NIST Technical Note 1747 

Factors Influencing the Smoldering

Performance of Polyurethane Foam

Mauro Zammarano 

Roland H. Krämer 

Szabolcs Matko 

Shivani Mehta 

Jeffrey W. Gilman 

Rick D. Davis 

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

 

 

 

NIST Technical Note 1747 

Factors Influencing the Smoldering

Performance of Polyurethane Foam

Mauro Zammarano 

Roland H. Krämer* 

Szabolcs Matko 

Rick D. Davis 

Fire Research Division 

Engineering Laboratory 

National Institute of Standards and Technology 

*Former Guest Researcher 

Jeffrey W. Gilman 

Polymers Division 

Material Measurement Laboratory 

National Institute of Standards and Technology 

Shivani Mehta 

Directorate for Engineering Sciences 

U.S. Consumer Product Safety Commission 

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

July 2012 

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 

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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 1747 

Natl. Inst. Stand. Technol. Tech. Note 1747, 70 pages (July 2012) 

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

CODEN: NTNOEF 

Acknowledgements 

Appreciation is extended to the Consumer Product Safety Commission for financial and 

technical support for this project under Interagency Agreement Number: CPSC-I-09-0015. 

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Abstract 

The objective of this study was to evaluate whether polyurethane foam (PUF) could be produced 

as a Standard Reference Material for ultimate use in a standard test intended to ensure the 

smoldering performance of commercially available upholstered furniture. For this purpose, PUF 

was produced by a commercial manufacturer and its smoldering propensity was evaluated. The 

experimental design was organized into four parts or “Iterations”. In Iteration 1, the results 

showed that smoldering intensity was dominated by foam morphology (cell size, strut thickness 

and length, open vs. closed cell structure, etc.), which overrode effects of chemical composition 

for the range of polyols and surfactants studied. In Iteration 2, the morphology of the foam was 

controlled by varying the processing parameters, that is, the tin catalyst content, water content 

and mixing head pressure. The results showed that smoldering increased with air permeability 

but that a better morphological descriptor of the foam structure was required to fully characterize 

smoldering in the high permeability range (i.e., in PUF with a dominantly open-cell structure). In 

Iteration 3, it was determined that for these PUFs, smoldering was controlled by cell size. The 

feasibility of a reference material with well-characterized and reproducible smoldering is linked 

to the ability to manufacture an open-cell PUF with a well-defined cell size and density. 

Preliminary data from Iteration 4 indicates that PUFs with consistent smoldering can be 

produced on a commercial scale manufacturing line. 

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Table of Contents 

Executive Summary ..................................................................................................................v

List of Figures.......................................................................................................................... xi

List of Acronyms, Abbreviations, and Symbols ....................................................................xvi

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

2. Experimental Description .....................................................................................................3

2.1. Materials ...........................................................................................................................3

2.2. Sample preparation ...........................................................................................................3

2.3. Sample identification ........................................................................................................5

2.4. Flammability Testing ........................................................................................................5

2.4.1. Box Smolder Test.......................................................................................................5

2.4.2. Mock-up Smolder Test .............................................................................................10

2.4.3. Mock-up Open-flame Test........................................................................................11

2.4.4. Flame Spread Test....................................................................................................14

2.5 Foam Characterization.....................................................................................................14

3. Results and Discussion ........................................................................................................ 16

3.1. Iteration 1: Impact of Polyols and Surfactants .................................................................16

3.1.1. Materials for Iteration 1............................................................................................17

3.1.2. Results for Smoldering in the Box Tests...................................................................19

3.1.3. Results for Flame Spread Tests.................................................................................28

3.1.4. Conclusions of Iteration 1.........................................................................................29

3.2. Iteration 2: Impact of Processing Parameters...................................................................29

3.2.1. Bun-to-bun variability and in-bun variability ............................................................33

3.2.2. Smoldering performance - Box test...........................................................................35

3.2.3. Correlation between smoldering in the box test and mockup test ..............................39

3.2.4. Effect of the processing parameters on smoldering ...................................................40

3.2.5. Effect of the processing parameters on air permeability ............................................43

3.2.6. Image analysis and cell size determination................................................................44

3.2.7. Effect of the processing parameters on cell size ........................................................47

3.2.8. Effect of the air permeability on smoldering .............................................................48

3.2.8 Conclusions of Iteration 2..........................................................................................50

3.3 Iteration 3: Impact of cell morphology in open-cell PUF..................................................51

3.3.1. Smoldering vs. air permeability ................................................................................52

3.3.2. Effect of Specific Surface Area on Smoldering.........................................................56

3.3.3. Image analysis and cell size determination................................................................60

3.3.4. Open-flame Tests (bare foam) ..................................................................................62

3.3.5. Open-flame Tests with barrier material.....................................................................65

3.3.6. Conclusions of Iteration 3.........................................................................................66

3.4 Iteration 4: From Pilot Plant to Production Line ...............................................................67

4. Summary ............................................................................................................................. 68

5. References............................................................................................................................ 69

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Executive Summary 

The objective of this project was to evaluate the potential for specifying and obtaining flexible 

polyurethane foam (PUF) that smoldered readily and consistently. Such a foam could then be 

used as a Standard Reference Material (SRM) for evaluating upholstery fabrics for their 

tendency to support smoldering and, thus, reduce the ignition potential of upholstered furniture 

by cigarettes. 

Smoldering is a self-sustaining oxidation process through which heat is released from surface 

reaction of the fuel with oxygen in the air. The chemical reaction is slow compared to a flaming 

reaction, and the heat released is far lower. To maintain the fuel at a smoldering temperature the 

released heat must be retained inside the fuel. Much of the heat from any reaction at the outer 

surface of the fuel tends to be dissipated to the surroundings. The surrounding-fuel-mass acts as 

a thermal insulator. Once oxygen has reached the heated interior surfaces of the fuel-mass, it is 

completely consumed in the smoldering reaction. The oxygen must be replenished in order for 

the smoldering to continue. Thus, fuels with a tendency to smolder have pathways for air to 

penetrate to their interior. The heat generation rate then depends on the chemistry of the fuel, its 

porosity (the magnitude of the internal surface area), and the permeability of the fuel to air. 

For this project, batches of PUF were prepared according to NIST specifications and were 

obtained from a commercial manufacturer (“foamer”). Each batch was characterized by 

combustion tests. In addition, the air permeability for each batch of foam was measured using 

the pressure drop across a slice of set thickness and area. The foam firmness was measured 

using a standard indentation force deflection device. The internal surface area per unit volume of 

the foam was calculated from the measured Brunauer-Emmett-Teller (BET) surface adsorption 

and the foam density. The cell size distribution was determined through microscopic analysis. 

PUFs were designed for ultimate use as a reference material in the proposed CPSC standard 

16 C.F.R. 1634 (herein referred to as “CPSC proposed standard”). The possible implications for 

open-flame configuration, which are part of the CPSC proposed standard, also needed to be 

considered. For this reason, even though smoldering propensity was the main focus of this study, 

open-flame combustion was also investigated and specimens from each of the batches were 

examined using four combustion methods: 

 The method to be used in the CPSC proposed regulation for testing the contribution of an 

upholstery fabric to the smoldering of a fabric/PUF composite (herein referred to as 

“mockup test”). The orientation of the materials tested in this method replicates a 

common scenario for the ignition of upholstered furniture. Two pieces of PUF are placed 

at right angles to one another, simulating the seat and back of a chair. The upholstery 

fabric to be tested covers the exposed surfaces of the foam. A lit cigarette is placed in the 

crevice formed by the two foam pieces, and is then covered by a piece of a standard 

lightweight fabric. The test result is the mass loss of the assembly during the 45 min 

duration of the test. For the development of the SRM/PUF, NIST used a cotton 

upholstery fabric with consistent high smoldering. 

 A box test method that measures the tendency of a padding material to smolder, with no 

complications from possible variability in the upholstery fabrics and the assembly of the 

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composite. A slab of foam is placed in a tight-fitting plywood box and cartridge heater is 

inserted into the center of the foam. Holes bored into the plywood provide a path for 

lateral air flow, while a piece of glass fiber cloth placed on top of the foam slab prevents 

vertical, buoyancy-driven air flow at the cavity formed around the cartridge heater during 

the test. Thermocouples are placed in the foam at several distances from the heater to 

measure the temperature rise. The heater is raised to a temperature high enough to initiate 

smoldering of the foam. “Self-sustained smoldering” is achieved when all the 

thermocouples reached temperatures above 100 oC. The test is of 100 min duration. Also 

measured are the onset temperature for self-sustained smoldering, the mass loss of the 

foam, the diameter of the smolder zone, and the power supplied to the heater. 

 The open-flame configuration method to be used in the CPSC proposed standard. The 

purpose of this test is to evaluate the effectiveness of barrier materials. Thus, the PUF 

needs to be prone to substantial mass loss from flaming ignition. There are two parts to 

this testing. 

o Bare foam test. This involves a two-piece mockup, similar in shape to the first 

test described above, but with larger foam slabs. A small gas burner is applied to 

the crevice for 5 s. The test is concluded at 120 s or when the mass loss reaches 

20 %. If the mass loss is lower, the PUF is not usable for the second part of the 

test. 

o Composite test. This test uses the same type of foam mockup as the bare foam 

test but the foam is covered with a barrier fabric and a standard upholstery fabric. 

A larger flame impinges on the crevice for 70 s. The mass loss is recorded for 

45 min or until no further mass loss is observed, whichever occurs first. 

 The measurement of flame spread rate method. A single, horizontal slab of the foam is 

laid on a drywall plate, which is located on a load cell. The foam is ignited along one 

edge by a gas burner. The flame spread rate is determined using the measured time for 

the flame front to move a fixed distance. The mass loss rate is determined using the 

change in specimen mass that occurred during the flame spread interval. This test uses 

less foam than the above described tests. 

The experimental design was organized into four parts or Iterations as follows. 

 Iteration 1: Evaluation of the effect of the chemistry of the foam. Five polyether polyols 

in combination with three surfactants were examined. All five polyols were reacted with 

a single mixture of isomeric toluene diisocyanates. A common mixture of catalysts and 

emulsifiers was used, and the blowing agent was deionized water. There were 

considerable differences in foam density and air permeability among the foams. 

The mass loss in the box tests indicated no correlation with foam density, air 

permeability, catalyst content, the relative humidity during the test, or the air flow above 

the enclosure. The onset temperature for smolder was between 320 oC and 340 oC for all 

the formulations. The average mass loss for all the formulations at a temperature of 

340 oC was the same as for a temperature of 360 oC. There was no effect of the surfactant 

type on the mass loss. For eight of the formulations, there was no effect of the polyol 

type on mass loss. For two chemically identical batches of the ninth formulation, there 

was a factor-of-two difference in mass loss; however, the two batches were foamed at 

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different temperature and relative humidity. Examination of the lower smoldering batch 

showed some formation of closed cells creating lower air permeability. Examination of 

the remaining two foams indicated larger pore sizes and thus a reduction in internal 

surface area. Combined, these results indicated at most a modest dependence of 

smoldering mass loss on foam chemistry (within the range of formulations) and a 

potential dependence on air permeability and internal surface area. 

The fire spread rate (FSR) measurements showed that the foam formulation had no 

significant effect on fire spread rate. The mass loss rate (MLR) values in the same tests 

showed no dependence on the polyol, but there was a systematic dependence on the 

surfactant. Surfactants with flame-retardant properties may suppress flaming over the 

liquid produced by thermal degradation of the foam. This might have an impact on the 

open-flame test to be used in the CPSC proposed standard. 

 Iteration 2: Evaluation of the effect of processing parameters. For a single 

polyol/surfactant formulation, a full factorial experimental design with high and low 

levels of water, tin catalyst, and mixing head pressure produced flexible foam samples 

with a range of densities (28.4 kg·m-3 to 34.3 kg·m-3) and air permeabilities (3.1 m·min-1 

to 74.2 m·min-1). 

An air permeability value above 70 m·min-1 was a required specification, but not 

necessarily sufficient to achieve a mass loss of 25 % in the mockup test. In this high 

range of air permeability, the data suggested that at least one other variable was 

impacting smoldering. Thus, a better morphological description of the foam structure was 

required. 

The highest air permeability and smoldering in these specimens was achieved at low tin 

catalyst and low pressure levels. 

 An increase in head pressure or tin content correlated with a decrease in smoldering. 

 An increase in water content correlated with an increase in smoldering in the box test 

and a decrease in the mockup test. 

 An increase in head pressure or tin content correlated with a decrease in permeability, 

while an increase in water content correlated with an increase in permeability. 

The cell size increased with increasing mixing head pressure and higher water content. 

At the higher water level, the cell size decreased with the tin catalyst content. 

 Iteration 3: Evaluation of the effect of cell morphology. The reference foam used for this 

Iteration was the foam from Iteration 2 which had the highest air permeability. A second, 

identical formulation was produced at a lower mixing-head pressure because it was found 

that the head pressure is the main processing parameter controlling smoldering at high 

water level. A third foam was produced the same way as the first, except that it did not 

contain the processing aid. 

The mass loss values in the mockup tests of the three foams were very different despite 

having similar air permeability values. This suggested that there may be other parameters 

influencing smoldering. It was proposed that two foams could have significantly 

different pores sizes and ratio of open/closed membrane and still yield the same air 

permeability. 

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Air access to the pores is critical. A high fraction of closed cells retards air movement 

through the foam. The surface-area measurement does not discriminate between closed 

and open cells, since krypton, the gas adsorbed in the measurement technique, can diffuse 

through the membranes. Thus, a specification for a smoldering foam must supplement a 

surface-area measurement with information from a micrograph or an air permeability 

measurement. 

For foams of high air permeability, as the internal surface area of the foam increased, the 

smoldering mass loss in the mockup test increased. As the cell cross-sectional area 

decreased, the smoldering mass loss increased. Holding the foam mass density constant, 

having smaller pores means having more pores and thus more internal surface area at 

which the smoldering reaction occurs. 

 Iteration 4: NIST is collecting, analyzing, and interpreting characterization and 

smoldering data of PUFs produced from the foamer’s production/manufacturing line. 

The data (which will be provided in a separate document) indicates it is possible to 

commercially produce foam with consistent smoldering. 

After working with an experienced foamer and performing extensive testing, NIST has come to 

the following conclusions. 

At the pilot scale, it was possible to make high-smoldering foam targeted to a particular 

smoldering mass loss using the proposed test method. However, the intrinsic low repeatability 

and limited size of the bun (block of foam obtained in a single foaming process) in the pilot scale 

would generate relatively high costs. Because more buns would be needed to generate a given 

volume, and because bun to bun variability is significant, a pilot plant is less than ideal in terms 

of the variability/quality of a final potential SRM. 

On a commercial production line, it may be feasible to produce a polyurethane foam with an 

approximate 30 % smoldering mass loss provided that the manufacturer can meet specifications 

for chemical formulation, air permeability, average cell cross-sectional area and mass density. 

However, it appears that on a manufacturing scale it may be difficult to achieve these 

specifications in a reproducible way within and between buns. Identifying conforming sections 

of a bun would require extensive testing, time and resources. 

Using the current manufacturing technologies, it may be feasible to produce a high smoldering 

foam with lower variability within the bun, but with a smoldering mass loss value closer to 

20 %. Since this mass loss value is less than the targeted 30 %, there is little information on the 

bun to bun variability and validation of the values for the targets parameters. The target for this 

type of foam should meet the following criteria: 

 Pure polyurethane foam based on formulation C1 (see the main text for more detail), with 

no significant visible defects or impurities, in which the processing parameters (amount 

of catalysts, water and head pressure) are adjusted to meet the criteria reported below for 

air permeability, cell size and mass density (uncertainties equal to one standard 

deviation). 

 Air permeability: (71 ± 8) m·min-1

 Average cell area: (0.31 ± 0.01) mm2

 Mass density: (27.4 ± 0.2) kg·m-3

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Additional effort will be required to identify a manufacturer with quality controls sufficient to 

produce a long-term supply of foam consistent with CPSC’s proposed test standard. 

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List of Tables 

Table 1. Polyol specifications provided by the manufacturer (uncertainties not provided)............4

Table 2. Surfactant specifications provided by the manufacturer (uncertainties not provided). .....4

Table 3. Other components used in PUF by the manufacturer (uncertainties not provided). .........4

Table 4. Influence of the secondary parameters on smoldering measured in the box test (78 tests).

 .......................................................................................................................................... 19

Table 5. Mass loss measured in the box test at heater set-point-temperatures of 320 C, 330 C, 

340 C and 360 C (90 tests). .............................................................................................21

Table 6. Onset temperature for self-sustained smoldering, mass loss and main properties of

formulations of Iteration 1.................................................................................................. 22

Table 7. Mass losses measured by the box test (ML340+360) and the mockup test (MLmockup) are 

compared for 4 formulations in Iteration 1 (uncertainty is shown as one standard deviation

calculated over at least 3 replicates). ..................................................................................27

Table 8. Summary of the selected high and low levels used for tin catalyst content, water content

and head pressure............................................................................................................... 31

Table 9. Formulations of Iteration 2 and corresponding levels for the full factorial experimental

design. ............................................................................................................................... 32

Table 10. Density (), air permeability (), mass loss in the mockup test (MLMockup) and mass 

loss in the box test (ML340+360) for the foams of Iteration 2. The average values (Avg) ± one 

standard deviation (StDev) are reported.............................................................................. 32

Table 11. Comparison between in-bun variability and bun-to-bun variability in terms of air

permeability (). The average values (Avg), the standard deviation (StDev) and relative

standard deviation (StDev%) are reported. .........................................................................34

Table 12. Mass loss for formulation of Iteration 2 measured in the box test at a heater-set-point

temperature of 320 °C (ML320), 330 °C (ML330), 340 °C (ML340), 360 °C (ML360) and the 

average of the mass loss measured at a temperature of 340 °C and 360 °C (ML340+360). ......36

Table 13. Average values of relative standard deviation (StDev%) for MLmockup (mockup test)

and ML340+360 (box test) calculated for the samples of Iteration 1, Iteration 2, Iteration 3 and 

Iteration 1 to 3.................................................................................................................... 40

Table 14. Composition and processing parameters for the formulations of Iteration 3................ 52

Table 15. Mass loss measured in the box test for formulation of Iteration 3 at a heater-set-pointtemperature of 320 °C, 330 °C, 340 °C, and 360 °C. ..........................................................52

Table 16. Mass loss measured in the mockup test for formulations of Iteration 2. ......................54

Table 17. Values of specific surface area (StDEv and AVG calculated over at least three 

replicates). ......................................................................................................................... 56

Table 18. Average cell area calculated from each view point..................................................... 61

Table 19. Summary of the open-flame tests on bare foams. The values of densities and

permeability shown here are the average of x values for each formulation measured by the 

foamer. The uncertainty is equal to one standard deviation................................................. 64

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List of Figures 

Figure 1. Schematic of the four-Iteration experimental design for the SRM/PUF specification

process. ................................................................................................................................2

Figure 2. Schematic drawing of the box test setup, showing the test specimen and the location of

the thermocouples. ...............................................................................................................6

Figure 3. Energy consumed by the heater to reach and sustain a set temperature of 360 oC. The 

black points indicate the energy consumption during the heating; the red points indicate the 

energy input at the set temperature. The red line is a least-squares regression fit to the red 

data points and its slope is used to calculate the average power necessary to stabilize the 

heater at the set temperature. ................................................................................................7

Figure 4. Enclosure for performing box tests. ..............................................................................8

Figure 5. Temperature profiles measured in the smolder box test for a foam showing selfsustained-smoldering (right) and a foam in which smoldering is not self-sustained (left). The 

temperature of the cylindrical heater is shown as a solid line (target temperature of 360 °C).

The averages of two thermocouple temperatures measured in the foam are shown for

distances from the heater of 50 mm (dash-dot), 76 mm (dash) and 102 mm (dot). ................8

Figure 6. Comparison of mass loss and average temperature of the six thermocouples at the end 

of the box test. Data are collected on foams from Iteration 1 and 2. ......................................9

Figure 7. Illustration of the charred material produced in the box test. The respective mass losses

were 2 % by mass (left), 4 % by mass (center) and 7 % by mass (right)................................9

Figure 8. Plot of total mass loss in the smolder box test versus the power required to maintain the 

temperature of the heater at the target temperature. For samples that intensely smoldered 

resulting in high mass loss, only low power was needed for the heater. Data are collected on

foams from Iteration 1 and 2. .............................................................................................10

Figure 9. Schematic drawing of the mockup test described in the CPSC proposed standard

(upholstery cover fabric smoldering ignition resistance test)............................................... 11

Figure 10. A set of foam mockups ready to be tested prior to the addition of the lighted cigarette.

