Plastics Materials 7 Episode 5 ppsx

Preparation
of
Monomer
207
revealed that over 950 patent applications had been filed on the subject by the
summer of 1996 and has since shown no signs of abating. Commercial
production commenced in the late 1990s and it is estimated that in 2000
metallocene-catalysed polyethylenes will comprise about 2% of the total
polyethylene market. This is somewhat less spectacular than achieved by LLDPE
and reflects the fact that although these materials may have many superior
properties in the finished product they are more expensive than the traditional
materials and in some respects more difficult to process. Whereas the
metallocene polymers can be of LDPE, LLDPE and HDPE types it is anticipated
that LLDPE types (referred to as mLLDPE) will take over
50%
of the market;
mainly for film application.
By the mid- 1990s capacity for polyethylene production was about
50
000
000
t.p.a, much greater than for any other type of plastics material. Of this
capacity about 40% was for HDPE, 36% for LDPE and about 24% for LLDPE.
Since then considerable extra capacity has been or is in the course of being built
but at the time of writing financial and economic problems around the world
make an accurate assessment of effective capacity both difficult and academic. It
is, however, apparent that the capacity data above is not reflected in consumption
of the three main types
of
material where usage of LLDPE is now of the same
order as the other two materials. Some 75% of the HDPE and LLDPE produced

is used for film applications and about 60% of HDPE for injection and blow
moulding.
Polymers of low molecular weight and of very high molecular weight are also
available but since they are somewhat atypical in their behaviour they will be
considered separately.
10.2
PREPARATION
OF
MONOMER
At one time ethylene for polymerisation was obtained largely from molasses,
a by-product of the sugar industry. From molasses may be obtained ethyl
alcohol and this may be dehydrated
to
yield ethylene. Today the bulk
of
ethylene
is
obtained from petroleum sources. When supplies of natural or
petroleum gas are available the monomer is produced in high yield by high-
temperature cracking of ethane and propane. Good yields of ethylene may also
be obtained if the gasoline (‘petrol’) fraction from primary distillation of oil
is ‘cracked’. The gaseous products of the reaction include a number of lower
alkanes and olefins and the mixture may be separated by low-temperature
fractional distillation and by selective absorption. Olefins, in lower yield, are
also obtained by cracking gas oil. At normal pressures (760mmHg) ethylene
is a gas boiling at -103.71”C and it has a very high heat of polymerisation
(3350-41
85
J/g). In polymerisation reactions the heat of polymerisation must
be carefully controlled, particularly since decomposition reactions that take

place at elevated temperatures are also exothermic and explosion can occur if
the reaction gets out of control.
Since impurities can affect both the polymerisation reaction and the properties
of the finished product (particularly electrical insulation properties and resistance
to heat aging) they must be rigorously removed. In particular, carbon monoxide,
acetylene, oxygen and moisture must be at a very low level. A number of patents
require that the carbon monoxide content be less than 0.02%.
208
Polyethylene
It was estimated in
1997
that by the turn of the century 185 million tonnes of
ethylene would be consumed annually on a global basis but that at the same time
production of polyethylene would be about 46000000t.p.a., i.e. about
25%
of
the total. This emphasises the fact that although polyethylene manufacture is a
large outlet for ethylene the latter is widely used for other purposes.
10.3 POLYMERISATION
There are five quite distinct routes to the preparation of high polymers of
ethylene:
(1)
High-pressure processes.
(2)
Ziegler processes.
(3) The Phillips process.
(4) The Standard Oil (Indiana) process.
(5)
Metallocene processes.
10.3.1

High-pressure Polymerisation
Although there are a number of publications dealing with the basic chemistry of
ethylene polymerisation under high pressure, little information has been made
publicly available concerning details of current commercial processes. It may
however be said that commercial high polymers are generally produced under
conditions of high pressure (1000-3000 atm) and at temperatures of 80-300°C.
A free-radical initiator such as benzoyl peroxide, azodi-isobutyronitrile
or
oxygen is commonly used. The process may be operated continuously by passing
the reactants through narrow-bore tubes or through stirred reactors or by a batch
process in an autoclave. Because of the high heat of polymerisation care must be
taken to prevent runaway reaction. This can be done by having a high cooling
surface-volume ratio in the appropriate part of a continuous reactor and in
addition by running water or a somewhat inert liquid such as benzene (which also
helps to prevent tube blockage) through the tubes to dilute the exotherm. Local
runaway reactions may be prevented by operating at a high flow velocity. In a
typical process
10-30%
of the monomer is converted to polymer. After a
polymer-gas separation the polymer
is
extruded into a ribbon and then
granulated. Film grades are subjected to
a
homogenisation process in an internal
mixer or a continuous compounder machine to break up high molecular weight
species present.
Although in principle the high-pressure polymerisation
of
ethylene follows the

free-radical-type mechanism discussed in Chapter
2
the reaction has two
particular characteristics, the high exothermic reaction and a critical dependence
on the monomer concentration.
The highly exothermic reaction has already been mentioned. It is particularly
important to realise that at the elevated temperatures employed other reactions
can occur leading to the formation of hydrogen, methane and graphite. These
reactions are also exothermic and it is not at all difficult for the reaction to get
out
of
hand. It is necessary to select conditions favourable to polymer formation
and which allow
a
controlled reaction.
Most vinyl monomers will polymerise by free-radical initiation over a wide
range
of
monomer concentration. Methyl methacrylate can even be polymerised
Polymerisation
209
by photosensitised catalysts in the vapour phase at less than atmospheric pressure.
In
the case of ethylene only low molecular weight polymers are formed at low
pressures but high molecular weights are possible at high pressures. It would
appear that growing ethylene polymer radicals have a very limited life available for
reaction with monomer. Unless they have reacted within a given interval they
undergo changes which terminate their growth. Since the rate of reaction of radical
with monomer is much greater with higher monomer concentration (higher
pressure) it will be appreciated that the probability of obtaining high molecular

weights is greater at high pressures than at low pressures.
At high reaction temperatures (e.g.
200°C)
much higher pressures are required
to obtain a given concentration or density of monomer than at temperatures of
say
25°C
and it might appear that better results would be obtained at lower
reaction temperatures. This is in fact the case where a sufficiently active initiator
is employed. This approach has an additional virtue in that side reactions leading
to branching can be suppressed. For a given system the higher the temperature
the faster the reaction and the lower the molecular weight.
By varying temperature, pressure, initiator type and composition, by
incorporating chain transfer agents and by injecting the initiator into the reaction
mixture at various points in the reactor it is possible to vary independently of
each other polymer characteristics such as branching, molecular weight and
molecular weight distribution over a wide range without needing unduly long
reaction times. In spite of the flexibility, however, most high-pressure polymers
are of the lower density range for polyethylenes (0.915-0.94g/cm3) and usually
also of the lower range of molecular weights.
10.3.2
Ziegler
Processes
As
indicated by the title, these processes are largely due to the work of Ziegler
and coworkers. The type of polymerisation involved is sometimes referred to as
co-ordination polymerisation since the mechanism involves a catalyst-monomer
co-ordination complex or some other directing force that controls the way in
which the monomer approaches the growing chain. The co-ordination catalysts
are generally formed by the interaction of the alkyls of Groups

