NBSIR 75-757

A TECHNIQUE FOR THE MEASUREMENT

OF FLASH FIRE POTENTIAL OF

POLYMERIC MATERIALS

James E. Brown and John J. Comeford

Center for Fire Research

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

December 1975

Progress Report

Feb. 1 - Aug. 31, 1974

Sponsored in part by:

Department of Transportation

Federal Aviation Administration

National Aviation Facilities Experimental Center

Atlantic City, N. J. 08405

U.S. DEPARTMENT OF COMMERCE, Rogers C.B. Morton, Secretary

James A. Baker, ill, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Acting Director

CONTENTS

Page

LIST OF TABLES iv

LIST OF FIGURES v

1. INTRODUCTION 1

2. EXPERIMENTAL 3

2.1. Materials 3

2.2. Flash-Fire Cell 3

2.3. Pyrolysis Rate Apparatus 3

2.4. Procedure - Flash-Fire Cell Experiments 4

2.5. Procedure - Evolution Rates of Pyrolysis Products . . 4

3. RESULTS AND DISCUSSION 4

3.1. Flash-Fire Cell Experiments 4

3.2. Rate of Pyrolyzate Evolution 7

4. CONCLUSION 10

5. ACKNOWLEDGEMENTS 11

6. REFERENCES 11

iii
LIST OF TABLES

Page

Table 1. Properties of Polyurethane Foams 13

Table 2. Results of Pyrolysis with Furnace at 500 °C and

Conditions of the Flash-Fire Cell at the Initial

Flash Fire for Sample No. 16. 14

Table 3. Pyrolysis of Polyurethane No. 16 at Low Temperatures

Where Flash Fires Were Not Observed 15

Table 4. The Results at the Initial Flash Fire of a Fire

Retarded Foam, No. 42 and Unretarded Foams No. 18

and 8 When Pyrolyzed With the Furnace Preset to

500 °C 16

Table 5. Minimum Weights of Foams for Production of Flash Fires

In the Flash-Fire Cell When the Pyrolysis Furnace

was Preset at About 500 °C 17

iv
LIST OF FIGURES

Page

Figure 1. Schematic of Flash-Fire Cell 18

Figure 2. Typical Recordings of Sample Temperature and

Oxygen Content In Flash-Fire Cell During the

Production of Flash Fires. 19

Figure 3. Typical Recording of Sample Temperature and Oxygen

Content During Pyrolysis of Sample 16 in the

Flash-Fire Cell Without the Production of

Flash Fires. 20

Figure 4. Effect of Temperature and Sample Weight on Production of Flash Fires 21

Figure 5. Recording of the Catalytic Detector Response to

Two Major Stages of Combustible Pyrolyzate

Evolved from Sample 16 as the Temperature is

Increased at About 60 "'C/min 22

Figure 5a. A Recording Showing the Catalytic Detector Response

to The Pyrolyzate Evolved from a Conventional

Flexible Foam as the Temperature was Raised at

about 60 °C/min 23

Figure 6. A Typical Recording Showing the Effect of Temperature

on the Two Major Stages of Evolution of Total

Combustibles from a Polyurethane Shown in Figure 5a. 24

Figure 7. An Arrhenius Plot of the Results Obtained from the

Second Major Stage of the Combustible Pyrolyzate

Shown in Figure 6 25

Figure 8. A Recording Showing a Reduced First Major Stage as a Result of Removing the "Cracking Coil." 26

Figure 9. A Recording Showing the Total Mass-Flow Rate of

Pyrolyzate from the Conventional Polyurethane as the Temperature was Increased at a Rate of 60 °C/min. 27

V

A TECHNIQUE FOR THE MEASUREMENT

OF FLASH FIRE POTENTIAL OF POLYMERIC MATERIALS

James E. Brown and John J. Comeford

This report summarizes work for the period February - August 1974 on a continuing program to characterize the chemical and physical parameters of importance in a flash fire and

to develop laboratory scale methods for measuring the flash

fire potential of materials.

