Irradiation cross-linked polymeric insulated electric cable

Insulation for an electric power cable, and a power cable including such insulation, the insulation having an improved dielectric strength and being irradiation cross-linked polymeric material having mixed therewith carbon black having a particle size in the range from about 200 to about 500 millimicrons, the carbon black content being about 10% to about 40% of the weight of the mixture of carbon black and the polymeric material. Also, the cable insulation may be layers of different density polyethylene, at least one of the layers being the described mixture of polyethylene and carbon black.

This invention relates to irradiation cross-linked, polymeric, electrical 
insulating material and particularly to polymeric insulation of electric 
cables which has been cross-linked by irradiation. 
The use of cross-linked polymeric insulation in electric power cables to 
produce certain desirable mechanical or electric characteristics is well 
known in the art. See, for example, U.S. Pat. Nos. 3,325,325; 3,749,817; 
3,769,085; 3,387,065; 3,725,230; and 3,852,518. In some cases, the 
cross-linking is caused by irradiating the polymeric material with high 
energy electrons. 
It is known in the art to incorporate carbon black in cross-linkable 
polymeric materials for filling or coloring purposes to make such 
materials semi-conductive. If the cross-linked material is to serve as 
insulation, it should have a volume resistivity of at least the order of 
1.times.10.sup.10 ohm-cm. at 23.degree. C. and preferably, 
1.times.10.sup.15 ohm-cm. at such temperature. To obtain such resistivity, 
a medium thermal (MT) type of carbon black having a particle size in the 
range of 200-500 millimicrons usually is mixed with the polymeric material 
in amounts of up to 2.5% of the total weight of the mixture. When the 
cross-linked material is to serve as a semi-conducting material, the 
volume resistivity generally is below 1.times.10.sup.5 ohm-cm. at 
23.degree. C., and to obtain such resistivity, channel black having a 
particle size in the range of 20-50 millimicrons usually is mixed with the 
polymeric material in various amounts, usually in the 30-40% range, 
depending upon the desired resistivity. In other words, if the 
cross-linked material is to serve as insulation, relatively small 
quantities of a relatively coarse carbon black is mixed with the 
cross-linkable material whereas if the cross-linked material is to be 
semi-conducting, rather than an insulator, relatively fine channel black 
is mixed with the cross-linkable material. 
It has been discovered that the dielectric strength of irradiation 
cross-linked, polymeric, insulating material can be increased by a factor 
of at least two without reducing the volume resistivity thereof below 
1.times.10.sup.15 ohm-cm. at 23.degree. C. by significantly increasing, 
the amount of coarse carbon black mixed with the cross-linkable polymeric 
material prior to its being extruded and subjecting it to radiation. Thus, 
in accordance with the invention, carbon black having a particle size in 
the range from about 200 to about 500 millimicrons and in the range of 
from about 10% to about 40% of the weight of the mixture of carbon black 
and polymeric material is mixed with the cross-linkable, polymeric 
material prior to its being extruded and subjecting it to irradiation. 
While not purporting to explain fully the reason for the significant 
improvement in the dielectric strength, it is believed that the increase 
in the amount of coarse carbon black, as compared to the amount normally 
used for filling or coloring purposes, substantially increases the 
diffusion of the electrons as they traverse the cross-linkable material 
and thereby minimizes the development of electron tracks or "trees". Such 
tracks or trees affect the dielectric strength of the insulating material, 
larger or more numerous trees reducing the dielectric strength. Amounts of 
such carbon black up to 40% of the total weight of the mixture of carbon 
black and polymeric material do not reduce the volume resistivity of the 
cross-linked material below 1.times.10.sup.10 ohm-cm. whereas larger 
amounts adversely affect the insulating properties of the cross-linked 
material. Preferably, the coarse carbon black content is about 20 to 30% 
of the weight of the mixture of the two. The carbon content of the 
irradiated insulating material is the same as the carbon content of the 
material prior to irradiation. 
One object of the invention is to provide a radiation cross-linked, 
polymeric insulating material which has a dielectric strength which is 
substantially higher than the dielectric strength of similarly irradiation 
cross-linked, prior art, insulating materials. 
A further object of the invention is to provide an electric power cable 
having a conductor which is insulated by one or more layers of an 
irradiation cross-linked, polymeric material which has an improved 
dielectric strength as compared to prior art cables with a conductor 
similarly insulated.

