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Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers
248 Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers
The application of metallocene catalysis to the preparation of polypropylenes
reached a commercial stage with the production by Exxon of their Achieve range
in 1996 and in 1997 by Targor, the BASF-Hoechst joint venture with the
introduction of Metocene. Such metallocene polypropylenes are, however, only
a small proportion of the total polypropylene market, predicted at only about 3%
of the total in 2005.
With a rapidly growing market many grades of polypropylene are available but
five main classes may be distinguished:
homopolymers produced by Ziegler-Natta catalysis;
block copolymers produced by Ziegler-Natta catalysis;
random copolymers produced by Ziegler-Natta catalysis;
rubber-modified blends of the above;
homopolymers and copolymers produced by metallocene catalysis.
Two interesting developments should also be noted; syndiotactic polypropylene
produced by a novel metallocene system and polypropylene grafted with styrene
and/or maleic anhydride marketed by Monte11 as Hivalloy.
Mention should be made of the nomenclature for the polymer. Industrially the
materially is invariably known in the English-speaking world as polypropylene.
However, the IUPAC name for the monomer is propene and until 1975 the
recommended IUPAC name was polypropene, a term very rarely used. The latest
IUPAC rules base the name of a polymer on the constitutional repeating unit,
which in this case is a propylene unit (c.f. a methylene unit for polyethylene) and
this leads to the name poly(propy1ene) (i.e. with brackets). In this volume the
more common, unbracketed but still unambiguous name will be used.
11.1.1 Preparation of Polypropylene
There are many points of resemblance between the production of polypropylene
and polyethylene using Ziegler-type catalysts. In both cases the monomers are
produced by the cracking of petroleum products such as natural gas or light oils.
For the preparation of polypropylene the C3 fraction (propylene and propane) is
the basic intermediate and this may be separated from the other gases without
undue difficulty by fractional distillation. The separation of propylene from
propane is rather more difficult and involves careful attention to the design of the
distillation plant. For polymer preparation impurities such as water and
methylacetylene must be carefully removed. A typical catalyst system may be
prepared by reacting titanium trichloride with aluminium triethyl, aluminium
tributyl or aluminium diethyl monochloride in naphtha under nitrogen to form a
slurry consisting of about 10% catalyst and 90% naphtha. The properties of the
polymer are strongly dependent on the catalyst composition and its particle shape
In the suspension process, which was the first method to be commercially
developed, propylene is charged into the polymerisation vessel under pressure
whilst the catalyst solution and the reaction diluent (usually naphtha) are metered
in separately. In batch processes reaction is carried out at temperatures of about
60°C for approximately 1-4 hours. In a typical process an SO-85% conversion
to polymer is obtained. Since the reaction is carried out well below the polymer
melting point the process involves a form of suspension rather than solution
polymerisation. The polymer molecular weight can be controlled in a variety of
ways, for example by the use of hydrogen as a chain transfer agent or by
variations in the molar ratio of catalyst components, the polymerisation
temperature, the monomer pressure or the catalyst concentration.
At this stage of the process the following materials are present in the
polymerisation vessel :
The first step in separating these ingredients involves the transfer of the
reaction mixture to a flash drum to remove the unreacted monomer, which is
purified (where necessary) and recycled. The residual slurry is centrifuged to
remove the bulk of the solvent together with most of the atactic material which
is soluble in the naphtha. The remaining material is then treated with an agent
which decomposes the catalyst and dissolves the residue. A typical agent is
methanol containing a trace of hydrochloric acid. The solution of residues in the
methanol is removed by a centrifuging operation and the polymer is washed and
dried at about 80°C. At this stage the polymer may be blended with antioxidants,
extruded and cut into pellets. There are a number of variations in this basic
process, many of which involve extra processes to reduce the atactic content of
the polymer. A typical flow sheet for the manufacture of polypropylene is given
in Figure I 1 .I.
