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TRANSCRIPT
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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.
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“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].
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Chapter 1
INTRODUCTION
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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
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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
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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.
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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
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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.
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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.
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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
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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-
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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].
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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
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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
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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
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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,
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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.
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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
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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
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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.
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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
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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
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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
75
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.