Fabrication of Fine-celled PP/Ground Tire Rubber Powder Composites Using Supercritical Carbon Dioxide
Fabrication of Fine-celled PP/Ground
Tire Rubber Powder Composites
Using Supercritical Carbon Dioxide
Zhen-Xiu Zhang1,2, Lin Li2, Zhen Xiang Xin1, and Jin Kuk Kim2*
1Key
Laboratory of Rubber-Plastics, Ministry of Education, Shandong Provincial
Key Laboratory of Rubber-Plastics, Qingdao University of Science and Technology,
Qingdao, 266042, China
2School
of Nano and Advanced Materials Engineering, Gyeongsang National
University, Gyeongnam, Jinju, 660-701, South Korea
Received: 23 February 2011, Accepted: 14 March 2011
Summary
PP/Ground Rubber Tire (GRT) powder microcellular blend foams were prepared
by supercritical carbon dioxide. The effects of blend composition like the
content of GRT and maleic anhydride-grafted polypropylene (PP-g-MA) on
crystallinity, solubility, diffusivity, morphology and mechanical properties of PP/
GRT microcellular composites were studied. The results showed that the PP/
GRT composite foams with a unique bimodal (large and small) cellular structure,
in which the large-cells embrace a GRT powder. Depending on the composition,
generally, the higher content of GRT results in the smaller cell sizes, higher cell
densities and relative densities, whereas the 20 wt% GRT composite shows
the lowest relative density. The mechanical properties of the microcellular PP/
GRT composite foams are directly related to the blend composition and the
processing conditions. The PP-g-MA/GRT (50/50) produced microcellular foams
with a very fine and uniform cell structure, lower relative densities and improved
mechanical properties.
Keywords: Polypropylene, ground rubber tire, foam, composite
*Corresponding author: Prof. Jin Kuk Kim, School of Nano and Advanced
Materials Engineering, Gyeongsang National University, Gyeongnam, Jinju,
660-701, South Korea. Tel: (+82) (0)55-751-5299; Fax: +82-55-753-6311;
Email: [email protected]
©Smithers
Rapra Technology, 2011
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Introduction
Due to its outstanding characteristics and low cost, polypropylene (PP) has
been considered as a substitute for other thermoplastic foam materials [1].
But it does not provide a high enough state of physical properties at a given
flexibility to fully compete in the cellular elastomer market [2]. To improve the
impact toughness and extend its application range, a number of studies on
toughening PP with rubber have been made in the last 20 years [3], Recycling
of ground rubber tire powder is our preference from the ecological and
economical point of view. The usage of GRT powder as dispersed phase
in PP matrix offers an interesting opportunity for recycling of scarp rubber,
so the PP/GRT composites foaming technology is to fulfill the requirement
of lower cost, lighter weight and better fuel economy, therefore presents an
important milestone in many applications.
Polymers are often blended to create new functions, which each polymer
alone cannot express, or to compensate for the weakness in the mechanical
properties of each polymer. In polymeric foaming also, polymer blends are
often used to create fine cell structures [4]. Han et al. [5] have reported the
well-mixed and poorly mixed PS/9 wt% PMMA blends were foamed with CO2,
and the results showed that the well-mixed blends were rather homogeneous,
whereas the poorly mixed blends clearly showed a dominant small cell phase
and larger cells spread as stripes through the foamed sample. Doroudinai
et al. [6] studied the foaming of high-density polyethylene (HDPE)/isotactic
polypropylene (PP) blends. They found that a fine cellular structure could be
created in the blend polymer by choosing a suitable temperature, whereas little
or no foaming took place in each neat polymer. Siripurapu et al. [7] investigated
PS/poly(vinylidene fluoride) (PVDF) and poly(methyl methacrylate) (PMMA)/
PVDF blends for foaming to expand the operating windows and concluded
that blending PS with PVDF was not suitable for foaming because of their
immiscibility, whereas PMMA/PVDF could be foamed at various operating
conditions. Related to the situation of poorly mixed blends is the case of blending
non-miscible polymers, foaming of these “blends” can result in remarkable
foam morphologies. Taki et al. [4] have produced foamed poly(ethylene glycol)
(PEG)/polystyrene (PS) blends showed a unique bimodal (large and small) cell
structure, in which the large-size cells embraced a PEG particle.
