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OCR for page 61
Fire Properties of Materials
Archie Tewarson*
ABSTRACT
Fire properties of materials associated with the pyrolysis, ignition, combustion, fire
propagation, and flame extinction processes are discussed. The relationships between the fire-
hardening of materials and fire properties are enumerated. Fire-hardening is defined as a process
where resistance to pyrolysis, ignition, combustion, and fire propagation is increased, and
release rates of heat and fire products are decreased.
INTRODUCTION
Flammability is an interaction of pyrolysis, ignition, combustion, fire propagation, and
flame extinction processes. The first four processes are brought about by the heat exposure of
the material. The heat exposure must be of sufficient strength to satisfy the requirements of the
pyrolysis process.
Pyrolysis is generally an endothermic process, characterized by the softening, melting,
discoloration, cracking, decomposition, and vaporization and so forth of the material and release
of products (i.e., smoke, toxic, and corrosive pyrolysis products). The boundary of the pyrolysis
process is defined as the pyrolysis front.
Ignition is a process in which the pyrolysis products mix with air and form a combustible
mixture, and the mixture ignites by itself (auto-ignition) or is ignited by a flame, a hot object,
an electrical spark, or similar means (piloted ignition).
Combustion is a process in which the pyrolysis products react with oxygen from air, with
a visible flame (flaming combustion). Heat and products (i.e., smoke and toxic and corrosive
combustion products) are released in this process.
Fire propagation is a process in which the pyrolysis front, accompanied by the flaming
or nonflaming combustion process, moves beyond the point of origin at a certain rate, defined
as the fire-propagation rate. Heat and products (i.e., smoke and toxic and corrosive combustion
products) are released at an increasing rate during the propagation process.
Flame extinction is a process in which the pyrolysis, ignition, combustion, and fire-
propagation processes are interrupted by external agents such as water, Malone, or alternatives.
Heat and products are released at a decreasing rate until flame extinction. Pyrolysis products
continue to be released past the flame extinction as long as the heat within the material continues
to satisfier the requirements of the pyrolysis process.
Flammability Section, Factory Mutual Research Corporation, Norwood, Massachusetts.
61
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62
Improved Fire- arm Smoke-Resistant Materials
The release of heat and products (i.e., smoke and toxic and corrosive pyrolysis and
combustion products) are hazardous to life and property. Hazard due to smoke and toxic and
corrosive products is deemed as nonthermal hazard ~ewarson, 1992). Hazard due to heat (i.e.,
high temperature and radiation) is defined as thermal hazard (Tewarson, 1992~.
For protection of life and property from fires, materials need to be fire-hardened, and
active and passive fire projections need to be provided. Fire-hardening is defined as a process
in which resistance to pyrolysis, ignition, combustion, and fire propagation is increased, and
release rates of heat and fire products are decreased. The fire-hardening requirements for the
materials are considered in terms of the fire properties listed in Table I. Fire-hardening can be
achieved by several techniques of modifying the fire properties.
TABLE 1 Fire Properties of Materials to Assess Degree of Fire-Hardening
Fire Property
Description of the Fire Property
Pyrolysis
Heat of gasification (AH')
Surface re-radiation loss (q ',,)
Yield of a product
Product generation parameter
Critical heat flux, (q ',,)
Thermal response parameter
Flame heat flux (q f3
Net heat of complete
combustion (AHT)
Chemical heat of combustion HAHN
Convective heat of
combustion (AHOY)
Radiative heat of combustion (AH,~
Yield of a product
Energy required to pyrolyze a unit mass of a material
origimally at ambient temperature
Heat lost to the environment from the hot surface
Amount of a product generated per unit mass of a
material pyrolyzed
Amount of a product generated in pyrolysis per unit;
amount of energy required to pyrolyze a unit mass of a material
Ignition Process
Minimum heat flux at or below which a flammable vapor-air
mixture is not created
Ease of in-depth penetration of the thermal wave and time delay to
reach the ignition temperature
Combustion Process
Heat flux transferred from the flame back to the surface
Amount of energy released in the complete combustion of a unit
mass of a material pyrolyzed with water as gas
Amount of energy actually released in a fire from the combustion
of a unit mass of a material pyrolyzed
Component of the chemical heat of combustion carried away from
the flame by flowing combustion product-air mixture
Component of the chemical heat of combustion transmitted away
from the flame by radiation
Amount of a product generated in the combustion per unit mass of
a material pyrolyzed
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Archie Tewarson
J TABLE 1 (continued)
63
Fire Property
Description of the Fire Property
Heat release parameter
Product generation parameter
Fire-propagation index
Visibility through smoke (not defined)
Smoke damage (not defined)
Toxic effects of products (not defined)
Corrosion damage by products-
· · .
corrosion index
Amount of energy generated in combustion per unit amount of
energy required to pyrolyze a unit mass of a material
Amount of a product generated in combustion per unit; amount of
energy required to pyrolyze a unit mass of a material
Fire Propagation
Extent and rate of fire propagation beyond the ignition zone
Nonthermal Damage
Maximum distance over which an observer can see
Smoke damage due to discoloration, smell, or electrical
malfunction
Toxic effects of products on humans
Rate of corrosion per unit mass concentration of a material
pyrolyzed
PYROLYSIS
When a material is exposed to heat flux, pyrolysis products are generated. The rate of
generation of the pyrolysis products is defined as the mass pyrolysis rate (Tewarson, 1988,
1994)
m"p = ~ or
. ~
(1)
where m p is the mass pyrolysis rate in (kg/m2 s), it e is the external heat flux (kW/m2), if ,~ is
the surface re-radiation loss (kW/m2), and AH5 is the heat of gasification (MJ/kg).
