The influence of silicone shell on double-layered microcapsules in intumescent flame-retardant natural rubber composites

Article abstract of DOI:10.1007/s10973-014-3965-2

Double-shell co-microencapsulated ammonium polyphosphate and mesoporous MCM-41 (M(A&M)) were prepared by using melamine-formaldehyde resin and organic silicon in situ polymerization. The structure of the microcapsules was characterized by Fourier-transform infrared spectroscopy, particle size analysis, and scanning electron microscopy. In particular, the influence of basic properties of flame-retardant natural rubber, such as flame-retardant and mechanical property, was investigated with the limiting oxygen index, UL-94 test, thermogravimetric analysis, tensile test, and rubber process analyzer (RPA). Due to the presence of organic silicone shell, the NR/M(A&M) composites had much better flame-retardant and mechanical properties than other flame-retardant natural rubber (NR) composites. The limited oxygen index value of the NR/M(A&M) composite reached the maximum, and the UL-94 ratings were increased to V-0. Furthermore, M(A&M) showed better compatibility in NR system by the RPA. The occurrence of a synergistic effect between MCM-41 and intumescent flame-retardant in the NR composites was proved. As a result, the thermal stability and flame retardancy of NR were enhanced.

Full text of DOI:10.1007/s10973-014-3965-2

J Therm Anal Calorim (2014) 118:349–357
DOI 10.1007/s10973-014-3965-2
The influence of silicone shell on double-layered microcapsules
in intumescent flame-retardant natural rubber composites
•
Na Wang Yuhu Wu Long Mi Jing Zhang
•
•
•
•
Xuri Li Qinghong Fang
Received: 19 March 2014 / Accepted: 14 June 2014 / Published online: 10 September 2014
´
´
Ó Akademiai Kiado, Budapest, Hungary 2014
Abstract Double-shell co-microencapsulated ammonium
polyphosphate and mesoporous MCM-41 (M(A&M)) were
prepared by using melamine–formaldehyde resin and
organic silicon in situ polymerization. The structure of the
microcapsules was characterized by Fourier-transform
infrared spectroscopy, particle size analysis, and scanning
electron microscopy. In particular, the influence of basic
properties of flame-retardant natural rubber , such as flame-
retardant and mechanical property, was investigated with
the limiting oxygen index, UL-94 test, thermogravimetric
analysis, tensile test, and rubber process analyzer (RPA).
Due to the presence of organic silicone shell, the NR/
M(A&M) composites had much better flame-retardant and
mechanical properties than other flame-retardant natural
rubber (NR) composites. The limited oxygen index value
of the NR/M(A&M) composite reached the maximum, and
the UL-94 ratings were increased to V-0. Furthermore,
M(A&M) showed better compatibility in NR system by the
RPA. The occurrence of a synergistic effect between
MCM-41 and intumescent flame-retardant in the NR
composites was proved. As a result, the thermal stability
and flame retardancy of NR were enhanced.
Introduction
Natural rubber (NR) has many unique advantages over
competitive synthetic rubbers by virtue of its unique
combination of physico-mechanical properties. And its
latitude of application is being widened day by day, by the
judicious design and compounding of formulations for
specific applications. However, despite a number of supe-
rior qualities, its inherent high flammability prevents NR
from being used in some highly demanding applications
such as coal mine convey or belts, power cables, aircraft
tire treads, and so on [1–3].
Because of the potential demand of flame-retardant
natural rubber (FRNR) for diverse applications, FRNR has
been a hot research topic. Among the various approaches to
confer flame retardancy to NR, intumescent flame retar-
dants (IFR) with little smoke release and low toxicity are
preferable additives in NR compared with traditional flame
retardants containing phosphorus or halogen [4, 5]. A
widely used IFR system is a mixture of ammonium poly-
phosphate (APP), pentaerythritol (PER), and melamine
(MEL). Synergism has been found between IFR and MCM-
41 or SBA-15 in PP composites (PP-IFR) in our previous
work [6]. Results show that mesoporous silica MCM-41
and SBA-15 exert the effective synergistic effect found in
intumescent flame-retardant systems of PP to obtain good
flame retardancy and satisfactory mechanical properties.