Significant modifications have been made to a flash fire

cell developed earlier to measure the flash fire potential

of materials by characterizing the conditions required to produce an ignitable pyrolyzate-air mixture by thermally degrading

a polymer. The furnace temperature, cell geometry and orientation, and sample size are specified. These modifications

have resulted in an improved technique, especially in terms of

reproducibility, for evaluation of flash fire potential of

materials

.

Experiments have been conducted on rates of combustible

gas formation from flexible polyurethanes to assist in the

optimization of the flash-fire cell operating conditions.

In the rate study, two successive major stages of degradation were found for polyurethane as the temperature approached

500° at a rate of about 60 °C/min. It was also found that the

products of the second stage appear to be responsible for flash

fires in the flash-fire cell. A minimum polyurethane weight to

enclosure volume ratio greater than 0.2 g/1 and a sample pyrolysis temperature greater than 380 °C were required to produce a flash fire in this apparatus.

Key words: Aircraft fires; fire hazard; flash fire; polyurethane; pyrolysis.

1 . INTRODUCTION

This report describes the research from February, 1974 to August,

1974 on a joint National Bureau of Standards — Federal Aviation Administration program directed towards ultimately reducing flash fire hazards

in inhabited enclosures such as aircraft cabins.

The purpose of this study was to evaluate the parameters which may

produce flash fires as a result of thermal degradation of polymeric

materials in an enclosed environment. We define a flash fire as a fire

(flame front) which propagates through a fuel-air mixture as a result of

the energy release from the combustion of that fuel, having required

only an ignition source. Since most organic polymers produce combustible

1
products during thermal degradation, we set out to determine the conditions under which a flash fire would result from the products of thermal

degradation in air of a variety of polyurethane foams, which are recognized to be susceptible to this fire hazard.

A laboratory flash fire cell was developed to permit observation of

flash fires resulting from the products of thermal degradation of polymeric material in an enclosed environment.

Observations of flash fires were made using a variety of flexible

polyurethane foams. The experimental technique for a given material involved heating the foam in a pyrolysis chamber while allowing the evolved

products of pyrolysis to pass into the cell. During the pyrolysis, the

temperature of the sample and the oxygen concentration in the cell are recorded. From the change in the oxygen concentration, the volume fraction of the pyrolysis products is inferred. Essentially then, the lower

limit of flammability of the pyrolyzate-air mixtures is also recorded

when a flash occurs.

Investigations on the products of pyrolysis as a function of temperature were carried out in nitrogen atmospheres by Wooley [l]"*". He

reported that at low temperatures (200 to 300 °C) there is a rapid and

complete loss of the toluene diisocyanate (TDI) units of the polyurethane

as a yellow smoke leaving a residue derived from the comonomer polyol.

This suggests that below 500 °C and perhaps below 750 °C the initial re- action involves ure thane linkage rupture to produce TDI and other aromatic

compounds. The familar yellow smoke which contains virtually all of the

nitrogen of the polyurethane, was reported to be stable at temperatures

up to 750 °C where other compounds including hydrogen cyanide are formed.

Thermal degradation of polyurethane was also studied by Tilley and

co-workers [2] and Napier and Wong [3], The mechanism of degradation

appears to closely follow the mechanism demonstrated by well characterized

monomeric urethane compounds [4,5]. Thermal degradation of urethanes generally produced isocyanates, amines, carbon dioxide and alkenes. It was suggested that polyure thanes also produce the nitrogen compounds, carbon

dioxide from the urethane groups in the polymer, and unsaturated compounds

as well as carbonyl compounds from the polyether-polyol fraction of the

copolymer. These latter compounds were identified in an earlier report

from our laboratories [6].

We report the technique and results of pyrolyzing several polyurethane

foams in the flash-fire cell. We also report an exploratory study of the

rate of production of combustible products from polyurethane pyrolysis in

nitrogen and the rate of production of volatile combustibles when polyurethane is pyrolyzed in air.

"Slumbers in brackets correspond with the literature references listed at

the end of this paper.

2
2 . EXPERIMENTAL

2.1. Materials

The polyurethane foams used in this study were all of the flexible

polyether type although the molecular weight of the polyether-polyol and

method of polymerization differed. The foams, their general formulations

and some physical properties are listed in table 1. The formulations

and densities were supplied by the manufacturers. The oxygen indices

(01) were kindly supplied by Dr. Robert McCarter [7].

