Correlation of Plasma Temperature in Laser-Induced Breakdown Spectroscopy with the Hydrophobic Properties of Silicone Rubber Insulators

Abstract

In this study, we have investigated the relationship between the plasma temperature in remote laser-induced breakdown spectroscopy (LIBS) experiments and the hydrophobic properties of silicone rubber insulators (SIRs). Contact angle and LIBS measurements were conducted on both artificially-aged (accelerated aging) and field-aged SIRs. This study reveals a clear connection between plasma temperature and the properties of aged SIRs on artificially-aged SIR specimens. Specifically, the plasma temperature exhibits a consistent increase with the duration of the accelerated aging test. The hydrophobicity of the artificially-aged SIRs was assessed by performing contact angle measurements, revealing a decrease in the hydrophobicity with increased aging test duration. Furthermore, we extended our investigation to the study of nine field-aged SIRs that had been in use on 150 kV overhead transmission lines for 0 to 21 years. We find that the laser absorption and hardness of the material do not relate to the plasma temperature. In summary, we observe a direct connection of plasma temperature to both contact-angle measurements and operation time of the in-service insulators. These results strongly suggest the potential use of LIBS for remotely evaluating the hydrophobicity and aging degree of silicone rubber insulators, thus assessing their real-time on-site operational quality.

1. Introduction

Silicone rubber insulators (SIRs) have been extensively used in high-voltage (HV) power systems during the last decades due to their exceptional insulating properties, even under heavily polluted conditions [1]. Their resistance to harsh environmental conditions such as pollution, UV radiation, and extreme temperatures makes them highly durable and reliable, ensuring prolonged service life even in challenging operational environments. Additionally, SIRs exhibit superior hydrophobicity, reducing the risk of surface contamination and tracking, which can compromise insulating performance. Moreover, their lightweight nature facilitates easier installation and maintenance compared to traditional porcelain and glass insulators. Furthermore, silicone rubber’s elasticity mitigates damage risk from mechanical stress, vibrations, and seismic events, enhancing grid resilience and reliability. These combined attributes appoint SIRs as the preferred choice for HV applications, offering optimal performance and cost-effectiveness over their counterparts [2,3].

Silicone rubber hydrophobicity plays a critical role in the preservation of the electrical integrity of SIRs, hence it leads to expansion of their lifespan and safeguards constant operation of HV power systems. Most importantly, SIRs exhibit the unique ability to recover their hydrophobicity, at a higher efficiency than other polymeric insulators, even in cases that it has been significantlyly suppressed due to aging [4,5]. Such a long-term silicone rubber water repellency prevents moisture ingress and wetting of the insulators’ surface and suppresses the formation of conductive pathways, which may cause leakage current to flow along the insulator (dry band arcing). By maintaining a dry and clean surface, hydrophobic SIRs reduce the risk of pollution flashover and ensure reliable insulation performance [6]. Additionally, hydrophobicity counteracts the accumulation of ice during cold weather conditions, further reinforcing the insulators’ ability to withstand environmental challenges.

Contact angle and spray methods have been applied in order to estimate hydrophobicity in SIRs. Although contact angle measurement is a reliable method, it is commonly employed in the laboratory, only for testing a limited number of areas along the insulator, leaving the rest unaccounted for. Researchers have studied the relation between hydrophobicity of SIRs and contact angle measurements, noting that a hydrophobicity reduction correlates with lower contact angles of water droplets [7,8,9,10,11,12,13,14]. Alternatively, the spray method involves spraying water on the insulator surface and observing the shape of the droplets formed and the percentage of wet regions. The results are classified into seven hydrophobicity grades, according to the Swedish Transmission Research Institute (STRI), which defined a hydrophobicity classification (HC) method in 1992 based on water droplet aggregation status [15]. The spray method has been proven to be an easily applicable test and is adopted by IEC 62073 technical specification [16], referred to as Method C, in the guidance on wettability measurements of insulator surfaces.

