Polymer International Polym Int 57:431–448 (2008)

ReviewNew developments in flame retardancy ofstyrene thermoplastics and foamsSergei V Levchik1∗ and Edward D Weil21Supresta US LLC, 430 Saw Mill River Rd, Ardsley, NY 10502, USA2Polytechnic University, Six Metrotech Center, Brooklyn, NY 11201, USA

Abstract: This review provides an insight into new developments in flame retardancy of the broad class ofstyrenic polymers but mostly focuses on commercially important styrene thermoplastics, on some blends basedon polystyrene as well as on polystyrene foams. Although halogen-based systems continue to dominate in flameretardancy of styrenic polymers, various alternative systems are being developed. Especially, activity is observedwith phosphorus-based flame retardants, where some systems are already commercially available. There is alsosignificant activity with nanocomposites, where good results in retarding flame spread have been achieved, butthe problem of ignition resistance has not been solved yet. Critical discussion of various flame-retardant systemsdeveloped for styrenics is given. 2007 Society of Chemical Industry

Keywords: polystyrene; HIPS; ABS; styrene foam; flame retardant; halogen; phosphorus; nanocomposite

INTRODUCTIONPolystyrene is a commodity plastic manufactured ona very large scale. Amorphous styrene homopolymer,prepared by radical polymerization, has a glass tran-sition temperature of ca 100 ◦C.1 The brittleness ofpolystyrene considerably limits its use in engineeringand high-performance products. However, the trans-parency of polystyrene makes it useful in applicationslike light coverings or the transparent parts of elec-tric appliances, and in such uses flame retardancy isusually required.

In order to overcome the brittleness characteristic,polystyrene has been extensively modified and itscopolymers and blends represent the most versatileclass of polymeric materials.2 The toughness ofpolystyrene is improved by copolymerization orblending with a butadiene elastomer or other rubber-like polymer, but the rubber should be present as aseparate dispersed phase.1 Crosslinking of the rubberparticles helps to prevent their undesired dissolutioninto the polystyrene matrix. In order to create strongbonds between polystyrene and rubber, grafting isused to anchor the rubber particles to the matrix.This polymer is known as high-impact polystyrene(HIPS). The biggest applications for HIPS are intelevision and computer cabinets, followed by smallappliances.3 Flame retardancy is required in mostof these products. Apart of flame retardancy, HIPSshould be light stabilized and chemically resistant inthese applications.

The name ABS polymer is derived from the initialletters of the three main monomers, acrylonitrile,

butadiene and styrene, used in its preparation;however, ABS is not a terpolymer. Industriallyimportant ABS resins are two-phase polymer systems,where the modifying rubber (usually polybutadiene) isdispersed as a discrete phase in the continuous phase ofa styrene–acrylonitrile copolymer (SAN).4 Thus, theABS polymer is a rubber-modified SAN copolymer,just as HIPS is rubber-modified polystyrene. Thefraction of rubber component in ABS is usuallygreater than in HIPS. Good impact is achieved ifthere is a good adhesion between the matrix andthe rubber. Similar to HIPS, grafting of acrylonitrileand styrene on the rubber phase helps to achievethis adhesion. ABS occupies an intermediate positionbetween commodity plastics and engineering plastics.5

ABS is a good insulator and its electrical propertiesdo not change much with change of temperatureand humidity. The largest application for flame-retardant ABS is molded housings for small kitchenappliances, tools and vacuum cleaners. In automotiveapplications, ABS is used in interior and exteriorparts. In the electrical and electronic markets, flame-retardant grades of ABS are used for telephones andcomputer housings.

Polystyrene foam is the most widely producedexpanded polymeric material after polyurethane foam.Its importance has been increased even further bycontemporary energy saving requirements as it is oneof the most important thermal insulators. Becauseof extensive use as a construction material,6 flame-retardant grades constitute a large fraction of the worldproduction of polystyrene foam. In some countries,

∗ Correspondence to: Sergei V Levchik, Supresta US LLC, 430 Saw Mill River Rd, Ardsley, NY 10502, USAE-mail: sergei.levchik@supresta.com(Received 4 September 2006; revised version received 15 January 2007; accepted 23 January 2007)Published online 13 December 2007; DOI: 10.1002/pi.2282

2007 Society of Chemical Industry. Polym Int 0959–8103/2007/$30.00

SV Levchik, ED Weil

laws require that flame-retardant polystyrene be usedin all construction applications.

Most of the problems related to flame retardancyof styrenic polymers were considered solved or almostsolved in the 1960s and 1970s with the broad intro-duction of brominated flame retardants. However,in the 1990s it was discovered that some of theseproducts may adversely affect the environment and,possibly, human health. Although risk assessments onsome flame retardants used in styrenic polymers arenot completed yet, active research and developmentof new, potentially safer products is in progress. Sincethe last comprehensive review of the subject was pub-lished in 1973,7 we believe it is timely to review newdevelopments in flame retardancy of styrenic poly-mers.

This review is primary focused on commerciallyimportant thermoplastic polymers, on some blendsbased on polystyrene as well as on polystyrene foams.In this review we will consider only polymers andblends where the styrenic part composes the majorityof the material. For example, some elastomers andunsaturated polyesters where styrene constitutes only20–30% are outside the scope of this review. Wehave already recently reviewed the flame retardancyof polycarbonate (PC)/ABS blends where ABS is aminor component.8,9 We believe that poly(phenyleneether) (PPE)/HIPS should be also reviewed separatelybecause of the major influence of the PPE componenton the flammability of the blend. The literature ofthe last ten years is mostly cited; however, if an olderpublication provides fundamentals for understandingrecent developments, it is also discussed. This reviewwill be helpful to compounders of styrenic polymersand it is also intended to be of assistance to researchand development personnel by providing an overviewof the state-of-the-art and perhaps by suggesting newapproaches.

HALOGENATED FLAME RETARDANTSHIPS and ‘crystal’ polystyreneHalogen-based flame retardants are the most widelyused for styrenic polymers and particularly forHIPS.10 The decomposition products of halogenatedflame retardants can initiate and strongly promotepolystyrene degradation.11 Fast decomposition con-tributes to dripping, especially when a low-meltingflame retardant is employed that also acts as an effec-tive internal viscosity reducer. This phenomenon moreoften is observed with aliphatic halogenated flameretardants and it can be an economical way to attainV-2 rating in the UL-94 test using a relatively lowconcentration of flame retardant. There is sometimesan advantage of using aliphatic halogenated flameretardants in HIPS because of better UV stability.V-2 HIPS formulations with aliphatic halogenatedcompounds may show UV stability similar to non-flame-retardant HIPS.

Some grades of chlorinated alkanes have suffi-cient thermal stability for use with styrenic polymers,but processing temperatures should be limited to220 ◦C.10 1,2,5,6,9,10-Hexabromocyclododecane (1)can be applied to HIPS and it provides a V-2 UL-94 or glow wire 960 ◦C rating at 1.6 mm at a dosageof only 3.3%.12 However, the processing stability ofthe polymer may decrease, leading to deteriorationof the mechanical properties and dark appearance.11

This happens because of release of HBr and strongdiscoloration of (1) upon thermal decomposition.Effective stabilizing systems can significantly retardthese processes.13–15 The stabilization additives areusually halogen acid acceptors, such as dialkyltin saltsof carboxylic acids, the oxides and hydroxides of somemetals or hydrotalcite,11 zeolites,16 etc. Combinationsof 1 with brominated epoxy oligomers were also founduseful in UL-94 V-2 formulations.17 Other aliphaticcommercial flame retardants, like tetrabromocyclooc-tane or dibromoethyldibromocyclohexane, can also beused in HIPS.18,19

Br Br

Br Br Br Br

(1)

CH2Br CHBr CH2 O C

CH3

CH3

O CH2 CHBr CH2Br

Br

Br

Br

Br

(2)

