U.S. patent number 3,666,876 [Application Number 05/055,776] was granted by the patent office on 1972-05-30 for novel compositions with controlled electrical properties.
This patent grant is currently assigned to Esso Research and Engineering Company. Invention is credited to Eric O. Forster.
United States Patent |
3,666,876 |
Forster |
May 30, 1972 |
NOVEL COMPOSITIONS WITH CONTROLLED ELECTRICAL PROPERTIES
Abstract
An electrical conductor shielded with an insulating coating
comprising a semi-conductive material of a polymer having a
dielectric constant of at least 2.0 and a filler selected from the
group consisting of electrically conductive metals and their
alloys, said filler having a particle size range of about 0.05 to
about 50 microns, said particles having an approximately linear
distribution of particle size with an average particle size of
about 1 to about 20 microns.
Inventors: |
Forster; Eric O. (Scotch
Plains, NJ) |
Assignee: |
Esso Research and Engineering
Company (N/A)
|
Family
ID: |
22000082 |
Appl.
No.: |
05/055,776 |
Filed: |
July 17, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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678655 |
Oct 27, 1967 |
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Current U.S.
Class: |
174/36;
174/102SC; 174/120SC; 174/105SC |
Current CPC
Class: |
H01B
9/027 (20130101); H01B 3/004 (20130101) |
Current International
Class: |
H01B
9/02 (20060101); H01B 3/00 (20060101); H01B
9/00 (20060101); H01b 009/02 () |
Field of
Search: |
;174/36,12SC,15SC,16SC,12SC,12R,127
;117/75,127,128,128.4,230,232 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Myers; Lewis H.
Assistant Examiner: Grimley; A. T.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. Pat.
application, Ser. No. 678,655, filed Oct. 27, 1967 now abandoned.
Claims
What is claimed is:
1. A shielded electrical conductor comprising:
a. an electrically conductive conductor;
b. a first insulating semi-conductor composition coating
surrounding said conductor comprising:
1. a polymer having a dielectric constant of at least 2.0, and
2. a filler selected from the group consisting of electrically
conductive metals and their alloys, said filler having a particle
size range of about 0.05 to about 50 microns, said particles having
an approximately linear distribution of particle sizes with an
average particle size of about 1 to about 30 microns,
wherein said first insulating semi-conductor coating comprises
about 20 to about 40 wt. percent, based on the total composition of
filler;
c. a series of additional insulating coatings laid one upon the
other over the first insulating coating, said additional coatings
comprising the semi-conductor composition of (b) (1) and (2); each
succeeding coating containing about 10 to about 30 wt. percent more
filler than the preceding coating up to a maximum of about 90 wt.
percent based on the total composition; and
d. a final outer conductive coating.
2. The composition of claim 1 wherein the final outer coating
comprises:
a. a polymer having a dielectric constant of at least 2.0; and
b. about 80 to about 90 wt. percent based on the total composition
of a filler selected from the group consisting of electrically
conductive metals and their alloys, said filler having a particle
size range of about 0.05 to about 200 microns.
3. The composition of claim 2 wherein the filler is present at
about 90 wt. percent and has a particle size range of about 0.05 to
about 50 microns, said particles having a linear distribution of
particle size with an average size of about 1 to about 30
microns.
4. An insulated conductor which comprises an electrically
conductive conductor insulated with a semi-conductor coating
composition comprising:
a. a first layer of the semi-conductor composition comprising:
1. a polymer having a dielectric constant of at least 2.0, and
2. a filler selected from the group consisting of electrically
conductive metals and their alloys, said filler having a particle
size range of about 0.05 to about 50 microns, said particles having
an approximately linear distribution of particle sizes with an
average particle size of about 1 to about 30 microns,
wherein said first insulating semi-conductor coating comprises
about 70 to about 90 wt. percent based on the total composition of
filler;
b. a series of additional layers of semi-conductor composition laid
one upon the other over said first layer, said additional layers
comprising the composition of (a) (1) and (2); each of said
additional layers containing about 10 to about 30 wt. percent less
filler than the preceding layers; and
c. a final outer layer of said semi-conductor composition
containing about 20 to about 40 wt. percent filler.
