FR2500950A1 - PROCESS FOR PRODUCING AN INSULATING LAYER FORMED WITH OLEFINIC POLYMERS WITHOUT LOAD FOR CABLES OF ELECTRICAL ENERGY TRANSPORT, METHOD FOR IMPROVING FLEXIBILITY, AND INSULATED CABLES OBTAINED - Google Patents

Abstract

The invention relates to an improved insulation for electrical cables and an improved process for producing the insulation. Cross-linked polyethylene insulation is too stiff, in particular in high-voltage cables which require a thick insulation layer. Polypropylene copolymers have electrical properties which are substantially identical to those of polyethylene, but do not have the necessary mechanical strength and must be filled with clay or other fillers which provide the required strength but impair the electrical properties. A novel feature of the process according to the invention is that the extruder screw is used to effect a mixing of the components in an improved polymer system for cable insulation.

Description

 The industry of energy transport cables, or power cables, has long accepted that chemically crosslinked and uncharged polyethylene constitutes an insulating material characterized by very low dielectric losses, great. dielectric strength and excellent physical properties. Its most inopportune properties are relatively high hardness and minimal flexibility. The installation costs increase sharply due to the extension of time required to produce junctions and ends. Compositions based on copolymers and terpolymers of ethylene and propylene and other elastomers have been used in the field of power cables due to their inherently greater flexibility and the greater ease of installation, which greatly reduces these costs.

 Ethylene and propylene rubbers are amorphous materials which must be reinforced by adding fillers such as hard calcined clays so that they acquire the physical resistance necessary for their use in the field of insulation. The polyethylene being of crystalline structure does not require the addition of a reinforcing material. Therefore, compositions without load are practical and are used in most applications for power cables for a nominal voltage greater than 2 kV. A charge-free polymer system provides the highest degree of electrical properties and, conversely, the addition of charges detracts, in proportion to the amount of charge used, from the inherent good electrical properties of ethylene and propylene rubbers.

 The present invention combines the highest degree of electrical characteristics of a load-free polymer system and the inherent flexibility of rubber by providing a composition suitable for applications in the manufacture of power cables, both in the field high voltages than low voltage. This is achieved by physically combining polyethylene and a copolymer of ethylene and propylene or a terpolymer of ethylene and propylene with a suitable antioxidant and a peroxide type curing agent to obtain a crosslinked composition.

The ratios of the constituents of the polymer system can be varied to obtain more or less flexibility as desired without appreciably altering other physical properties and without significant modification of the electrical characteristics. The resulting compositions are inherently endowed with toughness, flexibility and the greatest degree of electrical properties, in a manner comparable to that of a chemically crosslinked and typically filler-free polyethylene. The crosslinking itself can also be carried out in the absence of chemical crosslinking by an irradiation treatment.

 Based on the still limited data currently available, it can be stated that the time or service life of the compositions used in the present invention will exceed the normal service life of typical chemically crosslinked and uncharged polyethylene compositions. Other properties, such as flexibility at low temperatures, resistance to weathering, resistance to deformation and to mechanical damage, are at least equivalent to those of polyethylene and of typical crosslinked ethylene and propylene copolymers.

 A study of the new compositions indicated that their dielectric strength, in the presence of an alternating current, clearly exceeds that of the formulations of ethylene and propylene rubber and that their breakdown voltage slightly exceeds that of crosslinked uncharged polyethylene.

 The present invention also covers a new process for mixing the constituents of the composition in question. Crude polyethylene and crude copolymer of ethylene and propylene preferably having 15% crystallinity are purchased in the form of pellets. These original pellets remain intact during the entire preliminary mixing cycle which ensures both the kneading of the polymers themselves and the addition of the antioxidant and the peroxide-type ripening agent by absorption through the tablet surfaces. This constitutes a modification of the mixing process for obtaining a crosslinkable polyethylene according to US Pat. No. 3,455,752. Modifications in the manner of operating the mixing are necessary since the system comprises two polymers. The final mixture is produced by the screw of the extruder cylinder which homogenizes the ingredients of the polymer system to form the insulating material before extruding it onto the final cable to be produced. At this stage, the polymers merge into each other forming a complete matrix, and the additives are uniformly dispersed in the polymer blend. This is a clear difference compared to the process of the aforementioned patent no. 3,455,752, which does not require the melting of two polymers during the final extrusion operation.

Other objects, characteristics and advantages of the invention will emerge from the following detailed description, given by way of illustration and in no way limiting, with reference to the appended drawing in which
Figure 1 is a cross section of a cable made according to the present invention; and
FIG. 2 is a flow diagram schematically describing the steps of the method of the present invention.

