Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From silicon reactant having at least one...
Reexamination Certificate
1995-02-01
2001-09-25
Dawson, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From silicon reactant having at least one...
C524S403000, C525S474000, C525S475000
Reexamination Certificate
active
06294635
ABSTRACT:
FIELD OF INVENTION
The present invention relates to addition-crosslinking silicone rubber compositions which crosslink to give a silicone rubber of increased heat resistance because they comprise a palladium-containing heat stabilizer, and to the use of palladium and compounds thereof as a heat stabilizer in addition-crosslinking silicone rubber compositions.
BACKGROUND OF INVENTION
Silicone rubbers have a higher heat stability than organic elastomers. Nevertheless, relatively long open storage at temperatures above 200° C. leads to an increasing loss in mechanical strength and elasticity, which is called embrittlement. Furthermore, a rapid weight loss is recorded at temperatures above 300° C., this being accompanied by a silicification of the silicone rubber. For these reasons, silicone rubbers which have not been heat-stabilized are of only limited suitability for applications which include exposure to temperatures above 220° C. over a relatively long period of time. Possible causes of the loss in elastomeric properties of the silicone rubber at higher temperatures are, thermal and thermooxidative changes in the organic side groups of the polymer chains, crosslinking sites being re-formed and depolymerization processes, such as cleavage of SiOSi bonds and formation of rings.
However, the heat stability of silicones can be improved considerably by addition of so-called heat stabilizers, such as the elements and compounds of the transition metals and lanthanides. For example, EP-A-231 519 describes the use of acetylacetonates, as heat stabilizers for addition-crosslinking 2-component silicone rubber compositions, of copper, zinc, aluminum, iron, cerium, zirconium and titanium.
There are considerable differences in the activity of the metal-containing heat stabilizers used. These differences in the activity of the metals are, also the consequence of the various forms of application, such as metal dust, dispersion of insoluble metal compounds or silicone-soluble metal compounds, and are founded in the different composition of the silicone materials to be stabilized, such as silicone oils or peroxide-, condensation-, addition- or radiation-crosslinking silicone elastomers. Nevertheless, it can be said that above all the 3d transition metals, in particular Ti, Mn, Fe, Co, Ni and Cu, the sub-group elements of the 4th period, that is to say Ti, Zr and Hf, and the lanthanides, above all Ce, have proved to be metals having a heat-stabilizing action.
The publication “Izv. Vyssh. Uchebn. Zaved., Khim, Khim, Tekhnol., 1992, 35 (6), 66-71” describes metal complexes of a macrocyclic triisoindole-benzene compound, which are comparable structurally to phthalocyanine complexes and contain either Zr, Y, Pd or Al as metals, as stabilizers for increasing the heat stability and flame resistance of condensation-crosslinking silicone rubbers. The heat-stabilizing action of these complex compounds is evaluated only by the loss in weight during storage in the open at 450° C., the heat-stabilizing efficiency increasing in the sequence of metals Al<Pd<Y<Zr. However, the loss in weight of the silicone rubber which occurs during storage at 450° C. is not conclusive for the retention of the elastomer properties of stabilized silicone rubbers at typical upper use temperatures, which are between 200° C. and 300° C. The lower heat-stabilizing action of Ni
3
O
4
and Co
2
O
3
used for comparison and the virtually comparable action of the metal-free triisoindole-benzene compounds suggest that it is above all a stabilizing effect of the triisoindole-benzene component in the metal complexes.
The object was to provide particularly heat-stable addition-crosslinking silicone rubber compositions.
SUMMARY OF INVENTION
The present invention relates to addition-crosslinking silicone rubber compositions which crosslink to give a silicone rubber having an increased heat resistance, wherein the silicone rubber compositions comprise a palladium-containing heat stabilizer.
Palladium-containing addition-crosslinked silicone rubbers have a significantly higher heat stability than silicone rubbers which comprise other metals as the heat-stabilizing additive. It is thus possible for the use properties of silicone rubbers to be maintained for a longer time at higher temperatures than previously. In particular, the mechanical elastomer properties, such as Shore A hardness, tensile strength, complex dynamic modulus under compressive deformation, elongation at break and tear strength, are retained to a particularly high degree after exposure to heat.
Addition-crosslinked silicone rubbers which comprise palladium for the purpose of heat stabilization are pale yellowish, yellowish-pale brown to dark brown in color, depending on the palladium content. If the palladium-containing heat stabilizer is sufficiently finely distributed, the transparency of the silicone rubber is not adversely affected. In contrast to other heat stabilizers, such as carbon black, even very small amounts of palladium have proved to be sufficient to achieve the desired heat-stabilizing effect.
In view of the numerous metals and metal compounds of which a heat-stabilizing activity is known, it is astonishing that the exceptionally high heat-stabilizing activity of palladium and compounds thereof has so far remained ignored. The exceptional heat stability of palladium-containing addition-crosslinked silicone rubbers during storage in the open with access of fresh air is astonishing and unexpected, however, in as much as palladium is known as a catalyst for total oxidation of organic compounds.
The palladium-containing heat stabilizer is called constituent (I) below. The palladium in itself can be introduced into the silicone rubber composition in any desired form and in any desired manner. However, the activity as a heat stabilizer is better at a fine distribution.
Elemental palladium, such as palladium powder, colloidal palladium and palladium black, if appropriate fixed on finely divided support materials, such as active charcoal, carbon black, aluminum oxide, barium sulfate or carbonate, calcium sulfate or carbonate, quartz flour or pyrogenic or precipitated silica, can be used as constituent (I). The particle size of the palladium particles and of the carrier materials is preferably below 100 &mgr;m, in order to ensure fine distribution of the palladium in the silicone rubber composition.
Constituent (I) can also be palladium in the form of its compounds. In this case, the palladium compound, or a mixture of various palladium compounds, is dissolved, partly dissolved or dispersed in a constituent or a mixture of several constituents of the silicone rubber composition according to the invention, with the aid of a solvent or a solvent mixture and/or at elevated temperature. Any desired palladium compounds can be employed. The palladium compounds employed are preferably anhydrous or water-containing compounds of the general formulae PdX
2
, L
2
PdX
2
, L
2
Pd
2
X
2
, L
4
PdX
2
, L
2
PdX
4
, PdL
4
, M
2
PdX
4
or M
2
PdX
6
, in which X is a halide, such as Cl, Br and I, a complex inorganic acid radical, such as NO
2
, NO
3
, 0.5 SO
4
and CN, a carboxylic acid radical of the general formula OCOR′, in which R′ is an unsubstituted or substituted alkyl, cycloalkyl or aryl radical having 1-20 C atoms, in particular acetate and trifluoroacetate, or a complexing unsubstituted or substituted organic anion, such as acetylacetonate, hexafluoroacetylacetonate and 0.5 phthalocyaninato, or OH, L is a donor which is chosen from the group consisting of nitrogen-containing ligands, such as NH
3
, primary, secondary and tertiary amines of the formula NR″
3
, in which R″ can be a hydrogen atom or an alkyl, cycloalkyl or aryl radical, diamines, and bipyridyl, from the group consisting of phosphorus-containing ligands, such as PR″
3
and Ph
2
P—(CH
2
)n—PPh
2
, in which n has the values 1 or 2, CO, H
2
O, dimethylformamide, dimethylsulfoxide, nitriles, in particular dibenzonitrile and acetonitrile, and the dienes, such as 0.5 cyclooctadiene
Achenbach Frank
Barthel Herbert
Brooks & Kushman P.C.
Dawson Robert
Wacker-Chemie GmbH
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