Properties and mechanism of ceramizable silicone rubber with enhanced flexural strength after high-temperature

2.1 Materials

Methyl vinyl SR (molecular weight: 540,000) was obtained from Nanjing Dongjue Company. APP was purchased from Aladdin Reagent Company and ZB was also purchased from Aladdin Reagent Company. MP (400 mesh) was purchased from Hebei Longhao Company. Hydroxyl silicone oil was purchased from McLean Reagent Company and GP was purchased from Ami Fine Powder Company. 2,4-Dichlorobenzoyl peroxide (DCBP) was purchased from Nouryon Chemicals.

2.2 Sample preparation

According to Table 1, the methyl vinyl SR and the fumed silica were repeatedly mixed in an open mill until transparent. Then MP, APP, ZB, and GP were added and mixed evenly. Finally, 1.25 g of hardener was added and mixed evenly. The SR composite was cured in a flat vulcanization machine at 120°C for 10 min to obtain the ceramizable SR samples. SMG consisted of SR, MP, and GP. SMGA consisted of SR, MP, GP, and APP. SMGAZ consisted of SR, MP, GP, APP, and ZB. SMGZ consisted of SR, MP, GP, and ZB. The numbers behind the designations represent the different ratios of APP and ZB in the composite materials (Figure 1).

Figure 1

Ceramizable SR-based precursor vulcanization and sintering process.

2.3 Preparation of ceramic body

The samples with dimensions of 100 mm × 13 mm × 3 mm were prepared and calcined to 600, 800, and 1,000°C in a muffle furnace in air atmosphere with a heating rate of 10°C min⁻¹ and a holding time of 30 min. For comparison, the samples were also produced at 1,000°C and with the different heating rates of 5, 10, and 20°C min⁻¹.

2.4 Characterization

3.1 Mechanical properties of ceramizable SR composite

The mechanical properties of the ceramizable SR composite were determined by tensile tests. The results are shown in Table 2 and Figure 2. The tensile strength of SR is 3.65 MPa and the elongation at break is 479.26%. By adding APP or ZB, the elongation at break of SR improved significantly, while the tensile strength decreased slightly. This is because the incorporation of fillers disrupts the cross-linking network of the SR, thereby reducing the overall structural strength of the material [36]. It is noteworthy that when APP and ZB were simultaneously added to SR in different ratios, the mechanical properties of the SR/APP/ZB system were slightly worse compared to the samples in which only APP or ZB was loaded. Furthermore, with the increase of ZB content in SR/APP/ZB, both the elongation at break and tensile strength of the composite gradually decreased. However, the elongation at break of SR/APP/ZB remained slightly higher compared to SR without APP or ZB.

Figure 2

Tensile performance of SR composite: (a) stress–strain curves and (b) tensile strength and elongation at break.

The experiments also show that the density and hardness of SR are 1.363 g/cm³ and 51 Shore A, respectively, which are significantly lower than the values after adding APP/ZB. This indicates that the density and hardness of the material increased with the total amount of filler added. In particular, as the ZB content increased, the density and hardness of the SR/APP/ZB system gradually increased, indicating that the addition of ZB can effectively improve the density and hardness of the composite. However, due to the relatively small variation in filler content, the change in density was relatively small. Overall, the addition of APP and ZB improved the elasticity of the material while maintaining its flexibility.

Table 2 shows the changes in mechanical properties of the composite when the filler content and composition ratios were adjusted. This property allows the SMGAZ composite to maintain high shape quality and structural integrity while balancing elasticity and mechanical strength, providing potential advantages for applications in high temperature environments.

3.2 Volume resistance and surface resistance of ceramizable SR composite

Figure 3 shows the volume resistance and surface resistance of various composites. Compared to the original SMG without APP and ZB, the volume resistance of the composite with APP decreased significantly. This is because APP has relatively low electrical insulation properties. The ammonium salt structure of APP had the effect of an electrolyte, which, as a conductive medium in the composite, was not conducive to insulation performance, thereby reducing the overall electrical insulation [37]. Furthermore, the surface resistance also showed a similar decreasing trend, further confirming that the addition of APP weakened the insulation properties of the composite.

Figure 3

Volume resistance and surface resistance of SR composite.

