13.Foamed poly(lactic acid) composites with carbonaceous fillers for electromagnetic shielding
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Foamed poly(lactic acid)composites with carbonaceous fillers for electromagneticshieldingStanislaw Frackowiak a ,⇑,Joanna Ludwiczak a ,1,Karol Leluk a ,1,Kazimierz Orzechowski b ,2,Marek Kozlowski a ,3a Wroclaw University of Technology,Department of Environmental Engineering,Wybrzeze Wyspianskiego 27,Wroclaw 50-370,Poland bUniversity of Wroclaw,Department of Chemistry,Joliot-Curie 14,Wroclaw 50-370,Polanda r t i c l e i n f o Article history:Received 22July 2014Accepted 2October 2014Available online 21October 2014Keywords:Polymer compositesElectromagnetic shielding Polylactide Carbon fibersa b s t r a c tElectromagnetic shielding is one the key factors for electronic devices in their use and transportation.Polylactide (PLA)is a biodegradable polymer with a moderate biodegradability and decent mechanical properties.Replacement of traditional materials by biodegradable polymers brings about the fossil resources savings and helps solving problems related to the plastic packaging waste.In this work com-posites of PLA with carbon black and carbon nanofibers were described.Improvement of the material mass/electromagnetic interference SE (shielding effectiveness)ratio can be obtained by introducing foaming technology into the material preparation process.Microporous structure can greatly improve material properties such as thermal isolation,mechanical properties and in case of composites filled with carbonaceous fillers such as carbon black carbon fibres –also the electrical conductivity.In order to improve their application range and reduce density,a cellular structure was created using chemical blowing agent.It was found that in low loaded composites (although above the percolation level)the shielding effectiveness relies on the amount of a conductive filler but it may be additionally enhanced by the foaming process.Electrical properties,electromagnetic shielding effectiveness and mor-phology of cellular composites for described polymer-filler systems have been presented.Ó2014Elsevier Ltd.All rights reserved.1.IntroductionIn modern world full of the electronic hardware one of the most important but least noticeable issue is a need to shield the electric devices from electromagnetic interferences (EMI).Electric devices emit electromagnetic fields at various frequencies,which can cause a significant damage or poor performance of the equipment.Electromagnetic shielding can be addressed to the reflection and/or absorption of electromagnetic radiation by a material.Reflection is regarded as a primary shielding mechanism and in order to perform shielding,the shield must interact with electromagnetic fields in the radiation due to mobile charge carriers.Secondary shielding mechanism –absorption,is realised by electric or magnetic dipoles that interact with electromagnetic fields in the radiation [1].Therefore the shielding materials use to be conduc-tive and the most commonly they are metals.Although conductiv-ity is not necessary,it is advisable when shielding electronic devices.Electromagnetic radiation,especially that of high frequen-cies tends to interfere with electronics such as computers,contain-ing many microchips that can be harmed due to electrostatic discharges.Packaging/shielding material should be able to protect viable electronic goods both from the electromagnetic radiation and accidental electrostatic discharges.Both criteria meet metals but they are bulky and expensive in transportation.Polymer composites filled with electrically conductive carbon based materials (carbon black –CB,carbon nano fibers –CNF,carbon nanotubes –CNT)are by far lighter and easy to manufacture.In comparison with metals,polymer composites are semiconductors,but for good antistatic properties with a charge dissipation half-time of 2–10s the surface resistivity of 1010–1011[X cm]is required [2].Such resistivity can be obtained for electroconductive polymer composites with most of the common fillers at a volume concentration close to the percolation threshold [3–7]./10.1016/j.matdes.2014.10.0090261-3069/Ó2014Elsevier Ltd.All rights reserved.