Development of halogen-free flame retardant phosphazene and rice husk ash incorporated benzoxazine blended epoxy composites for microelectronic applications

Article abstract of DOI:10.1039/c5nj00364d

The present study is focused on the synthesis and characterization of flame retardant amine-terminated cyclophosphazene and silane functionalized rice husk ash reinforced benzoxazine blended epoxy composites as a halogen-free flame retardant material (ATC

Full text of DOI:10.1039/c5nj00364d

NJC
View Article Online
View Journal
PAPER
Development of halogen-free flame retardant
phosphazene and rice husk ash incorporated
benzoxazine blended epoxy composites for
microelectronic applications
Cite this:DOI: 10.1039/c5nj00364d
Krishnamoorthy Krishnadevi and Vaithilingam Selvaraj*
The present study is focused on the synthesis and characterization of flame retardant amine-terminated
cyclophosphazene and silane functionalized rice husk ash reinforced benzoxazine blended epoxy
composites as
a halogen-free flame retardant material (ATCP/FRHA/Bz-Ep). FT-IR spectroscopy,
scanning electron microscopy (SEM), X-ray diffraction analysis, contact angle measurements, dielectric
constant, DSC, TGA, UL-94, LOI and cone calorimetry were used to characterize the surface
morphology as well as the structural, electrical, thermal and flame retardant properties of the resultant
ATCP/FRHA/Bz-Ep composite material. The experimental results suggested that ATCP/FRHA/Bz-Ep
composites exhibit better flame retardant and dielectric performance compared to that of a neat Bz–Ep
material. A plausible mechanism of the fire retardant ATCP/FRHA/Bz-Ep composite material is hypothesized
based on the results of cone calorimetric, thermal and electrical analysis. From the abovementioned results,
it was concluded that the ATCP/FRHA/Bz-Ep composite can be used as an electrical resistant material for
electronic and microelectronic applications.
Received (in Montpellier, France)
10th February 2015,
Accepted 17th June 2015
DOI: 10.1039/c5nj00364d
www.rsc.org/njc
as heavy smoke during the combustion process. Though the
halogen containing flame retardants attract great attention,
Introduction
Polymer composites can be used in various high performance they still face significant challenges in both academic and
applications due to their light-weight, which is as an essential industrial applications. However, brominated flame retardants
and desirable property required for sustainable flame retardant in particular are restricted in many countries due to the migra-
polymers. Flame retardants are one of the most important tion of toxic by-products such us hydrogen bromide and dioxins7 to
additives for combustible polymers to improve their fire resistant the environment during combustion process.
properties.1–3 Flame retardant materials are used to reduce
The abovementioned drawbacks tempt researchers to develop
combustibility and toxic fume production of the polymers, innovative and eco-friendly halogen-free polymeric materials for
which are pivotal requirements for the development and appli- electronic, electrical, aerospace and transportation applications.
cation of new materials.
Amongst all the halogen-free flame retardants, scientists are
Epoxy resins are high performance thermosetting resins posses- introducing nitrogen-, phosphorous- and silica-based intumes-
sing many attractive properties such as high mechanical strength, cent flame retardants as additives for achieving flame retardant
good adhesion,4 good electrical insulation properties,5 low cost and performance.
ease of formation.6 However, epoxy resins are easily flammable and
Due to prominent processability, the excellent thermal and
they are unable to satisfy some high performance flame retardant mechanical properties of epoxy-modified benzoxazine blended
applications. In general, the flame retardant property of epoxy systems have attracted increasing attention in a wide range of
resins can be improved by adding flame retardant additives or by applications, including adhesives, filler-reinforced composite
incorporating reactive flame retardants.
materials, and coatings for electronic circuits.8–11 However, the
The most effective flame retardants are halogen containing absence of inorganic materials in the benzoxazine blended
chemicals, but they usually release harmful toxic gases as well epoxy systems means they have low flame retardant capacities
that are inadequate for the fire resistance properties of the
materials. Therefore, effective and eco-friendly inorganic flame
retardant materials should be incorporated in the Bz–Ep com-
Nanotech Research Lab, Department of Chemistry, University College of Engineering,
Villupuram (A Constituent College of Anna University, Chennai), Kakuppam,
posites in order to enhance their flame resistance properties.
Rice husk ash is a biomass waste material, and it contains a
Villupuram-605 103, Tamil Nadu, India. E-mail: rajselva_77@yahoo.co.in,
vselva@aucev.edu.in; Fax: +91-4146-224500
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015
New J. Chem.

