CA2039894A1 - Water-tree resistant cable formulations - Google Patents

Abstract

ABSTRACT
Compositions comprising crosslinked low density polyethylene and hydrolysed ethylene vinyl acetate terpolymers for use in the primary crosslinkable insulation for electrical power distribution cables.
Propagation of water trees in electrical insulation is inhibited. Stabilization of the composition is enhanced by the presence of an antioxidant phenolic ester and a hindered amine light stabilizer.

Description

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FIELD OF THE INVENTION
~ his invention relates to polyethylene compositions of use as cable insulation for high voltage power distribution cables, and more particularly to compositions comprising a polyethylene terpolymer additive to inhibit the propagation of water trees in the electrical insulation.

BACKGROUND OF THE INVENTION
High voltage distribution cable insulations comprised of crosslinked ethylene polymer compositions were initially expected to last for 20-25 years. ~owever, the actual failure rate of these cables has been found to be significantly greater than the anticipated failure rate.
Despite the fact that ethylene polymer is considered to be the most moisture resistant polymer available, water has been identified as a contributing factor to this unexpected failure in performance. Tree like patterns, named "wat~r treesn, occur on aging when the polymer insulation (dielectric) is exposed to water and elec~rical stress. These water trees comprise water filled micro cavities, which can originate inside the insulation usually from a~void, imperfection or a contaminant and grow in the electrical field direction. They can also orlginate at the insulation interface with the -, . , . :
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semiconductive polymer compounds applied as shields between the conductor and insulation and on top of the insulation. Water trees, once initiated, develop slowly and lead to a reduction in dielectric strength of the insulation and result in cable failure. Accordingly; it is widely accepted that water trees are responsible for the trend of increasing power distribution cable failure rates.
Several methods to improve the performance of the crosslinked ethylene polymer insulation against dielectric deterioration by wa~er tree generation and growth have been described. US Patent Mo. 4,144,202 and Canadian Patent No. 1,106,165 relate to the inhibition of water tree growth by employment of certain organosilane compounds. Suitable polymeric compositions, process procedures and descriptions of the test methods are described in these patents. US Patent No. 4,206,260 describes a composition containing an effective amount of an alcohol containing 6 to 24 carbon atoms as being an efficient water and electrical tree retardant insulation. German Patent No. 2,737,430 discloses that certain alkoxysilanes act as tree retardant additives in polyethylene insulation. European Patent No. 0,166,781 describes a blend of ethylene and vinyl acetate copolymer as a water tree retardant material.

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Certain aliphatic carhoxylic acid derivatives when incorporated in suitable amounts in crosslinkable ethylene insulation compounds are reported to suppress water tree growthO
Apart from the above attempts to improve the insulation performance, the cable industry has also sought other means of overcoming the water tree problem.
Rejuvenation of aged cables by pumping acetophenone or a polymerisable silane compound into the insulation, and moisture impervious cable designs are two of the most frequently used methods. Over the years, the United States governing body for cable specifications Association of Edison Illuminating Companies (A.E~I.C.) has also revised the requirements of the insulation, in order to minimise the number and size of the contaminants present in them, which are potential sites for water tree initiation.
SUMMARY OF THE INVENTION
It has now been widely accepted by researchers in the field of dielectrics that the ethylene polymer compositions are better protected if they, are inhibited against water tree growth by a suitable additive or other modification. The object of the pre~ent invention is therefore to provide an insulation composition that is resistant to wster tree growth.

