The use of model cellulose gel beads to clarify flame-retardant characteristics of layer-by-layer nanocoatings

Carbohydrate Polymers 255 (2021) 117468

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Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol

The use of model cellulose gel beads to clarify flame-retardant
characteristics of layer-by-layer nanocoatings
ăklỹkaya a, *, Rose-Marie Pernilla Karlsson a, c, Federico Carosio b, Lars Wồgberg a, c, *
Oruỗ Ko
a

Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 56, SE-100 44 Stockholm, Sweden
Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, Alessandria Site Viale Teresa Michel 5, 15121, Alessandria, Italy
c
Wallenberg Wood Science Center, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden
b

A R T I C L E I N F O

A B S T R A C T

Keywords:
Layer-by-Layer assembly
Flame-retardant
Thermal stability
Cellulose gel beads

Layer-by-Layer (LbL) assembled nanocoatings are exploited to impart flame-retardant properties to cellulosic
substrates. A model cellulose material can make it possible to investigate an optimal bilayer (BL) range for the

deposition of coating while elucidating the main flame-retardant action thus allowing for an efficient design of
optimized LbL formulations. Model cellulose gel beads were prepared by dissolving cellulose-rich fibers followed
by precipitation. The beads were LbL-treated with chitosan (CH) and sodium hexametaphosphate (SHMP). The
char forming properties were studied using thermal gravimetric analysis. The coating increased the char yield in
nitrogen to up to 29 % and showed a distinct pattern of micro intumescent behavior upon heating. An optimal
range of 10–20 BL is observed. The well-defined model cellulose gel beads hence introduce a new scientific route
both to clarify the fundamental effects of different film components and to optimize the composition of the films.

1. Introduction
Cellulose is the most abundant biopolymer on earth (Klemm et al.,
2005), the most common sources of cellulose being wood and cotton.
Cotton fibers have been one of the major constituents in textiles, and
wood fibers have a broad application in the pulp and paper industry.
Cellulose-based materials are inexpensive, biodegradable and recy­
clable, but the inherent flammable character of cellulose limits its
application or requires flame-retardant treatment for specific applica­
tions. Recent developments have shown that treatment of the fiber
surfaces with thin layers of polymers and nanoparticles, through the LbL
technique, can impart excellent flame protection both for textiles (Li,
Schulz, & Grunlan, 2009) and for wood fibers (Koklukaya et al., 2015).
The surfaces of fibers from both cotton and wood are however rough and
chemically heterogeneous and they are not suitable for fundamental
investigations of the assembly of multilayers and the effects of LbL
coatings. Different model cellulose surfaces with different degrees of
crystallinity have therefore been developed (Aulin et al., 2009). The
most commonly used films have been prepared by spin coating of dis­
solved cellulose onto smooth silica surfaces (Aulin et al., 2009;
ăklỹkaya et al., 2018). It is not possible to dissolve cellulose in con­
Ko
ventional solvents due to its relatively high molecular mass and close

packing of the glucan macromolecule in a crystalline structure. How­
ever, regenerated cellulose materials can be prepared using solvents
such as N-Methylmorpholine-N-Oxide (NMMO) (Johnson, 1969),
cupriethylenediamine (CED) (Schweizer, 1857) or lithium chloride in N,
N-dimethylacetamide (LiCl-DMAc) (McCormick, 1981). Through the
regeneration of cellulose in suitable liquids, materials with different
shapes can be prepared such as films (Wendler et al., 2012), fibers
(Woodings, 2003), hydrogels (S. Wang et al., 2016), spheres (Oliveira &
Glasser, 1996) etc., and the degree of crystallinity of these materials is
dependent on the choice of solvent. Carrick et al. (Carrick et al., 2014)
and Karlsson et al. (Karlsson et al., 2018) demonstrated the use of
LiCl-DMAc to prepare nm smooth cellulose spheres with a crystallinity
below 1%. Regenerated cellulose fibers widely used in textiles (i.e.,
lyocell, viscose, and rayon) are prepared using different regeneration
processes (Wendler et al., 2012), but the use of native and regenerated
cellulose can be limited by the thermal instability and flammability of
the cellulose. Flame-retardancy can be imparted to cellulosic materials
via chemical additives in the wet state (Hall et al., 1999), pad-cure
coating (Horrocks, 2011), spray coating (Helmstetter, 1998) and
recently, the LbL technique (Holder et al., 2017). The LbL assembly
technique has been employed to apply an efficient flame-retardant
coating on substrates such as textile (Li, Schulz, & Grunlan, 2009;

* Corresponding authors at: Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 56, SE-100 44 Stockholm, Sweden.
E-mail addresses: (O. Kă
oklỹkaya), (L. Wågberg).
/>Received 4 September 2020; Received in revised form 26 November 2020; Accepted 27 November 2020
Available online 2 December 2020
0144-8617/© 2021 The Authors.

