Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-03-24
2002-05-21
Moore, Margaret G. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C523S213000, C524S588000
Reexamination Certificate
active
06391944
ABSTRACT:
TECHNICAL FIELD
The field of the invention is that of silicone elastomers which can be obtained by polyaddition and the main components of which are silicone polymers and fillers.
More specifically, the present invention relates to the preparation of an intermediate product useful for obtaining these silicone elastomers and consisting of a suspension of a reinforcing filler in a polyorganosiloxane carrying Si-alkenyl—preferably Si-Vi—functional groups capable of reacting by polyaddition with the SiH crosslinking functional groups of another POS.
PRIOR ART
A distinction may be made between reinforcing and non-reinforcing fillers in silicone rubbers.
The most widely used reinforcing fillers are preferably pyrogenic silicas having a BET surface area >50 m
2
/g. They owe their reinforcing effect firstly to their morphology and secondly to the hydrogen bonds which form between the silanol groups on the surface of the silicas (3-4.5 SiOH groups/mm 2) and the polyorganosiloxane (POS) chains. These interactions between the filler and the polymer increase the viscosity and modify the behaviour of the polymer near the solid surface of the fillers. Moreover, the bonds between polymers and fillers improve the mechanical properties but may also cause prejudicial premature curing (“structuring”) of the precursor compositions of the elastomers.
Non-reinforcing fillers interact extremely weakly with the silicone polymer. These are, for example, chalk, quartz powder, diatomaceous earth, mica, kaolin, aluminas or iron oxides. Their effect is often to increase the viscosity of the uncured precursors of the elastomers, as well as the Shore hardness and the modulus of elasticity of these precursors.
Silicone elastomers may also contain, inter alia, catalysts, inhibitors, crosslinking agents, pigments, antiblocking agents, plasticizers and adhesion promoters.
These elastomers, curable by polyaddition and also called RTV elastomers, are formed, before curing, by casting, extrusion, calendering, or compression, injection or transfer moulding.
Silicone compositions made of elastomers, which can be cured by polyaddition at room temperature or at higher temperatures (generally <200° C.), are conventionally packaged in the form of two-component systems, that is to say comprising two parts which are packaged separately and have to be mixed at the time of use. In two-component systems, one of the components comprises the catalyst for the polyaddition reaction. This catalyst is preferably of the platinum kind. It may, for example, be a platinum complex like the one prepared from chloroplatinic acid and 1,3-divinyl-1,1,3,3-tetramethyldisiloxane, according to U.S. Pat. No. 3,814,730 (Karstedt catalyst). Other platinum complexes are described in Patents U.S. Pat. Nos. 3,159,601, 3,159,662 and 3,220,972. This component including the catalyst may also comprise only one of the POS fluids of type I having Si-alkenyl, preferably Si-vinyl, crosslinking functional groups or only one of the POS fluids of type II having an SiH crosslinking functional group. Generally, the POS fluids of type I and the POS fluids of type II comprise at least two Si-Vi and SiH groups per molecule, respectively, preferably in the &agr; and &ohgr; positions on the chain: at least one of the two having to comprise at least three crosslinking functional groups per molecule. These compositions comprise, in a known manner, POS fluids of type I and II, a platinum catalyst for crosslinking by polyaddition and a platinum inhibitor allowing the compositions to cure only once they have been removed from the package and mixed together, optionally after they have been heated slightly. As examples of inhibitors, mention may be made of:
polyorganosiloxanes, advantageously cyclic polyorganosiloxanes, substituted with at least one alkenyl, tetramethylvinyltetrasiloxane being particularly preferred,
pyridine,
organic phosphines and phosphites,
unsaturated amides,
alkylated maleates
and acetylenic alcohols (cf. FR-B-1,528,464 and FR-A-2,372,874).
Such compositions may also be in the form of one-component systems which cure only after having been heated.
The preparation of concentrated suspensions (pastes) of reinforcing silicas in vinyl silicone oils, these suspensions being intended to produce elastomers that can be cured by the reaction of a polyhydrogenated crosslinking molecule such as a POS with the vinyl silicone oil (SiH/SiVi addition), is widespread in the field of elastomers.
The commonest reinforcing particulate fillers are based on silica, but substances such as TiO
2
, Al
2
O
3
and kaolin, for example, may also be used in certain cases. These reinforcing fillers have a BET specific surface area of at least 50 m
2
/g, and generally up to 400 m
2
/g. These are ultrafine powders which may be dispersed in silicone, preferably SiVi, oils. This dispersion causes problems when mixing some of the pulverulent filler with the oil and particular care must be taken in order to obtain a uniform distribution of the fillers in the suspension.
Another difficulty to be overcome is associated with the rheology of the suspensions prepared. This is because it is clear that introducing a pulverulent particulate filler of very small particle size into the silicone oil necessarily causes an appreciable increase in the viscosity. However, this characteristic, although it accompanies the achievement of good mechanical properties for the silicon elastomers comprising the suspension as raw material, is prejudicial to the handling and forming of the suspension and of the silicone compositions containing the suspension. It is in fact more convenient, for moulding, extrusion or forming, to handle fluid compositions which readily lend themselves, inter alia, to pumping, flowing or mixing with functional additives.
The problematic considered here may therefore be summarized as how to find a technical compromise between a priori antinomic specifications for the suspensions of fine particulate fillers in silicone oils, namely: fine distribution of particles in the silicone matrix—uniformity of the dispersion—suitability of the rheology of the suspension to the handling constraints (processibility)—mechanical properties of the RTV silicone elastomers.
French Patent Application No. 2,320,324 falls within this problematic and describes a process for a homogeneous distribution in polyorganosiloxanes of a highly disperse active filler of BET specific surface area of at least 50 m
2
/g, this process being characterized in that the filler is treated during incorporation, in the presence of water, by a modifier or compatibilizer of the silazane type, hexamethyldisilazane being particularly preferred. The other compatibilizers mentioned are trimethylsilane, trimethylchlorosilane, trimethylethoxysilane, triorganosilyl mercaptans, triorganosilyl acylates or triorganosilyl amines. According to this process, described in this prior application, an &agr;,&ohgr;-trimethylsiloxy polydimethylsiloxane with hexamethyldisilazane (HMDZ) and with water. Once this mixture has been homogenized, some particulate silica is incorporated into it and mixing is continued until a homogeneous mixture is obtained. Next, the mixture is heated to 130° C. in order to remove the excess HMDZ and water by devolatilization. It is left to cool and, after measuring the viscosity of the suspension obtained, it is found that the latter is relatively high, which, of course, gives the elastomers capable of being prepared from this suspension good mechanical properties, but which proves to be unacceptable from a handling standpoint in an industrial context. This compatibilization treatment of the silica with the silicone oil may be termed “early” since the HMDZ is present as soon as the reinforcing silica is brought into contact with this silicone oil. The compatibilization treatment is a means of retarding or preventing reaction between the surface of a reinforcing filler and a siloxane polymer. This interaction causes what is called structuring and as a result the conversion of these mixtures is more difficult.
Pro
Canpont Dominque
Plantier Andre
Pouchelon Alain
Moore Margaret G.
Rhodia Chimie
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