1 Introduction

Silicone rubber (SR) composite materials play a crucial role in electrical insulation applications, particularly in demanding outdoor environments [1, 2]. This is attributed to their exceptional surface hydrophobic nature, significantly enhancing their resistance against electrical arcing and discharges [3,4,5]. SR-based insulation offers multiple advantages, such as low weight, easy transportation, resistance against vandalism, and excellent performance under highly polluted conditions [6,7,8]. Despite having several advantages, SR materials are flammable once they ignite, and this drawback limits their broad applicability in regions subjected to wildfire risks.

Intumescent additives are substantially employed by researchers to enhance the flame retardancy of polymeric materials [9,10,11,12,13,14]. Li et al. [15] successfully attached melamine (MEL) and phytate compounds to multi-walled carbon nanotubes (MWNCTs) and then incorporated them into the polyamide. It was reported that smoke release was impeded by 48%, and the limiting oxygen index (LOI) was enhanced to 26.4%. Zhao et al. [16] developed a magnesium-based curing agent as an intrinsic flame retardant for epoxy, using magnesium hydroxide (MDH), 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), maleic acid, and ammonium polyphosphate (APP) compounds. It was exhibited that the LOI of epoxy was improved to 26% with the addition of 7% of intumescent additives, whereas the peak heat release rate (HRR), total heat release (THR), smoke production rate (SPR), and total smoke production (TSP) were reduced by 54%, 35%, 43%, and 38%, respectively, as compared to pristine epoxy. Liu et al. [17] explored the flammability of polypropylene (PP) by using piperazine pyrophosphate (PAPP), and THR and peak HRR declined by 23% and 85%, respectively, relative to pristine PP. Ai et al. [18] investigated the flame resistance of a novel herbicide additive named amitrole modified through an organo-phosphorus compound incorporated into an epoxy polymer. It suggested that epoxy with only 7.5% content of flame retardant enhances the LOI of epoxy to 34%, and toxic gases emitted in the combustion process were impeded significantly. Zhu et al. [19] reported the fire retardancy of epoxy employing the P/N-containing oligomer and APP. LOI was improved to 30%, while peak HRR and TSP appeared with considerably lower values, relatively.

Non-intumescent and ceramifiable fire retardants were also developed to enhance the flame retardancy of polymers by generating a ceramic-like barrier against heat [20]. Anyszka et al. [21] explored the ceramisability of SR under fire using montmorillonite (MMT). SR created a compact, porous, and durable layer of ceramic, sticking MMT particles to the SR matrix with heat exposure. Lou et al. [22] investigated the flame retardancy of SR using APP, calcium carbonate, mica, and glass additives and reported that LOI increased to 31.2%, while peak HRR and THR were reduced compared to pristine SR. Hanu et al. [23] reported the thermal stability and fire retardancy of SR using iron oxide, glass, and mica additives. SR offered excellent thermal stability performance with the addition of 15% glass and mica powder, whereas SR showed a lower peak HRR by replacing the glass with iron oxide additives. Furthermore, Li et al. [24] explored the viability of glass and zinc borate, fumed silica, aluminium hydroxide (ATH) and MDH, calcium carbonate, and silicone oil in enhancing the flame characteristics of the SR matrix. Results showed that the LOI value increased to 34.8%, and flexural strength reached 9.70 MPa with the optimised addition of the above-mentioned fillers.

As far as the partial discharge (PD) resistance of electrically insulating material is concerned, Lee et al. [25] suggested that epoxy composites with micron and nano-sized silica offered better discharge resistance relative to their pristine form. Tanaka et al. [26] investigated the PD resistance of epoxy filled with modified clay particles, and results exhibited that micro/nano epoxy composites offered excellent resistance due to the impact of the interaction zone. Li et al. [27] studied the role of particle size on the discharge resistance and dielectric strength of epoxy. There was an obvious improvement seen in PD resistance with increasing micron particles, while nano-sized particles assisted slightly in enhancing the dielectric strength. Nazir et al. [28] found that the partial discharge inception voltage (PDIV) of EPDM polymer increased from 2.58 to 3.25 kVrms as the doping level of nano-boron (BN) powder rose from 0 to 7%, and there was a notable decrease in surface discharge parameters. Wang et al. [29] reported the thermal properties and PD resistance of SR using micron/nano BN powder. It exhibited that thermal conductivity and PDIV linearly increased with the increasing BN contents in the composites.

