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STYRENE POLYMERS
Introduction
Polystyrene (PS), the parent of the styrene plastics family, is a high molecular
weight, linear polymer. Its chemical formula [ CH(C6 H5 )CH2 ]n, where n (which
for commercial uses) is between 800 and 1400, tells little of its properties. The
main commercial form of PS (atactic PS) is amorphous and hence possesses high
transparency. The polymer chain stiffening effect of the pendant phenyl groups
raises the glass-transition temperature (T g ) to slightly over 100◦ C. Therefore,
under ambient conditions, the polymer is a clear glass, whereas above the T g it
becomes a viscous liquid which can be easily fabricated, with only slight degradation, by extrusion or injection-molding techniques. It is this ease with which
PS can be converted into useful articles that accounts for the very high volume
(>20 billion pounds per year) used in world commerce. Even though crude oil is
the source of the polymer, the energy savings and environmental impact accrued
during fabrication and use, compared to alternative materials, more than offsets
the short life of many PS articles (1).
Commercial manufacture of PS in North America began in 1938 by The Dow
Chemical Co. The polymerization process involved loading 10-gal cans with neat
styrene monomer and immersing the can in a liquid bath heated at progressively
higher temperatures over several days until the monomer conversion reached
99%. The solid polymer was removed from the cans and any rust spots chipped
off the cylinders of polymer with a hatchet (Fig. 1). The cylinders of polymer were
finally ground into a powder and packaged for shipment to customers.
Within a few years, Dow replaced the “can process” with the more economical continuous bulk polymerization process still used today. Generally, the key
problems associated with manufacture of the polymer are removal of the heat
Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.
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Fig. 1. Dow Chemical worker chipping rust from cylinders of PS made using the “can
process” (ca 1940).
of polymerization and pumping of highly viscous solutions. Conversion of the
monomer to the polymer is energetically very favorable and occurs spontaneously
on heating without the addition of initiators or catalysts. Because it is a continuous polymerization process, material-handling problems are minimized during
manufacture. By almost any standard, the polymer produced is highly pure and
is usually greater than 99 wt% PS; however, for particular applications processing aids are often deliberately added to the polymer. Methods for improving the
toughness, solvent resistance, and upper use temperature have been developed.
Addition of butadiene-based rubbers increases impact resistance, and copolymerization of styrene with comonomers such as acrylonitrile or maleic anhydride produces heat- and solvent-resistant plastics. Uses for these plastics are extensive.
Packaging applications, eg, disposable cups, meat and food trays, and egg cartons,
are among the largest area of use for styrene plastics. Rigid foam insulation in
various forms is being used increasingly in the construction industry, and modified styrene plastics are replacing steel or aluminum parts in automobiles. These
applications result in energy savings beyond the initial investment in crude oil.
The cost of achieving a given property, eg, impact strength, is among the lowest
for styrene plastics as compared to other competitive materials.
Properties
The general mechanical properties of styrene polymers are given in Table 1. Considerable differences in performance can be achieved by using the various styrene
plastics. Within each group, additional variation is expected. In choosing an appropriate resin for a given application, other properties and polymer behavior
during fabrication must be considered. These factors depend on the combination
of inherent polymer properties, the fabrication technique, and the devices, eg, a
mold used for obtaining the final object. Accordingly, consideration must be given
to such factors as the surface appearance of the part and the development of
anisotropy, and its effect on mechanical strength, ie, long-term resistance of the
molding to external strain.
Table 1. Mechanical Properties of Injection-Molded Specimens of Main Classes of Styrene-Based Plasticsa
Property
Poly
(styrene-coPolystyrene acrylonitrile)
(PS)
(SAN)b
249
CAS registry
[9003-53-6]
number
Specific gravity
1.05
Vicat softening
96
point, ◦ C
Tensile yield,
42.0
MPae
Elongation at
1.8
rupture, %
Modulus, MPae
3170
Impact strength
21
(notched Izold),
J/m f
Dart-drop impact Very low
strength
Relative ease of
Excellent
fabrication
a Ref.
High
impact
PS
(HIPS)d
[9003-54-7]
HIPS
Type 1
Type 2
[9003-53-6]
Standard
ABS
Super
ABS
[9003-56-9]
1.08
107
1.20
103
1.05
103
1.05
95
1.05
99
1.05
108
1.04
103
1.04
108
68.9
131
39.6
29.6
31.0
53.8
41.4
34.5
3.5
1.5
15
58
55
10
20
60
2690
96
2140
134
2620
193
2620
187
2070
267
1790
428
3790
21
7580
80
Very low
Medium high
Low
Excellent
Poor
Excellent
2
wt% acrylonitrile.
c 20% glass fibers.
d Medium molecular weight.
e To convert MPa to psi, multiply by 145.
f To convert J/m to ft·lbf/in., divide by 53.38.
b 24
Glassfilled
PSc
Acrylonitrile–
butadiene–
styrene
terpolymer
(ABS)d
Medium high Medium high
Excellent
Excellent
High
Very high
Very high
Good
Good
Medium good
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Fig. 2. PS tensile strength vs M w (5). To convert Mpa to psi, multiply by 145.
Physical. An extensive compilation of physical properties of PS is given
in References 2 and 3. In general, a polymer must have a weight-average molecular weight (M w ) about 10 times higher than its chain entanglement molecular
weight (M e ) to have optimal strength. Below M e , the strength of a polymer changes
rapidly with M w . However, at about 10 times M e , the strength reaches a plateau
region (Fig. 2). For PS, the M e is 18,100 (4). Thus, PS having a M w < 150,000 is
generally too brittle to be useful. This indicates why no general-purpose molding
and extrusion grades of PS having M w < 180,000 are sold commercially.
Stress–Strain. The strain energy, derived from the area under the stress–
strain curve, is considered to indicate the level of strength of a polymer. High
impact PS (HIPS) has a higher strain energy than an acrylonitrile–butadiene–
styrene (ABS) plastic, as shown in Figure 3. However, based on different impacttesting techniques, ABS materials are generally more ductile than HIPS materials
(6,7). The failure of the stress–strain curve to reflect this ductility can be related to
the fact that ABS polymers tend to show only localized flow or necking tendency
at low rates of extension and, therefore, fail at low elongation. HIPS extends
uniformly during such tests, and the test specimen whitens over all of its length
and extends well beyond the yield elongation. However, at higher testing speeds,
ABS polymers deform more uniformly and give high elongations.
Tensile strengths of styrene polymers vary with temperature. Increased temperature lowers the strength. However, tensile modulus in the temperature region
of most tests (−40–50◦ C) is affected only slightly. The elongations of PS and styrene
copolymers do not vary much with temperature (−40–50◦ C), but the elongation
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251
Fig. 3. Stress–strain curves for styrene-based plastics. To convert Mpa to psi, multiply
by 145.
of rubber-modified polymers first increases with increasing temperature but ultimately decreases at high temperatures.
The molecular orientation of the polymer in a fabricated specimen can significantly alter the stress–strain data as compared with the data obtained for an
isotropic specimen, eg, one obtained by compression molding. For example, tensile strengths as high as 120 MPa (18,000 psi) have been reported for PS films
and fibers (8). Polystyrene tensile strengths below 14 MPa (2000 psi) have been
obtained in the direction perpendicular to the flow.
Creep, Stress Relaxation, and Fatigue. The long-term engineering tests
on plastics in extended use environments and temperatures are required for predicting the overall performance of a polymer in a given application. Creep tests
involve the measurement of deformation as a function of time at a constant stress
or load. For styrene-based plastics, many such studies have been carried out (9,10).
Creep curves for styrene and its copolymers at room temperature show low elongation with only small variation with stress, whereas the rubber-modified polymers
exhibit a low elongation region, followed by crazing and increasing elongation,
usually to ca 20%, before failure (Fig. 4).
Creep tests are ideally suited for the measurement of long-term polymer
properties in aggressive environments. Both the time to failure and the ultimate
elongation in such creep tests tend to be reduced. Another test to determine plastic behavior in a corrosive atmosphere is a prestressed creep test in which the
specimens are prestressed at different loads, which are lower than the creep load,
before the final creep test (11).
Stress-relaxation measurements, where stress decay is measured as a function of time at a constant strain, have also been used extensively to predict the
long-term behavior of styrene-based plastics (9,12). These tests have also been
adapted to measurements in aggressive environments (13). Stress-relaxation measurements are further used to obtain modulus data over a wide temperature range
(14).
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Fig. 4. Typical creep behavior for rubber-modified styrene polymers.
Fatigue is another property which is of considerable interest to the design
engineer. Cyclic deflections of a predetermined amplitude, short of giving immediate failure, are applied to the specimen, and the number of cycles to failure is
recorded. In addition to mechanically induced periodic stresses, fatigue failure
can be studied when developing cyclic stresses by fluctuating the temperature.
Fatigue in polymers has been reviewed (15). Detailed theory and practice of
fatigue testing are covered. Fatigue tests are carried out for two main reasons: to
learn the inherent fatigue resistance of the material and to study the relationship
between specimen design and fatigue failure. Fatigue tests are carried out both
in air and in aggressive environments (16).
Melt Properties. The melt properties of PS at temperatures between 120
and 260◦ C are very important because it is in this temperature range that it is
extruded to make sheets, foams, and films, or is molded into parts. Generally, it
is desired to make parts having high strength from materials having low melt
viscosity for easy melt processing. However, increased polymer molecular weight
increases both strength and melt viscosity. The melt viscosity of PS can be decreased to improve its melt processability by the addition of a plasticizer such
as mineral oil. However, the addition of a plasticizer has a penalty, ie, the heatdistortion temperature is lowered. In applications where heat resistance is very
important, melt processability can be influenced, without a significant effect on
heat resistance, by control of the polydispersity (17), by branching (18), or by the
introduction of pendant ionic groups, eg, sodium sulfonate (19,20).
Impact Strength. When they fail, thermoplastic polymers typically dissipate energy either through microscopic shearing and/or crazing. Because of its
inherently high entanglement molecular weight (19,000 g/mol), PS under plain
strain conditions (thick parts) does not form shear bands. Thus, pure PS fails under tensile stress because of the formation of crazes (regions of microfibrils that
bridge the top and bottom of a precursor crack) that rapidly develop into cracks
and propagate catastrophically. Essentially, PS under plane strain conditions has
a high yield stress and a low break stress. There can be very high local strains
in the region of a craze, but very little material is deformed and little energy is
dissipated. The crazes usually initiate at imperfections, such as surface scratches
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or impurities in the bulk, and so as soon as one craze starts to form, the stress
is concentrated at that site, the craze grows into a crack, and failure soon follows
at very small macroscopic strain. Toughening PS involves finding ways to force
it to form more crazes, thus dissipating more energy, or forcing it to deform by
microscopic shear banding (21–24).
There are basically two different approaches used, and two different types of
materials employed to obtain toughened PS. The first involves the use of dispersed
rubber particles, which could have a variety of compositions including polybutadiene (PB) or various elastic block copolymers, in a continuous PS matrix. The
second involves the use of styrenic block copolymers, or blends of block copolymers with PS homopolymer, where the block copolymer forms the continuous
phase.
Dispersed rubber particles are the basis of toughening in HIPS. The toughening mechanism is quite complex, and the various ways in which dispersed rubber
particles toughen PS have been discussed in detail (21–26). There are several
ways in which the dispersed particles result in increased energy absorption during failure, but just two dominate. First, rubber particles result in the formation
of multiple crazes, and thus the absorption of more energy. A rubber particle (assumed to be approximately spherical in shape) has a much lower modulus than
PS. When a macroscopic tensile stress is applied to a medium containing soft,
dispersed spheres, there is a stress concentration at the equator of the spheres,
where the poles are in the direction of the applied stress. The stress at a distance
from the particle is proportional to the particle size. It has been pointed out that
for crazes to develop, a stress greater than that at the tip of a craze must extend
at least three craze fibril dimensions into the matrix (22). Thus, while all particles
result in stress concentration, small particles will not induce crazing because the
critical stress does not extend far enough into the matrix. For HIPS, in which the
rubber particles are composed of PB with PS occlusions, the minimum particle size
to induce crazing is about 0.8 µm (22). Optimum toughening occurs with rubber
particles from 1.0 to 2.0 µm.
The second major way in which rubber particles toughen PS is by allowing the
formation of shear bands. Depending on the rubber particle size and volume fraction, the ligaments of PS between the rubber particles can become small enough
(about 3.0 µm) so that if the rubber particles cavitate (due to triaxial stresses imposed by tensile deformation) the PS ligaments are under stress rather than strain
(26). Under these conditions, the PS can shear band, thus absorbing considerable
amounts of energy. Recent experimental work has emphasized the importance of
shearing in addition to crazing in contributing to toughness in rubber-modified
PS (24). Unfortunately, there are minimum rubber volume fractions and particle sizes for this mechanism to be effective, and those restrictions can result in
significant light scattering and opacity as well.
The particle size restrictions for toughening PS depend on the mechanical
properties of the rubber particle. For a HIPS rubber particle, with a Young’s modulus of 333 MPa, the minimum size is 0.8 mm, while for a pure PB particle, with
a Young’s modulus of 47 MPa, the critical size is 0.44 mm (27). The stiffer the particle, the larger it must be. HIPS systems with bimodal particle size distributions
can also be effective in toughening (28), but the bimodal particle size distribution
is generally detrimental to optical clarity.
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Dispersed rubber particles make other, less significant, contributions to the
toughening of PS. Provided they are reasonably well adhered to the matrix and
are somewhat tough, they can effectively terminate crazes and retard crazes from
developing into cracks. The act of cavitation, either internal to the particle or at the
particle–matrix interface, can absorb some energy, but an insignificant amount.
Some adhesion of the rubber particle to the PS matrix is needed, but the exact
role is not clear (23,25). In HIPS, the needed adhesion is provided by the PS that
is grafted to the rubber particles during the polymerization process.
In summary, in order for dispersed rubber particles to toughen PS, they must
be above a critical size, which is of the order of 0.8 µm, depending on the nature
of the rubber. Higher concentrations of rubber particles will increase toughness
by enabling more shear banding. Some adhesion between the rubber and matrix
is also required.
HIPS materials can have a glossy or a dull surface appearance. This surface appearance is a function of surface roughness, which is caused by how much
the rubber particles disrupt the surface regularity. The rubber particles near the
surface can disrupt the surface by causing either depressions or elevations. These
irregularities are caused not only by the nature of the rubber particles (eg, size and
shape) but also by processing conditions. For example, during injection molding,
the surface of the polymer is pressed against a very smooth polished mold surface
and quenched locking in the smooth surface. However, when HIPS is extruded
into sheets and thermoformed into parts, it is allowed to cool slowly giving the
rubber particles near the surface time to relax. Polybutadiene rubber particles
shrink upon cooling more than the PS matrix, thus causing depressions (29).
Material Types
General-Purpose (not rubber-toughened) PS (GPPS). GPPS is a high
molecular weight (M w = 2–3 × 105 ), crystal-clear thermoplastic that is hard, rigid,
and free of odor and taste. Its ease of heat fabrication, thermal stability, low specific
gravity, and low cost results in moldings, extrusions, and films of very low unit
cost. In addition, PS materials have excellent thermal and electrical properties,
which make them useful as low cost insulating materials.
Commercial PSs are normally rather pure polymers. The amount of styrene,
ethylbenzene, styrene dimers and trimers, and other hydrocarbons is minimized
by effective devolatilization or by the use of chemical initiators (30). Polystyrenes
with low overall volatile content have relatively high heat-deformation temperatures. The very low content of monomer and other solvents, eg, ethylbenzene,
in PS is desirable in the packaging of food. The negligible level of extraction of
organic materials from PS is of crucial importance in this application because of
taste and odor issues.
When additional lubricants, eg, mineral oil and butyl stearate, are added to
PS, easy-flow materials are produced. Improved flow is usually achieved at the cost
of lowering the heat-deformation temperature. Stiff-flow PS has a high molecular
weight and a low volatile level and is useful for extrusion applications. Typical
levels of residuals in PS grades are listed in Table 2. Differences in molecular
weight distribution are illustrated in Figure 5.
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Table 2. Residuals in Typical Polystyrene, wt%
Grade
Polystyrene
Styrene
Ethylbenzene
Styrene dimer
Styrene trimer
Extrusion
Injection molding
0.04
0.02
0.04
0.25
0.1
0.1
0.1
0.8
Fig. 5. Molecular weight distribution curves for representative PSs.
Specialty PS.
Ionomers. Polystyrene ionomers are typically prepared by copolymerizing
styrene with an acid functional monomer (eg, acrylic acid) or by sulfonation of PS
followed by neutralization of the pendant acid groups with monovalent or divalent alkali metals. The introduction of ionic groups into PS leads to significant
modification of both solid state and melt properties. The introduction of ionic interactions in PS leads to increasing T g , rubbery modulus, and melt viscosity (20).
For the sodium salt of sulfonated PS, it has been shown that the mode of deformation changes from crazing to shear deformation as the ion content increases
(31,32).
Tactic PS. Isotactic (iPS) and syndiotactic (sPS) PSs can be obtained
by the polymerization of styrene with stereospecific catalysts of the Ziegler–
Natta-type. Aluminum-activated TiCl3 yields iPS while soluble Ti complexes
[eg, (η5 -C5 H5 )TiCl3 ] in combination with a partially hydrolyzed alkylaluminum
[eg, methylalumoxane] yield sPS. The discovery of the sPS catalyst system was
first reported in 1986 (33). As a result of the regular tactic structure, both iPS
(phenyl groups cis) and sPS (phenyl groups alternating trans) are highly crystalline. Samples of iPS quenched from the melt are amorphous, but become
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crystalline if annealed for some time at a temperature slightly below the crystalline melting point. The rate of crystallization is relatively slow compared to
sPS and with other crystallizable polymers, eg, polyethylene or polypropylene.
This slow rate of crystallization is what has kept iPS from becoming a commercially important polymer even though it has been known for over 40 years. sPS, on
the other hand, crystallizes rapidly from the melt and is currently in the process
of being developed for commercial use in Japan by Idemitsu Petrochemical Co.
and in the United States and Europe by The Dow Chemical Co. In the amorphous
state, the properties of iPS and sPS are very similar to those of conventional atactic PS. Crystalline iPS has a melting temperature of around 240◦ C, while sPS
melts at about 270◦ C (34). In the crystalline state, both iPS and sPS are opaque
and are insoluble in most common organic solvents.
Stabilized PS. Stabilized PSs are materials with added stabilizers, eg, uv
screening agents and antioxidants. Early stabilization systems for PS included
alkanolamines and methyl salicylate (35). In recent years, improved stabilizing
systems have been developed; these involve a uv-radiation absorber, eg, Tinuvin
P (Ciba Specialty Chemicals) with a phenolic antioxidant. Iron as a contaminant,
even at a very low concentration, can cause color formation during fabrication;
however, this color formation can be appreciably retarded by using tridecyl phosphite as a costabilizer with the uv-radiation absorber and the antioxidant (36).
Rubber-modified styrene polymers are heat-stabilized with nonstaining rubber
antioxidants, eg, Irganox 1076 (Ciba). Typically, stabilizer formulations for PS
are designed by trial and error. However, recently a predictive model was developed for PS photodegradation allowing the prediction of weatherability of PS
containing a certain concentration of a uv absorber (37).
