33

Research Papers

1. B.R. Manjunath, P. Sadasivamurthy, P.V. Reddy, Karickal

R. Haridas, ‘Studies on improving performance of PVC

compositions for electrical cable sheathing

applications’, The Chemist, 2012. [Accepted (in

press)].

2. B.R. Manjunath, P. Sadasivamurthy, P.V. Reddy, Karickal

R. Haridas, ‘Studies on cenospheres as fillers for PVC

compounds for applications in electrical cables’, The

Chemist, 2012. [Communicated].

3. Presented paper ‘Cenospheres as possible fillers for

PVC compounds in electrical cable industry’ at 24th

Kerala Science Congress, Kottayam, Kerala, 2012; the

poster was conferred the Best Poster Award in

Chemical Sciences section.

4. Presented paper ‘Investigation of cenospheres as

possible fillers for PVC compounds in electrical cable

industry’, at National Seminar on Social Relevance of

Chemical Sciences, Kuvempu University, Shimoga,

Karnataka, 2011.

34

“I am inclined to think that the development of polymerization is, perhaps, the biggest

thing Chemistry has done, where it has had the biggest effect on everyday life. The world would be a totally different place

without artificial fibers, plastics, elastomers, etc. Even in the field of electronics, what

would you do without insulation? And there, you come back to polymers again!”.

Lord Todd, President of the Royal Society of London,

in answer to the question, ‘What do you think has been Chemistry’s biggest contribution to

Science, to Society?’

[Quoted in Chemical Engg. News, 58 (40) Pp. 29, 1980].

35

Chapter 1

INTRODUCTION

36

1.1. Introduction1.2. Types of flame-retardants1.3. Mechanism of action of flame-retardants1.4. Performance criteria for and choice of flame-retardants1.5. Production and uses of flame-retardants and flame-

retarded polymers1.6. Plasticizers1.7. Formation of toxic products on heating or combustion

of flame-retarded products1.8. Overview of exposure and hazards to humans and the

environment1.9. Regulations with respect to flame-retardants1.10. Recommendations for the protection of human health

and the environment1.11. Further research

37

1.1. Introduction

Accidental fire is an ever-present hazard. In present-day living, there

is a rapidly increasing development in the size and number of buildings,

skyscrapers, warehouses and methods of transport. Carpeting, furnishings,

equipment, increased presence of electrical cables, oil and gas for heating

and so on, all increase the fire load in a building. New technologies, new

processes and new applications introduce new fire hazards (e.g., new ignition

sources such as welding sparks and short circuits) [1]. Though modern fire-

fighting techniques, equipment and building design reduce the destruction

due to fires, a high fuel load in either a residential or a commercial building

can offset even the best of building constructions [2(i)]. There is an ever-

existing need to improve upon the flame-retardance of the basic material, the

polymer, used in various items, ranging from electrical cables to wall

coverings to clothes to furniture in buildings.

Each year, over three million fires leading to over 1,00,000 injuries

and 15,000 deaths are reported worldwide. The direct property losses exceed

$8 billion and the total annual cost has been estimated at over $100 billion.

Personal losses occur mostly in residences where furniture, wall coverings

and clothes are frequently the fuel. Large financial losses occur in

commercial structures such as office buildings and warehouses. Fires also

occur in aeroplanes, buses and trains [2(ii)].

In order to provide additional protection from fires and to increase

escape time when a fire occurs, methods to enhance the flame-retardance of

consumer goods have been developed. Flame-retardants are chemicals

added to polymeric materials, both natural and synthetic, to enhance flame-

retardance properties. They may be physically blended with or chemically

38

bonded to the host polymer. Generally, they either lower ignition

susceptibility or make the flame spread slower, once ignition has occurred.

Flame-retardant systems for synthetic or organic polymers act in five

basic ways: (i) gas dilution; (ii) thermal quenching; (iii) protective coating;

(iv) physical dilution; and (v) chemical interaction [3]; or through a

combination of these mechanisms.

1. Inert gas dilution involves using additives that produce large volumes

of non-combustible gases on decomposition. These gases dilute the

oxygen supply to the flame or dilute the fuel concentration below the

flammability limit. Metal hydroxides, metal salts and some nitrogen

compounds function in this way.

2. Thermal quenching is the result of endothermic decomposition of the

flame-retardant. Metal hydroxides, metal salts and nitrogen

compounds act to decrease surface temperature and the rate of

burning.

3. Some flame-retardants form a protective liquid or char barrier. This

limits the amount of polymer available to the flame front and/or acts

as an insulating layer to reduce the heat transfer from the flame to the

polymer. Phosphorus compounds and intumescent systems based on

melamine and other nitrogen compounds are examples of this

category.

4. Inert fillers (glass fibers and microspheres) and minerals (talc) act as

thermal sinks to increase the heat capacity of the polymer or reduce

its fuel content.

5. Halogens and some phosphorus flame-retardants act by chemical

interaction. The flame-retardant dissociates into radical species that

compete with chain-propagating steps in the combustion process.

39

Chemicals that are used as flame-retardants can be inorganic, organic,

mineral, halogen-containing or phosphorus-containing. The term ‘flame-

retardant’ represents a class of use and not a class of chemical structure [3].

Preventive flame protection, including the use of flame-retardants,

has been practiced since ancient times. Some examples of early historical

developments in flame-retardants are shown in Table 1.1 [4].

Table 1.1. Early historical fire-retardant developments

Development Date

Alum used to reduce the flammability of wood by the EgyptiansThe Romans used a mixture of alum and vinegar on woodMixture of clay and gypsum used to reduce flammability of theatre curtainsMixture of alum, ferrous sulfate and borax used on wood and textiles by Wyld in BritainAlum used to reduce flammability of balloonsGay-Lussac reported a mixture of (NH4)3PO4, NH4Cl and borax to be effective on linen and hempPerkin described a flame-retardant treatment for cotton using a mixture of sodium stannate and ammonium sulfate

About 450 BCAbout 200 BC

1638

173517831821

1912

The advent of synthetic polymers in the last century was of special

significance, since the water-soluble inorganic salts in use up to that time

were of little or no utility in these largely hydrophobic materials. This led to

modern research concentrating on the development of polymer-compatible

flame-retardants.

With the increasing use of thermoplastics and thermosets on a large

scale for applications in electrical engineering and electronics, building,

transportation, new flame-retardant systems came to be developed. They

40

mainly consisted of inorganic and organic compounds based on bromine,

chlorine, phosphorus, nitrogen, boron, and metallic oxides and hydroxides.

Today, there are flame-retardant systems developed to fulfill the

multiple flammability requirements of the above-mentioned applications.

1. 2. Types of flame-retardants

A distinction is made between reactive and additive flame-retardants.

Reactive flame-retardants are reactive components, chemically built into a

polymer molecule. Additive flame-retardants are incorporated into the

polymer either prior to, during or (most frequently) following

polymerization.

There are three main families of flame-retardant chemicals [1,2(ii)-

(iv),5,6].

1. The main inorganic flame-retardants are aluminum trihydroxide

(ATH), antimony trioxide, magnesium hydroxide, ammonium

polyphosphate and red phosphorus. This group represents about

50% by volume of the worldwide flame-retardant production.

Some of these chemicals are also used as flame-retardant

synergists, of which antimony trioxide is the most important [7].

2 Halogenated products are based primarily on chlorine and bromine.

This group represents about 25% by volume of the worldwide

production [8].

3. Organophosphorus products are primarily phosphate esters and

represent about 20% by volume of the worldwide production.

Products containing phosphorus, chlorine and/or bromine are also

important.

In addition, nitrogen-based flame-retardants are used for a limited

number of polymers.

41

1.2.1. Inorganic flame-retardants

Very few inorganic compounds are suitable for use as flame-

retardants in plastics. This is because they are usually too inert to be

effective in the range of decomposition temperatures of plastics (between

150 and 400 oC). One major disadvantage of inorganic flame-retardants is

hygroscopicity- this is sometimes sought to be overcome by adding fillers

such as clay which reduce water absorption.

Metal hydroxides form the largest class of all flame-retardants used

commercially today and is employed alone or in combination with other

flame-retardants to achieve necessary improvements in flame-retardancy.

Antimony compounds are used as synergistic co-additives in combination

with halogen compounds, facilitating a cut in total flame-retardant levels

needed to achieve a desired level of flame-retardancy. To a limited extent,

compounds of other metals also act as synergists with halogen compounds.

They may be used alone but are most commonly used with antimony trioxide

to enhance other characteristics, for example, smoke reduction or afterglow

suppression. Ionic compounds have a very long history as flame-retardants

for wool- or cellulose-based products. Inorganic phosphorus compounds are

primarily used in polyamides and phenolic resins, or as components in

intumescent formulations.

1.2.1.1. Metal hydroxides

Metal hydroxides function in both the condensed and gas phases of a

fire by absorbing heat and decomposing to release their water of hydration.

