The Electromagnetic Shielding Properties of Biodegradable Carbon Nanotube–Polymer Composites

Abstract

In this article, the electromagnetic shielding properties of carbon nanotube–polymer nanocomposites are presented. The composite fabrication technique is spray-drying, the usage of which leads to a uniform dispersion of carbon nanotubes (CNTs) in a polymer matrix. Obtaining good filler dispersion is necessary to form a continuous, electrically conductive network of CNTs inside the polymer matrix. In the described nanocomposites, the network of conductive filler particles acts as an electromagnetic radiation barrier. For this reason, developing a highly effective fabrication method is very important. Also, the method should be simple enough to be easily adopted in an industrial environment. The authors shows in this text that both goals mentioned are achieved. The obtained nanocomposite material not only has electrostatic shielding capabilities but comprises electromagnetic shielding properties, which fulfills the main goal of the presented work. It is also worth mentioning that the developed manufacturing method allows for the usage of different fillers and polymers and thus the fabrication of materials capable of meeting a wide range of requirements.

1. Introduction

In today’s digital age, electronic devices have become integral to a myriad of sectors, including healthcare, industry, and transportation, shaping how we live, work, and connect. In healthcare, advanced monitoring systems and wearable technologies have revolutionized patient care by enabling real-time health metric tracking. Industrial sectors leverage sensors and automation to enhance manufacturing processes, improving both efficiency and safety. In transportation, sophisticated electronic systems are crucial for the operation and management of both passenger and freight services. However, the deployment of these technologies faces significant challenges, particularly due to the environmental conditions in which they operate. Devices must withstand extreme temperatures, which can impair their performance and reduce their lifespan, necessitating the development of advanced cooling technologies and materials with greater thermal tolerance. High-humidity environments pose risks of corrosion to electronic circuits, driving the need for protective measures such as hermetic seals and corrosion-resistant coatings. Additionally, the mechanical stresses of vibration and shock, especially prevalent in transportation applications, require robust mechanical designs and the integration of shock-absorbing materials to ensure device integrity and reliability. The rapid expansion of electronic applications also brings to the forefront the issue of electromagnetic and electrostatic interference. Protective housing or specialized layers are essential to shield sensitive components from such disruptions, ensuring consistent performance across diverse settings. Moreover, as the environmental impact of technology gains global attention, the push for sustainable manufacturing practices becomes increasingly urgent. The electronics industry is particularly scrutinized for its use of industrial polymers, which are often not biodegradable, and the generation of considerable toxic waste. In response to these environmental concerns, significant strides are being made in the realm of materials science, particularly through the development of nano-materials that enhance electrical and thermal properties and biodegradable polymers, aimed at reducing electronic waste. Innovations such as flexible electronics open new avenues for applications like bendable displays and smart textiles, marrying functionality with adaptability. Most of the polymers used in industry are polyolefins made from petroleum, which cause thousands of tons of wastes that pollute the environment—these polymers are not easily biodegradable (the decomposition process can take hundreds of years). The material that meets the requirements described above is a polymer composite with carbon nanotubes. The chosen polymer is polylactide obtained from natural renewable resources, like corn or cellulose [1]. In terms of wastes, it is fully biodegradable [2].

Using carbon nanotubes (CNTs) as the electro-conductive phase (filler) guarantees the acquisition of material comprising both electrostatic [3,4] and electromagnetic shielding. Carbon nanotubes, apart from their tremendous mechanical and thermal properties [5,6,7], have great electrical properties [8] and thus are a natural choice for making conductive polymer composites [3,9,10]. To obtain nanocomposites with the desired properties, there is a need for proper fabrication method usage. The main problem limiting carbon nanotubes’ application is their tendency to form aggregates (bundles), which are very hard to disintegrate. This is caused by the van der Waals forces between the carbon atoms forming carbon nanotubes, combined with the large specific surface area of CNTs, reaching meters per gram [11,12,13]. The authors’ solution to the mentioned problem is a developed method facilitating the acquisition of nanocomposites with a good, uniform dispersion, as the formation of a continuous electro-conductive network is needed to manufacture materials used as shields against both static electricity and electromagnetic radiation. One of the major goals achieved by the authors of the presented work was developing a method universal enough to be easily adapted to the different expectations of industry and allowing for the application of a protective layer onto a wide possible range of surfaces. This method is a spray-drying technique. Its advantage over popular extrusion is working with a solution facilitating the disintegration of the carbon nanotube bundles, combined with the ease of the protective layer’s application. Using different filler and polymer types is also acceptable as long as solution formation is possible. The method also allows for the formation of a conductive network of filler particles with a very low filler content, 0.25% by weight as the percolation threshold [3] since that characterizes the shielding capabilities of the obtained material.