Description
BACKGROUND OF INVENTION
Elastomers such as butyl rubber, ethylene propylene copolymers,
ethylene propylene terpolymers (EPDM) and SBR have been used for
wire insulation and generally as electrical insulators because of
their inability to conduct electrical currents. In many
applications involving high electrical stress of 10 kv or greater,
it becomes difficult if not impossible to join cables without
enhancing electrical breakdown at the splice.
When high voltage electric current flows through a conductor, a
field is set up in the insulation surrounding the conductor. In
order to splice a second conductor onto the first conductor, it is
necessary to remove a portion of the insulation. Reinsulating the
stripped wire with the same type of insulation is unsatisfactory
since the discontinuity between the insulations results in field
breakdown and arcing across the insulated splice. Similarly, at
terminal points where the insulation of the cable has been removed
for contact purposes, it is necessary to protect the terminal and
base wire from corona discharges to ground. Most insulating
materials are inadequate, at conventionally used thicknesses, in
preventing such discharge and breakdown. Theory predicts that one
could avoid these difficulties if one could produce a material, the
electrical conductivity of which is a function of applied stress
provided that such dependency would be reversible.
High voltage conductors are commonly insulated with crosslinked
polyethylene. This material, however, has the disadvantage that it
is prone to bubble formation during curing and hence breakdown at
high voltages. Further heat generation within the polymer as a
result of the high voltage field surrounding the conductor causes
thermal degradation. If the acceleration potential, i.e., voltage
drop, across the insulator could be reduced, arcing and breakdown
could substantially be eliminated.
Semi-conductive tapes have been formed by incorporating silicon
carbide into PVC polymers. These tapes have been useful in avoiding
field breakdown in splices in high voltage lines.
Pseudo-semi-conductive tapes have also been formed by incorporating
carbon black in all types of polymers, including suitably
crosslinked polyethylene. These types of tapes have found wide use
as separators between the central metallic conductor and the
insulation layer particularly where the metallic conductor is
formed by strands of metal lines. Because of their rather low
resistance value, these carbon black filled polymers are also used
at times as an electrical ground at the outside of a cable since it
is less prone to corrosion than would be metallic grounds.
SUMMARY OF INVENTION
It has surprisingly been found that semi-conductors suitable for
use as insulating materials for high voltage line splices may be
prepared by dispersing particles of metal and magnetic or
non-magnetic alloys, into a polymer, the polymer preferably having
a dielectric constant of at least 2.0. Additionally, by controlling
the concentration of the particles as a function of thickness of
the insulator, an insulation material may be prepared having
controlled electrical properties that are not subject to breakdown
at high voltages.
In the practice of this invention metal or alloy particles are
dispersed in a polymer, preferably an elastomer, and compounded
with various curing agents and bonding agents. The compounded
material is then extruded onto an electrical conductor and cured in
place.
The compositions of this invention may be formed into tapes for
wrapping splices. Similarly, heat shrinkable tubes may be formed to
be placed over splices and shrunk into place.
In a particularly preferred embodiment, these compositions are used
to prepare insulated conductors having unusually high breakdown
voltage comprising a conductor coated with multiple layers of
insulation, each layer, in an outward direction from the conductor,
containing a lesser amount of filler than the previous layer. The
coating nearest the conductor may contain as much as 90 wt. percent
filler. The outermost layer preferably contains about 20 to 40 wt.
percent filler.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 presents a comparison of high voltage current
characteristics for metal filled synthetic rubbers.
FIG. 2 is a plot of the high voltage current characteristics of
heat shrinkable tubing.
FIG. 3 illustrates the structure of multilayered insulated
conductors.
