styrene polymers

Vol. 4 STYRENE POLYMERS 247

STYRENE POLYMERS

Introduction

Polystyrene (PS), the parent of the styrene plastics family, is a high molecularweight, linear polymer. Its chemical formula [ CH(C6H5)CH2 ]n, where n (whichfor commercial uses) is between 800 and 1400, tells little of its properties. Themain commercial form of PS (atactic PS) is amorphous and hence possesses hightransparency. The polymer chain stiffening effect of the pendant phenyl groupsraises the glass-transition temperature (Tg) to slightly over 100◦C. Therefore,under ambient conditions, the polymer is a clear glass, whereas above the Tg itbecomes a viscous liquid which can be easily fabricated, with only slight degra-dation, by extrusion or injection-molding techniques. It is this ease with whichPS 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 isthe source of the polymer, the energy savings and environmental impact accruedduring fabrication and use, compared to alternative materials, more than offsetsthe short life of many PS articles (1).

Commercial manufacture of PS in North America began in 1938 by The DowChemical Co. The polymerization process involved loading 10-gal cans with neatstyrene monomer and immersing the can in a liquid bath heated at progressivelyhigher temperatures over several days until the monomer conversion reached99%. The solid polymer was removed from the cans and any rust spots chippedoff the cylinders of polymer with a hatchet (Fig. 1). The cylinders of polymer werefinally ground into a powder and packaged for shipment to customers.

Within a few years, Dow replaced the “can process” with the more econom-ical continuous bulk polymerization process still used today. Generally, the keyproblems 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.

248 STYRENE POLYMERS Vol. 4

Fig. 1. Dow Chemical worker chipping rust from cylinders of PS made using the “canprocess” (ca 1940).

of polymerization and pumping of highly viscous solutions. Conversion of themonomer to the polymer is energetically very favorable and occurs spontaneouslyon heating without the addition of initiators or catalysts. Because it is a contin-uous polymerization process, material-handling problems are minimized duringmanufacture. By almost any standard, the polymer produced is highly pure andis usually greater than 99 wt% PS; however, for particular applications process-ing aids are often deliberately added to the polymer. Methods for improving thetoughness, solvent resistance, and upper use temperature have been developed.Addition of butadiene-based rubbers increases impact resistance, and copolymer-ization of styrene with comonomers such as acrylonitrile or maleic anhydride pro-duces 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 invarious forms is being used increasingly in the construction industry, and modi-fied styrene plastics are replacing steel or aluminum parts in automobiles. Theseapplications result in energy savings beyond the initial investment in crude oil.The cost of achieving a given property, eg, impact strength, is among the lowestfor styrene plastics as compared to other competitive materials.

Properties

The general mechanical properties of styrene polymers are given in Table 1. Con-siderable differences in performance can be achieved by using the various styreneplastics. Within each group, additional variation is expected. In choosing an ap-propriate resin for a given application, other properties and polymer behaviorduring fabrication must be considered. These factors depend on the combinationof inherent polymer properties, the fabrication technique, and the devices, eg, amold used for obtaining the final object. Accordingly, consideration must be givento such factors as the surface appearance of the part and the development ofanisotropy, and its effect on mechanical strength, ie, long-term resistance of themolding to external strain.

Table 1. Mechanical Properties of Injection-Molded Specimens of Main Classes of Styrene-Based Plasticsa

Acrylonitrile–butadiene–

styrenePoly High terpolymer

(styrene-co- Glass- impact (ABS)d

Polystyrene acrylonitrile) filled PS Standard SuperProperty (PS) (SAN)b PSc (HIPS)d HIPS Type 1 Type 2 ABS ABS

CAS registry [9003-53-6] [9003-54-7] [9003-53-6] [9003-56-9]number

Specific gravity 1.05 1.08 1.20 1.05 1.05 1.05 1.05 1.04 1.04Vicat softening 96 107 103 103 95 99 108 103 108

point, ◦CTensile yield, 42.0 68.9 131 39.6 29.6 31.0 53.8 41.4 34.5

MPae

Elongation at 1.8 3.5 1.5 15 58 55 10 20 60rupture, %

Modulus, MPae 3170 3790 7580 2690 2140 2620 2620 2070 1790Impact strength 21 21 80 96 134 193 187 267 428

(notched Izold),J/m f

Dart-drop impact Very low Very low Medium high Low Medium high Medium high High Very high Very highstrength

Relative ease of Excellent Excellent Poor Excellent Excellent Excellent Good Good Medium goodfabrication

aRef. 2b24 wt% acrylonitrile.c20% glass fibers.dMedium molecular weight.eTo convert MPa to psi, multiply by 145.f To convert J/m to ft·lbf/in., divide by 53.38.

249


Fig. 2. PS tensile strength vs Mw (5). To convert Mpa to psi, multiply by 145.

Physical. An extensive compilation of physical properties of PS is givenin References 2 and 3. In general, a polymer must have a weight-average molec-ular weight (Mw) about 10 times higher than its chain entanglement molecularweight (Me) to have optimal strength. Below Me, the strength of a polymer changesrapidly with Mw. However, at about 10 times Me, the strength reaches a plateauregion (Fig. 2). For PS, the Me is 18,100 (4). Thus, PS having a Mw < 150,000 isgenerally too brittle to be useful. This indicates why no general-purpose moldingand extrusion grades of PS having Mw < 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. Highimpact PS (HIPS) has a higher strain energy than an acrylonitrile–butadiene–styrene (ABS) plastic, as shown in Figure 3. However, based on different impact-testing 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 tothe fact that ABS polymers tend to show only localized flow or necking tendencyat low rates of extension and, therefore, fail at low elongation. HIPS extendsuniformly during such tests, and the test specimen whitens over all of its lengthand 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 tem-perature lowers the strength. However, tensile modulus in the temperature regionof most tests (−40–50◦C) is affected only slightly. The elongations of PS and styrenecopolymers do not vary much with temperature (−40–50◦C), but the elongation


Fig. 3. Stress–strain curves for styrene-based plastics. To convert Mpa to psi, multiplyby 145.

of rubber-modified polymers first increases with increasing temperature but ulti-mately decreases at high temperatures.

The molecular orientation of the polymer in a fabricated specimen can sig-nificantly alter the stress–strain data as compared with the data obtained for anisotropic specimen, eg, one obtained by compression molding. For example, ten-sile strengths as high as 120 MPa (18,000 psi) have been reported for PS filmsand fibers (8). Polystyrene tensile strengths below 14 MPa (2000 psi) have beenobtained in the direction perpendicular to the flow.

Creep, Stress Relaxation, and Fatigue. The long-term engineering testson plastics in extended use environments and temperatures are required for pre-dicting the overall performance of a polymer in a given application. Creep testsinvolve the measurement of deformation as a function of time at a constant stressor 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 elonga-tion with only small variation with stress, whereas the rubber-modified polymersexhibit 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 polymerproperties in aggressive environments. Both the time to failure and the ultimateelongation in such creep tests tend to be reduced. Another test to determine plas-tic behavior in a corrosive atmosphere is a prestressed creep test in which thespecimens 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 func-tion of time at a constant strain, have also been used extensively to predict thelong-term behavior of styrene-based plastics (9,12). These tests have also beenadapted to measurements in aggressive environments (13). Stress-relaxation mea-surements are further used to obtain modulus data over a wide temperature range(14).


Fig. 4. Typical creep behavior for rubber-modified styrene polymers.

Fatigue is another property which is of considerable interest to the designengineer. Cyclic deflections of a predetermined amplitude, short of giving imme-diate failure, are applied to the specimen, and the number of cycles to failure isrecorded. In addition to mechanically induced periodic stresses, fatigue failurecan be studied when developing cyclic stresses by fluctuating the temperature.

Fatigue in polymers has been reviewed (15). Detailed theory and practice offatigue testing are covered. Fatigue tests are carried out for two main reasons: tolearn the inherent fatigue resistance of the material and to study the relationshipbetween specimen design and fatigue failure. Fatigue tests are carried out bothin air and in aggressive environments (16).

Melt Properties. The melt properties of PS at temperatures between 120and 260◦C are very important because it is in this temperature range that it isextruded to make sheets, foams, and films, or is molded into parts. Generally, itis desired to make parts having high strength from materials having low meltviscosity for easy melt processing. However, increased polymer molecular weightincreases both strength and melt viscosity. The melt viscosity of PS can be de-creased to improve its melt processability by the addition of a plasticizer suchas mineral oil. However, the addition of a plasticizer has a penalty, ie, the heat-distortion temperature is lowered. In applications where heat resistance is veryimportant, melt processability can be influenced, without a significant effect onheat resistance, by control of the polydispersity (17), by branching (18), or by theintroduction of pendant ionic groups, eg, sodium sulfonate (19,20).

Impact Strength. When they fail, thermoplastic polymers typically dis-sipate energy either through microscopic shearing and/or crazing. Because of itsinherently high entanglement molecular weight (19,000 g/mol), PS under plainstrain conditions (thick parts) does not form shear bands. Thus, pure PS fails un-der tensile stress because of the formation of crazes (regions of microfibrils thatbridge the top and bottom of a precursor crack) that rapidly develop into cracksand propagate catastrophically. Essentially, PS under plane strain conditions hasa high yield stress and a low break stress. There can be very high local strainsin the region of a craze, but very little material is deformed and little energy isdissipated. The crazes usually initiate at imperfections, such as surface scratches


or impurities in the bulk, and so as soon as one craze starts to form, the stressis concentrated at that site, the craze grows into a crack, and failure soon followsat very small macroscopic strain. Toughening PS involves finding ways to forceit to form more crazes, thus dissipating more energy, or forcing it to deform bymicroscopic shear banding (21–24).

There are basically two different approaches used, and two different types ofmaterials employed to obtain toughened PS. The first involves the use of dispersedrubber particles, which could have a variety of compositions including polybuta-diene (PB) or various elastic block copolymers, in a continuous PS matrix. Thesecond involves the use of styrenic block copolymers, or blends of block copoly-mers with PS homopolymer, where the block copolymer forms the continuousphase.

Dispersed rubber particles are the basis of toughening in HIPS. The toughen-ing mechanism is quite complex, and the various ways in which dispersed rubberparticles toughen PS have been discussed in detail (21–26). There are severalways in which the dispersed particles result in increased energy absorption dur-ing failure, but just two dominate. First, rubber particles result in the formationof multiple crazes, and thus the absorption of more energy. A rubber particle (as-sumed to be approximately spherical in shape) has a much lower modulus thanPS. 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 distancefrom the particle is proportional to the particle size. It has been pointed out thatfor crazes to develop, a stress greater than that at the tip of a craze must extendat least three craze fibril dimensions into the matrix (22). Thus, while all particlesresult in stress concentration, small particles will not induce crazing because thecritical stress does not extend far enough into the matrix. For HIPS, in which therubber particles are composed of PB with PS occlusions, the minimum particle sizeto induce crazing is about 0.8 µm (22). Optimum toughening occurs with rubberparticles from 1.0 to 2.0 µm.

The second major way in which rubber particles toughen PS is by allowing theformation of shear bands. Depending on the rubber particle size and volume frac-tion, 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 im-posed by tensile deformation) the PS ligaments are under stress rather than strain(26). Under these conditions, the PS can shear band, thus absorbing considerableamounts of energy. Recent experimental work has emphasized the importance ofshearing in addition to crazing in contributing to toughness in rubber-modifiedPS (24). Unfortunately, there are minimum rubber volume fractions and parti-cle sizes for this mechanism to be effective, and those restrictions can result insignificant light scattering and opacity as well.

The particle size restrictions for toughening PS depend on the mechanicalproperties of the rubber particle. For a HIPS rubber particle, with a Young’s mod-ulus of 333 MPa, the minimum size is 0.8 mm, while for a pure PB particle, witha Young’s modulus of 47 MPa, the critical size is 0.44 mm (27). The stiffer the par-ticle, the larger it must be. HIPS systems with bimodal particle size distributionscan also be effective in toughening (28), but the bimodal particle size distributionis generally detrimental to optical clarity.


Dispersed rubber particles make other, less significant, contributions to thetoughening of PS. Provided they are reasonably well adhered to the matrix andare somewhat tough, they can effectively terminate crazes and retard crazes fromdeveloping into cracks. The act of cavitation, either internal to the particle or at theparticle–matrix interface, can absorb some energy, but an insignificant amount.Some adhesion of the rubber particle to the PS matrix is needed, but the exactrole is not clear (23,25). In HIPS, the needed adhesion is provided by the PS thatis grafted to the rubber particles during the polymerization process.

In summary, in order for dispersed rubber particles to toughen PS, they mustbe above a critical size, which is of the order of 0.8 µm, depending on the natureof the rubber. Higher concentrations of rubber particles will increase toughnessby enabling more shear banding. Some adhesion between the rubber and matrixis also required.

HIPS materials can have a glossy or a dull surface appearance. This sur-face appearance is a function of surface roughness, which is caused by how muchthe rubber particles disrupt the surface regularity. The rubber particles near thesurface can disrupt the surface by causing either depressions or elevations. Theseirregularities are caused not only by the nature of the rubber particles (eg, size andshape) but also by processing conditions. For example, during injection molding,the surface of the polymer is pressed against a very smooth polished mold surfaceand quenched locking in the smooth surface. However, when HIPS is extrudedinto sheets and thermoformed into parts, it is allowed to cool slowly giving therubber particles near the surface time to relax. Polybutadiene rubber particlesshrink upon cooling more than the PS matrix, thus causing depressions (29).

Material Types

General-Purpose (not rubber-toughened) PS (GPPS). GPPS is a highmolecular weight (Mw = 2–3 × 105), crystal-clear thermoplastic that is hard, rigid,and free of odor and taste. Its ease of heat fabrication, thermal stability, low specificgravity, and low cost results in moldings, extrusions, and films of very low unitcost. 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 minimizedby effective devolatilization or by the use of chemical initiators (30). Polystyreneswith low overall volatile content have relatively high heat-deformation temper-atures. 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 oforganic materials from PS is of crucial importance in this application because oftaste and odor issues.

When additional lubricants, eg, mineral oil and butyl stearate, are added toPS, easy-flow materials are produced. Improved flow is usually achieved at the costof lowering the heat-deformation temperature. Stiff-flow PS has a high molecularweight and a low volatile level and is useful for extrusion applications. Typicallevels of residuals in PS grades are listed in Table 2. Differences in molecularweight distribution are illustrated in Figure 5.


Table 2. Residuals in Typical Polystyrene, wt%

Grade

Polystyrene Extrusion Injection molding

Styrene 0.04 0.1Ethylbenzene 0.02 0.1Styrene dimer 0.04 0.1Styrene trimer 0.25 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 PSfollowed by neutralization of the pendant acid groups with monovalent or diva-lent alkali metals. The introduction of ionic groups into PS leads to significantmodification of both solid state and melt properties. The introduction of ionic in-teractions in PS leads to increasing Tg, rubbery modulus, and melt viscosity (20).For the sodium salt of sulfonated PS, it has been shown that the mode of defor-mation changes from crazing to shear deformation as the ion content increases(31,32).

Tactic PS. Isotactic (iPS) and syndiotactic (sPS) PSs can be obtainedby the polymerization of styrene with stereospecific catalysts of the Ziegler–Natta-type. Aluminum-activated TiCl3 yields iPS while soluble Ti complexes[eg, (η5-C5H5)TiCl3] in combination with a partially hydrolyzed alkylaluminum[eg, methylalumoxane] yield sPS. The discovery of the sPS catalyst system wasfirst 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 crys-talline. Samples of iPS quenched from the melt are amorphous, but become


crystalline if annealed for some time at a temperature slightly below the crys-talline melting point. The rate of crystallization is relatively slow compared tosPS and with other crystallizable polymers, eg, polyethylene or polypropylene.This slow rate of crystallization is what has kept iPS from becoming a commer-cially important polymer even though it has been known for over 40 years. sPS, onthe other hand, crystallizes rapidly from the melt and is currently in the processof 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 amorphousstate, the properties of iPS and sPS are very similar to those of conventional at-actic PS. Crystalline iPS has a melting temperature of around 240◦C, while sPSmelts at about 270◦C (34). In the crystalline state, both iPS and sPS are opaqueand are insoluble in most common organic solvents.

Stabilized PS. Stabilized PSs are materials with added stabilizers, eg, uvscreening agents and antioxidants. Early stabilization systems for PS includedalkanolamines and methyl salicylate (35). In recent years, improved stabilizingsystems have been developed; these involve a uv-radiation absorber, eg, TinuvinP (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 phos-phite as a costabilizer with the uv-radiation absorber and the antioxidant (36).Rubber-modified styrene polymers are heat-stabilized with nonstaining rubberantioxidants, eg, Irganox 1076 (Ciba). Typically, stabilizer formulations for PSare designed by trial and error. However, recently a predictive model was de-veloped for PS photodegradation allowing the prediction of weatherability of PScontaining a certain concentration of a uv absorber (37).

