US 3666876 A
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.
Description (OCR text may contain errors)
United States Patent [151 3,666,876
Forster 1 51 May 30, 1972 541 NOVEL COMPOSITIONS WITH 3,096,210 7/1963 BOOl'lSll'a .1 ..174/120 sc CONTROLLED ELECTRICAL 3,287,489 11/1966 Hvizd, Jr. ..174/ 102 SC P OPERTIES 2,377,153 5/1945 Hunter et al. ..174/120 SC X 72 inventor: Eric 0. Forster, Seotch Plains, NJ. FOREIGN PATENTS OR APPLICATIONS  Assignee: Esso Research and Englneerlng Company 824,861 12/1959 Great Britain ..174/102 SC  Filed: 1970 Primary Examiner-Lewis H. Myers  App]. No.: 55,776 Assistant Examiner-A. T. Grimley Related us. Application Dam Attorney-Chasan and Sinnock and'A. Lagani, J r.
 Continuation-impart of Ser. No. 678,655, Oct. 27, ABSTRACT 1967 abandoned An electrical; conductor shielded with an insulating coating comprising a semi-conductive material of a polymer having a  us I'M/I02 ag 321% dielectric constant'of at least 2.0 and a filler selected from the  Int. Cl. ..H0lb 9/02 group consisting of electrically conductive metals and their 58 Field 61 Search ..174/36, 102 SC, 105 sc, 106 sc, W said having Panicle Size range of 4 120 SC 120 R 12 7 5 2 12 2 about 50 microns, said particles having an approximately I 230 232 linear distribution of particle size with an average particle size I of about 1 to about 20 microns. 56 R f C'ted 1 e erences l i 4 Claims, 4 Drawing Figures UNITED STATES PATENTS 3,433,891 3/1969 Zysk et a] ..174/102 SC Patented May 30,1972 I 3 Shun-Shoo 1 FIG. I
' HIGH VOLTAGE CURRENT CHARACTERISTICS OF METAL FILLED SYNTHETIC RUBBERS 1 I I I I I l l I. EPT 4504 FILLED WITH AI POWDER I 2. ENJAY BUTYL 035 FILLED WITH AI POWDER 3. EPT4505 FILLED WITH Fe POWDER- 4. ENJAY BUTYL 035 FILLED WITH Fe POWDER All in form of 70 mil pad 5. SIMPLEX TAPE (20mil) 6. NON-FILLED ELASTOMER vi a E 'DECREASING 0-" I VOLTAGE 2 u: 4 4 cc m U INCREASING c VOLTAGE 2 6/ 1 I I 4L, -6 I I 1 1 l I I I I APPLIED VOLTAGE V Patented May 30, 1972 3,666,876
' s Shuts-Sh. 2
HIGH VOLTAGE CURRENT CHARACTERISTICS OF HEAT SHRINKABLE TUBING-VULCANIZED PAD I 1 I l I I I I 40% OF (70% POLYETHYLENE so ENJAY EPR 404) a 60% 500 MESH IRON POWDER PAD THICKNESS 0.032"
RUNS ON 2 PADS AVERAGE o VOLTAGE INCREASING [1 VOLTAGE DECREASING log CURRENT, amps.
I IIIIII I I I I I I I I O 2000 4000 6000 V 8000 I0000 APPLIED VOLTAGE 5. 0.13m!" lnv Ior Patented May 30, 1972 3 Shuts-Sh! S E. 0. F yrs/er Inventor NOVEL COMPOSITIONS WITH CONTROLLED ELECTRICAL PROPERTIES CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of US. Pat. application, Ser. No. 678,655, filed Oct. 27, 1967 now abandoned.
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 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 tenninal 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 anelectrical 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 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 and about 10 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 1-8 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 MnAl, and Cu,MnSn. Illustrative of suitable nonmagnetic 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 to 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 10l00 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 fillcrs 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 observationis 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 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-l418-64 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 norbomene, 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 F. of about 40 prepared from a monomer blend having an ethylene content of about 6 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 to about 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 fil1ed" 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 unsatu rated 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 is a C -C 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 and R are independently selected from the group consisting of hydroxyl, methyl, alkoxy, acryloxy and R 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 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 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 l. 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 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.