Ignition-Resistant PS. Polymers containing flame retardants have been
developed. The addition of flame retardants does not make a polymer noncombustible, but rather increases its resistance to ignition and reduces the rate of
burning with minor fire sources. The primary commercial developments are in the
areas of PS foams (see FOAMED PLASTICS) and television and computer housings.
Both inorganic (hydrated aluminum oxide, antimony oxide) and organic (alkyl
and aryl phosphates) additives have been used (38). Synergistic effects between
halogen compounds and free-radical initiators have been reported (39). Several
new halogenated compounds and corrosion inhibitors are effective additives (40)
(see CORROSION AND CORROSION INHIBITORS). The polymer manufacturer’s recommendations with regard to maximum fabrication temperature should be carefully
observed to avoid discoloration of the molded part or corrosion of the mold or the
machine.
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Antistatic PS. Additives or coatings are utilized to minimize primarily dustcollecting problems in storage (see ANTISTATIC AGENTS). Large lists of commercial
antistatic additives have been published (38). For styrene-based polymers, alkyl
and/or aryl amines, amides, quaternary ammonium compounds, anionics, etc are
used (see ADDITIVES).
Branched PS. Random-chain branching is found, to some extent, in all
commercial free-radically produced polymers. Branching takes place during manufacture by chain transfer to polymer and can also take place in the polymer after
manufacture by exposure to uv or other forms of radiation. Chain branching in
some polymers is known to improve certain properties and is practiced commercially (eg, polyethylene and polycarbonate). In PS, many types of branch structures
have been synthesized and the effect of branch structure on properties studied.
Some of the branch structures possible in PS are shown in Figure 6.
Most of the recent studies of branched PS have focused on new synthetic
methodologies (41–52) and on rheological properties (53–56).
Anionic polymerization has been the polymerization mechanism most widely
used to make and study branched PSs. This is likely due to the ability to control the
termination step, which is required for making many of the branch architectures
shown in Figure 6. However, all large-scale PS production plants utilize freeradical polymerization chemistry. This is likely due to the cost associated with
monomer purification needed for the anionic chemistry (57).
The PS business is extremely cost competitive. Oftentimes, an improvement
in properties which is possible with a modification that would add a few percentage to the manufacturing cost is not practical because the customer will not pay
the increased cost. Any property advantage must be very significant before the
customer will pay more. It is likely that branching in PS may lead to improved
performance of PS in certain applications. However, it is uncertain whether the
Fig. 6. Several possible PS branch architectures.
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Fig. 7. Two Continuous stirred tank reactors (CSTR) and a continuous plug flow reactor
(CPFR) configuration utilized for continuous mass polymerization of styrene.
increased performance will be significant enough to allow an increase in the selling
price of the resin.
Since all commercial PS is currently manufactured using continuous
free-radical bulk polymerization, most industrial research aimed at producing
branched PS have been limited to free-radical chemistry. In general, the type
of branch architecture that is possible using free-radical chemistry is limited
to random-branched structures. Three reactor types are used commercially for
continuous free-radical bulk polymerization of styrene (Fig. 7). These three reactor types are characterized as being either plug-flow or backmixed. Continuous
plug-flow reactors (CPFR) typically have excellent radial mixing but virtually no
backmixing, unless they are recirculated. These reactors are usually described as
stratified agitated tower reactors. Continuous stirred tank reactors (CSTR), on the
other hand, have high degrees of backmixing. They are usually single-staged and
operated isothermally and at constant monomer conversion. CPFR-type reactors,
on the other hand, are multistaged, having a temperature profile of typically 100–
170◦ C. Two general configurations of CSTR reactors are utilized commercially, ie,
recirculated coil and ebullient.
Typically, a few percentage by weight of a chain-transfer solvent (eg, ethylbenzene) is utilized during continuous free-radical bulk polymerization. The addition of a chain-transfer solvent to the polymerization decreases polymer molecular weight and therefore overall plant capacity when making high molecular
weight products. However, the presence of some chain-transfer solvent is essential in keeping the polymerization reactor from becoming eventually fouled with
gel and insoluble cross-linked polymer. The mechanism of buildup during continuous free-radical bulk polymerization without a chain-transfer solvent is likely
chain transfer to polymer.
Peroxide initiators typically utilized for the manufacture of PS form t-butoxy
(58) and/or acyloxy radical intermediates (59). These radicals have a strong
propensity to abstract H-atoms. This propensity has found utility when making
HIPS. For example, the use of initiators which generate t-butoxy radicals increases
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Fig. 8. Formation of t-butoxy radicals and their use to measure H-abstraction.
the level of grafting of PS onto PB rubber. The increase in grafting is generally
thought to be due to H-abstraction from the rubber backbone by t-butoxy radicals (60). This raises questions regarding the extent of H-abstraction from the
PS backbone during polymerization; that is, the use of initiators that generate
potent H-abstracting radicals may increase the extent of long-chain branching
during PS manufacture. This question has been investigated by decomposing bis(tbutylperoxy)oxalate in benzene solutions of PS (61,62). Bis(t-butylperoxy)oxalate
decomposes upon heating to form two t-butoxy radicals and carbon dioxide. Once
the t-butoxy radicals are formed they either abstract an H-atom or decompose to
form acetone and a methyl radical (Fig. 8).
The extent of H-abstraction was determined by measuring the ratio of tbutyl alcohol (TBA)/acetone produced. Using cumene as a model for PS, a high
level of H-abstraction was observed. However, PS showed a very low level of Habstraction, which decreased further as the degree of polymerization (DP) of the
PS was increased (Fig. 9). This was explained by the coil configuration of the PS
chains restricting access of the t-butoxy radicals to the labile tertiary benzylic
H-atoms on the PS backbone.
Fig. 9. H-abstraction as the ratio of t-butyl alcohol (TBA) to acetone (Ac) vs DP of PS
(61,62).
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The long-chain branching that takes place during PS manufacture in using
continuous free-radical bulk polymerization reactors is extremely small. However,
manufacturers of PS for film applications, where gel particles are a big problem,
constantly monitor their product for gel. If the level of gel gets too high, the polymerization reactor must be cleaned to remove the coating of gel on the reactor
walls. The mechanism of gel formation is not certain, but it is generally believed
that a polymer layer forms on metal surfaces inside the polymerization reactor
(63). The layer is dynamic, but the polymer chains that are in the layer are exposed to a free-radical environment for a longer period of time than are polymer
chains in solution. The longer PS is exposed to free radicals, the more backbone Habstraction can take place. Furthermore, the concentration of PS in the absorbed
layer is very high. Thus, the “gel” effect (Trommsdorff effect) (64) may contribute to
increased molecular weight growth in the layer. In any event, the layer of polymer
ends up becoming higher in molecular weight than other chains. Eventually, crosslinks form between the chains leading to infinite molecular weight. This effect not
only can lead to the formation of gel, but can eventually result in reactor fouling
if not controlled. Suspension polymerization does not have this problem. Addition
of low levels (ie, 100–500 ppm) of a divinyl monomer to suspension styrene polymerization leads to the formation of branched PS without problems with reactor
fouling. However, even though suspension polymerization can be used to produce
branched PS, it is no longer practiced commercially for economic reasons.
The approaches to branching during continuous bulk free-radical polymerization that have been reported include the addition of a small amount of divinyl monomer (65), vinyl functional initiator (66), polyfunctional initiator (67),
and vinyl functional chain-transfer agent (68) to the polymerization mixture. Researchers at Dow have investigated all of these approaches in using continuous
free-radical bulk polymerization and found that they all lead to gel and eventual
reactor fouling after a few days of continuous operation, even in the presence of
a chain-transfer solvent. They found that only if a chain-transfer agent is added
to the continuous free-radical bulk polymerization to maintain a high level of
termination, could branched PS be produced without reactor fouling (69).
More recently, Dow researchers developed a new approach to making
branched PS using continuous free-radical bulk polymerization, which eliminates
the potential of reactor fouling by carrying out the branching after the polymer exits the polymerization reactor (70). The concept is to incorporate latent functional
groups into the polymer backbone during the polymerization, which subsequently
react to form a cross-link after the polymer exits the polymerization reactor
(Fig. 10). The requirement is that the functional groups be inert at polymerization temperatures (100–160◦ C) and then during the devolatilization step while
Fig. 10. General approach to post-polymerization reactor branching (cross-linking).
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Fig. 11. Branching (cross-linking) chemistry of pendant BCB-functionalized PS.
the polymer is heated to 240◦ C, the groups must efficiently couple in the viscous
polymer melt.
The functional group that worked best was benzocyclobutene (BCB) (48,71).
BCB functional initiators were used to place BCB functional groups on the PS
chain ends and BCB functional comonomers were used to place BCB groups pendant along the PS backbone. BCB is a very unique molecule in that it is totally
inert at temperatures <180◦ C. However, at temperatures >200◦ C the strained
four-membered ring opens resulting in the formation of a highly reactive orthoquinonemethide intermediate (1), which couples with another orthoquinonemethide on another PS chain resulting in the formation of a cross-link (Fig. 11).
One of the key reasons that it is difficult to long-chain branch PS without
formation of cross-linked gel is due to the fact that the main mechanism of termination taking place in normal free-radical polymerization processes is radical
coupling. Thus, dynamic (growing) high molecular weight branched polystyryl
radicals couple together. This problem can be virtually eliminated using new controlled radical polymerization techniques. Soluble hyperbranched PS, previously
only prepared using anionic polymerization chemistry, has been prepared using
this technique. The controlled radical polymerization process was carried out using styrene having an alkoxyamine substituent (2) making it an A2 B monomer,
which is well known to yield soluble hyperbranched polymers (72).
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Styrene Copolymers. Acrylonitrile, butadiene, α-methylstyrene, acrylic
acid, and maleic anhydride have been copolymerized with styrene to yield commercially significant copolymers. Acrylonitrile copolymer with styrene (SAN) is the
largest volume styrenic copolymer and is used in applications requiring increased
strength and chemical resistance over PS. Most of these polymers have been prepared at the crossover or azeotrope composition, which is ca 24 wt% acrylonitrile
(see COPOLYMERIZATION).
Copolymers are typically manufactured using well-mixed CSTR processes to
eliminate composition drift that causes a loss in transparency. SAN copolymers
prepared in batch or continuous plug-flow processes, on the other hand, are typically hazy because of composition drift. SAN copolymers with as little as 4 wt%
difference in acrylonitrile composition are immiscible (73). SAN is extremely incompatible with PS. As little as 50 ppm of PS contamination in SAN causes haze.
Copolymers with over 30 wt% acrylonitrile are available and have good barrier
properties. If the acrylonitrile content of the copolymer is increased to >40 wt%,
the copolymer becomes ductile. SAN copolymers constitute the rigid matrix phase
of the ABS engineering plastics.
Unlike PS homopolymers, SAN copolymers turn yellow upon heating. The
extent of discoloration is proportional to the percentage of acrylonitrile in the
copolymer (74). Generally, the mechanism of discoloration is thought to involve cyclization of acrylonitrile polyads in the polymer backbone, as depicted in Figure 12.
However, recent studies have revealed that oligomers also contribute to discoloration. Specifically, one of the SAN trimer structures containing one styrene and
two acrylonitrile units was found to thermally decompose resulting in the formation of a naphthalene derivative (Fig. 13), which readily oxidizes to form a
highly colored species. The relative contribution of oligomers and polymer backbone polyad cyclization to overall discoloration is about equal.
Fig. 12. Formation of red/yellow upon heating SAN triad in polymer backbone.
Fig. 13. Formation of red/yellow upon heating SAN trimer.
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Styrene–butadiene copolymers are mainly prepared to yield rubbers (see
ELASTOMERS; STYRENEUTADIENE COPOLYMERS). Many commercially significant latex paints are based on styrene–butadiene (weight ratio usually 60:40 with high
conversion) copolymers (see COATINGS; PAINT). Most of the block copolymers prepared by anionic catalysts, eg, sec-butyl lithium, are also elastomers. However,
some of these block copolymers are thermoplastic rubbers, which behave like
cross-linked rubbers at room temperature but show regular thermoplastic flow at
elevated temperatures (75,76). Diblock (styrene–butadiene) and triblock (styrene–
butadiene–styrene) copolymers are commercially available. Typically, they are
blended with PS to achieve a desirable property, eg, improved clarity/flexibility
(76) (see POLYBLENDS). These block copolymers represent a class of new and interesting polymeric materials (77,78). Of particular interest are their morphologies
(79–82), solution properties (83,84), and mechanical behavior (85,86).
Maleic anhydride readily copolymerizes with styrene to form an alternating
structure. Accordingly, equimolar copolymers are normally produced, corresponding to 48 wt% maleic anhydride. However, by means of CSTR processes, copolymers
with random low maleic anhydride contents can be produced (87). Depending on
their molecular weights, these can be used as chemically reactive resins, eg, epoxy
systems and coating resins, for PS-foam nucleation, or as high heat-deformation
molding materials (88).
Recently, it has been discovered that styrene forms a linear alternating
copolymer with carbon monoxide using palladium(II)–phenanthroline complexes.
The polymers are syndiotactic and have a crystalline melting point ∼280◦ C (89).
Currently, Shell Oil Co. is commercializing carbon monoxide α-olefin plastics based
upon this technology (90).
Currently, Dow is commercializing ethylene–styrene interpolymers (ESI).
Dow uses what they call a constrained-geometry metallocene catalyst (91).
The classical heterogeneous Ziegler–Natta catalysts are generally ineffective for
copolymerizing ethylene with styrene. ESI made by Dow have a “pseudorandom”
structure meaning they do not contain any regioregularly arranged SS sequences,
even at styrene contents approaching 50 mol%. Dow claims that the constrainedgeometry of the catalyst (caused by the chelate ligand) plays a critical role in
favoring the incorporation of the styrene. The technical significance of Dow’s new
catalyst is that the ESI resins are not contaminated with polyethylene or PS homopolymers, which generally results using alternate catalyst systems. A wide
range of ESI resins have been prepared depending upon the ethylene/styrene ratio in the polymerization feed. Incorporation of high amounts of styrene into the
copolymer requires the use of very high ratios of styrene. This presents a challenge at the end of the process because the unreacted styrene is removed by high
temperature vacuum devolatilization, which can lead to some contamination by
atactic PS (by spontaneous free-radical polymerization) if not done properly. ESI
copolymers containing low levels of styrene are crystalline thermoplastics (92)
while increasing the styrene content leads to a rapid decrease in crystallinity affording materials displaying excellent elastomeric properties (93). ESI containing
about 20 mol% styrene are excellent compatibilizers for PE–PS blends (94).
Hydrogenated PSs. Dow researchers have successfully developed a new
hydrogenation catalyst that allows complete hydrogenation of the phenyl rings
on PS without degrading the molecular weight of the polymer (95). Complete
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Table 3. Glass-Transition Temperatures of Substituted Polystyrene
Polymer
Polystyrene
Poly(o-methylstyrene)
Poly(m-methylstyrene)
Poly(p-methylstyrene)
Poly(2,4-dimethylstyrene)
Poly(2,5-dimethylstyrene)
Poly(p-tert-butylstyrene)
Poly(p-chlorostyrene)
Poly(α-methylstyrene)
CAS registry number
Tg ◦ C
[9003-53-6]
[25087-21-2]
[25037-62-1]
[24936-41-2]
[25990-16-3]
[34031-72-6]
[26009-55-2]
[24991-47-7]
[25014-31-7]
100
136
97
106
112
143
130
110
170
hydrogenation of the phenyl rings of PS increases the glass-transition temperature
(T g ) to ∼140◦ C (96). Because of the hydrophobicity and low birefringence of the
saturated polymer, it is well suited for use in optical media applications. Dow is
currently planning to commercialize hydrogenated PS for use as a substrate for
DVD discs.
Polymers of Styrene Derivatives. Many styrene derivatives have been synthesized and the corresponding polymers and copolymers prepared (97). The glasstransition temperatures for a series of substituted styrene polymers are shown in
Table 3. The highest T g is that of poly(α-methylstyrene), which can be prepared
by anionic polymerization. Because it has a low ceiling temperature (61◦ C), depolymerization can occur during fabrication with the produced monomer acting
as a plasticizer and lowering the heat distortion to 110–125◦ C (98). The polymer
is difficult to fabricate because of its high melt viscosity and is more brittle than
PS, but can be toughened with rubber.
Some polymers from styrene derivatives seem to meet specific market demands and to have the potential to become commercially significant materials. For
example, monomeric chlorostyrene is useful in glass-reinforced polyester recipes
since it polymerizes several times as fast as styrene (98). Poly(sodium styrenesulfonate) [9003-59-2] is a versatile water-soluble polymer and is used in waterpollution control and as a general flocculant (99,100). Poly(vinylbenzyl ammonium
chloride) [70504-37-9] has been useful as an electroconductive resin (101). (see
ELECTRICALLY-CONDUCTING POLYMERS).
Transparent Impact PS (TIPS). Rubber is incorporated into PS primarily
to impart toughness. The resulting materials are commonly called high impact
PS (HIPS) and are available in many different varieties. In standard HIPS resins,
the rubber is dispersed in the PS matrix in the form of discrete particles. The
mechanism of rubber-particle formation and rubber reinforcement and several
general reviews of HIPS and other heterogeneous polymers have been published
(102–108). The photomicrographs in Figure 14 show the different morphologies
possible in HIPS materials prepared using various types of rubbers (109,110). If
the particles are much larger than 5–10 µm, poor surface appearance of moldings,
extrusions, and vacuum-formed parts are usually noted. Although most commercial HIPS contains ca 3–10 wt% PB or styrene–butadiene copolymer rubber, the
presence of PS occlusions within the rubber particles gives rise to a 10–40% volume
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265
Fig. 14. Electron photomicrographs of several HIPS resins prepared using different types
of rubbers.
fraction of the reinforcing rubber phase (108,111). Accordingly, a significant portion of the PS matrix is filled with rubber particles. Techniques have been published for evaluating the morphology of HIPS (110,112,113).
Polystyrene, by itself, is low in cost, relatively clear (90% transparency), has a
moderate tensile modulus (3.3 GPa), and is relatively brittle (G1c about 150 J/m2 ).
For many commercial applications, the optical clarity is a significant advantage,
but the pure polymer is far too brittle for many applications and so some means
must be taken to toughen it. Approaches to toughening include modifying the backbone structure by introducing comonomers (for example making SAN), adding
plasticizers, and blending with various rubbers and elastomers. All of these options for toughening can have a deleterious effect on the desirable properties;
comonomers raise the cost, plasticizers lower the modulus, and rubber fillers can
destroy transparency. HIPS resins are opaque. The opacity is due to the scattering of light as it passes through the sample. This scattering is caused because the
PB rubber particles and PS phase have different refractive indices. There are two
ways to solve this problem and stop the light scattering: (1) adjust the relative
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refractive indices of the two phases so they are nearly the same or (2) shrink the
sizes of the PS and rubber phases to <20 nm. Most companies that want to market
products for TIPS applications utilize the latter technique and have designed high
styrene content microphase separated block copolymer rubbers that when blended
with PS result in very small phase domain sizes. The refractive indices of PS and
PB are 1.59 and 1.51, respectively. The technique of refractive index matching can
be accomplished by either raising the refractive index of PB by copolymerizing refractive index enhancing comonomers with butadiene (eg, styrene, vinylbiphenyl,
vinylnaphthalene) or by reducing the refractive index of PS by copolymerizing
methyl methacrylate into the PS backbone (114).
