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Preparation and characterisation of flame retardant encapsulated with
functionalised silica-based shell
Doan-Trang Hoanga*, Diane Schorrb, Véronic Landrya, Pierre Blancheta,
Stéphanie Vanslambroucka, Christian Dagenaisb
aNSERC Industrial Research Chair on Ecoresponsible Wood Construction, Department of
Wood and Forest Sciences, Université Laval, Quebec, QC, G1V 0A6, Canada
bFPInnovations, 1055 rue du PEPS, Quebec, QC, G1V 4C7, Canada
*Corresponding author: Doan-Trang Hoang, NSERC Industrial Research Chair on
Ecoresponsible Wood Construction, Department of Wood and Forest Sciences, Université
Laval, Quebec, QC, G1V0A6, Canada.
Tel.: +1 581 922 1206
E-mail address: [email protected] (T.D.T. Hoang).
1
Preparation and characterisation of flame retardant encapsulated with
functionalised silica-based shell
Abstract
Intumescent fire retardant (IFR) coatings are nowadays considered as the most effective flame
retardant (FR) treatment. Nevertheless, the principal compound in an IFR system, ammonium
polyphosphate (APP), is highly sensitive to moisture and IFR coating effectiveness decreases quickly.
The main objective of this study is to encapsulate APP in a hybrid silica-based membrane by sol-gel
process using alkoxysilane tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) precursor.
The morphology and structure of APP and microencapsulated ammonium polyphosphate (MAPP)
were assessed by scanning electron miscroscopy and Fourier transform infrared spectroscopy (FTIR).
X-ray photoelectron spectroscopy (XPS) results revealed that APP was well encapsulated inside the
polysiloxane shells. The thermal degradation of APP and MAPP was evaluated by thermogravimetric
analysis. At 800 °C, the MAPP had higher char residue (70.49 wt%) than APP (3.06 wt%). The
hydrophobicity of MAPP increased significantly with the water contact angles up to 98°, in
comparison to 20° for APP.
Keywords: Flame retardants; ammonium polyphosphate; polysiloxane shell; hydrophobicity; thermal
stability
2
1. Introduction
The use of timber products in construction has been growing over the past years due to their
outstanding properties (low density, good physical and mechanical properties, availability,
etc.), environmental benefits and as they are aesthetically pleasing (Lowden and Hull, 2013).
However, wood products are considered as combustible materials in construction and they are
strictly ruled by building codes, mainly because of their inherent flammability and propensity
of spreading fire (Goldsmith, 2011). In order to reduce wood flammability and to delay fire
propagation over its surface, flame retardant (FR) additives are commonly impregnated into
the wood structure or dispersed into coatings applied over wood surfaces (Lowden and Hull,
2013). Impregnation treatments are expensive and are not adapted to all wood-based
materials. Besides, they generate wood swelling and shrinking (Goldsmith, 2011). On the
other hand, fire retardant coatings possess all the advantages of a regular decorative film, such
as easy to apply, they do not affect the intrinsic properties of wood, low cost, but also develop
an insulation barrier that isolates the heat flux from the wood substrate and maintains its
thermal degradation, ignition, or combustion properties (Mariappan, 2016).
There are two different groups of fire retardant coatings: non-intumescent and intumescent
(Dahm, 1996; Weil, 2011). Intumescent fire retardant (IFR) coating is the easiest, more
economical and efficient way to protect wood substrates from fire (Kumar, Kumar and Arora,
2013; Mariappan, 2016). Moreover, the IFR mixture is environmental friendly compared to
some conventional flame retardants, especially, halogenated flame retardants, which were
found to be persistent, bioaccumalative and toxic. A classical IFR coating is composed of
three principal active components including an inorganic acid source/catalyst (e.g. ammonium
polyphosphate), a carbon-rich source/carbonific (e.g. pentaerythritol) and a blowing
agent/spumific (e.g. melamine) which can be hold together in the formulation by a polymeric
3
binder (e.g. polyvinyl acetate-ethylene) (Weil, 2011; Derakhshesh et al., 2012; Mariappan,
2016). When heated beyond a critical temperature, an IFR coating starts swelling and then
expanding to a thick multicellular charred layer to protect the underlying substrate against the
heat generated by the heat source (e.g. fire or flame). Therefore the structural integrity is
maintained (Vandesall, 1971; Alongi, Han and Bourbigot, 2015).
