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Flame retardants for electrical equipment

Introduction

Accurate figures are difficult to obtain but it has been estimated that fires killup to 100,000 people annually worldwide, with more than 4000 in theEuropean Union (EU). It is also estimated that 10% of fires are attributed toelectrical faults, due to mains wiring in buildings and in electrical equipment,with these fires accounting for 19% of injuries. Flame retardants are used veryeffectively to prevent fires, reduce their seriousness and also to delay onset toallow people more time to evacuate. US research has shown that flameretardants can give as much as 15 times longer to escape than plasticswithout flame retardants. Since they were introduced, 1000s of lives have

been saved and so there is no doubt about their value. Figures for the UKestimate that 3000 lives were saved between 1988 and 2000 as a direct resultof using flame retardants.

Why are flame retardants used?

Flame retardants are used as additives to plastics in a wide variety ofelectrical equipment. Many types of plastics burn very easily and it has beenestimated that the plastics in a typical TV set are equivalent to 6 litres ofpetrol, not something consumers want in their living rooms! However, onlyaround 12% of plastics contain flame retardants. Some types are inherently

resistant to fire, such as rigid PVC, and so do not need flame retardantadditives. Some equipment is not at risk such as battery powered productslike mobile phones because of the low voltages used, and therefore flameretarded plastics are not needed. Mobile phone battery chargers however doneed to have flame retardants as they are powered at mains voltages and soarcing and high temperatures can occur if there is a defect.

Equipment manufacturers use flame retardants mainly because they arerequired to by legislation. In the EU, the General Product Safety Directive andthe Low Voltage Directive require products to be safe and this includes notcreating a risk of fire to consumers or business users. Compliance with thesedirectives is through many different mandatory Standards which are written forspecific types of equipment. Fire safety is usually specified by standards fromthe Underwriters Laboratory (UL) referred to as UL94 which has various fireprotection levels such as V0, 1, 2 or HB. IT and telecom equipment in the EUusually complies with UL94 V0 whereas consumer goods such as televisionsneed to meet UL94 HB which is a considerably lower level of protection thanUL94 V0. Televisions and other consumer products in the USA must meetUL94 V0 which explains why TV fires in US are far less common than inEurope.


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What types are available? (refer to glossary)

There are well over 100 different types of flame retardants available includingmore than 70 brominated flame retardants. The main types and theircharacteristics are:

Brominated flame retardants these account for 21% by weight(32% by value) of all flame retardants worldwide and are suitable formany types of plastic, and are the only types that can be used withHIPS and ABS. PBT is used for connectors and is also difficult to flameretard effectively without brominated compounds. This family includesreactive flame retardants that react with other ingredients in plastics tocreate brominated polymers which are very stable and do not leach outinto the environment. Tetrabromobisphenol A (TBBP-A) is used in thelargest quantities of the brominated flame retardants with 90% beingused reactively in FR4 epoxy resin PCB laminates. Most of theremainder is used as an additive flame retardant in ABS where it has

replaced octabromodiphenyl ether which is now banned in the EU andsome US States. Brominated compounds together with some of thephosphorous based compounds are the most effective flame retardantsand so can be used at the lowest concentrations. However, brominatedflame retardants are under pressure from environmentalists. Thereasons are complex; very few are harmful and a very small numberare classified as toxic such as HBCDD. Many are persistent whichmeans that if they escape into the environment, they remain there for along time. Some have been found in womens breast milk and also inPolar Bears. This would not be a cause for concern if they are totallyharmless but research on many chemicals is incomplete and so thepossibility that they might have some toxicity cannot always be ruledout. Another concern where real harm does occur is due touncontrolled recycling of electronic scrap in Asia and Africa. If thesematerials are recycled by modern, well-controlled processes no harmfulby-products are produced. However, in Asia and Africa, backyardrecycling is carried out to recover metals by burning on open fires. Thiscreates very toxic brominated furans and dioxins which arecarcinogens and so poison the local populations. Efforts to preventthese practices have so far been unsuccessful and so EnvironmentalGroups are demanding that these materials are banned. This however

would not completely solve this problem as burning all types of plasticproduces toxic emissions, the most toxic being polycyclic aromatichydrocarbons (PAH) which are also carcinogens. Also, the toxicity offumes from burning organophosphorous flame retarded plastics is notknown.

