saleem et al-2007-polymer composites

High Performance Thermoplastic Composites: Study onthe Mechanical, Thermal, and Electrical ResistivityProperties of Carbon Fiber-ReinforcedPolyetheretherketone and Polyethersulphone

Anjum Saleem, Lars Frormann, Azhar IqbalInstitut fur Polymerwerkstoffe und Kunststofftechnik, Technische Universitat Clausthal, Agricola Str. 6, 38678Clausthal-Zellerfeld, Germany

High temperature processing thermoplastic polymers,polyetheretherketone (PEEK) and polyethersulphone(PES), were melt blended with carbon bers (CFs) tomake composites. These composites were investigatedfor their mechanical, thermal, and electrical properties.Mechanical properties that are expressed in terms ofstorage modulus, loss, and damping were enhancedwith the addition of CFs. Thermal properties were deter-mined by DSC and TGA. These methods help to under-stand the effects of ber content and bermatrix adhe-sion in the composites. Composites were also tested fortheir electrical and thermal conductivity because CFsleave the composites thermally and electrically conduc-tive. CFs enhanced the crystallinity of the PEEK appre-ciably that in turn inuenced thermal conductivity, elec-trical resistivity, and the stiffness of PEEK/CF(composites of PEEK with CFs). PES/CF (composites ofPES with CF) shows a different behavior due to theamorphous nature of PES. The work involves one llerand two different matrices, and so it provides an inter-esting comparison of how matrix morphology can inu-ence the properties of composites. POLYM. COMPOS., 28:785796, 2007. 2007 Society of Plastics Engineers

INTRODUCTION

More and more requirements in nearly all applicationsaddress the need for high temperature resistance and highstrength. Polymers that can fulll these requirements aregrouped together as high temperature resins. Polyethere-therketone (PEEK) and polyethersulphone (PES) are tworepresentative members of this group of polymers. PES is ahigh temperature amorphous thermoplastic polymer havingall the strengths and weaknesses of an amorphous polymer.PEEK on the other hand is a high temperature semicrystal-

line thermoplast whose crystalline nature admits it to theorientation processes to provide high strength and chemicalresistance. The success of these resins is mainly due to themolecular rigidity of the repeat unit [1, 2]. Both resins canbe compression molded, extruded, or injection molded us-ing conventional equipment and have high processing tem-perature (up to 300C).

Thermoplasts are attractive for high strain to break, in-denite shelf life, ability to thermoform, and light weight,etc. But the problem is they lack strength. Conventionalengineering materials, metals, and their alloys are strongand tough, but not light. The reinforcement of thermoplas-tics with the high modulus bers to improve strength andrigidity has been in practice from many years. PES andPEEK have the excellent properties of the thermoplasts andalso they offer a high strength over a wide temperaturerange. To maximize the usefulness of these resins, they areoften available as reinforced composites [3]. However, theuse of these resins is limited by cost and processing con-cerns. The resins can cost considerably more than the stan-dard commercial resins. Furthermore their high processingtemperature may not be suitable for all plastics-processingequipment. Still, their high purity, good wear, low amma-bility, and outstanding physical properties have made themimportant components for automatic-transmission and en-gine parts [4, 5].

Although the majority of ber-reinforced resins containglass bers, the attractive properties of carbon bers (CFs)have made them a material of choice in various applications.CFs have higher stiffness, excellent electrical and thermalconductivity, and high resistance to fatigue and creep [6] incomparison to the glass bers. CF-reinforced compositeshave all the ideal properties, leading to their rapid develop-ment and successful use for many applications over the lastdecade [7].

Present work deals with the study of CF-reinforced com-posites of PEEK and PES. Mechanical, thermal, and elec-trical properties of these composites have been presented

Correspondence to: Prof. Dr. Lars Frormann; e-mail: [email protected] grant sponsor: AiF, Germany.DOI 10.1002/pc.20297Published online in Wiley InterScience (www.interscience.wiley.com). 2007 Society of Plastics Engineers

POLYMER COMPOSITES2007

here. Two different matrices and one ller were used in thiswork. So the article provides a comparative study on howmatrix morphology can change the properties of the com-posites. Composites were analyzed by DMA (dynamic me-chanical analysis), DSC (differential scanning calorimetry),TGA (thermogravimetric analysis), and for their electricalas well as thermal conductivity properties. The bulk prop-erties of all the composites are very much dependant uponthe matrix, reinforcement, interface, surface properties ofthe llers and matrix, crystallinity, size of the spherulites,and transcrytallinity, etc. [8]. Mechanical properties wereanalyzed by monitoring property changes in material withrespect to the temperature by DMA. Thermal analysis wasused to study the effects of ber content and bermatrixadhesion in the composite. The interaction and adhesionbetween the ber and matrix has a signicant effect indetermining the mechanical and physical behavior of theber composites [9].