1-111
metals with
halides and other derivatives of transition metals in Groups IV-VI11 of the
Periodic Table. In a typical process the catalyst is prepared from titanium
tetrachloride and aluminium triethyl or some related material.
In a typical process ethylene is fed under low pressure into the reactor which
contains liquid hydrocarbon to act as diluent. The catalyst complex may be
first prepared and fed into the vessel or may be prepared
in situ
by feeding
the components directly into the main reactor. Reaction is carried out at some
temperatures below
100°C
(typically
70°C)
in the absence of oxygen and
water, both of which reduce the effectiveness of the catalyst. The catalyst
remains suspended and the polymer, as it is formed, becomes precipitated from
the solution and a
slurry
is
formed which progressively thickens as the reaction
proceeds. Before the slurry viscosity becomes high enough to interfere
seriously with removing the heat of reaction, the reactants are discharged into
a catalyst decomposition vessel. Here the catalyst is destroyed by the action
of
ethanol, water
or
caustic alkali. In order to reduce the amount of metallic
catalyst fragments to the lowest possible values, the processes of catalyst

decomposition, and subsequent purification are all important, particularly
where the polymer
is
intended for use in high-frequency electrical insulation.
2
10
Polyethylene
A
number of variations in this stage of the process have been described
in
the
literature.
The Ziegler polymers are intermediate in density (about 0.945 g/cm3) between
the high-pressure polyethylenes and those produced by the Phillips and Standard
Oil (Indiana) processes.
A
range of molecular weights may be obtained by
varying the AI-Ti ratio in the catalyst, by introducing hydrogen as a chain
transfer agent and by varying the reaction temperature.
Over the years, considerable improvements and extensions of the Ziegler
process have taken place. One such was the advent of metallocene single-site
catalyst technology in the late 1980s. In these systems the olefin only reacts at a
single site on the catalyst molecules and gives greater control over the process.
One effect is the tendency to narrower molecular weight distributions. In a
further extension
of
this process Dow in 1993 announced what they refer to as
constrained geometry homogeneous catalysts.
The catalyst is based on Group
IV

transition metals such as titanium, covalently bonded to a monocyclopentadiene
group bridged with a heteroatom such as nitrogen. The catalyst is activated by
strong Lewis acid systems. These systems are being promoted particularly for use
with linear low-density polyethylene (see Section 10.3.5).
10.3.3
The
Phillips Process
In this process ethylene, dissolved in a liquid hydrocarbon such as cyclohexane,
is polymerised by a supported metal oxide catalyst at about 130-160°C and at
about 200-500 Ibf/in2 (1.4-3.5 MPa) pressure. The solvent serves to dissolve
polymer as it
is
formed and as a heat transfer medium but
is
otherwise inert.
The preferred catalyst is one which contains 5% of chromium oxides, mainly
Cr03, on a finely divided silica-alumina catalyst (75-90% silica) which has been
activated by heating to about 250°C. After reaction the mixture is passed to a gas-
liquid separator where the ethylene is flashed off, catalyst is then removed from the
liquid product of the separator and the polymer separated from the solvent by either
flashing off the solvent or precipitating the polymer by cooling.
Polymers ranging in melt flow index (an inverse measure
of
molecular weight)
from less than 0.1 to greater than 600 can be obtained by this process but
commercial products have a melt flow index of only 0.2-5 and have the highest
density of any commercial polyethylenes
(-
0.96 g/cm3).
The polymerisation mechanism is largely unknown but

no
doubt occurs at or
near the catalyst surface where monomer molecules are both concentrated and
specifically oriented
so
that highly stereospecific polymers are obtained. It is
found that the molecular weight of the product is critically dependent on
temperature and in a typical process there
is
40-fold increase in melt flow index,
and a corresponding decrease in molecular weight, in raising the polymerisation
temperature from 140°C to just over
170°C.
Above 4001bf/in2 (2.8MPa) the
reaction pressure has little effect
on
either molecular weight or polymer yield but
at lower pressures there is a marked decrease in yield and a measurable decrease
in molecular weight. The catalyst activation temperature also has an effect on
both yield and molecular weight. The higher the activation temperature the
higher the yield and the lower the molecular weight. A number of materials
including oxygen, acetylene, nitrogen and chlorine are catalyst poisons and very
pure reactants must be employed.
In a variation of the process polymerisation is carried out at about 9O-10O0C,
which is below the crytalline melting point and at which the polymer has a low
Polymerisution
21
1
solubility in the solvent. The polymer is therefore formed and removed as a slurry
of granules each formed around individual catalyst particles. High conversion

rates are necessary to reduce the level of contamination of the product with
catalyst and in addition there are problems of polymer accumulation on reactor
surfaces. Because of the lower polymerisation temperatures, polymers of higher
molecular mass may be prepared.
10.3.4
Standard
Oil
Company (Indiana) Process
This process has many similarities to the Phillips process and is based on the use
of a supported transition metal oxide in combination with a promoter. Reaction
temperatures are of the order of 230-270°C and pressures are
40-80
atm.
Molybdenum oxide is a catalyst that figures in the literature and promoters
include sodium and calcium as either metals or as hydrides. The reaction is
carried out in a hydrocarbon solvent.
The products of the process have a density of about
0.96
g/cm3, similar to the
Phillips polymers. Another similarity between the processes is the marked effect
of temperature on average molecular weight. The process is worked by the
Furukawa Company of Japan and the product marketed as Staflen.
10.3.5
Processes for Making Linear Low-density Polyethylene and
Metallocene Polyethylene
Over the years many methods have been developed in order to produce
polyethylenes with short chain branches but no long chain branches. Amongst the
earliest of these were a process operated by Du Pont Canada and another
developed by Phillips, both in the late
1950s.

More recently Union Carbide have
developed a gas phase process. Gaseous monomers and a catalyst are fed to a
fluid bed reactor at pressures of 100-300 Ibf/in2 (0.7-2.1 MPa) at temperatures
of 100°C and below. The short branches are produced by including small
amounts of propene, but-1-ene, hex-1-ene or oct-l -ene into the monomer feed.
Somewhat similar products are produced by Dow using a liquid phase process,
thought to be based
on
a Ziegler-type catalyst system and again using higher
alkenes to introduce branching.
As
mentioned in Section 10.3.2, there has been recent interest in the use of
the Dow constrained geometry catalyst system to produce linear low-density
polyethylenes with enhanced properties based, particularly, on ethylene and
oct-I-ene.
LLDPE materials are now available in a range of densities from around
0.900
g/cm3 for VLDPE materials to 0.935 g/cm3 for ethylene-octene copoly-
mers. The bulk of materials are of density approx. 0.920g/cm3 using butene in
particular as the comonomer.
In
recent years the market for LLDPE has increased substantially and is now
more than half the total for LDPE and for HDPE.
Mention has already been made
in
this chapter of metallocene-catalysed
polyethylene (see also Chapter 2). Such metallocene catalysts are transition metal
compounds, usually zirconium or titanium, incorporated into a cyclopentadiene-
based structure. During the late
1990s