2.2. Flash-Fire Cell

A diagram of the flash-fire cell is shown in figure 1. The cell

is constructed of a Pyrex cylinder 50 cm in length and 5 cm in diameter.

The volume of the cell is approximately one liter. Each end of the

cylinder has integral "0" ring flanges fitted with neoprene "0" rings

and poly (methyl methacrylate) windows which are fastened with springloaded clamps. The cell is equipped with a pyrolysis chamber which is

connected to the main cell body by an "0" ring flange. The chamber is

heated by a regulated electric furnace. The cell also contains an ignition source consisting of a lOKV AC arc 5 to 10 mm in length between

platinum electrodes and a continuous polarographic oxygen analyzer. The

ignition source and detector of the oxygen analyzer are located near the ' end of the cell farthest from the pyrolysis chamber. During the experiments chromel-alumel thermocouples are used to measure the temperatures

of the sample and the furnace.

2.3. Pyrolysis Rate Apparatus

The pyrolysis apparatus is designed so that the kinetics of the

pyrolysis of polymeric materials may be derived from the evolution of

the pyrolysis products. The design and principles of operation of the

apparatus are described elsewhere [8]. Essentially, the apparatus con- sists of a tube furnace allowing arbitrary temperature variations, a quartz pyrolysis tube and a detector to monitor the evolution of pyrolysis products. One of two detectors was used; a combustible gas indicator

or a meter measuring the mass flow rate.

The apparatus may be operated with a high- temperature zone near the

exit end of the quartz tube. The high- temperature zone is maintained

with an electrically heated "cracking coil" which serves to reduce the

molecular weight of heavier vapors evolved from the sample and thus prevent condensation. A fine wire (chromel-alumel) thermocouple embedded

in the sample measures the temperature of the sample during the pyrolysis.

The thermocouple and detector responses are continuously recorded on a two channel recorder.

3
2.4. Procedure - Flash-Fire Cell Experiments

The flash-fire cell was mounted in either a vertical or horizontal

position. A weighed sample of the polyurethane foam having a thermocouple (T/C) inserted into it, was placed in the pyrolysis chamber. The

cell was assembled as shown in figure 1. The preheated furnace was then

brought into position surrounding the pyrolysis chamber so that the foam

would quickly reach the pyrolysis temperatures. During the time the

sample was being heated, the temperature of the sample and the concentration of oxygen in the cell were continuously recorded on a twochannel recorder.

As the products of pyrolysis evolved from the sample, the lOKV AC arc was turned on for about 2 seconds at chosen intervals, generally

every 60 seconds. The sample was heated until a flash occurred or until pyrolysis was complete.

2.5. Procedure - Evolution Rates of Pyrolysis Products

In the evolution rate of total combustible gas experiments, a weighed

sample of about 10 mg of the polyurethane foam contained in aluminum foil

was placed into the quartz tube for insertion into the tube furnace. The

air was then flushed from the quartz tube by a flow of nitrogen at a rate

of about 15 ml/min and the "cracking coil" was activated and maintained at

a temperature in the range of 550 to 600 °C. The gas evolving from the

quartz tube entered the combustible gas detector after mixing with a larger

volume of air. The air was pumped through the detector at a rate on the

order of 2000 ml/min.

With the activated system in a stable condition, power to the furnace

was then turned on and advanced manually at a predetermined rate so as to

obtain an increase in the furnace temperature of about 60 °C/min. The

response from the combustible gas indicator which resulted from the products evolving from the sample undergoing pyrolysis was simultaneously

recorded with the sample temperature.

In a similar manner, the total pyrolysis products were detected by

passing the gases from the quartz pyrolysis tube directly into the mass

flow rate meter.

3. RESULTS AND DISCUSSION

3.1. Flash-Fire Cell Experiments

All experiments in the flash-fire apparatus were carried out at

atmospheric pressure. As the pyrolysis products evolved into the cell

in these experiments, the air was displaced. Hence, before ignition

occurs, the decrease in oxygen concentration in the cell is attributed

to displacement of the air by the pyrolyzate.