Figure 11. An example of a mockup assembly without a barrier used in the open-flame test for

 .......................................................................................................................................... 11

bare foam. .......................................................................................................................... 12

Figure 12. Photo of test with formulation C1 a few seconds before extinguishing the sample. At

this stage, flaming liquid material is dripping to the underlying tray, but there is no pool fire

(which would sustain flaming and boost the heat release rate through a feedback effect).... 12

Figure 13. A photo of the 240 mm butane flame impinging on the standard rayon material

covering the foam for 70 s in the open-flame test with barrier material............................... 13

Figure 14. Photo of the open-flame test with barrier material. The 240 mm butane flame was

removed from the crevice after 70 s of flame impingement. ...............................................13

Figure 15. Schematic drawing showing the setup used for measuring flame spread. ..................14

Figure 16. Photograph of the electronic high differential pressure instrument for measuring air

permeability. ...................................................................................................................... 16

Figure 17. Clamping of the PUF samples during a permeability measurement........................... 16

Figure 18. Design of experimental parameter space for Iteration 1. ...........................................18

Figure 19. Percentage of samples from Table 6 showing self-sustained smoldering at various setpoint temperatures of the heating element. .........................................................................22

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Figure 20. Mass loss vs. set point temperature of the heater in the box test using data from Table 

5. The uncertainty bars shown here are equal to the average of the absolute deviations of

data points from their mean (Dev). Data points that do not show an uncertainty bar are

single measurements. .........................................................................................................23

Figure 21. Average mass loss vs. set point temperature of the heater in the box test calculated for

the samples in Figure 20 (uncertainty bars shown here are equal to one standard deviation).

 .......................................................................................................................................... 24

Figure 22. Average mass loss at 340 C and 360 C (ML340+360) in the box test for the 

formulations of Table 7 (uncertainty bars equal to one standard deviation). The ML340+360 for 

the replicates of formulation A1 and A11 (A1R and B2, respectively) are shown in Figure 

24. ...................................................................................................................................... 25

Figure 23. Average mass loss at 340 C and 360 C (ML340+360) in the box test grouped by

surfactant or polyol type. For example, ML340+360 shown for S1 is an average of the 

ML340+360 measured for A1, A6 and A9 (uncertainty bar equal to one standard deviation). .25

Figure 24. Average mass loss at 340 C and 360 C (ML340+360) in the box test for formulations

A1, A1R, A4, A5, B2 and A11........................................................................................... 25

Figure 25. Optical micrographs for formulations A1 (on the left) with an air permeability of

(77.2±2.3) m·min-1 and its replicate formulation A1R (on the right) with an air permeability

of (45.9±7.7) m·min-1. Bar size in the right bottom corner is 1 mm..................................... 27

Figure 26. Optical micrographs for: A4 (bio-polyol formulation) with an air permeability of

(89.2±3.2) m·min-1 (on the left); and A5 (graft-polyol formulation) with an air permeability

of (76.3±10.9) m·min-1 (on the right). Bar size in right bottom corner is 1 mm. ..................27

Figure 27. Mass loss in the box test (ML340+360) vs. mass loss in the mockup test (MLmockup) for 

four formulations of Iteration 1. .........................................................................................28

Figure 28. MLR measured for Iteration 1 formulations. There is a significant reduction of MLR 

for surfactants S1 and S2. Uncertainty shown as one standard deviation calculated over at

least 3 replicates. ................................................................................................................ 28

Figure 29. FSR measured for Iteration 1 formulations. Uncertainty is shown as one standard

deviation calculated over at least 3 replicates. ....................................................................29

Figure 30. Schematic diagram showing the 2k

3 experimental design with high and low settings of

Figure 31. Schematic diagram showing the 2k3 experimental design with the corresponding

the amount of water, amount of tin catalyst and mixing head pressure. ...............................30

formulations. ...................................................................................................................... 32

Figure 32. Air permeability vs. distance from the pour start for formulations of Iteration 1 

(uncertainty bars are equal to one StDev over at least 3 replicates). ....................................33

Figure 33. Air permeability vs. distance from the pour start for formulations of Iteration 2 

(uncertainty bars are equal to one StDev over at least 3 replicates). ....................................34

Figure 34. Correlation between the air permeability measured by the foamer in the pilot plant and 

air permeability measured by NIST collected from the middle section of the foam bun

(30 cm middle section between pour-start and pour-end). Uncertainty bars for NIST data are

equal to the StDev over at least 3 replicates. The foamer's data are based on a single

measurement per bun. The red line is a least-squares regression fit to the data points. ........35

Figure 35. Percentage of samples showing self-sustained smoldering in the box test vs. set-pointtemperatures of the heater for formulations of Iteration 1 and Iteration 2. The percentage of

smoldering samples is reduced in Iteration 2 as compared to Iteration 1 for a given

temperature. ....................................................................................................................... 37

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Figure 36. Average mass loss vs. set point temperature of the heater in the box test calculated for

samples of Iteration 1 and 2. The mass loss is significantly reduced and the onset

temperature increased in Iteration 2 as compared to Iteration 1 (uncertainty bars shown here

are equal to one StDev). .....................................................................................................38

Figure 37. Average mass loss at 340 C and 360 C (ML340+360) in the box test for the

formulations of Table 12 (uncertainty bars equal to one StDev and are calculated over at

least 3 replicate measurements). .........................................................................................38

Figure 38. Smoldering measured as mass loss in the box test (ML340+360) vs. smoldering

measured as mass loss in the mockup test (MLmockup) (uncertainty bars shown are equal to

one StDev calculated over at least 3 replicates). .................................................................39

Figure 39. 2-D contour plot for ML340+360 vs. tin content and pressure at a low water level 

(2.70 php). ......................................................................................................................... 41

Figure 40. 2-D contour plot for ML340+360 vs. tin content and pressure at a high water level

(2.95 php). ......................................................................................................................... 41

Figure 41. 2-D contour plot for MLmockup vs. tin content and pressure at a low water level

(2.70 php). ......................................................................................................................... 42

Figure 42. 2-D contour plot for MLmockup vs. tin content and pressure at a high water level

(2.95 php). ......................................................................................................................... 42

Figure 43. 2-D contour plot for permeability vs. tin content and pressure at a low water level

(2.70 php). Permeability is slightly more sensitive to the tin variation rather than the 

pressure variation in the range investigated here and at this water level. .............................43

Figure 44. 2-D contour plot for permeability vs. tin content and pressure at a high water level

(2.95 php). Permeability is equally affected by tin and pressure variations in the range 

investigated here and at this water level.............................................................................. 44

Figure 45. Image analysis and cell area calculation for a B9 foam: a) an optical slice with an area 

of (4.757 x 4.757) mm2 and a thickness of 197.5 m is acquired by a confocal microscope;

b) a Gaussian filter is applied to remove noise and prevent over-segmentation; c)

segmentation by Euclidian distance map calculation; d) each segmented area that is not on

the edge of the image is labeled and measured....................................................................45

Figure 46. Cell area distribution and cumulative percentage for formulation B9. ....................... 46

Figure 47. Mean and median values of cell area calculated for all foams of the 2k

3 experimental

design. ............................................................................................................................... 46

Figure 48. 2-D contour plot for average cell area  vs. tin content and pressure at a low water

level (2.70 php).................................................................................................................. 47

Figure 49. 2-D contour plot for average cell area  vs. tin content and pressure at a high water

level (2.95 php).................................................................................................................. 48

Figure 50. ML340+360 (box test) vs. air permeability for all available data from Iteration 1 and 

Iteration 2 (uncertainty bars equal to the StDEv calculated over at least three replicates). The 

black line is a least-squares regression fit to the data points................................................ 49

Figure 51. MLmockup (mockup test) vs. air permeability for all available data from Iteration 1 and 

Iteration 2. No smoldering is observed in the mockup test below a threshold value of

permeability (uncertainty bars equal to the StDEv calculated over at least three replicates).

The red line and black line are least-squares regression fits to the red and blue data points,

respectively. ....................................................................................................................... 49

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Figure 52. Average mass loss at 340 C and 360 C (ML340+360) in the box test for the 

formulations of Iteration 3 (uncertainty bars equal to the StDev and are calculated over at 

least 3 replicate measurements). ......................................................................................... 53 

Figure 53. ML340+360 (box test) vs. air permeability for all available data from Iteration 1, 

Iteration 2 (blue data points), and Iteration 3 (red data points). The black line is a leastsquares regression fit to the blue data points (uncertainty bars equal to one StDEv calculated 

over at least three replicates). ............................................................................................. 53 

Figure 54. MLmockup (mockup test) vs. air permeability for all available data from Iteration 1, 

Iteration 2 and Iteration 3. The formulations of Iteration 3 had similar air permeability but 

dramatically different smoldering. This is indicating that at least for foams with high 

permeability there is another parameter controlling smoldering (uncertainty bars equal to 

one StDEv calculated over at least three replicates). The red line is a least-squares regression 

fit to the red data points. ..................................................................................................... 54 

Figure 55. MLmockup (mockup test) as a function of air permeability for formulation C1 measured 

for several foam buns. In this high range of permeability smoldering increases when air 

permeability decreases. The red line is a least-squares regression fit to the data points 

(uncertainty bars are equal to one StDEv calculated over at least three replicates). ............. 55 

Figure 56. MLMockup vs. specific surface area. Uncertainty bars are equal to one StDev and 

calculated from at least 3 replicate measurements for datasets 1 (blue dots) and 2 (green 

dots); for dataset 3 (black dots), as previously described, the StDev were assumed equal to 

20 % and 30 % of the mean value of SSA and MLmockup, respectively. The red line is a leastsquares regression fit to the black data points. .................................................................... 57 

Figure 57. MLmockup vs. specific surface area for formulation C1 measured on several buns. The 

red line is a least-squares regression fit to the data points. In this high range of permeability, 

smoldering increased when the specific surface area increased (uncertainty bars equal to one 

StDEv calculated over at least three replicates). The data of MLmockup vs. air permeability for 

the same foam buns are shown in Figure 55. ...................................................................... 58 

Figure 58. Schematic drawing illustrating two possible mechanisms promoting an increase in air 

permeability (): a) increase in fraction of open membranes; b) increase in cell size. Both 

mechanisms induce also a decrease in SSA but only a) promotes an increase in smoldering 

due to an increase in oxygen supply. .................................................................................. 59 

Figure 59. Effect of cell size () on smoldering in the mockup test (uncertainty bars equal to one 

StDev). The black line is a least-squares regression fit to the data points. ........................... 62 

Figure 60. A test with formulation C1 few seconds before extinguishing the sample. At this stage 

flaming liquid material is dripping to the underlying tray but there is no pool fire with 

sustained flaming that can boost the heat release rate through a feed-back effect. ............... 63 

Figure 61. Ignition was not observed after the first 5 s of flame impingement with formulations 

B5 and C2; a second attempt was carried out by applying the open-flame for other 5 s in a 

new location 5 in. apart from the original ignition point. Here, it is shown a mockup 

assembly for formulation C2 after attempting ignition twice. ............................................. 63 

Figure 62. Mass loss vs. time measured for three mockups prepared with formulation C1. ........ 64

Figure 63. Mass loss vs. time measured for three mockups prepared with formulation C3. ........ 65

Figure 64. Photo of the residue of the mockup after the open-flame test with barrier material.... 65 

Figure 65. Photo of the residue of the mockup at the end of the test after removing the barrier 

material, the polyester fabric and the cover fabric............................................................... 66 

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Figure 66. Total mass loss in barrier test as a funciton of time measured for three mockups 

prepared with formulation C1. ............................................................................................ 66 

Figure 67. Photo of the comparison between residues of mockups (formulation C1) tested with 

barrier material (on the left) and without barrier material (on the right). ............................. 66 

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List of Acronyms, Abbreviations, and Symbols 

2-D 2 Dimensional 

 Air permeability 

threshold Threshold value for air permeability 

 Mean values of area per cell calculated in a selected cross-section 

Avg Average value 

atm Standard atmosphere (pressure) 

BET Method used for the calculation of surface area 

BS British Standard 

CFM Cubic feet per minute 

CFR Code of Federal Regulations 

CPSC Consumer Product Safety Commission 

Dev Average of the absolute deviations of data points from the mean value measured 

Dev% Average relative deviation expressed as a percentage of the mean value measured 

EDM Euclidian distance map 

FSR Flame spread rate 

GA State of Georgia 

IFD25 Indentation force deflection 25 % 

IFD Indentation force deflection 

MA Commonwealth of Massachusetts 

MD State of Maryland 

ML120s Mass loss measured in % by mass 120 s after ignition during the open-flame 

mock up test 

ML340+360 Average mass loss percentage measured in the box test at a heater temperature of 

340 C and 360 C 

MLmockup Mass Loss percentage measured in the cigarette smoldering mockup test 

MLR Mass loss rate 

NIST National Institute of Standards and Technology 

NY State of New York 

PA Commonwealth of Pennsylvania 

php Parts (by mass) per hundred parts of polyol for a specific component in a foam 

formulation (e.g., 10 php of X means that 10 g of component X are used in 

combination with 100 g of polyol). 

psi Pound-force per square inch 

PUF Flexible Polyurethane Foam 

R.H. Relative humidity 

RM Reference Material 

SRM/PUF Standard Reference Material for Flexible Polyurethane Foam 

SFPE Society of Fire Protection Engineers 

SRM Standard Reference Material 

SSA Specific surface area (area per unit volume) 

StDev Standard deviation 

StDev% Relative standard deviation expressed as a percentage of the mean 

TC Thermocouple 

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Tmax Maximum temperature measured 102 mm away from the center of the cartridge 

heater during the box test 

U.S. United States 

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This page is purposely left blank. 

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1. Introduction 

A review of fire statistics from the last two decades shows a reassuring decline in home fire 

deaths.1

 Yet despite decline, upholstered furniture and bedding remain the most frequent “first 

items to ignite” that result in residential fire deaths in the United States.2

 According to estimates 

by the United States (U.S.) Consumer Product Safety Commission (CPSC), a large number of 

these fire deaths can be attributed to smoldering materials commonly found in upholstered 

furniture and bedding. Despite the promising introduction of Reduced Ignition Propensity 

cigarettes in all 50 states3, smoldering of upholstered furniture and bedding remains a threat to 

life and property. 

Initiation of smoldering furniture fires is a complicated process that depends on the material 

properties of the ignition source (e.g., smoking material), the fabric, the filling, the interactions 

between the materials, and the design of the upholstered furniture. To date, the propagation 

mechanism of smoldering in furniture, which is composed of textile fabrics and porous fuels, 

such as polyurethane foam (PUF), remains largely unstudied. The variety of material 

formulations and the intrinsically low reproducibility of the foaming process have effectively 

prevented a thorough understanding of the properties controlling smoldering in polyurethane 

foams. Ihrig et al. came the closest to explaining the impact of polyurethane substrates on the 

probability of smolder initiation by determining that the air permeability of the polyurethane 

foams was the overriding material property controlling smoldering propensity.4,5 Unfortunately, 

no further analysis or morphological description of the foams was given. Empirical evidence 

showed that air permeability was very important to smoldering, but there was not enough 

information provided to fully explain the differences in the smoldering propensity of different 

foam formulations. 

The factors affecting open-flame flammability in flexible polyurethane foam have been 

extensively studied; however, quantification of the key properties that determine smoldering 

propensity of such materials has been largely neglected.6,7 This is mainly due to the inability to 

procure foams with consistent and homogeneous properties. Ohlemiller and Rogers described the 

effect of the fundamental properties of porous fuels on smoldering and attempted to better 

understand the relationship of smoldering to PUF properties; however, their work was limited 

because of difficulties in obtaining foam samples produced under well-controlled conditions.8,9,10 

A great body of evidence has been established through numerical simulation of smoldering 

combustion of PUF and indicated the significance of air permeability on the rate of smolder 

propagation.11,12,13,14 Thermal analysis of the foams has been performed in great detail in order to 

obtain multi-step models of foam pyrolysis and char oxidation that provided input data for 

models.13 However, the relationship of the input parameters and actual foam formulation 

parameters has never been established. In many studies, morphological description of the PUF 

has been limited to the simplest terms. 

The purpose of the research presented here was to understand the effect of basic formulation 

parameters and foam morphology on the initiation and extent of smoldering in PUF. The 

research then evaluated whether PUF with reproducible and well characterized smoldering could 

be used to produce a reference material (RM) for ultimate use in a standard test. For this purpose, 

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PUF was produced under controlled conditions and its smoldering propensity was evaluated 

using two test methods: 

 The upholstery cover fabric smoldering ignition resistance test. This is a well-established 

test described in proposed CPSC 16 CFR 163415 (herein referred to as “CPSC proposed 

standard”); 

 The semi-quantitative test method. This is a new, robust test developed at NIST (the box 

smolder test, described in Section 2.4.1 of this report).16 

The experimental design was organized in four parts, (referred to here as Iterations): 

 Iteration 1: evaluation of the effect of raw materials; 

 Iteration 2: evaluation of the effect of processing parameters; 

 Iteration 3: evaluation of the effect of cell morphology in open-cell PUF; 

 Iteration 4: scale-up from pilot plant to a production line. 

Each Iteration started with material/formulation selection and sample preparation (e.g., foaming, 

cutting and conditioning). The samples were then conditioned and tested. Finally, the output data 

were analyzed for selecting the PUF specifications for the next Iteration. At the end of this 

process, a potential SRM for PUF (SRM/PUF) was specified (Figure 1). 

Figure 1. Schematic of the four‐iteration experimental design for the SRM/PUF specification

process.

The aim of the Iterations was to separately evaluate, in a controlled fashion, the effect of a single 

set of parameters (raw materials, processing parameters, or cell morphology) on smoldering. 

These parameters are not independent of each other; changing a single parameter may precipitate 

a change in another parameter. For example, a variation in raw materials may require an 

adjustment in processing parameters and/or induce a different cell morphology. Furthermore, 

climatic conditions (temperature, humidity and atmospheric pressure) during manufacturing may 

impact the cell morphology (cell size, strut thickness and length, open vs. closed cell structure, 

etc.) so that fine-tuning of processing parameters (water content, catalyst content and mixing 

head pressureI

) is required on a daily basis to ensure consistent foam quality. This implies that, 

I

 Mixing-head pressure is the pressure inside the mixing head. See Section “Sample preparation” 

for more details. 

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even for a given formulation, a bun-to-bun variability in the foam properties is somewhat 

inevitable for typical manufacturing systems. Significant variations in PUF properties were 

observed in the same foam bun (in-bun variability) as a function of the specific location due to 

surface-proximity effects (a relatively high density layer, the so-called “skin”, is generated on the 

surface of a bun during foaming), temperature gradient effects (core-to-periphery decrease in the 

temperature of the bun) and gravity (top-to-bottom pressure increase). 

The aforementioned phenomena, coupled with the intrinsic complexity of smoldering 

mechanisms, outline the challenges that need to be addressed for developing a potential 

SRM/PUF with a reproducible and well-characterized smoldering behavior. 

2. Experimental DescriptionII 

This study required NIST to procure foams with specified chemical and morphological 

properties. Foam samples were custom made by a commercial foamer, according to NIST 

specifications. The materials used for preparing these PUF samples are listed below. 

2.1. Materials 

All materials were used as-received unless otherwise indicated. Five different polyether polyols

(P1-P5) were selected for investigating the effect of polyols on smoldering. Their specifications,

as provided by the manufacturer, are listed in Table 1. Similarly, three different surfactants (S1­

S3) were selected for investigating the effect of surfactants on smoldering. Their specifications,

as provided by the manufacturer, are listed in

Table 2. The other reagents used in the custom formulations were an isocyanate, water, catalysts

and processing aids. Their specifications, as reported by the manufacturer, are shown in Table 3.

Stannous octoate is referred to hereafter as “tin catalyst” or “tin”.

2.2. Sample preparation 

Samples were prepared in a small pilot plant or in a production line. In both cases, all reagents 

were pumped at a controlled rate into a fixed mixing chamber (mixing head). The pressure in the 

mixing head was adjusted by controlling a valve at the outlet. In the pilot plant, the material was 

transferred from the mixing head to a foaming box through a feeding tube. After 15 minutes at 

room temperature, the foams were cured in an oven at 110 C for one hour and post-cured at 

room temperature for an additional 24 hours. In the production line, the ingredients of the foam 

formulation, discharged through the nozzle of the mixing head, fell onto the front of a conveyor 

belt. The temperature of the bun reached typically about (150 to 170) C in water-blown foams. 

Curing was completed in air, and no post-curing was required. Samples were cut with an 

automatic laser system. All samples were conditioned at a temperature of (21±3) C and between 

50 % and 66 % relative humidity for at least 24 hours prior to testing. 

II The policy of NIST is to use SI units of measurement in all its publications, and to provide 

statements of uncertainty for all original measurements. In this document however, data from 

organizations outside NIST are shown, which may include measurements in non-SI units or 

measurements without uncertainty statements. 

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Table 1. Polyol specifications provided by the manufacturer (uncertainties not provided).

Specifications P1 P2 P3 P4 P5 

Polyether Polyether triol Biotriol produced by based 

without double metal polyol 

end cap cyanide catalyst 

Polyether 

triol with 

ethylene 

oxide end 

cap 

Molar mass1 3,000 3,200 3,000 1,700 

Polyether polyol 

with 

copolymerized 

styrene and 

acrylonitrile 

­

(g·mol-1)

OH number1 54.5-57.5 50.5-53.5 54.5-57.5 56 31.1

(mg KOH·g-1) 

Acid number1 <0.02 <0.02 <0.02 <0.50 ­

(mg KOH·g-1) 

Water content <0.05 <0.05 <0.05 <0.05 <0.07 

 (%)1 

Viscosity1 at 480 520 580 3,200 4,750 

25 C (mPa·s) 

Density1

 at 1.02 1.02 1.02 1.00 1.04 

25 C (kg·L-1) 

Table 2. Surfactant specifications provided by the manufacturer (uncertainties not provided).

