POLYMERTESTING

Polymer Testing 27 (2008) 129–134

ARTICLE IN PRESS

0142-9418/$ - see

doi:10.1016/j.po

�CorrespondiE-mail addre

www.elsevier.com/locate/polytest

Material Properties

Dynamic rheology and microstructure of polypropylene/claynanocomposites prepared under Sc-CO2 by melt compounding

Yang Zhao, Han-Xiong Huang�

Center for Polymer Processing Equipment and Intellectualization, College of Industrial Equipment and Control Engineering,

South China University of Technology, Guangzhou 510640, PR China

Received 25 August 2007; accepted 5 November 2007

Abstract

Polypropylene (PP)/clay nanocomposites were prepared using a twin-screw extruder with the aid of supercritical carbon

dioxide (Sc-CO2). The dynamic rheological properties were measured using a rheometer in the oscillatory mode. X-ray

diffraction and transmission electron microscopy were used to characterize the microstructure of extruded

nanocomposites. Results showed that an optimized CO2 concentration existed. When the CO2 concentration increased

up to the optimized level, the nanocomposites tended to be more viscous, especially at low frequency, whereas further

increasing the CO2 concentration resulted in a decrease in the complex viscosity and dynamic moduli. The presence of Sc-

CO2 with a concentration not higher than the optimized level was helpful to promote the degree of dispersion of the nano-

clay in PP matrix, but overloading of CO2 would have a negative effect on the clay dispersion.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Polymer nanocomposite; Supercritical carbon dioxide; Melt compounding

1. Introduction

Polymer/clay nanocomposites (PCNs) have beenthe subject of intensive research in recent years.Typically, PCNs are prepared via in-situ intercala-tive, sol-gel methods or a direct melt-intercalationapproach such as melt extrusion [1–4]. No matterwhich method is used, the extent of dispersion ofclay in the polymer is mainly decided by theinterfacial interaction between the polymer matrixand the clay.

In the preparation of polypropylene (PP)/claynanocomposite (PPCN) by melt-intercalation, PP

front matter r 2007 Elsevier Ltd. All rights reserved

lymertesting.2007.11.006

ng author. Tel./fax: +8620 2223 6799.

ss: mmhuang@scut.edu.cn (H.-X. Huang).

has an unfavorable interaction with the clay becausePP does not include any polar group in itsbackbone, and silicate layers even modified bynon-polar alkyl groups are incompatible with PP.So, some chemical methods such as using compa-tibilizers are frequently used to promote the nano-clay dispersion [5–7]. However, the addition of thecompatibilizers results in lower mechanical proper-ties of the final composite [7].

Recently, a great deal of attention has been givento the preparation of nanocomposites with the aidof supercritical carbon dioxide (Sc-CO2) to expandthe clay intergallery and promote polymer inter-calation. Sc-CO2 as a kind of green solventoffers many advantages compared with othersolvents. Direct injection of Sc-CO2 into a molten

.

ARTICLE IN PRESSY. Zhao, H.-X. Huang / Polymer Testing 27 (2008) 129–134130

nanocomposite during melt blending was proved tobe useful for the dispersion of clay. Treece et al. [8]used a single-screw extruder to prepare PPCNs andfound that Sc-CO2 was useful to improve the claydispersion. Garcia-Leiner et al. [9,10] used poly-ethylene and clay to prepare nanocomposite in asingle-screw extruder with a modified hopperequipped with a Sc-CO2 injection pump. Theirresults showed a 40–100% increase in basal spacingof clay with the aid of Sc-CO2. Han et al. [11]employed a two-time extrusion process to preparethe PP/clay nanocomposites with the aid of Sc-CO2,and found that 2wt% is the best concentration forSc-CO2 to promote the dispersion of clay.

However, some researchers found that using Sc-CO2 did not improve the clay dispersion. Yang et al.[12] found a negative effect of Sc-CO2 on the claydispersion for nylon 6/clay nanocomposites. Theythought that, with the addition of supercriticalfluids, the free volume of polymer melt increasedand the melt viscosity decreased, which meant thatinteraction between the molecule chains decreased.These factors did not contribute towards improvingclay dispersion.

