WO1999024504A1

WO1999024504A1 - Water tree retarding additive

Info

Publication number: WO1999024504A1
Application number: PCT/SE1998/001977
Authority: WO
Inventors: Perry Nylander
Original assignee: Borealis A/S
Priority date: 1997-11-12
Filing date: 1998-11-02
Publication date: 1999-05-20
Also published as: CN1281483A; EP1030879A1; KR20010052099A; SE9704137D0; AU9773698A; JP2001522919A; SE9704137L

Abstract

A water tree retarding additive and a composition for an electric cable comprising it are disclosed. The additive has general formula (I) where n = 1-3, m = 0-3, R1 = -H, -CH3, -(CH2)nCOOH, R2 = (a) a linear or branched, saturated C4-C20 alkyl, (b) a linear or branched, unsaturated C4-C20 alkyl, (c) a linear or branched, saturated C4-C20 acyl, if m=0, (d) a linear or branched, unsatured C4-C20 acyl, if m=0, (e) an aromatic group, (e) in formula (II) where R3 = (a)-(d), and m = 1-3. The additive preferably is N-lauroyl ethylenediamine-N,N',N'-triacetic acid.

Description

WATER TREE RETARDING ADDITIVE

Field of the invention

The present invention relates to a water tree retarding additive for electric cables. More specifically, the invention relates to a water tree retarding additive for electric cables and a composition for an electric cable, such as a composition for an insulating layer or a semiconductive layer which comprises the additive. Background of the Invention Electric cables and particularly electric power cables for medium and high voltages may be composed of a plurality of polymer layers extruded around the electric conductor. In power cables the electric conductor is usually coated first with an inner semiconductor layer followed by an insulating layer, then an outer semiconductor layer followed by water barrier layers, if any, and on the outside a sheath layer. The layers of the cable are based on different types of ethylene plastics which usually are crosslinked. The insulating layer of an electric cable is composed of ethylene plastic. By the expression "ethylene plastic" is meant, generally and in connection with the present invention, a plastic based on polyethylene or a copolymer of ethylene, wherein the ethylene monomer con- stitutes the major part of the mass. Thus, polyethylene plastics may be composed of homopolymers or copolymers of ethylene, wherein the copolymers may be graft copolymers or copolymers of ethylene and one or more monomers which are copolymerisable with ethylene. LDPE (low density polyethylene, i.e. polyethylene prepared by radical poly^¬ merisation at a high pressure) cross-linked by adding peroxide, for instance dicumyl peroxide, in connection with the extrusion of the cable, is today the predominant cable insulating material. A limitation of conventional LDPE is its tendency to be exposed, in the presence of water and under the action of strong electric fields, to the formation of dendritically branched defects, so- -called water trees, which can lead to breakdown and possible electric failure. This tendency is strongly affected by the presence of inhomogeneities, microcavi- ties and impurities in the material. Water treeing has been studied carefully, especially since the 1970 's, when polymer materials and, in particular, cross-linked polyethylene became the predominant insulating material for electric cables for medium and high voltages. In the past years, these studies have entailed improvements in the construction of the cables, the manufacturing procedure and the quality and cleanliness of the used materials. These improvements have resulted in an increased service life of the manufactured cables. Yet there is a pronounced need of still more improved materials in respect of resistance to water treeing. Such an improved resistance to water treeing is desirable not only for insulating layer materials, but also for semiconductor layer mate- rials of electric cables.

European Patent Application EP-A-0 027 300 discloses a water tree retarding agent for high-voltage electric cables which consists of a metal complex of a diketone, salicylic acid which may be substituted with 1-2 lower alkyl groups of a Schiff's base formed from an amine and salicylaldehyde which may be substituted with 1-2 lower alkyl groups.

From European Patent Specification EP-A-0 057 604 it is known to inhibit water treeing by adding to a semi- conducting composition, which mainly consists of a poly- olefin and 5-50% by weight carbon black based on the weight of the total composition, a polyethylene glycol having a molecular weight of about 1000-20000 in an amount of 0.1-20% by weight. This composition is intended for semiconducting layers of electric cables and by adding polyethylene glycol, it is said to be possible to eliminate water trees which grow into the insulating layer from the interface between the insulating layer and the semiconducting layer.

Moreover, US Patent Specification US-A-4 , 812, 505 discloses a composition, which is usable as insulating layer in electric cables and which is resistant to water treeing. The composition comprises a copolymer of ethylene and at least one alpha-olefin having 4-8 carbon atoms, such as 1-butene, 1-hexane or 1-octene, and besides comprises a polyethylene glycol having a molecular weight in the range of about 1000-20000 in an amount of 0.1-20% by weight. Summary of the invention

It has now surprisingly been found that excellent results with regard to water tree reduction can be achieved with a specific type of water tree retarding additive which can be included in the polymer composition as a separate additive or be copolymerised or grafted into the composition.

