CA1046453A - Connection or terminal portions for cross-linked polyethylene cables - Google Patents

Connection or terminal portions for cross-linked polyethylene cables

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Publication number
CA1046453A
CA1046453A CA237,713A CA237713A CA1046453A CA 1046453 A CA1046453 A CA 1046453A CA 237713 A CA237713 A CA 237713A CA 1046453 A CA1046453 A CA 1046453A
Authority
CA
Canada
Prior art keywords
ethylene
connection
cross
terminal portions
vinyl acetate
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
CA237,713A
Other languages
French (fr)
Inventor
Toshiyuki Hayami
Hironaga Matsubara
Shousuke Yamanouchi
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Sumitomo Electric Industries Ltd
Original Assignee
Sumitomo Electric Industries Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Sumitomo Electric Industries Ltd filed Critical Sumitomo Electric Industries Ltd
Priority to CA237,713A priority Critical patent/CA1046453A/en
Application granted granted Critical
Publication of CA1046453A publication Critical patent/CA1046453A/en
Expired legal-status Critical Current

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Abstract

ABSTRACT OF THE DISCLOSURE
In the connection or terminal portions of cross-linked polyethylene cables produced by winding irradiated tapes around the connection or terminal portions of cross-linked polyethylene insulated cables and heat molding the same, the irradiated tapes comprise polyethylene and/or an ethylene copolymer as a base resin plus an ethylene-vinyl acetate copolymer containing more than about 15% vinylacetate (VA) in amount which provides more than a 2.5% VA content in the irradiated tapes.

Description

; l ~

"~ 1 BACKGROUND OF THE INVENTION

-' 1. _Field of _he_Invent'ion . ' ` The present invention relates to molded connection or terminal portions of cross-linked polyethylene insulated cables.
- 2. Description of the Prior Art ~ - . .
~ To form molded connections for cross-linked poly-.,^ ,.
ethylene insulated cables, known methods generally comprise '~ subjecting a polyethylene tape to electron beam irradiation to 10 effect cross-linking, winding the irradiated polyethylene tape ~' ' ~- around a cable and then heat molding the resulting assembly.
, .; , .
''; The use of irradiated polyethylene which has been subjected to `
partial cross-linking permits satisfactory molding since no ,- eccentricity, deformation or the like is caused by heating, and the initial breaking or separation characteristics of the molds are superior as compared with the use of uncross-linked ~ - polyethylene. On the other handl complete fusion between -~' layers of the tape or between the tape and the cable insulators .~, :, . .. .
cannot be expected by this method. As a result, after long times - ' under high temperature, in particular after immersion in water for long times under an applied voltage, those portions which are not ~'i complete by fused some time produce water tree (or electrochemical ' .. . . .
trees as they are sometimes termed) to cause dielectr~c breakdown Most particularly, foreign'particles wound between ~'' layers of tape tend to form nuclei around which water ,~,, ~ . .
~ trees develop into the tapes remarkably.
~,.'~', 2`.; SUMMARY OF THE INVENTION

~ -- The object of the present invention is, therefore, to ~:..
overcome the a'bove defects and to provide good molded connection ' ~``' or terminal portions of cables.
:: .

." -, .

` ~46~53 - 1 The primary feature of the present invention lies in .
the use, as a molded dielectric layer, of a composition which comprises polyethylene and/or ethylene copolymer blended with an ethylene-vinyl acetate copolymer of more than about 15% by weight VA content the ethylene-vinyl acetate copolymer being present in such an amount as to provide a more than about 2.5% by weight VA content in the composition. The composition is shaped into a tape and then irradiated with electron beams.

