NIST Technical Note 1859

Evaluating Smoldering Behavior of 

Fire-Blocking Barrier Fabrics 

Shonali Nazaré

William Pitts

Szabolcs Matko

Rick D. Davis

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.TN.1859

January 2015
NIST Technical Note 1859

Evaluating Smoldering Behavior of 

Fire-Blocking Barrier Fabrics

Shonali Nazaré 

William Pitts

Szabolcs Matko

Rick D. Davis

Fire Research Division

Engineering Laboratory

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.TN.1859

January 2015

U.S. Department of Commerce

Penny Pritzker, Secretary

National Institute of Standards and Technology

Willie May, Acting Under Secretary of Commerce for Standards and Technology and Director
Certain commercial entities, equipment, or materials may be identified in this

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

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

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

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

National Institute of Standards and Technology Technical Note

Natl. Inst. Stand. Technol. Tech. Note 1859, 35 pages (January 2015)

CODEN: NTNOEF

This publication is available free of charge from: 

http://dx.doi.org/10.6028/NIST.TN.1859
iii

Abstract

This study reports on the smoldering propensity of commercially available barrier fabrics (BFs) 

in a small-scale mock-up configuration. Most BFs are smolder resistant when tested alone over a 

standard flexible polyurethane foam (FPUF). However, when covered with a smolder-prone 

cover fabric (CF), most BFs failed the smoldering ignition test described in the Consumer 

Product Safety Commission’s proposed standard 16 CFR 1634. Results of this study suggest 

that the smolder-prone CFs, when placed on top of a number of BFs, are capable of releasing 

sufficient heat to initiate the char-oxidation smoldering process of some of the BFs and 

subsequently transmit the heat to the underlying FPUF. A smoldering index for BFs was derived 

from the measured char volume fraction (CVF) of the FPUF by varying the BF component in the 

FPUF/BF/CF mock-up systems, while holding the other two components constant. 

Of the 20 BFs tested in this study, five BFs self-extinguished and passed the smoldering ignition 

test criterion as described in the proposed 16 CFR 1634. The smoldering index for these BFs 

was zero. BFs with smoldering index of 1 or more resulted in sustained smoldering in the FPUF. 

The smoldering propensity of BFs and the amount of heat transmitted to the FPUF varied 

depending on the BF structure, fiber content, air permeability, and bulk density. Flame retardant 

treatments and use of char forming fibers showed a greater tendency for BF smolder in the 

presence of the smolder-prone CF. BFs with char-forming fiber blends had greater smoldering 

propensity as compared to BFs with low charring fiber blends. The lower the smoldering 

propensity of the BF, the less likely was the development of sustained smoldering in the FPUF.

Keywords: upholstered furniture; barrier fabrics; smoldering; air-permeability; char length; mass 

loss
iv

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v

Contents

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

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

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

List of Acronyms and Abbreviations ....................................................................................... viii

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

1.2. Existing test methods for assessing smoldering ignitability of BFs.................................... 2

2. Experimental Materials and Test Methods........................................................................... 4

3. Results and Discussion............................................................................................................. 7

3.1 Cigarette-induced FPUF smolder ......................................................................................... 7

3.2 Cigarette-induced BF smolder.............................................................................................. 8

3.3 Cover fabric-induced FPUF smolder................................................................................... 8

3.4. Cover fabric-induced BF smolder...................................................................................... 12

3.4.1 Highloft barriers........................................................................................................... 18

3.4.3 Knitted barriers ............................................................................................................ 20

3.4.4 Woven barriers............................................................................................................. 20

3.5 Char length on BF and CVF of FPUF............................................................................ 21

3.6 Air permeability of BF and CVF of FPUF..................................................................... 22

4 Conclusions and Future Work.............................................................................................. 23

References.................................................................................................................................... 25
vi

List of Tables

Table 1. Standards and test methods for assessing smoldering ignitability of BFs........................ 3

Table 2. Description of BFs used in soft furnishings. Uncertainties are reported as Type A 

uncertainties with experimental standard deviations. Values are reported with 2σ uncertainty ... 5

Table 3. Digital images and char lengths of charred BFs tested in a mock-up configuration with 

and without smolder-prone CF ....................................................................................................... 9

Table 4. Digital images of FPUF/CF mock-up assemblies and char damage on FPUF at the end 

of 15 min, 30 min, and 45 min test durations ............................................................................... 11

Table 5. Mass loss, CVF and derived indices from smoldering ignition resistance test for 

FPUF/BF/CF composites. Uncertainties are Type A uncertainties49, 50 based on experimental 

standard deviations. Values are reported as 2σ (i.e, coverage factor of 95%).............................. 15
vii

List of Figures

Figure 1. Real-time mass loss (A) and mass loss rate (B) curves for a FPUF/CF mock-up 

assembly........................................................................................................................................ 10

Figure 2. Real-time percent mass loss of complete mock-up assemblies during smoldering 

ignition resistance tests for: (a) highloft, (b) nonwoven, (c) woven, and (d) knitted barrier 

materials........................................................................................................................................ 14

Figure 3. Char volume fraction (CVF) of FPUF during smoldering ignition resistance test for 

BFs in the presence of a smolder-prone cover fabric. .................................................................. 16

Figure 4. Relative changes in char volume fraction (CVF) of FPUF with and without BFs....... 17

Figure 5. CVF of FPUF versus char length on BF ...................................................................... 22

Figure 6. CVF of FPUF at the end of 45 min smoldering ignition tests versus air permeability. 23
viii

List of Acronyms and Abbreviations

ASTM ASTM International

BEARHFTI California Bureau of Electronic and Appliance Repair, Home Furnishings 

and Thermal Insulation

BF Barrier fabric

BHFTI California Bureau of Home Furnishings and Thermal Insulation

CAL TB California Test Bulletin

CFM Cubic Feet per square Meter of sample

CFR Code of Federal Regulations

CPSC Consumer Product Safety Commission

CVF Charred Volume Fraction

FPUF Flexible Polyurethane foam

FR Flame Resistant

HTP Heat Transfer Properties

NFPA National Fire Protection Association

NIST National Institute of Standards and Technology 

SPUF Standard Polyurethane Foam

SRM Standard Reference Material

RUF Residential upholstered furniture

UFAC Upholstered Furniture Action Council

w/o Without
1

1. Introduction

Fire-blocking barrier fabrics (BFs) are predominantly used in soft furnishing products that are required to 

comply with open-flame flammability regulations, for e.g., the 16 CFR 1633,1 Cal TB 129,2 Cal TB 603,3

Cal TB 133,4 FAR 25.853,

5

etc. These materials interposed between a cover fabric (CF) and the soft 

cushioning modify the thermal response of the upholstery by limiting heat transfer and gas flow in the 

upholstered assembly. This essentially limits the product involvement in a fire by preventing and/or 

significantly delaying the ignition of interior cushioning materials, lowering the heat release rate, reducing 

the rate of flame spread and/or extinguishing the flames from an exterior CF or ticking.6 Flexible 

polyurethane foam (FPUF) is one of the most flammable materials that contribute to the high heat release 

rates observed when upholstered products catch fire. Generally, the flammability of FPUF has been 

addressed by adding flame retardants. However, concerns have recently been raised about potentially 

harmful human health and ecological effects of fire retardants, and the effectiveness of fire retardants at the 

levels typically used in residential upholstered furniture (RUF) meeting the California TB 117 standard has 

been questioned.7,8 As a result of such concerns, California TB 117 has been updated to remove an openflame test requirement for RUF soft cushioning materials, which was the primary driver for adding fire 

retardants to RUF in the United States.9 A promising approach for reducing the flammability of residential 

upholstered products, without compromising aesthetics and comfort, is to incorporate a BF. BFs, also 

known as interliners, have been previously used to comply with Cal TB 133 requirements4for upholstered 

seating used in public places and vehicles of mass transport.5,10 Most standards issuing authorities in the 

United States, including the Upholstery Furniture Action Council (UFAC), ASTM, and the National Fire 

Protection Association (NFPA), require that BFs pass a smoldering ignition test when a smolder-prone 

cover fabric, referred to as a Class II fabric by UFAC, is used in RUF. 

Generally, a polyester batting is used under the cover fabric to provide a barrier to smoldering ignition 

sources and to achieve durability, aesthetic, and comfort effects.

11 Polyester battings are less susceptible 

to smoldering as most of the heat from the smoldering cigarette and cover fabric is consumed in melting 

the polyester fibers. Polyester melts and shrinks away from the heat source12 and moreover, the low density 

structure of polyester batting increases its effectiveness as a thermal insulator.13

Following the introduction of an open-flame regulation for mattresses (16 CFR 16331) in 2007, several new 

fire-barrier technologies, discussed elsewhere,

6

have been commercialized. One of the objectives of this 

study is to assess the potential of incorporating similar BFs to those used by the residential mattress industry 

to improve the fire performance of RUF. 