Literature suggests that intumescent fire retardants are excellent flame retardants but result in an increment in the electrical conduction capability of polymers due to the swelling and moisture absorption nature of additives [30, 31]. In addition, powders such as inorganic silica, alumina, and BN are excellent in providing adequate PD resistance to electrical insulation and thermal characteristics [7, 32] but lack in providing flame retardancy. Here, the authors have synthesised the SR co-filled composites by adopting the novel strategy of ATH/MMT/chopped glass fibre (GF) hybrid additives and explored the flame and surface discharge resistance of the co-filled composites to be used as industrial electrical insulating material.

2 Experimental

2.1 Materials

A pure form of liquid silicone (SYL-GARDTM184) was received from DOW chemical in two parts ((A) base polymer and (B), curing agent) with a base viscosity of 5100 cP and density of 1.03 g/cm3. Moreover, the ATH (density = 2.42 g/cm3) is natural flame resistive and was procured from Redox, Australia with a mean grain size of 5 microns, while GF (E- glass) was purchased from Allnex, Australia, with a filament length of 6000 microns and an average diameter of 13 microns. MMT nanoclay (CLOSITE Na+, density = 2.86 g/cm3) is received from ResChem, Australia, with a mean size of 25 microns. Figure 1 exhibits the micron structure of all additives used in this work.

Fig. 1
figure 1

SEM micrographs of additives used for the fabrication of co-filled SR composites

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2.2 Composites’ fabrication

A block diagram in Fig. 2 illustrates the process used for the fabrication of SR co-filled composites. The pertinent amount of all additives was weighed after drying at 110 °C for 12 h in an oven [33]. Initially, the required amount of SR part A was taken in a lab polyethylene cup. In the first stage, ATH particles were mixed with SR part A using a hand stirrer, followed by a sharp-blade mechanical mixer for 30 s. A similar process was repeated with the MMT and GF additives as per the required formulation. Later, the cross SR composites were reacted with part B of the matrix in a 10:1 ratio. The matrix was again mechanically mixed for 30 s with care to ensure that the composites were not heated up. Afterward, the composites were placed in a lab vacuum pump chamber and degassed until no trapped air was seen on the top surface in the form of bubbles. The moulds were preheated in the oven to prevent the settling of additives on the surface of the moulds. Finally, the moulds were placed in the lab oven, and the curing was performed at 120 °C for 1800s. SR composites with five different formulations were prepared, as shown in Table 1. All specimens were thoroughly washed with ethanol and water, then dried in an oven at 80 °C for a couple of hours.

Fig. 2
figure 2

A block diagram for the fabrication of SR co-filled composites

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Table 1 A description of SR co-filled composites
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2.3 Characterisation

The microstructure of the SR composites was evaluated via SEM (Hitachi S3400, Mito, Japan). SEM was operated at an accelerated voltage level of 15.0 kV. Before SEM imaging, the specimen was sputter-painted with a fine coating of gold to achieve the required conductivity. Furthermore, a PerkinElmer-STA 6000 was engaged to analyse the chemical functional groups with the addition of the additives.

The thermal stability and degradation of the SR composite are evaluated via thermogravimetric analysis (TGA) using the TGA/STA6000 (PerkinElmer) instrument. All TGA tests were conducted in a compressed air environment from 50 until 800 °C with an increasing rate of 10 °C per minute. Moreover, the electrical bulk conductivity of the specimens was assessed at 20 °C using the Keithley resistivity test setup as per ASTM D257. Additionally, the surface hydrophobic characteristic is evaluated via static water contact angle using the DMs-200, Kyowa, Japan. The size of the droplet was controlled at ~ 4.5 µL for each reading.

