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Influence of synthesized nano-ZnO on cure and physico-mechanical properties of SBR/BR blends

Influence of synthesized nano-ZnO on cure and physico-mechanical properties of SBR/BR blends Int J Ind Chem (2017) 8:273–283 DOI 10.1007/s40090-016-0107-7 RESEARCH Influence of synthesized nano-ZnO on cure and physico- mechanical properties of SBR/BR blends 1 1 1 • • • Madhuchhanda Maiti Ganesh C. Basak Vivek K. Srivastava Raksh Vir Jasra Received: 6 June 2016 / Accepted: 7 November 2016 / Published online: 17 November 2016 The Author(s) 2016. This article is published with open access at Springerlink.com Abstract This study focuses on the synthesis of zinc oxide out using DSC. The results differ slightly from static curing (ZnO) nanoparticles by high temperature calcination as except PEG modified nano-ZnO. Use of ZnO nanoparticles well as low-temperature hydrolysis methods and their could provide faster crosslinking, better reinforcement at efficiency as cure activator in styrene-butadiene rubber/ lower concentration compared to reference ZnO. polybutadiene rubber blend. The synthesized nano-ZnO samples were characterized by means of X-ray diffraction, Keywords Nano-ZnO  SBR  BR  Curing  Cure BET surface area and transmission electron microscopy. properties The synthesized nano-ZnO samples had wurtzite structure and average particle size in the ‘nm’ range. ZnO nanoparticles, synthesized on sepiolite template, were of Introduction smallest particle size (maximum number of particles in the 2 -1 range of 7–12 nm) and highest surface area (104 m g ). The rubber industries, specifically tire industries, contribute Polyethylene glycol (PEG)-6000 coated ZnO nanoparticles significantly to economy of a nation where automobile had rod-like structure; average diameter of the rods was industry is growing at a very fast pace. Improvement in 50 nm. In the case of PEG-coated ZnO containing com- quality and safety of rubber products can have significant pounds, optimum cure time of the blend was decreased by impact on this industry [1, 2]. Zinc oxide (ZnO) is primarily 5 min compared to that of standard rubber grade-ZnO used as an activator for sulfur vulcanization of rubbers. containing compound (used as reference). Optimum cure Besides, inclusion of ZnO in the rubber compound brings time was lowered by 7–10 min in the case of synthesized other benefits viz., reduction in heat build-up, improvement nano-ZnO containing compounds compared to the refer- of abrasion resistance and heat resistance of the vulcanizates. ence ZnO based compound in presence of conventional Furthermore, its high thermal conductivity helps to dissipate filler, carbon black. It was also observed from ICP-OES local heat concentrations in rubber products. Zinc oxide is a analysis that the presence of very little amount of magne- necessary ingredient in rubber compounds for bonding rub- sium in one of the synthesized ZnO has noticeable impact ber to reinforcing steel cord, etc. Besides improving the on cure properties. PEG-coated ZnO increased the tensile properties of vulcanized rubbers, ZnO also assists in the strength of gum vulcanizates by 28% compared to the processing of uncured rubbers. ZnO is added to rubber for- reference ZnO, acting as nanofiller at 3 phr loading. The mulation to reduce shrinkage of molded rubber products and study of curing behavior in dynamic condition was carried maintain the cleanliness of molds [3]. The road transport emission of zinc due to tire wear is the main sources of zinc pollution after iron and steel & Ganesh C. Basak production and non-ferrous metals manufacture. This arises ganesh.basak@ril.com from the zinc content (1 wt%) of the tire-tread material [4, 5]. But some adverse environmental effects of zinc Reliance Technology Group, Vadodara Manufacturing exposure have been reported. In view of the upcoming Division, Reliance Industries Ltd., Vadodara, legislation and eco-labeling requirements for tires, it can be Gujarat 391346, India 123 274 Int J Ind Chem (2017) 8:273–283 stated that it is desirable to keep the ZnO content in rubber reported that nano-zinc oxides are effective activators and compounds as low as possible. reinforcing agents in rubber systems. The ‘‘little size In rubber industry, various kinds of vulcanization acti- effect,’’ ‘‘surface effect’’ and ‘‘quantum effect’’ of nano- vators like CaO, MgO, CdO, CuO, PbO and NiO have been ZnO governs the properties of the composites [21]. used in order replace conventional ZnO due to its toxic and Although considerable amount of work has been done so far fouling characteristics for aquatic flora and fauna. Although on the use of nano-ZnO in place of conventional ZnO as a among the various activators studied, MgO shows most cure activator and for enhancing the mechanical properties promising candidate in terms of activating properties in of elastomer, the study on SBR/BR-nano-ZnO composites is comparison to ZnO but maximum crosslinking can be scarcely available in the literature [22]. In the tire industry, achieved in the presence of ZnO only [6]. Moreover, few SBR/BR blend is of considerable importance as it is widely reports are also available that describe the effect of layered used in passenger car tire-tread compound. Hence, investi- double hydroxide (LDH) on elastomeric materials in the gation of nanocomposite based on SBR/BR blends and nano- place of ZnO. According to the literature reports, LDH ZnO would not only be providing valuable information but material can be used as an alternative cure activator in place also have wide applications. Typically SBR/BR blend shows of ZnO and stearic acid combo in the conventional cure slower curing rate than other general purpose rubbers such as package for the preparation of rubber composites, and NR and BR [3]. Hence, it will be of interest to study the cure simultaneously can provide a strong platform for reduction properties of this blend with nano-ZnO. of ZnO level in elastomer vulcanizate system [7]. In this work, we have studied the influence of mor- In another approach, the concentration of ZnO can be phology, specific surface area and dispersibility of ZnO minimized if the efficiency of ZnO during vulcanization can nanoparticles on the static and dynamic vulcanization of be enhanced by the maximization of the contact between the SBR/BR blends. We have studied the effect of sepiolite ZnO particles and the accelerators in the compound. This template and ‘eco-friendly’ metal oxide, magnesium oxide contact is dependent on the size, shape, specific surface area (MgO) on nano-ZnO in the crosslinking of the rubber and dispersibility of the ZnO particles. Nano-sized ZnO blend. The influence of nano-ZnO on the properties of particles have been paid more attention for their unique SBR/BR vulcanizates in the absence as well as in the properties, even though there are limited open literatures presence of conventional filler was also evaluated. available on nano-ZnO as cure activators. ZnO nanoparticles were studied as a cure activator and curing agent in natural rubber (NR), nitrile rubber (NBR), carboxylated nitrile rub- Experimental ber (XNBR) and chloroprene rubber (CR) by Bhowmick and his coworkers [8–10]. Similarly, it was used as cure activator Materials in NR and CR by Joseph et al. [11, 12]. Nanostructured zinc oxide was used in crosslinking of hydrogenated butadiene- Zinc nitrate [Zn(NO ) 6H O] [molecular weight (M.W.) 3 2 2 acrylonitrile elastomer and XNBR by Przybyszewska and 297.48, 98% purity], ammonium carbonate [(NH ) CO ] 4 2 3 Zaborski [13–15]. Guzman et al. synthesized mixed metal (M.W. 157.13, 31% purity), acetone (M.W. 58.08, 99.5% oxide nanoparticles of zinc and magnesium to reduce the ZnO purity), methanol (M.W. 32.04, 99.5% purity), sodium levels in rubber compounds [16]. Heideman et al. studied the hydroxide pellets (M.W. 40.00, 98% purity), 1-octanol influence of nano-ZnO on the cure properties of solution (M.W. 130.23, 99% purity), Stearic acid (M.W. 284.48, styrene-butadiene rubber (SBR) and ethylene–propylene– 98% purity), sulfur powder (M.W. 32.06, 99% purity), N- diene rubber [17]. Kim et al. investigated the effect of nano- cyclohexyl-2-benzothiazole sulfenamide (CBS) (M.W. ZnO on the cure characteristics and mechanical properties of 264.42, 97% purity), microcrystalline wax, magnesium the silica-filled natural rubber/butadiene rubber compounds oxide (MgO) were procured from Labort Fine Chem. Pvt. [18]. Jincheng and Yuehui studied the application of nano- Ltd., India. Standard rubber grade zinc oxide (ZnO), used ZnO master-batch in SBR [19]. as