Thermally conductive h-BN/EHTPB/epoxy composites with enhanced toughness for on-board traction transformers

Shuai Yuan; Lijun Zhou; Tiandong Chen; Dongyang Wang; Lujia Wang

doi:10.1515/epoly-2022-0076

Article Open Access

Thermally conductive h-BN/EHTPB/epoxy composites with enhanced toughness for on-board traction transformers

Shuai Yuan , Lijun Zhou , Tiandong Chen , Dongyang Wang and Lujia Wang

Published/Copyright: October 31, 2022

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From the journal e-Polymers Volume 22 Issue 1

Abstract

The dry-type on-board traction transformer (DOBTT) has the characteristics of huge heat generation and high heat dissipation requirements, so it has higher requirements for heat dissipation performance of epoxy resin (EP) insulation. However, the toughness of the existing high thermal conductivity EP composite after being modified by inorganic particles is greatly reduced, and it is very easy to crack under the occasion of frequent vibration such as electric multiple units or electric locomotives, so it cannot be directly applied to the DOBTT. In this article, the composite using h-BN to improve the thermal conductivity, and epoxidized hydroxy-terminated polybutadiene (EHTPB) liquid rubber to improve the toughness was prepared. After characterization and testing, it was found that when the EHTPB content was between 10 and 15 phr, the elongation at break of the EP/h-BN/EHTPB composite could be increased by 47.9% and the impact strength could be increased by 47.8% compared with that without EHTPB. The thermal and electrical performances were still satisfactory, which has a potential in application of on-board electrical equipment.

Keywords: epoxy resin; toughness; thermal conductivity; on-board electrical equipment; electric multiple units

1 Introduction

Epoxy resin (EP) is the main insulation material for power equipment because of its excellent electrical performance (1–3). However, the low thermal conductivity (TC) (∼0.20 W·m⁻¹·K⁻¹) of EP limits its application in high power density equipment and devices. Nowadays, the electric multiple units (EMUs) are developing towards the goal of lightweight. The lightweight dry-type on-board traction transformer (DOBTT) uses EP as the casting insulation of the winding, so the weight can be reduced by nearly 50%. However, since the Joule heat caused by the load loss of the on-board traction transformer is much larger (242 kW) than the other transformers (generally less than 30 kW), its heat dissipation is challenging (4–6). In the previous research, it was found that improving the TC of EP can significantly improve the heat dissipation performance of DOBTT (7,8), since the EP tightly wrapped on the outside is the only way to dissipate heat before convection. The hotspot temperature of windings can be reduced by 45.92 K by increasing the TC of EP from 0.25 to 0.5 W·m⁻¹·K⁻¹. Therefore, developing a kind of high TC EP insulation material suitable for DOBTT is the key to its successful design and application.

It is feasible to improve the TC of EP composites by adding inorganic fillers with high intrinsic TC. Since boron nitride (BN, ∼300 W·m⁻¹·K⁻¹) has much higher intrinsic TC than alumina (Al₂O₃) and aluminum nitride, it has been widely used in the research of high TC modification of EP in recent years (9–12). Hou et al. modified EP with 3-aminopropyl triethoxy silane-treated hexagonal boron nitride (h-BN) (13). The TC of the obtained material reached 1.178 W·m⁻¹·K⁻¹ when adding 30 wt% of h-BN, which is 6.14 times higher than that of the neat EP. Yetgin et al. prepared double fillers EP composites using h-BN and Al₂O₃ (14). It is found that the TC of the 22.5 wt% h-BN + 7.5 wt% Al₂O₃ composites reached 0.35 W·m⁻¹·K⁻¹. Tuo et al. investigated the effect of KH-560-treated h-BN on the TC and electrical properties of the composite, and found that when the mass fraction of h-BN is 30 wt%, the TC reached 0.858 W·m⁻¹·K⁻¹, and still maintained good insulation performance (15). However, when h-BN is added, the mechanical properties of the composites will decrease, especially the properties characterizing the toughness of the composites. Gong et al. prepared h-BN/EP composites and found that when the content of h-BN was 40 wt%, the elongation at break decreased by 59.5% and the impact strength decreased by 61.1% compared with the neat EP (16). Nayak et al. improved the TC by blending reduced graphene oxide, h-BN and EP. It is found that the impact strength of the composites was reduced by about 50% under the ideal ratio for TC improvement (17). It can be seen that although the addition of h-BN can greatly improve the TC of EP, the sharp decrease of its toughness makes it unable to be applied to the high-speed EMU, which vibrates frequently when in operation.