Specifications S1 S2 S3 

Alkyl-pendant organo/silicone Organo/silicone Conventional 

surfactant designed for low surfactant designed polysiloxane 

flammability PUF, blown for supercritical-CO2- block 

with water and/or methylene blown slabstock PUF copolymer 

chloride in a conventional and reduced surfactant for 

flexible slabstock process flammability slabstock and 

molded PUF 

Viscosity at 25 C 

(mPa·s )1 

750 945 1,150 

Density at 25 C 1.04 1.03 1.05 

(kg·L-1)

 1 

Table 3. Other components used in PUF by the manufacturer (uncertainties not provided).

Components Description 

TDI Mass ratio mixture of 2,4- (80%) and 2,6-isomers (20%) 

Deionized water Blowing agent 

Tin catalyst Stannous salt of ethyl-hexanoic acid (stannous octoate) 

Amine catalyst Mass ratio of triethylene diamine (33%) and dipropylene glycol (67%), 

soluble in water and polyol. 

Polyether catalyst Polyether based catalyst 

Processing aid Mixture of emulsifiers based on a fatty ester used as a processing aid to 

decrease the viscosity; soluble in the polyol and insoluble in water. 

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2.3. Sample identification 

The properties of a foam sample may vary depending on the specific formulation, the foam bun 

and the location in the bun; thus, it is useful to specify these parameters with an identification 

code for each custom made sample. The formulations are identified by a code as X# with 

X=A,B,C,D and the symbol ‘#’ a number between 1 and 11. Formulations ‘A’ are formulations 

prepared for Iteration 1, formulations ‘B’ for Iteration 2, ‘C’ for Iteration 3 and “D’ for Iteration 

4. The symbol ‘#’ indicates the formulation number in a given Iteration; ‘1’ indicates the first 

formulation prepared for a given Iteration, ‘2’ the second one, etc. For example, C2 and A11 

indicate the 2nd formulation from the 3rd Iteration and 11th formulation from the 1st Iteration, 

respectively. A generic formulation X# can be foamed multiple times. The specific foam bun for 

a given formulation is identified by a number after the formulation code. For example, C2-14 

indicates the 14th foam pouring from formulation C2. The foam location in a specific bun is 

indicated by a letter after the foam bun code; ‘S’, ‘M’, and ‘E’ are used to indicate samples that 

are cut from a region close to the pouring start position, middle position or end position, 

respectively. For example, C2-14S indicates a foam sample from formulation C2, 14th pouring, 

and cut from the pouring start region. For the samples prepared in the production line, a lower 

case letter (t, m, b) is also used to indicate samples that are cut from a region close to the top (t), 

middle (m) or bottom (b) of the bun. For example, D2-1Sb indicates a foam sample from 

formulation D2, 1st pouring, and cut from the pouring start region at the bottom of the bun. 

A detailed description of foam properties, pouring conditions, and date of foaming for each bun 

is provided in Appendix 1. 

2.4. Flammability Testing 

The smoldering of PUF was evaluated by two tests. The first one, the Mockup Test (described in 

the CPSC proposed standard), aims to mimic a realistic ignition scenario for upholstered 

furniture; the second one, the Box Smolder Test (see Section 2.4.1) is designed to measure 

smoldering in a fire scenario with a setup that is not affected by variability in the testing 

materials (e.g., barrier fabric, cotton sheeting, smoking material) other than the foam itself. Both 

of these tests provide information regarding the smolder propensity of the foam under different 

configurations. 

In this project, formulation and processing of PUF were experimented targeting a specific 

smoldering behavior, suitable for the proposed CPSC standard. In doing so, the possible 

implications for open-flame configuration, which are also part of the CPSC proposed standard, 

needed to be considered. For this reason, even though smoldering propensity was the main focus 

of this study, open-flame combustion was also investigated. Open-flame resistance was 

measured by two methods: the Mock-up Open-flame Test, described in the CPSC proposed 

standard, and Flame Spread Test, a newly developed method intended as a smaller scale 

alternative to the Mock-up Open-flame Test. Both methods are discussed in detail below. 

2.4.1. Box Smolder Test 

The ‘box’ in the Box Smolder Test is a newly developed apparatus for assessing smoldering in 

PUF.16 A heater (a copper cylinder, 19 mm in diameter and 95 mm in length with a 

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concentrically inserted cartridge heater, 6 mm in diameter and a maximum power of 85 W) is 

inserted into the core of a (280 × 280 × 153) mm3 foam sample contained in a plywood box with 

13 mm thick walls (Figure 2). 

Three holes, 13 mm in diameter were cut into each side of the box to allow lateral air flow into 

the foam block. The top surface of the foam was covered by a glass fiber cloth (planar density of 

558 g·m-2 ± 12 g·m-2) in order to prevent chimney effects due to cavities formed in the foam 

block around the cylindrical heater. The glass fiber cloth also served to minimize the risk of 

transition to open flaming by reducing the oxygen supply. 

The temperature of the heater was increased from room temperature to a set temperature between 

320 C and 360 C at a heating rate of 6 C·min-1. The temperature in the foam was monitored 

with six thermocouples (0.5 mm thick K-type thermocouples, KMQSS-020G-6, Omega 

Engineering Inc.) at distances of 50 mm (TC1 and TC4), 75 mm (TC2 and TC5) and 102 mm 

(TC3 and TC6) from the center of the heater. The thermocouples were held by 47 mm long 

syringe needles with the exposed thermocouple tip positioned at a height of 57 mm from the 

bottom of the plywood box. A thermocouple was also used to monitor the temperature of the 

heater; the power necessary to keep the temperature of the heater at the target temperature was 

recorded (Figure 3). The cylindrical heater was powered by a custom fabricated power supply 

with a proportional–integral–derivative controller (model CN77343, Omega Engineering Inc.). 

The power supplied to the heater was recorded using a power meter with an accuracy of 

± 0.1 W·h (Watt ́s Up? pro 99333, Electronic Educational Devices). Thermocouple readings 

were recorded with either of two types of digital data loggers (OM-Daqpro-5300, Omega 

Engineering Inc. and CR23X, Campbell Scientific Inc.). 

Figure 2. Schematic drawing of the box test setup, showing the test specimen and the location

of the thermocouples.

6
                                  

                           

                                  

                               

       

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3. Energy consumed by the heater to reach and sustain a set temperature of 360 oC. The

black points indicate the energy consumption during the heating; the red points indicate the

energy input at the set temperature. The red line is a least‐squares regression fit to the red

data points and its slope is used to calculate the average power necessary to stabilize the

heater at the set temperature.

Two smolder box setups were run concurrently. Both boxes were contained in a custom 

fabricated enclosure with inner dimensions of (165 × 86 × 66) cm3 and open top (Figure 4). A 

perforated aluminum grid was installed 280 mm above the counter to ensure sufficient 

ventilation by natural convection, and the smolder boxes were placed on a platform, 85 mm 

above the grid. An air flow of (19 ± 4) m3·min-1 measured at the top rim of the enclosure was 

maintained by a canopy hood positioned atop the enclosure. Lower levels of ventilation could 

not be used since a removal of the toxic exhaust gases was required. The heater was set to 

maintain a specified temperature until it was switched off 80 min after the start of the test 

(t=80 min). The thermocouple measurements were continued for an additional 20 min, for a total 

of 100 min. The test was then terminated and the mass loss of the foam was determined by 

weighing the sample with the box. If smoldering persisted, the sample was quenched after the 

mass determination by adding water with a spray bottle equipped with silicone tubing and a 

syringe needle. 

The box test was designed to investigate foam smoldering in a well-characterized fashion by 

measuring the following set of parameters: 

 mass loss, 

 temperature profiles, 

 diameter of smolder zone, 

 power supplied to the heater, and 

 onset temperature for self-sustained smoldering. 

Mass loss was measured at the end of the test (t=100 min) by measuring the mass of the foam 

sample in the box without removing the char that was produced by smoldering. This is a major 

difference between the box test and the CPSC proposed standard (upholstery cover fabric 

smoldering ignition resistance test), where all char is removed and included in the mass loss. 

7
             

 

 

 

 

 

 

 

 

                           

                         

                               

                       

                              

Figure 4. Enclosure for performing box tests.

An example of temperature profiles for two commercial foams is shown in Figure 5. As 

discussed above, the heater is turned off at t=80 min, once the heating ramp and isothermal stage 

are completed, and data are collected for an additional 20 minutes after turning off the heater, 

resulting in a 100 minute total test duration. In the measurements shown in Figure 5, the heater 

was tuned to a set point of 360 C. The temperature of the smolder-prone foam continued to rise 

in the outer parts of the sample independent of the heater temperature. Comparing this result to a 

sample that did not smolder intensively demonstrated that the temperature rise was due to the 

smoldering reaction and not thermal diffusion from the heater into the sample. Temperature rise 

throughout the foam block is an indicator of smoldering intensity. 

A self-sustained smoldering foam is defined in the context of this test as a foam that reaches a 

temperature above 100 C for all six thermocouples at the end of the box test. In self-sustained 

smoldering, generally, the heater temperature drops well below the temperature of the smolder 

zone. The foam continues to smolder on its own, as indicated by smoke generation, and the 

sample has to be extinguished with water. Samples that did not exhibit sustained smoldering had 

a mass loss below 2 %. Figure 6 shows the correlation between mass loss and average 

temperature for the six thermocouples reached at the end of the box test. 

Figure 5. Temperature profiles measured in the smolder box test for a foam showing self‐

sustained‐smoldering (right) and a foam in which smoldering is not self‐sustained (left). The

temperature of the cylindrical heater is shown as a solid line (target temperature of 360 °C).

The averages of two thermocouple temperatures measured in the foam are shown for

distances from the heater of 50 mm (dash‐dot), 76 mm (dash) and 102 mm (dot).

8
                             

                             

 

 

 

                             

                                    

 

 

 

 

 

 

 

Figure 6. Comparison of mass loss and average temperature of the six thermocouples at the

end of the box test. Data are collected on foams from Iteration 1 and 2.

The diameter of the smolder zone was measured by cross-sectioning the foam residue. After the 

sample had cooled down and dried, if extinguished with water, the foam block was cut into two 

parts and the diameter of charred material inside the foam was determined (at about mid-depth). 

A visual comparison of the residue of samples that smoldered with different intensity is shown in 

Figure 7. Even though the mass loss was only a single digit percentage of the sample mass, the 

diameter of the charred region was substantial for smolder-prone samples. The diameter of the 

smolder zone was only measured on few representative samples from Iteration 1. 

Figure 7. Illustration of the charred material produced in the box test. The respective mass

losses were 2 % by mass (left), 4 % by mass (center) and 7 % by mass (right).

Power supplied to the heater can also be used to assess the intensity of the smoldering reaction. 

Figure 8 shows the mass loss in the box test versus the power required to maintain the 

temperature of the heater at the target temperature during the test. For all samples that attained a 

high mass loss, the power supplied to the heater was much lower than for samples that did not 

significantly lose mass. This established that the heater was not driving the temperature rise in 

smoldering samples. 

The smolder box test was also designed to determine the onset temperature for self-sustained 

smoldering in the PUF samples. The onset temperature for self-sustained smoldering is defined 

as the lowest set temperature of the heater at which the foam shows self-sustained smoldering. 

The onset temperature was investigated by conducting box tests at four different target 

9
 

 

 

 

 

 

                                   

                         

                                 

           

 

 

 

 

 

 

 

temperatures of the heater (320 C, 330C, 340 C, and 360 C), and observing the lowest 

temperature at which sustained smoldering was achieved. 

The box test provides a characterization of smoldering propensity in a specific fire scenario. 

There is no direct relationship to all possible fire scenarios. The presence of the box itself 

reduces smoldering, likely by limiting the buoyant convection (i.e., oxygen supply).III Therefore, 

the mass loss data reported here apply to this specific setup and cannot be considered intrinsic 

material properties. Modifications to this testing device and protocol are currently being 

considered (e.g.; improving air flow by removing the wooden sides and bottom). Initial 

experiments without the box sides have shown this modification significantly increases mass 

loss. 

Figure 8. Plot of total mass loss in the smolder box test versus the power required to maintain

the temperature of the heater at the target temperature. For samples that intensely smoldered

resulting in high mass loss, only low power was needed for the heater. Data are collected on

foams from Iteration 1 and 2.

2.4.2. Mock-up Smolder Test 

This test method was designed to measure the resistance of an upholstery cover fabric to a 

smoldering ignition source when the fabric is placed over a standard polyurethane foam 

substrate. The objective here was to develop a PUF with well characterized and reproducible 

smoldering to be used as a standard substrate for fabric testing. To accomplish this, the mockup 

test was used to measure smoldering of a specific PUF sample for a specific upholstery cover 

-2)IV fabric (100 % cotton, indigo twill weave and an average mass of 0.68 kgm . 

For a full description of the operation procedures and test set-up refer to the CPSC proposed 

standard.15 Contained here is a brief description with a schematic drawing of the test setup 

(Figure 9) and a set of foam mockups that are ready to be tested (Figure 10). The mass of the 

III A comparison of the mass loss of a foam formulation measured with a box (three replicates)

and without a box (three replicates), showed that the mass loss increased by a factor of two when

a box was not used. IV Average mass data measured by the manufacturer.

10
 

 

 

 

 

 

 

                           

              

                                   

  

 

 

(203 × 203 × 76) mm3

 vertical and (127 × 203 × 76) mm3 horizontal PUF is recorded and is later 

used in calculating the smoldering mass loss value. The PUF is covered with the upholstery 

fabric (Indigo Buckaroo Denim purchased from Jo Ann Fabrics was used in this study), and the 

entire ensemble is placed in the specimen holder. A lighted cigarette (Standard Cigarette for 

Ignition Resistance Testing, NIST SRM 1196)17 is placed in the crevice formed by the 

intersection of vertical and horizontal panels of each test assembly. Each cigarette is covered 

with a piece of sheeting fabric, a 100 % cotton, white plain weave of (19 to 33) threads/cm2, and 

a weight of (125 ± 28) gm

-2. The cigarette is allowed to burn its entire length. After 45 min, the 

PUF (char and non-char) is separated from the other testing components (e.g., cigarette ash and 

fabric) and the mass of this PUF is measured. The char is removed and the mass of the 

remaining non-charred foam is measured. The smoldering mass loss value is calculated as the 

difference of between the non-charred PUF mass and the original PUF mass divided by the 

original PUF mass. 

Figure 9. Schematic drawing of the mockup test described in the CPSC proposed standard

(upholstery cover fabric smoldering ignition resistance test).

Figure 10. A set of foam mockups ready to be tested prior to the addition of the lighted

cigarette.

2.4.3. Mock-up Open-flame Test 

The open-flame resistance was measured according to the CPSC proposed standard.15 This 

proposed standard includes open-flame tests on foams with or without a barrier fabric (bare 

foam) that uses a mock-up assembly similar toBS 5852.18 Two blocks of foam per mockup 

11
 

 

 

 

 

 

 

 

                                 

   

                               

                                 

                           

 

assembly are used. The vertical block is (457±5) mm x (305±5) mm x (76±2) mm; the horizontal 

block is (457±5) mm x (83±5) mm x (76±2) mm. An example of a mockup-assembly without a 

barrier is shown in Figure 11. 

The open-flame test setup for bare foams (without barrier) used here is described in the CPSC 

proposed standard. It is intended to define the flammability performance requirements for 

standard polyurethane foam in an assembly of mockups. This test is a slab of bare foam (no 

cover or barrier fabric) impinged by a 35 mm butane flame for 5 seconds. According to CPSC 

proposed standard, 120 s after removing the open-flame the mass loss of the bare foam should be 

greater than 20 %. The bare PUF mockup assembly and frame was placed on a metal tray and 

mounted on a scale; the mass loss of the sample was measured and monitored in real time. The 

open-flame ignition source was applied to the seat/back crevice of the mockup, as seen in Figure 

12. A hand-held carbon dioxide extinguisher extinguished the flames after 120 s or when the 

mass loss reaches 20 % by mass. 

Figure 11. An example of a mockup assembly without a barrier used in the open‐flame test for

bare foam.

Figure 12. Photo of test with formulation C1 a few seconds before extinguishing the sample. At

this stage, flaming liquid material is dripping to the underlying tray, but there is no pool fire

(which would sustain flaming and boost the heat release rate through a feedback effect).

The open-flame test with barrier material is intended to measure the open-flame ignition 

12
 

 

 

 

 

 

                               

                         

                               

                    

resistance of interior fire-barrier materials to be used over standard polyurethane foam in a 

mockup assembly. This test is detailed in the CPSC proposed standard. Briefly, the interior firebarrier material (a fabric 153 gm-2 product composed of a fiberglass base needle-punched with 

polyester and modacrylic fibers, covered with a polyester batting, nominally 153 gm

-2, 9.5 mm 

thick, and nonwoven) is placed between a standard cover fabric (100 % bright regular rayon, 

scoured, 20/2 ring spun basket weave construction, (271 ± 17) gm

-2) and the foam, and 

assembled on a metal test frame. 

The open-flame ignition source was a 240 mm butane flame, impinged onto the seat/back crevice 

of the sample for 70 s. The mass loss was recorded for 45 min or until the mockup selfextinguished and no mass change was observed, whichever occurred first. The ignition source at 

the beginning and end of flame impingement is shown in Figure 13 and Figure 14, respectively. 

After ignition the flame propagated on the fabric covering the top vertical block of foam and the 

top side of the horizontal block. Flame-out occurred between 7 min 30 s and 15 min, but the 

samples continued smoldering for several minutes more, inducing further mass loss. 

Figure 13. A photo of the 240 mm butane flame impinging on the standard rayon material

covering the foam for 70 s in the open‐flame test with barrier material.

Figure 14. Photo of the open‐flame test with barrier material. The 240 mm butane flame was

removed from the crevice after 70 s of flame impingement.

13
 

 

 

 

 

 

 

 

                       

 

 

 

 

 

                 

2.4.4. Flame Spread Test 

Open-flame resistance was measured by the Flame Spread Test, a newly developed method 

intended as a smaller scale alternative to the Mock-up Open-flame Test. The setup is shown in 

Figure 15. The foam sample (28.0 × 7.5 × 3.0) cm3, insulated underneath by a drywall plate, was 

placed on an aluminum-foil catch-pan to retain flaming liquid and prevent dripping. The sample 

mass was monitored in real time by a load cell. Ignition was achieved by impinging the samples 

for 20 s with a T-burner. Any pool developed during the tests was fully contained within the pan. 

The pool was immediately behind the flame front and was rapidly pyrolyzed. A video acquisition 

system was used to record the entire test and monitor the position of the flame front. The speed 

at which the flame front spread over the surface of the PUF (flame spread rate) was measured. 

The average flame spread rate (FSR) was calculated by measuring the time (tspread) required by 

the flame front to travel from a mark placed at 76.2 mm from the pilot ignited end to a location at 

228.6 mm. An average mass loss rate (MLR) was also calculated for each test by dividing the 

mass loss measured at t= tspread (designated as mspread) by tspread. 

Sample size: 28 cm × 7.5 cm × 3 cm

aluminum pan 

Figure 15. Schematic drawing showing the setup used for measuring flame spread.

2.5 Foam Characterization 

Foams are three-dimensional structures containing gas bubbles (cells). In PUF, cells are 

polyhedrons, most commonly dodecahedrons with pentagonal faces, separated from each other 

by thin sections of polymer: the foam struts (polymer at the shared edge of the polyhedrons) and 

the foam membrane or window (polymeric thin film connecting the struts on a face of the 

polyhedron). An open-cell is a cell with only open windows, i.e., no residual membrane. A 

closed-cell is a cell completely separated from the adjacent ones by windows. PUFs typically 

contain closed and open cells, as well as partially open cells (cell with few residual membranes). 

The openness or porosity of PUF is generally described by measuring the air permeability. 

Briefly, the differential pressure is measured for a PUF sample with a given thickness and area as 

air flows through the sample. An electronic high differential pressure air permeability-measuring 

14
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

instrument (FAP 5352 F2, Frazier Precision Instrument Co. Inc., Hagerstown, MD) was used in 

this study ( 

Figure 16). Foam was cut into samples (90x90x13) mm3

 and placed in a circular clamp, exposing 

38.5 cm

2

 to perpendicular air flow. 

The target pressure-drop through the 13 mm thick foam slice was set to 127 Pa (13 mm of 

water). Nozzles with orifice diameters of 1.0 mm, 1.4 mm, 2.0 mm, 3.0 mm, 4.0 mm, 6.0 mm, 

8.0 mm, or 11.0 mm were used in order to reach the target pressure drop. The test was conducted 

at room temperature. The value of permeability () in terms of volumetric air flow was read in 

cubic feet of air per square foot of sample area per minute (CFM) at about 20 C and 1 atm and 

was converted to cubic meters per square meter of sample per minute (or simply meters per 

minute) at a temperature of 0 C and a pressure of 100 kPa. 