EXPERIMENTAL

Materials

Two polymers were used in this work as matrix material.Table 1 lists the characteristics of these polymers.

PEEK is a Victrex PEEK 450G and was provided byVictrex. PEEK is a semicrystalline polymer and the repeatunit is

PES is an Ultrason E2010 and it was provided by BASF.It is an amorphous polymer with high thermal durability.This polymer is resistant to boiling water, chemical corro-sion as well as ignition and can be easily adapted forprocessing. Its repeat unit is

As llers, CF were used. Table 2 lists the importantcharacteristics of this ller. CF were obtained by TOHOTenax with grade HTA 5131 with specic electrical resis-tance 1.6 103 cm, tensile modulus 238 GPa, andelongation at break 1.7%.

Sample Preparation

The composite components were dry blended in desiredratios. Their blending was done in a Thermo Haake Rheo-mix 600 mixing head operating at 50 rpm. The mixing ofPEEK with CF was done at 380C and the mixing of PESand CF was done at 360C. The components were mixed fora period of 10 min. The resulting extrudates were milled atroom temperature and then injection molded using an in-jection molding machine (ARBURG-220S, ALL-ROUNDER, 15060). All the samples were dried in anoven before injection molding at 100C for 3 hr to removethe moisture. Two types of molds were obtained by theinjection molding machine, round and bone shape. Thedimensions of bones were 30 2 2 mm3 and they wereused for the mechanical analysis. The round samples of 2mm thickness and 50 mm diameter were used for thethermal and electrical conductivity measurements.

Instrumentation

Mechanical Analysis. The dynamic mechanical analysis(DMA) was done by TA Instrument Thermal Analysis andRheology DMA-2980 1.7B in single cantilever mode. Adynamic force of 0.000118 N was used with a frequency of10 Hz and the amplitude of 20 m. The temperature scanranged from 80 to 180C with 2C/min. A constant nitro-gen ow of 40 ml/min was used to purge the instrument.

TABLE 1. Characteristics of the polymers.

Material Grade SourceDensity(g/cm3)

Melting temperature(C) Tg (C)

Polyether-ether-ketone (PEEK) Victrex PEEK 450G Victrex 1.3 340 143Polyether-sulphone (PES) Ultrason E2010 BASF 1.37 340390 225

TABLE 2. Characteristics of the ller.

Material Grade Source FormDensity(g/cm3)

Diameter(m)

Length(mm)

Aspectratio

Carbon bers (CF) HTA5131 TOHO Tenax Fiber 1.76 7 6 857

786 POLYMER COMPOSITES2007 DOI 10.1002/pc

Small bones were used for the measurement of storage, loss,and tan moduli as a function of temperature.

The E-moduli were determined by Zwick material test-ing machine, Z 2.5/TN1S under a speed of 5 mm/min.

Thermogravimetry. Weight loss as a function of tem-perature was analyzed by TA Instruments, TGA-2950. Thesamples weighing 38 mg were heated from 0 to 1000Cwith 20C/min under a nitrogen purge. The instrumenttemperature calibration was performed by using the curietemperature of various metals according to the manufactur-ers recommendations.

DSC Analysis. Differential scanning calorimetry analy-sis (DSC) of the composites and pure polymers was done byTA Instruments, DSC-2920. Samples of3 mg were placedand sealed in open aluminum pans. A constant nitrogen owof 40 ml/min was used to purge the instrument. The heatingrange was from 0 to 390C at a rate of 10C/min. Beforestarting each test the instrument was calibrated according tothe manufacturers recommendations

For PEEK composites, degree of crystallinity was calcu-lated by the following way

XcQm QcmQx 100 (1)

In this equation, Xc is the degree of crystallinity in percent-age, Qm and Qc are the experimentally obtained endother-mic melting and exothermic crystallization enthalpies, re-spectively, m is the experimental weight content of thePEEK matrix in the composite, and Qx is equal to 130 J/g isthe melting enthalpy value deduced for fully crystallizedPEEK [10].

Electrical Resistivity. Electrical resistivity was mea-sured by the instrument, Agilent 4339B High ResistanceMeter. This model is designed for measuring very highresistance and related parameters of insulation materials. Allthe measurements were made at 100 mV. The instrumentwas calibrated according to manufacturers recommenda-tions before use.