Ignition-Resistant PS. Polymers containing flame retardants have beendeveloped. The addition of flame retardants does not make a polymer noncom-bustible, but rather increases its resistance to ignition and reduces the rate ofburning with minor fire sources. The primary commercial developments are in theareas of PS foams (see FOAMED PLASTICS) and television and computer housings.Both inorganic (hydrated aluminum oxide, antimony oxide) and organic (alkyland aryl phosphates) additives have been used (38). Synergistic effects betweenhalogen compounds and free-radical initiators have been reported (39). Severalnew halogenated compounds and corrosion inhibitors are effective additives (40)(see CORROSION AND CORROSION INHIBITORS). The polymer manufacturer’s recom-mendations with regard to maximum fabrication temperature should be carefullyobserved to avoid discoloration of the molded part or corrosion of the mold or themachine.


Antistatic PS. Additives or coatings are utilized to minimize primarily dust-collecting problems in storage (see ANTISTATIC AGENTS). Large lists of commercialantistatic additives have been published (38). For styrene-based polymers, alkyland/or aryl amines, amides, quaternary ammonium compounds, anionics, etc areused (see ADDITIVES).

Branched PS. Random-chain branching is found, to some extent, in allcommercial free-radically produced polymers. Branching takes place during man-ufacture by chain transfer to polymer and can also take place in the polymer aftermanufacture by exposure to uv or other forms of radiation. Chain branching insome polymers is known to improve certain properties and is practiced commer-cially (eg, polyethylene and polycarbonate). In PS, many types of branch structureshave 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 syntheticmethodologies (41–52) and on rheological properties (53–56).

Anionic polymerization has been the polymerization mechanism most widelyused to make and study branched PSs. This is likely due to the ability to control thetermination step, which is required for making many of the branch architecturesshown in Figure 6. However, all large-scale PS production plants utilize free-radical polymerization chemistry. This is likely due to the cost associated withmonomer purification needed for the anionic chemistry (57).

The PS business is extremely cost competitive. Oftentimes, an improvementin properties which is possible with a modification that would add a few percent-age to the manufacturing cost is not practical because the customer will not paythe increased cost. Any property advantage must be very significant before thecustomer will pay more. It is likely that branching in PS may lead to improvedperformance of PS in certain applications. However, it is uncertain whether the

Fig. 6. Several possible PS branch architectures.


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 sellingprice of the resin.

Since all commercial PS is currently manufactured using continuousfree-radical bulk polymerization, most industrial research aimed at producingbranched PS have been limited to free-radical chemistry. In general, the typeof branch architecture that is possible using free-radical chemistry is limitedto random-branched structures. Three reactor types are used commercially forcontinuous free-radical bulk polymerization of styrene (Fig. 7). These three reac-tor types are characterized as being either plug-flow or backmixed. Continuousplug-flow reactors (CPFR) typically have excellent radial mixing but virtually nobackmixing, unless they are recirculated. These reactors are usually described asstratified agitated tower reactors. Continuous stirred tank reactors (CSTR), on theother hand, have high degrees of backmixing. They are usually single-staged andoperated 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, ethyl-benzene) is utilized during continuous free-radical bulk polymerization. The ad-dition of a chain-transfer solvent to the polymerization decreases polymer molec-ular weight and therefore overall plant capacity when making high molecularweight products. However, the presence of some chain-transfer solvent is essen-tial in keeping the polymerization reactor from becoming eventually fouled withgel and insoluble cross-linked polymer. The mechanism of buildup during contin-uous free-radical bulk polymerization without a chain-transfer solvent is likelychain 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 strongpropensity to abstract H-atoms. This propensity has found utility when makingHIPS. For example, the use of initiators which generate t-butoxy radicals increases


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 generallythought to be due to H-abstraction from the rubber backbone by t-butoxy rad-icals (60). This raises questions regarding the extent of H-abstraction from thePS backbone during polymerization; that is, the use of initiators that generatepotent H-abstracting radicals may increase the extent of long-chain branchingduring PS manufacture. This question has been investigated by decomposing bis(t-butylperoxy)oxalate in benzene solutions of PS (61,62). Bis(t-butylperoxy)oxalatedecomposes upon heating to form two t-butoxy radicals and carbon dioxide. Oncethe t-butoxy radicals are formed they either abstract an H-atom or decompose toform acetone and a methyl radical (Fig. 8).

The extent of H-abstraction was determined by measuring the ratio of t-butyl alcohol (TBA)/acetone produced. Using cumene as a model for PS, a highlevel of H-abstraction was observed. However, PS showed a very low level of H-abstraction, which decreased further as the degree of polymerization (DP) of thePS was increased (Fig. 9). This was explained by the coil configuration of the PSchains restricting access of the t-butoxy radicals to the labile tertiary benzylicH-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).


The long-chain branching that takes place during PS manufacture in usingcontinuous 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 poly-merization reactor must be cleaned to remove the coating of gel on the reactorwalls. The mechanism of gel formation is not certain, but it is generally believedthat 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 ex-posed to a free-radical environment for a longer period of time than are polymerchains in solution. The longer PS is exposed to free radicals, the more backbone H-abstraction can take place. Furthermore, the concentration of PS in the absorbedlayer is very high. Thus, the “gel” effect (Trommsdorff effect) (64) may contribute toincreased molecular weight growth in the layer. In any event, the layer of polymerends up becoming higher in molecular weight than other chains. Eventually, cross-links form between the chains leading to infinite molecular weight. This effect notonly can lead to the formation of gel, but can eventually result in reactor foulingif not controlled. Suspension polymerization does not have this problem. Additionof low levels (ie, 100–500 ppm) of a divinyl monomer to suspension styrene poly-merization leads to the formation of branched PS without problems with reactorfouling. However, even though suspension polymerization can be used to producebranched PS, it is no longer practiced commercially for economic reasons.

The approaches to branching during continuous bulk free-radical polymer-ization that have been reported include the addition of a small amount of di-vinyl monomer (65), vinyl functional initiator (66), polyfunctional initiator (67),and vinyl functional chain-transfer agent (68) to the polymerization mixture. Re-searchers at Dow have investigated all of these approaches in using continuousfree-radical bulk polymerization and found that they all lead to gel and eventualreactor fouling after a few days of continuous operation, even in the presence ofa chain-transfer solvent. They found that only if a chain-transfer agent is addedto the continuous free-radical bulk polymerization to maintain a high level oftermination, could branched PS be produced without reactor fouling (69).

More recently, Dow researchers developed a new approach to makingbranched PS using continuous free-radical bulk polymerization, which eliminatesthe potential of reactor fouling by carrying out the branching after the polymer ex-its the polymerization reactor (70). The concept is to incorporate latent functionalgroups into the polymer backbone during the polymerization, which subsequentlyreact to form a cross-link after the polymer exits the polymerization reactor(Fig. 10). The requirement is that the functional groups be inert at polymeriza-tion temperatures (100–160◦C) and then during the devolatilization step while

Fig. 10. General approach to post-polymerization reactor branching (cross-linking).


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 viscouspolymer melt.

The functional group that worked best was benzocyclobutene (BCB) (48,71).BCB functional initiators were used to place BCB functional groups on the PSchain ends and BCB functional comonomers were used to place BCB groups pen-dant along the PS backbone. BCB is a very unique molecule in that it is totallyinert at temperatures <180◦C. However, at temperatures >200◦C the strainedfour-membered ring opens resulting in the formation of a highly reactive ortho-quinonemethide intermediate (1), which couples with another orthoquinoneme-thide 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 withoutformation of cross-linked gel is due to the fact that the main mechanism of ter-mination taking place in normal free-radical polymerization processes is radicalcoupling. Thus, dynamic (growing) high molecular weight branched polystyrylradicals couple together. This problem can be virtually eliminated using new con-trolled radical polymerization techniques. Soluble hyperbranched PS, previouslyonly prepared using anionic polymerization chemistry, has been prepared usingthis technique. The controlled radical polymerization process was carried out us-ing styrene having an alkoxyamine substituent (2) making it an A2B monomer,which is well known to yield soluble hyperbranched polymers (72).


Styrene Copolymers. Acrylonitrile, butadiene, α-methylstyrene, acrylicacid, and maleic anhydride have been copolymerized with styrene to yield commer-cially significant copolymers. Acrylonitrile copolymer with styrene (SAN) is thelargest volume styrenic copolymer and is used in applications requiring increasedstrength and chemical resistance over PS. Most of these polymers have been pre-pared at the crossover or azeotrope composition, which is ca 24 wt% acrylonitrile(see COPOLYMERIZATION).

Copolymers are typically manufactured using well-mixed CSTR processes toeliminate composition drift that causes a loss in transparency. SAN copolymersprepared in batch or continuous plug-flow processes, on the other hand, are typ-ically hazy because of composition drift. SAN copolymers with as little as 4 wt%difference in acrylonitrile composition are immiscible (73). SAN is extremely in-compatible 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 barrierproperties. If the acrylonitrile content of the copolymer is increased to >40 wt%,the copolymer becomes ductile. SAN copolymers constitute the rigid matrix phaseof the ABS engineering plastics.

Unlike PS homopolymers, SAN copolymers turn yellow upon heating. Theextent of discoloration is proportional to the percentage of acrylonitrile in thecopolymer (74). Generally, the mechanism of discoloration is thought to involve cy-clization of acrylonitrile polyads in the polymer backbone, as depicted in Figure 12.However, recent studies have revealed that oligomers also contribute to discol-oration. Specifically, one of the SAN trimer structures containing one styrene andtwo acrylonitrile units was found to thermally decompose resulting in the for-mation of a naphthalene derivative (Fig. 13), which readily oxidizes to form ahighly colored species. The relative contribution of oligomers and polymer back-bone 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.


Styrene–butadiene copolymers are mainly prepared to yield rubbers (seeELASTOMERS; STYRENEUTADIENE COPOLYMERS). Many commercially significant la-tex paints are based on styrene–butadiene (weight ratio usually 60:40 with highconversion) copolymers (see COATINGS; PAINT). Most of the block copolymers pre-pared by anionic catalysts, eg, sec-butyl lithium, are also elastomers. However,some of these block copolymers are thermoplastic rubbers, which behave likecross-linked rubbers at room temperature but show regular thermoplastic flow atelevated temperatures (75,76). Diblock (styrene–butadiene) and triblock (styrene–butadiene–styrene) copolymers are commercially available. Typically, they areblended with PS to achieve a desirable property, eg, improved clarity/flexibility(76) (see POLYBLENDS). These block copolymers represent a class of new and inter-esting 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 alternatingstructure. Accordingly, equimolar copolymers are normally produced, correspond-ing to 48 wt% maleic anhydride. However, by means of CSTR processes, copolymerswith random low maleic anhydride contents can be produced (87). Depending ontheir molecular weights, these can be used as chemically reactive resins, eg, epoxysystems and coating resins, for PS-foam nucleation, or as high heat-deformationmolding materials (88).

Recently, it has been discovered that styrene forms a linear alternatingcopolymer 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 basedupon 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 forcopolymerizing 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 constrained-geometry of the catalyst (caused by the chelate ligand) plays a critical role infavoring the incorporation of the styrene. The technical significance of Dow’s newcatalyst is that the ESI resins are not contaminated with polyethylene or PS ho-mopolymers, which generally results using alternate catalyst systems. A widerange of ESI resins have been prepared depending upon the ethylene/styrene ra-tio in the polymerization feed. Incorporation of high amounts of styrene into thecopolymer requires the use of very high ratios of styrene. This presents a chal-lenge at the end of the process because the unreacted styrene is removed by hightemperature vacuum devolatilization, which can lead to some contamination byatactic PS (by spontaneous free-radical polymerization) if not done properly. ESIcopolymers containing low levels of styrene are crystalline thermoplastics (92)while increasing the styrene content leads to a rapid decrease in crystallinity af-fording materials displaying excellent elastomeric properties (93). ESI containingabout 20 mol% styrene are excellent compatibilizers for PE–PS blends (94).

Hydrogenated PSs. Dow researchers have successfully developed a newhydrogenation catalyst that allows complete hydrogenation of the phenyl ringson PS without degrading the molecular weight of the polymer (95). Complete


Table 3. Glass-Transition Temperatures of Substituted Polystyrene

Polymer CAS registry number Tg◦C

Polystyrene [9003-53-6] 100Poly(o-methylstyrene) [25087-21-2] 136Poly(m-methylstyrene) [25037-62-1] 97Poly(p-methylstyrene) [24936-41-2] 106Poly(2,4-dimethylstyrene) [25990-16-3] 112Poly(2,5-dimethylstyrene) [34031-72-6] 143Poly(p-tert-butylstyrene) [26009-55-2] 130Poly(p-chlorostyrene) [24991-47-7] 110Poly(α-methylstyrene) [25014-31-7] 170

hydrogenation of the phenyl rings of PS increases the glass-transition temperature(Tg) to ∼140◦C (96). Because of the hydrophobicity and low birefringence of thesaturated polymer, it is well suited for use in optical media applications. Dow iscurrently planning to commercialize hydrogenated PS for use as a substrate forDVD discs.

Polymers of Styrene Derivatives. Many styrene derivatives have been syn-thesized and the corresponding polymers and copolymers prepared (97). The glass-transition temperatures for a series of substituted styrene polymers are shown inTable 3. The highest Tg is that of poly(α-methylstyrene), which can be preparedby anionic polymerization. Because it has a low ceiling temperature (61◦C), de-polymerization can occur during fabrication with the produced monomer actingas a plasticizer and lowering the heat distortion to 110–125◦C (98). The polymeris difficult to fabricate because of its high melt viscosity and is more brittle thanPS, but can be toughened with rubber.

Some polymers from styrene derivatives seem to meet specific market de-mands and to have the potential to become commercially significant materials. Forexample, monomeric chlorostyrene is useful in glass-reinforced polyester recipessince it polymerizes several times as fast as styrene (98). Poly(sodium styrene-sulfonate) [9003-59-2] is a versatile water-soluble polymer and is used in water-pollution control and as a general flocculant (99,100). Poly(vinylbenzyl ammoniumchloride) [70504-37-9] has been useful as an electroconductive resin (101). (seeELECTRICALLY-CONDUCTING POLYMERS).

Transparent Impact PS (TIPS). Rubber is incorporated into PS primarilyto impart toughness. The resulting materials are commonly called high impactPS (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. Themechanism of rubber-particle formation and rubber reinforcement and severalgeneral reviews of HIPS and other heterogeneous polymers have been published(102–108). The photomicrographs in Figure 14 show the different morphologiespossible in HIPS materials prepared using various types of rubbers (109,110). Ifthe particles are much larger than 5–10 µm, poor surface appearance of moldings,extrusions, and vacuum-formed parts are usually noted. Although most commer-cial HIPS contains ca 3–10 wt% PB or styrene–butadiene copolymer rubber, thepresence of PS occlusions within the rubber particles gives rise to a 10–40% volume


Fig. 14. Electron photomicrographs of several HIPS resins prepared using different typesof rubbers.

fraction of the reinforcing rubber phase (108,111). Accordingly, a significant por-tion of the PS matrix is filled with rubber particles. Techniques have been pub-lished for evaluating the morphology of HIPS (110,112,113).

Polystyrene, by itself, is low in cost, relatively clear (90% transparency), has amoderate 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 meansmust be taken to toughen it. Approaches to toughening include modifying the back-bone structure by introducing comonomers (for example making SAN), addingplasticizers, and blending with various rubbers and elastomers. All of these op-tions for toughening can have a deleterious effect on the desirable properties;comonomers raise the cost, plasticizers lower the modulus, and rubber fillers candestroy transparency. HIPS resins are opaque. The opacity is due to the scatter-ing of light as it passes through the sample. This scattering is caused because thePB rubber particles and PS phase have different refractive indices. There are twoways to solve this problem and stop the light scattering: (1) adjust the relative


refractive indices of the two phases so they are nearly the same or (2) shrink thesizes of the PS and rubber phases to <20 nm. Most companies that want to marketproducts for TIPS applications utilize the latter technique and have designed highstyrene content microphase separated block copolymer rubbers that when blendedwith PS result in very small phase domain sizes. The refractive indices of PS andPB are 1.59 and 1.51, respectively. The technique of refractive index matching canbe accomplished by either raising the refractive index of PB by copolymerizing re-fractive index enhancing comonomers with butadiene (eg, styrene, vinylbiphenyl,vinylnaphthalene) or by reducing the refractive index of PS by copolymerizingmethyl 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 thefirst company (1972) to market these resins (115). Phillips calls their styrene–butadiene block copolymer resins for TIPS blending K-resins. They have a radial orstar 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 materialsare produced by polymerizing styrene first with sec-butyl lithium (sBL). Once thedesired 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 livingPS chains. The result is a bimodal mixture of polystyryl lithium. Once the styreneis all converted, butadiene is added to form a mixture of styrene–butadiene blockcopolymers having the same butadiene block length and two different PS blocklengths. At this point, the butadiene segment is still living. Silicone tetrachlorideis added as a coupling agent resulting in the formation of the star block copolymer(3).

