Porous,low thickness carbon-fiber reinforced epoxy composites with excellent flexibility and superior electromagnetic radiation protection

Abstract

Fiber reinforced polymers, especially carbon fiber reinforced polymers, are expected in the future to contribute more than 50% of the structural mass of an aircraft. With increasing application and experience increased attention is paid to carbon-fiber reinforced composites with improved advanced properties, such as mechanical, structural and electrical, allowing them to displace the more conventional materials, such as metal alloys. In this paper, we used the Design of experiment methodology to investigate the design and properties of newly developed unique porous carbon-fiber reinforced composites intended for electromagnetic shielding purposes. The main goal was therefore to fabricate a composite with low thickness, high permeability to air and water vapor with a satisfactory ability to shield electromagnetic fields, whereas the investigation of the influential variables of the reinforcement and the investigation of the influence of the matrix on the overall shielding efficiency of the composite belongs to the sub-objectives. Furthermore, other important properties of the composites including heat and mass transfer, mechanical and electrical properties were evaluated. A quality index evaluation approach using weighted and normalized data was implemented to choose a composite with properties, which best fits to its intended use. The highest quality index was achieved by the composite containing reinforcement with warp and weft sett 18 dm⁻¹ using carbon tape 2 mm wide. This composite provides electromagnetic shielding 36 dB at 1.5 GHz, having high air permeability 1000 mm/s, relatively low bending rigidity of around 2.5 Nmm and thickness of only 0.37 mm.

Keywords

Carbon fibres polymer-matrix composites electromagnetic interference shielding mechanical properties transport properties

Introduction

Carbon fiber reinforced epoxy composites are very often used in the avionics industry now. The aeronautical industry has replaced classic flight instrumentation with electronic equipment making it more vulnerable to its electromagnetic environment. Lightning and high intensity radiated fields both within the aircraft (onboard avionics or passenger mobile phones) and external to the aircraft representing this environment are met in a regular flight service with a given probability. Strong and variable external electromagnetic fields may penetrate into the fuselage and the internal wiring installations, causing interfering and possibly destructive voltages and currents that may endanger vulnerable airborne equipment.¹ That is why there is an increasing demand to not only reduce aircraft weight using innovative materials, but also provide advanced properties such as reliable mechanical properties combined with a high level of electromagnetic interference shielding ability.²

Electromagnetic interference shielding is a technique of creating a barrier that prevents leakage of strong electromagnetic fields that can interfere with sensitive electronic devices or signals. At the same time, it represents an electromagnetic shell made of the shielding materials (electrically conductive or magnetic), which forms a close region and shields incoming electromagnetic wave, wheareas reflection, absoption and attenuation are the main mechanisms to reduce the effects of electromagnetic field of the radiation sources.³ Electromagnetic shielding effectiveness (SE), usually expressed in decibels (dB), is a measurement of the attenuation of an electromagnetic signal through the shielding material, and it is commonly used to evaluate the performance of this material.

In addition to traditionally used metal sheets, carbon fiber (CF) reinforced polymer composites are now preferred for electromagnetic shielding applications, mainly due to their low density. The CF is electrically conductive due to the planar layered structure of carbon atoms.⁴ Previous studies ^5–11 confirmed that concentration, orientation, electrical characteristics, and aspect ratio (the ratio of length to diameter) of discontinuous CF are the determining parameters for the level of the electromagnetic shielding ability of the composite. However, the randomly dispersed discontinuous CF does not have to provide a sufficiently continuous conductive network. That is why the most practical method of producing a conductive network with fillers at a low concentration is to use continuous carbon fibers (CCF). It has been reported that composites with CCF show better electromagnetic shielding efficiency than those with discontinuous CF. As described in ¹², the mean SE over the whole measured frequency range of polymer-matrix composites with CCF was about 115 dB and that of polymer matrix composites with discontinuous CF was about 98 dB. The dominant mechanism of electromagnetic shielding effectiveness of CCF composites is through reflection.^4,13 This is the reason why woven CCF were used to construct a predesigned conductive network as a composite reinforcement in this paper.

The preparation and characterization of continuous CF-reinforced composites have been reported in several studies.^4,12–21 If we focus on the SE ability of the CCF composites, the literature review revealed that to obtain a fabric with a high capacity to shield electromagnetic field, it is advantageous to use continuous, highly electrically conductive CFs, with the plain-weave-type being the preference. For example, CCF laminated composites were studied in terms of their ability to shield the electromagnetic field in.¹⁶ An excellent SE (more than 80 dB) was achieved by a composite slab made of four layers of prepregs 3 mm thick, whereas the reflection mechanism is dominant. Another paper ¹⁹ describes SE evaluation and prediction of CCF laminate, providing SE around 70 dB for frequency 12 GHz and having thickness around 1 mm. As described in ⁴, the woven CCF composite’s SE depends on the weave type, the number, and the angle of overlapped plates. It was found that the SE of the single, double, and triple plain and balanced-twill woven CCF composite plates was 50 dB, 60 dB, and 70 dB, respectively. The authors mainly focus on mechanical properties and electromagnetic shielding ability; however, less attention is generally paid to the structure of the CCF composites, especially in terms of porosity and air/moisture permeability. Despite the important fact that the development of flexible air/water vapor permeable composites designed for shielding the electromagnetic field has received increasing attention in recent years. This is because the porosity and the associated air permeability can be one of the important requirements, especially with regard to the removal of generated heat from important aircraft electronic equipment.

It should be mentioned that some approaches have been found to manufacture porous carbon films or CF reinforced composites,^22–25 but these approaches do not consider electrical conductivity or electromagnetic shielding of the structure. They are fabricated for thermal protection, gas separation, purification, or liquid phase processing.

All the above research indicates the need for exploration of electromagnetic shielding of lightweight, low thickness CCF woven reinforced composites in terms of their structure, especially the porosity and heat and mass transfer. With this in mind, the current research builds on the findings already published by the authors in the paper ²⁶ and aims to fabricate an epoxy composite with low thickness, high permeability to air and water vapor and with a satisfactory ability to shield electromagnetic fields, whereas the investigation of the influential variables of the reinforcement and the analysis of the influence of the matrix on the overall shielding efficiency of the composite belongs to the sub-objectives of this study. Design of experiement methodology was used to investigate the design and properties of newly developed unique porous carbon-fiber reinforced composites made of Aksaca electrically conductive CCF tapes intended for electromagnetic shielding purposes. Eleven composite samples were prepared in total with a different sett of warp and weft threads and different thread thickness of the plain weave CCF reinforcement. To maintain low thickness, high porosity, and flexibility of the composites, carbon fiber woven fabrics reinforcements were only impregnated with epoxy resin, which endows composite mechanical robustness by stabilization of binding points. The main attention was paid to the electromagnetic shielding ability of the composites measured by the ASTM D4935-18 standard. Furthermore, other important properties of the composites such as thickness, density, thermal conductivity, thermal resistivity, air permeability, the ultimate mechanical properties, bending rigidity and electrical properties represented by elecrical resistivities and relaive permittivity were evaluated. Finally, a Quality index evaluation approach using weighted and normalized data was implemented to choose a composite with properties that best fit its intended use.

Geometrical structure of multifilament tapes

Carbon multifilaments in the form of prepreg tapes are increasingly used in high-performance composites. The advantages of using carbon multifilament tapes with a rectangular cross-section over threads (rovings) with a circular cross-section (see Figure 1) include the following: (a) constant thickness and width, (b) reduced thickness compared to rovings, (c) evenly distributed tensile stress.²⁷ Assuming that the tape consists of carbon fibers of circular cross section of diameter d, two packing arrangements can be considered; open packing and hexagonal close packing.²⁸ Close packing is one of the basic forms of packing models in which the filaments fit into a hexagonal pattern as illustrated in Figure 1 and its maximum value is 0.785. Close packing has been analyzed over the last few decades, for example in papers.^29–31

Figure 1.

