An overview of ame retardancy of polymeric materials:application, technology, and future directions

KEY WORDS: ame retardants; sustainability; re safety

we need to start with the basics and explain what is meant by re hazard, re risk, and why ameretardant approaches are used today.

FIRE AND MATERIALSFire Mater. 2013; 37:259279Published online 19 March 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/fam.2128*Correspondence to: Alexander B. Morgan, Multiscale Composites and Polymers Division, University of DaytonResearch Institute, Dayton, OH 45469, USA.

E-mail: alexander.morgan@udri.udayton.edu{This work was carried out by the National Institute of Standards and Technology (NIST), an agency of the U.S. govern-ment and, by statute, is not subject to copyright in the USA. Certain commercial equipment, instruments, materials, orFire hazard is best dened as the potential of a material to contribute meaningfully to a re, whichcould get out of control and cause damage [1]. This fundamental force of nature that makes repossible on this planet also ensures that a lot of the materials we use today have the potential toburn. All carbon-based materials, from wood to plastics, can be combusted as long as heat and1. FIRE HAZARD, FIRE RISK, AND FLAME RETARDANCY

Fire as a hazard to society is something that has been around for all of recorded history. In the modernera, re hazards have changed because we rarely have open res in the home and those which arepresent are well controlled (furnaces, ovens, etc.). Further, we have smoke alarms, better awarenessin large buildings of exit signs, sprinkler systems, and many other re safety engineering controls.One could argue that, by looking at re statistics in the modern era, re as a problem is decreasing,but that is because of a keen awareness of re hazard, re risk, and the need for ame retardancy inmodern materials brought into use by re safety engineers and re scientists worldwide. So, beforedelving into the focus of this article, which is ame retardancy of polymeric materials (aka plastics),Alexander B. Morgan1,*, and Jeffrey W. Gilman2,{

1Multiscale Composites and Polymers Division, University of Dayton Research Institute, Dayton, OH 45469, USA2National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

SUMMARY

Flame retardancy of polymeric materials is conducted to provide re protection to ammable consumergoods, as well as to mitigate re growth in a wide range of res. This paper is a general overview ofcommercial ame retardant technology. It covers the drivers behind why ame retardants are used today,the current technologies in use, how they are applied, and where the eld of ame retardant research isheaded. The paper is not a full review of the technology, but rather a general overview of this entire eldof applied science and is designed to get the reader started on the fundamentals behind this technology. Thispaper is based upon presentations given by the authors in late 2009 at the Flame Retardants and Fire Fightersmeeting held at NIST. Copyright 2012 John Wiley & Sons, Ltd.

Received 14 July 2011; Revised 4 February 2012; Accepted 6 February 2012companies are identied in this paper. This in no way implies endorsement or recommendation by NIST. The policy ofNIST is to use metric units of measurement in all its publications, and to provide statements of uncertainty for all orig-inal measurements. In this document, however, data from organizations outside NIST, which may include measurementsin non-metric units or measurements without uncertainty statements, are shown.

Copyright 2012 John Wiley & Sons, Ltd.

260 A. B. MORGAN AND J. W. GILMANoxygen are present, and because oxygen is plentifully available, combustion is a constant force ofnature on our planet. Certainly, some materials are easier to burn than others, but given enoughoxygen and heat, any carbon-based material will burn, including thermally stable forms of carbonlike graphite and diamond. Modern polymeric materials t into the broad class of carbon-basedammable materials and, in some cases, can be far more ammable than natural materials likewood, cotton, or other cellulosic mass. Whereas these materials may be more ammable than naturalbased materials, they are in use because they are far superior in other ways, such as lower cost, andease of fabrication. However, the recent move by mankind to a more sustainable society hasmotivated all segments of society to critically evaluate our use of natural resources (energy, water,etc.), toxicity issues, and environmental impact. This has created a paradigm shift in how thematerials industry does strategic planning. The previous singular emphasis of evaluating futureinvestment plans based on costs using return on investment economic is shifting to the use ofmodels, which emphasize the triple bottom line (social value, economic, and environmentaloutcomes) and employ both environmental and economic models, using life-cycle-assessmentsoftware such as Building for Environmental and Economic Sustainability. Life-cycle-assessmentuses standardized (ISO14040) methods to compare the effects of use of bio-based polymers, greenchemistry approaches, sustainable manufacturing and recycling on the development of sustainableproducts. This has resulted in resurgence in the interest in developing products using bio-basedpolymers. Whereas the ammability of polymers found in nature, such as wool and cellulose, areknown, the ammability of new bio-based polymers where the monomers are bio-derived, such asthose from corn, soy, castor, and so on, are not well known, nor have the best ame retardantapproaches been developed for these new materials. If use of plastics, either petroleum or bio-based,is increasing in all aspects of daily life, one must realize that these materials bring with them rehazard: if they catch re, they are likely to burn and set other things on re.

Now, let us talk about re risk scenarios. Fire risk is dened as the potential for something to catchre in a particular situation [2]. The re risk of a aming meteor landing on your house and burning itto the ground is pretty low, but if you have a wood clapboard house in a dry area of California(wildlandurban interface), your home environment is a part of a high re risk scenario frompossible wildland res. Likewise, if you have any piece of electronic equipment in your possession,you have the potential for re risk. Specically, if the battery shorts out, or the power supply failsand causes arcing on the circuit board, this re risk scenarios could ignite the electronic device. Onemethod to minimize re risks is to prevent the material from igniting and lower the rate of heatrelease once the ignition occurs. This paper focuses on how ame retardancy of polymeric materialsis accomplished. We discuss the results of a study, which sought to answer the question: Do ameretardants really work? That is, do they provide people more time to escape from a re?

In 1988, the National Bureau of Standards conducted a study on the ammability and re hazards/re risks associated with polymeric materials in a wide range of common products [3]. This includedproducts found in ofces and homes. Products containing no ame retardants were compared with theidentical products with ame retardants. The ame retardants used in these products varied anddesigned to address specic re risk scenarios. The use of non-ame retardant materials led to rapidincreases in heat release, short times to room ashover, and large emissions of toxic carbonmonoxide. The use of ame retardant materials greatly delayed the time to ashover and, in somecases, resulted in products, which were difcult to ignite. Indeed, the results justied approachesthat had been driven by the insurance industry and re safety regulators for some time; polymericmaterials are ammable and will contribute to re risk and hazard, and short of using non-ammable materials, ame retardant additives are an effective way to mitigate that hazard andminimize re risk.

To put the re risk/re hazard picture into a more modern perspective, one can use combinedreported re statistics from 1980 to 2008 (Figure 1) and 20032006 re statistics (Table I). Whenstudying these numbers, one can see that the number of reported res has greatly decreased since1980, and civilian deaths/injuries as well as re ghter injuries have decreased as well over thisperiod. The decreases in res, deaths, and injuries can be attributed to better education, better resafety building codes and standards, wider use of smoke alarms, and, to some extent, continued useof ame retardants in potentially ammable materials used in various re risk scenarios. However, ifCopyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

AN OVERVIEW OF FLAME RETARDANCY OF POLYMERIC MATERIALS 261one looks at the total cost of re over this time period, despite the decrease in actual reported res andinjuries/deaths, the total cost has greatly increased over this period from $74 billion in 1980 to $138billion in 2008. So why would re costs go up whereas actual number of res decrease? It could bethat whereas res have decreased, there is a price for re safety vigilance, including paying for there ghter service, and building re protection equipment. This paper is not meant to serve as an in-depth economic study, but rather to point out that there is a societal cost to re protection, and itwill only increase with time given current trends.

The 20032006 re statistics, which summarize data on the rst item ignited, point out why ameretardants were used; the rst items ignited are nearly always polymeric materials. Hence, the need toutilize ame retardant in these type of products. Other than Californias Technical Bulletin 117, mostupholstered furniture and mattress/bedding items are not currently ame retarded. However,examination of the number of res where these materials are the rst item ignited and the associatedfact that these res result in nearly one-third of the re deaths, one could argue that less ammableupholstered furniture, mattresses, and bedding would improve re safety. This might beaccomplished using ame retardants, using barrier fabrics or inherently re safe polymers, althoughthis later option is usually cost prohibitive. Looking at other items though, such as thermoplastics,

Figure 1. Reported re and re losses from 19802008 (Table and plot courtesy Anthony Hamins, NIST).

Table I. 20032006 US re statistics.

First item ignited Fires Deaths Injuries Property damage ($B)

Upholstered furniture 7400 590 900 0.4Mattress/bedding 11,200 380 1390 0.4Thermoplastics* 29,400 280 1160 0.7Structural member, component, insulation 32,500 240 620 1.3Other furniture or utensils 6000 170 500 0.2Conned cooking re/materials 134,900 130 3670 0.3Interior wall covering 8200 120 340 0.3Subtotal of above categories 229,600 1910 8560 3.6Totals{ 378,600 2850 13,090 6.1

Note: Table I comes from: Ahrens, M. M. Home Structural Fires, NFPA, Quincy, MA Jan 2003*It is assumed that the overriding reason that the items (in the categories for curtains, wire insulation, carpeting,and appliance housings).rst ignited was due to thermoplastic content.Cooking could also lead to ignition of cabinetry and interior wall coverings (not included here).{Includes results for all sources, not just those listed here; does not include unknown sources.

Copyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

insulation, and interior wall coverings, all of these today have re test standards that very often requiresome sort of ame retardant for the material to pass various re safety test.

With the case for ame retardants to protect polymeric materials now explained, we can focus on thespecic ame retardant approaches and chemistries used today. The approaches are highly varied andthe chemistry equally so. To best explain how these approaches and ame retardant chemistries work,some basics of polymer combustion need to be discussed because the ame retardant chemistries aredesigned to address aspects of polymer combustion.

2. FLAME RETARDANT FUNDAMENTALS AND APPROACHES

Polymer combustion is driven by the thermally induced decomposition (pyrolysis) of solid polymer

262 A. B. MORGAN AND J. W. GILMANSolidPolymer

PyrolysisZone

FlameFrontMelting, Flow

Direction of Feed to Fire

Figure 2. General schematic of polymer decomposition and combustion behavior.into smaller fragments, which then volatilize, mix with oxygen, and combust. This combustionreleases more heat, which reradiates onto the unburned polymer, thus continuing to drive pyrolysisand combustion until a lack of heat/fuel/oxygen causes the re to extinguish. This is admittedly asimplistic explanation, but it holds basically true for just about all polymeric materials.Thermoplastic polymers have a tendency to drip and ow under re conditions, which can lead toadditional mechanisms of ame spread or propagation whereas thermoset polymers tend to not dripand ow and instead produce pyrolysis gases from the surface of the sample directly into thecondensed phase. A general schematic of this behavior is shown in Figure 2.

The exact physics and chemistries that occur in polymer combustion are dictated by the polymer thatis burning. The chemical structure of the polymer and how it behaves upon exposure to heat willdetermine how much heat, smoke, and other gases are released when that polymer burns. Going intothe details of polymer decomposition chemistry is beyond the scope of this paper, but it sufces tosay it is important to realize that polymer decomposition chemistry is very important when trying toaddress the re hazard of a polymer through ame retardant approaches. Extensive sources ofinformation on polymer decomposition chemistry and how the details of polymer decompositionaffect ame spread and growth can be found elsewhere [4].

A simplistic way to look at polymer decomposition chemistry in relation to re hazard is to look atthe chemical structure repeat units of the polymer and realize what the heat release potential of thoserepeat units are. This has been done in a study by Lyon and others at the Federal AviationAdministration. This group additivity approach to polymer ammability showed that highly aliphaticmaterials (polymers with mostly sp3 carbon bonds) have the potential for high heat release whereasthose with more aromatic character (sp2 bonds) have lower heat release [5,6]. Interestingly, Lyonalso found that the cost of polymers and their inherent ammability (heat release capacity) can beinversely related, that is, the more expensive highly aromatic engineering polymers (with highthermal stability) have very low ammability whereas inexpensive commodity aliphatic polymers(with low thermal stability) had much higher ammability (Figure 3). However, classifying rehazard just by chemical structure alone can miss things, such as high smoke release, tendency todrip while burning, and release of corrosive or toxic gases that are equally important re hazards.So, when considering re protection approaches, one must look at all aspects of the burning

DiffusionZone

ThermalDecomposition

Condensed Phase Vapor PhaseCopyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

AN OVERVIEW OF FLAME RETARDANCY OF POLYMERIC MATERIALS 263polymer: heat release, smoke release, mechanical integrity under re conditions (such as ow orsoftening), and how that polymer behaves in a particular re risk scenario.

When considering how to ame retard a polymeric material or protect that same material from a re,there are three main approaches one can take. These are: engineering approaches, use of inherently lowammable polymers, and ame retardant additives.

The rst approach, re safety through an engineering approach, is one of the cheapest to implement.It is a solution, which seeks to nd a way to get the polymer out of the re risk scenario. This can be donewith a re protection shield or changing how the construction of the entire product is used such that it isremoved from the re risk scenario completely. However, this approach, whereas easily implementedand often very cost effective, can be easily defeated. For example, if one uses a re proof fabric toprotect ammable foam in furniture or mattresses, if the fabric ever gets ripped or torn, the re willbe able to get at the foam. Likewise, if a metal protection shield for a polymer loses adhesion andfalls off, or because of corrosion a hole appears in the heat shield, the re can burn the material onthe other side. Even intumescent re protection barriers (to be discussed in more detail next),although giving outstanding re performance for polymers and metal, can be easily defeated if thebarrier is scratched away (intumescent paint) or falls off upon impact or with time when left in placefor years, thus, leaving the underlying material completely exposed to re damage.

Use of low heat release (inherently ame retardant) polymers is another way to address the re

Fire

Haz

ard,

c

(J/g-

K)

0.1 1 10 100Materials Cost ($/lb)

1000

10

100

11000

Figure 3. Plot of re hazard (heat release capacity) versus materials cost. Plot courtesy: Richard E. Lyon,Federal Aviation Administration.safety of polymeric materials in a wide range of re risk scenarios. This tends to be a rather robustmethod of re protection, as it does not matter what the re risk scenario is the polymer is alreadyof minimal re hazard to begin with, and so will do well regardless of the re it is exposed to. Lowheat release polymers can be fabricated into a wide range of forms, making them relatively easy toimplement in a wide range of applications. However, these same low ammability polymers comewith a high cost (see Figure 2), and so their use can be limited for economic reasons. Further, withan increasing drive for recycling and sustainability of polymeric materials, many of the lowammability polymers are difcult to recycle, especially ber reinforced polymer composites usedin aircraft, mass transport, and maritime constructions. So, for mostly economic reasons, inherentlylow ammability polymers are not often used except in applications demanding their use (militaryand aerospace primarily).

The third option, ame retardant additives, is the primary focus of this paper. The reason for thelarge use of this solution is because it is a proven approach, tends to be very cost effective, and isan approach that is relatively easy to incorporate into a polymer because of the wide range ofknowledge on this subject. However, as will be discussed in this paper, the use of ame retardantadditives has its own problems including potential for leaching into the environment, difculty withrecycling, and often a compromise in reaching a balance in the properties of the polymer. Still, theadditives are used because they work in spite of their drawbacks. In the next sections of the paper,

Copyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

material scientist chooses the ame retardant based upon polymer thermal decomposition

264 A. B. MORGAN AND J. W. GILMANchemistry, re risk scenario/regulatory test, and the whole laundry list of material commercialrequirements such as cost, process type, color, environmental stability, and now end-of-liferecycling/sustainability issues. This last requirement, recycling and sustainability is a very newrequirement, and so many ame retardants were not designed for this requirement and are nowunder rereview for their suitability and use to meet this need. In the next section, the differentame retardant classes are discussed, starting with the most widely used class, halogenatedame retardants.the wide range of ame retardant chemistries will be discussed, along with details on their history, howthey work, their pros and cons, and how they are used today. It should be pointed out here that a ameretardant is a chemical applied for a particular application. It is not any different than a chemicalapplied for curing a disease (a pharmaceutical) or a chemical applied to provide color to a fabric(a pigment). What makes it different than pharmaceutical or pigment chemicals is that its solepurpose is to minimize the ammability.

3. FLAME RETARDANT ADDITIVE TECHNOLOGY

There are six general classes of ame retardant chemistries available today. Some general chemicalstructures of these additives will be discussed along with their mode of action. Of nal note,normal additives and reactive additives will be discussed in each section should both types beavailable. Normal additives are those which do not chemically bond to the polymer and aremixed into the polymer either during polymerization or during melt compounding of thethermoplastic material. Normal additives are part of polymer formulations and tend to be themost common type of ame retardant additive. Reactive additives are those added to the polymerduring polymerization or a post-processing step that chemically bond to the polymer backbone.Either they copolymerize as a new polymer monomer or they graft onto the polymer via apost-polymerization reaction. Normal additives are typically more available than reactive ameretardant additives.

All types of ame retardant chemistries fall into one (or more) of three mechanisms of ameretardant action. These three mechanisms are gas phase ame retardancy, endothermic ameretardants, and char forming ame retardants. These can be summed up as follows:

1. Gas phase ame retardants (ex. halogen, phosphorus)

These materials reduce the heat released in the gas phase from combustion by scavenging reactivefree radicals. To understand where the gas phase is, see Figure 1.

2. Endothermic ame retardants (ex. metal hydroxides, carbonates)

These materials function in the gas phase and condensed phase by releasing non-ammable gases(H2O, CO2), which dilute the fuel and cool the polymer through endothermic decomposition of theame retardant additive. The lower substrate temperature slows the pyrolysis rate. These materialsalso leave behind a ceramic-like residue, which protects the underlying polymer.

3. Char-forming ame retardants (ex. intumescents, nanocomposites)

These materials operate in the condensed phase by preventing fuel release through binding up fuelas non-pyrolyzable carbon (char) and providing thermal insulation for underlying polymer through theformation of char protection layers.

