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COMPOS I T E MATER I A L S

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Kunststoffe international 1/2014 www.kunststoffe-international.com

CHRISTOF HÜBNER ET AL.

Multifunctional thermoplasticpolymer composites combiningelectrical conductivity with good

mechanical properties are suitable for awide range of applications in manybranches of engineering (e.g. for dissi-pation of static charges, electrostaticpainting processes, and electromagneticshielding). The potential of carbon nan-otubes (CNTs) as a functional filler to im-part electrical conductivity to both ther-mosetting and thermoplastic polymers iswell known and is being utilized. Buttheir potential – because of their excep-tionally high elastic modulus (up to1 TPa) – to improve the mechanicalproperties of thermoplastic composites,in particular, has not so far been exploit-ed. The main reasons for this are the ten-dency of individual CNTs to agglomer-ate and form CNT clumps and theirlargely chemically inert surface that pre-vents effective bonding to the polymermatrix. In recent years, there have beenmany attempts to chemically modify thesurface of CNTs to make them compati-ble with different thermoplastic poly-mers. But these functionalizations havethe disadvantage of damaging the struc-ture of the CNTs, which can lead to bothlength shortening and loss of electricalconductivity, so running counter to thetwo target properties of electrical con-ductivity and improved mechanicalproperties.

In comparison with the most com-monly used functional filler for electricalconductivity – carbon black – CNTs have

the great advantage of being effective atsignificantly lower filler contents in therange of just a few percent by weight andso to embrittle the composite far less.

Polypropylene (PP) continues to be apopular commodity material, which isvalued for its good mechanical propertiesand relatively low price.However,becauseof the respective surface properties ofCNTs and the PP melt, PP is very unsuit-able for uniform dispersion of CNTs inthe matrix. This adversely affects both theelectrical conductivity and mechanicalproperties of PP-CNT composites. Vari-ous compounding additives have im-proved the dispersion of CNTs and shift-ed the percolation threshold – the fillerloading at which the composite becomesconductive – to lower filler contents(down to 0.4 % in PP). However, thesemeasures did not achieve a substantialimprovement in the mechanical proper-ties of the composites.

PP glass fiber (GF) composites are usedin many sectors on account of their im-pressive price/performance ratio. To at-tain the goal of electrically conductivecomposites with high mechanical prop-erty values at a moderate cost, it wouldtherefore seem a promising approach todevelop composites with a filler/reinforc-ing material mix of CNTs and glass fibers.

Materials and Processing

In the study reported here, a PP of theR352-08R type from Dow DeutschlandInc., Schwalbach, Germany, was used asthe matrix material.The CNTs came fromNanocyl S.A., Sambreville, Belgium,(multiwalled CNTs, type NC 7000) andthe glass fibers from European OwensCorning Fiberglas SPRL, Brussels, Bel-gium (type 968/968A). The additive usedto improve dispersion of the CNTs wasthe organic peroxide Peroxan Bec fromPergan GmbH, Bocholt, Germany. Thetest specimen designations in the graphsare explained in Table 1.

The materials were compounded on aLeistritz ZSE 27HP-52D co-rotatingtwin-screw extruder with an L/D ratio of52 from Leistritz AG, Nuremberg, Ger-many. First of all, PP-CNT composite ma-terials with 2.5 % CNT content were pro-duced. Then the glass fibers were fed invia a side port.

The composite materials were injec-tion molded into test specimens (ISO527-1 and ISO 179-1/1eA) on an EngelES 200/60 HL ST injection moldingmachine from Engel Austria GmbH,Schwertberg, Austria, and their mechan-ical properties were tested. Other testspecimens were compression moldedfrom pellets into test specimens measur-

Multifunctional Composites Reinforcing Materials. The combination of modern nanofillers with conventional

glass fibers is paving the way to multifunctional composites with outstanding elec-

trical and mechanical properties. The following article demonstrates this in tests

with a polypropylene matrix.

Abbreviation Meaning

PP polypropylene

Px peroxide

C2.5 2.5 wt.-% CNTs

G20 20 wt- % glass fibers

Fraunhofer Institut für Chemische Technologie ICTPolymer EngineeringD-76327 PfinztalGermanyTEL +49 721-4640-511> www.ict.fraunhofer.de

Contacti

Translated from Kunststoffe 1/2014, pp. 62–65Article as PDF-File at www.kunststoffe-international.com; Document Number: PE111577

Table 1. Test specimen designations used in thegraphs

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COMPOS I T E MATER I A L S

© Carl Hanser Verlag, Munich Kunststoffe international 1/2014

ing 60 × 10 × 1 mm3 on a Collins P 200P/M laboratory press from Dr. CollinGmbH, Ebersberg, Germany. The electri-cal properties of both types of test speci-men were tested.

