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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