A Green and Structure-Controlled Approach to the Generation of Silicone Rubber Foams by Means of
Carbon Dioxide
A Green and Structure-Controlled
Approach to the Generation of
Silicone Rubber Foams by Means of
Carbon Dioxide
Hao Xua, Yunchuan Hea, Xia Liaoa,*, Tinggang Luob, Guangxian Lia,
Qi Yanga, and Chuanjian Zhouc,*
aCollege
of Polymer Science and Engineering, State Key Laboratory of Polymer
Materials Engineering, Sichuan University, Chengdu, Sichuan 610065, China
bChina Bluestar Chengrand Research Institute of Chemistry Industry Co. Ltd,
Chengdu, Sichuan 610065, China
cKey Laboratory of Special Functional Aggregated Materials, Ministry of Education,
Shandong University, Jinan, Shandong 250100, China
Received: 23 March 2005, Accepted: 10 August 2015
Summary
Silicone rubber foams were successfully generated by environmentally friendly
blowing agent, supercritical carbon dioxide (scCO2), in this research. Firstly,
the effect of the saturation time on the cellular structure was investigated. The
diffusion of scCO2 into the rubber matrix would be enhanced thus decreasing
the viscosity as increasing the saturation time. It would further promote the cell
growth, which has a close connection with the cellular structure. After that, the
effect of pre-curing time on cellular morphology of silicone rubber foams was
further researched in detail. When increasing pre-curing time in the short time
range, cell nucleation would be affected more than cell growth in the foaming
process. If continuously increasing pre-curing time, both cell nucleation and
growth would be restricted thus resulting in the formation of silicone rubber
foams with small cell density and small cell size. This investigation not only
provided a green way to produce silicone rubber foams, but also guided us to
control cellular morphology via the saturation time and pre-curing time.
Keywords: Silicone rubber foam; Supercritical carbon dioxide; Saturation time; Open
cell; Pre-curing time; Cellular structure
*Corresponding authors: Xia Liao, Tel.:+86-28-8540 8361; E-mail: [email protected]
Chuanjian Zhou, Tel.:+86-531-81696515; E-mail: [email protected]
©Smithers
Information Ltd. 2016
Cellular Polymers, Vol. 35, No. 1, 2016
19
Hao Xu, Yunchuan He, Xia Liao, Tinggang Luo, Guangxian Li, Qi Yang, and Chuanjian Zhou
Introduction
Silicone rubbers are synthetic polymers based on polyorganosiloxanes with high
molecular weight. Their basic backbone is the silicon-oxygen (Si-O) bond and
organic groups attached directly to the silicon atom via silicon-carbon (Si-C)
bonds [1]. As a result of this special structure, silicone rubbers exhibit a list of
superior performance including superior temperature and chemical resistance,
good electrical insulation, excellent ozone resistance and biocompatibility,
etc. [2]. These characteristics make the silicone rubber widely applied in many
areas, such as cure oven, mechanical shock absorbers in the aerospace
industry [3-5]. Silicone rubber foams combine the characteristics of silicone
rubber and foam materials, for example, a light weight, good resilience, high
thermal stability, shape conformance and low compression set [4]. They also
provide enhanced temperature range suitability, which offers a wider operating
temperature range compared to other organic rubber foams [6]. Hence, silicone
rubber foams have been widely used in challenging applications ranging from
joint sealants, insulators, aircraft and transportation industry, to biomaterials
for wound dressings [3].
The foaming process of silicone rubbers usually bases on the expansion
of gaseous phase dispersed through the rubber matrix. Solvent foaming
methods and chemical foaming methods are mostly used ways in generating
silicone rubber foams [3, 5]. Silicone rubber compounds are mixed with an
inert component in solvent foaming methods, which can be dissolved in a
specific solvent. In a result, this kind method could cost more blowing agents
and need a time-consumed washing process leading to the low production
efficiency [7]. In chemical foaming methods, foaming agents usually consist
of an organic and inorganic thermally unstable component, which would
decompose to gas components upon heating above a certain temperature
[7]. The products in the decomposition are harmful to the environment and
this method can lead to generating solid residue and sublimate, which
remain in the rubber matrix. In addition, the cell size is nonuniform and it
is too hard to control the cellular structure of silicone rubber foams via the
above two methods.