several systems were developed where the
new catalysts could be employed in existing polymerisation processes for
producing LLDPE-type polymers. These include high pressure autoclave and
2 12
Polyethylene
solution processes as well as gas phase processes. At the present time it remains
to be seen what methods will become predominant.
Mention may also be made of catalyst systems based on iron and cobalt
announced in 1998 by BP Chemicals working in collaboration with Imperial
College London and, separately, by DuPont working in collaboration with the
University of North Carolina. The DuPontNNC catalysts are said to be based on
tridentate pyridine bis-imine ligands coordinated to iron and cobalt. These are
capable of polymerising ethylene at low pressures (200-600 psi) yielding
polymers with very low branching (0.4 branches per 1000 carbon atoms) and
melting points as high as 139°C. The BP/ICL team claim that their system
provides many of the advantages of metallocenes but at lower cost.
10.4 STRUCTURE AND PROPERTIES
OF
POLYETHYLENE
The relationship between structure and properties
of
polyethylene
is
largely in
accord with the principles enunciated in Chapters
4,
5
and 6. The polymer is
essentially a long chain aliphatic hydrocarbon of the type
and would thus be thermoplastic. The flexibility of the C-C bonds would be

expected to lead to low values for the glass transition temperature. The
Tg,
however, is associated with the motion of comparatively long segments in
amorphous matter and since in a crystalline polymer there is only a small number
of
such segments the
Tg
has little physical significance. In fact there is
considerable argument as to the position of the
Tg
and amongst the values quoted
in the literature are -130"C, -120"C, -105"C, -93"C, -81"C, -77"C, -63"C,
48"C, -3O"C, -20°C and +60"C! In one publication Kambour and Robertson
and the author* independently concluded that -20°C was the most likely value
for the
Tg.
Such a value, however, has little technological significance. This
comment also applies to another transition at about -120°C which is currently
believed to arise from the Schatzki crankshaft effect. Far more important is the
crystalline melting point
T,,
which is usually in the range 108-132°C for
commercial polymers, the exact value depending on the detailed molecular
structure. Such low values are to be expected of a structure with a flexible
backbone and no strong intermolecular forces. Some data on the crystalline
structure of polyethylene are summarised in Table
10.1.
There are no strong
intermolecular forces and most of the strength of the polymer
is

due to the fact
that crystallisation allows close molecular packing. The high crystallinity also
leads to opaque structures except in the case of rapidly chilled film where the
development of large crystalline structures is prevented.
Polyethylene, in essence a high molecular weight alkane (paraffin), would be
expected to have a good resistance to chemical attack and this is found to be the
case.
The polymer has a low cohesive energy density (the solubility parameter
6
is
about 16.1 MPa'/*) and would be expected to be resistant to solvents of solubility
parameter greater than 18.5 MPa'I2. Because it is a crystalline material and does
*
JENKINS,
A.
D.
(Ed.),
Polymer
Science,
North-Holland,
Amsterdam
(1972).
Structure
and
Properties
of
Polyethylene
2
13
Table

10.1
Crystallinity data
for
polyethylene
Molecular disposition
Unit cell dimensions
Cell density (unbranched polymer) (25°C)
Amorphous density (20°C)
planar zigzag
a
=
1.368,
b
=
4.928,
c
=
2.548,
1.014
0.84
not enter into specific interaction with any liquids, there is no solvent at room
temperature. At elevated temperatures the thermodynamics are more favourable
to solution and the polymer dissolves in a number of hydrocarbons of similar
solubility parameter.
The polymer, in the absence of impurities, would also be expected to be an
excellent high-frequency insulator because of its non-polar nature. Once again,
fact is in accord with prediction.
At the present time there are available many hundreds of grades of
polyethylene, most
of

which differ in their properties in one way or another. Such
differences arise from the following variables:
(1)
Variation in the degree of short chain branching in the polymer.
(2)
Variation in the degree
of
long chain branching.
(3)
Variation in the average molecular weight.
(4)
Variation in the molecular weight distribution (which may in part depend on
(5)
The presence of a small amount
of
comonomer residues.
(6)
The presence of impurities or polymerisation residues, some of which may
the long chain branching).
be combined with the polymer.
Further variations can also be obtained by compounding and cross-linking the
polymer but these aspects will not be considered at this stage.
Possibilities of
brunching
in high-pressure polyethylenes were first expressed
when investigation using infrared spectroscopy indicated that there were about
20-30
methyl groups per
1000
carbon atoms. Therefore in a polymer molecule

of molecular weight
26
000
there would be about
40-60
methyl groups, which is
of course far in excess of the one or two methyl groups to be expected from
normal chain ends. More refined studies have indicated that the methyl groups
are probably part of ethyl and butyl groups. The most common explanation is that
these groups arise owing to a ‘back-biting’ mechanism during polymerisation
(Figure
10.1).
Polymerisation could proceed from the radical in the normal way or
alternatively chain transfer may occur by a second back-biting stage either to the
butyl group
(Figure
10.2(a))
or
to the main chain
(Figure
10.2(b)).
According to this scheme a third back-bite is also possible
(Figure
10.3).
In the
first stage a tertiary radical is formed which could then depolymerise by
p-scission. This will generate vinylidene groups, which have been observed and
found to provide about
50%
of the unsaturation in high-pressure polymers, the

rest being about evenly divided by vinyl and in-chain
trans
double bonds. (There
may be up to about three double bonds per
1000
carbon atoms.)
214
Polyethylene
CH,
f
*CH,-CH
CH2
/
CHz
\
CH,
-
CH
*
I
I I
/CH2
CH,
*CH2
FH2
1
+C,H,
-
CH,
-

CH
-
(CH2)?-
CH,
I
CH2
I
CH,*
Figure
10.1
/
CH2
-
/
cH,
-
CH3
-
CH
H
I
-CH /CHz-cH*
(a)
/CH2
-
CH
CH,
-
CH,
\

CH,
-
CH,*
\
/
Bu
/
Bu
\ /
cHz-
CH
/cH2-cH
\
CH,
-
CH,
CH,
+
-CH
-
CH
I
(b)
Figure
10.2.
(a)
Transfer to the
butyl
group.
(b) Transfer to the

main chain
/
\H
*CH,
/Et
/Et
\
/CHz-cH
\
*
FH2
-C-Et CH,
-
-C-Et
/
\H *CH,
/
yy
*C-CH2-
*CH
I
I
Et Et
/
CH2-
-
CHI*
-
CH,=C
Et

I
Figure
10.3
Short chain branching is negligible with Ziegler and Phillips homopolymers
although it is possible
to
introduce deliberately
up
to
about seven ethyl side
chains per
1000
carbon atoms in the Ziegler polymers.
The presence of these branch points is bound
to
interfere with the ease of
crystallisation and this is clearly shown in differences between the polymers. The
branched high-pressure polymers have the lowest density (since close-packing due
Structure and Properties
of
Polyethylene
215
mCH,-CH,*
+
H-3
-&CH,-CH,
+
3-*
Growing
Radical