 FIG. 1 shows a cable 10 for transporting high voltage energy (sometimes called hereinafter "power cable" or "high tensile cable # '), comprising a conductive core 12 formed from several twisted wires or strands and covered a semiconductor layer 14 for shielding the conductive core, preferably an anti-radiation screen 16 is applied to the outside of the shield layer 14.

 The insulation of the cable, which is designated by the reference 20, is applied to the radiation shield 16 or to the shield 14 if there is no radiation shield. The insulation sheath 20 consists of a mixture of polyethylene and a copolymer of ethylene and propylene.

 The ethylene-propylene copolymer does not contain a filler, and in particular no mineral filler, while the ethylene-propylene rubber (EPR) described in the United States patent has 3,579,610 in included one. The EPR rubber of this aforementioned patent contained 40% ethylene, while the ethylene-de-propylene copolymer used in the present invention comprises -78 + 3% ethylene and has 15% crystallinity, while the caou # tchouc EPR of the aforementioned patent is entirely amorphous. Ordinarily, ethylene and propylene rubber cannot be used for the insulation of electric cables if a charge is not added to this ethylene rubber and propylene to give it the physical properties necessary for its use in a cable, such as abrasion resistance, resistance to deep cuts and certain characteristics of behavior as a function of temperature. When subjected to a rise in temperature for their maturation, the polyethylene and the ethylene and propylene rubber crosslink on themselves and with each other.

 Polyethylene is crystalline and it gives the copolymer of ethylene and propylene the properties necessary to be able to be used for the insulation of a cable.

These better physical properties are obtained without harming the electrical characteristics of the copolymer of ethylene and propylene. It is known to use charges to impart the necessary physical properties, but this is achieved at the expense of the essential electrical characteristics for obtaining the best isolation results. The advantages of the present invention in the electrical field are due to the fact that the polymer system does not include mineral reinforcing fillers. It is on the crystallinity of the copolymer of ethylene and propylene and of polyethylene that one bases oneself to obtain the physical resistance and tenacity necessary for the insulation of a cable of transport of electrical energy.

 The ratio of polyethylene to the copolymer of ethylene and propylene can be 1: 1.

However, this ratio can be varied over a wide range. The proportion of polyethylene must be sufficient to impart the necessary physical strength to the insulating layer, but the copolymer of ethylene and propylene must be present in a proportion sufficient to significantly increase the flexibility of the polyethylene. Indeed, an insulating layer of crosslinked polyethylene is too rigid, especially in high-voltage cables whose insulating layer must be thick. The propylene copolymers have electrical properties substantially equal to those of polyethylene but do not have the necessary physical strength and must contain a clay filler or another filler conferring the required mechanical strength but causing a degradation of the electrical characteristics. The invention proposes to use a mixture of polyethylene and a copolymer of ethylene and propylene in a ratio between the polyethylene and the copolymer of ethylene and propylene which can vary between 80:20 and 20:80. However, the preferred range is between 60:40 and 40:60.

 It can be seen in FIG. 1 that the insulation sheath 20 is covered with a semiconductor insulation screen 22 to which a corrugated metal armature 24 is applied. This frame 24 is preferably obtained by longitudinal folding, around the core of the cable, of a metal strip with transverse undulations. Other types of armouring or shielding screens can be used, such as tape or wire mesh. An outer sheath 26 is extruded over the metal shielding armature 24.

 Figure 2 is a flow diagram schematically showing the successive steps of the preferred method of the invention. To economically manufacture the cables of FIG. 1, polyethylene (PE) pellets, which are hard at room temperature, and ethylene-propylene copolymer (EPR) pellets, which are soft and rubbery at room temperature, in a ribbon mixer which mixes them in the form of pellets, that is to say forms a non-homogeneous mixture. This mixing is carried out in the absence of any mineral reinforcing filler and it is then heated and discharged into a heated extruder.

 The heat prevailing in the cylinder of the extruder and the work carried out on the pellets by the screw of the extruder soften the pellets and cause a thorough mixing of the materials of the pellets, so that they melt one in the other by forming an entirely homogeneous matrix.

 If additional ingredients, such as an antioxidant and a peroxide-type curing or crosslinking agent, are added to the pellets while they are in the ribbon blender, these additional ingredients pass through the walls of the pellets and mix polyethylene and ethylene-propylene copolymer without having to wait for it to merge the pellets into each other under the action of the extruder screw. But the mixture of polyethylene and copolymer d ethylene and propylene can also be introduced into the body or casing of an extruder in which are also introduced xs the antioxidant and / or curing or cross-linking agents that the screw advances in the extruder.

 The conductor 12 passes through the head of the extruder passing through a guide end, and the insulating material is extruded over the external surface of the conductor, at the end of the machine, according to conventional extrusion practice for the coating of the electrical conductor.

