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Abstract

Many articles in the literature discuss about the harmful effects of electromagnetic waves emitted by electrical and electronic devices on human health. The composite panels, which are the subject of this study, are designed with the inspiration of the absence of a protective wall covering material from the effects of electromagnetic waves in the market. This study aims to develop a material with electromagnetic insulation as well as heat and sound insulation for especially civil engineering and space engineering applications. In the study, panels are produced by adding carbon fibre and carbon particles into the polyurethane matrix. Heat transmission coefficient tests, sound absorption measurement tests and electromagnetic protection efficiency tests have been made for panels. As a result of the tests, electromagnetic protection values ranging from 22 to 83 dB were obtained from panels and heat transfer coefficient values were found to be less than the standard values of 0.06 W/mK and sound insulation values were found to be having the lowest 0.22 and the highest 0.90 Hz sound absorption coefficient. In addition to the above findings, the composite panels were elastic, lightweight and smooth. Finally, it is determined that the composite panels developed in this study satisfy the conditions required for heat, sound and electromagnetic insulation for engineering applications.

Keywords

Insulation carbon fibre composite electromagnetic carbon particle

Introduction

In recent years, composites have been a type of material that researchers and producers have emphasised because of their advantages like lightness, flexibility, durability, economy, smooth production processes and their diversity [1,2]. These materials, which are entirely different from the properties of the constituent materials, are composed of at least two different materials, usually reinforcement and matrix elements. The desired physical properties such as lightness, elasticity, heat and electrical conductivity, economy, etc. of the composite material play an essential role in the selection of the reinforcement element, while features like the interaction of additive with matrix material, binding, appearance, compliance with ambient conditions, ensuring load and voltage transfer, and impact prevention are the determiners of the matrix material [3 –5].

Electrically conductive polymer composites are obtained from either of compounding carbon particles, carbon fibres or metal particles, for instance, gold, copper, silver, aluminium into a polymer matrix, or, by the direct usage of electrically conductive polymers like polyaniline. In particular, the polymer composites those reinforced with carbon fibres and/or carbon particles have a more extensive usage at aerospace, construction, automotive, and maritime industries to provide thermal and electromagnetic insulation in consequence of their features such as low density, resistance to humidity, high friction, and better wear and fatigue rates [6 –13].

Electromagnetic pollution is the electromagnetic signals transmitted or electromagnetic radiation emitted by electrical circuits of the devices, which may interfere with the proper functioning of the surrounding electrical devices or damage the living organisms. The comprehensive development of digital information processing systems and telecommunication devices has led to an unprecedented level of electromagnetic pollution [14 –16]. This situation has led to the search for new material with effective electromagnetic shielding in a wide range of applications. These materials include conductive paints, metal-reinforced rubbers, foams and multi-layered topologies, etc. [15]. In recent years, studies on electromagnetic shielding materials are focused on conductive reinforced polymeric materials that prevent interference through absorption rather than reflecting, which include a considerable number of carbon particles or carbon fibres reinforced composites [17 –23]. This study discusses a comparative analysis between electromagnetic protection and, heat and sound insulation of, carbon fibre and/or carbon particle-reinforced, conductive, polyurethane (PU) polymer composites. PU is a commercially important polymer among the other reinforced composites because of its easy machinability, dimensional stability, low density, low thermal conductivity, good sound insulation and low cost [24,25]. Carbon particles are commonly used filler material to improve dimensional stability, providing conductivity and prolonging the life of plastics by having antioxidant features. Meanwhile, in the last decade, carbon fibres have created extensive uses for bringing in features to composites such as high rigidity, excellent electrical and thermal conductivity, and high fatigue resistance [26].

This study aims to assess the composites of carbon fibre and carbon particle additive materials with polyurethane concerning heat, sound and electromagnetic insulation. When the literature is examined, in addition to the heat and sound insulation properties, it is seen that a material that also provides electromagnetic shielding is not designed and there is no such product in the market. In this context, the study is thought to be innovative and original.