reference (designated as SZ), was supplied by Labort In our previous work, we have studied the effect of nano- Fine Chem. Pvt. Ltd., India. Polyethylene glycol-6000 ZnO on the cure properties of polybutadiene rubber (BR) (PEG) was obtained from Alfa Biochem, Greece. Zinc [20]. It was observed that the nano-ZnO reduces curing time acetate dihydrate [Zn (CH COO) 2H O] (M.W. 219.50, 3 2 2 and also enhances physico-mechanical as well thermal 98.5% purity) and Oxalic acid (M.W. 126.07, 99.8% pur- stability properties of butadiene rubber compound at lower ity) were procured from S. D. Fine Chem. Ltd., India. N- concentration compared to the conventional micro-ZnO. (1,3-dimethyl butyl)-N -phenyl-p-phenylenediamine However, to the best of our knowledge, the effect of nano- (6PPD) was obtained from John Baker Inc., USA. ZnO as cure activator has not yet been explored for SBR/BR Polybutadiene rubber (BR; Cisamer 01; ML at 1?4 rubber blend. In the open literature, it has already been 100 C = 45; cis-content 96%) was collected from 123 Int J Ind Chem (2017) 8:273–283 275 Reliance Industries Ltd., India. Styrene-butadiene rubber Method-4 [20] (SBR 1502; ML at 100 C = 48) was supplied by Japan 1?4 Synthetic Rubber, Japan. Sepiolite (Pangel S9) was gen- Equivalent volume of Zn (CH COO) 2H O (0.5 M) and 3 2 2 erously supplied by Tolsa, Spain. Carbon black (N330) was sodium hydroxide (1.5 M) were mixed to obtain a solution A. 2.5 g of PEG-6000 was dissolved in 10 ml of water to procured from Philips Carbon Black Ltd., India. obtain solution B. The solution B was then added into solution A to obtain solution C. 50 ml of 1-octanol was Preparation of nano-ZnO added to solution C under stirring at room temperature to obtain solution D. Then solution D was transferred to Nano-ZnO was synthesized by high temperature calcina- tion as well as low-temperature hydrolysis methods. The Teflon lined stainless steel autoclave which was then typical procedures are described below. maintained at 180 C for 4 h under autogenous pressure. The ZnO powder was obtained after filtering, washing and drying in oven at 120 C. The sample is designated as Z4. Method-1 [9] Method-5 Zn(NO ) 6H O and ammonium carbonate (NH ) CO 3 2 2 4 2 3 were, respectively, dissolved in distilled water at a con- The solution of Zn (CH COO) 2H O (0.1 M) was pre- 3 2 2 centration of 1.0 M. Zinc nitrate solution was then slowly pared in 50 ml methanol under stirring. 25 ml of NaOH dropped into the vigorously stirred (NH ) CO solution 4 2 3 (0.3 M) solution, prepared in methanol, was mixed with with molar ratio of 2:1 to prepare the precursor. A white above solution under continuous stirring to get the pH of precipitate occurred immediately on mixing of the two reactants between 8 and 11, and then 4 g of sepiolite was solutions. Stirring was done for 3 h to have complete added with vigorous stirring. It was then transferred into a precipitation. The white precipitate thus obtained was fil- Teflon lined sealed stainless steel autoclave and maintained tered and repeatedly washed with distilled water to remove at 150 C for 12 h under autogenous pressure. Subse- impurities and dried at 105 C for 6 h. Calcination of the quently, it was allowed to cool naturally to room temper- dried sample was carried out at 450 C in a muffle furnace. ature. After the reaction was complete, the resulting white The sample thus obtained is designated as Z1. solid product was washed with methanol, filtered and then dried in a laboratory oven at 100 C. The sample is des- Method-2 [23] ignated as Z5. 0.1 M aqueous solution of oxalic acid was added to 0.1 M Method-6 aqueous solution of Zn(CH COO) 2H O and the solution 3 2 2 was stirred for 4 h. The white precipitates thus obtained 0.1 M solution of Zn(CH COO) 2H O in 50 ml methanol 3 2 2 were filtered and washed with acetone and distilled water was prepared. To this solution, 0.5 mol of MgO was added. to remove impurities and dried at 120 C for 6 h. The dried 25 ml of NaOH (0.3 M) solution, prepared in methanol, sample was calcined at 450 C in a muffle furnace to was mixed with above solution under continuous stirring to remove CO and CO from the compound. The sample is get the pH of reactants between 8 and 11. After that it was designated as Z2. transferred into Teflon lined sealed stainless steel autoclave and maintained at 150 C for 12 h under autogenous Method- 3 [24] pressure. It was then allowed to cool naturally to room temperature. After the completion of the reaction, resulting The solution of Zn (CH COO) 2H O (0.1 M) was pre- white solid products were washed with methanol, filtered 3 2 2 pared in 50 ml methanol under stirring. 25 ml of NaOH and dried in a laboratory oven at 100 C. The sample is (0.3 M) solution, prepared in methanol, was mixed with designated as Z6. above solution under continuous stirring to get the pH of reactants between 8 and 11. These solutions were trans- Characterization of zinc oxide particles ferred into a Teflon lined sealed stainless steel autoclave and maintained at 150 C for 12 h under autogenous X-ray diffraction (XRD) pressure. It was then allowed to cool naturally to room temperature. After the reaction was complete, the resulting X-ray diffraction analysis was done using X-ray diffrac- white solid product was washed with methanol, filtered and tometer, Rigaku ‘‘Mini flex’’ model in the range of 10 to then dried in a laboratory oven at 100 C. The sample is 80 (=2h). The zinc oxide powder was deposited on the designated as Z3. sample holder uniformly. 123 276 Int J Ind Chem (2017) 8:273–283 Brunauer Emmet Teller (BET) surface area measurement (volume 50 cm ) for 3 min at 80 C and 60 rpm. The formulation is given in Table 1. It was chosen as a typical BET surface area determination was done from N tire-tread formulation. Amount of ZnO used (0.5, 1.5 and adsorption data measured at 77.4 K using micromeritics- 3 phr) was lower than that used (5 phr) in the conven- ASAP-2020 instrument. The samples were activated at tional formulations. The sample was then passed through 200 C for 20 min under vacuum (10 mmHg) prior to a cold two roll open mixing mill at a friction ratio 1:1.2. measurements. Five point BET surface area and total pore The curing studies were followed with an Oscillating Disc volume were measured. The average of five reading is Rheometer (ODR-2000, FLEXSYS) at 145 C tempera- reported here. ture and oscillating arc of 3 for 1 h. The samples were then compression molded at 145 C at optimum cure Transmission electron microscopy (TEM) time. Morphology of different ZnO samples was investigated by Physico-mechanical properties of rubber composites transmission electron microscopy (TEM) (JEOL 2010) having LaB filament, operating at an accelerating voltage Tensile test of 200 kV. ZnO powder samples were dispersed by ultra- sonication in acetone for 30 min. A copper grid was Tensile test of the sample was carried out according to immersed in and taken out of the suspension and dried at ASTM D412-98a on dumbbell shaped specimens using room temperature. Image analysis of the microphotographs Instron 3367 universal testing machine at ambient tem- was performed using UTHSCSA Image Tool for Windows -1 perature at a crosshead speed of 500 mm min . Average Version 3.00. It was used to determine the particle size of five samples is reported here. distribution. Hardness Differential scanning calorimetry (DSC) Hardness of each composition was obtained using Shore A Cure-studies were done using differential scanning calori- metric analysis. It was carried out using modulated DSC Durometer tester as per ASTM D 2240-97. (DSC 2910, TA Instruments, USA). The samples were -1 heated from ambient temperature to 250 C (at 5 C min Volume fraction of rubber (V ) and crosslink density heating rate) in air. 5 mg of each sample was taken for the measurement. The error limit in the ‘weighing measure- The cured samples were immersed in toluene for 72 h at ments’ was within ±5%. 25 C temperature. The volume fraction of rubber in the swollen gel, at equilibrium swelling, was calculated using Elemental analysis Eq. (1): ðÞ D  FT q Elemental analysis was done using a Perkin Elmer (Model: V ¼ ; ð1Þ 1 1 ðÞ D  FT q þ A q Optima 4300 DV) inductively coupled plasma-optical 0 r s emission spectroscopy (ICP-OES). where D Deswollen weight, F weight fraction of the insoluble component, T initial weight of the test specimen, Scanning electron microscopy (SEM) -3 q density of rubber, 0.89 g cm , q density of solvent, r s -3 0.86 g cm , A amount of solvent absorbed. SEM samples were fractured in liquid nitrogen immersion -1 Further, the crosslink density, ; in mol g of rubber 2M and mounted with carbon tape wrapping. The images were hydrocarbon was calculated using the Flory–Rehner studied with a Nova NanoSEM 650 instrument, FEI, USA, Eq. (2): operating at 1 and 10 kV for the micro and synthesized nano-ZnO samples, respectively. 