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Article

Application of Magnesium Hydroxide/Diphenoxy Phosphate in Silicone Rubber Flame Retardant Cable Material

by
Wei Wang
1,
Fan Yang
2,
Yunlai Lu
3,
Zhi Luo
2,
Fangya Li
2,
You Wu
2,
Jianbing Zhang
2,
Zufeng Xiao
1,*,
Wei Li
1,3,* and
Caiqin Qin
1
1
School of Chemistry and Materials Science, Hubei Engineering University, Xiaogan 432000, China
2
State Grid Hubei Electric Power Co., Ltd., Xiaogan Electric Power Supply Company, Xiaogan 432000, China
3
Hubei Yunlai Plastic Technology Co., Ltd., Xiaogan 432000, China
*
Authors to whom correspondence should be addressed.
Coatings 2023, 13(5), 934; https://doi.org/10.3390/coatings13050934
Submission received: 26 April 2023 / Revised: 13 May 2023 / Accepted: 15 May 2023 / Published: 16 May 2023
(This article belongs to the Special Issue Organic Synthesis and Characteristics of Thin Films Second Volume)
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Abstract

:
Deketoxime–type room–temperature vulcanized silicone rubber cable materials were prepared using α, ω–dihydroxy polydimethylsiloxane, carbon black, calcium carbonate, magnesium hydroxide, piperazine bis (diphenoxy phosphate) salt (PBDP), and melamine diphenoxy phosphate (MDP). The effects of carbon black and flame retardants on the mechanical properties, flame–retardant properties, and electrical insulation properties of silicone cable coatings were investigated. The research results showed that the products obtained had good mechanical and electrical insulation properties, with tensile strength greater than 3.0 MPa, dielectric strength greater than 22 kV/mm, and volume resistivity higher than 6.5 × 1014 Ω·cm. When 30 parts of Mg(OH)2:MDP = 2:1 are added to 100 parts of resin, the flame–retardant performance of wire and cable materials can be significantly improved. Under the thermal radiation illumination of 50 kW/m2, the ignition time (TTI) of the Mg(OH)2/MDP coating increased by 16 s, and the maximum heat release rate (pkHRR) and total heat release rate (THR) decreased by 29.7% and 68.8%, respectively, compared with silicone rubber without flame retardant. The silicone rubber coatings prepared were flame retardant up to the FV–1 level.

1. Introduction

High–voltage transmission is an important method for reducing electricity consumption and transmitting electricity over long distances. Overhead bare wires [1,2,3] are susceptible to short–circuiting under high–voltage electric fields under the influence of complex weather and geography, causing accidents, such as the tripping of power lines, which poses a serious challenge to the operation of distribution networks. Compared with traditional bare wire, overhead insulated wire [4,5,6] has higher safety, the line occupies less space, there is a reduction in wire corrosion, and other characteristics used in the distribution network transformation project can effectively reduce the incidence of line safety accidents. Compared with the direct replacement of insulated lines or the use of sleeves, direct insulation coating wrapping on overhead bare wire effectively reduces the intensity of manual work, shortens the transformation time, and enables the insulation transformation of bare wire under energized conditions.
Of the many polymeric materials available, silicone rubber is the most suitable for extrusion as an insulating coating material. Compared to other polymers, silicone rubber with Si–O structural units has excellent resistance to high and low temperatures, corrosion, and weathering, and it exhibits insulation properties. It is widely used in the aerospace [7,8], rail transportation [9], electronic appliance [10], automotive [10,11], biomedical [12,13,14], and other fields. Room–temperature vulcanized rubber (RTV) [15,16,17] is one of the varieties of liquid silicone rubber. It is usually made from α, ω–dihydroxy polydimethylsiloxane (PDMS), which undergoes cross–linking reactions at room temperature and atmospheric pressure when it encounters moisture in the air to produce a three–dimensional structure. Room–temperature vulcanized rubber maintains elasticity over a wide temperature range and also has good adhesion to various substrates; it has been widely used as an anti–flash coating material in high–voltage transmission insulators [18,19,20]. So far, only a few products meeting the requirements for overhead bare wire wrapping have been reported. Wang Hualing et al. [21] prepared room–temperature cured coating materials with dielectric strengths greater than 20 kV/mm and tensile strengths greater than 3.5 MPa using hydroxyl–terminated dimethylpolysiloxane, silica, aluminum nitride, and carbon black. Junting Yang et al. [22] chose silica as a reinforcing filler to prepare a good, low–density, and high–strength silicone rubber material. Ruiqi Shang et al. [4] analyzed the effects of the dielectric properties of silicone rubber with different aluminum hydroxide content additions using a modified Cole–Cole model.
Although room–temperature vulcanized rubber has the advantages of a high oxygen index at the combustion limit and a low heat release rate, which can slow down the propagation of flame, silicone rubber itself is combustible and not easily self–extinguished [23,24], making it difficult to meet the flame–retardant requirements of high–voltage transmission lines. Therefore, flame retardants need to be added to room–temperature vulcanized rubber to improve its flame–retardant properties. There are three main ways to improve the flame–retardant properties of silicone rubber: adding flame retardants, changing material composition, and blending with other polymer materials. The addition of flame retardants is the simplest and easiest way to improve the flame retardancy of silicone rubber. Magnesium hydroxide (MH) is a widely used inorganic flame retardant [25,26]. Mg(OH)2 facilitates the formation of a charred layer on the surface of the material, preventing the entry of oxygen and heat; at the same time, the MgO generated by decomposition is a good refractory material that can improve the resistance of polymeric materials to flames. Due to the poor compatibility of MH with silicone rubber, it is difficult to disperse uniformly, which may affect the mechanical properties and extrusion performance of the coating. To achieve compatibility between MH and polymer, certain synergists or phosphorus–based flame retardants can be added to improve the flame–retardant properties of the composite while reducing the amount of MH added. Phosphorus and nitrogen flame retardants [27,28,29] have the advantages of high flame retardant efficiency, low toxicity, and good compatibility with polymer materials. In this experiment, we compounded synthesized phosphate with magnesium hydroxide for use as a flame retardant for silicone rubber. Using methods such as combustion experiments and thermogravimetric analysis, we initially explored the flame–retardant effect of the synthesized flame retardant and the compounding system on the silicone rubber material, expecting to find a formulation with excellent flame–retardant properties for silicone rubber.

