Liquid silicone rubber composition for application to high-voltage electrical insulators and articles

A high voltage electrical insulator formed from a curable liquid silicone rubber composition, and subsequently cured, a composition consisting essentially of PA0 (A) 100 weight parts of a fluid mixture of polyorganosiloxane comprising PA1 (a) at least 5 weight percent of a polyorganosiloxane resin comprising repeating units selected from the group consisting of SiO.sub.4/2 and RSiO.sub.3/2, where R is a monovalent hydrocarbon group, and PA1 (b) polydiorganosiloxane having a viscosity in the range from about 100 mPa.multidot.s to 100,000 mPa.multidot.s, that contains at least 2 silicon-bonded alkenyl groups in each molecule, PA0 (B) 1 to 100 weight parts microparticulate silica, PA0 (C) polyorganohydrogensiloxane that contains at least 2 silicon-bonded hydrogen atoms in each molecule, in a quantity that provides a value from 0.5:1 to 20:1 for the ratio of the number of moles of silicon-bonded hydrogen in the instant composition to the number of moles of silicon-bonded alkenyl in component (A), and PA0 (D) a catalytic quantity of a platinum catalyst. The liquid silicone rubber composition, prior to its cure, evidences a desirable fluidity and excellent moldability. Upon its cure the composition produces very mechanically strong silicone rubber moldings that have excellent high-voltage insulating properties tracking resistance, erosion resistance, arcing resistance without the use of fillers such as aluminum oxide, aluminum hydroxide or quartz.

FIELD OF THE INVENTION 
This invention relates to high-voltage electrical insulators and articles 
formed from liquid silicone rubber compositions. More particularly, this 
invention relates to a high-voltage insulating (HVI) liquid silicone 
rubber composition that, prior to its cure, exhibits a suitable fluidity 
and an excellent moldability. The compositions of this invention cure to 
give silicone rubber moldings that have excellent mechanical strength and 
excellent high-voltage insulating properties, including resistance to 
tracking, resistance to erosion, and resistance to arcing. Compositions of 
this invention are well-suited for application to high-voltage insulators 
and articles such as anode caps, plug boots, insulators, flame-retardant 
wire and cable, and so forth. 
BACKGROUND OF THE INVENTION 
Silicone rubbers have excellent electrical characteristics and as a result 
are frequently used for high-voltage insulators and articles such as anode 
caps, plug boots, insulators, flame-retardant wire and cable, and the 
like. The silicone rubber compositions used for these applications have 
typically contained large amounts of inorganic filler, e.g., aluminum 
hydroxide powder, aluminum oxide powder, or quartz powder. See, for 
example, Japanese Patent Publication 53-35982, Japanese Patent Publication 
62-26124, and Japanese Patent Application Laid Open Number 4-209655. 
However, liquid silicone rubber compositions that are highly loaded with 
these inorganic fillers have very high viscosities. This gives these 
compositions poor moldability, and makes them difficult to use in 
applications that require the fluidities appropriate for injection molding 
or casting materials. In addition, the silicone rubber moldings afforded 
by the cure of highly filled silicone rubber compositions have low 
mechanical strengths. Under conditions of exposure to severe soiling or 
the weather, these compositions will also suffer from such deterioration 
phenomena as tracking and erosion due to the high electrical stresses, 
which results in a drastic decline in their high-voltage insulating 
properties and has prevented these compositions from being completely 
acceptable. 
Therefore, there is a need for a liquid silicone rubber composition that 
prior to its cure has a desirable fluidity and excellent moldability, and 
that can be cured to give very mechanically strong silicone rubber 
moldings that have excellent high-voltage insulating properties. 
