ABSTRACT
The effectiveness of alum and potassium sesquicarbonate was
studied by incorporating various concentrations of the flame
retardants into the polyurethane foam sample. The
flammability tests were carried out and the results showed
that as the concentration of the flame retardants increased,
the flame propagation rate, after glow time, burn length and
flame duration decreased for both flame retardants, while
ignition time, add-on and char formation increased for both
flame retardants. Thermogravimetric analysis shows that both
alum and potassium sesquicarbonate functions as flame
retardants on the foam samples at low percentage
concentration but the polyurethane foam filled with potassium
sesquicarbonate required a higher activation energy than alum
for the pyrolysis / combustion of the samples. Also the onset
of degradation time was more delayed in potassium
sesquicarbonate than alum.
vi
TABLE OF CONTENTS
Title page – – – – – i
Certification – – – – – ii
Dedication – – – – – iii
Acknowledgements – – – – – iv
Abstract – – – – – v
Table of contents – – – – – vi
List of table – – – – – xi
List of figures – – – – – xii
CHAPTER ONE
1.0 INTRODUCTION – – – – – 1
LITERATURE REVIEW – – – – – 3
1.1 Flame retardants – – – – – 3
1.2 History of flame retardants – – – – 5
1.3 Types of flame retardants – – – – – 7
1.3.1 Inorganic flame retardants – – – – 8
1.3.2 Halogenated organic flame retardants – – 11
1.3.3 Organophosphorous flame retardants – – 13
1.4 Mechanism of action of flame retardants – – 14
vii
1.4.1 Physical action – – – – – 18
1.4.2 Chemical action – – – – – 19
1.5 Improvement of the flame retardancy – – 20
1.6 Co-additives for use with flame retardant – – 22
1.7 Smoke suppressants – – – – – 24
1.7.1 Condensed phase – – – – – 24
1.7.2 Gas phase – – – – – 26
1.8 Performance criteria and choice
of flame retardants – – – – – 26
1.9 Uses of flame retardants – – – – – 28
1.10 Formation of toxic products on heating or
combustion of flame retarded products – – 30
1.10.1 Toxic products in general – – – – 30
1.11 Human exposure to flame retardants – – 31
1.11.1 Environmental exposure – – – – 32
1.12 Polyurethane foam polymer – – – – 33
1.13 History of polyurethane foam polymer – – 34
1.14 Basic chemicals of polyurethane foam – – 37
1.15 Raw materials used for polyurethane
foam polymer – – – – – 41
viii
1.15.1 Isocyanates – – – – – 41
1.15.2 Polyols – – – – – 44
1.15.2.1 Polyethers – – – – – 46
1.15.2.2 Polyesters – – – – – 48
1.15.3 Surfactants – – – – – 49
1.15.4 Chain extenders and cross linkers – – 50
1.15.4.1 Catalysts – – – – – 51
1.16 Physical properties of polyurethane foams- 53
1.17 Mechanical properties of
polyurethane foam – – – – – 54
1.18 Chemical properties of polyurethane
foam polymer – – – – – 54
1.19 Polyurethane foam polymer structures- – 55
1.20 Applications of polyurethane
foam polymer – – – – – 57
1.21 Alum – – – – – 58
1.21.1 Crystal chemistry of alums – – – 58
1.22 Origin of alum – – – – – 59
1.23 Production of alum – – – – – 61
1.23.1 Alum from alunite – – – – – 61
ix
1.23.2 Alum from clay or bauxite – – – – 62
1.24 Types of alum – – – – – 63
1.25 Alum solubility – – – – – 64
1.26 Properties of alum – – – – – 65
1.27 Uses of alum – – – – – 66
1.28 Potassium sesquicarbornate – – – – 66
1.29 Production of potassium sesquicarbonate
(mild vegetable caustic) – – – – – 67
1.30 The aim of this research – – – – – 68
CHAPTER TWO
2.0 EXPERIMENTAL – – – – – 69
2.1 Materials and methods – – – – – 69
2.2 Apparatus – – – – – 69
2.3 Characterization of the foam samples – – 72
2.3.1 Determination of the ignition time – – – 72