By adding flexible rubber components to EP, especially liquid rubber, the brittleness and toughness of composites can be improved (18–20). This modification method usually does not significantly reduce other properties while enhancing toughness. Chikhi et al. found that the elongation at break increased after adding amino-terminated butadiene acrylonitrile to the EP (21). Fakhar et al. found that adding vinyl-terminated butadiene acrylonitrile to EP can improve the toughness and impact strength of the composites without reducing the other mechanical properties (22). Robinette et al. prepared epoxy-terminated butadiene acrylonitrile (ETBN)-modified composites and found that when the ETBN content was 8 wt%, the toughness was increased by nine times, and the other mechanical properties were only slightly decreased (23). However, while the toughness of the EP is improved by adding the above-mentioned strongly polar liquid rubbers, the electrical insulation performance of the composite will be reduced (24). This limits its application in situations where high electrical insulation, low dielectric constant and dielectric loss are required.

Epoxidized hydroxy-terminated polybutadiene (EHTPB) is a new type of liquid rubber with good electrical insulation performance and weak polar obtained by epoxidizing partial double bonds of hydroxy-terminated polybutadiene (HTPB). It has the characteristics of low dielectric constant, low dielectric loss and high insulation resistance. Since EHTPB retains the flexible segment of HTPB, the modified composites based on EHTPB have good toughness. Meanwhile, the epoxy group will increase the compatibility between EHTPB and EP in accordance with the similar compatibility principle. In this way, EHTPB may improve the composite toughness if it is incorporated into the EP matrix. Zhou et al. prepared liquid rubber toughened EP based on the nonpolar EHTPB in order to simultaneously improve the impact strength and dielectric properties of EP (25). The properties of the composite all reached a maximum at an EHTPB content of 10 phr (parts per hundreds of resin). However, the study of using EHTPB to toughen h-BN/epoxy high TC composites has not been reported. For the development of DOBTT, research studies on h-BN/EHTPB/epoxy composites are urgent need.

To prepare the EP-based insulation materials with both TC and toughness, in this article, the composite using h-BN to construct the path for heat conduction and EHTPB liquid rubber to improve the toughness was prepared. First, the mechanisms of toughening and heat conduction improvement are explained based on scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR). Furthermore, the mechanical properties, electrical properties and TC of EP/h-BN/EHTPB composites at different EHTPB contents were studied and compared with the corresponding properties of EP/h-BN composites. Finally, the composite with the best toughness improvement and the least impact on other properties is obtained.

2 Materials and methods

2.1 Materials

The industrial grade diglycidyl ether of bisphenol-A (DGEBA) with epoxy value of 0.51 mol·100 g⁻¹ and viscosity of 12,460 MPa·s purchased from Shanghai Youpeng Chemical Co., Ltd is adopted as the EP in this study. The curing agent used in this article is methyltetrahydrophthalic anhydride (MeTHPA) purchased from Shanghai Yuanye Biotechnology Co., Ltd, China. The molecular formula of EP and its curing agent is shown in Figure 1. The accelerator used in this study is 2,4,6-tris(dimethylaminomethyl)phenol provided by Hubei Changxinsheng Chemical Co., Ltd, China. The h-BN powder with an average particle diameter of 10 μm is purchased from Hefei ZhongHang Nanometer Technology Development Co., Ltd, China. The γ-(2,3-epoxypropoxy)propytrimethoxysilane (KH-560) silane coupling agent is provided by China Shandong Jinan Yi Sheng Resin Co., Ltd, China. The EHTPB with epoxy value of 0.18 mol·100 g⁻¹ is purchased from Zibo Qilong Chemical Co., Ltd, China.

Figure 1

Molecular formula of (a) EP and (b) its curing agent MeTHPA.

2.2 Preparations

2.2.1 Surface treatment of h-BN

First, 10 g of KH-560 was added to 30 mL of ethanol aqueous solution (the volume ratio of ethanol to water was 9:1), and the amount of KH-560 was 1 wt% of the total weight of h-BN. Then the above mixture was ultrasonically dispersed in an ultrasonic disperser for 10 min, and then dropped into 1 kg of h-BN. Next, the h-BN dropped with KH-560 mixture was mixed at high speed for 10 min in a high-speed mixer. Finally, after high-speed mixing, it was taken out and vacuum dried at 80°C for 2 h.