In recent years, supercritical carbon dioxide (scCO2) has been broadly
investigated in the production of microcellular foams because of its many
unique properties [8, 9]. One advantage is that foams generated by scCO2
own uniform cell size which even can be controlled by adjusting experiment
parameters [10]. Furthermore, scCO2, as a green physical blowing agent, also
avoids environmental problems generated by chemical blowing agents [10, 11].
To date, most studies on high pressure CO2 polymer foaming have focused
on plastic polymers and very little attention has been paid to crosslinked
resins such as rubbers [12, 13]. During the foaming of rubbers, viscoelastic
20
Cellular Polymers, Vol. 35, No. 1, 2016
A Green and Structure-Controlled Approach to the Generation of Silicone Rubber Foams by Means of
Carbon Dioxide
properties of elastomer matrix have a great effect on the saturation process.
Comparing with chemical foaming technology, the simultaneously control
of foaming and vulcanizing is difficult in scCO2 foaming, because these
two processes progress separately [14]. It also increases the complexity of
cellular structure control comparing with the foaming of plastic polymers by
scCO2. In this study, the effect of the saturation time and pre-curing time on
the cellular structure (cell density and cell size) of silicone rubber foams has
been figured out in detail. On the other hand, open porous silicone rubber
foams were successfully generated by the scCO2 foaming technology. All
these investigations could guide us to control the cellular structure of silicone
rubber foams more effectively in the manufacture process.
Experimental
Preparation of Silicone Rubber Foams
Polymethylvinylsiloxane (PMVS) with 0.15-0.18% vinyl was supplied by
China Bluestar Chengrand Research & Design Institute of Chemical Industry.
The molecular weight of the silicone rubber was about 500,000-700,000.
Precipitated silica (921, specific surface area = 150 m2/g) was obtained from
Nanchang Nanji Chemical Industry Co. Limited.
PMVS, precipitated silica and hydroxyl silicone oil were mixed by the internal
mixer at 100oC for about 30 min. After cooling to room temperature, silicone
rubber was finally formulated with the addition of dicumyl peroxide (DCP) at
ambient temperature for 15 min. Pre-curing specimens with a 2 mm thickness
were prepared by compression molding at 120oC for different time. Then disk
specimens were placed in an autoclave linked to a CO2 cylinder and they were
saturated with CO2 at a specific saturation temperature and pressure. After
the saturation, the high pressure CO2 was rapidly released to atmosphere
conditions. Finally foamed sheets were quickly placed into a heated oven for
a complete post-curing.
Cell Structure Observation
The foamed samples’ morphology was observed with a Quanta 250
(FEI Company, America) scanning electron microscope (SEM). First, the
samples were freeze-fractured in liquid nitrogen and then sputter-coated
with gold. The cell size and its distribution were determined from the SEM
micrographs.
Cellular Polymers, Vol. 35, No. 1, 2016
21
Hao Xu, Yunchuan He, Xia Liao, Tinggang Luo, Guangxian Li, Qi Yang, and Chuanjian Zhou
Statistics of Cell Density (No), Average Cell Diameter and Open
Cell Content
No is the cell density defined as the number of bubbles per cubic centimeter
of the non-foamed silicone rubber and is given as follows [15]:
No = (nM 2/A) 3/2·1/(1－Vf) (1)
Vf = 1－ρ/ρf (2)
where n is the number of bubbles in the micrograph of the foam sample, A
is the area of the micrograph (cm2), Vf is the volume fraction occupied by the
voids, ρ is the density of silicone rubber, ρf is the density of silicone rubber
foams and M is the magnification factor of the micrograph. Average cell
diameter was determined from SEM micrographs, and it was analyzed by
software. The image analysis software (Image-Pro Plus) was used to determine
cell diameter and count the number of cells in the SEM image. For a single
asymmetric cell, the average length of 90 diameters passing through cell’s
centroid was regarded as the cell diameter. The angle between neighboring
diameters was 2 degree. All cells in the SEM images were used to calculate the
average cell diameter. The open cell content was measured by ULTRAFOAM
1000 (Quantachrome Corporation, America).