DETAILED DESCRIPTION
Throughout the specification and claims, the term "electrically
conductive metals" means those materials which have a resistivity
of between about 10.sup.-.sup.6 and about 10.sup.-.sup.1
ohm-centimeters. Typical of such electrically conductive metals are
magnesium, the metals of Group VIII of the Periodic Table of the
Elements (E. H. Sargent & Co., 1966) such as iron, cobalt,
nickel and platinum, the metals of Group I-B of the Periodic Table
such as silver, gold and copper or the metals of Group II-B of the
Periodic Table of the Elements such as zinc and aluminum.
Illustrative of the magnetic materials suitable for use in the
practice of this invention are alloys such as Fe-Ni, Co-Ni,
Cu.sub.2 MnAl, and Cu.sub.2 MnSn. Illustrative of suitable
non-magnetic materials are alloys such as Cu-Sn, Cu-Zn, and
Al-Mg.
The term "semi-conductor" as used in the specification and claims
means a material whose conductivity is a non-linear function of the
applied stress, i.e., voltage potential, and which has a
resistivity of between 10.sup.1 to 10.sup.10 ohm-centimeters. While
metals tend to have increasing resistivity with increasing
temperature, the semi-conductors show a decrease in resistivity
with increase in temperature.
Under low voltage stresses, the semi-conductors act as insulators
and conduct no electricity. As the voltage increases to the order
of 10-100 kilovolts, the semi-conductors begin to conduct
electricity, the conductance being a function of the stress applied
and increasing as the applied stress increases.
This behavior is quite unique and cannot be achieved with carbon
black filler polymers. The reason for this difference is believed
to result from the different structure of metallic fillers and
carbon black. The latter forms "structural units," i.e. chains of
particles which cannot be broken up under normal mixing conditions.
Metal fillers as contemplated in this invention do not possess such
a structure but are known to consist of individual, discrete
particles unless they are magnetic in which case they can form
agglomerates which can be readily broken up in normal mixing
operations. The basis of this invention therefore appears to be the
formation of a uniform dispersion of discrete metal particles all
essentially insulated from each other by very thin polymer films.
As the voltage increases beyond a certain critical value, the
electrons emanating from one particle can tunnel through the thin
film into the next particle and thus cause a rapid increase in
current flow without causing permanent dielectric breakdown. In the
case of the carbon black filled polymers, it has been noted that
under similar conditions, permanent breakdown results. This
observation is attributed to the inability of forming uniform
dispersions of carbon black in polymers so that the number of
direct carbon-carbon contacts and the separation of carbon chains
from each other are hard to control. In these carbon filled
polymers, sudden voltage surges lead to heavy current flow through
the few existing contacts and hence to overheating which results in
the formation of carbonization of the adjacent polymeric material.
The net result of such a surge is the formation of highly
conductive tracks and permanent damage to the material.
Any polymer which may be readily extruded or coated onto an
electrical conductor is suitable for use in the practice of this
invention. Preferably, the polymer has a dielectric constant
greater than 2.0. Illustrative of such polymers are polyethylene,
butyl rubber, ethylene-propylene copolymers, ethylene-propylene
terpolymers (EPDM), styrene butadiene rubber, polyvinyl chloride
and mixtures thereof.
The expression "butyl rubber" as employed in the specification and
claims is intended to include copolymers made from a polymerization
reaction mixture having therein about 70 to about 99.5 percent by
weight of an isoolefin which has about 4 to 7 carbon atoms and
about 30 to 0.5 percent by weight of a conjugated multiolefin
having about four to about 14 carbon atoms. The resulting copolymer
contains 85 to 99.5 percent of combined isoolefin and 0.5 to 15
percent of combined multiolefin. The term "butyl rubber" is
described in an article by R. M. Thomas et al. in Industrial
Engineering and Chemistry, vol. 32, pages 1,283 et seq., Oct.,
1940.
