NIST Technical Note 1799

Impact of Test and Foam Design on 

Smoldering 

Mauro Zammarano

Szabolcs Matko

Rick D. Davis

http://dx.doi.org/10.6028/NIST.TN.1799
i

NIST Technical Note 1799

Impact of Test and Foam Design on 

Smoldering

Mauro Zammarano

Szabolcs Matko

Rick D. Davis

Fire Research Division

Engineering Laboratory

http://dx.doi.org/10.6028/NIST.TN.1799

May 2013

U.S. Department of Commerce 

Rebecca Blank, Acting Secretary

National Institute of Standards and Technology 

Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and Director
ii

Certain commercial entities, equipment, or materials may be identified in this

document in order to describe an experimental procedure or concept adequately.

Such identification is not intended to imply recommendation or endorsement by the

National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose.

National Institute of Standards and Technology Technical Note

Natl. Inst. Stand. Techn. Techn. Report 1799, 26 pages (May 2013)

CODEN: NTNUE2 
iii

Abstract

The performance of an upholstery cover fabric in the promotion of smoldering combustion of

polyurethane foam (PUF) can be assessed by a previously developed bench scale test. The 

purpose of the research reported here was to determine whether the test apparatus and the 

polyurethane foam could be modified to increase the total assembly mass loss (more severe 

smoldering) while also increasing experimental repeatability.

The results show that the replacement of the non-air-permeable assembly frame with an airpermeable substrate in the test apparatus induces a significant increase in smoldering mass loss 

and mass loss repeatability. Similar results are achieved by increasing the air permeability of 

PUF with a process that removes the residual membranes in the foam (reticulation). Ultimately, 

both the reticulation process of PUF and the modification of the test apparatus aim to promote 

oxygen transport to the smolder front by natural convection.

These findings indicate the importance of natural convection on ignition, magnitude, and 

repeatability of smoldering combustion; they might be useful for future development of benchscale smoldering test methods with improved correlation with full-scale smoldering tests.

Keywords

Polyurethane foam; smoldering; furniture; flammability; SRM 1202
iv

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v

Contents

Abstract..........................................................................................................................................iii

Contents.......................................................................................................................................... v

List of Tables ................................................................................................................................. vi

List of Figures............................................................................................................................... vii

List of Acronyms ........................................................................................................................... ix

Introduction..................................................................................................................................... 1

Experimental................................................................................................................................... 2

Mockup Designs: Standard (Closed) and Modified (Open) Mockup......................................... 2

Test Enclosure............................................................................................................................. 4

Experimental Setup..................................................................................................................... 4

NIST Box Test ............................................................................................................................ 5

Materials ..................................................................................................................................... 6

Results............................................................................................................................................. 6

Mockup Smoldering Tests Run in the Enclosure without a Lid................................................. 6

Mockup Smoldering Tests Run in the Enclosure with Lid......................................................... 9

Smoldering in the NIST Box Test ............................................................................................ 14

Conclusions................................................................................................................................... 15

References..................................................................................................................................... 17
vi

List of Tables

Table 1. Mass loss measured according to CPSC’s proposed regulation2(MLCPSC) for the closed 

and open mockup. ........................................................................................................................... 6

Table 2. Average (xˉ ), standard deviation () and relative standard deviation (rel) values for the 

peak of MLRGAS (PMLR), time to PMLR (TTP), and MLGAS at t=TTP (ML@TTP) calculated 

for the three tests of Figure 7 and Figure 8..................................................................................... 9

Table 3. Mass loss, sustained smoldering and Tav/Tav................................................................ 15
vii

List of Figures

Figure 1. In the presence of a non-air permeable substrate the oxygen supply to the smoldering 

front is enhanced by natural convection in a thick sample (a), and it is severely limited in a thin 

foam sample (b); oxygen supply can be promoted by natural convection in thin foam samples 

replacing the non-permeable substrate with an air-permeable substrate (c)................................... 2

Figure 2. Schematic drawing of the standard (closed) mockup test (a) versus the modified (open) 

mockup test (b): a gap of 13 mm (about 0.5 in) between the foam and the sample holder and a 

partial fabric removal promote natural convection in the open mockup. Five thermocouples (TC1 

to TC5) are installed in both configurations. .................................................................................. 3

Figure 3. Photos of two sample-holder prototypes for the open mockup test. In the first prototype 

(left), metal screws and a metal wire mesh are used to suspend the foam at a distance of 13 mm 

from the wood sample holder. In the second prototype (right), thermocouples are also added to 

monitor the foam temperature......................................................................................................... 3

Figure 4. Schematic drawing of the enclosure................................................................................ 4

Figure 5. Picture of the actual experimental setup. The mockups (open on the left and closed on 

the right) are placed inside polycarbonate enclosures. Two scales are used for monitoring in real 

time the mass loss due to the volatiles released by both the fabric and the foam (MLGAS). MLCPSC

was calculated at the end of the test (t=45 min) by first removing all charred foam and then 

weighing the residual uncharred foam only.................................................................................... 5

Figure 6. Schematic drawing of the box test. The box test is a smoldering test that does not 

require the use of a fabric or a cigarette.......................................................................................... 5