On the other hand, with the gradual increase in ZB content (as seen in SMGAZ-1 to SMGAZ-5), both volume resistivity and surface resistivity show an increasing trend. This is mainly because ZB has high electrical insulation properties and its introduction into the composite effectively improves the overall insulation properties of the material. In particular, for the composites with higher ZB content, both volume and surface resistivity values increase significantly, indicating that ZB has a positive influence on improving the electrical insulation properties of the composite. It is worth noting that the volume and surface resistance of SMGAZ-3 exceeded 10¹³ Ω cm, which meets the national standards for insulation materials. This suggests that by adjusting the proportions of APP and ZB, a ceramizable SR composite with excellent electrical insulation properties can be produced, which is suitable for high insulation application scenarios.

The addition of APP reduced the insulation performance of the composite, while ZB effectively compensated for this deficiency and significantly improved both the volume and surface resistance of the material. Therefore, by rationally designing the content ratio of APP and ZB, the electrical performance of the composite can be optimized by balancing high resistivity with good mechanical properties to meet the requirements of specific applications. Insulating properties suitable for high insulation application scenarios can be created.

3.3 Flexural strength of the ceramics of ceramizable SR composites

The SMG, SMGA, and SMGZ samples did not retain their original shape after firing at 1,000°C and showed significant surface deformation. In contrast, the ceramic residue of SMGAZ showed a dense appearance and a smooth white surface, indicating significantly improved dimensional stability and smoothness due to the combination of APP and ZB in the SR composite, as shown in Figure 4. The flexural strength of the SMGAZ samples calcined at different temperatures is shown in Figure 5a. In the temperature range of 600–1,000°C, the flexural strength increases with the increase in the sintering temperature. At 1,000°C, the flexural strength of the ceramic residue reaches a maximum value of 39.47 MPa and shows excellent mechanical properties.

Figure 4

Surface topography of the sample calcined at 600–10°C min⁻¹ (a), 800–10°C min⁻¹ (b), 1,000–10°C min⁻¹ (c), 1,000–5°C min⁻¹ (d), and 1,000–20°C min⁻¹ (e).

Figure 5

Flexural strength (a) of the sample after calcination at 600–10°C min⁻¹, 800–10°C min⁻¹, 1,000–10°C min⁻¹, and the flexural strength (b) at 1,000–10°C min⁻¹, 1,000–5°C min⁻¹, 1,000–20°C min⁻¹.

Figure 5b shows the flexural strength of the samples calcined at 1,000°C at different heating rates. The samples with lower heating rates generally have better flexural strength. When comparing the samples produced with the heating rates of 5 and 20°C min⁻¹, it can be seen that the flexural strength decreases slightly with the increase in the heating rate. Specifically, the flexural strength of SMGA decreases from 10.19 ± 0.97 to 8.66 ± 2.56 MPa, while the flexural strength of SMGAZ-3 decreases from 42.19 ± 2.69 23.28 ± 3.32 MPa and that of SMGAZ-2 from 45.39 ± 3.67 MPa decreases to 6.80 ± 0.89 MPa when the heating rate increased from 5 to 20°C min⁻¹.

It is observed that at the heating rate of 20°C min⁻¹, the lower ZB content in SMGAZ-1 resulted in insufficient liquid phase formation during ZB melting, resulting in incomplete bonding of the ceramic body and the formation of irregular blocks [14]. In contrast, SMGAZ-5, which had higher ZB content, maintained continuity but lacked the self-supporting properties provided by the gas produced during APP decomposition, resulting in significant deformations and irregular shapes, which could affect fire protection performance in practical applications.

We compared the flexural strength of SMGAZ-3 with those reported [7,11,15,25,38–40] in previous studies (Table 3). For single-filler systems, Hu et al. [11] reported a flexural strength of 9.99 MPa for the SRAAM sample with APP filler, while Song et al. [38] achieved 11.58 MPa for the SR-5 sample using ZB filler. In contrast, dual-filler systems (APP/ZB) showed improved performance: the EVA-based [25] ceramizable material reached 21.20 MPa and the polyethylene-based [39] material attained 30.50 MPa. The flexural strength of SMGAZ-3 in this work is increased by 29.4–86.2% compared with the above results; this demonstrates that the synergy between the SR matrix and the APP/ZB composite filler significantly enhances high-temperature flexural performance.

These results indicate that the flexural strength of the samples with higher APP content tends to fluctuate significantly at high heating rate because the rapid decomposition of APP creates many irregular pores, which significantly reduces the strength of the samples. The SMGAZ-3 maintained its good shape and showed good flexural strength at various heating rates at 1,000°C, reaching 39.47 MPa, fully meeting the requirements for fire protection applications.