Despite many advantages, IFR coatings (but also some non-intumescent types) are
extremely sensitive to the moisture in the air, and its effectiveness is reduced under long time
exposure to high humidity conditions. This is due to the hygroscopic character of the flame
retardant additives (Truax, 1956; Goldsmith, 2011; Mariappan, 2016). Ammonium
polyphosphate (APP), one of the principal compounds in IFR coatings, tends to hydrolyze to
water-soluble monoammonium phosphate when exposed to relative high humidity. Inorganic
salts from the intumescent coating migrate towards the surface of the polymer matrix, which
deteriorates fire protection performance (Daniliuc et al., 2012; Qu et al., 2012; Deng et al.,
2014). The hydrophilic elements in IFR coating cause non-uniform of the charred structure,
which reduces the thermal stability and interaction of the components in the coating and leads
to the loss of mechanical strength and oxidation resistance of char at high temperature (Lv et
al., 2009; Mariappan, 2016). For this reason, the IFR coatings are not widely used in exterior
wood siding. It is also problematic for some indoor uses (relative humidity < 70%) (Daniliuc
et al., 2012).
In order to overcome these disadvantages, different techniques have been developed, such
as ultrafine processing, surface modification with coupling agents and encapsulation with a
hydrophobic shell material (Wang et al., 2015). The encapsulation of hydrolysis-sensitive
components in IFR coating with water insoluble polymers is an effective method mentioned
in the literature (Lv et al., 2009; Wang et al., 2015). The APP encapsulated with different
polymers (e.g. melamine-formaldehyde resin, urea-formaldehyde resin/polyurethane, etc.)
4
have been widely investigated. However, the evolvement of formaldehyde and 2,4-
disocyanatotoluene during the preparation process and utilization of microcapsules causes
health hazards and environmental problems (Deng et al., 2014).
Recently, it has been observed that APP encapsulated by silicon-containing compounds
applied on synthetic polymers can considerably reduce its degradation caused by moisture.
Furthermore, the combination of phosphorus and silicon based compounds can improve the
performance of fire retardant (Qu et al., 2012; Deng et al., 2014). The synergistic effect
between silicon and IFR can be explained by the following mechanism: phosphorus promotes
char formation, nitrogen releases gases as diluents, and silicon forms a smooth layer that
protects the forming char from oxidation ( Agrawal and Narula, 2014). As demonstrated in
various researches (Lv et al., 2009; Qu et al., 2012; Deng et al., 2014), coating the surface of
APP with an organic-inorganic hybrid polysiloxane not only allows enveloping the FR
additive in a hydrophobic shell, but also provides higher fire retardant properties of IFR
coating.
In this study, APP was successfully encapsulated in an organic-inorganic hybrid sol
prepared initially from a solution of tetraethoxysilane (TEOS) by using methyltriethoxysilane
(MTES) as hydrophobic modifier under alkaline condition. The sol-gel method was chosen to
prepare the APP microcapsules as it has already proved its great potential in preparing hybrid
polysiloxane materials. The sol-gel process involves hydrolysis and condensation of the
silicon alkoxide precursors, which is a simple, economical and ecological technique (Qian et
al., 2014). Silicon-based systems are relatively new FR additives for wood (Lowden and Hull,
2013). In this project, the combination of TEOS/MTES was used to encapsulate APP, which
will be added in intumescent formulation for wood substrate.
5
2. Materials and methods
2.1. Materials
The commercially available APP ((NH4PO3)n, crystalline form II, n 1000, average particle
size 15 µm), was provided by Inortech Chimie Inc. (Quebec, Canada). TEOS (Si(OC2H5)4,
98% purity), MTES (99% purity) and polyoxyethylene-2-oleyl ether surfactant were
purchased from Sigma Aldrich (Germany). Anhydrous ethanol (CH3CH2OH) and ammonium
hydroxide (NH4OH, 28% purity) were purchased from Commercial Alcohols (Ontario,
Canada) and from Anachemia (Montreal, Canada), respectively. The nanopure water was
used from the ultrapure water system (Thermo Scientific Barnstead International D50280,
Iowa, USA). All raw materials were used without the need of further purification.
2.2. Encapsulation of ammonium polyphosphate by the hydrophobic polysiloxane shell
Throughout the entire procedure, the temperature was kept constant at 40 °C and under
ambient atmospheric pressure. 150 mL ethanol and 50 mL nanopure water were poured into a
500 mL erlenmeyer flask. The mixture was first stirred at 700 rpm for 10 min with a
mechanical stirrer. Then, 50 g APP was added and stirred at 1000 rpm for 15 min. After that,
1 g of surfactant and 17 g of ammonia water were added subsequently and stirred for at least
20 min. Then, 10 g TEOS was slowly added dropwise into the mixture by the use of a funnel.
