# **Polymer Composites for Electrical and Electronic Engineering Application**

Edited by Shaojian He Printed Edition of the Special Issue Published in *Polymers*

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## **Polymer Composites for Electrical and Electronic Engineering Application**

## **Polymer Composites for Electrical and Electronic Engineering Application**

Editor

**Shaojian He**

MDPI ' Basel ' Beijing ' Wuhan ' Barcelona ' Belgrade ' Manchester ' Tokyo ' Cluj ' Tianjin

*Editor* Shaojian He School of New Energy North China Electric Power University Beijing China

*Editorial Office* MDPI St. Alban-Anlage 66 4052 Basel, Switzerland

This is a reprint of articles from the Special Issue published online in the open access journal *Polymers* (ISSN 2073-4360) (available at: www.mdpi.com/journal/polymers/special issues/Polym Compos Electr Electron Eng Appl).

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 He is currently an associate professor at the School of New Energy at North China Electric Power University. He was a visiting scholar at State University of New York, University at Buffalo, USA, during March 2019 – August 2020. His current research is focused on high-performance rubber nanocomposites and high voltage insulation material. He has undertaken several research projects including two research funds from the National Natural Science Foundation of China (NSFC). He is a recipient of Alan Glanvill Award by the Institute of Materials, Minerals and Mining (IOM3) in 2012. He has published more than 60 papers in academic journals and applied more than 20 Chinese invention patents. He is also a member of the Rubber & Plastic Green Manufacturing Professional Committee of the Chemical Industry and Engineering Society of China (CIESC), a guest editor of the SCI journal *Polymers*, an academic dissertation evaluation expert of Academic Degree Center of Ministry of Education, a communication evaluation expert of the National Natural Science Foundation of China, and an evaluation expert of the Beijing Science and Technology Commission.

## *Article* **Study on Microwave Absorption Performance Enhancement of Metamaterial/Honeycomb Sandwich Composites in the Low Frequency Band**

**Songming Li 1,2, Hao Huang 1 , Sibao Wu 1 , Jiafu Wang 3 , Haijun Lu 1,2 and Liying Xing 1,2, \***

**Abstract:** With the rapid development of electronic technology and modern radar detection system, there is increasingly urgent demand for microwave absorbing composites working efficiently in the low frequency range (e.g., 1–2 GHz). In this work, a type of metamaterial/honeycomb sandwich composite (MHSC) was proposed and fabricated, which exhibited a light weight structure and excellent wave-absorbing performance in the low frequency band. The relationship between the wave-absorbing properties and the design parameters of the composite, such as the thickness of the wave-transmitting skin, the thickness and dielectric properties of the wave-absorbing honeycomb, was systematically investigated. The electromagnetic coupling interference between the honeycomb absorber and metamaterial resonator proved to be a crucial factor that affects synergistic waveabsorbing performance in the low-frequency band. Under the rational design, the incorporation of subwavelength-sized phase-gradient metamaterial units in the composite can significantly improve low-frequency wave-absorbing performance for greater than 5 dB (an increment larger than 100%); and the obtained MHSC exhibits averaged reflectivity (*R*a) less than −10 dB in the low frequency band of 1–2 GHz as well as outstanding performance (*R*a < −14.6 dB) over an extremely wide frequency range (1–18 GHz). The MHSC reported in this study could be a promising candidate for the key material in high-performance radar stealth and other related applications.

**Keywords:** honeycomb sandwich composites; metamaterial; radar stealth; microwave absorbing material; low frequency

#### **1. Introduction**

In recent years, microwave-absorbing materials (MAMs) have attracted much attention in view of their practical applications in electromagnetic shielding, camouflage, radar stealth and other advanced technologies for both commercial and military purposes [1–8]. Compared with other types of MAMs (e.g., wave-absorbing coating), structural wave-absorbing composites show comprehensive superiorities, including high mechanical strength, heat resistance and broadband absorption ability [9]. They exhibit dual functions for load-bearing and wave-absorbing in an integral structure and are widely used in radar stealth equipment [10]. Nowadays, with the rapid evolution of electronic technology, modern radar systems can effectively detect a target within a broad frequency band (e.g., 1–18 GHz). In particular, low frequency early warning radar (1–2 GHz) has become the most predominant threat for conventional military planes and even stealth aircraft [11–14]. As a result, there is an increasingly urgent demand for structural waveabsorbing composites having light weight structure, broadband absorption ability, and

**Citation:** Li, S.; Huang, H.; Wu, S.; Wang, J.; Lu, H.; Xing, L. Study on Microwave Absorption Performance Enhancement of Metamaterial/ Honeycomb Sandwich Composites in the Low Frequency Band. *Polymers* **2022**, *14*, 1424. https://doi.org/ 10.3390/polym14071424

Academic Editor: Bozena Jarz ˛abek ˙

Received: 19 February 2022 Accepted: 28 March 2022 Published: 31 March 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

high performance low-frequency radar stealth function at the same time [14]. This presents a great challenge in the design and fabrication of such material systems within the bounds of current knowledge.

Honeycomb sandwich composites (HSCs) are widely used in aeronautical structures and automobiles due to their advantages, such as being light weight, having high stiffnessto-mass ratio, good heat-insulation performance and so on [15]. By coating or filling hexagonal honeycomb cores with lossy agents, an HSC can become an efficient MAM [16]. The wave-absorbing performance of an HSC is affected by a series of design parameters, such as thickness, core size and orientation of the honeycomb [17–19], coating thickness and dielectric properties of the lossy agents [20–22], and even the fabrication/molding process [23]. Vast work and progress have been made by using varied lossy agents, including dielectric materials (e.g., carbon black, graphene and multi-wall carbon nanotube) and magnetic materials (e.g., carbonyl iron, ferrite and nickel metal) to fabricate waveabsorbing HSCs with low reflectivity and high absorptivity [17,21,24–31]. Nevertheless, early studies mainly focused on enhancing the wave-absorbing properties of HSCs in a relatively high frequency band within 2–18 GHz (mostly 8–18 GHz). By contrast, there is still a lack of effective methodologies to realize satisfactory radar stealth performance (averaged reflectivity less than −10 dB) in the low frequency band of 1–2 GHz for conventional HSC. Although increasing the thickness of honeycomb, using a high content of lossy agents, or employing magnetic materials with high complex permeability may be helpful in improving low-frequency wave-absorbing performance [14,32], this will inevitably result in drastic weight increase for the total structure, which is infeasible for practical applications. Consequently, it is necessary to introduce new technical approaches and mechanisms to fundamentally solve this problem.

Metamaterials are kinds of artificial materials which consist of subwavelength structural units and possess many peculiar electromagnetic properties unattainable with natural materials [33–35]. By arranging resonant units in a periodic, quasi-periodic or disordered/coded manner, the design of metamaterials can exhibit artificially customized permittivity and permeability following predetermined spatial distribution, resulting in full control of the amplitude, phase, polarization, propagation and dispersion of the electromagnetic wave [36–38]. Compared with the traditional microwave absorber, metamaterial not only provides new perspectives on the wave-absorbing mechanism but also offers more practical approaches to realize high performance radar stealth with a light-weight structure [39–41]. Among metamaterials, phase-gradient metasurface (PGM), which is based on generalized laws of reflection and refraction (Snell's Law), is a promising candidate to solve the bottleneck problem of traditional wave-absorbing material in the low frequency region [42–45]. Through the abnormal reflection properties of PGM, the main lobe of a reflected radar wave can be deviated in a non-threatening direction to reduce radar cross section (RCS); meanwhile, due to abnormal refraction, the effective thickness of the absorber below PGM is significantly increased to enhance the energy dissipation of the electromagnetic wave [46,47]. Although it is expected that the incorporation of metamaterial can possibly improve absorption efficiencies, or widen the absorption band of conventional MAMs, at the current stage, there is still lack of study concerning the fabrication and wave-absorbing performance of honeycomb sandwich composites loaded with metamaterials [48,49]. Huang et al. [48] proposed a type of wave-absorbing honeycomb containing "H-shaped" metamaterial, where the inner wall of wave-transmitting honeycomb cores was first posted with metal units and then impregnated with wave-absorbing resin. The reflectivity of the metamaterial-loaded honeycomb is less than −5 dB in the frequency band of 2~18 GHz. However, electromagnetic interaction and coupling interference between the metamaterial units and honeycomb absorber, especially in the low-frequency microwave region, is still ambiguous. It is of vital significance and urgent demand to investigate the synergistic/conflict effect of the two absorbing mechanisms and to optimize design parameters to the composites, in order to achieve satisfactory low-frequency and broadband wave-absorbing performance within a light-weight structure.

In this work, we reported on a type of metamaterial/honeycomb sandwich composite (MHSC) which exhibited excellent microwave absorption performance in the low frequency band of 1–2 GHz. The metamaterial units designed, following the phase-gradient principle, were incorporated into MHSC via a pattern-transfer method and molded together with the composite. The influences of the design parameters (including the thickness of skin, and thickness and dielectric properties of honeycomb) on the low-frequency wave-absorbing properties of the composites were systematically investigated, and the related mechanisms were elucidated. It was demonstrated that coupling interference between the honeycomb absorber and metamaterial resonator was the most crucial factor that affected their synergistic wave-absorbing performance in the low-frequency band. On the basis of the above understanding, an optimized radar stealth performance with averaged reflectivity (*R*a) less than −10 dB in the band of 1–2 GHz was successfully obtained for MHSC, while there was no significant weight increase compared with the original HSC. On this occasion, the MHSC also exhibited excellent radar stealth performance (*R*a < −14.6 dB) over an extremely wide frequency range of 1–18 GHz, which shows great opportunities for real applications.

#### **2. Materials and Methods**

#### *2.1. Materials*

Quartz fiber/bismaleimides wave-transmitting prepregs (QW280/5429), wave-absorbing honeycomb (SKuF), wave-transmitting Nomex honeycomb (NH-1-2.7-48) and carbon fiber/bismaleimides prepregs (ZT7H/5429) were purchased from AVIC Composite Corporation Ltd., Beijing, China. The poly(ether-ether-ketone) (PEEK) film with the thickness of 10–15 µm was fabricated by solution casting, following the literature [50]. Epoxy glue film (J-116) was purchased from the Institute of Petrochemistry, Heilongjiang Academy of Sciences, Harbin, China. All the reagents and materials were purchased from commercial sources and used as received without further purification.

#### *2.2. Fabrication of MHSC*

The fabrication process of the metamaterial/honeycomb sandwich composites (MH-SCs) is schematically shown in Figure 1. First, the PEEK film was plated with a thin copper layer by magnetron sputtering, and the periodic metal units were then fabricated by wet-etching, according to the designed patterns, to obtain the metamaterial layer. The wave-transmitting skin and the reflection skin were prepared by the autoclave vacuum bag molding method. To fabricate the wave-transmitting skin, several layers of the QW280/5429 prepregs were stacked together with the metamaterial layer, which were cured at 180 ◦C for 3 h, followed by post-curing at 200 ◦C for 5 h. During the curing process, a vacuum and a pressure of 0.7 MPa was applied. A photograph of the fabricated wavetransmitting skin containing metamaterial is shown in Figure S1 in the Supplementary Materials, where the dark patterns are the periodic metal units. The reflection skin was prepared by curing ZT7H/5429 prepregs using the same procedure. Finally, the wavetransmitting skin, containing metamaterial units, the wave-absorbing honeycomb and the reflection skin were bonded together by J-116 glue film at a temperature of 180 ◦C under a pressure of 0.3 MPa for 2 h [51], by which means the MHSCs were successfully fabricated. To investigate the effect of metamaterial on wave-absorbing properties, conventional HSC was also fabricated, which uses the same procedure as above, except that the wave-transmitting prepregs were cured alone without adding the metamaterial layer.

#### *2.3. Characterization*

The dielectric property (i.e., the complex relative permittivity, *ε*r = *ε*r ′ + i*ε*r ′′) of the wave-absorbing honeycomb was measured by the free space method in a frequency range of 1–2 GHz. Two spot-focusing horn lens antennae (transmitting and receiving antennae) were placed face-to-face at a distance of twice the focal length. A honeycomb sample with an area of 600 mm × 600 mm was mounted on the focal plane of both the antennae. The *S*-parameters (Scattering parameters) of the sample in free space was measured by a vector

network analyzer (Keysight PNA-L). The complex relative permittivity was calculated from the input reflection scattering parameter (*S*11) and the forward transmission scattering parameter (*S*21), by using the Nicholson-Ross algorithm [52]. The reflectivity (*R*, unit: dB) of the MHSC and HSC was measured by the NRL-arc method in a frequency range of 1–2 GHz. The transmitting antenna and receiving antenna, connected to a vector network analyzer (Keysight PNA-L), were mounted on top of the sample at normal incidence. Wave-absorbing pyramids were placed beneath the sample to suppress the reflection of the background. The value of *R* was calculated using the equation *R* = 10log(*P*/*P*m), where *P* and *P*m are respectively the reflection power of the composite with an area of 600 mm × 600 mm and the reflection power of the metal sheet with the same dimensions. All the tests were conducted at a temperature of 25 ◦C.

