In this article, the composite insulator silicone rubber shed, naturally retired after 15 years of operation, was used as the raw material to degrade and prepare nano-silica materials. The typical appearance of some shed samples is shown in Figure 1. Cracks appeared on the surface of some sheds of waste composite insulators, and the sheds were seriously whitened, faded, and hardened. The color of the shed surface and the interior of some samples are very different, and the fading is obvious, which can be directly identified with the naked eye. It means that its aging is very serious and it is no longer suitable for continuing to run on the network. The Shore A of waste silicone rubber was 73.10, and its average hydrophobic angle was 88.73°.

Pretreatment of waste silicone rubber sheds: the silicone rubber sheds of the waste composite insulators are peeled off as a whole, thinned, and cut to obtain silicone rubber particles. Then, the silicon rubber particles obtained by cutting are fully stirred with a sufficient amount of anhydrous ethanol to separate silicone oil and silicone grease, and the filtrate is filtered to remove the filtrate to obtain once-washed silicone rubber particles, which are repeatedly washed several times to obtain clean silicone rubber particles. The clean silicone rubber particles are dried in a blast drying oven at 60°C for 0.5–2 h to make them lose moisture completely.

When the transmission line passes through the humid areas with dense vegetation, it is usually easy to breed microorganisms such as algae and fungi on the surface of the silicone rubber (Figure 2). After genetic identification, the epiphytes on the surface of the silicone rubber are mainly Streptomyces and Xanthomonas. Dominant flora including Streptomyces and Xanthomonas were extracted and added to a mineral salt medium. The formula of the culture medium (Jansomboon et al., 2020) is entered in the following having 0.1 mL of the trace element (FeSO₄·7H₂O, 50 mg; ZnSO₄·7H₂O, 1 mg; MnCl₂·4H₂O, 0.3 mg; H₃BO₃, 3 mg; CoCl₂·6H₂O, 2 mg; CuCl₂·2H₂O, 0.1 mg; NiCl₂·6H₂O, 0.2 mg; and Na₂MoO₄·H₂O, 0.3 mg) in 100 ml of media adjusted to pH 7.0, including (g/L) Na₂HPO₄·12H₂O (9.0), KH₂PO₄ (1.5), NH₄NO₃ (1.0), MgSO₄·7H₂O (0.2), CaCl₂·2H₂O (0.02), and Fe(III)[NH4] citrate (0.0012).

The ternary mixtures from solvents including diethylamine, hexane, and methanol or in a binary mixture of hexane and methanol were prepared in a 250 ml flask. A small piece (approximately 5 mm × 3 mm × 4 mm in size) of the vulcanized silicone rubber shed containing filler was mixed with acetone into the solvent. At room temperature or under solvent reflux conditions, the mixture is fully contacted via a magnetic or mechanical stirrer to dissolve the silica in the silicone rubber into the solvent, the mixed solution is filtered, and the filtrate is transferred to a 250 ml beaker. If adjusting the pH value is necessary, a small amount of KOH is added, and then, the solution is heated and distilled to remove the solvent. The filtration residue was treated again with a mixture of diethylamine and hexane to obtain the packing.

After the reaction, a white powder, namely, silicon dioxide, was collected, and 0.5 g of the recovered filler was added to 10 ml of a mixture of methanol and H₂O (v/v = 1:1) and placed in a planetary ball mill for wet grinding at room temperature for 24 h. Then, it is dried and put into a glass bottle with an aluminum cover to obtain nano-silica. The removal of this antimicrobial substance by organic solvent extraction would lead to increased growth of several microorganisms on this substrate in the next experiment.

The schematic diagram of the established biodegradation is shown in Figure 3. The bioreactor column (I.D. 5 cm × l80 cm) is made of borosilicate glass. The lower part of the bioreactor (up to 28 cm in height) was used as a suspended growth bio scrubber, and the upper part (up to 70 cm in height) was used as a conventional biofilter. The biofilter unit was filled with blocks of polyurethane foam and held at a height of over 34 cm by using a stainless steel frit (0.1 mm × 0.1 mm). Bacterial cultures are cultured by packaging media via an upper nutrient tank. Excess bacterial culture is collected at the bottom of the bioreactor through a bed of polyurethane foam forming a suspended growth bioscrubber. The OD value of liquid samples from the device is periodically monitored. When the OD value exceeds 0.9, 10% culture medium is added. The gas product produced in the bubbler is injected into the scrubber at a flow rate of 0.06–0.48 m³/h. The carbon dioxide concentration was measured by gas chromatography, and two sampling ports were appropriately set in each unit to extract samples. The biodegradation took 30 days. v e = − ln ( 1 − φ r ) + φ r + χ φ r 2 V 0 ( φ r 1 3 − 2 φ r f ) = ρ 0 M c . (1)