The combination of aromatic (34%) and aliphatic(34%) bromine in bis(2,3-dibromopropyl ether) oftetrabromobisphenol A (2) helps to reach high flame-retardant efficiency.13 It is recommended for use inHIPS and ABS when class UL 94 V-2 is required. Thetemperature processing window of 2 is wide from 200to above 280 ◦C because of low melting temperature(113–117 ◦C), but relatively high thermal stability ofthe additive. Typically, 5 wt% of 2 combined with 1wt% Sb2O3 results in a V-2 UL-94 rating or glow wire960 ◦C rating at 1.6 mm.12

Tris(tribromoneopentyl) phosphate (3) combinesbromine (70%) and phosphorus (3%) in the samemolecule. This molecule shows excellent fire-retardantefficiency with excellent UV and visible lightstability.13 In HIPS and ABS V-2 formulations, 3is applied on exposed sections, which require light sta-bility, brightness and good initial colors. Compound 3melts at 181 ◦C and therefore it is easy to compound.Because of high thermal stability, the processing win-dow is wide between 200 and 260 ◦C. In combinationwith 1 wt% hindered amine as a synergist, 14 wt% 3 inHIPS ensures passing of UL 1694 for small electricalparts.20

Although early evolution of hydrogen halide isimportant for high flame-retardant efficiency, the

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flame retardant needs to be sufficiently thermallystable to be melt compounded with polystyrene.Aromatic bromine compounds are the most widelyused and about 10 wt% of bromine is requiredto pass the UL 94 V-0 rating. Antimony trioxideis usually used as a synergist with brominatedcompounds. Because many aromatic flame retardantcompounds melt at a temperature higher than themelting temperature of HIPS, this affects the physicalproperties of the compounded plastic.21 High-meltingadditives can phase-separate and act as inert fillersin both the rubber and polystyrene phases, whichdecreases impact strength but may result in higherheat distortion temperature and higher modulus. Thelow-melting additives can raise the glass transitiontemperature (Tg) of the rubber phase and plasticizethe polystyrene phase. Such plasticizing action leads toa lower heat distortion temperature and modulus. Theadditives that are compatible with, and localized inthe polystyrene phase help retain the impact strength.Soluble flame retardants may cause blooming, wherethe flame retardant migrates and deposits on thesurface of molded parts.10

P

O

OH2CC

CH2Br

CH2Br

BrH2C

O

CH2

CBrH2C

CH2Br

CH2Br

O CH2 C

CH2Br

CH2Br

CH2Br

(3)

Br

Br

Br Br

Br

O

Br Br

Br

BrBr

(4)

Decabromodiphenyl oxide (Deca) (4) is the mostwidely used flame retardant in HIPS, because of itslow cost and relatively high bromine content (83wt%).10 Solubilization of Deca in HIPS increaseswith longer processing time and higher processingtemperature.22 The correlation was derived betweenthe degree of solubilization and physical properties ofHIPS. Usually about 75 to 90% of Deca dissolves inthe polystyrene phase which results in decrease of Tg

of 12 ◦C.21 The portion soluble in rubber phase leadsto an increase of the glass transition temperature ofthe polybutadiene.23 Antimony oxide antiplasticizesthe grafted rubber phase but act as inert filler in thepolystyrene phase.21 Interestingly, addition of somehigh-density polyethylene helps prevent dripping whenDeca + Sb2O3 are used in HIPS.24

Some brominated flame retardants show relativelypoor photo-oxidative stability and Deca is one them.The strength of Ar-Br bonds lies in the range

293–326 kJ mol−1 and they can be broken by theenergy of light in the wavelength range 370–410 nm.11

This means that bromine can be dissociated not onlyby direct sunlight but also by scattered daylight oreven under indoor exposure to fluorescent lighting.Under certain conditions, antimony trioxide can actas a strong photosensitizer. Silica can also acceleratephotodegradation of Deca.25

Antos and Sedlar26 found that photochemicaldecomposition of Deca is a radical process leadingto the formation of HBr, where hydrogen is abstractedfrom the polymer. The stepwise debromination ofDeca leads to lower brominated diphenyl ethers witha number of byproducts. The photodecompositionof Deca cannot be retarded by the presence ofUV stabilizers such as sterically hindered aminelight stabilizers (HALS). In contrast, HALS canpromote a deeper stepwise debromination of Decaallowing release of more than one bromine radicalper molecule of Deca. Furthermore, HBr ruinsthe stabilizer performance through the formation ofinactive ammonium salt with HALS. Therefore, theuse of this type of antioxidant alone, even at a veryhigh concentration, provides insufficient protection. Ablack pigment (carbon black) is more effective and alsomasks discoloration resulting from aging. Where blackpigment is unacceptable, a suitable concentration ofUV absorber must be used.

There are conflicting reports in the literatureregarding the fate of Deca in HIPS during servicelife. For example, spectral and chemical analyseshave demonstrated11 that the bromine content in thesurface layer of HIPS exposed to photo-oxidationdecreases. A similar conclusion was reached inanother study27 where Deca/Sb2O3-containing HIPSwas exposed to high-energy UV irradiation and thenDeca was extracted from the surface layer. Onlyafter 76 h exposure was about 30% of the originalDeca decomposed. One may assume that in theextreme case it will result in loss of flame resistance.But in reality, the flame retardancy actually showssome improvement over time.11 Weathering affectsDeca through more complicated processes such asphotohydroxylation reactions leading to changes indeeper layers. However, even in these cases the flame-retardant performance of HIPS was only slightlyaffected. At the time of submitting this review forpublication use of Deca was limited in Europe due toits content of lower brominated impurities.

Because in some geographical regions like Europeand the Far East there is a special concern ofpreventing brominated flame retardants from gettinginto the environment, significant efforts have been putinto the study of aging and potential recyclability offlame-retarded HIPS. For example, HIPS containingDeca has been subjected to an accelerated thermo-oxidative aging corresponding to a normal indoorservice time of about ten years and no significantloss of Deca has been detected.28 It was concludedthat Deca-containing HIPS is suitable for recycling

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because no significant changes in physical propertieswere found. On the other hand, a recent study29

showed that detectable levels of toxic polybrominateddibenzofurans and dibenzodioxins derived from Decawere found near extruders in the course of recyclingof Deca-containing HIPS. It was noticed thatSb2O3, Fe2O3 or water can catalyze formation ofdibenzofurans and dibenzodioxins.

Ethylenebis(pentabromobenzene) (5) was devel-oped as an alternative to Deca, which does not producebrominated dibenzofurans and dibenzodioxins uponheating.10 Compound 5 has a high bromine content,82%, and it melts at 380 ◦C.30 Because of such a highmelting point it behaves as an insoluble filler in HIPS.Similar to Deca, about 12 wt% 5 with 4 wt% Sb2O3

synergist typically gives a UL-94 V-0 rating at 1.6 mmsample thickness.12 Compound 5 shows lower impactstrength, but better thermal stability and better UVstability than Deca. Dolomite, hydrotalcite or zeo-lite provide further improved thermal stability, whichhelps in the recycling of items containing 5.31 In con-trast to Deca which is mostly used in black-coloredplastics, 5 can be used in light gray business electronichousings. This additive is very efficient in syndiotacticpolystyrene, e.g. only 6 wt% 5 with 1 wt% Sb2O3 isneeded for a V-0 rating in the glass-filled polymer.32

Although brominated polystyrene usually is not used inHIPS, it can be applied in combination with 5, and thishelps with melt flow properties during processing.33

Combinations of 5 with 2 and some Sb2O3 are veryeffective at low levels (such as 6 wt% total) in providingV-2 ratings to HIPS.34 It was recently discovered that5, as well as some other brominated flame retardants,can give V-0 ratings in HIPS without Sb2O3, but withvery small amounts of free radical initiator and iron orcobalt complex instead.35

CH2 CH2Br

Br

Br

Br

Br

Br Br

Br

BrBr

(5)

N CH2 CH2 N

Br

Br

Br

Br

Br

Br

Br

Br

O

O

O

O

(6)

Another even higher melting brominated productis ethylene bis(tetrabromophthalimide), 6 (Br content67 wt%, melts at 445 ◦C).12,30 The low solubility of6 in HIPS probably relates to the high polarity ofthe phthalimide, which makes it incompatible withpolystyrene.23 Because of much better UV stability

than Deca and 5, this product is used in applicationswhere color stability is critical. Both 5 and 6 havegood recyclability in HIPS because of good thermalstability. These two additives are also used for thelower UL-94 grades, such as V-2 ratings (8 wt% of theflame retardant combined with 3 wt% Sb2O3) or theglow wire flammability test.