TABLE I.-POWDERED METAL FILLED ELASTOME RS g A B O D E F Enjay Vistalon 4504 100 100 100 Enjoy buty1035 100 100 100 Zinc oxidc 5 5 .5 5 5 5 iron powder 500 mesh 200 200 0.5 fired iron oxide powdcr 200 200 Aluminum powder 200 200 A-172 silane 1 2 2 2 2 2 2 AgeRitc resin D 2 .1. 5 1. 5 1. 5 ERD 90 (Pba04 dispersed). 5. 6 5.6 5. 6 5.6 5.6 5.6 Drimix TAC 3 2 2 2 Di-Cup 40 HAF 7 7 7 Stearlc acid .5 0. 5
5 5 4 4 2. 85 Press cures 207320 Hardness Shore A 64 71 g 80 48 45 .63
100% modulus, p.s.i 220 220 130 so 160 Tensile strength, p.s.i. 380 260 370 780 140 320 Percent elongation 280 210 80 790 600 710 1 Vinyl trls (methoxy ethoxy) silane.
1 Polymerized trlmethyl dihydroquinoline.
3 25% dispersion of triallyl cyanurate on mlcrocel.
4 Dlcumyl peroxide 40% on carbon black.
4 Parafiin wax having a melting point of 133 F.
Benzothiazyl disulfide (R. 'l. Vanderbilt (30.).
7 P-quinone dloxime.
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:
Cup R using a cool mill.
The finished blend was vulcanized at 500 psi and 320 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.
TABLE III-ELECTRIC PROPERTIES OF METAL FILLED ELASTOMERS A B C D E F EPT EPT EPT Butyl Butyl Butyl Fe Al F8103 Fe Al F 03 4. 6 9. 83 5. 2 4. 8
1. 9X10 1. 2X10 10' l0- 1. 1X10 4X10' 5. 8X10 7Xl0 3. 8X10- 4Xl0' 6X10' 4. 6X10- 10- 3X10 6X10' 6. 2X10 2X10' 1. 2X10' Ill- 10' 1. 1X10 10 1 Measured with a Keithley 610B electrometer using a N.J.E. re ulated power supply with continously variable voltage output from 0 to 30,000 VDC, shielded connectors an a 2" diameter measuring electrode. Samples were measured at 100, 200, 500, 1
000, 1,500, 2,000, 2,500, 3,000 and 3,500 volts, keeping the voltage at these values or at least five minutes. The voltage was reduced in the same manner. Each sample was subjected to at least 3 uch cycles.
These data are shown graphically in FIG. 1. Curve 5 is a typical curve for a Simplex tape (a silicon-carbide containing PVC) of mil thickness, whereas curve 6 is a typical curve for an elastomer having no filler. it will be noted that the nonfilled material and those filled with nonconductive mo, show only the current gain as a function of applied voltage predictable from ohms 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 Formulation Parts Master batch 60 Polyethylene 4O AgeRite D 0.4 Zinc Oxide 5.0 ERD 90 (Pb O 90%) 5-6 Drimix TAC dispersion of triallyl cyanurate on microcel) 2 500 mesh iron powder 150 A-l 72 silane (vinyl tris methoxyethoxy)silane) l Di-Cup R (Dicumyl peroxide) Mixing was accomplished in a Banbury blender according to the time/temperature schedule shown below. The final It is often desirable to shield current carrying conductors such as television antennas and auto electrical coil wirings and plug wirings to prevent disruptive efiects 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 tiller particles. The succeeding layers, 3, 4 and S, 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 to wt. percent filler. The outer layer, 6, contains about wt. percent filler and in this case the filler material may consist of particles larger than those specified for the semiconducting 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 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 Eis 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 5 80-90 wt. percent filler particles. The succeeding layers, 3,
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.
What is claimed is:
l. 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 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 compositigrr ot:
8 (b) (l) 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 to about wt. percent based on the total composition of a filler selected from the group consisting of elecuically 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 comprismg: l. 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) (l) 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.