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Seediscussions,stats,andauthorprofilesforthispublicationat:/publication/229136779Influenceofinjectionmoldingparametersontheelectricalresistivityofpolycarbonatefilledwithmulti-walledcarbon.ArticleinCompositesScienceandTechnologyMarch2008DOI:10.1016/pscitech.2007.08.031CITATIONS106READS1314authors,including:Someoftheauthorsofthispublicationarealsoworkingontheserelatedprojects:PiezoresistiveSensors(MasterThesis)ViewprojectPetraPtschkeLeibnizInstituteofPolymerResearchDresden303PUBLICATIONS10,882CITATIONSSEEPROFILEUdoWagenknechtLeibnizInstituteofPolymerResearchDresden143PUBLICATIONS2,628CITATIONSSEEPROFILEAllcontentfollowingthispagewasuploadedbyPetraPtschkeon20March2015.Theuserhasrequestedenhancementofthedownloadedfile.Allin-textreferencesunderlinedinblueareaddedtotheoriginaldocumentandarelinkedtopublicationsonResearchGate,lettingyouaccessandreadthemimmediately.Influence of injection molding parameters on the electrical resistivityof polycarbonate filled with multi-walled carbon nanotubesTobias Villmow, Sven Pegel, Petra Po tschke*, Udo WagenknechtLeibniz Institute of Polymer Research Dresden, Hohe Strasse 6, 01069 Dresden, GermanyReceived 7 May 2007; received in revised form 23 August 2007; accepted 25 August 2007Available online 14 September 2007AbstractTwo polycarbonate (PC) composites with 2 and 5 wt% multi-walled carbon nanotube (MWNT) content were injection molded using atwo-level, four-factor factorial design to evaluate the influences of holding pressure, injection velocity, mold temperature, and melt tem-perature on the electrical surface and volume resistivities. For both composites variations in resistivity of the injection-molded plates upto six orders of magnitude were found. The highest impact was determined for the injection velocity followed by the melt temperatureand the interaction of both. The resistivity varied also locally within the plates showing differences up to five orders of magnitude for2 wt% and up to two orders for 5 wt% MWNT. Thereby, areas of equal resistivity are formed in a semicircular shape with values increas-ing with the flow path. Transmission electron microscopy (TEM) investigations indicated a skin layer with highly oriented nanotubes incase of high injection velocity and low melt temperature, but a network-like structure even in the skin area at low injection velocity andhigh melt temperature.? 2007 Elsevier Ltd. All rights reserved.Keywords: A. Nanostructures; A. Polymermatrix composites; B. Electrical properties; E. Injection molding; Multi-walled carbon nanotubes1. IntroductionCarbon nanotubes (CNT) 13 constitute a novel typeof electrically conductive filler to modify insulating poly-mers. Due to the fact that CNT offer a high length/diame-ter ratio (aspect ratio) of about 1001000 4 percolationcan be achieved with relatively low CNT contents in com-parison to carbon fibers or carbon black. Particularly theirunique electrical 510 and thermal 1114 properties, incombination with high mechanical strength 1519, pre-destine CNT for applications as filler in polymeric matri-ces. It is reported that polymer/CNT composites showhigh strength and stiffness combined with electrical conduc-tivity at relatively low concentrations of CNT 2023.The majority of experimental results concerning thermal,mechanical, and electrical properties of melt mixed poly-mer/CNT composites have been generated using small-scalesize mixing equipments, due to the scarcity and high price ofCNT materials. In order to study electrical or mechanicalproperties mainly compression molding is used, which leadsto results that may be not comparable to industrial pro-cesses like injection molding 24. So far, only a limitednumber of papers describe injection molding as a methodof electrically conductive nanocomposite shaping 25,26.At the moment, different companies constantly work onthe optimization of CNT syntheses, as a result of whichtheir prices have started to fall remarkably and industrialapplications be realized in some cases. Polymer/CNT com-posites can be used for electrostatic dissipation (ESD)4,27, electromagnetic interference- shielding (EMI- shield-ing) 2830, electrostatic painting 31, and mechanicalreinforcement 32,33. In the presented work 20 kg of PC/CNT composite were produced diluting a masterbatch con-taining MWNT on a twin screw extruder. By means of thistechnique it was possible to produce high quality compositematerials with well-dispersed MWNT within the PC matrix.0266-3538/$ - see front matter ? 