The water tree retarding additive according to the present invention is a compound having the general formula (I)

(CH₂)_nCOOH R₂(CH₂)_m-N (I)

where n = 1-3, m = 0-3, Rx = -H, -CI-₃, -(CH₂)_nCOOH,

R₂ = (a) a linear or branched, saturated C₄-C₂o alkyl,

(b) a linear or branched, unsaturated C₄-C₂₀ alkyl

(c) a linear or branched, saturated C₄-C₂₀ acyl, in which case m=0, (d) a linear or branched, unsaturated C₄-C₂o acyl, in which case m=0,

(e) an aromatic group, ( CH₂ ) _nCOOH ( f ) -N where R₃ = ( a ) - (d) , and m=l-3 .

"A

The present invention thus provides a water tree retarding additive for electric cables, characterised in that the additive is a compound with the general formula (I) defined above.

The present invention also provides a composition for an electric cable, characterised in that it comprises an ethylene plastic and a compound of the general formula (I) defined above.

Other distinguishing features and advantages of the present invention will appear from the following specifi- cation and the appended claims.

Detailed description of the invention

In the above defined formula (I) n = 1-3, i.e. the group -(CH₂)_nCOOH represents -CH₂COOH, -(CH₂)₂COOH or - (CH₂)₃COOH. It is preferred at present that n = 1, i.e the group -(CH₂)_nCOOH represents -CH₂COOH.

The group Ri can be -H, -CH₃, or - (CH₂) _nCOOH . Preferably Ri is -(CH₂)_nCOOH, and more preferably Ri is -CH₂COOH, i.e. n = 1.

The group R₂ may be bonded directly to the nitrogen atom in the above formula (1), i.e. m can be 0, and if R₂ is of type (c) or (d) this is a must, i.e. m should then be 0. If R₂ is of type (a), (b) or (e) it is immaterial whether is 0, 1, 2 or 3, whereas if R₂ is of type (e) m should be at least 1 and preferably 2. As is seen from formula (I), R₂ may be selected from C4-C20 alkyl groups, C₄-C₂o acyl groups, aromatic groups, or a group -N (R₃) - (CH₂) _nCOOH where R₃ is a C₄-C₂o alkyl group, or a C₄-C₂o acyl group. The alkyl groups and the acyl groups can be linear or branched and may be saturat- ed or unsaturated. The aromatic group may comprise one or more aromatic rings, such as phenyl and naphthyl, and the aromatic rings may be substituted on the nuclei with one or more, preferably aliphatic substituents . The substi- tuent(s) may be saturated or unsaturated.

Preferably the compound of the formula (I) is compatible with the polymer composition of the electric cable. As the base polymer of the polymer composition normally is an ethylene plastic this means that R₂ should be oleophilic.

When R₂ is unsaturated it may have one or more double bonds, and preferably has at least a terminal double bond, such as a vinyl group which makes the com^¬ pound of formula (I) useful as a comonomer for incorporation in the polymer composition by copolymerisation or grafting. As an example of an unsaturated group R₂ p-vinyl phenylene may be mentioned. When R₂ is unsaturat- ed and the compound of formula (I) is included in the polymer composition by copolymerisation or grafting the water tree retarding additive is securely attached to the composition by chemical bonding which inhibits any migration of the additive from the composition. At present it is preferred that Ri is -CH₂COOH, m = 2, and R₂ is -N (R₃) -CH₂COOH, i.e. the compound of formula (I) is a derivative of ethylenediamine triacetic acid. Preferably, because of commercial availability, R₃ is then a C₄-C₂o acyl group, more preferably a linear, saturated C₄-C₂o acyl group, such as a C₁₀-C1₄ acyl group, and most preferably R₃ is lauroyl (Cι₂) . Thus, the most preferred compound of formula (I) at present is N-lauroyl ethylenediamine-N, N ' ,N ' -triacetic acid.

The water tree retarding additive of the present invention should be included in the polymer composition of an electric cable in an amount that is effective for retarding or eliminating the occurence of water trees under the prevailing operating conditions of the electric cable. More particularly, the compound of formula (I) should generally be incorporated in the composition in an amount of about 0.01-10 % by weight, preferably about 0.01-5 % by weight, and most preferably about 0.05-3 % by weight of the composition.

The compound of formula (I) may be added as a water tree retarding additive to the semiconductive layers and/or the insulating layer of an electric cable, and is preferably added at least to the insulating layer.