⇑Corresponding author.Tel.:+48714686.E-mail addresses:stanislaw.frackowiak@.pl (S.Frackowiak),joanna.ludwiczak@.pl (J.Ludwiczak),karol.leluk@.pl (K.Leluk),kazimierz.orzechowski@chem.uni.wroc.pl (K.Orzechowski),marek.a.kozlowski@.pl (M.Kozlowski).1Tel.:+48713204690.2Tel.:+48713757114.3Tel.:+48713206538.Further improvement of the material mass/EMI SE(shielding effectiveness)ratio can be obtained by introducing foaming tech-nology into the material preparation process.Microporous struc-ture can greatly improve material properties such as thermal isolation,mechanical properties and in case of compositesfilled with CB or CNF–also the electrical conductivity[8–10].Filler par-ticles have less polymer volume to occupy which allows to obtain lower percolation threshold when compared to conventional solid composites.Polymer foams with carbon-based conductivefillers can be an alternative to currently used materials,such as high cost conductive polymers.They combine low weight and good EMI shielding with low cost and can be tailored for specific applications [11–15].Zhang et al.[16]developed a novel composite material consisting of a syntactic foam(hollow carbon microspheres in resole resin)reinforced with CNF.Thefiller content was0.7;1.4;2.1and 2.8vol.%.The composite preparation took place in a high-shear homogenizer and the shielding effectiveness was eval-uated as a ratio between the incoming and outgoing power of an electromagnetic wave.Maximum EMI shielding was ca.25dB for a composite containing2.0%of thefiller.Ameli et al.[17]presented composites with a polypropylene matrix,filled with CNF for EMI shielding.Their work focused on the effect of foaming on the carbonfibers orientation,aggregation and on the percolation threshold.The EMI SE was measured in the X-band frequency range(8.0–12.4GHz).They observed that a physical foaming of PP-CF composite caused a change in its microstructure due to the biaxial stretching of cells with thefibers and by the plasticizing effect of a dissolved gas on the viscosity of composites.Also the density of composites was reduced by25%as a result of foaming. Electrical properties were also improved due to higherfibers inter-connectivity as a result of the biaxial stretching during foam-ing.The percolation threshold was reduced from8.75to7vol.%, while EMI SE increased for over65%.Biopolymers have attracted recent years a great attention for replacing the petrochemical based plastics due to the fossil fuel depletion,ecological hazards by the non-degradable plastics and an overall public concern.One of the most widely used alternative for conventional plastics is polylactide(PLA)which has good mechanical properties(similar to polystyrene),is fully biodegrad-able,biocompatible,of good chemical resistance and derived from renewable resources(e.g.corn starch).It can be processed with conventional processing methods(injection moulding,extrusion, spinning and others)[18,19].Range of applications varies from medicine implants and compostable packaging,to agriculture. When developing a novel packaging material for EMI shielding one has to take in mind the ecological aspect of such.Packaging has a very short life span and tends to be discarded soon after use and according to the best knowledge of the authors there are no papers concerning implementation of biodegradable polymers for EMI shielding material.Therefore the aim of the work reported was to develop a lightweight biodegradable polymer compositefilled with cheap conductivefiller mainly for packaging application,with good EMI SE properties along with ability to protect from electrostatic dis-charges,combining good mechanical and functional behaviour with low density.2.Materials and methods2.1.MaterialsComposites were prepared by melt mixing of polymer matrix, PLA3052D from NatureWorks,with carbon basedfillers.Mixing was performed using a Thermo Scientific Polylab QC equipped with internal mixer with chamber temperature of180°C and rotor speed of60RPM.First was carbon black(CB)Ketjenblack EC-300J from Akzo Chemicals which according to manufacturer specifications has a very high specific surface area(approx. 