The mechanical properties of microcellular foams processed in batch method
have rarely been reported. Kumar et al. [8] studied the tensile behaviors
of microcellular foamed polymer polycarbonate foams, Matuana et al. [9]
studied the mechanical properties of microcellular polyvinyl chloride (PVC)
foams, Sun et al. [10-11] examined the mechanical properties of polysulfone,
polyethersulfone and polyphenylsulfone microcellular foams. Recently, Fu et
al. [12] investigated the effect of nanoclay on the mechanical properties of
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PMMA/clay nanocomposite foams. The tensile behaviors of TPO were also
investigated by Wong et al. [13], they reported the effects of foaming conditions
and relative density on the mechanical properties of microcellular TPO foams.
The purpose of this study was to investigate the relationships of processing,
structure and properties of microcellular PP/GRT composites, and at last
generation of fine-celled PP/GRT composite. When polymer blends are foamed,
the cellular structure is determined not only by the morphology, crystallinity
and viscosity of the blend polymers but also by the solubility and diffusivity of
the physical foaming agent in the polymers. Therefore, there is a possibility of
creating various cell structures by the blending of polymers. In this study, PP/
GRT composites were foamed by pressure-quench method described by Goel
and Beckman [14] using CO2 as foaming agent, the effect of blend composition
on the desorption behavior of CO2 and crystallinity of PP/GRT composite, and
its relationship with cell structure as well as mechanical properties of foamed
PP/GRT composites were investigated, the mechanism of a unique cellular
structure of foamed PP/GRT composite was also investigated.
Experimental
Materials
Polypropylene (R520Y) supplied by SK Corporation, which has a melt flow
index (MFI) of 1.8 g/10 min (ASTM D1238), a density of 0.9 g/cm3, and melting
point of 153°C was used as a matrix in this experiment. Maleic anhydride
grafted polypropylene (PP-g-MA) was prepared by our research group [30],
which has MAH grafting degree of 0.592 wt%, melting point of 151°C. The
GRT powder was ground by wet grinding method and was supplied by
Hongbok Industry, Korea. The composition of scrap rubber is: polymer content
of 48.5% with natural rubber (NR) and styrene-co-butadiene rubber (SBR) in
25% and 75% ratio respectively. The other composition of waste rubber was
organic additives, carbon block and ash content 13.4%, 27.7% and 10.4%,
respectively. Its particle size was characterized to be 30-50 µm as shown in
Figure 1. Commercial grade CO2, an environmentally friendly PBA, with a
purity of 99.95% was supplied by Hyundai Gas Inc.
Preparation PP/ GRT Composites by Twin Screw Extruder
PP/GRT composite samples were prepared at different ratios, as shown in
Table 1. All experiments were performed by using a modular intermeshing
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Figure 1. SEM microphotograph of 30-50 μm GRT powder (scale bar is 50 µm)
Table 1. Formulation and blend conditions of PP/GRT composites
GRT
PP
PP-g-MA
0
100
0
20
80
0
40
60
0
50
50
0
50
0
50
Barrel Temperature
Screw Speed
200-230°C
100 rpm
twin screw extruder (D=19 mm, L/D=40/19, BauTech). The screw speeds
were kept at 100 rpm, the barrel temperature was maintained at 200, 210,
220, and 230°C from the hopper to the die. The extrudate was pelletized and
dried under vacuum at 80°C for 24 h to remove any residual water. The plate
samples of the blends with 2.0 mm thickness were compression-molding at
180°C for 6 min.
Preparation and Analysis of PP/GRT Composites Foams
Microcellular foaming experiments were performed in a batch process. A
schematic of the batch-foaming process is shown in Figure 2. Plate samples
that were 2.0 mm thick, 60.0 mm long, and 4.0 mm wide were enclosed
high-pressure vessel. The vessel was flushed with low-pressure CO2 for
about 3 min and pressurized to the saturated vapor pressure CO2 at room
temperature and preheated to desired temperature. Afterward, the pressure
was increased to the desired pressure by a syringe pump (ISCO260D) and
maintained at this pressure for 2 h to ensure equilibrium absorption of CO2
by the samples. After saturation, the pressure was quenched atmospheric
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Figure 2. The schematics of batch-foaming process
pressure within 3 s and the samples were taken out. Then foam structure was
allowed to full growth during rapid depressurization.
