ra
l
ias
02, I
Received in revised form
Keywords:
Hydrogenated nitrile rubber
Organically modified nanoclay
e re
te it
osition
ature can be extended with the addition of an inorganic filler.
Polymer nanocomposites have long been a crucial topic for
advanced scientific research and industrial development, presenting
a variety of applications in transportation, construction and food
with weak inter-atomic forces. In fact, MMT-nanocomposite was the
first successful example of a polymereclay nanocomposite [11e22].
Natural Na-montmorillonite is hydrophilic in nature and is not
compatible with most of the organic polymers. However, sodium
cations in the interlayer space of montmorillonite can be exchanged
with organic cations to yield organophilic montmorillonite (e.g.,
Cloisite 30B, used in this study), which is compatible with such
polymers. The formation of polymer-filler nanocomposite affects the
thermal behaviour of the matrix because well dispersed nanofillers
* Corresponding author. Present address: Indian Institute of Technology, Patna
800013, India. Tel.: þ91 3222 283180/91 612 2277380; fax: þ91 3222 220312/91 612
2277384.
Contents lists availab
Polymer Degradat
ev
Polymer Degradation and Stability 95 (2010) 2555e2562
E-mail addresses: anilkb@rtc.iitkgp.ernet.in, director@iitp.ac.in (A.K. Bhowmick).
temperature for an extended period of time is a paramount
requirement for a large number of applications. The introduction of
hydrogenated nitrile rubber has provided elastomers with superior
resistance to degradation even in hot air environment and a good
temperature performance up to the range of about 150 �C, combined
with good oil and fluid resistance, [1,2]. Nevertheless, industry still
sees the need to extend the upper operating range of the polymer.
Increasing the heat resistance of a composition would indeed allow
it to be used at higher operating temperatures, or alternatively, it can
be used at a particular temperature expectedly with a longer useful
lifetime. The useful service life of an elastomer at elevated temper-
lation properties from the viewofmodification efficiency and product
cost [3,9,10]. Besides, economic and environmental factors, its natural
abundance, high mechanical strength and chemical resistance make
clay a suitable filler for polymer composites. Moreover, it is believed
that the superior properties of the elastomereclay nanocomposites
are not only because of the nanoscale dispersion of clay particles
within the elastomer matrix, but are also the result of a strong inter-
action between the elastomer matrix and the clay layers.
The claymostly used ismontmorillonite (MMT)which belongs to
the general family of 2:1 layered silicates. These are composed of
regular staking of two-dimensional plate like layers bound together
Accelerated heat aging
Life time prediction
1. Introduction
The ability of an elastomeric comp
0141-3910/$ e see front matter � 2010 Elsevier Ltd.
doi:10.1016/j.polymdegradstab.2010.07.032
hydrogenated nitrile rubber (HNBR) compounds undergo cross-linking reactions that lead to embrittle-
ment and ultimately failure. Incorporation of clay filler, however, resulted in significant improvement of
the degradation profile of the nanocomposite at elevated temperatures. Loss of ductility during aging of
the nanocomposite was also milder, relative to the unfilled polymer, indicating a restricted degradation by
the clay filled rubber, thus prolonging the durability. From the scanning electron microscopy and atomic
force microscopy studies, it was found that nanofillers protected the elastomer from surface rupture that
took place on oxidation. Life prediction of both virgin elastomer and the nanocomposite indicated a three-
fold increase in the effective service temperature range of the HNBR using 8 parts organically modified
nanoclay.
� 2010 Elsevier Ltd. All rights reserved.
to perform at elevated
packaging [3e8]. Though many organic or inorganic additives can be
incorporated for the reinforcement of a polymer, clay is considered to
be an optimum choice because of its small particle size and interca-
Accepted 28 July 2010
Available online 5 August 2010
spectroscopic analysis of the degraded products revealed that under aerobic hot aging conditions,
18 July 2010
with that of the virgin polymer for the first time. Changes in technical properties such as tensile strength,
modulus and elongation at breakwere studied as a function of time and temperature of aging. The infrared
Effect of organo-modified clay on accele
nitrile rubber nanocomposites and their
Anusuya Choudhury a, Anil K. Bhowmick a,*, Matth
aRubber Technology Centre, Indian Institute of Technology Kharagpur, Kharagpur 7213
b LANXESS Deutschland GmbH, 41538 Dormagen, Germany
a r t i c l e i n f o
Article history:
Received 3 April 2010
a b s t r a c t
Organically modified clay
erated heat aging to estima
journal homepage: www.els
All rights reserved.
ted aging resistance of hydrogenated
ife time prediction
Soddemann b
ndia
inforced hydrogenated nitrile rubber vulcanizate was subjected to accel-
s long-term thermo-oxidative stability and its useful lifetimewas compared
le at ScienceDirect
ion and Stability
ier .com/locate /polydegstab
The surface morphology of the samples was studied by using
JEOL JSM-800 scanning electron microscope (JEOL JSM-800, Tokyo,
Japan) operating at an accelerating voltage of 20 kV. The samples
were sputter coated with gold before analysis.
