CN1281483A - Water tree retarding additive - Google Patents

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 Inhibitor Additives

field of invention

The present invention relates to water tree inhibiting additives for electrical cables. In particular the invention relates to water treeing inhibiting additives and compositions for electrical cables, for example compositions for insulating or semiconducting layers comprising such additives.

Background of the invention

Electrical cables, especially medium and high voltage power cables, may contain multiple polymer layers extruded around electrical conductors. For power cables, the conductor is usually coated with a semiconductive inner layer, an insulating layer, a semiconductive outer layer, a waterproof layer, and as required, a layer outside the waterproof layer. layers of sheathing. These coverings of cables are based on different types of generally cross-linked vinyl plastics.

The insulation of the cable is made of vinyl plastic. The term "ethylene plastics" generally and for the purposes of this invention refers to plastics mainly composed of polyethylene or ethylene copolymers, wherein the majority of the plastic body is composed of ethylene monomers. Thus, polyethylene plastics may include homopolymers or copolymers of ethylene, where the copolymer may be a copolymer or graft copolymer of ethylene and one or more monomers that are copolymerizable with ethylene. For extruded cables, LDPE (low density polyethylene, that is, polyethylene prepared by high-pressure free radical polymerization) cross-linked by adding peroxides (such as dicumyl peroxide) is currently the main cable insulation material. A limitation of conventional LDPE is the tendency in the presence of moisture and under the action of strong electric fields to form defects known as dendritic branches (so-called water trees), which can cause the polymer material to crack and have the potential to fail electrically . The presence of inhomogeneities, microporosity and impurities in the material can have a serious influence on the tendency to form dendrites. Especially since the 1970s, when polymeric materials, especially cross-linked polyethylene, became insulating materials for medium and high voltage cables, the phenomenon of water treeing has been carefully studied. Over the past few years, this research has led to improvements in cable construction, manufacturing processes, and the quality and cleanliness of the materials used. These improvements have resulted in increased service life of the cables produced, but further improvements in polymerization are still needed. Water tree resistance performance of biomaterials. Not only the cable insulation layer material needs to improve the water tree resistance, but also the cable semi-conductor layer material needs to improve the water tree resistance.

European patent application EP-A-0027300 discloses a metal complex of a diketone, salicylic acid which may be substituted by 1-2 lower alkyl groups in a Schiff base formed from an amine and which may be substituted by 1-2 Water tree inhibitor in high voltage cables composed of lower alkyl substituted salicylaldehyde.

From the European patent specification EP-A-0057604 it is known to add 0.1-20 wt.% Polyethylene glycol with a molecular weight of about 1000-20000 can inhibit the formation of water trees. The semiconducting composition is used as a semiconducting layer of an electric cable by adding polyethylene glycol, which is said to avoid the formation of water trees in the insulating layer at the interface between the insulating layer and the semiconducting layer.

Furthermore, US patent specification US-A-4812505 discloses a composition which can be used as an insulating layer for electric cables and which is resistant to water trees. The composition comprises a copolymer of ethylene and at least one alpha-olefin having 4-8 carbon atoms such as 1-butene, 1-hexene or 1-octene, in addition to 0.1-20 (weight) %, polyethylene glycol with a molecular weight ranging from about 1000-20000.

Summary of the invention

It has surprisingly been found that excellent results in reducing water tree formation can be achieved with a specific type of water tree inhibiting additive which can be added to a polymer composition as a stand alone additive or in combination with The polymer phase copolymerization or phase grafting in the compound.

Water tree inhibitory additive of the present invention is a kind of compound with following general formula (I): In the formula: n=1-3;

m=0-3;

R ₁ =-H, -CH ₃ , -(CH ₂ ) _n COOH;

R ₂ =(a) linear or branched saturated C ₄ -C ₂₀ alkyl,

(b) linear or branched unsaturated C ₄ -C ₂₀ alkyl,

(c) linear or branched saturated C ₄ -C ₂₀ acyl, when m=0,

(d) linear or branched unsaturated C ₄ -C ₂₀ acyl, when m=0;

(e) aromatic groups;

In the formula, R ₃ =(a)-(d), and m=1-3.