800m2/g(BET)),low ash content and apparent bulk density of 125–145kg/m3Secondfiller constituted nanosized carbon nanofi-bres(CNF)PR-19XT-HHT from Pyrograf Inc.According to the pro-ducer their CNF are heat-treated to temperatures up to3000°C. This high heat treatment creates the most graphitic carbon nanofi-bres and reduces the iron catalyst content to very low levels.Filler concentration for composites with CB was2,4,6and8vol.%and4, 8,12and16vol.%for CNF.2.2.Preparation and foaming of compositesFoaming of composites was carried out using the exothermic blowing agent Luvomax AZ-C1(Lehmann&Voss&Co.),which con-sists of azodicarbonamide(ADC)and ethylene propylene rubber (EPM)carrier.Thermal decomposition of azodicarbonamide occurs at200°C resulting in the evolution of nitrogen,carbon monoxide, carbon dioxide,and ammonia gases.The chemical blowing agent was used in this study in an amount of0.5wt.%.Polymer matrix and nanocomposites with a dimensions of 20Â20mm and a thickness of3mm were foamed by thermal decomposition of the ADC in a vacuum oven at220°C for10min.2.3.Cellular structure characterizationScanning electron microscopy(SEM)was used to characterize morphology of samples.Sputtering with gold was performed prior to SEM observations which were carried out with VEGA TESCAN microscope.The foam density was evaluated by a buoyancy method with the density kit mounted on a balance from Mettler Toledo.2.4.Thermal propertiesThe thermal analysis was performed using DSC Q20from TA Instruments.All experiments were conducted at the heating rate of10°C/min,in the temperature range from20up to220°C.The samples of about8mg weight were placed in aluminum pans. After thefirst heating run,the samples were cooled down,and sub-sequently heated again with same rate.The glass transition tem-perature(T g)and the melting temperature(T m)were determined from the second heating run.2.5.Volume resistivity measurementsIn order to estimate precisely the percolation threshold of each polymer/filler system,electrical measurements of the volume resistivity as a function of thefiller concentration were performed. Samples were prepared by press moulding in a form of plates and for minimizing the contact resistance,gold electrodes were vapour deposited on each sample.The currentflow was measured using Keithley6512electrometer from Keithley Instruments,while the voltage supply was High Voltage Power Supply GPR-30H100(GW Instek).2.6.Electromagnetic shielding effectivenessThe measurements of real and imaginary part of electric per-mittivity were performed with HP4282A(Agilent)Precision LCR Meter in the frequency range100Hz–1MHz.The apparatus was set in Cp-D mode,the sample capacity(in the absence of electricalfield)was calculated on the geometrical basis.All mea-surements were performed in ambient conditions(room tempera-ture:24°C,humidity level:55%).Samples were produced in a form750S.Frackowiak et al./Materials and Design65(2015)749–756of pellets diameter of14.1mm,2mm thick.No further processing was applied to the investigated material.The electromagnetic shielding effectiveness was calculated using formulae below.It was assumed that total SE is a sum of reflective(R)and absorption(A)part:S:E:¼20log g4g sþ20log e t=d¼RðdBÞþAðdBÞð1ÞWhere two contributions can be written out as:AðdBÞ¼20log eÀt=dð2ÞRðdBÞ¼20log ggsð3Þ3.Results and discussion3.1.Thermal propertiesDSC curves of the PLA,PLA/CB,and PLA/CNF composites areshown in Fig.1.The thermal properties such as glass transitiontemperature(T g),crystallization temperature(T c),cold crystalliza-tion temperature(T cc),percentage of crystallization(X c)obtainedfrom the DSC studies are summarized in Table1.Polymer matrixis characterized by a glassy transition of approximately62°C.The results show that with the addition of CB and CNF to PLAmatrix the T g of the composites did not change significantly.PLAexhibited an exothermic peak at114°C due to cold crystallizationand a single endothermic peak due to melting.An intensive,non-Fig.1.DSC curves of(a)PLA/CB and(b)PLA/CNF composites with differentfiller content.S.Frackowiak et al./Materials and Design65(2015)749–756751matrix.Addition of carbon black caused a decrease of the noniso-thermal crystallization peak,however during a cooling rate of 5°C/min the isothermal crystallization