. By winding the irradiated tape obtained as above around a connection or terminal portion of a cable, for example, ,.: .
~i as shown in the drawing, and heat molding, a molded dielectric ... .
-- connection layer 4 which is completely fused to cable conductor . :.
; 1, connection sleeve 2 and dielectric cross-linked polyethylene --~ layer 3 can be obtained in which no trees (such as electric . . .or water trees) are present or will develop to thereby produce a connection portion with excellent characteristics for long-.,.:~; ' term immersion in water under applied voltages.
~j DESCRIPTION OF THE ACCOM@ANYING DRAWIMG

The drawing is a schematic cross-sectional view of a ,....................................................................... .
~ 2~ connection portion of a cable according to the present invention~
....
. DETAILED DESCPIPTION OF THE INVENTION
The polyethylene used in the present invention is ~-~ not especially limited, but highly superior results are obtained `~ with polyethylene of a melt index of about 0.1 to about 40, .
preferably 1 to 2 and a density (p) of 0.91 to 0.96, preferably 0.91 to 0.94. In a similar manner, highly preferred ethylene-vinyl acetate copolymer used in the present invention have a melt -~ index of about 0.6 to about 70 and a density of 0.92 to 0.98.
- .
In the present invention, a vinyl acetate content in the ethylene-vinyl acetate copolymer is preferably about 15~ by ~ . .
~ weight to about 50% by weight and the vinyl acetate content in ,.." .:
.. .:
-~ - 2 -',.'s ~

.:, : .
.~- ~. ..
t, . ;.

~ 6~S3 ` 1 the finc~ blend is preferably about 2.5% by weight to about 13%
; by weight. The ratio of ethylene-vinyl acetate copolymer to polyethylene (EVA/PE) is preferably about 1/20 to about 6/1 on weight basis.
`~ The term "ethylene copolymer" as used herein as a base .. ...
~ resin means an ethylene copolymer comprising predominantly the .. ~ .
ethylene component, for example, more than 95% by weight ethylene, such as an ethylene-propylene copolymer, an ethylene-.
i. ~ acrylate copolymer, eg. ethylene-methyl acrylate copolymer, . :..
`~ 10 ethylene-ethyl acrylate copolymer or the like, an ethylene-.....
`;~ styrene copolymer and the like preferably an ethylene-acrylic ,,~,, . ' .
;~ acid copolymer.
There is no special limitation on tha comonomer used to form a copolymer with ethylene when high ethylene percentages are used, e.g., more than about 95~ by weight ethylene.
The composition of this invention can be compounded in ~-~ a conventional procedure, for example, using rolls which are ~-; well known in the art.
~~ The tape produced in accordance with the present 20 invention for most commonly encountered commercial applications ' generally has a thickness of about 0.1 to 10 mm, prefer~bly 0.1 to 0.2 mm. The tape is cross-linked by irradiation with, for example, electron beams at an intensity of about 1 to about 10 ;- ,, " , i` Mrad preferably 1 to 2 Mrad, to a degree of gelation of about 20 to about 50~.
... ~ ~, .
'i The resulting tape is then wound around a cable and -, ,. ;
~- can be heat molded in a conventional manner, for example, at a temperature of about 120C to about 200C for a period of =~ about 2 to about 8 hours under a pressure of more than about ~-; 30 3 ~g/cm . For example, in XLPE 3-6 KV cables, heat molding " ,;;::
~ is effected at a temperature of 130C for 2 hours and at a . . :;
.,~,r . ., 3 , .,~