Figure 7. MLGAS (mass loss due to the volatiles released by both the fabric and the foam, 

normalized by the total mass of the fabric and the foam) for three Foam F samples in a closed 

mockup............................................................................................................................................ 8

Figure 8. Mass loss rate calculated as the time derivative of MLGAS for the samples of Figure 7. 8

Figure 9. Photograph of the closed (right) and open (left) mockups after about (39 to 40) min 

from the beginning of the test. Foam F is used for both mockups. .............................................. 10

Figure 10. Temperature and mass loss profiles for a sample of Foam F in the open mockup test 

with enclosure lid. Transition to flaming was observed at about t = 44 min................................ 11

Figure 11. Temperature and mass loss profiles for a sample of Foam F in the closed mockup test 

with enclosure lid during the first 60 min..................................................................................... 11

Figure 12. Temperature and mass loss profiles for Foam F in the closed mockup test with 

enclosure lid after t = 60 min. ....................................................................................................... 12

Figure 13. Side view of the closed mockup after testing. The bottom of the vertical foam slab in 

contact with the sample holder shows sign of charring (labeled as charred foam/fabric). Charring 

in this region was generated by a smoldering front propagating on the fabric and around 

uncharred foam. At this point (t = 70 min), the temperature in all thermocouples increased 

rapidly possibly due to an increase in the foam-substrate gap (especially at the bottom corners of 

the vertical foam slab), that promoted natural convection............................................................ 13

Figure 14. Photograph showing the residues for the foams tested in the open mockup (right) and 

closed mockup (left). The test duration was about 100 min in the closed mockup. In the open 

mockup, transition from smoldering to flaming occurred at about t = 44 min............................. 13

Figure 15. Comparison between the temperature profiles of the thermocouples in the box test 

without box (left) and with the box (right). A sample shows unambiguously sustained smoldering 
viii

(left only) if the temperature of at least one thermocouple is higher than the temperature of the 

heater (in blue) at any time during the test. .................................................................................. 14
ix

List of Acronyms

CPSC U.S. Consumer Product Safety Commission

MLCPSC Mass loss of the foam expressed as % of the initial mass of the 

foam; charred material is removed and accounted as mass loss

(according to the CPSC proposed regulation)

2

MLGAS Mass loss of the mockup measured in real time and expressed as % 

of the initial mass of the foam plus the fabric; charred material is 

not removed and is not accounted as mass loss

(MLCLOSED)GAS Value of MLGAS in the closed mockup configuration

(MLOPEN)GAS Value of MLGAS in the open mockup configuration

(MLOPEN/MLCLOSED)CPSC Ratio between MLCPSC in the open mockup and MLCPSC in the 

closed mockup

MLR Mass loss rate

ML@TTP Value of MLGAS at t=TTP

NIST National Institute of Standards and Technology

PMLR Peak of mass loss rate

PUF Polyurethane foam

SRM Standard Reference Material

SSS Self-sustained smoldering

TTP Time to peak of mass loss rate

xˉ Average value

 Standard deviation



rel Relative standard deviation

(

rel

CLOSED /

rel

OPEN)CPSC Ratio between 

rel calculated for MLCPSC in the closed mockup and 



rel calculated for MLCPSC in the open mockup
1

Introduction

The influence of an upholstery cover fabric to promote smoldering in polyurethane foams can be 

assessed using a foam mockup test.

1

The methodology is based on the test described in the

Consumer Product Safety Commission (CPSC) proposed regulation (73 FR 11702).2This 

method requires a polyurethane foam (PUF) with repeatable and well-characterized smoldering.

The National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 

1202 is a kit that includes a smoldering standard polyether-based polyurethane foam, a denim 

cover fabric, a cotton fabric, and SRM 1196 cigarettes. The denim fabric supplied in the kit is 

used to cover the exposed portion of the polyurethane foam in the test method. The denim fabric 

is a control or a reference for the test, which is used to determine the performance of other 

fabrics in the promotion of smoldering mass loss of the composite assembly as described on the 

SRM certificate. In order to assess the smoldering propensity of other cover fabrics, the denim 

fabric must be replaced with this other fabric. The test measures the mass loss of the foam during 

a 45 min period. During the development of SRM 1202, the mass loss values drifted drastically 

from test to test for the same batch of foam and, even more significantly, from batch to batch. 

The highest repeatable mass loss values were in the low twenties.

The purpose of the research reported here was to determine whether the mockup test device and 

the polyurethane foam could be modified to increase the total mass loss values (more severe 

smoldering) while also increasing experimental repeatability. 

Numerical simulation and experimental data indicated the key role of oxygen supply on 

smoldering combustion of PUF.