 The present invention improves the extrusion in a way that was not possible when the insulating material consisted entirely of ethylene and propylene rubber with the addition of a filler.

 The insulating material of the present invention can be discharged through a filter pack or a finely perforated screen, located in the extruder between the end of the screw and the end of the extruder. The mixture of the two basic polymers, polyethylene and the copolymer of ethylene and propylene, has a viscosity at extrusion temperature allowing it to be discharged through a packet of filters whose orifices or openings equivalent to 0.044 mm, this which removes impurities larger than 43 microns from the insulating material.

The elimination of particles larger than 43 microns, or the rupture of particles crossing the screen, greatly improves the efficiency of the insulating material by allowing it to withstand a greater electrical voltage per unit thickness of the insulator.

A preferred copolymer of ethylene and propylene can be obtained under the designation "Vistalon 70 #" from
Exxon Chemical Co., New Jersey, United States of America. This material is approximately 15% crystalline. Exxon company
Chemical Co. manufactures another copolymer of ethylene and propylene, called "Vistalon 404" and which has essentially no crystallinity. This amorphous copolymer cannot be used in the present invention, because it requires the presence of a filler to have the solidity necessary to be able to constitute or form part of the insulating material.

A material equivalent to a copolymer of ethylene and propylene is a copolymer of ethylene and propylene which the addition of a diene has transformed into a terpolymer. This material, called "Nordel 2722", is available at DuPont, Wilmington, Delaware (United States
United States of America). For the purposes of the present invention, this terpolymer "Nordel 2722", without filler, is to be considered as equivalent, from the mechanical point of view, to a copolymer of ethylene and propylene having a little crystallinity. Likewise, it is possible to use in the implementation of the invention of other products which are chemical equivalents of copolymers of ethylene and of propylene whose crystallinity is sufficient for the production of an insulating lining by mixing with polyethylene, without incorporating 'Any filler, including a mineral filler and not polymer, in the insulating material.

 It should be understood that the expression "copolymer of ethylene and propylene" is taken here in a broad sense to also designate and include such copolymers even in the presence of an additional copolymerized monomer, as in the terpolymer "Nordel "of ethymene, propylene and a diene described above.

 It can be seen that all of the polymers used to constitute the insulation sheath or lining have the quality desired for producing electrical insulation. These materials are "uncharged or" without charge ", that is to say that the insulating composition does not comprise the addition of a material, usually clay, intended to increase the mechanical resistance of the insulating composition. Indeed, the presence of such a charge leads to the degradation of the electrical characteristics of the insulating material.

 It goes without saying that, without departing from the scope of the invention, numerous modifications can be made to the process for producing an insulating layer, to the process for improving the flexibility of the insulation layer of electric cables and to electric cables. described and represented.

Claims (16)