Polybrominated trimethylphenyl indan (7) is solublein HIPS, but does not lower the heat resistance becausethe softening temperature of the additive is close tothat of polystyrene.10 Compound 7 was designed asa material not related to Deca and not producingbrominated dioxins and furans. This was confirmed bythe analysis of the volatile products of decompositionof this flame retardant, pure or compounded inHIPS.36 Compound 7 was also recommended foruse in ‘crystal’ polystyrene, because transparencyof the polymer is not compromised.37 The degreeof commercial development of this material is notclear. Other decabromodiphenyl compounds whichhave linkages different from Deca are also underdevelopment, e.g. 8.38

CH3

H3C CH3

Brn

Brn

(7)

CH2 X CH2Br

Br

Br

Br

Br

Br Br

Br

Br Br

X = O or S

(8)

Tris(tribromophenyl) cyanurate (10) with 67 wt%bromine content is a relatively new flame retardantdeveloped for HIPS. It is soluble in the polymer,but does not much affect the thermal properties.Compound 10 provides a V-0 rating at 15 wt% incombination with 3 wt% Sb2O3.39 This is anotherproduct with relatively good UV stability. It issynergistic with products having 2,3-dibromopropylgroups, e.g. 2 or tris(2,3-dibromopropyl)isocyanurate(9) (available in Japan) and it can be used to providea V-2 rating in HIPS.40

A series of pentabromobenzyl alkyl ethers41 (11),or pentabromobenzyl alkyl esters42 (12), was recentlysuggested as effective flame retardants for HIPS, whichprovide V-0 rating at 15–17 wt% with 5–6 wt%Sb2O3. High-purity pentabromobenzyl bromide (13)has been developed for ‘crystal’ polystyrene, where itprovides V-2 rating at only 2 wt% loading.43

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Flame retardancy of styrene thermoplastics and foams

N

N

N

O OH2C

HCH2C

CH2CH

CH2

O

CH2

CH

CH2Br

Br

BrBr

BrBr

(9)

N

NN

O

Br Br

Br

OO

Br

BrBr

Br

BrBr

(10)

CH2 O

BrBr

Br

Br Br

R

(11)

CH2 O

BrBr

Br

Br Br

C

O

R

(12)

CH2Br

BrBr

Br

Br Br

(13)

Mahdavian et al.44 studied the flame-retardantaction of small (synergistic) amounts of aluminatrihydrate (ATH) in combination with tetrabromo-bisphenol A (14). Although ATH was not as effectiveas Sb2O3 (increase of oxygen index (OI) by 20) itstill shows some positive effect by increasing OI by 8in the presence of 4.5% ATH. It was suggested thatATH can provide gas-phase flame-retardant action byreacting with HBr and forming AlBr3 which volatilizessimilar to SbBr3 to the flame. This mechanism seemshighly speculative. More likely is that Al2O3, a knownLewis acid catalyst, formed during decomposition ofATH catalyzes debromination of 14.

Brominated epoxy oligomers provide V-0 rat-ing at 15 wt% loading with combination of5 wt% Sb2O3.45 The benefit of Br/Cl syn-ergism to flame retard HIPS was shown ona mixture of brominated epoxy oligomers and

C

CH3

CH3

HO OH

Br Br

BrBr

(14)

CCl2CCl2

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

(15)

bis(hexachlorocyclopentadieno)cyclooctane (15).46

The highest oxygen index and V-0 rating was obtainedusing 12 wt% of 15 and 7.66 wt% brominated epoxywithout Sb2O3. If a 1:1 mixture of chlorinated andbrominated flame retardants is used (5.85% of eachhalogen) the most effective level of Sb2O3 seems tobe 4% for low-molecular-weight epoxy and 3% forhigh-molecular-weight epoxy, in order to reach UL-94 V-0. The addition of 1% of an organic siliconematerial decreases the afterglow and helps to reachV-0 at 1.6 mm thickness.

In view of the existence of well-developed tech-nology for making ring-substituted bromostyrenemonomers, it would seem attractive to copolymerizesuch bromostyrenes with non-brominated styrene inorder to produce inherently flame-retardant polymers.However, this approach has not found any commercialuse. Examples of halogenated monomers publishedin the literature include chlorinated or brominatedstyrene, m-(2,2-dibromocyclopropyl) styrene or m-(2,2-dichlorocyclopropyl) styrene.47 More complexsystems involve the use of non-styrene comonomerssuch as bromoethyl acrylate, (β-(methacryloxy)ethyldiphenyl thiophosphinate, pentachloroethyl allyl ure-thane and dibromopropyl methacrylate. Significantattention in the early developments was given to thedirect bromination of polystyrene in an aqueous sus-pension of the polymer or reaction of polystyrene withchlorine in a solvent. None of these approaches weresuccessful because of side reaction of halogenationof the polymer backbone resulting in loss of thermalstability.

The efficiency of chlorinated, brominated andiodinated polystyrenes as flame-retardant additiveswas compared in ‘crystal’ polystyrene.48 It wasfound that brominated and iodinated polystyrenes arealmost identical either alone or in combination withSb2O3. On the other hand chlorinated polystyreneat the same weight loading level was less efficientthan the bromine or iodine analogs and showedvery little synergism with Sb2O3. Polystyrenes withdifferent degree of chlorination were prepared byAlekseev et al.49 Significant decrease of flammabilitywas observed which was attributed to the dilution ofcombustible products with non-flammable hydrogenchloride. It was also found that chlorination increasesconsiderably the compression strength, which wasattributed to the partial crosslinking of the polymerduring processing.

The mode of flame-retardant action in the gasphase (flame zone) was studied by Petrella50 usingmixture of monomeric styrene with hydrogen halidesor small halogenated molecules representing the fuel

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composition of the flame-retarded polystyrene. Hefound that HBr mostly provides a chemical mechanismof action by strongly affecting the initiation steps andto a lesser degree the propagation steps of combustion,whereas HCl was mostly functioning as an inert diluentto the styrene pyrolysate–oxygen combustion mixturesuppressing only propagation steps.

Costa and Camino51,52 studied the flame-retardanteffect of chlorinated paraffins (CP) in ‘crystal’polystyrene by comparing the oxygen and nitrousoxide indices. Since CP increased the nitrous oxideindex more efficiently than the OI, the primaryaction was attributed to the condensed phase effect.It has been shown that, in the presence of CP,depolymerization of polystyrene starts earlier, but ingeneral thermal decomposition proceeds more slowly.The early depolymerization results in melt flow andremoval of the heat from the combustion zone. Onthe other hand, slow thermal decomposition due tointeraction of polystyrene with the charred residuefrom dehydrochlorination of CP results in a lowersupply of flammable products.

ABSSome flame retardants used in HIPS can to acertain extent be applied in ABS. Uhlmann et al.53

systematically studied mechanical and flammabilityperformance of more than 20 different flame retardantsin ABS and found that chlorinated flame retardantsare less effective than brominated materials. Themore thermally stable flame retardants, such as 4,5 and 6, appear to be more efficient than the solubleflame retardants reported. The DTUL and Vicat arehighest with this group of flame retardants. Flameretardants that are soluble, even if they have a meltingpoint above the compounding temperature, produceDTULs and Vicats that are lower. On the other hand,the Izod and Gardner impact strengths are significantlygreater with flame retardants that are soluble andmelt blendable than with those that are insoluble.Most soluble flame retardants bloom and plate out,with the exception of polymeric materials such asbrominated polystyrene and the brominated epoxyoligomers. Large insoluble flame-retardant moleculessuch as 5 and 6 do not plate out or bloom. Because ofall these considerations, preference is generally givento oligomers with molecular weights below 1200 gmol−1 which are soluble in ABS and produce goodimpact properties.