4
It is possible that some of the oxygen in the cell may be consumed by

oxidation of the polyurethane during the pyrolysis, but the extent of this

oxidation is expected to be small since the air surrounding the sample

would be rapidly displaced by the products evolving from the sample. No

significant gas phase consumption of oxygen is expected in the cell except

during the actual burning. Consequently, it is assumed that the oxygen

analyzer reading can be related to the volume fraction of the air in the

air-pyrolysis product mixture, at least near the ignition source where the

flash-fire begins.

Figure 2 shows a typical recording of the oxygen concentration and

the temperature of the sample versus time during an experiment in the

flash-fire cell. In this experiment flash fires did occur. The flash

fires occurred where the two large deflections are seen on the curve showing the concentration of the oxygen.

The oxygen concentration (measured near the ignition source) is

expressed as volume percent. The time of heating the sample is in minutes. The temperature is the apparent temperature of the sample during

the heating period since thermal gradients and variations in thermal

contact between the thermocouple and sample probably exist.

At the onset of a flash fire, a flame front generally propagated from

the igniter throughout the cell in the order of 1 second. Ignition of the

pyrolyzing sample has not been noted, possibly because of the short duration of the flame in the atmosphere around the sample or because this

atmosphere, being nearly oxygen depleted, is near the upper flammability

limit. The response of the oxygen analyzer is rapid compared to the

duration of the experiment. Thus, the instrument gives a good indication

of the diffusion of pyrolysis products to the probe. It also gives an accurate indication of the onset of the flash. The response to the flash

fire is reflected by the abrupt change in the curve of oxygen concentration. The magnitude of these deflections appears to be related to the

intensity of the flash itself. On the other hand, the deflections probably are not a true representation of the actual variations in the oxygen

concentration during the flash fire since response of the instrument to

the heat released and other effects of the flash fire are not presently

known.

Listed in table 2 are the results and the conditions at which flash

fires were reproduced in the flash-fire cell by pyrolysis of sample No. 16

This sample was used to evaluate the effect of varying the conditions in

the flash-fire cell.

The conditions evaluated in these experiments were, (a) orientation

of the flash-fire cell, (b) the use of a glass wool filter in the pyrolysis chamber for aerosols and particulates, and (c) the effect of adding

air to the pyrolysis product-air mixture. In all cases flash fires were

produced regardless of the state or presence of these conditions. In most

instances several flash fires were produced during the pryolysis when the

sample weight was of the order of 0.8 g. A flash was not produced when

the sample pyrolysis temperature was less than 380 °C. We did observe,

5
however, some flaming at the electrodes from condensed pyrolysis products.

The results of the pyrolysis of the polyurethane foams at the lower temperature are shown in table 3. The temperatures listed are the maxima

obtained by the foams during the pyrolyses and the times Indicate the

length of time the foams were pyrolyzed. The percent oxygen listed is the

minimum volume fraction of oxygen measured.

In the course of the flash-fire experiments, we observed that the

initial pyrolyzate entering the cell was a smoke, yellow to dark brown in

appearance [1]. We attempted to filter this product, particulates and

possibly aerosol of heavy oils and tars, from the pyrolyzate. With a glass wool filter placed in the pyrolysis chamber so that the pyrolyzate

which entered the cell first passed through the glass wool plug, less

condensation of tars and oil was observed on the wall of the flash-fire

cell.

The technique derived for the production of flash fires from polyurethane sample 16 was applied to additional foams. A polyurethane foam

containing a fire retardant additive and two other foams without fire

retardant additives were selected. These foams are identified in table

1 as samples 42, 18, and 8. The results of the pyrolyses of these foams

in the flash-fire cell are reported in table 4. All of the runs were made

with the flash-fire cell in the vertical position except runs 1 through 3

of sample 42. A glass wool filter was also used in each experiment except

runs 2 and 3 of sample 8. In the latter two runs air was added to the

cell at rates of 100 and 50 ml/min, respectively. Other than the exceptions mentioned, all experiments reported in table 4 were performed in the same manner. A flash fire was produced in each of the experiments reported

in table 4. Although the fire-retarded foam (sample 42) may be more

resistant to combustion than any of the other foams used in this study,

based on the oxygen indices, flash fires were produced from it by the

least amounts of pyrolysis products in the cell.

