Multiple insulating layer high voltage wire insulation

An electrical insulation composition comprising two or more crosslinked insulating layers wherein the layers comprise compatible polymer materials wherein crosslinking is present between the compatible layers to bond said layers together so that the crosslinked layers, when applied to a conductor, strip from said conductor without substantial separation. In addition, the crosslinked layers provide at least one of the following continuous service ratings when applied to 10 AWG or smaller conductor: i. greater than about 10 kV DC; ii. greater than about 5 kV AC.

FIELD OF THE INVENTION
 This invention relates to multiple layer high voltage wire insulation. More
 particularly, this invention relates to a two or more layer
 crosslinked/bonded insulation construction suitable for continuous use at
 elevated DC and/or AC voltages. The insulation can be made with excellent
 flame resistant properties and is particularly suitable for fine
 performing television set receiver cable.
 BACKGROUND OF THE INVENTION
 Over the years, a large number of prior art disclosures have focused on the
 development of cable insulation wherein said cable is of a multiple layer
 design. For example, as early back as 1935-1940, a two layer insulating
 cable design was disclosed, the first layer made from natural rubber, and
 an outer flame retardant layer containing polycholoroprene or neoprene
 rubber. By the mid 1940's, first layers reportedly contained
 poly(ethylene) followed by an outer layer of poly(vinyl chloride). In the
 1960's first layers were manufactured from ethylene-propylene diene
 terpolymer, or ethylene propylene copolymers, and the outer layer
 comprised chlorosulfontated polyethylene, neoprene, or Hypalon
 (chlorosulphonated polyethylene) rubber. In the mid 1960's first layers
 were manufactured from crosslinked polyethylene, and the outer layers of
 crosslinked poly(vinylidine fluoride).
 For example, in U.S. Pat. No. 3,269,862 there is disclosed an electrical
 insulation material which comprises a first inner layer of a polyolefin
 and a second outer layer of poly(vinylidine fluoride) in which the polymer
 comprising each of the layers is crosslinked.
 In U.S. Pat. No. 3,546,014, there is disclosed a method of manufacturing
 thin wall wire by first providing a insulation layer of chemically
 crosslinked polyethylene over a metal conductor. The surface of the
 insulation is etched over its circumference, and a flame retardant coating
 of a thermosetting halogenated polyolefin is applied uniformly over the
 polyethylene insulation.
 In U.S. Pat. No. 4,051,298, there is disclosed the combination of a
 previously cured copolymer of ethylene and propylene adjoined to a
 subsequently cured elastomeric blend of ethylene and propylene mixed with
 chlorosulfonated polyethylene. The combination of materials is said to
 provide various advantages when used in electrically conducting wire and
 cable products while also providing an overlying strippable semiconductive
 layer.
 In U.S. Pat. No. 4,184,001, there is disclosed an insulation system for
 electrical conductors, which comprises a layer of crosslinked flurocarbon
 polymer, selected from ethylene-tetrafluroethylene copolymer, ethylene
 chlorotrifluoroethylene copolymer and ethylene-tetrafluoroethylene
 terpolymer. This layer of polymer is then covered with a polyimide
 coating.
 In U.S. Pat. No. 4,789,589 there is disclosed a conductor wire with an
 inner layer of insulation comprising cellular polyolefin and an outer
 layer of poly(vinyl chloride). The poly(vinyl chloride) is said to include
 a material compatible with the polyolefin (such as chlorinated PE), and is
 said to bond to the cellular polyolefin to hold the layers together.
 In U.S. Pat. No. 5,059,483, there is disclosed shaped articles of
 crosslinked polymers comprising a first component having little or no
 crosslinking and high relative elongation, and a second component having a
 relatively high level of crosslinking and low elongation. The articles are
 described as being useful in the form of electrical insulation, the first
 component being adjacent to a wire or other conductor.
 In U.S. Pat. No. 5,162,609, there is disclosed a fire resistant cable
 suitable for the transmission of high frequency signals which includes a
 core which contains a plurality of twisted pairs of insulated conductors
 and a jacket. The insulation system includes dual layers, the outer of
 which is a flame retardant plastic material. Specifically, the insulation
 system includes an inner layer of polyolefin plastic material and an outer
 layer of flame-retardant polyolefin plastic material characterized by a
 suitable low dissipation factor and dielectric constant. The outer layer
 of flame retardant polyethylene is about 0.003 inch.