3,4,5,6,7,8,9,10 We previously reported on foam characteristics that 

influence smoldering performance.11 For a foam of given formulation, smoldering is controlled 

by the cell morphology, in particular, smoldering increases with an increasing fraction of open 

versus closed cells, and decreasing cell size. Such a foam morphology boosts oxygen supply to 

the smoldering front by promoting natural convection (open cell structure), and an increase of 

surface area available for thermo-oxidative reactions (small cell size).12

Based on this information, commercial foams were identified that should have higher and more 

repeatable mass loss values. These foams were standard polyether-based polyurethane foams but, 

in addition, they were post-processed with a reticulation procedure. The reticulation process 

removes residual membranes in the foam and provides a highly permeable foam with a 

homogeneous fully-open cell structure. This morphology is expected to increase smoldering 

magnitude, due to the increase in air permeability and natural convection, and increase the 

smoldering repeatability, due to the removal of residual membranes.*

Here, the SRM 1202 mockup test and the NIST box test11 have been modified to suppress the 

limiting effect on natural convection of the wood substrate used as sample holder. As depicted in 

the schematic drawing of Figure 1, this effect is particularly important for thin foam samples 

(Figure 1b) rather than thicker foam samples (Figure 1a), where natural convection can still 

efficiently supply oxygen to the smoldering front. In other words, the closer the smoldering front 

 

*

In non-reticulated polyurethane foams it is technically impossible to control the fraction of open versus closed cells 

throughout the same foam batch and between different batches. The resulting variations in air permeability and 

surface area affect smoldering significantly.
2

gets to the substrate, the less oxygen reaches the smoldering front by natural convection and, 

ultimately, the smoldering front might become unstable. Thus, it is reasonable to speculate that 

the higher the natural convection (controlled by foam thickness, foam air-permeability, substrate 

permeability and, eventually, fabric permeability) and the higher the smoldering rate and 

repeatability.

Figure 1. In the presence of a non-air permeable substrate the oxygen supply to the smoldering 

front is enhanced by natural convection in a thick sample (a), and it is severely limited in a thin 

foam sample (b); oxygen supply can be promoted by natural convection in thin foam samples 

replacing the non-permeable substrate with an air-permeable substrate (c).

The aforementioned effect of foam thickness might explain the lack of correlation between 

bench-scale (i.e., thin foam samples) performance and full-scale smoldering performance (i.e., 

thick foams samples).13 This inconsistency can be simply overcome by using an air-permeable 

substrate that would allow a uniform oxygen supply to the smoldering front through natural 

convection (Figure 1c).

Ultimately, the two approaches used here, the reticulation process of PUF and the modification 

of the testing configuration, aim to promote oxygen supply to the smoldering front by natural 

convection. The findings reported in this work indicate the key role of natural convection on 

ignition, magnitude and repeatability of smoldering.

Experimental

Uncertainties are reported as one standard deviation ().

Mockup Designs: Standard (Closed) and Modified (Open) Mockup

The standard mockup test1(closed mockup) was modified to boost natural convection by 

introducing a 13 mm (about 0.5 in) gap between the foam and the sample holder and by 

removing part of the fabric (Figure 2 and Figure 3).

a)

Foam

Substrate

Smoldering 

Front Smoldering Front

Foam

Substrate

b)

Smoldering Front

Foam

Permeable Substrate

c)

Convection Convection

Convection
3

Figure 2. Schematic drawing of the standard (closed) mockup test (a) versus the modified

(open) mockup test (b): a gap of 13 mm (about 0.5 in) between the foam and the sample holder 

and a partial fabric removal promote natural convection in the open mockup. Five 

thermocouples (TC1 to TC5) are installed in both configurations.

Figure 3. Photos of two sample-holder prototypes for the open mockup test. In the first 

prototype (left), metal screws and a metal wire mesh are used to suspend the foam at a 

distance of 13 mm from the wood sample holder. In the second prototype (right), 

thermocouples are also added to monitor the foam temperature. 

An obvious advantage of the open mockup is that the wood sample holder is not exposed to 

charring and can always be reused, even with highly smoldering foams.

a) b)
4

Test Enclosure

Smoldering tests were run inside polycarbonate enclosures in order to improve the repeatability 

of the test by:

• minimizing the effect of airflow variations over time in the hood;

• suppressing turbulence.

A schematic drawing of the enclosure is shown in Figure 4. Tests were run with or without the 

removable lid. The presence of the lid promoted a pure convective laminar flow of the smoke 

produced by smoldering, and, likely, increased the temperature and the concentration of 

combustible volatiles in the enclosure.

Figure 4. Schematic drawing of the enclosure.

Experimental Setup

Each mockup with either the standard (closed) or modified (open) set-up was placed inside a 

polycarbonate enclosure. Tests were run with and without the removable lid. Smoldering was 

assessed for all mockups as foam mass loss at the end of the test (MLCPSC), according to the

CPSC proposed regulation.

2

The test duration was 45 min. All charred material was removed 

from the foam at the end of the test and accounted as mass loss. MLCPSC was expressed as % of 

the initial mass of the foam.

In some cases, the mass of each mockup and the temperature of the thermocouples TC1 to TC5 

were also recorded in real time throughout the test. The mass loss measured here was due to the 

volatiles released by both the fabric and the foam (MLGAS). MLGAS was expressed as % of the 

initial mass of the foam plus the fabric. The charred foam was accounted for mass loss 

calculations in MLCPSC but not in MLGAS, therefore, MLGAS at t=45 min was different from 

MLCPSC. The actual experimental set up is shown in Figure 5.
5

Figure 5. Picture of the actual experimental setup. The mockups (open on the left and closed on

the right) are placed inside polycarbonate enclosures. Two scales are used for monitoring in 

real time the mass loss due to the volatiles released by both the fabric and the foam (MLGAS).

