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Evaluation of the physico-mechanical and
electrical properties of styrene-butadiene
rubber/aluminum powder and styrene-
butadiene rubber/cerium sulfate composites
Article

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Polimery -Warsaw-
· March 2015
DOI: 10.14314/polimery.2015.100
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Evaluation of the physico-mechanical and electrical
properties of styrene-butadiene rubber/aluminum powderand styrene-butadiene rubber/cerium sulfate composites
Amira Nassar1),/c42), A.A. Yehia2), S.H. El-Sabbagh2)
DOI: dx.doi.org/ 10.14314/polimery.2015.100
Abstract : Different concentrations of powdered aluminum (Al) or cerium sulfate in the range between 10
and 60 phr were incorporated into styrene-butadiene rubber (SBR) matrices. The physico-mechanical andelectrical properties of vulcanizates were measured and evaluated. An improvement in the mechanicalproperties according to increased filler loading in SBR composites was noted. This finding is reinforcedby filler dispersion values (Lee equation), reinforcing index and thermal analysis.The dielectric properties, namely the relative permittivity ( /c101’) and dielectric loss ( /c101”), were measured as
function of both frequency and concentration of the SBR/filler composites. The results showed an enhan –
cement in the dielectric and mechanical properties, especially for Al powder. This is due to the enhance –
ment of filler-rubber interactions and the continuity of the conductive phase through the composites. The
SEM micrograph showed that the filler agglomerates were dispersed within the SBR matrix.
Keywords : aluminum powder, cerium sulfate, styrene-butadiene rubber, mechanical properties, image
analysis, electrical properties.
Ocena fizyko-mechanicznych i elektrycznych w³aœciwoœci kauczuku butadie-
nowo-styrenowego nape³nionego proszkami aluminium lub siarczanu(VI)ceru(IV)
Streszczenie : Do matrycy kauczuku butadienowo-styrenowego (SBR) wprowadzano proszki alumi-
nium (Al) lub siarczanu(VI) ceru(IV) w iloœci 10—60 phr. Oceniono w³aœciwoœci fizyko-mechanicznei elektryczne wytworzonych wulkanizatów. Stwierdzono, ¿e wraz ze zwiêkszeniem zawartoœci cz¹steknape³niacza w matrycy SBR poprawia³y siê w³aœciwoœci mechaniczne kompozytów, co potwierdzonometod¹ analizy termograwimetrycznej, na podstawie indeksu wzmocnienia ( RI) oraz stopnia dyspersji
nape³niacza. Badano te¿ zale¿noœæ w³aœciwoœci dielektrycznych — wzglêdnej przenikalnoœci ( /c101’) i strat
dielektrycznych ( /c101”) — od czêstotliwoœci zewnêtrznego pola magnetycznego i od stê¿enia cz¹stek na –
pe³niacza w matrycy SBR. Wyniki wskazuj¹ na polepszenie w³aœciwoœci zarówno dielektrycznych, jak
i mechanicznych zw³aszcza w przypadku kauczuku nape³nionego proszkiem aluminium. Obserwowa –
n¹ poprawê przypisano interakcjom miêdzy cz¹stkami nape³niacza i kauczuku, skutkuj¹cym przewod –
noœci¹ fazy ci¹g³ej kompozytów. Mikrofotografie SEM wskazuj¹ na obecnoœæ aglomeratów cz¹stek
nape³niacza rozproszonych w matrycy kauczuku SBR.
S³owa kluczowe : aluminium w postaci proszku, siarczan(VI) ceru(IV), kauczuk butadienowo-styreno –
wy, w³aœciwoœci mechaniczne, w³aœciwoœci elektryczne.
Industrial activities reveal a continuous demand for
improved materials that satisfies increasingly stringentrequirements such as high physico-mechanical characte –
ristics with cost reduction. These requirements can be
achieved through the use of composite materials whoseconstituents synergistically comply with the needs forspecific applications [1]. In most industrial applications,
elastomers are used as a matrix for very fine particulatesfor the production of composite materials. Such particlescan interact physically and/or chemically with the elasto –
mers creating high performance polymeric composites
[2]. Metallic powders are a special type of particle fillerthat impart special qualities to rubber composites. Thesefillers enhance properties such as thermal conductivity,electrical conductivity, response to magnetic fields, heatcapacity, etc. Polymeric, metal-filled composites are used
as antistatic materials in tires and wind blades to dissi –
pate the accumulated electrostatic charges [3].100 2015 ,,60nr 2
1)National Research Centre, Solid State Physics Department, Metal
Physics Laboratory, Dokki, Giza, Egypt.