1=3 qVV  V =2 r r lnðÞ 1  V þ V þ vV ¼ ; ð2Þ r r Preparation of rubber composites 2M v Flory–Huggins interaction parameter, 0.46 for BR- Compounding and vulcanization toluene system [25], V molar volume of swelling solvent, toluene, M number average molecular weight of the chain ZnO was mixed with rubber by melt mixing method using between two crosslinks. Brabender Plasticorder (PL2000, Germany) internal mixer 123 Int J Ind Chem (2017) 8:273–283 277 Results and discussion Characterization of nano-ZnO XRD Figure 1 shows the XRD patterns of different zinc oxide (ZnO) samples. The sharp intense peaks, confirming the good crystalline nature of synthesized ZnO, correspond to (100), (002), (101), (102), (110), (103), (200), (112), (201) and (004) planes. All of the indexed peaks in the obtained diffractograms match with that of the bulk ZnO (JCPDS card # 79-0207) which confirm that the synthesized sam- ples are of wurtzite hexagonal structure [26]. Any other peak related to impurities was not detected in the diffrac- togram within the detection limit of the XRD. Absence of any extra peak in the diffractograms of final products indicates the purity of the products. In Z5, additional peaks can be observed, other than the earlier mentioned peaks for ZnO. These are for (060), (131) [at 20], (260) [at 24] and (080, 331) planes [at 27,28] of sepiolite clay [27]. It proves that ZnO particles are formed on sepiolite without distorting the crystal structure of either material. The average crystal size was calculated by Scherrer Eq. 28]: Kk L ¼ ; ð3Þ b cos h where, b is the full-width at half maximum (FWHM) of the peak corresponding to (100) plane, K is a constant (0.89), k is the incident wavelength of CuK radiation (k = 0.154 nm), L is the crystallite size, and h is the diffraction angle at a certain crystal plane. The average crystallite size of Z1, Z2, Z3, Z4, Z5 and Z6 was calculated using Eq. (3) and was found to be 23, 20, 18, 27, 27 and 16 nm, respectively. It should be noted that crys- tallite size is assumed to be the size of a coherently diffracting domain. It is not necessarily the same as particle size [28]. BET surface area BET surface area of different prepared nano-ZnO is reported in Table 2. Surface area and pore volume both increase in the synthesized ZnO samples compared to the reference one. Highest surface area and pore volume can be observed in the case of Z5. This could be due to dispersion of ZnO particles on the fibrous sepiolite template surface. For the same sample, smallest particle size was also observed through TEM (Fig. 2e). So the surface area results corroborate well with the microscopic study. The sample Z4 shows minimum surface area among the synthesized samples, as it is coated with PEG. The organic coating of PEG resists the nitrogen to be absorbed on the surface of ZnO particles. Table 1 Formulation and designation of different rubber compounds Ingredients SBWZ SBSZ SBZ1 SBZ2 SBZ3 SBZ4 SBZ5 SBZ6 0.5SBZ4 1.5SBZ4 0.5SBZ5 1.5SBZ5 SBSZF SBZ4F SBZ5F SBR 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 BR 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 Sulfur 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 CBS 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 ZnO 0 3 3333 330.5 1.5 0.5 1.5 3 3 3 6PPD 2 2 2222 222 2 2 2 2 2 2 Wax 1 1 1111 111 1 1 1 1 1 1 Stearic acid 0 2 2222 222 2 2 2 2 2 2 Carbon black 0 0 0000 000 0 0 0 60 60 60 Naphthenic oil 0 0 0000 000 0 0 0 8 8 8 Formulation of compounds is expressed in phr (parts per hundred rubber) 278 Int J Ind Chem (2017) 8:273–283 Fig. 1 X-ray diffractogram of the different zinc oxide samples Sample Z6 has maximum particles in the range of Table 2 Crystallite size of different zinc oxide samples 10–18 nm. Sample FWHM of (100) Average crystallite plane (degree) size (nm) Application of synthesized nano-ZnO Z1 0.3444 23 Z2 0.3936 20 Cure properties Z3 0.4428 18 Z4 0.2952 27 The effect of synthesized nano-ZnO as cure activator has Z5 0.2952 27 been studied on SBR/BR blend. A representative rheo- Z6 0.4920 16 graphic profile of SBR/BR blends at 145 C is shown in Fig. 4 and cure time is tabulated in Table 3. In the absence of ZnO, the curing is extremely slow in the sample SBWZ TEM and modulus is also lowest. The optimum cure time is faster with synthesized nano-ZnO samples by complex Figure 2a–f portrays TEM photo-micrographs of different formation with acceleration compared to the reference one. zinc oxide (ZnO) samples. TEM image exhibits the mor- ZnO helps in producing vulcanization precursor, hence phology of synthesized particles to be in nano region. faster curing can be observed in the presence of ZnO [29]. Samples Z1, Z2, Z3, Z5 and Z6 show hexagonal structure. Due to decrease in particle size of ZnO, the area of contact The particle size distribution curves for these samples are increases which helps to react better with accelerator. This shown in Fig. 3. It shows that for Z1, maximum particles leads to the generation of vulcanization precursor quicker. are in the range of 26–50 nm; for Z2, it is also in the range It results in a faster curing rate and lower cure time. Fastest of 26–50 nm and for Z3, it is in the range of 15–28 nm. curing can be seen with the use of organo-coated ZnO, Z4, The sample Z4 evinces rod-like structures grown on PEG- followed by Z5. Due to the presence of long-chain organic sheets. The average rod diameter is *50 nm. These nano- PEG molecules, it has more compatibility with elastomeric rods are of 100–200 nm in length. Z5 exhibits an inter- matrix leading to better dispersion. The curing reaction is esting morphology; it consists of smallest ZnO particles. not affected and slowed down in the presence of Z4 at Figure 2e infers that ZnO nanoparticles are grown on long lower dose, i.e., 0.5 and 1.5 phr. It indicates that dispersion bundles of sepiolite nanofibers. The ZnO particles are very of ZnO plays a major role in efficient vulcanization, rather small in size; most of the particles are in 7–12 nm range. 123 Int J Ind Chem (2017) 8:273–283 279 Fig. 2 TEM image of the sample a Z1, b Z2, c Z3, d Z4, e Z5 and f Z6 Fig. 4 Representative rheographic profiles of SBR/BR blends con- Fig. 3 Particle size distribution curves of different ZnO samples taining different ZnO samples 123 280 Int J Ind Chem (2017) 8:273–283 than higher loading of ZnO. The difference in minimum based sample. Increased DS indicates resistance to polymer (M ) and maximum (M ) torque value, [DS = (M - chain mobility [30]. Due to the formation of increased L H H M )] has increased in SBR/BR blend with the use of syn- crosslinks, chain mobility is restricted which will lead to thesized nano-ZnO compared to that of reference ZnO higher crosslink density. In the case of Z5, the template, sepiolite which is fibrous clay with high aspect ratio, can help in better distribution of ZnO particles. As mentioned Table 3 BET value of different nano-ZnO samples earlier, the area of contact increases in such case and helps 2 -1 3 -1 Sample Surface area (m g ) Pore volume (cm g ) in generation of vulcanizing precursors faster. These nano- ZnO samples impart better co-curing of both the rubbers in Reference ZnO, SZ 5 0.025 SBR/BR blend. Z1 18 0.123 The presence of very little amount of magnesium in Z6 Z2 11 0.084 (Zn:Mg 90:1, as revealed from ICP-OES analysis) has visi- Z3 39 0.110 ble impact on cure properties. It increases the optimum cure Z4 5 0.039 time but at the same time it also increases DS values. It Z5 104 0.230 produces maximum number of crosslinks but at a slower rate. Z6 55 0.110 Most encouraging effect of synthesized nano-ZnO on curing can be observed in the presence of filler. Optimum cure time is lowered by 7–10 min in the case of synthe- sized nano-ZnO containing compounds compared to the reference ZnO based compound (which even contains higher dose of ZnO). Thus, ZnO nanoparticles can help in the reduction of the production cycle and also in mini- mizing the Zn-pollution due to lower dose. Cure behavior in dynamic condition is shown in the representative plots (Fig. 5). The results differ slightly from static curing, though Z4 shows most efficient -1 crosslinking activities (DH = 123.70 Jg ) in dynamic curing, too (Table 4). Physico-mechanical properties of different rubber composites The nano-ZnO particles may also act as nano-fillers. Phy- Fig. 5 DSC curves of different