The fire-hardening of materials requires that the values of surface re-radiation loss and
heat of gasification be as high as possible.
Heat of Gasification
For a meldng type of material, the heat of gasification is expressed as:
T
m
AHg = |CpradT~ Him +
T.,
T
v
|cp,ldT + AHv
T
nit
(2)
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64
Improved Fire- arm Smoke-Resistant Materials
where IBM add AHv are the heats of melting and vaporization at the respective melting and
vaporization temperatures in MI/kg; Cp a, and Cp ~ are the specific heats of the solid and molten
solids in MJ/kg, respectively; and Ta, To, and Tv are the ambient, melting, and vaporization
temperatures in K, respectively. For materials that do not melt, but sublime, decompose, or
char, Equation 2 is motived accordingly. Table 2 lists examples of the heat of gasification
values taken from Tewarson 198S, 1994. The values are measured by differential scanning
calorimetry (DSC) and by the mass pyrolysis technique using the Factory Mutual Research
Corporation (FMRC) Flammability Apparatus shown in Figure 1.
Modifications in the pyrolysis behavior of the materials to increase the Cp, AHm, and AHv
values and the melting and vaporization temperatures would increase the heat of gasification and
reduce Me mass pyrolysis rate (Equation I) and other related fire properties.
Surface Re-Radiation Loss
Surface re-radiation loss is proportional to the fourth power of the pyrolysis temperature
of the material. Stronger chemical bonds and pyrolysis mechanisms favoring retention of carbon
in the solid phase (charring) would result in higher pyrolysis temperature and surface re-
radiation. Mass pyrolysis rate decreases with increase in the surface re-radiation loss (Equation
I). Table 2 lists examples of the surface re-radiation loss taken from Tewarson 1988, 1994. The
values are quantified by the mass pyrolysis technique in the FMRC Flammability Apparatus
(Figure I).
COMBUSTION
In the combustion process the pyrolysis products burn with air; a flame is established
over the surface; and heat transferred from the flame back to the surface sustains the combustion
process, with or without the external heat flux. For the combustion process, Equation ~ is
expressed as (Tewarson, 1988, 19941:
n _ d/ e+ ~ f ~ or
OHS
(3)
where 7h p is the mass pyrolysis rate in the combustion process (kg/m2 s), and Of is the flame
heat flux transferred back to the surface (kW/m21.
The f~re-hardening of materials requires that the flame heat transferred back to the
surface be reduced as much as possible.
Results from numerous small- and large-scale fires show that, as the surface area of the
burning material increases, the flame radiative heat flux increases and reaches an asymptotic
limit, whereas the flame convective heat flux decreases and becomes much smaller than the
flame radiative heat flux at the asymptotic limit (Hottel, 19591. In small-scale experiments with
fixed surface area, flame radiative heat flux increases and flame convective heat flux decreases
with increase in the oxygen mass fraction (Ye), as shown in Figure 2 (Tewarson et al., 1981~.
OCR for page 65
Archie Tewarson
TABLE 2 Surface Re-Radiation Loss and Heat of Gasification of Polymers
65
Heat of Gasification (MJ/kg)
Surface Re-Radiation Mass Pyrolysis
Polymer Loss (kW/m2) Technique. DSC
Polypropylene 15 2.0 2.0
Polyethylene (low density) 15 1.8 1.9
Polyethylene (high density) 15 2.3 2.2
Plasticized polyvinylchoride
(PVC), LOIb = 0.20 10 2.5
Plasticized PVC, LOI = 0.30 - 2.1 -
Plasticized PVC, LOI = 0.35 - 2.4 -
Rigid PVC, LOI = 0.50 - 2.3 -
Polyoxymethylene 13 2.4 2.4
Polymethylmethacrylate 11 1.6 1.6
Polystyrene (granular3 13 1.7 1.8
Expanded polyurethane (flexible) 16-19 1.2-2.7 1.4
From FbIRC Flammability Apparatus (Figure 1). See Tewarson (1988, 1994) for other materials.
bLOI: Limiting Oxygen Index.
SOURCE: Data from Tewarson (1988, 19941.
For YO > 0.30, the flame radiative heat flux reaches an asymptotic limit comparable to the limit
for normal air burning in large-scale fires (Tewarson et al., 1981; Tewarson, 1988, 19941. Thus,
large-scale flame radiative heat flux conditions can be simulated in small-scale experiments.
The technique to simulate large-scale flame radiative heat flux conditions in small-scale
flammability experiments by the oxygen mass fraction variations is defined as the Flame
Radiation Scaling Technique (Tewarson, 1988, 1994).