2)National Research Centre, Polymers and Pigments Department,
Dokki, Cairo, Egypt.
/c42)Author for correspondence; e-mail: fatmohmar@yahoo.com

The incorporation of metal fillers not only improves
the electrical properties of the rubbers but also enhancesthe thermal conductivity of the rubber composites [4].Studies on the performance of rubber-metal pairs havebeen an important area in the tribology research field.There are many interrelated reports on determining theway to decrease the friction coefficient and improve themechanical properties of rubber composites [5]. Variousmethods are available to increase the conductivity of rub –
bers. One of the best and easily applied methods is the
incorporation of metal powders. Poor adhesion andnon-uniform dispersion of the discrete phase in the mat –
rix cause fluctuations in the composite properties. This
can be overcome by using coupling/bonding agents,which increase the rubber-filler interactions [6].
The application of rare earth salts in rubber is a new
research area. Related studies showed that rare earthcompounds had a particular function in rubber proces –
sing and application, such as heat stability, strengthening
action, etc. Cerium sulfate and cerium oxide are conside –
red the most important rare earth compounds. They are
extensively applied in glass, ceramics, catalysis, poli –
shing powders, phosphor, absorption ultraviolet mate –
rial, etc. [3, 7].
The aim of the present work is to evaluate the effect of
powdered aluminum (Al) and cerium sulfate [Ce(SO
4)2]
as reinforcing fillers for a styrene-butadiene rubber (SBR)matrix through the measurement of the mechanical, rhe-ological, scanning electron microscope (SEM) and dielec-tric properties. A secondary aim is to study the effect ofdifferent loadings (10—60 phr) of the investigated fillerson the properties of SBR.
EXPERIMENTAL PART
Materials
— SBR, is a styrene-butadiene rubber (1502) with
23.5 % styrene content, a molecular weight MWof
140 000 g/mole, Mooney viscosity ML(1 + 4) at 100 °C =
52 ± 3, and glass transition Tg= – 60 °C. Supply is Kumho,
LG, Zeon, ENI, and Sinopec CQ. Made in China.com con –
necting Buyers with china suppliers. Trade terms: FOB.
— Fine aluminum (Al) powder, particle size in the
range 1.74—2.12 µm, 99 % trace grade of metals basis,molecular weight 26.97 g/mol, boiling point 2327 °C,flash point 645 °C and density 2.7 g/cm
3, was supplied by
Aldrich (USA 18530. 1M). The chemical composition ofAl powder is presented in Table 1.
— Cerium(IV) sulfate Ce(SO
4)2powder, oxydimetric
min. 98 %, particle size in the range 4.88—13.9 µm wassupplied by E. Merck, Darmstadt, Germany.
— Accelerators: N-cyclohexyl-2-benzothiazole sulfen –
amide (CBS), trade name: Rhenogran
®CBS-80, Vulka –
cit®CZ. From Rheinehemie Germany.
— Antioxidants TMQ — polymerized 2,2,4-trime –
thyl-1,2-dihydroquinoline, trade name: PILnox®TDQ.From NOCIL LIMITED Navi Mumbai, Maharashtra
400705, India.
— Activators: stearic acid with density at 15 °C of
0.9—0.97 g/cm3, zinc oxide (ZnO) with density at 15 °C of
5.55—5.61 g/cm3, were supplied by Aldrich Company,
Germany.
— Curing agent: elemental sulfur as vulcanizing
agent with fine pale yellow powder and density of2.04—2.06 g/cm
3at room temperature (25 °C ± 1) was sup –
plied by Aldrich Company, Germany.
The above-mentioned ingredients are generally used
in the rubber industry. The solvents and chemicals wereof pure grade.
Sample preparation
Different concentrations of aluminum (Al) or cerium
sulfate [Ce(SO
4)2] powder at 10, 20, 30, 40, 50 or 60 phr
were mixed with SBR and the other ingredients accor-ding to ASTM D 15-72 (2007) in a two-roll mill. The speedof the slow roller was 24 rpm and the gear ratio was 1:1.4.Vulcanization was carried out in a single-daylight, elec-trically heated, auto controlled by hydraulic press at152 °C under a pressure of 4 MPa at the curing time,which was determined by an oscillating disc rheometer.The compounded rubber and vulcanizates were testedaccording to standard methods.