There have also been a number of quite substantial changes in the method of
polymerisation over the years. For example, newer catalyst systems, such as
those containing magnesium compounds, give an appreciable improvement in
the yield of isotactic material and this enables the washing stage to be dispensed
with. In particular, both liquid (bulk process) and gas-phase processes have been
developed, including methods which avoid the need for separate stages for the
removal of catalyst residues and/or atactic material. Thermal and chemical aftertreatments have also been developed to reduce the width of the molecular mass
One such system is that developed by Himont, which uses three
(1) A titanium component supported on a magnesium halide.
(2) An organo-aluminium component.
(3) A Lewis base.
Detailed modifications in the polymerisation procedure have led to continuing
developments in the materials available. For example in the 1990s greater
understanding of the crystalline nature of isotactic polymers gave rise to
developments of enhanced flexural modulus (up to 2300 MPa). Greater control of
molecular weight distribution has led to broad MWD polymers produced by use
of twin-reactors, and very narrow MWD polymers by use of metallocenes (see
below). There is current interest in the production of polymers with a bimodal
MWD (for explanations see the Appendix to Chapter 4).
Another technical development is that of high impact isotactic polypropylene
in which rubber droplets are produced in situ during the polymerisation stage.
After propylene homopolymerisation ethylene is added to the reacting mass in a
second reactor and finely dispersed ethylene-propylene rubber droplets are
formed by polymerisation in the porous homopolymer polypropylene pellets.
Polypropylenes produced by metallocene catalysis became available in the late
1990s. One such process adopts a standard gas phase process using a metallocene
catalyst such as rac.-dimethylsilylenebis(2-methyl-1-benz(e)indenyl)zirconium
dichloride in conjunction with methylaluminoxane (MAO) as cocatalyst. The
exact choice of catalyst determines the direction by which the monomer
approaches and attaches itself to the growing chain. Thus whereas the isotactic
material is normally preferred, it is also possible to select catalysts which yield
syndiotactic material. Yet another form is the so-called hemi-isotactic polypropylene in which an isotactic unit alternates with a random configuration.
Metallocene catalysis can also make possible the production of copolymers of
propylenes with monomers such as long-chain olefins, cyclic olefins and styrene
which is not possible with more conventional Ziegler-Natta catalysts.
11.1.2 Structure and Properties of Polypropylene
Polypropylene is a linear hydrocarbon polymer containing little or no
unsaturation. It is therefore not surprising that polypropylene and polyethylene
have many similarities in their properties, particularly in their swelling and
solution behaviour and in their electrical properties. In spite of the many
similarities the presence of a methyl group attached to alternate carbon atoms on
the chain backbone can alter the properties of the polymer in a number of ways.
For example it can cause a slight stiffening of the chain and it can interfere with
the molecular symmetry. The first effect leads to an increase in the crystalline
melting point whereas the interference with molecular symmetry would tend to
depress it. In the case of the most regular polypropylenes the net effect is a
melting point some 50°C higher than that of the most regular polyethylenes. The
methyl side groups can also influence some aspects of chemical behaviour. For
example the tertiary carbon atom provides a site for oxidation so that the polymer
is less stable than polyethylene to the influence of oxygen. In addition, thermal
and high-energy treatment leads to chain scission rather than cross-linking.
The most significant influence of the methyl group is that it can lead to
products of different tacticity, ranging from completely isotactic and syndiotactic
structures to atactic molecules (see Chapter 4). The isotactic form is the most
regular since the methyl groups are all disposed on one side of the molecule.
