Abstract
The objective of this study is to explore the impact of a nanofiller, hexagonal boron nitride (h-BN), on the main physical, electrical, and thermal characteristics of unsaturated polyester (UPE) resin. To obtain a homogeneous dispersion, h-BN nanoparticles were surface-modified using 3-glycidoxypropyltrimethoxysilane to give S/h-BN nanoparticles. UPE-S/h-BN composites were prepared by using various ratios (1, 5, 10 wt%) of these modified nanoparticles. Thermogravimetric analysis studies showed that the presence of S/h-BN nanoparticles boosted the thermal stability of the UPE resin. The electrical volume resistivity value increased from 1.3 × 1013 to 1.38 × 1014 Ω cm with the addition of 10 wt% S/h-BN. The contact angle results indicated that the hydrophobicity of UPE-S/h-BN composites increased and the value of 110° was obtained for UPE-S/h-BN10. The findings revealed that incorporating S/h-BN nanoparticles into UPE resin, in specific ratios, improved its properties and the resulting product has the potential to be used as an insulation varnish.
1 Introduction
In recent years, polymer nanocomposites have attracted significant interest from researchers due to their unique capacity to manipulate thermal, mechanical, electrical, and optical characteristics (1). Numerous industries, including automotive, sensors, medical devices, injection molding, membranes, coatings, adhesives, refractory additives, aerospace, packaging and consumer goods, and pharmaceutical distribution, extensively use this emerging class of materials (2,3). Among these industries, the automotive industry widely uses these lightweight polymer nanocomposites in interior parts, fuel system components, tires, and electrical components due to their improved properties (4). Electrical equipment such as motors, transformers, generators, and power electronics need to be coated with superior materials to provide electrical insulation, prevent current leakage from these equipment, protect them against corrosion, ensure mechanical protection, enhance thermal performance, reduce electrical noise, ensure safety, and extend their life (5). Insulation varnishes, also known as impregnating varnishes or insulating resins, can be used to coat and impregnate electrical components.
Thermoset polymer materials can be utilized in various application areas due to their superior electrical insulating qualities, despite their low intrinsic thermal conductivity. Unsaturated polyester (UPE) resins are commonly used thermoset resins in insulation varnishes due to their cost-effectiveness, low density, excellent corrosion resistance, improved thermal and mechanical strength, electrical insulation, and ease of processing (6). UPE resins are formed through a traditional esterification reaction between a saturated dicarboxylic acid (aromatic or aliphatic type) and an unsaturated monomer (e.g., maleic anhydride) in the presence of a reactive diluent, known as a co-monomer. During the esterification process, high temperatures are used, and an azeotropic distillation solvent is employed to eliminate the water generated (7). This reaction results in a long-chain polymer with unsaturated bonds along its backbone, which can further crosslink when cured with radicals from a chemical or photoinitiator, heat, or light (8).
Based on their molecular structures, UPE resins are classified into several groups such as anhydrides of isophthalic, terephthalic, orthophthalic and chlorendic, and bisphenol-fumarate. The majority of these resins contain anhydrides as well as co-monomers such as styrene, ethylene glycol, toluene, maleic anhydride, and other additives. Thus, the choice of monomer components can have a major impact on the final characteristics of UPE resins (9). To enhance the flexibility and impact characteristics, fatty acids with aliphatic tails consisting of long carbon chains like sebacic acid and adipic acid are generally used. The two most common types of geometric isomers, fumaric acid, and maleic acid both containing a ring structure are utilized to improve the thermal properties or corrosion performance of the UPE resin. To further improve the flexibility and air-drying properties of UPE resins, tetrahydrophthalic anhydride can also be used in the production of unsaturated polyester resins (10).
Despite being the most common thermoset resin, the utilization of UPE resin is restricted in many fields due to less impact properties with average thermal characteristics. Various additives or fillers can be used in unsaturated polyester resin to enhance its overall properties, such as mechanical, thermal, electrical, barrier, and flammability properties (11–15). In previous studies, various fillers, such as nanosilica, titanium dioxide, fumed nanosilica, zinc oxide, montmorillonite in polyester varnish (16), boron nitride (BN) and a silane coupling agent (17), and micro silica were used (18). Because of their superior qualities compared to neat polymers, nanoparticle-reinforced polymer resins have emerged as significant possibilities for a variety of applications (19). Also, these fillers can tailor the inherent properties and impose new characteristics on the UPE resin to give a new high-performance material with better applicability.