The specimens were made into disks of diameter
10 mm and thickness 2 mm for the electrical measure –
ments.
Methods of testing
— The mechanical properties were determined accor –
ding to ASTM D 412-06a (2013) and ASTM D 6204-12
(2012). The rheometric characteristics were measured at152 °C using a Monsanto Rheometer (model 100) accor –
ding to ASTM D 2084-11, hardness was determined ac –
cording to ASTM 2240-07 (2007) and swelling was deter –
mined according to ASTM D 3616-95 (2009).
The swelling data were utilized to determine the mo –
lecular weight between two successive crosslinks ( M
c)
through the Flory-Rehner relationship [7]:
/c91/c93 1
21
21
1
202
13 MVVV V
VVCrr r
rr/c61/c45/c45/c43 /c43
/c45/c230
/c232/c231/c246
/c248/c247/c114/c109 ln( )
/(1)POLIMERY 2015, 60,n r2 101
Table 1 . Chemical composition of aluminum powder
Component Concentration, ppm
Iron (Fe) /c1633500
Silicon (Si) /c1632500
Copper (Cu) /c163200
Zinc (Zn) /c163500
Titanium (Ti) /c163200

where: /c114— the density of rubber, V0— the molar volume
of solvent (toluene), Vr— the volume fraction of the swol –
len rubber, and µ — the interaction parameter between
the rubber and toluene (0.446) [8].
The cross linking density ( N) can be calculated from
the equation:
N= (1/2 Mc) (2)
— Scanning electron microscopy (SEM) — Phase mor –
phology was studied using a JEOL JXA-840A electron
probe micro analyzer supplied by JEOL, Japan. The frac –
ture surfaces were gold coated to avoid electrostatic char –
ging during examination.
— Electric properties — The dielectric properties of
the specimens were measured using a SchlumbergerSolartron (1260) in the frequency ( f) range from f=1 0
-1Hz
tof= 5·106Hz, at room temperature 30 °C.
— Thermogravimetric analysis (TGA) of the composi –
tes was carried out using Perkin Elmer analyzer equip –
ment, USA. Sample weights between 13 and 32 mg were
scanned from 50 to 1000 °C using a nitrogen air flow of50 cm
3/min and a heating rate of 10 °C/min.
RESULTS AND DISCUSSION
Morphology of SBR/powder composites
Conventional SEM is used to study the surface mor-
phologies of the prepared composites. The SEM micro-graphs are shown in Fig. 1. It was observed that the SEMmicrophotograph of SBR without fillers revealed a gooddistribution of rubber ingredients such as zinc oxide and
curative sulfur in the SBR matrix, as shown in (Fig. 1a).The SEM micrographs for SBR/Al composites loadedwith 10 and 60 phr are shown in Fig. 1b and 1c. A uniformdistribution of Al powder was observed in the conti –
nuous SBR phase. It is worth noting the agglomeration of
the Al particles as a function of increased concentration.The SEM micrographs for the composite of SBR/ceriumsulfate [SBR/Ce(SO
4)2] composites loaded with 10 and
60 phr of cerium sulfate powder are shown in (Fig. 1d, e);one can clearly see the intense agglomeration of ceriumsulfate powder. Also, the boundaries between ceriumsulfate particles and SBR matrix are clearly seen. Thismay explain the low mechanical properties of such com –
posites described below.