Antimony oxide this is a synergistic flame retardant which is alwaysused with either brominated flame retardants, chlorinated flameretardants or chlorinated polymers such as PVC. It is very effective atfairly low concentrations so that the amounts of brominated flameretardant used can be halved. Typically about 3% antimony oxide canreplace about 10% of brominated flame retardant.

Phosphorous based flame retardants these account for about 14%by weight of flame retardants used worldwide. These are increasingly


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used as replacements for brominated flame retardants and can beequally effective in some types of polymer but are not suitable for HIPSor ABS. HIPS and ABS can however be replaced by other plasticssuch as ABS/PC which can use phosphorous compounds. Most areregarded as being non-hazardous although a few older types have

been found to have some hazardous properties. However, mostorganophosphorous based flame retardants are relatively new andhave not been thoroughly tested. In general they are less persistent inthe environment than brominated flame retardants but more testing isrequired to determine whether they pose a risk to health or theenvironment. One constraint on the potential for replacing brominatedcompounds by phosphorous compounds is the limitations in worldwideproduction capacity, as this is far less than would be required toreplace all brominated flame retardants.

Chlorinated the main types are chlorinated paraffins. These areavailable as short chain, medium chain and long chain. Short chain are

PBTs (persistent, bioaccumulate and toxic) and endocrine disruptors(cause sex changes in aquatic animals) and are already restricted inthe EU. Medium chain have similar toxicity and the EU is planning torestrict this material. The risks from long chain chlorinated paraffins areas yet not known. The use of these flame retardants has declinedsignificantly over recent years due to concerns over their toxicity. Theirmain use is as additives to PVC in which they provide both plasticisingproperties and flame retardancy.

Metal hydroxides the most common of these is alumina trihydrate(ATH) which accounts for 43% by weight of all flame retardants

worldwide and an even higher percentage in Europe (>50%). It can beused in many types of plastic and is one of the cheapest options.However to achieve UL94 V0 requires very high loadings so thatplastics are more ATH than polymer with 60% ATH typically beingused. High loadings of ATH, which is a white powder, can bedetrimental to the plastics physical properties and colour. It is used inpolyethylene wire insulation to give a material which emits very littlesmoke in fires and so is safe to use in enclosed environments such asin tunnels and in vehicles. Magnesium hydroxide is more expensive butit can be used at lower loadings.

Others there are several other types of flame retardant used in

smaller quantities which are suitable for certain types of plastic. Theseinclude melamine based compounds, borates and stannates.

How do flame retardants work?

To understand how flame retardants work, it is necessary to consider whathappens in a fire. Fires start when solid materials are exposed to heat. Thiscauses decomposition to smaller molecules which include flammable gases.

These gases mix with oxygen from the air and, if the temperature is highenough, they react very rapidly giving out more heat which continues theprocess. The chemical reactions between oxygen and flammable gases

involve charged molecules called free-radicals H+and OH- which areessential for the reaction with oxygen molecules.


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Flame retardants function in several different ways depending on the type.

The main types are:Halogenated flame retardants, when heated these emit Br- or Cl- as freeradicals which scavenge the H+and OH- free radicals and so effectively stop

the chemical reactions that create the high temperatures that cause the fire tocontinue. Brominated flame retardants are more effective than chlorinatedcompounds and are even more effective when combined with antimonycompounds which create volatile antimony bromides.Antimony oxide is not a flame retardant on its own and is always used withorganohalogen compounds. It allows the quantity of brominated flameretardant to be halved and is also used in plasticised PVC. In fires, it releasesantimony bromide or antimony chloride which are very effective free radicalscavengers.Phosphorous flame retardants function in a completely different way. Whensubjected to heat, they react to form polymeric phosphates that form a hard

