Note: Descriptions are shown in the official language in which they were submitted.
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Addition-crosslinkin~ silicone rubber blends, a process for their preparation
and
' their use
The present invention relates to addition-crosslinking silicone rubbers, a
process for
their preparation and their use.
Addition-crosslinking silicone rubbers are distinguished by a broad range of
use. This
also includes, for example, use as high-voltage insulators, arresters, etc.
For this field of use, it is necessary for the organopolysiloxane elastomers
to have high
arc resistance and high creep resistance in combination with good mechanical
properties.
It is known that the arc resistance and creep resistance are improved by the
addition of
aluminum dioxides and/or aluminum hydroxides but the mechanical properties are
relatively poor. In addition, these materials cannot be processed on injection
molding
machines, owing to the high viscosity.
DE-A-3 831 478 discloses the use of addition-crosslinking silicone elastomers
which
may contain titanium dioxide and zinc oxide in addition to the extender
fillers described
above. These materials may be crosslinked under platinum catalysis but they
have the
disadvantage that they do not have a long shelf life in the uncrosslinked
state. DE-A-
3 048 207 furthermore discloses flameproof materials which also contain
magnesium
oxide and optionally further additives, such as, for example, Ti02, in
addition to
hydrated alumina.
However, these materials disadvantageously have a short shelf life and too low
a creep
resistance.
It was therefore the object of the present invention to provide addition-
crosslinking
polysiloxane materials which have creep resistance and arc resistance, do not
have the
disadvantages of the prior art and can be processed by the injection molding
technique.
It has now been found that addition-crosslinking organopolysiloxane materials
which,
in addition to oxidic aluminum compounds, also contain zinc oxide and
optionally
titanium dioxide have the desired property profile.
The invention therefore relates to addition-crosslinking silicone rubber
blends
essentially consisting of
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(a) 20 to 40% by weight of at least one linear or branched organopolysiloxane
containing alkenyl groups and having a content of 0.0002 to 5% by weight of
alkenyl groups with a viscosity of 0.1 to 1000 Pas at 25°C,
(b) at least one hydrogensiloxane having at least 3 SiH functions per molecule
in an
amount such that the molar ratio of SiH groups to the total amount of Si-
bonded
alkenyl groups is at least 2.0:1,
(c) 0.01 to 250 ppm of at least one Pt catalyst and optionally an inhibitor,
(d) 35-55% by weight of aluminum hydroxide, alumina and/or mixed
oxides/hydroxides thereof,
(e) 5 to 25% by weight of at least one, optionally surface-modified filler
having a
specific surface area of from 150 to 500 m2/g
(fj 1 to 5% by weight of at least one metal oxide of zinc having a BET surface
area
of 30 to 70 m2/g, optionally in combination with 0 to 5% by weight of at least
one metal oxide of titanium, having a BET surface area of 35 to 65 m2/g
and optionally
(g) further auxiliaries, the sum of all components being 100% by weight.
The term organopolysiloxane (a) includes all polysiloxanes used to date in
organopolysiloxane materials. It is preferably a siloxane comprising units of
the
general formula (I)
(R)ySiO~a-y>iz
in which
R denotes a monovalent aliphatic radical having 1 to 8 carbon atoms and an
alkyl
radical having 2 to 8 carbon atoms,
y is 1.95 to 2.01, and which contains at least 0.0002 to 5% by weight of
alkenyl
groups.
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Preferably, (a) is dimethylvinylsilyloxy-terminated.
In a preferred embodiment of the invention, the organopolysiloxanes (a)
according to
the invention have viscosities of 0.01 to 1000 Pas, very particularly
preferably 10 to
80 Pas.
The viscosities are determined according to DIN 53 019 at 20°C.
In the context of the invention, hydrogensiloxanes (b) are preferably linear,
cyclic or
branched organopolysiloxanes comprising units of the general formula (II)
(Ri)W(H)ZSi0~4_W_Z~i2 (II),
in which
R1 is a monovalent aliphatic radical having 1 to 8 carbon atoms,
w is0,l,2or3,
z is 0, 1 or 2
and the sum w+z is 0, 1, 2 or 3,
with the proviso that on average at least 3, preferably 10 to 20, Si-bonded
hydrogen
atoms are present per molecule.
The hydrogensiloxanes (b) preferably have a viscosity of 0.03 to 1 Pas.
The hydrogensiloxanes (b) may additionally contain organopolysiloxanes whose
number of SiH groups is less than 3.
Catalysts (c) for the crosslinking reaction are preferably Pt (0) complexes
with alkenyl
siloxanes as ligands in catalytic amounts of 0.01 to 250 ppm of Pt.
In the context of the invention, inhibitors are all customary compounds which
have
been used to date for this purpose. Examples of such preferred inhibitors are
e.g.
1,3-divinyl-tetramethyldisiloxane, 1,3,5,7-tetravinyl-1,3,5,7-
tetramethylcyclotetrasilox-
ane, 2-methylbutin-2-of or 1-ethinylcyclohexanol in amounts of 50 to 10,000
ppm.
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Aluminum hydroxide, alumina and/or the mixed oxides/hydroxides thereof, such
as, for
example, Al0(OH), are preferably those compounds which have a BET surface area
of
3 to 6 m2/g, measured by means of N2 absorption.
In the context of the invention, fillers (e) are preferably reinforcing
fillers, such as, for
example, pyrogenic silica having a BET surface area between 50 and 400 m2/g,
measured by means of N2 absorption, which may also be surface-treated, and/or
extender fillers, such as, for example, quartz powder.
The surface treatment may also be carned out in situ by the addition of
silazanes, such
as hexamethylsilazane and/or divinyltetramethyldisilazane, and
vinylalkoxysilanes,
such as, for example, vinyltrimethoxysilane, and water or other customary
filler loading
compositions.
