NBSIR 75-901

Smoke and Carbon Monoxide

Formation from Materials Tested

in the Smoke Density Chamber

Thomas Y. King, Research Associate

Armstrong Cork Company

Lancaster, Pennsylvania

Center for Fire Research

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

October 1 975

Final Report

U S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 75-901

SMOKE AND CARBON MONOXIDE

FORMATION FROM MATERIALS TESTED

IN THE SMOKE DENSITY CHAMBER

Thomas Y. King, Research Associate

Armstrong Cork Company

Lancaster, Pennsylvania

Center for Fire Research

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

October 1975

Final Report

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

James A. Baker, III, Under Secretary

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

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Acting Director

CONTENTS

Page

LIST OF FIGURES iv

LIST OF TABLES v

£. INTRODUCTION 1

2. THEORETICAL ASPECTS 2

3. EXPERIMENTAL DETAILS 3

3.1. Smoke Density Chamber 3 3.2. Test Materials 3

4. SMOKE AND CARBON MONOXIDE GENERATION 5 4.1. Sample Probe Position 5 4.2. Material Composition 5 4.3. Heat Flux . 6

4.4. Weight Loss 8

4.5. Oxygen Depletion 8

5. SMOKE MASS DENSITY VERSUS OPTICAL DENSITY 9

6. SMOKE PARTICULATE SIZE AND SHAPE 11

7. CONCLUSIONS AND RECOMMENDATIONS 12

8. ACKNOWLEDGEMENT 13

9 . REFERENCES 13

iii
LIST OF FIGURES

Page

Figure 1. Probe Position and Carbon Monoxide Concentration from

Red Oak and Rigid PVC (Flaming Exposure) 14

Figure 2. Smoke and Carbon Monoxide from Various Materials

(Smoldering Exposure) 15

Figure 3. Smoke and Carbon Monoxide from Various Materials

(Flaming Exposure) 16

Figure 4. Variations in Maximum Specific Optical Density with

Irradiance Level (Flaming Exposure) 17

Figure 5. Effect of Heat Flux on Carbon Monoxide Formation - ABS (Flaming Exposure) 18

Figure 6. Effect of Heat Flux on Carbon Monoxide Formation - Plasticized PVC (Flaming Exposure) 19

Figure 7. Effect of Heat Flux on Carbon Monoxide Formation - Red Oak (Flaming Exposure) 20

Figure 8. Effect of Heat Flux on Carbon Monoxide Formation - Polystyrene (Flaming Exposure) 21

Figure 9. Effect of Heat Flux on Carbon Monoxide Formation - Rigid PVC (Flaming Exposure) 22

Figure 10. Variations in Maximum Specific Optical Density with

Irradiance Level (Smoldering Exposure) 23

Figure 11. Carbon Monoxide Formation for Various Materials at

4.2 W/cm^ (Smoldering Exposure). . 24

Figure 12. Smoke and CO Variations with Chamber OxygenPlasticized PVC (Flaming Exposure) 25

Figure 13. Smoke and CO Variations with Chamber Oxygen-Red Oak

(Flaming Exposure) 26

Figure 14. Smoke and CO Variations with Chamber Oxygen-Rigid PVC

(Flaming Exposure) 27

Figure 15. Smoke and CO Variations with Chamber Oxygen-ABS

(Flaming Exposure) 28

Figure 16. Smoke and CO Variations with Chamber Oxygen-Polystyrene

(Flaming Exposure) 29
LIST OF FIGURES (cont'd)

Page

Figure 17. Optical Density vs Smoke Mass Density 30

Figure 18. Rigid PVC "Flaming Exposures" Smoke Particulate,

Scanning Electron Microphotographs 31

Figure 19. Plasticized PVC "Flaming Exposure" Smoke Particulate,

Scanning Electron Microphotographs 32

Figure 20. Red Oak "Flaming Exposure" Smoke Particulate, Scanning

Electron Microphotographs 33

Figure 21. ABS "Flaming Exposure" Smoke Particulate, Scanning

Electron Microphotographs 34

Figure 22. Polystyrene "Flaming Exposure" Smoke Particulate,

Scanning Electron Microphotographs 35

LIST OF TABLES

Table 1. Materials Studied 4

Table 2. Average Percent Total Weight Loss (Flaming Exposure). . . 8

Table 3. Ratio of Particulate Smoke Mass to Optical Density - K Factors 11

Table 4. Estimated Minimum Particle Diameters 12

v

SMOKE AND CARBON MONOXIDE FORMATION FROM

MATERIALS TESTED IN THE SMOKE DENSITY CHAMBER

Thomas Y. King 1

The effects of physical and chemical factors on the smoke

and carbon monoxide generated by burning polymers and red oak

in the NBS Smoke Density Chamber are reported.

