Indian Journal of Chemical Technology
Vol. 11, November 2004, pp. 853-864
Crosslinked polyethylene
S M Tamboli, S 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.
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°C1-3.
Most polyethylene compounds contain reasonably
good amount of fillers. Polyethylenes are
thermoplastic in nature and therefore they can be
reprocessed repeatedly. Polyethylene, however, will
soften and flow, and lose critical physical properties
at
elevated
temperature
thereby
limiting
its applications4,5. Therefore, crosslinking of
polyethylene is carried out to retain desirable
properties at high temperature. Crosslinking will
change the nature of polymer from thermoplastic to
thermoset to yield a non melting, more durable
polymer matrix.
All types of important polyethylenes are
crosslinked, like Linear low density polyethylene
(LLDPE), Low density polyethylene (LDPE), High
density polyethylene (HDPE) and Ethyl vinyl acetate
copolymer (EVA) and Polyolefinic elastomer (POE).
Branched structure is more suitable for crosslinking.
Therefore, crosslinking of LLDPE and HDPE requires
more attention.
Crosslinking leads to the formation of insoluble and
infusible polymers in which polymer chains are
joined together to form three-dimensional network
structure6-8. In thermoset, crosslinking (curing) takes
place through reaction between polymer chains with
_________
*For correspondence (E-mail: ddkale@udct.org)
several functional groups. These functional groups are
capable of forming chemical bonds to convert
thermoplastics into thermosets9,10. McGrins11 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.
Crosslinked polyethylenes are either extruded or
injection moulded. When degree of crosslinking is
deliberately maintained very low, the resulting
compound is termed as crosslinkable polymer.
Crosslinking can be combined with foaming also.
Crosslinking of biopolymers and foaming is very
common in food industry. Crosslinking for partially
crosslinked extruded profile is commonly employed
in furniture. Although crosslinking of thermoplastics
such as nylon, polypropylene and styrenics has
received attention in literature. Present review is
directed to the crosslinking of polyethylenes only.
Crosslinked polyethylene forms a dense network of
high molecular weight, which improves impact
strength, environmental stress crack resistance
(ESCR), creep and abrasion resistance without
influencing tensile strength and density to any
appreciable extent. Crosslinked polyethylene finds
wide applications in packaging and electrical
insulation
applications
and
rotomoulding
12,13
applications . The degree of crosslinking can
change
considerably
from
applications
to
applications. Some aspects of crosslinking are
reviewed here.
854
INDIAN J. CHEM. TECHNOL., NOVEMBER 2004
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
structure14-16. 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 atoms18.
The main difference between thermoplastic and
crosslinked polymer is 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 documented in literature16,17.
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.
The crosslinking of polyethylene takes place in four
stages: initiation, propagation, branching and
termination. The principal reaction involved in each
step is discussed below.
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 highenergy radiations abstracts hydrogen atom from the
backbone of polymer chain to produce free radicals.
a) Peroxide decomposition
ROOR
RO* + PH
b) High energy radiation
hv
PH
2 RO*
ROH + P*
H* + P*
Propagation and branching
The free radicals react with atmospheric oxygen to
generate peroxide radicals and through series of
reaction crosslinking takes place, these are described
by Peacock19. Crosslinking causes a dense network of
different polymer chains through chemical bonding.
O2
P*
POO*
POO*+PH
POOH + P*
Fig.1⎯Schematic view of crosslinked and
uncrosslinked polyethylene
Branching
POOH
PO* + PH
PO* + OH*
POH + P*
When P* on two sites join, it leads to branching or
network formation.
Termination
Termination takes place by quenching of free
radicals due to presence of additives, impurities etc.
P* + P*
POO* + POO*
PO* + H*
P–P
POOP + O2
POH
Presence of side branches in a polyethylene chain is
a reason for variation in number of important physical
properties such as density, hardness, flexibility and
melt viscosity. Presence of branches is the point in the
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 polymers20.
TAMBOLI et al.: CROSSLINKED POLYETHYLENE
855
Table 2⎯Types of radiation sources
Particulate
Non particulate
α - particles
β - particles
High energy electron
Protons
Deuterons
Neutron
Microwave
Infrared
X-ray
γ-ray
Light energies (UV)
The relative scission to crosslinking ratio is given
by 22
Fig.
