Sol–Gel CaCO₃/SiO₂ Boost Anti-Flashover Silicones

Abstract

This study developed high-performance anti-flashover silicone coatings using sol–gel-synthesized CaCO₃/SiO₂ hierarchical fillers optimized via L₁₆(4⁵) orthogonal design. The optimal filler (Sample 5) was prepared under 70 vol% ethanol, with nTEOS:nCaCO₃ = 1:1 and 0.2 mol/L NH₃·H₂O, at 45 °C, for 18 h, featuring covalent Si-O-Ca bonding, a dual-scale microstructure (2–4 μm CaCO₃ cores + 20–40 nm SiO₂ nodules), a 14.44 m²/g specific surface area, and bimodal porosity (8–80 nm). Composite C7 (30 wt% filler, 3 wt% KH-570, 1:2 resin-to-filler ratio) achieved superhydrophobicity (a 153° contact angle via Cassie-Baxter stabilization), ultrahigh electrical insulation (3.20 × 10¹⁴ Ω·cm volume resistivity, 1.60 × 10¹³ Ω surface resistivity), and robust mechanical properties (Shore 3H hardness, 5B adhesion). Standardized IEC 60507:2020 tests showed that C7’s flashover voltages (14.8 kV for KMnO₄, 14.3 kV for NaCl/KMnO₄, 13 kV for NaCl) exceeded that of neat silicone resin (NSR) and conventional CaCO₃-filled composite (SR-CC) by >135%. Additionally, C7 retained superhydrophobicity after 500 h UV aging and maintained a 124° contact angle after 12 months of outdoor exposure. The superior performance stems from synergistic hierarchical topology, tortuous discharge paths, and interfacial passivation. This work establishes a microstructure-driven design paradigm for grid protection materials in harsh environments.

1. Introduction

High-voltage electrical insulators are critically vulnerable to pollution flashover (PF), a catastrophic failure mode triggered by the conductive leakage currents that form across insulator surfaces under contaminated, humid conditions [1,2,3]. This phenomenon jeopardizes grid stability worldwide, especially in coastal [4], industrial [5], or arid regions prone to airborne salt/dust deposition [6]. Silicone rubber SR coatings [7,8], prized for their intrinsic hydrophobicity and hydrophobicity transfer capability, have become the frontline defense against PF. However, conventional SR formulations are loaded with micron-sized mineral fillers [9,10]. Furthermore, aluminum trihydrateand silica exhibit inherent limitations: insufficient hydrophobicity retention under severe wet pollution stress, moderate electrical insulation, and inadequate mechanical robustness—particularly when high filler loadings compromise interfacial adhesion [11] and introduce defect pathways for discharge initiation [12].

The pivotal role of extreme hydrophobicity in suppressing leakage currents is well-established [13,14]. Surface wettability fundamentally dictates the formation of continuous conductive water films: hydrophobic surfaces induce discontinuous water droplet formation via high contact angles, i.e., CAs > 120, significantly suppressing leakage currents [15]. Critically, the Cassie-Baxter C−B [16,17] wetting state, where air pockets are stably trapped within microscale and nanoscale surface asperities, creates an energy barrier that prevents droplet spreading and minimizes the solid–liquid contact area [18,19]. Achieving and sustaining this metastable non-wetting state necessitates precisely engineered hierarchical micro-nano topographies integrated with intrinsically hydrophobic chemistry. This demands filler architectures far beyond conventional particulate morphologies.

