CA2022312C - Tree resistant compositions - Google Patents

Abstract

A water tree resistant composition comprising:
(a) a copolymer of at least the two comonomers, ethylene and 4-methyl-1-pentene, said copolymer having a density no greater than about 0.920 gram per cubic centimeter; or (b) a copolymer of at least the two comonomers, ethylene and 1-octene, said copolymer having a density no greater than about 0.920 gram per cubic centimeter; or (c) the copolymer of (a) or (b) grafted with a hydrolyzable vinyl silane, the copolymers of (a) and (b) being produced by contacting the relevant comonomers, under polymerization conditions, with (i) a catalyst system containing a catalyst precursor comprising magnesium, titanium, a halogen, and an electron donor, and a hydrocarbyl aluminum cocatalyst or (ii) a catalyst system containing a catalyst precursor comprising vanadium an electron donor, and a hydrocarbyl aluminum halide; a hydrocarbyl aluminum cocatalyst; and a halogen substituted lower alkane promoter.

Description

TREE RESISTANT COMPOSITIONS
Technical Field This invention relates to compositions which are useful in low to high voltage insulation because of their resistance to water trees.
Background Art Power cables insulated with extruded dielectrics are known to suffer from shortened life when installed underground where water contact is likely. The shortened life has been attributed to the formation of water trees, which occur when an organic polymeric material is subjected to an electrical field over a long period of time in the presence of water in liquid or vapor form. The net result is a reduction in the dielectric strength of the insulation.
Many solutions have been proposed for increasing the resistance of organic insulating materials to degradation by water treeing. These include, for example, the addition to polyethylene of (i) a polar copolymer such as a copolymer of ethylene and vinyl acetate; (ii) a voltage stabilizer such as dodecanol; and (iii) a filler, e.g., clay. These solutions all have shortcomings of some kind such as an increase in dielectric loss, i.e., the power factor, volatility, or cost.
Disclosure of the Invention An object of this invention, therefore, is to provide a water tree resistant composition adapted for use in low to high voltage insulation, 16,157 --: 2022312 which does not depend on the use of modifying additives such as polar copolymers, voltage stabilizers, or fillers to achieve treeing resistance, and which will take advantage of the desirable electrical characteristics of "pure"
polyethylene, for example, its low dissipation f actor .
Other objects and advantages will become apparent hereinafter.
According to the present invention, the above object is met by a water tree resistant insulation composition comprising:
(a) an ethylene copolymer having a density of not greater than about 0.920 gram per cubic centimeter produced by polymerizing a mixture of ethylene and 4-methyl-1-pentene and, optionally, one or more other alpha-olefin comonomers in the presence of a polymerization catalyst; or (b) an ethylene copolymer having a density of not greater than about 0.920 gram per cubic centimeter, produced by polymerizing a mixture of ethylene and 1-octene and, optionally, one or more other alpha-olefin comonomers in the presence of a polymerization catalyst; or (c) an ethylene copolymer according to paragraphs (a) or (b) grafted with a hydrolyzable vinyl silane compound: or (d) a mixture of any two or more of the copolymers described in paragraphs (a), (b), and (c) in any proportion, wherein said polymerization catalyst is comprised of (A) magnesium, titanium, halogen, and an electron donor together with one or more aluminum containing compounds; or 16,157 (B) vanadium, halogen, and an electron donor together with one or more aluminum containing compounds and, optionally, a lower haloalkane compound.
Detailed Description In this specification, the term "copolymer"
is considered to mean a polymer based on two or more comonomers. Either of the above-mentioned copolymers, i.e., the ethylene/4-methyl-1-pentene copolymer and the ethylene/1-octene copolymer, can include additional comonomers such as alpha-olefins having 3 to 12 carbon atoms or dienes. The dienes can be conjugated or non-conjugated dienes containing 5 to 25 carbon atoms such as 1,4-pentadiene, 1,4-hexadiene, 1,5-hexadiene, dicyclopentadiene, 4-vinyl cyclohexene, 1-vinyl-1-cyclopentene, and the alkylbicyclononadienes, indenes, and norbornenes. Ethylidene norbornene is an example of the latter. The non-conjugated dienes are preferred.
The ethylene/4-methyl-1-pentene or 1-octene copolymers can be advantageously blended with one or more of the following:
(i) polyethylenes having densities in the range of about 0.91 to 0.93 prepared by conventional high pressure techniques;
(ii) ethylene copolymers wherein at least one comonomer is a vinyl acid, a vinyl acid ester, or a vinyl ester of an organic acid;
(iii) ethylene terpolymers based on at least two comonomers referred to in items (ii) and (viii);
16,157 (iv) ethylene terpolymers based on alpha-olefins having 3 to 8 carbon atoms;
(v) ethylene/propylene rubbers;
(vi) ethylene/propylene/diene monomer rubbers;
(vii) hydrolyzable graft polymers produced by grafting silane to any of items (i) to (vi); or (viii) ethylene/hydrolyzable silane ' copolymers.
The high pressure technique referred to in item (i) is described in Introduction to Polymer Chemistry, Stille, Wiley and Sons, New York, 1982, at pages 149 to 153. The ethylene/hydrolyzable silane copolymer can be prepared by the process described in United States patent 3,225,018 and the terpolymer by the process described in United States patent 4,291,136.