2.2.2 Preparations of the h-BN/EP composites

First, the E51-EP, the curing agent and the accelerator are manually mixed and stirred evenly, wherein the amounts of the curing agent and the accelerator are 85 wt% and 1 wt% of E51, respectively. Then the surface-treated h-BN of 0–40 phr was added to the above mixture. After the addition of fillers, it was first ultrasonically dispersed at room temperature for 10 min, and then the mixture was pre-cured by stirring in an oil bath at 85°C with a high-speed disperser at 300 rad·min⁻¹ for 50 min. The pre-cured EP was then poured into a 85°C preheated mold and then a flat vulcanizer was used to mold and cure the mixture for 1 h, and the molding pressure is set to 5 MPa. Finally, further curing at 120°C and 160°C for 2 h was carried out in a vacuum drying oven, respectively, to obtain h-BN/EP composite samples. According to the above process, circular discs, dumbbell-shaped samples and rectangular samples were prepared for subsequent electrical, thermal and mechanical properties tests, as shown in Figure 2.

Figure 2

Samples of different shapes for different property tests.

2.2.3 Preparations of the hBN/EHTPB/EP composites

The preparation procedure of the hBN/EHTPB/EP composites is shown in Figure 3. First, E51-EP, a certain amount of EHTPB (0–20 phr) and 1 wt% of 2-methylimidazole were manually stirred to prepare EHTPB/EP prepolymer. Then, the curing agent was added, mixed manually again and stirred evenly, and then 40 phr of surface-treated h-BN was added. The subsequent operations were the same as the preparation process of h-BN/EP composites.

Figure 3

Procedure of EP/h-BN/EHTPB composites preparation.

2.3 Characterizations

FTIR spectra of the sample were obtained with Nicolet Nexus 670 infrared spectrophotometer to study the structure change of composites at the molecular level. The powder of the sample is compressed together with potassium bromide and then put into the infrared spectrophotometer for scanning. The wavelength range is set to 400–4,000 cm⁻¹, and the scanning times are set to 40.

The micrographs of the sample were obtained with Quanta FEG 250 SEM to study the microstructure and morphology of the composites. First, the sample was placed in liquid nitrogen for 2 min, and then it was brittle broken with a clamp. Then, the gold sputtering treatment of the fracture surface was carried out by ion sputtering. The voltage of the electron gun is set to 10 kV.

The DRL-III TC tester provided by Wuhan Gelaimo Testing Equipment Co., Ltd was used to measure the TC of the samples based on heat flux method.

The UTM4104X electronic universal testing machine is used for tensile test, and the XJU-22 pendulum impact testing machine is used for impact strength test. The standards for tensile test and impact strength test are ISO 527: 2012, ISO 180: 2019 and ASTM D-256, respectively.

The dielectric breakdown strength test was carried out using the power frequency withstand voltage test system according to IEC 60243-1:2013. The system consists of two identical 200 mm spherical copper electrodes, an autotransformer and a voltage monitoring system, as shown in Figure 4. In order to prevent flashover or surface discharge during the test, the sample and electrodes are be placed in the oil tank filled with 45# transformer insulation oil. First, place the sample with thickness d in the middle of the two electrodes and clamp it. Then, the short-time (rapid-rise) testing method is used to apply voltage to the sample, and monitor the change of voltage at both ends of the sample U during the test. When U suddenly changes to zero, it means that the sample has been broken down, and the system automatically records the maximum voltage across the sample before breakdown, which is the breakdown voltage U _b of the sample. Finally, the breakdown strength E _B can be calculated according to Eq. 1:

(1) E B = U b d

Figure 4

Power frequency withstand voltage test system.

The volume resistivity ρ _V measurement was carried out using HIOKI SM7120 super megohm meter. According to the standard IEC 62631-3-1: 2016, the applied voltage for the volume resistance R _V measurement is 1,000 V. According to Eq. 2, the ρ _V can be obtained:

(2) ρ V = π D 2 R V 40 d

where D is the main electrode diameter.

The dielectric properties test was performed using the OMICRON DIRANA insulation diagnostic analyzer, and the electrodes used for the test were three electrodes. The voltage applied during the test was 200 V and the frequency range was 10⁻³–10³ Hz.

3 Results and discussion

3.1 FTIR analysis

The FTIR spectra of the prepared composites are depicted in Figure 5. For the FTIR spectrum of neat EP, the broad peak near 3,448 cm⁻¹ is mainly the stretch vibration of –OH. It shows that there are a lot of hydroxyl groups in the system, and a lot of intermolecular hydrogen bonds are formed after curing. The peaks near 3,060 and 3,031 cm⁻¹ are mainly stretch vibration of ═CH. The peaks near 2,963, 2,926 and 2,878 cm⁻¹ are mainly attributed to the stretch vibration of –CH. The peak at 1,737 cm⁻¹ results from the stretch vibration of C═O. The signals near 1,610, 1,581 and 1,510 cm⁻¹ are contributed by the stretching vibration of C═C. The peaks at 1,460 and 1,384 cm⁻¹ are in-plane bending vibration peaks of –CH. The peaks around 1,244, 1,182 and 1,045 cm⁻¹ are mainly the stretching vibration peaks of CO, including C–O, C–OH and other structures. The absorption peak near 830 cm⁻¹ is attributed to the out-of-plane bending vibration of the ═CH of para substituted benzene ring. The peak at 562 cm⁻¹ is C═O in-plane bending vibration peak.