Published by Elsevier Ltd.
This is an open access
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O. Kă
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Carbohydrate Polymers 255 (2021) 117468

Srikulkit et al., 2006), polyurethane foam (Kim et al., 2011), wood fibers
ăklỹkaya et al., 2020; Ko
ăklỹkaya et al., 2018),
(Koklukaya et al., 2015; Ko
and cellulose nanofibril based aerogels (Koklukaya et al., 2017). LbL
treatment is based on the consecutive adsorption of layer constituents at
the solid-liquid interface (Decher, 1997). Different systems have been
investigated to impart micro-intumescent flame-retardant nanocoatings
on cotton fabrics using CH as a carbon source/blowing agent and SHMP

as an acid source (Guin et al., 2014; Leistner et al., 2015; Mateos et al.,
2014). However, there is limited fundamental understanding of the
flame-retardant mechanism and the effects of the LbL-assembled mul­
tilayers prior to their deposition as coatings on wood fiber/cellulose
surfaces. The conventional approach is to perform the depositions on the
selected substrate with variable parameters, perform the complete
characterization and then trace back to optimal layer number and
coating mode of action. In this way, the mechanisms behind the
improved flame-retardant characteristics of LbL films composed of poly
(allylamine hydrochloride) (PAH) and montmorillonite clay (MMT) on
polyamide 6 as well as for LbL systems comprising of CH and ammonium
polyphosphates (APP) applied to a cotton substrate could be indirectly
identified (Apaydin et al., 2014; Jimenez et al., 2016). It was shown that
the flame-retardant mode of action of (PAH/MMT) coating occurred in
the condensed phase. The 40 BLs of (PAH/MMT) coating protected the
underlying polyamide 6 from an external heat flux of 25 kW/m2
(Apaydin et al., 2014). It was also shown for a (CH/APP) multilayer
coating that the flame-retardant behavior was due to a combination of a
condensed phase forming an aromatic char layer and a gas phase
releasing non-flammable volatiles that promote the micro-intumescence
phenomenon (Jimenez et al., 2016). Based on earlier investigations it is
apparent that the common approach is to investigate the optimal
deposition conditions and flame-retardant mechanism after a complete
characterization of the treated substrates (Apaydin et al., 2014; Guin
et al., 2014; Jimenez et al., 2016; Mateos et al., 2014). Within this
context, model substrates such as silicon oxide have also been employed
in order to investigate the compositional and morphological changes
occurring within the coating after the exposure to a flame or to a heat
flux (Koklukaya et al., 2017; Maddalena et al., 2018). This approach has
the limitation of focusing only on the coating disregarding the effects of

the substrate that is replaced by silicon oxide. Thus, although the earlier
studies helped to identify the overall flame-retardant effect of LbL
multilayers, the use of a small scale and a controlled preliminary
screening approach involving the substrate of interest would have
allowed for the optimal design of the coating architecture and compo­
sition while providing a meaningful insight on the crucial interactions
occurring during combustion between the deposited LbL coating and the
substrate. In order to address such questions, we propose a simple and
yet effective strategy for the study and design of a flame-retardant LbL
assembly of nanocoatings directly on model cellulose beads. To this aim,
we have used dissolved carboxymethylated fibers to prepare
cellulose-based hydrogel beads to be used as model cellulose substrates
in combination with the LbL technique to deposit intumescent coatings
of CH and SHMP. This system is a good candidate for studying the
fundamental processes behind the development of intumescence coating
as it has already been used to confer flame-retardant properties to cotton
(Guin et al., 2014). These authors reported an improved cellulose char
formation combined with the formation of a barrier consisting of
sub-micron-sized bubbles as the main mechanism for optimal
flame-retardancy (Guin et al., 2014; Jimenez et al., 2016). In the present
work we are using our model system to investigate a much deeper un­
derstanding of the fundamentals behind the flame-retardant action of
the CH/SHMP system. Model experiments were also performed using
silicon oxide model surfaces (Carosio et al., 2018; Koklukaya et al.,
ăklỹkaya et al., 2018) to
2017) and flat model cellulose surfaces (Ko
investigate the molecular details of the morphology, thickness, and
roughness of the formed films. A correlation between optimal char
forming ability and deposited BL range is identified. The smooth surface
texture of cellulose gel beads provided a clear view on the structural

changes of the nanocoating during degradation pointing out a
micro-intumescent behavior. The results also demonstrate the excellent
applicability of the cellulose beads as model substrates for cellulose rich
materials in a variety of fundamental studies.
2. Experimental
2.1. Materials
The cellulose fibers employed in this study were obtained from a
ă
dissolving grade pulp supplied by Domsjă
o Fabriker AB, Ornskă
oldsvik,
Sweden. The cellulose content of the pulp was 93 % and the degree of
polymerization was about 780 (provided by the manufacturer). N,Ndimethylacetamide (DMAc) (>99.5 %, GC grade), lithium chloride
(LiCl), and acetic acid (Sigma-Aldrich) were used as received. CH (Mw =
60 000, 95 % deacetylation) was supplied by GTC Union Corp., Qingdao,
China, and SHMP (crystalline, +200 mesh, 96 %) was obtained from
Sigma-Aldrich, Stockholm, Sweden. Poly(vinyl amine) (PVAm), com­
mercial name Xelorex 6300, was supplied by BASF. PVAm was dialyzed
and freeze-dried prior to use. Monochloroacetic acid, methanol, iso­
propanol, ethanol, HCl, NaOH, and NaCl were all analytical grade pur­
chased from Merck, Stockholm Sweden.
2.2. Cellulose gel bead preparation
The cellulose fibers were first carboxymethylated following to the
method previously described by Wågberg et al. (Wågberg et al., 2008)
and the degree of substitution (D.S) was calculated by conductometric
titration (Katz & Beatson, 1984) to be 0.13 which corresponds to a
charge density of 795 μeqv/g. 1 g of the carboxymethylated pulp was
then dissolved in 100 mL solution of 7 wt% LiCl/DMAc following the
steps previously described by Karlsson et al. (Karlsson et al., 2018) and