SUMMARY OF THE INVENTION 
The invention is a high voltage electrical insulator formed from a curable 
liquid silicone rubber composition, and subsequently cured, a composition 
consisting essentially of 
(A) 100 weight parts of a fluid mixture of polyorganosiloxane comprising 
(a) at least 5 weight percent of a polyorganosiloxane resin comprising 
repeating units selected from the group consisting of SiO.sub.4/2 and 
RSiO.sub.3/2, where R is a monovalent hydrocarbon group, and 
(b) polydiorganosiloxane having a viscosity in the range from about 100 
mPa.multidot.s to 100,000 mPa.multidot.s, that contains at least 2 
silicon-bonded alkenyl groups in each molecule, 
(B) 1 to 100 weight parts microparticulate silica, 
(C) polyorganohydrogensiloxane that contains at least 2 silicon-bonded 
hydrogen atoms in each molecule, in a quantity that provides a value from 
0.5:1 to 20:1 for the ratio of the number of moles of silicon-bonded 
hydrogen in the instant composition to the number of moles of 
silicon-bonded alkenyl in component (A), and 
(D) a catalytic quantity of a platinum catalyst 
DETAILED DESCRIPTION OF THE INVENTION 
The inventors have discovered that high voltage electrical insulators can 
be formed from an addition reaction-curable liquid silicone rubber 
composition whose base ingredient is a particular polyorganosiloxane has 
unexpectedly good electrical properties without the use of inorganic 
fillers typically used for electrical resistance, such as aluminum oxide, 
aluminum hydroxide, or quartz. The components are formed from a high 
voltage insulating (HVI) liquid silicone rubber composition consisting 
essentially of: 
(A) 100 weight parts of a fluid mixture of polyorganosiloxane comprising 
(a) at least 5 weight percent of a polyorganosiloxane resin comprising 
repeating units selected from the group consisting of SiO.sub.4/2 and 
RSiO.sub.3/2, where R is a monovalent hydrocarbon group, and 
(b) polydiorganosiloxane having a viscosity in the range from about 100 
mPa.multidot.s to 100,000 mPa.multidot.s, that contains at least 2 
silicon-bonded alkenyl groups in each molecule, 
(B) 1 to 100 weight parts microparticulate silica, 
(C) polyorganohydrogensiloxane that contains at least 2 silicon-bonded 
hydrogen atoms in each molecule, in a quantity that provides a value from 
0.5:1 to 20:1 for the ratio of the number of moles of silicon-bonded 
hydrogen in the instant composition to the number of moles of 
silicon-bonded alkenyl in component (A), and 
(D) a catalytic quantity of a platinum catalyst. 
The HVI liquid silicone rubber composition according to the present 
invention, because it comprises components (A) to (D) and in particular 
because it contains (a) polyorganosiloxane resin containing the 
SiO.sub.4/2 unit and/or RSiO.sub.3/2 unit in its component 
(A), evidences a desirable fluidity and excellent moldability prior to its 
cure and upon its cure provides very mechanically strong silicone rubber 
moldings that have excellent high-voltage insulating properties. 
Component (A) is the base ingredient of the composition. This component (A) 
preferably has a viscosity at 25.degree. C. in the range from 100 
centipoise to 100,000 centipoise and more preferably from 100 centipoise 
to 50,000 centipoise. Viscosities below the given range result in a 
decline in the mechanical strength of the cured silicone rubber, while 
viscosities in excess of the given range result in a reduced fluidity and 
hence in a reduced moldability for the liquid silicone rubber composition. 
