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61
In 1942 the Japanese overran Malaya and the then Dutch East
Indies to cut off the main sources of natural rubber for the United States
and the British Commonwealth. Because of this the US Government
initiated a crash programme for the installation of plants for the
manufacture of a rubber from butadiene and styrene. This product, then
known as GR-S (Government Rubber-Styrene), provided at that time an
inferior substitute for natural rubber but, with a renewed availability of
natural rubber at the end of the war, the demand for GR-S slumped
considerably. (Today the demand for SBR (as GR-S is now known) has
increased with the great improvements in quality that have been made and
SBR is today the principal synthetic rubber).
Because of such desirable characteristics as low cost, good
mouldability, excellent colour range, transparency, rigidity and low water
absorption, polystyrene became rapidly developed. For many purposes,
however, it was considered to be unacceptably brittle and this led to the
development of the rubber-modified high-impact polystyrene (HIPS) and to
the complex ABS, AMBS and MBS materials. These developments,
together with the considerable success of expanded polystyrene as both
insulation and packaging materials, have led to the plastics materials based
on styrene becoming a most important group of thermoplastics.
Whilst there is a large number of manufacturers, two companies,
Dow and BASF, have a significant share of the market, with 16.2% and
11.4% respectively for their products Styron and Polystyrol.
In addition to polystyrene and high-impact polystyrene there are
other important styrene-based plastics. Most important of these is ABS,
with a global capacity of about 5∙106 t.p.a. and production of about 3∙10
6
t.p.a. Data for the more specialized styrene -acrylonitrile copolymers are
difficult to obtain but consumption estimates for Western Europe in the
early 1990s were a little over 60000 t.p.a.
In the late 1990s a crystalline form of polystyrene, syndiotactic
polystyrene became commercially available but unless otherwise stated
references to polystyrene in this chapter will refer to the traditional
amorphous polymer.
The rarely used systematic IUPAC name for polystyrene is poly-(1phenylethylene).
62
Preparation of the monomer
CH
CH
COOH
D
ry
Distil
CH
CH
2
+ CO
2
CHO
CH
CH3CO
O
CH3CO
+
CH3COONa
180
0
C
CH
COOH
+
CH
3
COOH
C
2H5
+
C
2H4
Laboratory preparation
The principal constituent of storax is cinnamic acid and for
laboratory purposes styrene is still most easily obtained in high purity but
dry distillation of cinnamic acid and its salts under atmospheric pressure:
The cinnamic acid is readily prepared by heating benzaldehyde
with acetic anhydride and sodium acetate (the Perkin Reaction):
Commercial preparation
The bulk of commercial styrene is prepared by the Dow process or
some similar system. The method involves the reaction of benzene and
ethylene to ethylbenzene, its dehydrogenation to styrene and a final
finishing stage. It is therefore useful to consider this process in each of the
three stages.
Preparation of ethylbenzene
Ethylbenzene is prepared by reaction of ethylene and benzene in
the presence of a Friedel-Crafts catalyst such as aluminium chloride at
about 95°C:
To improve the catalyst efficiency some ethyl chloride is added
which produces hydrochloric acid at the reaction temperatures.The purity
of the ethylene is not critical providing that acetylene is not present. The
normal purity of ethylene used is about 95%. The purity of t he benzene is
somewhat higher at about 99% and it is important here that sulphur, as
impurity, should be below 0.10%.
In order that the amount of side reaction should be reduced and to
minimize the production of polyethylbenzene, the molar ratios of feedstock
and products are approximately as indicated in the following equation:
0,58CH2=CH2 + 1C6H6→0.41 Ethylbenzene
0.51 Benzene
0.08 Polyethylenbenzenes
63
After passing through the reaction chamber the products are cooled
CH
C2H
5
CH
2
630
0
C
and the aluminium chloride, which is in the form of a complex with the
hydrocarbons, settles out. The ethylbenzene, benzene and polyethylbenzene
are separated by fractional distillation, the ethylbenzene having a purity of
over 99%. The polyethylbenzene are dealkylated by heating at 200°C in the
presence of aluminium chloride and these products together with the
unchanged benzene are recycled.
Plants have now been installed by some manufacturers to produce
ethylbenzene via catalytic reforming processes. The reforming process is
one which converts aliphatic hydrocarbons into a mixture of aromatic
hydrocarbons. This may be subsequently fractionated to give benzene,
toluene and a “xylene fraction” from which ethylbenzene may be obtained.