But due to its high water solubility and low compatibility
with matrix material, the IFR system is not durable, and
microencapsulation of IFR system has been regarded as an
effective strategy. Chen, Xilei et al. [7, 8] successfully
microencapsulated APP with polyurethane (PU), urea-
melamine-formaldehyde (UMF) resin, and PVA-MEL-
formaldehyde resin shell. To improve the FR properties of
composites, microencapsulated ammonium polyphosphate
Keywords Double-shell co-microencapsulated ꢀ Organic
silicon ꢀ Synergism ꢀ Mesoporous MCM-41 ꢀ Intumescent
flame retardant ꢀ Natural rubber
N. Wang (&) ꢀ Y. Wu ꢀ L. Mi ꢀ J. Zhang ꢀ X. Li ꢀ Q. Fang
College of Materials Science and Engineering, Shenyang
University of Chemical Technology, Shenyang 110142, China
e-mail: iamwangna@sina.com
N. Wang
College of Materials Science and Chemical Engineering, Harbin
Engineering University, Harbin 150001, China
123

350
N. Wang et al.
N
C
N
C
(a)
N
C
N
C
N
C
N
N
C
N
C
N
NHCH N
2
C
N
HOH CHN
C
N
C
N
NHCH OH
2
2
H O
H N
NH
2
C
N
C
N
2
2
CH OH
2
CH OH
2
Condensation
3HCHO
N
C
HNH CHN
N
2
C
NHCH OH
C
N
C
NHCH OH
2
NH
2
2
N
C
C
N
OH
Si
OH
OH
OH
(b)
N
HO
OH
(1)
HO
Si OH
HO
Si
O
Si OH
H O
2
C
OH
OH
OH
OH
OH
N
C
C
N
C
N
N
N
C
OH
OH
OH
C
N
(2) HO Si OH
OH
HO
Si
Si
O
OH
C H OH
C H O
Si OC H
2 5
2
5
N
C
2
5
C
C
OH
OH
OH
Si
N
N
HO
(3)
n(Si
O
Si)
O
Si
OH
n
n
HO
Si
Si
Si
O
O
(c) OH
OH
Si
O
O
HO
Si
Si
O
Si
NH2CHN
N
N
C
C
O
OH
C
C
OH
Si
O
N
N
O
OH
HO
Si
Si
N
N
C
O
C
OH
Si
O
O
O
O
HO
O
OH
Si
C
O
Si
O
N
C
HO
Si
N
Si
N
O
OH
N
O
C
O
O
C
C
N
C
O
N
OH
Si
Si
Si
Si
N
C
O
HO
OH
C
N
O
OH
O
N
C
OH
Si
O
Si
O
O
O
Si
OH
Si
OH
OH
Si
OH
Fig. 1 a Schematic of the double-shell MC(A&M), b schematic of the shell formation process of the MF, c schematic of the shell formation
process of the OS
with melamine–formaldehyde resin (MCAPP) prepared by
in situ polymerization that showed decreased water solu-
bility and particle size, which was better than ammonium
polyphosphate (APP), was used as additive to NR together
with mesoporous silica MCM-41 as a synergistic agent to
form IFR composite in our recent study [9]. However, the
ideal mechanical and FR properties of NR composites
cannot be achieved by the single-layered microencapsula-
tion of IFR. And then, we studied the double-layered co-
microencapsulated APP and MCM-41 fillers (M(A&M))
with MEL-formaldehyde (MF) resin and zinc borate (ZB)
in intumescent flame-retardant natural rubber composites
[10] to solve the problem mentioned. As the outer shell is
made of inorganic ZB materials, the compatibility and
material mechanical performance are still not ideal.
stereochemical configuration and binding sites that
enhance compatibility with NR matrix which improves
thermal stability [11–14].
