Effect of graphene on the thermophysical properties of melamine-urea-formaldehyde/N-hexadecane microcapsules

Article abstract of DOI:10.1039/c5ra12566a

Heat transfer performance remains a key factor for the potential application of microencapsulated phase change materials (microPCMs). The purpose of this work is to explore the effect of different amounts of graphene added on the thermophysical properties of microPCMs. Microcapsules containing n-hexadecane (C16) with a melamine-urea-formaldehyde (MUF) shell were synthesized by emulsion polymerization. The spectra of Fourier transform infrared spectroscopy (FTIR) confirmed that the MUF shell is fabricated on the surface of C16. Graphene is successfully mixed with microPCMs and the crystal type of microPCMs is not changed, which is detected by X-ray Diffraction (XRD). The results indicated that heat enthalpy of microPCMs gradually decreases with the rising addition of graphene, but the heat transfer performance and thermal stabilities are enhanced. In addition, graphene in microPCMs obviously reduces the degree of supercooling of C16. MicroPCMs with 0.05 wt% added graphene have the best anti-osmosis performance.

Full text of DOI:10.1039/c5ra12566a

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PAPER
Effect of graphene on the thermophysical
properties of melamine-urea-formaldehyde/N-
Cite this: RSC Adv., 2015, 5, 74024
hexadecane microcapsules
ab
Bingyang Wu,ab Guo Zheng
and Xu Chencd
*
Heat transfer performance remains a key factor for the potential application of microencapsulated phase
change materials (microPCMs). The purpose of this work is to explore the effect of different amounts of
graphene added on the thermophysical properties of microPCMs. Microcapsules containing
n-hexadecane (C16) with a melamine-urea-formaldehyde (MUF) shell were synthesized by emulsion
polymerization. The spectra of Fourier transform infrared spectroscopy (FTIR) confirmed that the MUF
shell is fabricated on the surface of C16. Graphene is successfully mixed with microPCMs and the crystal
type of microPCMs is not changed, which is detected by X-ray Diffraction (XRD). The results indicated
that heat enthalpy of microPCMs gradually decreases with the rising addition of graphene, but the heat
transfer performance and thermal stabilities are enhanced. In addition, graphene in microPCMs obviously
reduces the degree of supercooling of C16. MicroPCMs with 0.05 wt% added graphene have the best
anti-osmosis performance.
Received 29th June 2015
Accepted 27th August 2015
DOI: 10.1039/c5ra12566a
www.rsc.org/advances
materials are used in traditional microPCMs. However, the poor
thermal conductivity of organic wall materials lead to the low
Introduction
heat absorbing and releasing efficiency of microPCMs, limiting
the large-scale application of microPCMs in industry.
In recent years, phase change energy storage materials (phase
change materials, PCMs) have become a hot topic in the elds
of energy utilization and the material sciences.1–3 Phase change
energy storage technology efficiently improves the utilization
ratio of energy.4,5 Microencapsulated phase change materials
(microPCMs) have spawned a new research eld by taking
microcapsule technology and applying it to the phase change
materials.6 In addition, microPCMs enhance the utilization
efficiency of phase change materials and broaden its potential
areas of application.7 The microencapsulated phase change
materials can not only reduce the reaction of the phase change
materials and improve the environment to effectively increase
the heat transfer area but can also control the volume of phase
change materials in phase transition.8,9
MicroPCMs are mainly made of core materials (such as
straight-chain alkanes, fatty acids, or composite phase change
materials) and wall materials (such as urea-formaldehyde
resin,10,11 polyurethane12 and melamine-formaldehyde resin,13,14
etc.). Straight-chain alkanes are used as the core material given
that it is non-toxic, non-corrosive, and affordable.15 Organic wall
Graphene is known as the thinnest material in the world,16,17
having a thickness of 0.3354 nm.18 Electrons face little resis-
tance and interference during electron transmission in gra-
phene layers.19,20 The electron mobility of graphene can be
achieved 2 ꢀ 105 cm2 (V s)ꢁ1 and is about 100 times the electron
mobility in silicon with the use of graphene transfering plane
semiconductor operations.21,22 In addition, the tensile modulus
or Young's modulus (1100 GPa) and the ultimate strength (116
GPa) of graphene match single-walled carbon nanotubes.23
Graphene is light in weight, with good thermal conductivity and
contains a large specic surface area.24 Thus, graphene has
broad applications due to its excellent properties, and it has
broad potential applications in biomedicines,25–28 solar cell,
sensor, nanoelectronics, high-performance electronic devices,
crystallization behavior, eld emission materials, gas sensors
and energy storage.29,30 However, to the best of the author's
knowledge, very little work has been done on the application of
graphene to microPCMs. Toward this end, microcapsules con-
taining n-hexadecane (C16) with melamine-urea-formaldehyde
(MUF) shell were synthesized by emulsion polymerization.
