Indian Journal of Chemical Technology
`Vol. 11, November 2004, pp. 853-864
`
`Crosslinked polyethylene
`
`
`SM Tamboli, § T Mhaske & D D Kale*
`
`Institute of Chemical Technology, University of Mumbai, Matunga, Mumbai 400 019, India
`
`Received 12 June 2003: revised received 22 July 2004; accepted 4 August 2004
`
`
`Properties of polyolefins can be modified by crosslinking process. Different methods of crosslinking and effect of
`process parameters, selection of crosslinking agents and applications are briefly discussed.
`
`IPC Code: C 08 F 2/00
`
`Keywords: Crosslinking, polyethylene, crosslinking agents.
`
`several functional groups. These functional groups are
`capable of
`forming chemical bonds
`to convert
`thermoplastics
`into thermosets®’°. McGrins’’ has
`described various commercially important crosslinked
`thermoset materials and their curing reactions. These
`are not of much relevance in the present study. For
`thermoplastics, crosslinking is
`a process in which
`high molecular weight thermoplastics are converted
`into thermosets.
`
`Polyethylenes are commodity plastics. They account
`for more
`than
`70% of
`total plastics market.
`Polyethylene is easily available, at relatively low cost
`and easily processable.
`It
`finds
`applications
`in
`household items, packaging,
`insulation, net
`ropes,
`fishing rods or medical applications, etc. Polyethylene
`is processed at temperature in the range 150-250°C’°.
`Most polyethylene compounds contain reasonably
`good
`amount
`of
`ffillers.
`Polyethylenes
`are
`thermoplastic in nature and therefore they can be
`Crosslinked polyethylenes are either extruded or
`reprocessed repeatedly. Polyethylene. however, will
`injection moulded. When degree of crosslinking is
`soften and flow, and lose critical physical properties
`deliberately maintained very low,
`the
`resulting
`at
`elevated
`temperature
`thereby
`limiting
`compound is
`termed as
`crosslinkable polymer.
`its
`applications’.
`Therefore.
`crosslinking
`of
`Crosslinking can be combined with foaming also.
`polyethylene
`is
`carried out
`to
`retain desirable
`Crosslinking of biopolymers and foaming is very
`properties at high temperature. Crosslinking will
`common in food industry. Crosslinking for partially
`change the nature of polymer from thermoplastic to
`crosslinked extruded profile is commonly employed
`thermoset
`to yield a non melting. more durable
`in furniture. Although crosslinking of thermoplastics
`polymer matrix.
`such as nylon, polypropylene and styrenics has
`are
`polyethylenes
`important
`of
`All
`types
`received attention in literature. Present
`review is
`crosslinked,
`like Linear
`low density polyethylene
`directed to the crosslinking of polyethylenes only.
`(LLDPE), Low density polyethylene (LDPE), High
`Crosslinked polyethylene forms a dense network of
`density polyethylene (HDPE) and Ethyl vinyl acetate
`high molecular weight, which improves
`impact
`copolymer (EVA) and Polyolefinic elastomer (POE).
`strength,
`environmental
`stress
`crack
`resistance
`Branched structure is more suitable for crosslinking.
`(ESCR).
`creep and
`abrasion resistance without
`Therefore, crosslinking of LLDPE and HDPE requires
`influencing tensile
`strength and density to any
`moreattention.
`appreciable extent. Crosslinked polyethylene finds
`Crosslinking leads to the formationof insoluble and
`wide
`applications
`in packaging
`and_
`electrical
`infusible polymers
`in which polymer chains are
`insulation
`applications
`and
`rotomoulding
`joined together to form three-dimensional network
`applications’? The degree of crosslinking can
`structure®*. In thermoset, crosslinking (curing) takes
`
`change applications—toconsiderably from
`
`
`place through reaction between polymer chains with
`applications.
`Some
`aspects of crosslinking
`are
`reviewedhere.
`
`
`*For correspondence (E-mail: ddkale@udct.org)
`
`Yita v. MacNeil IP, IPR2020-01139, Page 1
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`MacNeil Exhibit 2145
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`MacNeil Exhibit 2145
`Yita v. MacNeil IP, IPR2020-01139, Page 1
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`Crosslinking process
`Crosslinking is a process in which carbon atoms of
`same or different polyethylene chains are joined
`together
`to form the three-dimensional network
`structure’*'®. The crosslinking process essentially
`forms bonds between the polymer chains, which
`could be directly between carbon to carbon or a
`chemical bridge linking two or more carbon atoms".
`The main difference between thermoplastic and
`crosslinked polymeris that, at temperature above the
`crystalline melting point crosslinked polymer behaves
`as a soft rubber while thermoplastic has no significant
`strength above melting temperature. The changes in
`the properties of polyethylene due to crosslinking
`have been compared and documentedinliterature’®’”.
