Biopolymer-coated composites for enhanced dielectric and electromagnetic interference shielding applications - a green initiative

The utilization of natural fibre-reinforced polymer composites has been tremendously growing in various applications of automotive and aerospace components. In this aspect, the researcher’s community is approaching the global market with new ideas for developing a complete eco-friendly, sustainable, and green composite. Plant-based composites have received great interest from the initial stage due to their unique features, such as lightweight, corrosion resistance, specific properties, excellent mechanical and thermal properties. This research article attempts a novel technique of coating the fibres with polylactic acid (PLA) as a part of surface modification which improves fibre properties. Then the fibres were reinforced with various weight percentages of conductive fillers, such as Copper (Cu), Alumina (Al2O3), and Graphene (Gr), to improve the electrical properties using the hand layup technique. Then the fabricated samples were tested for dielectric and electromagnetic interference (EMI) shielding effectiveness (SE) using resonance and open shielded method. Based on the test results, it was noted that the dielectric strength (K) and shielding effectiveness (SE) of the composites started to increase with the increase of weight percentage of conductive fillers, which highlighted that by incorporating conductive fillers, the fibres started losing their insulation properties. The composites with 0.9 wt% of nanofillers achieved maximum SEabs of −19.61 dB and a SEtotal of −22.67 dB at a frequency range of 8–12 GHz.