MLCPSC was calculated at the end of the test (t=45 min) by first removing all charred foam and 

then weighing the residual uncharred foam only.

NIST Box Test

This is a smoldering test method developed at NIST.

11 The foam sample is placed in a wood box 

and covered with a glass fiber cloth; an electrical heater is used to initiate smoldering (Figure 6). 

The presence of the box and the fiber glass cloth limit natural convection. Tests on the same 

foam were run with and without the box to evaluate the effect of the presence of the box on 

smoldering.

Figure 6. Schematic drawing of the box test. The box test is a smoldering test that does not 

require the use of a fabric or a cigarette. 
6

Materials

Three different foams were tested in the mockup test. The foam used in NIST SRM 1202 (SRM 

1202 foam, for the remainder) is a slabstock polyether-based polyurethane foam.1Foam C and 

Foam F are both reticulated polyether-based polyurethane foams. Foam C contains carbon 

black. A polyether-based flexible polyurethane foam (Foam B11)

11 was used for the box test.

The foam densities were (26.7 ± 0.7, 22.4 ± 0.3, 22.3 ± 0.3 and 29.2 ± 0.6) kgm

-3

for SRM 1202 

foam, Foam C, Foam F and Foam B11, respectively.

Results

Mockup Smoldering Tests Run in the Enclosure without a Lid

Smoldering tests were run in the closed and open mockup configuration in the enclosure without

a lid. Mass loss was calculated on the foam only according to the CPSC proposed regulation2

(MLCPSC). The results are summarized in Table 1.

Table 1. Mass loss measured according to CPSC’s proposed regulation2(MLCPSC) for the closed

and open mockup.

SRM 1202 Foam

(Non-reticulated)

Foam C

(Reticulated)

Foam F

(Reticulated)

Mockup setup Mockup setup Mockup setup

Closed Open Closed Open Closed Open 

MLCPSC (%) 17.1 48.9 19.5 57.8 67.5 92.4

 (%) 13.0 10.8 4.2 3.0 6.0 0.9



rel(%) 76.1 22.1 21.5 5.2 8.9 1.0

Number of Tests 12 4 4 4 4 3

(MLOPEN/MLCLOSED)CPSC 2.9 3.0 1.4

(

rel

CLOSED /

rel

OPEN)CPSC 3.4 4.1 8.9

: standard deviation.



rel

: relative standard deviation.

(MLOPEN/MLCLOSED)CPSC: ratio between MLCPSC in the open mockup and closed mockup.

(

rel

CLOSED /

rel

OPEN)CPSC: ratio between 

rel calculated for MLCPSC in the open mockup and closed mockup.

The data of Table 1 for SRM 1202 foam and Foam C indicate that:

1. In both the open and standard mockup test, the average mass loss for the Foam C is about 

15 % higher than the average mass loss for SRM 1202 foam. This suggests that relative 

smoldering performance between different foams is not affected by the test configuration.

2. In both configurations, the relative standard deviation for Foam C is 3-4 times smaller 

than the relative standard deviation for SRM 1202 foam. This indicates that a reticulated 

foam with its regular open cell structure induces a more repeatable smoldering.

3. The average mass loss increases by a factor of about 3 for both SRM 1202 foam and 

Foam C when the open mockup test is used rather than the standard closed mockup test.

The open test configuration induces a proportional variation in average mass loss in both 

foams.
7

4. The relative standard deviation decreases by a factor of 3 to 4 when the open mockup is 

used (3.4 for SRM 1202 foam and 4.1 for Foam C). In the standard mockup the 

smoldering front might become unstable when approaching the sample holder due to 

insufficient oxygen supply; this phenomenon might affect the repeatability of the test.

The results for Foam F in Table 1 show qualitatively the same trend observed for SRM 1202 

foam and Foam C, i.e., an increase in smoldering mass loss and a decrease in the relative 

standard deviation when the open mockup configuration is used. However, the magnitude of 

these variations was different. There was only a 1.4 fold increase (instead than about 3 fold 

increase as for Foam C and SRM 1202 foam) in mass loss because Foam F already lost most of 

its mass (about 67 %) in the closed mockup. Similarly, the relative standard deviation in the open

mockup is particularly low because smoldering was close to 100 % after 45 min, however the 

mass loss rate might vary significantly between one test and the other.

Mass loss, measured as MLGAS, was recorded in real time on three Foam F samples (same 

formulation as the Foam F used in Table 1 but different batch) in a closed mockup configuration.

Figure 7 shows the MLGAS profiles measured for these samples. MLGAS at t = 45 min is 

obviously different from the mass loss measured according to the CPSC proposed regulation

(MLCPSC), where only the mass loss in the foam is measured and all charred foam is accounted as 

mass loss. As an example, the three tests of Figure 7 have an average MLCPSC of (81.7 ± 3.1) %

and an average MLGAS of (42.6 ± 7.4) %.