Figure 5

FTIR spectra of the prepared composites.

Compared with the neat EP, the absorption intensity of EP/h-BN increases at some positions in the spectrum. The greatly enhanced absorption at 1,378 cm⁻¹ corresponds to the stretching vibration of B–N, which is the characteristic absorption peak of h-BN. The peak enhancement near 1,018 cm⁻¹ is mainly contributed by the stretching vibration of Si–O–Si, and the peak at 463 cm⁻¹ is the in-plane bending vibration peak of Si–O–Si. This confirmed that the surface of h-BN was grafted with groups after being surface treated, and the surface affinity of inorganic particles was improved. There are two kinds of functional groups, inorganic-philic and organic-philic, in the silane coupling agent KH-560 molecule, which can connect two different chemical structure types or materials that are incompatible with each other at the interface, thereby improving the bonding force between the inorganic material and the resin matrix. The untreated h-BN has a large surface tension at the interface with the EP matrix, and thus the wettability of both is poor, forming a large thermal resistance to the propagation of ablative phonons. After the surface modification of nanoparticles, the surface energy is reduced, the wettability is improved and it can be better bonded with the substrate, which reduces the interfacial thermal resistance (26). The mechanism of surface modification to improve the performance is shown schematically in Figure 6. The signal enhancement at the peak position of 670 cm⁻¹ is mainly due to the increased substitution of the benzene ring.

Figure 6

Mechanism of surface modification to improve the thermal performance.

The structure of EHTPB contains both hydroxyl and epoxy groups, as shown in Figure 7. Because of the existence of epoxy groups and active terminal hydroxyl groups, EHTPB can react with curing agents and EP matrix molecules, thus participating in the crosslinking and curing process of EP. During the curing of the system, in addition to the DGEBA molecules being linked by the curing agent, the DGEBA and EHTPB and EHTPB are also linked to each other by the curing agent molecules. Therefore, EHTPB can be embedded into the rigid EP cross-linked network in the form of elastic links. Compared with neat EP and EP/h-BN composites, the absorption peak of EP/h-BN/EHTPB composites decreased at 830 cm⁻¹, but the signal of –OH was significantly enhanced at 3,448 cm⁻¹. There is no absorption peak near 915 cm⁻¹ (characteristic absorption peak of epoxy group) in the three FTIR curves, indicating that the curing agent has fully reacted with the epoxy groups in EP and EHTPB, and the cross-linking system has been completely cured.

Figure 7

Molecular formula of EHTPB.

3.2 Morphologies

The SEM micrographs of neat EP and EP/h-BN (40 phr h-BN) composites are shown in Figure 8a and b, respectively. After comparison, it is found that the particles are still well dispersed without obvious agglomeration. Although the void between the particles will inhibit the improvement of TC, h-BN fillers have been overlapped in the EP matrix to form a heat conduction path, which can effectively improve the TC of the composites.

Figure 8

SEM micrographs of prepared composites. (a) Neat EP, (b) EP/h-BN (100:40), and (c) EP/h-BN/EHTPB (100:40:20).

On the premise that the content of h-BN is 40 phr, the SEM micrograph of the EP/h-BN/EHTPB composites with 20 phr of EHTPB is shown in Figure 8c. It can be seen that h-BN particles are still uniformly dispersed in the composite, and EHTPB is also dispersed well in the matrix, indicating that the addition of EHTPB has no obvious obstacle to the formation of heat conduction path. The fracture surfaces of neat EP and EP/h-BN composites are relatively smoother, and the cracks are relatively more regular, showing the typical glass fracture characteristics. This indicates that the energy absorbed by samples in the fracture process of neat EP and EP/h-BN samples is low, which is the brittle fracture characteristic of thermosetting resins. This result is consistent with the characteristics of low toughness of thermally conductive EP composites, since it lacks energy dissipation channels when it is stressed. After adding EHTPB, the fracture surface becomes rough and shows the characteristics of ductile fracture. The microcavities and protrusions on the fracture surface are caused by deformation and cavitation of EHTPB during fracture. They will play the role of energy consumption in the fracture process of composites. As observed in the SEM micrograph, the cracks tend to disperse and terminate after passing through the microcavities and protrusions, and more folds and gullies gather around the cavities, indicating that the combination of rubber particles and the EP matrix reduces the growth rate of cracks. The mechanism of EHTPB to improve the mechanical properties is shown schematically in Figure 9.

