APPAREILS ELECTRIQUES RENFERMANT DES FLUIDES DIELECTRIQUES AMELIORES

ELECTRICAL DEVICES CONTAINING IMPROVED DIELECTRIC FLUIDS

Abrégé anglais

Abstract of the Disclosure
Improved electrical devices are disclosed
such as transformers, and capacitors containing an improved
dielectric fluid consisting essentially of a major amount
of a liquid polyorganosiloxane and a minor amount of
certain selected nitro substituted organic compounds
which are soluble in said siloxane.

Revendications

The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:
1. A dielectric fluid having a viscosity of 5 to
500 cs. at 25°C. and consisting essentially of 50 to 99.5 percent by
weight of a liquid polyorganosiloxane and 0.5 to 50 percent by
weight of a compound soluble in said siloxane, which compound
is selected from the group consisting of nitro substituted
aromatic hydrocarbon, nitro substituted aliphatic hydrocarbons,
nitro substituted cycloaliphatic hydrocarbons and mixtures thereof.
2. A dielectric fluid as defined in claim 1 consisting
essentially of 80 to 99.5 percent by weight of the siloxane and
0.5 to 20 percent by weight of the nitro substituted hydrocarbon.
3. A dielectric fluid as defined in claim 2 wherein
the siloxane has the general formula R3Si0(R2Si0)xSiR3 wherein
R is a hydrocarbon or a halogenated hydrocarbon radical and
x is an integer, and the compound is a mononitro-substituted
aliphatic or aromatic hydrocarbon containing from 1 to 10 carbon
atoms.

4. A transformer or capacitor containing as the dielectric
fluid a composition having a viscosity of 5 to 500 cs. at 25°C. and
containing 80 to 99.5 percent by weight of a liquid polyorganosiloxane
and 0.5 to 20 percent by weight of a nitro substituted aromatic,
aliphatic or cycloaliphatic compound or mixtures thereof soluble
in said polyorganosiloxane.
5. The transformer or capacitor of claim 4 wherein the
liquid polyorganosiloxane is a trimethylsilyl endblocked
dimethylpolysiloxane.