View Article Online
Paper
NJC
rich source of silica. In general, the utilization of biomass rice (LY556) (DGEBA) was purchased from Ciba-Geigy Ltd, India.
husk ash in the composites has brought several advantages 4-Acetamido phenol, bisphenol-A, triethylamine, potassium carbo-
such as low density, greater deformability, less abrasiveness to nate, aniline, sodium hydroxide, paraformaldehyde and other
equipment, biodegradability and low cost. Hence, the present solvents (AR grade) were purchased from SRL Chemicals, India.
study attempts to obtain low k values, improved thermal and
high performance flame retardant properties and improved
hydrophobicity using eco-friendly bio-waste materials like rice
husk ash.
Synthesis of hexa(acetamidophenyl)cyclotriphosphazene
(CPAC)
A mixture of 4-acetamido phenol (15.65 g, 0.1035 mol) and
K2CO3 (21.09 g, 0.15824 mol) in dry acetone (200 ml) was
agitated at room temperature for 30 minutes. To this mixture,
hexachlorocyclotriphosphazene (CP) (5 g, 0.0143 mol) was
added and refluxed at 60 1C for 4 days. The resulting product
was then cooled to room temperature and filtered. The crude
product (CPAC) was further purified using hexane and dried
in a vacuum oven at 50 1C for 2 h. The yield of the product
and melting point were found to be 62.95% and 252–255 1C,
respectively.
Phosphazenes are a well-known class of versatile functional
materials with an inorganic backbone structure. When phos-
phazenes are appropriately substituted, the resultant materials
can be employed as flame resistant materials, elastomers,
membranes, solid ionic conductors and inert biomaterials.12
Such compounds containing both phosphorous and nitrogen
can display enhanced flame retardancy when compared to
similar compounds containing phosphorous alone. In addition,
these materials yield relatively minor amounts of toxic combustion
products in fire situations. Therefore, the properties of organic
polymers can be modified to significantly improve their fire
resistance, ionic conductivity and biological compatibility by
incorporating small amounts of specially modified eco-friendly
phosphazene.13–15 Hence, this makes phosphazenes particu-
larly good candidates for the development of fire resistant eco-
friendly materials in electrical and electronic applications.
With this in mind, the present study is focused on the
synthesis and characterization of ATCP/FRHA/Bz-Ep compo-
sites for flame retardant applications. The improved thermal
properties of ATCP/FRHA/Bz-Ep composites were confirmed
by TGA and DSC techniques. The flame retardant properties
of synthesized composites were studied by UL-94 test, LOI and
cone calorimetry measurements. Furthermore, the dielectric
and hydrophobic nature of the composites are determined by
BDS and contact angle measurements, respectively. From these
results, it was concluded that the presence of N, P and silica
(three in one) material can promote thermal, flame retardant,
dielectric (low k) and hydrophobic properties.
Synthesis of amine-terminated cyclophosphazene (ATCP)
A
mixture of hexa(acetamidophenyl)cyclotriphosphazene
(CPAC) (9 g), methanol (180 ml) and sulfuric acid (108 ml)
was refluxed for 4 h at 80 1C and then cooled to room
temperature followed by the dropwise addition of ammonia
in an ice bath until the pH reached 8. The resultant grey solid
product (Fig. 1) was filtered, washed with excess water and
dried in vacuum at 50 1C for 48 h. The yield of the product
and melting point were found to be 90% and 172–174 1C,
respectively.
Preparation of rice husk ash
Rice husk was collected from a local rice mill in Tamil Nadu,
India, and washed several times with de-ionized water. It was
dried in sunlight for 7 days. The dried rice husk was burned in
open air. After burning, the residue was crushed to obtain rice
husk ash (RHA). The crushed rice husk ash (40 g) was
immersed in 1 M HCl (1 liter) solutions for 4 h and then
washed repeatedly with de-ionized water and dried in an oven
at 110 1C for 2 days. Rice husk ash was calcined (in a ceramic
Methods and materials
Hexachlorocyclotriphosphazene (CP) and 3-aminopropyltri- crucible) at 350 1C for 3 h and 600 1C for 4 h in a muffle furnace.
methoxysilane (3-APTMS) (99%) were purchased from Sigma The resultant rice husk ash was white in color and was stored
Aldrich, India. Diglycidyl ether of bisphenol-A epoxy resin for further use.
Fig. 1 Synthesis of amine-terminated cyclotriphosphazene.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015