In a study conducted by Wang et al., nano-silica was modified by using a silane coupling agent KH570 and was used to modify the UPE resin. The results revealed that when the high dispersibility nanosilica powder was added into the UPE resin matrix with a mass fraction ratio of 1.5%, mechanical properties were greatly enhanced and an improvement of 9.6% was obtained in the impact strength of nano-silica/UPE resin polymer composite (20). Sharma et al. have prepared silica-UPE resin composites by adding nano/micro-silica into UPE resin and evaluated the electrical properties of composites. They compared the results of the surface, volume resistivity, dielectric strength, dissipation factor, and dry arc resistivity of nano- and microcomposites and found that the addition of nano-silica greatly enhances the electrical properties as compared to micro-silica (18). Swain et al. used two different chemically modified carbon nanotubes (CNT), allyl ester functionalized carbon nanotubes (ACNT) and silane functionalized carbon nanotubes (SCNT), to modify the UPE resin. The results demonstrated that the addition of ACNT/SCNT to the UPE resin matrix resulted in a great enhancement in the electrical performance with respect to surface and volume resistivity, dielectric strength, and dry arc resistance (21). Calabrese et al. compared the thermal and electrical properties of three commercial unsaturated polyester imide resins and demonstrated the best resin that could better perform the insulation function of electric motors (22).
BN is favored in polymer composites due to its superior properties such as its thermal stability, oxidation resistance, thermal conductivity, electrical/dielectric characteristics, low toxicity, and cost-effectiveness (17,23,24,25,26). BN, composed of boron and nitrogen atoms, exists in different crystalline forms, with the two most common ones being hexagonal boron nitride (h-BN) and cubic BN (c-BN). h-BN, a ceramic material, is also known as “white graphite” due to its layered crystal structure, similar to that of graphite, and has high thermal conductivity, inertness, and tribological properties that render it interesting as a lubricant and high-temperature material. The direct addition of h-BN to the polymer matrix can lead to an increase in the overall material properties as well as various difficulties and limitations such as poor dispersion, poor interfacial adhesion, reduced mechanical properties, and some processing difficulties. To address these challenges, various strategies were employed to improve the dispersion and interfacial adhesion of h-BN in the polymer matrix. Surface modification of BN with different coupling agents enhances its compatibility with various matrices, improves the dispersion and interfacial adhesion within the composite, and imparts desirable properties, making it a versatile and valuable material for a wide range of industrial applications (27). Silane coupling agents are widely employed for filler treatment in the fabrication of polymer composites (28). According to research findings, silane coupling agents significantly impact the thermal conductivity of these composites (29). At low silane content, the interfacial adhesion strength improves with increasing content. However, when the silane coating becomes excessively thick, the effectiveness of the interface diminishes, and it may even act as a barrier for heat conduction (23,24,25).
In our previous study, we prepared h-BN/UPE composites without any surface modification of nanoparticles and we could not obtain a homogenous dispersion before and after the crosslinking and the formation of separate filler phases (30). In this study, we aimed to design an insulation varnish material to be used in electrical equipment in the automotive industry. For this reason, we added various amounts of silane surface-modified h-BN nanoparticles (S/h-BN) to the UPE resin to obtain a high-quality composite material. To the best of our knowledge, no literature work has been found on the usage of modified h-BN nanoparticles as filler for the UPE resin. We have also examined the structural, physical, and thermal properties of both neat UPE resin and S/h-BN-containing composite UPE resin by various characterization methods.
2 Materials and methods
2.1 Materials
UPE resin is a product of Betek Boya ve Kimya San, A.S., Türkiye. Table 1 lists the properties of the neat UPE resin. Nanohexagonal BN powder (specific surface area of 29.5 m2·g−1 and particle size of 80–100 nm) was obtained from BORTEK Bor Teknolojileri Ltd. Sti. The silane coupling agent, 3-glycidoxypropyltrimethoxysilane (97%, GENIOSIL® GF 80), was obtained from Wacker.
