ElEctron BEam crosslinking of WirE and caBlE ... - IBA Industrial
ElEctron BEam crosslinking of WirE and caBlE ... - IBA Industrial ElEctron BEam crosslinking of WirE and caBlE ... - IBA Industrial
IBA Industrial - White Papermotivated other organizations to develop wire andcable products using crosslinked PVC insulation [12-16]. Crosslinked PE and other ethylene copolymerswere also being developed at that time for highperformancewires and cables [17, 18].The Union Carbide Corporation (now part of DowChemical) developed an EB crosslinkable low smoke,low toxicity, flame retardant, non-halogen containing,cable jacketing compound that consists mainly ofethylene vinylacetate (EVA) and aluminum trihydrate(ATH). Similar EB crosslinked cables were specified bythe New York City Transit Authority for their buses andsubway trains during the 1980s [9].Following a tragic fire aboard the aircraft carrier USSForrestall in 1967, the United States Navy realizedthat toxic fumes and dense smoke from burning PVCjacketed cables created a dangerous firefightingenvironment within the confines of a ship [19]. Inresponse to this, the US Navy developed a newspecification for a family of low smoke, low toxicityshipboard cables. The conductors were insulated withpolyolefins and jacketed with polyvinylidene fluoride(PVDF-Kynar) [9]. Both of these materials werecrosslinked with EB processing.EB crosslinked ethylene tetrafluoroethylene (ETFE-Tefzel) wire is also used in aircraft airframes. Thisinsulation is thinner and lighter than the materialsused for comparable wires. It can be used attemperatures up to 200 degrees Celcius and willperform temperatures as low as -65 °C [9]. In general,fluoropolymers are superior to the polyolefins, such aspolyethylene, for very high temperature applications[20-24].Another application of EB crosslinking is theproduction of anti-lock brake cables for automobiles.These cables must be flexible and resistant toabrasion from sand and gravel that are thrown upfrom the road. Resistance to salt, oil and gasoline isalso important. EB crosslinking is also used in theproduction of welding cables wherein EB crosslinksa rubber jacketing. In many areas, EB processing isTHE alternative to CV curing, as in this case. Theadvantage of EB curing is a higher production throughputrate and lower energy consumption than alternativecuring systems [9].Crosslinkable FormulationsPolyethylene (PE) is the most common polymerused for EB crosslinkable insulation. PE has lowcost, has a favorable response to EB processingand minimal toxicity when exposed to a fire. Blendsof PE and ethylene-propylene copolymers (EPM) orethylene-propylene-diene (EPDM) elastomers areused if greater flexibility is needed. Flame retardantcompositions of PE/EPM or PE/EPDM are replacingpolyvinyl chloride (PVC) because of concerns overthe release of toxic chlorinated byproducts when PVCcompounds are exposed to a fire. PE, EPM and EPDMcan be made flame retardant by adding aluminumhydrates or other compounds to the formulation.Hydrates absorb energy and release water vaporwhen decomposed in a flame and thereby retard itspropagation.A typical flame retardant EB crosslinkable formulationis given below in Table 1. PE/EPM or PE/EPDM areused as the polymer base. This combination providesflexibility while not lowering dielectric propertiesand not increasing moisture permeability as wouldethylene copolymers, such as ethylene vinyl acetate(EVA) and ethylene acrylate copolymers, such asethylene ethylacrylate (EEA). In this illustrativeformulation, Hydral 710 is aluminum trihydrate(Al(OH)3), a compound that releases water whenexposed to combustion conditions. Zinc oxide (ZnO)is commonly used to enhance the aging propertiesof the material. A process aid, such as mineral oil,facilitates compound extrusion around the conductor.Silane A-172 is a wetting agent used to enhance theinteraction between the polymer and the Hydral 710.The antioxidant also reduces aging effects. Trimethylolpropane triacrylate (TMPTA) and tri-allyl cyanurate(TAC) are multi-functional monomers that enhance thecrosslinking response and thus reduce the amount ofdose needed to crosslink the wire or cable jacketing.This effect increases the processing speed. One orthe other of these agents can be used. This type ofinsulation can tolerate temperatures as high as 150 °C[7,25-30]. The weight fraction of Hydral 710 is 250/373= 0.67 and the calculated density of the insulation is1.61 g/cu cm. The density and atomic compositionof this type of flame retardant insulation should betaken into account when determining the maximumEB penetration. (See the section below on PhysicalAspects, etc.)2
