dispersion of entangled carbon nanotube by melt extrusion · dispersion of entangled carbon...

Korea-Australia Rheology Journal June 2010 Vol. 22, No. 2 89

Korea-Australia Rheology JournalVol. 22, No. 2, June 2010 pp. 89-94

Dispersion of entangled carbon nanotube by melt extrusion

Joo Seok Oh1, Kyung Hyun Ahn

1 and Joung Sook Hong

2,*1School of Chemical and Biological Engineering, Seoul National University

2Department of Chemical and Environmental Engineering, Soongsil University(Received January 29, 2010; final revision received February 18, 2010; accepted March 3, 2010)

Abstract

This paper investigates the dispersion of carbon nanotube (CNT) in a polymer melt during iterative extrusionby measuring electrical and rheological properties. 2 wt% CNT as received was mixed with polymer (lowdensity polyethylene) through twin screw extruder. The extrusion was iteratively performed at the same pro-cess condition. At the same time, the rheological and electrical properties were measured. We expected themixing energy applied on entangled CNT increases with process time, which improves the CNT dispersion.The electrical property of intermediate composite was effectively improved by iterative extrusion. After thefifth extrusion, CNT/LDPE composite reached to the conductive electric level (surface resistance ≤E+5 Ω/sq). Also, the rheological properties of composite were increased according to CNT dispersion. Especiallythe rheological properties over lower frequency region were significantly increased by the dispersed nan-otube. This paper suggests that the dispersion by only iterative mixing process results in a disentanglementof CNT and they forms an electrically useful structure. The rheological and electrical measurements indicatethat the CNT disentangled by the iterative mixing method forms a percolation structure.

Keywords : carbon nanotube, melt extrusion, surface resistance, rheology, dispersion

1. Introduction

CNT has been regarded as attractive as a reinforcing

agent by its mechanical strength and good conductivity.

CNT with no defect has a high mechanical strength as

much as 1000 GPa which is comparable to that of metal,

glass fiber, and graphite fiber. In addition, the electrical

properties of CNT can be used to induce conductivity to

non-conducting materials(Chen et al. 2006; Harris 1999;

Lau and Hui 2002). As an electrical filler in polymer, car-

bon black has been used in industry due to its low cost and

easy process. Practically, it needs to be contained over

25 wt% to realize the conductivity in an insulating poly-

mer, which makes much difficult to maintain the mechan-

ical property of polymer by high filler content. Also they

come out from the fractured surface during usage as well.

Instead of carbon black, CNT has been studied because

CNT can provide an electrical property with its low con-

tent by its high aspect ratio and high electrical prop-

erty(Harris 1999). However, previous studies on mechanical

and electrical performance in several CNT applications did

not obtain a consistency in properties of CNT-composite

because of the CNT dispersion is much different from the

dispersion of spherical particles such as carbon black. A

nanoscale diameter of CNT and high aspect ratio gives

much larger surface area, which induces significant entan-

glement between tubes even from synthesis step by a van

der Waals force. Then a disentanglement of entangled CNT

must be worked out to realize its high potential in practical

applications.

To improve the dispersion of CNT, a number of methods

have been introduced to produce CNT/polymer composites

by using a chemical modification of incorporated CNT

(Chen et al. 2006; Harris 1999; Lau and Hui 2002; Krause

et al. 2009; Bauhofer and Kovacs 2009; Li and Shimizu

2007; Chen et al. 2007; Zhu et al. 2003; Lee et al. 2008),

in-situ polymerization(Kumar et al. 2002; Deng et al.

2002; Funck and Kaminsky 2007), surfactant-assisted dis-

persion(Vigolo et al. 2002; Rastogi et al. 2008; Huang et

al. 2009), interfacial polymerization(Haggenmueller et al.

2006), electro-spinning(Wang et al. 2005), and solution

mixing(Qian et al. 2000). Those methods provide suc-

cessful progress in performance with the incorporation of

small amount of CNT (<1% CNT). For example, solution

mixing of CNT and molten polystyrene(PS) led to 30~40%

increase in elastic stiffness and 25% increase in tensile

strength with 1% CNT addition(Qian et al. 2000). But

those dispersion methods demand a usage of solvent and

are not practically applicable due to the formation of severe

aggregation of nanotubes as the content of CNT increases.

All methods are based on the chemical treatment to modify

surface functionality or to remove the impurity of CNT.

*Corresponding author: [email protected]© 2010 by The Korean Society of Rheology

Joo Seok Oh, Kyung Hyun Ahn and Joung Sook Hong

90 Korea-Australia Rheology Journal

Without any chemical modification on CNT, the direct

mixing of CNT and matrix did not result in any progress

in the CNT dispersion so far.