But how does one know what chemistry to use? That is much harder to dene and is beyond thescope of this paper, but it sufces to say that there is no universal ame retardant approach.Because ame retardants are molecules designed for a particular application, those same ameretardant additives were optimized for a particular polymer in a specic re risk scenario. Whatworks for one polymer in that specic regulatory test may not (and often does not) work forother polymers in the same test. Further, what works for a polymer in one test may not workfor the same polymer in other regulatory tests/re risk scenarios. So, the ame retardant chemist/Copyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

3.1. Halogenated ame retardants

Halogenated ame retardants, like their name suggest, are molecules that incorporate elements fromgroup VII of the periodic table F, Cl, Br, and I. They can vary widely in chemical structure, fromaliphatic to aromatic carbon substrates that have been per-halogenated (all hydrogen replaced withhalogen) or can come in inorganic forms, but it is the organohalogen compounds that nd the mosteffectiveness as ame retardant additives for polymers [7]. Of the halogens in question,organochlorine and organobromine are used most, with organobromine compounds by far the mostcommonly used. The reasons for organobromine compound use can be simply explained in that theC-Br bond is just right for preventing res. The bond is stable enough for environmental exposureand yet unstable enough that heat can easily break the bond, releasing the bromine under reconditions to inhibit combustion free radical reactions. Examples of the chemistry that bromine canundertake in the vapor phase to inhibit combustion are shown in Figure 4. These unique bondstrengths make halogen-based ame retardants strictly vapor phase ame retardants as the halogenswork in the vapor phase to inhibit combustion.

Industrially speaking, producing organobromine compounds as ame retardants, turns out to be avery efcient chemistry, making it possible to produce large amounts of organobromine compounds,cost effectively. This again is because of the unique aspects of bromine and carbon chemistry thatmake these compounds easy to perbrominate, and so one small organic molecule can deliver a highpayload of effective bromine to the re. As indicated in the previous paragraph, the range ofstructures is widely varied for brominated ame retardants, and just some of the more commonstructures are shown in Figure 5. It should be noted that not all organobromine compounds makecost effective ame retardants. The ame retardant must be tailored to be compatible with thepolymer, must have the right cost, and it must release its bromine under the right re conditions

AN OVERVIEW OF FLAME RETARDANCY OF POLYMERIC MATERIALS 265not too soon before the onset of polymer decomposition, but not too far after the polymer has begunto completely decompose. So, the structures in Figure 4 exist for a reason they were found towork and have optimal properties for the polymers they are used in today. Halogenated ameretardants are sometimes used with synergists such as antimony oxide, zinc borate, or otherphosphorus chemistry, as these other elements help make the halogens more efcient in the vaporphase [6].

H + O2 HO + O HO + H H2O HO + CO CO2 + HR-Br R + Br Br + R-H HBr + R H + Br HBr HO + HBr H2O + Br H + HBr H2 + Br SbBr3 + 3H Sb + 3HBr Sb + HO SbOH SbOH + HO SbO + H2O

Figure 4. Free radical combustion reactions with bromine.

OBr5Br5

N

BrBr

BrBr O

ON

O

O

BrBr

BrBr

Decabromodiphenyl ether

1,2-ethylene bis(tetrabromophthalimide)

OHHO

Br

Br

Br

Br

BrBr

BrBr

Br

Br

Tetrabromo bisphenol A

Hexabromocyclododecane

OO

Br

Br

Br

Br

Bis(2,3-dibromopropylether)ofTetrabromo bisphenol A

BrBr Br

Br

PO

OOO

Br

BrBr

Br

Br

Br

Br

Br

Br

Tris(tribromoneopentyl)phosphate

Figure 5. Bromine FR chemistry structures.Copyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

266 A. B. MORGAN AND J. W. GILMAN H+O2!HO +O HO+H!H2O HO+CO!CO2 +H R-Br!R+Br Br+R-H!HBr +R H+Br!HBr HO+HBr!H2O+Br H+HBr!H2 +Br SbBr3 + 3H! Sb + 3HBr Sb +HO! SbOH SbOH+HO! SbO+H2OHalogenated ame retardant technology has been in use since the 1930s, which makes it a proven

cost effective technology for ame retarding a wide range of polymers. However, it does have itsdrawbacks including an increase in smoke release under re conditions, a release of corrosive gasesduring burning, and environmental scrutiny. The increase in smoke and release of corrosive gases(namely HBr) during burning is an effect of its ame retardant chemistry. Because it is inhibitingcombustion through the formation of HBr, naturally it will cause the formation of partiallycombusted polymer decomposition products as well as CO. Additionally, once the bromine isconsumed by the re, any remaining polymer will burn if exposed to additional heat, and sohalogenated ame retardants do not always perform well under very high heat ux conditions unlessa lot of halogen is present in the polymer. So, this can mean higher smoke and corrosive gas releasebut not always superior re performance under very strict re risk scenarios. The last drawbackmentioned in this section, environmental scrutiny, is related to how it has been used to date, as anormal additive. Because the additive is just put into the polymer and is not chemically bound to thepolymer, it can leave the polymer over time and get into the environment. Because these compoundswere designed to be stable for years while waiting passively to provide protection to the polymer incase of re, they are stable in the environment as well once out of the plastic resulting in theirpersistent discovery in the environment. Whether this appearance in the environment is caused byindustrial misuse or poor recycling of ame retardant products by society cannot be determined atthis time. Both or neither may be responsible, but it is clear that these compounds are in theenvironment, and some toxicology tests and other environmental science studies seem to indicatethat these materials are not welcome outside their original polymer matrices. Therefore, any productcontaining halogenated ame retardant additives will be under greater scrutiny and regulation inglobal markets, possibly even leading to delisting of these materials. So, newer approaches withhalogenated ame retardants are either focusing on easily recycled versions of those additives orreactive ame retardant approaches. Some reactive brominated ame retardants exist, but they arelimited to just a few polymers (styrenics, acrylates, urethanes, and epoxies) or are not widely usedbecause of cost. One exception to this is for epoxies, where all of the tetrabromobisphenol A(Figure 4) is converted into an epoxide, which copolymerizes into the epoxy matrix, and so thismaterial is an example of a widely used reactive ame retardant for halogen. Still, this material hasits problems especially with the use of non-lead-based soldering compounds. Specically, the non-lead-based material is of higher melting temperatures, and so the non-lead-based solder can causethe tetrabromobisphenol A to begin decomposing right on the circuit board as the solder is applied.

3.2. Phosphorus-based ame retardants

As the name describes, phosphorus-based ame retardants incorporate phosphorus into their structure,and the structure can vary greatly from inorganic to organic forms, and between oxidation states(0, +3, +5) [8]. Making direct phosphoruscarbon bonds tends to be expensive and/or difcult toachieve, so the majority of phosphorus-based ame retardants are assembled with phosphorusoxygen bonds with any organic groups attached at the oxygen, although some phosphoruscarbonbonds do exist in ame retardant structures. Inorganic forms of phosphorus ame retardants alsotend to be phosphates, but one material, red phosphorus, has its own unique structure and is the onlyphosphorus (0) ame retardant in use. Interestingly, the other form of elemental phosphorus, whitephosphorus, is a pyrophoric material, and so a simple change in molecular structure allows amaterial that, in one form, starts res to serve as a ame retardant in another form. Some examplestructures of common phosphorus-based ame retardants are shown in Figure 6.

Phosphorus compounds are unique in that they can be vapor phase or condensed phase ameretardants, depending upon their chemical structure and their interaction with the polymer under reCopyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

PO

OOOO O

PPO O

OO O O O OPP

O O

OO

O O

Resorcinol DiphosphateBisphenol A Diphosphate Triphenylphosphate

PO

R-OM+PO

OH

AN OVERVIEW OF FLAME RETARDANCY OF POLYMERIC MATERIALS 267conditions. It is not uncommon for a particular phosphorus ame retardant to be predominantly vaporphase in one polymer but condensed phase in another [912]. Phosphorus compounds are sometimescombined with other ame retardants to gain synergistic effects, but only show vapor phase synergywhen combined with halogen. When they are combined with other materials, it is typically toenhance char formation or oxidative durability of the chars formed by the phosphorus ameretardant. Some examples of the vapor phase phosphorus chemistry are shown in Figure 7, andcondensed phase char formation provided by phosphorus is discussed more in the intumescentsection of this paper. Some of these reactions can be quite complex, and so, in some cases, productsand educts not balanced by stoichiometry (reaction #2 for example) are shown.

