CA1041189A - Styrene in dielectric layer of electric wires or cables - Google Patents
Styrene in dielectric layer of electric wires or cablesInfo
- Publication number
- CA1041189A CA1041189A CA239,384A CA239384A CA1041189A CA 1041189 A CA1041189 A CA 1041189A CA 239384 A CA239384 A CA 239384A CA 1041189 A CA1041189 A CA 1041189A
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- Prior art keywords
- styrene
- wires
- electric cables
- copolymer
- polystyrene
- Prior art date
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Abstract
ABSTRACT OF THE DISCLOSURE
Electric wires or cables having a dielectric layer or layers of an uncross-linked or cross-linked mixture of (1) a polyolefin and (2a) polystyrene or (2b) a styrene copolymer.
The polystyrene is present in such an amount as to provide a 10 to 30% styrene content, based on the entire mixture weight, while if the styrene copolymer is used it is present in such an amount as to provide a 3 to 30% styrene content, based on the entire mixture weight. The mixture may comprise, alter-natively, both polystyrene and a styrene copolymer in an amount so as to provide a 10 to 30% and a 5 to 20% styrene content, due to the former and the latter, respectively,based on the entire mixture weight.
Electric wires or cables having a dielectric layer or layers of an uncross-linked or cross-linked mixture of (1) a polyolefin and (2a) polystyrene or (2b) a styrene copolymer.
The polystyrene is present in such an amount as to provide a 10 to 30% styrene content, based on the entire mixture weight, while if the styrene copolymer is used it is present in such an amount as to provide a 3 to 30% styrene content, based on the entire mixture weight. The mixture may comprise, alter-natively, both polystyrene and a styrene copolymer in an amount so as to provide a 10 to 30% and a 5 to 20% styrene content, due to the former and the latter, respectively,based on the entire mixture weight.
Description
1 R~CKG~OUND OF THE INVENTION
Field o~ the Invention The present inventlon relates to electric wires or cables.
Description of the Prior Art Electric wires or cables having dielectric layers of polyolefins such as polyethylene, cross-linked polyethylene and the like are in wide use. Such polymers have in recent 10 years been improved so that they are able to withstand high ~ -applied voltages. One important problem at present with such electric wires and cables is that they undergo a gradual de-gradation with time during under continuous applied voltages, resulting in a reduction in their capability to withstand applied voltages. Since the service life of electric wires or cables per se is as long as thirty ~ears, the initial thickness of dielectric layers therefor must be increased over initial re-quirements with present designs so as to oompensate for the above ~ --reduction in their ability to withstand applied voltages with ~ -i 2C the passage of time. On the other hand, recent urban develop- ~ -ments have required increased power transmission capacity for electric wires or cables while keeping volume as small as possible. This has led to the difficult problem, with electric wires or cables having a dielectric layer essentially consisting of a polyolefin, of increasing the transmission voltage - without increasing the thickness of the dielectric layer.
Various approaches to overcome the above problem have been considered, includlng preventlng the occurrence of the above described degradation, whereby the decrease in capability ~-to withstand applied voltages during a thlrty year service life need not be taken into account, and the requirements of urban ~ ' -, ' , . , ' 1 areas regardin~ high transmission capabillty and low volume can be met.
SU~ARY ~P T~E I~VENTION
The inventors, takIng note of the above points, performed extensive studies on t~e aging of electric wires or cables of the foregoing type, and found that aging is primarily caused by trees ~or electrochemical trees, as they are sometimes termed) generated in the polyolefin dielectric from voids ` 10 which develop in the polyolefin dielectric material, and that the degradation described above can be avoided by preventing the generation of trees from the voids in dielectric polyolefins material while significantly decreasing the thickness of the dielectric layer.
The primary object of the present invention is thus to provide electric wires or cables in which the development , of trees is prevented and longer service life is obtained.