The air permeability for all samples prepared in the pilot plant was provided by the 

manufacturer. The value of  measured for each foam bun is reported in Appendix 1. Air 

permeability was measured by the manufacturer on a foam slice collected near the center point 

between pour start and pour end and at a depth of about 2.5 cm from the top surface of the bun. 

The foam slice was cut parallel to the bottom surface of the bun. 

Air permeability measurements were also conducted by NIST to assess potential variations in 

permeability along the direction of pouring. Foam slices were cut perpendicular to the direction 

of pouring. Eight measurements per slice were performed. 

The slices used for the NIST and vendor measurements were from planes orthogonal to each 

other. Due to anisotropy in the foam, cells looked roughly like circles in the slice used by the 

vendor and ovals in the slices used by NIST. This anisotropy also generated differences in the air 

permeability measured respectively by NIST and by the vendor. 

The manner in which the sample is clamped is critical due to possible leaks. An example of a 

clamped PUF specimen is shown in Figure 17. This type of clamping was originally developed 

for textiles, but has proved to be more reliable than the concentric-cylinders mount 

(conventionally used for foams), likely due to improved sealing. 

PUF foam firmness and load bearing capacity is described using a standard measurement value 

called IFD (Indentation Force Deflection). An IFD number represents the force in pounds that is 

applied by a circular indenter to induce a given deflection expressed as a percentage of the initial 

thickness of the PUF sample. The values of IFD reported in this study were measured by a Zwick 

IFD Testing Unit equipped with a circular indenter (area 323 cm2

) at 25 % deflection (IFD25) on 

PUF samples with a 10.2 cm thickness and a square base of (30.5 × 30.5) cm2

. The IFD25 value 

is reported for each formulation in Appendix 1. 

The surface area of PUF was calculated by Brunauer-Emmett-Teller (BET) measurements19 

carried out by an experienced testing laboratory. The BET values of surface per unit mass of 

sample are then converted in mass per unit volume (specific surface area) dividing the BET 

values by the density of the sample. The specific surface area (SSA) is defined here as surface 

15
 

                         

   

                    

 

 

per unit volume and it is expressed in inverse meters. The samples used for BET measurements 

were about (1x1x23) cm3

. 

The thermal conductivity and thermal diffusivity were measured on three inch thick samples of 

foams by the transient plane source measurement technique, previously described in detail. 

20 

Figure 16. Photograph of the electronic high differential pressure instrument for measuring air

permeability.

Figure 17. Clamping of the PUF samples during a permeability measurement.

3. Results and Discussion 

3.1. Iteration 1: Impact of Polyols and Surfactants 

The purpose of Iteration 1 was to determine if the chemical composition (within the parameters 

chosen for this project) impacts the PUF smoldering and open-flame behavior. The typical 

precursor components used in a PUF formulation are polyether polyols, a monomeric or 

oligomeric isocyanate, water, surfactants, catalysts, and additives (e.g., fire retardants and 

colorant). Typical commercial grade PUF formulations were used and additives were excluded to 

avoid unnecessarily increasing the number of composition variables. 

In Iteration 1 the smoldering performance of the foams was mainly evaluated with the box test 

because the repeatability in the mockup test may be affected by the variability in the mockup 

components (fabric, cotton sheeting, etc.).; The mockup test was used only on selected 

formulations to verify, as suggested by the box test results, that smoldering performance was 

dominated by morphology and not chemistry (within the experimental parameter space 

investigated in this project). The open-flame behavior of the foams was investigated by the 

16
 

 

 

 

 

 

 

 

 

small scale flame spread test described previously in order to limit the amount of foam prepared 

in Iteration 1. 

3.1.1. Materials for Iteration 1 

Polyols are the largest mass-fraction component in PUF. Polyether polyols are used in about 

75 % of the global market for slabstock PUF.21 This study investigated the effect of chemical 

structure of polyols on smoldering by using three polyether polyols (P1, P2, and P3). The three 

polyether polyols were all glycerol initiated, poly(propylene oxide) / poly(ethylene oxide) mixed 

block polyether triols. All three products had comparable OH numbers (52 g·mol-1 to 58 g·mol-1), 

similar molar masses (3000 g·mol-1 to 3200 g·mol-1), and acid numbers lower than 0.02 g·mol-1, 

according to the manufacturer. Polyols P1 and P2 differed by the existence of an ethylene oxide 

end cap for polyol P1. Both polyols were produced with a conventional process employing KOH 

as the catalyst. The third polyol P3 was produced in a double metal cyanide catalyzed process 

and had no ethylene oxide cap. Additionally, a bio-derived polyol (P4) and a graft-polyol (P5) 

were used as a possible replacement to conventional petroleum-based polyols. The specifications 

of polyols and all other materials used in this study are described in the Experimental Section. 

The second largest component by mass fraction in PUF is the isocyanate. In Europe, PUF is 

often fabricated using a supercritical CO2 process with diphenylmethane diisocyanate (MDI) and 

MDI oligomers. Outside Europe, a water based foaming process with toluene diisocyanate (TDI) 

is more common.22 In this study, TDI was used exclusively to reduce the experimental parameter 

space. 

Surfactants largely impact morphological-cell-structure parameters (e.g., density, airflow, and 

surface area), which can play a significant role in the smoldering process. This study includes 

three silicone surfactants (S1, S2 and S3); S1 is a conventional silicon-based surfactant, and S2 

and S3 are believed to impart some reduction in the severity of PUF flammability. Surfactant S2 

is also commonly used in the supercritical CO2 process. 

Based on the above considerations, the impact of the chemical composition on PUF smoldering 

was assessed by characterizing 11 formulations containing five different polyols and three 

different surfactants. The experimental design is shown in Figure 18 with each dot representing a 

specific formulation (PiSj) for a given combination of polyol (Pi) and surfactant (Sj) type 

(i=1,2,3 and j=1,2,3,4,5 refer to polyol and surfactant type, respectively). In formulations 

containing the uncommon commercial polyols (P4 and P5), 25 % by mass of a standard polyol 

(P1) was replaced by P4 or P5. Formulations with all possible combinations of polyols P1, P2 

and P3 and surfactants S1, S2 and S3 were foamed in order to investigate whether or not there 

was a systematic effect of either one of the components. A detailed description of all 

formulations of Iteration 1 (A1, A1R, A2, A3, A4, A5, A6, A7 A8, A9, A10, A11) is reported in 

Appendix 1, including type and quantity of reagents, atmospheric conditions during 

manufacturing, mixing head pressure, rise profile during foaming, air permeability, density, and 

indentation factor (IFD25). Formulation A1R was a repetition of A1 at a different temperature 

and humidity. Four replicate buns were foamed for each formulations of Iteration 1. 

17
                    

 

 

 

 

 

 

S1: FR grade 

S2: CO

S3: conventional 

Polyether polyols 

 processing Surfactants 2

Figure 18. Design of experimental parameter space for Iteration 1.

The hydroxyl content for the polyols used in Iteration 1, expressed as OH number in Table 1, 

ranged between (31 to 57) mg KOH·g-1. The TDI index (percentage ratio between the actual 

amount of TDI used in a formulation and the theoretical stoichiometric amount of TDI required 

to react with any reactive additive, e.g., water and polyols) was kept constant at a value of 105 

for all formulations by controlling the content of TDI, polyol, water, and reactive catalysts. The 

formulations were also designed to minimize differences in density (by tuning the water content) 

and air permeability (by tuning the catalyst content). 

Nevertheless, significant differences in foam density and air permeability, observed among the 

foams of Iteration 1 (Table 4), were likely to influence smoldering and therefore the results in 

Iteration 1. For these reasons, the effect of secondary parameters deserved closer scrutiny. The 

secondary parameters investigated were foam density, air permeability, amount of tin catalyst, 

relative humidity during testing (R.H.-Testing), and air flow above the enclosure during testing. 

All these parameters may potentially affect smoldering.4-11 These parameters were monitored and 

correlated to the final mass loss measured in 78 smolder box tests of Iteration 1. 

Table 4 lists the range of values tested for these parameters along with the coefficients of 

correlation of the values of the secondary parameters to smoldering (assessed in the box test as 

the average mass loss measured at a heater target temperature of 340 C and 360 C). All of the 

coefficients were found to be very small. This demonstrates that the secondary parameters do not 

have a controlling effect on the outcome of the box tests. 

18
                             

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 4. Influence of the secondary parameters on smoldering measured in the box test (78

tests).

Parameter Unit Min. Max. Avg StDev 

Foam Density 

Air Permeability 

Tin Catalyst Content 

R.H.-Testing

Air Flow above Enclosure 

kg · m-3

m·min-1 

phpV

 % 

m·min-1 

 27.5 

45.9 

 0.12 

48 

0.07 

30.8 

90.9 

0.21 

65 

0.12 

29.1 

80.2 

§ 

58 

0.08 

1.0 

29.1

§

4 

0.02

R2† 

< 0.01 

 0.02 

 0.02 

< 0.01 

 0.05 

† Coefficient of determination of a linear least square fit to mass loss after 100 min of test time.

§ The uncertainty of the dosing unit was not determined by the foam manufacturer. 

3.1.2. Results for Smoldering in the Box Tests 

The box test was designed to determine a minimum temperature required to initiate PUF 

smoldering and quantify smolder intensity. The minimum temperature required to initiate 

smoldering could be a potentially valuable property to describe the smoldering propensity of a 

given type of material. However, it is unclear if a distinct temperature of smolder initiation exists 

and, if so, how it would depend on foam properties and test conditions. In order to find the range 

of temperature at which smoldering is initiated in this particular setup, all foam samples were 

measured with heater set point temperatures of 320 °C, 330 °C, 340 °C, and 360 °C. Such a 

characterization required a large number of replicates for good statistical certainty on the 

minimum heater temperature required to assure self-sustained smoldering. Considering the 

number of formulations investigated, it was too time-consuming and unrealistic (due to the 

limited amount of foam available) to do more than replicate tests at each selected set 

temperature. It was decided that since the onset temperature of a formulation was unknown apriori, there would be more information obtained by measurements at four temperatures with 

limited replicates than by measurements at one temperature with more replicates, but no 

information on the onset temperature. 

Table 5 reports the mass loss measured in the box test for all formulations of Iteration 1 at the 

different set point temperatures. A total of 90 box tests were performed during Iteration 1. For 

each formulation, two or three tests were performed at 320 °C, one or two tests at 330 °C, two 

tests at 340 °C, and one or two tests at 360 °C. As previously mentioned, the number of 

replicates was limited by time and by foam-availability constraints. 

Formulation A1R and B2 in Table 5 were repetitions of formulations A1 and A11, respectively. 

Sustained smolderingVI was generally not observed at 320 °C except for A11 (in one test out of 

two) and A1R (single test only). At 330 °C, approximately 60 % of the samples were smoldering 

V

 Parts (by mass) per hundred parts of polyol for a specific component in a foam formulation 

(e.g., 10 php of X means that 10 g of component X are used in combination with 100 g of 

polyol). For clarity and convenience, php is used in place of mass fraction in foam formulations 

(a variation in the amount of a single additive in a formulation causes a variation in the mass 

fraction of all additives).

VIA mass loss above 2 % was observed in foams with sustained smoldering. 

19
 

 

 

 

 

 

 

and at 340 °C and 360 °C, all of the samples smoldered (see Figure 19). Preliminary tests, run 

on a set of five commercial foams, gave similar results: a heater target temperature of 330 °C 

was sufficient to initiate self-sustained smoldering for four out of five samples. 

In Table 6, the average percent mass loss and onset temperature for sustained smoldering are 

shown together with the polyol/surfactant type, density and air permeability. There is no 

significant correlation between density and permeability (secondary parameter) and onset 

smoldering temperature. Formulations based on polyol P3 always show an onset temperature 

between 320 °C and 330 °C, whereas, for all other polyols, the onset temperature appears to fall 

into a wider interval between 320 °C and 340 °C. As discussed below, the relatively low onsettemperature observed for P1 based formulations do not appear to significantly affect smoldering 

intensity, measured as mass loss in the box test. 

20
3.3 

1.0 9.0 

1.1 

1.2 5.7 7.2 1.0 8.6 3.5 7.7 

Tabl

e

5. Mass loss measure

d in th

e bo

x test at heater set‐point‐temperatures of 32

0 C, 33

0 C, 34

0 C an

d 36

0 

Formulation Heater at 320 C Heater at 330 C Heater at 340 C Heater 

Test Mass loss Avg Dev Mass loss Avg Dev Mass loss Avg Dev Mass l# (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) 

A1 1 1.0§ 1.1 0.1 13.8 13.8 - 10.1 11.5 1.4 9.7 

(P1-S1) 2 1.2§ - 12.9 -

A1R 1 3.3 - 1.8§ 2.7 0.9 5.6 5.5 0.1 7.9 

(P1-S1) 2 - 3.5 5.4 - 

A2 1 0.8§ 0.9 0.1 1.4§ 1.3 0.2 5.4 5.5 0.1 8.0 

(P1-S2) 2 1.0§ 1.1§ 5.5 -

3 0.8§ - - -

A3 1 1.0§ <0.1 6.2 2.8 5.4 6.3 0.9 10.0 

(P1-S3) 2 1.0§ 11.8 7.1 - 

A4 1 1.0§ 1.2 0.2 0.8§ 0.7 0.2 6.6 6.2 0.4 4.3 

(P1/P4-S1) 2 1.3§ 0.4§ 5.8 -

3 - 0.9§ - -

A5 1 1.0§ 1.0 0.1 1.1§ 0.1 3.9 4.4 0.5 6.7 

(P1/P5-S1) 2 0.9§ 1.0§ 4.9 -

A6 1 1.1§ 1.1 <0.1 4.2 5.2 1.0 7.0 6.6 0.5 7.2 

(P2-S1) 2 1.1§ 6.2 6.1 -

A7 1 1.2§ 0.1 5.6 0.1 7.2 <0.1 8.4 

(P2-S2) 2 1.1§ 5.7 7.2 -

A8 1 0.8§ 0.9 0.1 1.2§ 1.2 <0.1 8.0 8.0 <0.1 4.7 

(P2-S3) 2 0.9§ 1.2§ 8.0 -

A9 1 1.1§ 0.1 6.8 1.8 7.7 7.8 0.1 11.4 

(P3-S1) 2 0.9§ 10.3 7.8 -

A10 1 1.0§ 1.0 - 6.4 6.5 0.1 7.3 7.7 0.4 8.2 

(P3-S2) 2 - 6.6 8.1 10.8 A11 1 6.1 2.6 7.2 0.5 8.5 7.7 0.8 6.9 (P3-S3) 2 0.9§ 8.1 6.9 -

B2 1 0.1§ 0.1 - 6.7 5.8 1.3 7.0 6.9 0.1 8.3 

(P3-S3) 2 - 4.8 6.8 -

§Non-smoldering sample. The mass loss in non-smoldering samples is less than 2 %. 
   

0

20406080

100120

Smoldering samples (%)

320°C 330°C 340°C 360°C

Figure 19. Percentage of samples from Table 6 showing self‐sustained smoldering at various set‐point temperatelement.Table 6. Onset temperature for self‐sustained smoldering, mass loss and main properties of formulations of Iter†Average mass loss in the box test for a heater set temperature of 340 C and 360 C (three replicates).Formulation Polyol-Surfactant Density 

(kg·m-3) 

Permeability (m·min-1) 

Onset temperature for self-sustained smoldering (C) 

AVG mass loss a340 C+360 C (

A1 P1-S1 30.1±0.8 77.2±2.3 320 to 330 10.9±1.7 A1R P1-S1 28.4±0.4 45.9±7.7 320 to 330 6.3±1.4 

A2 P1-S2 30.8±0.4 87.5±15.2 330 to 340 6.3±1.5 

A3 P1-S3 30.1±0.9 70.9±8.6 320 to 330 7.5±2.3 

A4 P1/P4-S1 29.1±0.6 89.2±3.2 330 to 340 5.6±1.2 

A5 P1/P5-S1 28.7±0.2 76.3±10.9 330 to 340 5.2±1.4 

A6 P2-S1 28.8±0.5 81.8±11.8 320 to 330 6.8±0.6 A7 P2-S2 29.4±0.8 90.9±9.8 320 to 330 7.6±0.7 

A8 P2-S3 29.1±0.3 81.8±5.7 330 to 340 6.9±1.9 

A9 P3-S1 27.7±0.3 90.4±5.2 320 to 330 9.0±2.1 

A10 P3-S2 27.5±0.3 82.9±3.4 320 to 330 8.6±1.5 

A11 P3-S3 28.8±0.2 77.7±8.6 320 to 330 7.4±0.9 

B2 P3-S3 30.3±0.6 89.8±8.3 320 to 330 7.4±0.8 
 

 

 

 

 

                                   

                                 

                                 

 

 

 

 

 

 

 

 

 

Figure 20 summarizes the mass loss vs. set point temperature of the heater in the box test. The 

value ranges (shown as uncertainty bars) are equal to the mean deviation.VII Data points that do 

not show an uncertainty bar are single measurements. The maximum mass loss was observed for 

sample A1 (13.8 %) at 330 C; the mass loss measured for the same sample A1 at 340 C and 

360 C was lower but there are not enough data available to conclude if this variation in mass 

loss between 330 C and (340 or 360) C is statistically significant. 

0

3

6

9

12

15

315 320 325 330 335 340 345 350 355 360 365

Massloss(%)

Temperature (C)

A1 A1R A2 A3 A4 A5 A6

A7 A8 A9 A10 A11 B2

Figure 20. Mass loss vs. set point temperature of the heater in the box test using data from

Table 5. The uncertainty bars shown here are equal to the average of the absolute deviations of

data points from their mean (Dev). Data points that do not show an uncertainty bar are single

measurements.

Figure 21 shows the average mass loss vs. set point temperature of the heater in the box test 

calculated for all samples in Table 6. Uncertainty bars shown here represent the range of the 

values measured during the test. This graph appears to be a sigmoid curve with an "S" shape. 

The mass loss is negligible below the onset temperature and then increases asymptotically with 

temperature. The high standard deviation observed at 330 C is due to the fact that 330 C is 

close to the smoldering onset temperature; about 60 % of the samples showed sustained 

smoldering at this temperature. The average difference in mass loss between 340 C and 360 C 

is about 1 % and is not statistically significant. 

VII The mean deviation for the mass loss was determined as follows. The mass loss was 

determined in two replicate tests for each of nine PUF formulations. The difference between 

each pair of values was calculated and divided by two (deviation). The mean deviation, obtained 

as the average of the nine deviations, was equal to about 8 % of the mean mass loss. 

23
                                    

                               

 

 

 

 

 

 

 

 

 

 

0

2

4

6

8

10

12

310 320 330 340 350 360 370

Massloss(%)

Temperature (C)

Figure 21. Average mass loss vs. set point temperature of the heater in the box test calculated

for the samples in Figure 20 (uncertainty bars shown here are equal to one standard deviation).

Since only two tests at 340 C and one test at 360 C were conducted, an average of the mass 

loss measured at 340 C and 360 C (temperatures at which all samples demonstrated 

smoldering) was calculated for a more reliable assessment of the smoldering intensity (Table 6). 

The average mass losses at 340 C and 360 C (ML340+360) for all the formulations are shown in 

Figure 22. Formulation A1 has the highest ML340+360 value. Figure 23 reports the values of 

ML340+360 grouped by surfactant or polyol type. For example, ML340+360 shown for S1 is an 

average of the ML340+360 measured for A1, A6, and A9 and ML340+360 shown for P2 is an average 

of the ML340+360 measured for A6, A7, and A8. The analysis of variance indicates that there is no 

significant difference between the mean values of groups P1, P2 and P3 or groups S1, S2 and S3 

(Figure 23). This means that in terms of mass loss there is no systematic effect of the polyol or 

the surfactant on smoldering. 

The repeatability of the foaming process was verified by manufacturing replicates of the 

formulations. For example, A1R and B2 are chemical replicates (same formulation but foamed at 

different temperature and relative humidity, see Appendix 1 for details on atmospheric 

conditions, processing parameters and foam properties) of A1 and A11, respectively. Polyol P3 

used in formulation A1 and A1R appeared to be insensitive to variations in atmospheric 

conditions and smoldering performance was unaffected. However, there was a significant 

variation between the two formulations based on polyol P1 (A1R and B2) in terms of average 

mass loss (ML340+360 = 10.9±1.7 % for A1, and ML340+360 = 6.3±1.4 % for A1R), detailed in 

Figure 24.VIII Noticeably, even though A1 and A1R are identical formulations, they show the 

highest and lowest ML340+360, respectively, in the conventional PUF formulation (i.e., not 

including graft-polyol P5 or bio-polyol P4) (Table 6). As also reported in Table 6, the air 

permeabilities and densities were largely different for A1 and A1R. These results indicate that 

atmospheric conditions during manufacturing may cause substantial variation of foam 

morphology that overshadows any effect on smoldering created by the chemical differences 

associated with the three primary polyols and/or three surfactants selected for this study. 

VIII The relatively higher sensitivity of polyol P1 to variations in atmospheric conditions was 

confirmed by the foamer on the basis of its multi-year experience. 