1 S Metal oxides (f) of zinc are preferably those compounds which have a BET
surface area
of 45 to 55 m2/g, and oxides of titanium which have a BET surface area of 45
to
55 m2/g, measured by means of N2 absorption.
In a further preferred embodiment of the invention, the blend contains further
auxiliaries (g), such as color-imparting, inorganic pigments, such as iron
oxides or
cobalt spinets.
The invention also relates to a process for the preparation of the addition-
crosslinking
silicone rubber blends according to the invention, according to which 1. at
least one
organopolysiloxane (a), catalyst (c), aluminum hydroxide (d) and metal oxides
(f),
optionally fillers (e) and/or auxiliaries (g) are mixed and 2, at least one
organopolysiloxane (a), at least one hydrogensiloxane (b) and optionally
fillers (e) and
aluminum hydroxide (d) auxiliaries (g) and/or inhibitor (c) and metal oxides
(f) are
mixed as a separate mixture, and these two mixtures are combined only in the
injection
molding machine [lacuna] in an upstream mixing head with subsequent static
mixer.
Mixing is preferably effected by means of mixers suitable for high-viscosity
materials,
such as, for example, kneaders, dissolvers or planetary mixers.
In one embodiment of the process according to the invention, the filler is
rendered
hydrophobic, the imparting of hydrophobic properties preferably being effected
in situ
by the addition of hexamethyldisilazane and/or divinyltetramethyldisilazane
and water.
In the in situ imparting of hydrophobic properties, preferably
organopolysiloxane (a),
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filler (e) and aluminum hydroxide (d) and the water repellent, preferably
' hexamethyldisilazane and/or divinyltetramethyldisilazane, are stirred,
preferably at
temperatures of 90-100°C for at least 20 minutes in a mixing unit
suitable for high-
viscosity materials, such as, for example, a kneader, dissolver or planetary
mixer, and
then freed at Temperatures 150-160 C from excess loading compositions and
water,
initially at atmospheric pressure and then in vacuo at a pressure of 100 to 20
mbar. The
further components (b and f) or (c), (f) and optionally (g) are then mixed in
over 10 to
30 minutes.
The invention also relates to the use of the addition-crosslinking silicone
rubber blends
according to the invention for the preparation of silicone elastomers having
creep
resistance and arc resistance.
The following Examples, in which all parts denote parts by weight, illustrate
the
invention but without restricting it.
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Examples
Example 1
In a dissolves having a planetary gear, 75 parts by weight of a vinyl-
terminated
polydimethylsiloxane mixture having an average viscosity of 40 Pa.s at
25°C (a) were
mixed with 10 parts by weight of hexamethyldisilazane and 5 parts by weight of
water
and then stirred with 31 parts by weight of pyrogenically prepared silica
having a
specific surface area of 300 m2/g according to BET (e), 110 parts by weight of
low-
sodium aluminum trihydroxide (d) and 2 parts by weight of pyrogenically
prepared
titanium dioxide (fj to give a homogeneous material. The mixture was first
heated to
100°C and stirred for 2 hours in the closed dissolves and then freed
from water and
excess silazane at 160°C in vacuo.
After the material had been cooled to 90°C, 33 parts by weight of a
vinyl-terminated
polydimethylsiloxane mixture having an average viscosity of 5 Pa.s at
25°C (a) and
0.05 parts by weight of a platinum catalyst in the form of a complex of
chloroplatinic
acid with symmetrical divinyltetramethyldisiloxane and containing 0.15% of
platinum
(c) were added. The material thus obtained is referred to as component A
below.
In a dissolves having a planetary gear, 75 parts by weight of a vinyl-
terminated
polydimethylsiloxane mixture having an average viscosity of 40 Pa.s at
25°C (a) were
mixed with 10 parts by weight of hexamethyldisilazane and 5 parts by weight of
water
and then stirred with 32 parts by weight of pyrogenically prepared silica
having a
specific surface area of 300 m2/g according to BET (e), 110 parts by weight of
low-
sodium aluminum trihydroxide (d) and 2 parts by weight of pyrogenically
prepared
titanium dioxide (f) to give a homogeneous material. The mixture was first
heated to
100°C and stirred for 2 hours in the closed dissolves and then freed
from water and
excess silazane at 160°C in vacuo.
After the material had been cooled to 90°C, 22 parts by weight of a
vinyl-terminated
polydimethylsiloxane mixture having an average viscosity of 10 Pa.s at
25°C (a), 4
parts by weight of activated zinc oxide (f), 22 parts by weight of a mixture
of
trimethylsilyl-terminated polyorganosiloxane crosslinking agent having on
average 8
methylhydrogensiloxane units and 100 dimethylsiloxane units per molecule (b)
and
about 0.2 parts by weight of ethinylcyclohexanol as inhibitor (c) were added.
The
material thus obtained is referred to as component B below.
50 parts of each of the components A and B described above were stirred
together and
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sheets having a thickness of 2 or 6 mm were produced by vulcanization at
175°C for
minutes. The sheets were then heated for 4 hours at 200°C in a forced-
circulation
oven. For all vulcanized products thus produced, the reactivity, the
mechanical
properties and the arc resistance according to DIN 57 441 and creep resistance
5 according to IEC Publ. 587 were tested.
Table 1
Reactivity t60 after 1 day 3.7 sec
Reactivity t60 after 21 days 3.9 sec
Reactivity t60 after 183 days 4.2 sec
Tensile strengthDIN 53 504 S2 4.0 N/mm2
Elongation DIN 53 504 S2 600%
Shore A hardnessDIN 53 505 40
Arc resistance DIN 57 441 HL 2
Creep resistanceIEC Publ. 587 1 A 3.5