’Bad’
Polymer
’Bad’ Radical
Polymer
CH,
=
CH,
3
-
*
-
CH,
=
CH,
-
-CH,
-
CH,*
-
etc. etc
Figure
10.4
to crystallisation is reduced), the least opacity (since the growth of large crystalline
structures is impeded) and a lower melting point, yield point, surface hardness and
Young’s modulus in tension (these properties being dependent
on
the degree of
crystallinity).
In
addition the more the branching and the lower the crystallinity, the

greater will be the permeability to gases and vapours.
For
general technological
purposes the
density
of the polyethylene (as prepared from the melt under standard
conditions) is taken as a measure of short chain branching.
In
addition to the short chain branches there is some evidence in high-pressure
polyethylenes for the presence per chain of a few long branches which are
probably several tens of carbon atoms long. These probably arise from the
transfer mechanism during polymerisation shown in
Figure
10.4.
Such side
chains may be as long as the original main chain and like the original main chain
will produce a wide distribution of lengths. It is therefore possible to obtain fairly
short chains grafted
on
to short main chains, long side chains
on
to long main
chains and a wide variety of intermediate situations.
In addition, subsequent chain transfer reactions may occur
on
side chains and
the larger the resulting polymer, the more likely will it be
to
be attacked. These
features tend to cause a wide molecular weight distribution for these materials

and
it
is sometimes difficult to check whether an effect is due inherently to a wide
molecular weight distribution or simply due to long chain branching.
One further effect of long chain branches is
on
flow properties. Unbranched
polymers have higher melt viscosities than long-branched polymers of similar
weight average molecular weight. This would be expected since the long-
branched molecules would be more compact and be expected to entangle less
with other molecules.
The more recently developed so-called linear low-density polyethylenes are
virtually free of long chain branches but do contain short side chains as a result
of copolymerising ethylene with a smaller amount of a higher alkene such as oct-
I-ene. Such branching interferes with the ability of the polymer to crystallise as
with the older low-density polymers and like them have low densities. The word
linear in this case is used to imply the absence of long chain branches.
For reference purposes the polymer produced from diazomethane is particularly
useful in that it is free from both long and short branches and apart from the end
groups consists only
of
methylene groups. This material is generally known as
polymethylene, which is
also
the name now being recommended by IUPAC
to
describe polyethylenes in general. The diazomethane polymer has the highest
density of this family of materials, it being about
0.98
g/cm3. Copolymerisation

with diazoethane and higher homologues provides an alternative method for
producing a polymer with short chains but with
no
long ones.
Differences in
molecular weight
will also give rise to differences in properties.
The higher the molecular weight, the greater the number of points of attraction and
2
16
Polyethylene
entanglement between molecules. Whereas differences in short chain branching
and hence degree of crystallinity largely affect properties characterised by small
solid displacement, molecular weight differences will affect properties that
involve large deformations such as ultimate tensile strength, elongation at break,
melt viscosity and low-temperature brittle point. There is also an improvement in
resistance to environmental stress cracking with increase in molecular weight.
Before the advent of Ziegler and Phillips polymers it was common practice to
characterise the molecular weight for technological purposes by the
melt
flow
index
(MFI), the weight in grams extruded under a standard load in a standard
plastometer at 190°C in
10
minutes. This test had also proved useful for quality
control and as a very rough guide to processability. From measurements of MFI
various workers have calculated the apparent viscosity of the polymer and
correlated these figures with both number average and weight average molecular
weight. (It should be noted that estimation of apparent viscosities from melt flow

index data is rather hazardous since large corrections have to be made for end
effects, pressure losses in the main cylinder and friction of the plunger. It would
be better to use a high shear viscometer designed to minimise the sources of error
and to compare results at equal shear rate.) Suffice it to say that the higher the
melt flow index, the lower the molecular weight.
With the availability
of
the higher density polymers the value of the melt flow
index as a measure of molecular weight diminishes. For example, it has been
found* that with two polymers of the same weight average molecular weight (4.2
X
lo')),
the branched polymer (density
=
0.92 g/cm3) had only
1/50
the viscosity
of the more or less unbranched polymer (density
=
0.96g/cm3). This is due to
long chain branches as explained above.
Commercial polyethylenes also vary in their
molecular weight distribution
(MWD). Whilst for some purposes a full description of the distribution is
required, the ratio-of _weight average molecular weight to number average
molecular weight
(M,/M,)
provides a useful parameter. Its main deficiency is
that it provides
no

information about any unusual high or low molecular weight
tail which might have profound significance. For polymethylenes the ratio is
about 2 whilst with low-density polymers values varying from
1.9
to
100
have
been reported with values of 20-50 being said to be typical. High-density
polymers have values of 4-15.
The very high figures for low-density materials are in part a result of long
chain branching and, as has already been stated, it is sometimes not clear if an
effect is due to branching or to molecular weight distribution. It is generally
considered, however, that with other structural factors constant a decrease in
M,/M,
leads to an increase in impact strength, tensile strength, toughness,
softening point and resistance to environmental stress cracking. There
is
also a
pronounced influence
on
melt flow properties, the narrower distribution
materials being less sensitive to shear rate but more liable to sharkskin effects.
The general principles outlined in the previous paragraph (which has been
unchanged since the first edition of this book) have been found to be particularly
relevant for the metallocene polyethylenes being introduced in the late 1990s.
These have
Mw/Mn
ratios in the range
2-3
and while they do exhibit enhanced

toughness they show higher melt viscosities at high shear rates than correspond-
ing traditional polymers and suffer from problems with melt defects.
Much of recent development in polymerisation technology has been devoted to
establishing control of the MWD
of
LLDPE polymers. With such polymers,
narrowing the MWD confers higher toughness, greater clarity, lower heat seal
Properties
of
Polyethylene
217
initiation temperatures and, where this is important, higher cross-link efficiency.
As
with LDPE there is lower melt shear sensitivity and poorer melt strength.
Catalyst systems have been used which result in polymers with a bimodal
(double-peaked) molecular weight distribution in an attempt to improve flow
properties, whilst another approach combines the use of polymers with narrow
molecular weight distribution but with a broad side-chain length distribution.
A
number of
comonomers
have been used in conjunction with ethylene. Such
comonomers are either hydrocarbons such as propylene or but-1 -ene non-
hydrocarbons such as vinyl acetate. Small amounts of a second alkene
are
sometimes used to produce a controlled degree of short chain branching and
some retardation in the growth of large crystal structures.
As
will be described in
the next chapter, copolymers of this type produced by the Phillips process have

better creep, environmental stress cracking and thermal cracking resistance than
the corresponding homopolymer. The use of hydrocarbon comonomers such as
oct-1-ene became very common with the development of LLDPE and this
approach is also being used with metallocene polyethylenes. Properties of
metallocene polyethylenes such as low density (cf. standard homopolymers),
lower melt temperatures, clarity and heat sealability would be expected to be
more related to the presence of copolymers than the narrow molecular weight
distribution (which has a more significant effect
on
toughness and melt flow
properties). Small amounts of vinyl acetate also impede crystallisation and, as
with the alkene copolymers, substantial amounts of the second comonomer lead
to rubbery materials.
The final variable to be mentioned here is the presence of impurities. These
may be metallic fragments residual from Ziegler-type processes or they can be
trace materials incorporated into the polymer chain. Such impurities as catalyst
fragments and carbonyl groups incorporated into the chain can have a serious
adverse influence on the power factor of the polymer, whilst in other instances
impurities can have an effect
on
aging behaviour.
10.5 PROPERTIES
OF
POLYETHYLENE
Polyethylene is a wax-like thermoplastic softening at about 80-130°C with a
density less than that of water. It is tough but has moderate tensile strength, is an
excellent electrical insulator and has very good chemical resistance. In the mass
it is translucent or opaque but thin films may be transparent.
10.5.1
Mechanical Properties

The mechanical properties are very dependent
on
the molecular weight and on
the degree of branching of the polymer.
As
with other polymers these properties
are also dependent
on
the rate of testing, the temperature of test, the method of
specimen preparation, the size and shape of the specimen and, to only a small
degree with polyethylene, the conditioning of samples before testing. The data in
Table
10.2,
although not all obtained from the same source, has been obtained
using only one test method for each property. The figures given show clearly the
general effects
of
branching (density) and molecular weight on some polymer
properties but it should be remembered that under different test conditions
different results may be obtained. It should also be remembered that polymers of
different density but with the same melt flow index do not have the same
molecular weight. The general effects of changing rate of testing, temperature
Properties
of
Polyethylene
219
6
12
18