Figure 9

Mechanism of EHTPB to improve the mechanical properties.

The above results prove that after EHTPB participates in the curing network system of EP/h-BN, a large number of flexible segments of rubber are introduced into the whole system, which can absorb more energy during fracture and improve the toughness.

3.3 Mechanical properties

3.3.1 Mechanical properties of EP/h-BN composites

The stress–strain curves of EP and EP/h-BN composites were obtained through tensile tests, and the tensile strength and elongation at break of EP/h-BN composites at different h-BN contents were further calculated, as shown in Table 1. It can be seen that the tensile strength and elongation at break of EP/h-BN composites decrease with the increase of h-BN content. The tensile strength of EP/h-BN composites containing 40 phr h-BN decreases by 31.9% compared with EP, while the elongation at break decreases by 39.5%. In the aspect of impact strength, it decreased with the increase of h-BN content, indicating that the toughness of EP/h-BN composites decreased with the addition of h-BN. The impact strength of the composite containing 40 phr h-BN decreases by 38.3%. In conclusion, the addition of h-BN decreases the toughness thus increases the risk of cracking of insulation under the action of frequent vibration of trains.

Table 1

Mechanical properties of EP/h-BN composites

Mechanical properties	Content of h-BN (phr)
Mechanical properties	0	10	20	30	40
Tensile strength (MPa)	59.16	56.19	49.28	41.20	40.26
Elongation at break (%)	3.52	3.18	2.83	2.21	2.13
Impact strength (kJ·m⁻²)	12.83	11.00	9.63	8.94	7.91

The reasons for the above changes in mechanical properties are as follows. The interior of neat EP is a continuous cross-linked network structure. However, after the addition of h-BN, inorganic particles will occupy part of the space, so that a continuous cross-linked structure cannot be generated in this part of the space. When EP/h-BN composites are subjected to tensile stress, the part occupied by h-BN particles has no cross-linking structure to transmit stress, and h-BN itself does not have the ability to absorb energy. Therefore, the accumulation of energy makes EP/h-BN composites more prone to fracture. In addition, the addition of h-BN will introduce defects in the EP matrix, and the higher the h-BN content, the more defects in the EP/h-BN composites. The defects will cause the crack growth rate in the material to accelerate, and aggravate the damage of EP/h-BN composite. Therefore, all the mechanical properties decrease with the increase of h-BN content.

3.3.2 Mechanical properties of EP/h-BN/EHTPB composites

Tensile test and impact test were carried out for EP/h-BN/EHTPB composites with EHTPB contents of 5, 10, 15 and 20 phr, respectively, and the results of their mechanical properties are shown in Table 2. With the addition of EHTPB, the tensile strength of the composite first increased and then decreased. When the content of EHTPB is 10 phr, the tensile strength reaches the maximum value of 48.72 mPa, which is 21.0% higher than that without EHTPB. This is due to the reaction between the active terminal hydroxyl group of rubber particles and the epoxy group of EP after adding EHTPB, which effectively increases the interface force. At the same time, EHTPB can also fill some defects in EP/h-BN composites, effectively preventing the development of cracks in the tensile process. However, further increasing the content of EHTPB will reduce the tensile strength of the composite. This is mainly because when the content of EHTPB is too high, the polymer material properties of EHTPB occupy a dominant position, and the large amount of EHTPB will further enhance the degree of crosslinking of the system, resulting in a decline in the tensile strength of the composite.

Table 2

Mechanical properties of EP/h-BN (40 phr)/EHTPB composites

Mechanical properties	Content of EHTPB (phr)
Mechanical properties	0	5	10	15	20
Tensile strength (MPa)	40.26	44.57	48.72	43.49	41.64
Elongation at break (%)	2.13	2.30	2.59	3.15	2.77
Impact strength (kJ·m⁻²)	7.91	11.00	11.69	10.31	9.17

With the increase of EHTPB content, the elongation at break of the composites first increased and then decreased. When the content of EHTPB is 15 phr, the elongation at break of the composite is the largest, reaching 3.15%, which is 47.9% higher than that without EHTPB, and only 10.5% lower than that of neat EP. The impact strength of EP/h-BN/EHTPB composites still increases first and then decreases with the increase of EHTPB content. When the content of EHTPB is 10 phr, the impact strength of the composite is the highest, reaching 11.69 kJ·m⁻², which is 47.8% higher than that without EHTPB, and only 8.8% lower than that of neat EP. The improvement of the toughness of composites by adding EHTPB is mainly due to the fact that its elastic links are embedded in the rigid EP crosslinked network in the form of chemical bonds. When the sample is subjected to an external force, the elastic chain molecules of the liquid rubber are first stretched by the impact of the external force, effectively bearing the external force. This process slows down the external impact on the rigid cross-linked network, thereby effectively buffering the deformation of the EP cross-linked molecules and significantly improving the impact toughness of the composite.

