Advances in Ma terials Physics and Che mist ry, 2012, 2, 260-262
doi:10.4236/ampc.2012.24B066 Published Online December 2012 (htt p://www.SciRP.org/journal/ampc)
Tensile Properties of Natural Rubber Nanocomposites
Affected by Crosslink De nsi t y
Mohammad Reza Saeb1, Ra ha Sarami2, Bijan Iranpak3, Ramtin Gaffari4
1Department of Resin and Additives, Institute for Color Science and Technology, Tehran, Iran
2Techni cal Faculty, Polymer Group, Tarbiat Modares University, Tehran, Iran
3Islamic Azad University, Tehran North Branch, Tehran, Iran
4Islamic Azad University, Central Tehran Branch, Tehran, Iran
Received 2012
ABSTRACT
The aim of this study was to investigate the tensile properties of the natural rubber nanocomposites containing surface modified cal-
cium carbonate nanofiller. All nanocomposites were prepared at various nanofiller contents, using a laboratory scale two-roll mill.
The results revealed that the ultimate tensile properties altered by changing nanofiller content throughout the elastomeric matrix,
which co uld be ascr ibed to th e particular i nteractio ns at the filler/ matrix inter face. At 10 parts by weigh t of nano filler, base d on 100
parts by weight of natural rubber, the highest value of tensile properties was achieved as a result of crosslink density being at its
highest level, as measured by swelling method.
Keywords: Natural Rubber Nanocomposites; Calcium Carbonate; Tensile Proper ties
1. Introduction
For a long time, natural rubber (NR) has been considered as an
important elastomer from both industrial and academic points
of views. Also, there have been many works on dealing with
incorporation of inorganic fillers, e.g., silica, calciu m carbonate
(CC), and clay to diminish the production costs as well as en-
hancement of ultimate properties of elastomeric matrices. The
main issue, however, was to load high levels of this matter
within rubber compounds owing to agglomeration. Among
micron and nano size CC, the latter was found to be the better
one, regarding mechanical and curing characteristics. Employ-
ing surface modified CC filler in various elastomeric com-
pounds, in particular in nano scale, has been already reported
by several authors [1-3]. Surface modification inhibits the un-
desired filler-filler interaction; at the same time, enhances the
polymer-filler adhesion. The thermo-mechanical properties of
butadiene rubber nanocomposites reinforced with modified CC
has been shown that both thermal stability and mechanical
properties of the prepared nanocomposites highly depend on the
size and amount of CC nanofillers. [3] The tensile properties in
these systems were considerably improved by increasing the
amount of filler up to an optimum content which is ascribed to
acceptable intermolecular interactions between the rubber
chain s and the filler su rfaces. Similar t rend s have been reported
by other researchers regarding to incorporation of CC with
thermoplastics [4].
In the current work, NR nanocomposites containing sur-
face-coated calcium carbonate (MCC) were prepared by a la-
boratory two-roll mill and then their tensile and curing charac-
teristics were studied. The MCC nanofiller has already been
surface modified with stearic acid, according to the supplier.
Tensile strength, elongation at break, and crosslink density data
demonstrated considerable changes while employing various
amounts of MCC nanofillers.
2. Experimental
2.1. Materials
NR SMR20 with Mooney viscosity of 65, under brand name of
TAYTEX, was purchased from the AH YAU rubber factory
and used, as received. The nano-sized precipitated calcium
carbonate (NPCCA-201); namely MCC, was purchased from
Shandong Haize Nanomaterials Co., Ltd. This matter is a sort
of coated calcium carbonate with stearic acid, according to
supp lier. The mean par ticle size a nd specific surface ar ea of th e
MCC were roughly 50 nm and 24.51 m2/g, according to the
supplier.
2.2. Sample Preparation and Characterization
Based on 100 part of NR rubber, three compounds containing
various amounts of MCC (5, 10, 15 phr) were prepared. Table
1 represents the formulation of NR/MCC nanocomposites. In
o0rder to better control of curing process, two accelerators are
simultaneously used while compounding, the tetramethylthi-
uram disulfide (TMTD) with melting point of 148°C and den-
sity of 1.43 g/cm3 and dibenzothiazyl disulfide (MBTS) with
melting point of 166°C and density of 1 g/cm3. The former is
purchased from Akrochem Corporation, USA and the latter
from Meyors, China Chemical Ltd.
The mixing of rubber, zinc oxide, TMTD antioxidant, and
stearic acid was carried out using a laboratory open two-roll
mill at 60°C for 5 min. The gear friction of the mill was 1:1.2.
Furthermore, the filler was added to the compound and milled
at 60°C for 10 min. The prepared sheets, of about 2 mm in

M. R. SAE B ET AL.
261
thickness, were vulcanized at 160°C and 70 bars by using a
hydraulic press. Mechanical data were gathered using a 20 kN
Zwick apparatus, according to ASTM D412, on the way in
which, elongation at break and tensile strength of the produced
samples were si multan eo usl y measur ed. All vulcani zed samples
were also swollen by toluene to calculate their crosslink densi-
ties.
3. Results and Discussions
The particle size and particle size distribution of MCC through
the NR matrix play important role on ultimate properties. The
higher the homogeneity through dispersion of MCC nanofiller
into the rubber matrix the better is the interfacial bonding of
filler-polymer. Normally, the nanocomposite behavior can be
affected by the nature of either matrix or filler [5]. Figure 1
demonstrates the tensile strength and elongation at break data
versus MCC content, simultaneously. All tests were repeated
five times for every sample and then averaged .