Description

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In numerous electrical devices, lt i9 necessary
to provide a liquid insulating medlum which i8 called
a ''dielectrical fluid.'' This liquld has a substantlal
higher breakdown strength than alr and by displacing
air from spaces between conductors in the electrical
s equipment or apparatus, materially raises the breakdown
voltage of the electrical device. With the ever increaslng
sophistication of electrical equipment, the varlous
s electrical devices are operating at hlgher and higher
voltages. This means that the dielectric fluids used
in such devices are sub~ected to greater and greater
stresses. These problems have, of course, necessitated
the search for improved dielectric fluids.
With the exception of certain special
~ applications, the polychlorinated biphenyl compounds
s (generally known as "PCB's") have been the standard
dielectric fluid in electrical devices since the 1930 ~ s
when the PCB's replaced mineral oil. Various other
liquids including some siloxanes have also been suggested
V 20 for use as dielectric fluids. See, for example, U.S.
Patents 2,377,689 and 3,838,056 and British Patents
899,658 and 899,661. Recently, the PCB's have been found
to have a negative impact on the environment and efforts
4 are being made worldwide to find suitable replacements
for the PCB's.
By way of illustration, corona or partial
discharge is a ma~or factor causing deterioration and
failure of capacitors or other power factor correction
devices. A capacitor operating in corona will have
a life of only minutes or hours instead of the expected
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twenty years. A capacitor properly impregnated with
a suitable dielectric fluid will be essentially free
of corona discharge to a voltage of at least twice
- the rated voltage. During use when a dielectric fluld
is placed under increasing stress, a point is reached
where breakdown occurs. The voltage at which the
capacitor will suddenly flash into corona is known
in the art as the corona inception voltage (CIV). This
voltage is dependent upon the rate at which the voltage
is applied. There is considerable difference between
the sensitivity of different fluids to the rate of
, rise of voltage. The corona will, however, extinguish
with a reduction of voltage. The corona extinctlon
voltage (CEV) is not a fixed value for each fluid
but is a function of the intensity of corona before
' the voltage is reduced. For best results, both the CIV
.~ and CEV should be as high and as close together as
possible.
It has been discovered in accordance with
this invention that when certain select nitro substituted
organic compounds are incorporated into liquid poly-
organosiloxanes that the resulting composition is
useful as a dielectric fluid in electrical devices.
~S; It is further believed that these compositions when
used as dielectric fluids provide suitable replacements
for the PCB's which are currently being employed in
the marketplace.
More specifically, this invention relates
to an electrical device containing a dielectric fluid
wherein the improvement comprises employing as the
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J
: dielectrlc fluid a composltion conslsting essentlally
of a ma~or amount of a liquid polyorganosiloxane and
a mlnor amount of a compound soluble in said siloxane
; whlch compound is selected from the group consistlng
of nitro substituted aromatic hydrocarbons, nitro
substituted allphatic hydrocarbons and nitro substituted
cycloaliphatic hydrocarbons.
This invention further relates to a dielectric
fluid con~isting essentially of a ma~or amount of a
liquid polyorganosiloxane and a minor amount of a compound
soluble ln said siloxane which compound is selected
from the group consisting of nitro substituted aromatic
hydrocarbons, nitro substituted aliphatic hydrocarbons,
nitro substituted cycloaliphatic hydrocarbons and mixtures
thereof.
The liquid polyorganosiloxanes useful in
this invention wlll be composed predominately of
~$~ siloxane units of the formula R2SiO and may al~o contain
small amounts of siloxane units of the formulae R3SiO~/2,
RSiO3/z, and SiO4/2. Of particular interest are
liquid polyorganosiloxanes of the general formula
R3SiO(R2SiO)XSiR3. In the foregoing formulae, the
R radicals preferably represent hydrocarbon radicals
and halogenated hydrocarbon radicals. Illustrative
examples of suitable R radlcals are the methyl, ethyl,
propyl, butyl, hexyl, decyl, dodecyl, octadecyl, vinyl,
allyl, cyclohexyl, phenyl, xenyl, tolyl, xylyl, benzyl,
2-phenylethyl, 3-chloropropyl, 4-bromobutyl, 3,3,3-
trifluoropropyl, dichlorophenyl, and alpha,alpha,
alpha-trifluorotolyl radicals. Preferably, R contains
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from 1 to 6 carbon atoms wlth the methyl, vlnyl and
; phenyl radlcal~ belng the most preferred.
The llquld polyorganosiloxane portlon of the
dielectric fluld compositlon of thls lnventlon constltutes
a maJor portlon thereof, that ls to say, more than
50 percent of the composltlon and preferably the llquid
polyorganoslloxane constltutes from 80 to 99.5 percent
by welght of the dlelectric fluld compositlon of thls
lnvention. These llquid polyorganosiloxanes are well
known materials which are commercially available throughout
the world.
The dielectrlc fluld composltlon of this
~i lnvention also contalns a mlnor amount of a compound
selected from the group consisting of nitro substituted
aromatic hydrocarbons, nltro substituted aliphatic hydrocarbons,
nltro substituted cycloaliphatic hydrocarbons and mixtures
thereof. It is believed essential that these nitro
compounds be soluble in the llquid polyorganosiloxane
portion of the composition. Whlle any of the above indicated
nitro compounds are useful in accordance with the present
invention, the mononitro compounds containing from 1
to 10 carbon atoms are the preferred embodiment at this
time. The nitro hydrocarbon compounds used herein constitute
a minor portion, that is, less than 50 percent of the
composition of this invention. It is generally preferred,