`Thus,
`crosslinking reduces
`the melt
`index and
`elongation at break, while improves the impact
`strength, creep resistance, resistance to slow crack
`growth and also environmental stress crack resistance
`(ESCR). The
`density
`and
`tensile
`strength
`of
`polyethylene are not influenced by crosslinking.
`Thecrosslinking of polyethylene takes place in four
`stages:
`initiation,
`propagation,
`branching
`and
`termination. The principal reaction involved in each
`step is discussed below.
`
` Non Crosslinked LDPE
`
`Crosslinked LDPE
`
`Fig.1—Schematic viewofcrosslinked and
`uncrosslinked polyethylene
`
`Branching
`=———— PO*+OH*
`POOH
`PO* +PH ————* POH+P*
`
`When P* ontwosites join, it leads to branching or
`network formation.
`
`Initiation
`The first step in crosslinking process is generation
`of free radicals, which can be through a chemical
`reaction or
`radiation energy. Decomposition of
`initiators which are normally peroxides, or high-
`————» p-P
`p* +p*
`energy radiations abstracts hydrogen atom from the
`POO* +POO* —————*
`POOP +0;
`backbone of polymerchain to producefree radicals.
`PO*+H*=————+ POH
`
`Termination
`Termination takes place by quenching of free
`radicals due to presence of additives, impurities etc.
`
`a) Peroxide decomposition
`ROOR ————* 2 RO*
`RO* +PH —————» ROH + P*
`
`b) High energy radiation
`hv———$_—_\_+>
`
`PH
`
`H* + p*
`
`Propagation and branching
`The free radicals react with atmospheric oxygento
`generate peroxide radicals and through series of
`reaction crosslinking takes place, these are described
`by Peacock’®. Crosslinking causes a dense network of
`different polymerchains through chemical bonding.
`O2
`P* ——* POo*
`POO*+PH ——\——~» POOH -+ P*
`
`Presence of side branches in a polyethylene chainis
`a reason for variation in numberof important physical
`properties such as density, hardness, flexibility and
`melt viscosity. Presence of branchesis the point inthe
`molecular network where oxidation may take place.
`Crosslinking takes place between carbon atom in
`neighboring chains or chain branches joined together
`with other branches of chain or with the same chain of
`polymer. This is depicted schematically in Fig. 1.
`The
`polyethylenes
`have
`different
`structures
`depending upon manufacturing process. Low density
`polyethylene is highly branched, while high density
`polyethylene and linear low density polyethylene are
`linear polymers. In general, branched polymers are
`easy to crosslink as compared to linear polymers,
`since formation of network is more probable for
`branched polymers”.
`
`Yita v. MacNeil IP, IPR2020-01139, Page 2
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`MacNeil Exhibit 2145
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`MacNeil Exhibit 2145
`Yita v. MacNeil IP, IPR2020-01139, Page 2
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`855
`
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`H
`
`H
`
`DDRAYWAA ce DARAYAA
`
`H
`
`a)
`
`Polyethylene
`
`energyradiation
`
`Free Radical
`
`et(——
`
`—
`PP PPBOO
`
`epee
`
`b)
`
`Crosslinked PE
`
`by
`
`Fig. 2—Schematic representation of radiation crosslinking
`
`
`
`Table 2—Types ofradiation sources
`
`Particulate
`
`Non particulate
`
`o = particles
`B - particles
`High energy electron
`Protons
`Deuterons
`Neutron
`
`
`
`Microwave
`Infrared
`X-ray
`y-ray
`Light energies (UV)
`
`ae)
`The relative scission to crosslinking ratio is given
`
`B+ AS)
`
`2 G(X)
`
`a@
`
`@
`
`Table 1—G(X) and G(S) values of some polymers
`
`Where,
`
`Polymers
`
`
`
` DPE
`
`
` HDPE
`
`
`Atactic PP
`sotactic PP
`
`G(X)
`
`G(S)
`
`1.4
`2.1
`012-027
`0.07-0.14
`
`0.8
`1.3
`0.10-0.24
`0.10-0.27
`
`
`
`-
`
`2.15
`Polyvinyl chloride
`22
`0.15
`Polypropylene oxide
`0.6
`0.5
`Nylon (6 & 6,6)
`0.06 —0.17
`0.1-0.3
`Polyvinyl acetate
`-
`3.8
`Polybutadiene
`= 0.018
`0.045
`Polystyrene
`0.18
`0.55
`Polymethyl acrylate
`
`Polymethyl methacrylate .22-3.5 -
`
`
`Crosslinking process is carried out by using (1)
`Physical or(11) Chemical crosslinking methods.
`
`Physical crosslinking
`In this method, crosslinking is obtained by free
`radical mechanism. The free radical is generated in
`polymerchain by using high energy radiations”’. This
`process is shownschematically inFig. 2.
`
`Thus, a free radical is generated by the high energy.
`Twoor more chains, then, join together where the free
`radical is generated.