2. Methods

The polylactide (PLA) used as the polymer matrix was provided by Cargill-Dow. It contains 95.9% L-lactide and 4.1% D-lactide. Reagent-grade trichloromethane (CHCl₃) was used as the solvent for the polylactide and was provided by Chempur Poland. The same reagent was used as the solvent to prepare the solution of CNTs and the PLA. The carbon nanotubes used in the described experiments were synthesized in our laboratory using the liquid source chemical vapor deposition (LSCVD) technique [3,14]. The apparatus was a three-zone oven with a temperature controller for each zone, connected to a liquid catalyst solution feeding system and a vapor carrying gas source. The setup allowed us to easily change all of the synthesis parameters—the temperature in each furnace zone, the catalyst concentration, and the carrying gas flow. By alternating these parameters, the properties of the synthesized material could be tailored, and so could the nanocomposite material [15,16,17]. CNTs after each synthesis, prior to further stages of nanocomposite preparation, they were examined using the thermogravimetric technique (TGA) and scanning electron microscope (SEM) imaging. The carbon nanotubes grew in the form of a carpet, growing perpendicularly to the carpet’s base (Figure 1).

It is crucial to properly steer the synthesis parameters since the dimensions of the filler particles, such as their length, are important factors in the electrically conductive network’s formation inside the polymer matrix. This makes synthesis one of the factors with the biggest influence on composite performance. Also, tuning the process is the key to obtaining high-purity material, eliminating the need for one or two stages in the purification process and importantly lowering the cost of the final product. Examination using the thermogravimetric technique was carried out for each sample used in the experiments. The apparatus used was a TA Instruments Q50. Then, SEM micrographs were taken using the JEOL IT200 apparatus. The TGA gave us information about the CNTs’ purity (Figure 2a), a factor which is important in the process of obtaining homogeneous composites both in terms of electrical conductivity and electromagnetic shielding, and SEM had the role of confirming the TGA results. The analysis allowed us to check for the appearance of soot and other amorphic carbon forms in the examined specimens (Figure 2b).

The SEM micrographs, both in Figure 1 and Figure 2a, show no impurities nor other allotropic forms of carbon in the examined samples. This is also the conclusion of the TGA analysis, as only one decomposition peak can be observed in the temperature, expected for multiwalled carbon nanotubes in air [18,19]. If other forms of carbon were found in the synthesized material, the used technique would confirm this—another decomposition peak would appear. It is worth mentioning the high thermal stability of the filler allows for the use of the material as an organic conductor, as a high current can be carried through the nanocomposite material. The thermal limitation on the electric current conductance is the matrix, as decomposition of the PLA occurs at 517.15 K [20]. After the described analysis, which was carried out for each sample, polylactide nanocomposite samples were prepared using the same routine each time. The developed nanocomposite manufacturing technique was a solution-based process, which raises another problem, i.e., the use of an appropriate solvent. First of all, the solvent should achieve good dissolution of the polymer at room temperature and be easily removed by evaporation from the surface of the layer made of the nanocomposite. The latter assumption has great importance since no expensive vacuum heating oven installation is needed to obtain solvent-free material as the process outcome. The second problem at the solvent choice stage is the need to disintegrate the carbon nanotube aggregates formed during synthesis. The solvent with the described necessary parameters is trichloromethane (CHCl₃)—its parameters comprise both good PLA solubility and Hansen solubility parameters [21,22]. These parameters of trichloromethane allowed us to achieve a good dispersion state when preparing the CNT solution, which was the first step of the solution preparation. The dispersion time for each specimen was 90 min. The examination process for the dispersion effectiveness involved using optical microscopy and scanning electron microscopy, accordingly. The initial examination was carried out using a scientific-grade optical microscope working in transmission mode. Right after the end of sonication, a droplet of CNT/CHCl₃ solution was placed between a slide glass and a coverslip. Observation at this scale allowed us to investigate the state of dispersion, but examination of individual nanotubes was impossible due to the wavelength—the length of the light used for observation must fall within the range of visible radiation, i.e., λ = 400 ÷ 700 nm. For confirmation of good dispersion, which corresponds to effective agglomerated disintegration and effective use of sonication, an electron microscope was used. Before observation, a droplet of the solution was placed on the Si wafer and left until the solvent evaporated. An example of observation of the dispersion state is visible in the SEM micrograph in Figure 2a. After examining the dispersion, the CNTs/PLA solution was prepared. PLA granules were added to the solution and ultrasonicated until the polymer was fully dissolved. Adding the polymer to the dispersed CNT solution caused a rise in viscosity and behavior similar to surfactant particles, preventing the rapid reaggregation of the filler particles and thus an increase in the dispersion stability. This effect is not permanent, and reaggregation occurs nevertheless, as the van der Waals forces between the carbon atoms on the surface of CNTs are great. Slowing the process is significant enough to make the process of composite fabrication more convenient and less challenging. The time needed until the sonication effect is lost is significantly longer, and thus working with the spray-drying technique is possible [3].