Smoke and carbon monoxide formation was found to depend

on irradiance level and oxygen concentration in the chamber.

Smoke optical density correlations with particulate mass density showed that the ratio of these two varied with

material composition and burning conditions.

Key words: Carbon monoxide; electrostatic precipitation;

heat flux; oxygen depletion; particulate mass; scanning

electron microscope; smoke.

1. INTRODUCTION

The most important consideration during a fire is life safety. Smoke,

toxic gases, ease of ignition, heat release, and flame spread are some of

the factors which can contribute to a fire hazard. All of these factors

should be considered in the final assessment of a material's potential

hazard. In past years, studies dealing with smoke and noxious gas production from burning materials were considered less significant than ignition

and flame propagation studies. Statistics show that inhalation of products

of combustion from the sustained burning of materials is a contributing

factor in a large number of fire deaths. Poor visibility caused by the

smoke compounds the problem since it impedes escape for the victim and causes him to remain in the fire environment for longer periods of time.

The density of smoke and concentration of toxic gases present at any

given time in an actual fire depends on variables such as the amounts and

types of materials burned, heat exposure conditions, air temperatures, air

velocities, enclosure volumes and dimensions, and ventilation. Many of

these variables are interrelated and time dependent. In order to evaluate

and compare the smoke generating potential of different materials it is

necessary to control as many of the variables as possible.

This paper presents the results of smoke and carbon monoxide measurements from burning selected materials under various controlled fire

environments in a smoke density chamber.

1 The author was a Research Associate at the National Bureau of Standards,

Washington, D.C. when the work reported here was performed.

1
2. THEORETICAL ASPECTS

Smoke can be defined as solid or liquid particulate matter in suspension arising from the burning or decomposition of any material. Smoke is

most commonly measured in terms of its light attenuation in a photometer.

This method assumes the application of Bouguer's or Beer's law to the

attenuation of light by the smoke, and results are given as optical density

rather than light absorption or transmittance. Foster [l] 2 has described

the relationship:

I = Tp e

where (I) is the transmitted light intensity, (Io) is the incident light

intensity, (Ke) is the extinction area coefficient, and (B) is a function

of mass concentration (c), particle mass mean radius (a), density (d) , and

optical light path length (s), as given by:

4 ad

The extinction area coefficient (Ke) can be defined in terms of the absorption cross section area (A) and scattering cross section area (S) , so that

S + A

Ke = 5—. The optical density (D) then can be described by:

2

iTa

Io KeB

D = lo8in t~~ =

or

10 I - 2.303 (1)

S + A j /3cs

Tra z / \ 4ad/ _ 3cs (S + A)

2.303 4-^^(2.303) (2)

For systems with nonuniform particle size, such as smoke, (S) and (A) are expressed as complicated functions of particle size distribution, refractive index, and wavelength of incident light.

Smoke measurements can also be expressed in terms of specific optical

density (Ds

) in which light path length (L) , chamber volume (V) , and

exposed specimen surface area (A^) are taken into consideration. Equation

(1) then becomes:

Numbers in brackets correspond with the literature references listed at

the end of this paper.

- ( at> log

io r

= KeB

2.303

2
D j being dimensionless , allows direct comparison of smoke densities

regardless of chamber volume, light path length, and exposed specimen

surface area, provided other parameters remain constant.

3. EXPERIMENTAL DETAILS

3.1. Smoke Density Chamber

Data reported in this paper were obtained from measurements in the

^ National Bureau of Standards Smoke Density Chamber [2], This is a 0.51 m

(18-ft 3

) enclosed chamber in which a vertically positioned photometer with

0.9m path length measures the amount of light attenuation (or scattering)

by the smoke. The smoke emanates from burning a vertical specimen 3 by 3

inch and with a 2-9/16 inch square exposed surface area. The specimen can be exposed to heat under either flaming or non-flaming (smoldering) con- ditions. In the standard test method the specimen surface is exposed to a heat flux of 2.5 W/cm2 (2.2 Btu/s/ft 2

) f rom a radiant source (electrically

heated) with or without a piloted ignition source consisting of a 6 jet

air-propane burner. The wall temperature of the chamber is maintained at

35+2 °C at the start of the test. The specific optical smoke density is

dependent upon specimen variables of thickness and density, the exposed

sample area being constant as defined by the specimen holder.