Such molecules cannot crystallise in a planar zigzag form as do those of
polyethylene because of the steric hindrance of the methyl groups but crystallise
in a helix, with three molecules being required for one turn of the helix. Both
right-hand and left-hand helices occur but both forms can fit into the same crystal
structure. Commercial polymers are usually about 90-95% isotactic. In these
products, atactic and syndiotactic structures may be present either as complete
molecules or as blocks of varying length in chains of otherwise isotactic
molecules. Stereo-block polymers may also be formed in which a block of
monomer residues with a right-handed helix is succeeded by a block with a lefthanded helix. The frequency with which such changes in the helix direction occur
can have an important influence on the crystallisation and hence the bulk
properties of the polymer. In practice it is difficult to give a full description of a
specific propylene polymer although there has been marked progress in recent
years. Many manufacturers simply state that their products are highly isotactic,
others quote the polymer crystallinity obtained after some specified annealing
Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers
Figure 11.2. Effect of isotacticity on tensile properties. (Reproduced by permission of IC1 Plastics
treatment, whilst others quote the so-called ‘isotactic index’, the percentage of
polymer insoluble in n-heptane. Both of these last two properties provide some
rough measure of the isotacticity but are both subject to error. For example the
isotactic index is affected by high molecular weight atactic polymer which is
insoluble in n-heptane and by the presence of block copolymers of isotactic and
atactic structures which may or may not dissolve, according to the proportion of
each type present.
In spite of these problems the general effects of varying the degree of
isotacticity are well known. Whereas the atactic polymer is an amorphous
somewhat rubbery material of little value, the isotactic polymer is stiff, highly
crystalline and with a high melting point. Within the range of commercial
polymers, the greater the amount of isotactic material the greater the crystallinity
and hence the greater the softening point, stiffness, tensile strength, modulus and
hardness, all other structural features being equal (Figure 11.2).
The influence of molecular weight on the bulk properties of polypropylene is
often opposite to that experienced with most other well-known polymers.
Although an increase in molecular weight leads to an increase in melt viscosity
and impact strength, in accord with most other polymers, it also leads to a lower
yield strength, lower hardness, lower stiffness and softening point. This effect is
believed to be due to the fact that high molecular weight polymer does not
crystallise so easily as lower molecular weight material and it is the differences
in the degree of crystallisation which affect the bulk properties. It may also be
mentioned that an increase in molecular weight leads to a reduction in brittle
point (see Table 11.1).
Published data on com-mercial polypropylenes indicate that their molecular
weights arcinthe range M , = 38 000-60000 and M , = 220000-700000, with
values of M,/M, from about 5.6 to 11.9. These averages are somewhat higher
than those encountered normally with polyethylene and may help to explain the
difference in molecular weight dependence. It is in fact the case that the very high
molecular weight polyethylenes also have some difficulty in crystallising and
they too have lower tensile strength and stiffness than more conventional
polymers of lower molecular weight.
Only a limited amount of information is available concerning the effects of
molecular weight distribution. There is, however, evidence that the narrower the
distribution the more Newtonian are the melt flow properties. It has been noted
Table 11.1 Some mechanical and thermal properties of commercial polypropylenes
Melt flow index
Elongation at break (%)
Vicat softening point
Impact strength (ft lbf)
2.16kg at 230°C.
(b) Straining rate 18 mdmin.
(c) Falling weight test on 14 in diameter moulded bowls at 20°C
(a) Standard polyethylene grader: load
that with polymers of molecular we&hts suitable for moulding and extrusion,
polymers of wide distribution (e.g. M J M , about 6 ) are stiffer and more brittle
than those with a M J M , ratio of about 2.
The morphological structure of polypropylene is rather complex and at least
four different types of spherulite have been observed. The properties of the
polymer will depend on the size and type of crystal structure formed and this will
in turn be dependent on the relative rates of nucleation to crystal growth. The
ratio of these two rates can be controlled by varying the rate of cooling and by
the incorporation of nucleating agents. In general the smaller the crystal
structures the greater the transparency and flex resistance and the less the rigidity
and heat resistance.
One unfortunate characteristic property of polypropylene is the dominating
transition point which occurs at about 0°C with the result that the polymer
becomes brittle as this temperature is approached. Even at room temperature the
impact strength of some grades leaves something to be desired. Products of
improved strength and lower brittle points may be obtained by block
copolymerisation of propylene with small amounts (4-1 5%) of ethylene. Such
materials are widely used (known variously as polyallomers or just as propylene
copolymers) and are often preferred to the homopolymer in injection moulding
and bottle blowing applications.
