# Physical Hybrid of Nanographene/Carbon Nanotubes as Reinforcing Agents of NR-Based Rubber Foam
## 2 Materials and Methods  
Standard Malaysian NR (SMR) with a trade name of SMR-20 (Mooney viscosity of ML (1 + 4) @100 \({}^{\circ}\)C) was obtained from the Malaysian Rubber Company, Sungai Laling, Malaysia. Non-functionalized (pristine) carbon nanotubes (CNTs) with an outer diameter of 8-15 nm (95% \(<\) purity) was purchased from Notrino, Iran. C-750 graphene nanosheets (GNSs) with a surface area of 750 m\({}^{2}\)/g and an average thickness of 1-5 nm were obtained from XG Science (Lansing, MI, USA). Sulfur, Carbon black, DMF, paraffin oil, MBTS(2-2'-Dithiobis benzothiazole), CBS (N-cyclohexyl-2-benzothizolesulfenamide), stearic acid, and ZnO were commercial grades. The chemical blowing agent azodicarbonamide (ADC) was provided by the Letai Chemical Plant (Beijing, China).  
Before compounding, CNTs and GNSs were kept in an oven at 90 \({}^{\circ}\)C for 15 h to eliminate trace moisture. We dispersed 3.5 g of the dried CNTs and GNSs in 200 mL DMF (solvent) in a separate beaker. Each sample was exposed to an ultrasonic process (400 W) in three 5-min steps to improve the dispersion quality.  
An open two-roll mill was then used for compounding the formulation at an ambient temperature. For this, 20 g of NR was masticated in a two-roll mill for 1 min and dissolved in 200 cc of DMF under stirring. The pre-prepared CNTs/DMF solution was then added to half of the NR solution, and the GNSs/DMF solution was added to the other half of the NR solution and placed separately in an ultrasonic bath for two 30-min steps. The two dispersions were then mixed and placed in the same ultrasonic bath for 5 min. To obtain a masterbatch, the samples were dried in a vacuum oven at 70 \({}^{\circ}\)C until all DMF was removed and the samples' weight remained unchanged. Table 1 shows the compounding formulations. First, 57 g NR was masticated in a two-roll mill for 5 min, and after sufficient mastication, the masterbatch was carefully added, and the compounds were milled for 3 min. Other compounding ingredients, except for the curing agent, were then fed gradually into the mill, and the compounding continued for 2 min. In the end, the curing agent was added, and the compounding continued for another 2 min.  
The cure behavior of the compounds was determined using an Oscillating Disk Rheometer (ODR) model 4308 (Zwick Co., Ulm, Germany) at a temperature of 160 \({}^{\circ}\)C, according to ASTM D22044. An optical microscope was employed to study the foam morphology, cell size, and cell size distribution. Resilience was measured at a temperature of 25 \({}^{\circ}\)C using a resilience tester (Frank GMBH, Birkenau, Germany), in accordance with ASTM D1054. A Shore-A type hardness test was performed at room temperature to measure the stiffness of the foams by a hardness tester (Zwick Co. 3100, Germany) according to ASTM D2240. To study the nanofiller's effect on the NR foams' mechanical properties, a compression test was performed using (Hiwa-400, Urmia, Iran) according to ASTM D412. Thermogravimetric analysis (TGA) was carried out to estimate the thermal stability of the foams using a Pyris TGA (Perkin-Elmer, Waltham, MA, USA) in the temperature range of 25 \({}^{\circ}\)C to 1000 \({}^{\circ}\)C under an oxygen atmosphere at a heating rate of 10 \({}^{\circ}\)C/min. To study the dynamical mechanical properties of samples, DMTA test was carried out using (DMA 2000 TRITEC, Edinburgh, England). The test was performed by applying a bending force with a frequency of 4 Hz at a heating rate of 10 \({}^{\circ}\)C/min in the temperature range of \(-\)100 \({}^{\circ}\)C to 100 \({}^{\circ}\)C.