View Article Online
NJC
Paper
Fig. 2 Synthesis of functionalized rice husk ash.
Functionalization of RHA
heating rate of 10 1C minÀ1. Differential scanning calorimetric
(DSC) analysis was performed on a Netzsch DSC-200. The
samples (each about 10 mg) were heated from 25 1C to 500 1C
and the thermograms were recorded at a heating rate of 10 1C
minÀ1. The dielectric constant and dielectric loss of the samples
were measured using a broad band dielectric spectrometer
(BDS-NOVOCONTROL Technologies, Germany) at 35 1C in the
frequency range from 1 Hz to 2 MHz. Contact angle measure-
ments were carried out by a goniometer using the sessile drop
method. Before measuring the contact angle, the samples
should be dried at 60 1C in a vacuum oven for 24 h. The vertical
burning tests were carried out using a UL-94 vertical flame
chamber instrument with a sample dimension of 130 Â 13 Â
3 mm according to ASTM D3801. The limiting oxygen index (LOI)
was analyzed with a sample dimension of 150 Â 37.5 Â 12 mm
using an Atlas HVUL2 according to ASTM D2863. The cone
calorimetry tests were conducted following the ISO 5660 proce-
dures on a cone calorimeter (Fire Testing Technology Ltd).
Specimens [dimensions: 100 Â 100 Â 2.9 (Æ0.1) mm3] were
irradiated with a heat flow of 50 kW mÀ2. From this analysis, the
time to ignition (TTI), heat release rate (HRR), total heat release
(THR), total smoke production (TSP), char residues, and CO/CO2
emission rate ratio can be acquired.
RHA silica is functionalized with 3-aminopropyltrimethoxysilane
(3-APTMS) according to the reported method.16 50 g of rice husk
ash was mixed with 400 ml of dried toluene. The mixture was
refluxed under a nitrogen atmosphere for 2 h and then 20 ml of
3-aminopropyltrimethoxysilane (3-APTMS) was slowly added.
Finally, the resultant product was then filtered, washed with
ethanol and dichloromethane several times and dried at 25 1C
for 24 h. The resultant functionalized rice husk ash (FRHA) was
stored for further use (Fig. 2).
Synthesis of flame retardant composites
According to our previous report, 15 wt% of CPA (hexa(amino-
phenyl)cyclotriphosphazene) is more effective than the other
weight percentage ratios used.17,18 Hence, DGEBA epoxy resin
and benzoxazine (Bz)19 were mixed with 15 wt% of amine-
terminated cyclotriphosphazene and various weight percen-
tages (1, 3 and 5%) of functionalized rice husk ash (Table 1).
The resultant reaction mixture was agitated for 5 h using a
mechanical stirrer. The amine-terminated cyclophosphazene,
silane functionalized rice husk ash, benzoxazine and epoxy
material (ATCP/FRHA/Bz-Ep) composites (Fig. 3) were thoroughly
mixed and then cured using preheated moulds at 120 1C for 1 h,
180 1C for 1 h and 250 1C for 2 h in order to get ATCP/FRHA/Bz-Ep
composites.
Results and discussions
Characterization
Structural characterization
The two broad bands observed at 3438 and 3338 cmÀ1 corre-
spond to –NH2 stretching vibrations, in fact they also ascertain
the presence of the amine group in the ATCP material (Fig. 4).
The peaks observed at 3043 and 2925 cmÀ1 correspond to ring
C–H stretching vibrations. The aromatic C–C stretching vibra-
tions were noticed at 1507 and 1621 cmÀ1. The characteristic
stretching vibrations of P–O–Ph, P–N–P and P–O–C were seen
at 1256, 1178 and 952 cmÀ1, respectively. The aromatic C–H
FT-IR spectra were obtained on a Perkin Elmer 6Â spectrometer.
The surface morphologies of the samples were examined using a
scanning electron microscope (SEM; JEOL JSM Model 6360). X-ray
diffraction patterns were acquired at 25 1C using a Rich Seifert,
Model-3000 X-ray powder diffractometer (monitoring the diffrac-
tion angle at 2y from 101 to 701). Thermogravimetric analysis
(TGA) was performed on a Netzsch STA 409 thermogravimetric
analyzer under a continuous flow of nitrogen (20 ml minÀ1) at a
bending vibrations appeared at 832 and 695 cmÀ1
.
In the 13C NMR spectrum (Fig. 5a), the presence of aromatic
carbon was confirmed by the peaks observed at 111.5 and
120.3 ppm. From the 1H NMR spectrum (Fig. 5b), the presence
of aromatic protons in ATCP was confirmed by the appearance
of the peaks at 6.52 and 7.2 ppm. The presence of NH2 protons
was confirmed by the peak at 2.3 ppm. The appearance of the
peak at 30.1 confirms the presence of carbon that was attached
to a NH2 group in the ATCP material. A single peak appearing at
Table 1 Preparation of amine-terminated cyclophosphazene and function-
alized rice husk ash based flame retardant (ATCP/FRHA/Ep-Bz) composites
Sample code
ATCP (g) FRHA (g) Bz (g) Ep (g) THF (ml)
Bz–Ep
ATCP/Bz-Ep
ATCP/FRHA(1%)/Bz-Ep 15
ATCP/FRHA(3%)/Bz-Ep 15
ATCP/FRHA(5%)/Bz-Ep 15
—
—
1
3
5
50
50
20
20
20
20
20
15
42.5 42.5
42
41
40
42
41
40
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015
New J. Chem.

View Article Online
Paper
NJC
Fig. 3 Proposed schematic of amine-terminated cyclophosphazene and functionalized rice husk ash reinforced benzoxazine blend epoxy composites.
corresponds to OH stretching vibrations. The stretching bands
observed at 2961 and 2865 cmÀ1 correspond to C–H stretching
vibrations. The peak observed at 1041 cmÀ1 confirms the presence
of C–O stretching vibrations. The aromatic C–C stretching vibra-
tions were observed at 1514 and 1613 cmÀ1. The characteristic
stretching vibrations of P–O–Ph, P–N–P and P–O–C were observed
at 1257 cmÀ1, 1186 cmÀ1 and 946 cmÀ1, respectively. The peak
at 827 cmÀ1 confirms the presence of aromatic C–H bending
vibrations. The Si–O–Si peak was observed at 1000–1130 cmÀ1. The
abovementioned results confirm the complete curing of ATCP/
FRHA/Bz-Ep composites.
Surface morphology
From the SEM image of the Bz–Ep composite [Fig. 8(a)], it was
noticed that the synthesized composite material has a homo-
Fig. 4 FT-IR spectrum of amine-terminated cyclophosphazene (ATCP).
geneous glassy microstructure.
Furthermore, the ATCP/Bz-Ep/FRHA composite [Fig. 8(b–e)]
shows a platelet-like surface morphology and exhibits the
10.1 ppm in the 31P NMR spectrum (Fig. 5c) confirms the
non-phase separated homogeneous morphology owing to cross-
linking between the benzoxazine-based epoxy resin with amine-
terminated cyclophosphazene and the functionalized rice husk
ash materials.
presence of phosphorous in the ATCP material.
Fig. 6 shows the FT-IR spectra of rice husk ash and function-
alized rice husk ash. The peak observed at 3473 cmÀ1 indicates the
presence of both hydrogen bonded –OH and Si–OH groups. The
Si–OH stretching vibration appears due to both the silanol group
and adsorbed water on the surface.20 Similarly, the peak observed
X-ray diffraction analysis
at 2937 cmÀ1 indicates the presence of –CH2 stretching vibration. The X-ray diffraction studies were used to ascertain the struc-
The peaks appearing at 1095 and 800–798 cmÀ1 confirm the tural modification of Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/
presence of the Si–O–Si bond. Thus, these results show that the Bz-Ep, ATCP/FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep com-
aminopropyl groups have become an integral part of RHA and are posites. The resultant XRD patterns of synthesized composites
covalently bonded with the RHA material.
are shown in Fig. 9. The cured composites have almost similar
The FT-IR spectra of Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/ prominent peaks at an angle of about 17.951, which is a
Bz-Ep, ATCP/FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep characteristic of amorphous21,22 composites corresponding to
composites are shown in Fig. 7. The peak observed at 3433 cmÀ1 the interaction at a d spacing of 4.9 Å calculated from the Bragg
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015