Octabromodiphenyl oxide (Octa) is soluble in ABSflame retardant and it was one of the preferredflame retardants in ABS for over a decade. However,recently it was removed from the market14 becauseof environmental concerns. After removal of Octa,tetrabromobisphenol A (14) is the most widelyused flame retardant for ABS, because of itsrelatively low cost. Discoloration due to thermalinstability can be a problem if the processingtemperature is higher than 240 ◦C. Compound 14is not recommended when high impact and/or high

thermal properties are needed. Compatibilization withgrafted styrene–maleic anhydride copolymer helps topreserve good physical properties and provides betterdispersion of Sb2O3 which is reflected in a higherOI.54 Although metal-containing coupling agents alsohelp with dispersion, OI actually decreases,55 probablybecause of interference with evolution of HBr.

Brominated epoxy oligomers based on tetrabromo-bisphenol A with molecular weights below 1200 gmol−1 have bromine content of about 50–60%, soft-ening temperature of about 110 ◦C and they arethermally stable up to 300 ◦C.56 These oligomers pro-vide acceptable impact strength, heat distortion andUV stability.36,57 A very important additional func-tion of epoxy oligomers in ABS is to provide betterflow properties during processing.36 Tribromophenol-end-capped brominated epoxies (16) were speciallydesigned to overcome metal adhesion problems dur-ing lengthy injection molding operations, which areobserved with regular grade epoxy oligomers.58 Theyhave advantages in thermal stability over regular epoxyoligomers. Flame retardant manufacturers have madefurther proprietary modifications of 16 for improve-ment in Gardner impact properties.

O CH2 CH

OH

CH2 OBr

Br Br

BrBr

(16)

CH3

CH3

O CH2 CH

OH

CH2 O

Br

Br

Br

Br

Brn

C

Bis(tribromophenoxy) ethane (17) is a soluble flameretardant used mostly in ABS, but not in HIPS. Itrequires Sb2O3 as a synergist, but zinc stannate or zinchydrostannate could be used as well,59 which helps toreduce smoke. In order to prevent undesirable drips,a small amount (<0.5 wt%) of polytetrafluoroethylene(PTFE) is added to the compound. Apart fromthe well-known physical effect which PTFE plays inholding molten polymer, some chemical interactionwith Sb2O3 was detected, which may also contributeto flame retardancy.60 Apparently, some chlorinatedparaffins provide a synergistic effect to 17.59 Similarsynergism was observed between 17 and 15.46 Theoptimum ratio of chlorine and bromine was found tobe 1:1 on a molar basis.

Poly(pentabromobenzyl acrylate) (18) is a polymericflame retardant suitable for ABS. Polymerization ofpentabromobenzyl acrylate can be performed in thepresence of an inorganic filler, e.g. CaCO3, so theadditive is deposited on the surface of the inorganicparticles.61 This approach helps to produce mineral-filled ABS, where the inorganic filler does not interfere

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Br

Br

Br

O CH2 CH2 O

Br

Br

Br

(17)

CH2 CH

C

O

O

CH2

Br Br

BrBr

Br

(18)

with the reaction between the brominated flameretardant and Sb2O3. In contrast CaCO3 positivelycontributes to the flame retardancy by producingmore coherent char during combustion. Althoughbrominated polystyrene in general is not compatiblewith ABS it can be combined in a 1:1 proportion withDeca (4) or other flame retardants (2, 6, 14) whichresults in good physical properties.62

Tris(tribromophenyl) cyanurate (10) provides anattractive cost and optimal balance of propertiesin ABS: e.g. high melt flow during injectionmolding, light stability, good impact propertiesand high heat distortion temperature (HDT).57 Aseries of pentabromobenzyl alkyl ethers (11)41 orbis(pentabromobenzyl) ethers (8)38 developed forHIPS are also effective in ABS at 15 wt% incombination with 6.0 wt% Sb2O3.

The effect of different grades of antimony trioxide(average particle sizes of 0.1, 0.52 and 1.31 µm) eitheralone or with 17 or 14 was studied in ABS.63 Theexperiments showed that standard grades of 0.52 and1.31 µm had a detrimental effect on impact and flexuralproperties, whereas a new submicron particle sizeproduct (0.1 µm) had little effect on impact strengthand a slight detrimental effect on the flexural modulusand flexural strength. The addition of the 0.1 µm graderesulted in improvements in flame retardancy as well,compared to standard grades.

Luda et al.64 studied the mechanism of fire-retardant action of three brominated flame retardants,nonabromobiphenyl (obsolete in the 1990s), Octaand bis(tribromophenoxy) ethane (17). It was foundthat the more thermally stable nonabromobiphenyland Octa show mostly condensed phase action whenapplied without Sb2O3 synergist, whereas 17 showscondensed-phase action at low concentrations, whichchanges to gas-phase action at higher concentrations.The condensed-phase action was attributed to theinteraction of Br radicals with ABS which causescharring. Addition of Sb2O3 triggered the gas-phasemechanisms with all three additives, presumably

because SbBr3 poisons the flame, but the condensedphase contribution remained significant as well.

Polystyrene foamsBecause the primary application of flame-retardantpolystyrene foams is in building insulation, the ASTME84-04 Steiner Tunnel test65 is used to assess theflammability of the foam. The concentrations of flameretardants used are about one order lower than formolded parts.

There are two main processes of manufacturingpolystyrene foam.6 The first is a one-step process basedon the extrusion of a polystyrene mixture with a low-boiling blowing agent that expands at the processingtemperatures in the melt. The cellular structure isfixed by fast cooling of the material. This extrusionprocess is mostly practiced in the USA and is veryimportant for the manufacture of polystyrene sheetand film (extruded polystyrene foam, XPS). In thisprocess flame retardant is fed in the extrusion processsimilar to thermoplastics, but the heat exposure is veryshort.

An alternative two-step process was developed inEurope. In this process, the first step consists of prepa-ration of particles (beads) containing a homogeneouslydispersed blowing agent by suspension polymerizationof styrene. The beds are then expanded in the molds atthe second stage (expanded polystyrene foam, EPS).In this process the flame retardant is incorporated intothe reaction mixture during polymerization.11 Flameretardants can also be applied to the surface of thebeads after completion of polymerization. This proce-dure does not ensure homogeneous dispersion of theflame retardant throughout the polymer and thereforenot broadly practiced.

The amount of flame retardant needed dependson the efficiency and varies from about 0.5 to 1%.11

Almost all commercially produced flame retardantsdecrease the Tg value of polystyrene which is reflectedin a decrease in heat resistance of the foam. Thiseffect is sometimes compensated by the addition ofcrosslinking agents, such as divinylbenzene. Someflame retardants tend to decrease the molecular weightof polystyrene.

Because expanded polystyrene foam is processed ata lower temperature, aliphatic bromine compoundscan be used for this application.10 Pentabromophenyl2,3-dibromopropyl ether (19), bis(2,3-dibromopropylether) of tetrabromobisphenol A (2) and bis(2,3-dibromopropyl) tetrabromophthalate (20)66 foundsome utility in expanded polystyrene foam. However,hexabromocyclododecane (1) is the most widely usedflame retardant for expanded polystyrene foam.