In this paper, the double-shell co-microencapsulated
APP and MCM-41 fillers M(A&M) with MEL-formalde-
hyde (MF) resin and OS (Fig. 1c) were prepared by in situ
polymerization. The structure of the microcapsules was
characterized by Fourier-transform infrared spectroscopy,
particle size analysis, and scanning electron microscopy. In
particular, the influence of basic properties of FRNR, such
as flame-retardant and mechanical property, was investi-
gated with the limiting oxygen index, UL 94 test, ther-
mogravimetric analysis, tensile test, and rubber process
analyzer (RPA).
With the advantages of both the inorganic material and
organic material, organic silica (OS) has a good application
prospect. The chemical modification or functionalization of
OS and attachment of the modified OS to PUs are of great
interest owing to their improved properties and potential
use in several different applications. This system with
covalently anchored functional groups on the silica causes
Experimental
Materials
Natural rubber SMR-20 was provided by Petrochemical
Co., Jilin. The commercial product APP (phase II, average
degree of polymerization n [ 1000, soluble fraction in
its surface to have specific attributes, such as
a
123

Influence of silicone shell on FRNR
351
H2O \ 0.5 mass%) was supplied by Shifang Changfeng
Chemical Co., Ltd., China. PER and MEL were supplied
by Sinopharm Chemical Reagent Co., Ltd. The mixture of
APP, PER, and MEL was set at a mass ratio of 3:1:1, at
which the FR properties were the maximum [4]. Bis-(c-
triethoxysilylpropyl)-tetrasulfide (TESPT) was provided by
Evonik Degussa Co. Ltd. Analytical-grade tetraethyl sili-
cate, stearic acid, sulfur, and other chemicals were used as
received.
Table 1 Formulation of FRNR composites
Sample
NR/ APP/ MCAPP/ MCM- M(A&M)/
phr phr
phr
41/phr phr
NR
100
/
/
/
/
/
/
/
/
/
NR/APP
100 40
/
NR/MCAPP
NR/M(A&M)
100
100
/
/
40
/
40(APP/MCM-
41 39/1)
NR/MCAPP/
MCM-41
100
/
39
1
/
Spherically shaped mesoporous MCM-41 particles with
uniform diameters within the range of 80–100 nm were
prepared in the laboratory following a previously described
method [15]. Infrared analysis of the synthesized nanosized
mesoporous MCM-41 particles has been reported in a
previous work [16].
laboratory two-roll mill. The components of NR com-
pounds included stearic acid, PER, MEL, TESPT, carbon
soot, zinc oxide, sulfur, age inhibitor 4010, electric insu-
lating oil, accelerant CZ, and tetramethylthiuram disulfide,
which had fixed values of 35, 13, 13, 5, 5,4, 1.2, 1, 1, 0.8,
0.35, (phr), respectively. All NR samples were vulcanized
at 145 °C in a hot press for the optimum cure time t90
determined by a GT-M2000-A rheometer (GaoTie Limited
Co., Taiwan). All specimens were then cut from the vul-
canized sheets and conditioned at 20 °C for 24 h before
testing.
Preparation of microcapsules
Preparation of MF pre-polymer: 15 g of MEL, 34 g of
37 mass% formaldehyde solution, 0.3 g of Na2CO3, and
40.0 mL of deionized water were added to a three-necked
bottle with a stirrer (230 rpm). The mixture was heated to
80 °C and kept at that temperature for 1 h. The MF pre-
polymer solution was then prepared, and it was ready for
microencapsulation of APP and MCM-41.
Characterization
Preparation of single-shell microcapsules: a mixture of
APP and MCM-41 (60 g) and 150 mL of ethanol were
added to a three-neck bottle, under stirring (300 rpm,
20 min). Then, the MF pre-polymer solution, whose pH
was adjusted to 5–6 with hydrochloric acid dilution. The
resulting mixture was heated to 75 °C for 2 h. The con-
densation reaction mechanism of the MF is shown in
Fig. 1a.
