A novel application of titanium suboxide to enhance the flame retardancy,thermal stability,and thermal and electrical conductivity of silicone rubber

Abstract

This study investigates the impact of Titanium Suboxide (Ti₄O₇) on the flame retardancy, mechanical properties, thermal stability, and electrical and thermal conductivity of silicone rubber composites. The addition of 1.0 part of Ti₄O₇ to the silicone rubber composites results in a total heat release (THR) of 45 MJ/m², which is 45.7% lower than the silicone rubber without Ti₄O₇, leading to a decrease in the peak of heat release rate (PHRR). Furthermore, Ti₄O₇ exhibits temperature stability at low temperatures and acts as a physical barrier, inhibiting the early decomposition of silicone rubber and enhancing the thermal stability of the composites. However, at temperatures exceeding 450°C, the catalytic activity of Ti₄O₇ is activated, promoting the decomposition of the silicone rubber composites and generating more residue. The addition of an appropriate amount of Ti₄O₇ can further improve the mechanical properties of the silicone rubber composites and enhance their electrical and thermal conductivity.

Keywords

Silicone rubber titanium suboxide flame retardant electric properties thermal property

Introduction

Silicone rubber (SR) is a versatile synthetic elastomer that consists of silicon, oxygen, and organic side groups. It has become a valuable material for a variety of industries due to its unique combination of physical and chemical properties.¹ While its excellent temperature resistance, flexibility, and chemical stability make it a popular choice for coatings and adhesives applications, attention must be paid to its poor flame retardancy and potential release of toxic fumes.² To ensure the safety and durability of silicone rubber in practical applications, the incorporation of flame retardants is paramount. Zhang et al.² introduce ammonium polyphosphate (APP), aluminium diethylphosphonate (ADP), and octaphenylpolyhedral zwitterionic sesquioxane (OPS) into room temperature vulcanized silicone rubber insulation composites. The results show that the addition of the composite flame retardants significantly improves the ablative and flame-retardant properties of the Room Temperature Vulcanizing (RTV) silicone rubber composites.

In conjunction with its suboptimal flame retardancy, the comparatively modest electrical conductivity exhibited by silicone rubber imposes constraints on its viability across several applications necessitating favorable electrical attributes. Consequently, enhancing the electrical conductivity of silicone rubber has emerged as a noteworthy subject commanding attention within the realm of scientific inquiry.^3,4 One of the main benefits of improving the conductivity of silicone rubber is that it can reduce the contact thermal resistance between the heat source surface and the silicone rubber contact surface. For example, conductive silicone rubber can be used as an insulator and electromagnetic interference (EMI) shield in electronic devices.^5,6 In addition, conductive silicone rubber can be used in heating elements, sensors, and other applications where electrical properties are important.⁷ Another benefit of increasing the conductivity of silicone rubber is its ability to dissipate static electricity. Static electricity can cause damage to sensitive electronic components, especially in manufacturing environments.⁸ Wang et al.⁶ choose carbon nanotubes (CNTs) and conductive sponges (CS) to construct a 1D-3D synergistic network in a silicone matrix. They prepare new silicone rubber composites with better thermal and electrical conductivity properties. Such silicone resin-based composites with high thermal and electrical conductivity have a wide range of applications in the thermal management of emerging electronic devices.

