Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B3/00—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties
  • H01B3/18—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances
  • H01B3/30—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes
  • H01B3/44—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes vinyl resins; acrylic resins
  • H01B3/441—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes vinyl resins; acrylic resins from alkenes

Definitions

• the present invention relates to an electric cable, insulated with compositions of low density polyethylene, which are crosslinked by moisture. More precisely, it refers to the application of compositions with a low density polyethylene base, and which are cross-linked by moisture for insulating and covering cables. The process is known as "SIOPLAS".
• the oldest one consists in applying to the polyethylene as additives organic peroxides, with an activating temperature above the softening temperature for the polyethylene used.
• organic peroxides with an activating temperature above the softening temperature for the polyethylene used.
• These composites are applied in the case of cables manufactured through an extrusion process followed by a thermo-chemical cross-linking process instantaneous simultaneous with the heating of the insulated cable above the peroxide activating temperature, for instance, through saturated vapor heating or through radiant heat produced by electric resistances, followed by a process corresponding to water cooling.
• the process is done under pressure, since the decomposition gases generated by the chemical reactions cause the existence of bubbles in the extruded insulation.
• This process is still widely used for the production of medium/high tension cables, because it is the only way to obtain large extruded and crosslinked insulation sections without empty spots. This process requires high cost industrial installations.
• Another process for obtaining cross-linked polyethylene insulation is irradiation by means of electron beams or sources of gamma radiation.
• the cable is insulated by means of a normal extrusion process that may be applied to thermoplastic polyethylene, followed by a crosslinking process, in a subsequent phase, by means of high energy electron beam radiation or gamma radiation sources.
• the process is mainly used for cables with a small extruded section, since the penetration capacity of the electrons is limited.
• Both processes, the electron beam radiation and gamma sources require high cost installations, mainly because of special protection needed for the operator and for the environment.
• the production speed of cables is limited by the amount of energy that these sources can liberate and by the amount of material (insulation) to be cross-linked. These processes are usually indicated and applied to special cables.
• Another process is the chemical cross-linking done by means of moisture.
• an organo-silane that can be hydrolysed is introduced to the polyethylene molecule which still maintains its thermoplastic characteristics, being applied on the cable by means of a usual extrusion process that can be applied to the thermoplastic polyethylene, where the extrusion speed is only limited by the extrusion and the material characteristics.
• the polyethylene cross-linking in the cable occurs in the reel, either in the environmental temperature and humidity conditions or by exposing the reel to a "sauna" type environment, depending on the material.
• This last process for obtaining cross-linked polyethylene as described above is, nowadays, widely used by the manufacturers of cables, mainly for insulation of power cables for low tension.
• the process is widely mastered by the manufacturers of cables, with at least two ways of obtaining the composite for extrusion:
• extrusion processes commonly used for insulating conductors differ according to the type of tools used for the formation of insulation on the conductor.
• the keywords used to differentiate the processes are "Extrusion under Pressure”, “Extrusion under Semi-tubes” and “Extrusion under Tubes”.
• the tools used have, approximately, the final insulation dimensions (the polyethylene is placed over the conductor without stretching); for the “Extrusion under Semi-tubes”, the tools are slightly bigger than the dimensions of the insulation and they are generally used when the composite tends to flow backwards along the male die; and for the “Extrusion under Tubes”, the male and female dies are, generally, much bigger than the insulation dimension of the conductor, and they are used when the profile of the conductor is not round, which makes it impossible to use tight tools, or in the case of composites applied on very thin materials (typical example: FEP "Teflon”). In this case, the final shape of the insulation is obtained by stretching the tube up to the final dimension.