2. Materials

Dihydroxy polydimethylsiloxane (average Mn ~1500, viscosity ~35 cst), melamine (RG, ≥99%), diphenyl phosphate (RG, ≥99%), 1,4–diazacyclohexane (AR), nano calcium carbonate (≥99%), dimethyl silicone oil (viscosity: 1000 mPa.s), Mg(OH)2 (AR), and carbon black (particle size: 7–40 nm) were purchased from West Asia Chemical Technology (Shandong, Co., Ltd., Linyi, China). Methyl tris–butanone oxime silane (KH–301, ≥95%, MOS), dibutyltin laurate (AR), and N–aminoethyl–γ–aminopropyltrimethoxysilane (KH–792, ≥95%) were purchased from Nanjing Jingtianwei Chemical Co., Ltd., Nanjing, China.

2.1. Analytical Methods and Test Standards

The coating was pressed into a thin film material with a press vulcanizer, and its properties were tested after curing. An electronic universal testing machine (UTM, Shimadzu, AG–IC50kN, Kyoto, Japan) was used to test the mechanical properties of the films. A cone calorimeter (Cone, Firemana, PX07007, Beijing, China) was used to test several performance parameters of the combustion specimens, with a thermal radiation illumination setting of 50 kW/m2. A Horizontal Vertical Flame Chamber Tester (HVFT, Cots, CFZ–3, Kunshan, China) was used to test the performance of the combustion specimens with a flame height of 20 mm ± 2 mm. A thermogravimetric analyzer (TG, Netzsch, STA449F5, Bayern, Germany) was used to test the thermal stability of the films, with test conditions set at a heating rate of 10 °C/min at N2 flow (50 mL/min). Scanning electron microscopy (SEM, JEOL, JSM–6510, Tokyo, Japan) was used to test the morphology of the coatings after curing. The purified diphenyl phosphates and their structures were confirmed using nuclear magnetic resonance (NMR, Bruker, Avance III 400 Hz, Karlsruhe, Germany).
The surface drying performance and depth of cure performance indexes were tested under standard laboratory conditions at 25 +/− 2 °C and 50 +/− 5% humidity.
Surface drying time: tested according to standard GB/T 13477.5–2002.
Curing depth: tested according to standard GB/T 32369–2015.
Hardness: tested according to GB/T 2411–2008.
Tensile strength and elongation at break: tested according to GB/T 528–2009.
Flame–retardant performance: tested according to GB/T10707–2008.
Tensile shear strength: tested according to GB/T–13936–2014.
Electrical breakdown strength: tested according to ASTM D1498–2008.
Volume resistivity: tested according to GB/T1692–2008.

2.2. Synthesis of Piperazine bis (Diphenoxy Phosphate) Salt (PBDP) and Melamine Diphenoxy Phosphate (MDP)

The reaction equation for the synthesis of diphenoxy phosphate is shown in Scheme 1.
Synthesis of PBDP: Weigh 8.61 g (0.1 mol) of 1,4–diazacyclohexane solid and add to a three–necked flask containing 100 mL of water to dissolve by heating. Add diphenyl phosphate in batches with constant stirring until the whole solution becomes neutral and the reaction stops. Filter and dry the product to obtain a white solid (48.6 g, 83% yield), and identify the structure of the product using NMR.
Synthesis of MDP: Add 25.0 g (0.1 mol) of diphenyl phosphate powder to 100 mL of water, stir to dissolve, and heat to 90 °C. After 5 min, when all the solids are dissolved, add 13.6 g (0.11 mol) of melamine and react for 0.5 h. After the pH of the solution is measured to be neutral, filter the suspension solution to obtain a white solid (33.4 g, yield 89%). Identify the structure of the melamine diphenyl phosphate salt using NMR.

