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Microcellular foams of thermoplastic vulcanizates (TPVs) based on waste ground
rubber tire powder
Zhen Xiu Zhang a, Shu Ling Zhang a,b, Sung Hyo Lee a, Dong Jin Kang c, Dae-Suk Bang c, Jin Kuk Kim a,a School of Nano and Advanced Materials Engineering, Gyeongsang National University, Jinju, 660-701, South Koreab Alan G. MacDiarmid Lab, College of Chemistry, Jilin University, Changchun, 130012, People's Republic of Chinac Department of Polymer Engineering and Science, Kumoh National Institute of Technology, Gyungbuk, 730-701, South Korea
a b s t r a c ta r t i c l e i n f o
Article history:
Received 1 October 2007
Accepted 24 July 2008
Available online 26 July 2008
Keywords:
Foam
Thermoplastic vulcanizates
Waste ground rubber tire powder
Microstructure
With the increased adoption of thermoplastic vulcanizates (TPVs) in automotive weather seal systems, the
foams of TPVs present an important milestone in providing key applications such as trunk and door seals. In
this study, microcellular foams of TPV based on waste ground rubber tire powder (WGRT) were investigated.
In order to investigate the relationship between processing conditions and structure of TPV foams, we first
prepared the thermoplastic vulcanizates of PPgMA/WGRT, then the samples were saturated with carbon
dioxide and the saturated specimens were expanded during the pressurequench process. The results
indicated that the microcellular structure was dependent on the processing conditions. Cell size increased
with saturation temperature, whereas cell density and relative density decreased. Different nucleation
processes were produced with saturation pressure.
Crown Copyright 2008 Published by Elsevier B.V. All rights reserved.
1. Introduction
To date the best choice for automotive weather seal materials is
ethylenepropylene diamine monomer (EPDM) rubbers [1]. Over the
last decade, thermoplastic vulcanizates have gained increasing
acceptance in numerous fields as a replacement for thermoset rubber
due to its rubberlike performance coupled with the ease of
processing associated with thermoplastics.
Despite these advantages compared to thermoset EPDM, the lack
of foaming technology for TPVs has limited their use. In order to solve
the acute problem of waste ground rubber tires, our lab is researching
the thermoplastic vulcanizates from WGRT [2]. The aim of the work is
to prepare the microcellular foams based on thermoplastic vulcani
zates of PPgMA/WGRT and discuss the influence of processing
conditions on the structure of the foams. By providing a closed cell
structure, microcellular foams of PPgMA/WGRT offer an opportunityto incorporate the advantage of TPVs into foam materials, which
enlarges their application fields.
2. Experimental
2.1. Materials and sample preparation
Polypropylene (R520Y) was manufactured by SK Corporation.
Maleic anhydride (MA) and dicumyl peroxide (DCP) were reagent
grade and obtained commercially from Aldrich. Maleic anhydride
grafted polypropylene (PPgMA) was prepared by our lab according
to the earlier literature [3]. The particle size of waste ground rubber
tire powder was 50 meshes. CO2 with a purity of 99.95% was supplied
by Hyundai Gas Inc. Blends of PPgMA/WGRT (50/50) were extruded
in a twinscrew extruder at the screw speed of 120 rpm and
temperature profile of 130/150/160/160/180/180/180/180/190 C. The
resulting blends were molded at temperature profiles of 190/200/210/
220 C by injection.
Materials Letters 62 (2008) 43964399
Corresponding author. Tel.: +82 55 751 5299; fax: +82 55 753 6311.
E-mail address: [email protected] (J.K. Kim).
Table 1
Processing conditions and foam characteristics for PPgMA/WGRT (50/50) samples
Pressure MPa Temperature C Average cell size
D m
Cell density N0cells/cm3
Relative
density %
16 140 0.6080
16 145 31 1.77 108 0.4187
16 150 75 8.41 107 0.0989
16 154 86 7.56 107 0.0752
10 150 25 2.85 108 0.4685
12 150 149 6.21 106 0.1610
14 150 191 5.41 106 0.0952
18 150 51 1.98 108 0.1294
0167577X/$ see front matter. Crown Copyright 2008 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.matlet.2008.07.039
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2.2. Foam preparation
The foams were prepared with a pressurequench method
described by Goel and Beckman [4]. Plate samples (2312 mm)
were enclosed highpressure vessel. The vessel was flushed with low
pressure CO2 for 3 min and preheated to desired temperature.
Afterward, the pressure was increased to the desired pressure by ahighpressure pump and maintained at this pressure for 1 h. After
saturation, the pressurewas quenched to atmospheric pressurewithin
3 s and the samples were taken out. Then foam structure was allowed
to full growth during rapid depressurization. The processing condition
is listed in Table 1.
2.3. Foam characterization
The foamed samples were fractured in liquid nitrogen, coated with
an approximately 10nm thick layer of gold on the fractured surface,
and observed with a Philips XL 30SFEG scanning electron micro
scope (SEM). The cell diameter (D) is the average of all the cells on the
SEM photo, usually more than 100 cells were measured.
D d= =4 1
Where d is the measured average diameter in the micrograph.
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 relativedensity (r). The volume fraction occupied by the microvoids (Vf)
was calculated as
Vf 1f
m2
Where m and f are the density of the unfoamed polymer and
foamed polymer respectively.
Thecelldensity(N0) based on theunfoamedsamplewas calculated as
Nf Vf
6 D
33
N0 Nf
1Vf 4
Where Vf is the volume fraction occupied by the microvoids, Nf is
the cell density based on the foamed sample.