Introduction
Universally, after the outbreak of the COVID pandemic, automotive industries have faced a more severe crisis than ever in history.But, since the past few months, the situation has been slowly recovering, and the demand for the automobile industry and automotive components has increased by 25%.After replacing conventional materials like cast iron and aluminium in automotive components, thermosetting plastics have occupied the worldwide market.Recently, researchers have predicted that nearly 18% of global wastage will be occupied by automobile parts such as rubber tyres, glass, plastic, and metals in the future [1][2][3].Sharma et al (2016) presented a review article concerning automobile waste and its management methods.This research article states that 20 million tons of plastics and polymers from automotive industries have been dumped annually into the land, making them unfit for agricultural purposes and other uses [4].So, the entire research community evinced their boundless interest and massive concern towards waste management and environmental protection primarily caused by plastics [5][6][7].However, finding a better replacement material than plastics and conventional materials is another challenging task for all the researchers as the existing material has firmly rooted its prints with their excellent properties [8,9].Therefore, natural fibres, also called plant-derived fibres, have started making their footmarks as a suitable replacement material.Features like eco-friendly, cheap, non-toxic, lightweight, durable, and sustainable make natural fibres more attractive when compared with synthetic material.Also, natural fibres equally compete with synthetic fibres with their stunning mechanical and thermal properties [1,[10][11][12].
Conversely, on the other hand, it was also noticed that there was a tremendous shift in the growth of the electronic and electrical industries [13].A Global Consumer Electronic Market (GCEM) report documented that the market shares grew by 38% in 2020-2022.The main reason behind this dramatic change is due to the work-from-home trend resulting in the high adoption of consuming computing products [14].It is also estimated that the advent of technology and digitalization is further likely to increase the demand for electronic items and domestic appliances by 15% by the end of 2027 [15].But due to this rapid usage of electronic devices, a unique form of pollution is produced from these devices, known as Electromagnetic Interference [16].This form of pollution has become a serious threat to electronic gadgets as they can reduce the performance of the circuits [17,18].Sometimes this kind of EMI radiation can significantly damage human life.From this perspective, various electronic components are built to improve advanced technological features after the arrival of electric vehicles [19].Hence it is predicted that these devices can play a massive part in creating electromagnetic radiation in a vehicle which can eventually cause damage to the electronic components creating colossal damage to electronic circuits and components [20].So, to overcome this issue, a suitable shielding material should be adopted for the vehicle's interior and exterior parts, including radio, navigation systems and other electronics [21].There are different shielding materials in the market, which include metals, conductive coatings and laminates which are immensely used in automotive industries.Hence, EMI shielding helps protect these devices from interference caused by an EM wave [20,22].
Kenaf, scientifically called Hibiscus Cannabinus from the family of Malvaceous, derived from the stem part of the plant, has been widely considered the most deserved alternative to jute fibres [1,3,23].Unique features like short plantation cycles, flexibility, superior mechanical and thermal properties, and high performance compared with the other fibres make them more suitable as a sustainable composite material.Though natural fibres have many advantages, their disadvantages are poor interfacial bonding, moisture absorption, sensitivity to climatic changes and poor electrical properties that make them stop achieving great heights in advanced applications [24,25].So, researchers are making necessary modifications in the matrix and reinforcement phases which can reduce properties like hydrophilic nature and improves electrical properties making natural fibres into a synergic composite material [2,26].
Alkaline treatment, also known as the surface modification technique adopted in reinforcement material, enhances the interfacial bonding between the matrix and the reinforcement [27,28] and redefines the fibres' topography [29][30][31].Ismail et al (2021) investigated the effect of alkali treatment on kenaf fibres' physical, mechanical, and thermal properties.This study revealed that the alkali-treated fibres exhibited higher thermal stability at high temperatures than untreated fibres [32].Sathish Kumar et al (2020) noticed that fibres treated with 6% alkali-treated fibres improved the surface adhesive nature by increasing the surface roughness of the fibre as a result of improving the strong interfacial bonding of fibres by alkali treatment [33].To make composites more efficient and biocompatible, few researchers have attempted biopolymer coating over the surface of fibres to boost their hydrophobicity, which eventually enhances the mechanical properties of the biocomposites [34][35][36].Gupta et al (2018) studied the effect of PLA-coated sisal fibre reinforced in the polyester matrix on various static and dynamic mechanical properties fabricated via the hand layup technique.This study established that alkali-treated and PLA-coated sisal composites showed excellent mechanical, dynamic mechanical, and water absorption properties [34].Gupta et al (2018) used jute fibres that had undergone an alkali treatment and a PLA coating to overcome jute composites' drawbacks.This work claimed that composite materials made of coated and treated fibres performed best in improving mechanical, thermal, and water resistance qualities [37].Vikas et al (2023) has studied the dry sliding wear characteristics of natural fibre reinforced with PLA composites for advanced engineering applications.The experimental results depicted that incorporation of natural fibres enhanced the tribological properties of neat PLA matrix [38].
After modifications in reinforcement material, it is known that the matrix plays the most potent part in any polymer composite as they bind the fibre layers and transfer the load between them.Also, the matrix gives a proper net shape and determines the surface quality.So, improving the matrix's performance by incorporating fillers has been implemented, which directly enhances the properties of polymer composite [2].Vishnu et al (2017) investigated the mechanical, flexural and impact of flax fibre-reinforced epoxy composites with nano TiO 2 fillers.This article incorporated 6% TiO 2 filler, which projected the best mechanical, flexural and impact properties [39].Jayamani et al (2020) investigated the dielectric properties of natural coconut fibres filled with conductive copper fillers and reinforced with a Poly Lactic Acid (PLA) matrix.This work showed that adding copper fillers to the matrix could improve the dielectric properties and make them more suitable for electrical applications [40].
Anu et al (2022) investigated the EMI shielding properties of various natural fibres reinforced with multilayered epoxy composites.This research resulted that kenaf fibres exhibited in SE of −27 to −29 dB at 8-12 GHz (X-band) [41].Kritika et al (2020) investigated the influence of Iron oxide on graphene nanoparticles with epoxy composites.This study established that the overall nanocomposites, which had a higher amount of graphene and Iron oxide, exhibited enhanced EMI shielding performance with 89% attenuation of the incident wave and SE of 9.6 dB in 8-12 GHz (X-band) [42].Ning Li et al (2006) developed a lightweight EMI shielding composite of an epoxy-based single-walled carbon nanotube.Based on the results, it was found that the maximum shielding effectiveness was 15 wt% of single-walled carbon nanotubes (SWCNT)s with 49 dB at 10 MHz and SE around 15-20 dB in the 500 MHz to 1.5 GHz range [43].Deeraj et al (2022) investigated the electro-spun carbon fibres embedded with titanium carbide and titanium dioxide with epoxy composites.This study achieved effective EMI shielding properties of SE −33.31 dB at 12-18 GHz (Ku band) [44].
From what has been stated above, it can be concluded that only a few works have been carried out on natural fibre composites as electromagnetic interference shielding materials.Also, it can be noted that most of the experimental results are limited only to surface modifications such as alkaline treatment with NaOH and KOH etc. So, in this research article, we have proposed a novel technique of adopting biopolymer coating using PLA after alkaline treatment to improve the topography of the fibre, which helped eliminate the hydroxyl group and eventually reduce porosity and voids.Further, these fibres were reinforced with various percentages of conductive fillers to improve the electrical properties of the composite.Finally, these fabricated samples were tested for electromagnetic Interference (EMI) shielding effectiveness (SE) using a Vector Network Analyser at an X-band frequency range of 8-12 GHz.Also, these materials were subjected to testing for dielectric properties using the resonance method.