A laboratory cone calorimeter (FFT iCone Classic, UK) was used to study the flame-resistive parameters of the specimens. All the cone tests were conducted at a heat flux of 35.0 kW/m2, and specimens with a height and width of 10 cm were used, while the thickness was controlled at 0.3 cm for each sample.

The electrical field enhancement was studied using the finite element analysis (FEA) tool of ANSYS Maxwell. PD is a flashover or partial breakdown of the electrically insulating material because of electrical field enhancement relative to insulation withstand capability [34, 35]. The partial surface discharge occurred across the surface of the insulation, and it was measured using the IEC 60270 [36]. A circuit used for the surface discharge measurement is illustrated in Fig. 3. Surface discharge was measured using the instrument OMICRON MPD 600 coupled with a 1.0-nF coupling capacitor (Cb) and a quadripole unit. The voltage was supplied from a step-up transformer with the low voltage side energised from 0.240 kVrms via a variac [37]. The parallel plate electrode (high-voltage (HV) electrode, thickness = 10 mm; diameter = 70 mm, and ground electrode, thickness = 25 mm; diameter 125 mm) geometry was used for surface discharge experiments. Figure 3 shows the schematic of an electrical circuit and parallel plate electrode system used in this work.

Fig. 3
figure 3

Schematic of surface discharge resistance measuring circuit

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3 Results and discussion

3.1 Characterisation of composites

Figure 4a illustrates the microstructure of SR composites captured through SEM. Results suggest that all the additives are mixed with a high degree of homogeneity. ATH and MMT particles are exhibited with non-uniform shapes, as seen in SRB and SRC. Interestingly, the GF additives are uniformly distributed in a 3D pattern, while ATH and MMT particles are observed in between and on the outer side of the threads penetrating the SR material. Moreover, Fig. 4b shows the critical functional groups found in the FTIR spectra of the SR matrix. The peaks found at 790 cm−1, 1014 cm−1, 1259 cm−1, and 2960 cm−1 represent Si (CH3)2, Si–O-Si, Si-CH3, and CH3(C-H), respectively [7]. An interesting fact of the FTIR result is observed in the wavenumber range of 3250 cm−1 to 3700 cm−1. No peak is observed in the pure SRA, but interestingly, a group of four identical peaks is seen in counterparts, as shown in Fig. 4c. These peaks represent the presence of hydroxyl functional groups in the moisture content of ATH particles.

Fig. 4
figure 4

Characterisation studies of SR co-filled composites a SEM microstructure of composites, b FTIR profiles of composites, c hydroxy functional groups in magnified FTIR profiles, d TGA studies, e bulk electrical conductivity of composites, and f static water contact angle of composites

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Figure 4d shows the TGA profiles of the composites. The first degradation in filled composites is observed in the range of 219 to 380 °C, which could be due to the deterioration of ATH additives in this range, while the second dip in the range of 400 to 610 °C shows the full depolymerisation of polymeric chains of the composites. Interestingly, SRA appears with a minor first deterioration dip, while it is found major in counterparts due to ATH decomposition. This is the reason that the starting decomposition temperature of the composites in the second stage is delayed in the filled SR composites. The initial decomposition temperature at the second stage is measured at 398 °C, 428 °C, 435 °C, 432 °C, and 440 °C in SRA, SRB, SRC, SRD, and SRE, respectively.

Figure 4e exhibits the electrical bulk conductivity of the SR specimens. The bulk conductivity of the SR composites is measured at 1.777 × 10−11 S/cm, 1.434 × 10−11 S/cm, 1.527 × 10−11 S/cm, 1.596 × 10−11 S/cm, and 1.808 × 10−11 S/cm for SRA, SRB, SRC, SRD, and SRE, respectively. It is found that ATH addition reduces the electrical conductivity of the composite, while it linearly increases with the addition of MMT and GF, as seen in SRC, SRD, and SRE.

