Horrocks / Price Fire Retardant Materials

1. Auflage 2001
ISBN: 978-1-85573-746-4
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 448 Seiten

This is a comprehensive source of information on all aspects of fire retardancy. Particluar emphasis is placed on the burning behaviour and flame retarding properties of polymeric materials and textiles. It covers combustion, flame retardants, smoke and toxic products generally and then goes on to concentrate on some more material-specific aspects of combustion in relation to textiles, composites and bulk polymers. Developments in all areas of fire retardant materials are covered including research in new areas such as nanocomposition.Fire retardant materials is an essential reference source for all those working with, researching into, or designing new fire retardant materials. - Detailed analysis of the burning behaviour and flame retarding properties of ploymers, composites and textiles - Covers smoke and toxic gas generation - Analysis of material performance in fire

Autoren/Hrsg.

Horrocks, A. Richard

Price, D.

Inhaltsverzeichnis

1;Front Cover;1
2;Fire Retardant Materials;4
3;Copyright Page;5
4;Table of Contants
;6
5;Preface;12
6;List of contributors;14
7;Chapter 1. Introduction: polymer combustion, condensed phase pyrolysis and smoke formation;18
7.1;1.1 Polymers;18
7.2;1.2 Condensed-phase processes;28
7.3;1.3. Smoke;36
7.4;1.4 Conclusions;45
7.5;1.5 Bibliography;45
7.6;References;45
8;Chapter 2. Mechanisms and modes of action in flame retardancy of polymers;48
8.1;2.1 Introduction;48
8.2;2.2 General considerations;48
8.3;2.3 Gas-phase mechanisms;49
8.4;2.4 The condensed-phase mechanism;52
8.5;2.5 Modes of action of halogen-based flame retardants: synergistic systems;56
8.6;2.6 Modes of action of phosphorus-based flame retardants;60
8.7;2.7 Modes of action of borates;72
8.8;2.8 Modes of action of metal hydroxides and other hydrated inorganic additives;73
8.9;References;74
9;Chapter 3. Toxicity of fire retardants in relation to life safety and environmental hazards;86
9.1;3.1 Introduction;86
9.2;3.2 Toxic combustion products from fires: general;88
9.3;3.3 Toxic effects of smoke products;93
9.4;3.4 Methods available and hazards to be assessed;95
9.5;3.5 Effects of specific fire-retardant systems;103
9.6;3.6 Conclusions;138
9.7;References;140
10;Chapter 4. Textiles;145
10.1;4.1 Introduction;145
10.2;4.2 Hazards and risks;145
10.3;4.3 Burning behaviour of textiles;148
10.4;4.4 Flammability testing of textiles;157
10.5;4.5 Burning and flame-retardant mechanisms;163
10.6;4.6 Cellulosic textiles;170
10.7;4.7 Flame-retarded wool and blends;174
10.8;4.8 Flame-retardant synthetic fibres;177
10.9;4.9 High heat and flame resistant fibres and textiles;183
10.10;4.10 Methods of flame retardancy;186
10.11;4.11 Environmental issues;191
10.12;4.12 Conclusions;194
10.13;References;195
11;Chapter 5. Composites;199
11.1;5.1 Introduction;199
11.2;5.2 The properties of the constituents of composites;200
11.3;5.3 Flammability of composite structures;210
11.4;5.4 Methods of imparting flame retardancy to composites;214
11.5;5.5 Conclusions: some important considerations;218
11.6;References;218
12;Chapter 6. Nanocomposites;221
12.1;6.1 Introduction;221
12.2;6.2 The structure and properties of layered silicates;222
12.3;6.3 The structure of nanocomposites;224
12.4;6.4 Synthesis methods;225
12.5;6.5 Characterisation;227
12.6;6.6 Properties of nanocomposites;227
12.7;6.7 The mechanism of flame retardance in nanocomposites;231
12.8;6.8 Conclusion;234
12.9;References;234
13;Chapter 7. Recent developments in flame-retarding thermoplastics and thermosets;237
13.1;7.1 Introduction;237
13.2;7.2 Thermoplastics versus thermosets;238
13.3;7.3 Factors affecting flammability and its reduction;238
13.4;7.4 Testing procedures and hazard assessments: general aspects;244
13.5;7.5 Flame-retardant thermoplastics;246
13.6;7.6 Flame-retardant elastomers;259
13.7;7.7 Flame-retardant thermosets;262
13.8;7.8 Inherently flame-retardant polymers;265
13.9;7.9 Conclusions;266
13.10;References;266
14;Chapter 8. Applications of halogen flame retardants;281
14.1;8.1 Introduction;281
14.2;8.2 Mechanism of flame retardancy with halogenated compounds;282
14.3;8.3 Guidelines for choice of halogenated flame retardants;293
14.4;8.4 New trends in halogenated flame retardants;306
14.5;8.5 Conclusions;307
14.6;Acknowledgements;308
14.7;References;308
15;Chapter 9. Natural polymers, wood and lignocellulosic materials;310
15.1;9.1 Combustion of natural polymers, wood and lignocellulosic materials;310
15.2;9.2 Chemistry of fire retardancy;314
15.3;9.3 Fire retardancy of lignocellulosic boards and panels;322
15.4;9.4 Test methods;326
15.5;9.5 Recent progress in fire retardants for wood and lignocellulosics;328
15.6;References;329
16;Chapter 10. Intumescent materials;335
16.1;10.1 Introduction;335
16.2;10.2 Physical modelling of intumescent polymer behaviour in fires;336
16.3;10.3 Chemical aspects of intumescence;342
16.4;10.4 Intumescent systems;349
16.5;10.5 Conclusion;349
16.6;References;352
17;Chapter 11. Graft copolymerisation as a tool for flame retardancy;354
17.1;11.1 Introduction: the production of char from polymers;354
17.2;11.2 The importance of char;355
17.3;11.3 Conclusion;368
17.4;References;368
18;Chapter 12. Performance-based test methods for
material flammability;372
18.1;12.1 Introduction;372
18.2;12.2 Testing and ranking systems;373
18.3;12.3 Performance-based test methods;379
18.4;12.4 End-use scenarios, critical conditions and controlling processes;383
18.5;12.5 Models for calculating upward flame spread and fire growth;385
18.6;12.6 Conclusions and recommendations;391
18.7;References;392
19;Chapter 13. Fire safety design requirements of flame-retarded materials;395
19.1;13.1 Introduction: drawbacks of tests and measurements;395
19.2;13.2 Some fundamentals: flame spread and flashover;399
19.3;13.3 Selection of materials as part of the design process;403
19.4;13.4 Conclusions;411
19.5;References;412
20;Chapter 14. Mathematical modelling;415
20.1;14.1 Introduction;415
20.2;14.2 A basic model for single-step pyrolysis;416
20.3;14.3 Extensions of the simple model;424
20.4;14.4 Ignition;432
20.5;14.5 Discussion;435
20.6;References;437
21;Index;442