 U.S. Pat. No. 5,281,766 describes a motor lead wire that is overcoated with
 a primary insulation layer of polyolefin, protected by a jacket of
 poly(vinylidine fluoride) or a poly(vinylidine fluoride) copolymer having
 an approximate maximum thickness of 0.005 inches. The primary insulation
 is crosslinked and stabilized with a zinc salt of
 methylmercaptobenzimidazole and a hindered phenol anti-oxidant.
 U.S. Pat. No. 5,358,786 describes a fire-resistant plenum-type electrical
 cable that is insulated with an inner layer of a flurocarbon copolymer and
 an outer layer of an abrasion resistant and flame resistant poly(vinyl
 chloride).
 U.S. Pat. No. 5,514,837 describes a plenum cable having a plurality of
 insulated conductors enclosed by a jacket. In a preferred structure, one
 of the conductors is covered with an insulation layer of a flame retardant
 polyethylene or polypropylene resin, and one of the other conductors is
 insulated with fluorinated ethylene propylene (FEP).
 As can be seen from the above review of the prior art, most of the multiple
 layer insulating cable designs reported to date, wherein the outer layer
 provides flame retardant characteristics, feature dissimilar material
 (resin) systems, which can contribute to incompatibility at the resin
 interfaces. In addition, none of the systems reported disclose multiple
 layer insulating design which can be crosslinked in a single crosslinking
 pass so that the multiple layers can be made to consistently stick
 together and strip together without significant separation. Furthermore,
 to date, a multiple layer insulating design of the aforementioned combined
 characteristics, that also provides product flexibility and a continuous
 service rating of greater than 10 kilovolts (kV) DC, or 5 kV AC such as is
 required in fine performing television set receiver cable, has not been
 available.
 Accordingly, it is an object of this invention to overcome the
 disadvantages of the prior art and provide a multilayer insulating
 material, particularly suitable for high voltage wire insulation, wherein
 the layers are made compatible such that they will
 It is also an objective of the present invention to manufacture such
 multilayer construction from at least two layers, wherein the layers as
 noted have crosslinking at the interface, and wherein the outer layer is
 also made flame retardant, with good flexibility, and wherein the inner
 layer is chosen for outstanding dielectric breakdown characteristics.
 Finally, it is also an object of this invention to optimally crosslink such
 a two or more layer type insulation construction in a single crosslinking
 pass so that the layers not only adhere to one another, but strip together
 without noticeable separation, and wherein the crosslinked layers also
 minimize moisture penetration.
 SUMMARY OF THE INVENTION
 In composition form, the present invention discloses an electrical
 insulation composition for DC voltage leads, comprising two or more
 crosslinked insulation layers wherein said layers comprise compatible
 polymer materials wherein crosslinking is present between said layers to
 bond said layers together so that said crosslinked layers, when applied to
 a conductor, strip from said conductor without substantial separation, and
 wherein said crosslinked layers provide, when applied to about 10 AWG or
 smaller conductor, at least one of the following continuous service
 ratings: i. greater than about 10 kV DC; ii. greater than about 5 kV AC.
 In product form, the present invention discloses an insulated conductor
 wire comprising a conducting wire and surrounding insulation comprising
 two or more crosslinked layers bonded together, characterized in that said
 surrounding insulation provides, when applied to about 10 AWG or smaller
 conductor, at least one of the following continuous service ratings: i.
 greater than about 10 kV DC; ii. greater than about 5 kV AC, and wherein
 said bonded layers strip from said conductor without substantial
 separation and substantially prevent diffusion by air, moisture,
 electrolytes, and said insulated conductor wire passes a CSA FT1 or UL
 VW-1 vertical flame test.