This process cools both the polymer and the flame and dilutes the flammable

gas mixture. The very high concentrations (50 to 80%) required to impart

flame-retardancy often adversely affect the mechanical properties of the

polymer into which they are incorporated.

42

ATH is the largest volume flame-retardant in use today. It

decomposes when exposed to temperatures over 200°C, which limits the

polymers into which it can be incorporated. Magnesium hydroxide is stable

to temperatures above 300°C and can be processed into several polymers.

1.2.1.2. Antimony compounds

Antimony trioxide is not a flame-retardant per se, but is used as a

synergist. It is utilized in plastics, rubbers, textiles, paper and paints,

typically 2-10% by weight, with organochlorine and organobromine

compounds to diminish the flammability of a wide range of plastics and

textiles [9].

Antimony oxides and antimonates must be converted to volatile

species. This is usually accomplished by release of halogen acids at fire

temperatures. The halogen acids react with the antimony-containing

materials to form antimony trihalide and/or antimony halide/oxide. These

materials act both in the substrate (condensed phase) and in the flame to

suppress flame propagation. In the condensed phase, they promote char

formation, which acts as a physical barrier to flame and inhibits the

volatilization of flammable materials. In the flame, the antimony halides and

halide oxides, generated in sufficient volume, provide an inert gas blanket

over the substrate, thus excluding oxygen and preventing flame spread.

These compounds alter the chemical reactions occurring at fire temperatures

in the flame, thus reducing the ease with which oxygen can combine with the

volatile products. It is also suggested that antimony oxychloride or

trichloride reduces the rate at which the halogen leaves the flame zone, thus

increasing the probability of reaction with the reactive species. Antimony

trichloride probably evolves heavy vapors which form a layer over the

condensed phase, stop oxygen attack and thus choke the flame. It is also

43

assumed that the liquid and solid antimony trichloride particles contained in

the gas phase reduce the energy content of the flames by wall or surface

effects [10].

Other antimony compounds include antimony pentoxide, available

primarily as a stable colloid or as a redispersible powder. It is designed

primarily for highly specialized applications, although manufacturers suggest

it has potential use in fiber and fabric treatment. Sodium antimonate is

recommended for formulations in which deep tone colors are required or

where antimony trioxide may promote unwanted chemical reactions.

1.2.1.3. Boron compounds

Within the class of boron compounds, by far the most widely used is

boric acid. Boric acid (H3BO3) and sodium borate (borax) (Na2B4O7.10H2O)

are the two flame-retardants with the longest history, and are used primarily

with cellulosic material, e.g., cotton and paper. Both products are effective,

but their use is limited to products for which non-durable flame-retardancy is

acceptable since both are very water-soluble.

Zinc borate, however, is water-insoluble and is mostly used in

plastics and rubber products. It is used either as a complete or partial

replacement for antimony oxide in PVC, nylon, polyolefin, epoxy, EPDM,

etc. In most systems, it displays synergism with antimony oxide. Zinc

borate can function as a flame-retardant, smoke-suppressant and anti-arcing

agent in condensed phase. Recently, zinc borate has also been used in

halogen-free, fire-retardant polymers.

1.2.1.4. Other metal compounds

Molybdenum compounds have been used as flame-retardants in

cellulosic materials for many years and more recently with other polymers,

mainly as smoke-suppressants [1]. They appear to function as condensed-

44

phase flame-retardants [11]. Titanium and zirconium compounds are used

for textiles, especially wool [12].

Zinc compounds, such as zinc stannate and zinc hydroxy-stannate,

are also used as synergists and as partial replacements for antimony trioxide.

1.2.1.5. Phosphorus compounds

Red phosphorus and ammonium polyphosphate (APP) are used in

various plastics. Red phosphorus was first introduced in polyurethane foams

and found to be very effective as a flame-retardant. It is now used

particularly for polyamides and phenolic applications. The flame-retarding

effect is due, in all probability, to the oxidation of elemental phosphorus

during the combustion process to phosphoric acid or phosphorus pentoxide.

The latter acts by the formation of a carbonaceous layer in the condensed

phase. The formation of fragments that act by interrupting the radical chain

mechanism is also likely.

Ammonium polyphosphate is mainly applied in intumescent coatings

and paints. Intumescent systems puff up to produce foams. Because of this

characteristic, they are used to protect materials such as wood and plastics

that are combustible and those like steel that lose their strength when

exposed to high temperatures. Intumescent agents have been available

commercially for many years and are used mainly as fire-protective coatings.

They are now used as flame-retardant systems for plastics by incorporating

the intumescent components in the polymer matrix, mainly polyolefins,

particularly polypropylene [1].

1.2.1.6. Other inorganic flame-retardants

Other inorganic flame-retardants, including ammonium sulfamate

(NH4SONH2) and ammonium bromide (NH4Br), are used primarily with

cellulose-based products and in forest fire-fighting [5].

45

1.2.2. Halogenated organic flame-retardants

Halogenated flame-retardants can be divided into three classes:

aromatic, aliphatic and cycloaliphatic. Bromine and chlorine compounds are

the only halogen compounds having commercial significance as flame-

retardant chemicals. Fluorine compounds are expensive and, except in

special cases, are ineffective because the C-F bond is too strong. Iodine

compounds, although effective, are expensive and too unstable to be useful

[2(ii), 13]. The brominated flame-retardants are much more numerous than

the chlorinated types because of their higher efficacy [14].

With respect to processability, halogenated flame-retardants vary in

their thermal stability. In general, brominated aromatic flame-retardants are

thermally more stable than chlorinated aliphatics, which, in turn, are

thermally more stable than brominated aliphatics. Brominated aromatic

compounds can be used in thermoplastics at fairly high temperatures without

the use of stabilizers and at very high temperatures with stabilizers. The

thermal stability of the chlorinated and brominated aliphatics is such that,

with few exceptions, they must be used with thermal stabilizers, such as a tin

compound.

Halogenated flame-retardants are either added to or reacted with the

base polymer. Additive flame-retardants are those that do not react in the

application designated. There are a few compounds that can be used as an

additive in one application and as a reactive in another; tetrabromobisphenol

A is the most notable example. Reactive flame-retardants become a part of

the polymer either by becoming a part of the backbone or by grafting onto

the backbone. The choice of a reactive flame-retardant is more complex than

the choice of an additive type. The development of systems based on

reactive flame-retardants is more expensive for the manufacturer, who in

effect has to develop novel co-polymers with the desired chemical, physical

46

and mechanical properties, as well as the appropriate degree of flame-

retardance [2(i),13]. Synergists such as antimony oxides are frequently used

with halogenated flame-retardants.

1.2.2.1. Brominated flame-retardants

Bromine-based flame-retardants are highly brominated organic

compounds with a relative molecular mass ranging from 200 to that of large

molecule polymers. They usually contain 50 to 85% (by weight) of bromine

[14].

The highest volume brominated flame-retardant in use today is

tetrabromobisphenol-A (TBBPA) [15], followed by decabromodiphenyl ether

(DeBDE) [16]. Both of these flame-retardants are aromatic compounds.

The primary use of TBBPA is as a reactive intermediate in the production of

flame-retarded epoxy resins, used in printed circuit boards [15]. A secondary

use for TBBPA is as an additive flame-retardant in ABS systems. DeBDE is

the second largest volume brominated flame-retardant and is the largest

volume brominated flame-retardant used solely as an additive. The greatest

use (by volume) of DeBDE is in high-impact polystyrene, which is primarily

used to produce television cabinets. Secondary uses include ABS,

engineering thermoplastics, polyolefins, thermosets, PVC and elastomers.

DeBDE is also widely used in textile applications as the flame-retardant in

latex-based back coatings [2(ii)].

Hexabromocyclododecane (HBCD), a major brominated

cycloaliphatic flame-retardant, is primarily used in polystyrene foam. It is

also used to flame-retard textiles.

1.2.2.2. Chlorinated flame-retardants

Chlorine-containing flame-retardants belong to three chemical

groups: aliphatic, cycloaliphatic and aromatic compounds. Chlorinated

47

paraffins are by far the most widely used aliphatic chlorine-containing flame-

retardants. They have applications in plastics, fabrics, paints and coatings

[17].

Bis(hexachlorocyclopentadieno)cyclo-octane is a flame-retardant

having unusually good thermal stability for a chlorinated cycloaliphatic. In

fact, this compound is comparable in thermal stability to brominated

aromatics in some applications. It is used in several polymers, especially

polyamides and polyolefins for wire and cable applications. Its principal

drawback is the relatively high use levels required, compared to some

brominated flame-retardants [2(ii)].

1.2.3. Organophosphorus flame-retardants

One of the principal classes of flame-retardants used in plastics and

textiles is that of phosphorus, phosphorus-nitrogen and phosphorus-halogen

compounds. Phosphate esters, with or without halogen, are the predominant

phosphorus-based flame-retardants in use.

For textiles, phosphorus-containing materials are by far the most

important class of compounds used to impart durable flame-resistance to

cellulose. These textiles flame-retardant finishes usually also contain

nitrogen or halogen, or sometimes both [5,12].

