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http://jrp.sagepub.com/content/27/16-17/1893The online version of this article can be found at:

 DOI: 10.1177/0731684407082545

November 2007 2008 27: 1893 originally published online 12Journal of Reinforced Plastics and Composites

S.T. Sam, H. Ismail, M.N. Ahmad Fauzi and A. Abu BakarComposites

The Effect of Carbon Black on the Properties of Magnetic Ferrite Filled Natural Rubber  

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The Effect of Carbon Black on the Propertiesof Magnetic Ferrite Filled Natural

Rubber Composites

S. T. SAM, H. ISMAIL,* M. N. AHMAD FAUZI AND A. ABU BAKAR

School of Materials and Mineral Resources EngineeringUniversiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia

ABSTRACT: The effect of carbon black loading and type on the properties of ferrite filled naturalrubber (NR) composites was investigated. The carbon black loading used was from 0 to 50 phr andferrite loading was fixed at 80 phr. Carbon black grades N330 and N660 were used in this work. Thescorch time decreased with the addition of carbon black in ferrite filled NR composites whereas thecure time increased. The tensile strength and elongation at break of ferrite filled natural rubbercomposites were found to decrease with the increase of both types carbon black loading. However,the stress at 100% elongation (M100) and stress modulus at 300% elongation (M300) exhibit anincreasing trend. The microstructure showed that filler dispersion of ferrite filled natural rubbercomposites become poorer with increasing carbon black loading. The thermal stability was found toenhance with carbon black loading. The swelling test indicated that the swelling percentage reducedwith increasing carbon black loading while at low loading enhanced the magnetic properties of thecomposites. The N330 carbon black filled NR composites showed longer scorch and cure times andbetter tensile properties, swelling resistance and magnetic properties but lower thermal stability thanN660 carbon black filled NR composites.

KEY WORDS: rubber ferrite composites, nickel zinc ferrite, magnetic filler, carbon black.

INTRODUCTION

FERRITES ARE MAGNETIC materials which are physically dark or black in color, andvery hard, brittle and inert to a lot of chemicals. In the form of ceramic, ferrite can be

used in producing magnetic memories, flexible magnets, microwave absorbers, TV yokes,and some other useful devices. The incorporation of ferrite in natural rubber is importantin producing an easily moldable and flexible composite materials called ferrite rubbercomposites (RFCs) or polymer bonded magnets (PBMs). RFCs have wide industrialapplications as they can be tailored in various shapes and properties. According to Maliniet al. [1], RFCs can be applied in wave absorbers and electromagnetic interference (EMI)shielding material. Several researches [1–8] have reported the work on RFCs by usingdifferent types of ferrite filler in various rubber matrices.

*Author to whom correspondence should be addressed. Email: hanafi@eng.usm.my

Journal of REINFORCED PLASTICS AND COMPOSITES, Vol. 27, No. 16–17/2008 1893

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In this study, the ferrite powder used was waste ferrite with zero economic value. Thistype of ferrite was useful in producing many types of electronic devices such as highfrequency transformers and inductors due to their naturally low current loss [9]. Two typesof carbon blacks used in hybrid with ferrite were N330 and N660 grades. The aim of thestudy is to investigate the effect of carbon black loading and two different types of carbonblacks on tensile properties, thermal stability, swelling percentage and magnetic propertiesof ferrite filled natural rubber composites.

EXPERIMENTAL

Compounding Ingredient and Mixing Procedure

Natural rubber (SMR L) was obtained from the Rubber Research Institute of Malaysia(RRIM). The waste ferrite used in this study was Ni-ZnFe2O4, which was obtained fromACME Ferrite Products Sdn. Bhd. The ferrite powder was a by-product of grinding thefully sintered Ni-ZnFe2O4. Its specific gravity is 4.90 g/cm

3 with mean particle size between4–8 mm. The N330 and N660 carbon black were obtained from Cabot Company(Malaysia) with the mean particle size of 28 and 70 nm respectively. The surface area ofN330 and N660 are 80 and 35m2/g, respectively. Others chemicals such as sulphur, zincoxide, stearic acid, n-cyclohexylbenthiazyl sulpheamide (CBS) and sterically hindredbisphenol (Vulkanox NKF) were purchased from Bayer (M) Ltd. The carbon blackloadings used in this study were 10, 20, 30 and 50 phr. The formulation used is shown inTable 1. The ferrite was dried in a vacuum oven at 808C for 24 hours to expel moisture.Mixing of the raw materials was carried out on a conventional laboratory two roll mill(160� 320mm). The mixing time was kept below 30 minutes to avoid prematurevulcanization of excess heat. The cure times, t90 were determined using a MonsantoRheometer, Model MDR 2000 (moving die rheometer) at 1508C according to ISO 3417.The rubber compounds were then compression molded at 1508C using t90 and thevulcanizates were conditioned for 24 hours prior to testing.