"t';' ' `
,, ~''."' .: :'' ', ~ ' , . '' .' ' ' . ' ; 1 temperature of 200C for 4 hours under a pressure of 4.6 Kg/cm2.
PREFERRED EMBODIMENTS OF THE INVENTION
~-~`;; The present invention will now be described in detail by .; - .
~"- way of preferred embodiments thereof and with reference to several .~; comparison examples. All percentages in Table 1 are by weight.
c~' ~ ., .
,~ ; Examples 1 - 11 ....~
. . .
~- Polyethylene and ethylene-vinyl acetate copolymer as shown in Table 1 were compounded in the ratio specified in Table 1 using a roll at a temperature of 120C for 20 minutes ;:
and formed into tapes having a thickness of 0.2 mm and a width of ~;; 25 mm in a conventional manner. The resultant tapes were irradi-, ated with electron beams to a degree of gelation of 30~ in a con-ventional manner, wound around 3.3 Kv cables of a 100 mm2 diameter insulated with cross-linked polyethylene and molded under heating .,~ ~, ............... . .
at 130C for 2 hours plus at 200C for 4 hours u~der a pressure ~! of 3.4 Kg/cm2 to form connection portions having a total thickness (overall dimensions of the layer of one tape on the conductor) of 3~2 mm.
, ~ .
~ The cables were then immersed in water at normal ;,.................... .
i - ~ temperature for 1000 hours under an applied voltage of 60 Kv.

The cables were then disassembled, and thereafter the development ~-~ of trees examined, the results of which are shown in Table 1 ~" Compar~son Examples 1, 2 and 5.

The connection portions ~ cables were prepared in the ,;; same manner as in Example 1, except that polyethylene alone was `,~ substituted for the mixture of polyethylene and the ethylene-. -~ vinyl acetate copolymer as shown in Table 1, and the character-....
;,` istics of the resultant connection portions were measured as .~, -~, for Examples 1-11. The results are shown in Table 1.

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~,.~.,.~ , ,~; ~ . , ~'` l Comparison Examples 3 and 4 The connection portions of cables were prepared in the same manner as in the Example l, except that an ethylene-vinyl acetate copolymer alone was substituted for the mixture .: .
of polyethylene and ethylene-vinyl acetate copolymer as shown in Table 1, and the characteristics of the connection portions ' . :
;~ measured, the result of which are shown in Table l.
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~ ~09~69~53 ~ 1 The connection portions produced as in Example 1 and ::~ as in Comparison Example 1 were lmmersed in water at 70C and ~ . , a voltage of 100 Kv applied thereto, and the time required for . breakdown observed. While the connection portions of Comparison xample 1 showed dielectric breakdown in 334 hours, the connec- -~- tion portions of Example 1 withstood the indicated voltage for . more than 1,000 hours.

- As is apparent from the foregoing results, while compo-: sition containing only polyethylene or only an ethylene-vinyl acetate copolymer provide unsatisfactory characteristics, those .. containing a blend of polyethylene and an ethylene-vinyl acetate - copolymer within the present invention pro~ide highly preferred results.

- A VA content in the ethylene-vinyl acetate copolymer : less than 15% does not provide the advantageous effects of . the present invention; the VA content preferably ranges between : .:
:~ about 20 and about 40%.

. .: , ,. . .
.~ The proportion of the ethylene~vinyl acetate to the .: polyethylene should not be too low in view of the VA content '~ ~o required to attain the objects of this invention, and the VA
.. content in the composition should be at least 2.5%, preferably, .. in the range of from 2.5 to 20% and most preferably from 10-12.5~.
Within the VA content range specified above, satisfactory cable connection or terminal portions having excellent electric characteristics as well as being free of the development of ., .-trees can be obtained.
" -, ,~............................................. . .
'.''`-,, .
.-. 30 - 7 -~,-, . , . ,~ .

,',.:~ ;;, ;`, .