Results

Rheology: An important considerationin incorporating nanoscale fillers inpolymer melts is the change in viscosityof the melt. Because of the very large spe-cific surface area of the fillers, melt vis-cosity generally rises very steeply. Thisis particularly true of compoundingprocesses, where the aim is to break upagglomerates of nanoparticles to in-crease the specific surface area of thenanofillers. Current work on the disper-sion of CNTs in thermoplastic melts hasshown that a low matrix viscosity is ad-vantageous for the dispersion of CNTs.To counter an excessively high viscosityincrease, an organic peroxide was addedto the composite in the present study. Itseffect on melt viscosity can be seen in Fig-ure 1. While the peroxide significantly re-duces the viscosity of the PP on its own,it significantly increases viscosity on ad-dition to the PP-CNT composite. Thiscan be attributed to the better dispersionand correspondingly enlarged effectivesurface area of the CNTs (Fig. 2). The ad-dition of 20 % glass fibers again increas-es viscosity significantly but the perox-ide has an overall viscosity-lowering ef-fect in the 2-filler system (CNTs + glassfibers).

The decrease in the viscosity of the ma-trix material achieved by the peroxide notonly results in better dispersion of theCNTs but also reduces the specific ener-gy input during compounding and henceenergy costs. Figure 3 shows thin sectionsof the composites containing 2.5 % CNTs

without (left) and with (right) the addi-tion of peroxide. Besides the reductionfrom 3.5 % to 3 % in the area of visibleand therefore undispersed CNT agglom-erates relative to the total observed sur-face area, a significant reduction in spe-cific energy input (from 1.2 kWh/kg to0.55 kWh/kg) was also noted.

Electrical Properties: Figure 4 showsthe volume resistivity of the composites.This was determined directly on extrud-ed strands, injection molded specimens,and compression molded bars. Lookingat the values for extruded strands, start-ing with the pure PP-CNT compositematerial, a significant reduction in vol-ume resistivity can be seen with the useof peroxide, with the addition of glassfibers, and with the addition of both per-oxide and glass fibers. Through the re-duction in viscosity, the peroxide obvi-ously achieves better deagglomeration ofthe CNTs, making more of them avail-able to form the conductive network.The unexpected reduction in volume re-

sistivity when glass fibers are added tothe PP-CNT composite can be attributedto better dispersion of the CNTs due tothe double shearing of the material intwo consecutive processing steps – pro-duction of the CNT composites and in-corporation of the glass fibers in theCNT composite.

The test specimens produced by injec-tion molding all show significantly high-er volume resistivity values than the ex-truded strands. To simulate real-life con-ditions, the skin was not removed fromthe injection molded specimens beforethe electrical properties were tested. Di-rectly on the surface, the flash has a filler-depleted layer with a higher electrical re-sistivity than the inside of the specimen.There is also the possibility that the CNTsare shortened because of the strong shearforces in the injection molding operation.The use of peroxide has no significant ef-fect on resistivity. Through the additionof glass fibers, the resistivity value isslightly increased, as expected.

PPPP-PxC2.5C2.5-PxC2.5-G20C2.5-G20-Px

Frequency

105

104

103

102

Pa·s

η*

10-1 100 101 rad/s 102

Fig. 1. Complex vis-cosities of the testedmaterials (figures: ICT)

© Kunststoffe

Fig. 2. Typical morphology of the PP-CNT-GF composites without peroxide (left) and with peroxide (right)

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COMPOS I T E MATER I A L S

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Virtually all the compression molded ma-terials show the lowest resistivity values.Compression molding offers the dis-persed CNTs the best conditions forforming a conductive network, thanks tothe very long cooling times and low shear.Any negative influence by the glass fibersis scarcely perceptible.

Mechanical Properties: The elasticmodulus of the materials is increased byaround 30 % through the addition of2.5 % CNTs, and by about 380 %through the addition of 20 % glassfibers. The combination of the two fillersleads to an increase in the elastic modu-lus that is slightly greater than the sum

of the two individual effects. The chain-shortening effect of the peroxide is re-flected in a slight decrease in the elasticmodulus of all the composite materialsas compared with the peroxide-free vari-ants.

The maximum stresses follow the samepattern as the elastic moduli. A 2.5 % ad-dition of CNTs achieves a slight increaseof about 16 %, while with 20 % glassfibers, the maximum stress goes up byabout 160 %, and with a combination ofthe two by about 200 %. Here again, thedifferences between the peroxide-con-taining and peroxide-free composites aresmall.

