i

NIST Internal Report 7805

Interagency Agreement Report: 

Exposure and Fire Hazard Assessment 

of Nanoparticles in Fire Safe 

Consumer Products

Rick D. Davis

Yeon Seok Kim

Richard H. Harris

Marc Nyden

Nasir Uddin

Treye Thomas

August 2011
ii

NIST Internal Report 7805

Interagency Agreement Report: 

Exposure and Fire Hazard Assessment 

of Nanoparticles in Fire Safe 

Consumer Products

Rick D. Davis

Yeon Seok Kim

Richard H. Harris

Marc Nyden

Nasir Uddin

Fire Research Division

Engineering Laboratory

National Institute of Standards and Technology

Treye Thomas

Directorate of Health Services

Consumer Products Safety Commission

August 2011

U.S. Department of Commerce 

Gary Locke, Secretary 

National Institute of Standards and Technology 

Patrick D. Gallagher, Director 
iii

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 and Internal Report

Natl. Inst. Stand. Techn. Internal Report 7805, 57 pages (August 2011)

CODEN: NTNUE2
iv

Abstract

An innovative technology was evaluated as a potential approach to generate flammability, 

environmental, health, and safety compliant soft furnishings. Nanoparticle-based nanometer thin 

film coatings on standard polyurethane foam and non-woven fire blocking barrier fabrics were 

applied using a waterborne approach called Layer-by-layer (LbL) assembly. This is the first 

reportof LbL on a complex three dimensional substrate to improve the fire resistance of foam 

and barrier fabrics using sodium montmorillonite clay (MMT), carbon nanofibers (CNF), and 

multi-walled carbon nanotubes (MWCNT). The LbL process and recipe was tailored for each 

nanoparticle type (CNF, MWCNT, and MMT) in order to fabricate durable coatings that 

completely covered the entire internal and external surfaces of the substrate. The CNF and 

MWCNT coatings on foam were thinner and contained fewer nanoparticles, but resulted in the 

greatest reduction in peak heat release rate (a measure of material flammability). The reduction 

in foam peak heat release due to the nano-fire retardant\LbL coatings is as high as 1100 % 

greater than 17 other commercial fire retardants commonly used to reduce foam flammability. 

This technology has strong commercial viability for foam due to ease and flexibility of the LbL 

process and the significant reduction in foam flammability caused by the coatings. However, 

does not appear to be a valid approach for barrier fabrics as the non-woven structure 

disassembled during the fabrication process.

In order to enable other agencies to assess the environmental, health, and safety risk of using this 

nanoparticle-based technology for reducing the flammability of soft consumer products, this 

project developed the methodology to promote, collect, and quantify nanoparticles released from 

various substrates when exposed to various conditions. In general, the release of nanoparticles 

was an order of magnitude higher from simulated chewing than simulated wear and tear, highest 

from the barrier fabric, and lowest for MMT. The release was between 0.50 mass fraction % to 

0.0003 mass fraction % of the total nanoparticle loading on the substrate. The measurement 

methodology developed enables the quantification of nanoparticles with greater accuracy and at 

an order of magnitude lower concentration than current measurement methods.

Keywords

Nanoparticle; flammability; foam; mattress; upholstered furniture; service-life; fire retardants; 

Layer-by-layer assembly; cone calorimetry; heat release rate; carbon nanofiber; carbon 

nanotubes; clay, barrier fabric; Consumer Product Safety Commission
v

Acknowledgements

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

technical support for this project. Interagency Agreement Number: CPSC-I-08-0008
vi

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vii

Contents

Abstract.............................................................................................Error! Bookmark not defined.

Acknowledgements ..........................................................................Error! Bookmark not defined.

List of Tables................................................................................................................................ ix

List of Figures................................................................................................................................ x

List of Acronyms..............................................................................Error! Bookmark not defined.

1. Introduction ......................................................................................................................... 15

2. Experimental ....................................................................................................................... 17

2.1. Materials ................................................................................................................................ 17

2.1.2. CNF-based bilayered nanocoatings............................................................................. 19

2.1.3. MWCNT-based trilayered nanocoatings..................................................................... 20

2.1.4. MMT-based trilayered nanocoatings........................................................................... 21

2.2. Fire performance testing ........................................................................................................ 22

2.3. Simulated aging ..................................................................................................................... 23

2.2. Characterization ..................................................................................................................... 24

3. Results and Discussion ........................................................................................................ 26

3.1. Characterization of nanocoatings........................................................................................... 28

3.1.1. CNF-based nanocoatings............................................................................................. 29

3.1.2. MWCNT-based nanocoatings ..................................................................................... 32

3.1.2. MMT-based nanocoatings........................................................................................... 36

3.2. Fire performance.................................................................................................................... 39

3.2.1. CNF-based nanocoatings............................................................................................. 39

3.2.2. MWCNT-based nanocoatings............................................................................................. 40

3.2.3. MMT-based nanocoatings........................................................................................... 41

4.4. Comparison to other flame retarding technologies......................................................... 44

Flame Retardant Location ......................................................................................................... 46

% Reduction relative to PUF..................................................................................................... 46

In coating on foam ...................................................................................................................... 46

Embedded in foam...................................................................................................................... 46

3.5. Nanoparticle release from stressing....................................................................................... 46

3.5.1. Measurements methodology........................................................................................ 46

3.5.2. Stress induced nanoparticle release ............................................................................. 50
viii

4. Conclusion............................................................................................................................ 52

5. Future Research................................................................................................................... 54

6. References............................................................................................................................. 54
ix

List of Tables

Table 1. Provided are the average physical characteristics of nanoparticle coated substrates 

organized by substrate type and highest to lowest in nanoparticle content. BFs coating thickness 

is assumed to be similar to that measured on PUF. MMT coatings are sufficiently different on 

PUF and BF that the coating thickness on PUF is believed to not be a good estimate of the BF 

coating thickness. Values reported with 2σ uncertainty. ............................................................. 28

Table 2. Cone Calorimetry data of the washed uncoated and coated PUF organized from highest 

to lowest in PHRR (PHRR in bold). Values reported with 2σ uncertainty................................ 42

Table 3. Cone Calorimetry data of the uncoated and coated BF organized from highest to lowest 

in PHRR. Values reported with 2σ uncertainty. .......................................................................... 43

Table 4. Reduction in Cone data (relative to pure PUF) caused by FR in a LbL coating on and 

embedded in foam. Adjusting for different FR loadings, the LbL approach can result in the 

largest reduction in PHRR. No uncertainty was reported for the literature values. Experimental 

values reported with 2σ uncertainty.............................................................................................. 46

Table 5. Simulated wear and tear nanoparticles released relative to the total nanoparticle content 

on the specimen (organized from highest to lowest in nanoparticle release). Values reported with 

2σ uncertainty. .............................................................................................................................. 52

Table 6. Simulated chewing nanoparticles released relative to the total nanoparticle content on 

the specimen (organized from highest to lowest in nanoparticle release). Values reported with 2σ

uncertainty..................................................................................................................................... 52

 

 
x

List of Figures

Figure 1. The CNF/polymer coating process was an alternating submersion in a cationic 

(CNF/PEI) and an anionic (PAA) solution with washing (rinse and wring) between each solution. 

After creating 4 BLs (BL is CNF:PEI/PAA), the specimen was dried in a convection oven for. 19

Figure 2. The MWCNT/polymer coating process was an alternating submersion in an anionic 

(PAA), MWCNT cationic (MWCNT-PEI) and polymer cationic (PEI only) solutions with 

washing (rinse and wring) between each solution. After creating four TLs (TL is PAA\MWCNTPEI\PEI), the specimen was dried in a convection oven for 12 h at 70 °C ± 1 °C to remove excess 

water.............................................................................................................................................. 21

Figure 3. The MMT/polymer coating process was an alternating submersion in a polymer 

cationic (PEI), clay anionic (MMT), and polymeric anionic (PAA) solutions with washing (rinse 

and wring) between each solution. After creating five TLs (TL is PEI\MMT\PAA), the specimen 

was dried in a convection oven for 12 h at 70 °C ± 1 °C to remove excess water. ...................... 22

Figure 4. Simulated saliva extractions were performed using bottles containing a simulated saliva 

and a substrate that were rotated at 6.3 rad/s (60 rev/min) for 30 min. ........................................ 24

Figure 5. Simulated normal wear and tear stressing was performed by pounding substrates at 1 

cycle/s for 100,000 cycles at 20682 Pa ± 69 Pa of pressure......................................................... 24

Figure 6. SEM images of as-received BF (a) 20,000x, (b) 50,000x, (c) 100,000x, and (d) 

200,000x. The 27 μm ± 3 μm diamter strand is a bundle of several fibers that “welded”during 

manufacturing. .............................................................................................................................. 26

Figure 7. SEM images of as-received PUF at (a) 1,000x, (b) 2,000x, (c) 5,000x, (d) 10,000x, (e) 

20,000x, (f) 50,000x and (g) 100,000x and washed PUF at (h) 5,000x. The PUF surface was 

smooth and featureless after debris (dust, etc.) was removed by washing (h).............................. 27

Figure 8. SEM images of the inside section of a CNF/PUF at (a) 1,000x, (b) 5,000x, (c) 10,000x, 

and (d) 20,000x, of a thicker island at (e) 50,000x (f) 100,000x, and (g) 200,000x, and of an 

aggregate at (h) 200,000x. ............................................................................................................ 30

Figure 9. SEM images of a fractured edge of CNF/PUF at (a) 20,000x, (b) 100,000x, (c) 

200,000x, and (d) 500,000x. The CNF coating is 359 nm ± 36 nm. ........................................... 31

Figure 10. SEM images of a delaminated CNF/PUF at (a) 10,000x, (b) 50,000x, (c) 100,000x, 

and (d) 200,000x. The CNFs below the surface are welded together with polymer. The root 

cause of delimitation may be the freeze fracture process or poor adhesion to the PUF due to the 

high CNF concentration................................................................................................................ 31
xi

Figure 11. SEM images of the inside section of a CNF/BF at (a) 20,000x, (b) 50,000x, (c) 

100,000x, and (d) 200,000x and (e) 200,000x. The CNF coatings on the PUFs and BFs appear to 

be similar....................................................................................................................................... 32

Figure 12. SEM images of the inside section of a MWCNT/PUF at (a) 1,000x, (b) 5,000x, (c) 

10,000x, (d) 20,000x, (e) 50,000x (f) 100,000x, (g) 200,000x, and (h) 500,000x. The MWCNTs 

were well dispersed and distributed throughout the polymer coating. The coating was smooth 

and featureless except for a few small larger aggregates and a few 10 nm ± 5 nm wide cracks.. 34

Figure 13. SEM images of a fractured edge of MWCNT/PUF at (a) 50,000x, (b) 100,000x, (c) 

200,000x, and (d) 500,000x. The MWCNT coating thickness is 440 nm ± 47 nm..................... 35

Figure 14. SEM images of the inside section of a MWCNT/BF (a) 20,000x, (b) 50,000x, (c) 

100,000x, and (d) 200,000x. The MWCNT coatings appear similar on PUF and BF. ............... 35

Figure 15. SEM images of the inside section of a MMT/PUF at (a) 1,000x, (b) 5,000x, (c) 

10,000x, (d) 20,000x, (e) 50,000x (f) 100,000x, and (g) 200,000x.............................................. 37

Figure 16. SEM images of a fractured edge of MMT/PUFat (a) 10,000x, (b) 20,000x, (c) 

50,000x, and (d) 100,000x. The MMT coating was 1000 nm ± 450 nm..................................... 38

Figure 17. SEM images of a MMT/BF (a) 10,000x, (b) 20,000x, (c) 50,000x, (d) 100,000x and 

(e) 200,000x. The MMT coatings are on PUF and BF. ............................................................... 38

Figure 18. HRR curves indicate the MWCNT and CNF coatings significantly reduce PHRR, 

THR, and total burn time, but MMT coatings increase flammability, as compared to pure PUF. 