In consideration of the aforementioned contraryresults, the objectives of this work are to demon-strate the effect of Sc-CO2 on the clay dispersionand further investigate the relationship between theconcentration of Sc-CO2 and the final microstruc-ture of PPCNs. An industrial-scale twin-screwextruder (TSE) was used for the continuous extru-sion of PPCNs with the Sc-CO2 injected into thebarrel. The prepared PPCN pellets were thencompression molded into samples to be used forcharacterization.

Fig. 1. Schematic of the extrusion process

2. Experimental

2.1. Materials and equipment

The PP used was grade J501 (fiber extrusiongrade) with a melt index of 2.7 g/10min (230 1C,2.16 kg), manufactured by Sinopec Group Guangz-hou Co. The organically treated clay used was acommercial product of Nanocor USA, octadecylamine modified Nanocor I30P, having particle sizewithin a range 16–20 mm and basal spacing of2.1 nm. Industrial carbon dioxide was used withpurity of 99.5%. The clay was dried under vacuumat 80 1C for 12 h before use.

The experimental equipment is schematicallyshown in Fig. 1. The equipment mainly included aco-rotating TSE (35mm diameter, 40 length-to-diameter ratio) and a CO2 injection system. Themetered CO2 injection system had a cylinder, apositive displacement syringe pump (500D, ISCO)and back pressure regulators.

The melt and Sc-CO2 were mixed using a screwconfiguration as shown in Fig. 2. The screw wasarranged with conveying, kneading, mixing, andreverse conveying elements. The polymer/CO2 solu-tion can only be obtained at pressures above thesolubility pressure of CO2, so the screw configura-tion was arranged to generate the required pressure.There were six reverse conveying elements insertedto elevate the pressure in the barrel. Between theCO2 injection port and the CO2 vent port, kneadingand mixing elements were added to improve themixing efficiency. At the same time, these kneadingand reverse conveying elements helped to generatemelt seal and prevent CO2 from leaking. During the

to prepare the PPCNs using Sc-CO2.

ARTICLE IN PRESS

Fig. 2. Schematic of screw configuration used in this work.

Y. Zhao, H.-X. Huang / Polymer Testing 27 (2008) 129–134 131

experiments, the pressure in the cylinder was keptabove 8MPa.

2.2. Experimental procedure and sample preparation

A mixture of 97wt% PP and 3wt% clay was fedto the hopper. The compounding was carried out ata feed rate of 4.4 kg/h, a screw speed of 100 rpm andthe temperature profile of 30–100–160–160–110–90–90–170–170–170 1C from the hopper to thepelletizing die. The Sc-CO2 was injected into theextruder at a position about 22 times the screwdiameter (D) from the hopper. The CO2 in thesyringe pump was compressed to 20MPa, and theflow rate used to inject CO2 was adjusted to keepthe CO2 concentration at 0%, 2.5%, 4%, and 7% ofthe feed rate of the polymer/clay mixture. At aposition of about 34D from the hopper, CO2 in thenanocomposite was vented by a vacuum pump. Theextruded nanocomposite strand was then pelletizedafter solidification in a water bath. Finally, thepellets were dried overnight at 80 1C prior topreparing samples for rheological tests and micro-structure characterization.

2.3. Characterizations

A Bohlin Gemini 200 Rheometer equipped with aparallel-plate fixture (25mm diameter) was used inan oscillatory mode to conduct dynamic frequencysweep measurement of the nanocomposites. Dy-namic complex viscosity (Z�) and the storage andloss moduli (G0 and G00) as functions of angularfrequency (o) ranging from 0.1 to 100 rad/s weremeasured at 190 1C. A fixed strain of 1% was usedto ensure that measurements were carried out withinthe linear viscoelastic range of the materialsinvestigated. Samples with a thickness of about1mm for the test were prepared by compressionmolding the PPCN pellets.

An X-ray diffractometer was used to measure thebasal spacing between silicate layers in the nano-composites. The X-ray diffraction (XRD) wasperformed using a Japan Rigaku D/max-IIIA

diffractometer at room temperature. The sampleswere scanned in 2y ranges 1.6–101 at a rate of11/min. The generator was operated at 40 kV and30mA. Samples with a thickness of about 0.15mmwere prepared by compression molding the PPCNpellets.