The mixture was stirred for 10 min before adding dropwise 2.5 g of MTES. The resulting
mixture was then equipped with a condenser and stirred at 1000 rpm for 4 hr. After that, the
mixture was cooled to room temperature. The final mixture was filtered using a Büchner
funnel through a 0.1 µm nylon filter (disk diam. 90 mm, Magna,GVS,USA), washed with
water, and dried at 80 °C until weight equilibrium. 51 g of product were obtained and noted as
MAPP-1.
6
2.3. Elimination of the non-encapsulated APP by water
The purpose of this protocol was to remove the APP particles that were not encapsulated in
the MAPP-1 specimen. A preliminary study was conducted to determine the solubility
conditions of APP: 5 g of APP was solubilized in 150 mL water at 75 °C from at least 1 hr.
Therefore, 40 g of the aforementioned MAPP-1 and 1500 mL of ultrapure water were put into
a 2000 mL erlenmeyer flask. The mixture was stirred at 1000 rpm for 2 hr at 75 °C with a
mechanical stirrer to eliminate all of the non-encapsulated APP. After that, the mixture was
cooled to room temperature, filtered by a Büchner funnel through a 0.1 µm nylon filter (disk
diam. 90 mm, Magna, GVS, USA), washed with hot water (around 100 °C), and dried at 80
°C until weight equilibrium. Approximately 10 g of product were obtained and noted as
MAPP-2. A small amount of the MAPP-2 obtained was finely ground using an agate mortar
in order to crack the silica shells for further XPS analysis.
2.4. Preparation of the pure sol-gel without APP
The pure sol-gel was used as the reference specimen to compare with the chemical
composition of MAPP-1 and MAPP-2. The pure sol-gel was prepared like the preparation of
aforementioned MAPP-1 without the addition of APP and surfactant. After cooling to room
temperature, the mixture was dried directly at 80 °C until constant weight without filtering
and washing. 2 g of product were obtained and noted as SG.
2.5. Characterisation methods
2.5.1. Scanning electron miscroscopy (SEM)
The morphology and average particle size of APP particles before and after encapsulation was
studied using a JSML-6360LV (JEOL Corporation, Tokyo, Japan) microscope. The SEM
images were recorded at two different points for each sample with an acceleration voltage of 7
15 kV at different magnifications (1000, 5000 and 10000X). According to the scale bar in the
SEM image at 1000X magnification, three separated particles were selected to be measured.
The average particle size was established from the measurements taken at each point of the
samples. The samples were sputter-coated with a conductive layer (gold palladium) to obtain
a maximum magnification of textural and morphological characteristics. The shell thickness
was determined using the SEM-FIB (focused ion beam) (Quanta 3D FEG, FEI Company,
USA) for MAPP-1 specimen operating at 4 kV. Prior to SEM-FIB investigation, a small
quantity of MAPP-1 powder was directly deposited on the double-sided conductive carbon
tape. Three measurements were taken at 30000X magnification.
2.5.2. Contact angle measurement
Water contact angles (WCA) were measured using a goniometer (First Ten Angstroms USA,
FTA200 series) to compare the hydrophobicity of APP, MAPP-1 and MAPP-2. For this
analysis, the powder samples were first compacted using a pellet press under 30 kN for 20 s
(MTS Alliance RT/50, USA). An auto-syringe generated 3 µL water droplets on the surface
of samples with a volumetric flow of 3 µL/s. Images were captured and analyzed by the Fta32
Video 2.0 software. Image taken at 0.5 s after the water droplet encountered the pellet surface
was recorded to determine WCA of samples. The contact angle measurements were repeated
five times for each sample and the average value of these 5 measurements was then taken as
the final WCA of sample.
2.5.3. Fourier transform-infrared (FTIR) Spectroscopy
The presence of important characteristic absorption bands of APP, MAPP-1, MAPP-2 and SG
was studied by FTIR spectroscopy (Spectrum 400 model from Perkin Elmer, USA). An
attenuated reflection (ATR) crystal diamond accessory was used to record the spectra. The
8
powdered samples were directly placed on the ZnSe crystal. Good sensitivity was achieved
using high pressure contact against diamond (all samples were pressed at the same pressure
controlled by a gauge). Absorption spectra were recorded for a wavelength range from 4000
to 650 cm-1. Thirty-two scans were taken and the resolution was set to 4 cm-1. Three analyses
were realized for each sample.