2.3. Production Processes for Insulating Coatings

The insulating coating was produced according to reference [17,30]. α, ω–dihydroxy polydimethylsiloxane, carbon black, calcium carbonate, magnesium hydroxide, and diphenyl phosphate were added to the reactor and then stirred at 120 °C for 1 h. The stirring was continued under a vacuum for 3 h. Finally, dimethyl silicone oil, a coupling agent, a cross–linking agent, and dibutyltin laurate were added, and stirring was continued for 3 h for encapsulation. The sample formulations are shown in Table 1 (phr: parts per hundred parts of resin).

3. Results

3.1. Structural Characterization of Phosphorus–Based Flame Retardants PBDP and MDP

NMR spectra were used to determine the structure of the products. Figure 1 shows the NMR spectrum of pyridine diphenoxy phosphate. In Figure 1a, the triple peak at δ = 7.35–7.31 ppm corresponds to the chemical shifts of the ortho– and para–hydrogen atoms in the benzene ring. The multiple peaks at δ = 7.18–7.14 ppm correspond to the chemical shifts of the meta–hydrogen atoms in the benzene ring. The single peak at δ = 3.48 ppm corresponds to the chemical shifts of the hydrogen atoms in the piperazine ring. The ratio of the integrated area of the three hydrogen atoms is 8:12:8, which is consistent with the theoretical one of PBDP. Figure 1b shows the NMR carbon spectrum of the salt. There are five different chemical environments of carbon atoms (C1–C5) in the salt, which correspond to peaks of 129.71, 120.15, 124.46, 120.11, and 40.02 ppm in the spectra. The NMR data indicated that we had successfully synthesized piperazine bis (diphenoxy phosphate) salt (PBDP).
The NMR spectrum of melamine diphenoxy phosphate (MDP) is shown in Figure S1. Similar to PBDP, MDP has two sets of peaks for the hydrogen atom on the benzene ring that appear in the NMR spectrum. As shown in Figure S1a, the triple peak at δ = 7.35–7.31 ppm corresponds to the chemical shifts of the ortho–and para–hydrogen atoms in the benzene ring. The multiple peaks at δ = 7.17–7.13 ppm correspond to the chemical shifts of the meta–hydrogen atoms in the benzene ring. The active hydrogen in the amino group is not shown. The ratio of the integrated area of the two hydrogen atoms was 8:12, which is consistent with the theoretical one of MDP. In Figure S1b, the NMR carbon spectrum of the melamine salt shows six chemical shifts (151.51, 151.44, 129.69, 124.45, 120.14, and 120.10), corresponding to six carbon atoms (C1–C6) in different chemical environments in the melamine salt. The NMR data indicated that melamine diphenoxy phosphate (MDP) was also successfully synthesized.
PBDP: 1H NMR (400 MHz, D2O, ppm), δ = 7.36–7.32 (t, J = 8.0 Hz, 8 H), 7.18–7.14 (m, 12 H), and 3.48 (s, 8 H). 13C NMR (100 MHz, D2O, ppm), δ = 129.71, 124.46, 120.15, 120.11, and 40.02.
MDP: 1H NMR (400 MHz, D2O, ppm), δ = 7.35–7.31 (t, J = 7.8 Hz, 8 H), 7.18–7.14 (m, 12 H), and 3.48 (s, 8 H). 13C NMR (100 MHz, D2O, ppm), δ = 151.51, 151.44, 129.69, 124.45, 120.14, and 120.10.

3.2. Effect of Different Amounts of Carbon Black on the Mechanical Properties of the Samples

Carbon black performs functions such as coloring and reinforcement in rubber products. The smaller the particle size of the carbon black, the larger the effective contact area between the carbon black and the rubber, and the more it can bond with the rubber physically or chemically, thus improving the mechanical properties of room–temperature vulcanized rubber [31]. Therefore, we chose carbon black with a particle size less than 30 nm as reinforcing material. The effects of different amounts of carbon black on the mechanical properties of room–temperature vulcanized rubber were investigated, and the specific data are shown in Table S1 and Figure 2.
The tensile strength and elongation at the break of sample 1 without the addition of carbon black were 0.81 MPa and 162%, respectively (Table S1, Entry 1). Compared with sample 1, the room–temperature vulcanized rubber with the addition of carbon black (samples 2–6) showed significant improvements in terms of tensile strength and elongation at break (Table S1, Entries 2–6). Sample 2 showed a 188% increase in tensile strength and a 59% increase in elongation at break compared to sample 1. The other samples (3–6) showed increases in tensile strength of 227%–284% and elongation at break of 83%–91% compared to sample 1. The silicone rubber with the addition of carbon black had good adhesion to the experimental substrate. The tear strength of the rubber with the addition of carbon black increased between 111% and 172% compared to sample 1 (Figure 2a,b). The hardness of the coatings increased with the addition of carbon black, which indicated that the carbon black dispersed into RTV had an excellent reinforcement effect. The principle of carbon black reinforced silicone rubber can be explained by the “Bonded rubber shell layer structure model” [32]. The strong interaction between the carbon black and the rubber molecules results in the rubber molecules being adsorbed on the surface of the carbon black to form a shell structure. When the rubber is deformed, this shell structure acts as a skeleton in the vulcanized rubber and distributes the stresses evenly, thus increasing the strength of the rubber.