The infrared spectra of the microcapsules were obtained on
a Nicolet MNGNA-IR 560 with 4 cm-1 resolution. Mean
particle size and distribution of microcapsules were eval-
uated by a laser particle size analyzer (Rise-2008, Shan-
dong, China). The SEM images of the microcapsules and
NR composites were obtained with a scanning electron
microscope JEOL JSM-6360LV. The specimens were
previously coated with a conductive gold layer. The sam-
ples for testing the morphology of the burnt composites
were prepared using burnt samples of oxygen index.
Complete burning of the samples was ensured. LOI data
were obtained at room temperature on an oxygen index
instrument (JF-3) produced by Jiangning Analysis Instru-
ment Company, China, according to ISO 4589-1984
standard. The dimensions of the specimens were
126 9 6.5 9 3 mm3. The vertical test was carried out on a
CFZ-2 type instrument (Jiangning Analysis Instrument
Company, China) according to the UL-94 test standard.
The specimens used had dimensions of 126 9 13 9
3 mm3. Room-temperature tensile tests of the composites
were conducted following GB/T528-1998 on an Instron
Preparation of double-shell microencapsules: tetraethyl
silicate (40 g), anhydrous ethanol (40 g), and distilled
water (80 g) were added to a three-necked bottle, and the
pH of the mixture was adjusted to pH 2–3 with sulfuric
acid. The resulting mixture was heated at 60 °C for 3 h
with stirring (350 rpm). After the hydrolysis reaction of the
tetraethyl silicate had taken place, the sol solution as
encapsulation precursor was obtained. The single-wall
microcapsules were added into mixture solution with stir-
ring (400 rpm, 2 h), and the temperature of the sol solution
was controlled at 30 °C using a constant temperature bath.
The condensation reaction mechanism and hydrolysis
reaction mechanism of the tetraethyl silicate onto the sur-
face of single-wall microcapsules are shown in Fig. 1b.
1211 testing machine at
a
crosshead speed of
Preparation of FRNR composites
500 mm min-1. The thermal stability of the NR compos-
ites was examined by TG using a Perkin-Elmer TGA 7
thermal analyzer at a heating rate of 10 °C min-1. Nitrogen
was used as a carrier gas at a constant flow rate during
analysis. Dynamic mechanical properties were measured
Flame-retardant natural rubber composites were mixed
with the change of the amount of IFR agents and rubber
compounds into NR according to the recipe in Table 1 by a
123

352
N. Wang et al.
100.0
90.0
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
a
b
c
APP
MCM-41
MC(A&M)
(a)
c
800
1083
880
1568
b
3390
960
1250
880
3200
800
1256
a
1020
1000
0.1
1.0
10.0
100.0
Size/μm
1080
1500
100.0
90.0
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
(b)
4000
3500
3000
2500
2000
Wavenumbers/cm–1
Fig. 2 FTIR spectra of APP, MCM-41 and MC(A&M)
with a dynamic mechanical thermal analyzer (DMTAV,
Rheometrics Science Corp.). The dynamic storage modulus
was determined at a frequency of 10 Hz and a heating rate
of 3 °C min-1 from -100 to 100 °C. Curing was per-
formed on Rubber Process Analyzer RPA-8000 (GaoTie
Limited Co., Taiwan) at the temperature of 145 °C, fre-
quency of 100 cpm, and strain amplitude of 0.5°. For strain
sweep, the temperature was 60 °C, the frequency was
60 cpm, and the strain was between 1 and 100 %.
0.1
1.0
10.0
100.0
Size/μm
Fig. 3 Size distribution of APP (a) and M(A&M) (b)
of double-shell microcapsules increases to 28.05 lm
compared to the 8.43 lm of APP. And Fig. 3 clearly shows
that the particle size distributions of M(A&M) become
more concentrated than APP without microencapsulated.
