ElEctron BEam crosslinking of WirE and caBlE ... - IBA Industrial

IBA Industrial - White PaperElectron Beam Crosslinkingof Wire and Cable InsulationTechnical Information Series TIS 01812Marshall R. Cleland and Richard A. GallowayIBA Industrial, Inc.Edgewood, New York, USACrosslinking provides significant commercial benefits to wire and cable insulation. Ionizing energy, as providedby an accelerated electron beam (EB), is an efficient means of crosslinking the polymers that are used for wireand cable jacketing. In this process, chemical bonds are formed between polymer molecules to produce athree-dimensional insoluble network. This can be done without heat. In most instances, ionization causes theabstraction of hydrogen from a polymer to produce active sites along a polymer chain that can bond to similarsites on adjacent chains without the use of crosslinking agents. In some instances, specific activators may alsobe used to enhance the crosslinking process. EB processing is faster, more controllable and more economicalthan thermal and/or chemical crosslinking when used in the production of insulated wires and cables.EB crosslinked wire and cable insulation exhibits several desirable properties. It will not melt and flow at elevatedambient temperatures, nor melt and flow should the conductor become heated due to a shorted electrical circuit.EB crosslinking protects wire and cable insulation during a soldering operation, when a short circuit conditionoccurs or at high-temperatures, as near the engine or exhaust pipe of an automobile. EB crosslinking reducesthe risk of flame propagation should a fire occur in electrical equipment. Tensile strength, especially at elevatedtemperatures, is increased, as are abrasion resistance, stress crack resistance and solvent resistance [1-9].ApplicationsThe use of crosslinked polyvinyl chloride (PVC)and polyethylene (PE) insulation for thin wall, lowvoltage wires that are found in motors, automobiles,household appliances, computers and other electronicequipment was introduced and gained commercialacceptance more than thirty years ago. The EBcrosslinking process has been proven to be lessexpensive, requiring less factory floor space, allowinga greater selection of insulating materials, permittingproduction over a wider range of wire gauges andgiving faster processing rates than continuousvulcanization (CV), which requires heat and time.The CV process is impractical for small diameterwires because of their limited tensile strength. Theuse of moisture to cure silane crosslinked PE is aneven slower process than CV with heat. The nearlyinstantaneous EB crosslinking process facilitatesshorter runs with various product specifications andjust-in-time production schedules.The first company to produce EB crosslinked wire andother crosslinked plastic products was the RaychemCorporation, which was founded by Paul Cook in1957. Its main offices are in Menlo Park, California,but it has operations in 40 countries and sales in 85countries. It produces about 50,000 different products,and its annual sales are well over 1.0 billion dollars.This pioneering company is now an important part ofTyco Electronics Corporation [10].One of the largest commercial applications ofelectron beam crosslinked wire was the production ofdistributing frame wire for the central exchanges of theBell Telephone system. Crosslinked PVC was adoptedin the early 1970s to replace a more complex andcostly wire consisting of uncrosslinked PVC which wascovered with a cotton braid that had to be coated witha special lacquer to promote fire retardance. Tetraethyleneglycoldimethacrylate (TEGDM) was addedto the PVC to aid in EB crosslinking. A stabilizer,a processing lubricant and a polymeric plasticizerwere also included [11]. This successful application1