Carrick et al. (Carrick et al., 2014). The water in the pulp was first sol­
vent exchanged by displacement with ethanol and the ethanol was
subsequently exchanged with DMAc using a filtration procedure. Each
solvent was displaced over a period of two days during which the solvent
was changed at least twice per day. After this first step, the DMAc in
which the pulp was to be dissolved was dehydrated by heating and
keeping it for 30 min at a temperature of 105 ◦ C. The LiCl was also
dehydrated during this 30 min in an oven at 105 ◦ C. After the dehy­
dration, the DMAc was allowed to cool and the LiCl was added and
dissolved. The pulp was added to the DMAc/LiCl solution at a temper­
ature of ca. 40 ◦ C and then instantly placed in a 5 ◦ C fridge and stirred
with a magnetic stirrer overnight. After about 24 h, the solution was
clear. The solution was then filtered using a 0.45 μm PTFE syringe filter
and the filtrate was employed to form gel beads by drop-wise precipi­
tation through a needle of 0.64 mm into about 95 mL of a non-solvent
consisting of 80 mL 0.03 M HCl (aq) with 15 mL ethanol. The gel
beads formed were allowed to rest in the bottom of the beaker at 5 ◦ C for
24 h. The non-solvent was then replaced with deionized water by
decanting about 80 mL of the non-solvent four times during two days
and stepwise decreasing the concentration of HCl. The gel beads were
then washed with deionized water for one week in order to remove any
residual DMAc/LiCl. The wet cellulose gel beads have an average
diameter of 2.7 mm and after drying the average diameter of beads was
0.6 mm. Prior to LbL treatment, the gel beads were dried at 23 ◦ C and 50
% RH.
2.3. Model LiCl/DMAc cellulose films
Cellulose dissolved in a solution of DMAc/LiCl was used to prepare
non-crystalline cellulose films according to a method similar to that
presented by Eriksson et al. (Eriksson et al., 2005). P-type, boron doped,
thickness 625 μm, single side polished flat silicon wafers (Addison En­

gineering, Inc. San Jose, CA) were used as a substrate for the model
cellulose surfaces. The silicon wafers were cut (10 × 60 mm) into strips.
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The silicon wafers and QCM-D crystals were cleaned by rinsing them in a
sequence of Milli-Q water, ethanol and Milli-Q water and then drying
them with nitrogen gas. The surfaces were plasma treated for 3 min in a
plasma oven (PDC-002, Harrick Scientific Inc.) at 30 W under reduced
air pressure. Prior to spin coating of the cellulose solution, silicon wafers
were treated with 0.1 g/L PVAm solution at pH 7.5 for 15 min to form an
anchoring layer for the cellulose and then rinsed with Milli-Q water and
dried with a stream of nitrogen. Spin-coating was performed using a spin
coater (KW-4A-2, Chemat Technology, Northridge, CA, USA) and the
cellulose solution was placed onto a PVAm treated silicon surface on the
spin-coater disk. Spin coating was performed at 3 000 rpm for 30 s.
These model cellulose surfaces were precipitated by immersion in
Milli-Q water. The substrates were then placed in Milli-Q water to
remove excess solvent and LiCl. Finally, the substrates were dried with
nitrogen and stored in a desiccator until further use. The dry thickness of
the non-crystalline cellulose film was measured by AFM to be 38 ± 2 nm.

represents one bilayer (BL). This process was repeated with 1 min
adsorption times until the desired number of bilayers had been depos­
ited. Containers and filters were replaced after deposition of every 20 BL

to avoid any complex formation. Coated beads were placed on a Teflon
surface after LbL-treatment and dried at 23 ◦ C and 50 % RH. Photograph
of cellulose gel bead before and after the LbL-treatment is shown in
supporting information Figure S1.
2.5. Thin film characterization
2.5.1. Quartz crystal microbalance with dissipation
A Quartz Crystal Microbalance with Dissipation (QCM-D, Q-Sense
ăteborg, Sweden) was used to estimate both the amount of
AB, Go
adsorbed polyelectrolyte with associated water and the viscoelastic
properties of the adsorbed film. The normalized frequency change can
be related to the adsorbed mass of polyelectrolyte and water and the
energy dissipation can be related to the viscoelastic properties of the film
(Rodahl et al., 1995). The adsorption of polyelectrolytes and the rinsing
steps were monitored until saturation was reached.