TIPS resins made by blending high styrene content (∼75 wt%) styrene–
butadiene block copolymers with GPPS have good slow speed toughness (ie,
30% elongation), but poor high speed toughness. Phillips Petroleum became the
first company (1972) to market these resins (115). Phillips calls their styrene–
butadiene block copolymer resins for TIPS blending K-resins. They have a radial or
star block structure with polybutadiene at the core and PS at the tips of the arms.
The PS segments have a bimodal molecular weight distribution. These materials
are produced by polymerizing styrene first with sec-butyl lithium (sBL). Once the
desired molecular weight is reached, more sBL is added followed by more styrene.
The added sBL initiates new chains while styrene also adds to the existing living
PS chains. The result is a bimodal mixture of polystyryl lithium. Once the styrene
is all converted, butadiene is added to form a mixture of styrene–butadiene block
copolymers having the same butadiene block length and two different PS block
lengths. At this point, the butadiene segment is still living. Silicone tetrachloride
is added as a coupling agent resulting in the formation of the star block copolymer
(3).
Several other companies also market transparent styrene–butadiene block
copolymers for TIPS applications. BASF markets styrene–butadiene block copolymers having a similar structure (Styrolux); however, rather than pure PB segments, they make a gradient or taper styrene–butadiene block having a T g of
−30 ◦ C (116). Their process to make this special architecture is similar to Phillips
in that they prepare a bimodal mixture of polybutadienyl lithiums. However, then
a mixture of butadiene and styrene are added to make the taper block. After
the taper block is made, more styrene is added to add a short PS block. A tetrafunctional coupling agent is then added to form the star structure. BASF teaches
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that the purpose of the long PS block is to enhance compatibility in blends with
PS (116). Fina markets linear styrene–butadiene triblock materials under the
trade name Finclear. These linear triblock resins are not just simple high styrene
content styrene–butadiene–styrene triblock materials. Rather, they use the taper
block concept; that is, rather than having pure PS and PB segments in the block
copolymer, there is a gradient change over or tapering of the styrene to butadiene
segments.
Unlike for PS, very little work has been published on the fracture of block
copolymers. There is a substantial amount of literature on PS toughened by blending with block copolymers. In those systems, the block copolymer is dispersed, and
provided the proper particle size and modulus are obtained, substantial improvements in toughness are obtained, as described above for HIPS. For the neat block
copolymers, however, fracture mechanisms are fundamentally different than for
PS.
Results on the fracture of KR01 and KR03, both Phillips K-resins, have been
reported (117). KR01 has an overall molecular weight of 180,000 (M w ), while for
KR03 the molecular weight is 220,000 (M w ). KR03 is asymmetrical, with one
long PS block, while KR01 is fairly symmetrical. The different block structures
result in quite different morphologies, which in turn impart quite different fracture properties. KR01 has basically a cylindrical morphology consisting of wavy
rods of polybutadiene, about 20 nm in diameter, in a PS matrix. The cylindrical
morphology is expected based on the volume fraction of PB. In KR03, the PB is
in a disrupted lamellar morphology. The rubber phase is in sheets about 20 nm
thick, but the sheets are neither flat nor continuous, although annealing sharpens the morphology somewhat. KR01 fractures in a manner similar to PS. There
are well-defined crazes, and no deformation outside of the craze region. In KR03,
there is significant microcracking (better termed nanocracking), voiding in the PB
phase, and significant deformation. Argon refers to the failure of KR03 as a form
of crazing and he reports strains to failure of 9% and 110% for KR01 and KR03,
respectively, while Phillips product literature gives corresponding values of 20%
and 160%. Notched Izod impact strengths have been reported as 0.4 ft·lb/in. and
0.75 ft·lb/in. for KR01 and KR03, respectively. The change in morphology has an
appreciable effect on toughness (117).
The individual components (PS and styrene–butadiene block rubber) that
make up TIPS are optically transparent. Ignoring the effects of surface imperfections (scratches, roughness), blends and block copolymers become hazy and the
transparency is reduced due to light scattering from regions of differing refractive index. In very qualitative terms, significant light scattering will occur when
regions of differing refractive index have dimensions of the order of the wavelength of visible light. For systems with dispersed rubber particles, like HIPS,
that condition means that the rubber particles must be less than about 100 nm in
diameter for the material to be transparent. Unfortunately, this requirement for
clarity is inconsistent with the requirement for toughness of a minimum rubber
particle size of 0.8 µm. The domain sizes in block copolymers are much smaller, of
the order of 20 nm, and so block copolymers tend to be highly transparent. When
homopolymers are blended with blocks, however, inhomogeneities in composition
and refractive index can develop, and if the dimensions are large enough, light
scattering can be significant.
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Fig. 15. Transmittance vs particle size for PB particles in a PS matrix with a path length
of 1.0 mm. The calculation assumes 10 vol% of spherical particles and is for light with 0.55
µm wavelength.
Block copolymers of styrene and butadiene are highly transparent, despite
the large refractive index difference between PS and PB and the fact that the
blocks are phase separated. Styrolux and KR03 resins have transparencies around
90%. The transparency is due to the very small domain size, which is typically
of the order of 20 nm. Spheres of equivalent size would scatter very little light,
as shown from the data in Figure 15. In block copolymers the domain sizes are
determined primarily by block molecular weights, and for commercial polymers
the domains will always be small enough that they will not be a significant source
of scattering. As long as block copolymer composition is uniform, degradation is
avoided, smooth surfaces are maintained and fabricated articles are generally
clear.
As a means of reducing cost and increasing modulus, GPPS is often blended
with block copolymers, despite the fact that the blends tend to have inferior optical
clarity than the pure block copolymers. There is some evidence that the addition
of GPPS can raise the T g of the block copolymer system (118). Several examples of how adding GPPS to block copolymers effects clarity have been provided
(116,119). The solubility of the GPPS in the PS blocks depends on the relative
molecular weights. With identical molecular weights the solubility is expected to
be unlimited, and depending on block copolymer geometry (star vs linear) solubility may be maintained with a GPPS/block ratio of up to 1.5 (118). The reason
that Styrolux and KR03 are asymmetrical, with on average one long PS block per
molecule, is probably to enhance solubility of GPPS.
When GPPS of the proper molecular weight is added to a lamellar block
copolymer, like KR03 or Styrolux, it is absorbed by the PS phase and hence swells
it. With no added homopolymer, the block copolymer does not scatter light because
all of the domains are much smaller than the wavelength of light, even though
the PS domains will be larger than the butadiene domains, and to visible light
the material has a uniform refractive index. If the GPPS is uniformly absorbed
by the PS blocks the material remains transparent, even if the PS domains become comparable in size to the wavelength of light. In this case, the system is
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transparent because the PB domains are still very small and evenly dispersed.
Optically, the system is very similar to HIPS, except that it is easier to maintain
the dispersion due to the block structure. As GPPS is added, the average refractive
index of the material increases, but to visible light there is not a refractive index
gradient that would result in scattering. The blend may begin to scatter light for
one of two reasons. First, if large volumes of GPPS are added, even if miscibility
with the blocks is maintained, variations in PS domain sizes can occur due to
random fluctuations in morphology. When these domains become large, they will
begin to scatter. If the GPPS begins to phase separate, then it will very quickly
grow into domains that will cause significant scattering. The key to maintaining
clarity is solubility of the added GPPS in the PS blocks. It has been shown how in
a lamellar Styrolux system, added GPPS swells the PS blocks but leaves the butadiene block domains essentially unchanged (116). A blend with 60 vol% Styrolux
and 40% GPPS has about 78% transmittance, compared to 90% for the pure block
copolymer. Transmission electron micrographs show that the butadiene domains
are essentially the same size in the blend and pure block copolymer (Fig. 16),
but that the PS domains grow. Eventually, regions that are relatively high in PS
develop that are large enough to scatter light. Individual PS domains are not
that large by themselves. In contrast, it has also been shown that in a styrene–
butadiene–styrene triblock system, added GPPS readily phase separates, forms
relatively large domains of pure PS, and results in much higher light scattering.
In summary, optical clarity in TIPS systems depends on minimizing light
scattering, which arises from having regions of differing refractive index. The dimensions of these scattering centers is dictated by the size of dispersed rubber
particles (as in HIPS), by the morphological features in block copolymers, or by
compositional fluctuations in blends of block and homopolymers. The conundrum
for HIPS-type materials is that rubber particles large enough to toughen PS are
also large enough to cause significant light scattering. In block copolymers, the
morphological features are too small to scatter significantly and so these systems are highly transparent. Blends of high styrene content (>70 wt% styrene)
Fig. 16. TEM of KR03 and 50:50 blend of KR03 and PS.
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block and GPPS remain transparent as long as gross phase separation can be
prevented.
Besides transparency and toughness, modulus is also a key property for TIPS.
The two approaches to toughening PS involve the addition of a relatively low
modulus rubber phase. Adding rubber unavoidably lowers the modulus of the
resulting composite; exactly how much depends on the resulting morphology. For
systems with dispersed rubber particles, in which the PS remains the continuous
phase, simple mechanical models do a good job of predicting modulus as a function
of rubber phase volume fraction. In fact, measuring the dynamic modulus at small
strains can be used to quantitatively evaluate the volume fraction of elastomer.
The modulus of rubber-modified PS is given approximately by (120)
1/G = 1/G1 [1 + 1.86(φ1 /φ2 )]
where the assumption is made that the elastomer is in dispersed spherical domains, and Poisson’s ratio for PS is 0.35. G is the shear modulus of the composite,
G1 the modulus of the PS matrix, and φ 1 and φ 2 the volume fractions of the PS
and elastomer phases, respectively. In the case of HIPS with occluded PS in the
rubber phase, the volume fraction of elastomer phase φ 2 is the total volume fraction of particles, not the volume of PB. A plot of modulus ratio vs effective volume
fraction of rubber is given in Figure 17. At effective elastomer volume fractions
up to 0.30 (including occluded and grafted PS), an upper limit on what is typically
used for HIPS, the reduction in modulus is only 45%.
For lamellar morphologies, the situation is quite different. There are no good
mechanical models for predicting the modulus of these systems, mostly because
there is not an adequate way to describe the morphology. The modulus of a lamellar
system depends on the relative volume fractions of the two phases, their individual
moduli, and the orientation and aspect ratio of the lamellae. The last two parameters are almost impossible to determine in real systems; so, predicting moduli
based on volume fractions and individual phase moduli is virtually impossible. If
the rubber phase is co-continuous, as it is in lamellar systems like the K-resins,
the modulus is significantly reduced relative to PS or HIPS. For KR03, the ratio
G/G1 is typically 0.42 (121).
Fig. 17. Modulus ratio vs soft phase volume fraction for systems like HIPS.
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Acrylonitrile–Butadiene–Styrene (ABS) Polymers. ABS polymers have
become important commercial products since the mid-1950s. The development
and properties of ABS polymers has been discussed in detail in Reference 122.
ABS polymers, like HIPS, are two-phase systems in which the elastomer component is dispersed in the rigid SAN copolymer matrix. The electron photomicrographs in Figure 18 show the difference in morphology of mass versus emulsion
ABS polymers. The differences in structure of the dispersed phases are primarily
a result of differences in production processes, types of rubber used, and variation
in rubber concentrations.
Because of the possible changes in the nature and concentration of the
rubber phase, a wide range of ABS polymers are available. Generally, they are
rigid [modulus at room temperature, 1.8–2.6 GPa, ie, (2.6–3.8) × 105 psi] and
have excellent notched impact strength at room temperature (ca 135–400 J/m, ie,
Fig. 18. Electron photomicrographs of some commercial ABS resins produced by bulk,
emulsion, or a mixture of the two processes.
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2.5–7.5 ft·lb/in.) and at lower temperatures, eg, at −40◦ C (50–140 J/m, ie, 0.94–
2.6 ft·lb/in.). This combination of stiffness, impact strength, and solvent resistance
makes ABS polymers particularly suitable for demanding applications. Another
important attribute of several ABS polymers is their minimum tendency to orient
or develop mechanical anisotropy during molding (123,124). Accordingly, uniform
tough moldings are obtained. In addition, ABS polymers exhibit good ease of fabrication and produce moldings and extrusions with excellent gloss, which can
be decorated by many techniques, eg, lacquer painting, vacuum metallizing, and
electroplating (125–128). In the case of electroplating, the strength of the molded
piece is significantly improved (123). When an appropriate decorative coating or
a laminated film is applied, ABS polymers can be used outdoors (129).
ABS can be blended with bisphenol A polycarbonate resins to make a material having excellent low temperature toughness. The most important application
of this blend is for automotive body panels.
When inherent outdoor weatherability is important, acrylonitrile–ethylene–
styrene (AES) or acrylonitrile–styrene–acrylate (ASA) materials are typically
used. These materials utilize ethylene–propylene copolymers and polybutylacrylate as the rubber phase, respectively. Ethylene–propylene and polybutylacrylate
rubbers are inherently more weatherable than polybutadiene because they are
more saturated leaving fewer labile sites for oxidation (130). Even though AES and
ASA polymers are more weatherable than ABS, additives are needed to stabilize
the materials against outdoor photochemical degradation for prolonged periods.
Additive packages for AES and ASA weatherable materials generally contain both
uv absorbers and hindered amine light stabilizers. Typical outdoor applications for
weatherable polymers include recreational vehicles, camper tops, and swimming
pool accessories. The extremely hostile environments where these stabilized AES
materials are utilized still take their toll on the materials. Over very prolonged
periods of use, the surface gains a chalky appearance as the polymer degrades.
Serious discoloration can also result and was found to be caused by degradation
of the additives (131).
High heat ABS resins are produced by adding a third monomer to the styrene
and acrylonitrile to “stiffen” the polymer backbone, thus raising the T g . Two
monomers used commercially for this purpose are α-methylstyrene (132) and Nphenylmaleimide (133).
Not only are ABS polymers useful engineering plastics, but some of the high
rubber compositions are excellent impact modifiers for poly(vinyl chloride) (PVC).
Styrene–acrylonitrile-grafted butadiene rubbers have been used as modifiers for
PVC since 1957 (134).
New Rubber-Modified Styrene Copolymers. Rubber modification of
styrene copolymers other than HIPS and ABS has been useful for specialty purposes. Transparency has been achieved with the use of methyl methacrylate as
a comonomer; styrene–methyl methacrylate copolymers have been successfully
modified with rubber. Improved weatherability is achieved by modifying SAN
copolymers with saturated, aging-resistant elastomers (135).
Inorganic-Reinforced Styrene Polymers. Glass reinforcement of PS and
SAN markedly improves their mechanical properties. The strength, stiffness, and
fracture toughness are generally at least doubled. Creep and relaxation rates are
significantly reduced and creep rupture times are increased. The coefficient of
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thermal expansion is reduced by more than half, and generally, response to temperature changes is minimized (136). Normally, ca 20 wt% glass fibers, eg, 6 mm
long, 0.009 mm dia, E-glass, can be used to achieve these improvements. PS, SAN,
HIPS, and ABS have been used with glass reinforcement.
Four approaches are currently available for producing glass-reinforced parts.
These include use of preblended (reinforced molding compound; blending of reinforced concentrates with virgin resin), a direct process, in which the glass is
cut and weighed automatically and blended with the polymer at the molding
machine, and general inplant compounding (123). The choice of any of these
processes depends primarily on the size of the operation and the corresponding
economics.
Reinforcement of PS with inorganic materials other than glass has been
under intense study in recent years. The inorganic materials of highest interest
have a lamellar (layered) structure with at least one dimension in the nanometer scale range. Hence, addition of these inorganic materials to polymers form
what are called nanocomposite materials. Many natural and synthetic lamellar inorganic materials are commercially available. The molecular structures of
these materials vary widely. The biggest challenge encountered when blending
nanolayered inorganic with hydrophobic polymers like PS is to end up with the
inorganic nanolayers separated and evenly dispersed throughout the polymer matrix. The grouping of the layers of lamellar pieces can generally be visualized
as a cabbage-like or book-like structure in which, for the purpose of making a
nanocomposite, the task is to scatter the leaves or book pages uniformly throughout the polymer matrix (Fig. 19). The dimensions of the individual lamellae are
on the nanometer scale and are important for enhancement of polymer physical
properties.
Several approaches for the exfoliation of platelets exist in producing the
PS nanocomposite. The actual mixing of the inorganic with PS can take place via
compounding or by introducing it into the polymerization process. The approaches
include (1) using the inorganic material with no modification, (2) chemical treatment of the inorganic material surface to alter hydrophilicity or to attach reactive
functional groups, (3) use of compatibilizers/surfactants when mixing, (4) copolymerization styrene with a polar comonomer (ie, maleic anhydride) to make PS
more compatible with the polar inorganic material, (5) physical manipulation of
inorganic material (ie, freeze-drying or ball milling) (Fig. 20), and (6) use of polymerization mechanism to do the work of exfoliating the platelets.
Fig. 19. Illustration of dispersed/intercalated platelets and exfoliated platelets.
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Fig. 20. Use of freeze-drying to separate lamellae.
There are advantages and disadvantages to these approaches. Method 1
is simple; however, unless working with a highly polar polymer, exfoliation and
dispersion of the mismatched systems is unlikely without using a great deal of
energy to mechanically separate and disperse the layered material. Methods 2–4
require the addition of compatibilizing additives, and have been shown to be quite
effective. Use of these additives often results in compromising the physical properties of the parent polymer system, and benefits that might be provided by the
inorganic platelets can easily be outweighed by the addition of these extra materials. Use of these additives can also add substantial cost to the composite system.
Approach 5 is a useful tool when used in combination with the other approaches,
but when used alone, it is difficult to obtain fully exfoliated dispersions. Using the
polymerization mechanism to exfoliate the platelets (method 6) is generally not as
straightforward as the other methods; however, it can be an ideal way to exfoliate
and disperse platelets without the use of unwanted additives or the added expense
of compounding approaches.
The synthesis of a PS/Montmorillanite clay nanocomposite prepared by combining methods 2 and 6 described earlier was published (137) (Fig. 21). They
utilized living free-radical polymerization (LFRP) of styrene to provide exfoliated
platelets in a PS matrix. Transmission electron micrographs and x-ray diffraction patterns support the idea of preparing nanocomposites via polymerization
initiation.
Only recently have some elegant approaches for the synthesis of finely dispersed composites been reported in the academic literature (137,138). The styrenebased systems were SAN, ABS, and PS. Blends of these materials were prepared
either by compounding with organically modified inorganic materials or by mixing
organically modified (with or without an additional compatibilizer) with monomer
prior to polymerization. Physical properties of the composites were also examined
in several reports, and some changes in modulus and T g reported. Many of these
studies focused on attempts to disperse these ionic inorganic materials in the
incompatible organic polymer systems. Good dispersions were obtained in some
cases; however, complete exfoliation of the inorganic lamellar systems was generally not obtained. As a result, only intercalated products were made. Having
exfoliated platelets of the proper aspect ratio and in the correct alignment in the
polymer matrix should provide the maximum impact on physical properties; however, dispersing such a material in relatively nonpolar systems, such as styrenic
polymers, remains a challenge.