Table 3 compares the results from the flame radiation scaling technique used in the small-
scale experiments in the FMRC Flammability Apparatus and results from large-scale fires. The
data show that the asymptotic flame heat flux values from the FMRC Flammability Apparatus
are in good agreement with the values derived from the mass pyrolysis rate in large-scale fires.
The asymptotic flame heat flux values vary from 22 kW/m2 to 77 kW/m2. dependent cr~mar~lv
~ ~ ,
on the pyrolysis mode rather than on the chemical structures. For example, for liquids, which
vaporize primarily as monomers, the asymptotic flame heat flux values are in the range of 22
kW/m2 to 44 kW/m2, irrespective of their chemical structures. For polymers, which vaporize
as high molecular weight oligomers, the asymptotic flame heat flux values increase substantially
to the range of 49 kW/m2 to 71 kW/m2, irrespective of their chemical structures. The
independence of the asymptotic flame heat value from the chemical structure is consistent with
the dependence of the flame radiation on optical thickness, soot concentration, and flame
temperature.
Modifications in the pyrolysis behavior to enhance release of higher monomer fraction
relative to oligomer fraction and reduction in the carbon atom fraction relative to other atoms
in the pyrolysis products (enhanced surface charring) would reduce the flame heat flux
transferred back to the surface and the mass pyrolysis rate (Equation 3~.
OCR for page 66
66
In
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NOI103S d~ddn
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m<:
Improved Fire- aru] Smoke-Resistant Materials
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OCR for page 67
Archie Tewarson
60
50
x
-
cot
I ~ 30
3
_
~0
20
10
o
67
Aft Radiative
[///~ Convective
l
=~
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0
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...._... ,
........
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P; .
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.... i.
Do
i
// ~ ~
FIGURE 2 Flame radiative and convective heat fluxes at various oxygen mass fractions for the stP~dy-state
combustion of 100 x 100 x 25-mm-thick slab of polypropylene. Data are from the FMRC Flammability
Apparatus. Numbers are the oxygen mass fractions.
The mass pyrolysis rate is directly proportional to the heat release rate and the generation
rates of products. Decrease in the mass pyrolysis rate, thus, would reduce the thermal and
nonthermal hazards.
IGNITION
Ignition is a process in which the pyrolysis products are generated at a certain rate, mix
with air, and form a combustible mixture that ignites by itself (auto-ignition) or is ignited by a
flame, a hot object, or similar means (piloted ignition). The rate of generation of the pyrolysis
products leading to ignition is defined as the critical mass pyrolysis rate. Minimum heat flux at
or below which the critical mass pyrolysis rate is not achieved and there is no ignition is defined
as the critical heat flux (CHF). The CHF value is very close to the surface re-radiation loss.
Relationships have been developed between the time to ignition and external heat flux
(Tewarson, 1988). These relationships are as follows: it) for thermally thick materials, the
surface is at the ignition temperature and the back is close to the ambient temperature at the
ignition condition; and (2) for thermally thin materials, the surface is at the ignition temperature
and the back is close to the ignition temperature at the ignition condition.
OCR for page 68
68
Improved Fire- Art Smoke-Resistant Materials
TABLE 3 Asymptotic Mass Pyrolysis Rate and Flame Heat Flux in Combustion
Mass Pyrolysis Rate Flame Heat Flux
(kg/m2.s) x 103 (kW/m2)
Flame Radiation Large- Flame Radiation Large
Polymers/Liquidsa Scaling Techniques Scale Scaling Techniques Scale
Aliphatic Carbon-Hydrogen Atoms
Polyethylene 26 - 61
Polypropylene 24 - 67
Heavy fuel oil (2.6-23 m)b 36 29
Kerosene (30-80 m) - 65 - 29
Crude oil (6.5-31 m) - 56 - 44
n-Dodecane (0.94 m) - 36 - 30
Gasoline (1.5-223 m) - 62 - 30
JP-4(1.0-5.3m) - 67 - 40
JP-5 (10.60-17 m) - 55 - 39
n-Heptane (1.2-10 m) ~66 75 32 37
n-Hexane (0.75-10 m) - 77 - 37
Transformer fluids (2.37 m) 27-30 25-29 23-25 22-25
Aromatic Carbon-Hydrogen-Oxygen Atom
Polystyrene (0.93 m) 36 34 75 71
Xylene (1.22 m) - 67 - 37
Benzene (0.75-6.0 m) - 81 - 44
Al iphatic Carbon-Hydrogen- Oxygen Atoms
Polyoxymethylene 16 - 50
Polymethylmethacrylate 28 30 57 60
(2.37 m)
Methanol (1.2-2.4 m) 20 25 22 27
Acetone (1.52 m) - 38 24
- Aliphatic Carbon-Hydrogen- Oxygen-Nitrogen Atoms
Expanded polyurethanes 21-27 - 64-76
(flexible)
Expanded polyurethanes 22-25 - 49-53
(rigid)
Aliphatic Carbon-Hydrogen-Halogen Atom
Polyvinylchloride (PVC) 16 - 50
Ethylenetetrafluoroethylene (ETFE) 14 - 50
(Tefzel)
Fluannated ethylene-propylene (FEP) 7 - 52
(Teflon)
Aflame Radiation Scaling Technique: Pool diameter fixed at 0.10 m, Yo20.30.
bNumbers in parentheses are the pool diameters in meters.