Thermal Conductivity. The thermal conductivity wasmeasured by an instrument, NETZSCH TCA 200-LT-A asa function of temperature. This model is a computer-con-trolled instrument used to measure the thermal conductivityof materials by the guarded heat ow meter method. Thetest sample is placed between two heated surfaces con-trolled at different temperatures with a heat ow from thehotter to the colder. The thermal resistance of the interfacesbetween the sample and adjacent surfaces is reduced byapplying a coupling agent i.e. silicone heat sink compound.

RESULTS AND DISCUSSION

Composite Morphology

Figure 1 shows the morphology of the polished surface(normal direction) of a small portion of the round injectionmolded sample of PEEK with 35 wt% of CF. The gure alsoshows that the bers are dispersed uniformly within thematrix and are oriented in the ow direction (FD). Theaverage length of the CFs has been reduced because extru-sion causes the ber breakage.

As the molded specimens are anisotropic, an illustrationof the directions in which various property measurementswere taken is also shown in Fig. 1.

Mechanical Properties

DMA is an excellent technique to measure the propertiesof the polymers. It provides information about the ability ofthe materials to store and dissipate mechanical energy upondeformation.

Figure 2a shows the storage modulus as a function oftemperature for pure PEEK and PEEK/CF (composites ofPEEK with CF) and Fig. 3 shows the storage modulus ofpure PES and PES/CF (composites of PES with CF) as afunction of temperature. PEEK is a semicrystalline polymerand crystallinity makes the polymers stiffer. PEEK showsgood retention of modulus up to its Tg (143C), beyond thistemperature the modulus drops sharply but some rigidity ismaintained up to its melting temperature (340C). PES onthe other hand shows a rapid fall in properties after 210Cwith a little strength or rigidity maintained beyond 250C.

Both gures show that the storage modulus increases bythe addition of CF. This is due to the reason that bers arecombined with the matrix and are giving support to thematrix by reducing the movement of chains [10]. Twodistinct variations of storage modulus with temperature canbe observed in both cases:

FIG. 1. Optical microscope of the injection molded sample of the PEEKwith 35 wt% of CFs and an illustration of the directions in which variousproperty measurements were taken on an injection molded sample andexplanation of the effect of extrusion on ber length.

DOI 10.1002/pc POLYMER COMPOSITES2007 787

A sharp drop in storage modulus near 140C in Fig. 3 andnear 220C in Fig. 2.

A reduction in the rate of drop in storage modulus after170C in Fig. 2 and after 230C in Fig. 3.

The rst drop in the storage is due to the relaxationassociated with the amorphous phase ( relaxation). Inthis case, the glassy state of the amorphous phase goesthrough its glass transition and there is a sharp drop in thestorage modulus [11]. The reduction in modulus is morepronounced for pure polymers. The second observation

for a reduction in the rate of drop is due to the softeningof the matrix until the melt region is reached [12].

In Fig. 2a after 160C a rise is observed in storagemodulus. This appears due to cold crystallization. Toconrm this observation some specimens were heatedabove Tg till 180C and then were analyzed again andthey showed no rise in storage modulus after 160C (Fig.2b). With the addition of the bers, the height of thesepeaks is increasing that could be due to the tendency ofshort bers to enhance the crystallization [13, 14].

FIG. 2. (a) Storage modulus of pure PEEK and the composites with CF. (b) Storage modulus of CF-reinforcedPEEK composites after heating above Tg.


The loss moduli represent the energy lost during defor-mation of the polymer. It is related to the viscous portionof the elastomer. It provides information about the overallexibility and the interactions between the components ofcomposite material.

Figure 4 shows the loss modulus for PEEK and PEEK/CFwhere relaxation peak is located between 140 and160C while a peak is located between 165 and 180Crepresenting cold crystallization. With the increase of theCF in the matrix, the peaks of loss moduli are becomingnarrow and extended to the higher temperature side. The

increase in the peak temperature indicates the decrease inchain mobility. Peaks are becoming narrow as the bercontent is increasing. This indicates that amorphous por-tion of the composites is decreasing as the incorporationof the bers in the matrix considerably enhances stiffnessand strength of the composites.

Furthermore Fig. 5 presents the loss modulus of PES andPES/CF. There is only relaxation peak located between220 and 260C because PES is an amorphous polymer.Glass transition of PES is quite higher than PEEK. This isbecause of the sulphone groups present in the chemical

FIG. 3. Storage modulus of pure PES and the composites with CF.