The geometry of a carbon (a) roving and (b) tape.

Carbon roving with the fineness T [tex] composed from individual carbon filaments with fineness t [tex] has a total number n [-] of filaments

n = \frac{T}{t} .

(1)

Diameter D [mm] of the carbon roving with the circular cross-section can be estimated as

D = \sqrt{\frac{4 T}{π ρ_{F} μ}},

(2)

where ρ_F [kg/m³] is density of carbon fiber and μ [-] is packing density (μ_lim = 0.907, μ_real = 0.7).³² The equation (2) can also be used for carbon filament diameter calculation when considering carbon filament fineness t and μ = 1.

When flattening the carbon roving having diameter D due to the position of the fibers, it is assumed that the fibers are not deformed. The assumption is that the multifil flattens from the original circular cross-section to the so-called Kemp’s cross-section, which is rectangular with semi-circular ends.³³ The width of the multifilament a with a rectangular cross-section is increased, followed by a decrease in thickness b. The a and b parameters are changed depending upon the number of layers l_n [-].³² Relative width α and relative thickness β, and relative flatness γ $(γ \geq 1)$ of the carbon multifilament are defined as $α = \frac{a}{D}; β = \frac{b}{D}; γ = \frac{α}{β} = \frac{a}{b} .$

There are two types of geometrical assumptions (based on constant area or constant circle circumference) when predicting the relation between relative width and relative thickness.³²

Electromagnetic shielding effectiveness

The electromagnetic shielding effectiveness (SE or SE_T) of a material is defined as the ratio of the transmitted power to incident power and is expressed in the form of

{S E}_{T} = - 10 \log \frac{P_{T}}{P_{I}} = {S E}_{A} + {S E}_{R} + {S E}_{M} [d B],

(3)

where, P_T and P_I refer to transmittance power and incident power, respectively. SE_A, SE_R, and SE_M describe the shielding effectiveness due to absorption, reflection, and multiple reflections, respectively, see Figure 2.

Figure 2.

Diagram of electromagnetic wave attenuation when passing through a material, where R, and T are reflection and transmission components, respectively.

Scattering parameters S11* (or S22*) and S21* (or S12*) in [dB], which are obtained by measurement using a two-port network analyzer, give the reflection (R), transmission (T), and absorption (A) components [-], where

{S 11}^{*} = 10 \log {| S_{11} |}^{2}

(4)

{S 21}^{*} = 10 \log {| S_{21} |}^{2} = {S E}_{T}

(5)

R = {| S_{11} |}^{2}; T = {| S_{21} |}^{2}; A = 1 - {| S_{11} |}^{2} - {| S_{21} |}^{2}

(6)

In the case of SE_T >10 dB, shielding effectiveness due to multiple reflections can be considered negligible. Thus effective reflection could be shown as (1-R). Since, after reflection, the remaining waves can be described as (1-R) which will be subjected to either absorption into the material or transmission through the material, thus the effective absorption should be equal to (1-T-R)/(1-R). The effective values can be described in [dB] such as: SE_Reff = −10 log (1−R) dB and SE_Aeff = − 10 log (T/1 – R).

The significance of reflection and absorption components can also be expressed as a portion of reflection (R) or absorption (A) to not transmitted energy (1-T) expressed in [%] or [dB]

R_{p e r c} = 100 \frac{R}{1 - T}; A_{p e r c} = 100 \frac{A}{1 - T} [%],

(7)

{S E}_{R} = {S E}_{T} \frac{R}{1 - T}; {S E}_{A} = {S E}_{T} \frac{A}{1 - T} [dB],

(8)

Quality index evaluation

According to,³⁴ quality means fitness for purpose. In other words, whatever is produced – a good or service – it must fit its purpose. To be fit for purpose, every good and service must have the suitable properties to satisfy customer needs and must be carried out with only a few failures. A quality index QI was chosen to evaluate the total performance of the fabricated composites. In the first step, the broken line function is constructed to estimate quality grade u_i(x) ( $0 \leq u_{i} \leq 1$ ) for each property

u_{i} (x) = f (x, L, H),

(9)

where H is entirely satisfactory, L is just unsatisfactory, and x is the current measured value under the same property, see Figure 3.

Figure 3.

Diagram of utility grade u_i(x) construction.

Based on the quality grade of all essential features that were singled out as pivotal, the quality index QI ( $0 \leq Q I \leq 1$ ) can be calculated using a weighted geometrical average

Q I = e^{\sum_{i = 1}^{m} (w_{i} \ln (u_{i}))},

(10)

where w_i is the weight of each property, i is the summation index, and m is the total number of properties.

Materials and methods

Continuous carbon-fiber tape

To prepare the woven reinforcements, we used Aksaca carbon roving (more precisely carbon tape, due to its cross-section shape), type A-35, which is comprised of continuous carbon fibers of a polyacrylonitrile precursor (producer DowAksa, Turkey). The main physical, chemical and geometrical properties are shown in Table 1 and Table 2. Equations mentioned in Chapter 1 were used for the assessment of geometrical characteristics. Microscopic images of the CFs in carbon tape A-35 can be found in.²⁶

Table 1.

Physical and chemical properties of Aksaca carbon tape A-35.³⁵

Property	Value
Odor	NO
Water solubility	NO
Fineness T [tex]	196
No of filaments n [-]	3000
Fiber diameter d [μm]	7
Density ρ_T [g/cm³]	1.75
Strength S [MPa]	3500
Young’s modulus E [GPa]	230
Elongation ε [%]	1.5
Electrical conductivity σ [S/m]*	5.80·10⁴

Table 2.

Geometrical properties of Aksaca carbon tape A-35.

Property	Value
Diameter of ideal circular roving D [mm]**	0.45
Cross section shape	Rectangular
Width of carbon tape a [mm]	1.95
Thickness of carbon tape b [mm]	0.19
Relative width α [-]	4.32
Relativne thickness β [-]	0.42
Relativne flatness γ [-]	10.26

*accroding to,²⁶ **according to ³² having μ = 0.7.

Carbon-fiber woven reinforcements

Using the carbon tape, we manufactured eleven types of reinforcement in the form of a plain weave fabric with a different total warp and weft sett as described in,²⁶ whereas warp and weft sett is identical for each sample type. As the main goal of the present study was to explore the influence of the weft and warp sett and the widths of the carbon tapes creating the woven reinforcement on the essential properties of epoxy composites, a two-factor factorial design ³⁶ was chosen to comprehensively and systematically study the interaction between various factors in addition to identifying any significant factors. The variable of interest was the SE effectiveness measured at a frequency of 1.5 GHz, bending rigidity represented by bending force, air permeability, thermal insulation properties, and ultimate mechanical properties. We decided to test three types of weft and warp setts and three widths of carbon tape; thus, a 3² factorial design was used. Coarser warp and weft threads with higher width were obtained by combining single carbon tapes of 2× and 3×, which corresponds to the tape width 3.12 mm and 4.18 mm (details can be found in).²⁶ Five samples were tested for each combination of warp and weft sett (spacing) and tape width, with all 45 tests run in random order. The factor levels are shown in Table 3. These factors and levels were chosen based on the previous experience of the authors, taking into account the required functionality and the lowest possible consumption of carbon roving. Since the warp and weft sett was expected to have the highest effect on the final SE ability of the sample, the sample set was expanded by two more samples with higher and lower setts for improved investigation of the effect of the sett on the SE and other related properties of the composite. Table 4 lists the properties of all the samples in the series, while Figure 4 shows a diagram of the reinforcement properties.

Table 3.

Input variables, units and levels.

Variable name	Variable unit		Levels
Spacing	mm	5	10	15
Width	mm	2	3	4

Table 4.

Main properties of the carbon-fiber woven sample set.

