High-temperature behavior of silicone rubber composite with boron oxide/calcium silicate

Xiaotian Wang; Yan Qin; Chenglong Zhao

doi:10.1515/epoly-2022-0051

Article Open Access

High-temperature behavior of silicone rubber composite with boron oxide/calcium silicate

Xiaotian Wang , Yan Qin and Chenglong Zhao

Published/Copyright: July 25, 2022

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From the journal e-Polymers Volume 22 Issue 1

Abstract

The ceramifiable silicone rubber (SR) composite is prepared using boron oxide, calcium silicate, and kaolin as ceramifiable fillers. The effects of the content of CaSiO₃/B₂O₃ on the high-temperature properties of composites are investigated. In the process of decomposition and oxidation of the ceramifiable SR composite in air, B₂O₃, and low-melting-point glass frit that participate in the formation of the residue network structure in different temperature regions, it continuously produces a liquid phase during the process of the ceramifying transformation. Microscopic images reveal that different structures are formed at different temperatures. The network structure of the ceramic residue becomes increasingly compact with the increase in temperature from 600°C to 800°C, which has a better protective effect on heat transfer and mass loss. At 900°C, with the lattice reconstruction of calcium silicate and the change of crystal structure, volume expansion occurs after cooling, alleviating the volume shrinkage caused by ceramic phase formation in the process of ablation. When the ratio of CaSiO₃/B₂O₃ reaches 1:1 (both are 15 phr), the bending strength and linear shrinkage of the composites reach a satisfactory balance, the bending strength and the shrinkage reach 18.5 MPa and 12.1%, respectively.

Keywords: silicone rubber; thermal decomposition; shape stability; ceramization; formula design

1 Introduction

Silicone rubber (SR), as one of the promising alternatives to the traditional polymer flame retardant materials, has attracted much attention for the heat-shielding field of the aerospace industry, civil building, and power transmission (1,2). Due to its special molecular structure, it has the characteristics of both organic polymer and inorganic polymer, excellent chemical stability, high- and low-temperature resistance, and insulation performance after ablation. At the same time, the combustion process of low smoke and less poison can effectively slow down the flame propagation speed. Nevertheless, SR still has the problems of serious pulverization of residues after combustion, which is difficult to form effective protection for the interior, and the unsatisfactory balance between mechanical properties and flame retardant properties (3). Because the ceramifiable polymer composites can overcome this shortcoming, the development of ceramizable SR with higher sintering strength, lower forming temperature, and better shape stability has become a research hotspot.

The ceramifiable polymer composites, unlike common polymers, have excellent performance at room temperature and can form hard ceramic shells at a temperature above 600°C. In recent years, ceramifiable SR composites have developed rapidly. Ceramifiable SR composites have relatively lower heat-release rates and minimal sensitivity to external heat flux in comparison to most organic polymer composites (4). Based on the in-depth study of SR composite, the ceramic forming mechanism of ceramicable SR has been widely recognized. By adding various inorganic filler, dispersant agents, and other additives to SR, the mechanical properties of the composite are enhanced at room temperature. Moreover, eutectic reactions take place at high temperatures between mineral fillers and combustion products of SR, and then, the hard, durable ceramic layer with a porous structure is formed (5).