The foam morphology was characterized by utilizing a scanning electron
microscope (SEM, Philips XL 30S). The foamed samples were cooled in liquid
nitrogen and fractured to produce a clean and intact surface with minimum
plastic deformation. They were then gold coated by using a sputter coater for
enhanced conductivity. The average cell size and cell density were analyzed
by utilizing the ImageJ software. The cell sizes, cell densities and relative
densities were characterized. The cell diameter (D) is the average of all the
cells on the SEM photo, usually more than 100 cells were measured.
D=
∑dn
∑n
i i
i
(1)
Where ni was the number of cells with a perimeter-equivalent diameter of di.
The density of foam and unfoamed samples was determined from the sample
weight in air and water respectively, according to ASTM D 792 method A. Then
the density of the foamed sample is divided by the density of the unfoamed
sample to obtain the relative density (ρr). The volume fraction occupied by
the microvoids (Vf) was calculated as:
Vf = 1−
ρf
ρm
(2)
Where rm and rf are the density of the unfoamed polymer and foamed polymer
respectively.
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The cell density (N0) based on the unfoamed sample was calculated as:
Nf =
N0 =
Vf
π 3
D
6
(3)
Nf
1− Vf
(4)
Where Vf is the volume fraction occupied by the microvoids, Nf is the cell
density based on the foamed sample.
To study the effect of GRT on CO2 solubility and diffusivity, experiments of CO2
sorption and desorption were performed. At high temperatures, temperature
control and the accurate reading of pressure and volume are very difficult,
so the absorption measurement was carried out at 45°C and desorption was
characterized at room temperature. Although the testing temperatures are
not the actual foaming temperature, the results can provide some general
trend. The experimental procedure was similar to the employed by Berens
et al. [15] flat plate samples 60 mm in length, 20 mm in width, and 2 mm in
thickness of PP or PP/GRT composites were placed in a high-pressure vessel
that was connected to a syringe pump. The samples were saturated with
CO2 at 15 MPa and 45°C for 24 hours. They were then quickly taken out of
the high-pressure vessel and placed on a high-resolution balance. The CO2
desorption curve (weight loss with time) was recorded.
The crystallization temperature and crystallinity of the PP/GRT composites
were investigated using a differential scanning calorimeter (DSC, Q20, TA
Instruments). For each sample, 6–10 mg was sliced from the compressionmolded specimens and placed in a hermetic aluminum pan under 50 mL/min
nitrogen flow. The samples were heated from room temperature to 200°C at
10°C/min. The crystallinity was calculated from the specific heat required
for melting (ΔHm) by integrating the area under the corresponding peak and
dividing this value by the heat of fusion for the pure crystalline phase of
polypropylene (ΔHmo), 207.1 J/g.
The tensile mechanical properties of microcellular PP/GRT composite foams
were tested, following the ASTM D638 procedure on a Tensometer 2000
(Bong Shin) mechanical testing machine at room temperature. Foamed
samples were allowed to desorb the gas for at least 2 weeks before property
characterization. PP/GRT composites foam samples used for tensile testing
had a thickness ranging from 2 mm to 4 mm, depending on the expansion
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ration of the sample under different foaming conditions. The displacement
rate of the crosshead was 10 mm per min. Tensile strength was calculated
as the load force divided by the initial cross-sectional area of the specimen.
The modulus of elasticity was obtained by calculating the slope of the stressstrain curves in the elastic region. The elongation at break of the sample was
calculated in terms of percent elongation. A minimum of five specimens was
tested for each set of foaming conditions.
Results and Discussion
Effect of Blending on Crystallinity
The results of the effect of GRT content and compatibilizers on crystallinity
are summarized in Table 2. Random PP is a semi-crystalline polymer, and
its mechanical properties are greatly affected by its overall crystallinity,
the foaming behavior of a semi-crystalline polymer largely depends on the
crystallization temperature [16]. Therefore, it is important to investigate
the effects of GRT content and PP-g-MA on the crystallization behavior of
blends. The crystallization temperature (Tc) and crystallinity of all PP/GRT
composites decreased when the GRT content was increased. This effect
could be explained by the lowered mobility of polymer chains in the PP/GRT
matrix, which resulted from the presence of dispersed GRT particles. The
presence of the rubber powder restricts the growth of the crystalline phases
and thereby leading to the amorphous regions. The PP-g-MA and PP-g-MA/
GRT composite showed reduced crystallinity compared with PP and PP/
GRT composite. This was ascribed to the reduced perfection of the crystals,
deriving from the anhydride groups, and the presence of possible branching.
