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101
nHOROH + nR
1
OCOOR
1
(ROCO
O
)
n
+ 2nR
1
OH
(I)
nHOROH + nCOCl
2
(ROCO)
n
O
+2nHCl
(II)
the dihydroxybenzenes, hydroquinone and resorcinol, separately with
phosgene in solution in pyridine. The hydroquinone polycarbonate was an
infusible and insoluble crystalline power whereas the resorcinol polymer
was an amorphous material melting at about 200°C. The third
dihydroxybenzene, catechol, yields a cyclic carbonate only, which is not
surprising bearing in mind the proximity of the two hydroxy groups to each
other. By the use of diphenylcarbonate, Bischoff and von Hedenström
prepared similar products by an ester-exchange reaction in 1902.
polycarbonates using ester-interchange reactions. These materials had a
low melting point, were easily hydrolyzed and did not achieve commercial
significance.
these did not fulfil his requirements for a fibre-forming polymer which
were eventually met by the polyamide, nylon 66. As a consequence the
polyesters were discarded by Carothers. However, in 1941 Whinfield and
Dickson working at the Calico Printers Association in England announced
the discovery of a fibre from poly(ethylene terephthalate). Prompted by the
success of such a polymer, Farbenfabriken Bayer initiated a programme in
search of other useful polymers containing aromatic rings in the main
chain. Carbonic acid derivatives were reacted with many dihydroxy
compounds and one of these, bis-phenol A, produced a polymer of
immediate promise.
was being carried out in search of thermally and hydrolytically stable
thermosetting resins. As a by-product from this work the research team at
General Electric also produced polycarbonates from bis-phenol A so that
by 1958 production of bis-phenol A polycarbonates was being carried out
in both Germany and the USA.
Figure 29
Polycarbonates were first prepared by Einhorn in 1898 by reacting
In 1930 Carothers and Natta prepared a number of aliphatic
Carothers also produced a number of aliphatic linear polyesters but
Independently at the General Electric Company in America, work
Today about 75% of the market is held by General Electric and
102
Bayer with their products Lexan and Makrolon respectively. Other
OH
+ (CH2)2C
O
HO C
CH
3
CH
3
OH
+ H2O
2
manufacturers are ANIC (Italy), Taijin Chemical Co., Mitsubishi Edogawa
and Idemitsu Kasei in Japan and, since 1985, Dow (USA) and
Policarbonatos do Brasil (Brazil). Whilst this market is dominated by bisphenol A polycarbonates, recent important developments include alloys
with other thermoplastics, polyester carbonates and silicone-polycarbonate
block copolymers.
Production of intermediates
Polycarbonates may be produced from a wide range of
polyfunctional hydroxy compounds. In practice only the diphenyl
compounds have proved of much interest and the only polycarbonate of
commercial significance is derived from bis-phenol A 2,2-bis-(4hydroxyphenyl) propane.
Bis-phenol A may be produced by the condensation of phenol with
acetone under acidic conditions:
Schnell states that the initial product is isopropenylphenyl which
then reacts with a further molecule to form the bis-phenol A.
At elevated temperatures the second stage of the reaction takes
place in the reverse direction and so reactions are carried out below 70°C.
In order to achieve a high yield, an excess of phenol is employed and the
initial reaction product is a bis-phenol A–Phenol adduct. The bis-phenol A
may be separated from the adduct by crystallization from appropriate
solvents or by distilling off the phenol.
Improvements in the rate of the condensation reaction have been
claimed with the use of co-catalysts such as an ionisable sulphur
compound and by pre-irrad ia t ion w ith ac tinic light.
Unless great care is taken in control of phenol/acetone ratios,
reaction conditions and the use of catalysts, a number of undesirable byproducts may be obtained such as the o-, p- and o-,o- isomers of bis-phenol
A and certain chroman-type structures. Although tolerable when the bisphenol A is used in epoxy resins, these have adverse effects on both
physical properties and the colour of polycarbonate resins.
Residual traces of these impurities must thus be removed by some
103
technique such as recrystallization from chlorobenzene or acqueous
alcohol. The melting point is a useful measure of purity and for
polycarbonate resins the melting point should be in the range 154-157°C
compared with values of 140-150°C for epoxy resin grade bis-phenol A.
Phosgene, employed in both of the main processes, is prepared
commercially from carbon monoxide and chlorine.