Ceramifiable SR composites are composed of SR, inorganic ceramic fillers, fibers, fluxing agents, and other functional additives, with a more comprehensive performance than ordinary SR (6). Tang et al. (7) prepared ceramic SR composites with low-melting point glass powder as the co-solvent and studied the influence of glass powder content on its mechanical properties and ceramization process. The results show that glass powder can significantly reduce the thermal decomposition temperature of SR. However, as its content continues to increase, the surface of the composite material is further transformed into a porous pyrolysis layer, and a eutectic reaction occurs between the fillers to form new crystals to form a denser ceramic layer, thereby increasing the bending strength after ceramization. There are also many studies regarding the effects of silicate mineral fillers on SR ceramization. Hao et al. (8) prepared ceramic SR with wollastonite with different aspect ratios and studied the effect of its content on the mechanical properties and pyrolysis process of the composite material. The results show that different wollastonite has an effect on the mechanical properties of SR. Acicular wollastonite will slow down the thermal decomposition of SR, having more excellent flame retardant properties. Compared with thermal stability, the sintered body of ordinary wollastonite has a higher bending strength. This is because of its smaller particle size, which is beneficial to sintering. Hu et al. (9) studied the phase composition of silicon rubber/ammonium polyphosphate/aluminum hydroxide/mica composites during ceramization at different sintering temperatures. The results show adhesive phosphate is formed between ammonium polyphosphate and the thermal decomposition products of aluminum hydroxide, and phosphate fills the pores at 800°C. The density of the ceramic residue formed between mica and phosphate is improved, and the composite has high bending strength and excellent self-support. This happens to coincide with the research of Hanu et al. (10), who had performed numerous studies on ceramic mica/SR and frit mica/SR composites. They find that the ceramization reaction is accomplished by eutectic reactions between mica and organosilicone-fired products that required temperatures of at least 800°C. However, the addition of sinter blocks can reduce the temperature of the ceramization reaction. Furthermore, Guo et al. (11) studied the effect of the glass fit with the softening temperature of 480°C on the properties of ceramizable composites, and also investigated the formation mechanism of the ceramic polysiloxane elastomer. The structure of the ceramic residue is converted from the sea-island phase to a uniform matrix with the increased content of the glass frit and the linear shrinkage, as well as flexural strength and impact strength of the ceramic residue obviously increase. Anyszka et al. (12) added borate into SR composites to improve ceramifying reaction at low temperatures, due to the thermal decomposition of the borate producing boron oxide as an adhesive. Although the thermal stability of the SR decreases significantly, the fluxing agent (B₂O₃) microparticles are removed from the ceramic phase into the volatiles. It has a certain guiding significance for low-temperature sintering molding of ceramifiable SR.

Many researchers have focused their attention on the properties of ceramifiable SR composite with low softening temperature fluxing agents, while ignoring the damage of volume shrinkage, which is caused by matrix combustion and liquid phase. Thus, herein, we have demonstrated a novel packing combination strategy, adjusting the proportion of high-temperature resistant fillers with different melting points, to form a compact ceramic shell at a lower ablation temperature to obtain a satisfactory balance of bending strength and linear shrinkage after ablation. Specifically, the processes of thermal decomposition and ceramifying transformation are discussed. The synergistic effect of boron oxide and calcium silicate, acting as fluxing agents and high-temperature resistant packing, on the ceramifying properties of SR composite is investigated.

2 Experimental

2.1 Materials

Commercial methyl vinyl SR containing 0.13–0.18% of vinyl groups, reinforced by fumed silica with a Brunauer–Emmett–Teller surface area of 300 m²·g⁻¹, was used as the matrix and produced from Chengdu Zhonghao Chenguang Technology Co. Ltd (Chengdu, China). Kaolin (Al₂O₃·2SiO₂) with an average particle size of 11 µm and calcium silicate were used as ceramifiable fillers. Magnesium hydroxide was used as refractory fillers, enhancing ceramization process. The purity of the calcium oxide is greater than 98%. Low-melting-point glass frits, which could melt within the range of 450–650°C to form a liquid phase, and boron oxide were the fluxing agents. Hydroxyl silicone oil could soften the SR to improve the processability of the composites. 2,5-Dimethyl-2,5-di(tert-butylperoxy) hexane (DBPH) used as the vulcanizing agent of the SR was purchased from Guizhou Weidun Crystal Phosphorus New Material Co. Ltd (Guizhou, China). The silane coupling agent KH-550 (the chemical name is 3-amino propyl triethoxy silane), low-melting-point glass frit, and hydroxyl silicone oil were also from the same company. All other raw materials were purchased from Shanghai Macklin Biochemical Co. Ltd (Shanghai, China).

2.2 Sample preparation

2.2.1 Surface modification of fillers

To modify the surface of all fillers except low-melting-point glass frit, we used KH-550 3% aqueous ethanol solution; 3.0 g of KH-550 was mixed in 100 g of 95 wt% aqueous ethanol using a mechanical agitator at a rotation speed of 1,000 rpm, stirring the mixture evenly for 4 h at room temperature until the fillers were evenly dispersed. Finally, the mixture was placed in a vacuum-drying oven for 12 h at 120°C to ensure that all of the solvents were removed.