The 2σ uncertainty is ± 5% in HRR and ± 2 s in time.................................................................. 43

Figure 19. HRR curves of the uncoated and coated BF. All nanoparticle coatings deteriorate the 

fire performance of the BF. The 2σ uncertainty is ± 5% in HRR and ± 2 s in time.................... 44

Figure 20. UV-VIS absorbance plot of a CNF/DI/SDS suspension prepared without sodium 

chloride, with sodium chloride, and the sodium chloride removed by dialysis. Dialysis was 

critical to obtaining accurate and repeatable absorbance values at the MWCNT and CNF peak 

maximum (267 nm) for the simulated chewing suspensions........................................................ 47

Figure 21. A Beer’s Law curve was constructed from the UV-VIS absorbance value of several 

CNF or MWCNT calibration standards (CNF provided here). CNF and MWCNT concentration 

was quantified based on the measured UV-VIS absorbance value of a stressed suspension and a 

Beer’s Law curve. The 2σ uncertainty is ± 5 % of the CNF or MWCNT mass fraction value... 48

Figure 22. ICP-OES intensity as a function of element type and concentration using a diluted 

nitric acid/DI solution containing 0.0010 mass fraction % ± 0.0005 mass fraction % MMT and 
xii

five different concentrations of each element. The concentration at an intensity of 0 cps is the 

amount of each element in MMT. The 2σ uncertainty is 5 % of the value (error bars are smaller 

than the data markers)................................................................................................................... 49

Figure 23. The amount of each element in MMT in a diluted nitric acid/DI solution containing 

0.0010 mass fraction % ± 0.0005 mass fraction % MMT in diluted. The error bars represent a 2σ

standard uncertainty in the concentration. .................................................................................... 49

Figure 24. Beer’s Law calibration curve of ICP-OES intensity as a function of MMT 

concentration. The 2σ uncertainty is 5 % of the value (error bars are smaller than the data 

markers). ....................................................................................................................................... 50
xiii

List of Acronyms

ANPR Announced notice of proposed rulemaking

ASTM American Society for Testing and Materials

BF Barrier fabric

BL Bilayer

CNF Carbon nanofibers

CNF/BF Carbon nanofiber coated barrier fabric

CNF/PUF Carbon nanofiber coated standard polyurethane foam

CPSC Consumer Product Safety Commission

DI Deionized (water)

DMF N,N-dimethylformamide

ICP-OES Inductively Coupled Plasma – Optical Emission Spectrometer

LbL Layer-by-layer assembly/coating

MMT Montmorillonite clay

MMT/BF Montmorillonite clay coated barrier fabric

MMT/PUF Montmorillonite clay coated standard polyurethane foam

MWCNT Multi-walled carbon nanotubes

MWCNT/BF Multi-walled carbon nanotubes coated barrier fabric

MWCNT/PUF Multi-walled carbon nanotubes coated standard polyurethane foam

NIST National Institute of Standards and Technology

PAA Poly(acrylic acid)

PEI Branched Polyethyleneimine

PHRR Peak heat release rate

PUF Standard polyurethane foam

SEM Scanning Electron Microscopy

TGA Thermal gravimetric analysis

THR Total heat released

TL Trilayer
xiv

Page Intentionally Left Blank
15

1. Introduction

The estimated annual total societal cost of fire to the United States economy is $360 billion [1] 

with fires in structures, such as single and multi-family dwellings, and fixed mobile homes,

accounting for an estimated $90 billion of this total cost [2]. There are a reported 135,000 

residential home fires annually, 5 % of which are accounted for by soft furnishings (mattresses, 

bedclothes, upholstered furniture) as the first item ignited. However, first item ignited soft 

furnishings are annually estimated to account for a disproportionately high amount of fire losses 

(33 % of the civilian fatalities, 18 % of civilian injuries, and 11 % of the property losses) [3,4]. 

Even though no amount of money can adequately represent personal injury and deaths, the 

estimated annual societal cost of fire associated with soft furnishings as the first item ignited is 

estimated at $5 billion based on $5 million per fatality and $230,000 per injury as used by Hall

[1] in calculating the total societal cost of fire.

The Consumer Products Safety Commission (CPSC) is responsible for existing and proposed 

United States flammability regulations of soft furnishings [5,6,7]. These regulations, as well as 

the introduction of Reduced Ignition Propensity cigarettes [8], are expected to significantly 

reduce the $5 billion annual cost of soft furnishing fires. In order to comply with the 2007 open 

flame mattress regulation (CPSC 16 CFR 1633 [6]), manufacturers inserted fire blocking barrier 

fabrics around the soft polyurethane foam core. CPSC research (unpublished) suggests that 

upholstered furniture manufacturer’s will likely also require fire blocking barrier fabrics in order 

to comply with the proposed open flame/smoldering ignition regulation for upholstered furniture 

(CPSC ANPR 16 CFR 1634 [7]). Similar to manufacturer of other fire safe products, the 

technical and engineering options to comply with national and/or international fire performance 

regulations are quickly diminishing because of mandated sustainability regulations for consumer 

products (e.g., REACH [9] and EcoLabel [10]).

Layer-by-Layer (LbL) assembly has been extensively studied for the past 20 year as a 

methodology to create multifunctional films generally less than 1μm thick [11,12,13]. The thin 

film coatings were commonly fabricated by alternate deposition of a positively charged layer and 

negatively charge layer (called a bilayer, BL). By taking advantage of electrostatic, H-bonding

[14], covalent bonds [15], and/or donor/acceptor interactions, these bilayers are continuously 

assembled on the surface of flat substrates. The LbL process is quite flexible and robust, which 

allows it to be tuned for specific coating characteristics and for coating a range of substrate

types. For example, altering the concentration, pH, and/or temperature of the LbL solutions can

result in a 1 nm rather than 100 nm thick BL [16,17]. LbL thin films have been used in an 

extensive breadth of applications, such as oxygen barriers [18] and sensors [19], and have useful 

properties, such as antimicrobial [20] and antireflection [21]. A more recent application, which 

is directly aligned with the research presented in this manuscript, was LbL montmorillonite 

(MMT) coatings (sodium exchanged MMT) of cotton fabric to improve the fire performance 

characteristics of this textile [13]. MMT has been extensively studied in LbL thin films 

[18,22,23] and, when used as an additive filler, has been shown to simultaneously improve the 

mechanical and fire performance attributes of polymers [24,25,26]. The uniqueness this 

approach is the concept of improving fire performance by using LbL assembly and creating LbL

MMT coatings on cotton fabric (MMT/cotton) [13]. The results are exciting in that Li achieved 

complete and uniform high quality MMT based coatings on cotton. In addition, the MMT
16

coatings resulted in a significant retention of fabric like char after conducting vertical burn tests 

and there was no or less ember afterglow when the flame was removed. These results suggest 

the coating may better prevent thermal and flame penetration from reaching and igniting the 

foam (PUF), and therefore, the MMT/cotton may reduce the risk of fire spread in residential 

homes if used in soft furnishings.

Natural and synthetic clays are one of the most widely studied additives to improve polymer fire 

performance through the formation of a nanocomposite [27]. The ease of functionalizing the 

clay with a range of different organic treatments is the main reason for clay’s versatility and 

popularity as a polymer additive. Smectite clays are perhaps the most commonly used because 

they are water dispersible making them ideal for organic modification. In contrast to other fire 

retardants, clay at a minimum can often improve fire performance while maintaining other 

polymer physical and mechanical attributes of the polymer. This is attributed to both the 

significantly lower loading levels of clay (compared to other flame retardants) to obtain the same 

performance and the interfacial interactions between the clay and polymer. Synergistically 

combining flame retardants with clay is not necessary intuitive. For example, a 2 mass fraction 

% MMT added to polystyrene with (10, 15, or 20) mass fraction % ammonium polyphosphate 

and pentaerythriol (APP/PER) resulted in a reduce flame spread rate as compared to a similar 

loading level of just APP/PER. In contrast, all other combinations ((4, 6, 8, and 10) mass 

fraction % MMT with (5, 10, 15, 20, 25, or 30) mass fraction % APP/PER) resulted in an 

increased flame spread rate (as high as 20% faster) as compared to a similar loading level of just 

APP/PER [28].

Carbon nanofibers (CNFs) are cylindrical nanostructures constructed of stacked graphitic cones 

or cups. Compared to carbon nanotubes (CNT), CNFs can be at least an order of magnitude 

larger with a diameter and length in the range of 5 nm to 300 nm and 0.1 μm to 1000 μm, 

respectively. Due to the intrinsic electrical, thermal, and mechanical properties of CNFs, the 

thermal and electrical conductivity, tensile and compressive strength, ablation resistance, 

damping properties, and flammability of polymers [29] have been significantly altered with 

incorporation of CNF [30]. 

Recently, a reduction in PUF flammability was reported by the incorporation of CNFs directly 

into the PUF [29]. At a 4 mass fraction % CNF loading, the CNFs formed a network structure 

that reduced the peak heat release rate (PHRR) by 35% and prevented melt dripping. The 

approach of incorporating CNFs into the PUF has a few potential drawbacks. For example, 

commercialization may be difficult as the foam manufacturing process is extremely sensitive to 

small changes in recipe, especially the presence of solid particles. Another potential drawback is 

based on the mechanism by which nanoparticles are believed to reduce polymer flammability

[31]. It is has been proposed the reduction in flammability primarily results from the formation 

of a char at the surface that thermally protects the polymer and prevents volatilization of polymer 

degradation products. Perhaps placing the nanoparticles at the surface could accelerate char 

formation.

Compared to CNFs, CNT are significantly smaller in dimension. Single-walled carbon 

nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT) have received a lot of 

attention in the science community because their inherent characteristics, such as small size and 
17

high aspect ratio (diameter of 1 nm to 100 nm and length of 1 μm to 100 μm for SWCNT and 

MWCNT, respectively) [32,33], high modulus (approximately 1 TPa) [34], high intrinsic 

electrical conductivity (σ > 104 S/cm) [35], and high thermal conductivity (k > 1000 W/m·K) 

[36], are expected to impart electrical conductivity, mechanical strength, and thermal 

conductivity to a polymer when the CNTs are incorporated into the polymer. However, CNTs 

continue to be underutilized partially due to the difficulties in generating stabilized CNT 

suspensions. To improve stability researchers have used non-covalent stabilizing agents (e.g.,

surfactants [37,38,39], water-soluble polymers [40,41,42], and inorganic nanoparticles [43,44]) 

and chemically modified CNTs. For electrical conductivity, non-covalent stabilization of CNTs 

is preferred over covalent functionalization because chemical modification has been shown to 

reduce conductivity [45,46,47]. On the other hand, covalent functionalization shows better 

solubility due to the strong interfacial interaction between the nanotubes and polymer matrix via 

direct chemical bonding. 

The research presented in this report is unique in that it is the first published report of fabricating 

CNF, MWCNT, and MMT based thin films/coatings on foam and barrier fabrics with the intent 

to improve their fire performance, and measuring the release of nanoparticles from these coated 

substrates under conditions that mimic the end-use conditions of soft furnishings. The LbL

approach, we believe, is ideal for reducing the flammability of the substrates, and by extension 

soft furnishing, as it will more quickly form the char-like armor needed to reduce the 

flammability of soft furnishings because the high concentration of nanoparticles are already at 

the surface (of the foam and barrier fabric) rather than randomly mixed throughout the polymer. 

This approach is also more commercially viable than impregnating the substrates with 

nanoparticles as it is a post-manufacturing process that uses equipment commonly used in the 

manufacturing of these substrates. Industry has expressed interest in using this technique.

2. Experimental [48,49,50]

Unless otherwise indicated all values are reported with 2σ uncertainty.

2.1. Materials

All materials were used as-received from the supplier unless otherwise indicated. Branched 

polyethylenimine (PEI, branched, Molecular Mass = 25,000 g/mol) and poly(acrylic acid) (PAA, 

Molecular Mass = 100,000 g/mol) were obtained from Sigma-Aldrich (Milwaukee, WI). These 

polymers were selected primarily because their behavior in LbL assembly is well documented 

and understood. Baytubes C150HP multi-walled carbon nanotubes (MWCNT, average diameter 

was 14 nm, length was 1 μm to 10 μm) were obtained from Bayer MaterialScience AG

(Pittsburgh, Pennsylvania). PR-24-XT-PS carbon nanofibers (CNF, average diameter = 100 nm, 

length was 30 μm to 100 μm) were obtained from Pyrograf Products Incorporated. Sodium 

montmorillonite clay (MMT, trade name is Cloisite Na

+

) was obtained from Southern Clay

Products Inc. (Gonzales, TX). The standard (untreated) polyurethane foam [51] coated in this 

study was stored as-received from the supplier (cardboard box with no packaging material at 

25 °C ± 2 °C). On the day of coating, nine substrates (length/width//height of (10.2 cm / 10.2 cm 

/ 5.1 cm) ± 0.1 cm) were cut from a single substrate (length/width//height of (30.6 cm / 30.6 cm / 

5.1 cm) ± 0.1 cm). These smaller substrates (0.6 mass fraction % ± 0.1 mass fraction %) were 
18

rinsed and wringed out (discussed below in the coating process) to remove debris and other

extractables. After drying, the post-extraction mass of these substrates was 12.7 g ± 0.3 g. The 

barrier fabric (BF) coated in this study was a poly(melamine-co-formaldehyde) based non-woven

(density of 222 g/m2). The BFs were not washed prior to coating because there was no visible

evidence of dust or debris, and the integrity of the BF deteriorates after 10 washings.