Rheological properties
The determined rheological properties of SBR/Al and
SBR/Ce(SO
4)2composites are given in Table 2. It can be
deduced that the incorporation of such fillers in the SBRmatrix increased the M
Hand MLvalues, as well as /c68Mva-
lues up to 50 phr for Al and 40 phr for Ce(SO4)2, before
decreasing. The decrease of torque value at high loadingof the investigated fillers may be attributed to a reactionbetween the aluminum (Al) or cerium sulfate with the cu-rative, which translates into a small increase in the cross-link density and decrease in torque. On the other hand,the increment of torque values can be attributed to a phy-sical interaction between SBR and filler phase surfacesand the hydrodynamic effect [9—11]. The maximum tor-102 POLIMERY 2015, 60,n r2
Table 2 . Formulation, rheometric characteristics and filler dispersion parameters of SBR/filler composites at 152 °C
Filler
content, phrML, dNm MH,dNm tC90, min tS2, min CRI, min-1/c68M,dNm /c97f /c104r Mr L
Aluminum powder
0 7.0 41.0 17.5 7.75 10.25 34.0 –-
10 7.5 43.0 17.0 7.75 10.81 35.5 0.441 1.07 1.05 0.0220 8.0 45.0 13.0 7.00 16.67 37.0 0.441 1.14 1.09 0.0530 8.5 46.0 12.0 7.13 20.51 37.5 0.343 1.21 1.12 0.0940 8.8 47.3 11.8 6.19 17.98 38.5 0.331 1.25 1.15 0.1050 9.0 48.5 11.5 5.00 15.38 39.5 0.324 1.28 1.18 0.1060 8.8 43.0 13.5 6.88 15.11 34.3 0.012 1.25 1.05 0.20
Cerium sulfate powder
10 4.0 47.8 8.4 3.00 18.60 43.8 15.735 1.14 2.33 – 1.1920 5.0 40.5 11.3 3.00 12.12 35.5 5.441 1.43 1.98 – 0.5530 5.5 40.5 12.8 2.38 9.64 35.0 3.529 1.57 1.98 – 0.4140 5.3 31.0 15.3 3.63 8.60 25.8 0.367 1.50 1.22 0.2850 5.0 23.5 17.0 4.00 7.70 18.5 0.176 1.43 1.15 0.2860 4.5 22.5 18.0 4.50 7.41 18.0 0.024 1.36 1.10 0.26
Base recipe (in phr): SBR — 100; stearic acid — 2; zinc oxide — 5; CBS ( N-cyclohexyl-2-benzothiazole sulfenamide) — 0.8; TMQ — polymeri –
zed 2,2,4-trimethyl-1,2-dihydroquinoline — 1; sulfur — 2; /c68M— the difference between maximum torque MHand minimum torque ML,
tS2— scorch time; tC90— optimum cure time; CRI — cure rate index, /c97f— specific constants for the fillers; /c104r— the relative viscosity; Mr—
the relative modulus, L=/c104r—Mr,where phr is part per hundred parts of rubber.

que MHcan be regarded as a measure of the composites’
modulus [4], while the minimum torque ( ML) is an indi –
rect measure of the viscosity of the rubber composites [10,
12]. Therefore, the incorporation of Al or Ce(SO4)2pow –
der into the SBR matrix increased the MLvalues and, con –
sequently, the viscosity of the composites as shown in
Table 2. This can be due to the fact that the filler tends toimpose extra resistance to the flow of the mixes [13]. The/c68Mvalues show a similar trend since /c68Mcan be conside –
red a measure of the dynamic shear modulus, which wasascribed to the cross linking of the rubber phase [14]. The
increment of /c68Mvalues may be due to the additional
physical cross links created in the rubber matrix. This canbe proved by equilibrium swelling ( Q)d a t aa n dt h e
calculated cross linking density ( N), which is given in
Table 3.
On the other hand, the presence of aluminum partic –
les in the SBR matrix has accelerated the curing process as
shown in (Table 2), i.e. decreased the scorch time ( ts
2) and
optimum cure time ( tC90), up to 50 phr of Al and thenPOLIMERY 2015, 60,n r2 103
20 m/c109a)
20 m/c109b)
20 m/c109c)
20 m/c109d)
20 m/c109e)
Fig. 1. SEM images of: a) SBR without filler, b) SBR/10 phr Al, c) SBR/60 phr Al, d) SBR/10 phr Ce(SO4)2, e) SBR/60 phr Ce(SO4)2compo –
sites at magnification 2000×

increased. This may be due to the chemical amphoteric
nature of Al. The presence of cerium sulfate particles inthe composite increased the optimum cure time ( t
C90) and
consequently decreased the cure rate. This increase maybe due to the acidic nature of cerium sulfate [3] (as shownin Table 2).
Filler dispersion
The filler dispersion and formation of filler agglome –
rates in polymer matrices have been studied by Lee [15].