**Figure 1.** Schematic illustration about the fabrication of the metamaterial/honeycomb sandwich composites (MHSCs).

#### **3. Results and Discussion**

#### *3.1. Wave-Absorbing Properties of HSC*

*δ ε* ″ *ε* ′ Honeycomb sandwich composite (HSC) is generally composed of a wave-transmitting skin, a wave-absorbing honeycomb and a reflection skin. The wave-absorbing mechanism of HSC is schematically illustrated in Figure S2 in the Supplementary Materials. When the electromagnetic wave is incident onto the HSC, it will first interact with the wavetransmitting skin. The material of this layer often exhibits low dielectric constant and low loss tangent (tan *δ* = *ε*r ′′/*ε*r ′ ) [53,54], in order to realize good impedance matching with the free space and to decrease reflection loss (increase transmittance) on the surface of the HSC. With good performance of the wave-transmitting skin, the electromagnetic wave energy can enter into the HSC as much as possible. As the electromagnetic wave enters into the wave-absorbing honeycomb, it will be scattered in the periodic hexagonal structure and attenuated by the wave-absorbing resin adhering to the honeycomb surface (Figure S2). To obtain rapid and sufficient attenuation of the electromagnetic wave, the honeycomb should have relatively large dielectric loss and sufficient thickness. Meanwhile, impedance matching between the skin layer and the honeycomb is also important to suppress reflection on the interface and realize the maximum absorption ratio of the radar waves. The reflection skin consists of carbon fiber reinforced bismaleimides resin, which is the bottom layer of the HSC. Due to the high conductivity of carbon fiber, this layer can be regarded as a reflection layer and shields electromagnetic signals on the back side of the HSC. According to the above analysis, it is realized that the rational design of the wave-transmitting skin and the wave-absorbing honeycomb are very important for wave-absorbing performance of the HSC. The effects of thickness of wave-transmitting skin, dielectric properties of waveabsorbing honeycomb and thickness of wave-absorbing honeycomb on wave absorbing properties of the HSC are presented as follows.

#### 3.1.1. Effect of the Thickness of the Wave-Transmitting Skin

For wave-absorbing HSC, the wave-transmitting skin is the first layer of material that interacts with the incident electromagnetic wave. The impedance matching between the skin and free space determines direct reflection on the surface and affects the integral wave-absorbing performance of HSC. Herein, the quartz fiber (*ε*r ′ = 3.7, tan *δ* = 0.0005) and bismaleimides (*ε*r ′ = 3.3, tan *δ* = 0.01) with low dielectric constant and low loss tangent were respectively selected as the reinforcing fiber and resin matrix of the wave-transmitting skin. Under the same composition of material, the thickness of the skin layer is the main factor in influencing transmittance of the electromagnetic wave and wave-absorbing performance. Wave-transmitting skins with different thicknesses were fabricated by adjusting the number of plies of the stacked prepregs in the curing step. Figure 2 shows the reflectivity of the HSC with different thicknesses (0.5–2.5 mm) of the wave-transmitting skin in the low frequency range (1–2 GHz), where the same type and thickness of honeycomb is used. The results illustrate that the wave-absorbing performance of HSC is slightly enhanced by increasing the thickness of the wave-transmitting skin. This phenomenon is ascribed to the long wavelength of the electromagnetic wave in this frequency band, which can sufficiently penetrate through the skin layer with negligible dielectric loss. However, increasing the thickness of the wave-transmitting skin can also weaken wave-absorbing properties in the high frequency range (e.g., 12–18 GHz) [13], and increase the total weight of the HSC structure, which works against the demand for wide-band radar stealth using light-weight material. To balance wave-absorbing performance, weight and mechanical property of the HSC, a moderate thickness (1 mm) of wave-transmitting skin is selected and used in the following sections. *ε* ′ *δ ε* ′ *δ*

**Figure 2.** Reflectivity of the honeycomb sandwich composites (HSCs) with different thicknesses of the wave-transmitting skin.

#### 3.1.2. Effect of Dielectric Properties of Wave-Absorbing Honeycomb

*ε* ′ The honeycomb structure is the core functional layer of the wave-absorbing HSC, whose dielectric property and thickness show significant influence on radar stealth performance of the composite. Wave-absorbing honeycombs were fabricated by repeatedly immersing the Nomex honeycomb in a dispersion of conductive carbon black mixed with bismaleimides resin and acetone for several cycles to reach a predetermined weight increment, and finally cured at an elevated temperature in the oven. The Nomex honeycomb is a kind of wave-transmitting material with small dielectric constant (*ε*r ′ = 1.1) and low loss tangent (tan *δ* = 0.003). Conductive carbon black can attenuate the radar wave through electronic polarization, interfacial polarization and other effects, during which electromagnetic energy is converted into conduction current and finally dissipated into heat. The effective dielectric constant and loss tangent of honeycomb can be promoted by increasing the loading amount of wave-absorbing agent. Figure 3a,b respectively show the real part and imaginary part of the complex relative permittivity of the eight wave-absorbing honey-

combs (1#~8#) used for this study. In the frequency range from 1 GHz to 2 GHz, *ε*r ′ of the honeycombs show the order of 1# < 2# < 3# < 4# ≈ 6# < 5# < 7# < 8# and *ε*r ′′ show the order of 1# < 2# < 3# < 4# < 5# < 6# < 7# < 8#. The reflectivity of the HSC with honeycombs 1#~8# was measured in the frequency range of 1–2 GHz, and the results are shown in Figure 3c. The thickness of wave-absorbing honeycomb is fixed at 30 mm. The results demonstrate that, by increasing the dielectric properties (*ε*r ′ and *ε*r ′′) of honeycomb, the wave-absorbing performance of HSC gradually increases at first, while rapidly decreasing when complex permittivity exceeds a certain level. The effect of the dielectric property of honeycomb on wave-absorbing performance originated from the following mechanisms. When the real part and imaginary part of permittivity are both small (e.g., 1#~5#), the impedance difference between the wave-transmitting skin and the honeycomb is relatively low, and the electromagnetic wave can effectively pass through their interface and enter into the honeycomb. In this condition, the increasing of *ε*r ′′ enhances the attenuation rate and absorption ratio of the radar wave in the honeycomb structure, which contributes to decrease in the reflectivity of HSC. However, when the dielectric property of honeycomb is sufficiently large (e.g., 7# and 8#), impedance mismatching between the skin and honeycomb leads to considerable reflection of the incident radar wave on the interface, which decreases energy transmission efficiency and weakens wave-absorbing performance of the HSC. Limited by the above reasons, even for honeycomb 6#, which has the best performance among the overall samples, reflectivity in 1–1.3 GHz is still larger than −10 dB, which is unsatisfactory in satisfying the demand for high-performance radar-stealth in the low frequency band. Moreover, wave-absorbing honeycomb with high dielectric properties loads a large amount of the wave-absorbing agent and shows a large density (Table 1), which significantly increases the total weight of the composite structure. Consequently, increasing the dielectric properties of honeycomb is of limited effect in improving low-frequency wave-absorbing performance of the composites in line with the demand for a light weight structure.

*ε* ′ *ε* ″ **Figure 3.** (**a**) Real part and (**b**) imaginary part of the complex relative permittivity (*ε*r ′ + i*ε*r ′′) of different wave-absorbing honeycombs (1#~8#). (**c**) Reflectivity of the honeycomb sandwich composites fabricated from the wave-absorbing honeycomb 1#~8#.

− −

**Table 1.** Density of the wave-absorbing honeycomb.

1 The surface density was calculated for the honeycomb with the thickness of 30 mm.

#### 3.1.3. Effect of the Thickness of Wave-Absorbing Honeycomb

As the low-frequency radar wave has a long wavelength and large penetration depth, the thickness of wave-absorbing honeycomb also affects the wave-absorbing property of HSC to a large extent. The influence of honeycomb thickness was investigated using honeycomb 6#, which exhibits optimal dielectric property (Figure 3c) in the low frequency range compared with the others. Figure 4 shows the reflectivity of HSC with the thickness of honeycomb (6#) ranging from 10 mm to 50 mm. It demonstrates that the honeycombs with moderate thickness of 30 mm and 40 mm possess the best wave-absorbing performances among the samples, where the averaged reflectivity of the corresponding HSC is respectively −11.5 dB and −11.2 dB in the frequency band of 1–2 GHz. For the fixed dielectric property, the honeycombs with lower thickness (e.g., 10 mm and 20 mm) could not sufficiently attenuate the incident radar wave in the transmission length. In this case, increasing honeycomb thickness is beneficial to enhance wave-absorbing performance. However, when honeycomb thickness reaches a much larger level (e.g., 50 mm), the resonant absorption peak of honeycomb moves to a lower frequency region (*f* < 1 GHz), which weakens its wave-absorption ability in the band of interest (1–2 GHz). By balancing these two opposite factors and considering the demand for a light-weight wave-absorbing composite, the honeycomb with moderate thickness of 30 mm was chosen to fabricate the wave-absorbing MHSC in the following sections.

**Figure 4.** Reflectivity of HSC-6# with different thicknesses of the wave-absorbing honeycomb.

#### *3.2. Wave-Absorbing Properties of MHSC*

According to the above discussion, increasing the dielectric property (Figure 3) and increasing the thickness (Figure 4) of honeycomb both have limited effect in improving the wave-absorbing property of HSC in the low-frequency band. To solve this problem, we designed a type of MHSC that contains metamaterial structural units with unique electromagnetic response in the frequency range of 1–2 GHz. MHSC was fabricated by incorporating a PEEK layer loading the periodic metal arrays into the conventional HSC structure, which is schematically illustrated in Figure 1. The PEEK film acts as the supporting substrate of the periodic metal units and guarantees position accuracy in the

pattern-transfer process. Moreover, during the curing step, the PEEK film can be dissolved in the resin matrix and fused into wave-transmitting skin, which prevents mechanical property loss brought about by the redundant film layer.

As shown in Figure 5, the metamaterial layer consists of open square metal rings with subwavelength sizes, which were designed on the basis of the phase gradient principle [45]. These anisotropic metal units can effectively resonate in the electromagnetic field under the specific wavelength, inducing a phase shift to the incident radar wave and producing a phase gradient on the meta-surface. In consequence, when the radar wave is incident on to the metamaterial units, both the reflected wave and the transmitted wave will be deflected with an abnormal reflection/refraction angle. The mechanism is schematically shown in Figure 5. On one hand, abnormal reflection can make the reflected wave deviate from the incident direction, which changes the direction and shape of the reflected lobe and reduces the RCS (Radar Cross Section) of the target. On the other hand, the abnormal transmission can increase the propagation distance (i.e., the effective thickness) of the radar wave in the honeycomb, which can significantly enhance electromagnetic loss and improve the wave-absorbing property of the composites in the low-frequency band.

**Figure 5.** Schematic illustration of the wave-absorbing mechanism of metamaterial/honeycomb sandwich composites (MHSCs).

#### 3.2.1. Properties of MHSC with Wave-Transmitting Honeycomb

*θ θ* Firstly, the effect of metamaterial units on the radar stealth performance of composites was investigated using the wave-transparent Nomex honeycomb with different thicknesses ranging from 10 mm to 50 mm. The thickness of the wave-transmitting skin of MHSC was fixed at 1 mm. As shown in Figure 6, increasing the thickness of Nomex honeycomb can significantly decrease the reflectivity of HSCM in the frequency range of 1–2 GHz. This phenomenon is ascribed to the abnormal transmission of the electromagnetic wave on the metamaterial surface. As demonstrated in Figure 5, the effective thickness of the honeycomb (*d*/cos(*θ*), propagation distance of the refracted wave) shows positive correlation with its actual thickness *d*. Consequently, for a certain electromagnetic response (cos(*θ*)) of metamaterial units, a relatively large thickness (*d*) of honeycomb is conducive to fully utilize the radar stealth function of the metamaterials in MHSC. According to Figure 6, when the thickness of Nomex honeycomb is larger than 30 mm, the composite with metamaterial shows relatively small reflectivity (less than −5 dB) in the band. However, the wave-transparent Nomex honeycomb without loading absorbing agents has low dielectric loss and cannot effectively attenuate the electromagnetic wave in the high frequency range (e.g., 8–18 GHz). Consequently, in the next section, wave-absorbing honeycombs with different dielectric constants and dielectric loss were complexed with metamaterial structure to fabricate MHSC, in order to realize more efficient wave-absorbing property in the low frequency range and balanced performance in a wide-frequency range.