3.4 TGA of ceramizable SR composite

Figure 6 shows the TG and DTG curves of the samples on the thermal decomposition behavior of SMG. The thermal degradation behavior of the SMGA and SMGAZ-3 composites is similar to that of the original SMG, but both the initial decomposition temperature (T _5%) and the temperature at the maximum degradation rate (T _max) are reduced. This is mainly because the added inorganic filler APP generated gases such as ammonia and water vapor during thermal decomposition, which affected the thermal stability of the SR matrix and reduced the overall stability of the composite [15]. Table 3 summarizes the TGA data for various samples, where the residual mass of SMGA at 1,000°C is 39.2%, which is lower than the residual mass of SMG at the same temperature (42.9%), further suggesting that the decomposition of APP phosphates produced gases that catalyzed and accelerated the decomposition process of the SR matrix.

Figure 6

TGA (a) and DTG (b) of the sample.

As summarized in Table 4, the SMGZ composite exhibited a residual mass of 49.6% at 1,000°C, representing a 6.7% increase compared to the SMG sample (42.9%). This result shows that the addition of ZB effectively slowed the thermal decomposition of SR and increased the thermal stability of the composite. Under high-temperature conditions, ZB undergoes melting and transitions into a liquid phase, forming a protective layer that acts as a thermal barrier to block external heat transfer, reduce mass loss, and increase the residual char weight [38]. In addition, the initial decomposition temperature of SMGAZ-3 was 308°C lower than that of the original SMG of 374°C, but its residual mass at 1,000°C was 46.8%, higher than that of SMG of 42.9%. The good morphology was retained, which means that the sample can maintain its structural integrity and surface smoothness after calcination at high temperatures. This shows that the synergistic effect of APP and ZB brings significant advantages in terms of morphology stability. This indicates that the simultaneous addition of APP and ZB effectively improves the thermal stability and morphology stability of the composite and avoids the problem of excessive morphology changes when adding ZB alone.

As shown in Figure 7, DSC analysis reveals significant endothermic peaks in the 300–350°C range for both SMGA and SMGAZ-3 systems containing APP, a phenomenon attributed to APP’s thermal decomposition. Given the higher APP concentration in SMGA, its endothermic peak exhibits greater intensity, indicating a positive correlation between APP’s endothermic effect and its concentration [11]. This finding aligns with the TG data, in which SMGA shows a lower initial decomposition temperature (287°C) compared to SMGAZ-3 (308°C). A comparison of the DSC curves of SMG and SMGZ systems demonstrates that the notable increase in SMGZ’s heat flow value, manifested as an upward curve shift, suggests that ZB effectively suppresses the kinetic process of thermal degradation [41]. This conclusion is corroborated by the TG test results.

Figure 7

DSC curve of sample.

Comparing the DSC curves of SMG and SMGZ systems, the notable increase in the heat flow value of SMGZ, manifested as an upward shift of the curve, suggests that ZB effectively suppresses the kinetic process of material thermal degradation. This conclusion is corroborated by TG test results, as the decomposition residue of SMGZ (49.8%) exceeds that of SMG (42.9%), jointly confirming the synergistic enhancement of material thermal stability by ZB.

The addition of APP lowered the initial decomposition temperature of the composite, but its synergistic effect with ZB significantly increased the residual mass and morphology stability of the material at high temperatures, thereby improving the overall thermal stability of the composite. The synergistic effect of APP and ZB plays an active role in improving the thermal stability of polymer-based materials [42]. By rationally adjusting the ratio of APP and ZB, the thermal decomposition behavior and high-temperature performance of SR composites can be significantly improved, making them suitable for applications requiring high thermal stability.

3.5 Microstructure of the formed ceramics

Figure 8 shows the cross-sectional morphology of SMGA, SMGZ, and SMGAZ-3 after calcination at 1,000°C observed using SEM. Figure 8a1 shows the cross-sectional morphology of SMGA. The SMGA sample has large and uneven pores and the internal structure is relatively loose. This phenomenon is attributed to the thermal decomposition of APP at elevated temperatures, which generates gaseous products including ammonia and water vapor. These released gases create internal pores within the composite matrix during the initial degradation stage. Subsequent calcination at 1,000°C induces structural reorganization through sintering, during which the accumulated pores coalesce and propagate toward the surface, ultimately resulting in the formation of interconnected surface cracks [43]. The lamellar structure of mica can be clearly seen in Figure 8a3, indicating that there was no reaction between APP and mica when APP was used alone.