The hydrophobic water contact angle of selected composites is shown in Fig. 4f. Hydrophobicity is a key surface property for high-voltage outdoor insulation, which renders excellent pollution flashover performance to SR insulation [38]. The contact angle is measured at 109.2°, 106°, 108.3°, 116.5°, and 115.5° in SRA, SRB, SRC, SRD, and SRE. The high hydrophobicity in the SRD and SRE composites could be due to the presence of GF, which may enhance the surface roughness of composites, and hence, it may increase the static water drop angle [39].

3.2 Cone calorimeter test

To understand the flame resistance of the SR co-filled composites, Fig. 5 illustrates the profiles of HRR, average rate of heat emission (ARHE), THR, SPR, TSP, and total oxygen consumed (TOC) produced from the cone calorimeter test. Table 2 shows the key data points extracted from the profiles. Results suggest that the peak HRR, maximum average rate of heat emission (MARHE), THR, peak SPR, TSP, and TOC are decreased with the addition of solely ATH, ATH/MMT, and ATH/GF in SR, as seen in SRB, SRC, and SRD. MMT and GF help in improving the fire resistance of composites in SRC and SRD, as MARHE and TSP reduce to 46.62 kW/m2 and 1.49 m2 in SRC and decline to 33.11 kW/m2 and 1.03 m2 in SRD, respectively, relative to both measured at 61.29 kW/m2 and 6.02 m2 in SRB. Moreover, it is evidenced that the presence of MMT/GF jointly with ATH in SRE can further aid in aggrandising the fire resistance of SR. Therefore, MARHE and TSP are computed at 24.93 kW/m2 and 0.48 m2 in SRE. A similar trend is also observed in the time to ignition (TI) and time to peak heat release (TP). The higher TI and TP of SRC, SRD, and SRE relative to SRB confirm the excellent fire retardancy of co-filled composites compared to SRB and SRA.

Fig. 5
figure 5

Cone calorimeter data profiles of SR composites a HRR, b ARHE, c THR, d SPR, e TSP, and f TOC

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Table 2 Cone calorimeter data of SR composites
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The excellent performance of composites in the cone calorimeter test could be attributed to the natural inherent flame retardancy characteristics of ATH and MMT. ATH is a natural compound with three hydrated water molecules present in its structure [40]. Upon decomposition, ATH releases its water vapours, reduces the combustion process of polymers, and results in the formation of char residue of aluminium oxide. MMT is a natural inorganic flame retardant for polymers without the production of toxic gases [41]. MMT decomposes after flame exposure and generates protonic catalytic sites, which help in creating a stronger interaction with the char residue [33]. Hence, it is highly likely that ATH/MMT jointly form a compact char residue that acts as a heat barrier against combustible gases, resulting in better flame resistance.

To understand the flame resistance mechanism, an SEM study is conducted for the char residue of the composites, as shown in Fig. 6. The char residue of SRA is observed as highly fluffy and fragile. Interestingly, the char residue of the filled composites is found solid, with a few weak points and cracks seen in SRB and SRC. Interestingly, the char residue of SRE is found compact and continuous relative to its counterparts; therefore, cone parameters are improved relatively. It is believed that GF is a non-flammable additive and acts as a fire barrier and reinforcing agent [42, 43]. Moreover, the SEM study of SRE confirms that GF assists in the formation of compact char, which is interlocked by the GF threads, and hence, it renders extraordinary protection to SRE against flame.

Fig. 6
figure 6

Microstructure of char residue of SR composites

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3.3 Electrical surface discharge

To explain the electrical surface resistance of SR composites, the net electric field enhancement is studied through FEA simulation. A three-dimensional parallel plate electrode with the sample is simulated in ANSYS Maxwell, as shown in Fig. 7a. Figure 7b shows the net electrical field enhancement on the surface of the sample on a logarithmic scale with an applied voltage of 5.0 kV. A significant field enhancement is observed at the triple junction point of the top electrode, sample surface, and air media. The maximum field strength is computed at 4.66 × 106 V/m.