Normally, in routine testing, only the maximum specific optical smoke

density (Dm) is reported. The recorded maximum specific optical smoke

density is corrected by subtracting the photocell residue reading from the

Dm

after the test has been terminated and the chamber exhausted. The rate

of increase of D_ and the time to reach a specific D value may also be of m r . m

interest

.

Carbon monoxide (CO) concentration was recorded simultaneously with

the smoke density using an infrared analyzer connected in line to the top

of the chamber. Results are reported as volume concentration in parts per

million (ppm) . In these studies flow rates from the chamber to the analyzer were maintained at about 1 liter/min by using a critical orifice under

vacuum. A dry ice trap and filter assembly was necessary to remove most

of the smoke ahead of the analyzer. The sample line dead volume resulted

in a delayed time response of approximately 30 s to reach 95% of the

steady-state value. The carbon monoxide curves reported were not cor- rected for this factor.

3.2. Test Materials

The five materials used in this study (table 1) were: red oak, rigid

polyvinyl-chloride, plasticized polyvinyl-chloride , acrylonitrile-butadienestyrene (ABS), and polystyrene.

3
Table 1. Materials Studied

Material Abbreviation

Approximate

Thickness

(in) (mm)

* Approximate

Density

(lb/ft 3

) (kg/m3

)

Rigid

Polyvinylchloride R-PVC 0.023 0.58 77.3 123.7

Plasticized

Polyvinylchloride

i

P-PVC 0.022 0.56 80.6 129.0

AcrylonitrileButadieneStyrene ABS 0.031 0. 79 57.4 91.8

Red Oak Red Oak 0.045 1.14 39.6 63.4

Polystyrene Polystyrene 0.026 0.66 65.0 104.0

Density measurements were calculated from the approximate thickness and

weights of the actual samples.

In order to compare the smoke and carbon monoxide generating potentials of

these particular materials, several sample design criteria were used:

1. Samples were compared at similar weights (approximately

4.15 grams average) since total smoke and carbon monoxide

depend upon the weight of sample combusted, especially in

the case of non-charring materials.

2. The samples used in the study contained no fillers Or

flame retardants and a minimum of other components

necessary for fabrication. This permitted a better

comparison of the data resulting from the basic polymer

rather than from the many additives possible; in practical applications, additives could comprise a large

percentage of the actual composition and have a significant effect on the results.

3. Sample thickness and weight were relatively small to

meet the following criteria:

a. The heat transfer into the sample and resultant smoke generation would not be restricted

by excessive char formation. The time for

particulate deposition to occur during the

buildup stage was thereby limited.

4
b. Oxygen depletion would be minimized in the chamber by burning a relatively small sample.

c. The burning characteristics of the material

resulting from variable sample densities would

be minimized for thin compared to thick samples.

4. SMOKE AND CARBON MONOXIDE GENERATION

Factors which can govern a material's tendency to smoke include:

composition, density, weight, thickness, heat flux, airflow, oxygen

availability, pressure, surface characteristics, and geometry. This

study was conducted primarily to obtain data on the relative concentration of smoke and carbon monoxide generated from the 5 typical materials

and to determine if a relationship exists between smoke optical density

and carbon monoxide formation for various materials under a range of

exposure conditions.

Since the chamber is equipped with a vertically arranged photoelectric cell and light source to eliminate errors in measurements due

to smoke stratification, it was necessary to determine what effect smoke

stratification might have on carbon monoxide measurements made at a single location in the chamber. It was conceivable that due to convective heat flow patterns smoke and hot gases would tend to concentrate at

the top of the chamber before mixing. To determine whether this occurred,

carbon monoxide was monitored at the top, middle, and bottom of the

chamber. Carbon monoxide produced from red oak and rigid PVC samples

was measured at these three positions under flaming conditions, as shown in

figure 1. Results indicated that a significant carbon monoxide concentration gradient existed only during the first few minutes of the test.

