Chiang Mai J. Sci. 2013; 40(2) 261

Chiang Mai J. Sci. 2013; 40(2) : 261-273http://it.science.cmu.ac.th/ejournal/Contributed Paper

Effects of Reprocessing on the Structure andProperties of Polycarbonate/Multi-WalledCarbon Nanotube Based ElectrostaticDissipative CompositesDarunee Aussawasathien*, Natcha Prakymoramas and Dumrong ThanomjitrPlastics Technology Lab, Polymer Research Unit, National Metal and Materials Technology Center,Pathumthani 12120, Thailand.*Author for correspondence; e-mail: daruneea@mtec.or.th

Received: 6 February 2012Accepted: 2 July 2012

ABSTRACTPolycarbonate (PC)/multi-walled carbon nanotube (MWCNT) based electrostatic

dissipative (ESD) composite was reprocessed by repeated injection molding to investigate ifthis material can be successfully recycled. The varied parameter was the number of cycles(1-6) at the processing temperature of 235-280oC. The effects of reprocessing on thestructure, rheological, electrical and mechanical properties were studied. The ESDproperties such as electrical resistance, tribo-charge voltage, and decay time were in thestatic dissipative range for ESD application after reprocessing. The decreases in heatdeflection temperature were insignificant. The tensile strength was slightly increasedand almost no change was observed at the higher number of injection molding cycles(4-6 cycles). In contrast, the elongation at break and impact strength were decreased andbegan to slightly change at injection molding cycles of 4-6. The changes in mechanicalproperties partly resulted from the larger exposure to the shear stresses in the melt as thenumber of injection molding cycles increased, leading to better dispersion level ofMWCNTs in the polymer matrix. There was no observable chemical change as well asmaterial degradation after reprocessing for 1-6 cycles. However, the polymer chain scissionwas observed since the melt flow index increased and the storage modulus, loss modulus,and complex viscosity decreased when the number of injection molding cycles increased.Overall, it was concluded that the PC/MWCNT composite had high possibility forreprocessing because there were only minor material property changes after 6 injectionmolding cycles, except for its mechanical and rheological properties.

Keywords: polycarbonate, muti-wall carbon nanotube, reprocessing, injection molding

1. INTRODUCTIONPolymer recycling is of great interest in

respect of economical and environmentalconcerns [1, 2]. For economic reasons, thereprocessing of scrap material from molding

processes such as sprues, runners, andunwanted parts is usually performed especiallyfor high performance and expensive polymers.Moreover, polymer recycling is currently used

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to reduce the environmental problemscaused by the increasing quantity of post-consumer plastic products in the wastestream. Increasing environmental concernsand legislation have resulted in significantpressure to reduce, reuse or recyclevarious waste polymer products. Mostly,polymer recycling is carried out byreprocessing, involving several hightemperature shearing cycles that lead tomaterial deterioration such as the occurrenceof thermal and/or mechanical degradation,the consequence of chemical and physicalchanges, and the damage of final properties.Therefore, knowledge of the nature andintensity of the degradation is significantlyimportant to prevent such materialdetriments by selecting a suitable stabilizerand/or the maximum amount of recycledmaterial which may be added to the virginmaterial with an acceptable level in theultimate properties.

A number of studies on reprocessingand its effects on material properties havebeen published for many kinds of polymerssuch as polyolefins [3, 4], polycarbonate (PC)[5], poly(ethylene terephthalates) [6], andpolyamides (PAs) [7-11]. Reprocessing ofpolymer blends such as PC/ poly (butylenesterephthalate) (PBT) [12], PA6/ polypropylene(PP) [13], and PP-(ethylene-propylene (EP))copolymer/ poly(ethylene-co-vinyl acetate)(EVA) [14] has been conducted as well asthat of reinforced materials [15-18] andnanocomposites [19-22].

Electronic components are susceptible todamage from electrostatic discharge (ESD).A variety of materials has been developed topackage sensitive electronic devices andprevent damage during storage and shipping.For many articles in ESD protectedenvironments, the optimal surface resistivityis in the range of 106-109 Ω/sq [23]. Evenhigher values may be accepted if the article is

capable of dissipating charge fast enough.Too high surface resistivity results in anuncontrolled discharge [23]. Static dissipativematerials are often used to slow down thecharge removal process and prevent adamaging ESD event. Electrostatic dissipatingthermoplastic compounds have successfullyeliminated ESD failures in many applicationsin the electronics industry. A variety ofconductive fillers is presently available tomaterial engineers, including carbon black(CB), carbon fibers (CF), carbon nanotubes(CNT), metallic powders, flakes or fibers,and glass spheres or glass fibers coatedwith metals [23]. Among of these fillers, CNTis considered as a perfect candidate forthe development of electrically/thermallyconductive polymer-based composites due toits exceptional electrical and thermalconductivity, high strength and stiffness andlarge aspect ratio [24-30]. The conductivity ofESD compounds depends not only on thefiller type and concentration, but also on thespecific polymer matrix used and on thegenerated morphology [31].