The organopolysiloxane resin (a) encompassed by component (A) functions to 
improve the high-voltage insulating properties while at the same time also 
improving the mechanical strength of the cured silicone rubber. The 
subject organopolysiloxane resin must contain the SiO.sub.4/2 siloxane 
unit and/or the RSiO.sub.3/2 siloxane unit within the molecule. R in the 
preceding formula represents substituted and unsubstituted monovalent 
hydrocarbon groups, for example, alkyl groups such as methyl, ethyl, and 
propyl; alkenyl groups such as vinyl, allyl, and propenyl; aryl groups 
such as phenyl and tolyl; and haloalkyl groups such as 
3,3,3-trifluoropropyl and 3-chloropropyl. In addition to the required 
siloxane units as described above, this organopolysiloxane resin may 
contain the R.sub.3 SiO.sub.1/2 siloxane unit and the R.sub.2 SiO.sub.2/2 
siloxane unit where R is defined as above. The subject organopolysiloxane 
resin (a) is preferably soluble in the organopolysiloxane constituting 
component (b). Component (a) can be specifically exemplified by 
methylpolysiloxane resin composed of the SiO.sub.4/2 and (CH.sub.3).sub.3 
SiO.sub.1/2 units; methylpolysiloxane resin composed of the SiO.sub.4/2, 
(CH.sub.3).sub.3 SiO.sub.1/2, and (CH.sub.3)SiO.sub.3/2 units; 
polyvinylmethylsiloxane resin composed of the SiO.sub.4/2, 
(CH.sub.3).sub.3 SiO.sub.1/2, and (CH.sub.2 .dbd.CH)(CH.sub.3).sub.2 
SiO.sub.1/2 units; polyvinylmethylsiloxane resin composed of the 
SiO.sub.4/2, (CH.sub.2 .dbd.CH)(CH.sub.3).sub.2 SiO.sub.1/2, and (CH.sub.2 
.dbd.CH)SiO.sub.3/2 units; methylphenylpolysiloxane resin composed of the 
PhSiO.sub.3/2 (Ph.dbd.phenyl) and (CH.sub.3).sub.2 SiO.sub.2/2 units; and 
methylphenylvinylpolysiloxane resin composed of the PhSiO.sub.3/2 and 
(CH.sub.2 .dbd.CH)(CH.sub.3)SiO.sub.2/2 units. Although component (a) can 
range from a liquid at ambient temperature to a solid at ambient 
temperature, it should generally have a viscosity at 25.degree. C. from 
100 to 100,000 centipoise and preferably from 100 to 50,000 centipoise. 
When silicone resin that is solid at ambient temperature is used, it is 
preferably used dissolved in the diorganopolysiloxane (b). The proportion 
of component (a) in component (A) can be from 5 to 100 weight % and is 
preferably from 10 to 70 weight %. When the component (a) content below 5 
weight percent, there is no improvement in high-voltage insulating 
properties and a reduced strength on the part of the cured silicone 
rubber. 
The polydiorganosiloxane constituting component (b) must contain at least 2 
alkenyl groups in each molecule in order for cure of the composition 
according to the present invention to provide a rubbery elastic silicone 
rubber. The subject alkenyl is exemplified by vinyl, allyl, and propenyl. 
The non-alkenyl organic groups in component (b) are exemplified by 
substituted and unsubstituted monovalent hydrocarbon groups, e.g., alkyl 
such as methyl, ethyl, and propyl; aryl such as phenyl and tolyl; and 
haloalkyl such as 3,3,3-trifluoropropyl and 3-chloropropyl. The molecular 
structure of this component will generally be straight chain, but some 
branching may be present. The alkenyl in component (b) may be present at 
terminal or pendant position on the molecular chain or at both positions. 
Moreover, only a single type of alkenyl group may be present or a 
combination of two or more types of alkenyl groups may be present. 
Component (b) can be specifically exemplified by 
dimethylvinylsiloxy-endblocked polydimethylsiloxanes, 
dimethylvinylsiloxy-endblocked dimethylsiloxane-methylvinylsiloxane 
copolymers, and dimethylvinylsiloxy-endblocked 
dimethylsiloxane-methylphenylsiloxane copolymers. The viscosity of 
component (b) at 25.degree. C. should be in the range from 100 to 100,000 
centipoise and preferably falls in the range from 100 to 50,000 
centipoise. 
The microparticulate silica (B) is a reinforcing filler and functions 
mainly to impart mechanical strength to the composition according to the 
present invention. The subject microparticulate silica can be exemplified 
by dry-process silicas such as fumed silicas and by wet-process silicas 
such as precipitated silicas. Preferred among these are fumed silicas with 
specific surface areas of at least 50 m.sup.2 /g. Particularly preferred 
are fumed silicas with specific surface areas of at least 100 m.sup.2 /g 
whose surface has been treated with an organosilicon compound selected 
from the group consisting of organosilanes, organosilazanes, 
organosiloxane oligomers, and mixtures of the preceding. Component (B) is 
admixed at from 1 to 100 weight parts per 100 weight parts component (A) 
and preferably at from 10 to 40 weight parts per 100 weight parts 
component (A). A high level of mechanical strength will not be obtained 
when the addition of component (B) falls below the given range, while the 
addition of component (B) in excess of the given range causes the 
viscosity of the composition according to the present invention to become 
excessively large and causes a loss of the fluidity that is a 
characteristic feature of liquid silicone rubber compositions. 
