Polymer International Polym Int 54:401–405 (2005)DOI: 10.1002/pi.1706

Effect of nano-barium titanate on themechanical, electrical and solubilityperformance of ferrite-filled poly(etherether ketone) compositesMayank Dwivedi,1∗ Sarfaraz Alam2 and G L Verma3

1O/o CCR&D & DS, R No 117 D, B-Wing, Sena Bhavan, DRDO Headquarters, New Delhi-110 011, India.2Defence Materials and Stores Research and Development Establishment, DMSRDE PO, GT Road, Kanpur-208 013, India3Department of Applied Chemistry, Delhi College of Engineering, Shahbad-Daulatpur, Bawana Road, Delhi-110 042, India

Abstract: Ferrite and nano-barium titanate reinforced poly(ether ether ketone) (PEEK) composites weremade by compression moulding technology. The effects of reinforcement of the ferrite and nano-bariumtitanate on the PEEK matrix were studied with the aim assessing its potential as a radar-absorbingstructural material (RASM). With an increase in the percentage filling of ferrite in the PEEK matrix, thespecific gravity, hardness, tensile and flexural moduli, brittleness, dielectric constant, loss tangent andreflection losses were increased, while the tensile and flexural strengths, and elongation at break, weredecreased. With increasing nano-barium titanate reinforcement in the ferrite-filled PEEK composites,the specific gravity, hardness, tensile and flexural strengths, tensile modulus and dielectric constant werecomparatively increased, while the elongation at break, flexural modulus and loss tangent (marginally)were decreased. Reinforcement of the nano-barium titanate caused a comparatively marginal reductionin the loss tangent, although no effect on reflection losses was observed. The solubility resistance of theferrite-filled PEEK composites was improved with reinforcement by the nano-barium titanate. 2004 Society of Chemical Industry

Keywords: composites; ferrite; mechanical properties; nano-barium titanate; poly(ether ether ketone); scanningelectron microscopy

INTRODUCTIONPoly(ether ether ketone) (PEEK) is a known high-performance thermoplastic which is used as a neatpolymer, in blends and in matrix material for variousapplications. It is a semicrystalline and highly aromaticthermoplastic belonging to the class of polymersknown as poly(aryl ether ketone)s.1,2

At room temperature, unfilled PEEK is a tough andstrong material with excellent abrasion resistance andlong-term load-bearing properties.3 PEEK has gooddimensional stability, parts/products produced fromPEEK have good dimensional accuracy.4 PEEK alsoshows good resistance to fatigue failure and cyclicloading can be carried out for many millions of cyclesat high stresses without failure. The properties5 of neatPEEK are shown in Table 1.

The natural grade of PEEK is 35 % crystalline.The chief effect of the crystallinity of PEEK is toincrease the properties related to stress and to decreasethe properties related to deformation.6 The crystallinemelting point is reported to be 334 ◦C, and the glass

transition temperature (Tg) is 143 ◦C.7 Good thermalstability and excellent electrical properties up to its Tg

make PEEK suitable for most electrical applications.PEEK is resistant to most common chemicals,8

with one notable exception being concentratedsulfuric acid. It also dissolves in p-chlorophenol,benzophenone and diphenylsulfone, but at hightemperatures. In addition, PEEK does not hydrolysein water, even at 280 ◦C. PEEK is resistant tostress cracking, with hairline cracks only occurringin acetone. High mechanical stresses may shorten itsservice life.

PEEK is an inert polymer under normal storageconditions and there are no known dust hazards inthe handling of fully compounded granules (suchas those supplied by ICI Americas, Inc.).9 Whenhandling PEEK powders or in other situations wheredust may be generated, precaution should be takento prevent dust accumulation and the threshold limitvalue (TLV), for nuisance particulates, of around 106

particles m−3 should be observed.10

∗ Correspondence to: Mayank Dwivedi, O/o CCR&D & DS, R No 117 D, B-Wing, Sena Bhavan, DRDO Headquarters, New Delhi-110 011,IndiaE-mail: mayank@drdohq.res.in(Received 30 April 2004; revised version received 30 June 2004; accepted 5 August 2004)Published online 6 December 2004

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M Dwivedi, S Alam, GL Verma

Table 1. Physical and mechanical properties of neat PEEKa

Property Value

Density (g ml−1) 1.32Tensile strength (Mpa) 97.0Tensile modulus (Gpa) 2.3Flexural strength (Mpa) 170Flexural modulus (Gpa) 3.6Elongation at break (%) >60Rockwell hardness (F scale) 112Dielectric constant (50 Hz, 0–150 ◦C) 3.30Loss tangent (1 MHz) 0.003Reflection losses (dB) <1

a At 23 ◦C, unless otherwise stated.