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As is apparent from Comparison Examples 3 and 4, the '~ use of ethylene-vinyl acetate copolymer alone shows substantially no effect even with the same VA content as in the compositions - of the present invention comprising two components, and, - therefore, an ethylene-vinyl acetate copolymer of a higher VA conten-t is required to o~tai~ a similar effect. A higher - VA content, however, degrades other electrical properties of the connection portions such as dielectric tan ~, inpulse withstanding voltage and the like, rendering them impractical ... ,~ 10 ; for commercial use.
:
:.
. It is clear from Table l that the gelation rate has ~- no substantial effect on the characteristics, and further that high density polyethylene and ethylene-vinyl , acetate copolymers as well as low density polyethylene can be ,'i,f.`. used as the base resin.
.. :~. - ~ , Ethylene copolymers that can also be used in this invention as a substitute for polyethylene include ethylene-.~,................................................. .
acrylate copolymers, ethylene-propylene copolymers, ethylene-.. . .
styrene graft copolymers and the like.
-: 20 As will be apparent to those skilled in the art, the compositions used to form the tape can contain, if desired, :'~
- anti-oxidants such as 4,4'-thiobis(6-tert-butyl-m-cresol), .,.;............... . .
~ stabilizers, cross-linking agents, fillers, and the like, i; -,. .
~;~ generally in an amount of 0.1 to 0.~% by weight depending upon ,. ~,-....
- the characteristics desired.
;.-~.~ .
Although the foregoing description has been made for application to cable connection portions, it should be under-~. .
stood that the present invention can also be applied to the terminal portions of cables.

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1 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 therei.n without departing from the spirit and scope thereof.

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Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
1. In the connection or terminal portions of cross-linked polyethylene-insulated cables produced by winding a tape around the connection or terminal portions of the cables and heat-molding the same, the improvement wherein said tapes comprise a blend of polyethylene and/or an ethylene copolymer plus an ethylene-vinyl acetate copolymer having a vinyl acetate of content of more than about 15% present in an amount as to provide a vinyl acetate content of more than about 2.5% in said tape, which tape has been electron beam irradiated to a degree sufficient to inhibit the formation of water trees.
2. Connection or terminal portions of cross-linked polyethylene insulated cables as claimed in claim 1, wherein said blend comprises polyethylene and/or an etheylene copolymer having a melt index of about 0.1 to about 40 and a density of 0.91 to 0.98 plus an ethylene-vinyl acetate copolymer having a vinyl acetate content of about 15% to about 50% present in an amount as to provide a vinyl acetate content of about 2.5% to about 13% in said tapes, and said tape has a thickness of about 0.1 to about 1.0 mm and has been irradiated with electron beams at about 1 Mrad to about 10 Mrad.
3. Connection or terminal portions of cross-linked polyethylene insulated cables as claimed in claim 2, wherein the wound tape is heat molded at a temperature of about 120°C to about 270°C.
4. Connection or terminal portions of cross-linked polyethylene insulated cables as claimed in claim 1, wherein said ethylene-vinyl acetate copolymer has a melt index of about 0.6 to about 70 and a density of 0.92 to 0.98.
5. Connection or terminal portions of cross-linked polyethylene insulated cables as claimed in claim 4, wherein the ratio of ethylene-vinyl acetate copolymer to polyethylene is from about 1/20 to about 6/1, weight basis.
6. Connection or terminal portions of cross-linked polyethylene insulated cables as claimed in claim 1, wherein said ethylene copolymer comprises more than about 95% by weight ethylene.
7. Connection or terminal portions of cross-linked polyethylene insulated cables as claimed in claim 7, wherein said ethylene copolymer is selected from the group consisting of ethylene-propylene copolymers, ethylene-methyl acrylate copolymers, ethylene-ethyl acrylate copolymers, ethylene-styrene copolymers and ethylene-acrylic acid copolymers.
8. Connection or terminal portions of cross-linked polyethylene insulated cables as claimed in claim 1, wherein said tape has a degree of gelation of from about 20 to about 50
CA237,713A 1975-10-15 1975-10-15 Connection or terminal portions for cross-linked polyethylene cables Expired CA1046453A (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
CA237,713A CA1046453A (en) 1975-10-15 1975-10-15 Connection or terminal portions for cross-linked polyethylene cables

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
CA237,713A CA1046453A (en) 1975-10-15 1975-10-15 Connection or terminal portions for cross-linked polyethylene cables

Publications (1)

Publication Number Publication Date
CA1046453A true CA1046453A (en) 1979-01-16

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