P4 + 2O2! 4PO H3PO4!HPO2 +HPO+PO H+PO!HPO H+HPO!H2 +PO 2OH+ PO!HPO+H2O OH +H2 + PO!HPO+H2OPhosphorus based ame retardant technology has been around since the 1940s1950s, and is

becoming more commonly used as halogenated materials are deselected for the reasons describedpreviously. As mentioned, they can be effective in both vapor and condensed phases, meaning thatthey can be useful in low loading levels when combined with polymers that inherently char on theirown. Further, phosphorus ame retardants tend to do well in high heat ux re conditions, andthrough char formation, can provide superior re protection in combination with other ameretardants (see intumescent section later). However, like all ame retardant technologies, they havetheir drawbacks. They are not as widely useful as halogenated ame retardants, and sometimes will

RPhosphinate SaltsM = Al, ZnR = Alkyl 9,10-dihydro-9-oxa-10-phosphaphenanthrene-

10-oxide (DOPO)

[NH4PO4]nAmmonium Polyphosphate

Figure 6. Typical phosphorus FR additive chemical structures.have little to no effectiveness by themselves in styrenic or polyolen polymers. Further, they alsogenerate more smoke and CO during re conditions because they are helping inhibit polymercombustion, and nally they also are starting to be under regulatory scrutiny for environmentalimpact. Indeed, because of changing regulations for all chemicals within Europe (REACH), justabout all ame retardants, including phosphorus compounds, are being reviewed for environmentalimpact. In some cases, the phosphorus compounds can be made to be reactive ame retardants,

P4 + 2O2 4PO H3PO4 HPO2 + HPO + PO H + PO HPO H + HPO H2 + PO 2OH +PO HPO + H2O OH + H2 + PO HPO + H2O

Figure 7. Vapor phase ame retardant reactions for phosphorus.

Copyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

also tend to be fairly inexpensive and can be readily coated with surfactants to make their use in

268 A. B. MORGAN AND J. W. GILMANpolymers easier. These materials however suffer from two main drawbacks. The rst is that theyhave a limited re performance window. Specically, once enough heat has consumed all themineral ller and all the water/CO2 has been released, the metal oxide left behind provides noadditional protection to the polymer. So, the mineral ller can delay ignition and slow initial amegrowth, but it cannot stop it completely if enough constant external heat is applied. The otherwhich remove a lot of the scrutiny, but in other cases, this is not possible thus eliminating phosphorusas a non-halogenated ame retardant for some strict customer applications. There are not as manyreactive versions of phosphorous ame retardants, but one commercial one is notable (DOPO inFigure 6) for its growing use in epoxy circuit boards.

3.3. Mineral ller ame retardants

Mineral ame retardants fall into the endothermic cooling mechanism of ame retardancy, and have aunique vapor/condensed phase activity. Specically, under re conditions, the mineral llerendothermically decomposes upon exposure to heat. This cools the condensed phase, thus, slowingthermal decomposition of the polymer. Further, the decomposition products of the mineral ller arenon-ammable, and so the residue left behind from thermal decomposition (usually a metal oxide)dilutes the total amount of polymer fuel available (condensed phase), and the release ofnon-ammable gas from the mineral ller helps dilute the fuel available in the vapor phase forignition. The most commonly used mineral llers in use today as ame retardants are metalhydroxides and metal carbonates. Some of these materials are synthesized whereas others are minedand rened for use as ame retardant additives. Not any metal carbonate or metal hydroxide can beused as a ame retardant; the hydroxide or carbonate needs to be able to release its water or carbondioxide at elevated temperatures, but not too high of a temperature that the polymer decomposesbefore the mineral ller activates. So, hydroxides and carbonates, which decompose between 180and 400 C tend to be the only materials in use today as ame retardant additives. For thehydroxides, this includes the widely used aluminum (Al(OH)3) and magnesium (Mg(OH)2)hydroxides. Aluminum hydroxide is often called alumina trihydrate (Al2O33H2O) because thewater is hydrated on the aluminum oxide surface rather than predominated by Al-OH bonds, but thenet stoichometry is the name and so the structures are used interchangeably for ame retardantchemistry discussions. For the carbonates, magnesium carbonate is sometimes used, althoughcalcium carbonate is also used in combination with other llers (silicone) and activating materials tobe active as a ame retardant for wire and cable compounds [1317]. Magnesium carbonate is alsoused in a slightly different form called hydromagnesite, which can release water and CO2 at lowertemperatures. Mineral llers do not typically get used with synergists, but in some cases arecombined with other ame retardants to reduce smoke release. Some general schemes on these mainclasses of mineral llers are shown next:

Aluminum hydroxide (or alumina trihydrate ATH) 2Al(OH)3 +Heat (180200 C) ! Al2O3 + 3H2O (g)" Or: Al2O33H2O+Heat (180200 C) ! Al2O3 + 3H2O (g)"

Magnesium hydroxide

Mg(OH)2 +Heat (300320 C) ! MgO+H2O (g)" Hydromagnesite

3MgCO3Mg(OH)23H2O+Heat (220240 C) ! Mg4(CO3)3(OH)2 + 3H2O (g)" Mg4(CO3)3(OH)2 +Heat (320350 C) ! 4MgO+ 3CO2 (g)" +H2O (g)" Net gas released: 4moles H2O, 3moles CO2.

Mineral llers are very old technology, with denite references from the 1920s and some referencessuggesting that they may have been in use as far back as the 17th century. Regardless, they are aproven technology and are perceived to be very environmentally friendly. Further, under reconditions, they tend to greatly lower smoke and reduce overall toxic gas emissions because themineral ller is replacing ammable polymer fuel with non-ammable inorganic mass. These llersCopyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

drawback is that, for mineral llers to be effective, high loadings of mineral llers are needed to ameretard the plastic, often at the expense of polymer mechanical properties. For example, polyolen wireand cable compounds ame retarded with mineral llers will often have 50 to 70-wt.% loading toachieve acceptable regulatory re performance. This may be acceptable for a exible wire and cablejacket that can become stiffer in use and still be acceptable for sale, but it will not work for acomputer casing or circuit board that requires higher levels of mechanical durability that high levelsof mineral ller would interfere with. Therefore, mineral llers have some practical limits ofusefulness to just a few polymer classes, namely polyolens, rubbers, and polymeric materials thatcan tolerate high loading levels. Indeed, one could really classify ame retardant systems withmineral llers as ceramic composites with polymeric binders rather than as lled polymer systems.There are no reactive versions of this ame retardant class, and so these materials are strictly normaladditives.

AN OVERVIEW OF FLAME RETARDANCY OF POLYMERIC MATERIALS 2693.4. Intumescent ame retardants

Intumescent ame retardants get their name from their mode of ame retardancy during re conditions.Specically, they create a protective carbon foam under re conditions; they rise up in response to heat(intumesce). This class of ame retardants is strictly condensed phase in its activity, and either providesits own carbon char or uses the polymer as a carbon char source. Intumescents are typically composedof three components that make the carbon char form. The rst is an acid catalyst, which causes thecarbon source (the second component) to crosslink and form a thermally stable form of carbon. Thelast component is the spumic or gas former, which causes the carbon source to become a carbonfoam. These three materials work together to make the intumescent work; by themselves, theyprovide some ame retardancy, but it is the combination, which provides the real protection whenthese materials are exposed to heat and ame. Examples of these chemical reactions are shown inFigure 8 (how the carbon char forms) and in Figure 9 (specic condensed phase reactions). Aclassic intumescent system is ammonium polyphosphate (acid source), pentaerythritol (carbonsource), and melamine (gas former/spumic). Sometimes, each of the three components in anintumescent is a separate chemical, which is combined to make an intumescent formulation, and, inother cases, all three components are combined into the same structure to make a single intumescentame retardant. Therefore, intumescent ame retardancy can be rather varied in structure andformulation [18,19]. A related intumescent is expandable graphite, which is its own carbon sourceand therefore requires no acid catalyst [20,21]. Instead the graphite expands under re conditions asit releases gas trapped in between the graphite layers, and this high surface graphite then providesthermal protection caused by its highly fragmented/high surface area structure of thermallyinsulating plates. At this time, there are no commercial reactive intumescent ame retardants, theintumescent additive has to be compounded or added into the polymer, in some way, to be effective.

Intumescent ame retardant are typically used to provide re protection for re barriers, steel,rewall holes, and applications requiring a high level of re safety. Very commonly, theintumescents are incorporated into a paint or barrier form, which is applied to another substrate, and

Carbon char barrierProtects underlying material(thermal barrier)Slows release of gases

Heat (Fire)Heat (Fire)

Heat (Fire)

Decomposing polymer, Ammonium polyphosphatePolyol (pentaertytritol)

Water, Ammonia"Carbon Foam Blowing Agents"

Figure 8. General schematic of intumescent formation.Copyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

Figure 9. Intumescent ame retardant chemical reactions.