275
Fig. 21. LFRP of styrene to produce nanocomposites.
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Degradation
Like almost all synthetic polymers, styrene plastics are susceptible to degradation
by heat, oxidation, uv radiation, high energy radiation, and shear. However, in
normal use, only uv radiation imposes any real limit on the general usefulness of
these plastics (139,140). Thus, it is generally recommended that the use of styrene
plastics in outdoor applications be avoided.
Thermal Degradation. Typically, PS loses about 10% of its molecular
weight when it is fabricated. A significant amount of research has been carried
out to determine the nature of the “weak links” in PS (141–144). Various initiation of degradation mechanisms have been proposed: (1) chain-end initiation
(unzipping), (2) random scission initiation, and (3) scission of weak links in the
polymer backbone. It has been suggested that chain-end initiation is the predominant mechanism at 310◦ C while random scission produces stable molecules.
Evidence for weak link scissions comes mainly from studies showing loss of molecular weight vs. degradation time. These plots usually show a rapid initial drop in
molecular weight indicating initial rapid weak link scission. However, the picture
is also complicated by the fact that the mechanism of degradation is temperaturedependent. It appears that weak link scissions taking place at high temperatures
initiate depolymerization while at lower temperatures, scissions simply cause a
decrease in molecular weight. In any case, a clear difference in thermal stability
has been shown between PS produced using a peroxide initiator and PS made
without an initiator polymerization. Figure 22 shows the Arrhenius activation
Fig. 22. Comparison of thermal stability of the PS backbone made with and without peroxide initiator – –◦– – Peroxide initiated Ea = 94.6 kJ/mol (22.6 kcal/mol); -—No initiator
added Ea = 226 kJ/mol (54 kcal/mol).
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Fig. 23. Thermogravimetric analysis of polymers and copolymers of styrene in nitrogen
at 10◦ C/min. A, PS; B, poly(vinyl toluene); C, poly(α-methylstyrene); D, poly(styrenecoacrylonitrile), 71.5% styrene; E, poly(styreneco-butadiene), 80% styrene; F, poly(styrenecomethylstyrene), 75% styrene.
energy for chain scission in PS made with and without a peroxide initiator. This
difference in stability of the resins is likely due to the initiator derived fragments
that remain in the polymer after isolation.
Poly(α-methylstyrene) unzips to monomer exclusively. Figure 23 is a comparison of the thermal stability of several copolymers. Thermal-oxidative degradation
of PS occurs much faster, leading to additional volatile components consisting of
aldehydes and ketones, yellowing of the polymer with a very dramatic drop in
molecular weight, and some cross-linking. Rates and yields are highly oxygenand temperature-sensitive. Figure 24 shows the magnitude of oxidative attack on
PS and the extent to which an antioxidant can protect the polymer.
Environmental Degradation. In the past several years, PS has received
much public and media attention. Polystyrene has been described by various environmental groups as being nondegradable, nonrecyclable, toxic when burned,
landfill-choking, ozone-depleting, wildlife-killing, and even carcinogenic. These
misconceptions regarding PS have resulted in boycotts and bans in various localities. Actually, PS comprises less than 0.5% of the solid waste going to landfills
(Fig. 25) (145).
The approach which the plastics industry has taken in managing plastics
waste is an integrated one of source reduction, recycling, incineration for energy
recovery, reduction of litter, development of photodegradable plastics for specific
litter prone applications, the development of biodegradable plastics, and increasing public awareness of the recyclability and value of PS. The balance of these
approaches varies from year to year depending upon public concerns, political
pressures, legislation, technology development, and the development of an understanding of the actual contribution of plastics to total solid waste generation.
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Fig. 24. Thermal and thermooxidative degradation of PS.
Currently, recycling is being the most heavily researched and developed. The National Plastics Recycling Company (NPRC) was established in 1989 through the
combined efforts of the top eight U.S. PS producers (ie, Huntsman, Dow, Polysar,
Fina, Arco, Mobil, Chevron, and Amoco). Its charter is to facilitate plastic recycling with the ultimate objective of recycling 25% of the PS produced in the United
States each year (146).
Fig. 25. Approximate composition of solid waste going into landfills in the United States
(145)
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Fig. 26. Energy of the solar spectrum vs the wavelength sensitivity of PS.
Photodegradation. Natural sunlight emits energy only in wavelengths
above 290 nm. Therefore, any polymer that does not absorb light energy at wavelengths above 290 nm should not be photodegradable. The activation spectrum
of PS versus the intensity of the solar spectrum is shown in Figure 26. Since PS
degradation is activated by radiation at wavelengths above 290 nm, it is quite
photodegradable and must be stabilized by the addition of uv-absorbing additives
if it is to be used in outdoor applications where durability is important.
Even though PS is naturally quite photodegradable, there have been considerable efforts to accelerate the process to produce photodegradable PS (147–152).
The approach is to add photosensitizers (typically ketone-containing molecules),
which absorb sunlight (eg, benzophenone). The absorbed light energy is then
transferred to the polymer to cause backbone scission via an oxidation mechanism (Fig. 27). Photodegradable PS would be useful in litter prone applications
(eg, fast food packaging).
A more effective approach to enhancing the rate of photodegradation of PS is
to copolymerize styrene with a small amount of a ketone-containing monomer.
Thus, the ketone groups are attached to the polymer during its manufacture
(Fig. 28) (147,149).
Attaching the ketone groups to the polymer backbone is more efficient, on
a chain scission/ketone bases, because some of the light energy that the pendant
ketone absorbs leads directly to chain scission via the Norrish type II mechanism,
as well as photooxidation via the Norrish type I mechanism (Fig. 29) (149).
A key problem with the manufacture of photodegradable PS containing low
levels of methyl vinyl ketone and methyl isopropenyl ketone is their human
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Fig. 27. The chemistry of molecular weight breakdown of PS during outdoor exposure.
Fig. 28. Incorporation of photosensitive ketone groups into PS during manufacture.
Fig. 29. Polystyrene backbone scission resulting from sunlight exposure of PS containing
attached ketone groups.
toxicity. This problem has been solved by adding the ketoalcohol intermediate,
formed during the vinyl ketone manufacture, to the styrene polymerization and
generating the vinyl ketone in situ. (Fig. 30) (151,152).
A concern in the use of photodegradable PS is the environmental impact of
the products of photooxidation. However, photodegraded PS is expected to be more
susceptible to biodegradation because the molecular weight has been reduced,
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Fig. 30. Manufacture of photodegradable PS via in situ vinyl ketone process.
the PS chains have oxidized end groups, the incorporation of oxygen as alcohol
and ketone groups has increased the hydrophilicity of the PS fragments, and the
surface area has increased (153–155).
Biodegradation. Polystyrene is inherently resistant to biodegradation
mainly because of its hydrophobicity. Efforts have been made to enhance the
biodegradability of PS by inserting hydrolyzable linkages (eg, ester and amide)
into its backbone. This was accomplished by adding monomers to the polymerization, which are capable of undergoing ring-opening copolymerization (Fig. 31)
(156).
Environmental Effect of Blowing Agents. Until the mid-1980s, the
most common blowing agents for extruded PS foams were chlorofluorocarbons
(CFCs). However, when these materials were shown to contribute to the ozone
depletion problem, there became a considerable effort to find alternative blowing
agents for the manufacture of extruded PS foam. Most of the research has focused
on the development of carbon dioxide foaming technology for PS (157). By contrast, PS bead foam uses hydrocarbon blowing agents (eg, pentane). Hydrocarbon
blowing agents are extremely flammable and add volatile organic compounds to
the atmosphere. In Europe, all PS foam is manufactured using carbon dioxide as
the blowing agent.
Polymerization
Styrene and most derivatives are among the few monomers that can be polymerized by all four distinct mechanisms, ie, anionic, cationic, free-radical, and
Fig. 31. Preparation of biodegradable PS by incorporating ester linkages into the backbone via ring-opening copolymerization of styrene with a cyclic ketene acetal.
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Ziegler–Natta. These include processes dependent on electromagnetic radiation,
which is usually a free-radical mechanism, or high energy radiation, which is either a cationic or free-radical mechanism depending on the water content of the
system. All mechanisms, other than Ziegler–Natta, generally yield polymers with
a high degree of random placement of the phenyl group relative to the backbone,
ie, the polymers are classified as atactic and amorphous. Anionically made PS is
usually atactic and amorphous, but in some cases, eg, at low temperatures, iPS
has been prepared.
Each of the mechanisms used to polymerize styrene has its own unique advantages/disadvantages as summarized below.
Anionic:
Initiation, propagation, and termination steps are sequential resulting in
the formation of narrow polydispersity (M w /M n < 1.1); termination step is
controlled allowing control of end-group structure; polymerization feed must
be purified.
Cationic:
High molecular weight polymers are difficult to make because of the instability of polystyrylcarbocation giving fast termination; polymerization feed
must be purified.
Free-Radical:
Initiation, propagation, and termination steps are simultaneous resulting
in the formation of broad polydispersity (M w /M n > 2); multiple termination paths lead to a variety of end groups; polymerization feed need not be
purified.
Ziegler–Natta:
Metal complexes allowing stereospecific polymerization resulting in the formation of high melting crystalline tactic PS; polymerization feed must be
purified.
Free-radical polymerization is the preferred industrial route because:
(1) monomer purification is not required (158) and (2) initiator residues need not
be removed from polymer as they have minimal effect on polymer properties. The
exceptions are the styrene–butadiene block copolymers and very low molecular
weight PS. These polymers are manufactured using anionic and cationic polymerization chemistry, respectively (159). Analytical standards are available for PS
prepared by all four mechanisms (see INITIATORS).
Free Radical. The styrene family of monomers are almost unique in their
ability to undergo spontaneous or thermal polymerization merely by heating to
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Fig. 32. Flory mechanism for spontaneous initiation of styrene polymerization
>100◦ C. Styrene in essence acts as its own initiator. The mechanism by which
this spontaneous polymerization occurs has been studied extensively and has
challenged researchers for over 50 years. Two mechanisms explaining spontaneous styrene polymerization have been proposed and supported by considerable
circumstantial evidence. The oldest mechanism (160) involves a bond-forming reaction between two molecules of styrene to form a 1,4-diradical (·D·) (Fig. 32).
Evidences favoring this mechanism include (1) the identification of cis and trans
1,2-diphenylcyclobutanes as major dimers (161,162), and (2) the large differences
between spontaneous and chemically initiated (azobisisobutyronitrile) styrene
polymerizations in the presence of the free-radical scavenger 1,1 -diphenyl-2picrylhydrazyl (DPPH). The rate of consumption of DPPH is 25 times faster
than that expected from the rate of polymerization measurements. This difference was explained by the spontaneous formation of ·D·, many of which become
self-terminated before initiating polymer radicals (163). However, experiments to
test the mechanism showed that there was no significant difference in the molecular weight of PS initiated by monoradicals compared with the spontaneously
initiated polymerizations taken to the same monomer conversion (164).
The second mechanism (Fig. 33) (165) involves the Diels–Alder reaction of two styrene molecules to form a reactive dimer (DH) followed by a
Fig. 33. Mayo mechanism for spontaneous initiation of styrene polymerization.
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Fig. 34. Formation of the Mayo Dimer via the Flory Diradical.
molecular-assisted homolysis (MAH) between the DH and another styrene
molecule. The Mayo mechanism has been generally preferred even though critical
reviews (162,166) have pointed out that the mechanism is only partially consistent with the available data. Also, the postulated intermediate DH has never
been isolated. Evidences supporting the mechanism include kinetic investigations
(167,168), isotope effects (166), and isolation/structure determination of oligomers
(166,169). Even though the reactive dimer intermediate DH has never been isolated, the aromatized derivative DA has been detected in PS (169). Also, D· has
been indicated as an end group in PS using 1 H NMR and uv spectroscopy (170).
The Flory and Mayo proposals could be combined by the common diradical
·D·, which collapses to either DH or 1,2-diphenylcyclobutane (Fig. 34). Nonconcerted Diels–Alder reactions are permissible for two nonpolar reactants (171).
The spontaneous polymerization of styrene was studied in the presence of
various acid catalysts (172) to see if the postulated reactive intermediate DH could
be intentionally aromatized to form inactive DA. The results showed that the rate
of polymerization of styrene is significantly retarded by acids (eg, camphorsulfonic
acid) accompanied by increases in the formation of DA. This finding gave further
confirmation for the intermediacy of DH since acids would have little effect on the
cyclobutane dimer intermediate in the Flory mechanism.
A potentially important commercial benefit of adding an acid catalyst to
the spontaneous free-radical polymerization of styrene is that a significant shift
results in the rate/molecular weight curve for PS. This shift is most pronounced at
high molecular weights allowing formation of high molecular weight PS at a much
faster polymerization rate (Fig. 35). An explanation for this phenomenon is that
the intermediate DH has a high chain-transfer constant (ie, ∼10) (173). Addition
of acid immediately causes DH to aromatize, thus lowering its concentration and
hence decreasing its availability to participate in chain-transfer processes. Also,
the main mechanism of termination is chain coupling (174), the rate of which is
most affected by radical concentration. Since the polymerization temperature can
be raised in the presence of acid without increasing free-radical concentration,
the propagation rate is increased relative to termination rate, thereby raising the
molecular weight.
The addition of a fugitive acid such as camphorsulfonic acid to styrene polymerizations contaminates the polymer with a substance that could lead to corrosion of equipment being used to process the polymer. To solve this problem, Dow
researchers added vinyl functional sulfonic acids (eg, 2-sulfoethyl methacrylate)
to the polymerization (175). The acid then becomes copolymerized into the polymer, thus immobilizing it. Also, they found that the addition of as low as 10 ppm
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Fig. 35. Polymerization rate vs molecular weight relationship for spontaneous bulk
styrene polymerization under neutral and acidic conditions.
of 2-sulfoethyl methacrylate significantly increases the production rate of high
molecular weight PS (176).
Polystyrene produced by the spontaneous initiation mechanism is typically
contaminated by dimers and trimers (1–2 wt%). These oligomers are somewhat
volatile and cause problems during extrusion (vapors) and molding (mold sweat)
operations. The use of chemicals to generate initiating free radicals significantly
reduces the formation of oligomers. Oligomer production is reduced because the
polymerization temperature can be lowered to slow down the Diels–Alder dimerization reaction. A wide range of free-radical initiators are available. They differ
mainly in the temperature at which each generates free radicals at a useful rate.
Typically, in continuous bulk polymerization processes it is best to polymerize
styrene at about the 1-h half-life temperature of the initiator it contains to maximize initiator efficiency.
Initiators that have been utilized to initiate styrene polymerization can be
generally categorized into three types: peroxides, azo, and carbon–carbon (177).
Peroxides are thermally unstable and decompose by homolysis of the O O bond,
resulting in the formation of two oxy radicals. Azo compounds decompose by concerted homolysis of the N C bonds on either side of the azo linkage, resulting
in extrusion of nitrogen gas and the formation of two carbon-centered radicals.
Carbon–carbon initiators decompose by homolysis of a sterically strained C C
bond, resulting in the formation of two carbon-centered radicals. The radicals initiate styrene polymerization and end up attached to the chain ends and may have
an effect upon polymer stability (178–181).
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Since most of the bulk PS reactors were originally designed to produce spontaneously initiated PS in the 100–170◦ C temperature range, the peroxide initiators used generally have 1-h half-lives in the range of 90–140◦ C. If the peroxide
decomposes too rapidly, a run-away polymerization could result, and if it decomposes too slowly, peroxide exits the reactor. Since organic peroxides are significantly more expensive than styrene monomer (5–20×), it is economically prudent
to choose an initiator that is highly efficient and is entirely consumed during the
polymerization.
Another economically driven objective is to utilize initiators that increase
the rate of polymerization of styrene to form PS having the desired molecular
weight. The commercial weight-average molecular weight (M w ) range for GPPS
is 200,000–400,000. For spontaneous polymerization, the M w is inversely proportional to polymerization rate (Fig. 36).
The main reason that the M w decreases as the polymerization temperature
increases is the increase in the initiation and termination reactions leading to
a decrease in the kinetic chain length (Fig. 37). At low temperature, the main
termination mechanism is polystyryl radical coupling, but as the temperature increases, radical disproportionation becomes increasingly important. Termination
by coupling results in higher M w PS than any of the other termination modes.
There are typically several different product grades produced in a singlepolymerization reactor, and transitioning between these products in the minimum
time maximizes production yield. Most PS producers rely on the use of kinetic
modeling and computer simulation to aid in the manufacture of PS to minimize
Fig. 36. Polymerization rate for PS using different types of initiators.
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Fig. 37. General chemistry of free-radical styrene polymerization.
transition time between product grades. Kinetic models have been developed for
styrene polymerization without added initiators (182–184), using one monofunctional (185–189), two monofunctional initiators with different half-lives (190),
symmetrical difunctional (4), (191) and (5), (6), (192) and the unsymmetrical difunctional initiators (7), (193,194) and (8) (195,196). These models clearly show
the polymerization rate advantage of using initiators for the manufacture of PS
using continuous bulk polymerization processes (Fig. 36).
One of the key reasons for the polymerization rate advantage of using difunctional initiators is their theoretical ability to form initiator fragments, which
can initiate polymer growth from two different sites within the same fragment,
ultimately leading to “double-ended PS.” If double-ended PS chains are produced,
given that the main mechanism of termination is chain coupling, it is clear why
higher M w PS can be produced at faster rates using difunctional initiators (197).
One of the goals of PS researchers has been the discovery of an initiator that truly
initiates only double-ended PS chains. Because the main mechanism of termination is radical coupling, the production of pure double-ended chains would lead
to the formation of ultrahigh molecular weight PS because termination by chain
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Fig. 38. Double-ended PS initiated by diradical formed during Bergman Cyclization.
coupling would only lead to a higher molecular weight double-ended chain. There
have been many attempts to develop initiators that form only diradicals. However, the discovery of diradicals that efficiently initiate styrene polymerization
is a challenge (198). Dow researchers recently discovered that the p-phenylene
diradical formed during “Bergman Cyclization” of certain enediynes can initiate
double-ended PS, but commercialization of these initiators remains a challenge
(Fig. 38) (199). To date, the only clear demonstration of double-ended PS has been
with difunctional LFRP initiators (200).
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Below 80◦ C, radical combination is the primary termination mechanism
(201). Above 80◦ C, both disproportionation and chain transfer with the Diels–
Alder dimer are increasingly important. The gel or Trommsdorff effects, as manifested by a period of accelerating rate concomitant with increasing molecular
weight, is apparent at below 80◦ C in styrene polymerization; although, subtle
changes during the polymerization at higher temperature may be attributed to
variation of the specific rate constants with viscosity (201,202).
In some cases, inhibition of polymerization can be regarded as a special type
of chain transfer. This is of importance in commercial-scale operations involving
styrene storage for extended periods. The majority of inhibitors are of the phenolic/
quinone family. All of these species function as inhibitors only in the presence of
oxygen. 4-tert-Butylcatechol (TBC) at 12–50 ppm is the most universally used inhibitor for protecting styrene. At ambient conditions and with a continuous supply
of air, TBC has a half-life of 6–10 weeks (203). The requirement of oxygen causes
complex side reactions, resulting in significant oxidation of the monomer, which
causes yellow coloration especially in the vapor phase. An inert gas blanket reduces this problem and flammability hazards, but precautions must be taken to
ensure an adequate level of dissolved oxygen in the liquid phase. Another family
of inhibitors is that characterized by the N O bond; these do not seem to require
oxygen for effectiveness. They include a variety of nitrophenol compounds, hydroxylamine derivatives, and nitroxides (204,205). Nitric oxides are particularly
useful. The unpaired electron associated with the N O bond is very stable and
yet nitroxides couple with carbon-centered radicals at diffusion-controlled rates.