A blend of hexabromocyclododecane (1) andbis(2,3-dibromopropyl ether) of tetrabromobisphenolA (2) in combination with peroxide was recently foundsynergistic in polystyrene foams.67 Combinations of1 with other aromatic high-melting flame retardantslike Deca (4)68 or ethylene bis(pentabromobenzene)(5)69 were found helpful in passing Japanese JIS

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Br

Br

Br Br

Br

O CH2 CH CH2

(19)

Br

Br

Br

Br

C

O

O CH2 CH CH2

Br Br

C

O

O CH2 CH CH2

Br Br

(20)

9511 or German DIN 4102 tests for buildingmaterials. Non-halogenated materials like triphenylphosphate68,70,71 or isocyanuric acid68 are also syn-ergistic with hexabromocyclododecane (1). Bromi-nated terpenes with a bromine content of 70wt% were developed as a substitute for 1.72

Recently, dibromohexahydrophthalimides,73 e.g. N-2-3-dibromopropyl-4,5-dibromohexahydrophthalimide(21),74,75 were found to have the right solubility inpolystyrene (about 7–8 wt% at 25 ◦C) and togetherwith their high bromine content are comparable inefficiency with 1.

N

Br

Br

O

O

CH2 CH

Br

CH2

Br

(21)

Antimony trioxide is not necessary for polystyrenefoam and it cannot be used because of interfer-ence with the foaming process. Instead, synergisticagents such as stable peroxides (e.g. dicumyl per-oxide, di-tert-butyl peroxide) or azo compounds areused.11 The ratio of flame retardant to synergisticagent is about 2:1. Many other free radical-generatingcompounds have been disclosed as synergists that canbe used in place of peroxides to enhance the self-extinguishing action of bromine compounds.76 Exam-ples of some of these materials are the disulfides, sulfe-namides, N-nitroso-N-methylaniline, tetraphenylhy-drazine, pentaphenylphosphorane α, α′-diphenyl-α-methoxybibenzyl, 2,3-dimethyl-2,3-diphenylbutane(believed to be used commercially), polymers of diiso-propylbenzene, tetraethyllead and heavy-metal saltsor chelates. Recently highly thermally stable hinderedamines were recommended to the same purpose.77

It is believed11 that the main mechanism offlame retardancy in polystyrene foams is no longertermination of the radical reaction in the gas phase,but primarily depolymerization of polystyrene byradicals formed in the course of flame retardantdecomposition. The depolymerization is accompanied

by a decrease in the polymer viscosity so that thepolymer melts and withdraws from the flame front.

PHOSPHORUS-BASED FLAME RETARDANTSPhosphorus-containing copolymersCamino and co-workers78–80 studied combustion ofpolystyrenes end-capped with methoxychlorophos-phonate groups (22). These polymers were made byquenching of living polymerization of styrene withPOCl3 and then reacting with methanol. These chainends helped increase OI of the polymer by 50%.Comparison of OI and nitrous oxide index (NOI)trends pointed to the condensed phase mechanism ofaction.78 Further investigation showed that this strongflame-retardant effect is related to the changes in themechanism of thermal decomposition.79,80 A conden-sation reaction with formation of –P–O–P–bondsbetween chain ends with elimination of CH3Cl wasobserved at relatively low temperature (ca 200 ◦C)prior to extensive degradation of the polymericchains (280–430 ◦C).79 The condensation led to anincrease in the molecular weight. Although the mainstage of thermal depolymerization of phosphorylatedpolystyrene occurred through the same mechanismas regular polystyrene, the rate of volatilization wassignificantly slower.80 It was believed that slow decom-position was responsible for the flame-retardant effect.Interestingly, phosphorus end-capping of poly(α-methylstyrene) did not cause any improvement inflame retardancy.81

It was found82 that 2,4,4,5,5-pentaphenyl-1,3,2-dioxapholane (23) thermally opens the ring at aC–C bond and produces a biradical, which wasused to initiate polymerization of polystyrene. Thus,phosphorus was incorporated in the polymer chain,and even at very low concentration suppresseddripping of polystyrene.

CH2 CH P

O

Cl

OCH3

(22)

OP

O

(23)

Ebdon et al.83 copolymerized styrene with a varietyof comonomers containing covalently bound phospho-rus, including vinylphosphonic acid, several dialkylvinylphosphonates and various vinyl and allyl phos-phine oxides. The flame retardance of these copoly-mers was assessed by measuring char yields and OIs.All phosphorus-containing copolymers produce charon burning (and also on heating in air or nitrogen) and

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Flame retardancy of styrene thermoplastics and foams

have OIs higher than polystyrene. However, despitegeneral correlations between OI, char yield and phos-phorus content, some copolymers have higher thanexpected OI and/or char yield, whilst others havelower, indicating that the phosphorus environment isimportant. For example, the raising of OI to 27.4was especially pronounced in the case of pyrocatecholvinylphosphonate (24), yet char yield was modest (5.4wt%). On the other hand, char production seemed tobe at a maximum for diethyl p-vinylbenzylphosphonate(26) (15.0 wt%), yet OI at 24.0 was not high.

OP

O

CHCH2 O

(24)

CH2 CH

CH2

PC2H5O OC2H5

O

(26)

In other studies84,85 diethyl alkylphosphonate-substituted acrylates were copolymerized with styreneand these copolymers were compared with tri-ethyl phosphate, diethyl ethylphosphonate or tri-n-butylphosphine oxide added to polystyrene. At the3.5 wt% phosphorus content chosen for comparison,only a slight increase of OI from about 18 to about21 was observed. On the other hand, cone calorime-try experiments gave much clearer evidence showingadvantage of having phosphorus in the system, butsimilar to OI measurements there was little differ-ence between the additive and reactive approaches.The main advantage of the reactive approach was themaintenance of the physical and chemical propertiessimilar to those of the homopolymer.

A series of copolymers of the diethyl phosphorylatedderivative of N-(p-hydroxyphenyl)maleimide (27, 28and 30) with styrene were studied by thermogravimetryand OI.86 Although the char yield increased withphosphorus content, the copolymers surprisinglyshowed a maximum of char yield which was not at themaximum concentration of phosphorus. Copolymers27 and 28 had maximum char yields of 50 and 55%respectively at 90 mol% of phosphorus units, whereascopolymer 30 had maximum only at 33 mol% ofphosphorus units. OI data followed the char yieldtrend and it was speculated that there is synergismbetween styrene and the phosphorylated monomers.

Copolymers with cyclophosphazene pendant groups(31) were prepared by reacting azidocyclophosp-hazenes with diphenylstyrylphosphine groups incor-porated in the copolymer structures.87 A significantincrease in char yield was observed and it was believedthat the fire resistance of polystyrene is improveddue to a condensed phase mechanism. The ary-loxyphosphazene was more effective than the triflu-oroethoxyphosphazene in this system. Phosphazenes

N

CH2 CH

OO

O

P

O

C2H5O OC2H5

(27)

N

CH2 CH

OO

O

P

O

O O

(28)

N

CH2 CH

OO

O

P

O

C2H5O OC2H5

Br Br

(30)

R =

R = CH2CF3

CH

P

N

PN

PN

P

N

OR

RO

OR OR

OR

CH2 CH CH2

(31)

not included in the polymer structure but used asadditives were less effective.

In the recent patent literature there are reportedattempts of copolymerization of phosphorus-contain-ing monomers with polystyrene. For example, themonomer made by reacting bisphenol A diglycidylether with 9,10-dihydro-9-oxa-10-phosphaphenan-threne-10-oxide and methacrylic acid (32) was copoly-merized with styrene and this resulted in a V-0 ratedcopolymer.88 Another recently reported89 monomeris the product of reaction of cyclic neopentylglycolhydroxymethylphosphonate with fumaric acid (33).