1.2.3.1. Non-halogenated compounds

Although many phosphorus derivatives have flame-retardant

properties, the number of those with commercial importance is limited. Some

are additive and some, reactive. The major groups of additive

organophosphorus compounds are phosphate esters, polyols, phosphonium

derivatives and phosphonates. The phosphate esters include trialkyl

derivatives such as triethyl or trioctyl phosphate, triaryl derivatives such as

48

triphenyl phosphate and aryl-alkyl derivatives such as 2-ethylhexyl-diphenyl

phosphate.

The flame-retardancy of cellulosic products can be improved through

the application of phosphonium salts. The flame-retardant treatments

attained by phosphorylation of cellulose in the presence of a nitrogen

compound are also of importance [12].

Plasticizers are mixed into polymers to increase flexibility and

workability. The esters formed by reaction of the three functional groups of

phosphoric acid with alcohols or phenols are excellent plasticizers. The

phosphoric acid esters are also remarkable flame-retardants, and for this

reason are extensively used in plastics [17].

Aryl phosphate plasticizers are used in PVC-based products. They

are also used as lubricants for industrial air compressors and gas turbines.

Miscellaneous uses of aryl phosphates are as pigment dispersants and

peroxide carriers, and as additives in adhesives, lacquer coatings and wood

preservatives [18].

1.2.3.2. Halogenated phosphates

In addition to the above types, flame-retardants containing both

chlorine and phosphorus or bromine and phosphorus are used widely.

Halogenated phosphorus flame-retardants combine the flame-

retardant properties of both the halogen and the phosphorus groups. In

addition, the halogens reduce the vapor pressure and water solubility of the

flame-retardant, thereby contributing to the retention of the flame-retardant

in the polymer.

One of the largest selling members of this group, tris(1-chloro-2-

propyl) phosphate (TCPP) is used in polyurethane foam. Tris(2-chloroethyl)

phosphate is used in the manufacture of polyester resins, polyacrylates,

49

polyurethanes and cellulose derivatives. The most widely used bromine- and

phosphorus-containing flame-retardant used to be tris(2,3-

dibromopropyl)phosphate, but it was withdrawn from use in many countries

due to carcinogenic properties in animals [2(iii),18].

1.2.4. Nitrogen-based flame-retardants

Nitrogen-based compounds can be employed in flame-retardant

systems or form part of intumescent flame-retardant formulations. Nitrogen-

based flame-retardants are used primarily in nitrogen-containing polymers

such as polyurethanes and polyamides. They are also utilized in PVC and

polyolefins and in the formulation of intumescent paint systems [19].

Melamine, melamine cyanurate, other melamine salts and guanidine

compounds are currently the most used group of nitrogen-containing flame-

retardants. Melamine is used as a flame-retardant additive for polypropylene

and polyethylene. Melamine cyanurate is employed commercially as a

flame-retardant for polyamides and terephthalates (PET/PBT) and is being

developed for use in epoxy and polyurethane resins. Melamine phosphate is

also used as a flame-retardant for terephthalates (PET/PBT) and is currently

being developed for use in epoxy and polyurethane flame-retardant

formulations. Also in the development stages for use as flame-retardant

additives are melamine salts and melamine formaldehyde for their

application in thermoset resins [20].

1.2.5. Requirements of an ideal flame-retardant

Following are some of the requirements of an ideal flame-retardant:

1. It should be compatible with the base polymer.

2. It should be easy to incorporate.

3. It should not alter the mechanical properties of the polymer.

50

4. It should not bloom or bleach and possess good resistance to aging.

5. It must be stable at processing and service temperatures.

6. It should be effective in small quantities and must be non-

corrosive.

7. It must be odor-free and free from harmful effects on human

physiology and environment, and

8. It must emit low smoke and must be cost-effective.

1.3. Mechanism of action of flame-retardants

The mechanism of action of flame-retardants and smoke suppressants

is indeed quite complex. However, a general outline of the same is given in

the ensuing paragraphs.

1.3.1. General aspects

To understand flame-retardants, it is necessary to first understand

fire. Fire is a gas-phase reaction. Thus, in order for a substance to burn, it

must become a gas. In the case of a candle, the wax melts and migrates up

the wick by capillary action. The wax is pyrolysed to volatile hydrocarbon

fragments on the wick's surface at 600-800°C. There is no oxygen at the

nucleus of the flame. Some of the hydrocarbon fragments aromatize to soot

particles and, in the luminescent region of the flame, react with water and

carbon dioxide to form carbon monoxide. Most of the pyrolysis gases are

carried to the exterior of the flame and encounter oxygen diffusing inwards.

They react exothermically to produce heat, which melts and decomposes

more wax, maintaining the combustion reaction. If there is adequate oxygen,

the combustion products from the candle are carbon dioxide and water [21].

Natural and synthetic polymers can ignite on exposure to heat.

Ignition occurs either spontaneously or results from an external source such

51

as a spark or flame. If the heat evolved by the flame is sufficient to keep the

decomposition rate of the polymer above that required to maintain the

evolved combustibles within the flammability limits, then a self-sustaining

combustion cycle will be established, figure 1.1.

Figure 1.1. The combustion process

This self-sustaining combustion cycle occurs across both the gas and

condensed phases. Fire-retardants act to break this cycle by affecting

chemical and/or physical processes occurring in one or both of the phases.

There are a number of ways in which the self-sustaining combustion cycle

can be interrupted. Whatever the method used, the end effect is to reduce the

rate of heat transfer to the polymer and thus remove the fuel supply.

Troitzsch [1] describes the general physical and chemical mechanisms of

flame-retardant action, in both the gas and condensed phases and the

behavior of flame-retardants. Fundamentally, four processes are involved in

polymer flammability: preheating, decomposition, ignition and

combustion/propagation. Preheating involves heating of the material by

means of an external source, which raises the temperature of the material at a

rate dependent upon the thermal intensity of the ignition source, the thermal

conductivity of the material, the specific heat of the material, and the latent

heat of fusion and vaporization of the material. When sufficiently heated, the

52

material begins to degrade, i.e., it loses its original properties as the weakest

bonds begin to break. Gaseous combustion products are formed, the rate

being dependent upon such factors as intensity of external heat, temperature

required for decomposition, and rate of decomposition.

The concentration of flammable gases increases until it reaches a

level that allows sustained oxidation in the presence of the ignition source.

The ignition characteristics of the gas and the availability of oxygen are two

important variables in any ignition process. After ignition and removal of the

ignition source, combustion becomes self-propagating if sufficient heat is

generated and is radiated back to the material to continue the decomposition

process. The combustion process is governed by such variables as rate of

heat generation, rate of heat transfer to the surface, surface area, and rates of

decomposition. Flame-retardancy, therefore, can be achieved by eliminating

(or improved by retarding) any one of these variables. A flame-retardant

should inhibit or even suppress the combustion process. Depending on their

nature, flame-retardants can act chemically and/or physically in the solid,

liquid or gas phase. They interfere with combustion during a particular stage

of this process, i.e. during heating, decomposition, ignition or flame spread

[1].

1.3.1.1. Physical action

There are several ways in which the combustion process can be

retarded by physical action [1].

(a) By cooling. Endothermic processes triggered by additives cool the

substrate to a temperature below that required to sustain the

combustion process.

(b) By formation of a protective layer (coating). The condensed

combustible layer can be shielded from the gaseous phase with a

53

solid or gaseous protective layer. The condensed phase is thus

cooled, smaller quantities of pyrolysis gases are evolved, the oxygen

necessary for the combustion process is excluded and heat transfer is

impeded.

(c) By dilution. The incorporation of inert substances (e.g., fillers) and

additives that evolve inert gases on decomposition and dilute the fuel

in the solid and gaseous phases so that the lower ignition limit of the

gas mixture is not exceeded.

1.3.1.2. Chemical action

The most significant chemical reactions interfering with the

combustion process take place in the solid and gas phases [1]. Usually,

reactions occur in two phases:

(a) Reaction in the gas phase. The free radical mechanism of the

combustion process which takes place in the gas phase is interrupted

by the flame-retardant. The exothermic processes are thus stopped,

the system cools down, and the supply of flammable gases is reduced

and eventually completely suppressed.

(b) Reaction in the solid phase. Here two types of reaction can take

place. Firstly, breakdown of the polymer can be accelerated by the

flame-retardant, causing pronounced flow of the polymer and, hence,

its withdrawal from the sphere of influence of the flame, which

breaks away. Secondly, the flame-retardant can cause a layer of

carbon to form on the polymer surface. This can occur, for example,

through the dehydrating action of the flame-retardant generating

double bonds in the polymer. These form the carbonaceous layer by

cyclizing and cross-linking.