The aim of this present study was to assessthe potential of nano-barium titanate- and ferrite-reinforced composites as radar-absorbing structuralmaterials (RASMs). Towards this aim initial experi-ments were carried out to study the effects of fillingthe PEEK matrix with ferrite on the mechanical, elec-trical and solubility performances. Further changes insuch performances were then investigated after rein-forcement with nano-barium titanate. Ferrite, beinga radar-absorbing material (RAM),11 when filled intothe PEEK matrix is likely to increase the absorption ofelectromagnetic radiation by the composite, althoughat the cost of reduced mechanical and solubility per-formances. However, reinforcement by nano-bariumtitanate in ferrite-filled PEEK is likely to improve themechanical and solubility performances. Particles ofnano-barium titanate have diameters ranging from 5to 70 nm, and lengths exceeding 10 µm.12

EXPERIMENTALMaterialsA fine-powder grade of PEEK (Grade, 450 PF),supplied by ICI Inc., UK, was used in this study.Both the ferrite and nano-barium titanate used inthese experiments were indigenous materials. Theferrite (supplied by DMSRDE, Kanpur, India) wasa cobalt–silicon (Co–Si)-doped barium hexaferrite,having the molecular formula, BaCoxSixFe12−2x.13

The barium titanate (supplied by DMSRDE, Kanpur,India) had a particle size of ∼25 nm. Fast extrusionfurnace (FEF) grade carbon black, as per ASTM D1765, No 550, was employed in the production of allof the composites.

Sample preparationThe moulding of samples was carried out ona compression moulding machine (supplied byBEMCO, Belgaum, India, capacity of 30 tons). Thecharge was weighed, mixed and then dried at 200 ◦Cfor 2 h. A matched die mould, made from tool-gradesteel, was used to fabricate the composite sheets.Charging of the mould cavity was one of the mostimportant steps in the moulding process as powderedPEEK has a high bulk factor. The charge was

compacted many times to remove entrapped air andobtain void-free composite sheets. The moulding wascarried out according to the cycle shown in Figure 1.

Table 2 lists the sample numbers, designations andconstituents for the ferrite-filled PEEK compositesand the ferrite- and nano-barium titanate-filled PEEKcomposites moulded in this study.

To assess the trend in mechanical, electricaland solubility performances, these experiments werelimited to composites containing up to 10 % of ferriteand 1 % (constant) of nano-barium titanate andcarbon black, respectively.

CharacterizationSpecific gravities were measured by using a chemicalbalance and distilled water. The Rockwell hardness (Fscale) for all of the specimens was tested on a Rockwellhardness tester (Buehler Rockwell Type, Macromet 1,Buehler, Illinois, USA).

The tensile strengths, moduli and elongation atbreak data were obtained from measurements ona Universal Testing Machine (Star Testing System,Mumbai, India, capacity of ca 104 kg). The sampleswere tested according to ASTM 638 D, with thecrosshead speed maintained at 5 mm min−1.

The flexural strengths and moduli were measuredon a Universal Testing Machine (Zwick 1773, Model

400

300

200

100

0

Tem

pera

ture

(°C

)

Application ofthe compaction pressure of 60 kg cm–2 at 390 °Cfor 4 times at an interval of 30 sec

Application ofthe compaction pressure of30 kg cm–2 at 345 °Cfor 5 times at an intervalof 30 sec

Final application of thecompaction pressure of 100 kg cm–2 at 390 °C for 1 time

Hold for 20 min at a pressure of 100 kg cm–2 at 400 °C

Cooling

150 200 250 3000 50 100

Time (Min)

Pressure:

Pressure:contact pressureat 140 °C

(ii)

(i)

Figure 1. Moulding cycle used to produce the composite sheets(heating rate, 2 ◦C min−1).