In the formula, v e is the cross-linking density, mol cm⁻³; ρ 0 is the density of the silicone rubber before swelling, mg·m⁻³; V ₀ is the molar volume of the solvent, generally 105.91 L mol⁻¹; φ r is the volume fraction of the silicone rubber in the swelling body, that is, the reciprocal of the swelling degree; χ is the interaction parameter between the silicone rubber and the solvent, generally 0.43; f is the functionality of the adhesive network; and M c is the average relative molecular mass between the cross-linking points, g mol⁻¹.

In order to explore the biodegradation law of waste silicone rubber and the formation mechanism of nano-silica, a gas chromatography-mass spectrometer (GC-MS, US, Agilent) was used to test the composition of gas-phase products with a detection accuracy of 0.01 × 10⁻⁶. Also, a thermogravimetric-mass spectrometer (TG-MS) was used to analyze the gas generation process and reasons. To analyze the physical and chemical properties of the solid product, X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) and Fourier transform infrared spectroscopy (FTIR, Nicolet iS5) were used to study its elements and functional groups, respectively. At the same time, the particle size and particle size distribution were detected by using a laser particle size analyzer (LPSA, Mastersizer 2000), and the morphology was observed by using a scanning electron microscope (SEM, JEOL JSM-7800F, Japan).

The degradation test lasted 30 days. During the test, an analytical balance was used to monitor the quality change of the silicone rubber, the weight loss in the silicone rubber was detected, and different factors and levels of the orthogonal test were set. Three parallel samples were made for each experimental condition, and the average value was taken as the experimental result.

According to the experimental results in Table 1, when the temperature is the same, the primary and secondary relationship between the factors affecting the degradation experiment is the concentration of bacteria, followed by the ventilation. The optimal experimental condition obtained by the orthogonal experiment is B3C2; that is, the biomass concentration is 1500 CFU/ml and oxygen ventilation is 15 L/min while keeping the room temperature at 25°C, with pH adjusted to 7.0.

The degradation experiment of waste silicone rubber was carried out under optimized conditions, and the calculated cross-linking density of silicone rubber was only the total cross-linking density of silicone rubber, or the apparent cross-linking density. In silicone rubber systems, cross-linking points can be divided into two categories: one type is formed by the crosslinking on the molecular chain and the entanglement between the molecular chains, which is called chemical crosslinking; another type is formed by the relatively weak chemical bond between the filler and the siloxane polymer, such as the adsorption of the filler and the siloxane molecule, which is called physical crosslinking. Jansomboon et al. (2020) proposed that the silicone rubber filled with fillers was fully swollen in toluene dissolved with ammonia gas, and the ammonia gas could enter the silicone rubber to destroy the adsorption between the filler and the siloxane molecules, thereby destroying the physical crosslinks in the silicone rubber system. Under this condition, the measured cross-linking density of silicone rubber is only the chemical cross-linking density, which is lower than the apparent cross-linking density, and the difference between the physical cross-linking density and the chemical cross-linking density is the apparent cross-linking density of the silicone rubber. The cross-linking density reflects the number of cross-linking bonds in the rubber. Under optimized conditions, after 60 days of degradation, the cross-linking density of waste silicone rubber is as shown in Figure 4. After degradation, the cross-linking density decreased by 47.6%, indicating that mold destroyed part of the cross-linking bonds in the rubber, which was helpful for the further degradation of natural rubber.

According to the GC-MS monitoring results during the experiment, the dimethicone rings (D₃–D₈) are the main cracking products of silicone rubber. Because the structure of the lysis products is relatively similar, the chromatographic retention time is also relatively close. The specific product mass fraction and its share in the PDMS are shown in Table 2, and small-molecule siloxane rings (D₃, D₄, and D₅) are numerous compared to the slightly larger molecular weights of D₆, D₇, and D₈.