30
A
INCREASING DENSITY
OR
DECREASING TEMPERATURE
t
v(
\
II)
Y
I-
VI
STRAIN
C
Figure
10.5.
Effect of polymer density, testing rate and temperature
on
the shape of the stress-strain
curve
for polyethylene’
18.48
11.03
18.96 10.90
20.00 10.34
22.07
9.66
and density
on
the tensile stress-strain curves are shown schematically in

Figure
10.5.
It is seen in particular that as the test temperature is lowered or the testing
rate increased, a pronounced ‘hump’ in the curve becomes apparent, the apex of
the hump
A
being the yield point. Up to the yield point deformations
are
recoverable and the polymer is almost Hookean in its behaviour. The working of
the sample, however, causes ‘strain softening’ by, for example, spherulite
breakdown or in some cases by crystal melting
so
that the polymer extends at
constant stress. This cold drawing, however, causes molecular orientation and
induces crystallisation
so
that there is a stiffening of the sample and an upward
sweep of the stress-strain curve. The effect of temperature on a sample of low-
density polyethylene with an MFI
of
2
is shown in
Figure
10.6.
The varying
influence of rate
of
strain
on
tests results can be clearly shown from figures

obtained with two commercial polyethylene samples
(Table
10.3).
It is seen that
in one case an increase in rate of strain is accompanied by increase in tensile
strength and in the other case, reduction.
6
12
18
30
Table
10.3
Effect of straining rate on the
measured tensile strength
and
elongation
at
break of
two
samples of polyethylene
(idmin)
380
450
300
490
200 490
180
500
I
Elongation

at
break
(%)
I
I
I
I
220
Polyethylene
6*ooo1
EXTEN
IN
IN
0
6
i
2.35
in
Figure
10.6.
Effect of temperature on the tensile stress-strain curve for polyethylene. (Low-density
polymer -0.92g/cm3.
MFI
=
2.) Rate of extension 190%
per
minute”
The elongation at break
of
polyethylene is strongly dependent on density

(Figure
10.7),
the more highly crystalline high-density materials being less
ductile. This lack
of
ductility results in high-density polymers tending
to
be
brittle, particularly with low molecular weight materials. The tough-brittle
dependence on melt flow index and density is shown in
Figure
10.8.
Under load polyethylene will deform continuously with time (‘creep’).
A
knowledge
of
creep behaviour is important when considering load-bearing
applications, water piping being a case in point with polyethylene. In general
there will be an increase in creep with increased load, increased temperature and
decreased density.
A
large amount
of
creep data has been made available in
specialised monographs and in trade literature.
Properties
of
Polyethylene
22
1

Figure
10.7.
Effect of density and melt
flow
index on elongation at break. (Separation rate
45
cm/min
on specimen of lin
gauge
length.)
A,
constant density
(0.92
glcm’).
B,
constant
MFI (0.7).
C,
constant density
(0.94
g/cm3).’ (Reproduced by permission of
ICI)
10.5.2
Thermal Properties
As
mentioned in Section 10.4 there are conflicting data on the position of the
T,
of polyethylene. It is the author’s belief that
a
transition at about -20°C is

probably the true
T,
but another transition at about -120°C is also to be observed.
Tough at room temperature, the polymers become brittle on cooling but some
specimens
do
not appear to become brittle until temperatures as low
as
-70°C
have been reached. In general the higher the molecular weight and the more the
branching the lower the brittle point. Measured brittle points also depend on the
method
of
sample preparation, thus indicating that the polymer is notch sensitive,
i.e.
sensitive to surface imperfections.
The specific heat of polyethylene is higher than for most thermoplastics and is
strongly dependent on temperature. Low-density materials have
a
value
of
about
2.3
J/g at room temperature and
a
value of 2.9 J/g at
120-140°C.
A
somewhat
schematic representation

is
given in
Figure
10.9.
The peaks in these curves may
u u
090
091
092
093
094
095
096
DENSITY AT
23
‘C
IN
g/cmJ
Figure
10.8.
Effects of melt
flow
index and density on the room temperature tough-brittle transition
of polyethylene.’ (Reproduced by permission
of
ICI)
222
Polyethylene
0
Figure

10.9.
Specific heat-temperature relationships
for
low-density polyethylene, high-density
polyethylene and polystyrene." (The Distillers
Company
Ltd.)
be considered to be due to a form of latent heat of fusion of the crystalline zones.
Melting point
(T,)
data are given in
Table
10.2
and the
T,
is seen to vary with
density.
Flow properties of polyethylene have been widely studied. Because
of
the
wide range of average molecular weights amongst commercial polymers the
viscosities vary widely. The most commonly used materials, however, have
viscosities lower than for unplasticised PVC and poly(methy1 methacrylate) and
higher than for the nylons.
Typical of thermoplastics (see Chapter
8)
the melts are pseudoplastic and also
in common with most thermoplastics the zero shear rate apparent viscosity
of
linear

polyethylene is related to the weight average molecular weight by the
relationship
log
(Y~,~)
=
K
+
3.4
log
G,
for polymers with
a
molecular weight in excess of about
5000.
Polymers with long branches do
not
fit these equations and different relations
exist with polymers
of
different degrees
of
long branching.
In
many cases the
equation
log
(qa,o)
=
A
+

BG,,1'2
gives a good fit to the data.
It is interesting to note that so-called linear low-density polyethylenes are said
to be less pseudoplastic than conventional low-density polyethylenes. Thus
on
Properties
of
Polyethylene
223
comparing the two materials at the same melt flow index the ‘linear’ polymer
will be found to be more viscous at the higher shear rates usually encountered
during processing.
As
usual, an increase in temperature reduces melt viscosity and equations of
the type discussed in Chapter
8
fit data very well. Melt processing is usually
carried out in the range
150-210°C
but temperatures as high as 300°C may be
used in some paper-coating applications. In an inert atmosphere the polymer is
stable at temperatures up to 300°C
so
that the high processing temperatures do
not lead to severe problems due to degradation, providing contact
of
the melt
with oxygen is reduced to a minimum.
The elastic melt effects mentioned briefly in Chapter
8

are commonly
encountered with polyethylene. Some typical experimental results
on
die swell
are shown in
Figure
8.8.
The phenomenon of elastic turbulence (waviness,
bambooing, melt fracture) is also observed in low-temperature processes (e.g.
bottle blowing) and when extruding at very high rates (wire covering). This
situation is generally aggravated by high molecular weights and low temperature
but reduced by long chain branching and increasing the molecular weight
distribution.
Effect
of
increase
of
Table
10.4
Effect of polymer structure on flow properties
On
On
flow
On
critical
On
viscosity behaviour shear rate sharkskin
index
Branching (long)
Molecular weight