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This object is achieved by employing hydrolysed ethylene vinyl acetate copolymer as an additive in the dielectric co~position.
Accordingly, the invention provides an electrically insulating cross-linked polyethylene composition as a component of high voltage electrical cables, said cross-linked polyethylene composition being obtained by cross-linking a composition comprising approximately 95% w/w of a low density cross-linkable polyethylene and at least 1~
w/w of a hydrolysed terpolymer of ethylene, vinyl acetate and vinyl alcohol;
said terpolymer being obtained from the at least g5~
hydrolysis of an ethylene-vinyl acetate copolymer having a 20-30~ w/w vinyl acetate content; and wherein said polythylene and said terpolymer have substantially similar melt index.
The low density ethylene polymers (LDP~) of value in the practice of the invention have a density (ASTM 1505 test procedure with conditioniny as in ASTM Dl248) of aboot 900 to 950 kg/m3 and a melt index (MI) ~ASTM D1238 test procedure) of about 0.5 to 10 decigrams per minute.
The ethylene polymers may be made under high pressure ùsing a tubular or autoclave reactor wi$h any of the known free radical initiators employed in olefin polymerization technology.

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The ethylene-vinyl acetate-vinyl alcohol terpolymer of use in the practice of the invention will, hereinafter, be referred to as EVA(OH) terpolymer~
The EVA(OH) terpolymer is obtained by hydrolysis of ethylene-vinyl acetate copolymer having a vinyl acetate conten~ of approximately 25% w/w. Hydrolysis to give the EVA(OH) terpolymer may be carried out on the EVA copolymer in solution, emulsion or suspension, and in a batch process or in a reactive extrusion as a continuous process. ~ydrolysis of the acetate ester group to the hydroxyl radical must be to effect 95% hydrolysis and preferably 98% hydrolysis, under hydrolysis conditions known to the art. The amount of EVA(OH) terpolymer used in the composition according to ~he invention is determined by ~he final concentration of OH groups in the crosslinked composition.
EVA(OH) terpolymer used in the present composition should have a mel~ index matching as close as possible that of the ethylene polymer. It should not contain residual ionic impurities which are detrimental for the electrical properties of the crosslinked insulation.
Chemical crosslinking agents either alone or in combination with a coagent are used in appropriate amounts to crosslink the polymer insulation. The preferred crosslinking agent is dicumyl peroxide. Other organic peroxides which can be employed are, for example, ',"~ ~ ' ' ' ' ' ,:
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ditertiarybutyl peroxide (D~BP) and tertiaryb~yl peracetate (TBPA).
The ins~lation composition preferably includes the commonly used hindered phenolic es~ers and/or amines as antioxidants and scorch inhibitors, and thioesters and organic phosphites as synergists. The most pre~erred are phenolic esters and hindered amine light stabilisers.
These are used in convent~onal and appropriate amounts to obtain the intended crosslink network with the intended thermal stability.
Other additives which may be employed in the composition of the present invention will include plasticisers, coupling agents, colorants and chelating agents.
The evaluation of the insulation composition was performed as follows.
The effectiveness of the insulation composition was judged by performing accelerated water tree gr~wth (AWTG) tests on molded samples of the insulation. The test utilised compression molded dish shaped speci~ens with built in protrusion defects of 5 micrometer tip radius.
The design of the test sample was the same as described in US Patent No. 4,144,202 and Canadian Patent No.
1,106,165. The molded and cured dish specimen was pretreated in running vacuum for 72 hours at 75C to drive away the volatile by products produced during the !

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crosslinking step. ~he dish was then filled with ~.lM
NaCl electrolyte solution~ A platinum electrode dipped in this electrolyte W3S connected to a 1 K~z high volt2ge source. The average electric field with an applied voltage of 6kV was 2 kV/mm; and the calculated electrical stress enhancement at the protrusion tip was approximately 200. Under these conditions crosslinked polyethylene insulation quickly developed water trees at the tip of the conical protrusion. A smaller dish with 5 protrusions and a larger dish with g protrusions were employed for the tests.
After a preset test period, the samples were removed from the test chamber and each protrusion wa~ punched out using a rectangular or circular die and an arbor press.
The resulting discs were sectioned into slices 200 micrometers thick and placed in a boiling solution consisting of 6g methylene blue and 0.5 9 sodium carbonate in 200 ml. of distilled water for 5 minutes. The slices were then mounted on microscopic glass slides fo~
examination and measurement of the size of the water trees. The average tree size was then calculated.
Tests at an insulation temperature of 65C were also carried out to simulate the operating conditions of a commercial-size cable~ Accelerated water absorption measurements were made at 90C to determine the wate~
content at saturation.