24
                                   

                             

                             

                                   

                               

                         

                                 

                           

0

2

4

6

8

10

12

14

ML340+360 (%)

A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11

Figure 22. Average mass loss at 340 C and 360 C (ML340+360) in the box test for the

formulations of Table 6 (uncertainty bars equal to one standard deviation). The ML340+360 for the

replicates of formulation A1 and A11 (A1R and B2, respectively) are shown in Figure 24.

0

2

4

6

8

10

12

ML340+360 (%)

S1 S2 S3 P1 P2 P3

Figure 23. Average mass loss at 340 C and 360 C (ML340+360) in the box test grouped by

surfactant or polyol type. For example, ML340+360 shown for S1 is an average of the ML340+360

measured for A1, A6 and A9 (uncertainty bars equal to one standard deviation).

0

2

4

6

8

10

12

14

MLmockup (%)

A1 (P1‐S1) A1R (P1‐S1) A4 (P1/P4‐S1) A5 (P1/P5‐S1) B2 (P3‐S3) A11 (P3‐S3)

Figure 24. Average mass loss at 340 C and 360 C (ML340+360) in the box test for formulations

A1, A1R, A4, A5, B2 and A11 (uncertainty bars equal to one standard deviation).

25 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The non-conventional formulations, A4 and A5, are also compared in Figure 22. Replacing 25 % 

of polyol P1 with a bio-polyol in A4 or graft-polyol in A5 produced a large reduction in 

ML340+360 as compared to A1 but no significant reduction as compared to A1R. The effect of 

adding P4 and P5 to the formulation on smoldering might be explained by a change of foam 

morphology rather than foam chemistry. Therefore, the morphology of formulations A1, A1R, 

A4 and A5 was investigated by means of a confocal microscope (Zeiss LSM 510). An optical 

section with an identical thickness of 1 mm was collected for all samples for comparison of cell 

structure. 

The micrographs of Figure 25 show heterogeneities in A1R produced by large clusters of 

residual membranes (likely fully closed cells) in the right and bottom area and a generally more 

open and homogeneous cell structure for A1. This is consistent with the significantly different 

measured values of air permeability and smoldering propensity for A1 and A1R. 

Since A1 and A5 have similar air permeabilities, it was expected that the smoldering behavior 

would also be similar; however, the smoldering intensity of the A5 foam was significantly 

smaller than that of the A1 foam. The air permeability of the A4 foam was greater than the A1 

foam, but A4 showed a lower smoldering propensity than the A1 foam (Table 6). Figure 26 

shows that the cell structures of formulations A4 and A5 were very similar. There were a very 

limited number of residual membranes and the average cell size was larger than in A1. Both of 

these characteristics caused a reduction in surface area of the foam. These data suggest that 

surface area also plays a key role in smoldering. 

The idea that smoldering performance is dominated by morphology and not chemistry for the set 

of formulations tested here is confirmed by the mockup tests. Tests were conducted with three 

mockups each for formulations A1, A1R, A8, and A11. The mass loss values measured by the 

box test (ML340+360) and the mockup test (MLmockup) for these formulations are compared in 

Table 7. The same data are also plotted in Figure 27. The observed morphologic differences 

between the chemically identical A1 and A1R formulations result in an even larger difference in 

smoldering intensity in the mock-up test as compared to the box test; the average ML340+360 

increased from 6.3 % for A1R to 10.9 % for A1 (the ratio between ML340+360 for A1 and 

ML340+360 for A1R is about 1.7), and the average MLmockup increased from 0.5 % for A1R up to 

30.5 % for A1 (the ratio between MLmockup for A1 and MLmockup for A1R is 61). 

The results in Figure 27 show that the box test is not as sensitive as the mockup test to 

morphological variations. Formulations A11 and A8 have a comparable mass loss in the box test 

to formulation A1R but drastically different mass loss in the mockup test. This is due to the fact 

that, as discussed in Section 3.2.8, there is a threshold value for air permeability (Φ45 m·min-1), 

below which no smoldering is observed in the mockup test. A detailed comparison of data 

outputs from the two tests is discussed in Section 3.2.3. 

26
                             

                             

                        

                         

                         

                           

                               

                           

           

Formulation 

 

A1 A1R 

Figure 25. Optical micrographs for formulations A1 (on the left) with an air permeability of

(77.2±2.3) m∙min‐1 and its replicate formulation A1R (on the right) with an air permeability of

(45.9±7.7) m∙min‐1

. Bar size in the right bottom corner is 1 mm.

A4 A5 

Figure 26. Optical micrographs for: A4 (bio‐polyol formulation) with an air permeability of

(89.2±3.2) m∙min‐1 (on the left); and A5 (graft‐polyol formulation) with an air permeability of

(76.3±10.9) m∙min‐1 (on the right). Bar size in right bottom corner is 1 mm.

Table 7. Mass losses measured by the box test (ML340+360) and the mockup test (MLmockup) are

compared for 4 formulations in Iteration 1 (uncertainty is shown as one standard deviation

calculated over at least 3 replicates).

MLmockup 

(%) 

ML340+360 

(%) 

Air Permeability, Φ

(m·min-1) 

A1 30.5±12.1 10.9±1.7 77.2±2.3 

A1R 0.5±0.2 6.3±1.4 45.9±7.7 

A8 32.0±2.3 6.9±1.9 81.8±5.7 

A11 29.3±6.5 7.4±0.9 77.7±8.6 

27
                                    

                      

 

 

 

 

 

 

                             

                             

   

A8

A11

Figure 27. Mass loss in the box test (ML340+360) vs. mass loss in the mockup test (MLmockup) for

four formulations of Iteration 1 (uncertainty bars equal to one standard deviation).

3.1.3. Results for Flame Spread Tests 

Flame spread tests were conducted to examine the role of surfactants on the burning behavior of 

the foams and evaluate the possible effects on the CPSC proposed standard. All formulations 

from Iteration 1 were tested, each in three replicates. Typically, the flame spread over the surface 

of the sample, and then the foam started to collapse and form a liquid pool. The results 

demonstrated that the surfactant had a strong impact on the burning behavior of the pool. 

Surfactants S1 and S2 are designed to decrease the flammability of PUF and S3 is a conventional 

non-flame-retardant surfactant. Foams containing S1 and S2 showed reduced mass loss rate 

(MLR) for all three polyols as compared to S3 (Figure 28). Surfactants with flame-retardant 

action may suppress flaming over the liquid produced by thermal degradation of the foam. This 

suggests that surfactant type can also have an impact on the open-flame test according to the 

CPSC proposed standard, where a pool fire may occur. 

Figure 28. MLR measured for Iteration 1 formulations. There is a significant reduction of MLR

for surfactants S1 and S2. Uncertainty shown as one standard deviation calculated over at least

3 replicates.

28
 

                           

             

 

 

 

 

 

 

 

 

However, in terms of average flame spread rate (FSR) a fairly low variation for different foam 

formulations was observed (Figure 29). The formation of pool fires with S3 led to a small 

increase in FSR with polyols P1 and P2, but not with P3. These results do not show a significant 

systematic effect of a specific polyol on mass loss or flame spread. 

Figure 29. FSR measured for Iteration 1 formulations. Uncertainty is shown as one standard

deviation calculated over at least 3 replicates.

3.1.4. Conclusions of Iteration 1 

In Iteration 1, three types of commercially available polyols and surfactants were selected in 

order to investigate the effect of chemical composition on smoldering. Formulations with all 

possible combinations of three different polyols with three different surfactants were 

manufactured in order to investigate whether there is a systematic effect of either one of the 

polyols or surfactants. In terms of smoldering (measured as mass loss in the box test), there is no 

systematic effect on smoldering for any of the polyols or surfactants (within the parameters 

chosen for this project). The results demonstrated that the variations in foam morphology due to 

variation in manufacturing conditions (e.g., temperature and humidity) may override any 

potential effect of the chemical composition. In terms of the open-flame test, surfactants with a 

physical flame-retardant action may prevent flame propagation over the liquid pool and decrease 

the mass loss rate. 

3.2. Iteration 2: Impact of Processing Parameters 

The purpose of Iteration 2 was to determine if the processing parameters (within the parameters 

chosen for this project) impact PUF smoldering and open-flame performance. Typical processing 

parameters with a strong impact on foam morphology are water content, tin catalyst content and 

head pressure. They are routinely adjusted during PUF manufacturing to compensate for 

morphological variations caused by climatic conditions (i.e., variations in atmospheric 

temperature, humidity and pressure) and deliver a PUF with consistent specifications throughout 

the year. Water controls the blowing action by reacting with TDI and releasing CO2, the tin 

29
 

 

 

 

 

 

                           

                         

 

 

 

 

 

catalyst controls the rate of the polyol/TDI reaction, and the head pressure controls nucleation 

and cell growth at the exit of the mixing head.21 

A 2k3 factorial experimental design (two levels for three factors) with high and low levels of 

water, tin catalyst, and mixing head pressure was chosen in order to produce flexible foam 

samples with a range of densities (28.4 kg·m-3 to 34.3 kg·m-3) and air permeabilities (3.1 m·min-1 

to 74.2 m·min-1). Figure 30 shows a schematic diagram with the experimental design and effect 

of each factor on foaming. The choice of the experimental-design factors was limited to the three 

parameters expected to lead to the most significant changes in foam morphology. This does not 

exclude the possibility that other parameters could also significantly affect smoldering. 

Figure 30. Schematic diagram showing the 2k3 experimental design with high and low settings

of the amount of water, amount of tin catalyst and mixing head pressure.

Iteration 1 showed that there was no systematic effect on smoldering of any specific polyol or 

surfactant, so the selection of the formulation for Iteration 2 was not critical. Originally, 

formulation A8 (P2-S3) was selected, because it is a standard PUF based on a conventional 

surfactant without any flame retardant action and a widely used commercial polyol. However, a 

different formulation, A11 (P3-S3) was ultimately used in combination with a processing aid, 

because formulation B1 (replicate of A8 during Iteration 2) yielded foams with poor quality due 

to splitting (cracks in the bun) because it was manufactured in a different climate (see Appendix 

1). The average manufacturing plant temperature and relative humidity decreased from 

(25.0 ± 1.8) C and (59.3 ± 10.7) % during Iteration 1, to (21.6 ± 0.7) C and (28.1 ± 4.0) % 

during Iteration 2, respectively. According to the foamer and to our results from Iteration 1, 

polyol P1 is more sensitive to temperature/humidity variations than polyol P3. As already 

mentioned, the choice of the polyol is not critical so polyol P1 was replaced by polyol P3. 

However, the use of polyol P3 did not solve the splitting problem as formulation B2 gave rise to 

the same foam cracks observed in B1 foam. 

30
 

 

 

 

 

 

 

 

 

 

 

 

 

                               

       

 

 

 

 

At this point, due to the inability to control temperature and humidity in the pilot plant the only 

possible solution was to vary the formulation. The foamer suggested the addition of 0.15 phpIX 

of a processing aid (Table 3), which is a mixture of emulsifiers based on a fatty ester. This 

processing aid was used to decrease the viscosity and enhanced flow of the foaming material in 

the tube connecting the mixing head with the foaming box. This was sufficient to prevent foam 

splitting in formulation B3. 

With a suitable formulation agreed upon, the next step was to investigate the effect of processing 

parameters on smoldering. Ideally, the processing parameters should be varied over a wide 

range, but this was not possible due to limited foam stability at the extreme processing 

conditions. A series of preliminary experiments was performed to identify the processing 

window, i.e., the maximum range of variability for water content, tin catalyst content and mixing 

head pressure (referred to hereafter as “pressure”) that ensures successful foaming. 

Formulation B3 was the first of Iteration 2 that yielded good quality foams and was not affected 

by splitting. In the B3 foam, the tin catalyst content was 0.2 php, the water content 2.95 php and 

pressure 34.5 kPa (5 psi) (see Appendix 1.) Initially, 0.2 php was selected as the high level for 

the tin catalyst, 2.95 php the high level for water and 34.5 kPa as the low level for pressure. 

When the water level was decreased from 2.95 php to 2.7 php (low level for water content) there 

was excessive shrinkage and the high level of tin was decreased to 0.19 php. For this reason 

formulation B3 was superseded and formulation B4 was used instead in the 2k

3 experimental 

design. Similar iterations were carried out for identifying feasible low/high levels for pressure 

and tin content. 

The selected high and low levels for the three processing parameters are shown in Table 8. 

Outside of this processing window foaming was not reproducible or yielded poor quality foams 

due to foam collapse, shrinking, or splitting. 

Table 8. Summary of the selected high and low levels used for tin catalyst content, water

content and head pressure.

Processing parameter Low Level High level 

Tin Catalyst (php) 0.16 0.19 

Water (php) 2.70 2.95 

Head Pressure (kPa) 34.5 55.2 

The eight formulations of Iteration 2 used for the full factorial experimental design are provided 

in Figure 31 and Table 9. The average air permeability and density for the eight formulations 

used in the experimental design and three additional formulations from Iteration 2 (B2, B3 and 

B6) were calculated over at least four buns (Table 10). For convenience, the smoldering 

performance is also reported in Table 10 and discussed in the following sections. The average 

permeability calculated for these samples from Iteration 2 was (31 ± 28) m·min-1 while the 

average permeability of the samples from Iteration 1 was (80 ± 12) m·min-1. This reduction in 

IX Parts per hundred polyol (php) is the mass of a component in a foam recipe defined relative to 

the largest mass component of the formulation; the polyol. 

31
 

                     

                         

 

 

 

                               

                                   

         

 

 

 

 

 

 

permeability was thought to be mainly due to a variation in climatic conditions, but the 

processing aid might also have a small effect on air permeability (see Iteration 3). 

Figure 31. Schematic diagram showing the 2k3 experimental design with the corresponding

formulations.

Table 9. Formulations of Iteration 2 and corresponding levels for the full factorial experimental

design.

B4 B5 B7 B8 B9 B10 B11 B12 

Water High High Low Low Low Low High High 

Tin catalyst Low Low High Low Low High High High 

Pressure Low High High High Low Low Low High 

Table 10. Density (), air permeability (), mass loss in the mockup test (MLMockup) and mass

loss in the box test (ML340+360) for the foams of Iteration 2. The average values (Avg) ± one

standard deviation (StDev) are reported.

 (kgm-3)  (mmin-1) ML340+360 (%) MLmockup (%) 

B2* 30.3±0.6 89.8±8.3 7.4±0.8 -

B3* 30.2±0.6 49.3±10.6 2.4±1.6 -

B4 30.1±0.6 74.2±8.4 6.8±0.5 8.1±3.4 

B5 30.4±0.3 44.8±4.1 2.2±1.9 0.2±0.1 

B6* 33.8±0.8 33.4±5.5 2.9±1.1 0.5±0.2 

B7 31.6±0.4 4.7±1.7 0.8±0.2 0.5±0.6† 

B8 33.6±0.5 16.6±4.1 1.4±0.5 0.7±0.2† 

B9 34.3±0.6 59.7±3.4 5.1±2.3 15.2±5.6 

B10 30.7±0.7 3.1±0.4 0.8±0.6 1.2±0.5† 

B11 29.2±0.6 43.3±5.9 5.4±2.2 2.5±2.2 

B12 28.4±0.6 3.4±3.3 0.8±0.2 0.3±0.3† 

*This formulation is not part of the experimental design. 

† Formulations that did not smolder in the box test at the highest heater set temperature (360 C) 

did not smoldered in the mockup test either. 

32 
 

 

 

 

 

 

 

 

 

 

 

 

                             

                       

Three replicates of each of the formulations were smoldered in the box test and three replicates 

were smoldered in the mockup test. The values of average mass loss at a heater temperature of 

340 C or 360 C (ML340+360) and the average mass loss in the mockup test (MLmockup) are also 

shown for convenience in Table 10 and discussed further in the following sections. 

3.2.1. Bun-to-bun variability and in-bun variability 

As discussed above, climatic conditions (temperature, humidity and atmospheric pressure) affect 

the cell morphology so a fine tuning in processing parameters (water content, catalyst content 

and head pressure) is required on a daily basis for a consistent foam quality. This implies that 

even for a given formulation, a bun-to-bun variability in the foam properties is likely. Significant 

variations in PUF properties are also observed in the same foam bun (in-bun variability) as a 

function of the specific location due to surface effects and pressure/temperature gradients. 

For in-bun variability, multiple measurements of air permeability for each bun were performed 

by NIST to assess potential variations in permeability along the direction of pouring. Foam slices 

were cut perpendicular to the direction of pouring. Eight measurements per slice were 

performed. The foam buns were about 90 cm long. The results are shown in Figure 32 

(Iteration 1) and Figure 33 (Iteration 2). Air permeability varied dramatically along the pouring 

direction. For formulations from Iteration 1, it generally reached a maximum in the center of the 

bun and decreased towards the pour start and pour end. For formulations from Iteration 2, there 

was no clear trend. The processing conditions for these foams were purposely selected on the 

edges of the processing window. They are relatively instable formulations with irregular cell 

structure and, consequently, irregular air permeability. 

The bun-to-bun variability was assessed by calculating the standard deviation of the air 

permeability measured by the foamer for four or more different buns of the same formulation. 

The bun-to-bun variability calculated for the same foams of Figure 32 and Figure 33 are reported 

in Table 11 and compared to the in-bun variability. 

Figure 32. Air permeability vs. distance from the pour start for formulations of Iteration 1

(uncertainty bars are equal to one StDev over at least 3 replicates).

33
                             

                       

 

 

 

 

                         

                       

         

 

 

 

Figure 33. Air permeability vs. distance from the pour start for formulations of Iteration 2

(uncertainty bars are equal to one StDev over at least 3 replicates).

The in-bun variability in terms of relative standard deviation (StDev%) of the air permeability 

() is generally higher than the bun-to-bun variability. In other words, the properties of a foam 

sample are more affected by the position of the sample in the bun, rather than the specific bun 

used. This is true for the small buns used here, prepared in a pilot plant, but much lower in-bun 

variability is expected for large size buns from a production line. 

Table 11. Comparison between in‐bun variability and bun‐to‐bun variability in terms of air

permeability (). The average values (Avg), the standard deviation (StDev) and relative

standard deviation (StDev%) are reported.

In-Bun variability Bun-To-Bun variability 

Formulation Permeability, † Permeability, ‡ 

Bun Avg StDev StDev% Avg StDev StDev% 

# (m·min-1) (m·min-1) (%) (m·min-1) (m·min-1) (%) 

A1 3 59.6 4.7 7.9 77.1 5.2 6.7 

A3 1 44.8 11.7 26.0 70.8 8.6 12.1 

A9 2 51.5 22.0 42.7 90.3 5.2 5.8 

A11 3 57.6 12.0 20.9 77.7 8.6 11.1 

B4 2 32.9 20.2 61.6 74.2 8.4 11.3 

B10 1 2.9 1.4 47.5 3.1 0.4 12.9 

B12 1 9.2 11.1 121.2 3.4 3.3 97.1 

†Values of  measured in different locations of the same foam bun by NIST. 

‡Values of  measured in different foam buns and same middle section of the bun by the 

foamer. 

34 
                             

                             

                           

                                 

                             

 

 

 

 

 

 

       

0

20

40

60

80

100

120

0 20 40 60 80 100

Foamer Air Permeability (m∙min‐1)

NIST Air Permeability from middle section (m∙min‐1)

Figure 34. Correlation between the air permeability measured by the foamer in the pilot plant

and air permeability measured by NIST collected from the middle section of the foam bun

(30 cm middle section between pour‐start and pour‐end). Uncertainty bars for NIST data are

equal to the StDev over at least 3 replicates. The foamer's data are based on a single

measurement per bun. The red line is a least‐squares regression fit to the data points.

The foam slices used for the air permeability measurements by the foamer and NIST were cut 

from planes that were orthogonal to each other (see Section 2.5). Due to anisotropy in the foam, 

cells often looked roughly like circles in the foamer's slices and ovals in the NIST slices. As a 

result of the anisotropy in the foam, the air permeability was measured to be 30 % higher in the 

foamer's samples than in the NIST samples (Figure 34). The NIST air permeability data reported 

here were calculated only from the middle section of the foam bun (30 cm middle section 

between pour-start and pour-end) to minimize in-bun variability. Similarly, all the foamer's data 

were collected from the middle section. Except for one outlier, there is a good correlation 

between the two sets of measurements. The values of permeability measured by the foamer were 

used to assess foam permeability in the remaining tests described below (unless otherwise 

specified). The permeability values appeared to be reliable for the middle section of the foam 

buns; however, outside of this region, the variation in permeability can be drastic. 

3.2.2. Smoldering performance - Box test 

The complete set of smoldering data from the box test for all formulations of Iteration 2 at a 

heater-set-point temperature of 320 C, 330 C, 340 C, and 360 C are reported in Table 12. A 

drastic decrease in smoldering was observed in Iteration 2 as compared to Iteration 1, likely due 

to the lower values of foam air permeability. The average value of ML340+360 was 3.3 ± 2.5 % for 

the formulations of Iteration 2 and 7.3 ± 1.5 % for the formulations of Iteration 1 (Table 12). 