In order to improve upholstered product fire safety, BFs must protect the cushioning layer from both flaming 

ignition and smoldering ignition sources. In a previous study, we evaluated the potential fire-blocking 

performance of barrier fabrics when exposed to open-flame ignition sources.14 Intrinsic properties that 

influence the heat transfer properties (HTP) of BFs, as related to thermal protection of cushioning 

components in upholstered products, were identified. When tested for heat transfer characteristics, the area 

density and thickness of BFs had a strong influence. However, when tested as a composite in a mock-up 

assembly, the performance of active and passive barrier materials showed clear differences. For 

chemically-active BFs, the construction parameters and material properties such as thickness, air 

permeability, and heat transfer were of little significance. In the case of passive BFs, however, these 

parameters became decisive. Results from this study14 suggested that if a BF is not an active fire barrier, 
2

then the amount of heat transferred through the BF is critical, i.e., the material should be thermally thick to 

protect the underlying cushioning layers.

BFs used in mattresses generally pass the smoldering ignition test largely because of the use of 

thermoplastic tickings (i.e., covering fabric), and/or polyester batting under the ticking. Moreover, 

mattresses pass the smoldering ignition test due to the fact that, in a flat configuration, most of the thermal 

energy from the smoldering cigarette used as an ignition source is lost to the environment at ambient 

conditions. However, smoldering conditions in a crevice formed between two or more cushions, such as 

commonly found in RUF, can be very different from the horizontal configuration of a mattress. In a crevice 

configuration, the maximum amount of thermal energy from a smoldering ignition source is directed into 

the cushioning layers which, depending on the structure and composition of the cushioning material, can 

act as an insulator to retain heat. Moreover, in a crevice configuration, the fabric surface contact with a 

smoldering cigarette is roughly twice that of flat configurations15.

This study reports on the smoldering propensity of commercially available BFs in a small-scale upholstered 

seating mock-up configuration. The focus is to rank the smoldering tendencies of different BFs. UFAC, 

ASTM, NFPA, BEARHFTI (Bureau of Electronic and Appliance Repair, Home Furnishings and Thermal 

Insulation), and the US Consumer Product Safety Commission (CPSC), have defined tests for barrier fabrics 

wherein a BF component is tested in similar mock-up configurations and pass/fail criteria based on CF char 

length, and/or mass loss of the components and/or of the complete mock-up assembly. This report provides

insight into the quantitative properties of the BFs that define their smoldering propensity when used in 

combination with FPUF and CF. Existing test methods for assessing smoldering ignitability of a BF 

component in upholstered products are discussed in the following section.

1.2. Existing test methods for assessing smoldering ignitability of BFs

Various test methods that are used to assess smoldering ignitability of BFs are listed in Table 1. All test 

methods essentially require testing of a BF component interposed between FPUF and a smolder-prone cover 

fabric (Class II cover fabrics16). The smoldering ignition assessment of a barrier component, as described 

in the test methods listed in Table 1, is generally conducted on a small scale mock-up composite that is 

voluntarily used by many upholstered furniture and fabric manufacturers. 

It should be noted that Cal TB 116,

17 ASTM 1352,18 and NFPA 26119 require the use of real-scale mockups for smoldering ignition testing. The small-scale mock-up facilitates evaluation of smoldering propensity

in multi-component systems of complex design. The mock-up consists of a wooden frame, upholstery 

components such as FPUF, BF, CF, and a lit cigarette covered by a sheeting material. A covered cigarette 

smolders less intensely, increases the duration of glowing oxidation, and helps the cigarette burn 

completely, thereby increasing the likelihood of sustained smoldering of the FPUF substrate and the 

reproducibility of the test method20. The pass/fail criteria and the classification norms vary between the 

standardizing authorities. The barrier test method (Method 321) defined by the UFAC, specifies a maximum 

allowable upward spread of smoldering, i.e., char length on the back cushion CF. The BF fails if there is 

an obvious smoldering ignition or the vertical char length on the CF exceeds 38 mm upward from the 

crevice21. The BF passes the test only if it is capable of preventing charring of the CFs. The ASTM E1353 

standard test method for cigarette ignition resistance of components of upholstered furniture22, classifies 

BFs based on a critical upward char length of 51 mm on the back CF. 
3

Table 1. Standards and test methods for assessing smoldering ignitability of BFs.

Issuing 

authority

Standard/Test 

methods

Test specimen Pass/fail/classification criteria.

UFAC UFAC Method 321

Voluntary test methods

Small-scale mock-up

Standard cotton velvet cover fabric and standard FPUF 

substrate

 Vertical char length on CF< 38 mm.

NFPA NFPA 26023

Standard

Small-scale mock-up

Standard cotton velvet cover fabric and standard FPUF 

substrate

 Class I barrier material: Vertical char length on CF

< 51 mm.

 Class II barrier material: Vertical char length on 

CF > 51 mm

ASTM ASTM E135322

Standard

Small-scale mock-up

Standard cotton velvet cover fabric and standard FPUF 

substrate

 Class A barrier material: Vertical char length on 

CF < 51 mm.

 Class B barrier material: Vertical char length on 

CF > 51 mm or obvious smoldering ignition.

ASTM ASTM D523827

Standard test method

(now withdrawn)

Cotton battings  Char length on BF < 25.4 mm.

CPSC 16 CFR Part1634 

(proposed)25

Test method in proposed 

federal regulation

Small-scale mock-up

Standard cotton velvet cover fabric and standard FPUF 

substrate

 No transition to flaming during 45 min test 

duration.

 FPUF mass loss < 1 %.

BHFTI Cal TB 117-20139

Test method required in 

the state of California.

Small-scale mock-up For BF to pass:

 No smoldering after the 45 min test duration.

 No transition to flaming during 45 min test 

duration.

 Char length on CF < 51 mm.

BHFTI Cal TB 11617

Test method required in 

the state of California.

Real-scale RUF  No transition to flaming during test duration.

 Char length on CF < 51 mm.
4

According to ASTM E1353, materials that develop char lengths of 51 mm or less on the CF are classified 

as Class A barriers and those which develop obvious ignition or have char lengths in excess of 51 mm are 

classified as Class B barriers. NFPA 26023 classifies barrier materials as Class I or II based on a char 

length similar to that prescribed by ASTM E1353. The most recent version of Cal TB 117-20139, also 

uses the vertical char length (ASTM E1353) criterion to assess smoldering tendency of barrier materials. 

A BF passes the test if the vertical char length on Type II CF is less than 51 mm. A failure is recorded if 

the mock-up continues to smolder after the 45 min test duration or transitions to open-flaming. A Type II 

cover fabric is a smolder-prone cover fabric that fails the cover fabric test (without a barrier fabric) as 

described in Section 1 of Cal TB 117-20139. None of the test methods described above specifies 

measurement of the char length on the BF. Moreover, char length measurements tend to have poor 

reproducibility, and possible effects of textile structures, fiber content, and air permeability on smoldering 

combustion are uncertain.24 The BF smoldering resistance test described in the proposed CPSC 16 CFR 

part 163425 is the only test method that uses mass loss of FPUF as its ‘pass/fail’ criterion. It is, therefore, 

the only test method that uses the smoldering behavior of the cushioning materials to characterize the 

smoldering behavior of a mock-up. The BF, when tested with a cotton velvet cover fabric (a smolderprone (Class II) fabric), passes the test if the mass loss of FPUF is less than 1 % following the removal of 

any charred material and the mock-up does not transition to flaming during the 45 min test duration. This 

is a very stringent criterion as it requires minimal smoldering within the FPUF. This mass loss criterion 

provides a more quantitative characterization of FPUF smoldering ignition than the criteria described in 

the other test methods26. 

ASTM had a barrier component test (now withdrawn) to assess the smoldering ignition resistance of cotton 

battings. In the ASTM D523827 test method, a lighted cigarette was placed between precut and 

preconditioned pieces of cotton battings and the length of the char was measured as soon as smoke was 

observed. This test method fails to specify the duration of testing. Failure was defined as char lengths of 

25.4 mm or greater. This test method was often used by cotton batting manufacturers to spot check their 

products. This is a severe test as the ignition source is completely enclosed, permitting the cotton batting 

to act as an insulator to retain heat while being porous enough to allow adequate air flow to support 

smoldering combustion. Only boric acid-treated cotton battings generally passed this test.

28

2. Experimental Materials and Test Methods 1

A range of commercially available BFs were included in this study. The structural and physical properties 

of BFs such as mass per unit area (aerial density), bulk density, thickness, and air permeability were 

evaluated and are given in Table 2. For the purpose of this study, the mass per unit area is associated with 

the quantity of material available per unit area, while bulk density is related to the ability of oxygen to 

penetrate to ignition site. The exact fiber blend compositions are proprietary and thus were not available. 

No chemical analysis of barrier materials was made. The BFs varied in average thicknesses from 0.1 mm 

to 7.8 mm. 