2.3.2 Determination of burn length – – – – 73
2.3.3 Determination of flame propagation rate- – 74
2.3.4 Determination of flame duration – – – 74
2.3.5 Determination of char formation – – – 75
x
2.3.6 Determination of After-glow time – – – 75
2.3.7 Determination of Add-on – – – – 76
2.3.8 Determination of thermogravimetric analysis – 76
CHAPTER THREE
3.0 RESULTS AND DISCUSSION – – – – 78
3.1 Effects of flame retardants on ignition time – 78
3.2 Effects of flame retardants on burn length- – 80
3.3 Effects of flame retardants on
flame propagation rate – – – – – 82
3.4 Effects of flame retardants on flame duration – 86
3.5 Effects of flame retardants on char formation – 88
3.6 Effects of flame retardants on After-glow time – 90
3.7 Effects of flame retardants on Add-on – – 92
3.8 Effects of flame retardants on degradation profile- 95
xi
LIST OF TABLES
Table 1. : Solubility of the compounds.
Table 2. : Foam formulation using Alum as flame
retardant.
Table 3. : Effect of flame retardants on ignition time.
Table 4. : Effects of flame retardants on burn length.
Table 5. : Effects of flame retardants on flame
propagation Rate.
Table 6. : Effects of flame retardants on flame duration.
Table 7. : Effects of flame retardants on char formation
Table 8. : Effects of flame retardants on after glow time.
Table 9. : Effects of flame retardants on Add-on.
Table 10. : Effects of flame retardants on degradation
Profiles.
xii
LIST OF FIGURES
Fig. 1: The combustion process.
Fig. 2: Basic unit in a urethane block copolymer.
Fig. 3: Structure-property relationships in polyurethane.
Fig. 4: Thermogravimetric analyzer
Fig. 5: Effects of flame retardants on Ignition time.
Fig. 6: Effects of flame retardants on burn length
Fig. 7: Effects of flame retardants on flame propagation
rate
Fig. 8: Effects of flame retardants on flame duration
Fig. 9: Effects of flame retardants on char formation
Fig. 10: Effects of flame retardants on After glow time
Fig. 11: Effects of flame retardants on Add – on
Fig. 12: Effects of flame retardants on the maximum
temperature of degradation
Fig. 13: Effects of flame retardants on onset of thermal
degradation time.
Fig. 14a: Effects of alum on the 1st step of thermal
degradation.
xiii
Fig. 14b: Effects of potassium sesquicarbonate on the 1st
step of thermal degradation.
Fig. 15a: Effects of alum on the 2nd step of thermal
degradation.
Fig. 15b: Effects of potassium sesquicarbonate on the 2nd
step of thermal degradation.
Fig. 16: Effects of flame retardants on duration of thermal
degradation.
Fig. 17a: Effects of alum on the 1st step of energy of
degradation.
Fig. 17b: Effects of potassium sesquicarbonate on the 1st
step of energy of degradation.
Fig. 18a: Effects of alum on the 2nd step of energy of
degradation.
Fig. 18b: Effects of potassium sesquicarbonate on the 2nd
step of energy of degradation.
1
CHAPTER ONE
1.0 INTRODUCTION
In every day to day activity, foam materials are all around
our homes, vehicles, schools and industries. It is the
cushioning material of choice in almost all furniture and
bedding. It is used as carpet cushions. It is the material used
for pillows, roof liners, sound proofing, car and truck seats.
Foam has become such a widely used material because it
provides a unique combination of form and function [1].
Types of foam such as neoprene, polystyrene,
polyethylene, polyurethane, polyether and polyester based
polyurethane are synthetic plastics that have very desirable
properties; easily malleable and shapeable. They are also
capable of “giving” and returning to its original shape [2].