2.2.2 Preparation of composites

The SR and all the inorganic fillers were mixed in a two-roll open mill at room temperature for 45 min until a homogeneous batch was obtained. And then the vulcanizing agent DBPH was incorporated into the composites until a visually good dispersion was achieved. The samples were molded to platens by press vulcanizer at 175°C with the pressure of 10 MPa for 10 min. Then, they were put in a blast drying oven at 200°C for 2 h for additional vulcanization. The compounding formulas of prepared samples are given in Table 1.

Table 1

Chemical compositions of silicone rubber composites

Sample	Ingredients (phr)a
Sample	Silicone rubber	Fumed silica	Kaolin	Magnesium hydroxide	Boron oxide	Calcium silicate	Glass frit	DBPH
SR1-1	100	35	30	15	5	5	15	2
SR1-2					5	10
SR1-3					5	15
SR1-4					5	20
SR2-1					10	5
SR2-2					10	10
SR2-3					10	15
SR2-4					10	20
SR3-1					15	5
SR3-2					15	10
SR3-3					15	15
SR3-4					15	20
SR4-1					20	5
SR4-2					20	10
SR4-3					20	15
SR4-4					20	20

a
Parts per hundreds of rubber.

2.3 Performance characterization

The density of the SR composites is measured, according to Chinese Standard GB/T 533-2008 at room temperature, using DK-300 automatic electronic densitometer. The shore A hardness of the composites is conducted by LX-A Shore Durometer at room temperature.

The tensile strength and elongation at break tests of the composites are performed using an Instron-5967 universal testing machine. The loading speed is 100 mm·min⁻¹. The linear shrinkage is calculated as Eq. 1:

(1) L = L 1 − L 2 L 1 × 100 %

where L is the linear shrinkage (%). L ₁ and L ₂ are the length of the samples before and after heat treatment, respectively.

The bending strength of ceramic residue is assessed using an Instron-5967 universal testing machine. The loading speed is 2 mm·min⁻¹. The span is 64 mm. The bending strength is calculated as Eq. 2:

(2) σ f = 3 p ⋅ l 2 b ⋅ h 2

where σ _f is the bending strength (MPa). p is the load (N). l, b, and h are the span length (mm), width (mm), and thickness (mm) of the ceramic residue, respectively.

Thermal gravimetric analysis (TGA, STA2500, NETZSCH, Selb, Germany) is conducted to investigate the thermal stability of the samples under air. Then, a series of samples are heated at a rate of 10°C·min⁻¹, from room temperature to 1,000°C. Fourier transform infrared spectroscopy (FTIR) is obtained in the range of 400–4,000 cm⁻¹ on a Nexus FTIR spectrophotometer (Thermo Nicolet, Waltham, MA, USA).

The crystal phases of the ceramic residue are identified using an X-ray diffraction (XRD) method (PANalytical B.V, Heracles Almelo, Netherland). The scan is conducted from a 2θ angle of 5° to 80° with a step interval of 4°. The morphology of the ceramic residue after combustion at different temperatures is characterized using field emission scanning electron microscopy (FESEM, JSM-7500F, JEOL (BEIJING) Co., Ltd, Beijing, China).

3 Result and discussion

3.1 Mechanical properties at room temperature

Density and shore A hardness of composites with different proportions of boron oxide and calcium silicate are shown in Figure 1. Both of them show a stepwise upward trend with the increase of fillers from 5 to 20 phr. The density of the composites increases from 1.35 to 1.66 g·cm⁻³. This phenomenon is due to the addition of high-density components. Calcium silicate (2.9 g·cm⁻³) and boron oxide (2.46 g·cm⁻³) have an obviously higher density compared with SR (1.1–1.2 g·cm⁻³). Meanwhile, the shore A hardness value of the composites also shows the same trend. The peak hardness reaches 87 with 20 phr fillers, which means both of them to reduce the flexibility of SR molecular chains. In fact, due to the larger particle size of B₂O₃ (200 mesh), the addition of it results in less flexibility and greater hardness of the composites compared to calcium silicate.

Figure 1

Density and shore hardness of composites.

