MXPA99008513A - Tree resistant cable - Google Patents

Abstract

A cable comprising one or more electrical conductors or a core of one or more electrical conductors, each conductor or core being surrounded by an insulating composition comprising a mixture of:(i) about 20 to about 50 percent by weight of a homogeneous polyethylene having a polydispersity in the range of about 1.5 to about 3.5 and an essentially uniform comonomer distribution;and (ii) about 50 to about 80 percent by weight of a homopolymer of ethylene made by a high pressure process.

Description

"CABLE RESISTANT TO THE RAMIFIED DOWNLOADS" TECHNICAL FIELD This invention relates to insulated electrical power cable with a polyethylene composition having improved resistance to branched discharges of water.

BACKGROUND INFORMATION A typical electrical power cable usually comprises one or more conductors in a cable core that is surrounded by several layers of polymeric material including a first semiconductive protective layer, an insulating layer, a second semiconductive protective layer, a protective of tape or metallic wire and a shirt. Surrounding the conductor or core can be achieved, for example, by extrusion, coating or wrapping. These isolated cables are known to suffer from shortened duration when installed in an environment where the insulation extends to water, e.g. underground or high humidity locations. The cut life has been attributed to the formation of branched discharges of water, which occur when an organic polymer material is subjected to an electric field over a prolonged period of time in the presence of water in the form of liquid or vapor. The net result is a reduction in the dielectric strength of the insulation. Many solutions have been proposed to increase the resistance of organic insulation materials to degradation by branching water discharges. The most recent solutions involve the addition of polyethylene glycol, a branched water discharge growth inhibitor, to a heterogeneous low density polyethylene such as that described in U.S. Patent Nos. 4,305,849; 4,612,139; and 4,812,505. Another solution is the use of a homogeneous polyethylene per se as the organic insulation material, that is, without the addition of a branched water discharge growth inhibitor. See U.S. Patent Number 5,246,783. Both of these solutions seem to be steps in the right direction, but there is a continuing industrial demand for improvement partly because the power cable is increasingly exposed to harsher environments, and partly because consumers are more concerned with the longevity of the cable, eg a service life of 30 to 40 years.

- - EXHIBITION OF THE INVENTION An object of this invention, therefore, is to provide an insulated cable that exhibits a highly improved branched water discharge resistance. Other objects and advantages will be evident below. In accordance with the invention, an insulated cable that fills the aforementioned object has been discovered. The cable comprises one or more electrical conductors or a core of one or more electrical conductors, each conductor or core being surrounded by an isolation composition consisting of a mixture of (i) from about 20 percent to about 50 percent by weight of a homogeneous polyethylene having a polydispersity within the range of about 1.5 to about 3.5, and an essentially uniform comonomer distribution; and (ii) from about 50 percent to about 80 weight percent of an ethylene homopoly produced by a high pressure process.