Table 2. Crystallization temperature, crystallinity of PP and PP/GRT
composites
Sample
PP
Tc, °C
Crystallinity of Blends (%) Crystallinity of Polymer (%)
125.33
41.15
41.15
PP-g-MA
114.28
37.32
37.32
PP/GRT (80/20)
115.12
33.35
47.64
PP/GRT (60/40)
114.43
26.21
43.68
PP/GRT (50/50)
113.93
20.44
40.88
PP-g-MA/GRT
(50/50)
113.41
20.21
40.42
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Solubility and Diffusivity of PP/GRT Composites
It is known that the foamability of polymers is affected by the sorption of gas
in the polymer and that the mechanisms of cell nucleation and cell growth
are influenced by the amount of gas dissolved in the polymer and the rate
of gas diffusion. Desorption isotherm curves for the PP/GRT composites are
illustrated in Figure 3. After releasing the pressure in the high-pressure vessel,
the samples still remained unfoamed due to the low temperature applied, and
thus the dimensions of the samples keep unchanged. The initial stage of the
desorption curve of CO2 from the polymer is linear with respect to time [16],
and the y-intercept back to time zero yields the solubility of the samples in
the experiment. By extrapolating from desorption curves, the solubility of CO2
in PP and PP/GRT composites was obtained (see Table 3). The composites
Figure 3. Desorption curves of PP/GRT composites
Table 3. Solubility and diffusivity of foamed PP and PP/GRT
composites
Solubility (wt%)
Diffusivity (cm2/s)
PP
6.61
4.72×10-5
PP/GRT(80/20)
6.64
5.46×10-5
PP/GRT(60/40)
6.76
1.0×10-4
PP/GRT(50/50)
7.2
2.5×10-4
PP-g-MA/GRT(50/50)
6.9
1.7×10-4
Sample
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exhibited a slightly higher solubility than pure PP. This can be explained that
the poor adhesive between PP and GRT, there exist some microvoids in the
blends and it can hold amount of gas, this is one reason, another reason is
because of the lower crystallinity of the blends.
Diffusivities of CO2 in the samples were determined using the following
equation for Fickian diffusion [16] through a flat plate:
0.5
D 
= 4 
M∞
π
Mt
 t0.5 


 L 
(5)
where Mt is the mass gain by the sample at time t, M∞ the maximum mass
gain, D the diffusivity of CO2 in the polymer, and L the thickness of the sample.
M∞ is the equilibrium solubility of CO2 at the conditions of the experiment.
The diffusion coefficient of CO2 in the sample was obtained from the slope
of Mt/M∞ plotted against t0.5/L. As our expectation that blends exhibit high
diffusivities because of the addition of GRT to the polymer leads to poor
adhesion between the GRT and the polymer matrix. The poor surface adhesion
of the polar GRT to the nonpolar polymer provides a channel through which
gas can quickly escape from the blends.
Effect of GRT Content
The effect of GRT powder on the cell structure of PP/GRT composites were
identified by comparing the results from experiments with various GRT
contents of 0 wt%, 20 wt%, 40 wt% and 50 wt%, the foaming temperature and
saturation pressure were fixed at 155°C and 12 MPa, respectively. Figure 4
are the scanning electron micrographs of foamed PP/GRT composites
with different concentration of the scrap rubber powder. The comparison
shows that existence of GRT powder caused deterioration of microcellular
structure, larger cells, and the cells become non-spherical and non-uniform.
The figures suggest that foaming PP/GRT composites, promotes further
detachment of GRT from the matrix, caused by bubbles formation at the
interface. This is because the poor adhesion between PP and GRT easily
led to the coalescing and a two-phase separation at the boundary, and
higher solubility and higher diffusivity make the bubble growth rate faster.
As illustrated in Figure 3, the solubility and diffusivity of CO2 in PP/GRT
composites were larger than those in pure PP. So during the manufacturing
process at foaming PP/GRT composites, big cells occurred, the foaming
effect caused by CO2 is so extreme that it causes the cells to “explode”
during depressurization.