Leseproben

2 Mechanisms and modes of action in flame retardancy of polymers
Menachem Lewin; Edward D. Weil    Polymer Research Institute, Polytechnic University, Brooklyn, New York 2.1 Introduction
Some basic mechanisms of flame retardancy were recognised as early as 1947 when several primary principles were put forward.1 These included the effect of the additive on the mode of the thermal degradation of the polymer in order to produce fuel-poor pyrolytic paths, external flame retardant coatings to exclude oxygen from the surface of the polymer, internal barrier formation to prevent evolution of combustible gases, inert gas evolution to dilute fuel formed in pyrolysis and dissipation of heat away from the flame front. Discovery of the flame-inhibiting effect of volatile halogen derivatives subsequently led to the postulation of the radical trap-gas-phase mechanism.2 The gas-phase and the condensed-phase proposals have long been generally considered as the primary, though not the only, effective mechanism of flame retardancy. This situation is now being modified as new mechanisms of new flame-retarding systems, especially those based on physical principles, evolve and as new insights into the performance of flame retardants is being gained. In many cases several mechanistic principles operate simultaneously and consequently it is difficult to identify one dominant mechanism. In such cases modes of action of particular flameretarding formulation may be defined and described. This paper attempts to review some of the principles, mechanisms and modes of action which prevail at present in the field of flame retardancy of polymers. 2.2 General considerations
Pyrolysis and combustion of polymers occur in several stages. The polymeric substrate heated by an external heat source is pyrolysed with the generation of combustible fuel. Usually, only a part of this fuel is fully combusted in the flame by combining with the stoichiometric amount of atmospheric oxygen. The other part remains and can be combusted by drastic means, e.g. in the presence of a catalyst and by an excess of oxygen. A part of the released heat is fed back to the substrate and causes its continued pyrolysis, perpetuating the combustion cycle. Another part is lost to the environment. The energy needed to heat the polymer to the pyrolysis temperature and to decompose and gasify or volatilise the combustibles and the amount and character of the gaseous products determines the flammability of the substrate. A flame retardant acting via a condensed-phase chemical mechanism alters the pyrolytic path of the substrate and reduces substantially the amount of gaseous combustibles, usually by favouring the formation of carbonaceous char and water.3 In this case the heat released in the combustion decreases with an increase in the amount of the flame-retarding agent. In the gas-phase mechanism the amount of combustible matter remains constant but the heat released in the combustion usually decreases with an increase in the amount of the flame-retarding agent. The amount of heat returned to the polymer surface is therefore also diminished and the pyrolysis is retarded or halted as the temperature of the surface decreases. The flame-retarding moiety has to be volatile and reach the flame in the gaseous form. Alternatively it has to decompose and furnish the active fraction of its molecule to the gaseous phase. The char remaining in the substrate will contain less of the active agent. The pyrolysis of the polymer should, in the limiting case, proceed as if there would have been no flame-retarding agent incorporated in it. In addition presence of the gas-phase active agent should not influence the composition of the volatiles reaching the flame.3 2.3 Gas-phase mechanisms