DESCRIPTION OF THE PREFERRED MODALITY (S) - - The homogeneous polyethylenes are copolymers of ethylene, of one or more alpha-olefins and optionally, a diene. A copolymer is a polymer formed from the polymerization of two or more monomers and includes terpolymers, tetramers, etc. The alpha-olefins may have from 3 to 12 carbon atoms and preferably have from 3 to 8 carbon atoms. Examples of the alpha-olefins are propylene, 1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene. As mentioned above, the polymers have a polydispersity (Mw / Mn) within the range of about 1.5 to about 3.5, and a comonomer distribution essentially uniform. The homogeneous polyethylenes are characterized by relatively low and unique DSC melting temperatures. The heterogeneous polyethylenes on the other hand have a polydispersity (Mw / Mn) greater than 3.5 and do not have a uniform comonomer distribution. Mw is defined as the weight average molecular weight and Mn is defined as the number average molecular weight. Homogeneous polyethylenes can have a density within the range of 0.86 to 0.94 gram per cubic centimeter, and preferably have a density within the range of 0.87 to about 0.93 gram per cubic centimeter. It also has a melt index within the scale of about 0.5 to about 30 grams per 10 minutes, and preferably - They have a melt index within the scale of about 0.5 to about 5 grams per 10 minutes. Homogeneous polyethylenes can be prepared, for example, with vanadium-based catalyst systems, such as those described in U.S. Patent Nos. 5,332,793 and 5,342,907, and can also be prepared, and preferably prepared with single-site metallocene catalyst systems, such as those described in U.S. Patent Nos. 4,937,299 and 5,317,036. The ethylene homopolymer can be prepared, for example, by a high pressure process described in the Introduction to Polymer Chemistry, Stille, Wiley and Sons, New York, 1962 on pages 149 to 151. The density of the homopolymer can be within the range of 0.916 to 0.930 gram per cubic centimeter, and preferably within the range of 0.920 to 0.928 gram per cubic centimeter. The melt index may be within the range of about 1 to about 10 grams per 10 minutes, and preferably falls within the range of about 1.5 to about 5 grams per 10 minutes. The melt index is determined in accordance with method D-1238 of the American Society for the Testing of Materials, Condition E, which is measured at 190 degrees centigrade. The amount of the ethylene homopolymer that may be in the isolation composition may be within the range of from about 50 to about 80 weight percent, and preferably falls within the range of from about 60 percent to about 75 percent by weight. weight. The amount of the homogeneous polyethylene that can be in the insulation composition can be within the range of about 20 percent to about 50 percent by weight, and preferably falls within the range of about 25 percent to about 40 percent in weigh. The percentages are based on the weight of the homopolymer mixture and the homogeneous polyethylene. Conventional additives, which can be introduced into the polyethylene formulation, are exemplified by antioxidants, coupling agents, ultraviolet light absorbing agents or stabilizers, antistatic agents, pigments, dyes, nucleating agents, fillers or reinforcing fillers. or polymer additives, slip agents, plasticizers, processing aids, lubricants, viscosity control agents, tackifiers, antiblocking agents, surfactants, extender oils, metal deactivators, voltage stabilizers, fillers or fillers flame retardants and additives, crosslinking agents, reinforcers and catalysts and smoke suppressors. The filler or filler materials and additives may be added in amounts ranging from less than about 0.1 to more than about 200 parts by weight per 100 parts by weight of the base resin, in this case, polyethylene. Examples of antioxidants are: hindered phenols such as tetrakis [methylene (3, 5-di-tert-butyl-4-hydroxyhydroxycinnamate)] -methane, bis [beta- (3,5-diter-butyl-4-hydroxybenzyl) - methylcarboxyethyl)] sulfide, 4,4'-thiobis (2-methyl-6-tert-butylphenol), 4-4'-thiobis (2-tert-butyl-5-methylphenol), 2'-thiobis (4- methyl-ß-tert-butylphenol) and thiodiethylene bis (3, 5-di-tert-butyl-4-hydroxy) hydroxynamate; phosphites and phosphonites such as tris (2,4-di-tert-butylphenyl) phosphite and di-tert-butylphenyl phosphonite; thio compounds such as dilaurylthiopropionate, dimyristylthiodipropionate and distearylthiodipropionate; several siloxanes; and various amines such as polymerized 2,2,4-trimethyl-1,2-dihydroquinoline. The antioxidants can be used in amounts of about 0.1 to about 5 parts by weight per 100 parts by weight of the polyethylene.