−

*θ*

*θ*

**Figure 6.** Reflectivity of the MHSC with the different thicknesses of the wave-transmitting Nomex honeycomb.

−

#### 3.2.2. Properties of MHSC with Wave-Absorbing Honeycomb

Figure 7a–e show the reflectivity curves of five different MHSCs (colored solid lines) in the frequency band of 1–2 GHz, which were compared with corresponding HSCs without metamaterials (black dash lines). The HSCs and MHSCs were fabricated from wave-absorbing honeycombs (1#~5#) with different dielectric properties (see Figure 3a,b) and with a fixed thickness of 30 mm. The results illustrate that, for the five honeycomb samples, incorporating the metamaterial layer in the composite can decrease reflectivity in the low frequency band to a large extent. In fact, the metamaterial designed in this paper is a kind of subwavelength anisotropic structure. Due to the distinct phase-frequency response of the resonant units with different rotational orientation, the metamaterial can generate a gradual phase distribution (i.e., phase gradient) for the reflected and transmitted waves [44–46]. This unique electromagnetic effect equivalently changes the shape and direction of the reflected lobe and increases the effective thickness of the honeycomb absorber, which significantly enhances the low-frequency radar stealth performance of the composites. However, as revealed in Figure 7f, the wave-absorbing performance of MHSC is still correlated with the dielectric property of the wave-absorbing honeycomb in a complex manner. To quantitatively evaluate this issue, the averaged reflectivity (*R*a) of the HSC and MHSC in the frequency range of 1–2 GHz is extracted from the curves and listed in Table 2. HSC without metamaterial increasing the dielectric property of the honeycomb (1#~5#) within a certain range can significantly enhance radar-stealth performance, where *R*a is decreased from −1.35 dB to −6.29 dB from 1# to 5#. This effect is attributed to the more intensified wave absorption ability of the honeycomb with the larger dielectric loss. By contrast, when incorporated with metamaterials, it is the MHSC-4# with the moderate dielectric property of honeycomb that exhibits the best wave-absorbing performance (*R*a = −10.59 dB) among the five MHSC samples. As shown in the second row of Table 2, for honeycombs 1#, 2#, 3#, 4# and 5# with increased dielectric property, averaged reflectivity increases first and then decreases. In other words, the wave-absorbing performance of the metamaterial units is strongly restricted by the dielectric properties of the honeycomb structure. The value of (*R*a,HSC − *R*a,MHSC) is the reflection reduction of MHSC compared with HSC, which represents the enhancing effect of the radar stealth performance contributed by the metamaterial layer in the composite. As shown in the last row of Table 2, this value is negatively correlated with the dielectric property of the wave-absorbing honeycomb. Specifically, for the wave-transmitting Nomex honeycomb with the lowest dielectric constant and lowest dielectric loss, the metamaterial decreases the averaged reflectivity for 8.14 dB in the low frequency band, while the value is only 4.09 dB for the wave-absorbing honeycomb 5# with the largest dielectric property among

the samples. The mechanism is that, although the honeycomb with larger dielectric loss exhibits better wave-absorbing property, it will also show more significant interference on the electromagnetic response of the metamaterial layer. To achieve optimal radar-stealth performance in the low frequency band, the metamaterial units should be complexed with the wave-absorbing honeycomb that exhibits well-matched dielectric properties.

− **Figure 7.** (**a**–**e**) Reflectivity of HSC (dash line) and MHSC (solid line) in the low frequency range (1–2 GHz). The wave-absorbing honeycomb is respectively (**a**) 1#, (**b**) 2#, (**c**) 3#, (**d**) 4# and (**e**) 5#, whose dielectric properties show an ascending order of 1# < 2# < 3# < 4# < 5#. The shadow area in each figure represents the reduction of reflectivity by inserting the metamaterial units in the honeycomb sandwich composite. (**f**) Comparison of the wave-absorbing performance of MHSC fabricated from the honeycomb with different dielectric properties.

− **Table 2.** Averaged reflectivity of HSC and MHSC with different honeycombs in the frequency band of 1–2 GHz.

− − − − − According to the above investigation, the fabricated honeycomb sandwich composite with metamaterial (e.g., MHSC-4#) can realize excellent low-frequency wave-absorbing performance with averaged reflectivity as low as −10.59 dB in the band of 1–2 GHz. Compared with HSC-4# with the same thickness of wave-transmitting skin (1 mm) and wave-absorbing honeycomb (30 mm), incorporating the phase-gradient metamaterial into the composite can significantly enhance the wave-absorbing performance for 5.43 dB, with an increment larger than 100%. Meanwhile, the thin metamaterial layer with a surface density of only 0.19 kg/m2 (much lower than that of the wave-absorbing honeycombs, see Table 1) shows no obvious weight-increase to the total structure. Moreover, under dielectric match between the honeycomb and metamaterial, the MHSC also possesses a more satisfactory and enhanced wide-band wave-absorbing property than the corresponding HSC. The reflectivity of MHSC-4# and HSC-4# in the frequency range of 1–18 GHz was also

characterized, and the curves are shown in Figure 8. According to the results, averaged reflectivity in the frequency band of 1–4 GHz is −11.18 dB for MHSC-4# but only −7.51 dB for HSC-4#. The −10 dB bandwidth in the frequency band of 1–8 GHz is 5.4 GHz for MHSC-4# but only 3.16 GHz for HSC-4#. Moreover, over an extremely wide frequency range of 1–18 GHz, the averaged reflectivity of the HSCM-4# is lower than −14.8 dB, with the lowest reflectivity of −33.4 dB at the frequency of 9.5 GHz. These results indicate that, when matched with a suitable honeycomb, incorporation of the phase-gradient metamaterial layer can significantly enhance the wave-absorbing properties of the composites in the low frequency band while effectively maintaining original wave-absorbing performance in the high frequency range. It is strongly believed that the MHSCs reported in this study show promising potential to be applied as high-performance radar stealth composite materials.

**Figure 8.** Wide-band wave-absorbing performance of the MHSC (solid line) and HSC (dot line) in the frequency range from 1 GHz to 18 GHz. Both the composites used the wave-absorbing honeycomb 4# with a thickness of 30 mm. From left to right, the five regions with the colors of red, yellow, green, blue and purple respectively represent frequency bands of 1–2 GHz, 2–4 GHz, 4–8 GHz, 8–12 GHz and 12–18 GHz.

#### **4. Conclusions**

− In summary, we have fabricated a new type of metamaterial/honeycomb sandwich composite (MHSC) and systematically investigated wave-absorbing performance in the low frequency band of 1–2 GHz. It is proved that, simply increasing dielectric properties and heights of the wave-absorbing honeycomb both show a limited effect in improving low-frequency radar stealth performance of the conventional HSC, which is ascribed to the long wavelength of the electromagnetic wave in the band. This challenging problem is successfully solved by incorporating a phase-gradient metamaterial layer in the composite via a pattern-transfer method and optimizing the designing parameters of the fabricated MHSC. When the height of the wave-absorbing honeycomb and the height of the wavetransmitting skin are respectively 30 mm and 1 mm, the fabricated MHSC can possess averaged reflectivity less than −10 dB in the frequency band of 1–2 GHz, whose waveabsorbing performance is promoted for larger than 5 dB compared with the corresponding HSC. It is discovered that enhancement of radar stealth performance in the low frequency band contributed by the metamaterial layer shows negative correlation with dielectric properties of the wave-absorbing honeycomb, which is caused by coupling interference between the two response structures. The optimal low-frequency wave-absorbing property of MHSC is achieved by elaborately balancing the dielectric loss of the honeycomb and the electromagnetic response of the metamaterial units. Under rational design, the MHSC loaded with a thin and light-weight metamaterial layer not only possesses more excellent low-frequency (1–2 GHz) wave-absorbing properties compared with the conventional HSC but also shows more satisfactory radar stealth performance over a wide frequency range

of 1–18 GHz. This study both proposes an effective methodology to enhance the lowfrequency wave-absorbing performance of conventional HSCs and provides deep insight into electromagnetic coupling between the honeycomb absorber and metamaterials. It is highly expected that the discoveries and design methods of this work can be extended to fabricate light-weight and wide-band MAMs based on metamaterials and other composite structures, such as laminates, foams, fabrics, aerogels, and so on.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/polym14071424/s1, Figure S1: Photograph of the fabricated wave-transmitting skin containing metamaterial units; Figure S2: Schematic illustration about the structure and wave-absorbing mechanism of the honeycomb sandwich composites (HSC).

**Author Contributions:** Conceptualization, S.L. and L.X.; methodology, S.L. and H.L.; formal analysis, J.W.; investigation, S.L., H.H. and S.W.; resources, H.L. and L.X.; writing—original draft preparation, S.L. and H.H.; writing—review and editing, L.X.; visualization, H.H.; supervision, L.X.; project administration, L.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Article* **Bio-Based** *Eucommia ulmoides* **Gum Composites with High Electromagnetic Interference Shielding Performance**

**Hailan Kang 1,2 , Sen Luo 1,2 , Hongyang Du 1,2 , Lishuo Han 1,2 , Donghan Li 1,2 , Long Li 1,2, \* and Qinghong Fang 1,2, \***

**Abstract:** Herein, high-performance electromagnetic interference (EMI) shielding bio-based composites were prepared by using EUG (*Eucommia ulmoides* gum) with a crystalline structure as the matrix and carbon nanotube (CNT)/graphene nanoplatelet (GNP) hybrids as the conductive fillers. The morphology of the CNT/GNP hybrids in the CNT/GNP/EUG composites showed the uniform distribution of CNTs and GNPs in EUG, forming a denser filler network, which afforded improved conductivity and EMI shielding effect compared with pure EUG. Accordingly, EMI shielding effectiveness values of the CNT/GNP/EUG composites reached 42 dB in the X-band frequency range, meeting the EMI shielding requirements for commercial products. Electromagnetic waves were mainly absorbed via conduction losses, multiple reflections from interfaces and interfacial dipole relaxation losses. Moreover, the CNT/GNP/EUG composites exhibited attractive mechanical properties and high thermal stability. The combination of excellent EMI shielding performance and attractive mechanical properties render the as-prepared CNT/GNP/EUG composites attractive candidates for various applications.

**Keywords:** *Eucommia ulmoides* gum; carbon nanotubes; graphene; electromagnetic shielding

#### **1. Introduction**

With the widespread application of advanced electronic devices, our lives are enhanced by ever greater convenience. However, the development of electronic devices has created considerable quantities of electromagnetic (EM) pollution due to electromagnetic radiation and EM interference; this pollution affects human health and the normal operation of other devices [1–6]. In order to avoid these issues, many novel materials with high EMI shielding performance were explored with the aim of addressing problems related to EMI and radiation [7]. Conductive polymer composites (CPCs) consisting of polymer and conductive fillers were explored as promising alternative EMI shielding materials; these materials are generally metal-based materials [8–12]. Research on CPCs was motivated by their low weight, flexibility, resistance to corrosion and good processability [13–16].

Numerous efforts were devoted to the design and construction of CPCs using compounding polymers with carbon-based nanofillers, such as carbon black (CB), carbon fibers (CFs), carbon nanotubes (CNTs) and graphene [17–23]. For example, Cheng et al. [24] prepared CPCs using an ultrahigh-molecular-weight polyethylene/polypropylene (PP) blend with conductive CB, which exhibits an absorption-dominated EMI shielding effectiveness (EMI SE) as high as 27.29 dB over the X-band frequency range. Lin et al. [25] developed CF/PP composites with an EMI SE of ~20 dB via adding 15 wt% CFs. Since both CNTs and graphene have exceptional mechanical, electric transport properties and extremely high

**Citation:** Kang, H.; Luo, S.; Du, H.; Han, L.; Li, D.; Li, L.; Fang, Q. Bio-Based *Eucommia ulmoides* Gum Composites with High Electromagnetic Interference Shielding Performance. *Polymers* **2022**, *14*, 970. https://doi.org/ 10.3390/polym14050970

Academic Editor: Ahmed I. A. Abd El-Mageed

Received: 14 January 2022 Accepted: 26 February 2022 Published: 28 February 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

aspect ratios, they make excellent conducting fillers in CPCs for EMI shielding applications [26–31]. Wu et al. [32] prepared CNT/PP composites with an EMI SE of 43.1 dB by adding 5 wt% CNTs. Abraham et al. [33] produced CNT/styrene-butadiene rubber composites with ionic liquids, and the EMI SE of the composites was found to reach 35.06 dB at 18 GHz when adding 10 wt% CNTs. Lu et al. [34] fabricated graphene nanoplatelet (GnP)/ethylene propylene diene monomer rubber (EPDM) composites via a combination process that included mixing, ultrasonication and compression. The GnP/EPDM composites exhibit a high EMI SE of 33 dB in the range 8.2–12.4 GHz and 35 dB in the range 12.4–18 GHz.

The EMI shielding performance of CPCs is heavily dependent on the conductivity, dispersion and connectivity of fillers in the matrix [35,36]. Three-dimensional (3D) hybrid filler materials were successfully constructed via the hybridization of one-dimensional (1D) CNTs with two-dimensional (2D) graphene, which can overcome the problem of dispersion faced by the individual fillers. The π–π interaction between the CNTs and the graphene is beneficial in the nanoscale separation of the individual CNTs and graphene sheets [37,38]. Moreover, the long CNTs were found to be entangled with the graphene nanosheet, which inhibited their aggregation and provided efficient pathways for electron transfer. Zhao et al. [39] produced a material with an improved EMI shielding performance using polydimethylsiloxane with CNT/graphene hybrids; this material achieved an EMI SE of 31 dB. Kim et al. [40] fabricated CNT/GNP/PP composites with an EMI SE of 36.5 dB at 1.25 GHz. Mohammed et al. [41] prepared PP/polyethylene composites using GNP/CNT hybrid nanofillers. Across this research, it can be seen that using CNTs and graphene as hybrid fillers yield an effective route to the fabrication of CPCs with excellent EMI shielding performance.

With concern regarding the price fluctuations of fossil fuel reserves and the increasing environmental awareness, eco-friendly polymeric materials have attracted increasing research attention. *Eucommia ulmoides* gum (trans-1,4-polyisoprene, EUG) is extracted from the *Eucommia ulmoides* Oliv and an isomer of natural rubber. Due to its unique dual nature combining properties of both plastics and rubber, EUG was used as a thermoplastic material, thermoelastic shape memory material and as a highly elastic material in many fields. To date, there is little research regarding the use of EUG as an EMI shielding matrix. In the present work, we used EUG with a crystalline structure as a matrix material and CNT/GNP hybrids as conductive fillers to construct a shielding composite with a high EMI shielding performance. The effect of the CNT/GNP hybrids on the morphology, electrical conductivity and EMI shielding properties was investigated. Furthermore, the EMI shielding mechanism of the CNT/GNP/EUG composites was analyzed.