Accordingly, the present invention provides a water-tree inhibiting additive for electric cables, characterized in that the additive is a compound defined by the above general formula (I).

The present invention also provides a composition for cables, characterized in that the composition comprises vinyl plastic and the compound defined by the above general formula (I).

Other advantageous features and advantages of the present invention will appear from the following description and appended claims.

Detailed Description of the Invention

In the above formula (I), n=1-3, that is, the group -(CH ₂ ) _n COOH represents -CH ₂ COOH, -(CH ₂ ) ₂ COOH or -(CH ₂ ) ₃ COOH, when n=1 That is, the group -(CH ₂ ) _n COOH is preferred when it represents -CH ₂ COOH.

The group R ₁ can be -H, -CH ₃ or -(CH ₂ ) _n COOH, preferably R ₁ is -(CH ₂ ) _n COOH, more preferably R ₁ is -CH ₂ COOH, ie n=1.

The group R ₂ can be directly bonded to the nitrogen atom of the above formula (I), that is, m can be 0, if R ₂ is type (c) or type (d), then m must be 0. If _R2 is of type (a), (b) or (e), it is immaterial whether m is 0, 1, 2 or 3, whereas if _R2 is of type (e), m should be at least 1, Preferably 2.

As seen in formula (I), R ₂ can be selected from C ₄ -C ₂₀ alkyl groups, C ₄ -C ₂₀ acyl groups, aromatic groups or -N(R ₃ )-(CH ₂ ) _n COOH group, wherein R ₃ is a C ₄ -C ₂₀ alkyl group or a C ₄ -C ₂₀ acyl group. Alkyl groups and acyl groups can be linear or branched, and saturated or unsaturated. Aromatic groups may contain one or more aromatic rings such as phenyl and naphthyl, and the aromatic rings may be aromatic rings substituted on the nucleus by one or more preferably aliphatic substituents. Substituents can be saturated or unsaturated.

Preferred compounds of formula (I) are compatible with the polymer composition for cables. When the base polymer in the polymer composition is usually vinyl, this means that _R2 should be oleophilic.

When _R is an unsaturated group, it may contain one or more double bonds, preferably a group containing at least one terminal double bond such as a group that enables the compound of formula (I) to be used as a compound by copolymerization or graft polymerization. The vinyl groups of the comonomers incorporated in the polymer composition. P-vinylphenylene can be used as an example of the unsaturated group R ₂ . When _R is an unsaturated group and the compound of formula (I) is combined in the polymer composition by copolymerization or grafting, the water tree inhibitory additive is firmly connected with the composition through chemical bonding and can inhibit the additive in the composition migration.

Presently, it is preferred that R ₁ is -CH ₂ COOH, m=2 and R ₂ is -N(R ₃ )-CH ₂ COOH, ie the compound of formula (I) is a derivative of ethylenediamine triacetic acid. Considering commercial availability, preferred R ₃ is a C ₄ -C ₂₀ acyl group, more preferably a linear, saturated C ₄ -C ₂₀ acyl group such as a C ₁₀ -C ₁₄ acyl group, and most preferred R ₃ is lauroyl (C ₁₂ ). Accordingly, the presently most preferred compound of formula (I) is N-lauroylethylenediamine-N,N',N'-triacetic acid.

The dosage of the water tree inhibiting additive of the present invention in the polymer composition for cables should be able to effectively suppress or avoid the occurrence of water tree phenomenon under prevailing cable operating conditions. More specifically, the amount of the compound of formula (I) in the composition should generally be about 0.01-10 (weight)%, preferably about 0.01-5 (weight)%, most preferably about 0.05- 3 (weight)% (by weight of the composition).

The compounds of formula (I) can be added as water treeing inhibitory additives in the semiconducting layer of the cable and/or in the insulating layer of the cable, but preferably at least in the insulating layer.