Lee assumed that the relative viscosity ( /c104
r) and the rela –
tive modulus ( Mr) could be determined from rheometric
data through the expressions:
/c104rLf
LrHf
HM
MMM
M/c61/c6100(3)
where: MH,ML— denotes the torque maximum and mi –
nimum, respectively, and the superscripts, fand 0 — rela –
ted to the loaded and the unloaded polymer, respectively.
Also, Lee introduced a parameter L, defined as:
L=/c104r–Mr (4)
The torque variation for the loaded and unloaded
composites is directly proportional to filler loading, astraight line is obtained, its’ slope was defined by Wolf as/c97
f[16, 17].
MM
MMm
mHf
Lf
HLff
p/c45
/c45/c233
/c235/c234/c249
/c251/c250/c45/c61/c230
/c232/c231/c231/c246
/c248/c247/c247 001/c97 (5)
where: mp— the mass of polymer in the composites, mf—
the mass of filler in the same composites and /c97f—i sa
specific constant for the filler, which is independent of thecure system and closely related to the morphology of the
filler. Also, /c97fcan be calculated from the changes in the
torque, which occur during vulcanization of the twocompounds, the loaded and unloaded one (Fig. 2).
Table 2 shows the computed /c104
r,Mrand Lvalues of
SBR/Al and SBR/Ce(SO4)2composites. One can see that in
the case of filled SBR composites with aluminum (Al), thefiller particles are well dispersed in the matrix except athigher loadings. Both /c104
rand Mrvalues increased with
aluminum loading, indicating an increase in the relativeviscosity and relative modulus of the elastomer except at60 phr of Al loading, where the values of /c104
rorMrdecrea –
sed [14].
In SBR/cerium sulfate composites, there is a large dif –
ference between /c104rand Mrvalues, since for 10, 20 and104 POLIMERY 2015, 60,n r2
0.8 0.6 0.4 0.2 0.0
m/ mf paluminiumcerium sulphate1.5
1.0
0.5
0.0
-0.5
-1.0Maximum change in torque
Fig. 2. Maximum changes in cure meter torque during vulcaniza –
tion as a function of mass of investigated filler/mass of polymer
(mf/mp)Table 3 . Physico-mechanical properties of the SBR/filler composites
Al content, phr
Property0 1 02 03 04 05 06 0
Se100, MPa 0.81±0.01 1.43±0.01 1.49±0.05 1.55±0.02 1.68±0.01 1.58±0.01 1.49±0.04
TSb, MPa 1.98±0.17 3.64±0.04 3.95±0.07 4.04±0.09 4.55±0.06 5.78±0.06 4.39±0.09
Eb, % 385±3 372±2 360±2 345±2 339±2 334±2 324±2
H, Sh A 40±1.03 44±1.05 49±1.01 51±1.03 53±1.08 56±1.06 57±1.06
Qmin toluene, % 359±0.4 321±0.1 316±1.0 312±1.1 312±1.0 309±1.0 299±0.9
N·1 05, mole/cm3 6.8±0.8 8.5±0.3 8.7±0.1 8.9±0.4 8.9±0.3 9.1±0.6 9.7±0.2
MC, g/mol 7396±1 5916±2 5735±2 5591±1 5591±2 5484±2 5138±3
Ce(SO4)2content,
phr
Property0 1 02 03 04 05 06 0
Se100, MPa 0.81±0.01 0.92±0.02 1.18±0.01 1.29±0.01 1.34±0.01 1.15±0.05 1.15±0.03
TSb, MPa 1.98±0.17 1.56±0.06 2.72±0.01 2.92±0.009 4.02±0.12 3.96±0.05 3.16±0.04
Eb, % 385±3 114± 1 224±1 226±2 227±2 225±2 220±2
H, Sh A 40±1.0 39±0.2 44±0.1 46±0.3 50±0.4 51±0.2 51±0.1
Qmin toluene, % 359±0.4 234±0.6 227±0.8 225±0.7 202±0.3 259±0.5 259±1.1
N·1 05, mol/cm3 6.8±0.8 15.8±0.5 16.7±0.4 17.0±0.2 20.9±0.4 12.9±0.1 12.9±0.6
MC, g/mol 7396±1 3173±1 2991±1 2940±1 2389±1 3869±1 3869±1

30 phr the values of /c104ris less than that of Mr. This may be
due to agglomeration of the filler in the elastomer andthis is in good agreement with the SEM data. On the otherhand, /c97
fvalues of SBR/filler composites are found to de –
crease with filler loading. This indicates that the investi –
gated filler particles are well dispersed in the SBR matrix
[15].