Figure 8 shows the mass loss rate (MLRGAS) calculated as the time derivative of MLGAS for the 

same samples of Figure 7. Self-sustained smoldering (SSS) is defined here as persisting 

smoldering after the removal of the heat generated by the ignition source. In the mockup test, 

SSS can be unambiguously identified by increasing values of MLRGAS after the complete 

consumption of the ignition source; in fact, at this stage, there is no more heat generated by the 

cigarette, and an increase in MLRGAS can be only induced by an increase in heat generated by 

smoldering of the foam and/or the fabric. For the ignition source used here (a NIST SRM 1196 

cigarette), complete consumption is achieved in 30 min (typically between 25 min and 28 min).

All three tests in Figure 8 showed SSS, in fact, MLRGAS kept increasing for all samples after 

t = 30 min and the peak of MLGAS (PMLR) occurred at t > 30 min. These closed mockup tests 

were run with highly permeable reticulated foams. In general, no SSS was observed when the air 

permeability of the foam was below a threshold value, and MLCPSC was lower than 0.2 %.

11,†

In 

other words, when natural convection was suppressed by reducing the air permeability of the 

foam, SSS did not occur and the smoldering mass loss was significantly lower.

 

†

In this study SSS was assessed by smoke production, i.e., no visible smoke production at t > 30 min implies no 

SSS.
8

Figure 7. MLGAS (mass loss due to the volatiles released by both the fabric and the foam, 

normalized by the total mass of the fabric and the foam) for three Foam F samples in a closed 

mockup.

Figure 8. Mass loss rate calculated as the time derivative of MLGAS for the samples of Figure 7.

The values of time to PMLR (TTP), PMLR and MLGAS at t=TTP (ML@TTP) are summarized in 

Table 2. The relative standard deviation (rel) for TTP, ML@TTP and PMLR are about 3 %, 

18 % and 33 %. The relative standard deviation for MLCPSC in the same set of foam samples is 

about 4 %. This implies that even if the MLCPSC values are similar, the maximum smoldering rate 

(that is proportional to PMLR), might vary significantly between one foam sample and another.

In Figure 8, the reduction in MLRGAS for t > TTP ( 40 min) indicates a reduction in smoldering 

rate. It can be argued that for foams with high values of MLCPSC, like Foam F, the test duration 

should be decreased to prevent a reduction in smoldering rate due to sample size limitations (i.e., 

0

10

20

30

40

50

60

0 10 20 30 40 50

MLGAS (%)

Time (min)

Test 1

Test 2

Test 3

0

1

2

3

4

5

6

0 10 20 30 40 50

MLRGAS (%/min)

Time (min)

Test 1

Test 2

Test 3
9

foam depletion and/or approach of the smoldering front to the sample holder). For t < TTP, 

MLRGAS is still increasing, the differences in mass loss (measured as MLGAS) between different 

samples might not be significant and the mass loss ranking of the samples might be different 

from the mass loss rate ranking at t = 45 min. For example, at t = 30 min, MLGAS values are 

9.5 %, 15.1 % and 11.1 % for test 1, test 2 and test 3, respectively. At t = 45 min, MLGAS values 

are 34.7 %, 43.5 % and 49.5 % for test 1, test 2 and test 3, respectively (see Figure 7). In general, 

a prolonged smoldering time is preferable in terms of sensitivity but increases the 

aforementioned effect of sample size limitations. As a result, the selection of an optimal testing 

time for MLCPSC type calculations (where the test needs to be stopped to remove the char and 

calculate the mass loss at a given time) remains questionable and a continuous real-time massloss measurement, as MLGAS, appears to be generally more robust and meaningful.

The PMLR calculated from MLGAS might be a better indicator for smoldering propensity than 

MLCPSC, in particular for evaluating the probability of smoldering-to-flaming transition.14 The 

severity of smoldering increases with an increase in PMLR and ML@TTP, and decreases with 

an increase in TTP.

Table 2. Average (xˉ ), standard deviation () and relative standard deviation (rel) values for the

peak of MLRGAS (PMLR), time to PMLR (TTP), and MLGAS at t=TTP (ML@TTP) calculated for the 

three tests of Figure 7 and Figure 8.

TTP 

(min)

PMLR 

(%/min)

ML@TTP 

(%)

xˉ 39.3 4.0 33.6

 1.2 1.3 6.1

rel 2.9 32.9 18.2

Mockup Smoldering Tests Run in the Enclosure with Lid

The previous paragraph has shown the importance of natural convection on smoldering in terms 

of both magnitude and repeatability. In this section the closed and open mockup tests were run in 

a similar configuration to the one used before with the exception of the enclosure that, instead of 

being open on the top, was closed with a lid (Figure 4 and Figure 5). An open and a closed 

mockup test were run in parallel with Foam F. Figure 9 shows the two samples at about 39 min 

to 40 min from the beginning of the test. In the open mockup (Figure 9, on the left), the amount 

of smoke in the enclosure was observed to be larger and the smoldering front reached the side of 

the foam. These observations indicate a higher smoldering rate for the open mockup test.
10

Figure 9. Photograph of the closed (right) and open (left) mockups after about (39 to 40) min 

from the beginning of the test. Foam F is used for both mockups.

This conclusion was supported by the values of mass loss and temperature of the thermocouples 

(Figure 10 and Figure 11) that were higher in the open mockup test than the closed mockup test. 