The surface morphologies of microPCMs were characterized
using a eld-emission scanning electron microscope (FE-SEM)
and optical microscope (BA2000). The crystallographic behav-
iours of microPCMs were obtained using X-ray diffraction
(XRD). The heat transfer performance was measured using the
Hot Disk Thermal Constant Analysers. The thermal properties
aSchool of Material Science and Engineering, Tianjin Polytechnic University, Tianjin,
China. E-mail: zhengguo0703@126.com
bTianjin Engineering Research Center of Textile Fiber Interface Treatment Technology,
Tianjin, China
cKey Laboratory of Advanced Textile Composites, Ministry of Education, Tianjin
Polytechnic University, Tianjin, China
dSchool of Textiles, Tianjin Polytechnic University, Tianjin, China
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of microPCMs were investigated by differential scanning calo- melamine-formaldehyde (MF) prepolymer solution was
rimetry (DSC) and thermal gravimetric analysis (TGA).
obtained. This mixture was subsequently cooled down to room
temperature (RT ¼ 25 ꢂC). Urea was added into the MF pre-
polymer solution until complete dissolution, which was
melamine-urea-formaldehyde (MUF) pre-polymer solution.
Graphene was dispersed in an aqueous solution of SDS and PVA
in a certain proportion. The solution was mildly sonicated for 30
min using an ultrasonic cleaner to obtain stable graphene
suspension. Herein the SDS functioned as the surfactant
reducing the surface tension of the aqueous solution and
assisting in graphene dispersion; the PVA functioned as a
stabilizer, inhibiting the reunion of graphene in the solution. By
adding C16 and graphene suspension to the beaker of the MUF
pre-polymer solution, the reaction solution was dispersed by the
XHF-D type high speed disperser for 20 min to a form stable oil-
in-water (O/W) emulsion. Subsequently, the reaction beaker was
removed into the 80 ꢂC water bath and allowed to react for 4 h.
The obtained suspension solution of microPCMs was washed
with distilled water to remove unreacted polymers and C16 aer
the suspension was placed for 24 h. Finally, microPCMs were
obtained with vacuum ltration. Reaction conditions were
shown in Table 1; the schematic fabrication of the microPCMs
and the fabricated process of the microcapsule shell at the
surface of C16 droplets was shown in Fig. 1.
Experimental
Materials
N-Hexadecane (C16) used as core material was purchased from
Alfa Aesar, USA. Melamine (M) (Sigma-Aldrich, USA), urea (U)
(Sigma-Aldrich, USA) and formaldehyde solution, 37 wt% in
H2O (F) (Sigma-Aldrich, USA) were used as shell materials.
Sodium dodecyl sulfate (SDS) and polyvinyl alcohol (PVA) used
as emulsifying agents were obtained from Guangzhou Tehsun
Chemical Technology Co., Ltd, China. Graphene (thickness:
1.0–1.77 nm; layers: 1–5; specic area: 360–450 m2 gꢁ1) was
obtained from Suzhou Hengqiu Graphene Science and Tech-
nology Co., Ltd, China.
