Note: Descriptions are shown in the official language in which they were submitted.
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Description
ELASTOMERIC COMPOSITION FOR PROVIDING
ELECTR CAL STRESS CONTROL
Technical Field
The invention relates to a dielectric material of
permanent resilience, for influencing electrical fields
in power current and high voltage systems, the material
ccntaining a dielectric base material having permanent
resilience, pre~erably silicone rubber or polyethylene
or EPDM, with a content:of a finely distributed effect
~ material increasing the relative dielectric constant
: ~ (permittivity).
Background Art
A description of the general prior art with respect
15 to stress control can be found, for example, in the :
~ German Offenlegungsschrift 28 21 017.
:~ Mater-lals of the general kind indicated above are
known, for example, from U.S. Patent No. 4,053,702. They
: contain titanium dioxide as the effect material. That
known substance makes possible, inter alia, the manufac-
~: ture of permanently resilient stress control elements
~ of definite ~eometric configuration, which may be simply
: ~ shifted on, while yielding resiliently, at the site of
- application, typically a cable connection. Due to thelr
resllience, they then fit in a gap free manner That
strong and gap-free fitting is retaired, due t~o the per-
manently resilient properties, over very long time
: periods, for instance many years, and particularly over
~: ~ the usual operational li~e of power~current systems. ;:~
30 The application~of such permanently resilient stress ~:~
control elements requ~res less knowledge and skill than ~:
: the application of other stress control devices, such asg
: for example, metallic stress control cones, the gap-fre0
wrapping of tapes Or stress-controlllng material, the
: 35 molding or modelling and subsequent hardening of flowable ?
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or shapeable masses having stress-controlling propertles
at the site o~ operation, etc.
Materials of the kind described inltially therefore
have made possible a considerable advance in the field of
stress control ln power current and high voltage systems.
The materials described inltially act, together with
cable insulating materials of low dielectric constant,
upon electric fields in the sense of a refraction. For
the sake of completeness, it should be mentioned that for
the manufacture o~ stress-controlling devices, other
materials are known which mainly act-in a resistive
manner; they contain electrically conductive or semi-
conductive effect materials which provide to the material
a desired (mostly voltage-dependent) electrical conduc-
tivity (U.S. Patent Nos. 3,673,305; 3,666,876). In these
cases, permittivity is also increased by the embedding of
particles of electrically conductive or semi-conductive
e~ect material; for example, it may be up to 11 (U.S.
Patent No. 3,666,876). With resilient materials having
resistive stress control properties, however, the active
current, which due to the function flows continuously,
rnay gra~ually ~ive rise to changes of electrical conduc-
tivity, and to a premature agein~ of the materlal. Thus,
other modes of operation are preferred for permanently
resilient materials, particularly the refractive mode of
operation which also applied to the material of the
present invention.
It will be appreclated that a relatively hlgh per-
mittivity o~ the material i8 desirable. Thus, lt is
desired to use substances having a permittivity as high
as possible as the e~fect material for materials showing
; refractive stress control action, e.g. the titanium
dioxide already mentioned, but also other known substances
o~very high permittivity, e.g. barium titanate. The use
of such materials as an effect material for materials
having a refractive stress-controlling action has been
known for a long time, but without paying part~cular
attention to the requirement Or permanent resilience
(U.S. Patent Nos 3,673,305; 3,823,334; 3,287,489). In
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this connection, however, it was found also that when
utilizing effect materials o~ very high permittlvity, e.g.
barium titanate having permittivities of appro~imately
lO,000, the permittivity of the material cannot be
increased beyond approximately 25 if the material is to
retain the permanently resillent propertles of the base
material. The reason for this therefore is that in
mixtures of that kind, the permittivity ~r mix f the
mixture has to be calculated according to a logarithmic,
and not according to an additive~ mixture formula from
the permittivities ~rn f the components of the mixture:
log ~r mix = ~Xn log ~rn
in which Xn is the volume ratio, and ~rn is the relative
dielectric constant (permittivity) of the component ~L.