The present invention provides electric wires or cables having a dielectric layer or layers of an uncross-linked or e` 20 cross-linked mixture of ~1) a polyolefin and (2a) polystyrene or (2b) a styrene copolymer. The polystyrene copolymer is ' present in such an amount as to provide a 10 to 30% styrene 'f content on the basis of the entire mixture weight, while if the styrene copolymer is used it is present in such an amount as to provide a 3 to 30~ styrene content on the basis of the entire mixture weight. The mixture may comprise, alternatively, both polystyrene and a styrene copolymer in an amount so as to provide a 10 to 30% and a 5 to 20~ styrene content, due to the former and the latter, respectively, based on the entire r 30 weight of the polymers in the mixture.
By the use of the electric wires or cables of the ... .. .
Field o~ the Invention The present inventlon relates to electric wires or cables.
Description of the Prior Art Electric wires or cables having dielectric layers of polyolefins such as polyethylene, cross-linked polyethylene and the like are in wide use. Such polymers have in recent 10 years been improved so that they are able to withstand high ~ -applied voltages. One important problem at present with such electric wires and cables is that they undergo a gradual de-gradation with time during under continuous applied voltages, resulting in a reduction in their capability to withstand applied voltages. Since the service life of electric wires or cables per se is as long as thirty ~ears, the initial thickness of dielectric layers therefor must be increased over initial re-quirements with present designs so as to oompensate for the above ~ --reduction in their ability to withstand applied voltages with ~ -i 2C the passage of time. On the other hand, recent urban develop- ~ -ments have required increased power transmission capacity for electric wires or cables while keeping volume as small as possible. This has led to the difficult problem, with electric wires or cables having a dielectric layer essentially consisting of a polyolefin, of increasing the transmission voltage - without increasing the thickness of the dielectric layer.
Various approaches to overcome the above problem have been considered, includlng preventlng the occurrence of the above described degradation, whereby the decrease in capability ~-to withstand applied voltages during a thlrty year service life need not be taken into account, and the requirements of urban ~ ' -, ' , . , ' 1 areas regardin~ high transmission capabillty and low volume can be met.
SU~ARY ~P T~E I~VENTION
The inventors, takIng note of the above points, performed extensive studies on t~e aging of electric wires or cables of the foregoing type, and found that aging is primarily caused by trees ~or electrochemical trees, as they are sometimes termed) generated in the polyolefin dielectric from voids ` 10 which develop in the polyolefin dielectric material, and that the degradation described above can be avoided by preventing the generation of trees from the voids in dielectric polyolefins material while significantly decreasing the thickness of the dielectric layer.
The primary object of the present invention is thus to provide electric wires or cables in which the development , of trees is prevented and longer service life is obtained.
The present invention provides electric wires or cables having a dielectric layer or layers of an uncross-linked or e` 20 cross-linked mixture of ~1) a polyolefin and (2a) polystyrene or (2b) a styrene copolymer. The polystyrene copolymer is ' present in such an amount as to provide a 10 to 30% styrene 'f content on the basis of the entire mixture weight, while if the styrene copolymer is used it is present in such an amount as to provide a 3 to 30~ styrene content on the basis of the entire mixture weight. The mixture may comprise, alternatively, both polystyrene and a styrene copolymer in an amount so as to provide a 10 to 30% and a 5 to 20~ styrene content, due to the former and the latter, respectively, based on the entire r 30 weight of the polymers in the mixture.
By the use of the electric wires or cables of the ... .. .
- 2 -.,, ~
, . . . . . . .
1 present invention, the disadvantages of the prior ar-~ are overcome in that the development of the trees is prevented to thereby attain service life several to several tens of times as long as that of a conventional polyolefin dielectric insulated wire or cable. As a result, we have succeeded in significantly decreasing the initial thickness of the polyolefin dielectric layer in such wires on cables.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The term styrene content ~) as is used herein means the percent by weight of styrene on the weight of the entire - -polymer mixture. Polystyrene, for example, has a 100% styrene content, while a polyolefin blended with 30~ by weight of polystyrene has a 30% styrene content. As a further example, assuming that 70 parts by weight of polyolefin is blended with 30 parts by weight of SBR which contains 50~ by weight styrene, in this instance the styrene content would be 15~
, The term "polyolefin" as is used herein refers to a polyolefin which is predominantly polyethylene, which can contain, if desired, a small amount of a component such as propylene, butene, butadiene, isobutylene, etc. Preferred of such polyolefins are those which have a melt index of 0.1 or higher and a density of 0.90 to 1.0, and include both low density and high density ethylene homopolymers.