Sol–gel synthesis [20] offers unparalleled potential for constructing such advanced functional fillers. This technique enables atomic-level control over hybrid inorganic–inorganic interfaces through sequential hydrolysis–condensation reactions, thereby facilitating the formation of covalent bonds between dissimilar phases such as SiO₂ and CaCO₃. By adjusting precursor ratios, catalysts, solvents, temperature, and reaction duration, one can rationally design multi-scale architectures encompassing nanoporous structures [21], core–shell motifs [22], and controlled surface roughness [23,24]. For anti-flashover applications, sol–gel-derived hybrid fillers thus represent a promising frontier, potentially outperforming conventional fillers via the following: (1) synergistic surface engineering, i.e., integrating micro-scale CaCO₃ cores with nano-scale SiO₂ features to create dual-scale roughness optimal for C-B state stabilization [25]; (2) covalent interface construction, which entails generating robust Si–O–Ca bonds via dehydration condensation, replacing physically adsorbed interfaces prone to hydrolysis and delamination [26]; (3) tailored porosity architecture, which involves creating bimodal meso-/macro-pores to disrupt electrical discharge paths [27] and enhance contaminant shedding [28]; (4) multi-functionality, i.e., simultaneously enhancing electrical insulation, mechanical reinforcement, and hydrophobic durability within a single composite particle.

However, integrating such engineered fillers into SR matrices poses significant challenges: Filler microstructure, surface chemistry, polymer-filler coupling, and loading fraction profoundly yet non-linearly influence bulk coating properties. Traditional trial-and-error approaches struggle to navigate the vast multi-parameter design space necessary for maximal PF suppression. Furthermore, comprehensive correlation studies linking hierarchical filler synthesis parameters directly to electrical and pollution performance under standardized test conditions are scarce.

This work establishes a pioneering materials design paradigm where hierarchical micro-nano architectures, precisely controlled via orthogonal–optimized sol–gel synthesis and functionalized silane coupling, unlock unprecedented levels of hydrophobicity, electrical insulation, and mechanical durability in silicone composites—a critical advancement for protecting critical power infrastructure against the persistent threat of environmental pollution-induced flashover.

2. Experimental

2.1. Materials

All chemicals were used as received. Industrial-grade calcium carbonate (CaCO₃) was purchased from Anhui Jiangdong Technology Powder Co., Ltd. (Xuancheng, China); the absolute ethanol we used was obtained from Chengdu Kelong Chemical Co., Ltd. (Chengdu, China) with ≥99.7% purity; the tetraethyl orthosilicate, TEOS, was from Chengdu Kelong Chemical Co., Ltd. with ≥98% purity; the ammonia hydroxide, NH₃⋅H₂O, was from Chengdu Kelong Chemical Co., Ltd., with a 25–28 wt% concentration; the γ-Methacryloxypropyltrimethoxysilane (KH-570) was from Chengdu Kelong Chemical Co., Ltd., with ≥98% purity; methyl silicone resin (SH−3021), a low-molecular-weight solvent-borne silicone resin, was purchased from Hubei Longsheng Sihai New Materials Co., Ltd. (Zaoyang, China)

2.2. Synthesis of CaCO₃/SiO₂ Composite Particles

CaCO₃/SiO₂ particles were synthesized via the sol–gel method. TEOS hydrolysis/condensation, catalyzed by NH₃·H₂O, generated SiO₂. Dehydration condensation between SiO₂ silanol groups, i.e., Si–OH, and CaCO₃ surface hydroxyls formed covalent –Si–O–Ca– bonds. For the orthogonal experimental design of L₁₆(4⁵), five factors at four levels were optimized, as shown in

Table 1. Orthogonal array (L₁₆(4⁵)) for the synthesis of CaCO₃/SiO₂ composite particles.

Sample	Ethanol (vol)	nCaCO3: nTEOS	NH3·H2O (mol/L)	Temp. (°C)	Time (h)
1	65	1:0.5	0.2	35	9
2	65	1:1	0.3	40	12
3	65	1:1.5	0.4	45	15
4	65	1:2	0.5	50	18
5	70	1:1	0.2	45	18
6	70	1:0.5	0.3	50	15
7	70	1:2	0.4	35	12
8	70	1:1.5	0.5	40	9
9	75	1:1.5	0.2	50	12
10	75	1:2	0.3	45	9
11	75	1:0.5	0.4	40	18
12	75	1:1	0.5	35	15
13	80	1:2	0.2	40	15
14	80	1:1.5	0.3	35	18
15	80	1:1	0.4	50	9
16	80	1:0.5	0.5	45	12

.