270 A. B. MORGAN AND J. W. GILMANthe intumescent then protects that underlying material from thermal damage for some period of time.This versatility and their mode of ame retardant action mean that these materials are capable ofproviding very robust re safety for highly demanding applications, and these features lead to theirincreasing use today. However, they do have some drawbacks, including water absorption issues(important if the underlying structure needs to be protected against corrosion or electrical shortcircuit), and low thermal stability. Intumescent systems work by activating well before the polymerobtains a chance to thermally decompose, and so most intumescent materials activate around180200 C, with some that now go up to 240 C before activating. This unfortunately eliminatestheir use in higher melting thermoplastics as they would activate during melt compounding into thethermoplastic. Therefore, intumescent materials, whereas they provide excellent re protection, tendto be limited to lower temperature materials and re protection barriers.O P O

O

O- NH4+

PO

O OO-

NH4+

*

**

*

Heat

-2 NH3

n

n

O P O

O

OH

PO

O OOH

*

**

*

n

n

Heat

- H2O

O P O

O

PO

O O

*

**

*

n

n

O

O P O

O

O- NH4+**

n

Heat

- H2O OP O

O

NH2

**

n

Polyphosphoric Acid Poly phosphoric acid anhydrideAmmonium Polyphosphate

Poly phosphoramide

OP

O

O

O- NH4+**

n

H3PO4HO

HO OH

OH Heat

-H2O, -NH3

Pentaerythritol

OH

OH

OP

O

O

OPO

O

O- NH4+

Heat

-H2O, -NH3 Crosslinked Carbon Char3.5. Inorganic ame retardants

This category is a bit of a catch-all category in that it covers a wide range of chemical structures thatcan act in both vapor and condensed phases, but ultimately are niche ame retardants. They eitherassist other ame retardants to work better, address a particular effect of ame retardancy (such assmoke formation), or have a niche ame retardancy effect in a select few polymers. As the nameimplies, these ame retardants have no carbon in their structure, and cover a wide range of elementsfrom the periodic table. Whereas there are numerous literature reports on various metal oxides ormetal complexes providing ame retardancy in very select systems, none of these materials arecommercially used today [22]. The only commercial inorganic ame retardants are borates,stannates, and silicates, each of which will be discussed in turn here.

Borates used include zinc borates (2ZnO3B2O33.5H2O), which are used as synergists forhalogenated ame retardants, mineral llers, and phosphorus to help the systems work better. Theyalso address afterglow conditions in highly mineral-lled systems where even after the re is out,the remaining material can be quite hot and could reignite other objects. Stannates used include zincstannate and zinc hydroxyl stannate (ZnSnO3, ZnSnO33H2O), which help lower smoke formation inthe presence of halogenated ame retardant systems. Silicates are more varied in structure and use,and often get used as a combination ller/preceramic system for providing thermal protection to theunderlying polymer through formation of a protective ceramic barrier on top of the polymer. Onelast example of silicate ame retardancy is actually a silicone, which is an exception to the point

Copyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

made earlier about inorganic ame retardants containing no carbon. These silicones (typicallypolydimethyl siloxane polymers) have been successfully used to ame retard polycarbonate resins[23,24] and also seem to show some effectiveness in wire and cable formulations when combinedwith calcium carbonate [13,14].

The main advantage of using these ame retardants is that they address some aw/weakness ofanother ame retardant and because of their inorganic structure, are mostly perceived to haveminimal environmental impact. However, because they are niche materials, they are used onlysparingly and because of the low use levels, can be rather expensive. Because of the scrutiny beingshown to halogen and phosphorus, this class of ame retardants is getting more attention, and itmay be that some classes (especially silicon based) start to become more widely available as ameretardants. Likewise, metal oxides and other inorganics capable of additional char formation andunique modes of ame retardant action may also nd more use as they are scaled up and made morecommercially viable. There are no known reactive ame retardants in this class of inorganicadditives because of the incompatibility between inorganic and organic chemistry (formation ofstable carbon-inorganic bonds during processing or polymerization).

3.6. Polymer nanocomposites

Of all the technologies reviewed in this paper, the polymer nanocomposite approach to ameretardancy is the newest technology now in use. Polymer nanocomposites are polymers lled with

AN OVERVIEW OF FLAME RETARDANCY OF POLYMERIC MATERIALS 271PolymerClay

UndecomposedPolymer + Clay

Char (Clay Rich)Barrier

Final Clay + CarbonChar

Figure 10. Polymer nanocomposite ame retardant mechanism.nanoscale particles nely dispersed in the polymer matrix such that the majority of the polymer inreally all interfacial polymer. This differs from a traditional polymer composite in that interfacialpolymer is usually only a minor component and the bers/llers in that traditional composite arequite large in size meaning that the polymer is usually bulk material at the microscale andmacroscale. A polymer nanocomposite is all interfacial polymer at the macroscale, microscale, andnanoscale. Additional details on what makes a polymer nanocomposite unique and the variety ofstructures available can be found in several excellent review articles and books [2528]. For ameretardancy, polymer nanocomposites are condensed phase ame retardants that slow (but do notstop) the mass loss rate of the polymer during re conditions through formation of a nanoparticle-rich re protection barrier (Figure 10) [2931]. This results in a lowering of peak heat release rateand inhibition of polymer ow (melting/dripping) during a re, but it does not lower the total heatrelease of the fuel; it just spreads it out over a longer time and makes it burn less intensely(Figure 11) [32]. Additionally, nanocomposites show earlier time to ignition as can be seen in

Fire Fire Fire FireCopyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

272 A. B. MORGAN AND J. W. GILMANFigure 11, but many other ame retardants show early time to ignition as well, so this effect may ormay not be a negative feature [33]. By themselves, the polymer nanocomposites are indeed retardingame growth, but not enough to pass regulatory tests by themselves. Still, this reduction in heatrelease rate and re growth is notable, and with more polymer nanocomposites coming intocommercial use for other properties (enhanced thermal, mechanical, electrical properties), one canargue that everywhere a polymer nanocomposite is used, one is using a lower ammability/ameretardant polymer system. So, because polymer nanocomposites lower the base ammability of thepolymer, they can be combined with other traditional ame retardants to produce new ameretardant materials with a better balance of ammability/mechanical properties. To be more specic,one can use less of the traditional ame retardant in combination with the nanocomposite to achieve

Figure 11. Heat release rate reduction typical for ame retardant polymer nanocomposites. PE, polyethylene ;FR, ame retardant; PP, polypropylene; and VGCNF, vapor grown carbon nanober.the same level of re safety (in some cases better) than that from using the traditional ameretardant alone. This has been shown in numerous polymer systems [34] and has worked its wayinto a few commercial systems as well [35,36]. In effect, polymer nanocomposite technology,regardless of nanoller, seems to be a nearly universal class of ame retardant synergist. Bythemselves, they certainly lower ammability, but they are best when combined with other ameretardants to produce a superior ame retarded and multi-functional material.

Like all the ame retardants discussed in this report, polymer nanocomposites as ame retardantshave their own pros and cons. The most commonly used nanoparticles for commercialnanocomposite formation are clay nanoparticles (organically treated layered silicates) and carbonnanotubes/nanobers. Costs for clays tend to be relatively inexpensive, but they are limited for whatpolymers they can be put into because of thermal instability issues associated with the organictreatment on the clay surface [3739]. Carbon nanotubes/nanobers do not have this thermalinstability problem but tend to be much more expensive or do not have a good interface with manypolymers leading to difculties in producing the desired nanocomposite structure. So, whereaspolymer nanocomposites can greatly improve ame retardancy, while at the same time bringingenhanced material properties, their biggest drawback is their newness and unknowns about thetechnology. Because the technology is new, it is not fully proven in some peoples minds forreliable re safety performance. Further, there is concern about nanoparticle safety andenvironmental disposition just as there is with halogenated or phosphorus-based ame retardants.Finally, the newness of the technology and the uniqueness of the polymer nanocomposite structuremean that the technology can require great care and skill to produce a successful polymernanocomposite, and this care and skill can add even more costs to the product. Polymer

Copyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

group and so this change will not come easily, but appears likely to happen as more regulatorsbecome aware of inherently high heat release / highly ammable polymers being used in more and

AN OVERVIEW OF FLAME RETARDANCY OF POLYMERIC MATERIALS 273more building products, vehicles, and consumer goods. Ultimately though, as re safety regulationschange, it will force material suppliers to redesign their products to meet these regulations and tests,which will in turn force a change on FR chemistry and its application because FR is not a universalscience and instead must be tailored to work for a specic test.

4.2. Sustainability requirements for polymeric materials

Along with improved durability and ame retardancy for a modern polymeric material, sustainabilityand ecological impact of the polymeric materials are starting to drive research and productnanocomposites are not as easy to implement as the other additives previously described. For the mostpart, provided you can melt the additive into the polymer in a uniform manner, you will get a ameretardant product. This is not true for polymer nanocomposites, and without paying attention to howthe nanocomposite is prepared, the polymer/nanoparticle interface, and quality control involvingcomplex scientic analysis, one will not succeed in making a ame retardant polymernanocomposite. Despite these drawbacks and changes though, polymer nanocomposites representthe newest class of ame retardant technology available and have a lot of promise to produce newre safe materials for commercial and non-commercial use.

4. FUTURE DIRECTIONS

The past section has dened the current technology and has hinted at changes and challenges facingame retardant technology, and so this section focuses on likely and predicted future directions thatone will see ame retardancy (either as additive, engineering solution, or low ammability polymer)progress. There are three likely future directions that will greatly change ame retardancy as weknow it today, and this includes regulatory changes in re tests, sustainability requirements forplastics, and new ame retardant chemistry.

