Investigation of effects of alum and potassium sesquicarbonate on the fire characteristics of flexible polyurethane foam

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

Project 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.

Project Overview

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 polymer compatible 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. Sb2O 3 + 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 flame retardant 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].16This 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/propagationNon – combustible gasesPyrolysis Plastic Combustible gases air gas mixture flame combustion Q1 ignites +Q2 Products(Endothermic) (Exothermic)Liquid productsSolid charred residue air embersThermal feedbackFig. 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 + HOâ—¦ → Brâ—¦ +H2O HBr + Oâ—¦ → HOâ—¦ +Br HBr + Hâ—¦ → H2 + Brâ—¦ HBr + ROH2 → ROH3 + Brâ—¦ RBr → Râ—¦ + Brâ—¦ 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 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 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- in place 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 – R2Although, 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’ R’ – N – CONHR – Biuret CONHR –40 – RNCO + R’NHCOOR RNCOOR – CONHR – AllophanateThe initial studies on polyurethane synthesis were based on simple diisocyanates and diols but the main importance of the reaction is now concerned with the use of intermediates which are often themselves polymeric in character (polyesters, polyethers) and carry terminal groups (usually – OH or – NCO) capable of further reaction and thus of increasing the molecular size during actual fabrication, processing, chain extension etc. some of the reactions are reversible under the action of heat, thus introducing the possibility of molecular rearrangement during processing [39]. The “polyurethanes” can have a preponderance of other linking groups and the whole macro-molecular system in these polymers can accordingly be designed so as to incorporate links which provide the required molecular flexibility, branching or cross linking necessary to give the properties sought in the finished product [42].41 1.15 Raw materials used for polyurethane foam polymers [43] In manufacturing polyurethane polymers, two groups of at least bifunctional substances are needed as reactants; compounds with isocyanate groups and compounds with active hydrogen atoms. The physical and chemical character, structure and molecular size of these compounds influence the polymerization reaction as well as ease of processing and final physical properties of the finished polyurethane. In addition additives such as catalysts, surfactants, blowing agents, cross linkers, flame retardants, light stabilizers and fillers are used to control and modify the reaction process and performance characteristics of the polymer [39]. The raw materials include the following:1.15.1 Isocyanates Isocyanates with two or more functional groups are required for the formation of polyurethane polymers. Only the diisocyanates are of interest for polyurethane manufacture and relatively few of these are employed commercially. Volume wise, aromatic isocyanates account for the vast majority of 42 global diisocyanate production. Aliphatic and cycloaliphatic isocyanates are also important building blocks for polyurethane materials but in much smaller volumes. There are a number of reasons for this; first, the aromatically linked isocyanate group is much more reactive than the aliphatic one. Secondly, aromatic isocyanates are more economical to use. Aliphatic isocyanates are used only if special properties are required for the final product. Even within the same class of isocyanates, there is a significant difference in reactivity of the functional group based on steric hindrance. In the case of 2, 4-toluene diisocyanate, the isocyanate group in the para position to the methyl group is much more reactive than the isocyanate group in the ortho position. The most important ones used in elastomer manufacture are the 2, 4- and 2, 6 toluene diisocyanates (TDI), 4,4-dicyclohexylmethane diisocyanates (MDI) and its aliphatic analogue 4, 4 dicyclohexylmethane diisocyanate (H12MDI) xylene diisocyanate (XDI) etc. Some various monoisocyanates used commercially are n – butyl, n – propyl, n – phenyl and 4 – 43 NaN3 -N2 NaOBr -HBr chloro and 3, 4 -dichlorophenyl isocyanates which are used for substituted ureas and carbamates [44]. Isocyanates can be made in many ways using the Curtius, Hoffman and Lossen rearrangements which may involve nitrene as an intermediate but are not satisfactory for large scale operation. a. Curtius Reaction RCOCl RCON3 RCON RNCOb. Hoffman RearrangementRCONH2 RCONHBr RCON RNCOc. Lossen rearrangement R’COOR2 NH2OH R2OH + RCONHOH H2O RCON RNCOThe use of azides in the Curtius reaction is hazardous and the utility of Hoffman and Lossen rearrangement is limited to preparation of isocyanates. An isocyanate takes part 44 in very many reactions but are difficult to prepare in high yield and purity. Aromatic isocyanates are made by phosgenation of the corresponding amines or amine hydrochlorides in an inert medium (o- dichlorobenzene) the reaction proceeding in two stages: first, at room temperature or some what higher temperature to generate the carbamyl chloride and HCl; further treatment with phosgene at temperature of 150 – 170ËšC then forms the isocyanate.RNH2 COCl2 RNHCOCl + HCl RNH2 RNH2HCl + RNCO HCl RNH2HCl COCl2 RNCO + 3HCl1.15.2 Polyols Polyols are higher molecular weight materials manufactured from an initiator and monomeric building blocks. They are easily classified as polyether and polyester polyols. Polyether polyols contain