The butyl rubber generally has a Staudinger molecular weight
between 20,000 to 500,000; preferably about 25,000 to about
200,000; especially 45,000 to 60,000; and a Wijs Iodine No. of
about 0.5 to about 50; preferably 1 to 15. The preparation of butyl
rubber is described in U.S. Pat. No. 2,356,128 which is
incorporated herein by reference.
Typical of such butyl rubber is Enjay Butyl 035 (Enjay Chemical
Co.) a polymer having a Mooney viscosity at 212.degree. F. of about
38-47 and a mole percent unsaturation of about 0.6 to about
1.0.
The term "EPDM" is used in the sense of the definition as found in
ASTM-D-1418-64 and is intended to mean a terpolymer containing
ethylene and propylene in the backbone and a diene in the side
chain. Illustrative methods for producing these terpolymers are
found in U.S. Pat. No. 3,280,082 and British Pat. No. 1,030,989
which are incorporated herein by reference. Any EPDM may be used in
the practice of this invention.
The diene monomer is preferably a nonconjugated diene. Illustrative
of these nonconjugated diene monomers which may be used in the
terpolymer (EPDM) are hexadiene, dicyclopentadiene, ethylidene
norbornene, methylene norbornene, propylidene norbornene, and
methyl tetrahydroindene. The particular diene used does not form a
critical part of this invention and any EPDM fitting the above
description may be used. A typical EPDM is Vistalon 4504 (Enjay
Chemical Co.) a polymer having a Mooney viscosity at 212.degree. F.
of about 40 prepared from a monomer blend having an ethylene
content of about 56 wt. percent and a non-conjugated diene content
of about 3.3 wt. percent.
The electrically conductive metal particles and ferromagnetic
particles used in this invention should have a particle size fine
enough to pass through a 425 mesh screen, i.e. less than 50
microns, preferentially through a 500 mesh screen, i.e. less than
45 microns. The particle size may vary between about 0.05 to about
50 microns, more preferably about 0.05 to about 45 microns, most
preferably about 0.05 to 30 microns. The average particle size is
preferably 1 to about 30 microns; more preferably about 10 to about
20 microns. To produce satisfactory results, in terms of
elongation, tensile strength and electrical properties, it is
advisable to use a powder of broad but uniform particle size
distribution. This can be achieved by intentionally mixing powders
produced by grinding particles and sieving them through a 500 mesh
screen, for example, and admixing to this material a fraction of
colloidally produced powder (i.e. ca 0.05 micron) of the same
material. The methods for producing colloidal particle size
powdered metals is well known to the art.
By controlling the ratio of the two fractions, one can maximize
either electrical or mechanical properties or produce a compromise
which satisfies the particular needs under consideration. The
concentration of the electrically conductive metal powders in the
polymer matrix may be from about 20 to about 90 wt. percent,
preferably from about 40 to about 80 wt. percent, more preferably
from 50 to 70 wt. percent.
The terms "filler" or "filled" as used throughout the specification
and claims refer to the electrically conductive particles of metal
and magnetic or non-magnetic alloys of this invention.
In the practice of this invention, it has been found advantageous
to use a coupling agent which serves to bring about a better bond
between the metal particles and the polymer matrix. Preferably the
coupling agent is an unsaturated organosilane which is employed in
amounts ranging from about 0.1 to about 5, preferably about 1 to
about 4 parts per weight per hundred parts of polymer mix. Although
the metal particles may be treated with the organosilane rather
than adding the organosilane to the polymer mix, the latter
technique has been found to be more convenient.