3.3. Effects of Different Quantities of Carbon Black on the Surface Drying Time and Deep Curing Performance of the Coating

As shown in Figure 3 and Table S2, the surface drying time of sample 1 without the addition of carbon black was 31.2 min, while the surface drying time of sample 1 with the addition of 50 g of carbon black (sample 6) was shortened to 22.5 min. The curing depth at 24 h after the addition of carbon black also increased, up to 3.8 mm (sample 4). The experimental results show that reactive groups, such as hydroxyl, carboxyl, and ester groups on the surface of carbon black, can interact with the molecular chains of silicone rubber through hydrogen bonding and positively contribute to the deep curing rate of the coating. However, as the addition of carbon black continues, the surface drying time of the coating increases, and the depth of cure decreases. The probable reason for this is that the carbon black particles and silicone rubber are cross–linked with each other to form a tight three–dimensional mesh structure that acts as a barrier to the penetration of water vapor and prevents further diffusion of moisture into the interior of the coating. According to the field experiment, when the amount of carbon black added per 100 parts of resin was greater than 30, the flow properties of the rubber decreased, and it became difficult to extrude. Therefore, we finally chose to add 30 phr of carbon black to the insulation coating.

3.4. Effects of Different Flame Retardants on the Flame–Retardant Properties of Room–Temperature Vulcanized Rubber

We added Mg(OH)2, PBDP, MDP, or mixed flame retardants to the rubber coatings and investigated their effects on the combustion performance, flame–retardant performance, and thermal stability of room–temperature vulcanized rubber. The combustion performance of RTV silicone rubber was characterized by a conical calorimeter with the heat radiation intensity set to 50 kW/m2 and sample size 100 mm × 100 mm × 2 mm. The dimensions of the sample strip for the vertical burn test were 13 mm × 13 mm × 3 mm.

3.4.1. Ignition Time of Adding Different Flame Retardants

The combustion experimental data are shown in Table S3 and Figure S2. Figure 4 shows the ignition time (TTI) of room–temperature vulcanized rubber with different flame–retardant coatings. The ignition time (TTl) of the warm vulcanized rubber(sample 7) without added flame retardant was 23 s. The ignition times (TTl) of the room–temperature vulcanized rubber with added flame retardants (samples 4 and 8–14) were prolonged compared to sample 7. From the experimental results, the ignition time of vulcanized rubber was further prolonged by the addition of phosphorus flame retardant compared with that of Mg(OH)2. The addition of Mg(OH)2 prolonged the ignition time by 7 s. The addition of PBDP prolonged the ignition time by 8–20 s. The addition of MDP prolonged the ignition time by 16–26 s. The addition of phosphate was effective in extending the ignition time of normal–temperature vulcanized rubber. The reason for this may be that silicone rubber releases substances such as phosphoric acid and polyphosphoric acid during the heating process, which causes the polymeric material to dehydrate carbonization, forming a protective carbon film on the surface of the material and thus delaying the combustion process.

3.4.2. Heat Release Rate (HRR) with Different Flame Retardants

The heat release rate (HRR) is the rate of heat release per unit area of material when ignited. The peak of the heat release rate (pkHRR) represents the peak of the heat release rate. The higher the values of HRR and pkHRR, the more combustible the material. Figure 5 shows the heat release rate curves for rubber coatings containing different flame retardants. The silicone rubber without flame retardant (sample 7) had a higher value of 219.54 kW/m2. The addition of Mg(OH)2 or MDP to the samples was effective in reducing the peak heat release rate of the rubber, with pkHRR values of 103.78 kW/m2 (sample 4) and 153.68 kW/m2 (sample 13), respectively. Compared to sample 7, the pkHRRs of samples 4 and 13 were reduced by 52.7% and 29.7%, respectively. In contrast, the peak heat release rate increased to 232.72 kW/m2 for sample 10 with the addition of PBDP. Other samples with the addition of Mg(OH)2 or MDP were also effective in reducing the peak heat release rate of the rubber.