Table 2. Sample numbers, designations and compositions of the

composite samples used in this study

Content (%)

Number Designation PEEK FerriteBariumtitanate

Carbonblack

1a PFM2 97 2 0 11b PFBM2 96 2 1 12a PFM4 95 4 0 12b PFBM4 94 4 1 13a PFM6 93 6 0 13b PFBM6 92 6 1 14a PFM8 91 8 0 14b PFBM8 90 8 1 15a PFM10 89 10 0 15b PFBM10 88 10 1 1

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Performance of ferrite-filled poly(ether ether ketone) composites

No 9275, Ulm, Germany, load capacity of 100 kN).The samples were tested according to ASTM 790 D,with the crosshead speed maintained at 5 mm min−1.

Scanning electron micrographs of the fracturedsamples, in the flexural mode, were obtained (at amagnification of 200) on a scanning electron micro-scope (Jeol, Model no JEL-JSM-35F, Massachusetts,USA).

Dielectric constant and loss tangent data wereobtained on a microwave bench at a frequency of10 GHz at room temperature.

The solubility (resistance) behaviour for all of thesamples was investigated by using the following sol-vents: 20 % sulfuric acid, 20 % sodium hydroxide,N,N-dimethylformamide (DMF) and kerosene. Thesamples were exposed to these materials at roomtemperature for 7 d, and the weight loss/gain mea-surements recorded.

RESULTS AND DISCUSSIONThe colour of all of the samples was black due toincorporation of carbon black.

Mechanical performanceSpecific gravity and hardnessThe specific gravity and hardness of all of the samplesincreased with increasing ferrite content due to itshigher weight and hardness (Table 3). In addition, thespecific gravities and hardness of the PFBM sampleswas found to be higher than the corresponding PFMsamples. This is attributed to filling of the micro-voidsby the nano-barium titanate component.

Tensile strength, modulus and elongation at breakThe decreases in tensile strength and elongation atbreak, and increase in tensile modulus, were examinedas a function of the increase in ferrite content for thePFM and PFBM samples (Table 3). Representativetensile stress–strain curves for composites PFM4 andPFBM4 are shown in Figure 2.

The effects observed were attributed to thedispersive action and non-reinforcing nature of ferrite

0 0.01

100

80

60

Str

ess

(MP

a)

40

20

00.02

Strain

0.03

PFM4PFBM4

0.04 0.05

Figure 2. Comparison of tensile stress–strain curves of PFM4 andPFBM4.

which causes discontinuity among the polymericchains of PEEK. This also results in a decreasein strain, which, in turn, increases the modulus.The percentage elongation at break of the PFBMsamples was seen to be lower than those of thePFM samples with 2, 4 and 6 % ferrite content.This might be attributed to the high reinforcingeffect of the nano-barium titanate. The PFM samples,containing 8 and 10 % ferrite, showed no differencein percentage elongation at break when comparedwith the corresponding PFBM samples. This might beattributed to the dominance of the ferrite content overthe nano-barium titanate, which leads to the slightreduction in reinforcing effect of the nano-bariumtitanate.

Flexural strength and modulusThe flexural strength showed a decreasing trend withincrease in ferrite content, for all of the PFM andPFBM samples, while the flexural modulus was foundto increasing with increase in ferrite content, in allcases. However, in contrast to the tensile moduli data,it was found that the flexural moduli of the PFBMsamples were correspondingly lower than their PFBManalogues. This effect might be attributed to the higherstrain caused in the PFBM samples in the bendingmode (Table 3). Representative flexural stress–strain

Table 3. Data obtained for the mechanical properties of the composite samples used in this study

Number DesignationSpecificgravity

Rockwellhardnessa

Tensilestrength(MPa)

Tensilemodulus

(GPa)

Elongationat break

(%)

Flexuralstrength(MPa)

Flexuralmodulus

(GPa)

1a PFM2 1.3396 113 92 2.5 4.4 151 5.351b PFBM2 1.3535 118 96 3.0 4.0 164 4.962a PFM4 1.3540 114 89 2.6 4.0 141 5.392b PFBM4 1.3624 119 92 3.0 3.6 154 4.983a PFM6 1.3654 115 84 2.8 3.4 131 5.493b PFBM6 1.3763 120 86 3.2 3.2 144 5.064a PFM8 1.3821 115 80 2.9 3.2 118 5.524b PFBM8 1.3942 121 84 3.2 3.2 139 5.125a PFM10 1.3983 116 75 3.0 2.8 112 5.575b PFBM10 1.4179 121 78 3.3 2.8 134 5.15

a F scale.