After this initial time period concentration was essentially uniform

throughout the chamber under these conditions.

The relationship between simultaneous smoke (dotted curves) and

carbon monoxide (solid curves) measurements with time for the five

materials burned under non-flaming (2.5 W/cm^) exposures is shown in

figure 2. Curves shown are averages of three separate determinations

and are approximations of the actual curves recorded, since data points

were plotted every 2 minutes for smoke and 2.5 minutes for carbon monoxide.

Results indicate that the four plastic materials generated carbon

monoxide very slowly (2-3 ppm/min) at a constant cumulative rate over

the time interval defined under these test conditions. Since there were

substantial differences, in certain cases in maximum smoke densities,

4.1. Sample Probe Position

4.2. Material Composition

5
but little differences in the rates of CO buildup, the detection of a simple correlation between smoke density and carbon monoxide concentration for different materials appears doubtful. The six-fold difference

in maximum smoke density between rigid and plasticized PVC was attributed

to the large contribution of the plasticizer to the smoldering smoke

evolution. The magnitude of this difference, however, may change with

plasticizers of different chemical composition and physical properties.

Compared to the plastics, red oak liberated large quantities of

carbon monoxide under non-flaming burning conditions. The relatively

rapid rate of CO generation (approximately 80 ppm/min) occurred concurrently with the onset of glowing combustion.

Figure 3 illustrates similar curves for the five materials burned

under flaming conditions. Under this exposure, the rate of carbon

monoxide generation from the pilot burners (~5-6 ppm/min) was subtracted

from actual measured CO curves. With the exception of red oak, generally

more smoke and CO were produced under flaming, compared to smoldering,

conditions. In most cases, the maximum carbon monoxide levels for red

oak and ABS reached a plateau after the maximum smoke density had been

reached, which was consistent with the observation that little char

remained at that point. For rigid and plasticized PVC, the CO concentration continued to increase well after the maximum smoke density was attained, which apparently resulted from slow oxidation of the remaining

char

.

The carbon monoxide curve for polystyrene was not typical of those

previously described. The melting and dripping characteristics of the

material when burned caused a substantial portion of the sample to flow

into a holding pan at the base of the sample holder. Delayed ignition

of this material resulted in a second burning stage and accounts for the

erratic shape of the CO curve.

4.3. Heat Flux

In an actual fire a material is subjected to a broad range of irradiance levels, from pre-ignition to flashover. Studies relating smoke density

and carbon monoxide concentration to variations in thermal flux levels for

the five materials were made under flaming and non-flaming exposure conditions .

Under flaming exposures, the radiant heat flux from the electric

heater was varied from 0 to approximately 4.2 W/cm2 with the same pilot

burner exposure. Irradiance levels under non-flaming conditions were

typically 1.0, 2.5, and 4.2 W/cm^. (A more efficient radiant heater

assembly was used to attain the 4.2 W/cm2 irradiance level.) A Gardner

thin foil air-cooled radiometer was used to calibrate the heater at flux

levels ranging from 1.9 to 3.1 W/cm^, while a water-cooled thin foil

radiometer was used for calibrations at 1.0 and 4.2 W/cm^.

6
In figure 4, the average maximum specific optical smoke densities

(uncorrected) are shown at various irradiance levels under flaming con- ditions for the five, materials. At a relatively high heat flux (~4.2

W/cm^) the plastic materials tested generate similar smoke densities

under this particular test configuration. Total weight loss after a 20-

minute period ranged from 93 - 99%. At the lower irradiance levels

smoke density is strongly dependent upon material composition. The

largest change in maximum smoke generation occurred at heat fluxes ranging

from 2.0 to 3.0 W/cm^ for polystyrene and rigid PVC. ABS reached the

highest smoke buildup at about 3.0 W/cm^, and a further increase in radiant

flux level resulted in an apparent lowering of the maximum smoke density.

Variations in irradiance levels had little effect on smoke density for

red oak samples under these conditions. The results for materials such as polystyrene which melt and drip when burned should be qualified since they are strongly dependent upon the physical properties of the sample and test

geometry.

