8. MAJOR COMMERCIAL POLYMERS
8.1 POLYETHYLENE
Development
The conversion of ethylene to high molecular weight polymer
was first accomplished in 1933 by Fawcett and Gibson of Imperial
Chemical Industries Ltd. (U.K.) during an investigation of the
reaction between ethylene and benzaldehyde at high pressure.
The first commercial high pressure polyethylene plant began
production in 1939 and until 1955 all commercial polyethylene was
produced by high pressure processes. In 1953 Ziegler in Germany,
Phillips Petroleum Co. (U.S.A.) and Standard Oil Co.
(Indiana) (U.S.A.) almost simultaneously discovered methods
whereby high pressures could be avoided.
The first plant using the Ziegler process came on stream in
1955 (Farbwerke Hoechst A.G.) whilst the first Phillips and
Standard Oil plants began production in 1957 and 1961
respectively. At the present time, however, the bulk of commercial
polyethylene is still produced by high pressure methods.
Polyethylene is now one of the major commercial polymers
and is used in such diverse applications as chemical plant,
domestic goods, electrical insulation, packaging film and toys.
Preparation
High pressure processes
Commercial high pressure polyethylene processes are
generally operated at pressures of 1000-3000 atmospheres and
temperatures of 80-3000C.
Polymerization is initiated by free radical producing agents
such as azobisobutyronitrile, benzoyl peroxide or oxygen (which
probably leads to in situ peroxide formation), and the choice of
initiator determines the appropriate reaction conditions.
For the oxygen-initiated reaction, for example, the optimum
conditions are about 1500 atmospheres and 2000C with 0.03-0.1%
oxygen.
Generally, a continuous process is operated and
polymerization is carried out either in narrow-bore tubular
reactors or in autoclaves fitted with stirrers.
The reaction is highly exothermic and efficient heat
dissipation is essential. Insufficient control of reaction variables is
liable to lead to explosive formation of carbon, hydrogen and
methane. In some processes a diluent such as benzene or water is
added, mainly to serve as a heat-exchange medium but also to
assist in the removal of polymer from the reactor. In a typical
process, 10-30% of the monomer is converted to polymer per
cycle. After pressure letdown, the polymer is separated from
unreacted ethylene (which is recycled) and diluent, if any, and
then the polymer is extruded as ribbon and granulated.
The most note worthy feature of the above type of process is,
of course, the very high pressure involved. It is necessary to use
high pressure in order to obtain high molecular weight polymer. It
seems that growing polyethylene radicals have a very limited life
available for reaction with monomer and unless they have reacted
within a certain interval termination occurs. Thus high molecular
weights are favored by the increased monomer concentration
which obtains at high pressures. It may be noted that whilst
commercial processes involve what is strictly the gas phase (since
temperatures are above the critical temperature of 9.70C), at the
high pressures used the gas resembles a liquid and contains
polymer in solution. Thus these processes resemble conventional
bulk polymerization.
In the previous chapter the polymerization of ethylene is
shown as leading to a linear polymer as follows:
n CH2 = CH2
[~[CH2-CH2~]n
Whilst this is an essentially correct representation of the high
pressure processes, the polymer obtained by these methods is, in
fact, somewhat branched. Examination of the infrared spectrum
of the polymer shows the presence of about 30 methyl groups per
1000 carbon atoms in the chain. Methyl groups must be terminal
groups, but this number is larger than can be accounted for by
methyl chain ends in a linear polymer; hence branches must be
present.
In 1940 infrared spectroscopy revealed that low-density
polyethylene contains branched chains. These branches are
of two distinct type. Branching due to intermolecular chain
transfer, arising from reactions of the type.
R1CH2CH2 . + R2CH2CH2R3
.
Intermolecular
R1CH2CH3+ R2CHCH2R3
Hydrogen transfer
The second branching mechanism in polyethylene is
postulated to produce short-chain branching by
intramolecular chain transfer(backbite).
RCH2CH2CH2CH2CH2CH2 .
Transient six-membered
ring formation
H2
C
RH2C
HC
H
CH2 Intermolecular
.C
H2
CH2 Hydrogen transfer
.
RCH2CHCH2CH2CH2CH3
Ziegler processes
Commercial Ziegler polyethylene processes are generally
operated at pressures only slightly above atmospheric, namely 2-4
atmospheres and at temperatures of 50-750C.
For the polymerization of ethylene the catalyst is usually
based on titanium tetrachloride/aluminium alkyl (e.g.,
diethylaluminium chloride). The catalyst may be prepared in situ
by adding the components separately to the reactor as solutions
in diluents such as diesel oil, heptane or toluene or the
components may be pre-reacted and the catalyst added as a
slurry in a liquid diluent.