The term "polyolefin" also includes copolymers of ethylene and another comonomer other than styrene, e.g., an ethylene-vinyl acetate copolymer, an ethylene-propylene copolymer, and an ethylene-acrylate copolymer (e.g., an ethylene-methyl-acrylate copolymer, an ethylene-ethylacrylate copolymer), etc. ~ "
Preferred copolymers of ethylene and another comonomer have a melt index of 0.6 or higher and a density of 0.90 to 1.2. If - - ... ..
.
- ~ ~
1~4~39 1 desired, of course, a blend of polyolefins and olefin copolymers can be used in the present invention. An example of a system based on such is a polyethylene: styrene-butadiene rubber:
ethylene-propylene rubber = 95:4:1.
It should be clear from the above discussion that the copolymers of ethylene and another comonomer include, for example, a copolymer of a polyolefin which is predominantly ethylene, e.g., polyethylene with a small amount of polypropy-lene with, e.g., vinyl acetate. While technically a terpolymer, such material still equivalent in this invention to a poly-ethylene homopolymer.
When cross-linking is desired for the dielectric layer, it can be effected through electron beam irradiation, for example, at 3 Mrad to 40 Mrad, after extrusion coating of the dielectric layer or, alternatively, through heating to activate a conventional cross-linking agent, for example, --using about 2% dicumyl peroxide, with heating at 200C to 300C or by like means after extrusion coating.
As well be apparent to one skilled in the art, the dielectric layer or layers of the electric wires or cables of ~ the present invention can have added thereto, if desired, `~ conventional additives such as anti-aging agents and the like in conventionally used amounts, for example, 4,4'-thiobis(6-tert-butyl-m-cresol) and N',N-di-~-naphthyl-p-phenylenediamine in an amount of 0.1 to 0.5~.
When either polystyrene or a styrene copolymer is blended alone with the polyolefin, no remarkable increase in service life can be obtained with a less than 10% styrene ~ ~ ~ , .". ~ , content, while on the other hand, with a more than 30% styrene ~; 30 content, the physical properties of the composition are signi--ficantly degraded, primarily failing the bending characteristics ,, .
~. .
,: , -: . . , ~
1 required in electric wires or ca~les.
When both polystyrene and one or more styrene copolymers are used, if the styrene content attributable to the polystyrene is less than 10~, and the styrene content attributable to the styrene copolymer is less than 5~, based on the entire polymer weight, a remarkable increase in service life is not achieved.
On the other hand, with a more than 30~ styrene content attributable to the polystyrene or with a more than 20~ ;
styrene content attributable to the styrene copolymer, the physical properties of the composition are significantly de-graded, primarily the bending characteristic required in electric wires or cables. Preferred styrene copolymers are styrene-butadiene copolymers containing 20 to 80% styrene.
Preferred polystyrenes are those having a low melting point;
most styrene copolymers used contain at least about 10~ styrene. -~
Preferred cross-linked materials used in the present invention-have a degree of gelation of from about 70% to about 95%.
The present invention will now be described with reference to preferred embodiments and several comparison examples.
EXAMPLES
:
Onto twisted conductors 100 mm2 in cross section, an inner semiconductor layer of a thickness of about 0.8 mm was extruded at 120C and then compositions comprising various compounds as shown in Tables 1 and 2 were extruded at 120C (2 mm thickness) thereover to provide electric wires or cables. -~
As is shown in Tables 1 and 2, where cross-linking by electron beam irradiation is desired, it is eff~cted in a conventional manner at 30 Mrad after extrusion. When chemical cross-linking .. :
iS 9 1 is desired, a cross-linking agent or agents is/are is blended with the composition prior to extrusion and cross-linking efrected by heating at 200C after extrusion coating.