Sample code	Warp/weft sett [dm⁻¹]	Carbon tape spacing s [mm]	Number of tapes n [-] making up weft/warp thread
1	5/5	20	1/1
2	7/7	15	1/1
3	7/7	15	2/2
4	7/7	15	3/3
5	9/9	10	1/1
6	9/9	10	2/2
7	9/9	10	3/3
8	18/18	5	1/1
9	18/18	5	2/2
10	18/18	5	3/3
11	100/100	1	1/1

Figure 4.

Diagram of carbon tape woven fabric having plain weave with different settings such as carbon tape spacing s and carbon tape width a.

These specific spacings of warp and weft threads were chosen regarding the carbon material consumption and the air and water vapor permeability of woven structures often required for the final application. The sample with the lowest warp and weft sett had five threads per 100 mm, corresponding to a 20 mm carbon tape spacing (s, see Figure 4) in both the warp and weft. Meanwhile, the densest sample reinforcement was 100 threads per 100 mm.

Carbon-fiber woven reinforced composites

Epoxy resin EPIKOTE^TM MGS LR285 (ρ_ER = 1200 kg/m³, η_ER = 600–900 mPa.s) and hardener 500 (ρ_H = 1030 kg/m³, η_H = 200–350 mPa.s) was used to manufacture composites.³⁷ Epoxy and hardener were firstly mixed using a mixing ratio: 100:40 (by weight) by magnetic stirring at room temperature for 5 min. Textile reinforcements were impregnated by a mixture of epoxy and hardener assisted by vacuum infiltration at room temperature for 24 h, and then treated by curing at 60°C for 15 h. Images of carbon woven fabric reinforced composites representatives are shown in Figure 5 using an HP ScanJet Pro 2500 f1.

Figure 5.

Images of selected samples in the set: (a)–(e): sample 1–5, (f) sample 8, (g) sample 11.

Vacuum infiltration was applied using a flexible vacuum bag system. The initiated epoxy resin was manually applied over the entire sample area using foam rollers. After uniform application of the resin, the vacuum bag system was closed and the pressure (p = 600 mbar) was applied to ensure uniform application of the resin over the entire sample area. Redundant matrix was aspirated using pressure through the perforated bottom film into the suction layer formed by the non-woven fabric. Inter-thread pores were completely filled with the resin in case of samples having very low spacing of carbon tapes (sample 11). The inter-thread pores remained almost free - containing air in case of samples with higher carbon tape spacing (sample 1 – 10). Microscopic details of pores of chosen samples are shown in Figure 6, whereas light gray color represents matrix and the black color is black background. Images were captured by PROMICAM 3-5CP Digital Camera. Size of images is 1224 × 1024 px (1 px = 19.4 μm).

Figure 6.

Microscopic images of inter-thread pores of: (a) sample 7, (b) sample 10.

This type of epoxy was chosen due to hydrophilic character of the laminating resin system, advantageous mixing viscosities adjusted such that the resin will not run out of wide-meshed fabrics on vertical surfaces. The used epoxy system meets the standards for gliders and motor gliders. Due to its excellent physiological compatibility is the most commonly used system in the aerospace industry today.

Epoxy impregnation along with CF fabric reinforcing can endow composite mechanical robustness (mechanical stability of pure woven reinforcement, especially with high carbon tape spacing, is poor) as well as retaining especially flexibility and porosity of the carbon woven fabric reinforced composite.

Methods of measurements

Thickness of composites (h) was evaluated according to ASTM D1777-96 standard ³⁸ using a measuring head with a diameter of 50 mm and 1 kPa pressure.

Mass per unit area (GSM) of composites was measured according to ASTM D3776 ³⁹ (option C) standard.

To measure weight fraction, volume fraction, and composite density, standard ASTM D3171-22 was used

W_{f} = \frac{w_{f}}{w_{c}}

(11)

V_{m} = (1 - W_{f}) \frac{ρ_{c}}{ρ_{m}}

(12)

V_{f} = 1 - V_{m}

(13)

where W_f is a fiber weight fraction, w_f and w_c are the mass of fibers used and mass of the composite, respectively, V_f is the volume fraction of fibers, and V_m is the volume fraction of the matrix in the composite. ρ_m is a density of the matrix, ρ_c,teor is a density of the composite calculated using a mixing rule

ρ_{c, t e o r} = \frac{1}{\frac{W_{f}}{ρ_{f}} + \frac{W_{m}}{ρ_{m}}}

(14)

The derived relations apply to an ideal composite material without cavities. It is possible to express the volume fraction of cavities V_cav by the following equation

V_{c a v} = \frac{ρ_{c, t e o r} - ρ_{c, e x}}{ρ_{c, t e o r}}

(15)

where ρ_c,ex is a density of a composite determined experimentally.

The electromagnetic shielding effectiveness of the samples was experimentally measured via a far-field electromagnetic plane wave using the ASTM D4935-18 standard method ⁴⁰ for planar materials. This measurement method is valid over a frequency range of 30 MHz to 1.5 GHz; however, as these limits are not exact, the measurements were performed over a frequency range of 30 MHz to 3 GHz, which corresponded to a wavelength of 10–0.1 m. More attention was paid to the SE at 1.5 GHz since this is within the frequency range defined in the standard and also since it is close to the frequency of frequently used sources of electromagnetic liability, such as mobile phones, wireless LAN systems, radars, and GPSs.

The setup consisted of a sample holder with its input and output connected to a network analyzer. An SE test fixture (model EM-2107A; Electro-Metrics, Inc.) was used to hold the sample, with the design and dimensions of the sample holder following the standard method noted above. The measured sample was in the shape of a circle with a diameter of 13.31 cm. To generate and receive the electromagnetic signals, we used a Rohde & Schwarz ZN3 network analyzer, while the insertion–loss method was used to determine the SE of the fabric. The samples were air-conditioned prior to testing (T = 23 ± 3°C, RH = 50 ± 10%), with the measurements performed (n = 5) at five different randomly chosen sample locations to facilitate the subsequent statistical analysis.

Air permeability was measured with the Air Permeability Tester FX 3300 from Textest Instruments. The test pressure was 100 Pa on an area of 20 cm². Five measurements were performed for each composite sample according to the standard ISO 9237 standard.⁴¹ Air permeability was measured under standard laboratory conditions (T = 22 ± 3°C, RH = 50 ± 10%).

Thermal properties were evaluated using the Alambeta instrument,⁴² and tests were performed according to standard ISO EN 31,092–1994. Five measurements of thermal properties were performed for each composite type. Before the measurement, samples were air-conditioned under standard laboratory conditions (T = 22 ± 3°C, RH = 50 ± 10%). In all cases, the measuring head temperature was approximately 32°C, and the measuring head pressure was 200 Pa. Thermal properties of composites were characterized by these two properties: thermal conductivity λ [Wm⁻¹K⁻¹] and thermal resistivity RT [Km²W⁻¹], which are directly connected with thermal insulation properties.

Bending rigidity of samples was measured using a TH-7 device ⁴³ according to the CSN 80 0858 standard.⁴⁴ The tests were performed with 40 mm width of the samples in the warp direction. During the measurement by TH-7 forces required to bend a sample to certain angles are recorded. The output value is bending force Fm [Nm]. The measurement conditions were as follows: deformation rate v = 6 mm/min, bending angle α = 30°, temperature T = 23 ± 2°C, RH = 50 ± 5%. The measurement was performed at five different sample locations for subsequent statistical analysis.

Tensile properties of the composite samples, including strength and deformation, were examined under quasi-static loading conditions using the testing systems TIRAtest 2300 and Testometric M-350-5CT with load capacity ranging from 10 kN according to the ASTM D5035 standard.⁴⁵ The tests were performed with 40 mm width of the samples in the warp direction. The measurement conditions were as follows: deformation rate (speed of the clamps) v = 100 mm/min, distance between the clamps of the tensile strength tester l₀ = 100 mm, and temperature T = 23 ± 2°C, RH = 50 ± 5%. The measurement was performed at five different sample locations for subsequent statistical analysis.