At t = 40 min, the mass loss in the closed mockup, (MLCLOSED)GAS, was 13.3 % and the mass loss 

in the open mockup, (MLOPEN)GAS, was 45.8 %. The ratio between MLGAS in the open mockup 

and MLGAS in the closed mockup, (MLOPEN/MLCLOSED)GAS, was about 3. The temperature 

measured by thermocouples TC1 to TC5 reaches a maximum of 390 C for the closed mockup 

and about 670 C for the open mockup, enough to induce autoignition. Transition to flaming was 

observed at t  44 min and the sample was suppressed with a CO2 fire extinguisher after about 

10-15 s from the initiation of flaming ignition. Interestingly, the temperatures of all 

thermocouples appeared to decrease rather than increase after ignition, possibly, due to the 

oxygen depletion inside the foam (no more heat produced in the foam due to smoldering) and the 

low thermal conductivity of the charred foam (the increase in temperature due to the fire is 

negligible in the foam core, where the thermocouples are placed).

Transition to flaming was never observed in the open mockup test without the enclosure lid

(eight tests). Even though more validation tests are necessary, this result suggests that the 

enclosure lid promotes transition to flaming, likely, by increasing the temperature and the 

concentration of combustible volatiles in the enclosure. Also, the presence of the enclosure lid 

helped to decrease the rate of combustion by limiting the amount of oxygen available to the 

smoldering front, another beneficial effect of the lid.
11

Figure 10. Temperature and mass loss profiles for a sample of Foam F in the open mockup test 

with enclosure lid. Transition to flaming was observed at about t = 44 min.

Figure 11. Temperature and mass loss profiles for a sample of Foam F in the closed mockup test 

with enclosure lid during the first 60 min.

0

20

40

60

0

100

200

300

400

500

600

700

0 10 20 30 40 50 60

MLGAS (%)

Temperature (

oC)

Elapsed time (min)

TC#1 (bottom)

TC#2

TC#3

TC#4

TC#5 (top)

Mass Loss (%)

0

20

40

60

0

100

200

300

400

500

600

700

0 10 20 30 40 50 60

MLGAS (%)

Temperature (

oC)

Elapsed time (min)

TC#1 (bottom)

TC#2

TC#3

TC#4

TC#5 (top)

Mass Loss (%)
12

Figure 12. Temperature and mass loss profiles for Foam F in the closed mockup test with 

enclosure lid after t = 60 min.

For the closed mockup, the test duration was about 100 min. In Figure 12 the temperature and 

mass loss profiles for t > 60 min are shown (mass loss data reading became unreliable after 

70 min). The temperature of TC1 (closest thermocouple to the bottom of the sample holder) is 

the lowest throughout the test. The temperature of thermocouples TC3 to TC5 started decreasing 

after t = 60 min and started rising again at about t = 70 min, when the smoldering fabric reached 

the bottom of the vertical foam and induced some charring in the foam in contact with the 

sample holder (Figure 13). The rapid increase in smoldering observed at this stage might be due 

to the formation of an air gap between the sample holder and the bottom corners of the vertical 

foam slab; in fact, such an air gap might promote natural convection. The final foam residues in 

the closed and open mockups are compared in Figure 14.

The temperature of the thermocouples, TC3 and TC4, reached a maximum temperature of about 

600 C at t = 80 min in the closed mockup test (maximum temperature in the open mockup 

670 C), therefore, autoignition and transition to flaming cannot be excluded in this test 

configuration, even though it would require more time. The temperature peak for TC5 at 

t = 86 min (478 C) occurred when the smoldering front on the fabric reached the top of the 

vertical foam.

The data of Figure 11 and Figure 12 show also that the foam temperature can increase 

significantly even without a significant MLGAS. For example in Figure 11 at t = 12 min, the TC#3 

thermocouple reached a temperature of about 120 C without any significant MLGAS. This 

suggests that temperature measurements rather than mass loss measurements are preferable in 

terms of sensitivity.

0

30

60

0

100

200

300

400

500

600

700

60 70 80 90 100 110 120

Mass Loss (%)

Temperature (

oC)

Elapsed time (min)

TC#1 (bottom)

TC#2

TC#3

TC#4

TC#5 (top)

Mass Loss (%)
13

Figure 13. Side view of the closed mockup after testing. The bottom of the vertical foam slab in 

contact with the sample holder shows sign of charring (labeled as charred foam/fabric). 

Charring in this region was generated by a smoldering front propagating on the fabric and 

around uncharred foam. At this point (t = 70 min), the temperature in all thermocouples 

increased rapidly possibly due to an increase in the foam-substrate gap (especially at the 

bottom corners of the vertical foam slab), that promoted natural convection.

Figure 14. Photograph showing the residues for the foams tested in the open mockup (right) 

and closed mockup (left). The test duration was about 100 min in the closed mockup. In the 

open mockup, transition from smoldering to flaming occurred at about t = 44 min.

Uncharred foam

Charred foam/fabric
14

Smoldering in the NIST Box Test

The effect of natural convection on smoldering was also investigated with the NIST box test.