The methodologies for assessing hydrophobicity in silicone rubber insulators (SIRs) present several limitations, including those related to laboratory testing, cost, and operational disruptions. Conducting this analysis in a laboratory may not fully replicate real-world conditions, potentially leading to discrepancies between laboratory results and actual field performance. Additionally, the equipment and expertise required for precise measurements can incur substantial costs, hindering widespread adoption, especially for smaller organizations or developing regions. Furthermore, implementing these methods often requires temporarily halting power grid operations for testing insulators, resulting in downtime.

Therefore, the development and implementation of remote methods to monitor hydrophobicity on insulators are of significant importance in safeguarding the reliability and efficiency of electrical grids. Remote monitoring allows for continuous and real-time assessment of insulator conditions with no need for manual intervention or grid shutdowns, minimizing operational disruptions and downtime. This approach enables activities to implement timely maintenance or mitigation strategies, preventing costly outages, equipment failure, and safety hazards. Additionally, remote monitoring facilitates the optimization of maintenance schedules and resource allocation, ultimately enhancing grid resilience and performance while reducing operational costs.

Among the methods, recently developed for aging evaluation and quality assessment of SIRs [17], Laser-Induced Breakdown Spectroscopy (LIBS) has concentrated great scientific interest and its use has been demonstrated in various studies. Kokkinaki et al. [18] performed remote LIBS analysis of field-aged polymeric insulators, on the basis of a spectral indicator, appropriately defined to reflect the carbon-to-silicon content, serving as a tool to identify SIRs among other types of insulators and to assess the physical condition of SIRs in real time. Homma et al. [19] showcased depth profiling of chemical composition changes in composite insulators via remote LIBS, indicating the potential of the method for evaluating the type or extend of degradation, based on emission intensity ratios. Thangabalan et al. [20] explored the relationship between plasma temperature and filler content or gamma irradiation dose in SIR, suggesting LIBS for classifying aging and degradation levels. Qin et al. [21] investigated aluminum phosphate contamination using LIBS for rapid detection, while Wang et al. [22] focused on ablation and spectral characteristics of high-temperature vulcanized (HTV) materials, proposing LIBS as a tool for on-site hardness characterization. Liu et al. [23], in a review article, highlighted LIBS’s non-destructive and contactless inspection capabilities for assessing insulator surfaces, despite challenges related to detecting glass and ceramic insulators or inaccuracies due to surface state variations. These studies highlight the potential of LIBS to address basic operational features of SIRs, ranging from physical properties to aging and contamination levels, despite existing challenges and areas for improvement.

In this paper, we report on remote LIBS measurements on SIRs subjected to accelerated ageingand field-aged ones and investigated the correlation of the developed plasma temperature with the insulator hydrophobicity. By concurrently examining the hardness and optical properties of SIRs, our objective was to discern potential interconnection between these parameters. The motivation behind this research stemmed from our interest in examining the feasibility of utilizing LIBS as a remote monitoring tool for assessing hydrophobicity in SIRs. Our efforts were directed towards advancing methods for effectively and efficiently evaluating the hydrophobic performance of insulators in HV power systems, aiming at upgrading their reliability and longevity through proactive maintenance strategies.

2. Materials and Methods

2.1. Silicone Rubber Insulators (SIRs)

The basic structure of SΙRs consists of a load-bearing glass-fiber-reinforced epoxy rod (core), which is responsible for the tensile strength of the insulators, a surrounding SΙR housing, and metal end fittings at both ends (HV and ground) for the transmission of the mechanical load to the core (Figure 1). SΙR is an elastomeric polymer, poly-dimethylsiloxane (PDMS), with its backbone chain consisting of several thousand dimethylsilanoxy- monomeric units, linked together with covalent -Si-O- bonds, with each Si atom bearing two methyl (-CH₃) groups. Cross-linking (vulcanization) gives rise to formation of a chain network, responsible for the physical properties of the elastomer, as explained next. SΙR preserves its properties such as elasticity, extreme heat resistance, chemical stability, and hydrophobicity, the latter being provided by the methyl groups.