The polyorganosiloxane (C) is a crosslinker for the composition according 
to the present invention. More specifically, the composition according to 
the present invention is crosslinked and thereby cured by the addition 
reaction of the silicon-bonded hydrogen in component (C) with the 
silicon-bonded alkenyl in component (A) in the presence of the platinum 
catalyst (D). The organopolysiloxane (C) must contain at least 2 
silicon-bonded hydrogen atoms in each molecule. The organic groups present 
in (C) in addition to the silicon-bonded hydrogen can be nonexhaustively 
exemplified by alkyl groups such as methyl, ethyl, and propyl; aryl groups 
such as phenyl and tolyl; and substituted alkyl groups such as 
3,3,3-trifluoropropyl and 3-chloropropyl. The molecular structure of 
component (C) can be straight chain, branched straight chain, cyclic, or 
network. While the molecular weight of component (C) is not critical, this 
component preferably has a viscosity at 25.degree. C. from 3 to 10,000 
centipoise. Component (C) is added in a quantity that provides a value 
from 0.5:1 to 20:1 and preferably from 1:1 to 3:1 for the ratio of the 
number of moles of silicon-bonded hydrogen in the composition to the 
number of moles of silicon-bonded alkenyl. The composition according to 
the present invention will not undergo adequate cure when this molar ratio 
falls below 0.5:1, while a value in excess of 20:1 will cause the 
generation of excess hydrogen gas and foaming. 
The platinum catalyst (D) is a curing catalyst for the composition 
according to the present invention. This platinum catalyst can be 
exemplified by chloroplatinic acid, alcohol solutions of chloroplatinic 
acid, complexes between chloroplatinic acid and olefins, complexes between 
chloroplatinic acid and divinylsiloxanes, platinum black, platinum, and 
supported platinum. The addition of component (D) will vary as a function 
of the particular type of platinum catalyst and thus cannot be strictly 
specified. However, as a general rule component (D) will be added at from 
1 to 1,000 weight parts and preferably at from 5 to 100 weight parts, in 
each case as platinum metal proper for each 1,000,000 weight parts 
component (A). 
The composition according to the present invention can be prepared simply 
by mixing the above-described components (A) to (D) to homogeneity in 
their specified quantities. The mixing sequence for components (A) to (D) 
is not crucial to this process, but in a preferred embodiment components 
(A) and (B) are first mixed with heating under reduced pressure, the 
resulting mixture is then cooled to afford a silicone rubber base 
compound, (C) and (D) are subsequently mixed into this silicone rubber 
base compound. 
The composition according to the present invention may optionally contain a 
heretofore known addition-reaction inhibitor, for example, 
ethynylcyclohexanol, dimethylformamide, triphenylphosphine, cyclic 
methylvinylsiloxane, 3-methyl-1-butyn-3-ol, 3,5-dimethyl-1-hexyn-3-ol, 
cyclohexylbutynol, 3-phenyl-1-butyn-3-ol, diphenylethynyl carbinol, and 
3,5-dimethyl-3-hexen-1-yne. The composition according to the present 
invention may also optionally contain the heretofore known reinforcing 
fillers, semireinforcing fillers, nonreinforcing fillers, flame 
retardants, heat stabilizers, and adhesion promoters. While the 
composition according to the present invention basically must be a fluid 
silicone rubber composition that manifests liquidity at ambient 
temperature, its viscosity at 25.degree. C. preferably falls in the range 
from 500 to 100,000 poise.

