# Influence of Magnetic Wood on Mechanical and Electromagnetic Wave-Absorbing Properties of Polymer Composites

Authors: Satish Geeri; B. Krishna Murthy; Aditya Kolakoti; M. Murugan; P. V. Elumalai; N. R. Dhineshbabu; S. Prabhakar
Journal: International Journal of Polymer Science
 (2023)

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## Abstract

The application of wireless electronic devices is increasing nowadays; hence, there is a necessity for electromagnetic wave-absorbing material, which is mechanically stable. Polymer composites with magnetic wood as filler material were fabricated by hand lay-up methods of 6 mm thickness. For the proposed immersion duration, magnetic wood was developed by in situ chemical co-precipitation methods. The microwave absorbing phenomenon is evaluated based on the complex permeability, complex permittivity, dielectric tangent, and magnetic tangent losses. The experimentation was done by vector network analyzer in the frequency range of 8.2–12.4 GHz by X-band and Through-reflect-line (TRL) calibration. It was observed that the reflection losses increase as the immersion time increases from −8.70 dB to −9.30 dB at the frequency range of 10.2 GHz. A similar trend is also followed for the mechanical properties like tensile strength, bending strength, and impact strength with respect to the immersion time. The results revealed that the best mechanical and electromagnetic absorption properties are obtained for the specimens with immersion time of 72 hours. Validation is done for the electromagnetic wave-absorbing properties and mechanical properties by regression analysis, and the experimental data are in close agreement with the regression data.


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## Body

## 1. Introduction

The usage of the wireless communication devices is escalating day by day, and also the rapid growth of electronic appliances leads to generate electromagnetic (EM) waves. These EM waves have an impact on human health and also on the durability of electronic instruments. Hence, there is a need to reduce the intensity of EM waves by using a shielding material. There are two different approaches for processing shielding material for EM waves, and the first one is that the material should absorb or reflect the EM waves when placed in specific applications. The second one is reinforcing the dielectric and magnetic particles in the material to act as a shielding material. Mechanical strength is also an essential parameter for this shielding material when placed in application [1–3]. The shielding material as a composite can be preferable, as the composites can be fabricated by involving the properties like dielectric and/or magnetic losses. Hence, these are influenced by the reflection losses (RLs), which reflect the EM wave’s performance.The fabrication process was done based on the second method, by adding magnetic wood to the polymer-based composites. Still, there is a requirement for EM wave-absorbing material with lightweight, more strength, and easy to process [1–4].The works carried out for improving the absorbing properties of EM waves are by synthesis of magnetic wood [4], nanoparticles [5–7], and coated materials [8, 9]. Wood is a naturally available resource and degradable material; hence, there is a scope for research to be carried out on magnetic wood-based polymer composites to absorb the EM waves generated by wireless communication systems.Even though these composites have some limitations as mentioned in the previous works, they are still used in many applications [10]. It has been proved that magnetic wood has strong magnetic properties of absorbing nature by Oka et al. [11–13]. The multi-layer magnetic composites were fabricated using in situ chemical co-precipitation method and followed by a hot pressing procedure. The EM wave-absorbing performance improved from −14.14 dB to −60.16 dB as the layers increased from 3 to 7 [14]. The EM absorbing properties were studied by in situ method with magnetic nanoparticles developed for poplar wood flours. The best results are obtained for different weight ratios of virgin wood floors and magnetic wood flours [15]. Depending on the thickness of the composites, the EM absorbing properties will be influenced [16].The composites were fabricated based on the porous Fe3O4 decorated graphene, and the developed Fe3O4 decorated graphene specimens exhibit an enhanced dielectric loss [17]. Studies were carried out on magnetic CO Fe3O4 and hydroxylapatite through a hydrothermal process, and the results revealed that the specimen had improved mechanical properties. It is also concluded that these composites have influenced lift, drag, alignment, and actuation by magnetic fields [18]. The composites were fabricated with iron oxide with a facile hydrothermal method with ferric trichloride and ferrous chloride as the precursors at 90°C. It is concluded that these composites have strong absorbing nature of EM waves [19].Oka et al. [20] investigated the properties of powder-type magnetic wood composites for different mixed ratios of Mn–Zn ferrite and Ni–Zn ferrite powder. The experimental data reveals that the mixed ratio influences by EM wave-absorbing properties and the thickness of the board [20]. Wang et al. [21] fabricated a lightweight with soft magnetic wood-based composites. It is stated that EM absorbing properties are influenced by the weight percentage of FeNi3 [21].Gao et al. [22] prepared a Fe3O4 wood composite by in situ chemosynthesis at room temperature by processing the specimen with X-ray powder diffraction (XRD), Value stream mapping (VSM), and Fourier transform infrared spectroscopy (FTIR) techniques. The results showed that the average grain size of Fe3O4 was approximately 14 nm. The vibrating sample magnetometry showed that the composites have saturation magnetisation Ms values from 4.7 to 25.3 emu/g, and concluded that interaction between wood and Fe3O4 turned stronger when less Fe3O4 was introduced in the composition [22].Based on the above literature study, it is clear that only a few researchers reported on the hand lay-up technique in magnetic wood-based polymer composites. It was also observed that most of the works focused purely on EM wave-absorbing behaviour. But in the present work, an attempt has been made to improve the EM properties as well as mechanical strength. The experimental results have been validated with the regression analysis, and the values have high accuracy.In the present work for the development of magnetic wood, the impregnated method is chosen [23], i.e., Fe2+ and Fe3+ solutions are used to immerse specimens. Hence, Fe3O4 particles were developed using the wood’s in situ chemical reaction method [24]. The main aim of the fabricated composites should have a strong absorbing nature and more strength.