3.4.3. Total Heat Release (THR) with Different Flame Retardants

Total heat release (THR) is the total amount of heat released from the ignition of a material until the flame is extinguished. Figure 6 shows the total heat release (THR) for adding different flame–retardant coatings. With the addition of 30 phr of Mg(OH)2 (sample 4), the THR of the room–temperature vulcanized rubber decreased from 35.43 MJ/m2 (samples 7) to 9.69 MJ/m2. The THRs of the room–temperature vulcanized rubber with 30 phr of PBDP (sample 10) or 30 phr of MDP (sample 13) also decreased to 16.93 MJ/m2 and 20.31 MJ/m2, respectively. The THR of sample 11 (20 phr Mg(OH)2 and 10 phr MDP) was reduced to 11.05 MJ/m2. Combining the HRR and THR data, sample 4 and sample 13 both had good flame–retardant properties. Acidic groups such as polyphosphoric acid and phosphoric acid formed by the decomposition of phosphate under heating conditions captured the debris from the decomposition of silicone rubber. As can be seen in Figure S2, the uneven distribution of fine bubbles on the surface of the burned material indicates that the diphosphate has good char formation properties.

3.4.4. Vertical Combustion Test with Different Flame Retardants

Table 2 and Figure S3 show the vertical burning times of room–temperature vulcanized rubber containing different amounts of flame–retardant fillers. During the course of the experiment, it was found that samples 1 and 7 without flame retardant burned continuously, with the flame spreading to the fixture and largely free of residual char, while accompanied by the phenomenon of dripping, and igniting skimmed cotton. The experimental phenomenon showed that the flame–retardant performance of PBDP was lower than that of MDP and Mg(OH)2, while Mg(OH)2 had a synergistic flame–retardant effect with MDP. In sample 10 containing PBDP, the after–flame burn time after ignition was reduced, and the sample drip phenomenon and igniting the skimmer phenomenon occurred. The flame–retardant grade of the material was FV–2. The flame retardancies of the samples with the addition of Mg(OH)2 and diphenyl phosphate flame retardants were significantly improved. Sample 4 with 30 phr of Mg(OH)2 was self–extinguishing after the first ignition, and the sum of the two after–flame times was less than 10 s. The flame retardancy of this material reached the FV–0 level. For sample 11 with 20 phr Mg(OH)2 and 10 phr MDP, a protective charcoal layer was observed on the surface of the specimen during ignition. However, due to the presence of the charcoal layer, the second after–flame burn time was slightly longer, and the flame retardant rating was only FV–1. The following are possible reasons for flame retardancy: the water vapor released by the pyrolysis of magnesium hydroxide takes away more heat from the substrate and reduces the surface temperature of the substrate, and the foam–like material formed by the thermal decomposition of MDP covers the surface of the material, preventing the further transfer of heat and oxygen, thus achieving a synergistic flame–retardant effect.

3.5. Thermogravimetric Analysis with Different Flame Retardants

Figure 7 shows the thermal weight loss graphs of silicone rubber samples 4, 7, 10, 11, 13, and 14. The silicone rubber (sample 7) without the addition of magnesium hydroxide started to lose weight at 175 °C and ended at 680 °C, with a residual mass of 33.32% for the silicone rubber coating. The initial decomposition temperature of the added flame retardant was delayed in all cases compared to sample 7 without the flame retardant. The thermal decomposition curve of silicone rubber (sample 4) was divided into two intervals. The first interval (230–320 °C) was the decomposition of small molecules of the material by heat. During the second interval (370–590 °C), high thermal weight loss occurred, probably due to the decomposition of magnesium hydroxide at 390 °C to produce water vapor and high–temperature–resistant solid magnesium oxide. Magnesium hydroxide absorbs a large amount of heat during the decomposition process, which can curb the initial decomposition of silicone rubber effectively. The residual mass was 36.31% for the silicone rubber coating. The rubber with MDP added (sample 13) started to lose weight at 243 °C and ended at 650 °C. The residual mass was 33.36% for the silicone rubber coating. The thermogravimetric curves of the composites (samples 11 and 14) with the addition of mixed flame retardants of magnesium hydroxide and diphenyl phosphate were similar to those of sample 4, but the material had a lower temperature in the first decomposition interval, while the temperature in the second decomposition interval was higher. The possible reason for this is that MDP catalyzes the decomposition of magnesium hydroxide, bringing forward the first decomposition temperature. Heated MDP produces phosphoric acid, which promotes the carbonization of hydroxyl compounds and forms a dense carbonized film on the surface of the substrate. The carbonized film covering the surface of the substrate prevents heat and oxygen transfer from inhibiting the thermal decomposition of the material until the decomposition temperature of the macromolecules in the rubber is reached. The temperature of the second decomposition interval was delayed. The results of the thermal decomposition experiments showed that the addition of magnesium hydroxide with diphenyl phosphate resulted in better thermal stability performance of the room–temperature vulcanized rubber.

3.6. Microscopic Morphology of Coatings Containing Different Flame Retardants

Figure 8 shows SEM photographs of the surfaces containing different flame–retardant coatings. It can be seen from the figure that the surface of sample 7 is smoother. Sample 4 with the addition of Mg(OH)2 showed more solid particles. The samples with the addition of PBD and PMDP (11, 13, and 14) were able to produce a better miscible system mix with the silicone rubber substrate compared to sample 4, resulting in a smoother surface.