Tests on the same type of foam were run with and without the box to evaluate the effect of the 

presence of the box on smoldering (Figure 6). The temperature of the electric heater, used as the 

smoldering ignition source, was set to 330 oC or 340 oC. Three tests were run per each 

combination of temperature and testing configuration (with or without box) for a total of 12 tests.

Figure 15 shows a comparison between the temperature profiles of the thermocouples with and 

without the box with the source set to 340 oC. A sample shows unambiguously self-sustained 

smoldering (SSS) in the box test if the temperature of at least one thermocouple is higher than 

the temperature of the heater (in blue) at any time during the test; in this scenario smoldering is 

not driven by the heater but is self-sustained by the heat generated in the foam by char 

oxidation.14 According to this definition, without box, 100 % of the samples showed SSS,

however, with a box, only 33 % and 66 % of the samples showed SSS at a heater set temperature 

of 330 C and 340 C, respectively. Independent of the heater temperature, 50 % of the total 

samples tested with the box demonstrated SSS behavior. The results are summarized in Table 3.

Figure 15. Comparison between the temperature profiles of the thermocouples in the box test 

without box (left) and with the box (right). A sample shows unambiguously sustained 

smoldering (left only) if the temperature of at least one thermocouple is higher than the 

temperature of the heater (in blue) at any time during the test.

Self-sustained and non-self-sustained smoldering samples showed an average mass loss value of 

(8.0 ± 3.1) % (9 tests) and (2.7 ± 0.8) % (3 tests), respectively; thus, when natural convection 

was suppressed by reducing the air permeability of the test apparatus, sustained smoldering was 

less likely to occur and the mass loss was significantly lower. As already mentioned, the air 

permeability of the foam had a similar effect on SSS in the mockup test.

Data in Table 3 suggest that the average mass loss at both 330 C and 340 C increases when the 

box is removed. This difference in mass loss increased with the heater temperature. More 

replicate tests are needed to verify that these differences in mass loss are significant (unknown 

uncertainty for smoldering mass loss in presence of the box).
15

Table 3. Mass loss, sustained smoldering and Tav/Tav.

Heater 

Temperature 

(C)

Samples with SelfSustained Smoldering

(%)

Average Mass Loss in 

Samples with SSS 

(%)

Tav/Tav in 

Samples with SSS

Without 

Box

With

Box

Without 

Box

With Box Without 

Box

With 

Box

340 100 66 12.0±1.3 6.8* 0.25±0.11 0.48*

330 100 33 7.3±2.8 4.2* 0.28±0.10 0.33*

*Data with unknown uncertainty (less than three samples with SSS are available).

The asymmetry of the smoldering front can be used to evaluate the repeatability of the 

smoldering process in a specific scenario (i.e., with or without box). The differences in 

temperature between the thermocouples at the same distance from the heater (TC1 and TC4, TC2 

and TC5, TC3 and TC6) are used to evaluate the asymmetry of the smoldering front. In

smoldering samples, the asymmetry of the smoldering front can be evaluated by the following 

expression:

Tav/Tav= Abs(TTC1(t)- TTC4(t)) / ((TTC1(t)+TTC4(t))/2) + Abs(TTC2(t)- TTC5(t)) / 

((TTC2(t)+TTC5(t))/2) + Abs(TTC3(t)- TTC6(t)) / ((TTC3(t)+TTC6(t))/2)

where TTCi(t)is the temperature of the generic thermocouple TCi (with i integer between 1 and 6)

at a generic time t. For a perfectly symmetric smoldering front Tav/Tav= 0.

At a heater temperature of 330 C, the foam was barely smoldering in presence of the box and 

the test configuration had an insignificant effect on asymmetry of the smoldering front; however 

Tav/Tav increased by almost a factor of two at a heater temperature of 340 C when the box was 

used. More replicate tests are necessary for calculating standard deviation values and confirm 

these preliminary data.

Conclusions

We identified new commercial foams that have higher and more repeatable mass loss values as 

compared to SRM 1202 foam in the CPSC mockup test. These foams were standard polyetherbased polyurethane foams post-processed with a reticulation procedure which removes residual 

membranes in the foam and provides high permeability foams with a homogeneous fully-open 

cell structure. This morphology is expected to increase the magnitude and repeatability of 

smoldering mass loss in the mockup test by promoting natural convection and suppressing air 

permeability variation in PUF.

The use of substrates impermeable to air prevents natural convection and generates instability in 

the smoldering front in the proximity of the substrate due to oxygen “starvation.” This effect is 

particularly important for thin foam samples rather than thicker foam samples, in which natural 

convection can efficiently supply oxygen to the smoldering front. In general, the higher the 

natural convection (controlled by foam thickness, foam air-permeability, substrate permeability 
16

and fabric permeability) and the higher the smoldering rate, percentage of self-sustained

smoldering samples and repeatability.

The aforementioned effect of foam thickness is also important, because it might explain the lack 

of correlation between bench-scale (i.e., thin foam samples) performance and full-scale 

smoldering performance (i.e., thick foams samples) whenever air-impermeable substrates are 

used. A better correlation might be achieved using a more realistic test configuration by the 

incorporation of air-permeable substrates in the test, allowing a uniform oxygen supply to the 

smoldering front through natural convection, ultimately producing a more intense and repeatable

smoldering.