• thermoplastic materials are polymeric (macromolecules) which, in the extrusion process, are squeezed and stretched.
• the polymeric materials require a relatively long time for relaxing the tensions stored during the passage through the tools. Since the insulation is rapidly cooled, to obtain productivity and to avoid deformities in the reel, a large part of the tensions is stored.
• the power cables for low tension are covered by international specifications, such as IEC 502(83), NBR ABNT Project 3.20.3-026(90). Under the service conditions, these cables are classified for continuous service at a 90°C temperature (in the conductor), and at an overload process, during a short period of time, up to 130°C, and under a short circuit condition up to 250°C.
• the cross-linked polyethylene obtained by the "Sioplas" process when heated above its softening temperature (95 to 115°C), tends to relax the tensions left by the extrusion process, creating a contraction, especially in the ends of the cable exposing the conductor.
• the rules described above include a qualification test. Such a test consists in cutting 200mm from the central part of a sample that has at least 1200mm of the isolated cable, then exposing this sample to heat for 1 hour at 130°C in an air stove, and measuring the isolation contraction at both ends. The value found must not exceed 4%.
• the current invention has the purpose of solving the current technical problems, through the modification of the polyethylenes that are usually employed in the "Sioplas" process, with the addition of polypropylene (PP).
• PP polypropylene
• the polypropylene has a softening temperature between 155 and 165°C, which guarantees, up to 130°C. the dimensional stability of the extruded material.
• the mixture of the PP with the low density polyethylene (LDPE) or linear low density polyethylene (LLDPE) or co-polymeric polyethylene of vinyl ethylene acetate (EVA) in an adequate concentration is the solution for the contraction problem.
• the low density polyethylene is a homopolymeric or copolymeric vinyl acetate (VA) or methylacrylate (MA) up to 10%.
• the linear low density polyethylene (LLDPE) is a butene or hexane copolymer.
• the polypropylene is a propylene-ethylene copolymer with a co-monomer level (ethylene) over 6%, being used in a preferred proportion between 15 and 25%.
• compositions with polyethylene base which can be crosslinked through the "Sioplas" process.
• compositions can be easily reproduced in a laboratory using the technique of humectation for the granulated polyethylene with the silane, peroxide and DBTL.
• the above compositions were extruded with tube extrusion tools over a flat profile conductor with a rate of section reduction of 2 (DDR - draw down ration).
• a fusion temperature (Melt) of 220°C was used. After treating them in waterbath at 70°C for 16 hours, in a contraction controlled way, they became faded and a fracture in the flection occurred.
• Examples 6 and 7 Compositions of polyethylene modified with copolymeric polypropylene.
• Ingredient Example 6 Example 7: LDPE (MFI 1,3) 80 - LLDPE (MFI 1,0) - 80 Copolymeric PP (8% ethylene) MFI 4; EPT30RSF Spheripol from HIMONT 20 20 Antioxidant 0.2 0.2 VTMO 1.5 1.5 Cumila Peroxide (96-100%) 0.12 0.12 DBTL 0.05 0.05 0.05
• Examples 6 and 7 were processed the same way as examples 1, 2, 3, 4 and 5.
• Examples 8, 9 and 10 were processed the same way as examples 1, 2, 3, 4, 5, 6 and 7.

Landscapes

• Physics & Mathematics (AREA)
• Spectroscopy & Molecular Physics (AREA)
• Compositions Of Macromolecular Compounds (AREA)
• Processes Of Treating Macromolecular Substances (AREA)
• Organic Insulating Materials (AREA)
• Extrusion Moulding Of Plastics Or The Like (AREA)

Applications Claiming Priority (2)

Publications (2)

Family

ID=4054935

Family Applications (1)

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Cited By (3)

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• 1992
  • 1992-09-11 BR BR929203610A patent/BR9203610A/pt not_active Application Discontinuation
• 1993
  • 1993-09-09 AU AU46265/93A patent/AU4626593A/en not_active Abandoned
  • 1993-09-10 CA CA 2105896 patent/CA2105896A1/fr not_active Abandoned
  • 1993-09-13 EP EP9393307197A patent/EP0587453A3/en not_active Withdrawn

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