Polyurethane foams which have been in use for almost
40 years, offer a wide variety of product suitable for various
applications. It appears to be a simple product but actually
very complex. The market place for polyurethane has
witnessed innovations and improvement which have led to
2
great usage. Polyurethane is a good example of traditional
organic polymer system that has useful structural and
mechanical properties in foam but it is limited by its low
thermo-oxidative stability [3].
New technologies , new processes and new applications
introduce new fire hazards (e.g. new ignition sources such as
welding sparks and short circuits) [4]. Modern fire fighting
techniques and equipments have reduced the destruction due
to fires. However, a high fuel load in either a residential or a
commercial building can offset even the best of building
construction [5]. Wood, paper, textiles and synthetic textiles
all burn under the right conditions, many burn rigorously and
ignite readily. The ability to control or reduce flammability of
materials have engaged the mind of scientists. Fire hazards
may be reduced by either retarding the fire or initiating a
chemical reaction that stops the fire. It has been observed that
some of the fire retardant chemicals have adverse effects on
the properties of materials on which they are imparted [6]. The
choice of suitable polymeric flame retardants is restricted to
3
species that allow the retention of advantageous properties of
the polyurethane.
LITERATURE REVIEW
1.1 Flame retardants
Flame retardants are materials that resist or inhibit the
spread of fire. They are chemicals added to polymeric
materials, both natural and synthetic to enhance flame
retardant properties [7]. A fire retardant is a material that is
used as a coating on or incorporated into a combustible
product to raise the ignition or to reduce the rate of burning of
product [8].
Chemicals used as flame retardants can be inorganic,
organic, mineral, halogen or phosphorus-containing
compounds. In general, fire retardants reduce the flammability
of materials by either blocking the fire physically or by
initiating a chemical reaction that stops the fire. Flame
retardant systems used in synthetic or organic polymers act in
five basic ways [7].
4
1. Gas dilution:- This 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. Examples are metal salts, metal
hydroxides and some nitrogen compounds.
2. Thermal quenching:- This is the result of endothermic
decomposition of the flame retardant. Metal
hydroxides and metal salts act to decrease the surface
temperature and rate of burning.
3. Protective coating:- Some flame retardants form a
protective liquid or char barrier which limits the
amount of polymer available to the flame front and
also act as an insulating layer to reduce the heat
transfer from the flame to the polymer. This includes
phosphorus compounds.
4. Physical dilution:- Inert fillers (glass fibres) and
minerals act as thermal sinks to increase the heat
capacity of the polymer or reduce its fuel content.
5
5. Chemical interaction:- Some flame retardants such as
halogens and phosphorus compounds dissociate into
radicals species that compete with chain propagating
steps in the combustion process.
Flame retardants have faced renewed attention in recent
years, aside from various conventional alternatives such as
antimony or phosphorus based retardants which have
toxicological problems of their own, nanoadditive flame
retardants such as carbon nano tubes, nanographites, layered
double hydroxides (LDH) have been shown to enhance a
number of polymer properties, thermal stability, strength,
oxidation resistance, processing, rheology and flammability in
polyurethane foams [9].
1.2 History of flame retardants [10]
In 450BC, alum was used to reduce the flammability of
wood by the Egyptians while the Romans used a mixture of
vinegar and alum on wood in about 200BC. In 1638, a mixture
of clay and gypsum was used to reduce the flammability of
6
theatre curtains. Alum was also used to reduce the
flammability of balloons in 1783.
Gay Lussac reported a mixture of ammonium phosphate,
ammonium chloride and borax to be effective on linen and
hemp. In 1821 and 1912, Perkins described a flame retardant
treatment for cotton using a mixture of sodium stannate and
ammonium sulphate [6]. The advent of synthetic polymers
earlier this century was of special significance, since the water
soluble inorganic salts used up to that time were of little or no
utility in hydrophobic materials. Modern developments were
therefore concentrated on the development of polymercompatible flame retardants.