2.3. Preparation of Anti-Flashover Coatings

KH-570 was selected as the optimized coupling agent. The KH-570 coupling agent was pre-hydrolyzed in an ethanol/water solution at pH = 4.5 and at 50 °C for 1 h. This modified agent was then incorporated with CaCO₃/SiO₂ particles and methylphenylsilicone resin, and the mixture was homogenized by ball-milling at 400 rpm for 2 h. After degassing, the suspension was blade-coated onto glass substrates and subjected to a two-stage thermal cure: initial preheating at 80 °C for 1 h followed by main curing at 120 °C for 2 h. Final coatings exhibited a uniform thickness of 0.30 ± 0.02 mm, as verified by ISO 2808 standards. As for the orthogonal experimental design of L₉(3³), three factors at three levels were studied, as shown in

Table 2. Orthogonal array (L₉(3³)) for coating formulation.

Sample	Filler Loading (wt)	Resin-Filler Mass Ratio	KH-570 (wt)
C1	10	2:1	0
C2	10	1:1	1
C3	10	1:2	3
C4	20	2:1	1
C5	20	1:1	3
C6	20	1:2	0
C7	30	2:1	3
C8	30	1:1	0
C9	30	1:2	1

.

2.4. Characterization Techniques

Characterization techniques were conducted following standardized protocols and instrumentation. For morphology, Scanning Electron Microscopy (SEM) with JEOL JSM-IT800 (Tokyo, Japan) was used; for surface analysis, Contact Angle Measurement with JCY-2 Contact Angle Meter from Shanghai Fangrui Instrument Co., Ltd., China, was employed, and Specific Surface Area (BET) was measured using BSD-PS Surface Area Analyzer from BSD Instruments (Guangdong, China); for crystal structure analysis, X-ray Diffraction (XRD) was performed with the DX-2700BH X-ray Diffractometer from Dandong Haoyuan Instrument Co., Ltd., (Dandong, China). The test conditions were as follows: Cu Kα radiation (λ = 1.5406 Å); tube voltage, 40 kV; tube current, 30 mA; 2θ scanning range, 10°~80°; scanning speed, 5°/min; step size, 0.02°. For UV aging resistance, the Xenon Lamp Aging Test Chamber was used, following the ASTM G155 standard; for electrical properties, Volume/Surface Resistivity was tested in accordance with IEC 62631-3 [29], and AC Flashover Voltage was measured in accordance with IEC 60507:2020 [30]; for mechanical properties, shore hardness was tested using ASTM D3364-20 [31], and cross-cut adhesion was tested using ASTM D3359-23 [32]; for chemical analysis, Fourier Transform Infrared Spectroscopy (FTIR) was carried out with the Bruker Vector-22 FTIR Spectrometer from Bruker Corporation, Germany; for particle analysis, Dynamic Light Scattering (DLS) was performed with Zetasizer Nano ZS from Malvern Panalytical, (Worcestershire, UK) (Model: ZTS1240). All tests adhered to the referenced standards to ensure data reliability. The test conditions were as follows: irradiance, 1.2 kW·m⁻²; wavelength range, 290~800 nm; relative humidity, 50%; temperature, 50 °C; aging duration, 500 h [33]. For outdoor exposure performance, the outdoor natural exposure test was conducted under ambient weather conditions, with an exposure period from September 2024 to September 2025.