The resins in the formulation can be crosslinked by adding a crosslinking agent to the composition or making the resin hydrolysable, which is achieved by adding hydrolyzable groups such as -Si (0R) 3, wherein R is a hydrocarbyl radical to the resin structure through copolymerization or grafting. Suitable crosslinking agents are organic peroxides such as dicumyl peroxide; 2,5-dimethyl-2,5-di (t-butylperoxy) exan; t-butylcumyl peroxide; alpha, alpha-bis (tert-butylperoxy) diisopropylbenzene; and 2,5-dimethyl-2,5-di (t-butylperoxy) hexane-3. Dicumyl peroxide is preferred. Hydrolyzable groups may be added, for example, copolyzing (in the case of homogeneous polyethylene) the ethylene and the comonomer (s) with an ethylenically unsaturated compound having one or more -Si (OR) 3 groups such as vinyltrimethoxysilane, vinyltriethoxysilane and gamma-methacryloxypropyltrimethoxysilane or by grafting these silane compounds to either the resin in the presence of the aforementioned organic peroxides. The hydrolysable resins are then crosslinked by moisture in the presence of a silanol condensation catalyst, such as dibutyltin dilaurate, dioctyltin maleate, dibutyltin diacetate, stannous acetate, lead naphthenate and zinc caprylate. Dibutyltin dilaurate is preferred. Examples of hydrolysable copolymers and hydrolysable graft copolymers are the ethylene / comonomer / vinyltrimethoxysilane copolymer, the ethylene / comonomer / gamma-methacryloxypropyltrimethoxysilane copolymer, the ethylene / comonomer vinyltrimethoxysilane copolymer, the low density ethylene grafted vinyltrimethoxysilane copolymer linear / 1-butene and vinyltrimethoxysilane homopolymer grafted with low density polyethylene or ethylene. The cable of the invention can be prepared in various types of extrusion apparatuses e.g. of single screw types or cufflinks. The stirring can be carried out in the extrusion apparatus or before extrusion in a conventional mixer TM such as a BRABENDER mixer or a BANBURY TM mixer. A description of a conventional extrusion apparatus can be found in U.S. Patent Number 4,857,600. A typical extrusion apparatus has a hopper at its end upstream and a die at its downstream end. The hopper is fed into a tubular body containing a screw. At the downstream end between the end of the screw and the matrix there is a sieve pack and a breaker plate. The screw portion of the extrusion apparatus is considered as being divided into three sections. The feeding section, the compression section and the regulated supply section and two zones, the rear thermal zone and the frontal thermal zone, running the sections and zones from upstream to downstream. In the alternative, there may be multiple heating zones (more than two) along the axis running from upstream to downstream. If it has more than one tubular body, the tubular bodies are connected in series. The diameter length ratio of each tubular body is within the range of about 15: 1 to about 30: 1. In the wire coating, where the material is cross-linked after extrusion, the cross-head die is fed directly into a heating zone and this zone can be maintained at a temperature within the range of about 130 ° C to about 260 ° C, and preferably within the range of about 170 ° C to about 220 ° C. The advantages of the invention are in (I) the highly improved growth regime of the branched discharge of water); (II) that the additives used to improve the resistance to the branched discharge of water can be avoided; (III) that "all" polyethylene composition takes full advantage of the desirable electrical characteristics of polyethylene, for example, its low dissipation factor; and (IV) the composition is useful in low, medium and high voltage applications. The patents and the publication that are mentioned in this specification are incorporated herein by reference. The invention is illustrated by the following examples. Examples 1 to 12 The resistance of the insulating compositions to branched discharges of water is determined by the method described in US Pat. Number 4, 144,202. This measure leads to a value for resistance to the branched discharge of water in relation to a normal polyethylene insulation material. The term used for the value is "branched water discharge growth regime" (TGR). The length of the branched water discharge (TL) is also measured. The WTL is the fraction of the thickness through which the branched discharge of water has grown. The lower the values of WTGR and WTL, the better the resistance to the branched discharge of water. The values of WTGR and WTL are expressed in percentage. 100 parts by weight of each of the polyethylene blends described below are stirred in a twin screw extruder with 0.18 part by weight of the primary antioxidant, bis (3,5-di-tert-butyl-4) -hydroxy) thiodiethylethylene hydrocinnamate; 0.18 part by weight of the secondary antioxidant, distearyl thio dipropionate; and a sufficient amount of dicumyl peroxide to provide an oscillatory disk torque reading (5 degree arc at 182 degrees centigrade with a 20 second preheat) of 48 inch-pound torque. The Twin Screw Assembly Team is a BRABENDER Twin Screw Rotation Device TM. It has two counter-rotating gear screw. The temperature of the graduation temperatures in the tubular body is 100 degrees centigrade in the area of the throat of the hopper increasing to 150 degrees centigrade in the outlet region downstream of the stirring apparatus. The melting temperature is from about 150 degrees to about 155 degrees centigrade. The stirring apparatus yields resin strands, each strand having a diameter of approximately 3.18 millimeters which is then cut into boxes as granules. The stirring to the same conditions is carried out to ensure a uniform additive composition. The final granules are molded into 2.54-centimeter discs that are 6.35 millimeters thick in a two-step press. initial step final step pressure (kg / cm2) low high temperature (° C) 120 175 dwell time (minutes) 9 from 15 to 20 Each plate is tested for WTGR and the results are compared to a control polyethylene homopolymer, which exhibits 100 percent WTGR. The WTL is also measured. The variables and the results are shown in Table 1: TABLE I Example of polyethylene mixture WTGR WTL (% HPPE /% of (percentage) * (percentage) * metallocene PE) 1 95/5 484 26.00 2 90/10 220 18.00 3 80/20 53 7.75 4 80/20 34 7.34 75/25 4 3.00 6 75/25 24 5.64 7 70/30 25 5.73 8 70/30 18 5.62 9 60/40 14 5.05 50/50 15 5.04 11 50/50 37 8.00 12 0/100 101 10.00 * Variations in WTGR and WTL from one set of experiments to the next should be expected.