 Furthermore, in process form, the present invention discloses a process for
 preparing an insulated conductor wire containing a conductor wire and a
 surrounding insulation of two or more insulating layers, said process
 comprising selecting a polymer resin composition as a first insulating
 layer for coating said conductor wire, and coating said conductor wire
 with said first polymer resin. This is followed by selecting a second
 polymer resin composition compatible with said first insulating layer so
 that said second polymer resin will crosslink with said first insulating
 layer at the layer interface. The first insulating layer is then coated
 with this selected and compatible second polymer resin to form a second
 insulating layer followed by crosslinking said first and second insulating
 layers between said layers to bond said layers together to form the
 insulated conductor wire of the present invention.
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 As noted above, the present invention first comprises an electrical
 insulation composition for high DC and AC voltage leads, comprising two or
 more crosslinked insulating layers wherein said layers comprise compatible
 polymer materials wherein crosslinking develops between said layers to
 bond said layers together. Preferably, this crosslinking is effected by
 irradiation, and one can also preferably employ accelerators/promotors to
 enhance the amount of crosslinking which develops herein.
 Preferably said crosslinking accelerators/promotors contain at least one
 allyl or vinyl group selected from the group consisting of esters of
 methacrylic acid, polyfunctional vinyl monomers, and mixtures thereof.
 More preferably, the crosslinking accelerators/promotors are selected from
 the group consisting of triallyl isocyanurate (TAIC), triallylcyanurate,
 trimethylpropane trimethacrylate (TMPTMA), decamethylene glycol
 dimethacrylate, divinyl benzene, diallylphthalate, and mixtures thereof.
 The electrical insulation herein is further characterized in that said
 bonding which is present between said layers substantially limits air,
 moisture or electrolytes from diffusing therein. Those skilled in the art
 will recognize that by limiting such diffusion, the present invention
 avoids the collection of air, moisture or electrolytes between the layers,
 which often leads to service problems such as dielectric breakdown or
 corona discharge.
 Furthermore, with respect to the crosslinking between layers noted herein,
 such interlayer crosslinking is further defined and characterized in that
 the layers will strip or remove themselves from a conductor without
 substantial separation, and this has been found to be the case in the
 present invention even after sharp bending or repeated flexing. This
 stripping behavior therefor eliminates a long-standing problem in the
 insulation stripping operation for multiple layer designs when the layers
 separate.
 In the broad context of the present invention, the polymer materials are
 therefore selected according to the compatibility criterion noted; i.e.,
 they are selected so that the two or more layers will adhere to one
 another and crosslink across or between their surfaces when irradiated,
 chemically crosslinked or thermally crosslinked to provide the
 aforementioned electrical insulation characteristics. Preferably, it has
 been found that the polymer material in each layer is selected from the
 group consisting of thermoplastic elastomers, polyolefin polymers,
 chlorinated polyolefins polymer, alpha-olefin polymers, poly(vinyl
 chloride), ETFE copolymers, ECTFE copolymers, silicone elastomers,
 chlorinated polyethylene and mixtures thereof.
 In a particular preferred embodiment, one of the layers, and preferably the
 outer layer, is also made flame retardant. This is done by selecting the
 outer layer according to the value of limiting oxygen index, which is a
 basic measure of the relative amount of oxygen necessary to sustain
 burning. Preferably, the limiting oxygen index of the outer layer is
 characterized by a limiting oxygen index greater than that of air; i.e,
 greater than about 21%, and therefor falling between 21-100%. With regards
 to such flame retardance, it is also preferred to flame retard the
 composition so that the layered composition also passes a CSA FT1 or UL
 VW-1 vertical flame test. Towards such goal, preferable flame retardants
 include antimony type compounds, such as antimony trioxide.
 Alternatively, flame retardant polymer material can be used for the outer
 layer, which would effectively be the case when the outer layer comprises
 halogen containing resins such as poly(vinyl chloride) or chlorinated
 polyethylene.
 In addition, with the above in mind, preferably one of the layers, such as
 the inner layer, is selected for outstanding dielectric breakdown
 characteristics, in some cases having flame retardance. By reference to
 outstanding dielectric breakdown characteristics, it is meant that the
 insulating layers are selected to provide a continuous service rating of
 greater than about 10 kV DC or 5 kV AC for the finished, crosslinked
 insulated wire.

WORKING EXAMPLES
 Example 1
 Wire insulation compounds of the following compositions were obtained by
 melt mixing of the ingredients.