As mentioned earlier, the insulating layer of an electric cable is usually comprised of an ethylene plastic and more particularly a low density ethylene polymer (LDPE) , which preferably is crosslinked. In addition to the compound of the present invention the insulating layer may include other conventional additives, such as antioxidants to counteract decomposition due to oxidation, radiation, etc.; lubricating additives, such as stearic acid; crosslinking additives such as peroxides which decompose upon heating and initiate crosslinking; and other water tree retarding agents besides the compound of formula (I) according to the present invention. When the polymer composition comprises a semicon- ducting layer of an electric cable the ethylene plastic usually is an ethylene copolymer such as an ethylene- methyl acrylate copolymer (EMA) , an ethylene-ethyl acrylate copolymer (EEA) , an ethylene-butyl acrylate copolymer (EBA) , or an ethylene- vinyl acetate copolymer (EVA) . The polymer composition further includes carbon black in an amount sufficient to make the composition semiconductive, preferably about 15-45 % by weight of the composition.

To facilitate the understanding of the invention, some illustrating, non-restrictive examples will be given below. All parts and percentages refer to weight, unless otherwise stated.

Example 1 In this Example, three semiconducting polymer compositions of an electric cable were tested, viz. Polymer A, B, and C. The first composition (Polymer A) consisted of 60.35 parts by weight of an ethylene-butyl acrylate copolymer (EBA) having 17% by weight butyl acrylate, 0.65 parts by weight of 1, 2-dihydro-2, 2, 4-trimethylquinoline polymer (trade name Vulcanox HPG) , and 39 parts by weight of carbon black (Furnace black; Cabot CSX254) in order to^" make the composition semiconducting. This composition was used as reference.

The second composition (Polymer B) consisted of 60.15 parts by weight of the same EBA polymer as in the first composition, 0.65 parts by weight of polytrimethyl- quinolin, and 39 parts by weight of carbon black. Polymer B also contained 0.2 parts by weight of N-lauroyl ethylenediamine-N, N ', N ' -triacetic acid. This composition was a composition according to the invention.

The third composition (Polymer C) was the same as Polymer B, except that the N-lauroyl ethylenediamine- -N, N' ,N' -triacetic acid was substituted by 0.2 parts by weight of ethylene diaminetetraacetic acid (EDTA) . The formula of EDTA is not covered by the general formula (I) and Polymer C is thus not a composition according to the invention .

Each of the three compositions above was incorporated as inner semiconducting layers in electric cables, which, seen from inside and outwards, consisted of a

1.4 mm copper conductor, an inner semiconducting layer having an outer diameter of 2.8 mm, an insulating layer having an outer diameter of 5.8 mm and an outer semiconducting layer having an outer diameter of 6.1 mm. The insulating layer consisted of low-density polyethylene having a density of 0.923 g/cm3 and an MFR of 2 g/10 min, and the outer semiconducting layer consisted of an ethylene-butyl acrylate copolymer with an addition of about 40% by weight carbon black. The testing of the dielectric strength was carried out on these test cables in accordance with a method developed by Alcatel AG & Co, Hannover, Germany, and described in an article by Land H.G., Schadlich Hans, "Model Cable Test for Evaluating the Ageing Behaviour under Water Influence of Compounds for Medium Voltage Cables", Conference Proceedings of Jlcable 91, 24-28 June 1991, Versaille, France. As a value of the dielectric strength is stated 63% of E_max from Weibull diagram in kV/mm. After conditioning the cables for 16 h at 90°C the cables were aged for 1000 h at 9 kV/mm in 70°C water heated via the inner conductor, which was kept at 85°C, whereupon the dielectric strength was determined

(T=1000 h) . In addition to the dielectric strength the number of vented trees/mm2, and the longest found bow-tie tree were determined for Polymers A and B. The results of the testing are stated in the Table 1.

Table 1

Polymer A Polymer B Polymer C Dielectric strength (kV/mm) at T = 1000 h 34.3 78.1 47.4 Number of vented trees/mm² 0.34 0.00 Max. length of vented trees, L₉₀% (μm) 1190 0

Longest found bow-tie tree (μm) 511 120

As appears from Table 1, the composition including the water tree retarding additive according to the invention had superior properties with regard to every one of the tested properties. It is also evident from Table 1 that Polymer C, which included an additive similar to that of the invention, but outside formula (I) did not achieve the same high dielectric strength as Polymer B according to the invention. Example 2 The water tree resistance (WTR) was determined for four insulating polymer compositions of an electric cable, Polymers 1-4, by so-called Ashcraft testing. Ashcraft testing, which is a testing method for determining the WTR properties of polymers, has been described by Ashcraft, A. C, "Water Treeing in Polymeric Dielectrics", World Electrotechnical Congress in Moscow, USSR, 22 June 1977. By Ashcraft testing, well character- ised effects are provided, viz. sharp, water-filled indentations, by means of a needle in compression-moulded cups. A voltage of 5 kV/6 kHZ is applied across the water, whereas the bottom of the cup is connected to earth. The temperature is constantly kept at 65 °C. The average length of the water trees after 72 h ageing is considered as a measure of the growth rate of the water trees in the specific insulating material.

