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

• This invention relates to compositions useful in the preparation of cable insulation, semiconducting shields, and jackets.
• a typical electric power cable generally comprises one or more conductors in a cable core that is surrounded by several layers of polymeric materials including a first semiconducting shield layer (conductor or strand shield), an insulating layer, a second semiconducting shield layer (insulation shield), a metallic tape or wire shield, and a protective jacket. Additional layers within this construction such as moisture impervious materials are often incorporated. Other cable constructions such as plenum and riser cable omit the shield.
• crosslinking of the polymeric materials is essential to the particular cable application, and, in order to accomplish this, useful compositions generally include a polymer; a crosslinking agent, usually an organic peroxide; and antioxidants, and, optionally, various other additives such as a scorch inhibitor or retardant and a crosslinking booster.
• Crosslinking assists the polymer in meeting mechanical and physical requirements such as improved thermal aging and lower deformation under pressure.
• the crosslinking of polymers with free radical initiators such as organic peroxides is well known.
• the organic peroxide is incorporated into the polymer by melt blending in a roll mill, a biaxial screw kneading extruder, or a BanburyTM or BrabenderTM mixer at a temperature lower than the onset temperature for significant decomposition of the peroxide.
• Peroxides are judged for decomposition based on their half life temperatures as described in Plastic Additives Handbook, Gachter et al, 1985, pages 646 to 649.
• An alternative method for organic peroxide incorporation into a polymeric compound is to mix liquid peroxide and pellets of the polymer in a blending device, such as a HenschelTM mixer or a soaking device such as a simple drum tumbler, which are maintained at temperatures above the freeze point of the organic peroxide and below the decomposition temperature of the organic peroxide and the melt temperature of the polymer.
• a blending device such as a HenschelTM mixer or a soaking device such as a simple drum tumbler, which are maintained at temperatures above the freeze point of the organic peroxide and below the decomposition temperature of the organic peroxide and the melt temperature of the polymer.
• the polymer/organic peroxide blend is then, for example, introduced into an extruder where it is extruded around an electrical conductor at a temperature lower than the decomposition temperature of the organic peroxide to form a cable.
• the cable is then exposed to higher temperatures at which the organic peroxide decomposes to provide free radicals
• a substituted hydroquinone has been suggested as a scorch inhibitor in United States patent 5,292,791 at levels of at least 0.1 percent by weight based on the weight of the polymer.
• substituted hydroquinones seriously decrease the cure density in high pressure low density polyethylene (HP-LDPE) and are therefore not suitable for use at levels above about 0.1 weight percent for thermosetting wire and cable formulations based on HP-LDPE.
• higher melting additives such as the substituted hydroquinones above about 0.1 percent often exhibit sweatout leading to dust handling and extrusion complications.
• An object of this invention is to provide a crosslinkable resin composition, which can be processed at high temperatures; is improved in terms of scorch inhibition and cure rate; and exhibits high temperature heat stability.
• composition comprises:
• the VLDPE can be a copolymer of ethylene and one or more alpha-olefins having 3 to 12 carbon atoms and preferably 3 to 8 carbon atoms.
• alpha-olefins are propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene.
• the density of the VLDPE can be in the range of 0.860 to 0.915 gram per cubic centimeter.
• the melt index of the VLDPE can be in the range of about 0.1 to about 20 grams per 10 minutes and is preferably in the range of about 0.3 to about 5 grams per 10 minutes.
• the portion of the VLDPE attributed to the comonomer(s), other than ethylene, can be in the range of about 1 to about 49 percent by weight based on the weight of the copolymer and is preferably in the range of about 15 to about 40 percent by weight.
• a third comonomer can be included, e.g., another alpha-olefin or a diene such as ethylidene norbornene, butadiene, 1,4-hexadiene, or a dicyclopentadiene.
• the third comonomer can be present in an amount of about 1 to 15 percent by weight based on the weight of the copolymer and is preferably present in an amount of about 1 to about 10 percent by weight. It is preferred that the copolymer contain two or three comonomers inclusive of ethylene.
• the VLDPE can be homogeneous or heterogeneous.
• the homogeneous polyethylenes usually have a polydispersity (Mw/Mn) in the range of about 1.5 to about 3.5 and an essentially uniform comonomer distribution, and are characterized by single and relatively low DSC melting points.
• 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 weight average molecular weight and Mn is defined as number average molecular weight.
• the VLDPEs are produced by low pressure processes. They are preferably produced in the gas phase, but they can also be produced in the liquid phase in solutions or slurries by conventional techniques. Low pressure processes are typically run at pressures below 1000 psi.