Titanium suboxide (Ti₄O₇), recognized as titanium suboxide, constitutes a ceramic compound characterized by the coexistence of titanium oxide (TiO) and titanium dioxide (TiO₂) phases. Distinguished by its singular attributes encompassing catalytic prowess, elevated thermal stability, and a composite electrical conductivity profile, this material assumes a position of intrigue for a diverse array of application domains.⁹ Ti₄O₇ exhibits high catalytic activity and selectivity for the oxidation of carbon monoxide (CO) and volatile organic compounds (VOCs), making it a promising material for use as a catalyst in the automotive industry for the removal of CO from exhaust gases. Sun et al.¹⁰ synthesized micron-sized Ti₄O₇ powder using thermal reduction under H-2 atmosphere and successfully used it as a carrier material for Pt-based catalysts. Ti₄O₇ also exhibits mixed conductivity, making it useful in applications such as solid oxide fuel cells and oxygen sensors. Ti₄O₇ has been shown to have high ionic conductivity at high temperatures, which is important for efficient operation in these devices. Additionally, Ti₄O₇ has interesting electron transport properties that may have potential applications in electronics and optoelectronics. Tan et al.¹¹ proposed the preparation of Ti₄O₇-doped polypyrrole coatings on metal bipolar plates, resulting in conductive Ti₄O₇ particles that enhance the electrical conductivity and hydrophobicity of the composite coating while maintaining high chemical stability in simulated cathode proton exchange membrane fuel cell environments at 25°C and 70°C. In conclusion, Ti₄O₇ has unique properties that make it useful in a variety of applications, such as catalysis, high temperature environments, solid oxide fuel cells, and oxygen sensors. Further research may reveal even more potential applications for Ti₄O₇ due to its unique properties.^12,13

Upon analyzing the properties of Ti₄O₇, we conclude that this ceramic material holds great potential in enhancing the properties of silicone rubber, making it a topic of interest. A key advantage of incorporating Ti₄O₇ into silicone rubber is its catalytic carbonization effect, which makes it a useful flame retardant. Silicone rubber is known to ignite and burn when exposed to heat, making it hazardous in certain applications.¹⁴ Through the incorporation of Ti₄O₇ into silicone rubber, the material gains an elevated level of flame retardancy. This augmentation can be attributed to the catalytic attributes inherent to Ti₄O₇, which manifest when subjected to heat. This catalytic effect instigates the generation of a carbonaceous layer at the material's surface. Functioning as a safeguarding shield, this carbon layer effectively impedes the initiation of ignition and subsequent combustion processes.¹⁵ Moreover, Ti₄O₇ has the potential to enhance the thermal and electrical conductivity of silicone rubber. It boasts a high thermal conductivity, which improves the heat dissipation properties of silicone rubber, making it ideal for thermal management applications such as electronics and high temperature environments. Additionally, Ti₄O₇ has hybrid conductivity, which enables it to make silicone rubber electrically conductive. This attribute is useful in applications that require electrical properties such as sensors or electromagnetic interference (EMI) shielding.^16,17 Moreover, the efficacy of Ti₄O₇ in augmenting the thermal stability of silicone rubber is widely acknowledged. Prolonged exposure to elevated temperatures often prompts silicone rubber to undergo a decline in its desirable attributes, resulting in material degradation. Through the integration of Ti₄O₇, the composite material attains heightened resilience in high-temperature conditions. This fortification enables its application within industries demanding robust performance under extreme thermal circumstances, such as the automotive and aerospace sectors.¹⁸ To investigate the effects of Ti₄O₇ on various properties of SR composites, this paper focuses on mixing Ti₄O₇ into SR in varying proportions, using fumed silica and silicone oil to enhance the mechanical properties of SR. The key properties of SR composites including flame retardancy, thermal and electrical conductivity, mechanical properties, and thermal stability were tested, and the catalytic carbonization effect of Ti₄O₇ was revealed through residue analysis.

Material and characterization

Materials

Silicone rubber (SR, 110-2, Relative molecular weight: 45–85, Vinyl content: 0.13–0.18) was brought from Nanjing Dongjue Silicone Co., Ltd. (China). Titanium suboxide (Ti₄O₇, the average size is about 5 μm, Figure 1) was offered by Hubei Xinghengye Technology Co., Ltd. (China). 2,5-dimethyl-2,5-di(tert-butylperoxy) hexane (C₁₆H₃₄O₄, Molecular weight: 290.44) was attained from Sinopharm Chemical Reagents Co., Ltd. (China). Fumed silica (R9200) was offered by Evonik. (Germany). Silicone oil (JP-203, HO[(CH₃)₄SiO]_nH) was brought from Shenzhen Ji Peng Silicone Fluorine Material Co., Ltd.

Figure 1.

The TEM images of Ti4O7 at low and high magnification.