Several other companies also market transparent styrene–butadiene blockcopolymers for TIPS applications. BASF markets styrene–butadiene block copoly-mers having a similar structure (Styrolux); however, rather than pure PB seg-ments, they make a gradient or taper styrene–butadiene block having a Tg of−30 ◦C (116). Their process to make this special architecture is similar to Phillipsin that they prepare a bimodal mixture of polybutadienyl lithiums. However, thena mixture of butadiene and styrene are added to make the taper block. Afterthe taper block is made, more styrene is added to add a short PS block. A tetra-functional coupling agent is then added to form the star structure. BASF teaches


that the purpose of the long PS block is to enhance compatibility in blends withPS (116). Fina markets linear styrene–butadiene triblock materials under thetrade name Finclear. These linear triblock resins are not just simple high styrenecontent styrene–butadiene–styrene triblock materials. Rather, they use the taperblock concept; that is, rather than having pure PS and PB segments in the blockcopolymer, there is a gradient change over or tapering of the styrene to butadienesegments.

Unlike for PS, very little work has been published on the fracture of blockcopolymers. There is a substantial amount of literature on PS toughened by blend-ing with block copolymers. In those systems, the block copolymer is dispersed, andprovided the proper particle size and modulus are obtained, substantial improve-ments in toughness are obtained, as described above for HIPS. For the neat blockcopolymers, however, fracture mechanisms are fundamentally different than forPS.

Results on the fracture of KR01 and KR03, both Phillips K-resins, have beenreported (117). KR01 has an overall molecular weight of 180,000 (Mw), while forKR03 the molecular weight is 220,000 (Mw). KR03 is asymmetrical, with onelong PS block, while KR01 is fairly symmetrical. The different block structuresresult in quite different morphologies, which in turn impart quite different frac-ture properties. KR01 has basically a cylindrical morphology consisting of wavyrods of polybutadiene, about 20 nm in diameter, in a PS matrix. The cylindricalmorphology is expected based on the volume fraction of PB. In KR03, the PB isin a disrupted lamellar morphology. The rubber phase is in sheets about 20 nmthick, but the sheets are neither flat nor continuous, although annealing sharp-ens the morphology somewhat. KR01 fractures in a manner similar to PS. Thereare well-defined crazes, and no deformation outside of the craze region. In KR03,there is significant microcracking (better termed nanocracking), voiding in the PBphase, and significant deformation. Argon refers to the failure of KR03 as a formof 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. and0.75 ft·lb/in. for KR01 and KR03, respectively. The change in morphology has anappreciable effect on toughness (117).

The individual components (PS and styrene–butadiene block rubber) thatmake up TIPS are optically transparent. Ignoring the effects of surface imperfec-tions (scratches, roughness), blends and block copolymers become hazy and thetransparency is reduced due to light scattering from regions of differing refrac-tive index. In very qualitative terms, significant light scattering will occur whenregions of differing refractive index have dimensions of the order of the wave-length 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 indiameter for the material to be transparent. Unfortunately, this requirement forclarity is inconsistent with the requirement for toughness of a minimum rubberparticle size of 0.8 µm. The domain sizes in block copolymers are much smaller, ofthe order of 20 nm, and so block copolymers tend to be highly transparent. Whenhomopolymers are blended with blocks, however, inhomogeneities in compositionand refractive index can develop, and if the dimensions are large enough, lightscattering can be significant.


Fig. 15. Transmittance vs particle size for PB particles in a PS matrix with a path lengthof 1.0 mm. The calculation assumes 10 vol% of spherical particles and is for light with 0.55µm wavelength.

Block copolymers of styrene and butadiene are highly transparent, despitethe large refractive index difference between PS and PB and the fact that theblocks are phase separated. Styrolux and KR03 resins have transparencies around90%. The transparency is due to the very small domain size, which is typicallyof 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 aredetermined primarily by block molecular weights, and for commercial polymersthe domains will always be small enough that they will not be a significant sourceof scattering. As long as block copolymer composition is uniform, degradation isavoided, smooth surfaces are maintained and fabricated articles are generallyclear.

As a means of reducing cost and increasing modulus, GPPS is often blendedwith block copolymers, despite the fact that the blends tend to have inferior opticalclarity than the pure block copolymers. There is some evidence that the additionof GPPS can raise the Tg of the block copolymer system (118). Several exam-ples 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 relativemolecular weights. With identical molecular weights the solubility is expected tobe unlimited, and depending on block copolymer geometry (star vs linear) solu-bility may be maintained with a GPPS/block ratio of up to 1.5 (118). The reasonthat Styrolux and KR03 are asymmetrical, with on average one long PS block permolecule, is probably to enhance solubility of GPPS.

When GPPS of the proper molecular weight is added to a lamellar blockcopolymer, like KR03 or Styrolux, it is absorbed by the PS phase and hence swellsit. With no added homopolymer, the block copolymer does not scatter light becauseall of the domains are much smaller than the wavelength of light, even thoughthe PS domains will be larger than the butadiene domains, and to visible lightthe material has a uniform refractive index. If the GPPS is uniformly absorbedby the PS blocks the material remains transparent, even if the PS domains be-come comparable in size to the wavelength of light. In this case, the system is


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 maintainthe dispersion due to the block structure. As GPPS is added, the average refractiveindex of the material increases, but to visible light there is not a refractive indexgradient that would result in scattering. The blend may begin to scatter light forone of two reasons. First, if large volumes of GPPS are added, even if miscibilitywith the blocks is maintained, variations in PS domain sizes can occur due torandom fluctuations in morphology. When these domains become large, they willbegin to scatter. If the GPPS begins to phase separate, then it will very quicklygrow into domains that will cause significant scattering. The key to maintainingclarity is solubility of the added GPPS in the PS blocks. It has been shown how ina lamellar Styrolux system, added GPPS swells the PS blocks but leaves the buta-diene block domains essentially unchanged (116). A blend with 60 vol% Styroluxand 40% GPPS has about 78% transmittance, compared to 90% for the pure blockcopolymer. Transmission electron micrographs show that the butadiene domainsare 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 PSdevelop that are large enough to scatter light. Individual PS domains are notthat large by themselves. In contrast, it has also been shown that in a styrene–butadiene–styrene triblock system, added GPPS readily phase separates, formsrelatively large domains of pure PS, and results in much higher light scattering.

In summary, optical clarity in TIPS systems depends on minimizing lightscattering, which arises from having regions of differing refractive index. The di-mensions of these scattering centers is dictated by the size of dispersed rubberparticles (as in HIPS), by the morphological features in block copolymers, or bycompositional fluctuations in blends of block and homopolymers. The conundrumfor HIPS-type materials is that rubber particles large enough to toughen PS arealso large enough to cause significant light scattering. In block copolymers, themorphological features are too small to scatter significantly and so these sys-tems are highly transparent. Blends of high styrene content (>70 wt% styrene)

Fig. 16. TEM of KR03 and 50:50 blend of KR03 and PS.


block and GPPS remain transparent as long as gross phase separation can beprevented.

Besides transparency and toughness, modulus is also a key property for TIPS.The two approaches to toughening PS involve the addition of a relatively lowmodulus rubber phase. Adding rubber unavoidably lowers the modulus of theresulting composite; exactly how much depends on the resulting morphology. Forsystems with dispersed rubber particles, in which the PS remains the continuousphase, simple mechanical models do a good job of predicting modulus as a functionof rubber phase volume fraction. In fact, measuring the dynamic modulus at smallstrains 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 do-mains, 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 φ1 and φ2 the volume fractions of the PSand elastomer phases, respectively. In the case of HIPS with occluded PS in therubber phase, the volume fraction of elastomer phase φ2 is the total volume frac-tion of particles, not the volume of PB. A plot of modulus ratio vs effective volumefraction of rubber is given in Figure 17. At effective elastomer volume fractionsup to 0.30 (including occluded and grafted PS), an upper limit on what is typicallyused for HIPS, the reduction in modulus is only 45%.

For lamellar morphologies, the situation is quite different. There are no goodmechanical models for predicting the modulus of these systems, mostly becausethere is not an adequate way to describe the morphology. The modulus of a lamellarsystem depends on the relative volume fractions of the two phases, their individualmoduli, and the orientation and aspect ratio of the lamellae. The last two param-eters are almost impossible to determine in real systems; so, predicting modulibased on volume fractions and individual phase moduli is virtually impossible. Ifthe 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 ratioG/G1 is typically 0.42 (121).

Fig. 17. Modulus ratio vs soft phase volume fraction for systems like HIPS.


Acrylonitrile–Butadiene–Styrene (ABS) Polymers. ABS polymers havebecome important commercial products since the mid-1950s. The developmentand properties of ABS polymers has been discussed in detail in Reference 122.ABS polymers, like HIPS, are two-phase systems in which the elastomer compo-nent is dispersed in the rigid SAN copolymer matrix. The electron photomicro-graphs in Figure 18 show the difference in morphology of mass versus emulsionABS polymers. The differences in structure of the dispersed phases are primarilya result of differences in production processes, types of rubber used, and variationin rubber concentrations.

Because of the possible changes in the nature and concentration of therubber phase, a wide range of ABS polymers are available. Generally, they arerigid [modulus at room temperature, 1.8–2.6 GPa, ie, (2.6–3.8) × 105 psi] andhave 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.


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 resistancemakes ABS polymers particularly suitable for demanding applications. Anotherimportant attribute of several ABS polymers is their minimum tendency to orientor develop mechanical anisotropy during molding (123,124). Accordingly, uniformtough moldings are obtained. In addition, ABS polymers exhibit good ease of fab-rication and produce moldings and extrusions with excellent gloss, which canbe decorated by many techniques, eg, lacquer painting, vacuum metallizing, andelectroplating (125–128). In the case of electroplating, the strength of the moldedpiece is significantly improved (123). When an appropriate decorative coating ora laminated film is applied, ABS polymers can be used outdoors (129).

ABS can be blended with bisphenol A polycarbonate resins to make a mate-rial having excellent low temperature toughness. The most important applicationof this blend is for automotive body panels.

When inherent outdoor weatherability is important, acrylonitrile–ethylene–styrene (AES) or acrylonitrile–styrene–acrylate (ASA) materials are typicallyused. These materials utilize ethylene–propylene copolymers and polybutylacry-late as the rubber phase, respectively. Ethylene–propylene and polybutylacrylaterubbers are inherently more weatherable than polybutadiene because they aremore saturated leaving fewer labile sites for oxidation (130). Even though AES andASA polymers are more weatherable than ABS, additives are needed to stabilizethe materials against outdoor photochemical degradation for prolonged periods.Additive packages for AES and ASA weatherable materials generally contain bothuv absorbers and hindered amine light stabilizers. Typical outdoor applications forweatherable polymers include recreational vehicles, camper tops, and swimmingpool accessories. The extremely hostile environments where these stabilized AESmaterials are utilized still take their toll on the materials. Over very prolongedperiods of use, the surface gains a chalky appearance as the polymer degrades.Serious discoloration can also result and was found to be caused by degradationof the additives (131).

High heat ABS resins are produced by adding a third monomer to the styreneand acrylonitrile to “stiffen” the polymer backbone, thus raising the Tg. Twomonomers used commercially for this purpose are α-methylstyrene (132) and N-phenylmaleimide (133).

Not only are ABS polymers useful engineering plastics, but some of the highrubber compositions are excellent impact modifiers for poly(vinyl chloride) (PVC).Styrene–acrylonitrile-grafted butadiene rubbers have been used as modifiers forPVC since 1957 (134).

New Rubber-Modified Styrene Copolymers. Rubber modification ofstyrene copolymers other than HIPS and ABS has been useful for specialty pur-poses. Transparency has been achieved with the use of methyl methacrylate asa comonomer; styrene–methyl methacrylate copolymers have been successfullymodified with rubber. Improved weatherability is achieved by modifying SANcopolymers with saturated, aging-resistant elastomers (135).

Inorganic-Reinforced Styrene Polymers. Glass reinforcement of PS andSAN markedly improves their mechanical properties. The strength, stiffness, andfracture toughness are generally at least doubled. Creep and relaxation rates aresignificantly reduced and creep rupture times are increased. The coefficient of


thermal expansion is reduced by more than half, and generally, response to tem-perature changes is minimized (136). Normally, ca 20 wt% glass fibers, eg, 6 mmlong, 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 re-inforced concentrates with virgin resin), a direct process, in which the glass iscut and weighed automatically and blended with the polymer at the moldingmachine, and general inplant compounding (123). The choice of any of theseprocesses depends primarily on the size of the operation and the correspondingeconomics.

Reinforcement of PS with inorganic materials other than glass has beenunder intense study in recent years. The inorganic materials of highest interesthave a lamellar (layered) structure with at least one dimension in the nanome-ter scale range. Hence, addition of these inorganic materials to polymers formwhat are called nanocomposite materials. Many natural and synthetic lamel-lar inorganic materials are commercially available. The molecular structures ofthese materials vary widely. The biggest challenge encountered when blendingnanolayered inorganic with hydrophobic polymers like PS is to end up with theinorganic nanolayers separated and evenly dispersed throughout the polymer ma-trix. The grouping of the layers of lamellar pieces can generally be visualizedas a cabbage-like or book-like structure in which, for the purpose of making ananocomposite, the task is to scatter the leaves or book pages uniformly through-out the polymer matrix (Fig. 19). The dimensions of the individual lamellae areon the nanometer scale and are important for enhancement of polymer physicalproperties.

Several approaches for the exfoliation of platelets exist in producing thePS nanocomposite. The actual mixing of the inorganic with PS can take place viacompounding or by introducing it into the polymerization process. The approachesinclude (1) using the inorganic material with no modification, (2) chemical treat-ment of the inorganic material surface to alter hydrophilicity or to attach reactivefunctional groups, (3) use of compatibilizers/surfactants when mixing, (4) copoly-merization styrene with a polar comonomer (ie, maleic anhydride) to make PSmore compatible with the polar inorganic material, (5) physical manipulation ofinorganic material (ie, freeze-drying or ball milling) (Fig. 20), and (6) use of poly-merization mechanism to do the work of exfoliating the platelets.

Fig. 19. Illustration of dispersed/intercalated platelets and exfoliated platelets.


Fig. 20. Use of freeze-drying to separate lamellae.

There are advantages and disadvantages to these approaches. Method 1is simple; however, unless working with a highly polar polymer, exfoliation anddispersion of the mismatched systems is unlikely without using a great deal ofenergy to mechanically separate and disperse the layered material. Methods 2–4require the addition of compatibilizing additives, and have been shown to be quiteeffective. Use of these additives often results in compromising the physical prop-erties of the parent polymer system, and benefits that might be provided by theinorganic platelets can easily be outweighed by the addition of these extra mate-rials. 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 thepolymerization mechanism to exfoliate the platelets (method 6) is generally not asstraightforward as the other methods; however, it can be an ideal way to exfoliateand disperse platelets without the use of unwanted additives or the added expenseof compounding approaches.

The synthesis of a PS/Montmorillanite clay nanocomposite prepared by com-bining methods 2 and 6 described earlier was published (137) (Fig. 21). Theyutilized living free-radical polymerization (LFRP) of styrene to provide exfoliatedplatelets in a PS matrix. Transmission electron micrographs and x-ray diffrac-tion patterns support the idea of preparing nanocomposites via polymerizationinitiation.

Only recently have some elegant approaches for the synthesis of finely dis-persed composites been reported in the academic literature (137,138). The styrene-based systems were SAN, ABS, and PS. Blends of these materials were preparedeither by compounding with organically modified inorganic materials or by mixingorganically modified (with or without an additional compatibilizer) with monomerprior to polymerization. Physical properties of the composites were also examinedin several reports, and some changes in modulus and Tg reported. Many of thesestudies focused on attempts to disperse these ionic inorganic materials in theincompatible organic polymer systems. Good dispersions were obtained in somecases; however, complete exfoliation of the inorganic lamellar systems was gen-erally not obtained. As a result, only intercalated products were made. Havingexfoliated platelets of the proper aspect ratio and in the correct alignment in thepolymer matrix should provide the maximum impact on physical properties; how-ever, dispersing such a material in relatively nonpolar systems, such as styrenicpolymers, remains a challenge.

Fig. 21. LFRP of styrene to produce nanocomposites.

275


Degradation

Like almost all synthetic polymers, styrene plastics are susceptible to degradationby heat, oxidation, uv radiation, high energy radiation, and shear. However, innormal use, only uv radiation imposes any real limit on the general usefulness ofthese plastics (139,140). Thus, it is generally recommended that the use of styreneplastics in outdoor applications be avoided.