The alkoxyamines produced when nitroxides couple with styryl radicals are thermally labile. At elevated temperatures (generally >100◦ C), the C O bond dissociates back to the precursor radicals in equilibrium with the alkoxyamine. Thus, if
alkoxyamines are added to styrene, they can initiate polymerization. The nitroxide moiety is retained on the propagating chain-end and effectively supresses the
termination mechanisms (ie, polystyryl radical coupling, chain transfer, and disproportionation) typical of normal free-radical polymerization. The result is that
narrow polydispersity PS is produced. Once the unreacted styrene is removed from
the nitroxide-terminated PS and a second monomer is added, a block copolymer
can be produced.
Other miscellaneous compounds that have been used as inhibitors are sulfur
and some sulfur compounds, picrylhydrazyl derivatives; carbon black, and some
soluble transition-metal salts (206). Both inhibition and acceleration have been
reported for styrene polymerized in the presence of oxygen. The complexity of
this system has been clearly demonstrated (207). The key reaction is the alternating copolymerization of styrene with oxygen to produce a polyperoxide, which
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Table 4. Chain-Transfer Constants (K ct ) in
Free-Radical Styrene Polymerization
Compound
Benzene
Toluene
Ethylbenzene
Isopropylbenene
α-Methylstyrene dimer
l-Dodecanethiol
l,l-Dimethyl-l-decanethiol
l-Hexanethiol
K ct
0.00002
0.00005
0.0002
0.0002
0.3
13
1.1
15
T,◦ C
100
100
100
100
120
130
120
100
above 100◦ C decomposes to initiating alkoxy radicals. Therefore, depending on
the temperature, oxygen can inhibit or accelerate the rate of polymerization.
Chain Transfer in Free-Radical Styrene Polymerization. The concept
of chain transfer is depicted in Figure 37. Chain-transfer agents are occasionally
added to styrene to reduce the molecular weight of the polymer, although for many
applications this is unnecessary as polymerization temperature alone is generally sufficient to achieve molecular weight control. Some typical chain-transfer
agents for styrene polymerization are listed in Table 4. The chain-transfer agents
of commercial significance are α-methylstyrene dimer, terpinolene, dodecane-1thiol, and 1,1-dimethyldecane-1-thiol. Chain transfer to styrene monomer has
been reported, but recent work strongly suggests that this reaction is negligible
and transfer with the Diels–Alder dimer is the actual inherent transfer reaction
(208,209). Chain transfer to PS has received some attention, but experiments indicate that it is minimal (210,211). If it did occur at a significant extent, then
the polymer would be branched. The chain-transfer constants of several common
solvents and chain-transfer agents are shown in Table 4.
High levels of chain-transfer agents can be used to control the termination
process. If the chain-transfer agent has a functional group attached to it, the functional group ends up becoming attached to the end of the PS chain. If the functionalized chain-transfer agent operates by donating an H-atom to the polystyryl
radical, the functional group ends up becoming attached to only one end of the PS
chain. However, this technique does not quantitatively functionalize the polymer
chains because not all chains get initiated by the functionalized chain-transfer
agent fragment formed by loss of the H-atom. However, if both the initiator and
the chain-transfer agent contain the functional group (F), high purity one-end
functional PS can be produced (Fig. 39) (212).
Another approach to control end-group structure is the use of chain-transfer
agents that operate by an addition–fragmentation mechanism. This approach can
lead to the formation of PS having functional groups at both ends if both the
initiating and terminating fragments contain functional groups (Fig. 40).
Recently, it was found that the common chain-transfer agent αmethylstyrene dimer operates by an addition–fragmentation mechanism (Fig. 41)
(213).
The use of addition–fragmentation chain-transfer agents also places a reactive double bond on one end of the polymer chain and thus yields a macromonomer.
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Fig. 39. Preparation of mono-end-functional PS using both a functionalized initiator and
a functionalized chain-transfer agent (CTA).
Fig. 40. Generic addition–fragmentation chain-transfer structure and the mechanism of
action.
Fig. 41. Mechanism of action of α-methylstyrene dimer.
Copolymerization of the chain-end double bond with more styrene leads to
branched PS.
Ionic. Instead of a neutral unpaired electron, styrene polymerization can
proceed with great facility through a positively charged species (cationic polymerization) or a negatively charged species (anionic polymerization). The polymerization reaction is more sensitive to impurities than the free-radical system, and
pretreatment of the monomer is generally required (214). n-Butyllithium (NBL)
is the most widely used initiator for anionic polymerization of styrene. In solution,
it exists as six-membered aggregates, and a key step in the initiation sequence is
dissociation yielding at least one isolated molecule.
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If the initiation reaction is very much faster than the propagation reaction,
then all chains start to grow at the same time, and since there is no inherent termination step (living polymerization), the statistical distribution of chain lengths
is very narrow. The average molecular weight is calculated from the mole ratio of
monomer-to-initiator sites. Chain termination is usually accomplished by adding
proton donors, eg, water or alcohols, or electrophiles such as carbon dioxide.
Anionic polymerization, if carried out properly, can truly be a living polymerization (215). Addition of a second monomer to polystyryl anion results in the
formation of a block polymer with no detectable free PS. This technique is of considerable importance in the commercial preparation of styrene–butadiene block
copolymers, which are used either alone or blended with PS as thermoplastics.
Anionic polymerization offers very fast polymerization rates because of the
long lifetime of polystyryl carbanions. Early research focused on this attribute,
most studies being conducted at short reactor residence times (<1 h), at relatively low temperatures (10–50◦ C), and in low chain-transfer solvents (typically
benzene) to ensure that premature termination did not take place. Also, relatively
low DPs were typically studied. Continuous commercial free-radical solution polymerization processes to make PS, on the other hand, operate at relatively high
temperatures (>100◦ C), at long residence times (>1.5 h), utilize a chain-transfer
solvent (ethylbenzene), and produce polymer in the range of 1000–1500 DP.
Two companies (Dow and BASF) have devoted significant research efforts
to develop continuous anionic polymerization processes for PS. However, to date
no PS is produced commercially using these processes. Dow researchers utilized
conventional free-radical polymerization reactors of the CSTR-type (216–220) to
study the anionic polymerization of styrene while BASF focused upon continuous reactors of the linear-flow-type (221). In an anionic CSTR process, initiation,
propagation, and termination occur simultaneously and thus the polydispersity
of the resulting PS is ∼2. A chain-transfer solvent (ethylbenzene) and relatively
high temperatures (80–140◦ C) were also used. Under these conditions, the chain
transfer to solvent (CTS) is extremely high since the high monomer conversions
(>99%) achieved under steady-state operation results in a large ratio (typically,
500:1) of solvent to monomer. The result of high CTS is that very high yields of
PS based on NBL (the most costly raw material) and PS having high clarity (low
color) are produced (Fig. 42). Under very stringent feed purification conditions, as
high as an 8000% yield based on NBL initiator (158) can be achieved. According to
Figure 42, without high levels of CTS, it would be impossible to make a PS having
sufficient clarity to meet the current color requirements to be sold as “prime” resin
(with no CTS, 640 ppm of NBL is required to produce a PS of 1000 DP).
One of the key benefits of anionic PS is that it contains much lower levels of
residual styrene monomer than free-radical PS (222). This is because free-radical
polymerization processes only operate at 60–80% styrene conversion, while anionic processes operate at >99% styrene conversion. Removal of unreacted styrene
monomer from free-radical PS is accomplished using continuous devolatilization
at high temperature (220–260◦ C) and vacuum. This process leaves about 200–
800 ppm of styrene monomer in the product. Taking the styrene to a lower level
requires special assisted devolatilization processes such as steam stripping (223).
The most recent process research aimed at anionic PS is that of BASF. Unlike
the Dow process, they utilize CPFR with virtually no backmixing to make narrow
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Fig. 42. Polystyrene color vs amount of n-butyllithium (NBL) consumed for its production
in a CSTR.
polydispersity resins. In the Dow CSTR anionic process, the fast polymerization
rate achieved with anionic polymerization is not a problem because the polymerization exotherm is controlled by the rate of monomer addition to the reactor; that
is, the reactor is operated in a monomer-starved condition. However, in CPFR,
the rate of polymerization of NBL-initiated styrene polymerization far exceeds
the ability to remove heat resulting in run-away polymerization. This problem
was recently solved by Asahi (224) and BASF with the addition of electron deficient metal alkyls (eg, dibutylmagnesium, diethylzinc, and triethylaluminum) to
assist the butyl lithium initiator (225,226). Addition of metal alkyls accomplish
two things: (1) the propagation rate is slowed and (2) the polystyryl lithium chain
end is stabilized to high temperatures. The rate of the polymerization can actually
be slowed to match the rate typically observed during free-radical polymerization
of styrene by the addition of the proper amount of metal alkyl (Fig. 43). As the
conversion of monomer to polymer proceeds, the viscosity rapidly increases forcing
the need to finish the polymerization at high temperatures to be able to handle
the high viscosity. It is well known that at temperatures >100◦ C, PS lithium is
unstable and decomposes by elimination of lithium hydride, resulting in the formation of dead polymer. The presence of electron deficient metal alkyls increases
the thermal stability of the polystyryl chain ends so that polymerization up to
high conversion and temperatures can be achieved.
The mechanism (226) involves the formation of a metal complex or aggregate
between the lithium and the electron deficient metal. Styrene then inserts in the
complex. This hypothesis is supported by the change in uv spectrum as the Li/Mg
ratio changes. There is a shift of the λmax to lower wavelength as the Mg/Li ratio
approaches unity. As the Mg/Li ratio is increased to >1, the λmax shifts back until
at 20/1, the λmax returns to match that of pure polystyryl lithium.
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Fig. 43. Propagation rate retarding effect of adding dibutylmagnesium to butyllithium
initiated anionic polymerization of styrene. · · ·◦· · · Lithium alone; -—Mg/Li = 0.8;
——Mg/Li = 2; – –•– – Mg/Li = 4; · · ·· · · Mg/Li = 20.
Continuous anionic polymerization, conducted above the ceiling temperature (61◦ C), is also useful for making thermally stable styrene-co-α-methylstyrene
(SAMS) having high α-methylstyrene (AMS) content. Preparation of SAMS having >20 wt% of AMS units using bulk free-radical polymerization leads to very
slow polymerization rates, low molecular weight polymer, and the formation of
high levels of oligomers. The preparation of SAMS using an anionic CSTR polymerization reactor operating at 90–100◦ C allows the production of SAMS having
up to about 70 wt% AMS units (Fig. 44) (227). By conducting the polymerization
above the ceiling temperature, no AMS polyads of more than two units are possible. SAMS copolymers having polyads of greater than two units in length are
thermally unstable (228).
Cationic polymerization of styrene can be initiated either by strong acids,
eg, perchloric acid, or by Friedel-Crafts reagents with a proton-donating activator, eg, boron trifluoride or aluminum trichloride, with a trace of a protonic acid
or water. Cationic polymerization of styrene can also be initiated using heterogeneous acid catalysis such as acidic clays and strong acid ion exchange resins.
The solvent again plays an important role, and chain-transfer reactions are very
common where the reactants are polymer, monomer, solvent, and counterion. As a
result, high molecular weights are more difficult to achieve and molecular weight
distributions are often comparable to those obtained from free-radical polymerizations. Commercial use of cationic styrene polymerization is reported only where
low molecular weight polymers are desired (110). In recent years, considerable
advances have taken place with regard to cationic polymerization of styrene. Its
use in making block copolymers and even living cationic polymerizations have
been reported (229).
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Fig. 44. Comparison of continuous (CSTR) and batch anionic production of SAMS.
Ziegler–Natta. Ziegler–Natta-initiated styrene polymerization yields
stereoregular tactic PS. The tacticity can be isotactic (phenyl rings cis to each
other) or syndiotactic (phenyl rings trans to each other) depending upon the initiator structure (230). Currently, all commercially important styrenic plastics are
amorphous, since the vast excess of these polymers is made by a free-radical mechanism. However, Dow Chemical and Idemitsu Petrochemical Companies are working together to commercialize crystalline sPS. Dow and Idemitsu are currently
manufacturing sPS using a bulk polymerization process. Since the sPS crystallizes as it forms, the polymerization process is very challenging because it requires
mixing and removing the heat of polymerization from a sticky solid. Proper reactor design is required to keep the sticky mass from solidifying. The properties of
sPS are quite different from atactic PS because of its crystallinity (mp = 270◦ C).
Since sPS must be fabricated above its melting point and the polymer decomposes
at 300◦ C, there is a very narrow fabrication temperature range. This problem is
improved by copolymerizing a small amount of comonomer (eg, p-methylstyrene)
into the polymer to lower the crystalline melting point. Because of its crystallinity,
sPS finds utility in applications requiring heat resistance and solvent resistance
(231) (see SYNDIOTACTIC POLYSTYRENE).
Living Free-Radical Styrene Polymerization. The requirements for a
polymerization to be truly “living” are that the propagating chain-ends must not
terminate during the polymerization. If the initiation, propagation, and termination steps are sequential (ie, all of the chains are initiated and then propagate
at the same time without any termination), then monodisperse (ie, M w /M n =
1.0) polymer is produced. In general, anionic polymerization is the only mechanism that yields truly living styrene polymerization and thus monodisperse
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PS. However, significant research has been conducted with the goal of achieving “living” styrene polymerization using cationic, free-radical, and Ziegler–Natta
mechanisms.
Since Otsu first proposed living free-radical polymerization (LFRP) in 1982,
(232,233) there has emerged a multitude of examples (234–242). However, significant controversy exists over the nature of the living chain-end and whether the
process can really be called “living.” Instead, terms such as pseudo-living, quasiliving, resuscitatable, stable free-radical-mediated, and dormant, have been used
to better describe the process. Currently, most researchers prefer to use the term
controlled radical polymerization. The process generally involves the addition of
relatively stable free radicals to the polymerization. The growing polystyryl radicals are intercepted and terminated by coupling with the stable radicals thereby
minimizing termination by chain transfer and the coupling of polystyryl radicals
with themselves. Prior to the discovery of LFRP, stable free radicals were only
added to styrene as polymerization inhibitors. They acted as radical scavengers
to stop the propagation process. However, the bond formed between the polystyryl
radical and some stable freeradical is somewhat labile, and styrene can insert into
the bond once it is activated by heat or photolysis. The main evidences used to support the livingness of the polymerization are the increase of molecular weight with
monomer conversion, the formation of narrow polydispersity PS, and the ability to
prepare block copolymers. Normally, free-radical polymerizations show relatively
flat molecular weight versus conversion plots. Although no one has yet demonstrated the preparation of truly monodisperse (M w /M n =1.0) PS using LFRP, polydispersities approaching 1.05 have been reported for very low molecular weight
(M w < 20,000) PS prepared in the presence of stable nitroxy radicals.
A variant of the LFRP of styrene is called atom transfer radical polymerization (ATRP). In ATRP the initiator is an activated halide. The halogen–carbon
bond is broken by transfer of the halogen from the chain end to a chelated transition metal (eg, copper, iron, or ruthenium). After the resulting polystyryl radical adds a few monomer units, the halogen transfers back onto the chain-end.
This process repeats itself many times resulting in the formation of PS having
a controlled molecular weight. If at some point a second monomer is added to
the polymerization, block copolymers are produced. Although the chemistry is potentially low in cost due to the nature of the low cost of the initiators and metal
catalysts, there is a serious drawback; ie, the transition-metal catalyst is difficult
to remove from the final polymer. Since the presence of traces of transition metals
tend to promote degradation of polymers, some means must be developed for the
easy elimination of the catalyst residues from the polymer before ATRP becomes
a commercial reality.
Currently, most of the LFRP research is focused on the use of nitroxides
as the stable freeradical. The main problems associated with nitroxide mediated
radical polymerizations (NMRP) are slow polymerization rate and the inability
to make high molecular weight narrow polydispersity PS. This inability is likely
due to side reactions of the living end leading to termination rather than propagation (243). The polymerization rate can be accelerated by the addition of acids
or anhydrides to the process (244). The mechanism of the accelerative effect of
the acid is not certain. Another problem with nitroxides is that they work well
for vinylaromatic monomers, but not for acrylate and diene monomers. This has
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greatly limited their use for making styrene containing block copolymers. A way
around this problem is to make the nonstyrene block using some other chemistry
and then place an alkoxyamine group on one of the chain-ends. The alkoxyamine
functional polymer is then dissolved in styrene and heated to >100◦ C where the
alkoxyamine functional polymer becomes a macroinitiator for the styrene polymerization (245). There are several examples of the tandem polymerization concept, including the preparation of polymethylmethacrylate (PMMA) via normal
radical polymerization using an alkoxyamine functional azoinitiator (9). Subsequently, styrene polymerization is initiated using the alkoxyamine functional
PMMA (10) as a macroinitiator to make S-MMA block copolymer (11). The synthetic scheme is displayed in Figure 45.
The successful application of NMRP to the synthesis of pure block copolymers requires that the termination processes that typically take place during
free-radical polymerization (ie, radical coupling, chain transfer, and disproportionation) be virtually eliminated.
There have been two approaches to the study of the NMRP of styrene: (1) in
situ formation of the NMRP initiator (Fig. 46) (246,247) and (2) presynthesis of
the initiator (200,248).
Fig. 45. Normal-living tandem polymerization approach to make styrenic block
copolymers.
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Fig. 46. In situ synthesis of alkoxyamine initiator during nitroxide mediated radical polymerization (NMRP) of styrene.
The in situ approach leads to the formation of multiple nitroxide initiating
species and there is not perfect stoicheometry between the initiating and mediating species. With the presynthesis approach, a pure compound can be used to
initiate polymerization which should lead to “cleaner chemistry.” However, it has
been believed that excess nitroxy radicals is important to achieve narrow polydispersity and good end-group purity (249).
There has been debate over the extent that NMRP eliminates termination
processes. The thermal stability of a small molecule (12) that models the propagating chain-end of the NMRP of styrene has been synthesized and studied (250).
Thermolysis of (12) in an ESR spectrophotometer showed continuous formation
of 2,2,6,6-tetramethylpiperdinyloxy (TEMPO). Analysis of the residue left in the
ESR tube showed decomposition products consisting primarily of styrene. They
went on to study the kinetics of the decomposition and found that (12) decomposes
in the temperature range utilized for NMRP of styrene at a rate comparable to
the styrene conversion rate. Based on this observation, they concluded that endgroup purity achieved by NMRP of styrene should decrease with both M n and
monomer conversion. They further concluded that this termination process would
likely seriously limit the ability of NMRP for the preparation of high molecular
weight (>100,000) PS having a narrow polydispersity. These conclusions have
been disputed by other research groups who have attempted to prove that NMRP
chemistry virtually eliminates all termination processes. These claims are supported by measuring the amount of nitroxyl moiety on the terminal chain-end.