This work investigates the impact ofreprocessing on properties of the PC/MWCNT composite such as chemicalstructures, ESD properties, and rheologicaland mechanical properties, by means ofrepeated injection molding cycles. The numberof injection molding cycles was 1-6.

2. MATERIALS AND METHODS2.1 Materials and Sample Preparation

The commercial PC/MWCNTcomposite (EA-3240, injecting moldinggrade for ESD applications) was kindlyprovided by Cheil Industries Inc., Korea.The virgin PC/MWCNT pellets were driedin hopper dryer at 80°C for 3 h prior toprocessing. The injection molding of thePC/MWCNT composite was carriedout using a Fanuc Roboshot 2000i 100B

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reciprocating screw injection moldingmachine to obtain tensile, impact, andHDT/VICAT specimens. The plasticizationunit consists of a standard screw with adiameter of 32 mm, L/D ratio of 20/1, andcompression ratio of 4D. The temperatureprofile was 235, 260, 290 and 280oCfrom rear zone to the nozzle and themold temperature was set at 100oC. Thescrew rotation speed was 50 rpm. Theinjection speed and pressure were 20 cm3/sand 80 bar, respectively. After the firstinjection cycle, some specimens were keptfor properties evaluation, and others wereground into appropriate size by a plasticcrusher. The granulated material was driedand injected using the same conditions asabove and the resultant specimens wererepeatedly processed until the 6th cycle ofinjection molding was completed. Specimensfrom each reprocessing cycle were keptto condition for 24 h in a desiccator beforecharacterization.

2.2 CharacterizationThe electrical resistance of injection-

molded samples was measured accordingto ANSI/ESD STM11.13-2004 using aresistance meter (3M-701). The tribo-chargevoltage and decay time were determinedbased on ESD ADV11.2-1995 using anelectrostatic voltage meter (Trek-520), and acharged plate monitor system (Trek Model150A), respectively. Tensile properties werecarried out according to ASTM D638 usinga Universal testing machine (Instron 4502),at a crosshead speed of 5 mm/min andtest temperature of 25oC. The specimendimensions were 12.7 mm × 165 mm × 3.2mm (W × L × T). The Young’s modulus andelongation at break were determined fromthe load-displacement curves. Izod impacttests were performed on an impact tester(Radmana model ITR 2000) based on ASTM

D256. The notched sample had a size of12.7 mm × 63.5 mm × 3 mm (W × L × T).A HDT/VICAT softening temperaturetester (Yasuda model HD-PC) was used formeasuring the HDT of specimen. Thesamples were prepared with the dimensionsof 13 mm × 127 mm × 5 mm (W × L × T).Tests were in accordance with ASTMD648 at a load of 1.82 MPa. TGAmeasurements were carried out using a TAinstruments thermobalance (TGA Q500)under nitrogen atmosphere and at aheating rate of 20oC/min. Dynamicmeasurements were carried out using anadvanced rheometric expansion system(ARES). The frequency sweep from 0.1to 100 rad/s was performed at 260oCunder dry nitrogen condition. For all themeasurements, the PC/MWCNT sampleswere tested within the linear viscoelasticstrain range. The MFI was measured with anextrusion plastometer (Davenport model 10,Lloyd Instruments), according to ASTMD1238 at 260oC, with a 2.16 kgf weight.TEM samples were ultra-thin-sectioned at70 nm using an ultramicrotome. Themorphology of the nanocomposites wasobtained using a JEOL JEM-2010, LaB6filament at an accelerating voltage of 200 kV.To verify the effect of reprocessing onchemical structures and bonding betweenMWCNTs and PC matrix, attenuated totalreflectance ATR-FTIR measurement wascarried out on the samples at differentinjection molding cycles. FTIR spectra wererecorded using a Thermo Nicolet 6700spectrometer. Single beam spectra werecollected from 16 scans at resolutionof 4 cm-1 and the measurement range was4000-600 cm-1. All experiments wereperformed at ambient temperature,25 ± 2oC. Five specimens were tested foreach set of measurements.