Notes to Table I: 1. A description is given below of the metallocene polyethylenes (metallocene PE) (each polyethylene is prepared with a single-site metallocene catalyst): The metallocene PEs used in Examples 1, 2, 3, 5, 6, 7, 11 and 12 = a copolymer of ethylene and 1-octene having a ratio of Mw / Mn of about 2; a critical comonomer distribution; 24 weight percent 1-octene; a melt index of 1 gram per 10 minutes; and a density of 0.870 gram per cubic centimeter. The metallocene PEs used in Examples 4, 8, 9 and 10 = a copolymer of ethylene and 1-octene having a ratio of Mw / Mn of about 2; a critical comonomer distribution; 24 weight percent 1-octene; a melt index of 5 grams per 10 minutes; and a density of 0.870 gram per cubic centimeter. Note: A description of the metallocene resins as well as molecular weight, molecular weight distribution and comonomer distribution with respect to this resin can be found in US Patent Number 5,246,783 in column 4, line 13 through column 5 , line * 9. 2. HPPE = an ethylene homopolymer prepared by a high pressure process that has a melt index of 2 grams per 10 minutes and a density of 0.923 gram per cubic centimeter. The high pressure preparation is achieved through the use of a tubular reactor that operates between 1,968.40 and 2,810 kilograms per square centimeter and from 220 degrees to 230 degrees centigrade. Organic peroxide initiators are used to provide free radicals for the reaction and a chain transfer agent is used. 3. Percentage - of HPPE / percentage of metallocene PE = the first number is the percentage of HPPE and the second number is the percentage of metallocene polyethylene in the mixture. 4. WTGR and WTL are described above. Examples 13 and 14 The resin for Example 13 is prepared in the following manner. A 25 weight percent mixture of a homogeneous metallocene-catalyzed copolymer having a melt index of 1 gram per 10 minutes and a density of 0.91 gram per cubic centimeter, made up of 24 weight percent 1-octene and 76 weight percent. percent by weight of ethylene and 75 percent by weight of an ethylene homopolymer produced at high pressure having a melt index of 2 grams per minute and a density of 0.923 gram per cubic centimeter are stirred with 0.18 part by weight of the antioxidant primary, thioethylene bis (3, 5-di-tert-butyl-4-hydroxy) hydrocinnamate; 0.18 part by weight of the secondary antioxidant, distearyl thiodipropionate; and a sufficient amount of dicumyl peroxide to provide a reading of the oscillating disc rheometer (arc of 5 degrees to 182 degrees centigrade with a preheat of 20 seconds) of 48 pumping-pound of torque in a ZSK revolving apparatus with screws co-meshing gears consisting of modular screw elements at a melting temperature of 180 degrees centigrade. The granules formed after passing through the underground granulator and dryer are used to coat a conductor. Example 14 is the same than Example 13 with the exception that the homogenous metallocene-catalyzed copolymer is omitted. The wire coating is achieved through the use of a 6.35-centimeter ROYLE TM extrusion apparatus. A 14 AWG (American Wire Gauge) copper conductor with a diameter of 0.064 is used. A triple cross head allows all three layers to be extruded at one time. The first layer (the layer closest to the copper conductor) of a semiconducting polymeric material is extruded with a thickness of 0.7 millimeter. The second insulation or intermediate layer made with the material of Example 13 or Example 14 is extruded to a thickness of 1.5 millimeters and the outer layer of a semiconducting polymeric material is extruded to a thickness of 0.15 millimeter. The cables are cured in a steam environment for 5 minutes at atmospheric pressure. Two cables are prepared for each example. A cable for each example will be aged and a cable for each example will not age. After the extrusion, the two cables that are going to be aged are stored at 90 degrees centigrade for 16 hours in an air oven. Each of the cables to be aged is cut into 12 pieces, each having a length of 3.2 meters. The copper conductor is then removed and inserted into the conductor channel and a conductor diameter of 0.75 mm. Demineralized water is used to fill the driver's extra space. All pieces of cable are placed in a water bath of the key at 70 degrees Celsius. The 12 pieces of each of the cables to be aged are connected in series and a current of 18 amperes is applied to maintain a conductor temperature of 18 degrees centigrade. An effort is also applied - lí 9 kilovolts electric After aging for 1000 hours, the pieces of wire are allowed to cool. The two cables, which have not been aged, are cut into pieces, each piece being 3.8 meters in length, the pieces are stored for 1000 hours in ambient air, then treated for 16 hours at 90 degrees centigrade. a test of electric disintegration in the pieces using a ramp-up test at 2 kilovolts per second The results are shown in Table II: TABLE II RESISTANCE TO DISINTEGRATION CHARACTERISTIC WEIBULL (VALUE TO 63%) Example • 13 14 No añej (kV / m) 95.0 93.4 With aging (kV / mm) 90.1 42.1 Resistance Retention of 94.8 45.1 Original BD (%) Notes to Table II: 1. kV / mmm = kilovolts per millimeter 2. BD = disintegration The results (the highest number is the best) clearly show the significant advantage for Example 13 with the mixture of the two resins with respect to Example 14 with a resin.