Thermal Degradation. Typically, PS loses about 10% of its molecularweight when it is fabricated. A significant amount of research has been carriedout to determine the nature of the “weak links” in PS (141–144). Various ini-tiation of degradation mechanisms have been proposed: (1) chain-end initiation(unzipping), (2) random scission initiation, and (3) scission of weak links in thepolymer backbone. It has been suggested that chain-end initiation is the pre-dominant mechanism at 310◦C while random scission produces stable molecules.Evidence for weak link scissions comes mainly from studies showing loss of molec-ular weight vs. degradation time. These plots usually show a rapid initial drop inmolecular weight indicating initial rapid weak link scission. However, the pictureis also complicated by the fact that the mechanism of degradation is temperature-dependent. It appears that weak link scissions taking place at high temperaturesinitiate depolymerization while at lower temperatures, scissions simply cause adecrease in molecular weight. In any case, a clear difference in thermal stabilityhas been shown between PS produced using a peroxide initiator and PS madewithout an initiator polymerization. Figure 22 shows the Arrhenius activation

Fig. 22. Comparison of thermal stability of the PS backbone made with and without per-oxide initiator ––◦–– Peroxide initiated Ea = 94.6 kJ/mol (22.6 kcal/mol); -�—No initiatoradded Ea = 226 kJ/mol (54 kcal/mol).


Fig. 23. Thermogravimetric analysis of polymers and copolymers of styrene in nitrogenat 10◦C/min. A, PS; B, poly(vinyl toluene); C, poly(α-methylstyrene); D, poly(styreneco-acrylonitrile), 71.5% styrene; E, poly(styreneco-butadiene), 80% styrene; F, poly(styreneco-methylstyrene), 75% styrene.

energy for chain scission in PS made with and without a peroxide initiator. Thisdifference in stability of the resins is likely due to the initiator derived fragmentsthat remain in the polymer after isolation.

Poly(α-methylstyrene) unzips to monomer exclusively. Figure 23 is a compar-ison of the thermal stability of several copolymers. Thermal-oxidative degradationof PS occurs much faster, leading to additional volatile components consisting ofaldehydes and ketones, yellowing of the polymer with a very dramatic drop inmolecular weight, and some cross-linking. Rates and yields are highly oxygen-and temperature-sensitive. Figure 24 shows the magnitude of oxidative attack onPS and the extent to which an antioxidant can protect the polymer.

Environmental Degradation. In the past several years, PS has receivedmuch public and media attention. Polystyrene has been described by various en-vironmental groups as being nondegradable, nonrecyclable, toxic when burned,landfill-choking, ozone-depleting, wildlife-killing, and even carcinogenic. Thesemisconceptions regarding PS have resulted in boycotts and bans in various local-ities. 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 plasticswaste is an integrated one of source reduction, recycling, incineration for energyrecovery, reduction of litter, development of photodegradable plastics for specificlitter prone applications, the development of biodegradable plastics, and increas-ing public awareness of the recyclability and value of PS. The balance of theseapproaches varies from year to year depending upon public concerns, politicalpressures, legislation, technology development, and the development of an un-derstanding of the actual contribution of plastics to total solid waste generation.


Fig. 24. Thermal and thermooxidative degradation of PS.

Currently, recycling is being the most heavily researched and developed. The Na-tional Plastics Recycling Company (NPRC) was established in 1989 through thecombined 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 recy-cling with the ultimate objective of recycling 25% of the PS produced in the UnitedStates each year (146).

Fig. 25. Approximate composition of solid waste going into landfills in the United States(145)


Fig. 26. Energy of the solar spectrum vs the wavelength sensitivity of PS.

Photodegradation. Natural sunlight emits energy only in wavelengthsabove 290 nm. Therefore, any polymer that does not absorb light energy at wave-lengths above 290 nm should not be photodegradable. The activation spectrumof PS versus the intensity of the solar spectrum is shown in Figure 26. Since PSdegradation is activated by radiation at wavelengths above 290 nm, it is quitephotodegradable and must be stabilized by the addition of uv-absorbing additivesif it is to be used in outdoor applications where durability is important.

Even though PS is naturally quite photodegradable, there have been consid-erable 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 thentransferred to the polymer to cause backbone scission via an oxidation mecha-nism (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 isto 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, ona chain scission/ketone bases, because some of the light energy that the pendantketone 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 lowlevels of methyl vinyl ketone and methyl isopropenyl ketone is their human


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 containingattached ketone groups.

toxicity. This problem has been solved by adding the ketoalcohol intermediate,formed during the vinyl ketone manufacture, to the styrene polymerization andgenerating the vinyl ketone in situ. (Fig. 30) (151,152).

A concern in the use of photodegradable PS is the environmental impact ofthe products of photooxidation. However, photodegraded PS is expected to be moresusceptible to biodegradation because the molecular weight has been reduced,


Fig. 30. Manufacture of photodegradable PS via in situ vinyl ketone process.

the PS chains have oxidized end groups, the incorporation of oxygen as alcoholand ketone groups has increased the hydrophilicity of the PS fragments, and thesurface area has increased (153–155).

Biodegradation. Polystyrene is inherently resistant to biodegradationmainly because of its hydrophobicity. Efforts have been made to enhance thebiodegradability of PS by inserting hydrolyzable linkages (eg, ester and amide)into its backbone. This was accomplished by adding monomers to the polymer-ization, which are capable of undergoing ring-opening copolymerization (Fig. 31)(156).

Environmental Effect of Blowing Agents. Until the mid-1980s, themost common blowing agents for extruded PS foams were chlorofluorocarbons(CFCs). However, when these materials were shown to contribute to the ozonedepletion problem, there became a considerable effort to find alternative blowingagents for the manufacture of extruded PS foam. Most of the research has focusedon the development of carbon dioxide foaming technology for PS (157). By con-trast, PS bead foam uses hydrocarbon blowing agents (eg, pentane). Hydrocarbonblowing agents are extremely flammable and add volatile organic compounds tothe atmosphere. In Europe, all PS foam is manufactured using carbon dioxide asthe blowing agent.

Polymerization

Styrene and most derivatives are among the few monomers that can be poly-merized by all four distinct mechanisms, ie, anionic, cationic, free-radical, and

Fig. 31. Preparation of biodegradable PS by incorporating ester linkages into the back-bone via ring-opening copolymerization of styrene with a cyclic ketene acetal.


Ziegler–Natta. These include processes dependent on electromagnetic radiation,which is usually a free-radical mechanism, or high energy radiation, which is ei-ther a cationic or free-radical mechanism depending on the water content of thesystem. All mechanisms, other than Ziegler–Natta, generally yield polymers witha 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 isusually atactic and amorphous, but in some cases, eg, at low temperatures, iPShas been prepared.

Each of the mechanisms used to polymerize styrene has its own unique ad-vantages/disadvantages as summarized below.

Anionic:

Initiation, propagation, and termination steps are sequential resulting inthe formation of narrow polydispersity (Mw/Mn < 1.1); termination step iscontrolled allowing control of end-group structure; polymerization feed mustbe purified.

Cationic:

High molecular weight polymers are difficult to make because of the insta-bility of polystyrylcarbocation giving fast termination; polymerization feedmust be purified.

Free-Radical:

Initiation, propagation, and termination steps are simultaneous resultingin the formation of broad polydispersity (Mw/Mn > 2); multiple termina-tion paths lead to a variety of end groups; polymerization feed need not bepurified.

Ziegler–Natta:

Metal complexes allowing stereospecific polymerization resulting in the for-mation of high melting crystalline tactic PS; polymerization feed must bepurified.

Free-radical polymerization is the preferred industrial route because:(1) monomer purification is not required (158) and (2) initiator residues need notbe removed from polymer as they have minimal effect on polymer properties. Theexceptions are the styrene–butadiene block copolymers and very low molecularweight PS. These polymers are manufactured using anionic and cationic polymer-ization chemistry, respectively (159). Analytical standards are available for PSprepared by all four mechanisms (see INITIATORS).

Free Radical. The styrene family of monomers are almost unique in theirability to undergo spontaneous or thermal polymerization merely by heating to


Fig. 32. Flory mechanism for spontaneous initiation of styrene polymerization

>100◦C. Styrene in essence acts as its own initiator. The mechanism by whichthis spontaneous polymerization occurs has been studied extensively and haschallenged researchers for over 50 years. Two mechanisms explaining sponta-neous styrene polymerization have been proposed and supported by considerablecircumstantial evidence. The oldest mechanism (160) involves a bond-forming re-action 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 trans1,2-diphenylcyclobutanes as major dimers (161,162), and (2) the large differencesbetween spontaneous and chemically initiated (azobisisobutyronitrile) styrenepolymerizations in the presence of the free-radical scavenger 1,1′-diphenyl-2-picrylhydrazyl (DPPH). The rate of consumption of DPPH is 25 times fasterthan that expected from the rate of polymerization measurements. This differ-ence was explained by the spontaneous formation of ·D·, many of which becomeself-terminated before initiating polymer radicals (163). However, experiments totest the mechanism showed that there was no significant difference in the molec-ular weight of PS initiated by monoradicals compared with the spontaneouslyinitiated polymerizations taken to the same monomer conversion (164).

The second mechanism (Fig. 33) (165) involves the Diels–Alder reac-tion of two styrene molecules to form a reactive dimer (DH) followed by a

Fig. 33. Mayo mechanism for spontaneous initiation of styrene polymerization.


Fig. 34. Formation of the Mayo Dimer via the Flory Diradical.

molecular-assisted homolysis (MAH) between the DH and another styrenemolecule. The Mayo mechanism has been generally preferred even though criticalreviews (162,166) have pointed out that the mechanism is only partially con-sistent with the available data. Also, the postulated intermediate DH has neverbeen 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 iso-lated, the aromatized derivative DA has been detected in PS (169). Also, D· hasbeen indicated as an end group in PS using 1H 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). Noncon-certed Diels–Alder reactions are permissible for two nonpolar reactants (171).

The spontaneous polymerization of styrene was studied in the presence ofvarious acid catalysts (172) to see if the postulated reactive intermediate DH couldbe intentionally aromatized to form inactive DA. The results showed that the rateof polymerization of styrene is significantly retarded by acids (eg, camphorsulfonicacid) accompanied by increases in the formation of DA. This finding gave furtherconfirmation for the intermediacy of DH since acids would have little effect on thecyclobutane dimer intermediate in the Flory mechanism.

A potentially important commercial benefit of adding an acid catalyst tothe spontaneous free-radical polymerization of styrene is that a significant shiftresults in the rate/molecular weight curve for PS. This shift is most pronounced athigh molecular weights allowing formation of high molecular weight PS at a muchfaster polymerization rate (Fig. 35). An explanation for this phenomenon is thatthe intermediate DH has a high chain-transfer constant (ie, ∼10) (173). Additionof acid immediately causes DH to aromatize, thus lowering its concentration andhence decreasing its availability to participate in chain-transfer processes. Also,the main mechanism of termination is chain coupling (174), the rate of which ismost affected by radical concentration. Since the polymerization temperature canbe raised in the presence of acid without increasing free-radical concentration,the propagation rate is increased relative to termination rate, thereby raising themolecular weight.

The addition of a fugitive acid such as camphorsulfonic acid to styrene poly-merizations contaminates the polymer with a substance that could lead to corro-sion of equipment being used to process the polymer. To solve this problem, Dowresearchers added vinyl functional sulfonic acids (eg, 2-sulfoethyl methacrylate)to the polymerization (175). The acid then becomes copolymerized into the poly-mer, thus immobilizing it. Also, they found that the addition of as low as 10 ppm


Fig. 35. Polymerization rate vs molecular weight relationship for spontaneous bulkstyrene polymerization under neutral and acidic conditions.

of 2-sulfoethyl methacrylate significantly increases the production rate of highmolecular weight PS (176).

Polystyrene produced by the spontaneous initiation mechanism is typicallycontaminated by dimers and trimers (1–2 wt%). These oligomers are somewhatvolatile and cause problems during extrusion (vapors) and molding (mold sweat)operations. The use of chemicals to generate initiating free radicals significantlyreduces the formation of oligomers. Oligomer production is reduced because thepolymerization temperature can be lowered to slow down the Diels–Alder dimer-ization reaction. A wide range of free-radical initiators are available. They differmainly in the temperature at which each generates free radicals at a useful rate.Typically, in continuous bulk polymerization processes it is best to polymerizestyrene at about the 1-h half-life temperature of the initiator it contains to maxi-mize initiator efficiency.

Initiators that have been utilized to initiate styrene polymerization can begenerally 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 con-certed homolysis of the N C bonds on either side of the azo linkage, resultingin extrusion of nitrogen gas and the formation of two carbon-centered radicals.Carbon–carbon initiators decompose by homolysis of a sterically strained C Cbond, resulting in the formation of two carbon-centered radicals. The radicals ini-tiate styrene polymerization and end up attached to the chain ends and may havean effect upon polymer stability (178–181).


Since most of the bulk PS reactors were originally designed to produce spon-taneously initiated PS in the 100–170◦C temperature range, the peroxide initia-tors used generally have 1-h half-lives in the range of 90–140◦C. If the peroxidedecomposes too rapidly, a run-away polymerization could result, and if it decom-poses too slowly, peroxide exits the reactor. Since organic peroxides are signifi-cantly more expensive than styrene monomer (5–20×), it is economically prudentto choose an initiator that is highly efficient and is entirely consumed during thepolymerization.

Another economically driven objective is to utilize initiators that increasethe rate of polymerization of styrene to form PS having the desired molecularweight. The commercial weight-average molecular weight (Mw) range for GPPSis 200,000–400,000. For spontaneous polymerization, the Mw is inversely propor-tional to polymerization rate (Fig. 36).

The main reason that the Mw decreases as the polymerization temperatureincreases is the increase in the initiation and termination reactions leading toa decrease in the kinetic chain length (Fig. 37). At low temperature, the maintermination mechanism is polystyryl radical coupling, but as the temperature in-creases, radical disproportionation becomes increasingly important. Terminationby coupling results in higher Mw PS than any of the other termination modes.

There are typically several different product grades produced in a single-polymerization reactor, and transitioning between these products in the minimumtime maximizes production yield. Most PS producers rely on the use of kineticmodeling and computer simulation to aid in the manufacture of PS to minimize

Fig. 36. Polymerization rate for PS using different types of initiators.


Fig. 37. General chemistry of free-radical styrene polymerization.

transition time between product grades. Kinetic models have been developed forstyrene polymerization without added initiators (182–184), using one monofunc-tional (185–189), two monofunctional initiators with different half-lives (190),symmetrical difunctional (4), (191) and (5), (6), (192) and the unsymmetrical di-functional initiators (7), (193,194) and (8) (195,196). These models clearly showthe polymerization rate advantage of using initiators for the manufacture of PSusing continuous bulk polymerization processes (Fig. 36).

One of the key reasons for the polymerization rate advantage of using di-functional initiators is their theoretical ability to form initiator fragments, whichcan 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 whyhigher Mw 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 trulyinitiates only double-ended PS chains. Because the main mechanism of termina-tion is radical coupling, the production of pure double-ended chains would leadto the formation of ultrahigh molecular weight PS because termination by chain


Fig. 38. Double-ended PS initiated by diradical formed during Bergman Cyclization.

coupling would only lead to a higher molecular weight double-ended chain. Therehave been many attempts to develop initiators that form only diradicals. How-ever, the discovery of diradicals that efficiently initiate styrene polymerizationis a challenge (198). Dow researchers recently discovered that the p-phenylenediradical formed during “Bergman Cyclization” of certain enediynes can initiatedouble-ended PS, but commercialization of these initiators remains a challenge(Fig. 38) (199). To date, the only clear demonstration of double-ended PS has beenwith difunctional LFRP initiators (200).


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 man-ifested by a period of accelerating rate concomitant with increasing molecularweight, is apparent at below 80◦C in styrene polymerization; although, subtlechanges during the polymerization at higher temperature may be attributed tovariation of the specific rate constants with viscosity (201,202).