Typical properties of UPE resin
| Properties | Value |
|---|---|
| Percentage solid content (%) | 84–85 |
| Viscosity (cP) | 1,030 |
| Thermal conductivity (W·mK−1, 23°C) | 0.182 |
| Flashpoint (°C) | 49.4 |
2.2 Surface modification of h-BN nanoparticles
First, 50 g of h-BN nanoparticles were surface-modified with 3-glycidoxypropyltrimethoxysilane (1.5% of the total h-BN mass) by adding them to 750 mL of 95% weight (wt.) aqueous ethanol solution containing predetermined amount of silane coupling reagent. The resulting mixture was heated to 80°C while stirring and refluxed for 6 h. After cooling to room temperature, the mixture was washed with ethanol at least three times. Finally, the h-BN nanoparticles were dried in an oven at 120°C for 12 h (31).
2.3 Preparation of modified composite samples
S/h-BN nanoparticles (1%, 5%, and 10 wt% of UPE resin) were incorporated into the UPE resin and stirred continuously (with a mechanical stirrer) at 1,200 rpm until a homogeneous appearance was achieved (at approximately 30 min). The viscosity and solid content of all samples were determined. The dispersions were transferred into Teflon Petri dishes and followed by a thermally crosslinking reaction in an oven cured at 200°C for 4 h. The film samples were collected, and all additional characterization procedures were performed (Figure 1). The observations shown in Figure 1 clearly illustrate that an increase in the h-BN ratio is directly correlated with the emergence of bubbles. Note that these bubble formations were meticulously considered and factored in during the compilation and interpretation of the test outcomes.
Digital photos of (a) UPE-S/h-BN0, (b) UPE-S/h-BN1, (c) UPE-S/h-BN5 and (d) UPE-S/h-BN10 composite samples after curing.
2.4 Characterization
The viscosity was determined using a Brookfield viscometer with a No. 3 spindle rotating at 20 rpm at 25°C. The percentage solid content of the samples was calculated according to the ASTM D 115 standard, in which preweighed samples (1.2–1.5 g) were put into special cups and placed in an oven at 135°C for 3 h. For structural evaluation of the prepared cured samples, a Fourier transform infrared (FTIR) spectrometer equipped with an attenuated total reflectance apparatus (Shimadzu Corp. Iraffinity-1S FTIR Spectrometer) was used. The spectra were collected over a scan range of 600–4,000 cm−1 with an average of 16 scans. To examine the thermal degradation phenomenon of the cured UPE resin samples, a thermogravimetric analyzer (Perkin Elmer, TGA 4000) was used. The measurements were carried out between 30°C and 900°C with a heating speed of 10°C·min−1 under a nitrogen atmosphere. The scanning electron microscope (SEM; Quanta 400F field emission SEM) was used to examine the surface morphology of the UPE/S/h-BN composite samples. The gold-palladium coated samples (a thickness of 3 nm) were chopped into 1 cm × 1 cm squares. Thermal constant analyzer-hot discs were used to estimate the thermal conductivity of the UPE/S/h-BN composite samples. The measurement parameters were a temperature of 100°C, a measurement time of 80 s, and an output power of 25 mW; an isotropic technique was used. The contact angle measurements of all composite samples were made with the contact angle analyzer (data physics contact angle system, OCA). About 1 mL of distilled water was dropped onto the surface of the film samples using an automatic micrometer syringe. Values were taken as the average value of five individual measurements taken from different locations of each film surface. Volumetric resistivity and surface resistivity values were measured using Keithley 6517B and Keithley 8009 instruments. The average thickness measurements of the samples (Figure 1) were calculated by taking measurements from ten different regions of each sample with an Asimeto brand thickness gauge.