Compound A
 HDPE 55.0% by weight
 SEBS block copolymer 36.0
 TMPTMA 2.5
 Antimony trioxide 6.0
 Antioxidants 0.5
 Compound B
 HDPE 13.2% by weight
 SEBS block copolymer 22.5
 EPDM 4.5
 TMPTMA 3.3
 Antimony trioxide 23.6
 DBBPO 16.9
 Antioxidants 2.2
 Fillers 13.8
 Where SEBS: styrene-ethylene-butadiene-styrene
 TMPTMA: trimethylpropoane trimethacrylate
 DBBPO: decabromobiphenyl oxide
 An inner layer insulation, Compound A, was melt extruded over a solid 20
 AWG tin coated copper conductor followed by melt extrusion of an outer
 layer insulation, Compound B, to give insulation wall thickness of 0.030
 and 0.037 inch, respectively. The finished insulated wire gave an outside
 diameter of 0.165 inch. Subsequently this insulated wire was irradiated to
 15 MR under an electron beam accelerator.
 The two layers of insulation, after crosslinking by irradiation, were
 stripped off together from the conductor by using mechanical hand
 strippers and could not be separated by mechanically peeling off one layer
 from the other. When the insulation tubing was subjected to testing of
 tensile strength and elongation at break, the two layers broke
 simultaneously together. If the two layers are not bonded, the two layers
 would break separately, unless the two layers have identical elongation
 characteristics. Typical values of tensile strength are 2300 psi and 250%
 elongation when tested at 20 inches/minute.
 An experimental test, Delamination Resistance, was devised to demonstrate
 the bonding of the two layers of insulation.
 After subjecting this irradiation crosslinked insulated wire to 180 degree
 repeated bending for 1000 cycles at rate of 30 bendings per minute, the
 bent portion of the insulation tubing showed no separation of the two
 layers when tested for tensile strength and elongation.
 The irradiation crosslinked insulated wire met and exceeded all the
 requirements and qualification by UL Style 3239, UL Subject 758 and CSA
 Standard 22.2 No. 16-1980 (TV-50) for 50 KV DC rating at 105.degree. C.
 Among the stringent tests are vertical flame test (UL VW-1, CSA FT-1) and
 high voltage cut through test at 105.degree. C. Other typical
 characteristics of this wire are as follows:
 100% modulus at RT: 1600 psi
 Dielectric breakdown in water:&gt;150 KV DC
 LOI of Compound B: 30%
 When this type of wire is used as high voltage hook-up and lead wire in
 color TV receivers or projection TV, the absence of layer
 separation/delamination eliminates the arcing between layers seen on
 conventional dual layer construction of a non-bonded polyethylene inner
 layer and flame retarded PVC outer layer. This condition occurs as
 "treeing" from the anode cap, between the layers for as much as 1/2 inch
 up the cable and then arcing to the core conductor or through the entire
 insulation. This arcing can cause premature cable and/or system failure.
 Example 2
 An inner layer insulation, Compound C (TAIC, triallyl isocyanurate, to
 replace TMPTMA in Compound A), was melt extruded over a solid 20 AWG tin
 coated copper conductor followed by melt extrusion of an outer layer
 insulation, Compound B, to give insulation wall thickness of 0.024 and
 0.028 inch, respectively. The finished insulated wire gave an outside
 diameter of 0.136 inch. Subsequently this insulated wire was irradiated to
 15 MR under an electron beam accelerator.
 This irradiation crosslinked insulated wire met all the requirements for 30
 KV DC, 105.degree. C. under UL Style 3239, UL Subject 758 and CSA
 (Standard 22.2 No. 16-1980).
 The two layers of insulation, after crosslinking by irradiation, were
 stripped off together from the conductor with mechanical hand strippers
 and could not be separated by mechanically peeling off one layer from the
 other. When the insulation tubing was subjected to testing of tensile
 strength and elongation at break, the two layers broke simultaneously to
 give tensile strength of 2094 psi and elongation of 204% at break, at 20
 inches/minute.