Mol.
Wt.
distribution
decreases little effect increases
?
decreases decreases increases decreases
increases slightly decreases decreases
?
In
addition to elastic turbulence (characterised by helical deformation) another
phenomenon known as ‘sharkskin’ may be observed. This consists of a number
of ridges transverse to the extrusion direction which are often just barely
discernible to the naked eye. These often appear at lower shear rates than the
critical shear rate for elastic turbulence and seem more related to the linear
extrudate output rate, suggesting that the phenomenon may be due to some
form
of slip-stick at the die exit. It appears to be temperature dependent (in a complex
manner) and is worse with polymers
of
narrow molecular weight distribution.
Melt elasticity is of considerable importance in understanding much of the
behaviour of polyethylene when processing by film extrusion techniques and
when blow moulding. The complex relationships observed experimentally here
have been summarised by the author elsewhere.
’*
10.5.3
Chemical Properties
The chemical resistance of polyethylene is, to a large measure, that expected
of
an alkane. It is not chemically attacked by non-oxidising acids, alkalis and many

aqueous solutions. Nitric acid oxidises the polymer, leading to a rise in power
factor and to a deterioration
in
mechanical properties.
As
with the simple alkanes,
halogens combine with the hydrocarbon by means of substitution mechanisms.
224
Polyethylene
Cl
Figure
10.10
S0,Cl
When polyethylene is chlorinated in the presence of sulphur dioxide, sulphonyl
chloride as well as chlorine groups may be incorporated into the polymer
(Figure
10.10).
This reaction is used to produce a useful elastomer (Hypalon, see Chapter
11).
Oxidation of polyethylene which leads to structural changes can occur to a
measurable extent at temperatures as low as 50°C. Under the influence of
ultraviolet light the reaction can occur at room temperature. The oxidation
reactions can occur during processing and may initially cause a reduction in melt
viscosity. Further oxidation can cause discolouration and streaking and in the
case of polymers rolled for 1-2 hours on a two-roll mill at about 150°C the
product becomes ropey and incapable of flow. It is rare that such drastic
operating conditions occur but it is found that at a much earlier stage in the
oxidation of the polymer there is
a
serious deterioration in power factor and for

electrical insulation applications in particular it is necessary to incorporate
antioxidants. It is to be expected that the less branched high-density polyethyl-
ene, because of the smaller number of tertiary carbon atoms, would be more
resistant to oxidation. That this is not always the case has been attributed to
residual metallic impurities, since purer samples of high-density polymers are
somewhat superior to the low-density materials.
Since polyethylene is a crystalline hydrocarbon polymer incapable of specific
interaction and with a melting point of about 1OO"C, there are
no
solvents at room
temperature. Low-density polymers will dissolve in benzene at about 60°C but
the more crystalline high-density polymers only dissolve at temperatures some
20-30°C higher. Materials of similar solubility parameter and low molecular
weight will, however, cause swelling, the more
so
in low-density polymers
(Table
10.5).
Low-density polyethylene has
a
gas permeability in the range normally
expected with rubbery materials (Table
5.11).
This is because in the amorphous
zones the free volume and segmental movements facilitate the passage
of
small
molecules. Polymers of the Phillips type (density
0.96
g/cm3) have a permeabil-

ity of about one-fifth that of the low-density materials.
Exposure of polyethylene to ultraviolet light causes eventual embrittlement of
the polymer. This is believed to be due to the absorption of energy by carbonyl
groups introduced into the chain during polymerisation andlor processing. The
carbonyl groups absorb energy from wavelengths in the range 220-320 nm.
Fortunately very little energy from wavelengths below
300
nm strikes the earth's
surface and
so
the atmosphere offers some protection. However, in different
climates and in different seasons there is some variation in the screening effect
of the atmosphere and this can give rise to considerable variation in the outdoor
weathering behaviour
of
the polymer.
Properties
of
Polyethylene
225
Table 10.5
Absorption of liquids by polyethylenes of density 0.92 and
0.96 g/cm3 at 20°C after 30 days immersion
Solvent
Carbon tetrachloride
Benzene
Tetrahydrofuran
Petrol
(BP
60-100°C)

Diethyl ether
Lubricating oil
Cyclohexanone
Ethyl acetate
Oleic acid
Acetone
Acetic acid
Ethanol
Water
Solubility
purumeter
6
MPa'I2
17.5
18.7
19.4
15.1
20.3
18.6
20.4
26.0
48.0
-
-
-
-
470
increase in weight in
polymers
0.92

g/cm3
42.4
14.6
13.8
12.8
8.5
4.9
3.9
2.9
1.81
1.24
1.01
0.7
<0.01
0.96
gfcm3
13.5
5.0
4.6
5.8
2.6
0.95
2.4
1.6
1.53
0.79
0.85
0.4
<0.01
When polyethylene is subjected to high-energy irradiation, gases such as

hydrogen and some lower hydrocarbons are evolved, there is an increase in
unsaturation and, most important, cross-linking occurs by the formation of
C-C
bonds between molecules. The formation of cross-link points interferes with
crystallisation and progressive radiation will eventually yield an amorphous but
cross-linked polymer. Extensive exposure may lead to colour formation and in
the presence of air surface oxidation will occur. Oxygen will cause polymer
degradation during irradiation and this offsets the effects of cross-linking. Long
exposure to low radiation doses on thin film in the presence
of
oxygen may lead
to serious degradation but with short exposure, high radiation doses and thicker
specimens the degradation effects become less significant. Since cross-linking is
accompanied by a loss of crystallisation, irradiation does not necessarily mean an
increased tensile strength at room temperature. However, at temperatures about
130°C
irradiated polymer still has some strength (it is quite rubbery), whereas the
untreated material will have negligible tenacity. It is found that incorporation of
carbon black into polyethylene which is subsequently irradiated can give
substantial reinforcement whereas corresponding quantities in the untreated
product lead to brittleness.
If polyethylene is exposed to a mechanical stress in certain environments,
fracture of the sample occurs at stresses much lower than in the absence of the
environment.
As
a corollary if a fixed stress, or alternatively fixed strain, is
imposed on a sample the time for fracture is much less in the 'active
environment' than in its absence. This phenomenon is referred to as environmen-
tal
stress

cracking. An example of this effect can be given by considering one of
the tests used (the Bell Telephone Laboratory Test) to measure the resistance
of
a specific polymer
to
this effect.
A
small moulded rectangle is nicked to a fixed
length and depth with a sharp blade and the nicked sample is then bent through
180 degrees
so
that the nick is on the outside of the bend and at right angles to
the line of the bend. The bent sample is held in a jig and immersed in a specific
226
Polyethylene
detergent, usually an alkyl aryl polyethylene glycol ether (e.g. Igepal CA) and
placed in an oven at 50°C. Low-density polymers with an MFI of
20
and above
will often be observed to crack in an hour or two. Amongst materials which
appear to be active environments are alcohols, liquid hydrocarbons, organic
esters, metallic soaps, sulphated and sulphonated alcohols, polyglycol ethers and
silicone fluids. This is rather a formidable list and at one time it was thought that
this would lead to some limitation in the use of polyethylene for bottles and other
containers. However, for a number of reasons this has not proved a problem
except with high-density homopolymers and the main reason for concern about
the cracking phenomenon is in fact associated with cables when the polyethylene
insulator is in contact with greases and oils.
The reason for the activity of the above named classes of liquids is not fully
understood but it has been noted that the most active liquids are those which