Mechanical properties
The mechanical properties, namely: stress at 100 %
elongation ( Se100), tensile strength ( TSb), elongation at
break ( Eb) and hardness ( H) of SBR/Al and SBR/Ce(SO4)2
composites versus filler content are given in (Table 3). It is
clear from these data that the mechanical properties im –
prove with increasing of filler content up to 50 phr Al and
40 phr Ce(SO4)2.This can be related to the interfacial ad –
hesion and physical bonding between the filler and SBR
matrix. These interactions facilitate the stress-transferfrom the SBR matrix to the fillers under investigation. Afurther increase in filler content deteriorates the mechani –
cal properties of the composites. The decrease in elonga –
tion and swelling is the typical characteristic for inorga –
nic filled composites [18].
On the other hand, hardness, which depends on the
distribution of the rigid filler in the SBR matrix, is increa-sed with higher contents of Al or Ce(SO
4)2. It is well
known that incorporation of the filler particles in the softmatrix reduces the elasticity of the polymer chains resul-ting in more rigid composites [19—21].
The reinforcing index ( RI) is an empirical parameter
representing a reinforcing effect and can be calculatedfrom the mechanical properties according to the follo-wing equation:
RI={N/[ N0· (filler content % /100)]} (6)where: Nand N0are nominal values obtained from the
mechanical measurements of the sample, with or withoutfiller, respectively. The calculated RIvalues are given in
Table 4. The data indicated that the efficiency of powde –
red aluminum metal (Al) as filler is more than that of
Ce(SO
4)2since the RIof Al is greater than the RIof
Ce(SO4)2. The obtained data was confirmed by published
results [22, 23].
Thermal analysis
Thermogravimetric analysis (TGA) is one of the most
accepted methods for studying the thermal properties ofpolymer composites. TGA curves provide informationabout the thermal stability and extent of degradation ofthe polymeric material. The data obtained from TGA forSBR composites in the absence and presence of 40 phr ofaluminum and cerium sulfate powder, respectively, areshown in Fig. 4 and summarized in Table 5. The thermalstability of SBR increases with loading of the investigatedfillers and also the decomposition temperature of theSBR/aluminum and SBR/cerium sulfate powder becomehigher than SBR. This can be attributed to a decrease inthe diffusion of volatile gases in the polymer/filler matrixdue to the homogeneous distribution of the filler [24, 25].The thermal stability of the filled samples is higher, attemperatures T/c179500 °C, i.e. after the decomposition of
SBR.POLIMERY 2015, 60,n r2 105
120
100
80
60
40
20
0
0 200 400 600 800 1000
Temperature, °CMass, %
SBR/Ce(SO )42SBR/Al
SBR
Fig. 3. Thermogravimetric analysis of SBR/investigated filler
compositesT a b l e 4. The value of reinforcing index ( RI) for aluminum and
cerium sulfate powders
Filler content, phr RI,%
Aluminum powder
0–
10 22.1820 13.0430 9.5640 8.8550 9.3960 6.31
Cerium sulfate powder
10 9.5020 8.9830 6.9140 7.6450 6.4360 4.54T a b l e 5. Thermogravimetric data for SBR and SBR/filler com-
posites
SampleInitial thermal
decomposition
temperature
(Ti), °CFinal thermal
decomposition
temperature
(TF),°CResidual
mass, %
SBR 144.50 454.81 5.00
SBR/Al 190.38 598.45 27.37SBR/CeSO
4 177.41 657.00 13.72

Electrical properties
To perform a dielectric measurement, the sample is
placed between two metallic electrodes, which form a ca –
pacitor.
The permittivity /c101’, measures the capacitance whereas
dielectric loss, /c101”, measures conductance. These terms
were calculated with the contribution of the followingequation [26]:
/c162/c61/c101/c101cd
A0(7)
/c101”=/c101’ tan /c113 (8)
where: c— the measured capacitance, /c101o— the permitti –
vity of space, /c113— the phase angle, A— the surface area
and d— its thickness.