2.5.4. X-ray photoelectron spectroscopy (XPS)
X-ray photoelectron spectroscopy was used to analyze the chemical composition on the
outermost surface (maximum 10 nm depth) of samples. The XPS spectrometer used is a PHI
5600-ci spectrometer (Physical Electronics, Chanhassen, MN, USA). All data were collected
at a nominal photoelectron angle of 45°. The size of the analytical X-ray spot was 0.5 mm 2.
The C1s peak at 285.0 eV (aliphatic carbon) was used as a reference to correct the binding
energies scale. For APP, MAPP-1, MAPP-2, ground MAPP-2 and SG samples, the survey
spectra at low-resolution recorded using a monochromatic Al X-ray source (1486.6 eV) at
300 W with a charge neutralizer. The XPS analysis of MAPP-2 residue also performed in the
same conditions.
2.5.5. Thermogravimetric and differential thermogravimetric analysis (TG/DTG)
To compare the thermal stability and the decomposition behaviour of APP before and after
encapsulation, the thermogravimetric analysis were performed from 25 to 800 °C in a
thermogravimetric analyzer Mettler Toledo TGA/DTA 851e (UK). The measurements were
carried out at a linear heating rate of 10 °C.min-1 under air, which is similar to the
environment of a real fire exposure (the atmosphere). The weight of APP and MAPP-2
samples were kept within 1.5-3.0 mg in open aluminum crucibles of 70 µL. The temperature
values corresponding to the maximum mass loss rate have an uncertainty of ± 2 °C. Three
9
measurements were performed on each sample. The MAPP-2 residues obtained at the end of
the TGA measurement were kept to perform the XPS analysis.
3. Results and discussions
3.1. Surface morphology
The surface morphology of APP, MAPP-1 and MAPP-2 was presented in Figure 1. The SEM
images showed that APP particles (Figure 1(a)) had irregular cubic shapes with an average
particle size of around 10 ± 2 µm. After encapsulation (Figure 1(b)), the shape and the
dimension of APP did not change significantly. However, the surface of MAPP-1 (Figure
1(e)) appeared to be rougher than the APP (Figure 1(d)) surface due to the layer of
polysiloxane coated on the APP surface. Indeed, an excess of silica-based particles was
noticeable on the surface of the coating whereas the APP surface was very smooth and
uniform. As observed on Figure 1(c) of MAPP-2, a partial coating was removed from the APP
surface. The washing step eliminated the non-encapsulated APP, some excess of silica-based
particles and a part of the silica coating. Therefore, the surface of MAPP-2 (Figure 1(f)) was
slightly smoother than that of MAPP-1. This observation explained the greater weight loss of
MAPP-2 after water treatment.
[Figure 1 here]
The SEM micrograph of MAPP-2 at magnification of 10000X was recorded to
determinate the sol-gel coating thickness. As displayed in Figure 1(g), the flame retardant
particles were covered by a 150 nm thick monolayer film. In addition, the obtained SEM-FIB
image of MAPP-1 (Figure 1(h)) also proved that the shell thickness is around 153 ± 2.4 nm,
which was quite similar to the conventional SEM image of MAPP-2. The results of SEM
analysis indicated that APP surface might be coated by the silica-based compound.
10
3.2. Hydrophobicity of sol-gel coating
The hydrophobicity is an essential property of the coating that directly concerns its humidity
resistance. Figure 2 showed the hydrophilic/hydrophobic properties of the APP, MAPP-1 and
MAPP-2 samples, as assessed by the contact angle measurements. As seen on Figure 2(a), the
non-encapsulated APP has a hydrophilic surface (20 ± 1°) as the water droplet sank into
pellets completely after a short time. On the contrary, after encapsulation, the MAPP-1
(Figure 2(b)) yielded hydrophobicity to APP surface with the WCA increasing up to 96 ± 4°.
After washing, without the presence of non-encapsulated APP, the MAPP-2 specimen (Figure
2(c)) showed a slight increase of WCA (98 ± 5°), even though part of the shell was removed.
The water-droplet dispersed slowly into pellets for both MAPP-1 and MAPP-2 and their
hydrophobic properties maintained over 10 s (hydrophobicity considered as WCA
approximately 90°). This strong hydrophobicity property could be explained by the core-shell
particle growth mechanism which presented in Figure 2(d).
In sol-gel process, the ethyl groups of TEOS precursor were first hydrolyzed and
condensed to form a silica network with large amount of hydroxyl groups on the surface.
