An experimental study on one-step and two-step foaming of natural rubber/silica nanocomposites

The curing and cellular structure of natural rubber (NR)/silica composite foams were investigated. The presence of an activator in the rubber formulation significantly lowered the decomposition temperature of the azodicarbonamide foaming agent, which allowed foaming before NR curing. Therefore, two foam methods were designed: foaming initially at 90°C and then curing at 140°C, and foaming and curing simultaneously at 140°C. Two-step foaming generated a lower cell density and higher cell size. Incorporation of nano silica into NR increased the foam density, but decreased the cell size. The higher foaming temperature restricted the bubble growth because of a higher curing rate and inhibited cell coalescence.


Introduction
Nanomaterials have received widespread attention for numerous applications . Additionally, there is a critical need for solid materials, which simultaneously provide the processing of polymers and the high electrical properties of metals [27][28][29][30][31][32][33][34][35][36][37][38]. Today, polymeric foams have attracted much attention from different industries. Elastomeric foams have greater flexibility than plastic foams, and thus, they are applied in specific uses such as soft insulation. The properties of foams are influenced by their density and cellular structure, which in turn is governed by the foaming process and conditions [39]. The foaming of rubber is most often conducted by physical or chemical techniques. The control of cell size and cell distribution in rubber foams is more challenging than in thermoplastic foams since simultaneous foaming and curing occur [40][41][42]. Therefore, a balance between foaming and curing progress is essential. In the chemical method, bubble growth and curing are developed by heating. Consequently, suitable thermal conditions should be determined.
Commercially available rubber foams are produced from ethylene propylene diene monomer rubber (EPDM), styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), natural rubber (NR), and polyurethane (PU). NR-based foams have exceptional properties such as low-temperature flexibility, excellent tear strength, and very good resilience [43,44]. Several studies have been performed on the properties and performance of NR foams. In most studies in this field, NR latex was used to produce foam [45,46]. However, a limited number of studies have been reported on the preparation of foams from solid NR. Najib et al. investigated mechanical performance and sound absorption of NR foams [47]. The foams with smaller cell sizes displayed higher sound absorption performance. Similar work on these properties was performed by Harnnarongchai [48]. The impact of temperature and rubber type on the properties of NR foams was also reported [49]. Moreover, it was stated that adding carbon black decreased the cell size and increased the cell density of NR foams [44,50]. The impact of blowing agent content on cure characteristics and mechanical and morphological properties of NR foams was reported by Pechurai et al. [51] who used 4,4′-oxydibenzenesulfonyl hydrazide (OBSH) as a foaming agent. Additionally, a combination of amino alcohol and carbonic acid [52] and a combination of sodium bicarbonate and azodicarbonamide were also considered as blowing agents [53,54]. A high level of the blowing agent led to more cell coalescence [54]. The effect of adding silica nanofiller on the curing and cellular structure of NR foams has been studied in our previous work [55,56]. It was found that the curing system of NR acted as an efficient activator for the blowing agent and promoted its decomposition. In addition, the blowing agent in the formulation of NR can promote the curing rate because of its exothermic decomposition. This finding allowed us to design one-step and two-step foaming processes, which were not studied in our previous work.
In this work, the cellular characteristics of one-step and two-step foaming were compared. In the one-step route, foaming and curing were performed simultaneously at 140°C. In the two-step route, foaming was performed at 90°C and curing was accomplished at 140°C. Moreover, the effect of incorporated silica particles was investigated.

Materials
Natural rubber (NR) ribbed smoked sheet 1 (RSS1) as the matrix was supplied by Thomson Enterprise Co. (India). Precipitated silica (ULTRASIL VN3) was purchased from Evonik Industries (Germany) as nanofiller with a specific surface area of 180 m 2 /g and an average particle size of 15 nm. Tetramethyl thiuram disulfide (TMTD) and N-cyclohexyl-2-benzothiazole sulfonamide (CBS) as accelerators and activated azodicarbonamide (porofor ADC/K) as the blowing agent were received from Lanxess Co. (Germany). Zinc oxide (ZnO) as the activator, sulfur as the curing agent, stearic acid, and paraffin oil were obtained from local companies.

Compounding of samples
The mixing of components was performed by an internal compounder (Brabender W50 EHT) at 60°C at a rotating speed of 80 rpm. First, NR was added into the mixer, and then silica and paraffin oil were gradually added within about 4 min. Subsequently, other ingredients consisting of the activator, accelerators, sulfur, blowing agent, and stearic acid were added into the mixer. The total mixing time for all samples was set at 15 min. Sample codes and compositions are introduced in Table 1.

Foam preparation
Two routes for preparing NR foams were designed. In the one-step method (route 1), foaming and curing of samples were conducted at 140°C for 20 min in a hot oven. The samples were cooled at room temperature after foaming. In the two-step method (route 2), first foaming was conducted for 10 min at 90°C, and then curing was performed at 140°C for 20 min using a hot oven.