## 2. Experimental Details

### 2.1. Materials

The core region of the oak wood tree’s main branch was considered a filler material for the present work. The other materials are epoxy resin (Aralide AW 106), E-glass fibre (woven fabric 310 gsm), and hardener (HV 953 U) with the required proportions were considered.


### 2.2. Synthesis of Magnetic Wood

Oak particleboard was considered to fabricate the composites with300×300×5 mm dimensions for three proposed specimens. After washing with distilled water, it was immersed in the mixed solution 2 : 1 molar ratio of Fe3+ and Fe2+ solutions for 24 hours, 48 hours, and 72 hours at room temperature. Later, the three specimens are soaked in ammonia solution for the same duration and followed by cleaning with distilled water and drying in sunlight. As the immersion time increases, the thickness swelling also increases. After going through these stages, the wood appears like a light shade of red colour with partially semi-solids in nature. This is used as a filler material for these composites during the fabrication processes. The magnetic/oak wood is placed at the centre of the composites along with the specification, as shown in Table 1.Table 1
Specification of composites.
S. noImmersion time of oakwood in solution (hours)No. of E-glass matComposites1243Specimen 12486Specimen 23729Specimen 3


### 2.3. Fabrication of Magnetic Wood-Based Polymer Composites

The E-glass mat is placed one after the other for all three specimens with polymer as per the proposal. But, placing the magnetic/oak wood at the centre of composites with a sequence of order after 2nd, 3rd, and 5th glass mat for specimens 1, 2, and 3, respectively. A hand lay-up technique was chosen for the fabrication process for the proposed specimens, as shown in Figure 1. Finally, three composites were fabricated with 300×300 mm dimensions.Figure 1
Flowchart of fabrication process.


### 2.4. Characterization

The tensile test was done at room temperature on a universal testing machine based on American Society for Testing and Materials (ASTM) D 638-89 standards. The stiffness property was determined by conducting the flexural test on a universal testing machine with ASTM D 790-86 standards, and the impact test was performed using ASTM D 7136-15 standards. The mechanical properties were determined for the three specimens by choosing five different locations of each composite for experimentation, and the specimens are shown in Figures2(a), 2(b), and 2(c). The specimens with the dimensions 22.86×10.16 mm were considered to evaluate the EM wave-absorbing properties by WR 90 waveguide method, and the specimens are as shown in Figure 2(d). These properties were calculated in the frequency range of 8.2–12.4 GHz with Through-reflect-line (TRL) calibration by a vector network analyzer (R&S ZVB 20). The EM wave-absorbing properties measure based on Equations (1) and (2).
(1)RLdB=20Log10Zi−Z0Zi+Z0,(2)Z=Z0μrεrtanh2πfμrεrμ0ε0×t.HereZi is input impedance, Z0 is absorber interface 377 Ω, f is the frequency in GHz, and εrand μr are relative complex permittivity and permeability, where space ε0=8.854×10−12 F/m−1, μr relative permeability of dielectric material, εr relative permittivity of magnetic material, t thickness, μ0=4Π×10−7 H/m−1.Figure 2
(a) Fabricated specimens for tensile testing, (b) fabricated specimens for bending testing, (c) fabricated specimens for impact testing, (d) fabricated specimens for electromagnetic wave-absorbing testing.