β 1 G (S)
=
α 2 G (X)
2⎯Schematic representation of radiation crosslinking
Table 1⎯G(X) and G(S) values of some polymers
Polymers
G(X)
G(S)
LDPE
HDPE
Atactic PP
Isotactic PP
Polyvinyl chloride
Polypropylene oxide
Nylon (6 & 6,6)
Polyvinyl acetate
Polybutadiene
Polystyrene
Polymethyl acrylate
Polymethyl methacrylate
1.4
2.1
0.12 – 0.27
0.07 – 0.14
2.15
0.15
0.5
0.1 – 0.3
3.8
0.045
0.55
-
0.8
1.3
0.10 – 0.24
0.10 – 0.27
0.22
0.6
0.06 – 0.17
< 0.018
0.18
1.22 – 3.5
Crosslinking process is carried out by using (i)
Physical or (ii) Chemical crosslinking methods.
Physical crosslinking
In this method, crosslinking is obtained by free
radical mechanism. The free radical is generated in
polymer chain by using high energy radiations21. This
process is shown schematically in Fig. 2.
Thus, a free radical is generated by the high energy.
Two or more chains, then, join together where the free
radical is generated.
High energy 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 number of
crosslinking and chain scission per 100eV radiant
energy absorption for different polymers.
… (1)
Where,
α = probability of crosslinking of chains after one
electron volt of energy absorbed.
β = probability of chain scission after one electron
volt of energy absorbed.
G(X) = number of crosslinking per 100eV radiant
energy absorbed.
G(S) = number of scission per 100eV of energy
absorbed.
It is known that 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 23.
Crosslinking of polymers by radiation and their
technology involve four main variables.
(i)
(ii)
(iii)
(iv)
Type of radiation and its sources.
The nature of polymer structure to be irradiated.
Mechanism and theories of reactions.
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 higher crosslinking is obtained. The photon
INDIAN J. CHEM. TECHNOL., NOVEMBER 2004
856
energy gained by polymer in UHF field is given by
the following equation31,
Table 3⎯Wavelength and photon energy of
some radiations
Type of
radiation
Wavelength
(nm)
Photon
[MeV]
Infrared
UV
Soft X - ray
1250
125
12.5
1.25
0.125
0.0125
0.001
100
101
102 103
X - ray
γ - rays
energy
… (3)
Where,
104 105
1 2 × 106
energy of radiation is relatively dependent on
wavelength. Table 3 depicts the relationship between
radiation sources, wavelength and photon energies.
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 penetration is given by21,
r = k c1/2 e–1.151εcx
N = E2. 2 f εt tan ζ
… (2)
Where, r: rate of crosslinking reaction, c:
concentration of photoinitiator, ε: extinction
coefficient of photoinitiator, x: thickness of reactive
polymer layer.
Thermoplastic crosslinking by UV radiation 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 millimeters24,25.
Therefore UV light is used for crosslinking of thin
parts only26. Ketones such as benzophone, and benzil
dimethyl ketal are suitable photo initiator for
crosslinking of mainly polyethylene27,28.
Electron beams will penetrate up to few centimeters
of thermoplastic polymers. The crosslinking 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 29,30.
A microwave represents very high electromagnetic
spectrum [109 to 1012 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
N = loss or gain of energy.
E = field intensity.
f = frequency (Hz) of the alternating field.
εt = dielectric coefficient.
tan ζ = 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 ζ 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-triazine32,33.
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:
(i)
(ii)
(iii)
(iv)
(v)
crosslinking reaction takes place at room
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 one step.
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,
(iii) Safety precautions are needed to protect
operators from radiation.
TAMBOLI et al.: CROSSLINKED POLYETHYLENE
857
Table 4⎯Commercial uses of radiation-processing techniques
Substrate
Radiation process
Commercial use
Polyolefins and PVC
Cross-linking with high-energy radiation
sources in 0.4-3 Mev range.
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
high traffic areas
Adhesive products for modification of wood,
textiles, paper, film and metal substrates
Polyolefins and PVC foams
Polytetra fluoroethylene (Teflon)
Wood impregnated with acrylic
or methacrylic monomers
Curing of coating and adhesives
Degradation by high energy electron or
cobalt-60 in 0.2-0.4 May
Polymerization with cobalt-60 source
Low energy electron processing equipment in
100-500 Kev range
Chemical crosslinking
Chemical crosslinking is a method, in which
chemicals or initiators are used to generate free
radicals, which in turns leads to crosslinking. In this
method, crosslinking takes place through direct
carbon-to-carbon bonds or through the chemical
bridges which connect different polyethylene
molecules34-36.