ZnO containing SBR/BR compounds depicting dynamic curing sico-mechanical properties of different rubber composites Table 4 Cure properties of Sample Cure time, t (min) M (lb-in) M (lb-in) M - M (lb-in) 90 H L H L different elastomeric compounds a SBWZ 38.2 (0.08) 20.28 (0.07) 3.97 (0.14) 16.31 SBSZ 18.5 (0.10) 27.01 (0.10) 4.87 (0.13) 22.14 SBZ1 17.0 (0.09) 27.43 (0.15) 3.62 (0.09) 23.81 SBZ2 16.9 (0.07) 28.03 (0.20) 3.72 (0.10) 24.31 SBZ3 16.4 (0.12) 28.35 (0.04) 4.12 (0.08) 24.23 SBZ4 12.9 (0.07) 27.85 (0.13) 4.05 (0.02) 23.80 SBZ5 13.0 (0.11) 28.74 (0.09) 4.48 (0.04) 24.46 SBZ6 19.2 (0.08) 30.16 (0.07) 5.03 (0.05) 25.13 0.5SBZ4 12.2 (0.07) 27.93 (0.13) 4.42 (0.04) 23.51 1.5SBZ4 12.5 (0.10) 27.87 (0.14) 4.43 (0.03) 23.44 SBSZF 25.3 (0.07) 35.83 (0.21) 6.95 (0.12) 28.88 SBZ4F 15.6 (0.09) 43.76 (0.23) 8.01 (0.14) 35.75 SBZ5F 18.0 (0.04) 39.88 (0.24) 7.45 (0.11) 32.43 Values in parentheses are standard deviations 123 Int J Ind Chem (2017) 8:273–283 281 are reported in Table 5. Z2, Z3, Z4 and Z5 containing 3 phr loading. Tensile strength, 100% modulus and hard- SBR/BR blend based nanocomposites show better tensile ness decrease with decreasing ZnO loading. strength and hardness compared to the reference one. The Nano-ZnO containing compounds show slightly better sample containing organo-coated ZnO (Z4) displays high- properties than those of reference ZnO based compound. est tensile strength. This is ascribed to the better compat- Z5 based compound exhibits highest overall properties. ibility and in turn better dispersion of Z4 in the rubber This may be due to some synergistic effect between matrix. This leads to better curing as observed in the pre- nanofiller, sepiolite and conventional filler, carbon black vious experiments and higher value of volume fraction of [31]. The similar kind of effect has also been studied in our rubber (V ) as well as crosslink density. Z4 imparts highest previous study using mesoporous silica as reinforcing filler reinforcement. From these results it can be concluded that in the poly butadiene rubber matrix in the presence of dispersion of nanoparticles plays the major role in nanoclays, silica and carbon black [32]. enhancement of properties. Though lower filler loading maintains the cure proper- Morphology ties unaltered (as observed in 0.5SBZ4 and 1.5SBZ4) but it does not provide the same amount of reinforcement as The topographical images of SEM shown in Fig. 6 of different SBR/BR blend either having synthesized nano- ZnO or standard rubber grade-ZnO is studied to evaluate the extent of dispersion of ZnO within the rubber blend. Table 5 Dynamic cure properties of different elastomeric The black phase implies the rubber matrix, whereas the compounds white dot is the reflection of ZnO particles. In SEM images -1 Sample T (C) DH (J g ) of SBSZ (Fig. 6a), some agglomeration of ZnO nanopar- max ticles in the form of white dot can be seen. Figure 6b SBSZ 261 63.66 indicates that uniform dispersion of nano-ZnO occurs SBZ2 261 88.39 throughout the entire blend in comparison to standard SBZ3 262 81.06 rubber grade-ZnO (Fig. 5a). In case of SBZ4 (Fig. 6c), the SBZ4 258 123.70 rod-like structure as observed in TEM photo-micrographs SBZ5 254 86.58 (Fig. 2d) can also be seen. However, in the image of SBZ6 260 92.92 sepiolite based synthesized nano-ZnO (Fig. 5d), the homogenous distribution of ZnO with minimum particle Fig. 6 SEM images of a SBSZ, b SBZ3, c SBZ4 and d SBZ5 123 282 Int J Ind Chem (2017) 8:273–283 Table 6 Physico-mechanical properties of different rubber composites Sample Tensile strength Modulus at 100% Elongation at Hardness V Crosslink -5 -3 (MPa) elongation (MPa) break (%) (shore A) density 9 10 mol cm SBWZ 1.73 (0.02) 0.70 (0.02) 445 (10) 37 (0.3) 0.112 (0.003) 2.50 (0.01) SBSZ 1.89 (0.02) 0.84 (0.03) 310 (12) 41 (0.2) 0.165 (0.002) 6.31 (0.02) SBZ1 1.82 (0.03) 0.73 (0.03) 320 (15) 43 (0.3) 0.164 (0.002) 6.22 (0.03) SBZ2 2.12 (0.03) 0.78 (0.04) 335 (18) 43 (0.4) 0.168 (0.003) 6.60 (0.01) SBZ3 2.07 (0.04) 1.09 (0.03) 265 (22) 45 (0.3) 0.164 (0.004) 6.22 (0.01) SBZ4 2.54 (0.01) 1.05 (0.02) 295 (14) 44 (0.4) 0.171 (0.002) 6.90 (0.02) SBZ5 2.05 (0.05) 1.07 (0.02) 250 (18) 45 (0.2) 0.167 (0.002) 6.50 (0.03) SBZ6 1.90 (0.03) 0.90 (0.03) 280 (12) 46 (0.1) 0.169 (0.003) 6.70 (0.04) 0.5SBZ4 1.61 (0.02) 0.72 (0.02) 335 (10) 37 (0.3) 0.165 (0.002) 6.31 (0.02) 1.5SBZ4 2.15 (0.01) 0.94 (0.03) 290 (17) 41 (0.2) 0.169 (0.004) 6.70 (0.01) SBSZF 14.30 (0.5) 1.62 (0.02) 560 (09) 61 (0.1) 0.187 (0.005) 8.65 (0.04) SBZ4F 14.84 (0.2) 1.67 (0.01) 565 (08) 63 (0.2) 0.191 (0.005) 9.13 (0.03) SBZ5F 15.24 (0.3) 1.82 (0.01) 645 (10) 63 (0.3) 0.190 (0.007) 9.01 (0.02) Values in parentheses are standard deviations size is observed (as seen in TEM images too) (Fig. 2e). As uniform dispersion of synthesized nano-ZnO over standard a result, the improvement of mechanical properties of the rubber grade-ZnO within rubber blend and this fact account SBR/BR blend can be noticed (Table 5). From the SEM for better mechanical properties. Nano-ZnO imparted faster images, it can be distinguished that the distribution of ZnO curing even in the presence of conventional filler, carbon in the SBR/BR blend is comparatively better for nano-ZnO black compared with reference ZnO. Thus, the use of ZnO which in turn is reflected in mechanical properties nanoparticles can provide faster curing, better reinforce- (Table 6). ment at lower dosing compared to standard ZnO, which can lead to shorter production cycles and less zinc pollution. Acknowledgements The authors are highly thankful to Ms. Hetal Conclusions Patel and Mr. Chirag S. Shah for their kind cooperation. Authors are grateful to Reliance Industries Ltd. for its consent to publish this Six different nano-ZnO samples were synthesized by both work. Authors are also thankful to colleagues from catalyst, analytical high temperature calcination and low-temperature hydrol- and elastomer groups of RTG-VMD for their support. ysis methods. All the samples had wurtzite structure and Open Access This article is distributed under the terms of the Creative average particle size in the ‘nm’ range. ZnO, grown on Commons Attribution 4.0 International License (http://creative sepiolite nanofiber, showed smallest particle size as well as commons.org/licenses/by/4.0/), which permits unrestricted use, distri- highest surface area. PEG-coated ZnO nanoparticles were bution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the rod-like in structure. Effect of these nano-ZnO samples on Creative Commons license, and indicate if changes were made. cure properties of SBR/BR blends was studied by both static and dynamic curing methods. PEG-coated nano-ZnO sample exhibited maximum positive impact on cure prop- References erties. For PEG-coated ZnO, cure properties remained unaltered even at lower loadings (0.5 and 1.5 phr) of ZnO. 1. Frohlich J, Niedermeier W, Luginsland HD (2005) The effect of filler–filler and filler–elastomer interaction on rubber reinforce- From the observed results, it can be concluded that the cure ment. Compos A 36:449–460 properties are governed primarily by dispersion of cure- 2. Schuater RH (2001) The challenge a head-new polymer filler activator rather than its concentration and morphology. systems. Rubber World 224:24–28 Nano-ZnO can act as nanofiller also. The sample contain- 3. Morton M (1959) Introduction to rubber technology. Reinhold Publishing Corporation, New York ing organo-coated ZnO (Z4) displays highest tensile 4. Councell TB, Duckenfield KU, Landa ER, Callender E (2004) strength due to better compatibility and in turn better dis- Tire-wear particles as a source of zinc to the environment. persion of Z4 in the rubber matrix. Dosing (of nano-ZnO) Environ Sci Technol 38:4206–4214 lower than 3 phr could not impart any reinforcement. 5. Smolders E, Degryse F (2002) Fate and effect of zinc from tire debris in soil. Environ Sci Technol 36:3706–3710 Topographical images of SEM study indicates more 123 Int J Ind Chem (2017) 8:273–283 283 6. Heideman G, Noordermeer JWM, Datta RN, Baarle BV (2005) 19. Jincheng W, Yuehui CJ (2006) Application of nano-zinc oxide Effect of metal oxides as activator for sulphur vulcanisation in master batch in polybutadiene styrene rubber system. J Appl various rubbers. Kautschuk Gummi Kunststoffe 58:30–42 Polym Sci 101:922–930 7. 