 Example 3
 An inner layer insulation, Compound A, was melt extruded over a solid 20
 AWG tin coated copper conductor followed by melt extrusion of an outer
 layer insulation, Compound B, to give insulation wall thickness of 0.016
 and 0.019 inch, respectively. The finished insulated wire gave an outside
 diameter of 0.102 inch. Subsequently this insulated wire was irradiated to
 15 MR under an electron beam accelerator. When the irradiation crosslinked
 insulation tubing was subjected to testing of tensile strength and
 elongation at break, the two layers broke simultaneously to give tensile
 strength of 1880 psi and elongation of 175% at break.
 The irradiation crosslinked insulated wire met all the requirements for 20
 KV DC, 105.degree. C. under UL Style 3239, UL Subject 758 and CSA TV-20
 (CSA Standard 22.2 No. 16-1980). This cable also qualifies to these
 Standards for 15V DC, 105.degree. C. rating.
 Example 4
 An ethylene-octene-copolymer was melt extruded over a 22 AWG (7 strands of
 30 AWG) tin coated copper conductor as the inner layer insulation,
 followed by melt extrusion of an outer layer insulation, Compound D, to
 give insulation wall thickness of 0.035 and 0.043 inch, respectively. The
 finished insulated wire gave an outside diameter of 0.186 inch.
 Subsequently this insulated wire was irradiated to 12 MR under an electron
 beam accelerator. When the irradiation crosslinked insulation tubing was
 subjected to testing of tensile strength and elongation at break, the two
 layers broke simultaneously to give tensile strength of 2480 psi and
 elongation of 250% at break. This wire is suitable for 50 KV DC
 application according to UL Style 3239, UL Subject 758 and CSA Standard
 22.2 No. 16-1980.

Compound D
 CPE (Blend of 25%/35% Cl) 39.8% by weight
 PVC 10.7
 Antimony trioxide 28.8
 Lead phthalate 8.8
 TOTM 1.7
 TMPTMA 3.9
 Antioxidants 3.2
 Red Color concentrate 3.1
 Example 5
 Compound A (inner layer) and Compound B (outer layer) were melt extruded
 over a 18 AWG (19 strands of 30 AWG) tin coated copper conductor to give
 insulation wall thickness of 0.040 and 0.055 inch, respectively. The
 finished insulated wire gave an outside diameter of 0.236 inch.
 Subsequently this insulated wire was irradiated to 15 MR under an electron
 beam accelerator. This wire is suitable for UL Standard 814 GTO-15
 105.degree. C. application, rated 15 KV AC, and also meets CSA Standard
 C22.2. No 127-95 requirements for 15 KV AC cable. This same cable
 qualifies to UL Style 3239, UL Subject 758 for rating at 60 KV DC,
 105.degree. C. The crosslinked insulation tube measured 1650 psi tensile
 strength and 200% elongation at break when tested at 20 inches/minutes,
 and there was no separation of layers.
 Example 6
 Compound A (inner layer) and Compound B (outer layer) were melt extruded
 over a 20 AWG solid tin coated copper conductor to give insulation wall
 thicknesses of 0.012 and 0.014 inch, respectively. The finished insulated
 wire gave an outside diameter of 0.084 inch. Subsequently this insulated
 wire was irradiated to 15 MR under an electron beam accelerator. The
 crosslinked insulation gave a tensile strength of 2200 psi and elongation
 at break of 230% at 20 inches/minute. This wire met the requirements for
 10 KVDC 105.degree. C. application, when tested according to UL 3239, UL
 Subject 758 and the layers would not separate when stripped from the
 conductor.
 Delamination Resistance Test
 Two-inch cable specimen with its conductor removed is bench marked for
 0.2515 inch on the center section of insulation. A cutting tool which is
 adjusted to extrude 0.015-0.018 inch of sharp blade is passed across the
 cable insulation between the 0.2517 inch bench marks.
 This notch cut section of the specimen shall be placed between two grips of
 an Instron tensile machine, which grips are 1.0 inch apart at the start of
 the test. The specimen is pulled at a rate of 20 inches per minute until
 rupture of both jacket and primary insulations.
 Any separation (delamination) of the one insulation layer from the other
 insulation layer during the test indicates the absence of or weak bonding
 between the two or more layers. By definition, if there is no separation
 of the layers at break, the two layers are bonded. All of the Examples
 reported above showed no separation.