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
processes37. Accordingly, radiation crosslinking 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% degree of crosslinking.
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,
(i) organic peroxide based and
(ii) silane based (moisture cured).
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
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
point38,39. Schematic representation of this reaction is
shown in Fig. 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. Number of peroxides,
which are suitable for crosslinking of thermoplastic
and their dissociation temperatures, are listed in
Table 5.
Dicumyl peroxide (DCP) is widely used for
crosslinking of thermoplastics, and crosslinking
INDIAN J. CHEM. TECHNOL., NOVEMBER 2004
858
Table 5⎯Peroxide decomposition rates [kd] and curing temperatures
Initiator
Solvent
Temperature
(°C)
kd ( S-1)
Dicumyl peroxide
Benzene
115
130
145
112
132
154
128
138
148
138
158
100
115
130
108
128
150
125
115
130
145
115
134
156
115
130
145
120
141
164
100
115
130
85
100
115
75
80
100
70
80
91
80
80
2.05* 10-5
1.05* 10-5
6.86* 10-4
1.93* 10-5
1.93* 10-4
1.93* 10-3
8.75* 10-5
2.31* 10-4
5.37* 10-4
2.57* 10-4
1.52* 10-3
8.75* 10-7
5.66* 10-6
3.22* 10-2
1.93* 10-5
1.93* 10-4
1.93* 10-3
2.8* 10-5
1.15* 10-5
6.86* 10-5
4.75* 10-4
1.93* 10-5
1.93* 10-4
1.93* 10-3
3.91* 10-6
2.35* 10-5
1.14* 10-4
1.93* 10-5
1.93* 10-4
1.93* 10-3
5.83* 10-6
3.53* 10-5
2.91* 10-4
6.9* 10-6
5.05* 10-5
2.71* 10-4
2.62* 10-5
2.5* 10-5
2.28* 10-5
1.35* 10-6
4.64* 10-5
1.93* 10-4
2.53* 10-5
6.7* 10-4
Chlorobenzene
Dodecane
Cumene
Di–t–butyl peroxide
Di-t-amyl peroxide
2,5-Dimethyl-2,
5-di (t-butyl-peroxy) hexane
Benzene
Chlorobenzene
Decalin
benzene
Chlorobenzene
2,5-Dimethyl-2,5-di (t-butyl–
peroxy) hexynes
Benzene
Chlorobenzene
n-Butyl-4,4-bis (t-butyl
peroxy) valerate
Dodecane
1,1-Bis (t-butyl peroxy)-3,3,5tri methylcyclohexane
Benzene
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.
Curing temperature
(°C)
160
175
185
195
160
150
175
Thermal decomposition of peroxide
according to the following equation20,40,41,
r = kd .[c] with kd = ko . e-E/RT
Where,
Kinetics of crosslinking by peroxide
Decomposition of peroxide, that is, generation of
free radical is slowest reaction and it is the ratedetermining step of crosslinking reaction of PE.
r = rate of decomposition
kd = rate constant
ko = rate constant at base temp.[0°C]
proceeds
… (4)
TAMBOLI et al.: CROSSLINKED POLYETHYLENE
859
Table 6⎯Various peroxides used for crosslinking
Name
Dicumyl peroxide
Group
Half Life Data
10H
1H
Dialkyl peroxide
117
137
120
140
131
152
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
1,1-Di(t-butyl peroxy)
3,3,5 - trimethyl
cyclohexane
n-butyl 4, 4-bis
(t-butylperoxy) valerate
Peroxy ketals
E = activation energy
[c] = concentration of peroxide
T= absolute temperature
R = universal gas constant.
Table 6 lists the curing parameter for various
peroxides used for crosslinking alongwith their half
life periods and active oxygen content. The crosslinking of PE depends upon the type of peroxide and
the temperature. The reactions are of first order and
depend also on the chemical nature of the polymer to
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.
The cross-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 shown that the order of reaction
depends on the temperature and it increases at high
temperatures.