Preparation of SR and its composites

First, the raw materials were placed on a two-roll mill for 10 min at room temperature, mixed evenly and then transferred into a mold. Then, the sheet was obtained by hot pressing (at 175°C and 10 MPa for 8 min) and cold pressing and then sliced into specimens with a desired size for investigating its flame retardant, mechanical properties and other key properties. The formula was shown in Table 1.

Table 1.

The formula.

Samples	SR	SR/Ti₄O₇0.5	SR/Ti₄O₇1.0	SR/Ti₄O₇2.0
Silicone rubber/g	100	100	100	100
Silicone oil/g	4	4	4	4
Fumed silica/g	40	40	40	40
Ti₄O₇/g	0	0.5	1	2
2,5-dimethyl-2,5-di (tert-butylperoxy) hexane/g	2	2	2	2

Characterization

The morphologies of SR composites were investigated by Scanning electron microscopy (SEM, Phenom ProX, Phenom, China).

A Cone calorimeter test (CCT) was performed on an FTT cone calorimeter according to ISO 5660 at the heat flux of 50 kW/m² (sample size: 100 mm × 100 mm × 3 mm).

The mechanical properties were obtained by using a universal testing machine (Rate: 250 mm/min. UTM4204, SUNS, China).

Thermal stability was performed using a Thermal Gravimetric Analyzer (TGA, Q50, TA, USA). About 10.0 mg of sample was put in an alumina crucible and heated from ambient temperature to 800°C and the heating rate was set as 10°C/min in nitrogen atmosphere or air.

The dielectric frequency domain spectrum test shall be conducted according to GB/T 1409-2006 Recommended Method for Measuring the Permittivity and Dielectric Loss Factor of Electrical Insulating Materials at Power Frequency, Audio Frequency and High Frequency. The test temperature is 25°C, and the frequency is 0.01–1000 Hz.

The sample of volume resistance is prepared into a round sheet with a diameter of 100 mm, and tested on a ZC-36 type impedance meter according to GB/T 1410-2006. The test temperature is 25°C and the test voltage is 1000 V.

The sample of dielectric strength is prepared into a round sheet with a suitable diameter and tested on an AC dielectric strength tester. The dielectric strength at breakdown is recorded and measured five times to obtain the average value.

The residues collected from cone calorimeter tests were evaluated by Raman spectra (RS, ATR3110-1064, OPTOSKY, China).

Results and discussion

Characterization of SR and its composites

SEM analysis (Figure 2) of the silicone rubber (SR) shows that in the absence of titanium black, only a few white particles are present in the cross-section of SR, indicating slight agglomeration of the silica. The majority of the silica combines with the silicone rubber, providing good reinforcement. Energy-dispersive X-ray spectroscopy (EDS) analysis of SR indicates that it mainly consists of silicon and oxygen, with a small amount of carbon, suggesting that silica is the primary filler in the silicone rubber without impurities mixed during the process. However, the fracture surface of SR/Ti₄O₇ presents a different phenomenon, with a large number of particles observed. EDS analysis has unequivocally identified the predominant composition of these particles to be Ti₄O₇. Concerning the SR, the majority of the silica content synergistically amalgamates with the polymer matrix, thereby engendering efficacious reinforcement. EDS analysis conducted on the SR reveals a predominant composition of silicon and oxygen, with a marginal occurrence of carbon, signifying that silica constitutes the principal filler in the silicone rubber, devoid of any impurities stemming from the fabrication process. However, a distinctive phenomenon unfolds upon scrutinizing the fracture surface of silicone rubber composites infused with Ti₄O₇ (SR/Ti₄O₇). This phenomenon is characterized by the conspicuous presence of numerous particles. EDS analysis corroborates that these particles are primarily comprised of Ti₄O₇. Nonetheless, a noteworthy observation arises: the majority of these particles display limited compatibility with the silicone rubber matrix. This lack of compatibility results in their segregation on the fracture surface, leading to an impaired affinity with the silicone rubber matrix.¹⁹ The presence of large amounts of Ti₄O₇ promotes agglomeration, further diminishing the compatibility between Ti₄O₇ and silicone rubber. Therefore, a significant effect on the mechanical properties of the silicone rubber may result from filling content of Ti₄O₇ beyond a certain value.

Figure 2.