`Highenergy radiation on polymeric material gives
`chain scission or
`crosslinking. The
`changes
`in
`physical and chemical properties depend upon the
`efficiency of crosslinking reaction and its relative
`ratio with degradation. Table 1 shows the numberof
`crosslinking and chain scission per 100eV radiant
`energy absorption fordifferent polymers.
`
`a = probability of crosslinking of chains after one
`electron volt of energy absorbed.
`B = probability of chain scission after one electron
`volt of energy absorbed.
`G(X) = numberof crosslinking per 100eV radiant
`energy absorbed.
`G(S) = number of scission per 100eV of energy
`absorbed.
`
`It is knownthat bond energy for cleavage of C-H
`bond is 364 kJ/mol. The electron beam having energy
`sufficient
`to
`break C-H bond is
`suitable
`for
`crosslinking’.
`Crosslinking of polymers by radiation and their
`technology involve four mainvariables.
`
`(1) Type of radiation andits sources.
`(ii) The nature of polymerstructure to be irradiated.
`(111) Mechanismand theories of reactions.
`(iv) Physical, chemical and mechanical properties of
`network formation.
`
`Some of these radiation crosslinking are described
`briefly.
`Radiation induced crosslinking of thermoplastics
`can be carried out using particulate or non-particulate
`radiations. These are listed in Table 2.
`Particulate radiation sources are not commercially
`used. Only non-particulate radiation sources are used
`for commercial crosslinking of thermoplastics by
`radiation. Crosslinking by radiation mainly depends
`upon photon energy of radiation sources. The higher
`the photon energy. the more the penetration taking
`place and highercrosslinking is obtained. The photon
`
`Yita v. MacNeil IP, IPR2020-01139, Page 3
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`MacNeil Exhibit 2145
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`MacNeil Exhibit 2145
`Yita v. MacNeil IP, IPR2020-01139, Page 3
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`
`Table 3—Wavelength and photon energy of
`some radiations
`
`energy gained by polymerin UHF field is given by
`the following equation*’.
`
`Type of
`radiation
`
`Wavelength
`(nm)
`
`Photon
`[MeV]
`
`energy
`
`N=E.2fetanc
`
`.. B)
`
`Infrared
`UV
`Soft X - ray
`
`X - ray
`
`10°
`10°
`10° 10°
`
`10° 10°
`
`1250
`125
`12.5
`1.25
`0.125
`0.0125
`y - tays 12x 10° 0.001
`
`
`
`
`relatively dependent on
`radiation is
`energy of
`wavelength. Table 3 depicts the relationship between
`radiation sources, wavelength and photonenergies.
`Selection of radiation sources mainly depends upon
`availability,
`the radiation penetration rate required.
`the dose rate and impact on manufacturing process
`(product handling, shielding. safety, equipment cost
`and maintenance).
`The depth of high energy penetrationis given by”’.
`r= ke? eh blecx
`. (2)
`
`c:
`reaction,
`crosslinking
`of
`rate
`7:
`Where,
`extinction
`¢:
`photoinitiator,
`of
`concentration
`coefficient of photoinitiator, x:
`thickness of reactive
`polymerlayer.
`Thermoplastic crosslinking by UVradiation is a
`very slow process. Thermoplastic is mixed with
`photo-initiators, which makes it suitable to use UV
`light for crosslinking. UV radiation penetrates the
`polymer up to a depth of only a few millimeters*”’.
`Therefore UVlight is used for crosslinking of thin
`parts only~®. Ketones such as benzophone, and benzil
`dimethyl ketal
`are
`suitable photo
`initiator
`for
`crosslinking of mainly polyethylene’’”*.
`Electron beamswill penetrate up to few centimeters
`thermoplastic polymers. The
`crosslinking of
`of
`moulded parts having thick wall results in variable
`crosslink density. Therefore,
`this process is mainly
`used for thin wall products such as films, shrinkable
`insulating parts and crosslinking of insulating cables
`and foams ~*~".
`
`A microwaverepresents very high electromagnetic
`spectrum [10° to 10’ Hz]. Therefore,
`it is called as
`ultra
`high
`frequency
`(UHF)
`radiation
`source.
`Microwave crosslinking is independent on the part
`thickness. It is applicable to parts of any size. The
`
`Where.
`
`N= lossorgain of energy.
`E = field intensity.
`f= frequency (Hz) ofthe alternating field.
`é, = dielectric coefficient.
`tan € = dielectric loss factor.
`
`The drawback of microwave field is that, only
`components with polar group are excitable in this
`field. Thermoplastic
`such
`as
`polyethylene
`or
`polypropylene is non polar compound with very low
`tan € value. Therefore, crosslinking of polyethylene in
`UHF field becomes very difficult. Crosslinking of
`polyethylene in microwave field is possible only by
`using intensely polar
`additives
`such as
`carbon
`black,
`peroxide, metallic
`powders
`and_
`triallyl
`oxy-s-triazine*?,
`Most of the applications of radiation crosslinked
`polymers are in electrical
`insulation and packaging
`films. These are briefly described in Table 4.