An interesting picture emerges in thecase of notched impact strength. First ofall, there is a very significant increase withthe addition of only 2.5 % CNTs. 20 %glass fibers increases the notched impactstrength more than 2.5 % CNTs. Thecombination of the two produces the bestvalues, according to the familiar pattern.While the use of peroxide reducesnotched impact strength in the case of PPon its own and the PP-CNT composite,this value is increased in the case of the

Fig. 3. Micrographs showing thin sections of the PP-CNT composites (2.5 % CNTs) without (left) and with (right) peroxide

103

102

101

100

10-1

10-2

Ωm

C2.5 C2.5-Px C2.5.G20 C2.5-G20-Px

Volu

me

resi

stiv

ity

Injectionmolded bar

Extrudedstrands

Compressionmolded bars

Fig. 4. Volumeresistivities ofthe tested com-posites as afunction of themethod of pro-cessing

© Kunststoffe

a)

4.0

GPa

3.0

2.5

2.0

1.5

1.0

0.5

0PP C2.5 G20 C2.5-G20

Elas

tic m

odul

us

b)

70

50

40

30

20

10

0

MPa kJ/m2

PP C2.5 G20 C2.5-G20

Max

imum

str

ess σ m

ax

c)

7.0

6.0

5.5

5.0

4.5

4.0

3.5

3.0PP C2.5 G20 C2.5-G20

Notc

hed

impa

ct s

tren

gth

I not

ch

Without peroxideWith peroxide

Fig. 5. Mechanical properties of the composites

© Kunststoffe

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COMPOS I T E MATER I A L S

two composites with a combination ofCNTs and glass fibers (Fig. 5).

Thermal Properties: Figure 6 shows theresults of thermogravimetric measure-ments (TGA) on composites in air. Theuse of peroxide reduces the thermal sta-bility of the PP slightly due to shorteningof the polymer chains, while the additionof 2.5 % CNTs increases the thermal sta-bility of the PP-CNT composites consid-erably as compared with PP alone. In the2-filler system (CNTs + glass fibers), ther-mal stability drops somewhat as com-pared with the composites with CNTsalone but is still significantly higher thanin the case of the matrices alone. In viewof the minimal differences between theperoxide-free and peroxide-containing

composites, it was concluded that therewas no systematic effect of the peroxideon the thermal stability of the compos-ites.

The increase in the thermal stability ofthe composites containing CNTs can beattributed to the radical-intercepting ac-tion of the CNTs described in the litera-ture, which delays oxidative decomposi-tion of the polymer matrix. This is par-ticularly important in terms of theincreased processing stability of CNTcomposites.

Conclusion

In the reported study, it was shown thatskilful combination of glass fibers with

CNTs as fillers makes it possible to pro-duce multifunctional composites with anexcellent property profile. The propertiesresulting from the addition of both typesof filler, such as mechanical stability andelectrical conductivity, can be combinedwith virtually no mutually adverse effect.The negative influence of peroxide usedas a processing aid on the thermal stabil-ity of the matrix material PP is very muchmore than compensated by the CNTs. Atthe same time, the peroxide enables thespecific energy input during compound-ing to be reduced by 50 %.�

ACKNOWLEDGMENTThis study was funded by the European Commissionunder the FP7-People-ITN-2008 program (grant no.238363).

THE AUTHORSDR.-ING. CHRISTOF HÜBNER, born in 1962, is

Manager of the Nanocomposites Group at the Fraun-hofer Institute for Chemical Technology (ICT), Pfinztal,Germany; Christof.huebner@ict.fraunhofer.de

DIPL.-ING. (FH) PATRICK WEISS, born in 1979, is aProject Manager at the Fraunhofer Institute for Chem-ical Technology (ICT), Pfinztal.

DR.-ING. SHYAM SATHYANARAYANA, born in1987, works in Advanced Materials & Systems Re-search at BASF SE, Ludwigshafen, Germany.

PROF. DR.-ING. FRANK HENNING, born in 1969, isProduct Group Manager Polymer Engineering andDeputy Director of the Fraunhofer Institute for Chemi-cal Technology (ICT), Pfinztal.

Temperature

100

80

60

40

20

0

%

200

Wei

ght

loss

250 300 350 400 °C 500

PPC2.5C2.5-G20PP-PxC2.5-PxC2.5-G20-Px

Fig. 6. Thermalproperties of thecomposites

© Kunststoffe

© Carl Hanser Verlag, Munich Kunststoffe international 1/201444

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