SOURCE: Data from Tewarson (1988, 1994~.
OCR for page 69
Archie Tewarsor~
69
Ignition of Thermally Thick Materials
-
_ ~ 4
_ d/ e ~ or
~ Tig~P up
(4)
where ti' is the time to ignition (s), q `~ is the critical heat flux (kW/m2), ATig is the ignition
temperature of the material above the ambient temperature (K), k is the thermal conductivity of
the material (kW/m K), p is the density of the material (kg/m3), and cp is the specific heat of
the material - /kg K). Still ~kpcp is defined as the thermal response parameter (TRP) for the
thermally thick material (kW s''2/m2).
For thermally thick materials, the square root of time to ignition is directly proportional
to TRP and inversely proportional to the external heat flux. Figure 3 shows a typical example
of the data for a thermally thick polymethy~methacrylate (PMMA) slab at various velocities (I)
of the co-flowing air.
0.35
0.30
0.25
0.20
o
· _
·F~
c 0.15
o
T 0.10
._
~ 0.05
Gym 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Natural Flow
-I Co-Flow; v' = 0.~S m/s
-I- Co-Flow; v' = 0.09 m/s
- ~ Co-Flow; v' = 0.05 m/s
0.00 ~ ~ 1 ~ 1 ~ 1 ~ 1 1 ~
0 1 0 20 30 40 50 60
-
70 80 90 1 00
External Heat Flux (kW/m2)
FIGURE 3 Ignition data for 100 x 100 x 25-mm thick polymethylmethacrylate (PMMA) slab with blackened
surface. Data measured in the FMRC Flammability Apparatus.
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Representative terms from entire chapter:
heat release
70
Improved Fire- aM Smoke-Resistant Materials
Ignition of Thermally Thin Materials
1 ~
Archie Tewarson
71
composite systems. The TRP values increase with decrease in the resin fraction and increase in
the fiber fraction. For the same resin fraction, the TRP value is highest for the graphite fiber
systems, intermediate for the glass fiber systems and lowest for the KevIar0 fiber system,
following the trends in the thermal conductivities of the fibers, that is, graphite > glass ~
KevIai.. For higher thermal conductivity fibers, a larger fraction of the heat applied to the
surface is transferred to the interior, and time required to reach the ignition temperature is
longer, resulting in the higher TRP value.
The residual flexural strength retained (RF SR) is one of the parameters used to assess the
structural performance of the composite systems (Sorathia et al., 19931. The dependency of
RFSR on the properties of the composite systems is very similar to the dependency of TRP. A
relationship between the RFSR and the TRP has thus been postulated (Tewarson and Haskell,
19941.
Variations in the chemical bonds within similar generic resins and additives also play a
major role in the ignition behavior of the composite systems by affecting the TRP values, as
indicated by the data in Table 4 for f~berglass-reinforced polyester and epoxy composite systems.
FIRE PROPAGATION
Fire propagation is a process in which the pyrolysis front moves beyond the ignition
zone, accompanied by the sustained combustion process. The rate of the movement of the
pyrolysis front is defined as the fire-propagation rate. For a sustained fire-propagation process,
flame or external heat sources need to transfer heat flux ahead of the pyrolysis front to satisfy
the CHF and TRP values.
The upward fire-propagation rate in the direction of air flow for thermally thick materials
is expressed as (Sibulkin and Kim, 19771:
l/2 _ 61/2 ~
U -
~ Tig~kp cp)
(6)
where u is the fire-propagation rate in m/s; of is an effective flame heat transfer distance (m),
assumed to be constant; Of is the flame heat flux transferred ahead of the pyrolysis front
(kW/m21; and ~T,,:~kpcp is the TRP for the thermally thick materials in kW · s''2/m2 (Equation
41.
The flame heat flux transferred ahead of the pyrolysis front is a function of the rate of
heat actually released in the fire-propagation process, defined as the chemical heat release rate.
Figure 5 shows an example of the chemical heat release rate for the downward fire propagation
for a 300-mm long, 100-mm wide, and 25-mm-thick vertical slab of PMMA in an oxygen mass
fraction of 0.446 (Tewarson and Ogden, 1992~. The slope of the curve is the fire-propagation
rate. The figure also shows the combustion of the entire slab in normal air and in reduced
oxygen mass fractions. The flame extinction occurs at an oxygen mass fraction of 0. 178.
82
Imp roved Fire- and Smoke-Resistant Materials
matic structure). A similar trend is found for the liquids and gases. The presence of O and N
atoms in the chemical structures of the materials with aliphatic C-H structure appears to enhance
the preferential carbon atom conversion to CO. The order could be due to preferential pyrolysis
of the material to CO and/or preference for the reactions between OH and CO compared to the
reactions between OH and C. A decrease in the OH concentration with increase in the
equivalence ratio is also suggested by the order.