Accordingly, that formula shows particularly that with a
material consisting of two components, namely,
Component l: Elastomeric base material havlng an
~r f about 3,
Component 2: Barium titanate having an
~r of about lO,000,
one would have to employ a proportion of abouk 35 volume
percent or about 75 weight per cent barium titanate to
obtain a permittivity of the mixture of about 50. Wi.th
the proportion o~ er~ect rnaterial bein~ so high, the
resilient properties, however, of the material are
insufficient to permit the manufacturing there~rom of
practicable gap-free fitting stress control elements having
permanent resilience. The resilience itself, as well as
the maintenance in time of an elastic tension once
produced in the material (the so-called permanent
resetting force), are insufficient. It was generally true
for the prior art (e.g., U.S. Patent No. 4,053~702) that
permanently re~ilient materials of the kind initially
described could bc manufactured with permittivities of
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only up to about 25, even when employing the kno~n e~fectmaterials having ve~y high permittivitie~. The permanent
reslliency of dielectric materials can only be observed
and malntained for the technical practice of testing
results achleved i~ th~ residual stress (permanent set)
under constant deflection according to the specification
DIN 53517 (ISO-STANDARD R 815-69), particularly after
accelerated ageing for 72 hours at 150C are always less
than about 35~ and results Or SHORE-A Hardness according
to the spec~fication DIN 53505 (ISO~STANDARD R 868-68)
are less than 65. If these requirements characterizing
permanent resiliency cannot be met it cannot be warranted
that such materials can be used to produce elastic stress
control elements. All described materials known until
now, which have a permittivity above 25 do not pass this
important criteria of permanent resiliency.
Disclosure of Invention
The present invention provides a dielectric, perman-
ently resilient material which is suitable for a highly
effective refractive stress control.
According to the invention, there is provided a
material o~ the kind initially described, wherein the
effect material comprises strongly structurized dust-fine
particles of an electrically polarizable material, such
as carbon black, in a concentration at which in the range
Or usual power frequencies, e.g. 50 cycles per second,
the following properties of the material are present in
combination:
a) The volume reslstivity has at least a
3 minimum value which i8 still sufficient
rOr purposes of electrical insulation~
b) The relative dielectric constant
(permittivity) ~s greater than 30,
and up to about 300, and
c) The dielectric loss factor is not greater
than approximately 1.5
and further comprises a supplemental effect material in
the form of finely divided particles possessing metal
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conductivity to improve the electric stre~s d-issipation
and consequently the dielectric strengths at high f~equen-
cies as they are typical for impulse waves 5 and to hold
the permittivity at high ~requencies (e.g. 105 Hz) to at
5 least about 20.
Detailed Description
The composition accordlng to the invention utilizes
the fact that ~ith effect materials like carbon black,
which comprise strongly structurized or cleft dust-fine
lO polarizable particles~ a relatively small range of mean
concentrations in the base material can be found in whlch
there is no disturbing electrical conductivity and a
relatively high permittivlty with satisfactory properties
of permanent resilience. The material further comprises a
15 supplemental effect material of metal conductivity which
performs in such a way that the impulse resistance
required for high voltage accessories is maintained. It
is surprising that in this manner, permanently resilient
insulating materials can be produced which are dielectrics,
20 i.e. insulators, and which have substantially higher
permittivities than hitherto possible, and which exhibit
the necessary high electric impulse strength required for
high voltage equipment.
rrrue, it is known that the permittivity of an insul- ?
25 ating material can be increased by the incorporation of
finely divided particles of electrically conductlve or
semi~conductive material, as long as excessive concen-
trations Or such particles are avoided because they can
provide an excessive specific electrical conductlvity
30 which is not suitable for the use as an insulating
material. Ilowevcr~ it has been stated in respect thereto
that by the addition of finely divided titanium dioxide or
carbon black, the permittivity of natural or synthetic
rubber could be increased to values from lO to about 25,
35 and that it would be appropriate to use titanium dioxide
because that material had a less adverse effect upon the
dielectric strength and the specific volume resistivity
(U.S Patent No. 3~2871489). That statement is in
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correspondence with the known te~ching stated in ~.S.
Patent No. 4,053,702 that elastomeric materials o~ the
kind indicated compr~sing titanium dioxide, titanates or
the like as ef~ective materials, can be produced with a
permittivity of up to only about 25~ and that, ~or example,
a permlttivity of at least 50 could not be reached with
elastomeric materials.
Furthermore, stress-controlling materials are known
which contain carbon black as a filler for improving the
mechanical properties of the material (U.S. Patent Nos.
2,515,7~8; 3,349,164), or as an effect material for
obtaining a desired electrical conductivity ~U.S. Patent
No. 3,673,305); however, no suggestions towards the
present invention can be found in this connectionO
The composition according to the invention in prin-
ciple is very simple and can be produced at minimum
expense. In combination, it shows good properties of
permanent resilience, good chemical durability, good
electrical insulating capability, fully satisfactory
dielectric and impulse strength, and high values of
permittivity which had not been thought possible hitherto
with compositions displaying permanent resilience. Thus~
the composition according to the invention can particu-
larly be employed at great advantage in stress control
elements whlch then can have substantially smaller
dlmensions as compared with elements made of known re-
frac~ive materlals of lower permittivlty. Such a stress
control element, e.g. ln the ~orm of a shaped body, like
a sleeve, which can be shifted-on reslllently, ls
designed with respect t,o its electrical properties and
its geometric con~iguration, in correspondence with the
desired modification o~ an electric field existing at its
slte of application. Additionally, depending upon the
strength of the electric field, a constltuent of elec-
tr1cally conductive resilient material may be insertedin the stress control element to make contact with a
cable shield.