The averaged carbon monoxide curves for ABS, plasticized PVC, red oak,

rigid PVC, and polystyrene at the various heat fluxes under flaming exposures are shown in figures 5-9. The rate of carbon monoxide formation for

all materials tested is increased with successively higher irradiance

levels. After this initial period of rapid CO generation, concurrent

with observed rapid burning and smoke production, further carbon monoxide

formation probably depends upon the nature and amount of char remaining

after flaming combustion and the total heat flux level (including the con- tribution from the burner flames) of the particular test. For example, the

total CO generated by burning ABS at different irradiance levels (fig. 5)

tends to converge after 8 minutes. At the higher heat fluxes and temperatures, more smoke is produced with little char remaining, accounting for

the rapid leveling of the CO buildup after Dmis reached. The converse

occurs at the lower irradiance levels , and the carbon monoxide concentration after an extended period of time can be as high or higher than that

obtained at the higher irradiance levels at similar times, probably due to

the slow formation of CO from oxidation of the char.

Under non-flaming conditions the five materials show a similar

increase in Dmwith higher irradiance levels (fig. 10). The curves shown

here are based on limited data. Red oak and rigid PVC produced substantially less smoke density, over the range of heat flux levels explored,

than ABS, plasticized PVC, or polystyrene.

Carbon monoxide formation was negligible for all materials tested at

a 1.0 W/cm^ radiant flux level over a 40-minute period. At 4.2 W/cm^ the

averaged carbon monoxide curves (fig. 11) do show appreciable differences

in the tendency of the plastic materials to form carbon monoxide. The

induction period of between 4 to 6 minutes before the CO concentration is

measurable suggests that CO formation is linked to char oxidation rather

than smoke production, since a significant amount of smoke generated occurs

in all cases prior to any measurable carbon monoxide. Furthermore, the

relative ranking of the plastics based on carbon monoxide concentration at

any given time (R-PVC>P-PVC , ABS>polystyrene) might be expected if one assumes a similar ranking for the amount of char remaining at any given

7
time. Some CO contribution from the oxidation of smoke and decomposition

products in close proximity to the radiant heat source also is possible.

At 4.2 W/cm2

, red oak produced larger quantities of carbon monoxide at a more rapid rate (~280 ppm/min) , and the induction period was shorter

C30 s).

4.4. Weight Loss

Although the initial weight of the samples in the previous studies

was similar, the total weight loss was found to depend upon the composition of the material and the thermal exposure conditions. In table 2 the

average percent total weight loss occurring after a 20 minute period and

initial average sample weights (grams) are shown for the five materials at

various irradiance levels under flaming conditions.

Table 2. Average Percent Total Weight Loss* (Flaming Exposure)

0 W/cm2 1.0 W/cm2 1.9 W/cm2 3.1 W/cm2 4.2 W/cm2

R-PVC (4. 30) 16.5 (4.19) 61.1 (3.97) 72. 6 (3.92) 85.6 (4.19) 93.4

ABS (4. 38) 36.6 (4.00) 74.9 (4.00) 82. 1 (4.47) 94.4 (4.11) 97.9

P-PVC (4. 08) 39.5 (4.00) 71.5 (4.12) 83. 2 (3.95) 90.4 (4.02) 94.5

Red Oak (4. 04) 41.5 (4.01) 73.3 (4.32) 79. 6 (4.13) 87.6 (4.05) 94.8

Polystyrene (4. 00) 20.0 (3.92) 45.7 (3.96) 69. 3 (4.01) 91.4 (3.99) 98.5

(Range 3

,

.92 - 4. 47 g, av 4.08 g)

[Initial sample weight av (g) given in parenthesis.]

*The heading gives irradiance levels from the electric heater in W/cm2 . In

the first case where the value is 0 W/cm2 the radiance heater was off with

only the pilot burners ignited.

While total weight loss increases with irradiance level, the data give

no indication of the relationship between smoke density and weight loss

rate. Others [3] have shown through continuous weight loss measurements in

the chamber that for certain materials, the smoke density for a given mass

loss of sample is greater in the latter stages of sample consumption. The

early degradation products, because of their physical and chemical nature,

could have less tendency to obscure a light beam path than a similar weight

of degradation products generated later in the combustion process.

4.5. Oxygen Depletion

Another important factor affecting smoke and carbon monoxide generation in a developing fire is oxygen depletion. In some large-scale fire
tests it is not uncommon for the oxygen level to drop to such a low value

that the fire is extinguished.