Preferential Conversion of Carbon ire the Material to Smoke with Decrease in Fire Ventilation
With decrease in fire ventilation during the combustion of the nonhalogenated polymers,
the preferential conversion of the carbon in the material to smoke follows the order: PS > wood
~ PE = PP ~ nylon > PMMA. The order for the preferential conversion of the carbon atom
to smoke is opposite to the order for the conversion of the carbon to CO, except for wood. The
order could be clue to preferential pyrolysis of the material to carbon and/or preference for the
reactions between OH and CO compared to the reactions between OH and C, and/or decrease
in the concentration of OH.
THERMAL AND NONTHERMAL DAMAGE
Damage due to heat is defined as thermal damage; and damage due to smoke, toxic, and
corrosive products is defined as nonthermal damage ~ewarson, 1992~. Nonthermal damage
depends on the chemical nature and deposition of products on the walls, ceilings, building
furnishings, equipment, and components, and so forth, and on the environmental conditions. The
seventy of the nonthermal damage increases with time. Some examples of nonthermal damage
to property are corrosion damage, electrical malfunctions, and damage due to discoloration and
odors. Toxic effects of fire products on the human body that result in an injury or loss of life
are examples of nonthermal damage to life. The subject of toxicity has been discussed (NRC,
1986~. This paper deals with the subject of nonthermal damage in industrial and commercial
occupancies due to smoke and corrosive fire products.
The subject of corrosion for commercial and industrial occupancies has been reviewed
based on the knowledge denved from the telephone central office (TCO) experience for the
deposition of atmospheric pollutants and fire products on equipment, severity of corrosion
damage, and ease of cleaning the equipment (Reagor, 1992; FCC, 1993~.
In TCO fires involving PVC-based electrical cables, contamination levels in the range
of about 5 ,ug/cm2 to 900 ~g/cm2 have been observed (Reagor, 1992; FCC, 19931. In general,
an electronic switch would be expected to accumulate zinc chloride levels in the range of about
5 ~g/cm2 to 9 ,ug/cm2 from the interaction with the environment over its expected lifetime of 20
or more years. Clean equipment is expected to have less than about 2 ~g/cm2 of chloride
contamination; whereas, contaminated equipment can have as high as 900 ~g/cm2. Thus,
equipment contamination levels due to chloride ions and ease of restoration have been classified
into four levels (Reagor, 1992), which are listed in Table 8.
Archie Tewarson
TABLE 8 Contamination Levels for the Surface Deposition of Chloride Ions for Electronic Equipment
83
Chloride Ion
(,ug/cm2)
Level Damage/Cleaning/Restoration
2
One No damage expected. No cleaning and restoration required.
< 30 Two Equipment can be easily restored to service by cleaning without little
impact on long-term reliability.
30 to 90 Three Equipment can be restored to service by cleaning, as long as no unusual
corrosion problems arise, and the environment is strictly controlled soon
after the fire.
< 90 Four The effectiveness of cleaning the equipment dwindles and the cost of
cleaning quickly approaches the replacement cost. Equipment
contaminated with high chloride levels may require severe
environmental controls even after cleaning in order to provide
potentially long-term reliable operation.
SOURCE: Data from Reagor (1992~.
CORROSION
Corrosion is defined as an unwanted chemical reaction and/or destruction or deterioration
of a material because of reaction with its environment. Most of the knowledge on corrosion
damage has been based on air pollution, for example, that due to acid rain. and on laboratorv-
scale pyrolysis and combustion experiments.
_ _ , _ _ _ _ ,
In fires mew surfaces are exposed to fire products that include water (generated in the
combustion process and present in the ambient airs. The exposure is of short duration, a few
minutes to a few days. Figure 9 shows an example of corrosion of a thin copper film (5,000 A'
exposed to the combustion products of PVC homopolymer and commercial materials as
measured in the FMRC Flammability Apparatus (Figure I). The slopes of the lines represent the
corrosion rate. The corrosion rate from the PVC homopolymer is significantly higher than the
rate from the PVC commercial materials, indicating dilution and/or partial neutralization of
hydrogen chloride (MCI) by the pyrolysis products of nonhalogenated additives in commercial
materials. The corrosion is faster in the initial stages and becomes slower in later stages due to
protective oxide film formation on the surface.
The corrosion rate of a metal exposed to the pyrolysis and combustion products is found
to satisfy the following relationship (Tewarson, 19941:
z? = I[ -
-'corr r ~ core
(21 )
where Renoir is the corrosion rate (A/min), ,u is corrosion constant [(A/min)/(kglnI)], and CCorr is
the average concentration of the corrosive product (kg/m3). In the gas phase the average
concentration of the corrosive product is equal to the ratio of the total mass of the product in kg
to the total volume of water in the gas phase in of.
84
Improved Fire- am Smoke-Resistant Materials
500
400
300
200
100
a
. ~
! ,
. ,
. ,
. ~
.,
1
,.,
~ 1!
,
l
l
...................
~-
................... ~
..
0 250 500
: :
: :
... .., ~- ,
PVC
Homopolymer
. .
I
. ! _. ~- i
_. I'.= an_
~ -
................... 1
,... 1
. . i !
PVC Commercial Materials
1 ' 1 . 1 .
750 1 000 1 250 1 500
Time (second)
-
FIGURE 9 Gas-phase corrosion from the combustion products of PVC homopolymer and commercial materials.