These operations must be conducted in an inert atmosphere
(usually of nitrogen) since oxygen and water reduce the
effectiveness of the catalyst and may even cause explosive
decomposition. In a typical process, ethylene and the catalyst and
diluent are fed continuously into the reactor. At the reaction
temperatures generally used, the polymer is only sparingly
soluble in the hydrocarbon diluent and therefore forms as a
slurry, which is continuously removed. The reaction is quenched
by the addition of alcohols such as methanol, ethanol or
isopropanol and the resulting metallic residues are extracted with
alcohol hydrochloric acid. This purification is extremely
important when the polymer is intended for use in high frequency
electrical insulation. Finally, the polymer is centrifuged, dried,
extruded and granulated.
The mechanism of vinyl polymerization effected by ZiegerNatta catalysts is discussed in the previous chapter. The
polyethylene obtained by Ziegler processes differs significantly
from the polymer obtained by high pressure processes in that
much less branching is present. Spectroscopic evidence indicates
the presence of 57 ethyl groups per 1000 carbon atoms; butyl side
groups appear to be absent.
Phillips processes
Phillips processes are usually operated under conditions
which are intermediate between those used in high pressure
processes and Ziegler processes.
Pressures are generally 30-40 atmospheres and temperatures
are 90-1600C. Polymerization is effected by means of a
chromium oxide catalyst. Typically, the catalyst is prepared by
impregnating silica or silica-alumina (which is the support) with
an aqueous solution of a chromium salt and heating the product
in air at 400-8000C. The final product contains about 5% of
chromium oxides, mainly chromium trioxide. In a typical
process, ethylene is passed into a suspension of the catalyst in a
hydrocarbon such as cyclohexane.
Depending mainly on the temperature at which
polymerization is carried out, the polymer is obtained either in
solution or as a slurry. Solution processes are normally run at
120-l600C, at which temperatures the polymer is soluble in the
diluent. Hot polymer solution is continuously drawn from the
reactor; unreacted ethylene is flashed off and suspended catalyst
is removed by filtration or centrifuging. The solution is then
cooled to precipitate the polymer which is separated by
centrifuging. Slurry processes, on the other hand, are usually
run at 901000C, at which temperatures the polymer has low
solubility in the diluent. The slurry, which consists of polymer
granules each formed around separate catalyst particles, is
drawn off continuously. Unreacted ethylene is flashed off and
then the polymer is separated by centrifuging. The polymer so
obtained is contaminated with a small amount of catalyst which
is not usually removed.
At the present time the nature of polymerization with Phillips
catalysts is incompletely understood. Whatever their mode of
reaction may be, these catalysts lead to almost completely linear
polyethylene. No ethyl or butyl branches have been definitely
detected although the number of methyl groups (up to about 3
per 1000 carbon atoms) may be somewhat greater than the
number of main chain ends.
Properties
As has been noted in the preceding section, the polyethylenes
obtained in the various commercial processes differ in the
amount of branching which is present.
The presence of branches reduces the ability of polymer
chains to pack together closely and regularly, i.e., to crystallize.
As a result the various types of polyethylene have somewhat
different properties.
Thus the more highly branched polymers have the lower
density, crystalline melting point, stiffness, surface hardness, and
softening temperature and greater permeability to gases and
vapors.
To a first approximation, these properties are dependent only
on the degree of branching. Other physical properties are more
difficult to correlate since they are also affected by variations in
average molecular weight and molecular weight distribution.
Comparative values for some properties of typical commercial
grades of polyethylenes are given in Table 2.2.
The electrical insulating properties of polyethylene are
outstanding. Since it is a non-polar material, properties such as
dielectric constant and power factor are almost independent of
the frequency and temperature. Dielectric constant increases
slightly with increasing density.
Chemically, polyethylene can be regarded as a high molecular
weight paraffin and as may be expected it is a rather inert
material. Since there is no specific interaction, such as hydrogen
bonding, with any solvent and since the polymer is crystalline,
there is no solvent for polyethylene at room temperature. At
elevated temperatures, however, there is appreciable solubility
in hydrocarbons and halogenated hydrocarbons, e.g., toluene,
xylene and dichloroethylene. The temperature necessary to
dissolve polyethylene in a given solvent increases as the
crystallinity of the polymer increases and ranges from about
600C to 800C for commercial materials. It may be noted here
that some liquids cause environ-mental stress cracking of
polyethylene. If the polymer is stressed in the presence of liquids
such as alcohols, esters and ketones, fracture occurs at stresses
much lower than those required to cause failure in the absence
of the liquid.