Ultra-thin films about 100 nm thick were cut fromthe samples by an ultramicrotome. The ultra-thinfilms were then examined by transmission electronmicroscopy (TEM, Jeol JEM-100CX II), operatedat an accelerating voltage of 100 kV, to observe thedispersion state of the nano-clay in PPCNs preparedwithout and with 2.5wt% CO2.

3. Results and discussion

The rheological response of filled polymers haspreviously been shown to be quite sensitive to filledparticle loading, size and dispersion [1,6,7]. There-fore, rheological tests, as an effective indirect probeof microstructure of PCNs, can be employed as apowerful tool for characterizing the state of claydispersion in the polymer.

Fig. 3 gives the G0 and G00 versus o curves ofPPCNs prepared with and without Sc-CO2. As canbe directly seen, the G0 of PPCNs prepared with Sc-CO2 is higher than those prepared without Sc-CO2.The PPCNs prepared with Sc-CO2 have a greaterincrease in G0 in the low-frequency region than thosein the high-frequency region. Moreover, it isinteresting to note that at low-frequency G0 in-creases significantly with adding 2.5wt% CO2, but afurther increase in the CO2 concentration up to4wt% does not increase G0 much more. When7wt% CO2 is added, G0 is lower than that of PPCNsprepared with 2.5 and 4wt% Sc-CO2.

Similar to G0, G00 increases with the aid of Sc-CO2

over the whole frequency region, but the incrementfor G00 is less than that for G0 when CO2 is added.This means that the Sc-CO2 has a more significanteffect on the elastic behavior than the viscousbehavior.

Fig. 4 gives the Z� versus o curves of PPCNsprepared with and without Sc-CO2. Z� is calculated

ARTICLE IN PRESS

Fig. 4. Comparison of the complex viscosity Z� of PPCNs prepared with different CO2 concentrations at 190 1C.

Fig. 3. Comparison of the dynamic moduli G0, G00 of PPCNs prepared with different CO2 concentrations at 190 1C.

Fig. 5. XRD patterns for PPCNs prepared with different CO2 concentrations.

Y. Zhao, H.-X. Huang / Polymer Testing 27 (2008) 129–134132

ARTICLE IN PRESSY. Zhao, H.-X. Huang / Polymer Testing 27 (2008) 129–134 133

using the following equation:

Z� ¼ ½ðG0=oÞ2 þ ðG00=oÞ2�1=2. (1)

Table 1

XRD results for PPCNs prepared with different CO2 concentra-

tions

2y (deg.) Basal spacing

(nm)

Peak intensity

(counts/s)

Clay (I30P) 4.2 2.1 –

PPCNs

0wt% CO2 3.4 2.6 2500

2.5wt% CO2 3.2 2.8 1300

4wt% CO2 3.2 2.8 1300

7wt% CO2 3.4 2.6 2000

Fig. 6. TEM photomicrographs of PPCNs prep

The addition of Sc-CO2 results in the increase inZ� of the nanocomposites. Similar to G0, when theCO2 concentration increases up to 7wt%, Z� ofPPCNs is lower than that prepared with 2.5 and4wt% Sc-CO2.

The rheological behavior of PCNs at lowfrequency is not only influenced by the clay loadingbut also related to the clay networks formed in thepolymer matrix [8]. Because the clay content waskept constant at 3wt% in this work, the rheologicalbehavior shown in Figs. 3 and 4 suggests that thepresence of Sc-CO2 in the extrusion process changesthe final microstructure of the PPCNs. Further-more, the Sc-CO2 reduces the viscosity of thepolymer matrix melt and, therefore, promotes

ared (a) without and (b) with 2.5% CO2.

ARTICLE IN PRESSY. Zhao, H.-X. Huang / Polymer Testing 27 (2008) 129–134134

diffusion of polymer chains between the silicategalleries [13].

Fig. 5 shows the XRD patterns for PPCNsprepared under Sc-CO2 with concentrations varyingfrom 0 to 7wt%. The results of basal spacinginferred from Bragg’s law and the d(001) peakintensity are summarized in Table 1. The increasein basal spacing (d(001) ¼ 2.6 nm) for the PPCNswithout CO2 compared with dry I30P clay(d(001) ¼ 2.1 nm) shows that the gap between silicatelayers expands because of intercalation of PP chainsinto the gallery spaces.