Dehydrogenation
Styrene is produced from the ethylbenzene by a process of
dehydrogenation:
This is an endothermic reaction in which a volume increase
accompanies dehydrogenation. The reaction is therefore favoured by
operation at reduced pressure. In practice steam i s passed through with the
ethylbenzene in order to reduce the partial pressure of the latter rather than
carrying out a high-temperature reaction under partial vacuum. By the use
of selected catalysts such as magnesium oxide and iron oxide a conversion
of 35-40% per pass with ultimate yields of 90-92% may be obtained.
Styrene purification
The dehydrogenation reaction produces “crude styrene” which
consists of approximat ely 37.0% styrene, 61% ethylbenzene and about 2%
of aromatic hydrocarbon such as benzene and toluene with some tarry
matter. The purification of the styrene is made rather difficult by the fact
that the boiling point of styrene (145.2°C) is only 9°C higher than that of
ethylbenzene and because of the strong tendency of styrene to polymerize
at elevated temperatures. To achieve a successful distillation it is therefore
necessary to provide suitable inhibitors for the styrene, to distil under a
partial vacuum and to make use of specially designed distillation columns.
In one process the crude styrene is first passed through a pot
containing elemental sulphur, enough of which dissolves to become a
64
polymerization inhibitor. The benzene and toluene are then removed by
distillation. The elthylbenzene is then separated from the styrene and tar by
passing this through two distillation columns, each with top temperatures of
about 50°C and bottom temperatures of 90°C under a vacuum of about 35
mmHg. The tar and sulphur are removed by a final distillation column and
the styrene is permanently inhibited by addition of 10 ppm of t-
butylcatechol, which has less adverse effects on the final polymer than
sulphur.
Styrene is a colourless mobile liquid with a pleasant smell when
pure but with a disagreeable odour due to traces of aldehydes and ketones if
allowed to oxidize by exposure to air. It is a solvent for polystyrene and
many synthetic rubbers, including SBR, but has only a very limited mutual
solubility in water.
Styrene takes part in a very large number of chemical reactions. In
particular it has a strong tendency to polymerize on heating or on exposure
to ultraviolet light.
Polymerization
Polystyrene was first made by E.Simon in 1839 who at the time
believed he had produced an oxidation product, which he called styrol
oxide. Since that time the polymer ization of styrene has been extensively
studied. In fact a great deal of the work which now enables us to understand
the fundamentals of polymerization was carried out on styrene.
The polymer may be prepared by mass, suspension, solution, and
emulsion methods, the first two being the most important. Mass
polymerization has the advantage of apparent simplicity and gives a
polymer of high clarity and very good electrical insulation characteristics.
There are, however, severe problems due to the exothermic reaction and the
product has a broad molecular weight distribution. Polymerization in
solution reduces the exotherm but may lead to problems of solvent recovery
and solvent hazards. The solvent may also act as a chain transfer agent and
cause a reduction in molecular weight. Suspension polymerization avoids
most of these problems but there is some contamination of the polymer by
water and the suspension agent. Furthermore the polymer must be dried and
aggregated before being sold as pellets suitable for injection moulding and
extrusion. Emulsion polymerization techniques are seldom used with
polystyrene since the large quant ities of soap used seriously affects clarity
and electrical insulation characteristics. This process is therefore used only
for the production of polystyrene latex.
65
Mass polymerization
Continuous mass polymerization units are extensively used for
making polystyrene. Great care is necessary to prevent the heat of reaction
accelerating the polymerization to such an extent that the reaction gets out
of control. The problem is made particularly difficult by the fact that heat
can only be taken away from the points of higher temperature by
conduction because of the very high viscosity of the reacting material, and
also the low thermal conductivities of both styrene and polystyrene.
Most mass processes used today are a variation of that developed
by Wolff in Germany before Word War II. In this process the styrene is
prepolymerized by heating (without initiators) in a prepolymer ization kettle
at 80°C for two days until a 33-35% conversion to polymer is reached (see
Figure 9).
Figure 9 – Tower process for mass polymerization of styrene
The monomer-polymer mixture is then run into a tower about 25 ft
66
high. The tower is fitted with heating and cooling jackets and internally
with a number of heating and cooling coils. The top of the tower is
maintained at a temperature of about 100°C, the centre at about 150°C and
the bottom of the tower at about 180°C. The high bottom temperature not
only ensures a higher conversion but boils off the residual styrene from the
polymer. The base of the tower forms the hopper of an extruder from which
the melt emerges as filaments which are cooled, disintegrated and packed.