Figure 2 shows the variety in tensile strength and elongation at the break of the composites at room temperature. The tensile strength is evidently improved by the increase of calcium silicate, while the counterpart is relatively impaired with boron oxide added (Figure 2a). For different amounts of calcium silicate, the tensile strength decreases by an average of 0.84 MPa of about 16%, while the counterpart for boron oxide improves by an average of 0.56 MPa of about 11%. However, for the elongation at break, the addition of both led to a uniform downward trend (Figure 2b). With the increase in the amount of fillers, the elongation at break decreases by 34.6% (CaSiO₃) and 72.9% (B₂O₃), respectively. This phenomenon may be caused by the formation of multiple hydrogen bonds between the silanol functional groups on the surface of calcium silicate and the SR, which has a relatively stable binding ability to achieve the reinforcement of materials. Adding boron oxide, by contrast, harms the continuity of the matrix and the poor bonding between the surface of the filler particles and the SR matrix led to the formation of weak interfacial interactions, resulting in the increment of defects and the reduction of the tensile strength of the composites. On the other hand, in the process of the composites being stretched, particles hinder the elongation of SR molecular chains, decreasing the elongation at break. This influence may be related to the particle size of fillers. The average particle size of boron oxide (200 mesh) used in this article is larger than that of calcium silicate (1,250 mesh), which is not conducive to the dispersion of boron oxide in the SR matrix, which also illustrates the increase of the hardness of the composites. It has already been reported that the use of fillers with small particle sizes can increase the tensile strength for better reinforcement (13).

Figure 2

Tensile strength (a) and elongation (b) of the composites, at break at room temperature.

3.2 Bending strength and size stability

Figure 3 shows the differences in the linear shrinkage of the ceramic residue at four representative temperatures. Under 800°C, boron oxide plays a leading role in the linear shrinkage of the composites. The ceramic residue has no obvious shrinkage at 600°C, while counterparts reach 16.5% and 19.8% at 700°C and 800°C, respectively. A more liquid phase is generated by the melted complex compound or solid solutions form from boron oxide and other oxides (e.g., MgO and CaO) with a low melting point during the sintering process (14) (about 700°C). The liquid products flow at high temperatures and fill the microholes formed by the high-temperature decomposition of SR. Then, during cooling, the liquid products shrunk under the action of surface tension, resulting in a more compact structure, whereas, at 900°C, the linear shrinkage of composites is significantly reduced by the addition of calcium silicate. With the increase in calcium silicate dosage, the linear shrinkage of ceramic residue decreases by 33.1% on average. Studies have shown that calcium silicate undergoes lattice reconstruction, accompanied by crystal transformation, and disordered amorphous structure begins to transform into ordered low-temperature single-chain 2M-wollastonite structure around 830°C (8), which can also be confirmed in the XRD pattern. During the cooling process, β-dicalcium silicate is transformed into γ-dicalcium silicate. Due to the large density difference, the crystal transformation will cause a large volume effect, with a volume expansion of about 12% (15,16).

Figure 3

Linear shrinkage of ceramic residue calcined at (a) 600°C, (b) 700°C, (c) 800°C, and (d) 900°C.

Figure 4 illustrates a distinct variety in the bending strength of ceramic residue obtained from 600°C to 900°C. As the content of both increases, from 5 to 20 phr, at 600°C, the bending strength has no obvious change. In contrast, the mechanical property has improved gradually, from 7.28 MPa (700°C) and 10.51 MPa (800°C) to 14.78 MPa and 16.05 MPa, respectively. More boron oxide could produce more liquid phase at high temperature, which is conducive to form the more compact structure of the ceramic residue, resulting in the higher mechanical properties of the residue. However, low-melting-point glass frits begin to melt at 450°C and melted completely at 700°C. Therefore, under 700°C, the bending strength of SR composite material does not reach the highest theoretical level. As the temperature increases, due to the complete melting of the glass frits, more liquid phase, generated to fill the porous structure in the ceramic residue, undergoes a eutectic reaction to form new crystals with ceramic fillers, resulting in a further increase in bending strength. At higher temperatures (above 800°C), with fillers increasing, the bending strength changes to a peak shape. When the ratio reaches 1:1, the peak intensity reaches 18.5 MPa. Due to the phase transformation of calcium silicate in the sintering process, irregular dense microcellular is formed inside. When calcium silicate is present in a relatively excessive amount, this would reduce the bending strength of composites (17). However, the addition of boron oxide inhibits the rate of crystal transformation, making it stable and slowing down the volume expansion. When they reach the ideal balance, the bending strength and size shrinkage reach 18.5 MPa and 10%, respectively (18).