In this study, we investigated the dispersion of CNT by

means of polymer melt compounding. Polymer melt com-

pounding with no CNT-chemical treatment is useful, espe-

cially in industry because it does not demand additional

process step. The melt compounding studied so far (Chen

et al. 2006; Harris 1999; Lau and Hui 2002; Krause et al.

2009; Bauhofer and Kovacs 2009; Li and Shimizu 2007;

Chen et al 2007; Zhu et al. 2003) has been performed in

micro-compounder of lab-scale, but we increased the scale

of compounding extruder (L/D~38.7). Furthermore, the

composites were subjected to the extrusion iteratively.

Then, the applied mixing energy on CNT dispersion would

be further increased. The rheological and electrical prop-

erties of polymer composite were measured after each

extrusion. Electrical, rheological and morphological

approaches have shown how CNT dispersion is accom-

plished depending on iterative process.

2. Experimental

2.1. MaterialsLow density polyethylene (LDPE 5322) used in this

study was provided by Hanwha Co. Ltd (Korea). The den-

sity of LDPE is 0.921 g/cm3(ASTM D1505) and it has a

melt flow index of 3.2 g/10 min (ASTM D1238, 190oC/

2.16 kg). CNT used in this study is multiwalled carbon

nanotube purchased from Nanocyl Co. Ltd (NC7000). It is

produced by the chemical vapor decomposition method

and the purity is higher than 87.5%. Fig. 2 presents the

SEM image of CNT powders (NC7000).

2.2. Preparation of CNT/LDPE compositesCNT/LDPE composites were prepared by melt com-

pounding of LDPE and CNT. Melt compounding was per-

formed by using a twin screw extruder (APV Co. D=19 mm,

L/D=38.7) with two kneading zones. Extruder has seven

heating zones and its temperature profile was set to 170,

180, 190, 200, 210, 210, 210oC from feeding zone to die,

respectively. The rotation speed of screw was fixed to

200 rpm. The composition of CNT was fixed to 2 wt% in

this study. As shown in Fig. 1, extrusion was repetitively

performed under the same extrusion condition. After each

extrusion, the extrudate was pelletized and dry-mixed by

mild-tumbler. To investigate the effect of extrusion on

CNT dispersion, the extrudates were then compression-

molded for rheological and electrical measurement using a

Fig. 1. Schematic diagram of iterative mixing process: extrusion-

pelletizing-dry mixing. The CNT/LDPE extrudate is pel-

letized and dry-mixed. Mixing process is repetitively per-

formed at the same process condition. As the iteration

goes on, the entangled CNT disperse more, which

increase rheological property of composite and decreases

electrical resistance.

Fig. 2. SEM picture of CNT powder (Nanocyl NC7000).

Fig. 3. Comparison of rheological properties(complex viscosity

η*, storage modulus G’) of LDPE depending on the num-

ber of extrusion.



hot press molder (PHI Pasadena Hydraulics, Inc.). It was

molded at 190oC for 10 min and then annealed at room

temperature. Because the iterative extrusion of polymer may

cause degradation by shear and heat, the rheological prop-

erties of LDPE were also checked out as shown in Fig. 3.

The rheological properties of LDPE maintains constant for

fifth extrusion, which means there is no change in molec-

ular structure by iterative extrusion.

2.3. CharacterizationThe composite morphology was examined by scanning

electron microscopy (SEM) using a JEOL model Hitachi-

S4800 apparatus operating at an accelerating voltage of

7 kV to 20 kV. The samples for SEM were fractured in liq-

uid nitrogen after each extrusion and then sputtered with

palladium to avoid charging on the fractured surface.

The thermal analysis was measured by differential scan-

ning calorimeter(DSC) using a DSC Q2000 (TA instru-

ments). DSC was performed with sample of about 2 to

5.0 mg between 20 to 150oC by 10oC/min.

The rheological properties were measured at 190oC by

means of RDSII (Rheometrics Inc.) with a parallel plate fix-

ture (25 mm diameter). The complex viscosity (η*(Pas)),

storage modulus (G’(Pa)), and loss modulus (G”(Pa)) were

measured as a function of frequency(ω(rad/s)) using the

dynamic oscillatory mode. As an electrical property, the

surface resistance was measured by Hiresta-Up (Mitsubishi

Chemical Co.) with bar-type probe.