#### **2. Materials and Methods**

#### *2.1. Materials*

*Eucommia ulmoides* gum (EUG) was supplied by Shandong Beilong Eucommia Chemical Industry Co., Ltd., Weifang, China. Carbon nanotubes (CNTs, with a diameter in the range of 8–15 nm and length of 1–10 µm) were purchased from the Shanghai Kajite Chemical Technology Co., Ltd., Shanghai, China. Graphene nanosheets (GNPs) were provided by Shenzhen Hongda Changjin Technology Co., Ltd., Shenzhen, China. Triton X-100 (polyethylene glycol tertoctylphenyl ether) used in this work was provided by Shanghai Youdao Aoba Chemical Co., Ltd., Shanghai, China.

#### *2.2. Methods*

#### 2.2.1. Preparation

CNTs and GNPs were modified via the use of Triton X-100. An amount of 0.1 g of GNPs (or CNTs) was mixed with 1 mL of Triton X-100 and subsequently grounded manually in an agate bowl for 30 min. The product was then dispersed in deionized water and magnetically stirred in a glass beaker for 20 min. The suspension was ultrasonicated for 60 min in ice bath conditions, filtrated and vacuum dried to obtain *m*-GNPs (or *m*-CNTs).

The calculated amounts of *m*-GNPs (0 wt%, 0.80 wt%, 2.35 wt% and 3.85 wt%) and *m*-CNTs (9.62 wt%) were dispersed in toluene at 2 mg/mL under the action of mechanical stirring; this was followed by sonication for 60 min. The *m*-GNPs and *m*-CNTs suspensions were subsequently mechanically stirred for 10 min and ultrasonicated for 30 min. Then CNT/GNP suspension was added to the EUG toluene solution; the mixture was then mechanically stirred for 30 min and ultrasonicated for 2 h. The mixture was precipitated using ethanol and then dried at 40 ◦C. The dried mixture was mixed with curing agents (ZnO, 4 phr; SA, 2 phr; sulfur, 2 phr; NOBS, 1.2 phr) on a two-roll mill at 65 ◦C. Then, the blends were cured into 1-mm-thick sheets under 150 ◦C at the vulcanization time. The CNT/GNP/EUG composites are referred to as ECGx, where x represents the content of GNPs added to the composites.

#### 2.2.2. Characterization

The morphology of the composites was investigated via scanning electron microscopy (SEM, SU8010, Hitachi Co., Ltd., Tokyo, Japan) and transmission electron microscopy (TEM, FEI Talos F200, Thermo Fisher Scientific Inc, Shanghai, China). The samples used for SEM were fractured in liquid nitrogen and had a surface coated with a thin gold layer. The TEM samples were ultramicrotomed at −100 ◦C to produce sections with a thickness in the range of 50–60 nm. Dynamic mechanical rheology measurements were performed using a rubber process analyzer (RPA-8000, Goteh Testing Machines Co., Ltd., Qingdao, China). A strain sweep was undertaken from 1% to 200% at 60 ◦C and 1 Hz.

The electrical conductivity of the composites was measured using a four-probe conductometer (RTS-9, Guangzhou Four-Probes Technology Co. Ltd., Guangzhou, China). The EMI shielding performance of the composites was measured in the frequency range of 8.2–12.4 GHz (X band) at room temperature, using a vector network analyzer (E5071C, Agilent Technologies, Santa Clara, CA, USA); the test samples were of length 22.86 mm, width 10.16 mm and thickness 2.00 mm. The EMI performance parameters, including the SE total (*SET*), SE reflection (*SER*) and SE absorption (*SEA*), were calculated from the scattering parameters (*S*11 and *S*21).

$$R = \left| \mathbf{S}\_{11} \right|^2 \tag{1}$$

$$T = \left| \mathbf{S\_{21}} \right|^2 \tag{2}$$

$$SE\_R = -\log\_{10}(1 - R) \tag{3}$$

$$SE\_A = -\log\_{10}(T/(1-R))\tag{4}$$

$$SE\_T = SE\_R + SE\_A \tag{5}$$

The tensile tests were carried out using an INSTRON 3365 testing machine (Instron Co., Ltd., Norwood, MA, USA) according to the standard ASTM D412 using a crosshead speed of 500 mm/min at 23 ± 2 ◦C. Cut the EUG/CNTs/GNPs composite sample into strips with a section width of 6 mm and thickness of 2 mm with a dumbbell cutter, and clamp both ends of each sample. For each data point, at least five specimens were tested, and an average value was taken. The thermal stability of the samples was measured via thermogravimetric analysis (TGA, Q50, TA Instruments, Rutherfordton, NC, USA) from 40 ◦C to 600 ◦C using a heating rate of 10 ◦C/min in an N2 atmosphere. Differential scanning calorimetric (DSC) measurements were obtained using a DSC-Q200 (TA Instruments, Rutherfordton, NC, USA) under N2 from −70 ◦C to 100 ◦C at a heating/cooling rate of 10 ◦C/min. The crystallinity of the EUG was determined, as

$$X\_{\mathfrak{c}} = \frac{\Delta H\_m}{\Delta H\_m^\*} \times 100\% \tag{6}$$

where ∆*H*∗ *m* is the theoretical value of 100% crystallized EUG (125.6 J/g).

#### **3. Results and Discussion**

#### *3.1. The Morphology of the CNT/GNP/EUG Composites*

It is generally accepted that the properties of composite material are closely correlated with the dispersion of the filler material used. Figures 1 and 2 show the SEM and TEM images of CNT/GNP/EUG composites. As shown in Figures 1a and 2a, the CNTs are uniformly dispersed within the EUG matrix without obvious agglomeration or entanglement. As shown in Figures 1b–d and 2b, the CNTs and GNPs are uniformly dispersed across the matrix after the shear force was applied during the open mill process. CNTs were observed to show an individual distribution state and are seen to be closely attached to the GNPs, further demonstrating that the GNPs lead to an efficient dispersion of the CNTs and implying that there exists a high-strength binding interaction between the GNPs and CNTs. Because of the π–π interaction between the CNTs and GNPs, GNPs can improve the dispersibility of CNTs and inhibit their re-aggregation due to their larger surface area and steric hindrance; rod-shaped CNTs can also reduce stacking effects of the GNPs [42]. Furthermore, with an increase in hybrid loading, the GNPs exhibit increased agglomeration; these were caused by the effect of the π–π bond between the GNPs and the strong Van der Waals force. Thus, the composites form a more complete CNT/GNP conductive network, which provides a favorable condition for the electrical conductivity of the composites.

**Figure 1.** SEM images of CNT/GNP/EUG composites: (**a**) ECG0; (**b**) ECG1; (**c**) ECG2; (**d**) ECG3.

**Figure 2.** TEM images of CNT/GNP/EUG composites: (**a**) ECG0; (**b**) ECG3.

The Payne effect is widely used to characterize filler dispersion and filler networks. Figure 3 shows the storage modulus (G′ ) as a function of the strain for the CNT/GNP/EUG composites. G′ of the CNT/GNP/EUG composites is seen to decrease rapidly with in-

creasing dynamic strain amplitudes; it is also seen to exhibit high sensitivity to strain, an indication of a typical Payne effect. The decrease in G′ is due to the destruction of filler networks. G′ increases significantly with increasing GNP loading of the CNT/GNP hybrids, demonstrating that denser filler networks are formed between CNTs and GNPs at higher CNT/GNP loading. The difference between G′ at 200% and G′ at 1% (referred to as ∆G′ ) also increases with the increase in the GNP loading; this phenomenon indicates an increase in the Payne effect. In summary, with increasing GNP loading within CNT/GNP hybrids, the filler networks possess a higher modulus and a higher sensitivity to strain. The strong filler networks also demonstrate that the CNT/GNP hybrids are highly dispersed within the EUG matrix.

**Figure 3.** Storage modulus versus strain of CNT/GNP/EUG composites.

#### *3.2. Electrical Conductivity and EMI Shielding Properties of the CNT/GNP/EUG Composites*

Electrical conductivity is a key factor that determines the EMI shielding properties of a given composite [43]. Materials with high electrical conductivity usually exhibit good EMI shielding properties. The volume electrical conductivity values of the CNT/GNP/EUG composites fabricated here are shown in Figure 4. CNTs are observed to be non-directionally dispersed and distributed but in a directional distribution in local microbeams, which results in the composites having anisotropic characteristics across different micro-domains; these properties have an effect on the electrical conductivity of the rubber. The electrical conductivity of CNTs/EUG composites with 9.62 wt% CNTs is seen to be around 2.31 S/cm. This result indicates that the CNTs in the composites form a conductive network at this CNT loading. The electrical conductivity of the CNT/GNP/EUG composites slightly increases with increasing of GNP loading within CNT/GNP hybrids. As shown in Figure 2, the CNT/GNP hybrids were well dispersed within the EUG matrix, which is conducive to the formation of perfect conductive networks and leads to an improved EMI shielding performance.

**Figure 4.** Electrical conductivity of CNT/GNP/EUG composites.

The outstanding electrical properties endow CNT/GNP/EUG composites with high EMI shielding properties [44]. The EMI shielding properties of the CNT/GNP/EUG composites were assessed by measuring the values of *SET*, *SER* and *SEA*. Figure 5 shows the EMI *SET*, *SER* and *SEA* of the CNT/GNP/EUG composites in the X-band frequency range. The pure EUG exhibits a very small *SET* value of less than 0.5 dB, indicating that EUG is almost completely transparent to the electromagnetic waves over the whole X-band frequency range. It can be seen from Figure 5a that the value of *SET* of the CNT/GNP/EUG composites gradually increased from 30.29 dB to 42.49 dB with the increase in the GNPs loading within the CNT/GNP hybrids. The enhanced EMI shielding properties are attributed to the increased conductive networks created by the synergistic effect of GNPs and CNTs.

**Figure 5.** (**a**) EMI *SET* (**b**) EMI *SER*, (**c**) EMI *SEA* and (**d**) EMI *SET* at 10 GHz of CNT/GNP/EUG composites.

In order to state the synergistic effect of 2D GNPs and 1D CNTs, the same weight loading of GNP and CNT composites were fabricated, and the comparative results of electrical conductivity and EMI shielding efficiency are summarized in Figure 6. The electrical conductivity and EMI *SET* of the CNT/GNP/EUG composite are all higher than CNT/EUG composite and GNP/EUG composite. As a 1D material, CNTs contact each other predominantly through point contacts. As a 2D material, GNPs contact each other via surface contacts. In CNT/GNP hybrids, GNPs function as the 'spacers' between the polymers, and the CNTs function as the 'wires' making up the conductive network. The combination of the two fillers creates much denser conductive networks than either filler used alone. Such perfect conductive networks permit the dissipation of electromagnetic waves through both reflection and absorption. Therefore, the CNT/GNP/EUG composites exhibit enhanced electrical conductivity and EMI shielding efficiency because of the synergistic effect of GNP and CNTs.

**Figure 6.** Comparison of (**a**) electrical conductivity and (**b**) EMI *SET* of CNT/EUG composite (13.47 wt% CNT), GNP/EUG composite (13.47 wt% GNP) and CNT/GNP/EUG composite (9.62 wt% CNT and 3.85 wt% GNP, ECG3).

The maximum EMI SET value of the CNT/GNP/EUG composite is seen to be as high as ~42 dB, a value that meets the EMI shielding requirement for commercial products. Table 1 compares the results obtained in this work with other EMI shielding polymeric composites reported in previous work [17,18,20,28,30–32]; it is seen that the CNT/GNP/EUG composites created in this work exhibit superior EMI *SET* compared with previously studied composites. The results indicate that this work provides a practical and effective method for the preparation of high-performance EUG-based EMI shielding composites.

**Table 1.** Comparison of EMI shielding properties of CNT/GNP/EUG composites with other CPCs.

PBT: Polyb; PBT: Polybutylene terephthalate; ASA: Acrylonitrile Styrene acrylate copolymer; PDMS: Polydimethylsiloxane; SBR: 1,3-butadiene polymer.

#### *3.3. EMI Shielding Mechanism of the CNT/GNP/EUG Composites*

In order to fully elucidate the EMI shielding mechanism of the CNT/GNP/EUG composites, the values of *SEA*, *SER* and the relative values at 10 GHz were calculated and plotted in Figure 5b,c,d, respectively. The values of *SEA* and *SER* of the CNT/GNP/EUG composites all increase with increases in GNP loading of the CNT/GNP hybrids. More importantly, the *SEA* values greatly increase with the GNP loading of the CNT/GNP hybrids. In all the CNT/GNP/EUG composites, the *SEA* values are much greater than the *SER* values; this is because the conductive networks provide a sufficient number of interfaces that form multiple reflections and thus attenuate the incident electromagnetic waves [45]. For instance, the *SET*, *SER* and *SEA* of ECG3 are 42 dB, 7.4 dB and 34.6 dB, respectively. Furthermore, the *SEA* value that represents absorption makes up 82% of the *SET* value, whereas the *SER* value that represents reflection accounts for only 18% of the *SET* value. Thus, the results of this work indicate that absorption is the predominant shielding mechanism of the CNT/GNP/EUG composites.