3 Results and discussion
3.1 FTIR analysis
A comparison of the FTIR spectra of unmodified h-BN and silane-modified h-BN nanoparticles is depicted in Figure 2. In the FTIR spectrum of unmodified h-BN, two broad peaks were observed at around 1,336 and 765 cm−1, which were attributed to the in-plane stretching vibration and the out-of-plane bending vibration of hexagonal BN, respectively. However, some significant changes are observed when the spectrum of unmodified h-BN is compared with modified h-BN. As shown in Figure 2, the disappearance of the absorption band observed between 3,000 and 3,500 cm−1 in the FTIR spectrum of h-BN in the S/h-BN band can be attributed to the valence vibration of the −OH group of the S/h-BN nanoparticles, suggesting that a reaction has occurred after surface modification of h-BN with 3-glycidoxypropyltrimethoxysilane. Furthermore, the B–N bond vibration in the modified h-BN shows a frequency shift from 1,336 cm−1 (in unmodified h-BN) to 1,323 cm−1. The shoulder of B–N bond vibration at 1,336 and 765 cm−1 weakens (32). Based on the above results, it can be inferred that the silane coupling agent effectively reacts with the h-BN particle surface, leading to the modified h-BN nanoparticles. Figure 3 presents the IR spectrum of the unmodified UPE and modified samples. This comparison could be to identify any differences in the spectra, which could provide insights into the structural changes caused by the modification process. These peaks are essential for identifying and characterizing the chemical structure of modified samples, as they correspond to specific types of bond vibrations. The stretching vibration absorption peaks of Si–O are observed at 785 and 1,155 cm−1. As shown in Figure 3, as the amount of h-BN increases, the peak intensity also increases. The stretching vibration absorption peak of Si–O–H is observed at 650 cm−1, and that of O–H at 3,450 cm−1. Analyzing these peaks allows researchers to confirm the presence of key functional groups and better understand the molecular arrangement in unmodified samples (20).
FTIR spectra of S/h-BN and h-BN nanoparticles.
FTIR spectra of UPE-S/h-BN0, UPE-S/h-BN1, UPE-S/h-BN5, and UPE-S/h-BN10 composites.
3.2 Percentage solid content and viscosity measurement
The physical characterization of all UPE/S/h-BN composite samples was carried out by determining their percentage solid content and viscosity. The summary of the measurement results is given in Table 2. It was clear that the addition of S/h-BN nanoparticles increased the percentage of solid content and viscosity values, probably due to the higher co-condensation degree of the resins. When compared to the UPE/S/h-BN0 sample without any nanoparticles, viscosity values of 1,030, 1,190, 2,050, and 5,180 cP were obtained for UPE-S/h-BN0, UPE-S/h-BN1, UPE-S/h-BN5, and UPE-S/h-BN10, which received increasing amounts of S/h-BN, respectively.
Composition, percentage solid content, and viscosity of UPE-S/h-BN composites
| Sample code | S/h-BN content (wt%) | Solid content (%) | Viscosity (cP) |
|---|---|---|---|
| UPE-S/h-BN0 | 0 | 85.00 | 1,030 |
| UPE-S/h-BN1 | 1 | 87.13 | 1,190 |
| UPE-S/h-BN5 | 5 | 91.20 | 2,050 |
| UPE-S/h-BN10 | 10 | 93.61 | 5,180 |
3.3 TGA
The thermal degradation of the h-BN and S/h-BN nanoparticles, neat UPE resin, and its S/h-BN-containing composites was investigated by TGA. Figures 4 and 5 show the TGA thermograms and the correlative data, and the temperature of 5 weight loss (T d5), the temperature of 50% weight loss (T dmax,50), and the char residue percentage at 900°C are presented in Table 3.
TGA thermograms of UPE-S/h-BN composites.
TGA thermograms of S/h-BN and h-BN nanoparticles.
Thermal decomposition data of UPE-S/h-BN composites
| Sample | T d5 (°C) | T dmax,%50 (°C) | Residue at 900°C (%) |
|---|---|---|---|
| UPE-S/h-BN0 | 292.67 | 440.83 | 2.02 |
| UPE-S/h-BN1 | 286.17 | 440.33 | 3.20 |
| UPE-S/h-BN5 | 292.83 | 446.00 | 6.79 |
| UPE-S/h-BN10 | 275.33 | 443.00 | 9.88 |
| h-BN | — | — | 98.38 |
| S/h-BN | — | — | 97.81 |
As seen in Figure 4, a typical thermal decomposition behavior was observed for h-BN nanoparticles as no significant weight reduction was detected until a temperature of 900°C, and a weight loss of 1.69 wt% was obtained at this temperature, due to humidity loss. Compared with h-BN, the TGA thermogram of silane-modified h-BN nanoparticles showed a weight loss of 2.19 wt% caused by the decomposition of organic molecules used in the modification reaction on the surface of S/h-BN nanoparticles. As seen in the TGA thermograms given in Figure 5, all samples exhibited a single-step decomposition process. The initial decomposition temperature of the neat UPE resin (UPE/S/h-BN0) was recorded at 292.67°C. In comparison, the h-BN containing composite samples displayed slightly different initial decomposition temperatures as 286.17°C, 292.83°C, and 275.33°C for UPE-S/h-BN1, UPE-S/h-BN5, and UPE/S/h-BN10, respectively. It was clearly observed that the addition of h-BN nanoparticles had a positive influence on the maximum decomposition temperature (T dmax,50%) of the composites. The neat UPE resin exhibited a T dmax,50% of 440°C, while the highest values were achieved in the composites with UPE-S/h-BN10 (443°C) and UPE-S/h-BN5 (446°C). As previously stated in the literature (16,17), the integration of BN nanoparticles in polymer composites is advantageous due to the improved thermal conductivity and stability features. Table 3 also demonstrates the char residue for neat UPE resin at 900°C is 2.02% and it increased proportionally with increasing amounts of h-BN, suggesting that the addition of h-BN endowed the polymer matrix more thermally stable.