FIG. 4. Loss modulus of PEEK and composites with CF.


structure of PES that make it so stiff. Relaxation peaks inPES/CF are also shifting to the higher temperature indi-cating a good association between the ber and the ma-trix.

tan is the indicative of the materials ability to dissipateenergy. tan is equal to loss modulus/storage modulus.The relaxation peak is located at 240C for PES (Fig. 7)and at 150C for PEEK (Fig. 6). Peak temperature is anindicative of the Tg of the matrix. Maximum heat dissi-pation occurs at this temperature. Figure 6 shows tan forPEEK and PEEK/CF. When bers are added to the matrix

and the crystallinity is increased, peaks become loweredand spread out. Nucleation of the crystallization of matrixby the bers can provide transverse crystals in the bulk ofthe matrix. These transcrystalline regions further enhancethe adhesion between matrix and llers [15]. So reductionin peak heights indicates the good interfacial bonding anda decrease in the chain mobility. PES/CF also shows thesame results (Fig. 7) but peaks are not as lowered as theyare in the case of PEEK. It means that the modulus of thecrystalline polymers increase a lot by the addition of thebers and this is obvious by the results.

FIG. 5. Loss modulus for PES and composites with CF.

FIG. 6. Tan of PEEK and composites with CF.


E-moduli that are measured in the FD are presented inFig. 8. E-modulus of pure PEEK is higher than PES.Addition of CF increases the E-modulus signicantly inboth polymers. However, E-modulus of PEEK compos-ites is lower than the E-modulus of PES composites. This

behavior could be attributed to the increased crystallinityof PEEK by the addition of CF that makes the compositesbrittle [16, 27]. PES is an amorphous polymer and so ithas more ability to bend without breaking as compare toPEEK i.e. it is tougher than PEEK.

FIG. 7. Tan of PES and the composites with CF.

FIG. 8. E-moduli of the composites of PEEK and PES with CF.


Thermal Properties

Figure 9 shows the DSC thermograms of PEEK andPEEK/CF. These curves are the rst heating scans of thematrix and the composites. At 140C plots shift upward.This indicates the approach of the glass transition point ofthe polymer and composites. Figure also shows one exo-thermic peak between 150 and 175C, and an endothermicpeak between 300 and 350C [17]. The exothermic peaksappear due to the cold crystallization. In semicrystallinepolymers, above Tg, when chains have a lot of mobility,they wiggle and squirm and try to gain enough energy tomove into very ordered arrangements. When chains are inthese crystalline arrangements they give off heat and anexotherm can be seen in the DSC curve. Temperature at thelowest point of this exothermic dip is called crystallizationtemperature Tc and it is an indication that the material cancrystallize. For PEEK/CF this temperature lies between 150and 160C. When heating is continued past Tc then anendothermic peak can be seen. This peak indicates thedisappearance of the crystals [18]. Chains come out of theirordered arrangement and begin to move around freely. Thepeak temperature is the melting temperature (Tm) of thematerial. For PEEK and PEEK/CF this temperature liesbetween 340 and 350C. Both crystallization and meltingpeaks are decreasing by the addition of bers, indicating aneffective association between ller and matrix. The percentcrystallinity calculated for pure PEEK, PEEK with 10% CF,and PEEK with 35% CF is 10, 16, and 17%, respectively, byDSC. It seems percentage of crystallinity is increasing bythe addition of bers. However, from rst heating scans ofthe composites, it is not possible to have some nal state-ment about the effect of bers on crystallinity. But theprevious work of several authors has been reported that the

quality and quantity of crystallization depends upon thethermal history and ber characteristics (length, content, ortype). CF have a tendency to nucleate the crystallizationprocess of PEEK under favorable thermal conditions [14,28]. The effect of thermal history and ber characteristicson the crystallinity of composites will be presented in futurework.

The DSC thermogram for PES/CF are presented in Fig.10. PES is an amorphous polymer. That is any crystalliza-tion peak or any sharp melting peak cannot be found [19].The curve rises near 213C, indicating the approach of theglass transition temperature. Melting of the polymer is notsignicantly inuenced by the addition of the ller [20]. Inthis case we cannot develop any solid conclusion about theeffect of llers on the melting temperature of the compos-ites.

Thermogravimetric curves of the neat polymers and thecomposites are very much in agreement with other results.Figure 11 shows that the weight loss has reduced signi-cantly by the addition of bers as compare to the purePEEK. Semicrystalline nature of PEEK and increase of thiscrystallinity with the addition of bers give a lot of strengthto the composites. However Fig. 12 shows the weight lossas a function of temperature for the pure PES and PES/CF.It is quite obvious that the bers are more efcient inreducing weight loss in the case of PEEK than in PEScomposites. This is due to the lack of crystallinity in PES.