Material and method

Material

For the composite panels produced in this study, polyurethane was preferred as the matrix material, while carbon fibre (6 mm length) and carbon particles (400 micron) were used as reinforce materials. For the production of composite panels, metal molds shown in Figure 1 are used. In this study, three different composite panels namely; carbon fibre/polyurethane matrix, carbon particle/polyurethane matrix and carbon fibre/carbon particle/polyurethane matrix were obtained. Their thickness and density values and the carbon fibre and carbon particle amounts are given in Table 1.

Figure 1.

Pouring into metal molds and curing process of composites.

Table 1.

Some features of the composite panels.

Sample code	Thickness (mm)	Density(g/cm²)	Carbon fiber amount in 100 g polyurethane (g)	Carbon particle amount in 100 g polyurethane (g)
S1	4.68	0.655	–	–
S2	6.68	1.33	–	4
S3	8.4	1.14	0.5	–
S4	8.16	1.6	1	–
S5	4.15	0.755	0.5	0.5
S6	3.76	0.612	–	1

Method

Production process of composite panels

Carbon fibre and carbon particle supplementation in the matrix polymer was made by hand depositing method. The mixture is homogeneous by mixing with the help of a laboratory type mixer. The resulting homogeneous material was poured into the metal moulds of 35 × 25 cm, and solid composite panels were obtained as a result of 14 days of curing [27]. The end products subsequently have been tested for thermal conductivity, sound absorption, as well as electromagnetic shielding.

Determining thermal conductivity properties of composite panels

Heat transfer coefficient tests were carried out according to ASTM C177, C518 standard, with the KEM – Quick Thermal Conductivity Meter (Figure 2) which uses the hotwire method and has a measuring range between 0.023 and 11.63 W/MK. According to hotwire method, first, the sample is placed on both sides of a heated plate and then the average thermal conductivity is measured; then the panels were conditioned at a temperature of 20 ± 2°C and 65 ± 5% relative humidity and formed in the dimensions of 120 mm × 50 mm. For each panel, a total of six measurements, three on the front and three on the back, were averaged. Table 2 shows the results of the heat transmission test applied to the composite samples.

Figure 2.

KEM – Quick thermal conductivity meter.

Table 2.

Heat coefficient test findings.

Sample no.	S1	S2	S3	S4	S5	S6
Heat transfer coefficient (λ) W/mK	0.047	0.13	0.046	0.042	0.048	0.092

Determining sound insulation properties of composite panels

Sound absorption tests were performed to determine the sound insulation properties of the panels. The frequency values of the sound absorption coefficients and the loss of sound transmission values of the composites were measured in the Brüel & Kjaer Tube-Type 7758 impedance tube device (Figure 3) in the frequency range of 100–6300 Hz according to the TS EN ISO 10534-2 standard.

Figure 3.

Brüel & Kjaer tube-type 7758 impedance tube.

This device can make measurements for each frequency instantaneously and give the instantaneous frequency change value.

Under the conditions of 20 ± 2°C temperature and 65 ± 5% relative humidity, from each composite panel, three pieces of 28 mm and three pieces of 100 mm in diameter were cut to be placed inside the large and small impedance tube. The sound pressures P1, P2 were measured with the device's microphones 1 and 2 at frequencies 100, 125, 130, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150, 4000, 5000, 6300 Hz separately.

Determining electromagnetic shielding performance of composite panels

The measurement of the electromagnetic shielding parameters of the panels is carried out in the electromagnetic activity test box (clean room, Faraday cage, test box) between the frequency range of 30 MHz–6 GHz, according to the 'plate-panel measurement' method [27]. The effectiveness and accuracy of the test box were calibrated by the measurements done with the NBS test assembly (NBS Flanged coaxial test fixture) due to the ASTM D 4935-89 standard.