however, that these materials be employed in an amount
in the range from 0.5 to 20 percent by weight of the
composition.
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Thus, in accordance with the pres~nt teachings,
a dielectric fluid is provided which has a viscosity of
5 to 500 cs. at 25C. and which consists essentially of 50 to
99.5~ by weight of a liquid polyorganosiloxane and 0.5 to 50%
by weight of a compound soluble in said siloxane, which
compound is selected from the group consisting of nitro
substituted aromatic hydrocarbon, nitro substituted aliphatic
hydrocarbons, nitro substituted cycloaliphatic hydrocarbons and
mixtures thereof.
. 10 In accordance with a further embodiment, a trans-
. former or capacitor is provided which contains as the dielectric
' fluid, a composition which has a viscosity of 5 to 500 cs. at
, 25C and contains 80 to 99.5% by weight of a liquid poly-
.. organosiloxane and 0.5 to 20% by weight of a nitro substituted
aromatic, aliphatic or cycloaliphatic compound or mixtures
thereof soluble in the polyo~-ganosiloxane.
: The dielectric fluid compositions of this
invention may also contain small amounts of conventional
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addltlves such as HCl scavengers, corrosion lnhibitors
and other conventional addltives normally employed
- in such compositions so long as they do not have an
adverse effect of the performance of the composition9
of this lnventlon.
The two most important electrical devlce~
in which the dielectric fluids of thls lnventlon are
~i useful are ln capacltors and transformers. They are
s also very useful dlelectrlc fluids ln other electrlcaldevices such as electrical cables, rectlflers, eletro-
magnets, swltches, fuses, circuit breakers and as
coolants and insulators for dielectrlc devices such
as transmitters, recelvers, fly-back coils, sonar
; bouys and toys. The methods for employing the dielectric
¦ fluids in these various applications (for example, as
a reservolr of liquid or as an impregnant) are well known
to those skllled in the art. For best reqults, the
4 vlscoslty of the dlelectrlc fluid composition of this
invention should be in the range of 5 to 500 centistokes
at 25C. If the viscosity exceeds 500 centistokes, they
are difficult to use as impregnants and at less than
- 5 centistokes their volatility becomes a problem unless
they are used in a closed system.
Now in order that those skilled in the art
may better understand how the present invention can
be practiced, the followlng examples are given by
way of illustration and not by way of limitation. All
parts and percents referred to herein are by weight
and all viscosities measured at 25C. unless otherwise
specified.
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Example 1
, A screening test for dielectric fluids was developed
which it is believed correlates well with results obtained in
test capacitors. The main piece of equipment required for
this test is a commercial corona detector with a manual variable
; voltage control. The test cell consists of a glass cylindrical
container. The base of the cell is a ceramic filled plastic
which has a stainless steel metal plate which is connected
directly to ground. There is a stainless steel top for the
container which has attached thereto a micrometer adjustable
high voltage electrode with a steel phonograph needle on the
end. The tip of this needle is positioned 0.0635 cm. (25 mils)
above the grounded base. In the high voltage line attached to
the electrode, there is a 1.67 x 108 ohm resistance. This is
a current limiting resistor.
~- During the test, a few cubic centimeters of the
test fluid is placed in the container and the top set in place.
As the voltage is increased, partial discharge occurs between
' the tip of the electrode and the ground plate. This draws
current which reduces the applied voltage below the discharge
,~ level. When no cuxrent is being drawn, the applied voltage
is again at partial discharge potential. Current is drawn by
discharges again and the process is repeated. Thus, the
current is in effect turned on and off very rapidly and
the total breakdown of the fluid can never occur.
In operation, the applied voltage is slowly increased by
f adjustment of the variable voltage control. The partial
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dlscharges are observed on the oscllloscope of the
corona ~etector. The polnt at which the ellptical
lissa~ous pattern on the screen becomes flooded wlth
; discharges, and there is a constant audlble crackllng
.,.
from the cell, ls recorded as the corona inceptlon voltage
(CIV). The rate of rise of the applied voltage ls perhap~
a few hundred volts per second. When the CIV has been
determined, the voltage ls slowly decreased untll the
eliptical lissa~ous pattern on the screen can be seen
again due to the partial cessation of discharges.
The polnt at whlch this occurs is recorded as the corona
' extinction voltage (CEV).
A number of dielectric fluid compositions
s were prepared which consisted essentially of a liquid
trimethylsilyl endblocked polydimethylsiloxane having
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a vlscoslty of 50 cs. and varlous nitro substltuted
hydrocarbons in varylng amounts. These composltlons were
tested in the screening test described above. The
specific nltro substituted hydrocarbons employed,
the amount used (the balance being the siloxane) and the
test results are set forth in the table below. A
number with a plus (+) behind it indicates that the
test was terminated at that point and the actual
value is somewhat greater than the value~reported.
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.
: Nitro Amount CIV CEV
ComDound (Wgt ~) (ln KV) (ln KV)
A* None None 15.6 14.8
B Nitrobenzene 5 18,0 14.5
2.5 20.0 18.8
1 19.2 17.6
C 2-Nitropropane 10 19.8 17.0
18.4 17.6
2.5 20.0+ 18.0
~: 10 1 20.0+ 19.0
D o-nitrotoluene 1 19 18
r
E** o-nltrodl- <1 18 16.4
` phenylamlne
. ,
p; ~
. *Included for comparison
. **Saturated solution
i ' .
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r ~ :
i:
,:
r
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