By the out break of the Second World War, flame proof
canvas for outdoor use by the military was produced by a
treatment with chlorinated paraffins and an insoluble metal
oxide, mostly antimony oxide as a glow inhibitor together with
a binder resin [11].
After the war, non-cellulosic thermoplastic polymers
became more and more important as the basic fibres used for
flame retardant applications. In 1971, cotton supplied 78% of
7
the fibres used to produce children’s sleepwear whereas in
1973, it supplied less than 10% in the U.S.A [12].
With the increasing use of thermoplastics and thermosets
on a large scale for applications in building, transportation,
electrical engineering and electronics, new flame retardant
systems were developed. They mainly consist of inorganic and
organic compounds based on bromine, chlorine, phosphorus,
nitrogen, metallic oxides and hydroxides.
Today, these flame retardant systems fulfill the multiple
flammability requirements developed for the above mentioned
applications.
1.3 Types of flame retardants
A distinction is made between reactive and additive flame
retardants. Reactive flame retardant are reactive components
chemically built into a polymer molecule while additive flame
retardants are incorporated into the polymer during
polymerization [4, 7].
Reactive – type of flame retardants is preferable because they
produce stable and more uniform products, such flame
8
retardants are incorporated into the polymer structure of some
plastics. Additive -type of flame retardants, on the other hand,
are more versatile and economical. They can be applied as a
coating to woods, woven fabrics, and composites or as
dispersed additives in bulk materials such as plastics and
fibres.
There are three main families of flame-retardant
chemicals; [12, 13].
1.3.1 Inorganic flame retardants
(a) Metal hydroxides form the largest class of all flame
retardants used commercially today and are 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. 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
such as smoke reduction. Ionic compounds are used as flame
9
retardants for wool or cellulose based products. Inorganic
phosphorus compounds are primarily used in polyamides and
phenolic resins or as components in intumescent
formulations.
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 dilutes the flammable gas mixture. The very high
concentrations (50 – 80%) required to impart flame retardancy
often adversely affect the mechanical properties of the polymer
into which they are incorporated.
(b) Antimony trioxide is used as a synergist. It is utilized in
plastics, rubbers, textiles, papers typically, 2 – 10% by weight
with organochlorine and organobromine compounds to
diminish the flammability of a wide range of plastics and
textiles. 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 trihalides or
10
halide oxides. These materials act both in the substrate
(condensed phase) and in the flame to suppress flame
propagation. Other antimony compounds include antimony
pentoxide available primarily as a stable colloid or as
redispersible powder.
Sb2O3 + 6HCl → 2SbCl3 + 3H2O
Sb2O3 + 2HCl → 2SbOCl + H2O
(c) Within the class of boron compounds by far the most
widely used is boric acid. Boric acid (H3BO3) and sodium
borate (Na2B4O7. 10H2O) are the two flame retardants with the
longest history and are used primarily with cellulose material
e.g. cotton and paper. Both products are effective but their use
is limited to products for which non durable flame retardancy
is accepted since both are very water soluble.
Zinc borate 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 etc., for
example,
Sb2O5 + 6NH4BF3 → 6NH3 + 6BF3 + 2SbF3 + 3H2O
11
(d) Red phosphorus and ammonium polyphosphate (APP) are
used in various plastics. Red phosphorus was first
investigated 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 to the oxidation of elemental phosphorus during
the combustion process to phosphoric acid or phosphorus
pentoxide [12-13].
Ammonium polyphosphate is mainly applied in
intumescent coatings and paints. Intumescent systems puff
up to produce foams. Because of these characteristics, 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.
1.3.2 Halogenated organic flame retardants [14]
These 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
12
compounds are expensive and are ineffective because the C – F
bond is too strong. Iodine compounds although effective are
expensive and too unstable to be useful.
Halogenated flame retardants vary in their thermal
stability. In general, aromatic brominated flame retardants are
more thermally stable than chlorinated aliphatics, which are
more thermally stable than brominated aliphatics.