Rheology Measurements
Prepared disk-shaped samples were used to measure the dynamic shear
rheological properties. Dynamic frequency sweep tests of silicone rubber with
various pre-curing time were carried out using a strain-controlled Rheometric
Scientific ARES rheometer (TA Instruments, USA). The strain amplitude was set
at 1%, large enough to give a reliable signal while keeping the measurement
in the linear viscoelastic regime. The rheological functions, such as complex
viscosity (η*), storage (G′) and loss modulus (G′′) were measured in an angle
frequency range of 0.02 to 100 rad/s.
Modifying the geometry of the commercially available high-pressure
rheometer (MCR-102, Anton Paar) to the parallel plate, we measured the
complex viscosity (η*) of the silicone rubber compounds in a high pressure
CO2 atmosphere.
22
Cellular Polymers, Vol. 35, No. 1, 2016
A Green and Structure-Controlled Approach to the Generation of Silicone Rubber Foams by Means of
Carbon Dioxide
Results and Discussion
Effect of Saturation Time on Cellular Morphology of Silicone
Rubber Foams
As is well-known, the saturation time is a key factor during the foaming
process [16]. The diffusion of CO2 into the rubber matrix has a close relation
with the saturation time. As a result of the scCO2’s plasticization effect on
polymers, the saturation of CO2 could have an effect on the molecular motility
and the foaming process [17-19]. It may lead to the differences in the cell
morphology of foams saturated for different time. Silicone rubber sheets
with same ingredients and pre-curing time (6 min) were saturated at 50oC,
10 MPa for different time (1 h, 2 h, 3 h and 4 h). As shown in Figure 1, there
were some differences in the cellular morphology of samples generated for
different time. It seemed that cell size gradually became larger and more
bubble coalescence occurred with increasing saturation time from1 h to 4 h.
Figure 1. SEM photographs of silicone rubber foam saturated at 50oC, 10 MPa for
different time: (a) 1 h; (b) 2 h; (c) 3 h; (d) 4 h
After that, image analysis was carried out on a series of foamed samples
generated for different time to obtain the trend of cell size distribution. The
cell size and its distribution were statistically analyzed and shown in Figure 2.
When the saturation time was 1 h, cell size distribution was the narrowest and
the diameter of most cells was approximately between 30~40 μm. Hence, the
silicone rubber foam owned the good uniformity of cell size. After the saturation
Cellular Polymers, Vol. 35, No. 1, 2016
23
Hao Xu, Yunchuan He, Xia Liao, Tinggang Luo, Guangxian Li, Qi Yang, and Chuanjian Zhou
for a longer time, more and more large cells were gradually generated. The
curve of cell size distribution moved to the right (large cell size zone) and
the size of some large cells was even above 70 μm when saturated for 4 h.
Figure 2. Statistical cell size distribution of silicone rubber foam saturated for different
time
Only judging from the SEM photographs, the effect of saturation time on
cellular structure could not be clearly figured out. Hence, the cell density and
average cell diameter of silicone rubber foams saturated for different time are
calculated and listed in Table 1. It could be found that the cell density dropped
from 3.76×106 to 1.75×106 cells/cm3, and average cell diameter enlarged
from 37.09 to 49.44 μm when prolonging the saturation time from 1 h to 4 h.