The composition according to the invention preferably
has a permittivity between about 50 and 150, at low
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frequencies (e.g., 50 Hz) and at least about 20 at hlgh
frequencies (e.g., 105 Hz). In this range, the properties
of permanent resilience are particularly good~ at a good
electrical insulating capabili~y.
The effectiveness of the strongly structurized of
cleft dust-flne polarizable particles employed in accor-
dance with the invention is dependent upon the morphology
of the particles. Therefore, the concentration of effect
material to be employed in the production of the compo-
sition is determined by producing for every given charge
o~ unitary quality of the effect material, a plurality
of test compositions having different contents of effect
material, determining the permittivity ~r at low fre-
quencies ~e.g. 50 Hz) and the specific electrical volume
resistivity ~ for each of the test compositions, and
determining the concentration of effect material at which
a desired pair of values ~r~ ~ is present. It is parti-
cularly advantageous to determine a concentration as the
optimum one which is associated in the function log
~r = f (log ~) with a medium range in which the absolute
value of the slope is higher than in the area adJacent on
both sides. In this manner, the suitable concentrations
and particularly also the optimum concentrations in the
base materials can be evaluated for every charge of an
effect materlal exhlbiting a macroscopically unitary
quality.
It has been found that with the preferred ef~ect
material, i.e carbon black, the commercially available
qualities show very dlfferent effectiveness in the sense
of the present invention. For example~ the optimum
concentration may be approximately 3 parts by weight,
bu~ also approximately 30 parts by weight per 100 parts
by weight base material. Thus~ the teaching to determine
the concentrations to be used separately for every
charge of an effect material exhibiting a macroscopi-
cally unitary quality constitutes an essential part of
the present invention.
The minimum value of the specific electrlcal volume
reslstivity of the composition according to the invention
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should be approximately 106 ohm,cm ln order for the compo-
sition to still be considered an insulating material;
preferably, the minimum value is approximately 10 ohm.cm.
Furthermore, khe composltion according to the invention is
characterized in that it has, as a rule~ a positive tem-
perature coefficient o~ specific electrical volume resis-
tivity. In other words, the temperature coefficient of the
specific electrical volume resistivity should be at least
approximately zero in the temperature range of from about
0 to 100C. This of~ers the known advantage that the
proportion of losses caused by action currents becomes
smaller with an increasing temperature of tha composition;
this counteracts undesirable heating up. Preferably~ the
temperature coef~icient of the specific electrical volume
resistivity is approximately 0.01 per degree wlthin the
temperature range o~ about 0 to 100C.
Within the range of permittivities stated, the
dielectric loss factor at low and high rrequencies ls not
~reater than about 1.5, and not greater than about 1 in
the range Or the permittivities preferably employed. Thls
is completely suf'ficient for the intended use as stress
control elements for high voltage accessories.
Furthermore~ it has been found that the addition of a
conductive material showing metal conductivity, as known
per se from the German Offenlegungschri~t, 28 21 017, in the
form o~ ~inely divided particles, the load carrying
capabllity and the st,ress control action are strongly
improved at high frequencies, e.g,, as may occur with loads
produced b~ shock waves or lightening strikes in high
voltage llnes. Experiments with impulse voltage loads o~
a duration of 1.2 per 50 ~s have shown that the stability
against such impulses can be increased by up to 100 per
cent by the additional use of platelet-shaped conductlve
matérial. The conductive material may consist simply o~
aluminum which is easily and commercially available in
form of thin platelets or flakes. The conductive material
may also consist of vacuum metalized microspheres based on
glass spheroids or plastic spheroids. In the latter case,
it is generally sufficient if the microspheres are only
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super~icially conductive. The area size of the conductive
microspheres should ~e ln the same range as the area of
conductive platelets, which means that the conductive
surface of both types Or particles will be comparable.
