The Cure and Thermal Behaviour of Mg(OH)2/Epoxy Resin/DEEA Nanocomposites Prepared by a Novel Method
The Cure and Thermal Behaviour of Mg(OH)2/Epoxy Resin/DEEA
Nanocomposites Prepared by a Novel Method
Brief Communication
Cheng Yiyun1,2, He Pingsheng1* and Cui Ronghui1
Department of Polymer Science and Engineering, University of Science and Technology of China,
Hefei, Anhui 230026, People’s Republic of China
2
School of Life Science, University of Science and Technology of China, Hefei, Anhui 230027,
People’s Republic of China
1
Received: 27 January 2005 Accepted: 12 April 2005
INTRODUCTION
Polymeric-inorganic nanocomposites are a class
of composites in which the inorganic filler phase
dimensions are of the order of nanometres1. They
contain only a few percent of well-dispersed
inorganic components of nanometre size dispersed
in an organic polymer, but they exhibit better
properties than pure polymers. Magnesium
hydroxide Mg(OH)2 is a smoke-reducing and nontoxic additive that has been extensively used in
halogen-free flame-retardant polymeric materials.
However, its low flame-retardant efficiency and
the very high loadings that are consequently
required when used in its ordinary form lower the
mechanical properties of a flame-retarded polymeric
composition sharply2. It has been reported that
nanosized Mg(OH)2/polymeric composites have
the potential to solve the above problems because
of the mechanically reinforcing and flame-retardant
functions of nanosized composite materials3.
We have already presented a novel, simple
method to prepare Mg(OH)2/epoxy resin/DEEA
nanocomposites4. However, no details about their
characteristics and potential applications were
mentioned. In this present study, we investigate
their cure and their thermal behaviour.
EXPERIMENTAL
Materials
The diglycidyl ether of bis-phenyl A, epoxy resin
E51 with an epoxy value 0.48-0.54 and an average
epoxy equivalent of 196, was obtained from the
Shanghai Resin Factory, China. Diethylenetriamine
(DEEA, C.R. grade), produced by the Development
Center for Special Chemical Agents in the Huabei
Region, was used as the curing agent. Methanol,
acetone, magnesium acetate and sodium hydroxide
were all A.R. grades and were used without further
treatment.
Preparation of Mg(OH)2/Epoxy Resin
Nanocomposite
Magnesium acetate Mg(Ac)2 (1.0 g) was dissolved
in 8 mL methanol, and 5 g epoxy resin E51 together
with 10 mL acetone were added to the solution with
agitation. Then, the mixed solution was added slowly
to an aqueous solution of NaOH (2 g in 100 mL),
again with agitation. An emulsion was formed.
Epoxy resin droplets began to precipitate because the
Mg(OH)2 particles formed in the epoxy resin matrix
had a high density. The product was vacuum-filtered
and washed several times with water, and dried in
an oven at 70 °C for 2 h. It was necessary to agitate
the materials every 10 min during drying. Finally, a
homogeneous viscous liquid was obtained.
Isothermal Cure Experiment
*Author to whom correspondence should be addressed.
Email: [email protected]
Polymers & Polymer Composites, Vol. 13, No. 8, 2005
The uncured nanocomposite and the DEEA were
mixed in the stoichiometric ratio of 100/15 (by
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Cheng Yiyun, He Pingsheng and Cui Ronghui
weight). The resulting ointment-like composite
was cured by HLX-I Resin Curemeter (a homemade experimental equipment)5 at 90, 100, 110
and 120 °C.
Dynamic Torsional Vibration Method
(DTVM)
Dynamic torsional vibration is a non-resonant
forced vibration5-7. A schematic representation
of a home-made experimental setup, the “HLXI Resin Curemeter”, is shown in Figure 1. The
lower mould 3, containing a heater, was filled with
resin and used as the torsional vibrator. When the
motor 6 was switched on, the upper mould 2, also
containing a heater, was moved down to approach
the mould 3, leaving a small adjustable gap. During
the isothermal cure, the temperature was controlled
by thermistors. As soon as the upper mould 2 and
the lower mould 3 were closed to each other, the
motor 5 was turned on and the lower mould 3 started
a torsional vibration with a frequency of 0.05 Hz
at an angle below 1°. The torsional vibration could
be changed according to the hardness of the cured
resin material by adjusting the eccentric disc 4 on
the speed-change gear 7. The torque amplitude of
the torsional vibration was transformed into an
electrical signal by the strain gauge load cell 1,
amplified through the amplifier 10 and recorded
by the recorder 11.
The resin systems have different properties at different
degrees of cure, e.g. torque, viscosity, modulus etc.
The degree of cure of the resin system was monitored
and determined by measuring the change in torque.