54

Flame-retardancy is improved by flame-retardants that cause the

formation of a surface film of low thermal conductivity and/or high

reflectivity, which reduces the rate of heating. It is also improved by flame-

retardants that might serve as a heat sink by being preferentially decomposed

at low temperature. Finally, it is improved by flame-retardant coatings that,

upon exposure to heat, intumesce into a foamed surface layer with low

thermal conductivity properties. A flame-retardant can promote

transformation of a plastic into char and thus limit production of combustible

carbon-containing gases. Simultaneously, the char will decrease thermal

conductivity of the surface. Flame-retardants can also chemically alter the

decomposition products, resulting in a lower concentration of combustible

gases. Reduced fuel will result in less heat generation by the flame and may

lead to self-extinction.

Structural modification of the plastic, or use of an additive flame-

retardant, might induce decomposition or melting upon exposure to a heat

source so that the material shrinks or drips away from the heat source. It is

also possible to significantly retard the decomposition process through

selection of chemically stable structural components or structural

modifications of a polymer. In general, anything that will prevent the

formation of a combustible mixture of gases will prevent ignition. However,

one may also distinguish those cases in which the flame-retardant or the

modified polymer unit, upon exposure to a heat source, will form gas

mixtures that will react chemically in the gas phase to inhibit ignition. The

goal of flame-retardance in the combustion and propagation stages is to

decrease the rate of heat generated or radiated back to the substrate. Any or

all of the above-mentioned mechanisms could function to prevent a self-

sustaining flame [22].

55

Flame-retardancy occurs both as already stated in the vapor phase (by

interfering with oxidation through removal of free radicals) and in the

condensed phase (charring or altering thermal degradation processes).

Phosphorus acts primarily in the condensed phase by promoting charring,

presumably through the formation of phosphoric acid and a decreased release

of flammable volatiles. However, some reports indicate that certain organic

phosphorus compounds may also work in the gas phase by scavenging free

radicals. Antimony (which functions only in the presence of a halogen) is

believed to work similarly to phosphorus in the condensed phase and

combine with halogens in the gas phase to scavenge free radicals that are

necessary for combustion. The role of nitrogen is not completely

understood. Nitrogen is known to impart flame-retardancy in combination

with phosphorus and also by itself, as in polyamides and aminoplasts.

Bromine and chlorine act in the gas phase by reacting with free radicals [23].

The mechanism for imparting durable flame-retardance to cellulose is that of

increasing the quantity of carbon, or char, formed instead of volatile products

of combustion, and flammable tars. Salts that dissociate to form acids or

bases upon heating are usually effective flame-retardants. Salts of strong

acids and weak bases are the most effective compounds. Ammonium and

amine salts are generally effective, as are Lewis acids and bases, either by

themselves or when formed in combustion.

1.3.2. Condensed phase mechanisms

The role of phosphorus compounds has been extensively studied. In

both cellulose and thermoplastics, phosphorus salts of volatile metals and

most organophosphorus compounds are known to be effective flame-

retardants. The formation of char appears to be the key. For example,

although triphenyl phosphate, triphenyl phosphite and triphenyl phosphine

56

are all equivalent on a phosphorus basis, the more effective flame-retardant

compounds act by forming phosphoric acid, which changes the course of the

decomposition of cellulose to form carbon and water [24].

The flame-retardant action of phosphorus compounds in cellulose is

believed to proceed by way of initial phosphorylation of the cellulose,

probably by initially formed phosphoric or polyphosphoric acid. The

phosphorylated cellulose then breaks down to water, phosphoric acid and an

unsaturated cellulose analogue, and eventually to char by repetition of these

steps. Certain nitrogen compounds such as melamines, guanidines, ureas and

other amides appear to catalyze the steps forming cellulose phosphate and

are found to enhance or synergize the flame-retardant action of phosphorus

on cellulose. In polyethylene terephthalate and polymethyl methacrylate, the

mechanism of action of phosphorus-based flame-retardants has been shown

to involve both a similar decrease in the amount of combustible volatiles and

a similar increase in the amount of residues (aromatic residues and char).

The char formed also acts as a physical barrier to heat and gases. In rigid

polyurethane foams the action of phosphorus flame-retardants also appears

to involve char enhancement. In flexible foam, the mechanism is less well-

understood [25].

1.3.3. Gas-phase mechanisms

In addition to the condensed-phase mechanism, phosphorus flame-

retardants can exert gas-phase flame-retardant action. It has been

demonstrated that trimethyl phosphate retards the velocity of a methane-

oxygen flame with about the same molar efficiency as antimony trioxide

[2(ii)]. The mechanisms of action can differ depending on the type of

compound used as a flame-retardant. The mechanism affects the generation

57

of products of combustion, some of which are potentially corrosive and

toxic.

One of the methods for improving the flame-retardancy of

thermoplastic materials is to lower their melting point. This results in the

formation of free radical inhibitors in the flame front and causes the material

to recede from the flame without burning.

Free radical inhibition involves the reduction of gaseous fuels

generated by burning materials. Heating of combustible materials results in

the generation of hydrogen, oxygen, and hydroxide and peroxide radicals

that are subsequently oxidized with flame. Certain flame-retardants act to

trap these radicals and thereby prevent their oxidation. Bromine is more

effective than chlorine. If the resulting compound is less readily oxidized

than the radical that is removed, the result is reduced flammability.

Measurements of the limiting oxygen index of polymers show that, in

contrast to the situation with chlorine, the effect of bromine does depend on

the gaseous oxidant involved. This suggests that bromine compounds act to

some extent by interfering with the flame reactions and it is generally

believed that this is probably their principal mode of action, although they

can also affect the condensed-phase decomposition of the polymer.

Any gas-phase mechanism of flame-retardancy by bromine

compounds must by definition involve the release of volatile bromine-

containing species, which then inhibit the flame reactions. In the case of

brominated flame-retardants, it is generally assumed that hydrogen bromide

is liberated and reacts with the free radicals responsible for the propagation

of combustion, replacing them by the relatively unreactive bromine atom.

The mechanism operating in a particular polymer system will depend

on the mode and ease of breakdown of the brominated flame-retardant

present. Some of these compounds are thermally stable and volatilize when

58

the associated polymer is heated to sufficiently high temperatures. Others

decompose to give substantial amounts of either lower molecular weight

organic bromine compounds or hydrogen bromide [25,26].

The presence of chemically bound bromine can also affect the rates

and modes of thermal decomposition of organic polymers in the condensed

phase. Brominated flame-retardants vary considerably in both their

volatility and thermal stability. Although some very stable compounds

volatilize chemically unchanged, others break down within the polymer or

react directly with it in the condensed phase. Hydrogen bromide is often a

product and can significantly influence the rate and course of polymer

decomposition, although its effect is small in comparison with those which it

exerts on polymer combustion as a whole. However, even thermally stable

brominated flame-retardants can affect the decomposition of polymers in the

condensed phase, causing the original polymer breakdown stage to be

replaced by two separate stages, both of which involve polymer and additive.

Thus, it is clear that hydrogen bromide is not the only bromine-containing

compound which affects condensed-phase polymer decomposition and that

organic bromine compounds can also markedly change the rate and mode of

breakdown of organic polymers [13].

A critical factor governing the effectiveness of brominated flame-

retardants and indeed their mechanism of action is their thermal stability

relative to that of the polymers with which they are associated. The most

favorable situation for a flame-retardant to be effective will be one in

which its decomposition temperature lies 50°C or so below that of the

polymer. In general, decomposition at this temperature with the liberation of

substantial quantities of hydrogen bromide or elemental bromine is likely to

enhance flame-retardant activity. Owing to the relatively low C-Br bond

energy, bromine compounds generally breakdown at quite low temperatures

59

(typically 200-300°C). Temperatures in this range overlap well with the

decomposition of many common polymers. This is probably a factor

determining the superior flame-retardant effectiveness of bromine

compounds compared with that of chlorine compounds [26].

1.3.4. Co-additives for use with flame-retardants

Brominated flame-retardants are in some cases used on their own, but

their effectiveness is increased by a variety of co-additives, so that in

practice they are more often used in conjunction with other compounds or

with other elements incorporated into them. Thus, for example, the addition

of small quantities of organic peroxides to polystyrene greatly reduces the

amount of hexabromocyclododecane needed to give flame-retardant foam;

other free radical initiators behave in a similar fashion. These compounds

appear to act by promoting depolymerization of the hot polymer, giving a

more fluid melt. More heat is therefore required to keep the polymer alight,

because there is a greater tendency for the more molten material to drip away

from the neighborhood of the flame [1,27]. The flame-retardant properties of

bromine compounds, like those of chlorine compounds, will be considerably

enhanced when they are used in conjunction with other hetero-elements,

notably phosphorus, antimony and certain other metals.

The simultaneous presence of phosphorus in bromine-containing

polymer systems usually serves to improve their degree of flame-retardance,

with bromine and phosphorus exerting effects that are largely additive rather

than synergistic.

Sometimes the two elements are present in the same molecule, e.g.,

tris(2,3,-dibromopropyl)phosphate. In other systems, however, it is more

convenient to use mixtures of a bromine compound and a phosphorus

compound so that the ratio of the two elements can be readily adjusted. It

60

has already been pointed out that brominated flame-retardants on their own

act predominantly in the gas phase. In contrast, phosphorus compounds act

mainly in the condensed phase, especially with oxygen-containing polymers.