At least one of the layers 3 and 4 is a layer of insulating polymeric 
maerial having the composition of the invention, that is, it is a 
radiation cross-linked, polymeric material with a volume resistivity of at 
least 1.times.10.sup.10 ohm-cm. and containing carbon black having a 
particle size in the range of 200-500 millimicrons and in an amount, by 
weight, in the range of 10-40% of the total weight of the polymeric 
material and carbon black. Preferably, the carbon black is a carbon black 
known commercially as a "medium thermal" type. Although carbon black 
having a particle size outside the range of 200-500 millimicrons may also 
be present in small amounts, the amount of carbon black having a particle 
size smaller than 200 millimicrons must be less than an amount which will 
cause the volume resistivity to be less than 1.times.10.sup.10 ohm-cm. The 
polymeric material may be any of the known materials which are 
cross-linkable by radiation treatment and may, for example, be 
polyethylene, polyvinyl chloride, silicone rubber, styrene butadiene 
rubber, ethylene copolymers including ethylene propylene rubber, ethylene 
terpolymers, mixtures of such polymers, etc. 
Preferably, the carbon black is present in an amount of 20-30% by weight 
and most preferably, in an amount of about 28% and the volume resistivity 
of the insulating layer is at least 1.times.10.sup.15 ohm-cm. 
In a preferred embodiment of the cable of the invention, both of the layers 
3 and 4 are made of the irradiated, cross-linked polymeric material of the 
invention, and for example, the layer 3 may be low density polyethylene 
and the layer 4 may be either high density or medium density polyethylene, 
each layer containing carbon black in the amounts and of the particle size 
described. However, one of the two layers 3 and 4 may be of a different 
material, and if desired one of the two layers 3 and 4 may be omitted, the 
remaining layer being of the irradiated cross-linked polymeric material of 
the invention. As used herein, the terms "low", "medium" and "high" 
density polyethylene refer to the ASTM Type I, Type II and Type III 
standards, namely, low density polyethylene has a density from about 0.910 
to about 0.925 gms/cm.sup.3, medium density polyethylene, about 0.926 to 
about 0.940 gms/cm.sup.3 and high density polyethylene, about 0.941 to 
0.965 gms/cm.sup.3. 
Alternatively, the layer 3 may be semi-conductive layer, such as a layer of 
radiation cross-linked, polymeric material, having a volume resistivity of 
1.times.10.sup.5 ohm-cm. or less, for conventional stress distribution 
purposes, and the layer 4 would be a layer of the radiation cross-linked, 
polymeric material of the invention. 
Of course, the cable 1 may have additional layers of various materials 
either intermediate a layer 4 made of the insulating material of the 
invention and the conductor 2 or externally of the layer 4, e.g. an 
armoring or shielding layer. In other words, the insulating material of 
the invention may be used as electrical insulation wherever such is 
required. 
The insulating material of the invention may be prepared by prior art 
processes and may include, in addition to the carbon black and the 
polymeric material, other materials conventionally employed in making 
radiation cross-linked, polymeric, insulating materials. In the 
manufacture of an electric cable, such as the cable 1, one or more layers 
of the prepared polymeric material are extruded separately or 
simulataneously over the conductor 2 in a conventional manner, and 
thereafter, the layer or layers of the material are subjected to radiation 
in the appropriate doses and as required to produce the cross-linking, 
such as is described in said patents. 
Although preferred embodiments of the present invention have been described 
and illustrated, it will be understood by those skilled in the art that 
various modifications may be made without departing from the principles of 
the invention.