4.1. Regulatory changes and their effect on ame retardancy

Returning to the introduction, ame retardants exist solely because someone discovered that they couldbe applied to the problem of providing passive re safety to a polymeric material. Likewise, thesechemicals are designed to provide re safety for a particular polymer in a particular re risk scenarioand their acceptance is determined by a particular regulatory test. Today, many of the tests are pass/fail tests designed to mimic a specic re risk scenario, but the new change that is coming is to lookat the inherent ammability of a material, not just how it behaves in one specic test. This means thatre safety engineers and regulators are spending more time studying ammability behavior such asheat release rate, smoke release/toxicity, and how the material behaves under a constant external heatux. Possible change coming to regulatory tests involves new emphasis on inherent ammability,which will dictate the ame retardancy approach to be used with polymeric materials. Rather thansimply address the pass/fail tests, which mimic specic re risk scenarios, the polymeric material willalso need to conrm that it can perform when it encounters different heat sources, that is, it will be oflow ammability and will not contribute meaningfully to a growing re. This likely means that testssuch as the cone calorimeter will become more commonly used as a regulatory tool rather than just ascientic re safety engineering tool. However, the cone calorimeter cannot measure all aspects ofmaterial ammability and so it may become a supplemental tool for measuring inherent heat releaseof material while more specic re risk scenarios are used to study physical phenomena (dripping,vertical ame spread, etc.) that the cone calorimeter cannot measure [40]. Further, this new emphasison overall ammability may result in some approaches of ame retardancy (namely char formation)becoming more prevalent as they tend to be more robust across a wide range of heat uxes. Thoseame retardants, which are only good for one test in one type of re test, are likely to becomedeselected or will only be used when combined with other ame retardants that address theshortcoming of that niche (FR) product. Of course, re safety regulators are a very conservativeCopyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

Reduction of Hazardous Substances laws, which dictate what happens to a product at the end of its

274 A. B. MORGAN AND J. W. GILMANeffective use. Products containing polymer are very often disassembled and where possible, allpolymers in that product are recycled (regrinding and remelting for thermoplastics). When apolymer cannot be recycled, they are sent to incinerator for energy recovery. There can besubstantial cost savings if the recycler can reuse the polymers they recycle, and so one can foreseedirectives coming from the industrial as well as government sector that dictate that ame retardantpolymers (so widely used in electronics) will need to be fully recyclable or can be burned cleanlyvia incineration if not recyclable. Therefore, the need to recycle the polymer or incinerate it cleanlywill likely drive new ame retardant development or deselection certain ame retardants are foundto not recycle well or not incinerate cleanly. The broader issue of sustainability for that ameretardant product may be addressed through bio-renewable feedstocks for the base polymer or forthe ame retardant itself, but ultimately one will have to look at that specic products life cycleanalysis to determine what factors beyond recyclability and clean incineration affect ame retardantdesign and use. To date, what continues to drive the recycle issues for ame retardant materials isthat the additives put into the plastic typically are not bound to the polymer; they are free to migrateif given enough energy or time. This migration can occur during regrinding of the polymer (exposedparticles during grinding) or during remelting (phase separation) and, to some extent, even duringnormal use of the polymer. If the materials migrate out of the polymer, the ecological impact of theame retardant outside its polymer shell needs to be considered. If the additive mitigates and isreleased into the environment, how does this potential ecological impact compare with the largeamounts of toxic gases released during a re? There have been several studies looking at thisparticular issue, which indicate that providing re safety does less damage to the environment thannot using the ame retardant in the product [4144]. Each individual products life cycle analysisshould be looked at separately to ensure that this logic is correct across all materials. Part of theenvironmental and life-cycle analysis of polymers may drive ame retardant research to focus moreon the nal fate/location of the additive itself, and to ensure that if it does get into the environment.

One way to address the issue of ame retardant additives leaving the polymer during the end-of-lifefor that polymer (landll, incinerator, regrind/recycle) is to ensure that the ame retardant additive isbound to the polymer rather than just mixed in. This means that the types of ame retardants mostlikely to be used in the future will be reactive ame retardants, not just additives mixed into thepolymer. The reactive additives will either be something that copolymerizes with the polymer duringits original synthesis, or will be something that grafts onto the polymer backbone during processingof the polymer into its nal form. The use of additives for polymers are not likely to go away evenwith these new changes in environmental regulations as some things cannot be easily accomplishedwithout the use of additives (color for example), but it may be that when additives are mixed intothe polymer, they come in forms that are far less likely to come out of the plastic over time. Thiswould include polymeric versions of ame retardants that are completely miscible with the polymer,encapsulated ame retardants, or ame retardants known to have minimal to no environmentalimpact should they escape the polymer matrix. Therefore, one should expect more research anddevelopment to be spent on reactive and polymeric ame retardants in the future.

4.3. New ame retardant technology

The need to protect against re is not going to go away anytime soon, and so if some ame retardantsare deselected because of environmental impact or the fact that they cannot meet new re safety tests,what will replace them? One can see from the chemistry outlined in this paper that there are six broadclasses of ame retardant chemistries available today, but certainly there are other technologies up andcoming that may become commercial realities in the future.requirements. For example, in Europe, there is little land left for landlls so their garbage is sent toincinerator or is recycled under laws and mandates. Additionally, the lack of proper recyclingsystems (incentives as well as infrastructure) in Europe led the countries within the European Unionto export their hazardous electronic waste with some political consequences that have been reportedin the media. These factors led to the creation of laws in the European Union to deal with electronicwaste (e-waste). Examples of the laws are the Waste Electronics and Electrical Enclosures andCopyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

AN OVERVIEW OF FLAME RETARDANCY OF POLYMERIC MATERIALS 275One possible new area of ame retardant chemistry would be catalysis. Some transition metals atelevated temperatures can form more thermally stable carbon/carbon bonds and so the use of acatalyst to cause a polymer to crosslink and form a more thermally stable char, rather than breakapart into smaller monomer pieces may be a promising new direction. There are already somestudies out there showing that this technology may have promise, including the use of specic metalcomplexes, [22,45,46] nanoscale metal oxides, [47,48] and waste catalysts from olen crackers[49,50], which help improve char formation and lower the ammability of the polymer.

Another area of new ame retardant chemistry would be using ceramic/glass precursors, which meltunder re conditions to form a protective ceramic/glass shield on the top of the burning polymer.Obviously, because glasses and ceramics are already in their highest oxidation state, they cannot beburned further and so if successfully implemented would provide very robust re safety to apolymeric material. The key to making this technology successful though is to get these inorganicprecursors to fuse together at low temperatures (< 400 C) so that they are available to protect thepolymer before the polymer undergoes vigorous thermal decomposition. Further, the low meltingglass must maintain a high enough viscosity through a wide temperature range so that as externalheat is applied to the glass surface, it does not crack open/ow away, which would expose theunderlying material to re damage. To date, there are only a handful of these systems, which showsome promise [51,52], but there is still much to be done with these materials before they are moresuccessful outside their current niche applications.

The last new technology, which may come in the future, would be new vapor phase ame retardants.To date, this is dominated by halogen, but some other elements have been found to show some vaporphase ame retardant activity, namely phosphorus, and a few metals (tin, iron, manganese) [53,54] inselect forms. However, that appears to be the limit of what has been discovered to date as vapor phaseame retardants. So, if halogen is deselected from use and phosphorus is likewise limited, this justabout depletes the choices of vapor phase ame retardants available for use. The aforementionedmetal-based vapor phase ame retardants in this paragraph unfortunately are only lab curiositiesbecause they are toxic metal carbonyl compounds. To eliminate an entire ame retardant mechanism(vapor phase) would severely limit the ability to ame retard polymeric materials, and so research inthis area is sorely needed. Because this area is so crucial, it seems that it is likely that eventuallysome new vapor phase ame retardants will be discovered in the future, or someone will discoverenvironmentally friendly and economically viable versions of halogenated ame retardants that canbe used instead. Admittedly, this prediction of future ame retardant technology is the hardest topredict and dene, but still it seems that something in this area is likely to come out in the comingyears.

While there is some new ame retardant research being conducted, the only way it will become anindustrial reality is if there is commercial demand for the technology, and it will have to compete withinherently low ammability plastics. One simple solution to the entire ame retardant technology issueis to stop using ame retardants completely and to shift everything to low ammability engineeringplastics. Of course, this means shifting polymer costs from the ranges of $1$10/lb to $20$500/lbof material, not to mention the additional costs associated with processing these higher temperaturehigh performance polymers. Many of these materials have high melting temperatures or requirespecialized equipment to process, and so cost will be the dominating factor determining if any newame retardant technology comes into use or not. So, with such a high cost associated withengineering polymers, it seems likely that the use of ame retardants is here to stay, and in somecases, may increase rather than decrease in use. However, how these ame retardants are used willchange both from a handling/disposal point of view and from how they are incorporated into thepolymer. Some ame retardants in use today are likely to be removed from use as new and betterreplacements come into being whereas others are likely to remain unchanged if they already have agood environmental prole. Change will occur in this eld, but right now, the exact direction thatthe technology will head in or if it will head into multiple directions, remains unclear. Funding forthe eld also remains at an all time low in some parts of the world (USA) whereas is growing inother areas (Europe, Asia), and so re concerns in other parts of the world may dominate what newtechnologies are discovered because, again, ame retardancy is an applied eld driven by specicre protection needs.Copyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

respective chemistries; new ame retardant chemistry is likely to be discovered and exploited asmore time and money are spent researching this area. It should be pointed out that, for the most

276 A. B. MORGAN AND J. W. GILMANpart, ame retardancy is not a topic that most people think about because of successful protection ofsociety provided by the current use of passive ame retardant additives. Fire damages and losses inthe USA have been reduced by the use of ame retardant additives [55]. If the current trend is toincorporate more rather than less polymeric materials into our modern civilizations, what shouldhappen next? This is something for the regulators and society to decide if the drive is for more resafety, then new ame retardants will have to be created to address these new tests. Likewise, ifimproved recycling and lower environmental impact is desired, then new ame retardants will haveto be made to meet these requirements. Flame retardant technology is driven by external forces, andthese new external forces will greatly change the technology as we know it.