Figure 4

Bending strength of ceramic residues calcined at (a) 600°C, (b) 700°C, (c) 800°C, and (d) 900°C.

3.3 Microstructure and phase evolution

3.3.1 Comprehensive thermal analysis

Figure 5 shows the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of nine typical samples and magnesium hydroxide in air. T ₅, T _max1, T _max2, and residue weight at 1,000°C are listed in Table 2. It can be seen that composites undergo two stages of thermal decomposition process in air. The first stage at 302–415°C is attributed to the decomposition of magnesium hydroxide, and the second stage at 415–583°C is due to SR (Figure 5b). T ₅ and T _max2 of SR1-1, SR2-1, SR3-1, and SR4-1 are lower than those of SR0-0. With the increase in boron oxide, T ₅ decreases from 400.7°C to around 345.2°C and T _max2 shifts from 491.9°C to around 449.8°C. However, adding calcium silicate has little effect on T ₅ and T _max2 of composites. The low decomposition temperature indicates the reduction in the thermal stability of the composites under air. This is mainly attributed to boron oxide being an acidic substance, which accelerates the random chain decomposition of SR matrix backbone at the low-temperature region (19,20). On the other hand, through endothermic decomposition, water release, and poor interface connection between the flame retardant fillers and molecule chains of SR, the heating rate of the composite material is reduced, resulting in the side chain oxidation decomposition of the SR, which could cause the decrease in thermal stability of the SR composites (21,22,23).

Figure 5

TG and DTG curves for composites and magnesium hydroxide: (a) TG curves and (b) DTG curves.

Table 2

Thermal decomposition parameters of silicone rubber composites

Sample code	T ₅ a (°C)	T _max1 a (°C)	T _max2 a (°C)	Residue weight (wt%)
SR0-0	401	375	492	61
SR0-1	405	379	483	63
SR0-2	407	366	480	61
SR0-3	397	371	482	65
SR0-4	399	377	483	63
SR1-1	344	368	444	64
SR2-1	341	351	455	64
SR3-1	346	359	444	66
SR4-1	35	356	457	68
Mg(OH)₂	334	359	—	—

a
T ₅, T _max1, and T _max2 are the temperatures of 5% weight loss, first and second peak decomposition, respectively.

Decomposition reaction of magnesium hydroxide occurs due to heat absorption during 300–385°C, liberating water, magnesium oxide (Figure 5b). T _max1 is the maximum decomposition temperature of magnesium hydroxide. In this process, crystal water from the magnesium hydroxide is liberated and gasified, which causes the main mass loss of the SR composites. From Figure 5a, the value of the mass loss is about 31.05% for magnesium hydroxide, which is consistent with the theoretical water content of magnesium hydroxide (31.03%). Boron oxide, as a flux in connection with other ceramic fillers, starts to soften at 450°C and turned completely into liquid at 700°C (24), which could form a perfect protective layer to cut off the pervasion of external heat and decrease mass loss, improving residue weight. It is obvious that the residue weight increases from 62.78% to 68.82% (Table 2). With increasing temperature, the decomposition reaction continues in the second stage at 415–583°C. The macromolecular chains of SR are ruptured to form cyclic oligomers, including cyclic trimers and tetramers (25). Cyclic oligomers are decomposed and oxidized at high temperature, producing CO₂, H₂O_(g), and an overwhelming amount of white silica with a small amount of black residue (e.g., silicon carbide and silicon nitride), that is difficult to be detected, which results in the second main mass loss of composites (21).

