Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
1999-07-07
2001-02-27
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S154000, C523S219000
Reexamination Certificate
active
06194476
ABSTRACT:
The present invention relates to use of a compressible silicone composition, in particular use of a compressible silicone composition formable from a silicone gel-forming composition and hollow compressible filler for providing an electrically insulating seal.
Silicones have, in general, many well known advantages over organic polymers, for example high temperature resistance, low surface tension, high chemical resistance, low toxicity, high moisture resistance, high dielectric strength and high flame resistance. However, a disadvantage silicones have, in general, versus organic polymers is their high coefficient of thermal expansion. For example, when a silicone elastomer is used to fill a space of fixed volume, an increase in environmental temperature will result in the silicone elastomer causing undue mechanical stress on the surface(s) defining the space. It is known to fill silicone rubber compositions with hollow compressible microspheres to counteract effects caused by thermal expansion; when the silicone rubber expands within a space of fixed volume the microspheres compress under the force exerted thus reducing the stress on the surface(s) defining the space. For example, U.S. Pat. No. 5,258,212 discloses a curable liquid silicone rubber-forming composition which comprises fine hollow microspheres and which has good vibration damping characterisics for use in packing for electronic components, and EP-A-0771842 discloses a silicone rubber composition for forming articles and sealing bodies, which composition comprises a cross-linkable silicone rubber-forming composition and hollow bodies of plastic. Such silicone compositions filled with hollow bodies can be from 20 to 40 times more compressible than the corresponding unfilled composition, and thus provide a silicone composition which has the aforementioned advantages of silicones as well as high compressibility to counteract the effects of high thermal expansion. For brevity, compositions containing compressible hollow bodies will hereinafter be referred to as “compressible” compositions.
However, the present inventors have found disadvantages in using compressible silicone rubber compositions. For example, if a cavity is required to be 100% filled with a compressible silicone composition (for example, where a gas-tight seal is required within the cavity or where the compressible silicone is acting as an electrical insulator within the cavity) over a wide temperature range, environmental temperature decrease will cause a compressible silicone rubber composition to shrink within the cavity. As shrinkage occurs, separation of the compressible silicone rubber from the cavity walls can occur resulting in gaps appearing between the compressible silicone composition and the cavity walls. The cavity would thus no longer be gas-tight or electrically insulated.
We have now found that if a silicone gel-forming composition is used in the formation of a compressible silicone composition instead of a prior art silicone rubber-forming composition then the aforementioned advantages of the prior art compositions are kept but the disadvantages of compressible silicone rubbers are reduced.
According to the present invention there is provided use of a silicone gel-forming composition for providing an electrically insulating seal, which silicone gel-forming composition comprises a mixture of organosilicon compounds which can crosslink to form a silicone gel and hollow compressible microspheres.
Silicone gels are well known in the art, and are often characterised by their physical properties, as they usually have a relatively high flexibility and penetration. For example, U.S. Pat. No. 4,861,804 discloses a silicone gel composition containing hollow microspheres for use as a shock absorbing material or as a sound and vibration-proof material. They tend to be flowable under pressure, have some tackiness and sometimes are self-healing. Such physical properties result from the low crosslink density of silicone gels relative to the much higher crosslink densities of other types of silicone elastomers, such as silicone rubbers. Crosslinkable silicone gel-forming compositions usually comprise a siloxane polymer and organosilicon crosslinker, and an organosilicon crosslinker both having reactive groups which allow reaction of the polymer with the crosslinker. The crosslink density the number of reactive groups of the siloxane polymer which react with reactive groups of the organosilicon crosslinker.
Thus, the aforementioned physical properties of a silicone gel are desirable and may be achieved by controlling the crosslink density to keep it at a suitably low level, and this may be achieved in a number of ways. For example, where said organosilicon compounds comprise a siloxane polymer and an organosilicon crosslinker, a low crosslink density may be achieved by having an excess of siloxane polymer reactive groups compared to organosilicon crosslinker reactive groups. This ensures that some siloxane polymer reactive groups will always remain unreacted. An organosilicon crosslinker reactive group/siloxane polymer reactive group ratio of 0.5/1 to 1/1 is typical for silicone gel formation.
Another way of achieving a low crosslink density is to have a low total number of reactive groups available for crosslinking, i.e. substantially all of the available reactive groups do undergo a crosslinking reaction but are only present in sufficient numbers to result in a low crosslink density. For example, the siloxane polymer may have only terminal reactive groups together with a relatively high viscosity giving a greater distance between reactive groups, or the crosslinker may have a reduced number of reactive sites.
A further way of achieving a low crosslink density is to employ a suitable inhibitor or catalyst deactivator to inhibit crosslinking from proceeding once a desired crosslink density has been reached. Which particular inhibitor to use will depend on the particular crosslinking mechanism in question. Such inhibitors are well known to the skilled person and include acetylenic alcohols, alkyl maleates, alkyl fumarates, organic peroxides, sulphoxides, amines, amides, phosphines, phosphites, nitrites and oximes.
Preferred organosilicon compounds for use in the silicone-gel forming composition according to the present invention comprise a siloxane polymer an organosilicon crosslinker.
Useful siloxane polymers comprise units of the general formula R
a
R′
b
SiO
(4-a-b)/2
(I), wherein R is a monovalent hydrocarbon group having up to 18 carbon atoms, R′ is a monovalent hydrocarbon or hydrocarbonoxy group, a hydrogen atom or a hydroxyl group, a and b each have a value of from 0 to 3, with the sum of a+b being no more than 3.
Preferably the siloxane polymers are substantially linear polyorganosiloxanes having the general structure (II)
wherein R and R′ have the same meaning as above, and wherein x is an integer, preferably having a value of from 10 to 1500. It is particularly preferred that R denotes an alkyl or aryl group having from 1 to 8 carbon atoms, e.g. methyl, ethyl, propyl, isobutyl, hexyl, phenyl or octyl. More preferably at least 50% of all R groups are methyl groups, most preferably substantially all R groups are methyl groups. R′ may be an alkoxy group, for example having up to 3 carbon atoms, but is preferably selected from an aliphatically unsaturated hydrocarbon group or a hydrogen atom. More preferably R′ denotes an alkenyl group having up to 6 carbon atoms, more preferably vinyl, allyl or hexenyl, suitable for addition reactions. A mixture of siloxane polymers may be used, for example a mixture of higher and lower visosity polymers, e.g. a first siloxane having a visosity at 25° C. of from approximately 500 mm2/s to 2500 mm
2
/s and a second siloxane having viscosity at 25° C. of from 5000 mm
2
/s to 50000 mm
2
/s.
Organosilicon crosslinkers may be selected from silanes, low molecular weight organosilicon resins and short chain organosiloxane polymers. The crosslinker has at least 3 silicon-bonded substi
De Ridder Lucrece
Mollie Jean-Paul
Dow Corning S.A.
Foelak Morton
Milco Larry A.
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