 

1 Certain commercial equipment, instruments or materials are identified in this paper in order to specify the experimental procedure 

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 materials or equipment identified are necessarily the best available for this purpose.
5

Table 2. Description of BFs used in soft furnishings. Uncertainties are reported as Type A uncertainties49,50 with experimental 

standard deviations. Values are reported with 2σ uncertainty

Serial 

No

Sample 

code

Fiber Blend Structure FR system Thickness, 

mm

Area 

Density, 

g/m2

Bulk 

Density, 

g/cm3

Air Permeability

CFM m/s

1 BF-1 FR rayon/polyester Highloft Passive 4.1 ± 0.1 155 0.038 550 ± 47 2.8 ± 0.2

2 BF-2 FR rayon/polyester Highloft Passive 6.7 ± 0.2 230 0.034 339 ± 18 2.0 ± 0.1

3 BF-3 FR rayon/polyester Highloft Passive 7.8 ± 0.6 240 0.031 450 ± 28 2.3 ± 0.1

4 BF-4 Boric acid treated cotton/ FR 

rayon/polyester

Stratified Highloft Passive 5.7 ± 0.1 230 0.040 428 ± 52 2.2 ± 0.2

5 BF-5 Boric acid treated cotton Highloft Passive 6.9 ± 0.8 230 0.033 248 ± 

20

1.3 ± 0.1

6 BF-8 FR rayon/polyester Needle punched 

nonwoven

Passive 4.3 ± 0.1 237 0.055 429 ± 16 2.2 ± 0.1

7 BF-9 FR rayon/polyester

(needlepunched)

Needle punched

nonwoven

Passive 2.2 ± 0.1 240 0.109 301 ± 8 1.5 ± 0.1

8 BF-10 FR polyester /FR rayon Stitchbond

nonwoven

Active/Passive 0.7 ± 0.1 165 0.236 212 ± 13 1.1 ± 0.1

9 BF-11 Glass fiber core/ FR acrylic 

fiber (core spun yarn)

Knitted Active 0.9 ± 0.1 186 0.207 445 ± 8 2.3 ± 0.1

10 BF-12 Glass fiber core/ FR acrylic 

fiber (core spun yarn)

Knitted Active 1.6 ± 0.1 237 0.148 564 ± 8 2.9 ± 0.1

11 BF-13 FR rayon/glass fiber/PLA 

fiber (core spun yarn)

Knitted Active 1.4 ± 0.1 165 0.118 380 ± 14 1.9 ± 0.1

12 BF-15 Glass fiber core/ FR acrylic 

fiber 

Woven Active 0.5 ± 0.1 170 0.340 406 ± 2 2.1 ± 0.1

13 BF-16 FR rayon/glass fiber/PLA Nonwoven Active 2.9 ± 0.1 290 0.097 381 ± 38 1.9 ± 0.2

14 BF-17 Glass fiber Woven Passive 0.2 ± 0.1 150 0.750 0 0

15 BF-18 Glass fiber Woven Passive 0.1 ± 0.1 170 1.700 0 0

16 BF-19 Glass fiber Woven Passive 0.3 ± 0.1 320 1.067 0 0

17 BF-20 Para-aramid/melamine Woven Passive 0.77 ± 0.02 264 0.343 41 ± 3 0.2 ± 0.1

18 BF-21 Para-aramid Nonwoven Passive 0.67 ± 0.02 69 0.103 414 ± 15 2.1 ± 0.1

19 BF-22 Meta aramid/para aramid Woven/nonwoven 

composite 

Passive 1.61 ± 0.11 267 0.166 187 ± 27 0.9 ± 0.1

20 BF-23 Cotton/glass fiber Knitted/

backcoated

Active/Passive 1.5 ± 0.1 284 0.189 0 0
6

The air permeability of components in upholstered products is known to have a significant effect on their 

smoldering propensity.20,29 Air permeability is described as the rate of air flow passing perpendicularly 

through a known area, under a prescribed air pressure differential between the two surfaces of the material. 

Air permeabilities of BFs were measured using an electronic high differential pressure air permeabilitymeasuring instrument (FAP 5352 F2, Frazier Precision Instrument Co. Inc., Hagerstown, MD).

30

 The air 

pressure differential between the two surfaces of the material was set to 125 Pa (13 mm of water). Fabrics 

were clamped in a circular specimen holder, exposing 6.45 cm2 to the perpendicular air flow. Nozzles with 

orifice diameters of 2.0 mm, 3.0 mm, and 6.0 mm were used in order to reach the target pressure drop (125 

Pa). The value of permeability (ɸ) 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 second or simply meters per second) at a temperature of 20 °C and atmospheric 

pressure of 100 kPa. Ten readings were taken for each of the BFs and calculated averages are reported.

The smoldering tendencies of BFs were studied in two different experiments. First, a smoldering cigarette 

was used as an ignition source to initiate smolder directly in the BF. The second set of experiments was 

designed to determine the ability of a BF to act as a smoldering ignition source of FPUF. The smoldering 

propensity of a BF in the presence of a smolder-prone cover fabric was tested using the test method 

described in CPSC’s proposed standard 16 CFR 1634,25 which is an adaptation of the UFAC Test Method 

3.16 The test method is designed to assess the barrier performance when placed between a smolder-prone 

cover fabric and a FPUF substrate. The test method essentially evaluates the likelihood of FPUF smoldering 

in the presence of a BF. The mock-up consists of a wooden frame, a FPUF, BF, cover fabric, and a lit 

cigarette under a sheeting material. Note that it has recently been shown that such mock-up assemblies 

where the vertical (back) and horizontal (seat) pieces of FPUF are pressed tightly against the wood 

frame9,16,18,25 do not smolder as readily as mock-ups which are raised above the wooden frame and have 

greater access to the surrounding air.29

Smoldering behavior may be examined in several ways, depending on whether it is characterized by 

smoldering temperatures, smoldering times, char lengths, char areas, or mass loss.

31 For the purpose of this 

study, we considered the mass loss of the complete mock-up assembly and the fraction of the FPUF used 

in the mock-up assembly which smoldered during a set time period. Rate of mass loss is reported in this 

study and has been previously used by Damant et al24 to evaluate the smoldering propensity of upholstery 

fabrics under conditions of cigarette ignition. Char length measurements on the BFs are also reported. Char 

lengths on the CFs are not reported since different standards prescribe different methods of measuring char 

lengths as described earlier. 

All materials were conditioned, specimens were assembled, and tests were performed at a relative humidity 

(RH) of 55 % ± 5 % and a temperature of 22 °C ± 3 °C. A detailed description of the equipment used for 

maintaining these environmental conditions is available elsewhere.32 The FPUF foam used in this study 

was a well characterized flexible polyurethane foam32 especially developed for evaluating upholstery 

materials and their tendency to support smoldering or limit flaming combustion.33 

The masses of the vertical (203 × 203 × 76) mm3 and horizontal (127 × 203 × 76) mm3 FPUF sections were 

recorded and used later to calculate the fraction that smoldered. The FPUF was covered with the BF, and, 

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

and the entire ensemble was placed in the wooden frame. The 100 % cotton CF had an average aerial 

density of 445 g/m2 ± 3 g/m2 and air permeability of 0.10 m/s ± 0.02 m/s. The BF and the cover fabric were 
7

carefully placed over the FPUF, and particular attention was paid to avoid formation of any air pockets 

between layers since such pockets tend to behave as insulators and can result in poor reproducibility.

34 The 

complete mock-up assembly along with the specimen holder was placed on a mass balance accurate to ± 

0.01g within the normal operating environment of the laboratory used for material conditioning, sample 

assembly, and sample testing. The smoldering ignition test was carried out inside a poly(methyl 

methacrylate) enclosure, in order to prevent excessive changes in surface ventilation, located under a 

laboratory canopy-type hood. A lighted cigarette (Standard Cigarette for Ignition Resistance Testing, NIST 

SRM 1196)35 was placed in the crevice formed by the intersection of the vertical (back) and horizontal (seat) 

panels of each test assembly. Each cigarette was covered with a piece of sheeting fabric (UF-400; 

Testfabrics, Inc.; West Pittston, PA, USA.), a 100 % cotton, white plain weave with 19 threads/cm2 to 33 

threads/cm2, and an aerial density of 115 g/m2 ± 1 g/m2. The cigarette was allowed to burn its entire length

(82 mm). Three specimens were tested for each barrier fabric.

The real-time mass of the complete assembly was recorded manually every two min. After the 45 min test 

duration, the FPUF (char and non-char) was separated from the other test components (e.g., cigarette ash, 

CF and BF), and the mass of this FPUF was measured. The char was then removed and the mass of the 

remaining non-charred foam was measured. The smoldering mass loss ratio was calculated as the difference 

between the non-charred FPUF mass and the original FPUF mass divided by the original FPUF mass.25 The 

percent mass loss of FPUF was defined by Zammarano et al29 as the char volume fraction (CVF). They 

assumed that the density of virgin FPUF was homogeneous and did not vary during the test. They also 

showed that the CVF is proportional to the average volumetric rate of smoldering propagation during the 

test and was, therefore, used in this work as a measure of the average smoldering propagation rate.