Electrical Resistivity

Presence of CFs render the polymers electrically conduc-tive. Figure 13 shows the electrical resistivity, which is theinverse of conductivity, of pure PEEK, PES, and their

FIG. 9. DSC thermogram of PEEK and the composites of PEEK with CF.


CF-reinforced composites. For PEEK composites a perco-lation threshold is observed near 35 wt% CF loading. Asudden decrease in resistivity is observed at this point. ForPES composites a percolation threshold is observed near 10wt% ller loading.

At lower ller loadings, ller particles act like conduc-tive islands in a sea of electrically insulating polymer. Asmore particles are introduced, the conductive particles be-come more crowded and are more likely to come in contactwith each other. At percolation threshold, a majority of ller

particles are in contact with at least two of their nearestneighbors, thereby forming a continuous conductive chainor network. An electrical charge now can ow withoutencountering the high resistance polymer resin. At percola-tion threshold, the volume resistivity decreases sharply likea jump function, but beyond percolation concentration ad-ditional ller loading does not greatly reduce the resistanceof the composite [21, 25, 26]. In PEEK composites theformation of conducting network is not as easily as in thecase of PES composites, and this might be due to the

FIG. 10. DSC thermogram of PES and the composites of PES with CF.

FIG. 11. TGA curves of PEEK and the composites with CF.


presence of crystallinity in PEEK that is not there in PES[24]. So we see a percolation at higher ller loading inPEEK/CF than in PES/CF.

Thermal ConductivityCFs are very effective and common llers that are used

to increase the thermal conductivity of the composites [21].

Figure 14 shows the thermal conductivity of PEEK and itscomposites with CF. Thermal conductivity is increasingwith temperature as well as with increasing ller concen-tration. It can be observed that thermal conductivity in-creased signicantly at the lower ller loading but at higherloadings after 20 wt% CF the increase is not so high. Thiscould be due to the increased crystallinity at these concen-

FIG. 12. TGA curves of PES and composites of PES with CF.

FIG. 13. Electrical resistivity of the composites of PEEK and PES with CF at room temperature.


trations. Mathis [22] shows for his data a linear relationshipbetween thermal conductivity and Youngs modulus. Ther-mal conductivity is very sensitive to the orientation, crystalfraction, and the degree of crystal perfection [22]. However,the discussion about the orientation of crystals and amor-phous segments is beyond the scope of this work.

Figure 15 gives the thermal conductivity of PES and itscomposites with CF. There is a continuous increase ofthermal conductivity with the increase of ller and theincrease of temperature [19]. Thermal conductivity of thespecimens was measured in the normal direction. CFs aremore efcient in increasing thermal conductivity in PES

FIG. 14. Thermal conductivity of PEEK and the composites with CF.

FIG. 15. Thermal conductivity of PES and its composites with CF.


composites than in PEEK composites. The reason could bethe presence of crystal fractions and orientations in PEEKthat hinder the establishment of a network for conductivity.

CONCLUSION

The work deals with the study of the effect of CFs on themechanical, thermal, and electrical properties of two poly-mers, semicrystalline PEEK, and amorphous PES. Compos-ites were prepared by melt compounding. CFs were used asllers.

Storage modulus showed a signicant increase with theincreasing amount of the llers. This was due to the reasonthat the llers allowed stress transfer. The loss moduluspeaks became lower and shifted to higher temperaturesindicating the improved llermatrix adhesion. DSC ther-mograms are also supporting DMA results. Melting peakshave lowered by the addition of llers because of the goodassociation between matrix and reinforcement. ForPEEK/CF exothermic peaks are indicating the cold crystal-lization.

Addition of CF makes the composites electrically con-ductive. So we can observe a percolation threshold in thecase of both llers. For PEEK/CF this threshold lies near 35wt% of CF, while for PES/CF this threshold lies near 10wt% of CP. Percolation threshold indicates the presence ofconductive bridges in the composites.

Thermal conductivity was measured as a function oftemperature and ller concentration. It was observed thatthermal conductivity increases both with the increase oftemperature and the ller concentration.

PES and PEEK are high temperature processing engi-neering polymers. Their properties can be improved signif-icantly by the addition of CFs. Presence of CFs give a lot ofstrength to the PEEK composites but on the other handincreased crystallinity renders these composites brittle. PEScomposites have more toughness i.e. ability to bend withoutbreaking and better thermal conductivity due to the lack ofcrystallinity but have less strength. These composites areused in practical applications with a control of their prop-erties according to the requirement. They offer an unrivalledselection of properties to the design engineers.

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