Having established the minimum pyrolysis temperature for the production of flash fires from the various foams, the sample size was decreased in order to determine the minimum sample weight necessary to

produce a flash fire. The foams used in this effort were samples 42, 16

and 8. The oxygen index of each sample respectively indicates high,

moderate and low resistance to combustion. The results for these samples

are listed in table 5. The total volume of the flash fire apparatus is

about 1.1 liters. For this apparatus, the minimum sample weight appears

to be about 0.22 g. This finding suggests that a sample to volume ratio

of 0.2 g/1 can produce sufficient fuel to result in a flash fire when the

polyurethane foam is heated to at least 390 °C.

A graphic representation of the flash-fire potential of a flexible

polyurethane foam is shown in figure 4 with the results obtained from

sample 16. This figure demonstrates the flash-fire potential in terms

of sample weight and pyrolysis temperature. The data plotted were taken

from tables 2, 3 and 4.

6
As far as the effect of the glass-wool filter is concerned, no firm

conclusions can be reached. Nevertheless, flash-fire occurrence after a substantial reduction of heavy oils and tars in the flash-fire cell suggested that the flash fire was initiated by the low molecular weight

moeities in the pyrolyzate. Moreover, the higher molecular weight moeities evolved during the early stage of the pyrolysis, and at lower sample

temperatures, did not alone support flash fires when ignited at or near

room temperature. The latter substances appear to be composed chiefly of

tars, oils and particulates. Although the lower molecular weight moeities

may be primarily responsible for the flash fire, some at least of the

heavier components experience combustion during the flash fire. Low

molecular weight species of three carbon atoms or less, which are apparently necessary to support a flash fire such as methane, ethane, propylene, and acetone, were shown in a previous report [6] to be present.

In table 2, horizontal cell orientation, the experiments to be compared

are runs 9 and 14 (filter) with run 15 (no filter) ; for vertical orientation, runs 6, 7 and 8 are to be compared with run 5. Run 2 is unaccountably atypical from several points of view, so we shall exclude it from

the discussion. In table 4, runs 2 and 3 of sample 8, both with vertical

orientation, were the only ones in which no filter was used. Inspection

of the two tables makes it quite apparent that runs with and without filters show no difference in the percentage of O2 just before the flash.

Also, there is no difference in time to flash for the horizontal orientation (runs 9, 14 and 15, table 2). On the other hand, all runs without

filter in the vertical orientation have significantly shorter time to

flash. The times to flash represent diffusion and mixing phenomena which ' are not closely controlled. Therefore it is premature to attach much importance to the time data.

We foresee experiments in the flash-fire cell from which a measure

of the energy released during flash fires will be obtained. It has been

shown [9] that the product- of the lower limit of flammabillty L expressed

as volume fraction and the heat of combustion AH^, of a large number of

organic compounds of diverse classifications approximate a constant,

llKcal/mole of mixture. Using the argument for the energy released by

combustion at the lower limit of flammabillty and considering that the

flash-fire cell contains about 4.5 x 10"^ moles of gas (1.1 liters), we estimate that the energy released from the flash fire is of the order of

0.5 Kcal from the pyrolysis products.

3.2. Rate of Pyrolyzate Evolution

To a large degree, the temperatures selected for the furnace of the

flash-fire apparatus were predicated on preliminary findings of the

combustible-pyrolyzate rate experiments.

Qualitatively we have determined with the pyrolysis rate apparatus

[8] that there are two major stages in the total combustible-pyrolyzate

evolved during thermal degradation of polyurethane foams. And we have

7
also determined thaL the first major stage contains essentially all of

the readily condensable pyrolysis products. These stages of course would

be expected when the proposed degradation mechanism of the ure thane group

is considered [1,2,3].