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3. RESULTS AND DISCUSSION3.1 Morphology

Figure 1 (a-f) shows the TEM imagesof the PC/MWCNT injection-moldedsamples at different injection molding cycles.As can be seen, MWCNTs were wellseparated and uniformly distributed in the PCmatrix. The structure and dimension ofMWCNTs for each injection molding cyclewere very similar, indicating that the aspectratio of MWCNTs remained the same afterreprocessing and MWCNT agglomerationwas not observed. Reprocessing mayimprove dispersion due to the longerexposure to the shear stresses. This occurredin the case of polyolefin-clay nanocomposites[22] because exfoliation and dispersion aredifficult to attain for such kind ofnanocomposites. In this study, the lack of

agglomeration upon reprocessing by theinjection molding method was attributed tothe high screw speed in the injection machinewhich favors MWCNT dispersion.

3.2 ESD MeasurementAn ESD control system should be

introduced in the production and handlingof electronic parts and devices. Staticdissipative materials are often used to slowdown the charge removal process andprevent a damaging ESD event. TheElectronic Industry Association (EIA)specifies that the typical requirements forsurface resistivity, tribo-charge voltage, anddecay time are 106-109 Ω/sq, less than 25 V,and less than 5 sec, respectively [23]. In thisexperiment as presented in Table 1, theinjection-molded PC/MWCNT composite

Figure 1. TEM photographs of the PC/MWCNT injection-molded samples at differentinjection molding cycles: (a) 1st cycle, (b) 2nd cycle, (c) 3rd cycle, (d) 4th cycle, (e) 5th cycle, and (f)6th cycle.

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Table 1. ESD results for injection-molded specimens using PC/MWCNT composites atdifferent injection molding cycles.

had ESD properties such as electrical resistance(3-8 × 109 Ω/sq), tribo-charge voltage(2-3 V), and decay time (< 0.1 s), suitable forESD protection. As the injection moldingcycles increased from 1 to 6, the ESDproperties of specimens were slightlychanged but remained in the acceptablerange for ESD protection. Based on TEMtopology (see Figure 1 (a-f)), there was noobservable change in the MWCNT aspectratio since the tube length was unchanged.The slight change in filler dispersion did notaffect the conductivity of the composite sincethe resistivity change may be still in thesteepness range (the percolation threshold)of the percolation curve. The percolationthreshold indicates the critical amount offiller necessary to initiate a continuousconductive network, which varies frompolymer to polymer for a given conductivefiller type [23].

3.3 RheologyFigures 2-4 show the storage modulus

(G′), the loss modulus (G′′), and the complexviscosity (η*) of the PC/MWCNT compositeat different injection molding cycles processedat 260oC. The G′ and G′′ of the PC/MWCNT composites tended to decreasewith increasing injection molding cycles.The G′ and G′′ of the 6th injection moldingcycle significantly decreased compared with

the 1st injection molding cycle, whichsuggested that the rheological properties ofthe PC/MWCNT composite were influencedby the injection molding cycle. The effect ofpolymer chain scission might be larger thanthe effect of MWCNTs distribution.Therefore, the rheological properties ofPC/MWCNT composites decreased withincreasing injection molding cycles. TheG′ and G′′increased with increasing thefrequency for all injection molding cycles.It is suggested that the increase ofrheological properties of the PC/MWCNT composites at high frequency isrelated to an increase of the MWCNT-MWCNT network structure because oflow degree of aggregation of MWCNTsat high frequency [30]. Moreover, theincrease of the rheological data of thePC/MWCNT composites at highfrequency suggests that the interactionbetween PC and MWCNT may exist inthe PC/MWCNT composites. Therheological properties of polymercomposites at high frequency region reflectthe dynamics of polymer entanglement [30].

In Figure 4, shear-thinning behaviorwas observed for the η* of the PC/MWCNT composites at different injectionmolding cycles. The shear-thinningbehavior and long time relaxation suggesta pseudo-solid-like behavior of the

Injectionmolding

cycle

Tribo-chargevoltage (V)

Standarddeviation

Surfaceresistivity

(Ω/sq)

Standarddeviation

Decay time (sec) from 1000 to 100 V

Positivecharge

Standarddeviation

Negativecharge

Standarddeviation

123456

2.72.62.02.12.22.6

0.150.260.800.350.740.23

7.83 × 109

5.94 × 109

7.01 × 109

6.74 × 109

7.96 × 109

2.82 × 109

0.0290.0420.0370.0170.0100.015

0.100.020.010.010.050.11

0.0160.0250.0510.0220.0640.048

0.020.040.060.010.020.04

0.0330.0560.0170.0240.0490.036

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Frequency (rad/s)

Frequency (rad/s)

Figure 3. Effect of reprocessing on the loss modulus (G′′) of the PC/MWCNT compositesat 260oC.

