical properties Final treatment A Final treatment B Final treatment C
Shore hardness 62 65 69
Breaking strength 62 kg / cm2 97 kg / cm2 109 kg / cm2
Elongation at break 400 oxo 380 / o 300 O / o
AFNOR compression test 13.2 13.2 9
 The seals thus produced exhibit low swelling during a sudden decompression of 60 bars at atmospheric pressure and return to their initial dimension with a very reduced residual swelling (0.220 / o after 15 minutes).

 

   The favorable properties of the materials obtained can probably be attributed to the association of the good desorption properties of the silicone elastomer with the mechanical properties of the hydrocarbon elastomers envisaged, the diffusion coefficients of which are on the other hand relatively low.

 

Claims (1)

CLAIM Device comprising an enclosure containing gases undergoing rapid and significant decompressions at the time of dismantling and a seal ensuring the tightness of said enclosure, characterized in that the material constituting said seal comprises at least one silicone elastomer and one hydrocarbon elastomer, said elastomers being grafted and crosslinked. SUB-CLAIMS 1. Device according to claim, characterized in that the hydrocarbon elastomer is chosen from the group consisting of an ethylenepropylene copolymer, an acrylonitrile butadiene, polybutadiene, polyisoprene and a terpolymer based on ethylene and propylene. 2. Device according to claim, characterized in that the silicone elastomer is a polysiloxane. 3. Device according to claim, characterized in that the weight proportions of silicone elastomer and of hydrocarbon elastomer are substantially equal.