Volume and surface resistivity of the composite and reinforcement were measured according to the standard ASTM D257-07⁴⁶ using a concentric ring probe technique at temperature T = 22.3°C and relative humidity RH = 40.7%. Volume resistivity is measured by applying a voltage potential across opposite sides of the sample and measuring the resultant current through the sample. Surface resistivity is measured by applying a voltage potential between two electrodes of specified configuration that are in contact with the same side of a material under test. The measurement was performed at ten different sample locations for a subsequent statistical analysis.

Dielectric properties were measured using the non-resonant method, more precisely parallel-plate method according to the ASTM D150-18⁴⁷ using Keysight 16,453A Dielectric Material Text Fixture on the Keysight E4991 A Impedance Analyzer, at temperature T = 23°C and relative humidity RH = 41%. The relative permittivity of the dielectric materials can be written in the complex form as

ε_{r}^{*} = ε_{r}^{'} - {j ε}_{r}^{″},

(16)

where

ε_{r}^{'}

and

ε_{r}^{″}

are the real and imaginary parts of the relative permittivity, respectively.⁴⁸ The dielectric constant is connected to the relative part of the permittivity, and it is a measure of the ability of a material to store electric energy by polarization. The imaginary part is referred to as the relative dielectric loss factor. The tangent of the angle between the storage and loss components is known as the loss tangent, and it is expressed as

\tan δ = \frac{ε_{r}^{″}}{ε_{r}^{'}} .

(17)

Microscopic analysis Composite samples were analyzed for their structure using a TESCAN VEGA TS 5130 scanning electron micro-scope having a resolution 3.5 nm and magnification 20 – 500 000 × (TESCAN s. r.o., Czech Republic). Samples were coated with gold using an SCD 030 auto sputter coating device (Balzers Union FL 9496, Balzers).

Results and discussion

Basic physical properties of the carbon tape reinforced composites

Table 5 presents the main physical properties of carbon fabric reinforced composites, including GSM, density, thickness, fiber weight fraction, fiber volume fraction, and cavity volume fraction. Mean values together with 95% confidence interevals of means of evaluated properties are shown. As the table shows, all parameters (except volume fraction of cavities) are increasing with an increase in warp and weft setts. At the same time, an increase in values is also observed when using carbon tapes with increasing width, which is caused by the increase in the weight of the reinforcement when both factors are increased. On the contrary, the cavity volume fraction of the fabric reinforced composite decreases both with an increase in the warp and weft setts and with an increase in the width of the carbon tape.

Table 5.

Physical properties of composites.

Sample code	GSM [g.m⁻²]	ρ_c,ex [g.cm⁻³]	h [mm]	W_f [%]	V_f [%]	V_cav [%]
1	34.55 ± 0.67	0.17 ± 4.82E-03	0.20 ± 0.01	58.68 ± 1.53	48.58 ± 1.02	88.21 ± 0.97
2	42.77 ± 2.44	0.19 ± 5.72E-03	0.22 ± 0.02	59.27 ± 4.40	49.10 ± 2.87	86.77 ± 2.39
3	78.28 ± 3.71	0.24 ± 3.15E-03	0.33 ± 0.02	60.38 ± 1.60	50.25 ± 1.18	83.51 ± 1.23
4	113.54 ± 2.48	0.28 ± 7,99E-03	0.39 ± 0.02	62.40 ± 1.21	52.38 ± 0.82	80.33 ± 1.47
5	67.90 ± 1.30	0.25 ± 3.98E-02	0.28 ± 0.06	57.09 ± 1.04	46.86 ± 0.93	82.93 ± 3.36
6	122.65 ± 4.79	0.30 ± 9.67E-03	0.41 ± 0.03	67.19 ± 3.87	57.58 ± 1.27	79.95 ± 3.43
7	175.54 ± 1.90	0.38 ± 3.40E-02	0.46 ± 0.05	66.86 ± 0.65	57.21 ± 0.33	74.41 ± 2.90
8	127.62 ± 0.80	0.34 ± 1.55E-02	0.37 ± 0.02	62.53 ± 0.91	52.52 ± 0.62	76.59 ± 1.88
9	228.89 ± 4.03	0.40 ± 6.64E-03	0.57 ± 0.02	68.52 ± 1.14	59.07 ± 0.46	73.56 ± 1.66
10	344.30 ± 6.55	0.49 ± 1.10E-02	0.71 ± 0.03	69.42 ± 3.46	60.07 ± 0.83	67.79 ± 4.85
11	568.12 ± 12.67	0.59 ± 5.18E-03	0.96 ± 0.03	69.04 ± 1.54	59.64 ± 0.56	60.77 ± 2.78

Analysis of the electromagnetic shielding effectiveness of the carbon tape reinforced composites

This evaluation aimed to determine the effect of the two factors mentioned above on the resulting electromagnetic shielding ability of the composite. Another goal was to find out how the SE of composite changes due to epoxy impregnation compared to the SE of the reinforcement itself.

Table 6 summarizes the experimentally measured mean values, standard deviations, and margins of error (half of the width of 95% confidence interval of mean) of the SE means for all the samples and the frequencies of 600 MHz, 1, 1.5, and 3 GHz. Here, it was clear that carbon tape reinforcements with a SE ranging from 11 to 51 dB at a frequency of 1.5 GHz were prepared.

Table 6.

Mean values, standard deviations, and margins of error of composite SE measured at frequency: 600 MHz, 1 GHz, 1.5 GHz and 3 GHz.

Sample code	Measured electromagnetic shielding effectiveness SE [dB]
Sample code	f = 600 MHz			f = 1 GHz			f = 1.5 GHz			f = 3 GHz
	$\bar{x}$	std	MOE_0.95	$\bar{x}$	std	MOE_0.95	$\bar{x}$	std	MOE_0.95	$\bar{x}$	std	MOE_0.95
1	18.83	2.20	2.53	13.17	1.15	1.00	11.90	0.85	0.98	3.60	0.78	0.90
2	23.69	2.23	2.57	18.19	2.11	1.83	16.76	1.43	1.64	7.31	1.47	1.69
3	27.10	1.29	1.48	21.37	0.67	0.58	19.98	0.61	0.70	10.98	1.80	2.07
4	28.93	2.53	2.91	23.96	3.19	2.78	22.63	2.92	3.35	13.43	2.89	3.32
5	31.02	0.61	0.71	25.24	0.96	0.84	23.79	0.90	1.03	14.19	0.92	1.06
6	33.93	0.14	0.17	28.79	1.52	1.32	27.16	1.68	1.94	17.20	1.80	2.07
7	36.70	0.70	0.80	31.90	0.07	0.06	30.56	0.20	0.23	21.39	0.82	0.94
8	39.90	0.87	1.00	36.17	1.01	0.88	35.53	0.81	0.93	25.95	1.47	1.69
9	45.11	0.32	0.37	42.09	1.31	1.14	41.29	1.32	1.51	30.20	1.62	1.86
10	49.96	0.61	0.70	47.64	1.14	0.99	47.23	1.19	1.37	34.87	1.13	1.30
11	58.86	5.18	5.96	54.58	5.46	4.75	51.65	4.26	4.90	38.37	4.00	4.60

The results of the analysis of variance (ANOVA) for the factorial design are presented in Table 7. Here, SS is the sum of squares, df is the degrees of freedom, MS is the mean squared error, which is SS/df for each source of variation, the F-statistic is the ratio of the mean squared errors, and the p-value is the probability that the test statistic can take a value greater than the value of the computed test statistic. The SE was measured at a frequency of 1.5 GHz.

Table 7.

Analysis of variance for the factorial design.