3.7. Mechanical Properties and Electrical Properties of the Samples

Table 3 shows that the tensile strength and elongation at the break of the silicone rubber samples with flame–retardant additions were slightly lower compared to sample 7 without flame–retardant additions. The sample with the addition of diphenyl phosphate had better compatibility with silicone rubber and better mechanical properties compared to sample 4. Sample 10 had an elongation at a break of 280% and a tensile strength of 3.12 MPa. Sample 13 had an elongation at a break of 297% and a tensile strength of 3.41 MPa. Variations in the mechanical properties of the samples can be verified in the SEM images. The surface drying times for the samples with flame retardants (4, 7, 10, 11, 13, and 14) ranged from 20.5 to 24.7 min, and the depth of cure for 24 h ranged from 3.4 to 3.7 mm.
The electrical properties of all six samples (4, 7, 10, 11, 13, and 14) were excellent. The volume resistivity of silicone rubber for all samples was greater than 5.0 × 1014 Ω·cm, while the breakdown voltage was above 20 kV/mm, and the electrical properties met the requirements for laying 10 kV overhead cables. Combining the above experimental results, we used the formulation of sample 11 as the final formulation.

3.8. Field Trials and Applications

As shown in Figure 9, we first completed a simulation experiment of wrapping bare cables with normal–temperature vulcanized rubber in the laboratory using the wrapping robot independently developed by Xiaogan Power Supply Company of State Grid Hubei Province, and there was almost no drip hanging phenomenon during the wrapping process. As shown in Figure 9b, the vulcanized rubber coating can finish wrapping closely on the cable. The performance of the product after the experiment, such as flame retardancy and electrical insulation, met the requirement of wrapping 10 kV cable. The coating has been applied to 10 kV cable line transformation in Hubei Province, Guangdong Province, and Yunnan Province. The flow of the live insulation coating operation on–site is shown in Figure 9c–f. The coating was first filled on the ground, then the robot was sent to the cable by lifting the insulation rope, followed by controlling the robot to wrap the bare wire cable, and the coating work can be monitored in real time by the monitor.

4. Conclusions

The effect of carbon black on the mechanical properties of room–temperature vulcanized rubber was investigated, and insulating coatings with flame–retardant properties were prepared through the addition of Mg(OH)2 and MDP. Carbon black improved the tensile strength and elongation at the break of room–temperature vulcanized rubber, accelerated the surface drying time, and deepened the curing depth of the samples. Compared to the sample without the flame retardant (sample 7), sample 11 with 30 phr of Mg(OH)2:MDP = 2:1 showed a greater increase in TTI, HRR, and THR. The flame–retardant properties reached the FV–1 level, the tensile strength was greater than 3.2 MPa, its volume resistivity was greater than 6.5 × 1014 Ω·cm, and the breakdown voltage was greater than 20 KV/mm. The coating is suitable for mass production and can be used in retrofit projects for 10 kV overhead conductors.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/coatings13050934/s1: Figure S1: The NMR spectrum of Melamine diphenoxy phosphate (MDP): (a) 1HNMR, and (b) 13C NMR; Figure S2: Burning test diagram of some materials: (a) Before burning, (b)After burning; Figure S3: Vertical combustion test diagram of some materials; Table S1: Mechanical properties of different quality of carbon black; Table S2: Effect of different quality of carbon black on surface drying time and depth of cure; Table S3: Experimental data of cone calorimeter for room temperature vulcanized rubber containing different flame retardants.

Author Contributions

Designed the experiment, W.L.; experiments and writing draft of the manuscript, W.W.; review and editing, Z.X. and C.Q.; data curation, F.Y.; writing—review and editing, Y.L., Z.L., F.L., Y.W. and J.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the Hubei Province 2023 Science and Technology Talent Service Enterprise Project (Grant Number: KJRQ2023000167).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Supporting data can be obtained from the corresponding authors.

Acknowledgments

The authors are grateful for the Department of Science and Technology of Hubei Province, China.

Conflicts of Interest

The authors declare no conflict of interest.