The volume of the smoke chamber was designed [4] so that sufficient

oxygen would be available for complete combustion of most specimens. To

determine general trends, smoke and carbon monoxide concentrations from the

five materials were evaluated under an artificially oxygen depleted chamber

atmosphere. Nitrogen was used to purge the chamber and the oxygen concen- tration was monitored continuously (paramagnetic oxygen analyzer) until a desired 15% oxygen level was obtained. The sample was attached by a spring

clip to a dummy blank located in front of the radiant heater so that it

could be positioned in front of the heater at the desired time without

opening the chamber. Thermal radiation was blocked from the sample during

purging by means of an aluminum* foil shield positioned between the radiant

heater and the sample. Under flaming exposures, the pilot burners were

ignited prior to purging the chamber.

Comparison of the resultant smoke and carbon monoxide measurements at

normal (21% 0£ initially) and oxygen lean (15% O2 initially 3

) chamber atmospheres are shown in figures 12-16 for plasticized PVC, rigid PVC, ABS, red

oak, and polystyrene, respectively, under flaming conditions. By reducing

the oxygen level in the chamber, the duration of flaming combustion for all

materials was noticeably reduced, as expected. The times at which the

maximum smoke densities were reached were usually increased. Although more

data over a range of oxygen levels are necessary, some general observations

can be made. For those materials which generate similar maximum smoke

densities under standard flaming and non-flaming burning conditions (e.g.,

plasticized PVC and polystyrene), artificially reducing the oxygen in the

chamber to an initial 15% level initially does not appear to change the

maximum smoke density significantly although the time to reach this value

is lengthened. For materials which have considerably higher maximum smoke

densities under flaming compared to non-flaming exposures (e.g., rigid PVC

and ABS), burning similar samples in an oxygen deficient atmosphere appears

to result in a significant reduction of the maximum smoke density.

Carbon monoxide concentrations were found to be similar or lower initially at the lower chamber oxygen level, but were enhanced in all cases after this initial burning period.

5. SMOKE MASS DENSITY VERSUS OPTICAL DENSITY

Smoke particulates present a dual fire hazard — visual obscuration

and particulate inhalation by potential victims. Optical density measurements appear to be sufficient for relating to visual obscuration, but

little has been reported on smoke concentration in terms of mass density,

another useful parameter to characterize "quantity of smoke."

The oxygen concentration was not measured after the start of the test.

During the test the oxygen concentration fell below the 15% level due to

consumption by the burning sample as well as by the pilot burners.

9
Correlations between smoke mass density and optical density were

studied in the smoke chamber using both gravimetric filtration and electrostatic precipitation as collection techniques. All smoke was sampled

at the midpoint of the side wall of the chamber opposite the photometer.

For smoke samples composed primarily of carbonaceous soot, a 0.6 micron

(average pore diameter) millipore PVC filter was employed to collect the

larger particle size mass fraction. An electrostatic mass monitor [5] was

used as a backup for the smaller particles and/or condensible liquid droplets which might pass through the filter. For smoke generated under non- flaming (smoldering) type conditions (consisting primarily of suspended

liquid droplets); and red oak under flaming conditions (consisting of both

solid and liquid particulate matter), the electrostatic mass monitor was

the sole collection technique. This latter technique has several advantages over filtration methods. Smoke mass is continuously monitored by a shift in resonance frequency of a small quartz plate which collects smoke

on its surface. Smoke in the sampling stream is deposited electrostatically on the quartz plate, thus changing the mass of the plate. The reson- ance frequency of the plate is proportional and very sensitive to the mass

of the plate. A smoke deposition rate on the order of 0.1 mg/min can be

easily detected. Measurements were made under steady-state conditions

after the maximum optical smoke density had been reached. With collection

by filtration, a 1.0 liter/min critical orifice was used and the smoke was sampled over a 5-min period after reaching the maximum density. The average optical density had to be determined over this 5-min time period.

Because of problems with moisture absorption, it was necessary to allow the

smoke on the filter paper to approach a steady weight during micro-weighing.

A comparison of optical smoke densities (per metre path length) versus smoke mass densities (milligram per cubic metre) for the five materials

under flaming and non-flaming conditions is shown in figure 17. Initial

sample weights were varied by changing the sample area exposed while keeping

thickness constant, to obtain a range of optical densities in the chamber.