Table 1. Effect of saturation time on cell density and average cell
diameter
Saturation time
(h)
Cell density
(cells/cm3)
Average cell diameter
(μm)
1
3.76×106
37.09
2
2.27×106
42.83
3
2.23×106
45.57
4
1.75×106
49.44
As we all know, the dissolution and the diffusion of scCO2 into silicone rubber
matrix has a close connection with the saturation time, which could further
affect the viscoelastic properties of the specimen. In order to find out how the
24
Cellular Polymers, Vol. 35, No. 1, 2016
A Green and Structure-Controlled Approach to the Generation of Silicone Rubber Foams by Means of
Carbon Dioxide
saturation time influences the silicone rubber matrix, we tested the complex
viscosity of silicone rubber compounds during the saturation process. As
shown in Figure 3, the viscosity of silicone rubber decreased a lot in the first
hour as a result of the plasticization effect of scCO2. During the next three
hours, the viscosity of the rubber matrix still decreased and the decreasing
rate of η* gradually became smaller. This may because that a great amount
of scCO2 was dissolved and diffused into the silicone rubber in the first hour.
At this moment, the saturation degree of scCO2 in the specimen got to a high
level. Then the plasticization effect of scCO2 was the strongest in this period.
In the next three hours, only a very small amount of scCO2 could be dissolved
and diffused into the rubber matrix in corresponding with the changes of
viscosity in Figure 3. The decrease of viscosity could enhance the motility of
molecules chains thus increasing the cell growth. In a result, silicone rubber
foams with larger cell size and smaller cell density could be easily generated
when increasing saturation time from 1 h to 4 h.
Figure 3. Time dependence of complex viscosity of silicone rubber under 10 MPa at
50oC for 4 h
Morphology of Open Porous Silicone Rubber Foams
Open porous silicone rubber foams were successfully produced by CO2 and the
cellular morphology was observed by scanning electron microscope. Figure 4
shows morphology of open porous silicone rubber foams with different precuring time (4 min and 6 min) which were saturated under 5 MPa for 1 h at
Cellular Polymers, Vol. 35, No. 1, 2016
25
Hao Xu, Yunchuan He, Xia Liao, Tinggang Luo, Guangxian Li, Qi Yang, and Chuanjian Zhou
40oC ,50oC and 60oC, respectively. The cell structure shown in Figure 4 had a
remarkable difference with silicone rubber foams produced by Lee and Song
[20, 21]. Most bubbles in these foams were coalesced and cell walls were
broken and linked each other. The small and large cells appeared together
in the open porous foams. Then the open cell content of different foamed
specimens was tested so as to figure out the structural difference of these
open porous foams. The testing results showed that open cell content of
the specimen a-1 was 68.18%, a-2 was 72.36%. After increasing pre-curing
time to 6 min, open cell content of the specimen b-1 was 54.26%, b-2 was
57.75%, b-3 was 55.98%. It indirectly reflected that the pre-curing time had
an effect on cellular structure. However, it was not clear to find the specific
morphology difference caused by different pre-curing time as a result of the
open porous structure. Hence the next section could help us to clearly figure
out how the pre-curing time affects cellular structure in detail.
Figure 4. Morphology of open-celled silicone rubber foam with different pre-curing
time under 5 MPa for 1 h at different temperatures: (a-1) 4 min 40oC; (a-2) 4 min 60oC;
(b-1) 6 min 40oC; (b-2) 6 min 60oC; (b-3) 6 min 50oC
Effect of Pre-curing Time on Cellular Morphology of Silicone
Rubber Foams
All silicone rubber specimens were not fully crosslinked before the foaming
process in this research. So the pre-curing time could affect the crosslinked
network in the rubber matrix thus acting as a vital role in the foaming process
of silicone rubber. In order to figure out the relation between pre-curing time
and cellular morphology, silicone rubber with different pre-curing time (6 min,
18 min, 24 min and 48 min) were foamed at 50oC, 10 MPa for 1 h. As shown
26
Cellular Polymers, Vol. 35, No. 1, 2016
A Green and Structure-Controlled Approach to the Generation of Silicone Rubber Foams by Means of
Carbon Dioxide
in Figure 5, there were distinct differences among the foamed specimens
with different pre-curing time. When pre-curing time was 6 min, it could form
uniform and well defined cells. After increasing pre-curing time to 18 min,
larger cells were obviously generated but the uniformity of cell size became
worse. The cellular morphology changed again while the pre-curing time of
specimen was 24 min. Silicone rubber foams with pre-curing time 24 min
owned many irregular shaped cells and even the shrinkage of some cells
could be observed. After increasing to 48 min, the unique phenomenon was
observed that there were no cells generated in the corresponding specimen.