The last process step was using spray-drying to apply a layer of the nanocomposite material onto a Si wafer to create samples suitable for SEM analysis, as the dispersion state was checked for each sample prior to electric measurements and electromagnetic shielding examinations. An example of the described analysis is presented in Figure 3.

The used technique allowed us to achieve a good dispersion of carbon nanotubes. This statement is confirmed by the low-magnification SEM image in Figure 3, as no agglomerates nor big CNT bundles can be observed. Also, there are no surface charging affects, which indicates the electric conductivity of the nanocomposite layer. This also confirms that an electric conductive network of filler particles was formed during the process. The sonication time for all the filler concentrations was 90 min, as previously mentioned, and our experiments showed no need for a longer sonication time. It should also be mentioned that a longer sonication time can cause the degradation of the polymer matrix [23], as can long exposition to high-temperature solvent treatment [24], due to the heating effect of sonication and result in worse mechanical properties in the protective layer. The obtained solution of PLA/CNTs was sprayed onto a glass surface, prior to electric conductivity and electromagnetic shielding examinations. Electric conductivity measurements were made using a four-electrode technique, and each concentration was examined ten times before calculation of the conductivity.

The last stage of the experiments and the most important for the applications described at the start of this publication was checking the electromagnetic radiation shielding properties of the nanocomposites. The experiments were carried out using a GTEM 1000 chamber and a measuring receiver SMR 4503 with GTEM software - COMPLIANCE-3 software version 3.90. The uncertainty in the SMR 4503 receiver’s measurements is ±1.5 dB. The estimated uncertainty for electromagnetic emissions in the GTEM chamber is ±6 dB. A diagram of the measurement system is presented in Figure 4.

This diagram represents the setup used to evaluate the electromagnetic emissions and shielding properties of the carbon nanotube (CNT)–polymer nanocomposites. The setup includes a GTEM 1000 chamber and a measuring receiver (SMR 4503) equipped with GTEM software (COMPLIANCE-3 software version 3.90), ensuring precise monitoring of electromagnetic radiation within a frequency range of 30 to 300 MHz. The testing methodology involved spraying the nanocomposite layers onto a glass surface, with each test consistently using the same setup and an antenna emitter to ensure the reliability and reproducibility of the results. This methodological rigor is crucial for accurately determining the material’s capability to shield against electromagnetic interference, a key property for materials used in electronic housing and other protective applications.

3. Results and Discussion

The electric conductivity of the nanocomposite material was examined at a wide range of filler concentrations, and the obtained results vary from 10⁻¹³ S/cm for neat polymer to 2.7 S/cm for a 10% weight concentration. The results are shown on the plot below (Figure 5).

