Heat-resistant antiflaming and friction mechanisms in nano-Fe₂O₃-reinforced silicon rubber

2.1 Preparation of material

To avoid affecting the physico-mechanical properties of SR, the nano-Fe₂O₃ particles were added to the original formula. The size of the added nano-Fe₂O₃ particles was between 50 and 100 nm. The SR and all additional ingredients were mixed in an XK-150 (Qingdao Ltd., Shandong province of China) mixer at 50±5°C and 35 rpm for 15 min. The basic formulation was as follows: 80 phr nonvulcanized SR, 2 phr zinc oxide, 1 phr sulphur, 1 phr accelerant, 1 phr stearic acid, 20 phr carbon black, and 10, 20, or 30 phr nano-Fe₃O₄ particles were added as a fraction of the total raw material used. Then, the mixed Fe₂O₃/SR composites were weighed. The mass difference between the mixed SR and all the original materials used had to be within of ±1%, or the rubber was remixed. Then, the specimens were put on a smooth, clean, metal plate for 6 h before 30-min vulcanization at 150.0°C. The Fe₂O₃/SR composites were magnetized using a U5-10 magnetizer (Dongguan, Guangdong province of China) for 15 min.

2.2 Performance test

The ambient temperature was 23±2°C, and the relative humidity was 30% to 50%. The tensile strength and maximum elongation of the specimens were measured (after vulcanization) with a DXLL-10000 (Jiangdu, Jiangsu province of China) electronic tensile tester. The size of the samples was 6 mm×2 mm. The Shore A hardness was characterized using a MC010-TH200 rubber durometer with a sample size of ϕ 24.5×8 mm. The microstructure of the Fe₂O₃/SR and the distribution of nano-Fe₂O₃ particles in SR and magnesium hydroxide were analyzed using a JSM-5600LV electron microscope (JEOL Company, Japan) and energy dispersive X-ray spectrometry. Thermo-gravimetric analysis was carried out using a thermo-gravimetric analyzer (TGA) 27 type thermo-gravimetric analyzer with an N₂ gas shield and a heating rate of 10°C/min. The combustion performance was tested by standard tapered calorimeter with a thermal power of 35 kW/m^-2. The friction properties of the composites were tested using a universal test machine (UTM) with a sample size of 6 mm×2 mm. The reciprocating frequency of the UTM was 10 Hz, and over a travel of 10 mm, a test duration of 5 min was used.

3.1 Microstructure

The microstructure of Fe₂O₃/SR composites is shown in Figure 1. Nano-Fe₂O₃ particles were uniformly distributed in the rubber, and no obvious aggregation was observed when the nano-Fe₂O₃ content was 10 phr, as shown in Figure 1A. However, with increased addition of nano-Fe₂O₃, the aggregation of nano-Fe₂O₃ particles increased because they were are drawn together due to magnetism and nonuniform mixing, as shown in Figure 1B. As expected, the aggregation of nano-Fe₂O₃ particles was intensified by adding more nano-Fe₂O₃ particles, as shown in Figure 1C.

Figure 1

SEM images of the Fe₂O₃/SR composites at various nano-Fe₂O₃ contents.

As shown in Figure 1, the SR-nano-Fe₂O₃ interfaces were fuzzy, indicating that they had good mutual affinity. The specific surface area of the Fe₂O₃ nanoparticles was high, so the atomic ratio on the surfaces was also relatively high, which explains why nano-Fe₂O₃ particles had a high-energy surface and SR had low-energy surfaces. When mixing the two materials, the high-energy SR surface was inclined to be strongly adsorbed on the high-energy surface of the nano-Fe₂O₃ particles. This kind of adsorption can compensate for the imbalanced force field on the high-energy surface; meanwhile, nano-Fe₂O₃ can fill surface defects on SR. Thus, nano-Fe₂O₃ can greatly reinforce the SR; also, the double dipole role between nano-Fe₂O₃ and the SR matrix was beneficial for a closer-knit combination between the two phases.

Table 1 shows that percentage composition, atomic percentage composition of Fe elements, and X-ray counts were basically in line with the addition of nano-Fe₂O₃. With increased filler concentration, the region of aggregated nano-Fe₂O₃ was enlarged. However, in terms of the overall effect, mixing was relatively uniform. It could be expected that the uniform nano-Fe₂O₃ particles were conducive to improvements in the composites’ performances.

3.2 Physical and mechanical properties

The tensile strength and maximum elongation of magnetic Fe₂O₃/SR composites filled with different nano-Fe₂O₃ contents are shown in Figures 2 and 3.

Figure 2

Tensile strength of Fe₂O₃/SR composites.

Figure 3

Maximum elongation of Fe₂O₃/SR composites.