Research findings

When the surface images of all panels compared, it could be detected that the surfaces of the carbon particle reinforced composites were smoother than the surfaces of the fibre reinforced composite panels [27]. This was due to the fact that carbon fibers absorbed a certain amount of matrix material into their bodies. This problem has been fixed by determining the optimum amount of matrix and fiber additive by repeated experiments.

Figure 4 illustrates the SEM images of the composite panels. In Figure 4(a), the carbon particles can be seen and in Figure 4(b), both the carbon particles and the carbon fibers can be seen.

Figure 4.

SEM image of (a) Sample 3 and (b) Sample 5.

Heat transmission coefficient findings of composites

To make the quantitative comparison of thermal conductivity capabilities of different materials, the heat transmission coefficient (λ) is used, which basically is the amount of heat passing through a material in 1 h with unit thickness of 1 m and area of 1 m² to the upright direction, while the temperature difference between the parallel surfaces of the material is 1°C. As the heat transmission coefficient rises, the material's thermal insulation feature decreases [10], and thus according to the standards, the heat transmission coefficient of an insulating material must be less than 0.060 W/mK.

Heat transmission coefficient tests were carried out according to ASTM C177, C518 standard in KEM ASTQuick Thermal Conductivity Meter. Table 2 shows the results of the heat transmission test applied to composite samples. The measurements were made as a total of six measurements, as three front and three back sides for each sample and these values were averaged.

Sound absorption coefficient findings of composites

The sound absorption coefficient of a material is the sound absorbing performance of that material, which depends on the materials natural frequency. The sound absorption coefficient indicated by α can take values ranging from 0 to 1, which calculated by taking the ratio of the absorbed sound energy on the related surface to the total sound energy that beats on [28]. The sound insulation of a structural element is directly proportional to the intensity of the material.

The sound absorption coefficients of composite specimens produced in the project are given in Table 3.

Table 3.

Sound absorption measurement results of composites.

Composite		Sound absorption coefficient
Sample no.	Composite density (g/cm²)	250 Hz	500 Hz	1000 Hz	2000 Hz
S1	0.655	0.22	0.48	0.75	0.53
S2	1.33	0.31	0.52	0.78	0.84
S3	1.14	0.45	0.59	0.80	0.87
S4	1.6	0.55	0.75	0.85	0.90
S5	0.755	0.47	0.62	0.79	0.86
S6	0.612	0.26	0.49	0.76	0.51

Electromagnetic shielding performance results of composites

The tests were first performed with a metal plate filling the gap of the measurement device, then when the gap is empty, for calibration purposes. Later on, the samples were placed in order and tested for different specific frequencies. The electromagnetic shielding performance findings of composite panels produced in the project are shown in Table 4.

Table 4.

Electromagnetic shielding results of composites.

Test order	Frequency (Mhz)	27	50	100	250	500	900	1000	1800	2400	2600	5000	6000
Test order	Signal strength (dBm)	2.5	3.3	3.5	3.3	3.8	3.1	3.4	2.9	2.1	0.8	−6	−8.6
1	Sample 1	−37	−43	−31	−21	−34	−62	−72	−25	−21	−31	−48.5	−43
2	Sample 2	−39	−47	−32	−21	−31.5	−73	−74	−26	−32	−32	−51.5	−48
3	Sample 3	−37.5	−43	−33	−22	−42.5	−81	−83	−37	−56	−47	−73	−69
4	Sample 4	−37	−43	−32	−22	−41.5	−80	−83	−45	−63	−47	−90	−60
5	Sample 5	−37.5	−44	−32	−22	−44	−83	−85	−48	−44	−44	−75	−60.5
6	Sample 6	−37.5	−43	−32	−21	−32.5	−62	−72	−24	−26	−32	−54	−54
7	Empty	−36.5	−43	−33	−21	−35.1	−63.8	−72.3	−20	−20	31.7	−47.6	−50.6
8	Fully loaded	−38.7	−43	−33	−21	−44.5	−86.1	−90	−61	−46	−56	−93	−84.4
	Frequency (Mhz)	27	50	100	250	500	900	1000	1800	2400	2600	5000	6000

The values in Table 4 are shown graphically in Figure 5. As can be seen from the graph, S4 is the material with the most effective shielding value at almost all frequencies and S4 is followed by S3 and S5, respectively. The reason for this is thought to be due to the carbon fibers contained in S3, S4 and S5.