Then, the previous spherical silica particles were deposited on the surface of APP (Hussain et
al., 2018). The addition of organic co-precursor MTES into the silica sol offered hydrophobic
property to the coating surface. MTES hydrolysis and co-condensed on the silica particles, the
hydroxyl groups on the silica clusters were replaced by the methyl groups of MTES which
transformed Si-OH to Si-O-Si-CH3 groups (Cai et al., 2014). The reactants further
polymerized and spread all over the initial particle’s surface to form an organic-inorganic
hybrid coating and completed the encapsulation of APP. The hydrophobicity of the sol-gel
coatings was attributed to the presence of methyl groups (–CH3) attached on the silica sol,
which might provide the humidity resistance to the APP flame retardancy.
11
[Figure 2 here]
3.3. Chemical composition by FTIR Spectroscopy and XPS
Figure 3 presented FTIR spectra obtained for APP, MAPP-2 and SG samples. The
assignments of the peaks were reported in Table 1. Figure 3(a) showed the presence of
important absorption peaks of APP, such as the wide band in the range 3200-3000 cm -1
(elongation vibration of O-H bond), 1433 cm-1 (stretching vibration of N-H bonds), 1055 and
1013 cm-1 (stretching vibration of PO2 and PO3) and 882 cm-1 (P-O asymmetric vibration). The
strongest and sharp peaks at 1245 cm-1 (stretching vibration of P=O) and 798 cm-1 (P-O
symmetric vibration) characterized the polyphosphate chain. The infrared spectra of MAPP
before washing (data not shown) was similar to that of APP as there was remaining non-
encapsulated APP left on the sample surface, except for the substantial increased of the peak
at 1055 cm-1, which could be explained by the presence of Si-O-Si asymmetric stretching
vibration belong to the silica shell.
[Figure 3 here]
After washing, the spectrum of MAPP-2 (Figure 3(b)) showed new absorption peak at 893
cm-1 corresponding to the stretching vibration of O-H in Si-OH groups. The obvious increased
in intensity of the absorption peak around 1055 cm-1 was related to the existence of Si-O-Si
groups, indicating the presence of polysiloxane. On the other hand, the typical absorption
peaks of APP such as NH4+, P=O, PO3 and P-O-P disappeared. These observations suggested
that APP was encapsulated into the silica-based shell.
Both infrared spectra of MAPP-2 and SG exhibited the characteristic peaks of
polysiloxane including stretching absorption peak of Si-O-Si in the 1040-1055 cm -1 region
and stretching vibration of Si-CH3 near 790 cm-1. A shift of Si-CH3 peak from 778 cm-1 in
12
hybrid silica sample to 798 cm-1 in MAPP-2 was observed. This shift to higher wavenumbers
may be caused by an increase in chain length for polymers, which suggested that TEOS and
MTES have been successfully bonded onto the surface of APP (Belva et al., 2006). As
illustrated in Figure 3(c), the intensity of Si-CH3 and Si-O-Si stretching vibration peaks
increased due to the high amount of polysiloxane in the pure sol-gel. The absorption band at
2985 cm-1 of C-H in methyl groups of polysiloxane increased slightly. An additional
absorption signal appeared at 1276 cm-1, corresponding to the stretching vibrations of Si-CH3.
These observations combined with the disappearance of O-H stretching vibration at 893 cm-1
confirmed that the –Si-CH3 groups replaced the hydroxyl groups on the silica clusters through
oxygen bonds. The previous remarks illustrated the reduction of polysiloxane amount in
MAPP-2, which might be caused by the water washing. These comments demonstrated that
the polysiloxane shell covered the whole APP surface, despite the rinsing step.
[Table 1 here]
In addition to the FTIR results, XPS analyses were performed to determine the surface
chemical composition. Figure 4 presented XPS survey spectra of the APP, MAPP-1, MAPP-2
and ground MAPP-2 samples. The surface elemental compositions of these samples were also
listed in Table 2. The outer surface concentration changed proportionally to the peak intensity.
The spectrum of pure APP (Figure 4(a)) presents signals of oxygen (at 530 eV), carbon (at
285 eV), nitrogen (at 399 eV), and phosphorus atoms (132 eV for P2p and 193 eV for P2s).
After encapsulation (Figure 4(b)), APP was covered by the polysiloxane shell but there
were the remnant non-encapsulated APP on the MAPP-1 surface. Therefore, the N and P
atoms contents dropped greatly from 19.7 to 1.6% and from 14.7 to 2.0%, respectively.
Meanwhile, the surface of oxygen content was 44.6%, which was lower than that of APP
(54.4%). On the other hand, the intensity of C1s peak increased sharply (from 11.2 to 32.5%).