2007 Elsevier Ltd. All rights reserved.doi:10.1016/pscitech.2007.08.031*Corresponding author. Tel.: +49 3514658395; fax: +49 3514658565.E-mail address: poeipfdd.de (P. Po tschke)./locate/compscitechAvailable online at Composites Science and Technology 68 (2008) 777789COMPOSITESSCIENCE ANDTECHNOLOGYDue to the fact that injection molding is the most impor-tant processing procedure besides (profile-) extrusion it isthe aim of this work to investigate the influence of the injec-tion molding conditions on the electrical resistivity of PC/CNT composites.One characteristic for injection-molded parts is ananisotropic character in structure and, thus, propertiesdue to molecular chains and filler particles orientationresulting from high processing velocities. If the melt iscooled down rapidly to ensure short cycle times, these ori-entations are frozen and remain in the molded part. In thecase of conductive filler particles having a high aspect ratio,such as CNT, orientations lead to an impairment of net-work formation and, thus, to a shift of the percolationthreshold towards higher filler contents 34,35. These ori-entations are unavoidable and have to be minimized by aprocess optimization to ensure electrical percolation ofthe CNT within the polymer matrix. Therefore, two nano-composites with different MWNT contents were injectionmolded varying holding pressure, injection velocity, moldtemperature, and melt temperature. As discussed in litera-ture on other polymer/filler composites 36, these fourparameters have the highest influence on the degree of ori-entation in injection-molded parts. In order to evaluatetheir effect they have been varied systematically using thetwo-level, four-factor factorial experiment design, whichenables a regression analysis of the generated electricalresistivity values. Thereby a direct conclusion betweeninjection molding parameters and the specific electricalresistivity of the samples can be drawn.Besides commonly used integral resistivity measure-ments, a new and promising local resolution measurementmethod was performed to get a precise insight in the inho-mogeneous and anisotropic character of the injection-molded PC/CNT composites.Additional morphological investigations were executedto determine the possible reasons for the changes in theelectrical resistivity. To investigate the different states ofCNT dispersion and orientation that could be responsiblefor the differences in resistivity TEM was applied.2. Materials and experimental methods2.1. Composite preparationAs a first step a masterbatch with 15 wt% MWNT,obtained in the form of pellets (Hyperion Catalysis Inter-nationals, USA), was diluted with PC Makrolon?2600(Bayer Material Science, Germany). Makrolon?2600 isan injection molding grade with a medium viscosity(MVR 10 cm3/ 10 min) and a density of 1.2 g/cm3. Hype-rion uses vapor grown MWNT consisting of 815 graphiticlayers, wrapped around a hollow 5 nm core, for the mast-erbatches. The typical MWNT diameter ranges from 10to 15 nm, while lengths are between 1 and 10 lm. The den-sity after incorporation into a composite is approximately1.75 g/cm337.Two nanocomposites, namely experiment series I and II(2 and 5 wt% MWNT content, respectively), were pro-duced using a ZE 25 extruder with two co-rotating screws(Berstorff, Germany) at a barrel temperature of 295 ?C, ascrew speed of 200 rpm, and a throughput of 10 kg/h.The extruded material was cooled down to room tempera-ture in a water bath and granulated afterwards. All thematerials were dried in a vacuum oven at 120 ?C for at least4 h before each processing step. The materials showed vol-ume resistivities of 37 X cm (2 wt%) and 6 X cm (5 wt%) asmeasured on pressed plates indicating that both samplesare electrically percolated.2.2. Injection moldingInjection molding was used to produce specimens withthe dimensions 80 80 2 mm3by means of an Ergotech100/420-310 (Demag, Germany) using a two-cavity mold,fed by a flash gate. A filling study was executed for the5 wt% sample to get an impression of the contour thatthe melt takes sequentially while it fills the cavity. Injectionmolding was performed using an injection velocity of80 mm/s, a mold temperature of 80 ?C, a melt temperatureof 300 ?C without executing holding pressure and varyingthe feeding stroke between 58 and 13 mm with steps of5 mm (Fig. 1).Experiment series I and II were designed as 16 experi-ments based on the two-level, four-factor factorial designto evaluate the influence of the holding pressure (z1), injec-tion velocity (z2), mold temperature (z3), and melt temper-ature (z4) on the electrical resistivity of the samples, asshown in Table 1. After reaching steady state conditionssix specimens were taken for further investigations for eachof the 16 experiments.The stated values for all experiments (Table 2) are setpoints with an accuracy of ca. 5%. The cycle times of theexperimentsdifferwithdifferentinjectionvelocities,Fig. 1. Development of the melt front as obtained by a filling study forexperiment series II, 5 wt% MWNT content.778T. Villmow et al. / Composites Science and Technology 68 (2008) 777789whereas a constant cooling time of 25 s was applied for allexperiments.Due to the fact that no previous knowledge about suit-able injection molding conditions of PC/CNT compositeswas available, the intervals between the two levels werechosen as large as possible.The low and high level of melt and mold temperatureswere defined according to the values indicated in the datasheet of Makrolon?2600. The levels for injection velocityand holding pressure were determined experimentally inpreliminary investigations. The holding pressure varied inboth experiment series because of the different MWNTcontents and the subsequently differences in the flowabilityof the melts.The pure PC was injection molded using a holding pres-sure of 60 bar, an injection velocity of 80 mm/s, a moldtemperature of 80 ?C, and a melt temperature of 300 ?C.2.3. Specimen characterization2.3.1. Electrical resistivityTwo different kinds of measurements have been per-formed to get a comprehensive impression of the electricalvolume and surface resistivity of the injection-moldedplates.To obtain the resistivities of the 16 6 samples for eachexperiment series an integral measurement method using aHiresta-UP electrometer in combination with an U-Type J-Box test fixture (both from Mitsubishi Chemical Corpora-tion, Japan) was used. The U-Type J-Box was equippedwith ring electrodes having an inner electrode diameter of50 mm and an outer electrode diameter of 70 mm (mea-surement method A, measurement range: 1041013X).Samples having an electrical resistivity below 104X weresputter-coated with thin gold electrodes having the samedimensions like those of the U-Type J-Box. These elec-trodes were contacted with test prods that were connectedto a Model 2000 Multimeter (Keithley, USA, measurementmethod B, measurement range: 10?6to 106X).Local resolution measurements were realized using theHiresta-UP electrometer in connection with a URS-probe(Mitsubishi Chemical Corporation, Japan). The URS-probe was equipped with ring electrodes having an innerelectrode diameter of 5.9 mm and an outer electrode diam-eter of 11 mm (measurement method C). The measurementarrangement shown in Fig. 2 indicates the nine symmetri-cally positioned measurement positions, the electrodedimensions of the U-Type J-Box, and the different areasinvolved in the measurement processes. For volume resis-tivity measurements, a resitable UFL plate (MitsubishiChemical Corporation) was connected to the Hiresta-UP,so that the outer electrode of the URS-probe acted likethe guard electrode.Samples having an electrical resistivity below 104X weremeasured by means of a Loresta-GP electrometer in com-bination with an ASP-probe (both from Mitsubishi Chem-icalCorporation,Japan,measurementmethodD,measurement range: 10?3107X). The distances betweenthe outer and inner pins of this four-pin probe were15 mm and 5 mm, respectively, with pin diameters of0.75 mm. With this method only surface resistivity couldbe measured. The values shown in the diagrams and tablesare the mean values of six specimens measured for eachexperiment.2.3.2. MorphologyThe morphology of selected injection-molded plates wasstudied by means of two different analysis methods tocheck the state of dispersion in the micro- and nano-scale.Samples were taken at two sampling positions in the meltflow direction (Fig. 3).Light transmission microscopy (LM) was performed onthin sections cut on a RM 2055 microtome (Leica, Ger-many) at room temperature. A diamond knife with a cutangle of 35? (Diatome, Switzerland) was used. The micro-graphs have been imaged with an Axioplan 2 (Zeiss,Germany).TEM was performed by means of an EM 912 (Zeiss,Germany) on ultra thin sections of around 150 nm thick-ness cut with a Reichert Ultracut S ultramicrotome (Leica,Table 1Experiment design using the two-level, four-factor factorial designcreating 16 experiments with systematically varied injection moldingparameters (with z1= holding pressure, z2= injection velocity, z3= moldtemperature and z4= melt temperature); here, + and, - refer to thehigh and low levelFactorsExperimentz1z2z3z41+2+?3+?