peak occurred.Addition of CB reduced the cold crystallization temperature,which indicates a nucleating effect of thefiller.The isothermal crystallization was observed during cooling nanocompositesfilled with carbonfibers.Two melting peaks were observed for all nanocomposites.According to[20–22]the double melting behaviour is related to the presence of crystals with different crystal forms.The lamellae of different size and perfection are created during the cold and primary crystallization.The peak at the lower temperature may be associated with melting of smaller crystallites formed during a cold crystallization while at a higher temperature melt the crystallites formed during a primary crystal-lization.The intensity of melting peaks is dependent on the nano-differences in cellular structures formed in PLA/CB and PLA/CNF nanocomposites.The addition of2%CB(Fig.3a)into PLA provided a cellular structure with an average pore size of280l m.An increase of the nanofiller content to4%CB(Fig.3b)resulted in a reduction of the pore size to approximately140l m.Further, increase of thefiller content(6%,8%CB)(Fig.3c and d)has no ben-eficial effect on the cellular structure of nanocomposite,because of the cells coalescence.This may be due to a higher melt viscosity caused by the presence of nanofiller[23],making it difficult to expand,resulting in concentration of the gas formed during the decomposition of the blowing agent inside the sample or may be due to the agglomeration of carbon black particles.Addition of the lowest nanofiber content(4%CNF)(Fig.3e)resulted in creation of pores size of about210l m.An increase in CNF content to8%, 12%,16%(Fig.3f–h)caused a reduction in the average cell sizeFig.2.SEM images of:(a)CB,(b)CNF,(c)6%CB dispersed in PLA matrix,(d)6%CNF dispersed in PLA matrix. 752S.Frackowiak et al./Materials and Design65(2015)749–756exhibited a similar density in the range of 0,55–0,68g/dm 3.That connected with an observation that with higher CNF content a pore dimension decreased,but their number increased,so that density was not significantly changed.The carbon fibers have contributed to a decrease in PLA density by over 50%referring to unfoamed poly-mer matrix.3.4.Volume resistivity measurementsFor estimation of the percolation threshold for each polymer/fil-ler system,plots of the volume resistivity versus the filler content were presented on Fig.5and 6.Estimation of the percolation threshold for composite materials designed to protect against the electromagnetic radiation is very important.It provides an information on the filler concentration at which the damping mechanism changes from the reflection EM waves into their absorption.Additionally,packaging materials for electronic devices should be able to discharge electrostatic charge which otherwise may damage or even destroy electronic components.SEM images of PLA/CB with filler content:(a)2%CB,(b)4%CB,(c)6%CB,(d)8%CB and PLA/CNF:(e)4%CNF,(f)8%CNF,(g)12%CNF,(h)16%Fig.4.Density of PLA (unfoamed,foamed)and foamed PLA nanocomposites.Fig.5.Volume resistivity measurements of composites containing carbon black.Fig. 6.Volume resistivity measurements of composites containing carbon nanofibers.absorption peaks.The increase in e 0and e 00is most impressive low frequency region ($5times,below 1kHz for e 0and for e 00)but also in 100kHz –1MHz e 0is still enhanced (almost tripled).Also a smooth dispersion/absorption curves appeared deflection point about 20kHz.The tailing is much more empha-(comparing to the unfilled,reference sample)but the be rather attributed to the Maxwell–Wagner phenomenon spatial polarization related to the filler (a conductive material)to water molecules.Well emphasized dispersion indicates creation of CNF domains (with their own relaxation time)in foamed,continuous (referring to the basic local filed theories).Obviously,the discussed system surely cannot be treated as a ‘‘nanoscaled’’one,as single CNF (nano)species,even totally persed in the polymer matrix,are embedded in a micro-sized (created after foaming process).Thus,it should be assumed observed dispersion (and corresponding absorption)curves 4%CNF samples are related rather to a bunch of CNF’s placed single micro-shell than