EXAMPLES 
The invention is explained below through working examples, in which "parts" 
denotes "weight parts" and the values reported for organopolysiloxane 
viscosity were measured at 25.degree. C. The physical properties of the 
silicone rubber moldings were measured by the methods stipulated in JIS 
K-6301. To evaluate the high-voltage insulating properties, an inclined 
plane antitracking test was carried out in accordance with the method 
stipulated in IEC Publication 587 using a model HAT-520 from Hitachi Kasei 
Kogyo Kabushiki Kaisha. The test voltage was 4.5 kV. The evaluation A 
referenced in the table refers to the time in minutes required until the 
current flowing in a high-voltage circuit passing through the test 
specimen exceeded 60 mA. The evaluation B referenced in the table refers 
to the time in minutes for tracking to reach a mark placed on the surface 
of the test specimen at a position 25 mm from the lower electrode. The 
erosion was evaluated visually and scored on the following 5-level scale: 
micro, small, moderate, strong, and deep. 
Example 1 
20 parts surface-treated fumed silica with a specific surface of 200 
m.sup.2 /g (surface pretreated with hexamethyldisilazane) was mixed to 
homogeneity into 100 parts of a liquid mixture of organopolysiloxane 
composed of 35 weight percent polyvinylmethylsiloxane resin 
(weight-average molecular weight=4,000, vinyl group content=2 weight %, 
consisting of 30 mole % SiO.sub.4/2 units 68.4 mole % (CH.sub.3).sub.3 
SiO.sub.1/2 units, and 1.6 mole % of (CH.sub.2 .dbd.CH)(CH.sub.2).sub.2 
SiO.sub.1/2 and 65 weight % dimethylvinylsiloxy-endblocked 
polydimethylsiloxane (viscosity=2,000 centipoise, silicon-bonded vinyl 
content=0.23 weight %). The mixture was additionally heated for 1 hour at 
170.degree. C. under a vacuum. The following were then homogeneously 
incorporated into the resulting mixture to give a liquid silicone rubber 
composition: 1.4 parts trimethylsiloxy-endblocked 
dimethylsiloxane-methylhydrogensiloxane copolymer (silicon-bonded hydrogen 
content=0.7 weight %) and, as curing catalyst, chloroplatinic acid 
sufficient to provide 5 ppm platinum metal. This liquid silicone rubber 
composition was cured by heating for 5 minutes at 150.degree. C. to give a 
silicone rubber molding in the form of a sheet. This silicone rubber 
molding was submitted to measurement of its physical properties and 
high-voltage insulating properties. The obtained measurement results are 
reported in Table 1. 
Example 2 
20 parts fumed silica with a specific surface of 200 m.sup.2 /g and, as 
surface treatment agent for the silica, 3 parts hexamethyldisilazane and 1 
part water, were mixed to homogeneity into 100 parts of a liquid mixture 
of polyorganosiloxane composed of 35 weight % polyvinylmethylsiloxane 
resin (weight-average molecular weight=4,000, vinyl group content=2 weight 
%, consisting of 30 mole % SiO.sub.4/2 units 68.4 mole % (CH.sub.3).sub.3 
SiO.sub.1/2 units, and 1.6 mole % of (CH.sub.2 .dbd.CH)(CH.sub.2).sub.2 
SiO.sub.1/2 and 65 weight % dimethylvinylsiloxy-endblocked 
polydimethylsiloxane (viscosity=2,000 centipoise, silicon-bonded vinyl 
content=0.23 weight %). The mixture was additionally heated for 3 hours at 
170.degree. C. under a vacuum. The following were then homogeneously 
incorporated into the resulting mixture to give a liquid silicone rubber 
composition: 1.4 parts trimethylsiloxy-endblocked 
dimethylsiloxane-methylhydrogensiloxane copolymer (silicon-bonded hydrogen 
content=0.7 weight %) and, as curing catalyst, chloroplatinic acid 
sufficient to provide 5 ppm platinum metal. This liquid silicone rubber 
composition was cured by heating for 5 minutes at 150.degree. C. to give a 
silicone rubber molding in the form of a sheet. This silicone rubber 
molding was submitted to measurement of its physical properties and 
high-voltage insulating properties. The obtained measurement results are 
reported in Table 1. 