In some cases, inhibition of polymerization can be regarded as a special typeof chain transfer. This is of importance in commercial-scale operations involvingstyrene 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 ofoxygen. 4-tert-Butylcatechol (TBC) at 12–50 ppm is the most universally used in-hibitor for protecting styrene. At ambient conditions and with a continuous supplyof air, TBC has a half-life of 6–10 weeks (203). The requirement of oxygen causescomplex side reactions, resulting in significant oxidation of the monomer, whichcauses yellow coloration especially in the vapor phase. An inert gas blanket re-duces this problem and flammability hazards, but precautions must be taken toensure an adequate level of dissolved oxygen in the liquid phase. Another familyof inhibitors is that characterized by the N O bond; these do not seem to requireoxygen for effectiveness. They include a variety of nitrophenol compounds, hy-droxylamine derivatives, and nitroxides (204,205). Nitric oxides are particularlyuseful. The unpaired electron associated with the N O bond is very stable andyet nitroxides couple with carbon-centered radicals at diffusion-controlled rates.The alkoxyamines produced when nitroxides couple with styryl radicals are ther-mally labile. At elevated temperatures (generally >100◦C), the C O bond dissoci-ates back to the precursor radicals in equilibrium with the alkoxyamine. Thus, ifalkoxyamines are added to styrene, they can initiate polymerization. The nitrox-ide moiety is retained on the propagating chain-end and effectively supresses thetermination mechanisms (ie, polystyryl radical coupling, chain transfer, and dis-proportionation) typical of normal free-radical polymerization. The result is thatnarrow polydispersity PS is produced. Once the unreacted styrene is removed fromthe nitroxide-terminated PS and a second monomer is added, a block copolymercan be produced.

Other miscellaneous compounds that have been used as inhibitors are sulfurand some sulfur compounds, picrylhydrazyl derivatives; carbon black, and somesoluble transition-metal salts (206). Both inhibition and acceleration have beenreported for styrene polymerized in the presence of oxygen. The complexity ofthis system has been clearly demonstrated (207). The key reaction is the alter-nating copolymerization of styrene with oxygen to produce a polyperoxide, which


Table 4. Chain-Transfer Constants (K ct) inFree-Radical Styrene Polymerization

Compound Kct T,◦C

Benzene 0.00002 100Toluene 0.00005 100Ethylbenzene 0.0002 100Isopropylbenene 0.0002 100α-Methylstyrene dimer 0.3 120l-Dodecanethiol 13 130l,l-Dimethyl-l-decanethiol 1.1 120l-Hexanethiol 15 100

above 100◦C decomposes to initiating alkoxy radicals. Therefore, depending onthe temperature, oxygen can inhibit or accelerate the rate of polymerization.

Chain Transfer in Free-Radical Styrene Polymerization. The conceptof chain transfer is depicted in Figure 37. Chain-transfer agents are occasionallyadded to styrene to reduce the molecular weight of the polymer, although for manyapplications this is unnecessary as polymerization temperature alone is gener-ally sufficient to achieve molecular weight control. Some typical chain-transferagents for styrene polymerization are listed in Table 4. The chain-transfer agentsof commercial significance are α-methylstyrene dimer, terpinolene, dodecane-1-thiol, and 1,1-dimethyldecane-1-thiol. Chain transfer to styrene monomer hasbeen reported, but recent work strongly suggests that this reaction is negligibleand transfer with the Diels–Alder dimer is the actual inherent transfer reaction(208,209). Chain transfer to PS has received some attention, but experiments in-dicate that it is minimal (210,211). If it did occur at a significant extent, thenthe polymer would be branched. The chain-transfer constants of several commonsolvents and chain-transfer agents are shown in Table 4.

High levels of chain-transfer agents can be used to control the terminationprocess. If the chain-transfer agent has a functional group attached to it, the func-tional group ends up becoming attached to the end of the PS chain. If the func-tionalized chain-transfer agent operates by donating an H-atom to the polystyrylradical, the functional group ends up becoming attached to only one end of the PSchain. However, this technique does not quantitatively functionalize the polymerchains because not all chains get initiated by the functionalized chain-transferagent fragment formed by loss of the H-atom. However, if both the initiator andthe chain-transfer agent contain the functional group (F), high purity one-endfunctional PS can be produced (Fig. 39) (212).

Another approach to control end-group structure is the use of chain-transferagents that operate by an addition–fragmentation mechanism. This approach canlead to the formation of PS having functional groups at both ends if both theinitiating and terminating fragments contain functional groups (Fig. 40).

Recently, it was found that the common chain-transfer agent α-methylstyrene dimer operates by an addition–fragmentation mechanism (Fig. 41)(213).

The use of addition–fragmentation chain-transfer agents also places a reac-tive double bond on one end of the polymer chain and thus yields a macromonomer.


Fig. 39. Preparation of mono-end-functional PS using both a functionalized initiator anda functionalized chain-transfer agent (CTA).

Fig. 40. Generic addition–fragmentation chain-transfer structure and the mechanism ofaction.

Fig. 41. Mechanism of action of α-methylstyrene dimer.

Copolymerization of the chain-end double bond with more styrene leads tobranched PS.

Ionic. Instead of a neutral unpaired electron, styrene polymerization canproceed with great facility through a positively charged species (cationic polymer-ization) or a negatively charged species (anionic polymerization). The polymer-ization reaction is more sensitive to impurities than the free-radical system, andpretreatment 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 isdissociation yielding at least one isolated molecule.


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 ter-mination step (living polymerization), the statistical distribution of chain lengthsis very narrow. The average molecular weight is calculated from the mole ratio ofmonomer-to-initiator sites. Chain termination is usually accomplished by addingproton donors, eg, water or alcohols, or electrophiles such as carbon dioxide.

Anionic polymerization, if carried out properly, can truly be a living poly-merization (215). Addition of a second monomer to polystyryl anion results in theformation of a block polymer with no detectable free PS. This technique is of con-siderable importance in the commercial preparation of styrene–butadiene blockcopolymers, which are used either alone or blended with PS as thermoplastics.

Anionic polymerization offers very fast polymerization rates because of thelong lifetime of polystyryl carbanions. Early research focused on this attribute,most studies being conducted at short reactor residence times (<1 h), at rela-tively low temperatures (10–50◦C), and in low chain-transfer solvents (typicallybenzene) to ensure that premature termination did not take place. Also, relativelylow DPs were typically studied. Continuous commercial free-radical solution poly-merization processes to make PS, on the other hand, operate at relatively hightemperatures (>100◦C), at long residence times (>1.5 h), utilize a chain-transfersolvent (ethylbenzene), and produce polymer in the range of 1000–1500 DP.

Two companies (Dow and BASF) have devoted significant research effortsto develop continuous anionic polymerization processes for PS. However, to dateno PS is produced commercially using these processes. Dow researchers utilizedconventional free-radical polymerization reactors of the CSTR-type (216–220) tostudy the anionic polymerization of styrene while BASF focused upon continu-ous reactors of the linear-flow-type (221). In an anionic CSTR process, initiation,propagation, and termination occur simultaneously and thus the polydispersityof the resulting PS is ∼2. A chain-transfer solvent (ethylbenzene) and relativelyhigh temperatures (80–140◦C) were also used. Under these conditions, the chaintransfer 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 ofPS based on NBL (the most costly raw material) and PS having high clarity (lowcolor) are produced (Fig. 42). Under very stringent feed purification conditions, ashigh as an 8000% yield based on NBL initiator (158) can be achieved. According toFigure 42, without high levels of CTS, it would be impossible to make a PS havingsufficient 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 ofresidual styrene monomer than free-radical PS (222). This is because free-radicalpolymerization processes only operate at 60–80% styrene conversion, while an-ionic processes operate at >99% styrene conversion. Removal of unreacted styrenemonomer from free-radical PS is accomplished using continuous devolatilizationat 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 levelrequires special assisted devolatilization processes such as steam stripping (223).

The most recent process research aimed at anionic PS is that of BASF. Unlikethe Dow process, they utilize CPFR with virtually no backmixing to make narrow


Fig. 42. Polystyrene color vs amount of n-butyllithium (NBL) consumed for its productionin a CSTR.

polydispersity resins. In the Dow CSTR anionic process, the fast polymerizationrate achieved with anionic polymerization is not a problem because the polymer-ization exotherm is controlled by the rate of monomer addition to the reactor; thatis, the reactor is operated in a monomer-starved condition. However, in CPFR,the rate of polymerization of NBL-initiated styrene polymerization far exceedsthe ability to remove heat resulting in run-away polymerization. This problemwas recently solved by Asahi (224) and BASF with the addition of electron defi-cient metal alkyls (eg, dibutylmagnesium, diethylzinc, and triethylaluminum) toassist the butyl lithium initiator (225,226). Addition of metal alkyls accomplishtwo things: (1) the propagation rate is slowed and (2) the polystyryl lithium chainend is stabilized to high temperatures. The rate of the polymerization can actuallybe slowed to match the rate typically observed during free-radical polymerizationof styrene by the addition of the proper amount of metal alkyl (Fig. 43). As theconversion of monomer to polymer proceeds, the viscosity rapidly increases forcingthe need to finish the polymerization at high temperatures to be able to handlethe high viscosity. It is well known that at temperatures >100◦C, PS lithium isunstable and decomposes by elimination of lithium hydride, resulting in the for-mation of dead polymer. The presence of electron deficient metal alkyls increasesthe thermal stability of the polystyryl chain ends so that polymerization up tohigh conversion and temperatures can be achieved.

The mechanism (226) involves the formation of a metal complex or aggregatebetween the lithium and the electron deficient metal. Styrene then inserts in thecomplex. This hypothesis is supported by the change in uv spectrum as the Li/Mgratio changes. There is a shift of the λmax to lower wavelength as the Mg/Li ratioapproaches unity. As the Mg/Li ratio is increased to >1, the λmax shifts back untilat 20/1, the λmax returns to match that of pure polystyryl lithium.


Fig. 43. Propagation rate retarding effect of adding dibutylmagnesium to butyllithiuminitiated 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 tempera-ture (61◦C), is also useful for making thermally stable styrene-co-α-methylstyrene(SAMS) having high α-methylstyrene (AMS) content. Preparation of SAMS hav-ing >20 wt% of AMS units using bulk free-radical polymerization leads to veryslow polymerization rates, low molecular weight polymer, and the formation ofhigh levels of oligomers. The preparation of SAMS using an anionic CSTR poly-merization reactor operating at 90–100◦C allows the production of SAMS havingup to about 70 wt% AMS units (Fig. 44) (227). By conducting the polymerizationabove the ceiling temperature, no AMS polyads of more than two units are pos-sible. SAMS copolymers having polyads of greater than two units in length arethermally 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 activa-tor, eg, boron trifluoride or aluminum trichloride, with a trace of a protonic acidor water. Cationic polymerization of styrene can also be initiated using hetero-geneous acid catalysis such as acidic clays and strong acid ion exchange resins.The solvent again plays an important role, and chain-transfer reactions are verycommon where the reactants are polymer, monomer, solvent, and counterion. As aresult, high molecular weights are more difficult to achieve and molecular weightdistributions are often comparable to those obtained from free-radical polymeriza-tions. Commercial use of cationic styrene polymerization is reported only wherelow molecular weight polymers are desired (110). In recent years, considerableadvances have taken place with regard to cationic polymerization of styrene. Itsuse in making block copolymers and even living cationic polymerizations havebeen reported (229).


Fig. 44. Comparison of continuous (CSTR) and batch anionic production of SAMS.

Ziegler–Natta. Ziegler–Natta-initiated styrene polymerization yieldsstereoregular tactic PS. The tacticity can be isotactic (phenyl rings cis to eachother) or syndiotactic (phenyl rings trans to each other) depending upon the ini-tiator structure (230). Currently, all commercially important styrenic plastics areamorphous, since the vast excess of these polymers is made by a free-radical mech-anism. However, Dow Chemical and Idemitsu Petrochemical Companies are work-ing together to commercialize crystalline sPS. Dow and Idemitsu are currentlymanufacturing sPS using a bulk polymerization process. Since the sPS crystal-lizes as it forms, the polymerization process is very challenging because it requiresmixing and removing the heat of polymerization from a sticky solid. Proper reac-tor design is required to keep the sticky mass from solidifying. The properties ofsPS 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 decomposesat 300◦C, there is a very narrow fabrication temperature range. This problem isimproved 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 apolymerization to be truly “living” are that the propagating chain-ends must notterminate during the polymerization. If the initiation, propagation, and termina-tion steps are sequential (ie, all of the chains are initiated and then propagateat the same time without any termination), then monodisperse (ie, Mw/Mn =1.0) polymer is produced. In general, anionic polymerization is the only mech-anism that yields truly living styrene polymerization and thus monodisperse


PS. However, significant research has been conducted with the goal of achiev-ing “living” styrene polymerization using cationic, free-radical, and Ziegler–Nattamechanisms.

Since Otsu first proposed living free-radical polymerization (LFRP) in 1982,(232,233) there has emerged a multitude of examples (234–242). However, signif-icant controversy exists over the nature of the living chain-end and whether theprocess can really be called “living.” Instead, terms such as pseudo-living, quasi-living, resuscitatable, stable free-radical-mediated, and dormant, have been usedto better describe the process. Currently, most researchers prefer to use the termcontrolled radical polymerization. The process generally involves the addition ofrelatively stable free radicals to the polymerization. The growing polystyryl rad-icals are intercepted and terminated by coupling with the stable radicals therebyminimizing termination by chain transfer and the coupling of polystyryl radicalswith themselves. Prior to the discovery of LFRP, stable free radicals were onlyadded to styrene as polymerization inhibitors. They acted as radical scavengersto stop the propagation process. However, the bond formed between the polystyrylradical and some stable freeradical is somewhat labile, and styrene can insert intothe bond once it is activated by heat or photolysis. The main evidences used to sup-port the livingness of the polymerization are the increase of molecular weight withmonomer conversion, the formation of narrow polydispersity PS, and the ability toprepare block copolymers. Normally, free-radical polymerizations show relativelyflat molecular weight versus conversion plots. Although no one has yet demon-strated the preparation of truly monodisperse (Mw/Mn=1.0) PS using LFRP, poly-dispersities approaching 1.05 have been reported for very low molecular weight(Mw < 20,000) PS prepared in the presence of stable nitroxy radicals.

A variant of the LFRP of styrene is called atom transfer radical polymer-ization (ATRP). In ATRP the initiator is an activated halide. The halogen–carbonbond is broken by transfer of the halogen from the chain end to a chelated tran-sition metal (eg, copper, iron, or ruthenium). After the resulting polystyryl rad-ical 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 havinga controlled molecular weight. If at some point a second monomer is added tothe polymerization, block copolymers are produced. Although the chemistry is po-tentially low in cost due to the nature of the low cost of the initiators and metalcatalysts, there is a serious drawback; ie, the transition-metal catalyst is difficultto remove from the final polymer. Since the presence of traces of transition metalstend to promote degradation of polymers, some means must be developed for theeasy elimination of the catalyst residues from the polymer before ATRP becomesa commercial reality.

Currently, most of the LFRP research is focused on the use of nitroxidesas the stable freeradical. The main problems associated with nitroxide mediatedradical polymerizations (NMRP) are slow polymerization rate and the inabilityto make high molecular weight narrow polydispersity PS. This inability is likelydue to side reactions of the living end leading to termination rather than propa-gation (243). The polymerization rate can be accelerated by the addition of acidsor anhydrides to the process (244). The mechanism of the accelerative effect ofthe acid is not certain. Another problem with nitroxides is that they work wellfor vinylaromatic monomers, but not for acrylate and diene monomers. This has


greatly limited their use for making styrene containing block copolymers. A wayaround this problem is to make the nonstyrene block using some other chemistryand then place an alkoxyamine group on one of the chain-ends. The alkoxyaminefunctional polymer is then dissolved in styrene and heated to >100◦C where thealkoxyamine functional polymer becomes a macroinitiator for the styrene poly-merization (245). There are several examples of the tandem polymerization con-cept, including the preparation of polymethylmethacrylate (PMMA) via normalradical polymerization using an alkoxyamine functional azoinitiator (9). Sub-sequently, styrene polymerization is initiated using the alkoxyamine functionalPMMA (10) as a macroinitiator to make S-MMA block copolymer (11). The syn-thetic scheme is displayed in Figure 45.

The successful application of NMRP to the synthesis of pure block copoly-mers requires that the termination processes that typically take place duringfree-radical polymerization (ie, radical coupling, chain transfer, and dispropor-tionation) be virtually eliminated.

There have been two approaches to the study of the NMRP of styrene: (1) insitu formation of the NMRP initiator (Fig. 46) (246,247) and (2) presynthesis ofthe initiator (200,248).

Fig. 45. Normal-living tandem polymerization approach to make styrenic blockcopolymers.


Fig. 46. In situ synthesis of alkoxyamine initiator during nitroxide mediated radical poly-merization (NMRP) of styrene.

The in situ approach leads to the formation of multiple nitroxide initiatingspecies and there is not perfect stoicheometry between the initiating and medi-ating species. With the presynthesis approach, a pure compound can be used toinitiate polymerization which should lead to “cleaner chemistry.” However, it hasbeen believed that excess nitroxy radicals is important to achieve narrow polydis-persity and good end-group purity (249).