3.4 SEM analysis
SEM analysis was used to examine the surfaces of the fabricated UPE-S/h-BN composite samples. Figure 6(a) shows the SEM micrograph of the UPE-S/h-BN0 sample and examination of this micrograph indicates that the surface of the neat UPE resin is smooth. With the addition of 1% S/h-BN, it was seen that the surface of the composite completely changed, and a rough surface was obtained (Figure 6(b)). This difference can be assigned to the active role of added S/h-BN nanoparticles in the curing of UPE resin. The increase in the amount of S/h-BN nanoparticles led to the formation of separate two phases, the UPE resin phase and nanoparticles phase, and homogenous distribution cannot be obtained as shown in Figure 6(c) and (d). Compared to the neat UPE sample, some agglomerations were observed in the SEM micrographs of UPE-S/h-BN5 and UPE-S/h-BN10 samples. Also, as shown in Figure 1, the visual appearance of cured UPE-S/h-BN5 and UPE-S/h-BN10 samples showed a void structure created by air bubbles during crosslinking.
SEM micrographs of (a) UPE-S/h-BN0, (b) UPE-S/h-BN1, (c) UPE-S/h-BN5, and (d) UPE-S/h-BN10.
3.5 Thermal conductivity
The thermal conductivity results of the neat UPE resin and UPE-S/h-BN composites are given in Table 4. As given, the thermal conductivity constant of the neat UPE resin was found to be 0.182 W·mK−1. With the addition of 1% by weight of S/h-BN to the UPE resin matrix, a thermal conductivity value of 0.188 W·mK−1 was obtained, which is higher than the neat UPE resin. h‐BN, which has a two-dimensional honeycomb structure similar to graphene, has a thermal conductivity value of 360 W·mK−1 and is known as an excellent heat-conducting additive (33,34,35). Thus, the thermal conductivity increase obtained with the addition of S/h-BN is compatible with the literature (36). However, for UPE-S/h-BN5 and UPE-S/h-BN10 samples, the thermal conductivity values of 0.171 and 0.138 W·mK−1 were obtained. This decrease observed with increasing h-BN amount can be attributed to the heterogeneous distribution of h-BN nanoparticles in the UPE resin matrix at these higher loading ratios, which has a deleterious impact on the resin structure as seen in the SEM micrographs of these samples.
Thermal conductivity data of UPE-S/h-BN composites
| Sample | Thermal conductivity constant (W·mK−1) | Standard deviation |
|---|---|---|
| UPE-S/h-BN0 | 0.182 | 0.200 |
| UPE-S/h-BN1 | 0.188 | 0.002 |
| UPE-S/h-BN5 | 0.171 | 0.001 |
| UPE-S/h-BN10 | 0.138 | 0.001 |
3.6 Contact angle
The water contact angle test was used to determine the effect of S/h-BN nanoparticles on the hydrophobicity of the neat UPE resin sample. According to the results shown in Figure 7, the integration of S/h-BN nanoparticles increased the hydrophobicity of the neat UPE resin due to the chemical stability of the B–N hexagonal structure and hydrophobicity of the nanoparticles, which inhibited water absorption. Higher S/h-BN nanoparticle content increased surface roughness, which improved the water contact angle. While the water contact angle of the neat UPE resin sample was 91.00°, the degree of hydrophobicity increased with the addition of S/h-BN nanoparticles and the contact angle of the composite samples increased as the S/h-BN nanoparticle ratio increased (98.32°, 110.5°, and 114.4° for the UPE-S/h-BN1, UPE-S/h-BN5 and UPE-S/h-BN10, respectively).