Source	SS	Percent contribution	df	MS	F	p-value
Spacing	2161.18	86.62	2	1080.59	2464.76	1.1e-22
Width	296.37	11.88	2	18.18	338	5.3e-15
Spacing*Width	29.64	1.19	4	7.41	16.9	6.5e-06
Error	7.89	0.32	18	0.44
Total	2495.08		26

Here, it was clear that both factors, Spacing and carbon tape Width, were statistically significant since all of the p-values were below the significance level (α = 0.05). Meanwhile, a two-factor interaction was also significant. The main effect of Spacing dominated this process, accounting around 87% of the total variability, while the main effect of Width accounted around 12% and the percentage contribution of the interaction was ∼1%. It was confiremed that Spacing corresponding to warp and weft sett of woven fabric was the main influencing factor in this case which was expected and this finding is consistent with published results in the field of electromagnetic shielding fabrics development.^18,49–51 In addition, decreasing the Spacing by one level while keeping other variables constant increases the carbon roving area by 100%, while increasing the roving thickness by one level increases the area of electrically conductive material by only 59%. A normal probability plot of the residuals and the other usual diagnostics indicated that both the normality and the constant variance assumptions were met. The spacing–width plot is shown in Figure 7, with the main effect plots indicating that variable Spacing had a negative main effect, i.e., an increase in the variable moved the electromagnetic SE downward, while variable Width had a positive main effect. Overall, the main effect of the spacing was higher than that of the width.

Figure 7.

Dependence of SE on: (a) carbon tape spacing and (b) carbon tape width forming the weft and warp threads in the composite structure.

The regression model that describes this experiment (first-order model with interactions) can be written in the following form

\hat{S E} = β_{0} + β_{1} S p a c i n g + β_{2} W i d t h + β_{12} S p a c i n g W i d t h + r,

(18)

where SE [dB] is the function fitted to a response,

β_{i}

is the regression coefficient to be estimated, Spacing and Width are the factors, and r is the residual. The regression coefficients can be estimated using the least square method in this model, with the final equation having the following form

\hat{S E} = 30.09 - 1.28 S p a c i n g + 6.96 W i d t h - 0.29 S p a c i n g W i d t h + r .

(19)

Figure 8 shows the case’s three-dimensional response surface and the contour plot. It should be mentioned that the simplest possible model was chosen, which adequately describes the experiment with a relatively high coefficient of determination (R² = 0.97).

Figure 8.

The response surface graph for dependent variable SE (f = 1.5 GHz).

The Table 8 shows the difference between the measured and the predicted value called residual for whole sample set. The residual is defined as

r = S E - \hat{S E} .

(20)

Table 8.

Error between measured and predicted SE data (f = 1.5 GHz).

Sample code	SE [dB]
Sample code	Measured $S E$	Predicted $\hat{S E}$	r
1	11.90	6.81	5.09
2	16.76	16.11	0.65
3	19.98	18.72	1.26
4	22.63	21.33	1.30
5	23.79	25.41	1.62
6	27.16	29.47	2.31
7	30.56	33.53	2.97
8	35.53	34.71	0.82
9	41.29	40.22	1.07
10	47.23	45.73	1.50
11	51.65	42.15	9.50

As Table 8 shows, r < 5 for all samples except for the sample 11 which confirms the suitability of the proposed model, especially taking into account the fact that the random error of the coaxial transmission line method, as reported in the standard (see Ref. ⁴⁰), is ±5 dB.

The obtained model (Eq. (19)) could also be used to create a graphical representation of the experimental region. The resultant response-surface contour plot (Figure 8) indicates how the response related to two continuous design variables behaves. Here, it was clear that the higher the carbon tape width and the lower the carbon tape spacing (i.e., higher warp and weft sett), the higher the SE.

Effect of carbon tape spacing and carbon roving width on the SE of composite

Figure 9(a) shows the dependence of the mean SE values on the frequency for samples 1, 2, 5, 8, and 11, with the shading around the mean values indicating their 95% intervals. Here, it was clear that the SE decreased slightly with an increase in frequency (decreasing wavelength) for all the samples, which was in accordance with the theoretical assumptions. It should be noted that the lowest examined frequency (30 MHz) corresponded to a wavelength of 10 m, while the highest measured frequency corresponded to a wavelength of 0.1 m, meaning the longest aperture dimensions of all samples was small (L <28 mm) concerning the wavelength of the electromagnetic wave for the entire studied frequency range. It was also observed that the wider the carbon thread spacing s (the lower the aperture dimension L), the lower the SE ability of the sample. The sample with the lowest warp and weft sett was found to have the lowest SE (SE = 12 dB for f = 1.5 GHz), while that with the highest warp and weft sett had the highest SE (SE = 52 dB for f = 1.5 GHz) comparing samples having carbon tape width a = 2 mm. Figure 9(b) shows the dependence of the SE on the carbon tape spacing s at a particular frequency, i.e., 1.5 GHz, where, with an increase in s, the SE behavior was more visible. The solid line in this figure represents the data approximation using an exponential function with a high coefficient of determination (R² = 0.99). It should be noted that the specific spacing of the carbon threads, or the sett of the warp and the weft of the fabric, must be chosen in view of the requirements of the final application.

Figure 9.

Dependence of SE on (a) frequency and (b) carbon tape spacing (f = 1.5 GHz) for samples 1,2,5,8 and 11.

Figure 10(a) shows the dependence of the SE on the frequency for samples 2, 3, and 4, i.e., those with different widths of carbon tape in their warp and weft threads (s = 15 mm). Again, it was clear that the SE decreased slightly with an increase in frequency and that this was distinct over the entire frequency range, i.e., the greater the number of tapes forming the warp and weft threads, the greater the SE. Meanwhile, Figure 10(b) shows the relationship between the SE and carbon tape width making up the warp and weft threads for the frequency of 1.5 GHz, which could be approximated using a logarithmic function with a very good fit (R² = 0.99). The same behavior was also observed with the other sets of samples (s = 10 mm, s = 5 mm) having different widths of carbon tape making up the warp and weft threads.

Figure 10.

Dependence of SE on (a) frequency and (b) a number of carbon tapes (f = 1.5 GHz) for samples 2, 3, and 4.

In Figure 11(a), there is a comparsion of SE of all sample set for the frequency 1.5 GHz, which corresponds to the behavior of composites described above.

Figure 11.

(a) Comparison of SE for all sample for f = 1.5 GHz, (b) SE throught absorption and reflection for sample 9.

Furthermore, the mechanism behind the shielding ability of the prepared carbon reinforcements intended for the formation of composites was investigated in terms of the chosen sample set. The SE effectiveness through absorption (SE_A) and reflection (SE_R) was calculated (in dB) based on the measured scattering parameters S₂₁ and S₁₁ according to equations (4)–(6), and (8) for chosen sample no. 9 (s = 5 mm, a = 4 mm) with the results presented in Figure 11(b). When comparing the mean values of SE_A and SE_R over the entire frequency range, it is visible that SE_A was the dominant mechanism for very low frequencies (f < 80 MHz), while at higher frequencies, SE_R was the main contributor to the SE.

