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Effect of Ozone, Thermo, and Thermo-oxidative Aging on the Physical Property of Styrene Butadiene Rubber-Organoclay Nanocomposites Sugata Chakraborty, Saptarshi Kar, Saikat Dasgupta, Rabindra Mukhopadhyay, Narendra P.S. Chauhan, Suresh C. Ameta and Samar Bandyopadhyay Journal of Elastomers and Plastics 2010 42: 443 originally published online 8 September 2010 DOI: 10.1177/0095244310374226 The online version of this article can be found at: http://jep.sagepub.com/content/42/5/443

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Effect of Ozone, Thermo, and Thermo-oxidative Aging on the Physical Property of Styrene Butadiene Rubber–Organoclay Nanocomposites SUGATA CHAKRABORTY,* SAPTARSHI KAR, SAIKAT DASGUPTA AND RABINDRA MUKHOPADHYAY Hari Shankar Singhania Elastomer and Tyre Research Institute (HASETRI), Jaykaygram, P.O. Tyre Factory, Rajsamand – 313 342 Rajasthan, India

NARENDRA P. S. CHAUHAN AND SURESH C. AMETA Department of Polymer Science, M.L.S. University, Udaipur, Rajasthan, India

SAMAR BANDYOPADHYAY R&D Centre, J.K. Tyre, Jaykaygram, P.O. Tyre Factory, Rajsamand Rajasthan, India

ABSTRACT: The present study describes the effect of thermo, thermo-oxidative, and ozone exposure on the retention of physical property of organoclay nanocomposites. Accelerated thermal aging is carried out at 1308C for 30 h. Accelerated thermooxidative aging is carried out at 1058C for 7 days. Samples are exposed to 50 pphm ozone atmosphere for 24 and 48 h. The result indicates that the retention of physical property is better in nanocomposites compared to carbon black-filled compounds under thermal and thermo-oxidative aging. Besides, ozone resistance is also relatively superior for nanocomposites. The superior barrier property of the nanocomposites is attributed to the better retention of the physical property after aging.

*Author to whom correspondence should be addressed. E-mail: [email protected]

JOURNAL OF ELASTOMERS AND PLASTICS Vol. 42–September 2010 0095-2443/10/05 0443–10 $10.00/0 DOI: 10.1177/0095244310374226 ß The Author(s), 2010. Reprints and permissions: http://www.sagepub.co.uk/journalsPermissions.nav


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KEY WORDS: nanocomposites, ozone, accelerated aging, rubber.

INTRODUCTION

fillers into elastomer matrices leads to a significant improvement in the physical, mechanical, and electrical properties of cross-linked elastomeric composites. This reinforcing effect is primarily due to interactions between the rubber and filler surfaces. Traditionally, carbon black has been the primary filler used by the rubber industry. Even today, carbon black continues to be the most important reinforcing agent in the rubber industry. About 5 million metric tons of carbon black is globally consumed each year, while only 250,000 tons of the different silica grades (including the highly dispersible silica) are used each year. But, due to its polluting nature, the ubiquitous black color of the compounded rubber material and its dependence on petroleum feedstock (for synthesis) caused researchers to look out for other ‘white’ reinforcing agents. Since the 1950s, non-black fillers, such as precipitated silica, have been increasingly used. At present, nanometer-scale reinforcing particles like clay have attracted considerable attention from polymer scientists. For a filler to behave as a good reinforcing agent, the three main factors are particle size, structure, and surface characteristics. Researchers succeeded only recently in intercalating polymers into the clay layers and thereby prepared polymer clay nanocomposite, which exhibit not only outstanding mechanical properties but also very good barrier and thermal properties. Different methods for synthesizing polymer-layered silicate nanocomposites have been typically described, for example, in situ intercalative polymerization, polymer intercalation from solution, and direct polymers melt intercalation. Several studies have shown the possibility of preparing intercalated or exfoliated rubber nanocomposites by different methods [1–5]. It has been reported that the nanolayered silicate dispersed into a rubber matrix provides an effective reinforcement [6–10]. However, there is hardly any work on the aging behavior and ozone resistance of organoclay elastomer nanocomposites. Elastomeric composites are very much prone to ozone attack due to the presence of unsaturation in the back bone. The goal of the present study is to understand the aging behavior of the organoclay-filled

T

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Aging of Organoclay Nanocomposites

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styrene-butadiene rubber nanocomposites. From the application point of view, it is of prime importance to understand the effect of filler on the physical property in the long run. Retention of physical property after aging is the tool to understand such parameter. In the present study, the relative retention of the physical strength of organoclay and carbon black-filled compounds after accelerated aging and ozone exposure has been carried out.