Diphenyl carbonate, an alternative source of the carbonate group to
phosgene, may be obtained by reacting phenol with phosgene in acqueous
caustic soda solution, the reaction being accelerated by tertiary amines. The
diphenyl carbonate can be purified by redistillation.
Polymer preparation
There are four possible practical routes to linear polycarbonates:
(1) Ester exchange of dihydroxy compounds with diesters of
carbonic acid and monofunctional aromatic or aliphatic hydroxy
compounds.
(2) Ester exchange of bis-alkyl or bis-aryl carbonates of dihydroxy
compounds with themselves or with other dihydroxy compound.
(3) Reaction of dihydroxy compounds with phosgene in the
presence of acid acceptors.
(4) Reaction of the bis-chlorocarbonic acid ester of dihydroxy
compounds with dihydroxy compounds in the presence of acid acceptors.
Of these four routes the first and third have been studied
intensively, in particular in the preparation of the bis-phenol A
polycarbonates.
Ester exchange
The equation for the ester-exchange reaction (1) is shown in Figure
30.
CH
3
n
Diphenyl Carbonate
O
C O
O
CH
3
C
CH
3
+
O
C
O
HO C
O
+ 2n
n
CH
3
OH
OH
Figure 30
104
In this method the reaction is typically carried out at 180-220°C at
20-30 mmHg pressure until 80-90% of the phenol of condensation has
been removed. The temperature is then gradua lly rai sed to 2 90-300°C and
the pressure reduced to 1 mmHg or below. The melt viscosity increases
considerably during this period and the reaction is stopped while the
material can still be forced out of the kettle by an inert gas.
The high melt viscosity limits the molecular weights obtainable
and although number average molecular weights of 50000 can be obtained
it is difficult to attain values of above 30000 without special equipment.
Because bis-phenol A is somewhat unstable at elevated
temperature it is desirable to work with an excess of diphenyl carbonate so
that the bis-phenol A is rapidly used up. The reaction may be conveniently
carried out using twice or more than twice the theoretical quantity of
diphenyl carbonate so that the initial reaction product is the bis(phenyl
carbonate) of bis-phenol A (Figure 31 (a)).
Polymerization then proceeds by splitting out of diphenyl
carbonate to give the polycarbonate resin (Figure 31 (b)).
CH
3
2
O C O
O
O
O C
O
C
O
O
O
HO C
+
CH
3
C
CH
3
CH
3
C
CH
3
CH
+
3
O
OH
O
C
O
O
C
O
(a)
O
(b)
Figure 31
This variation has the obvious disadvantage that the less volatile
diphenyl carbonate is more difficult to remove than phenol.
A number of basic materials such as hydroxides, hydrides and
amides of alkaline and alkaline earth metals and met al oxides such as zinc
oxide and antimony oxide are useful catalysts for the reaction. Acid esterexchange catalysts such as boric acid, p-toluene sulphonic acid and zinc
chloride are less effective. Catalyst systems are not essential when diaryl
carbonates are used as carbonate sources and do in fact cause problems in
their subsequent removal.
Although of importance in the early days of polycarbonate
105
production the transesterification process was virtually abandoned in the
HO C
CH
3
CH
3
OH
n
+ nCOCl
2
O
C
CH
3
CH
3
O
C
O
+ 2nHCl
n
mid-1970s. This was because although it avoids the use of solvents (as
required in the phosgenation process described below) it has the
disadvantages of special equipment being required for the high reaction
temperatures, good vacuum systems in order to work at very low pressures,
the need to handle highly viscous melts and a tenden cy to give a yell ow
colour due to high temperature side reactions. Interestingly in the mid1990s improvements were made to the process details and a plant using the
transesterification process has been reported to be in operation in Japan
(GE/Mitsui)
Phosgenation process
High molecular weight polycarbonates may be produced without
undue difficulty by the phosgenation process. The basic reaction is as
shown in Figure 32.
Figure 32
For this reaction to proceed it is obviously necessary to remove the
hydrochloric acid formed, preferably by means of hydrohalide acceptor.
The attractive possibility of dissolving the bis-phenol A in caustic
soda solution and bubbling phosgene into it is not practical since the
polymer is insoluble in the caustic soda and precipitates out at a low and
variable molecular weight.
Greater success has been achieved with organic solvents which are
also hydrohalide acceptors, pyridine being a specific example.
Typically in such a process the bis-phenol A is dissolved in about
ten times its weight of pyridine and vigorously stirred at 25-35°C.
