Flammability.pdf

Encyclopedia of Polymer Sceince and Technology

FLAMMABILITY

Introduction

Plastics (polymers) are a large and growing fraction of the ﬁre load in homes, com-

mercial environments, and transportation (1–5). Moreover, the plastics that are

most widely used are those that are least expensive and these tend to be the most

ﬂammable. Flammability , which generally refers to the propensity of a substance

to ignite easily and burn rapidly with a ﬂame, is one indicator of ﬁre hazard. Fig-

ure 1 shows the relationship between two measures of ﬂammability—heat release

capacity (see under Heat Release Rate) and ﬂame resistance (see under Testing

for Regulatory Compliance) versus the truckload price of commercial polymers.

Flammability and cost span over 2 orders of magnitude but the commodity poly-

mers costing less than about $1/pound comprise over 95% of the polymers in use

and these will continue to burn after brief exposure to a small ﬂame. Engineering

and specialty plastics costing over $2/pound are typically polymers with aromatic

backbones and ﬂuoropolymers, and these will self-extinguish or resist ignition be-

cause of high thermal stability or low fuel value. Figure 1 shows that the ﬂame

resistance of polymers does not always correlate with cost but does correlate rea-

sonably well with heat release capacity.

Regulations (1–5) governing the ﬂammability of plastics (eg, see section Test-

ing for Regulatory Compliance) used in consumer electronics, electrical equip-

ment, building and construction, home furnishings, automobiles, and public trans-

portation have created an annual worldwide market of tens of billions of pounds

for ﬂame-retardant plastics (1). The designation ﬂame retardant, ﬂame resistant,

or ignition resistant as it applies to these plastics typically refers to the tendency

of a thin (2–3 mm) strip of the material to withstand a brief exposure to a Bunsen

burner ﬂame or a hot wire without continuing to burn. Flame or ignition resis-

tance is a low level of ﬁre safety that is intrinsic to heat-resistant polymers but can

be achieved with commodity polymers by adding ﬂame-retardant chemicals (1–4).

The economic incentive to add ﬂame retardants to commodity polymers in order

to pass ﬂammability requirements (ie, the slope in Fig. 1) has focused polymer

ﬂammability research over the past few decades on the mechanisms and efﬁcacy

of ﬂame-retardant additives (6–10), rather than on polymer ﬂammability as an

intrinsic material property. This trend in research combined with the fact that

ﬂaming combustion of solids is a highly coupled, multiphase process, and that ﬁre

test results depend on the apparatus, test conditions, and sample geometry, has

limited the understanding of polymer ﬂammability to a descriptive/qualitative

nature.

FLAMMABILITY

Fig. 1. Heat release capacity, cost, and ﬂame resistance of commercial polymers.

In this article simplifying assumptions are made about the burning processes

under well-deﬁned (standardized) conditions in order to provide mathematical

relationships between polymer chemical structure and ﬂammability properties.

Experimental data are used in lieu of, and in support of, analytic results drawn

from the broad subject material that is referenced in the bibliography. The goal

of this article is to provide a consistent, mechanistic interpretation of the burning

process as it relates to synthetic polymers and to describe currently accepted test

methods to quantify burning behavior.

The Burning Process

1 kJ/kg) secondary chemical bonds. These volatile compounds spontaneously

form combustible mixtures with air that ignite easily and burn with a high ve-

locity. Polymers are very large (macro) molecules with the same intermolecular

and intramolecular forces as low molecular weight compounds, but their boiling

temperature is essentially inﬁnite because of their high molecular weight. Con-

sequently, both intermolecular and intramolecular (backbone) chemical bonds of

polymers must be broken to generate volatile fuel species, and this process re-

quires a large (

2 MJ/kg) and continuous supply of thermal energy for ignition

and sustained burning.

Flaming combustion can be roughly divided into physical and chemical pro-

cesses taking place in each of three separate phases: gas, mesophase, and con-

densed (liquid/solid) phase (6,7,10–16). The mesophase is the interface between

≈

Gases and volatile liquids are small molecules that are held together by weak

(

FLAMMABILITY

Fig. 2. Physical and chemical processes in the ﬂaming combustion of polymers.

the gas and condensed phase during burning. Figure 2 is a schematic diagram of a

horizontal polymer slab that is burning with a diffusion ﬂame. Physical processes

are shown on the left-hand side of Figure 2, and these include energy transport

by radiation and convection between the gas phase (ﬂame) and the mesophase;

energy loss from the mesophase by mass transfer (vaporization of the pyrolysis

gases); and conduction into the solid. At typical burning rates the polymer surface

(mesophase) recedes at a velocity of about 10 − 6 m/s. Conservation of momentum

at the gas–mesophase boundary shows that fuel gases evolve at a relatively low

velocity (

≈

10 − 3 m/s) compared to the burning velocity of these gases when mixed

1 m/s). Consequently, fuel generation is the rate-limiting step in poly-

mer ﬂaming combustion and it is governed primarily by the rate at which heat

and mass are transported to and from the polymer, respectively.