Figure 5.

Shielding effectiveness grafic of composite samples.

When Table 1 is examined, the sample containing the highest amount of carbon fiber is S4. S4 provided more effective protection than S6 containing the same amount of carbon particles and even S2 containing four times as many carbon particles. This suggests that the fiber form provides more effective shielding support than metal powder or particle forms.

In addition, the frequency range in which the S4 sample is most effective is the 900–1000 MHz range and the shielding value is above 80 dB. This value is sufficient for electric and electronic devices for shielding applications in UHF band especially for televisions, cellular phones and global positioning systems.

Conclusion

When the surface morphologies of the composite panels were examined, the surfaces of the carbon fibre-reinforced panels were more jagged than the carbon particle reinforced ones, even though both include the same amount of matrix material. It is determined that the carbon fibres were able to soak only a certain amount of matrix material and therefore homogeneous dispersion within the matrix materials could not be achieved. This situation was eliminated by increasing the amount of matrix material used for fibre reinforced composite panels at the subsequent panel productions.

In Table 2, it could be seen that the heat transmission coefficient values of all composite panel samples are smaller than 0.060 W/MK except for S6 and S2, indicating the four composites produced in this study can be used as heat insulation material.

It was determined that the composite panel with the best heat insulation characteristics among the samples had been the fibre-reinforced panel; concordantly, the carbon particle additive had reduced the thermal insulation of composite panels.

In Table 3, which contains the sound absorption coefficients of composite panels produced in this project, it could be seen that sample S4, which has the highest density and the highest fibre content, has the best sound insulation and followed by S3, S5, S2, S6, and S1 order. It is comprehensible that the composite panels deliver sound insulation in proportion to their densities, merely, sample S6, which embodies carbon particles, has the lowest density, but has better sound insulation than sample S1, in the meantime, a similar result was observed between the samples S2 and S3. Among all the results, it could be comprehended that the lowest sound absorption coefficient is 0.22 Hz; moreover, the highest is 0.90 Hz.

Finally, the electromagnetic shielding effectiveness values of composite panels have been examined, and it is seen that each panel is active at different frequencies. At 27 MHz and 50 MHz, sample S2 is determined as the most useful material with a protection value of 47 dB and 39 dB respectively. At 100 MHz, S3 was the most striking specimen with 33 dB, while sample S3, S4 and S5 were the most useful composites with 22 dB at 250 MHz. S5 provides the best protection at 500 MHz, 900 MHz, 1000 MHz and 1800 MHz frequencies. S4 composite seems to have the best electromagnetic shielding with values like 63 dB at 2400 MHz, 47 dB at 2600 MHz, and 90 dB at 5000 MHz. The composite panel under the alias S3 is designated as the best shielding material at 6000 MHz with 69 dB outcome. When the results of all electromagnetic shielding efficacy tests were evaluated, it reveals that each material is active at different frequencies and that the materials have different behaviours as a result of the properties they possess [27].

As a result, the composite materials produced in this study have been evaluated as materials with significant values for their thermal insulation, sound insulation and electromagnetic shielding values; that being said, in future studies, these materials are going to be examined for their mechanical properties, and hopefully, more advanced materials would be designed.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The author thank to University of Burdur Mehmet Akif Ersoy for financial support for the research of this article referenced to project number 194-NAP-13.

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High structural insulation composite material development with electromagnetic protection effect reinforced with carbon fibers and particles