• Typical catalyst systems which can be used to prepare these polyethylenes, are magnesium/titanium based catalyst systems, which can be exemplified by the catalyst system described in United States patent 4,302,565 (heterogeneous polyethylenes); vanadium based catalyst systems such as those described in United States patents 4,508,842 (heterogeneous polyethylenes) and 5,332,793; 5,342,907; and 5,410,003 (homogeneous polyethylenes); a chromium based catalyst system such as that described in United States patent 4,101,445; a metallocene catalyst system such as that described in United States patents 4,937,299 and 5,317,036 (homogeneous polyethylenes); or other transition metal catalyst systems.
• Catalyst systems which use chromium or molybdenum oxides on silica-alumina supports, can be included here. Typical processes for preparing the polyethylenes are also described in the aforementioned patents.
• the scorch inhibitor is a substituted hydroquinone.
• it is a hydroquinone substituted at the 2 position with a tertiary alkyl group or at the 2 and 5 positions with the same or different tertiary alkyl groups.
• tertiary alkyl groups are tertiary butyl and tertiary amyl.
• the alkyl group can have 1 to 18 carbon atoms.
• the cure (crosslinking) booster can be any one, or a mixture, of a broad selection of boosters.
• An important point here is to select a cure booster with good heat stability.
• it can be an ester, ether, or ketone containing at least 2 , and preferably 3, unsaturated groups such as a cyanurate, an isocyanurate, a phosphate, an ortho formate, an aliphatic or aromatic ether, or an allyl ester of benzene tricarboxylic acid.
• the number of carbon atoms in the ester, ether, or ketone can be in the range of 9 to 40 or more, and is preferably 9 to 20.
• Preferred esters, ethers, and ketones are essentially non-volatile at storage temperatures, and the unsaturated groups are preferably allyl groups.
• Specific examples are triallyl cyanurate (TAC); triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione also known as triallyl isocyanurate (TAIC); triallyl phosphate; triallyl ortho formate; tetra-allyloxy-ethane; triallyl benzene-1,3,5-tricarboxylate; diallyl phthalate; zinc dimethacrylate; ethoxylated bisphenol A dimethacrylate; methacrylate terminated monomer with average chain lenght of C 14 or C 15 ; pentaerythritol tetraacrylate; dipentaerythritol pentaacrylate; pentaerythritol triacrylate; dimethylolpropane tetraacrylate;
• Preferred boosters are triallyl cyanurate (TAC); 3,9-divinyl-2,4,8,10-tetra-oxaspiro[5.5]undecane (DVS); triallyl isocyanurate (TAIC); and triallyl trimellitate (TATM).
• TAC triallyl cyanurate
• DVS 3,9-divinyl-2,4,8,10-tetra-oxaspiro[5.5]undecane
• TAIC triallyl isocyanurate
• TATM triallyl trimellitate
• the weight ratio of scorch inhibitor to cure booster can be in the range of about 0.2:1 to about 0.5 :1, and is preferably in the range of about 0.05:1 to about 0.25:1.
• the organic peroxide preferably has a one hour half life decomposition temperature measured in benzene of about 125 to about 150 degrees C and can be exemplified by the following compounds [the numbers set off by the parentheses are their one hour half life decomposition temperatures (in degrees C)]: t-butyl peroxy benzoate (125); dicumyl peroxide (135); alpha, alpha'-bis-t-butylperoxy-1,4-diisopropylbenzene (137); 2,5-dimethyl-2,5-di(t-butyl-peroxy)hexane (138); t-butyl cumyl peroxide (138); t-butyl hydroperoxide (140); di-t-butyl peroxide (149); and 2,5-dimethyl-2,5-di(t-butyl peroxy)hexane-3 (149).
• the proportions of the compounds can be about as follows (in parts by weight): Component Broad Range Preferred Range (b) scorch inhibitor 0.02 to 0.09 0.05 to 0.09 (c) cure booster 0.2 to 1 0.3 to 0.6 (d) organic peroxide 0.4 to 4 0.6 to 2
• composition of the invention can be processed in various types of extruders, e.g., single or twin screw types.
• extruders e.g., single or twin screw types.
• a description of a conventional extruder can be found in United States patent 4,857,600.
• a typical extruder has a hopper at its upstream end and a die at its downstream end. The hopper feeds into a barrel, which contains a screw. At the downstream end, between the end of the screw and the die, is a screen pack and a breaker plate.
• the screw portion of the extruder is considered to be divided up into three sections, the feed section, the compression section, and the metering section, and two zones, the back heat zone and the front heat zone, the sections and zones running from upstream to downstream.
• extruder includes, in addition to conventional extruders, the combination of an extruder, crosshead, die, and a heating or cooling zone where a further forming of the material can be accomplished.
• the heating or cooling follows the die and may be, for example, an oven.
• the die of the crosshead feeds directly into a heating zone, and this zone can be maintained at a temperature in the range of about 130 to about 260 degrees C, and preferably in the range of about 170 to about 220 degrees C.
• the extrudate is then crosslinked by exposing it to a temperature greater than the decomposition temperature of the organic peroxide.
• the crosslinking can be accomplished in, for example, an oven or a continuous vulcanizable (CV) tube.
• CV continuous vulcanizable
• a pressure rated vulcanizing tube is mechanically coupled to the extruder crosshead such that the polymer melt exits the crosshead/die assembly into a vulcanizing pipe running perpendicular to the extruder.
