Note: Descriptions are shown in the official language in which they were submitted.
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Silicone rubber material
FIELD OF THE INVENTION
The present invention relates to a silicone rubber material.
Such a material is particularly useful for widely varying
applications at low temperatures. One particularly useful
such application is as an insulator for electrical apparatus
placed outdoors. The invention also relates to a method for
manufacturing such a material.
BACKGROUND OF THE INVENTION
Silicone rubber material is usually based on polyalkyl- =
siloxane, most often polydimethylsiloxane, designated PDMS
in this application, which in the temperature range of
about -35 - 150 C is a chemically and physically stable
material, with good mechanical and electrical properties,
that may be used for many different technical purposes. 5i-
licone rubber material is cross-linked, for example, by
supplying a suitable organic peroxide that reacts with
groups on the main chain of PDMS and hence bond the macro-
molecules together. The fundamental chemistry for silicone
rubber and its crosslinking is clear, for example, from F
Billmeyer, Textbook of polymer science, John Wiley & Sons
Ltd, pp. 482-484. Another way to achieve crosslinking is to
add a platinum catalyst that breaks up double bonds on
vinyl groups and renders them reaction-prone with respect
to adjoining siloxane chains.
It is well known that silicone rubber material is used for
different electrical insulation purposes, which is clear,
for example, from R Hackham, Outdoor HV Composite Polymeric
Insulators, IEEE Trans Dielectrics and Elec. Insul., Vol. 6
(1999), pp. 557-585.
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To the pure polysiloxanes there may be mixed fillers, ,both
so-called bulk fillers such as, for example, silicon dioxi-
de or quartz, and fibre fillers such as, for example, short
or long glass fibres. Examples of silicone rubber materials
with bulk fillers are clear from EP 1 052 655 Bl.
Silicone rubber material, wholly based on PDMS, starts
crystallizing at about -40 C and hence the material
stiffens and experiences brittleness. For non-crosslinked
silicones with a molecular weight of about 2000 u, the so-
called freezing point by be lowered by replacing methyl
groups on the main chain of PDMS by phenyl groups. Phenyl
groups are larger than methyl groups and hence suppress
structural order. This has been described in Warrick et
al., Polymer Chemistry of the Linear Siloxanes, Industrial
Engineering Chemistry, 1952, p. 2199. However, the cost of
polydiarylsiloxane is high relative to polydialkylsiloxane,
and for that reason polydiarylsiloxane cannot be used for
many electrical applications.
EP 0 470 745 A2 describes that 100 parts by weight of an
organopolysiloxane gum, with the formula for the average
repeating unit being RaSiO4-A/2, where R is a monovalent
=
hydrocarbon group, with the proportion of alkyl being at
least 50 per cent, and a is a number between 1.98 and 2.02,
have been mixed with inorganic fillers and 1-20 parts by
weight of an organosilane or organosiloxaneoligomers
according to the formula
R1 CH3
OH- [-SiO - [-SO - H
R2 CH3
where m is between 1 and 20 and n is between 0 and 20. In
addition, an organic peroxide is added.
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By organopolysiloxane gum is meant a substituted or un-
substituted monovalent hydrocarbon group. Examples of such
groups are alkyl, alkenyl, cykloalkyl and aralkyl groups.
According to C.A. Hampel and G.G. Hawley, Glossary of
Chemical Terms, Van Nostrand Reinhold Company, 1976, p.
196, oligo is a "prefix derived from Greek, meaning
"several" or "slight"; in chemistry it appears in such
terms as oligosaccharides (containing from three to ten
monosaccharide units) and oligodynamic (slight bactericidal
ability)".
EP 1 079 398 A2, Example 1, p. 10, describes that 80 parts
by weight dimethylpolysiloxane is grafted with dimethyl-
vinylsiloxy groups at both ends of the respective dimethyl-
polysiloxane molecule, whereby 40 parts by weight of Si02
filler was also added. 40 parts by weight of this liquid
silicone rubber has then, in its turn, been mixed with 60
parts by weight of dimethylpolysiloxane with a lower vis-
cosity and a lower degree of polymerization than the one
mentioned above and 140 parts by eight aluminium hydroxide.
This latter mixture has then in turn, according to example
4, p. 12, been mixed with 120 parts by weight aluminium
hydroxide and 10 parts by weight of a dimethylpolysiloxane-
diphenylsiloxane copolymer grafted with dimethylvinylsiloxy
groups at both ends of the respective dimethylpolysiloxane-
diphenylsiloxane copolymer molecule. The diphenylsiloxane
groups make up 20% of the sum of dimethylsiloxane and the
diphenylsiloxane groups in the copolymer. Out of the sum of
polydimethylsiloxane and polydiphenylsiloxane, the poly-
diphenylsiloxane thus constitutes between 2 and 3 per cent
by weight. The object of EP 1 079 398 A2 is to create a
material that may be used for electrical applications and
that have tixotropic properties suitable for sealing and
repairing polymeric insulants, p. 2 lines 34-37.