Figure 5. SEM photographs of silicone rubber foam saturated at 50oC, 10 MPa for 1 h
with different pre-curing time: (a) 6 min; (b) 18 min; (c) 24 min; (d) 48 min
The statistical analysis of cell size of specimens with different pre-curing time
was made and shown in Figure 6. The cell size distribution became wider
firstly when pre-curing time increased from 6 min to 18 min. Then it turned to
get narrower while continuously increasing to 24 min. The cell size distribution
of specimen with 48 min pre-curing time was not shown in Figure 6 because
no cells were generated in this kind specimen.
In order to figure out the effect of pre-curing time on cellular structure more
accurately, cell density and average cell diameter of different specimens are
calculated and shown in Table 2. The cell density decreased from 3.76×106 to
2.50×106 cells/cm3 when increasing pre-curing time from 6 to 24 min. At the
same time, the average cell diameter became smaller from 37.09 to 29.09 μm.
If go on increasing pre-curing time to 48 min, both the cell density and average
cell diameter could not be calculated as a result of the disappearance of all
Cellular Polymers, Vol. 35, No. 1, 2016
27
Hao Xu, Yunchuan He, Xia Liao, Tinggang Luo, Guangxian Li, Qi Yang, and Chuanjian Zhou
Figure 6. Statistical cell size distribution of silicone rubber foams with different precuring time
cells. The calculation results in Table 2 corresponded well to the cellular
morphology changes of the foamed specimens shown in Figure 5.
Table 2. Effect of pre-curing time on cell density and average cell
diameter
Pre-curing time
(min)
Cell density
(cells/cm3)
Average cell diameter
(μm)
6
3.76×106
37.09
18
2.99×106
49.73
24
2.50×106
29.09
48
/
/
The rheology analysis of silicone rubber compounds with different pre-curing
time could help us to understand how the pre-curing time influences the
structure of silicone rubber compounds (shown in Figure 7). When pre-curing
28
Cellular Polymers, Vol. 35, No. 1, 2016
A Green and Structure-Controlled Approach to the Generation of Silicone Rubber Foams by Means of
Carbon Dioxide
time was 6 min, there would be a very small part of curing agent that was
decomposed. Then the complex viscosity (η*) of specimen was the smallest
and the storage modulus (G′) exceeded the loss modulus (G′′) by a quite
small amount. It demonstrated that the crosslinking of silicone rubber was
inadequate and then the elasticity of the silicone rubber compounds was so
small, which was benefit for the cell nucleation and growth. Hence, silicone
rubber foams were generated with uniform and well defined cells. As the
pre-curing time increased to 18 min, more curing agents were decomposed
to form the crosslinking network in silicone rubber matrix. The viscosity of
the specimen was increased with pre-curing time. Meanwhile, the elasticity
was also enhanced, which was reflected in Figure 7b where the increase
rate of G′ was larger than the G′′ and the gap between them was enlarged.