The polyelectrolyte (0.1 mass fraction % ± 0.03 mass fraction %) and DI (< 0.5 μS) water 

solutions were prepared as follows. A 2 L glass container was charged with DI water (1300 mL) 

and PEI (0.1 mass fraction % ± 0.03 mass fraction %, 1.3 g ± 0.4 g). This PEI cationic stock

solution was slowly agitated for 6 h at room temperature before using. The preparation of the 

PAA anionic stock solution was similar to the PEI cationic solution, except PAA (0.1 mass 

fraction % ± 0.03 mass fraction %, 1.3 g ± 0.4 g) was used instead of PEI. The pH value was 10 

and 3 for the PEI and PAA solutions, respectively.

The CNF/PEI suspension in DI water was prepared by charging a plastic bottle (250 mL) with 

the PEI cationic stock solution (150 mL ± 1 mL) then adding CNF powder (0.050 mass fraction 

% ± 0.003 mass fraction % relative to total PEI stock solution (600 mL), 0.30 g ± 0.02 g). The 

suspension was sonicated at 40 W using a Sonics VCX130 sonicator with a 13 mm probe for 1 h 

with the temperature never exceeding 70 °C ± 1 °C. The sonicated suspension was diluted with 

more PEI stock solution (450 mL) and was manually agitated for 3 min ± 1 min. The CNF/PEI

suspension was immediately used for coating.

The MWCNTs were first functionalized with PEI to facilitate dispersion and distribution in DI 

water and to improve retention of the MWCNTs in the coating. Amination of MWCNTs was 

prepared according to the procedure by Liao et al [52]. A plastic vial (500 mL) was charged 

with N,N-dimethylformamide (200 mL ± 1 mL, DMF), PEI (5.0 mass fraction % ± 

0.1 mass fraction relative to DMF, 10 g ± 0.1 g), and MWCNT (20.0 mass fraction % ± 

0.1 mass fraction % relative to PEI, 2.0 g ± 0.1 g), The mixture was sonicated at 40 watts using 

a Sonics VCX130 sonicator with a 13 mm probe for 1 h then agitated with a stir bar for 2 d at 

50 °C ± 2 °C. The functionalized MWCNT product (MWCNT-PEI) was isolated from the 

suspension by filtering through a 0.20 μm nylon membrane and washing four times with 

alternating methanol and water washes to remove excess PEI and DMF. The MWCNT-PEIs 

were dried in a dessicator with anhydrous calcium sulfate for at least 3 d prior to use. Once dried, 

MWCNT-PEIs were grounded using mortar and pestel. 

The MWCNT-PEI in DI water suspension was prepared by charging a plastic bottle (250 mL) 

with DI water (150 mL ± 1 mL) and MWCNT-PEI (0.10 mass fraction % ± 0.03 mass fraction % 

relative to total DI water, 0.60 g ± 0.02 g). The suspension was sonicated at 40 W using a Sonics 

VCX130 sonicator with a 13 mm probe for 1 h with the temperature never exceeding 70 °C ±

1 °C then was diluted with more DI water (450 mL) and shaken by hand for 3 min ± 1 min. The 

MWCNT-PEI suspension was used immediately for coating.

The MMT in DI water suspension was prepared by charging a 2 L glass container with DI water 

(1300 mL) then adding MMT (0.2 mass fraction % ± 0.03 mass fraction %, 2.6 g ± 0.8 g). This 

MMT suspension was stirred for 12 h using a magnetic stirrer before coating. The MMT

suspension was used immediately for coating.
19

2.1.2. CNF-based bilayered nanocoatings

The four BL CNF coatings on PUF and BF took approximately 30 min per specimen (14 min for 

first BL and 15 min for the remaining three BLs). In general, the fabrication process was 

alternately depositing cationic (CNF and PEI) and anionic (PAA) layers on the surface of the 

substrate and removing unbound material (polymer and CNF) by rinsing with DI water and 

wringing out the excess water several times (

Figure 1). The process of removing excess water using a convection oven and dessicator 

extended over a period of 3 d. 

Figure 1. The CNF/polymer coating process was an alternating submersion in a cationic 

(CNF/PEI) and an anionic (PAA) solution with washing (rinse and wring) between each solution. 

After creating 4 BLs (BL is CNF:PEI/PAA), the specimen was dried in a convection oven for 

12 h at 70 °C ± 1 °C to remove excess water.
20

More specifically, a plastic container (2 L) was charged with the CNF:PEI cationic suspension

(600 mL ± 10 mL), a similar container was charged with the PAA anionic solution 

(600 mL ± 10 mL), and six more containers were charged with deionized water 

(600 mL ± 10 mL each). The substrate was submersed into the CNF:PEI cationic suspension

and after squeezing and releasing the substrate four times in the CNF:PEI suspensions, the 

substrate was soaked in the suspension for an additional 5 min. The substrate was removed and 

the excess solution was squeezed back into the cationic dipping container. To remove unbound

PEI and/or CNF, the substrate was thoroughly rinsed in three separate containers. Since most of 

the cationic materials were typically removed in the first rinsing container, the rinsing water in 

this container was replaced with fresh deionized water after each washing cycle. Excess water

was removed by passing the substrate twice through a manually powered wringer. The PAA 

anionic layer was then deposited and the unbound PAA was removed using the same procedure 

as described above, except the washing was performed using different rinsing containers. This 

deposition of the CNF:PEI layer followed by the PAA layer created a single BL

(CNF:PEI/PAA). The procedure for depositing the next three BLs was similar to the first BL, 

except the substrate was only submersed in the coating solutions for 1 min rather than 5 min. 

After the four BLs were deposited, the specimen was dried in a convection oven (70 °C ± 1 °C, 

12 h) and stored in a dessicator (at least 3 d) with anhydrous calcium sulfate before weighing and 

analyzing.

2.1.3. MWCNT-based trilayered nanocoatings

The four trilayer (TL) MWCNT coatings on PUF and BF took approximately 40 min per 

specimen (20 min for the first TL and 20 min for the remaining three TL). In general, the 

fabrication process was alternately depositing an anionic layer (PAA), a functionalized MWCNT 

only cationic layer (MWCNT-PEI), and a polymer only cationic layer (PEI) on the surface of the 

substrate and removing unbound material (MWCNT and polymer) by rinsing with DI water and 

wringing out the excess water several times (Figure 2). The process of removing excess water 

using a convection oven and dessicator occurred over a period of 3 d. 

More specifically, three plastic containers (2 L) were charged with the coating solutions. One 

container (2 L) was charged with the PAA anionic solution (600 mL ± 10 mL), a second with the 

MWCNT-PEI cationic suspension (600 mL ± 10 mL), and a third with the PEI cationic solution 

(600 mL ± 10 mL). Three rinsing containers (2 L) per coating solution were charged with DI 

water (600 mL ± 10 mL, each). The substrate was submersed into the PAA anionic solution and 

after squeezing and releasing the substrate four times, the substrate was soaked in the PAA 

solution for an additional 5 min. The substrate was removed and the excess solution was 

squeezed back into the anionic dipping container. To remove unbound PAA, the substrate was 

thoroughly rinsed in three separate containers. Since most of the PAA was typically removed in 

the first rinsing container, the rinsing water in this container was replaced with fresh deionized 

water after each washing cycle. Excess water was removed by passing the substrate twice 

through Dyna-Jet BL-44 hand wringer (Dyna-Jet Products, Overland Park, KS).

The MWCNT-PEI cationic layer was then deposited onto the PAA/substrate and the unbound 

MWCNT-PEI was removed using the same procedure described above, except the washings 

were performed using different rinsing containers. The polymer only cationic layer (PEI only) 
21

was then deposited onto the MWCNT-PEI/PAA/substrate and washed using the same procedures 

described above, except using different rinsing containers. This deposition of a PAA layer, a 

MWCNT-PEI layer, and a PEI layered created a single TL (PEI/MWCNT-PEI/PAA/substrate). 

The procedure for depositing the next three TLs was the same as the first TL, except the 

substrate was only soaked in the coating solutions for 1 min rather than 5 min. After the coating 

was complete, the specimen was dried in a convection oven (70 °C ± 1 °C, 12 h) and then stored 

in a dessicator (at least 3 d) with anhydrous calcium sulfate before weighing and analyzing. 

Figure 2. The MWCNT/polymer coating process was an alternating submersion in an anionic 

(PAA), MWCNT cationic (MWCNT-PEI) and polymer cationic (PEI only) solutions with 

washing (rinse and wring) between each solution. After creating four TLs (TL is PAA\MWCNTPEI\PEI), the specimen was dried in a convection oven for 12 h at 70 °C ± 1 °C to remove excess 

water.

2.1.4. MMT-based trilayered nanocoatings

The eight TL MMT coatings on PUF and BF took approximately 50 min per specimen (20 min 

for the first TL and 30 min for the remaining seven TL). In general, the fabrication process was 

alternately depositing a cationic layer (PEI), an anionic clay layer (MMT), and an anionic layer 

(PAA) on the surface of the substrate and removing unbound material (MMT and polymers) by 

rinsing with DI water and wringing out the excess water several times ( 3). Similar to the CNF 

and MWCNT coating processes, the excess water was removed over 3 d using a convection oven 

and a dessicator. 

More specifically, three plastic containers (2 L) were charged with the coating solutions. One 

container (2 L) was charged with the PEI cationic solution (600 mL ± 10 mL), a second with the

MMT anionic suspension (600 mL ± 10 mL), and a third with the PAA anionic solution
22

(600 mL ± 10 mL) ( 3). Three rinsing containers (2 L) per coating solution were charged with 

DI water (600 mL ± 10 mL, each). A substrate was submersed into the PEI cationic soultion and 

after squeezing and releasing the substrate four times, the substrate soaked in the PEI solution for 

an additional 5 min. The substrate was removed and the excess solution was squeezed back into 

the anionic dipping container. To remove unbound PEI, the substrate was thoroughly rinsed in 

three separate containers. Since most of the PEI was typically removed in the first rinsing 

container, the rinsing water in this container was replaced with fresh deionized water after each 

washing cycle. Excess water was removed by passing the substrate twice through a Dyna-Jet 

BL-44 hand wringer (Dyna-Jet Products, Overland Park, KS).

The MMT anionic layer was then deposited onto the PEI/substrate and the unbound MMT was 

removed using the same procedure described above, except the washings were performed using 

different rinsing containers. The second anionic (PAA only) layer was then deposited onto 

MMT/PEI/substrate and washed using the same procedures described above, except using

different rinsing containers. This deposition of a PEI layer, a PAA layer, and a MMT layered 

created a single TL (PAA/MMT/PEI/substrate). The procedure for depositing the next seven 

TLs was the same as the first TL, except the substrate was only soaked in the coating solutions 

for 1 min rather than 5 min. After the eight TLs were deposited, the specimen was dried in a 

convection oven (70 °C ± 1 °C, 12 h) and then stored in a dessicator (at least 3 d) with anhydrous 

calcium sulfate before weighing and analyzing. A more conventional MMT/PEI BL coating was 

also evaluated in this study. The process is identical to the TLs, except no PAA and 20 BL.

Figure 3. The MMT/polymer coating process was an alternating submersion in a polymer 

cationic (PEI), clay anionic (MMT), and polymeric anionic (PAA) solutions with washing (rinse 

and wring) between each solution. After creating five TLs (TL is PEI\MMT\PAA), the specimen 

was dried in a convection oven for 12 h at 70 °C ± 1 °C to remove excess water.

2.2. Fire performance testing 
23

A Cone Calorimeter (Cone, Fire Testing Technology, East Grinstead, United Kingdom), 

operating at 35 kW/m2 with an exhaust flow of 24 L/s, was used to measure the fire performance 

of uncoated and nanoparticle coated PUF. The experiments were conducted according to 

standard testing procedures (ASTM E-1354-07). A (10.2 cm / 10.2 cm / 5.1 cm) ± 0.1 cm 

sample was placed in a pan constructed from aluminum foil. The pan was slightly larger than the 

test sample and the pan sides were flared away from the sample. This allowed the sides as well 

as the top of the sample to be exposed during testing. When exposed to the external heat flux, 

the PUF and MMT/PUF melted to form a pool of burning polymer. In contrast, the 

MWCNT/PUF and CNF/PUF shrank slightly. As described by Zammarano et al. [53], it is 

critical to normalize the data according to surface because the PUF samples rapidly form a melt 

pool, whereas the, PUF filled with CNF and MWCNT did not collapse and maintained a two 

times larger surface area during the test. The data presented here was not normalized in terms of 

surface area since the surface areas for CNF/PUF and MWCNT/PUF were not quantitatively 

measured and changed significantly throughout the test. 

All values are reported with a 2σ uncertainty of ± 5 % in HRR and ± 2 s in time. BF Cone 

testing was performed using the same methodology and sample size as PUF, except the BF was 

0.4 cm ± 0.1 cm thick. There was no adjustment for surface area since there was no difference in 

the coated and uncoated BF dimensions during the test.