(a) Bromine-based flame retardants are highly brominated
organic compound which usually contain 50 – 85% by weight
of bromine. The highest volume brominated flame retardant in
use today is tetrabromobis – phenol A(TBBPA) followed by
decabromodiphenyl ether(DeBDE). Both of these flame
retardants are aromatic compounds. TBBPA is used as a
reactive intermediate in the production of flame retarded epoxy
resins used in printed circuit boards. It is also used as an
additive flame retardant in ABS systems. DeBDE is solely used
as an additive [15].
(b) Chlorinated paraffins are by far the most widely used
aliphatic chlorine-containing flame retardants. They have
13
applications in plastics, fabrics, paints and coatings. Aromatic
chlorinated flame retardants are not used for flame retarding
polymers.
1.3.3 Organophosphorus flame retardants
One of the principal classes of flame retardant 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.
Although, many phosphorus derivatives have flame
retardant properties, the number of these with commercial
importance is limited. Some are additive and some reactive.
The major groups of additive organophosphorus compounds
are phosphate esters, polyols, phosphonates, etc. The flame
retardancy of cellulosic products can be improved through the
application of phosphonium salt. The flame retardant
treatments attained by phosphorylation of cellulose in the
presence of a nitrogen compound are also of importance.
14
Halogenated phosphorus flame retardants combine the
flame retardant properties of both the halogen and the
phosphorus group [13]. In addition the halogens reduce the
vapour 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.
(a) Nitrogen-based compounds can be employed in flameretardant systems or form part of intumescent flame retardant
formulations [16]. 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.
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
15
flame retardant additive for polypropylene and polyethylene.
Melamine cyanurate is used in polyamides and terepthalates.
1.4 Mechanism of action of flame retardants
To understand flame retardants; it is necessary to
understand fire. Fire is a gas-phase reaction. Thus, in order
for a substance to burn, it must become a gas.
Natural and synthetic polymers can ignite on exposure to
heat. Ignition occurs either spontaneously or results from an
external source such 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 [17-19].
16
This self sustaining combustion cycle occurs across both
the gas and condensed phases. Fire retardants act to break
this cycle by affecting chemical and physical processes
occurring in one or both of the phases.
Fundamentally, four processes are involved in polymer
flammability
a. Preheating
b. Decomposition
c. Ignition
d. Combustion/propagation
Non – combustible gases
Pyrolysis
Plastic Combustible gases air gas mixture flame combustion
Q1 ignites +Q2 Products
(Endothermic) (Exothermic)
Liquid products
Solid charred residue air embers
Thermal feedback
Fig. 1: The combustion process
17
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
material begins to degrade, that is, 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 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 [17]. Combustion process is governed by such
variables as rate of heat generation, rate of heat transfer to the
18
surface, surface area, rates of decomposition [19]. Flame
retardancy can be achieved by eliminating (or improved by
retarding) any one of these variables.
Depending on their nature, flame retardants can act
chemically or physically in the solid, liquid or gas phase.
1.4.1 Physical action
There are several ways in which the combustion process
can be retarded by physical action [4];
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:- The condensed
combustible layer can be shielded from the gaseous
phase with a 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
impeded.
c. By dilution:- The incorporation of inert substances (e.g.
19
fillers) and additives that evolve inert gases on
decomposition dilutes the fuel in the solid and gaseous
phases so that the lower ignition limit of the gas mixture
is not exceeded.
1.4.2 Chemical action
a. Reaction in the gas phase:- The free 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 through
the dehydrating action of the flame retardant generating
20
double bonds in the polymer. These form the
carbonaceous layer by cyclizing and cross linking.
1.5 Improvement of the flame retardancy
Flame retardancy is improved by flame retardants that
cause the formation of a surface film of low thermal
conductivity and 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, form 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 [18-20].
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
21
shrinks or drips away from the heat source [21]. It is also
possible to significantly retard the decomposition process
through selection of chemically stable structural components.