the repeating ether linkage – R-O-R- and have two or more hydroxyl groups as terminal functional groups. They are manufactured commercially by 45 the catalyzed addition of epoxies (cyclic ethers) to an initiator (active hydrogen containing compounds) such as water, glycols. Polyester polyols are made by the polycondensation of multifunctional carboxylic acids and hydroxyl compounds. They can be further classified according to their end use as flexible or rigid polyols depending on the functionality of the initiator and their molecular weight. Flexible polyols have molecular weights from 2,000 to 10,000 (OH group from 18 to 56) while rigid polyols have molecular weights from 250 to 700 (OH group from 300 to 700). Polyols for flexible applications use low functionality initiators such as dipropylene glycol (f = 2) or glycerine (f = 3) while polyols for rigid applications use high functionality initiators such as sucrose (f = 8), sorbitol (f = 6) and, mannich bases (f = 4). Graft polyols (also called filled or polymer polyols) contain finely dispersed styrene acrylonitrile or polyurea (PHD) polymer solids chemically grafted to a high molecular weight polyether backbone. They are used to add toughness to microcellular foams and cast elastomers PHD polyols are used to modify the combustion properties of HR flexible foam. 46 Polyester polyols fall into two distinct categories according to composition and application Conventional polyester polyols are based on virgin raw materials and manufactured by the direct polyesterification of high purity diacids and glycols such as adipic acid and 1, 4- butanediol. Other polyester polyols are based on reclaimed raw materials and are manufactured by transesterification (glycolysis) of recycled polyethyleneterephthalate (PET). They bring excellent flammability characteristics to polyisocyanurate (PIR) board stock and polyurethane spray foam insulation [44].1.15.2.1 Polyethers Polypropylene glycols and poly tetramethylene glycols are the polyethers commonly used in solid polyurethanes. The manufacturing process in both cases involves the addition polymerization of the monomeric epoxide.47 n nCH2 – CH2 catalyst H O (CH2)4 OH Polytetramethylene glycol CH2 CH2O CH3 CH2 – CHCH3 base catalyst H OCH2CH OHOThe manufacture of polypropylene glycol is usually carried out in stainless steel or glass line reactors and similar to the polyesters by essentially batch process. A polymerization initiator is employed to control the type of polyether produced. Ethylene glycerol, propylene glycol, diethylene glycol and dipropylene glycol can be used as initiator in the manufacture of difunctional polyethers whereas glycerol is a general purpose initiator for trifunctional polyethers. Polyether based 48 polyurethanes have better hydrolytic stability and lower temperature flexibility than polyester based polyurethanes.1.15.2.2 Polyesters The manufacture of polyesters is usually carried out as a batch process in glass lined or stainless steel reactors as a condensation polymerization. For preparation of the polyesters, conventional methods of polyesterification i.e. reaction between acid and diol or polyol are used, the water of condensation being removed by distillation and the reaction assisted, if necessary by use of an azeotrope or vacuum. The molecular weight can be controlled by the molar ratio of the reactants and the reaction conditions, but it is essential that the terminal groups should be hydroxyl so as to ensure facility for ultimate reaction with isocyanates. Caprolactone polyester is another type of polyester which is of interest in the field of solid polyurethanes and it is obtained by the addition polymerization of caprolactone in the presence of an initiator.49 n 2nCH2 (CH2)4CO + HOROH O HO (CH2)5COO R OOC(CH2)5 OHThe reaction is rapid and has the advantage that no water is produced as a by product. Low molecular weight polyester with functionalities e.g. f = 2.4 or f =3 have been made of cross linking agent for use with polyurethanes. Polyester based polyurethanes are less expensive and have better oxidative and high temperature stability than polyether based urethanes. 1.15.3 Surfactants [45] Surfactants are added to the foam formulation to decrease the surface tension of the system and facilitate the dispersion of water in the hydrophobic medium. They are used to modify the characteristics of both foam and non foam polyurethane polymers. In foams, they also aid in nucleation, stabilization and regulation of the cell structure. The choice of surfactants depends upon the type of foam preparation. 50 Both ionic and non ionic surface active agents have been employed. Anionic surfactants have been used for the preparation of polyester and polyether prepolymer foams. Non ionic surfactants are used in polyester and polyether urethanes. Examples of surfactants are block or graft copolymers, polymethylsiloxanes, polyalkylene oxides etc.1.15.4 Chain extenders and cross linkers Chain extenders (f=2) and cross linkers (f=3 or greater) are low molecular weight hydroxyl and amine terminated compounds that play an important role in the polymer morphology of polyurethane fibers, elastomers, adhesives and certain integral skin and micro cellular foams. The elastomeric properties of these materials are derived from the phase separation of the hard and soft copolymers segments of the polymer, such that the urethane hard segment domains serve as cross links between the amorphous polyether (or polyester) soft segment domains. This phase separation occurs because the mainly non-polar, low melting soft segments are incompatible with the polar, high melting hard segments. 