2.3. Simulated aging 

A Head Over Heels shaker (HOH) was used to simulate the type of stresses expected from 

chewing on a piece of soft furnishing (Figure 4) [54]. A polymer coated glass bottle (300 mL) 

was charged with a simulated saliva solution (100 mL, 0.9 mass fraction % ± 0.08 mass fraction 

% sodium chloride in deionized water) and four substrates. The size of each substrate was a 

mass of 0.6 g ± 0.06 g with dimensions (length / width / thickness) of (4.0 / 5.0 / 1.0) cm ± 

0.01 cm for the PUF and a diameter of 5.5 cm ± 0.01 cm for the BF. The extraction bottle was 

secured to a clamp positioned 8.0 cm ± 0.1 cm from the horizontal turning shaft of the HOH then 

spun at a rate of 6.3 rad/s (60 rev/min) for 30 min. The four substrates were removed after 

squeezing all the simulated saliva solution back into the bottle. Four new substrates were added 

to this bottle and the stressing and squeezing process was repeated. This process was repeated a 

total of 10 times; therefore, the simulated saliva extraction suspension to be analyzed by UV-VIS 

contained nanoparticles released from 40 PUF or BF specimens.

These suspensions contained sodium chloride, which was found to cause nanoparticle 

aggregation. In order to obtain accurate and reproducible measurements of the nanoparticles,

sodium chloride was removed from the simulating chewing extracts using a Pierce Snakeskin 

pleated dialysis tube (10,000 g/mol relative molecular mass cut-off) suspended in deionized 

water. The dialysis tube was charged with approximately half of the extract (actual mass 

recorded) then the bag was placed in a deionized water bath (2 L, 0,2 μS), which was 

continuously agitated at room temperature. During every hour of the day the bath conductivity 

was measured and the water was replaced with fresh deionized water. At night, the dialysis 

proceeded unattended. In the morning of the following day, the conductivity was measured and 

the water replaced again. Throughout the day, the same process of hourly measuring 

conductivity and replacing the water continued until the measured conductivity was ≤ 0.5 μS 
24

(approximately 24 h), which was similar to the measured conductivity for the pure distilled

water. As a comparison, the measured conductivity of the simulated saliva solution was 

16,000 μS ± 300 μS. The dialysis tube was removed and the dialyzed simulated chewing extract 

was transferred to a beaker and the mass measured. The dialysis tubes were smaller than the 

volume of simulated chewing extract; therefore, the extract was dialyzed in two approximately 

equal mass parts. These dialyzed extracts were combined and then sodium dodecyl sulfate (0.23 

mass fraction % ± 0.02 mass fraction %, SDS) was added to stabilize the nanoparticles in the 

suspension. 

A mechanical pounder was used to simulate the type of “wear and tear” expected from routine 

use of soft furnishings (Figure 5). The “Pounder” was constructed of a hard plastic (diameter of 

8.9 cm ± 0.1 cm) pounding element that had a slight convex curvature attached to a pneumatic 

driven vertical piston. The controller was a Trumeter 7922 counter and Trumeter 7951 timer. 

The substrate dimensions (length / width / thickness) for the pounding studies were (10.2 / 10.2 / 

5.0) cm ± 0.1 cm for the PUF and the same surface dimensions for the BF, but the BF was only 

0.4 cm ± 0.1 cm thick. The substrate was placed in a polyolefin bag then secured under the 

pounder by set screws positioned around the edge of the substrate. A substrate was pounded for 

100,000 cycles (approximately 28 h) at 1 cycle/s at a force pressure of 20682 Pa ± 69 Pa (3 

lbf/in2 ± 0.1 lbf/in2). After pounding was complete, the substrate was removed from the bag and 

the bag was washed with deionized water (100 mL) containing SDS (0.23 mass fraction % ± 

0.02 mass fraction %) SDS. The same wash solution was used for 10 experiments; therefore, the 

100 mL simulated wear and tear suspension contained the nanoparticles released from 10 

substrates. The suspension was stored in a 300 mL glass bottle until analysis.

Figure 4. Simulated saliva extractions were performed using bottles containing a simulated saliva

and a substrate that were rotated at 6.3 rad/s (60 rev/min) for 30 min. 

Figure 5. Simulated normal wear and tear stressing was performed by pounding substrates at 1 

cycle/s for 100,000 cycles at 20682 Pa ± 69 Pa of pressure.
25

2.2. Characterization

A Zeiss Ultra 60 Field Emission-Scanning Electron Microscope (FE-SEM, Carl Zeiss Inc., 

Thornwood, NY) was used to collect images of the nanocoatings, from which, the coating 

thickness was approximated (Table 1) and the distribution of nanoparticles and overall quality of 

the LbL coating was inspected. All SEM samples were sputter coated with 4 nm of Au/Pd 

(60 mass fraction %/40 mass fraction %) prior to SEM imaging.

A Q-500 GA Thermal Gravimetric Analyzer (TGA, TA Instruments, New Castle, DE) was used 

to measure the concentration of nanoparticles on the substrates (Table 1, Mass fraction % 

MWCNT on MWCNT/PUF). The samples (20 mg ± 3 mg) were placed on a ceramic pan 

(250 μL, TA Instruments) then loaded into the furnace by the autosampler. Under a nitrogen 

atmosphere, the temperature was stabilized at 90 °C ± 1 °C (30 min) then ramped to 800 °C ± 

2 °C at 10 °C/min. The reported nanoparticle content was based on the remaining mass fraction 

% at 600 °C (CNF and MWCNT) and 800 °C (MMT). All values are reported with 2σ standard 

uncertainty.

A Lambda 950 Ultraviolet-Visible (UV-VIS, Perkin Elmer, Waltham, MA) spectrometer with a 

10 mm ES quartz cuvette [55] was used to measure the mass fraction of CNF and MWCNT in 

suspensions generated from stressing the specimens. The absorbance spectrum was collected 

from 185 nm to 1800 nm at 1 nm increments with an instrument integration time of 0.2 s per 

increment. The incident light was circularly polarized prior to the sample compartment, and the 

instrument was corrected for both the dark current and the background. The relationship 

between the measured maximum absorbance band for the MWCNT and CNF (267 nm), after 

subtracting the spectrum of the simulate saliva/SDS solution, and the MWCNT or CNF

concentration was determined from a Beer’s Law calibration curve constructed from the 

absorbance of four different concentration calibration standards. The highest concentration 

calibration standard was 0.5 mg ± 0.01 mg of CNF or MWCNT suspended in SDS/DI (0.23 mass 

fraction % ± 0.02 mass fraction %) that was sonicated at 40 watts using a Sonics VCX130 

sonicator with a 13 mm probe for 1h. The three lower concentration standards were aliquots of 

the highest concentration standard diluted with the same SDS/DI solution and sonicated for 10 

min.

An Optima 5300 DV Inductively Coupled Plasma – Optical Emission spectrometer (ICP-OES, 

PerkinElmer Inc., Shelton, CT) was used to measure the mass fraction of MMT collected from 

stressing the clay coated substrates. The relationship between the measured ICP-OES intensity 

and MMT concentration in the stressed suspensions was determined from a series of Beer’s Law

calibration curves of four elements in the clay (Al, Fe, Mg, and Na). The intensity of Al, Fe, Mg, 

and Na in MMT was measured by selectively exciting each element at the element specific 

wavelengths (394 nm, 238.2 nm, 285 nm, and 589 nm, respectively). To determine the amount of 

each element in MMT, a 5 mL aliquot of a MMT/DI solution (0.020 mass fraction % ± 0.005 

mass fraction % MMT, 100 mL) was first diluted with nitric acid (14 M, 100 mL) to give a 

MMT concentration of 0.0010 mass fraction % ± 0.0005 mass fraction % then known amounts

of Al, Fe, Mg and Na were added and the ICP-OES intensity was measured. An additional 

amount of each element was added to this solution and the intensity was again measured. This 
26

process was repeated three more times and the ICP-OES intensities of each element in these five 

solutions were plotted as a function of concentration and element type. The concentration at an

intensity value of 0 cps, is the amount of each element in MMT (0.0010 mass fraction % ± 

0.0005 mass fraction %). The Beer’s Law calibration curves were created by plotting the ICPOES intensity of a 0.0010 mass fraction % ± 0.0005 mass fraction % MMT and three dilutions of 

this solution.

3. Results and Discussion

This project is investigating cutting edge technology that is based on decades of LbL research on 

flat, pseudo one dimensional substrates. Research, such as presented here, is required to 

understand how the parameters involved in fabricating these coatings will impact the quality and 

performance attributes of the substrate. To that point, the data provided here should be placed in 

the context of these coatings fabricated with a specific process, and type of nanoparticle, 

polymer, and substrate. All values are reported with a 2σ uncertainty. 

SEM images of the as-received BF shows the “fiber bundle” surface is smooth (Figure 6) and 

contains micron sized grooves along the bundler axis. It is assumed that these grooves represent 

channels between fibers within the bundle not completely welded together during manufacturing. 

It appears that the 27 μm ± 3 μm bundles contained at least five fibers with a diameter of 9 μm ± 

3 μm. The BF was not washed prior to coating because the non-woven construction was not 

durable enough to handle both prewashing and the coating process. 

(a) (b)

(c) (d)

Figure 6. SEM images of as-received BF (a) 20,000x, (b) 50,000x, (c) 100,000x, and (d) 

200,000x. The 27 μm ± 3 μm diamter strand is a bundle of several fibers that “welded”during 

manufacturing.

Other than dust and debris on the surface of the PUF, the as-received PUF surface appeared

smooth and featureless even at high magnification (Figure 7). Prior to depositing the first layer, 
27

the PUF was washed with DI water, which completely removed all of the debris (Figure 7h). 

The wavy edges of the PUF walls were a result of the manufacturing process. The PUF was 

initially closed cell with a very thin polyurethane membrane connecting the walls. When the 

membrane was “popped” to form this open cell structure, there was a slight relaxation of the 

strained edges of the walls and the membrane snapped back onto the walls, which created the 

wavy appearance observed in Figure 7h. 
28

(a) (b)

(c) (d)

(e) (f)

(g) (h)

Figure 7. SEM images of as-received PUF at (a) 1,000x, (b) 2,000x, (c) 5,000x, (d) 10,000x, (e) 

20,000x, (f) 50,000x and (g) 100,000x and washed PUF at (h) 5,000x. The PUF surface was 

smooth and featureless after debris (dust, etc.) was removed by washing (h).
29

3.1. Characterization of nanocoatings

The physical characteristics of the nanoparticle coated substrates are provided in Table 1. The 

increase in substrate mass due to the coating (Mass fraction % coating) was measured using a 

laboratory microbalance. The amount of nanoparticles in the coating (Mass fraction % 

Nanoparticles in coating) was calculated from TGA and microbalance values. The four BL CNF 

coatings of PUF increases the mass of the substrate by 3.2 mass fraction % ± 

0.4 mass fraction %, of which, 51 mass fraction % ± 3 mass fraction % is CNFs. The total CNF 

content relative to PUF mass is 1.6 mass fraction % ± 0.1 mass fraction %. The physical 

characteristics of the four TL MWCNT coatings on PUF (increase the mass of the substrate by 

3.4 mass fraction % ± 0.4 mass fraction %, of which, 50 mass fraction % ± 3 mass fraction % is 

MWCNTs) are similar to that of the CNF coatings. The total MWCNT content relative to the 

substrate mass is 1.7 mass fraction % ± 0.1 mass fraction %. The MMT coatings on PUF are

significantly thicker (1000 nm ± 450 nm) with a higher nanoparticle concentration in the coating 

and on the substrate (66 mass fraction % ± 13 mass fraction % and 2.1 mass fraction % ±

0.2 mass fraction %, respectively). These nanoparticle loading levels on the substrate were

similar to what is used to improve the fire performance of polymers [27,31]. In contrast to these 

more conventional nanocomposites, the nanoparticles in these coatings were concentrated at the 

surface rather than randomly dispersed and distributed throughout the polymer matrix

The characteristics measured for the BFs are similar to what was measured for the PUF 

specimens of the same nanoparticle-based coatings, except for MMT. For example, the CNF 

content in the coatings on PUF and BF are 50 % ± 3 %. Since the MMT coating process 

deposited 2.2 times more coating (by mass fraction) on the BF than the PUF, the thickness of the 

coating on the BF is presumably thicker. The freeze fracture process used to measure the coating 

thickness on the PUF did not work on BF. Therefore, the coating thickness values on PUF are 

used as an estimate of the BF values for the MWCNT and CNF. Justification for this assumption 

is that both substrates were coated using the same processes and the TGA and microbalance 

values indicate both substrates contained the same mass of nanoparticles and coating.

Table 1. Provided are the average physical characteristics of nanoparticle coated substrates

organized by substrate type and highest to lowest in nanoparticle content. BFs coating thickness 

is assumed to be similar to that measured on PUF. MMT coatings are sufficiently different on 

PUF and BF that the coating thickness on PUF is believed to not be a good estimate of the BF 

coating thickness. Values reported with 2σ uncertainty.