Data from the FMRC Flammability Apparatus.
The total mass of the corrosive product is equal to YCO,7WT, where YCO!T iS the yield of the
corrosive product (kg/kg) and WT is the total mass of the material pyrolyzed (kg). If VT is the
total volume of the fire product-air mixture, then the volume of water isfwVT, where fw is the
volume fraction of water in the fire product-air mixture. The concentration of the corrosive
product then becomes YCo~,WT/fw VT, and from Equation 21:
. _ IlYcorr WT
R _
corr fwVT
Rearranging Equation 22:
(22 ~
Corrosi on Index = 11 tore = RCorz/ ( WT/ VT) ( 2 3 )
w
The corrosion index (CI) is the rate of corrosion per unit average mass concentration of the
material pyrolyzed (A/min)/(kg/m). The CT values have been reported (Tewarson, 1994~. The
typical CT value for gas-phase corrosion for a highly halogenated polymer with hydrogen atoms
Archie Tewarson
85
in the structure, such as PVC, is 4 x 103 (A/min)/(kg/nf). For a highly halogenated polymer
with no hydrogen atoms in the structure, such as tetrafluoroethylene (TEE), the CI value is 0.6
x 103 (A/min)/(kg/m3), indicating the importance of the formation of water in the combustion
and the inefficiency of the hydrolysis process with water from the ambient air to generate acids.
The CT values suggest that:
.
For significant gas-phase corrosion it is necessary to have hydrogen atoms in the
structure of the halogenated polymers. For example, the C! values for PVC
(hydrogen atoms in the structure) and TFE (no hydrogen atoms in structure) differ
by factor of seven. The difference is probably due to (~) the inefficiency of the
hydrolysis process in the gas phase during the conversion of fluorocarbon products
generated from TEE to hydrogenfluoride; and (2) the high water solubility of
hydrogen chloride generated from PVC.
Fire retardation of nonhalogenated polymers by halogenated compounds increases the
CT values.
Presence of water in the environment is not necessary for the gas-phase corrosion
from the products of halogenated polymers with hydrogen atom in the structure as
water is generated in the combustion process.
Increase in the oxygen concentration of the environment increases the C! values.
Fire-hardening requires that within each fire-propagation group, the C be reduced to
values as low as possible.
SMOKE DAMAGE
Smoke is a mixture of black carbon (soot) and aerosol (Siegla and Smith, 1981;
GoIciberg, 19851. It has been suggested that soot nucleation and growth occur near the highly
ionized regions of the flames in combustion processes and that some of the charges are
transferred to smoke particles.
Smoke damage in industrial and commercial occupancies is considered in terms of
discoloration and odor of the property exposed to smoke; interference in the electric conduction
path and corrosion of the parts exposed to smoke is a carrier of the corrosive products.
FLAME EXTINCTION
Flame extinction is achieved by applying fire extinguishing agents, such as water,
Halon@, or alternates, which interrupt the pyrolysis, combustion, and f~re-propagation processes
by: (~) interacting with the burning material in the solid phase (mainly removal of heat), (2)
reducing the availability of oxygen to the fire (creation of nonflammable mixture), and (3)
removing the heat from the flame and interfering with the chemical reactions within the flame.
86
Improved Fire- arm Smoke-Resistant Materials
The~?ame extinction requirements are lowerformatenals with a higher degree of fire-hardening.
For example, group N-] materials with FP! < 7 do not require fire protection.
When the extinguishing agents, active in the gas phase, are applied to a flame, the HRP
values decrease; the PGP values of the products of incomplete combustion, such as CO, smoke,
and, mixture of hydrocarbons, increase. These results are very similar to the results for the
ventilation-controlled fires (figures 6-~. Figures 10 and ~ ~ show the ratios of the HRP and PGP
values for well-ventilated combustion of a polyester-70 percent glass composite system in the
presence and absence of Malone 1301.
In Figure 10, the PGP ratio for hydrocarbons increases to Il5, for CO it increases to
10, and for smoke it increases to 2. These results suggest a possible interruption by Malone 1301
of the reactionts) in which CO and hydrocarbons are consumed, rather than the reactions in
which smoke is consumed. This type of behavior is also found for the ventilation-controlled
combustion of materials with oxygen atoms in the structure (figures 7 and 8~.
As shown in Figure ~ I, the HRP ratio decreases to 0.5S, below which the flame becomes
unstable, leading to flame extinction. This is similar to the behavior shown in Figure 6, which
results from the increase of the equivalence ratio.
SUMMARY
Fire-hardened materials offer resistance to pyrolysis, ignition, combustion, and fire
propagation and would be materials of choice for commercial aircraft interiors to reduce hazards
due to heat (thermal hazard) and smoke, toxic, and corrosive products (nonthermal hazard).
The resistance to pyrolysis and ignition would be increased by increasing the values of
(~) the gasification temperature or surface re-radiation loss and heat of gasification to r~luce the
mass pyrolysis rate, and (Z) the ignition temperature or the critical heat flux (CHF) and the
thermal response parameter ~RP) to delay ignition and increase removal of heat from the
surface to the interior. Stronger chemical bonds, pyrolysis mechanisms favoring retention of
carbon in the solic! phase (charring), enhancement of thermal conductivity, density, and specific
heat of the materials are some of the factors expected to be effective in this endeavor. Some of
the commercial materials introduced recently satisfy these requirements.