reduce the molecular cohesion to the greatest extent. It is also noticed that the
effect is far more serious where biaxial stresses are involved (a condition which
invariably causes a greater tendency to brittleness). Such stresses may be frozen
in as a result of molecular orientation during processing
or
may be due to
distortion during use.
Different polyethylenes vary considerably in the environmental stress cracking
resistance. It has been found that with low-density polymers the Bell Test
generally shows that the higher the molecular weight the greater the resistance,
low-density polymers with a melt flow index of
0.4
being immune to the
common detergents. Narrow molecular weight distributions considerably
improve resistance of a polymer of given density and average molecular weight.
Large crystalline structures and molecular orientations appear to aggravate the
problem. The effect of polymer density is somewhat complicated. The Bell Test
is performed at constant strain and hence much higher stresses will be involved
in the high-density polymers. It is thus not surprising that these materials often
appear to be inferior by this test but in constant stress tests different results may
be expected. Paradoxically, Phillips-type homopolymers have often been less
satisfactory in service than indicated by the Bell Test.
It may seem surprising that low-density, comparatively low molecular weight
(MFI
20)
materials have been successfully used for detergent bottles in view of
the stress cracking phenomenon. (Nevertheless higher molecular weight
materials are usually used here, i.e. with an MFI
<0.7.)
The reason for this lies

in the fact that good processing conditions and good design result in low stresses
being imparted to the products. Under these conditions stress cracking times are
invariably longer than the required service life
of
the product.
10.5.4
Electrical Properties
The insulating properties of polyethylene compare favourably with those of any
other dielectric material.
As
it is a non-polar material, properties such as power
factor and dielectric constant are almost independent of temperature and
frequency. Dielectric constant is linearly dependent
on
density and a reduction of
density
on
heating leads to a small reduction in dielectric constant. Some typical
data are given in
Table
10.6.
Oxidation
of
polyethylene with the formation of carbonyl groups can lead to
a serious increase
in
power factor. Antioxidants are incorporated into compounds
for electrical applications
in
order to reduce the effect.

Properties
of
Polyethylene
227
Table
10.6
Electrical properties of polyethylene
Volume resistivity
Dielectric strength
Dielectric constant
density
=
0.92 g/cm’
density
=
0.96
g/cm3
Power factor
>1Oz2nm
700 kV/m
2.28
2.35
-1-2
x
10-4
10.5.5
Properties
of
LLDPE and VLDPE
As with LDPE and HDPE materials, there is

a
wide range of linear low-density
polyethylenes (LLDPEs). Primarily competitive with LDPE, the ‘linear low’
materials have found rapid acceptance because of their high toughness (at low,
normal and high temperatures), tensile strength, elongation
at
break and puncture
resistance compared to LDPE materials of similar melt flow index and density.
More specifically the improved resistance
to
environmental stress cracking has
been emphasised by suppliers
as
also has the ability to use dishwashers to clean
LLDPE kitchen utensils,
a
consequence of the higher heat deformation
resistance.
The very low density materials (VLDPEs) introduced
in
the mid-1980s are
generally considered
as
alternatives to plasticised PVC (Chapter 12) and
ethylene-vinyl acetate (EVA) plastics (see Chapter 11). They have
no
volatile or
extractable plasticisers as in plasticised PVC nor do they have the odour or
moulding problems associated with EVA. Whilst VLDPE materials can match
the flexibility of EVA they also have better environmental stress cracking

resistance, improved toughness and
a
higher softening point.
Some comparative data for
a
VLDPE copolymer based
on
ethylene and oct-
1-ene and
an
EVA material
(91%
ethylene,
9%
vinyl acetate) are given in
Table
10.7.
Table
10.7
Comparison of VLDPE and EVA (9%VA)
VLDPE
EVA
Density (g/cm3)
0.910
MFI
7
Tear
strength (N/mm2) 11.4
Elongation at break
(%)

710
Vicat temperature
(“C)
78
Low-temperature brittle point (“C) -135
Hardness (Shore
D)
42
Stress
crack time
(h)
600
0.926
9
6.1
475
51
-130
32
240
10.5.6
Properties
of
Metallocene-catalysed Polyethylenes
Metallocene-catalysed polyethylenes exhibit the general characteristics
of
polyethylene as noted in the introductory paragraph
of
Section 10.5. Furthermore
228

Polyethylene
they are more like low density polyethylenes (LDPE and LLDPE) than HDPE.
As with LLDPE they are usually copolymers containing small quantities of a low
molecular weight a-olefin such as but- 1 -ene, hex- I-ene and oct- 1 -ene. The
property differences largely arise from the narrow molecular weight distribution,
the more uniform incorporation of the a-olefin and the low level
of
polymerisation residues (about one-tenth that of Ziegler-Natta catalysed
LLDPE).
It is generally claimed that metallocene polyethylenes (often abbreviated to
m-PE) exhibit superior mechanical and optical properties as well as better
organoleptic properties (resulting from the lower residue levels). As an example
m-LLDPE is particularly favoured as a stretch film for wrapping because of the
good prestretchability, high puncture resistance and tear strength, all
of
which are
claimed to be better than with conventional LLDPE.
As previously mentioned, narrow molecular weight distribution polymers such
as m-PE are less pseudoplastic in their melt flow behaviour than conventional
polyethylenes
so
that given an m-LLDPE and a conventional LLDPE of similar
melt index (measured at low shear rates), the m-LLDPE will have a much higher
melt viscosity at the high shear rates involved in film processing. The polymers
are also more susceptible to melt fracture and sharkskin. This difference requires
that such steps be taken as to use more highly powered extruders, to use special
processing aids such as fluoroelastomers or to make compromises in the polymer
structure which may, however, reduce the advantages
of
m-PE materials. One

obvious approach would be to produce bi-, tri-
or
other polymodal blends (see the
Appendix to Chapter
2
for explanations) to overcome the inherent disadvantages
of narrow molecular weight distribution polymers. It is of interest that ‘bimodal’
polymers produced by a two-reactor system have become available which have
enhanced resistance to cracking and are rapidly finding use in pipe
applications.
Metallocene-catalysed very low density polyethylene (m-VLDPE) has become
available with densities of as
low
as
0.903.
This is
of
use for sealing layers of
multi-layer films since sealing can commence at lower temperatures than with
conventional materials such as LLDPE and EVA (see Section 11.6) with the
polymer seal exhibiting both cold strength and hot tack strength.
10.6 ADDITIVES
Although polyethylene can be, and indeed often is, used without additives a
number may be blended into the polymer for various reasons. These additives can
be classified as follows:
(1)
Fillers.
(2)
Pigments.
(3)