3.3.2 FTIR analysis

Figure 6 shows the FTIR spectra of ceramic residue obtained by the typical sample (SR3-3) at different temperatures. A broad peak between 3,100, and 3,600 cm⁻¹ is assigned to the vibration of hydroxyl (–OH). Hydroxyl is mainly from crystal water in the SR composite before heat treatment, and after combustion, the porous char layer tended to absorb moisture from the release of crystal water and air (26). FTIR peaks are assigned as follows: 3,696 cm⁻¹ for Mg–OH asymmetrical stretching, 2,964 cm⁻¹ for –CH₃ asymmetric stretching, 1,414 cm⁻¹ for –CH₃ deformation, and 1,261 cm⁻¹ for –CH₃ wagging. Due to the disappearance of these peaks after combustion at 600°C, this means that an oxidation reaction of methyl and methylene on the side chains and thermal decomposition of magnesium hydroxide has occurred. Characteristic absorption peaks of the Si–C bond at 802 cm⁻¹ also become noticeably weak after decompositions above 600°C, which illustrates the destruction of the network structure of SR, resulting in fracture of the Si–C bond. The peaks observed at 1,076 and 464 cm⁻¹ are associated with the symmetrical stretching and bending deformation vibration of Si–O bond and the vibrations of calcium ions (17), respectively. At 900°C, the absorption band 1,076 cm⁻¹ moves to the low frequency 1,047 cm⁻¹ and becomes slightly wider. Meanwhile, a new absorption band 607 cm⁻¹ appears and tends to strengthen, which is the result of the phase transformation of calcium silicate at this temperature, indicating that its crystallinity and ordering degree gradually improve. Moreover, the peak marked at 1,392 cm⁻¹ is the characteristic absorption peak of the B–O bond. As the temperature increases, SR gradually decomposes and boron oxide and other oxides with low melting points continuously soften and melt, as a new bridge connecting the ceramic filler and SR pyrolysis residue, according to the disappearance of methyl groups (–CH₃), the fracture of Si–C bond, and the weakening of the Si–O bond. The peak at 673 cm⁻¹ is ascribed to the vibration of Al–O–Si groups and enhances above 800°C, which indicates that some eutectic reactions between Al₂O₃ and SiO₂ happen to form aluminosilicate (11). The appearance of aluminosilicate has positive significance for the enhancement of the bending strength of the composites.

Figure 6

FTIR spectra for ceramic residue: original, 600°C, 700°C, 800°C, and 900°C.

3.3.3 XRD analysis

The XRD patterns of the ceramic residue of the sample SR3-3 are illustrated in Figure 7. A large hump appears at around 2θ = 10–15° at room temperature, indicating the presence of amorphous SiO₂. Amorphous SiO₂ is generated from kaolin and might be the original additional SiO₂. The diffraction peaks of magnesium hydroxide and calcium oxide are obvious. Due to the influence of amorphous SiO₂ and other phases, the diffraction peaks of calcium silicate are weak. When the combustion temperature is 600°C, with the dehydration of magnesium hydroxide and thermal decomposition of SR, the large hump of amorphous SiO₂ disappears while the new phase appeared, namely, clathrasil ((SiO)_x), and the diffraction peaks of calcium silicate are gradually prominent. With temperature continuously increasing, the lattice reconstruction of calcium silicate occurs along with the fracture of part of silyl groups, which leads to the transformation of disordered amorphous structure into ordered low-temperature single-chain structure (wollastonite-2M (PDF#27-0088)). From the XRD pattern, the crystal structure of partial silicon dioxide is changed to a hexagonal lattice, diffraction peaks of phosphorus quartz appear at the temperature of 700°C and 800°C. Moreover, there are also a few diffraction peaks of silicon nitride and silicon carbide (27).

Figure 7

XRD patterns of combustion residue at different temperatures.

When the combustion temperature is 900°C, the peaks of wollastonite-2M continue to increase while the new phrase appears, namely, mullite (3Al₂O₃·2SiO₂), which indicates that some eutectic reactions between Al₂O₃ and SiO₂ happen to form aluminosilicate. The diffraction peaks of cristobalite (SiO₂(PDF#39-1425)) appear for the first time, indicating that some silica crystals change from hexagonal lattice systems to cubic lattice systems. This is because the content of glass powder is relatively insufficient, and silica cannot fully participate in the porcelain-forming process but is converted into cristobalite at high temperatures (7). According to the XRD pattern analysis, many thermal chemical reactions have occurred during the process of SR composite combustion, which is consistent with the results of the FTIR spectra (28).