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(a)
(b)
(c)
(d)
Figure 4. Effect of GRT content on microcellular structure of PP/GRT composite
foams at 12 MPa, 155°C, (a) 0 wt%, (b) 20 wt%, (c) 40 wt%, (d) 50 wt%
It is well known that PP and GRT are immiscible with each other, and islandsea morphology was established at these weight fractions: GRT became
islands, and PP made the matrix. Some GRT was lost when the samples were
prepared for SEM observations, and this resulted in voids in the matrix. As it
can be seen in the picture, an increase in the GRT weight fraction made the
number of the island increase.
Figure 5 shows an enlarged SEM micrograph of the PP/GRT (50/50). It
clearly shows a bimodal cell size structure, in which the average diameter of
the smaller cell was less than 55 µm and the cell size of the larger cell was
about 420 µm, a GRT was located at a large cell in the foams. Counting the
cells whose diameters were larger than 55 µm at the six different positions
in each micrograph and averaging the counted numbers gave the average
number of large cells. The number density of large cells (Nf) was calculated
by the Equation (3).
Figure 6 shows the relationship between the number density of large cells
and the weight fraction of GRT in the blend. As the fraction of the GRT
increased, the number of large cells increased. This indicated that the large
cells originated at the boundary between the PP and GRT, the small cells
originated in the PP phase of the blend.
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Figure 5. Enlarged SEM micrograph of the PP/GRT composite foam
Figure 6. Relationship between the GRT content, average size of large cells and
number density of large cells of PP/GRT composite foams at 12 MPa and 155°C
As it can be seen in the micrographs of the PP/GRT composite foams that
existence of many smaller cells around larger cells. The diameter of the
smaller cells was calculated from the cross-sectional area of the cells under
the assumption that a cell took a spherical shape. Figure 7 represents how
the cell diameter and density of the cells in the blend foam were changed by
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Figure 7. Effect of the GRT content on the average cell size and cell density of small cells
the concentration of GRT. As illustrated in Figure 4 and 7, the size of cells in
the foamed PP/GRT (80/20) composite became bigger than that of pure PP
foamed under the same conditions. Blending GRT with PP created numerous
large cells at the boundary between the matrix polymer and the dispersed GRT
powder, and at the same time, it enlarges the size of the cells nucleating at the
PP matrix. This is because the foaming behavior of semicrystalline polymers
is affected significantly by their crystallinity [16-17]. The aforementioned DSC
results demonstrated that the total crystallinity of the PP/GRT composites
decreases with increasing of content of GRT. Moreover, GRT powder plays
the role of a nucleating agent. Therefore the PP/GRT (80/20) composite shows
the big cell size, however further increase in GRT content, leads to a smaller
cell size in the foamed PP/GRT, this may be due to that the higher viscosity
of blends. The overall cell density is much lower than that of pure PP; this
could be due to the heterogeneous nucleation induced in the presence of
GRT. In heterogeneous nucleation, the number of nucleated cells depends
strongly on the number of nucleating site (or the distribution of nucleating
agent) [18]. The filler distribution and size, together with mixed nucleating
regimes (heterogeneous and homogeneous) are deemed to affect the final
cell distribution and size.
A formation mechanism of the cellular structure observed in the PP/GRT
composite foaming is summarized in Figure 8. Before the foaming, the
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(a)
(b)
(c)
(d)
Figure 8. Schematic diagram of the formation mechanism of the bimodal cellular
structure: (a) initial state, (b) bubble nucleation and growth, (c) bubble “explode” and
(d) Formation of the bi-modal structure
island (GRT powder)-sea (PP) morphology was established because of the
immiscibility of both the polymers. The dispersibility of GRT in the PP matrix
was changed by the weight fraction and could be controlled by blending. When
the PP/GRT composite polymer was foamed, because of the poor adhesion
of PP and GRT, bubble nucleation and growth occurred at the interface of PP
and GRT at the initial stage of foaming. Because of the higher diffusivity of
CO2 in PP/GRT composite, the gas accumulates at the boundary of PP and
GRT, and bubbles become larger than that nucleated in PP matrix. At last,
the bubble to “explode” during depressurization. This phenomenon created
a bimodal cell structure, in which the larger cell embraced a GRT.