The SEM and EDS of SR and SR/Ti₄O₇2.0 composite;(a) SR, (b) SR/Ti₄O₇2.0.

Mechanical properties of SR and its composites

To evaluate the effect of Ti₄O₇ on the mechanical properties of silicone rubber, a series of tests are conducted and the results are presented in Figure 3. The initial tensile strength and elongation at break of the unfilled silicone rubber are measured to be 7.5 MPa and 317%, respectively. Upon the addition of 0.5 part of Ti₄O₇, both the tensile strength and elongation at break increase to 7.9 MPa and 374%, respectively, indicating that a small amount of Ti₄O₇ can have a positive complementary effect. However, as the amount of Ti₄O₇ increases, the elongation at break gradually decreases, possibly due to agglomeration of excessive Ti₄O₇, which impedes the transfer of internal forces within the silicone rubber.^20,21 This agglomeration phenomenon can be observed in Figure 2(b). The agglomeration of Ti₄O₇ particles introduces the possibility of stress concentration when subjected to external stretching forces. This phenomenon can exacerbate crack formation and heighten the material's susceptibility to fracture. Interestingly, counter to this anticipated behavior, the inclusion of Ti₄O₇ paradoxically augments the tensile strength of the silicone rubber, albeit with a constrained improvement. This outcome can be ascribed to the entwining of a significant portion of Ti₄O₇ particles within the molecular chains of the silicone rubber.^22,23 Consequently, Ti₄O₇ can act as physical cross-linking points, further reinforcing the tensile strength of the silicone rubber after cross-linking.

Figure 3.

The mechanical properties of SR and its composites.

Thermal properties of SR and its composites

The thermal decomposition process of the samples is investigated by conducting TGA tests under a nitrogen atmosphere. The relevant data is shown in Table 2. Figure 4(a) shows that the starting thermal decomposition temperature (T_5%) gradually increases with increasing Ti₄O₇ content. The T_5% of SR without Ti₄O₇ addition is 465.2°C, while the T_5% of SR/Ti4O₇2.0 after the addition of 2 phr (part per hundred rubber) is raised to 472.1°C. This is likely due to the presence of Ti₄O₇ hindering the diffusion of the internal gas of SR/Ti₄O₇2.0 to the outside, affecting the early degradation rate of the sample. The addition of Ti₄O₇ also promotes an increase in the residue content of the silicone rubber composite. The addition of 2 phr of Ti₄O₇ increases the residue content from 33.8 to 38.5%, indicating that Ti₄O₇ may have some adsorption effect in addition to acting as a physical barrier to gas diffusion, further inhibiting the diffusion of gas.²⁴ The maximum weight loss temperature (T_m) of the sample is also changed by the addition of Ti₄O₇, as observed in Figure 4(b). The T_m of SR without Ti₄O₇ addition is 603°C, while the addition of Ti₄O₇ up to 0.5 significantly increases the Tm to 664°C. The continued addition of Ti₄O₇ increases the Tm and decreases the peak mass loss rate (MLR_m). This observation suggests that the incorporation of Ti₄O₇ not only hinders the premature decomposition of the silicone rubber but also reduces its rate of mass loss.^25,26 An intriguing phenomenon is noted: while the thermal decomposition of pure silicone rubber exhibits a single decomposition phase, the addition of Ti₄O₇ introduces two decomposition phases, particularly in the temperature range of 660–690°C, where a distinctive peak of thermal weight loss emerges.²⁷ This phenomenon is likely attributed to the presence of Ti₄O₇, which exhibits the capacity to absorb a substantial amount of gases released during the fracture of silicone rubber molecular chains, particularly in the intermediate and later phases of decomposition. This absorption effect impedes alterations in the sample's mass, subsequently mitigating the abrupt degradation of the silicone rubber.

Table 2.

The data of TGA and DTG in N₂.

Samples	T_5% (°C)	T_m (°C)	MLR_m (%/°C)	Residue (%)
SR	465.2	603	0.49	33.8
SR/Ti₄O₇0.5	467.3	664	0.32	36.4
SR/Ti₄O₇1.0	469.9	682	0.33	37.3
SR/Ti₄O₇2.0	472.1	685	0.29	38.5

Figure 4.