3. Result and Discussion

3.1. Microstructural Evolution and Chemical Bonding Validation of CaCO₃/SiO₂ Hybrid Fillers

FTIR was employed to elucidate the chemical bonding and structural characteristics of the sol–gel-synthesized CaCO₃/SiO₂ hybrid composites. For pristine CaCO₃, the FTIR spectrum exhibited characteristic peaks of CO₃²⁻, including asymmetric stretching at 1410–1450 cm⁻¹, out-of-plane bending at 875 cm⁻¹, and in-plane bending at 712 cm⁻¹ [34]. In contrast, for SiO₂ derived from TEOS hydrolysis, the spectra showed characteristic Si-O-Si asymmetric stretching at ~1100 cm⁻¹, symmetric Si-O-Si stretching at ~800 cm⁻¹, and in-plane bending at 450–500 cm⁻¹ [35]. Crucially, for the CaCO₃/SiO₂ hybrid composites, a distinct peak in the 950–1000 cm⁻¹ range was observed, which was absent in the spectra of pristine CaCO₃ or pure SiO₂. This peak confirms the formation of covalent Si-O-Ca bonds, arising from the dehydration condensation between the surface hydroxyl groups of CaCO₃ and the silanol groups from SiO₂ oligomers. Additionally, the intensity of the CO₃²⁻ peaks in the hybrid particles was reduced compared to that of pristine CaCO₃, and a broadened Si-O-Si band at ~1100 cm⁻¹ was observed, indicating the effective encapsulation of CaCO₃ microparticles by the SiO₂ matrix. The FTIR spectra corresponding to the above structural and bonding analyses are shown in Figure 1.

Figure 2 presents the comparative XRD patterns of pristine CaCO₃ and CaCO₃/SiO₂ composite particles, which were utilized to verify the crystal phase structure and interfacial interaction between the two components. For pristine CaCO₃, distinct and sharp diffraction peaks can be observed at 2θ values of 29.4°, 36.0°, 39.4°, 43.2°, and 48.5°, which are characteristic of calcite-type CaCO₃ (JCPDS No.05-0586) and correspond to the (104), (110), (113), (202), and (116) crystal planes, respectively. Notably, the XRD pattern of the CaCO₃/SiO₂ composite particles retains all the characteristic diffraction peaks of calcite-type CaCO₃ without the emergence of new diffraction peaks, indicating that the introduction of SiO₂ via sol–gel synthesis does not alter the crystal structure of CaCO₃. Meanwhile, a slight reduction in the intensity of the diffraction peaks of the composite particles is observed compared to that of pristine CaCO₃, which can be attributed to the uniform coating of amorphous SiO₂ on the surface of CaCO₃ microparticles. Amorphous SiO₂ exhibits no distinct diffraction peaks, only a broadened “hump” around 2θ ≈ 22°, which is not obvious here due to the low content and uniform dispersion, and its insulating layer weakens the diffraction signal of the underlying CaCO₃ crystals. This result is consistent with that of the FTIR analysis (confirming Si-O-Ca covalent bonding) and SEM0 observations (revealing the core–shell microstructure), collectively verifying the successful synthesis of CaCO₃/SiO₂ hybrid particles where amorphous SiO₂ is tightly combined with CaCO₃ without causing phase transformation.

The orthogonal experimental design (L₁₆(4⁵)) effectively revealed the synergistic control of sol–gel parameters on filler morphology. As shown in

Table 3. Orthogonal experimental samples: SEM Particle Size-BET SSA Correlation.

Sample	Specific Surface Area (m2/g)	SEM Image Location	Sample	Specific Surface Area (m2/g)	SEM Image Location
	Median Particle Size (μm)			Median Particle Size (μm)
CaCO3	SSA: 21.35 D50: 0.2		9	SSA: 11.32 D50: 4.5
1	SSA: 5.97 D50: 2.53		10	SSA: 3.07 D50: 2.98
2	SSA: 6.63 D50: 4.21		11	SSA: 1.64 D50: 2.34
3	SSA: 2.54 D50: 2.83		12	SSA: 2.97 D50: 2.84
4	SSA: 5.85 D50: 4.45		13	SSA: 11.90 D50: 4.25
5	SSA: 14.44 D50: 4.22		14	SSA: 3.77 D50: 4.18
6	SSA: 2.51 D50: 2.97		15	SSA: 6.91 D50: 4.89
7	SSA: 4.18 D50: 3.5		16	SSA: 3.90 D50: 2.83
8	SSA: 3.22 D50: 3.63

, all composites exhibited hierarchical micro-nano architectures featuring micron-scale agglomerates (D₅₀ = 2.34–4.89 μm) decorated with nano-features, consistent with sol–gel self-assembly mechanisms. BET analysis demonstrated significant specific surface area (SSA) variations (1.64–14.44 m²/g), with distinct microstructure categories emerging: a high-SSA group (>10 m²/g, e.g., S5/S9/S13), where there were loose nanoparticle assemblies or porous nanofiber networks with well-developed pore channels; a medium-SSA group (5–10 m²/g, e.g., S1/S2/S4/S15), with rough-surfaced nanosphere aggregates or stacked lamellae; a low-SSA group (<5 m²/g, e.g., S3/S6/S7), with densified monolithic structures where surface nanostructures vanished due to sintering.