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curves comparing PFM4 and PFBM4 are shown inFigure 3.

The reason for this might be that the extent ofalignment of the reinforcing nano-barium titanate inthe flexural mode was lower than in the tensile mode.In the flexural mode, only half of the thickness ofthe sample below the neutral axis was under tensionhence causing some alignment of the reinforcement(nano-barium titanate), while the upper half of thethickness to the neutral axis of the sample was undercompression, and here the nano-barium titanate couldmainly cause a filler effect but not a reinforcing effect.Whereas, in the tensile mode, the entire sample is intension, so causing a maximum possible alignmentof reinforcement in a situation where short anddiscontinuous reinforcements eg from nano-bariumtitanate, are involved.

Scanning electron micrographs of fractured sur-faces, in the flexural mode, of the PFM4 and PFBM4samples are shown in Figures 4 and 5, respectively.It is evident from the micrograph (Figure 4) thatthe fractured surface shows some smooth regions inthe case of the PFM4 sample. This indicated thatthere was discontinuity among the polymeric chainsof the PEEK matrix due to the presence of the ferritefiller, whereas, in the case of the PFBM4 sample, the

0 0.04

200

160

120

Str

ess

(MP

a)

80

40

00.08

Strain

0.12

PFBM4PFM4

0.16 0.2

Figure 3. Comparison of flexural stress–strain curves of PFM4 andPFBM4.

Figure 4. Scanning electron micrograph of PFM4.

Figure 5. Scanning electron micrograph of PFBM4.

Table 4. Electrical properties of the composite samples used in this

study

Number DesignationDielectricconstant

Reflectionlosses (dB)

Losstangent

1a PFM2 3.66 < 1 0.0321b PFBM2 4.43 < 1 0.0272a PFM4 3.68 < 1 0.0482b PFBM4 4.67 < 1 0.0363a PFM6 3.72 ∼= 1 0.0673b PFBM6 4.88 ∼= 1 0.0484a PFM8 3.80 > 1 0.0754b PFBM8 4.90 > 1 0.0505a PFM10 4.23 ∼= 2 0.0785b PFBM10 4.93 ∼= 2 0.063

micrograph of the fractured surface shows a disruptivepattern (Figure 5). This confirms that incorporationof nano-barium titanate in the PEEK matrix causes areinforcing effect with good interfacial bonding.

Electrical performanceDielectric constant, reflection losses and loss tangentAn increasing trend was observed for the dielectricconstant with increase in ferrite content (Table 4).

This trend was more pronounced in the filledcomposites containing both ferrite and nano-bariumtitanate. Reflection losses were observed from ≤1 to∼=2 dB in all of the PFM and PFBM samples, with thevalues increasing with the increase in ferrite content.This was attributed to the high electromagneticradiation absorbing character of ferrite. No significanteffects of the nano-barium titanate component onreflection loss were observed. The loss tangent showsan increasing trend with increase in ferrite contentin the composites, while, in contrast, reinforcementwith nano-barium titanate leads to a reduction inthe loss tangent. Such a reduction was attributed tothe higher electromagnetic transparency of the nano-barium titanate when compared to ferrite.

404 Polym Int 54:401–405 (2005)

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Solubility performanceAlthough concentrated H2SO4 is a good solvent forPEEK, this is not the case for the dilute acid (20 %).The weight gain in both of the sample systems, PFMand PFBM, was due to moisture absorption. Thepresence of nano-barium titanate in the compositeshad a negligible effect (Table 5).

The effect of NaOH on these samples was noticeableand there was some weight loss which increased withincreasing ferrite content. This was attributed to thereaction of ferrite with the alkali and degradationof the PEEK in an NaOH environment, as it wasevident that some light brown powdered deposits ofPEEK had settled at the bottom of the container.Nano-barium titanate reduced the effect of NaOH, asthe reinforcing properties of the nano-barium titanateimproves the morphological structure, hence reducingthe interstitial space for NaOH to enter and reactwith the ferrite, as well as reducing dissolution of thePEEK.