2.4. Layer-by-Layer deposition
CH solution (1 g/L) was prepared in 1 v/v% acetic acid, and a SHMP
solution (5 g/L) was prepared in Milli-Q water (18.2 MΩ cm Milli-Q
grade water Synergy 185, Millipore Bellerica, USA). Both solutions
were stirred with a magnetic stirrer for 24 h to ensure complete disso­
lution and the pH of the solutions was then adjusted to pH 5 using 5 M
NaOH for CH and 1 M HCl for SHMP and the electrolyte concentration
was adjusted to 10 mM NaCl. The silicon wafers were cleaned according
to the method previously described with Milli-Q water, ethanol and
Milli-Q water and dried with nitrogen gas (Aulin et al., 2008). The sil­
icon wafers were then placed in an air plasma cleaner (PDS 002, Harrick
Scientific Corp.) for 3 min in order to clean and activate the surface prior
to LbL deposition. The silicon wafers or model cellulose surfaces were
alternately dipped into polyelectrolyte solutions in the order CH and

SHMP using an automatic dipping robot (StratoSequence VI, nanoStrata
Inc., Tallahassee, Florida, USA). The adsorption time for the first bilayer
was 5 min in each solution in order to achieve a uniform deposition,
while the time for the rest of the depositions was 1 min. The substrates
were rinsed with Milli-Q water (pH 5) three times between each depo­
sition for 1 min in each rinsing step without intermediate drying. For LbL
deposition on cellulose gel beads, a home-made filtration set up was
used (Fig. 1). Prior to LbL deposition, the cellulose beads were washed
according to the previously described procedure, the carboxyl groups
ărklund, 1993), and
were converted to their sodium form (Wågberg & Bjo
the beads were dried at 23 ◦ C and 50 % RH. Dried cellulose beads were
immersed in cationic CH solution for 5 min, after which the solution was
filtered by suction and the beads were rinsed twice with Milli-Q water
(pH 5) to ensure removal of loosely adhered and excess polymer. The
beads were then exposed to anionic SHMP solution by filling the
container with a solution and allowing adsorption for 5 min. The solu­
tion was then filtered by applying vacuum pressure and the beads were
rinsed twice with Milli-Q water (pH 5). One such sequence of deposition

2.5.2. Atomic force microscopy
An Atomic force microscope (AFM), Nanoscope IIIa (Bruker AXS,
Santa Barbara, CA) was used to investigate the surface topography,
roughness, and thickness of the multilayer films deposited on model
cellulose surfaces prepared on silicon wafers. The films were scratched
with a sharp blade in the dry state. The thickness was measured before
and after LbL treatment to determine the thickness of the films formed. E
and J-type piezoelectric scanners and Scanasyst cantilevers with a
nominal resonance frequency of 70 kHz and a 0.4 N/m spring constant
were used to scan the surfaces in air. The surface roughness value was

calculated from acquired images with an area of 2 × 2 μ m2.
2.5.3. Nitrogen analysis
The ANTEK 7000 nitrogen analyzer (Antek Instruments, Houston,
TX, USA) was used to measure the nitrogen content of chitosan adsorbed
on the cellulose beads. The method is based on combustion of the sample
(c.a. 5 mg) at 1050 ◦ C in an oxygen-poor atmosphere where nitrogen is
oxidized to NO before being further oxidized to excited NO2 in ozone.
The light emitted when the excited NO2 is converted to its standard state
is detected by a photomultiplier tube. The system is calibrated with a
known amount of CH (Supporting information Figure S2).
2.5.4. Fourier transform infrared spectrometry
Spectra of cellulose beads were obtained using a Perkin-Elmer
Spectrum 2000 FTIR with an attenuated total reflectance crystal acces­
sory (Golden Gate). ATR-FTIR spectra were recorded in the 4000− 600
cm− 1 region at a resolution of 4.0 cm− 1 and using 16 scans.
2.5.5. Thermogravimetric analysis
The thermal degradation of the untreated and LbL-treated cellulose
Fig. 1. a) Photograph of cellulose gel bead in wet swollen state (on
the right) and once dried then again swollen in water (on the left),
b) Schematic description of the LbL assembly on cellulose gel
beads. Beads were treated with cationic chitosan (CH) and anionic
sodium hexametaphosphate (SHMP). The process was repeated in
order to deposit 100 BL. The polyelectrolyte concentration was 1
g/L for CH and 5 g/L for SHMP, both in a 10 mM NaCl aqueous
solution at pH 5. The rinsing solution was Milli-Q water at pH 5
and c) Schematic of cellulose bead before and after LbL assembly.

3

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beads was investigated by thermogravimetric analysis (TGA) (Mettler
Toledo TGA/DSC, Stockholm, Sweden). The samples (5 ± 1 mg) were
placed in 70 μL aluminium oxide crucibles and heated at a rate of 10 ◦ C/
min from 40 to 800 ◦ C in nitrogen at a flow rate of 50 mL/min.

sputter coated with Pt/Pd.
3. Results and discussion
3.1. Monitoring the build-up of multilayer films on flat model surfaces
and on cellulose beads

2.5.6. Heating element
The thermal degradation behavior of untreated and LbL treated
cellulose beads were monitored using a high-speed camera (IDT N4MS3) with a 2× magnification microscope lens. The samples were
placed in a small ceramic crucible and covered with a microscope slide
cover slip. A light emitting diode light source (IDT 7 LED) was used to
illuminate the samples. The ceramic crucible was placed on a flat
heating element (d 10.8 × 2 mm/24 V/50 W/750 ◦ C/Button heater,
Rauschert Steinbach GmbH, Germany) with heating rate of 300 ◦ C/min
when the temperature was set to ~370 ◦ C. The changes in the structure
of the samples were recorded using high-speed camera at a frame rate of
100 frames per second. A schematic of the experimental setup is shown
in Figure S3.