### 2.5. Morphology of Composites

The SEM image of Figure3(a) was taken after the tensile test conducted on composite, and it is evident that fibre is pulled out from the polymer as load acting in the fibre direction. As per the scanning electron microscope (SEM) images (Figure 3(a)), it is also evident that wood, polymer, and E-glass mats have a good bonding. During the magnetic treatment procedure, the specimen is placed in the solution for 24 hours, 48 hours, and 72 hours separately. Hence, the specimen colour, thickness, and state of nature (solid to semi-solid) changed. It is observed that the pores present in the wood are reduced after the magnetic treatment process, as the swelling in the specimen takes place, as shown in Figure 3(b). Due to the presence of pores in the wood/polymer, the stress transfer cannot occur uniformly, which leads to failure. The image in Figure 3(b) was taken from the flexural tested specimen.Figure 3
SEM image showing: (a) E-glass mat and polymer, (b) pores in wood, (c) the crack in wood and polymer, and (d) fibre pull-out from polymer.
(a)(b)(c)(d)It is also observed that as immersion time increases, the pores in the wood will reduce further. It has more impact on the performance of EM wave-absorbing nature, as shown in Figure3(c). But at the same time, the presence of pores in wood/polymer material leads to failure of the specimen, as shown in Figure 3(c), and the particular wood has less strength.The crack propagation starts from the pores present in the wood; due to the acidic nature of ferrous chloride, ferric, and ammonia solution, there was partial decomposition of wood, and the shape/texture of wood changes. After the experimentation and the specimen from Figure3(d), it is clear that the fibre is pulled out from the polymer material at the failure of the specimen.


## 2.1. Materials

The core region of the oak wood tree’s main branch was considered a filler material for the present work. The other materials are epoxy resin (Aralide AW 106), E-glass fibre (woven fabric 310 gsm), and hardener (HV 953 U) with the required proportions were considered.


## 2.2. Synthesis of Magnetic Wood

Oak particleboard was considered to fabricate the composites with300×300×5 mm dimensions for three proposed specimens. After washing with distilled water, it was immersed in the mixed solution 2 : 1 molar ratio of Fe3+ and Fe2+ solutions for 24 hours, 48 hours, and 72 hours at room temperature. Later, the three specimens are soaked in ammonia solution for the same duration and followed by cleaning with distilled water and drying in sunlight. As the immersion time increases, the thickness swelling also increases. After going through these stages, the wood appears like a light shade of red colour with partially semi-solids in nature. This is used as a filler material for these composites during the fabrication processes. The magnetic/oak wood is placed at the centre of the composites along with the specification, as shown in Table 1.Table 1
Specification of composites.
S. noImmersion time of oakwood in solution (hours)No. of E-glass matComposites1243Specimen 12486Specimen 23729Specimen 3


## 2.3. Fabrication of Magnetic Wood-Based Polymer Composites

The E-glass mat is placed one after the other for all three specimens with polymer as per the proposal. But, placing the magnetic/oak wood at the centre of composites with a sequence of order after 2nd, 3rd, and 5th glass mat for specimens 1, 2, and 3, respectively. A hand lay-up technique was chosen for the fabrication process for the proposed specimens, as shown in Figure 1. Finally, three composites were fabricated with 300×300 mm dimensions.Figure 1
Flowchart of fabrication process.


## 2.4. Characterization

The tensile test was done at room temperature on a universal testing machine based on American Society for Testing and Materials (ASTM) D 638-89 standards. The stiffness property was determined by conducting the flexural test on a universal testing machine with ASTM D 790-86 standards, and the impact test was performed using ASTM D 7136-15 standards. The mechanical properties were determined for the three specimens by choosing five different locations of each composite for experimentation, and the specimens are shown in Figures2(a), 2(b), and 2(c). The specimens with the dimensions 22.86×10.16 mm were considered to evaluate the EM wave-absorbing properties by WR 90 waveguide method, and the specimens are as shown in Figure 2(d). These properties were calculated in the frequency range of 8.2–12.4 GHz with Through-reflect-line (TRL) calibration by a vector network analyzer (R&S ZVB 20). The EM wave-absorbing properties measure based on Equations (1) and (2).
(1)RLdB=20Log10Zi−Z0Zi+Z0,(2)Z=Z0μrεrtanh2πfμrεrμ0ε0×t.HereZi is input impedance, Z0 is absorber interface 377 Ω, f is the frequency in GHz, and εrand μr are relative complex permittivity and permeability, where space ε0=8.854×10−12 F/m−1, μr relative permeability of dielectric material, εr relative permittivity of magnetic material, t thickness, μ0=4Π×10−7 H/m−1.Figure 2
(a) Fabricated specimens for tensile testing, (b) fabricated specimens for bending testing, (c) fabricated specimens for impact testing, (d) fabricated specimens for electromagnetic wave-absorbing testing.