The item (i) high pressure polymers, which are blended with the base ethylene/alpha-olefin copolymer, are preferably blended to provide an average density for the blend of no higher than about 0.920 gram per cubic centimeter. The weight ratio of base copolymer to added copolymer is usually in the range of about 3:1 to about 1:3. If polar copolymers such as ethylene/vinyl acetate or ethylene/ethyl acrylate are used in the blend, the concentration of the polar comonomer should be kept at low levels to avoid high dielectric losses, e.g., less than about 10 percent by weight of the blend.
The ethylene/4-methyl-1-pentene or 1-octene copolymers are produced using either a titanium or vanadium containing catalyst system 16,157 With respect to the titanium containing catalyst system, the respective comonomers are contacted with a catalyst system containing a catalyst precursor comprising magnesium, titanium, a halogen, and an electron donor as well as one or more aluminum containing compounds cocatalyst such as triethylaluminum and triisobutylaluminum. The catalyst system and the preparation of the copolymer are described in United States patent 4,302,565.
The copolymer produced in the presence of the titanium containing catalyst system can be prepared as follows: Into a flask equipped with a mechanical stirrer are placed anhydrous MgCl2 and tetrahydrofuran (THF). To this mixture, TiCl4 is added. Porous dehydrated silica is added to the solution and stirred. The mixture is dried to provide a dry, free-flowing powder having the particle size of the silica. The desired weight of impregnated precursor composition and activator compound, e.g., triethylaluminum, is added to a mixing tank with a sufficient amount of anhydrous aliphatic hydrocarbon diluent such as isopentane to provide a slurry system. The activator compound and precursor compound are used in such amounts as to provide a partially activated precursor composition which has an A1/Ti ratio of up to 10:1. The contents of the slurry system are then mixed and dried. The resulting catalyst is in the form of a partially activated precursor composition which is impregnated within the pores of the silica. It is injected into, and fully activated within, the polymerization reactor. Activator compound is added 16,157 -~ - 6 - 2022312 to the polymerization reactor so as to maintain the A1/Ti ratio in the reactor at a level of about 10:1 to 400:1. Ethylene is then copolymerized with 4-methyl-1-pentene, preferably, in the gas phase in a fluidized bed. The reaction is conducted, after equilibrium is reached, for 1 hour at 85°C and under a pressure of 300 psig, a gas velocity of about 3 to 6 times Gmf, and a space time yield of about 4.4 to 6.3 in a fluid bed reactor system.
With regard to the vanadium containing catalyst system, the respective comonomers are preferably contacted with a supported catalyst system containing a catalyst precursor comprising a vanadium trihalide, an electron donor, and a hydrocarbyl aluminum halide together with a hydrocarbyl aluminum cocatalyst and a halogen substituted lower alkane promoter, the lower alkane promoter having 1 to 7 carbon atoms. The catalyst system and a process for preparing the copolymer are described in European Patent Application 0 120 501 published on October 3, 1984.
The copolymer produced in the presence of the vanadium containing catalyst system can be prepared as follows: To a flask containing anhydrous tetrahydrofuran is added VC13. The mixture is stirred until the VC13 is dissolved.
To this solution is added dehydrated silica and stirring is continued. The flask is vented and the solution is dried to the mud stage. The impregnated silica is a free-flowing solid which has 0.25 millimole of vanadium per gram. The solid is removed from the flask and stored under nitrogen.
16,157 _ 7 _ Then, the modifier is introduced using the following procedure. To a flask containing anhydrous isopentane is added the impregnated silica described above. To this mixture is added with stirring, diethylaluminum chloride, as modifier, in anhydrous hexane. The amount of modifier is employed in an amount sufficient to provide 1.7 mole of modifier per mole of tetrahydrofuran. This mixture is heated until the product is a free-flowing powder. The vanadium precursor is then removed from the flask and stored under nitrogen. The ethylene copolymer is produced in the gas phase in a fluidized bed reactor under the following operating conditions:
promoter/cocatalyst molar ratio: 1.0;
aluminum/vanadium atomic ratio: 40; temperature:
70°C: gas velocity: 1.5 feet per second; nitrogen pressure (mole percent): 50; comonomer/ethylene molar ratio: 0.24; hydrogen/ethylene molar ratio:
0.007; and pounds/hour-cubic feet: 4.7.
Trisobutylaluminum cocatalyst is added during polymerization. Chloroform, CHC13, is added as the promoter as a 5 Weight percent solution in isopentane. The polymerization is conducted for more than one hour after equilibrium is reached under a pressure of about 300 psig.
While the gas phase polymerization, particularly one carried out in one or more fluidized beds, is preferred, the copolymers can be produced in a conventional solution phase process such as the process described in the Introduction to Polymer Chemistry, referred to above.
16,157 _8-The amount of 4-methyl-1-pentene or 1-octene, in the case where the comonomers are the primary comonomers along with ethylene, is preferably in the range of about 5 to about 50 percent by weight based on the weight of the copolymer and is most preferably in the range of about 15 to about 40 percent by weight. The amount of ethylene is preferably greater than about 50 percent by weight and is preferably more than 60 percent by weight. Where additional comonomers are used, the amount of these comonomers is preferably in the range of about 1 to about 15 percent by weight.