Claims (7)

R E I V I N D I C A C I O N S

1. A cable comprising one or more electrical conductors or a core of one or more electrical conductors, each conductor or core is surrounded by an insulation composition comprising a mixture of (i) from about 20 percent to about 50 percent by weight of a homogeneous polyethylene having a polydispersity within the range of about 1.5 to about 3.5, and an essentially uniform comonomer distribution; and (ii) from about 50 percent to about 80 weight percent of an ethylene homopolymer made by a high pressure process. The cable according to claim 1, wherein the homogeneous polyethylene is a copolymer of ethylene, one or more alpha-olefins, each having from 3 to 12 carbon atoms and optionally a diene. 3. The cable according to claim 1, wherein the homogeneous polyethylene is made with a single-site metallocene catalyst system. The cable according to claim 1, wherein the homogeneous polyethylene has a density within the range of 0.86 to 0.94 gram per cubic centimeter and a melt index within the scale of about 0.5 to about 30 grams per 10 minutes . 5. The cable according to claim 2, wherein the alpha-olefin is 1-butene, 1-hexene, 4-methyl-1-pentene or 1-octene. The cable according to claim 1, wherein the ethylene homopolymer has a density within the range of 0.916 to 0.930 gram per cubic centimeter, and a. melt index within the scale of about 1 to about 10 grams per 10 minutes. 7. A cable comprising one or more electrical conductors or a core of one or more electrical conductors, each conductor or core being surrounded by an insulation composition comprising a mixture of (i) from about 25 percent to about 40 percent by weight of a homogeneous polyethylene made with a single-site metallocene catalyst system having a polydispersity within the range of from about 1.5 to about 3.5, and an essentially uniform comonomer distribution; and (ii) from about 60 percent to about 75 weight percent of an ethylene homopolymer made by a high pressure process.