37 Microcellular Foams of Thermoplastic Vulcanizates (TPVs) Based on Waste Ground Rubber Tire Powder

7/31/2019 37 Microcellular Foams of Thermoplastic Vulcanizates (TPVs) Based on Waste Ground Rubber Tire Powder

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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

Contents lists available at ScienceDirect

Materials Letters

j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
mailto:[email protected]://dx.doi.org/10.1016/j.matlet.2008.07.039http://www.sciencedirect.com/science/journal/0167577Xhttp://www.sciencedirect.com/science/journal/0167577Xhttp://dx.doi.org/10.1016/j.matlet.2008.07.039mailto:[email protected]


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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).

4397Z.X. Zhang et al. / Materials Letters 62 (2008) 43964399


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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.

[6] Stafford CM, Russell TP, McCarthy TJ. Macromolecules 1999;32:7610.[7] Lee KN, Lee HJ, Kim JH. Polym Int 2000;49:712.[8] Liang MT, Wang CM. Ind Eng Chem Res 2000;39:4622.[9] Xu ZM, Jiang XL, Liu T, Hu GH, Zhao L, Zhu ZN, et al. J Supercrit Fluids 2007;41:299.

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37 Microcellular Foams of Thermoplastic Vulcanizates (TPVs) Based on Waste Ground Rubber Tire Powder