Characterizations
An oscillating disc rheometer (ODR; Hiwa 100, Iran) was used to measure the rheology of the samples at temperatures of 90°C and 140°C in accordance with the ASTM D2084. Foam density was determined by the displacement method according to the ASTM D792. Scanning electron microscopy (SEM; VegaII Tescan, Czech) was applied to analyze the cell morphology. Samples were coated before this test with a thin layer of gold for better resolution. The average cell size of the bubbles was calculated by the ImageJ software from at least 50 different cell measurements. The cell density of foams was determined using equation (1): where N is the cell density, d is the cell size (diameter), ρ r is the solid rubber density, and ρ f is the foam density. The cell wall thickness, δ, of NR foams was calculated using equation (2): 3 Results and discussion 3.1 Curing behavior Figure 1 shows the curing plots of all samples at 90°C and 140°C. The Y-axis in this figure is the difference between the minimum torque, M L , and maximum torque, M H , in the ODR test. As can be seen, the compounds at 90°C were not cured until 30 min, and the torque for all samples was close to zero. However, at 140°C, the materials cured at different rates. By addition of silica content from 0 to 40 phr, torque was enhanced due to increased viscosity from the added nanofiller. The degree of conversion for all samples was calculated as follows: where M t is the torque value at time t. Figure 2 displays the conversion curves of samples at 140°C. As can be seen, the curing reaction of foam samples (S0-S40) was  faster than the PR (non-foam) sample. This result was due to the exothermic decomposition of blowing agent that accelerated the curing reaction of rubber [56]. The torque for the PR sample was slightly higher than S0 because S0 was a foam, while PR was not; therefore, the finial modulus of PR was higher. Nonetheless, the scorch time and curing time of SX were explicitly lower than PR. This observation confirmed that the addition of nanofiller had a negligible impact on the curing reaction, while the blowing agent clearly promoted curing [56]. Figure 3 shows the density of the foams versus silica content for different foam processing. The addition of silica content up to 40 phr increased the foam density from approximately 0.3 to 0.6 g/cm 3 . This result might be related to the higher density of mineral particle when compared with the natural rubber. Moreover, compounds containing a higher silica level had higher viscosity (Figure 1b), which could restrict the bubble growth. However, the densities of samples from the two-step foaming process were 7% to 22% lower than foams prepared in one-step at 140°C. In two-step foaming, bubbles first grew and then the rubber cured. Thus, there was no limitation such as crosslinking on the bubble growth. Viscosity, which is a barrier to cell growth, was higher at 90°C when compared with 140°C. Therefore, the bubble growth rate might be slower in the two-step foaming process. However, the curing reaction did not occur at 90°C. As the curing reaction progressed, the viscosity increased and bubble growth was prevented. The scorch time of samples at 140°C was between 3 and 7 min (Figure 2). Hence, in the one-step foaming at 140°C, bubbles had less time to grow and foams with higher density were produced. Figure 4 shows the cell structure of samples foamed by one-step and two-step foaming. According to the SEM  figures, many closed cells in addition to a few open cells were observed. The addition of silica resulted in smaller cell size and more uniform distribution. However, the cell diameter from the two-step foaming process was not uniform and was larger when compared with the one-step foaming. Additionally, cell coalescence even at 40 phr silica appeared. Apparently, natural rubber at 90°C does not have enough strength to hold the gas inside bubbles and prevent cell coalescence over a long time. Figure 5 illustrates the cell size of samples versus silica content. A higher silica content led to a lower cell diameter. For instance, the cell size of S0, which excluded silica, was about 580 µm, while for S40 the cell size was 200 µm. The cell sizes of foams prepared by route 1 were between 15% and 30% smaller than foams prepared by route 2. Figures 6 and 7 show the prepared cell size distribution of foams prepared by routes 1 (one-step) and 2 (two-step), respectively. As seen, adding silica resulted in a smaller cell size and a narrower and more uniform cell distribution.  An experimental study on one-step and two-step foaming of natural rubber/silica nanocomposites  431  On the other hand, the cell density and cell wall thickness increased by adding silica due to the nucleating effect of silica particles (Figure 8). The cell density of S40 prepared by route 1 was approximately 10-fold higher than S0 and two-fold higher than S40 prepared by route 2. Therefore, the performance of silica as a nucleating agent in route 2 was inferior due to the lower temperature providing less energy for nucleation.

Conclusions
The curing and cell structure of natural rubber filled silica composite foams prepared by one-step and twostep foaming processes were compared. The addition of blowing agent to the NR compound noticeably promoted the curing rate and lessened the scorch time. The presence of silica had a negligible impact on the curing behavior. Two-step foaming caused a decrease in foam density and an increase in cell density. Moreover, the two-step foaming route caused the larger cell size and nonuniform cell distribution. The average cell size from route 1 was 15-30% lesser than route 2. At the higher temperature, faster curing restricted the bubble growth. In contrast, unstable grown bubbles at 90°C due to the lack of curing generated larger cells with a nonuniform distribution along with increased cell coalescence.