Here, the mockup test and the NIST box test have been modified to suppress the limiting effect 

on natural convection of the wood substrate, used as the sample holder. In the mockup test the 

wood substrate is replaced with a metal wire mesh and in the NIST box test the wood box is 

removed.

The findings from this research show that:

 the reticulated foams used here have a more repeatable smoldering behavior (3- to 4-fold 

reduction in relative standard deviation for mass loss) and a higher average mass loss (2-

to 3-fold increase in mass loss) as compared to SRM 1202 foam;

 the use of a permeable substrate in the open mockup test induced roughly a 3-fold 

increase in smoldering mass loss;

 smoldering is more repeatable when a permeable substrate is used in the mockup test (the 

relative standard deviation for smoldering mass loss decreases by a factor of 3 to 4);

 the combination of a permeable substrate, an external enclosure and a reticulated foam 

promoted the transition from smoldering to flaming in the mockup test;

 in the NIST box test the percentage of self-sustained smoldering samples decreases from 

100 % to 50 % when natural convection was limited by the wood substrate, used as 

sample holder;

 continuous real-time mass-loss and temperature monitoring appears to be a more 

meaningful, sensitive and robust approach for smoldering evaluation than a single mass 

loss calculation at an arbitrarily defined length of time.

These findings indicate the importance of natural convection on ignition, and the magnitude and 

repeatability of smoldering combustion. This information will be useful for the future 

development of bench-scale smoldering tests with an improved correlation of the rate of 

smoldering with full-scale smoldering tests using actual furniture.
17

References

 

1. Davis R.D., Zammarano M., Matko S., Kraemer R.H. NIST TN 1775: Standard Operating 

Procedures for Smolder Ignition Testing of Upholstery Fabrics, National Institute of Standards 

and Technology Technical Note 1775, November 2012, 24 pgs. available at 

http://www.nist.gov/manuscript-publication-search.cfm?pub_id=912584

2. Proposed standard for the flammability of upholstered furniture, CPSC 1634, 73 FR 11702 

(March 4, 2008).

3. Rogers F.E., Ohlemiller T.J., Kurtz A., and Summerfield M., Studies of the smoldering 

combustion of flexible polyurethane cushioning materials. Journal of Fire and Flammability, 

1978, 9, p. 5-13.

4. Dodd A.B., Lautenberger C., and Fernandez-Pello C., Computational modeling of smolder 

combustion and spontaneous transition to flaming. Combustion and Flame, 2012. 159(1): p. 448-

461. 

5. Ortiz-Molina M.G., Toong T-Y., Moussa N.A., and Tesoro G.C., Smoldering combustion of 

flexible polyurethane foams and its transition to flaming or extinguishment. Symposium 

(International) on Combustion, 1979. 17(1): p. 1191-1200. 

6. Rein G., Lautenberger C., Fernandez-Pello A.C., Torero, J.L., and Urban, D.L., Application of 

genetic algorithms and thermogravimetry to determine the kinetics of polyurethane foam in 

smoldering combustion. Combustion and Flame, 2006. 146(1-2): p. 95-108. 

7. Torero J.L. and Fernandez-Pello A.C., Forward smolder of polyurethane foam in a forced air 

flow. Combustion and Flame, 1996. 106(1-2): p. 89-109. 

8. Rein G., Smouldering combustion phenomena in science and technology. International 

Review of Chemical Engineering, 2009, p. 3-18. 

9. Torero J.L., Fernandez-Pello A.C., Natural Convection Smolder of Polyurethane Foam, 

Upward Propagation, Fire Safety Journal, 1995, 24, 35-52.

10. Bar-Ilan A., Rein G., Walther D.C., Fernandez-Pello A.C., The Effect of Buoyancy on 

Opposed Smoldering, Combust. Sci. and Tech, 2004, 176 2027-2055.

11. Zammarano M., Matko S., Kraemer R.H., Davis R.D., Gilman J.W., Mehta S.N. NIST TN 

1747: Factors Influencing the Smoldering Performance of Polyurethane Foam, National Institute 

of Standards and Technology Technical Note 1747, July 2012, 89 pgs. available at 

http://dx.doi.org/10.6028/NIST.TN.1747

12. Zammarano M., Matko S., Kraemer R.H., Davis R.D., Gilman J.W., Sung Li Piin, Fox D.M.,

Mehta S.N., Smoldering in Flexible Polyurethane Foams: The Effect of Foam Morphology, in

Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science

Chapter 29, pp 459–479, ACS Symposium Series, Vol. 1118, December 18, 2012, American 

Chemical Society.

13. 16 CFR Part 1634, Upholstered Furniture Fire Safety Technology Meeting and Request for 

Comments (March 1 2013), available at 

http://www.cpsc.gov/Global/Newsroom/FOIA/CommissionBriefingPackages/2013/upholstmeetF

R.pdf

14. Chai C. Y. H., Wang J. H., Transition from Smoldering to Flaming Combustion of 

Horizontally Oriented Flexible Polyurethane Foam with Natural Convection, Combustion and 

Flame 127, 2001, p. 2252-2264.