Because of the narrow particle size distribution of
M(A&M), it has better compatibility in NR than APP when
blended with NR in this study, which expects for better
flame retardancy and mechanical properties.
Results and discussion
Characterization of microcapsules
The FTIR spectra of APP and M(A&M) are shown in
Fig. 2. Figure 2a shows the typical absorption peaks of
APP at 3200 (N–H), 1256 (P = O), 1020 (symmetric
vibration of PO2), 880 (P–O asymmetric stretching vibra-
tion), and 800 cm-1 [17]. Figure 2b shows that the
1080 cm-1 band represented the asymmetric stretching
vibration of Si–O–Si from MCM-41. As shown in Fig. 2c,
the main absorption peaks appeared at 3390, 1568, 1250,
1083, 960, 800, and 463 cm-1. The peaks at 1083, 798, and
463 cm-1 signify the bending vibration of the Si–O func-
tional group, and the peak at 960 cm-1 is assigned to the
Si–OH functional group [18]. The absorption at 1568 cm-1
is due to the ring vibration of MEL from MF resin. Con-
sequently, the spectrum of M(A&M) reveals the charac-
teristic bands of APP/MCM-41 and the well-defined
absorption peaks of MF resin and OS.
The surface morphologies of the APP and M(A&M)
particles are shown in Fig. 4. The surface of APP particle is
obviously very smooth. It is observed from the SEM
micrographs that the presence of OS shell does affect the
morphological makeup of the microparticles. M(A&M) is
observed to have uneven surfaces that are likely due to the
fact that the methylolation reaction in the synthesis of MF
resin produced different addition products. A second step
of polycondensation results in different degrees of poly-
merization of the inner layer MF resin to produce this
phenomenon. Thus, double-layered microencapsulation is
achieved. As expected, the functionalized organic group
after crosslinking make the M(A&M) be incorporated into
NR three-dimensioned network, which can enhance the
dispersion of M(A&M) and the compatibility between NR
matrix and M(A&M).
Figure 3 shows the number-average particle size distri-
bution of APP and M(A&M). Both APP and M(A&M) are
in the size range of 0.1–120 lm, but the mean particle size
123

Influence of silicone shell on FRNR
353
Table 2 Flame retardancy and mechanical property of flame-retar-
dant NR composites
Flame-retardant
composite
Flame
retardancy
Mechanical property
LOI/% UL-94
Tensile
Elongation
strength/ at break/%
MPa
NR
16
No rating 22.25
452.6
NR/APP
26.2
28.7
29.9
No rating
V-0
6.62
7.50
8.23
7.68
276.6
272.6
309.6
348.2
NR/MCAPP
NR/M(A&M)
V-0
NR/MCAPP/MCM-41 29.2
V-0
stereochemical configuration and binding sites that
enhance compatibility with organic binders which improve
thermal stability. That is the reason why NR/M(A&M)
system has a relatively high LOI value compared with our
previous double-layered microencapsulation with MF resin
and ZB outside APP and MCM-41 [10].
Moreover, MCM-41 can reduce not only the amount of
amorphous char protecting but also the intumescent char
strengthening. The above results show that the presence of
MF resin and OS outside APP and MCM-41 enabled
M(A&M) to be an effective flame retardant in NR com-
pared with APP/MCM-41. This enhancement in flame re-
tardancy is thought to be due to the combined effect of
higher char yield, physical dilution of the combustible
gases through the decomposition processes of M(A&M),
and the heat-sink effect of the synergist MCM-41.