`
`Advantages of radiation induced crosslinking
`Advantages of radiation induced crosslinking are
`briefed below:
`
`(1)
`
`(ii)
`
`(iii)
`(iv)
`
`(v)
`
`at
`
`room
`
`crosslinking reaction takes place
`temperature,
`reaction is completed in fraction of seconds,
`hence high output is obtained,
`reaction can take place without any additives,
`highly suitable for
`relatively thin insulating
`layers,
`crosslinking takes place in only onestep.
`
`Disadvantages of radiation induced crosslinking
`Some of the disadvantages of radiation induced
`crosslinking are given below:
`
`(i) high capital cost,
`(ii) difficult
`to
`cross-link article with irregular
`shapes,
`are
`precautions
`(1ii) Safety
`operators fromradiation.
`
`to
`
`protect
`
`needed
`
`Yita v. MacNeil IP, IPR2020-01139, Page 4
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`MacNeil Exhibit 2145
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`MacNeil Exhibit 2145
`Yita v. MacNeil IP, IPR2020-01139, Page 4
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`Table 4—Commercial uses ofradiation-processing techniques
`
`Substrate
`
`Radiation process
`
`Commercial use
`
`Polyolefins and PVC
`
`Polyolefins and PVC foams
`
`Cross-linking with high-energy radiation
`sources in 0.4-3 Mevrange.
`Cross-linking with high energy electron
`
`Wire insulation for computers, and
`communication application
`Improved thermal stability for insulating and
`packaging application
`Conversions of waste Teflon material into easily
`moldable powder or waxes of commercial value
`No-wear high performance wood floors for
`hightraffic areas
`Adhesive products for modification of wood,
`Lowenergy electron processing equipmentin
`
`Curing of coating and adhesives
`textiles, paper, film and metal substrates
`100-500 Kevrange
`
`Polytetra fluoroethylene (Teflon)
`
`Wood impregnated with acrylic
`or methacrylic monomers
`
`Degradation by high energy electron or
`cobalt-60 in 0.2-0.4 May
`Polymerization with cobalt-60 source
`
`Chemical crosslinking
`in which
`a method,
`Chemical crosslinking is
`chemicals or
`initiators are used to generate free
`radicals, which in turns leads to crosslinking. Inthis
`method,
`crosslinking takes place through direct
`carbon-to-carbon bonds or
`through the chemical
`bridges which
`connect
`different
`polyethylene
`molecules****.
`Degree of crosslinking in thermoplastic resin varies
`according
`to
`crosslinking
`process.
`Chemical
`crosslinking by using peroxide gives highest and
`uniform degree of crosslinking as compared to
`physical crosslinking method. Kim and White have
`reported the difference in degree of crosslinking
`between
`physical
`and
`chemical
`crosslinking
`processes’. Accordingly,radiationcrosslinking yields
`between 34-75% degree of crosslinking. In chemical
`crosslinking method, peroxide gives much high
`degree of crosslinking (up to 90%), while silane based
`crosslinking can be 45-70%degreeofcrosslinking.
`Peroxide initiated crosslinking process depends on
`several variables, namely operating temperature, type
`and concentration of peroxide,
`and molecular
`characteristics of virgin resin such as, molecular
`weight, molecular weight
`distribution,
`branch
`distribution and concentration of
`terminal vinyl
`groups.
`The two main chemical crosslinking methods are,
`organic peroxide based and
`silane based (moisture cured).
`
`(1)
`(ii)
`
`Crosslinking of thermoplastic by peroxide
`Peroxide crosslinking has been in use for more than
`40 years and is
`the most common method for
`crosslinking
`of
`thermoplastics
`especially
`polyethylenes.
`In this method, organic peroxide is
`used as initiator. Usually, organic peroxide is used in
`its
`original
`unprocessed
`structure. Downstream
`
`R-O-O-R
`
`2R-0
`
`eeee eNO Isa ROTI
`
`CT SNSL
`ad
`
`|
`SH
`NyN_AA_
`cHN
`
`Fig. 3—Schematic representation of crosslinking of polyethylene
`
`processing equipment operates at higher temperature.
`The compounding of polyethylene and peroxide must
`be carried out at low temperature. below the peroxide
`decomposition temperature. Crosslinking is carried
`out
`in the downstream equipment at significantly
`higher
`temperature
`and
`pressure.
`The
`higher
`temperature decomposes the initiator and liberates a
`free radical that will abstract a hydrogen atom from
`polymer chain. This abstraction site then becomes
`reactive radical,
`forming a crosslinked bond with
`another reactive radical of same or different chain.
`This reaction occurs until all peroxide is consumed or
`the
`temperature
`falls below the decomposition
`point’**°. Schematic representation of this reaction is
`showninFig. 3.