The complex permittivity of the CNT/GNP/EUG composites was also measured in order to understand their EM absorption properties better. Figure 7 shows the real part (*ε'*), the imaginary part (*ε"*) and the dielectric loss tangent (tan *δε* = *ε"*/*ε* ′ ) in the X-band frequency range. The values of *ε'* and *ε"* represent the storage and loss capability of electric energy within a material, respectively, whereas tan *δε* is used to evaluate the dielectric loss capacity of a material. Both *ε'* and *ε"* decrease with increasing frequency; this decrease is related to the interface polarization [46]. It is noted that the values of *ε'* and *ε"* of the CNT/GNP/EUG composites increase with an increase in the GNP loading of the CNT/GNP hybrids. This is ascribed to the formation of perfect conducting networks by the CNTs and GNPs within the EUG matrix. The enhanced *ε'* value is predominantly due to the creation of more dipoles within the composite. The uniform distribution of the CNT/GNP hybrids without agglomeration can create many interfaces and hence induce polarization at the interfaces of CNT/GNP, CNT/EUG and GNP/EUG. Moreover, the introduction of more layers of GNPs with a large specific surface area induces an increase in the interface polarization [47]. The increase in the value of *ε"* with an increase in the GNP loading is attributed to the dielectric relaxation and electrical conductivity of the composites. The incorporation of GNPs enhances the electrical conductivity, resulting in high conduction loss. Furthermore, the presence of a high number of interfaces between the EUG matrix and the fillers results in further increases in the dielectric loss within the composite. The value of tan *δε* is significantly increased by increases in GNP loading, indicating a superior dielectric loss capacity. The enhanced value of tan *δε* represents significant dissipation of electrical energy and attenuation of wave absorption.

**Figure 7.** (**a**) *ε*', (**b**) *ε*" and (**c**) tan *δε* of CNT/GNP/EUG composites.

Based on the preceding analysis, we established the shielding mechanism of the CNT/GNP/EUG composites, which is illustrated in Figure 8. When the EM waves reach the interfaces within the composite, a proportion of the EM waves are reflected at the surface of the CNT/GNP/EUG composite due to the impedance mismatch between the composites and the air, while a proportion of EM waves is absorbed by the CNT/GNP/EUG composite due to the conduction losses, multiple reflections from interfaces and the interfacial dipole relaxation losses. The CNT/GNP/EUG composites form more effective conductive networks due to the synergistic effect of 2D GNPs and 1D CNTs, thereby causing the attenuation of EM waves via Joule heating, which increases the EM absorption. Moreover, the numerous EUG/CNT, EUG/GNP and CNT/GNP interfaces facilitate substantial attenuation of the EM waves via multiple reflections and scattering. The interfacial polarizations also attenuate EM waves and enhance the EM wave absorption effect. Importantly, the boundary of crystalline–amorphous phases within EUG matrix can accumulate electrical

charges, thus resulting in polarization loss, while the crystalline regions of the EUG matrix also increase the propagation of EM waves within the material and hence enhances EM absorption [48,49].

**Figure 8.** Proposed electromagnetic shielding mechanism for CNT/GNP/EUG composites.

#### *3.4. Crystallinity of the CNT/GNP/EUG Composites*

Thermal and crystalline behaviors were characterized by the DSC curves, and the crystallinity of EUG was calculated. As shown in Figure 9a, all samples exhibit one peak upon heating curves, which corresponds to the melting peak of crystalline EUG. Compared with pure EUG, the melting temperature (*Tm*) of the CNT/EUG composite shifts to a higher temperature, and the melting enthalpy (∆*Hm*) increases from −23.95 to −35.48 J/g. Accordingly, the crystallinity (*XC*) of the CNT/EUG composite also increases from 19.1% to 28.2%, implying that CNTs acted as the nucleating agent and promoted the crystallization of EUG segments. However, the *Tm*, ∆*Hm* and *XC* of CNT/GNP/EUG composite all decrease with the increase in GNP loading of the CNT/GNP hybrids. These results indicate that the EUG segmental mobility was restricted by strong networks formed by the synergistic effect of CNTs and GNPs.

**Figure 9.** (**a**) DSC curves of CNT/GNP/EUG composites, (**b**) the crystallinity (Xc) of CNT/GNP/EUG composites.

#### *3.5. Thermal and Mechanical Properties of the CNT/GNP/EUG Composites*

Figure 10 depicts the TGA curves of the CNT/GNP/EUG composites; the related data are shown in Table 2. As shown in Figure 8, the thermal decomposition of CNT/GNP/EUG composites takes place in a single stage between 260 ◦C and 300 ◦C, predominantly due

to the pyrolysis of the EUG macromolecules. The decomposition temperatures of the CNT/GNP/EUG composites with 5% and 30% weight loss (*T5*, *T30*) and the calculated heat resistance index (*THRI*) are all higher than 303 ◦C, 410 ◦C and 180 ◦C, respectively. The value of *THRI* of the CNT/GNP/EUG composites all increase with the increase in the GNP loading of the CNT/GNP hybrids. The value of *THRI* for the CNT/GNP/EUG is 191 ◦C, which is 11 ◦C higher than that of pure EUG (180 ◦C). The reason for this increase is due to the CNT and GNP fillers possessing high heat resistance, which improves the thermal stability of the EUG matrix.

**Figure 10.** (**a**) Thermogravimetric curves of CNT/GNP/EUG composites. (**b**) Enlarged view of thermogravimetry at 100–90%.

**Table 2.** Thermal characteristics data of CNT/GNP/EUG composites.

T5 and T30 correspond to the decomposition temperature of 5% and 30% weight loss, respectively.; THeat-resistance index = 0.49 ∗ [T5 + 0.6 ∗ (T30 − T5)].

The mechanical properties of the CNT/GNP/EUG composites are shown in Figure 11. The tensile strength of the CNT/GNP/EUG composites is seen to increase with an increase in the GNP loading of the CNT/GNP hybrids. The elongation at break initially increases and subsequently decreases with increasing GNP loading of the composite. The incorporation of GNPs in the CNT/GNP/EUG composite improves the dispersion of CNTs, resulting in enhanced tensile strength and elongation at break. On the other hand, the introduction of more CNT/GNP hybrids increases the filler–filler and filler–polymer networks, which further reinforce the composites and resists the removal of polymer chains; this results in improved tensile strength and decreased elongation at break. The Young's modulus of ECG0, ECG1, ECG2 and ECG3 are 0.05 GPa, 0.051 GPa, 0.052 GPa and 0.057 GPa, respectively. Besides, the shore A hardness of ECG0, ECG1, ECG2 and ECG3 are 79, 81, 82 and 84. The Young's modulus and the hardness of the CNT/GNP/EUG composite show a slight increase with the increase in the CNT/GNP hybrids, an indication of the reinforcement of CNT/GNP hybrids. ECG3 exhibits a tensile strength of 20.9 MPa and elongation at a break of 304%. Therefore, the combination of the excellent electromagnetic shielding performance and high mechanical properties makes the as-prepared CNT/GNP/EUG composite appropriate for a wide range of applications.

**Figure 11.** Stress-strain curves of CNT/GNP/EUG composites.

#### **4. Conclusions**

In summary, we have successfully fabricated bio-based composites with a high EMI SE and attractive mechanical properties by combining a matrix of EUG with a crystalline structure and CNT/GNP hybrid structures used as conductive fillers. The synergistic effect of the CNT/GNP hybrid originates from the bridging of CNTs between GNPs, which facilitates the enhanced filler networks, electrical conductivity, EMI shielding properties and mechanical properties of the CNT/GNP/EUG composites. The maximum electrical conductivity and EMI SE values of CNT/GNP/EUG composites are observed to be up to 2.71 S/cm and 42.49 dB, respectively. Furthermore, the tensile strength of the CNT/GNP/EUG composites was found to increase with an increase in the GNP loading of the CNT/GNP hybrids. These results indicate that the method studied here provides a practical and effective route towards the creation of high-performance EUG-based EMI shielding composites.

**Author Contributions:** Conceptualization, H.K., Q.F. and L.L.; methodology, L.L., S.L., H.D. and L.H.; formal analysis, D.L.; investigation, S.L. and H.D.; resources, H.K.; data curation, S.L. and L.H.; writing—original draft preparation, L.L.; writing—review and editing, D.L. and H.K.; supervision, Q.F.; project administration, H.K.; funding acquisition, H.K. and Q.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Natural Science Foundation of China (grant number 52073178); Natural Science Foundation of Liaoning, China (grant number 2019-MS-263); and Outstanding Young Talent Projects of the Shenyang University of Chemical Technology, China (grant number 2019YQ003).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Article* **Streamer Propagation along the Insulator with the Different Curved Profiles of the Shed**

**Xiaobo Meng 1 , Liming Wang 2, \* , Hongwei Mei 2, \* , Bin Cao 2 and Xingming Bian 3**

**Abstract:** The flashover along the insulator endangers the reliable operation of the electrical power system. The reasonable curved profiles of the shed could improve the flashover voltage, which would reduce power system outages. The research on the influence of the curved profiles of the shed on the streamer propagation along the insulator made of polymer was presented in the paper. The streamer propagation "stability" field, path, and velocity affected by the curved profiles of the shed, were measured by ultraviolet camera, ICCD camera, and photomultipliers. The "surface" component of the streamer is stopped at the shed with the different curved profiles, while the "air" component could go round the shed and reach the cathode. The streamer propagation "stability" fields are inversely proportional to the curved profiles of the shed. The streamer propagation velocities are proportional to the curved profiles of the shed. The relationship between the streamer propagation and the flashover propagation was discussed in depth. The subsequent flashover propagation is greatly affected by the streamer propagation path and "stability" field. Furthermore, the influence of the material properties on the streamer propagation path was also discussed in depth.

**Keywords:** streamer discharge; curved profiles; streamer propagation "stability" field; streamer propagation path; streamer propagation velocity

#### **1. Introduction**

The flashover along the insulator surface causes many widespread power outages every year. It endangers the stability and safety of electrical power system, resulting in a large number of social and economic losses. The reasonable shed design might improve the flashover voltage, which would reduce power system outages. The test results of the flashover along the insulator with the different shed configurations have been used to optimize the design of the insulator by many researchers at home and abroad [1–4].

The flashover along the insulator was supported by many researchers [5–7]. However, the streamer discharge along the insulator with a shed were little investigated [8–10]. The streamer propagation along the insulator is the important physical process before the flashover. The inhibited streamer discharge along the insulator can lead to the subsequent flashover process to disappear. Hence, the research on the streamer discharge along the insulator with the different sheds is helpful for designing the shed configuration of insulator.

The streamer discharge along the smooth insulator have been investigated widely, but the characteristics of streamer discharge along the insulator with a shed were rarely researched [11–15]. In the paper [12,13], the influences of the shed on the streamer discharge were discussed, the "surface" component of streamer propagation along the insulator was blocked at the shed, but the "air" component could cross the shed and reach the cathode

**Citation:** Meng, X.; Wang, L.; Mei, H.; Cao, B.; Bian, X. Streamer Propagation along the Insulator with the Different Curved Profiles of the Shed. *Polymers* **2022**, *14*, 897. https:// doi.org/10.3390/polym14050897

Academic Editors: Shaojian He and Alexey Bubnov

Received: 25 January 2022 Accepted: 22 February 2022 Published: 24 February 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

plate. In the paper [15], the influences of the shed configuration on the streamer discharge were discussed and summarized, such as the diameter, location, and combination of the shed. However, the influences of the curved profiles of the shed on the streamer discharge was not studied in depth. The photograph of streamer discharge along the profiled insulator surfaces was not shot by advanced high-definition camera [13]. The relation between the streamer discharge and flashover along the insulator with the shed was still not very clear [16]. "stability"

"surface"

"air"

In the paper, the streamer propagation "stability" field, path, and velocity affected by the curved profiles of the shed, were measured by ultraviolet camera, ICCD camera, and photomultipliers. The streamer energy loss at the shed was estimated by a method, which was used to interpret physical phenomenon in the test. Furthermore, the relationship between the characteristics of the streamer and the flashover was discussed in depth. It provided theoretical and experimental basis for optimization design of the curved profiles of the shed. "stability"

#### **2. Experimental Arrangement and Measurement System**

The streamer propagation "stability" field, path, and velocity affected by the curved profiles of the shed, were measured by ultraviolet camera, ICCD camera, and photomultipliers in a three-electrode arrangement. The experiment arrangement and measurement system could be found in our past literature [15], as shown in Figure 1. The streamer discharge was triggered by a square pulse voltage applied to the needle electrode. A negative DC voltage was applied to two flat electrodes to produce a uniform electric field. The photographs of the streamer discharge paths were shot by the UV imaging detector (Ofil Corporation, Tel Aviv-Yafo, Israel). The micromorphology of streamer discharge was taken by an ICCD camera (Princeton Instruments, Trenton, NJ, USA). The photons radiated from the streamer discharge could be acquired by the photomultiplier (ET Enterprises Limited, Uxbridge, UK), so three photomultipliers were adopted to monitor the development process of the streamer discharge.