Preparation of the MicroC16
MicroC16 were synthesized by emulsion polymerization. A
mixture of melamine and formaldehyde aqueous solution with
distilled water was heated at 70 ꢂC for 20 min until a clear
Table 1 Reaction conditions
Characterization
Reagent (wt%)
MicroC16
MicroC161
MicroC162
MicroC163
Fourier transformed infrared (FTIR) spectra of C16 and
microPCMs were obtained using an FTIR spectroscopy
(TENSOR37 Bruker, Germany) in the range of 4000–500 cmꢁ1 at
room temperature. The surface morphologies of microPCMs
containing different amounts of graphene added were charac-
terized using a eld-emission scanning electron microscope
(FE-SEM, S-4800 Hitachi, Japan) and optical microscope
(BA2000) obtained by Chongqing Photoelectric Instrument Co.,
Graphene
C16
Melamine
Formaldehyde
Urea
PVA
SDS
Distilled water
—
0.05
7.6
2.4
4.5
0.8
0.4
1.2
82.05
0.1
7.6
2.4
4.5
0.8
0.4
1.2
82.0
0.2
7.6
2.4
4.5
0.8
0.4
1.2
81.9
7.6
2.4
4.5
0.8
0.4
1.2
83.1
Fig. 1 Schematic formation of the graphene microPCMs based on n-hexadecane core and MUF resin shell by emulsion polymerization.
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Ltd, China. The diameters of the microcapsules were measured
on the OM micrographs, and more than 200 microcapsules were
counted. The diffraction patterns of C16, graphene and
microPCMs were obtained using an X-ray diffractometer (XRD,
D8 Bruker, Germany, 40 kV, 150 mA) at 0 ꢂC, with the scanning
range at 5–40ꢂ. The thermal conductivity coefficient and
thermal diffusivity of microPCMs containing different amounts
of graphene added were measured by using the Hot Disk
Thermal Constant Analysers (TPS-2500S Hot Disk, Sweden) at
room temperature. The phase change properties of the C16 and
microPCMs were measured using a Differential Scanning
Calorimetry (DSC, 200-F3 NETZSCH, Germany) in the range of
ꢁ10–60 ꢂC at a rate of ꢃ10 ꢂC minꢁ1 in a nitrogen atmosphere.
Thermal stability of C16 and the prepared microPCMs with
different amounts of graphene added were investigated using a
thermogravimetric analyzer (TGA, STA449F3 NETZSCH, Ger-
many) at a scanning rate of 10 ꢂC minꢁ1 in the range of 25–
600 ꢂC in a nitrogen atmosphere. The anti-osmosis measure-
ments of microPCMs were characterized in 0.96 g mlꢁ1 density
ethyl alcohol by means of a 723PC spectrophotometer obtained
by Shanghai Cany Precision Instrument Co., Ltd, China.
Fig. 2 FTIR spectra of C16 and microPCMs: (a) C16; (b) graphene
microPCMs; (c) pure microPCMs.
micrographs of microPCMs (Fig. 3(e)–(h)) showed that the
surfaces of microPCMs without graphene were relatively rough
and adhered to many irregular MUF polymers particles. More-
over, microPCMs surfaces were increasingly smooth and irreg-
ular particles were signicantly reduced with the rising addition
of graphene. The reason is that graphene in solution hinders
the self-aggregation of MUF pre-polymers in the reaction solu-
tion, thus forming smooth surfaces of microPCMs. However,
the larger autopolymer particles on the surface of microPCMs
were observed as the amount of graphene added continued to
increase to 0.2 wt%. This is because the graphene adhered to
each other, providing a nucleating agent for MUF pre-polymers
when the graphene concentration in the solution reached a
certain level. This led to polymerization and precipitation of
pre-polymers in the solution.
In addition, C16 was solidied and exothermic when the
environmental temperature was below the freezing point of C16,
accompanied by obvious microPCMs volume change. This
change made the microPCMs wall unsmooth. This phenomenon
explained that the C16 were successfully encapsulated into the
microPCMs, and the microPCMs shell had excellent elasticity
and toughness. Therefore, it would not burst due to the core
material phase change. Particle size distributions of microPCMs
are shown in Fig. 3(i)–(l). The particle size distribution of
MicroC16 is relatively concentrated and the average particle size
is 9.85 mm. Particle size distribution of microPCMs with the
rising addition of graphene is broadened and the average
particle size decreases slightly. Taken together, the morphology
of microPCMs is the most ideal and particle size distribution is
relatively uniform with 0.05–0.1 wt% graphene addition.