The TGA (a) and DTG (b) of SR and its composites in N2.

In addition to exploring the effect of Ti₄O₇ on the thermal decomposition stage of the silicone rubber composites in a nitrogen atmosphere, we are also conducting thermogravimetric tests on the silicone rubber composites in an air atmosphere to investigate the effect of oxygen on the thermal stability of the silicone rubber composites. From Figure 5(a) and Table 3, we can see that the T_5% of SR without Ti₄O₇ addition is 424.2°C. As the Ti₄O₇ content continues to increase, the T_5% of the silicone rubber composites gradually increases, and when the number of parts added reaches 2 phr, the T_5% of SR/Ti₄O₇2.0 increases to 436.1°C. This pattern is similar to the change in thermal weight loss in a nitrogen atmosphere, but the overall T_5% is reduced compared to it because the presence of oxygen accelerates the thermal decomposition reaction of the silicone rubber composite, and the addition of Ti₄O₇ acts as a physical barrier to inhibit the early decomposition of the silicone rubber to some extent. In addition, the residue content of the silicone rubber composites gradually increases with the increase of Ti₄O₇ content, and when the addition of Ti₄O₇ reaches 1 part, the residue content increases to 44.2%, which is 49.0% compared to the residue content of SR.²⁸ Indeed, this phenomenon can be attributed to the catalytic activity of Ti₄O₇, which is activated at high temperatures. The elevated temperature stimulates the catalytic reaction between oxygen and the silicone rubber composite, leading to the formation of a solid-phase structure and ultimately promoting the generation of residues. Figure 5(b) shows the Tm and MLR_m of the silicone rubber composites, from which we can see that the T_m of all the silicone rubber composites is around 550°C, which is different from the T_m pattern of the samples under nitrogen atmosphere, probably because Ti₄O₇ has more of a catalytic effect than an adsorption effect in the presence of oxygen, and therefore cannot inhibit the thermal oxidative decomposition of the silicone rubber at high temperatures, but instead acts as a promoter.²⁹ This can be explained by the change in MLR_m. The MLR_m of the silicone rubber composites under air atmosphere is generally higher compared to that of the silicone rubber composites under nitrogen atmosphere, which proves that the presence of oxygen promotes the decomposition of the silicone rubber composites, and the addition of Ti₄O₇ further increases the MLR_m, which increases to 1.62%/°C when the number of Ti₄O₇ additions reaches 1.0 part, indicating that Ti₄O₇ promotes the degradation of SR composites. However, it should be noted that the MLR_m decreases when the Ti₄O₇ addition fraction reaches 2 phr, due to the decomposition of SR/Ti₄O₇2.0 as early as around 501°C and the unique decomposition peak in the graph. This phenomenon demonstrates the catalytic ability of Ti₄O₇ to be activated at high temperatures.

Figure 5.

The TGA (a) and DTG (b) of SR and its composites in air.

Table 3.

The data of TGA and DTG in air.

Samples	T_5% (°C)	T_m (°C)	MLR_m (%/°C)	Residue (%)
SR	424.2	550.1	1.25	29.6
SR/Ti₄O₇0.5	428.3	550.2	1.5	38.5
SR/Ti₄O₇1.0	433.4	550.5	1.62	44.2
SR/Ti₄O₇2.0	436.1	550.4	1.2	45.5