**Figure 1.** Experimental equipment.

The experiment was taken under standard atmospheric pressure, and the relative humidity was about 60% and temperature was about 20 ◦C. The insulators with the different curved profiles of the shed were made of polymer (nylon). The diameters of the sheds were 70 mm. The fillet diameters of the curved profiles of the sheds were 25 mm, 16 mm, and 5 mm respectively, as shown in Figure 2. The surface resistivity of the polymer (nylon) is 6.8 × 10 9 Ω under the test condition.

Ω under the test condition

**Figure 2.** Insulators with the different fillet diameters of the curved profiles of the sheds.

#### **3. Experimental Results**

#### *3.1. Streamer Propagation Fields*

The streamer propagation probability under the different electric fields is an important parameter in the development of streamer, which is useful to reveal streamer characteristics. The detailed measurement method of the streamer propagation probability was based on our past literature [17]. The streamer propagation probability distribution is shown in Figure 3.

**Figure 3.** Streamer propagation probability.

"stability"

"stability"

"stability"

ity"

bility"

ity" bility" "stability" The formula of Gaussian distribution could be used to calculate the streamer "stability" propagation fields *E*st corresponding to streamer propagation probability of 97.5% [15]. Figure 4 shows that the relation between the pulse amplitude and the streamer "stability" propagation fields is linear. There is inverse proportional relationship between the streamer "stability" propagation fields and the pulse amplitude, as same as the result in the literature [18]. The streamer "stability" propagation fields for the insulator with a shed are larger than that for the smooth insulation surface and the air alone. Specifically, the streamer "stability" propagation fields are inversely proportional to the fillet diameters of the curved profiles of the sheds.

"stability"

"stability"

"stabil-

"stabil-

"sta-

"sta-

**Figure 4.** Stability fields for streamer propagation.

#### *3.2. Light Emission*

The photomultipliers were used to measure the streamer discharge along the insulators with the different curved profiles of the sheds. The double light peaks were detected by the photomultiplier 2 at the shed, whereas single light peak at the cathode were detected by the photomultiplier 3. The test results measured by the photomultiplier have been discussed in depth in the literature [15]. The "surface" component of streamer propagation along the insulator is blocked at the shed, but the "air" component could cross the shed and reach the cathode plate. cussed in depth in the literature [15]. The "surface" component of streamer propagation along the insulator is blocked at the shed, but the "air" component could cross the

cussed in depth in the literature [15]. The "surface" component of streamer propagation along the insulator is blocked at the shed, but the "air" component could cross the The photographs of the streamer discharge along the insulators were shot by the ultraviolet camera (Figures 5–7). The shooting method of the ultraviolet camera was listed in the literature [17]. Consistent with the measurement result of the photomultipliers, the "air" propagation path bypasses the shed and reaches the cathode, while the "surface" propagation path stops at the shed. – "air" "surface"

**Figure 5.** Streamer discharge photographs (Insulator A). (**a**) 570 kV/m, (**b**) 590 kV/m, (**c**) 630 kV/m, (**d**) 680 kV/m, (**e**) 710 kV/m, (**f**) 740 kV/m.

–

**Figure 6.** Streamer discharge photographs (Insulator B). (**a**) 610 kV/m, (**b**) 630 kV/m, (**c**) 650 kV/m, (**d**) 690 kV/m, (**e**) 710 kV/m, (**f**) 750 kV/m.

cussed in depth in the literature [15]. The "surface" component of streamer prop-

agation along the insulator is blocked at the shed, but the "air" component could cross the

"air" "surface"

**Figure 7.** Streamer discharge photographs (Insulator C). (**a**) 630 kV/m, (**b**) 650 kV/m, (**c**) 690 kV/m, (**d**) 720 kV/m, (**e**) 750 kV/m, (**f**) 780 kV/m.

"air" "surface" "air" "surface" "surface" "air" "surface" "air" "surface" propagation path due "air" "surface" "air" "surface" "surface" "air" "surface" "air" "surface" propagation path due The ICCD camera (PI MAX3) made by Princeton Instruments was used to take photographs of the micromorphology of the streamer propagation along the insulators. Due to the weaknesses of the streamer discharge, each photograph recorded the three-times process of streamer discharges in order to make the photograph clearer. Figures 8–10 shows the streamer discharge photographs. It is found that there are also two streamer paths: the "air" propagation path and the "surface" propagation path. The "air" propagation path crosses the shed and arrives at the cathode. Different from the measurement results of the ultraviolet camera, the "surface" propagation path also crosses the shed and arrives at the cathode. However, the photomultiplier 3 detected single light peak at the cathode, which meant that the "surface" component was blocked by the shed, and only the "air" component crossed the shed. The reason to explain this contradiction is that the "surface" component is blocked by the shed. When the "air" component of streamer bypasses the shed, sometimes it might propagate along the "surface" propagation path due to the attraction of the surface charge [19–21], or sometimes it propagated along the "air" propagation path, or both cases existed in the one streamer discharge. Due to the larger fillet diameters of the curved profiles of the insulators A and B, the air component is more easily attracted

"surface" "air"

"surface" "air"

–

–

– "air"

– "air"

"air"

"air"

"surface" "air"

"surface" "air"

"air"

"air"

"air"

"air"

by the surface charge and develops along the insulating material surface. This situation is not obvious on the surface of the insulator C due to the smallest fillet diameters of the curved profiles. In the literature [15], the "air" component of streamer might propagate along either the "surface" propagation path or the "air" propagation path. Therefore, the "surface" propagation path and the "air" propagation path behind the shed both belong to the "air" component of streamer due to the attraction of the surface charge. The greater the fillet diameters of the curved profiles, the easier it is for the "air" component of streamer to develop along the surface of the insulating material. The streamer propagation paths along the insulator with a shed are described in Figure 11.

**Figure 9.** Streamer propagation photographs measured by ICCD camera (Insulator B). (**a**) 620 kV/m, (**b**) 660 kV/m, (**c**) 700 kV/m, (**d**) 740 kV/m.

**Figure 10.** Streamer propagation photographs measured by ICCD camera (Insulator C). (**a**) 640 kV/m (**b**) 680 kV/m, (**c**) 720 kV/m (**d**) 760 kV/m.

**Figure 11.** Streamer propagation paths along the insulators.

#### *3.3. Streamer Propagation Velocity*

" " of the "air" component " " The streamer "stability" propagation velocities *V*st were defined as the velocities of streamer propagation at the stability fields [22]. The velocities of streamers propagating along the insulators with a shed decreases linearly but slowly with the pulse amplitude in Figure 12. The velocities of the "air" component along the insulators with a shed are higher than that along the smooth insulator in Figure 13. The reason could be found in the literature [15]. It is obvious that the "air" component velocities are proportional to the fillet diameters of the curved profiles of the sheds.

he "air" component velocit

he "air" component velocit

of the "air" component

" " "

"

" "

of the "air" component

he "air" component velocit

**Figure 12.** Streamer stability propagation velocity.

**Figure 13.** Streamer propagation velocity ("air" component).

#### velocity ("air" component) *3.4. Evolution of Streamer to Flashover*

" " " " " " " " The characteristics of flashover along the insulator were investigated through improving the applied voltage between the plates. The flashover "stability" propagation fields *E*50 corresponding to flashover propagation probability of 50% could be acquired by the method of acquiring the streamer "stability" propagation fields *E*st . Table 1 shows that the flashover "stability" propagation fields *E*50 increase with streamer "stability" propagation fields *E*50. The flashover "stability" propagation fields are also inversely proportional to the fillet diameters of the curved profiles of the sheds. Hence, the applied electric field has the same effect on the streamer propagation and the subsequent flashover propagation.

" " **Table 1.** The comparison of streamer "stability" propagation fields Est and flashover "stability" propagation fields E50.

along the "air" "surface" The photographs of the flashover path were acquired by a high-speed camera in Figure 14. It shows that the flashover propagates along the insulator with only one propagation path. In front of the shed, the flashover propagates along either the "surface" or "air" propagation path of the previous streamer. The flashover propagated along the

Insulator A 585 824 Insulator B 605 842 Insulator C 630 863

" " " "

**Shed Configuration**

"air" propagation path of the previous streamer behind the shed due to the interruption of the "surface" propagation path at the shed. Hence, the streamer channel has a great influence on the subsequent flashover. The reasonably optimized curved profiles of the sheds could improve the electric fields for the streamer propagation, thereby, the electric fields for the flashover propagation are also improved. The perspective of restraining the streamer discharge to prevent the flashover discharge would be a new idea to design the curved profiles of the shed.

(**a**)

(**b**)

(**c**)

**Figure 14.** Photographs of flashover along the insulator with the shed. (**a**) Insulator A. (**b**) Insulator B. (**c**) Insulator C.

#### **4. Discussion**

*4.1. Tangential Electric Field along Streamer Propagation Path*

The reason why the "surface" component of streamer would not cross this small electric Ansoft Maxwell was adopted to calculate the electric fields along the insulator. Figure 15 shows the tangential electric field along the whole insulator surface. The tangential electric field is small and even negative in the regions of the curved profiles of the shed. The reason why the "surface" component of streamer would not cross this small electric field region is that the energy from this small electric field could not maintain the streamer propagation. Hence, the "surface" propagation path fails to reach the cathode.

"surface" propagation path fails to reach the cathode.

**Figure 15.** Tangential electric field along the whole insulator surface. " " "

However, the "air" component could cross the shed and reach the cathode plate. Hence, the streamer propagation characteristic parameters, such as the streamer "stability" propagation fields and the streamer "stability" propagation velocities, are all determined by the "air" component of streamer. The distribution of the tangential electric field along the "air" path is important to explain the streamer propagation characteristic parameters. Figure 16 shows that the tangential electric field along the "air" path before and behind the shed is proportional to the fillet diameters of the curved profile of the shed; whereas, the tangential electric field at the shed is inversely proportional to the fillet diameters. " " "air" path is important to explain the streamer propagation characteristic pa-Figure 16 shows that the tangential electric field along the "air" path before and

The reason why the "surface" component of streamer would not cross this small electric

"surface" propagation path fails to reach the cathode.

"

**Figure 16.** Tangential electric field along the "air" path.

Tangential electric field along the "air" path the tangential electric field along the "air" pa The streamer has not yet obtained much energy from the electric field in front of the shed, so the applied electric field has the greatest impact on it. When the streamer propagates along the insulator with the largest fillet diameters of the curved profile of the shed, the tangential electric field along the "air" path is the largest, so it is easy for the streamer to develop and to cross the shed where the tangential electric field is greatly reduced. Therefore, lower electric field is required for the streamer to propagate along the insulator with the larger fillet diameters of the curved profile of the shed. Under the same electric field, the "air" component velocities along the insulator are proportional to the fillet diameters of the curved profiles of the sheds.

field, the "air" component velocities along the insulator are proportional to the fillet di-

sh = shfc + shsc − (stfe − stse) = shc − ste

–

" "

#### *4.2. Streamer Propagation Energy Loss at the Shed*

The streamer discharge would lose much energy at the shed due to the low tangential electric field and the charges in the streamer attaching to the shed [23–26]. The method in the literature [15] was used to estimate the energy loss at the shed. The calculated energy loss at the shed (*L*shc) differed by an error (*Q*ste) from the real energy loss (*L*shc) as shown in Equation (1).

$$L\_{\rm sh} = L\_{\rm shfc} + L\_{\rm shsc} - (Q\_{\rm stfc} - Q\_{\rm stse}) = L\_{\rm shc} - Q\_{\rm ste} \tag{1}$$

In Table 2, *E*st1 is the stability fields for streamer propagation along insulators with a shed. *v*1/*v*2 is the "air" component velocity before/after the shed at *E*st1. *Q*Ef1/*Q*Es1 is the energy obtained from ambient electric field before/after the shed. *E*f2 is the ambient field corresponding to the same value of *v*1 when streamer propagates along smooth insulators during the first half part of air gap. *E*s2 is the ambient field corresponding to the same value of *v*2 when streamer propagates in air alone during the second half part of air gap. *Q*Ef2/*Q*Es2 is the energy obtained from ambient electric field *E*f2/*E*s2 in its case.

**Table 2.** Energy loss at the shed.

Table 2 showed the energy loss at the shed (*L*shc) is inversely proportional to the fillet diameters of the curved profile of the shed. From the perspective of the energy loss, the streamer propagation energy loss at the shed is inversely proportional to the fillet diameters of the curved profile of the shed. Therefore, the streamer "stability" propagation fields are inversely proportional to the fillet diameters, and the streamer propagation velocities are proportional to the fillet diameters under the same electric field.

#### *4.3. Influence of the Material Properties*

In the literature [12,13], the authors found that the "air" component of streamer discharge could bypass the shed and reach the cathode, the "surface" component of streamer discharge could also bypass the shed with the large fillet diameters of the curved profile and reach the cathode. However, we used more advanced experimental equipment and experimental testing methods in this paper, such as the ICCD camera and the UV imaging detector was adopted to shoot the streamer discharge photographs. The different test conclusion was obtained that the "surface" component of streamer discharge could not bypass the shed, even the fillet diameters of the curved profile of the shed is much large. The "surface" propagation path behind the shed does not belong to the "surface" component of streamer discharge, but belongs to the "air" component of streamer discharge.

Our conclusion is closer to the actual situation, in addition to our more advanced measuring equipment, it is more important that we consider the influence of material properties on the streamer discharge. There are much microporous defects (physical defect and chemical defect) on the surface of the polymer in Figure 17 measured by the scanning electron microscope. The surface adsorbed charges on the polymer increase with the increase of the microporous defects [27]. Therefore, the surface of the polymer would adsorb a lot of negative charge with the negative DC voltage applied on the gap.