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Coatings 13 00934 sch001 550
Scheme 1. The reaction equation for the synthesis of diphenoxy phosphate: (A) PBDP and (B) MDP.
Scheme 1. The reaction equation for the synthesis of diphenoxy phosphate: (A) PBDP and (B) MDP.
Coatings 13 00934 sch001
Coatings 13 00934 g001 550
Figure 1. The NMR spectrum of piperazine bis (diphenoxy phosphate) salt (PBDP): (a) 1H NMR and (b) 13C NMR.
Figure 1. The NMR spectrum of piperazine bis (diphenoxy phosphate) salt (PBDP): (a) 1H NMR and (b) 13C NMR.
Coatings 13 00934 g001
Coatings 13 00934 g002 550
Figure 2. Effects of different quantities of carbon black on the mechanical properties of samples 1–6: (a) Tensile strength of samples 1–6, and (b)Elongation at the break of samples 1–6.
Figure 2. Effects of different quantities of carbon black on the mechanical properties of samples 1–6: (a) Tensile strength of samples 1–6, and (b)Elongation at the break of samples 1–6.
Coatings 13 00934 g002
Coatings 13 00934 g003 550
Figure 3. Effects of different quantities of carbon black on the surface drying time and deep curing performance of the coating.
Figure 3. Effects of different quantities of carbon black on the surface drying time and deep curing performance of the coating.
Coatings 13 00934 g003
Coatings 13 00934 g004 550
Figure 4. The ignition time of adding different flame retardants. 7: 0 phr Mg(OH)2, 4: 30 phr Mg(OH)2, 8: 20 phr Mg(OH)2 and 10 phr PBDP, 9: 10 phr Mg(OH)2 and 20 phr PBDP, 10: 30 phr PBDP, 11: 20 phr Mg(OH)2 and 10 phr MDP, 12: 10 phr Mg(OH)2 and 20 phr MDP, 13: 30 phr MDP, and 14: 10 phr Mg(OH)2, 10 phr PBDP, and 10 phr MDP.
Figure 4. The ignition time of adding different flame retardants. 7: 0 phr Mg(OH)2, 4: 30 phr Mg(OH)2, 8: 20 phr Mg(OH)2 and 10 phr PBDP, 9: 10 phr Mg(OH)2 and 20 phr PBDP, 10: 30 phr PBDP, 11: 20 phr Mg(OH)2 and 10 phr MDP, 12: 10 phr Mg(OH)2 and 20 phr MDP, 13: 30 phr MDP, and 14: 10 phr Mg(OH)2, 10 phr PBDP, and 10 phr MDP.
Coatings 13 00934 g004
Coatings 13 00934 g005 550
Figure 5. HRR curves of vulcanized rubber with different flame retardants. 7: 0 phr Mg(OH)2, 4: 30 phr Mg(OH)2, 10: 30 phr PBDP, 11: 20 phr Mg(OH)2 and 10 phr MDP, 13: 30 phr MDP, 14: 10 phr Mg(OH)2, 10 phr PBDP, and 10 phr MDP.
Figure 5. HRR curves of vulcanized rubber with different flame retardants. 7: 0 phr Mg(OH)2, 4: 30 phr Mg(OH)2, 10: 30 phr PBDP, 11: 20 phr Mg(OH)2 and 10 phr MDP, 13: 30 phr MDP, 14: 10 phr Mg(OH)2, 10 phr PBDP, and 10 phr MDP.
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Coatings 13 00934 g006 550
Figure 6. THR curves of vulcanized rubber with different flame retardants. 7: 0 phr Mg(OH)2, 4: 30 phr Mg(OH)2, 8: 20 phr Mg(OH)2 and 10 phr PBDP, 9: 10 phr Mg(OH)2 and 20 phr PBDP, 10: 30 phr PBDP, 11: 20 phr Mg(OH)2 and 10 phr MDP, 12: 10 phr Mg(OH)2 and 20 phr MDP, 13: 30 phr MDP, and 14: 10 phr Mg(OH)2, 10 phr PBDP, and 10 phr MDP. Coatings 13 00934 i001 for PBDP was added to the flame retardant, Coatings 13 00934 i002 for MDP was added to the flame retardant.
Figure 6. THR curves of vulcanized rubber with different flame retardants. 7: 0 phr Mg(OH)2, 4: 30 phr Mg(OH)2, 8: 20 phr Mg(OH)2 and 10 phr PBDP, 9: 10 phr Mg(OH)2 and 20 phr PBDP, 10: 30 phr PBDP, 11: 20 phr Mg(OH)2 and 10 phr MDP, 12: 10 phr Mg(OH)2 and 20 phr MDP, 13: 30 phr MDP, and 14: 10 phr Mg(OH)2, 10 phr PBDP, and 10 phr MDP. Coatings 13 00934 i001 for PBDP was added to the flame retardant, Coatings 13 00934 i002 for MDP was added to the flame retardant.
Coatings 13 00934 g006
Coatings 13 00934 g007 550
Figure 7. TG curves of different silicone rubbers: 7: 0 phr Mg(OH)2, 4: 30 phr Mg(OH)2, 10: 30 phr PBDP, 11: 20 phr Mg(OH)2 and 10 phr MDP, 13: 30 phr MDP, 14: 10 phr Mg(OH)2, 10 phr PBDP, and 10 phr MDP.
Figure 7. TG curves of different silicone rubbers: 7: 0 phr Mg(OH)2, 4: 30 phr Mg(OH)2, 10: 30 phr PBDP, 11: 20 phr Mg(OH)2 and 10 phr MDP, 13: 30 phr MDP, 14: 10 phr Mg(OH)2, 10 phr PBDP, and 10 phr MDP.
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Coatings 13 00934 g008 550
Figure 8. SEM images of six different samples (500×).
Figure 8. SEM images of six different samples (500×).
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Coatings 13 00934 g009 550
Figure 9. Field trials and applications: (a) Insulation coating robot (tester) performing insulation coating wrapping experiments in the laboratory. (b) Overmolded room–temperature vulcanized rubber. (c) Machine commissioning and adding coatings in field work. (d) Hoisting the insulated coating robot onto the cable in field work. (e) Field work diagram of room–temperature vulcanized rubber coating with insulation coating robot completing wrapping on 10 kV cable. (f) Real–time monitoring of insulation coating wrapping work.
Figure 9. Field trials and applications: (a) Insulation coating robot (tester) performing insulation coating wrapping experiments in the laboratory. (b) Overmolded room–temperature vulcanized rubber. (c) Machine commissioning and adding coatings in field work. (d) Hoisting the insulated coating robot onto the cable in field work. (e) Field work diagram of room–temperature vulcanized rubber coating with insulation coating robot completing wrapping on 10 kV cable. (f) Real–time monitoring of insulation coating wrapping work.
Coatings 13 00934 g009
Table
Table 1. Sample formulations.
Table 1. Sample formulations.
EntryPDMS
(g)		Carbon Black
(phr)		CaCO3
(phr)		Mg(OH)2
(phr)		PBDP
(phr)		MDP
(phr)
110001003000
2100101003000
3100201003000
4100301003000
5100401003000
6100501003000
710030100000
81003010020100
91003010010200
10100301000300
111003010020010
121003010010020
13100301000030
1410030100101010
Table
Table 2. Flame–retardant properties of room–temperature vulcanized rubber with different flame retardants.
Table 2. Flame–retardant properties of room–temperature vulcanized rubber with different flame retardants.
EntryFirst after–Flame Burn Time for 5 Specimens (t1,i)/sSecond
after–Flame Burn Time for 5 Specimens (t2,i)/s		Total after–Flame Burn Time
Per Specimen
(t1,i + t2,i)/s		Total after–Flame Burn Time for 5 Specimens (tf)/sDrip Ignition of Skimmed CottonFlame
Retardant Grade
1BurnBurnBurnBurnYesNR
7BurnBurnBurnBurnYesNR
43/4/2/3/26/6/5/7/59/10/8/10/744NoFV–0
1022/16/19/23/2026/25/25/27/3448/41/44/50/54237YesFV–2
115/4/5/3/77/9/11/6/1212/13/16/9/1969NoFV–1
136/6/5/7/710/9/11/13/1416/15/16/20/2188NoFV–1
1414/11/13/13/1213/10/15/17/1427/28/28/30/266/6/5/7/4NoFV–1
Table
Table 3. Mechanical properties with different flame retardants.
Table 3. Mechanical properties with different flame retardants.
Entry7
0 phr Mg(OH)2		4
30 phr Mg(OH)2		10
30 phr PBDP		11
20 phr Mg(OH)2 + 10 phr MDP		13
30 phr MDP		14
10 phr Mg(OH)2 + 10 phr PBDP + 10 phr MDP
Tensile strength (MPa)3.68 ± 0.323.07 ± 0.313.12 ± 0.273.28 ± 0.233.41 ± 0.223.14 ± 0.27
Elongation at break (%)327 ± 29244 ± 18280 ± 14291 ± 25297 ± 31253 ± 21
Surface drying time (min)28.223.020.324.720.522.1
Depth of cure (mm/24 h)3.33.73.43.53.53.6
Volumetric resistivity (Ω·cm)9.16 × 10145.21 × 10147.19 × 10146.52 × 10147.36 × 10145.06 × 1014
Breakdown voltage (KV/mm)24.321.422.522.122.721.9
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MDPI and ACS Style