3. Results

The addition of 2 wt% CNT to polymer melt leads to a

significant increase in viscosity by a physical networking

of the dispersed CNTs (Huang et al. 2006). However, this

increase in viscosity of composite is able to be expected

when 2wt% CNT is dispersed sufficiently. From a hydro-

dynamical point of view to accomplish the CNT disper-

sion, the composite needs to be under process for several

times under simple shear flow to apply more energy

against whole van der Waals force between entangled

CNTs (Huang et al. 2006). In fact, it is difficult to expect

the dispersion of CNT by simple extrusion of short pro-

cessing time. If the extrusion time is longer by iterative

extrusion, the CNT dispersion is able to be expected as

shown in Fig. 1.

First of all, 2 wt% CNT/LDPE composite and LDPE

were checked their degradation by iterative extrusion based

on rheology and thermal analysis (DSC). From Fig. 3 and

Fig. 4, it is known that iterative extrusion of short pro-

cessing time does not modify LDPE and composite. Fig. 4

compares the thermal analysis of composite and LDPE

with the number of extrusion. As expected from Fig. 3, the

thermal analysis of LDPE is not changed during repetitive

extrusion. In the case of 2 wt%CNT/LDPE, the slope of

beginning stage of endotherm peak looks changed a little

by iterative extrusion but the maximum peaks are the same

at 108oC, which means the molecular weight of PE is not

changed during iterative extrusion.

Fig. 5 represents the variation of rheological properties

of 2 wt% CNT/LDPE composite depending on the number

of extrusion. Because LDPE was confirmed its consis-

tency of rheological properties over repetitive extrusions

as shown in Fig. 3, it can be considered the increment of

rheological properties is purely caused by the dispersion

of CNT. The storage modulus reflects the structural

change of composite by the influence of relaxation time

Fig. 4. Comparison of thermal analysis (DSC) of LDPE,

2 wt%CNT/LDPE depending on the number of extrusion.

For the DSC measurement, heat scan was performed

twice between 20oC and 150oC by 10oC/min. The DSC

data shown above is on second round.

Fig. 5. Comparison of rheological properties(complex viscosity

η*, storage modulus G’) of 2 wt%CNT/LDPE depending

on the number of extrusion.



( , here λ is relaxation time and

H(λ) is relaxation time spectrum)(Larson 1999). The stor-

age modulus of the first extrudate simply increases all over

the observation frequency region compared to that of

LDPE, which is expected by the filling effect in composite.

However, when extruded three times, the storage modulus

increases while the slope at lower frequency region starts

to decrease. Even after the fifth extrusion, the storage mod-

ulus of composite increases further and they present the

plateau at lower frequency region. We believe that this pla-

teau of storage modulus is originated from the network

structure of the disentangled CNTs (Huang et al. 2006).

The disentangled CNT initially increases the relaxation

times like an ultra molecule, which is reflected in the

increases in storage modulus at low frequency region (Lar-

son 1999). Table 1 lists the slope of G’ at lower frequency

depending on the number of iteration. The slope of 2 wt%

CNT/LDPE shows non-terminal behavior as the number of

extrusion increases. The slope of composite after fifth

extrusion significantly decreases from 17.82 to 4.6. As

CNT disentangles further over several extrusions, they

form a physical network structure. Then, it dominates the

rheological behavior of the composite, as can be seen with

the plateau at low frequency region. Meanwhile, the struc-

tural change of CNT/LDPE composite is confirmed

through the surface resistance. Fig. 6 shows the surface

resistance of extrudates with extrusion time. The surface

resistance is simply decreased over the extrusion time,

which means the number of CNT interconnection effec-

tively increases as the extrusion goes on. The surface resis-

tance is much lower when CNT in polymer forms an

electric path homogeneously. With the fifth extrusion, the

composite reaches to the conducting electric level (surface

resistance ≤E+5Ω/sq). Above fifth extrusion of compos-

ite, the decrease of surface resistance becomes slow.

Fig. 7 compares the morphology of CNT/LDPE com-

posite. The morphology of the first extrudate is not so dif-

ferent from that of composite after the fifth extrusion. Over

iterative extrusions, the change of rheological and elec-

trical properties of the composite can be strong evidence to

the change of the CNT dispersion, while the morphological

observations are difficult to discriminate the effect of mul-

tiple extrusions on CNT dispersion.

To ensure those observations, the iterative extrusions were

performed with other commercial multi-walled CNTs (JEIO

(purity 94.1%), CM95 (purity, 95%)). All CNTs show the

same effect of multiple extrusions on rheological properties

and surface resistance of CNT composite. Table 2 lists the

surface resistance and storage modulus of composite of dif-

ferent CNTs depending on extrusion time. The storage

modulus at frequency 0.1 rad/s increases as much as 300%

G′ ω( )H λ( )ω2λ2

1 ω2λ2+

------------------------ λln( )d∞–

∞

∫=

Table 1. Slope of G’ of 2 wt%CNT/LDPE

MaterialsSlope of G’/G’

ω=0.1 at

lower frequency

LDPE 17.8

2 wt%CNT/LDPE(# iteration=1) 14.9



Fig. 7. Comparison of morphologies of 2 wt%CNT/LDPE after

the first (a-c) and the fifth extrusion (d-f).