As previously indicated, the insulation in electrical cables is usually made of ethylene plastic, more specifically of low density ethylene polymers (LDPE), preferably crosslinked LDPE. In addition to the compounds of the present invention, the insulating layer may contain other conventional additives such as antioxidants capable of preventing decomposition due to oxidation, radiation, etc., lubricants such as stearic acid, crosslinking agents such as those that decompose when heated and initiate crosslinking. Peroxides etc. and other water tree inhibitors than the compounds of formula (I) according to the invention.

When the polymer composition constitutes the semiconducting layer of the cable, the ethylene plastic is usually an ethylene copolymer such as ethylene-methyl acrylate copolymer (EMA), ethylene-ethyl acrylate copolymer (EEA), ethylene-butyl acrylate copolymer ( EBA) or ethylene-vinyl acetate copolymer (EVA). The polymer composition also includes carbon black in an amount sufficient to render the composition semiconductive, preferably in an amount of about 15-45% by weight of the composition.

In order to facilitate the understanding of the invention, several illustrative and non-limiting examples are given below. All parts and percentages are by weight unless otherwise indicated.

Example 1

In this example, three semiconductive polymer compositions for electrical cables, Polymers A, B and C, were tested.

The first composition (polymer A) is composed of 60.35 parts by weight of ethylene-butyl acrylate copolymer (EBA) containing 17% by weight of 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 are made of carbon black (furnace black, Cabot CSX 254) that makes the composition semiconductive. This composition was used as a 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 polytrimethylquinoline and 39 parts by weight of carbon black. Polymer B also contained 0.2 parts by weight of N-lauroylethylenediamine-N,N',N'-triacetic acid. The composition is a composition according to the invention.

The third composition (polymer C) except that N-lauroyl ethylenediamine-N, N', N'-triacetic acid is replaced by 0.2 parts by weight of ethylenediaminetetraacetic acid (EDTA), the rest are mixed with the polymer B is the same. The general formula (I) does not include the formula for EDTA, therefore polymer C is not a composition according to the invention.

Each of the above three compositions is used as a semiconductive inner layer in a cable, which includes a 1.4mm copper conductor from the inner core to the outer surface, a semiconductive inner layer with an outer diameter of 2.8mm, An insulating layer with an outer diameter of 5.8 mm and a semiconductive outer layer with an outer diameter of 6.1 mm, the insulating layer is a low-density polyethylene with a density of 0.923 g/ ^cm3 and an MFR of 2 g/10 min , while the semiconductive outer layer is ethylene-butyl acrylate copolymer with about 40 (weight)% carbon black added.

The dielectric strength of the above test cables is in accordance with the method developed by Alcatel AG & CO, Hannover, Germany and Land H. G． , Schdlich Hans in "Model Cable Test for Evaluating the Aging Behavior under Water Influence of Compounds for Medium Voltage Cables" (Conference Proceedings of J1 Cable 91, 24 -28 June 1991, Versaille, France) The description made in the paper is measured. The 63% of Emax expressed in kV/mm in the Weibull diagram is specified as the dielectric strength value. The cable is subjected to an environment of 16 hours at 90°C After adjustment, age for 1000 hours in 70°C water with an electric field strength of 9 kV/mm and heated by the inner conductor, then keep the cable at 85°C and measure its dielectric strength (T=1000 hours). In addition, the number of vented trees per square millimeter and the length of the longest bow-tie tree of polymers A and B were also determined. The test results are shown in Table 1.

Table 1 Polymer A Polymer B Polymer C Dielectric strength (kV/mm) T=1000h 34.3 78.1 47.4 Number of exposed water trees/mm ² 0.34 0.00 Maximum length L of exposed water tree ₉₀ % (μm) 1190 0 The maximum length of bow-shaped cross water tree (μm) 511 120

As shown by the data in Table 1, each tested performance of the composition comprising the water tree inhibiting additive according to the present invention is very excellent.

The data in Table 1 also show that Polymer C, which contains additives similar to the present invention but not within the scope of formula (I), does not achieve as high a dielectric strength as Polymer B according to the present invention.

Example 2

The water tree resistance (WTR) of four insulating polymer compositions for electric cables was determined by means of the so-called Ashcraft test.