Measurement of Tensile Properties

The tensile tests were conducted using an Instron Universal Testing Machine, model3366 according to ASTM D412 at a 500mm/min crosshead speed. Parameters obtainedincluding tensile strength, tensile modulus and elongation at break.

Table 1. A typical formulation of rubber compound.

Ingredient Recipe (phr)

SMR L 100Ferrite 80Zinc Oxide 5Stearic Acid 3CBS 0.5NKF 1Sulfur 2.5Carbon Black 0, 10, 20, 30 and 50

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Measurement of Thermal Stability

Thermal degradation of RFCs was analyzed using a Perkin Elmer thermalanalyzer. About 10mg of sample was needed for analyzing. The heating rate of thesystem was 108C/min and the test was conducted from 50 to 6008C in a nitrogenatmosphere.

Swelling Test

The swelling test was carried out according to ISO1817. The dimension of the sampleswas 30mm� 5mm� 2mm. The samples were immersed in toluene for 48 hours in a darkcondition. The excess toluene was removed by using tissue paper. The swollen weight ofthe samples was weighted by using an electrical balance. The swelling percentage of thesample can be calculated with the equation below:

Swelling ð%Þ ¼M2 �M1

M1� 100:

The M1 and M2 refer to original weight and swollen weight respectively.

Scanning Electron Microscopy

The tensile fracture surface of the RFCs was examined using a Scanning ElectronMicroscope (SEM), model Leica Cambridge S-200. The micrograph obtained from SEMwas used to observe the filler distribution of the fracture surface and the micro defect ofthe samples. All the surfaces were initially sputter coated with gold in order to avoidelectrostatic charging and poor image resolution.

Magnetic Properties

The initial permeability, quality factor and inductance of RFC samples weredetermined using a Hewlett Packard impedance/gain phase analyzer model HP-4194 A.The samples were cut in the form of discs having an inner diameter (ID) of �3.7mm,outer diameter (OD) of �20mm and a thickness of �2mm. The initial permeability,quality factor and inductance were measured using 10 Cu wire windings at frequency rangeof 1 kHz–35MHz.

RESULTS AND DISCUSSION

Cure Characteristic

Figures 1 and 2 show the scorch time (t2) and cure time (t90) for carbon black filledRFCs using two types of carbon black, N330 and N660. It can be observed that scorchtime decreased with the increase of carbon black loading for both composites. Geethammaet al. [10] reported that as the filler loading increased, the incorporation time of the fillerinto rubber matrix also increased and consequently generated more heat due to theadditional friction. The decrease of the scorch time indicated that the composites took lesstime to reach the onset time of vulcanization as the carbon black loading increased.

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Figure 2 shows that the cure time of both RFCs decreased with increasing carbon blackloading. As reported by Ismail et al. [11], the curing rate of rubber composites depends onpresence metal oxide. Thus, the reduction of metal oxide content (ferrite) in rubbercomposites with increasing carbon black loading had reduced the curing rate consequently

10.5

11

11.5

12

12.5

13

0 10 20 30 40 50 60

Carbon black loading (phr)

Cur

e tim

e (m

in)

N330N660

Figure 2. Variation of cure time with carbon black loading in RFCs.

2.5

3

3.5

4

4.5

5

0 10 20 30 40 50 60

Carbon black loading (phr)

Sco

rch

time

(min

)N330N660

Figure 1. Variation of scorch time with carbon black loading in RFCs.