The orthogonal design revealed synergistic control of sol–gel parameters on filler architecture, where ethanol concentration (65–80 vol%) inversely regulated polycondensation kinetics—high alcohol content (e.g., 70 vol% in S5) retarded gelation to favor loose networks with uniformly dispersed SiO₂ nanoparticles. Notably, Sample 13 (80 vol% ethanol), despite belonging to the high-SSA group (11.90 m²/g), exhibited obvious SiO₂ agglomeration as observed by SEM: instead of anchoring uniformly on CaCO₃ cores as discrete 20–40 nm nodules (like S5), its SiO₂ nanoparticles aggregated into large clusters (>200 nm), leading to uneven surface topology and compromised structural homogeneity. This confirms that excessive ethanol (80 vol%) disrupts the balanced hydrolysis–condensation process, inducing unwanted particle agglomeration. Conversely, elevated NH₃·H₂O concentration (0.4 mol/L in S7) accelerated coagulation-induced densification, while the nTEOS: nCaCO₃ ratio critically modulated pore distribution, with a 1:1 stoichiometry maximizing meso-/macroporosity synergy. This parametric interplay culminated in Sample 5 (70 vol% EtOH, nTEOS: nCaCO₃ = 1:1, 0.2 mol/L NH₃·H₂O, 45 °C, 18 h) exhibiting an ideal hierarchical structure: 20–40 nm SiO₂ nanoparticles uniformly anchored on 2–4 μm CaCO₃ clusters—free of agglomeration and with optimal dual-scale roughness. The resultant structure facilitated Cassie–Baxter state formation, while interconnected bimodal pores (8–25 nm mesopores + 20–80 nm macropores) suppressed discharge channels through elongated electron tunneling paths—collectively enhancing hydrophobicity and delivering >28% higher flashover voltage versus controls (Section 3.5). In contrast, the agglomerated structure of Sample 13 failed to achieve such performance synergy, further validating the optimality of Sample 5’s synthesis parameters. Given these superior properties, Sample 5 was exclusively selected as the functional filler for all subsequent silicone resin composites (C1-C9), with variations limited to resin/filler/KH570 ratios.

3.2. Hierarchical Filler-Induced Hydrophobicity Leap in Silicone Composites

The contact angle (CA) results in Figure 3 confirm C9 as the optimal composite with the highest hydrophobicity: the neat silicone resin (NSR) showed a baseline CA of 92°, the conventional 30 wt% CaCO₃-filled composite (SR-CC) reached ~113°, while C9 achieved a peak CA of 153°—surpassing NSR by 61° and SR-CC by 40°. This superior hydrophobicity originates from the CaCO₃/SiO₂ hierarchical filler’s dual-scale roughness (micron clusters decorated with nano-nodules) in C9, which stabilizes the Cassie–Baxter wetting state by trapping air pockets at the surface. This effectively reduces the solid–liquid contact area, inhibiting water droplet spreading and enhancing hydrophobicity—critical for mitigating pollution flashover on high-voltage insulators.

3.3. Dual Resistivity Analysis

Figure 4 quantifies the exceptional electrical insulation performance of the optimized composite, C7, with its volume resistivity reaching 3.20 × 10¹⁴ Ω·cm and surface resistivity at 1.60 × 10¹³ Ω—values that far surpass that of the neat silicone resin (NSR, ~10¹³ Ω·cm volume resistivity, ~10¹² Ω surface resistivity) and conventional CaCO₃-filled composite (SR-CC, similar order to NSR) and fully meet the stringent requirements of the IEC 62631-3 standard. This remarkable enhancement stems from the synergistic effects of the CaCO₃/SiO₂ hierarchical filler’s unique structure and interfacial design: the amorphous SiO₂ shell encapsulated on CaCO₃ cores acts as a robust insulating barrier, inherently impeding electron tunneling and ionic conduction; the filler’s bimodal porosity (8–80 nm) physically elongates charge migration paths within the silicone matrix, disrupting continuous conductive pathways; meanwhile, covalent Si–O–Ca bonding and KH-570-mediated interfacial cross-linking minimize micro-voids and defect sites, further suppressing leakage current formation.