`Elimination of hydrogen atom converts tertiary
`hydrogen atoms of polypropylene and polyethylene to
`tertiary radical chain with low reactivity. Tertiary
`radical
`sites are not very reactive and are not
`converted
`easily
`into more
`reactive
`secondary
`radicals. The
`shifting of the radical
`site along
`branched chain is hindered, and dimerization of chain
`radical becomes more difficult. Numberof peroxides,
`which are suitable for crosslinking of thermoplastic
`and their dissociation temperatures,
`are listed in
`Table 5.
`is widely used for
`Dicumyl peroxide (DCP)
`crosslinking of
`thermoplastics,
`and
`crosslinking
`
`Yita v. MacNeil IP, IPR2020-01139, Page 5
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`MacNeil Exhibit 2145
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`MacNeil Exhibit 2145
`Yita v. MacNeil IP, IPR2020-01139, Page 5
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`nitiator
`
`Table 5—Peroxide decompositionrates [kj] and curing temperatures
`Solvent
`Temperature
`kg(s
`CC)
`
`Curing temperature
`CC)
`
`
`
`Dicumy! peroxide
`
`Benzene
`
`Chlorobenzene
`
`Dodecane
`
`Cumene
`
`Benzene
`
`Chlorobenzene
`
`Decalin
`benzene
`
`Chlorobenzene
`
`
`
`Di-t-butyl peroxide
`
`Di-t-amyl peroxide
`
`2,5-Dimethyl-2,
`5-di (¢-butyl-peroxy) hexane
`
`2.05* 107
`115
`1.05* 10°
`130
`6.86* 107
`145
`1.93* 10°
`112
`1.93* 107
`132
`1.93* 107
`154
`8.75* 10°
`128
`2.31* 107
`138
`5.37* 107
`148
`2.57* 107
`138
`1.52* 107
`158
`8.75* 10°!
`100
`5.66* 10°
`115
`3.22* 10°
`130
`1.93* 10°
`108
`1.93* 107
`128
`1.93* 107
`150
`2.8* 107.
`125
`1.15* 10°
`115
`6.86* 10°
`130
`4.75* 10°
`145
`1.93* 107
`115
`1.93* 107
`134
`1.93* 10°
`156
`3.91* 10°
`115
`2.35* 10°
`130
`1.14* 104
`145
`1.93* 10°
`120
`1.93* 107
`141
`1.93* 10°
`164
`5,83* 10°
`100
`3.53* 10°
`115
`2.91* 107
`130
`6.9% 10°
`85
`§.05* 10°
`100
`2.71* 107
`115
`2.62* 10°
`75
`2.5* 10°
`80
`2.28* 10°
`100
`1.35* 10°
`70
`4.64* 10°
`80
`91
`1.93* 10%
`80
`2.53* 107
`
`80
`6.7* 107
`
`160
`
`175
`
`185
`~
`
`195
`~
`
`160
`
`150
`
`175
`»"
`
`2,5-Dimethyl-2,5-di
`peroxy) hexynes
`
`(r-butyl- Benzene
`
`Chlorobenzene
`
`n-Butyl-4,4-bis (t-butyl
`peroxy) valerate
`
`Dodecane
`
`1,1-Bis (t-butyl peroxy)-3,3,5- Benzene
`tri methyleyclohexane
`
`
`Benzoyl peroxide
`
`Benzene
`
`Chlorobenzene
`
`Decane
`Dioxane
`
`efficiency of DCP is more than the other peroxides.
`During
`reaction
`of
`dicumyl
`peroxide with
`polyethylene, gas generated in reaction contains 98%
`methane.
`
`peroxide
`of
`decomposition
`Thermal
`accordingto the following equation™’**”’.
`DRT
`_—
`: _ -BAKL
`r= kg [ec] with kg = k, .€
`
`proceeds
`
`a (4)
`
`Kinetics of crosslinking by peroxide
`Decomposition of peroxide, that is, generation of
`free radical
`is slowest reaction and it
`is the rate-
`determining step of crosslinking reaction of PE.
`
`r=rate of decomposition
`kg = rate constant
`k, =rate constant at base temp.[0°C]
`
`Where.