6. ADDITIONAL FLAME RETARDANT INFORMATION SOURCES

Most of the information in this paper is not heavily cited because it is mostly general knowledge tothose working in the ame retardant additive eld and is best summarized in the list of books andadditional references. This paper is again meant to serve as an initial overview, not to be the singlesource key reference on ame retardant chemistry and additive technology.

A review of current ame retardant systems for epoxy resins Weil, E. D.; Levchik, S. J. Fire.Sci. 2004, 22, 2540.

Commercial ame retardancy of thermoplastic polyesters: review. Weil, E. D.; Levchik, S. J.Fire Sci. 2004, 22, 339350.

Commercial ame retardancy of unsaturated polyester and vinyl resins: review Weil, E. D.;Levchik, S. J. Fire Sci. 2004, 22, 339350.

Thermal decomposition, combustion and re-retardancy of polyurethanes a review of therecent literature Levchik, S. V.; Weil, E. D. Polym. Int. 2004, 53, 15851610.

Thermal decomposition, combustion and ame-retardancy of epoxy resins a review of therecent literature Levchik, S. V.; Weil, E. D. Polym. Int. 2004, 53, 19011929.

New developments in ame retardancy of epoxy resins Levchik, S.; Piotrowski, A.; Weil, E.;Yao, Q. Polym. Degrad. Stab. 2005, 88, 5762.

Developments in ame retardant textiles a review Horrocks, A. R.; Kandola, B. K.; Davies,P. J.; Zhang, S.; Padbury, S. A. Polym. Degrad. Stab. 2005, 88, 312.5. CONCLUSION

The eld of ame retardancy today is dynamic. Regulations that govern how ame retardants are used,what they must protect against, and where they should go at the end of a products life are all beingreevaluated. This paper is meant to serve as an introductory review to ame retardant technologyand all the changes that are now occurring. It is not the denitive review on the technology nor canit cover everything about this applied eld. It is meant to be an introduction for those not familiarwith the eld. Flame retardancy is a very applied eld and so understanding all the nuances of thetechnology is essential to understanding why a particular ame retardant is in use today, as well aswhat its specic strengths and weaknesses are in its current use. Fundamental studies are stillneeded, but an understanding of the importance of balancing properties and very complex re riskscenarios and polymer combustion concepts need to be mastered by researchers before they candevelop new ame retardant technologies. However, if the reader is just interested in learning thehows and whys of ame retardant technology, rather than trying to develop a new technology, it isour hope that this review will serve that role nicely.

Stopping the burning of materials is an endeavor as old as recorded history, and as long as we see aneed to provide protection against re, we will have to use ame retardancy in one way or another as acivilization. The six ame retardant additive technologies listed in this paper are what we use today,but they may not be what will be used in the future, nor are they perfect. They are suitable andproven for ame retardancy in their respective polymers, but they are not the pinnacles of theirCopyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

J. Fire Sci. 2008, 26, 542. New developments in ame retardancy of styrene thermoplastics and foams Levchik, S. V.;

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AN OVERVIEW OF FLAME RETARDANCY OF POLYMERIC MATERIALS 277

CoWeil, E. D. Polym. Int. 2008, 57, 431448. Flame retardants in commercial use or development for textiles Weil, E. D.; Levchik, S. V. J.Fire Sci. 2008, 26, 243281.

Combined re retardant and wood preservative treatments for outdoor wood applications areview of the literature Marney, D.C.O.; Russell, L.J. Fire Technology 2008, 44, 1 14

Zinc borates as multifunctional polymer additives Shen, K. K.; Kochesfahani, S.; Jouffret,F. Polym. Adv. Technol. 2008, 19, 469474.

Ignition, combustion, toxicity, and re retardancy of polyurethane foams: a comprehensivereview Singh, H.; Jain, A. K. J. App. Polym. Sci. 2009, 111, 11151143.

Fire Properties of Polymer Composite Materials Eds. Mouritz, A. P.; Gibson, A. G. Springer-Verlag, The Netherlands, 2006. ISBN 978-1-4020-5355-9.

Flame retardancy of silicone-based materials Hamdani, S.; Longuet, C.; Perrin, D.; Lopez-cuesta, J-M.; Ganachaud, F. Polym. Degrad. Stab., 2009, 94, 465495.

Fire Retardancy of Polymeric Materials, 2nd Edition. Eds. Wilkie, C. A.; Morgan A. B. 2009,Taylor and Francis. ISBN 9781420083996.

Flame Retardants for Plastics and Textiles: Practical Applications Weil, E. D.; Levchik, S. V.2009, Hanser Gardner Publications, ISBN 9781569904541.

Fire Retardancy of Polymers: New Strategies and Mechanisms Hull, T. R.; Kandola B. K. Eds.2009, Royal Society of Chemistry, ISBN 9780854041497.

REFERENCES

. NFPA 550 Fire Safety Concepts Tree. National Fire Protection Association: Quincy, MA, 2012.

. DiNenno PJ, Drysdale DD, Beyler CL,WaltonWD, Custer RLP, Hall JR, Watts JM. (eds). Society for Fire ProtectionEngineering Handbook. National Fire Protection Association: Quincy, MA, 2008, ISBN 978-0-87765-821-4.

. Babrauskas V, Harris R H, Gann RG, Levin BC, Lee BT, Peacock RD, Paabo M, Tilley W, Yoklavich MF, ClarkHM. Fire hazard comparison of re-retarded and non-re-retarded products. US Department of Commerce NationalBureau of Standards Special Publication #749, 1988.

. Hirschler MM, Morgan AB. Thermal decomposition of polymers. In Society for Fire Protection EngineeringHandbook, DiNenno PJ, Drysdale DD, Beyler CL, Walton WD, Custer RLP, Hall JR, Watts JM (eds). National FireProtection Association: Quincy, MA, 2008, ISBN 978-0-87765-821-4.

. Walters RN, Lyon RE. Molar group contributions to polymer ammability. Journal of Applied Polymer Science2002; 87:548563.

. Lyon RE, Takemori MT, Safronava N, Stoliarov SI, Walters RN. A molecular basis for polymer ammability.Polymer 2009; 50:26082617.

. Grand AF, Wilkie CA (eds). Fire Retardancy of Polymeric Materials. Marcel Dekker, Inc. NY: NY, 2000. ISBN0-8247-8879-6.

. Levchik SV, Weil ED. A review of recent progress in phosphorus-based ame retardants. Journal of Fire Sciences2006; 24:345364. Flammability Tewarson, A. Chapter 42 in Physical Properties of Polymers Handbook, Mark J.E. ed. AIP Press, NY 1996. pp 577604.

Overview of recent developments in the ame retardancy of polycarbonates Levchik, S. V.;Weil, E. D. Polym. Int. 2005, 54, 981998.

Flame retardants in commercial use or in advanced development in polycarbonates andpolycarbonate blends Levchik, S. V.; Weil, E. D. J. Fire Sci. 2006, 24, 137151.

Flame and smoke retardants in vinyl chloride polymers commercial usage and currentdevelopments Weil, E. D.; Levchik, S.; Moy, P. J. Fire Sci. 2006, 24, 211236.

Thermal decomposition, combustion and re-retardancy of polyurethanes a review of therecent literature Levchik, S. V.; Weil, E. D. Polym. Int. 2004, 53, 15851610.

A review of ame retardant polypropylene bres Zhang, S.; Horrocks, A. R. Prog. Polym. Sci.2003, 28, 15171438.

Flame retardants for polystyrenes in commercial use or developmentWeil, E. D.; Levchik, S. V.J. Fire Sci. 2007, 25, 241264.

Flame retardants in commercial use or development for polyolens Weil, E. D.; Levchik, S. V.pyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

278 A. B. MORGAN AND J. W. GILMAN9. Pawlowski KH, Schartel B. Flame retardancy mechanisms of triphenyl phosphate, resorcinol bis(diphenylphosphate) and bisphenol A bis(diphenyl phosphate) in polycarbonate/acrylonitrile-butadiene-styrene blends.Polymer International 56:14041414.

10. Braun U, Balabanovich AI, Schartel A, Knoll U, Artner J, Ciesielski M, Doring M, Perez R, Sandler JKW, Alstadt V,Hoffmann T, Pospiech D. Inuence of oxidation state of phosphorus on the decomposition and re behavior ofame-retarded epoxy resin composites. Polymer 2006; 47:84958508.