View Article Online
NJC
Paper
Fig. 5 (a) 13C NMR, (b) 1H NMR and (c) 31P NMR spectra of synthesized ATCP.
Fig. 6 FT-IR spectra of rice husk ash and functionalized rice husk ash materials.
Fig. 7 FT-IR spectra of Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/Ep-Bz, ATCP/
FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep flame retardant composites.
equation (l = 2d sin y). Hence, the XRD studies confirm the
complete incorporation of flame retardant additive-like ATCP
and FRHA materials in the Bz–Ep blends.
were determined through contact angle measurements using
a goniometer. The surface contact angles were measured with
5 ml of water as the probe liquid and the results are presented
in Fig. 10. The values of the contact angles of Bz–Ep, ATCP/
Contact angle measurements
The wettabilities of Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/Bz-Ep, Ep-Ep, ATCP/FRHA(1%)/Bz-Ep, ATCP/FRHA(3%)/Bz-Ep and
ATCP/FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep composites ATCP/FRHA(5%)/Bz-Ep composites are 681, 821, 861, 981
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015
New J. Chem.

View Article Online
Paper
NJC
Fig. 10 Contact angle measurements of Bz–Ep, ATCP/Bz-Ep, ATCP/
FRHA(1%)/Bz-Ep, ATCP/FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep
flame retardant composites.
Fig. 8 Fracture morphology of (a) Bz–Ep, (b) ATCP/Bz-Ep, (c) ATCP/
FRHA(1%)/Bz-Ep, (d) ATCP/FRHA(3%)/Bz-Ep and (e) ATCP/FRHA(5%)/
Bz-Ep composites.
Fig. 11 Dielectric constant of Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/
Bz-Ep, ATCP/FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep composites.
given in Table 2. From Fig. 11, it was noticed that the ATCP/
FRHA(5%)/Bz-Ep hybrid has a lower value of dielectric constant
(B1.62) than that of the Bz–Ep (B4.63) composite.
Fig. 9 XRD analysis of Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/Bz-Ep, ATCP/
FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep flame retardant composites.
Furthermore, the polybenzoxazine network structure con-
structed via multiple cross-links and subsequent hydrogen
bonding (–OHÁ Á ÁO inter- and –OHÁ Á ÁN intramolecular hydrogen
and 1061, respectively. From the results, it was noticed that
the hydrophobic nature can be increased with the addition of
phosphazene and various percentages of functionalized rice
husk ash into the Bz–Ep composite. This is due to the less
polar nature of the Si–O–Si linkages in the network system,
which increases the hydrophobic nature of the resulting
hybrid systems.
Table 2 The dielectric constant, dielectric loss, charge transfer resistance
and contact angle values of flame retardant composites
Charge
Dielectric Dielectric transfer
Contact
Sample name
constant loss resistance (O) angle value
Dielectric properties
Bz–Ep
ATCP/Bz-Ep
ATCP/FRHA(1%)/Bz-Ep 3.32
ATCP/FRHA(3%)/Bz-Ep 2.65
4.63
3.64
0.215
5.27 Â 109
5.95 Â 109
7.11 Â 109
8.52 Â 109
9.55 Â 109
681
821
861
981
1061
0.194
0.173
0.135
0.091
The dielectric constants with respect to the applied AC frequen-
cies ranging from 1 MHz to 20 MHz are presented in Fig. 11.
The obtained dielectric constant and dielectric loss values are ATCP/FRHA(5%)/Bz-Ep 1.62
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015

View Article Online
NJC
Paper
bonds) may be the reason for the observed dielectric
features.23–25 This inter- and intramolecular hydrogen bonding
can influence the change in polarization throughout the polymer
network. In addition, the intermolecular hydrogen bonding can
obviously enhance the polarization and dielectric constant,
whereas the intramolecular hydrogen bonding contributes toward
reducing the dielectric constant.25 Furthermore, the composites
have a higher volume fraction of intramolecular hydrogen bonding,
which can help reduce the interfacial polarizability and hence
the dielectric constant values are reduced. In addition to that
ATCP/FRHA/Bz-Ep hybrid composites have siloxane and a highly
cross-linked structure, which can also decrease the dielectric
constant and dielectric loss.
The dielectric loss is another important factor to apply the
material for commercial microwave electronic devices. An
almost negligible dielectric loss is desirable for practical appli-
cations. Frequency-dependent dielectric loss spectra of Bz–Ep,
ATCP/Bz-Ep, ATCP/FRHA(1%)/Bz-Ep, ATCP/FRHA(3%)/Bz-Ep
and ATCP/FRHA(5%)/Bz-Ep composites are shown in Fig. 12
and their resulting values are presented in Table 2. Similar to
the dielectric constant, the values of dielectric loss also follow
the decreasing trend from Bz–Ep to ATCP/FRHA/Bz-Ep and
their results are presented in Table 2. ATCP/FRHA/Bz-Ep sys-
tems have low dielectric constants, which is due to high cross-
linking densities, siloxane structure, intramolecular hydrogen
bonding and reduced surface free energy that promotes the
hydrophobic nature26,27 of the composites.
Apart from this, the dielectric constants mainly depend on
the polarizability of the materials, and the materials with less
polar functional groups favour a low k value. Furthermore, the
incorporation of the less polar Si–O–Si containing rice husk ash
also contributes to reduce the dielectric constant. In addition,
the cross-links in ATCP/FRHA/Bz-Ep maintain sufficient distance
between the matrixes, which can significantly reduce the inter-
facial polarization that results in a very low k value for the hybrid
polymer system. The phenomena such as less polar functional
groups, large volume fraction of intramolecular hydrogen bonds,
Fig. 13 Nyquist plot of Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/Bz-Ep,
ATCP/FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep flame retardant composites.
and low surface free energy are attributed to the low dielectric
constant value (1.62 d) with a high thermal stability as well as
flame retardant ability for the ATCP/FRHA/Bz-Ep composites.
Thus, the resultant ATCP/FRHA(5%)/Bz-Ep composite has a
low k value, which can be used for high performance dielectric
applications.
Impedance measurements were used to assess the dielectric
behaviour of the polymer composites. From the Nyquist plot
(Fig. 13), typical capacitance response was observed with semi-
circles for Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/Bz-Ep, ATCP/
FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep composites. The
large semicircle is related to high charge transfer resistance
(Rct) and constant phase element (CPE2). The decreased size of
the semicircle indicates lower electrical resistivity due to the
creation of conductive paths. This technique is based on the
consideration of a linear electrical behaviour of the system
under study. However, it has been reported that the polymeric
composites can also show a non-linear electrical behaviour.28,29
From this plot, it is seen that the charge transfer resistance
values are 5.27 Â 109, 5.95 Â 109, 7.11 Â 109, 8.52 Â 109 and
9.55 Â 109 O for Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/Bz-Ep,
ATCP/FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep composites,
respectively (Table 2). From the results, it was concluded that the
ATCP/FRHA(5%)/Bz-Ep composites exhibit higher charge transfer
resistances (Rct) than the Bz–Ep composite.
Thermal properties
The glass transition temperature (Tg) of Bz–Ep, ATCP/Bz-Ep,
ATCP/FRHA(1%)/Bz-Ep, ATCP/FRHA(3%)/Bz-Ep and ATCP/
FRHA(5%)/Bz-Ep composites were determined by DSC mea-
surements and the data obtained are listed in Table 3. The
Bz–Ep matrix shows a lower Tg value than that of the other
composites (Fig. 14). The increased value of Tg for ATCP/FRHA/
Bz-Ep composites is may be due to the presence of high cross-
linking density among the amine-terminated cyclophosphazene
functionalized rice husk ash (which exists as a thermally stable
Si–O–Si skeleton in the composite) and benzoxazine-based epoxy
composites.30 Therefore, the introduction of rigid groups into the
Fig. 12 Dielectric loss of Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/Bz-Ep,
ATCP/FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep composites.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015
New J. Chem.