Analyzing the graphs in Figure 9, it is clearly seen that the relative density
decrease initially and then increased with increasing GRT weight fraction
at all processing conditions. Addition of 20 wt% GRT powder promote
heterogeneous nucleation, however the higher initial CO2 concentration may
lead to bubble coalescence at the weak surface between PP and GRT during
bubble growth and eventually makes bubbles “explode” causing lower relative
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Figure 9. Effect of GRT weight fraction on the relative density of foamed PP/GRT
blends at different processing conditions (a) 12 MPa, (b) 16 MPa, (c) 20 MPa
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density. Further increase of GRT content will increase the melt viscosity and
diffused rate of CO2 from PP matrix, it also causes more resistance to cell
nucleation and growth. In addition, since GRT powder cannot be foamed,
the more the amount of GRT in the sample is, the larger the relative density.
The effect of GRT on the elastic modulus and tensile strength of foamed PP/
GRT blends at different foaming conditions can be observed from Figures 10
and 11. The mechanical properties such as elastic modulus and tensile strength
decrease, as the GRT weight fraction in the PP/GRT blends increased except
for the 20 wt% GRT blend. These results were expected since the PP act as
a plastic segment which contributes most of the tensile mechanical strength
of the PP/GRT blends. However, for 20 wt% GRT blend, the results are not
favorable. A proposed reason is that the cell nucleation and growth may occur
at the interface region of the GRT and PP. Consequently, it could detach the
GRT from the PP matrix causing deterioration of PP and GRT adhesion, the
bubble “explode” and caused some larger bubbles than high weight fraction
of GRT blend.
PP-g-MA/GRT Composite Foam
Thermoplastic vulcanizate (TPV) is a special class of thermoplastic elastomers
(TPEs) made of a rubber/plastic polymer mixture in which the rubber phase
is highly vulcanized. One of major criteria for a thermoplastic vulcanizate is
that elongation at break is more than 100%. For PP/GRT (50/50) composite,
tensile strength is 11.5 MPa and elongation is only 39.4%. In order to obtain
TPV based on GRT, PP is replaced by PP-g-MA. The reactivity of MA group
in PP-g-MA and phenolic OH group in GRT can enhance the compatibility
between PP and GRT.23 As shown in Figure 12, it can be observed that tensile
strength is 11.9 MPa and elongation is 210.4% for PP-g-MA/GRT (50/50)
composite, namely we have successfully prepared TPV based on GRT.
The cell morphologies of microcellular PP/GRT (50/50) and PP-g-MA/GRT
(50/50) composite are presented in Figure 13, the foaming temperature and
saturation temperature were fixed at 155°C and 20MPa respectively. As shown
in Figure 13, significant improvement on the cell morphology can be seen in
the PP-g-MA/GRT composite. As the aforementioned, the foaming PP/GRT
(50/50) composite, promotes further detachment of GRT from the matrix,
which resulted from the fact that there was almost no interfacial adhesive
between GRT powder and PP matrix, and the majority of GRT powder were
pulled out and the big bubbles remained. However, PP-g-MA made the
morphology undergo a considerable change in the interfacial behavior. The
cell size becomes smaller and uniform, it clearly shows that the connection
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Figure 10. Effect of GRT weight fraction on the tensile strength of foamed PP/GRT
blends at different processing conditions (a) 12 MPa, (b) 16 MPa, (c) 20 MPa
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Figure 11. Effect of GRT weight fraction on the tensile modulus of foamed PP/GRT
blends at different processing conditions (a) 12 MPa, (b) 16 MPa, (c) 20 MPa
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Figure 12. Comparison of mechanical properties between PP/GRT (50/50) composite
and PP-g-MA/GRT (50/50) composite
(a)
(b)
Figure 13. Microcellular structure of PP/GRT composite foams at 20 MPa, 155°C: (a)
PP/GRT=50/50, (b) PP-g-MA/GRT=50/50
of GRT and PP matrix, because chemical interactions between the phenolic
hydroxyl groups in GRT and maleic anhydride groups in PP-g-MA in which
an interphase between the dispersed phase (GRT) and the polymer matrix
(PP) was formed, the changes should be attributed to the compatibilization
imparted by PP-g-MA/GRT binary blend.
From Figures 14-15, it can be observed that the relative density and cell size
of PP/GRT composites are higher than, whereas the average cell density is
lower than the PP-g-MA/GRT composite. It is known that the mechanism of
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cell growth is governed by the stiffness of the gas/polymer matrix, the rate
of gas diffusion, and the amount of gas loss, the poor surface adhesion of
PP-GRT provides a channel through which CO2 can quickly escape from the
composites [25]. However, the presence of PP-g-MA/GRT decreases the
possibility so as to facilitate cell nucleation and growth. So the PP-g-MA/
GRT composite has lower relative density, smaller average cell size, higher
cell density and uniform cell structure.