We show curves of typical combustion-rate experiments in figures 5 (sample 16) and 5A (a conventional foam similar to sample 16, not listed

in table 1) for the rate of evolution of the total combustible pyrolyzate

and the temperature of the sample, versus time of heating the sample.

Although the output voltage of the combustion detector has not been

calibrated in terms of energy, the area under the rate curve is proportional to [7] the energy (heat) released by the catalytic combustion of

the products from the decomposing polymer. Hence, the magnitude of the

response by the detector at any point on the curve is the result of the

energy released from combustion of the pyrolyzate at that moment. As it

may be seen in figures 5 and 5A, the first stage of degradation of the

foam was not complete before the onset of the second stage though the

second stage, occurring at higher temperatures, contains the bulk of the

combustible products.

The temperature at which the onset of combustibles is observed and

the temperatures for the respective maxima of the first and second stages

of degradation are of the same magnitude for the various polyurethane

foams. These results indicate that significantly different chemical bonds

are broken in the respective stages of pyrolyzate evolution. However,

this is not to imply that within the respective stages, the bonds cleaved

are necessarily identical chemically but that the cleavage involves bonds

of similar energy.

We propose that the stages of the degradation reactions involve different energetics since these stages rriay be separated by first limiting

the pyrolysis temperature of the sample to the temperature of the first

stage until the rate of combustible product evolution approaches zero; and

then increasing the pyrolysis temperature of the sample through the range

for the second stage. This experiment is typified in figure 6. The shape

of the curves in figure 6 suggests that the first stage or degradation

consists of more than one product each of which is produced at a different rate. The second stage is expected to be multicomponeuu but these

products seem to be produced at about the same rates. The reason for the

variations seen in the two stages could be clarified by rate measurements

of the individual components.

Following the example of McCarter [8], we performed an exercise and

calculated the rate constants k at several temperatures from about 400 to

475 °C for the second stage in an experiment as shown in figure 6. A plot was made of the natural logarithm of k versus the reciprocal of the absolute temperature. A plot of these data, figure 7, is linear. This suggests a first order reaction for the second stage where the slope is the

ratio of the apparent activation energy to the gas constant, E*/R. An

apparent activation energy of 52 Kcal/mole was calculated. Because this

value obtained for the activation energy is similar in magnitude to the

values found for other materials with oxygen containing functional groups

8
[2,8], this excercise serves as a check on the ability of the Instrumentation to function as a crude probe for the degradation kinetics. An attempt at estimating the activation energy of the first major stage by the

procedure above was not successful. The lack of success may be due to

the nature of the various reactions involved in the thermal degradation.

The rate of evolution of combustible pyrolyzate from polyurethane

pyrolyzed in air approximates more closely the conditions of the pyrolysis

in the flash fire experiments. To achieve these rate studies, the "cracking coil" was removed from the pyrolysis tube. An example of the record

from this study is shown in figure 8. As a result of removing the "cracking coil", a large amount of tars and oils condensed in the unheated area

of the pyrolysis tube. Therefore, the relative magnitude of the first

stage of pyrolyzate evolution is significantly reduced though the onset

temperature and the temperature at which the maximum rates occur are es- sentially the same as in figure 5. This result again suggests that the

second stage is composed primarily of low molecular weight combustibles.

We have previously shown that flash fires occurred in the flash-fire cell

only when the pyrolysis temperature was within the temperature range cor- responding to the second stage of the evolution of combustible pyrolyzate.

In the postulated mechanism [1,2] for degradation of polyurethane the

products include inert substances such as CO2 as a major component which

is produced in the first stage of the degradation process. If a diluent

such as CO2 is indeed produced in the early stage of polyurethane degradation, then a substantial increase in the magnitude of the first major

stage of pyrolyzate evolution would be observed if the total flow of the

pyrolyzate evolved is measured, rather than measuring the merely combustible gases.