Through the comprehensive consideration of tensile strength, elongation at break and impact strength, when the EHTPB content is 10–15 phr, the mechanical properties of EP/h-BN composites, especially the toughness, are best improved.

3.4 Electrical properties

3.4.1 Volume resistivity

The average value of ten samples under each ratio was taken as the final volume resistivity test result, as shown in Figure 10. The volume resistivity is inversely proportional to h-BN content, since h-BN particles provide a path for carrier migration, which increases the conductivity current inside the composite and decreases the volume resistivity. However, the decreasing trend gradually slow down when the filler content continues to rise. This may be mainly because the stacked h-BN at high adding amount will hinder the migration of carriers. The volume resistivity of EP/h-BN composites containing 40 phr h-BN decreases to 1.93 × 10¹⁵Ω·cm compared to 1.27 × 10¹⁷Ω·cm of neat EP. Even so, the volume resistivity is still at a high level, which can meet the requirements of on-board traction transformer in terms of insulation resistance.

Figure 10

Volume resistivity of prepared composites.

The volume resistivity under different EHTPB contents when the content of EHTPB is fixed at 40 phr is also shown in Figure 10. The volume resistivity of EP/h-BN/EHTPB composites first increases and then decreases with the increase of EHTPB content, but the volume resistivity of the composites with EHTPB is always higher than that without EHTPB. The volume resistivity of the composite reaches the maximum value of 6.50 × 10¹⁵ Ω·cm after adding 15 phr EHTPB. The main reasons for the increase of the volume resistivity of the composite after adding EHTPB are as follows: first, the EHTPB itself has a high volume resistivity, which can well hinder the migration of carriers and inhibit the accumulation of space charges at the phase interface in the composite. Second, EHTPB rubber particles can create deep traps in the EP cross-linking network, thus effectively inhibiting the formation of internal current in the composites. Third, the reaction between EHTPB and EP significantly improves the interface interaction, thereby effectively reducing and suppressing interface defects, shallow traps and scattered moving charges. However, when the EHTPB content continues to increase to 20 phr, rubber particles begin to aggregate and form large particles, thus introducing defects into the composite again, resulting in a decrease in the volume resistivity of the composites.

3.4.2 Breakdown strength

The samples after breakdown are shown in Figure 11. Each sample has an obvious through breakdown channel, which indicates that the composite is completely broken down. There are black carbide residues around the breakdown channel, which is caused by the arc burning at the moment of breakdown. The breakdown strengths at different h-BN contents are depicted in Figure 12. When the filling amount of h-BN is less than or equal to 10 phr, the breakdown strength of EP/h-BN composite decreases compared with that of neat EP, but when the content of h-BN continues to increase, the breakdown strength of EP/h-BN composite gradually increases. The breakdown strength reaches the maximum value of 35.7 kV·mm⁻¹ after adding 40 phr of EHTPB, which is 14.3% higher than that of neat EP.

Figure 11

Samples and their breakdown channels after breakdown strength test.

Figure 12

Breakdown strength of EP/h-BN and EP/h-BN/EHTPB composites.

The reason for sample breakdown caused by short-time (rapid-rise) testing is the development of electrical tree inside the material, so the development path of electrical tree inside the EP/h-BN composite directly affects its breakdown strength (27). When the filling amount of h-BN is less than or equal to 10 phr, due to the small amount of h-BN, it is uniformly dispersed in the EP matrix and wrapped by the resin, forming isolated “island” like structures (28). At this time, h-BN has little effect on the development of branches of electrical tree. However, h-BN particles will introduce defects in the EP matrix, thus reducing the breakdown strength of EP/h-BN composites. With the further increase of h-BN content, the “island” structure disappeared, and the h-BN particles began to stack each other in the matrix to form a pathway. Since the h-BN particles are in the form of flakes, the electrons will be scattered when the electric field is perpendicular to the plane of the particles. This leads to the fact that the electrical tree cannot continue to move forward after encountering the h-BN plane, but can only change its direction and move forward along the h-BN surface, and cannot continue to move forward until it bypasses the particle surface. This prolongs the breakdown path of the composite, resulting in an increase in the breakdown strength.