SIR is a highly adhesive gel or liquid and must be cured/vulcanized in order to become solid. In general, the manufacturing process of SIR housing includes mixing, shaping, and vulcanization [1]. Designing the geometrical characteristics of the housing in a shed-like shape (Figure 1) increases the creepage distance that is the shortest distance along the housing surface, between the metal ends of the insulator. Thus, this shape improves the performance of the insulators and facilitates the washout of pollutants by rain. Vulcanization uses a curing agent, which generates heat in order to cross-link the PDMS chains and may be performed at either high temperatures (High-Temperature Vulcanization, HTV) or at room temperature (Room-Temperature Vulcanization, RTV) depending on the preferable properties of the end product. According to the vulcanization method, the viscosity of the base polymer, and the temperature applied, SIR HTV insulators are classified into two main types: (a) solid SIR, vulcanized at approximately 200 °C, the cured rubber being enriched with fillers in order to reinforce their mechanical strength, and (b) LSR insulators, which are liquid SIR; vulcanized at 100–200 °C, characterized by lower viscosity than solid SIR, and are filler free.

Several polymer-based field-aged insulators have been removed from different locations of the 150 kV power transmission networks of Crete and Rhodes islands, Greece. The selection of insulators in Crete was based on the pollution mapping of the network as well as operation time in the field, whereas the insulators in Rhodes were removed all from the same location, and were covered with coal dust, attributed to wildfires. Information on two pristine reference insulators (No. 1 and 2) that have been manufactured using the LSR fabrication method, six field-aged insulators from Crete (No. 3 to 8), and one field-aged insulator (No. 9) from Rhodes that have been manufactured using the HTV fabrication method is given in

Table 1. Field-aged SIRs selected from the 150 kV transmission network of Greece.

Number	Type/Location	Site Pollution Severity (SPS) a	Operation Time (y)
1	LSR	N/A	0
2	LSR	N/A	0
3	HTV/Crete	Heavy	21
4	HTV/Crete	Medium	17
5	HTV/Crete	Heavy	10
6	HTV/Crete	Medium	17
7	HTV/Crete	Heavy	10
8	HTV/Crete	Heavy	8
9	HTV/Rhodes	Too heavy	11

^a Site’s pollution severity as determined in [24].

. In general, the SIR insulators examined herein were manufactured by several companies and may contain different fillers in a variety of concentrations. The insulators were 1.9 m in length and comprised 18 to 40 silicone rubber sheds. Shed diameters ranged from 100 to 150 mm, with both the number of sheds and their diameters varying among manufacturers (Figure 1). Unfortunately, the precise chemical composition of the insulators’ SIR housing material has not been disclosed by the manufacturing companies.

A number of sheds were removed from all three parts of the insulators along the polymeric housing (ground, middle, and HV as shown in Figure 1). Specimens were collected from the sheds by cutting them crosswise. Small, 2 × 2 cm² samples were extracted from various locations on each shed, with thicknesses ranging from 3 to 5 mm. The specimens were examined by LIBS both on their surface (upper and lower) and their cross-section (bulk). The specimens were not subjected to any cleaning prior to irradiation.

2.2. Accelerated Aging of SIRs

A number of sheds were removed from the reference SIR insulator (No 1) and were subjected to accelerated (artificial) aging in the laboratory, via the inclined plane test (IPT) method, according to IEC 60587 standard [25]. In general, this method evaluates the tracking and erosion resistance of polymeric insulators under contamination conditions. The experimental IPT arrangement, in this work, is described elsewhere [11]. Briefly, the sheds were mounted between two stainless steel electrodes (HV and ground) at an angle of 45 degrees from the horizontal. A contaminant NaCl solution (2.53 mS/cm conductivity) was allowed to flow over each shed surface with a constant flow rate of 36 mL/h from the high voltage towards the ground electrode side. A test power frequency voltage of 4.5 kV was applied to each shed for 1 to 6 h, respectively. Moreover, a 33 kΩ resistor was connected in series with each shed at the HV side. During the tests, dry band arcing [26] took place on the surface of the shed, mainly in the area closer to the ground electrodes, and leakage current was measured through a shunt resistor, connected in series with the ground electrodes. Current waveforms were recorded, and the leakage charge Q and energy E of the discharges for each shed can be calculated using the equations:

$$Q={\int }_0^t{I}_{rms}dt$$

(1)

and

$$E={\int }_0^t\left(V_{rms}-V_{res}\right)I_{rms}dt$$

$$={\int }_0^t\left(4500-33000I_{rms}\right)I_{rms}dt$$

(2)

where I_rms represents the average leakage current, V_rms is the applied voltage, V_res is the voltage of the 33 kΩ resistance of the circuit, and t is the testing time. Plots of the average charge and energy of the dry band arcs as a function of testing time (Figure 2a) indicate that, indeed, the properties of the insulators degrad with increasing testing time.

2.3. Hydrophobicity Measurements

Static contact angle measurements of the artificially- and field-aged SIR sheds were performed according to the IEC-62073 standard [16], which includes spray and contact angle methods. The spray method is based on the appearance of the insulator surface after water mist exposure and wettability classes are determined. Contact angle measurements were performed using the CAM 101 instrument (KSV Instruments Ltd., Espoo, Finland). Water droplets, almost 10 μL in volume, were applied on all (field-aged and artificially-aged) insulator sheds, and average static contact angles were recorded. It should be noted that measurements were performed without previously removing surface pollutants of the field-aged insulator specimens.

The static contact angle measurements were conducted on both the upper and lower surfaces of the sheds for the accelerated SIR sheds. However, for the field-aged samples, these measurements were extended to both surfaces as well as to the cross-section of the sheds to examine the bulk material of the insulator.

Figure 2b illustrates a consistent decrease in the contact angle measurement over the accelerated aging process. This observation validates the effectiveness of the method in simulating aging processes and reducing the hydrophobicity of SIR.

2.4. Remote LIBS Measurements

In Figure 3, the experimental LIBS setup is depicted. The laser beam from a Q-switched Nd: YAG laser (λ = 1064 nm; τ_pulse = 10 ns; BM Industries, Series 5000) passed through a beam expander, before being focused by a plano-convex lens (0.05 m in diameter, F = +5 m) on the insulator specimen surface (target) resulting in plasma formation. The beam expander consisted of a two-lens system, an image plano-concave lens (0.013 m in diameter, f₁ = −0.150 m) and an objective plano-convex lens (0.025 m in diameter, f₂ = +1.0 m). The distance between the image and the objective lens was approximately 0.850 m and the magnification, defined as the ratio f₀/f₁, was almost 7×. The plasma formed on the target surface had a conical shape of approximately 2 mm in length, as estimated with the naked eye. The light emitted by the plasma plume was collected by a parabolic mirror (300 mm in diameter) located at approximately 8 m distance from the insulator target. An optical fiber (600 μm in diameter, 2 m in length) was positioned and aligned at the focus (f = +1.1 m) of the collecting mirror, transmitting light into a portable spectrometer (Avaspec-2048-2-USB2, Avantes). The spectrometer is equipped with a diffraction grating (1200 gr/mm, spectral resolution in the range of 0.2–0.3 nm), and a CCD detector for the recording of the LIBS spectra (in the 195.1–465.8 nm wavelength range), with a minimum time delay (τ_DELAY = 1.28 μs, set by default by the manufacturer) with respect to the arrival of the laser pulse on the target. The spectrometer is triggered by the laser Q-switch TTL pulse, which precedes the laser pulse by almost 120 ns. Typical integration time was adjusted to the minimum value (τ_GATE = 1.050 ms). All LIBS spectra have been corrected according to the spectrometer’s grating diffraction efficiency and wavelength sensitivity with the use of a calibrated light source (Deuterium Halogen lamp, AvaLight-DH-S, Avantes). The laser beam irradiation area on the target had an elliptical shape, with characteristic dimensions of approximately 300 μm and 700 μm axes length, and the laser fluence (F_LASER), employed, was almost 35 J/cm². It should be noted that specimens were not subjected to any cleaning prior to irradiation.