View Article Online
Paper
Table 3 Thermal properties of Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/Bz-Ep, ATCP/FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep composites
NJC
TGA in nitrogen atmosphere
TGA in air
Char yield at
800 1C (wt%)
Char yield at
800 1C (wt%)
a
b
a
b
Thermoset sample
Tg (1C)
Tonset (1C)
Tmax (1C)
Tonset (1C)
Tmax (1C)
Bz–Ep
ATCP/Bz-Ep
ATCP/FRHA(1%)/Bz-Ep
ATCP/FRHA(3%)/Bz-Ep
ATCP/FRHA(5%)/Bz-Ep
211
218
223
233
240
318
322
328
333
338
370
374
378
385
393
22.57
29.47
30.42
39.46
44.25
313
317
325
329
334
367
370
374
382
390
20.84
24.97
28.64
34.54
41.20
a
b
The onset decomposition temperature at which the thermoset undergoes 5 wt% of weight loss. The characteristic temperature at which
maximum rate of weight loss of 40 wt% occurs.
Fig. 14 DSC analysis of Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/Bz-Ep,
ATCP/FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep flame retardant
composites.
Fig. 15 TGA analysis for Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/Bz-Ep,
ATCP/FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep flame retardant
composites under a nitrogen atmosphere.
backbone of a benzoxazine-based epoxy composite can increase
the Tg value of the resulting cured composite.
Thermogravimetric analysis (TGA) technique is another
powerful tool to evaluate the thermal stability of composites.
TGA traces of cured benzoxazine-based epoxy composites can
provide important information about their thermal stability
and thermal degradation behavior. Fig. 15 and 16 show the TGA
thermograms and DTG curves of hybrid composites in the
nitrogen atmosphere and the results are presented in Table 3.
The 5% weight loss temperature of Bz–Ep, ATCP/Bz-Ep, ATCP/
FRHA(1%)Bz-Ep, ATCP/FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/
Bz-Ep composites were 318, 322, 328, 333 and 338 1C, respec-
tively. The variation in degradation values observed is due to
the presence of phosphorous- and nitrogen-containing amine-
terminated cyclophosphazene and silicon-containing function-
alized rice husk ash in the epoxy composites. Similar trends
have also been observed for maximum weight loss (Fig. 16) of
the composites. Furthermore, it was noticed that the presence
of amine-terminated cyclophosphazene and increasing weight
percentages of functionalized rice husk ash can increase the
thermal stability of the resulting composites. The decom-
position temperature of phosphorus-containing composites
is higher than that of the conventional epoxy matrices, which
is due to the presence of P–C bonds, since the P–C bond has a
higher thermal stability than the C–C bond.31 The major
weight loss observed above 300 1C is associated with the
decomposition of the polymer network and finally yields a
residual char at 800 1C.
Fig. 16 DTG curve of Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/Bz-Ep, ATCP/
FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep flame retardant composites.
From Table 3, it can be seen that the value of char yield
increases with increase in wt% of reinforcement, which is due
to the presence of phosphorous, nitrogen and (1%, 3% and 5%)
silicon in the composite systems. A relatively higher char yield
is obtained for the ATCP/Bz-Ep/FRHA(5%) composite compared
to that of the Bz–Ep composite. Furthermore, the higher char
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015