Fig. 4 SEM images of APP particles (a) and M(A&M) particles (b)
Flame retardancy
Mechanical properties
LOI and UL-94 tests are widely used to determine the
flammability of flame-retardant materials. From the
experimental results shown in Table 2, NR is easily flam-
mable polymeric material with a LOI value of 16, and it
cannot pass UL-94 test. The improvements in flame re-
tardancy of the vulcanizates at higher concentrations of
M(A&M) are clear from the increase in value of LOI from
26.2 to 29.9 as given in Table 2. The experimental results
of vertical burning rate show that the NR/M(A&M) sys-
tems give a V-0 rating. It can be clearly found that NR/
M(A&M) system has higher flame retardancy than that of
NR/MCAPP/MCM-41. This enhancement in flame retar-
dancy is thought to be due to the fact that when the NR
composites containing M(A&M) were heated, MF resin as
the inner shell of MCAPP released water vapor and NH3
gases, which reduced the concentration of air and made the
NR materials swell to form intumescent char [10]. This
system with covalently anchored functional groups on the
silica causes its surface to have specific attributes, such as a
Table 2 shows the mechanical properties of NR and FRNR
composites. The tensile strength value and elongation at
break were 6.62 MPa and 276.6 % for the NR/APP system,
as well as 22.5 MPa and 452.6 % for NR, respectively. It is
clear that addition of IFR filler had markedly decreased the
mechanical properties of the composites. However, com-
pared with un-microencapsulated samples, NR/M(A&M)
system evidently increases the tensile strength and elon-
gation at break. This finding was due to the fact that OS as
the outer shell of double-shell microencapsulation is
organic and can reinforce the strength of microencapsula-
tion and the formation of M(A&M)-rubber coupling as
shown in Fig. 5. Complications arise during mixing com-
pounds as several chemical reactions tend to take place,
such as silica and silane reaction, M(A&M)-rubber cou-
pling and crosslinking between the rubber chain reaction.
This kind of coupling or crosslinking makes the double-
shell microcapsule get better dispersion within NR matrix
and let NR/M(A&M) composites to achieve proper
123

354
N. Wang et al.
stronger char layer. The Si–O-Si network structure of
double-shell microcapsule plays an important role in con-
densed phase. On one hand, silicon-containing compounds
have good trapping ability of active free radicals in the
vapor phase, leading to decreased smoke and materials. On
the other hand, the char layer can be further strengthened
by the strong ceramic network [19]. Furthermore, the
synergist MCM-41 can effectively slow down the degra-
dation of underlying materials and stop the transfer of heat
and flammable volatiles. For NR/APP/MCM-41 (Fig. 6d),
the char layer is non-uniformly inflated, but a compact
structure is formed. The bigger inflated bubble can easily
fracture. Analysis of these images suggests that the addi-
tion of MCM-41 and double-layered microencapsulation
with MF resin and OS outside APP and MCM-41 posi-
tively affects the FR properties of NR composites.
HO
Si
O
O
Si
O
Si
O
O
Si
O
O
Si
OH
Thermal stability
Figure 7 shows the TG curves of NR and FRNR compos-
ites. The related TG data are listed in Table 3. Thermal
degradation of pure NR under N2 atmosphere involved two
steps, in which the thermal degradation takes place in the
range of 220–470 °C, and steadily decomposed at 470 °C.
In the presence of APP, the NR/APP system significantly
decreased T20 % to 352.07 °C compared with pure NR;
moreover, residue is improved. This improvement may be
due to the fact that the decomposition of IFR took place in
the first place, and then isolates NR from heat and oxygen
to enter. The decomposition processes of NR/M(A&M)
composite show T50 % of 421 °C, which is 27 and 21 °C
higher than NR and NR/MCAPP, respectively. The deg-
radation of NR/M(A&M) ends with a mass loss of 36.76 %
higher than the other samples. The enhancement in the char
residue at 600 °C is connected to the improvement in the
anti-flammability of NR composites. This phenomenon
may be due to the fact that the OS shell materials are
advantageous to form carbonaceous-silicate charred layer
building up on the surface, which insulates the core
material and slows the escape of the volatile products
generated during thermal degradation. The carbonaceous-
silicate charred layers may create a physical protective
barrier on the surface of the microencapsulated composites.