**Figure 17.** Scanning electron microcopy images.

The method of the thermally stimulated current was used to measure the trap charges (nC) on the surface of the polymer. The trap charges on the polymer could be calculated by the result of the thermally stimulated current in Figure 18. The trap charge on the polymer used in the test is 1879 nC, which is greater than other materials. The more trap charges means that the more negative charge is accumulated or deposited on the polymer surface. The result of the thermally stimulated current proves the result of the scanning electron microscope.

**Figure 18.** The result of the thermally stimulated current.

As shown in Figure 19, the vector of electric fields is parallel to the surface of the polymer with no surface charge. However, the vector of electric fields points to the surface of the polymer with negative surface charge. Therefore, the air component of streamer discharge behind the shed is more easily attracted by the polymer surface and develops along the "surface" propagation path. It could explain reasonably that why the "surface" propagation path and the "air" propagation path behind the shed both belong to the "air" component of streamer in the Section 3.2.

− 0 μC/m

− 0 μC/m

− 0 μC/m **Figure 19.** Vector plots of the electric field distribution along the surface of the polymer. (**a**) no surface charge, (**b**) −10 µC/m2 surface charge. The darker part is polymer material, the white part is air.

This contradiction of the measurement result from the ICCD camera and the UV imaging detector has given us great enlightenment. It is easy to produce confusing results or conclusions for a single test equipment or measurement method to study the physical process of the temporal and spatial evolution of streamer discharge. Combining a variety of test equipment or measurement methods, and then comprehensively analyzing the test results, can accurately reveal the physical development process of streamer discharge.

The test results tell us that the smaller fillet diameters of the curved profile of the shed would be adopted to obtain the larger flashover voltage in electrical power system. However, the outdoor insulators of the transmission lines are prone to accumulating pollution on the surface in the atmospheric environment, and the larger fillet diameters of the curved profile of the shed would remove pollutants easily through rain and wind. Hence, the fillet diameters of the curved profile of the shed not only affect the flashover voltage of the insulator, but also the removal of pollutants through rain and wind. In engineering practice, the fillet diameters of the curved profile of the shed should be chosen to consider the impact on both sides, taking into account both flashover voltage and cleaning capacity. Sometimes in order to obtain high cleaning ability, the flashover voltage can be appropriately reduced to obtain better environmental adaptability. These results provide a theoretical basis for promoting the shape of the shed.

#### **5. Conclusions**

From typical photomultiplier signals and the streamer discharge photographs, the "surface" component of streamer discharge was blocked at the shed. Only the "air" component crossed the shed, and the "air" component behind the shed might propagate along either the "surface" propagation path or the "air" propagation path.

The streamer "stability" propagation fields were inversely proportional to the fillet diameters of the curved profiles of the sheds. The streamer propagation velocities were proportional to the fillet diameters.

The applied electric field had the same effect on the streamer propagation and the subsequent flashover propagation. Furthermore, the streamer channel had a great influence on the development path of the subsequent flashover. The perspective of restraining the streamer discharge to prevent the flashover discharge would be a new idea to design the curved profiles of the shed.

The tangential electric field along the streamer propagation path and the streamer propagation energy loss at the shed could be used to explain the characteristics of the streamer discharge. From the perspective of the tangential electric field, when the streamer propagates along the insulator with the largest fillet diameters of the curved profile of the shed, the tangential electric field along the "air" propagation path is the largest, so it is easy for the streamer to develop and to cross the shed where the tangential electric field is greatly reduced. From the perspective of the streamer propagation energy loss, the streamer propagation energy loss at the shed is inversely proportional to the fillet diameters of the curved profile of the shed. Therefore, lower electric field is required for the streamer to propagate along the insulator with the larger fillet diameters of the curved profile of the shed.

The surface of the polymer would adsorb a lot of negative charge with the negative DC voltage applied on the gap. The vector of electric fields points to the surface of the polymer with negative surface charge. Hence, the air component of streamer discharge behind the shed is more easily attracted by the polymer surface and develops along the "surface" propagation path. It could explain reasonably that why the "surface" propagation path and the "air" propagation path behind the shed both belong to the "air" component of the streamer.

**Author Contributions:** Conceptualization, L.W.; methodology, L.W.; formal analysis, X.B.; software, B.C.; data curation, X.M.; writing—original draft preparation, X.M.; writing—review and editing, B.C.; supervision, X.B.; project administration, H.M.; funding acquisition, L.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by the State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources (Grant No. LAPS20004), the GuangDong Basic and Applied Basic Research Foundation (2021A1515110568).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of the data; in the writing of the manuscript; nor in the decision to publish the results.

#### **References**

## *Article* **The Dynamic Behaviour of Multi-Phase Flow on a Polymeric Surface with Various Hydrophobicity and Electric Field Strength**

**Qi Li 1,\*, Rui Liu 1 , Li Li 2 , Xiaofan Song 3 , Yifan Wang 2 and Xingliang Jiang 1**

**Abstract:** The dynamic behaviour of rain droplets on the insulator surface is a key measure to its reliability and performance. This is due to the fact that the presence and motion of rain droplets cause intensive discharge activities, such as corona and low current arcing, which accelerate the ageing process and flashovers. This article aims to investigate and characterize the movement of a rain droplet placed on an inclined insulator surface subject to an intensive electric field. The rain droplets' movement on hydrophobic surfaces in the absence of an electric field is investigated. A high speed camera is used to capture the footage and finite element method (FEM) is used to simulate the multi-physics phenomenon on two polymeric surfaces, namely, silicon rubber (SiR) and PTFE (polytetrafluoroethylene). A 'creepage' motion was observed. The inception of motion and the movement speed are analysed in correlation with various surface conditions. Models are established to estimate the moisture and potential discharge characteristics on the inclined polymeric surfaces. They are further utilized to analyse the actual insulators subject to wet conditions.

**Keywords:** droplet vibration; high voltage insulator; polymeric surface; corona discharge; arcing; creepage distance

#### **1. Introduction**

The distortion and movement of rain drops in an electric field has been the subject of a number of experimental research studies. High voltage AC outdoor insulators can be distinct by their bulk insulation material as ceramic insulators (glass or porcelain) or composite insulators (polymeric or non-ceramic insulators). Composite insulators consist of pultruded glass cores and polymeric shed sheaths are fast becoming the main choice of materials because of their improved wetting behaviour and greater degree of hydrophobicity [1,2]. The discharge activity on the surface of polymeric insulator surfaces is one of the ageing mechanisms leading to insulator failure. Such activity between the rain droplets causes the production of radicals that chemically react with the surface of the insulator, thereby altering the properties of the insulator material. The practical aspect of the subject seems to be an important factor in justifying the extensive work carried out on electrical discharges from rain drops on polymeric surfaces.

The surface of operational polymeric insulator experiences hydrophobicity weakness and even loss due to the surface discharge of the rain droplet. Due to the reduced hydrophobicity, the rain droplets coalesce and form 'rivulets' of conductivity, which bridge the insulation path on the insulator surface leading to the flashover between HV and ground. Most experimentation revolves around discharges between rain drops on the top of flat horizontal surfaces. As the weather-proof sheds of insulators are neither horizontal nor planar, it is worth studying the behaviour of rain droplets in more complex situations.

**Citation:** Li, Q.; Liu, R.; Li, L.; Song, X.; Wang, Y.; Jiang, X. The Dynamic Behaviour of Multi-Phase Flow on a Polymeric Surface with Various Hydrophobicity and Electric Field Strength. *Polymers* **2022**, *14*, 750. https://doi.org/10.3390/ polym14040750

Academic Editor: Shaojian He

Received: 15 December 2021 Accepted: 25 January 2022 Published: 15 February 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

This article aims to investigate the behaviour of a rain droplet on an inclined insulator surface under the influence of intense AC electric field. This unique experiment and its results are necessary to further understand the behaviour of moisture [3] and ageing process for polymeric surfaces, especially their application within power systems for insulators and bushings.

#### **2. Literature Review**

The study of thunderstorms inspired the early work on the impact of electrical fields on rain drops and much of the activity was concerned with the distortion and break-up of individual drops in high fields. The discharges on the surface of polymeric insulators are important in the ageing mechanisms of insulators. Since the introduction of hydrophobic polymeric insulators, a considerable amount of work has been conducted addressing the electrical discharges from rain droplets on polymeric surfaces. Rowland [4] highlighted the critical role played by the local ageing of the polymers' surface in the formation of 'wet fingers' of conductivity. Karady et al., for example, describes the formation of the filaments between rain droplets on aged commercial silicon rubber insulators and a mechanism by which this led to flashover of single sheds [5–7].

The vibration and distortion of rain drops in the presence of an alternating field (ac) is well established [8–11]. A rain droplet in a strong electric field deforms as a result of the interaction of electrostatic force and the surface tension of the rain droplet [12]. The movement of rain droplet in alternating field (ac) is characterized by periodic vibration or fluctuation. Higashiyama and Yamada investigated in detail the behaviour of a rain droplet placed on the surface of the hydrophobic polymeric sheet in presence of an ac field. They demonstrated the change in droplet shape during vibration, showing that the droplet is deformed and synchronized with the ac field [8]. Their study revealed that the frequency and the volume of rain droplet greatly affect the amplitude of vibration.

An electric field alone can force raindrops up an incline in order to move away from a high-stress region pointing out the importance of gravity on inclined surfaces [13]. Phillips et al. and Cheng et al. have shown elongation of single raindrops [14,15]. They observed that raindrops do not change their shape gradually, but in a series of steps, and do not necessarily recover their shape after the field is removed. Krivda and Birtwhistle [13] explained that natural vibrations of a rain drop change its shape during the ac cycle and hence can reduce the insulation path. This increases the risk of flashover. It was shown that when several raindrops coalesce they can bridge a significant distance of insulation and hence discharges appear between them. The corona discharge takes place at the triple junction area of the rain–air–solid interface as the electric field at this point is intensified due to the difference in permittivity of silicon rubber, air and the rain droplet [16].

The corona discharge destroys the hydrophobicity of the insulator and thus a long chain of elongated drops is formed. Under the influence of electric field, this can lead to the flow of currents over the surface of the insulation [5].

The discharge activity between rain droplets on the insulator surface plays an important role in its ageing mechanisms. It is also critical in the processes leading up to the flashover of these insulators in highly polluted or marine environments. However, limited research has been conducted on the impact of surface shapes and dimensions on surface hydrophobicity and water sliding behaviour.

#### **3. Methodology**

To address this gap in the literature, this article aimsu to design a specific experimental set up to investigate the factors affecting the rain droplets' dynamic movement on an inclined insulation surface. To simplify, a flat inclined surface was used with the capability to vary its inclined angle. The first step was to investigate the behaviour of the rain droplets on an inclined plane in the absence of electric field [17]. This will facilitate the design process while identifying the key variables that need to be controlled. In the second step, the experiments were conducted in a high voltage environment. The aims and objectives of the study can change depending upon need and setup availability/feasibility.

#### *3.1. In the Absence of an Electric Field*

Silicon rubbers with different additives are materials of high interest for high voltage composite insulators. A number of other materials were also chosen for investigation in order to examine the role of surface energy or hydrophobicity. These materials included glass, PTFE and a commercial silicon rubber-based material (SiR). A drop of de-ionised water was placed on the sample surface by a pipette and the angle of inclination gradually increased until the droplet started to move. The experiment was repeated with three different sizes of droplet (50, 90 and 140 µL) and material types (SiR, PTFE and Glass). The diagram of the test setup is shown in Figure 1. μ μ μ

**Figure 2.** Syringe unit of the optical contact angle measuring device (Data Physics OCA Series).

#### *3.2. In the Presence of an Electric Field*

The basic understanding developed through investigating rain droplet behaviour on different materials (without E-field) helped to design the experiment. The experimental setup consists of two large cylindrical metal electrodes with rounded edges. The electrode surfaces were smooth with little or no asperities in order to obtain uniform electric field. A silicon rubber sample with an inclination of 30 was used as a test material. The metal electrodes were separated by about 50 mm. A 50 µL drop of de-ionised water was placed in the central region on top of the insulator sample. The droplet size (volume) was controlled by the electronic syringe unit of the optical contact angle measuring device. A high speed camera (imaging rate 3902 fps) was used to observe and record the movement of the rain droplet under the influence of high AC electric field. A diagram of the experimental setup is shown in Figure 3. μ μ

**Figure 3.** Diagrammatic overview of the experimental setup.

Ω The electrical diagram of the same experiment is shown in Figure 4. The components include a high voltage transformer, the rated voltage of which is 80 kV AC (frequency 50 Hz), and a 125 kΩ current limiting resistor. The circuit also contains a surge protection device and a voltage divider. A digital storage oscilloscope was used to monitor the voltage waveforms. Ω

**Figure 4.** Diagram of the test circuit.

A 'break down test' was conducted in order to find the maximum voltage that can be safely applied to the test setup before the dielectric (air between electrodes) breaks down. A fairly large rain droplet with the volume 100 µL was applied to the sample surface and the voltage was gradually increased. The breakdown occurred at 55 kV and hence it was decided to perform all experimentation below the 35 kV voltage level.

The silicon rubber used in the experiment was designed using basic geometry. The thickness of the material was 7 mm. A 30◦ inclined silicon rubber sample was made using a sharp cutting tool as shown in Figure 5. The dimensions of the sample are shown in Table 1.