to 1+1(CNF and void)system.Similar observations can be made for samples filled with carbon with one exception.In spite of absorption maxima around kHz a shift to 1kHz was observed.Also no low-frequency tailing recorded in the sample filled with carbon black.Despite,high-frequency process was slightly emphasized.Basing on this two sets of plots it may be concluded that carbon and carbon black behave differently during processing.Although both composites are filled well beyond their percolation clear dispersion curves characterize materials with different absorption frequency.The question is,what are the parameters responsible for the specific value of frequency.Is it only material property (i.e.geometry)or/either the filling degree?Analyzing the plots from Figs.8and 9it may be supposed filling degree is not the main factor responsible for shiftingFig.7.Real (a)and imaginary (b)part of complex electric permittivity measured for solid and foamed PLA.Fig.8.e 0(a)and e 00(b)curves measured for PLA-CNF (4%)composites.Fig.9.e0(a)and e00(b)curves measured for PLA-CB(2%)composites.Fig.10.Dispersion(a),(c)and absorption(b),(d)curves of PLA+4%CB and PLA+8%CNF samples.enhancement in shielding properties as the material gains in con-ductive properties.The increase corresponds tofiller/polymer matrix ratio and presence of voids due to foaming process but is irrespective to thefiller shape.Despite using fractal-type carbon black orfibered carbon one can easily notice about15dB gain in attenuation due to formation of free volumes(4%CNF/2%CB:À25dB vs.same foamed samples:À40dB).The difference appears when comparing highly loaded samples:PLA+2%CB/PLA+4%CB and PLA+4%CNF/PLA+8%CNF(foamed and not foamed).In case of nanofiberedfiller(CNF),increasing its content twice(from4% to8%)leads to more than2,5times gain in S.E.,much greater com-paring to PLA+CB samples.It may be concluded that in low loaded composites(although above the percolation level)the shielding effectiveness relies on the amount of a conductivefiller but it may be additionally enhanced by the foaming process.Further increase of thefiller amount leads to conductive composites,whereas foaming has no remarkable influence on the composite’s electrical properties.Con-sidering only the value of S.E.at given frequency(not analysing the frequency dependency)the influence of thefiller on the electric properties is irrespective to its morphology(shape,aspect ratio, surface area,etc.).4.ConclusionsBased on results obtained,the most important factors influenc-ing material design for EMI SE are listed below.(1)Foaming of conductive composites results in an increasedelectromagnetic shielding effectiveness for materials with afiller concentration close to the percolation threshold.(2)Absorption peak for frequencies in the radio range wasobserved for composites with lowfiller content.That accounted to1kHz and10kHz for materialsfilled with car-bon black and carbon nanofibres,respectively.(3)Higher shielding effectiveness at lowfiller concentrationwas obtained for composites containing rather carbon black than carbon nanofibres.(4)More homogeneous and well dispersed cell structure wasobtained for materialsfilled with carbon nanofibres.Carbon black exhibited tendency to form agglomerates which pro-moted overgrowing of cells.That was observed particularly forfiller concentrations high above the percolation thresh-old(6–8%).AcknowledgmentsThe research was supported by Wroclaw Research Center EIT+ under the project‘The Application of 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poly(lactic acid)composites.Polym.Degrad.Stab.2008;93:1044–52.Table2Shielding effectiveness calculated for selected frequencies.f(Hz)100100010,000100,000Solid PLA5,64,8734Foamed PLA123,96,531PLA+4%CNF232551>150PLA+4%CNF foamed4242138>150PLA+8%CNF7092>150>150PLA+8%CNF foamed88140>150>150PLA+12%CNF>150>150>150>150PLA+12%CNF foamed>150>150>150>150PLA+16%CNF>150>150>150>150PLA+16%CNF foamed>150>150>150>150PLA+2CB212348>150PLA+2CB foamed263863>150PLA+4%CB>150>150>150PLA+4%CB foamed495187>150PLA+6%CB>150>150>150>150PLA+6%CB foamed126>150>150>150PLA+8%CB>150>150>150>150PLA+8%CB foamed>150>150>150>150756S.Frackowiak et al./Materials and Design65(2015)749–756。