The gas-phase activity of the active flame retardant consists in its interference in the combustion train of the polymer. Polymers, like other fuels, produce upon pyrolysis species capable of reaction with atmospheric oxygen and produce the H2–O2 scheme which propagates the fuel combustion by the branching reaction:4 •+O2=OH•+O•   [2.1] •+H2=OH•+H•   [2.2] The main exothermic reaction which provides most of the energy maintaining the flame, is: H•+CO=CO2+H•   [2.3] To slow down or stop the combustion, it is imperative to hinder the chain-branching reactions [2.1] and [2.2]. The inhibiting effects of halogen derivatives, usually chlorine and bromine, is considered to operate via the gas-phase mechanism. This effect in the first instance occurs either by releasing a halogen atom, if the flame-retardant molecule does not contain hydrogen, or by releasing a hydrogen halide: X=M•+X•   [2.4] X=HX+M•   [2.5] where M• is the residue of the flame-retardant molecule. The halogen atom reacts with the fuel, producing hydrogen halide: H+X•=HX+R•   [2.6] The hydrogen halide is believed to be the actual flame inhibitor by affecting the chain branching: •+HX=H2+X•   [2.7] H•+HX=H2O+X•   [2.8] Reaction [2.7] was found to be about twice as fast as [2.8] and the high value of the ratio H2/OH in the flame front indicates that [2.7] is the main inhibiting reaction.5 It is believed that the competition between reactions [2.7] and [2.1] determines the inhibiting effect. Reaction [2.1] produces two free radicals for each H atom consumed, whereas reaction [2.7] produces one halogen radical which recombines to become the relatively stable halogen molecule. 2.3.1 Comparing flame-retardant activity of halogen derivatives
Equation [2.7] represents an equilibrium with a forward reaction and a reverse reaction. The equilibrium constants of equation [2.7] for HBr and HCl are:6 HCI=0.583?exp?1097/RT;KHBr=0.374?exp?(16760/RT The equilibrium constants decrease strongly with increase in temperature, which explains the decreasing effectivity of halogen derivatives in large hot fires.6 Petrella5 calculated that in the temperature range 500–1500 K the forward reaction predominates and KHBr is much higher than KHCl. Both are highly effective at the ignition temperature range of polymers. The flame-retardant effectivity of the halogens was stated to be directly proportional to their atomic weights, i.e. F:Cl:Br:I = 1.0:1.9:4.2:6.7.7 On a volumetric basis 13% of bromine was found to be as effective as 22% of chlorine when comparing the tetrahalophthalic anhydrides as flame retardants for polyesters.8,9 A similar effect was found for PP, PS and PAN3 and when comparing NH4Cl to NH4Br in cellulose.9 The activity of the halogens is also strongly affected by the strength of the respective carbon-halogen bonds. The low bond strength of I–C and consequently the low stability of the iodine compounds virtually exclude their use. The high stability of the fluorine derivatives and the high reactivity of the fluorine atoms in reactions [2.7] and [2.8] will prevent the radical quenching processes in the flame. The lower bond strength and stability of the aliphatic compounds, their greater ease of dissociation as well as the lower temperature and earlier formation of the HBr molecules are responsible for their higher effectivity as compared to the aromatic halogen compounds. The higher stability of the latter along with their higher volatility allow these compounds to evaporate before they can decompose and furnish the halogen to the flame. 2.3.2 Physical modes of action of halogenated flame retardants
The radical trap activity is not the only activity of the halogenated flame retardants. The physical factors such as the density and mass of the halogen and its heat capacity, have a profound influence on the flame-retarding activity of the agent. In addition, its dilution of the flame which thus decreases the mass concentration of combustible gases present are effective. Larsen7,199,200 demonstrated the important role of the heat capacity of the flame retardant. In flame retardant polymer systems the halogens appear to work by reducing the heat evolved in the combustion of the gases given off by the decomposing polymer (low or zero fuel value plus action as a heat sink) such that to sustain burning the mass rate of gasification must be increased by the application of an increased external heat flux.10 Other authors11 showed by thermochemical computation that most of the action of a wide variety of halocarbon flame inhibitors could be correlated to a combination of heat capacity and endothermic bond dissociation. A physical effect, often mentioned but rarely demonstrated or evaluated, is the ‘blanketing’ effect of excluding oxygen from the surface of the pyrolysing polymer. Ignition generally takes place in the vapour phase adjacent to the condensed phase, when an ignitable fuel–air mixture is reached. There is, however, evidence that the rate of pyrolysis...