+4+?5+?+6+?+?7+?+8+?9?+10?+?11?+?+12?+?13?+14?+?15?+16?Table 2Set points of the injection molding parameters for experiment series I andII (with z1= holding pressure, z2= injection velocity, z3= mold temper-ature and z4= melt temperature)z1(bar)z2(mm/s)z3(?C)z4(?C)Series IHigh level (+)80150100320Low level (?)401060280Series IIHigh level (+)70150100320Low level (?)351060280T. Villmow et al. / Composites Science and Technology 68 (2008) 777789779Germany). A diamond knife with a cutting angle of 35?and a tub (Diatome, Switzerland) was used. The slicingdirection was angle shifted (15?) perpendicular to the injec-tion direction of the plates. Prior microtoming, samplepieces (2 2 15 mm3) were cut from the plates andembedded in epoxy resin. This was necessary to stabilizethe cut edges, to avoid deformations of the cuts, and toenable an accurate trimming procedure in order to obtainan assignable resin/composite interface parallel to theinjection velocity. In addition, an irradiation of the borderarea of the thin sections could be prevented to improve thecontrast in that most interesting area.3. Results3.1. Influence of injection molding parameters on theelectrical resistivity of the injection-molded plates3.1.1. Experiment series I (2 wt% MWNT content)Electrical volume and surface resistivity values of theinjection-molded plates with a MWNT content of 2 wt%measured using method A are shown in Fig. 4. The electri-cal resistivity drops at least by four orders of magnitude incomparison to the unfilled PC and is in the range between1012and 1014X cm or X/h. However, the plates of theexperiments 5, 7, 13, and 15 show volume and surface resis-tivities in the range of 107X cm or X/h. These four exper-iments were all injection-molded with a combination of lowinjection velocity and high melt temperature, indicatingthat these parameters have the biggest influence.The level of influence of the different parameters wasevaluated exactly using the regression analysis method.To make the data processing and the calculation of theregression coefficients convenient according to the experi-mental level codes, the investigated parameters z1to z4were encoded to give the following normalized variablesx1to x4oscillating within the range of 1 and 1 (Eqs.(1)(4). The interactive parameters were calculated bymultiplication of the involved main factors. One exampleis given in Eq. (5).Fig. 3. Sampling positions for morphological investigations (numericalvalues in mm).Fig. 2. Arrangement of the nine measurement points for the local resolution measurements and the electrode dimensions of the integral measurementmethod (numerical values in mm).780T. Villmow et al. / Composites Science and Technology 68 (2008) 777789x12z1? z1D1?2z1?6040?with z1?holding pressure1x22z2? z2D2?2z2?80140?with z2?injection velocity2x32z3? z3D3?2z3?8040?with z3?mold temperature3x42z4? z4D4?2z4?30040?with z4?melt temperature4x12 2z1? z1D1? 2z2? z2D2? 2z1?6040? 2z2?80140?5Here, D is the difference between the high and low level ofthe investigated factors and z is its mean value. The cod-ing of the parameters is summarized in Table 3 with thestatistical calculation and regression coefficients bj, whereBj, Djand bjare expressed as follows:BjX16i1Zi? yi6DjX16i1x2ji7bj Bj=Dj8In the equations shown above (Eqs. (6)(8), Bjis the sta-tistical parameter signifying the role of the investigatedfactors on the electrical resistivity. Djsignifies the net nor-malized magnitude of the operating variables over allexperiments, while bjis the regression coefficient. Thecomplete regressions equation can be written asFig. 4. Volume and surface resistivity of the experiment series I; 2 wt%MWNT content (measurement method A).Table 3Statistical analysis table for experiment series I, 2 wt% MWNT contentExperimentx0x1x2x12x3x13x23x123x4x14x24x124x34x134x234x1234Logarithmic values yi(vol)(sur)1111111111111111112.4912.91211111111?1?1?1?1?1?1?1?112.8113.6531111?1?1?1?11111?1?1?1?112.4712.8541111?1?1?1?1?1?1?1?1111112.8113.75511?1?111?1?111?1?111?1?17.317.74611?1?111?1?1?1?111?1?11113.8314.22711?1?1?1?11111?1?1?1?1117.736.17811?1?1?1?111?1?11111?1?113.5313.9991?11?11?11?11?11?11?11?112.8113.26101?11?11?11?1?11?11?11?1113.0814.14111?11?1?11?111?11?1?11?1112.7812.77121?11?1?11?11?11?111?11?113.0013.86131?1?111?1?111?1?111?1?117.919.88141?1?111?1?11?111?1?111?113.5413.81151?1?11?111?11?1?11?111?17.247.78161?1?11?111?1?111?11?1?1113.5314.04Dj16161616161616161616161616161616Bj186.85?0.9217.63?1.270.68?0.89?0.420.72?25.41?0.5723.090.22?0.09?1.320.021.43(vol)bj11.68?0.061.10?0.080.04?0.06?0.030.05?1.59?0.041.440.01?0.01?0.080.000.09Bj194.80?4.2519.552.524.39?0.88?2.93?0.74?28.12?3.7820.904.454.03?1.03?3.310.95(sur)bj12.18?0.27?0.06?0.18?0.05?1.76?0.241.310.280.25?0.06?0.210.06T. Villmow