`
`Yita v. MacNeil IP, IPR2020-01139, Page 6
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`MacNeil Exhibit 2145
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`MacNeil Exhibit 2145
`Yita v. MacNeil IP, IPR2020-01139, Page 6
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`

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`TAMBOLIetal.: CROSSLINK
`
` Table 6—Various peroxides used for crosslinking
`
`859
`
`Name
`
`Group
`
`HalfLife Data
`10H
`1H
`
`% Active O)
`
`
`Description
`
`
`Dicumy! peroxide
`
`Dialkyl peroxide
`
`2,5-Dimethyl-2,5-di-
`(t-butylperoxy) hexane
`2,5-Dimethyl-2,
`5-Di-(t-buytlperoxy)
`hexyane-3
`Di-t-amyl peroxide
`
`Di-t-butyl peroxide
`
`
`
`Peroxy ketals
`
`117
`137
`120
`140
`131
`152
`
`123
`143
`129
`149
`96
`115
`
`2.13- 5.92
`
`5-10
`10— 10.6
`5 -— 5.36
`
`Powder orflake
`Solid or liquid
`
`92%liquid
`45% solid on inert
`filler
`
`$.8-9
`
`96%liquid
`
`10.8
`9.73
`4.1-4.34
`
`98.5%liquid
`92%liquid
`1,1-Di(t-butyl peroxy)
`40% solid on inert
`3,3,5 - trimethyl
`filler
`cyclohexane
`n-butyl 4, 4-bis
`109
`40% solid on inert
`
`(t-butylperoxy) valerate
`120
`filler
`
`E = activation energy
`[c] = concentration of peroxide
`T= absolute temperature
`R= universal gas constant.
`
`Where,
`
`A = numberof peroxide molecules per two PE chains,
`Mpz, M,,; = molecular weight of PE and peroxide
`respectively,
`A= thickness of reactive polymer layer OR the mass
`concentration of peroxide per one gram of PE.
`The decrease of XLPE density is due to additional
`branching introduced by peroxide and the maximum
`density
`can
`be
`obtained
`at
`0.5% peroxide
`concentration. The drop in crystallinity as a function
`of peroxide concentration and increase in crosslinking
`impose somerestraint on mobility of polymerchains
`in molten state preventing them from arranging into
`lamellae fold®.
`Relative degree of crosslinking and degradation of
`polymers is discussed by Kwei™ with the help of
`following equation,
`
`S+S°° = {Po/ Qo} + {1/ Oy Yn D}
`
`.
`
`(6)
`
`Where,
`
`for various
`Table 6 lists the curing parameter
`peroxides used for crosslinking alongwith their half
`life periods and active oxygen content. The cross-
`linking of PE depends uponthe type of peroxide and
`the temperature. The reactions are of first order and
`depend also on the chemical nature of the polymerto
`be cross-linked. For LDPE, the value for the order of
`reaction is 0.90-0.99.
`for HDPE,
`1.06
`and for
`copolymer of ethylene between 0.88 and 1.22. The
`decomposition of peroxide is a rate-determining step.
`Thecross-linking reaction is exothermic. The value of
`activation energy for various crosslinking reaction of
`different polymers
`such
`as LDPE, HDPE or
`copolymer is practically same if same peroxide is
`used. It has been shownthat the order of reaction
`depends on the temperature and it increases at high
`temperatures.
`Po/Qo= ratio of degradationto crosslinking
`Numberof bonds between PE chains as a function
`D=radiation dose (Mrad)
`of peroxide
`content may be
`stoichiometrically
`S= percent of soluble molecule in network
`calculated by assuming that one peroxide molecule
`Y, = initial number average molecular weight of
`independently is
`responsible only for one bond
`polymer.
`between two PE chains. The calculation is based on
`the following equation”,
`
`X.M,.
`M,,
`
`A=
`
`65
`
`©
`
`Crosslinking of copolymers
`Crosslinking of PE-PP has been studied by Braun™.
`They observed that PP did not crosslink and grafting
`
`Yita v. MacNeil IP, IPR2020-01139, Page 7
`
`MacNeil Exhibit 2145
`
`MacNeil Exhibit 2145
`Yita v. MacNeil IP, IPR2020-01139, Page 7
`
`

`

`860
`
` T
`
`
`
` MBER 2004
`INDIAN J. CHEM. TECHNOL., NOV
`
`
`
`of PP onto PE could take place. They have reported
`that as PP content
`increased, the heat resistance of
`crosslinked compound decreased.
`is blended with
`Ethylene vinyl acetate (EVA)
`polyethylene quite widely. Ethylene vinyl acetate
`copolymer is more rapidly crosslinked and accepts
`highfiller loading, withoutsignificant loss of physical
`properties, making themwell suited for lowervoltage
`applications and in automobile etc.
`
`when exposed to excessive heat or light. Therefore.
`polyethylene compounds used for coating wire and
`cable application may contain anti-oxidants. Cables
`exposed to ultraviolet
`rays in sunlight must also
`contain carbon black or other UV inhibitors. Such
`additives
`also
`assure
`that
`the
`electrical
`and
`mechanical properties of the resin are preserved under
`the high temperatures prevailing in the extruder’.
`Carbon black of fine particle size can be used as a
`thermal antioxidant. Carbon black is used as
`a
`Peroxide coagent crosslinking method
`
`conducting and__plastics.filler for elastomer
`
`
`
`Coagents are low molecular weight molecules with
`Incorporation of carbon black increases brittle point
`two or more reactive double bonds. Coagents work in
`and yields stress with concentration. without affecting
`a free radical cure system. Free radicals are generated
`tensile strength?’