The LbL formation of multilayer films consisting of CH and SHMP on
model cellulose surface was investigated using QCM-D. Fig. 2 shows the

results from the QCM-D measurements, where the normalized frequency
shift for the third overtone and the change in the dissipation are shown
as functions of the number of adsorbed layers.
The frequency shift during the adsorption of CH showed an increase
followed by a decrease during SHMP adsorption, as shown in Fig. 2a.
The energy dissipation data (ΔD) showed a significant decrease during
adsorption of the CH layer followed by an increase during the adsorption
of the SHMP layer. Benselfelt et al. reported a similar behavior for the
adsorption of a multilayer film of PDADMAC/PSS on a model cellulose
surface (Benselfelt et al., 2017). Earlier studies have also clearly shown a
deswelling of cellulose film due to polyelectrolyte adsorption (Benselfelt
et al., 2017; Enarsson & Wågberg, 2008; Notley, 2008; Wang et al.,
2011; Vuoriluoto et al., 2015). It can therefore be suggested that the
detected changes in the QCM-D measurements following the adsorption
of CH are due to a deswelling of the highly charged cellulose film on the
QCM crystal due to a neutralization of the charges of the cellulose by the
adsorbed CH similar to earlier results (Benselfelt et al., 2017). The
decrease in the pH accompanying the addition of CH will naturally add
to this effect but, as noted earlier (Xie & Granick, 2002), the charges of
the CH will also increase the degree of dissociation of the carboxyl
groups in the cellulose, which means that the effect of the pH will
probably not be the dominating cause of the deswelling. When the SHMP

2.5.7. Scanning Electron microscopy and energy dispersive X-ray analysis
A field emission scanning electron microscope (FE-SEM, Hitachi S4800) was used to investigate the surface morphology of cellulose beads
before and after the LbL treatment. The residues from the heating
element test were also investigated with FE-SEM to study the change in
morphology. Test pieces were coated with a 5 nm thick platinum/
palladium layer using a Cressington 208 HR high-resolution sputter
coater. The presence of phosphorus in the LbL-treated cellulose beads

before and after the heat element test and TGA was performed using an
Inca (Oxford Instruments, X-MAX N80) energy dispersive X-ray spec­
trometer (EDX). Dried cellulose beads were mounted on the specimen
holder and then cut in half using a razor blade. Samples for EDX were not

Fig. 2. The LbL build-up of five bilayers of (CH/SHMP) thin film on model cellulose surfaces (i.e., 795 μeq/g) using QCM-D measurements. a) Change in normalized
frequency, b) change in energy dissipation, c) total adsorbed mass calculated using the Sauerbrey equation, and d) chemical structures of polyelectrolytes used. The
polyelectrolyte concentrations were 1 g/L for CH and 5 g/L for SHMP, both in a 10 mM NaCl aqueous solution at pH 5. The rinsing solution was Milli-Q (MQ) water at
pH 5. The adsorption sequence and rinsing were continued until a steady state signal was reached.
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was added, there was a significant decrease in frequency and a
concomitant increase in the dissipation, which indicate that when the
SHMP is adsorbed, the charge of the chitosan is efficiently compensated
and probably over-compensated, which means that the cellulose film is
again able to swell, and this swelling is much more significant compared
to earlier results (Benselfelt et al., 2017). The trends for the subsequent
layers are the same, showing both a steady build-up of the LbL film and a
reversible swelling and deswelling of the cellulose film which is a clear
demonstration of the dynamics of the LbL assembly during the build of
the films which indeed is important for the development of the prop­
erties of the films.
To add further information to the QCM-D measurements and to
quantify the amount of polymers in the adsorbed layers the cellulose

beads were also used as a substrate for LbL deposition and the amount of
CH adsorbed was determined using nitrogen analysis. The results
showed a steady increase in adsorbed amount during the build-up of 100
BL supporting the results of the QCM-D measurements regarding the
steady build-up of LbLs on the cellulose surface (Fig. 3). The initial value
for the reference sample is probably due to residual nitrogen containing
cellulose solvent in the prepared beads and this value could be sub­
tracted from the other layers since the amount of adsorbed polymer was
determined from a calibration curve using the CH (Figure S2).
The chemical composition of the LbL coatings deposited on cellulose
beads has been assessed qualitatively using FTIR spectroscopy in an
attenuated total reflection (ATR) configuration. Fig. 4 shows the spectra
of cellulose beads, CH, SHMP and LbL-treated cellulose beads. The
characteristic peaks of cellulose are described in supporting informa­
tion. The peak observed at 1734 cm− 1 was ascribed to protonated car­
boxylic acids. The LbL-treated cellulose beads showed peaks at 1655
cm− 1 ascribed to the carbonyl (C=O), at 1592 cm− 1 ascribed to NH2, and
at 1153 cm− 1, 1063 cm− 1, and 1028 cm− 1 ascribed to stretching vi­
brations of C–O–C in glucosidic bonds of CH (Osman & Arof, 2003). The
presence of SHMP in the coating was shown by two strong signals at
1250 cm− 1 and 865 cm− 1 corresponding to stretching of P=O and P–O–P
groups (Drevelle et al., 2005). The absorbance intensity of two peaks
increased as the number of bilayers increased indicating the build-up of
the multilayer. Further evidence of the thin film growth was provided by
the fact that the weak signal at 1734 cm− 1, related to C=O stretching
vibrations in the carboxylic group present on the beads slowly dis­
appeared during the LbL deposition. The absorbance peaks of LbL thin
film starts to dominate over the absorbance peaks of cellulose already at
10 BL of deposition.