The term "organosilane" as employed herein includes the silane, its
silanols (the corresponding partially or completely hydrolyzed
forms of the silane), its siloxanes (the corresponding condensation
products of the silanols) and mixtures thereof. The organosilane
may be represented by the formula:
wherein R.sub.1 is a C.sub.2 -C.sub.16 radical containing vinyl
type unsaturation selected from the group consisting of alkenyl
styryl, alkenyl alkyl, alkenoloxalkyl; X is selected from the group
consisting of hydroxyl, alkoxy acyloxy; R.sub.2 and R.sub.3 are
independently selected from the group consisting of hydroxyl,
methyl, alkoxy, acryloxy and R.sub.1. Nonlimiting useful compounds
which may be employed are the following: vinyl
tri(beta-methoxy-ethoxy)-silane, vinyl triethoxy silane, divinyl
diethoxy silane, allyl triacetoxy silane; in place of the vinyl and
allyl groups of the above named compounds, the corresponding
styryl, acryloalkyl, methacryloalkyl, acryloxy propyl and
methacryloxy propyl compounds may be used. All of the silanes are
convertible into the useful corresponding silanols by partial or
complete hydrolysis with water. The preferred organosilanes are
gamma-methacryloxypropyl trimethoxy silane and vinyl
tri-beta-methoxyethoxy silane.
The following examples serve to further illustrate how the
processes of this invention may be carried out as well as the
benefits derived from its use.
EXAMPLE 1
A series of compounded mixes designated as samples A-F were
prepared using Vistalon 4504 and Enjay Butyl 035 as the polymers.
The metal or ferromagnetic powders used were iron powder, iron
oxide powder and aluminum powder. The exact formulation of these
blends is shown in Table I.
In the preparation of these blends, the polymer was milled on a
cool (i.e. below 130.degree. F.) mill and allowed to band. The
metal powder or magnetic material was gradually added to the banded
polymer at a rate sufficient to prevent destruction of the polymer
band. After the filler material was added, the other compounds were
then added starting with the silane. The vulcanizing agents were
added last and the mixture was allowed to blend for about 20 to
about 30 minutes. The samples were then press cured at 320.degree.
F. for 20 minutes to form 70 mil pads, the physical properties of
which are shown in table I. The electrical properties of these
filled metal elastomers are shown in Table I. The electrical
properties of these filled metal elastomers are shown in Table
II.
It will be noted that as the applied voltage is increased from 100
volts to 3,500 volts, the current increases. It is noted further
that there is no hysteresis effect, that is in going from the lower
voltage to the higher voltage and back to the lower voltage, there
is essentially no change in electrical properties. ##SPC1##
##SPC2##
These data are shown graphically in FIG. 1. Curve 5 is a typical
curve for a Simplex tape (a silicon-carbide containing PVC) of 20
mil thickness, whereas curve 6 is a typical curve for an elastomer
having no filler. It will be noted that the non-filled material and
those filled with nonconductive Fe.sub.2 O.sub.3 show only the
current gain as a function of applied voltage predictable from
ohm's law. Hence, the electrical stress or voltage drop across any
insulating material made of any such materials will be very large,
whereas in contrast, the electrical stress is decreased in the
semi-conductors of this invention due to increased current
flow.
EXAMPLE 2
A heat shrinkable composition was prepared in the following
manner:
A blend of polyethylene and ethylene-propylene copolymer were mixed
for 1 to 2 minutes in a Banbury mill. Fifty parts of copolymer were
used to 50 parts of ethylene and 1 part of AgeRite D stabilizer
were mixed at 260.degree. F. for 3 minutes. The temperature was
then raised to 300.degree. F. while mixing was continued. At this
temperature the blend was discharged. An aliquot of this master
batch was used in producing a final formulation which
comprised:
Formulation Parts Master batch 60 Polyethylene 40 AgeRite D 0.4
Zinc Oxide 5.0 ERD 90 (Pb.sub.3 O.sub.4, 90%) 5-6 Drimix TAC (25%
dispersion of triallyl cyanurate on microcel) 2 500 mesh iron
powder 150 A-172 silane (vinyl tris methoxyethoxy)silane) 1 Di-Cup
R (Dicumyl peroxide) 2.8
Mixing was accomplished in a Banbury blender according to the
time/temperature schedule shown below. The final composition of the
polymer blend contained 70 parts polyethylene, 30 parts
ethylene-propylene rubber. The ingredients listed above were added
to the Banbury mixer to according to the following time
schedule:
Start:
0 minutes Master batch plus AgeRite resin D charged; 1 minute Add
polyethylene, raise temperature to 220.degree. F. and flux for 3
minutes; 4 minutes Add all other ingredients in sequence shown
except the Di-Cup R; 8 minutes Dump and allow to cool; add Di-Cup R
using a cool mill.