Both NMR (251) and nitrogen analyses (252) have been utilized to determine the
nitroxyl content of PS made using NMRP. However, these techniques are not very
sensitive and provide only an approximation of the level of nitroxide groups in
the polymer. Further, it has not been conclusively established that the nitroxide
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is at the chain terminus. Furthermore, because of the insensitivity of these analytical techniques, TEMPO-end capped PS samples to be analyzed have always
had molecular weights <10,000. Most researchers have reported levels of nitroxide on the terminal chain end at >90% (251). The quality of nitroxide-mediated
polymerization is directly linked to maintenance of alkoxyamine functionality on
the PS chain end during the polymerization.
The level of alkoxyamine on the PS chain end during the polymerization
up to high conversions and molecular weight has been measured (253). A very
sensitive gpc-uv analysis technique was used to precisely quantitate the level of
chromophore attached to polymers (254). They compared the end-group purity of
PS made by the in situ and presynthesis of initiator approaches, as well as demonstrating the impact of excess nitroxide on nitroxide end-group yield. The gpc-uv
technique for polymer end-group analysis requires that the initiator/mediator be
tagged with a chromophore having a unique absorbance; that is, absorb at a wavelength at which PS is totally transparent (>280 nm). The chromophore chosen for
study was the phenylazophenyl chromophore because of its intense absorption at
>300 nm. The chromophore was attached to either the initiating or terminating
fragment of the alkoxyamine initiator allowing direct quantitative comparison of
both the initiating and terminal ends of PS initiated/mediated using these materials. The data show that the number of polymer chains having chromophores
attached to a chain-end decreases with the number-average molecular weight
(M n ). This trend is not as dramatic if the chromophore is attached to the initiating radical. M n and styrene conversion are directly proportional. Therefore, it
follows that chain-end purity also decreases rapidly with monomer conversion.
This was predicted from earlier work on the thermal decomposition of a small
molecule which models the dormant end during NMRF (250). Polystyrene having
both a M n >10,000 and at the same time >90% of one of its chain-ends having
a chromophore were never obtained. This data has significant implications for
those trying to use NMRP to make functionalized polymers and block copolymers;
that is, it is impossible to make polymers having perfect structures by using this
chemistry. Therefore, unless you are willing to accept styrenic polymers having
some defects in the structure, this chemistry should not be used. However, this
chemistry may be the best way to make certain block copolymers even though the
structure produced is not perfect. For example, S-co-S-alt-MA copolymers have
been synthesized using NMRP (255). At the present time, no alternative chemistry for making these block copolymers exists.
To date, no one has yet commercialized any polymers produced using
an LFRP process, although several companies are aggressively evaluating the
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Fig. 47. Dow in situ block copolymer process for making HIPS/ABS toughened (256).
potential of the chemistry for commercial use. One of the applications of highest potential volume is the preparation of styrene–butadiene block copolymers in
situ during the manufacture of HIPS (256). In this process (Fig. 47), butadiene is
polymerized to PB using conventional anionic chemistry. Normally, after the polymerization is complete, the resulting polybutadienyl lithium (PB-Li) is terminated
by the addition of a proton donor (an alcohol) to quench the carbanion. However,
in the tandem process, a species capable of initiating LFRP that is connected to
a group capable of terminating PB-Li is added to terminate the PB rubber (257).
This results in the formation of PB that has an LFR functional group on one of
its chain ends (13). The LFR-functionalized PB is then dissolved in styrene. The
rubber styrene solution is then pumped into a typical continuous bulk PS reactor.
During the polymerization process, the LFR-functionalized PB acts as a macroinitiator, resulting in the formation of a styrene–butadiene block polymer (14) in
situ, simultaneously with the formation of PS. The final product is a blend of PS
and a styrene–butadiene block rubber. The resulting HIPS product has properties
similar to HIPS made by polymerizing a solution of styrene containing a styrene–
butadiene block rubber (14). The same chemistry can be used to make ABS
resins where the rubber phase ends up being a SAN–butadiene block copolymer
(15).
Recently, a universal alkoxyamine initiator (16) was reported that allows
LFRP of monomers other than styrene. This new alkoxyamine was discovered
utilizing high speed combinatorial synthesis techniques. It has been used to make
styrene–butadiene, S–BA, and S-MMA block copolymers directly without the need
for tandem polymerization techniques (258).
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The newest LFRP system involves reversible addition fragmentation chain
transfer (RAFT) (259). PS, PMA, and PMMA having a polydispersity of <1.3 and
block copolymers containing these monomers have been produced utilizing this
novel chemistry. The initiators are either thioesters or thiocarbonates (260). The
RAFT process is shown in Figure 48.
Amazingly, CSIRO researchers (261) used the RAFT chemistry to make an
S-nBA-S triblock copolymer having an M n of 162,000 and a polydispersity of 1.16.
This was accomplished using a difunctional RAFT reagent (thiocarbonate (17)).
The chemistry is depicted in Figure 49.
Copolymerization. Styrene spontaneously copolymerizes with many
other monomers. The styrene double bond is electronegative because of the donating effect of the phenyl ring. Monomers that have electron withdrawing substituents (eg, acrylonitrile, maleic anhydride) tend to most readily copolymerize
with styrene because their electropositive double bonds are attached to the electronegative styrene double bond. Spontaneous copolymerization experiments of
many different monomer pair combinations indicate that the mechanism of initiation changes with the relative electronegativity difference between the monomer
pairs (262).
Copolymerization makes possible dramatic improvements in one or more
properties. There is a vast amount of literature on styrene-containing copolymers
Fig. 48. Reversible addition fragmentation chain transfer (RAFT) process developed by
CSIRO researchers.
Fig. 49. Synthesis of S-BA block copolymers using the RAFT process.
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Table 5. Free-Radical Copolymerization
Reactivity Ratios for the Monomer Pairs
Monomer 2
Acrylonitrile
Butadiene
m-Ethyl methacrylate
m-Divinylbenzene
a Monomer
r1
r2
T, ◦ C
0.4
0.5
0.54
0.65
0.04
1.40
0.49
0.60
60
50
60
60
1 = Styrene
(263). The reactivity ratios for the monomer pairs are listed in Table 5. Styrene–
acrylonitrile (SAN) copolymers are large-volume thermoplastics with improved
mechanical properties and heat and solvent resistance with some loss of color
stability (see ACRYLONITRILE AND ACRYLONITRILE POLYMERS). The disparity of reactivity ratios shown in Table 5 implies an unequal insertion rate of the monomers
into the copolymer; Figure 50 illustrates manifestations of this effect. Most SAN
copolymers are manufactured at or near the azeotrope or crossover point (A in
Fig. 50) to avoid significant drift in composition (Fig. 51) (264,265). SAN copolymers having as little as a 4 wt% difference in acrylonitrile content are not miscible
(73). Therefore, the preparation of SAN under conditions where composition drift
can occur leads to haze. Reactors have been designed to overcome composition
drift problems on a commercial scale (266). All SAN copolymers produced in the
United States are made in a mass or solution continuous process. The greater
tendency of these copolymers to develop yellow coloration requires more attention
to techniques of polymerization and devolatilization (267–270). The discoloration
Fig. 50. Relationship between feed composition and copolymer composition of styrene
acrylonitrile copolymerization.
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Fig. 51. Drift in monomer composition (—) and copolymer composition (—) with conversion for three initial monomer mixtures; ratios are based on wt%.
mechanism has been linked to both acrylonitrile sequence distribution in the polymer backbone (271) and the thermal decomposition of oligomers (272–275).
Copolymers with butadiene, ie, those containing at least 60 wt% butadiene,
are an important family of rubbers. In addition to synthetic rubber, these compositions have extensive uses as paper coatings, water-based paints, and carpet backing. Because of unfavorable reaction kinetics in a mass system, these copolymers
are made in an emulsion polymerization system, which favors chain propagation
and disfavors termination (276). The result is economically acceptable rates with
desirable chain lengths. Usually, such processes are run batchwise in order to
achieve satisfactory particle-size distribution.
Styrene–maleic anhydride (SMA) copolymers are used where improved resistance to heat is required. Processes similar to those used for SAN copolymers are
used. Because of the tendency of maleic anhydride to form alternating copolymers
with styrene, composition drift is extremely severe unless the polymerization is
carried out in CSTR reactors having high degrees of backmixing.
Divinylbenzene copolymers with styrene are produced extensively as supports for the active sites of ion-exchange resins and in biochemical synthesis.
About 1–10 wt% divinylbenzene is used, depending on the required rigidity of
the cross-linked gel, and the polymerization is carried out as a suspension of the
monomer-phase droplets in water, usually as a batch process. Several studies have
been reported recently on the reaction kinetics (277,278).
Polymerization of styrene in the presence of PB rubber yields a much tougher
material than PS. Although the material is usually opaque and has a somewhat
lower modulus, the gain in impact resistance has placed this plastic in very wide
commercial usage. The polymerization is complex because the rubber has sufficient reactivity, ie, at the allylic hydrogen and addition to 1,2-vinyl units, to
graft and cross-link and the polymerization system is comprised of two phases.
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Fig. 52. Ternary phase diagram for the system styrene PS–PB rubber.
The graft copolymer of PS on polybutadiene acts as an emulsifier, stabilizing the
dispersion against coalescence. Events taking place during this polymerization
can be followed with the ternary phase diagram shown in Figure 52. For a feed
mixture made up of 8 wt% rubber in styrene (point A) on an increment of conversion to PS (to D), phase separation occurs. Small droplets of styrene/PS, which
are stabilized by the graft copolymer, are dispersed in the styrene/polybutadiene
solution. The composition of the two phases is given by points B and C for the PSrich phase and polybutadiene-rich phase, respectively. The phase-volume ratio,
ie, rubber phase/PS phase, is given by the ratio DB/DC. As the reaction proceeds
along the line AE, the phase composition and phase-volume ratio can be read
from the tie lines. Larger droplets of PS solution form and the small, original ones
remain. When the phase-volume ratio is about unity (point F), provided there
is sufficient shearing agitation, the larger PS-containing droplets, which have
mostly coalesced into large pools, become the continuous phase and polybutadienecontaining droplets are dispersed therein (279,280). However, the latter contain
the small, original PS particles as occlusions. The particle size and size distribution
are largely controlled by the applied shear rate during and after phase inversion,
the viscosity of the continuous phase, the viscosity ratio of the two phases, and
the interfacial tension between the phases (81,279). The viscosity parameters depend on phase compositions, polymer molecular weights, and temperatures; the
interfacial tension is largely controlled by the amount and structure of the graft
copolymer available at the particle surface. Phase-contrast micrographs of this
phase-inversion sequence, wherein the PS phase is dark, are shown in Figure 53.
If shearing agitation is insufficient, then phase inversion does not occur and
the product obtained is an inter network of cross-linked rubber with PS, and
its properties are much inferior. The morphology of the dispersed rubber phase
remains much as formed after phase inversion, unless there is a step change in
phase concentration by, eg, blending streams. Rubber-phase volume, including the
occlusions, largely controls performance; for example, Table 6 illustrates Izod impact strength dependence (281).
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Fig. 53. Phase-contrast photomicrographs showing particle formation via phase
inversion.
Once the desired particle size and morphology with sufficient graft to provide
adequate adhesion between the rubber particles and the PS phase have been
achieved, the integrity of the particle must be protected against deformation or
destruction during fabrication by cross-linking of the polybutadiene chains. These
chemical changes taking place on the rubber begin with polymerization. Either
anionic or Ziegler–Natta-polymerized polybutadiene is used at 5–10 wt% styrene.
If peroxide initiators are used, about half of the rubber is grafted by the time
phase inversion occurs; less is grafted in the absence of peroxides (282).
Hydrogen abstraction by alkoxy radicals at any of the allylic sites yields a
site for styrene polymerization, resulting in a graft (282,283). Model studies based
on styrene–butadiene block copolymers show a strong influence of graft structure
on particle morphology (81,82). The double bonds in polybutadiene are much less
Table 6. Dependence of Izod Impact
Strength on Rubber Particle Size
Rubber particle weight
average dia., µm
0.6
0.8
1.0
1.5
2.0
2.5
3.0
3.5
48
53
64
75
91
93
99
100
a ASTM
b To
Izod impact
strengtha , J/mb
D256.56.
convert J/m to ft·lbf/in., divide by 53.38.
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reactive than those in styrene, but as conversion exceeds ca 80% and temperature
approaches or exceeds 200◦ C, copolymerization through the polybutadiene (1,2
units) chain seems to occur and leads to cross-linking and, hence, gel formation
(284).
Since the extent of grafting of PS onto PB has such a profound effect on the
properties of HIPS, there has been a significant amount of research carried out
to develop an understanding of the grafting process and also the development of
ways to increase the extent of grafting (60,285–288).
Commercial Polymerization Processes
Polymerization. There are two problems in the manufacture of PS: removal of the heat of polymerization (ca 700 kJ/kg (300 Btu/lb)) of styrene polymerized and simultaneously handling a partially converted polymer syrup with
a viscosity of ca l05 mPa (= cP). The latter problem strongly aggravates the former. A wide variety of solutions to these problems have been reported. For the four
mechanisms described earlier, ie, free radical, anionic, cationic, and Ziegler–Natta,
several processes can be used. Table 7 summarizes the processes that have been
used to implement each mechanism for liquid-phase systems. Free-radical polymerization of styrenic systems, primarily in solution, is of principal commercial
interest. Suspension processes are declining in importance (289,290). Emulsion
processes have been reviewed (291). The historical development of styrene polymerization processes has been reviewed (289,292,293).
Two types of reactors are used for continuous solution polymerization. The
first is the continuous plug-flow reactor (CPFR), approximating in the ideal
case a plug-flow reactor, and the second is the continuous-stirred tank reactor
(CSTR), which ideally is isotropic in composition and temperature (see REACTOR
TECHNOLOGY). The CPFR usually involves conductive heat transfer to many tubes
through which a heat-transfer fluid flows. Multiple temperature zones can easily be achieved in a single reactor; agitation is provided by a rotating shaft with
arms down the central axis of the tubular vessel. Reactors of this type operate for
long periods and handle very high viscosity partial polymers with reliable heattransfer coefficients. CSTRs are either of the recirculated coil configuration and
rely upon conduction to the cooled jacket for heat removal or are ebullient making
use primarily of evaporative cooling to achieve temperature control. The limitation
Table 7. Process vs Mechanism for Styrene Polymerization
Mechanism
Process
Free radical
Anionic
Cationic
Zeigler
Mass/bulk
Solution
Used for all styrene plastics
Used for all styrene plastics
Yes
Styrene–butadiene
block copolymers
Yes
Yes
Yes
Suspension
Precipitation
Emulsion
Used for all styrene plastics
Yes
Used for styrene–butadiene
latices and ABS plastics
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Fig. 54. Typical continuous plug-flow reactor (CPFR) polymerization process.
of this reactor occurs at high viscosity, when the rate of vapor disengagement is
exceeded by the rate of vapor generation and the ability to mix in the condensed
vapor. Typically, this limit occurs at 60–70 wt% solids. Figures 54 and 55 illustrate
typical processes based on these two types of reactors. Most GPPS is manufactured
in the United States in such facilities. The CPFR process (Fig. 54) typically uses
up to 20 wt% ethylbenzene mainly to reduce viscosity, but also to eliminate reactor
fouling. The three reactors may each be subdivided into three temperature-control
zones, which allows for flexible control of the molecular weight distribution of the
polymer as well as optimum output per reactor volume. Temperatures are 100–
170◦ C. At ca 80 wt% solids, the partial polymer is heated from 180◦ C to 240◦ C
before being devolatilized upon entering a vacuum tank. The recovered solvent
and monomer are continuously recycled to the feed. The molten polymer leaves the
vacuum tank and is stranded, cooled, and chopped into pellets for shipping. The
CSTR process (Fig. 7) differs in that it is typically a single reactor and, therefore,
Fig. 55. Typical CSTR polymerization process.
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involves a single temperature zone and operates at ca 60 wt% solids, requiring a
large polymer heater to achieve adequate devolatilization (294,295). The need for
a large polymer heater to achieve adequate devolatilization can be eliminated by
adding a CPFR after the CSTR to carry the conversion to high solids.
There has been some research aimed at continuously converting monomer
completely to polymer using reactive extrusion (296). However, this process has
not yet been commercialized.
Rubber-modified PS manufacture places several additional demands: (1) dissolving ca 5–10 wt% polybutadiene in styrene, often a 10- to 20-h process; (2) shear
conditions to achieve phase inversion and the desired particle size for a given product; (3) control of phase compositions to produce the desired particle morphology;
and (4) sufficient time and temperature at the end of the process to achieve the necessary cross-linking, gel formation, and swelling index. Graft copolymer formation
occurs throughout the polymerization, and sufficient amount of graft copolymer
for phase stability is necessary at phase inversion. In the case of a CPFR system
(Fig. 54), only addition of a rubber dissolver is needed to meet these requirements.
For a CSTR-based system (Fig. 55), multiple reactors in series are required to
avoid rapid shifts in phase compositions on mixing of one reactor effluent with
the next reaction contents with loss of desirable morphology (297,298). Figure 56
shows a multizone CSTR reactor system. Phase inversion and particle sizing is
accomplished in the first-stage reactor with the effluent entering a single-tank,
baffled reactor operating as five stages with a common vapor space. An alternative is three CSTR reactors in series, followed by an adiabatic reactor to achieve
high solids content (297). The multiple stages are necessary to achieve favorable
Fig. 56. Typical staged CSTR process for HIPS.
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rubber-particle morphology with CSTRs; CPFR systems inherently avoid sudden
phase-composition changes, unless severe internal backmixing is allowed to occur.
Generally, phase inversion and particle sizing occurs in the first reactor.
Grafting rate is highest initially, and the amount formed is adequate to stabilize
the particles. The shear field in the reactor is critical to the control of particle size
as well as to meet the minimum shear rate for phase inversion, without which
the product consists of a continuous cross-linked rubber network with markedly
inferior properties (279,280). After the particles are formed and sized, only gentle
agitation is applied to aid heat transfer without disturbing the particle structure.
Conditions that produce the desired PS-phase molecular weight are used. Prior to
devolatilization, the partial polymer is heated to ca 200◦ C to cross-link the rubber
chains in order to protect the rubber particles from the very high shear fields
during fabrication. Stranding, cooling, and chopping into pellets follows.
Polybutadiene rubbers are used almost exclusively as reinforcing agents for
styrenic plastics. The very low glass-transition temperature (−80◦ C or lower) gives
good low temperature properties, and the allylic hydrogen atom and weakly active
double bonds can provide the desired degree of grafting and cross-linking. Other
rubbers have been used; acrylates, ethylene–propylene–diene rubbers, polyisoprene, and polyethylene have been reported, but with limited commercial success
because of their low chemical reactivity, not withstanding their acceptable glasstransition temperature (299).
Devolatilization. Current bulk polymerization processes convert styrene
to about 80%. The polymer syrup exits the polymerization reactor at about 170◦ C.