There has been debate over the extent that NMRP eliminates terminationprocesses. The thermal stability of a small molecule (12) that models the propa-gating chain-end of the NMRP of styrene has been synthesized and studied (250).Thermolysis of (12) in an ESR spectrophotometer showed continuous formationof 2,2,6,6-tetramethylpiperdinyloxy (TEMPO). Analysis of the residue left in theESR tube showed decomposition products consisting primarily of styrene. Theywent on to study the kinetics of the decomposition and found that (12) decomposesin the temperature range utilized for NMRP of styrene at a rate comparable tothe styrene conversion rate. Based on this observation, they concluded that end-group purity achieved by NMRP of styrene should decrease with both Mn andmonomer conversion. They further concluded that this termination process wouldlikely seriously limit the ability of NMRP for the preparation of high molecularweight (>100,000) PS having a narrow polydispersity. These conclusions havebeen disputed by other research groups who have attempted to prove that NMRPchemistry virtually eliminates all termination processes. These claims are sup-ported by measuring the amount of nitroxyl moiety on the terminal chain-end.Both NMR (251) and nitrogen analyses (252) have been utilized to determine thenitroxyl content of PS made using NMRP. However, these techniques are not verysensitive and provide only an approximation of the level of nitroxide groups inthe polymer. Further, it has not been conclusively established that the nitroxide


is at the chain terminus. Furthermore, because of the insensitivity of these an-alytical techniques, TEMPO-end capped PS samples to be analyzed have alwayshad molecular weights <10,000. Most researchers have reported levels of nitrox-ide on the terminal chain end at >90% (251). The quality of nitroxide-mediatedpolymerization is directly linked to maintenance of alkoxyamine functionality onthe PS chain end during the polymerization.

The level of alkoxyamine on the PS chain end during the polymerizationup to high conversions and molecular weight has been measured (253). A verysensitive gpc-uv analysis technique was used to precisely quantitate the level ofchromophore attached to polymers (254). They compared the end-group purity ofPS made by the in situ and presynthesis of initiator approaches, as well as demon-strating the impact of excess nitroxide on nitroxide end-group yield. The gpc-uvtechnique for polymer end-group analysis requires that the initiator/mediator betagged with a chromophore having a unique absorbance; that is, absorb at a wave-length at which PS is totally transparent (>280 nm). The chromophore chosen forstudy was the phenylazophenyl chromophore because of its intense absorption at>300 nm. The chromophore was attached to either the initiating or terminatingfragment of the alkoxyamine initiator allowing direct quantitative comparison ofboth the initiating and terminal ends of PS initiated/mediated using these ma-terials. The data show that the number of polymer chains having chromophoresattached to a chain-end decreases with the number-average molecular weight(Mn). This trend is not as dramatic if the chromophore is attached to the initi-ating radical. Mn and styrene conversion are directly proportional. Therefore, itfollows that chain-end purity also decreases rapidly with monomer conversion.This was predicted from earlier work on the thermal decomposition of a smallmolecule which models the dormant end during NMRF (250). Polystyrene havingboth a Mn >10,000 and at the same time >90% of one of its chain-ends havinga chromophore were never obtained. This data has significant implications forthose trying to use NMRP to make functionalized polymers and block copolymers;that is, it is impossible to make polymers having perfect structures by using thischemistry. Therefore, unless you are willing to accept styrenic polymers havingsome defects in the structure, this chemistry should not be used. However, thischemistry may be the best way to make certain block copolymers even though thestructure produced is not perfect. For example, S-co-S-alt-MA copolymers havebeen synthesized using NMRP (255). At the present time, no alternative chem-istry for making these block copolymers exists.

To date, no one has yet commercialized any polymers produced usingan LFRP process, although several companies are aggressively evaluating the


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 high-est potential volume is the preparation of styrene–butadiene block copolymers insitu during the manufacture of HIPS (256). In this process (Fig. 47), butadiene ispolymerized to PB using conventional anionic chemistry. Normally, after the poly-merization is complete, the resulting polybutadienyl lithium (PB-Li) is terminatedby 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 toa 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 ofits chain ends (13). The LFR-functionalized PB is then dissolved in styrene. Therubber styrene solution is then pumped into a typical continuous bulk PS reactor.During the polymerization process, the LFR-functionalized PB acts as a macro-initiator, resulting in the formation of a styrene–butadiene block polymer (14) insitu, simultaneously with the formation of PS. The final product is a blend of PSand a styrene–butadiene block rubber. The resulting HIPS product has propertiessimilar to HIPS made by polymerizing a solution of styrene containing a styrene–butadiene block rubber (14). The same chemistry can be used to make ABSresins where the rubber phase ends up being a SAN–butadiene block copolymer(15).

Recently, a universal alkoxyamine initiator (16) was reported that allowsLFRP of monomers other than styrene. This new alkoxyamine was discoveredutilizing high speed combinatorial synthesis techniques. It has been used to makestyrene–butadiene, S–BA, and S-MMA block copolymers directly without the needfor tandem polymerization techniques (258).


The newest LFRP system involves reversible addition fragmentation chaintransfer (RAFT) (259). PS, PMA, and PMMA having a polydispersity of <1.3 andblock copolymers containing these monomers have been produced utilizing thisnovel chemistry. The initiators are either thioesters or thiocarbonates (260). TheRAFT process is shown in Figure 48.

Amazingly, CSIRO researchers (261) used the RAFT chemistry to make anS-nBA-S triblock copolymer having an Mn 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 manyother monomers. The styrene double bond is electronegative because of the do-nating effect of the phenyl ring. Monomers that have electron withdrawing sub-stituents (eg, acrylonitrile, maleic anhydride) tend to most readily copolymerizewith styrene because their electropositive double bonds are attached to the elec-tronegative styrene double bond. Spontaneous copolymerization experiments ofmany different monomer pair combinations indicate that the mechanism of initi-ation changes with the relative electronegativity difference between the monomerpairs (262).

Copolymerization makes possible dramatic improvements in one or moreproperties. There is a vast amount of literature on styrene-containing copolymers

Fig. 48. Reversible addition fragmentation chain transfer (RAFT) process developed byCSIRO researchers.

Fig. 49. Synthesis of S-BA block copolymers using the RAFT process.


Table 5. Free-Radical CopolymerizationReactivity Ratios for the Monomer Pairs

Monomer 2 r1 r2 T, ◦C

Acrylonitrile 0.4 0.04 60Butadiene 0.5 1.40 50m-Ethyl methacrylate 0.54 0.49 60m-Divinylbenzene 0.65 0.60 60aMonomer 1 = Styrene

(263). The reactivity ratios for the monomer pairs are listed in Table 5. Styrene–acrylonitrile (SAN) copolymers are large-volume thermoplastics with improvedmechanical properties and heat and solvent resistance with some loss of colorstability (see ACRYLONITRILE AND ACRYLONITRILE POLYMERS). The disparity of reac-tivity ratios shown in Table 5 implies an unequal insertion rate of the monomersinto the copolymer; Figure 50 illustrates manifestations of this effect. Most SANcopolymers are manufactured at or near the azeotrope or crossover point (A inFig. 50) to avoid significant drift in composition (Fig. 51) (264,265). SAN copoly-mers having as little as a 4 wt% difference in acrylonitrile content are not miscible(73). Therefore, the preparation of SAN under conditions where composition driftcan occur leads to haze. Reactors have been designed to overcome compositiondrift problems on a commercial scale (266). All SAN copolymers produced in theUnited States are made in a mass or solution continuous process. The greatertendency of these copolymers to develop yellow coloration requires more attentionto techniques of polymerization and devolatilization (267–270). The discoloration

Fig. 50. Relationship between feed composition and copolymer composition of styreneacrylonitrile copolymerization.


Fig. 51. Drift in monomer composition (—) and copolymer composition (—) with conver-sion for three initial monomer mixtures; ratios are based on wt%.

mechanism has been linked to both acrylonitrile sequence distribution in the poly-mer 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 composi-tions have extensive uses as paper coatings, water-based paints, and carpet back-ing. Because of unfavorable reaction kinetics in a mass system, these copolymersare made in an emulsion polymerization system, which favors chain propagationand disfavors termination (276). The result is economically acceptable rates withdesirable chain lengths. Usually, such processes are run batchwise in order toachieve satisfactory particle-size distribution.

Styrene–maleic anhydride (SMA) copolymers are used where improved resis-tance to heat is required. Processes similar to those used for SAN copolymers areused. Because of the tendency of maleic anhydride to form alternating copolymerswith styrene, composition drift is extremely severe unless the polymerization iscarried out in CSTR reactors having high degrees of backmixing.

Divinylbenzene copolymers with styrene are produced extensively as sup-ports for the active sites of ion-exchange resins and in biochemical synthesis.About 1–10 wt% divinylbenzene is used, depending on the required rigidity ofthe cross-linked gel, and the polymerization is carried out as a suspension of themonomer-phase droplets in water, usually as a batch process. Several studies havebeen reported recently on the reaction kinetics (277,278).

Polymerization of styrene in the presence of PB rubber yields a much toughermaterial than PS. Although the material is usually opaque and has a somewhatlower modulus, the gain in impact resistance has placed this plastic in very widecommercial usage. The polymerization is complex because the rubber has suf-ficient reactivity, ie, at the allylic hydrogen and addition to 1,2-vinyl units, tograft and cross-link and the polymerization system is comprised of two phases.


Fig. 52. Ternary phase diagram for the system styrene PS–PB rubber.

The graft copolymer of PS on polybutadiene acts as an emulsifier, stabilizing thedispersion against coalescence. Events taking place during this polymerizationcan be followed with the ternary phase diagram shown in Figure 52. For a feedmixture made up of 8 wt% rubber in styrene (point A) on an increment of con-version to PS (to D), phase separation occurs. Small droplets of styrene/PS, whichare stabilized by the graft copolymer, are dispersed in the styrene/polybutadienesolution. The composition of the two phases is given by points B and C for the PS-rich 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 proceedsalong the line AE, the phase composition and phase-volume ratio can be readfrom the tie lines. Larger droplets of PS solution form and the small, original onesremain. When the phase-volume ratio is about unity (point F), provided thereis sufficient shearing agitation, the larger PS-containing droplets, which havemostly coalesced into large pools, become the continuous phase and polybutadiene-containing droplets are dispersed therein (279,280). However, the latter containthe small, original PS particles as occlusions. The particle size and size distributionare 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, andthe interfacial tension between the phases (81,279). The viscosity parameters de-pend on phase compositions, polymer molecular weights, and temperatures; theinterfacial tension is largely controlled by the amount and structure of the graftcopolymer available at the particle surface. Phase-contrast micrographs of thisphase-inversion sequence, wherein the PS phase is dark, are shown in Figure 53.If shearing agitation is insufficient, then phase inversion does not occur andthe product obtained is an inter network of cross-linked rubber with PS, andits properties are much inferior. The morphology of the dispersed rubber phaseremains much as formed after phase inversion, unless there is a step change inphase concentration by, eg, blending streams. Rubber-phase volume, including theocclusions, largely controls performance; for example, Table 6 illustrates Izod im-pact strength dependence (281).


Fig. 53. Phase-contrast photomicrographs showing particle formation via phaseinversion.

Once the desired particle size and morphology with sufficient graft to provideadequate adhesion between the rubber particles and the PS phase have beenachieved, the integrity of the particle must be protected against deformation ordestruction during fabrication by cross-linking of the polybutadiene chains. Thesechemical changes taking place on the rubber begin with polymerization. Eitheranionic 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 timephase inversion occurs; less is grafted in the absence of peroxides (282).

Hydrogen abstraction by alkoxy radicals at any of the allylic sites yields asite for styrene polymerization, resulting in a graft (282,283). Model studies basedon styrene–butadiene block copolymers show a strong influence of graft structureon particle morphology (81,82). The double bonds in polybutadiene are much less

Table 6. Dependence of Izod ImpactStrength on Rubber Particle Size

Rubber particle weight Izod impactaverage dia., µm strengtha, J/mb

0.6 480.8 531.0 641.5 752.0 912.5 933.0 993.5 100aASTM D256.56.bTo convert J/m to ft·lbf/in., divide by 53.38.


reactive than those in styrene, but as conversion exceeds ca 80% and temperatureapproaches or exceeds 200◦C, copolymerization through the polybutadiene (1,2units) 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 theproperties of HIPS, there has been a significant amount of research carried outto develop an understanding of the grafting process and also the development ofways to increase the extent of grafting (60,285–288).

Commercial Polymerization Processes

Polymerization. There are two problems in the manufacture of PS: re-moval of the heat of polymerization (ca 700 kJ/kg (300 Btu/lb)) of styrene poly-merized and simultaneously handling a partially converted polymer syrup witha viscosity of ca l05 mPa (= cP). The latter problem strongly aggravates the for-mer. A wide variety of solutions to these problems have been reported. For the fourmechanisms described earlier, ie, free radical, anionic, cationic, and Ziegler–Natta,several processes can be used. Table 7 summarizes the processes that have beenused to implement each mechanism for liquid-phase systems. Free-radical poly-merization of styrenic systems, primarily in solution, is of principal commercialinterest. Suspension processes are declining in importance (289,290). Emulsionprocesses have been reviewed (291). The historical development of styrene poly-merization processes has been reviewed (289,292,293).

Two types of reactors are used for continuous solution polymerization. Thefirst is the continuous plug-flow reactor (CPFR), approximating in the idealcase 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 tubesthrough which a heat-transfer fluid flows. Multiple temperature zones can eas-ily be achieved in a single reactor; agitation is provided by a rotating shaft witharms down the central axis of the tubular vessel. Reactors of this type operate forlong periods and handle very high viscosity partial polymers with reliable heat-transfer coefficients. CSTRs are either of the recirculated coil configuration andrely upon conduction to the cooled jacket for heat removal or are ebullient makinguse 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 Used for all styrene plastics Yes YesSolution Used for all styrene plastics Styrene–butadiene Yes Yes

block copolymersSuspension Used for all styrene plasticsPrecipitation YesEmulsion Used for styrene–butadiene

latices and ABS plastics


Fig. 54. Typical continuous plug-flow reactor (CPFR) polymerization process.

of this reactor occurs at high viscosity, when the rate of vapor disengagement isexceeded by the rate of vapor generation and the ability to mix in the condensedvapor. Typically, this limit occurs at 60–70 wt% solids. Figures 54 and 55 illustratetypical processes based on these two types of reactors. Most GPPS is manufacturedin the United States in such facilities. The CPFR process (Fig. 54) typically usesup to 20 wt% ethylbenzene mainly to reduce viscosity, but also to eliminate reactorfouling. The three reactors may each be subdivided into three temperature-controlzones, which allows for flexible control of the molecular weight distribution of thepolymer 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◦Cbefore being devolatilized upon entering a vacuum tank. The recovered solventand monomer are continuously recycled to the feed. The molten polymer leaves thevacuum tank and is stranded, cooled, and chopped into pellets for shipping. TheCSTR process (Fig. 7) differs in that it is typically a single reactor and, therefore,

Fig. 55. Typical CSTR polymerization process.


involves a single temperature zone and operates at ca 60 wt% solids, requiring alarge polymer heater to achieve adequate devolatilization (294,295). The need fora large polymer heater to achieve adequate devolatilization can be eliminated byadding a CPFR after the CSTR to carry the conversion to high solids.

There has been some research aimed at continuously converting monomercompletely to polymer using reactive extrusion (296). However, this process hasnot yet been commercialized.

Rubber-modified PS manufacture places several additional demands: (1) dis-solving ca 5–10 wt% polybutadiene in styrene, often a 10- to 20-h process; (2) shearconditions to achieve phase inversion and the desired particle size for a given prod-uct; (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 nec-essary cross-linking, gel formation, and swelling index. Graft copolymer formationoccurs throughout the polymerization, and sufficient amount of graft copolymerfor 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 toavoid rapid shifts in phase compositions on mixing of one reactor effluent withthe next reaction contents with loss of desirable morphology (297,298). Figure 56shows a multizone CSTR reactor system. Phase inversion and particle sizing isaccomplished in the first-stage reactor with the effluent entering a single-tank,baffled reactor operating as five stages with a common vapor space. An alterna-tive is three CSTR reactors in series, followed by an adiabatic reactor to achievehigh solids content (297). The multiple stages are necessary to achieve favorable

Fig. 56. Typical staged CSTR process for HIPS.


rubber-particle morphology with CSTRs; CPFR systems inherently avoid suddenphase-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 stabilizethe particles. The shear field in the reactor is critical to the control of particle sizeas well as to meet the minimum shear rate for phase inversion, without whichthe product consists of a continuous cross-linked rubber network with markedlyinferior properties (279,280). After the particles are formed and sized, only gentleagitation is applied to aid heat transfer without disturbing the particle structure.Conditions that produce the desired PS-phase molecular weight are used. Prior todevolatilization, the partial polymer is heated to ca 200◦C to cross-link the rubberchains in order to protect the rubber particles from the very high shear fieldsduring fabrication. Stranding, cooling, and chopping into pellets follows.