## 2.5. Morphology of Composites

The SEM image of Figure3(a) was taken after the tensile test conducted on composite, and it is evident that fibre is pulled out from the polymer as load acting in the fibre direction. As per the scanning electron microscope (SEM) images (Figure 3(a)), it is also evident that wood, polymer, and E-glass mats have a good bonding. During the magnetic treatment procedure, the specimen is placed in the solution for 24 hours, 48 hours, and 72 hours separately. Hence, the specimen colour, thickness, and state of nature (solid to semi-solid) changed. It is observed that the pores present in the wood are reduced after the magnetic treatment process, as the swelling in the specimen takes place, as shown in Figure 3(b). Due to the presence of pores in the wood/polymer, the stress transfer cannot occur uniformly, which leads to failure. The image in Figure 3(b) was taken from the flexural tested specimen.Figure 3
SEM image showing: (a) E-glass mat and polymer, (b) pores in wood, (c) the crack in wood and polymer, and (d) fibre pull-out from polymer.
(a)(b)(c)(d)It is also observed that as immersion time increases, the pores in the wood will reduce further. It has more impact on the performance of EM wave-absorbing nature, as shown in Figure3(c). But at the same time, the presence of pores in wood/polymer material leads to failure of the specimen, as shown in Figure 3(c), and the particular wood has less strength.The crack propagation starts from the pores present in the wood; due to the acidic nature of ferrous chloride, ferric, and ammonia solution, there was partial decomposition of wood, and the shape/texture of wood changes. After the experimentation and the specimen from Figure3(d), it is clear that the fibre is pulled out from the polymer material at the failure of the specimen.


## 3. Results and Discussions

An EM wave absorber is needed to protect humans and the environment. The polymer composites of EM wave-absorbing ability with stable mechanically are essential in the application. Based on Equations (1) and (2), the EM wave-absorbing properties were calculated in terms of relative complex permeability, relative complex permittivity, tangent losses, and RLs. After that, a morphological study is required to study the failure of the composites.


### 3.1. Electromagnetic Properties

The EM properties of wood-based composites have been evaluated experimentally by using a vector network analyzer with waveguide methods. The relative complex permittivity and permeability of composites are shown in Figure4(a).Figure 4
(a) Real part of permittivity, (b) imaginary part of permittivity, (c) real part of permeability, (d) imaginary part of permeability.
(a)(b)(c)(d)To investigate the EM wave-absorbing properties, it is essential to know the complex permittivity and permeability behaviour in the frequency range of 8.2–12.4 GHz by using Equations (1) and (2). The real and imaginary part of complex permittivity represents the electric storage capability and loss capability [4]. These complex permittivity and permeability are generated from electronic polarization, ion polarization, intrinsic electric polarization, and magnetic properties and influence on the EM absorbing behaviour.As the conductivity nature of samples gradually increases by increasing the submerged time of the samples from 24 hours to 72 hours, the specimens have the highest value in both real and imaginary part permittivity. Hence with the combination of good impedance matching behaviour and strong ability for EM attenuation, the absorbing nature is achieved for 72 hours submerged specimen [4, 25].As shown in Figure4(a), the real part of permittivity is improved as the immersion time of the specimen increases. The complex permittivity values are higher for the 72 hours submerged specimen due to the influence of electric polarization and electric conductivity. The chemical bonds have oxygen content when a function with the exterior surface of wood, –OH, and –CO– will develop electric dipolar polarization. This polarization mechanism depends on the different valances of ions availability and electron transfer between Fe3+ and Fe2+ ions position [26, 28, 29].The trend followed in the graphs of the real and imaginary part of permittivity developed due to interfacing (interface polarization) between Fe3O4 cluster and the carbohydrates from the wood, as shown in Figures 4(a) and 4(b). Hence, due to the interference of polarization and dipole relaxation, there was an improvement in the EM properties [17, 29].The fabricated composites show a strong magnetic loss in the frequency range, and the real part of permeability is more than the imaginary part of complex permeability. It is evident from Figures4(c) and 4(d) that the real and imaginary part of complex magnetic permeability follows the same trend for the three specimens; as the immersion time increased, the permeability properties also improved.It can be clearly seen that the real part of the permeability decreased up to 10.4 GHz and later increased at a higher frequency. Due to the impact of Fe3+ and Fe2+ions in the wood, the real part of permeability increases above unity at a higher frequency and increases with increasing frequency. The imaginary part of permeability is close to zero in the frequency range of 9.6–12.4 GHz, as shown in Figure 4(d).