IBA Industrial - White PaperTable 1.Typical Flame Retardant Wire and Cable FormulationComponentParts by WeightPE-EPDM 100Hydral 710 250Zinc Oxide 5Process Aid 10Silane A-172 2Antioxidant 1TMPTA or TAC 5Processing MethodsSmall diameter wires or cables are usually EBcrosslinked by passing them many times througha wide, scanning beam. This method has severalbenefits. (1) A narrow, high-current electron beammust be scanned to increase its width and reducethe average current density to avoid overheating thethin metallic beam window of the accelerator; (2) Thescanning beam will be much wider than the wire orcable diameter, so that such products can pass throughthe beam many times to intercept most of the beamcurrent; (3) Multiple passes also avoid the possibilityof overheating the insulated wire or cable by allowingsome of the heat from EB processing to dissipatebetween passes.Small wires can be EB processed while passing backand forth through the beam by using a wire handlingfixture that consists of two rows of sheaves or two soliddrums displaced on opposite sides of the scanningbeam, as shown in Figure 1.Figure 1.A figure-eight multiple sheavefixture for electron beamprocessing of small size insulatedwire and plastic tubing.This arrangement is usually called the figure-eightmethod of under beam exposure. Larger wires andcables require a larger bending radius and can beprocessed with a four-drum fixture, as shown in Figure2.Figure 2.A four-drumfixture for electronbeam processingof medium sizeinsulated wire,cable and plastictubing.This is a modification of the simpler two-drum race trackmethod, which has the disadvantage when processingthicker wires and cables with larger drums in thatthe forward pass is closer to the beam window andreceives a higher dose than the more distant reversepass. In addition to using larger outer drums for thickerwires and cables, the two inner drums of the four-drumfixture bring the reverse pass closer to the forward passso that the doses on opposite sides of the products arenearly the same. This method also keeps the forwardand reverse passes perpendicular to the plane of thescanning electron beam so that the beam strikes thewires and cables at right angles to their direction ofmotion, thereby providing the greatest penetration intothe insulating material [30-32].The EB energy and consequently the electron rangeshould be at least sufficient to penetrate the radialthickness of the insulation and preferably more thanthis. Some of the beam will strike near the sides of thewire or cable where the chordal thickness is greaterthan the radial thickness. The electrons need notpenetrate the full chordal thickness since the wires willbe treated from opposite sides and small wires will twistslightly between successive passes on the under-beamhandling system when passing through the beam. Also,the divergence of the scanning beam causes the sidesof the wire or cable to be treated as the wire progressesfrom one end to the other of the multiple pass underbeamfixture [32].The outer sheaths on very large cables can be treatedby rotating the cables as they pass through theelectron beam along the scanning direction. With thismethod, both the pay-off and take-up reels are rotatedoutside of the shielded treatment room. The electronenergy needs to be just enough to penetrate the radialthickness of the sheath. In this way, a nearly uniformdose distribution around the cable will result. Largecables may have to be cooled inside and outside of thetreatment room when a relatively high dose is deliveredin a single pass through the beam [33]. However,proper formulation of the cable jacketing can reduce thedose needed to impart crosslinking and thereby reducethe temperature rise.3
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<strong>IBA</strong> <strong>Industrial</strong> - White Papermotivated other organizations to develop wire <strong>and</strong>cable products using crosslinked PVC insulation [12-16]. Crosslinked PE <strong>and</strong> other ethylene copolymerswere also being developed at that time for highperformancewires <strong>and</strong> cables [17, 18].The Union Carbide Corporation (now part <strong>of</strong> DowChemical) developed an EB crosslinkable low smoke,low toxicity, flame retardant, non-halogen containing,cable jacketing compound that consists mainly <strong>of</strong>ethylene vinylacetate (EVA) <strong>and</strong> aluminum trihydrate(ATH). Similar EB crosslinked cables were specified bythe New York City Transit Authority for their buses <strong>and</strong>subway trains