Enhancing Flame Retardancy and Thermal Insulation Performance of Silicone Rubber Composites Using Silicon Carbide

This study focuses on modifying silicone rubber using silicon carbide (SiC) to enhance the flame retardancy and thermal insulation properties of the composites. The study characterizes the thermal stability, mechanical properties, and flame retardancy of the composite materials. The results demonstrate the effective improvement in thermal stability achieved by incorporating SiC into the silicone rubber composites. At a SiC addition level of 6 wt.%, the composite material shows optimal thermal stability with an initial decomposition temperature of 487°C, which is 84°C higher compared to the pure matrix. Additionally, the SiC/SR composites system exhibits a char yield of 72.13% at 900°C, representing an approximately 9% increase compared to the pure SR matrix. Simultaneously, the incorporation of SiC significantly enhances the flame retardancy of silicone rubber. At a SiC addition level of 10 wt.%, the LOI (Limiting Oxygen Index) value increases by 10.5% to reach 30.4%, surpassing the non-additive sample. Moreover, the UL-94 classification of the composite material is V-0, meeting the standard for flame retardant materials.


Introduction
Silicone rubber, known for its remarkable chemical stability, excellent electrical insulation properties [1], high-temperature resistance, and non-toxic characteristics, is widely employed in diverse fields including the food and pharmaceutical industries, biotechnology, electronics, electrical equipment [2], and aerospace.Despite its numerous advantageous properties, the inherent deficiency in flame retardancy poses a significant limitation to its broader application.Silicone rubber lacks the ability to self-extinguish once ignited, necessitating the urgent need to enhance its flame retardancy.Addressing this challenge has become a paramount concern demanding effective solutions.
Physical blending of flame retardants with silicone rubber is a widely employed approach to enhance its flame retardancy [3].The performance of silicone rubber composites with superior flame retardancy and mechanical properties is typically achieved through optimized compounding of various flame retardants [4].For instance, Chai [5] et al. conducted research on a synergistic flame-retardant system utilizing gum arabic, layered double hydroxide, and carbon nanotubes, resulting in simultaneous improvements in flame retardancy and mechanical properties at low loading levels.Similarly, Zhang [6] et al. demonstrated the effectiveness of an aluminum diethylphosphinate and octaphenyl polyhedral oligomeric silsesquioxane synergistic flame-retardant system, which imparted excellent flame retardancy to silicone rubber composites.Certain individual flame retardants, such as diatomite [7], sepiolite [8], montmorillonite [9], and halloysite [10], may exhibit limited flameretardant effectiveness when used alone.However, their excellent flame retardancy can serve as a foundation for optimizing compounding with multiple flame retardants [11].
During silicone rubber modification, SiC is often incorporated as an additive due to its high surface area, remarkable mechanical properties, and good compatibility.It enhances the strength of the char layer, and the migration of silicon compounds to the char layer contributes to the protection of the char residue [12].Zhang [13] et al. observed a significant improvement in the tensile strength of montmorillonite/silicone rubber composites upon the addition of silicon carbide whiskers.
This study aims to investigate the influence of silicon carbide as a flame-retardant additive on the flame retardancy and thermal stability of silicone rubber.The flame-retardant modification of silicone rubber is achieved by incorporating SiC as a functional filler.Comprehensive burning tests are conducted to evaluate the fire behavior of the silicone rubber composites containing SiC.The results reveal that the inclusion of SiC functional filler leads to rapid self-extinguishing during the combustion process, characterized by minimal flame propagation, limited mass loss, and the formation of an intact "shell" structure on the burning surface.Consequently, understanding the impact of SiC on the properties of silicone rubber holds significant importance in enhancing the thermal stability and flame-retardant performance of silicone rubber composites.