The effect of concentration of conducting Al, and fre –
quency on the permittivity of SBR/Al composites, are
shown in Fig. 4. It is noted that permittivity /c101’ increased
with higher Al content while it decreased at higher app –
lied frequencies. The increase in /c101’ may be due to inter –
facial polarization arising in electrically heterogeneous
materials like composites due to a difference in the con –
ductivity of input raw materials, namely rubber and Al
powder [26]. As the Al content increases, the interfacialpolarization and formation of a network of conducting Alparticles increases in the composites and this could be thereason for an increase of /c101’.
The dielectric loss ( /c101”), as a function of frequency, of
SBR/Al composites with different Al content is shownin Fig 4b. It is noted that the specimens filled with Alalways showed greater losses than the pure SBR in thewhole frequency range. Figure 4b also shows that thedielectric loss with frequency follows two steps. The
first and second regions lie in the frequency range10—10
4and 104—107Hz, respectively. A gradual de –
crease in dielectric loss was noted in the first region,
followed by a marginal increase in dielectric loss va –
lues in the second region.
Concerning dielectric losses ( /c101”) as a function of fre –
quency, data in the low frequency range depict a relaxa –
tion process that must be attributed to an interfacial pola –
rization known as Maxwell-Wagner-Sillar (MWS). This
phenomenon appears in heterogeneous media, whichconsists of phases with different dielectric permittivittiesand conductivities and is due to the accumulation ofcharges at interfaces [21]. The molecules present in theunfilled composites are nonpolar in nature and the rela –
xation is not due to dipole orientation. Hence, a very low
dissipation factor and, consequently, very low dielectriclosses are noted in unfilled composites [19]. In the filledspecimens, there exists one or more interfaces betweenthe filler and the elastomer. Increases in filler contentshifts the relaxation peaks to higher frequencies with a si –
multaneous increase in their abscissa. Heterogeneity is
greater in the filled specimens and the MWS effect arisingfrom the filler-polymer interface is superimposed on thatof the elastomer [21]. As frequency is increased, the di-electric properties increase considerably.
When the frequency of the applied field is increased,
the bound charges (dipoles) present in the composite can-not reorient themselves quickly enough to respond to theapplied electric field and, as a result, the dielectric con-stant decreases. Furthermore, at higher frequencies, thepolarizability (electric, ionic and orientation polariza-tion) and electric displacement of dielectric materials106 POLIMERY 2015, 60,n r2
/c101”
f,Hz f,Hz20406080100
/c101’
10-210010210410610810 wt %
20 wt %30 wt %
40 wt %
50 wt %
60 wt %0w t%a)
10-210010210410610810-2100102
101
10-110 wt %
20 wt %30 wt %
40 wt %
50 wt %
60 wt %0w t%b)
Fig. 4. a) Permittivity ( /c101’), b) dielectric loss ( /c101”) versus applied frequency with various Al contents

were not maintained with the vibration electromagnetic
field [27]. For Ce(SO4)2filler, (Fig. 5) it appears that both
the magnitude of permittivity and dielectric loss decreaseat higher frequencies and a slight deviation from linearbehavior. From these figures it was noted the increasing(/c101’) and ( /c101”) values with the increasing of Ce(SO
4)2con-
tent at different frequencies. On the other hand, the di-electric property results showed an improvement, as wellas mechanical properties, especially with the addition ofAl filler. Such fillers enhance the viscoelastic response todeformation and increase the electrical conductivity anddielectric constant [28].
CONCLUSIONS
From the above study, it could be concluded that:— The incorporation of aluminum metallic powder
slightly reduces the scorch and cure times of SBR compo –
sites, while cure times are increased for SBR/cerium sul –
fate composites.
— The torque difference /c68Mincreases with higher fil –
ler content for aluminum filler due to the increment in
crosslink density.
— The tensile strength, S
e100and hardness increased at
higher filler content in the SBR matrix, while the elonga –
tion at break decreased.
— The swelling percentage decreases with higher alu –
minum powder and cerium sulfate powder content in
SBR composite.
— The experimental results indicate that the dielectric
constant and dielectric loss are increased in SBR/Al andSBR/Ce(SO
4)2composites.— The dielectric constant decreases at higher frequen-
cies, which can be attributed to the orientation polarization.
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Received 20 I 2014.108 POLIMERY 2015, 60,n r2
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