The breakdown strength results of composites containing EHTPB are depicted in Figure 12. After adding EHTPB, the breakdown strength of the composite will increase. When the EHTPB content is 5 phr, the maximum breakdown strength of the EP/h-BN/EHTPB composite reaches 38.5 kV·mm⁻¹, which is 7.8% higher than that without EHTPB and 23.2% higher than that of neat EP. EHTPB liquid rubber has the characteristics of high insulation resistance, low dielectric constant and low dielectric loss. When EHTPB is dispersed in the EP matrix and combined with the chemical bond of EP and curing agent, a strong interface force is formed in the composite, which can play a role in preventing carrier movement. This effectively inhibits the development of electrical tree in the composite during the breakdown process, thereby improving the breakdown strength of the composite. It can also be seen from the results that although the breakdown strength of the EP/h-BN/EHTPB composite increases first and then decreases after the addition of EHTPB, it is always greater than that of the composite without EHTPB in the range of 0–20 phr.

Figure 13 shows the Weibull distribution of the breakdown strength of the composites. For the h-BN/EHTPB/EP composites, the breakdown strength values with 63.2% probability for composites with EHTPB content of 0, 5, 10, 15 and 20 phr are 36.06, 39.10, 37.13, 38.60 and 36.99 kV·mm⁻¹, respectively, and their corresponding shape parameters β are 59.18, 34.07, 33.57, 34.71 and 32.53, respectively. In the Weibull distribution, β represents the dispersion factor of the dielectric strength, and higher β indicates less dispersion of the dielectric strength results.

Figure 13

Weibull distribution of breakdown strength values of composites.

3.4.3 Dielectric permittivity and dissipation factor

According to the dielectric response theory, the real part of the complex dielectric constant represents the full current capacitive current density, which can be used to characterize the dielectric constant of a material. The dielectric constant indicates the magnitude of the material’s polarizability and reflects the material’s ability to bind charges (29). The imaginary part of the complex dielectric constant, on the other hand, refers to the full-current resistive current density, which correspondingly can be used to characterize the dielectric loss of the material. The dielectric loss is the energy loss due to the charge movement inside the material during the conductivity, relaxation polarization.

The variation of dielectric permittivity with frequency for the composites with different EHTPB contents is shown in Figure 14a. It can be seen that the real part of the composite complex permittivity tends to decrease as the frequency increases, which is mainly because the dipole polarization and interfacial polarization cannot keep up with the change of electric field frequency as the frequency increases, so it leads to the weakening of the polarization of the composite at high frequencies. The real part of the complex dielectric permittivity of h-BN/EHTPB/EP composites decreases after the addition of EHTPB, which is mainly because the addition of EHTPB can improve the interfacial interaction and make the interfacial polarization weaker. At the same time, the addition of EHTPB will fill some voids and defects in the EP/h-BN composites and weaken the polarization of the composites. In addition, EHTPB itself has a low dielectric constant, so it will lead to a decrease in the real part of the composite complex dielectric permittivity.

Figure 14

Dielectric permittivity and dissipation factor of EP/h-BN/EHTPB composites. (a) Dielectric permittivity, (b) dissipation factor.

The variation of dissipation factor with frequency for the h-BN/EHTPB/EP composite is shown in Figure 14b. It can be seen that the dissipation factor gradually decreases with the increase of EHTPB content, which is mainly because the EHTPB rubber particles in the EP matrix will hinder the migration of carriers, leading to the decrease of conduction current in the composite and the consequent decrease of conductivity loss. At the same time, the dielectric loss of EHTPB itself is small, so the addition of EHTPB leads to the reduction of dielectric loss of the composite material. The dissipation factor of the composite increases and then decreases with the increase of frequency, mainly because the relaxation polarization is close to the external electric field frequency in the middle frequency band. Therefore, the relaxation polarization gradually dominates in the low to medium frequency band, leading to a gradual increase in dielectric loss with increasing frequency. The relaxation polarization in the middle to high frequency band gradually cannot keep up with the change of electric field, and the relaxation polarization cannot occur, so the dissipation coefficient gradually decreases again with the increase of frequency.

3.5 TC

The influence of h-BN filling amount on the TC of EP/h-BN composite is depicted in Figure 15. It can be seen that the TC of EP/h-BN composites gradually increases with the increase of h-BN content. When the content of h-BN is below 30 phr, the TC of the composite increases relatively slowly, but when the content of h-BN is between 30 and 40 phr, the TC of the composite increases significantly. The TC reaches 0.58 W·m⁻¹·K⁻¹ after adding 40 phr of h-BN, which is 2.59 times that of neat EP. Before h-BN particles fully contact each other, the improvement mainly depends on the extremely high intrinsic TC of h-BN. When the content is more than 30 phr, h-BN particles begin to contact each other to form a heat conduction path. At this time, the heat can flow along the heat conduction path with low thermal resistance, so the TC is significantly increased.