Experimental
Firstly, SiC powder was placed in a drying oven at 120°C for 3 hours.Then, the dried SiC was blended with silicone rubber in the specified proportion.Next, hydrogen-containing silicone oil curing agent, inhibitor, and platinum catalyst were added in their respective proportions and meticulously mixed.The Si-H/CH=CH 2 ratio was 1.5, with the platinum catalyst constituting 5‰ of the base SR polymer by weight, and the inhibitor added in half a drop.The mixture was mechanically stirred at room temperature for 10 minutes, followed by vacuum degassing for 10 minutes.Subsequently, the mixture was poured into molds and subjected to curing at room temperature for 24 hours, followed by 1 hour at 80°C and 1 hour at 120°C, resulting in the formation of the silicone rubber composite material.1 depicts the impact of varying SiC contents on the thermal stability of silicone rubber, while table 1 presents the corresponding temperature and char yield at the initial decomposition temperature of the material for different SiC concentrations.The thermal degradation curves of the composite system generally exhibit similar trends, wherein the thermal stability initially improves and subsequently diminishes with the addition of SiC.Notably, due to the considerably higher decomposition temperature of SiC exceeding 900°C, each sample undergoes a single decomposition event within the 900°C range, namely the decomposition of the silicone rubber component.At a SiC content of 6 wt.%, the composite system demonstrates optimal thermal stability, evidenced by an initial decomposition temperature of 487°C, representing an 84°C increase compared to the pure matrix.Simultaneously, the composite system exhibits the highest char yield at 900°C, reaching 72.13%, which is approximately 9% greater than that of the pure matrix.The incorporation of silicon carbide facilitates the accumulation of the char layer during the thermal decomposition process, while the migration of silicon-containing compounds into the char layer enhances the structural integrity of the residue [12].Consequently, compared to the pure silicone rubber matrix, SiC/SR composite materials exhibit enhanced char layer formation on the material surface, leading to increased char yield and improved flame retardancy.

Effect of SiC on the thermal stability of silicone rubber.
Figure 1(b) shows the DTG curves of silicone rubber composite systems with different SiC contents.It can be observed from the graph that the addition of SiC results in a more pronounced peak in weight loss for the flame-retardant silicone rubber system.Furthermore, it is evident from the curves that the inclusion of SiC leads to a slight advancement in the maximum decomposition rate of the composite materials.This phenomenon, combined with an increase in char yield, indicates a more concentrated thermal decomposition process in the presence of SiC.It is worth mentioning that within the temperature range of 400-500°C, the thermal decomposition rate of silicone rubber composites with lower SiC contents is significantly lower than that of pure SR.Simultaneously, the thermal decomposition rates of composites with 2 wt.% and 6 wt.% SiC are essentially the same, but higher than those of composites with 4 wt.%SiC.However, in the temperature range of 500-650°C, the decomposition rate of the SR composites surpasses that of pure SR, and the weight loss can be ascribed to the breaking of Si-O-Si bonds [14].The provided data indicates that the introduction of SiC enhances the thermal stability of silicone rubber by SiC's high melting point and thermal stability [15].It further demonstrates that the incorporation of SiC improves the char yield at 900°C within the flame-retardant system of silicone rubber.At this specific temperature, the combined effect of silicon carbide and the resulting ceramic residue from the silicone rubber matrix forms a compact char layer.This layer acts as a barrier against oxygen and heat, effectively impeding the thermal decomposition of the inner materials.However, an excessive addition of SiC exceeding 6 wt.% leads to agglomeration, thereby causing a decrease in the thermal stability of the composite material rather than an increase.