Example 3 
20 parts surface-treated fumed silica with a specific surface of 200 m2/g 
(surface pretreated with hexamethyldisilazane) was mixed to homogeneity 
into 100 parts of a liquid mixture of organopolysiloxane composed of 15 
weight percent polyvinylmethylsiloxane resin (weight-average molecular 
weight=4,000, vinyl group content=2 weight %, consisting of 30 mole % 
SiO.sub.4/2 units 68.4 mole % (CH.sub.3).sub.3 SiO.sub.1/2 units, and 1.6 
mole % of (CH.sub.2 .dbd.CH)(CH.sub.2).sub.2 SiO.sub.1/2 and 85 weight % 
dimethylvinylsiloxy-endblocked polydimethylsiloxane (viscosity=2,000 
centipoise, silicon-bonded vinyl content=0.23 weight %). The mixture was 
additionally heated for 1 hour at 170.degree. C. under a vacuum. The 
following were then homogeneously incorporated into the resulting mixture 
to give a liquid silicone rubber composition: 1.4 parts 
trimethylsiloxy-endblocked dimethylsiloxane-methylhydrogensiloxane 
copolymer (silicon-bonded hydrogen content=0.7 weight %) and, as curing 
catalyst, chloroplatinic acid sufficient to provide 5 ppm platinum metal. 
This liquid silicone rubber composition was cured by heating for 5 minutes 
at 150.degree. C. to give a silicone rubber molding in the form of a 
sheet. This silicone rubber molding was submitted to measurement of its 
physical properties and high-voltage insulating properties. The obtained 
measurement results are reported in Table 1. 
Comparative Example 1 
A liquid silicone rubber composition was prepared as in Example 1, but in 
this case using 100 parts dimethylvinylsiloxy-endblocked 
polydimethylsiloxane (viscosity=2,000 centipoise, silicon-bonded vinyl 
content=0.23 weight %) in place of the 100 parts liquid mixture of 
organopolysiloxane composed of 35 weight % polyvinylmethylsiloxane resin 
(weight-average molecular weight=4,000, vinyl group content=2 weight %, 
consisting of 30 mole % SiO.sub.4/2 units 68.4 mole % (CH.sub.3).sub.3 
SiO.sub.1/2 units, and 1.6 mole % of (CH.sub.2 .dbd.CH)(CH.sub.2).sub.2 
SiO.sub.1/2 and 65 weigh % dimethylvinylsiloxy-endblocked 
polydimethylsiloxane (viscosity=2,000 centipoise, silicon-bonded vinyl 
content=0.23 weight %) that was used in Example 1. The resulting liquid 
silicone rubber composition was cured by heating for 5 minutes at 
150.degree. C. to give a silicone rubber molding in the form of a sheet. 
This silicone rubber molding was submitted to measurement of its physical 
properties and high-voltage insulating properties. The obtained 
measurement results are reported in Table 1. 
TABLE 1 
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Com- 
Exam- Exam- Exam- parative 
ple 1 ple 2 ple 3 Example 1 
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polyvinylmethylsiloxane 
35 35 15 -- 
resin (wt %) 
dimethylvinylsiloxy- 65 65 85 100 
endblocked 
polydimethylsiloxane (wt %) 
surface-treated 20 -- 20 20 
fumed silica (parts) 
fumed silica (parts) -- 20 -- -- 
hexamethyldisilazane/water -- 3/1 -- -- 
(parts) 
trimethylsiloxy-endblocked 1.4 1.4 1.4 1.4 
dimethylsiloxane-methylhydro- 
gensiloxane copolymer (parts) 
chloroplatinic acid 5 5 5 5 
(ppm platinum metal) 
composition viscosity (poise) 1,000 3,000 900 800 
physical properties 
durometer (JIS A) 65 64 40 25 
tensile strength (kgf/cm.sup.2) 100 90 60 25 
elongation (%) 200 200 250 450 
tear strength (A) (kgf/cm) 15 14 10 3 
high voltage insulating 
properties 
(antitracking) 
evaluation A (minutes) .gtoreq.360 .gtoreq.360 .gtoreq.360 80 
evaluation B (minutes) .gtoreq.360 
.gtoreq.360 .gtoreq.360 50 
erosion micro micro small deep 
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