The finished blend was vulcanized at 500 psi and 320.degree. F. for
20 minutes. The resulting 0.032 inch pad was tested electrically,
the results of which test are shown in FIG. 2. It will be noted
that the product had excellent reversible response to electrical
stresses. The material had a breakdown voltage of about 120
kilovolts per centimeter, whereas the unfilled material is known to
fail at stresses below 100 kilovolts per centimeter.
It is often desirable to shield current carrying conductors such as
television antennas and auto electrical coil wirings and plug
wirings to prevent disruptive effects from stray currents. Such
shielded conductors generally have a central conductor covered with
an insulator and an outer conductive shield which is connected to
ground. A similar shielded material having equivalent shielding
protection properties may be prepared as shown in FIG. 3a. The
central conductor, 1, is coated with an insulating composition, 2,
which comprises the semi-conducting material of this invention
containing about 25 percent of the filler particles. The succeeding
layers, 3, 4 and 5, are composed of the semi-conducting material of
this invention, each succeeding layer having a higher percentage of
filler. For example, layer 3 may have from about 30 to 40 wt.
percent filler, layer 4 may have 50 to 60 wt. percent filler and
layer 5 may have 70 to 80 wt. percent filler. The outer layer, 6,
contains about 90 wt. percent filler and in this case the filler
material may consist of particles larger than those specified for
the semi-conducting material of this invention and may be as large
as 200 microns in order to insure that the outer coating is
conductive. The term "conductive coating" as used in the
specification and claims is one which has a resistivity less than
about 10.sup.2 ohm-centimeters. The particle size range in the
conductive coating may vary from about 0.05 to about 200 microns.
Where the larger particles are used, the filler may be present at
about 80 to 90 wt. percent. This outer coating acts as a shielding
and is connected to ground just as in the prior art shielded
terminals the outer sheath is connected to ground.
Using the semi-conductive compositions of this invention, it is
possible to prepare an insulated electrical conductor which has an
unusually high breakdown voltage and the surprising characteristic
that current leakage may occur without actual damage to the
insulating material. Referring now to FIG. 3b, a center electrical
conductor, 1, is coated with a layer of the semi-conducting
material, 2, of this invention having about 80-90 wt. percent
filler particles. The succeeding layers, 3, 4 and 5, each have a
lesser amount of filler, for example, layer 3 has from about 70 to
80 percent filler, layer 4 has from about 50 to 60 percent filler
and layer 5 has about 25 to 50 percent filler.
The semi-conductive characteristics of the coating permit a limited
current flow within the insulator at high voltage stresses and
therefore reduces the acceleration potential across the insulator,
thereby giving the material an unusually high breakdown voltage. As
has been pointed out earlier, the current flow under these
conditions involves many particles all separated by very thin films
so that in no individual pair of large number of electrons is
flowing and hence no significant localized heating occurs. As the
voltage reaches very high values under an overload, tunneling will
become also probable between particles having larger separations,
thus preventing excessive electron passage through a limited number
of tunneling positions. Thus, in the event of overvoltage loads, a
sufficient current flow may occur to prevent actual rupture of the
insulator as would normally be the case in conventional insulation
materials.
Since many different embodiments of this invention may be made
without departing from the spirit and scope thereof, it is to be
understood that the present invention is not limited to the
embodiments specifically disclosed in the specification
thereof.
* * * * *