et al. / Composites Science and Technology 68 (2008) 777789781 y b0Xmi1bixiXmi1Xmji1bijxixj:9From the regression coefficients bj, which are shown in Ta-ble 3, it becomes obvious that the parameter x2(normal-ized injection velocity), the parameter x4(normalizedmelt temperature), and the interactive parameter x24havethe highest impact on the volume and surface resistivity,whereas x4having a negative value, indicating resistivitydecrease and x2and x24having positive values indicatingresistivity increase.Based on the calculated coefficients bj, using measuredvalues in logarithmic mode, the regressions equationbetween the electrical surface and volume resistivity andeach of the investigated injection molding factors wasobtained. The regression equations enable the predictionof the volume and surface resistivity for the particularinjection-molded PC/CNT composite for any set valuesof injection velocity and melt temperature. These equationswere taken afterwards to check how the resistivity changes,when one parameter is kept constant while the second oneis varied between the two investigated levels. Neglectingthose coefficients with a marginal impact (in our case allcoefficients bij 0.3jbijmaxj) the two regression equationscan be written as yvolume;log 11:681:10x2?1:59x41:44x24and10 ysurface;log 12:181:22x2?1:76x41:31x24:11These equations have to be re-normalized using the Eqs.(1)(4) to establish a direct connection between injectionmolding parameters and volume and surface resistivity.The re-normalized equations can be written asyvolume;log 15:816 ? 0:019z2? 0:027z4 1:714 ? 10?4z2z4and12ysurface;log 16:214 ? 0:1376z2? 0:027z4 1:560 ? 10?4z2z4:13It can be calculated with Eqs. (12) and (13) that the volumeand surface resistivity increases by ca. 4.6 orders of magni-tude when the injection velocity is increased from 10 to150 mm/s at a constant melt temperature of 300 ?C. Keep-ing the injection velocity constant at 80 mm/s, an increaseof the melt temperature from 280 ?C to 320 ?C leads to adecrease in volume and surface resistivity by around 0.5 or-ders of magnitude.3.1.2. Experiment series II (5 wt% MWNT content)The results of the resistivity measurements using mea-surement method A and B on the injection-molded plateswith 5 wt% MWNT are summarized in Fig. 5. Here, twogroups of experiments can be classified. In most of theexperiments the electrical resistivity drops by about 13orders of magnitude compared to the neat PC. All thesesamples were injection-molded with low injection velocityor/and high melt temperature. The determined resistivityvalues of these experiments were in the range of 105X cmor X/h. However, four experiments led to values whichwere significantly higher (1081010X cm or X/h). Theseexperiments were processed with low melt temperatureand high injection velocity. Experiment 16 with low levelof all parameters also shows significantly higher values(108109X cm or X/h).The regression analysis was performed analogically tothe experiment series I using the Eqs. (1)(9). Based onthe calculated coefficients bj(table not shown), the regres-sions equations for the experiment series II, establishing aconnection between the electrical surface and volume resis-tivity and investigated injection molding factors with thehighest impact, can be written as yvolume;log 6:53 1:14x2? 1:50x4? 1:02x24and14 ysurface;log 5:83 1:03x2? 1:51x4? 0:68x24:15Eqs. (14) and (15) indicate again that the parameter x2(normalized injection velocity), the parameter x4(normal-ized melt temperature), and the interactive parameter x24have the highest impact on volume and surface resistivity.Again the parameter x2had a positive value, indicatingresistivity decrease with higher values, however, besidesx4now also x24had a negative value, indicating resistivityincrease and dominance of the influence of melt tempera-ture versus injection velocity in this interactive parameter.The re-normalized equations, establishing a direct con-nection between injection molding parameters and electri-cal volume and surface resistivity, can be written asyvolume;log 5:78 0:04z2? 