Then the diffusion of scCO2 into rubber matrix would be restricted and the
concentration of gas in the specimen decreased thus restricting cell nucleation
rate [22-24]. Although the elasticity was increased, its effect on cell growth
was not enough strong. In conjunction with the analysis of cellular morphology,
we can make a conclusion that the effect of elasticity increase on the cell
nucleation was much stronger than on the cell growth during this progress
(increasing pre-curing time from 6 to 18 min). Consequently, it would lead to
the formation of silicone rubber foams with large cell size and small cell density.
When continuously increasing pre-curing time, most parts of the curing agent
would be decomposed and the viscosity of silicone rubber compounds was
increased to a high level. The elasticity of silicone rubber compounds was also
enhanced a lot because the G′ continued to increase and it was much higher
than the G′′, thus causing the great increase of elastic resilience ability of the
specimen. Then it resulted in the shrinkage of cells, which led to the irregular
shaped cells and the decrease of the cell size. When pre-curing time was
increased to 48 min, the curing agent was almost completely decomposed and
the chemical crosslinking network became integrated in the silicone rubber
matrix. At this time, both the viscosity of silicone rubber compounds and the
gap between the G′ and G′′ were the largest. On the other hand, the storage
modulus of the specimen did not vary much with the frequency comparing
with other specimens. It demonstrated that the silicone rubber compounds
owned a very strong elasticity when increasing pre-curing time to 48 min.
Then the high elastic resilience prevented the cell growth and accelerated
the shrinkage of cells. In a result, there were no cells generated in the silicone
rubber. According to the above research, the effect of the pre-curing time
on silicone rubber foam’s structure is a complex process. When pre-curing
time is not too long, cell nucleation would be affected more than cell growth.
If continually increasing pre-curing time, both the cell nucleation and growth
would be restricted. All these investigations could guide us to generate silicone
rubber foams with specific cellular structure by controlling pre-curing time.
Cellular Polymers, Vol. 35, No. 1, 2016
29
Hao Xu, Yunchuan He, Xia Liao, Tinggang Luo, Guangxian Li, Qi Yang, and Chuanjian Zhou
Figure 7. Frequency dependence of (a) complex viscosity; (b) storage and loss
modulus of silicone rubber compounds with various pre-curing time at 50oC
Conclusions
A green and structure-controlled approach for the manufacture of silicone
rubber foams was reported in this research. Firstly, the effect of saturation time
on the cellular structure was investigated. When prolonging the saturation time,
more and more scCO2 molecules could diffuse into the silicone rubber matrix
thus enhancing its plasticization effect. Hence, the viscosity of silicone rubber
compounds would decrease with the saturation time. It would promote the
cell growth and lead to foams with large cell size and small cell density. Open
celled silicone rubber foams with different pre-curing time (4 min and 6 min)
were successfully produced by CO2 at 40oC, 50oC and 60oC, 5 MPa for 1 h.
Combining with the testing results of open cell content, the pre-curing time
30
Cellular Polymers, Vol. 35, No. 1, 2016
A Green and Structure-Controlled Approach to the Generation of Silicone Rubber Foams by Means of
Carbon Dioxide
had a stronger effect on the cellular structure than the saturation temperature.
Then the effect of the pre-curing time on cellular structure was discussed in
detail. Pre-curing time also has a strong effect on cellular morphology. When
increasing pre-curing time in the short time range, cell nucleation would be
affected more than cell growth in the foaming process. The foams with large
cell size and small cell density could be generated easily. If continuously
increasing pre-curing time, the viscosity and elasticity of the rubber matrix
would increase at the same time. Both cell nucleation and growth would be
restricted thus resulting in the formation of silicone rubber foams with small
cell density and small cell size. Hence, this study not only provides us a clean
and sustainable way to produce silicone rubber foams, but also guides us to
control cellular morphology more effectively, which can make silicone rubber
foams widely applied in many more fields.
Acknowledgments
This research was supported by the National Natural Science Foundation
of China (No. 51373103 and 51103091), and the Key Laboratory of Special
Functional Aggregated Materials, Ministry of Education, China.
References
1.