Materials and methods
The overall methodology carried out in this research article is shown in figure 1.The entire research work was carried out in two phases.In the first phase, surface modification, such as alkaline treatment and biopolymer coating, was carried out on the kenaf fibre mats.Then in the second phase, various weight percentages of conductive fillers were incorporated into the matrix and fabricated via the hand layup technique.Finally, all these samples were evaluated for dielectric constant and EMI shielding performance.Morphological characteristics such as Field Emission Scanning Electron Microscopy (FE-SEM) were carried out to evaluate the microstructure of nanofillers and kenaf fibres.Also, FE-SEM analysis was used for evaluating the dispersion of fillers into the nanocomposites.

Materials
Kenaf fibre mats of 280 gsm were purchased from Go Green Products Pvt. Ltd., Chennai, India.Epoxy resin [LY556], Hardener [HY951], Chloroform and Sodium pellets (NaOH) were purchased from Herenba Pvt.Ltd., Chennai, India.Biodegradable PLA pellets were purchased from Nature Work, Chennai, India.Fillers like graphite, copper oxide and aluminium oxide were purchased from Lab Chemicals Pvt. Ltd., Chennai, India.All these materials were directly used without further processing.

Kenaf fibre mats
Initially, non-woven 280 GSM Kenaf bidirectional fibre mats were cut to the necessary sizes, maintaining a 40% fibre weight percentage.The weights of each of these mats were also recorded for later processing once they were all separately weighed.The properties of kenaf fibres are presented in table 1.

Epoxy resin and hardener
Epoxy can be called the building block for composite materials.Reinforcement materials such as glass, carbon, plant fibres, and other materials maintain their structural integrity under tough conditions.When properly cured, epoxy resins provide several valuable qualities.Epoxy is resistant to heat and chemicals, has excellent tensile and compression strengths, gives minimal shrinkage during curing, and corrosion protection.The properties of the epoxy resin and the hardener are presented in table 2.

Polylactic acid pellets
Polylactic acids (PLA) are usually considered sustainable, biodegradable and environment-friendly biopolymers derived from renewable raw materials such as corn starch, potatoes, agricultural and sugarcane wastages.PLA can exhibit excellent mechanical properties with flexible processing at a low cost.PLA is widely used in the application of compostable food packaging materials and biomedical appliances.The properties of PLA pellets are given as the supplier details and tabulated in table 3.

Selection of nanofiller materials
Nanofillers such as Alumina (Al 2 O 3 ), Copper (Cu), and Graphene (Gr) were chosen for the fabrication of kenaf nanocomposites.Graphene is a two-dimensional carbonaceous crystalline allotrope which are known for its unique properties containing high optical transparency, conductivity with large flexibility, lightweight and high resistance within a strong nano size [45,46].It is estimated that graphene is 200 times more resistant than steel and five times light than aluminium.It is estimated that the size of graphene varies from 1.5-3 nm with thickness of 1.5 μm and conductivity of 1500-1980 S m −1 .Graphene fillers have excellent mechanical, thermal, and electrical properties and are widely used in biosensors, batteries, and multifunctional polymer composites [47,48].Copper oxide acts an excellent antimicrobial agent which are relatively cheap, and stable in regards with Mixing ratio Parts by weight 100 10 their chemical and physical properties.The nanoparticles of CuO are spherical with a size distribution varying from 10 to 80 nm with a thickness of 1.6-3.1 nm respectively.In material science aluminium oxide is extensively used as a non-metallic filler with excellent electrical and mechanical properties.Aluminium oxide is highly used as fillers, abrasives, catalysis, gas purification.