A typical experimental curve obtained by DTWM as
shown in Figure 2. The torque required to turn the
resin system by a small angle, which corresponds
to the modulus or viscosity of the resin system, was
chosen as a relative parameter to characterize the
degree of cure. The time of closure of the moulds
was defined as the start of cure. When the curing
time was in the range OA, the network structure
formed during the cross-linking reaction was not
able to cause a forced vibration of the upper mould.
As a result, the strain gauge load cell did not register
any signal to input. At point A, the viscosity of the
resin system was high enough (i.e. the degree of cure
was high enough) for gelation to occur in the resin
system. Thus, OA was defined as the gel time, tg, for
the resin system. After point A, the torque increased
with increasing cure time. The increasing amplitude
of the torque reflected the rate of the curing reaction.
With further increasing cure times, the increase in
torque became smaller and smaller until finally the
equilibrium torque G∞, was reached at point C, the
curing reaction was complete as far as was possible at
that temperature. Since the “cup-like” experimental
curve was symmetric with respect to the time axis, for
convenience we were able just to take the upper-half
of the Figure to analyze the cure process.
Figure 1. Schematic representation of the dynamic torsional vibration apparatus
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Polymers & Polymer Composites, Vol. 13, No. 8, 2005
The Cure and Thermal Behaviour of Mg(OH)2/Epoxy Resin/DEEA Nanocomposites Prepared by a Novel Method
Figure 2. Analysis of isothermal cure curve
Figure 3. The cure curves of E51/DEEA and Mg(OH)2/
E51/DEEA nanocomposite
Thermogravimetric Analysis
Thermogravimetric analysis (TGA) was carried
out using a TGA instrument (Shimadzu TGA50H) in air at a heating rate of 10 °C/min. Thermal
decomposition of each sample was performed over
a temperature range of 20-800 °C. Residual weight
of the samples as a function of temperature was
recorded.
Figure 4. Cure curves of Mg(OH) 2 /E51/DEEA
nanocomposites at various temperatures
RESULTS AND DISCUSSION
Cure Curves of the Nanocomposite
Figure 3 shows the isothermal cure curves of the
E51 and the Mg(OH)2/E51composite systems using
DEEA as the curing agent. The addition of Mg(OH)2
nano-particles reduced the gelation times tg of the
composite. Figure 4 shows the isothermal cure curves
of the Mg(OH)2/E51composites measured at 90, 100,
110 and 120 °C, respectively. Obvious differences
in gelation times, curing rates and maximum
torque were observed at different temperatures.
The gelation time decreased and the curing rate
accelerated with increasing temperature.
Analysis of Cure Curves by Flory’s Gelation
Theory
According to Flory’s gelation theory8, the chemical
conversion at the gel point is constant and is
not related to the reaction temperature or other
experimental conditions. As a result, the apparent
activation energy of the cure reaction, Ea, can
be calculated from the gel time, tg, by using the
following equation:
ln tg =C + Ea /RT
(1)
Polymers & Polymer Composites, Vol. 13, No. 8, 2005
where T is the curing temperature, R is the gas
constant, and C is a constant. Figure 5 shows a plot
of ln tg versus 1/T. The apparent activation energy
Ea (calculated from the slope of the line) was about
82.39 KJ/mol.
TGA Analysis of the Nanocomposite
Typical weight loss for the E51/DEEA composite
and Mg(OH)2/E51/DEEA nanocomposite in an air
atmosphere was measured by TGA and is shown in
Figure 6. A double-stage decomposition was found
for both the resin and the nanocomposite (the first
weight loss for the nanocomposite just below 100 °C
was attributed to loss of water). The degradation
trends in air were similar for both materials. This
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Cheng Yiyun, He Pingsheng and Cui Ronghui
Figure 5. Plot of lntg versus 1/T for nanocomposite
system
Figure 6. The TGA curves of E51/DEEA and Mg(OH)2/
E51/DEEA nanocomposite
is reasonable since the thermal weight loss of
the nanocomposite was from the organic epoxy
resin part. However, the residues left behind after
decomposition were different. While it was only
about 2.5% in the case of E51/DEEA, it was 6.7%
for the nanocomposite in air. Furthermore, the
onset temperature of the main degradation process
was higher by 23 °C for the nanocomposite. It was
deduced that the presence of Mg(OH)2 nano-particles
enhanced the thermal stability of the resin.
REFERENCES
For those Mg(OH)2/E51/DEEA nanocomposite with
good compatibility, it was not only the difference in
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In addition, the enhanced thermal stability is partly
attributable to the prevention of out-diffusion of
the volatile gas from the thermally decomposed
products because Mg(OH)2 nano-particles, when
well dispersed in epoxy networks, act as gas
barriers, reducing the permeability of the volatile
gas10. Therefore, the dispersion of Mg(OH)2 nanoparticles effectively enhaces the thermal stability
of the nanocomposite.
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ACKNOWLEDGEMENTS
This work was financially supported by the
National Natural Science Foundation of China
(No.20174038).
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