Especially, the new peak attributed to silicon component of polysiloxane shell appeared at
13
105.2 eV. The presence of O-CH3 bonds in polysiloxane increased the atomic percentage of
carbon in the MAPP-1 specimen. These observations confirmed that APP was encapsulated
into silicon-based particles.
[Figure 4 here]
For the MAPP-2 specimen (Figure 4(c)), the nitrogen and phosphorus signals disappeared
while the silicon and carbon peak intensities increased slightly. The washing step completely
eliminated the non-encapsulated APP. As demonstrated in the SEM analysis, the shell
thickness is around 150 nm, which was much higher than the XPS penetration depth
(approximately 3-10 nm), which made it impossible to identify the nitrogen and phosphorus
contents of APP inside the polysiloxane shell.
For the ground MAPP-2 specimen (Figure 4(d)), the relative concentration of oxygen and
carbon was almost similar than MAPP-2 sample. However, a small amount of nitrogen (0.7%)
and phosphorus (0.7%) appeared again on the sample surface and the silicon content
decreased from 20.7 to 16.6%. These observations suggested that APP was entirely
encapsulated by the silica-based particles, then the grinding step ruptured the thick shell and
APP were expelled to the sample surface. Combining the results of FTIR and XPS analyses, it
confirmed that APP is encapsulated inside the polysiloxane membrane.
[Table 2 here]
3.4. Thermogravimetric behaviour by TG/DTG
Thermograms presented in Figure 5 were used to evaluate the thermal behaviour of APP,
MAPP-1 and MAPP-2 specimens. The thermal degradation of pure APP consisted in two
successive steps. The first one started at 305 °C with a slight mass loss of about 4.53%. The
mass loss in this step was due to the release of small molecules such as H 2O and NH3, which
then formed a highly crosslinked polyphosphoric acid layer (Gu et al., 2007; Duquesne et al.,
14
2013). Then the second step occurred at temperatures ranging from 500 to 700 °C with the
maximum weight loss rate at 557 °C. The mass loss of this decomposition step was up to
52.80%. This is due to the release of phosphoric acid, polyphosphoric acid (PPA),
metaphosphoric acid and polymetaphosphoric acid coming from the APP decomposition (Gu
et al., 2007; Sun, Qu and Li, 2013). In this stage, PPA acid could evaporate and/or dehydrate
to phosphorus oxides (P4O10) that sublime (Duquesne et al., 2013). At 800 °C, the thermal
degradation of APP left behind a low amount of residue (3.06%).
The thermal behaviour of MAPP-1 was quite similar to APP. MAPP-1 degradation also
had two main steps, which the maximum mass loss rate occurred at 295 and 525 °C, and the
weight loss of each degradation step is 6.45 and 44.06%, respectively. As seen in the
thermograms, the degradation of MAPP-1 was observed at lower temperature than that of
pure APP. This might be explained by the reaction between APP and polysiloxane shell. The
silanol coming from the degradation of polysiloxane accelerated the thermal depolymerisation
of APP as acid catalyzer. However, after 540 °C, MAPP-1 was thermally more stable than
APP and promoted higher residue up to 29.94%.
[Figure 5 here]
Compared to APP and MAPP-1, the thermal degradation of MAPP-2 consisted of three
consecutive stages. The first step took place sooner at the temperature range between 130 and
160 °C. The decrease of MAPP-2 initial decomposition temperature was attributed to
endothermic reaction due to the removing of solvent (ethanol) and water molecules trapped in
the specimen (Innocenzi, Abdirashid and Guglielmi, 1994; Yu et al., 2003). Therefore, only
1.97% maximum mass loss occurred at around 150 °C. This first step was not essential for
sample characterisation as it was affected by the moisture during sample preparation and
measurement. The second step is the main degradation stage, which occurred at 230 °C and
reached the maximum mass loss rate at 280 °C (7.60%). In this stage, the mass loss was
15
mainly due to the elimination of silanol groups from the powder surface. The remaining of
ethanol and water could be continuously eliminated from the sample due to the condensation
reaction. The sharp exothermic peaks noticed could be attributed to the combustion of –OCH3
groups until 380 °C. Beyond 400 °C, the degradation rate slowed down greatly and was
mostly constant until 570 °C. Then, above 600 °C, the material degraded slightly with a broad
peak of maximum decomposition rate at 610 °C due to the oxidation of –CH3 groups. The
maximum mass loss of this step was 21.50%. The encapsulated APP yielded a high char
residue at 800 °C (70.49%) showing an excellent thermal stability of APP combined with
polysiloxane (in TGA condition). Like MAPP-1, MAPP-2 also started to degrade earlier than
APP. However, above 380 °C, the thermal behaviour of MAPP-2 was clearly more stable in
comparison with APP and MAPP-1 and possessed a lower mass loss rate. Moreover, MAPP-2
had a significant effect on char formation with 70.49 wt% residue left at 800 °C,
demonstrating that MAPP-2 had the best heat resistance especially at high temperature in air
atmosphere. Polysiloxane’s decomposer such as silanol could react with the polyphosphoric
acids formed from APP degradation and increased the char formation by carbonization
process. Then the crosslinking reaction occurred and led to the formation of a three
dimensional network. During this process, silanols might also form a compact silica-based
shell which improved the thermal stability of char (Deng et al., 2014). From these results, the
encapsulated APP led to a better thermally stable compound than pure APP.