According to C. A. Hampel and G. G. Hawley, Glossary of
Chemical Terms, Van Nostrand Reinhold Company, 1976, p. 69,
a copolymer is a "high-polymer substance, usually an elas-
tomer, made up of two or more different kinds of monomer,
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for example, styrene and butadiene. Copolymers are made by
simultaneous polymerization of monomers in the same opera-
tion, usually in emulsion form; as a result, the consti-
tuent monomers are combined into a common macromolecule, in
contrast to the blending of two separately polymerized
monomers".
In S. Wang and J. E. Mark, Reinforcement of elastomeric
poly(dimethylsiloxane) by glassy poly(diphenylsiloxane), J.
of Material Science 25 (1990), p. 66, 26.8 per cent by
weight poly(diphenylsiloxane) has been mixed into a
poly(dimethylsiloxane) network for research reasons.
Similarly, it has been established that there was 6.5 per
cent by weight of poly(diphenylsiloxane) in a
poly(dimethylsiloxane) network which has been manufactured
by in-situ polymerization. No industrial application has
been described.
In C. M. Kuo and S. J. Clarson, Investigation of the
Interactions and Phase Behavior in Poly(dimethylsiloxane)
and Poly(methylphenylsiloxane) Blends, Macromolecules
25(1992), p. 2193, linear poly(dimethylsiloxane) and linear
poly(diphenylsiloxane) have been mixed, for research
reasons, with a fraction of a volume of poly(dimethyl-
siloxane) up to 0.93. No crosslinking is described. No
industrial application is described for the use of these
mixtures.
For electrical outdoor applications in countries having
winter climate, it is required that the material should
maintain good mechanical properties down to temperatures of
-50 C. This applies, for example, to insulators for elec-
trical apparatuses that are placed in the open. Currently,
porcelain insulators are used, the mechanical properties of
which do not change significantly when the temperature
drops from about 100 C to about -50 C. Porcelain insulators
have the disadvantage that the material is brittle. This
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implies, for one thing, that if the insulator due to un-
fortunate circumstances, for example by a rapid increase in
pressure, is burst form inside, then sharp parts may spread
in a neighbouring region, and, for another, that a high
5 mechanical safety factor must be used during mechanical
dimensioning. Since porcelain has a relatively high densi-
ty, the latter means that porcelain insulators become
heavy. Porcelain insulators generally also entail a high
cost and the possibilities of tailor-making a geometry for
a porcelain insulator requires a considerable contribution
in material and process development.
OBJECTS OF THE INVENTION
It is a main object of the present invention to suggest a
silicone rubber material that withstands temperatures down
to -50 C, and reduces the above-mentioned disadvantages of
the prior art.
It is another object of the present invention to obtain
better mechanical properties for the silicone rubber
material over considerable parts of the temperature range
used than hitherto known materials. .
SUMMARY OF THE INVENTION
According to one embodiment of the present invention,
there is provided a silicone rubber material for
electrical insulation, wherein said material comprises
a mixture of a polyalkylsiloxane (A) and a
polyarylsiloxane (B), wherein
said mixture consists of polyarylsiloxane (B) in an
amount of 3-10 parts by weight per 100 parts by weight
of polyalkylsiloxane (A).
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5a
The material according to the invention is
characterized in that a component A consisting of a
polyalkylsiloxane is mixed with a component B
consisting of a polyarylsiloxane. In several respects,
polyarylsiloxanes have good properties but are
expensive and therefore it is an advantageous solution,
also from the cost point of view, to make a mixture.
Polyalkylsiloxane in this application means an
organopolysiloxane with the following average
composition:
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leriS3_0 (4_n) /2
where R is a substituted or unsubstituted monovalent
hydrocarbon group and n is a number between 1.98 and 2.02.
The number of repeating units may be from 100 to 20000. One
example of a polyalkylsiloxane is polydimethylsiloxane
which constitutes the main part of Powersillb 318.
Powersilisi 318 also contains an organic peroxide for cross-
linking
Polyarylsiloxane, in this application, means a polydi-
methylsiloxane where methyl groups on the main chain in the
molecules have been replaced by substituted or unsubstitu-
ted phenyl groups according to the below:
R5 R1
0
R4 R2
R3
where Rri, where n=1..5, are aryl or alkyl groups.
One example of a polyarylsiloxane is the main ingredient in
Wacker Elastosil R490/55 but for the invention also other
phenylated silicone rubber materials may be used.
Elastosil R490/55 also contains an organic peroxide for
crosslinking.
A mixture of 1-15 parts by weight of component B per 100
parts by weight of component A gives an improvement of the
compliance of the material at -50 C while at the same time
reducing the cost compared with a material that only con-
tains component B. Experiments and investigations have
surprisingly shown that a mixture of 3-10 parts by weight
of component B on 100 parts by weight of component A gives
a very advantageous reduction of the stiffness coefficient
at -50 C while at the same time the cost advantage is par-
ticularly great. Nor does such a mixture influence the
process properties, as for example the viscosity, when
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manufacturing components, which is an advantage. Stiffness
coefficient in this application means the stiffness co-
efficient, so-called storage modulus G', which is measured
with an instrument called Dynamical Mechanical Analyser,
with the generally accepted abbreviation DMA. The mixture
has a viscosity according to Mooney ML (1+4), at 23 C,
according to the standard DIN 53523, in the interval of 50-
65M.