2.6. Atomic force microscopy (AFM)
MultiMode Scanning ProbeMicroscopemodelwith a Nanoscope
IIIa controller by Digital Instruments Inc. (Veeco Metrology Group),
Santa Barbara, CA, USA, was used for the AFM studies. The AFM
measurements were carried out in air at ambient conditions (25 �C),
using tapping mode probes with constant amplitude (40 mV). The
rotated tapping mode-etched silicone probe (RTESP) [square
pyramid in shape with a spring constant of 20 N/m, nominal radius
ation
result in the modification of their degradation pathways. Previously,
several reports have shown that presence of nanoclay improves the
degradation temperature of plastics, viz. nylon 6, polylactides,
polystyrene, polypropylene [23e27]. Bhowmick et al. have done
extensive work to improve the thermal stability of polymers like
polybutadiene rubber (BR), acrylonitrile butadiene rubber (NBR),
styrene butadiene rubber (SBR), fluoroelastomer (FKM) and nylon
6 by using different types of nanoclay [28e30]. The thermal
degradation pathway of virgin HNBR has also been studied by other
researchers [31,32]. However, any detailed information regarding
the aging mechanism of HNBR-clay nanocomposite at elevated
temperature is still lacking. Nevertheless, there is a shortage of
information about the long-term thermal stability of HNBR-clay
nanocomposite. It has also not yet been suggested how the use of
nanoclay can improve the lifetime of an article.
Alongwith the development of science and technology, there has
been an increased demand for materials with higher temperature
classification and excellent thermal performance. Performance
during outdoor application is a critical issue for the commercial
exploitation of the end products. The useful life time of an article, at
a given service temperature, is often assessed by determining the
time required for a particular property of interest to degrade to
a critical value [1,26]. The properties of rubber are subject to changes
ultimately to the point where the material is no longer capable of
fulfilling its functions. The life of a rubber component is of critical
importance in many applications and hence it becomes necessary to
predict its longevity. The comparative prediction of the full life time
of an article is often based on accelerated aging tests, conducting
characterizations at ordinary time interval of exposure. In this way,
change in bulk properties can be recorded quickly.
The presentwork aimed to develop a better understanding of the
effect of clay on the aging performance of HNBR nanocomposite. The
focus of this research is to study the effect of thermal aging in air, at
different time intervals and temperatures, on the physico-mechan-
ical properties of HNBR and its nanocomposite. The present article
reports, for the first time, a comparison of the predicted life span of
unfilled and nanoclay-filled rubbers. The results are explained in
terms of morphology analysis of the aged sample by scanning elec-
tron microscopy and atomic force microscopy. Such type of analysis
is also reported for the first time. The aging mechanism of the
elastomer and its nanocomposite has further been investigated by
FTIR studies.
2. Experimental
2.1. Materials
Therban C3467, having an acrylonitrile content of 34%, diene
content of 5.5%, Mooney Viscosity, ML(1þ4) at 100 �C [where, M
stands for Mooney number, L indicates the use of large (i.e., stan-
dard) rotor, 100 �C is the test temperature, sample preheating time
before the start of shearing is 1 min and shearing duration is 4 min]
of 68 and specific gravity of 0.95 were obtained from Lanxess,
Germany. The clay used was Cloisite 30B (produced by ion exchange
of a methyl tallow bis-2-hydroxy quaternary ion) purchased from
Southern Clay Products, Gonzales, Texas, USA. Designation of the
nanofiller is reported in Table 1. Solvents used in this study were
supplied by Merck Limited, Mumbai, India.
2.2. Preparation of rubbereclay nanocomposites
2.2.1. Vulcanized sample
The rubber was first dissolved in chloroform (10% rubber solu-
tionw/v). 8 phr of clay was dispersed inmethyl ethyl ketone (MEK),
A. Choudhury et al. / Polymer Degrad2556
by sonicating in an ultrasonicator for 30min. (1 g in 50 ml) at 25 �C.
1 phr of dicumyl peroxide dispersed in MEK was then added into
the rubber solution. The amount of the filler and the solvent was
selected from the knowledge of our earlier studies [33,34]. The filler
dispersion was then poured into the prepared rubber solution and
stirred for 3 h in a magnetic stirrer at room temperature followed
by 30 min vigorous stirring with a mechanical stirrer to make
a homogenous mixture. The solution was finally cast on a Petri
dish to get a thin film. The solvent was allowed to evaporate at
room temperature and dried in a vacuum oven at 50 �C, till there
was no weight variation. The samples were then cured at 150 �C for
1 h under 5 MPa pressure in a hydraulic press to prepare 1 mm
thick sheets. The designation of rubber, nanofiller and their nano-
composites vulcanizate is represented in Table 1.