The most common devolatilization technology involves increasing the temperature of the syrup to >230◦ C by passage through a heat exchanger connected to a
vacuum tank. The syrup foams, forming strands which fall into a vacuum tank
at 5–10 mm Hg as they exit the heat exchanger. An equilibrium of styrene in the
polymer and in the vapor phase takes place, which is dependent upon the temperature and the pressure in the tank. Figure 57 shows the vapor–polymer equilibrium
partitioning data (calculated) for styrene in PS at various levels of vacuum (onestage devolatilizer) (300). Typically, devolatilizers operate at ca 230–240◦ C and
5–10 mm Hg. Therefore, the level of residual styrene monomer left in the polymer
is typically in the range of 300–500 ppm. If the polymer temperature is increased
to greater than 240◦ C, polymer degradation begins to take place resulting in the
formation of styrene monomer. Asahi researchers have solved this problem by stabilizing the polymer against unzipping by the addition of a special stabilizer (18),
thus allowing higher temperature devolatilization to be carried out (301).
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Fig. 57. Vapor-polymer equilibrium partitioning data for styrene in PS vs temperature
and pressure. · · ·◦· · · 133.3 Pa; -—667 Pa;——1.33 kPa; - -•- - 667 Pa + 10% water. To
convert Pa to mm Hg, divide by 133.3.
Some foods packaged in PS are extremely sensitive to taste (eg, chocolate
chip cookies). These sensitive applications require PS having residual styrene
monomer levels below 200 ppm. It is not feasible to consistently lower the styrene
level to <200 ppm without special techniques.
A new technology for lowering residuals in PS being commercialized by Dow
is centrifugal devolatilization in a rotating packed bed (302,303). The basic concept
involves pumping the molten PS into the center of an evacuated spinning rotor
packed with metal mesh. Centrifugal force quickly pushes the polymer through
the high surface area mesh where it coats the metal and volatiles quickly diffuse
from the thin polymer film. The centrifugal force continues to force the polymer out
of the metal mesh packing against the exterior surface of the centrifuge having
a circumferential set of die holes. After being extruded from the die holes, the
polymer strands are cut into pellets as the rotor rotates past a slowly moving,
continuous knife blade. A drawing of the centrifugal devolatilizer is shown in
Figure 58.
A very effective way to get residual styrene monomer levels below equilibrium is to replace the styrene in the vapor phase with something else. Generally,
it is preferred that the material in the vapor phase be condensable, nontoxic, low
cost, easily separable from the recycle, and inert (eg, water). However, simply
bleeding the replacement material into the vapor space of the devolatilizer would
still require significant residence time to reach equilibrium. A way to expedite the
equilibrium process is to mix the replacement material with the molten polymer
(304). Then as the material vaporizes in the molten polymer, it nucleates bubbles stripping styrene from the polymer while creating a vapor phase composed
mainly of the stripping material. Stripping materials that have been used include
nitrogen, (305), methanol (306), and water (307).
Another technique to expedite the transport of the volatile components from
the molten polymer is to increase the number and rate of bubbles formed (308).
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STYRENE POLYMERS
N2 Purge
Vacuum
311
Cutter
(Vapor)
Hollow shaft
Polymer melt
Packing
Drive
Seal
Seal housing
Drive base
Fig. 58. Drawing of centrifugal devolatilizer being used by Dow.
Techniques that have been used to increase the number of bubbles and their rate
of formation (nucleation) are the addition of chemical nucleating agents (309) and
ultrasound (310). Nucleation of bubbles in the molten polymer can help expedite
achievement of equilibrium in conventional falling strand devos. However, this facilitation mechanism can not get below equilibrium and thus has marginal value.
Chemically Removing Residual Monomer. Some researchers have
tried to lower the level of residual styrene monomer by chemical means. The idea
of adding something to molten PS that scavenges residual styrene monomer has
been the subject of extensive R&D efforts over the past several decades. However,
there is an inherent difficulty in this concept; that is, small molecules in very low
concentration must diffuse together, find each other, and couple together in a viscous polymer. The obvious way to overcome this difficulty is to use the scavenger in
higher than stoicheometric concentration. This introduces several new problems
including contamination of the polymer with a new material and increased raw
material costs. Styrene scavenger compounds that have been patented include
unsaturated fatty acids (311) and aldehydes (312), sulfonyl hydrazides (313), cyclopentadienes (311), K-resin (314), peroxides (315), and terpenes (316).
Another option for lowering residual styrene monomer is to carry the polymerization up to high conversion. Typically, continuous bulk polymerization processes produce partial polymer syrups at about 65–80% solids. The unreacted
styrene that remains is removed by evaporation. If the solids content of the polymer could be taken higher, the level of residual styrene in the polymer would
be lower, especially when using a one-stage devolatilizer. In suspension polymerization, devolatilization is not even required because styrene is polymerized to
>99.9% monomer conversion. If it were possible to polymerize styrene to very high
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STYRENE POLYMERS
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conversion in bulk polymerization processes, significantly lower residual styrene
monomer could be achieved. Because of viscosity constraints, bulk polymerization
reactors cannot operate at >80% solids. To achieve >99% monomer conversion
while maintaining the solids <80% would require high levels of solvent. The problem with solvents is chain transfer. Going above 95% conversion in ethylbenzene
results in an exponential increase in polydispersity due primarily to chain transfer effects. A high conversion bulk styrene polymerization process resulting in the
formation of low residual PS has been patented, which solves the viscosity problem and allows going to >99.9% solids by finishing the polymerization off in an
extruder (317). Extruders are not very effective heat exchangers, yet are designed
for handling high viscosity materials. Thus, Kelly carried out the polymerization
of pure styrene monomer without the use of a solvent in a conventional polymerization reactor to normal solids levels and then fed the partial polymer into an
extruder where he finished off the polymerization. He used a mixture of initiators
having different half-lives so that radicals were continuously generated. Recently,
free-radical polymerization of styrene directly in an extruder using 1 wt% peroxide initiator was analyzed (318). They were not successful in making PS having
a molecular weight >100,000. They got around this problem by attaching a prepolymerization reactor to the front end of the extruder to polymerize to 25% solids
to make high M w PS. The low M w PS then made in the extruder giving them a
bimodal PS. The conversion they achieved in the extruder was 98–99%.
A final technique that has been utilized to chemically remove residual
styrene in PS is radiation treatment. Both beta (e-beam) and gamma radiation
have been tried. E-beam appears to be the most effective form of radiation and
is most suitable for continuous use (319,320). The e-beam ruptures C H bonds,
resulting in the formation of PS radicals. These radicals are very reactive and
scavenge unreacted monomer. However, if no styrene is in the vicinity of the PS
radical, it can do other things such as couple with another PS radical or react with
oxygen. Currently, electron beam treatment of polymers is used commercially for
elastomer cross-linking (wire/cable coatings) but not for monomer reduction.
Polystyrene treated with 1–4 Mrad of e-beam radiation at temperatures between 25 and 200◦ C shows an optimum irradiation temperature between 80 and
150◦ C (Fig. 59). It is interesting that all dosages tested have a temperature range
for maximum effectiveness, which drifts toward lower temperature as the dosage
increases (321). At all temperatures studied, weight-average molecular weight
increases with e-beam dose (Fig. 60) (321).
Fabrication.
Injection Molding. There are two basic types of injection-molding machines
in use: the reciprocating screw and the screw preplasticator (322). Their simple design, uniform melt temperature, and excellent mixing characteristics make them
the preferred choice for injection molding. Machines with shot capacities up to
25 kg for solid injection-molded parts and 65 kg for structural foam parts are available (323,324). Large solid moldings include automotive dash panels, television
cabinets, and furniture components. One-piece structural foam parts weighing
35 kg or more are molded for increased rigidity, strength, and part weight reduction (325) (see PLASTICS PROCESSING).
The injection-molding process is basically the forcing of melted polymer into
a relatively cool mold where it freezes and is removed in minimum time. The shape
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313
Fig. 59. Performance of different e-beam doses vs temperature (321). - -◦- - 10 kGy; -—20
kGy; -—30 kGy; - -•- - 40 kGy. To convert kGy to Mrad, divide by 10.
of a molding is defined by the cavity of the mold. Quick entry of the material into
the mold followed by quick setup results in a significant amount of orientation in
the molded part. The polymer molecules and, in the case of heterogeneous rubbermodified polymers, the rubber particles tend to be highly oriented at the surface
of the molding. Orientation at the center of the molding tends to be significantly
less because of the relaxation of the molten polymer.
Fig. 60. Effect of dose on M w at ambient and elevated temperature (321). - -◦- - 25◦ C;
-—85◦ C. To convert kGy to Mrad, divide by 10.
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The anisotropy that develops during the molding operation is detrimental
to the performance of the fabricated part in several ways. First, highly oriented
moldings, which form particularly if low melt temperatures are used, exhibit good
gloss, have an abnormally narrow use temperature range causing early warping,
and, perhaps most importantly, tend to be brittle even though the material is
inherently capable of producing tough parts (326). However, this development of
polymer orientation during molding can also be used to advantage, as in the case
of rotational orientation during the molding operation (327).
The achievement of isotropic moldings is also important when the molded
part is to be decorated, ie, painted, metallized, etc Highly oriented parts that have
a high frozen-in internal stress memory tend to give rise to rough or distorted
surfaces as a result of the relaxing effect of the solvents and/or heat. For electroplating ABS moldings, it is particularly important to obtain isotropic moldings.
Isotropy can be achieved by the use of a high melt temperature, a slow fill speed,
a low injection pressure, and a high mold temperature (127,128,328,329).
Injection molding of styrene-based plastics is usually carried out at 200–
300◦ C. For ABS polymers, the upper limit may be somewhat less, since these
polymers tend to yellow somewhat if too high a temperature and/or too long a
residence time are imposed.
To obtain satisfactory moldings with good surface appearance, contamination, including that by moisture, must be avoided. For good molding practice,
particularly with the more polar styrene copolymers, drying must be part of the
molding operation. A maximum of 0.1 wt% moisture can be tolerated before surface imperfections appear.
For achieving appropriate economics, injection-molding operations are
highly automated and require few operating personnel (322). Loading of the hopper is usually done by an air-conveying system; the pieces are automatically
ejected, and the rejects and sprues are ground and reused with the virgin polymer.
Also, hot probes or manifold dies are used to eliminate sprues and runners.
Extrusion. Extrusion of styrene polymers is one of the most convenient
and least expensive fabrication methods, particularly for obtaining sheet, pipe,
irregular profiles, and films. Relatively small extruders, eg, 11.5 cm diameter, can
produce well over 675 kg/h of polymer sheet. Extrusion is also the method for plasticizing the polymer in screw injection-molding machines and is used to develop
the parison for blow molding. Extrusion of plastics is also one of the most economical methods of fabrication, since it is a continuous method involving relatively
inexpensive equipment. The extrusion process has been studied in great detail
(330,331). Single-screw extruders work extremely well with styrene-based plastics. Machines are available with L/D (length-to-diameter) ratios of 36:1 or more.
Some of the longer L/D extruders are used with as many as three vent zones for
removal of volatiles, often eliminating the necessity for predrying as is practiced
with hygroscopic materials, eg, SAN and ABS. Where venting is inadequate, these
polymers must be predried to a maximum moisture content of 0.03–0.05 wt% to
obtain high quality sheet (see FILMS AND SHEETING).
Many rubber-modified styrene plastics are fabricated into sheet by extrusion
primarily for subsequent thermoforming operations. Much consideration has been
given to the problem of achieving good surface quality in extruded sheet (332,333).
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315
Excellent surface gloss and sheet uniformity can be obtained with styrene-based
polymers.
Considerable work has been done on mathematic models of the extrusion
process, with particular emphasis on screw design. Good results are claimed for
extrusion of styrene-based resins using these mathematical methods (331,334).
With the advent of low cost computers, closed-loop control of the extrusion
system has become commonplace. More uniform gauge control at higher output
rates is achievable with many commercial systems (335,336).
Lamination of polymer films, both styrene-based and other polymer-types,
to styrene-based materials can be carried out during the extrusion process for
protection or decorative purposes. For example, an acrylic film can be laminated
to ABS sheet during extrusion for protection in outdoor applications. Multiple
extrusion of styrene-based plastics with one or more other plastics has grown
rapidly in the last 10 years.
Thermoforming and Orientation. Thermoforming of HIPS and ABS extruded sheet is of considerable importance in several industries. In the refrigeration industry, large parts are obtained by vacuum-forming extruded sheet. Vacuum
forming of HIPS sheet for refrigerator-door liners was one of the most significant
early developments promoting the rapid growth of the whole family of HIPS. When
a thermoplastic polymer film or sheet is heated above its glass-transition temperature, it can be formed or stretched. Under controlled conditions, new shapes can
be controlled; also, various amounts of orientation can be imparted to the polymer
film or sheet for altering its mechanical behavior.
Thermoforming is usually accomplished by heating a plastic sheet above its
softening point and forcing it against a mold by applying vacuum, air, or mechanical pressure. On cooling, the contour of the mold is reproduced in detail. In order to
obtain the best reproduction of the mold surface, carefully determined conditions
for the plastic-sheet temperature, ie, heating time and mold temperature, must
be maintained.
Several modifications of thermoforming plastic sheet have been developed. In
addition to straight vacuum forming, there are vacuum snapback forming, drape
forming, and plug-assist-pressure-and-vacuum forming. Some combinations of
these techniques are also practiced. Such modifications are usually necessary to
achieve more uniform wall thickness in the finished deep-draw sections. Vacuum
forming can also be continuous by using the sheet as it is extruded. An example of
this technology is practiced with several high speed European lines in operation
in the United States. Precise temperature conditioning allows carefully controlled
levels or orientation in the finished part (337).
Thermoforming is perhaps the process with the lowest unit cost. Examples of
thermoformed articles are refrigerator-door and food-container liners, containers
for dairy products, luggage, etc Some of the largest formed parts are camper/trailer
covers and liners for refrigerated-railroad-car doors (338).
Orientation of styrene-based copolymers is usually carried out at temperatures just above T g . Biaxially oriented films and sheet are of particular interest.
Such orientation increases tensile properties, flexibility, toughness, and shrinkability. PS produces particularly clear and sparkling film after being oriented
biaxially for envelope windows, decoration tapes, etc Oriented films and sheet of
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STYRENE POLYMERS
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styrene-based polymers are made by the bubble process and by the flat-sheet or
tentering process. Fibers and films can be produced by uniaxial orientation (339)
(see FILMS AND SHEETING).
Blow Molding. Blow molding is a multistep fabrication process for manufacturing hollow symmetrical objects. The granules are melted and a parison is
obtained by extrusion or by injection molding. The parison is then enclosed by
the mold, and pressure or vacuum is applied to force the material to assume the
contour of the mold. After sufficient cooling, the object is ejected.
Styrene-based plastics are used somewhat in blow molding but not as much
as linear PE and PVC. HIPS and ABS are used in specialty bottles, containers, and
furniture parts. ABS is also used as one of the impact modifiers for PVC. Clear,
tough bottles with good barrier properties are blow-molded from these formulations.
Polystyrene or copolymers are used extensively in injection blow molding.
Tough and craze-resistant PS containers have been made by multiaxially oriented
injection-molded parisons (340). This process permits the design of blow-molded
objects with a high degree of controlled orientation, independent of blow ratio or
shape.
Additives. Processing aids, eg, plasticizers and mold-release agents (see
ABHERENTS), are often added to PS. Even though PS is an inherently stable polymer, other compounds are sometimes added to give extra protection for a particular application. Rubber-modified polymers containing unreacted allylic groups
are very susceptible to oxidation and require carefully considered antioxidant
packages for optimum long-term performance. Ziegler–Natta-initiated polybutadiene rubbers are especially sensitive in this respect, since they often contain
organocobalt residues from the catalyst complex. For food-contact applications,
the additives must be FDA approved. Important additives used in styrene plastics are listed in Table 8.
Economic Aspects
Most of the styrene monomer manufactured globally goes into the manufacture
of PS and its copolymers; thus the price of the two tend to parallel each other
(Fig. 61).
Polystyrene is a global product with North America, Western Europe, and
the Pacific consuming most of the world’s production (Fig. 62). The global PS
production capacity generally parallels the demand for the material (Fig. 63).
However, the trend over the last 15 years has been toward narrowing the gap
between capacity and demand in an effort to maximize the profitability of the
business.
Characterization
Four modes of characterization are of interest: chemical analyses, ie, qualitative and quantitative analyses of all components; mechanical characterization,
ie, tensile and impact testing; morphology of the rubber phase; and rheology at
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317
Table 8. Additives Used in Styrene Plastics
Type
Plasticizers
Mold-release
agents
agents
Antioxidants
Compounds
Ignition
supression
agents
Comments
<4
all cause loss of
heat-distortion
temperature
Phthalate esters
Adipate esters
Steric acid
0.1
Many mold-release
agents cause
yellowness of haze
Metal sterates
Sterate esters
Silicones
Amide waxes
Alkylated phenols
Thioesters
uv stabilizers
Amount
(wt%)
Mineral oil
Phosphite esters
Antistatic
agents
Examples
Quaternary
ammonium
compounds
Benzotriazoles
Benzophenones
Halogenated
compounds
Ionol
Tris(nonylphenyl)
phosphite
Dilaurylthiodipropionate
3
1
Synergism sometimes
achieved with
multiple use
1
1
2
Tinuvin P
0.25
Synergism sometimes
achieved with
multiple use
Antimony oxide
Aluminum oxide
Phosphate esters
a range of shear rates. Other properties measured are stress-crack resistance,
heat-distortion temperatures, flammability, creep, and others depending on the
particular application (341).
Since plastics are almost invariably modified with one or more additives,
there are three components of chemical analysis: the high molecular weight portion, ie, the polymer; the additives, ie, plasticizer and mold-release agent; and the
residuals remaining from the polymerization process. The high molecular weight
portion is best characterized using gpc which gives molecular weight and polydispersity information. If the gpc is equipped with a diode-array detector, chromophores responsible for polymer discoloration can be located (177). The additives
in the polymer are best characterized by extracting them from the polymer and analyzing the extract using high performance liquid chromatography (342). Figure 5
illustrates typical molecular weight distribution curves from gpc, and Figure 64
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Fig. 61. Historical comparison of styrene monomer and GPPS pricing. · · ·◦· · · Styrene
monomer; -—Polystyrene.
Fig. 62. Global consumption of PS by geographical area (145).
shows a capillary gas chromatogram for determining residuals. Chemical
characterization techniques utilized for styrenic polymers are summarized in
Table 9. Mechanical testing is carried out according to the methods listed in
Table 10.
Rubber particle-size distribution is usually measured with a Coulter counter
or directly from electron photomicrographs (110,343); the latter also gives details
of particle morphology. Rheological studies are made with a modified tensile tester
with capillary rheometer (ASTM D1238-79) or with the powerful Rheometrics
mechanical spectrometer (344). For product specifications, a simple melt-flow test
is used (ASTM D1238 condition G) with a measurement of the heat-distortion
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319
Fig. 63. Global demand vs capacity for PS since 1985 (145).
temperature (ASTM D1525). These two tests, with solution viscosity as a measure
of molecular weight, have been used historically to characterize PS. However,
molecular weight distribution and residuals analyses, in general, have replaced
them.
Health and Safety Factors
In pellet form, styrene-based plastics have a very low degree of toxicity (345).