Number of bonds between PE chains as a function
of peroxide content may be stoichiometrically
calculated by assuming that one peroxide molecule
independently is responsible only for one bond
between two PE chains. The calculation is based on
the following equation42,
X .M pe
… (5)
A=
M pr
% Active O2
Description
2.13- 5.92
Powder or flake
Solid or liquid
5-10
10 – 10.6
5 – 5.36
123
143
129
149
96
115
92% liquid
45% solid on inert
filler
8.8-9
96% liquid
10.8
9.73
4.1-4.34
98.5% liquid
92% liquid
40% solid on inert
filler
40% solid on inert
filler
109
129
Where,
A = number of peroxide molecules per two PE chains,
Mpe, Mpr = molecular weight of PE and peroxide
respectively,
X= 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 some restraint on mobility of polymer chains
in molten state preventing them from arranging into
lamellae fold43.
Relative degree of crosslinking and degradation of
polymers is discussed by Kwei42 with the help of
following equation,
S + S0.5 = {P0 / Q0} + {1 / Q0 Yn D}
… (6)
Where,
P0/Q0 = ratio of degradation to crosslinking
D = radiation dose (Mrad)
S = percent of soluble molecule in network
Yn = initial number average molecular weight of
polymer.
Crosslinking of copolymers
Crosslinking of PE-PP has been studied by Braun44.
They observed that PP did not crosslink and grafting
860
INDIAN J. CHEM. TECHNOL., NOVEMBER 2004
of PP onto PE could take place45. They have reported
that as PP content increased, the heat resistance of
crosslinked compound decreased.
Ethylene vinyl acetate (EVA) is blended with
polyethylene quite widely. Ethylene vinyl acetate
copolymer is more rapidly crosslinked and accepts
high filler loading, without significant loss of physical
properties, making them well suited for lower voltage
applications and in automobile etc.
Peroxide coagent crosslinking method
Coagents are low molecular weight molecules with
two or more reactive double bonds. Coagents work in
a free radical cure system. Free radicals are generated
by using peroxide or high energy radiation sources,
coagent reacts with these free radicals to increase the
efficiency of cure i.e. to give more crosslinks. The
presence or addition of a coagent in crosslinking
provides more reactive sites where the crosslinking
reaction can occur46. Addition of coagents reduces
cure time, improves resistance to oils and fuels,
improves heat aging, improves peroxide efficiency,
improves flexibility and gives higher tensile strength
and hardness47,48.
Advantages of peroxide-coagent system
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 extruder50,51.
Carbon black of fine particle size can be used as a
thermal antioxidant. Carbon black is used as a
conducting filler for elastomer and plastics.
Incorporation of carbon black increases brittle point
and yields stress with concentration, without affecting
tensile strength52.
Crosslinking by silane
There are two types of processes for crosslinking 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
The advantages of peroxide-coagent system are:
(i) excellent heat stability
(ii) simple compounding
(iii) better hot tear
(iv) good shelf life stability.
Disadvantages of peroxide-coagent system
The disadvantages of peroxide-coagent system are:
(i) higher cost
(ii) surface tackiness in presence of oxygen.
Applications
Peroxide-coagent systems are widely used in
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 are49,
ethylene glycol dimethylacrylate, 1,3–butylene glycol
dimethylacrylate, poly (ethylene glycol) dimethylacrylate and trially cyanurate dimethylacrylate.
Effect of anti-oxidants and carbon black fillers
The power factor of polyethylene may be
unfavorably affected by oxidation of the insulator
Fig. 4⎯Schematic representation of two-step crosslinking
process of polyethylene by using silane
TAMBOLI et al.: CROSSLINKED POLYETHYLENE
place at the site where free radical is generated. The
grafted polyethylene is mixed with silanol
condensation catalyst such as dibutyltin dilauarate,
and extruded or injection moulded profile is
produced. Up to this stage the polyethylene retains the
thermoplastic nature. Extruded or injection moulded
article is crosslinked with the help of water or at
elevated temperature or at room temperature53,54. The
two-step silane crosslinking process is schematically
represented in Fig. 4.
In one-step process, a copolymer of silane and
polyethylene may be formed or free radical generating
and grafting take place in single step only. The
chemical reaction of silane crosslinking is shown in
Fig. 5. The reaction continues until all grafted
copolymer is converted into crosslink chains. In the
peroxide and irradiation cross-linking processes, the
links between macromolecule consist of carboncarbon bond. The silane process gives -Si-O-Sicrosslinks. These siloxane bridges are weaker than
carbon-carbon bond, and this will have effect on the
attainable strength and long term chemical
stability55,56.
Silane grafting of polyethylene
The most common silane 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 group57,58.
Various types of silanes used in crosslinking are listed
in Table 7.