Specimens Specimen 

Mass (g)

Mass fraction 

% coating

Mass fraction % 

Nanoparticles

Coating 

thickness (nm)

on substrate in coating

PUF

MMT 15.9 ± 0.3 3.2 ± 0.6 2.1 ± 0.2 66 ± 13 1000 ± 450

MWCNT 13.2 ± 0.3 3.4 ± 0.4 1.7 ± 0.1 50 ± 3 440 ± 47

CNF 13.1 ± 0.3 3.2 ± 0.4 1.6 ± 0.1 51 ± 3 359 ± 36

BF

MMT 5.5 ± 0.5 6.9 ± 1.4 4.6 ± 1.0 66 ± 15 ---

MWCNT 5.0 ± 0.2 3.6 ± 0.5 1.8 ± 0.3 51 ± 3 440 ± 47

CNF 5.5 ± 0.4 3.5 ± 0.3 1.7 ± 0.2 49 ± 3 359 ± 36
30

3.1.1. CNF-based nanocoatings

The images in Figure 8 indicate that the CNFs are well distributed along the walls of the PUF. 

At low magnification (Figure 8a), the wall surfaces appear to be sparsely populated, with 

approximately 10 μm by 10 μm sized aggregates of CNFs. The areas between the aggregates are 

populated with a network of CNF whiskers and regions that appear to be free of CNFs. At

higher magnifications (Figure 8c to Figure 8g), it becomes apparent that a portion of these 

regions actually do contain CNFs and these are not visible at lower magnifications because they 

are completely embedded in the polymer coating. Along the surface there are also “islands” of 

CNFs that appear to have dewetted from the surface (Figure 8d to Figure 8g). The larger islands 

(approximately 10 μm) appear to contain more single CNFs, rather than the bundles observed in 

the larger aggregates (Figure 8h). The smaller islands (less than 3 μm) either contain no CNFs 

or what appears to be short individual CNFs (less than 1 μm). 

SEM images of a fractured CNF/PUF were taken with the fracture surface in the plane of the 

image, which provides cross section views of the PUF and the coating (Figure 9). The CNF 

coatings are 359 nm ± 36 nm (based on 10 measurements of five different CNF/PUF specimens. 

The surface morphology at low magnification (Figure 9a) is consistent with that observed in 

Figure 8 (large aggregates, CNF network, and areas without CNF). Based on all the images 

taken of fractured CNF/PUFs, the CNF coating appears to cover the entire surface, although the 

thickness is not completely uniform. 

The thicker islands, one of which is shown in Figure 9c, are 374 nm ± 100 nm and are 

constructed of at least twenty fibers randomly oriented in the plane of the coating. The “hills and 

valleys” topography of the islands are created by overlapping CNFs. The fairly uniform (34 nm 

± 2 nm) polymer coating covering the CNFs, and the small gaps between them, suggest that 

these fibers were probably deposited at the same time. Analysis of other islands reveals similar 

characteristics, which suggests the fibers in the islands were probably deposited as loosely 

interacting groups rather than isolated and independent CNFs. The last layer of polymer 

covering the CNFs varied from island to island, but never exceeded 34 nm ± 2 nm. This is the 

same thickness measured for the regions between the thicker islands, which appear to contain 

little to no CNFs. Figure 10 is the only example of coating delamination. The delamination may 

have resulted from the freeze fracture process or may indicate a section of poor adhesion. These 

images illustrate that the coating can contain regions of high CNF concentration welded together 

by polymer. 

The SEM images of the CNF/BFs indicate the CNF coatings are similar on PUF and BF (Figure 

11). More specifically, there are regions of high fiber aggregation separated by regions with 

little (or no) CNFs and islands that contain high CNF aggregation or individual CNFs. The 

entire BF surface is coated with this non-uniform CNF distribution.
31

(a) (b) 

(c) (d)

(e) (f)

(g) (h)

Figure 8. SEM images of the inside section of a CNF/PUF at (a) 1,000x, (b) 5,000x, (c) 10,000x, 

and (d) 20,000x, of a thicker island at (e) 50,000x (f) 100,000x, and (g) 200,000x, and of an 

aggregate at (h) 200,000x.
32

(a) (b) 

(c) (d)

Figure 9. SEM images of a fractured edge of CNF/PUF at (a) 20,000x, (b) 100,000x, (c) 

200,000x, and (d) 500,000x. The CNF coating is 359 nm ± 36 nm.

(a) (b)

(c) (d)

Figure 10. SEM images of a delaminated CNF/PUF at (a) 10,000x, (b) 50,000x, (c) 100,000x, 

and (d) 200,000x. The CNFs below the surface are welded together with polymer. The root 

cause of delimitation may be the freeze fracture process or poor adhesion to the PUF due to the 

high CNF concentration. 
33

(a) (b)

(c) (d)

Figure 11. SEM images of the inside section of a CNF/BF at (a) 20,000x, (b) 50,000x, (c) 

100,000x, and (d) 200,000x and (e) 200,000x. The CNF coatings on the PUFs and BFs appear to 

be similar. 

3.1.2. MWCNT-based nanocoatings

Uniform coatings with well dispersed and distributed MWCNTs were only achieved when the 

MWCNTs were well stabilized in the MWCNT depositing solution over the entire coating 

process. Critical to fabricating the MWCNT coatings is to functionalize the MWCNTs with PEI 

using a procedure by Liao [52] and to use TL, rather than BL, coatings. PEI and PAA were 

selected for these coatings because their well documented LbL behavior makes them an idea 

starting point for evaluating new additives and substrates for the LbL process. 

Initially, a stable MWCNT suspension was attempted using PEI or sodium deoxycholate 

surfactant. After sonicating for one hour, the MWCNTs appeared well dispersed and stable; 

however, for sonication over an hour, the MWCNTs appeared to destabilize as the bottom of the 

suspension was darker than the top. A couple of the more stable suspensions were used to 

fabricate a coating. Even though the PUF was washed well between depositions, because it is a 

three dimensional porous substrate, there was still unbound PAA present in the PUF when the 

MWCNTs were deposited. This PAA caused the MWCNT suspension to rapidly destabilize as 

indicated by a settling of the MWCNTs on the bottom of the suspension. The result from these 

attempts was a coating with a very non-uniform distribution of MWCNTs.

A drastic improvement in the process and therefore the coatings resulted from using MWCNTs 

functionalized with PEI. The MWCNT-PEI formed a stable suspension in DI over several days 
34

without the need for a stabilizing surfactant. The high dispersion and distribution of MWCNTs 

and the high stability of the suspension was indicated by the very uniform black color (caused by 

MWCNT-PEIs) across the PUF. However, the black MWCNT-PEIs were easily transferred 

from the PUF into the PAA and, to a lesser extent, to the rinsing solutions. This suggested that 

the MWCNT-PEIs had a relatively weak charge, as compared to PAA, which resulted in poor 

adhesion between these layers. The retention was addressed by depositing a PEI layer after the 

MWCNT-PEI (creating a TL) as the adhesion was then based more on pure PAA and PEI 

interactions, which strongly adhere to each other. 

At low SEM magnifications, the LbL process appears unsuccessful (Figure 12a through Figure 

12e). The MWCNT/PUF surface appears void of MWCNTs other than a few sparsely populated 

MWCNT aggregates that are on the order of tens of micron in size. However, images at higher 

magnifications (Figure 12e through Figure 12h) revealed that the LbL process had worked 

extremely well. The PUF surface was completely covered with a uniform coating, which 

contained well dispersed and distributed MWCNTs that are completely embedded in the polymer 

coating. All of the MWCNT/PUFs analyzed by SEM had a completely intact MWCNT 

protective layer over the PUF. A few small surface cracks (10 nm ± 5 nm) were observed and 

were believed to result from the drying process. These types of cracks were common in thicker 

LbL coatings. It was assumed that these rare nanometer scale surface cracks would not 

deteriorate the fire performance of the MWCNT/PUF; therefore, there was no further 

investigation of the cracks.

SEM images of a fractured MWCNT/PUF were taken with the fracture surface in the plane of 

the image, which provides cross sectional views of the PUF and the coating (Figure 13). The 

MWCNT coating was 440 nm ± 47 nm thick based on seven measurements taken on each of 12 

different MWCNT/PUF specimens. Other than the MWCNTs exposed at the fracture surface, all 

MWCNTs were completely embedded in the polymer coating. The cracks in the MWCNT 

coating were likely there prior to fracturing, but the size was increased by the strain resulting 

from the freeze fracture process.

The SEM images of the MWCNT/BFs indicate the MWCNT coatings were similar on PUF and 

BF (Figure 14). More specifically, the entire surface of the BF was coated with uniform 

distribution and high concentration of MWCNT.
35

(a) (b)

(c) (d)

(e) (f)

(g) (h)

Figure 12. SEM images of the inside section of a MWCNT/PUF at (a) 1,000x, (b) 5,000x, (c) 

10,000x, (d) 20,000x, (e) 50,000x (f) 100,000x, (g) 200,000x, and (h) 500,000x. The MWCNTs 

were well dispersed and distributed throughout the polymer coating. The coating was smooth 

and featureless except for a few small larger aggregates and a few 10 nm ± 5 nm wide cracks. 
36

(a) (b)

(c) (d)

Figure 13. SEM images of a fractured edge of MWCNT/PUF at (a) 50,000x, (b) 100,000x, (c) 

200,000x, and (d) 500,000x. The MWCNT coating thickness is 440 nm ± 47 nm. 

(a) (b)

(c) (d)

Figure 14. SEM images of the inside section of a MWCNT/BF (a) 20,000x, (b) 50,000x, (c) 

100,000x, and (d) 200,000x. The MWCNT coatings appear similar on PUF and BF.
37

3.1.2. MMT-based nanocoatings

The images in Figure 15 indicated the entire PUF surface was coated with a non-uniform 

distribution of clay. The surface was covered with regions of high MMT aggregation that can be 

as large as 100 μm by 100 μm. Fracture surface images (Figure 16) indicated these regions can 

be several microns thick. The coating thickness was 1000 nm ± 450 nm. The large uncertainty 

in the thickness stems from a large variation in the degree of clay aggregation, which can be seen 

in Figure 16b. The entire left wall in image Figure 16b is covered with a high aggregation of 

MMT with a thickness of 1250 nm ±150 nm. On the top wall, the coating is 1000 nm ± 100 nm 

on the left, but thins out to 450 nm ± 56 nm on the right. On the right wall, the coating appears 

to be very similar in thickness to the top right, but the part of this wall in the background has the 

same rough, highly aggregated appearance that is observed on the left wall. This disparity is also 

shown in several images in Figure 16 and Figure 15. The smooth and featureless regions 

between these large aggregates (Figure 15a and Figure 15b) are actually completely filled with 

clay (Figure 15c to Figure 15f). The coating thickness in these regions is closer to what was 

expected from this process (500 nm ± 120 nm). A cross section of a thickness average region of 

the coating (1000 nm) indicates the coating is highly filled with MMT sheets stacked upon each 

other similar to a deck of cards (Figure 16c and Figure 16d). 

The SEM images of the MMT/BFs indicate that the MMT coatings were similar on PUF and BF 

(Figure 17), except there were more regions with highly aggregated clay on the BF. The large 

variation in aggregate size is well illustrated in these BF images. On the front of the bundle, the 

MMT aggregates appear as flakes that could easily be peeled away. In contrast, the top is 

covered with both very tall and completely embedded clay aggregates and smooth, thinner 

coated regions (similar to what is presented on PUF in Figure 15 and Figure 16). The root cause 

of the variation in aggregation, regions of extremely thick and highly aggregated clay, the high 

clay concentration (relative to the nanoparticles in the other coatings), and the higher coating 

mass on BFs (relative to PUF), is currently under investigation. It is assumed using a TL rather 

than the more traditional BL approach contributed to these characteristics.
38

(a) (b)

(c) (d)

(e) (f)

(g) 

Figure 15. SEM images of the inside section of a MMT/PUF at (a) 1,000x, (b) 5,000x, (c) 

10,000x, (d) 20,000x, (e) 50,000x (f) 100,000x, and (g) 200,000x.
39

(a) (b)

(c) (d)

Figure 16. SEM images of a fractured edge of MMT/PUFat (a) 10,000x, (b) 20,000x, (c) 

50,000x, and (d) 100,000x. The MMT coating was 1000 nm ± 450 nm. 

(a) (b)

(c) (d)

Figure 17. SEM images of a MMT/BF (a) 10,000x, (b) 20,000x, (c) 50,000x, (d) 100,000x and 

(e) 200,000x. The MMT coatings are on PUF and BF.
40

3.2. Fire performance

The Cone Calorimeter is a routine bench scale fire test that simulates a developing fire scenario 

on a small specimen and is used to measure the forced burning fire performance of polymers. 