Alkaline treatment of kenaf fibres
After weighing the mats individually, the mats were carefully cleaned with distilled water and then dried in the sunlight to remove external impurities and specks of dirt.Meanwhile, 6% NaOH pellets were dissolved in 250 grams of distilled water.The cleaned mats were immersed in the prepared alkaline solution for 8 h.Then these mats were removed from the alkaline solution and washed with hydrogen peroxide solution to neutralise the alkali solution and remove the excess NaOH that could be adhered to the surface of the fibre mats.Finally, all the treated mats were dried for 4 h in a hot air oven at 60 °C.

Biopolymer coating of kenaf fibres
After the completion of the alkaline treatment, the treated fibre mats were taken for a biopolymer coating using polylactic acid pellets.Soya-based PLA pellets were chosen, and the fibre-to-pellet ratio was maintained at 100:16 grams.PLA solution was prepared by immersing PLA pellets into Acetone solution at 60 °C for 30 min in a fume hood chamber.To obtain an evenly dispersed PLA solution, the solution was manually stirred at regular intervals and heated to 60 °C.Care was taken to prevent the acetone solvent's evaporation by covering the beaker with paraffin film.Further alkali-treated kenaf mats were immersed in PLA solution for 3 min on each side, taken off from the solution, dried at room temperature for 48 h, and then dried again in a hot air oven for 4 h at 40 °C.

Fabrication of Kenaf fibre nanocomposites
Kenaf mats maintaining a weight fraction of 40 wt% were selected to prepare composite laminates.Initially, a stainless-steel plate was wrapped with a silicon-coated Teflon sheet to fabricate kenaf fibre nanocomposites.A thin layer of resin hardener was applied to the mat's surface for uniform wetting, and another mat was placed over it.The process was repeated for all the mats.To eliminate pores and voids, the laminates were taken further for degassing in the compression moulding machine for 45 min at 75 °C and 5 bar pressure.The entire set-up is once more cured for 48 h at room temperature under constant dead weights.Similarly, for the fabrication of kenaf nanocomposites, various proportions of fillers were reinforced with treated, PLA-coated fibres maintaining a weight fraction of 40% were chosen.Fillers such as aluminium oxide (Al 2 O 3 ), copper (Cu), and graphene (Gr) of weight percentages 0.3, 0.5, 0.7 and 0.9 wt% with an overall weight of the composite were individually weighed using a sensitive weighing balance.Then these fillers were added to epoxy resin and mechanically stirred for 40 min at 1000 rpm, followed by sonication for 30 min using an ultrasonicator for uniform dispersion of fillers.A similar procedure stated above is used to fabricate kenaf fibre nanocomposites.Table 4 represents the chemical composition of kenaf fibre nanocomposites.

Testing and morphological analysis
Electromagnetic interference (EMI) shielding effectiveness (SE) using vector analyser set-up The shielding effectiveness of the fabricated composite samples was measured.Measurement methods, such as an open field or free space method, shielded box method, shielded room method etc, are generally adopted for calculating the EMI shielding effectiveness of any material.However, in our research work, the open field or free space method, as shown in figure 2. was adopted to measure the shielding effectiveness of the fabricated samples.The mechanical set-up of EMI shielding consisted of a vector analyser of the model (N5230A PNA-L) with an operating range of 10 MHz to 50 GHz.The fabricated samples were tested with a frequency range of 8-12 GHz.The EM waves emitted from the source (VNA) recorded the transmitting and reflected radiation signals.
Generally, this measures the scattering parameters defined as S 11 and S 21 .
Usually, the main mechanism of EMI shielding theory states that when an EM wave from the source strikes the material's surface, it may undergo absorption, reflection or transmission.The total shielding effectiveness (SE total ) reflection coefficient (R), absorption (A) and Transmission (T) can be calculated using the below equations: where S 11 : spectrum parameter in open-air medium where S 12 : spectrum parameter of samples