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Bhattacharyya S, Gedanken A (2008) A template-free, sono- 13. Przybyszewska M, Zaborski M (2009) New coagents in chemical route to porous ZnO nano-disks. Microporous Meso- crosslinking of hydrogenated butadiene-acrylonitrile elastomer porous Mater 110:553–559 based on nanostructured zinc oxide. Compos Interfaces 27. Yalcin H, Bozkaya O (1995) Sepiolite-palygorskite from the 16:131–141 Hekimhan region clay. Clay Miner 43:705–717 14. Przybyszewska M, Zaborski M (2010) Effect of ionic liquids and 28. Monshi A, Foroughi MR, Monshi MR (2012) Modified Scherrer surfactants on zinc oxide nanoparticle activity in crosslinking of equation to estimate more accurately nano-crystallite size using acrylonitrile butadiene elastomer. J Appl Polym Sci 116:155–164 XRD. World J Nano Sci Eng 2:154–160 15. Przybyszewska M, Zaborski M (2009) The effect of zinc oxide 29. Blow CM, Hepburn C (1982) Rubber technology and manufac- nanoparticle morphology on activity in crosslinking of carboxy- ture. Butterworth Scientific, London lated nitrile elastomer. Express Polym Lett 3:542–552 30. Shamugharaj AM, Bae JH, Lee KY, Noh WH, Lee SH, Rye SH 16. Guzman M, Reyes G, Agullo N, Borros S (2011) Synthesis of Zn/ (2007) Physical and chemical characteristics of multiwalled Mg oxide nanoparticles and its influence on sulfur vulcanization. carbon nanotubes functionalized with aminosilane and its influ- J Appl Polym Sci 119:2048–2057 ence on the properties of natural rubber composites. Comp Sci 17. Heideman G, Datta RN, Noordermeer JWM, Van Baarle B Technol 67:1813–1822 (2005) Influence of zinc oxide during different stages of sulfur 31. Maiti M, Sadhu S, Bhowmick AK (2005) Effect of carbon black vulcanization. Elucidated by model compound studies. J Appl on properties of rubber nanocomposites. J Appl Polym Sci Polym Sci 95:1388–1404 96:443–451 18. Kim I, Kim W, Lee D, Kim W, Bae J (2010) Effect of nano zinc 32. Maiti M, Basak GC, Srivastava VK, Jasra RV (2016) Mesoporous oxide on the cure characteristics and mechanical properties of the silica reinforced polybutadiene rubber hybrid composite. Int J Ind silica-filled natural rubber/butadiene rubber compounds. J Appl Chem 7:131–141 Polym Sci 117:1535–1543 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png International Journal of Industrial Chemistry Springer Journals

Influence of synthesized nano-ZnO on cure and physico-mechanical properties of SBR/BR blends

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Abstract

Int J Ind Chem (2017) 8:273–283 DOI 10.1007/s40090-016-0107-7 RESEARCH Influence of synthesized nano-ZnO on cure and physico- mechanical properties of SBR/BR blends 1 1 1 • • • Madhuchhanda Maiti Ganesh C. Basak Vivek K. Srivastava Raksh Vir Jasra Received: 6 June 2016 / Accepted: 7 November 2016 / Published online: 17 November 2016 The Author(s) 2016. This article is published with open access at Springerlink.com Abstract This study focuses on the synthesis of zinc oxide out using DSC. The results differ slightly from static curing (ZnO) nanoparticles by high temperature calcination as except PEG modified nano-ZnO. Use of ZnO nanoparticles well as low-temperature hydrolysis methods and their could provide faster crosslinking, better reinforcement at efficiency as cure activator in styrene-butadiene rubber/ lower concentration compared to reference ZnO. polybutadiene rubber blend. The synthesized nano-ZnO samples were characterized by means of X-ray diffraction, Keywords Nano-ZnO  SBR  BR  Curing  Cure BET surface area and transmission electron microscopy. properties The synthesized nano-ZnO samples had wurtzite structure and average particle size in the ‘nm’ range. ZnO nanoparticles, synthesized on sepiolite template, were of Introduction smallest particle size (maximum number of particles in the 2 -1 range of 7–12 nm) and highest surface area (104 m g ). The rubber industries, specifically tire industries, contribute Polyethylene glycol (PEG)-6000 coated ZnO nanoparticles significantly to economy of a nation where automobile had rod-like structure; average diameter of the rods was industry is growing at a very fast pace. Improvement in 50 nm. In the case of PEG-coated ZnO containing com- quality and safety of rubber products can have significant pounds, optimum cure time of the blend was decreased by impact on this industry [1, 2]. Zinc oxide (ZnO) is primarily 5 min compared to that of standard rubber grade-ZnO used as an activator for sulfur vulcanization of rubbers. containing compound (used as reference). Optimum cure Besides, inclusion of ZnO in the rubber compound brings time was lowered by 7–10 min in the case of synthesized other benefits viz., reduction in heat build-up, improvement nano-ZnO containing compounds compared to the refer- of abrasion resistance and heat resistance of the vulcanizates. ence ZnO based compound in presence of conventional Furthermore, its high thermal conductivity helps to dissipate filler, carbon black. It was also observed from ICP-OES local heat concentrations in rubber products. Zinc oxide is a analysis that the presence of very little amount of magne- necessary ingredient in rubber compounds for bonding rub- sium in one of the synthesized ZnO has noticeable impact ber to reinforcing steel cord, etc. Besides improving the on cure properties. PEG-coated ZnO increased the tensile properties of vulcanized rubbers, ZnO also assists in the strength of gum vulcanizates by 28% compared to the processing of uncured rubbers. ZnO is added to rubber for- reference ZnO, acting as nanofiller at 3 phr loading. The mulation to reduce shrinkage of molded rubber products and study of curing behavior in dynamic condition was carried maintain the cleanliness of molds [3]. The road transport emission of zinc due to tire wear is the main sources of zinc pollution after iron and steel & Ganesh C. Basak production and non-ferrous metals manufacture. This arises ganesh.basak@ril.com from the zinc content (1 wt%) of the tire-tread material [4, 5]. But some adverse environmental effects of zinc Reliance Technology Group, Vadodara Manufacturing exposure have been reported. In view of the upcoming Division, Reliance Industries Ltd., Vadodara, legislation and eco-labeling requirements for tires, it can be Gujarat 391346, India 123 274 Int J Ind Chem (2017) 8:273–283 stated that it is desirable to keep the ZnO content in rubber reported that nano-zinc oxides are effective activators and compounds as low as possible. reinforcing agents in rubber systems. The ‘‘little size In rubber industry, various kinds of vulcanization acti- effect,’’ ‘‘surface effect’’ and ‘‘quantum effect’’ of nano- vators like CaO, MgO, CdO, CuO, PbO and NiO have been ZnO governs the properties of the composites [21]. used in order replace conventional ZnO due to its toxic and Although considerable amount of work has been done so far fouling characteristics for aquatic flora and fauna. Although on the use of nano-ZnO in place of conventional ZnO as a among the various activators studied, MgO shows most cure activator and for enhancing the mechanical properties promising candidate in terms of activating properties in of elastomer, the study on SBR/BR-nano-ZnO composites is comparison to ZnO but maximum crosslinking can be scarcely available in the literature [22]. In the tire industry, achieved in the presence of ZnO only [6]. Moreover, few SBR/BR blend is of considerable importance as it is widely reports are also available that describe the effect of layered used in passenger car tire-tread compound. Hence, investi- double hydroxide (LDH) on elastomeric materials in the gation of nanocomposite based on SBR/BR blends and nano- place of ZnO. According to the literature reports, LDH ZnO would not only be providing valuable information but material can be used as an alternative cure activator in place also have wide applications. Typically SBR/BR blend shows of ZnO and stearic acid combo in the conventional cure slower curing rate than other general purpose rubbers such as package for the preparation of rubber composites, and NR and BR [3]. Hence, it will be of interest to study the cure simultaneously can provide a strong platform for reduction properties of this blend with nano-ZnO. of ZnO level in elastomer vulcanizate system [7]. In this work, we have studied the influence of mor- In another approach, the concentration of ZnO can be phology, specific surface area and dispersibility of ZnO minimized if the efficiency of ZnO during vulcanization can nanoparticles on the static and dynamic vulcanization of be enhanced by the maximization of the contact between the SBR/BR blends. We have studied the effect of sepiolite ZnO particles and the accelerators in the compound. This template and ‘eco-friendly’ metal oxide, magnesium oxide contact is dependent on the size, shape, specific surface area (MgO) on nano-ZnO in the crosslinking of the rubber and