0:0028z4? 1:21 ? 10?4z2z4and16ysurface;log 5:87 0:03z2? 0:006z4? 8:10 ? 10?5z2z4:17Eqs. (16) and (17) led to the conclusion that a variation ofthe injection velocity from 10 to 150 mm/s at constant melttemperature of 300 ?C results in a decrease of volume andsurface resistivity by around 0.6 and 0.9 orders of magni-Fig. 5. Volume and surface resistivity of the experiment series II; 5 wt%MWNT content (measurement method A and B).782T. Villmow et al. / Composites Science and Technology 68 (2008) 777789tude, respectively. In comparison to the experiment series Ithe injection velocity has now a clearly lower influence onthe resistivity.Keeping the injection velocity constant at 80 mm/s, anincrease of the melt temperature from 280 ?C to 320 ?Cdecreases the volume and surface resistivity by around0.5 orders of magnitude, which corresponds to the valuedetermined for experiment series I.3.2. Local resolution resistivity measurements3.2.1. Experiment series I (2 wt% MWNT content)Local resolution measurements on plates of the experi-ments 5, 7, 13, and 15 as shown in Table 4 revealed big gra-dients of the specific volume and surface resistivity in arange of five orders of magnitude. The lowest resistivityvalues were determined at the measurement points 7, 9,and 5. For experiment 15, surface and volume resistivityare in addition shown in Fig. 6, where four areas of similarresistivity values are marked. The lowest values were foundin the area II, next to the area I, directly behind the flashgate. With an increase of the flow path, areas III and IVare formed, which have higher resistivity values. Thereby,areas of equal resistivities seem to have a semicircularshape. This arrangement corresponds to the flow lines ofthe melt during injection process as seen by means of thefilling study (Fig. 1).An influence of holding pressure on resistivity in thenear of the sprue (measurement point 8) could not beobserved. Independent from the level of the holding pres-sure the specific volume resistivity was in a range of 1081010X cm, what is above the values measured for area II.3.2.2. Experiment series II (5 wt% MWNT content)Also at 5 wt% MWNT content, local resolution surfaceresistivity values of the experiments 5, 7, 13, and 15revealed certain gradients, even if they are much smalleras compared to series I. The values varied in a range oftwo orders of magnitude (Table 5) exhibiting again semicir-cular areas of equal surface resistivities. The lowest resistiv-ity values were again determined at the measurementpoints 7, 9, and 5 and the highest values at point 8 in theTable 4Data table of local resolution resistivity measurements for experiment series I, 2 wt% MWNT contentExperiment571315qsur(X/h)qvol(X cm)qsur(X/h)qvol(X cm)qsur(X/h)qvol(X cm)qsur(X/h)qvol(X cm)Measurement points11.1E+121.7E+113.3E+111.1E+104.2E+121.5E+115.4E+138.5E+1123.3E+101.8E+105.6E+092.6E+092.2E+121.3E+106.6E+111.5E+1032.2E+125.8E+114.8E+125.8E+113.2E+121.8E+111.8E+134.3E+1141.4E+109.3E+091.1E+091.1E+101.6E+122.1E+111.0E+105.4E+1152.3E+074.9E+072.0E+089.8E+065.0E+098.3E+082.4E+085.8E+0761.3E+108.2E+099.2E+091.6E+093.8E+121.8E+113.0E+096.5E+1174.4E+087.5E+061.4E+091.4E+076.6E+089.1E+074.5E+081.1E+0782, 3E+075.0E+108.6E+092.4E+083.7E+091.3E+103.1E+083.8E+0993.5E+085.0E+062.0E+081.8E+071.1E+094.2E+077.1E+084.3E+06Fig. 6. Local resolution resistivity for experiment 15, series I; 2 wt% MWNT content, (measurement method C).T. Villmow et al. / Composites Science and Technology 68 (2008) 777789783near of the sprue. The resistivity increased again with anincrease of the flow path from area II to IV.4. DiscussionThe investigations have shown that the electrical resis-tivity of the injection-molded plates was reduced signifi-cantly by incorporation of MWNT. The samples with aMWNT content of 2 wt% showed volume and surfaceresistivities down to 107X cm and 106X/h, respectively.The lowest resistivity values for the experiment series IIwith a MWNT content of 5 wt% were 105X cm or104X/h. The volume resistivity of the neat PC wasaround 1018X cm. Interestingly, in most experiments with2 wt% MWNT surface resistivities were slightly higherthan volume resistivities, whereas at 5 wt% surface resis-tivities were mostly lower than volume resistivities. Incase of experiment series I these differences could berelated to the different geometrical parameters involvedin the calculations from resistance to resistivity values,because also the neat PC shows a higher surface resistiv-ity than volume resistivity. In the case of