Fig. 7
figure 7

FEA simulation study a ANSYS Maxwell 3D model and b electric field distribution on the surface of the sample

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Moreover, the electric field is plotted across a line bisecting the sample fixed on the surface. Figure 8 illustrates the electric field strength on the surface of the sample along the line. It shows that there is no significant variation in the electric field with the change in the permittivity of the SR composites. Furthermore, Fig. 8 confirms that there is a significant field enhancement in the air gap between the sample and the bottom electrode, leading to high ionisation and initiating partial surface discharge activity.

Fig. 8
figure 8

A variation in the electric field across a line on the surface of the sample

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For the surface discharge resistance study of SR composites, the partial discharge patterns are recorded at the PDIV of composites and a fixed voltage level of 5.0 kVrms. PDIV is measured by ramping up the voltage for each sample until PD pulses are detected. For PDIV, the phase-resolved partial discharge pattern (PRPD) is recorded for 30 s with a threshold of 5 pC. Figure 9a and b show the PRPD pattern of SRA and SRE at PDIV. Figure 9c and d show the PRPD patterns of SRA and SRE at a constant root mean square voltage of 5.0 kV recorded for 3 min with a threshold of 50 pC. PDIV and key electrical discharge parameters at 5.0 kV are extracted from the recorded PD data, as shown in Table 3.

Fig. 9
figure 9

Phase-resolved partial discharge pattern of composites a SRA at PDIV, b SRE at PDIV, c SRA at 5.0 kV, and d SRE at 5.0 kV

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Table 3 Key surface discharge data of SR composites
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Results show that the addition of particles in SR composites results in a reduction of PDIV relative to pristine SRA. PDIV of SRC, SRD, and SRE is measured at 1.48 kVrms, 2.30 kVrms, and 2.32 kVrms, respectively, compared to 1.96 kVrms in SRB. PDIV increases in SRD and SRE, indicating excellent electrical discharge resistance, while PDIV drops in SRC. This could be due to a significant increment in the capacitance, the permittivity of SRC with high dielectric loss, and reduced electrical dielectric strength [44].

At 5.0 kV, discharge parameters are analysed to further explore the discharge resistance of composites. As shown in Table 3, Qmax, Qaverage, and repetition rate are computed at 7095 pC, 436 pC, and 31,919 pulses/s in SRE, respectively, compared to 7746 pC, 441 pC, and 43,187 pulses/s in SRB. The excellent surface discharge resistance of SRE with improved PDIV and lower surface discharge parameter values could be attributed to the excellent resistance provided by ATH/MMT/GF jointly embedded in SR [28, 45]. The co-filled additives act as a barrier against the movement of free electrical charges along the surface of the samples, which tends to increase the PDIV and reduce Qmax, Qaverage, and repetition rate.

4 Conclusion

This work addresses the fire resistance parameters, electrical field modelling, and electrical surface discharge resistance of SR matrix co-filled with ATH, MMT, and GF additives. It is concluded that ATH, ATH/MMT, and ATH/MMT/GF significantly increase the flame resistance of SR by decreasing the peak HRR, MARHE, THR, peak SPR, TSP, and TOC of the SR composites. SRE offers a 59% and 92% reduction in MARHE and TSP relative to SRB, respectively. A linear increment in TP and TI also validates the extraordinary resistance of composites against flame. It is also summarised that a significant electrical field enhancement is seen at the triple junction point of the top HV plate, sample, and air gap, which tends to start the surface discharge activity on the surface of the composites. Furthermore, it is concluded that the addition of the additives results in enhanced PDIV values, while there is a considerable reduction seen in the Qmax, Qaverage, and repetition rate of ATH/MMT/GF co-filled composites. SRE presents an 8%, 1%, and 7% reduction in Qmax, Qaverage, and repetition rate, respectively, relative to SRB. It is concluded that the addition of MMT and GF within the only ATH-filled SR could result in enhancing the flame and electrical surface discharge resistance of the SR material for use as industrial electrical insulating material.