View Article Online
NJC
Paper
flame retardancy in the form of a condensed phase. Such a
category of a flame retarding mechanism is well known as
an intumescent mechanism. In this mechanism, an organic
material can swell when exposed to fire or heat to form a
foamed mass usually carbonaceous in the condensed phase,
promoting char formation on the surface as a barrier to inhibit
gaseous products from diffusing to the flame and shielding the
polymer surface from heat.
Cone calorimetry is often used to evaluate the fire resistance
behaviour of materials and its measurements provide several
useful burning parameters.32–37 Fig. 18 and 19 show time to
ignition (TTI), the heat release rate (HRR), total heat release
rate (THR), total smoke production (TSP), carbon monoxide
production (COP), and smoke production rate (SPR) for Bz–Ep,
ATCP/Bz-Ep, ATCP/FRHA(1%)/Bz-Ep, ATCP/FRHA(3%)/Bz-Ep
and ATCP/FRHA(5%)/Bz-Ep composites. Time to ignition was
used to determine the influence on ignitability, which can be
measured from the onset of the HRR curve (Table 4). TTI
showed clear differences in the ignition behaviour among the
composite materials developed, and TTI of the cured ATCP/
Fig. 17 TGA curves of Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/Bz-Ep,
ATCP/FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep flame retardant com-
posites in air.
yield confirms higher flame retardant properties. Hence, the FRHA(5%)/Bz-Ep system was smaller than that of Bz–Ep thermo-
introduction of phosphorous, nitrogen and silicon into the sets, which may be due to the presence of flame retarded ATCP
modified Bz–Ep shows better flame retardancy than that of and FRHA (Table 4) in the composite materials.
a pure Bz–Ep matrix. Data obtained from thermal analysis
HRR is the measurement of heat release per unit surface
indicates that ATCP/FRHA/Bz-Ep composites exhibit higher area of a burning specimen. This is considered as a typical
thermal stability and better flame retardancy than those of a case for thin non-charring materials. The ATCP/FRHA/Bz-Ep
benzoxazine-based epoxy matrix.
composites show significant flame retardant effects with a
The TGA curves of Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/ drastic decrease in peak heat release rate (PHRR) from 713 to
Bz-Ep, ATCP/FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep flame 289 kW mÀ2. The combination of both amine-terminated
retardant composites in an air atmosphere are shown in Fig. 17. cyclophosphazene (ATCP) and functionalized rice husk ash
The decomposition processes of flame retardant composites in air (FRHA) leads to the improvement in the flame retardant
were similar to those in an N2 atmosphere.
performance of the materials when compared with Bz–Ep and
ATCP/Bz-Ep composites.
Flame retardant properties
The peak originates from the full heat releasing of the
The flammability characteristics of the composites were evalu- insulating layer on the surface of the polymer composite.
ated by UL-94 vertical burning experiment; LOI values and their Herein, the phosphorus groups form an insulating protective
results are tabulated in Table 4. From the results, it was noticed layer, which prevents the volatiles from transferring to the
that the prepared composites achieved the V-0 classification surface of the material and thus increases the thermal stability
except the Bz–Ep composite and their corresponding LOI values of the char at higher temperatures. Furthermore, nitrogen in
increased from 28% to 51%. On this basis, it was concluded polymers can release non-flammable gases or decompose
that the ATCP- and FRHA-containing Bz–Ep composites have a endothermically to cool the pyrolysis zone at the combustion
virtually non-flammable nature. Such good inherent flame retar- surface,38 and silicon in the FRHA is likely to form thermally
dancy is ascribed to the presence of a unique combination stable silica that has a tendency to migrate onto the char
of phosphorous, nitrogen and siloxane networks in ATCP and surface, which serves as a protection layer to prevent the further
rice husk ash incorporated Bz–Ep materials. The occurrence degradation of char at high temperatures.39,40
of the abovementioned inorganic elements can promote the
Fig. 18(b) shows the total heat release (THR) curves of typical
formation of intumescent carbonaceous char, which enhances flame retarded composites. ATCP/FRHA(5%)/Bz-Ep shows the
Table 4 Cone calorimetry and UL-94 vertical burning test results of flame retardant composites
Sample names
UL-94 test LOI value (%) TTI (s) PHRR (KW mÀ2
)
THR (MJ mÀ2
)
PCOP (g sÀ1
)
PSPR (m2 sÀ1
)
TSP (m2)
Bz–Ep
ATCP/Bz-Ep
ATCP/FRHA(1%)/Bz-Ep V-0
ATCP/FRHA(3%)/Bz-Ep V-0
ATCP/FRHA(5%)/Bz-Ep V-0
V-1
V-0
28
34
39
45
51
57
52
48
45
40
713
610
435
374
289
64
58
51
43
31
0.033
0.025
0.021
0.017
0.013
0.16
0.13
0.10
0.07
0.05
12.98
11.33
8.77
7.36
5.52
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015
New J. Chem.

View Article Online
Paper
NJC
Fig. 18 (a) Heat release rate (HRR), (b) total heat release rate (THR) and (c) carbon monoxide production (COP) of Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/
Bz-Ep, ATCP/FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep composites.
Fig. 19 (a) Smoke production rate (SPR) and (b) total smoke production (TSP) of Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/Bz-Ep, ATCP/FRHA(3%)/Bz-Ep
and ATCP/FRHA(5%)/Bz-Ep flame retardant composites.
lowest THR value among all the flame retardant samples, which
The evolution of smoke is considered as another important
further evidences the good flame retardant performance. The parameter in the halogen-free flame retardant materials. The
decrease in THR value of the ATCP/FRHA(5%)/Bz-Ep composite formation of CO in fires takes place at low temperatures in the
indicates that a part of the polymer is not completely burnt. early stage of fire development, which is primarily attributed to
Hence, the compact protective layers on the surface serve as the incomplete combustion of the pyrolyzed polymer volatile
a thermal insulation layer that can inhibit polymer pyrolysis, products. When fire develops, higher temperatures favour the
which is the evolution of combustible gases to feed the formation of CO2, which is particularly dependent on oxygen
flame, and act as a barrier to separate oxygen from the burning availability to the fire.42,43 Fig. 18(c) shows the CO releasing rate
materials.41
during the combustion of composites. It can be seen that the
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015