V-2 or V-1 rated styrenics through an additiveapproachA single phosphorus-based additive or combinationsof phosphorus with traditional nitrogen-based syner-gists usually result in a V-2 or V-1 rating in the UL-94test and some improvement in the OI. For example,about 8–10 wt% resorcinol bis(di-2,6-dimethylphenylphosphate) (35) is required for a V-2 rating inABS.90,91 Compound 35 is a commercial flame retar-dant manufactured in Japan and it is believed being

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CH2 C

CH3

C

O

O CH2 CH

OH

CH2 O C

CH3

CH3

O CH2 CH

OH

CH2 O P

O

O

(32)

CHC

O

O

CH2P

O

O O

H3C CH3

CH C

O

O

CH2P

O

O O

CH3H3C

(33)

used in ABS V-2 formulations for the Japanesemarket. Some other bisphosphates, including com-mercially available resorcinol bis(diphenyl phosphate)(36) and bisphenol A bis(diphenyl phosphate)91,92

(37) or biphenyl bis(diphenyl phosphate),93 or evenaryl monophosphates92 perform similarly or better interms of flame retardancy, but thermal properties suf-fer. Use of novolac in the formulations helps withthermal properties and provides high OI values up to28–31.94

O P

OO

O

H3C

H3C

CH3H3C

O

CH3

CH3H3C

CH3O

OO

P

(35)

O P

O

O

O O P

O

O

O

(36)

O P

O

O

O C

CH3

CH3

O P

O

O

O

(37)

V-2 formulations based on aromatic phosphates arealso known for HIPS. However, HIPS needs strongplasticizers like diarylalkyl phosphates95 which accel-erate dripping and extinguishing. Another approachwould be the use of bicyclic phosphates, e.g. spiro-bisphosphate made from pentaerythritol and phenol

(39), in combination with peroxide in order to pro-mote dripping.96 This spirobisphosphate as well assome other mono- and diphosphates were found syn-ergistic with elemental sulfur in ‘crystal’ polystyrene.97

Piperazine bis(di-2,6-dimethylphenyl phosphorami-date) (41) is not a plasticizing additive, but it reducesafterflaming time in the UL-94 test.98 Co-additionof some poly(phenylene oxide) (PPO; usually at thelevel of 10–20 wt%) to the formulations with aromaticphosphates helps improve thermal stability99 or evenpushes the UL-94 rating to V-1.100–106 Halogen-freeV-1 HIPS, presumably based on bisphosphate tech-nology, is being marketed in Europe107 and probablyin Asia.

P

O

O

OP

OO

O O

O

(39)

O P

O

O

N P

O

O

O

(41)

N

A commercially available cyclic alkylphosphonatemade from trimethylolpropane (42) is rich inphosphorus and thermally stable. This phosphonatealone or in combination with aromatic phosphatesprovides a V-2 rating in ABS108,109 or HIPS110

at relatively low loading (<5 wt%) which helpsto maintain good thermal properties. Addition ofsome PPO (<10 wt%) further improves thermalproperties of HIPS.111 Similar performance is given byanother commercial product, 2-methyl-2,5-dioxo-1-oxa-2-phospholane (44), which reportedly gives a V-2rating in ABS at only 1 wt% loading.112 Experimentalalkyl or aryl spirobisphosphonates, e.g. pentaerythritolbis(methylphosphonate) (45), give a V-2 rating inHIPS at 5 wt% loading.113 When this type of productis used in combination with aromatic bisphosphates athigher loading of 25 wt%, a V-0 rating is achievable inABS.114

P

O

O C2H5

CH2 O

O

CH3

(OCH3)x

H3C

O2-x

(42)

P

O O

CH3

O

(44)

P

O

O

O O

P

OO

H3C

CH3

(45)

P

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Flame retardancy of styrene thermoplastics and foams

Metal salts of alkylphosphonic or dialkylphosphinicacids were also found efficient in providing a V-2 ratingor increasing the OI of styrenic polymers. For example,the aluminium salt of methyl methylphosphonicacid (46) which was in commercial development inthe early 1990s gives a V-2 rating and increasesOI by 2–4 units in ABS and HIPS at 11 wt%loading.115,116 The commercially available aluminiumsalt of diethylphosphinic acid (48) allows one toachieve a V-1 rating in ABS when combined witha small amount (1 wt%) of certain minerals likeATH, hydrotalcite or zinc borate117 or with a nitrogen-containing synergist such as melamine cyanurate.118

Use of ammonium polyphosphate in combinationwith the alkylphosphinate salts results in a V-0 rating in ABS.119,120 The ammonium salt ofnitrilotris(methylphosphonamidic acid) (50) showeda V-1 rating in ABS at 15 wt% loading.121 Becausethis salt is water soluble it needs to be encapsulated.It is believed that this product is being introducedto the market in Europe.122 The commerciallyavailable ethylenediamine phosphate in combinationwith vermiculite was also reported to give a V-2 ratingin styrenic polymers.123

CH3

P

O

O OH3C

3

Al

(46)

Al

3

C2H5 P

O

C2H5

O

(48)

(50)

N

CH2

PH2N ONH4

O

CH2 P

O

NH2

ONH4CH2P

O

NH2

NH4O

Stabilized red phosphorus in combination withnitrogen-containing synergists, which are presumablymelamine salts, is recommended for HIPS for EIC60950 rated TV sets.124 Red phosphorus can providea V-0 rating in ABS; however, the presence ofanother resin, e.g. polyester125 or nylon,126 is required.These resins, although not particularly charrable,probably can provide enough char to flame-retardABS. The mechanism of fire-retardant action ofred phosphorus and its combinations either withMg(OH)2 or melamine polyphosphate was studied byBraun and Schartel.127,128 It was found that neither ofthese additives nor their combinations dramaticallychange the mechanism of thermal decompositionof HIPS, but they affect the composition of thevolatile products, shifting it towards higher oligomericfragments.

V-0 rated styrenicsIn order to push the UL-94 rating of ABS or HIPS toV-0 without use of halogens, a charring agent should

be added to the system. This principle is widely usedon a commercial scale in PC/ABS and PPO/HIPSblends, but in these blends the charrable polymer,PC or PPO, is present as the major component ofthe resin. The challenge with pure ABS or HIPS isusing as low an amount as possible of charring agent.General principles of optimization of the combinationsof one or few phosphorus-based products with one orfew charring agents with the use of regression analysishave been described in detail.129

Tris(2,6-dimethylphenyl) phosphate (51) combinedat 12 wt% with 20 wt% PPO and 5 wt% novolacprovides a V-0 rating in ABS,130 but about 10 wt%novolac is needed for HIPS.131 Di(p-hydroxy-o-tert-butylphenyl)phenyl phosphate (52) in combinationwith PPO helps significantly to increase OI in HIPS.132

Aromatic bisphosphates, which are more thermallystable (or less volatile) than monophosphates, canalso be used for V-0 in ABS or HIPS. For example,18 wt% resorcinol bis(2,6-dimethylphenyl phosphate)(35), 22 wt% PPO and 5 wt% novolac133,134 or 16wt% resorcinol bis(diphenyl phosphate), with 21 wt%PPO and 3 wt% novolac135 show a V-0 rating inHIPS. An even higher PPO content of up to 33 wt%is needed in syndiotactic polystyrene136 or ‘crystal’polystyrene.137 These examples show that a relativelyhigh total content of additives is required for a V-0rating. If a lower loading is used, only a V-1 rating isachievable.101,105

P

OO

O

O

(51)

HO P

O

O

OO OH

(52)

Because PPO is not compatible with ABS,other charring agents like PC138 or thermoplasticpolyurethane TPU139 or poly(ethylene terephthalate)(PET)140 were suggested instead. A spirobisphosphatemade from pentaerythritol and phenol (39) was foundspecifically advantageous in ABS.138,139 Use of organ-oclay helps with physical, most of all, and thermalproperties.141 In general, organoclays are useful co-additives to overcome the plasticization effect due to ahigh loading of phosphate.142–144

Lee et al.145,146 studied the effect of monophos-phates and bisphosphates combined either with epoxyresin or phenolic novolac in ABS and found thatthe combinations of any phosphate with the charringagent give a synergistic effect. Resorcinol bis(diphenylphosphate) (36) was more effective than triphenylphosphate. On the other hand, a triple combination ofbisphosphate + monophosphate + charring agent waseven more beneficial.129 Using thermal analysis andinfrared spectroscopy it was shown that phosphates

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react with OH groups present in the charring agentand thus involve it in more complete charring, whereasphosphorus is also retained in the condensed phase.