It is therefore of interest to discover whether, when both elements are present

together, each continues to act in the usual way or new mechanisms come

into operation. However, the evidence here is somewhat conflicting. Studies

of the effects of phosphate esters, with or without bromine present, on the

combustion of polyesters show that more char is formed when the flame-

retardant contains bromine, and that most of this bromine remains in the

char. This suggests that the bromine-phosphorus compound affects primarily

the condensed-phase processes. However, studies of the flammability of

rigid polyurethane foams show that the inhibiting effect of tris(2,3-

dibromopropyl)- phosphate on combustion depends on the nature of the

gaseous oxidant, suggesting that the flame-retardant acts here, at least in

part, by interfering with reactions in the gas phase. With hydrocarbon

polymers, such as polyolefins and polystyrene, the major part of the

phosphorus present volatilizes and acts in the gas phase, being apparently

converted to simple species, such as phosphorus and phosphorus oxide free

radicals. These species can then interfere chemically with the reactions

responsible for flame propagation by catalyzing the recombination of the

active free radicals involved. In such cases, any bromine present

simultaneously is presumably converted to species such as Br.e and HBr

which function in the gas phase in the usual way [13].

Antimony is a much more effective co-additive than phosphorus,

generally in the form of its oxide, Sb2O3. On its own, this compound has no

flame-retardant activity and is therefore almost always used in conjunction

with a halogen compound. In general, bromine-antimony mixtures are more

effective than the corresponding chlorine-antimony systems. The use of

61

antimony trioxide greatly reduces the high levels normally needed for

effective flame-retardance of bromine compounds on their own. The

principal mode of action is in the gas phase. If bromine and antimony are

present simultaneously in a burning organic polymer, the major part of the

antimony is volatilized, probably as SbBr3 or SbOBr. These compounds

then provide a ready source of hydrogen bromide and they also produce in

the middle of the combustion zone a mist of fine particles of solid SbO,

which can catalyze the recombination of the free radicals responsible for

flame propagation, via the formation of transient species like SbOH. A

number of other metal oxides have been investigated as partial or total

replacements for antimony trioxide. Their use, however, has a number of

disadvantages. The most important point is that volatilization of the

bromine occurs at the right stage of the combustion cycle. With zinc oxide,

volatilization takes place too early and the bromine has disappeared from the

system before it can become effective [28].

It can be concluded that in many, if not most, polymer systems in

which bromine and phosphorus are both present, the two elements tend to act

independently and therefore additively. The important mode of action of

metal oxides as co-additives for brominated flame-retardants is to catalyze

the breakdown of the bromine compound and therefore the release of volatile

bromine compounds into the gas phase. However, metal-bromine compounds

may also be formed, and these may have more specific modes of action in

inhibiting polymer combustion [29,30].

1.3.5. Smoke suppressants

Smoke production is determined by numerous parameters. No

comprehensive theory yet exists to describe the formation and constitution of

smoke.

62

Smoke suppressants rarely act by influencing just one of the

parameters determining smoke generation. Ferrocene, for example, is

effective in suppressing smoke by oxidizing soot in the gas phase as well as

by pronounced charring of the substrate in the condensed phase.

Intumescent systems also contribute to smoke suppression through creation

of a protective char. It is extremely difficult to divide these multifunctional

effects into primary and subsidiary actions since they are so closely

interwoven. At present, no uniform theory on the mode of action of smoke

suppressants has been established [1].

1.3.5.1. Condensed phase

Smoke suppressants can act physically or chemically in the

condensed phase. Additives can act physically in a similar fashion to flame-

retardants, i.e., by coating (glassy coatings, intumescent foams) or dilution

(addition of inert fillers), thus limiting the formation of pyrolysis products

and hence of smoke. Chalk (CaCO3), frequently used as filler, acts in some

cases not only physically as a dilutent but also chemically (in PVC, for

example) by absorbing hydrogen chloride or by effecting cross-linking so

that the smoke density is reduced in various ways. The processes

contributing to smoke suppression can be extremely complex.

Smoke can be suppressed by the formation of a charred layer on the

surface of the substrate, e.g., by the use of organic phosphates in unsaturated

polyester resins. In halogen-containing polymers, such as PVC, iron

compounds, e.g., iron (III) chloride, cause charring by the formation of

strong Lewis acids.

Certain compounds such as ferrocene cause condensed-phase

oxidation reactions that are visible as a glow. There is pronounced evolution

63

of carbon monoxide (CO) and carbon dioxide (CO2), so that less aromatic

precursors are given off in the gas phase.

Compounds such as MoO3 can reduce the formation of benzene

during the thermal degradation of PVC, probably via chemisorption

reactions in the condensed phase. Relatively stable benzene-MoO3

complexes that suppress smoke development are formed [1].

1.3.5.2. Gas phase

Smoke suppressants can also act chemically and physically in the gas

phase. The physical effect takes place mainly by shielding the substrate with

heavy gases against thermal attack. They also dilute the smoke gases and

reduce smoke density. In principle, two ways of suppressing smoke

chemically in the gas phase exist: the elimination of either the soot

precursors or the soot itself. Removal of soot precursors occurs by oxidation

of the aromatic species with the help of transition metal complexes. Soot can

also be destroyed oxidatively by high-energy OH radicals formed by the

catalytic action of metal oxides or hydroxides. Smoke suppression can also

be achieved by eliminating the ionized nuclei necessary for forming soot

with the aid of metal oxides. Finally soot particles can be made to flocculate

by certain transition metal oxides [1].

1.4. Performance criteria for and choice of flame-retardants

At present, the selection of a suitable flame-retardant depends on a

variety of factors that severely limit the number of acceptable materials.

Many countries require extensive information on human and

environmental health effects for new substances before they are allowed to

be put on the market. For existing chemicals, such data are not always

64

available but several national and international programs are in the process

of gathering this information.

For most chemicals, including flame-retardants, the following

information regarding human and environmental health is essential to

understanding a chemical's potential hazards:

1. Data from adequate acute and repeated dose toxicity studies is needed

to understand potential health effects.

2. Data on biodegradability and bioaccumulation potential is needed as

a first step in understanding a chemical's environmental behavior

and effects.

3. Information on the chemical's possible breakdown and/or combustion

products may also be needed.

4. Since flame-retardants are often processed into polymers at elevated

temperatures, consideration of the stability of the material at the

temperature inherent to the polymer processing is needed, as well as

on whether or not the material volatilizes at that temperature or

during use.

5. Consideration should be given to the need for information on the

possible formation of toxic and/or persistent breakdown products

during accidental fires or incineration.

Successfully achieving the desired improvement in flame-retardancy

is a necessary precursor to other performance considerations. The basic

flammability characteristics of the polymer to be used play a major role in

the flame-retardant selection process, as some polymers burn much more

readily than others.

65

Flame-retardant selection is also affected by the test method to be

used to assess flame-retardancy. Some tests can be passed with relatively

low levels of many flame-retardants, while high levels of very powerful

flame-retardants are needed to pass other tests.

There are many performance issues other than flame-retardancy that

must be considered during the selection of a flame-retardant for any use.

Just as in applications not needing improved flame-retardancy, a long list of

processing and performance requirements must be met before a material can

be accepted for use. The development of a polymer formulation that meets

all of these requirements involves finding the optimum combination of

polymer(s), flame-retardant(s), synergist(s), stabilizer(s), processing aid(s),

and all other additives. This is complex and difficult work requiring a great

deal of time, effort and expense.

Flame-retardants may adversely affect the processing characteristics

of polymers. Changes occurring in the viscosity of liquid systems or in the

flow of polymers that are melted during processing can cause major

problems. Significant alteration of the rate of reaction of thermoset polymers

or the speed and degree of crystallization of thermoplastic polymers may

result from the use of some flame-retardants. The temperatures routinely

used to process many polymers severely restrict the number of flame-

retardants suitable for incorporation.

Since flame-retardants are frequently used at high levels, they often

have a dramatic effect on the basic mechanical properties of polymers in

which they are used. Reduction of strength (tensile, compression), rigidity,

toughness and/or heat resistance are common problems.

66

When flame-retardants are added to polymers their appearance

(colour, gloss, transparency) and physical properties (density, hardness,

melting and glass transition temperatures, thermal expansion) often change

significantly. Electrical properties (resistance, dielectric, and tracking) are

frequently altered, and aging due to factors such as oxidation, UV radiation

and high temperature may be reduced.

The chemical properties of a flame-retardant are often of great

importance in its selection. Resistance on exposure to water, solvents, acids,

bases, oils or other substances may be a requirement for use. Issues related

to solubility, hydrolysis resistance or reactivity with other formulation

components may prevent the use of an otherwise desirable flame-retardant.

The relationship between cost and performance is an essential

consideration in the selection of a flame-retardant. In addition, the durability

(resistance to cleaning with water or by other techniques) of the flame-

retardant system is critical [30].