This protective barrier can limit the transfer of flammable
molecules to the gas phase, the transfer of heat from the
flame to the condensed phase, and oxygen diffusion in the
condensed phase [20]. On the other hand, MCM-41 dis-
persed in the composites shows good barrier properties and
high thermal stability, which may slow down heat trans-
mission to the polymer, consequently delaying the
decomposition of the composites [21]. Therefore, the
thermal stability of the NR/M(A&M) composite is
remarkably better than those of the other samples.
Fig. 5 Silica-silane-rubber coupling
mechanical properties. The tensile strength value and
elongation at break are 8.23 MPa and 309.6 % for the NR/
M(A&M) composites. On the other hand, these evident
enhancements in tensile strength and elongation at break
are likely to due to the synergist MCM-41, which results in
the reinforcement of NR composites.
Morphology of burnt composites
As mentioned above, the residue of char plays a significant
role in improving flame-retardant property of NR; thus, the
investigation of combustion residues could help to under-
stand the synergistic effect of the flame-retardant system.
Figure 6 shows the SEM images of carbonaceous residues
of burnt NR/APP (NRAPP), NR/MCAPP, NR/M(A&M),
and NR/APP/MCM-41 after LOI test. As displaced in
Fig. 6a, a relative loose structure, lot of cracks, cavities,
and some incomplete hollow-fractured bubbles can be
observed on the surface of the char residue in NR/APP.
Consequently, heat and flammable volatiles can easily
penetrate through the char layer into the interior of the
composites during combustion. After being single-layered
microencapsulated with MF resin, NR/MCAPP forms a
relatively complete char layer that had fewer, smaller
cracks and cavities than NR/APP. Regarding the NR/
M(A&M) composite (Fig. 6c), the microstructure of the
char residue displays a more compact structure and almost
no cavities compared with signal-layered microencapsu-
lated system (Fig. 6b). This finding indicates that double-
layered microencapsulation is effective in forming a
123

Influence of silicone shell on FRNR
355
Fig. 6 SEM images of flame-
retardant composites (1,0009):
a NR/APP, b NR/MCAPP,
c NR/M(A&M), and d NR/
MCAPP/MCM-41
110
100
90
Table 3 TG data of flame-retardant NR composites
a
b
c
d
e
NR
NR/APP
NR/MCAPP
NR/M(A&M)
NR/MCAPP/MCM-41
Sample
T20 %/8Ca
T50 %/8Cb
Char residue
at W600/%
NR
362.61
352.07
353.73
366.72
350.84
394.06
403.77
400.01
421.50
399.79
29.18
32.54
35.15
36.76
35.58
c
80
70
60
50
40
30
20
NR/APP
NR/MCAPP
NR/M(A&M)
NR/MCAPP/MCM-41
a
d
e
Temperature at 20 % mass loss/and
b
Temperature at 50 % mass loss
b
a
NR/M(A&M) composites compared to other samples,
which indicates that double-shell microencapsulation
enhances the compatibility between the IFR and the matrix
system. The glass transition temperature (Tg) of the sam-
ples determined from the temperature at the maximum
tan d is as follows: NR/APP (-51.82 °C), NR/MCAPP
(-49.80 °C), NR/M(A&M) (-48.36 °C), and NR/APP/
MCM-41 (-50.08 °C). In Fig. 9 the loss factor tan d is
shown for different vulcanizates. The maximum of tan d
peak reduces significantly suggesting a strong adhesion
between NR and microencapsulation. Sliding along the
intercalated interlayer is suppressed. In addition, MCM-41
100
200
300
400
500
600
700
Temperature/°C
Fig. 7 TG curves of a NR, b NR/APP, c NR/MCAPP, d NR/
M(A&M), and e NR/MCAPP/MCM-41
DMTA
Figures 8 and 9 depict the dynamic mechanical spectra
(dynamic storage modulus and loss factor tan d) as a
function of temperature for the NR and FRNR composites.