Figure 4. Effect of reprocessing on the complex viscosity (η*) of the PC/MWCNT compositesat 260oC.

Frequency (rad/s)

Figure 2. Effect of reprocessing on the storage modulus (G′) of the PC/MWCNT compositesat 260oC.

1st cycle2nd cycle3rd cycle4th cycle5th cycle6th cycle

1.0E+0.5

1.0E+0.4

1.0E+0.30.01 0.1 1 10 100 1000

G′′

(Pa)

1st cycle2nd cycle3rd cycle4th cycle5th cycle6th cycle

0.01 0.1 1 10 100 1000

1.0E+0.4

1.0E+0.3

1st cycle2nd cycle3rd cycle4th cycle5th cycle6th cycle

η* (P

a−s)

0.01 0.1 1 10 100 1000

1.0E+0.3

1.0E+0.2

1.0E+0.4

1.0E+0.5

G′′

(Pa)

1.0E+0.5

1.0E+0.6

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PC/MWCNT composites. The η* insignifi-cantly decreased with the increase ofinjection molding cycle. From the resultsof the G′ and G′′ shown in Figures 2 and3, the decrease of the η* of the PC/MWCNT composites at high injectionmolding cycle is mainly due to the decreaseof the G′ and G′′ since the η* is related toboth the G′ and G′′ (η* = [(G′)2 + (G′′)2]1/2/ω, where ω = frequency) [32].

The rheological results corresponded tothe increasing of MFI of the PC/MWCNTcomposite when the injection molding cycleincreased as shown in Figure 5. The MFIincreased from 29.57 to 79.75 g/10 min asthe injection molding cycle increased from1 to 6. This may result from the molecularchain scission of PC due to high shearstress during the injection molding process.Liu and et.al. [5] reported that the occurrenceof polymer chain scissions mostly occurrednear the chain ends. This might be the reasonwhy there were no large increases in MFI anddecrease in G′ , G′′, and η* upon reprocessing.

3.4 Mechanical PropertiesThe tensile strength and elongation at

break of injection-molded specimens are

given in Figure 6. The tensile strength valuesafter reprocessing insignificantly increased(1-4 cycles) from 60.66 to 62.02 MPa andremained at constant level of about 62 MPaat higher recycled process (4-6 cycles).The ductility results measured as the elongationat break and impact strength are presented inFigures 6 and 7, respectively. The elongationat break decreased from 45.32 to 13.37%(1-4 cycles) and slightly reduced at higherinjection molding cycles of 4-6 from 13.37to 7.66%. The impact strength decreased from221.69 to 93.86 J/mm at injection moldingcycles of 1-4 and slightly reduced at higherinjection molding cycles of 4-6 from 93.86to 67.97 J/mm. The tensile strengthand ductility results corresponded to thedegree of MWCNT fillers dispersion due tothe larger exposure to the shear stresses inthe melt. The dispersion of MWCNTs wasslightly increased together with the lack ofchange of MWCNT aspect ratio at injectionmolding cycles of 1-4, but the improvementof MWCNTs distribution at injectionmolding cycles of 4-6 was hardly observedfrom TEM topologies. Furthermore, thispossibility indicated that full dispersion ofMWCNT powder in PC matrix was not

Figure 5. Effect of reprocessing on the melt flow index of the PC/MWCNT compositesat 260oC.

Number of injection molding cycles

Mel

t flo

w in

dex

(g/1

0 m

in)

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achieved after the first injection moldingcycle because of the low interfacialinteraction between MWCNTs andpolymer leading to poor dispersion ofMWCNTs in common polymer matrices.This agreed with the TEM results whereslight change in the extent of dispersion wasobserved as injection molding cycleincreased. For a glassy, amorphous materiallike PC, the key parameters controlling themechanical performance are expected to bethe molecular weight, the free volume,the chemical nature, as well as the

molecular orientation. In the case of PCcomposite, the characteristic and distributionof fillers in the polymer matrix also play animportant role on mechanical properties ofthe composite.