Fire properties of SR and its composites

Cone calorimetry testing is employed to evaluate the combustion performance of the silicone rubber composite and the relevant data is shown in Table 4. The graph in Figure 6(a) portrays the heat release rate versus time, showing that the peak heat release rate (pHRR) of SR was remarkably high, reaching 544 kW/m², with the time required for ignition (TTI) being 49 s. The results demonstrate that the pHRR and TTI decrease gradually with the increase in Ti₄O₇ content. When the Ti₄O₇ addition reaches 1 part, the pHRR and TTI drop to 344 and 36 s, respectively, which are 36.8% and 26.5% lower than those of SR. These findings indicate that an appropriate amount of Ti₄O₇ can promote the early combustion decomposition of silicone rubber composites. Moreover, the catalytic attributes of Ti₄O₇ come to fruition during the combustion process, potentially influencing catalytic carbonization and contributing to cohesive-phase flame retardation mechanisms.^30,31 The graph in Figure 6(b) reveals that the inhibition of the heat release rate of the silicone rubber composite by Ti₄O₇ results in a significant reduction in the total heat release rate (THR). When 1 part of Ti₄O₇ is added, the THR reduces from 83 to 45 MJ/m², representing a 45.7% reduction. This finding confirms that Ti₄O₇ has a certain effect of inhibiting heat release in silicone rubber composites. However, it is important to study smoke release from silicone rubber composites. Figure 6(c) indicates that the addition of Ti₄O₇ not only fails to inhibit smoke production but also shows a negative effect by significantly increasing the total smoke release (TSP) with the increase in Ti₄O₇ content. When Ti₄O₇ is added at 2 phr, the TSP increases to 3.7 m², indicating a 105% increase compared to the SR without Ti₄O₇. This outcome may be due to the fact that Ti₄O₇ promotes the decomposition of silicone rubber composites by producing large layers of carbon, which impede the combustion of internal combustibles while increasing the degree of incomplete combustion of combustibles, thus generating large amounts of smoke. When 2 phr of Ti₄O₇ are added, the total smoke production (TSP) increases to 3.7 m², representing a remarkable 105% increase compared to the silicone rubber (SR) without Ti₄O₇. This outcome can be ascribed to the function of Ti₄O₇ in facilitating the decomposition of the silicone rubber composites, thereby facilitating the development of expansive carbon layers. These carbon layers impede the combustion of internal combustible constituents, while concurrently intensifying the extent of incomplete combustion. This interplay culminates in the production of substantial smoke volumes. The interpretation of Figure 6(c) is further verified by the change in residue content in Figure 6(d). It can be observed from Figure 6(d) that the residue content decreases to 29.2% after complete combustion of the SR, with the main components of the residue being silica from the decomposition of the main chain of the silicone rubber and a small amount of carbon layer from the decomposition of the side chain.³² While the residue content gradually increases with the increase of Ti₄O₇ content, when the addition of Ti₄O₇ reaches 2 phr, the residue content increases to 52.2%, demonstrating the good catalytic carbonization effect of Ti₄O₇. However, this effect generates a large amount of smoke when inhibiting the heat release. Therefore, the addition of Ti₄O₇ needs to be reasonably controlled in terms of the number of parts added. The findings highlight the importance of balancing the catalytic carbonization effect of Ti₄O₇ and its negative impact on smoke production in developing effective flame-retardant silicone rubber composites.

Table 4.

The data of CCT.

Samples	TTI (s)	pHRR (kW/m²)	THR (MJ/m²)	TSP (m²)	Residue (%)
SR	49	544	83	1.8	29.2
SR/Ti₄O₇0.5	41	504	76	2.3	37.4
SR/Ti₄O₇1.0	37	345	45	2.8	38.2
SR/Ti₄O₇2.0	36	344	44	3.7	52.2

Figure 6.

HRR (a), THR (b), TSP (c), WL (d) and THR/ML (e) of SR and its composites.

By exploring the co plotted curves of THR and ML, it can be found that the addition of Ti₄O₇ has an effect on the gas-phase and condensed phase flame retardancy of SR composite materials. The slope of the curve in Figure 6(e) represents the product of combustion efficiency and effective combustion heat in CCT. When the addition of Ti₄O₇ reaches 1 part, the flame-retardant effect of the condensed phase is significant, and the mass loss rate decreases while the total heat release also decreases. This indicates that the addition of Ti₄O₇ can effectively promote the formation of coke, increase its compactness, reduce the decomposition and volatilization of combustibles, and thereby reduce heat release.³³ Continuing to escalate the incorporation of Ti₄O₇ continues to decrease the rate of mass loss, albeit with marginal alterations in the overall heat release. This tendency might arise from the limitation of Ti₄O₇'s capacity to yield further enhancements in the density of the resulting coke structure.