The parameters reported from the test, such as time to ignition of the combustion gases (TTI), the 

time to peak, the peak maximum heat release rate (PHRR), and the total heat release (THR), are 

directly related to the potential fire threat of the burning polymer and, therefore, the values of 

these parameters are the bases of the performance metrics for several existing or proposed 

national fire regulations. 

3.2.1. CNF-based nanocoatings

TGA was used to determine the actual mass of CNFs and coating deposited onto the substrates. 

The four BL CNF coatings increase the mass of the PUF and BF substrates by 3.4 mass fraction 

% ± 0.1 mass fraction %, of which, 50 mass fraction % ± 4 mass fraction % are CNFs. The total 

CNF content relative to the substrate mass is 1.6 mass fraction % ± 0.3 mass fraction %, which is 

a typical loading level of carbon nanotubes or nanofibers incorporated (embedded) into polymers 

to improve the polymer’s fire performance. Unlike these other nanoparticle embedded 

nanocomposites, the CNFs in these coatings are concentrated at the surface rather than randomly 

dispersed and distributed throughout the polymer matrix [29,31].

The HRR data indicates the CNF coatings significantly improved the fire performance of foam 

(Figure 18 and Table 2). The HRR curves for CNF/PUF and PUF consist of two peaks. 

However, the attributes of the curves are quite different with both peaks of CNF/PUF being of 

similar HRR values (371 kW/m2 ± 10 kW/m2 and 348 kW/m2 ± 10 kW/m2) whereas in PUF the 

second peak is 2.8 times larger than the first peak (224 kW/m2 ± 12 kW/m2 and 620 kW/m2 ± 

26 kW/m2). The PHRR, which is often considered as one of the more critical values in accessing 

the flammability of a material, is 40 % ± 3 % lower for CNF/PUF than for PUF. The THR, 

which reflects the total size of the fire threat, and the total burning time of the foam was 21 % ± 

3 % smaller for CNF/PUF. However, the time to PHRR, which is often considered a critical 

value in accessing the amount of time for escaping a fire, is 66 % ± 2 % earlier for CNF/PUF. In 

other words, the Cone data suggests the CNF coatings may result in smaller fires and reduced 

fire spread, but create an initially larger fire that reduces the time to escape. In a real fire 

scenario, the CNF/PUF would likely perform significantly better than the HRR data suggests, but 

before going into this discussion it important to understand how the CNF coating altered the 

burning behavior of foam.

The attributes of the first HRR peak for both CNF/PUF and PUF are defined by pyrolysis of 

polyurethane decomposition gases (increase in HRR) and decrease in substrate surface area 

(decrease in HRR) (Figure 18). During the first peak the CNF coating forms a protective char 

and enables the foam to maintain its shape (and surface area) that is qualitatively similar to an 

untested foam specimen. At 63 s ± 2 s, the flames penetrate this protective char, which causes the 

foam to shrink as the remaining polymer is pyrolyzed. At the end of the experiment, a brittle 

char remains that has a surface area 90 % ± 5 % smaller than the untested foam specimen. In 

contrast to CNF/PUF, during the first HRR peak PUF collapses to form a liquid-like pool of 

degraded polyurethane. The surface area of this pool (defined by the sample pan) is qualitatively 

two times smaller than CNF/PUF surface area, which is the reason the HRR maximum for the 
41

first peak is 66% ± 2% higher for CNF/PUF (Table 2). The PUF pool rapidly pyrolyzes because 

there is no protective char and because the contents of the pool are volatile/combustible 

compounds (isocyanate and polyol based degradation products of polyurethane).

In a real fire scenario, the reduction in flammability due to replacing PUF with CNF/PUF will 

likely be greater than suggested by the Cone data (Figure 18 and Table 2) for two main reasons. 

First, this CNF/PUF Cone data (Figure 18 and Table 2) was not normalized, as suggested by 

Zammarano et al [53], since the surface area for CNF/PUF was not quantitatively measured and 

changed significantly throughout the test. Based on the qualitative observations, the HRR for the 

first peak could be reduced by a factor of two while the second peak may only be slightly 

reduced. The result is the first peak for CNF/PUF would have a maximum HRR and time to 

peak similar to PUF and the second peak would then become the PHRR, which has a time to 

peak similar to PUF. Secondly, the Cone data does not capture the real impact of the PUF pool 

fire. Since a pool fire can approximately increase the fire threat (as calculated from HRR, THR, 

and burn time) of a burning product (e.g. upholstered furniture) by 35 % [59] and the CNF 

coating prevents pool formation, it is hypothesized that in a real fire replacing PUF with 

CNF/PUF would decrease the HRR from a product by at least 35 %. 

The CNF coating on the BF increases the BF flammability (Figure 18 and Table 3). More 

specifically, a 2.6 times increase in PHRR (149 kJ/m2 ± 15 kJ/m2 as compared to 57 kJ/m2 ± 

9 kJ/m2) and a 3.9 times increase in THR (3.1 MJ/m2 ± 0.3 MJ/m2 as compared to 0.8 MJ/m2 ± 

0.1 MJ/m2) was measured for the CNF/BF as compared to pure BF (Figure 19 and Table 3). 

This increase is attributed to the coating polymers being more flammable then the BF itself and 

the coating process causing the BF to fall apart. The non-woven design is not appropriate for 

this process because depositing, washing, and rinsing creates regions of low fabric density, loss

of fibers, and collapse of the air pocket design.

3.2.2. MWCNT-based nanocoatings

TGA was used to determine the actual mass of MWCNTs and coating deposited onto the 

substrates. The five TL MWCNT coatings increase the mass of the PUF and BF substrates by 

3.5 mass fraction % ± 0.5 mass fraction %, of which, 50 mass fraction % ± 3 mass fraction % is 

MWCNTs. The total MWCNT content relative to the substrate mass is 1.8 mass fraction % ± 

0.4 mass fraction %, which is a typical loading level of carbon nanotubes or nanofibers 

incorporated (embedded) into polymers to improve the polymer’s fire performance. However, 

unlike these other nanoparticle embedded nanocomposites, the MWCNTs in these coatings are 

concentrated at the surface rather than randomly dispersed and distributed throughout the 

polymer matrix [29,31].

The HRR data indicated that the MWCNT coatings significantly improved the fire performance 

of foam (Figure 18 and Table 2). The HRR curves for MWCNT/PUF and PUF consist of two 

peaks. The time to maximum HRR for each peak was 21% ± 10 % earlier for PUF (32 s ± 2 s 

and 75 s ± 3 s as compared to 34 s ± 2 s and 78 s ± 2 s for MWCNT/PUF). The maximum 

measured HRR during the test (PHRR) was 35 % ± 3 % lower for MWCNT/PUF (620 kW/m2 ± 

26 kW/m2 as compared to 403 kW/m2 ± 10 kW/m2 ). The THR and total burn time was also 

lower for MWCNT/PUF (21 % ± 3 % and 25 % ± 3 %, respectively).
42

The improvement in fire performance of the MWCNT/PUF was a result of foam shape retention 

and char formation caused by the MWCNT coating network. Similar to CNF/PUF, the attributes 

of the first peak for both MWCNT/PUF and PUF were defined by an increase in HRR due to 

pyrolysis of polyurethane decomposition gases and a decrease in HRR due to collapse (PUF) or 

shrinkage (MWCNT/PUF) of the foam and char formation (MWCNT/PUF). At (45 ± 2) s, the 

PUF had completely collapsed into a pool of pyrolyzing polyurethane foam decomposition 

products with a surface area equivalent to the dimensions of the sample pan (10 cm by 10 cm). 

The subsequent rapid HRR increase for PUF (that resulted in the second peak and the PHRR) 

was a result of pyrolyzing the degradation products in this pool. Unlike PUF, during the first 

peak the MWCNT/PUF only shrank. Qualitatively, the MWCNT/PUF was comparable in 

dimensions to the untested foam, which was a surface area two times larger than PUF. At (54 ± 

2) s, the flames penetrated the protective MWCNT based char layer allowing for the remaining 

polyurethane to pyrolyze. The result was a second HRR peak, significant shrinkage (10% ± 5% 

of the original surface area by the end of the test), and brittle char residue. 

Similar to CNF/PUF, a potential concern of the MWCNT coating was the 75 % ± 2 % higher 

HRR for the first peak as it suggests in a real fire scenario, the MWCNT/PUF would result in a 

more rapidly developing fire. This is likely not true for two reasons. First, this MWCNT/PUF 

Cone data (Figure 18 and Table 2) was not normalized, as suggested by Zammarano et al [53], 

since the surface area for MWCNT/PUF was not quantitatively measured and changed 

significantly throughout the test. Based on qualitative observations, the HRR for the first peak 

could be reduced by a factor of two while the second peak may only be slightly reduced. The 

result is the first peak for MWCNT/PUF would have a maximum HRR and time to peak similar 

to PUF and the second peak would then become the PHRR, which has a time to peak similar to 

PUF. Secondly, the Cone data does not capture the real impact of the PUF pool fire since there 

is no pool fire. Since a pool fire can approximately increase the fire threat (as calculated from 

HRR, THR, and burn time) of a burning product (e.g. upholstered furniture) by 35 % [59] and 

the MWCNT coating prevents pool formation than it is assumed that in a real fire replacing PUF 

with MWCNT/PUF would decrease the HRR from a product by 35 %. 

Within the measurement uncertainty, the CNF and MWCNT coatings have a similar impact on 

the fire performance of the BF (Figure 19 and Table 3). The MWCNT coating causes a 2.8 times 

increase in PHRR (157 kW/m2 ± 17 kW/m2 as compared to 57 kW/m2 ± 9 kW/m2) and 3.6 times 

increase in THR (2.9 MJ/m2 ± 0.3 MJ/m2 as compared to 0.8 MJ/m2 ± 0.1 MJ/m2) (Figure 19 and 

Table 3). 

3.2.3. MMT-based nanocoatings

TGA was used to determine the actual mass of MMT and coating deposited onto the substrates. 

The MMT coatings increased the mass of the PUF substrate by 3.2 mass fraction % ± 0.6 mass 

fraction % and the BF substrate by 6.9 mass fraction % ± 1.4 mass fraction %, of which, 66 mass 

fraction % ± 15 mass fraction % is MMT. The total CNF content relative to the substrate mass is 

4.6 mass fraction % ± 1.0 mass fraction % for PUF and 2.1 mass fraction % ± 0.2 mass fraction 

% for PUF, which is a typical loading level of MMT incorporated into polymers (not a coating) 

to improve the polymer’s fire performance. Unlike these other nanocomposites, the CNFs in 
43

these coatings are concentrated at the surface rather than randomly dispersed and distributed 

throughout the polymer matrix [31].

The HRR data indicated that the MMT coatings improved the fire performance of foam (Figure 

18 and Table 2). The HRR curves for MMT/PUF and PUF consist of two peaks. The time to 

maximum HRR each peak was 12 % ± 3 % earlier for PUF (21 s ± 2 s and 75 s ± 3 s as 

compared to 34 s ± 2 s and 78 s ± 2 s for MWCNT/PUF). The maximum measured HRR during 

the test (PHRR) was 35 % ± 3 % lower for MWCNT/PUF (620 kW/m2 ± 26 kW/m2 as compared 

to 403 kW/m2 ± 10 kW/m2). The THR and total burn time was also lower for MWCNT/PUF 

(21 % ± 3 % and 25 % ± 3 %, respectively).

Similar to PUF, MMT/PUF collpased to form a pool fire. However, the collapse was slower 

than PUF, but fast enough that the shape was retained for less time than the CNF/PUF and 

MWCNT/PUF. The result of the slower collapse is a first peak that was more similar to what 

was observed for CNF/PUF. The higher HRR minimum value between the first and second 

HRR peak, a significantly higher HRR maximum for the second peak (compared to 

MWCNT/PUF and CNF/PUF), and a second peak with attributes more similar to PUF are a 

reflection of the MMT providing little protection in the form of a char and the MMT/PUF is 

burning similar to a normal PUF pool fire. The inability of the MMT to form a good protective 

char stems from the pool fire being too shallow to allow char formation [56]. Unlike, CNF/PUF 

and MWCNT/PUF it is unlikely in a real fire scenario the MMT/PUF would have the potentially 

35% further decrease in HRR because MMT/PUF collapses to form a pool fire [59].

Within measurement uncertainty, the CNF, MWCNT, and MMT coatings have a very similar 

impact on the fire performance of the BF (Figure 19 and Table 3). The MMT coating causes a 

2.7 times increase in PHRR (155 kJ/m2 ± 16 kJ/m2 as compared to 57 kJ/m2 ± 9 kJ/m2) and 3.7 

times increase in THR (155 MJ/m2 ± 16 MJ/m2 as compared to 0.8 MJ/m2 ± 0.1 MJ/m2) (Figure 

19 and Table 3). 