2.3. X-ray diffraction (XRD)
For the characterization of the nanocomposites, XRD studies
were performed on a PHILIPS X-PERT PRO diffractometer in the
range of 2e9�(2q) using Cu target (l ¼ 0.154 nm). The d-spacing of
the clay particles was calculated using the Bragg’s law.
2.4. Transmission electron microscopy (TEM)
The samples for TEM analysis were prepared by ultra-
cryomicrotomy with a Leica Ultracut UCT (Leica Microsystems
GmbH, Vienna, Austria). Freshly sharpened glass knives with cutting
edges of 45� were used to obtain cryosections of about 100 nm
thickness at e90 �C. The cryosections were collected individually
in sucrose solution and directly supported on a copper grid of
300 mesh size. Microscopy was performed with JEOL 2100, Japan.
Transmission electron microscope was operated at an accelerating
voltage of 200 kV.
2.5. Scanning electron microscopy (SEM)
Table 1
Designation and characteristics of the rubber and clay.
Sample name Designation Characteristics
Therban C3467 S1 e
Cloisite 30B 30B Substituted dimethyl
dihydrogenated tallow
(w65% C18; 30% C16; w5% C14,
like stearic acid) quaternary
ammonium. Cation-Exchange
Capacity (CEC) ¼ 0.90 mequiv/g.
Moisture content ¼ 2%
Di cumyl peroxide (DCP) x e
Therban C3467 þ 1 phr DCP S1-1x e
Therban C3467 þ 8 phr
Closite 30B þ 1 phr DCP
S1-30B-8-1x e
and Stability 95 (2010) 2555e2562
of curvature of 10 nm] with resonance frequency of 270 kHz was
Fig. 1. a) Tensile strength and b) elongation at break vs. time of ageing at 70 �C.
A. Choudhury et al. / Polymer Degradation and Stability 95 (2010) 2555e2562 2557
used. Height and phase images were recorded simultaneously at the
resonance frequency of the cantilever with a scan rate of 1Hz
and a resolution of 512 samples per line. This allowed the resolution
of individual primary particle measurements. The images were
analyzed using a nanoscope image processing software (5.30r1).
2.7. Roughness analysis
The root-mean-square roughness, RRMS is defined as the stan-
dard deviation feature height, Z, within a given area is represented
in Equation (1) as follows-
RRMS ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPðZi � ZAVÞ2
N
s
(1)
The mean surface level is defined as the line about which
roughness is measured. ZAV is the average Z values and Zi is the
current Z value and N is the number of points within the given area.
Roughness has been calculated from the images of same scan size.
2.8. Mechanical properties
Tensile specimens were punched out from the cast sheets using
ASTM Die-C. The tests were carried out as per the ASTM D 412-98
method in a Universal Testing Machine (Zwick/Roell Z010) at
a crosshead speed of 500 mm/min at 25 �C. The average of three
tests is reported here. The error was �2% in the measurements of
tensile strength and modulus and �5% for elongation at break.
2.9. Fourier transform infrared (FTIR) spectroscopic analysis
Attenuated total reflection (ATR)-FTIR spectra of the sampleswere
taken at room temperature using an infrared spectrophotometer
Fig. 2. a) Tensile strength and b) elongation
Nicolet Nexus, USA in the range 650 to 4000 cm�1 at a resolution of
4 cm�1. The average of three scans for each samplewas taken for peak
identification.
2.10. Aging studies
Air aging was performed with three sets of tensile specimens in
an air circulating oven [Blue M Electric Co., IL, USA] at three pre-
determined temperatures for various time periods (24 h, 48 h, 72 h,
96 h, 120 h and 168 h at 70 �C, 24 h, 48 h, 72 h, 96 h, 120 and 168 h
at 100 �C and 24 h, 48 h, 72 h and 96 h at 135 �C). Lifetime was
predicted from these data following ISO Standard 11346.
3. Results and discussion
The aging properties of neat HNBR and HNBR-filler nano-
composites were studied at various temperatures over different
periods. Plots of the change in properties with aging time at
a particular temperature (70 �C, 100 �C or 135 �C) for both the
samples are shown in Figs. 1 and 2. The properties of neat HNBR
on aging at longer times are observed to be affected with drastic
reduction in tensile strength and elongation at break. It can be
visualized from Fig.1a, b that at 70 �C, the neat HNBR showsmarginal
change in properties at the early stage of degradation. A marginal
increase in tensile strength is observed up to 24 h of aging (Fig. 1a),
beyondwhich there is a continuous decrease inproperties of the neat
elastomer. At 168 h of aging, the tensile strength reduces to as much
as 40% of its original value. These results can be explained as follows:
Partial cross-linking of the elastomer backbone at the initial stage of
aging takes place [31,35]. On further aging for prolonged time period
at 70 �C, the main chain starts degrading, which results in drastic
reduction of strength and elongation properties, as reflected in
Fig. 1a, b. These are also apparent from the FTIR studies (discussed
at break vs. time of ageing at 135 �C.