Further variations in the properties of polyethylenes may be achieved by
incorporating additives. These include rubber, antioxidants and glass fibres and
their effects will be discussed further in Section 11.1.4.
11.1.3 Properties of Isotactic Polypropylene
Although very similar to high-density polyethylene, isotactic polypropylene
differs from the former in a number of respects of which the following are among
the most important:
254 Aliphatic Polyolejins other than Polyethylene, and Diene Rubbers
(1) It has a lower density (0.90g/cm3).
(2) It has a higher softening point and hence a higher maximum service
temperature. Articles can withstand boiling water and be subject to many
steam sterilising operations. For example mouldings have been sterilised in
hospitals for over 1000 hours at 135OC in both wet and dry conditions
without severe damage.
( 3 ) Polypropylene appears to be free from environmental stress cracking
problems. The only exception seems to be with concentrated sulphuric and
chromic acids and with aqua regia.
(4) It has a higher brittle point.
( 5 ) It is more susceptible to oxidation.
As shown in the previous section the mechanical and thermal properties of
polypropylene are dependent on the isotacticity, the molecular weight and on other
structure features. The properties of five commercial materials (all made by the
same manufacturer and subjected to the same test methods) which are of
approximately the same isotactic content but which differ in molecular weight and
in being either homopolymers or block copolymers are compared in Table I 1 .I.
The figures in Table I1 .I show quite clearly how an increase in molecular
weight (decrease in melt flow index) causes a reduction in tensile strength,
stiffness, hardness and brittle point but an increase in impact strength. The
general effects of isotactic index and melt flow index on some mechanical and
thermal properties are also shown graphically in Figures 11.3-11.6.' Both
random and block copolymers are now available and these show interesting
differences as indicated in Table 11.2.
Many features of the processing behaviour of polypropylene may be predicted
from consideration of thermal properties. The specific heat of polypropylene is
lower than that of polyethylene but higher than that of polystyrene. Therefore the
plasticising capacity of an injection moulding machine using polypropylene is
lower than when polystyrene is used but generally higher than with a highdensity polyethylene.
LOG (MELT INDLX)
Figure 11.3. Variation of tensile yield stress with melt flow index (10kg load at 1 9 0 T ) and isotactic
index. (After Crespi and Ranalli')
-zI 6 0
M J I. =I
3 and isotactic index.
and melt flow index.
isotactic index (After
256 Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers
Table 11.2 Comparison of random and block copolymers (data based on three grades of Novolen
Melt Flow Ratio
230°C/2. 16 kg
This data indicates that the random copolymer has greater transparency but inferior low temperature impact strength
Studies of melt flow properties of polypropylene indicate that it is more nonNewtonian than polyethylene in that the apparent viscosity declines more rapidly
with increase in shear rate. The melt viscosity is also more sensitive to
temperature. Van der Wegt2 has shown that if the log (apparent viscosity) is
plotted against log (shear stress) for a number of polypropylene grades differing
in molecular weight, molecular weight distribution and measured at different
temperatures the curves obtained have practically the same shape and differ only
The standard melt flow index machine is often used for characterising the flow
properties of polypropylene and to provide a rough measure of molecular weight.
Under the conditions normally employed for polyethylene (2.16 kg load at
190OC) the flow rate is too low for accurate measurement and in practice higher
loads, e.g. 10 kg, and/or higher temperatures are used. It has been found3 that a
considerable pressure drop exists in the barrel so that the flow towards the end
of a test run is higher than at the beginning.