log T dB or SE SE dB vi Total Shielding effectiveness SE 10 6
ref abs Dielectric constant (K) using the resonance method In our study, a novel dielectric device called the Portable Dielectric Measurement Kit (PDMK), shown in figure 3, has been used to measure the permittivity of a variety of semi-solid solids, granular, and liquid materials at or near the ISM frequency of 2.45 GHz.The variable capacitor's (C1) (100 pF) capacitance point throughout the experiment.After that, the sample is firmly placed between the plates.The sensitivity measurement will revert, at which point the variable capacitance will be set to its lowest value, in our case, 10.The reading will eventually cross the resonance point, and then, abruptly, it will bounce back once to the resonance point.Next, the variable capacitor is inspected.This is recorded as C2.The variable resistor is stopped at that point, and the reading is recorded as C3.Next, the (K) value is determined using the formula (dielectric constant).The same procedure is conducted for the remaining samples, keeping C1 constant.Similarly, these steps are repeated to  measure variable capacitors for the remaining samples, and the dielectric constant (K) is calculated as per equation (7).
The dielectric constant using capacitance can be calculated from the below equation ( 7) where, • C 1 -Capacity of the standard variable capacitor at resonance condition.
• C 2 -Capacity of the standard variable capacitor at resonance condition containing test capacitor with sample.
• C 3 -Capacity of the standard variable capacitor at resonance condition containing test capacitor without sample.

Morphological characterisation Field Emission scanning electron microscopy (FE-SEM)
The microstructure of nanofillers and fabricated kenaf fibre-reinforced polymer composites was evaluated using a Field Emission Scanning Electron Microscope.These morphological characterisations were used to evaluate the nano size and dispersion of nanofillers.Also, FE-SEM is used to evaluate the morphological structure of fibre composites after fabrication.

Results and discussion
Field emission scanning electron microscopy (FE-SEM) Field Emission Scanning Electron Microscope (FE-SEM) of model QUANTA 250 FEG was used to analyse the microstructure of nanofillers and fabricated kenaf fibre polymer composites.For the analysis of the samples, initially, all the fabricated samples were gold coated before the scanning process.showing that the fibres are free from impurities and voids.Also, we observe that the fibre has no pull-outs or breakage.Figure 4(e) shows the microstructure of the same sample, where can observe that though the composites are with few agglomerations of fillers, the fibres do not have pull-outs or debonding.Moreover, it is observed that the fibres are free of voids and porosity, which eventually increases the interfacial strength between the layers of the matrix and reinforcement which is clearly depicted in figures 4(f)-(g).Also, this microstructure supports that after the alkali treatment and PLA coating the fibres do not undergo any kind of breakage or debonding and surrounded with nanofillers.