These values guarantee good static electricity shielding properties in the nanocomposites. The low percolation threshold—0.25% weight percent—allows us to reduce the cost of protective layer fabrication because a low amount of filler is enough to obtain the desired static electricity protection. The process of applying the protective layer allows us to effectively fix the good dispersion state of the filler, and that is the key to the low percolation threshold obtained. Also, the manufacturing technique can be adopted in an industrial environment with no need for high-cost installation.

The electromagnetic radiation shielding properties of carbon nanotube (CNT)–polymer nanocomposites at filler concentrations of 2% and 5% by weight were examined, within a range of 30 to 300 MHz. This range is critical, as it encompasses frequencies common in both consumer electronics and industrial applications, making the findings highly relevant to practical implementations. The plotted results, which are discussed in detail in the corresponding sections, demonstrate the material’s effectiveness in signal damping, highlighting how different filler concentrations influence the shielding efficiency. The data gathered from these experiments not only validate the shielding capabilities of the nanocomposites but also inform potential optimizations in the filler content for cost-effective production without compromising on performance. This comprehensive setup diagram and description are intended to provide clarity on the experimental process, helping to correlate the material’s composition with its functional properties in electromagnetic radiation shielding. Examination of the samples was conducted at two different filler concentrations—2% and 5% by weight—so that the goals of high electrical conductivity (0.13 and 1.7 S/cm, accordingly [3]), allowing for effective electric charge dissipation, and electromagnetic radiation shielding could be achieved. The plot of the signal level versus the frequency of the transmitted signals shown below shows good damping properties at both examined filler concentrations—see Figure 6.

This figure visually represents the results on the signal damping effectiveness across different concentrations of carbon nanotube (CNT) filler in polymer nanocomposite layers deposited on a glass surface. The data plotted in the figure reflect experiments conducted with filler concentrations of 2% and 5% by weight. These concentrations were chosen to assess the shielding performance at lower and moderately increased filler levels, demonstrating the material’s ability to dampen electromagnetic signals effectively within the frequency range of 30 to 300 MHz. The graph highlights a notable observation: increasing the filler content from 2% to 5% does not proportionately enhance the shielding properties within the tested frequency range. This finding is crucial, as it suggests that even at lower filler concentrations the nanocomposite material achieves sufficient electromagnetic interference (EMI) shielding, making it both cost-effective and efficient in applications requiring EMI protection. The absence of a significant shielding improvement at higher concentrations could guide future material engineering by optimizing the balance between the material cost and performance. By demonstrating effective signal damping at a lower filler concentration, this study supports the development of lightweight, cost-efficient shielding materials suitable for widespread use in electronic devices, potentially lowering the manufacturing costs while maintaining high performance in electromagnetic protection. For more desirable (from an industrial application point of view) results, the shielding properties were examined against the layer thickness. For the experiment, two different layer thicknesses were examined—100 μm and 200 μm. The results can be seen in Figure 7.

This graph provides a detailed analysis of the electromagnetic shielding effectiveness of nanocomposite layers with CNTs at a concentration of 2% by weight, measured at two distinct layer thicknesses—100 μm and 200 μm. The experimental setup was designed to test the material’s ability to dampen electromagnetic signals across a frequency range of 30 to 130 MHz, a spectrum relevant to many commercial and industrial electronic applications. The data reveal that the 100-micrometer layer exhibited slightly superior damping properties compared to the 200-micrometer layer within this frequency range. This counterintuitive result suggests that the effectiveness of the shielding does not linearly scale with an increased material thickness. A possible explanation for this phenomenon could be related to the microstructural organization within the composite.

When discussing the shielding mechanism of composites, it is essential to understand how they interact with electromagnetic waves. The effectiveness of a material in shielding electromagnetic radiation is typically described according to three main coefficients: absorption (A), reflection (R), and transmission (T). These coefficients help in quantifying how much of the incident electromagnetic wave is absorbed, reflected, or transmitted by the composite material.

The reflection coefficient (R) can be designated using the following formula:

$$R=\frac{E_r}{E_i}$$

(1)

where:

• E_r is the reflected electric field strength.
• E_i is the incident electric field strength.