### 3.2. Tangent Losses by Dielectric and Magnetic Parameters

The absorbing nature of EM waves is usually influenced by the dielectric loss tangents and magnetic loss tangents [4]. Based on Figures 5(a) and 5(b), it is evident that the dielectric loss tangent value improves by the submerged time from 24 hours to 72 hours; a similar trend is followed for the magnetic loss tangent also. The magnetic loss tangent value for 72 hours submerged specimen has more RLs than the remaining specimens, and is increased with increasing frequency, as shown in Figures 5(a) and 5(b).Figure 5
(a) Dielectric loss tangents, (b) magnetic loss tangents.
(a)(b)The 72 hours submerged specimen has the highest loss tangent value of 0.44 at 8.2 GHz and goes on to improve in the frequency range. Hence, it is the highest ability to convert the EM wave to energy in other forms, which is essential in EM wave-absorbing properties. Based on Figures5(a) and 5(b), it is clear that the major contribution of magnetic parameter develops the EM wave-absorbing property then dielectric tangent loss due to eddy current loss [17, 29].Based on Figure5(a), the dielectric loss tangents are constant throughout the frequency range and are increased by increasing the submerged time. These losses are developed by having more content of Fe3O4 as conductive nature in the specimens.


### 3.3. Microwave Absorbing Properties

The RLs depend on parameters like filler material, the thickness of composites, the number of layers, etc. In the present work, wood-based polymer was present. Figure6 shows magnetic wood-based polymer composites’ measured/calculated RL properties for different submerged times.Figure 6
Reflection loss (RL).It is clear that, as the submerged time increased, the EM wave-absorbing nature also improved from 24 hours to 72 hours at the frequency range from 8.2 GHz to 12.4 GHz, respectively. Many existing works on magnetic wood-based composites will have 90% attenuation of EM wave, and wide absorbing bandwidth was attained when the |RL| is more than 10 dB [4, 26, 29]. From Figure 6 and Table 2, it is evident that the RL values are close to 10 dB.Table 2
Reflection loss (RL) for different composites.
Submerged time (hours)Absorbing peck (GHz)Peck value (dB)Band value (dB)2410.2−8.70−1.264810.2−8.99−0.257210.2−9.40−2.31The magnetic property was developed in the composites due to iron salt impregnation duration of 24 hours, 48 hours, and 72 hours. It is clear that the submerged time influences on the EM wave-absorbing ability. As the submerged time increases, a sharp improvement in RL is noticed as it affects EM wave-absorbing nature. This behaviour is achieved due to the well-ordered structure of Fe3O4, which is in good agreement with research [25, 17, 29], and present results are compared with the previous works as reflected in Table 3.Table 3
The reflection losses compared with the literature.
Ref.Filler material and methodReflection losses and frequencyPresent work4Oak wood and co-precipitation chemical interactions−64.26 dB at 14.36 GHzBy co-precipitation chemical interactions for magnetic wood synthesis process and by hand lay-up method, the best results of RL are obtained at 10.2 GHz of −9.40 dB.11Wood and (1) coating type (Ni–Zn ferrite powder coated onto a fibre board), (2) powder type (Ni–Zn ferrite powder and cider wood powder mixed and pressed into boards), (3) impregnated type (cider wood impregnated with a water-based magnetic fluid), (4) sandwich type (combination structure consisting of a fibre board layer/magnetic binder layer/fibre board layer).At 1.93 GHz, the RL as −16.69 dB, −13.83 dB, −3.64 dB.13Coating-type magnetic wood−14.56 dB at 10.0 GHz14Oak wood and co-precipitation chemical interactions−14.14 dB at 7 GHz20Powder-type magnetic wood−20 dB over the 2.5–6.2 GHz