Results and discussion
Chemical characterization
The FTIR spectrums of C16 and microPCMs are shown in
Fig. 2(a). This gure shows that the peaks at 2910 and
2850 cmꢁ1 present at –CH2 stretching vibration. The peaks at
1469 cmꢁ1 present –CH3 asymmetric bending vibration. The
peak at 2910 and 2850 cmꢁ1 are not obvious in the curve (c) of
Fig. 2, compared with curve (a). This reects that the coating
effect of microPCMs for the core material is excellent. A wide
and strong absorption peak is observed at 3404 cmꢁ1, which is
formed with the N–H and O–H stretching vibration peak
superposition (combination). The peak at 1562 cmꢁ1, and the
peak at 813 cmꢁ1 collectively reects the triazine ring bending
characteristic and presents the C]N stretching vibration. The
C–H stretching vibration peak is found at 1342 cmꢁ1. The peak
at 1161 cmꢁ1 represents the C–O–C stretching vibration. Alco-
hols C–O stretching vibration peak appears near the 1015 cmꢁ1
.
As seen in the curve (b) of Fig. 3, the peaks at 1620 and 1440
cmꢁ1 present C]C stretching vibration, and C]C is formed by
the reduced graphene oxide. The characteristic peak in the
curve (d) and (c) of Fig. 2 is not found near the 1720 cmꢁ1, which
indicates an absence of C]O in microPCMs. The results of
FTIR show that the ether bond is formed by a polycondensation
reaction of methylol melamine in prepolymer, which illustrates
the formation of cross-linked prepolymer.
Morphology of microPCMs
Crystallography
Optical and FE-SEM micrographs of MicroC16, MicroC161,
MicroC162 and MicroC163 are shown in Fig. 3. As shown in The X-ray diffraction patterns of C16, graphene, MicroC16,
Fig. 3(a)–(d), microPCMs are spherical and have uniform size. It MicroC161, MicroC162 and MicroC163 are shown in Fig. 4. Both
can be clearly seen that the graphene is successfully coated in the C16, graphene, and microPCMs are highly crystallized,
microPCMs. In addition, the content of graphene in microPCMs given that the MUF copolymer shell of microPCMs is amor-
increased with the rising addition of graphene. FE-SEM phous,31 it is convenient to analyze the crystallographic forms of
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Fig. 3 Optical micrographs, FE-SEM images and particle size distribution of (a, e and i) MicroC16, (b, f and j) MicroC161, (c, g and k) MicroC162 and
(d, h and l) MicroC163.
encapsulated C16. The XRD pattern of C16 shows four charac- inhibits the motion of the C16 chains to some extent, resulting
teristic peaks of triclinic (010), (011), (100), and (111), as in a decrease of the crystallinity. The crystallization peak posi-
observed in the pattern of all samples at 19.2ꢂ, 19.6ꢂ, 23.3ꢂ and tion of MicroC161, MicroC162 and MicroC163, compared with
24.7ꢂ, which are the characteristic peaks of C16, respectively.32 MicroC16, does not indicate migration. This demonstrates that
XRD pattern shows that there are characteristic peaks of gra- the addition of graphene has no remarkable impact on the
phene at 2q of 26.6ꢂ in XRD curve of graphene, MicroC161, crystal type of microPCMs.
MicroC162 and MicroC163, indicating that graphene is mixed
with the microPCMs. The crystal structure of the C16 in the
microPCMs is the same as that of the C16. Nevertheless, the
intensities in the diffractions patterns at the low 2q (5 to 15ꢂ)
Heat transfer performance
Fig. 5 illustrates that the heat transfer performance of
decreased dramatically aer the microencapsulation. These
ndings indicate that the small space of the microPCMs
microPCMs with different amounts of graphene added at room
temperature. The thermal conductivity coefficient and thermal
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Fig.
4 XRD patterns of C16, graphene, MicroC16, MicroC161,
MicroC162 and MicroC163.
Fig.