Crosslinking of grafted polyethylene is completed
only in presence of moisture. A catalyst is used to
activate and speed up the crosslinking process. The
crosslinking is also enhanced by high temperature, but
silane crosslinking is usually performed at 50 to 80oC
and at atmospheric pressure.
Polypropylene is degraded due to peroxide and
therefore, it cannot be crosslinked similar to
861
polyethylene. The degradation is mainly due to βchain scission. Crosslinking of polypropylene is
possible only by silane grafting method.
Advantages of silane-grafted crosslinking
Various advantages of silane-grafted crosslinking
are:
(i) crosslinking can be done at room temperature
(ii) low cost
(iii) higher gel percentage obtained as compared to
physical crosslinking.
Disadvantage of silane-grafted crosslinking
Some disadvantages of silane-grafted crosslinking
are given below:
(i)
curing time is very high as compared to peroxide
crosslinking
(ii) extra downstream equipments are required (for
condensation)
(iii) bond strength of crosslinking is weaker than
bond strength in peroxide crosslinking system.
Effects of crosslinking
Polyethylene is crosslinked to improve its
dimensional stability at elevated temperature, to
Fig. 5⎯Silane grafted polyethylene crosslinking reaction
Table 7⎯Various types of silane used for crosslinking
Silane
Tetramethoxysilane
Tetraethoxysilane
Methyltriethoxysilane
Methyl tris methyltriethoxysilane
MolecularWeight
Colour
Boiling Point (°C)
Flash Point (°C)
152.2
208.3
136.3
Colourless
Colourless
Colourless
Yellowish
liquid
122
168
101
26
46
5
110
105
301.46
INDIAN J. CHEM. TECHNOL., NOVEMBER 2004
862
Table 8⎯Comparison between crosslinking processes
Crosslinking process
Peroxide
Silane
Radiation
No. of steps
Crosslinking mechanism
Gel %
Curing mechanism
Two step
Grafting
> 65
Condensation reaction
One step
Free radical
>60
Curing temperature
Curing time
Additives
Equipment cost
Bond strength
One step
Free radical
> 75
Homolysis temperature of
peroxide
150-160 °C
Less
No
Medium
Strong
Room temperature
Very low
Peroxide for initiating
High
Strong
Degree of crosslinking
Constant throughout the article
80-90 °C
Very high
Peroxide for grafting
Low
Weak as compared to
peroxide
Varies with residence
time in water bath and
temperature of bath
improve its impact resistance or to reduce its
propensity to stress crack. Due to crosslinking
polyethylene changes from ductile semi-crystalline
solid to a non-crystalline elastomer59.
As crosslinked density increases, a degree of
crystallinity and crystallite thickness decreases. This
was studied by Badar60. Reduction in crystallinity
occurs because of crosslinking taking place in
amorphous phase. There is also some breakdown of
crystallinity. Decrease in degree of crystallinity and
crystalline thickness, decreases Young’s modulus,
yield stress, elongation at break and peak melting
temperature of polyethylene61.
Harper62 studied the effect of crosslinking on melt
index and concluded that the melt flow index
decreases uniformly with increase in degree of
crosslinking.
Crosslinking increases the impact strength,
environmental stress crack resistance, creep resistance
without affecting tensile strength and flexural
modulus. Comparison of different crosslinking
methods is presented in Table 8.
Disadvantages of crosslinking
As mentioned earlier, crosslinking of polyethylene
changes its nature from thermoplastic to thermoset.
This enhances the viscosity to a very high value. To
control the degree of crosslinking in a continuous
process is very tricky. During extrusion of a profile, if
excessive crosslinking takes place stresses are
developed and, these do not get relaxed due to dense
network of polyethylene. Selection of proper
crosslinking agent is very critical. Blending uniformly
crosslinking agents with polyethylene beds can lead to
uneven distribution of crosslinking agent. Melting of
Varies with thickness of
article
polyethylene prior to crosslinking is therefore, key
function in processing.
Due to 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.
Application of crosslinked polymers 63,64
Cable insulation
The most advanced area of application for
crosslinked polyethylene is in the electrical cable
industry. Crosslinking, whilst not interfering with the
dielectric properties of polyethylene, introduces
resistance to flow and permanent deformation above
the softening point. This permits higher conductor
operating temperature and reduces the level of short
circuit and overload protection required.
Flame retardant properties of pipe are also
improved by irradiation (60% gel) by introducing
double layer structure to the jacket, in which inner
layer is adjacent to polyethylene insulation which is
crosslinked more densely than the outer layer. The
wire exhibits markedly improved resistance to flame
and heat deformation.