## 3.1. Electromagnetic Properties

The EM properties of wood-based composites have been evaluated experimentally by using a vector network analyzer with waveguide methods. The relative complex permittivity and permeability of composites are shown in Figure4(a).Figure 4
(a) Real part of permittivity, (b) imaginary part of permittivity, (c) real part of permeability, (d) imaginary part of permeability.
(a)(b)(c)(d)To investigate the EM wave-absorbing properties, it is essential to know the complex permittivity and permeability behaviour in the frequency range of 8.2–12.4 GHz by using Equations (1) and (2). The real and imaginary part of complex permittivity represents the electric storage capability and loss capability [4]. These complex permittivity and permeability are generated from electronic polarization, ion polarization, intrinsic electric polarization, and magnetic properties and influence on the EM absorbing behaviour.As the conductivity nature of samples gradually increases by increasing the submerged time of the samples from 24 hours to 72 hours, the specimens have the highest value in both real and imaginary part permittivity. Hence with the combination of good impedance matching behaviour and strong ability for EM attenuation, the absorbing nature is achieved for 72 hours submerged specimen [4, 25].As shown in Figure4(a), the real part of permittivity is improved as the immersion time of the specimen increases. The complex permittivity values are higher for the 72 hours submerged specimen due to the influence of electric polarization and electric conductivity. The chemical bonds have oxygen content when a function with the exterior surface of wood, –OH, and –CO– will develop electric dipolar polarization. This polarization mechanism depends on the different valances of ions availability and electron transfer between Fe3+ and Fe2+ ions position [26, 28, 29].The trend followed in the graphs of the real and imaginary part of permittivity developed due to interfacing (interface polarization) between Fe3O4 cluster and the carbohydrates from the wood, as shown in Figures 4(a) and 4(b). Hence, due to the interference of polarization and dipole relaxation, there was an improvement in the EM properties [17, 29].The fabricated composites show a strong magnetic loss in the frequency range, and the real part of permeability is more than the imaginary part of complex permeability. It is evident from Figures4(c) and 4(d) that the real and imaginary part of complex magnetic permeability follows the same trend for the three specimens; as the immersion time increased, the permeability properties also improved.It can be clearly seen that the real part of the permeability decreased up to 10.4 GHz and later increased at a higher frequency. Due to the impact of Fe3+ and Fe2+ions in the wood, the real part of permeability increases above unity at a higher frequency and increases with increasing frequency. The imaginary part of permeability is close to zero in the frequency range of 9.6–12.4 GHz, as shown in Figure 4(d).


## 3.2. Tangent Losses by Dielectric and Magnetic Parameters

The absorbing nature of EM waves is usually influenced by the dielectric loss tangents and magnetic loss tangents [4]. Based on Figures 5(a) and 5(b), it is evident that the dielectric loss tangent value improves by the submerged time from 24 hours to 72 hours; a similar trend is followed for the magnetic loss tangent also. The magnetic loss tangent value for 72 hours submerged specimen has more RLs than the remaining specimens, and is increased with increasing frequency, as shown in Figures 5(a) and 5(b).Figure 5
(a) Dielectric loss tangents, (b) magnetic loss tangents.
(a)(b)The 72 hours submerged specimen has the highest loss tangent value of 0.44 at 8.2 GHz and goes on to improve in the frequency range. Hence, it is the highest ability to convert the EM wave to energy in other forms, which is essential in EM wave-absorbing properties. Based on Figures5(a) and 5(b), it is clear that the major contribution of magnetic parameter develops the EM wave-absorbing property then dielectric tangent loss due to eddy current loss [17, 29].Based on Figure5(a), the dielectric loss tangents are constant throughout the frequency range and are increased by increasing the submerged time. These losses are developed by having more content of Fe3O4 as conductive nature in the specimens.


## 3.3. Microwave Absorbing Properties

The RLs depend on parameters like filler material, the thickness of composites, the number of layers, etc. In the present work, wood-based polymer was present. Figure6 shows magnetic wood-based polymer composites’ measured/calculated RL properties for different submerged times.Figure 6
Reflection loss (RL).It is clear that, as the submerged time increased, the EM wave-absorbing nature also improved from 24 hours to 72 hours at the frequency range from 8.2 GHz to 12.4 GHz, respectively. Many existing works on magnetic wood-based composites will have 90% attenuation of EM wave, and wide absorbing bandwidth was attained when the |RL| is more than 10 dB [4, 26, 29]. From Figure 6 and Table 2, it is evident that the RL values are close to 10 dB.Table 2
Reflection loss (RL) for different composites.
Submerged time (hours)Absorbing peck (GHz)Peck value (dB)Band value (dB)2410.2−8.70−1.264810.2−8.99−0.257210.2−9.40−2.31The magnetic property was developed in the composites due to iron salt impregnation duration of 24 hours, 48 hours, and 72 hours. It is clear that the submerged time influences on the EM wave-absorbing ability. As the submerged time increases, a sharp improvement in RL is noticed as it affects EM wave-absorbing nature. This behaviour is achieved due to the well-ordered structure of Fe3O4, which is in good agreement with research [25, 17, 29], and present results are compared with the previous works as reflected in Table 3.Table 3
The reflection losses compared with the literature.
Ref.Filler material and methodReflection losses and frequencyPresent work4Oak wood and co-precipitation chemical interactions−64.26 dB at 14.36 GHzBy co-precipitation chemical interactions for magnetic wood synthesis process and by hand lay-up method, the best results of RL are obtained at 10.2 GHz of −9.40 dB.11Wood and (1) coating type (Ni–Zn ferrite powder coated onto a fibre board), (2) powder type (Ni–Zn ferrite powder and cider wood powder mixed and pressed into boards), (3) impregnated type (cider wood impregnated with a water-based magnetic fluid), (4) sandwich type (combination structure consisting of a fibre board layer/magnetic binder layer/fibre board layer).At 1.93 GHz, the RL as −16.69 dB, −13.83 dB, −3.64 dB.13Coating-type magnetic wood−14.56 dB at 10.0 GHz14Oak wood and co-precipitation chemical interactions−14.14 dB at 7 GHz20Powder-type magnetic wood−20 dB over the 2.5–6.2 GHz