Polybutadiene rubbers are used almost exclusively as reinforcing agents forstyrenic plastics. The very low glass-transition temperature (−80◦C or lower) givesgood low temperature properties, and the allylic hydrogen atom and weakly activedouble bonds can provide the desired degree of grafting and cross-linking. Otherrubbers have been used; acrylates, ethylene–propylene–diene rubbers, polyiso-prene, and polyethylene have been reported, but with limited commercial successbecause of their low chemical reactivity, not withstanding their acceptable glass-transition temperature (299).

Devolatilization. Current bulk polymerization processes convert styreneto about 80%. The polymer syrup exits the polymerization reactor at about 170◦C.The most common devolatilization technology involves increasing the tempera-ture of the syrup to >230◦C by passage through a heat exchanger connected to avacuum tank. The syrup foams, forming strands which fall into a vacuum tankat 5–10 mm Hg as they exit the heat exchanger. An equilibrium of styrene in thepolymer and in the vapor phase takes place, which is dependent upon the tempera-ture and the pressure in the tank. Figure 57 shows the vapor–polymer equilibriumpartitioning data (calculated) for styrene in PS at various levels of vacuum (one-stage devolatilizer) (300). Typically, devolatilizers operate at ca 230–240◦C and5–10 mm Hg. Therefore, the level of residual styrene monomer left in the polymeris typically in the range of 300–500 ppm. If the polymer temperature is increasedto greater than 240◦C, polymer degradation begins to take place resulting in theformation of styrene monomer. Asahi researchers have solved this problem by sta-bilizing the polymer against unzipping by the addition of a special stabilizer (18),thus allowing higher temperature devolatilization to be carried out (301).


Fig. 57. Vapor-polymer equilibrium partitioning data for styrene in PS vs temperatureand pressure. · · ·◦· · · 133.3 Pa; -�—667 Pa;——1.33 kPa; - -•- - 667 Pa + 10% water. Toconvert Pa to mm Hg, divide by 133.3.

Some foods packaged in PS are extremely sensitive to taste (eg, chocolatechip cookies). These sensitive applications require PS having residual styrenemonomer levels below 200 ppm. It is not feasible to consistently lower the styrenelevel to <200 ppm without special techniques.

A new technology for lowering residuals in PS being commercialized by Dowis centrifugal devolatilization in a rotating packed bed (302,303). The basic conceptinvolves pumping the molten PS into the center of an evacuated spinning rotorpacked with metal mesh. Centrifugal force quickly pushes the polymer throughthe high surface area mesh where it coats the metal and volatiles quickly diffusefrom the thin polymer film. The centrifugal force continues to force the polymer outof the metal mesh packing against the exterior surface of the centrifuge havinga circumferential set of die holes. After being extruded from the die holes, thepolymer strands are cut into pellets as the rotor rotates past a slowly moving,continuous knife blade. A drawing of the centrifugal devolatilizer is shown inFigure 58.

A very effective way to get residual styrene monomer levels below equilib-rium 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, lowcost, easily separable from the recycle, and inert (eg, water). However, simplybleeding the replacement material into the vapor space of the devolatilizer wouldstill require significant residence time to reach equilibrium. A way to expedite theequilibrium process is to mix the replacement material with the molten polymer(304). Then as the material vaporizes in the molten polymer, it nucleates bub-bles stripping styrene from the polymer while creating a vapor phase composedmainly of the stripping material. Stripping materials that have been used includenitrogen, (305), methanol (306), and water (307).

Another technique to expedite the transport of the volatile components fromthe molten polymer is to increase the number and rate of bubbles formed (308).


N2 Purge

Vacuum

(Vapor)

Hollow shaft

Polymer melt

Seal

Seal housing

Drive base

Drive

Packing

Cutter

Fig. 58. Drawing of centrifugal devolatilizer being used by Dow.

Techniques that have been used to increase the number of bubbles and their rateof formation (nucleation) are the addition of chemical nucleating agents (309) andultrasound (310). Nucleation of bubbles in the molten polymer can help expediteachievement of equilibrium in conventional falling strand devos. However, this fa-cilitation mechanism can not get below equilibrium and thus has marginal value.

Chemically Removing Residual Monomer. Some researchers havetried to lower the level of residual styrene monomer by chemical means. The ideaof adding something to molten PS that scavenges residual styrene monomer hasbeen 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 lowconcentration must diffuse together, find each other, and couple together in a vis-cous polymer. The obvious way to overcome this difficulty is to use the scavenger inhigher than stoicheometric concentration. This introduces several new problemsincluding contamination of the polymer with a new material and increased rawmaterial costs. Styrene scavenger compounds that have been patented includeunsaturated fatty acids (311) and aldehydes (312), sulfonyl hydrazides (313), cy-clopentadienes (311), K-resin (314), peroxides (315), and terpenes (316).

Another option for lowering residual styrene monomer is to carry the poly-merization up to high conversion. Typically, continuous bulk polymerization pro-cesses produce partial polymer syrups at about 65–80% solids. The unreactedstyrene that remains is removed by evaporation. If the solids content of the poly-mer could be taken higher, the level of residual styrene in the polymer wouldbe lower, especially when using a one-stage devolatilizer. In suspension polymer-ization, devolatilization is not even required because styrene is polymerized to>99.9% monomer conversion. If it were possible to polymerize styrene to very high


conversion in bulk polymerization processes, significantly lower residual styrenemonomer could be achieved. Because of viscosity constraints, bulk polymerizationreactors cannot operate at >80% solids. To achieve >99% monomer conversionwhile maintaining the solids <80% would require high levels of solvent. The prob-lem with solvents is chain transfer. Going above 95% conversion in ethylbenzeneresults in an exponential increase in polydispersity due primarily to chain trans-fer effects. A high conversion bulk styrene polymerization process resulting in theformation of low residual PS has been patented, which solves the viscosity prob-lem and allows going to >99.9% solids by finishing the polymerization off in anextruder (317). Extruders are not very effective heat exchangers, yet are designedfor handling high viscosity materials. Thus, Kelly carried out the polymerizationof pure styrene monomer without the use of a solvent in a conventional polymer-ization reactor to normal solids levels and then fed the partial polymer into anextruder where he finished off the polymerization. He used a mixture of initiatorshaving different half-lives so that radicals were continuously generated. Recently,free-radical polymerization of styrene directly in an extruder using 1 wt% perox-ide initiator was analyzed (318). They were not successful in making PS havinga molecular weight >100,000. They got around this problem by attaching a pre-polymerization reactor to the front end of the extruder to polymerize to 25% solidsto make high Mw PS. The low Mw PS then made in the extruder giving them abimodal PS. The conversion they achieved in the extruder was 98–99%.

A final technique that has been utilized to chemically remove residualstyrene in PS is radiation treatment. Both beta (e-beam) and gamma radiationhave been tried. E-beam appears to be the most effective form of radiation andis 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 andscavenge unreacted monomer. However, if no styrene is in the vicinity of the PSradical, it can do other things such as couple with another PS radical or react withoxygen. Currently, electron beam treatment of polymers is used commercially forelastomer cross-linking (wire/cable coatings) but not for monomer reduction.

Polystyrene treated with 1–4 Mrad of e-beam radiation at temperatures be-tween 25 and 200◦C shows an optimum irradiation temperature between 80 and150◦C (Fig. 59). It is interesting that all dosages tested have a temperature rangefor maximum effectiveness, which drifts toward lower temperature as the dosageincreases (321). At all temperatures studied, weight-average molecular weightincreases 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 de-sign, uniform melt temperature, and excellent mixing characteristics make themthe preferred choice for injection molding. Machines with shot capacities up to25 kg for solid injection-molded parts and 65 kg for structural foam parts are avail-able (323,324). Large solid moldings include automotive dash panels, televisioncabinets, and furniture components. One-piece structural foam parts weighing35 kg or more are molded for increased rigidity, strength, and part weight reduc-tion (325) (see PLASTICS PROCESSING).

The injection-molding process is basically the forcing of melted polymer intoa relatively cool mold where it freezes and is removed in minimum time. The shape


Fig. 59. Performance of different e-beam doses vs temperature (321). - -◦- - 10 kGy; -�—20kGy; -—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 intothe mold followed by quick setup results in a significant amount of orientation inthe molded part. The polymer molecules and, in the case of heterogeneous rubber-modified polymers, the rubber particles tend to be highly oriented at the surfaceof the molding. Orientation at the center of the molding tends to be significantlyless because of the relaxation of the molten polymer.

Fig. 60. Effect of dose on Mw at ambient and elevated temperature (321). - -◦- - 25◦C;-�—85◦C. To convert kGy to Mrad, divide by 10.


The anisotropy that develops during the molding operation is detrimentalto the performance of the fabricated part in several ways. First, highly orientedmoldings, which form particularly if low melt temperatures are used, exhibit goodgloss, have an abnormally narrow use temperature range causing early warping,and, perhaps most importantly, tend to be brittle even though the material isinherently capable of producing tough parts (326). However, this development ofpolymer orientation during molding can also be used to advantage, as in the caseof rotational orientation during the molding operation (327).

The achievement of isotropic moldings is also important when the moldedpart is to be decorated, ie, painted, metallized, etc Highly oriented parts that havea high frozen-in internal stress memory tend to give rise to rough or distortedsurfaces as a result of the relaxing effect of the solvents and/or heat. For electro-plating 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 thesepolymers tend to yellow somewhat if too high a temperature and/or too long aresidence time are imposed.

To obtain satisfactory moldings with good surface appearance, contamina-tion, including that by moisture, must be avoided. For good molding practice,particularly with the more polar styrene copolymers, drying must be part of themolding operation. A maximum of 0.1 wt% moisture can be tolerated before sur-face imperfections appear.

For achieving appropriate economics, injection-molding operations arehighly automated and require few operating personnel (322). Loading of the hop-per is usually done by an air-conveying system; the pieces are automaticallyejected, 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 convenientand least expensive fabrication methods, particularly for obtaining sheet, pipe,irregular profiles, and films. Relatively small extruders, eg, 11.5 cm diameter, canproduce well over 675 kg/h of polymer sheet. Extrusion is also the method for plas-ticizing the polymer in screw injection-molding machines and is used to developthe parison for blow molding. Extrusion of plastics is also one of the most econom-ical methods of fabrication, since it is a continuous method involving relativelyinexpensive equipment. The extrusion process has been studied in great detail(330,331). Single-screw extruders work extremely well with styrene-based plas-tics. 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 forremoval of volatiles, often eliminating the necessity for predrying as is practicedwith hygroscopic materials, eg, SAN and ABS. Where venting is inadequate, thesepolymers must be predried to a maximum moisture content of 0.03–0.05 wt% toobtain high quality sheet (see FILMS AND SHEETING).

Many rubber-modified styrene plastics are fabricated into sheet by extrusionprimarily for subsequent thermoforming operations. Much consideration has beengiven to the problem of achieving good surface quality in extruded sheet (332,333).


Excellent surface gloss and sheet uniformity can be obtained with styrene-basedpolymers.

Considerable work has been done on mathematic models of the extrusionprocess, with particular emphasis on screw design. Good results are claimed forextrusion of styrene-based resins using these mathematical methods (331,334).

With the advent of low cost computers, closed-loop control of the extrusionsystem has become commonplace. More uniform gauge control at higher outputrates 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 forprotection or decorative purposes. For example, an acrylic film can be laminatedto ABS sheet during extrusion for protection in outdoor applications. Multipleextrusion of styrene-based plastics with one or more other plastics has grownrapidly in the last 10 years.

Thermoforming and Orientation. Thermoforming of HIPS and ABS ex-truded sheet is of considerable importance in several industries. In the refrigera-tion industry, large parts are obtained by vacuum-forming extruded sheet. Vacuumforming of HIPS sheet for refrigerator-door liners was one of the most significantearly developments promoting the rapid growth of the whole family of HIPS. Whena thermoplastic polymer film or sheet is heated above its glass-transition temper-ature, it can be formed or stretched. Under controlled conditions, new shapes canbe controlled; also, various amounts of orientation can be imparted to the polymerfilm or sheet for altering its mechanical behavior.

Thermoforming is usually accomplished by heating a plastic sheet above itssoftening point and forcing it against a mold by applying vacuum, air, or mechani-cal pressure. On cooling, the contour of the mold is reproduced in detail. In order toobtain the best reproduction of the mold surface, carefully determined conditionsfor the plastic-sheet temperature, ie, heating time and mold temperature, mustbe maintained.

Several modifications of thermoforming plastic sheet have been developed. Inaddition to straight vacuum forming, there are vacuum snapback forming, drapeforming, and plug-assist-pressure-and-vacuum forming. Some combinations ofthese techniques are also practiced. Such modifications are usually necessary toachieve more uniform wall thickness in the finished deep-draw sections. Vacuumforming can also be continuous by using the sheet as it is extruded. An example ofthis technology is practiced with several high speed European lines in operationin the United States. Precise temperature conditioning allows carefully controlledlevels or orientation in the finished part (337).

Thermoforming is perhaps the process with the lowest unit cost. Examples ofthermoformed articles are refrigerator-door and food-container liners, containersfor dairy products, luggage, etc Some of the largest formed parts are camper/trailercovers and liners for refrigerated-railroad-car doors (338).

Orientation of styrene-based copolymers is usually carried out at tempera-tures just above Tg. Biaxially oriented films and sheet are of particular interest.Such orientation increases tensile properties, flexibility, toughness, and shrink-ability. PS produces particularly clear and sparkling film after being orientedbiaxially for envelope windows, decoration tapes, etc Oriented films and sheet of


styrene-based polymers are made by the bubble process and by the flat-sheet ortentering 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 manu-facturing hollow symmetrical objects. The granules are melted and a parison isobtained by extrusion or by injection molding. The parison is then enclosed bythe mold, and pressure or vacuum is applied to force the material to assume thecontour of the mold. After sufficient cooling, the object is ejected.

Styrene-based plastics are used somewhat in blow molding but not as muchas linear PE and PVC. HIPS and ABS are used in specialty bottles, containers, andfurniture 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 formula-tions.

Polystyrene or copolymers are used extensively in injection blow molding.Tough and craze-resistant PS containers have been made by multiaxially orientedinjection-molded parisons (340). This process permits the design of blow-moldedobjects with a high degree of controlled orientation, independent of blow ratio orshape.

Additives. Processing aids, eg, plasticizers and mold-release agents (seeABHERENTS), are often added to PS. Even though PS is an inherently stable poly-mer, other compounds are sometimes added to give extra protection for a partic-ular application. Rubber-modified polymers containing unreacted allylic groupsare very susceptible to oxidation and require carefully considered antioxidantpackages for optimum long-term performance. Ziegler–Natta-initiated polybuta-diene rubbers are especially sensitive in this respect, since they often containorganocobalt residues from the catalyst complex. For food-contact applications,the additives must be FDA approved. Important additives used in styrene plas-tics are listed in Table 8.

Economic Aspects

Most of the styrene monomer manufactured globally goes into the manufactureof 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, andthe Pacific consuming most of the world’s production (Fig. 62). The global PSproduction capacity generally parallels the demand for the material (Fig. 63).However, the trend over the last 15 years has been toward narrowing the gapbetween capacity and demand in an effort to maximize the profitability of thebusiness.

Characterization

Four modes of characterization are of interest: chemical analyses, ie, qualita-tive and quantitative analyses of all components; mechanical characterization,ie, tensile and impact testing; morphology of the rubber phase; and rheology at


Table 8. Additives Used in Styrene Plastics

AmountType Compounds Examples (wt%) Comments

Plasticizers Mineral oil <4 all cause loss ofheat-distortiontemperature

Phthalate estersAdipate esters

Mold-release Steric acid 0.1 Many mold-releaseagents agents causeagents yellowness of haze

Metal steratesSterate estersSiliconesAmide waxes 3

Antioxidants Alkylated phenols Ionol 1 Synergism sometimesachieved withmultiple use

Phosphite esters Tris(nonylphenyl) 1phosphite

Thioesters Dilaurylthio- 1dipropionate

Antistatic Quaternary 2agents ammonium

compoundsuv stabilizers Benzotriazoles Tinuvin P 0.25

BenzophenonesIgnition Halogenated Synergism sometimes

supression compounds achieved withagents multiple use

Antimony oxideAluminum oxidePhosphate esters

a range of shear rates. Other properties measured are stress-crack resistance,heat-distortion temperatures, flammability, creep, and others depending on theparticular application (341).

Since plastics are almost invariably modified with one or more additives,there are three components of chemical analysis: the high molecular weight por-tion, ie, the polymer; the additives, ie, plasticizer and mold-release agent; and theresiduals remaining from the polymerization process. The high molecular weightportion is best characterized using gpc which gives molecular weight and poly-dispersity information. If the gpc is equipped with a diode-array detector, chro-mophores responsible for polymer discoloration can be located (177). The additivesin the polymer are best characterized by extracting them from the polymer and an-alyzing the extract using high performance liquid chromatography (342). Figure 5illustrates typical molecular weight distribution curves from gpc, and Figure 64


Fig. 61. Historical comparison of styrene monomer and GPPS pricing. · · ·◦· · · Styrenemonomer; -�—Polystyrene.