Effect of SiC on the flame retardancy of silicone rubber.
Table 2 showcases the flame-retardant performance data for various SiC/SR composite systems.The data presented in the table 2 clearly illustrates that the incorporation of a minor quantity of SiC results in an enhanced LOI value for the composite materials based on silicone rubber.However, it is noteworthy that the flame-retardant properties of the composite system remain inadequate, potentially attributable to the char conversion efficiency.Nevertheless, a notable improvement in the flameretardant performance of the silicone rubber matrix becomes evident when the SiC contents exceed 6 wt.%. Figure 2: Before-and-after comparison of the vertical burning experiments for the 8% SiC/SR composites.At a SiC content of 6 wt.%, the composite system exhibits an LOI value of 28.9%, surpassing the threshold for flame-retardant materials.Furthermore, it attains a UL-94 flame-retardant rating of V-0.When the SiC content is increased to 10 wt.%, the LOI value further improves to 30.4%, marking a 10.5% increase compared to the non-additive state.Notably, the UL-94 flame-retardant rating remains at V-0.These compelling data provide clear evidence that the addition of SiC significantly enhances the flame-retardant performance of silicone rubber materials.
SiC predominantly operates through a condensed phase flame-retardant mechanism, which facilitates the crosslinking and carbonization of decomposition products, consequently strengthening the residual char.The formation of silicon-containing compounds on the surface of the sample serves as a highly effective insulation barrier [16], displaying notable thermal insulation properties and oxygen isolation effects.As a result, the thermal degradation of the internal structure is suppressed, leading to a noteworthy improvement in the flame-retardant performance of the composite material.

Effect of SiC on the mechanical properties of silicone rubber
Figure 3 illustrates the impact of different SiC addition levels on the mechanical properties of silicone rubber.SiC is commonly utilized to enhance the mechanical characteristics of ceramic-based [17], metal-based [18], and polymer-based [19] composite materials.In general, the incorporation of SiC results in varied degrees of reduction in the mechanical strength of the composite material while maintaining favorable mechanical properties.Specifically, at an addition level of 4 wt.%, the tensile strength reaches its maximum value of 3.158 MPa.However, with further increases in SiC content, the tensile strength experiences a rapid decline, dropping to 2.276 MPa at a 10% addition level.This

Conclusion
The addition of SiC flame retardant proves highly effective in improving the thermal stability of silicone rubber, primarily by enhancing the char yield.The characterization results of the material highlight a significant increase in the composite system's char yield at 900°C, rising from 63.82% to 72.13% with a SiC addition of 6%.This increase is attributed to the interaction between SiC and the decomposition products during the thermal decomposition process, leading to the formation of a dense char layer.Consequently, the incorporation of SiC flame retardant substantially enhances the heat resistance of silicone rubber.Moreover, the introduction of SiC flame retardant also enhances the flame retardancy of the silicone rubber material.At a 10 wt.% addition level, the LOI value reaches 30.4%, representing a notable 10.5% improvement compared to the non-additive scenario.At this level, the material achieves a UL-94 rating of V-0, meeting the required classification for flame retardant materials.

Figure 1 :
Figure 1: Effect of SiC content on the thermal stability of silicone rubber (a)TG；(b)DTG；(c)T 5 、T max 、Char yield at 900℃ Figure1depicts the impact of varying SiC contents on the thermal stability of silicone rubber, while table 1 presents the corresponding temperature and char yield at the initial decomposition temperature of the material for different SiC concentrations.The thermal degradation curves of the composite system generally exhibit similar trends, wherein the thermal stability initially improves and subsequently diminishes with the addition of SiC.Notably, due to the considerably higher decomposition temperature of SiC exceeding 900°C, each sample undergoes a single decomposition event within the 900°C range, namely the decomposition of the silicone rubber component.At a SiC content of 6 wt.%, the composite system demonstrates optimal thermal stability, evidenced by an initial decomposition temperature of 487°C, representing an 84°C increase compared to the pure matrix.Simultaneously, the composite system exhibits the highest char yield at 900°C, reaching 72.13%, which is approximately 9% greater than that of the pure matrix.The incorporation of silicon .1088/1742-6596/2706/1/012003 5 decline can be attributed to the significant inclusion of SiC, which introduces internal defects within the silicone rubber, consequently diminishing its mechanical performance.

Table 1 :
Effect of SiC content on the thermal stability of the matrix

Table 2 :
Formulation and flame-retardant performance of SR/SiC composites