The Ashcraft test is a test method for determining the WTR performance of polymers, which has been adopted by Ashcraft, A. C． Described in "Water Treeing in Polymeric Dielectrics" (World Electrotechnical Congress in Moscow, USSR, 22 June 1977). Good characterization is provided by the Ashcraft test method, which uses a depressed compression-molded cup-conical specimen, fills the cup with water, and applies 5 kV between the water surface and the outer surface of the cup bottom (which is grounded) /6kHz voltage, the sample temperature is constant at 65°C, and the average length of the water tree formed after 72 hours of aging is taken as the measure of the growth rate of the water tree in the insulating material.

For the test, compression molded specimens were prepared from different polymers, namely Polymer 1, Polymer 2, Polymer 3 and Polymer 4, wherein Polymer 1 consisted of 99.8 parts by weight and had a density of 0.923 g/ Cubic centimeters, low density polyethylene (LDPE) with a melt flow rate (MFR) of 2 g/10 minutes and 0.2 parts by weight of thiobisphenol, the polymer 1 is used as a reference; the polymer 2 is A composition according to the invention consisting of 99.3 parts by weight of low density polyethylene of the same type as Polymer 1, 0.2 parts by weight of Irganox® ¹⁰³⁵ and 0.5 parts by weight of N-lauroylethylenediamine-N , N', N'-triacetic acid; Polymer 3 is a composition according to the present invention, consisting 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 oleoyl sarcosine (N-oleoyl-N-methyl-N-monoacetic acid); Polymer 4 is also a composition according to the present invention, consisting 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. Each polymer composition also contained about 2 parts by weight dicumyl peroxide for crosslinking purposes. The results of the Ashcraft test are summarized in Table 2.

Table 2 combination Average length of water trees (%) Polymer 1 (reference) 100 Polymer 2 (invention) 60 Polymer 3 (invention) 61 Polymer 4 (invention) 70

These test results clearly show that the compositions according to the invention have high WTR performance.

Claims (10)

1. A water tree inhibiting additive for cables, characterized in that the additive is a compound of general formula (I): In the formula: n=1-3; m=0-3; R ₁ =-H, -CH ₃ , -(CH ₂ ) _n COOH; R ₂ =(a) linear or branched saturated C ₄ -C ₂₀ alkyl, (b) linear or branched unsaturated C ₄ -C ₂₀ alkyl, (c) linear or branched saturated C ₄ -C ₂₀ acyl groups, if m=0; (d) linear or branched unsaturated C ₄ -C ₂₀ acyl groups, if m=0; (e) aromatic groups; In the formula, R ₃ =(a)-(d), and m=1-3. 2. The additive according to claim 1, wherein n=1. 3. Additive according to claim 1 or 2, wherein R ₁ =-(CH ₂ ) _n COOH. 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 general formula (I) is N-lauroylethylenediamine-N,N',N'-triacetic acid. 6. A composition for cables, characterized in that the composition comprises vinyl plastic and a compound of general formula (I): In the formula: n=1-3; m=0-3; R ₁ =-H, -CH ₃ , -(CH ₂ ) _n COOH; R ₂ =(a) linear or branched saturated C ₄ -C ₂₀ alkyl, (b) linear or branched unsaturated C ₄ -C ₂₀ alkyl, (c) linear or branched saturated C ₄ -C ₂₀ acyl groups, if m=0; (d) linear or branched unsaturated C ₄ -C ₂₀ acyl groups, if m=0; (e) aromatic groups;

In the formula, R ₃ =(a)-(d), and m=1-3. 7. The composition according to claim 6, wherein the content of the compound of general formula (I) in the composition is 0.01-10% by weight. 8. A composition according to claim 6, wherein the compound of general formula (I) is N-lauroylethylenediamine-N,N',N'-triacetic acid. 9. A composition according to claim 6 or 7, wherein the composition forms an insulating layer for electric cables. 10. 6. A composition according to claim 6 or 7, wherein the composition comprises carbon black in an amount sufficient to render the composition semiconductive and capable of being formed into a semiconductive layer for an electrical cable.