The moulding shrinkage of polypropylene is less than that experienced with
polyethylenes but is dependent on such processing factors as mould temperature,
melt temperature and plunger dwell time. In general, conditions which tend to
reduce the growth of crystal structures will tend to reduce shrinkage; for
example, low mould temperatures will encourage quenching of the melt. It is also
found that low shrinkage values are obtained with high melt temperatures. This
is probably due to the fact that high melt temperatures lead to a highly disordered
melt whereas some molecular order may be present in melts which have not been
heated much above the crystalline melting point. Such regions of order would
provide sites for crystal nucleation and hence crystallisation would be more rapid
when cooling was carried out.
The electrical properties of polypropylene are very similar to those of highdensity polyethylenes. In particular the power factor is critically dependent on
the amount of catalyst residues in the polymer. Some typical properties are given
in Table 11.3 but it should be noted that these properties are dependent on the
antioxidant system employed as well as on the catalyst residues.
Table 11.3 Some typical electrical properties of a high
heat stability grade of polypropylene4
Dielectric constant at 5 X lo6 Hz
Volume resistivity ( a m )
Power factor at 10’ Hz
5 X lo6 Hz
As with electrical properties the chemical resistance of polypropylene shows
many similarities to high-density polyethylene. The two polymers have similar
solubility parameters and tend to be swollen by the same liquids. In both cases the
absence of any possible interaction between the crystalline polymer and the liquid
prevents solution of the polymers in any liquids at room temperature. In some
instances polypropylene is more affected than polyethylene but in other cases the
reverse is true. Similar remarks may be made concerning the permeability of the
two polymers to liquids and gases. With many permeants polypropylene shows the
lowest permeability but not, for example, with hexane. It may be mentioned in this
context that although high-density polyethylene is usually intermediate between
low-density polyethylene and polypropylene, where the permeant causes stress
cracking (as with a silicone oil), the high-density polyethylenes often have the
highest permeability. The fact that polypropylene is resistant to environmental
stress cracking has already been mentioned.
Polypropylene differs from polyethylene in its chemical reactivity because of
the presence of tertiary carbon atoms occurring alternately on the chain
backbone. Of particular significance is the susceptibility of the polymer to
oxidation at elevated temperatures. Some estimate of the difference between the
two polymers can be obtained from Figure 11.7, which comparess the rates of
oxygen uptake of each polymer at 93°C. Substantial improvements can be made
by the inclusion of antioxidants and such additives are used in all commercial
compounds. Whereas polyethylene cross-links on oxidation, polypropylene
degrades to form lower molecular weight products. Similar effects are noted
TIME AT 200.F
Figure 11.7. Comparison of oxidation rates of unstabilised polyethylene and polypropylene (After
258 Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers
Table 11.4 Effect of percentage stretch on tensile properties of
when the polymer is exposed to high-energy radiation and when heated with
peroxides (conditions which will cross-link polyethylene).
Although a crystalline polymer, polypropylene mouldings are less opaque
when unpigmented than corresponding mouldings from high-density polyethylene. This is largely due to the fact that the differences between the amorphous
and crystal densities are less with polypropylene (0.85 and 0.94g/cm3
respectively) than with polyethylene (see Chapter 6). Clarity may also be affected
by the use of nucleating agents (see also Sections 3.3 and 11.1.4). Biaxially
stretched film has a high clarity since layering of the crystalline structures
reduces the variations in refractive index across the thickness of the film and this
in turn reduces the amount of light scattering.6
Biaxial stretching also leads to polymers of improved tensile strength. The
effect of increasing the amount of stretching on the tensile strength and breaking
elongation are given in Table 11.4.
There are other differences between cast, monoaxially oriented, and balanced
biaxially oriented film. Typical figures illustrating these effects are given in Table
When film is produced by air-cooled tubular blowing methods cooling rates
are slower and larger degrees of crystallinity result. Hence tubular film is slightly
Table 11.5 Comparison of cast, monoaxially oriented and biaxially oriented polypropylene film'
Tensile strength (Ibf/in*)
Tensile strength (MPa)
Elongation at break (%)
ASTM D.523 gloss (45°C head)
Low temperature brittleness
Coefficient of friction
brittle at "C