View Article Online
NJC
Paper
Fig. 20 Char morphology and inner layer morphology of (a) and (a1) Bz–Ep, (b) and (b1) ATCP/Bz-Ep and (c) and (c1) ATCP/FRHA(1%)/Bz-Ep flame
retardant composites.
Analysis of residual char
SEM micrographs of the outside and the inside features of the
char obtained from the UL-94 test are shown in Fig. 20 and 21.
The outside aspect of the char was observed to present a
carbonaceous structure with a more compact and dense char
layer, while the inside shows a multi-porous structure.
This result indicates the typical morphology associated with
intumescent char formation. It was well known that the char-
ring structure is one of the most important factors that deter-
mine the flame retardancy on the basis of SEM observation.
The char layer looks rigid and compact with many integrated
closed honeycomb pores in the inside structural features that
could impart a temperature gradient to the char layer during
combustion. Such a protective char layer can serve as a barrier
against heat and oxygen diffusing items that protects the inside
matrix. From the FE-SEM images, it was concluded that the use
of cyclophosphazene and rice husk ash as a flame retardant
Fig. 21 Char morphology and inner layer morphology of (a) and (a1)
material can lead to remarkably important flame retardant
properties of the resultant composite materials.
ATCP/FRHA(3%)/Bz-Ep and (b) and (b1) ATCP/FRHA(5%)/Bz-Ep flame
retardant composites.
Fig. 22 shows the FTIR spectra of the residual chars collected
from the UL-94 test of Bz–Ep, ATCP/Bz-Ep, ATCP/FRHA(1%)/Bz-
CO productions of the flame retardant ATCP/FRHA(5%)/Bz-Ep Ep, ATCP/FRHA(3%)/Bz-Ep and ATCP/FRHA(5%)/Bz-Ep. The
sample are much lower than that of Bz–Ep composites during peak observed at 3433 cmÀ1 corresponds to NH stretching
combustion.
vibrations. The peaks at 2961 and 2865 cmÀ1 correspond to
Development of flame retardant materials with low-smoke C–H stretching vibrations. The peak observed at 1041 cmÀ1
polymers is a long-pursued target (Fig. 19(a)). The ATCP/FRHA(5%)/ confirms the presence of C–O stretching vibrations. The aro-
Bz-Ep sample has a much lower smoke production rate than that matic C–C stretching vibration was observed at 1514 cmÀ1. The
of other Bz–Ep composites. From the results, it was concluded that characteristic stretching vibrations observed at 1257 cmÀ1
,
the accumulation of ATCP intumescent flame retardant together 1186 cmÀ1 and 946 cmÀ1 suggest that P–O–Ph, P–N–P and P–
with FRHA near the regressing sample surface acts as not only as O–C bonds exist in the residual chars of all the tested thermo-
a heat insulation layer but also as a barrier that prevents the sets. Furthermore, the peak observed between 1000 and 1130 cmÀ1
decomposed volatile products migrating to the sample surface.
confirms the presence of the Si–O–Si bond. From the overall
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015
New J. Chem.

View Article Online
Paper
NJC
burning test. During the UL-94 vertical burning flame retardant
test, all specimens were ignited with an alcohol lamp for 30 s. The
amount and shape of residues formed during combustion were
closely observed. Fig. 23(a) shows that a rare char was formed,
which confirms the poor flame retardancy and melt dripping.
Fig. 23(e) shows that a char layer was formed, and the thickness of
this char layer increased with increasing mass ratio of ATCP and
FRHA materials.
Conclusion
In this study, ATCP and FRHA were utilized together as a novel
combined halogen-free flame retardant system to improve the
flame retardance of benzoxazine blended epoxy composites.
The addition of FRHA enhances both the flame retardance and
dielectric properties of amine-terminated cyclophosphazene-
reinforced benzoxazine-based epoxy matrix. The fracture surface
exhibits a non-phase separated platelet-like homogeneous
morphology. The dielectric constant and dielectric loss decreased
due to the presence of intramolecular hydrogen bonding, high
cross-linking density and orientation of the molecules. The
improvement in the electrical resistivity was confirmed by
Nyquist plots with high charge transfer resistance for the
composite materials. The improved hydrophobic nature was
confirmed by contact angle measurements. Furthermore, the
introduction of a small amount of ATCP and FRHA were found
to improve the degradation properties, leading to a compact
and unbroken N, P and Si protective layer. Therefore, drastically
depressed peak heat release rates were achieved. This novel
flame retardant composite will open up new possibilities for
high performance halogen-free flame retardant polymeric
materials, which could be applied to micro-electronic devices.
Fig. 22 FT-IR spectra of residues obtained from the UL-94 test.
results, it was concluded that synergistic effects occurred and
strengthened with increasing FRHA mass ratio; this plays a very
important role in the self-extinguishing performance of flame
retardant ATCP/FRHA/Bz-Ep composites.
Fig. 23 shows the digital images of Bz–Ep, ATCP/Bz-Ep,
ATCP/FRHA(1%)/Bz-Ep, ATCP/FRHA(3%)/Bz-Ep and ATCP/
FRHA(5%)/Bz-Ep residues obtained after carrying out the vertical
Acknowledgements
The authors like to thank DST/Nanomission, New Delhi, India,
for financial support to carry out this study and to establish
Nanotech Research Lab through the grant no. SR/NM/NS-05/
2011(G).
References
1 A. Riva, G. Camino, L. Fomperie and P. Amigouet, Polym.
Degrad. Stab., 2003, 82, 341.
2 M. Fu and B. Qu, Polym. Degrad. Stab., 2004, 85, 633.
3 S. M. El-Sayed, H. M. Abdel Hamid and R. M. Radwan,
Radiat. Phys. Chem., 2004, 69, 339.
4 J. Gu, Q. Zhang, J. Dang and C. Xie, Polym. Adv. Technol.,
2012, 23, 1025.
5 J. Gu, J. Dang, Y. Wu, C. Xie and Y. Han, Polym.–Plast.
Technol. Eng., 2012, 51, 1198.
6 J.-W. Gu, G.-C. Zhang, S.-L. Dong, Q.-Y. Zhang and J. Kong,
Surf. Coat. Technol., 2007, 201, 7835.
7 G. C. Stevens and A. H. Mann, Risks and benefits in the use
of flame retardants in consumer products: a report for the
Fig. 23 Digital photos of (a) Bz–Ep, (b) ATCP/Bz-Ep, (c) ATCP/FRHA(1%)/
Bz-Ep, (d) ATCP/FRHA(3%)/Bz-Ep and (e) ATCP/FRHA(5%)/Bz-E residue
obtained after carrying out UL-94 vertical burning flame retardant test.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015