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increasing cure time. As can be observed in Figure 2, N330 carbon black displayed longercure time than N660. Generally, factors such as low surface area, high moisture contentand high metal oxide content can decrease the cure time of the compounds [12]. The slowercure time of RFCs with N330 might be due to the high surface area of N330 carbon black.Variation of torque difference (MH-ML) is shown in Figure 3. The MH-ML of thecomposites was found to increase with increasing of carbon black loading. As reported byCiesielski [13], the torque difference is an indirect indicator of crosslink density of rubbercomposites. The trend indicated that the crosslink density increased with the incorporationof carbon black in RFCs. Carbon black can act as a crosslink agent in the vulcanizationprocess, hence the crosslink density increased with the increase of carbon black [14]. It canbe seen in Figure 3 that N330 carbon black exhibits higher MH-ML than N660.The smaller particle size of a filler with larger surface area normally has greater interactionwith a rubber matrix [12].

Tensile Properties

The effect of carbon black loading on tensile strength in RFCs is shown in Figure 4.The tensile strength decreased about 10% and 20% with addition of 50 phr N330 andN660 respectively. The reduction of the tensile strength was due to the agglomeration ofthe filler and dilution effect at high filler loading. Thus, the filler was unable to transfer thestress to polymer matrix since the rubber-filler interaction is weak [5]. The phenomenon isin agreement with some of our previous work using rice husk ash in natural rubbercomposites [15], oil palm wood flour in natural rubber composites [16] and bamboo fibrein natural rubber composites [17]. Comparing two types of carbon black, tensile strengthof N330 is found to be higher than N660 in RFCs. Again, this must be attributed to the

4

6

8

10

12

14

16

18

20

0 10 20 30 40 50 60

Carbon black loading (phr)

Tor

que

diffe

renc

e (d

Nm

)N330N660

Figure 3. Relationship between torque difference and carbon black loading in RFCs.

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better interaction between N330 carbon black and rubber as a result of the smaller particlesize of N330. A similar trend can be observed in the elongation at break as shown inFigure 5. At a similar filler loading, N330 exhibits higher elongation at break than N660.The result indicated that the smaller particle size of filler has better interaction with rubbermatrix. Figure 6(a)–(c) and Figure 7(a)–(c) show the SEM micrograph with differentloading of N330 and N660 in RFCs. As the carbon black loading in RFCs increased, thefracture surface of the RFCs exhibits more holes or detachment of filler from rubbermatrix. This indicated the rubber-filler interaction becomes poorer with increase of carbonblack loading. It can be seen that, at a similar filler loading, the size of holes is bigger inN660-filled than N330-filled RFCs. Figures 8 and 9 show the effect of both carbon blacksloading on stress at 100% elongation (M100) and stress at 300% elongation (M300) ofRFCs. According to our previous report on bamboo filled natural rubber composites [18],the increase in filler loading would increase the restriction of macromolecular motion ofrubber chains. Another reason might be due to the nature of the fillers. Carbon black iscomposed of rigid particles that cause the overall increment of the stiffness of rubbercomposites. N330, which has smaller particles size, was found to exhibit higher M100and M300 than N660. The larger surface area of carbon black N330 has better interactionwith rubber matrix [19].

Thermal Stability

Figure 10 shows the TGA curves of RFCs with different carbon black loading. Sincethere was only one type of rubber matrix (i.e. NR) used in the composites, the curvesdisplayed a singe step degradation process for all RFCs produced. Table 2 shows detailedsummary variation of 5% weight loss (T-5%) and 30% weight loss (T-30%) of RFCs at

5

7

9

11

13

15

17

19

21

23

25

0 10 20 30 40 50 60

Carbon black loading (phr)

Ten

sile

str

engt

h (M

Pa)

N330N660

Figure 4. The effect of carbon black loading on elongation at break and tensile strength of RFCs.

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different loading of N330 and N660. T-5% indicates the initial thermal stability whereasT-30% shows the higher degradation rate of the rubber composites. As shown in Table 2,the incorporation of the carbon black resulted in overall improvement of thermal stability.Char residue content increased with higher carbon black loading. According to Ramesan[20], the higher content of char residue can insulate the undecomposed polymer from thedegradation process. The results show that there is a slightly higher degradationtemperature of RFCs with N660 compared to N330, particularly at the higher fillerloading. The degradation temperature can be related to the volatile material in the filler.The carbon black N660 which has less volatile matter might enhance the degradationtemperature of RFCs. The enhancement of the thermal stability with increasing fillerloading in polymer matrix have been reported by some researchers using different fillers,such as TiO2 and Fe2O3 filled poly (methyl methacrylate) [21], ferric oxide in siliconepolymer composites [22] and Al2O3 and ZnO filled silicone rubber [23].