The tensile strength and maximum elongation of magnetic Fe₂O₃/SR composites initially increased but then decreased with increasing nano-Fe₂O₃ content. This arose because the nano-Fe₂O₃ particles were uniformly distributed and have the interface structure of a rigid particle’s surface within the rubber matrix upon the addition of a small amount of nano-Fe₂O₃. However, with increased nano-Fe₂O₃ content, the interfacial thickness decreased due to a more narrow interparticle spacing. This caused weakness in the SR matrix, and imbalanced forces were concentrated, causing a decrease in strength. With increasing nano-Fe₂O₃ content, the homogeneity of the nanoparticles gradually declined and agglomeration increased, as seen in Figure 1. Binding forces decreased due to a larger area of phase interface existing between agglomerations of nano-Fe₂O₃ particles and the rubber matrix. However, the tensile strength and maximum elongation of magnetic rubber were improved due to the reinforcing effect of these nanoparticles.

The change in Shore A hardness of magnetic Fe₂O₃/SR composites filled with different nano-Fe₂O₃ contents is shown in Figure 4.

Figure 4

Shore A hardness of Fe₂O₃/SR composites.

The Shore A hardness of Fe₂O₃/SR composites was higher than that of ordinary SR. At 30 phr nano-Fe₂O₃ content, the Shore A hardness of unfilled SR was 64 degrees, whereas that of Fe₂O₃/SR composites was 77 degrees. The reason for this hardness increase, on one hand, was that the nano-Fe₂O₃ was harder than the unfilled SR; this increased the hardness of the composites. Moreover, the hardness of Fe₂O₃/SR composites also increased when nano-Fe₂O₃ particles filled the cavities in the SR. On the other hand, around evenly distributed rigid particles, there is interfacial layer of a certain thickness and good interfacial bonding. It can cause crazing upon damage, so that significant impact energy can be absorbed, leading to stress transmission. However, with increased concentrations of nano-Fe₂O₃ particles, uneven distribution thereof was more apparent. Thereby, the hardness increases of the composite became less significant, albeit not so as to reverse, or indeed stop, this increasing trend completely.

3.3 Heat-resistance performance

TGA data for these Fe₂O₃/SR composites are shown in Figure 5.

Figure 5

Residual mass of Fe₂O₃/SR composites.

It can be observed that pure SR started to decompose at 320°C; thereafter, the residue was reinforcing agent and other impurities. Mass loss had stopped by 420°C. At 10 phr nano-Fe₂O₃ addition, the SR’s decomposition temperature increased markedly to 350°C, and the mass loss rate decreased slightly. This was because the decomposition of Fe₂O₃/SR composites was delayed owing to the heat resistance of nano-Fe₂O₃. The decomposition temperature of the composites was further increased with increasing nano-Fe₂O₃ content. This further proved that the heat resistance of the composites was improved by the addition of nano-Fe₂O₃ particles.

3.4 Combustibility

The combustion performance of the magnetic Fe₂O₃/SR composites was tested in a standard tapered calorimeter. Results are given in Table 2.

It can be observed that the time to ignition (TTI) of Fe₂O₃/SR composites was longer than that of ordinary SR, and the average effective heat combustion (AvEHC) of Fe₂O₃/SR composites was reduced with the increase in nano-Fe₂O₃ content. Heat release rate data for the composites are shown in Figure 6.

Figure 6

Heat release rates of the composites.

The peak heat release rate (PHRR) and average heat release rate (AvHRR) of the composites were both reduced, and the PHRR time was delayed after adding nano-Fe₂O₃ (see Figure 6). This was because nano-Fe₂O₃ played a heat insulation role in the rubber matrix, and the antiflaming layer of nano-Fe₂O₃ was formed on the surface of the composites during combustion, which effectively hindered heat transfer to the unexploded part and prevented breakdown products from spreading to the flame region.

3.5 Tribological properties

Figure 7 shows friction coefficient data for magnetic Fe₂O₃/SR composites, filled with different concentrations of nano-Fe₂O₃, under a load of 2 N. The friction coefficient decreased gradually with increased nano-Fe₂O₃ level. When the nano-Fe₂O₃ content was 20 phr, the friction coefficient reached its minimum.

Figure 7

Friction coefficient vs. nano-Fe₂O₃ content.

The addition of nano-Fe₂O₃ enhanced the bonding force between the transfer film and the grinding surface; this therefore contributed to the formation of a complete, solid friction transfer film. Friction always occurred between the transfer film and the rubber matrix; the friction coefficient remained relatively low. Meanwhile, increased hardness enhanced the resistance to external pressure and made it difficult for loosened abrasive to penetrate the rubber matrix, thus effectively reducing the amount and depth of penetration of the abrasive on and into the composite’s surface.

Nano-Fe₂O₃ particles were hard to mix uniformly, especially at higher concentrations. Therefore, the friction coefficients of the composite improved with different filler concentrations because the increase in nano-Fe₂O₃ content caused a decrease in bond between the nano-Fe₂O₃ and the SR matrix [2, 3]. Meanwhile, large particle agglomerations in the composite resulted in roughness enhancement on the composite’s surface; thus, their friction coefficients increased.