One mechanism of 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,
hydroxide and provides radicals that are subsequently
oxidized with flame [22]. Certain flame retardants act to trap
these radicals and thereby prevent their oxidation. Bromine is
usually more effective than chlorine, for example;
HBr + HO◦ → Br◦ +H2O
HBr + O◦ → HO◦ +Br
HBr + H◦ → H2 + Br◦
HBr + ROH2 → ROH3 + Br◦
RBr → R◦ + Br◦
22
1.6 Co-additives for use with flame retardant [23]
Brominated flame retardants are in some cases used on
their own but their effectiveness is increased by a variety of coadditives, 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 hexabromocyclodecane needed to give a
flame retardant foam [15]. 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 neighbourhood of the flame.
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, sometimes the two elements are present in
23
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 ratios of the elements are readily adjusted. Brominated
flame retardants on their own act predominantly in the gas
phase while phosphorus compounds act mainly in the
condensed phase especially with oxygen containing polymers.
Bromine-phosphorus compounds affect 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 gaseous phase.
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 always used in conjunction with a halogen
compound [16]. The use of antimony trioxide reduces the high
levels normally needed for effective flame retardance of
24
bromine compounds on their own. The principal mode of
action is in the gas phase [7].
1.7 Smoke suppressants
Smoke production is determined by numerous
parameters. No comprehensive theory yet exists to describe
the formation and constitution of smoke. 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 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 [17].
1.7.1 Condensed phase
Smoke suppressants can act physically or chemically in
the condensed phase [24]. Additives can act physically in a
25
similar fashion to flame retardants, that is, by coating or
dilution thus limiting the formation of pyrolysis products and
hence of smoke. Chalk (CaCO3) frequently used as a filler acts
in some cases not only physically by effecting cross-linking so
that the smoke density is reduced in various ways. Smoke can
be suppressed by the formation of a charred layer on the
surface of the substrate, for example, by the use of organic
phosphates in unsatwurated polyester resins. In halogen
containing polymers such as PVC, iron compounds 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 of carbon (ii) oxide and carbon (iv) oxide,
so that less aromatic precursors are given off in the gas phase.
Compounds such as molybdenum oxide can reduce the
formation of benzene during the thermal degradation of PVC,
probably via chemisorption’s reactions in the condensed phase
[24].
Relatively stable benzene-MoO3 complexes that suppress
smoke development are formed.
26
1.7.2 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 [25]. 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.
1.8 Performance criteria 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
27
which are acceptable materials [26].
Many countries require extensive information on human
and environmental health effects for new substances before
they are allowed to be put on the market.
The following information regarding human and
environmental health is essential in understanding a chemical
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 behaviour and effects.
3. Since flame retardants are often processed into
polymers at elevated temperatures, consideration of
the stability of the material at the temperature
inherent to the polymers processing is needed as well
as on whether or not the material volatilizes that
temperature.
28
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.
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.
The chemical properties of a flame retardant are often of
great importance in its selection. Resistance to exposure to
water, solvents, acid, and bases may be a requirement for use.
The relationship between cost and performance is an
essential consideration in the selection of a flame retardant.
1.9 Uses of flame retardants [11] [27]
a. Plastics
The plastic industry is the largest consumer of flame
retardants estimated at about 95% for the USA in 1991 [28].
29
About 10% of all plastics contain retardants. The main
applications are in building materials and furnishings
(structural elements, roofing films, pipes, foamed plastics for
insulation, furniture and wall, floor coverings) transportation
(equipment and fillings for air craft, ships, automobiles and
railroad cars) and in electrical industry (cable housing and
components for television sets, office machines, household
appliances and lamination of printed circuits).
b. Textile/furnishing industry
In contrast to the plastics industry, the textile industry is
much smaller market for flame retardants. However, rather
than employing just one flame retardant, the use of a
combination of chemicals is usually necessary for textiles.