A single LIBS measurement involves recording 15 single-shot consecutive spectra upon laser irradiation at a single spot. Based on previous studies, the first 5 spectra are ignored and only the last 10 spectra are analyzed [18]. Six measurements (m = 6) were performed at adjacent spots for each area of interest (upper/lower surface and bulk) on all insulator specimens resulting in a total of 180 (3 × 6 × 10) single-shot spectra for each insulator. The spectral analysis concerns the calculation of the mean integrated intensity of a selected emission line, for each measurement, calculated as $I_{mean}={\displaystyle \frac{1}{m}}\sum _{j=1}^N{I}_j$, where Ij is the mean integrated emission intensity corresponding to the j-th measurement (j is any integer number between 1 and m) and $I={\displaystyle \frac{1}{s}}\sum _{k=6}^{15}{p}_k$, where p_k is the integrated intensity of the selected emission line for every single-shot spectrum recorded in a series of s = 10 consecutive single-shot spectra. All calculations were performed automatically using a homemade program in GNU Octave language. At the end, the program outputs three sets of values (I_mean, standard deviation) corresponding to the upper surface, the lower surface, and the bulk of the specimens.

2.5. Diffuse Reflectance and Shore Hardness Measurements

A portable spectrometer custom-made at IESL-FORTH was used to acquire diffuse reflectance spectra in the 1060–1070 nm spectral range [27]. Briefly, this device employs an external tungsten-halogen lamp (Osram Decostar 35, 10 W), placed at the front of the spectrometer optical head, to illuminate the object under examination. The diffuse reflectance light from the object’s surface is collected through the optical detection line and directed into the spectrometer via an optical fiber. To ensure a broader spectral coverage, a low-resolution spectrometer is employed, operating from 200 to 1100 nm with a resolution of approximately 1.4 nm (Avaspec-2048L-USB2, Avantes). The calibration of the y-axis was conducted using a Spectralon^® diffuse reflectance standard, known to display very high and uniform reflectance across the visible and near-infrared.

A homemade Shore durometer was built based on ASTM D2240 standard [28] that measures the depth of an indentation in the material created by a given force on a standardized presser. Two different durometer scales (type A and D) have been used to measure shore hardness of the SIRs.

3. Results and Discussion

The plasma temperature during LIBS measurements of SIRs can be calculated from the emission of Si lines or the CN molecular band, both present in the PDMS polymer LIBS spectra. While plasma temperature can also be determined using emission lines from filler elements such as Al and Ti, this approach is not universally applicable, as different manufacturers use varying types of fillers. Although recent advances have improved the calculation of plasma temperature from CN emission [29,30], the intensity of the CN band in our measurements was weak. Consequently, we opted to calculate the plasma temperature using the more reliable Si emission lines. The plasma temperature was determined using the Boltzmann plot method [31], employing Si emission lines from the recorded LIBS spectra, using the equation:

$$\ln \left({\displaystyle \frac{I_{Si}{\mathsf{\lambda }}_{Si}}{{\mathrm{A}}_{Si}{\mathrm{g}}_{Si}}}\right)=-{\displaystyle \frac{{\mathrm{E}}_{Si}}{kT}}+\ln \left({\displaystyle \frac{hc{N}_{Si}}{4\pi U_{Si}\left(T\right)}}\right)$$

(3)

where I_Si is the intensity of Si I lines, λ_Si is the center wavelength, A_Si is the probability of the transition (Einstein coefficient), g_Si is the degeneracy of the upper state of the transition, and E_Si is the energy of the upper state; U_Si(T) is the Si partition function; k, h, and c represent the Boltzmann constant, the Planck constant, and the speed of light, respectively; and N_Si is the atomic density of Si in the plasma. The plasma excitation temperature can be calculated, by the slope of the Boltzmann plot which is equal to −1/kT (see Figure 5).