In order to increase the dielectric strength of the
composition accordlng to the invention, it may be advan-
tageous to additionally intermix, as an additional effect
material, an insulating material which is different from
the base material, in finely divided form as platelet-
shaped particles ~See German OLS 2821017). Thereby, the
concentration of bridges between directly contacting par-
ticles of the e~fect material and of the additional con-
ductive ef~ect material is strongly reduced. Preferably,
the insulating material has a higher dielectric strength
than the base material to thereby increase dielectric
strength of the composition. In order to not adversely
affect, and possibly increase the refractive action of the
composition, it is advantageous to use an insulating
matcrial havin~ a permlttlvity which is at least equal to
that of the base material. An insulating material which
is particularly suited as an additional effect material is
mica which inherently is of platelet structure. Also,
when employing an insulating material different from the
base material, the boundary conditions stated above are
maintained.
The size of the platelets of the additional effect
materlal is o~ ~mportance ~or dielectric homogeneity of
the composition ln relation to the dimensions of the
structural parts made therefrom, With the dimensions and
flash-over distances whlch are appropriate for alternating
voltages from about 3 k~, the platelets of the conductive
material may have a transverse dlmension~ measured trans-
verse of their thickness, of about 5 to 75 ~m; an
advantageous intermediate range is 10 to 40 ~m. The
thickness of the platelet-shaped particles should be not
more than about one tenth of the trans~erse dimension to
retain the character of a platelet. Of course~ the same
also holds for the platelets of insulating material, and
they may be somewhat larger than plakelets of conductlve
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material, particularly with respect to their trans~e~se
dimensions.
It is particularly advantageous that, as known from
German Offenlegungsschrift 28 21 017, the platelets of
conductive material need not be oriented in definite
directions but, rather, may be distributed with an essen-
tially random-dlstribution orientation of their platelet
planes. Thus, particular manufacturing steps to orient
the platelets~ e.g., calendering, pasting-on, and the like,
are not necessary when compositions containing platelet-
shaped particles according to the invention are produced.
The dielectric composition according to the invention
can be prepared by inter-mixing the effect material, the
conductive ef~ect material and, where desired, the addi-
t~onal insulating effect materials with a liquid or pasteof flowable or die-castable base compound which is capable
of being hardened to the permanently resilient composition,
for example by cold or hot vulcanizing.- The hardening can
be performed in molds, whereby permanently resilient
bodies of a desired shaped configuration can be directly
obtained which are suitable as stress control elements.
Injection mol~ing, casting, die casting, etc., are also
satisfactory manufacturing techniques.
With particular kinds of carbon black which are
particularly suited for the purposes of the present inven-
tion, i e. which even at low concentrations cause a strong
increase in permlttivity, it has been found to be
appropriate to introduce the carbon black in a phase of
the preparation process where the viscosity is as low as
possible. Then, the dispersion of the carbon black is more
uniform.
Brief Description of Drawings
Figs. 1 to 5 illustrate the properties o~ dielectric
composi~ions which have been prepared with the e~fect
material carbon black, type N 75~, as set forth below.
Figs. 6 to 9 illustrate the properties Or dielectric
compositions according to the invention, which contain an
additional ef~ect material in the form o~ conductive
platelets.
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~ig. 10 illustrates composltions according to the
invention, prepared with silicone rubber type 101~30 and
as the e~fect material carbon black, type N 762, as set
forth below.
The invention will now be more speclfically illus- !
trated by the following examples, wherein all parts are
by weight unless otherwise specified, and whlch are
discussed in conjunction with the attached drawings.
Examples
One hundred parts by weight of silicone rubber base
material of the type S 2351 of Dow Corning were intermixed
with different parts by weight carbon black ~N 754 of
Columbian Carbon Company, New York) and various parts by
weight aluminum platelets (No-. 4-501 of Reynolds Metal
Company, Richmond, Va., USA.).
In each case, the mixtures were intermixed wlth 0.4
parts by weig~lt of the catalyst dicumylperoxide ~of the
type~'Dicup R" Or Herkules~, and cured in molds to form
permanently resilient test bodies of about 3 mm thickness.
Fig. 1 lllustrates the relationship between the
logarithms of the relative dielectric constant (permit-
tivity) and the electrical volume resistivlty for
different proportions of carbon black. It is apparent that
in the medium range of curve A, i.e., with the proportion
of carbon black where the absolute value of the ~lope i5
higher than in the adJacent ranges (i.e. the ~irst
deriva~ive ha~ an extremum), relatively high permittivities
are evident at still very high volume resistivitles, for
instance ~r = 5 at ~ = lolor~ cm. Only from ~r = 200
does the volume reslstivlty commence to drop to values
which are critical with respect for use as an insulating
material; the readily useful range extends to about
~r = 150.