Effect of electromagnetic interference shielding of kenaf fibre-reinforced nanocomposites
The experimental EMI set-up, as shown in figure 2 comprises three major components: the source antenna, which transmits incident EM waves; the receiver antenna, which receives the waves; and the vector network analyser, which helps to measure the scattering parameters of the wave.The fabricated samples are placed in the path between the source and the receiver to calculate the shielding effectiveness of the composites.So as per the EM shielding mechanism, when the EM wave is passed through the specimen, it is reflected and transmitted.As it is a porous composite laminate, multiple reflections are achieved, which is more similar to the absorption mechanism.S 11 parameters from the reflected wave and S 21 parameters from the transmitted wave are noted using the vector analyser, which helps in calculating the Shielding effectiveness of the material.Analysis of reflection, transmission and calculation of absorption coefficient is done from the above equations (2)- (7).
Table 5 presents the analytical calculation of reflection coefficient and shielding effectiveness resulting at free space medium, calculated from equations (1) and (4).Table 6 furnishes the EMI shielding values of different kenaf fibre-reinforced polymer composites, which can be derived from equations (2)-( 7).It can be observed from table 3 that samples S0, S1 and S2 have achieved significantly less EMI SE in terms of absorption and total shielding effectiveness at a frequency range of 8-12 GHz.This is due to the natural insulating property of natural fibres, which does not allow them to shield any EM waves transmitted from the source.Initially, in the case of untreated fibre sample S0, the shielding effectiveness in absorption and total shielding effectiveness was (SE abs )−7.97 and (SE total ) −14.35 dB, respectively.This concludes that the material could not shield the incident EM wave due to the absence of conductivity and permeability.Next, in the case of samples S1 and S2 we can observe a slight increase in shielding effectiveness in absorption and total shielding effectiveness, which were [(SE abs )−8.45 (SE total ) −14.65 dB] and [(SE abs )−9.01 (SE total ) −16.26 dB].This is because after alkaline treatment and biopolymer coating, these fibres are free from the hydroxyl groups, and the laminate's porosity and voids are reduced.Also, alkaline treatment and PLA coating are responsible for decreasing the interaction between the polar hydroxyl groups of lignocellulose and water molecules, which eventually increases the orientation of polarization.Further, it is observed that there is a dramatic increase in shielding effectiveness with an increase in the weight percentage of conductive fillers.Generally, it is known that an EM wave consists of magnetic and electric components which travel at the same frequency and do not exist individually.So, EMI shielding works under three primary mechanisms: absorption, reflection, and multiple reflections.Hence, when a material is placed in the path of an EM wave, the path is usually altered either by absorbing or reflecting the EM wave through conductive or ferromagnetic materials [49].
The composites which were incorporated with metals and conductive materials as nanofillers have the capability of blocking the electric components which are present in EM waves.Additionally, carbon allotropes such as graphene and carbon black can operate through multiple reflections mechanism as these materials have a high aspect ratio.Literature reports have mentioned that graphene and carbon allotropes are highly adopted for shielding applications because these materials' dimensions are less than the skin depth, making them penetrate more easily than metals, especially in the GHz range [50].This is the main reason that the filler content increases the conductivity of the composites, which are responsible for the attenuation of EM waves.Hence, as the fillers' weight percentage increases, the shielding effectiveness is also enhanced, as shown in figures 5 (a) and (b).That is at sample S6 with 0.9 wt% of alkali-treated biopolymer-coated nano filled laminates with maximum SE abs of −19.61 dB and SE total of −22.67 dB at a frequency range of 8-12 GHz.

Effect of dielectric constant on biopolymer-coated kenaf fibre-reinforced nanocomposites
The resistance that a material can provide against electric shock is inversely correlated with its dielectric constant.Therefore, finding the essential component of each parameter that can fulfil the composite attractiveness and result in a lower dielectric constant is crucial to produce suitable applications.With the aid of constant dielectric equipment, the varying capacitance of the fabricated samples was measured using the resonance method.First, the test sample's variable capacitor was measured, followed by the values of standard capacitors with and without dielectric material.Then, using the above equation (7), the average value of the dielectric constant was determined.Table 7 shows the dielectric constant values of all the laminated samples.The calculations showed that the dielectric constant of untreated kenaf fibre laminates was indicated as 4.1.This shows that the sample acts as a pure insulator as the fibre contains many hydrogen bonds and is hydrophilic.It is said that treated fibres' dielectric constant (1.6) is less than that of the untreated fibres as there is a decrease in polarisation direction, and this is also almost suitable for insulation purposes.This phenomenon increases the hydrophobicity of the treated fibres as it is well-known that alkaline treatment is carried out to remove hydroxyl groups in the cellulose of the fibre.Further, the dielectric constant of the treated and PLA-coated fibres sample  S3 has increased.This is due to the alkaline and PLA coating, which significantly reduces the fibres' moisture content and absorption ability, decreasing the interaction between the polar hydroxyl groups of lignocellulose and water molecules of the fibres.So, in the case of alkaline treatment, Na + ions can penetrate easily into fibres and destroy the hydrogen bonds, making them more reactive.Further, performing PLA coating reduces the hydrophobicity and increases the orientation of polarization.Also, literature reports that alkaline treatment followed by PLA coating or PLA as matrix increases the interfacial bonding between the fibre and the matrix.Also, it is noted that they play a massive role in reducing the voids and fibre debonding.This helps the treated and the PLA-coated samples become more favourable towards electrical characteristics.Finally, the dielectric constant of filler-filled composites tends to increase gradually with the weight percentage of fillers.It is a fact that the addition of conductive fillers is responsible for allowing ionic conductive into the fibres where polarisation takes place inside the composite, which may be electronic or dipole, which can cause a gradual increase in dielectric constant.Also, the dielectric properties shift towards the threshold percolation region, from insulation to conductive, making them more suitable for electrical applications.