[Table 3 here]
It was found that the most significant difference between APP before and after
encapsulation was their residual weight at the end of the TGA experiments. As described in
the literature (Yu et al., 2003; Qian et al., 2014), the char layer could slow down heat and
mass transfer between the gases and condensed phases, hence, an effective protection of the
char layer enable to improve the flame retardant performance during combustion. Therefore,
16
the residue corresponding to the char played an important role in the TGA analysis. The
increased residue amounts of encapsulated APP could be explained by the synergistic effect
between silicon with phosphorus and nitrogen of APP. The silicon compounds migrated to the
char surface and enhanced the char layer during the thermal degradation. Besides, visual
observation of the residues in the crucible at the end of the TGA measurement showed white
color for APP (Figure 6(a)), whereas a grey residue was obtained for MAPP-2 (Figure 6(b)).
The white residue could be explained by the formation of inorganic compounds as APP
sample is carbon-free. Meanwhile, it was demonstrated that the thermal degradation of
polysiloxane in air forms white silica powder and black silicon-oxycarbide (Belva et al.,
2006). Therefore, the grey residue of MAPP-2 could be attributed to the presence of carbon
from CH3 of organic precursor MTES on sample. It could also suggest that the interaction
between APP and polysiloxane shell occurred and their decomposition products promoted the
formation of char (Deng et al., 2014). The presence of carbon in MAPP-2 residue was
confirmed by XPS survey analysis (Figure 6(c)). As expected from the MAPP-2 residue
structure, the silicon (10.40%) and carbon (1.17%) attributed to polysiloxane degradation
were detected. The appearance of phosphorus (12.19%) and nitrogen (0.42%) belonged to the
APP core detected after the themal decomposition of polysiloxane shell. These results
indicated that coating APP surface by organic-inorganic hybrid silica-based compound
enhanced the char yields, which offer a better protection of the substrate from the thermal
degradation.
[Figure 6 here]
17
4. Conclusion
This paper successfully demonstrated the encapsulation of APP by silica-based compounds
via sol-gel process. The SEM, FTIR and XPS results clearly confirmed that the APP particles
were encapsulated inside the polysiloxane shells. Introducing organic silica precursor bearing
hydrophobic groups (MTES) into the pure silica particles (TEOS) offered the hydrophobic
property to APP surface. Indeed, the WCA of APP increased significantly from 20° to 98°.
Owing to the synergistic effect between APP and silicon-based compound, the encapsulated
APP had a better thermal behaviour and a high yield of char residue (70.49 wt%).
Acknowledgments
The authors are grateful to Natural Sciences and Engineering Research Council of Canada for
the financial support through its ICP and CRD programs (IRCPJ 461745-12 and RDCPJ
445200-12) as well as the industrial partners of the NSERC industrial chair on eco-
responsible wood construction (CIRCERB). The authors would like to thank Mr. Yves
Bedard for his technical assistance and helpful discussions during this study.
Declaration of interest statement
The authors declared no potential conflict of interest.
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Table 1. Main FTIR peaks attribution for APP, MAPP-2 and SG (n = 3) (Yu et al., 2003; Cai
et al., 2014; Deng et al., 2014; Bellayer et al., 2016).