Phosgene is then bubbled into the solution and in a few minutes the
pyridine hydrochloride starts to precipitate. As polymer is formed the
viscosity of the solution increases and eventually becomes too great for
stirring. The polymer is then recovered by the addition of a solvent such as
methyl alcohol which dissolves the pyridine hydrochloride but precipitates
the polymer.
A variation of this process involves the formation of a preformed
106
pyridine-phosgene complex. Polymerization will then be effected by
adding a solution of bis-phenol A.
Because of the cost of pyridine the phosgenation process may be
carried out with a mixture of pyridine and a non-hydrohalide-accepting
solvent for the polymer and the growing complexes. Suitable solvents
include methylene dichloride, tetrachlorethane and chloroform. Although
unsubstituted aromatic hydrocarbons may dissolve the solvent they are not
effective solvents for the acid chloride-pyridine complexes.
Today the most important process is that of interfacial
polymerization. In a typical process the disodium salt of bisphenol A in an
alkaline aqueous solution or suspension is reacted with phosgene in the
presence of an inert organic solvent such as methylene chloride,
chlorobenzene, tetrahydrofuran or dioxane. While initially a solvent for the
phosgene it dissolves arylchloroformates and oligocarbonates formed as
the polycondensation reaction proceeds at the interface or just inside the
aqueous phase. The arylchloroformates and oligocarbonates then condense
to form a high molecular weight polymer in the presence of a catalyst such
as triethylamine or tri-n-propylamine. Reaction temperatures are in the
range 10-35°C. The polymer is recovered by washing the organic phase
with water, neutralization of the caustic soda and either precipitation of the
polymer by a non-solvent or evaporation of the solvent by thorough
washing. The molecular weight is controlled by the use of monofunctional
phenols, particularly branched phenols such as iso-octyl phenol.
While the interfacial polycondensation-by-phosgenation process
has hazards involving the use of phosgene and solvents, the need for
solvent recovery and the requirement for purifying and densifying the
polymer it does have the advantage of being able to produce colourless
high molecular weight polymers using fairly simple processing equipment
under moderate preparation conditions and as indicated above is the most
important process for making polycarbonates on a commercial scale.
Relation of structure and properties
A study of the molecular structure of bis-phenol A polycarbonates
enables one to make fairly accurate predictions of the bulk properties of
the polymer. The relevant factors to be considered are:
(1) The molecule has a symmetrical structure and therefore
questions of stereospecificity do not arise.
(2) The carbonate groups are polar but separated by aromatic
hydrocarbon groups.
107
(3) The presence of benzene rings in the chain restricts flexibility
Crystal type
rhombic
of the molecule.
(4) The repeating unit of the molecule is quite long.
Because of its regularity it would be expected that the polymer
would be capable of crystallization. In practice, however, the X-ray pattern
characteristics of crystalline polymer is absent in conventionally fabricated
samples. On the other hand films which have been prepared by slow
evaporation from solvent or by heating for several days at 180°C do exhibit
both haziness and the characteristic X-ray diagram. The amount of
crystallization and the size of the crystallite structures decrease with an
increase in the molecular weight of the polymer. These effects are no doubt
associated with both the stiffness of the molecule and its long repeat unit.
The crystalline structure of bis-phenol A polymers has been
thoroughly studied by Prietschk and some of the data he obtained on the
crystal structure are summarized in Table 5.
Table 5 – Crystal structure date of bis-phenol A polycarbonates
cell as determined by Prietschk. It will be seen that in the crystalline zone
the molecules pack in such a way that the methyl groups attached to the
pivotal carbon atom extend toward the back of the carbonate group of the
neighbouring chain.
Cell constants
No. of units in elementary cell
Crystal density
Macroscopic density
a = 11.9, b = 10.1, c = 21.5
8
1.3 g/cm
1.2 g/cm
3
3
Figure 33 shows the disposition of the molecules in the elementary
108
Figure 33 - Unit cell diagrams for polycarbonate based on bis-phenol A
The rigid molecular backbone of the bis-phenol A polycarbonates
leads to a high melting temperature (Tm = 225-250°C) and glass transition
temperature (Tg = 145°C). That this molecular rigidity and not the polar
interchain attraction of the ester group or of the benzene rings is the
predominating influence may be inferred by comparison of the
polycarbonate resin with poly(ethylene terephthalate). This latter polymer
has similar chemical groups but has much lower values for Tg and Tm.