With the exception of red oak, ABS, and polystyrene under non-flaming

burning conditions, smoke mass density appeared to be linearly related to

optical density over the range explored. The observed cases of non- linearity are not fully understood. They might result from the particular

properties of the pyrolysis products contributing to agglomeration [6] or backward light scattering. The ratio of particulate smoke to optical

density, expressed as K (grams per cubic metre/optical density per metre)

shown in table 3, indicates little difference among the materials which

produce sooty smoke (ABS, R-PVC, P-PVC, and polystyrene) under flaming

conditions. This implies that regardless of the initial composition of

these materials, the same mass of carbonaceous smoke, will provide similar

light obscuration. Optical density/mass density relationships are affected

substantially by burning conditions. For all five materials smoke generated under non-flaming conditions resulted in half the obscuration per unit mass of smoke than under flaming conditions. Comparison of the smoke mass density from red oak under smoldering exposures and from any of the plastics

under flaming exposure at an arbitrary 0.2 optical density per metre value

shows that there is approximately a four-fold mass difference at the same optical density.

10
Table 3. Ratio of Particulate Smoke Mass to Optical

„ _ , Grams per Cubic Metre x Density - K Factors (- : —e ; - ) Optical Density per Metre

Material Exposure Collection Techniques K Factor

ABS Flaming Gravimetric Filtration 0. 27

Polystyrene Flaming Gravimetric Filtration 0. 27

R-PVC Flaming Gravimetric Filtration 0. 28

P-PVC Flaming Gravimetric Filtration 0. 30

Red Oak Flaming Electrostatic Precipitation 0. 40

Average 0

.

30

P-PVC Non- flaming Electrostatic Precipitation 0. 46

R-PVC Non- flaming Electrostatic Precipitation 0. 52

ABS Non- flaming Electrostatic Precipitation 0.,33--0. 56' Polystyrene Non-flaming Electrostatic Precipitation 0. 38--0. 51

Red Oak Non-flaming Electrostatic Precipitation 0. 83--1. 21

Average 0. 60

approximate range

6 . SMOKE PARTICULATE SIZE AND SHAPE

Smoke particle size to a large degree determines whether smoke is

entrapped in the lungs, filtered out in the nose, or inspired and expired

without depositing on the lung tissue [7]. Furthermore, particle size

undoubtedly plays an important role in the optical density versus mass density relationships.

In this study, smoke particulate matter from the five materials was examined with a scanning electron microscope. Samples were prepared by

electrostatic precipitation (aerosol sampler) [8] onto an electropolished

titanium metal surface. By using a pulsed precipitating electric field

in the sampler, smoke deposition on the metal surface was relatively uniform without preferential size separation.

Photographs of the particles from smoke generated under flaming

burning conditions from rigid and plasticized PVC, red oak, ABS, and polystyrene are shown in figures 18-22, respectively. The smoke samples from

the plastic materials under these burning conditions appeared as discrete

spherical units forming chains or "stringers." Smoke from the red oak

sample appeared as a mixture of solid particles and elliptical-shaped

liquid residue. The diameter of the smoke particles varied widely, ranging

from a fraction of a micron for monospherical particles up to 30 or more microns in diameter for the larger aggregates. Although a size distribution

analysis was not felt to be within the scope of this study, estimates were

made of the range of particle diameters assuming single, non-aggregated

spherical units (table 4)

.

11
Table 4. Estimated Minimum Particle Diameters

Material

Estimated Minimum

Diameter Range (ym)

R-PVC

P-PVC

Red Oak

ABS

Polystyrene

0.1

0.1

0.2

0.1

0.1

0.4

0.2

0.7

0.3

0.3

Similar attempts to evaluate smoke samples from these materials under

non-flaming burning conditions proved unrewarding. The smoke samples were

coated with a thin (~200 A) layer of tin metal before microscopic examination to improve conductivity and to eliminate possibilities of interior

microscopic contamination. The lack of particle definition upon examination indicated that the samples did not survive this treatment and

suggested that the smoke generated under these conditions was primarily

liquid

.

This project has been primarily of a basic nature. The materials

studied were selected to represent a group of typical polymers rather

than building materials" and commercial products; the results are specific

for the chemical composition and physical characteristics of these particular samples.

This study has demonstrated that the relationship between optical density and smoke mass density can change depending upon material composition

and burning conditions. Because of this changing relationship, optical

density and mass density of smoke should be evaluated jointly. Secondly,

since there is no simple relationship between smoke density and carbon

monoxide generation among materials of different composition, one cannot assume that a smoke density measurement gives a direct measure of the

relative amount of carbon monoxide. Finally, as the oxygen concentration

is depleted, combustion can change from a flaming to a smoldering mode.