The electric wires or cables thus obtained were immersed in water at 70C, a 20 KV AC voltage applied thereto, -and kept under these conditions for a certain period of time. -~
The development of trees was examined and the time required for dielectric bxeakdown in the insulating layers determined relative to the standard of a polyethylene layer (melt index: l.0, density: 0.92) set as "l". The results are shown in Tables l and 2.
In Table l, polystyrene or a styrene copolymer was used, as ind;cated therein, whereas ;n Table 2, polystyrene plus a styrene copolymer were used, as ;ndicated therein.
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1 present invention, the disadvantages of the prior ar-~ are overcome in that the development of the trees is prevented to thereby attain service life several to several tens of times as long as that of a conventional polyolefin dielectric insulated wire or cable. As a result, we have succeeded in significantly decreasing the initial thickness of the polyolefin dielectric layer in such wires on cables.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The term styrene content ~) as is used herein means the percent by weight of styrene on the weight of the entire - -polymer mixture. Polystyrene, for example, has a 100% styrene content, while a polyolefin blended with 30~ by weight of polystyrene has a 30% styrene content. As a further example, assuming that 70 parts by weight of polyolefin is blended with 30 parts by weight of SBR which contains 50~ by weight styrene, in this instance the styrene content would be 15~
, The term "polyolefin" as is used herein refers to a polyolefin which is predominantly polyethylene, which can contain, if desired, a small amount of a component such as propylene, butene, butadiene, isobutylene, etc. Preferred of such polyolefins are those which have a melt index of 0.1 or higher and a density of 0.90 to 1.0, and include both low density and high density ethylene homopolymers.
The term "polyolefin" also includes copolymers of ethylene and another comonomer other than styrene, e.g., an ethylene-vinyl acetate copolymer, an ethylene-propylene copolymer, and an ethylene-acrylate copolymer (e.g., an ethylene-methyl-acrylate copolymer, an ethylene-ethylacrylate copolymer), etc. ~ "
Preferred copolymers of ethylene and another comonomer have a melt index of 0.6 or higher and a density of 0.90 to 1.2. If - - ... ..
.
- ~ ~
1~4~39 1 desired, of course, a blend of polyolefins and olefin copolymers can be used in the present invention. An example of a system based on such is a polyethylene: styrene-butadiene rubber:
ethylene-propylene rubber = 95:4:1.
It should be clear from the above discussion that the copolymers of ethylene and another comonomer include, for example, a copolymer of a polyolefin which is predominantly ethylene, e.g., polyethylene with a small amount of polypropy-lene with, e.g., vinyl acetate. While technically a terpolymer, such material still equivalent in this invention to a poly-ethylene homopolymer.
When cross-linking is desired for the dielectric layer, it can be effected through electron beam irradiation, for example, at 3 Mrad to 40 Mrad, after extrusion coating of the dielectric layer or, alternatively, through heating to activate a conventional cross-linking agent, for example, --using about 2% dicumyl peroxide, with heating at 200C to 300C or by like means after extrusion coating.
As well be apparent to one skilled in the art, the dielectric layer or layers of the electric wires or cables of ~ the present invention can have added thereto, if desired, `~ conventional additives such as anti-aging agents and the like in conventionally used amounts, for example, 4,4'-thiobis(6-tert-butyl-m-cresol) and N',N-di-~-naphthyl-p-phenylenediamine in an amount of 0.1 to 0.5~.
When either polystyrene or a styrene copolymer is blended alone with the polyolefin, no remarkable increase in service life can be obtained with a less than 10% styrene ~ ~ ~ , .". ~ , content, while on the other hand, with a more than 30% styrene ~; 30 content, the physical properties of the composition are signi--ficantly degraded, primarily failing the bending characteristics ,, .
~. .
,: , -: . . , ~
1 required in electric wires or ca~les.
When both polystyrene and one or more styrene copolymers are used, if the styrene content attributable to the polystyrene is less than 10~, and the styrene content attributable to the styrene copolymer is less than 5~, based on the entire polymer weight, a remarkable increase in service life is not achieved.