The density of the ethylene/4-methyl-1-pentene copolymer is no greater than about 0.920 gram per cubic centimeter and is preferably in the range of about 0.88 to about 0.920 gram per cubic centimeter. The melt index is preferably in the range of about 0.5 to about l0 grams per ten minutes. Melt index is determined in accordance with ASTM D-1238, Condition E, and is measured at 190°C. The density of the ethylene/1-octene copolymer is no greater than about 0.920 gram per cubic centimeter and is preferably in the range of about 0.88 to about 0.920 gram per cubic centimeter. The melt index is preferably in the range of about 0.5 to about 20 grams per ten minutes and is most preferably in the range of about 0.5 to about 10 grams per ten minutes.
Various processes for preparing silane grafted polyethylene and ethylene/silane copolymers and numerous unsaturated silanes suitable for use in 16,157 -° 2022312 - g _ preparing these polymers and bearing hydrolyzable groups such as alkoxy, oxy aryl, oxyaliphatic, and halogen are mentioned in United States patents 3.075.948: 3.225.018; 4.412,042; 4.413.066;
4,574,133; and 4,593,071. In the silane grafted copolymer, the amount of incorporated silane monomer is preferably from about 0.5 percent to about l0 percent by weight based on the total weight of the copolymer. The silane monomer is most preferably incorporated into the copolymer in an amount of about 0.5 to about 10 percent by weight. The silane grafted to the copolymer can be, among others, a vinyl trialkoxy silane such as vinyl trimethoxy silane or vinyl triethoxy silane. Generally speaking, any unsaturated monomeric silane containing at least one hydrolyzable group can be used. If slower water cure or better shelf stability are desired, vinyl triisobutoxy silane, vinyl tris-(2-ethyl-hexoxy) silane or vinyl trisopropoxy silane can be used.
A free radical generator or catalyst is used in the preparation of the silane grafted polymer. Among the most useful free radical generators are dicumyl peroxide, lauroyl peroxide, azobisisobutyronitrile, benzoyl peroxide, tertiary butyl perbenzoate, di(tertiary-butyl) peroxide.
cumene hydroperoxide, 2,5-dimethyl-2,5-di(t-butyl-peroxy) hexyne. 2,5-dimethyl-2,5-di(t-butylperoxy)-hexane tertiary butyl hydroperoxide, and isopropyl percarbonate. The organic peroxides are preferred.
Rbout 0.01 to about 5 percent by weight of free radical generator based on the weight of the 16.157 - to - 2022312 copolymer is used, and preferably about 0.01 to about 0.1 percent by weight.
The organic peroxides are also used to cross-link or cure the ethylene/4-methyl-1-pentene and ethylene/1-octene copolymers. The hydrolyzable silane grafted copolymer can be cross-linked using an organic peroxide or with moisture in the presence of a conventional silanol condensation catalyst such as dibutyl tin dilaurate. The amount of organic peroxide used for cross-linking is in the range of about 0.5 to about 5 percent by weight based on the weight of the copolymer.
Various conventional additives can be added in conventional amounts to the insulation compositions. Typical additives are antioxidants, ultraviolet absorbers, antistatic agents, pigments, fillers, slip agents, fire retardants, stabilizers, cross-linking agents, halogen scavengers, smoke inhibitors, cross-linking boosters, processing aids, lubricants, plasticizers, and viscosity control agents.
Wire and cable is generally constructed of one or more metal conductors insulated with a polymeric material. In cable, these elements form a core and are protected by another polymeric sheath or jacket material. In certain cases, added protection is afforded by inserting a wrap between the core and the sheath. Subject composition is typically used as the insulating or jacketing layer, and is coated on or extruded about the electrical conductor. The hydrolyzable composition is generally cross-linked after it is in place on the wire or cable.
16,157 -m - 2022312 The invention is illustrated by the following examples.
Examples 1 to 7 The resistance of insulating compositions to water treeing is determined by the method described in United States patent 4.144,202. This measurement leads to a value for water treeing resistance relative to a standard polyethylene insulating material. The term used for the value is "water tree growth rate" (WTGR). From experience in laboratory tests of materials and for accelerated tests of cables, it has been established that the value for WTGR should be equal to or less than about percent of the standard to provide a useful improvement in cable performance, i.e., in the life of a cable which is in service and in contact with water during the period of service.
The compositions of examples 1 to 3 and 5 to 7 are based on ethylene/1-octene copolymers and are prepared by a gas phase process similar to the typical procedure for titanium based catalysts, mentioned above. The compositions of examples 4 and 6 are also based on ethylene/1-octene copolymers and are prepared by a gas phase process similar to the typical procedure for vanadium based catalysts, mentioned above. The copolymers contain antioxidant, calcium stearate, and dicumyl peroxide as the cross-linking agent. All specimens are 16,157 compression molded, cured, i.e., cross-linked, in the press, and vacuum treated for seven days at 80°C
prior to testing. Variables and results are set forth below.
Exampla density 0.89950.90180.90500.90600.91050.91100.9165 (g/cc) melt index0.73 0.40 1.08 0.28 0.93 0.42 1.2 WTGR (%) 0.20 0.05 3.0 0.60 0.2 3.5 3.9 Example 8 The procedure of example 3 is repeated except that it is not cross-linked. The density is 0.905 gram per cubic centimeter; the melt index is 1.08 grams per 10 minutes; and the WTGR (%) is 6.4.
Examples 9 and 10 The procedure of example 1 is repeated except that the copolymer is prepared in the solution phase.
Example 1 2 density (g/cc) 0.905 0.912 melt index 0.9 1.0 WTGR (%) 16.0 g,g Examples 11 to 14 In these examples, the effect of density on WTGR of cross-linked ethylene/4-methyl-1-pentene copolymer produced using a catalyst prepared by a procedure similar to the typical procedure for titanium based catalysts, mentioned above. The test 16,157 specimens are example 1, above.Density made as for and WTGR are set forth below.