View Article Online
NJC
Paper
department of trade and industry, Polymer Research Centre, 26 C. S. Liao, C. F. Wang, H. C. Lin, H. Y. Chou and F. C.
University of Surrey, Surrey, 1999. Chang, Langmuir, 2009, 25, 3359.
8 H. Ishida and T. Agag, Handbook of benzoxazine, Elsevier, 27 J. Liu, X. Lu, Z. Xin and C. Zhou, Langmuir, 2013, 29, 411.
Amsterdam, 1st edn, 2011, ch. 7, p. 169.
28 T. Alizadeh, Mater. Chem. Phys., 2012, 135, 1012.
29 J. Guo, A. Sun, X. Chen, C. Wang and A. Manivannan,
Electrochim. Acta, 2011, 56, 3981.
30 M. Sponton, L. A. Mercado, J. C. Ronda, M. Galia and
V. Cadiz, Polym. Degrad. Stab., 2008, 93, 2025.
9 S. Rimdusit and H. Ishida, Polymer, 2000, 41, 7941.
10 S. Tragoonwichian, N. Yanumet and H. Ishida, J. Appl.
Polym. Sci., 2007, 106, 2925.
11 H. R Allcock, Chemistry and applications of phosphazenes,
Wiley-Interscience, Hoboken, NJ, 2003, p. 19.
31 U. Quittmann, L. Lecamp, W. E. Khatib, B. Youssef and
C. Bunel, Macromol. Chem. Phys., 2001, 202, 628.
12 S. Fei and H. R. Allcock, J. Power Sources, 2010, 195, 2082.
13 H. R. Allcock, J. Inorg. Organomet. Polym. Mater., 2006, 32 A. B. Morgan and M. Bundy, Fire Mater., 2007, 31, 257.
16, 277.
33 C. Siat, M. L. Bras and S. Bourbigot, Fire Mater., 1998,
22, 119.
14 J. Ding and W. Shi, Polym. Degrad. Stab., 2004, 84, 159.
15 D. Kumar, G. M. Fohlen and J. A. Parker, Macromolecules, 34 B. Schartel and T. R. Hull, Fire Mater., 2007, 31, 327.
1983, 16, 1250.
35 G. Gallina, E. Bravin, C. Badalucco, G. Audisio,
M. Armanini, A. D. Chirico and F. Provasoli, Fire Mater.,
1998, 22, 15.
36 D. Y. Wang, Y. Liu, Y. Z. Wang, C. P. Artiles, T. R. Hull and
D. Price, Polym. Degrad. Stab., 2007, 92, 1592.
37 P. J. Elliot and R. H. A. Whiteley, Polym. Degrad. Stab., 1999,
64, 577.
16 I. Apivhat and P. Eakachai, J. Hazard. Mater., 2010, 184, 775.
17 K. Krishnadevi, A. Nirmala Grace, M. Alagar and V. Selvaraj,
High Perform. Polym., 2014, 26, 1.
18 K. Krishnadevi, V. Selvaraj and D. Prasanna, RSC Adv., 2015,
5, 913.
19 H. Ishida and D. P. Sanders, Macromolecules, 2000, 33, 8149.
20 E. Rafiee, S. Shahebrahimi, M. Feyzi and M. Shaterzadeh, 38 F. Gao and L. Tong anad Z. Fang, Polym. Degrad. Stab., 2006,
Int. Nano Lett., 2012, 2, 29. 91, 1295.
21 A. Proctor, P. K. Clark and C. A. Pareker, J. Am. Oil Chem. 39 L. Y. Ling, W. W. Lung, H. K. Ying and H. W. Hsuan,
Soc., 1995, 72, 459. Thermochim. Acta, 2004, 412, 139.
22 M. Rozainee, S. P. Ngo, S. S. Slem, K. G. Tan, M. Ariffin and 40 G. L. Nelson, Fire retardancy of polymeric materials, Marcel
Z. N. Zainura, Bioresour. Technol., 2008, 99, 703. Dekker, New York, 2000, p. 1.
23 J. Fan, X. Hu and C. Y. Yue, J. Polym. Sci., Part B: Polym. 41 S. Bourbigot, M. LeBras, R. Delobel and L. Gengembre, Appl.
Phys., 2003, 41, 1123. Surf. Sci., 1997, 120, 15.
24 H. D. Kim and H. Ishida, J. Phys. Chem. A, 2002, 106, 3271. 42 Y. Y. Yen, H. T. Wang and W. J. Guo, Polym. Degrad. Stab.,
25 C. F. Wang, Y. C. Su, S. W. Kuo, C. F. Huang, Y. C. Sheen and
2012, 97, 863.
F. C. Chang, Angew. Chem., Int. Ed., 2006, 45, 2248.
43 M. Z. Fu and B. J. Qu, Polym. Degrad. Stab., 2004, 85, 633.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015
New J. Chem.