Effect of epoxy resin impregnation on the SE of composite

As mentioned above, one of the aims of this study was to find out how the applied epoxy resin affects the shielding ability of the composite. That is why SE of composite and reinforcement were compared for all samples and all frequencies; see frequency-dependent SE graphs in Figure 12 for chosen samples. It is visible that composite SE is relatively close to the SE of reinforcement for f > 1 GHz. The composite appears to have a higher shielding ability for f < 1 GHz compared to the SE of reinforcement alone. To objectively evaluate the positive effect of impregnation using epoxy resin on SE, i. e to determine if there is a significant difference between the means of two groups (reinforcement, composite), two-sample t-test was realized for all sample’s SE at selected frequencies (600 MHz, 1 GHz, 1.5 GHz, and 3 GHz). Null and alternative hypotheses are as follows

H_{0} : µ_{R} = µ_{C}; H_{1} : µ_{R} \neq µ_{C}

(20)

where μ_R and μ_C are the mean values of SE of reinforcement and composite, respectively, and the results of the tests (0 – does not reject the null hypothesis, 1 – rejects null hypothesis) are summarized in Table 9. Performed t-tests confirm that composites have higher shielding efficiency for f

\leq

1 GHz compared to their reinforcement. For higher frequencies, the shielding ability of the composite is higher or the same compared to the reinforcement itself, which is desirable. This phenomenon is probably caused by following reasons: the higher thickness of the material in the case of the composite compared the thickness of reinforcement, non-disturbance of the conductive paths formed by the carbon tapes after the impregnation of the reinforcement with the matrix. SEM images in Figure 13 show reinforcement binding point in composite fracture capturing the level of saturation of the weft and warp thread fibers by the matrix. From the image, it seems that there is a pretty good saturation of reinforcement by matrix; however, the evaluation of the composite electromagnetic shielding confirms that the conductive paths have not been violated. In summary, the matrix, in this case, provides mechanical robustness and protective function to the composite while maintaining the carbon electro-conductive network.

Figure 12.

Comparison of SE of composite and reinforcement itself for (a) sample 1, (b) sample 7, and (c) sample 11.

Table 9.

The result of two-sample t-test.

Sample Code	Frequency [GHz]
Sample Code	0.6	1	1.5	3
1	1	1	1	0
2	1	1	1	0
3	1	1	1	0
4	1	1	0	0
5	1	1	1	1
6	1	1	1	0
7	1	1	0	0
8	1	1	1	1
9	1	1	1	1
10	1	1	1	1
11	1	1	0	1

Figure 13.

SEM images the composite fracture at the binding point with magnification (a) 500 × and (b) 1000 ×.

Air permeability and thermal properties

The aim of this evaluation was to analyze other properties of composites besides the electromagnetic shielding efficiency, which could also be essential for concrete practical use. In this subchapter the following descriptive quantities will be discussed: air permeability A_p [mm/s], which is directly related to water vapor permeability and the ability of the material to dissipate heat – thermal conductivity λ [Wm⁻¹K⁻¹], and thermal resistivity RT [Km²W⁻¹]. Mean values, standard deviations and margins of error for these characteristics are summarized in Table 10.

Table 10.

Mean values, standard deviations, and margins of error for composite air permeability A_p, thermal conductivity λ, and thermal resistivity RT.

Sample code	A_p [mm/s]			λ 10⁻³ [Wm⁻¹K⁻¹]			RT 10⁻³ [Km²W⁻¹]
Sample code	$\bar{x}$	std	MOE_0.95	$\bar{x}$	std	MOE_0.95	$\bar{x}$	std	MOE_0.95
1	10,000.00	200.00	496.83	21.13	3.61	4.09	8.37	0.47	0.53
2	6910.00	805.79	2001.70	22.57	1.27	1.43	7.47	0.35	0.40
3	4106.67	354.45	880.50	28.57	1.63	1.84	9.87	0.45	0.51
4	3173.33	660.40	1640.53	31.47	1.52	1.72	11.30	0.36	0.41
5	3653.33	418.61	1039.88	26.50	0.50	0.57	9.97	1.50	1.70
6	1763.33	535.38	1329.96	30.67	1.00	1.13	12.83	1.91	2.16
7	1420.00	181.93	451.95	32.63	1.07	1.21	12.03	0.59	0.66
8	1041.33	249.29	619.27	30.47	0.75	0.85	10.80	0.35	0.39
9	677.67	191.89	476.68	35.77	0.61	0.69	15.37	0.76	0.86
10	641.33	165.77	411.80	39.43	1.53	1.73	17.03	1.71	1.93
11	90.40	14.36	35.66	45.73	1.71	1.93	19.03	0.55	0.62

Air circulation and moisture vapor transport are frequently emerging requirements for composite structures. When evaluating air permeability A_p, it can be seen (Figure 14(a), Table 10) that air permeability is decreasing strongly with decreasing spacing between carbon tapes forming the warp and weft of the woven reinforcement and ranges between 100 – 10 000 mm/s. As expected, the decrease in air permeability can also be observed with increasing carbon tape width while maintaining the same spacing.

Figure 14.

Comparison of (a) air permeability, (b) thermal conductivity, and thermal resistivity for all samples.

An increase in the content of the conductive component in the unit volume of the sample also positively affects the thermal conductivity of the sample, which is in accordance with the theoretical analysis. The smaller the content of well-thermal insulating air in the sample, the higher its thermal conductivity; see Figure 14(b) and Table 10. It can also be seen from the performed analysis that the thermal resistance increases with the increasing content of carbon tape in the composite sample, which is probably caused by the increasing thickness of the sample, see Figure 14(c) and Table 10.

Mechanical properties

In this subchapter the following descriptive quantities will be discussed: ultimate mechanical properties – breaking force F [N/40 mm] and breaking elongation ε_b [%] and bending force Fm [Nm]. Mean values, standard deviations and margins of error for these characteristics are summarized in Table 11. The fabricated composite samples have a relatively high tensile strength, between 500 – 7000 N, see tensile force displacement curves at Figure 15(a), breaking force comparison at Figure 15(b) and Table 10. The more carbon tapes a composite sample contains, the higher its tensile strength, whereas the width of the carbon tape appears to have a more significant effect on strength. The elongation of the sample is also related to the tensile strength. The higher the tensile strength, the higher its elongation to break, see Figure 15(c). The bending force of composites samples varies in a relatively wide range from 0.5 to 27 Nmm, see bending force displacement curves at Figure 16(a) and bending force comparison at Figure 16(b). The stiffness of sample 11 was so high that it was beyond the capacity of the measuring device. It can also be seen here that the more dense the reinforcing fabric is, the higher the resulting bending stiffness of the composite.

Table 11.

Mean values, standard deviations, and margins of error for composite breaking force F, breaking elongation ε_b, and bending force Fm.

Sample code	F [N/40 mm]			ε_b [%]			Fm [Nmm]
Sample code	$\bar{x}$	std	MOE_0.95	$\bar{x}$	std	MOE_0.95	$\bar{x}$	std	MOE_0.95
1	467.29	60.80	59.58	1.43	0.32	0.32	0.63	0.18	0.15
2	524.88	46.68	45.74	2.47	0.54	0.53	0.42	0.11	0.10
3	1042.18	18.90	18.52	2.31	0.95	0.93	2.07	0.65	0.57
4	1633.53	83.85	82.17	2.29	0.12	0.11	3.89	0.47	0.41
5	1139.88	27.58	27.03	2.20	0.02	0.02	0.71	0.06	0.06
6	2161.64	108.64	106.46	2.49	0.19	0.19	2.59	0.62	0.54
7	3717.81	269.16	263.77	3.07	0.17	0.17	8.04	0.56	0.49
8	2482.67	197.24	193.29	2.70	0.18	0.18	2.55	0.41	0.36
9	4228.29	440.14	431.33	3.12	0.23	0.22	10.75	0.57	0.50
10	5230.46	53.87	52.79	3.51	0.07	0.07	27.27	2.90	2.54
11	6770.04	758.96	743.77	3.47	0.22	0.21	-	-	-

Figure 15.

(a) The tensile force displacement curves; comparison of (b) breaking force, (c) breaking elongation for all samples.

Figure 16.

(a) The bending force dispacement curves, comparison of (b) bending force for all samples.

Electrical properties

The electrical conductivity and permittivity of the reinforcement itself and the composite will be disucussed in this subchaper. In addition to other composites evaluations, the electrical conductivity, more precisely the surface and volume resistivity of the carbon reinforcement and the composite, were also compared. Sample 5 was chosen for this comparison. The comparison shows that the application of a non-conductive matrix causes a significant (by several orders of magnitude) increase in both resistivities, see Figure 17(a). However, this does not affect the electromagnetic shielding ability in any way, as it was demonstrated during the SE analysis. The matrix ensures the necessary robustness and mechanical endurance of the sample and does not impair the shielding ability of the composite. Figure 17(b) also compares the permittivity of the reinforcement and the composite for sample 5.