μ

**Figure 5.** Silicon rubber sample with 30 degrees inclination.

**Table 1.** Sample dimensions.

#### *3.3. Numerical Simulation Using the Finite Element Method (FEM)*

When the rain droplet is under the electric field, the electric field force drives the movement and deformation of the rain droplet, which leads to the distortion of the field strength and changes the size of the electric field force [18,19]. The interaction between the two is a coupling problem of electricity and fluid dynamics, which is defined as electrohydrodynamics. After a sensitivity study of various simplified versions of the simulation in comparison with the measurements, the FEM was identified as the most effective tool to understand the physical mechanism behind the droplets' dynamics.

#### **4. Results and Discussion**

#### *4.1. Contact Angles and Inclination Angle of Samples*

The experimentation conducted in absence of electric field was quite useful in developing a basic understanding about key concepts, such as surface hydrophobicity and water contact angles. The contact angles between rain drops on different surfaces are shown in Table 2; each value given is an average of three measurements.

**Table 2.** Contact angles between water droplets on different materials.

Table 3 shows the inclination angle at run-off for different materials and droplet sizes (volume). In the experiments carried out, a fresh sample of material was used for all readings. The line of sight was kept perpendicular to the scale while taking the readings in order to avoid a parallax error.

The results in Table 2 show that the highest contact angles were seen for silicon rubber sample. This can be explained by the excellent hydrophobicity of silicon rubber. Water molecules were seen to form discrete droplets on the surface of the insulation when the material is highly hydrophobic.

**Table 3.** Inclination angle at run-off.

Table 3 shows that, for a given volume, a greater degree of inclination was required to run off a water droplet from the SiR surface compared to PTFE and glass. The surface properties of these materials, such as surface roughness and hydrophobicity, are important in water droplet movement. The results were important in the design of the experiment conducted in presence of high electric field (AC). It provided the range of the droplet sizes and materials that can be used to meet the research specifications.

#### *4.2. Creepage Phenomenon on the Insulator Surface*

The main highlight of the experiment was the identification of a 'Creepage Phenomenon', which has not been observed earlier in the literature. The vibration and distortion of water drops on horizontal surfaces under the influence of an alternating field (AC) is well established. It was observed that, if the water droplet placed on an inclined insulator surface is subjected to an ac field, it tends to creep along the surface. Additionally, the movement is affected by numbers of factors, including the strength of field, volume of water droplet, type of the insulating material and the roughness of insulator surface. This dynamic behaviour of water droplet can play an important role in the ageing mechanisms of composite systems.

The voltage was uniformly increased from zero kV in steps of 5 kV. The 'Creepage Phenomenon' was seen at 30 kV (AC) using the experimental setup shown in Figure 3. The high-speed camera captured the movement and distortion of the water droplet placed on the inclined surface, which is shown in Figure 6. It can be clearly seen that the droplet creeps down the insulator surface during the course of the experiment.

Figure 6 shows the movement of the water droplets corresponding to footage of 8 s. It was found that:

The water droplet in strong electric field deforms as a result of the interaction of electrostatic force and the surface tension of the water droplet. Additionally, it vibrates with many vibration modes throughout the creepage movement. The deformation and vibration modes of the water droplet depend on a number of factors. The factors include the magnitude of the applied field, the surface tension, droplet size (volume) and the water density of the water droplet [13,20,21]. Additionally, the actual insulator surface has a non-uniform electric field distribution. This means that it is possible to find many vibration modes occurring simultaneously on its surface.

Upon careful observation of the high-speed camera footage, the water droplet was seen to elongate in the action of electric field. The change of water droplet shape would have a significant influence on the insulation performance of insulators.

#### *4.3. Speed of Creeping Movement*

The speed of the water droplet creeping down the insulator surface was calculated using MATLAB. This helped to quantify the experiment. The relationship between time and the speed of movement is shown in Figure 7. The graph expresses the variation of instantaneous speed against time. The average speed was also calculated, which is seen to increase with time.

**μ μ μ**

**Figure 6.** Still pictures from high-speed camera footage.

It was seen that the water droplet placed on the SiR sample under the influence of ac electric field vibrates. This results in the deformation of the water droplet shape. The change in shape can cause the enhancement of a local electric field in the triple junction area of water–air–solid. Cheng [15] points out that the surface of composite insulators can experience hydrophobicity loss as a result of local electric field enhancement. Corona discharge may appear at the triple junction as the electric field is most intensified in that area [22]. This can facilitate the movement of the water droplet on the sample surface and thereby an increase in instantaneous speed is observed.

The energy transfer mechanism can also play an important role in justifying the speed variation. The energy (surface tension) of vibrating water droplet is transferred to the kinetic energy of the moving droplet and vice versa. After each subsequent movement, the dynamics change, resulting in different conditions needed to initiate the movement again.

#### *4.4. The Occurance of the Creeping Phenomenon*

It was quite interesting to observe that the phenomenon did not recur, despite many attempts to reproduce it. Both old and new silicon rubber samples were employed to see the reproducibility of the experiment. Later, the angle of inclination was also increased to 40 degrees to see the effect of level of inclination on water droplet movement. This had no impact on the water droplet movement as the creepage phenomenon was not observed at all. An increase in the electric field resulted only in severe vibration and distortion of the water droplet, but no movement. The same experiment was repeated on PTFE (another hydrophobic insulator) in order to examine the effect of surface energy or hydrophobicity. This experiment again did not see any creeping of the water droplet subject to same experimental conditions.

The last stage involved changing the droplet size (volume) to investigate its effect on the water droplet movement. In this case, the 'Creepage Phenomenon' was successfully repeated four times with a water droplet of about 75 µL. This indicates two main possibilities: the water droplet size measured as 50 µL in the initial experiment was inaccurately dispensed by the electronic syringe system, or there are some other unrevealed factors governing the movement of water droplets on inclined surfaces.

#### *4.5. Electric Field with an Insulator*

The experimental set up was designed in a way to simulate the electric stress of real insulators in service. This was quite important in developing a useful experiment whose relevance can be directly related to the practical world of overhead line composite insulators. The finite element method was employed as an effective tool to compute the electric field. Figure 8a shows the electric field distribution of an insulator in service. It can be seen that the electric field distribution on a real insulator varies from 0 to 9.754 kV/cm along the insulator length.

The electric field distribution on the silicon rubber sample surface is shown in Figure 8b. The top electrode is connected to the high voltage end, so the electric field intensity increases in magnitude from bottom to top on the sample surface. The graph shown in Figure 8 confirms that the electric field on the sample surface simulates the electric stress experienced by real insulators. This shows that a great proportion of sample surface experiences the same electric stress as that of real high voltage composite system insulator.

**Figure 8.** (**a**) Electric field distribution on a real insulator; (**b**) Electric field distribution on a sample (SiR).

#### **5. Conclusions**

It can be concluded that the water droplet placed on an inclined insulator surface vibrates with various modes. This is due to the reason that the insulators in service experience different electric field distribution on its surface and form wide range of droplet sizes. A 'Creepage Phenomenon' was observed where the water droplet (placed on an inclined polymeric surface) tends to creep along its surface in presence of high AC electric field. The experiment proved that this phenomenon is water-droplet-size dependent. A water droplet of about 75 µL is needed to observe the creepage movement of the water droplet on a 30 degrees inclined silicon rubber sample. The average speed of movement is seen to increase with time. The effect of hydrophobicity (for fixed volumes) did not have an important impact on the creepage movement. The outcomes from the indicated that this new phenomenon, so called 'creepage', plays a significant role in the ageing process of high voltage composite insulators.

μ **Author Contributions:** Conceptualization, Methodology, Writing—review and editing, Q.L. Writing original draft preparation, Formal analysis, R.L. Validation, Resources, L.L. Visualization, Software, X.S. Investigation, Data curation, Y.W. Supervision, X.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Natural Science Foundation of China (51637002).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data sharing not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Article* **Investigation of the Compatibility and Damping Performance of Graphene Oxide Grafted Antioxidant/Nitrile-Butadiene Rubber Composite: Insights from Experiment and Molecular Simulation**

**Meng Song 1,\* , Xiulin Yue 1 , Chaokang Chang 1 , Fengyi Cao 1 , Guomin Yu 1 and Xiujuan Wang 2, \***

**Abstract:** Rubber damping materials are widely used in electronics, electrical and other fields because of their unique viscoelasticity. How to prepare high-damping materials and prevent small molecule migration has attracted much attention. Antioxidant 4010NA was successfully grafted onto graphene oxide (GO) to prepare an anti-migration antioxidant (GO-4010NA). A combined molecular dynamics (MD) simulation and experimental study is presented to investigate the effects of small molecules 4010NA, GO, and GO-4010NA on the compatibility and damping properties of nitrile-butadiene rubber (NBR) composites. Differential scanning calorimetry (DSC) results showed that both 4010NA and GO-4010NA had good compatibility with the NBR matrix, and the *Tg* of GO-4010NA/NBR composite was improved. Dynamic mechanical analysis (DMA) data showed that the addition of GO-4010NA increased the damping performance of NBR than that of the addition of 4010NA. Molecular dynamics (MD) simulation results show GO-4010NA/NBR composites have the smallest free volume fraction (FFV) and the largest binding energy. GO-4010NA has a strong interaction with NBR due to the forming of hydrogen bonds (H-bonds). Grafting 4010NA onto GO not only inhibits the migration of 4010NA but also improves the damping property of NBR matrixes. This study provides new insights into GO grafted small molecules and the design of high-damping composites.

**Keywords:** molecular dynamics simulation; damping performance; nitrile-butadiene rubber; graphene oxide; antioxidant 4010NA

#### **1. Introduction**

Rubber damping materials are widely used in electronics, electrical, aerospace, and automobiles for vibration and noise reduction due to their unique viscoelastic properties [1–3]. As some special fields have higher requirements for rubber damping materials, the preparation of high-performance damping materials has become a research hotspot. In recent years, the preparation, and research of organic hybrid damping materials have attracted much attention in the field of rubber damping [4,5]. Rubber damping materials with a dynamic hydrogen bond (H-bond) network can be prepared by adding small molecular compounds with polar functional groups, such as hydroxyl and amino into the polar rubber matrix, which can significantly improve the damping properties of the material [6,7]. In the previous studies, researchers often used nitrile-butadiene rubber (NBR) as the polar rubber matrix and added hindered phenol antioxidants (such as AO-80 or AO-60, etc.) to prepare organic hybrid damping materials with high-damping performance [8–10].

NBR has good damping performance due to the strong polarity of the -CN functional groups. However, NBR molecular chain contains a large number of unsaturated carbon-

**Citation:** Song, M.; Yue, X.; Chang, C.; Cao, F.; Yu, G.; Wang, X. Investigation of the Compatibility and Damping Performance of Graphene Oxide Grafted Antioxidant/Nitrile-Butadiene Rubber Composite: Insights from Experiment and Molecular Simulation. *Polymers* **2022**, *14*, 736. https://doi.org/10.3390/polym 14040736

Academic Editor: Brendan Howlin

Received: 21 January 2022 Accepted: 12 February 2022 Published: 14 February 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

carbon double bonds, which is easy to age in heat, oxygen, ozone, light, and other external conditions, thus shortening the service life and affecting the damping effect. At present, adding amine antioxidants into a rubber matrix is one of the easiest ways to prevent rubber aging. For most rubbers, amine antioxidants are more effective in preventing long-term oxidative degradation [11]. Many researchers improve the phenomenon of rubber aging easily by adding a small molecular antioxidant, but the antioxidant tends to migrate from the rubber matrix due to their low molecular weight, thus affecting the damping effect, resulting in shortened service life and environmental pollution [12–14]. At present, there are two solutions: the first is to improve the molecular weight of antioxidants [15,16]; the second is to graft antioxidants to the polymer chain or filler surface [17–21], which is an effective method for anti-migration of antioxidants.

Graphene oxide (GO) is a two-dimensional carbon material, arranged in a honeycomb shape, with an extremely high specific surface area and high stability. Compared with graphene, its surface contains a large number of oxygen-containing functional groups, such as hydroxyl (-OH), carboxyl (-COOH), and epoxy functional groups (-CH (O) CH-), which make GO have excellent mechanical, thermal, and electrical properties [22–25] and more practical application value and broad prospects. It can usually show a significant enhancement effect on some polar rubbers [26–29].

Grafting antioxidants on the surface of GO is an effective method to reduce the migration of antioxidants. At present, experimental studies have been carried out to graft antioxidants onto GO and other fillers. For example, Zhong et al. [30] modified GO with the antioxidant p-phenylenediamine (PPD), and added the modified GO into the NBR matrix, which significantly improved the thermal stability of the rubber matrix. Yao et al. [31] functionalized GO nanosheets through an acyl chloride reaction and then reacted the product with hindered phenol (HP) to obtain GO-g-HP with excellent anti-aging effects. Zhong et al. [32] grafted antioxidant 4020 to the surface of GO, which showed better anti-migration performance in styrene-butadiene rubber (SBR) than free antioxidants. By grafting the antioxidant onto the GO surface, the antioxidant can be fixed to prevent the migration in the rubber matrix, and the dispersity in the rubber can be improved. Generally, amine antioxidants contain polar functional groups, the addition of antioxidants can also form an H-bonds network with a polar rubber matrix, which can improve the damping and aging properties of rubber.