3.3.4 Micro-morphology

Figure 8 gives the microstructure images of the interior structure of combustion residue at different calcined temperatures. After calcination at 600°C, tiny microcracks and pores appear on the interior of the composites; due to the lack of liquid phase, it is apparently difficult for ceramic residue to form a continuous whole. Hence, the ablated composites have almost no strength at this temperature.

Figure 8

Micromorphology of combustion residue under different temperatures: (a) 600°C, (b) 700°C, (c) 800°C, (d) 900°C; (L) B₂O₃:CaSiO₃ = 5:20%; (R) B₂O₃:CaSiO₃ = 20:5%.

Under 700°C, SR has been decomposed and crystalline water is thoroughly released, resulting in the appearance of a mass of micropores, as shown in the right image of Figure 8b, and some radially distributed dense glassy phases appear locally in the residue. These local areas are formed by the melting of low-melting-point substances in the system. For instance, B₂O₃ and low-melting-point glass frits start to melt at approximately 450°C and flow and fill in the direction of the gap at higher temperatures. After being cooled to room temperature, local strong connections are formed, connecting ceramic residue through through-holes. At the same time, calcium silicate still exists in granular form, lamellar in part, and flake fillers are embedded in the cavity formed by the decomposition of the matrix, which is particularly obvious in the SEM of the cross-section, and the overall structure tends to close contraction (17).

After thermal treatment at 800°C, as the boron oxide content increases, the local glass phases gradually connect to each other. The liquid phase is so abundant that it completely covers the ceramic fillers (Figure 8c). At 900°C, when boron oxide is abundant, this trend is further enhanced with the formation of a new crystal phase, resulting in the compact inside structure and smooth surface of the ceramic residues. This has also been demonstrated by infrared spectroscopy. However, when calcium silicate is in relative excess, due to the crystal transformation and lattice reconstruction, a large number of irregular clusters with molten exterior appeared (17). Although the volume expansion improves the volume shrinkage of the composites to a certain extent, plentiful micropores also reduce the bending strength of materials.

4 Conclusion

In this article, the self-supporting properties and shape stability of silicon rubber composites at high temperatures were improved by optimizing the ratio between calcium silicate and boron oxide. By studying the thermal decomposition and ceramization processes of the composites under air, we found that the composites undergo different reaction types in different temperature ranges. Before 800°C, B₂O₃ and low-melting glass frit participated in the formation of the slag network structure through continuous melting and filling of voids. After 800°C, the molten liquid phase reacts with the ceramic filler to form an amorphous glass phase through connecting holes to form a bridge structure, generating new crystals (mullite), which made the network structure of the ceramic residue more compact. In addition, by adjusting the ratio of CaSiO₃/B₂O₃, it is found that there are significant differences in the influence of fillers on flexural strength and linear shrinkage at different temperatures. Before 800°C, boron oxide plays a leading role in linear shrinkage, and the addition of boron oxide makes the linear shrinkage rate of the composite material reach 16.5% (700°C) and 19.8% (800°C), respectively; The volume expansion caused by the change of calcium silicate has a more significant effect on the linear contraction. At 900°C, when the calcium silicate concentration is increased to 20 phr, the linear shrinkage of the composites decreased by an average of 33.1%. When the ratio between boron oxide and calcium silicate reaches 1:1 (both 15 phr), the flexural strength and the shrinkage of the composites reach 18.5 MPa and 12.1%, respectively.

Funding information: Authors state no funding involved.
Author contributions: Xiaotian Wang: writing – original draft editing, chart editing, analyzing data, process management; Yan Qin: conceptualization, resource, supervision; Chenglong Zhao: writing – original draft, writing – draft review.
Conflict of interest: Authors state no conflict of interest.
Data availability statements: The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Received: 2022-04-13

Revised: 2022-05-06

Accepted: 2022-05-10

Published Online: 2022-07-25

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/epoly-2022-0051

Keywords for this article

silicone rubber; thermal decomposition; shape stability; ceramization; formula design

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BY 4.0