DMF caused swelling of all of these samples.The weight gain continued to reduce with increasingferrite content, as the latter reduces the content ofPEEK in the samples, which is responsible for theswelling. Reinforcement with nano-barium titanatereduced the swelling comparatively, as this improvesthe morphological structure and reduces the interstitialspaces in the composites.

Kerosene had a negligible effect on both samplesystems and thus it could be said that these compositeswere inert to kerosene.

CONCLUSIONSIt was established from this study that the mechan-ical and solubility performances were improved with

Table 5. Solubility resistance data obtained for the composite

samples used in this study

Weight loss (−)/gain (+) (%)

Number Designation20 %

H2SO4

20 %NaOH DMF Kerosene

1a PFM2 0.00 −0.06 +0.16 0.001b PFBM2 +0.01 −0.03 +0.13 0.002a PFM4 +0.01 −0.06 +0.12 0.002b PFBM4 0.00 −0.04 +0.10 0.003a PFM6 +0.01 −0.08 +0.12 −0.013b PFBM6 +0.02 −0.04 +0.08 −0.014a PFM8 +0.01 −0.09 +0.09 0.004b PFBM8 +0.01 −0.06 +0.08 −0.015a PFM10 +0.01 −0.10 +0.09 0.005b PFBM10 +0.01 −0.08 +0.06 0.00

the reinforcement of nano-barium titanate in a ferrite-filled PEEK matrix. However, a higher content offerrite in the PEEK matrix leads to higher reflec-tion losses reductions in mechanical and solubil-ity performances. Radar-absorbing materials (RAMs)are usually deployed in hostile environments wherethey encounter mechanical loads, aerodynamic loads,chemical exposure, etc. The suitability of RAMsdepends upon their electromagnetic absorbance cou-pled with good mechanical and chemical performancesto withstand the structural loads and chemical envi-ronment. A higher content of ferrite in the PEEKmatrix will lead to a higher electromagnetic absorbancebut also reductions in mechanical and chemical per-formances, which is detrimental to any mechani-cal/aerodynamic structure. Hence, reinforcement bynano-barium titanate has compensated for the lossesin mechanical and chemical performances. Reinforce-ment by nano-barium titanate in ferrite-filled PEEKmatrices offers the possibility of a potential radar-absorbing structural composite material.

ACKNOWLEDGEMENTSThe authors are thankful to Dr A Sivathanu Pillai,Chief Controller R&D, DRDO, New Delhi andProfessor GN Mathur, Director, DMSRDE, Kanpur,for their encouragement and support of this work. Theauthors also wish to thank Mr SM Abbas, Mr B Lal,Mr DK Behera and Mr KN Pandey of DMSRDE fortheir kind co-operation during the course of this work.

REFERENCES1 Rose J and Staniland P (to ICI Americas, Inc.), US Patent

4 320 224 (1982).2 Blundell DJ and Osborn BN, Polymer 24:953 (1983).3 Mc Grail PT, Polym Int 41:103 (1999).4 Harris R, Rapid Prototyping and Tooling—Industrial Applications,

newsletter, Vol 7, Danish Technological Institute, Denmark,pp 6–7 (2002).

5 Victrex PEEK Properties Guide, Victrex Technology Centre,Lancashire, UK, pp 6–7 (1998).

6 Arzak A, Eguiazabal JI and Nazabal J, Polym Eng Sci 31:586(1991).

7 Attwood TE, Dawson PC, Freeman JL, Hay LRJ, Rose JB andStaniland PA, Polymer 22:1096 (1981).

8 Reisch MS, Chemical and Engineering News, American ChemicalSociety, Washington, DC, pp 21–49 (1989).

9 Victrex PEEK, Health and Safety Information, VKT 2/0585, ICIAmericas, Inc., Wilmington, DE, USA (1986).

10 Mark HF, Bikales N, Overberger CG, Menges G andKroschwitz JI, Encyclopedia of Polymer Science and Engineering,Vol 12, 2nd Edn, Wiley, Chichester, UK, pp 313–320 (1990).

11 Ngo AT and Pileni MP, Adv Mater 12:276 (2000).12 Urban JJ, Spanier JE, Ouyang L, Yun WS and Park H, Adv

Mater 15:423 (2003).13 Abbas SM, Aiyer RPRC and Prakash O, Bull Mater Sci 21:279

(1998).

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