5
Heat transfer performance of microPCMs with different
amounts of graphene added.
diffusivity of microPCMs are dramatically increased with the
rising addition of graphene, since the graphene has excellent
thermal conductivity properties and can effectively reduce the
thermal resistance of wall material, thus enhance the heat
transfer performance of microPCMs. However, the change rate
(K) of thermal conductivity and thermal diffusivity of
microPCMs are reduced ceaselessly, namely, that the heat
transfer property liing range of microPCMs decreases gradu-
ally with the increasing graphene added. The explanation for
this is that the excessive graphene is free in solution and cannot
be completely mixed into microPCMs with the high concen-
tration of graphene in the solution. Thermal conductivity
coefficient and thermal diffusivity of microPCMs are 0.053 W
mꢁ1 Kꢁ1 and 0.238 m2 sꢁ1, respectively. The coefficient of
thermal conductivity and thermal diffusivity of microPCMs
171.23 and 105.08, respectively, which indicates that the two
models have high tting precision. The value of Sig. is less than
0.05 at the level of 95% condence interval, which shows that
addition of graphene is signicantly related to the thermal
diffusivity and thermal conductivity of graphene.
Phase change property
The phase change properties of the C16, MicroC16, MicroC161,
MicroC162 and MicroC163 are investigated by DSC, and their
thermograms are displayed in Fig. 6. The melting and crystal-
lization properties of all the samples are obtained from DSC
analysis and summarized in Table 3. As can be seen from
Fig. 6(a), there is a signicant decrease in enthalpy of the
prepared microPCMs with different amounts of graphene
added in comparison to those of the C16. This is likely due to
the phase change behaviors conned by the inner space of the
microPCMs. It is also notable that the crystallizing behavior of
microPCMs varies with the rising addition of graphene. For
MicroC16, two crystallization peaks, a and b are observed from
the DSC cooling thermograms. This is because the microen-
capsulation of phase change materials is coated in a small
space, the crystallization converts into homogeneous crystalli-
zation, and the degree of supercooling increases. Furthermore,
the phase change heat can not be released at once, and the
crystallization process must be nished by releasing heat
multiple many times. The peak a decreases gradually and its
shoulder shis towards higher temperature with the rising
addition of graphene because of the excellent thermal perfor-
mance of graphene. Dispersed graphenes are added into
adding 0.1 wt% graphene are 0.154 W mꢁ1 Kꢁ1 and 0.56 m2 sꢁ1
.
The coefficient of thermal conductivity and thermal diffusivity
of graphene microPCMs, compared with pure microPCMs, are
increased by 191% and 160%, respectively. Thus, it can be seen
that adding a small amount of graphene can remarkably
improve the heat transfer performance of microPCMs.
Empirical models of addition, thermal diffusivity and coef-
cient of thermal conductivity of graphene can be established,
respectively. It can be more intuitive performance quantitative
relationship of the two through the model. We use the quadratic
function model to approximate the functional relationship
between them:
y ¼ K + a1x + a2x2, x ˛ (0, +N)
(1)
where y is dependent variable, x is addition of graphene, K is
constant term, a1 and a2 are control coefficient, the following
functions are obtained by curve tting:
Table 2 Model summary and parameter estimates
y ¼ 0.28 + 2.42x ꢁ 3.76x2, x ˛ (0, +N)
y ¼ 0.09 + 1.04x ꢁ 0.43x2, x ˛ (0, +N)
(2)
(3)
Model summary
Parameter estimates
Fitting parameter R Square
F
Sig. Constant a1
a2
As shown in eqn (2) and (3), y is positively correlated with x
when x is small. y is negatively correlated with x when x is small.
As shown in Table 2, the F test values of two tting results are
Diffusivity
Conductivity
0.99
0.99
171.23 0.02 0.28
105.08 0.03 0.09
2.42 ꢁ3.76
1.04 ꢁ0.43
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Table
3
Phase change properties of C16 and microPCMs with
microPCMs wall and core material with poor thermal conduc-
tivity to accelerate the phase change heat transfer. In addition,
the graphene in the microPCMs plays the function of the
nucleating agent during the phase change process of the core
material so as to make the crystallization of internal core
material change from homogeneous crystallization to the
heterogeneous crystallization.