Figure 8

SEM micrographs of SMGA (a1, a2, a3), SMGZ (b2, b2, b3), and SMGAZ-3 (c1, c2, c3) calcined at 1,000°C.

Figure 8b1 shows the cross-sectional morphology of SMGZ. At high temperature, the ZB melted into a liquid phase that partially filled the voids in the matrix, creating a relatively dense structure. However, not all pores could be completely filled, resulting in some residual pores of uneven size. The areas with larger pores tend to become stress concentration points, making the ceramic body more susceptible to cracking or fracture under stress [44]. Furthermore, no lamellar mica structure is observed in Figure 8b3, suggesting that ZB facilitates the decomposition of mica at high temperatures. The image of SMGAZ-3 after calcination at 1,000°C shows that its pores are more uniform and smaller compared to those of SMGA and SMGZ. This explains the superior flexural strength of this material at high temperatures.

3.6 XRD of the formed ceramics

To further investigate the influence of APP and ZB on the ceramization process of SMG, XRD analysis was carried out. Figure 9 shows the XRD patterns of mica (MP), SMG, and SMGAZ-3 ceramics. The typical diffraction peaks of mica are clearly visible in the XRD pattern of MP. When the ceramization temperature is changed, no significant fluctuations in the diffraction peaks are observed in the SMG samples. After ceramization at 600, 800, and 1,000°C, the SMG samples show the diffraction peaks similar to MP as well as the additional diffraction peaks corresponding to SiO₂. This phenomenon is attributed to the decomposition of SR, which leads to the formation of SiO₂. The XRD analysis of SMGAZ-3 shows that the diffraction peaks of SMGAZ-3 at 600°C exhibit the presence of MP, SiO₂, and ZB. SiO₂ is observed in its amorphous form and the absence of phosphate peaks suggests that phosphates formed from the decomposition of polyphosphazene are in a liquid phase. At 800°C, the intensity of the MP peaks decreases, the ZB peaks disappear completely, and the SiO₂ peaks disappear. At the same time, the new Al₂O₃·2SiO₂ diffraction peaks appear, indicating that mica decomposes to form aluminum oxide, which undergoes a eutectic reaction with SiO₂ formed during the decomposition of SR, resulting in the formation of Al₂O₃·2SiO₂ crystals [45]. This explains the weakening of mica peaks, the disappearance of SiO₂, and the appearance of Al₂O₃·2SiO₂ while ZB melts and remains in a liquid phase. At 1,000°C, only the diffraction peaks of Al₂O₃·2SiO₂ and aluminum borate are observed, indicating that mica completely decomposes into alumina with increasing temperature. Part of the aluminum oxide reacts with SiO₂ to form Al₂O₃·2SiO₂ crystals, while another part reacts with ZB to form aluminum borate, resulting in the formation of a high-strength ceramic matrix.

Figure 9

XRD result of the ceramics of MP, SMG, and SMGAZ-3.

3.7 Mechanism of ceramization

As shown in Figure 10, at elevated temperature, the decomposition of SR produces SiO₂ while the GP melts and forms a liquid phase. At the same time, the decomposition of APP produces polyphosphates and gases. The liquid phases of the GP and polyphosphates act as agglomerating agents, binding SiO₂, MP, ZB, and other residues in the SR matrix and at the same time filling some of the pores that are created when the SR decomposes. As the temperature continues to rise, ZB melts and facilitates agglomeration, improving the cohesion of the remaining components. At about 800°C, the mica gradually decomposes and leads to a eutectic reaction between alumina and SiO₂, resulting in the formation of mullite. At a temperature of 1,000°C, mica completely decomposes into aluminum oxide, which reacts with the molten ZB to form crystalline aluminum borate phases.

Figure 10

Ceramization process of SMGAZ.

During this process, the gases produced by the decomposition of SR and APP create pores in the matrix, which are subsequently encapsulated by the molten liquid phase. This liquid phase fills large voids and penetrates the material, effectively reducing the formation of macropores. When cooled and solidified, the liquid phase together with the crystallized phases of aluminum borate and mullite (Al₂O₃·2SiO₂) contribute to the formation of a durable ceramic material. The synergistic interaction between the dense microstructure and the crystalline phases gives the ceramic material superior mechanical strength and thermal stability.