51 The soft segments, which are formed from high molecular weight polyols are mobile and are normally present in coiled formation, while the hard segments which are formed from the isocyanate and chain extenders are stiff and immobile [46]. The choice of chain extender determines flexural, heat and chemical resistance properties. The most important chain extenders are ethylene glycol, 1, 4-butanediol (1, 4 – BDO or BDO) 1, 6 – hexanediol, hydroquinone bis (2-hydroxy ether) ether (HQEE). All of these glycols form polyurethanes that phase separate well and form well defined hard segment domains and are melt processable. They are all suitable for thermoplastic polyurethanes with the exception of ethylene glycol since its derived bis – phenyl urethane undergoes unfavourable degradation at high hard segment levels.1.15.4 Catalysts The catalyst most widely used commercially in polyurethane processes are tertiary amines and organotin compounds, catalysts can be classified as to their specificity, balance and relative power on efficiency. Traditional amine 52 catalysts have been tertiary amines such as triethylenediamine (TEDA also known as 1, 4-diazobicyclo [2.2.2] octane or DABCO) and dimethylethanolamine (DMEA). Tertiary amine catalysts are selected based on whether they drive the urethane (polyol + isocyanate) or gel reaction, the urea (water + isocyanate or blow) reaction or the isocyanate trimerization reaction. Since most tertiary amine catalysts will drive all three reaction to some extent, they are also selected based on how much they favour one reaction over another. Molecular structure gives some clue to the strength and selectivity of the catalyst. The requirement to fill large, complex tooling with increasing production rates has led to the use of blocked catalysts to delay front end reactivity while maintaining back end cure. Increasing aesthetic and environmental awareness has led to the use of non-fumigitive catalyst for vehicle interior and furnishing applications in order to reduce odour [47]. Organometallic compounds based on mercury, lead, tin (dibutyltin dilaurate) and zinc are used as polyurethane catalysts. Mercury carboxylates such as phenylmercuric 53 neodeoconate are particularly effective catalysts for polyurethane elastomer, coating and sealants: lead catalysts are used in highly reactive rigid spray foam insulation applications. Since the 1990s, bismuth and zinc carboxylates have been used as alternatives to lead and mercury because of the toxicity but they have short comings of their own.1.16 Physical properties of polyurethane foams Generally, the physical properties of polyurethane foams depend on the method by which they are prepared. For example, the windows may or may not be ruptured in the final stage of expansion, depending on the relative rate of molecular growth (gelation) and gas reaction, giving rise to flexible or rigid foams [48]. In polyurethane foam preparation, the variety in choice of simple molecules is great and consequently, the properties of the product are wide. Choice of the polyol has a major effect on the foam properties especially on its rigidity and flexibility. The crosslink density of the urethane polymer determines whether the foam will be flexible (low cross-link density) or 54 rigid (high cross-link density). Rigid foams are prepared from highly branched resins of low molecular weight while flexible foams are prepared from polyols of moderately high molecular weight and low degree of branching.1.17 Mechanical properties of polyurethane foam The mechanical properties of polyurethane foam are highly dependent on the proportion of the allophanate linkage which increases the reaction time and temperature for toluene diisocyanate based urethane. They are influenced by the functionality and molecular shape.1.18 Chemical properties of polyurethane foam polymer The chemical properties of polyurethane foams are also a function of the preparation process. For example, solvent resistance of polyurethane structure increases at higher cross link densities, appears to be unaffected by the type of aromatic diisocyanate and is reduced with the use of a large excess of isocyanate. The aliphatic and cycloaliphatic isocyanate can produce a polymer with an outstanding resistance to sunlight 55 as aliphatic are normally less photosensitive than their aromatic counterpart [49-50].1.19 Polyurethane foam polymer structures A urethane elastomer can be regarded as a linear block copolymer of the type shown below [51].A (B B)n A C Polyol Mono or Chain Flexible polymeric extender Block Isocyanate may be Rigid block flexible or rigid.Fig. 2: Basic unit in a urethane block copolymerThe segmented polymer structure can vary. Its properties over a very wide range of strength and stiffness by modification of its three building blocks; the polyol, diisocyanate and chain extender (glycol). Essentially the 56 hardness range covered is that of soft jelly like structures to hard rigid plastics. Properties are related to flexibility, chain entanglement, interchain forces and crosslinking [52]. Evidences from x-ray diffraction, thermal analysis, mechanical properties strongly supports the view that these polymers should be considered in terms of long (1000-2000nm) flexible segments and shorter (150nm) rigid units which are chemical and hydrogen bonded togetherRigid foams Semi – rigid Flexible foams, Foams surface coating Degree of branching or cross linkingCast elastomers Textile coating Spandex Films Fibres Increasing Plastics Milliable elastomers Chain stiffness interchain attraction -NH–C–O– Crystallinity O Increasing The Urethane linkFig. 3: Structure – property relationships in polyurethane1.20 Applications of polyurethane foam polymer [52] Polyurethane foams, used for almost 40years, offer a wide variety of products suitable

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