For those materials which produce less smoke under smoldering exposure

conditions, the smoke density will probably be lower when the oxygen level

is reduced. Potential fire hazard assessments will be more meaningful when

these factors are taken into consideration.

7. CONCLUSIONS AND RECOMMENDATIONS

12
8 . ACKNOWLEDGEMENT

The author would like to express his gratitude to W. J. Keery of the

National Bureau of Standards for making photomicrographs of the smoke

samples in the scanning electron microscope, and to T. G. Lee of the

National Bureau of Standards for his many helpful suggestions during the course of this study.

9. REFERENCES

[1] Foster, W. W. , Attenuation of Light by Wood Smoke, British Journal of

Applied Physics, Vol. 10 (Sept. 1959), pp. 416-420.

[2] Lee, T. G. , Interlaboratory Evaluation of Smoke Density Chamber, Nat.

Bur. Stand. (U.S.), Tech. Note 708 (Dec. 1971).

[3] Seader, J. D. and Chien, W. P. , Quantification and Measurement of

Smoke, paper presented at the Polymer Conference Series on Flammability Characteristics of Materials, University of Utah, Salt Lake

City (June 1973).

[4] Gross, D. , Loftus, J. J. and Robertson, A. F. , Method for Measuring

Smoke from Building Material, Symposium on Fire Test Methods - Restraint and Smoke 1966 (American Society for Testing and Materials,

Philadelphia, Pa., 1967), pp. 166-204.

[5] Olin, J. G. and Sem, G. J., Piezoelectric Microbalance for Monitoring

the Mass Concentration of Suspended Particles, presented at Symposium:

Advances in Instrumentation for Air Pollution Control, Sponsored by

National Air Polution Control Administration, U. S. Department of

Health, Education, and Welfare, Cincinnati, Ohio, May 26-28, 1969.

[6] Pilat, M. J. and Ensor, D. S., Plume Opacity and Particulate Mass Concentration, Atmospheric Environment, Pergamon Press, Vol. 4 (1970),

pp. 163-173.

[7] Landahl, H. D. and Black, S., Penetration of Airborne Particulates

through the Human Nose, Journal of Industrial Hygiene and Toxicology,

Vol. 29, No. 4 (July 1947), pp. 269-277.

[8] Liu, B. Y. H. , Whitby, K. T. and Yu, H. H. S., Electrostatic Aerosol

Sampler for Light and Electron Microscopy, the Review of Scientific

Instruments, Vol. 38, No. 1 (Jan. 1967), pp. 100-102.

13
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FIG. 18 RIGID PYC 'FLAMING EXPOSURE' SMOKE PARTICULATE,

SCANNING ELECTRON MICROPHOTOGRAPHS

31
FIG. 19 PLASTICIZED PVC 'FLAMING EXPOSURE' SMOKE PARTICULATE

SCANNING ELECTRON MICROPHOTOGRAPHS

32
FIG. 20 RED OAK 'FLAMING EXPOSURE' SMOKE PARTICULATE,

SCANNING ELECTRON M ICROP KOTO G R A PHS

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FIG. 21 ABS 'FLAMING EXPOSURE' SMOKE PARTICULATE,

SCANNING ELECTRON MICROPHOTOGRAPHS

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FIB. 22 POLYSTYRENE 'FLAMING EXPOSURE' SMOKE PARTICULATE,

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35
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 75- 901

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Smoke and Carbon Monoxide Formation from Materials Tested

in the Smoke Density Chamber

5. Publication Date October 1975

6. Performing Organization Code

7. AUTHOR(S)

Thomas Y. King

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

same as No. 9

13. Type of Report & Period Covered

Final Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.)

The effects of physical and chemical factors on the smoke and carbon monoxide

generated by burning polymers and red oak in the NBS Smoke Density Chamber are reported.

Smoke and carbon monoxide formation was found to depend on irradiance level and

oxygen concentration in the chamber. (Smoke optical density correlations with

particulate mass density showed that the ratio of these two varied with material

composition and burning conditions.

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

Carbon monoxide; electrostatic precipitation; heat flux;

oxygen depletion; particulate mass; scanning electron microscope; smoke.

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