dispersibility of the ZnO particles. Nano-sized ZnO blend. The influence of nano-ZnO on the properties of particles have been paid more attention for their unique SBR/BR vulcanizates in the absence as well as in the properties, even though there are limited open literatures presence of conventional filler was also evaluated. available on nano-ZnO as cure activators. ZnO nanoparticles were studied as a cure activator and curing agent in natural rubber (NR), nitrile rubber (NBR), carboxylated nitrile rub- Experimental ber (XNBR) and chloroprene rubber (CR) by Bhowmick and his coworkers [8–10]. Similarly, it was used as cure activator Materials in NR and CR by Joseph et al. [11, 12]. Nanostructured zinc oxide was used in crosslinking of hydrogenated butadiene- Zinc nitrate [Zn(NO ) 6H O] [molecular weight (M.W.) 3 2 2 acrylonitrile elastomer and XNBR by Przybyszewska and 297.48, 98% purity], ammonium carbonate [(NH ) CO ] 4 2 3 Zaborski [13–15]. Guzman et al. synthesized mixed metal (M.W. 157.13, 31% purity), acetone (M.W. 58.08, 99.5% oxide nanoparticles of zinc and magnesium to reduce the ZnO purity), methanol (M.W. 32.04, 99.5% purity), sodium levels in rubber compounds [16]. Heideman et al. studied the hydroxide pellets (M.W. 40.00, 98% purity), 1-octanol influence of nano-ZnO on the cure properties of solution (M.W. 130.23, 99% purity), Stearic acid (M.W. 284.48, styrene-butadiene rubber (SBR) and ethylene–propylene– 98% purity), sulfur powder (M.W. 32.06, 99% purity), N- diene rubber [17]. Kim et al. investigated the effect of nano- cyclohexyl-2-benzothiazole sulfenamide (CBS) (M.W. ZnO on the cure characteristics and mechanical properties of 264.42, 97% purity), microcrystalline wax, magnesium the silica-filled natural rubber/butadiene rubber compounds oxide (MgO) were procured from Labort Fine Chem. Pvt. [18]. Jincheng and Yuehui studied the application of nano- Ltd., India. Standard rubber grade zinc oxide (ZnO), used ZnO master-batch in SBR [19]. as reference (designated as SZ), was supplied by Labort In our previous work, we have studied the effect of nano- Fine Chem. Pvt. Ltd., India. Polyethylene glycol-6000 ZnO on the cure properties of polybutadiene rubber (BR) (PEG) was obtained from Alfa Biochem, Greece. Zinc [20]. It was observed that the nano-ZnO reduces curing time acetate dihydrate [Zn (CH COO) 2H O] (M.W. 219.50, 3 2 2 and also enhances physico-mechanical as well thermal 98.5% purity) and Oxalic acid (M.W. 126.07, 99.8% pur- stability properties of butadiene rubber compound at lower ity) were procured from S. D. Fine Chem. Ltd., India. N- concentration compared to the conventional micro-ZnO. (1,3-dimethyl butyl)-N -phenyl-p-phenylenediamine However, to the best of our knowledge, the effect of nano- (6PPD) was obtained from John Baker Inc., USA. ZnO as cure activator has not yet been explored for SBR/BR Polybutadiene rubber (BR; Cisamer 01; ML at 1?4 rubber blend. In the open literature, it has already been 100 C = 45; cis-content 96%) was collected from 123 Int J Ind Chem (2017) 8:273–283 275 Reliance Industries Ltd., India. Styrene-butadiene rubber Method-4 [20] (SBR 1502; ML at 100 C = 48) was supplied by Japan 1?4 Synthetic Rubber, Japan. Sepiolite (Pangel S9) was gen- Equivalent volume of Zn (CH COO) 2H O (0.5 M) and 3 2 2 erously supplied by Tolsa, Spain. Carbon black (N330) was sodium hydroxide (1.5 M) were mixed to obtain a solution A. 2.5 g of PEG-6000 was dissolved in 10 ml of water to procured from Philips Carbon Black Ltd., India. obtain solution B. The solution B was then added into solution A to obtain solution C. 50 ml of 1-octanol was Preparation of nano-ZnO added to solution C under stirring at room temperature to obtain solution D. Then solution D was transferred to Nano-ZnO was synthesized by high temperature calcina- tion as well as low-temperature hydrolysis methods. The Teflon lined stainless steel autoclave which was then typical procedures are described below. maintained at 180 C for 4 h under autogenous pressure. The ZnO powder was obtained after filtering, washing and drying in oven at 120 C. The sample is designated as Z4. Method-1 [9] Method-5 Zn(NO ) 6H O and ammonium carbonate (NH ) CO 3 2 2 4 2 3 were, respectively, dissolved in distilled water at a con- The solution of Zn (CH COO) 2H O (0.1 M) was pre- 3 2 2 centration of 1.0 M. Zinc nitrate solution was then slowly pared in 50 ml methanol under stirring. 25 ml of NaOH dropped into the vigorously stirred (NH ) CO solution 4 2 3 (0.3 M) solution, prepared in methanol, was mixed with with molar ratio of 2:1 to prepare the precursor. A white above solution under continuous stirring to get the pH of precipitate occurred immediately on mixing of the two reactants between 8 and 11, and then 4 g of sepiolite was solutions. Stirring was done for 3 h to have complete added with vigorous stirring. It was then transferred into a precipitation. The white precipitate thus obtained was fil- Teflon lined sealed stainless steel autoclave and maintained tered and repeatedly washed with distilled water to remove at 150 C for 12 h under autogenous pressure. Subse- impurities and dried at 105 C for 6 h. Calcination of the quently, it was allowed to cool naturally to room temper- dried sample was carried out at 450 C in a muffle furnace. ature. After the reaction was complete, the resulting white The sample thus obtained is designated as Z1. solid product was washed with methanol, filtered and then dried in a laboratory oven at 100 C. The sample is des- Method-2 [23] ignated as Z5. 0.1 M aqueous solution of oxalic acid was added to 0.1 M Method-6 aqueous solution of Zn(CH COO) 2H O and the solution 3 2 2 was stirred for 4 h. The white precipitates thus obtained 0.1 M solution of Zn(CH COO) 2H O in 50 ml methanol 3 2 2 were filtered and washed with acetone and distilled water was prepared. To this solution, 0.5 mol of MgO was added. to remove impurities and dried at 120 C for 6 h. The dried 25 ml of NaOH (0.3 M) solution, prepared in methanol, sample was calcined at 450 C in a muffle furnace to was mixed with above solution under continuous stirring to remove CO and CO from the compound. The sample is get the pH of reactants between 8 and 11. After that it was designated as Z2. transferred into Teflon lined sealed stainless steel autoclave and maintained at 150 C for 12 h under autogenous Method- 3 [24] pressure. It was then allowed to cool naturally to room temperature. After the completion of the reaction, resulting The solution of Zn (CH COO) 2H O (0.1 M) was pre- white solid products were washed with methanol, filtered 3 2 2 pared in 50 ml methanol under stirring. 25 ml of NaOH and dried in a laboratory oven at 100 C. The sample is (0.3 M) solution, prepared in methanol, was mixed with designated as Z6. above solution under continuous stirring to get the pH of reactants between 8 and 11. These solutions were trans- Characterization of zinc oxide particles ferred into a Teflon lined sealed stainless steel autoclave and maintained at 150 C for 12 h under autogenous X-ray diffraction (XRD) pressure. It was then allowed to cool naturally to room temperature. After the reaction was complete, the resulting X-ray diffraction analysis was done using X-ray diffrac- white solid product was washed with methanol, filtered and tometer, Rigaku ‘‘Mini flex’’ model in the range of 10 to then dried in a laboratory oven at 100 C. The sample is 80 (=2h). The zinc oxide powder was deposited on the designated as Z3. sample holder uniformly. 