Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Compositions to be polymerized by wave energy wherein said...
Reexamination Certificate
2000-11-22
2002-03-19
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Compositions to be polymerized by wave energy wherein said...
C521S092000, C521S095000, C521S097000, C521S154000, C522S081000, C522S083000, C522S148000
Reexamination Certificate
active
06359026
ABSTRACT:
The present invention relates to a process for the production of silicone foams by means of ultrahigh-frequency waves.
Silicone foam materials have been known for some time for various areas of application via various production processes. The manifold areas of application include thermal insulation at elevated temperatures, the production of flexible seals, use as damping elements in the form of foams, inter alia. In these applications, use is made of the known properties of elastic silicone compositions: temperature stability, small change in mechanical properties at varying temperatures, good ageing stability, inter alia.
The various processes for the production of silicone foams are numerous and are used depending on the specific requirements.
Widespread use is made of silicone compositions containing silyl groups which simultaneously eliminate hydrogen during the crosslinking process. Suitable co-reactants are silanol groups, alcohols or even water. The hydrogen formed serves as blowing gas and generates the silicone foam having the desired pore structure, see, for example, EP-A 416 229. A further process is used in the production of silicone foam from heat-vulcanizing, peroxidically crosslinkable siloxane compositions. Here, substances which decompose at elevated temperatures are used as blowing agents which provide the expanded siloxane composition for the moment of vulcanization. This process has experienced widespread acceptance in industry and is described, for example, in U.S. Pat. No. 2,857,343.
Processes have also been described in which use is made of the gas solubility, for example in moisture-curing silicone compositions, in particular at elevated pressure. When the compositions are depressurized, the solubility drops suddenly, and the gas bubbles forming produce the desired silicone foam, which is then crosslinked by, for example, contact with water from atmospheric moisture, see, for example, U.S. Pat. No. 4,229,548.
Of these three systems, only the first allows the production of a solid, voluminous silicone foam; however, this satisfies only very limited demands regarding the mechanical properties, in particular with respect to strength and elasticity.
It has furthermore been attempted to use platinum-catalysed, addition-crosslinking silicone mixtures, in which, as is known, high elasticity and good rubber-mechanical properties can be achieved, to produce silicone foams and moulded silicone foam articles, see EP-A 751 173. The heat necessary to activate the blowing agent and for the vulcanization is supplied externally here, for example using a fan-assisted oven or, in the case of an injection moulding machine, using the heatable mould. A disadvantage here is that the foam volume which can be achieved is limited, since a heat-insulating effect sets in from the outside as the foam begins to form, hindering the passage of further energy inwards and thus excessively slowing or preventing further expansion and vulcanization in the core from a certain foam layer thickness, possibly after 2 to 3 cm. Thicker silicone foam boards or moulded shells, as could be used for thermal insulation at elevated temperatures, could therefore not be produced.
EP-A 497 565 has already described the production of silicone foams by means of ultrahigh-frequency (UHF) waves in the presence of azodicarboxamide as UHF-active substances. However, contamination of the crosslinking system through the use of azodicarboxamide and dinitrosopentamethylenetetramine has been observed (see U.S. Pat. No. 5,246,973).
There was therefore a need for a process for the production of voluminous silicone foams which does not have the disadvantages of the prior art.
Surprisingly, it has now been found that the above object can be achieved extremely well by means of addition- or condensation-crosslinking silicone compositions initiated by UHF waves if they contain certain carbonates and/or hydrogencarbonates and certain UHF-active substances. This combination allows the production, in a short time, of elastic, voluminous silicone foam elements.
The present invention relates to a process for the production of silicone foams by means of ultrahigh-frequency waves, characterized in that the addition- or condensation-crosslinking silicone compositions contain, as constituents, alkali metal and/or ammonium carbonates and/or alkali metal and/or ammonium hydrogencarbonates and UHF-active substances.
In a preferred embodiment of the invention, the addition-crosslinking silicone composition used is a mixture of
a
1
) 100 parts by weight of at least one vinyl group-containing linear or branched organopolysiloxane containing at least 2 vinyl groups and having a viscosity of from 0.1 to 1000 Pa.s,
b
1
) from 3 to 200 parts by weight, preferably from 5 to 50 parts by weight, of at least one, optionally surface-modified filler,
c
1
) from 0.5 to 10 parts by weight, preferably from 1 to 8 parts by weight, of hydrogensiloxane containing at least 3 SiH functions per molecule,
d
1
) from 0.01 to 100 ppm, preferably from 0.03 to 50 ppm, of platinum in the form of a platinum catalyst,
e
1
) from 0.01 to 5 parts by weight, preferably from 0.03 to 3 parts by weight, of an inhibitor.
The vinyl group-containing organopolysiloxanes (a
1
) are preferably linear or branched organopolysiloxanes containing at last 2 vinyl groups whose viscosity, measured at 20° C. using a rotational viscometer, is in the range from 0.1 to 1000 Pa.s. Particular preference is given to vinyl-terminated polydimethylsiloxanes having a viscosity of from 0.2 to 150 Pa.s, optionally mixed with the dimethylsiloxanes containing pendant vinyl groups.
All viscosity data were measured at 20° C. in accordance with DIN 53 019.
Examples of fillers (b
1
) are extender fillers, such as, for example, quartz sand or cristobalite flour and, precipitated or pyrogenic silicas, whose surface is preferably treated, before or during the mixing process, with substances known per se, such as, for example, silazanes, with or without addition of water.
Component b
1
) is preferably finely divided pyrogenic or precipitated silica, which has optionally been surface-modified with hexamethyldisilazane and/or tetramethyldivinyldisilazane.
For the purposes of the invention, component c
1
) comprises known polyorganosiloxanes carrying hydrogen atoms on at least three silicon atoms, such as, for example, compounds of the formula
where R=C
1
-C
8
alkyl or C
6
-C
8
aryl,
m is=3, and
m+n=3−1000, and where the —SiOR
2
and —SiRHO units are randomly distributed in the molecule.
The Pt catalyst (d
1
) is preferably a Pt complex which catalyzes the adduction of Si—H groups onto vinylsiloxanes. Preference is therefore given to Pt(0) complexes containing vinylsiloxanes as ligands. However, it is also possible to use other metal complexes, such as, for example, Rh compounds. Preference is given to Pt(0) complexes containing vinylsiloxane ligands which are soluble in the siloxane polymers.
Suitable inhibitors (e
1
) are all compounds which permit a targeted reduction in the crosslinking rate, but do not cause irreversible damage to the catalyst. Particular preference is given to short-chain or cyclic polydimethylsiloxanes containing a plurality of adjacent vinyl groups on the silicon atoms, such as, for example, tetramethyldivinyldisiloxane, ethinylcyclohexanol and/or tetramethyltetravinylcyclo-tetrasiloxane as component c).
In a further embodiment of the invention, the condensation-crosslinking silicone composition used is a mixture of
a
2
) 100 parts of a linear or branched organopolysiloxane containing at least two silanol groups, where the viscosity is preferably in the range from 100 to 1,000,000 mPa.s,
b
2
) from 3 to 200 parts, preferably from 5 to 50 parts, of at least one filler which has optionally been surface-modified,
c
2
) from 1 to 15 parts by weight of a silane crosslinking agent selected from the series consisting of carboxamide-eliminating silanes of the formula
CH
3
Si(OC
2
H
5
)X
2
,
where X is either C
6
H
5
CON(CH
3
)— or
or
oxime
Hurnik Helmut
Marquardt Gerwig
Naumann Thomas
Foelak Morton
General Electric Company
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