3. Results and discussion
3.1. Mechanical properties and microstructure of thermoplastic vulcanizates of PP-g-MA/
WGRT
Thermoplasticvulcanizate(TPV)is a special classof thermoplasticelastomers(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
Fig. 1. Microstructure of PPgMA/WGRT foam (scale bar is 50 m).
Fig. 2. SEM micrographs of the specimens produced at different saturation temperature (a) 140 C, (b) 145 C, (c) 150 C, (d) 154 C. (Scale bar is 50 m).
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break is more than 100%. For PP/WGRT (50/50) blend, tensile strength is 11.5 MPa and
elongation is only 39.4%. In order to obtain TPV based on WGRT, PP is replaced by PPg
MA.The reactivityof MAgroupin PPgMAand phenolicOH group in WGRTcan enhance
the compatibility between PP and WGRT [2]. The result is that tensile strength is
11.9 MPa and elongation is 210.4% for PPgMA/WGRT (50/50) blend, namely we have
successfully preparedTPV basedon WGRT. Fig.1 shows a typical microstructureof PPg
MA/WGRT foam. The specific feature of this structure is a microcellular matrix and an
expanded interphase between matrix and WGRT powder.
3.2. Influence of saturation temperature on the microstructure of PP-g-MA/WGRT foam
It has been reported saturation temperature is an important processing parameter
to control the foam structure of microcellular polymer [58]. Fig. 2 shows foam
structures of PPgMA/WGRT (50/50) blend obtained at different saturation tempera
ture for a given saturation pressure of 16 MPa, saturation time of 1 h and
depressurization time of 3 s, and foam characteristics for PPgMA/WGRT (50/50)
blend can be seen in Table 1. From Table 1, it can be found that the average cell size
increases with saturation temperature, whereas the cell density and relative density
decrease. For PPgMA/WGRT (50/50) blend, the cell density and relative density at
140 C are not included in Table 1 due to very inhomogeneous microstructure. As
saturation temperature increases, the expansion increases and the cell walls become
thinner and a structure with a polygonal cells is developed. When the saturation
temperature increases, the crystalline phase could be partly or totally disrupted. Thus
CO2 is expected to be dissolved in and diffuse through both the amorphous phase and
the disrupted part of the crystalline phase [9]. Thus conditions become more favorablefor PPgMA/WGRT (50/50) blend to foam and for the cells to grow bigger in size.
Accordingly, cell density and relative density decrease.
3.3. Influence of saturation pressure on the microstructure of PP-g-MA/WGRT foam
Fig. 3 shows foam structures of PPgMA/WGRT (50/50) blend obtained at different
saturation pressure for a given saturation temperature of 150 C, saturation time of 1 h
and depressurization time of 3 s, and foam characteristics for PPgMA/WGRT (50/50)
blend can be seen in Table 1. From Table 1, it can be found that the average cell size first
increases with saturation pressure until 14 MPa and decreases from 14 MPa to 18 MPa.
Accordingly, the cell density and relative densityfirst decrease with saturation pressure
until 14 MPa and increase from 14 MPa to 18 MPa.
The increase of saturation pressure usually brings about two effects on the cell
growth. On the one hand, when the saturation pressure increases, the extent of the CO2inducedmelting temperature depression increases, namely the amount of CO2dissolved in samples increases. Thus sample is relatively soft and its deformability
and foamability are large. The result is that increasing saturation pressure becomes
more favorable for sample to foam and for the cells to grow bigger in size. On the other
hand, when the saturation pressure increases, namely depressurization rate increases
due to the same depressurization time, the cells have no enough time to grow bigger in
size. As the saturation pressure is lower, both the CO 2 inducedmelting temperature
depression and the amount of CO2 dissolved in sample become very important, so the
average cell size first increases with saturation pressure until 14 MPa, whereas the
corresponding celldensityand relativedensityfirst decrease. As thesaturation pressure
is higher, theeffectof depressurization rateon cellgrowthbecomes veryobvious, so theaveragecell sizedecreases withsaturation pressurefrom 14MPa to 18MPa whereasthe
corresponding cell density and relative density increase.
4. Conclusion
By maleic anhydridegrafted polypropylene (PPgMA) replacing
polypropylene (PP), we successfully prepare thermoplastic vulcani
zate (TPV) based on waste ground rubber tire powder (WGRT) due to
the enhanced compatibility between PP and WGRT. The enhanced
compatibility results from the reactivity of MA group in PPgMA and
phenolic OH group in WGRT. The effects of processing conditions on
the structure of PPgMA/WGRT foams were investigated. Higher
saturation temperature resulted in larger cell size, lower cell density
and relative density, whereas different nucleation processes were
produced with the increase of saturationpressure. Namely the average
cell size first increases then decreases with saturation pressure,
whereas the cell density and relative density first decrease then
increase. In a word, the abundance of foaming technology for TPV
based on WGRT enlarges their application fields.
Acknowledgement
The authors are grateful to the support from the BK21 program in
South Korea.
References
[1] Harper CA. Handbook of Plastics, Elastomers and Composites. 3rd edition. New
York: McGrawHill; 1996. p. 5. 21.
Fig. 3. SEM micrographs of the specimens produced at different saturation pressure (a) 10 MPa, (b) 14 MPa, (c) 16 MPa, (d) 18 MPa. (Scale bar is 200 m).
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[2] Lee SH, Maridass B, Kim JK. J Appl Polym Sci 2007;106:3209.[3] Zhu LC, Tang GB, Shi Q, Cai CL, Yin JH. React Funct Polym 2006;66:984.[4] Goel SK, Beckman EJ. Polym Eng Sci 1994;34:1137.[5] Arora KA, Lesser AJ, McCarthy TJ. Macromolecules 1998;31:4614.
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