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

IBA Industrial - White PaperPhysical Aspects of Electron BeamProcessingElectron Energy RequirementThe amount of crosslinking increases with the dose,which is defined as the absorption of energy per unitmass. The common unit of dose for electron beamprocessing is the kilogray (kGy) or the absorption ofone joule (watt-second) per gram of material. Polymercrosslinking usually requires doses in the range of 50to 150 kGy, depending on the chemical composition ofthe material.The dose within the material is usually not uniform.With electron energies above 500 kiloelectron volts(keV), the dose tends to increase with depth in thematerial to about half of the maximum electron rangeand then decrease to nearly zero at a greater depthwhere the electrons have dissipated most of theirkinetic energy. A useful quantity is the depth where theexit dose has diminished to 50 percent of the entrancedose. The electron energy should be enough to makethis quantity, usually named R(50e), equal to or greaterthan the thickness of the insulation. Values of R(opt),the depth where the exit dose equals the entrancedose, R(50), the depth where the exit dose equals 50percent of the maximum dose, and R(50e) are given inTables 2A and 2B for a wide range of electron energiesin flat materials, assuming the material compositionlisted in Table 1 above [34-36].Table 2A.Electron Ranges in Flame Retardant Polyethylene InsulationThe normalized ranges (thickness x density in g/sqcm) can be obtained by multiplying the ranges in cm,as given above, by the volume density of the flameretardant insulation, which has been calculated tobe 1.614 g/cu cm. Materials with different volumedensities tend to have similar normalized ranges,which can also be called areal or area densities.The areal density of a flat sheet can be measured bydividing its weight by its area. (Thickness x density =weight / area in g/sq cm). The areal density is a usefulquantity for comparing electron ranges in materialswith different compositions and volume densities [34-36].Table 2B.Electron Ranges in Flame Retardant Polyethylene InsulationIn addition to specifying the average dose needed tocrosslink the insulation, knowing the absorbed dosedistribution within the wire or cable insulation canprovide more assurance that the electron energy issufficient. The absorbed dose distribution depends onthe thickness, density and atomic composition of theinsulation as well as the diameter and compositionof the metallic conductor. Because of the cylindricalgeometry and the shielding and backscattering effectsof the conductor, accurate calculations of the dosedistribution are not simple, but they can be done withsuitable Monte Carlo probability codes. Several codesare available, but most of them are not very userfriendly[37, 38]. IBA Industrial, Inc. can provide adviceregarding these types of calculations [35].Temperature RiseThe thermal capacity of polymeric insulating materialis typically about half that of water, e.g. about 2 joulesper gram per degree Celcius. With this value, a doseof 100 kGy absorbed quickly in a single pass of thewire through the electron beam would cause thetemperature of the insulation to rise about 50 degreesCelcius. However, with a multiple-pass underbeamfixture which could have more than 100 passes, thedose per pass would be less than 1 kGy and thetemperature rise per pass would be less than halfa degree Celcius. The multiple-pass method allowsmuch of this heat to dissipate between passes.Processing RateThe results of Monte Carlo calculations of electronenergy depositions can also be used to estimate theline speed of an electron beam treatment process.The absorbed dose is proportional to the numberof energetic electrons injected per unit area of theprocessed material, while the beam current is ameasure of the number of electrons emitted per unittime. Therefore, the processing rate or line speed ofthe wire or cable within the electron beam increaseswith the beam current and the number of passesand decreases with increasing beam width and doseaccording to the following equation:S = 6.0 D(e) I N / (W D)where S is the line speed in meters per minute andD(e) is the energy deposition per electron in MeV perunit areal density (thickness times volume density).This quantity can be calculated with a suitableMonte Carlo code. I is the electron beam current inmilliamperes, N is the number of passes through thebeam, W is the width of the scanning beam in metersand D is the total dose in kGy [34-36].4

IBA Industrial - White PaperValues of the energy deposition per electron D(e) for awide range of incident electron energies at the entranceof flat materials are given in Table 3, assuming thematerial composition listed in Table 1 above.Table 3.Electron Energy Deposition in Flame Retardant PE InsulationFigure 4.The interior of a Minitron TM steeland lead shielded enclosureshowing the vacuum pumps,beam scanning magnet, scanhorn, figure-eight multiple passfixture for small wire and tubing,and the water-cooled beam stop.For example, in flame retardant polyethylene, the valueof D(e) is about 3.0 MeV per unit areal density (in gramsper square centimeter) for an electron energy of 800keV. Assuming that the beam current is 65 mA, thenumber of passes is 140, the scan width is 0.91 meters(36 inches) and the total dose is 150 kGy on the sideof the wire facing the beam, then the line speed will beabout 1200 meters per minute.Figure 5 & 6.An Easy-e-Beam TM 800 keVself-shielded system forelectron beam processing ofmedium size insulated wire,cable and plastic tubing.Electron AcceleratorsElectron beam crosslinking of wire insulation andcable jackets is usually done with electron energiesin the range from 500 keV to 1.5 MeV, although somefacilities use higher energies up to 3 MeV. The energyis determined by the thickness, density and atomiccomposition of the insulation and the diameter of theconductor. The electron beam current is usually in therange of 25 to 100 mA. The beam current requirementis determined by the absorbed dose and the line speed.These relationships have been described briefly in theprevious sections.Facilities with electron energies less than 1.0 MeV canbe shielded with steel panels or a combination of steeland lead panels to protect operating personnel fromthe X-rays generated by the electron beam. Facilitieswith energies greater than 1.0 MeV are usually shieldedwith thick concrete walls, which are less expensive thanequivalent steel and lead panels for the higher energies.Photographs of a Minitron TM 500 keV self-shieldedsystem are shown in Figures 3 and 4. Drawings ofan Easy-e-Beam TM 800 keV self-shielded system areshown in Figures 5 and 6. Drawings of Dynamitron ®1.5 MeV and 3.0 MeV concrete shielded systems areshown in Figures 7 and 8. All of these systems aresuitable for electron beam crosslinking of wires andcables.Figure 3.A Minitron TM 500 keV selfshieldedsystem for electronbeam processing of smallsize insulated wire and plastictubing.SummationIBA Industrial, Inc. (formerly known as RadiationDynamics, Inc.) has made many industrial electronaccelerators, called Dynamitrons ® , which are usedto crosslink wire and cable insulation and for otherapplications as well. This equipment has been provento be reliable and long lasting. Some Dynamitrons ®have been in use for more than 40 years. New modelswith the latest automatic control systems are now beingproduced to meet the present requirements for theseapplicationswww.iba-cables.comFigure 7. A Dynamitron® 1.5MeV concrete shielded systemfor electron beam processingof medium and large sizeinsulated wire, cable and plastictubing.Figure 8. A Dynamitron® 3.0MeV concrete shielded systemfor electron beam processingof medium and large sizeinsulated wire, cable and plastictubing.5

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