Wang, W.; Yang, F.; Lu, Y.; Luo, Z.; Li, F.; Wu, Y.; Zhang, J.; Xiao, Z.; Li, W.; Qin, C. Application of Magnesium Hydroxide/Diphenoxy Phosphate in Silicone Rubber Flame Retardant Cable Material. Coatings 2023, 13, 934. https://doi.org/10.3390/coatings13050934

AMA Style

Wang W, Yang F, Lu Y, Luo Z, Li F, Wu Y, Zhang J, Xiao Z, Li W, Qin C. Application of Magnesium Hydroxide/Diphenoxy Phosphate in Silicone Rubber Flame Retardant Cable Material. Coatings. 2023; 13(5):934. https://doi.org/10.3390/coatings13050934

Chicago/Turabian Style

Wang, Wei, Fan Yang, Yunlai Lu, Zhi Luo, Fangya Li, You Wu, Jianbing Zhang, Zufeng Xiao, Wei Li, and Caiqin Qin. 2023. "Application of Magnesium Hydroxide/Diphenoxy Phosphate in Silicone Rubber Flame Retardant Cable Material" Coatings 13, no. 5: 934. https://doi.org/10.3390/coatings13050934

APA Style

Wang, W., Yang, F., Lu, Y., Luo, Z., Li, F., Wu, Y., Zhang, J., Xiao, Z., Li, W., & Qin, C. (2023). Application of Magnesium Hydroxide/Diphenoxy Phosphate in Silicone Rubber Flame Retardant Cable Material. Coatings, 13(5), 934. https://doi.org/10.3390/coatings13050934

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