The dependence is presented on the plot below—see Figure 8:

A higher reflection coefficient indicates that more of the incident wave energy is bounced back from the material’s surface, which can prevent the wave from penetrating the material. For the examined specimens, higher coefficient values could be observed in the frequency range between 30 and 130 MHz.

The transmission coefficient (T) can be designated using the following formula:

$$T=\frac{E_t}{E_i}$$

(2)

where:

• E_t is the transmitted electric field strength.
• E_i is the incident electric field strength.

A lower transmission coefficient value, which indicates better shielding effectiveness, as less energy from the wave passes through the material, could be observed in the range described above, as illustrated in Figure 9 below.

The absorption coefficient (A) can be designated using the following formula:

$$\mathrm{A}=1-\mathrm{R}-\mathrm{T}$$

(3)

A higher absorption coefficient indicates that more of the incident wave energy is converted into heat or other forms of energy within the material, thus reducing the amount of transmitted energy. The plot of the dependence in Figure 10 is consistent with the transmission and reflection coefficient plots—the highest absorption can be observed for the frequency range between 30 and 130 MHz.

A higher absorption coefficient indicates that more of the incident wave energy is converted into heat or other forms of energy within the material, thus reducing the amount of transmitted energy.

During the preparation phase, the thinner layer may experience enhanced solvent evaporation rates, leading to a denser packing of CNTs. This denser packing could facilitate more uniform electromagnetic field disruption, which is critical for effective shielding. Additionally, the graph highlights the importance of understanding the role of the composite thickness in designing electromagnetic shields. These findings could significantly impact the development of lightweight, efficient shielding materials for sensitive electronic equipment, potentially reducing the material costs and improving the adaptability of these technologies to various environmental conditions. By optimizing the concentration and distribution of the CNTs within the polymer matrix, manufacturers can tailor the shielding properties to specific applications, enhancing the performance and durability of electronic devices. The implications of this research extend into sectors such as aerospace, automotive, and consumer electronics, where effective electromagnetic shielding is crucial for both functionality and compliance with international safety standards.

4. Conclusions

This exploration into carbon nanotube–polymer nanocomposites has unearthed promising potential to advance material science, particularly in areas demanding robust electromagnetic shielding and static electricity protection. This research primarily focused on the efficacy of these nanocomposites as protective layers in electronic component housing—a critical need in environments subjected to high levels of electromagnetic interference and static charge. The utilization of ultrasound treatment to disintegrate carbon nanotube agglomerates has been a pivotal advancement in our methodology. This treatment promotes a more uniform distribution of nanotubes within the polymer matrix, resulting in a nanocomposite that is not only highly conductive but also more effective in shielding against electromagnetic interference. The process’s simplicity and cost-effectiveness are particularly beneficial for scaling to industrial production levels without necessitating sophisticated or prohibitively expensive equipment. Interestingly, our study revealed that increases in the filler content (carbon nanotubes) beyond a certain threshold did not proportionally enhance the electromagnetic shielding effectiveness. This plateau suggests that there is an optimal filler concentration that balances cost and performance, potentially lowering the material costs for manufacturers while still achieving the desired shielding properties. A significant aspect of this research was the selection of polylactide as the polymer matrix. As a biodegradable polymer made of renewable resources, polylactide not only enhances the environmental sustainability of the production process but also reduces the ecological footprint of the end products. This is particularly pertinent in the electronics industry, which faces increasing pressure to minimize its environmental impacts. Future research will aim to expand the range of biodegradable polymers used and explore various fillers to fine-tune the properties of these nanocomposites for broader applications, including in consumer electronics, where environmental impact and disposal are growing concerns. The implications of these findings extend well beyond the laboratory. They suggest a paradigm shift in material selection and utilization in the electronics industry, particularly in sectors where protection against electromagnetic and electrostatic disturbances is crucial. Additionally, the economic implications of using lower filler concentrations can make these nanocomposites a more attractive option for companies looking to innovate while keeping costs in check. Moreover, as the demand for more sustainable materials grows, our approach could lead to new standards in the materials used for electronic devices, potentially influencing regulatory frameworks and industry best practices. Future research will focus on lifecycle analysis of these materials, from production through to disposal, to better understand their long-term impacts on the environment and their role in the circular economy.