In order to characterize the difference of foamed PP/GRT and PP-g-MA/GRT
composites, the tensile properties were also measured. Figure 16 presents the
comparison of tensile strength of the PP/GRT and PP-g-MA/GRT microcellular
composites. It can be seen that foamed PP-g-MA/GRT composite has lower
tensile strength; this is caused by the lower relative density PP-g-MA/GRT
composite as aforementioned. The tensile moduli of the foamed composites
were similar to the tensile strength trends, which are presented in Figure 17.
The elongation at break for PP/GRT and PP-g-MA/GRT (50/50) microcellular
composites are summarized in Figure 18. It is can be seen that the foamed
PP-g-MA/GRT composite with higher elongation, it was increased almost
80% compared with PP/GRT composite. Because in the presence of PP-gMA which leads to the formation of chemical interactions between the carbon
black of GRT and maleic anhydride, thereby leading to formation of interphase
between the GRT and the PP matrix.
Figure 14. Comparison of the average cell size and cell density of foamed PP/GRT
and PP-g-MA/GRT composites
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Zhen-Xiu Zhang, Lin Li, Zhen Xiang Xin, and Jin Kuk Kim
Figure 15. Comparison of the relative density of foamed PP/GRT and PP-g-MA/GRT
composites at different processing conditions; (a) 12 MPa, (b) 16 MPa, (c) 20 MPa
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Figure 16. Comparison of The tensile strength of foamed PP/GRT with PP-g-MA/GRT
composites at different processing conditions; (a) 12 MPa, (b) 16 MPa, (c) 20 MPa
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Zhen-Xiu Zhang, Lin Li, Zhen Xiang Xin, and Jin Kuk Kim
Figure 17. Comparison of the tensile modulus of foamed PP/GRT with PP-g-MA/GRT
composites at different processing conditions; (a) 12 MPa, (b) 16 MPa, (c) 20 MPa
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Figure 18. Comparison of the Elongation at break of foamed PP/GRT with PP-g-MA/
GRT composites at different processing conditions; (a) 12 MPa, (b) 20 MPa
Conclusions
The microcellular foams of PP/GRT composites were produced by pressure
quench method using supercritical carbon dioxide as blowing agent, the
effects of blend composition on the crystallinity, sorption behavior of CO2, and
corresponding cellular morphology and mechanical properties of foamed PP/
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Zhen-Xiu Zhang, Lin Li, Zhen Xiang Xin, and Jin Kuk Kim
GRT composites were investigated. The experimental results showed that with
the addition of scrap rubber powder, the crystallinity of PP/GRT composites was
decreased, whereas the solubility and the diffusivity of CO2 increased in the PP/
GRT composites. The PP/GRT composite showed a unique cellular structure
in which large cells embraced a GRT powder and small cells existed around
large cells. The mechanism of creating such cell structure could be explained
by the morphology of the blends, the solubility and diffusivity of CO2 in PP/
GRT composites, this might lead to the development of an efficient nucleating
agent of polymer foaming. The mechanical properties of the microcellular
PP/GRT composite foams are directly related to the processing conditions
and composition. Under the same processing conditions, the mechanical
properties of foamed PP/GRT composites vary with the blend composition.
The PP/GRT (80/20) showed the lowest relative density and mechanical
properties. The PP-g-MA/GRT (50/50) produced microcellular foams with a
very fine and uniform cell structure, and enhanced the mechanical properties.
Acknowledgement
We are grateful for financial support from the Shangdong Natural Science
Foundation (Nos. ZR2010EM044) and Doctoral Found of QUST.
References
1. H.E. Naguib, C.B. Park, U. Panzer, and N. Reichelt, Strategies for Achieving
Ultra Low-density Polypropylene Foams, Polym. Eng. Sci., 42(7), 1481-1492
(2002).
2. N.C. Nayak and D.K. Tirpathy, Effect of aluminum silicate filler on morphology
and physical properties of closed cell microcellular ethylene–octene
copolymer, J. Mat. Sci., 37, 1347-1354(2002).
3. J.Z. Liang and R.K.Y. Li, Rubber Toughening in Polypropylene: A Review, J.
Appl. Polym. Sci., 77(2), 409–417 (2000).