There is a remarkable increase in the storage modulus for
123

356
N. Wang et al.
a
b
c
d
NR/APP
NR/MCAPP
NR/M(A&M)
NR/MCAPP/MCM-41
NR
3000
2800
2600
2400
2200
2000
1800
1600
1400
1200
1000
800
c
d
NR/MAPP/MCM-41
NR/APP
NR/M(A&M)
NR/MAPP
b
1000
100
10
a
600
400
–100
–50
0
50
100
0
20
40
60
80
100
Temperature/°C
Strain/%
Fig. 8 Temperature dependence of tan d of NR and FRNR compos-
ites: a NR/APP, b NR/MCAPP, c NR/M(A&M), and d NR/MCAPP/
MCM-41
Fig. 10 Dependence of G’ on strain of NR and FRNR compounds
a
b
c
d
NR/APP
NR/MCAPP
NR/MC(A&M)
NR/MCAPP/MCM-41
Table 4 DG0 values
1.0
0.8
0.6
0.4
0.2
DG0/MPa
a
Sample
d
b
NR
0.827
1.552
1.747
1.969
2.122
c
NR/APP
δ
NR/MCAPP
NR/M(A&M)
NR/MCAPP/MCM-41
0.0
The Payne effect of the compounds decreases in the
order NR/APP (2.122 MPa) [ NR/MCAPP(1.969 M-
–100
–50
0
50
100
Pa) [ NR/APP/MCM-41
(1.747 MPa) [ NR/M(A&M)
Temperature/°C
(1.552 MPa) [ NR (0.827 MPa). Because the G’ value
decreases according to the reduced filler network strength
[23], NR/M(A&M) has the best dispersion compared to
other FRNR composites. Due to the existence of MCM-41,
the dispersion of NR/APP/MCM-41 system is improved.
This phenomenon may be due to the fact that the coupling
or crosslinking between the OS shell materials and NR
makes the double-shell microcapsule get better dispersed
within NR matrix. Furthermore, the special channel struc-
ture of synergistic agent MCM-41 and its physical and
chemical adsorption improve the dispersion of fillers.
Fig. 9 Temperature dependence of storage modulus of NR and
FRNR composites: a NR/APP, b NR/MCAPP, c NR/M(A&M), and d
NR/MCAPP/MCM-41
with the NR matrix such as physical and chemical
adsorption to reduce chain mobility, chain slipping at the
outer surfaces of the aggregates is likely also hampered.
Therefore, the loss maximum is the smallest in case of the
system with the strongest filler matrix (NR/M(A&M)).
RPA
Figures 10 and Table 4 show the dependence of G’ on
strain of NR and FRNR composites to confirm the dis-
persion of fillers in the cured compounds. All the samples
show a typical Payne effect. The G’ decreases dramatically
at strain 1 %, and G’ almost reaches the same value as that
of unfilled rubber at the ultimate strain of 100 %; G’ is
used to indicate the strength of the filler network [22].
Conclusions
Double-shell co-microencapsulated APP and mesoporous
MCM-41 with MF resins and OS were successfully pre-
pared by in situ polymerization. Due to the fact that the
coupling or crosslinking between the OS shell materials
and NR, the M(A&M) shows better compatibility in NR
123

Influence of silicone shell on FRNR
357
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resin and OS as the double shell of the microcapsules
produce a synergism with IFR to form a mighty intumes-
cent char; the result shows that NR/M(A&M) has a
remarkable improvement in both mechanical properties
and flame retardancy. A lowered tan d peak value, an
increased Tg, and the highest storage modulus are obtained
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NR composites by acting as a synergistic agent compared
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(Grant No: 51103086, 51271188 and 51173110), State Key Lab of
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Technology (201304), China Postdoctoral Science Foundation (grant
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