Foams from Iteration 2 were less smolder-prone than samples from Iteration 1. The percentage 

of smoldering samples was also lower in Iteration 2 as compared to Iteration 1 for a given 

temperature. Sustained smoldering in foams from Iteration 2 was never observed at 320 °C; at 

330 °C about 35 % of the samples were smoldering; at 340 °C about 39 % were smoldering and 

even at the highest temperature (360 °C) only about 64 % of the samples were smoldering 

35
 

                               

                                   

                           

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(Figure 35). A higher heater set-point temperature would be required to fully investigate the 

dependence of smoldering vs. temperature in Iteration 2. Tests at a higher heater set-point were 

not conducted due to limited foam availability and the smoldering behavior previously observed 

in commercial foams.X 

Table 12. Mass loss for formulation of Iteration 2 measured in the box test at a heater‐set‐point

temperature of 320 °C (ML320), 330 °C (ML330), 340 °C (ML340), 360 °C (ML360) and the average of

the mass loss measured at a temperature of 340 °C and 360 °C (ML340+360).

MASS LOSS (%) 

B2* 

B3* 

B4 

B5 

B6* 

B7 

B8 

B9 

B10 

B11 

B12 

Test 

# 

1 

2 

1 

2 

1 

2 

1 

2 

1 

2 

1 

2 

1 

2 

1 

2 

1 

2 

1 

2 

3 

1 

2 

ML320 

One 

Test 

Avg 

(Dev) 

0.1§ 0.1 

-

0.7§

 0.7 

- -

0.9§ 0.9 

- -

0.9§ 0.9 

- -

0.8§ 0.8 

- -

0.5§ 0.5 

- -

0.6§ 0.6 

- -

0.5§ 0.5

- -

- -

- -

0.7§ 0.7

- -

- -

0.9§ 0.9 

- -

ML330 

One 

Test 

Avg 

(Dev) 

6.7 5.8 

4.8 (1.3) 

2.2§

 1.2 

0.2§

 (1.0) 

5.3 4.4 

3.5 (0.9) 

0.9§

 0.9 

0.9§

 (0.0) 

0.8§ 1.4 

1.9§ (0.6) 

0.5§

 0.6 

0.6§

 (0.1) 

0.9§ 0.8 

0.6§ (0.2) 

 5.6 5.8

5.9 (0.2)

0.7§ 0.7 

0.7§ (0.0) 

 4.2 3.4

2.3§

 (0.7) 

3.6

0.8§ 0.7 

0.6§ (0.1) 

ML340 

One 

Test 

Avg 

(Dev) 

7.0 6.9 

6.8 (0.1) 

2.0§ 1.6

1.1§

 (0.5) 

6.9 6.6 

6.2 (0.4) 

1.3§ 1.1

0.9§

 (0.2) 

1.7§ 2.4 

3.1 (0.7) 

0.6§ 0.7 

0.8§

 (0.1) 

2.0§ 1.6 

1.1§ (0.5) 

 3.6 3.8

 4 (0.2)

0.2§ 0.6 

0.9§ (0.4) 

 7.3 5.3

2.2§

 (2.0) 

 6.3

0.7§ 0.9 

1.0§ (0.2) 

ML360 

One 

Test 

Avg 

(Dev) 

8.3 8.3 

- -

 4.2 4.2

- -

7.2 7.2 

- -

 4.3 4.3

- -

3.9 3.9 

- -

0.9§

 0.9 

- -

1.2§ 1.2 

- -

 7.8 7.8

 - -

1.4§ 1.4 

- -

 5.6 5.6

- -

­

0.8§ 0.8 

- -

ML340+360 

Avg StDev 

7.4 0.8 

 2.4 1.6 

6.8 0.5 

 2.2 1.9 

2.9 1.1 

0.8 0.2 

1.4 0.5 

 5.1 2.3 

0.8 0.6 

 5.4 2.2 

0.8 0.2 

*This formulation is not part of the experimental design. 

†Average mass loss calculated at temperature heater of 340 C and 360 C. 

‡ Average of the absolute deviations of data points from their mean. 

§ Non-smoldering sample. 

X

 Preliminary tests on commercial grade PUF showed that some formulations that did not 

smolder at 360 °C did also not smolder at 400 °C. 

36
                         

                         

                               

 

 

 

 

 

 

 

 

 

 

Figure 35. Percentage of samples showing self‐sustained smoldering in the box test vs. set‐

point‐temperatures of the heater for formulations of Iteration 1 and Iteration 2. The

percentage of smoldering samples is reduced in Iteration 2 as compared to Iteration 1 for a

given temperature.

Figure 36 shows the average mass loss vs. set point temperature of the heater in the box test 

calculated for samples of Iterations 1 and 2. The average mass loss is significantly lower in 

Iteration 2 as compared to Iteration 1 at all temperatures. The plot for Iteration 1 is a sigmoid 

curve with an "S" shape whereas the same plot for Iteration 2 is almost perfectly linear in the 

temperature range investigated. The different trend can be explained by a wider distribution in 

onset temperature: in Iteration 1, self-sustained smoldering was always observed at an heater setpoint temperatures above 330 °C, but in Iteration 2 there were formulations with an onset 

temperature between 320 °C and 330 °C (B2, B4 and B9), formulations with an onset 

temperature above 360 °C (B7, B8, B10, B12), and a formulation (B11) with an onset spanning 

between 320 °C and 360 °C, likely due to foam heterogeneity.XI 

As reported in in Table 10, the formulations with the high air permeability (B2, B4 and B9) 

showed a low onset temperature (between 320 °C and 330 °C), the formulations with 

intermediate air permeability (B3, B5, B11 and B6) showed an intermediate onset temperature 

(between 340 °C and 360 °C), and formulations with a low air permeability (B8, B7, B12 and 

B10) showed a high onset temperature (360 °C and above). These data suggest that the onset 

temperature for sustained smoldering in the box test depends significantly on air permeability. 

This might be related to the increase in oxygen supply generated by buoyant convection when 

the temperature of the heating element increases. 

XI As discussed in Section 3.2.1, the processing conditions for Iteration 2 foams were purposely 

selected on the edges of the processing window. Consequently, they are relatively instable 

formulations with large variations in cell structure and air permeability. 

37
                               

                               

                           

         

 

 

 

 

 

                                   

                                 

  

Figure 36. Average mass loss vs. set point temperature of the heater in the box test calculated

for samples of Iteration 1 and 2. The mass loss is significantly reduced and the onset

temperature increased in Iteration 2 as compared to Iteration 1 (uncertainty bars shown here

are equal to one StDev).

In Iteration 2, as well as in Iteration 1, the average of the mass loss measured at 340 C and 

360 C (ML340+360) is used as an index of smoldering propensity for a given formulation. In 

Iteration 2, this average-mass-loss calculation includes also samples that did not show selfsustained smoldering. It can be argued that only self-sustained smoldering samples should be 

considered, but this does not appear to be feasible because even at higher heater set-point 

temperatures there would likely be formulations that do not show self-sustained smoldering.X 

The ML340+360 values for Iteration 2 formulations are compared in Figure 37. 

Figure 37. Average mass loss at 340 C and 360 C (ML340+360) in the box test for the

formulations of Table 12 (uncertainty bars equal to one StDev and are calculated over at least 3

replicate measurements).

38
 

 

 

 

 

                           

                               

              

 

 

 

3.2.3. Correlation between smoldering in the box test and mockup test 

The smoldering of formulations from Iteration 2 was also investigated by means of the mockup 

test; the results are listed in Table 10 (three replicates). A comparison of the smolder behavior 

measured in the box and the mockup test is shown in Figure 38. Formulations which smoldered 

moderately in the box test (i.e., ML340+360 < 5 %) did not smolder significantly in the mockup test 

and had an air permeability below 45 m·min-1 (red data points). As discussed later in Section 

3.2.7, an air permeability of 45 m·min-1 is a threshold value below which no smoldering is 

observed in the mockup test. The red line in Figure 38 is a least-squares regression fit to the data 

points for formulations with an air permeability above this threshold (blue data points). The low 

value of R2 (R2=0.4) for the linear fit might be due to an intrinsic poor correlation between the 

box test and the mockup test, and/or to an effect of in-bun and bun-to bun variability. The linear 

fit indicates that a mass loss of 30 % in the mockup test (target mass loss for a potential 

PUF/RM) is roughly equivalent to a mass loss in the box test (ML340+360) of 8 %. 

Figure 38. Smoldering measured as mass loss in the box test (ML340+360) vs. smoldering

measured as mass loss in the mockup test (MLmockup) (uncertainty bars shown are equal to one

StDev calculated over at least 3 replicates).

The slope of the linear fit of Figure 38 suggests that the mockup test is preferable for foams 

above the permeability threshold, because in this range it is more sensitive to smoldering 

variations than the box test. Whereas, below this threshold, the box test is a more useful tool for 

determining potential smoldering propensity, because smoldering can be boosted by increasing 

39
 

 

 

 

 

                           

                             

       

 

 

 

 

 

 

 

 

the heater temperature to achieve a significant mass loss even in formulations that are not 

smoldering in the mockup test. 

The relative standard deviation (StDev%) for MLmockup and ML340+360 were calculated for three 

formulations from Iteration 1 (A1, A1-R, A8), nine formulations from Iteration 2 (B4, B5, B6, 

B7, B8, B9, B10, B11, B12) and three formulations for Iteration 3 (C1, C2, C3). For each 

formulation, at least three mockup tests and three box tests were run. The average values of 

StDev% for MLmockup and ML340+360 calculated for the samples of Iteration 1 only, Iteration 2 

only, Iteration 3 only or all samples from Iteration 1 to 3, are shown in Table 15. In all three 

iterations, the StDev% in the box test was lower than in the mockup test. The average values of 

StDev% for all iterations are 34 % for ML340+360 and 51 % for MLmockup. The lower 

reproducibility of the mockup test might be related to variability in the testing materials (e.g., 

barrier fabric, cotton sheeting, smoking material) other than the foam itself. 

The high values of variability observed for these samples (produced in the pilot plant) were 

largely due to heterogeneity in the samples rather than the specific smoldering test used. As 

observed in Iteration 4, the relative standard deviations for MLmockup decreased from 51 % in the 

samples produced in the pilot plant (Iteration 1 to 3 reported in Table 13) to less than 5 % for the 

samples fabricated in the production line. 

Table 13. Average values of relative standard deviation (StDev%) for MLmockup (mockup test) and

ML340+360 (box test) calculated for the samples of Iteration 1, Iteration 2, Iteration 3 and

Iteration 1 to 3.

ML340+360 

(Box Test) 

MLmockup 

(Mockup Test) 

StDev% for samples from Iteration 1 only (%) 37 65 

StDev% for samples from Iteration 2 only (%) 42 59 

StDev% for samples from Iteration 3 only (%) 29 64 

StDev% for samples from Iteration 1 to 3 (%) 34 51 

3.2.4. Effect of the processing parameters on smoldering 

k

Tests conducted with the box and mockup tests per the 2 3 experimental design (Figure 31), 

were analyzed to investigate the effect of processing parameters (i.e., tin catalyst, pressure and 

water level) on smolder propensity. 

For better statistical certainty, such a study would require a larger number of replicates and the 

knowledge of the uncertainty for each of the processing parameters. Practically, this wasn’t 

feasible because the foamer did not provide uncertainties on the processing parameters and the 

number of tests was limited by time and sample availability. In spite of these limitations, from an 

engineering and decision-making perspective, this type of analysis was a necessary tool for 

understanding the critical parameters affecting smoldering and designing the formulations for the 

following iterations. Similar considerations apply also to Sections 3.2.5-3.2.7. 

40
 

 

 

 

 

                                 

  

                                 

   

Data analysis was performed using a software for data mining (FusionPro by S-Matrix) and a 

cubic regression model to generate 2-D contour plots. The effect of each processing parameter 

on the response parameter (smoldering performance) was evaluated using a Pareto ranking.23 

The values of ML340+360 and MLmockup were designed as the responses parameters for the box and 

mockup tests, respectively. The Pareto ranking for the box test data indicated that, in the 

investigated range, pressure was the parameter with the highest impact on ML340+360 and that tin 

and water had a smaller effect. According to this model, an increase in water level, or a decrease 

in head pressure and tin content caused an increase in ML340+360. A 2-D contour plot for 

ML340+360 vs. tin content and pressure at a low and high water level details these observations in 

Figure 39 and Figure 40. 

Figure 39. 2‐D contour plot for ML340+360 vs. tin content and pressure at a low water level

(2.70 php).

Figure 40. 2‐D contour plot for ML340+360 vs. tin content and pressure at a high water level

(2.95 php).

41
 

                                  

   

 

Similar analyses of the mockup data reveal an analogous effect of tin and pressure on smoldering 

(i.e., smoldering increases with a decrease in tin catalyst and pressure), and pressure is the 

parameter with the highest impact on MLmockup (Figure 41 and 

Figure 42). The water level, however, was had the opposite effect in each test; in the box test, 

smoldering increased with an increase in water content, whereas, in the mockup test, smoldering 

decreased with an increase in water content. Supposing that these findings are not biased by data 

uncertainty, the two tests were affected to a different extent by morphological and processing 

parameters: smoldering in the mockup test appeared to be more sensitive to a decrease in specific 

surface area (due to an increase in cell size at high water content), whereas, smoldering in the 

box test appeared to be more sensitive to an increase in oxygen supply (due to an increase in air 

permeability at high water content). This is supported by the limiting action on buoyant 

convection (i.e., oxygen supply) generated by the presence of a box in the box test. 

Figure 41. 2‐D contour plot for MLmockup vs. tin content and pressure at a low water level

(2.70 php).

Figure 42. 2-D contour plot for MLmockup vs. tin content and pressure at a high water level 

(2.95 php). 

42
 

 

 

 

                                 

                             

                     

3.2.5. Effect of the processing parameters on air permeability 

The effect of processing parameters on air permeability was investigated with a 2k3 experimental 

design (Figure 31). The values of permeability were designed as the response parameter (Table 

10) in this analysis. 

A cubic regression model was used to generate 2-D contour plots. The Pareto ranking data 

indicated that, in the investigated range, tin had a major impact on permeability, and that 

pressure and water had a lower effect. According to this model, 2-D contour plots were 

generated for air permeability versus tin and pressure at low water level (Figure 43) and high 

water level (Figure 44). 

The data presented in Figure 43 shows that at the low water level of 2.70 php, air permeability 

was more sensitive to the tin variation rather than the pressure variation. An increase in head 

pressure or tin content generated a decrease in air permeability. 

At a higher water level, 2.95 php, the effect of pressure and tin content on permeability was 

comparable (Figure 44). An increase in head pressure or an increase in tin content generated a 

decrease in air permeability. 

The comparison between the values of permeability at low water level (Figure 43) and high 

water level (Figure 44) shows that an increase in water content also promoted an increase in air 

permeability. 

Figure 43. 2‐D contour plot for permeability vs. tin content and pressure at a low water level

(2.70 php). Permeability is slightly more sensitive to the tin variation rather than the pressure

variation in the range investigated here and at this water level.

43
                                 

                           

             

 

 

 

 

 

 

 

Figure 44. 2‐D contour plot for permeability vs. tin content and pressure at a high water level

(2.95 php). Permeability is equally affected by tin and pressure variations in the range

investigated here and at this water level.

3.2.6. Image analysis and cell size determination 

The cell size distribution was measured for all formulations used in the 2k3 experimental design. 

First, an optical slice with an area of (4.757 x 4.757) mm2

 and a thickness of 197.5 m was 

acquired for each formulation by a confocal microscope (Zeiss LSM 510). The images were then 

processed by an ImageJ plug-in24 capable to directly segment a gray-level image using a 

watershed algorithm, also known as Euclidian distance map (EDM).25 A Gaussian filter blurring 

with a diameter of 20 pixels was applied to remove noise and prevent over-segmentation before 

applying the EDM algorithm. Finally, the ImageJ macro “analyze particle” was used for 

calculating the area of each segmented particle (i.e., cell) that is not on the edge of the image and 

has a combination of area above 0.02 mm2 and circularityXII above 0.4 (to remove small artifacts 

and minimize over-segmentation). 

As an example, the entire image analysis process for formulation B9 foam is shown in Figure 45. 

Even though in some cases under or over-segmentation was observed, in general the calculated 

average values of area per cell (cell area) appeared to be a reliable indicator for cell size. The 

typical average-cell-area repeatability for this methodology was estimated by calculating the 

standard deviation over five replicate measurements of  in a specific foam bun location. The 

relative standard deviation for  was 10 %. Inhomogeneity in the foam is likely to introduce a 

more substantial uncertainty. Multiple measurements of  would be necessary throughout 

different buns to account for in-bun and bun-to-bun variability. This was unrealistic at this stage, 

due to the large number of samples, but it was accomplished in Iteration 4 for those formulations 

that appeared to be potential SRM/PUF. 

XII Circularity of a particle is defined as 4(A/P2), where A and P are the area and the perimeter 

of the particle, respectively. 

44
                                     

                                     

                         

                               

         

 

 

 

 

 

a) b) 

c) d) 

Figure 45. Image analysis and cell area calculation for a B9 foam: a) an optical slice with an area

of (4.757 x 4.757) mm2 and a thickness of 197.5 m is acquired by a confocal microscope; b) a

Gaussian filter is applied to remove noise and prevent over‐segmentation; c) segmentation by

Euclidian distance map calculation; d) each segmented area that is not on the edge of the

image is labeled and measured.

Figure 46 shows the area-per-cell distribution (cell area distribution) and cumulative percentage 

of the distributions for the same B9 foam. Similar image analyses, cell area distribution and 

cumulative percentage were obtained for all other formulations (See Appendix 2). 

The data are summarized in terms of mean and median values in Figure 47. The mean values of 

cell area () are used for ranking the eight formulations in terms of cell size. The cell size 

ranking is as follows (10 % uncertainty): B12  B9  B10  B11 < B4  B7 < B8  B5, ranging 

from (0.24 ± 0.02) mm2 for formulation B12 to (0.60 ± 0.06) mm2 for formulation B5. 

45
                     

                               

 

 

 

Figure 46. Cell area distribution and cumulative percentage for formulation B9.

Figure 47. Mean and median values of cell area calculated for all foams of the 2k3 experimental

design.

It is reasonable to deduce that  can affect smoldering because as  increases, the specific 

surface (SSA) (area per unit volume of foam) available for oxidation decreases. For example, 

formulations B5 and B11 have a similar air permeability of about 44 m·min-1 but B11<B5, 

therefore SSAB11>SSAB5, demonstrating the expected result that B11foam is more smolder prone 

than B5 when tested with both the box and mockup tests (Table 10). The effect of air 

permeability and SSA will be discussed further in the following sections. 

46
 

 

 

 

 

                                   

      

 

 

 

 

 

 

 

 

 

3.2.7. Effect of the processing parameters on cell size 

The effect of processing parameters on cell size was investigated in terms of cell area with a 2k

3 

experimental design (Figure 31). The mean area per cell, , is specified as the response 

parameter (Figure 47). A cubic regression model was used to fit the data and generate 2-D 

contour plots for cell size versus pressure and tin content at low and high water content. The 

effect of each processing parameter on cell size was evaluated using a Pareto ranking. 

According to this model, pressure has the highest effect on cell size, followed by tin, whereas the 

effect of water is marginal. The 2-D contour plot in Figure 48 details the relationship of  vs. tin 

content and pressure at a low water level (2.70 php). At this water level, the cell size was 

dominated by the mixing head pressure; the cell size increased with increase in head pressure. 

(mm2) 

Figure 48. 2‐D contour plot for average cell area  vs. tin content and pressure at a low water

level (2.70 php).

Examining the 2-D contour plot in Figure 49 for cell size () vs. tin content and pressure at a 

high water level (2.95 php) revealed that an increase in head pressure induced an increase in cell 

size for relatively small values of tin content, but not for high values of tin content. An increase 

in tin content induced a decrease in cell size. 

A comparison of the values of air permeability at a low (Figure 48) and high water level (Figure 

49), shows that an increase in water content promoted a small increase in cell size. 

These trends can be explained as follows. Foaming in PUF is dominated by the expansion and 

coalescence of pre-existing air bubbles produced during mixing.26 The higher the mixing head 

pressure, the more rapid the expansion of these pre-existing air inclusions at the exit from the 

mixing chamber due to the increase in differential pressure between the pressure inside the cell 

and the atmospheric pressure. At a later stage during foaming, water also increases the rate of 

expansion by generating CO2 that diffuses inside a cell and increases the differential pressure. 

The tin catalyst can partially prevent cell expansion by promoting a fast crosslinking and it is 

reasonable to speculate that this effect is more relevant when the water blowing action is high 

47
                                     

      

 

 

 

 

 

 

 

 

 

(i.e., high water level). This might explain why the tin catalyst content has a significant effect 

only at a high water level. 

(mm2) 

Figure 49. 2‐D contour plot for average cell area  vs. tin content and pressure at a high water

level (2.95 php).

3.2.8. Effect of the air permeability on smoldering 

In this section, the effect of air permeability on smoldering is investigated using all available data 

from Iteration 1 and Iteration 2. These data include the foamer air permeability measurement 

data and smoldering data from both the box and the mockup tests. 