Figure 8 shows the relationship ofheat deflection temperature (HDT) versusnumber of injection molding cycles of thePC/MWCNT injection-molded samples.The HDT slightly decreased from 118 to113°C as the injection molding cycle increasedfrom 1 to 6. The slight variation of HDTvalues also implied that the polymer chain

Figure 6. Tensile strength and elongation at break curves of the PC/MWCNT injection-molded samples at different injection molding cycles.

Figure 7. Impact strength curve of the PC/MWCNT injection-molded samples atdifferent injection molding cycles.

Number of injection molding cycles

Ten

sile s

tren

gth

(MPa

) Elongation at break (%)

Number of injection molding cycles

Impa

ct st

reng

th (J

/mm

)

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Figure 8. Heat deflection temperatures of the PC/MWCNT injection-molded samplesas a function of injection molding cycles.

scission insignificantly occurred at the mainchain since the change of HDT is mainlydepended on the molecular weight ofbased polymer.

3.5 Chemical StructureThe TGA results on PC/MWCNT

injection-molded samples at differentnumbers of injection molding cycles from1- 6 are reported in Figure 9. All the TGAcurves were identical and they exhibitedfirst significant weight loss at about 400oC,

which was well above the experimentalprocessing temperature (235-280oC). Thisagreed with the excellent degree of stabilityobserved for the PC/MWCNT compositeafter reprocessing, resulting in only slightlychanges in its properties after reprocessing.

The FTIR result was used to detectpossible changes in the chemical structure ofthe PC/MWCNT composite as the injectionmolding cycle increased. As shown inFigure 10, the peaks at 1728 and 3000 cm-1

indicate the existence of an ester group

Figure 9. TGA curves of the PC/MWCNT injection-molded samples at differentinjection molding cycles.

Number of injection molding cycles

Hea

t def

lect

ion

tem

pera

ture

(°C

)

Temperature (°C)

Wei

ght (

%)

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(-O-C=O) and some stretching of C=Cin the aromatic ring of PC in the composite.The bands at 1015 and 1776 cm-1 are ascribedto internal planar bending vibrations ofaromatic rings and stretching of thecarbonyl groups, respectively [5]. However,the bands at 3673 and 3531 cm-1 assigned tovibrations of methyl groups and stretchingvibrations of hydroxyl groups [5] were notclearly seen. No significant change of FTIRspectra was observed indicating that anypossible change of the chemical structuremight be slight. This may be because eitherthe MWCNTs may obscure the peak beingsought, or the extent of the degradationcaused by high temperature and shear stresswas too low and thus the concentration ofthe functionalities was simply below thesensitivity of the instrument. In addition,reprocessing can break down the polymerchains but does not change the chemicalstructure significantly.

4. CONCLUSIONDetailed structural and property

characterizations suggested that the PC/MWCNT composite had high possibility forreprocessing since most of its properties didnot significantly change after six injection

molding cycles, except for its mechanicaland rheological properties. The PC/MWCNT composite had high thermalstability after reprocessing up to 6 cycles at235-280oC. Moreover, the electricalproperties and HDT were insignificantlyvaried. The G′ , G′′, and η* decreased withincreasing number of injection moldingcycles, relating to the increase in MFIvalues. This indicated a decrease of PCmolecular weight due to some degree ofthe polymer chain scission caused by highshear stress in the injection moldingprocess. The MWCNT structure was notdeteriorated by high shear stress duringseveral injection molding cycles since theaspect ratio of MWCNTs remainedunchanged. The tensile strengthinsignificantly increased whereas theelongation at break and impact strengthdecreased at injection molding cycles of 1-4,and then slightly changed at injectionmolding cycles of 4-6, corresponding tothe dispersion results of MWCNTs inthe composite as observed from TEMmicrographs. The effect of type and contentof antioxidants in the PC/MWCNTcomposite on rheological properties afterreprocessing cycles will be further investigated

Figure 10. FT-IR spectra of the PC/MWCNT composite at different injection moldingcycles.

Wavenumber (cm-1)

% T

rans

mita

nce

4000 3500 3000 2500 2000 1500 1000 500

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for a better understanding of thedegradation mechanism of the nanocom-posite.

ACKNOWLEDGEMENTSThe authors would like to acknowledge

National Science and TechnologyDevelopment Agency (NSTDA), Thailandand Western Digital (Thailand) CompanyLimited (WD) for their financial supportunder project code P0900060.

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