4. K. Taki, K. Nitta, S. Kirhara, and M. Ohshima, CO2 foaming of Poly(ethylene
glycol)/Polystyrene Blends: Relationship of the Blend Morphology, CO2 Mass
Transfer, and Cellular Structure, J. App. Polym. Sci., 97, 1899-1906 (2005).
5. X. Han, J. Shen, H. Huang, D.l. Tomasko, and L.J. Lee, CO2 foaming based
on polystyrene/poly(methyl methacrylate) blend and nanoclay, Polym. Eng.
Sci., 47, 103 (2007).
6. S. Doroudiani, C.B. Park, and M.T. Kortschot, Processing and
characterization of microcellular foamed high-density polyethylene/isotactic
polypropylene blends, Polym. Eng. Sci., 38, 1205-1215, (1998).
134
Cellular Polymers, Vol. 30, No. 3, 2011
Fabrication of Fine-celled PP/Ground Tire Rubber Powder Composites Using Supercritical Carbon Dioxide
7. S. Siripurapu, Y.J. Gay, J.R. Royer, J.M. Desimone, R.J. Spontak, and S.A.
Khan, Generation of Microcellular Foams of PVDF and Its Blends Using
Supercritical Carbon Dioxide in a Continuous Process, Polymer, 43, 55115520 (2002).
8. V. Kumar, M. VanderWel, J. Weller, and K.A. Seeler, Experimental
Characterization of the Tensile Behavior of Microcellular Polycarbonate
Foams, Journal of Engineering Material Technology, 116(4), 439–445
(1994).
9. L.M. Matuana, C.B. Park, and J.J. Balatinecz, Structures and Mechanical
Properties of Microcellular Foamed Polyvinyl Chloride, Cellular Polymers,
17(1), 1–16, (1998).
10. H. Sun, G.S. Sur, and E.J. Mark, Microcellular Foams from Polyethersulfone
and Polyphenylsulfone: Preparation and Mechanical Properties, European
Polymer Journal, 38(12), 2373–2381 (2002).
11. H. Sun and J.E. Mark, Preparation, Characterization, and Mechanical
Properties of Some Microcellular Polysulfone Foams, J. Appl. Polym. Sci.,
86(7), 1692–1701 (2002).
12. J. Fu and H.E. Naguib, Effect of nanoclay on the mechanical properties
PMMA clay nanocomposite foams, Journal of Cellular Plastics, 42(4), 325342 (2006).
13. S. Wong, H.E. Naguib, and C.B. Park, Effect of Processing Parameters on the
Cellular Morphology and Mechanical Properties of Thermoplastic Polyolefin
(TPO) Microcellular Foams, Advances in Polymer Technology, 26(4), 232246 (2007).
14. S.K. Goel and E.J. Beckman, Generation of microcellular polymeric foams
using supercritical carbon-dioxide.1. Effect of pressure and temperature on
nucleation, Polym. Eng. Sci., 34(14), 1137-1147 (1994).
15. A.R. Berens, G.S. Huvard, R.W. Korsmeyer, and F.W. Kunig, Application of
Compressed Carbon Dioxide in the Incorporation of Additives into Polymers,
J. Appl. Polym. Sci., 46, 231 (1992).
16. S. Doroudiani, C.B. Park, and M.T. Kortschot, Effect of the Crystallinity and
Morphology on the Microcellular Foam Structure of Semicrystalline Polymers,
Polym. Eng. Sci., 36(21), 2645-2662 (1996).
17. D.F. Baldwin, C.B. Park, and N.P. Suh, Microcellular Processing Study of Poly
(Ethylene Terephthalate) in the Amorphous and Semicrystalline States, Polym.
Eng. Sci., 36(11), 1437-1453 (1996).
18. S.H. Lee, B. Maridass, and J.K. Kim, Dynamic Reaction Inside Co-rotating
Twin Screw Extruder. II. Waste Ground Rubber Tire Powder/Polypropylene
Blends, J. Appl. Polym. Sci., 106(5), 3209-3219 (2007).
Cellular Polymers, Vol. 30, No. 3, 2011
135
Zhen-Xiu Zhang, Lin Li, Zhen Xiang Xin, and Jin Kuk Kim
136
Cellular Polymers, Vol. 30, No. 3, 2011