 1. A method for producing an insulating layer for a cable for transporting electrical energy, characterized in that a copolymer of ethylene and, in the absence of any mineral reinforcing filler, are mixed in a ribbon mixer and propylene, crystalline at about 15% and containing no filler capable of increasing the physical characteristics of the insulator, with polyethylene intended to impart additional crystallinity to the insulating material by forming with the copolymer of ethylene and propylene a polymer system whose electrical characteristics are substantially equal to those of polyethylene and whose physical characteristics are better because the system has greater toughness than the copolymer and greater flexibility than polyethylene, with antioxidant and a crosslinking agent to obtain a non-homogeneous mixture which is then introduced into the casing or the body of an extruder whose screw causes by s by rotation simultaneously advancing the ingredients, their homogenization and their extrusion to coat an electrical conductor using the mixture or system of polyethylene and the copolymer of ethylene and propylene.  2. Method according to claim 1, characterized in that pellets of the copolymer of ethylene and propylene are mixed with polyethylene pellets, each of the polymers being of the quality required for electrical insulation, and with a crosslinking agent and an antioxidant, the mixture which is extruded on a core of a cable is homogenized, then the flexible polymer system is crosslinked.  3. Method according to claim 1, characterized in that the insulating layer is produced without using a non-polymeric filler to increase the physical resistance of the insulator and in that it is based on the crystallinity of the system copolymer of ethylene and propylene and polyethylene to obtain the physical resistance and toughness characteristics necessary for the insulation of a cable for transporting electrical energy.  4. Method according to claim 1, characterized in that improves the flexibility of an electric cable insulated by crosslinked polyethylene by producing the insulating layer from a mixture, in proportions of between 80: 1 and 20 : 80, polyethylene and a copolymer of ethylene and propylene, and by applying as sheath or insulating lining the composition thus obtained around a conductive core of cable.  5. Method according to claim 1, characterized in that the ratio between the polyethylene and the copolymer or rubber of ethylene and propylene is between 60: 40 and 40: 60.  6. Method according to claim l, characterized in that it causes the mixing of polyethylene pellets and a copolymer of ethylene and propylene, and the mixture is sent to a heated extruder through which the conductive core, a thorough mixing of the polyethylene and the copolymer of ethylene and propylene is carried out in this extruder using a screw whose movement in the cylinder of the extruder agitates and works the polyethylene and the copolymer of ethylene and propylene by pushing the material towards the exit end of the cylinder of the extruder, where the mixture of polyethylene and copolymer of ethylene and propylene is extruded on the external surface of the conductive core of the cable.  7. Method according to claim 1, characterized in that one obtains, in an extruder, a molten mixture of poyethylene, a copolymer of ethylene and propylene, an antioxidant and a crosslinking agent on passes the melted mixture through a packet of filters which then passes through one end and through an extrusion die of the extruder.  8. Method according to claim 7, characterized in that the filter pack, disposed between the outlet end of the screw and the outlet end of the end of the extruder, has orifices or openings of 0.044 mm to remove foreign particles larger than 43 microns.  9. High voltage electrical cable having a conductive core (12), a layer (14) of extruded semiconductor material, an insulating layer (20), and another layer (22) of semiconductor material around the surface external of the insulating material (20), this cable being characterized in that the insulating layer (20) comprises a mixture, in proportions of between approximately 80: 1 and 20: 80, of polyethylene and of a copolymer of the ethylene and propylene which are both crosslinked, the polyethylene serving to give the copolymer of ethylene and propylene the necessary physical characteristics, in particular a fairly low viscosity for extrusion over the semiconductor layer (14) at the operating temperature of the extruder and the ethylene-propylene copolymer essentially comprising no filler intended to give it better physical characteristics, but this ethylene-propylene copolymer giving the material is olante better flexibility compared to that of an insulating material consisting of crosslinked polyethylene.  10. Electric cable according to claim 9, characterized in that it comprises a metal shielding armature (24) outside the insulating material (20) and an outer sheath (26) over the armature (24 ) metallic.  11. Electric cable according to claim 9, characterized in that it comprises, between the semiconductor layer (14) surrounding the conductive core (12), on the one hand, and the internal surface of the insulating layer (20), on the other hand, an anti-radiation screen (16) which surrounds the semiconductor layer (14).  12. Electric cable according to claim 9, characterized in that the insulating layer (20) comprises polyethylene and a copolymer of ethylene and propymene, present in proportions approximately between 60: 40 and 40: 60 and which are, each, cross-linked on themselves and to each other.  13. Electric cable according to claim 9, characterized in that at the extrusion temperature the insulating material has a viscosity allowing its discharge through a packet of filters with fine openings of 0.044 mm and in that this insulating material does not comprise no solid impurities larger than about 43 microns.  14. Electric cable according to claim 9, characterized in that # the ethylene and propylene rubber is a terpolymer of ethylene, propylene and a third co-monomer.  15. Process for producing an insulating layer that is not charged for cables for transporting electrical energy, this process being characterized in that an ethylene-propylene copolymer is produced, which is crystalline at around 15% and does not contain filler which increases the physical characteristics of the insulating material; pellets of this copolymer are mixed with polyethylene pellets (each polymer being of quality required for the manufacture of an electrically insulating layer and the polyethylene being intended to form with the copolymer a polymer system of greater crystallinity whose characteristics electric are substantially equal to those of polyethylene and whose physical characteristics include greater toughness than that of the copolymer and greater flexibility than that of polyethylene) with a crosslinking agent and an antioxidant; the mixture is introduced into a screw extruder and the mixture is homogenized by the movement of the screw in the body or the casing of the extruder, which advances the ingredients of the mixture; an electrical cable core for high voltage is passed through an extrusion die and a layer of the flexible polymer system is extruded on this core, the insulating material being thus produced without the use of a non-load polymer to increase the physical resistance of the insulating material, and the crystallinity of the copolymer and of the polyethylene ensuring the obtaining of the physical resistance and the toughness necessary for the insulation of a cable for transporting electrical energy.  16. Method for increasing the flexibility of electric cables insulated by crosslinked polyethylene, characterized in that the insulating composition is formed by mixing, in a ribbon mixer, in a proportion of between 60:40 and 40:60, pellets of polyethylene and pellets of a copolymer of ethylene and propylene; the mixture of pellets as well as at least one chemical crosslinking agent and antioxidant is sent to a heated extruder, which is caused to be distributed by dispersion through the walls of the pellets, then the thorough kneading and melting of this mixture of polyethylene, copolymer of ethylene and propylene, antioxidant and crosslinking agent by the kneading and working action exerted by the movement of a screw in the cylinder of the extruder, which pushes the material in this cylinder to a packet of filters then to an extrusion end and through an extrusion die on the external surface of the conductive core of the electric cable.