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: thi-doan-trang.hoang.1@ulaval.ca (T.D.T. Hoang).

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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

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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

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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.)

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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.

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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.

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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

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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

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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.

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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.

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[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

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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

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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.,

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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

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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,

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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]

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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.

23

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