Table 2. Cone Calorimetry data of the washed uncoated and coated PUF organized from highest 

to lowest in PHRR (PHRR in bold). Values reported with 2σ uncertainty.

Peak 1 Peak 2 THR 

(MJ/m2)

Residue Mass 

Fraction %

Burn 

time (s) HRR 

(kW/m2)

Time 

(s)

HRR 

(kW/m2)

Time 

(s)

PUF 224 ± 12 32 ± 2 620 ± 26 75 ± 3 33 ± 2 2.2 ± 0.1 140 ± 2

CNF/PUF 371 ± 10 27 ± 2 348 ± 10 83 ± 2 26 ± 1 11.0 ± 0.4 110 ± 2

MWCNT/PUF 391 ± 10 34 ± 2 403 ± 10 78 ± 2 26 ± 1 11.1 ± 0.4 105 ± 2

MMT/PUF 396 ± 15 36 ± 2 515 ± 15 86 ± 2 31 ± 2 2.6 ± 0.3 111 ± 2
44

Figure 18. HRR curves indicate the MWCNT and CNF coatings significantly reduce PHRR, 

THR, and total burn time, but MMT coatings increase flammability, as compared to pure PUF. 

The 2σ uncertainty is ± 5% in HRR and ± 2 s in time.

Table 3. Cone Calorimetry data of the uncoated and coated BF organized from highest to lowest 

in PHRR. Values reported with 2σ uncertainty.

Peak 1 THR 

(MJ/m2)

Burn time 

HRR (kW/m (s) 2) Time (s)

MWCNT/BF 157 ± 17 19 ± 2 2.9 ± 0.3 65 ± 2

MMT/BF 155 ± 16 15 ± 2 2.7 ± 0.4 60 ± 2

CNF/BF 149 ± 15 16 ± 2 3.1 ± 0.3 65 ± 2

BF 57 ± 9 25 ± 3 0.8 ± 0.1 50 ± 2

0

50

100

150

200

250

300

350

400

450

500

550

600

650

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

Heat Release (kW/m^2)

Time (sec)

PUF

MMT/PUF

CNF/PUF

MWCNT/PUF

Pool fire of collapsed PUF

PUF Collapse

CNF/PUF and 

MWCNT/PUF Shrinking

Flames burnt through char

MMT/PUF Slower Collapse
45

Figure 19. HRR curves of the uncoated and coated BF. All nanoparticle coatings deteriorate the 

fire performance of the BF. The 2σ uncertainty is ± 5% in HRR and ± 2 s in time.

4.4. Comparison to other flame retarding technologies

A previous study reported a 35 % reduction (Table 4) in PHRR for PUF using 4 mass fraction % 

CNFs embedded in the PUF (CNFs were added to the foam recipe) [29]. In comparison, the LbL

fabricated CNF/PUF specimen has a 20 % greater reduction in PHRR using 57 mass fraction % 

less CNFs. In other words, incorporating CNF as a coating rather than directly into the 

polyurethane foam yields a three times greater reduction in PUF flammability. This information 

may be of particular interest to foam manufacturers, as it is assumed the post-manufacturing 

coating of CNFs and using less CNFs will be easier and more cost effective to implement than 

incorporating CNFs into the foam recipe.

Najafi-Mohajeri used the Cone to measure the impact of 17 flame retardant additive packages 

(five non-halogen, four halogen, and seven halogen-phosphorous) on the flammability of a 

standard PUF [57]. These additive packages are commercially available and reported to be 

commonly used by the PUF industry. The five non-halogens reduced the PUF PHRR and THR 

by an average of 15 % and 14 %, respectively, at a 3.3 mass fraction % averaged loading (Table 

4). The four halogens reduced the PUF PHRR and THR by an average of 31 % and 16 %, 

respectively, at a 20 mass fraction % averaged loading. The seven halogen-phosphorous systems 
46

reduced the PUF PHRR and THR by an average of 14 % and 7 %, respectively, at a 28 mass 

fraction % averaged loading. 

Also using a Cone, Price [58] measured the flammability impact of incorporating melaminebased flame retardants into PUF (Table 4). The exact composition of the materials is unknown 

as they were purchased from a supplier. The melamine and melamine chlorate phosphate blend 

reduced the PHRR by 10 % and 15 %, respectively. As an alternative to using flame retardants, 

the authors also measured the impact of using fire blocking barrier fabrics to reduce the PUF 

flammability. Of the six specimens tested, the best performing combination was wrapping the 

standard PUF with zirconium hexafluoride flame retardant treated wool (FR-wool), which gave a 

29 % reduction in PHRR. This FR-wool also gave the greatest reduction in PHRR of the flame 

retardant foams (32 %), which was quite similar to what was reported for wrapping the standard 

PUF with this FR-wool. These results suggest the fire performance benefits gained by using 

these flame retardants are partially mitigated by the FR-wool.

Compared to these competitor flame retardant systems and the experimental CNF embedded in 

foam system, the nanoparticle LbL coatings developed in this project delivered a 14 % to 65 % 

larger reduction in PHRR and THR using 46 % to 95 % less FR [57,58]. To normalize the % 

PHRR reduction for the differences in FR content, the % PHRR values were divided by the FR 

loading level (Table 4). For example, a value of 0.5 % was calculated for the non-halogen FRs 

by dividing 15 % (% PHRR reduction relative to PUF) by 4 % (the FR loading level). This mass 

normalized % PHRR is the percent PHRR reduction measured for 1 mass fraction % loading of 

the FR. The FRs that yielded the lowest impact were the commercial non-halogen and halogenphosphorous FRs with a 0.5 % reduction in PHRR for 1 mass fraction % loading of these FRs. 

The commercial halogens FRs yielded a slightly better reduction with a value of 1.6 %. The best 

impact from an embedded FR was the experimental CNF system that yielded a 5 times to 17 

times larger reduction compared to the 17 commercial FRs, which is comparable to the impact of 

the worst LbL coating FR (MMT, 7.6 % reduction in % PHRR for 1 mass fraction % MMT). 

The greatest impact was measured for the CNF LbL coatings with a 17 times to 50 times greater 

% PHRR reduction (25 % reduction in % PHRR for 1 mass fraction % CNF) and the MWCNT 

LbL coatings with a 13 times to 39 times greater % PHRR reduction (19 % reduction in % 

PHRR for 1 mass fraction % MWCNT), relative to the commercial FRs. The only FR used in 

both the LbL coating and embedded was the CNF and the % PHRR reduction was 3 times when 

the CNF were used in the coating. The take message from this data is these nanoparticles yield a 

greater improvement in the fire resistance of polyurethane foam than the commonly used 

commercial FRs and using the nanoparticles in the LbL coating will give a greater improvement 

in the foam fire resistance than the embedded approach. Therefore, the LbL nanoparticle 

technologies deserve serious consideration for developing cost-effective fire safe polyurethane 

foam.
47

Table 4. Reduction in Cone data (relative to pure PUF) caused by FR in a LbL coating on and 

embedded in foam. Adjusting for different FR loadings, the LbL approach can result in the 

largest reduction in PHRR. No uncertainty was reported for the literature values. Experimental 

values reported with 2σ uncertainty.

Flame 

Retardant 

Location

FR (mass fraction % loading)

% Reduction relative 

to PUF

Mass FR 

normalized % 

PHRR THR PHRR reduction 

In coating 

on foam

CNF (1.6% ± 0.1%) (40 ± 3)% (21 ± 3)% 25 %

MWCNT (1.7% ± 0.1%) (35 ± 3)% (21 ± 2)% 19 %

MMT (2.1% ± 0.2%) (16 ± 3)% (6 ± 1)% 7.6 %

Embedded 

in foam

CNF (4%) 34 % 8.5 %

4 Halogen FRs (20%) 31 % 16 % 1.6 %

5 Non-halogen FRs (4%) 15 % 14 % 0.5 %

8 Halogen-Phosphorous FRs (28%) 14 % 7 % 0.5 %

3.5. Nanoparticle release from stressing

3.5.1. Measurements methodology

CNFs and MWCNTs released from stressing were quantified using UV-VIS. Collection and 

preparation of the nanoparticle containing suspensions for analysis had a significant impact on 

the accuracy and repeatability of the measurement. The initial methods developmed for these

measurements was based on analyzing the CNF containing suspensions as-received from the 

stressing experiments. After sonicating the simulated chewing/CNF suspension (0.050 mass 

fraction % ± 0.001 mass fraction % CNF and 0.9 mass fraction % ± 0.08 mass fraction % sodium 

chloride in DI) for 4 h, the UV-VIS absorbance value (using a 1 mm ES quartz cuvette) was 

significantly dependent on sonication time and the time between sonication and analysis. This 

dependence after long sonication times suggested the CNFs were not forming a well dispersed

and stable suspension. To facilitate better CNF dispersion and distribution, more stable 

suspension, and decrease in sonication time, SDS (of 0.0075 mass fraction % ± 0.02 mass 

fraction %) was added (prior to sonication) to this simulated chewing/CNF suspension. 

Incorporation of SDS resulted in a stable absorbance value after sonicating for 2 h at 90 % of 

maximum amplitude. The sonication time was not impacted by further increasing the SDS 

concentration to 0.23 mass fraction % ± 0.02 mass fraction %. In contrast, without sodium 

chloride in the suspension (simulated wear and tear), the higher SDS concentration yielded an 

equally stable absorbance value in only 1 h of sonciation. Since the only difference in the 

suspensions is sodium chloride, it was assumed the instability in the simulated chewing 

suspensions was a result of salt induced CNF aggregation.

Three suspensions were prepared and analyzed to demonstrate the impact of sodium chloride on

the absorbance value for CNF and MWCNT (267 nm) and the mitigation using dialysis (Figure 

20). The absorbance value of a CNF suspension prepared without sodium chloride (0.050 mass 

fraction % ± 0.001 mass fraction % CNF, 0.23 mass fraction % ± 0.02 mass fraction % SDS, and 

100 mL DI) is 2.18 ± 0.10 (Sodium chloride free spectrum). Adding sodium chloride (0.9 mass 

fraction % ± 0.08 mass fraction %) decreased the absorbance by 23% (1.66 ± 0.10, Before 

dialysis spectrum). Removing sodium chloride through 24 h of dialysis, reduced the 
48

conductivity to a value similar to DI (16,000 μS ± 300 μS reduced to < 0.5 μS) and resulted in an 

absorbance value similar to the sodium chloride free samples (2.16 ± 0.10, After dialysis 

spectrum). All spectra were collected after 1 h of sonication at 90 % maximum amplitude using 

a 10 mm ES quartz cuvette, except for the “Before Dialysis” which was sonicated for 4 h to 

obtain a stable absorbance value for this suspension. SDS was added immediately before 

sonication. 

By adding SDS and removing sodium chloride (simulated chewing extract only), repeatable and 

steady state UV-VIS absorbance values were obtained after 1 h of sonication. The CNF or 

MWCNT concentration in the stressed suspensions was based on a Beer’s Law calibration curve 

(CNF curve provided in Figure 21) constructed from the measured absorbance, at the maximum 

absorbance band (267 nm), of five prepared nanoparticle calibration suspensions. The 

absorbance values and the calibration curves constructed from a sodium chloride free and 

sodium chloride removed by dialysis suspensions were identical, which indicates that dialysis is 

an effective nondestructive method to remove sodium chloride. The lower limit detection of 

nanoparticles using this methodology is 0.00010 mass fraction % ± 0.00004 mass fraction %. 

Figure 20. UV-VIS absorbance plot of a CNF/DI/SDS suspension prepared without sodium 

chloride, with sodium chloride, and the sodium chloride removed by dialysis. Dialysis was 

critical to obtaining accurate and repeatable absorbance values at the MWCNT and CNF peak 

maximum (267 nm) for the simulated chewing suspensions. 
49

Figure 21. A Beer’s Law curve was constructed from the UV-VIS absorbance value of several 

CNF or MWCNT calibration standards (CNF provided here). CNF and MWCNT concentration 

was quantified based on the measured UV-VIS absorbance value of a stressed suspension and a 

Beer’s Law curve. The 2σ uncertainty is ± 5 % of the CNF or MWCNT mass fraction value.