During the combustion of the silicone rubber composite, an important factor to consider is the changes in the content of various gases detected by the CCT. To further analyze the state of the silicone rubber composite during combustion, we can study these gas variations. Figure 7(a) depicts a graph of the oxygen concentration versus time. It is observed that the silicone rubber composites without Ti₄O₇ and those with 0.5 part added consume less oxygen during combustion. As the combustion becomes more intense, the oxygen concentration decreases to 19.3%, indicating that the action of oxygen promotes the oxidation and carbonization of the silicone rubber composites. However, the addition of Ti₄O₇ can increase the degree of incomplete combustion, leading to the production of more CO when it accelerates the generation of thermal insulation.^22,23 It is also noteworthy that the peak COP of SR and SR/Ti₄O₇0.5 is delayed by approximately 50 s, indicating that controlling the carbon formation effect can increase the combustibility of the combustible material.³⁴ This, in turn, can reduce the CO content and increase the CO₂ content, thus improving the overall combustion performance of the silicone rubber composite.

Figure 7.

O₂ content (a), COP (b) and CO_₂P (c) of SR and its composites.

Characterization of char residues

To further understand the effect of Ti₄O₇ on the carbon formation properties of the silicone rubber composite, the CCT residue is examined. Figure 8 illustrates that the SR residue possesses a loose structure with numerous voids, which impedes the production of good thermal insulation, as both the flame and combustible gases can easily penetrate or diffuse into the interior, thus promoting the complete combustion of the combustible material within. This leads to the release of more heat and a significant rise in the flame hazard level. However, as the Ti₄O₇ content increases, the surface integrity of the insulation layer gradually improves, and the voids are reduced, thereby enhancing the insulation effect. However, when more than 1 part of Ti₄O₇ is added, the Ti₄O₇ tends to agglomerate, leading to a negative impact on the catalytic effect, thus producing more CO and smoke.³⁵ Therefore, it is essential to control the amount of Ti₄O₇ used to ensure that the total smoke release (TSP), carbon monoxide peak (COP), and total heat release (THR) are low at the same time. When the addition amount of Ti₄O₇ is controlled at 1–2 phr, TIO exhibits a significant catalytic carbonization effect during the combustion process of SR composite materials. At the same time as SR combustion produces silica flocs, Ti₄O₇ will transform the residual carbon in the combustible material into a more stable and dense carbon layer, which can effectively combine with silica flocs to form an effective insulation layer. On this basis, smoke particles and toxic gases generated internally during the combustion of SR composite materials can be blocked inside the insulation layer, making it difficult to release them externally. Therefore, the release of smoke and toxic gases from SR composite materials is reduced, and their flame retardancy is improved.³⁶

Figure 8.

Digital photos of the char residues of (a) SR, (b) SR/Ti₄O₇0.5, (c) SR/Ti₄O₇1.0, (d) SR/Ti₄O₇2.0.

The Raman spectra provide valuable insights into the degree of graphitization of the cone volume residues, which can be assessed by calculating the ID/IG ratio. As presented in Figure 9, the I_D/I_G values for SR, SR/Ti₄O₇0.5, SR/Ti₄O₇1.0, and SR/Ti₄O₇2.0 are 0.91, 0.85, 0.79, and 0.82, respectively. This trend suggests that increasing the Ti₄O₇ content has a positive effect on the graphitization of the residue. In a specific context, the introduction of 1 part of Ti₄O₇ yields an optimal quantity that facilitates effective catalytic carbonization. This transformation induces a shift in amorphous carbon to a more stable configuration, rendering it less susceptible to secondary combustion. However, it’s noteworthy that when surpassing this point, the graphitization of the residual material displays a decline. This phenomenon implies that an excessive surplus of Ti₄O₇ imparts an adverse impact on the graphitization process. One plausible explanation for this observation is that heightened agglomeration of Ti₄O₇ particles curtails their catalytic efficacy, thereby impeding the efficient development of graphitic structures within the residual material.³⁷ These findings underscore the importance of precisely controlling the amount of Ti₄O₇ added to silicone rubber composites to optimize their graphitization and catalytic properties.

Figure 9.

Raman spectrum of the char residues of SR, (b) SR/Ti₄O₇0.5, (c) SR/Ti₄O₇1.0, (d) SR/Ti₄O₇2.0.