123 276 Int J Ind Chem (2017) 8:273–283 Brunauer Emmet Teller (BET) surface area measurement (volume 50 cm ) for 3 min at 80 C and 60 rpm. The formulation is given in Table 1. It was chosen as a typical BET surface area determination was done from N tire-tread formulation. Amount of ZnO used (0.5, 1.5 and adsorption data measured at 77.4 K using micromeritics- 3 phr) was lower than that used (5 phr) in the conven- ASAP-2020 instrument. The samples were activated at tional formulations. The sample was then passed through 200 C for 20 min under vacuum (10 mmHg) prior to a cold two roll open mixing mill at a friction ratio 1:1.2. measurements. Five point BET surface area and total pore The curing studies were followed with an Oscillating Disc volume were measured. The average of five reading is Rheometer (ODR-2000, FLEXSYS) at 145 C tempera- reported here. ture and oscillating arc of 3 for 1 h. The samples were then compression molded at 145 C at optimum cure Transmission electron microscopy (TEM) time. Morphology of different ZnO samples was investigated by Physico-mechanical properties of rubber composites transmission electron microscopy (TEM) (JEOL 2010) having LaB filament, operating at an accelerating voltage Tensile test of 200 kV. ZnO powder samples were dispersed by ultra- sonication in acetone for 30 min. A copper grid was Tensile test of the sample was carried out according to immersed in and taken out of the suspension and dried at ASTM D412-98a on dumbbell shaped specimens using room temperature. Image analysis of the microphotographs Instron 3367 universal testing machine at ambient tem- was performed using UTHSCSA Image Tool for Windows -1 perature at a crosshead speed of 500 mm min . Average Version 3.00. It was used to determine the particle size of five samples is reported here. distribution. Hardness Differential scanning calorimetry (DSC) Hardness of each composition was obtained using Shore A Cure-studies were done using differential scanning calori- metric analysis. It was carried out using modulated DSC Durometer tester as per ASTM D 2240-97. (DSC 2910, TA Instruments, USA). The samples were -1 heated from ambient temperature to 250 C (at 5 C min Volume fraction of rubber (V ) and crosslink density heating rate) in air. 5 mg of each sample was taken for the measurement. The error limit in the ‘weighing measure- The cured samples were immersed in toluene for 72 h at ments’ was within ±5%. 25 C temperature. The volume fraction of rubber in the swollen gel, at equilibrium swelling, was calculated using Elemental analysis Eq. (1): ðÞ D  FT q Elemental analysis was done using a Perkin Elmer (Model: V ¼ ; ð1Þ 1 1 ðÞ D  FT q þ A q Optima 4300 DV) inductively coupled plasma-optical 0 r s emission spectroscopy (ICP-OES). where D Deswollen weight, F weight fraction of the insoluble component, T initial weight of the test specimen, Scanning electron microscopy (SEM) -3 q density of rubber, 0.89 g cm , q density of solvent, r s -3 0.86 g cm , A amount of solvent absorbed. SEM samples were fractured in liquid nitrogen immersion -1 Further, the crosslink density, ; in mol g of rubber 2M and mounted with carbon tape wrapping. The images were hydrocarbon was calculated using the Flory–Rehner studied with a Nova NanoSEM 650 instrument, FEI, USA, Eq. (2): operating at 1 and 10 kV for the micro and synthesized nano-ZnO samples, respectively. 1=3 qVV  V =2 r r lnðÞ 1  V þ V þ vV ¼ ; ð2Þ r r Preparation of rubber composites 2M v Flory–Huggins interaction parameter, 0.46 for BR- Compounding and vulcanization toluene system [25], V molar volume of swelling solvent, toluene, M number average molecular weight of the chain ZnO was mixed with rubber by melt mixing method using between two crosslinks. Brabender Plasticorder (PL2000, Germany) internal mixer 123 Int J Ind Chem (2017) 8:273–283 277 Results and discussion Characterization of nano-ZnO XRD Figure 1 shows the XRD patterns of different zinc oxide (ZnO) samples. The sharp intense peaks, confirming the good crystalline nature of synthesized ZnO, correspond to (100), (002), (101), (102), (110), (103), (200), (112), (201) and (004) planes. All of the indexed peaks in the obtained diffractograms match with that of the bulk ZnO (JCPDS card # 79-0207) which confirm that the synthesized sam- ples are of wurtzite hexagonal structure [26]. Any other peak related to impurities was not detected in the diffrac- togram within the detection limit of the XRD. Absence of any extra peak in the diffractograms of final products indicates the purity of the products. In Z5, additional peaks can be observed, other than the earlier mentioned peaks for ZnO. These are for (060), (131) [at 20], (260) [at 24] and (080, 331) planes [at 27,28] of sepiolite clay [27]. It proves that ZnO particles are formed on sepiolite without distorting the crystal structure of either material. The average crystal size was calculated by Scherrer Eq. 28]: Kk L ¼ ; ð3Þ b cos h where, b is the full-width at half maximum (FWHM) of the peak corresponding to (100) plane, K is a constant (0.89), k is the incident wavelength of CuK radiation (k = 0.154 nm), L is the crystallite size, and h is the diffraction angle at a certain crystal plane. The average crystallite size of Z1, Z2, Z3, Z4, Z5 and Z6 was calculated using Eq. (3) and was found to be 23, 20, 18, 27, 27 and 16 nm, respectively. It should be noted that crys- tallite size is assumed to be the size of a coherently diffracting domain. It is not necessarily the same as particle size [28]. BET surface area BET surface area of different prepared nano-ZnO is reported in Table 2. Surface area and pore volume both increase in the synthesized ZnO samples compared to the reference one. Highest surface area and pore volume can be observed in the case of Z5. This could be due to dispersion of ZnO particles on the fibrous sepiolite template surface. For the same sample, smallest particle size was also observed through TEM (Fig. 2e). So the surface area results corroborate well with the microscopic study. The sample Z4 shows minimum surface area among the synthesized samples, as it is coated with PEG. The organic coating of PEG resists the nitrogen to be absorbed on the surface of ZnO particles. Table 1 Formulation and designation of different rubber compounds Ingredients SBWZ SBSZ SBZ1 SBZ2 SBZ3 SBZ4 SBZ5 SBZ6 0.5SBZ4 1.5SBZ4 0.5SBZ5 1.5SBZ5 SBSZF SBZ4F SBZ5F SBR 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 BR 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 Sulfur 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 CBS 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 ZnO 0 3 3333 330.5 1.5 0.5 1.5 3 3 3 6PPD 2 2 2222 222 2 2 2 2 2 2 Wax 1 1 1111 111 1 1 1 1 1 1 Stearic acid 0 2 2222 222 2 2 2 2 2 2 Carbon black 0 0 0000 000 0 0 0 60 60 60 Naphthenic oil 0 0 0000 000 0 0 0 8 8 8 Formulation of compounds is expressed in phr (parts per hundred rubber) 278 Int J Ind Chem (2017) 8:273–283 Fig. 1 X-ray diffractogram of the different zinc oxide samples Sample Z6 has maximum particles in the range of Table 2 Crystallite size of different zinc oxide samples 10–18 nm. Sample FWHM of (100) Average crystallite plane (degree) size (nm) Application of synthesized nano-ZnO Z1 0.3444 23 Z2 0.3936 20 Cure properties Z3 0.4428 18 Z4 0.2952 27 The effect of synthesized nano-ZnO as cure activator has Z5 0.2952 27 been studied on SBR/BR blend. A representative rheo- Z6 0.4920 16 graphic profile of SBR/BR blends at 145 C is shown in Fig. 4 and cure time is tabulated in Table 3. In the absence of ZnO, the curing is extremely slow in the sample SBWZ TEM and modulus is also lowest. The optimum cure time is faster with synthesized nano-ZnO samples by complex Figure 2a–f portrays TEM photo-micrographs of different formation with acceleration compared to the reference one. zinc oxide (ZnO) samples. TEM image exhibits the mor- ZnO helps in producing vulcanization precursor, hence phology of synthesized particles to be in nano region. faster curing can be observed in the presence of ZnO [29]. Samples Z1, Z2, Z3, Z5 and Z6 show hexagonal structure. Due to decrease in particle size of ZnO, the area of contact The particle size distribution curves for these samples are increases which helps to react better with accelerator. This shown in Fig. 3. It shows that for Z1, maximum particles leads to the generation of vulcanization precursor quicker. are in the range of 26–50 nm; for Z2, it is also in the range It results in a faster curing rate and lower cure time. Fastest of 26–50 nm and for Z3, it is in the range of 15–28 nm. curing can be seen with the use of organo-coated ZnO, Z4, The sample Z4 evinces rod-like structures grown on PEG- followed by Z5. Due to the presence of long-chain organic sheets. The average rod diameter is *50 nm. These nano- PEG molecules, it has more compatibility with elastomeric rods are of 100–200 nm in length. Z5 exhibits an inter- matrix leading to better dispersion. The curing reaction is esting morphology; it consists of smallest ZnO particles. not affected and slowed down in the presence of Z4 at Figure 2e infers that ZnO nanoparticles are grown on long lower dose, i.e., 0.5 and 1.5 phr. It indicates that dispersion bundles of sepiolite nanofibers. The ZnO particles are very of ZnO plays a major role in efficient vulcanization, rather small in size; most of the particles are in 7–12 nm range. 