Phosphorus-containing materials are the most important
class of compounds to impart durable flame resistance to
cellulose. Flame retardant finishes containing phosphorus
compounds usually also contain nitrogen or bromine or
sometimes both. Another system is based on halogens in
conjunction with nitrogen or antimony.
30
1.10 Formation of toxic products on heating or
combustion of flame retarded products [26]
Natural or synthetic materials that burn produces
potentially toxic products. There has been considerable debate
on whether addition of organic 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 polybromintated dibenzodioxins
(PBDD) during manufacture, use and combustion of
brominated flame retardants.
1.10.1 Toxic products in general [29]
Combustion of any organic chemical may generate
carbon monoxide (CO) 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, HBr, phosphoric acid etc.
In general the acute toxicity of fire atmosphere is
determined mainly by the amount of CO, the source of which
31
is the amount of generally available flammable material [25].
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. Flame retardant significantly
decreases 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.
In brominated flame retardants, unless suitable metal
oxides, carbonates are also present, virtually all the bromine is
eventually converted to gaseous hydrogen bromide which is a
corrosive and powerful sensory irritant [15].
1.11 Human exposure to flame retardants
Potential sources of exposure include consumer
products, manufacturing and disposal facilities etc. Factors
affecting exposure of the general population include the
physical and chemical properties of the product, extent of
manufacturing and emission controls, end use etc. Potential
32
routes of exposure for the general population include the
dermal route (contact with flame- retarded textiles), inhalation
and ingestion.
1.11.1 Environmental exposure [26, 29 – 30]
Environmental exposure may occur as a result of the
manufacture, transport, use or waste disposal of flame
retardants. Routes of environmental exposure are water, air
and soil. Factors affecting exposure include the physical and
chemical properties of the product, emission controls,
disposal/recycling methods volume and biodegradability.
Environmental monitoring helps to determine the extent of
environmental exposure [31].
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
33
operating conditions of the incineration plant and plant’s
emission controls [32].
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. Some products such as
plastics containing flame retardants are suitable for
recycling [33].
1.12 Polyurethane foam polymer
A Polyurethane commonly abbreviated PU is any polymer
consisting of a chain of organic units joined by urethane links.
Polyurethane foams can also be defined as plastic materials in
which a proportion of solid phase is replaced by gas in the
form of numerous small bubbles (cells) [34]. The gas may be in
a continuous phase to give an open – cell material or it may be
discontinuous to give non-communicating cells. Low density
foams are dispersions of relatively large volumes of gas in
relatively small volumes of solids having for example, a density
less than 0.1 gcm-3. Medium foams are classified as having
34
density of 0.1 to 0.4gcm-3. High density foams; therefore have
a density higher than 0.4gcm-3 i.e. contain small volume of gas
in the matrix [35]. Polyurethanes are based on the exothermic
reaction of polyisocyanates and polyol molecules [36].
Many
different kinds of polyurethane materials are produced from a
few types of isocyanates and a range of polyols with different
functionality and molecular weights.
1.13 History of polyurethane foam polymer
The pioneering work on polyurethane polymers was
conducted by Otto Bayer and his co workers in 1937 at the
laboratories of I.G Farben in Leverkusen Germany [37]. They
recognized that using the polyaddition principle to produce
polyurethanes from liquid diisocyanates and liquid polyether
or polyester seemed to point to special opportunities especially
when compared to already existing plastics that were made by
polymerizing olefins or by poly condensation. The new
monomer combination also circumvented existing patents
obtained by Wallace Carothers on polyesters [24]. Initially,
work focused on the production of fibers and flexible foams
35
with development constrained by World War II (when PU’s
were applied on a limited scale as air crafting coating). It was
not until 1952 that polyisocyanates became commercially
available.