The limited degree of crystallinity is another factor contributing to
the toughness of the polymer. Interchain attraction, particularly in the
crystalline zones, contributes to a high yield strength but too much
crystallinity would not only reduce free volume but also lead to weak
planes at highly stressed spherulite interfaces. Again it is not surprising to
find that highly crystalline samples prepared by heating for prolonged
periods above their Tg or by precipating from solutions are quite brittle.
Aging of samples also leads to brittleness but at the present time
there appears to be no information on how this affects either free volume or
the secondary transitions.
Both the chemical solubility and the electrical properties are
consistent with those expected of a lightly polar polymer, whilst reactivity
is consistent with that of a polymer containing hydrolysable carbonate ester
109
linkages partially protected by aromatic hydrocarbon groupings. The
influence of these factors on specific properties is amplified in subsequent
sections.
Variations in commercial grades
The range of polycarbonates commercially available has increased
greatly in recent years. The main differences between these grades are
largely due to:
(1) Differences in molecular weight.
(2) The presence or otherwise of a second polyhydroxy compound.
(3) Differences in additives.
In the usual range of moulding and extrusion materials an increase
in molecular weight leads to comparatively small improvements in such
properties as tensile strength. It does, however, lead to the usual steep
increase in melt viscosity and to an increase in impact strength. Polymers
with number average molecular weights below 20000 are generally
unsuitable as plastics whilst those with molecular weights above 50000
(particularly those in range of Mn = ca 70000) are mainly processed into
film by solution casting methods.
Easy flow grades with Mn values in the range 18000-32000 are
particularly useful for thin wall mouldings and where low injection
moulding cycle times are of particular importance but as the molecular
weight is reduced toward the lower end of this range the polymers become
more brittle and more liable to stress cracking. The molecular weight is
controlled by the use of monofunctional additives such as iso-octyl
phenols to give branched alkyl phenyl end groups.
Structurally viscous grades are based on branched polymers
(branching being effected by the use of tri- or higher functional phenols).
These polymers exhibit a sharp decrease in viscosity with increasing shear
rate which makes them particularly suitable for extrusion and blow
moulding and also in reducing drip in case of fire.
Flame retardant grades usually employ additives, e.g. sodium
2,4,5-trichlorobenzene sulphonate, sometimes in conjunction with an anti-
dripping agent which cross-links the polymer as it burns thus reducing the
tendency to drip.
UV-stabilized grades exhibit reduced tendency to yellowing by the
use of stabilizers such as benzophenone and benzotriazole compounds.
Whilst remaining speciality materials, several copolymers have
been marketed over the years in order to enhance certain specific
110
properties whilst retaining the general characteristics of polycarbonates.
O
O
H
3
C
H
3
C
CH
3
C
O
H
There are also a number of homo polymers in which the bis-phenol A has
been replaced by another bis-phenol compound. The most important of
these are summarized below.
1) To enhance flame retardancy without use of additives, 2,2-bis-
(3,5-dibromo-4-hydroxyphenyl)propane (tetrabromobis-phenol A) has
been used in copolymers with bis-phenol A.
2) To enhance refractive index, copolymers of bis-phenol S
(thiodiphenol) with bis-phenol A have been employed. The refractive
index of the copolymer n0 is 1.610 compared with 1.590 for the
homopolymer.
3) To reduce melt viscosity an aliphatic dicarboxylic acid may be
used to partially replace the carbonic acid derivative. Easy-flow grades
with a melt flow rate of 80 are now available in order to meet the stringent
flow requirements involved in compact disc production. Such an
improvement in flow rate is, however, at the expense of heat distortion
temperature.
4) To produce resins for paints and for electrical insulation, film
cast from solution copolymers based on bis-phenol A and bis-phenol Z
(1,1-bis-(4-hydroxyphenyl)cyclohexane) have been used.
5) To enhance the resistance to heat softening bis-phenol A is
substituted by a stiffer molecule. In 1979 a polycarbonate in which the bisphenol A was replaced by tetramethylbis-phenol A was test marketed. This
material had a Vicat softening point of 196°C, excellent resistance to
hydrolysis, excellent resistance to tracking and a low density of about 1.1
g/cm3. Such improvements were obtained at the expense of impact strength
and resistance to stress cracking.
Another approach is to copolymerize bis-phenol A with the bisphenol of trimethylcyclohexanone (bis-phenol TMC); commercial
materials were introduced in 1992.
Bis-phenol TMC segment in copolymer