The possibility of intermixing different quantities of com-
ponent B provides possibilities of customizing process
properties, for example viscosity, and mechanical proper-
ties, for example the stiffness coefficient.
According to one embodiment of the invention, the material
comprises a component C, which is an organic peroxide whose
task is to crosslink components A and B in the mixture. An
example of an appropriate organic peroxide is bis(2,4-
dichlorobenzoyl)peroxide. Component C is added to the
mixture in an amount of 0.01-5 parts by weight on 100 parts
by weight of component A, preferably of 1-4 parts by weight
on 100 parts by weight of component A.
According to another embodiment of the invention, a plati-
num catalyst is used to crosslink components A and B in the
mixture. Examples of a platinum catalyst that may be uti-
lized for crosslinking are various types of a platinum com-
plex that are dissolved in, for example, alcohol, xylene,
divinylsiloxane, or cyclic vinylsiloxanes. One example of
such a platinum complex is a platinum carbonyl cyclovinyl
methylsiloxane complex. Preferably, an addition of 0.0005-
0.02 parts by weight of a platinum catalyst per 100 parts
by weight of components A-1-B may be used.
According to yet another embodiment of the invention, a
component D may also be added to the mixture of component
A, component B and component C. Component D comprises
different types of fillers to achieve the desired proper-
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ties. To improve the mechanical strength and the stiffness
coefficient, fibre fillers may be used. In this applica-
tion, fibre fillers mean a quantity of elongated particles
where the extent of the material in the transversal direc-
tion is smaller than 0.8 mm. Examples of fibre fillers are
short or long glass fibres as well as aramide fibres. In
this application, short fibres mean fibres whose length is
shorter than about 3 mm.
To improve the stiffness and the hardness, bulk fillers may
be used as fillers. In this application, bulk fillers mean
particles whose extent in three mutually perpendicular
directions does not differ by more than a factor of 10. The
mean particle size is smaller than 3 mm. Examples of bulk
fillers are silicon dioxide, aluminium trihydrate, quartz
and aluminium oxide. Fibre fillers and bulk fillers other
that those mentioned above may also be used to improve the
properties of the material.
Preferably, an addition of component D corresponding to a
part by weight of 0.3 to 2.5 times the part by weight of
component A 1- B gives a good stiffness of the material. An
advantageous embodiment is obtained with an addition of
component D corresponding to 0.5 to 1.5 parts by weight per
part by weight of components A B, which gives a very
useful combination of stiffness, viscosity and cost.
A particularly advantageous material is obtained if the
part by weight of component D is 0.8 to 1.2 times the part
by weight of components A B. This results in a material
. that has a good stiffness coefficient, especially taking
into consideration the need of electric insulators with
grooves, a viscosity that makes the material easy to shape,
and a low material cost.
A mixture of components according to the invention is par-
ticularly useful for electrical insulation.
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A particularly advantageous use of the invention relates to
electrical insulation of electrical apparatus for outdoor
use.
According to another embodiment of the present
invention, there is provided a method for manufacturing
a silicone rubber material for electrical insulation,
wherein
3-10 parts by weight of polyary1siloxane (B) and 100
= 10 parts by weight of polyalky1si1oxane (A) are mixed.
DESCRIPTION OF PREFERRED EMBODIMENTS
The technical effect of the invention was verified by the
following experiment.
A material was manufactured containing 100 parts by weight
Wacker Powersi1114 318 which was mixed with 25 parts by
weight of Wacker Elastosi10.4 R 490/55 and was extruded into
a flat object. The mixture was hardened in a furnace at
about 135 C for about 1 hour. The after-hardening was
carried out for about 6 hours. At -50 C, during a DMA test,
a stiffness coefficient of about 12 kPa was obtained.
The conclusion is that the material according to the in-
vention had a compliance that was about 40 per cent better
than a material wholly based on PDMS while at the same time
the viscosity properties were maintained at an advantageous
level.
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In a particularly advantageous embodiment, a material was
manufactured containing 100 parts by weight Wacker
Powersi lip 318 which was mixed with 5 parts by weight of
Wacker Elastosillb R 490/55 and extruded into a flat
object. The mixture was hardened in a furnace at about
130 C for about 1 hour. The after-hardening was carried out
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for about 5 hours. At -50 C, during a DMA test, a stiffness
coefficient of about 13 kPa was obtained.
The conclusion is that the material according to the in-
5 vention had a compliance that was about 35 per cent better
than a material wholly based on PDMS while at the same time
the viscosity properties were maintained at an advantageous
level. The cost of the material is somewhat higher than for
a material that is wholly based on PDMS but considerably
10 lower than for a material that is wholly based on a
phenylated silicone rubber.