In 1954, commercial production of flexible polyurethane
foam began based on toluene diisocyanate and polyester
polyols. The first commercially available polyether polyol was
introduced by Dufont in 1956 by polymerizing
tetrahydrofuran. In 1960, more than 45,000 tons of flexible
polyurethane foams were produced. As the decades progressed
the availability of chlorofluoroalkane blowing agents,
inexpensive polyether polyols and methylene diphenyl
diisocyanate (MDI) heralded the development and use of
polyurethane rigid foam as high performance insulation
materials. Urethane modified polyisocyanurate rigid foams
were introduced in 1967 offering even better stability and
flammability resistance to low density insulation products.
Also, during the 1960s, automotive interior safety components
such as door panels were produced by back filling
thermoplastic skins with semi-rigid foam.
36
In 1969, Bayer A.G exhibited an all plastic car in
Dusseldorf, Germany. Parts of this car were manufactured
using a new process called RIM (Reaction Injection Moulding)
[36]. Polyurethane RIM evolved into a number of different
products and processes. In 1980s, water blown micro cellular
flexible foam was used to mould gaskets for panel and radial
seal air filters in the automotive industry. Building on existing
polyurethane spray coating technology, extensive development
of two component polyurea spray elastomers took place in the
1990s.
During the same period, two new components
polyurethane and hybrid polyurethane polyurea elastomer
technology were used to enter the market place of spray- inplace load bed liners [38-39]. This technique creates a durable,
abrasion resistant composite with the metal substrate and
eliminates corrosion and brittleness associated with drop in
thermoplastic bed liners. The use of polyols derived from
vegetable oils to make polyurethane products began gaining
attention beginning around 2004, partly due to rising cost of
37
petrochemical feedstocks and partially due to an enhanced
public desire for environmentally friendly green products [40].
1.14 Basic chemical of polyurethane foam [41]
Polyurethanes belong to the class of compounds called
reaction polymers which include epoxies, unsaturated
polyesters and phenolics [38-39]. A urethane linkage is
produced by reacting an isocyanate group – N = C = O with a
hydroxyl (alcohol group) – OH.
H O
R1 – N= C = O + R2 – O – H → R1 – N – C – O – R2
Although, polyurethane synthesis can be effected by reaction
of chloroformic ester with diamines and of carbamic esters
with diols.
– RNH2 + ClCOOR’ → – RNHCOOR’ – + HCl – – – (i)
– ROH + ZOOCNHR1→ – ROOCNHR1 – + ZOH – – -(ii)
38
RNCO
Development has depended basically on the chemistry of
isocyanates, first investigated well over a hundred years ago by
Wurtz and by Hoffman but only directed to polymer formation
when Otto Bayer in 1938, during research on fibre forming
polymer analogous to the polyamides prepared a number of
linear polyurethane from diisocyanates and diols [1]. For
example, polyurethane from 1,4-butanediol and
hexamethylene diisocyanate:
HO (CH2)4 OH + OCN (CH2)6 NCO
[ O (CH2)4 OOCNH (CH2)6 NH COO ]
The NCO group can react generally with compounds
containing active hydrogen atoms i.e. according to the
following:
RNCO + R’OH → RNHCOOR urethane – – – (iii)
RNCO + R’NH2 → RNHCONHR urea – – – (iv)
RNCO + R’ COOH → RNHCOR’+CO2 Amide – – – (v)
RNCO + H2O → [RNHCOOH] → RNH2 + CO2
RNHCONHRUrea – – – (vi)
39
Thus, if the reagents are di or polyfunctional polymer,
formation can take place while these reactions normally occur
at different rates, they can be influenced appreciably and
controlled by the use of catalysts. Reactions (v) and (vi) give
rise to carbon (iv) oxide, a feature of value when forming
foamed products but introducing difficulty if bubble – free
castings and continuous surface coatings are required.
Linear products result if the reactants are bifunctional
but higher functionality leads to the formation of branched
chain or cross linked material. Chain branching or cross
linking then occurs due to the formation of acylurea, biuret
and allophanate links onto the main chain.
– RNCO + R’NHCOR’ → R’NCOR’ Acylurea
CONHR –
– RNCO + R’NHCONHR’
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