Figure 17.

Comparison of (a) electrical resistivity, (b) permittivity for reinforcement, and composite for sample 5.

Quality index evaluation

The total performance of advanced composites was evaluated using the Quality index, see chapter 3. From all tested characteristics, three properties were selected as critical, while their list, including the weights and the completely satisfactory H and unsatisfactory L values, is given in Table 12. The L, H values and weights were determined concerning the intended use of the composite as one of the components in manufacturing aircraft fuselage. These values were further determined using the acquired knowledge and experience of the author’s collective, which has been dedicated to the issue of electromagnetic shielding of a close space for a relatively long time.

Table 12.

Chosen properties for quality index estimation, their L, H values, and weights.

Property	L	H	Weight w_i [-]
Electromagnetic shielding effectiveness SE [dB] for f = 1.5 GHz	20	50	0.5
Air permeability Ap [mm/s]	50	200	0.3
Bending force Fm [Nmm]	10	5	0.2

Table 13 shows the values of the quality index for all composite samples taking into consideration their electromagnetic shielding ability, air permeability, and bending force. It is visible that sample 8 (a = 2 mm, s = 5 mm) has the highest quality index. It is followed by samples 10 and 9. It seems that the optimum spacing of carbon tapes in the sample is 5 mm. In this case, the electromagnetic shielding ability of the sample is balanced with other properties such as air and moisture permeability and bending force. The choice of defining properties strongly depends on the purpose of the use of the composite. The described relationship is universal. It is also possible to incorporate other characteristics desirable for the final purpose of using the composites.

Table 13.

Quality index including ranking (high to low) for the whole sample set.

Sample code	1	2	3	4	5	6	7	8	9	10	11
QI	0.32	0.32	0.32	0.28	0.34	0.48	0.48	0.71	0.53	0.60	0.43
Ranking	9	8	11	10	7	5	4	1	3	2	6

Conclusion

In this paper, we introduced porous and low thickness CCF reinforced epoxy composites with excellent flexibility and superior electromagnetic radiation protection. A total of eleven composite types were manufactured using plain-weave reinforcements made of carbon tapes (fineness of single carbon tape T = 196 tex) for the weft and warp threads. The samples differed in the two following respects of the reinforcement structure: warp and weft sett (spacing between threads) and warp and weft thread thickness (by combining two and three carbon tapes). To maintain low thickness, high porosity, and flexibility of the composites, CCF woven fabric reinforcements were only impregnated by epoxy resin, which endows composite mechanical robustness by stabilization of binding points. Using this simple approach, reinforcing CCF fabrics can be easily impregnated with matrix materials that are not easily obtainable in the form of prepregs to get composites for advanced application.

Composites with a density ranging form 0.2 – 0.6 g/cm³ were prepared; having a thickness from 0.2 to 0.96 mm. Manufactured composites showed shielding ability of 12–52 dB at 1.5 GHz, whereas the main effect of the carbon tape spacing was higher than that of the carbon tape width. As predicted, the higher the spacing, the lower the SE. It was found that impregnation of the CCF reinforcement with an epoxy matrix does not have a statistically significant effect on the SE of the composite. In addition, it was found that the dominant mechanism of SE for polymer-matrix CCF composites is reflection. The spacing of carbon tapes had a negative effect (an increase in the variable moved the dependent variable downward) on breaking strength and breaking elongation. In contrast, it had a positive impact on air permeability, bending rigidity and thermal properties.

The composite containing reinforcement with warp and weft sett 18 dm⁻¹ using thread 2 mm wide achieved the highest quality index. This composite provides electromagnetic shielding of 36 dB at 1.5 GHz, having a high air permeability of 1000 mm/s, relatively low force rigidity around 2.5 Nmm, and thickness of only 0.37 mm.

The chosen manufacturing process readily applies to reinforcing CCF fabrics with relatively low warp and weft sett, where binding points displacement is a limiting factor, and it represents a significant advantage over prepregs. Using this approach, low-cost, porous CCF woven reinforced composites that are permeable to air and water vapor with the desired SE can be designed and obtained for various applications in the aircraft industry.

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 author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by The European Union - European Structural and Investment Funds in the frames of Operational Programme Research, Development and Education - project Hybrid Materials for Hierarchical Structures and Ministry of Education Youth and Sports of the Czech Republic - project Hybrid Materials for Hierarchical Structures (HyHi, Reg. No. CZ.02.1.01/0.0/0.0/16_019/0000843).

ORCID iDs

Veronika Tunakova

Maros Tunak

References

Schroder

Rasek

Bruns

, et al. Analysis of high intensity radiated field coupling into aircraft using the method of moments. IEEE Trans Electromagn Compat 2014 Feb; 56(1): 113–122. DOI: 10.1109/TEMC.2013.2278208

Soutis

. Carbon fiber reinforced plastics in aircraft construction. Mater Sci Eng A 2005 Dec; 412(1–2): 171–176. DOI: 10.1016/j.msea.2005.08.064

Advanced high strength natural fibre composites in construction. Amsterdam: Elsevier, 2017.

Jou

. A novel structure of woven continuous-carbon fiber composites with high electromagnetic shielding. J Electron Mater 2004; 33(3): 162–170. DOI: 10.1007/s11664-004-0175-x

Jou

. The hight electromagentic shielding of woven carbon fiber composites appiled to optoelectronics devices. In: CLEO/Pacific Rim 2003. The 5th Pacific Rim Conference on Lasers and Electro-Optics (IEEE Cat. No.03TH8671). Taipei, Taiwan: IEEE, 2003, 755. DOI: 10.1109/CLEOPR.2003.1277291

Jou

Chiu

, et al. The influence of fiber orientation on electromagnetic shielding in liquid-crystal polymers. J Electron Mater 2002; 31(3): 178–184. DOI: 10.1007/s11664-002-0203-7

Wen

Wang

Zhang

. Ultrathin and anisotropic polyvinyl butyral/Ni-graphite/short-cut carbon fibre film with high electromagnetic shielding performance. Compos Sci Technol 2019 Jan; 169: 127–134. DOI: 10.1016/j.compscitech.2018.11.013

Unterweger

Mayrhofer

Piana

, et al. Impact of fiber length and fiber content on the mechanical properties and electrical conductivity of short carbon fiber reinforced polypropylene composites. Compos Sci Technol 2020; 188: 1079982020. DOI: 10.1016/j.compscitech.2020.107998

Hwang

. Tensile, electrical conductivity and EMI shielding properties of solid and foamed PBT/carbon fiber composites. Compos B Eng 2016 Aug; 98: 1–8. DOI: 10.1016/j.compositesb.2016.05.028

10.

Jana

Mallick

. Effects of sample thickness and fiber aspect ratio on EMI shielding effectiveness of carbon fiber filled polychloroprene composites in the X-band frequency range. IEEE Trans Electromagn Compat 1992; 34(4): 478–481. DOI: 10.1109/15.179281

11.

Das

N C

Khastgir

Chaki

T K

, et al.. Electromagnetic interference shielding effectiveness of carbon black and carbon fibre filled EVA and NR based composites. Compos Part A Appl Sci 2000 Oct; 31(10): 1069–1081. DOI: 10.1016/S1359-835X(00)00064-6

12.

Luo

Chung

DDL

. Electromagnetic interference shielding using continuous carbon-fiber carbon-matrix and polymer-matrix composites. Compos B Eng 1999 Apr; 30(3): 227–231. DOI: 10.1016/S1359-8368(98)00065-1

13.