That such a process is today commercially important is a measure
of the success of chemical engineers in overcoming heat transfer problems
involved with masses incapable of being stirred. An idea of the extent of
the problem can be gauged from the fact that it takes six hours to cool a
sample of polystyrene from 160°C using a cooling medi um at 15°C when
the heat transfer distance is two inches.
Solution polymerization
By polymerizing styrene in solution many problems associated
with heat transfer and the physical movement of viscous masses are
reduced, these advantages being offset by problems of solvent recovery and
the possibility of chain transfer reactions. In 1955 Distrene Ltd started a
plant at Barry in South Wales for the production of styrene by such a
solution polymerization process and some details have been made
available. The essential details of this proc ess are ind icated b y Figure 10.
Figure 10 – Flow diagram for commerical
solution polymerization of styrene
67
Styrene and solvent are blended together and then pumped to the
top of the first reactor which is divided into three heating zones. In the first
zone the solution is heated to start up the polymerization reaction but
because of the exothermic reaction in the second and third zones of the first
reactor and the three zones of the second reactor Dowtherm cooling coils
are used to take heat out of the system. By the time the reaction mixture
reaches the third reactor the polymerization reaction has started to slow
down and so the reaction mixture is reheated.
From the third reactor the polymer is then run into a devolatilising
(“stripping”) vessel in the form of thin strands. At a temperature of 225°C
the solvent, residual monomer and some very low molecular weight
polymers are removed, condensed and recycled. The polymer is t hen fed to
extruder units, extruded as filaments, granulated, lubricated and stored to
await dispatch.
Suspension polymerization
Suspension polymerization of styrene is widely practiced
commercially. In this process the monomer is suspended in droplets 1/321/64 in. in diameter in a fluid, usually water. The heat transfer distances for
the dissipation of the exotherm are thus reduced to values in the range 1/641/128 in. Removal of heat from the low-viscosity fluid medium presents
little problem. The reaction is initiated by mono mer-soluble initiators such
as benzoyl peroxide.
It is necessary to coat the droplets ef fectively with some suspension
agent, e.g. poly(vinyl alcohol), talc etc., to prevent them cohering. Control
of the type and quantity of suspension agent and of the agitation has a
pronounced effect on the resulting particles. It is not unknown for the
whole of the polymerizing mass to aggregate and settle to the bottom of the
reaction vessel because of such conditions being incorrect. Following
polymerization, unreacted monomer may be removed by steam distillation
and the polymer is washed and dried.
The disadvantages of the suspension process are that about 70% of
the volume of the kettle is taken up by water, the need for a drying stage
which could cause discolouration by degradation and the need to convert
the small spheres formed into a larger shape suitable for handling.
Furthermore, the suspension method cannot easily be converted into a
continuous process.
68
Emulsion polymerization
Because of the large quantities of soap left in the polymer, which
adversely affects clarity, electrical insulation characteristics and problems
in agitation and densification, this process is used only for making latices.
The techniques used are in many respects similar to those for
emulsion polymerized PVC.
Grades Available
In addition to the high-impact (toughened) polystyrene, polystyrene
is available in a number of gr ades. These may conveniently be grouped as
follows:
(1) General purposes grades. In these grades a balance is attempted
to obtain good heat resistance, reasonably high setting-up temperature,
good flow properties and reasonable im pact stren g th.
(2) High molecular weight grades. Polystyrene has little strength if
its molecular weight is below 50000 but increase rapidly, with molecular
weight up to 100000. An increase in molecular weight above 100000 has
little further effect on tensile strength but continues to have an adverse
effect on the ease of flow. Such higher molecular weight grades are
sometimes used where improved impact strength is required without the
loss of clarity that occurs with the toughened polystyrene.
(3) Heat-resistant grades. By reducing the amount of volatile matter
the softening point of the polystyrene can be raised. For example, by
reducing the monomer content from 5% to 0% the softening point may be
raised from 70°C to 100°C. Commercial heat-resisting grades usually have
a softening point about 7°C above the softening point of general purpose
polystyrene.
(4) Easy flow grades. By incorporating an internal lubricant such as
butyl stearate or liquid paraffin, by using a polymer of lower molecular
weight, by careful control of granule shape and size and by lubrication of
the granules with an external lubricant such as zinc stearate, the flow
properties of polystyrene may be improved with little effect on other
properties apart from reduction of up to 10°C in the softening point. These
materials are very useful for thin-wall mouldings, for moulding with
mini mum frozen-in strains or other products where the moulding is rather
intricate. They have not, however, replaced general purpose polystyrene
because of their lower setting-up temperature, which causes a prolongation
of the injection moulding cycle.