## 4. Mechanical Properties

The objective of the present work is to have both EM wave-absorbing properties and good mechanical properties for the fabricated composites. With reference to Table1, developing the good mechanical properties for the composites is done with two parameters: one by increasing the E-glass mat and the second by submerging process where the moisture amount is increased. As oak wood is immersed in the solution for 24 hours, 48 hours, and 72 hours, the moisture content significantly affects the composites’ mechanical performance. For the fabricated composites, the tensile, bending, and impact tests were conducted as per the ASTM standards [30].Based on the tensile strength, the basic information about the behaviour of the composites can be known. It is observed from Figure7(a) that the tensile strength gradually increased from specimen 1 to specimen 3. Specimen 2 is improved by 41.25% compared to specimen 1 and similarly, specimen 3 is enhanced by 5% compared to specimen 2. The strength gradually improved from specimen 1 to 3 due to the improved stage of stress transfer ability between wood, E-glass mat, and polymer. The improvement in the tensile strength occurs mainly for two reasons, i.e., one by increasing the number of E-glass mats from specimen 1 to 3, and warp fibres in composites are more in number to the direction of applied load in the universal testing machine [31–32]. The second reason is for having good bonding between polymer and wood, as shown in Figure 7(b). Hence, this study can conclude that wood filler-based polymer composites also exhibit a maximum tensile stress value.Figure 7
(a) Tensile strength for three different specimens, (b) bending strength for fabricated composites, (c) impact strength for fabricated composites.
(a)(b)(c)Among the three specimens, specimen 3 has more stiffness, as represented in Figure7(b). Specimen 2 is improved by 30.38% strength with reference to specimen 1, and the flexural strength of specimen 3 is improved by 68.05% compared to the previous specimen. The improvement was observed due to the following reasons. A more significant number of warp fibres are aligned along the specimen’s longitudinal direction; to these fibres, the load is applied in the transverse direction. By increasing the number of E-glass mats also, more stiffness is developed in specimen 3 compared with other specimens.A similar trend is followed in the case of the impact strength also. Specimen 2 is improved by 25.77%, and specimen 3 is improved by 22.95% compared to the previous specimens, as shown in Figure7(c). As the bending strength and impact strength improve from specimen 1 to 3, it is clear that the composites have the capability to transfer the stress between matrix and filler. Hence, this leads to improvement in mechanical properties. It can be stated that the moisture present in the polymer composites impacts the mechanical properties.


## 5. Regression Analysis

Regression analysis is one of the simple techniques for finding the functional relationship among the variables and the influence of the variables on the response. The relation between the variables and the response will be generated based on the equation. In the present work, the input variables reflect losses and mechanical properties. In that process, the regression equation was developed for tensile strength, flexural strength, hardness, and EM wave-absorbing properties.


### 5.1. Development of Regression Equation for Electromagnetic Wave Absorption Properties

Based on Figure8, it is clear that the experimental data is very close to the regression data and with high accuracy (R2=100%). The Equation (3) is generated for experimental data based on regression analysis, with immersion time as the input parameter and RL as the output parameter. The proposed regression equation for RL has been predicted and valued with 100% accuracy to experimental data.
(3)ReflectionlossdB=−8.3967−0.012500immersiontimehours,Figure 8
Fitted line plot of reflection loss.