Fig. 6. Variation of surface resistance of 2 wt%CNT/LDPE

depending on the number of extrusion.



in case of NC7000. Also, the surface resistance decreases

significantly with extrusion time. Those observations show

that the change of electrical and rheological property of

composite depending on the number of extrusion is not so

different with CNT though CNT has different morphology

or geometry according to the synthesis of CNT. Rather the

dispersion of CNT influences the property of composite

more significantly.

4. Discussion

Based on the observations in this study, it can be known

that the iterative extrusion of CNT composite results in the

increase in storage modulus and viscosity and the improve-

ment of electric conductivity by the enhanced CNT dis-

persion. We believe that the repetitive hybrid mixing of dry

mixing and melt compounding improves effectively the

dispersion of CNT. After extrusion, CNT composite extru-

date solidifies under air and it is continuously pelletized as

small as 3~4 mm of diameter, and then it is dry-mixed

again. The pelletizing and dry-mixing enhance the long

range dispersion of CNT particles as shown in Fig. 8. The

following melt compounding induces the long and short

range dispersion of CNT. To obtain CNT dispersion espe-

cially under viscous medium, it requires large energy

enough to unravel the individual CNT out of the entangled

CNT clumps because CNT has a significant van der Waals

force by large surface area. As can be known from Fig. 3,

LDPE is viscous as much as 100 to 1000 Pas at high shear.

CNT with the aspect ratio of 1000 will take a long time to

diffuse under the given flow condition. In fact, the resi-

dence time of extrusion at 200 rpm (apparent shear rate at

extruder clearance of 2 mm gap>10000 s-1) ranges only 2

to 5 minutes. Then, it is difficult to expect the complete

disentanglement of CNT clumps in a single extrusion

though the complex flow in an extruder is able to induce

dispersion effectively. Therefore, the iterative process of

dry-mixing and melt-compounding effectively induces the

long and short range dispersion of CNT and the property of

the composite changes significantly. Fig. 9 presents the

surface resistance and rheological property of CNT com-

posite over iterative extrusion. The surface resistance lin-

early decreases with iteration. After the fifth iteration, CNT

forms an electrical percolation over observation area and

Table 2. Complex viscosity, storage modulus and surface resis-

tance of composite with different CNT

CNT#

iteration

Viscosity

(Pas) at

shear rate

0.1 rad/s

Storage

Modulus

(Pa) at

shear rate

0.1 rad/s

Surface

resistance

(Ω/sq)

NC7000

1 6.6 K 0.1 K 1.0E+12

3 10.5 K 0.4 K 2.7E+08

5 20.6 K 1.4 K 4.3E+05

JEIO

1 6.6 K 0.1 K 1.0E+11

3 9.6 K 0.3 K 6.3E+09

5 15.5 K 0.8 K 2.2E+05

CM95

1 5.9 K 0.1 K 1.0E+12

3 7.7 K 0.2 K 1.0E+12

5 9.3 K 0.3 K 1.0E+10

Fig. 8. Schematic diagram of CNT dispersion during mixing:

long range dispersion occurs during dry mixing and extru-

sion, short range dispersion is an individual CNT dif-

fusion. A long- and short range dispersion simultaneously

occurs during extrusion and a dry mixing helps long range

dispersion of CNT.

Fig. 9. Variation of surface resistance and complex viscosity of

2 wt%CNT/LDPE depending on the number of extrusion.



the surface resistance almost reaches to a saturation value.

However, the rheological properties of the composite

change continuously over extrusions. Especially, the rheo-

logical properties at low shear rate increase significantly,

while those at higher shear rate increase a little. It means

that CNT maintains the diffusion over multiple extrusions

and the multiple extrusions enhance the dispersion con-

tinuously. The dispersed CNTs increase the complex vis-

cosity of CNT composite further. Then, we believe that the

rheological properties indicated the degree of CNT dis-

persion more precisely compared to the electrical property.

5. Conclusions

The iterative mixing process of dry mixing and melt

compounding was found to enhance CNT dispersion. Iter-

ative extrusion was found to apply more mixing energy on

CNT particles to induce CNT dispersion. During iterative

extrusions, the surface resistance of composite decreased

and their rheological properties were increased. The reduc-

tion of surface resistance and the increases in rheological

properties were caused by the disentangled CNT. This

paper shows that the intensive mixing process can induce

a disentanglement of CNT dispersion effectively.