experiment ser-ies II the reasons for the partial big differences betweensurface and volume resistivities (two orders of magnitudefor experiment 10) cannot be explained at the momentbut should be mentioned, as they could be the objectof further investigations in the field of injection moldingof polymer/CNT composites.Besides the MWNT content, the injection moldingconditions had an enormous influence on the electricalresistivities of the injection-molded PC/CNT composites.The electrical resistivities of the samples could be influ-enced in a wide range (up to 6 orders of magnitude)byvaryinginjectionmoldingvelocityandmelttemperature.The local resolution measurements on selected experi-ments revealed big gradients of the specific volume and sur-face resistivity in the range of five orders of magnitude inthe case of the experiment series I. The lowest resistivityvalues were determined in the near of the sprue and inthe middle of the sample.The reasons for these differences have to be assigned tochanges in the network structure of the MWNT within thePC matrix during the injection molding process.The materials as used for injection molding showedexcellent nanotube dispersion. Using light microscopyno agglomerates or remaining masterbatch clusters couldbe observed (images not shown) which was also observedin selected injection-molded plates. Also all TEM imagesindicated a very good dispersion of MWNT. In addition,bothmaterials(2and5 wt%MWNT)showedconductivity illustrating that a nanotube network wasformed.It may be assumed that the differences in resistivityobserved after injection molding result from changes ofthe nanotube network due to network orientation, net-work disruption and orientation of separated tubes,and cluster formation, which may be locally differentwithin the sample plate. In addition, skin effects, eitherby migration of nanofiller towards the core or by nano-filler orientation, may be expected. Furthermore, alsonanotube shortening due to the high shear and elonga-tionforcesduringpassingthenozzlemaynotbeexcluded.In order to get information on the nanotube arrange-ment, TEM investigations were performed at different sam-ple locations and especially at different sample depthsbelow the sample surface at which most effects wereexpected.TEM investigations on experiment 12/sampling position2 (2 wt% MWNT, injection velocity: 150 mm/s and melttemperature 280 ?C, volume resistivity 1013X cm) showedthat the MWNT were well separated but highly orientatedin injection direction in the area near to the mold wall up toa depth of around 10 lm (Fig. 7). That means that thesample had an insulating skin due to the fact that thenanotubes are orientated which led to an interruption oftube-tube contacts. However, the density of the nanotubesseemed not reduced in this skin layer, illustrating that nofiller depletion or migration, as observed i.e. in systemsof PS and PP with carbon black 38, took place in thissystem.This high degree of MWNT orientation results from thehigh injection velocity which leads to high shear forcesapplied to the melt when it passes the nozzle. Furthermore,this effect is increased when using low melt temperature dueto the increase of melt viscosity. If the melt with the orien-tated MWNT gets into contact with the cold mold wall, themelt cools down rapidly and the state of orientation isfrozen.The TEM images made in deeper sample depths of 17and 23 lm showed a lower state of MWNT orientationand a network like nanotube structure. Here, the nanotubenetwork reorientation or recombination could proceed eas-ier due to longer residence time in the molten state avail-able for relaxation processes.In contrast to that, the sample of the experiment 13/sampling position 2 (injection velocity: 10 mm/s andTable 5Data table of local resolution resistivity measurements for experimentseries II, 5 wt% MWNT contentExperiment5qsur(X/h)7qsur(X/h)13qsur(X/h)15qsur(X/h)Measurement points12.7E+043.6E+048.5E+041.8E+0421.1E+041.1E+041.5E+046.8E+0332.4E+043.8E+049.0E+041.6E+0447.8E+039.2E+031.2E+046.6E+0353.7E+033.6E+038.6E+033.2E+0367.4E+031.0E+041.2E+046.4E+0373.0E+032.7E+033.6E+032.1E+0383.1E+051.7E+051.7E+052.2E+0493.1E+032.9E+033.8E+032.0E+03784T. Villmow et al. / Composites Science and Technology 68 (2008) 777789melt temperature 320 ?C, volume resistivity 109X cm)showed a different morphology (Fig. 8). The degree ofMWNT orientation was much lower, thus, remainingtube connectivity. The nanotube network seemed to beless orientated and interrupted under these injectionmolding conditions. The higher melt temperature leadsto faster reorganization of the nanotube network inthe PC matrix. Thus, a percolated network structurewith many connection points between tubes could bedeveloped.The cognition about the highly orientated skin layer ofthe injection-molded sample explains the relatively lowinfluence of the melt temperature compared to the injectionvelocity as found in the experiment series I. Whereas theresistivity increased by ca. 4.6 orders of magnitude whenthe injection velocity is increased from 10 to 150 mm/s atFig. 