Swelling Percentage

A swelling test is used to observe the filler-matrix interaction. The swelling percentage isthe amount of toluene uptake per weight of the rubber matrix. According to our previousreport [24], the swelling percentage of the rubber composites was influenced by crosslinktype and density, filler loading and type of rubber matrix. Figure 11 shows the effect ofcarbon black loading on the swelling percentage of RFCs using two different carbonblacks. A decreasing trend can be seen as the carbon black loading increased. The resultsindicated that the penetration of the toluene into RFCs was reduced with the additionof carbon black loading. The RFCs with N330 carbon black had less swellingpercentage compared to N660 carbon black. The result is in agreement with our previous

300

400

500

600

700

800

900

1000

Carbon black loading (phr)

Elo

ngat

ion

at b

reak

(%

)N330N660

0 10 20 30 40 50 60

Figure 5. The effect of carbon black loading on elongation at break and tensile strength of RFCs.

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report which indicated that the smaller the particles size of filler, the better the rubber-fillerinteraction [12].

Magnetic Properties

The dependence of inductance upon frequency of two different carbon blacks in RFCs isshown in Figure 12. Generally, the inductance decreased slightly with increase of frequency.Thus, it can be reported that the RFCs were stable even at high frequency. This indicatesthat the RFCs are suitable to be used in high frequency application. The inductance of RFCs

Holes

Moreholes

Moreandbiggerholes

(a)

(b)

(c)

Figure 6. SEM micrograph of the tensile fracture surface of RFCs with different N330 loading (a) 10 phr(b) 30 phr and (c) 50 phr at 500X magnification.

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with carbon black N330 displayed higher inductance compared to N660. A similar reasonmust be attributed to the better dispersion of the carbon blackN330 in rubber matrix to thatdiscussed in the previous section. Figure 13 shows the variation of the inductance withdifferent carbon black loading in RFCs. At low and high applied frequency, the inductanceof the RFCs with N330 first increased up to 30 phr, and then decreased with higher loading.The decreasing trend in high loading was due to the increasing of magnetic interactionbetween the ferrite particles as the particles distance reduced [5]. RFCs with N660 carbonblack increased up to 20 phr at a frequency of 100 kHz and thereafter decreased with higherfiller loading. At higher frequency (i.e. 10MHz) the maximum loading of N660 carbon

Holes

Moreandbiggerholes

Moreholes

(a)

(b)

(c)

Figure 7. SEM micrograph of the tensile fracture surface of RFCs with different N660 loading (a) 10 phr(b) 30 phr and (c) 50 phr at 500X magnification.

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black to achieve optimum inductance was 10 phr. Thus, the ability to store energy in theform of magnetic field of N330 was better than N660 at low and high frequencies. The plotof initial permeability as a function of frequency is shown in Figure 14. The trend of theinitial permeability of all samples was the same with the inductance because the initial

0

0.5

1

1.5

2

2.5

3

3.5

4

0 10 20 30 40 50 60Carbon black loading (phr)

M10

0 (M

Pa)

N330N660

Figure 8. Effect of carbon black loading on stress at 100% elongation (M100).

0

2

4

6

8

10

12

0 10 20 30 40 50 60Carbon black loading (phr)

M30

0 (M

Pa)

N330N660

Figure 9. Effect of carbon black loading on stress at 300% elongation (M300) of RFCs.

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permeability depends on the inductance, dimension and number of coil winding of thesamples. It is known that permeability is the measure of the magnetic flux through a core[25]. Thus, the RFCs with N330 would allow magnetic flux more easily compared to N660.Figure 15 shows the dependence of quality factor (Q) on frequency at low (10 phr) and highloading (50 phr) of N330 and N660 carbon black. The resonance frequency of the RFCsoccurred at 30MHz. The result indicates that the types and loading of the carbon blackwould not affect the resonance frequency of Q factor of RFCs but was contributed by thepresence of ferrite in the composites. As can be observed in Figure 15, lower loading (10 phr)of both types of carbon black exhibit a better quality factor compared to higher loading(50 phr). This was due to the dilution effect which occurred when the filler loading wasincorporated up to 130 phr, i.e. 80 phr ferrite and 50 phr carbon black. As can be seen inFigure 16 at a similar carbon black loading, RFCs with N330 exhibited higher Q factor thanN660. This indicates that the larger particles size carbon black (i.e. N660) had caused thepoorer filler distribution and thus reduced the ratio of energy stored to the energy lost incomposites [25].