The phase change property of microPCMs can be described
with three parameters: encapsulated efficiency (h) and phase
change temperature Tc and Tm, which can be determined by
means of DSC analysis. The enthalpies of microPCMs from
solid–liquid phase change are important physical properties
affecting the working effect of the microPCMs, which strongly
depends on the encapsulated efficiency of PCMs. Moreover, h
can be calculated by the enthalpies obtained from DSC curves,
based on the formulation given below:
different amounts of graphene added
Tc (ꢂC)
Sample
Tm (ꢂC) DHm (J gꢁ1
)
a
b
DHc (J gꢁ1
)
h (%)
C16
MicroC16
MicroC161 15.9
MicroC162 15.8
MicroC163 14.6
15.2
14.7
199.6
167.4
154.2
136.5
104.7
—
7.7
—
198.4
—
2.1 169.8
84.7
77.8
69.1
52.2
10.7 6.4 155.3
—
—
—
—
138.3
103.6
because of further thermal decomposition of microPCMs wall.
Weight loss of C16 in microPCMs has obvious delay, compared
with pure C16, which shows that the capsule wall block vola-
tilization of C16. Consequently the thermal stabiꢂlity of
microPCMs is improved. As can be seen from 250–300 C and
450–500 ꢂC of Fig. 7(b), the mass attenuation rate of microPCMs
slightly declines and the thermal stability enhances slightly
with the rising addition of graphene, which is because graphene
has good heat transfer performance and makes the heat
distribution in the microPCMs more uniform, thereby
improving the thermal stability of microPCMs.
DHm;microPCMs þ DHc;microPCMs
h ¼
(4)
DHm;PCMs þ DHc;PCMs
where h is the microencapsulation ratio of C16; DHm,microPCMs
and DHc,microPCMs are melting enthalpy and crystallization
enthalpy of microPCMs; DHm,PCMs and DHc,PCMs are melting
enthalpy and crystallization enthalpy of n-alkane mixture,
respectively. The results indicated that the thermal properties of
the as-prepared microPCMs were satisfactory for energy storage
applications.
Anti-osmosis property
The anti-osmosis property is an important feature in the
application of microPCMs. Anti-osmosis measurement was
used to evaluate the sealing performance of microPCMs wall.
Thermal stability
Thermogravimetry (TG) diagrams and differential thermal Fig. 8 shows the release rates of microPCMs prepared by using
gravity (DTG) were shown in Fig. 7(a) and (b). As can be seen graphene, indicating that there is a signicant inuence of the
from Fig. 7(a), the C16 starts to lose weight at about 145 ꢂC and graphene content on microPCMs release behaviors. It can be
completes at about 270 ꢂC. The weight losing processes of seen that the anti-osmosis property of Sample MicroC161 is
microencapsules contains three steps. The rst step, the weight optimal, since a small amount of graphene added can reduce
loss of microPCMs occurs at 70 ꢂC, which is due to the fact that auto-polymerization of the MUF pre-polymers in the reaction
residual moisture in microPCMs is lost through drying. The solution to make the microcapsule wall more smooth and rm.
second step is from 210 to 230 ꢂC, which is caused by the However, the anti-osmosis property of microcapsule is poor
decomposition of C16. The third step is from 430 to 460 ꢂC with the rising addition of graphene. Osmosis rate of MicroC163
Fig. 6 DSC thermograms of C16(1), MicroC16(2), MicroC161(3), MicroC162(4) and MicroC163(5): (a) heating thermograms, and (b) cooling
thermograms.
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Fig. 7 TG spectra of C16, MicroC16, MicroC161, MicroC162 and MicroC163: (a) TG and (b) DTG curves.
microPCMs is similar to that of the C16. In addition, the heat
transfer performance, thermal stabilities and anti-osmosis
performance of microPCMs have the most remarkable
improvement. In summary, the addition of graphene endowed
microPCMs with enhanced features and expanded the potential
elds of application and scope of microPCMs. With the
improvement of the graphene preparation technology and the
growth of production, graphene microPCMs will play a more
important role in the future.
Acknowledgements
The authors would especially like to thank Tianjin Research
Program of Application Foundation and Advanced Technology
(NO. 15JCZDJC38400) and the National Natural Science Foun-
dation of China (NO. 51303131).
Fig.
8 Release curves of MicroC16, MicroC161, MicroC162 and
MicroC163.
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