Compared with the PPCN prepared without CO2,the PPCNs prepared with 2.5 and 4wt% CO2 havea lower angle of d(001) peaks (2y ¼ 3.21). Thisindicates that 2.5 and 4wt% CO2 concentrationsare helpful for the clay layer expansion. Moreover,the peak intensities for the PPCNs prepared with 2.5and 4wt% CO2 are lower than that of 0wt% CO2.This may mean that more clay layers are exfoliatedfrom clay stacks when adding 2.5 and 4wt% CO2.The PPCN prepared with 7wt% CO2 has the sameangle of d(001) peak as the one prepared withoutCO2, whereas the peak intensity of the former is alittle lower than that of the latter. This may mean aslight promotion in the clay dispersion when 7wt%CO2 is added, and there may exist an optimizedconcentration of CO2 to extremely promote the claydispersion. When the CO2 is overloaded, insolubleCO2 may have a negative effect on the dispersion ofclay. This may be due to the fact that the insolublegas occupies the high shearing space in the barreland the kneading or shearing effect on the melt isdecreased.

Fig. 6 illustrates the TEM photomicrographs ofPPCNs prepared without and with 2.5% CO2.Improved clay dispersion, more uniform distribu-tion of stacked clays and thinner silicate layers canbe seen in PPCN prepared with 2.5wt% CO2.

4. Conclusion

A Sc-CO2-assisted polymer extrusion setup wasdesigned to prepare PPCNs. The rheological prop-erties and microstructures of the PPCNs werecharacterized using a dynamic rheological test,

XRD and TEM. The results show that the Sc-CO2

has significant influence on the rheological behaviorand microstructure of PPCNs when the CO2

concentration is lower than an optimized level.Rheological test results show that the complexviscosity and dynamic moduli increase for PPCNsprepared with Sc-CO2. The XRD results indicatethat the addition of Sc-CO2 enhances the degree ofintercalation, and the basal spacing between silicatelayers increases with increase of the CO2 concentra-tion lower than the optimized level. Better disper-sion of clay is directly observed from TEM for thePPCNs prepared with the Sc-CO2 with a concentra-tion lower than the optimized level.

Acknowledgments

Financial support provided by the NationalNatural Science Foundation of China (10672061),Teaching and Research Award Program for Out-standing Young Teachers in Higher EducationInstitutions of MOE, P.R.C. is gratefully acknowl-edged.

References

[1] E.P. Giannelis., R. Krishnamoorti, E. Manias, Adv. Polym.

Sci. 138 (1992) 107.

[2] R.A. Vaia, E.P. Giannelis, Polymer 42 (2001) 1281.

[3] T.D. Fornes, D.R. Paul, Polymer 43 (2002) 5915.

[4] J.W. Cho, D.R. Paul, Polymer 42 (2001) 1083.

[5] M. Kawasumi, N. Hasegawa, M. Kato, A. Usuki, Macro-

molecules 30 (1997) 6333.

[6] J. Li, C.X. Zhou, G. Wang, W. Yu, Y. Tao, Polym. Compos.

24 (2003) 323.

[7] W. Lertiwimolnum, B. Vergnes, Polymer 46 (2005) 3462.

[8] M.A. Treece, P.J. Oberhauser, J. Appl. Polym. Sci. 103

(2007) 884.

[9] M. Garcia-Leiner, A.J. Lesser, J. Appl. Polym. Sci. 93 (2004)

1501.

[10] M. Garcia-Leiner, A.J. Lesser, ANTEC Tech Papers, 2004,

p. 1528.

[11] J.H. Han, S.M. Lee, Y.J. Ahn, H. Kim, J.G. Kim, J.W. Lee,

ANTEC Tech Papers, 2005, p. 1982.

[12] K. Yang, R. Ozisik, Polymer 47 (2006) 2849.

[13] Q.T. Nguyen, D.G. Baird, ANTEC Tech Papers, 2006,

p. 268.