`by using peroxide or high energy radiation sources,
`coagent reacts with these free radicals to increase the
`efficiency of cure ie. to give more crosslinks. The
`presence or addition of a coagent
`in crosslinking
`provides more reactive sites where the crosslinking
`reaction can occur®’. Addition of coagents reduces
`cure time,
`improves resistance to oils and fuels,
`improves heat aging,
`improves peroxide efficiency,
`improves flexibility and gives highertensile strength
`and hardness*"**.
`
`Advantages ofperoxide-coagent system
`The advantages of peroxide-coagent systemare:
`excellent heatstability
`(1)
`simple compounding
`(ii)
`(iii) better hot tear
`(iv) good shelflife stability.
`
`Disadvantages ofperoxide-coagent system
`The disadvantages of peroxide-coagent systemare:
`(i) higher cost
`(ii) surface tackiness in presence of oxygen.
`
`Applications
`are widely used in
`systems
`Peroxide-coagent
`automotive seals, automotive hoses, oil well packers,
`swab cups, golf balls, butterfly valves, belts, mats,
`shock absorber, cable, coating and foam packing.
`
`Some of the coagents used in crosslinking
`Some of the coagents used in crosslinking are”,
`ethylene glycol dimethylacrylate, 1.3—butylene glycol
`dimethylacrylate, poly (ethylene glycol) dimethyla-
`crylate and trially cyanurate dimethylacrylate.
`
`Effect of anti-oxidants and carbon black fillers
`be
`The power
`factor of polyethylene may
`unfavorably affected by oxidation of the insulator
`
`Crosslinking bysilane
`There are two types of processes forcrosslinking of
`polyethylene by silane grafting: two-step process and
`one-step process.
`In two-step process, silane molecule such as vinyl
`trimethoxysilane (VTO) is grafted on polyethylene
`chain. For grafting.
`the peroxide such as dicumyl
`peroxide is mixed with the polyethylene in small
`percentage. The peroxide initially generates
`free
`radical on polyethylene chain. Grafting of silane takes
`
`
`
`
`
`CROSSLINKING
`INITIATOR
`
`CATALYST
`
`
` TUMBLE MIXING
`
`
`POL
`
`THY |LENE.
`
`POLYETHYLENE
`
`Fe
`
`
`
`
`
`TUMBLE MIXING 4
`
`ANTI-
`OXIDANT
`
`
`Ff
`
`
`
`CROSS LINKABLE
`CATALYST
`
`
`GRAFT POLYMER
`
`MASTERBATCH
`‘GRANULES
`GRANULES
`
`
`
`
` TUMBLE MIXING
`
`CURINGAGENT
`
`"
`
`EXTRUSION
`
`
`=o
`CROSSLINKING
`
`
`
`Fig. 4—Schematic representation of two-step crosslinking
`process of polyethylene by using silane
`
`Yita v. MacNeil IP, IPR2020-01139, Page 8
`
`MacNeil Exhibit 2145
`
`MacNeil Exhibit 2145
`Yita v. MacNeil IP, IPR2020-01139, Page 8
`
`

`

`place at the site where free radical is generated. The
`polyethylene. The degradation is mainly due to B-
`
`grafted is mixed with—silanolpolyethylene
`
`chain scission. Crosslinking of polypropylene is
`condensation catalyst such as dibutyltin dilauarate,
`possible only by silane grafting method.
`and extruded
`or
`injection moulded profile
`is
`produced. Upto this stage the polyethyleneretains the
`thermoplastic nature. Extruded or injection moulded
`article is crosslinked with the help of water or at
`elevated temperature or at room temperature*’**. The
`two-step silane crosslinking process is schematically
`represented inFig. 4.
`In one-step process, a copolymer of silane and
`polyethylene may be formedorfree radical generating
`and grafting take place in single step only. The
`chemical reaction of silane crosslinking is shownin
`Fig.
`5. The reaction continues until all grafted
`copolymeris converted into crosslink chains. In the
`peroxide and irradiation cross-linking processes,
`the
`links between macromolecule consist of carbon-
`carbon bond. The silane process gives
`-Si-O-Si-
`crosslinks. These siloxane bridges are weaker than
`carbon-carbon bond, and this will have effect on the
`attainable strength
`and
`long
`term chemical
`stability???
`
` T
`
`
`
` ) POLYETHYLENE
`TAMBOLIetal.: CROSSLINK
`
`86]
`
`Advantages of silane-grafted crosslinking
`Various advantages of silane-grafted crosslinking
`are:
`
`crosslinking can be done at room temperature
`(1)
`low cost
`(ii)
`(iii) higher gel percentage obtained as compared to
`physical crosslinking.
`
`Disadvantage of silane-grafted crosslinking
`Some disadvantages of silane-grafted crosslinking
`are given below:
`
`(1)
`
`(ii)
`
`curing timeis very high as compared to peroxide
`crosslinking
`extra downstream equipments are required (for
`condensation)
`(iii) bond strength of crosslinking is weaker than
`bondstrength in peroxide crosslinking system.