, ~ 8 ~ 9 ~ 9 4 Commercially available insulation compound, meeting the cleanliness requirements as specified by A.E.I.C. was used for all comparisons. The performan-e of the compositions of the present invention was compared to a commercially available water tree retardant compo~nd described in US Patent No. 4,14~,202 and Canadian Patent No. 1,106,165.
DESCRIPTION OF THE PREFERRED EM30DIME~TS
In order that the invention may be ~etter understood a preferred embodiment will now be described with reference to the accompanying example and drawings, which drawing shows a graph of wa~er tree growth curves for a cross-linked polyethylene polymer and several cross-linked polyethylene - EVAtOH) copolymers.
The dielectric composition of the present invention prior to crosslinking of the polymers co~prises ethylene polymer, tree retarding component EVA(OB) at concentra~ions 0.5 to 5%, and optionally stabilizers, peroxide and/or other fillers and additives. All these components are described below in the Example.
EXAMPLE
Cable yrade ethylene homopolymer meeting the cleanliness requirements as specified by ~E.I.C., of MI =
2.5 dg/min and density - 918 kg/m3 was used as the main component o~ the composition. EVA(OH) terpolymer additive from four different sources (A,B,C,D) were used to - 9~ 39~

evaluate their effectiveness as tree retardant components. The general details of the compositions prior to crosslinking were:

COMPONENT WT %

Clean polyethylene approx. g5 (See Table 1) EVA(OH) 1-5 ~See Table 1) Hindered amine (Ciba Geigy CH944*) 0.2 Phenolic ester (Ciba Geigy Iryanox 0.3 245*) Dicumyl peroxide 1.9 * Trade mark Ten such compositions (Table 1) were tested for water tree growth. The EVA(OH) terpolymer used in each case was characterized for its ethylene ~nd vinyl acetate contents, % hydrolysis, melt index and melting points as determined by differential scanning calorimetry curves. The characteristics are reported in Table 2.

TABLB_l Composition No. 1 2 3 4 5 6 7 8 9 10 EVA(OH) A A A B B B C C D D
% EVA(OH) l 2 5 1 2 5 5 lO 5 10 polyethylene 99 98 95 99 98 95 95 90 95 90 !

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Characteci _ cs of EVA(OH) used ~ vinyl D.S.C.
% ethylene acetate ~ hydro- Melting Source Process MI b~_wt _ final lysis Pt. C
A Continuous 5 72 1.8 98 111 B Continuous 16 72 16 40 80 **
C Batch 100 72 2.2 98 106 D Continuous 0~5 72 6 95 98 Sources A, B and C Tw~re commercial grade EV~(OH) resins.
A & B were MELTHENE adhesive resin from Toya Soda Manufactu~i~g Co. Ltd., Tokyo, Japan; and C was "LEVASINT T resin from Bayer AG, Germany.
** Multiple melting peaks centered around 80C.
D was obtained from a pilot scale laboratory hydrolysis of ethylene-vinyl acetate copolymer with sodium methoxide in methanol as a continuous process in a twin screw extruder, and wherein the EVA(OH) was not purified.
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The water tree growth test results together with the accelerated water absorptions at 90C are given in Table 3.

Test time Water tree sizes in different Co~positions (hours~ (microns) Composition 72 110 ~5 76 140130 90 909~ 85 ~0 240 190 140 90 lg0190 200 120115125 130 Saturated Water absorption at 90~C***
(ppm) 1400 1900 3000 ~ 000 high high *** water ~bsorptions were measured on selective compositions; high in (9) is ~20000 ppm and in (10) 40000 ppm.