Polyethylene is unaffected by most acids, alkalis and aqueous
solutions. Strong oxidizing agents such as concentrated nitric
acid and concentrated solutions of hydrogen peroxide and
potassium permanganate oxidize the polymer, resulting in an
increase in power factor and a deterioration of mechanical
properties. Resistance to these reagents increases with increase
in density because of diminished permeability. Oxidation of
polyethylene also occurs in air on exposure to ultraviolet light
and/or elevated temperature.
8.2 Low-Density(Branched) Polyethylene
The first commercial ethylene polymer was branched poly
ethylene, commonly designated as low density or high pressure
material to distinguish it from the essentially linear material
described below.
After a period a relatively slow growth in the 1940’s, the
production of branched polyethylene expanded rapidly; this was
the first plastic with annual production exceeding 1 billion lb(in
1959)
Polymerization
Ethylene(b.p. –104) is made from the thermal(stream) and
catalytic cracking of a variety of hydrocarbons, ranging from
ethane derived from natural gas to fuel oil. About 25 billion lb was
produced in 1982, almost half of it used to produce of ethylene
polymers and copolymers.
High polymers of ethylene are made commercially at
pressures between 1000 and 3000 atm or possibly higher, and
temperatures as high as 250 ºC.
Traces of oxygen initiate the polymerization of ethylene
readily. Rapid exothermic reaction can occur, and violent
explosions have taken place. Many other possible impurities in the
monomer, such as hydrogen and acetylene, act as chain transfer
agents and must be carefully removed if high-molecular-weight
products are to be obtained. Besides oxygen peroxides(benzoyl,
diethyl), hydroperoxides, and azo compounds have been used as
initiators.
Ethylene polymerization can be carried out with benzene or
chlorobenzene as solvent.
Batch polymerization of ethylene can not be carried out
rapidly with reproducibility and good control. Long reaction
times, consistent with good control, are not economical. In
addition, chain branching becomes excessive at high conversion
and results in poor physical properties of the product. As a result,
balanced, continuos polymerization systems are preferred.
Emulsion polymerization has had little success.
Structure
Low density PE is a partially(50-60%) crystal-line solid
melting at about 115ºC, with density in the range 0.91-0.94.
It is soluble in many solvents at temperatures above 100ºC,
but only a few solvent mixtures provide borderline
solubility at or near room temperature.
Properties
The physical properties of low-density polyethylene are
function of three independent structural variables :
Molecular weight
Molecular-weight distribution or long chain branching and
short chain branching.
Short chain branching ha a predominant effect on the
degree of crystallinity and therefore on the density of
polyethylene.
The effect of molecular weight is largely evidence in
properties of the melt and properties involving large
deformations of the solid. As molecular weight increases, so
do tensile strength, tear strength, low- temperature
toughness, softening temperature, impact strength and
resistance to environmental stress cracking increases.
Applications
Almost two-thirds of the low-and medium density
branched polyethylene produced has gone into film and
sheeting uses for many years.
Polyethylene has filled a long-standing need for a material
that would effectively insulate electrical cables without
introducing electrical losses at high frequencies.
In addition to the bight-frequency uses, polyethylene is
being more generally utilised for mechanical protection of
wire and cables, where its chemical inertness and light
weight are advantageous. About 5% of the branched
polyethylene produced is used for wire and cable insulation.
8.3 POLYPROPYLENE
Development
It has been previously noted that co-ordination catalysts were
originally employed by Ziegler in 1953 to effect the polymerization
of ethylene. Natta extended the use of the catalysts to higher
olefins and obtained isotactic polypropylene in 1954. The first
commercial production of the polymer was by Montecatini in
1957. Polypropylene is now an important commercial polymer,
being used principally in the injection moulding of diverse articles
and for fibers and films.
Preparation
High pressure, free radical processes of the type used to
prepare polyethylene are not satisfactory when applied to
propylene and other n-olefins bearing a hydrogen atom on the
carbon atom adjacent to the double bond. This is attributed to
extensive transfer of this hydrogen to propagating centers
.
R + H-CH2-CH=CH2
.
RH + CH2-CH=CH2
CH2=CH-CH2
The resulting allyl radical is resonance stabilized and has a
reduced tendency to react with another monomer molecule.