On the other hand, with a more than 30~ styrene content attributable to the polystyrene or with a more than 20~ ;
styrene content attributable to the styrene copolymer, the physical properties of the composition are significantly de-graded, primarily the bending characteristic required in electric wires or cables. Preferred styrene copolymers are styrene-butadiene copolymers containing 20 to 80% styrene.
Preferred polystyrenes are those having a low melting point;
most styrene copolymers used contain at least about 10~ styrene. -~
Preferred cross-linked materials used in the present invention-have a degree of gelation of from about 70% to about 95%.
The present invention will now be described with reference to preferred embodiments and several comparison examples.
EXAMPLES
:
Onto twisted conductors 100 mm2 in cross section, an inner semiconductor layer of a thickness of about 0.8 mm was extruded at 120C and then compositions comprising various compounds as shown in Tables 1 and 2 were extruded at 120C (2 mm thickness) thereover to provide electric wires or cables. -~
As is shown in Tables 1 and 2, where cross-linking by electron beam irradiation is desired, it is eff~cted in a conventional manner at 30 Mrad after extrusion. When chemical cross-linking .. :
iS 9 1 is desired, a cross-linking agent or agents is/are is blended with the composition prior to extrusion and cross-linking efrected by heating at 200C after extrusion coating.
The electric wires or cables thus obtained were immersed in water at 70C, a 20 KV AC voltage applied thereto, -and kept under these conditions for a certain period of time. -~
The development of trees was examined and the time required for dielectric bxeakdown in the insulating layers determined relative to the standard of a polyethylene layer (melt index: l.0, density: 0.92) set as "l". The results are shown in Tables l and 2.
In Table l, polystyrene or a styrene copolymer was used, as ind;cated therein, whereas ;n Table 2, polystyrene plus a styrene copolymer were used, as ;ndicated therein.
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','' - ' ' -; ' - . . " ' ' ' ': . ' .~ .;: ~ ' ' ~ ` ' . ' ' . ' :- " . ', t *l The test specimens were remo~ed from the water after the application of the applied voltage and micro-scopically examined to observe the development of trees.
X: Considerable development of the trees O: No substantial development of the trees ~: No development of the trees *2 Determined by tensile testing using dumbbells according to JIS-3 (modified) O: With more than 200% elongation X: With less than 200~ elongation *3 Low density polyethylene: M.I. = 1.2 and p= 0.92 ; *4 SBR : styrene-butadiene copolymer; styrene content: 23%
*5 SBR : styrene-butadiene copolymer; styrene content: 70%
*6 Et-St : graft copolymer of polyethylene and polystyrene *7 ABS : acrylonitrile-butadiene-styrene copolymer *8 ACS : acrylonitrile-chlorinated ethylene-styrene copolymer - *9 HDPE: high density polyethylene with M.I. = 0.2 and p = 0.96 ~ -*10 EVA : ethylene-vinyl acetate copolymer *11 Cross-linked by electron beam irradiation *12 Chemîcally cross-linked by heating blend of polymers ~with 2 parts by weight of dicumyl peroxide therein as a cross-linking agent per 1~0 parts by weight of the polymer mixture.
- While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the ~ spirit and scope thereof.
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','' - ' ' -; ' - . . " ' ' ' ': . ' .~ .;: ~ ' ' ~ ` ' . ' ' . ' :- " . ', t *l The test specimens were remo~ed from the water after the application of the applied voltage and micro-scopically examined to observe the development of trees.