123 Int J Ind Chem (2017) 8:273–283 279 Fig. 2 TEM image of the sample a Z1, b Z2, c Z3, d Z4, e Z5 and f Z6 Fig. 4 Representative rheographic profiles of SBR/BR blends con- Fig. 3 Particle size distribution curves of different ZnO samples taining different ZnO samples 123 280 Int J Ind Chem (2017) 8:273–283 than higher loading of ZnO. The difference in minimum based sample. Increased DS indicates resistance to polymer (M ) and maximum (M ) torque value, [DS = (M - chain mobility [30]. Due to the formation of increased L H H M )] has increased in SBR/BR blend with the use of syn- crosslinks, chain mobility is restricted which will lead to thesized nano-ZnO compared to that of reference ZnO higher crosslink density. In the case of Z5, the template, sepiolite which is fibrous clay with high aspect ratio, can help in better distribution of ZnO particles. As mentioned Table 3 BET value of different nano-ZnO samples earlier, the area of contact increases in such case and helps 2 -1 3 -1 Sample Surface area (m g ) Pore volume (cm g ) in generation of vulcanizing precursors faster. These nano- ZnO samples impart better co-curing of both the rubbers in Reference ZnO, SZ 5 0.025 SBR/BR blend. Z1 18 0.123 The presence of very little amount of magnesium in Z6 Z2 11 0.084 (Zn:Mg 90:1, as revealed from ICP-OES analysis) has visi- Z3 39 0.110 ble impact on cure properties. It increases the optimum cure Z4 5 0.039 time but at the same time it also increases DS values. It Z5 104 0.230 produces maximum number of crosslinks but at a slower rate. Z6 55 0.110 Most encouraging effect of synthesized nano-ZnO on curing can be observed in the presence of filler. Optimum cure time is lowered by 7–10 min in the case of synthe- sized nano-ZnO containing compounds compared to the reference ZnO based compound (which even contains higher dose of ZnO). Thus, ZnO nanoparticles can help in the reduction of the production cycle and also in mini- mizing the Zn-pollution due to lower dose. Cure behavior in dynamic condition is shown in the representative plots (Fig. 5). The results differ slightly from static curing, though Z4 shows most efficient -1 crosslinking activities (DH = 123.70 Jg ) in dynamic curing, too (Table 4). Physico-mechanical properties of different rubber composites The nano-ZnO particles may also act as nano-fillers. Phy- Fig. 5 DSC curves of different ZnO containing SBR/BR compounds depicting dynamic curing sico-mechanical properties of different rubber composites Table 4 Cure properties of Sample Cure time, t (min) M (lb-in) M (lb-in) M - M (lb-in) 90 H L H L different elastomeric compounds a SBWZ 38.2 (0.08) 20.28 (0.07) 3.97 (0.14) 16.31 SBSZ 18.5 (0.10) 27.01 (0.10) 4.87 (0.13) 22.14 SBZ1 17.0 (0.09) 27.43 (0.15) 3.62 (0.09) 23.81 SBZ2 16.9 (0.07) 28.03 (0.20) 3.72 (0.10) 24.31 SBZ3 16.4 (0.12) 28.35 (0.04) 4.12 (0.08) 24.23 SBZ4 12.9 (0.07) 27.85 (0.13) 4.05 (0.02) 23.80 SBZ5 13.0 (0.11) 28.74 (0.09) 4.48 (0.04) 24.46 SBZ6 19.2 (0.08) 30.16 (0.07) 5.03 (0.05) 25.13 0.5SBZ4 12.2 (0.07) 27.93 (0.13) 4.42 (0.04) 23.51 1.5SBZ4 12.5 (0.10) 27.87 (0.14) 4.43 (0.03) 23.44 SBSZF 25.3 (0.07) 35.83 (0.21) 6.95 (0.12) 28.88 SBZ4F 15.6 (0.09) 43.76 (0.23) 8.01 (0.14) 35.75 SBZ5F 18.0 (0.04) 39.88 (0.24) 7.45 (0.11) 32.43 Values in parentheses are standard deviations 123 Int J Ind Chem (2017) 8:273–283 281 are reported in Table 5. Z2, Z3, Z4 and Z5 containing 3 phr loading. Tensile strength, 100% modulus and hard- SBR/BR blend based nanocomposites show better tensile ness decrease with decreasing ZnO loading. strength and hardness compared to the reference one. The Nano-ZnO containing compounds show slightly better sample containing organo-coated ZnO (Z4) displays high- properties than those of reference ZnO based compound. est tensile strength. This is ascribed to the better compat- Z5 based compound exhibits highest overall properties. ibility and in turn better dispersion of Z4 in the rubber This may be due to some synergistic effect between matrix. This leads to better curing as observed in the pre- nanofiller, sepiolite and conventional filler, carbon black vious experiments and higher value of volume fraction of [31]. The similar kind of effect has also been studied in our rubber (V ) as well as crosslink density. Z4 imparts highest previous study using mesoporous silica as reinforcing filler reinforcement. From these results it can be concluded that in the poly butadiene rubber matrix in the presence of dispersion of nanoparticles plays the major role in nanoclays, silica and carbon black [32]. enhancement of properties. Though lower filler loading maintains the cure proper- Morphology ties unaltered (as observed in 0.5SBZ4 and 1.5SBZ4) but it does not provide the same amount of reinforcement as The topographical images of SEM shown in Fig. 6 of different SBR/BR blend either having synthesized nano- ZnO or standard rubber grade-ZnO is studied to evaluate the extent of dispersion of ZnO within the rubber blend. Table 5 Dynamic cure properties of different elastomeric The black phase implies the rubber matrix, whereas the compounds white dot is the reflection of ZnO particles. In SEM images -1 Sample T (C) DH (J g ) of SBSZ (Fig. 6a), some agglomeration of ZnO nanopar- max ticles in the form of white dot can be seen. Figure 6b SBSZ 261 63.66 indicates that uniform dispersion of nano-ZnO occurs SBZ2 261 88.39 throughout the entire blend in comparison to standard SBZ3 262 81.06 rubber grade-ZnO (Fig. 5a). In case of SBZ4 (Fig. 6c), the SBZ4 258 123.70 rod-like structure as observed in TEM photo-micrographs SBZ5 254 86.58 (Fig. 2d) can also be seen. However, in the image of SBZ6 260 92.92 sepiolite based synthesized nano-ZnO (Fig. 5d), the homogenous distribution of ZnO with minimum particle Fig. 6 SEM images of a SBSZ, b SBZ3, c SBZ4 and d SBZ5 123 282 Int J Ind Chem (2017) 8:273–283 Table 6 Physico-mechanical properties of different rubber composites Sample Tensile strength Modulus at 100% Elongation at Hardness V Crosslink -5 -3 (MPa) elongation (MPa) break (%) (shore A) density 9 10 mol cm SBWZ 1.73 (0.02) 0.70 (0.02) 445 (10) 37 (0.3) 0.112 (0.003) 2.50 (0.01) SBSZ 1.89 (0.02) 0.84 (0.03) 310 (12) 41 (0.2) 0.165 (0.002) 6.31 (0.02) SBZ1 1.82 (0.03) 0.73 (0.03) 320 (15) 43 (0.3) 0.164 (0.002) 6.22 (0.03) SBZ2 2.12 (0.03) 0.78 (0.04) 335 (18) 43 (0.4) 0.168 (0.003) 6.60 (0.01) SBZ3 2.07 (0.04) 1.09 (0.03) 265 (22) 45 (0.3) 0.164 (0.004) 6.22 (0.01) SBZ4 2.54 (0.01) 1.05 (0.02) 295 (14) 44 (0.4) 0.171 (0.002) 6.90 (0.02) SBZ5 2.05 (0.05) 1.07 (0.02) 250 (18) 45 (0.2) 0.167 (0.002) 6.50 (0.03) SBZ6 1.90 (0.03) 0.90 (0.03) 280 (12) 46 (0.1) 0.169 (0.003) 6.70 (0.04) 0.5SBZ4 1.61 (0.02) 0.72 (0.02) 335 (10) 37 (0.3) 0.165 (0.002) 6.31 (0.02) 1.5SBZ4 2.15 (0.01) 0.94 (0.03) 290 (17) 41 (0.2) 0.169 (0.004) 6.70 (0.01) SBSZF 14.30 (0.5) 1.62 (0.02) 560 (09) 61 (0.1) 0.187 (0.005) 8.65 (0.04) SBZ4F 14.84 (0.2) 1.67 (0.01) 565 (08) 63 (0.2) 0.191 (0.005) 9.13 (0.03) SBZ5F 15.24 (0.3) 1.82 (0.01) 645 (10) 63 (0.3) 0.190 (0.007) 9.01 (0.02) Values in parentheses are standard deviations size is observed (as seen in TEM images too) (Fig. 2e). As uniform dispersion of synthesized nano-ZnO over standard a result, the improvement of mechanical properties of the rubber grade-ZnO within rubber blend and this fact account SBR/BR blend can be noticed (Table 5). From the SEM for better mechanical properties. Nano-ZnO imparted faster images, it can be distinguished that the distribution of ZnO curing even in the presence of conventional filler, carbon in the SBR/BR blend is comparatively better for nano-ZnO black compared with reference ZnO. Thus, the use of ZnO which in turn is reflected in mechanical properties nanoparticles can provide faster curing, better reinforce- (Table 6). ment at lower dosing compared to standard ZnO, which can lead to shorter production cycles and less zinc pollution. Acknowledgements The authors are highly thankful to Ms. Hetal Conclusions Patel and Mr. Chirag S. Shah for their kind cooperation. Authors are grateful to Reliance Industries Ltd. for its consent to publish this Six different nano-ZnO samples were synthesized by both work. Authors are also thankful to colleagues from catalyst, analytical high temperature calcination and low-temperature hydrol- and elastomer groups of RTG-VMD for their support. ysis methods. All the samples had wurtzite structure and Open Access This article is distributed under the terms of the Creative average particle size in the ‘nm’ range. ZnO, grown on Commons Attribution 4.0 International License (http://creative sepiolite nanofiber, showed smallest particle size as well as commons.org/licenses/by/4.0/), which permits unrestricted use, distri- highest surface area. PEG-coated ZnO nanoparticles were bution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the rod-like in structure. 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International Journal of Industrial ChemistrySpringer Journals

Published: Nov 17, 2016

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