The mass loss in the box test is expressed as the average mass loss measured at a heater-setpoint-temperature of 340 C and 360 C (ML340+360). The values of ML340+360 vs. air permeability 

are plotted in Figure 50 (three replicate test per data point). Despite a large data scattering it 

appeared that ML340+360 increases generally linearly with air permeability. An increase in 

smoldering with air permeability was expected because a higher air permeability induces a larger 

oxygen supply at the air-foam interface. 

Smolder propensity in the mockup test also increases with air permeability (Figure 51) (three 

replicate test per data point), but in this case, there is a threshold value of air permeability at 

about 45 m·min-1, below which no smoldering is observed. In this evaluation, a MLmockup greater 

than 25 % is considered the minimum level for denying a foam as sufficiently smolder prone. 

Only three formulations resulted in MLmockup above 25 % and have air permeabilities above 

70 m·min-1. 

The presence of an outlier in Figure 51 (MLmockup  8 % and permeability  75 m·min-1) implies 

that a permeability above 70 m·min-1 is a necessary but not sufficient specification by itself to 

achieve a MLmockup of 25 %. In this high range of air permeability, a better morphological 

description of the foam structure is required to determine other variables that are impacting 

smolder performance. This investigation is carried out in Iteration 3, which targets formulations 

with air permeability above 70 m·min-1. 

48
                               

                             

                     

                               

                             

                           

                               

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120

ML340+360 (%)

Air permeability (m∙min‐1)

Figure 50. ML340+360 (box test) vs. air permeability for all available data from Iteration 1 and

Iteration 2 (uncertainty bars equal to the StDEv calculated over at least three replicates). The

black line is a least‐squares regression fit to the data points.

0

10

20

30

40

50

0 10 20 30 40 50 60 70 80 90

MLmockup (%)

Air permeability (m∙min‐1)

Above Permeability Threshold

Below Permeability Threshold

Figure 51. MLmockup (mockup test) vs. air permeability for all available data from Iteration 1 and

Iteration 2. No smoldering is observed in the mockup test below a threshold value of

permeability (uncertainty bars equal to the StDEv calculated over at least three replicates). The

red line and black line are least‐squares regression fits to the red and blue data points,

respectively.

49
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3.2.8 Conclusions of Iteration 2 

At the end of Iteration 1, polyol P2 was chosen for continued study. Initial foam buns 

demonstrated that the variations in manufacturing conditions (specifically, a decrease in 

temperature and humidity in the pilot plant) caused splitting of the foam. Subsequent foams were 

made by replacing polyol P2 with polyol P3 and adding a processing aid to prevent foam 

splitting. The influence of the processing parameters (i.e., water content, catalyst content and 

head pressure) on smoldering, air permeability and cell size was investigated for a given 

formulation based on polyol P2 and surfactant S3. 

A series of preliminary experiments was performed to identify the processing window, i.e., the 

maximum range of variability for water content, tin catalyst content and head pressure that 

ensured successful foaming. A 2k

3 factorial experimental design (two levels for three factors) 

that examined a high and low level of water, tin catalyst and mixing head pressure was chosen in 

order to produce flexible foam samples within the specified processing window. 

Within the limits of this analysis (unknown uncertainty in the processing parameters, limited 

number of replicate tests and variability in the foam properties), which are considered acceptable 

at this stage for engineering and experimental design of the following iterations, the effects of the 

processing parameters can be summarized as follows: 

1. Smoldering behavior: 

 an increase in head pressure or tin content caused a decrease in smoldering; 

 an increase in water content caused an increase in smoldering in the box test and a 

decrease in the mockup test. 

2. Air permeability: 

 an increase in head pressure or tin content induced a decrease in permeability;

 an increase in water content induced an increase in permeability.

3. Cell size: 

 at a low water level, the mixing head pressure controlled the cell size and the cell size 

increased with the head pressure; at a high water level, the cell size increased with the 

head pressure and decreased with the tin catalyst content; 

 an increase in water content induced a marginal increase in cell size. 

Smoldering was measured in both the box test and the mockup test. In the box test, smoldering 

propensity increased with air permeability both in terms of mass loss and onset temperature for 

sustained smoldering (the higher the air permeability the lower the onset temperature). 

In the mockup test, no smoldering occurred below an air permeability threshold of about 

45 m·min-1. An air permeability value above 70 m·min-1 was a required but not necessarily 

sufficient specification to achieve a mass loss of 25 % in the mockup test. In this high range of 

air permeability at least one other variable was impacting smoldering. Thus, a better 

morphological description of the foam structure was required. This investigation was carried out 

in Iteration 3, targeting formulations with an air permeability above 70 m·min-1. 

50
 

 

 

 

 

 

 

 

 

 

 

 

 

 

For foams with an air permeability above 45 m·min-1, it appears to be a poor correlation 

(R2 = 0.4) between the box test and the mockup test when a least-squares regression fit is used. 

The low value of R2 might be due to an intrinsic poor correlation between the box test and the 

mockup test, and/or to an effect of in-bun and bun-to bun variability. In a range of permeability 

above 45 m·min-1, the mockup test appeared to be preferable to the box test due to its higher 

sensitivity to smoldering variations and a testing configuration that closely mimics a typical 

smoldering scenario. The box test has proved to be a versatile research tool that can differentiate 

smoldering propensity also in low permeability foams, provide additional information (onset 

temperature, temperature profile, power to sustain smoldering), and provide higher repeatability 

compared to the mockup test. 

3.3 Iteration 3: Impact of cell morphology in open-cell PUF 

The previous iterations examined the effect of chemistry and processing parameters on the 

smolder behavior of PUF foams. The third iteration examined the impact of cell morphology on 

smolder behavior. Data from iteration 2 demonstrated that an air permeability above 70 m·min-1 

is required for smoldering to occur but an outlier suggests that this is not the only specification 

required to produce a high smolder prone foam. Iteration 3 targeted PUF that have air 

permeability above this range to gain a better understanding of the morphological effect of the 

foam structure on smoldering prone foams. This high value of air permeability can be achieved 

only in foams with an open cell structure. 

Iteration 2 showed that the highest air permeability and smoldering (mockup test) in the 2k3 

experimental design was achieved at a high water level, low tin catalyst level and low pressure 

level (formulation B4). This is why, in Iteration 3, formulation B4 (P3-S3) was selected as 

potential SRM/PUF. The three formulations foamed for Iteration 3 are reported in Table 14. 

Formulation C1 (16 replicate buns) is the replicate of B4. Formulation C2 (14 replicate buns) is 

identical to C1, but it is foamed at a higher mixing-head pressure because it was found (see 

Iteration 2) that the head pressure is the main processing parameter controlling smoldering. 

Formulation C3 (12 replicate buns) is identical to C2 but does not contain the processing aid that 

was necessary due to the relatively cool temperatures registered during Iteration 2. For each 

formulation, fourteen replicate buns were foamed in Iteration 3. 

Table 14 also reports the average and standard deviation for density and air permeability data for 

the formulations of Iteration 3 (at least 12 replicates). 

In terms of density, the differences between C1 and C2, C1 and C3 are statistically significant (ttest for unpaired data)27. Similarly, in terms of permeability, the differences between C1 and C2, 

and C2 and C3 

It appears that an increase in head pressure in C2 and C3 as compared to C1 induces a decrease 

in the average values of density. Also the addition of the processing aid in C2 might slightly 

decrease its air permeability as compared to C3, where the processing aid was not used. 

51
                       

                         

 

 

 

 

 

 

                               

                     

 

 

 

 

 

Table 14. Composition and processing parameters for the formulations of Iteration 3

(unceratinties are equal to one standard deviation calculated over at least 12 replicates).

 Formulation 

Component 

Polyol P3 

Processing aid 

H2O 

Polyether catalyst 

Amine catalyst 

Tin catalyst 

Surfactant S3 

TDI 

Mixing head 

pressure 

Unit C1 C2 C3 

php 100 100 100 

php 0.15 0.15 0 

php 2.95 2.95 2.95 

php 0.06 0.06 0.06 

php 0.06 0.06 0.06 

php 0.16 0.15 0.15 

php 1 1 1 

php 39.09 39.09 39.09 

kPa 34.5 48.3 48.3

kg·m-3 

m·min-1 

 Properties 

30.9±0.9 30.1±0.8

78.4±4.6 70.9±10.7 

 29.9±0.6 

78.5±6.9 

Density 

Permeability 

3.3.1. Smoldering vs. air permeability 

All of the Iteration 3 foams were tested in the box test at multiple set point temperatures, the data 

are reported in Table 15. The average smoldering behavior measured in the box test, ML340+360, 

for all formulations of Iteration 3 is (5.9 ± 3.5) % and it is comparable to the average ML340+360 

measured in iteration 1 (7.3 ± 1.5) %. Sustained smoldering was never observed for C2, even at a 

heater temperature of 360 °C and the value of ML340+360 was significantly lower as compared to 

C1 and C3 (Figure 52). 

Table 15. Mass loss measured in the box test for formulation of Iteration 3 at a heater‐set‐

point‐temperature of 320 °C, 330 °C, 340 °C, and 360 °C.

MASS LOSS (%) 

C1 

C2 

C3 

320 C 330 C 340 C 360 C 340 C+360 C† 

Single AVG‡ Single AVG‡ Single AVG‡ Single AVG‡ 

Test (Dev) Test (Dev) Test (Dev) Test (Dev) AVG StDev 

1.0§ 1.0 8.1 6.7 10.1 9.0 9.1 9.1 9.1 1.1 

- - 5.3 1.4 7.9 1.1 - ­

0.9§ 0.9 1.0§ 0.8 1.5§ 1.3 3.9§,* 3.9 2.2 1.5 

- - 0.5§ 0.3 1.1§ 0.2 - -

0.9§

 0.9 0.8§ 1.0 6.2 6.3 7.0 7.0 6.5 0.4 

- - 1.1§ 0.2 6.5 0.2 - ­

†

Average mass loss calculated at 340 C and 360 C. ‡Average (AVG) and deviation (Dev) 

calculated at a given target temperature. §Non-smoldering sample. *This sample showed an 

anomalous behavior: it did not show self-sustained smoldering even if ML340+360 was above 2 %. 

52
 

                                   

                           

            

                           

                               

                           

            

The values of ML340+360 vs. air permeability are plotted for all available data from Iteration 1, 

Iteration 2 and Iteration 3 in Figure 53. The formulations of Iteration 3 had similar air 

permeability but dramatically different smoldering performance. This indicates that at least for 

foams with high permeability there is another parameter controlling smoldering. A similar 

conclusion can be drawn by looking at the smoldering data in the mockup test (Table 16). 

Figure 52. Average mass loss at 340 C and 360 C (ML340+360) in the box test for the

formulations of Iteration 3 (uncertainty bars equal to one standard deviation and are calculated

over at least 3 replicate measurements).

C2

Figure 53. ML340+360 (box test) vs. air permeability for all available data from Iteration 1,

Iteration 2 (blue data points), and Iteration 3 (red data points). The black line is a least‐squares

regression fit to the blue data points (uncertainty bars equal to one standard deviation

calculated over at least three replicates).

53
                           

 

 

 

 

 

 

 

 

                             

                             

                         

                       

                         

             

         

           

           

Table 16. Mass loss measured in the mockup test for formulations of Iteration 2.

C1 C2 C3 

MLmockup (%) 21.5±10.6† 10.0±2.5† 1.2±0.3†

†Uncertainty is equal to one standard deviation calculated over at least three replicates. 

The values of MLmockup as a function of air permeability are plotted for all available data from 

Iteration 1, Iteration 2, and Iteration 3 in Figure 54. Formulation C1 showed the highest mass 

loss in the box test (ML340+360) and mockup test (MLmockup), however, C3 had a higher mass loss 

than C2 in the box test and, vice versa, C2 had a higher mass loss than C3 in the mockup test. 

The differences in mass loss measured between C2 and C3 are significant. These results indicate 

that the box test cannot be used as a robust predictor for the mass loss in the mockup test 

(additional testing is required to avoid possible bias due to foam variability). The presence of a 

wood box in the box test might dramatically decrease buoyant convection and oxygen supply, 

and thus affect smoldering, especially in foams with high air permeability. 

0

10

20

30

40

50

0 10 20 30 40 50 60 70 80 90

MLmockup (%)

Air Flow by the foamer (m∙min‐1)

Iteration 1 & 2 (Permeability above Threshold)

Iteration 1 & 2 (Permeability below Threshold)

Iteration 3

C1

C3

C2

Figure 54. MLmockup (mockup test) vs. air permeability for all available data from Iteration 1,

Iteration 2 and Iteration 3. The formulations of Iteration 3 had similar air permeability but

dramatically different smoldering. This is indicating that at least for foams with high

permeability there is another parameter controlling smoldering (uncertainty bars equal to one

standard deviation calculated over at least three replicates). The red line is a least‐squares

regression fit to the red data points.

54
 

 

 

                             

                           

                           

                           

 

 

 

 

 

The values of MLmockup shown in Figure 54 are each an average value measured for a given 

formulation. Formulation C1 was selected to investigate the effect of bun-to-bun variability on 

smoldering behavior because it had the highest average MLmockup of Iteration 3. Sixteen different 

foams were prepared for formulation C1 (see Appendix 1) and characterized. Figure 55 shows 

the MLmockup for formulation C1 measured on several buns with different air permeability. In this 

high range of permeability, smoldering appeared to increase when air permeability decreased. 

This was an unexpected result, contrary to the common belief that smoldering always increases 

with air permeability.28 

Figure 55. MLmockup (mockup test) as a function of air permeability for formulation C1 measured

for several foam buns. In this high range of permeability smoldering increases when air

permeability decreases. The red line is a least‐squares regression fit to the data points

(uncertainty bars are equal to one standard deviation calculated over at least three replicates).

Reviewing the data in Figure 54 and Figure 55, infers that:

 to achieve a MLmockup greater than 2 %, an air permeability () above a threshold value of

about 45 m·min-1 (threshold) is a necessary but not sufficient specification, 

 for > threshold, there is no clear correlation between air permeability and MLmockup 

 at high air permeability (> 70 m·min-1), MLmockup increases with a decrease in  for 

formulation C1. 

To understand this phenomenon a morphological study of the foams from Iteration 3 was 

conducted in the following sections. 

55
 

 

 

 

 

                             

3.3.2. Effect of Specific Surface Area on Smoldering 

The data shown above emphasize the need for a better morphological description of the foam 

structure. In this section, the effect of specific surface area on smoldering in the mockup test is 

investigated. The specific surface area (SSA) on smoldering is defined here as surface per unit 

volume and is expressed in inverse meters. The total surface of PUF was calculated by density 

and BET measurements (see Section 2.5).29 

The calculated values of SSA for all formulations of Iteration 3 and some formulations of 

Iteration 1 and 2 are reported in Table 17 together with air permeability () and smoldering data 

(MLmockup). Each value of SSA was measured in a specific foam bun location. Two different 

foam samples, collected from the same bun and location, were used for replicate measurements 

in C1-3S and C1-5S. The maximum absolute deviation for SSA was about 15 % from the mean 

value. Since the typical uncertainty for BET measurements is below 1 %, significant 

heterogeneities existed in terms of SSA even in nearly the same foam bun location.30 

Table 17. Values of specific surface area (StDEv and AVG calculated over at least three

replicates).

Air permeability* 



(m·min-1) 

Specific surface area 

SSA 

(m-1) 

MLMockup 

(%) 

Avg StDev Avg StDev 

C1-3S 79 3839 19 24.6 3.4 

C1-3S 79 4898 18 24.3 0.2 

C1-4S 75 3693 15 25.5 5.4 

C1-5S 83 4994 58 26.9 9 

C1-5S 83 3316 3 26.9 8.6 

C1-6S 74 4132 16 31.8 2.8 

C1-11S 79 3880 16 38.4 15.5 

C1-11E 79 3264 16 24.8 2.7 

C1-12S 79 3105 19 9.1 3.7 

C1-14S 90 2928 15 7.1 5.2 

C1-14E 90 3320 6 3.4 0.6 

C1-15S 85 3496 28 14.9 10.2 

C2-6S 75 2677 15 11.8 13.2 

C2-7S 83 3316 24 8.2 6.1 

C3-9S 90 2783 9 1 0.1 

C3-8S 85 3339 9 1.6 1.9 

C3-12S 75 3144 18 1.1 0.5 

A1-1S 75 4543 81 30.5 12.1 

B5-2E 42 3844 28 0.2 0.1 

B11-2M 46 5720 188 1.6 0.9 

B12-3S 1 5173 270 0.3 0.3 

B4-4E 70 4742 31 6.4 4.9 

56
 

 

 

 

 

 

 

                             

                               

                           

                                       

               

 

     

     

     

     

*These values of air permeability were measured by the foamer in the middle section of the bun 

and do not take into account possible in-bun variations. 

The relative standard deviation calculated for SSA over all C1 samples of Table 17 (12 samples), 

affected by both bun-to-bun and in-bun variability, was about 20 %. This value of standard 

deviation was also assumed for formulations A1, C2 and C3 since the available data for SSA 

were not sufficient to calculate a standard deviation. Similarly, for MLmockup the standard 

deviation calculated for C1, 50 % was taken as a typical standard deviation for the other 

formulations. 

The value of MLmockup is plotted as a function of SSA in Figure 56. There are three different 

datasets: 

1. data for a specific foam bun and location with high air permeability ( > 70 m·min-1); 

2. data for a specific foam bun and location with low air permeability ( ≤ 70 m·min-1); 

3. average data for a specific formulation with high air permeability ( > 70 m·min-1). 

Dataset 1 shows that MLmockup increased with SSA even though there was a large scatter in the 

data, likely due to variability in foam properties. The averaged dataset 3 showed a similar trend 

with less scatter. Dataset 2 did not show any correlation with MLmockup. 

0

10

20

30

40

50

60

1000 2000 3000 4000 5000 6000 7000

MLmockup (%)

Specific surface area (m‐1)

A1

C2

C3

Specific foam location (>70min‐1)

Specific foam location (<70min‐1)

Formulation average (>70 min‐1)

C1

Figure 56. MLMockup vs. specific surface area. Uncertainty bars are equal to one StDev and

calculated from at least 3 replicate measurements for datasets 1 (blue dots) and 2 (green dots);

for dataset 3 (black dots), as previously described, the standard deviation were assumed equal

to 20 % and 30 % of the mean value of SSA and MLmockup, respectively. The red line is a least‐

squares regression fit to the black data points.

The highest value of specific surface area was observed for B11-2M (SSA = 5720 m-1). 

However, for sample B11-2M, as well as samples B5-2E, B12-3S and B4-4E (dataset 2) the air 

57
 

 

 

 

 

 

 

 

 

                             

                               

                         

                           

                     

permeability was relatively low, i.e.,  ≤ 70 m·min-1. For these foams, the fraction of closed cell 

membranes was likely higher than the one for foams with  > 70 m·min-1, and a large fraction of 

SSA was generated by residual membranes in the cells. In fact, a BET measurement accounts for 

the surface generated by the struts and the cell membranes. In particular, a BET measurement 

accounts also for the surface of the cell membrane in a closed cell foam because krypton gas will 

diffuse over time into a closed cell.30 Similarly, oxygen can diffuse through a cell membrane and 

sustain smoldering, but the oxygen supply rate will be lower in a closed cell as compared to an 

open cell, where oxygen transport is dominated by convection rather than diffusion. 

Furthermore, residual closed membranes in open cells can dramatically reduce oxygen supply by 

suppressing air convection. This is likely why formulations with high SSA but low  (e.g., B11­

2M) showed limited smoldering. 

The importance of  is confirmed in Figure 57 showing the values of MLmockup as a function of 

specific surface area for formulation C1 measured on several buns. In this high range of 

permeability, the number of residual cell membranes was minimal and smoldering increased as 

the specific surface area increased. Noticeably, data of MLmockup as a function of air permeability 

for the same Iteration 3 foam buns (Figure 55), showed that smoldering increased as air 

permeability decreased. 

Figure 57. MLmockup vs. specific surface area for formulation C1 measured on several buns. The

red line is a least‐squares regression fit to the data points. In this high range of permeability,

smoldering increased when the specific surface area increased (uncertainty bars equal to one

standard deviation calculated over at least three replicates). The data of MLmockup vs. air

permeability for the same foam buns are shown in Figure 55.

58
 

 

 

 

 

 

 

 

 

                         

                             

                               

           

 

 

In general, an increase in air permeability is promoted by an increase in fraction of open 

windows and an increase in cell size. In completely open-cell foams (i.e., no residual closed 

membrane), a further increase in permeability can be achieved by increasing the cell size, for 

example, by increasing the mixing head pressure. This effect is dominant in a high permeability 

range where most of the cells are open. 

This principle is illustrated in the schematic drawing of 

Figure 58. Either increasing the fraction of open membranes (case a) or increasing the cell size 

(case b) induces an increase in air permeability and a decrease in SSA. However, smoldering 

increases for case a, due to an increase in oxygen supply, and decreases for case b, due to a 

reduction in SSA. For a given formulation, the morphology of the foam that maximizes 

smoldering is characterized by a fine and largely open cell structure with a high value of air 

permeability (i.e.,  > 70 m·min-1). A high value of  is necessary to promote high oxygen 

supply through convective movements. In this high range of permeability a decrease in cell size 

promotes smoldering by increasing the amount of air/foam interface available for oxidation. 