Electrical properties of SR and its composites

From the results presented in Figure 10, it is evident that the silicone rubber composite exhibits high breakdown strength and volume resistivity. However, with an increase in the Ti₄O₇ content, the breakdown strength and volume resistivity show a gradual decrease, indicating a significant reduction in the insulation properties of the silicone rubber composite, and an increase in its conductivity. The addition of Ti₄O₇ to the silicone rubber composite results in the formation of a conductive network, which becomes denser with an increase in Ti₄O₇ content. This increased density of the conductive network facilitates the migration of charges and shortens the breakdown path, leading to a reduction in insulation properties.³⁸ Moreover, the inclusion of Ti₄O₇ instigates the emergence of interfacial imperfections within the silicone rubber composite. This phenomenon elevates charge accumulation and induces perturbations in the electric field configuration. Consequently, these factors collectively precipitate a reduction in breakdown strength and volume resistivity.

Figure 10.

Dielectric strength (a) and volume resistivity (b) of SR and its composites.

Figure 11(a) shows that the dielectric loss factor of each specimen is smooth in the 10¹–10³ Hz band with no significant loss peaks. However, as the addition of Ti₄O₇ increases, the loss values of the specimens in the low frequency band (0.01–10¹ Hz) rise rapidly. This is because the weakly polar silicone rubber lacks polar groups and therefore does not experience high frequency relaxation losses. However, Ti₄O₇ increases the conductivity current and polarization rate within the silicone rubber, leading to an increase in conductivity losses.³⁹ In contrast, Figure 11(b) demonstrates that the relative permittivity of each specimen follows essentially the same pattern of variation. This can be attributed to the fact that the addition of Ti₄O₇ increases the degree of interfacial polarization, promotes the formation of a conductive network in the SR and further enhances the electrical losses. Consequently, these enhance the permittivity of the SR composite.

Figure 11.

TThe tanδ (a) and dielectric constant (b) of SR and its composites.

Thermal conduction of SR and its composites

It is important to consider the thermal conductivity of the SR composite, as the addition of Ti₄O₇ increases the electrical conductivity and therefore the heat inside the SR composite with the passage of electric current. As can be seen in Figure 12, the thermal conductivity of SR without the addition of Ti₄O₇ is 0.388 W/mK. However, when 0.5 part of Ti₄O₇ are added, the thermal conductivity of the SR/Ti₄O₇0.5 composite increases to 0.447 W/mK, a significant increase of 15.2% compared to SR.²⁸ This observation highlights the favorable thermal conductivity properties of Ti₄O₇. The enhancement in thermal conductivity can be attributed to the creation of thermal conductivity channels within the silicone rubber (SR) composite by Ti₄O₇, which facilitate accelerated heat transfer. Moreover, as the amount of Ti₄O₇ added increases, the thermal conductivity further improves, indicating the potential of Ti₄O₇ to enhance the overall thermal conductivity of the SR composite.

Figure 12.

Thermal conduction of SR and its composites.

Conclusion

Ti₄O₇ imparts excellent flame retardancy, thermal stability, mechanical properties, and electrical and thermal conductivity to silicone rubber composites. The silicone rubber composites containing 1.0 part of Ti₄O₇ show a total heat release (THR) of 45 MJ/m², which is 45.7% lower than the SR without Ti₄O₇ addition. The peak heat release rate (PHRR) and time to ignition (TTI) decrease gradually with an increase in Ti₄O₇ content, reaching 36.8% and 26.5% lower, respectively, at 1.0 part of Ti₄O₇ compared to SR. In addition, Ti₄O₇ is more effective at low temperatures and can act as a physical barrier to inhibit the early decomposition of silicone rubber and enhance the thermal stability of silicone rubber composites. When the temperature surpasses 450°C, the catalytic activity of Ti₄O₇ becomes active, leading to the stimulation of the decomposition process in silicone rubber composites and resulting in increased residue formation. Moreover, the addition of Ti₄O₇ in suitable proportions can additionally enhance the mechanical properties of the silicone rubber composites and improve their electrical and thermal conductivity.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Jianxi Li

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