3. Results and Discussion

3.1 Cigarette-induced FPUF smolder

The cigarette-induced smoldering behavior of bare FPUF was tested in the mock-up configuration. A lit 

cigarette was placed in the crevice of the mock-up assembly and was covered with a sheeting material. It 

was found that the FPUF used in this study did not develop self-sustained smoldering (visually identified 

as continued production of smoke after the extinguishment of the ignition source). This is in agreement 

with earlier studies where flexible polyurethane foam had shown good resistance to sustained smoldering 

in the presence of a smoldering cigarette.29,36,37 Although the cigarettes burned their entire length, leaving 

behind a grey ash, the CVF for SRM foams was only 0.3 % ± 0.1 %. A minimal thermal decomposition of 

FPUF was noted. The cellular structure of FPUF in the immediate vicinity of smoldering cigarette collapsed 

forming a tar-like liquid instead of a porous char. It is appears that the FPUF followed the non-oxidative 

pathway prompted by an increase in heating rate in the presence of the smoldering cigarette29. Once the 

cellular structure, essential for smolder propagation, had collapsed, the thermal influence of the smoldering 

cigarette over the FPUF was lost. The loss of thermal contact with the smoldering ignition source led to 

smolder suppression. Previous studies38 have also shown that the rate of heat generated in the smoldering 

process is related to the size of the smoldering zone. Thus, localized thermal decomposition in FPUF caused 

by a smoldering cigarette was not sufficient to develop self-sustained smoldering in the FPUF. By 

definition, self-sustained smoldering occurs when the smoldering front has extended to a region beyond the 

thermal influence of the ignition source.29 
8

3.2 Cigarette-induced BF smolder

The cigarette-induced smoldering behavior of BFs was tested in a small-scale mock-up configuration with 

FPUF/BF composites. The main purpose of these experiments was to determine whether or not the intensity 

of heat flux from a smoldering cigarette in combination with a given BF was sufficient to initiate sustained 

smoldering in the FPUF. The SRM cigarette used in this study smolders with sufficient heat flux intensity 

to form char in some of the adjacent BF layers. However, none of the BFs evaluated in this study resulted 

in self-sustained smoldering in the FPUF. The insulating properties of charring BFs placed immediately 

under the glowing cigarette may cause excessive heat losses near the smoldering front, leading to 

extinguishment of smoldering within the BF layer. The mass loss of complete assemblies and the CVF of 

FPUF during these smoldering ignition tests were negligible and, hence, are not reported.

Typical digital images of the char formed and the measured char lengths on the BFs during these tests are 

shown in Table 3. Generally, most test methods discussed earlier require measurement of vertical char 

length from the crevice or length of char from the nearest point of the original location of the cigarette. In 

this study however, full length of char on the BFs are reported. Except for charring in the immediate vicinity 

of the cigarettes, the BFs remained intact and undamaged. Air permeability levels, fiber blends, and fabric 

structures had no significant effect on char areas on the BFs. Thus, BFs did not develop significant 

smoldering when placed in direct contact with the cigarette i.e., the heat flux from a smoldering cigarette 

was not sufficient to induce sustained smoldering in the BFs or the underlying FPUF.

3.3 Cover fabric-induced FPUF smolder

The real-time mass-loss and mass loss rate curves for FPUF covered only by the CF are shown in Figure

1(a) and Figure 1(b). It can be noted from Figure 1(a) that the mock-up assemblies did not begin to lose 

significant mass until almost 12 min from the start of the test. The smoldering process intensified as the 

cigarette burned, generating additional heat as more of the CF began to smolder. As the smolder front 

propagated laterally and downward into the crevice, the mass loss rate increased. At around 25 min - 28 

min into the test, the cigarette had burned completely; by this time the CF had become a self-sustained 

smoldering ignition source. At 35 min, the mock-up assembly mass loss rate began to rapidly increase (see 

Figure 1 (b)) indicating the start of sustained smoldering of the mock-up.29,32 From temperature 

measurements using thermocouples, Zammarano et al. established that the smoldering front reaches depth 

in the FPUF, where the heat losses are minimal and smoldering is likely to be self-sustained. Based on 

previous studies on smoldering of FPUF,

29,32,39 we propose that the acceleration in mass loss of the complete 

mock-up assembly occurs when the smoldering front has reached a depth in the FPUF where char oxidation 

becomes dominant.
9

Table 3. Digital images and char lengths of charred BFs tested in a mock-up configuration with and without smolder-prone CF

Highloft Barrier Fabrics Nonwoven Barrier Fabrics Knitted Barrier Fabrics Woven Barrier Fabrics

Sample

code

With CF w/o CF Sample

code

With CF w/o CF Sample

code

With CF w/o CF Sample

code

With CF w/o CF

Back Seat Back Seat Back Seat Back Seat Back Seat Back Seat Back Seat Back Seat

BF-1 BF-8 BF-11 BF-15

83 mm 105 mm 74 mm 74 mm 125 mm 119 mm 75mm 75 mm 165mm 175 mm 74 mm 74 mm 191 mm 171 mm 76 mm 78 mm

BF-2 BF-9 BF-12 BF-17

NT

110 mm 105 mm 74 mm 74 mm 220 mm 184 mm 73 mm 71 

mm

193 mm 155 mm 74 mm 74 mm 75 mm 73 mm

BF-3 BF-10 BF-13 BF-18 NT

132 mm 145 mm 67 mm 67 mm 100 mm 100 mm 80 mm 80 mm 123 mm 115 mm 74 mm 74 mm No char No char

BF-4 BF-16 BF-23

NT BF-19

NT

196 mm 200 mm 73 mm 72 mm 145 mm 141 mm 76 mm 76 mm No char No char No char No char

BF-5 BF-21 BF-20

143 mm 173 mm 73 mm 73 mm 129 mm 127 mm 68 mm 68 mm 63 mm 63 mm 63 mm 63 mm

BF-22

103 mm 100 mm 68 mm 68 mm
10

Figure 1. Real-time mass loss (A) and mass loss rate (B) curves for a FPUF/CF mock-up assembly.

At 45 min, the test was terminated, and the CVF was determined as described in the experimental section. 

The CVF for the FPUF was 10 % ± 5 % and the maximum char lengths on the seat and back CFs were

measured as 100 mm ± 2 mm. The development of smoldering and the extent of char in the FPUF as a 

function of time was characterized by stopping the tests after 15 min, 30 min, and 45 min. Results are 

shown in Table 4. The rapid growth of the CVF over the 30 min period is evident.
11

Table 4. Digital images of FPUF/CF mock-up assemblies and char damage on FPUF at the end of 15 min, 

30 min, and 45 min test durations

15 min* 30 min 45 min

Mock-up with FPUF/CF Mock-up with FPUF FPUF char

Seat panel Back panel Seat panel Back panel Seat panel Back panel

FPUF after 

removal of 

char

*For clarity, the cigarette has been removed from the crevice.

The results indicate that this smolder-prone CF and FPUF combination has a significant propensity to 

develop smoldering when in contact with a lit cigarette. Smoldering ignition of 100 % cellulosic CFs in 

combination with FPUF has been widely reported. 20, 24, 26, 34, 42 A smoldering cigarette produces sufficient 

heat to prompt thermal degradation in the cellulosic CF, followed by formation of a carbonaceous char. 

In presence of oxygen, this carbonaceous char undergoes further oxidation and produces more heat. This

excess heat is dissipated in the underlying FPUF and in part lost to the immediate surrounding atmosphere. 

The thermal degradation of FPUF in air starts at around 270 °C to 295 °C40 and is the result of two

competitive decomposition pathways: an oxidative pathway which produces a charred foam-like residue 
12

and a non-oxidative pyrolysis which generates liquid tar. The higher the likelihood of char formation, the 

greater will be the likelihood of the development of sustained smoldering. 15 A higher rate of char formation 

in a smolder-prone cover fabric is thus manifested in a higher smoldering propensity of FPUF. Furthermore, 

the shape of the cavities formed due to smoldering in FPUF (see FPUF images in Table 4) suggest a 

“concave” smolder front geometry. As pointed out by Ohlemiller,

41 the increasingly “concave” smolder 

front geometry reduces radiative heat losses to the surroundings leading to sustained smoldering in the 

FPUF. 

3.4. Cover fabric-induced BF smolder

The potential contribution of BFs to the smoldering ignition of FPUF was also studied for BFs covered by 

a smolder-prone cover fabric (CF). In order to assess the protective effectiveness of a BF, the smoldering 

behavior of the FPUF covered by both a BF and CF was characterized by recording the real-time massloss of mock-ups, BF char lengths, and char volume fraction (CVF) of FPUF. The mass loss curves for 

complete mock-up assemblies (FPUF/BF/CF) incorporating various BFs are shown in Figure 2. 
13
14

Figure 2. Real-time percent mass loss of complete mock-up assemblies during smoldering ignition 

resistance tests for: (a) highloft, (b) nonwoven, (c) woven, and (d) knitted barrier materials.

The mass loss data in Table 5 show that the smolder-prone CF when placed on top of a number of the BFs 

was capable of releasing sufficient heat to initiate smoldering and subsequently transmit the heat to the 

underlying FPUF. 
15

Table 5. Mass loss, CVF and derived indices from smoldering ignition resistance test for FPUF/BF/CF composites. Uncertainties are 

Type A uncertainties49, 50 based on experimental standard deviations. Values are reported as 2σ (i.e, coverage factor of 95 %).