2-Methyl-2,5-dioxo-1-oxa-2-phospholane is a verypotent flame retardant, because it gives a V-0 ratingat only 5 wt% loading in combination with 20wt% PPO147 and its loading can be decreasedto only 1 wt% when combined with triphenylphosphate, but in this case 35 wt% PPO is needed.148

Balabanovich and co-workers studied the mechanismof flame-retardant action of 44 in combination withPPO,149 or novolac,150 or melamine150 or ammoniumpolyphosphate.151 It was shown that 44 does notincrease the charring of PPO or novolac, so its maincontribution seems to be the gas-phase activity. Incontrast, 44 reacts with melamine or ammoniumpolyphosphate and a good part of it remains in thecondensed phase. The practicality of using 44 in HIPSeven at such low loading is doubtful because of its veryhigh volatility.

Cyclic or linear polyphosphazenes are high-meltingsolids not soluble in HIPS or ABS and therefore theydo not cause plasticization of the polymer.97,152,153

Functionalized tricyclophenylphosphazenes, e.g. 53,which are crosslinked after blending with ABS, providea V-0 rating at 25 wt% without charring agent.

P

NP

N

PN

O

O

O

O

O O

O

O

CC CH2

O

O

CC CH2

OC

O

CCH2

CH3

CH3

CH3

(53)

INTUMESCENT SYSTEMSAlthough the crosslinking of the rubber phase of HIPSor ABS can be catalyzed at high temperature by strongacids, in general styrenic polymers are not sensitiveto small amounts of acids during processing. Thisgives freedom to develop an intumescent system forstyrenics, which may include expandable graphite. Forexample, it was discovered that expandable graphiteincorporated in a foamed polystyrene at 15 wt% helpspassing the German standard for building materialsDIN 4102 with a B2 rating.154 Co-addition oftriphenyl phosphate, red phosphorus155 and chalk156

helped further improvement of the flame-retardantefficiency and the physical properties of the foam.

Expandable graphite is efficient not only in foambut also in non-foamed plastics. For example 10 wt%of the graphite with 10 wt% of a brominated flameretardant provides V-0 rating in ABS or HIPS.157

If the halogen-containing additive is substituted bymelamine cyanurate, higher loadings are needed(15 + 15 wt%).158 If one of the aromatic bisphosphates15 or 16 is added, it requires also addition of about 10wt% of charring agent in order to prevent exudationof phosphate and improve the flame retardancy.159

Expandable graphite can also be combined with otherintumescent additives like ammonium polyphosphate;this combination is very effective. For example,expandable graphite, ammonium polyphosphate andred phosphorus give a V-0 rating in ABS at a totalloading of only 10 wt%.160 Interestingly, ammoniumpolyphosphate without expandable graphite gives aV-1 rating in ABS at 15 wt%.161

NANOCOMPOSITESRecently, significant research activity has been directedto development of nanocomposite polymeric materialswith enhanced flame-retardant properties. ‘Crystal’polystyrene, ABS and HIPS were found compatiblewith the formation of nanocomposites. The mostcommonly used nanofiller is organically modifiedmontmorillonite-type clay. Upon processing withorganoclay, polystyrene penetrates into the galleriesof the clay and forms intercalated material. The clayplatelets can be further exfoliated by shear forcein the extruder. The flame-retardant performanceof nanocomposites is usually assessed by conecalorimetry, where mass loss and heat release rate(HRR) along with the rate of smoke generation andrate of evolution of some gases are recorded.

Comparative studies of natural sodium montmo-rillonite clay and organically modified clays wereperformed with polystyrene,162 HIPS,163 SAN164 andABS.165 It was shown that the peak of HRR forpolymers containing non-exfoliated sodium montmo-rillonite was almost the same as that of pure polymers.In the presence of 3 wt% nanoclay (presumably exfoli-ated), the peak HRR decreased by 48% in polystyrenebut only by 30% in HIPS. In SAN, the decrease ofpeak HRR was 36% at 4.6 wt% of organoclay, whereasin ABS it was only 28% at 5 wt% organoclay. Appar-ently, the decrease of peak HRR is more pronouncedin the resins not containing a rubber phase. With anincrease of the mass fraction of organoclay, the reduc-tion of peak HRR usually improves. Interestingly, itwas found that ABS with intercalated (not exfoliated)nanoclay shows better HRR reduction. The decreaseof peak HRR caused by nanoclay in polystyrene wascomparable to the polymer containing a total of 30%Deca (4) with Sb2O3.162

In another study166 natural montmorillonite anda fluorinated synthetic mica were converted intoorganoclays. Incorporated in polystyrene, these claysbehaved similarly in the cone calorimeter study. In

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Flame retardancy of styrene thermoplastics and foams

contrast, polystyrene nanocomposites prepared withsynthetic magadiite organoclay did not show anydecrease in peak HRR.167 It was speculated thatthe presence of aluminium in montmorillonite hasan important role in flame retardancy of polymer–claynanocomposites.

In order to improve the thermal stability oforganoclay, crown ethers168 or methoxysilyls169 wereused as modifiers instead of traditional long-chainammonium quaternary salts. Surprisingly, it was foundthat the flame-retardant effect was independent of thedegree of intercalation (no exfoliation was obtained)and ranged from 25 to 30% decrease of peak HRRwith the crown ether-modified clays. Methoxysilyl-modified clay nanocomposites showed results similarto traditionally modified clays.

Kinetic analysis of thermal decomposition ofpolystyrene nanocomposites showed170,171 an increaseof the activation energy. Since clay increases theglass transition temperature, it was suggested that itlowers the molecular mobility of the polymer chains.Kinetic analysis also confirmed that barrier formationlimiting diffusion may be responsible for the enhancedthermal stability of nanocomposites.171 Because thenanocomposites do not show any increase in smokeand CO yield, it was suggested that flame retardancydoes not occur by processes in the gas phase but due tothe change in the condensed phase.162 A mechanismwas suggested by which clay nanocomposite functioninvolves the formation of a surface layer that serves as apotential barrier to both mass and energy transport.172

Because the polymer is trapped between the plateletsof the clay, its volatilization is delayed and thusthe non-char-forming polystyrene has been convertedinto a charrable system. Furthermore, clay catalyzeschar formation. Very few other flame retardants arecapable of causing polystyrene to give carbonaceouschar especially at this low a loading.

Polymer nanocomposites have not only the uniqueadvantage of reduced flammability, but also improvedmechanical properties.172 The presence of clay leadsto a ‘filler effect’ that increases the stiffness but maydecrease the tensile strength of the nanocomposites.164

The heat distortion temperature is another parameterwhich can be improved in the presence of nanoclay.

Typical nanocomposites, once exposed to the flamein the UL-94 test, ignite (often, even more rapidly thanwithout the nanoclay), but burn very slowly.166,173

Flame retardancy achieved with nanocomposites aloneis not enough for the ignition resistance accessed by theUL-94 test. Increasing the total loading of organoclayis a possibility, but the nanocomposite benefits are lostand mechanical properties begin to suffer. Therefore,it is impractical to use nanocomposites alone toimpart ignition resistance to polymeric materials.174 Abetter approach is to combine the nanocomposite withanother flame retardant, such that the nanocompositeprovides the base reduction in flammability, andthe secondary flame retardant provides the ignitionresistance.

A copolymer of dibromostyrene (DBS) and styrenethat contains 40 mol% DBS gives a V-2 rating, butupon the addition of 3 wt% nanoclay the ratingimproves to V-0.175 A V-0 rating was also obtainedfor only 10% DBS (about 6 wt% Br) copolymerwhen antimony trioxide was further added. In anotherstudy176,177 nanosize silica was used to improve theflammability performance of brominated polystyrene.For example, 35 wt% of brominated polystyreneinstead of 40 wt% was used in combination with10 wt% silica for V-0 rating. For comparison,polystyrene–silica nanocomposites showed only aslight increase in the OI, but not enough to be classifiedin the UL-94 test.