1.5. Production and uses of flame-retardants and flame-retarded

polymers

It is difficult to obtain an accurate picture of market volumes of

flame-retardants as reports from different sources appear to conflict.

1.5.1. Production

The worldwide demand for flame-retardant chemicals in 1992 was

estimated to be 6,00,000 tonnes [6]. This includes over a hundred different

products.

The classification according to base chemical content is given in

table 1.2. [7] and the world market volume trends between 1986 and 1991 are

given in table 1.3 [2(i)].

67

Table 1.2. Demand for flame-retardants according to base chemical content.

Base chemicals Demand (tonnes)Bromine 150 000Chlorine 60 000

Phosphorus 100 000Antimony 50 000Nitrogen 30 000

Aluminium 170 000Others 50 000

Table 1.3. Flame-retardant market volume

Group 1986 (tonnes) 1991 (tonnes)Phosphate esters 20 000 18 000

Halogenated phosphates 13 000 16 000Chlorinated

hydrocarbons

15 000 15 000

Brominated

hydrocarbons

28 000 36 000

Brominated bisphenol A 16 000 18 000Antimony trioxide 22 000 25 000

Borates 8 000 8 000Aluminium trihydrate 140 000 170 000Magnesium hydroxide 2 000 3 000

Total 264 000 301 000

The annual consumption of different flame-retardants in Japan over

the period 1986 – 1994 is given in table 1.4. [6]. A comparable table of

68

global use was not available. The consumption of brominated flame-

retardants and antimony oxide in Japan has more than doubled over this

period, compared to the moderate increase in other flame-retardants. The

market for hydrated aluminium as a flame-retardant seems to have

decreased in Japan, whereas table 1.3. shows that an increase occurred

worldwide [2(i)].

Table 1.4. Trends in the annual consumption of flame-retardants

Type Compound Amount (tonnes)1986 1990 1994

Brominated

Tetrabromobisphenol A (TBBPA) 12 000 23 000 24 000

Decabromobiphenyl ether 3 000 10 000 5 500Octabromobiphenyl ether 600 1 100 500Tetrabromobiphenyl ether 1 000 1 000 0Hexabromocyclododecane 600 700 600

Bis(tetrabromophthalimido) ethane - 1 000 2 500Tribromophenol 100 450 3 500

Bis(tribromophenoxy)ethane 400 400 900TBBPA polycarbonate oligomer -- -- 2500

Brominated polystyrene -- -- 1 300TBBPA epoxy oligomer -- 3 000 7 000

Others 2400 -- 2150Subtotal 20000 40650 51450

Chlorinated

Chlorinated paraffins 4 000 4 500 4 300Others 850 700 900

Subtotal 4 850 5200 5200

Phosphoric

Halogenated ester 3 000 3 000 3 100Non-halogenated ester 4 000 4 400 4 400

Others 1 750 1 750 3 310

Subtotal 8 750 9 150 10810

Antimony oxide 8300 16000 17000

69

Inorganic

Hydrated aluminium 48 000 37 000 42 000Others 7200 8400 9000

Subtotal 63500 61400 68000Total 97100 116400 135460

A worldwide estimate of the consumption of flame-retardants

according to materials is not available but the figures for Europe listed in

table 1.5 reflects the world market in general [8,27,29,31].

Table 1.5. Estimated consumption of flame-retardants in Europe for 2005 and 2010

Product group

Consumption (104

tonnes)Product group

Consumption (104

tonnes)2005 2010 2005 2010

Polystyrene 4.0-4.5 4.5-5.0 Polyvinyl chloride

25.0-27.0

27.0-29.0

ABS 1.0-1.5 1.2-1.8 Polyurethanes 12.0-13.5

13.5-15.0

Polyesters 7.5-8.0 8.5-9.0 Engineering plastics

1.5-1.8 1.7-2.0

Epoxy resins

3.5-4.0 4.0-4.5 Paper and textiles

9.0-10.0 10.0-11.0

Polyolefins 10.0-12.0

11.0-13.0

Rubber and elastomers

5.0-6.0 6.0-7.0

Other 11.5-11.7

12.6-12.7

Total 90.0-100.0

100.0-110.0

1.5.2. Uses

The consumption of flame-retardants in plastics and other

combustible materials is closely linked to regulations covering fire

precautions. The principal regulations relate to the building, transportation,

electrical engineering, furnishing and mining sectors [1].

1.5.2.1. Plastics

70

The plastics industry is the largest consumer of flame-retardants,

estimated at about 95% in 1991. About 10% of all plastics contain flame-

retardants [6]. The main applications are in building materials and

furnishings (structural elements, roofing films, pipes, foamed plastics for

insulation, furniture and wall and floor coverings), transportation (equipment

and fittings for aircraft, ships, automobiles and railroad cars), and in the

electrical industry (cable housings and components for television sets, office

machines, household appliances and lamination of printed circuits).

The growth in the flame-retardant market reflects the enormous

expansion of the plastics industry in recent decades. Between 1988 and 1994,

there was a worldwide increase of 20%. Although the USA, Western Europe

and Japan are still the largest plastic producers (30, 24 and 12% of the

market, respectively), other countries showed the largest increases between

1988 and 1994, e.g., South Korea (170%); China (60%); Taiwan (54%) [32].

Examples of flame-retardants used in various plastics [6] are as

follows:

1. PVC: Chlorinated paraffins or phosphate esters, antimony trioxide,

aluminum hydroxide

2. Acrylonitrile-butadiene-styrene (ABS): Octabromodiphenyl ether,

antimony trioxide

3. Expandable polystyrene: Hexabromocyclododecane

4. High-Impact polystyrene (HIPS): Decabromodiphenyl ether or

tetrabromo- bisphenol A, antimony trioxide

5. Linear polyester: Brominated organics

6. Polypropylene: Tetrabromobisphenol A, bis(2,3-dibromopropyl

ether), antimony trioxide

71

7. Low-density polyethylene (LDPE) films: chlorinated paraffins,

antimony trioxide

8. High-density polyethylene (HDPE) and cross-linked polyethylene:

Brominated aromatics

9. Polyurethane foams: Organophosphates, brominated organic

compounds, aluminum trihydrate

10. Polyamides: Brominated aromatic compounds, chlorinated

cycloaliphatic compounds, antimony trioxide, red phosphorus,

melamine

11. Polycarbonates: Tetrabromobisphenol A, brominated organic

oligomers, sulfonate salts

12. Unsaturated polyesters: Chlorinated and brominated organic

compounds, antimony trioxide, aluminum trihydrate

13. Epoxy resins: Tetrabromobisphenol A

1.6 Plasticizers

Plasticizers are additives that increase the plasticity or fluidity of a

material. The dominant applications are for plastics, especially polyvinyl

chloride (PVC). The properties of other materials are also improved when

blended with plasticizers including concrete, clays, and related products. The

worldwide market for plasticizers in 2000 was estimated to be several

million tons per year [33,34].

1.6.1. Introduction

In 1951, the International Union of Pure and Applied Chemistry

(IUPAC) developed a universally accepted definition for a plasticizer as a

substance or material incorporated in a material to increase its flexibility,

workability, or distensibility. A plasticizer may reduce the melt viscosity,

72

lower the temperature of a second-order transition, or lower the elastic

modulus of the product.

The use of DOP prevailed as the preferred general-purpose plasticizer

for PVC until the late 1970s. Today, although there are about 70 plasticizers

available, about 80% of the worldwide consumption is comprised of three

plasticizers, di-2-ethylhexyl phthalate (DOP), diisononyl phthalate (DINP),

and diisodecyl phthalate (DIDP).

1.6.2. Mechanism of plasticization

For a plasticizer to be effective, it must be thoroughly mixed and

incorporated into the PVC polymer matrix. This is typically obtained by

heating and mixing until either the resin dissolves in the plasticizer or the

plasticizer dissolves in the resin. The plasticized material is then molded or

shaped into the useful product and cooled. Different plasticizers will exhibit

different characteristics in both the ease with which they form the plasticized

material and in the resulting mechanical and physical properties of the

flexible product [34].

Plasticization is described by three primary theories: the Lubricating

Theory, the Gel Theory and the Free Volume Theory. According to the

Lubricating Theory of plasticization, as the system is heated, the plasticizer

molecules diffuse into the polymer and weaken the polymer-polymer

interactions (van der Waals’ forces). Here, the plasticizer molecules act as

shields to reduce polymer-polymer interactive forces and prevent the

formation of a rigid network. This lowers the PVC Tg and allows the

polymer chains to move rapidly, resulting in increased flexibility, softness,

and elongation. The Gel Theory considers the plasticized polymer to be

neither solid nor liquid but an intermediate state, loosely held together by a

73

three-dimensional network of weak secondary bonding forces. The Free

Volume Theory considers that when small molecules such as plasticizers are

added, they lower the Tg by separating the PVC molecules, adding free

volume and making the PVC soft and rubbery [35].