Flame retarders.
(4)
Slip agents.
(5)
Blowing agents.
(6)
Rubbers.
(7)
Cross-linking agents.
(8)
Antioxidants.
(9)
Carbon black.
(10)
Antistatic additives.
Additives
229
Fillers,
important constituents of many plastics materials, are rarely used with
polyethylene since they interfere with the crystallinity
of
the polymer and often
give rather brittle products
of
low ductility. Carbon black has some reinforcing
effect and is of use in cross-linked polymers. It is also of some use in introducing
a measure of conductivity to the polymer. Somewhat better results with
non-
black fillers may be achieved with the use of silane and titanate coupling agents
and compounds with increased rigidity and tensile strength compared with

unfilled polymer may be obtained. However, unlike polypropylene, mineral-
filled polyethylene has remained unimportant.
A
number of
pigments
are
available for use in polyethylene. The principal requirements of a pigment are
that it should have a high covering power/cost ratio and that it should withstand
processing and service conditions.
In
the case of polyethylene special care should
be taken to ensure that the pigment does not catalyse oxidation, an effect
observed with a number of pigments based
on
cobalt, cadmium and manganese.
Other adverse effects have also been reported with hydrated chromic oxide, iron
blues, ultramarine and anatase titanium dioxide. For electrical insulation
applications pigments such as cobalt blues, which cause a rapid rise of power
factor
on
aging, should be avoided.
Polyethylene bums readily and a number of materials have been used
asflame
retarders.
These include antimony trioxide and a number of halogenated
materials.
Layers of low-density polyethylene film often show high cohesion, or
‘blocking’, a feature which is often a nuisance
on
both processing and use. One

way of overcoming this defect is to incorporate
anti-blocking
agents such as fine
silicas.
In
addition
slip agents
may be added to reduce the friction between layers
of film. Fatty acid amides such as oleamide and, more importantly, erucamide,
are widely used for this purpose. Polymers with densities of above 0.935 g/cm3
show good slip properties and slip agents are not normally required for these
products.
Products with very low dielectric constant (about
1.45)
can be obtained by the
use of cellular polymers.
Blowing agents
such as
4,4’-oxybisbenzenesulphono-
hydrazide and azocarbonamide are incorporated into the polymer.
On
extrusion the
blowing agent decomposes with the evolution of gas and gives rise to a cellular
extrudate. Cellular polyethylene is a useful dielectric in communication cables.
Although many
rubbery materials
show varying compatibility with poly-
ethylene the only elastomeric materials used in commercial compounds are
polyisobutylene (PIB) and butyl rubber. Polyisobutylene was originally used as
a ‘plasticiser’ for polyethylene but was later found also to improve the

environmental stress cracking resistance. Ten per cent PIB in polyethylene gives
a compound resistant to stress cracking as assessed by the severe BTL test. It has
been shown13 that within broad limits the higher the molecular weight of the PIB
the greater the beneficial effect. Very high molecular weight polyisobutylenes
are, however, less effective, possibly due to the difficulty in obtaining
satisfactory blends. PIB may or may not increase the ‘ease of flow’
of
polyethylene, this depending on the molecular weights of the two polymers.
Because
of
its lower cost butyl rubber is preferred to polyisobutylene
at
the
present time, use of the latter in polyethylene being largely restricted to cable
applications.
Polyethylene
is
sometimes blended with ethylene-propylene rubber (see
Chapter
11).
In this application it is most commonly used as an additive to the
rubber, which in turn is added to polypropylene to produce rubber-modified
230
Polyethylene
polypropylenes. In addition up to 20%
of
ethylene-propylene rubber may be
used in blown film applications.
Vulcanised (cross-linked) polyethylene is being used for cable application where
service temperatures up to

90°C
are encountered. Typical
cross-linking
agents
for
this purpose are peroxides such as dicumyl peroxide. The use of such agents is
significantly cheaper than irradiation processes for the cross-linking
of
the polymer.
An alternative process involves the use of vinyl silanes (see Section 10.9).
When polyethylene
is
to be used in long-term applications where a low power
factor
is
to be maintained and/or where it is desired to provide thermal protection
during processing,
antioxidants
are incorporated into the polymers. These were
discussed extensively in Chapter
7
but a few particular points with regard to their
use in polyethylene should be made. Although amines have been used widely in
the past phenols are now used almost exclusively.
For protection against degradation during processing 4-methyl-2,6-t-butyl-
phenol is widely used. It causes only a low level of staining and is also used
in
non-toxic formulations. Its volatility restricts its use for long-term and/or high-
temperature work. For service use
1,1,3-tris-(4-hydroxy-2-rnethyl-5-t-butyl-

pheny1)butane (Topanol CA) and
bis-[2-hydroxy-5-methyl-3-(
l-methylcyclo-
hexy1)phenyllmethane (Nonox WSP) are widely used. Only small amounts
(of
the order of
0.1
%)
are required of these chain-breaking antioxidants, which may
be used
in
conjunction with a peroxide-decomposing antioxidant such as
dilauryl-P$'-thiodipropionate
(DLTP). The phenols show little tendency to
bloom, bleed, discolour or stain but Topanol CA/DLTP blends cause some
discolouration which can be minimised by incorporation of certain phosphorus
compounds (e.g. 0.1-0.2%
of
Phosclere T268).
In the presence of carbon black the phenols and phenol/DLTP combinations are
much less effective whilst some phenolic sulphides (e.g. Santonox) show positive
synergism with carbon black. However, in general terms the phenol systems tend
to be reduced to about the same levels as to those to which the phenol sulphide
systems are raised. Some typical figures are given in
Table
10.8.
Chain-breaking antioxidant
Phenol alkane (0.1
%)
Phenol alkane (0.05%)

Phenol alkane (0.1%)
Phenol alkane (0.1
%)
Phenol alkane
(0.1
%)
Phenol sulphide (0.1
%)
Phenol sulphide (0.1
%)
Phenol sulphide
(0.2%)
Phenol sulphide (0.1
%)
Phenol sulphide
(0.1
%)
DLTP Carbon Copper Induction period
black powder
at
140°C
(h)
60
-
300
0.05%
-
-
450 0.1%
-

0.1% 3%
-
310
0.1%
-
0.1% 100
-
110
-
240
0.1%
-
-
3%
-
190
0.1%
-
0.1%
8-16
0.1
%
3%
-
No
data supplied
-
-
-
- -

Phcnol alkanc-Topanol
CA
(TcT)
Phenol
sulphide 4,4'-thiohia-(3-methyl-O-r-butylphenol)
Carhvn
black-Kosmui
CBY.
Additives 23
1
TIME
IN
h
Figure
10.11.
Oxidation of polyethylene in air at
105°C.
Effect
of
adding 0.1% antioxidant on power
factor.'
A,
blank.
B,
N,N'-diphenyl-p-phenylenediamine.
C, 4,4'-thiobis-(6-butyl-m-cresol).
D,
Nonox
WSP.
E,

N,N'-di-P-naphthyl-p-phenylenediamine
A
number of tests have been devised for measuring the efficiency of
antioxidants. Samples may be aged by hot rolling at about 160°C, by air oven
aging or by aging in aerated water. Changes in the polymer can be noted by
measurements of such properties as carbonyl content (by infrared measure-
ments), gel content, melt flow index, oxygen uptake and power factor. Since the
ruison d'&tre
for incorporating antioxidants is often to prevent an increase in
power factor
on
aging, power factor measurements are widely used. In addition
the property is very sensitive to oxidation.
Figure
10.11
shows the change in
power factor of polyethylenes containing various antioxidants after air aging.
Figure
10.12
shows the effect of varying the antioxidant concentration. The sharp
increase in power factor after an induction time during which little change occurs
is to be noted in particular. In practice about
0.1%
of antioxidant is employed in
electrical grade compounds.
The weathering properties of polyethylene are improved by the incorporation
of
carbon
blacks.
Maximum protection is obtained using blacks with a particle

size of
25
pm and below. In practice finely divided channel or furnace blacks are
used at
2-3%
concentration and to be effective they must be very well dispersed
into the polymer. The use of more than
3%
black leads to little improvement in
weathering resistance and may adversely affect other properties.
TIME
IN
h
Figure
10.12.
Oxidation
of
polyethylene
in
air at 105°C.
Effect
of
antioxidant concentration
(N,"
diphenyl-p-phenylenediamine).
A,
blank.
B,
50ppm. C, 100ppm.
D,

500ppm.
E,
lOWppm