Example li 12 13 14 density (g/cc) 0.895 0.902 0.910 0.917 WTGR ($) 0.1 0.4 0.9 8.2 16.157

Claims (10)

1. A water tree resistant insulation composition having a water tree growth rate equal to or less than about 0.9 percent consisting essentially of:
(a) a crosslinked ethylene copolymer having a density not greater than about 0.9105 gram per cubic centimeter and a melt index of about 0.40 to about 0.93 gram per 10 minutes produced by polymerizing, in the gas phase in a fluidized bed in the presence of a polymerization catalyst, a mixture of two comonomers, (i) one of which is ethylene and (ii) one of which is 4-methyl-1-pentene or 1-octene;
or (b) an ethylene copolymer according to paragraph (a) grafted with a hydrolyzable vinyl silane compound, wherein such polymerization catalyst is comprised of magnesium, titanium, halogen, and an electron donor together with one or more aluminum containing compounds.

2. The composition defined in claim 1 wherein comonomer (ii) is 4-methyl-1-pentene.

3. The composition defined in claim 1 wherein comonomer (ii) is 1-octene.

4. The composition defined in claim 1 wherein the ethylene copolymer is grafted with a hydrolyzable vinyl silane compound.

5. The composition defined in claim 1, paragraph (a), additionally containing a monomeric unsaturated silane containing at least one hydrolyzable group and a free radical generator.

6. The composition defined in claim 1 wherein the density is no less than about 0.88 gram per cubic centimeter.

7. The composition defined in claim 1 wherein the portion of the copolymer attributed to 4-methyl-1-pentene or 1-octene is in the range of about 5 to about 50 percent by weight based on the weight of the copolymer.

8. An electrical conductor coated with, or having extruded thereon, a composition as defined in claim 1.

9. A cable comprising a metal core conductor and at least one layer surrounding the core comprising the composition defined in claim 2.

10. A cable comprising a metal core conductor and at least one layer surrounding the core comprising the composition defined in claim 3.