7. TEM images of experiment 12/sampling position 2 taken at different sample depths (Experiment series I, 2 wt% MWNT content).T. Villmow et al. / Composites Science and Technology 68 (2008) 777789785constant melt temperature of 300 ?C, the increase of themelt temperature from 280 ?C to 320 ?C at constant injec-tion velocity of 80 mm/s results in resistivity raise by onlyaround 0.5 orders of magnitude. This impact was equalfor 2 wt% and 5 wt% MWNT. This finding is related tothe fact, that a skin layer once frozen cannot by influencedanymore by a higher melt temperature and, thus, the prin-cipal possibility of MWNT reorganization can only occurin the core region.In comparison to the results generated on samples of theexperiment series I, the influence of the injection velocitydecreases at 5 wt% MWNT content (experiment series II)and just leads to a decrease of volume and surface resistivityby around 0.5 and 1.0 orders of magnitude, respectively.Whereas the high injection velocity results in a high tubeorientation and thus, a loss of tubetube contacts in the caseof experiment series I, this process is constricted by the highMWNT loading that offers a large number of nanotubeFig. 8. TEM images of experiment 13/sampling position 2 taken at different sample depths (Experiment series I, 2 wt% MWNT content).786T. Villmow et al. / Composites Science and Technology 68 (2008) 777789entanglements. This is illustrated in Figs. 9 and 10, wherecomparable TEM images of experiments 12/sampling posi-tion 1 (5 wt% MWNT, injection velocity: 150 mm/s andmelt temperature 280 ?C, volume resistivity 1010X cm)and experiment 13/sampling position 1 (5 wt% MWNT,injection velocity: 10 mm/s and melt temperature 320 ?C,volume resistivity 104X cm) are shown. Interestingly, thesample with low melt temperature and high injection veloc-ity (experiment 12) shows a better dispersion as comparedto that injection molded at low velocity and high melt tem-perature, where some clustering may be observed, whichobviously has a positive effect on the sample conductivity.Fig. 9. TEM images of experiment 12/sampling position 1 taken at different sample depths (Experiment series II, 5 wt% MWNT content).T. Villmow et al. / Composites Science and Technology 68 (2008) 7777897875. ConclusionThe presented work provides a comprehensive overviewof the influence of selected injection molding parameters onthe electrical resistivity of MWNT/PC composites withfixed sample geometry. Thereby, the relations betweenthe most important injection molding parameters and theelectrical resistivity were evaluated, which can be regardedas an important step for further development with regardto industrial applications.The investigations have shown impressively that theresistivity of injection-molded MWNT/PC compositescan be influenced by selective variation of injection mold-ing parameters at a fixed MWNT content. By varying hold-ing pressure, injection velocity, mold temperature, and melttemperature according to a two-level, four-factor factorialFig. 10. TEM images of experiment 13/sampling position 1 taken at different sample depths (Experiment series II, 5 wt% MWNT content).788T. Villmow et al. / Composites Science and Technology 68 (2008) 777789experiment design, variations up to 6 orders of magnitudewere found as well for 2 wt% MWNT as for 5 wt% MWNTcontent in PC. The highest impact was determined for theinjection velocity followed by the melt temperature. Hold-ing pressure and mold temperature were found to haveonly very low influences.TEM investigations have shown that a high injectionvelocity led to a highly orientated skin layer with well-sep-arated MWNT, which acts electrically insulating due to theabsence of tube-tube contacts. Furthermore, it could beobserved that injection-molded MWNT/PC compositesexhibit a skin-core morphology with a decreasing MWNTorientation towards the core region. In a ce
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