Fig. 2 illustrates that at higher frequencies,
permittivity decreases relatively rapidly. Curve B
designates 1.0 kHz, Curve C 24 kHz and Curve D 53 Hz.
Fig. 3 illustrates that, as expected, the dielectric
loss factor increases with increasing proportions of
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carbon black; in contrast thereto, the dependence upon
frequency is less distinct. Curves B, C and D relate to
the same frequencies as ~ig, 2.
Fig, 4 illustrates - similarly as Fig. l - the
relation between the relative dielectric constant (permit-
tivity) ~r (,at 50 cps~ (Curve E) and the dielectric volume
resistivity ~CCurve ~) when the proportions of the effect
material carbon black (type N 754) are increasing. In
this instance lO parts of Type 4-501 aluminum flake were
also contained in the elastomer.
Fig. 5 illustrates the dependence of specific direct
current conductivity ~the reciprocal of the volume
resistivity) upon the field strength at various tempera-
tures for a test body having 32 phr carbon black of the
kype N 754. Curve G relates to 20C, Curve H 50C, Curve
I 800C and Curve J 100C.
Fig. 6 illustrates for a dielectric compositlon
having 32 phr carbon black of the type N 754 and lO phr
aluminum platelets for the type 4-501 (REYNOLDS), that the
direct current resistivity is not increased by the intro-
duction o~ the conductive effect material; a similar
dependence to that shown in Figure 5 exists. Curves G,
H, I and J relate to the same temperatures as Fig. 5. Both
figures, moreover, illustrate the posltive temperature
coefficient of volume resistivity.
Figure 7 illustrates the dielectric loss factcr
tan ~ and the dielectric constant (permittivlty) ~r
dependence upon the ~requency f at various temperatures
(same as F'ig. 5) for dielectric materials having 32 phr
carbon black of the type N 754 and lO phr aluminum
platelets of the type 4-50l. It can be recognized that ~t
ls easily possible to adjust the dielectric loss factor
close to values at least about l at low frequencies as
well as at high ~requencies and that the permittivity at
room, and even elevated temperatureg is characterized by
values between 50 up to lOO at low ~requencies (e.g.,
50 Hz) and to values at least about 20 at high frequencies
(,e.g., 105 Hz~.
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Flgure 8 illustrates that a dielectric mat~rial corl-
taining 32 phr carbon black (type N 754) provides, due to
the addition o~ 10 phr aluminum platelets an essentlal
increase of the relative dielectric constant (permittivity)
at low frequencies (e.g., 50 Hz) from about 250 up to 400
maximum, whereas at high frequencies (e.g., 105 Hz) the
relative dielectric constant is held at about 25. Curve
K represents 32 phr carbon black, and 10 phr aluminum,
Curve L with no aluminum~ and Curve M with no aluminum and
30 phr carbon black.
Figure 9 illustrates that this increase in relative
dielectric constant is only associated with a minor
increase in dielectric loss factor.
Figure 10 illustrates, in a manner similar to Fig. 4,
the properties of a dielectric material made with other
components. As a base material, silicone rubber type
101/30 of the firm Wacker~Chemle, Munchen~ was used; as an
ef~ect material carbon black of the brand N 762 of Columbian
Carbon Company; and as an additional e~ect material, the
already described aluminum platelets. One hundred parts
by weight of the base materials were intermixed with 6
parts by weight o~ the aluminum platelets, different
amounts of the carbon black, and 1.5 parts by welght of the
above mentioned catalyst dicumyl peroxide ~ Vicup 40C~',
filled into molds, and cured. It is apparent from Figure
10 that basically the same characteristics are obtained as
illustrated in Figs. 1 to 9. However, the first derivative
of the function log g - f (log ~r) reaches its extremum at
a different, i.e,~ lower, carbon black concentration than
shown ln ~ig. 4.
According to present understanding, it appears to be
predominantly important for the described invention that
the particles of ef~ect material have a surface area whi~h
is large in relation to their mass, and capable of offering
a certain resistance to the displacement of electrical
charges. It may be imagined that other effective materials,
which have properties similar to carbon black, may produce
similar or perhaps even better results than carbon black,
and the present invention provides a teac~ing to the skilled
4 expert how to test substances which may be suitable as
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e~ect materials. In cases where respectlve materlal data
for the effect materials are known whi¢h characterize the
structure and performance of its particles, and where these
data are useful also to characterize the effectiveness of
the present invention in accordance with the rating criteria
described in this specification, it may be su~ficlent to
simply apply such data for repeat orders for commercially
available brands of suitable effect materials. In the
above examples, the carbon black brands are characterized
by ASTM designations according to US standards, under which
they are also commercially available.
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