Fig. 3. SEM surface picture (in 5 mm) of a) S1-1x and b) S1-30B-8-1x before ageing and c) S1-1x and d) S1-30B-8-1x after ageing.
Fig. 4. AFM phase images of a) S1-1x, b) S1-30B-8-1x (before ageing) c) S1-1x and d) S1-30B-8-1x (after ageing).
A. Choudhury et al. / Polymer Degradation and Stability 95 (2010) 2555e25622558
later). No significant change in theseproperties of thenanocomposite
is observed under the same above conditions. A similar trend is
observed at an aging temperature of 100 �C (Figure not shown here).
Up to 24 h of aging, tensile strength of the neat elastomer increases
to 30%, while no such change is observed in the properties of the
nanocomposite. The overall reduction in properties for the neat
elastomer at 100 �C for 168 h is 80%, whereas the nanocomposite
retains almost all its properties even under similar aging conditions.
On aging at 135 �C, a remarkable drop in strength and elongation
properties for theneat elastomer takes place even at the early stage of
degradation is faster in the sample without the stabilizer. Or in the
other words, the clays do not destabilize the polymer by deactivating
the stabilizer. A similar observation is made when the samples have
been heated up to 800 �C in the furnace of the Perkin Elmer TGA
Instrument, under air atmosphere at a heating rate of 20 �C/min.
(not shown here).
This marked change in properties upon aging for the neat
elastomer is due to cross-linking and degradation of the rubber
as explained in detail later. Furthermore, changes in the surface
morphology of the system, on aging, reflect such remarkable drop in
properties of the neat elastomer. The surfaces of both HNBR and its
nanocomposite, subjected to different degrees of aging, have been
studied by scanning electron microscopy (SEM) and atomic force
microscopy (AFM). The surface morphology of the unaged sample of
S1 and its nanocomposite is similar before aging as shown in Fig. 3a,
b (SEM photograph). However, upon aging, a dramatic change on
surface morphology is observed as depicted in Fig. 3c, d. The surface
of the unfilled elastomer is cracked due to oxidative attack (Fig. 3c),
while the nanoclay protects the rubber surface (Fig. 3d). As the
nanoclays are not observed in the SEM photograph, further inter-
pretation is done using AFM.
Table 2
Quantitative parameters determined from roughness and power spectral density
analysis of unfilled elastomer and nanocomposite before and after ageing.
Sample Ra(nm) Rq(nm) Total power
(nm2)
Equivalent
RMS (nm)
S1-1x Before ageing 3.4 4.2 18.9 4.3
After ageing 16.1 18.8 368.0 19.2
S1-30B-8-1x Before ageing 8.7 12.9 123.0 11.0
After ageing 9.9 16.2 280.0 16.6
A. Choudhury et al. / Polymer Degradation and Stability 95 (2010) 2555e2562 2559
aging (24 h). In fact, S1-1x (unfilled elastomer vulcanizate) loses
its rubber-like properties within 24 h of aging at this temperature,
which can be clearly visualized from Fig. 2a, b. On the other hand, no
significant change in properties is observed, at this initial stage of
aging, for the nanocomposite. As reflected in Fig. 2a, b, the nano-
composite retains its properties up to 72 h of aging. However, beyond
this period, deterioration in properties of the nanocomposite takes
place. Thus, Figs. 1 and 2 explain the better retention of tensile
strength and elongation at break of the nanocomposite over the neat
elastomer upon aging in air. The stabilizer used in this study is BKF,
which is a Bi-Cresol derivative, and the amount present in Therban
C3467 is 0.1 phr. In order to find out the effect of clay without the
stabilizer, the stabilizer is removed from the rubber by ethanol
extraction and the rubber is then mixed with 8 phr of clay. Low
temperature (ageing at 135 �C for 48 h) degradation study is carried
out in absence of the stabilizer. The result showed that the unfilled
elastomer loses 90% of its properties upon ageing at 135 �C, while the
nanoclay-filled elastomer loses 15%of its properties. Thesefigures are
85% and 10% respectively for the samples with stabilizer. From these
results, it can be inferred that, the presence of the stabilizer has the
same effect on the degradation of HNBR. Only the speed of the
Fig. 5. Power spectral density of a) S1-1x, b) S1-30B-8-1x (before ageing
The phase images of the neat elastomer and the filled elastomer
vulcanizate are shown in Fig. 4a, b. In the case of clay filled sample
some distinct bright features are observed, which are absent in the
unfilled sample. This suggests that the filler appears as white
bright features (average
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