Fig. 62. Global consumption of PS by geographical area (145).

shows a capillary gas chromatogram for determining residuals. Chemicalcharacterization techniques utilized for styrenic polymers are summarized inTable 9. Mechanical testing is carried out according to the methods listed inTable 10.

Rubber particle-size distribution is usually measured with a Coulter counteror directly from electron photomicrographs (110,343); the latter also gives detailsof particle morphology. Rheological studies are made with a modified tensile testerwith capillary rheometer (ASTM D1238-79) or with the powerful Rheometricsmechanical spectrometer (344). For product specifications, a simple melt-flow testis used (ASTM D1238 condition G) with a measurement of the heat-distortion


Fig. 63. Global demand vs capacity for PS since 1985 (145).

temperature (ASTM D1525). These two tests, with solution viscosity as a measureof molecular weight, have been used historically to characterize PS. However,molecular weight distribution and residuals analyses, in general, have replacedthem.

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 prob-lems from ingestion, inhalation, or eye or skin contact. Heating of these polymersusually 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 verylow levels of degraded and oxidized hydrocarbons and additives. The warningproperties of styrene and any oxidized organics are such that the vapors normallycan be detected by humans at very low levels, and hazardous concentrations areusually irritating and obnoxious. Nevertheless, adequate ventilation should beprovided.

Styrene polymers burn under the right conditions of heat and oxygen supply.This can occur even when they are modified by addition of ignition-suppressionchemicals. Once ignited, these polymers may burn rapidly and produce a denseblack smoke. Combustion products from any burning organic material should beconsidered toxic. Ignition temperatures of several styrenic polymers are listed inTable 11. Fires can be extinguished by conventional means, ie, water and waterfog.


Fig. 64. Capillary gas chromatogram showing residuals in PS.

Table 9. Chemical Characterization Techniquesa

High molecularweight component Additive Residuals

Variable Method Variable Method Variable Method

Molecular weight Solution Mineral oil gc/hplc Styrene gcviscometry

Molecular weight gpc Phthalate gc Solvents gcdistribution esters

Copolymer composition ir Stearates ir Oligomers gcRubber content ir Other hplcGraft yield solubilityagpc = gel-permeation chromatography; gc = gas chromatography; hplc = high performance liquidchromatography; ir = infrared analysis.


Table 10. Mechanical Tests on PS

Test ASTM method

Stress–strainTensile D638Compression D695Flexural D790

ImpactIzod D256Dart D3029Hardness D785Creep D2990

Table 11. Ignition Temperatures and Burnhing Rates of Styrene-Based Polymersa

Minimum Minimumflash-ignition self-ignition Burning

Polymer temperature,◦C temperature, ◦C rateb, cm/s

PolystyreneGeneral–purpose 345–360 488–496 <0.06Rubber–modified 385–399 435–474 <0.06

Styrene–acrylonitrile copolymer 366 454 <0.06Acrylonitrile–butadiene–styrene 404 >404 <0.06

aTest specimens for burning rate data were 1.27 × 15.24 × 0.318 cm3. Descriptions of burning rateand other flammability characteristics developed from small-scale laboratory testing do not reflecthazards presented by these or any other materials under actual fire conditions.bIn a horizontal position.

Uses

PS Foams. The early history of foamed PS is given in Reference 346 andthe theory of plastic foams is discussed in Reference 347. Dow became the firstto commercially manufacture PS-extruded foam board for home insulation un-der the trade name Styrofoam (348). Foamable PS beads were developed in the1950s by BASF under the trademark of Styropor (349–351). These beads, madeby suspension polymerization in the presence of blowing agents such as pentaneor hexane, or by post-pressurization with the same blowing agents, have hadan almost explosive growth, with 200,000 metric tons used in 1980. Some typi-cal physical properties of PS foams are listed in Table 12 (see FOAMED PLASTICS).Some believe that there are unlimited new opportunities for the development ofnew foam materials (352).

The following are commercially significant foamed PSs. Extruded planksand boards in the density range of 29 kg/m3 and largely flame retardant are usedfor low temperature thermal insulation, buoyancy, floral display, novelty, packag-ing, 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 wellas automated molding of continuous planks are used. Extruded foamed PS sheet


Table 12. Characteristics Properties of Some Polystyrene Foams

Styrofoama, Bead, FoamProperty extruded board-molded sheet ASTM

Density, kg/m3b 35 32 96Compressive strength, kPac 310 207–276 290 D1621Tensile strength, kPac 517 310–379 2070–3450 D1623Flexural strenght, MPad 1138 379–517 D790Thermal conductivity, W/(m·K)e 0.030 0.035 0.035 C177Heat distortion, ◦C 80 85 85aRegistered trademark of The Dow Chemical Co.bTo convert kg/m3 to lb/ft3, multiply by 0.0624.cTo convert kPa to psi, multiply by 0.145dTo convert MPa to psi, multiply by 145.eTo convert W/(m·K) to (Btu·in.)/(h·ft·2◦F), multiple by 6.933.

17 mm thick with densities of 64–160 kg/m3 are used largely in packaging appli-cations. Extremely fine cell size is required for texture and strength. Special nu-cleating agents are employed for this purpose, eg, polymers of styrene and maleicanhydride, 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 aremade either by extrusion or from beads. High density (480–800 kg/m3) injection-molded objects are made from mixtures of FIP beads and PS granules or morecommonly 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 com-mercial significance is coextruded, foam-core ABS pipe (355,356). Reducing thedensity of the foam core by up to one-half and using a chemical blowing agentresults in about a 20% overall pipe weight and raw material usage reduction.Syntactic foams are combinations of foamable beads, eg, expandable PS or SANcopolymers. These are mixed with a resin, usually thermosetting, which has alarge exotherm during curing, eg, epoxy or phenolic resins (357). The mixture isthen placed in a mold and the exotherm from the resin cure causes the expandableparticles to foam and forces the resin to the surface of the mold. A typical exampleis expandable PS in a flexible polyurethane-foam matrix used in cushioning ap-plications (358). Sandwich panels are made either by foaming beads between theskin materials or by adhering skins to planks cut to precise dimensions. FoamedPS beads are admixed with concrete for lightweight masonry structures (359).

Extruded Rigid Foam. In addition to low temperature thermal insula-tion, foamed PSs are used for insulation against ambient temperatures in theform of perimeter insulation and insulation under floors and in walls and roofs.The upside-down roof system, in which foamed plastic such as Styrofoam plasticfoam is applied above the tar-paper vapor seal, thereby protecting the tar paperfrom extreme thermal stresses that cause cracking, has been patented (360). Thefoam is covered with gravel or some other wear-resistant topping (see ROOFING

MATERIALS).


In addition to such thermal-insulation uses in buildings, there is a poten-tially tremendous new area in the form of highway underlayment to prevent frostdamage. Damage to roadways, roadbeds, and airfields because of frost action is acostly and aggravating problem. Conventional treatments to prevent such damageare expensive and unreliable. During the 1960s, an improved and more economi-cal solution to the problem was developed (361). It is based on the use of thermalinsulation to reduce the heat loss in the frost-susceptible subgrade soil so that nofreezing occurs. Many miles of insulated pavements have been built in the UnitedStates, Canada, Europe, and Japan. The concept has proved valid, and the perfor-mance of extruded PS foam has been completely satisfactory. It is expected thatthe use of this concept will continue to grow as natural road-building materialscontinue to become more scarce and expensive and as the weight and speed ofland and airborne vehicles continue to increase.

A 2.54-cm Styrofoam plastic foam with thermal conductivity of ca0.03 W/(m·K) [(0.21 Btu·in.)/(ft·h·◦F)] is equivalent to 61 cm of gravel. Any syn-thetic foam having compressive strength sufficiently high and thermal conductiv-ity sufficiently low is effective. However, the resistance of PS-type foams to water,frost damage, and microorganisms in the soil makes them especially desirable. Aninteresting and important application of this concept was the use of Styrofoam inthe construction of the Alaska Pipeline. In this case, the foam was used to protectthe permafrost.

Rigid Foam from FIP Beads. Expandable PS (EPS) has been in wideuse in packaging since its introduction. Applications range from pallet shippingcontainers for computer terminals to material-handling stacking trays to packagesfor fresh fruits and vegetables. Expandable PS serves as a cushioning material, apackage 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 desireddensity 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 specificshape. Most parts are molded of standard white resins, although several pastelcolors are available.

Converters who manufacture EPS parts and components for packaging arecalled 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 withoutthe benefits of a mold; parts can be fabricated from billets by hot-wire cutting.Whether molded or fabricated, EPS packages and their components are typicallydesigned by careful consideration of the compression and cushioning propertiesof 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 andcontent, and bead size, have been analyzed (363). It is suggested that at leastpart 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 avalue of 1050 kg/m3 for pure PS beads and compared with a calculated density of1020 kg/m3 for a simple mixture in which n-pentane is dissolved in the PS. If all ofthe 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-Pentanepresent in voids has a reduced vapor pressure, which depends on the effective


Table 13. Loss of Isopentane from Foaming-in-Place Chlorostyrene Beadsa in OpenStorage, wt%b

After 20 h After 300 h

Temperature,◦C Polystyrene Poly(chlorostyrene) Polystyrene Poly(chlorostyrene)

25 18 1.3 40 1050 38 12 72 3380 48 24 80 54

a420-mm beads.bInitial concentration of isopentane = 6 wt%.

microcapillary diameter. Dissolved n-pentane in PS will also have a reduced vaporpressure, by an amount depending on the thermodynamic interaction betweensolvent 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 retainblowing agent as indicated in Table 13. The higher heat distortion of this polymerrequires steam pressures of 210–340 kPa (30–50 psi) for blowing.

Various other factors that influence the foaming of PS, especially cross-linking, are described (359). In Reference 359, foaming is discussed as a viscoelas-tic process in which there is competition between the blowing pressure, whichincreases with temperature, and the thinning and rupture of cell walls with con-sequent collapse of the foam. The presence of cross-links reduces the tendency forcell-wall rupture. Figure 65 shows foam volumes attained as a function of tem-perature for various amounts of divinylbenzene. The effects of temperature andcross-linking on the kinetics of foaming is also discussed in Reference 359.

Fig. 65. Maximum foaming volume of styrene divinylbenzene copolymers containing8.8 wt% CO2 as a function of divinybenzene content and temperature. Numeral besidecurve indicates wt% divinylbenzene. Vt/V ratio of final volume to initial volume at temper-ature t. Adapted from Ref. 362.


Another important factor affecting the foaming of PS beads is the differentialrate of diffusion of n-pentane compared to steam and air. It is estimated that thequantity of n-pentane normally used is sufficient to produce a foaming volumeof 30-fold and hence a foam density of no more than 32 kg/m3. However, thesteam 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 aspectof diffusion has been discussed in some detail (364). In cycle foaming in air, beadsare allowed to equilibrate with air after each heat-foaming step through a seriesof stages. Foaming volumes of 200:1 have been achieved in this manner, withdensities 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. Thelow cost of PS and its high thermal stability, which provides good recycle effi-ciency, as well as the possibility of using low cost blowing agents, are importantto the use of PS in rigid thermoplastic foams. The heat-distortion point of ca80◦C (in contrast to 65◦C for PVC foam) permits its use in most construction ap-plications. The low sensitivity of the polymer to moisture is another importantfactor.

Because of all of the aforementioned and possibly for other reasons, PS hasbeen the dominant material used in rigid thermoplastic foams. Poly(vinyl chloride)is economical and has good physical properties and is flame retarding, but it hasmarginal heat-distortion properties for some uses and does not readily lend itselfto the continuous extrusion process used for PS foam. So far, no FIP beads systembased on PVC has been perfected. This could well be a result of its high vaporbarrier compared to PS. Polyethylene is too permeable to retain a blowing agentand hence is ruled out as a FIP bead material, but it is commercially significantas extruded foam.

Flame Retardants. The growing use of PS foams in the constructionindustry has provided impetus for continuing research on flame-retardant ad-ditives (see FLAME RETARDANTS). Organic bromine compounds, eg, or pentabro-momonochlorocyclohexane, have long been used for this purpose in extruded PSfoam. Use of injection-molded structural foams, based on HIPS and ABS andcontaining flame-retardant additives, has grown rapidly since the late 1970s.A typical additive system to impart the required flame-retardant properties isbis(pentabromophenyl) oxide and a synergist, eg, antimony trioxide. Because ofenvironmental 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 researchedalternative.

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 thecorrect blowing agent for foam sheet is critical in determining ultimate density andphysical properties. Further, the choice of blowing agent may be dictated by safetyconsiderations, eg, flammability of hydrocarbons, or environmental concerns, eg,chlorofluorocarbons (366).


Table 14. Gas and Water Vapor Permeabilities of PSFilmsa

Composition Oxygen CO2 Water

Unmodified PS 3.50 14.00 160072:28 styrene-stat-acrylonitrile 0.09 4.62 2900

aGas permeability units = (cm3·cm)/(1010 cm2·s·kPa).

Oriented PS Film and Sheet. Oriented PS film, in addition to havingthe lowest cost of any of the rigid plastic materials, offers a high degree of opticalclarity, high surface gloss, and excellent dimensional stability (particularly withregard to relative humidity) and is heat shrinkable. It is not a barrier film; infact, one of its largest uses, that of packaging field-fresh produce, depends uponits 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 orien-

tation imparts some extremely desirable properties, particularly in regard to anincrease in elongation. Thus, the 1.5–2% of elongation normally associated withunoriented PS can become as high as 10%, depending on the exact conditions ofpreparation (371,372).

Several methods have been utilized in preparing biaxially oriented PS film.The bubble process is used commercially. The high softening point, coupled withthe tendency of oriented film to shrink above 85◦C, makes heat sealing difficult.Solvent, adhesive, and ultrasonic sealing are used in most applications. Scratchresistance, impact strength, and crease resistance are low. Although the bubbleprocess superficially resembles the common bubble process for making polyethy-lene, 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 polyethy-lene, for example, crystallinity stabilizes the blown bubble, whereas with a truethermoplastic, eg, PS, one must stabilize the blown film by cooling below the Tg.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 ca0.08 mm, which coincides with the defined difference between film and sheet. Filmis made more economically by the bubble method and sheet by the tenter-framemethod. The exact thickness for break-even depends on technological improve-ments, which can be made in both processes, in the degree of control which is usedin regulating them and in quality requirements.

PS film contains no plasticizers, absorbs negligible moisture, and exhibitsexceptionally good dimensional stability. It does not become brittle with age nordistort when exposed to low or high humidity. Excellent machinability, ie, abilityto 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 windowenvelopes. These same characteristics have also led to applications of film for win-dow cartons. An antifog film 0.025–0.032 mm thick has been used extensively forthe windows of cartons for bacon. Substantial amounts of film are also employedas sheet protectors and as inserts in wallets. A thickness of 0.060–0.130 mm iscustomary for these applications. PS film can be printed by means of flexographic,


rotogravure, and silk-screen methods. A lamination of reverse-printed film andpaper has been used for an attractive, high sparkle package for hand soap. Thefilm is an excellent base for metallizing because of the almost complete absence ofvolatiles. Oriented PS film is also widely used for lamination to PS foamed sheetfor preprinted decoration and/or property enhancement (373).

For PS sheet, the use responsible for its rapid growth is in meat trays, wherethe merchandising value is important, ie, the transparency of the package, as wellas its resistance to moisture and dimensional stability. A smaller but growinguse is as a photographic-film base where dimensional stability at all humiditiesand low gel count are both important. The thermoplastic nature of PS and thememory effects built into it through biaxial orientation make it especially suitablefor packaging applications. It must be processed under pressure rather than byvacuum-forming equipment to avoid the shrinkage, which would otherwise resultfrom 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 readingof characters through envelope windows, a significant technological trend which iscertain to enhance the use of PS film. In regard to produce overwrap, the transmis-sion characteristics of PS for water vapor and oxygen coincide more or less withthe metabolic requirements of the produce being packaged, eg, lettuce, so thatin 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 highoptical clarity are important. In addition, the feel of PS film gives an impressionof crispness as far as the product is concerned. PS film also has potential use as atype of synthetic paper (see PULP, SYNTHETIC). GPPS must be treated so that it isopaque, and this can take the form of either pigmentation, mechanical abrasion,or a surface treatment, eg, a chemical etch. HIPS is reasonably opaque. PS filmcan be given a suitable coating, eg, a latex-clay coating, of the kind that is used onordinary paper for the purpose of achieving a good printable surface. There areseveral general discussions of gas permeabilities in plastic films (6,8,9,375,376)(see BARRIER POLYMERS).

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