The resistance to combustion would be enhanced for materials with high resistance to
pyrolysis and ignition. In addition, the Fame treat flunk transferee back to the surface aru] the
heat of combustion need to be decreased to reduce the mass pyrolysis rate in the combustion and
the heat release rate. These two fire properties could be reduced by (~) modification of the
pyrolysis behavior to enhance release of higher monomer fraction relative to the oligomer
fraction, (2) reduction in the carbon atom fraction relative to other atoms in the pyrolysis
products (enhancing the char formation), (3) introduction of the oxygen atoms in the structure,
and (4) decrease in the chemical bond unsaturation, aromaticity, and others.
Initially the processes of pyrolysis, ignition, and combustion occur within the area where
the material is heated. The area is defined as the ignition zone. If the heat flux transferred
beyond the ignition zone satisfies the CHF and TRP values, fire propagation beyond the ignition
would be initiated. The thermal and nonthermal hazards depend on the rate and extent of fire
propagation beyond the ignition zone and are characterized by the fire-propagation index (FPl).
Archie Tewarson
8 ,o2
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4.0 4.5
FIGURE 10 Ratio of the POP in the presence and absence of Halon0 1301 for the well-ventilated combustion of
polyester-70 percent glass composite system exposed to 60 kW/m2 of external heat flux. Data are from the FMRC
Flammability Apparatus.
0.9
8
-
P~
_ 0.8
° 0.7
hi:
8
~ 0.6
-
0.5
0.4`
I
T
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t
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Halon (Volume I)
FIGURE 11 Ratio of the HRP in the presence and absence of Halon0 1301 for the well-ventilated combustion of
polyester-70 percent glass composite system exposed to 60 kW/m2 of external heat flux. Data are from the FMRC
Flammability Apparatus.
S_~E
Le~E,
4.0 4.5
88
Improved Fire- arm Smoke-Resistant Materials
Under high flame-radiation conditions, that is, large-scale fires, materials with FPI values <7
are nonpropagating, group N-1 materials. Materials with FPI values > 7 and < 10 show
decelerating propagation and are identified as group D-1 materials. Materials with FPI values
2 10, but ~ 20, show slowly propagating fire beyond the ignition zone and are identified as
group P-2 materials. Materials with FPI values 2 20 show rapidly propagating fire beyond the
ignition zone and are identified as group P-3 materials.
Fire-hardening requires materials to be group N-] materials. The FPT values would be
reduced by increasing the CHF and TRP values and decreasing the heat release rate. Within each
fire-orocaaation group. it is necessary that the heat release rate and the generation rates of fire
_# _ 4,
. ~ ~ . , ~ ~ .' ~ ~ ~ . ~ . ....
products be reduced to values as low as possible. rule neat release rate wlthm each ilre-
propagation group is characterized by the heat release parameter (HRP) (or the ratio of the heat
of combustion to heat of gasification). For group N-1 materials, HRP is <2. The generation
rates of products within each fire-propagation group are characterized by the product generation
parameter (POP) (or the ratio of the yield of the product to heat of gasification). The POP
values within each fire-propagation group need to be reduced to as low values as possible.
Parameters to characterize smoke and toxic damage have not been defined; for corrosion
damage, a corrosion index (CI) has been identified as the corrosion rate of a metal per unit
concentration of the material pyrolyzed.
NOMENCLATURE
CHF critical heat flux (kW/m2)
cco" average concentration of a corrosive product (kg/m3)
C} corrosion index
cp specific heat (MI/kg K)
ETFE ethylenetetrafluoroethylene (Tefzel)
fw volume fraction water (-)
FG fiberglass reinforced
FEP fluorinated ethylene-propylene Teflon)
FPT fire propagation index {10~ (0.42 Q'c0''3 / BATS ( - p)~/2~}
G"j mass generation rate of product j (kg/m2 s)
dHi heat of combustion, gasification, melting, or vaporization per unit mass of material
pyrolyzed (MI/kg)
HRP heat release parameter (AHCb/AHg)
k thermal conductivity (kW/m K)
mair mass flow rate of air (kg/s)
m"p mass pyrolysis rate (kg/m2 s)
M molecular weight (kg/mole)
PE polyethylene
POP product generation parameter (yj/AHg)(kg/MI)
PMMA polymethylmethacrylate
PP polypropylene
PS polystyrene
Archie Tewarson
PVC
q't
Q''i
Q'i
Qcorr
s
AT,g
TRIP
TOT
u
vg
VT
WT
Yj
Yo
Greek
c'
f.