Small-molecule cyclosiloxane fragments captured by GC-MS are characterized with obvious characteristics. For example, the characteristic ions of D₃ are m/z 133 and 191, the characteristic ions of D₄ are m/z 249 and 265 (Figures 5A,B), the characteristic ions of D₅ are m/z 267 and 201, the characteristic ions of D₆ are m/z 341, and the characteristic ions of D₇ are m/z 415, 327, and 281. D₈ and D₉ mass spectrograms such as m/z 73, 147, 221, 281, and 355 are more obvious, but when comparing D₈ with D₉, the proportion of m/z 355 fragments in the D₈ mass spectrogram is higher, and m/z 429 in D₉ is higher (Figures 5C,D); it may be that D₈ is more easily decomposed into D₅, and D₉ is more easily decomposed into D₆. Exploring its decomposition law, it can be found that the ring decomposition products all have D₃, indicating that it is easy to decompose at a high temperature of D₅–D₉ due to the instability, while D₃ is relatively stable, so it is decomposed into cyclosiloxane with a small molecule such as D₃, which is consistent with the trend of product proportion change. Comparing chain siloxanes to cyclic ones, the abundance of fragment ions produced by chain siloxane decreases with the increase in molecular weight mainly due to the occurrence of breakage of the main chain. For example, the content of m/z 73, 147, 221, 281, 355, and 429 in tetramethylhexasiloxane (C₁₄H₄₂O₅Si₆) (Figure 5E) is higher, and the possible cracking mechanism is discussed as follows.

In the bioreactor cavity, polysiloxanes are first broken into chain-like siloxanes with smaller molecules. However, as the lysis reaction progresses, the lysed chain siloxane undergoes rearrangement degradation to form a more stable small cyclic dimethicone. The possible cleavage mechanism of chain siloxane decomposition into D₃ and D₄ is shown in Figure 6. Similarly, other ring bodies can be rearranged from chain siloxanes.

Compared with C-O-C, the Si-O-Si chemical bond has higher energy; therefore, the biochemical cracking of silicone rubber is generally more difficult. The capability of producing fragments is due to the fact that the atoms of their molecular chains contain pairs of solitary electrons, while the other atoms contain empty orbitals, and at high temperatures, two adjacent atoms coordinate with each other, prompting bond breaks to form cyclic compounds (Kumar and Wyman, 2008; Jiang et al., 2009). The possible cleavage mechanism of the main fragment ions of D₄ and D₃ is shown in Figures 7, 8.

By pretreatment and separation of waste silicone rubber according to Section 2.2, a fumed silica is obtained; despite it being fully ground since the silica tends to form a three-dimensional cluster structure, the resulting silica contains not only extracted nano-silica but also micron-level particles. Through the test of the particle size distribution of the silica obtained by the test, it was found that its particle size was normally distributed, containing three peaks, including 111.4, 134.3, and 217.8 nm.

The relatively average specific surface area is around 217.8 m² g⁻¹, and the specific distribution values are shown in Table 3. The dispersibility of silica particle size is not conducive to its effective recycling. Therefore, it is necessary to use surfactants to modify silica. First, silica particles above 200 nm are separated, 0.2 g of nano-silica, 0.3 g of sodium dodecyl sulfate (SDS), and 0.5 g of co-surfactant (monohydric or dihydric alcohol) are weighed, and 250 m is added; deionized water is added to the conical flask to 100 g, mixed well, and stirred at 80°C for 2 h; heating is stopped; nano-silica dispersion is obtained; and a certain amount is taken and allowed to stand in a transparent glass bottle to observe the stability of the nano-silica dispersion (whether there is precipitation at the bottom). In the process of modifying silica, the ambient temperature is 25°C and the relative humidity is 60%. The particle size of nano-silica was monitored, and the stability of silica colloidal suspension was represented by Zeta potential, as shown in Figure 9; we found that the particle size gradually decreased with time and finally stabilized and the absolute value of Zeta potential increases and stabilizes above 60, indicating that the solution is in a state of excellent stability.

As shown in Figures 10A,B, the nano-silica dispersions under different magnifications were observed by using a scanning electron microscope (SEM), and it was found that the particle diameter was about 18 nm, which was consistent with the results of the particle size analyzer.

Meanwhile, the micrographs of fungi corrosion on the surface of waste silicone rubber were also taken as shown in Figures 10C,D; spherical cells secrete extracellular polymers to form biofilms on the surface of silicone rubber, the corrosive protease contacts the surface of the silicone rubber, causing the silicone rubber to become rough and uneven in appearance, and the fungus penetrates deep into the silicone rubber and disintegrates the macromolecular structure of the PDMS, resulting in irreversible degradation.