Sample

code

Initial 

Mass*, 

g

Mass loss for complete assembly (gaseous mass 

loss), g

Char volume fraction (CVF), % BF 

smoldering

index

Pass/Fail 

classification§

Avg. 

Foam

Back 

Foam

Seat

Foam

20 min 30 min 40 min 45 min Avg., %

No BF 221 0.91 2.06 3.90 6 ± 2 3 10 ± 5 9 ± 4 12 ± 5 - -

BF-1 255 0.91 1.88 3.33 7 ± 5 3 17 ± 13 13 ± 10 24 ± 18 1.7 Fail

BF-2 266 0.93 1.69 3.33 7 ± 5 3 17 ± 8 13 ± 6 22 ± 13 1.7 Fail

BF-3 269 0.59 1.94 5.77 10 ± 3 4 22 ± 5 18 ± 3 30 ± 8 2.2 Fail

BF-4 272 1.18 3.29 7.75 13 ± 6 5 33 ± 14 25 ± 10 45 ± 19 3.3 Fail

BF-5 295 0.93 2.82 7.75 14 ± 4 5 34 ± 8 23 ± 2 51 ± 17 3.4 Fail

BF-8 267 0.55 1.24 3.32 6 ± 4 2 16 ± 7 12 ± 5 21 ± 9 1.6 Fail

BF-9 255 0.01 30.0 14.26 31 ± 2 12 < 50 < 50 < 50 < 5.0 Fail

BF-10 253 0.62 1.13 1.98 4 ± 4 2 11 ± 6 9 ± 5 15 ± 7 1.1 Fail

BF-11 258 0.79 3.42 12.50 25 ± 5 10 47 ± 11 42 ± 9 51 ± 13 4.7 Fail

BF-12 274 0.23 1.43 7.09 17 ± 

13 6 35 ± 21 31 ± 20 42 ± 24 3.5 Fail

BF-13 281 0.56 1.49 4.64 8 ± 4 3 17 ± 8 15 ± 6 20 ± 11 1.7 Fail

BF-15 253 0.98 4.91 14.21 29 ± 

10 11 50 ± 22 45 ± 21 58 ± 24 5.0 Fail

BF-16 275 0.51 3.12 5.95 9 ± 5 3 22 ± 9 19 ± 7 26 ± 12 2.2 Fail

BF-21 233 1.63 4.17 8.46 13 ± 3 5 24 ± 1 21± 5 29 ± 8 2.4 Fail

BF-22 267 1.35 2.40 3.94 6 ± 4 2 13 ± 3 11 ± 2 16 ± 4 1.3 Fail

BF-17 253 0 0.50 1.01 1 ± 0.5 0 0 0 0 0.0 Pass

BF-18 253 0 0 0 0 ± 0 0 0 0 0 0.0 Pass

BF-19 277 0 0.65 0.82 1 ± 0.5 0 0 0 0 0.0 Pass

BF-20 271 1.15 1.84 2.35 2 ± 1 1 0.6 0 0 0.0 Pass

BF-23 319 0 0 0 0 0 0 0 0 0.0 Pass

*

Initial mass of mock-up components excluding wooden frame. Wooden frames weigh @ 1300 ± 20 g.

§ Pass/fail classification based on CPSC’s 16 CFR 1634 proposed rule25

.
16

The CVFs for mock-ups incorporating various BFs are shown in Figure 3. The average CVF at the end of 

a 45 min test depended on the smoldering propensity; which depended on the BF structure, fiber content, 

air permeability, and bulk density; of a given BF. The lower the smoldering propensity of the BF, the less 

likely was the development of sustained smoldering in the FPUF. The impact of a BF on smoldering 

likelihood of FPUF can be characterized in terms of changes in the CVF for mock-ups tested with and 

without a BF. Results are shown in Figure 4 as the CVF observed with the CF only minus the value for 

tests when a BF was included. Positive values indicate that the BF/CF combination resulted in reduced 

FPUF smoldering as compared to using the CF alone. Similarly, negative values indicate increased 

smoldering with the BF/CF combination. 

Figure 3. Char volume fraction (CVF) of FPUF during smoldering ignition resistance test for BFs in the 

presence of a smolder-prone cover fabric.

0

10

20

30

40

50

60

70

80 Char volume fraction (CVF) of FPUF, %
17

Figure 4. Relative changes in char volume fraction (CVF) of FPUF with and without BFs.

CVFs for FPUF/BF/CF combinations normalized by the CVF for the FPUF/CF combination provide a 

quantitative measure for the relative propensity of BFs to induce smoldering in FPUF and will be referred 

to as the smoldering index. The smoldering indices for various BFs are given in Table 5. Zero or close to 

zero values correspond to cases where the FPUF is protected by the BF, and sustained smoldering does not 

develop. BFs with smoldering index of 1 or more resulted in self-sustained smoldering in the FPUF. It is 

possible to get sustained smoldering for an smoldering index of less than one, but the smoldering would be

less intense than without the BF. 

In some cases (BF-18, BF-19 and BF-23), the cigarettes self-extinguished in less than 6 min and in others 

(BF-17 and BF-20), the observed burning rate was unusually slow. It is important to note that the burning 

behavior of a cigarette is known to vary with the type of substrate. Studies have shown that the cigarette 

linear burning rate varies between 1.3 mm/s to 2.1 mm/s, depending on physical and chemical properties of 

the substrate.42 Salig37 has shown that mock-up assemblies in which convective air flow at the crevice was 

blocked using aluminum foil recorded reduced temperatures by almost 300 °C. It is evident from this study 

that the air flow barrier provided by the BF can temporarily slow or even completely stop the smoldering 

process in the cigarette and the CF. All of the tests wherein cigarette self-extinguishment was observed 

included BFs that were almost impermeable to air. 
18

3.4.1 Highloft barriers

The transient mass loss curves for mock-up assemblies provide insight into the smoldering behavior of 

FPUF/BF/CF combinations. The curves for mock-up assemblies with highloft BFs are plotted in Figure 

2(a). All mock-ups tested with highloft BFs transitioned into self-sustained FPUF smoldering. The highloft 

barriers tested contained char-forming fibers, and their high porosity indicates greater surface areas 

favorable for surface oxidation reactions, which is conducive for enhanced smoldering. The heat flux 

provided by the smoldering BF/CF combinations was more efficient at inducing self-sustained smoldering 

in the FPUF than the smoldering CF alone. Figure 2(a) shows that different highloft BFs are more or less 

efficient at inducing FPUF smoldering depending on their fiber content and bulk densities. 

BF-1, BF-2 and BF-3 contain blends of polyester and flame-retarded rayon fibers. The smoldering index

for BF-1 and BF-2 was identical (1.7) and the mass losses of complete assemblies incorporating these BFs

at 45 min were approximately 7 g ± 5 g. The CVFs were 17 % ± 13 %. BF-3, which also contained a 

polyester and flame-retarded rayon blend, showed a slightly higher smoldering index (2.2) compared to BF1 and BF-2. The mass loss for the complete assembly was 10 g ± 3 g, and the CVF at the end of the 45 min 

test was 22 % ± 5 % . BF-3 has a lower bulk density (0.031 g/cm3) as compared to BF-1 and BF-2. A 

lower bulk density with a similar fiber blend suggests that BF-3 had a higher thickness and greater porosity. 

Porosity is defined as the ratio of the total amount of void space in a material to the bulk volume occupied 

by the fibers. It is worth noting here that the permeability value for BF-3 was intermediate to those for BF1 and BF-2. However, the porous nature of BF-3 may permit easier oxygen transport to the reaction site by 

diffusion and convection. Previous studies43have shown that the primary means of oxygen permeation 

through a fabric is diffusion, whereas air permeability measurements are based on air flow resistance. Since 

smoldering in systems such as those studied here is usually an oxygen-limited, surface-area-dependent 

process, the higher porosity provides an explanation for the more extensive smoldering of BF-3 (see Table 

3 for char damage) and intense smoldering of the FPUF as compared to BF-1 and BF-2.

Among the highloft barrier materials, BF-4 and BF-5 contained boric acid-treated cotton. BF-5 was made 

of 100 % boric-acid treated cotton, whereas BF-4 was a stratified blend of boric-acid treated cotton, FR 

rayon, and polyester. A layer of FR rayon/polyester blend was sandwiched between layers of boric acidtreated cotton battings. Mock-up with these BFs (BF-4 and BF-5) showed the high mass loss for the 

complete assemblies (13 g ± 6 g), and CVFs were 33 % ± 14 %. BFs with boric acid-treated cotton exhibit 

enhanced smoldering compared to BFs with only FR rayon and polyester fiber blends. The boric acidtreatment of cotton fibers appears to be ineffective in reducing smoldering propensity in the presence of a 

smolder-prone cover fabric. In the presence of the smolder-prone CF, the additional heat provided by char 

oxidation of the CF along with the heat from the smoldering cigarette was sufficient to induce piloted

smoldering of boric acid-treated cotton. The char formation promoted in the presence of boric acid may 

have also encouraged smoldering via oxidation of this char. Studies have shown that the addition of 

inorganic additives (e.g., boric acid, phosphorus compounds) to cellulose can lead to a wide range of 

reactivities from enhancement to total inhibition of smoldering combustion44,48. Moreover, Ohlemiller44

has also shown that the effectiveness of boric acid as a smolder retardant is substantially reduced when used 

over a flammable substrate. 