Sample Wavenumber
(cm-1)
Assignment Intensity*
APP 3200 – 3000 valence vibration of O-H in P-OH m (broad)
1433 stretching vibration of N-H in NH4+ m (sharp)
1245 stretching vibration of P=O s (sharp)
1055 – 1013 stretching vibration of PO2 and PO3 s (sharp)
882 asymmetric stretching vibration of P-O in P-O-P s (sharp)
798 symmetric stretching vibration of P-O in P-O-P s (sharp)
MAPP-2 2950 stretching vibration of C-H in CH3 w (sharp)
1050 stretching vibration of Si-O-Si s (sharp)
893 stretching vibration of O-H in Si-OH w (sharp)
798 stretching vibration of Si-CH3 s (sharp)
SG 2985 stretching vibration of C-H in CH3 w (sharp)
1276 stretching vibration of CH3 in Si-CH3 w (sharp)
1040 stretching vibration of Si-O-Si s (sharp)
778 stretching vibration of Si-CH3 s (sharp)
* s: strong; m: medium; w: weak.
.
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Table 2. Relative amount of atoms at surfaces of APP, MAPP-1, MAPP-2 and ground
MAPP-2 samples determined by low-resolution XPS scan (n = 1).
SampleAtomic %
0 C N P Si
APP 54.4 11.2 19.7 14.7 0.0
MAPP-1 44.6 32.5 1.6 2.0 19.4
MAPP-2 41.2 38.0 0.0 0.0 20.7
Ground MAPP-2 43.6 38.4 0.7 0.7 16.6
Table 3. Data for thermal degradation steps (temperature of maximum mass loss rate,
maximum mass loss quantity and residue quantity) of APP, MAPP-1 and MAPP-2 from 25 to
800 °C (n = 3).
Sample The first step The second step The third step Residue at
800°C (%)Tmax1
(°C)
ML1
(%)
Tmax2
(°C)
ML2
(%)
Tmax3
(°C)
ML3
(%)
APP 305 4.53 557 52.80 - - 3.06
MAPP-1 295 6.45 525 44.06 - - 29.94
MAPP-2 150 1.97 280 7.60 610 21.50 70.49
Tmax: Temperature of maximum mass loss rate; ML: Maximum mass loss corresponding to
Tmax.
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Figure 1. SEM images for morphology (n = 2) and particle size (n = 6) of (a and d) APP, (b
and e) MAPP-1 and (c and f) MAPP-2 at magnification of 1000X and 5000X and shell
thickness of microcapsules obtained from (g) SEM for MAPP-2 (n = 1) and (h) SEM-FIB for
MAPP-1 (n = 3).
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Figure 2. Images of water contact angle analysis for (a) APP, (b) MAPP-1 and (c) MAPP-2
(mean ± standard error) (n = 5) and (d) schematic illustration of reaction steps involved in
APP encapsulation by sol-gel process using TEOS and MTES as precursors in alkaline
condition.
25
Figure 3. FTIR spectra for (a) APP, (b) MAPP-2 and (c) SG (n = 3).
26
Figure 4. XPS survey scan spectra for (a) APP, (b) MAPP-1, (c) MAPP-2 and (d) ground
MAPP-2 (n = 1).
27
Figure 5. (a) TGA and (b) DTG curves for APP, MAPP-1 and MAPP-2 under air from 25 to
800 °C (n = 3).
APP MAPP-1 MAPP-2
28
Figure 6. Images of residue at the end of thermogravimetric analysis for (a) APP and (b)
MAPP-2 and (c) XPS survey scan spectra of MAPP-2 residue at the end of TGA analysis (n =
1).
29
Figure captions
Figure 1. SEM images for morphology (n = 2), particle size (n = 6) of (a and d) APP, (b and
e) MAPP-1 and (c and f) MAPP-2 at magnification of 1000X and 5000X and shell thickness
of microcapsules obtained from (g) SEM for MAPP-2 (n = 1) and (h) SEM-FIB for MAPP-1
(n = 3).
Figure 2. Images of water contact angle analysis for (a) APP, (b) MAPP-1 and (c) MAPP-2
(mean ± standard error) (n = 5) and (d) schematic illustration of reaction steps involved in
APP encapsulation by sol-gel process using TEOS and MTES as precursors in alkaline
condition.
Figure 3. FTIR spectra for (a) APP, (b) MAPP-2 and (c) SG (n = 3).
Figure 4. XPS survey scan spectra for (a) APP, (b) MAPP-1, (c) MAPP-2 and (d) ground
MAPP-2 (n = 1).
Figure 5. (a) TGA and (b) DTG curves for APP, MAPP-1 and MAPP-2 under air from 25 to
800 °C (n = 3).
APP MAPP-1 MAPP-2
Figure 6. Images of residue at the end of thermogravimetric analysis for (a) APP and (b)
MAPP-2 and (c) XPS survey scan spectra of MAPP-2 residue at the end of TGA analysis (n =
1).
30