• compositions incorporating peroxides are extrusion fabricated into insulation and cable jacketing at low melt extrusion temperatures to avoid premature crosslinking in the extruder.
• the fabricated melt shape exits the shaping die into the steam vulcanizing tube where post extrusion peroxide initiated crosslinking occurs.
• the steam tube is filled with saturated steam which continues to heat the polyolefin melt to the increased temperatures needed for crosslinking.
• Most of the CV tube is filled with saturated steam to maximize dwell time for crosslinking to occur.
• the final length before exiting the tube is filled with water to cool the now crosslinked insulation/jacketing.
• the insulated wire or cable passes through an end seal incorporating close fitting gaskets, which minimize the cooling water leakage.
• Steam regulators, water pumps, and valves maintain equilibrium of the steam and water and the respective fill lengths within the steam CV tube. Hot inert gases such as nitrogen can be used as an alternative to steam for the heating.
• additives can be added to the polymer either before or during processing.
• the amount of additive is usually in the range of about 0.01 to about 50 percent by weight based on the weight of the resin.
• Useful additives are antioxidants, ultraviolet absorbers, antistatic agents, pigments, carbon black, dyes, fillers, slip agents, fire retardants, plasticizers, processing aids, lubricants, stabilizers, smoke inhibitors, halogen scavengers, flow aids, lubricants, water tree inhibitors such as polyethylene glycol, and viscosity control agents.
• conductive particles are generally provided by particulate carbon black.
• Useful carbon blacks can have a surface area of about 50 to about 1000 square meters per gram. The surface area is determined under ASTM D 4820-93a (Multipoint B.E.T. Nitrogen Adsorption).
• the carbon black is used in the semiconducting shield composition in an amount of about 20 to about 60 percent by weight based on the weight of the composition, and is preferably used in an amount of about 25 to about 45 percent by weight.
• Examples of conductive carbon blacks are the grades described by ASTM N550, N472, N351, N110, and acetylene black.
• antioxidants are: hindered phenols such as tetrakis[methylene(3,5-di-tert- butyl-4-hydroxyhydrocinnamate)]methane, bis[(beta-(3,5-di-tert-butyl-4-hydroxybenzyl)methylcarboxyethyl)]sulphide, 4,4'-thiobis(2-tert-butyl-5-methylphenol), 2,2'-thiobis(4-methyl-6-tert-butylphenol), and thiodiethylene bis(3,5-di-tert-butyl-4-hydroxy hydrocinnamate); phosphites and phosphonites such as tris(2,4-di-tert-butylphenyl)phosphite and di-tert-butylphenyl-phosphonite; thio compounds such as dilaurylthiodipropionate, and dimyristylthiodipropionate; various siloxanes;
• Advantages of the invention are low scorch, high cure rate, high cure density, higher useful extrusion temperatures, less molecular weight degradation of copolymer, less dusting of resin due to peroxide sweat out, and, under suitable circumstances, higher throughput of wire or cable through the continuous vulcanizing oven.
• Another advantage relates to the solubility of the scorch inhibitor in the VLDPE. Its high effectiveness at about 0.09 part by weight and below allows use below its solubility limits. This reduces or eliminates blooming at the surface as well as crystallization in the resin matrix.
• the maximum extrusion temperature relates to the decomposition temperature of the organic peroxide, i.e., the extrusion temperature cannot be as high as the temperature at which significant decomposition of the peroxide takes place. Thus, it is advantageous to be able to use an organic peroxide having a higher decomposition temperature if the other components of the composition of the invention will tolerate a higher extrusion temperature.
• the term "surrounded” as it applies to a substrate being surrounded by an insulating composition, jacketing material, or other cable layer is considered to include extruding around the substrate; coating the substrate; or wrapping around the substrate as is well known by those skilled in the art.
• the substrate can include, for example, a core including a conductor or a bundle of conductors, or various underlying cable layers as noted above.
• composition temperature as it relates to organic peroxides is the onset temperature for significant decomposition of the organic peroxide. This temperature is based on the half life temperature of the organic peroxide.
• 100 parts by weight of the ethylene polymer are fluxed in a BrabenderTM mixer heated to 150 degrees C.
• the additives are added to the fluxed resin and mixed at a temperature of up to about 170 degrees C during a five minute period.
• the resulting composition is cooled and transferred to a heated two roll mill where the peroxide is added and blended at a temperature below 130 degrees C for three minutes.
• the hot sheeted peroxide composition is then fed to a granulator to provide a granulated product for use in the examples.
• Variables and results are set forth in the Table. Amounts of components are given in parts by weight.

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• Physics & Mathematics (AREA)
• Spectroscopy & Molecular Physics (AREA)
• Compositions Of Macromolecular Compounds (AREA)
• Organic Insulating Materials (AREA)

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Cited By (7)

Citations (5)

• 2000
  • 2000-03-29 EP EP00302623A patent/EP1041581A1/fr not_active Withdrawn
  • 2000-03-30 CA CA 2303580 patent/CA2303580A1/fr not_active Abandoned

Patent Citations (5)

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