Figure 58. Schematic drawing illustrating two possible mechanisms promoting an increase in air

permeability (): a) increase in fraction of open membranes; b) increase in cell size. Both

mechanisms induce also a decrease in SSA but only a) promotes an increase in smoldering due

to an increase in oxygen supply.

In general, in a typical PUF, where both open and closed cells coexist, there is no clear 

correlation between SSA and smoldering. In practice, this implies that SSA is a good 

59
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

morphological descriptor for smoldering propensity only in fully open PUF, like reticulated 

PUFXIII . 

3.3.3. Image analysis and cell size determination 

As discussed in the previous section, the specific surface area appears to be a good 

morphological descriptor for smoldering propensity only in fully open-cell PUF. In addition, 

BET measurements (necessary for SSA calculations) are extremely time-consuming and it is 

difficult to perform more than 2-3 measurements per day. Due to the bun-to-bun and in-bun 

variability, multiple measurements are necessary for a statistically sound approach. In practice, 

this is unrealistic. 

In this section, cell size is considered as alternative morphological descriptor for smoldering 

propensity. Noticeably, cell size is independent on the fraction of closed cells. The average cell 

structure is measured in terms of cross-sectional cell area by means of image analysis; then, the 

effect of cell size on smoldering is discussed. The assumption, here, is that average cell area is 

related to the SSA as illustrated in 

Figure 58 (case b), and a detailed study of cell size distribution is not required. 

The average cell area of C1, C2, and C3 formulations was measured with an approach similar to 

the one described in Iteration 2. However, in this case, the scanned area for each image was 

larger (142 mm2 instead than 50.4 mm2

) and for each foam bun, three confocal images were 

acquired from orthogonal planes to account for possible anisotropy in the foam: 

 image 1 (top view) is a confocal image acquired from the top surface of the bun after 

removing the foam skin; 

 image 2 (side view 1) and image 3 (side view 2) are confocal images acquired from the 

two planes which are orthogonal to the top surface plane and to each other. 

Even though in some cases an under/over-segmentation was observed, in general, the calculated 

average values of cell area appeared to be a reliable indicator for cell size. Inhomogeneity in the 

foam likely introduced a substantial variability in smoldering behavior. 

A complete cell-size analysis (including scan time and image analysis) required about an hour, 

rather than the several hours required for BET and density measurements. 

The buns selected for this study were C1-6MXIV and C1-14M (investigating the variation of cell 

structure in the same formulation), plus C2-3M and C3-4M (investigating the effect of mixing 

pressure and processing-aid). The confocal images from top view and side views with the 

relative image analysis and cell-area-distribution histograms for the four buns are reported in 

Appendix 2. 

XIII A reticulated foam is a foam that is post-processed to remove all residual membranes by

chemical etching or thermal treatment. XIV M indicates that the foam sample was collected from the middle section of the bun, see 

Section Formulation, bun and location identification.

60
 

 

 

 

 

 

                   

 

 

 

 

 

 

 

 

The typical average-cell-area uncertainty for this methodology was estimated by calculating the 

standard deviation over five replicate measurements of  in a specific foam bun location 

(formulation C1, sample size of approximately 125 cm3). The relative standard deviation for 

was 4 % in a specific foam location but, for a formulation in general, it can be significantly 

higher due to in-bun and bun-to-bun variability. 

The confocal micrograph images in Appendix 2 indicate that in this range of air permeability all 

foams have similar open-cell structures, but there are obvious variations in cell size. 

The following parameters are calculated for all foams and summarized in Table 18: 

 TOP: average cell area measured from top view; 

 Side1: average cell area measured from side 1 view; 

 Side2: average cell area measured from side 2 view; 

 Side: average of Side1 and Side2; 

 : average of Side1, Side2 and TOP. 

Table 18. Average cell area calculated from each view point.

Average cell area (mm2) 

TOP† Side1‡ Side2‡ Side* § 

C1-6M 0.21 0.26 0.25 0.26 0.24 

C1-14M 0.42 0.42 0.48 0.45 0.44 

C2-3M 0.34 0.31 0.43 0.37 0.36 

C3-4M 0.40 0.47 0.37 0.42 0.41 

†

Average cell area measured from top view. 

‡

Average cell area measured from side view 1 or side view 2. *

Average of Side1 and Side2. §

Average of TOP, Side1 and Side2. 

For a given bun, generally the average value of cell area is larger in the side view (Side) than in 

the top view (TOP) due to foam anisotropy. The average cell area , calculated as the average of 

Side1, Side2 and TOP, was used for cell size quantification. 

The cell size ranking is as follows: C1-6M <C2-3M <C3-4M < C1-14M. The cell area per cell varied 

from a minimum value of 0.24 mm2 for C1-6M to a maximum of 0.44 mm2 for C1-14M. The 

minimum and maximum values of  were observed for the same formulation (C1), which 

highlights the poor repeatability and large bun-to-bun variability in these formulations produced 

in the pilot plant. 

The effect of cell size on mass loss in the mockup test was evaluated with data (three replicates) 

from foam buns C1-6 and C1-14 of formulations C1 (subject to in-bun variability), and all data 

for formulations C2 and C3 in Figure 73 (subject to bun-to-bun and in-bun variability).XV The 

XV An average value of MLmockup for formulations C2 and C3 is reported because less than three 

replicate mockup tests were available for the specific foam buns C2-3 and C3-4. 

61
 

 

                                   

                         

 

 

 

 

 

 

 

 

 

 

 

   

standard deviation shown here for  is assumed to be equal to 4 % and it does not consider inbun and bun-to-bun variability. 

0

5

10

15

20

25

30

35

40

0.2 0.25 0.3 0.35 0.4 0.45 0.5

MLmockup (%)

Cell Area (mm2)

C1‐6

C3

C2 C1‐14

Figure 59. Effect of cell size () on smoldering in the mockup test (uncertainty bars equal to one

StDev). The black line is a least‐squares regression fit to the data points.

There is a clear effect of  on MLmockup: the larger the cell size, the lower the smoldering mass 

loss. The linear fit of Figure 59 was used to estimate the value of  generating a MLmockup of 

30 % (  0.239 mm2

) as a potential standard reference material foam (SRM/PUF). If a variation 

of ±15 % in smoldering mass loss is acceptable for a SRM/PUF then the corresponding 

acceptable variation in , calculated by using the linear fit of Figure 59, is approximately ±13 % 

(i.e., =0.239±0.031). 

At this stage, the aforementioned correlation between cell area and MLmockup requires further 

validation due to unknown in-bun and bun-to-bun variability. Given the large variability in the 

foams produced in the pilot plant, it was decided to postpone the validation of these data in 

Iteration 4, while foams with much more consistent properties were prepared on a production 

line. 

3.3.4. Open-flame Tests (bare foam) 

The open-flame resistance of bare foams was measured according to the open-flame resistance 

test described in the CPSC proposed standard.15 Three formulations from Iteration 3 (C1, C2, 

C3) and one formulation from Iteration 2 (B5) were tested. The tests qualify a foam’s standalone flaming performance in a bare foam configuration and evaluate a fire-barrier fabric with a 

cover fabric. 

A test conducted with formulation C1 demonstrated melt dripping of the foam. Figure 60 shows 

a test with formulation C1 a few seconds before extinguishing the sample. Although flaming 

liquid material was dripping to the underlying tray, there was no pool fire with sustained flaming 

that could have boosted the heat release rate though a feed-back effect (Figure 61).31 A similar 

62
 

 

 

 

 

 

                               

                                 

                       

                             

                                       

                                 

           

behavior was observed in all samples showing sustained combustion after removing the ignition 

source. Two formulations (B5 and C2) did not show sustained flaming after ignition source 

removal. When ignition was not observed after the first 5 s of flame impingement, a second 

attempt was carried out by applying the open-flame for another 5 s in a new location, 13 cm 

(5 in) away from the original ignition point. For both formulations (B5 and C2), the second 

attempt also failed. Figure 61 shows a mockup assembly for formulation C2 after attempting 

ignition twice. A plot of the mass loss as a function of time for formulations C1 and C3 are 

shown in Figure 62 and Figure 63, respectively. Three tests per formulation are reported. 

Specific data on each test are available in Appendix 3. 

Values for sustained flaming (ignition), final mass loss percentage 120 s after ignition source 

removal (ML120s) and foam properties (density and air permeability) are reported in Table 19. 

The ML120s values for C1 and C2 are identical and slightly below the target value of 20 % 

specified in CPSC proposed standard, section 1634.24 (Figure 81 and Figure 82, respectively). 

Three tests per formulation are reported. Specific data on each test are available in Appendix 3. 

Figure 60. A test with formulation C1 few seconds before extinguishing the sample. At this stage

flaming liquid material is dripping to the underlying tray but there is no pool fire with sustained

flaming that can boost the heat release rate through a feed‐back effect.

Figure 61. Ignition was not observed after the first 5 s of flame impingement with formulations

B5 and C2; a second attempt was carried out by applying the open‐flame for other 5 s in a new

location 5 in. apart from the original ignition point. Here, it is shown a mockup assembly for

formulation C2 after attempting ignition twice.

63
                             

                                   

                           

 

 

 

 

 

 

 

 

 

 

 

                           

Table 19. Summary of the open‐flame tests on bare foams. The values of densities and

permeability shown here are the averages of four values for B5, 16 values for C1, 14 values for

C2 and 12 values for C3. The uncertainty is equal to one standard deviation.

Formulation Buns tested Ignition ML120s (%) Density Air Permeability 

(kg·m-3) (m·min-1) 

B5 B5-3 NO 0 30.4±0.3 44.8±4.1 

C1 C1-8 & C1-9 YES 16.7±3.5 30.9±0.9 78.4±4.6 

C2 C2-9 NO 0 30.1±0.8 70.9±10.7 

C3 C3-3 & C3-1 YES 16.7±3.2 29.9±0.6 78.5±6.9 

Only two of four formulations ignited when exposed to the open-flame even though the 

composition of the foams was substantially the same. However, the structure and morphology 

(e.g., cell size, fraction of open/close cells, etc.) were different, indicating that these 

characteristics played an important role not only on smoldering but also on ignition. In particular, 

it is reasonable to conclude that in this fire scenario where convective heat transfer is dominant, 

ignition is more likely to occur in formulations with a high value of air permeability (Table 19). 

Figure 62. Mass loss vs. time measured for three mockups prepared with formulation C1.

64
                           

 

 

 

 

 

 

 

 

 

 

 

                               

Figure 63. Mass loss vs. time measured for three mockups prepared with formulation C3.

3.3.5. Open-flame Tests with barrier material 

Three samples from formulation C1 were tested in the Open-flame Test with barrier material (see 

Tests 7, 10 and 12 in Appendix 3). The final result of the ignition can be seen in Figure 64 and 

Figure 65, which show the entire mockup and only the foam of the same mockup assembly, 

respectively. The foam blocks appeared to be partially charred on the surface but they retained 

their shape and original mass. As seen in Figure 66, mass loss was about 1 % when the pilot 

flame was removed. According to the CPSC proposed standard, a barrier material fails the test if 

the final mass loss reaches 20 % of the original mass of the foam (fire barrier and fabric are not 

counted in the mass loss determination). The average final mass loss calculated from the 3 tests 

was (6.3 ± 1.2) % and was due only to mass loss in the fire barrier material, polyester fabric and 

cover fabric. The lack of a significant mass loss in the foam blocks demonstrates the 

effectiveness of the barrier material. This effectiveness is clearly shown in Figure 67 which 

compares the residue of mockups tested with the fire barrier material (no foam mass loss after 

45 min) and the residue of the mockups tested without the fire barrier material ((16.7 ± 3.5) % 

foam mass loss) after two min. 

Figure 64. Photo of the residue of the mockup after the open‐flame test with barrier material.

65
                                     

               

                                 

        

                           

                       

 

 

 

Figure 65. Photo of the residue of the mockup at the end of the test after removing the barrier

material, the polyester fabric and the cover fabric.

Figure 66. Total mass loss in barrier test as a funciton of time measured for three mockups

prepared with formulation C1.

Figure 67. Photo of the comparison between residues of mockups (formulation C1) tested with

barrier material (on the left) and without barrier material (on the right).

3.3.6. Conclusions of Iteration 3 

The data in Iteration 2 led to the conclusion that a variable other than air permeability was 

impacting smoldering and that a better morphological descriptor of the foam structure was 

required. This investigation was carried out in Iteration 3 on formulations with high air 

permeability (above 70 m·min-1). Both mockup and box tests indicated that high air permeability 

was a necessary but not sufficient condition to achieve intense smoldering. For these foams, the 

smoldering data measured by the box test could not be used to predict smoldering behavior in the 

mockup test. The smoldering scenario is substantially different in these two tests. It is presumed 

66
 

 

 

 

 

 

 

 

 

 

 

 

 

that the box in the box test significantly reduces the oxygen supply to the smoldering front, 

especially in high permeability foams. 

An increase in permeability caused a decrease in smoldering propensity for values of air 

permeability above 70 m·min-1. This was a new finding that required careful characterization of 

the cell morphology. NIST developed a confocal microscopy technique based on selffluorescence of PUF that allowed comparisons of optical slices of foams with identical optical 

thickness. This is a key requirement for a proper comparison of two dimensional cell sizes, 

where the apparent cell size is a function of the optical thickness of the slice. Image analysis was 

used for measuring the distribution of area cell and the average value of the surface area per cell 

was used as a morphological descriptor for cell size. 

The specific surface area data measured by BET analysis and density showed that smoldering is 

a function of specific surface area and that, at least for PUF with high air permeability (above 

70 m·min-1), the oxygen supply to the smoldering front is not a limiting factor. As an alternative 

to specific surface area, cell size can also be used as a morphological indicator for smoldering 

propensity. Cell size measurements are relatively fast and not affected by the fraction of closed 

cells. Mockup mass loss appeared to increase with decreased cell size in foams with high air 

permeability. 

PUF with reduced smoldering propensity can be prepared by controlling the head pressure 

during the foaming process; an increase in pressure increases cell size and a decrease in surface 

area, causing a decrease in smoldering propensity. Adjusting the head pressure during foaming 

enables tuning of the specific surface area, thus reducing the smoldering propensity in PUF 

without any required change in formulation. 

Open-flame tests showed that, in a fire scenario where convective heat transfer is dominant, 

ignition is more likely to occur in formulations with a high value of air permeability. This might 

also imply that is possible to increase ignition resistance of PUF by decreasing air permeability 

(i.e., promoting a closed cell structure). Open-flame tests, showed also that the use of a barrier 

fabric was extremely effective and prevented any mass loss in the foam during the test. 

At this stage, the feasibility of a standard reference material with a well characterized and 

reproducible smoldering is linked to the ability of manufacture to product a PUF with a welldefined and reproducible cell size. 

3.4. Iteration 4: From Pilot Plant to Production Line 

NIST is collecting, analyzing, and interpreting characterization and smoldering data of PUFs 

produced from several buns from the foamer’s production/manufacturing line. The results (a 

complete report will be provided in a separate document) indicate it is possible to commercially 

produce PUFs with reproducible smoldering. Preliminary data show that an approximate 18% 

smoldering mass loss PUF can be produced by targeting a C1 formulation with a permeability of 

(71 ± 8) m·min-1, cell area of (0.31 ± 0.01) mm2

, and density of (27.4 ± 0.2) kg·m-3 (uncertainties 

equal to one standard deviation). 

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4. Summary 

This 30 month project was broken into four iterations. In Iteration 1, NIST determined the 

smoldering attributes of PUF were not significantly impacted by the raw materials (i.e., polyol 

and surfactant) used to produce PUF in the foamer’s pilot plant. 

The foamer attempted to reproduce a promising formulation from Iteration 1, but due to changes 

in climate, an additional processing aid was required to produce foam without macroscopic 

defects in the pilot plant. 

In Iteration 2, NIST determined the processing parameters (i.e., tin catalyst content, water 

content, and equipment head pressure) strongly influenced the smoldering attributes of the PUF. 

NIST also determined how changes in these parameters impacted the PUF physical attributes, 

correlated these physical characteristics to smoldering and determined target values for these 

physical characteristics, which yield high smoldering. General trends were observed, such as 

decreased head pressure or tin catalyst results in high permeability and high permeability results 

in high smoldering. However, there were exceptions where PUF with similar permeability had 

significantly different smoldering performance. NIST measured cell size using confocal 

microscopy and showed smaller average cell sizes resulted in higher smoldering. 

NIST also developed and assessed a Box Smoldering test that was expected to enable measuring 

the PUF smoldering performance without the use of other materials (e.g., cover fabric). This is a 

robust test for measuring smoldering propensity but, in its current configuration, is not a good 

predictor for CPSC mockup test, and NIST is currently considering modifications. 

In Iteration 3, NIST determined the cell morphology was significantly impacted by the 

processing conditions and developed/utilized techniques (e.g., BET and confocal microscopy) to 

measure changes in the cell morphology. The data indicate the degree of PUF smoldering mass 

loss can be adjusted by targeting specific values of permeability, cell size, and foam density. 

Based on the data from this research project, two examples using a C1 type formulation are as 

follows. 

An approximate 30 % smoldering mass loss PUF can be produced by targeting a 

 permeability > 75 m·min-1, 

 cell size = (0.24 ± 0.02) mm2, and 

 density = (30.4 ± 1.6) kg·m-3. 

An approximate 18 % smoldering mass loss PUF can be produced by targeting a 

4. permeability = (71 ± 8) m·min-1, 

5. cell area = (0.31 ± 0.01) mm2

, and 

6. density = (27.4 ± 0.2) kg·m-3. 

Additional effort will be required to identify a manufacturer with quality controls sufficient to 

produce a long-term supply of foam consistent with CPSC’s proposed test standard. 

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5. References 

1

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MA. p. 118. 2

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Quincy, MA. p. 318. 3

 National Fire Protection Association, Coalition for Fire-Safe Cigarettes,

http://www.nfpa.org/categoryList.asp?categoryID=2254 or www.firesafecigarettes.org

4

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Honeycutt, R.H., and Bunch, J.E., The effects of upholstery fabric properties on fabric 

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1143. 7

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Combustion Science, 1985. 11(4): p. 277-310. 9

 Ohlemiller, T.J., Smoldering combustion. in the SFPE Handbook of Fire Protection

Engineering (3rd edition; Eds.: DiNenno, P.J., Drysdale, D., Beyler, C.L., and Walton, W.D.). 

2002, National Fire Protection Association: Massachusetts. p. 2.200–2.210. 10 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. 11 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. 12 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. 13 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. 14 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. 15 Proposed standard for the flammability of upholstered furniture, 73 FR 11702 (March 4,

2008). 16 Ohlemiller, T.J., On the Criteria for Smoldering Ignition in the CFR 1632 Cigarette Test for

Mattresses, in NIST Technical Note 1601. 2008, National Institute of Standards and Technology: 

Gaithersburg. p. 18. 17 http://www.nist.gov/customcf/get_pdf.cfm?pub_id=902075 

18 British Standard BS 5852:2006 – Methods of test for assessment of the ignitability of

upholstered seating by shouldering and flaming ignition sources (composite mock-ups). 

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19 Brunauer, S., Emmett, P.H., and Teller, E., Adsorption of gases in multimolecular layers.

Journal of the American Chemical Society, 1938. 60(2): p. 309-319. 20 Gustafsson, S.E., Transient plane source techniques for thermal conductivity and thermal 

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Eaves, D.), Rapra Technology Limited, 2004, p. 85-122. 22 O’Connor, J. M. and Russell, D., Understanding polyurethanes. Smithers Scientific Services

Inc., training seminar, Cleveland, Ohio, April 8-10, 2008. 23 Belegundu, A. D. Et al. Multi-objective Optimization of Laminated Ceramic Composites

Using Genetic Algorithms. Proceedings of the 5th AIAA/NASA/USAF/ISSMO Symposium on

Multidiscipinary Analysis and Optimization, Washington D.C.,1994, p. 1015-1022. 24 http://rsbweb.nih.gov/ij/ 

25 http://bigwww.epfl.ch/sage/soft/watershed/ 26 Klempner, D. and Sendijarevic, V., Fundamentals of foam formulation. in Handbook of

Polymeric Foams and Foam Technology (2nd edition; Eds.: Klempner, D. and Sendijarevic, V.).

Hanser Gardner Publications, Cincinnati, 2004, p. 5-14. 27 http://www.itl.nist.gov/div898/handbook/eda/section3/eda353.htm

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

Review of Chemical Engineering, 2009, p. 3-18. 29 Brunauer, S., Emmett, P.H., and Teller, E., Adsorption of gases in multimolecular layers.

Journal of the American Chemical Society, 60(2), p. 309-319 (1938). 30 Pendleton, P. and Badalyan A., Adsorption, 2005, 11 p. 61-66. 31 Ohlemiller T.J. and Shields J.R., Aspects of the fire behavior of thermoplastic materials. NIST

Technical Note 1493, 2008.

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