References

Bauhofer, W. and J. Z. Kovacs, 2007, A review and analysis of

electrical percolation in carbon nanotube polymer composites,

Composites Science and Technology 69, 1486-1498.

Chen, G.-X., H.-S. Kim, B. H. Park and J. S. Yoon, 2006, Multi-

walled carbon nanotubes reinforced nylon 6 composites, Poly-

mer 47, 4760-4767.

Chen, G.-X., Y. Li and H. Shimizu, 2007, Ultrahigh-shear pro-

cessing for the preparation of polymer/carbon nanotube com-

posites, Carbon 45, 2334-2340.

Deng, J., X. Ding, W. Zhang, Y. Peng, J. Wang, X. Long, P. Li,

and A. S. Chan, 2002, Carbon nanotube-polyaniline hybrid

materials, Eur Polym J. 38, 2497.

Funck, A. and W. Kaminsky, 2007, Polypropylene carbon nan-

otube composites by in situ polymerization, Composites Sci-

ence and Technology 67, 906-915.

Haggenmueller, R., F. Du, J. E. Fischer, and K. I. Winey, 2006,

Interfacial in situ polymerization of single wall carbon nan-

otube/nylon 6,6 nanocomposites, Polymer 47, 2381-2388.

Huang, Y. Y., S. V. Ahir, and E. M. Terentjev, 2006, Dispersion

rheology of carbon nanotubes in a polymer matrix, Phys. Rev.

B 73, 125422.

Ji,Y., Y. Y. Huang, A.R. Tajbakhsh, and E. M. Terentjev, 2009,

Polysiloxane surfactants for the dispersion of carbon nanotubes

in nonpolar organic solvents, Langmuir 25(20), 12325.

Krause, B., P. Potschke, and L. Haußler, Influence of small scale

melt mixing conditions on electrical resistivity of carbon nan-

otube-polyamide composites, Composites Science and Tech-

nology 69, 1505-1515.

Kumar, S., T. D. Dang, F. E. Arnold, A. R. Bhattacharyya, B. G.

Min, X. Zhang, R. A. Vaia, C. Park, W.W. Adams, R. H.

Hauge, R. E. Smalley, S. Ramesh, and P. A. Willis, 2002, Syn-

thesis, Structure, and Properties of PBO/SWNT Composites,

Macromolecules 35,9039-9043(2002).

Larson, R., 1999, The structure and rheology of complex fluids,

Oxford University Press, New York.

Lau, K.-T. and D. Hui, 2002, The revolutionary creation of new

advanced materials – Carbon nanotube composites, Compos-

ites Part B 33, 263-277.

Lee, G.-W., S. Jagannathan, H. G. Chae, M. L. Minus, and S.

Kumar, 2008, Carbon nanotube dispersion and exfoliation in

polypropylene and structure and properties of the resulting

composites, Polymer 49, 1831.

Li, Y. and H. Shimizu, 2007, High-shear processing induced

homogenous dispersion of pristine multiwalled carbon nan-

otubes in a thermoplastic elastomer, Polymer 48, 2203-2207.

Liu, J., T. Wang, T. Uchida, and S. Kumar, 2005, Carbon nan-

otube core-polymer shell nanofibers, J App. Poly. Sci. 96(5),

1992-1995.

Peter J. F. Harris, 1999, Carbon nanotubes and related structures

: new materials for the 21st century, Cambridge University

Press.

Qian, D., E.C. Dickey, R. Andrews, and T. Rantell, 2000, Load

transfer and deformation mechanisms in carbon nanotube-

polystyrene composites, Appl Phys Lett. 76, 2868.

Rastogi,R., R. Kaushal, S. K. Tripathi, A. L. Sharma, I. Kaur, and

L. Bharadwaj, 2008, Comparative study of carbon nanotube

dispersion using surfactants, J. Colloid Int. Sci. 328, 421-428.

Vigolo, B., P. Poulin, M. Lucas, P. Launois, and P. Bernier, 2002,

Improved structure and properties of single-wall carbon nan-

otube spun fibers, Appl Phys Lett. 81, 1210.

Zhu, J., J. Kim, H. Peng, J. L. Margrave, V.N. Khabashesku, and

E.V. Barrera, 2003, Improving the dispersion and integration of

single-walled carbon nanotubes in epoxy composites through

functionalization, Nano Lett. 3, 1107-1113.

dispersion of entangled carbon nanotube by melt extrusion · dispersion of entangled carbon...

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