The important chemical processes are shown on the right-hand side of Fig-

ure 2, and these are thermal degradation of the polymer in a thin surface layer

(the mesophase) as a consequence of the physical (energy transport) processes;

mixing of the volatile pyrolysis products with air by diffusion; and combustion

of the fuel/air mixture in a combustion zone that produces radiant energy over a

spectrum of wavelengths including visible. The combustion zone is bounded by a

fuel-rich region on the inside and fuel-lean region on the outside. Increasing the

concentration of oxygen in the environment is known to increase the ﬂame heat

ﬂux, because of either a higher ﬂame temperature, an increase in the volume of

the combustion zone, or an increase in the soot concentration (luminosity) of the

ﬂame. The chemical and physical processes of ﬂaming combustion particular to

each of the gas, meso, and solid phase are treated separately below.

The Gas Phase.

Kinetics. Condensed phases (solids and liquids) of combustible compounds

will only burn if they can be made to generate volatile fuel/air mixture, and so the

≈

with air (

FLAMMABILITY

phenomena that lead to ignition and the release of heat are those of the gas phase

(11–19). Although there are hundreds or thousands of chemical reactions going on

in the ﬂame that convert oxygen and fuel to stable combustion products, kinetic

modeling and experimental data have shown that the burning velocity is most

sensitive to the following reactions involving active radicals of fuel (R), hydroxyl

(OH), hydrogen (H), and oxygen (O), and halogen or phosphorus (X):

(1)

The set of seven radical reactions in equation 1 accounts for the key processes of

initiation, branching, propagation, and termination typical of hydrocarbon fuels as

well as the inhibition reactions that are important for polymers containing halogen

or phosphorus in a low oxidation state. Because the concentrations of radicals are

empirically related as [H]

≈

[OH]

≈

[H]

[O]

[OH]. Deﬁning a rate of

chain initiation,

θ =

k 1 [RH], a branching coefﬁcient f

(2/5) k 2 [O 2 ], and a linear

termination coefﬁcient g

(2/5)

{

k 5 [O 2 ]

( k 6 +

k 7 )[HX]

}

, the rate of change of

radical concentration at time t is

d[ n ]

d t

= θ +

f [n]

−

g [n]

(2)

The general solution of equation 2 for the total radical concentration at any time

[n]

f {

−

e − ( g − f ) t

}

(3)

−

Figure 3 is a schematic plot of equation 3 for three special cases:

g : In this case the branching reactions dominate and the radical concen-

tration increases exponentially as [n]

= {θ

exp(( f

−

g ) t )

−

}

/( f

−

g ), until

the reactants are depleted.

(2) g

f : In this case the radical concentration increases steadily with time as

[n]

= θ

t . It is the boundary between steady state and exponential growth.

2[O], the set of equations can be solved for the

total concentration of active radicals, [n]

(1) f

FLAMMABILITY

Fig. 3. Gas phase kinetics: accelerating ( f

g ), steady growth ( f

g ), and steady state ( f

g ) radical concentration histories at ignition.

f : In this case the rate of chain termination is greater than branching

and a steady-state concentration is reached at [n]

f ). This steady

state may be above or below the concentration for ﬂame propagation.

= θ

/( g

−

The molar inhibition efﬁciency of the halogen atoms X

bromine (Br), chlo-

10/2/1; ie, bromine

is 10 times more efﬁcient than ﬂuorine and 5 times more efﬁcient than chlorine

on a mole basis. On a mass basis the inhibitor efﬁciency is Br/Cl/F

2/1/1, which

explains the widespread use of bromine-containing monomers and additives as

ﬂame retardants (4–10).

Thermochemistry. If the kinetics are such that the combustion reactions

proceed to completion at a rate that sustains ﬂaming combustion, the chemical

reaction of a generic fuel with atmospheric oxygen yields carbon dioxide (CO 2 ),

water (H 2 O), nitrogen (N 2 ), and mineral acid (HX) in quantitative yield:

C c H h O m N n X x +

−

2 m

O 2 →

c CO 2 +

−

H 2 O

N 2 +

x HX

(4)

(3) g

rine (Cl), and ﬂuorine (F) is found to be in the ratio Br/Cl/F

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