### 5.2. Development of Regression Equation for Mechanical Properties

The experimental data are compared to the regression data and it is noticed that both are in good agreement, as shown in Figures9(a), 9(b), and 9(c). The error in the experimental data is also calculated and significantly less based on the R2 value. R2 is approximately close to 99.1%, which indicates that the accuracy level is more for the experimental data, as represented in Figures 10(a), 10(b), and 10(c).Figure 9
(a) Comparison of regression data and experimental data for tensile strength, (b) comparison of regression data and experimental data for bending strength, (c) comparison of regression data and experimental data for impact strength.
(a)(b)(c)Figure 10
(a) Tensile strength data fitted to line, regression equation andR2 value, (b) bending strength data fitted to line, regression equation and R2 value, (c) impact strength data fitted to line, regression equation and R2 value.
(a)(b)(c)Based on Table4, the equations were generated for experimental data by regression analysis with immersion time as input parameter and mechanical properties (tensile strength, bending strength, and impact strength) as output parameter. The proposed regression equations for mechanical properties have been predicted and valued with 99% accuracy to experimental data.Table 4
Regression equation for mechanical properties.
Mechanical propertiesRegression equationR2 valueTensile strengthTensile strength = 31.34 + 0.3828 immersion time0.991Bending strengthBending strength = 31.69 + 2.176 immersion time0.991Impact strengthImpact strength = 746.7 + 11.78 immersion time0.999


## 5.1. Development of Regression Equation for Electromagnetic Wave Absorption Properties

Based on Figure8, it is clear that the experimental data is very close to the regression data and with high accuracy (R2=100%). The Equation (3) is generated for experimental data based on regression analysis, with immersion time as the input parameter and RL as the output parameter. The proposed regression equation for RL has been predicted and valued with 100% accuracy to experimental data.
(3)ReflectionlossdB=−8.3967−0.012500immersiontimehours,Figure 8
Fitted line plot of reflection loss.


## 5.2. Development of Regression Equation for Mechanical Properties

The experimental data are compared to the regression data and it is noticed that both are in good agreement, as shown in Figures9(a), 9(b), and 9(c). The error in the experimental data is also calculated and significantly less based on the R2 value. R2 is approximately close to 99.1%, which indicates that the accuracy level is more for the experimental data, as represented in Figures 10(a), 10(b), and 10(c).Figure 9
(a) Comparison of regression data and experimental data for tensile strength, (b) comparison of regression data and experimental data for bending strength, (c) comparison of regression data and experimental data for impact strength.
(a)(b)(c)Figure 10
(a) Tensile strength data fitted to line, regression equation andR2 value, (b) bending strength data fitted to line, regression equation and R2 value, (c) impact strength data fitted to line, regression equation and R2 value.
(a)(b)(c)Based on Table4, the equations were generated for experimental data by regression analysis with immersion time as input parameter and mechanical properties (tensile strength, bending strength, and impact strength) as output parameter. The proposed regression equations for mechanical properties have been predicted and valued with 99% accuracy to experimental data.Table 4
Regression equation for mechanical properties.
Mechanical propertiesRegression equationR2 valueTensile strengthTensile strength = 31.34 + 0.3828 immersion time0.991Bending strengthBending strength = 31.69 + 2.176 immersion time0.991Impact strengthImpact strength = 746.7 + 11.78 immersion time0.999


## 6. Conclusion

Polymer-based composites with magnetic wood by hand lay-up method have been proposed to study the mechanical and EM absorbing properties. The EM absorbing phenomenon was investigated in terms of RL for a different submerged time of specimen. The mechanical behaviour was also studied for specimens as the moisture content absorbed and increased the number of E-glass mats as the influencing parameters. From the results, the following findings have been derived.The EM wave-absorbing properties are increased with an increase in the submerged period (24–72 hours) of wood. It is predicted that 72 hours submerged wood specimen has the maximum absorbing capability in X-band. The highest RL value of −9.40 dB is obtained at 10.2 GHz for specimen 3 due to the well-ordered structure of Fe3O4. The tensile strength is improved by 48.31% compared with specimen 1. This is the fact that good bonding between filler and matrix materials. Due to an increase in the number of E-glass mats and bonding parameters, the flexural strength is improved by 90% compared to specimen 1.The impact strength is increased by 54.64% due to moisture content absorbed and the bond between polymer and glass fibres. By the SEM image, it is clear about the failure analysis of composites. The experimental data (EM absorbing behaviour and mechanical properties) are very close to the regression data and are in good agreement with 99% accuracy.



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*Source: 1142654-2023-02-01.xml*