Crosslinked polyethylene pipes
The main applications for crosslinked polyethylene
hot water pipe are:
a) under floor or central heating
b) domestic or portable water piping system.
The benefits of crosslinking become obvious above
ambient temperatures where a reduction in the rate of
creep for the corresponding hoop stress is observed.
TAMBOLI et al.: CROSSLINKED POLYETHYLENE
Injection and blow moulded articles
Specially moulded articles such as containers with
significant improvement in ESCR and chemical
resistance characteristics can be made using
crosslinked polyethylene. Increasing the molecular
weight by crosslinking increases its solvent and creep
resistance. The increased dimensional stability at
evaluated temperatures allows the article to come in
contact with heated fluids.
Crosslinked film
Crosslinked polyethylene in packaging applications
is confined to multi-layer film constructions, in which
the cross linked layer provides a number of specific
effects including: increased temperature resistance
especially for hot filled or heat sterilization
applications; increased heat seal strength where a
thermoplastic seal is subsequently crosslinked;
increased impact, tear and abuse resistance capability
especially for packing irregular shaped items to
impart heat shrinking properties to the film.
Conclusions
Crosslinking of polyethylene and polypropylene is
practiced industrially for very interesting applications.
Polyethylene can be crosslinked by radiation energy
as well as by organic peroxide. Polypropylene can be
crosslinked by silane method. Very little information
exists on crosslinking of PVC. Various aspects of
crosslinking process have been studied earlier and are
reviewed in this article.
References
1
2
3
4
5
6
7
8
9
10
Crosslinked polyethylene foam
Crosslinked polyethylene (XLPE) foams have
various kinds of applications, such as thermal
insulation, floatation, automotive trim, and sports
goods.
The advantages of crosslinking, prior to foaming
are that high quality closed cell structure can be
achieved which gives the following benefits a)
increased mechanical properties, b) increased
resistance and recovery from deformation, c)
increased thermal resistivity, d) reduced vapour and
liquid transport, e) to improve elevated temperature
and load bearing characteristics of the foam, f) to
permit secondary moulding and foaming without
collapse of the cell structure.
11
12
13
14
15
16
17
18
19
20
21
Special application
High pressure crosslinked polyethylene tube shows
water clear transparency on high heat, while lower
heat produces translucency, but this effect of
temperature is not obtained with untreated PE.
High pressure crosslinked PE makes it feasible to
use the material for applications such as automobile
muffler tail pipe.
Crosslinked polyethylene fibre used as a filler in
denture base resin poly [methyl methacrylates],
improves impact strength without adverse aesthetic
effects.
863
22
23
24
25
26
27
28
Krupa & Luyt A S, Polymer Degradation and Stability, 70,
111 (2000).
Engel T, Modern Plastic, 44 (1967) 175.
Narkis M, Modern Plastic, 10 (1980) 68.
Samburski G & Narkis M, J Macromol Sci Phys B35(5)
(1996) 843.
Dakin Y I, J Appl Poly Sci, 59 (1996) 1355.
Narkis M, Modern Plastic, 47, 1982.
Sawatari Chie & Mastuo M, Polym J, 19 (12) (1987) 1365.
Houde M & Schreiber H P, J Appl Polym Sci, 46 (1992)
2049.
Stepto R F T, in Crosslinked Polymers, edited by Dickie Ray
A & Baver R S (ACS, Washington), 1988.
Shultz Allen R, Encyclopedia of Polymer Science, Vol.4,
edited by Mark H F & Gaylord N G (John Wiley & Sons,
NY), 1986, 350.
McGinnis V D, Encyclopedia, Vol. 4 (1972) 419.
Miltz J & Narkis M, Polymer, 9 (1968) 173.
Narkis M & Mitz J, J Appl Polym Sci,12 (1968) 1030.
Ghosh P, Polymer Science and Technology of Plastic and
Rubber (Tata McGraw Hill), 1993, 266.
Labana S S, Encyclopedia of Polymer Science and
Engineering,Vol.4, edited by Mark Herman (John Wiley &
Sons, New York), 1986, 350.
Syncure, Technical Service Report, Polyone Corp., No 66
1999.
Stepto R F T, in Crosslinked Polymers, edited by Dickie Ray
A, Labana S S & Baver R S (ACS, Washington), 1988.