3.2. Thickness and roughness of the films on model surfaces
Model cellulose surfaces prepared on silicon wafers were used as
substrates for multilayer film formation and the LbL-assembled films
were imaged using AFM to characterize the morphology, roughness, and
thickness of the dry films. The height images of the CH/SHMP films are
shown in Fig. 5. The thickness of the films was measured using AFM by
scanning the film scratched with a scalpel (Figure S4).
The thickness and roughness are shown in Fig. 6 as functions of the
number of deposited bilayers.
The films deposited on the model cellulose surfaces were somewhat
thicker than those deposited on the silicon wafers and the LbL film buildup showed a linear increase in the thickness up to 20 BL, after which the
increase in thickness was non-linear with additional BL deposition. It has
been suggested that the change in the LbL growth with the number of
BLs can be due to an in and out diffusion of polyelectrolytes in the LbLs
(Guin et al., 2014; Picart et al., 2001) or a type of island growth with
increasing number of BLs (Haynie et al., 2011) where an initial un­
evenness propagates and small islands grow into larger islands as the
number of BL increases. After passing 20 BL, there is a super-linear
growth (Abdelkebir et al., 2011) and the roughness was larger for 10
and 20 BL of films deposited on silicon wafers than those deposited on
model cellulose surfaces, but the roughness was similar for 50 and 100
BL of film on both surfaces and the formed layers were indeed very
smooth. A more detailed analysis of the surfaces also shows that there is
a granular morphology of 10 and 20 BL of CH/SHMP film on the silicon
oxide surfaces, as shown in Figure S5, and also on the cellulose model
surfaces as shown in Fig. 5. It can also be seen that the smaller granular
shape of the surfaces changes into a larger scale unevenness at around 50
BL, which results in a lower roughness value but also fits with the island
growth model (Haynie et al., 2011). This behavior has already been
observed for non-linear LbL systems and has been ascribed to the in and

out diffusion of polyelectrolytes in the LbL structure (Picart et al., 2002).
It is not possible to establish the molecular reason for the change in
growth detected when passing 20 BL for the present system, but it is
clear that there is a steady growth of the LbLs with the number of
deposition steps and that the granular structure of the surfaces changes
to a more even surface leaving a rather flat and flaw-free surface which
is probably essential for good flame-retardancy of the treated surfaces.
3.3. Thermogravimetric analysis of cellulose beads
Thermogravimetric analysis in nitrogen was used in order to eval­
uate the effect of the LbL coating on the char forming ability of the
cellulose gel beads in an oxygen depleted environment. This approach
can provide preliminary hints on the pyrolysis occurring in the
condensed phase of a burning material where the presence of a protec­
tive coating results in an essentially anaerobic atmosphere and reduced
heating rates. Fig. 7 shows the weight loss (TG) and the derivative of the
weight loss (dTG) as a function of temperature and Table 1 presents the
degradation temperatures and residual amounts of reference and LbLtreated beads.
The untreated and LbL-treated cellulose beads show similar thermal
degradation processes. The initial weight loss observed at 100 ◦ C is
attributed to the dehydration of water adsorbed by the coating. The first
significant loss of mass observed at 246 ◦ C is attributed to dehydration
due to water entrapped within the cellulose gel beads and depolymer­
ization of non-crystalline cellulose leading to an aliphatic char. The rate
of mass loss was lower in the LbL-treated than in the reference beads. It
is suggested that this is due to the charring properties of the LbL coating
since the rate at which thermal energy reaches the surface of cellulose
beads is reduced by the LbL film (Hribernik et al., 2007). A second
degradation step occurs at 308 ◦ C. The LbL-treated cellulose beads
exhibit no early degradation, which is generally reported to be due to the
presence of phosphorus (Guin et al., 2014). A possible explanation is

that the temperature at which the phosphorus compound catalyzes the

Fig. 3. Total amount of CH adsorbed on cellulose beads as a function of number
of bilayers deposited, determined by nitrogen analysis using a calibration curve
for the CH. The polyelectrolyte concentrations were 1 g/L for CH and 5 g/L for
SHMP, both in a 10 mM NaCl aqueous solution at pH 5. The rinsing solution
was Milli-Q water at pH 5.
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Fig. 4. FTIR spectra of a) cellulose gel beads, b) CH, c) SHMP and d) CH/SHMP treated cellulose beads (10, 20, 50 and 100 BL). The polyelectrolyte concentrations
were 1 g/L for CH and 5 g/L for SHMP, both in a 10 mM NaCl aqueous solution at pH 5. The rinsing solution was Milli-Q water at pH 5.