Although the Phillips and Standard Oil processes can be used to
prepare polypropylene. the polymer yields tend to be low and it
appears that these processes are not currently used for
commercial production of polypropylene. At the present time,
commercial production of polypropylene is exclusively by Zieglertype processes. Commonly a slurry process is used and is carried
out in much the same manner as described previously for the
preparation of polyethylene. In the case of polypropylene. some
atactic polymer is fo~ed in addition to the required isotactic
polymer; but much of this atactic material is soluble in the diluent
so that the product isolated is largely isotactic polymer. A common
method of characterizing polypropylenes is by 'isotactic index'
which is the percentage by weight of polymer remaining
undissolved after extraction by boiling n-heptane. Most
commercial polypropylenes have isotactic indices of 95~98%. This
index is not an accurate measure of the true isotactic content since
partially-ordered chains which are in part incorporated into
crystalline regions will not be extracted by n-heptane.
Properties
As a first approximation, the physical properties of
polypropylene resemble those of high density polyethylene but
there art some significant differences.
For comparison, typical values for some properties of a
standard commercial grade of polypropylene are given in Table
The most noticeable attribute of polypropylene is the increased
softening point and consequent higher maximum service
temperature. Properties such as tensile strength and stiffness,
whilst comparable to high density polyethylene at room
temperature, are maintained to a greater extent at elevated
temperatures. At 1400C polypropylene is still sufficiently stiff for a
strain-free article to retain its shape, whereas polyethylene is
molten at this temperature. At room temperature the impact
strength of polypropylene is comparable to that of high density
polyethylene; however, the impact strength of polypropylene falls
markedly as the temperature is reduced whereas polyethylene
shows little change.
The solubility characteristics of polypropylene are similar to those
of polyethylene. Thus polypropylene is insoluble at room
temperature but is soluble in hydrocarbons and chlorinated
hydrocarbons at temperatures above about 800C. Polypropylene
does not suffer environ mental stress cracking of the kind shown
by polyethylene.
In polypropylene, each alternate carbon atom in the
polymer chain bears a tertiary hydrogen atom which is relatively
labile. Thus, compared to polyethylene, polypropylene is more
prone to attack by oxidizing agents. Of particular significance is
the susceptibility of the polymer to oxidation by air at elevated
temperatures, and antioxidants are generally added. As has been
mentioned earlier, when polyethylene is exposed to high energy
radiation or is heated with a peroxide cross-linking occurs to give
a useful material. When polypropylene is similarly treated, the
cross-linking reactions are accompanied by an approximately
equal amount of chain scission and useful properties are not
developed.
Polypropylene can be chlorinated and chlorosulphonated in
much the same way as polyethylene, but the reactions are
accompanied by severe degradation and some cross-linking. These
processes have not, therefore, found commercial utilization.
8.4 POLYSTYRENE
Development
Commercial interest in polystyrene began in the 1930's
when the material was found to have good electrical insulation
characteristics, and limited production was started by I. G.
Farben industrie (Germany) and Dow Chemical Company
(U.S.A.) shortly before the Second World War.
During the war enormous quantities of styrene were
produced in the U.S.A. for use in the synthetic rubber
programme which was undertaken when supplies of natural
rubber were no longer available. Thus when the war ended and
natural rubber supplies were resumed, a large styrene capacity
was available for civilian use and new outlets were sought.
Applications for polystyrene were vigorously explored and the
material was quickly adopted in many fields. Polystyrene has
now become one of the major commercial plastics, being very
extensively used in such diverse applications as domestic
appliances, food containers, packaging, toys and, in expanded
form, thermal insulation.
Raw materials
The bulk of commercial styrene is prepared from benzene
by the following route :
CH2CH3
CH=CH2
C2H4
benzene
ethylbenzene
styrene
In the first stage, a Friedel-Crafts reaction is carried out by
treating benzene with ethylene in the liquid phase at 90-1000C at
slightly above atmospheric pressure. The catalyst is aluminium
chloride (with ethyl chloride as catalyst promoter). A molar excess
of benzene is used to reduce the formation of polyethylbenzenes;
the molar ratio of reactants is generally about I 0.6. The reactants
are fed continuously into the bottom of a reactor whilst crude
product is removed from near the top. The product is cooled and
allowed to separate into two layers; the lower layer, which consists
of an aluminium chloride-hydrocarbon complex, is removed and
returned to the reactor. The remaining ethylbenzene is then
separated by distillation from polyetkylbenzenes and benzene,
which are recycled.
Some ethylbenzene is obtained by direct recovery from
catalytic reforming processes. In these processes aliphatic
hydrocarbons are converted into mixtures of aromatic
hydrocarbons from which ethylbenzene may be separated.