1.6.3. Types of plasticizers

Plasticization is achieved by incorporating a plasticizer into the PVC

matrix through mixing and heat. The IUPAC definition of a plasticizer is

entirely focused on performance characteristics when combined with a

polymer; there is no implication of chemical structure or physical properties

of the plasticizer per se. But, the key performance properties are influenced

by plasticizer level (in phr) as well as the chemical type. So, an orderly

comparison of plasticizers is facilitated by separating them into three

subgroups, based on their performance characteristics in PVC, as given in

table 1.6. [36].

Table1.6 Plasticizer Family/Performance Grid

Family General purpose

Performance plasticizers Specialty plasticizers

Strong solvent

Low temp

Low volatility

Low diffusion

Stability

Flame-resistance

Pthalates × √ √ √ √ √Trimellitates √ × √Phosphates √ √ ×

X denotes the primary performance characteristics associated with each chemical family √ denotes the secondary functions associated with products in that class of plasticizers

Phthalates are the most widely used class of plasticizers in PVC. As

shown, they contribute the most complete array of required performance

properties in flexible PVC. In addition, their cost and availability supports

their preference. While historically DOP, di(2-ethylhexyl) phthalate, has

74

been the product of choice, the current market for GP plasticizers includes

dialkyl phthalates that are slightly different homologues of DOP, such as

diisoheptyl (C7), diisooctyl (C8), diisononyl (C9) and diisodecyl (C10)

phthalates; their combined usage totals more than 80% of the worldwide

plasticizer market.

General purpose plasticizers provide the desired flexibility to PVC

along with an overall balance of optimum properties at the lowest cost.

These include dioctyl (DOP), diisoheptyl (DIHP) to diisodecyl (DIDP)

phthalates. Phosphates and halogenated plasticizers provide fire-retardant

properties.

1.6.4. Plasticizer performance

Hardness/ softness is significantly influenced by plasticizer level, as

well as type of plasticizer, which controls plasticizer “efficiency”.

Tensile strength and ultimate elongation (% extension at failure) are

influenced by plasticizer level, but these properties are not significantly

altered as function of plasticizer type, with PVC formulated to specified

room temperature hardness.

The color of plasticized PVC compositions is typically not altered by

the plasticizer. This is because most commercial grade plasticizers are near

“water-white” in color. Highly colored (amber–brown) plasticizers would, of

course, impart undesired color to flexible PVC compositions.

1.7. Formation of toxic products on heating or combustion of flame-

retarded products

Natural or synthetic material that burns produces potentially toxic

products. There has been considerable debate on whether addition of organic

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flame-retardants results in the generation of a smoke that is more toxic and

may result in adverse health effects on those exposed. There has been

concern in particular about the emission of polybrominated dibenzofurans

(PBDF) and polybrominated dibenzodioxins (PBDD) during manufacture,

use and combustion of brominated flame-retardants.

1.7.1. Toxic products in general

Combustion of any organic chemical may generate carbon monoxide,

which is a highly toxic non-irritating gas, and a variety of other potentially

toxic chemicals. Some of the major toxic products that can be produced by

pyrolysis of flame-retardants are: CO, CO2, HCl, POX, ammonia vapor,

bromofurans, HBr, HCN, NOX and phosphoric acid [26].

In general the acute toxicity of fire atmospheres is determined mainly

by the amount of CO, the source of which is the amount of generally

available flammable material. Most fire victims die in post flash-over fires

where the emission of CO is maximized and the emission of HCN and other

gases is less. The acute toxic potency of smoke from most materials is lower

than that of CO [37].

Flame-retardants significantly decrease the burning rate of the

product, reducing heat yields and quantities of toxic gas. In most cases,

smoke was not significantly different in room fire tests between flame-

retarded and non-flame-retarded products [38].

Reports on toxicity studies on gases from full-scale room fires

involving fire-retardant materials are available in literature. Hydrogen

cyanide and carbon monoxide were the two major toxicants. There was no

evidence that the smoke from flame-retarded materials was more toxic to

rabbits than the smoke from non-flame-retarded materials.

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In case of brominated flame-retardant, unless suitable metal oxides or

metal carbonates are also present, virtually all the bromine is eventually

converted to gaseous hydrogen bromide. This is a corrosive and powerful

sensory irritant. In a fire situation however, it is always carbon monoxide or

hydrogen cyanide, rather than an irritant which causes rapid incapacitation.

Owing to its high reactivity, hydrogen bromide is unlikely to reach

dangerously high concentrations [39].

1.7.1.1. Formation of halogenated dibenzofurans and dibenzodioxins

PBDFs and PBDDs can be formed from polybrominated diphenyl

ethers (PBDEs), polybrominated phenols, polybrominated biphenyls (PBBs)

and other brominated flame-retardants under various laboratory conditions,

including heating. Because chlorinated derivatives are preferably formed

during pyrolysis, mixed halogen compounds will be predominantly produced

if a chlorine source is also available [40].

As in the case of PCDD/PCDF, it is the 2,3,7,8-substituted isomers

that are toxic.

1.7.1.2. Exposure to PBDD/PBDF from polymers containing

halogenated flame-retardants

The possibility of exposure of general public and the people in work

areas to these toxins is briefly considered below.

1.7.1.2.1. Exposure due to contact or emission from products containing

halogenated flame-retardants

Exposure of the general public to PBDD/PBDF impurities in flame-

retardant polymers is unlikely to be of significance. The possible exposure

to PBDD/PBDF from TV sets and computer monitors flame-retarded with

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halogenated flame-retardants has been discussed in Environmental Health

Criteria (EHC) 162: Brominated diphenyl ethers and is, as just stated,

unlikely to be of significance [41].

1.7.2. Workplace exposure studies

Several studies have been performed to determine whether

PBDD/PBDF is present in the fumes emitted during thermal processes, such

as the extrusion of resins containing halogenated flame-retardants under

normal processing conditions at temperatures in the range of 200 to 250°C

[15, 42].

Epidemiological studies of workers engaged in processing polymers

with PBDEs have been reported. Results of PBDD/PBDF workplace

monitoring during polymer processing have also been reported.

PBDD/PBDF personnel and room air levels during processing of PBDEs

were < 2 ng/m3 (TCDD equivalent) with the exception of two samples at the

extruder head (128 ng/m3, TCDD equivalent). Engineering controls were

successful in reducing these levels. Workplace control measures need to also

include appropriate industrial hygiene measures and monitoring of exposure

[15,42].

1.7.2.1. Formation of PBDD/PBDF from combustion

Early studies and their findings on flame-retardant combustion

products are discussed in the following section.

1.7.2.2. Laboratory pyrolysis experiments

In the late 1980s, many pyrolysis experiments (at temperatures of

400-900°C) on brominated flame-retardants and flame-retardant systems

were performed and the breakdown products measured.

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Flame-retardants or intermediates tested included PBBs, PBDEs,

2,4,6-tribromophenol, pentabromophenol, tetrabromobisphenol-A and

tetrabromophthalic anhydride. Pyrolysis of the flame-retardants alone, as

well as with polymer mixtures, was investigated. As different laboratories

carried out the experiments using a variety of testing methods and

conditions, a direct comparison of the many experiments was not possible

[15, 32].

Although they indicate which flame-retardants are likely to form

PBDF (and to a lesser extent PBDD), pyrolysis experiments are not

generally comparable to actual fire situations.

1.8. Overview of exposure and hazards to humans and the

environment

Since flame-retardants are a heterogeneous group of diverse

chemicals, the information presented in this section only provides a general

overview of possible routes of exposure to chemicals associated with flame-

retardant use. This section also provides a brief summary of the hazards to

human health and to the environment posed by chemicals connected with

flame-retardant use. For detailed information on the extent of exposure and

health and environmental effects of individual substances, the appropriate

specific EHC monographs may be consulted.

1.8.1. Human exposure

The possible routes through which human beings may come in

contact with flame-retardants are considered below.

1.8.1.1. General population

Potential sources of exposure include consumer products,

manufacturing and disposal facilities, and environmental media. Factors

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affecting exposure of the general population include the physical and

chemical properties of the product, the extent of manufacturing and

emission controls, the use made of the product (surface coating, durability of

fabric finishes, incorporation into a polymer, etc.), the end use, and the

method of disposal.

Potential routes of exposure for the general population include the

dermal route (contact with flame-retarded textiles), inhalation and ingestion.

1.8.1.2. Occupational exposure

Occupational exposure may occur during the manufacture, transport,

processing and disposal/recycling of flame-retardants. Routes of exposure

could include inhalation, dermal contact and ingestion. Factors affecting the

extent of exposure include industrial hygiene practices, engineering controls,

manufacturing processes and the type of product. As with any other

industrial chemical, workplace monitoring and good industrial practice can

delineate the extent of any exposure.

1.8.2 Exposure of the environment

Environmental exposure may occur as a result of the manufacture,

transport, use or waste disposal of flame-retardants. Routes of

environmental exposure can include water, air and soil. Factors affecting

exposure include the physical and chemical properties of the product,

emission controls, disposal/recycling methods, volume and

biodegradability/persistence. Environmental monitoring can determine the

extent of environmental exposure.