EXPERIMENTAL DETAILS

Materials The styrene-butadiene rubber (SBR) 1502 was supplied by M/S BST Elastomers, Bangkok, Thailand. SBR 1502 had a Mooney viscosity of 50 at 1008C, volatile matter 0.02%, specific gravity 0.91, and 24% bound styrene. Cloisite 20A organoclay was purchased from Southern Clay Products, USA. The rubber compounding ingredients used in this work were of commercial grade, viz. zinc oxide, stearic acid, sulfur, N330 carbon black, and N-t-butylbenzothiazole-2-sulfenamide (TBBS). Compound Mixing Mixing of the compounds was carried out in two stages, master and final. Master batch mixing was done at 908C and at a rotor speed of 60 rpm. First, SBR was masticated 60 s followed by the addition of clay or carbon black. It was mixed for additional 9 min. For final batch mixing, the temperature control unit (TCU) was kept at 608C and at a rotor speed of 60 rpm. The master batch was initially masticated for 60 s. Zinc oxide, stearic acid, sulfur, and accelerator were added and mixed for 4 min. The final batches were sheeted out in the laboratory two-roll mill. Characterization of the SBR–Coloisite Clay Nanocomposites Wide angle X-ray diffraction (WAXD) measurements were carried out in a Philips 1710 X-ray diffractometer at a scan rate of 0.58/min with Cu Ka target at 40 kV and 25 mA (wavelength ¼ 0.154 nm) with 2 scan range from 28 to 108. For transmission electron microscopy (TEM) measurements, 100 nm sections were microtomed at 1208C using Ultracut E Ultramicrotome (Reichert and Jung) with a diamond knife. Measurements were carried out with a Philips CM200 TEM at an acceleration voltage of 120 kV.


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The rheometric properties were determined in a moving die rheometer (MDR 2000E) from M/s Alpha Technologies, Akron, USA, at 1608C for 60 min keeping the rotor arc at 0.58 in accordance with ISO 6502 (ASTM D5289). The cure rate index (CRI) was measured according to ISO 6502 (ASTM D5289). Curing of tensile slabs was done using a compression molding technique in an electrically heated curing press from M/s Hind Hydraulics, New Delhi, India, at 1608C for 60 min. The tensile samples were prepared in accordance with ISO 37 (ASTM D412) type C die. The stress–strain properties were determined using a universal testing machine, Zwick UTM 1445 from M/s Zwick, Ulm, Germany, in accordance with ASTM D412. The hardness was determined in a Shore A durometer from M/s Prolific Industries, New Delhi, India, in accordance with ISO 7619 (ASTM D2240). Tensile specimens were air aged at 1058C for 7 days in a multicell aging oven from Tempo Industries Ltd., New Delhi, India for determining the accelerated thermo-oxidative aging property. The circulating fan of the oven was kept running till the aging was over to maintain uniformity of temperature. The aged samples were taken out of the oven after completion of the aging and were matured at room temperature for 24 h before any further testing. For accelerated thermal aging, the samples were aged inside the compression mold. The molding time was increased to 30 h for 1308C. The aged stress–strain properties were determined in the Zwick UTM, and the hardness was measured using the hardness tester as mentioned earlier. The swelling index of the cured samples was measured in accordance with ISO 1817 (ASTM D3616).The ozone testing of the samples was carried out according to ISO 1431 (ASTM D1149A) under static mode keeping a 50 5 pphm ozone concentration and 20% strain level at 508C. The photographs of the cracked samples were taken using an optical microscope Wild M10, Leica, Switzerland.

RESULTS AND DISCUSSION

WAXD Study Figure 1 shows the X-ray diffraction (XRD) patterns of Cloisite 20A and the organoclay-filled compound (OC), respectively. Cloisite 20A shows a characteristic diffraction peak at 2 3.648 corresponding to an



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200 3.5 nm, OC 2.4 nm, Cloisite I

0

2q

100

FIGURE 1. WAXD diffractograms of compound OC and Cloisite 20A.

FIGURE 2. TEM micrograph of compound OC.

inter-gallery distance of 2.4 nm. In the case of compound OC, the XRD patterns showed a broad peak at 2.58 corresponding to a layer spacing of 3.5 nm. The intercalation of the polymer and partial exfoliation of the clay layers led to disordering of the layered clay structure, causing the decrease in the XRD coherent layer scattering intensity of the compound OC. The findings are in line with the TEM study. TEM Study The TEM photomicrograph of compound OC shown in Figure 2 clearly points out the exfoliated as well as the intercalated nature of the SBR/ organoclay nanocomposite. Most of the clay platelets are observed to be


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Table 1. Rheometric and physical properties of the compounds. Parameter Maximum torque (Tmax) (dN-m) Minimum torque (Tmin) (dN-m) Torque ¼ Tmax Tmin (dN-m) TS02 (min) TC90 (min) CRI 100% mod (MPa) 300% mod (MPa) TS (MPa) EB (%) Hardness (S) Swelling index

CB

OC

9.5 0.8 8.7 8.0 14.9 14.5 1.2 3.0 5.5 390 49 4.22

7.9 0.6 7.30 2.4 7.9 18.2 1.1 2.2 7.0 620 52 4.59

Compound formulation: OC contains 10 phr Cloisite 20A organoclay and CB contains 10 phr N330 carbon black. Other ingredients: SBR – 100 phr, ZnO – 3 phr, stearic acid – 1 phr, TBBS – 1 phr, and S – 1.75 phr.