`
`Effects of crosslinking
`improve
`to
`crosslinked
`Polyethylene
`is
`dimensional
`stability at elevated temperature,
`“LL
`+
`RO)
`NN CHIN me NCHSCOHNLA
`| nccrsiociys
`etvtse |ind
`—_—
`ey -
`Hyc
`CH»S01 Dy
`HO
`Hc
`CHS¥OCH;),
`2
`.
`3CH,0H
`
`
`
`its
`to
`
`Silane grafting ofpolyethylene
`The most commonsilane used for cross-linking of
`polyethylene is vinyl trimethoxysilane (VTO). The
`silane is introduced into polyethylene by melt grafting
`using peroxide as an initiator. The silane-grafted
`polyethylene
`is
`then
`crosslinked
`through
`hydrolyzation of the methoxysilane group with water
`followed by condensation of the hydroxyl group*’**.
`Various types of silanes used in crosslinking are listed
`in Table 7.
`Crosslinking of grafted polyethylene is completed
`OH
`only in presence of moisture. A catalyst
`is used to
`|
`5 NNRL <I
`
`= —=ti—ie Hy)SiO}
`
`activate and speed up the crosslinking process. The
`HyC-——CHy S(OH),
`‘OH
`On
`crosslinking 1s also enhanced by high temperature, but
`silane crosslinking is usually performed at 50 to 80°C
`and at atmospheric pressure.
`Polypropylene is degraded due to peroxide and
`therefore,
`it
`cannot
`be
`crosslinked
`similar
`to
`
`a] Grafting of silane on PE,
`
`b] Condensation (crosslinking),
`
`oH
`
`(CL;I
`
`Fig. 5—Silane grafted polyethylene crosslinking reaction
`
`Silane
`
`Table 7—Various typesof silane used for crosslinking
`
`Boiling Point (°C)
`
`MolecularWeight
`
`Colour
`
`Flash Point (°C)
`
`Colourless
`
`Tetramethoxysilane
`Colourless
`Tetraethoxysilane
`Colourless
`Methyltriethoxysilane
`Yellowish
`Methyltris methyltrie-
`
`thoxysilane 105 301.46 liquid 110
`
`
`122
`168
`101
`
`
`
`26
`46
`5
`
`152.2
`208.3
`136.3
`
`
`
`Yita v. MacNeil IP, IPR2020-01139, Page 9
`
`MacNeil Exhibit 2145
`
`MacNeil Exhibit 2145
`Yita v. MacNeil IP, IPR2020-01139, Page 9
`
`

`

`862
`
` T
`
`
`
`
` MBER 2004
`INDIAN J. CHEM. TECHNOL., NOV
`
`
`
`Table 8—Comparison between crosslinking processes
`Silane
`
`Peroxide
`
`Radiation
`
`One step
`Free radical
`>60
`
`Two step
`Grafting
`>65
`Condensation reaction
`
`Room temperature
`Very low
`Peroxide for initiating
`High
`Strong
`
`compared to
`
`Crosslinking process
`
`No. of steps
`Crosslinking mechanism
`Gel %
`Curing mechanism
`
`Curing temperature
`Curing time
`Additives
`Equipment cost
`Bondstrength
`
` Degree of crosslinking
`
`Onestep
`Free radical
`= 75
`Homolysis temperature of
`peroxide
`150-160 °C
`Less
`No
`Medium
`Strong
`
`Constant throughoutthe article
`
`80-90 °C
`Veryhigh
`Peroxide for grafting
`Low
`Weak as
`peroxide
`residence Waries with thickness of
`Varies with
`time in water bath and
`article
`temperature of bath
`
`crosslinking is therefore, key
`
`polyethylene prior to
`function in processing.
`Dueto its thermoset nature recycling of crosslinked
`polyethylene cannot carried out by melting it with
`virgin polyethylene, since crosslinked polyethylene
`does not melt. The crosslinking process decreased the
`crystallinity.
`
`to reduce its
`resistance or
`impact
`its
`improve
`to crosslinking
`crack. Due
`propensity to stress
`polyethylene changes from ductile semi-crystalline
`solid to a non-crystalline elastomer’.
`a degree of
`As crosslinked density increases,
`crystallinity and crystallite thickness decreases. This
`was studied by Badar®’. Reduction in crystallinity
`occurs because of crosslinking taking place
`in
`Application of crosslinked polymers 83,64
`amorphous phase. There is also some breakdown of
`crystallinity. Decrease in degree of crystallinity and
`Cable insulation
`crystalline thickness, decreases Young’s modulus,
`advanced area of application for
`The most
`yield stress, elongation at break and peak melting
`crosslinked polyethylene is in the electrical cable
`temperature of polyethylene”.
`industry. Crosslinking, whilst not interfering with the
`Harper™ studied the effect of crosslinking on melt
`diel