3.4.2 Nonwoven barriers 
19

The mass loss curves for complete mock-up assemblies with nonwoven BFs covered with the smolderprone CF are shown in Figure 2 (b). All resulted in self-sustained smoldering of the FPUF by the end of 

the 45 min test. Among the nonwoven BFs, the mock-up with BF-9 exhibited a peculiar smoldering 

behavior. This mock-up did not start to lose mass until almost 22 min after the start of the test. However, 

the rate of mass loss for the mock-up assembly thereafter was very high. The smoldering was intense; so 

much so that the mock-up was manually extinguished before the end of the 45 min test. The CVF in this 

case was also higher (> 50 %) than found with the other nonwoven BFs evaluated in this study. The BF 

smolders along with the CF. BF-9 is a needle punched nonwoven material that has been treated to reduce 

flammability. The exceptionally high smoldering index of < 5.0 for BF-9 is in line with previous studies 

wherein flame-retarded fabrics have shown higher smoldering propensities.

45 The char of BF-9 was also 

very fragile and brittle. The digital images of charred BFs in Table 3 show that charred BF-9 lost its 

structural integrity as it decomposed during the smoldering process. 

The best performing nonwoven BF was BF-10 in terms of minimum mass loss of the mock-up and CVF 

(11 % ± 6 %). BF-10 is a blend of flame retarded polyester and flame retarded rayon. The flame retardant 

systems incorporated in BF-10 may be more effective in lowering the smoldering propensity than those

used in BF-8 and BF-9. The maximum char length on BF-10 was 100 mm ± 2 mm, which is similar to the 

maximum char length seen on the CF when tested with FPUF in the absence of a BF. However, the exact 

interaction between the smolder-prone CF and the BF is not clear at this stage and will be the subject of 

future studies. As noted from Table 3, BF-10 had a smaller charred area as compared to BF-9, and the 

smoldering index for BF-10 was 1.1. However, the char of BF-10 was so brittle that the charred portion of 

the BF completely disintegrated while separating components of the mock-up assembly after the test.

Mock-ups including nonwoven BFs (BF-21 and BF-22) consisting of highly flame-resistant para-aramid 

fibers also exhibited sustained smoldering behaviors. BF-21 is physically and thermally thin as compared 

to the other nonwoven BFs studied. As a result, heat transfer through this BF would be comparatively 

higher.

14 The mock-up assembly with BF-21 showed mass loss in just 5 min after the start of the test, and 

the average CVF was 24 % ± 1 %. The smoldering index for BF-21 was 2.4, whereas BF-22 yielded a 

value of 1.3. BF-22 is a composite fabric with two layers of nonwoven para-aramid fibers and a woven 

meta-aramid fabric quilted together. This type of fabric is used as a thermal liner in fire-fighter’s protective 

clothing and provides thermal insulation that protects the wearer from burn injury. The mass loss rate in 

mock-up assemblies with BF-22 was lower as compared to a BF with a similar thickness (BF-12). Better 

thermal insulation of BF-22 resulted in retaining a major proportion of heat within the layers of the BF and 

thereby restricting heat transfer to the FPUF. However, by the end of the 45 min test, the mock-up assembly 

with BF-22 covered with the smolder-prone CF had developed self-sustained smoldering.

BF-16 is a composite barrier with flame-retarded rayon and poly (lactic acid) fibers needlepunched on a 

woven glass-fiber fabric. BF-16 acts a passive fire barrier, and the flame-retarded rayon fibers char in place. 

The char forming ability of these fibers is favorable for the smoldering process. Thus, while the initial mass 

loss rate of the complete mock-up assembly was low, once the BF-16 started to smolder, the heat release 

from exothermic oxidation reactions of the BF char provided the required energy for smolder propagation 

through the barrier and into the FPUF. Beyond 30 min from the start of the test, significant smoldering of 

the FPUF (CVF = 22 % ± 8 %) was observed. The char of BF-16 (see Table 3) had better structural integrity 

compared to the other nonwoven BFs (BF-8, BF-9, BF-10, and BF-21) due to the woven glass-fiber 

substrate.
20

3.4.3 Knitted barriers

Knitted barrier fabrics (BF-11, BF-12 and BF-13) are produced by knitting core-spun yarn, which has an 

inherently fire-resistant fiber core (glass-fiber filament), and a sheath fiber that varies with performance 

requirements. BF-11 and BF-12 utilized flame-retarded acrylic fibers, whereas BF-13 had a flame-retarded 

rayon sheath. It can be noted from Table 2 that the knitted BFs have higher bulk densities and higher air 

permeabilities than highloft BFs. Since knitted fabrics have a loop structure, they have more open pores 

than woven fabrics; therefore, in general, the air permeability of knitted fabrics is higher than that of other 

fabrics of the same weight (compare BF-10 and BF-13). Thus, higher air permeabilities coupled with high 

bulk densities makes the structure more vulnerable to smoldering. The smoldering indices for BF-11, BF12, and BF-13 were 4.7, 3.5, and 1.7 respectively. The CVF increased with increasing area density of, BF13 (118 g/m3

), BF-12 (148 g/m3), and BF-11 (207 g/m3). The smoldering indices for BF-11 and BF-12 are 

consistent with the longer char lengths recorded (see Table 3). The lower smoldering propensity for the 

mock-ups with BF-13 may be associated with its lower bulk density and the type of sheath fibers. Mass 

loss curves for complete assemblies incorporating knitted barriers with smolder-prone cover fabrics are 

shown in Figure 2(d). BF-11 showed the highest mass loss rate followed by BF-12 and BF-13.

BF-23 is a knitted fabric with a cotton/glass fiber blend. The base fabric has an area density of 153 g/m2

and is backcoated with an intumescent flame-retardant system containing phosphorus and acrylic (85 wt % 

add-on). The coated fabric has an area density of 284 g/m2and is completely impermeable to air. With 

BF-23 interposed between FPUF and the smolder prone CF, the cigarette extinguished within 3 min from 

the start of the test, and no smoldering on the CF was observed. As mentioned earlier, the initial interaction 

between the smoldering cigarette and the substrate leads to reduced cigarette burn rate.46 The degree of 

reduction depends on the characteristics of the underlying materials (thermal conductivity, heat capacity, 

gas permeability, etc.). The reduction in cigarette burn rate may be minimal or the cigarette may extinguish. 

We propose that the impermeability of backcoated BF-23 to air and other gases is largely responsible for 

extinguishing the cigarette and lowering smoldering propensity. The mock-up with BF-23 showed no 

change in mass throughout the test duration, and the FPUF was completely protected in the presence of BF23. 

3.4.4 Woven barriers

Four out of five of the woven barrier fabrics examined in this study were comprised of woven glass

filaments. BF-20 is a para-aramid/melamine fiber-blend fabric generally used as an outer shell in fire fighter 

turnout gear. Such a fabric provides flame protection and serves as a primary defense to mechanical injury, 

heat, and fire. The inherently fire-resistant fibers in BF-20 have high decomposition temperatures (in excess 

of 350 °C), and it therefore takes longer times to initiate smoldering by a smoldering cigarette. Only a very 

small amount of localized smoldering was seen in the vicinity of the smoldering ignition source. The CF 

also did not smolder significantly. This observation can be attributed to an interaction between the 

smoldering propensities of the CF and the BF. As mentioned earlier, smoldering in the CF produces enough 

heat to char the adjacent pristine fabric to the point where smoldering can start. The heat generated is either 

dissipated into the underlying BF or conducted laterally along the CF. In the case of the FPUF/BF-20/CF 

combination, a large amount of heat was apparently dissipated to the BF, thereby reducing lateral heat 

transfer in the CF and limiting further smolder propagation. The CVF was negligible (0.6 % ± 0.1 %), and 

the mock-up assembly self-extinguished before the end of the 45 min test. BF-17, BF-18, and BF-19 consist 
21

of woven glass filaments. These BFs are completely impermeable to gases. With very small thicknesses 

(< 0.3 mm), these barrier fabrics have bulk densities in excess of 0.750 g/cm3. Mock-up assemblies 

consisting of such BFs self-extinguish as soon as the cigarette is completely burned or self-extinguished. 

The slight mass losses recorded for the complete mock-up assemblies (< 1 wt %) were either within the 

experimental error or due to gaseous mass loss as a consequence of CF smoldering.

In contrast, mock-ups incorporating BF-15, which is a woven fabric made from core-spun yarn, exhibited

a high smoldering propensity (smoldering index of 5.0). BF-15 is a physically and thermally thin barrier,

and the open weave results in high air permeability. Once sustained smoldering was established in the CF, 

around 25 min, the mass loss rate of the mock-up assembly increased rapidly (see Table 5). The FPUF 

smoldered significantly resulting in CVFs of 50 % ± 22 % at the end of 45 min test. 