Figure 15

TC of prepared composites.

The TC measurement results of EP/h-BN/EHTPB composites under different EHTPB contents are shown in Figure 15. When EHTPB content is 5 phr, the TC of EP/h-BN/EHTPB composite was 0.53 W·m⁻¹·K⁻¹, which decreased by 7.9% compared with that without EHTPB. When EHTPB content continues to increase to 10 and 15 phr, the TC fluctuated slightly and did not decrease significantly, still higher than 0.50 W·m⁻¹·K⁻¹, which meets the requirements of DOBTT (7,8). When the EHTPB content continues to increase to more than 15 phr, the TC of the composites decreases relatively obviously. When the EHTPB content is 20 phr, the TC is 0.47 W·m⁻¹·K⁻¹, which is 19.3% lower than that of the composites without EHTPB.

The reason for the above changes is mainly because the TC of EHTPB is lower than that of EP, so the average TC of EP/h-BN composites decreases after EHTPB is added. However, when EHTPB content is low, EHTPB rubber particles will fill some defects and voids in the EP/h-BN composites, reducing the thermal resistance due to defects and voids, which can slightly reduce the thermal resistance of composites. Therefore, when EHTPB content is low, the TC does not decrease much. With the gradual increase of the EHTPB content, the fillable defects and voids in the EP/h-BN composites have been completely filled, and the low intrinsic TC of the excess EHTPB particles makes the TC decrease rapidly. Finally, according to the test results, when the EHTPB content is between 0 and 15 phr, the TC of EP/h-BN/EHTPB composites can meet the requirements of DOBTT.

3.6 Consideration of comprehensive performance

Up to now, the influence of EHTPB content on mechanical properties, electrical properties and TC have been obtained, so the composite with relatively optimal comprehensive performance can be obtained. When the content of EHTPB is 10–15 phr, the elongation at break and impact strength can be improved most obviously, which are increased by 47.9% and 47.8%, respectively, compared with those before adding EHTPB. This means that compared with the existing EP insulation, the EP/h-BN/EHTPB composite material can maintain a high TC while the decrease in toughness does not exceed 10%. In the above range of EHTPB content, the breakdown strength and volume resistivity have also been improved to varying degrees. Although the TC decreases, its change is not obvious and is still higher than 0.5 W·m⁻¹·K⁻¹. Therefore, EP/h-BN/EHTPB composites with EHTPB content of 10–15 phr are considered to be suitable for DOBTT.

4 Conclusions

In this article, the EP/h-BN/EHTPB composite using h-BN to improve TC, and EHTPB liquid rubber to improve toughness was prepared. The following conclusions were drawn:

The addition of h-BN will significantly reduce the mechanical properties. After adding 40 phr of h-BN, the TC increased by 159% compared with neat EP while tensile strength, elongation at break and impact strength decreased by 31.9%, 39.5% and 38.3%, respectively.
The addition of h-BN will reduce the volume resistivity by two orders of magnitude, but can improve the breakdown strength of EP/h-BN composites. However, the insulation performance of EP/h-BN composites is still acceptable. The addition of EHTPB can slightly increase the volume resistivity and breakdown strength, but also slightly reduce the TC.
The addition of EHTPB can improve the mechanical properties of the composites, especially the elongation at break and impact strength. When the content of EHTPB is 15 phr, the elongation at break increased by 47.9% compared with that without EHTPB. When the content of EHTPB is 10 phr, the impact strength is increased by 47.8%. This makes the elongation at break and impact strength return to the level close to that of neat EP.

Funding information: This project was supported by National Natural Science Foundation of China (U1834203), Sichuan Science and Technology Program (Youth Science and Technology Innovation Research Team Project) (2020JDTD0009), Fundamental Research Funds for the Central Universities (2682022CX015) and National Natural Science Foundation of China (52207180).
Author contributions: Shuai Yuan: writing – original draft, conceptualization, formal analysis; Lijun Zhou: funding acquisition, writing – review and editing; Tiandong Chen: methodology, resources, data curation; Dongyang Wang: validation, writing – review and editing; Lujia Wang: visualization.
Conflict of interest: Authors state no conflict of interest.
Data availability statement: The data presented in this study are available on request from the corresponding author.

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Received: 2022-09-19

Revised: 2022-10-03

Accepted: 2022-10-09

Published Online: 2022-10-31

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

Articles in the same Issue

https://doi.org/10.1515/epoly-2022-0076

Keywords for this article

epoxy resin; toughness; thermal conductivity; on-board electrical equipment; electric multiple units

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