Under normal conditions of handling and use, they should pose no unusual problems from ingestion, inhalation, or eye or skin contact. Heating of these polymers
usually results in the release of some vapors. The vapors contain some styrene
[TLV = 100 ppm (1974); human detection, 5–10 ppm] and, in the case of styrene–
acrylonitrile copolymers, some acrylonitrile [TLV = 2 ppm (1978)] as well as very
low levels of degraded and oxidized hydrocarbons and additives. The warning
properties of styrene and any oxidized organics are such that the vapors normally
can be detected by humans at very low levels, and hazardous concentrations are
usually irritating and obnoxious. Nevertheless, adequate ventilation should be
provided.
Styrene polymers burn under the right conditions of heat and oxygen supply.
This can occur even when they are modified by addition of ignition-suppression
chemicals. Once ignited, these polymers may burn rapidly and produce a dense
black smoke. Combustion products from any burning organic material should be
considered toxic. Ignition temperatures of several styrenic polymers are listed in
Table 11. Fires can be extinguished by conventional means, ie, water and water
fog.
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Fig. 64. Capillary gas chromatogram showing residuals in PS.
Table 9. Chemical Characterization Techniquesa
High molecular
weight component
Additive
Residuals
Variable
Method
Variable
Method
Variable
Method
Molecular weight
Solution
viscometry
gpc
Mineral oil
gc/hplc
Styrene
gc
Phthalate
esters
Stearates
Other
gc
Solvents
gc
ir
hplc
Oligomers
gc
Molecular weight
distribution
Copolymer composition
Rubber content
Graft yield
ir
ir
solubility
= gel-permeation chromatography; gc = gas chromatography; hplc = high performance liquid
chromatography; ir = infrared analysis.
a gpc
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321
Table 10. Mechanical Tests on PS
Test
ASTM method
Stress–strain
Tensile
Compression
Flexural
Impact
Izod
Dart
Hardness
Creep
D638
D695
D790
D256
D3029
D785
D2990
Table 11. Ignition Temperatures and Burnhing Rates of Styrene-Based Polymersa
Polymer
Polystyrene
General–purpose
Rubber–modified
Styrene–acrylonitrile copolymer
Acrylonitrile–butadiene–styrene
Minimum
flash-ignition
temperature,◦ C
Minimum
self-ignition
temperature, ◦ C
Burning
rateb , cm/s
345–360
385–399
366
404
488–496
435–474
454
>404
<0.06
<0.06
<0.06
<0.06
specimens for burning rate data were 1.27 × 15.24 × 0.318 cm3 . Descriptions of burning rate
and other flammability characteristics developed from small-scale laboratory testing do not reflect
hazards presented by these or any other materials under actual fire conditions.
b In a horizontal position.
a Test
Uses
PS Foams. The early history of foamed PS is given in Reference 346 and
the theory of plastic foams is discussed in Reference 347. Dow became the first
to commercially manufacture PS-extruded foam board for home insulation under the trade name Styrofoam (348). Foamable PS beads were developed in the
1950s by BASF under the trademark of Styropor (349–351). These beads, made
by suspension polymerization in the presence of blowing agents such as pentane
or hexane, or by post-pressurization with the same blowing agents, have had
an almost explosive growth, with 200,000 metric tons used in 1980. Some typical physical properties of PS foams are listed in Table 12 (see FOAMED PLASTICS).
Some believe that there are unlimited new opportunities for the development of
new foam materials (352).
The following are commercially significant foamed PSs. Extruded planks
and boards in the density range of 29 kg/m3 and largely flame retardant are used
for low temperature thermal insulation, buoyancy, floral display, novelty, packaging, and construction purposes. Foamed boards and shapes from foaming-in-place
(FIP) beads (density 16–32 kg/m3 ) are used for packaging, buoyancy, insulation,
and numerous other applications. Batch molding of boards and shapes as well
as automated molding of continuous planks are used. Extruded foamed PS sheet
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Table 12. Characteristics Properties of Some Polystyrene Foams
Property
3b
Density, kg/m
Compressive strength, kPac
Tensile strength, kPac
Flexural strenght, MPad
Thermal conductivity, W/(m·K)e
Heat distortion, ◦ C
Styrofoama ,
extruded
Bead,
board-molded
Foam
sheet
35
310
517
1138
0.030
80
32
207–276
310–379
379–517
0.035
85
96
290
2070–3450
0.035
85
ASTM
D1621
D1623
D790
C177
a Registered
trademark of The Dow Chemical Co.
convert kg/m3 to lb/ft3 , multiply by 0.0624.
c To convert kPa to psi, multiply by 0.145
d To convert MPa to psi, multiply by 145.
e To convert W/(m·K) to (Btu·in.)/(h·ft·2◦ F), multiple by 6.933.
b To
17 mm thick with densities of 64–160 kg/m3 are used largely in packaging applications. Extremely fine cell size is required for texture and strength. Special nucleating agents are employed for this purpose, eg, polymers of styrene and maleic
anhydride, citric acid–sodium bicarbonate mixtures, and talc (353). High density
(35–64 kg/m3 ) extruded planks are used for heavy-duty structural applications.
Styrene copolymer foams containing, eg, acrylonitrile for gasoline resistance are
made either by extrusion or from beads. High density (480–800 kg/m3 ) injectionmolded objects are made from mixtures of FIP beads and PS granules or more
commonly HIPS or ABS with a chemical blowing agent, eg, azodicarbonamide.
These moldings have a high density skin of essentially PS and a foamed core.
Foamable ABS systems, eg, laminates with ABS skins and a heat-foamable core,
are used for structural applications, as in car body parts (354). Of growing commercial significance is coextruded, foam-core ABS pipe (355,356). Reducing the
density of the foam core by up to one-half and using a chemical blowing agent
results in about a 20% overall pipe weight and raw material usage reduction.
Syntactic foams are combinations of foamable beads, eg, expandable PS or SAN
copolymers. These are mixed with a resin, usually thermosetting, which has a
large exotherm during curing, eg, epoxy or phenolic resins (357). The mixture is
then placed in a mold and the exotherm from the resin cure causes the expandable
particles to foam and forces the resin to the surface of the mold. A typical example
is expandable PS in a flexible polyurethane-foam matrix used in cushioning applications (358). Sandwich panels are made either by foaming beads between the
skin materials or by adhering skins to planks cut to precise dimensions. Foamed
PS beads are admixed with concrete for lightweight masonry structures (359).
Extruded Rigid Foam. In addition to low temperature thermal insulation, foamed PSs are used for insulation against ambient temperatures in the
form of perimeter insulation and insulation under floors and in walls and roofs.
The upside-down roof system, in which foamed plastic such as Styrofoam plastic
foam is applied above the tar-paper vapor seal, thereby protecting the tar paper
from extreme thermal stresses that cause cracking, has been patented (360). The
foam is covered with gravel or some other wear-resistant topping (see ROOFING
MATERIALS).
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In addition to such thermal-insulation uses in buildings, there is a potentially tremendous new area in the form of highway underlayment to prevent frost
damage. Damage to roadways, roadbeds, and airfields because of frost action is a
costly and aggravating problem. Conventional treatments to prevent such damage
are expensive and unreliable. During the 1960s, an improved and more economical solution to the problem was developed (361). It is based on the use of thermal
insulation to reduce the heat loss in the frost-susceptible subgrade soil so that no
freezing occurs. Many miles of insulated pavements have been built in the United
States, Canada, Europe, and Japan. The concept has proved valid, and the performance of extruded PS foam has been completely satisfactory. It is expected that
the use of this concept will continue to grow as natural road-building materials
continue to become more scarce and expensive and as the weight and speed of
land and airborne vehicles continue to increase.
A 2.54-cm Styrofoam plastic foam with thermal conductivity of ca
0.03 W/(m·K) [(0.21 Btu·in.)/(ft·h·◦ F)] is equivalent to 61 cm of gravel. Any synthetic foam having compressive strength sufficiently high and thermal conductivity sufficiently low is effective. However, the resistance of PS-type foams to water,
frost damage, and microorganisms in the soil makes them especially desirable. An
interesting and important application of this concept was the use of Styrofoam in
the construction of the Alaska Pipeline. In this case, the foam was used to protect
the permafrost.
Rigid Foam from FIP Beads. Expandable PS (EPS) has been in wide
use in packaging since its introduction. Applications range from pallet shipping
containers for computer terminals to material-handling stacking trays to packages
for fresh fruits and vegetables. Expandable PS serves as a cushioning material, a
package insert for blocking and bracing, a flotation item, or as insulation.
The basic resin for EPS is in the form of beads that are expanded to a desired
density before molding. Densities for packaging parts typically are 20–40 kg/m3 .
Once expanded, the beads are fused in a steam-heated mold to form a specific
shape. Most parts are molded of standard white resins, although several pastel
colors are available.
Converters who manufacture EPS parts and components for packaging are
called shape molders. Others who mold large billets, eg, measuring 0.6 × 1.2 ×
2.4 m, are called block molders. The package user can obtain EPS parts without
the benefits of a mold; parts can be fabricated from billets by hot-wire cutting.
Whether molded or fabricated, EPS packages and their components are typically
designed by careful consideration of the compression and cushioning properties
of expanded PS (362).
A large number of factors that influence the foaming of foamable PS beads,
eg, the molecular weight of the polymer, polymer type, blowing-agent type and
content, and bead size, have been analyzed (363). It is suggested that at least
part of the pentane blowing agent is present in microvoids in the glassy PS. Thus,
the density of beads containing 5.7 wt% n-pentane is 1080 kg/m3 , compared to a
value of 1050 kg/m3 for pure PS beads and compared with a calculated density of
1020 kg/m3 for a simple mixture in which n-pentane is dissolved in the PS. If all of
the pentane were in voids, the calculated density would be 1120 kg/m3 . About 60%
of the pentane is held in voids, with the balance presumably dissolved. n-Pentane
present in voids has a reduced vapor pressure, which depends on the effective
324
STYRENE POLYMERS
Vol. 4
Table 13. Loss of Isopentane from Foaming-in-Place Chlorostyrene Beadsa in Open
Storage, wt%b
After 20 h
◦
After 300 h
Temperature, C
Polystyrene
Poly(chlorostyrene)
Polystyrene
Poly(chlorostyrene)
25
50
80
18
38
48
1.3
12
24
40
72
80
10
33
54
a 420-mm
b Initial
beads.
concentration of isopentane = 6 wt%.
microcapillary diameter. Dissolved n-pentane in PS will also have a reduced vapor
pressure, by an amount depending on the thermodynamic interaction between
solvent and polymer.
An unspecified experimental styrene copolymer, possibly with acrylonitrile,
shows a greatly reduced tendency to lose blowing agent on aging at 23◦ C (363).
FIP beads from chlorostyrene likewise have a greatly enhanced ability to retain
blowing agent as indicated in Table 13. The higher heat distortion of this polymer
requires steam pressures of 210–340 kPa (30–50 psi) for blowing.
Various other factors that influence the foaming of PS, especially crosslinking, are described (359). In Reference 359, foaming is discussed as a viscoelastic process in which there is competition between the blowing pressure, which
increases with temperature, and the thinning and rupture of cell walls with consequent collapse of the foam. The presence of cross-links reduces the tendency for
cell-wall rupture. Figure 65 shows foam volumes attained as a function of temperature for various amounts of divinylbenzene. The effects of temperature and
cross-linking on the kinetics of foaming is also discussed in Reference 359.
Fig. 65. Maximum foaming volume of styrene divinylbenzene copolymers containing
8.8 wt% CO2 as a function of divinybenzene content and temperature. Numeral beside
curve indicates wt% divinylbenzene. V t /V ratio of final volume to initial volume at temperature t. Adapted from Ref. 362.
Vol. 4
STYRENE POLYMERS
325
Another important factor affecting the foaming of PS beads is the differential
rate of diffusion of n-pentane compared to steam and air. It is estimated that the
quantity of n-pentane normally used is sufficient to produce a foaming volume
of 30-fold and hence a foam density of no more than 32 kg/m3 . However, the
steam and/or air that diffuses into the beads contributes to further expansion,
so that foaming volumes of 60-fold or more are commonly achieved. This aspect
of diffusion has been discussed in some detail (364). In cycle foaming in air, beads
are allowed to equilibrate with air after each heat-foaming step through a series
of stages. Foaming volumes of 200:1 have been achieved in this manner, with
densities down to 3.2–4.8 kg/m3 .
Lightweight concrete is made from prefoamed EPS beads, Portland cement,
and organic binders. Precast shapes are being used to provide structural strength,
thermal insulation, and sound deadening.
Polystyrene as a Raw Material for Rigid Thermoplastic Foams. The
low cost of PS and its high thermal stability, which provides good recycle efficiency, as well as the possibility of using low cost blowing agents, are important
to the use of PS in rigid thermoplastic foams. The heat-distortion point of ca
80◦ C (in contrast to 65◦ C for PVC foam) permits its use in most construction applications. The low sensitivity of the polymer to moisture is another important
factor.
Because of all of the aforementioned and possibly for other reasons, PS has
been the dominant material used in rigid thermoplastic foams. Poly(vinyl chloride)
is economical and has good physical properties and is flame retarding, but it has
marginal heat-distortion properties for some uses and does not readily lend itself
to the continuous extrusion process used for PS foam. So far, no FIP beads system
based on PVC has been perfected. This could well be a result of its high vapor
barrier compared to PS. Polyethylene is too permeable to retain a blowing agent
and hence is ruled out as a FIP bead material, but it is commercially significant
as extruded foam.
Flame Retardants. The growing use of PS foams in the construction
industry has provided impetus for continuing research on flame-retardant additives (see FLAME RETARDANTS). Organic bromine compounds, eg, or pentabromomonochlorocyclohexane, have long been used for this purpose in extruded PS
foam. Use of injection-molded structural foams, based on HIPS and ABS and
containing flame-retardant additives, has grown rapidly since the late 1970s.
A typical additive system to impart the required flame-retardant properties is
bis(pentabromophenyl) oxide and a synergist, eg, antimony trioxide. Because of
environmental concerns over the use of halogenated flame-retardant chemicals,
research is currently focusing upon the development of halogen-free systems.
Phosphorous containing compounds are currently the most heavily researched
alternative.
Foamed Sheet. PS foamed sheet is used for foamed trays, egg cartons,
disposable dinnerware, and packaging. Foam sheet manufacturing techniques,
manufacturers’ logistics, and markets are described in Reference 365. Choice of the
correct blowing agent for foam sheet is critical in determining ultimate density and
physical properties. Further, the choice of blowing agent may be dictated by safety
considerations, eg, flammability of hydrocarbons, or environmental concerns, eg,
chlorofluorocarbons (366).
326
STYRENE POLYMERS
Vol. 4
Table 14. Gas and Water Vapor Permeabilities of PS
Filmsa
Composition
Unmodified PS
72:28 styrene-stat-acrylonitrile
a Gas
Oxygen
CO2
Water
3.50
0.09
14.00
4.62
1600
2900
permeability units = (cm3 ·cm)/(1010 cm2 ·s·kPa).
Oriented PS Film and Sheet. Oriented PS film, in addition to having
the lowest cost of any of the rigid plastic materials, offers a high degree of optical
clarity, high surface gloss, and excellent dimensional stability (particularly with
regard to relative humidity) and is heat shrinkable. It is not a barrier film; in
fact, one of its largest uses, that of packaging field-fresh produce, depends upon
its being highly permeable to oxygen and water vapor (see Table 14).
PS films have been the subject of several reviews (367–370).
Although PS is normally considered a rather brittle material, biaxial orientation imparts some extremely desirable properties, particularly in regard to an
increase in elongation. Thus, the 1.5–2% of elongation normally associated with
unoriented PS can become as high as 10%, depending on the exact conditions of
preparation (371,372).
Several methods have been utilized in preparing biaxially oriented PS film.
The bubble process is used commercially. The high softening point, coupled with
the tendency of oriented film to shrink above 85◦ C, makes heat sealing difficult.
Solvent, adhesive, and ultrasonic sealing are used in most applications. Scratch
resistance, impact strength, and crease resistance are low. Although the bubble
process superficially resembles the common bubble process for making polyethylene, polypropylene, and Saran film, there are a number of substantive differences,
and the PS process is considerably more difficult to achieve. In the case of polyethylene, for example, crystallinity stabilizes the blown bubble, whereas with a true
thermoplastic, eg, PS, one must stabilize the blown film by cooling below the T g .
This involves extremely careful control of temperature in the blowing process.
As a general rule of thumb, there is an economic break-even point at ca
0.08 mm, which coincides with the defined difference between film and sheet. Film
is made more economically by the bubble method and sheet by the tenter-frame
method. The exact thickness for break-even depends on technological improvements, which can be made in both processes, in the degree of control which is used
in regulating them and in quality requirements.
PS film contains no plasticizers, absorbs negligible moisture, and exhibits
exceptionally good dimensional stability. It does not become brittle with age nor
distort when exposed to low or high humidity. Excellent machinability, ie, ability
to pass through packaging machinery at high speeds and to be cut and sealed,
etc, clarity, and stability are central factors in the use of oriented PS in window
envelopes. These same characteristics have also led to applications of film for window cartons. An antifog film 0.025–0.032 mm thick has been used extensively for
the windows of cartons for bacon. Substantial amounts of film are also employed
as sheet protectors and as inserts in wallets. A thickness of 0.060–0.130 mm is
customary for these applications. PS film can be printed by means of flexographic,
Vol. 4
STYRENE POLYMERS
327
rotogravure, and silk-screen methods. A lamination of reverse-printed film and
paper has been used for an attractive, high sparkle package for hand soap. The
film is an excellent base for metallizing because of the almost complete absence of
volatiles. Oriented PS film is also widely used for lamination to PS foamed sheet
for preprinted decoration and/or property enhancement (373).
For PS sheet, the use responsible for its rapid growth is in meat trays, where
the merchandising value is important, ie, the transparency of the package, as well
as its resistance to moisture and dimensional stability. A smaller but growing
use is as a photographic-film base where dimensional stability at all humidities
and low gel count are both important. The thermoplastic nature of PS and the
memory effects built into it through biaxial orientation make it especially suitable
for packaging applications. It must be processed under pressure rather than by
vacuum-forming equipment to avoid the shrinkage, which would otherwise result
from the high level of orientation.
Important uses of PS are as windows in envelopes, etc, as mentioned earlier,
and as produce overwrap. High optical clarity is required for mechanised reading
of characters through envelope windows, a significant technological trend which is
certain to enhance the use of PS film. In regard to produce overwrap, the transmission characteristics of PS for water vapor and oxygen coincide more or less with
the metabolic requirements of the produce being packaged, eg, lettuce, so that
in addition to giving mechanical protection to something like a head of lettuce,
the produce is able to metabolize in a normal manner (374). Low cost and high
optical clarity are important. In addition, the feel of PS film gives an impression
of crispness as far as the product is concerned. PS film also has potential use as a
type of synthetic paper (see PULP, SYNTHETIC). GPPS must be treated so that it is
opaque, and this can take the form of either pigmentation, mechanical abrasion,
or a surface treatment, eg, a chemical etch. HIPS is reasonably opaque. PS film
can be given a suitable coating, eg, a latex-clay coating, of the kind that is used on
ordinary paper for the purpose of achieving a good printable surface. There are
several general discussions of gas permeabilities in plastic films (6,8,9,375,376)
(see BARRIER POLYMERS).
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DUANE PRIDDY
Dow Chemical Company