Of
Xch
SCOT
Grad
~j
p
Hi
Subscript
a
ch
con
corr
cr
e
f
fc
fr
g
89
polyvinylchloride
heat flux (kW/m2)
heat release rate per unit sample surface area (m"AHch) (kW/m2)
heat release rate per unit sample width (kW/m)
corrosion rate (A/min)
stoichiometric mass air-to-fuel ratio (-)
time (s)
temperature (K)
ignition temperature above ambient (K)
thermal response parameter, thermally thick [AT,g (kocp)t'2] (kW s~'21m2)
thermal response parameter, thermally thin (AT,g bpcp) (LJ/m)
fire-propagation rate (m/s)
co-flow air velocity (m/s)
total volume of fire product-air mixture (m3)
total mass of material pyrolyzed (kg)
yield of product j (Wj/Wf) (kg/kg)
mass fraction of oxygen (-)
ventilation correlation coefficient for nonflaming region (-)
ventilation correlation coefficient for transition region (-)
ventilation correlation coefficient for the equivalence ratio (-)
equivalence ratio (Sm"pA/m,,,)
thickness or depth (m)
effective flame heat transfer distance (m)
combustion efficiency (Q"ch / m"~HT)
convective component of the combustion efficiency (Q"con /m"~HT3 (-)
radiative component of the combustion efficiency (Q"rad /m,'~HT) (~)
generation or consumption efficiency of a product (yj / I) (-)
corrosion constant (A/min)(kg/m3)
density (kg/m3)
stoichiometric yield for the maximum conversion of fuel to product j (-)
air or ambient
chemical
convective
corrosion
critical
external
flame
flame convective
flame radiative
gas or gasification
90 Improved Fire- arm Smoke-Resistant Materials
i chemical, convective, radiative
· · · ~
lg 1gmtlon
j fire product
m melting
n net
o initial
red radiation
stoich stoichiometric for the maximum possible conversion of the fuel to the product
rr surface re-radiation
s surface
th depth
v ventila~cion-controlled fire
w water
~well-ventilat~
Superscript
.
11
per unit time (s-l)
per unit width (m~l)
per unit area (mu)
REFERENCES
FCC (Federal Communications Commission). 1993. Network Reliability: A Report to the
Nation. Section G in Compendium of Technical Papers. Presentation by the Federal
Communications Commission's Network Reliability Council at the National Engineering
Consortium, Chicago, Illinois.
Goldberg, E.D. 1985. Black Carbon in the Environment-Properties and Distribution. New
York: John Wiley & Sons.
Hottel, H.C. 1959. Review: Certain laws governing the diffusive burning of liquids by Blinov
and Khudiakov (1957) (DokI Akad), Nauk SSSR, Vol. ~ 13, 1094, 1957. Fire Research
Abstract and Reviews (~:41-45.
NRC (National Research Council). 1986. Fire and Smoke: Understanding the Hazards. Board
on Environmental Studies and Toxicology, NRC. Washington, D.C.: National Academy
Press.
Reagor, B.T. 1992. Smoke corrosivity: Generation, impact, detection, and protection. Journal
of Fire Sciences (10~: 169-179.
Sibulkin. M., and I. Kim. 1977. The dependence of flame propagation on surface heat transfer.
, , as. ~ _
_ ~ T ~ ~ · ~ ~ . · ~ · ~ ~ ~ ~ ~ _ ~ ~ ~ ~
11: Upward burmng. Combustion science and technology 1 1:;~-4Y.
Siegla, D.C., and G.W. Smith, eds. 1981. Particulate Carbon Formation During Combustion.
New York: Plenum Press.
Sorathia, U., C. Beck, and T. Dapp. 1993. Residual strength of composites during and after fire
exposure. Journal of Fire Science ~ ~ :255-270.
Archie Tewarson
91
Tewarson, A. 1988. Generation of heat and chemical compounds in fires. Chapter I-13, Pp. I-
179 to I-199 in The SFPE Handbook of Fire Protection Engineering. Quincy,
Massachusetts: The National Fire Protection Association Press.
Tewarson, A. 1992. Non-thermal damage. Journal of Fire Science 10: ISS-241.
Tewarson, A. 1994. Flammability parameters of materials: Ignition, combustion, and fire
propagation. Journal of Fire Science 12:329-356.
Tewarson, A., en c! W.E. Haskell. 1994. Fire Hardening of Composite Systems. Presentation
at the Annual Conference on Fire Research at Gaithersburg, Maryland, the National
Institute of Standards and Technology, October 17-20.
Tewarson, A., and M.M. Khan. 1988. Flame propagation for polymers in cylindrical
configuration and vertical orientation. Pp. 1231-1240 in 22nd Symposium (international)
on Combustion. Pittsburgh, Pennsylvania: The Combustion Institute.
Tewarson, A., and D. Macaroni. 1993. Polymers and composites An examination of fire
spread and generation of heat and fire products. Journal of Fire Sciences Il:421-441.
Tewarson, A., and S.D. Ogden. 1992. Fire behavior of polymethy~methacrylate. Combustion
and Flame 89:237-259.
Tewarson, A., I.~. Lee, and R.F. Pion. 1981. The influence of oxygen concentration on fuel
parameters for fire modeling. Pp. 563-570 in I8th Symposium (International) on
Combustion. Pittsburgh, Pennsylvania: The Combustion Institute.
Tewarson, A., F.H. liang, and T. Morikawa. 1993. Ventilation-controlled combustion of
polymers. Combustion and Flame 95: 15 1-169.