Although the LIBS spectra present several Si emission lines, many are unsuitable for temperature calculation due to self-absorption effects, resulting in emission intensity decrease. Moreover, several emission peaks overlap due to the low resolution of the spectrometer. Therefore, we carefully selected three distinct emissions with clear spectral peaks (Figure 4) and listed their spectroscopic properties of the transitions in

Table 2. Spectroscopic parameters of Si I emission lines used for plasma temperature calculation [32].

λ (nm)	Elower (cm−1) a	Eupper (cm−1) b	A (s−1) c	Lower–Upper Level Configuration (Term)	g d
263.12	15,394.4	53,387.3	1.06 × 108	3s2 3p2 (1S)–3s2 3p3d (1P0)	3
298.85	6298.8	39,760.3	2.66 × 106	3s2 3p2 (1D3)–3s2 3p4s (3P0)	3
390.58	15,394.4	40,991.9	1.33 × 107	3s2 3p2 (1S)–3s2 3p4s (1P0)	3

^a The energy of the lower state of the transition; ^b the energy of the upper state of the transition; ^c transition probability for spontaneous emission from the upper to the lower energy state (Einstein’s coefficient); ^d degeneracy of the upper energy state.

(a comprehensive description of the line selection methodology is provided in the Supplementary Materials). Two of the selected lines exhibit a high energy level in the lower state of the transition (E_lower), experiencing minimal self-absorption, due to the low population in the lower state. The third line, while featuring a lower energy level in the lower state, exhibits a low Einstein coefficient, reducing its probability of absorbing light. A typical Boltzmann plot diagram is presented in Figure 5. The calculated plasma temperature values had a range from 7000 to 7700 K approximately.

The mean plasma temperature calculated from the LIBS spectra corresponding to the accelerated-aging series is plotted as a function of aging time in Figure 6. A distinct increase in plasma temperature over the duration of accelerated aging is observed. The fitting R-square value of 0.71 highlights a robust and consistent correlation between plasma temperature and accelerated aging time within the linear regression model. This result indicates that the plasma temperature is affected by the extent of hydrophobicity exhibited by the SIR insulator. Therefore, plasma temperature may be used as a measure of the hydrophobicity property of SIRs.

Similarly, we have investigated the connection between plasma temperature and the operational age or the contact angle measurement results of field-aged SIRs. Due to the different insulator sources and the resulting variations in their chemical composition, we conducted a comparative study of the plasma temperature and the relative contact angle expressed by the equation:

$${\theta }_{rel}=1-{\displaystyle \frac{{\theta }_{surface}}{{\theta }_{bulk}}}$$

(4)

where θ_surface and θ_bulk are the static contact angles measured on the surface and the bulk material, respectively, of the examined SIR sheds.

As seen in Figure 7a, a strong correlation between the plasma temperature and the operational years of field-aged SIRs is observed. This result highlights the significance of our study, as it potentially serves as an indicator tool for monitoring insulator performance and aging in HV systems. Such promising insights could pave the way for enhanced strategies in insulator design, maintenance, and performance optimization within HV contexts, ultimately contributing to improved reliability and safety in power systems.

Furthermore, a solid correlation emerges between plasma temperature and the relative contact angle of field-aged SIRs (Figure 7b). This observation is particularly significant since the relative contact angle serves as a crucial indicator of the hydrophobicity properties of the insulating material. Therefore, this result strongly suggests that remote LIBS analysis can actually monitor the hydrophobicity state of SIRs in-service, since it correlates well with an already established method, which is performed in the laboratory. Moreover, these data strongly support our proposal that fluctuations in plasma temperature correspond to observable alterations in the relative contact angle, signifying important surface characteristic changes. Further exploration of this correlation could unlock valuable insights into the impact of climatic conditions on insulator performance and aging, especially within HV applications, primarily directed toward enhancing electrical systems’ reliability and safety.