3.4. Mechanical Interlocking and Stress Dissipation Mechanisms in Hierarchical Composite Coatings

Table 4. Shore hardness: NSR, SR-CC, C1–C9.

Sample	NSR	SR-CC	C1	C2	C3	C4	C5	C6	C7	C8	C9
Hardness Value	3B	HB	HB	HB	HB	2H	H	H	3H	2H	2H

and Figure 5 collectively confirm the robust mechanical performance of C7, as evaluated via standardized tests. C7 achieves a Shore 3H hardness and 5B cross-cut adhesion, significantly outperforming that of the neat silicone resin (NSR, 3B hardness) and conventional CaCO₃-filled composite (SR-CC, HB hardness). The mechanical reinforcement originates from the hierarchical filler’s dual-scale architecture and enhanced interfacial interaction: micron-sized CaCO₃ cores provide bulk structural rigidity, while nano-scale SiO₂ nodules augment the interfacial contact area, enabling strong mechanical interlocking with the silicone matrix. Additionally, the KH-570 coupling agent covalently bridges the inorganic filler and organic resin, relieving interfacial stress concentrations and preventing filler detachment under external forces—ensuring the coating maintains structural integrity against environmental abrasion and thermal fluctuations.

3.5. Flashover Resistance Optimization via Hierarchical Filler Design

Figure 6 illustrates the standardized pollution flashover test platform compliant with IEC 60507:2020, featuring a needle–plane electrode configuration (1.5 cm air gap) and controlled environmental conditions (80% relative humidity, contaminant solutions with 200 μS/cm conductivity), while Figure 7 quantifies C7’s exceptional anti-flashover capability. Under three representative pollution scenarios (NaCl for coastal salt fog, NaCl/KMnO₄ mixture for industrial acid rain, KMnO₄ for oxidative pollutants), C7 exhibits peak flashover voltages of 14.8 kV (KMnO₄), 14.3 kV (NaCl/KMnO₄), and 13 kV (NaCl)—surpassing that of the NSR and SR-CC by >135%. This superiority arises from three synergistic mechanisms: the filler’s hierarchical micro-nano topology promotes contaminant shedding and stabilizes the Cassie–Baxter wetting state, reducing electrolyte film formation; the bimodal porosity and tortuous surface structure elongate discharge paths, dissipating arc energy; and the chemically passivated interface (Si–O–Ca bonds + KH-570 modification) resists oxidative degradation and ionic corrosion, preserving insulation performance under harsh pollution conditions.

3.6. UV Aging and Outdoor Exposure Durability of C7 Coating

Table 5. Contact angle (CA) and surface morphology changes in different coatings after 500 h UV irradiation.

Sample	Not Subjected to UV Irradiation	After 500 h of UV Irradiation
NSR	CA: 92°	CA: 78°
SR-CC	CA: 113°	CA: 105°
C7	CA: 150°	CA: 150°

presents the contact angle (CA) values and surface morphology changes in the NSR, SR-CC, and C7 coatings before and after 500 h of UV irradiation (per ASTM G155-23 standard). The data clearly demonstrate the superior UV aging resistance of the C7 coating compared to that of the control groups. For the neat silicone resin (NSR), the initial CA of 92° decreased by 15.2% to 78° after UV exposure, indicating significant degradation of surface hydrophobicity. This is attributed to the cleavage of Si-O-C bonds and oxidation of hydrophobic methyl (-CH₃) groups in the silicone resin under UV radiation, which increases surface energy and promotes water droplet spreading. The conventional CaCO₃-filled composite (SR-CC) showed a relatively milder CA reduction (from 113° to 105°, a decrease of 7.1%), as the physical dispersion of CaCO₃ particles provided limited shielding against UV penetration. However, the weak interfacial adhesion between CaCO₃ and the silicone matrix led to partial particle detachment during aging, resulting in reduced hydrophobicity retention.