Clay released during stressing was quantified using a Beer’s Law calibration curves constructed 

from the ICP-OES intensity of three mass fraction % MMT calibration standards in diluted nitric 

acid and DI. The amount of Al, Fe, Mg, and Na (elements) in MMT was determined using a 

standard addition method. More specifically, the ICP-OES intensity of five diluted MMT/DI 

solutions with different known amounts of each element was measured and plotted as a function 

of element type and concentration (Figure 22). The element concentration at zero intensity is the 

amount of that element in MMT in a diluted nitric acid solution containing 0.0010 mass fraction 

% ± 0.0005 mass fraction % MMT (Figure 23). The Beer’s Law calibration curves were 

constructed from the ICP-OES intensity of the diluted nitric acid solution containing 0.0010 

mass fraction % ± 0.0005 mass fraction % MMT and three lower mass fraction % MMT 

solutions that were created by dilution of this initial solution with more diluted nitric acid (Figure 

24). In contrast to the UV-VIS analysis, the stressed solutions were analyzed as-received by 

ICP-OES because the sodium chloride did not impact the analysis. However, the Na element is 

not used to quantify MMT concentration since the sodium chloride also contributes to the Na 

element ICP-OES intensity. Mg element was also not used to quantify MMT concentration since 

Mg can be found in PUF and BF. Therefore, MMT concentration is an average based on Fe and 

Al ICP-OES intensity.
50

Figure 22. ICP-OES intensity as a function of element type and concentration using a diluted

nitric acid/DI solution containing 0.0010 mass fraction % ± 0.0005 mass fraction % MMT and 

five different concentrations of each element. The concentration at an intensity of 0 cps is the 

amount of each element in MMT. The 2σ uncertainty is 5 % of the value (error bars are smaller 

than the data markers).

Figure 23. The amount of each element in MMT in a diluted nitric acid/DI solution containing 

0.0010 mass fraction % ± 0.0005 mass fraction % MMT in diluted. The error bars represent a 2σ

standard uncertainty in the concentration.
51

Figure 24. Beer’s Law calibration curve of ICP-OES intensity as a function of MMT 

concentration. The 2σ uncertainty is 5 % of the value (error bars are smaller than the data 

markers).

3.5.2. Stress induced nanoparticle release

The entire coated substrate (in a sealed plastic bag) was used for the simulated wear and tear 

experiments. The released nanoparticles are washed out of the bag using a SDS/DI solution 

(described in the experimental section). The same wash solution was used for 10 stressed 

samples so that the wash solution contained the nanoparticles released from 10 samples. This 

approach was used to increase the concentration of nanoparticles in the analyzed suspensions as 

the initial measurements indicated the nanoparticles released from a single substrate was below 

the detection limit of the instruments. Experiments conducted with known concentrations of 

nanoparticles indicated that within the detection limit of the measurement tools, all of the 

released nanoparticles were recovered using this methodology. 

The nanoparticle release values from the simulated wear and tear experiments are provided in 

Table 5. There were two general trends measured in the wear and tear studies. One was a higher 

release using CNFs and MWCNTs and a lower release using MMT. The other was more 

nanoparticles were released from the BF than the PUF substrate. More specifically, the release 

of CNFs and MWCNTs was 3.5 times higher from the BF than the PUF, which was an order of 

magnitude higher than MMTs released from either substrate. The highest and lowest release

values came from CNF/BF and MMT/PUF, respectively, which differ by more than two orders 

of magnitude. 

SEM images of all the released nanoparticles indicate the nanoparticles are embedded in the 

polymer coating and/or adhered to the substrate. In other words, the higher release values from 
52

BF may result from the BF being less durable than PUF to the mechanical stresses and not an 

indication of the nanoparticle coating durability. Presumably, the higher amount of CNFs 

released, as compared to the MMTs and MWCNTs, may result from the CNFs not being 

completely embedded in the polymer matrix (Figure 8 to Figure 11) and therefore easier to break 

away when the substrate is stressed. It is assumed that MMTs and MWCNTs were completely 

embedded in the coating because they are smaller and the TL is a more effective coating 

approach. Reducing the release of CNFs using a TL approach is currently under investigation. 

A small portion of the coated substrates were used for the simulated chewing experiments (4.6 

mass fraction % ± 0.1 mass fraction % of the entire coated substrate). The extraction solutions 

from 40 specimens were combined, sodium chloride was removed by dialysis and then the 

suspensions were stabilized by SDS. Similar to the simulated wear and tear experiments, this 

methodology yielded the most accurate and repeatable measurements, and combining the release 

from several substrates increased the nanoparticle content in the suspension to values above the 

LDL of the tool. 

The nanoparticle release values from the simulated chewing experiments are provided in Table 6. 

There were three general trends observed from the simulated chewing experiments. One was the 

highest release came when using MWCNTs and similar lower release values came from using 

CNFs and MMTs. Another was there appeared to be a bias, all be it smaller than in the wear and 

tear experiments, toward higher release from the BF. Lastly, compared to wear and tear, the 

release values were lower and closer in the chewing experiments.

For simulated chewing, the highest release values (MWCNTs) were an order of magnitude 

greater than the lowest release values (CNFs or MMTs on PUF) (Table 6). The MWCNTs 

release values were independent of the substrate and 3 times higher than the release of CNFs and 

MMTs from the BF. However, the CNFs and MMTs release values did depend on the substrate 

with a 3 times higher release from the BF than the PUF. Within the uncertainty of the 

measurements, the release of CNFs and MMTs were similar for a given substrate. The lowest 

chewing release values were comparable to the highest wear and tear release values. Therefore, 

as a general rule of thumb, the release from chewing was an order of magnitude greater than 

from wear and tear. 

Similar to wear and tear, the nanoparticles released from the simulated chewing were all 

embedded in the polymer matrix. The reason for higher CNF and MMT release from the BF 

may be a reflection of substrate durability. It is assumed that the simulated saliva causes the 

non-woven fabric to swell and separate. This combined with agitation may be sufficient stress to 

cause fragments of materials to wash out of the fabric. This “swelling” may also be the root 

cause tor the higher release values for the chewing experiments. All the materials used in the

coating can be dissolved in water and therefore if given sufficient time it is reasonable to expect 

that the coating in water will expand and potentially wash off the substrate. Reducing the release 

of nanoparticle by crossslinking the coating is currently under investigation. The higher release 

of MWCNT may be a result of its size and shape. The long fiber length of CNFs and the large 

flat surface of MMTs may provide sufficient interaction with the surrounding nanoparticles to 

help hold them in the coating.
53

Table 5. Simulated wear and tear nanoparticles released relative to the total nanoparticle content 

on the specimen (organized from highest to lowest in nanoparticle release). Values reported with 

2σ uncertainty.

Substrate Mass per specimen 

(mg)

Mass fraction % of total 

nanoparticle content

CNF/BF 0.037 ± 0.004 0.040 ± 0.004

MWCNT/BF 0.025 ± 0.003 0.027 ± 0.003

CNF/PUF 0.021 ± 0.002 0.010 ± 0.001

MWCNT/PUF 0.019 ± 0.002 0.0085 ± 0.0008

MMT/BF 0.0018 ± 0.0002 0.00095 ± 0.00009

MMT/PUF 0.0011 ± 0.0001 0.00028 ± 0.00002

Table 6. Simulated chewing nanoparticles released relative to the total nanoparticle content on 

the specimen (organized from highest to lowest in nanoparticle release). Values reported with 2σ

uncertainty.

Substrate Mass per specimen 

(mg)

Mass fraction % of 

total nanoparticle

MWCNT/PUF 0.047 ± 0.004 0.46 ± 0.04

MWCNT/BF 0.046 ± 0.005 0.42 ± 0.04

CNF/BF 0.014 ± 0.001 0.14 ± 0.01

MMT/BF 0.037 ± 0.004 0.13 ± 0.01

CNF/PUF 0.0052 ± 0.0005 0.053 ± 0.005

MMT/PUF 0.0054 ± 0.0005 0.043 ± 0.004

4. Conclusion

The characteristics of the CNF-based and MWCNT-based coatings on PUF were distinctly 

different with the MMT-based coatings having features similar to both of these nanocoatings. 

More specifically, the MWCNT coatings completely and uniformly covered the entire surface of 

the PUF and, other than a few sparsely distributed tens of micron sized MWCNT aggregates, the 

coating appeared smooth and featureless at lower magnifications. In contrast, the CNF-based 

coatings were rougher with an appearance more similar to a fibrous network rather than a 

smooth, uniform coating. These coatings covered all the PUF surfaces. Due to its large 

dimensions the CNFs tended to deposit as groups rather than individual fibers, which resulted in 

regions of high CNF concentration (tens of microns in size) and regions of no CNFs (less than a 

few microns in size). The highly aggregated regions contained fibers that were only partially 

embedded in the polymer coating. The regions of smaller aggregates (islands) generally 

contained fibers almost completely embedded in the polymer coating. The MMT coatings 

appeared rough due to regions of high clay aggregation (similar to CNF). These clay aggregates 

likely formed in the depositing solution and therefore deposited as large aggregates. The clay 

appeared to be completely embedded in the polymer matrix, which, in the non-aggregate regions, 

resulted in a coating that appeared smooth (similar to MWCNT). The CNF and MWCNT 

loading in the coatings were the same (50 mass fraction % ± 1 mass fraction %), but the CNF 

coating was 28% ± 8% thinner than the MWCNT coating (359 nm ± 36 nm as compared to 440 

nm ± 47 nm, respectively). In contrast, the MMT coatings were much thicker with a high 
54

variability of thickness (1000 nm ± 450 nm) and contained a higher concentration of 

nanoparticles (66 mass fraction% ± 16 mass fraction %).

One of the major factors impacting the physical characteristics of the coatings is the stability of 

the coating preparation suspensions. Within 4 h of stopping sonication, the CNFs begin to settle 

to the bottom of the depositing solution as indicated by a color gradient (darker at the bottom) in 

the CNF suspension. During deposition the CNF suspension appears very uniform. However, it 

is proposed that the reason the CNFs deposit as aggregates is because the CNFs had already 

begun to aggregate in the suspension. In comparison, the MWCNT suspensions remain visibly 

uniform over two days. This difference in quality of the suspensions is most likely a result of the 

smaller dimensions and/or the PEI functionalizing of the MWCNT. For the MMT, the 

suspension appeared to be fairly stable with no clay settling for several days. It is assumed the 

clay surface aggregation was more a result of poor clay dispersion in the DI than of poor 

suspension stability.

Even though the physical characteristics of the coatings were quite different, the improvement in 

PUF fire performance due to the CNF and MWCNT LbL fabricated coatings were similar and 10 

times to 50 times better (on a mass basis) than 17 commercial FRs commonly used in PUF. The 

MMT LbL fabricated coatings had less of an impact on fire performance (relative to the 

MWCNT and CNF); however, they were still 3 times to 17 times better than the commercial 

FRs, which is similar to what has been reported for CNFs embedded in the foam. This data 

suggests that these nanoparticles could potentially result in significantly less flammable PUF, but 

an at least 3 times greater improvement could be achieved if these nanoparticles were added 

through a LbL fabricated coating rather than embedded in the foam. This post-manufacturing

process route to increasing fire resistance performance of foam is attractive because it potentially 

has no impact of the foam manufacturing process.

The primary reason for the drastic decrease in flammability is that the coatings prevent foam 

melt dripping and increase char formation. In a real fire scenario, the formation of a pool fire, 

which is created by melting PUF, but not CNF/PUF or MWCNT/PUF, approximately increases 

the fire threat (as calculated from HRR, THR, and burn time) of the burning product by 35%

[59]. This impact is not captured in Cone data because there is no product (soft furnishing, etc.) 

for the pool fire to pose an additional flux upon. Therefore, the Cone data is a conservative 

measure of the improved fire performance created by the MWCNT and CNF coating; however, 

the actual benefit in real fires from using a CNF/PUF or MWCNT/PUF instead of PUF, could be 

35% greater than reported here. MMT coating also reduces the PHHR and THR, but not as 

significantly as CNF and MWCNT due to the inability of the MMT to maintain the shape and 

prevent the pool fire. For all nanoparticles, the nanoparticle coated BFs were more flammable 

than the pure BF. This may be due to the non-woven BF falling apart during the coating process.

The release of nanoparticles was in general an order of magnitude higher from simulated 

chewing than simulated wear and tear. These coatings (constructed of water dissolvable and 

suspendable materials) may be sensitive to moist stresses (e.g., washing and cleaning). To 

reduce nanoparticle release in simulated chewing, additional durability experiments should be 

conducted (e.g., crosslink the coating). Release was highest from the BF presumably due to the 

lower durability of the non-woven fabric design, as compared to the thermoset foam. A more 
55

durable barrier fabric should be considered for further investigation of this LbL technology (e.g., 

woven fabric). The simulated wear and tear release values were CNFs ~ MWCNTs (BF) > 

CNFs ~ MWCNTs (PUF) >> MMT (BF and PUF). The simulated chewing release values were 

MWCNT (BF and PUF) > CNF ~ MMT (BF) > CNF ~ MMT (PUF). To create a more durable 

coating (e.g., less release), the coating could be crosslinked or a TL approach could be used. It is 

important to keep in mind that a more durable coating may not be necessary from a risk exposure 

point of view since there has not been a toxicity evaluation of the materials released from these 

substrates. 

5. Future Research

This research has laid the foundation for using LbL to fabricate coatings on foam and barrier 

fabrics using a range of nanoparticles and other performance enhancing additives. We are 

currently fabricating and analyzing cellulosic fiber coatings and mixed additive coatings on both 

foam and barrier fabrics. 

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