P. Liu, D. Liu, H. Zou, P. Fan, X. Wen, J. Appl. Polym. Sci., 113, (2009), 35905.
2.
J. Wen, Y. Li, Y. Zuo, G. Zhou, J. Li, L. Jiang, W. Xu, Mater. Lett., 62, (2008),
3307-9.
3.
John B. Grand, Amanda S. Fawcett, Alex J. McLaughlin, Ferdinand Gonzaga,
Timothy P. Bender, Michael A. Brook, Polymer, 53, (2012), 3135-42.
4.
E-S Park. J, Appl. Polym. Sci., 110, (2008), 1723-9.
5.
R. Romanowski, B.A. Jones, T.J. Netto, Rubber Division, 10, Cleveland,
(2003), American Chemical Society.
6.
JJ. Chruściel, E. Leśniak, J. Appl. Polym. Sci., 119, (2011), 1696-703.
7.
M.A. Jacobs, M.F. Kemmere, J.T.- F. Keurentjes, Polymer, 45, (2004), 753947.
8.
Z.-M. Xu, X.-L. Jiang, T. Liu, G-H. Hu, L. Zhao, Z.-N. Zhu, W.-K. Yuan, J.
Supercrit. Fluids, 41, (2007), 299-310.
9.
D.L. Tomasko, A. Burley, L. Feng, S.-K. Yeh, K. Miyazono, S. Nirmal-Kumar,
I. Kusaka, K. Koelling, J. Supercrit. Fluids, 47, (2009), 493-9.
Cellular Polymers, Vol. 35, No. 1, 2016
31
Hao Xu, Yunchuan He, Xia Liao, Tinggang Luo, Guangxian Li, Qi Yang, and Chuanjian Zhou
10.
X. Liao, A.V. Nawaby, Ind. Eng. Chem. Res., 51, (2012), 6722-30.
11. Daofei Bao, Xia Liao, Ting He, Qi Yang, Guangxian Li, J. Polym. Res., 20,
(2013), 290-9.
12.
Y.-W. Chang, D. Lee, S-Y. Bae, Polym. Int., 55, (2006), 184-9.
13. J.J.A. Barry, S.N. Nazhat, F.R.A.J. Rose, A.H. Hainsworth, C.A. Scotchford,
S.M. Howdle, J. Mater. Chem., 15, (2005), 4881-8.
14. M. Shimbo, T. Nomura, K. Muratani, K. Fukumura, The Third International
Conference on Axiomatic Design, 6, Seoul, (2004), ICAD2004.
15. X. Xu, C.B. Park, D. Xu, R. Pop-Iliev, Polym. Eng. Sci., 43, (2003), 13781390.
16.
I.-K. Hong, S. Lee, Korean J. Chem. Eng., 31, (2013), 166-71.
17.
Lixian Song, Ai Lu, Peijie Feng, Zhongyuan Lu, Mater. Lett., 121, (2014), 1268.
18.
X. Liao, Y. Li, C.B. Park, P. Chen, J. Supercrit. Fluids, 55, (2010), 386-394.
19. G. Li, J. Wang, C.B. Park, R. Simha, J. Polym. Sci. B, 45, (2007), 24952508.
20.
Y. Li, C.B. Park, Ind. Eng. Chem. Res., 48, (2009), 6633-6640.
21. M.M. Hasan, Y.G. Li, G. Li, C.B. Park, P. Chen, J. Chem. Eng. Data, 55,
(2010), 4885-4895.
22.
S.K. Goel, E.J. Beckman, Polym. Eng. Sci., 34, (1994), 1137-1147.
23.
X. Liao, H. Zhang, Y. Wang, L. Wu, G. Li, RSC Adv., 4, (2014), 45109-45117.
24.
X. Liao, A. Victoria Nawaby, Y. Paul Handa, Cell. Polym., 26, (2007), 69-82.
32
Cellular Polymers, Vol. 35, No. 1, 2016