In stark contrast, the C7 coating maintained an unchanged CA of 150° even after 500 h of UV irradiation, retaining its superhydrophobic property. This exceptional stability stems from three synergistic protective effects: (1) the amorphous SiO₂ shell encapsulated on the CaCO₃ core acts as a physical barrier, effectively blocking UV radiation from reaching the silicone resin matrix and inhibiting chemical bond cleavage; (2) the covalent Si-O-Ca bonds between the CaCO₃ core and SiO₂ shell enhance interfacial stability, preventing filler detachment and preserving the hierarchical micro-nano surface structure; (3) the dual-scale roughness (2–4 μm CaCO₃ clusters decorated with 20–40 nm SiO₂ nodules) stabilizes the Cassie–Baxter wetting state, minimizing the impact of minor surface oxidation on overall hydrophobicity.

Table 6. Evolution of contact angle (CA) and surface morphology of different coatings during long-term outdoor exposure.

Sample	Not Subjected to Outdoor Exposure	3 Months of Outdoor Exposure	6 Months of Outdoor Exposure	9 Months of Outdoor Exposure	12 Months of Outdoor Exposure
NSR	CA:92°	CA:81°	CA:73°	CA:67°	CA:63°
SR-CC	CA:113°	CA:104°	CA:92°	CA:84°	CA:76°
C7	CA:150°	CA:143°	CA:135°	CA:129°	CA:124°

illustrates the evolution of CA and the surface morphology of the three coatings over 12 months of outdoor natural exposure. Consistent with the UV aging results, the C7 coating exhibited remarkable long-term hydrophobicity retention, outperforming NSR and SR-CC under complex natural weathering conditions (including UV radiation, rainfall erosion, temperature fluctuations, and atmospheric dust deposition).

The NSR coating showed the most severe hydrophobicity degradation, with the CA decreasing continuously from 92° to 63° over 12 months (a total reduction of 31.5%). This is due to the combined effects of prolonged UV oxidation and physical erosion, which compromise the surface integrity of the resin and eliminate residual hydrophobicity. The SR-CC coating displayed moderate durability, with the CA decreasing from 113° to 76° (a 32.7% reduction), as the lack of effective interfacial bonding and UV shielding led to gradual filler detachment and resin degradation. In comparison, the C7 coating maintained a high CA of 124° after 12 months of exposure, with only a 17.3% reduction from the initial 150°. The hierarchical micro-nano structure of C7 facilitates contaminant shedding and water droplet rolling, while the SiO₂ encapsulation layer resists UV oxidation and moisture penetration. Additionally, the covalent cross-linking between the filler and resin matrix (enhanced by KH-570) suppresses interfacial defects, ensuring structural integrity under long-term environmental stress.

4. Conclusions

This study successfully developed high-performance anti-flashover silicone coatings via sol–gel-engineered CaCO₃/SiO₂ hierarchical fillers, addressing conventional coatings’ limitations. Orthogonal optimization yielded Sample 5 with a dual-scale core–shell structure, covalent Si-O-Ca bonding, and bimodal porosity—key to multifunctional enhancement. Composite C7 achieved superhydrophobicity (a 153° contact angle), ultrahigh insulation (3.20 × 10¹⁴ Ω·cm volume resistivity, 1.60 × 10¹³ Ω surface resistivity), robust mechanics (shore 3H hardness, 5B adhesion), and >135% higher flashover voltage compared to the controls. It also exhibited exceptional durability, retaining superhydrophobicity after 500 h UV aging and maintaining a 124° contact angle post-12-month outdoor exposure. These improvements stem from synergistic hierarchical topology, tortuous discharge paths, and interfacial passivation. This work validates microstructure–property synergy via sol–gel synthesis and orthogonal optimization, providing a viable pathway for next-generation grid protection materials in harsh environments.