Such a consistent correlation between plasma temperature and the hydrophobicity of silicone rubber could possibly be attributed to several underlying factors/parameters, such as surface tension and optical properties of SIR, which may affect the interaction of the material with the laser pulse. Changes in the absorption coefficient due to the aging of silicone rubber, as well as other material properties such as energy bandgap and free electrons density, all play a significant role in the mechanisms of material absorption and the conversion of laser energy into thermal energy. These properties cause materials to respond differently to laser irradiation, adding to the complexity of the observed correlation. Moreover, in several cases, plasma temperature dependence on the changes in the hardness of the material has been reported [33,34,35,36]. Wang et al. studied the hardness of SIRs used in a 500 kV power grid in China [22], and they found a linear relation between the microhardness and the plasma temperature.

In order to monitor possible modifications in the SIR absorption properties induced throughout the accelerated aging test duration, diffuse reflectance values were recorded across the wavelength range of the laser beam from 1060 to 1070 nm, for both the upper and lower surfaces of the SIR sheds. Such changes may affect laser absorption and plasma temperature. Indeed, the diffuse reflectance values on the upper side are higher than the ones on the lower side of the insulator’s sheds (Figure 8a). However, in both cases, the diffuse reflectance values exhibit only minor variations within the measurement precision, indicating that the absorption properties of SIR remain constant throughout the accelerated aging process. This suggests that increased laser absorption does not influence plasma temperature.

Likewise, diffuse reflectance values were recorded for the sheds extracted from the field-aged SIRs deployed in the HV grid (Figure 8b). Clearly, the diffuse reflectance values do not exhibit a distinct pattern of variation. These results strongly indicate that the plasma temperature is unaffected by differences in the absorption of the material at the laser wavelength.

The emission lines observed in LIBS spectra have been associated with the hardness of non-metallic materials [37,38,39,40]. To explore potential correlations between the hardness of the silicone rubber with the calculated plasma temperature, the hardness of the artificially-aged SIR sheds was measured using the ASTM D2240 type A standard. Wang et al. revealed a positive correlation between plasma temperature and the hardness of sheds, extracted from SIRs operating in the 500 kV grid in China [22]. Contrary to their work, our measurements, presented in Figure 9a, fail to verify their observation, indicating the absence of correlation of the plasma temperature with the hardness of artificially-aged SIR sheds. This variance may be attributed to the fact that in this study we compared SIRs produced from various manufacturers having different filler compositions, while in their work Wang et al. studied SIRs from a single manufacturer. Additionally, differences in climate conditions between China and the Mediterranean region and insulator strain from the differences in the applied voltage (150 kV in Greece, 500 kV in China) may have affected the aging of the insulators producing this discrepancy.

Moreover, we estimated the hardness of the SIRs deployed in the HV grid. These insulator sheds exhibited greater hardness than the artificially-aged SIRs. Indeed they were fabricated by the HTV method, unlike the artificially-aged insulator, which was fabricated by the LSR method, and in fact showed a lower hardness (Table 1). Moreover, environmental factors such as temperature fluctuations and UV radiation might have contributed to the increased hardness of the material. Therefore, the ASTM D2240 Type D standard [28] was selected for measuring the hardness of the field-aged insulators, since Type A standard is inappropriate for harder materials. The outcome (Figure 9b) indicates a positive correlation of the hardness with the plasma temperature. However, this might be related to the differences in the material of the unused SIRs sheds (red dots) that have been manufactured using the LSR fabrication method, compared to the HTV-fabricated SIRs used in the grid (black dots) (Table 1). Excluding the LSR sheds, the positive correlation vanishes and a negative correlation emerges. Thus, it is difficult to derive consistent conclusions related to the correlation between plasma temperature and hardness.