Typical digital images of the char formed on various BFs during the smoldering ignition tests on mock-ups 

consisting of FPUF/BF/CF components are shown in Table 3. The qualitative analysis of chars on BFs 

suggests that typically highloft and nonwoven structures have very low residual strength, whereas the 

knitted and woven structures had better structural integrity compared to highloft and non-woven fabrics. 

Quantitative analysis of BF char is discussed below. 

3.5 Char length on BF and CVF of FPUF

Values of CVF for the FPUF are plotted against averages of maximum char length on the BFs in Figure 5. 

The CVFs show an almost linear relationship (R2 = 0.90) with non-zero char length values. It is also clear 

from the data in Figure 5 that highly significant differences in char lengths are produced due to variability 

in barrier fabric types. BF-9 produced the longest char length of 220 mm ± 44 mm followed by BF-15 

(191 mm ± 20 mm) and BF-11 (175 mm ± 14 mm). BF-15 and BF-11 have high air permeability levels 

with open woven and knitted structures respectively. However, both of the BFs are comprised of similar 

core-spun yarn. The high air permeability and thermal thinness of the two BFs facilitate increased oxygen 

supply to the smolder reaction zone, thereby supporting increased BF smolder rates, which result in greater 

heat release and peak temperature in the reaction zone.

41 The increased BF smoldering accelerates the rate 

of smolder spread both laterally over the fabric surface and into the FPUF. The longer char lengths on BF9 are likely attributable to the use of a flame retardant treatment that reduces smoldering resistance.

47
22

Figure 5. CVF of FPUF versus char length on BF

The shortest char lengths (< 75 mm) were produced on BFs having the lowest air permeability and which 

did not lead to sustained smoldering in the FPUF. Such lengths are comparable to the cigarette smolder 

length. The results of the char length analysis suggest that for a given BF structure, air permeability level 

has a significant effect on char lengths and smoldering propensity BF and thereby the FPUF. 

3.6 Air permeability of BF and CVF of FPUF

Figure 6 shows a plot of CVF of FPUF after the 45 min smoldering ignition test versus the air permeability 

for the BFs. The BF smoldering propensity and the CVF of FPUF are almost (R2 = 0.5) linearly related 

with air permeability, which suggests that additional parameters such as chemistry of fiber, fabric structure, 

and/or bulk density are responsible for the observed variations. Thus, air permeability is not a sufficient 

condition for smolder propagation in BFs, at least for configurations considered in this study. A low 

permeable BF structure restricts oxygen transport to the FPUF, thereby suppressing smolder propagation in 

the FPUF. The FPUF is mostly protected from smoldering with very little discoloration due to thermal 

impact. Studies20,42,48that have varied air permeability levels of selected upholstery fabrics while keeping 

the fiber composition the same have reported differences in smoldering times, char lengths and char areas. 

Donaldson et al42found that decreasing air permeability by a factor of 8 reduced the smoldering rate by 

14 %. 

y = 0.3x - 20.5

R2 = 0.90

0

10

20

30

40

50

60

0 50 100 150 200 250 Char volume fraction (CVF) of FPUF, %

Char length, mm 

Highloft BFs

Woven BFs

Knitted BFs

Nonwoven BFs
23

Figure 6. CVF of FPUF at the end of 45 min smoldering ignition tests versus air permeability.

4 Conclusions and Future Work

The smoldering propensity of BFs were assessed in terms of char length produced on BFs, mass loss of the 

standard mock-up assembly, and the CVF of the FPUF substrate when tested in a mock-up configuration 

using a cigarette ignition source. A smoldering index for BFs has been derived from CVF by varying the 

BF component in the FPUF/BF/CF mock-up systems, while holding the other two components constant. 

Smoldering could only be initiated in the FPUF when the BF was covered with a smolder-prone CF. Results 

of this study suggest that the smolder-prone CFs when placed over a number of BFs are capable of releasing 

sufficient heat to initiate the char-oxidation smoldering process in the BF, which can subsequently transmit 

the heat to the underlying FPUF. The smoldering propensity of BFs and thereby the amount of heat 

transmitted to the FPUF varies depending on the BF structure, fiber content, air permeability, and bulk 

density. The lower the smoldering propensity of the BF, the less likely is the development of self-sustained 

smoldering in the FPUF.

Of the 20 BFs tested in this study, mock-ups with five BFs self-extinguished and passed (see Table 5) the 

smoldering ignition test criterion described in CPSC’s proposed regulation for RUF (16 CFR 1634). The 

BFs that exhibited self-extinguishing behaviors contained inherently fire resistant fibers and had air 

permeability values of less than 0.2 m/s. BF-9, BF-11 and BF-15 had the highest smoldering propensities,

resulting in charring of a large fraction of the FPUF. 

BFs with highloft structures, which have higher porosity, favor surface oxidation reactions and hence 

extensive smoldering. Compared to their performance in residential mattresses, the boric acid treated cotton 

battings were less effective in reducing the smoldering propensity of the mock-up configuration in this 

0

5

10

15

20

25

30

35

40

45

50

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Charred volume fraction (CVF) of FPUF, %

Air permeability, m/s

Knitted BFs

Woven BFs

Highloft BFs

Nonwoven BFs
24

study, i.e., containing smolder-prone FPUF and CF. Other flame retardant treatments and use of char 

forming fibers showed a greater tendency for BF smolder in the presence of the smolder-prone CF. For 

BFs with similar fiber composition and similar structures, the smoldering propensity increased with an 

increase in bulk density. A less permeable BF structure restricts oxygen transport to the FPUF, thereby 

suppressing smolder propagation in the FPUF. However, air permeability is not a sufficient condition for 

smolder propagation. 

It is clear from the above discussion that smoldering ignition of FPUF by BFs is influenced by incident heat 

flux, oxygen availability, and BF properties. BF properties such as structure, density, type of weave, surface 

treatment, compressibility, air permeability, and fiber chemistry (charring, low charring and non-charring 

fibers) influence thermal properties such as thermal diffusivity, thermal resistance, and thermal 

conductivity. Both thermal diffusivity and thermal resistance are derived parameters and are functions of 

thermal conductivity. Thermal diffusivity is defined as the ratio between thermal conductivity and the 

volumetric heat capacity. Thermal resistance is the ratio of thickness and thermal conductivity of a fabric. 

Thermal conductivity is an intrinsic property of a material that indicates its ability to conduct heat (energy 

per unit area per unit time) divided by the temperature gradient. For textile materials, air within the fabric 

structure is the most important factor controlling the conductivity and is strongly influenced by the fabric 

structure. The fabric thickness, density, and heat capacity all play a role in determining the thermal 

conductivity and hence thermal resistance and thermal diffusivity of barrier fabrics. It is expected that the 

study of aforementioned thermo-physical properties will be useful in further understanding mechanisms of 

smoldering in barrier fabrics. Future work is planned to determine explicit relations between BF’s ability 

to convey heat from a CF/covered cigarette to the FPUF, such as to induce smoldering in the FPUF.
25

References 

[1] 16 CFR Part 1633 Standard for the flammability (open- flame) of mattress sets. Consumer Product

Safety Commission. March (2007). Available from: 

http://www.cpsc.gov/businfo/frnotices/fr06/mattsets.pdf.

[2] Technical Bulletin 129: Flammability Test Procedure for Mattresses for Use in Public Buildings, 

October 1992, State of California Department of Consumer Affairs Bureau of Home Furnishings 

and Thermal Insulation 3485 Orange Grove Avenue North Highlands, CA 95660-5595.

[3] Technical Bulletin 603: Requirements and Test Procedure for Resistance of a Mattress/Box Spring 

Set to a Large Open-Flame, July 2003, State of California Department of Consumer Affairs Bureau 

of Home Furnishings and Thermal Insulation 3485 Orange Grove Avenue North Highlands, CA 

95660-5595, available from http://www.peopleforcleanbeds.org/cal%20burn%20test%20603.pdf.

[4] Technical Bulletin 133, Flammability Test Procedure for Seating Furniture for Use in Public 

Occupancies, State of California, Department of Consumer Affairs, Bureau of Home Furnishings 

and Thermal Insulation, January (1991).

[5] Flammability Requirements for Aircraft Seat Cushions, Appendix F, Part II of Section 25.853 of 

the Federal Aviation Regulations (FAR), Federal Aviation Administration, see also Flammability 

Requirements for Aircraft Seat Cushions, Advisory Circular Number 25.853-1, Federal Aviation 

Administration, September 17, 1986.

[6] Nazaré S, Davis R. A review of fire-blocking technologies for soft furnishings, NIST Technical 

Note 1728, November 2011, National Institute of Standards and Technology, Gaithersburg MD. 

[7] Zota AR, Rudel RA, Morello-Frosch RA, Brody JG. Elevated house dust and serum 

concentrations of PBDEs in California: Unintended consequences of furniture flammability 

standards? Environmental Science and Technology. 2008; 42, (21):8158-8164.

[8] Chivas C, Guillaume E, Sainrat A, and Barbosa V. Assessment of risks and benefits in the use of

flame retardants in upholstered furniture in continental Europe, Fire Safety Journal. 2009; 44(5):

801-807.