The second stage of the styrene process involves the
dehydrogenation of ethylbenzene. The reaction is carried out in
the vapour phase at temperatures of 600-650~C over catalysts
based on either ferric or zinc oxides with lesser amounts of other
metallic oxides such as chromic, cupric and potassium oxides. The
reaction is favoured by low pressure and in order to reduce the
partial pressure of the ethylbenzene the feed is mixed with
superheated steam before passage over the catalyst. Normally, a
conversion of 35-40% per pass is achieved. The product is cooled
and allowed to separate into two layers; the aqueous layer is
discarded. The organic layer consists of styrene (about 37%),
ethylbenzene (about 61%) and benzene, toluene and tar (about
2%). The separation of styrene by distillation is difficult because
of the susceptibility of the monomer to polymerization at quite
moderate temperatures and because the boiling point of styrene
(1450C) is rather close to that of ethylbenzene (1360C). It is
necessary therefore to use specially designed columns and to add a
polymerization inhibitor (commonly sulphur) before distillation
and to distil under reduced pressure. In a typical process, a fourcolumn distillation train is used. In the first column benzene and
toluene are removed at atmospheric pressure; in the second and
third columns ethylbenzene is removed at about 35 mm Hg; in the
fourth column styrene is separated from sulphur and tar, also at
about 35 mm Hg. Finally, an inhibitor is added to the styrene; tbutyl catechol is preferred for this purpose rather than sulphur
which leads to discoloration of the final polymer. Styrene is a
colourless liquid with a characteristic odour.
Preparation
Styrene may be polymerized by means of all four i.e., by bulk,
solution, suspension and emulsion polymerization. Each of these
methods is practiced commercially, but bulk and suspension
polymerization are the more extensively used. The four processes
are described below.
Bulk poymerization
In a common type of process, styrene is partially polymerized
batch-wise by heating the monomer (without added initiator) in
large vessels at about 800C for 2 days until about 35% conversion
is attained. The viscous solution of polymer in monomer is then
fed continuously into the top of a tower which is some 25 feet high.
The top of the tower is maintained at a temperature of about
100C, the centre at about 1500C and the bottom at about 1800C.
As the feed material traverses the temperature gradient,
polymerization occurs and fully polymerized material emerges
from the base of the tower. The reaction is controlled by a complex
array of heating and cooling jackets and coils with which the
tower is fitted. The molten material is fed into an extruder,
extruded as filament and then cooled and chopped into granules.
Since the product contains few impurities, it has high clarity and
good electrical insulation properties. The polymer has a broader
molecular weight distribution than polymer prepared at one
temperature.
Solution polymerization
Continuous solution processes have found some commercial
utilization, the main advantage over bulk methods being a
lessening of the problems associated with the movement and heat
transfer of viscous masses. However, the technique does require
the added steps of solvent removal and recovery. In one process a
mixture of monomer, solvent (such as ethylbenzene or toluene)
and initiator is fed into a train of three polymerization reactors.
The first reactor has three heating zones; in the first zone the
solution is heated to start polymerization but because of the
exothermic reaction cooling is necessary in the second and third
zones. Cooling is also applied in the second reactor but by the time
the third reactor is reached reaction has abated and heat is again
applied to complete polymerization. The polymer solution is then
extruded as fine strands into a devolatilizing vessel. In this vessel,
which is at a temperature of 2250C, removal of solvent and
unreacted monomer takes place, being aided by the large surface
area of the strands. The molten material is fed into an extruder,
extruded as filament, cooled and chopped.
Suspension polymerization
Suspension processes are very widely used for the
polymerization of styrene. These processes simplify the heat
transfer problems associated with bulk methods and, unlike
solution methods, they do not involve solvent removal and
recovery. The disadvantages of the suspension technique are that
it requires the added step of drying and it does not readily lend
itself to continuous operation. Typically, polymerization is carried
out batch-wise in a stirred reactor, jacketed for heating and
cooling. A typical formulation might be as follows :
Styrene (inhibitor free)
Water (demineralized)
Tricalcium phosphate
Dodecylbenzene suTphonate
Benzoyl peroxide
100 parts by weight
70
0.8 (suspending agent)
0.003(suspending agent)
0.2 (initiator)
Reaction temperature is about 900C. When polymenzation is
complete, the product, in the form of a slurry, is washed with
hydrochloric acid and water to remove suspending agent,
centrifuged, dried in warm air (at about 600C), extruded and
chopped.
Emulsion polymerization
Emulsion processes are not used for making solid grades of
polystyrene. This is because these processes lead to polymer
containing large quantities of soap residues which impair the
electrical insulation properties and optical clarity. Emulsion
polymerization does, however, find limited application in the
production of polystyrene latex used in water-based surface
coatings. The techniques employed are very similar to those used
for other polymer latices, e.g. poly(vinyl acetate) latex.