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The results for the formulations (7) and (9) are shown in the Fig~re and are compared to standard ultra clean crosslinked polyethylene. It can be clearly seen that the EVA(OH) terpolymer with MI = 0.5 is more effective than EVA(OH) with MI = l00. In both cases there is a definite tree retardancy over the crosslinked polyethylene reference material.
Although the intended effect is obtained in compositions (9) and (l0), unusually higher water absorptions make them unsuitable for application as cable insulation because of the increase in the dielectric loss. Such a high water absorption is due to the residual impurities re~aining in the EVA(OH) terpolymer after the hydrolysis reaction. A comparison of the resul~s for (3) and (8) shows that although the moisture absorp~ions are equivalent, equal water retardancy effect is ob~ained at a lower concentration in the case o~ 5 MI (composition 3) than in the case of l00 MI (composition 8) EVA(O~). It has been found that blending an EVA(OH) terpoly~er having a significantly higher MI than the base polyethylene polymer results in a composition having poor physical propertiesO
In the case of MI 16, 40~ hydrolysed material (compositions 4,5,6) the improvement in water tree growth resistance is only marginal up to 5% concentration level.

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- 12 - ~ ~3 To achieve desired water tree growth resistance would necessitate use of higher percentage levels. This action would make the composition undesirable because o~
increased dielectric loss resulting from the higher ~inyl acetate content and decreased thermal stability d~e to the relatively poor oxida~ion resistance of ethylene vinyl acetate copolymer.
Increasing the concentration of the EVA(OH) terpolymer in the crosslinked insulation increases the effective number of hydroxyl groups which are responsible for increased water absorption. We have found that the size of the water trees decreases as the saturated water absorption of the crosslinked insulation increases (results for compositions 1,2,3 in Table 3). However whèn the saturated water absorption is increased beyond 4000 ppm, there is no additional improvement in water tree resistance (compare results for compositions 8,9,10).
Thus, the concentration of EVA(OH) terpolymer necessary to make the crosslinked insulation water tree retardant can be determined easily ~rom water absorption measurements by the skilled man.
The present discovery therefore brings out the importance of the narrow limits in the choice of the EVA(O~) terpolymer to make the ~inal insulation composition acceptable in baving the very demanding elect~rical and mechanical properties required of :
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insulation for high voltage electrical cables.
Table 4 compares the result of the accelerated water tree growth tests at 65C/1820 hour3 for composition ~7) to those for a commercially available water tree retardant compound wi~h silane as the water ~ree inhibitor (VCA
Natural EC grade TR4202) lUnion Carbide Inc.).

Accelerated Water Tree Tests at 65C for 1820 hours Compound Water tree size (microns) Composition 7 220 Commercial compound 400 The above results show the superiority of the present compositions according to the invention. It can be seen that water absorption in the commercial compound is 6000 ppm, which is nearly twice that of the successful composition described herein.

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WATER TREE GROWTH CURVES
5 micron tip, 6kV, 0.1M NaCl TREE LE~GTH (micron) 700 I _ ~ ~ ~ 8 o TIM~ (hours) Ultra clean XLPE ~ with 100 ~I EVA(OH) with 0.; MI EVA(OH) Figure ..
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Claims (5)

1. An electrically insulating cross-linked polyethylene composition for use in high voltage electrical cables, said cross-linked polyethylene composition being obtained from the cross-linking of a composition comprising approximately 95% w/w of a low density cross-linkable polyethylene and at least 1% w/w of a hydrolysed terpolymer of ethylene, vinyl acetate and vinyl alcohol;
said terpolymer being obtained from the at least 95%
hydrolysis of an ethylene-vinyl acetate copolymer having a 20-30% w/w vinyl acetate content; and wherein each of said polyethylene and said terpolymer has substantially similar melt index.

2. A composition as claimed in Claim 1 comprising 5% w/w terpolymer.

3. A composition as claimed in Claim 2 wherein said terpolymer has been obtained by the 98% hydrolysis of said ethylene-vinyl acetate copolymer.

4. A composition as claimed in Claim 3 wherein said polyethylene has a melt index of 2.5 and said terpolymer has a melt index of 5.

5. A high voltage electrical cable comprising an electrical conductor and an electrically insulating cross-linked polyethylene composition according to any one of Claims l to 4.