[9] Technical Bulletin 117-2013: Requirements, Test Procedure and Apparatus for Testing the 

Smolder Resistance of Materials Used in Upholstered Furniture, Department of Consumer Affairs, 

Bureau of Home Furnishings and Thermal Insulation, June (2013).

[10] Standard No. 302: Flammability of Interior Materials, Federal Motor Vehicle Safety Standards, 

U.S. Department of Transportation, September 24, 1998.

[11] Krasny JF, Parker WJ, Babrauskas V. Fire behavior of upholstered furniture and mattresses., 

William Andrew Publishing, LLC Norwich, New York, USA. 2001 Edition.

[12] Damant GH. Cigarette ignition of upholstered furniture. Journal of Fire Sciences. 1995; 13:337 - 

349.

[13] Mikelson DA, Evaluation of staple polyester/microfiberous polyolefin blended batting as a 

potential thermal insulating material for clothing. Technical Report NATICK/TR-81/007, United 

States Army Natick Research and Development Laboratories Natick, Massachusetts 01760. June 

1980.

[14] Nazaré S, Pitts W, Flynn S, Shields J and Davis R, Towards a New Approach for Evaluating FireBlocking Barrier Fabrics, NIST TN 1798, April 2013, National Institute of Standards and 

Technology, Gaithersburg MD. DOI http://dx.doi.org/10.6028/NIST.TN.1798.

[15] Spears AW, Rhyne AL, and Norman V. Factors for consideration in a test for cigarette Ignition 

Propensity on soft furnishings, Journal of Fire Sciences 1994; 1: 59-84.
26

[16] Upholstery Furniture Action Council, UFAC Central Box 2436 High Point, NC 27261 

http://www.ufac.org/method1.htm

[17] Technical Bulletin 116: Requirements, Test Procedure and Apparatus for Testing the Flame 

Retardance of Upholstered Furniture, State of California Department of Consumer Affairs 

Bureau of Home Furnishings and Thermal Insulation 3485 Orange Grove Avenue North 

Highlands, CA 95660-5595, available from http://www.bhfti.ca.gov/industry/116.pdf.

[18] ASTM E 1352-08a, Standard Test Method for Cigarette Ignition Resistance of Mock-Up 

Upholstered Furniture Assemblies, ASTM International, 100 Barr Harbor Drive, PO Box C700, 

West Conshohocken, PA 19428-2959.

[19] NFPA 261: Standard method of test for determining resistance of mock-up upholstered furniture 

material assemblies to ignition by smoldering Cigarettes, 2013 Edition , National Fire Protection 

Association, Quincy.

[20] Travers EB, and Olsen NF. Effect of air permeability on smoldering characteristics of cotton 

upholstery fabrics. Textile Research Journal. 1982; 52:598-604.

[21] Upholstery Furniture Action Council, UFAC Central Box 2436 High Point, NC 27261 

http://www.ufac.org/method3.htm

[22] ASTM E1353-08a, standard test method for cigarette ignition resistance of components of 

upholstered furniture. ASTM Standards for Upholstered Furniture, Mattresses and Bedding, 

ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-

2959.

[23] NFPA 260 Standard methods of tests and classification system for cigarette ignition resistance of 

components of upholstered furniture, National Fire Protection Association (NFPA), 1 

Batterymarch Park, Quincy, MA 02269-9101 USA.

[24] Williams SS, Damant G., Rate of weight loss as an aid to evaluate the smoldering propensity of 

upholstery fabrics. Journal of Consumer Product Flammability, 8, June; 89-104, 1981

[25] 16 CFR Part 1634, Standard for the flammability of residential upholstered furniture (proposed 

rule). Consumer Product Safety Commission. March (2008). Available from: 

http://www.cpsc.gov/businfo/frnotices/fr08/furnflamm.pdf.

[26] Madacsi JP, and Neumeyer JP. A comparison of smolder resistance standards for boric acid 

treated cotton batting, Journal of Consumer Product Flammability. 1982; 9:3-10.

[27] ASTM D5238 – 10 Standard test method for smoldering combustion potential of cotton-based 

batting. ASTM Standards for Upholstered Furniture, Mattresses and Bedding, ASTM 

International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959.

[28] Wakelyn PJ, Wolf S, Oliver K. Cotton batting fire-blocking barriers for soft furnishings. Presented 

at the 14th Annual Business Communications Company, Inc. (BCC) Conference on Flame 

Retardancy June 3, 2003. Stamford, CT.

[29] Zammarano M, Matko S, Pitts WM, Fox D M, Davis RD. Towards a Reference Polyurethane 

Foam and Bench Scale Test for Assessing Smoldering in Upholstered Furniture, Polymer 

Degradation Stability, In press, available at 

http://www.sciencedirect.com/science/article/pii/S0141391013004151

[30] http://www.frazierinstrument.com/products/fap/fap-faq.html#Air%20Permeability%20-

%20Size%20of%20Unit:

[31] Olsen, NF and Bollinger JR. Filtered and non-filtered cigarette ignition of cotton upholstery 

fabrics. Textile Research Journal. 1980; 50: 310-315.

[32] Zammarano M, Krämer R H, Szabolcs M, and Davis RD. Standard Operating Procedures for 

Smolder Ignition Testing of Upholstery Fabrics, TN1775, November 2012. 24 pgs. National 
27

Institute of Standards and Technology, Gaithersburg MD. Available at 

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

[33] Zammarano M, Krämer RH, Matko S, Mehta S, Gilman JW, Davis RD. NIST TN 1747 - Factors 

Influencing the Smoldering Performance of Polyurethane Foam. 2012.

[34] Ihrig AM, Spears AW, Rhyne AL, and Norman V. Factors involved in the ignition of cellulosic 

upholstery fabrics by cigarettes, Journal of Fire Sciences. 1986; 4: 237-260.

[35] http://www.nist.gov/customcf/get_pdf.cfm?pub_id=902075

[36] Damant GH, Flammability aspects of upholstered furniture. Journal of Consumer Product 

Flammability. 1976; 3; 21-61.

[37] Salig RJ. The Smoldering Behavior of Upholstered Polyurethane Cushionings and Its Relevance 

to Home Furnishing Fires. Thesis (M.S.). Cambridge, MA: Massachusetts Institute of Technology; 

1982.

[38] Ortiz-Molina MG, Mak AY, Tesoro GC, Toong TY. Report, Smoldering Combustion of Cellular 

Plastics and Its Transition to Flaming or Extinguishment. Final Report. Department of 

Mechanical Engineering, Massachusetts Institute of Technology for Products Research Committee 

(RP-76-U-3) June 1979.

[39] Rogers FE, Ohlemiller TJ, Kurtuz A, and Summerfield M. Studies of the smoldering combustion 

of flexible polyurethane materials. Journal of Fire and Flammability, 1978; 9:5-13.

[40] Chao CYH., and Wang JH., Comparison of the thermal decomposition behavior of a non-flame 

retarded and a fire retarded flexible polyurethane foam with phosphorus and brominated additives. 

Journal of Fire Sciences. 2001; 19:137-156.

[41] Ohlemiller TJ, Smoldering combustion. In chapter 11, Section 2, 171-179, SFPE Handbook of 

Fire Protection Engineering, 2nd Edition. Eds DiNenno PM et al. 

[42] Donaldson DJ, Yeadon DA, and Harper RJ Jr. Smoldering characteristics of cotton upholstery 

fabrics, Textile Research Journal. 1981; 51:196-202.

[43] Ohlemiller TJ, Villa KM, Braun E, Eberhardt KR, Harris RH Jr., Lawson JR, and Gann RG. Test 

methods for quantifying the propensity of cigarettes to ignite soft furnishings, NIST SP 851, 

August 1993, National Institute of Standards and Technology, Gaithersburg MD.

[44] Shafizadeh F, Bradbury AGW, DeGroot WF, and Aanerud TW. Role of inorganic additives in 

the smoldering combustion of cotton cellulose, Ind. Eng. Chem. Prod. Res. Dev. 1982; 21, (1).

[45] Tesoro GC, The fire safety of upholstered furniture: An evaluation of the state-of-the art and 

potential improvements, Journal of Consumer Product Flammability. 1981; 8:132-143.

[46] Lewis LS, Morton MJ, Norman V, Ihrig AM, and Rhyne AL. The effects of upholstery fabric 

properties on fabric ignitabilities by smoldering cigarettes II. Journal of Fire Sciences, 1995; 445-

471.

[47] Damant GH, and Young MA., Flammability classification of fabrics used as upholstery furniture 

coverings, Journal of Consumer Products Flammability, 4:60-113, 1977.

[48] Fresman G., A study in smoldering test behavior versus air flow variations in fabric, Journal of 

Coated Fabrics. 1984; 13 (3): 184-192.

[49] Taylor BN and Kuyatt CE, Guidelines for Evaluating and Expressing the Uncertainty of NIST 

Measurement Results, NIST Technical Note 1297, 1994 Edition (Supersedes 1993 Edition), 

Gaithersburg, MD, 20878. 

[50] Evaluation of measurement data — Guide to the expression of uncertainty in measurement. 

JCGM 100:2008. http://www.bipm.org/utils/common/documents/jcgm/JCGM_100_2008_E.pdf