Furthermore, we have explored the relationship between relative contact angle measurements and hardness. The outcome (Figure 9c) indicates a positive correlation of the hardness with the relative contact angle. However, excluding the LSR shed, the positive correlation vanishes, resulting in the absence of any correlation.

While a more detailed investigation of SIR material hardness is required to draw definitive conclusions, our findings support the argument that plasma temperature remains independent of material hardness. A reduction in material hydrophobicity signifies an increase in the number of polar groups within its molecular structure, which can influence the material’s absorption of laser light. Molecules with high polarity are more adept at interacting with the electromagnetic field of the laser. Moreover, the polymer’s hydrophobicity is known to be highly proportional to the cohesive energy density (defined as the potential energy per unit volume of a volatile or a non-volatile material) [41,42]. Both quantities potentially can enhance laser absorption and excitation processes leading to higher plasma temperatures, suggesting potential for a direct theoretical investigation into this correlation.

4. Conclusions

In this study, we examined the interplay between plasma temperature, as observed in remote laser-induced breakdown spectroscopy (LIBS) experiments, and the hydrophobic properties of silicone rubber insulators. Our findings unveiled a distinct correlation between plasma temperature and the aging process of silicone rubber insulators, particularly evident in accelerated aging tests conducted on SIR sheds. Notably, the plasma temperature consistently correlates positively with the duration of the aging tests, indicating its potential as a marker for insulator aging. To assess the hydrophobicity of silicone rubber insulators subjected to accelerated aging, contact angle experiments were performed, revealing a robust correlation with test duration. Furthermore, our investigation was extended to the examination of nine silicone rubber field-aged insulators operated in 150 kV high-voltage transmission lines of Crete with operational time ranging from 0 to 21 years. Additionally, we explored the interconnection between laser absorption, material hardness, and plasma temperature, yet no robust correlations were established. In summary, our study highlights a straightforward correspondence between plasma temperature and both contact angle measurements and operational time of in-service insulators, suggesting the feasibility of remotely evaluating the hydrophobicity of silicone rubber insulator hydrophobicity.

The measurement of plasma temperature from LIBS spectra suggests a potential for remote sensing applications to assess hydrophobic or hydrophilic properties of materials across diverse fields. While plasma temperature alone may not serve as a universal criterion for evaluating hydrophobicity in silicone rubber insulators, incorporating additional material properties, such as hardness and environmental factors, could improve its reliability. Machine learning techniques offer a promising approach to analyzing complex relationships between these properties and hydrophobicity, enhancing the accuracy of plasma temperature as an indicator of material aging. By integrating such methods, remote evaluation of hydrophobicity and the aging process in insulators can be made more robust and effective, allowing for better long-term performance monitoring.

Moreover, its application to high-voltage electric grids presents significant challenges. Key parameters of the LIBS instrument must be carefully studied and optimized, with a primary focus on preventing damage to the SIR material by the laser pulses during operation. To improve the field of view, incorporating large-diameter optics is essential, requiring an optimized optical collection and laser optics setup. Additionally, the equipment must be compact, stable, and resistant to external vibrations for effective use in the field. A mechanically guided aiming system is crucial to ensure accurate and consistent alignment, particularly when shifting between measurement points. The influence of the collection angle on the efficiency of remote LIBS measurements must also be evaluated, with empirical testing needed to determine the optimal angle for overhead power lines, considering variables such as instrument-to-insulator distance, line voltage, and environmental conditions. To enable rapid assessments by non-expert personnel, the development of automated LIBS spectra evaluation software is critical. Such software would provide direct assessments of insulator conditions, increasing the accessibility and practicality of remote LIBS for a broader range of users.

Nevertheless, employing plasma temperature as an indicator could enable rapid and non-destructive evaluation of surface characteristics, offering insights into material behavior and performance. This potential can be further enhanced by exploiting recent advances in plasma temperature calculations [43]. Such applications could cover industries ranging from electronics and aerospace to biomedical and environmental monitoring, fostering enhanced material selection, quality control, and process optimization.