during the 1980s [9].Following a tragic fire aboard the aircraft carrier USSForrestall in 1967, the United States Navy realizedthat toxic fumes <strong>and</strong> dense smoke from burning PVCjacketed cables created a dangerous firefightingenvironment within the confines <strong>of</strong> a ship [19]. Inresponse to this, the US Navy developed a newspecification for a family <strong>of</strong> low smoke, low toxicityshipboard cables. The conductors were insulated withpolyolefins <strong>and</strong> jacketed with polyvinylidene fluoride(PVDF-Kynar) [9]. Both <strong>of</strong> these materials werecrosslinked with EB processing.EB crosslinked ethylene tetrafluoroethylene (ETFE-Tefzel) wire is also used in aircraft airframes. Thisinsulation is thinner <strong>and</strong> lighter than the materialsused for comparable wires. It can be used attemperatures up to 200 degrees Celcius <strong>and</strong> willperform temperatures as low as -65 °C [9]. In general,fluoropolymers are superior to the polyolefins, such aspolyethylene, for very high temperature applications[20-24].Another application <strong>of</strong> EB <strong>crosslinking</strong> is theproduction <strong>of</strong> anti-lock brake cables for automobiles.These cables must be flexible <strong>and</strong> resistant toabrasion from s<strong>and</strong> <strong>and</strong> gravel that are thrown upfrom the road. Resistance to salt, oil <strong>and</strong> gasoline isalso important. EB <strong>crosslinking</strong> is also used in theproduction <strong>of</strong> welding cables wherein EB crosslinksa rubber jacketing. In many areas, EB processing isTHE alternative to CV curing, as in this case. Theadvantage <strong>of</strong> EB curing is a higher production throughputrate <strong>and</strong> lower energy consumption than alternativecuring systems [9].Crosslinkable FormulationsPolyethylene (PE) is the most common polymerused for EB crosslinkable insulation. PE has lowcost, has a favorable response to EB processing<strong>and</strong> minimal toxicity when exposed to a fire. Blends<strong>of</strong> PE <strong>and</strong> ethylene-propylene copolymers (EPM) orethylene-propylene-diene (EPDM) elastomers areused if greater flexibility is needed. Flame retardantcompositions <strong>of</strong> PE/EPM or PE/EPDM are replacingpolyvinyl chloride (PVC) because <strong>of</strong> concerns overthe release <strong>of</strong> toxic chlorinated byproducts when PVCcompounds are exposed to a fire. PE, EPM <strong>and</strong> EPDMcan be made flame retardant by adding aluminumhydrates or other compounds to the formulation.Hydrates absorb energy <strong>and</strong> release water vaporwhen decomposed in a flame <strong>and</strong> thereby retard itspropagation.A typical flame retardant EB crosslinkable formulationis given below in Table 1. PE/EPM or PE/EPDM areused as the polymer base. This combination providesflexibility while not lowering dielectric properties<strong>and</strong> not increasing moisture permeability as wouldethylene copolymers, such as ethylene vinyl acetate(EVA) <strong>and</strong> ethylene acrylate copolymers, such asethylene ethylacrylate (EEA). In this illustrativeformulation, Hydral 710 is aluminum trihydrate(Al(OH)3), a compound that releases water whenexposed to combustion conditions. Zinc oxide (ZnO)is commonly used to enhance the aging properties<strong>of</strong> the material. A process aid, such as mineral oil,facilitates compound extrusion around the conductor.Silane A-172 is a wetting agent used to enhance theinteraction between the polymer <strong>and</strong> the Hydral 710.The antioxidant also reduces aging effects. Trimethylolpropane triacrylate (TMPTA) <strong>and</strong> tri-allyl cyanurate(TAC) are multi-functional monomers that enhance the<strong>crosslinking</strong> response <strong>and</strong> thus reduce the amount <strong>of</strong>dose needed to crosslink the wire or cable jacketing.This effect increases the processing speed. One orthe other <strong>of</strong> these agents can be used. This type <strong>of</strong>insulation can tolerate temperatures as high as 150 °C[7,25-30]. The weight fraction <strong>of</strong> Hydral 710 is 250/373= 0.67 <strong>and</strong> the calculated density <strong>of</strong> the insulation is1.61 g/cu cm. The density <strong>and</strong> atomic composition<strong>of</strong> this type <strong>of</strong> flame retardant insulation should betaken into account when determining the maximumEB penetration. (See the section below on PhysicalAspects, etc.)2
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