Fig. 5. AFM Scanasyst height images of the reference model cellulose surface and LbL-treated surfaces with different numbers of BLs. The images are 2 × 2 μ m2 and
the z-range is indicated in the scale bar to the right of the images. The polyelectrolyte concentrations were 1 g/L for CH and 5 g/L for SHMP, both in a 10 mM NaCl
aqueous solution at pH 5. The rinsing solution was Milli-Q water at pH 5.

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Fig. 6. The average a) thickness and b) roughness values of CH/SHMP films deposited on silicon oxide and model cellulose surfaces as functions of the number of
bilayers deposited. The polyelectrolyte concentrations were 1 g/L for CH and 5 g/L for SHMP, both in a 10 mM NaCl aqueous solution at pH 5. The rinsing solution
was Milli-Q water at pH 5.

Fig. 7. a) Weight loss (TG) and b) derivative of weight loss (dTG) for untreated and CH/SHMP treated cellulose beads in a nitrogen atmosphere.

(~300 ◦ C/min) by using a specially designed heating element in which
the untreated and LbL-treated cellulose beads were heated to a high
temperature (~370 ◦ C) and monitored with a high speed camera in
order to assess the steep degradation curve observed in TGA evaluations
(Figure S6). 370 ◦ C was selected in order to ensure completion of all the
main degradation steps observed by TGA. All the samples immediately
began to exhibit thermal degradation resulting in the formation of a char
layer. During the degradation, sudden movements (e.g., jumping of the
sample) were observed which were attributed to the release of entrap­
ped water vapor within the cellulose beads and in the LbL structures.
The residues from this test were then investigated using SEM in order to
assess the coating morphology. Fig. 8 shows SEM images of untreated
and LbL-treated beads before and after heat application.
The untreated beads had a relatively smooth morphology. After
deposition of 10 BLs of CH/SHMP, the beads appeared to have a wrin­
kled morphology. A possible explanation of this structural change is the
difference in modulus of cellulose and of the coating (Nolte et al., 2005;
Stafford et al., 2004). More specifically, the cellulose beads were dry
prior to the LbL treatment but during the treatment process, they
became completely swollen due to the presence of water. Having
different moduli, the beads and the thin LbL films create a stress which
forms the wrinkled morphology (Chan & Crosby, 2011) upon drying as
shown in Fig. 8. A further increase in BL number results in an increased
coating thickness as shown by AFM, which consequently forms larger

wrinkles in the coating. After the application of heat, both the untreated
and LbL-treated beads maintain their shapes. The charring layer of the

Table 1
TGA data for untreated and CH/SHMP treated cellulose beads in a nitrogen
atmosphere.
Sample

Tmax1 [◦ C]

Tmax2 [◦ C]

Residue [%]

Cellulose bead
10 BL
20 BL
50 BL
100 BL

245
247
246
246
246

308
308
308
308

308

6
21
25
26
29

dehydration of cellulose overlaps the degradation temperature of
non-crystalline cellulose. The amount of residue found at 800 ◦ C
increased as the number of bilayers deposited increased. This behavior
can be attributed to the presence of phosphate groups in the SHMP
which favor the dehydration of CH towards the formation of an aromatic
char which acts as a thermal barrier to limit mass and heat transfer to the
cellulose beads (Carosio et al., 2015). In addition, an optimum in char
forming efficiency is clearly observable in the 10–20 BL range as the
increase of the deposited BL to 50 and 100 yields diminish returns in
terms of final residues (Table 1). This suggests that a flame-retardant
application of this system should target a BL number in between 10
and 20 BL and is in good agreement with a previous study where 17 BL
deposited on cotton yielded optimal flame-retardant properties (Guin
et al., 2014).
In order to mimic the exposure to a heating rate relevant to a fire
scenario, the beads were further characterized at high heating rates
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Fig. 8. SEM images of untreated and CH/SHMP treated cellulose beads before (the left-hand column) and after (the right-hand column) heat application. a) Un­
treated cellulose bead, b) 10 BL, c) 20 BL, d) 50 BL, and e) 100 BL. The higher magnification SEM images of indicated area by square frames are shown adjacent to
corresponding SEM image of cellulose bead. The polyelectrolyte concentrations were 1 g/L for CH and 5 g/L for SHMP, both in a 10 mM NaCl aqueous solution at pH
5. The rinsing solution was Milli-Q water at pH 5.

untreated cellulose bead exhibits obvious cracks and voids due to cel­
lulose pyrolysis and the release of volatile compounds. On the contrary,
LbL-treated beads show a unique structure which is generally defined as
micro-intumescent bubbling (Carosio et al., 2015; Li et al., 2011). The
formed sub-micronic bubbles become larger and more distinguishable as
the number of BLs deposited is increased. The reason for this change is
not exactly known but it can be suggested that the bubbles are formed as
a consequence of the release of volatile gases inside the beads, and that
this in turn creates a stress on the coating layer which yields by creating
bubbles. This behavior can be related to the thickness and barrier
properties of the films formed. More extensive model experiments are
needed to clarify these mechanisms, but the results show the potential of
using the beads to establish the molecular mechanism for different
LbL-treatments of cellulose surfaces. In addition, it is worth highlighting
that a nearly identical post combustion morphology has been observed
on cotton fabrics treated by the same CH/SHMP assembly (Guin et al.,
2014). This further shows the ability of the approach developed in this
work in predicting the coating behavior in real scale testing conditions.
Fig. 9 shows cross-sections of untreated and 100 BL treated cellulose
beads together with 50 BL treated cellulose beads after heat treatment
and their corresponding EDX spectra.
The elemental composition of the samples shows that phosphorus
was detected only in the LbL-treated beads, indicating the presence of