For the testing, compression-moulded test pieces were prepared from the various polymers, of which Poly- mer 1 consisted of 99.8 parts by weight of a low-density polyethylene (LDPE) having a density of 0.923 g/cm³ and a melt flow rate (MFR) of 2 g/10 min and 0.2 parts by weight of thiobisphenol . Polymer 1 was used as reference. Polymer 2, which was a composition according to the invention, consisted of 99.3 parts by weight of the same type of LDPE, 0.2 parts by weight of Irganox' 1035, and 0.5 parts by weight of N-lauroyl ethylenediamine-N, N ', N ' - -triacetic acid. Polymer 3, which was a composition according to the invention, consisted of 99.45 parts by weight of the above mentioned LDPE, 0.2 parts by weight of thiobisphenol (Rhodianox TBM6P) , and 0.35 parts by weight of oleoylsarcosine (N-oleoyl-N-methyl-N-monoacetic acid) . Polymer 4, which was also a composition according to the invention, consisted of 99.5 parts by weight of the above mentioned LDPE, 0.35 parts by weight of

Vulcanox HPG, and 0.15 parts by weight of N-lauroyl ethylenediamine-N, N' ,N' -triacetic acid. The polymer com- positions also contained about 2 parts by weight of dicumyl peroxide for crosslinking purposes. The results from the Ashcraft testing are compiled in Table 2.

Table 2

Composition Water tree

Average length (%)

Polymer 1 (reference) 100

Polymer 2 (inventive) 60

Polymer 3 (inventive) 61

Polymer 4 (inventive) 70

The test results clearly show the enhanced WTR properties of the compositions according to the invention.

Claims

PATENTKRAV

1. A water tree retarding additive for electric cables, c h a r a c t e r i s e d in that the additive is a compound with the general formula (I)

where n = 1-3, m = 0-3, Ri = -H, -CH₃, -(CH₂)_nCOOH,

R₂ = (a) a linear or branched, saturated C₄-C₂o alkyl,

(b) a linear or branched, unsaturated C₄-C₂o alkyl

(c) a linear or branched, saturated C₄-C₂0 acyl, if m=0, (d) a linear or branched, unsaturated C₄-C₂o acyl, if m=0,

(e) an aromatic group,

(CH₂)_nCOOH (f) - where R₃ = (a) -(d) , and m=l-3.

R₃

2. An additive according to claim 1, wherein n = 1.

3. An additive according to claim 1 or 2, wherein Ri = -(CH₂)_nCOOH.

4. An additive according to any one of claims 1-3, wherein m = 2 and R₂ = (f ) .

5. An additive according to any one of claims 1-4, wherein the compound of the general formula (I) is N-lauroyl ethylenediamine-N, N ' , N ^' -triacetic acid.

6. A composition for an electric cable, c h a r a c t e r i s e d in that it comprises an ethylene plastic and a compound of the general formula (I)

where n = 1-3, m = 0-3,

Ri = -H, -CH₃, -(CH₂)_nCOOH,

R₂ = (a) a linear or branched, saturated C₄-C₂₀ alkyl,

(b) a linear or branched, unsaturated C₄-C n alkyl (c) a linear or branched, saturated C₄-C₂₀ acyl, if m=0,

(d) a linear or branched, unsaturated C₄-C₂₀ acyl, if m=0,

(e) an aromatic group,

(CH₂)_nCOOH (f) -N where R₃ = (a) -(d), and m=l-3

^ R₃

7. A composition according to claim 6, wherein it comprises the compound of the general formula (I) in an amount of 0.01-10 % by weight.

8. A composition according to claim 6, wherein the compound of the general formula (I) is N-lauroyl ethylenediamine-N, N' ,N ^'-triacetic acid.

9. A composition according to claim 6 or 7, wherein the composition forms an insulating layer of an electric cable .

10. A composition according to claim 6 or 7, wherein the composition includes carbon black in an amount suffi^┬¼ cient to make the composition semiconducting and forms a semi-conducting layer of an electric cable.