Water contact angle of UPE-S/h-BN composite surfaces.
3.7 Electrical resistivity measurement
The electrical insulation strength of the film is the most significant factor in the evaluation of the properties of resin insulation varnishes.
The volume resistivity and surface resistivity are also important factors for evaluating the material’s insulation properties. The volume and surface resistivity of the composite films with different S/h-BN contents are shown in Table 5. As shown in Table 5, the volume resistivity of the films increased with increasing amounts of S/h-BN. As a result, with increasing the amount of S/h-BN, water absorption of the films decreased; meanwhile, the volume resistivity and surface resistivity of the films were improved significantly. Considering the data given in Table 5, when the amount of S/h-BN increases in the polymer matrix, the volumetric and surface resistances are increased; however, this value decreases due to the formation of agglomerates at 10 wt% of S/h-BN amount, which can be attributed to the formation of pores (37). In addition, the obtained electrical measurement results also confirmed the SEM and contact angle results.
Electrical resistivity measurement results
| Sample | Electrical surface resistivity (Ω cm) | Standard deviation | Electrical volume resistivity (Ω cm) | Standard deviation |
|---|---|---|---|---|
| UPE-S/h-BN0 | 7.19 × 1012 | 2.5 × 1010 | 1.30 × 1013 | 4.2 × 1011 |
| UPE-S/h-BN1 | 4.57 × 1012 | 8.0 × 1010 | 2.71 × 1013 | 5.0 × 1012 |
| UPE-S/h-BN5 | 9.34 × 1012 | 1.1 × 1011 | 7.23 × 1013 | 1.0 × 1013 |
| UPE-S/h-BN10 | 7.87 × 1012 | 2.0 × 1011 | 1.38 × 1014 | 1.0 × 1012 |
4 Conclusions
This study investigated the potential of a novel silane-modified nanoparticle containing UPE resin as an insulation varnish to be used in electrical components of the automotive industry. UPE-S/h-BN composites were prepared with the addition of silane-modified h-BN at various ratios. Several test methods were used to investigate the effect of S/h-BN nanoparticles on the physical, morphological, electrical, and thermal properties of the UPE resin.
The findings are as follows:
The physical characterization results indicated that increasing the amount of S/h-BN nanoparticles caused increase in viscosity and percentage solid content, and the UPE-S/h-BN10 composite showed the greatest increase in viscosity.
The water contact angle measurements demonstrate that the addition of S/h-BN to the UPE resin and increasing its amounts influenced the structure in a hydrophobic manner.
In SEM micrographs, the surface of the neat UPE resin sample presented a smooth appearance, while all UPE-S/h-BN composite samples showed a phase separation and overall heterogeneous distribution. The size of agglomerations increased with the additive ratio.
The FTIR analysis and TGA thermograms of h/BN and S/h-BN nanoparticles indicated that silane modification of h/BN nanoparticles was successfully done.
According to TGA measurements, the addition of S/h-BN and increasing its amounts significantly boosted the thermal stability of the UPE resin.
An increase in the thermal conductivity coefficient indicates that the material is more resistant to thermal shocks.
The addition of 1 wt% S/h-BN nanoparticle increased the thermal conductivity of the neat UPE resin but the addition of more nanoparticles led to a decreased thermal conductivity due to the agglomeration formation.
Overall, the obtained results show that this material can be used as impregnating varnishes or insulating resins to coat and impregnate electrical components.
Acknowledgements
The author expresses his sincere gratitude to Betek Boya Ve Kimya Sanayi A.Ş. Furthermore, he acknowledges the support provided by Assos. Prof. Aylin Karahan Toprakçı and METU Central Laboratory. Their contributions facilitated the realization of this research. The author also appreciates the insightful discussions with his colleague Selinay Gümüş, who provided valuable perspectives and suggestions that helped shape his ideas.
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Funding information: No financial or personal relationships with other people or organizations could inappropriately Q8 influence or bias the content of this manuscript.
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Author contributions: Kaan Aksoy: writing – original draft, writing – review and editing, methodology, formal analysis; visualization, project administration; resources.
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Conflict of interest: The author has no conflicts of interest related to this work.
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