According to this figure, by increasing MCC content from 5
to 10 phr, a drastic increase in both tensile and elongation at
break of NR nanocomposites were observed. Further increase
in the M CC conten t caused the mechani cal p roper ties to declin e.
In order to investigate the consequence of polymer-filler inte-
raction, crosslink density can be considered as an important
factor which tunes ultimate mechanical properties. Polymer
chains with restricted mobility are the result of improving
polymer-solid interface leading to excess physical crosslinks.
Diminishing the average number of intermolecular bonds per
unit volume, on account of the structural changes in polymer-
solid adhesion and also lowering the packing density of the
Tabl e 1. Compounding formulation.
Ingredient Loading (phr1)
NR rubber 100
MCC Var iable (5-15)
Parrafin wax 2
Zinc oxide 5
Stear ic Acid 1
Sulfur 2
MBTS+TMTD 2
Figure 1. Tensile strength and elongation at break data of NR na-
nocomposites at different conte nts of MCC.
polymer chai ns are other p ro bable circumstances.
Ignoring the effect o f fil ler-matrix i nteracti ons, all vulcanized
samples were swollen by toluene to calculate their crosslink
densities through Flory- Rehner equat ion [8]:
2
22 12
1/3
12 2
[ (1)]
(/ 2)
Ln
nV
υυ χυ
υυ
−− ++
=−
(1)
where n is the crosslink density, V1 is the molar volume of
utilized solvent (106.2752 cm3/mo l ) , υ2 is the volume fraction
of polymer in the swollen sample, and χ1 is the Flory–Huggins
polymer–solvent interaction parameter or the enthalpy of mix-
ing, which may be found in the literature[9]or determined by
the following equation:
2
11 2
11
()V
RT
δδ
χβ
−
= +
(2)
where β1 is the lattice constant of entropic origin and is often
assumed to be zero, T is the medium temperature, δ1 and δ2 and
are solubility parameters for solvent (18.2 MPa0.5 ) and NR
rubber (16.69MPa0.5), respectively. The alteration of crosslink
density, measured by swelling method, as a function of MCC
content is reported in Table 2. It should be mentioned that for
every sample, the swelling test was repeated 10 times and the
values then averaged.
Based on swelling method and using equations (1) and (2), it
was found that the highest value of crosslink density has been
achieved in case which the nanocomposite was filled with 10
phr of MCC. In can be speculated that as the number of cros-
slinks per unit volume increased, the elastic nature of prepare
samples got dominant to the viscous effect. In another word, a
3D network with small interlinks comes into exist at assigned
filler content. Thus, the tensile properties are definitely affected
by the interaction of MCC nanofiller and NR rubber chains at
their interface which is traced by crosslink density measure-
ments. I t means th at the in terface of MCC filler with NR matri x
is considerably higher, at 10 phr of filler than that of 5 phr.
However, by further increase up to 15 phr, the tensile properties
were slightly diminished, as a result of partial agglomeration
within the NR nanocomposite, which has been reported else-
where [5].
4. Conclusion
NR-based nanocomposites containing various amounts of sur-
face modified nano calcium carbonate (MCC) were prepared
using a laboratory-scale two roll mill. It was found that in-
creasing MCC from 5 to 10 phr within the NR nanocompound
results in higher values of tensile strength and elongation at
break. The consequences of particle size and particle size dis-
tribution were also investigated by measuring crosslink density
through the swelling test. All mechanical properties concerning
Tabl e 2. Crosslink density as a function of filler content.
Filler content (phr) Crosslink density*10-5 (mol/cm3)
5 9.23
10 9.89
15 9.63

M. R. SAE B ET AL.
262
15 phr of MCC were showed to be lower than those obtained
for samples containing 10 phr of MCC. According to the results
of the swelling test, the highest value of crosslink density was
achieved for the sample containing 10 phr of the MCC. There-
fore, it can be concluded that the tensile properties are highly
affected by the 3D network created in the presence of curing
agent and nanofil ler.
REFERENCES
[1] S. Mishra, N.G. shimpi, and U.D.Patil, “Effect on nano caco3 on
thermal properties of styrene butadiene rubber (SBR),” J Polym
Res, 2007, 14, 449.
[2] C.G.Ma, Y.A.Mai, M.Z.Rong, W.H.Ruan, and M.Q.Zhang,
“Phase structure and mechanical properties of ternary polypro-
pylene/elastomer/nano-CaCO3 composites ,” Comp Sci Tech.
2007, 67, 2997.
[3] Y.Zhou, S.Wang, Y.Zhang, X.Jiang, and D.Yi, “Rheological
properties of PDMS filled with caco3: The effect of filler parti cle
size and concen tration ,”J Appl Poly Sci, 2006, 101, 3395.
[4] H.Hanim, R.Zarina, M.Y.Ahmad Fuad, Z.A.Mohd.Ishak, and
Azman Hassan, “The effect of calcium carbonate nanofiller on
the mechanical properties and cristallization behavior of poly-
propylene,”Malay Polym J. 2008 , 3 , 38.
[5] M.R.Saeb, H.Ramezani-Dakhel, H.A.Khonakdar, G.Heinrich,
and U.Wagenknecht, “ A comp arative study on curing characte-
ristics and thermomechanical properties of elastomeric nano-
comp osites : The effect s of eggs hell an d calciu m carbo nate nan o-
fillers,”J Appl Poly Sci, 2012, DOI: 10.1002/app.38022.