69
Structure and properties of polystyrene
CHCH
2
n
Polystyrene has the simple repeating structure shown in Figure 11
and as might be expected from such a substantially linear polymer it is
thermoplastic. As with polypropylene, PVC and other vinyl compounds
there is the possibility of various stereo-regular forms. Because of its
amorphous nature the commercial polymer has for long been regarded as
atactic. As with poly (methyl methacrylate) subsequent work has, however,
indicated that the syndiotactic segments are more frequent than atactic
segments and it appears that this may be a common feature of most freeradical initiated vinyl polymers. The specific position of the benzene ring
is, however, sufficiently random to inhibit crystallization.
Figure 11 – Structure of polystyrene
Because of the chain-stiffening effect of the benzene ring the Tg of
commercial materials are in the range 90-100°C and isotactic polymers
have similar values (approx. 100°C). A consequence of this Tg value plus
the amorphous nature of the polymer is that we have a material that is hard
and transparent at room temperature. Isotactic polystyrene has been known
since 1955 but has not been of commercial importance. Syndiotactic
polystyrene using metallocene catalysis has recently become of commercial
interest. Both stereoregular polymers are crystalline with Tm values of
230°C and 270°C for the isotactic and syndiotactic materials respectively.
They are also somewhat brittle.
Being a hydrocarbon with a solubility parameter of 18.6 MPa it is
dissolved by a number of hydrocarbons with similar solubility parameters,
such as benzene and toluene. The presence of a benzene ring results in
polystyrene having greater reactivity than polyethylene. Characteristic
reactions of a phenyl group such as chlorination, hydrogenation, nitration
and sulphonation can all be performed with polystyrene. Chain rupture and
discolouration are frequently additional effects of such reactions.
The pure hydrocarbon nature of polystyrene gives it excellent
electrical insulation characteristics, as a result of both the fundamentally
good characteristics of the material and to the low water absorption of such
a hydrocarbon polymer. The insulation characteristics are therefore well
maintained in humid conditions.
70
Polystyrene is a hard, rigid transparent thermoplastic which emits a
characteristic metallic ring when dropped. It i s free from odour and taste,
burns with a sooty flame and has a low specific gravity of 1.054. Because
of its low cost, good mouldability, low moisture absorption, good
dimensi onal stability, good electric insulation properties, colourability and
reasonable chemical resistance it is widely used as an injection mouldi ng
and vacuum forming material. Additionally the low thermal conductivity
has been made use of in pol ystyrene foam used for thermal insulation. The
principal limitations of the polymer are its brittleness, inability to withstand
the temperature of boiling water and its mediocre oil resistance.
The mechanical properties of polystyrene depend to some ext ent on
the nature of the polymer (e.g. its molecular weight), on the method of
preparing the sample for testing and on the method of test, as is the case
with all plastics materials.
Amongst the optical properties of polystyrene of importance are its
high transmission of all wavelengths of visible light and its high refractive
index (1.592) which gives it a particularly high “brilliance”.
The electrical insulation characteristics of polystyrene are
extremely good.
The chemical resistance of polystyrene is not generally as good as
that of polyethylene. It is dissolved by a number of hydrocarbons such as
benzene, toluene and ethylbenzene, by chlorinated hydrocarbons such as
carbon tetrachloride, chloroform and o-dichlorobenzene, by a number of
ketones (but not acetone), and esters and by a few oils (e.g. oil of verbena
and ylang ylang oil). Many other materials, in particular acids, alcohols,
oils, cosmetic creams and foodstuffs, will cause crazing and cracking and in
some cases chemical decomposition. The extent of attack will also depend
on such factors as the grade of polystyrene, internal stresses in the
polystyrene product, external stresses to which the part is subjected, the
time and temperature of exposure and the concentration of the reagent.
Furthermore many materials do not attack polystyrene individually but do
so in combination. Cosmetic cream and patent medicines provide many
examples of this synergistic-type behaviour. Technical service bulletins
supplied by the manufacturer provide useful information on the subject of
chemical resistance.
Particular mention should be made of the influence of styrene
monomer. An increase of the residual monomer from 0 to 5% can cause a
30°C reduction in softening point. On the other hand there is a marked