glassy layer on the surface of the plastic. This inhibits access of oxygen to thecombustible material and prevents flammable gases from being released. Asa result, the plastic chars rather than burning.Metal hydroxides such as aluminium and magnesium hydroxides haveseveral effects. When heated, they release large volumes of water vapourwhich inhibits oxygen access to the surface and dilutes any releasedflammable gases. Also, they decompose endothermically which means thatthey adsorb heat, thereby cooling the material.Nitrogen based flame retardants such as melamine compounds. Themechanism of fire retarding is not fully understood and probably is acombination of effects. Inert nitrogen is released and stable barrier layers ofchar are formed on material surfaces.Intumescent materials are chemicals that produce a bulky porous ceramiccoating that covers the material surface preventing it from burning. Thesematerials release large amounts of inert gases and a thick viscous liquidwhich together form a foam. This foam loses its organic constituents when hotto leave a hard ceramic foam coating.Others Boron compounds function in a similar way as phosphorous flameretardants, Zinc borate works by a variety of mechanisms and zinc and tincompounds are added to PVC to reduce smoke emission and increase theeffectiveness of other types of flame retardant.

Flame retardant selection and design implications

Most equipment is designed without considering which type of flame retardantwill be incorporated and in most products, manufacturers are unaware ofwhich ones are used. Equipment is designed by selecting various parts whichare available in certain types of plastic and which meet the requiredregulations. However, it is possible to design equipment which complies withfire regulations and national safety standards with little or no fire retardant andsome manufacturers are adopting these ideas to minimise the use of

additives. The flammability of plastics varies considerably with some typesbeing highly flammable and there are others that will not sustain a fire. This is


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related to the amount of oxygen from the air that the plastic needs to burn andis referred to as the Limiting Oxygen Index or LOI. Air contains 21% oxygenand any plastic with a LOI value less than 21 will burn. The lower the LOIvalue, the more flammable the plastic. The table below gives some examples:

Plastic type LOIAcrylic 17Polypropylene (PP) 17Polyethylene (PE) 17Polystyrene (PS) 18Polycarbonate (PC) 26Polyphenylene sulphone (PPS) 34Polyvinyl chloride (PVC) 45Polytetrafluoroethylene (PTFE) >95

Plastics with LOI values below 21 may need flame retardants to meet safetystandards. However, plastics with higher values may also require flameretardants if they contain flammable additives that lower their LOI. Forexample, rigid PVC does not require flame retardants but plasticised PVC,used for cable insulation will require flame retarding additives, and antimonyoxide is often used. This is because the additives themselves are flammablematerials.

The types of plastics with high LOI may either not require flame retardantadditives or require smaller amounts than plastics with low LOI values. Thiscan be helpful to maintain the physical properties of the plastic such asstrength and colour. Most high LOI plastics are more expensive than low LOI

plastics, the exception being PVC. PVC is however under pressure fromenvironmental groups because toxic dioxins and furans are produced whenPVC is burned using unsuitable and uncontrolled recycling which is,unfortunately, fairly common in some parts of Asia and Africa. Variousfluorinated polymers are used as alternative wire and cable insulation asthese do not require flame retardant additives and they are more flexible andcan be used at higher temperatures than the other alternatives to PVC (flameretarded PE is a common choice). When fluorinated polymers burn however,they emit highly toxic gases.Design options One of the largest uses of flame retardants is in theenclosures of equipment. When these are adjacent to high voltage

components, safety standards specify that these must not support combustionand so most of the plastics used for enclosures need to contain flameretardant additives. However, changing the design of equipment can avoid theneed to use flame retardants. Some manufacturers now use metal enclosuresas these do not require flame retardants and also these are easier to recycleat end of life. If an inner metal enclosure is used, a decorative outer plasticenclosure will not require flame retardants if this is not at risk from fire. Thiscombination can be used where the appearance is important.

2008 Premier Farnell plc. Permission is granted for reproduction in

whole, or in part, provided Premier Farnell is credited.Written in collaboration with ERA Technology Ltd (www.era.co.uk/rfa)


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GlossaryABS Acetonitrile Butadiene styreneHIPS High impact polystyrene

PAH Polycyclic aromatic hydrocarbonsPC PolycarbonatePE PolyethylenePP PolypropylenePPS Polyphenylene sulphone

TBBPA Tetrabromobisphenol AHBCDD - HexabromocyclododecanePBT Polbutyl terephthalate or Persistent, Bioaccumulative and Toxic.ATH Alumina tri-hydrate

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