A measure of the total mass evolved from the decomposing polyurethane

was achieved. To achieve this measurement, the combustion detector was

removed from the apparatus and replaced with a mass flow rate meter. With

this arrangement, the thermal decomposition products, which were formed in

a stream of nitrogen, were swept directly into the mass flow rate meter

after passing over the "cracking coil." A graph of the total mass flow

rate, temperature, and pyrolysis time is shown in figure 9. The temperatures for the onset of mass flow from the pyrolyzing foam and the maximum

rates of evolution are similar to the temperatures of the combustible

pyrolyzate experiment for this sample. Since the magnitude of the first

stage is quite large and relatively distinct from the second stage, the

presence of inert substances are indicated, see also figures 5 and 5A.

The rate of mass flow experiments lend further qualitative support

to the idea that polyurethanes thermally degrade in stages, that is, the

polyurethane first loses the aromatic isocyanate units, and then the polyol undergoes fragmentation.

The two major stages of polyurethane degradation are illustrated

with the postulated mechanism [1-5] in the following scheme:

9
I. First major stage: Degradation of the urethane group at some temperature below 380 °C.

Polyurethane

Primary aromatic Amine + CO2 + Residue

derived from polyether

Secondary Amines + CO2

'Aromatic diisocyanate + Residue of PolyetherPolyol

II. Second major stage: Degradation of residues from first stage at

temperatures above 380 °C.

Residues of polyether-polyol Alkanes + alkenes + carbonyl

and compounds + etc.

Derivatives of polyethers

Although flexible foams may contain functional groups other than

urethane and other linkages as a result of cross-linking agents and ex- cess diisocyanate for example, this scheme is directed primarily at those

groups involved in the polyurethane molecules. Consequently, the above

scheme is only intended to illustrate a probable process by which the two

major stages of degradations may arise and not the complete process by

which all possible products are formed including further reactions of the

initial products. However, there are inferences that should be noted;

first, two of the three reactions in the first stage yield CO2; and second,

this scheme suggests that highly flammable compounds are produced only at

elevated temperatures, that is, in the second stage.

The production of CO2 in the early stages of degradation may well

account for the large volume fraction of pyrolysis products in the flashfire cell at the initial flash. Carbon dioxide, being inert, would reduce

the flammability of the fuel-air mixture by increasing the heat capacity

[10,11] of the gas mixture. In addition, the aromatic compounds evolved

in the first stage may also form aerosols and particulates when distilled

from the pyrolysis chamber or become a part of the residue. These distilled products condense on the walls and so are not likely to support

flash fires near room temperature.

y ^ 4. CONCLUSION

An apparatus has been developed in which flash fires can be reproduced from the pyrolysis products of polyurethane foams. And it seems apparent that the technique developed in this study is applicable to the

determination of the flash-fire potential of other polymeric materials.

Flash fires have been reproduced in this apparatus when the sample

to volume ratio is not less than 0.2 g/1 and the pyrolysis temperature

of the sample is not less than 380 °C. In these instances, it was also

shown that flash fires occur when the oxygen volume fraction is diluted

to about 17% by the pyrolysis products.

10
When the temperature of the polyurethane is raised above 390 °C at

60 °C/inin for example, the minimum temperature required to achieve a flash fire condition, two major stages of degradation are achieved. We

conclude then that the products of the second major stage are primarily

responsible for the flame front in the flash fires and that these products

include low molecular weight compounds derived from the polyol fraction

of the copolymer. The higher molecular weight components in the first

major stage, aromatic nitrogen compounds derived from and including the

diisocyanate fragments, are not expected to support a flash fire in the

flash-fire cell since the temperature in the cell remains near the

temperature of the room during the experiment.

5. ACKNOWLEDGEMENTS

The investigations described in this report were sponsored in part

by the National Aviation Facilities Experimental Center of the Federal

Aviation Administration—National Bureau of Standards Work Agreement

No. FA 67 NF-AP-21.

We are grateful to Dr. R. J. McCarter for assistance given in the use

of the pyrolysis rate apparatus and for making the polyurethane samples

available. We also wish to thank Mr. Henry Pierce for obtaining some of

the data on the flash-fire cell.

6. REFERENCES

[1] Woolley, W. D., Nitrogen-Containing Products from the Thermal

Decomposition of Flexible Polyurethane Foams, J. Br. PoI^th. , Vol. 4, 27 (1972).

