Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2001-08-27
2002-10-08
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...
C523S212000, C524S588000
Reexamination Certificate
active
06462104
ABSTRACT:
The field of the invention is that of polyorganosiloxane compositions which may be vulcanized cured into silicone elastomers by polycondensation actions, the main components of which are one or more reactive polyorganosiloxane(s) and fillers.
More specifically, the present invention elates to the preparation of an intermediate product which is useful in obtaining these silicone elastomers and which consists of a suspension of reinforcing filler in at least one reactive polyorganosiloxane bearing condensable or hydrolyzable functions, such as, for example, ≡Si—OH, which are capable of reacting together and/or with a vulcanizing agent chosen from organosilicon compounds bearing more than two condensable or hydrolyzable functions. The present invention also relates to the preparation of polyorganosiloxane compositions which may be vulcanized or cured into silicone elastomers by polycondensation reactions, comprising said reinforcing filler suspension as constituent element.
Reinforcing and non-reinforcing fillers are distinguished in silicone elastomers.
The reinforcing fillers most commonly used are pyrogenic silicas and precipitated silicas with a BET surface>40 m
2
/g. These are ultrafine powders which 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 (3-6 SiOH groups
m
2
) and the polyorganosiloxane (POS) chains. These interactions between the filler and the polymer increase the viscosity and modify the behavior of the polymer in the region of the solid surface of the fillers. Moreover, the bonds between polymers and fillers improve the mechanical properties, but may also cause a harmful premature curing (“structuring”) of the elastomer-precursor compositions.
Non-reinforcing fillers have an extremely weak interaction with silicone polymer. These fillers are, for example, chalk, quartz powder, diatomaceous earth, mica, kaolin, aluminas and iron oxides. Their effect is often to increase the viscosity of the non-vulcanized precursors of the elastomers, and also the Shore hardness and the modulus of elasticity of these elastomers.
Compositions capable of giving silicone elastomers also contain catalysts, vulcanizing agents and optionally pigments, antiblocking agents, plasticizers and/or adhesion promoters.
These polycondensation-vulcanizable elastomers, also known as polycondensation RTV elastomers, are formed before vulcanization by casting, extrusion, calendaring, or compression-, injection- or transfer-molding.
Silicone compositions which may be vulcanized or cured into elastomers by polycondensation at room temperature or at higher temperatures (generally<100° C.) are conventionally packaged in the form of one-component systems (i.e. systems comprising only one packaging) or two-component systems (i.e. systems comprising two parts packaged separately and needing to be mixed together at the time of use).
In the two-component systems, one of the components comprises the polycondensation reaction catalyst. This catalyst may be a metal compound such as, for example, an organotin compound. This component comprising the catalyst may also comprise the vulcanizing agent.
Such compositions may also be in the form of one-component systems which vulcanize at and above room temperature in the presence of water.
The preparation of concentrated suspensions (slurries) of reinforcing silicas in reactive silicone oils which are intended to produce polycondensation-vulcanized elastomers poses problems of mixing the pulverulent filler with the oil, and care must be taken particularly 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. Specifically, it is clear that the introduction of a pulverulent particulate filler of very small particle size into silicone oil necessarily induces an appreciable increase in viscosity. Now, this characteristic, although accompanying the production of good mechanical properties for vulcanized silicone elastomers comprising the suspension as starting material, is harmful to the handling and forming of the suspension and of the silicone compositions containing it. Specifically, it is more convenient, for molding, extrusion or forming, to work with fluid compositions which are easier to pump, flow or mix with functional additives, inter alia.
The problem considered herein may thus be summarized as the search for a technical compromise between specifications, which are in principle antinomic, for suspensions of fine particulate fillers in silicone oils, namely: (i) the fineness of distribution of the particles in the silicone matrix; (ii) the adaptation of the rheology of the suspension to the handling constraints, and (iii) the level of the mechanical properties of the vulcanized silicone elastomers.
Document FR-A-2 717 180 addresses this problem and discloses a process for preparing two-component filled compositions, which may be vulcanized into elastomers by polycondensation reactions, the characteristic of which lies in the fact that the filler used, which is usually a siliceous filler, and which is mixed with the reactive POS bearing condensable functions, is treated with a compatibilizer, in particular of silazane type, before being incorporated into the composition, this treatment being imperatively and exclusively performed in an unreactive POS, in particular a polydialkylsiloxane containing trialkylsilane end groups. This mode of treatment may be carried out in two stages, by first mixing the unreactive POS and the filler with a fraction of the treatment agent and, after homogenizing this first mixture, incorporating the remaining treatment agent therein. This mode of treatment gives relatively fluid filler suspensions, but the Applicant is of the opinion that it is not without adverse consequences on the elastomers prepared from compositions comprising this suspension: specifically, there is a fear that the unreactive POS, which is not bound to the surface of the silica and, consequently, is poorly attached to the silicone network created during the vulcanization, may exude from the elastomeric articles made.
In such a technical context, one of the essential objects of the present invention is to provide a process for preparing a suspension of a particulate filler treated with a compatibilizer, in a silicone oil, this suspension being potentially useful as a starting material for producing compositions which may be vulcanized or cured by polycondensation reactions into polycondensation RTV elastomer, said process needing to satisfy the following specifications:
uniformity and homogeneity of the distribution of the filler in the silicone oil,
optimization of the dispersion,
suspension which flows well (with no flow threshold) and viscosity suitable for handling and converting the suspension,
good stability on storage (flowability, viscosity, setting time) of curable compositions comprising the suspension,
elastomers with mechanical properties (in particular elongation at break and tear strength) of an acceptable level. Another essential object Qf the invention is to provide a process for preparing a reinforcing filler/silicone oil suspension for polycondensation RTV elastomers, which is simple to use, economical and applicable on an industrial scale.
Another essential object of the invention is to provide a process for obtaining a silicone composition which may be vulcanized or cured by polycondensation, comprising the suspension as obtained by the process intended above as constituent element, having good stability on storage and giving elastomers with mechanical properties of an acceptable level, whether this composition is in the form of a one-component composition or in the form of a two-component composition.
These objects, inter alia, are achieved by the present invention, which relates to a process for preparing a suspension of a particulate reinforcing filler, preferably of siliceous nature, in a silicone material (SM
Canpont Dominique
Howe Fabienne
Pouchelon Alain
Moore Margaret G.
Rhodia Chimie
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