40

50

60

70

80

90

100

110

0 100 200 300 400 500 600 700

Temperature (°C)

Wei

ght l

oss

(%)

50 phr carbon black N660

50 phr carbon black N330

10 phr carbon black N660

10 phr carbon black N330

Figure 10. Thermogravimetric analysis of RFCs with different type and carbon black loading.

Table 2. TGA data for RFCs with different carbon black loading.

Carbon black loading (phr) T-5% (8C) T-30% (8C) Char Residue (%)

N330 N660 N330 N660 N330 N6600 346 346 397 397 44.9 44.9

10 348 348 406 407 47.4 48.030 350 352 412 413 52.4 52.850 351 355 414 418 55.6 56.6

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0

50

100

150

200

250

0 10 20 30 40 50 60

Carbon black loading (phr)

Sw

ellin

g pe

rcen

tage

(%

)Carbon black N330Carbon black N660

Figure 11. The effect of carbon black loading on swelling percentage of RFCs.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.00E+00 5.00E+03 1.00E+04 1.50E+04 2.00E+04 2.50E+04 3.00E+04 3.50E+04 4.00E+04 4.50E+04

Frequency (kHz)

Indu

ctan

ce (

µH)

10 phr of carbon black N33050 phr of carbon black N33010 phr of carbon black N66050 phr of carbon black N660

Figure 12. Variation of inductance with frequency for different type and loading of carbon black.

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1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

0.00E+00 5.00E+03 1.00E+04 1.50E+04 2.00E+04 2.50E+04 3.00E+04 3.50E+04 4.00E+04 4.50E+04

Frequency (kHz)

Initi

al p

erm

eabi

lity

(Hm

−1)

10 phr carbon black N330 50 phr carbon black N33010 phr carbon black N66050 phr carbon black N660

Figure 14. Variation of initial permeability with frequency for different type and loading of carbon black.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0 10 20 30 40 50 60

Carbon black loading (phr)

Indu

ctan

ce (

µH)

Carbon black N330 with frequency 100 kHzCarbon black N330 with frequency 10 MHzCarbon black N660 with frequency 100 kHzCarbon black N660 with frequency 10 MHz

Figure 13. The effect of carbon black loading on inductance at different frequency of RFCs.

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0.00

20.00

40.00

60.00

80.00

100.00

120.00

0.00E+00 5.00E+03 1.00E+04 1.50E+04 2.00E+04 2.50E+04 3.00E+04 3.50E+04 4.00E+04 4.50E+04

Frequency (kHz)

Q fa

ctor

10 phr carbon black N33050 phr carboon black N66010 phr carbon black N66050 phr carbon black N660

Figure 15. Variation of quality factor (Q) with frequency for different type and loading of carbon black.

0.00

10.00

20.0

30.00

40.00

50.00

60.00

70.00

80.00

0 10 20 30 40 50 60

Carbon black loading (phr)

Q fa

ctor

Carbon black N330 with frequency 100 kHzCarbon black N330 with frequency 10 MHzCarbon black N660 with frequency 100 kHzCarbon black N660 with frequency 10 MHz

Figure 16. The effect of carbon black loading on Q factor at two different frequencies of RFCs.

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CONCLUSIONS

The incorporation of carbon black in RFCs had decreased the scorch time but increasedthe cure time of the composites. RFCs with N330 carbon black exhibit longer cure timeand scorch time compared to N660. On the other hand, the torque difference increasedwith increase of both types of carbon black loading whereas the tensile strength andelongation at break show the opposite trend. At a similar filler loading, RFCs with N330exhibit higher tensile strength and elongation at break than N660. However, the tensilemodulus (M100 and M300) increased gradually with the increase of both carbon blackloading. The SEM micrographs showed that the dispersion of the ferrite in NR matrixbecomes poorer by incorporation of both carbon blacks but the thermal stability hadimproved. The presence of carbon black at low loading enhanced the magneticproperties of RFCs and carbon black N330 imparts better magnetic properties thanN660 carbon black.

ACKNOWLEDGMENT

The authors acknowledge the materials and testing equipments provided by ACMEFerrite Products Sdn. Bhd., which has resulted in this article.

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