[2] Tilley, J. N., Nadeau, H. G. , Reymore, P. H., Waseciak, P. H.

and Sayigh, A. A. R. , Thermal Degradative Behavior of Selected

Urethane Foams Related to Variations of Constituents, Part II.

Chemical Reactions in Urethane Decomposition, J. Cellular Plastics,

Vol. 4, 45 (1972).

[3] Napier, D. H. and Wong, T. W., Toxic Products from the Combustion

and Pyrolysis of Polyurethane Foams, J. Br. Polym. , Vol. 4, 45

(1972).

[4] Dyer, E. and Wright, G. C, Thermal Degradation of Alkyl N-Pheylcarbamates, J. Am. Chem. Soc. , Vol. 31, 2138 (1959).

[5] Saunders, J. H., Thermal Degradation and Flammability of Urethane

Polymers, Rubber Chem. Tech., Vol. 32, 337 (1959).

11
REFERENCES, cont'd

[6] Paabo, M. and Comeford, J. J., A Study of the Decomposition

Products of Polyurethane Foam Related to Aircraft Cabin Flash

Fires, Nat. Bur. Stand. (U.S.) Report No. FAA-RD-73-46 (1973).

[7] McCarter, R. J., to be published.

[8] McCarter, R. J. , A New Technique for Thermal Analysis of VaporProduclng Reactions, J. Appl. Poly. Sc., Vol. 17, 1833 (1973).

[9] Kuchta, J. M. , Fire and Explosion Manual for Aircraft Accident

Investigators, Technical Report No. 4193, Bureau of Mines,

Pittsburgh, Pa. (1973).

[10] Huggett, C. , Combustion Process In the Aerospace Environment,

Aerospace Med., Vol. 40, 1176 (1969).

[11] Huggett, C, Habitable Atmospheres Which Do Not Support

Combustion, Combustion and Flame, Vol. 30, 140 (1973).

USCOMM-NBS-DC

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NBS-n4A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

I. KUnLIL^AllUN UK Kr.KUKl I\U.

NBSIR 75-757

^. (jov t Accession

No.

3. Recipient's Accession No.

i TITI F AND SIIRTITI F

A Technique for the Measurement of Flash Fire Potential

of Polymeric Materials

5. Publication Date December 1975

6» Performing OrganizdCion Code

7. AUTHOR(S)

James E. Brown and J. J. Comeford

8. Performing Organ. Report No. NBSIR 75-757

Q p p R Fnn MIMn OR r; A N 17 A T ION MAMF AMD AnDRF^;^

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. A914373

11. Contract/Grant No.

FA 67NF-AP-21

12a Spon s or ing anization Name and CIo mp Icte Address (S trGGt , Ci ty , S tstG , ZIP

)

Department of Transportation

Federal Aviation Administration

National Aviation Facilities Experimental Center

Atlantic City, N.J. 08405

13. Type of Report & Period Covered Progress Report

Feb. 1 - Aug. 31, 1974

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.) This report summarizes work for the period February - August 1974 on a continuing

program to characterize the chemical and physical parameters of importance in a flash

fire and to develop laboratory scale methods for measuring the flash lire potential

of materials.

Significant modifications have been made to a flash-fire cell develoi:id earlier

to measure the flash fire potential of materials by characterizing the condition

required to produce an ignitable pyrolyzate-air mixture by thermally degrading a polymer. The furnace temperature, cell geometry and orientation, and sample size are specified. These modifications have resulted in an improved technique, especially

in terms of reproducibility, for evaluation of flash fire potential of materials.

Experiments have been conducted on rates of combustible gas formation from flexible polyurethanes to assist in the optimization of the flash-fire cell operating

conditions.

In the rate study, two successive major stages of degradation were found for

polyurethane as the temperature approached 500° at a rate of about 60 °C/min. It was

also found that the products of the second stage appear to be responsible for flash

fires in the flash-fire cell. A minimum polyurethane weight to enclosure volume ratio

greater than 0.2 g/1 and a sample pyrolysis temperature greater than 380 °C were

required to produce a flash fire in this apparatus.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Aircraft fires; fire hazard; flash fire; polyurethane;

pyrolysis.

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