X: Considerable development of the trees O: No substantial development of the trees ~: No development of the trees *2 Determined by tensile testing using dumbbells according to JIS-3 (modified) O: With more than 200% elongation X: With less than 200~ elongation *3 Low density polyethylene: M.I. = 1.2 and p= 0.92 ; *4 SBR : styrene-butadiene copolymer; styrene content: 23%
*5 SBR : styrene-butadiene copolymer; styrene content: 70%
*6 Et-St : graft copolymer of polyethylene and polystyrene *7 ABS : acrylonitrile-butadiene-styrene copolymer *8 ACS : acrylonitrile-chlorinated ethylene-styrene copolymer - *9 HDPE: high density polyethylene with M.I. = 0.2 and p = 0.96 ~ -*10 EVA : ethylene-vinyl acetate copolymer *11 Cross-linked by electron beam irradiation *12 Chemîcally cross-linked by heating blend of polymers ~with 2 parts by weight of dicumyl peroxide therein as a cross-linking agent per 1~0 parts by weight of the polymer mixture.
- While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the ~ spirit and scope thereof.
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Claims (14)
1. Electric cables or wires having at least one dielectric layer consisting essentially of (1) a polyolefin and a member selected from the group consisting of (2a) polystyrene, wherein for polymer composition (1)-(2a) the polystyrene is present in such an amount as to provide a 10 to 30% styrene content; (2b) a styrene copolymer, wherein for polymer com-position (1)-(2b) the styrene copolymer is present in such an amount as to provide a 3 to 30% styrene content, and (2c) poly-styrene and a styrene copolymer, wherein for polymer composition (1)-(2c) the polystyrene is present so as to provide a 10 to 30% styrene content and the styrene copolymer is present in such an amount so as to provide a 5 to 20% styrene content;
all percentages being based on the entire polymer composition weight.
all percentages being based on the entire polymer composition weight.
2. Electric cables or wires as claimed in claim 1, wherein said mixture is cross-linked.
3. Electric cables or wires as claimed in claim 1, wherein said mixture is not cross-linked.
4. Electric cables or wires as claimed in claim 1, wherein said polyolefin is a polyethylene homopolymer of a density of 0.92 and a melt index of 1Ø
5. Electric cables or wires as claimed in claim 2, wherein the degree of cross-linking is represented by a degree of gelation of about 70 to about 95%.
6. Electric cables or wires as claimed in claim 1, wherein said polyolefin is an ethylene-vinyl acetate copolymer of a density of 0.93 and a vinyl acetate content of 5 wt.%.
7. Electric cables or wires as claimed in claim 1, wherein said styrene copolymer is a styrene-butadiene copolymer con-taining 20 to 80 wt.% styrene.
8. Electric cables or wires having at least one dielectric layer consisting essentially of a polyolefin, polystyrene and a styrene copolymer, the polystyrene and styrene copolymer being present in such amounts as to provide a 10 to 30% styrene contents from the polystyrene and a 5 to 20% styrene content from the styrene copolymer, respectively, based on the entire polymer mixture weight.
9. Electric cables or wires as claimed in claim 8, wherein said mixture is cross-linked.
10. Electric cables or wires as claimed in claim 8, wherein said mixture is not cross-linked.
11. Electric cables or wires as claimed in claim 8, wherein said polyolefin is a polyethylene homopolymer of a density of 0.92 and a melt index of 1Ø
12. Electric cables or wires as claimed in claim 9, wherein the degree of cross-linking is represented by a degree of gelation of about 70 to about 95%.
13. Electric cables or wires as claimed in claim 8, wherein said polyolefin is an ethylene-vinyl acetate copolymer of a density of 0.93 and a vinyl acetate content of 5 wt.%.
14. Electric cables or wires as claimed in claim 8, wherein said styrene copolymer is a styrene-butadiene copolymer con-taining 20 to 80 wt.% styrene.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA239,384A CA1041189A (en) | 1975-11-07 | 1975-11-07 | Styrene in dielectric layer of electric wires or cables |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA239,384A CA1041189A (en) | 1975-11-07 | 1975-11-07 | Styrene in dielectric layer of electric wires or cables |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1041189A true CA1041189A (en) | 1978-10-24 |
Family
ID=4104492
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA239,384A Expired CA1041189A (en) | 1975-11-07 | 1975-11-07 | Styrene in dielectric layer of electric wires or cables |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1041189A (en) |
-
1975
- 1975-11-07 CA CA239,384A patent/CA1041189A/en not_active Expired
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