Zhu

Shi

, et al. Simultaneously improved mechanical and electromagnetic interference shielding properties of carbon fiber fabrics/epoxy composites via interface engineering. Compos Sci Technol 2021 May; 207: 108696. DOI: 10.1016/j.compscitech.2021.108696

14.

Das

Yokozeki

. A brief review of modified conductive carbon/glass fibre reinforced composites for structural applications: lightning strike protection, electromagnetic shielding, and strain sensing. Compos Part C: Open Acc 2021; 5: 1001622021. DOI: 10.1016/j.jcomc.2021.100162

15.

Lin

M-C

Lin

J-H

Bao

. Applying TPU blends and composite carbon fibers to flexible electromagnetic-shielding fabrics: long-fiber-reinforced thermoplastics technique. Compos A: Appl. Sci. Manuf 2020; 138: 1060222020. DOI: 10.1016/j.compositesa.2020.106022

16.

Munalli

Dimitrakis

Chronopoulos

, et al.. Electromagnetic shielding effectiveness of carbon fibre reinforced composites. Compos B Eng 2019 Sep; 173: 106906. DOI: 10.1016/j.compositesb.2019.106906

17.

Chen

Wang

, et al. Electromagnetic interference shielding properties of electroless nickel-coated carbon fiber paper reinforced epoxy composites. J Wuhan Univ Technol-Mat Sci Edit 2014 Dec; 29(6): 1165–1169. DOI: 10.1007/s11595-014-1060-y

18.

Krishnasamy

Alagirusamy

Das

, et al. Electromagnetic shielding behaviour of conductive filler composites and conductive fabrics–A review. Ind J Fib & Tex Res (IJFTR) 2014; 39(3): 329–342.

19.

Rea

Linton

Orr

, et al. Electromagnetic shielding properties of carbon fibre composites in avionic systems. Microw Rev 2005; 29(1): 29–32.

20.

Lee

. Electro-mechanical properties of the carbon fabric composites with fibers exposed on the surface. Composite Structures 2016 Apr; 140: 77–83. DOI: 10.1016/j.compstruct.2015.12.066

21.

Paiva

JMFd

Mayer

Rezende

. Comparison of tensile strength of different carbon fabric reinforced epoxy composites. Mat Res 2006 Mar; 9(1): 83–90. DOI: 10.1590/S1516-14392006000100016

22.

Mayr

Plank

Sekelja

, et al. Active thermography as a quantitative method for non-destructive evaluation of porous carbon fiber reinforced polymers. NDT & E International 2011 Nov; 44(7): 537–543. DOI: 10.1016/j.ndteint.2011.05.012

23.

Peng

Burtovyy

Bordia

, et al. Fabrication of Porous carbon films and their impact on carbon/polypropylene interfacial bonding. J Compos Sci 2021 Apr; 5(4): 108. DOI: 10.3390/jcs5040108

24.

Banciu

Bara

Ion

, et al. Structural and functional properties of porous carbon fibers composites. Optoelectron Adv Mater Rapid Commun 2010; 4(11): 1647–1650.

25.

Chen

Wang

, et al. Porous carbon-bonded carbon fiber composites impregnated with SiO2-Al2O3 aerogel with enhanced thermal insulation and mechanical properties. Ceramics International 2018 Feb; 44(3): 3484–3487. DOI: 10.1016/j.ceramint.2017.11.064

26.

Tunakova

Tunak

. Carbon-fiber reinforcements for epoxy composites with electromagnetic radiation protection-prediction of electromagnetic shielding ability. Compos Sci Technol 2021; 215: 1090292021. DOI: 10.1016/j.compscitech.2021.109029

27.

Venkataraman

Militký

Samková

, et al. Hybrid prepreg tapes for composite manufacturing: a case study. Mater 2022 Jan; 15(2): 619. DOI: 10.3390/ma15020619

28.

Messiry

. Theoretical analysis of natural fiber volume fraction of reinforced composites. Alex Eng J 2013 Sep; 52(3): 301–306. DOI: 10.1016/j.aej.2013.01.006

29.

Iyer

Phatarfod

. 18—Some aspects of yarn structure. J Text Inst 1965 May; 56(5): T225–T247. DOI: 10.1080/19447026508662281

30.

Hearle

JWS

Merchant

. Relations between specific volume, count, and twist of spun nylon yarns. Tex Res J 1963 Jun; 33(6): 417–424. DOI: 10.1177/004051756303300603

31.

Gracie

. Twist geometry and twist limits in yarns and cords. J Text Inst 1960 Jul; 51(7): T271–T288. DOI: 10.1080/19447026008659767

32.

Neckar

. Yarns: creation, structure, properties. Prague, Czech Republic: SNL, 1990.

33.

Gurumurthy

B R

. Prediction of fabric compressive properties using artificial neural networks. Autex Res J 2007; 7(1): 19–31.

34.

Godfrey

(eds) Juran’s Quality Handbook. 5th ed. NY, USA: McGraw-Hill, 1999.

35.

‘Material safety data sheet in accordance to EEC-Regulation 91/155/EEC’. 2009. [Online] Available at: https://www.b2bcomposites.com/msds/icc/816019.pdf

36.

Montgomery

. Design and analysis of experiments. 10th ed. Hoboken, NJ: Wiley, 2020.

37.

‘Technical information for EPIKOTE resin MGS LR285’, 2010. [Online]. Available: https://www.swiss-composite.ch/pdf/t-Epoxyd-Harz-L-285-LF-e.pdf

38.

D13 Committee . Test method for thickness of textile materials. Conshohocken: ASTM International, 2019.

39.

D13 Committee . Test methods for mass per unit area (weight) of fabric. Conshohocken: ASTM International, 2020.

40.

ASTM 4935-18 . ‘Test method for measuring the electromagnetic shielding effectiveness of planar materials’. Conshohocken: ASTM International, 2018.

41.

‘Textiles. Determination of the permeability of fabrics to air. London: BSI British Standards, 1995.

42.

Hes

Bal

Dolezal

. Principles of clothing comfort and their use in evaluation of sensorial and thermal comfort of men’s casual jacket. Fibers Polym 2021 Oct; 22(10): 2922–2928. DOI: 10.1007/s12221-021-0425-z

43.

Fridrichová

. A new method of measuring the bending rigidity of fabrics and its application to the determination of the their anisotropy. Tex Res J 2013 Jun; 83(9): 883–892. DOI: 10.1177/0040517512467133

44.

Czech technical standard , ‘Textiles. Testing of stiffness and resiliance of textile fabrics’. Prague: Czech standardization agency, 1974 Jan.

45.

D13 Committee . Test method for breaking force and elongation of textile fabrics (strip method). Conshohocken: ASTM International, 2015.

46.

D09 Committee . Test methods for DC resistance or conductance of insulating materials. Conshohocken: ASTM International, 2021.

47.

D09 Committee . Test methods for AC loss characteristics and permittivity (dielectric constant) of solid electrical insulation. Conshohocken: ASTM International, 2022.

48.

Yamada

. Dielectric properties of textile materials: analytical approximations and experimental measurements—a review. Textiles 2022 Jan; 2(1): 50–80. DOI: 10.3390/textiles2010004

49.

Das

Krishnasamy

Alagirusamy

, et al. Electromagnetic interference shielding effectiveness of SS/PET hybrid yarn incorporated woven fabrics. Fibers Polym 2014 Jan; 15(1): 169–174. DOI: 10.1007/s12221-014-0169-0

50.

Duran

Kadoğlu

. Electromagnetic shielding characterization of conductive woven fabrics produced with silver-containing yarns. Tex Res J 2015 Jun; 85(10): 1009–1021. DOI: 10.1177/0040517512468811

51.

Cheng

K B

Lee

M L

Ramakrishna

, et al. Electromagnetic shielding effectiveness of stainless steel/polyester woven fabrics. Tex Res J 2001 Jan; 71(1): 42–49. DOI: 10.1177/004051750107100107