Conclusions
A biopolymer coating was adopted in fabricating a bio composite using polylactic acid (PLA) derived from soya beans.Further, to enhance the electrical properties, the fibres were reinforced with various weight percentages of metallic and conductive fillers and were fabricated using the Hand layup technique.Finally, the samples were tested to evaluate the Electromagnetic shielding effectiveness (SE) and dielectric constant (K), and the following conclusions were made.Initially, in the first stage, the fibres were treated with 6% of NaOH and coated with polylactic acid (PLA) to make them free from the hydroxyl group and reduce the interaction between the hydroxyl group and cellulose of fibres which could eventually decrease the absorption of synthetic matrix material and increase the polarization.Based on the test results of EMI shielding, it was noticed that the shielding effectiveness (SE) of untreated, alkali-treated and biopolymer-coated samples was low.But it was observed that there was a slight increase in SE in terms of absorption as the biopolymer coating enhanced the polarization, which decreased the insulating properties.Further incorporating conductive fillers enhanced the conductive nature of the composite as it was noticed that there was a dramatic increase in shielding effectiveness with an increase in wt% of fillers.The sample with 0.9 wt% of Cu, Al 2 O 3, and Gr fillers achieved maximum SE abs of −19.61 dB and a SE total of −22.67 dB at a frequency range of 8-12 GHz.This showed that the composites were the best shielding material for EM waves.Based on the dielectric testing results, the dielectric constant (K) gradually increased with the increase in wt% of conductive fillers.This was concluded as the wt% of conductive fillers improved the transition of threshold percolation graph shifts from dielectric nature to a conductor.So, we could observe that the dielectric constant (K) of an alkaline-treated sample was 4.1 and 1.6, which indicated that the S0 and S1 was purely an insulating material.In contrast, the dielectric constant (K) of sample S6 with 0.9 wt% of nanofillers was 2.9, clearly showing a material transition from insulating to conductive.Finally, we concluded that the above research fabricated a biopolymer-coated kenaf fibre polymer composite that is more sustainable and eco-friendlier.Also, this research work highlights that the composites act as a potential shielding material for electromagnetic interference applications in automotive and aerospace fields.

Figure 1 .
Figure 1.The methodology carried out in the research work for the fabrication of biopolymer kenaf fibre-reinforced nanocomposite.

Figure 2 .
Figure 2. Experimental set-up of EMI testing using vector network analyser.
Figures 4(a)-(c) depict the microstructure of Al 2 O 3 , Cu and Gr nanofillers.The flaky structure in the figure 4(b) indicates the graphene nanofillers whereas the figures 4 (a) and (c) ensure the alumina and copper are spherical shaped.This microstructure images ensures that all the nanofillers used in the research work ranges from 20-30 nm.Figures4(d)-(e) depict the microstructure of chemically treated PLA-coated 0.9 wt% of nanofiller-reinforced kenaf fibre epoxy composites.Figure4(d) represents the microstructure of chemically treated kenaf fibre,

Figure 3 .
Figure 3. Experimental set-up for dielectric constant determination.

Figure 5 .
Figure 5. (a) Shielding Effectiveness in terms of absorption of biopolymer coated Kenaf nanocomposites.(b) Total Shielding effectiveness of biopolymer coated kenaf nanocomposites.

Table 2 .
Properties of epoxy resin and hardener.

Table 5 .
Analytical calculation of reflection coefficient and shielding effectiveness of free air medium.

Table 6 .
Analytical calculation of EMI SE for biopolymer coated kenaf fibre reinforced epoxy composites.