11. Braun U, Bahr H, Sturm H, Schartel B. Flame retardancy mechanisms of metal phosphinates and metal phosphinatesin combination with melamine cyanurate in glass-ber reinforced poly(1,4-butylene terephthalate): the inuence ofmetal cation. Polymers for Advanced Technologies 2008; 19:680692.

12. Schartel B, Kunze R, Neubert D, Braun U. Mechanistic studies on PA-66 re retarded with red phosphorus. RecentAdvances in Flame Retardancy of Polymeric Materials 2002; 13:93103.

13. Kramer KH, Blomqvist P, Hees PV, Gedde UW. On the intumescence of ethyleneacrylate copolymers blended withchalk and silicone. Polymer Degradation and Stability 2007; 92:18991910.

14. Hermansson A, Hjertberg T, Sultan B-A. Linking the ame-retardant mechanisms of an ethyleneacrylate copolymer,chalk and silicone elastomer system with its intumescent behavior. Fire and Materials 2005; 29:407423.

15. Rigolo M, Woodhams RT. Basic magnesium carbonate ame retardants for polypropylene. Polymer Engineeringand Science 1992; 32:327334.

16. Laoutid F, Gaudon P, Taulemesse J-M, Lopez Cuesta JM, Velasco JI, Piechaczyk A. Study of hydromagnesiteand magnesium hydroxide based re retardant systems for ethylenevinyl acetate containing organo-modiedmontmorillonite. Polymer Degradation and Stability 2006; 91:30743082.

17. Morgan AB, Cogen JM, Opperman RS, Harris JD. The effectiveness of magnesium carbonate-based ame retardantsfor poly(ethylene-co-vinyl acetate) and poly(ethylene-co-ethyl acrylate). Fire and Materials 2007; 31:387410.

18. Bourbigot S, Le Bras M, Duquesne S, Rochery M. Recent advances for intumescent polymers. MacromolecularMaterials and Engineering 2004; 289:499511.

19. Bourbigot S, Duquesne S. Fire retardant polymers: recent developments and opportunities. Journal of MaterialsChemistry 2007; 17:22832300.

20. Duquesne S, Delobel R, Le Bras M, Camino G. A comparative study of the mechanism of action of ammoniumpolyphosphate and expandable graphite in polyurethane. Polymer Degradation and Stability 2002; 77:333344.

21. Modesti M, Lorenzetti A, Simioni F, Camino G. Expandable graphite as an intumescent ame retardant inpolyisocyanuratepolyurethane foams. Polymer Degradation and Stability 2002; 77:195202.

22. Morgan AB. A review of transition metal-based ame retardants: transition-metal oxide/salts, and complexes. In ACSSymposium Series 1013 Fire and Polymers V: Materials and Concepts for Fire Retardancy, Oxford UniversityPress NY: NY, 2009; 312328.

23. Nishihara H, Suda Y, Sakuma T. Halogen- and phosphorus-free ame retardant PC plastic with excellent moldabilityand recyclability. Journal of Fire Sciences 2003; 21:451464.

24. Nodera A, Kanai T. Flame retardancy of a polycarbonatepolydimethylsiloxane block copolymer: the effect of thedimethylsiloxane block size. Journal of Applied Polymer Science 2006; 100:565575.

25. Ray SS, Okamoto M. Polymer/layered silicate nanocomposites: a review from preparation to processing. Progress inPolymer Science 2003; 28:15391641.

26. Moniruzzaman M, Winey KI. Polymer nanocomposites containing carbon nanotubes. Macromolecules 2006;39:51945205.

27. Utracki LA, Sepehr M, Boccaleri E. Synthetic, layered nanoparticles for polymeric nanocomposites (PNCs).Polymers for Advanced Technologies 2007; 18:137.

28. Vaia RA, Maguire JF. Polymer nanocomposites with prescribed morphology: going beyond nanoparticle-lledpolymers. Chemistry of Materials 2007; 19: 27362751.

29. Zhu J, Uhl FM, Morgan AB, Wilkie CA. Studies on the mechanism by which the formation of nanocompositesenhances thermal stability. Chemistry of Materials 2001; 13:46494654.

30. Gilman JW, Jackson CL, Morgan AB, Harris R, Manias E, Giannelis EP, Wuthenow M, Hilton D, Phillips SH.Flammability properties of polymer-layered silicate nanocomposites. polypropylene and polystyrene nanocomposites.Chemistry of Materials 2000; 12:18661873.

31. Gilman JW, Harris RH, Shields JR, Kashiwagi T, Morgan AB. A study of the ammability reduction mechanism ofpolystyrene-layered silicate nanocomposite: layered silicate reinforced carbonaceous char. Polymers for AdvancedTechnologies 2006; 17:263271.

32. Bartholmai M, Schartel B. Layered silicate polymer nanocomposites: new approach or illusion for re retardancy?Investigations of the potentials and the tasks using a model system. Polymers for Advanced Technologies 2004;15:355364.

33. Morgan AB, Bundy M. Cone calorimeter analysis of UL-94V-rated plastics. Fire and Materials 2007; 31:257283.34. Morgan AB, Wilkie CA (eds). Flame Retardant Polymer Nanocomposites. Book published by John Wiley & Sons:

Hoboken, NJ; 2007. ISBN 978-0-471-73426-0.35. Morgan AB. Flame retarded polymer layered silicate nanocomposites: a review of commercial and open literature

systems. Polymers for Advanced Technologies 2006; 17:206217.36. Beyer G. Flame retardancy of nanocomposites from research to technical products. J. Fire Sci. 2005; 23:7587.37. Xie W, Gao Z, Pan W-P, Hunter D, Singh A, Vaia R. Thermal degradation chemistry of alkyl quaternary ammonium

montmorillonite. Chemistry of Materials 2001; 13:29792990.Copyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam

38. Dharaiya D, Jana SC. Thermal decomposition of alkyl ammonium ions and its effect on surface polarity oforganically treated nanoclay. Polymer 2005; 46:1013910147.

39. Shah RK, Paul DR. Organoclay degradation in melt processed polyethylene nanocomposites. Polymer 2006;47:4084.

40. Schartel B, Hull TR. Development of re-retarded materials Interpretation of cone calorimeter data. Fire andMaterials 2007; 31:327354.

41. Simonson M, Blomqvist P, Boldizar A, Mller K, Rosell L, Tullin C, Stripple H, Sundqvist JO. Fire-LCA model: TVcase study. SP Report 2000:13 ISBN 91-7848-811-7 Printed in 2000.

42. Simonson M, Andersson P, van den Berg M. Proceedings of Fire and Materials 2007 Conference, 2931 January2007, San Francsisco, CA. Interscience Communications.

43. Blomqvist P, Rosell L, Simonson M. Emissions from res part I: re retarded and non-re retarded TV sets. FireTechnology 2004; 40:3958.

44. Blomqvist P, Rosell L, Simonson M. Emissions from res part II: re retarded and non-re retarded TV sets. FireTechnology 2004; 40:5973.

45. Starnes WH Jr., Pike RD, Cole JR, Doyal AS, Kimlin EJ, Lee JT, Murray PJ, Quinlan RA, Zhang J. Cone calorimetricstudy of copper-promoted smoke suppression and re retardance of poly(vinyl chloride). Polymer Degradation andStability 2003; 82:1524.

46. Fontaine G, Turf T, Bourbigot S. Salen copper complexes: a novel ame retardant for thermoplastic polyurethane. InACS Symposium Series 1013 Fire and Polymers V: Materials and Concepts for Fire Retardancy, Oxford Univer-sity Press NY: NY, 2009; 329340.

47. Laachaci A, Chochez M, Leroy E, Ferriol M, Lopez-Cuesta JM. Polymer Degradation and Stability 2007; 92:6169.48. Laoutid F, Ferry L, Lopez-Cuesta JM, Crespy A. Polymer Degradation and Stability 2003; 82:357363.49. Estevao LRM, Le Bras M, Delobel R, Nascimento RSV. Polymer Degradation and Stability 2005; 88:444455.50. Le Bras M, Bourbigot S, Delporte C, Siat C, Le Tallec Y. Fire and Materials 1996; 20:191.51. Kashiwagi T, Gilman JW. Silicon based ame retardants. In Flame Retardancy of Polymeric Materials, Grand AF,

Wilkie CA (eds). Marcel Dekker, Inc. NY: NY, 2000; 353389.52. Gilman JW, Ritchie SJ, Kashiwagi T, Lomakin SM. Fire-retardant additives for polymeric materials I. Char

formation from silica gel-potassium carbonate. Fire and Materials 1997; 21:2332.53. Linteris GT, Knyazev VD, Babushok VI. Inhibition of premixed methane ames by manganese and tin compounds.

Combustion and Flame 2002; 129:221238.54. Linteris GT, Katta VR, Takahashi F. Experimental and numerical evaluation of metallic compounds for suppressing

AN OVERVIEW OF FLAME RETARDANCY OF POLYMERIC MATERIALS 279cup-burner ames. Combustion and Flame 2004; 138:7896.55. http://www.nfpa.org/categoryList.asp?categoryID=951&URL=Research/Fire%20statisticsCopyright 2012 John Wiley & Sons, Ltd. Fire Mater. 2013; 37:259279DOI: 10.1002/fam