Tal Horng-Jer, Polym Eng Sci, 39(9) (1999) 1577.
Peacock Andrew J, Handbook of Polyethylene (MarcelDekker Inc., New York), 2000.
Kresser O J, Polyethylene (Reinhold Corp., New York),
1957.
Pedernera N, Sarmoria C & Brandolin A, Polym Eng Sci,
39(10) (1999) 2085.
Nabio S V & Rangwalla I J, Radiation Curing of Polymeric
Materials, edited by Hole Charles E & Kinstie J F (ACS,
Washington), 1990, 534.
Sangester D F, The Effect of Radiation on High-Technology
Polymers, edited by Reichnanis E & O’Donnell J H (ACS,
Washington), 1989, 15.
Chapiro A, Radiation Chemistry of Polymeric Systems (John
Wiley & Sons), 1962, 1.
US-PS,3.219.566, Dow Chemical Co.
US-PS,2.484.529, DuPont.
Wu Quianghua & Qu Baogun, Polym Eng Sci, 41 (2001)
1220.
Mitsui H F & Ushairokawa Hosoi M, J Appl Polym Sci, 19
(1975) 361.
864
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
INDIAN J. CHEM. TECHNOL., NOVEMBER 2004
Ali Zl & Zahran Ah, Polym International, 49 (2000) 1555.
Amrhein E & Kolloid Z, J Polym Sci, (1967) 216.
Thomas Engel, Modern Plastics, (1967) 175.
Narkis M & Miltz J, J Appl Polym Sci, 13 (1969) 713.
Salyer I O & Davsion J E, J Appl Polym Sci, 28 (1983) 2903.
Sperling L H, Introduction to Physical Polymer Science
(John Wiley, New York), 2001, 363.
Bueche F, J Appl Phys, 44 (1973) 532.
Narkis M, Raiter I & Eyerer P, J Macromol Sci – Phys,
B26(1), 37 (1987).
Kim B & White J L, Polym Eng Sci, 37 (1997) 576.
Narkis M & Toboisky A V, J Appl Polym Sci, 14 (1970) 65.
Kumar Sen, De P P & Bhowmick A K, J Appl Polym Sci, 44
(1992) 183.
Berzin F & Veanes B, Polym Eng Sci, 40 (2) (2000) 344.
Kunert K A, Soszynska H & Pislewski N, Polymer, 22
(1981) 1355.
Wei K, J Appl Polym Sci, 42 (1991) 1939.
Stoyanov O V & Deberdeev R Ya, International Polym Sci
Technol, 14(7) (1987) 83.
Braun D, Ricter S & Hellmann G P, J Appl Polym Sci, 68
(1998) 2019.
Sherman R D & Jacobs S M, Polym Eng Sci, 23 (1983) 36.
Smoluk G, Mod Plast Int,12 (9) (1982) 46.
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
Bonotto Sergio, J Appl Polym Sci, 9 (1965).
U S Patent, 6.231.978 (2001).
Biggs B S & Hawkins W L, Modern Plastics, 31 (1953) 121.
Brandrup J & Immergut E H, in Polymer Handbook, 2nd edn
(John Wiley & Sons, New York), II-12 (1975).
Mayer J, Polym Eng Sci, 14 (1974) 705.
Hawkins W L, Hansen R H, Mareyek W & Winsow F H, J
Appl Polym Sci, I (1959) 37.
Narkis M, J Appl Polym Sci, 25 (1980) 1515.
Dennberg E M, Jordan M E & Cole H M, J Appl Polym Sci,
31 (1958) 127.
Befran M & Mijangos C, Polym Engg Sci, 40 (2000) 1534.
Yeong-tarang sheigh & Tsai T H, J Appl Polym Sci, 1 (1998)
255.
Hjerterberge T T, Palmof M M & Swatan A, J Appl Polym
Sci, 42 (1991) 1185.
Shieh Y T & Hsiao Kuo, J Appl Polym Sci, 70 (1998) 1075.
Marin J & Paul B, J Appl Polym Sci, 7 (1968) 133.
Badar Y & Ali Zl, Polym Int, 49 (2000) 1555.
Brody H, J Appl Polym Sci, 15 (1971) 987.
Harper B G, J Appl Polym Sci, 2 (1959) 363.
Hagiwara M, J Appl Polym Sci, 25 (1980) 1541.
Kumar Suresh & Pandya M V, J Appl Polym Sci, 64 (1997).