uniformly dispersed throughout the polymeric matrix with some intercalated clay platelets, which have thickness ranging from 20–40 nm and length of 100–400 nm. The findings are in line with the WAXD study. Rheometric Property The cure properties of the compounds are compiled in Table 1. The extent of curing (given by the Torque values) is higher in the case of compound CB (compound containing carbon black) compared to compound OC (compound containing organo-clay). This was probably due to the interaction of the accelerator with the highly exposed silanol groups of the silicate layers as well as with the organoamine of the silicate layers. It has been reported that fillers like silica reduce the extent of curing due to the interaction of the polar –OH groups with the accelerator molecules [11]. Song et al. [12] also reported the same type of observation. The TS02 (scorch time) and TC90 (optimum cure time) value of compound OC was lower when compared to compound CB. This was due to the presence of amine moiety in the Cloisite clay. Due to the same reason, the CRI was higher in compound OC. Physical Property The initial physical property of the compounds is reported in Table 1. It was found that compound OC exhibited a slightly lower modulus value when compared to compound CB. However, the tensile strength and


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Aging of Organoclay Nanocomposites (a) 80

(b) 80 OC

%

CB

% OC

40

40

(Mod +, TO) (Mod+, T)(TS–, TO) (TS–, T)

CB

(EB–, TO)(EB–, T)(HB+, TO)(Hd+, T)

FIGURE 3. (a) Relative decrease/increase of physical property after accelerated thermal and thermo-oxidative aging (300% modulus and Tensile strength) and (b) relative decrease/increase of physical property after accelerated thermal and thermo-oxidative aging (Elongation at break and hardness).

elongation at break of OC were, respectively, 30% and 60% higher when compared to compound CB. Better rubber to filler interaction was responsible for this. Swelling index of compound OC was higher in comparison to compound CB. The reason was probably the low extent of curing in compound OC. The low extent of curing in compound OC in comparison to compound CB was also reflected in the lower maximum Torque value from rheometer. Accelerated Thermal Aging

The relative retention of the physical properties of the thermally aged samples is shown in Figure 3(a) and (b). In Figure 3(a) and (b), ‘þ’ and ‘’ sign indicate the relative increase and decrease, respectively, while ‘Mod’ stands for 300% modulus, ‘TS’ stands for tensile strength, and ‘Hd’ stands for hardness. In the same figure, ‘TO’ stands for thermooxidative aging and ‘T’ stands for thermal aging. It was found that the relative increase in modulus and hardness was higher in compound OC. It was also found that the relative decrease in elongation at break was comparable for both OC and CB compounds. However, the relative decrease in tensile strength of compound OC was 10% in comparison to 17% of compound CB. After aging (anaerobic or aerobic), increase in modulus and decrease in tensile strength, and elongation at break is expected. The increase in modulus is mainly due to breakage of polysulfide cross-link and formation of monosulfide cross-link. Thus, the apparent cross-link density increases. This is reflected in the increasing hardness. During thermal or thermooxidative aging, polymer chain cession is considerably higher, which in turn decreases the elongation at break and ultimately the tensile strength [13].


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The better retention of the properties of compound OC in thermal aging was probably due to the slow release of the adsorbed curatives from the clay surface, which created additional cross-links. Thermo-Oxidative Accelerated Aging

After 7 days of accelerated thermo-oxidative aging at 1058C, the relative increase of 300% modulus of compound OC was 67%. Compound CB failed to cross the 300% elongation. The relative drop in tensile strength for OC and CB was 19% and 25% and that in elongation at break was 43% and 60%, respectively. The relative increase in hardness was comparable. The thermo-oxidative aging was carried out in the presence of air. Thus, the polymer degradation is a combination of thermo and thermooxidative processes. After aging, the polymer chain cession was severe enough to overcome the increase in apparent cross-link. The better performance of compound OC was probably due to the slow release of the adsorbed curatives as well as the barrier property of the compound. The exfoliated and intercalated clay layers slowed down the diffusion of the oxygen inside the rubber matrix [14]. Thus, the degradation process was less severe in compound OC compared to compound CB.

FIGURE 4. Ozone crack photograph of compound OC (A1 – 24 h and A2 – 48 h) and compound CB (A1 – 24 h and A2 – 48 h) at 30 magnification.



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Ozone Resistance

After 24 h of exposure, both the compounds generated cracks (Figure 4). However, the number of cracks, crack depth, and severity is much less in compound OC. After 48 h of exposure, the severity of cracks increased. Still, the compound OC exhibited relatively better performances in terms of crack length and number of cracks. The degradation of the polymer under ozone exposure starts from the surface [15]. The exposed double bends of the polymer chain are the sites of ozone attack. It starts from the surface and proceeds toward the depth. The superior barrier property of the dispersed organoclay in compound OC creates hindrance for the ozone attack on the surface and subsequent stages. Thus, compound OC exhibited better ozone resistance property compared to compound CB.

CONCLUSIONS

The above study clearly indicates that organoclay had no detrimental effect on accelerated aging property. Rather little improvement was observed due to the use of organoclay. Improvement in ozone resistance was also observed. The reason was attributed to the low permeability of the organoclay compound.

ACKNOWLEDGMENTS The authors thank HASETRI and JK Tyre management for the kind permission to publish this work.

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