SHMP in the thin films on the beads. Phosphorus is homogeneously
distributed along the surface of the bead, and it is still present as a

protective layer around the bead even after the heat application. This is
further shown by the elemental analysis line spectrum of the crosssection of a 20 BL treated cellulose bead after thermal gravimetric
analysis (Figure S7). Interestingly, no phosphorus was detected in the
core of the cellulose bead, indicating that the phosphate action upon
heating took place only on the surface of the cellulose bead where the
coating was located. This suggests that, upon heating, no migration of
the phosphate occurs inside the beads and that the main action of SHMP
is to mainly favor the coating char formation with limited effects on
cellulose. The formation of a protective barrier can then limit heat
transfer and promote cellulose charring as it is well known that the char
forming ability of cellulose is inversely proportional to the heating rate
(Alongi et al., 2013). This further explains the observed diminishing
returns in performances upon increasing the number of BL as observed
by TGA. In the 10–20 BL range the assembly produces a continuous and
thick enough coating capable of providing good thermal shielding per­
formances; since there is no phosphate migration, increasing the amount
of SHMP by adding more layers only slightly improves the coating
performances.
4. Conclusions
Based on the previously reported literature dealing with the use of
CH/SHMP LbL coating for cotton flame-retardancy (Guin et al., 2014;
Mateos et al., 2014), this work reported a novel approach employing
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Carbohydrate Polymers 255 (2021) 117468

Fig. 9. Cross-section SEM images and corresponding EDX elemental mapping of untreated, 100 BL treated and, after heat application, 50 BL treated cellulose beads.

well-defined, non-crystalline cellulose gel beads as a model substrate to
examine the molecular mechanism behind the effect of the multilayer
coating on the thermal degradation of cellulose. FTIR measurements
showed that absorbance peaks of pristine cellulose were dominated by
the absorbance peaks of coating after deposition of 10 BL. Thermogra­
vimetric analysis revealed that cellulose beads coated with CH/SHMP
films exhibit a degradation behavior different from that of the uncoated
reference beads. The multilayer coating of CH/SHMP due to synergetic
effect enhanced the char formation by favoring the dehydration of cel­
lulose and the char formed subsequently protected the underlying cel­
lulose resulting in a residue as high as 29 % at 800 ◦ C for 100 BL coated
beads under a nitrogen atmosphere. In addition, the amount of residue
significantly increased by a factor of 3.5 after only 10 BLs had been
deposited but a further increase in the BL number did not show a similar
increase. SEM images and EDX spectra show the formation of a
micro-intumescent swollen char layer located on the surface of the
LbL-treated beads. A correlation of the observed results with previously
reported literature (Apaydin et al., 2014; Holder et al., 2017; Jimenez
et al., 2016) dealing with the use of LbL assembled coatings for
flame-retardancy clearly demonstrates the effectiveness of the proposed
approach in providing meaningful insights on optimal BL range, coating
mechanism and microstructure changes upon heating. The proposed
colloidal approach has never been investigated before since common
practice of previously reported literature was to investigate the optimal
deposition conditions and flame-retardant mechanism after a complete

characterization of the treated substrates (Apaydin et al., 2014; Guin
et al., 2014; Jimenez et al., 2016; Mateos et al., 2014). Conversely, this
model material provides an excellent experimental platform for in­
vestigations aimed at a clear understanding of the effect of different
surface treatments on the thermal degradation of cellulose. Further
developments of the proposed approach might involve the design of
cellulose beads characterized by tunable degree of crystallinity (H. Li
et al., 2020) as well as the study of different LbL assembly encompassing
nanoparticles and the implementation of advanced characterization
techniques aiming at a deeper investigation of the molecular scale
mechanisms of the assembly.

authors. All the authors have given their approval to the final version of
the manuscript.
CRediT authorship contribution statement
ă klỹkaya: Investigation, Writing - original draft. Rose-Marie
Oruỗ Ko
Pernilla Karlsson: Investigation, Writing - review & editing. Federico
Carosio: Investigation, Validation, Writing - review & editing. Lars
Wågberg: Supervision, Validation, Writing - review & editing.
Declaration of Competing Interest
The authors declare no competing financial interest.
Acknowledgment
ăklỹkaya, and Federico Carosio acknowledge
Lars Wồgberg, Oruỗ Ko
financial support from SSF (The Swedish Foundation for Strategic
Research) and Lars Wågberg and Rose-Marie Pernilla Karlsson also
acknowledge The Wallenberg Wood Science Centre for financial
support.
Appendix A. Supplementary data

Supplementary material related to this article can be found, in the
online version, at doi: />References
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