On the basis of the estimated demand for flame-retardants, one

million ton-mark is being approached for flame-retardant polymers produced

each year.

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Most flame-retarded products eventually become waste. Municipal

waste is generally disposed of via incineration or landfill. Incineration of

flame-retarded products can produce various toxic compounds, including

halogenated dioxins and furans. The formation of such compounds and their

subsequent release to the environment is a function of the operating

conditions of the incineration plant and the plant's emission controls.

There is a possibility of flame-retardants leaching from products

disposed of in landfills. However, potential risks arising from landfill

processes are also dependent on local management of the whole landfill.

The significance of any release of flame-retardants from disposal sites has

yet to be determined.

Some products containing flame-retardants, including some plastics,

have been identified as suitable for recycling [43,44].

1.8.3. Hazards to humans

The hazards to humans associated with some flame-retardants have

been outlined in the relevant EHC monographs. For example, the use of

tris(2,3-dibromopropyl) phosphate and bis(2,3-dibromopropyl) phosphate

was banned in 1977 by the US Consumer Product Safety Commission and in

several other developed countries for use in children's clothing because of

concerns that the chemical might be a human carcinogen and because of the

possibility of significant human exposure through contact with treated

fabrics [16]. Delayed neurotoxicity due to tri- ortho-cresyl phosphate

(TOCP), one of the tricresyl phosphate isomers, has been observed in

humans. Some PBB congeners have been shown to produce chronic toxicity

and cancer in experimental animals. However, no definitive human health

effects, correlatable with exposure, were found in a population in Michigan,

USA, accidentally exposed to PBBs [45].

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1.8.4. Hazards to the environment

EHC monographs outline the hazards to the environment associated

with some flame-retardants. Some PBB congeners are persistent and

bioaccumulative and may pose a threat especially to higher levels of the food

chain. Hexachloro-cyclopentadiene is highly toxic to aquatic organisms.

However, information obtained under environmentally realistic conditions is

limited [46].

The potential hazard to the general environment is expected to be

low. Low concentrations of triphenyl phosphate have been detected in

environmental samples. Triphenyl phosphate is rapidly degraded in the

environment [47]. However, sediment-dwelling organisms near production

plants may have been exposed to concentrations high enough to exert toxic

effects. Tricresyl phosphate is also degraded rapidly in the environment, and

subsequent environmental concentrations are therefore low. The acute

toxicity of tricresyl phosphate to aquatic organisms is low [44].

Persistence of pentabromodiphenyl ether (PeBDE) and lower

brominated diphenylethers in the environment suggest that commercial

PeBDE should not be used [42].

Some flame-retardants have come under intense environmental

scrutiny. US EPA has called for additional testing [48].

The data on environmental levels of short-chain chlorinated paraffins

indicate that in areas close to release sources, there is a risk to both

freshwater and estuarine organisms. Recent data indicate that there is also a

potential risk to aquatic invertebrates from intermediate- and long-chain

chlorinated paraffin products [27].

1.9. Regulations with respect to flame-retardants

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Several national regulatory bodies have implemented regulations on

specific substances used in flame-retardant applications, as noted in table

1.7.

Table 1.7. Country-specific actions on PBBs, either taken or proposed [7]

Country Actions

Austria Prohibits the manufacture, placing on the market, import and use of PBBs and products containing these substances.

Canada Prohibits the manufacture, use, processing, offer for sale, selling or importation of PBBs for commercial, manufacturing or processing purposes.

Denmark Implements EC Directive 89/677 banning the use of PBBs in textiles.

Finland PBB may not be used in textile articles intended to come into contact with the skin (in accordance with EC Directive 83/264).

France Implements EC Directive concerning PBBs and their use on textiles.

Netherlands Proposed resolution would prohibit the storage of PBBs or products or preparations containing these substances or making them available to third parties. (Exports are excluded from the resolution).

Norway Ban on PBBs in textiles intended to come into contact with skin, implementation of EC Directives 76/769/EEC, 83/264 and 89/677.

Sweden Ban on PBBs in textiles intended to come into contact with skin by implementation of EC Directive 76/769.

Switzerland Prohibits manufacture, supply, import and use of PBBs and products containing these substances. Supply and import of capacitors and transformers containing PBBs is forbidden.

USA No current production or use. Companies intending to resume manufacture must notify US EPA 90 days in advance for approval.

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1.10. Recommendations for the protection of human health and

the environment

Wrapping up introduction, the following observations may be made.

Flame-retardants are a diverse group of compounds used to improve

the flame-retardancy of polymers and other materials. A large variety of

compounds, from inorganic to complex organic molecules, is used as flame-

retardants, synergists and smoke suppressants.

It is difficult to find accurate figures for the global use of flame-

retardants but estimates indicate that more than 6,00,000 tonnes are produced

annually. Available data indicate a substantial increase of brominated

organic product consumption during the last decade.

There are obvious benefits in using flame-retardants, as many human

lives and property are saved from fire. At present, knowledge of long-term

effects resulting from exposure to flame-retardants and their breakdown

products is limited. Most people that die in fires are killed by carbon

monoxide.

The majority of the organic flame-retardants are either covalently bound

into polymer molecules (reactive) or mixed into the polymer (additive).

They can act in several ways, either physically (by cooling, by formation of a

protective layer or by dilution of the matrix) or chemically (by reactions in

either the gas or the solid phase).

A number of factors govern the selection of the type of flame-retardant

to be used in a specific application. Some of these are the flammability of the

matrix, processing and performance requirements, chemical properties and

possible hazards to human and environmental health.

Exposure of the general population to flame-retardants can occur via

inhalation, dermal contact and ingestion. Potential sources of exposure are

consumer products, manufacturing/disposal facilities and environmental

84

media (including food intake). The same routes are possible for occupational

exposure, mainly during production, processing, transportation and

disposal/recycling of the flame-retardants or the treated products.

Occupational exposure to the breakdown products may also occur during fire

fighting. As several of the compounds used are lipophilic and persistent,

they may bioaccumulate. Some of the compounds have been shown to cause

organ damage, genotoxic effects and cancer.

There is also concern for occupational health and environmental

effects from combustion/pyrolysis products, especially the polyhalogenated

dibenzofurans and dibenzo-p-dioxins, from some organic flame-retardants.

Other breakdown products also need to be taken into account.

The properties of a number of flame-retardants make them persistent

and/or bioaccumulative, and they may therefore pose hazards to the

environment. Some of the compounds that have been evaluated so far

(polybrominated biphenyls, polybrominated diphenyl ethers and chlorinated

paraffins) have been found to belong to this group. Some of these have

therefore been recommended not to be used.

Several countries have developed regulations affecting the

production, use and disposal of flame-retardants. Some include restrictions

on the use of compounds because of potential toxic effects in humans.

Germany has developed rules for the maximum content of some 2,3,7,8-

substituted polychlorinated dibenzo-para-dioxins and dibenzofurans in

products.

The availability of relevant data on flame-retardants in the open

literature is limited, especially for some existing chemicals produced before

regulations for commercialization were strengthened in several countries.

International Programme on Chemical Safety, World Health

Organization, Geneva (IPCS) has issued evaluations for some flame-

85

retardants and has been continuously engaged in evaluating newly emerging

flame-retardants.

1.10.1. The recommendations

a) Information on the content and nature of flame-retardants,

including impurities in products, should be made available to

national authorities.

b) More complete information on the volume of flame-retardants

production and consumption should be made available.

c) In view of the increased recycling of flame-retarded products,

consideration could be given to harmonized labeling by an

international forum.

d) Compounds that present a toxic risk to humans and/or the

environment should not be used as flame-retardants.

e) Occupational exposure to flame-retardants and their breakdown

products should be minimized using appropriate engineering and

good industrial hygiene practices. The exposure of people working

in these operations should be monitored.

f) There is a need for proper assessment of occupational health and

environmental effects from combustion or pyrolysis products of

flame-retardants.

g) Emissions to the environment from manufacturing, processing,

transportation and disposal/recycling of products containing

persistent bioaccumulative compounds should be minimized using

best available techniques. The environment in the vicinity of such

operations should be monitored for the compounds used.

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h) The use of flame-retardants with properties that make them

persistent and bioaccumulative should be avoided.

i) The levels of the major persistent bioaccumulating flame-retardants

should be monitored routinely in environmental matrices (biota and

sediments). Some compounds that are no longer produced should

likewise be monitored, in order to indicate the long-term influence

of such products.

1.11. Further research

a) Further studies need to be undertaken to elucidate the fate of flame-

retardants in disposal/recycling operations.

b) There is a need for further evaluations of flame-retardants. Useful

criteria for setting priorities are volume of use, intrinsic toxic effects

on human health and the environment, exposure assessments, and

persistence and bioaccumulation/bio-magnification of flame-

retardants or their breakdown products.