Phosphorus may be either added to the polymeras an additive or else it may be chemicallycombined with the clay.174 Modified clays have beenprepared using an ammonium salt which contains anoligomeric material consisting of vinylbenzyl chloride,styrene and a vinylphenyl phosphate reacting withdimethylhexadecylamine (55).178 When phosphateswere incorporated as additives two materials whichshowed the best performance were tricresyl phosphateand resorcinol bis(diphenyl phosphate) (36).174 Therewas substantial reduction in the peak HRR up to70 and even 80% with these systems which ismuch larger than is usually seen for polystyrene–claynanocomposites (48%162), and the total heat releasedwas reduced by 57%, while it is unchanged with theclay alone. Phosphorus also helped to increase thetime to ignition which is unusual for nanocomposites.These changes were more than additive, suggestingthat there is some synergy between nanocompositeformation and the presence of phosphorus.

CH2 CH

O

OPO

O

CH2 CH CH2 CH

CH2

N+H3C CH3

C16H33

Cl-

x y z

(55)

In another study, triphenyl phosphate (TPP)nanocomposites were synthesized by intercalatingTPP into the galleries of organically modified mica-type silicate.142 It was found that ‘nano-TPP’ has ahigher volatilization temperature compared to plainTPP and the thermal stability of ABS was slightlyenhanced by adding nano-TPP. Next, epoxy resinand silane coupling agent were incorporated asflame co-retardants. A very large increase in OI(OI = 35) was observed with epoxy addition andfurther enhancement in thermal stability was obtained.

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Finally, when the phosphate 36 was added, OIreached a very high value of 45. It was found thatthe enhancement in flame retardancy was closelyrelated to the morphologies of the chars formed aftercombustion.

MISCELLANEOUS FLAME RETARDANT TYPES(NITROGEN, BORON, SILICON, SULFUR, IRON)Nitrogen-containing compounds are good synergistsfor phosphorus-containing or intumescent systems,but usually they are not effective in styrenic polymersalone. A rare example of an efficient system is acombination of 9 wt% diguanamine, e.g. 56, and10 wt% cyanuric acid which provides a V-1 rating inABS.179 Another class of products which is surprisinglyeffective is nucleotide bases.180 Reportedly, adenine(57) provides a V-0 rating in polystyrene at only 9wt%. Hindered amines were claimed to be synergistswith various melamine salts in polystyrene.181

H2N

N

N

N

H2N

CH2 N

N

N

NH2

NH2

(56)

N

NH2N

OH

NH

N

(57)

Polystyrenes were boronated predominantly in thepara-position on the phenyl rings182 (59) or a boron-containing styrenic monomer, 5-benzyl-2-phenyl-5-(4-vinylbenzyl)-[1,3,2]-dioxaborinane (61), and simi-lar monomer without boron were copolymerized withstyrene.183 The boron-containing polymers had higherOIs and gave greater yields of char, although it wasalso noticed that there was some contribution fromthe additional aromatic groups. The OI increasedwith increasing boron content in the copolymers. Itwas believed that the mechanism of flame retardancyinvolves, initially at least, the formation of boronatecrosslinks. At the later stages boric acid is releasedand creates a protective coating on the surface of theburning polymer.

Several silicon-containing methacrylates and acry-lates, e.g. 62, have been copolymerized with styrene.184

Surprisingly, very marginal improvement was found inOI and no correlation between OI values, char yieldsand silicon contents in these copolymers. Polystyrenesilylated on the benzene ring also gave very little charand did not show improvement in flame retardancy.182

It was speculated that on rapid heating during flam-ing combustion, the pendent polydimethylsiloxanechains are likely to depolymerize to produce a mixtureof flammable low-molecular-weight fragments. Thusrapid pyrolysis of these copolymers actually results inenhanced fuel production with little or no formationof char. In contrast, branched methylphenylsiloxaneswere effective in both ABS and HIPS, increasing OIfrom 18 to 25 at 9 wt% loading.185 Silica gel at 6 wt%

CH2 CH CH2 CH

BHO OH

(59)

OB

O

CH CH2

(61)

(CH2)3

SiH3C

CH3

(O Si)n

CH3

CH3

C4H9

O

O

CH2 C

CH3

C

(62)

in combination with 4 wt% K2CO3 caused polystyreneto produce a char yield of 6% (16% residue yield) incone calorimeter experiments which was reflected in adecrease of peak HRR.186

The blends and copolymers of styrene with sodiumstyrenesulfonate have been studied by thermal analysisand cone calorimetry.187 Cone calorimetry data showthat the peak HRR for the blends is always lessthan for polystyrene and it decreases as the fractionof PSSNa increases. On the other hand, the peakHRR is little changed for the copolymers, regardlessof the fraction of sodium styrenesulfonate that ispresent. The blend clearly shows enhanced thermo-oxidative stability relative to polystyrene attributed tothe formation of the graphite-like char which arisesfrom the interaction of adjacent sulfonate units. Sincethe adjacent sulfonate units are not present in thecopolymer, graphite-like char cannot form and thecopolymers do not have enhanced thermo-oxidativestability.

Combustion and thermal degradation of ABS wasstudied in the presence of Lewis acid-type transitionmetal chlorides (NiCl2, CoCl2, ZnCl2 and FeCl3).188

At 5 wt% loading, the OI increased from about 19to 22, which is less than the increase that mighthave commercial significance, but shows that thesechlorides have a strong charring effect. In fact in inertatmosphere, char formation in the range 10–30%was observed. This was explained as a catalyticcrosslinking effect of transition metal chlorides viaa macroradical coupling mechanism. On the otherhand, catalyzed char formation in air was unsuccessfuldue to the oxidative degradation of the char at ahigher temperature. In order to overcome oxidativeinstability, silicon was further incorporated.189 Amongvarious metal chlorides, ferric chloride was veryeffective in combination with silicon giving an OI

444 Polym Int 57:431–448 (2008)DOI: 10.1002/pi

Flame retardancy of styrene thermoplastics and foams

of 33. It was speculated that synergism is due tointeraction between ferric chloride and silicon atelevated temperatures, probably generating silicontetrachloride and hydrogen chloride. Another exampleof the flame-retardant effect of iron was found in theblend of ABS and poly(vinyl chloride) where FeOOHsignificantly increased char formation and decreasedsmoke generation.190

CONCLUSIONSThis paper has presented an overview of the recent lit-erature on flame retardancy of polystyrene, HIPS, ABSand polystyrene foams. Aromatic bromine compoundsare the most widely used in styrenic thermoplasticsbecause of the combination of high thermal stabilityand high efficiency. Aliphatic brominated compounds,although more efficient, are mostly used in polystyrenefoams because of limited thermal stability. New bromi-nated flame retardants have been developed becauseof the extensive risk assessment and uncertainty withpolybromodiphenyl ethers and hexabromocyclodode-cane.

Because styrenic polymers are not charrable,phosphorus is inherently less efficient in thesepolymers. In spite of this, many phosphorus-basedsystems are in the stage of very active development.In molded ABS and HIPS, V-2 and V-1 solutionsare already commercially available. There is a goodchance that V-0 solutions with the required set ofphysical properties will be developed soon. Flameretardant packages composed of charring agents andone or few phosphorus-based products are feasible.

There is a large body of academic and patent liter-ature on copolymerization of phosphorus-, halogen-, boron- or silicon-containing monomers withpolystyrene. Although some copolymers showed goodflame-retardant performance, no commercially impor-tant system has come as yet from this research. Becausepolystyrene and its copolymers in general are not sensi-tive to acids, very efficient intumescent flame-retardantsystems based on expandable graphite were developedand show some promise.

Nanocomposites can be easily produced withstyrenic polymers. Therefore there is significantactivity as regards flame-retardant systems basedon nanoclays. Unfortunately, the flame retardancyachieved with nanocomposites alone is not enough toachieve a good rating in the UL-94 test. Recentlyresearch has shifted towards systems combiningthe nanoclays with traditional flame retardants.Miscellaneous systems based on nitrogen, boron,silicon, sulfur or iron have been developed and theyhave various degrees of activity and promise in styrenicpolymers.

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